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Metal–Oxyl Species and Their Possible Roles in Chemical Oxidations

Yoshihiro Shimoyama
Yoshihiro Shimoyama
Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan
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Takahiko Kojima*
Takahiko Kojima
Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
*E-mail: [email protected]
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http://orcid.org/0000-0001-9941-8375

Cite this: Inorg. Chem. 2019, 58, 15, 9517–9542

Publication Date (Web):July 15, 2019

https://doi.org/10.1021/acs.inorgchem.8b03459

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Abstract

Metal–oxyl (Mⁿ⁺-O^•) complexes having an oxyl radical ligand, which are electronically equivalent to well-known metal–oxo (M⁽ⁿ⁺¹⁾⁺═O) complexes, are surveyed as a new category of metal-based oxidants. Detection and characterization of Mⁿ⁺-O^• species have been made in some cases, although proposals and characterization of the species are mostly done on the basis of density functional theory (DFT) calculations. The reactivity of Mⁿ⁺-O^• complexes will provide a way to achieve potentially difficult oxidative conversion of substrates. This Viewpoint will provide state-of-the-art knowledge on the Mⁿ⁺-O^• species in terms of the formation, characterization, and DFT-based proposals to shed light on the characteristics of the intriguing oxidatively active species.

Synopsis

Transition-metal−oxyl (Mⁿ⁺-O^•) complexes that are electronically equivalent to well-known metal−oxo (M⁽ⁿ⁺¹⁾⁺═O) complexes, with oxyl radical ligands, are surveyed as a new category of metal-based oxidants. A number of proposals on Mⁿ⁺-O^• species have been provided mostly on the basis of density functional theory calculations, although detection and characterization of Mⁿ⁺-O^• species are limited to several cases. The higher and unique reactivity of Mⁿ⁺-O^• species will allow us to expect the expansion of oxidation chemistry.

1. Introduction

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In biological and chemical oxidation reactions, high-valent metal–oxo (M═O) complexes have been known to act as reactive species.(1−12) So far, the first-, second-, and third-row transition-metal complexes with oxo ligands have been extensively studied with strong concerns on the structures,(13−20) electronic structures,(21−25) and reactivity in oxidation reactions of not only organic substrates(26−34) but also inorganic substrates including water.(35−39)

As previously described by Winkler and Gray, the “oxo wall” has been known to lie between the group 8 and 9 elements in the Periodic Table.(40) As for a metal–oxo complex in C_4v symmetry and the d⁴ electronic configuration, the complex can form a metal–oxo double bond; however, in the same symmetry, dⁿ (n ≥ 5) metal ions cannot afford metal–oxo species with double bonds because of occupation of the π* orbitals of the M═O bonds. Although some d⁵ and d⁶ metal–oxo complexes are in C_3v, C_2v, and C_∞v symmetry, these can form a metal–oxo double bond without obeying the “oxo wall” principle.(40b) In light of this concept, the elements earlier than group 8 in the tetravalent state can form double or triple bonds with an oxo (O^2–) ligand.(41) On the contrary, the elements later than group 9 in the tetravalent state cannot form multiple bonds and just form a single bond to afford metal–oxyl (M–O^•) complexes because of occupation of the π* orbital of the M–O bond.(42,43)

As for the metal–oxo complexes, the structures, properties, and reactivities in substrate oxidation reactions have been intensively investigated. For example, Fe^IV–oxo complexes bearing porphyrin π-radical cations as ligands have been investigated in light of modeling reactive species in oxidation reactions catalyzed by heme enzymes such as horseradish peroxidase (HRP)(44) and cytochrome P-450.(45) In addition, nonheme iron enzymes such as α-ketoglutarate-dependent taurine dioxygenase (TauD) have also been reported to generate Fe^IV–oxo intermediates as responsible species in oxidation reactions.(46) After a breakthrough report by Nam, Que, and their co-workers on the crystallographic characterization of a Fe^IV–oxo complex with a cyclic tetramine ligand,(47) a number of Fe^IV–oxo complexes have been prepared and characterized to shed light on the characteristics and reactivities of those complexes.(48) Not only Fe^IV–oxo complexes but also other metal–oxo complexes including Cr^V–oxo(49) and Mn^V–oxo(50) complexes have been studied as well.

Other than the first-row transition-metal ions, high-valent Ru–oxo complexes have been studied intensively. Major reasons for the investigation on Ru–oxo complexes should be the stability of the complexes to study the structures and reactivities in detail.(51) After the first report on the formation of a Ru^IV–oxo complex through proton-coupled electron-transfer (PCET) oxidation of the corresponding Ru^II–aqua precursor complex in water by Meyer and Moyer,(52) the procedure has been widely applied to various metal–oxo complexes including iron and manganese complexes.

Formal hydrogen-atom transfer (HAT) can be classified mechanistically as mentioned below into genuine HAT in a radical reaction and PCET.(53) PCET reactions can be elucidated in accordance with a thermochemical square scheme, as shown in Figure 1a:(54) PCET processes involve proton transfer followed by electron transfer, electron transfer followed by proton transfer, and concerted proton–electron transfer. Genuine HAT involves a single atomic site acting as a hydrogen-atom acceptor from a hydrogen-atom donor (Figure 1b). In PCET, however, the proton and electron are transferred to different atoms to accomplish a formal HAT reaction (Figure 1c).

Figure 1. (a) Thermochemical PCET square scheme for Y and X–H. The horizontal arrows represent electron-transfer (ET) processes, and the vertical ones represent proton-transfer (PT) processes. (b) Schematic representation of genuine HAT from H–X to Y^•. (c) Schematic representation of PCET from H–X to M–L.

Therefore, in hydrogen-atom abstraction by the metal–oxo species, the proton-acceptor site is the oxo ligand that is a formal O^2– ion and the electron-acceptor site is the metal center in affording the corresponding one-electron-reduced metal–hydroxo species, which can be classified as the PCET mechanism. On the other hand, hydrogen-atom abstraction by reactive species having spin density at the terminal oxygen atoms such as the metal–oxyl species can be classified as the HAT reaction because of the consistency of the acceptor site of the proton and electron. In terms of metal–oxyl complexes, however, there has been no rigorous definition for classification of the HAT/PCET mechanism. It should be noted that, concerning the reactivity of hydrogen-atom-accepting entities in HAT such as metal–oxo species, it has been commented that spin states of hydrogen-atom abstractors and the amount of the unpaired spin density on the abstracting atom do not affect the reactivity of the active species in formal HAT.(27b,55) In a very narrower definition of HAT, which is not separated as H⁺ and e^– from the “same bond” of a hydrogen-atom donor to an abstractor such as oxyl radicals including ^•OH and ^•OR, there should be a significant influence of the spin density at the oxygen atom upon a HAT process, as mentioned by Mayer and Saouma.(27b)

Besides a vast number of papers on high-valent metal–oxo complexes that have been published, metal–oxyl complexes, which are electronically equivalent species of the corresponding metal–oxo complexes with one-electron-reduced metal centers, have not been reported as often. Thus, examples of metal–oxyl complexes are limited; however, the species should exhibit different reactivities in substrate oxidation reactions compared to the metal–oxo complexes. We will provide a landscape of the transition-metal–oxyl complexes as potential reactive species with specific reactivity in oxidation reactions.

As for main-group elements, an oxygen ligand binds to the metal center in the highest valence states (Al^III, Mg^II, Ca^II, etc.); fulfilling the octet rule required in electronic configurations of the corresponding noble gases, the valency of the metal center does not change any more in most cases. Therefore, one-electron oxidation of main-group metal oxide species is expected to afford metal–oxyl species having oxygen-centered radicals, which enables one to discuss the reactivity of metal–oxyl species.(56) These characteristics allow easy access to information about HAT reactions as mentioned above. In this Viewpoint, however, we will focus on transition-metal–oxyl species and will not mention main-group metal–oxyl species.

2. Definition of a Metal–Oxyl Species

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As a contributing structure of a metal–oxo species having a double bond between the metal center and oxygen ligand, a metal–oxyl species should have nearly one unpaired electron or hole on the oxygen ligand and a metal center in a nearly one-electron-reduced oxidation state compared to the counterpart, as shown in Figure 2.(57) In the case of the oxyl ligand, the oxygen atom bound to the metal center does not satisfy the octet rule to possess formally seven electrons, as shown in Figure 2a. In this case, the π-bonding character of the metal–oxygen bond is reduced to single-bond character. It should be mentioned that the ratio of the contribution of each resonance structure influences the degree of oxyl character of the oxo ligand. Although the oxidation state of the metal center in the metal–oxygen species is not directly related to the M–O bond length, as observed for high-valent Ru–oxo complexes,(51) elongation of the M–O bond may be expected for the induction of metal–oxyl character to reduce π-bonding interactions (Figure 2b).

Figure 2. Schematic descriptions of the resonance structures of metal–oxo and metal–oxyl species with their corresponding Lewis structures. A red circle in part a represents a hole on the oxygen ligand.(57)

3. Experimentally Confirmed Metal–Oxyl Species

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Several metal–oxyl species have been satisfactorily characterized by various spectroscopic methods and supported by density functional theory (DFT) calculations. The following species can be recognized as satisfactorily characterized metal–oxyl species. The reactivities of the complexes will be also mentioned.

3.1. Zinc–Oxyl Species

Recently, Oda and co-workers have reported the formation of a Zn^II–oxyl (Zn^II–O^•) species stabilized in the MFI-type zeolite (Figure 3).(58a) As the precursor of the Zn^II–O^• species, a Zn^II-η²-ozonido (Zn^II–η²-O₃^•–) species has been formed by the reaction of dioxygen with a Zn^II–hydrido species, which is prepared by the Zn^II-exchanged MFI with dihydrogen via the H–H bond heterolysis, under UV irradiation. The Zn^II–ozonido species can be converted to the Zn^II–O^• species by removing dioxygen under vacuum at room temperature. The Zn^II–O^• species also reacts with dioxygen to form the Zn^II–ozonido species reversibly.

Figure 3. Schematic description of the formation of a Zn^II–oxyl species from a Zn^II–η²-O₃^•– species.(58a)

The Zn^II–O^• species has been characterized by various spectroscopic methods and DFT calculations. As shown in Figure 4, the species shows vibronic absorption bands around 12000 cm^–1 (∼830 nm) in the UV–vis–near-IR (NIR) spectrum. The weaker second band assigned to the E_0–1 vibronic transition shows an isotropic shift upon ¹⁸O labeling, and the separation changes from 605 to 575 cm^–1; the isotropic ratio is 1.05, consistent with the value calculated for ν(Zn–¹⁶O)/ν(Zn–¹⁸O) = {μ(Zn–¹⁸O)/μ(Zn–¹⁶O)}^1/2 = 1.05. This agreement supports the assignment of the absorption bands to the vibrational Franck–Condon progression in the Zn–O stretching mode.

Figure 4. UV–vis–NIR vibronic absorption spectra of the Zn^II–O^• complex formed in the MFI-type zeolite framework. (a) NIR regions for the Zn–¹⁶O^• (top) and Zn–¹⁸O^• species (bottom). (b) Schematic description of vibronic transitions of the Zn^II–O^• species. The dotted lines in part a represent the individual Gaussian contribution of the corresponding transition. This figure has been provided through a courtesy of Dr. A. Oda [PREST (JST)/Okayama University].

DFT calculations has been applied to optimize the ground state of Zn^II–O^• species in the MFI models, which consist of [Si₉₁Al₁O₁₅₁H₆₆]⁻ (Figure 5, left) and [Si₂Al₁O₄H₈]⁻ (Figure 5, right). The Zn^II–O^• moiety is coordinated by μ-oxo ligands bridging silicon and aluminum centers, as shown in Figure 5. The Zn–O bond lengths have been calculated to be 1.85 Å for part a and 1.86 Å for part b, respectively. Because the zinc(II) center in the d¹⁰ electronic configuration does not allow the formation of a Zn^II═O double bond, the spin distribution is completely localized on the oxygen ligand, suggesting oxyl character. Recently, Oda and co-workers have also reported on noncatalytic methane oxidation using the Zn^II–O^• species to form methanol in 94% selectivity at room temperature.(58b)

Figure 5. Optimized structures of Zn^II–oxyl species in the MFI models: in the [Si₉₁Al₁O₁₅₁H₆₆]⁻ (a) and [Si₂Al₁O₄H₈]⁻ (b) frameworks. This figure has been provided through a courtesy of Dr. A. Oda [PREST (JST)/Okayama University].

3.2. Ruthenium–Oxyl Complexes

Tanaka and co-workers have reported the first and structurally characterized example of metal-bound oxyl radical species, Ru^II–oxyl (Ru^II–O^•) complexes formed from ruthenium complexes having polypyridyl and semiquinonato ligands, [Ru^III(terpy)(4ClSQ)(OH₂)] (Ru^III_ClSQ–OH₂; terpy = 2,2′:6′,2″-terpyridine and 4ClSQ = 4-chloro-1,2-benzosemiquinonato) and [Ru^III(terpy)(DBSQ)(OH₂)] (Ru^III_DBSQ–OH₂; DBSQ = 3,5-di-tert-1,2-benzosemiquinonato) (Figure 6).(59) Upon the addition of a strong base such as KO^tBu into a CH₂Cl₂ solution of Ru^III_ClSQ–OH₂ or Ru^III_DBSQ–OH₂, UV–vis spectral changes due to deprotonation from the aqua ligand of the corresponding Ru^III–OH₂ complexes have been observed. On the basis of electron paramagnetic resonance (EPR), resonance Raman (rR), and X-ray photoelectron spectroscopy (XPS) analyses, the deprotonated products have been revealed to be [Ru^II(terpy)(DBSQ)(O^•)] (Ru^II_DBSQ–O^•) and [Ru^II(terpy)(4ClSQ)(O^•)] (Ru^II_ClSQ–O^•). The Ru^II–O^• complexes can be reversibly protonated with the addition of 2.0 equiv of HClO₄ to recover the corresponding Ru^III–OH₂ complexes. The EPR spectrum of Ru^II_DBSQ–O^• at 3.9 K exhibited an isotropic broad signal at g = 2.054 assigned to the Δm_s = 1 transition with a fine structure of |D| = 0.020 cm^–1 and |E| = 0.005 cm^–1 and an isotropic signal at g = 4.18 due to the forbidden Δm_s = 2 transition, indicating the triplet state of a biradical compound. On the basis of the |D| and g values, the spin–spin distance has been estimated to be 5.09 Å, which is consistent with the distance between the oxyl and DBSQ ligands. In the rR spectrum of Ru^II_DBSQ–O^• in CH₂Cl₂ upon photoexcitation at 704.3 nm, four peaks correlated with the ν(Ru–O) bands, which are coupled with the ν(C–C) bands of the DBSQ ligand, can be observed at 503, 521, 556, and 590 cm^–1.

Figure 6. Deprotonation of the Ru^III–OH₂ complexes to generate the corresponding Ru^II–O^• species.(59)

The Ru^II–O^• species Ru^II_DBSQ-O^• has been reported to be stable enough to form a single crystal suitable for X-ray crystallography. In the crystal structure of Ru^II_DBSQ–O^• (Figure 7), the Ru–O^• bond length was 2.043(7) Å, indicating a single-bond character of the Ru–O bond.(59a)

Figure 7. ORTEP drawing of the Ru^II–O^• species Ru^II_DBSQ–O^•.(59a) All hydrogen atoms are omitted for clarity. This figure has been provided through a courtesy of Prof. K. Tanaka and Dr. K. Kobayashi [Kyoto University].

The reaction of Ru^II_DBSQ–O^• with a spin-trapping reagent, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), affords a spin adduct showing an EPR signal with hyperfine splitting (hfs) derived from nitrogen and hydrogen nuclei at g = 2.006 at 193 K in CH₂Cl₂. This result supports the strong radical character of the Ru^II–O^• complex.(59b)

The Ru^II–O^• species have been applied to water oxidation(60) and organic substrate oxidation by a dinucleating strategy to connect the two Ru^II–O^• complexes mentioned above in a close distance.(61) The reaction mechanisms have been investigated experimentally or theoretically. In the water oxidation, Tanaka and co-workers have used quinone or 2,2′-bipyridine as bidentate ligands for a comparison of the water oxidation ability.(60b) Cyclic voltammograms of a semiquinonato complex, [Ru^II₂(btpyan)(Bu₂Sq)₂(OH)₂]²⁺ [Ru₂SQ; btpyan = 1,8-bis(2,2′:6′,2″-terpyridyl)anthracene and Bu₂Sq = 3,6-di-tert-butyl-1,2-benzosemiquinonato], and a 2,2′-bipyridine complex, [Ru^II₂(btpyan)(bpy)₂(OH)₂]²⁺ (Ru₂bpy), in the presence of water (10% v/v) in a CF₃CH₂OH/ether (1/1, v/v) solution showing large catalytic currents at potentials more positive than +1.0 V have been observed in both cases. In particular, the controlled-potential electrolysis of Ru₂SQ at +1.70 V evolved 0.69 mL of dioxygen in 91% current efficiency. Water oxidation using indium–tin oxide (ITO) electrodes modified with Ru₂SQ or Ru₂bpy showed a sharp contrast to each other in their catalytic reactivities. The controlled-potential electrolysis of Ru₂SQ supported on an ITO electrode at +1.70 V in acidic water, 1.5 mL of dioxygen with the current efficiency of 95%, and a turnover number of 2400 has been formed after 27.5 C passed in the electrolysis, whereas electrolysis of Ru₂bpy showed only a negligible amount of dioxygen evolution. Although it has been concluded that the distinct difference in the water oxidation abilities is associated with the function of the redox-noninnocent character of the quinone ligand, the significance of an oxygen-centered radical has been investigated by theoretical analyses as described below.

It is easily considered that two Ru^II–O^• species located closely in the ligand framework will enable radical coupling between these two oxyl radicals to form an O–O bond. However, upon the addition of 2 equiv of ^tBuOK to deprotonate from hydroxo ligands in Ru₂SQ, a decrease of absorption at 576 nm and the growth of a new band at 850 nm, which are indicative of the generation of Ru^II–O^• species, are observed. This result indicates that a tetraradical complex, [Ru^II₂(btpyan)(Bu₂Sq)₂(O^•)₂], still remains as a stable compound without undergoing O–O bond formation despite its remarkable catalytic activity toward water oxidation.(60f) The most plausible mechanism of O–O bond formation is considered to be the radical coupling between two oxyl radical species induced by spin inversion (SI) at the ruthenium(III) centers based on theoretical studies, as shown in Figure 8. Because the tetraradical species are in a local triplet diradical (LTD) state, facile O–O bond formation is suppressed, as explained by Hund’s rule. Then, further two-electron oxidation of [Ru^II₂(btpyan)(Bu₂Sq)₂(O^•)₂] will afford the open-shell ruthenium(III) complex [Ru^III₂(btpyan)(Bu₂Sq)₂(O^•)₂]²⁺ having six unpaired electrons. The spins on these ruthenium(III) centers will undergo facile SI because of the heavy-atom effect, which resulted in the formation of a local singlet diradical (LSD) state that proceeds radical coupling between the two oxyl radical species to form the O–O bond (Figure 8).(60e)

Figure 8. Plausible mechanism of O–O bond formation in Ru₂SQ proposed by Tanaka and co-workers.(60e)

The reactivities in substrate oxidation have been compared between dinuclear [Ru^II₂(OH)₂(Q)(btpyan)]²⁺ [Ru₂Q; Q = 3,6-di-tert-butyl-1,2-benzoquinone and btpyan = 1,8-bis(2,2′:6′,2″-terpyridyl)anthracene] and mononuclear [Ru^II(OH₂)(Q)(Ph-terpy)]²⁺ (RuQ; Ph-terpy = 4′-phenyl-2,2′:6′,2″-terpyridine).(61) The Ru^II–O^• complexes as reactive species can be formed from both complexes by using Ag⁺ as a mild oxidant and t-BuOK as a base, as shown in Figure 9. Ru₂Q could oxidize 1,3-cyclohexadiene and 1,2-dihydronaphthalene to afford benzene and naphthalene within 1 min, whereas RuQ could oxidize these substrates in low yields. On the other hand, Ru₂Q cannot oxidize 9,10-dihydroanthracene (DHA) at all, whereas RuQ shows relatively high reactivity in the DHA oxidation to afford anthracene in 42% yield. The inability of Ru₂Q to oxidize DHA apparently results from steric hindrance in the approach of the substrate to the oxo group in the cavity of the dimeric linkage.

Figure 9. Oxidation of hydrocarbons by di- and mononuclear Ru^III–hydroxo–quinone complexes (Ru₂Q and **RuQ**) in the presence of AgClO₄ and ^tBuOK.(61)

Recently, Kojima and co-workers have reported a novel Ru^III–oxyl (Ru^III–O^•) complex, an electronically equivalent structure of the corresponding Ru^IV–oxo (Ru^IV═O) complex, formed via PCET oxidation of a Ru^II–aqua (Ru^II–OH₂) precursor complex having an N-heterocyclic carbene (NHC) ligand, 1,3-bis(2-pyrdylmethyl)imidazolin-2-ylidene (BPIm), in acidic water (Figure 10).(62) Generally, in light of the “oxo wall” paradigm, it has been recognized that a double bond should be formed between the d⁴ ruthenium(IV) center in the octahedral geometry and the oxo ligand.(51) The key point of formation of the Ru^III–O^• complex is the strong trans influence of the NHC ligand bound to the position trans to the oxo ligand because it has been widely known that a strongly σ-donating ligand such as NHC makes coordination of a ligand at the trans position of the NHC moiety weaker because of its trans influence.(63) This concept has been applied to elongate the Ru–O double bond in a Ru^IV═O complex to generate the Ru^III–O^• complex.

Figure 10. PCET oxidation of the Ru^II–OH₂ complex to afford a Ru^III–O^• species.(62)

Characterization of the Ru^III–O^• complex has been conducted by several spectroscopic analyses. In the rR spectra, a Raman band derived from a stretching vibration of the Ru–O bond is observed at 732 cm^–1, which is shifted at 696 cm^–1 in ¹⁸O-labeled water. The value of 732 cm^–1 is the lowest value in comparison with those of any other Ru^IV═O complexes reported so far, which indicates a significantly weaker Ru–O bond in the Ru^III–O^• species. In X-ray absorption spectroscopy, an energy shift of a half-height energy of the Ru–K absorption edge has been reported to be 1.5 eV from the corresponding Ru^II–OH₂ to the Ru^III–OH₂ complex. On the other hand, the energy shift from Ru^III–OH₂ to Ru^III–O^• is only 0.5 eV. These results indicate that the valency of the ruthenium center is almost 3+ rather than 4+. The characteristics of the Ru^III–O^• complex could be revealed by DFT calculations at the UB3LYP/SDD (ruthenium) and D95** (carbon, hydrogen, nitrogen, and oxygen) levels of theory. In the optimized structure of Ru^III–O^• species in the most stable triplet state, the spin density has been estimated to be 0.96 at the ruthenium atom and 1.03 at the oxygen atom; thus, the species can be described as a Ru^III–O^• biradical. In addition, Mayer bond order (MBO) analysis has allowed us to estimate the bond order of the Ru–O bond as 1.3, supporting the single-bond character of the Ru–O bond.

The radical character of Ru^III–O^• species also reflected the catalytic oxidation of organic substrates in acidic water. The obvious difference of the reactivity was observed in benzaldehyde oxidation in water. Almost all Ru^IV═O complexes having polypyridyl ligands could not oxidize benzaldehyde and its derivatives.(64) However, the Ru^II(NHC)–aqua complex can catalyze oxidation of benzaldehydes effectively using (NH₄)₂[Ce^IV(NO₃)₆] (CAN) as a sacrificial oxidant in acidic water (pH ∼0.6) at 278 K (Figure 11).(61) The results indicate the high reactivity of the Ru^III–O^• complex in the oxidation of organic substrates compared to Ru^IV═O species. The catalytic oxidation proceeds via HAT from the C–H bond of the formyl group to the Ru^III–O^• moiety, which should be involved in the rate-determining step (RDS), as indicated by the kinetic isotope effect (KIE) on the oxidation of benzaldehyde-d₁ (k_cat^H/k_cat^D = 8.0). In addition, the electronic effect on the oxidation of para-substituted benzaldehyde by Ru^III–O^• showed a sharp contrast to that by Ru^IV═O in the Hammett plot (Figure 12). In the case of Ru^III–O^•, no electronic effect (ρ = −0.07) of the substituents is observed on the initial rates of catalytic oxidation of para-substituted benzaldehydes (blue line in Figure 12). In contrast, a relatively negative slope (ρ = −0.65) has been observed for the oxidation of benzaldehydes by [Ru^IV(O)(bpy)₂(py)]²⁺ as a reactive species in acetonitrile (MeCN; black line in Figure 12).(62,65) These results indicate that the catalytic benzaldehyde oxidation proceeds not via nucleophilic attack of an oxyl ligand to the formyl group (ρ > 0), not via a polarized transition state (TS; ρ < 0), but via a nonpolarized radical mechanism.

Figure 11. Oxidation of benzaldehydes by the Ru^II(NHC)–aqua complex using CAN as an oxidant.

Figure 12. Hammett plots for the oxidation of benzaldehyde derivatives: the Ru^III–O^• complex in water (blue line)(62) and [Ru^IV(O)(bpy)₂(py)]²⁺ in CH₃CN (black line).(65)

In addition, the Ru^III–O^• species has been revealed to show unique reactivity, which is totally different from that of Ru^IV═O complexes, that is, the oxidative cracking of aromatic rings. In acidic water (pH 1.6 or pD 1.2) at 283 K, benzene has been catalytically converted to formic acid (HCOOH) and CO₂ in the presence of the Ru^II–OH₂ complex shown in Figure 13 as a catalyst and CAN as an oxidant.(66) The origin of HCOOH has been confirmed to be benzene by ²H NMR spectroscopy using C₆D₆ as a substrate to observe DCOOH under the same conditions. HCOOH formed in the reaction has been used as a substrate to generate dihydrogen from the reaction mixture using [Rh^III(Cp*)(bpy)(H₂O)]²⁺ (Cp* = pentamethylcyclopentadienyl)(67) as a catalyst after simple pH adjustment to pH 3.3 by adding an aqueous NaOH solution. The scope and limitation of aromatic substrates have been examined. Benzene derivatives such as toluene, biphenyl, naphthalene, and anthracene can be easily converted to HCOOH. It should be noted that monosubstituted benzene derivatives having benzylic C–H bonds have been oxidized to afford the corresponding carboxylic acids holding substituents without oxidation of the benzylic positions (Figure 14). In the case of benzenesulfonate derivatives, because of the strong electron-withdrawing effect of the sulfonate group to make the aromatic ring electron-deficient, oxidation is switched to occur at the benzylic position (Figure 14).(62,66) Kinetic analysis on the oxidative benzene cracking has allowed us to conclude that the reactive species, the Ru^III–O^• complex, exhibits strong electrophilicity toward aromatic rings rather than the hydrogen-atom abstraction from benzylic positions or ring C–H bonds. The electrophilicity has been supported by a Hammett plot showing a ρ value of −1.41 for the oxidation of monosubstituted benzene derivatives.(66) The reaction mechanism of the oxidative benzene cracking has been proposed, as shown in Figure 15, on the basis of experimental results.(66) The Ru^III–O^• species attacks the aromatic ring to form benzene oxide, which undergoes rearrangement to afford oxepin. As in the NMR spectrum of the reaction mixture, 1,4-benzoquinone and muconic acid have been observed as intermediates to generate HCOOH. This unique reactivity of the Ru–O^• species may contribute to the development of a method to produce an energy source from environmentally harmful aromatic wastes in water.

Figure 13. Catalytic oxidative benzene cracking in water using the Ru^II(NHC)–aqua complex (Figure 10) as a catalyst and CAN as an oxidant at 283 K.

Figure 14. Change of the selectivity in benzene derivatives by the Ru^II–OH₂ complex in Figure 10 in water using CAN as an oxidant.(66)

Figure 15. Proposed mechanism of oxidative cracking of benzene by the Ru^III–O^• complex.(66)

4. Partially Characterized Metal–Oxyl Species

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The following metal–oxyl species have been proposed on the basis of some nondefinitive spectroscopic evidence. Mainly, the arguments have been made on the basis of DFT calculations.

4.1. Titanium–Oxyl Species

In the course of water oxidation by 0.1% niobium-doped perovskite n-SrTiO₃ (Figure 16a), a Ti^III–oxyl species (Ti^III–O^•) has been detected by ultrafast in situ attentuated-total-reflectance IR spectroscopy at an interface with water (Figure 16b).(68) Upon photoexcitation at 296 nm by a p-polarized (perpendicular to the surface) amplified Ti:sapphire laser beam, a signal was observed at 795 cm^–1, which was assigned to the Ti–O^• bond vibration on the basis of the known range of Ti–O stretches. The formation of the transient Ti–O^• species is derived from photoinduced charge separation. The species survives for >1 ns; however, the lifetime is shortened by increasing the methanol concentration. Also, the Ti–O^• species having a hole is indicated to be active in the water oxidation to evolve dioxygen. DFT calculations on the Ti–O^• species at the water interface has been applied to elucidate the characteristics of the Ti–O^• formed, indicating that the hole is located at O_x (see Figure 16b(68b)) in the lowest energy by 0.46 eV.

Figure 16. (a) Water-adsorbed n-SrTiO₃ surface (O_w, oxygen of adsorbed water; O_h, hydroxide). (b) Surface after photoexcitation. Changes of the electron density are described in yellow for a decrease and in cyan for an increase. Reprinted with permission from (68b). Copyright 2016 Springer Nature Publishing.

4.2. Nickel–Oxyl Complexes

Because nickel belongs to group 10 in the Periodic Table, in light of “the oxo wall”, Ni–oxyl complexes should be preferably formed rather than Ni–oxo complexes.

Recently, the preparation and spectroscopic characterization of Ni^III–oxyl (Ni^III–O^•) complexes have been reported. Company and co-workers have reported on the formation of a Ni^III–O^• complex in the reaction of a nickel(II) precursor with HmCPBA in CH₃CN at −30 °C, as depicted in Figure 17.(69) In the course of the reaction, the nickel(II)-bound mCPBA^– ligand undergoes heterolytic O–O bond cleavage to generate an oxidatively active species, which is proposed to be formulated as a Ni^III–O^• complex. The Ni^III–O^• complex has been characterized by X-ray absorption, rR, and EPR spectroscopies. Note that, however, the putative Ni^III–O^• complex is not selectively formed (35% yield) and thus not completely characterized. In particular, X-ray absorption spectroscopies should not be conducted on a complex mixture of nickel species to discuss the valency of the metal center and M–O bond distances. The dark-yellow Ni^III–O^• complex survives for 4.5 h at −30 °C while showing an absorption maximum at 420 nm and a shoulder at 520 nm; however, the reaction mixture contains other nickel(III) species to hamper the convincing characterization of the Ni^III–O^• complex. The Ni–O distance has been estimated to be 2.12 Å using extended X-ray absorption fine structure (EXAFS) analysis. DFT calculations have suggested that the Ni–O bond length of the Ni^III–O^• species is 1.95 Å and the MBO is 0.65 for the Ni–O bond.

Figure 17. Reaction of a nickel(II) complex with mCPBA to form a Ni^III–O^• complex.(69)

The reactivity of the Ni^III–O^• species has been examined using para-substituted thioanisoles, para-substituted styrenes, and aromatic compounds having benzylic C–H bonds as substrates in CH₃CN. In the oxidation of para-substituted anisoles and styrenes, products are the corresponding sulfoxides and epoxides, respectively. The second-order rate constants have been plotted against the Hammett parameters, affording Hammett plots showing a negative ρ value of −0.86 in both cases. This indicates the electrophilic character of the N^III–O^• species. Furthermore, Ni^III–O^• species could perform hydrogen-atom abstraction from C–H bonds of activated methylene moieties of organic compounds. As for the C–H oxidation of fluorine, 1,4-cyclohexadiene, DHA, and xanthene, a Bell–Evans–Polanyi plot, in which the logarithms of the normalized second-order rate constants are plotted against the bond-dissociation-energy (BDE) values of C–H bonds to be cleaved, affords a linear relationship with a slope of −0.23. These results strongly suggest that hydrogen-atom abstraction is involved in the RDS in the substrate oxidation reactions.

4.3. Copper–Oxyl Complex

Roithová and co-workers have reported the electronic structure of gas-phase [(CH₃CN)CuO]⁺ generated by the electron spray ionization of Cu^II(ClO₃)₂ in CH₃CN, using infrared photodissociation (IRPD) spectroscopy and its Franck–Condon analysis.(70) In the IRPD spectrum of the species at 5 K, the Cu–O stretching band can be observed at 693 cm^–1, which shifts to 672 cm^–1 upon ¹⁸O labeling at the ground state. DFT calculations have been applied to optimize the structure at the B3LYP/6-311+G** level and to elucidate the electronic structure using the multistate restricted active space second-order perturbation theory (MS-RASPT2). DFT analysis indicates that [(CH₃CN)CuO]⁺ involves a contribution from 56% of a triplet biradical made of a copper(I) center having the triplet oxygen atom and 36% of the Cu^II–O^• configuration.

4.4. Ruthenium–Oxyl Complexes

Kojima and co-workers reported a series of ruthenium(III) complexes having tris(2-pyridylmethyl)amine (TPA) derivatives and the catalytic reactivity toward cyclohexane by using HmCPBA as a terminal oxidant.(71) All complexes catalyzed cyclohexane oxidation to afford cyclohexanol (CyOH) and cyclohexanone (CyO). Attention has been focused on the reactions of Ru^III–TPA with unsubstituted TPA and Ru^III–TPA^2COOEt with TPA having two electron-withdrawing ethoxycarbonyl (COOEt) groups (Figure 18). These complexes show redox potentials of −0.26 V (vs Fc/Fc⁺, Ru^II/Ru^III) and 1.34 V (Ru^III/Ru^IV) for Ru^III–TPA and −0.05 V (Ru^II/Ru^III) and 1.53 V (Ru^III/Ru^IV) for Ru^III–TPA^2COOEt, respectively, which reflects that the electron density at the ruthenium center is governed by substituents on the pyridine rings. The KIE values of CyOH formation by Ru^III–TPA and Ru^III–TPA^2COOEt were determined to be 3.1 and 4.3, respectively. These KIE values are as low as those of hydrogen-atom abstraction by free radicals (5.6–6.3),(72) which indicates strong oxidation by Ru^III–TPA, the production of CyOH and CyO showed induction periods, and both products were formed at comparable rates. On the other hand, the reaction catalyzed by Ru^III–TPA^2COOEt proceeded efficiently without any induction period, in sharp contrast to that of Ru^III–TPA. In the isotope-labeling experiment in the presence of H₂¹⁸O, only 9% of ¹⁸O was incorporated into CyOH in the case of Ru^III–TPA, whereas 100% ¹⁸O incorporation was observed in the case of Ru^III–TPA^2COOEt. These results indicated that the reaction of Ru^III–TPA gave free-radical species derived from HmCPBA, followed by autoxidation of cyclohexane, whereas that of Ru^III–TPA^2COOEt gave radical character of the reactive species.(73) During catalysis, cyclohexane with HmCPBA affords a Ru–oxo species as a reactive species in cyclohexane oxidation. This reactive species generated by the reaction of Ru^III–TPA^2COOEt with HmCPBA in CH₃CN was characterized by rR spectroscopy. A Raman band derived from the Ru–O stretching frequency was observed at 752 cm^–1 in the presence of H₂¹⁶O, which completely shifted to 708 cm^–1 in the presence of H₂¹⁸O. This value of 752 cm^–1 was relatively weak compared to those reported for Ru^IV═O or Ru^V═O complexes, indicating a lower Ru–O bond energy. Such a high reactivity with a KIE value of 4.3, the feasibility of quantitative isotopic exchange, and the lower energy of Ru–O stretching vibration suggest a large contribution of Ru^III–O^• character for the resonance structures of the Ru^IV═O intermediate (Figure 18), which could have resulted from the higher redox potential of the Ru^III/Ru^IV redox couple of Ru^III–TPA^2COOEt.(71)

Figure 18. Ru^III–O^• complex in a larger contribution in the resonance structures.

Pushkar and co-workers reported the experimental evidence of radicaloid character of Ru^V═O intermediates in a diruthenium complex, [4,5]⁴⁺ (Figure 19), based on EPR measurements in ¹⁷O-enriched water.(74)[4,5]⁴⁺ also has been considered as a reactive intermediate in water oxidation by a “blue dimer” catalyst.(75) The EPR spectrum of the blue dimer BD[4,5]⁴⁺ in H₂¹⁶O showed an EPR signal derived from the S = ¹/₂ spin state with g tensors of g_xx = 2.03, g_yy = 1.98, and g_zz = 1.87. The simulated EPR spectrum clarified the ruthenium hfs of A_yy = 40 ± 5 G and A_zz = 25 ± 5 G, reflecting the isotopic composition for ^{99, 101}Ru (I = ⁵/₂), which was consistent with the reported apparent ^{99, 101}Ru hfs on the order of 40 G. In 50–60% ¹⁷O-enriched water, the BD [4,5]⁴⁺ intermediate having terminal Ru–O groups labeled with ¹⁷O was extracted by subtracting the contribution of [3,4]⁴⁺-prime, the spectrum showed a significant broadening of the EPR signals, and hfs due to ¹⁷O nuclei (I = ⁵/₂) was clearly observed on the low field side of the signal at g ∼ 2.1 with a splitting of 60 G. These experiments showed a high spin density on the oxygen in the d³ Ru^V═O moiety of the BD [4,5]⁴⁺ intermediate, which was considered to be a reactive species of water oxidation. The Ru^V═O unit in BD [4,5]⁴⁺ is the first to show large radicaloid character and is associated with large ¹⁷O hfs.

Figure 19. Structures of **[4,5]**⁴⁺ (left), **[3,4]**⁴⁺ (center), and **[3,4]**⁴⁺**-prime** (right).

5. Proposed Metal–Oxyl Species Based on DFT Calculations

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Other than the above-mentioned metal–oxyl species, metal–oxyl species have been proposed to be formed and argued on the basis of DFT calculations, as described in the following sections.

5.1. Manganese–Oxyl Complexes

From the viewpoint of theoretical chemistry, the formation of Mn–O^• species has been reported to be relavant to water oxidation by the oxygen-evolving complex in the photosystem II for natural photosynthesis. Such Mn–O^• species have been proposed to be involved in the O–O bond formation in the course of water oxidation to afford dioxygen.(76) Siegbahn and co-workers have reported on the involvement of Mn^IV–O^• species in the S₄ state to react with an oxo bridging ligand of a Mn–O–Ca moiety including the external manganese center attached to the Mn₃CaO₄ cubane cluster in the oxyl–oxo coupling to form the O–O bond for dioxygen evolution.(57,77) Batista and co-workers have also proposed a reaction mechanism of the O–O bond formation, in which the oxyl ligand on the manganese(IV) center undergoes the nucleophilic attack of a water molecule initially coordinated to the calcium(II) ion, on the basis of quantum mechanics/molecular mechanics (QM/MM) calculations, X-ray diffraction, and EXAFS analysis.(78)

Theoretical investigations on mononuclear Mn–oxo complexes also suggest Mn–oxyl character for a reactive species derived from [Mn^II(Py₂NR₂)(H₂O)₂]²⁺ via PCET oxidation. As reported by Baik and co-workers, the reactive species in the triplet spin state bears oxyl character on an oxo ligand, as shown in Figure 20.(79,80) The species is activated to form a quintet transition state (⁵TS) involving two oxyl ligands bound to the manganese(III) center (Figure 20, center) to form the O–O bond, affording a Mn^III–peroxo intermediate, which undergoes further oxidation to afford dioxygen.

Figure 20. Mn^IV–oxyl intermediate in water oxidation by [Mn^II(Py₂NR₂)(H₂O)₂]²⁺.(79)

Jackson and co-workers have described the electronic structure of [Mn^IV(O)(N4Py)]²⁺ for both the ground state and a ⁴E excited state derived from a d_xz, d_yz → d_x²–y² transition to elucidate the reactivity in HAT from a C–H bond of a substrate to the complex. The low-lying ⁴E excited state has been demonstrated to bear greater oxyl character (21%) with a longer Mn–O bond length, lowering the barrier for HAT.(81) Thus, Mn^III–O^• character can better facilitate HAT processes. In this excited state, the species has a hole in the lower-energy oxo orbital rather than a hole in the d_xz or d_yz orbital to accept an electron from the C–H bond of a substrate to generate a high-spin (HS) Mn^III–hydroxo complex as a product of HAT.

On the contrary, Borovik and co-workers have reported that Mn^V–oxo complexes having a tripodal triamido ligand (Figure 21) do not bear Mn^IV–oxyl character in light of EPR spectral data involving ¹⁷O-labeling experiments and DFT calculations to indicate that the Mn–oxo moiety has strong covalent character as an oxo ligand rather than an oxyl ligand.(82)

Figure 21. Schematic description of a Mn^V–oxo complex reported by Borovik and co-workers.(82)

5.2. Iron–Oxyl Complexes

Iron–oxyl complexes have yet to be formed and characterized. However, the occurrence of oxyl character in Fe–oxo complexes has been proposed and rationalized on the basis of theoretical calculations to explain the reactivity in C–H oxidation reactions.

An α-ketoglutarate-dependent mononuclear nonheme iron enzyme, syringomycin halogenase, can catalyze halogenation and hydroxylation of unreactive C–H bond through dioxygen activation. Solomon and co-workers have reported that an Fe^IV–oxo intermediate formed in the catalytic cycle bears 40% of Fe^III–oxyl character in the ground state and up to 65% in the TS of hydrogen abstraction, as indicated by variable-temperature magnetic circular dichroism spectroscopy and DFT calculations.(83) Another nonheme iron enzyme, taurine/TauD, hydroxylates taurine to afford amino acetaldehyde and sulfite. Concerning the reactive species in the catalytic cycle of TauD, a HS (S = 2) Fe^IV═O species called intermediate J has been observed and characterized spectroscopically.(84) Concerning the reaction of intermediate J with the substrate, the Fe^IV═O species has been proposed to be converted to a Fe^III–oxyl species en route to a ⁵TS on the basis of DFT calculations.(85) In the ⁵TS of HAT, the Fe–O bond is elongated from 1.62 to 1.76 Å and is interpreted as a HS iron(III) center (S = ⁵/₂) antiferromagnetically coupled with a three-centered C–H–O radical (S = ¹/₂).

Gunnoe and co-workers have reported on the formation of an Fe^III–oxyl complex in the course of oxygen migration from coordinated pyridine N-oxide (PyO) to the iron(II) center in [Fe^II(Cp*)(Ph)(CO)(PyO)] (Cp* = pentamethylcyclopentadienyl) on the basis of DFT calculations. The Fe^III–oxyl complex, [Fe^III(Cp*)(O^•)(Ph)(CO)], bears a large spin density (0.9 e^–) at the oxyl ligand together with 1.3 e^– at the iron center, as calculated at the B3LYP/CEP-31G(d) level of theory.(86)

Borovik and co-workers have reported on the electronic structure of an Fe^IV–oxo complex in the quintet (S = 2) spin state with the same ligand as that used for the Mn^V–oxo complex in Figure 21. The ¹⁷O-labeled Fe^IV–oxo complex has been scrutinized by EPR spectroscopy to reveal that the spin population on the oxygen ligand is 0.56, indicating a strong covalency in the Fe–oxo double bond without Fe–oxyl character.(87) Note that Pushkar’s report on the ruthenium catalyst as mentioned above also showed spin population at the terminal oxygen atom based on ¹⁷O-labeled EPR measurements.(74) In the report, the spin density at the terminal oxygen ligand was calculated to be 1.07 by DFT calculations. On the contrary, Borovik and co-workers have the spin density at the oxo ligand as only 0.56 based on electron-nuclear double resonance (ENDOR) measurements. Thus, the spin density at the oxygen ligand can also be a useful parameter for assuring metal–oxyl species by DFT calculations or EPR measurements.

On the other hand, Li and co-workers have reported a lowering of the activation energy of C–H activation of CH₄ by HS [Fe^IV(O)(F)₅]^3– on the basis of DFT analysis.(88) They propose that the strong π-donating equatorial ligands reduce the covalency of the Fe–O π bond, leading to Fe^III–O^• character involving a hole in the oxygen p_z orbital. In the case where a CH₄ molecule encloses to the Fe–O moiety from an equatorial (horizontal) direction, the Fe–O bond is significantly elongated in the TS. This elongation of Fe–O bonding involves two different electronic states. First, the valency of the iron center is trivalent, and there is a hole in the p_z orbital of the oxygen atom (Fe–O: <1.86 Å). Then, the hole on the p_z orbital switches to the p_x orbital (Fe–O: >1.86 Å). These electronic structures of iron–oxo species should reflect the feature of Fe^III–O^• character and can be described as Fe^III–O^• species. This switching of a hole in the p orbital of [FeO]⁺ in HS state in the TS causes the mixing of σ(89) and π(89) pathways and results in the proposal of a “hybrid pathway”.(88)

5.3. Cobalt–Oxyl Complexes

A cobalt(IV)–oxo complex should be difficult to form in a tetragonal geometry in light of the “oxo wall” paradigm because cobalt resides in group 9. A dinuclear Co^III–bis(μ-hydroxo) complex, [{Co^III(μ-OH)(TPA)}₂]⁴⁺, has been proposed to generate a dinuclear Co^III–bis(μ-oxyl) complex through PCET oxidation with CAN or [Ru^III(bpy)₃]³⁺ in water as a reactive species to perform O–O bond formation in the course of water oxidation (Figure 22).(90) Although any direct evidence has yet to be obtained, DFT calculations on the dinuclear Co^III–bis(μ-oxyl) complex have suggested that the unpaired electrons lie on the two oxygen bridges with a spin density of 0.830, while that on the cobalt center is 0.044 to support the formulation. The two oxyl ligands couple together in an intramolecular manner to afford a dinuclear Co^III–μ-peroxo complex as an intermediate, followed by two-electron oxidation of the dinuclear Co^III–μ-peroxo intermediate by CAN to afford dioxygen. Intramolecular O–O bond formation has been confirmed on the basis of ¹⁸O-labeling experiments for demonstrating the ¹⁸O distribution in the formed dioxygen.

Figure 22. Water oxidation by a Co^III–TPA complex via the formation of a dinuclear Co^III–bis(μ-oxyl) intermediate.(90a)

Co–oxo clusters and Co–oxides have been intensively investigated as effective water oxidation catalysts. In the course of water oxidation by Co–oxo clusters, it has been proposed that Co^IV–oxyl species act as reactive entities to perform O–O bond formation toward the generation of peroxo intermediates.(91) In addition, Dismukes and co-workers have reported on mechanistic insights into water oxidation by a cubane-type Co₄O₄ cluster on the basis of electrochemical and kinetic analyses.(92)

Terminal Co^II–oxyl (Co^III–oxo) or Co^III–oxyl (Co^IV–oxo) complexes have been proposed as active intermediates in intramolecular C–H abstraction or substrate oxidation. The Co^II–oxyl complex was generated from O–O bond homolysis of Co^II–μ-peroxo complexes having a Tp′ ligand [Tp′ = hydridotris(3-tert-butyl-5-methylpyrazolyl)borate], as shown in Figure 23.(93) When toluene-d₈ is used as a solvent, the incorporation of deuterium into the hydroxo ligand of Tp′Co^IIOH could not be observed. This indicates that the HAT reaction occurred not via intermolecular reaction but via intramolecular reaction such as ligand oxidation by a Co^II–O^• species, as described in Figure 23. Theopold and co-workers have reported a follow-up study on these Co^II–oxyl complexes.(94) They successfully trapped a dinuclear peroxo-bridged Co^II complex (Tp″Co^II(O₂)Co^IITp″) at −78 °C having hydrotris(3-isopropyl-5-methylpyrazolyl)borate (Tp″) as a precursor of a transient Co^II–oxyl complex. On the basis of a kinetic study by ¹H NMR spectroscopy, the absence of a significant solvent effect on the intramolecular HAT militates against a heterolytic cleavage of the O–O bond. In addition, a KIE value of 22 has been determined for the intramolecular HAT reaction in the use of Tp″-d₃₀ at 281 K. Such a large KIE value suggests that the HAT reaction by the Co^II–oxyl species proceeds via a tunneling mechanism.

Figure 23. Proposed mechanism of intramolecular HAT by the transient Co^II–oxyl (Co^II–O^•) species itself.(93)

Very recently, Maron and co-workers have proposed the formation of a Co^IV–O^• complex by O–O bond cleavage of a dinuclear Co^III₂(μ-O₂^2–) complex, as shown in Figure 24.(95) The Co^IV–O^• complex, [Co^IV(O^•)(B₂Pz₄Py)]⁺, has also been reported to form through the oxidation of a Co^III–hydroxo complex, [Co^III(OH)(B₂Pz₄Py)], by Magic Green [tris(2,4-dibromophenyl)aminium hexafluoroantimonate] as an oxidant in benzene. DFT calculations on the Co^IV–O^• complex have indicated that the species should be in the triplet ground state with a Co–O bond length of 1.671 Å and bears spin density at both the oxygen ligand and cobalt center. The Co^IV–O^• complexes react with each other to afford dioxygen and a Co^III species as products.(95) The dinuclear Co^III₂(μ-O₂^2–) complex does not react with toluene; however, it reacts with cyclohexadiene to produce benzene and a Co^III–OH₂ complex in toluene in the presence of a proton source. Because the reaction proceeds via formation of the Co^IV–O^• intermediate in resonance with the Co^V═O complex, it is suggested that the Co^IV–O^• complex may not possess very high reactivity in HAT. In addition, under the same conditions, the Co^III₂(μ-O₂^2–) complex can oxygenate triphenylphosphine (PPh₃) to afford OPPh₃ and the Co^III–OH₂ complex, together with an OPPh₃-bound Co^III complex.(95)

Figure 24. Formation of a Co^IV–O^• complex having pentadentate B₂Pz₄Py^2– as a ligand [Ar = p-methylphenyl (p-tolyl)].(95)

5.4. Nickel–Oxyl Complex

In the reaction of a Ni^II–alkyl metallacycle complex, [Ni(κ²-C,C-CH₂CH₂CH₂CH₂)(bpy)], with N₂O in benzene, oxygen-atom insertion into the Ni–C bond in the metallacycle is made to form an κ²-C,O-alkoxo metallacycle, as reported by Hillhouse and co-workers (Figure 25).(96) On the basis of DFT calculations to elucidate the reaction mechanism, a square-pyramidal Ni^III–oxyl complex has been postulated as an intermediate of the reaction through oxygen-atom transfer from N₂O to the nickel(II) center (Figure 25).(97)

Figure 25. Oxygen-atom insertion into a nickel(II) metallacycle complex to form a Ni^II–alkoxo metallacycle.(96,97)

5.5. Copper–Oxyl Complexes

Copper–oxygen species as reactive intermediates are highly important in substrate oxidation in vivo.(98) Although there are plenty of reports on reactive intermediates such as [Cu₂(O₂)]²⁺,(99) [Cu₂(μ-O)₂]²⁺,(100) and [CuOOH]⁺,(101) mononuclear Cu–oxyl species (Cu^II–O^• or Cu^III–O^•) have yet to be well-understood. Computational study(101) and synthetic copper(II) complexes(102) showed that the activation energies of substrate oxidation by Cu^II–oxyl species are predicted to be much lower than those of Cu^II–OOH or Cu–O₂ species as reactive intermediates. Monomeric Cu–oxyl species have been proposed as reactive intermediates in substrate oxidation reactions, catalyzed by copper-containing metalloenzymes such as dopamine β-monooxygenase,(103) peptidylglycine α-hydroxylating monooxygenase,(104) and lytic polysaccharide monooxygenases (LPMOs)(105) on the basis of theoretical study.

Schwarz reported the synthetic formation of [CuO]⁺ species and its reactivity toward methane in the gas phase, which showed high reactivity.(106) Triplet-ground-state [CuO]⁺ has a configuration of (1σ)²(2σ)²(1π_x)²(1π_y)²(1δ)⁴(3σ*)²(2π_x*)¹(2π_y*)¹(4σ*)⁰, which is comparable to the ³Σ_g^– ground state of molecular oxygen. Despite the fact that the electronic configuration of [CuO]⁺ in a triplet state should represent biradical character, DFT calculations estimate the spin density of 1.68 at oxygen and that of only 0.32 at the copper center.(106) This large population of spin densities at oxygen indicates the significantly large contribution of radicaloid character at the terminal oxygen atom. In addition, unless there is a much more strongly bonding interaction between the iron center and oxygen atom involved in the Fe^IV═O unit, the Cu–O bond in [CuO]⁺ is quite weaker, which is reflected in the low BDEs determined to be 31.1 kcal/mol (1.35 eV) experimentally(107) and estimated to be 25.3 kcal/mol based on a theoretical study at the CCSD(T) level.(108)

LPMOs utilize molecular oxygen and an electron donor to catalyze the oxidative cleavage of insoluble polysaccharides.(109) The active site of LPMO contains a mononuclear copper center ligated by two histidine residues. The reactive species is considered to be a Cu^II–O^• species, not a Cu^II–O₂^– one, based on theoretical studies. This LPMO catalysis can be activated also with the use of hydrogen peroxide (H₂O₂) as an oxidant in the presence of a substoichiometric amount of an external reductant. The Cu^I–H₂O₂ adduct in LPMO has a singlet ground state, and it undergoes homolytic O–O bond cleavage to form of a Cu^II–OH species and HO^• radical (Figure 26a) with an activation barrier of 5.8 kcal/mol.(109) Two possible pathways are considered for substrate oxidation in LPMO: HAT from the anomeric carbon of an adjacent sugar molecule by the trapped HO^• radical (¹TS2 in Figure 26b) and HAT from Cu^II–OH to HO^• to afford Cu^II–O^• species as the reactive species for hydrogen-atom abstraction from the anomeric carbon (¹TS3 in Figure 26b).

Figure 26. (a) Hydrogen-bonding network around the HO^• radical in an intermediate (1IC1 in part a) formed in the LPMO active site. (b) Relative energies calculated by the QM/MM methods (UB3LYP/B2, kcal/mol) for the reaction profile of the Cu^I–H₂O₂ intermediate formed in LPMO in the presence of polysaccharide. Reprinted with permission from (109). Copyright 2018 American Chemical Society.

The former pathway has relatively a large activation barrier compared to the latter pathway by 4.9 kcal/mol. It is suggested that the relatively lower activation barrier is derived from fixation of the HO^• radical by a hydrogen-bonding network in the active site (Figure 26a) to direct the oxygen-centered radical in HO^• to a C–H bond of the C4 position of a substrate (Figure 26a). Furthermore, this hydrogen-bonding network is assumed to prevent the HO^• radical from moving outside the hydrophobic site around the reaction center. The Cu^II–O^• formed has a high spin density of 0.83 on the oxygen atom with a Cu–O bond of 1.89 Å. Reflecting the strong radical character of the Cu^II–O^• species, HAT from the C4 position of the sugar requires it to overcome a small activation barrier of only 5.5 kcal/mol. This result suggests that Cu^II–O^• can be a highly reactive species for C–H activation.

Tolman et al. have reported the formation of Cu–oxyl (Cu–O^•) species with dioxygen as an oxidant.(110) In this case, an α-keto acid (α-ketophenylacetic acid) has been used as a ligand for the formation of Cu–O^• by reductive oxygen activation, followed by the loss of CO₂ and O–O bond homolysis of a peracid intermediate inspired by β-ketoglutarate-dependent nonheme iron enzymes. Exposing solutions of copper complexes L^HCu(O₂CC(O)Ph) (CuL^H) or L^m-OMeCu(O₂CC(O)Ph) (CuL^OMe) to dioxygen in acetone at −80 °C affords brown solutions within several hours. After warming and removal of the copper ion by the addition of a base, a mixture of ligands (L^H or L^m-OMe) and hydroxylated ligands (L^OH or L^m-OMe–OH) was identified by several spectroscopic analyses. In the case of using ¹⁸O₂, one ¹⁸O atom was incorporated into the hydroxylated ligand and the other into the benzoate based on electrospray ionization mass spectrometry, indicating that O–O bond cleavage occurs in the course of the reaction. The “peracid” as one of the possible intermediates is assumed to undergo two plausible reaction pathways based on DFT calculations (Figure 27).(110b) The first possible pathway goes through a singlet transition state (¹TS) structure involving oxidation of the ligand by the peracid itself, which is defined as “TS-Peracid” (Figure 27, top). Because the bond length of Cu–O in the structure of “TS-Peracid” is 1.72 Å, it seems to bear a strong “oxo-like” character. The second pathway involves a square-planar intermediate named “Oxo (sp)” followed by a triplet-transition-state structure “TS-Oxo” having a Cu–O bond length of 1.84 Å, which is described as a triplet Cu^II–oxyl species (Figure 27, bottom). Because these pathways have activation free energies of 17.8 and 10.4 kcal/mol relative to the peracid intermediate, respectively, the latter pathway is predicted to be more accessible than the former. Therefore, these theoretical calculations have yielded intriguing mechanistic notions for the process, implicating hydroxylation pathways that involve [Cu^I–OOC(O)R] and [Cu^II–O^•] species.

Figure 27. Calculated mechanism for arene substituent hydroxylation of the model.(110b)

Kodera and co-workers have recently reported selective hydroxylation of benzene by a dicopper(II) complex, having an ethylene-bridged polypyridyl ligand, [Cu₂(μ-OH)(6-hpa)](ClO₄)₃ (Cu₂(6-hpa); 6-hpa = 1,2-bis[2-[bis(2-pyridylmethyl)aminomethyl]-6-pyridyl]ethane), with H₂O₂ as a sacrificial oxidant (Figure 28).(111) The main product from the reaction was identified as phenol based on gas chromatography and ¹H NMR spectroscopy analysis. A time profile of phenol production catalyzed by Cu₂(6-hpa) revealed a high turnover frequency (TOF) of 1010 h^–1 and a turnover number of 12000 after 40 h. When a 1:1 mixture of C₆H₆ and C₆D₆ is used as a substrate, the KIE has been determined to be 1.04, indicating that C–H bond cleavage of benzene is not involved in the rate-limiting step. In addition, the active species of benzene hydroxylation obtained from Cu₂(6-hpa) is probably not be a hydroxyl radical (^•OH) because the KIE values reported for a Fenton-type reaction are in the range of 1.7–1.8.(112)

Figure 28. Proposed mechanism of H₂O₂ activation and benzene hydroxylation by Cu₂**(6-hpa)**.(111)

In the case of nitrobenzene, toluene, and phenol as substrates, the initial TOFs are in the following order: phenol ∼ toluene ≫ nitrobenzene. This indicates that the higher electron density of the aromatic ring in the substrate gives a higher initial TOF value. Therefore, the reactive species formed from the catalyst should bear electrophilic character in benzene hydroxylation. The regioselectivities of the oxidation sites for the substrates have been revealed to be the ortho and para positions for toluene, the ortho position for nitrobenzene, and the para position for phenol, which also implies that the active species generated from the catalyst bears electrophilic radical character, i.e., a metal-bound oxyl radical, as proposed in the reactions of nickel and copper complexes.(69,113)

Upon the addition of 1 equiv of H₂O₂ to Cu₂(6-hpa) in MeCN containing triethylamine (5 equiv) at −40 °C, an end-on trans-peroxodicopper(II) (Cu₂O₂) was obtained (Figure 28, middle). On the other hand, upon the addition of an excess amount of H₂O₂ to Cu₂(6-hpa), the Cu₂O₂ complex rapidly reacted to afford hydroperoxocopper(II) (CuO₂H; Figure 28). Because the reaction rate of this conversion process showed saturation behavior against the H₂O₂ concentration, an equilibrium involving the Cu₂O₂ complex and H₂O₂ should be operative. In addition, the two intermediates, Cu₂O₂ and CuOOH, have been characterized by rR spectroscopy. Cu₂O₂ and CuO₂H show Raman bands at 821 and 846 cm^–1, respectively, and then these bands are isotopically shifted by values of 43 and 53 cm^–1, respectively, when H₂¹⁸O₂ is used. These Raman bands are attributed to O–O stretching bands on the basis of those reported so far.(114) Although no direct evidence has been provided to support the formation of Cu^II–O^• or Cu^II–OO^• species, the reaction mechanism of benzene hydroxylation has been proposed, as shown in Figure 28.

Catalytic benzene hydroxylation reactions to afford phenol have also been reported by Pérez and co-workers, using Tp^XCu^I(NCMe) (Tp^X = hydrotrispyrazolylborate) complexes as catalysts and H₂O₂ as an oxidant in MeCN.(115) The copper(I) catalysts react with H₂O₂ to generate monomeric Cu^II–O^• species presumably via heterolysis of the O–O bond of H₂O₂ (Figure 29). The catalytic systems have provided 92% selectivity for phenol and benzoquinone in 8% yield. The KIE value of benzene hydroxylation has been determined to be 1.12 ± 0.01 with the use of an equimolar mixture of C₆H₆ and C₆D₆ as the substrate. As mentioned above, the KIE value is also out of the range (1.7–1.8) of those reported for the Fenton-type hydroxylation of benzene.(112) In this case, the formation of a bis(aryl) derivative, derived from the homocoupling of aryl radicals, as one of the typical products was not observed. Furthermore, benzene conversion into phenol is not significantly affected even in the presence of radical-trapping reagents such as CCl₄ or CBrCl₃, although chloro- or bromobenzene can be obtained in very low yields (ca. 0.5%). These results indicate that the main pathway in the benzene hydroxylation reaction should not involve the generation of phenyl radicals.

Figure 29. Schematic representation of benzene oxidation to phenol catalyzed by Tp^XCu^I(NCMe).(115)

In addition, electronic substituent effects on the benzene ring have been surveyed for benzene hydroxylation, and negative slopes were observed in the Hammett plots: ρ = −0.75 for Tp^*,BrCu(NCMe) and −1.1 for Tp^Br3Cu(NCMe). The negative slopes indicate electrophilic character of the Cu–O^• complexes as the reactive species.

In the course of electrocatalytic water oxidation by a Cu^II–aqua complex having triglycinato (TGG^4–) as a pentadentate ligand at pH 11 in 0.25 M phosphate buffer at 1.30 V (vs NHE), the Cu^II–aqua complex undergoes PCET oxidation to generate a putative Cu^III–O^• complex (Figure 30a), which reacts with a water molecule to form an O–O bond toward dioxygen evolution.(116) In addition, electrocatalytic water oxidation by a copper(II) complex having 2-pyridyl-2-propanolato (pyalk^–) as a ligand, [Cu^II(pyalk)₂], has been developed,(117a) and the reaction mechanism has been investigated on the basis of DFT calculations.(117b) In the proposed mechanism, a five-coordinated Cu^III–O^• complex (Figure 30b) has been formed through PCET oxidation of a Cu^III–OH intermediate and reacts with a water molecule as well, forming a Cu^II–OOH complex, followed by a further PCET oxidation to generate dioxygen.(117b)

Figure 30. Putative Cu^III–oxyl intermediates formed in electrocatalytic water oxidation: (a) [Cu^III(O^•)(TGG)]^2–;(116) (b) [Cu^III(O^•)(pyalk)₂].(117b)

5.6. Ruthenium–Oxyl Complexes

Ruthenium-bound oxyl radical intermediates are highly important not only in catalytic substrate oxidation but also in water oxidation. The blue diruthenium complex cis,cis-[(bpy)₂Ru(OH₂)]₂O⁴⁺ (bpy = 2,2′-bipyridine), which is called as the “blue dimer”, is one of the very few structurally well-defined molecular catalysts in water oxidation to afford molecular dioxygen.(75) Although the reactive species have been experimentally proposed as cis,cis-[(bpy)₂Ru^VO]₂O⁴⁺ ([5,5]⁴⁺ in Figure 31) for several decades, key plausible intermediates have recently been proposed as formally cis,cis-[(bpy)₂Ru^IVO^•]₂O⁴⁺ based on theoretical analysis.(118) The intermediates have two radicaloid oxyl moieties generated in situ that promote water oxidation at room temperature.

Figure 31. Diruthenium complexes **[3,3]**⁴⁺ and **[5,5]**⁴⁺ (top) and two possible mechanisms of O–O bond formation (bottom).

There are two possible mechanisms of water oxidation by diruthenium catalysts having radicaloid intermediates that have been considered in the past (Figure 31).(118) Pathway A indicates O–O bond formation between a metal-bound oxygen atom and a water molecule as a solvent, which affords a Ru^III–hydroperoxo intermediate. The direct coupling of the terminal oxo groups has been proposed to form a dinuclear (μ-oxo)(μ-peroxo)Ru^IV₂ complex, shown as pathway B in Figure 31.(118) Although the latter pathway is intuitively reasonable, it has been ruled out on the basis of experimental results.(119)

In the energetic analysis on the reactant [5,5]⁴⁺ ion, the most stable state has been proposed to be an antiferromagnetically coupled low-spin (LS) state in a staggered geometry. To propose the catalytic mechanism of water oxidation, a free water molecule was added to serve as the reference for all relative energies. It has been found that the formation of [4,4]*, modeled by adding one explicit water molecule to [5,5]⁴⁺, is an endothermic process with a relative energy of 20.2 kcal/mol (Figure 32, middle). [4,4]* is in an eclipsed geometry and is the antiferromagnetically coupled HS complex, where intramolecular electron transfer affords oxyl radical intermediates. Then the radicaloid oxyl moiety of [4,4]* attacks water in an electrophilic fashion. Spin densities of ±1.46 at ruthenium and ±1.07 at the terminal oxo ligands indicated that there is only a small electronic perturbation by the weak attachment of a water molecule to the eclipsed HS [5,5]⁴⁺. The first step of catalytic water oxidation involves O–H bond cleavage of water and the formation of hydroperoxo intermediate [3′,3]²⁺ from the reactant [4,4]*. This reaction has been considered to be RDS, having a free energy of activation of 25.9 kcal/mol in the solution phase (Figure 32). They concluded that the fundamental basis for the catalytic activity lied in a highly spin-polarized Ru^V═O core structure, which underwent intramolecular electron transfer to afford a Ru^IV–O^• species as a strong oxidant having an electrophilic reactivity toward a water molecule.

Figure 32. Optimized structures of diruthenium complex **[5,5]**⁴⁺ (antiferromagnetically coupled spin state) in staggered (a) and eclipsed (b) geometry.(118) (c) Formation of a precursor complex having two Ru^IV–O^• moieties triggered by a water molecule.

5.7. Rhenium–Oxyl Complexes

Strategies to obtain metal–oxyl species have been considered to require single-electron occupancy of a metal–oxo π*-symmetry antibonding orbital(120) or to remove an electron from a metal–oxo π -bonding orbital. The latter is applied by Soper and co-workers to a novel five-coordinate Re^VI–oxo complex, bearing 2,4-di-tert-butyl-6-(phenylamido)phenolate ([ap^Ph]^2–) and 2,4-di-tert-butyl-6-(phenylamido)semiquinolate ([isq^Ph]⁻) as the one-electron-oxidized species of [ap^Ph]^2– (Figure 33).(121) The unique properties of Re^VI(O)(ap^Ph)(isq^Ph)Cl (Re^VI(O)) are derived from symmetry-allowed mixing of a Re–O π bond with an orbital of the semiquinone-type ligand [isq^Ph]^•–. The mixing of a Re–O bond gives oxyl radical character of the oxo ligand. Intramolecular charge transfer at closed-shell oxo ligands is a new strategy for the formation of metal–oxyl species, showing unique reactivity with a lower activation barrier.

Figure 33. Reaction between Re^VI–oxo species Re^VI(O) and a trityl radical.(121)

Qualitative π-orbital interactions in Re^VI(O) are shown in Figure 34. As shown in Figure 34, the metal-based unpaired electron (d¹) does not satisfy single-electron occupancy of metal–oxo π*-antibonding orbital because of the orthogonality of the d_xy orbital to the Re–O_oxo π-bonding orbital. However, the [isq^Ph]^•– ligand at the trans position to the oxo ligand has its unpaired electron in a π-symmetry orbital, which overlaps with the d_xy orbital of the rhenium center and contributes to overlap with the Re–O π bonds (Figure 34b). Such symmetry-allowed mixing of the [isq^Ph]^•– ligand with a Re–O π bond provides an overlapping of the [isq^Ph]^•– singly occupied molecular orbital (MO) with the Re–O_oxo π-bonding orbital. These mixings of a π orbital of [isq^Ph]^•– and Re–O_oxo π-bonding orbitals cause a decrease of the bond order for the Re–O_oxo bond from 3.0 to 2.5. However, the oxidation potentials of [ap^Ph]^2– and O^2– imply that intramolecular electron transfer from O^2– to [isq^Ph]^•– has a large energy barrier, and thus it is not appropriate to describe the electronic structure of Re^VI(O) at the ground state as Re^VI(O^•)(ap^Ph)₂Cl, which absolutely has an oxyl radical ligand.(121)

Figure 34. Qualitative π-orbital interactions in Re^VI(O). Reprinted with permission from (121). Copyright 2011 American Chemical Society.

The contribution of oxyl radical character to the electronic structure of Re^VI(O) was reflected in a very slow reaction with the trityl radical Ph₃C^• to afford Ph₃COH and a deoxygenated metal complex at 25 °C. Two possible reaction mechanisms were proposed for formation of the alkoxide complex, i.e., initial outer-sphere electron transfer, as the first step to give Re^V(O) and the direct Ph₃C^• addition to Re^VI(O). However, the former mechanism is concluded as unlikely because the one-electron-reduced product of Re^VI(O), [Re^V(O)(ap^Ph)(isq^Ph)Cl]⁻ (Re^V(O)), performs rapid formation of a bis(μ-oxo) dimer, Re^V₂(μ-O)₂(ap^Ph)₂(isq^Ph)₂ (Re^V₂(μ-O)₂), which cannot react with Ph₃C⁺ to give Ph₃COH. Thus, the latter mechanism seems more reasonable. It is noteworthy that all of the closed-shell Re–oxo complexes reported so far, including complexes capable of oxo transfer reactions to other substrates, have been Ph₃C^•.(41,122) Consequently, Re^VI(O) unexpectedly reacted with Ph₃C^•, in spite of the fact that Re^VI(O) was neither a strong oxidant nor a ground-state oxyl radical. In addition, the reduction potential is lower by ca. 550 mV than that of ferrocenium, and thus Re^VI(O) cannot oxidize even PPh₃.

5.8. Rhodium–Oxyl Complexes

One example has been reported for a Rh–O^• complex so far.(123) A reaction of [Rh^IIIH{^tBu₂PCH₂SiMe₂CH₂P^tBu(CMe₂CH₂)}] (Rh–H) and N₂O in the 1:1 mole ratio at −78 °C in toluene-d₈ shows complete consumption of Rh–H accompanied by the production of [Rh(N₂)(PNP)] and the C_2v-symmetric, paramagnetic coproduct Rh–O, as evidenced by ¹H NMR spectroscopy analysis (Figure 35). Experimental observations such as no ³¹P NMR signals and paramagnetically shifted ¹H NMR signals suggest that Rh–O should be a paramagnetic species. DFT calculations of Rh–O show that Rh–O in the triplet state is more stable by 15.3 kcal/mol compared to that in the singlet state. The distribution of the spin density of Rh–O in the triplet state is 0.92 on the rhodium center, 0.08 on nitrogen atom, and 0.96 on the oxygen atom, indicating triplet biradical character as a formally Rh^II–O^• complex.

Figure 35. Reaction of a Rh^III–H (**Rh–H**) species with N₂O in toluene-d₈.(123)

5.9. Tungsten–Oxyl Complexes

Tungstate–oxyl species (W^(n–1)+–O^•) have been intensively investigated as the main players in the photoredox chemistry of polyoxometalates (POMs) for several decades.(124) POMs have been known as one of the metal oxide assemblies that have prominent subclasses of group 5 or 6 metal oxoanions such as tungstates, niobates, vanadates, molybdates, and tantalates.(125,126) In particular, heteropolyoxotungstates, [X_xW_yO_z]ⁿ⁻ (X = Si, P, S), have been widely investigated because they often exhibit high thermodynamic stability and high reactivity in the oxidation of water and organic substrates. The reactivity is derived from photoexcitation of the W═O moiety to produce W–O^• species, as shown in Figure 36. POMs exhibit high photoactivity under near-visible or UV-light irradiation because of the high molecular absorption coefficients (ε > 1 × 10⁴ M^–1 cm^–1) derived from O → W ligand-to-metal charge-transfer (LMCT) bands; photoexcitation affords W^(n–1)+–O^• as reactive species in the substrate oxidation reactions (Figure 36).(124) In the presence of water, it is widely considered that a lot of water molecules interact with the cluster shell by hydrogen bonding. Upon photoexcitation of the cluster, W^V–O^• species formed can abstract a hydrogen atom from an interacting water molecule homolytically to afford a hydroxyl radical. Various experiments have supported that such a strong oxidant acts as a reactive species in substrate oxidation in aqueous media. On the other hand, in the absence of associated water molecules, the substrates can be directly oxidized by the photoexcited POM cluster having W^V–O^• moieties.

Figure 36. Water (a) or alcohol (b) oxidation by W^V–O^• species generated from W^VI═O under photoirradiation.

The characterization of such a transient short-lived LMCT excited state and reactive charge-transfer intermediates has been conducted by picosecond flash excitation in MeCN.(127) After 355 nm pulse irradiation of the MeCN solution of [W₁₀O₃₂]^4–, a rise of the initial transient at 390 nm was observed with a lifetime of ca. 30 ps, assigned to an LMCT excited state. In the transient absorbance spectra measured at 0.5 or 14.5 ns delay, identical absorption at 780 nm and arelatively broad band from 400 to 560 nm were observed and decayed slowly with τ ≥ 80 ns. The latter species showing the transient broad absorption band with a long lifetime affords growth of the transient absorption band derived from the reduced species [W₁₀O₃₂]^5– in the presence of 4 M 2-butanol or 0.5 M cyclohexene.(126)

6. How To Stabilize Metal–Oxyl Species

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As in the case of stable Ru–O^• complexes that are fully characterized, π-accepting ligands such as bpy and semiquinonato derivatives coordinate to the ruthenium centers.

MOs of [Ru^III(O^•)(BPIm)(bpy)]²⁺ (Figure 10) related to the Ru–O^• bond are shown in Figure 37. As can be seen, frontier orbitals of the Ru–O^• bond consist of filled σ-bonding [93 and 94 (α-spin) and 96 (β-spin)] and π-bonding [113 (β-spin)] and π-antibonding orbitals (119). In the π-bonding orbitals (103 and 105 of the β-spin orbitals), delocalization of the MOs can be seen in the π* orbitals of the bpy ligand.

Figure 37. Selected MOs of [Ru^III(O^•)(BPIm)(bpy)]²⁺ (Figure 10) corresponding to the bonding and antibonding orbitals of the Ru–O bond. Values in parentheses are the MBO contribution from the corresponding MOs. Reprinted with permission from (62). Copyright 2016 Wiley-VCH.

Thus, electron delocalization from the Ru–O moiety to the bpy ligand should be indispensable for stabilization of the oxyl species. Also, the contribution of the NHC moiety to the π-antibonding orbital (119) of the Ru–O bond reduces the bond order of the Ru–O bond to 1.3. Thus, in [Ru^III(O^•)(BPIm)(bpy)]²⁺, strong electron donation from the NHC moiety coordinated at the trans position to the oxygen ligand is also important to generate the oxyl species.

In the cases of Ru^II_DBSQ-O^• and Ru^II_ClSQ-O^• in Figures 6 and 7, the complexes have two π-accepting ligands, semiquinonato and terpy. Although MO analysis on the complexes has yet to be reported, delocalization of the π-bonding orbitals involving the Ru–O bond can also be expected for the two complexes. The terpy ligand should have lower-energy π* orbitals compared to the bpy ligand because of the expansion of π conjugation. In addition, the semiquinonato ligand also has a hole in the π-bonding orbital to accept electron density from the ruthenium center, as discussed for the aforementioned the Re–O^• complex (Figure 34) having an iminosemiquinonato ligand. Note that Ru^II_DBSQ-O^• and Ru^II_ClSQ-O^• also have π-donating semiquinonato ligands at the positions trans to the oxygen ligands. It can be concluded that stabilization of the metal–oxyl complexes may require the introduction of a π-accepting ligand to reduce the electron density on a M═O bond and a strong donating ligand at the trans position to the oxygen ligand.

With regard to the metal center, ruthenium has been found to be one of the good candidates to generate oxyl complexes. The reason why ruthenium can stabilize the Ru–oxyl complexes may be the strong π-back-bonding ability to heteroaromatic ligands including π-conjugated polypyridyl ligands such as bpy and terpy and may be also semiquinonato ligands. As mentioned above, the π-bonding orbitals involving the Ru═O bond can delocalize into the bpy ligand in [Ru^III(O^•)(BPIm)(bpy)]²⁺ to reduce electron population of the Ru═O π bonding. In order to delocalize the π-bonding electrons of the M═O moiety, the second- and third-row metals may be appropriate because of the larger lobes of d_π orbitals to facilitate the π-bonding interaction with π* orbitals of the π-conjugated ligand to reduce the bond order (Figure 38). The reason why such M–O^• species are difficult to stabilize for the first-row transition-metal complexes is the smaller lobes of the 3d_π orbitals than the 4d_π and 5d_π orbitals.

Figure 38. Electron flow through MO formation to stabilize a metal–oxyl species: (a) strong σ donation from a ligand binding at the trans position to the oxyl ligand; (b) strong π-back bonding to a π-accepting ligand.

7. Spectroscopic Criteria for Experimental Characterization of Metal–Oxyl Species

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In this section, we provide some information on the methodologies for the characterization of metal–oxyl species. At the very least, metal–oxyl species should show spectroscopic characteristics: lower valency of the metal center of Mⁿ⁺–O^• than that of M⁽ⁿ⁺¹⁾⁺═O species based on XPS or XANES and lower frequency of the M–O vibration than M═O species based on IR or rR spectroscopy (Figure 39). In addition, longer M–O distances than the corresponding M═O bonds based on X-ray crystallography or EXAFS studies may be suggestive of M–O^• character. However, careful analysis on the oxidation state should be made using other spectroscopic methods such as XANES because the bond length is not always relevant to the oxidation state of the metal center.(51) The spin density at the oxygen ligand can be verified by ¹⁷O-enriched EPR and ENDOR measurements in possible cases. Despite indirect assignments, it is informative to use spin-trapping reagents, such as DMPO or (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, that afford stable products to be analyzed. In the cases where Mössbauer or EPR measurements are applicable, these spectroscopies should be effective in arguing the oxidation states of the metal centers in metal–oxyl species. In addition, computational studies on metal–oxyl species often show a large spin population on its oxygen atom (∼1.0) to support these experimental analyses based on DFT calculations. Finally, schematic descriptions of the proposed and determined structures of M–O^• species are summarized in Figures 40 and 41.

Figure 39. Spectroscopic and structural features and experimental methodologies to prove metal–oxyl species. Some methodologies in parentheses are applicable to the cases where they are effective.

Figure 40. Proposed and determined structures of spectroscopically and crystallographically characterized metal–oxyl complexes.

Figure 41. Proposed structures of partially characterized metal–oxyl species.

8. Summary

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In this Viewpoint, we surveyed M–O^• species as a new category of possible reactive species in oxidation reactions as an extreme resonance structure of the corresponding M═O species. Structural and spectroscopic parameters of selected M–O^• species are summarized in Table 1. Formation of the M–O^• species can be achieved through PCET oxidation of the corresponding lower-valent metal–aqua precursor complexes as observed for the Cu–O^• and Ru–O^• complexes, O–O bond cleavage of metal-bound peroxides as seen for the Cu–O^• and Ni–O^• complexes, and photoactivation and photoexcitation of metal–oxo complexes as observed for terminal oxo groups in POMs such as heteropolyoxotungstates. In addition, M–O^• species have been proposed en route to the TSs of C–H oxidation by the Fe–oxo complexes. As can be seen for the Ru^II–O^• and Re–O^• complexes, the use of π-accepting ligands such as bpy, terpy, and semiquinonato ligands can facilitate the formation of M–O^• complexes because of the delocalization of unpaired electron density. The introduction of strongly σ-donating (such as NHC and semiquinonato) ligands at the position trans to the oxo ligand is also effective for the M–O^• complexes, as seen in the Ru^II–O^• and Ru^III–O^• complexes. Thus, it is expected that the 4d and 5d transition metals should be preferred to the 3d counterparts because of the more facile formation of π-type interactions with π-accepting ligands. As can be seen in the Ru^III–O^• complex, M–O^• complexes can exhibit unique reactivity such as oxidative aromatic-ring cracking through the electrophilic attack on aromatic rings. Although the number of species observed is still limited, their pursuit is very attractive in terms of their characteristics and reactivity. M–O^• species show unique reactivity in the oxidation of organic substrates and water. Especially, a Ru^III–O^• complex has been demonstrated to perform unique oxidation reactions of aromatic compounds, and a Zn–O^• species has been reported to enable methane oxidation. Increasing numbers of proposals relevant to M–O^• species as reactive species and also their involvement in the TSs of oxidation reactions have been reported. Thus, a reliable methodology for the formation of M–O^• species and their characterization should become more important in oxidation chemistry on the basis of not only metal complexes but also solid catalysts. It can be expected that the reactivity of M–O^• complexes will provide a way of achieving potentially difficult oxidative conversion of substrates including methane oxidation.

Table 1. Summary of Parameters Observed and Estimated for Selected Metal–Oxyl Species by Experimental and Theoretical Methodsa

	experimental methods			theoretical methods
nomenclature	M–O bond, Å (method)	ν_M–O, cm^–1	g values in EPR	M–O bond, Å	ν_M–O, cm^–1	method	ref
[Ru^III(O^•)(BPIm)(bpy)]²⁺	1.77(1) (XAS)	732 (rR)	4.31	1.801	761	B3LYP/SDD, D95**	(62)
[Ru^II(DBSQ)(O^•)(terpy)]	2.043(7) (XC)	503, 521, 556, 590 (rR)	4.18, 2.054				(59a)
Ru^III(O^•)(TPA^2COOEt)		752 (rR)					(71)
[{Ru^V(O)(bpy)₂}(μ-O) {Ru^IV(OH)(bpy)₂}]⁴⁺	1.71 (XAS)	816–818 (rR)	2.03, 1.98, 1.87	1.76		B3LYP/dgdzvp	(74)
[{Ru^V(O)(bpy)₂}₂(μ-O)]⁴⁺ (eclipsed)				1.919		B3LYP/LACVP, 6-31G**	(118)
[Mn^IV(O^•)(O)(Py₂NR₂)]²⁺				1.620 (Mn–O^•), 1.591 (Mn═O)		M06/LACVP, 6-31G**	(79)
[Mn^III(O^•)₂(Py₂NR₂)]²⁺				1.818
[Mn^III(O^•)(N4Py)]²⁺				>1.85		TD-DFT, CASSCF	(81)
Fe^III–O^• model of TauD (intermediate J)				1.76		hybrid B3LYP/TZVP, SV(P)	(84)
[Fe^III(Cp*)(O^•)(Ph)(CO)]				1.673		B3LYP/CEP-31G(d)	(86)
[Co^III₂(μ-O^•)₂(TPA)₂]⁴⁺ b				1.843		B3LYP/Wachters–Hay, D95**e	(90)
[Co^IV(O^•)(B₂Pz₂Py)]⁺				1.671		B3PW91/Stuttgart–Köln ECP, 6-31G**	(95)
[Ni^III(L)(O^•)]c	2.12 (XAS)	450, 477 (rR)		1.95	433	B3LYP/TZVP	(69)
L^HCu^II(O^•)(OC(O)Ph) (TS-Oxo)				1.84		CASPT2/M06L	(110b)
Cu^II–O^• moiety in LPMO				1.89		QM/MM (B3LYP/B2)	(109)
[(CH₃CN)CuO]⁺		693 (IRPD)		1.7		B3LYP/6-311+G**MS-RASPT2	(70)
Zn^II–O^• in MFI zeolite		605 (UV–vis–NIR)	2.37, 1.98	1.858	643	B3LYP/LanL2DZ, 6-31G(d,p), 3-21G	(58a)
[Re^VI(O)(ap^Ph)(isq^Ph)Cl	1.7064(17) (XC)						(121)
[Rh^II(O^•)(PNP)]				1.814		B3LYP/LACVP, 6-31G**	(123)

^{^a}

Abbreviations used in the “nomenclature” of metal–oxyl species and their structures in this table have been described in main text. rR = resonance Raman spectroscopy. XAS = X-ray absorption spectroscopy (EXAFS in this table). XC = X-ray crystallography. IRPD = infrared photodissociation spectroscopy.

^{^b}

The O^• is not terminal but a bridging atom.

^{^c}

L = tetradentate dianionic macrocyclic ligand with two amidate, one pyridine, and one aliphatic amine groups.

^{^d}

Mössbauer spectral analysis is conducted instead of EPR spectroscopy, in which ΔE_Q = 0.60 mm/s and δ = 0.13 mm/s are obtained.

^{^e}

The (14s9p5d)/[9s5p3d] primitive set of Wachters–Hay with one polarization f function (α = 1.117) and D95** were used.

The authors declare no competing financial interest.

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Corresponding Author
- Takahiko Kojima, Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan, http://orcid.org/0000-0001-9941-8375, Email: [email protected]
Author
- Yoshihiro Shimoyama, Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan; Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan
Notes
The authors declare no competing financial interest.

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Acknowledgments

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This work has been supported by JST CREST (Grant JPMJCR16P1) and Grants-in-Aid 15H00915, 17H03027, and 18K19089 from the Japan Society of Promotion of Science of Japan (JSPS). T.K. also is grateful for financial support from the Yazaki Memorial Foundation for Science and Technology. Y.S. is thankful for support from a Research Fellowship for Young Scientists provided by JSPS (Grant 18J12050).

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Crystal structure of the non-heme iron halogenase SyrB2 in syringomycin biosynthesis
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A review. Oxoiron(IV) species are often implicated in the catalytic cycles of O2-activating non-heme iron enzymes. The paucity of suitable model complexes has stimulated the authors to fill this void, and their synthetic efforts have afforded a no. of oxoiron(IV) complexes. Here, the authors provide a chronol. perspective of the observations that contributed to the generation of the 1st non-heme iron(IV)-oxo complexes in high yield and summarizes their salient properties to date.
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A review. High-valent iron(IV)-oxo species have been implicated as the key reactive intermediates in the catalytic cycles of dioxygen activation by heme and non-heme iron enzymes. Our understanding of the enzymic reactions has improved greatly via investigation of spectroscopic and chem. properties of heme and non-heme iron(IV)-oxo complexes. In this Account, reactivities of synthetic iron(IV)-oxo porphyrin π-cation radicals and mononuclear non-heme iron(IV)-oxo complexes in oxygenation reactions have been discussed as chem. models of cytochrome P 450 and non-heme iron enzymes. These results demonstrate how mechanistic developments in biomimetic research can help our understanding of dioxygen activation and oxygen atom transfer reactions in nature.
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A review. Mononuclear nonheme iron enzymes generate high-valent Fe(IV)-oxo intermediates that effect metabolically important oxidative transformations in the catalytic cycle of O2 activation. In 2003, researchers 1st spectroscopically characterized a mononuclear nonheme Fe(IV)-oxo intermediate in the reaction of taurine-α-ketoglutarate dioxygenase (TauD). This nonheme Fe-contg. enzyme with a Fe active center was coordinated to a 2-His-1-carboxylate facial triad motif. In the same year, researchers obtained the 1st crystal structure of a mononuclear nonheme Fe(IV)-oxo complex bearing a macrocyclic supporting ligand, [(TMC)FeIV(O)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecene), in studies that mimicked the biol. enzymes. With these breakthrough results, many other studies have examd. mononuclear nonheme Fe(IV)-oxo intermediates trapped in enzymic reactions or synthesized in biomimetic reactions. Over the past decade, researchers in the fields of biol., bioinorg., and oxidn. chem. have extensively investigated the structure, spectroscopy, and reactivity of nonheme Fe(IV)-oxo species, leading to a wealth of information from these enzymic and biomimetic studies. Here, the authors summarize the reactivity and mechanisms of synthetic mononuclear nonheme Fe(IV)-oxo complexes in oxidn. reactions and examines factors that modulate their reactivities and change their reaction mechanisms. The authors focus on several reactions including the oxidn. of org. and inorg. compds., electron transfer, and O atom exchange with water by synthetic mononuclear nonheme Fe(IV)-oxo complexes. In addn., the authors recently obsd. that C-H bond activation by nonheme Fe(IV)-oxo and other nonheme metal(IV)-oxo complexes does not follow the H-atom abstraction/oxygen-rebound mechanism, which has been well-established in heme systems. The structural and electronic effects of supporting ligands on the oxidizing power of Fe(IV)-oxo complexes are significant in these reactions. However, the difference in spin states between nonheme Fe(IV)-oxo complexes with an octahedral geometry (with an S = 1 intermediate-spin state) or a trigonal bipyramidal (TBP) geometry (with an S = 2 high-spin state) does not lead to a significant change in reactivity in biomimetic systems. Thus, the importance of the high-spin state of Fe(IV)-oxo species in nonheme Fe-contg. enzymes remains unexplained. The authors also discuss how the axial and equatorial ligands and binding of redox-inactive metal ions and protons to the Fe-oxo moiety influence the reactivities of the nonheme Fe(IV)-oxo complexes. The authors emphasize how these changes can enhance the oxidizing power of nonheme metal(IV)-oxo complexes in O atom transfer and electron-transfer reactions remarkably. The authors demonstrate great advancements in the understanding of the chem. of mononuclear nonheme Fe(IV)-oxo intermediates within the last 10 yr.
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(d) Ray, K.; Pfaff, F. F.; Wang, B.; Nam, W. Status of Reactive Non-Heme Metal-Oxygen Intermediates in Chemical and Enzymatic Reactions. J. Am. Chem. Soc. 2014, 136, 13942– 13958, DOI: 10.1021/ja507807v
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7d
Status of reactive non-heme metal-oxygen intermediates in chemical and enzymatic reactions
Ray, Kallol; Pfaff, Florian Felix; Wang, Bin; Nam, Wonwoo
Journal of the American Chemical Society (2014), 136 (40), 13942-13958CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
A review. Selective functionalization of unactivated C-H bonds, water oxidn., and dioxygen redn. are extremely important reactions in the context of finding energy carriers and conversion processes that are alternatives to the current fossil-based oil for energy. A range of metalloenzymes achieve these challenging tasks in biol. by using cheap and abundant transition metals, such as Fe, Cu, and Mn. High-valent metal-oxo and metal-O2 (superoxo, peroxo, and hydroperoxo) cores act as active intermediates in many of these processes. The generation of well-described model compds. can provide vital insights into the mechanisms of such enzymic reactions. Here, the authors provide a focused rather than comprehensive review of recent advances in the chem. of biomimetic high-valent metal-oxo and metal-O2 complexes, which can be related to an understanding of the biol. systems.
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(e) Oloo, W. N.; Que, L., Jr. Bioinspired Nonheme Iron Catalysts for C-H and C═C Bond Oxidation: Insights into the Nature of the Metal-Based Oxidants. Acc. Chem. Res. 2015, 48, 2612– 2621, DOI: 10.1021/acs.accounts.5b00053
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7e
Bioinspired Nonheme Iron Catalysts for C-H and C=C Bond Oxidation: Insights into the Nature of the Metal-Based Oxidants
Oloo, Williamson N.; Que, Lawrence, Jr.
Accounts of Chemical Research (2015), 48 (9), 2612-2621CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
A review. Recent efforts to design synthetic iron catalysts for the selective and efficient oxidn. of C-H and C=C bonds have been inspired by a versatile family of nonheme iron oxygenases. These bioinspired nonheme (N4)FeII catalysts use H2O2 to oxidize substrates with high regio- and stereoselectivity, unlike in Fenton chem. where highly reactive but unselective hydroxyl radicals are produced. In this Account, we highlight our efforts to shed light on the nature of metastable peroxo intermediates, which we have trapped at -40 °C, in the reactions of the iron catalyst with H2O2 under various conditions and the high-valent species derived therefrom. Under the reaction conditions that originally led to the discovery of this family of catalysts, we have characterized spectroscopically an FeIII-OOH intermediate (EPR gmax = 2.19) that leads to the hydroxylation of substrate C-H bonds or the epoxidn. and cis-dihydroxylation of C=C bonds. Surprisingly, these org. products show incorporation of 18O from H218O, thereby excluding the possibility of a direct attack of the FeIII-OOH intermediate on the substrate. Instead, a water-assisted mechanism is implicated in which water binding to the iron(III) center at a site adjacent to the hydroperoxo ligand promotes heterolytic cleavage of the O-O bond to generate an FeV(O)(OH) oxidant. This mechanism is supported by recent kinetic studies showing that the FeIII-OOH intermediate undergoes exponential decay at a rate enhanced by the addn. of water and retarded by replacement of H2O with D2O, as well as mass spectral evidence for the FeV(O)(OH) species obtained by the Costas group. The nature of the peroxo intermediate changes significantly when the reactions are carried out in the presence of carboxylic acids. Under these conditions, spectroscopic studies support the formation of a (κ2-acylperoxo)iron(III) species (EPR gmax = 2.58) that decays at -40 °C in the absence of substrate to form an oxoiron(IV) byproduct, along with a carboxyl radical that readily loses CO2. The alkyl radical thus formed either reacts with O2 to form benzaldehyde (as in the case of PhCH2COOH) or rebounds with the incipient FeIV(O) moiety to form phenol (as in the case of C6F5COOH). Substrate addn. leads to its 2-e- oxidn. and inhibits these side reactions. The emerging mechanistic picture, supported by DFT calcns. of Wang and Shaik, describes a rather flat reaction landscape in which the (κ2-acylperoxo)iron(III) intermediate undergoes O-O bond homolysis reversibly to form an FeIV(O)(•OC(O)R) species that decays to FeIV(O) and RCO2• or isomerizes to its FeV(O)(O2CR) electromer, which effects substrate oxidn. Another short-lived S = 1/2 species just discovered by Talsi that has much less g-anisotropy (EPR gmax = 2.07) may represent either of these postulated high-valent intermediates.
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(a) Groves, J. T.; Haushalter, R. C.; Nakamura, M.; Nemo, T. E.; Evans, B. J. High-Valent Iron-Porphyrin Complexes Related to Peroxidase and Cytochrome P-450. J. Am. Chem. Soc. 1981, 103, 2884– 2886, DOI: 10.1021/ja00400a075
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8a
High-valent iron-porphyrin complexes related to peroxidase and cytochrome P-450
Groves, John T.; Haushalter, Robert C.; Nakamura, Mikio; Nemo, Thomas E.; Evans, B. J.
Journal of the American Chemical Society (1981), 103 (10), 2884-6CODEN: JACSAT; ISSN:0002-7863.
The oxidn. of chloro-5,10,15,20-tetramesitylporphinatoiron(III) (TMPFeCl) with m-chloroperoxybenzoic acid at -78° produced a green intermediate (I). The 1H NMR and Moessbauer spectra of I were consistent with an oxoiron(IV)-porphyrin radical cation structure for I. Treatment of I with Me4NOH or oxidn. of TMPFeCl with iodosylbenzene produced a red compd. (II). The 1H NMR spectrum of II showed a resonance which was assigned to the β-pyrrole H atoms. The magnetic susceptibility and the Moessbauer spectrum of II suggested an Fe(IV) or Fe(V) structure for II. Both I and II reacted with I- to produce TMPFe hydroxide and with olefins to regenerate TMPFeCl and to produce epoxides. O transfer to olefins in the presence of H218O produced epoxide with 99% incorporation of the label.
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(b) Nam, W.; Choi, S. K.; Lim, M. H.; Rohde, J.-U.; Kim, I.; Kim, J.; Kim, C.; Que, L., Jr. Reversible Formation of Iodosylbenzene-Iron Porphyrin Intermediates in the Reaction of Oxoiron(IV) Porphyrin π-Cation Radicals and Iodobenzene. Angew. Chem., Int. Ed. 2003, 42, 109– 111, DOI: 10.1002/anie.200390036
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8b
Reversible formation of iodosylbenzene-iron porphyrin intermediates in the reaction of oxoiron(IV) porphyrin π-cation radicals and iodobenzene
Nam, Wonwoo; Choi, Sun Kyung; Lim, Mi Hee; Rohde, Jan-Uwe; Kim, Inwoo; Kim, Jinheung; Kim, Cheal; Que, Lawrence, Jr.
Angewandte Chemie, International Edition (2003), 42 (1), 109-111CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
New [(porph)FeIIIOIPh]+ intermediates are generated in the reaction of oxoiron(IV) porphyrin π-cation radicals [(porph)FeIV:O]+ with PhI, and the electronic nature of iron porphyrin complexes and iodobenzene derivs. markedly influences the equil. between these two forms. These intermediates are converted back to the starting Fe(III) complexes ((porph)FeIII(CF3SO3)) upon addn. of olefins, which are epoxidized.
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(c) Groves, J. T.; Watanabe, Y. Oxygen Activation by Metalloporphyrins Related to Peroxidase and Cytochrome P-450. Direct Observation of the Oxygen-Oxygen Bond Cleavage Step. J. Am. Chem. Soc. 1986, 108, 7834– 7836, DOI: 10.1021/ja00284a058
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8c
Oxygen activation by metalloporphyrins related to peroxidase and cytochrome P-450. Direct observation of the oxygen-oxygen bond cleavage step
Groves, John T.; Watanabe, Yoshihito
Journal of the American Chemical Society (1986), 108 (24), 7834-6CODEN: JACSAT; ISSN:0002-7863.
In this abstr. TMP is 5,10,15,20-tetramesitylporphyrinato ligand. Treatment of Fe(III)TMP(OH) with RC6H4CO3H (I; R = p-NO2) at -46° gave the corresponding RC(O)OOFe(III)TMP (II; R = p-O2NC6H4) which, upon standing in soln., smoothly decompd. to the corresponding oxoiron(IV) porphyrin radical cation [RCO2Fe(O)TMP]+• (III). The 0.5-order dependence of the rate on the concn. of excess I indicated acid-catalyzed O-O bond cleavage. I contg. electron withdrawing groups facilitated the conversion of II to III and the relative rates, at const. acidity, had on LFER in σ with ρ 0.5. The temp. dependence for the conversion of II (R = m-ClC6H4) to III in the presence of excess I (R = m-Cl) showed that the conversion had a very low activation enthalpy and a large neg. activation entropy. The conversion of II to III involves acid catalyzed heterolytic O-O bond cleavage.
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(d) Yamaguchi, K.; Watanabe, Y.; Morishima, I. Direct Observation of the Push Effect on the Oxygen-Oxygen Bond Cleavage of Acylperoxoiron(III) Porphyrin Complexes. J. Am. Chem. Soc. 1993, 115, 4058– 4065, DOI: 10.1021/ja00063a026
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8d
Direct observation of the push effect on the oxygen-oxygen bond cleavage of acylperoxoiron(III) porphyrin complexes
Yamaguchi, Kazuya; Watanabe, Yoshihito; Morishima, Isao
Journal of the American Chemical Society (1993), 115 (10), 4058-65CODEN: JACSAT; ISSN:0002-7863.
The 1st direct observation of the push effect on heterolytic and homolytic O-O bond cleavage steps is reported for ligand dissocn. reactions in acylperoxoiron(III) meso-substituted porphyrin complexes (5). In transformation of 5 to the corresponding oxoferryl (O:FeIV) porphyrin cation radicals (6) in CH2Cl2 at -80°, heterolytic O-O bond cleavage is 1st order. Introduction of electron-donating substituents at the meso-positions of the porphyrin ring facilitates the O-O bond cleavage in 5. Addn. of 1 equiv of imidazole derivs. to a CH2Cl2 soln. of 5 immediately gave an acylperoxoiron(III) porphyrin-imidazole adduct (9). Heterolytic bond cleavage in 9 6 also is 1st order in [9], and was accelerated by the coordination of electron-rich imidazole derivs. However, the push effect on the homolytic O-O bond cleavage reaction was examd. in toluene at -6° to ∼-40°. The homolytic O-O bond cleavage of 9 afforded the imidazole adduct of oxoferryl porphyrin complex when phenylperacetic acid was employed. Homolysis of the O-O bond is enhanced by the imidazole ligation; however, the push effect on homolysis is much less than that on heterolysis. These results explain the biol. use of strong electron-donor ligands in heme enzymes such as peroxidase, cytochrome P 450, and catalase.
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(e) Fujii, H. Effects of the Electron-Withdrawing Power of Substituents on the Electronic Structure and Reactivity in Oxoiron(IV) Porphyrin π-Cation Radical Complexes. J. Am. Chem. Soc. 1993, 115, 4641– 4648, DOI: 10.1021/ja00064a027
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8e
Effects of the electron-withdrawing power of substituents on the electronic structure and reactivity in oxoiron(IV) porphyrin π-cation radical complexes
Fujii, Hiroshi
Journal of the American Chemical Society (1993), 115 (11), 4641-8CODEN: JACSAT; ISSN:0002-7863.
The effects of the electron-withdrawing power of the substituents bound to a porphyrin ring on the electronic structures and the reactivities of oxoiron(IV) porphyrin π-cation radical complexes were studied by using 2,7,12,17-tetramethyl-3,8,13,18-tetraarylporphyrins (I; aryl = mesityl, 2-chloro-6-methylphenyl, 2,6-dichlorophenyl, or 2,4,6-trichlorophenyl) and tetrakis-5,10,15,20-tetraarylporphyrins (II). The electronic structures of oxoiron(IV) porphyrin π-cation radicals were investigated by low-temp. UV-visible absorption spectra and 1H NMR measurements. The absorption spectra features of oxoiron(IV) porphyrin π-cation radicals of I changed with an increase of the electron-withdrawing power of ring substituents, while those of II did not. 1H NMR measurements demonstrated that oxoiron(IV) porphyrin radicals of I have an a1u radical character and that those of II are better described as an a2u radical species. The reactivities of oxygen atoms of oxoiron(IV) porphyrin π-cation radicals were examd. by competitive epoxidn. of cyclohexene by two oxoiron(IV) porphyrin π-cation radicals with different radical orbital occupancies or oxidn. potentials. The reactivity of the O atom of the oxoiron(IV) porphyrin π-cation radical depends on its oxidn. potential and is not affected by the a1u/a2u orbital occupancy.
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(a) Cussó, O.; Ribas, X.; Costas, M. Biologically Inspired Non-Heme Iron-Catalysts for Asymmetric Epoxidation; Design Principles and Perspectives. Chem. Commun. 2015, 51, 14285– 14298, DOI: 10.1039/C5CC05576H
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9a
Biologically inspired non-heme iron-catalysts for asymmetric epoxidation; design principles and perspectives
Cusso, Olaf; Ribas, Xavi; Costas, Miquel
Chemical Communications (Cambridge, United Kingdom) (2015), 51 (76), 14285-14298CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
Iron coordination complexes with nitrogen and oxygen donor ligands have long since been known to react with peroxides producing powerful oxidizing species. These compds. can be regarded as simple structural and functional models of the active sites of non-heme iron dependent oxygenases. Research efforts during the last decade have uncovered basic principles and structural coordination chem. motifs that permit us to control the chem. that evolves when these iron complexes react with peroxides, in order to provide powerful metal-based, but at the same time selective, oxidising agents. Oxidn. methodologies with synthetic value are currently emerging from this approach. The current review focuses on asym. epoxidn., a reaction which has large value in synthesis, and where iron/H2O2 based methodologies may represent not only a sustainable choice, but may also expand the scope of state-of-the-art oxidn. methods. Basic principles that underlay catalyst design as well as H2O2 activation are discussed, while limitations and future perspectives are also reviewed.
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(b) Kleespies, S. T.; Oloo, W. N.; Mukherjee, A.; Que, L., Jr. C-H Bond Cleavage by Bioinspired Nonheme Oxoiron(IV) Complexes, Including Hydroxylation of n-Butane. Inorg. Chem. 2015, 54, 5053– 5064, DOI: 10.1021/ic502786y
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9b
C-H Bond Cleavage by Bioinspired Nonheme Oxoiron(IV) Complexes, Including Hydroxylation of n-Butane
Kleespies, Scott T.; Oloo, Williamson N.; Mukherjee, Anusree; Que, Lawrence, Jr.
Inorganic Chemistry (2015), 54 (11), 5053-5064CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
The development of efficient and selective hydrocarbon oxidn. processes with low environmental impact remains a major challenge of the 21st century because of the strong and apolar nature of the C-H bond. Naturally occurring iron-contg. metalloenzymes can, however, selectively functionalize strong C-H bonds on substrates under mild and environmentally benign conditions. The key oxidant in a no. of these transformations is postulated to possess an S = 2 FeIV=O unit in a nonheme ligand environment. This oxidant has been trapped and spectroscopically characterized and its reactivity toward C-H bonds demonstrated for several nonheme iron enzyme classes. In order to obtain insight into the structure-activity relationships of these reactive intermediates, over 60 synthetic nonheme FeIV(O) complexes have been prepd. in various labs. and their reactivities investigated. This Forum Article summarizes the current status of efforts in the characterization of the C-H bond cleavage reactivity of synthetic FeIV(O) complexes and provides a snapshot of the current understanding of factors that control this reactivity, such as the properties of the supporting ligands and the spin state of the iron center. In addn., new results on the oxidn. of strong C-H bonds such as those of cyclohexane and n-butane by a putative S = 2 synthetic FeIV(O) species that is generated in situ using dioxygen at ambient conditions are presented.
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(a) Chan, S. L.-F.; Kan, Y.-H.; Yip, K.-L.; Huang, J.-S.; Che, C.-M. Ruthenium Complexes of 1,4,7-Trimethyl-1,4,7-triazacyclononane for Atom and Group Transfer Reations. Coord. Chem. Rev. 2011, 255, 899– 919, DOI: 10.1016/j.ccr.2010.11.026
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10a
Ruthenium complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane for atom and group transfer reactions
Chan, Sharon Lai-Fung; Kan, Yu-He; Yip, Ka-Lai; Huang, Jie-Sheng; Che, Chi-Ming
Coordination Chemistry Reviews (2011), 255 (7-8), 899-919CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
A review. With support by macrocyclic tertiary amine ligand 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3tacn), a no. of mononuclear metal-ligand multiple bonded complexes were isolated. Starting with a brief summary of these complexes, the present review focuses on ruthenium-oxo and -imido complexes of Me3tacn. A family of monooxoruthenium(IV) complexes [RuIV(Me3tacn)O(N-N)]2+ (N-N = 2,2'-bipyridines) and a cis-dioxoruthenium(VI) complex cis-[RuVI(Me3tacn)O2(CF3CO2)]+ were isolated, and the structures of [RuIV(Me3tacn)O(bpy)](ClO4)2 (bpy = 2,2'-bipyridine) and cis-[RuVI(Me3tacn)O2(CF3CO2)]ClO4 were detd. by x-ray crystallog. Oxidn. of [RuIII(Me3tacn)(NHTs)2(OH)] (Ts = p-toluenesulfonyl) with Ag+ and electrochem. oxidn. of [RuIII(Me3tacn)(H2L)](ClO4)2 (H3L = α-(1-amino-1-methylethyl)-2-pyridinemethanol) probably generate ruthenium-imido complexes supported by Me3tacn. DFT calcns. on cis-[RuVI(Me3tacn)O2(CF3CO2)]+ and proposed ruthenium-imido complexes were performed. [RuIV(Me3tacn)O(N-N)]2+ are reactive toward alkene epoxidn., and cis-[RuVI(Me3tacn)O2(CF3CO2)]+ efficiently oxidizes various org. substrates including concerted [3 + 2] cycloaddn. reactions with alkynes and alkenes to selectively afford α,β-diketones, cis-diols, or C=C bond cleavage products. Related oxidn. reactions catalyzed by ruthenium Me3tacn complexes include epoxidn. of alkenes, cis-dihydroxylation of alkenes, oxidn. of alkanes, alcs., aldehydes, and arenes, and oxidative cleavage of C≡C, C=C, and C-C bonds, all of which exhibit high selectivity. Ruthenium Me3tacn complexes are also active catalysts for amination of satd. C-H bonds.
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(b) Yip, W.-P.; Ho, C.-M.; Zhu, N.; Lau, T.-C.; Che, C.-M. Homogeneous [Ru^III(Me₃tacn)Cl₃]-Catalyzed Alkene cis-Dihydroxylation with Aqueous Hydrogen Peroxide. Chem. - Asian J. 2008, 3, 70– 77, DOI: 10.1002/asia.200700237
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10b
Homogeneous [RuIII(Me3tacn)Cl3]-catalyzed alkene cis-dihydroxylation with aqueous hydrogen peroxide
Yip, Wing-Ping; Ho, Chi-Ming; Zhu, Nianyong; Lau, Tai-Chu; Che, Chi-Ming
Chemistry - An Asian Journal (2008), 3 (1), 70-77CODEN: CAAJBI; ISSN:1861-4728. (Wiley-VCH Verlag GmbH & Co. KGaA)
A simple and green method that uses [Ru(Me3tacn)Cl3] (1; Me3tacn = N,N',N''-trimethyl-1,4,7-triazacyclononane) as catalyst, aq. H2O2 as the terminal oxidant, and Al2O3 and NaCl as additives is effective in the cis-dihydroxylation of alkenes in aq. tert-butanol. Unfunctionalized alkenes, including cycloalkenes, aliph. alkenes, and styrenes (14 examples) were selectively oxidized to their corresponding cis-diols in 70-96% yields based on substrate conversions of up to 100%. The prepn. of cis-1,2-cycloheptanediol (119 g, 91% yield) and cis-1,2-cyclooctanediol (128 g, 92% yield) from cycloheptene and cyclooctene, resp., on the 1-mol scale can be achieved by scaling up the reaction without modification. Results from Hammett correlation studies on the competitive oxidn. of para-substituted styrenes (ρ = -0.97, R = 0.988) and the detection of the cycloadduct [(Me3tacn)ClRuHO2(C8H14)]+ by ESI-MS for the 1-catalyzed oxidn. of cyclooctene to cis-1,2-cyclooctanediol are similar to those of the stoichiometric oxidn. of alkenes by cis-[(Me3tacn)-(CF3CO2)RuVIO2]+ through [3+2] cycloaddn.
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(c) Kojima, T.; Matsuo, H.; Matsuda, Y. Catalytic Hydrocarbon Oxygenation by Ruthenium-Pyridylamine Complexes with Alkyl Hydroperoxides: A Mechanistic Insight. Inorg. Chim. Acta 2000, 300–302, 661– 667, DOI: 10.1016/S0020-1693(99)00571-X
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10c
Catalytic hydrocarbon oxygenation by ruthenium-pyridylamine complexes with alkyl hydroperoxides: a mechanistic insight
Kojima, T.; Matsuo, H.; Matsuda, Y.
Inorganica Chimica Acta (2000), 300-302 (), 661-667CODEN: ICHAA3; ISSN:0020-1693. (Elsevier Science S.A.)
The use of TBHP (t-Bu hydroperoxide) or CHP (cumene hydroperoxide) with bis-μ-chloro Ru(II) dimers, [RuIICl(L)]2(ClO4)2 (L=tris(2-pyridylmethyl)amine and tris(5-methyl-2-pyridylmethyl)amine), gave catalytic alkane and alkene oxygenation at 40°C. These reactions were found to proceed via a Haber-Weiss-type electron-transfer reaction to generate alkoxo (RO√) and alkylperoxo (ROO√) radicals as reactive species. This electron transfer was mainly governed by the redox potentials of a complex employed as a catalyst. In addn., the environment around a ruthenium center(s) involving steric hindrance due to substituents on the pyridine rings and an intramol. π-π interaction, should be also significant to regulate the reactivity of the catalyst; probably in terms of shielding of the ruthenium center(s) against the approach of peroxides to undergo the electron transfer. For those reactions with alkyl hydroperoxides, O2 generated from peroxide decompn. plays an important role in promoting reactions as a radical chain carrier.
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Huynh, M. H. V.; Meyer, T. J. Proton-Coupled Electron Transfer. Chem. Rev. 2007, 107, 5004– 5064, DOI: 10.1021/cr0500030
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11
Proton-Coupled Electron Transfer
Huynh, My Hang V.; Meyer, Thomas J.
Chemical Reviews (Washington, DC, United States) (2007), 107 (11), 5004-5064CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review. Proton-Coupled Electron Transfer (PCET) describes reactions in which there is a change in both electron and proton content between reactants and products. It originates from the influence of changes in electron content on acid-base properties and provides a mol.-level basis for energy transduction between proton transfer and electron transfer. A review with 855 refs.
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(a) England, J.; Guo, Y.; Farquhar, E. R.; Young, V. G., Jr.; Münck, E.; Que, L., Jr. The Crystal Structure of a High-Spin Oxoiron(IV) Complex and Characterization of Its Self-Decay Pathway. J. Am. Chem. Soc. 2010, 132, 8635– 8644, DOI: 10.1021/ja100366c
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12a
The Crystal Structure of a High-Spin Oxoiron(IV) Complex and Characterization of Its Self-Decay Pathway
England, Jason; Guo, Yisong; Farquhar, Erik R.; Young, Victor G., Jr.; Munck, Eckard; Que, Lawrence, Jr.
Journal of the American Chemical Society (2010), 132 (25), 8635-8644CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
[FeIV(O)(TMG3tren)]2+ (1; TMG3tren = 1,1,1-tris{2-[N2-(1,1,3,3-tetramethylguanidino)]ethyl}amine) is a unique example of an isolable synthetic S = 2 oxoiron(IV) complex, which serves as a model for the high-valent oxoiron(IV) intermediates obsd. in nonheme iron enzymes. Congruent with DFT calcns. predicting a more reactive S = 2 oxoiron(IV) center, 1 has a lifetime significantly shorter than those of related S = 1 oxoiron(IV) complexes. The self-decay of 1 exhibits strictly first-order kinetic behavior and is unaffected by solvent deuteration, suggesting an intramol. process. This hypothesis was supported by ESI-MS anal. of the iron products and a significant retardation of self-decay upon use of a perdeuteromethyl TMG3tren isotopomer, d36-1 (KIE = 24 at 25°C). The greatly enhanced thermal stability of d36-1 allowed growth of diffraction quality crystals for which a high-resoln. crystal structure was obtained. This structure showed an Fe=O unit (r = 1.661(2) Å) in the intended trigonal bipyramidal geometry enforced by the sterically bulky tetramethylguanidinyl donors of the tetradentate tripodal TMG3tren ligand. The close proximity of the Me substituents to the oxoiron unit yielded three sym. oriented short C-D···O nonbonded contacts (2.38-2.49 Å), an arrangement that facilitated self-decay by rate-detg. intramol. hydrogen atom abstraction and subsequent formation of a ligand-hydroxylated iron(III) product. EPR and Mossbauer quantification of the various iron products, referenced against those obtained from reaction of 1 with 1,4-cyclohexadiene, allowed formulation of a detailed mechanism for the self-decay process. The soln. of this first crystal structure of a high-spin (S = 2) oxoiron(IV) center represents a fundamental step on the path toward a full understanding of these pivotal biol. intermediates.
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(b) Klinker, E. J.; Kaizer, J.; Brennessel, W. W.; Woodrum, N. L.; Cramer, C. J.; Que, L., Jr. Structures of Nonheme Oxoiron(IV) Complexes from X-ray Crystallography, NMR Spectroscopy, and DFT Calculations. Angew. Chem., Int. Ed. 2005, 44, 3690– 3694, DOI: 10.1002/anie.200500485
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12b
Structures of nonheme oxoiron(IV) complexes from X-ray crystallography, NMR spectroscopy, and DFT calculations
Klinker, Eric J.; Kaizer, Jozsef; Brennessel, William W.; Woodrum, Nathaniel L.; Cramer, Christopher J.; Que, Lawrence, Jr.
Angewandte Chemie, International Edition (2005), 44 (24), 3690-3694CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
From a combination of x-ray crystallog., NMR spectroscopy, and DFT calcns., the relative thermal stabilities of two oxoiron(IV) complexes with pentaaza ligands, [FeIV(O)(N4Py)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) and [FeIV(O)(Bn-TPEN)]2+ (Bn-TPEN = N-benzyl-N,N',N'-tris(2-pyridylmethyl)-1,2-diaminoethane) can be ascribed to the no. of pyridine rings that are oriented parallel to the Fe:O bond.
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13
(a) Mandimutsira, B. S.; Ramdhanie, B.; Todd, R. C.; Wang, H.; Zareba, A. A.; Czernuszewicz, R. S.; Goldberg, D. P. A Stable Manganese(V)-Oxo Corrolazine Complex. J. Am. Chem. Soc. 2002, 124, 15170– 15171, DOI: 10.1021/ja028651d
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13a
A Stable Manganese(V)-Oxo Corrolazine Complex
Mandimutsira, Beaven S.; Ramdhanie, Bobby; Todd, Ryan C.; Wang, Hailin; Zareba, Adelajda A.; Czernuszewicz, Roman S.; Goldberg, David P.
Journal of the American Chemical Society (2002), 124 (51), 15170-15171CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
I (R = p-tBuC6H4) reacted with Mn(acac)3 to give MnL (H3L = I) which was oxidized to MnO(L). Stable MnO(L) was characterized by resonance Raman spectra. The oxidn. of PPh3 or Me2S by MnO(L) was obsd. with the formation of MnL.
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(b) Zaragoza, J. P. T.; Siegler, M. A.; Goldberg, D. P. A Reactive Manganese(IV)-Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds. J. Am. Chem. Soc. 2018, 140, 4380– 4390, DOI: 10.1021/jacs.8b00350
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13b
A Reactive Manganese(IV)-Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds
Zaragoza, Jan Paulo T.; Siegler, Maxime A.; Goldberg, David P.
Journal of the American Chemical Society (2018), 140 (12), 4380-4390CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
High-valent metal-hydroxide species are invoked as crit. intermediates in both catalytic, metal-mediated O2 activation (e.g., by Fe porphyrin in Cytochrome P 450) and O2 prodn. (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear MIV(OH) complexes remain a rarity. Herein the authors describe the synthesis of MnIV(OH)(ttppc) (3) (ttppc = tris(2,4,6-triphenylphenyl) corrole), which was characterized by XRD. The large steric encumbrance of the ttppc ligand allowed for isolation of 3. The complexes MnV(O)(ttppc) (4) and MnIII(H2O)(ttppc) (1·H2O) were also synthesized and structurally characterized, providing Mn complexes related only by the transfer of H atoms. Both 3 and 4 abstr. an H atom from the O-H bond of 2,4-di-tert-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (3 = 90(2)%, 4 = 91(5)%). Complex 3 reacts with 2,4-DTBP with a rate const. of k2 = 2.73(12) × 104 M-1 s-1, which is ∼3 orders of magnitude larger than 4 (k2 = 17.4(1) M-1 s-1). Reaction of 3 with para-substituted 2,6-di-tert-butylphenol derivs. (4-X-2,6-DTBP; X = OMe, Me, tBu, H) gives rate consts. in the range k2 = 510(10)-36(1.4) M-1 s-1 and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, O-H cleavage occurs by a concerted H atom transfer (HAT) mechanism and the MnIV(OH) complex is a much more powerful H atom abstractor than the higher-valent MnV(O) complex, or even some FeIV(O) complexes.
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(c) Halbach, R. L.; Gygi, D.; Bloch, E.; Anderson, B. L.; Nocera, D. G. Structurally Characterized Terminal Manganese(IV) Oxo Tris(alkoxide) Complex. Chem. Sci. 2018, 9, 4524– 4528, DOI: 10.1039/C8SC01164H
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13c
Structurally characterized terminal manganese(IV) oxo tris(alkoxide) complex
Halbach, Robert L.; Gygi, David; Bloch, Eric D.; Anderson, Bryce L.; Nocera, Daniel G.
Chemical Science (2018), 9 (19), 4524-4528CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
A Mn(IV) complex featuring a terminal oxo ligand, [MnIV(O)(ditox)3][K(15-C-5)2] (3; ditox = tBu2MeCO-, 15-C-5 = 15-crown-5-ether) has been isolated and structurally characterized. Treatment of the colorless precursor [MnII(ditox)3][K(15-C-5)2] (2) with iodosobenzene affords 3 as a green free-flowing powder in high yields. The X-ray crystal structure of 3 reveals a pseudotetrahedral geometry about the central Mn, which features a terminal oxo (d(Mn-Oterm = 1.628(2) Å)). EPR spectroscopy, SQUID magnetometry, and Evans method magnetic susceptibility indicate that 3 consists of a high-spin S = 3/2 Mn(IV) metal center. 3 promotes C-H bond activation by a hydrogen atom abstraction. The [MnIV(O)(ditox)3]- furnishes a model for the proposed terminal oxo of the unique manganese of the oxygen evolving complex of photosystem II.
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(a) Qin, K.; Incarvito, C. D.; Rheingold, A. L.; Theopold, K. H. Hydrogen Atom Abstraction by a Chromium(IV) Oxo Complex Derived from O₂. J. Am. Chem. Soc. 2002, 124, 14008– 14009, DOI: 10.1021/ja028382r
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14a
Hydrogen Atom Abstraction by a Chromium(IV) Oxo Complex Derived from O2
Qin, Kun; Incarvito, Christopher D.; Rheingold, Arnold L.; Theopold, Klaus H.
Journal of the American Chemical Society (2002), 124 (47), 14008-14009CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The Cr(III) hydroxide [TptBu,MeCr(OH)(pz'H)]BARF (1, TptBu,Me = hydrotris(3-tert-butyl-5-methylpyrazolyl)borate, pz'H = 3-tert-butyl-5-methylpyrazole, BARF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) is produced by reaction of [TptBu,MeCr(pz'H)]BARF with [TptBu,MeCr(O2)(pz'H)]BARF or O atom donors ONMe3 or PhIO in Et2O. However, reaction of [TptBu,MeCr(pz'H)]BARF with PhIO in pure CH2Cl2 yields the Cr(IV) oxo complex [TptBu,MeCr(O)(pz'H)]BARF (2). 2 Abstrs. H atoms from org. mols. with weak C-H bonds to form 1. Both 1 and 2 were structurally characterized by x-ray crystallog.
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(b) Collins, T. J.; Slebodnick, C.; Uffelman, E. S. Chromium(V)-Oxo Complexes of Macrocyclic Tetraamido-N-Ligands Tailored for Highly Oxidized Middle Transition Metal Complexes: A New ¹⁸O-Labeling Reagent and a Structure with Four Nonplanar Amides. Inorg. Chem. 1990, 29, 3433– 3436, DOI: 10.1021/ic00343a030
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14b
Chromium(V)-oxo complexes of macrocyclic tetraamido-N ligands tailored for highly oxidized middle transition metal complexes: a new oxygen-18-labeling reagent and a structure with four nonplanar amides
Collins, Terrence J.; Slebodnick, Carla; Uffelman, Erich S.
Inorganic Chemistry (1990), 29 (18), 3433-6CODEN: INOCAJ; ISSN:0020-1669.
Me4N[CrOL] (LH4 = I) and Me4N[CrOL1] (L1H4 = II) were prepd. and characterized by x-ray crystallog. and IR and EPR spectroscopies. Because exchange of the oxo ligand with H2O is slow, the easily synthesized, stable, cryst. Me2(H18O18O)CCH2CH2C(18O18OH)Me2 was prepd. and used to conveniently synthesized 18O-labeled oxo complexes in high yields. The bonding of the 2 unique oxidn.-resistant macrocyclic tetraamides to Cr is compared. The structural and EPR properties are consistent with a Cr-centered radical in each case and suggest that a Cr(V) oxidn. state assignment is equally appropriate whether the ancillary ligand is the innocent [η4-L]4- or the potentially noninnocent [η4-L1]4-. Both oxo complexes contain nonplanar amide groups. The distortions of [Cr(O)(η4-L)]- are more marked, and it is a unique species in contg. 4 distinctly nonplanar amides. The discovery of these unusual structural parameters expands the class of nonplanar amides arising from ring constraint. Me4N[CrOL] is orthorhombic, space group P212121, Z = 4 whereas Me4N[CrOL1] is monoclinic, space group P21/c, Z = 4.
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(c) Srinivasan, K.; Kochi, J. K. Synthesis and Molecular Structure of Oxochromium(V) Cations. Coordination with Donor Ligands. Inorg. Chem. 1985, 24, 4671– 4679, DOI: 10.1021/ic00220a049
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14c
Synthesis and molecular structure of oxochromium(V) cations. Coordination with donor ligands
Srinivasan, K.; Kochi, J. K.
Inorganic Chemistry (1985), 24 (26), 4671-9CODEN: INOCAJ; ISSN:0020-1669.
CrOL+ (H2L = bis(salicylidene)ethylenediamine and its 5,5'-dichloro, 7,7'-dimethyl, 7,7'-diphenyl, 8,8,8',8'-tetramethyl, 5,5'-dichloro-8,8,8',8'-tetramethyl-, and 8,8'-benzo derivs.) were prepd. by O-transfer reactions of [CrL(H2O)2]X (X = triflate) with isodosylbenzene or m-ClC6H4CO2OH. X-ray crystallog. anal. of [CrOL]X (H2L = 7,7'-dimethyl deriv.) indicates that the 5-coordinate Cr atom is situated 0.53 Å above the Schiff base (mean) plane and describes a square-pyramidal configuration with the oxo ligand occupying the apical position. Isotopic 18O-substitution leads to a shift in the O:Cr stretching frequency from 1004 to 965 cm-1 in accord with theor. predictions. Similarly the magnetic susceptibility and the well-resolved isotropic ESR spectra reliably reflect the d1 electron configuration of the oxochromium(V) species in CH3CN solns. CrOL+ and various donor ligands such as pyridine N-oxide (Q), Ph3PO, and H2O form 1:1 assocn. complexes, the formation consts. K of which vary from 10-2 to 103 M-1, depending on the donor ligand and the substituent groups located on the Schiff base periphery. X-ray crystallog. detn. of [CrOLQ]X (H2L = 5,5'-dichloro-8,8,8'8'-tetramethyl deriv.) indicates that the donor ligand fills the apical position in CrOL+ to complete the octahedral coordination about Cr. Isotopic 18O-tracer studies of the formation of oxochromium(V) by O atom transfer to the Cr(III) complex are described. [CrOL]X is monoclinic, space group P21/n, with a 16.233(2), b 6.439(1), c 19.523(4) Å, β 94.44(1)°, Z = 4. [CrOLQ]X is tetragonal, space group P43212, with a 11.938(1), c 43.366(9) Å, Z = 8. The cyclic voltammetry of the oxochromium(V) cations is described.
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(a) Cundari, T. R.; Saunders, L.; Sisterhen, L. L. Molecular Modeling of Vanadium-Oxo Complexes. A Comparison of Quantum and Classical Methods. J. Phys. Chem. A 1998, 102, 997– 1004, DOI: 10.1021/jp972827u
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15a
Molecular Modeling of Vanadium-Oxo Complexes. A Comparison of Quantum and Classical Methods
Cundari, Thomas R.; Saunders, Leah; Sisterhen, Laura L.
Journal of Physical Chemistry A (1998), 102 (6), 997-1004CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
A force field for vanadium-oxos was developed and tested with a variety of complexes with coordination nos. of 5 or 6 and formal oxidns. states of +4 or +5 on the metal. Similarly, a semiempirical quantum mech. method for transition metals was extended to vanadium. In this research soft and hard ligands were studied, as were ligands coordinated through single, multiple, and dative bonds. Despite the diversity of vanadium coordination chem., generally good modeling is achieved in a fraction of the time with less computational resources using mol. mechanics and semiempirical quantum mechanics. The L4V4+O and L5V5+O groups were emphasized given their prevalence and importance. In general, the predictive ability was superior for the former structural motif. The combination of mol. mechanics and semiempirical quantum calcns. provide an effective and efficient tool for anal. of the steric and electronic energy differences between isomers.
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(b) Mchiri, C.; Amiri, N.; Jabli, S.; Roisnel, T.; Nasri, H. The (oxo)[2,3,7,8,12,13,17,18-octachloro-5,10,15,20-tetrakis(4-tolylporphyrinato)]vanadium(IV): Synthesis, UV-Visible, Cyclic Voltammetry and X-ray Crystal Structure. J. Mol. Struct. 2018, 1154, 51– 58, DOI: 10.1016/j.molstruc.2017.10.032
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15b
The (oxo)[(2,3,7,8,12,13,17,18-octachloro)-5,10,15,20-tetrakis(4-tolylporphyrinato)]vanadium(IV): Synthesis, UV-visible, Cyclic voltammetry and X-ray crystal structure
Mchiri, Chadlia; Amiri, Nesrine; Jabli, Souhir; Roisnel, Thierry; Nasri, Habib
Journal of Molecular Structure (2018), 1154 (), 51-58CODEN: JMOSB4; ISSN:0022-2860. (Elsevier B.V.)
The present work is concerned with the oxo V(IV) complex of 2,3,7,8,12,13,17,18-octachloro-5,10,15,20-tetrakis(4-tolylporphyrin) [V(Cl8TTP)O] (1), which was prepd. by reacting the (oxo)[5,10,15,20-tetrakis(4-tolylporphyrinato)]vanadium(IV) complex ([V(TTP)O]), under aerobic atm., with a large excess of thionyl chloride (SOCl2). The title compd. was characterized by UV-visible spectroscopy, cyclic voltammetry and x-ray crystal structure. The electron-withdrawing Cl substituents at the pyrrole carbons in the vanadyl-Cl8TTP deriv. produce remarkable red shifts of the Soret and Q absorption bands and an important anodic shift of the porphyrin ring oxidn. and redn. potentials. This is an indication that the porphyrin core of 1 is severely nonplanar in soln. The mol. structure of the vanadyl deriv. shows a very high saddle distortion and an important ruffled deformation of the porphyrin macrocycle. The crystal structure of 1 consists of 1-dimensional chains parallel to the c axis where channels are located between these chains.
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(c) Abernethy, C. D.; Codd, G. M.; Spicer, M. D.; Taylor, M. K. A Highly Stable N-Heterocyclic Carbene Complex of Trichloro-oxo-vanadium(V) Displaying Novel Cl-C_carbene Bonding Interactions. J. Am. Chem. Soc. 2003, 125, 1128– 1129, DOI: 10.1021/ja0276321
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15c
A Highly Stable N-Heterocyclic Carbene Complex of Trichloro-oxo-vanadium(V) Displaying Novel Cl-Ccarbene Bonding Interactions
Abernethy, Colin D.; Codd, Gareth M.; Spicer, Mark D.; Taylor, Michelle K.
Journal of the American Chemical Society (2003), 125 (5), 1128-1129CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Reaction of 1,3-dimesitylimidazol-2-ylidene and trichloro-oxo-vanadium(V) yields an air stable 1:1 adduct, which demonstrates the utility of N-heterocyclic carbenes to stabilize metal complexes in high oxidn. states. The stabilizing influence of the carbene ligand was further demonstrated electrochem. The mol. structure of this compd. reveals that the chloride ligands cis to the carbene are oriented toward the Ccarbene atom. D. functional theory calcns. on a hypothetical dimethyl- deriv. show that a bonding interaction occurs between lone pairs of these chlorides and the formally unoccupied p-orbital of the carbene. Previous studies indicated that this orbital was not involved in the bonding of N-heterocyclic carbenes to transition metals. The obsd. interaction therefore represents a new bonding mode for these widely used ligands.
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(a) Kojima, T.; Nakayama, K.; Ikemura, K.; Ogura, T.; Fukuzumi, S. Formation of a Ruthenium(IV)-Oxo Complex by Electron-Transfer Oxidation of a Coordinatively Saturated Ruthenium(II) Complex and Detection of Oxygen-Rebound Intermediates in C-H Bond Oxygenation. J. Am. Chem. Soc. 2011, 133, 11692– 11700, DOI: 10.1021/ja2037645
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16a
Formation of a Ruthenium(IV)-Oxo Complex by Electron-Transfer Oxidation of a Coordinatively Saturated Ruthenium(II) Complex and Detection of Oxygen-Rebound Intermediates in C-H Bond Oxygenation
Kojima, Takahiko; Nakayama, Kazuya; Ikemura, Ken-Ichiro; Ogura, Takashi; Fukuzumi, Shun-Ichi
Journal of the American Chemical Society (2011), 133 (30), 11692-11700CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
A coordinatively satd. ruthenium(II) complex having tetradentate tris(2-pyridylmethyl)amine (TPA) and bidentate 2,2'-bipyridine (bpy), [Ru(TPA)(bpy)]2+ (1), was oxidized by a Ce(IV) ion in H2O to afford a Ru(IV)-oxo complex, [Ru(O)(H+TPA)(bpy)]3+ (2). The crystal structure of the Ru(IV)-oxo complex 2 was detd. by X-ray crystallog. In 2, the TPA ligand partially dissocs. to be in a facial tridentate fashion and the uncoordinated pyridine moiety is protonated. The spin state of 2, which showed paramagnetically shifted NMR signals in the range of 60 to -20 ppm, was detd. to be an intermediate spin (S = 1) by the Evans' method with 1H NMR spectroscopy in acetone-d6. The reaction of 2 with various org. substrates in acetonitrile at room temp. afforded oxidized and oxygenated products and a solvent-bound complex, [Ru(H+TPA)(bpy)(CH3CN)], which is intact in the presence of alcs. The oxygenation reaction of satd. C-H bonds with 2 proceeds by two-step processes: the hydrogen abstraction with 2, followed by the dissocn. of the alc. products from the oxygen-rebound complexes, Ru(III)-alkoxo complexes, which were successfully detected by ESI-MS spectrometry. The kinetic isotope effects in the first step for the reaction of dihydroanthrathene (DHA) and cumene with 2 were detd. to be 49 and 12, resp. The second-order rate consts. of C-H oxygenation in the first step exhibited a linear correlation with bond dissocn. energies of the C-H bond cleavage.
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(b) Fackler, N. L. P.; Zhang, S.; O’Halloran, T. V. Stabilization of High-Valent Terminal-Oxo Complexes: Interplay of d-Orbital Occupancy and Coordination Geometry. J. Am. Chem. Soc. 1996, 118, 481– 482, DOI: 10.1021/ja953051i
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16b
Stabilization of High-Valent Oxo-Terminal Complexes: Interplay of d-Orbital Occupancy and Coordination Geometry
Fackler, Nathanael L. P.; Zhang, Songsheng; O'Halloran, Thomas V.
Journal of the American Chemical Society (1996), 118 (2), 481-2CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Pr4N[RuO(PHAB)] (I) (H4PHAB = 1,2-bis(2,2-diphenyl-2-hydroxyethanamido)benzene) was prepd. from RuO4- and H4PHAB and was oxidized by CeIV to [RuO(PHAB)] (II). The structures of I and II·Me2CO are reported. II represents the 1st structurally characterized mono-oxo RuVI complex. Comparison of I and II show that the preferred coordination geometry depends strongly on the formal occupancy of the metal-oxo π* orbitals. The stability of these complexes is estd. from reactivity comparisons. I and II facilitate C-H bond activation and O atom transfer reactions, I catalyzing the air oxidn. of PPh3. These results have important implications for the design of ligands that stabilize specific intermediates in catalytic reactions.
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(c) Che, C.-M.; Wong, K.-Y.; Mak, T. C. W. Oxo-Ruthenium(V) Complexes of Macrocyclic Tetradentate Tertiary Amines That Function as Active Electrochemical Oxidative Catalysts, and X-ray Crystal Structure of trans-[Ru^IV(tmc)O(Cl)]ClO₄ (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane). J. Chem. Soc., Chem. Commun. 1985, 988– 990, DOI: 10.1039/c39850000988
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16c
Oxoruthenium(V) complexes of macrocyclic tetradentate tertiary amines that function as active electrochemical oxidative catalysts, and x-ray crystal structure of trans-[Ru(IV)(tmc)O(Cl)]ClO4 (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane)
Che, Chi Ming; Wong, Kwok Yin; Mak, Thomas C. W.
Journal of the Chemical Society, Chemical Communications (1985), (14), 988-90CODEN: JCCCAT; ISSN:0022-4936.
trans-[Ru(IV)LO(Cl)]ClO4 (L = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (tmc), 1,4,8,12-tetramethyl-1,4,8,12-tetraazacyclopentadecane) were prepd. from trans-[Ru(VI)LO2](ClO4)2 suspended in acetone and an excess of PPh3. trans-Ru(V)LO(Cl)]2+, generated electrochem. from the corresponding trans-[Ru(IV)LO(Cl)]+ in MeCN contg. 1% C6H5CH2OH, catalyzed the in situ oxidn. of C6H5CH2OH to C6H5CHO. The structure of trans-[Ru(IV)(tmc)O(Cl)]ClO4 was detd. by x-ray crystallog.; crystals are orthorhombic, space group Pna21, with a 12.254(4), b 15.470(4), c 10.821(2) Å, d.(exptl.) = 1.63, d.(calcd.) = 1.646 g cm-3, and R = 0.077 for 1697 reflections.
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(a) Visentin, R.; Rossin, R.; Giron, M. C.; Dolmella, A.; Bandoli, G.; Mazzi, U. Synthesis and Characterization of Rhenium(V) Oxo Complexes with N-[N-(3-Diphenylphosphinopropionyl)glycyl]cysteine Methyl Ester. X-ray Crystal Structure of {ReO[Ph₂P(CH₂)₂C(O)-Gly-Cys-OMe(P, N, N, S)]}. Inorg. Chem. 2003, 42, 950– 959, DOI: 10.1021/ic025859r
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17a
Synthesis and Characterization of Rhenium(V) Oxo Complexes with N-[N-(3-Diphenylphosphinopropionyl)glycyl]cysteine Methyl Ester. X-ray Crystal Structure of {ReO[Ph2P(CH2)2C(O)-Gly-Cys-OMe(P,N,N,S)]}
Visentin, Roberta; Rossin, Raffaella; Giron, Maria Cecilia; Dolmella, Alessandro; Bandoli, Giuliano; Mazzi, Ulderico
Inorganic Chemistry (2003), 42 (4), 950-959CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
The PN2S chelate N-[N-(3-diphenylphosphinopropionyl)glycyl]-S-tritylcysteine Me ester [PN2S(Trt)-OMe] was synthesized and reacted with ReOCl3(PPh3)2 and Ph4P[ReOCl4]. The reactions of both tritylated and detritylated ligands with ReVO precursors gave two positional isomers, 9a and 9b, of the ReO(PN2S-OMe) complex. The two isomers, produced in a 1:1 molar ratio, are stable and do not interconvert. They were sepd. by reverse-phase HPLC and characterized by NMR, FTIR, and UV-visible spectroscopy and electrospray mass spectrometry. X-ray anal. established for 9a the presence in the solid of the syn isomer. Compd. 9a, C21H23N2O5PSRe, crystd. from warm MeCN in the triclinic space group P‾1, a 9.828(2), b 11.163(2), c 11.641(2) Å, α 106.48(3), β 109.06(3), γ 102.81(3)°, Z = 2. The PN2S coordination set is in the equatorial plane, and the complex shows a distorted square pyramidal coordination. The anti configuration assigned to 9b is consistent with all the available physicochem. data. Follow-up of the reaction of the detritylated ligand with Ph4P[ReOCl4] in EtOH or MeCN indicated that the P atom of the chelate binds first to the metal and that this bond acts as the driving force for coordination.
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(b) Most, K.; Köpke, S.; Dall’Antonia, F.; Mösch-Zanetti, N. C. The First Molybdenum Dioxo Compounds with η²-Pyrazolate Ligands: Crystal Structure and Oxo Transfer Properties. Chem. Commun. 2002, 1676– 1677, DOI: 10.1039/B205420E
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17b
The first molybdenum dioxo compounds with η2-pyrazolate ligands: crystal structure and oxo transfer properties
Most, Kerstin; Koepke, Sinje; Dall'Antonia, Fabio; Moesch-Zanetti, Nadia C.
Chemical Communications (Cambridge, United Kingdom) (2002), (16), 1676-1677CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
Mo dioxo compds. [MoO2Cl(η2-pz)] and [MoO2(η2-pz)2] with pz = η2-3,5-di-tert-butylpyrazolate were synthesized; crystallog. data, catalytic activity, and oxo transfer properties are described.
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(b1) Karunadasa, H. I.; Chang, C. J.; Long, J. R. A Molecular Molybdenum-Oxo Catalyst for Generating Hydrogen from Water. Nature 2010, 464, 1329– 1333, DOI: 10.1038/nature08969
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17b1
A molecular molybdenum-oxo catalyst for generating hydrogen from water
Karunadasa, Hemamala I.; Chang, Christopher J.; Long, Jeffrey R.
Nature (London, United Kingdom) (2010), 464 (7293), 1329-1333CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
A growing awareness of issues related to anthropogenic climate change and an increase in global energy demand have made the search for viable C-neutral sources of renewable energy one of the most important challenges in science today. The chem. community is therefore seeking efficient and inexpensive catalysts that can produce large quantities of hydrogen gas from H2O. Here the authors identify a Mo-oxo complex that can catalytically generate gaseous hydrogen either from H2O at neutral pH or from sea water. High-valency metal-oxo species can be used to create redn. catalysts that are robust and functional in H2O, a concept that has broad implications for the design of green' and sustainable chem. cycles.
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(c) Rayati, S.; Rafiee, N.; Wojtczak, A. cis-Dioxo-molybdenum(VI) Schiff Base Complexes: Synthesis, Crystal Structure and Catalytic Performance for Homogeneous Oxidation of Olefins. Inorg. Chim. Acta 2012, 386, 27– 35, DOI: 10.1016/j.ica.2012.02.005
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17c
cis-Dioxo-molybdenum(VI) Schiff base complexes: Synthesis, crystal structure and catalytic performance for homogeneous oxidation of olefins
Rayati, Saeed; Rafiee, Nasim; Wojtczak, Andrzej
Inorganica Chimica Acta (2012), 386 (), 27-35CODEN: ICHAA3; ISSN:0020-1693. (Elsevier B.V.)
The synthesis of two Mo(VI) tetradentate Schiff base complexes derived from 2,2'-dimethylpropylenediamine and 2-hydroxy-1-naphthaldehyde (hnaphnptnH2 = (2-HOC10H6CH:NCH2)2CMe2) or 3-methoxysalicylaldehyde (salnptn(3-OMe)2H2 = (2-HO-3-MeOC6H3CH:NCH2)2CMe2), (MoO2{hnaphnptn} (1) and MoO2{salnptn(3-OMe)2} (2)) is reported. Full characterization of these complexes was accomplished with elemental analyses, spectroscopic studies (1H NMR, IR, and UV-visible) and x-ray structure anal. X-ray crystallog. studies reveal that these complexes adopt a distorted octahedral six-coordinate configuration formed by tetradentate Schiff base ligand and two O atoms. Catalytic performance of the prepd. Mo complexes for oxidn. of different olefins with tert-Bu hydroperoxide was evaluated. These complexes are efficient and selective catalysts for the homogeneous oxidn. of various olefins. MoO2{salnptn(3-OMe)2} (2) with a methoxy groups on the salicylidene ring of the ligand promotes the effectiveness of the catalyst.
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(a) Dinda, S.; Drew, M. G. B.; Bhattacharyya, R. Oxo-Rhenium(V) Complexes with Bidentate Phosphine Ligands: Synthesis, Crystal Structure and Catalytic Potentiality in Epoxidation of Olefins Using Hydrogen Peroxide Activated Bicarbonate as Oxidant. Catal. Commun. 2009, 10, 720– 724, DOI: 10.1016/j.catcom.2008.11.028
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18a
Oxo-rhenium(V) complexes with bidentate phosphine ligands: Synthesis, crystal structure and catalytic potentiality in epoxidation of olefins using hydrogen peroxide activated bicarbonate as oxidant
Dinda, Subhajit; Drew, Michael G. B.; Bhattacharyya, Ramgopal
Catalysis Communications (2009), 10 (5), 720-724CODEN: CCAOAC; ISSN:1566-7367. (Elsevier B.V.)
Two oxorhenium(V) complexes with bidentate phosphine ligands were synthesized and isolated as [ReOCl3(dppm)] and [ReOCl3(dppp)] [dppm = 1,1-bis(diphenylphosphino)methane; dppp = 1,3-bis(diphenylphosphino)propane]. The dppp complex was structurally characterized. Both the complexes were used as catalysts in the epoxidn. of olefins using NaHCO3 as co-catalyst and H2O2 as terminal oxidant.
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(a) Galas, A. M. R.; Hursthouse, M. B.; Behrman, E. J.; Midden, W. R.; Green, G.; Griffith, W. P. The X-ray Crystal Structures of the Oxo-Osmium Complexes, OsO₂(OH)₂phen (1) and Os₂O₆py₄ (2). Transition Met. Chem. 1981, 6, 194– 195, DOI: 10.1007/BF00624344
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19a
The x-ray crystal structures of the oxoosmium complexes, OsO2(OH)2phen (1) and Os2O6py4 (2)
Galas, Anita M. R.; Hursthouse, Michael B.; Behrman, E. J.; Midden, W. R.; Green, G.; Griffith, William P.
Transition Metal Chemistry (Dordrecht, Netherlands) (1981), 6 (3), 194-5CODEN: TMCHDN; ISSN:0340-4285.
Os O2(OH)2 phen.5H2O (I) is monoclinic, space group P21/n, with a 8.125(2), b 18.259(2), c 11.197(1) Å, and β 90.18(1)°; d.(calcd.) = 2.10 for Z = 4. Os2O6Py4.6H2O (II) is triclinic, space group P‾1, with a 8.238(10), b 11.701(6), c 8.499(5) Å, α 109.91(5), β 110.54(8), and γ 67.74(7)°; d.(calcd.) = 2.28 for Z = 1. The structures were refined to final R's = 0.032 and 0.081, resp. The Os atoms in both I and II have octahedral coordination. In II the bridging O's occupy equatorial positions with an Os-Os bond of 3.018(2) Å. Raman spectral data are given and discussed based on the structure.
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(b) Bailey, A. J.; Bhowon, M. G.; Griffith, W. P.; Shoair, A. G.; White, A. J. P.; Williams, D. J. Oxo Osmium(VIII) Complexes in Oxidation: Crystal Structures of OsO₄·nmo (nmo = N-Methylmorpholine N-Oxode) and OsO₄·nmm (nmm = N-Methylmorpholine), and Use of cis-[OsO₄(OH)₂]^2– as an Oxidation Catalyst. J. Chem. Soc., Dalton Trans. 1997, 3245– 3250, DOI: 10.1039/a702965i
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19b
Oxo osmium(VIII) complexes in oxidation: crystal structures of OsO4·nmo (nmo = N-methylmorpholine N-oxide) and OsO4·nmm (nmm = N-methylmorpholine), and use of cis-[OsO4(OH)2]2- as an oxidation catalyst
Bailey, Alan J.; Bhowon, Minu G.; Griffith, William P.; Shoair, Abdel G. F.; White, Andrew J. P.; Williams, David J.
Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1997), (18), 3245-3250CODEN: JCDTBI; ISSN:0300-9246. (Royal Society of Chemistry)
The new complexes OsO4·nmo (nmo = N-methylmorpholine N-oxide) and OsO4·nmm (nmm = N-methylmorpholine) were made, their crystal structures detd., and their possible involvement in the catalyzed dihydroxylation of alkenes considered. The use of cis-[OsO4(OH)2]2- as a catalyst for the oxidn. of alcs., aldehydes and alkyl halides to carboxylic acids with [Fe(CN)6]3- and other cooxidants and also for the cleavage and dihydroxylation of alkenes with [Fe(CN)6]3- was investigated.
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(c) Barthazy, P.; Wörle, M.; Rüegger, H.; Mezzetti, A. Oxo Complexes of Osmium(IV) Formed via Dioxygen Activation. X-ray Structures of [OsX(dcpe)₂]PF₆ (X = Cl, Br), [OsCl(η-O₂)(dcpe)₂]BPh₄, and [OsCl(O)(dcpe)₂]BPh₄ (dcpe = 1,2-Bis(dicyclohexylphosphino)ethane). Inorg. Chem. 2000, 39, 4903– 4912, DOI: 10.1021/ic0002420
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19c
Oxo Complexes of Osmium(IV) Formed via Dioxygen Activation. X-ray Structures of [OsX(dcpe)2]PF6 (X = Cl, Br), [OsCl(η2-O2)(dcpe)2]BPh4, and [OsCl(O)(dcpe)2]BPh4 (dcpe = 1,2-Bis(dicyclohexylphosphino)ethane)
Barthazy, Peter; Woerle, Michael; Rueegger, Heinz; Mezzetti, Antonio
Inorganic Chemistry (2000), 39 (21), 4903-4912CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
Dioxygen addn. to the 16-electron complexes [OsX(P-P)2]+ gives the dioxygen adducts [OsCl(η2-O2)(P-P)2]+ (4), which in turn react with HCl gas to give the novel Os(IV) oxo complexes trans-[OsX(O)(P-P)2]+ (5) (X = Cl, Br; P-P = 1,2-bis(dicyclohexylphosphino)ethane (dcpe), 1,2-bis(diethylphosphino)ethane (depe), 1,2-bis((2R,5R)-2,5-dimethylphospholano)benzene (Me-duphos)). [OsX(dcpe)2]+ (X = Cl, Br) were studied by x-ray crystallog. and have a Y-shaped coordination geometry in the equatorial plane. The x-ray structural anal. of [OsCl(η2-O2)(dcpe)2]+ (4a) reveals an exceptionally short O-O bond (1.315(5) Å). Trans-[OsCl(O)(dcpe)2]+ (5a), the 1st oxo complex of Os(IV) studied crystallog., exhibits a long Os-O distance of 1.834(3) Å. The reactivity of 4 and 5 as oxidants is described. The dioxygen complex 4a transfers one O atom to PPh3 (to give Ph3PO) or oxidizes iodide ions to triiodide ions in the presence of anhyd. HCl. In both reactions, the corresponding oxo species 5a is quant. formed as the only metal-contg. product. Oxo complexes 5 are surprisingly stable and unreactive toward std. reducing agents such as phosphines.
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(d) Liu, Y.; Ng, S.-M.; Lam, W. W. Y.; Yiu, S.-M.; Lau, T.-C. A Highly Reactive Seven-Coordinate Osmium(V) Oxo Complex: [Os^V(O)(qpy)(pic)Cl]²⁺. Angew. Chem., Int. Ed. 2016, 55, 288– 291, DOI: 10.1002/anie.201507933
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19d
A Highly Reactive Seven-Coordinate Osmium(V) Oxo Complex: [OsV(O)(qpy)(pic)Cl]2+
Liu, Yingying; Ng, Siu-Mui; Lam, William W. Y.; Yiu, Shek-Man; Lau, Tai-Chu
Angewandte Chemie, International Edition (2016), 55 (1), 288-291CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
Seven-coordinate ruthenium oxo species are proposed as active intermediates in catalytic water oxidn. by a no. of highly active ruthenium catalysts, however such species have yet to be isolated. Reported herein is the 1st example of a seven-coordinate group 8 metal-oxo species, [OsV(O)(qpy)(pic)Cl]2+ (qpy = 2,2':6',2'':6'',2'''-quaterpyridine, pic = 4-picoline). The x-ray crystal structure of this complex shows that it has a distorted pentagonal bipyramidal geometry with an Os=O distance of 1.7375 Å. This oxo species undergoes facile O-atom and H-atom-transfer reactions with various org. substrates. Notably it can abstr. H atoms from alkylaroms. with C-H bond dissocn. energy ≤90 kcal mol-1. Probably highly active oxidants are designed based on Group 8 seven-coordinate metal oxo species.
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(a) Mondal, B.; Neese, F.; Bill, E.; Ye, S. Electronic Structure Contributions of Non-Heme Oxo-Iron(V) Complexes to the Reactivity. J. Am. Chem. Soc. 2018, 140, 9531– 9544, DOI: 10.1021/jacs.8b04275
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20a
Electronic Structure Contributions of Non-Heme Oxo-Iron(V) Complexes to the Reactivity
Mondal, Bhaskar; Neese, Frank; Bill, Eckhard; Ye, Shengfa
Journal of the American Chemical Society (2018), 140 (30), 9531-9544CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Oxo-iron(V) species have been implicated in the catalytic cycle of the Rieske dioxygenase. Its synthetic analog, [FeV(O)(OC(O)CH3)(PyNMe3)]2+ (1, PyNMe3 = 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3,6,9- trimethyl), derived from the O-O bond cleavage of its acetylperoxo iron(III) precursor, has been shown exptl. to perform regio- and stereo-selective C-H and C=C bond functionalization. However, its structure-activity relation is poorly understood. Herein we present a detailed electronic-structure and spectroscopic anal. of complex 1 along with well-characterized oxo-iron(V) complexes, [FeV(O)(TAML)]- (2, TAML = tetraamido macrocyclic ligand), [FeV(O)(TMC)(NC(O)CH3)]+ (4, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and [FeV(O)(TMC)(NC(OH)CH3)]2+ (4-H+) using wavefunction-based multireference complete active-space SCF calcns. Our results reveal that the x/y anisotropy of the 57Fe A-matrix is not a reliable spectroscopic marker to identify oxo-iron(V) species, and that the drastically different Ax and Ay values detd. for complexes 1, 4 and 4-H+ have distinctive origins compared to complex 2, a genuine oxo-iron(V) species. Complex 1, in fact, has a dominant character of [FeIV(O•••OC(O)CH3)2-•]2+, i.e. an SFe = 1 iron(IV) center antiferromagnetically coupled to an O-O σ* radical, where the O-O bond has not been completely broken. Complex 4 is best described as a triplet ferryl unit that strongly interacts with the trans acetylimidyl radical in an antiferromagnetic fashion, [FeIV(O)(•N=C(O-)CH3)]+. Complex 4-H+ features a similar electronic structure, [FeIV(O)(•N=C(OH)CH3)]2+. Owing to the remaining approx. half σ-bond in the O-O moiety, complex 1 can arrange two electron-accepting orbitals (α σ* O-O and β Fe-dxz) in such a way that both orbitals can simultaneously interact with the doubly occupied electron-donating orbitals (σC-H or πC-C). Hence, complex 1 can promote a concerted yet asynchronous two-electron oxidn. of the C-H and C=C bonds, which nicely explains the stereospecificity obsd. for complex 1 and the related species.
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(b) Ye, S.; Geng, C.-Y.; Shaik, S.; Neese, F. Electronic Structure Analysis of Multistate Reactivity in Transition Metal Catalyzed Reactions: The Case of C-H Bond Activation by Non-Heme Iron(IV)-Oxo Cores. Phys. Chem. Chem. Phys. 2013, 15, 8017– 8030, DOI: 10.1039/c3cp00080j
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20b
Electronic structure analysis of multistate reactivity in transition metal catalyzed reactions: the case of C-H bond activation by non-heme iron(iv)-oxo cores
Ye, Shengfa; Geng, Cai-Yun; Shaik, Sason; Neese, Frank
Physical Chemistry Chemical Physics (2013), 15 (21), 8017-8030CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
A review. This perspective discusses the principles of the multistate scenario often encountered in transition metal catalyzed reactions, and is organized as follows. First, several important theor. concepts (phys. vs. formal oxidn. states, orbital interactions, use of (spin) natural and corresponding orbitals, exchange enhanced reactivity and the connection between valence bond and MO based electronic structure anal.) are presented. These concepts are then used to analyze the electronic structure changes occurring in the reaction of C-H bond oxidn. by FeIVoxo species. The anal. reveals that the energy sepn. and the overlap between the electron donating orbitals and electron accepting orbitals of the FeIVoxo complexes dictate the reaction stereochem., and that the manner in which the exchange interaction changes depends on the identity of these orbitals. The electronic reorganization of the FeIVoxo species during the reaction is thoroughly analyzed and it is shown that the FeIVoxo reactant develops oxyl radical character, which interacts effectively with the σCH orbital of the alkane. The factors that det. the energy barrier for the reaction are discussed in terms of MO and valence bond concepts.
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(c) Ogliaro, F.; de Visser, S. P.; Groves, J. T.; Shaik, S. Chameleon States: High-Valent Metal-Oxo Species of Cytochrome P450 and Its Ruthenium Analogue. Angew. Chem., Int. Ed. 2001, 40, 2874– 2878, DOI: 10.1002/1521-3773(20010803)40:15<2874::AID-ANIE2874>3.0.CO;2-9
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20c
Chameleon states. High-valent metal-oxo species of cytochrome P450 and its ruthenium analogue
Ogliaro, Francois; De Visser, Samuel P.; Groves, John T.; Shaik, Sason
Angewandte Chemie, International Edition (2001), 40 (15), 2874-2878CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH)
D.-functional (DFT) calcns. were used to compare the electromeric states of the [(L)PorMzO] (M = Fe, Ru; Z = III-V, L = SH and SMe) complexes. The states, orbital occupancies, and relative energies are listed for the isolated mol. and for the mol. in a polarizing medium as well as the spin densities, resonance structures in the A2u and Πs states, and the π-π* orbital energy gaps. The catalytic activity of these complexes in monooxygenation reactions is discussed in relation to their electronic structure. The electronic structure of the Ru(V)O complex is distinctly different from that of the Fe complexes. The vacant π* orbital in the Ru(V)O catalyst heightened the electrophilicity of this compd. Such a heightened electrophilic nature was indeed found in the oxidn. reactions of substituted toluenes using a Ru porphyrin catalyst (Groves, 2000).
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(d) Decker, A.; Rohde, J.-U.; Que, L.; Solomon, E. I. Spectroscopic and Quantum Chemical Characterization of the Electronic Structure and Bonding in a Non-Heme Fe^IV═O Complex. J. Am. Chem. Soc. 2004, 126, 5378– 5379, DOI: 10.1021/ja0498033
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20d
Spectroscopic and Quantum Chemical Characterization of the Electronic Structure and Bonding in a Non-Heme FeIV:O Complex
Decker, Andrea; Rohde, Jan-Uwe; Que, Lawrence, Jr.; Solomon, Edward I.
Journal of the American Chemical Society (2004), 126 (17), 5378-5379CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
High valent FeIV:O species are key intermediates in the catalytic cycles of many mononuclear non-heme iron enzymes involving the binding and activation of dioxygen. Using variable temp. magnetic CD (VT MCD) spectroscopy and exptl. calibrated d. functional calcns., we are able to present the first detailed description of the electronic structure of a non-heme FeIV:O S = 1 complex. These studies define the nature of the FeIV:O bond and present the basis for understanding high-valent oxygen intermediates in non-heme iron enzymes.
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Ray, K.; Heims, F.; Pfaff, F. F. Terminal Oxo and Imido Transition-Metal Complexes of Groups 9–11. Eur. J. Inorg. Chem. 2013, 2013, 3784– 3807, DOI: 10.1002/ejic.201300223
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21
Terminal Oxo and Imido Transition-Metal Complexes of Groups 9-11
Ray, Kallol; Heims, Florian; Pfaff, Florian Felix
European Journal of Inorganic Chemistry (2013), 2013 (22-23), 3784-3807CODEN: EJICFO; ISSN:1434-1948. (Wiley-VCH Verlag GmbH & Co. KGaA)
A review. This review summarizes the properties of group 9-11 metal-oxo and metal-imido complexes, which have been either isolated or proposed as reactive intermediates in metal-catalyzed org. transformations. We begin with a general description of the bonding of transition-metal-oxo and -imido complexes in various geometries, followed by a discussion of complexes from groups 9-11. The focus of this review is to provide a clear picture of the state of the art as well as insight towards potential future synthetic endeavors.
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Carter, E. A.; Goddard, W. A., III Early- versus Late-Transition-Metal-Oxo Bonds: The Electronic Structure of VO⁺ and RuO⁺. J. Phys. Chem. 1988, 92, 2109– 2115, DOI: 10.1021/j100319a005
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22
Early- versus late-transition-metal-oxo bonds: the electronic structure of oxovanadium(1+) and oxoruthenium(1+)
Carter, Emily A.; Goddard, William A., III
Journal of Physical Chemistry (1988), 92 (8), 2109-15CODEN: JPCHAX; ISSN:0022-3654.
From all-electron ab initio generalized valence bond calcns. (GVBCI-SCF) on VO+ and RuO+, an accurate description of the bonding is obtained only when important resonance configurations are included self-consistently in the wave function. The ground state of VO+(3Σ-) has a triple bond similar to that of CO, with Decalcd(V-O) = 128.3 kcal/mol [Deexptl(V-O) = 131 ± 5 kcal/mol], while the ground state of RuO+(4Δ) has a double bond similar to that of O2, with Decalcd(Ru-O) = 67.1 kcal/mol. Vertical excitation energies for a no. of low-lying electronic states of VO+ and RuO+ are also reported. These results indicate fundamental differences in the nature of the metal-oxo bond in early and late metal-oxo complexes that explain the obsd. trends in reactivity (e.g., early metal oxides are thermodynamically stable whereas late metal oxo complexes are highly reactive oxidants). These results were used to predict the ground states of MO+ for other first-row transition-metal oxides.
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(a) Koizumi, K.; Shoji, M.; Nishiyama, Y.; Maruno, Y.; Kitagawa, Y.; Soda, K.; Yamanaka, S.; Okumura, M.; Yamaguchi, K. The Electronic Structure and Magnetic Property of Metal-Oxo, Porphyrin Manganese-Oxo, and μ-Oxo-Bridged Manganese Porphyrin Dimer. Int. J. Quantum Chem. 2004, 100, 943– 956, DOI: 10.1002/qua.20152
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23a
The electronic structure and magnetic property of metal-Oxo, porphyrin manganese-Oxo, and μ-Oxo-bridged manganese porphyrin dimer
Koizumi, K.; Shoji, M.; Nishiyama, Y.; Maruno, Y.; Kitagawa, Y.; Soda, K.; Yamanaka, S.; Okumura, M.; Yamaguchi, K.
International Journal of Quantum Chemistry (2004), 100 (6), 943-956CODEN: IJQCB2; ISSN:0020-7608. (John Wiley & Sons, Inc.)
Hybrid d. functional theory (HDFT) and post Hartree-Fock CCSD(T) methods are applied to elucidate the binding energies and the optimized M-O distances of transition metal oxides: MO (M = Cr, Mn, Fe, Co, Ni, Cu). The HDFT method can reproduce the CCSD(T) results, in agreement with the exptl. ones. The nature of the manganese-oxygen bonds in the Mn(VI)-O, Mn(IV)-O porphyrin (PP), and Mn(V)-O PP systems are examd. in relation to possible mechanisms of oxygen evolution from H2O2 and H2O in native and non-native manganese complexes. It is found that the radical character of the high-valent (PP)Mn(V)-O bond is remarkable, showing the strong potential to generate mol. oxygen because of its high reactivity. The electronic structure and magnetic property of μ-oxo-bridged manganese porphyrin dimer (PPMn(III)OMn(III)PP) are investigated for further discussion of structure and reactivity of PPMn(X)O (X = II-IV). The potential curve for release of mol. oxygen from PPMn(II)O2 is also examd. to show weak affinity of O2 in the Mn complex where the oxidn. no. (X) of Mn is low. Implications of the computational results are also discussed in relation to oxygen evolution reactions.
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(b) Yamaguchi, K.; Takahara, Y.; Fueno, T. Ab-Initio Molecular Orbital Studies of Structure and Reactivity of Transition Metal-OXO Compounds. Appl. Quant. Chem. 1986, 155– 184, DOI: 10.1007/978-94-009-4746-7_11
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23b
Ab-initio molecular orbital studies of structure and reactivity of transition metal-oxo compounds
Yamaguchi, K.; Takahara, Y.; Fueno, T.
(1986), (), 155-84CODEN: 55IFAJ ISSN:. (Reidel)
A review with 49 refs. deals with electronic properties, structure, and reactivity of transition metal oxo compds.
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Solomon, E. I. Geometric and Electronic Structure Contributions to Function in Bioinorganic Chemistry: Active Sites in Non-Heme Iron Enzymes. Inorg. Chem. 2001, 40, 3656– 3669, DOI: 10.1021/ic010348a
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24
Geometric and Electronic Structure Contributions to Function in Bioinorganic Chemistry: Active Sites in Non-Heme Iron Enzymes
Solomon, Edward I.
Inorganic Chemistry (2001), 40 (15), 3656-3669CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
A review with 75 refs. Spectroscopy has played a major role in the definition of structure/function correlations in bioinorg. chem. The importance of spectroscopy combined with electronic structure calcns. is clearly demonstrated by the non-heme iron enzymes. Many members of this large class of enzymes activate dioxygen using a ferrous active site that has generally been difficult to study with most spectroscopic methods. A new spectroscopic methodol. has been developed utilizing variable temp., variable field magnetic CD, which enables one to obtain detailed insight into the geometric and electronic structure of the non-heme ferrous active site and probe its reaction mechanism on a mol. level. This spectroscopic methodol. is presented and applied to a no. of key mononuclear non-heme iron enzymes leading to a general mechanistic strategy for O2 activation. These studies are then extended to consider the new features present in the binuclear non-heme iron enzymes and applied to understand (1) the mechanism of the two electron/coupled proton transfer to dioxygen binding to a single iron center in hemerythrin and (2) structure/function correlations over the oxygen-activating enzymes stearoyl-ACP Δ9-desaturase, ribonucleotide reductase, and methane monooxygenase. Electronic structure/reactivity correlations for O2 activation by non-heme relative to heme iron enzymes will also be developed.
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Yang, X.; Baik, M.-H. Electronic Structure of the Water-Oxidation Catalyst [(bpy)₂(OH_x)RuORu(OH_y)(bpy)₂]^z+: Weak Coupling between the Metal Centers is Preferred over Strong Coupling. J. Am. Chem. Soc. 2004, 126, 13222– 13223, DOI: 10.1021/ja0462427
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Electronic Structure of the Water-Oxidation Catalyst [(bpy)2(OHx)RuORu(OHy)(bpy)2]z+: Weak Coupling between the Metal Centers Is Preferred over Strong Coupling
Yang, Xiaofan; Baik, Mu-Hyun
Journal of the American Chemical Society (2004), 126 (41), 13222-13223CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
High-level DFT calcns. indicate that the singlet ground state of the water-oxidizing blue Ru dimer [(bpy)2(OH2)RuIIIORuIII(OH2)(bpy)2]4+ is not due to a strong coupling of the excess electrons from each of the low-spin d5 RuIII centers across the Ru-O-Ru moiety, as has been assumed to date. Instead, broken symmetry orbital calcns. suggest that a weak antiferromagnetically (AF) coupled singlet state is energetically more favorable by 10-35 kcal/mol. Exptl. obsd. redox potentials can only be reproduced if antiferromagnetic coupling is invoked.
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(a) Nam, W. High-Valent Iron(IV)-Oxo Complexes of Heme and Non-Heme Ligands in Oxygenation Reactions. Acc. Chem. Res. 2007, 40, 522– 531, DOI: 10.1021/ar700027f
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26a
High-Valent Iron(IV)-Oxo Complexes of Heme and Non-Heme Ligands in Oxygenation Reactions
Nam, Wonwoo
Accounts of Chemical Research (2007), 40 (7), 522-531CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
A review. High-valent iron(IV)-oxo species have been implicated as the key reactive intermediates in the catalytic cycles of dioxygen activation by heme and non-heme iron enzymes. Our understanding of the enzymic reactions has improved greatly via investigation of spectroscopic and chem. properties of heme and non-heme iron(IV)-oxo complexes. In this Account, reactivities of synthetic iron(IV)-oxo porphyrin π-cation radicals and mononuclear non-heme iron(IV)-oxo complexes in oxygenation reactions have been discussed as chem. models of cytochrome P 450 and non-heme iron enzymes. These results demonstrate how mechanistic developments in biomimetic research can help our understanding of dioxygen activation and oxygen atom transfer reactions in nature.
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(b) Nam, W.; Lee, Y.-M.; Fukuzumi, S. Tuning Reactivity and Mechanism in Oxidation Reactions by Mononuclear Nonheme Iron(IV)-Oxo Complexes. Acc. Chem. Res. 2014, 47, 1146– 1154, DOI: 10.1021/ar400258p
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26b
Tuning reactivity and mechanism in oxidation reactions by mononuclear nonheme iron(IV)-oxo complexes
Nam, Wonwoo; Lee, Yong-Min; Fukuzumi, Shunichi
Accounts of Chemical Research (2014), 47 (4), 1146-1154CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
A review. Mononuclear nonheme iron enzymes generate high-valent Fe(IV)-oxo intermediates that effect metabolically important oxidative transformations in the catalytic cycle of O2 activation. In 2003, researchers 1st spectroscopically characterized a mononuclear nonheme Fe(IV)-oxo intermediate in the reaction of taurine-α-ketoglutarate dioxygenase (TauD). This nonheme Fe-contg. enzyme with a Fe active center was coordinated to a 2-His-1-carboxylate facial triad motif. In the same year, researchers obtained the 1st crystal structure of a mononuclear nonheme Fe(IV)-oxo complex bearing a macrocyclic supporting ligand, [(TMC)FeIV(O)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecene), in studies that mimicked the biol. enzymes. With these breakthrough results, many other studies have examd. mononuclear nonheme Fe(IV)-oxo intermediates trapped in enzymic reactions or synthesized in biomimetic reactions. Over the past decade, researchers in the fields of biol., bioinorg., and oxidn. chem. have extensively investigated the structure, spectroscopy, and reactivity of nonheme Fe(IV)-oxo species, leading to a wealth of information from these enzymic and biomimetic studies. Here, the authors summarize the reactivity and mechanisms of synthetic mononuclear nonheme Fe(IV)-oxo complexes in oxidn. reactions and examines factors that modulate their reactivities and change their reaction mechanisms. The authors focus on several reactions including the oxidn. of org. and inorg. compds., electron transfer, and O atom exchange with water by synthetic mononuclear nonheme Fe(IV)-oxo complexes. In addn., the authors recently obsd. that C-H bond activation by nonheme Fe(IV)-oxo and other nonheme metal(IV)-oxo complexes does not follow the H-atom abstraction/oxygen-rebound mechanism, which has been well-established in heme systems. The structural and electronic effects of supporting ligands on the oxidizing power of Fe(IV)-oxo complexes are significant in these reactions. However, the difference in spin states between nonheme Fe(IV)-oxo complexes with an octahedral geometry (with an S = 1 intermediate-spin state) or a trigonal bipyramidal (TBP) geometry (with an S = 2 high-spin state) does not lead to a significant change in reactivity in biomimetic systems. Thus, the importance of the high-spin state of Fe(IV)-oxo species in nonheme Fe-contg. enzymes remains unexplained. The authors also discuss how the axial and equatorial ligands and binding of redox-inactive metal ions and protons to the Fe-oxo moiety influence the reactivities of the nonheme Fe(IV)-oxo complexes. The authors emphasize how these changes can enhance the oxidizing power of nonheme metal(IV)-oxo complexes in O atom transfer and electron-transfer reactions remarkably. The authors demonstrate great advancements in the understanding of the chem. of mononuclear nonheme Fe(IV)-oxo intermediates within the last 10 yr.
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(a) Bryant, J. R.; Mayer, J. M. Oxidation of C-H Bonds by [(bpy)₂(py)Ru^IVO]²⁺ Occurs by Hydrogen Atom Abstraction. J. Am. Chem. Soc. 2003, 125, 10351– 10361, DOI: 10.1021/ja035276w
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27a
Oxidation of C-H Bonds by [(bpy)2(py)RuIVO]2+ Occurs by Hydrogen Atom Abstraction
Bryant, Jasmine R.; Mayer, James M.
Journal of the American Chemical Society (2003), 125 (34), 10351-10361CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Anaerobic oxidns. of 9,10-dihydroanthracene (DHA), xanthene, and fluorene by [(bpy)2(py)RuIVO]2+ in acetonitrile soln. give mixts. of products including oxygenated and non-oxygenated compds. The products include those formed by org. radical dimerization, such as 9,9'-bixanthene, as well as by oxygen-atom transfer (e.g., xanthone). The kinetics of these reactions have been measured. The kinetic isotope effect for oxidn. of DHA vs DHA-d4 gives kH/kD ≥ 35 ± 1. The data indicate a mechanism of initial hydrogen-atom abstraction forming radicals that dimerize, disproportionate and are trapped by the oxidant. This mechanism also appears to apply to the oxidns. of toluene, ethylbenzene, cumene, indene, and cyclohexene. The rate consts. for H-atom abstraction from these substrates correlate well with the strength of the C-H bond that is cleaved. Rate consts. for abstraction from DHA and toluene also correlate with those for oxygen radicals and other oxidants. The rate const. for H-atom transfer from toluene to [(bpy)2(py)RuIVO]2+ appears to be close to that predicted by the Marcus cross relation, using a tentative rate const. for hydrogen atom self-exchange between [(bpy)2(py)RuIIIOH]2+ and [(bpy)2(py)RuIVO]2+.
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(b) Saouma, C. T.; Mayer, J. M. Do Spin State and Spin Density Affect Hydrogen Atom Transfer Reativity?. Chem. Sci. 2014, 5, 21– 31, DOI: 10.1039/C3SC52664J
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27b
Do spin state and spin density affect hydrogen atom transfer reactivity?
Saouma, Caroline T.; Mayer, James M.
Chemical Science (2014), 5 (1), 21-31CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
A review. The prevalence of hydrogen atom transfer (HAT) reactions in chem. and biol. systems has prompted much interest in establishing and understanding the underlying factors that enable this reactivity. Arguments have been advanced that the electronic spin state of the abstractor and/or the spin-d. at the abstracting atom are crit. for HAT reactivity. This is consistent with the intuition derived from introductory org. chem. courses. Alternative view on the role of spin state and spin d. in HAT reactions. After a brief introduction, the second section introduces a new and simple fundamental kinetic anal., which shows that unpaired spin cannot be the dominant effect. The third section examines published computational studies of HAT reactions, which indicates that the spin state affects these reactions indirectly, primarily via changes in driving force. The essay concludes with a broader view of HAT reactivity, including indirect effects of spin and other properties. It is suggested that some of the controversy in this area may arise from the diversity of HAT reactions and their overlap with proton-coupled electron transfer (PCET) reactions.
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Wang, B.; Lee, Y.-M.; Tcho, W.-Y.; Tussupbayev, S.; Kim, S.-T.; Kim, Y.; Seo, M. S.; Cho, K.-B.; Dede, Y.; Keegan, B. C.; Ogura, T.; Kim, S. H.; Ohta, T.; Baik, M.-H.; Ray, K.; Shearer, J.; Nam, W. Synthesis and Reactivity of a Mononuclear Non-Haem Cobalt(IV)-Oxo Complex. Nat. Commun. 2017, 8, 14839, DOI: 10.1038/ncomms14839
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28
Synthesis and reactivity of a mononuclear non-haem cobalt(IV)-oxo complex
Wang, Bin; Lee, Yong-Min; Tcho, Woon-Young; Tussupbayev, Samat; Kim, Seoung-Tae; Kim, Yujeong; Seo, Mi Sook; Cho, Kyung-Bin; Dede, Yavuz; Keegan, Brenna C.; Ogura, Takashi; Kim, Sun Hee; Ohta, Takehiro; Baik, Mu-Hyun; Ray, Kallol; Shearer, Jason; Nam, Wonwoo
Nature Communications (2017), 8 (), 14839CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
Terminal cobalt(IV)-oxo (CoIV-O) species have been implicated as key intermediates in various cobalt-mediated oxidn. reactions. Herein we report the photocatalytic generation of a mononuclear non-haem [(13-TMC)CoIV(O)]2+ (2) by irradiating [CoII(13-TMC)(CF3SO3)]+ (1) in the presence of [RuII(bpy)3]2+, Na2S2O8, and water as an oxygen source. The intermediate 2 was also obtained by reacting 1 with an artificial oxidant (i.e., iodosylbenzene) and characterized by various spectroscopic techniques. In particular, the resonance Raman spectrum of 2 reveals a diat. Co-O vibration band at 770 cm-1, which provides the conclusive evidence for the presence of a terminal Co-O bond. In reactivity studies, 2 was shown to be a competent oxidant in an intermetal oxygen atom transfer, C-H bond activation and olefin epoxidn. reactions. The present results lend strong credence to the intermediacy of CoIV-O species in cobalt-catalyzed oxidn. of org. substrates as well as in the catalytic oxidn. of water that evolves mol. oxygen.
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(a) Poverenov, E.; Efremenko, I.; Frenkel, A. I.; Ben-David, Y.; Shimon, L. J. W.; Leitus, G.; Konstantinovski, L.; Martin, J. M. L.; Milstein, D. Evidence for a Terminal Pt(IV)-Oxo Complex Exhibiting Diverse Reactivity. Nature 2008, 455, 1093– 1096, DOI: 10.1038/nature07356
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29a
Evidence for a terminal Pt(IV)-oxo complex exhibiting diverse reactivity
Poverenov, Elena; Efremenko, Irena; Frenkel, Anatoly I.; Ben-David, Yehoshoa; Shimon, Linda J. W.; Leitus, Gregory; Konstantinovski, Leonid; Martin, Jan M. L.; Milstein, David
Nature (London, United Kingdom) (2008), 455 (7216), 1093-1096CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
Terminal oxo complexes of transition metals have crit. roles in various biol. and chem. processes. For example, the catalytic oxidn. of org. mols., some oxidative enzymic transformations, and the activation of dioxygen on metal surfaces are all thought to involve oxo complexes. Moreover, they are believed to be key intermediates in the photocatalytic oxidn. of water to give mol. oxygen, a topic of intensive global research aimed at artificial photosynthesis and water splitting. The terminal oxo ligand is a strong π-electron donor, so it readily forms stable complexes with high-valent early transition metals. As the d orbitals are filled up with valence electrons, the terminal oxo ligand becomes destabilized. Here we present evidence for a dn (n > 5) terminal oxo complex that is not stabilized by an electron withdrawing ligand framework. This d6 Pt(IV) complex exhibits reactivity as an inter- and intramol. oxygen donor and as an electrophile. In addn., it undergoes a water activation process leading to a terminal dihydroxo complex, which may be relevant to the mechanism of catalytic reactions such as water oxidn.
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(b) Efremenko, I.; Poverenov, E.; Martin, J. M. L.; Milstein, D. DFT Study of the Structure and Reactivity of the Terminal Pt(IV)-Oxo Complex Bearing No Electron-Withdrawing Ligands. J. Am. Chem. Soc. 2010, 132, 14886– 14900, DOI: 10.1021/ja105197x
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29b
DFT Study of the structure and reactivity of the terminal Pt(IV)-oxo complex bearing no electron-withdrawing ligands
Efremenko, Irena; Poverenov, Elena; Martin, Jan M. L.; Milstein, David
Journal of the American Chemical Society (2010), 132 (42), 14886-14900CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The recently published [(PCN)Pt:O]+ complex is interesting as a unique example of a stable d6 terminal transition metal oxo complex not stabilized by electron withdrawing ligands and as a model of oxo complexes frequently implicated as key intermediates in various processes of oxygen transfer. In the present work, we report an extensive DFT study of its geometric and electronic structure, compn. in soln., and reactivity. The thermodn. data and calcd. 195Pt NMR chem. shifts reveal that one solvent mol. is weakly coordinated to the complex in acetone soln. This ancillary ligand is responsible for the diamagnetic state of the complex, retards intramol. oxygen transfer, and facilitates CO oxidn. Chem. transformations of the coordinated acetone mol., coordination of other ancillary ligands present in the reaction mixt., and protonation of the Pt-oxo group in nonacidic media are excluded based on thermodn. or kinetic considerations. Bonding of the terminal oxo ligand with strong electrophiles presents the key interaction in the mechanisms of intramol. oxygen insertion into the Pt-P bond, in CO oxidn. and in water activation mediated by microsolvation. Low affinity of the terminal oxo ligand toward "soft" covalent interactions brings about intermediate formation of agostic hydrido and hydroxo complexes along the reaction pathway of dihydrogen oxidn. Stabilization of the Pt-oxo bonding is attributed to bending of the terminal oxo ligand out of the plane of the complex and to significant transfer of electron d. from compact low lying Pt 5d orbitals to more diffuse 6s and 6p orbitals.
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(a) Zhou, M.; Schley, N. D.; Crabtree, R. H. Cp* Iridium Complexes Give Catalytic Alkane Hydroxylation with Retention of Stereochemistry. J. Am. Chem. Soc. 2010, 132, 12550– 12551, DOI: 10.1021/ja1058247
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30a
Cp* Iridium Complexes Give Catalytic Alkane Hydroxylation with Retention of Stereochemistry
Zhou, Meng; Schley, Nathan D.; Crabtree, Robert H.
Journal of the American Chemical Society (2010), 132 (36), 12550-12551CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
A series of Cp*Ir complexes can catalyze C-H oxidn., with ceric ammonium nitrate as the terminal oxidant and water as the source of oxygen. Remarkably the hydroxylation of cis-decalin and 1,4-dimethylcyclohexane proceeds with retention of stereochem. With H2O18, cis-decalin oxidn. gave 18O incorporation into the product cis-decalol.
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(b) Zhou, M.; Balcells, D.; Parent, A. R.; Crabtree, R. H.; Eisenstein, O. Cp* Iridium Precatalysts for Selective C-H Oxidation via Direct Oxygen Insertion: A Joint Experimental/Computational Study. ACS Catal. 2012, 2, 208– 218, DOI: 10.1021/cs2005899
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30b
Cp* iridium precatalysts for selective C-H oxidation via direct oxygen insertion: A joint experimental/computational study
Zhou, Meng; Balcells, David; Parent, Alexander R.; Crabtree, Robert H.; Eisenstein, Odile
ACS Catalysis (2012), 2 (2), 208-218CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
A series of Cp*Ir complexes are active precatalysts in C-H oxidn. of cis-decalin, cyclooctane, 1-acetylpyrrolidine, tetrahydrofurans, and γ-lactones. Moderate to high yields were achieved, and surprisingly, high selectivity for mono-oxidn. of cyclooctane to cyclooctanone was obsd. Kinetic isotope effect expts. in the C-H oxidn. of ethylbenezene to acetophenone yield kH/kD = 15.4 ± 0.8 at 23 °C and 17.8 ± 1.2 at 0 °C, which are consistent with C-H oxidn. being the rate-limiting step with a significant tunneling contribution. The nature of the active species was investigated by TEM, UV-vis, microfiltration, and control expts. DFT calcns. showed that the C-H oxidn. of cis-decalin by Cp*Ir(ppy)(Cl) (ppy = o-phenylpyridine) follows a direct oxygen insertion mechanism on the singlet potential energy surface, rather than the radical rebound route that would be seen for the triplet, in good agreement with the retention of stereochem. obsd. in this reaction.
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(a) Wu, X.; Yang, X.; Lee, Y.-M.; Nam, W.; Sun, L. A Nonheme Manganese(IV)-Oxo Species Generated in Photocatalytic Reaction Using Water as an Oxygen Source. Chem. Commun. 2015, 51, 4013– 4016, DOI: 10.1039/C4CC10411K
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31a
A nonheme manganese(IV)-oxo species generated in photocatalytic reaction using water as an oxygen source
Wu, Xiujuan; Yang, Xiaonan; Lee, Yong-Min; Nam, Wonwoo; Sun, Licheng
Chemical Communications (Cambridge, United Kingdom) (2015), 51 (19), 4013-4016CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
A nonheme manganese(IV)-oxo complex, [MnIV(O)(BQCN)(H2O)]2+ (where BQCN = N,N'-dimethyl-N,N'-bis(8-quinolyl)cyclohexanediamine), was generated in the photochem. and chem. oxidn. of [MnII(BQCN)(OTf)2] with water as an oxygen source, resp. The photocatalytic oxidn. of org. substrates, such as alc. and sulfide, by [MnII(BQCN)]2+ has been demonstrated in both neutral and acidic media.
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(b) Wu, X.; Seo, M. S.; Davis, K. M.; Lee, Y.-M.; Chen, J.; Cho, K.-B.; Pushkar, Y. N.; Nam, W. A Highly Reactive Mononuclear Non-Heme Manganese(IV)-Oxo Complex That Can Activate the Strong C-H Bonds of Alkanes. J. Am. Chem. Soc. 2011, 133, 20088– 20091, DOI: 10.1021/ja208523u
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31b
A Highly Reactive Mononuclear Non-Heme Manganese(IV)-Oxo Complex That Can Activate the Strong C-H Bonds of Alkanes
Wu, Xiujuan; Seo, Mi Sook; Davis, Katherine M.; Lee, Yong-Min; Chen, Junying; Cho, Kyung-Bin; Pushkar, Yulia N.; Nam, Wonwoo
Journal of the American Chemical Society (2011), 133 (50), 20088-20091CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
A mononuclear nonheme manganese(IV)-oxo complex was synthesized and characterized using various spectroscopic methods. The Mn(IV)-oxo complex shows high reactivity in oxidn. reactions, such as C-H bond activation, oxidns. of olefins, alcs., sulfides, and arom. compds., and N-dealkylation. In C-H bond activation, the Mn(IV)-oxo complex can activate C-H bonds as strong as those in cyclohexane. Probably C-H bond activation by the nonheme Mn(IV)-oxo complex does not occur via an oxygen-rebound mechanism. The electrophilic character of the nonheme Mn(IV)-oxo complex is demonstrated by a large neg. ρ value of -4.4 in the oxidn. of para-substituted thioanisoles.
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(a) Conte, V.; Coletti, A.; Floris, B.; Licini, G.; Zonta, C. Mechanistic Aspects of Vanadium Catalysed Oxidations with Peroxides. Coord. Chem. Rev. 2011, 255, 2165– 2177, DOI: 10.1016/j.ccr.2011.03.006
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32a
Mechanistic aspects of vanadium catalyzed oxidations with peroxides
Conte, Valeria; Coletti, Alessia; Floris, Barbara; Licini, Giulia; Zonta, Cristiano
Coordination Chemistry Reviews (2011), 255 (19-20), 2165-2177CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
A review. The enhancement of the reactivity of peroxides, particularly H2O2 and alkylhydroperoxides, in the presence of V catalysis is a very known process. The catalytic effect is detd. by the formation of an intermediate whose nature depends on the peroxides used and on its interaction with the metal precursor, high-valent peroxo V species being usually the reactive oxidants. During the last decades the mechanistic details for several types of oxidn. reactions have been elucidated. In a no. of cases theor. calcns. offered support to the proposed reaction pathways. In general, V(V) peroxo species behave as electrophilic O transfer reagents thus reacting preferentially with the more nucleophilic functional group present in the mol. In several instances the chemoselectivity obsd. in such processes is very high when not abs. As far as V peroxides are concerned, a radical oxidative reactivity toward alkanes and aroms. was also obsd.; also for this latter chem., diverse research groups studied in detail the mechanism. However, no clear-cut evidence of nucleophilic reactivity of V peroxo complexes was obtained. Here the authors collect a selection of recent achievements concerning the reaction mechanisms in the V catalyzed oxidn. and bromination reactions with peroxides.
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(b) Waidmann, C. R.; DiPasquale, A. G.; Mayer, J. M. Synthesis and Reactivity of Oxo-Peroxo-Vanadium(V) Bipyridine Compounds. Inorg. Chem. 2010, 49, 2383– 2391, DOI: 10.1021/ic9022618
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32b
Synthesis and Reactivity of Oxo-Peroxo-Vanadium(V) Bipyridine Compounds
Waidmann, Christopher R.; Di Pasquale, Antonio G.; Mayer, James M.
Inorganic Chemistry (2010), 49 (5), 2383-2391CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
The V(IV) compd. [VIVO(OH)(tBu2bpy)2]BF4 (VIVO(OH)) (tBu2bpy = 4,4'-di-tert-butylbipyridine) is slowly oxidized by O2 in ethereal solvents to give the oxo-peroxo compd. [VVO(O2)(tBu2bpy)2]BF4 (VVO(O2)) in excellent yield. This and related compds. were fully characterized by NMR, IR, and optical spectroscopies; mass spectrometry; elemental analyses; and an x-ray crystal structure of the 4,4'-dimethylbipyridine analog, [VVO(O2)(Me2bpy)2]BF4. Monitoring the reaction of VIVO(OH) with O2 in THF/MeCN mixts. by 1H NMR and optical spectroscopies surprisingly shows that the initial product is the cis-dioxo compd. [VV(O)2(tBu2bpy)2]BF4 (VVO2), which then converts to VVO(O2). Reaction of VIVO(OH) with 18O2 gives ∼60% triply 18O labeled VVO(O2). The mechanism of formation of VVO(O2) is complex and may occur via initial redn. of O2 at V(IV) to give a superoxo-V(V) intermediate, autoxidn. of the THF solvent, or both. That VVO2 is generated 1st appears to be due to the ability of VIVO(OH) to act as a hydrogen atom donor. For instance, VIVO(OH) reacts with VVO(O2) to give VVO2. VVO(O2) is also slowly reduced to VIVO(OH) by the org. hydrogen atom donors hydroquinone and TEMPOH (2,2,6,6-tetramethylpiperidin-1-ol) as well as by PPh3. Notably, the peroxo complex VVO(O2) is much less reactive with these substrates than the analogous dioxo compd. VVO2.
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33
(a) Liu, S.; Mase, K.; Bougher, C.; Hicks, S. D.; Abu-Omar, M. M.; Fukuzumi, S. High-Valent Chromoium-Oxo Complex Acting as an Efficient Catalyst Precursor for Selective Two-Electron Reduction of Dioxygen by a Ferrocene Derivative. Inorg. Chem. 2014, 53, 7780– 7788, DOI: 10.1021/ic5013457
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33a
High-valent chromium-oxo complex acting as an efficient catalyst precursor for selective two-electron reduction of dioxygen by a ferrocene derivative
Liu, Shuo; Mase, Kentaro; Bougher, Curt; Hicks, Scott D.; Abu-Omar, Mahdi M.; Fukuzumi, Shunichi
Inorganic Chemistry (2014), 53 (14), 7780-7788CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
Efficient catalytic two-electron redn. of dioxygen (O2) by octamethylferrocene (Me8Fc) produced hydrogen peroxide (H2O2) using a high-valent chromium(V)-oxo corrole complex, [(tpfc)CrV(O)] (tpfc = tris(pentafluorophenyl)corrole) as a catalyst precursor in the presence of trifluoroacetic acid (TFA) in acetonitrile (MeCN). The facile two-electron redn. of [(tpfc)CrV(O)] by 2 equiv of Me8Fc in the presence of excess TFA produced the corresponding chromium(III) corrole [(tpfc)CrIII(OH2)] via fast electron transfer from Me8Fc to [(tpfc)CrV(O)] followed by double protonation of [(tpfc)CrIV(O)]- and facile second-electron transfer from Me8Fc. The rate-detg. step in the catalytic two-electron redn. of O2 by Me8Fc in the presence of excess TFA is inner-sphere electron transfer from [(tpfc)CrIII(OH2)] to O2 to produce the chromium(IV) superoxo species [(tpfc)CrIV(O2•-)], followed by fast proton-coupled electron transfer redn. of [(tpfc)CrIV(O2•-)] by Me8Fc to yield H2O2, accompanied by regeneration of [(tpfc)CrIII(OH2)]. Thus, although the catalytic two-electron redn. of O2 by Me8Fc was started by [(tpfc)CrV(O)], no regeneration of [(tpfc)CrV(O)] was obsd. in the presence of excess TFA, regardless of the tetragonal chromium complex being to the left of the oxo wall. In the presence of a stoichiometric amt. of TFA, however, disproportionation of [(tfpc)CrIV(O)]- occurred via the protonated species [(tpfc)CrIV(OH)] to produce [(tpfc)CrIII(OH2)] and [(tpfc)CrV(O)].
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(b) Premsingh, S.; Venkataramanan, N. S.; Rajagopal, S.; Mirza, S. P.; Vairamani, M.; Rao, P. S.; Velavan, K. Electron Transfer Reaction of Oxo(salen)chromium(V) Ion with Anilines. Inorg. Chem. 2004, 43, 5744– 5353, DOI: 10.1021/ic049482w
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33b
Electron Transfer Reaction of Oxo(salen)chromium(V) Ion with Anilines
Premsingh, Sundarsingh; Venkataramanan, Natarajan Sathiyamoorthy; Rajagopal, Seenivasan; Mirza, Shama. P.; Vairamani, Mariappanadar; Rao, P. Sambasiva; Velavan, K.
Inorganic Chemistry (2004), 43 (18), 5744-5753CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
The kinetics of oxidn. of 16 meta-, ortho-, and para-substituted anilines with nine oxo(salen)chromium(V) ions have been studied by spectrophotometric, ESIMS, and EPR techniques. During the course of the reaction, two new peaks with λmax at 470 and 730 nm appear in the absorption spectrum, and these peaks are due to the formation of emeraldine forms of oligomers of aniline supported by the ESIMS peaks with m/z values 274 and 365 (for the trimer and tetramer of aniline). The rate of the reaction is highly sensitive to the change of substituents in the aryl moiety of aniline and in the salen ligand of chromium(V) complexes. Application of the Hammett equation to analyze kinetic data yields a ρ value of -3.8 for the substituent variation in aniline and +2.2 for the substituent variation in the salen ligand of the metal complex. On the basis of the spectral, kinetic, and product anal. studies, a mechanism involving an electron transfer from the nitrogen of aniline to the metal complex in the rate controlling step has been proposed. The Marcus equation has been successfully applied to this system, and the calcd. values are compliant with the measured values.
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Blakemore, J. D.; Crabtree, R. H.; Brudvig, G. W. Molecular Catalytic for Water Oxidation. Chem. Rev. 2015, 115, 12974– 13005, DOI: 10.1021/acs.chemrev.5b00122
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34
Molecular Catalysts for Water Oxidation
Blakemore, James D.; Crabtree, Robert H.; Brudvig, Gary W.
Chemical Reviews (Washington, DC, United States) (2015), 115 (23), 12974-13005CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review; mol. catalysts for water oxidn. are discussed.
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(a) Gilbert, J. A.; Eggleston, D. S.; Murphy, W. R., Jr.; Geselowitz, D. A.; Gersten, S. W.; Hodgson, D. J.; Meyer, T. J. Structure and Redox Properties of the Water-Oxidation Catalyst [(bpy)₂(OH₂)RuORu(OH₂)(bpy)₂]⁴⁺. J. Am. Chem. Soc. 1985, 107, 3855– 3864, DOI: 10.1021/ja00299a017
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35a
Structure and redox properties of the water-oxidation catalyst [(bpy)2(OH2)RuORu(OH2)(bpy)2]4+
Gilbert, John A.; Eggleston, Drake S.; Murphy, Wyatt R., Jr.; Geselowitz, Daniel A.; Gersten, Susan W.; Hodgson, Derek J.; Meyer, Thomas J.
Journal of the American Chemical Society (1985), 107 (13), 3855-64CODEN: JACSAT; ISSN:0002-7863.
The crystal and mol. structure of the water-oxidn. catalyst μ-oxobis[aquabis(2,2'-bipyridine)ruthenium(III)] perchlorate dihydrate, [(bpy)2(OH2)RuORu(OH2)(bpy)2](ClO4)4.2H2O [where bpy is C10H8N2] was detd. from 3-dimensional x-ray counterdata. The complex crystallizes in the monoclinic space group C2/c with 4 mols. in a cell with a 22.712(9), b 13.189(4), and c 20.084(5) Å, β = 122.08 (3)°. The structure was refined to a weighted R factor of 0.052 based on 2887 independent intensities with I ≥ 3σ(I). The structure shows that the bridging Ru-O-Ru angle is 165.4°, the Ru-O bond lengths being 1.869 (1) Å. Electrochem. studies show that the RuIII-RuIII dimer undergoes an initial 1-electron oxidn. to RuIII-RuIV and that the potential of the couple has a complex pH dependence because of the acid-base properties of the 2 redox states. Above pH 2.2, oxidn. to RuIII-RuIV is followed by a 2-electron oxidn. to [(bpy)2(O)RuIVORuV(O)(bpy)2]3+ followed by a pH-independent, 1-electron oxidn. to [(bpy)2(O)RuVORuV(O)(bpy)2]4+. In solns. with pH < 2.2, RuIV-RuV is unstable with respect to disproportionation, and oxidn. of the RuIII-RuIV dimer to [(bpy)2(O)RuVORuV(O)(bpy)2]4+ occurs via a 3-electron step.
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Tagore, R.; Chen, H.; Zhang, H.; Crabtree, R. H.; Brudvig, G. W. Homogeneous Water Oxidation by a Di-μ-Oxo Dimanganese Complex in the Presence of Ce⁴⁺. Inorg. Chim. Acta 2007, 360, 2983– 2989, DOI: 10.1016/j.ica.2007.02.020
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36
Homogeneous water oxidation by a di-μ-oxo dimanganese complex in the presence of Ce4+
Tagore, Ranitendranath; Chen, Hongyu; Zhang, Hong; Crabtree, Robert H.; Brudvig, Gary W.
Inorganica Chimica Acta (2007), 360 (9), 2983-2989CODEN: ICHAA3; ISSN:0020-1693. (Elsevier B.V.)
O2 evolution was obsd. upon mixing aq. [(terpy)(H2O)Mn(O)2Mn(H2O)(terpy)](NO3)3 (1, terpy = 2,2':6',6''-terpyridine) with aq. solns. of Ce4+. However, when the soln. of 1 was incubated at pH 1 (by dissolving in dil. HNO3) before mixing with Ce4+, very small amts. of O2 were obsd. This observation of acid-induced deactivation suggests an explanation, both for the previously reported lack of O2 evolution from aq. solns. of 1 with Ce4+ as oxidant, and the present observation of low amts. of O2 prodn. with the very acidic Ce4+ reagent. Evidence is provided for water being the source of evolved O2, and for the requirement of a high valent multinuclear Mn species for O2 evolution. We test the possibility of complications in the use of ceric ammonium nitrate (CAN) in oxidn. chem. due to the presence of the oxidizable NH4+ ion.
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Ellis, W. C.; McDaniel, N. D.; Bernhard, S.; Collins, T. J. Fast Water Oxidation Using Iron. J. Am. Chem. Soc. 2010, 132, 10990– 10991, DOI: 10.1021/ja104766z
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37
Fast Water Oxidation Using Iron
Ellis, W. Chadwick; McDaniel, Neal D.; Bernhard, Stefan; Collins, Terrence J.
Journal of the American Chemical Society (2010), 132 (32), 10990-10991CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Photolysis of water, a long-studied strategy for storing solar energy, involves two half-reactions: the redn. of protons to dihydrogen and the oxidn. of water to dioxygen. Proton redn. is well-understood, with catalysts achieving quantum yields of 34% when driven by visible light. Water oxidn., on the other hand, is much less advanced, typically involving expensive metal centers and rarely working in conjunction with a photochem. powered system. Before further progress can be made in the field of water splitting, significant developments in the catalysis of oxygen evolution are needed. Herein the authors present an iron-centered tetraamido macrocyclic ligand (Fe-TAML) that efficiently catalyzes the oxidative conversion of water to dioxygen. When the catalyst is combined in unbuffered soln. with ceric ammonium nitrate, its turnover frequency exceeds 1.3 s-1. Real-time UV-vis and oxygen monitoring of the active complex give insights into the reaction and decay kinetics.
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38
Brunschwig, B. S.; Chou, M. H.; Creutz, C.; Ghosh, P.; Sutin, N. Mechanisms of Water Oxidation to Oxygen: Cobalt(IV) as an Intermediate in the Aquocobalt(II)-Catalyzed Reaction. J. Am. Chem. Soc. 1983, 105, 4832– 4833, DOI: 10.1021/ja00352a050
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38
Mechanisms of water oxidation to oxygen: cobalt(IV) as an intermediate in the aquocobalt(II)-catalyzed reaction
Brunschwig, Bruce S.; Chou, Mei H.; Creutz, Carol; Ghosh, Pushpito; Sutin, Norman
Journal of the American Chemical Society (1983), 105 (14), 4832-3CODEN: JACSAT; ISSN:0002-7863.
The kinetics and product distribution of the Co(II)-catalyzed Ru(bpy)33+ (bpy = 2,2'-bipyridine) oxidn. of H2O were studied at pH 7. In accord with previous work, the O2 yield is stoichiometric (0.25 O2 per Ru(III)) when the initial [Ru(III)] is ∼10 [Co(II)]. With [Ru(III)]0 = (1-10) × 10-5 M, excess Ru(II), [Co(II)] = (1-10) × 10-6 M, the rate law is d[Ru(III)]/dt = a[Ru(III)]2[Co(II)]/[Ru(II)][H+]2 with a = (4 ± 1) × 10-10 M s-1 at 25°, pH 6.5-7.2 (0.025 M phosphate, 0.06 ionic strength). The results indicate rate-detg. formation of Co(IV) which then reacts with H2O (or OH-) to give H2O or O2 and regenerate Co(II). Depending on the [Co(II)] and the [Ru(III)]/[Co(II)] ratio, catalyst deactivation (via scavenging of Co(IV) and Co(II) or oligomerization of Co(III)) may deplete the active Co(II) pool and yield hydroxocobalt(III) solid, slow rates, and low O2 yields.
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Thomsen, J. M.; Huang, D. L.; Crabtree, R. H.; Brudvig, G. W. Iridium-Based Complexes for Water Oxidation. Dalton Trans. 2015, 44, 12452– 12472, DOI: 10.1039/C5DT00863H
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Iridium-based complexes for water oxidation
Thomsen, Julianne M.; Huang, Daria L.; Crabtree, Robert H.; Brudvig, Gary W.
Dalton Transactions (2015), 44 (28), 12452-12472CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
Organometallic Ir precatalysts have been found to yield homogeneous Ir-based water-oxidn. catalysts (WOCs) with very high activity. The Cp*Ir catalyst series can operate under a variety of regimes: it can either act as a homogeneous or a heterogeneous catalyst; it can be driven by chem., photochem., or electrochem. methods; and the mol. catalyst can either act in soln. or supported as a mol. unit on a variety of solid oxides. In addn. to optimizing the various reaction conditions, work has continued to elucidate the catalyst activation mechanism and identify water-oxidn. intermediates. This perspective describes the development of the Cp*Ir series, their many forms as WOCs, and their ongoing characterization.
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40
(a) Winkler, J. R.; Gray, H. B. Electronic Structures of Oxo-Metal Ions. Struct. Bonding (Berlin, Ger.) 2011, 142, 17– 28, DOI: 10.1007/430_2011_55
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(b) Winkler, J. R.; Gray, H. B. Living with Oxygen. Acc. Chem. Res. 2018, 51, 1850– 1857, DOI: 10.1021/acs.accounts.8b00245
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40b
Living with Oxygen
Gray, Harry B.; Winkler, Jay R.
Accounts of Chemical Research (2018), 51 (8), 1850-1857CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
A review. Work on the electronic structures of metal-oxo complexes began in Copenhagen over fifty years ago. This work led to the prediction that tetragonal multiply bonded transition metal-oxos would not be stable beyond the iron-ruthenium-osmium oxo wall in the periodic table, and that triply bonded metal-oxos could not be protonated, even in the strongest Bronsted acids. In this theory, only doubled bonded metal-oxos could attract protons, with basicities a function of the electron donating ability of ancillary ligands. Such correlations of electronic structure with reactivity have gained importance in recent years, most notably owing to the widespread recognition that high-valent iron-oxos are intermediates in biol. reactions crit. to life on Earth. In this Account we focus attention on the oxygenations of inert org. substrates by cytochromes P 450, as these reactions involve multiply bonded iron-oxos. We emphasize that P 450 iron-oxos are strong oxidants, so strong that they would destroy nearby amino acids if substrates are not oxygenated rapidly; it is our view that these high valent iron oxos are such dangerous reactive oxygen species that Nature surely found ways to disable them. Looking more deeply into this matter, mainly by examg. many thousands of structures in the protein data bank, we have found that P450s and other enzymes that require oxygen for function have chains of tyrosines and tryptophans that extend from active-site regions to protein surfaces. Tyrosines are near the heme active sites in bacterial P450s, whereas tryptophan is closest in most human enzymes. High-valent iron-oxo survival times taken from hole hopping maps range from a few nanoseconds to milliseconds, depending on the distance of the closest Trp or Tyr residue to the heme. In our proposed mechanism, multistep hole tunneling (hopping) through Tyr/Trp chains guides the damaging oxidizing hole to the protein surface, where it can be quenched by sol. protein or small mol. reductants. As the Earth's oxygenic atm. is believed to have developed about 2.5 billion years ago, the increase in occurrence frequency of tyrosine and tryptophan since the last universal evolutionary ancestor may be in part a consequence of enzyme protective functions that developed to cope with the environmental toxin - O2.
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41
Holm, R. H. Metal-Centered Oxygen Atom Transfer Reactions. Chem. Rev. 1987, 87, 1401– 1449, DOI: 10.1021/cr00082a005
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41
Metal-centered oxygen atom transfer reactions
Holm, R. H.
Chemical Reviews (Washington, DC, United States) (1987), 87 (6), 1401-49CODEN: CHREAY; ISSN:0009-2665.
A review with 545 refs. of O atom transfer reactions of transition metal complexes.
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O’Halloran, K. P.; Zhao, C.; Ando, N. S.; Schultz, A. J.; Koetzle, T. F.; Piccoli, P. M. B.; Hedman, B.; Hodgson, K. O.; Bobyr, E.; Kirk, M. L.; Knottenbelt, S.; Depperman, E. C.; Stein, B.; Anderson, T. M.; Cao, R.; Geletii, Y. V.; Hardcastle, K. I.; Musaev, D. G.; Neiwert, W. A.; Fang, X.; Morokuma, K.; Wu, S.; Kögerler, P.; Hill, C. L. Revisiting the Polyoxometalate-Based Late-Transition-Metal-Oxo Complexes: The “Oxo Wall” Stands. Inorg. Chem. 2012, 51, 7025– 7031, DOI: 10.1021/ic2008914
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42
Revisiting the Polyoxometalate-Based Late-Transition-Metal-Oxo Complexes: The "Oxo Wall" Stands
O'Halloran, Kevin P.; Zhao, Chongchao; Ando, Nicole S.; Schultz, Arthur J.; Koetzle, Thomas F.; Piccoli, Paula M. B.; Hedman, Britt; Hodgson, Keith O.; Bobyr, Elena; Kirk, Martin L.; Knottenbelt, Sushilla; Depperman, Ezra C.; Stein, Benjamin; Anderson, Travis M.; Cao, Rui; Geletii, Yurii V.; Hardcastle, Kenneth I.; Musaev, Djamaladdin G.; Neiwert, Wade A.; Fang, Xikui; Morokuma, Keiji; Wu, Shaoxiong; Kogerler, Paul; Hill, Craig L.
Inorganic Chemistry (2012), 51 (13), 7025-7031CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
Terminal oxo complexes of the late transition metals Pt, Pd, and Au are reported by the authors in Science and Journal of the American Chem. Society. Despite thoroughness in characterizing these complexes (multiple independent structural methods and up to 17 anal. methods in one case), the authors have continued to study these structures. Initial work on these systems was motivated by structural data from x-ray crystallog. and neutron diffraction and 17O and 31P NMR signatures which all indicated differences from all previously published compds. With significant new data, the authors now revisit these studies. New x-ray crystal structures of previously reported complexes K14[P2W19O69(OH2)] and K10Na3[PdIV(O)(OH)WO(OH2)(PW9O34)2] and a closer examn. of these structures are provided. Also presented are the 17O NMR spectrum of an 17O-enriched sample of [PW11O39]7- and a careful combined 31P NMR-titrn. study of the previously reported K7H2[Au(O)(OH2)P2W20O70(OH2)2]. These and considerable other data collectively indicate that previously assigned terminal Pt-oxo and Au-oxo complexes are in fact cocrystals of the all-tungsten structural analogs with noble metal cations, while the Pd-oxo complex is a disordered Pd(II)-substituted polyoxometalate. The neutron diffraction data were re-analyzed, and new refinements are fully consistent with the all-tungsten formulations of the Pt-oxo and Au-oxo polyoxometalate species.
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Stull, J. A.; Stich, T. A.; Hurst, J. K.; Britt, R. D. Electron Paramagnetic Resonance Analysis of a Transient Species Formed During Water Oxidation Catalyzed by the Ion [(bpy)₂Ru(OH₂)]₂O⁴⁺. Inorg. Chem. 2013, 52, 4578– 4586, DOI: 10.1021/ic4001158
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Electron Paramagnetic Resonance Analysis of a Transient Species Formed During Water Oxidation Catalyzed by the Complex Ion [(bpy)2Ru(OH2)]2O4+
Stull, Jamie A.; Stich, Troy A.; Hurst, James K.; Britt, R. David
Inorganic Chemistry (2013), 52 (8), 4578-4586CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
The Ru blue dimer [(bpy)2Ru(OH2)]2O4+-the 1st well-defined mol. complex able to catalyze H2O oxidn. at low overpotentials-was the subject of numerous exptl. and computational studies. However, elements of the reaction mechanism remain controversial. Of particular interest is the nature of the O-O bond-forming step. Herein, the authors report the 1st advanced EPR spectroscopic studies of a high-valent intermediate that appears under conditions in which the catalyst is actively turning over. Results from previous studies suggested that this intermediate is derived from [(bpy)2RuV(O)]2O4+, denoted {5,5}. Under photooxidizing conditions, the corresponding EPR signal disappears at a rate comparable to the turnover rate of the catalyst once the illumination source is removed. The electronic and geometric structures of this species were explored using a variety of EPR techniques. Continuous wave (CW) EPR spectroscopy was used to probe the hyperfine coupling of the Ru ions, while corresponding ligand 14N hyperfine couplings were characterized with electron spin echo envelope modulation (ESEEM) and hyperfine sublevel correlation spectroscopy (HYSCORE) methods. Finally, 1H/2H ENDOR was performed to monitor any exchangeable protons. The authors' studies strongly suggest that the accumulating transient is an S = 1/2 species. This spin state formulation of the so-called {5,5} species is consistent with only a limited no. of electronic structures, each of which is discussed. Notably, the obsd. large metal hyperfine coupling indicates that the orbital carrying the unpaired spin has significant ruthenyl-oxyl character, contrary to an earlier electronic structure description that had tentatively assigned the signal to formation of a bipyridine ligand radical.
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(a) Gajhede, M.; Schuller, D. J.; Henriksen, A.; Smith, A. T.; Poulos, T. L. Crystal Structure of Horseradish Peroxidase C at 2.15 Å Resolution. Nat. Struct. Biol. 1997, 4, 1032– 1038, DOI: 10.1038/nsb1297-1032
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44a
Crystal structure of horseradish peroxidase C at 2.15 Å resolution
Gajhede, Michael; Schuller, David J.; Henriksen, Anette; Smith, Andrew T.; Poulos, Thomas L.
Nature Structural Biology (1997), 4 (12), 1032-1038CODEN: NSBIEW; ISSN:1072-8368. (Nature America)
The crystal structure of horseradish peroxidase isoenzyme C (I) was solved to 2.15 Å resoln. An important feature unique to the class III peroxidases is a long insertion, 34 residues in I, between helixes F and G. This region, which defines part of the substrate access channel, is not present in the core conserved fold typical of peroxidases from classes I and II. Comparison of I and peanut peroxidase (II), the only other class III (higher plant) peroxidase for which an x-ray structure has been completed, reveals that the structure in this region is highly variable even within class III. For peroxidases of the I type, characterized by a larger FG insertion (7 residues relative to II) and a shorter F' helix, the authors identified the key residue involved in direct interactions with arom. donor mols. I is unique in having a ring of 3 peripheral Phe residues, 142, 68, and 179. These guard the entrance to the exposed heme edge. The authors predict that this arom. region is important for the ability of I to bind arom. substrates.
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(b) Rodríguez-López, J. N.; Lowe, D. J.; Hernández-Ruiz, J.; Hiner, A. N. P.; García-Cánovas, F.; Thorneley, R. N. F. Mechanism of Reaction of Hydrogen Peroxide with Horseradish Peroxidase: Identification of Intermediates in the Catalytic Cycle. J. Am. Chem. Soc. 2001, 123, 11838– 11847, DOI: 10.1021/ja011853+
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44b
Mechanism of reaction of hydrogen peroxide with horseradish peroxidase: Identification of intermediates in the catalytic cycle
Rodriguez-Lopez, Jose Neptuno; Lowe, David J.; Hernandez-Ruiz, Josefa; Hiner, Alexander N. P.; Garcia-Canovas, Francisco; Thorneley, Roger N. F.
Journal of the American Chemical Society (2001), 123 (48), 11838-11847CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The mechanism of the reaction of horseradish peroxidase isoenzyme C (HRPC) with H2O2 to form reactive enzyme intermediate compd. I was studied using electronic absorbance, rapid-scan stopped-flow, and ESR spectroscopies at both acid and basic pH. The roles of active site residues His-42 and Arg-38 in controlling heterolytic cleavage of the H2O2 O-O bond were probed with site-directed mutant enzymes H42L, R38L, and R38G. The biphasic reaction kinetics of H42L with H2O2 suggested the presence of an intermediate species and, at acid pH, a reversible 2nd step, probably due to a neutral enzyme-H2O2 complex and the ferric-peroxoanion-contg. compd. 0. ESR also indicated the formation of a protein radical situated more than ∼10 Å from the heme Fe. The stoichiometry of the reaction of the H42L/H2O2 reaction product and 2,2'-azinobis(3-ethylbenzothiazolinesulfonic acid) (ABTS) was concn.-dependent and fell from a value of 2 to 1 above 0.7 mM ABTS. These data could be explained if H2O2 underwent homolytic cleavage in H42L. The apparent rate of compd. I formation by H42L, while low, was pH-independent in contrast to wild-type HRPC where the rate fell at acid pH, indicating the involvement of an ionizable group with pKa of ∼4. In R38L and R38G, the apparent pKa was shifted to ∼8, but there was no evidence that homolytic cleavage of H2O2 occurred. These data suggest that His-42 acts initially as a proton acceptor (base catalyst) and then as a donor (acid catalyst) at neutral pH and predict the obsd. slower rate and lower efficiency of heterolytic cleavage obsd. at acid pH. Arg-38 was influential in lowering the pKa of His-42 and addnl. in aligning H2O2 in the active site, but it did not play a direct role in proton transfer.
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(a) Dubey, K. D.; Shaik, S. Cytochrome P450–The Wonderful Nanomachine Revealed through Dynamic Simulations of the Catalytic Cycle. Acc. Chem. Res. 2019, 52, 389– 399, DOI: 10.1021/acs.accounts.8b00467
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45a
Cytochrome P450-The Wonderful Nanomachine Revealed through Dynamic Simulations of the Catalytic Cycle
Dubey Kshatresh Dutta; Shaik Sason
Accounts of chemical research (2019), 52 (2), 389-399 ISSN:.
This Account addresses the catalytic cycle of the enzyme cytochrome P450 (CYP450) as a prototypical biological machine with automatic features. CYP450 is a nanomachine that uses dioxygen and two reducing and two proton equivalents to oxidize a plethora of molecules (so-called substrates) as a means of supplying bio-organisms with essential molecules (e.g., brain neurotransmitters, sex hormones, etc.) and protecting biosystems against poisoning. An enticing property of CYP450s is that entrance of an oxidizable substrate into the active site initiates a series of events that constitute the catalytic cycle, which functions "automatically" in a regulated sequence of events culminating in the production of the oxidized substrates (e.g., hydroxylated, epoxidized, etc.), oftentimes with remarkable stereo- and regioselectivities. It is timely to demonstrate how theory uses molecular dynamics (MD) simulations and quantum-mechanical/molecular-mechanical (QM/MM) calculations to complement experiments and elucidate the choreography by which the protein regulates the catalytic cycle. CYP450 is a heme enzyme that contains a ferric ion (Fe(III)) coordinated by a porphyrin ligand, a water molecule, and a cysteinate ligand that is provided by a strategic residue of the encapsulating protein. While many of the individual steps are sufficiently well-understood, we shall provide here an overview of the factors that cause all of the steps to be sequentially coordinated. To this end, we use examples from three different CYP450 enzymes: the bacterial ones CYP450BM3 and CYP450CAM and the mammalian enzyme CYP4503A4. The treatment is limited to the catalytic cycle, as aspects of two-state reactivity were reviewed previously (e.g., Shaik , S. ; et al. Chem. Rev. 2005 , 105 , 2279 ). What are the principles that govern the seeming automatic feature? For example, how do substrate entrance and binding gate the enzyme? How does the reductase attachment to the enzyme affect the next steps? What triggers the attachment of the reductase? How does the electron transfer (ET) that converts Fe(III) to Fe(II) occur? Is the ET coordinated with the entrance of O2 into the active site? What is the mechanism of the latter step? Since the entrance of the substrate expels the water molecules from the active site, how do water molecules re-enter to form a proton channel, which is necessary for creating the ultimate oxidant Compound I? How do mutations that disrupt the water channel nevertheless create a competent oxidant? By what means does the enzyme produce regio- and stereoselective oxidation products? What triggers the departure of the oxidized product, and how does the exit occur in a manner that generates the resting state ready for the next cycle? This Account shows that the entrance of the substrate triggers all of the ensuing events.
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(b) Ortiz de Montellano, P. R. Hydrocarbon Hydroxylation by Cytochrome P450 Enzymes. Chem. Rev. 2010, 110, 932– 948, DOI: 10.1021/cr9002193
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45b
Hydrocarbon hydroxylation by cytochrome P 450 enzymes
Ortiz de Montellano, Paul R.
Chemical Reviews (Washington, DC, United States) (2010), 110 (2), 932-948CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review. Here, the author focuses on cytochrome P 450-catalyzed hydrocarbon hydroxylation, the reaction that is most characteristic of P 450 isoforms. However, the principles that apply in these reactions also apply to other hydroxylation reactions, including those that occur on C atoms adjacent to N, S, or O atoms.
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Price, J. C.; Barr, E. W.; Tirupati, B.; Bollinger, J. M.; Krebs, C. The First Direct Characterization of a High-Valent Iron Intermediate in the Reaction of an α-Ketoglutarate-Dependent Dioxygenase: A High-Spin Fe(IV) Complex in Taurine/α-Ketoglutarate Dioxygenase (TauD) from Escherichia coli. Biochemistry 2003, 42, 7497– 7508, DOI: 10.1021/bi030011f
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46
The first direct characterization of a high-valent iron intermediate in the reaction of an α-ketoglutarate-dependent dioxygenase: A high-spin Fe(IV) complex in taurine/α-ketoglutarate dioxygenase (TauD) from Escherichia coli
Price, John C.; Barr, Eric W.; Tirupati, Bhramara; Bollinger, J. Martin, Jr.; Krebs, Carsten
Biochemistry (2003), 42 (24), 7497-7508CODEN: BICHAW; ISSN:0006-2960. (American Chemical Society)
The Fe(II)- and α-ketoglutarate (αKG)-dependent dioxygenases have roles in synthesis of collagen and sensing of oxygen in mammals, in acquisition of nutrients and synthesis of antibiotics in microbes, and in repair of alkylated DNA in both. A consensus mechanism for these enzymes, involving (i) addn. of O2 to a five-coordinate, (His)2(Asp)-facially coordinated Fe(II) center to which αKG is also bound via its C-1 carboxylate and ketone oxygen; (ii) attack of the uncoordinated oxygen of the bound O2 on the ketone carbonyl of αKG to form a bicyclic Fe(IV)-peroxyhemiketal complex; (iii) decarboxylation of this complex concomitantly with formation of an oxo-ferryl (Fe(IV):O2-) intermediate; and (iv) hydroxylation of the substrate by the Fe(IV):O2- complex via a substrate radical intermediate, has repeatedly been proposed, but none of the postulated intermediates occurring after addn. of O2 has ever been detected. An oxidized Fe intermediate in the reaction of one of these enzymes, taurine/α-ketoglutarate dioxygenase (TauD) from Escherichia coli, has been directly demonstrated by rapid kinetic and spectroscopic methods. Characterization of the intermediate and its one-electron-reduced form (obtained by low-temp. γ-radiolysis of the trapped intermediate) by Moessbauer and ESR spectroscopies establishes that it is a high-spin, formally Fe(IV) complex. Its Moessbauer isomer shift is, however, significantly greater than those of other known Fe(IV) complexes, suggesting that the iron ligands in the TauD intermediate confer significant Fe(III) character to the high-valent site by strong electron donation. The properties of the complex and previous results on related αKG-dependent dioxygenases and other nonheme-Fe(II)-dependent, O2-activating enzymes suggest that the TauD intermediate is most probably either the Fe(IV)-peroxyhemiketal complex or the taurine-hydroxylating Fe(IV):O2- species. The detection of this intermediate sets the stage for a more detailed dissection of the TauD reaction mechanism than has previously been reported for any other member of this important enzyme family.
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Rohde, J.-U.; In, J.-H.; Lim, M. H.; Brennessel, W. W.; Bukowski, M. R.; Stubna, A.; Münck, E.; Nam, W.; Que, L., Jr. Crystallographic and Spectroscopic Characterization of a Nonheme Fe(IV)═O Complex. Science 2003, 299, 1037– 1039, DOI: 10.1126/science.299.5609.1037
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47
Crystallographic and Spectroscopic Characterization of a Nonheme Fe(IV)=O complex
Rohde, Jan-Uwe; In, Jun-Hee; Lim, Mi Hee; Brennessel, William W.; Bukowski, Michael R.; Stubna, Audria; Muenck, Eckard; Nam, Wonwoo; Que, Lawrence, Jr.
Science (Washington, DC, United States) (2003), 299 (5609), 1037-1039CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
Following the heme paradigm, it is often proposed that dioxygen activation by nonheme monoiron enzymes involves an iron(IV)=oxo intermediate that is responsible for the substrate oxidn. step. Such a transient species has now been obtained from a synthetic complex with a nonheme macrocyclic ligand and characterized spectroscopically. Its high-resoln. crystal structure reveals an iron-oxygen bond length of 1.646(3) angstroms, demonstrating that a terminal iron(IV)=oxo unit can exist in a nonporphyrin ligand environment and lending credence to proposed mechanisms of nonheme iron catalysis.
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(a) Kryatov, S. V.; Rybak-Akimova, E. V.; Schindler, S. Kinetics and Mechanisms of Formation and Reactivity of Non-Heme Iron Oxygen Intermediates. Chem. Rev. 2005, 105, 2175– 2226, DOI: 10.1021/cr030709z
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48a
Kinetics and Mechanisms of Formation and Reactivity of Non-heme Iron Oxygen Intermediates
Kryatov, Sergey V.; Rybak-Akimova, Elena V.; Schindler, Siegfried
Chemical Reviews (Washington, DC, United States) (2005), 105 (6), 2175-2226CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review. Understanding the mechanisms of dioxygen activation at the metal centers is important for unraveling the mechanisms of metal-contg. oxidases and oxygenases, synthesizing new selective oxidn. catalysts and new drugs analogous to bleomycin, and suppressing free radical pathways of oxidative damage in biol. systems. Dioxygen-binding and -activating biomols. with nonheme iron centers include a unique glycopeptide antibiotic bleomycin and numerous proteins, which are generally grouped into two large families: mononuclear (having only one iron at the active site) and dinuclear (having two proximate irons connected by bridging ligands at the active site). Bleomycin and nonheme iron enzymes are briefly introduced. The only nonheme iron dioxygen carrier, hemerythrin, is considered in some detail. General aspects of model chem. are introduced, followed by detailed sections on the kinetics and mechanisms of dioxygen binding and activation with mono- and dinuclear nonheme iron complexes and related reactions. The last section of this review is devoted to issues of kinetic methodol. specific for dioxygen-binding studies.
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(b) Abu-Omar, M. M.; Loaiza, A.; Hontzeas, N. Reaction Mechanism of Mononuclear Non-Heme Iron Oxygenases. Chem. Rev. 2005, 105, 2227– 2252, DOI: 10.1021/cr040653o
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48b
Reaction mechanisms of mononuclear non-heme iron oxygenases
Abu-Omar, Mahdi M.; Loaiza, Aristobulo; Hontzeas, Nikos
Chemical Reviews (Washington, DC, United States) (2005), 105 (6), 2227-2252CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review. A large no. of non-heme iron enzymes are known to catalyze a wide range of reactions with O2. The major focus of this review is on reaction mechanisms of pterin-dependent arom. amino acid hydroxylases, followed by comparisons with 3 substrate hydroxylases, and ending with a discussion of intramol. dioxygenases.
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(c) McDonald, A. R.; Que, L., Jr. High-Valent Nonheme Iron-Oxo Complexes: Synthesis, Structure, and Spectroscopy. Coord. Chem. Rev. 2013, 257, 414– 428, DOI: 10.1016/j.ccr.2012.08.002
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48c
High-valent nonheme iron-oxo complexes: Synthesis, structure, and spectroscopy
McDonald, Aidan R.; Que, Lawrence
Coordination Chemistry Reviews (2013), 257 (2), 414-428CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
A review. High-valent iron-oxo intermediates have often been implicated, and in some cases identified, as the active oxidant in oxygen activating nonheme iron enzymes. Recent synthetic efforts have yielded pivotal insights into the generation of oxoiron(IV and V) complexes, and allowed thorough study of their spectroscopic, structural, and electronic properties. Furthermore, insight into the mechanisms by which nonheme iron sites activate dioxygen to yield high valent iron-oxo intermediates was obtained. This review covers the great successes in iron-oxo chem. over the past decade, detailing various efforts to obtain iron-oxo complexes in high yield, and to delve into their diverse structural and spectroscopic properties.
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(d) Fujii, H. Electronic Structure and Reactivity of High-Valent Oxo Iron Porphyrins. Coord. Chem. Rev. 2002, 226, 51– 60, DOI: 10.1016/S0010-8545(01)00441-6
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48d
Electronic structure and reactivity of high-valent oxo iron porphyrins
Fujii, Hiroshi
Coordination Chemistry Reviews (2002), 226 (1-2), 51-60CODEN: CCHRAM; ISSN:0010-8545. (Elsevier Science B.V.)
A review of high valent oxo Fe porphyrin complexes, their electronic structure and their reactivity as models for compds.-I and compds.-II in heme enzymes.
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49
(a) Kotani, H.; Kaida, S.; Ishizuka, T.; Mieda, K.; Sakaguchi, M.; Ogura, T.; Shiota, Y.; Yoshizawa, K.; Kojima, T. Importance of the Reactant-State Potentials of Chromium(V)-Oxo Complexes to Determine the Reactivity in Hydrogen-Atom Transfer Reactions. Inorg. Chem. 2018, 57, 13929, DOI: 10.1021/acs.inorgchem.8b02453
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49a
Importance of the Reactant-State Potentials of Chromium(V)-Oxo Complexes to Determine the Reactivity in Hydrogen-Atom Transfer Reactions
Kotani, Hiroaki; Kaida, Suzue; Ishizuka, Tomoya; Mieda, Kaoru; Sakaguchi, Miyuki; Ogura, Takashi; Shiota, Yoshihito; Yoshizawa, Kazunari; Kojima, Takahiko
Inorganic Chemistry (2018), 57 (21), 13929-13936CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
A new chromium(V)-oxo complex, [CrV(O)(6-COO--py-tacn)]2+ (1; 6-COO--py-tacn = 1-(6-carboxylato-2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane), was synthesized and characterized to evaluate the reactivity of CrV(O) complexes in a hydrogen-atom transfer (HAT) reaction by comparing it with that of a previously reported CrV(O) complex, [CrV(O)(6-COO--tpa)]2+ (2; 6-COO--tpa = N,N-bis(2-pyridylmethyl)-N-(6-carboxylato-2-pyridylmethyl)amine). Definitive differences of these two CrV(O) complexes were obsd. in resonance Raman scatterings of the Cr-O bond (ν = 911 cm-1 for 1 and 951 cm-1 for 2) and the redn. potential (0.73 V vs SCE for 1 and 1.23 V for 2); this difference should be derived from that of the ligand bound at the trans position to the oxo ligand, a tertiary amino group in 1, and a pyridine nitrogen in 2. When we employed 9,10-dihydroanthracene as a substrate, the second-order rate const. (k) of 1 was 4000 times smaller than that of 2. Plots of normalized k values for both complexes relative to bond dissocn. energies (BDEs) of C-H bonds to be cleaved in several substrates showed a pair of parallel lines with slopes of -0.91 for 1 and -0.62 for 2, indicating that the HAT reactions by the two complexes proceed via almost the same transition states. Judging from estd. BDEs of CrIV(OH)/CrV(O) (85-87 kcal mol-1 for 1 and 92-94 kcal mol-1 for 2) and the activation barrier in the HAT reaction of DHA (Ea = 7.9 kcal mol-1 for 1 and Ea = 4.8 kcal mol-1 for 2), the reactivity of CrV(O) complexes in HAT reactions depends on the energy level of the reactant state rather than the product state.
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(b) Kotani, H.; Kaida, S.; Ishizuka, T.; Sakaguchi, M.; Ogura, T.; Shiota, Y.; Yoshizawa, K.; Kojima, T. Formation and Characterization of a Reactive Chromium(V)-Oxo Complex: A Mechanistic Insight into Hydrogen-Atom Transfer Reactions. Chem. Sci. 2015, 6, 945– 955, DOI: 10.1039/C4SC02285H
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49b
Formation and characterization of a reactive chromium(V)-oxo complex: mechanistic insight into hydrogen-atom transfer reactions
Kotani, Hiroaki; Kaida, Suzue; Ishizuka, Tomoya; Sakaguchi, Miyuki; Ogura, Takashi; Shiota, Yoshihito; Yoshizawa, Kazunari; Kojima, Takahiko
Chemical Science (2015), 6 (2), 945-955CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
A mononuclear Cr(V)-oxo complex, [CrV(O)(6-COO--tpa)](BF4)2 (1; 6-COO--tpa = N,N-bis(2-pyridylmethyl)-N-(6-carboxylato-2-pyridylmethyl)amine) was prepd. through the reaction of a Cr(III) precursor complex with iodosylbenzene as an oxidant. Characterization of 1 was achieved using ESI-MS spectrometry, ESR, UV-visible, and resonance Raman spectroscopies. The redn. potential (Ered) of 1 is 1.23 V vs. SCE in acetonitrile based on anal. of the electron-transfer (ET) equil. between 1 and a one-electron donor, [RuII(bpy)3]2+ (bpy = 2,2'-bipyridine). The reorganization energy (λ) of 1 also is 1.03 eV in ET reactions from phenol derivs. to 1 on the basis of the Marcus theory of ET. The smaller λ value in comparison with that of an Fe(IV)-oxo complex (2.37 eV) is caused by the small structural change during ET due to the dπ character of the electron-accepting LUMO of 1. When benzyl alc. derivs. (R-BA) with different oxidn. potentials were employed as substrates, corresponding aldehydes were obtained as the 2e--oxidized products in moderate yields as detd. from 1H NMR and GC-MS measurements. One-step UV-visible spectral changes were obsd. in the oxidn. reactions of BA derivs. by 1 and a kinetic isotope effect (KIE) was obsd. in the oxidn. reactions for deuterated BA derivs. at the benzylic position as substrates. The rate-limiting step is a concerted proton-coupled electron transfer (PCET) from substrate to 1. In sharp contrast, in the oxidn. of trimethoxy-BA (Eox = 1.22 V) by 1, trimethoxy-BA radical cation was obsd. by UV-visible spectroscopy. Thus, the mechanism of the oxidn. reaction changed from one-step PCET to stepwise ET-proton transfer (ET/PT), depending on the redox potentials of R-BA.
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(c) Cho, J.; Woo, J.; Eun Han, J.; Kubo, M.; Ogura, T.; Nam, W. Chromium(V)-Oxo and Chromium(III)-Superoxo Complexes Bearing a Macrocyclic TMC Ligand in Hydrogen Atom Abstraction Reactions. Chem. Sci. 2011, 2, 2057– 2062, DOI: 10.1039/c1sc00386k
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49c
Chromium(V)-oxo and chromium(III)-superoxo complexes bearing a macrocyclic TMC ligand in hydrogen atom abstraction reactions
Cho, Jaeheung; Woo, Jaeyoung; Han, Jung Eun; Kubo, Minoru; Ogura, Takashi; Nam, Wonwoo
Chemical Science (2011), 2 (10), 2057-2062CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
A Cr(V)-oxo complex bearing a macrocyclic TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) ligand, [CrV(TMC)(O)(OCH3)]2+, was synthesized, isolated, and characterized by various physicochem. methods, including UV-visible, ESI-MS, resonance Raman, EPR and x-ray anal. The reactivity of the Cr(V)-oxo complex was studied in C-H and O-H bond activation reactions. The reactivity of a Cr(III)-superoxo complex, [CrIII(TMC)(O2)(Cl)]+, was studied in O-H bond activation reactions as well. By comparing reactivities of the Cr(III)-superoxo and Cr(V)-oxo complexes under the identical reaction conditions, the authors were able to demonstrate that the Cr(III)-superoxo complex is more reactive than the Cr(V)-oxo complex in the activation of C-H and O-H bonds. The present results provide strong evidence that under certain circumstances, metal-superoxo species can be an alternative oxidant for high-valent metal-oxo complexes in oxygenation reactions.
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50
(a) Mandimutsira, B. S.; Ramdhanie, B.; Todd, R. C.; Wang, H.; Zareba, A. A.; Czernuszewicz, R. S.; Goldberg, D. P. A Stable Manganese(V)-Oxo Corrolazine Complex. J. Am. Chem. Soc. 2002, 124, 15170– 15171, DOI: 10.1021/ja028651d
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50a
A Stable Manganese(V)-Oxo Corrolazine Complex
Mandimutsira, Beaven S.; Ramdhanie, Bobby; Todd, Ryan C.; Wang, Hailin; Zareba, Adelajda A.; Czernuszewicz, Roman S.; Goldberg, David P.
Journal of the American Chemical Society (2002), 124 (51), 15170-15171CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
I (R = p-tBuC6H4) reacted with Mn(acac)3 to give MnL (H3L = I) which was oxidized to MnO(L). Stable MnO(L) was characterized by resonance Raman spectra. The oxidn. of PPh3 or Me2S by MnO(L) was obsd. with the formation of MnL.
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51
Ishizuka, T.; Kotani, H.; Kojima, T. Characteristics and Reactivity of Ruthenium-Oxo Complexes. Dalton Trans. 2016, 45, 16727– 16750, DOI: 10.1039/C6DT03024F
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51
Characteristics and reactivity of ruthenium-oxo complexes
Ishizuka, Tomoya; Kotani, Hiroaki; Kojima, Takahiko
Dalton Transactions (2016), 45 (42), 16727-16750CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
A review. In this perspective, the authors have surveyed the synthetic procedure, characteristics, and reactivity of high-valent ruthenium-oxo complexes. The ruthenium-oxo complexes have served as ideal species to elucidate the characteristics of metal-oxo complexes in terms of not only geometrical and electronic structures but also oxidn. reactivity and mechanisms of oxidn. reactions. Due to the high stability and excellent reversibility of redox processes, ruthenium-oxo complexes provided significant mechanistic insights into the oxidn. of org. compds. including alcs., alkenes, and alkanes and also water from detailed kinetic anal.
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52
Moyer, B. A.; Meyer, T. J. Oxobis(2,2′-bipyridine)pyridineruthenium(IV) Ion, [(bpy)₂(py)Ru═O]²⁺. J. Am. Chem. Soc. 1978, 100, 3601– 3603, DOI: 10.1021/ja00479a054
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52
Oxobis(2,2'-bipyridine)pyridineruthenium(IV) ion, [(bpy)2(py)Ru:O]2+
Moyer, Bruce A.; Meyer, Thomas J.
Journal of the American Chemical Society (1978), 100 (11), 3601-3CODEN: JACSAT; ISSN:0002-7863.
The novel Ru(IV) complex [(bpy)2(py)Ru:O](ClO4)2 (bpy = 2,2'-bipyridine) contg. a single terminal oxo ligand was prepd. by the 2-electron oxidn. of [(bpy)2(py)RuOH2]2+ using Ce(IV). The complex was characterized by cond. studies in aq. soln., magnetic susceptibility, and IR and electronic spectroscopy. Spectrophotometric and electrochem. techniques were used to show that the conversion in 1 M HClO4. [(Bpy)2(py)Ru:O]2+-2H+-e- → [(bpy)2(py)RuOH2]3+ - e- → [(bpy)2(py)RuOH2]3+ is chem. reversible where the Ru(IV)/Ru(III) and Ru(III)/Ru(II) redn. potentials in 1 M HClO4 at 25° are +0.994V and +0.781V, resp., vs. the SCE. The reversible acid-base behaviors of the Ru(III), [(bpy)2(py)RuOH2]3+ .dblharw. [(bpy)2(py)RuOH]2+ + H+, and Ru(II) complexes, [(bpy)2(py)RuOH2]2+ .dblharw. [(bpy)2(py)RuOH]+ + H+, were studied by spectrophotometric titrns. The resp. pKa values were 0.83 and 10.8. As an oxidant, the [(bpy)2(py)Ru:O]2+ ion reacts rapidly with PPh3 in MeCN by a net O atom transfer reaction which gives the phosphine oxide complex [(bpy)2(py)Ru(OPPh3)]2+. The phosphine oxide complex undergoes a slow 1st-order solvolysis reaction (t1/2 = 100 min. at 25°) to give free OPPh3 and [(bpy)2(py)Ru(MeCN)]2+.
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53
Dietl, N.; Schlangen, M.; Schwarz, H. Thermal Hydrogen-Atom Transfer from Methane: The Role of Radicals and Spin States in Oxo-Cluster Chemistry. Angew. Chem., Int. Ed. 2012, 51, 5544– 5555, DOI: 10.1002/anie.201108363
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53
Thermal Hydrogen-Atom Transfer from Methane: The Role of Radicals and Spin States in Oxo-Cluster Chemistry
Dietl, Nicolas; Schlangen, Maria; Schwarz, Helmut
Angewandte Chemie, International Edition (2012), 51 (23), 5544-5555CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
A review. Hydrogen-atom transfer (HAT), as one of the fundamental reactions in chem., is investigated with state-of-the-art gas-phase expts. in conjunction with computational studies. The focus of this Minireview concerns the role that the intrinsic properties of gaseous oxo-clusters play to permit HAT reactivity from satd. hydrocarbons at ambient conditions. In addn., mechanistic implications are discussed which pertain to heterogeneous catalysis. From these combined exptl./computational studies, the crucial role of unpaired spin d. at the abstracting atom becomes clear, in distinct contrast to recent conclusions derived from soln.-phase expts.
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54
Usharani, D.; Lacy, D. C.; Borovik, A. S.; Shaik, S. Dichotomous Hydrogen Atom Transfer vs Proton-Coupled Electron Transfer During Activation of X-H Bonds (X = C, N, O) by Nonheme Iron-Oxo Complexes of Variable Basicity. J. Am. Chem. Soc. 2013, 135, 17090– 17104, DOI: 10.1021/ja408073m
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54
Dichotomous Hydrogen Atom Transfer vs Proton-Coupled Electron Transfer During Activation of X-H Bonds (X = C, N, O) by Nonheme Iron-Oxo Complexes of Variable Basicity
Usharani, Dandamudi; Lacy, David C.; Borovik, A. S.; Shaik, Sason
Journal of the American Chemical Society (2013), 135 (45), 17090-17104CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
We describe herein the hydrogen-atom transfer (HAT)/proton-coupled electron-transfer (PCET) reactivity for FeIV-oxo and FeIII-oxo complexes (1-4) that activate C-H, N-H, and O-H bonds in 9,10-dihydroanthracene (S1), DMF (S2), 1,2-diphenylhydrazine (S3), p-methoxyphenol (S4), and 1,4-cyclohexadiene (S5). In 1-3, the iron is pentacoordinated by tris-[N'-tert-butylureaylato-N-ethylene]-aminato ([H3buea]3) or its derivs. These complexes are basic, in the order 3 » 1 > 2. Oxidant 4, [FeIVN4Py-(O)]2+ (N4Py: N,N-bis-(2-pyridylmethyl)-bis-(2-pyridyl)-methylamine), is the least basic oxidant. The DFT results match exptl. trends and exhibit a mechanistic spectrum ranging from concerted HAT and PCET reactions to concerted-asynchronous proton transfer (PT)/electron transfer (ET) mechanisms, all the way to PT. The singly occupied orbital along the O···H···X (X = C, N, O) moiety in the TS shows clearly that in the PCET cases, the electron is transferred sep. from the proton. The Bell-Evans-Polanyi principle does not account for the obsd. reactivity pattern, as evidenced by the scatter in the plot of calcd. barrier vs reactions driving forces. However, a plot of the deformation energy in the TS vs the resp. barrier provides a clear signature of the HAT/PCET dichotomy. Thus, in all C-H bond activations, the barrier derives from the deformation energy required to create the TS, whereas in N-H/O-H bond activations, the deformation energy is much larger than the corresponding barrier, indicating the presence of a stabilizing interaction between the TS fragments. A valence bond model is used to link the obsd. results with the basicity/acidity of the reactants.
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55
(a) Ohzu, S.; Ishizuka, T.; Hirai, Y.; Jiang, H.; Sakaguchi, M.; Ogura, T.; Fukuzumi, S.; Kojima, T. Mechanistic Insight into Catalytic Oxidations of Organic Compounds by Ruthenium(IV)-Oxo Complexes with Pyridylamine Ligands. Chem. Sci. 2012, 3, 3421– 3431, DOI: 10.1039/c2sc21195e
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55a
Mechanistic insight into catalytic oxidations of organic compounds by ruthenium(IV)-oxo complexes with pyridylamine ligands
Ohzu, Shingo; Ishizuka, Tomoya; Hirai, Yuichirou; Jiang, Hua; Sakaguchi, Miyuki; Ogura, Takashi; Fukuzumi, Shunichi; Kojima, Takahiko
Chemical Science (2012), 3 (12), 3421-3431CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
A series of Ru(IV)-oxo complexes with tris(2-pyridylmethyl)amine derivs. [N(CH2C5H4)(CH2C5H3R)RuO(OH2)n]n+ (4-6, R = H, CO2; n = 0, 1) were synthesized from the corresponding Ru(II)-aqua complexes [N(CH2C5H4)(CH2C5H3R)Ru(OH2)m]n+ (1-3, same R, m = 1, 2) and fully characterized by 1H NMR and resonance Raman spectroscopies, and ESI-MS spectrometry. Based on the diamagnetic character confirmed by the 1H NMR spectroscopy in D2O, the spin states of 5 and 6 were detd. to be S = 0 in the d4 configuration, in sharp contrast to that of 4 being in the S = 1 spin state. The aqua-complexes 1-3 catalyzed oxidn. of alcs. and olefins using (NH4)2[CeIV(NO3)6] (CAN) as an electron-transfer oxidant in acidic aq. solns. Comparison of the reactivity of electrochem. generated oxo-complexes 4-6 was made in the light of kinetic analyses for oxidn. of 1-propanol and a water-sol. ethylbenzene deriv. The oxo complexes 4-6 exhibited no significant difference in the reactivity for the oxidn. reactions, judging from the similar catalytic rates and the activation parameters. The slight difference obsd. in the reaction rates can be accounted for by the difference in the redn. potentials of the oxo-complexes, but the spin states of the oxo-complexes have hardly affected the reactivity. The activation parameters and the kinetic isotope effects (KIE) obsd. for the oxidn. reactions of methanol indicate that the oxidn. reactions of alcs. with the RuIV:O complexes proceed via a concerted proton-coupled electron transfer mechanism.
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(b) Kojima, T.; Hirai, Y.; Ishizuka, T.; Shiota, Y.; Yoshizawa, K.; Ikemura, K.; Ogura, T.; Fukuzumi, S. A Low-Spin Ruthenium(IV)-Oxo Complex: Does the Spin State Have an Impact on the Reactivity?. Angew. Chem., Int. Ed. 2010, 49, 8449– 8453, DOI: 10.1002/anie.201002733
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55b
A Low-Spin Ruthenium(IV)-Oxo Complex: does the Spin State Have an Impact on the Reactivity?
Kojima, Takahiko; Hirai, Yuichirou; Ishizuka, Tomoya; Shiota, Yoshihito; Yoshizawa, Kazunari; Ikemura, Kenichiro; Ogura, Takashi; Fukuzumi, Shunichi
Angewandte Chemie, International Edition (2010), 49 (45), 8449-8453, S8449/1-S8449/23CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
A low-spin ruthenium(IV)-oxo complex and does the spin state have an impact on the reactivity are discussed.
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56
Schröder, D.; Roithová, J.; Alikhani, E.; Kwapien, K.; Sauer, J. Preferential Activation of Primary C-H Bonds in the Reactions of Small Alkanes with the Diatomic MgO^•+ Cation. Chem. - Eur. J. 2010, 16, 4110– 4119, DOI: 10.1002/chem.200902373
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57
Siegbahn, P. E. M. O-O Bond Formation in the S4 State of the Oxygen-Evolving Complex in Photosystem II. Chem. - Eur. J. 2006, 12, 9217– 9227, DOI: 10.1002/chem.200600774
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57
O-O bond formation in the S4 state of the oxygen-evolving complex in photosystem II
Siegbahn, Per E. M.
Chemistry - A European Journal (2006), 12 (36), 9217-9227CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
Based on recent X-ray structures of the oxygen-evolving complex in photosystem II, quantum chem. geometry optimizations of several thousand structures have been performed in order to elucidate the mechanism for dioxygen formation. Many of the results of these calcns. have been presented previously. The energetically most stable structure of the S4 state has been used in the present study to investigate essentially all the possible ways the O-O bond can be formed in this structure. A key feature, emphasized previously, of the S4 state is that an oxygen radical ligand is present rather than an MnV state. Previous studies have indicated that this oxygen radical can form an O-O bond by an attack from a water mol. in the second coordination shell. The present systematic investigation has led to a new type of mechanism that is significantly favored over the previous one. A calcd. transition-state barrier of 12.5 kcal mol-1 was found for this mechanism, whereas the best previous results gave 18-20 kcal mol-1. A requirement on the spin alignment for a low barrier is formulated.
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58
(a) Oda, A.; Ohkubo, T.; Yumura, T.; Kobayashi, H.; Kuroda, Y. Identification of a Stable Zn^II-Oxyl Species Produced in an MFI Zeolite and Its Reversible Reactivity with O₂ at Room Temperature. Angew. Chem., Int. Ed. 2017, 56, 9715– 9718, DOI: 10.1002/anie.201702570
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58a
Identification of a Stable ZnII-Oxyl Species Produced in an MFI Zeolite and Its Reversible Reactivity with O2 at Room Temperature
Oda, Akira; Ohkubo, Takahiro; Yumura, Takashi; Kobayashi, Hisayoshi; Kuroda, Yasushige
Angewandte Chemie, International Edition (2017), 56 (33), 9715-9718CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
Although a terminal oxyl species bound to certain metal ions is believed to be the intermediate for various oxidn. reactions, such as O-O bond generation in photosystem II (PSII), such systems have not been characterized. Herein, we report a stable ZnII-oxyl species induced by an MFI-type zeolite lattice and its reversible reactivity with O2 at room temp. Its intriguing characteristics were confirmed by in situ spectroscopic studies in combination with quantum-chem. calcns., namely analyses of the vibronic Franck-Condon progressions and the ESR signal features of both ZnII-oxyl and ZnII-ozonide species formed during this reversible process. MO analyses revealed that the reversible reaction between a ZnII-oxyl species and an O2 mol. proceeds via a radical O-O coupling-decoupling mechanism; the unpaired electron of the oxyl species plays a pivotal role in the O-O bond generation process.
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(b) Oda, A.; Ohkubo, T.; Yumura, T.; Kobayashi, H.; Kuroda, Y. Room-Temperature Activation of the C-H Bond in Methane over Terminal Zn^II-Oxyl Species in an MFI Zeolite: A Combined Spectroscopic and Computational Study of the Reactive Frontier Molecular Orbitals and Their Origins. Inorg. Chem. 2019, 58, 327– 338, DOI: 10.1021/acs.inorgchem.8b02425
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58b
Room-Temperature Activation of the C-H Bond in Methane over Terminal ZnII-Oxyl Species in an MFI Zeolite: A Combined Spectroscopic and Computational Study of the Reactive Frontier Molecular Orbitals and Their Origins
Oda, Akira; Ohkubo, Takahiro; Yumura, Takashi; Kobayashi, Hisayoshi; Kuroda, Yasushige
Inorganic Chemistry (2019), 58 (1), 327-338CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
Oxygenase reactivity toward selective partial oxidn. of CH4 to CH3OH requires an at. oxygen-radical bound to metal (M-O•: oxyl intermediate) that is capable of abstracting an H atom from the significantly strong C-H bond in CH4. Because such a reaction is frequently obsd. in metal-doped zeolites, it has been recognized that the zeolite provides an environment that stabilizes the M-O• intermediate. However, no exptl. data of M-O• have so far been discovered in the zeolite; thus, little is known about the correlation among the state of M-O•, its reactivity for CH4, and the nature of the zeolite environment. Here, we report a combined spectroscopic and computational study of the room-temp. activation of CH4 over ZnII-O• in the MFI zeolite. One ZnII-O• species does perform H-abstraction from CH4 at room temp. The resultant CH•3 species reacts with the other ZnII-O• site to form the ZnII-OCH3 species. The H2O-assisted extn. of surface methoxide yields 29 μmol g-1 of CH3OH with a 94% selectivity. The quantum mechanics (QM)/mol. mechanics (MM) calcn. detd. the central step as the oxyl-mediated hydrogen atom transfer which requires an activation energy of only 10 kJ mol-1. On the basis of the findings in gas-phase expts. regarding the CH4 activation by the free [M-O•]+ species, the remarkable H-abstraction reactivity of the ZnII-O• species in zeolites was totally rationalized. Addnl., the exptl. validated QM/MM calcn. revealed that the zeolite lattice has potential as the ligand to enhance the polarization of the M-O• bond and thereby enables to create effectively the highly reactive M-O• bond required for low-temp. activation of CH4. The present study proposes that tuning of the polarization effect of the anchoring site over heterogeneous catalysts is the valuable way to create the oxyl-based functionality on the heterogeneous catalyst.
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59
(a) Kobayashi, K.; Ohtsu, H.; Wada, T.; Kato, T.; Tanaka, K. Characterization of a Stable Ruthenium Complex with an Oxyl Radical. J. Am. Chem. Soc. 2003, 125, 6729– 6739, DOI: 10.1021/ja0211510
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59a
Characterization of a Stable Ruthenium Complex with an Oxyl Radical
Kobayashi, Katsuaki; Ohtsu, Hideki; Wada, Tohru; Kato, Tatsuhisa; Tanaka, Koji
Journal of the American Chemical Society (2003), 125 (22), 6729-6739CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The ruthenium oxyl radical complex, [RuII(trpy)(Bu2SQ)O•-] (trpy = 2,2':6',2''-terpyridine, Bu2SQ = 3,5-di-tert-butyl-1,2-benzosemiquinone) was prepd. for the first time by the double deprotonation of the aqua ligand of [RuIII(trpy)(Bu2SQ)(OH2)](ClO4)2. [RuIII(trpy)(Bu2SQ)(OH2)](ClO4)2 is reversibly converted to [RuIII(trpy)(Bu2SQ)(OH-)]+ upon dissocn. of the aqua proton (pKa 5.5). Deprotonation of the hydroxo proton gave rise to intramol. electron transfer from the resultant O2- to Ru-dioxolene. The resultant [RuII(trpy)(Bu2SQ)O•-] showed antiferromagnetic behavior with a RuII-semiquinone moiety and oxyl radical, the latter of which was characterized by a spin trapping technique. The most characteristic structural feature of [RuII(trpy)(Bu2SQ)O•-] is a long Ru-O bond length (2.042(6) Å) as the first terminal metal-O bond with a single bond length. To elucidate the substituent effect of a quinone ligand, [RuIII(trpy)(4ClSQ)(OH2)](ClO4)2 (4ClSQ = 4-chloro-1,2-benzosemiquinone) was prepd. and the authors compared the deprotonation behavior of the aqua ligand with that of [RuIII(trpy)(Bu2SQ)(OH2)](ClO4)2. Deprotonation of the aqua ligand of [RuIII(trpy)(4ClSQ)(OH2)](ClO4)2 induced intramol. electron transfer from OH- to the [RuIII(4ClSQ)] moiety affording [RuII(trpy)(4ClSQ)(OH•)]+, which then probably changed to [RuII(trpy)(4ClSQ)O•-]. The antiferromagnetic interactions (J values) between RuII-semiquinone and the oxyl radical for [RuII(trpy)(Bu2SQ)O•-] and for [RuII(trpy)(4ClSQ)O•-] were 2J = -0.67 cm-1 and -1.97 cm-1, resp.
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(b) Kobayashi, K.; Ohtsu, H.; Wada, T.; Tanaka, K. Ruthenium Oxyl Radical Complex Containing o-Quinone Ligand Detected by ESR Measurements of Spin Trapping Technique. Chem. Lett. 2002, 31, 868– 869, DOI: 10.1246/cl.2002.868
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60
(a) Wada, T.; Tsuge, K.; Tanaka, K. Electrochemical Oxidation of Water to Dioxygen Catalyzed by the Oxidized Form of the Bis(ruthenium-hydroxo) Complex in H₂O. Angew. Chem., Int. Ed. 2000, 39, 1479– 1482, DOI: 10.1002/(SICI)1521-3773(20000417)39:8<1479::AID-ANIE1479>3.0.CO;2-4
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60a
Electrochemical oxidation of water to dioxygen catalyzed by the oxidized form of the bis(ruthenium-hydroxo) complex in H2O
Wada, Tohru; Tsuge, Kiyoshi; Tanaka, Koji
Angewandte Chemie, International Edition (2000), 39 (8), 1479-1482CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH)
Electrochem. oxidn. of water is catalyzed by an ITO electrode modified with a bis(ruthenium-hydroxo) complex. This ruthenium complex is prepd. by reacting RuCl3 with 1,8-bis(terpyridyl)anthracene (btpyan) in MeOH, treating [Ru2Cl6(btpyan)] with 3,6-di-tert-butyl-1,2-benzenediol in the presence of KOAc in MeOH. This [RuII2(OAc)(3,6-tBu2sq)2(btpyan)]+ (3,6-tBu2sq = 3,6-di(tert-butyl)-1,2-semiquinone) was treated with triflic acid in MeOH contg. water and then sodium hexafluoroantimonate to give [Ru2(OH)2(3,6-tBu2qui)2(btpyan)](SbF6)2. When a controlled-potential electrolysis for this complex on ITO electrode was conducted at +1.7 V (vs. Ag/AgCl) in water, 1.1 mL of O2 evolved after 20.2 C had passed in the electrolysis. The current efficiency for O2 evolution was 95%.
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(b) Wada, T.; Tsuge, K.; Tanaka, K. Syntheses and Redox Properties of Bis(hydroxoruthenium) Complexes with Quinone and Bipyridine Ligands. Water-Oxidation Catalysis. Inorg. Chem. 2001, 40, 329– 337, DOI: 10.1021/ic000552i
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60b
Syntheses and Redox Properties of Bis(hydroxoruthenium) Complexes with Quinone and Bipyridine Ligands. Water-Oxidation Catalysis
Wada, Tohru; Tsuge, Kiyoshi; Tanaka, Koji
Inorganic Chemistry (2001), 40 (2), 329-337CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
The novel bridging ligand 1,8-bis(2,2':6',2''-terpyridyl)anthracene (btpyan) was synthesized by three reactions from 1,8-diformylanthracene to connect two [Ru(L)(OH)]+ units (L = 3,6-di-tert-butyl-1,2-benzoquinone (3,6-tBu2qui) and 2,2'-bipyridine (bpy)). An addn. of tBuOK (2.0 equiv) to a methanolic soln. of [RuII2(OH)2(3,6-tBu2qui)2(btpyan)](SbF6)2 ([1](SbF6)2) gave [RuII2(O)2(3,6-tBu2sq)2(btpyan)]0 (3,6-tBu2sq = 3,6-di-tert-butyl-1,2-semiquinone) due to the redn. of quinone coupled with the dissocn. of the hydroxo protons. The resultant complex [RuII2(O)2(3,6-tBu2sq)2(btpyan)]0 undergoes ligand-localized oxidn. at E1/2 = +0.40 V (vs. Ag/AgCl) to give [RuII2(O)2(3,6-tBu2qui)2(btpyan)]2+ in MeOH soln. Also, metal-localized oxidn. of [RuII2(O)2(3,6-tBu2qui)2(btpyan)]2+ at Ep = +1.2 V in CF3CH2OH/ether or H2O gives [RuIII2(O)2(3,6-tBu2qui)2(btpyan)]4+, which catalyzes H2O oxidn. Controlled-potential electrolysis of [1](SbF6)2 at +1.70 V in the presence of H2O in CF3CH2OH evolves dioxygen with a current efficiency of 91% (21 turnovers). The turnover no. of O2 evolution increases to 33,500 when the electrolysis is conducted in H2O (pH 4.0) by using a [1](SbF6)2-modified ITO electrode. However, the analogous complex [RuII2(OH)2(bpy)2(btpyan)](SbF6)2 ([2](SbF6)2) shows neither dissocn. of the hydroxo protons, even in the presence of a large excess of tBuOK, nor activity for the oxidn. of H2O under similar conditions. The structure of btpyan ligand was detd. by x-ray crystallog.
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(c) Muckerman, J. T.; Polyansky, D. E.; Wada, T.; Tanaka, K.; Fujita, E. Water Oxidation by a Ruthenium Complex with Noninnocent Quinone Ligands: Possible Formation of an O-O Bond at a Low Oxidation State of the Metal. Inorg. Chem. 2008, 47, 1787– 1802, DOI: 10.1021/ic701892v
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60c
Water Oxidation by a Ruthenium Complex with Noninnocent Quinone Ligands: Possible Formation of an O-O Bond at a Low Oxidation State of the Metal
Muckerman, James T.; Polyansky, Dmitry E.; Wada, Tohru; Tanaka, Koji; Fujita, Etsuko
Inorganic Chemistry (2008), 47 (6), 1787-1802CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
Tanaka and co-workers reported a novel dinuclear Ru complex, [Ru2(OH)2(3,6-Bu2Q)2(btpyan)](SbF6)2 (3,6-Bu2Q = 3,6-ditert-butyl-1,2-benzoquinone, btpyan = 1,8-bis(2,2':6',2''-terpyrid-4'-yl)anthracene), that contains redox active quinone ligands and has an excellent electrocatalytic activity for H2O oxidn. when immobilized on an In-Sn-oxide electrode (Inorg. Chem., 2001, 40, 329-337). The novel features of the dinuclear and related mononuclear Ru species with quinone ligands, and comparison of their properties to those of the Ru analogs with the bpy ligand (bpy = 2,2'-bipyridine) replacing quinone, are summarized here together with new theor. and exptl. results that show striking features for both the dinuclear and mononuclear species. The identity and oxidn. state of key mononuclear species, including the previously reported oxyl radical, were reassigned. The gas-phase theor. calcns. indicate that the Tanaka Ru-dinuclear catalyst seems to maintain predominantly Ru(II) centers while the quinone ligands and H2O moiety are involved in redox reactions throughout the entire catalytic cycle for H2O oxidn. The theor. study identifies [Ru2(O2-)(Q-1.5)2(btpyan)] as a key intermediate and the most reduced catalyst species that is formed by removal of all 4 protons before 4-electron oxidn. takes place. While the study toward understanding the complicated electronic and geometric structures of possible intermediates in the catalytic cycle is still in progress, the current status and new directions for kinetic and mechanistic studies, and key issues and challenges in H2O oxidn. with the Tanaka catalyst (and its analogs with Cl- or NO2-substituted quinones and a species with a xanthene bridge instead an anthracene) are discussed.
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(d) Wada, T.; Muckerman, J. T.; Fujita, E.; Tanaka, K. Substituents Dependent Capability of Bis(ruthenium-dioxolene-terpyridine) Complexes toward Water Oxidation. Dalton Trans. 2011, 40, 2225– 2233, DOI: 10.1039/C0DT00977F
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60d
Substituents dependent capability of bis(ruthenium-dioxolene-terpyridine) complexes toward water oxidation
Wada, Tohru; Muckerman, James T.; Fujita, Etsuko; Tanaka, Koji
Dalton Transactions (2011), 40 (10), 2225-2233CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
The bridging ligand, 1,8-bis(2,2':6',2''-terpyrid-4'-yl)anthracene (btpyan) was synthesized by the Miyaura-Suzuki cross coupling reaction of anthracenyl-1,8-diboronic acid and 4'-triflyl-2,2':6'-2''-terpyridine in the presence of Pd(PPh3)4 (5 mol%) with 68% in yield. Three ruthenium-dioxolene dimers, [Ru2(OH)2(dioxolene)2(btpyan)]0 (dioxolene = 3,6-di-tert-butyl-1,2-benzosemiquinone ([1]0), 3,5-dichloro-1,2-benzosemiquinone ([2]0) and 4-nitro-1,2-benzosemiquinone ([3]0)) were prepd. by the reaction of [Ru2Cl6(btpyan)]0 with the corresponding catechol. The electronic structure of [1]0 is approximated by [RuII2(OH)2(sq)2(btpyan)]0 (sq = semiquinonato). On the other hand, the electronic states of [2]0 and [3]0 are close to [RuIII2(OH)2 (cat)2(btpyan)]0 (cat = catecholato), indicating that a dioxolene having electron-withdrawing groups stabilizes [RuIII2(OH)2(cat)2(btpyan)]0 rather than [RuII2(OH)2(sq)2(btpyan)]0 as resonance isomers. No sign was found of deprotonation of the hydroxo groups of [1]0, whereas [2]0 and [3]0 showed an acid-base equil. in treatments with t-BuOLi followed by HClO4. Furthermore, controlled potential electrolysis of [1]0 deposited on an ITO (indium-tin oxide) electrode catalyzed the four-electron oxidn. of H2O to evolve O2 at potentials more pos. than +1.6 V (vs. SCE) at pH 4.0. On the other hand, the electrolysis of [2]0 and [3]0 deposited on ITO electrodes did not show catalytic activity for water oxidn. under similar conditions. Such a difference in the reactivity among [1]0, [2]0 and [3]0 is ascribed to the shift of the resonance equil. between [RuII2(OH)2(sq)2(btpyan)]0 and [RuIII2(OH)2(cat)2(btpyan)]0.
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(e) Tanaka, K.; Isobe, H.; Yamanaka, S.; Yamaguchi, K. Similarities of Artificial Photosystems by Ruthenium Oxo Complexes and Native Water Splitting Systems. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15600– 15605, DOI: 10.1073/pnas.1120705109
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60e
Similarities of artificial photosystems by ruthenium oxo complexes and native water splitting systems
Tanaka, Koji; Isobe, Hiroshi; Yamanaka, Shusuke; Yamaguchi, Kizashi
Proceedings of the National Academy of Sciences of the United States of America (2012), 109 (39), 15600-15605, S15600/1-S15600/11CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
The nature of chem. bonds of ruthenium(Ru)-quinine(Q) complexes, mononuclear [Ru(trpy)(3,5-t-Bu2Q)(OH2)](ClO4)2 (trpy = 2,2':6',2''-terpyridine, 3,5-di-tert-butyl-1,2-benzoquinone) (1), and binuclear [Ru2(bt pyan)(3,6-di-Bu2Q)2(OH2)]2+ (btpyan = 1,8- bis(2,2':6',2''-terpyrid-4'-yl)anthracene, 3,6-t-Bu2Q = 3,6-di-tert-butyl-1,2-benzoquinone) (2), has been investigated by broken-symmetry (BS) hybrid d. functional (DFT) methods. BS DFT computations for the Ru complexes have elucidated that the closed-shell structure (2b) Ru(II)-Q complex is less stable than the open-shell structure (2bb) consisting of Ru(III) and semiquinone (SQ) radical fragments. These computations have also elucidated eight different electronic and spin structures of tetraradical intermediates that may be generated in the course of water splitting reaction. The Heisenberg spin Hamiltonian model for these species has been derived to elucidate six different effective exchange interactions (J) for four spin systems. Six J values have been detd. using total energies of the eight (or seven) BS solns. for different spin configurations. The natural orbital analyses of these BS DFT solns. have also been performed in order to obtain natural orbitals and their occupation nos., which are useful for the lucid understanding of the nature of chem. bonds of the Ru complexes. Implications of the computational results are discussed in relation to the proposed reaction mechanisms of water splitting reaction in artificial photosynthesis systems and the similarity between artificial and native water splitting systems.
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(f) Kikuchi, T.; Tanaka, K. Mechanistic Approaches to Molecular Catalysts for Water Oxidation. Eur. J. Inorg. Chem. 2014, 2014, 607– 618, DOI: 10.1002/ejic.201300716
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60f
Mechanistic Approaches to Molecular Catalysts for Water Oxidation
Kikuchi, Takashi; Tanaka, Koji
European Journal of Inorganic Chemistry (2014), 2014 (4), 607-618CODEN: EJICFO; ISSN:1434-1948. (Wiley-VCH Verlag GmbH & Co. KGaA)
A review. Water oxidn., in which two water mols. undergo the coupled loss of four electrons and four protons to form an O-O bond, is one of the most appealing target reactions for mol. catalysts in view of hydrogen prodn. by electrolytic or photolytic water splitting to cope with urgent energy and environmental problems. Inspired by a natural oxygen-evolving multinuclear manganese cluster, a no. of water oxidn. catalysts based on multinuclear transition-metal complexes have been developed over the last three decades. In recent years, in parallel with the discovery of mononuclear oxygen-evolving complexes, both exptl. and theor. studies have yielded important insight into O-O bond-formation pathways in these water-oxidizing complexes. In this microreview, we will present an updated view of selected current literature focusing on the working mechanism of ruthenium-based water oxidn. catalysts and on the development of rationally designed ruthenium complexes that activate water at mild potentials.
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Wada, T.; Tsuge, K.; Tanaka, K. Oxidation of Hydrocarbons by Mono- and Dinuclear Ruthenium Quinone Complexes via Hydrogen Atom Abstraction. Chem. Lett. 2000, 29, 910, DOI: 10.1246/cl.2000.910
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62
Shimoyama, Y.; Ishizuka, T.; Kotani, H.; Shiota, Y.; Yoshizawa, K.; Mieda, K.; Ogura, T.; Okajima, T.; Nozawa, S.; Kojima, T. A Ruthenium(III)-Oxyl Complex Bearing Strong Radical Character. Angew. Chem., Int. Ed. 2016, 55, 14041– 14045, DOI: 10.1002/anie.201607861
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62
A Ruthenium(III)-Oxyl Complex Bearing Strong Radical Character
Shimoyama, Yoshihiro; Ishizuka, Tomoya; Kotani, Hiroaki; Shiota, Yoshihito; Yoshizawa, Kazunari; Mieda, Kaoru; Ogura, Takashi; Okajima, Toshihiro; Nozawa, Shunsuke; Kojima, Takahiko
Angewandte Chemie, International Edition (2016), 55 (45), 14041-14045CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
Proton-coupled electron-transfer oxidn. of a RuII-OH2 complex, having an N-heterocyclic carbene ligand, gives a RuIII-O. species, which has an electronically equiv. structure of the RuIV=O species, in an acidic aq. soln. The RuIII-O. complex was characterized by spectroscopic methods and DFT calcns. The oxidn. state of the Ru center was shown to be close to +3; the Ru-O bond showed a lower-energy Raman scattering at 732 cm-1 and the Ru-O bond length was estd. to be 1.77(1) Å. The RuIII-O. complex exhibits high reactivity in substrate oxidn. under catalytic conditions; particularly, benzaldehyde and the derivs. are oxidized to the corresponding benzoic acid through C-H abstraction from the formyl group by the RuIII-O. complex bearing a strong radical character as the active species.
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Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. An Overview of N-Heterocyclic Carbenes. Nature 2014, 510, 485– 496, DOI: 10.1038/nature13384
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63
An overview of N-heterocyclic carbenes
Hopkinson, Matthew N.; Richter, Christian; Schedler, Michael; Glorius, Frank
Nature (London, United Kingdom) (2014), 510 (7506), 485-496CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
A review. The successful isolation and characterization of an N-heterocyclic carbene in 1991 opened up a new class of org. compds. for investigation. From these beginnings as academic curiosities, N-heterocyclic carbenes today rank among the most powerful tools in org. chem., with numerous applications in com. important processes. Here we provide a concise overview of N-heterocyclic carbenes in modern chem., summarizing their general properties and uses and highlighting how these features are being exploited in a selection of pioneering recent studies.
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(a) Hirai, Y.; Kojima, T.; Mizutani, Y.; Shiota, Y.; Yoshizawa, K.; Fukuzumi, S. Ruthenium-Catalyzed Selective and Efficient Oxygenation of Hydrocarbons with Water an an Oxygen Source. Angew. Chem., Int. Ed. 2008, 47, 5772– 5776, DOI: 10.1002/anie.200801170
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64a
Ruthenium-catalyzed selective and efficient oxygenation of hydrocarbons with water as an oxygen source
Hirai, Yuichirou; Kojima, Takahiko; Mizutani, Yasuhisa; Shiota, Yoshihito; Yoshizawa, Kazunari; Fukuzumi, Shunichi
Angewandte Chemie, International Edition (2008), 47 (31), 5772-5776CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
Water is not only the solvent but also the sole oxygen source in the smooth and efficient oxidn. of org. compds. catalyzed by a RuII-pyridylamine-aqua complex with CeIV as the oxidant. An intermediate-spin RuIV-oxo complex is formed as the reactive species. This catalytic system is durable and able to gain high turnover nos. for various substrates.
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(b) Gerli, A.; Reedijk, J.; Lakin, M. T.; Spek, A. L. Redox Properties and Electrocatalytic Activity of the Oxo/Aqua System [Ru(terpy)(bpz)(O)]²⁺/[Ru(terpy)(bpz)(H₂O)]²⁺. X-ray Crystal Structure of [Ru(terpy)(bpz)Cl]PF₆·MeCN (terpy = 2,2′,2″-Terpyridine; bpz = 2,2′-Bipyradine). Inorg. Chem. 1995, 34, 1836– 1843, DOI: 10.1021/ic00111a035
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64b
Redox Properties and Electrocatalytic Activity of the Oxo/Aqua System [Ru(terpy)(bpz)(O)]2+/[Ru(terpy)(bpz)(H2O)]2+. X-ray Crystal Structure of [Ru(terpy)(bpz)Cl]PF6·MeCN (terpy = 2,2',2''-Terpyridine; bpz = 2,2'-Bipyrazine)
Gerli, Alessandra; Reedijk, Jan; Lakin, Miles T.; Spek, Anthony L.
Inorganic Chemistry (1995), 34 (7), 1836-43CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
[Ru(terpy)(bpz)X]n+, where terpy = 2,2',2''-terpyridine, bpz = 2,2'-bipyrazine, and X = Cl- (1), H2O (2), were prepd. and characterized by UV-visible and 1H NMR spectroscopies, and for the chloride deriv. also by x-ray diffraction. [Ru(terpy)(bpz)Cl]PF6·MeCN crystallizes in the triclinic space group P‾1 with the following crystallog. parameters: a 8.9173(4), b 12.6018(8), c 13.1743(8) Å, α 70.392(5), β 81.005(4), γ 76.954(5)°, Z = 2, R1 = 0.024 [for 5531 reflections Fo > 4σ(Fo)], and ωR2 = 0.061 for 6187 unique reflections. The redox properties of 2 were studied by electrochem. techniques over the pH range 0-12 in water. Only one reversible voltammetric wave (E1/2 = +0.66 V vs. SCE at pH = 7) is obsd. for 2 in the pH range 0-11, which was assigned to the Ru(II)/Ru(IV) couple. The two-electron nature of the redox process was confirmed by a spectrophotometric titrn. of 2 with Ce(IV). The 2nd-order rate const., kcat, for the oxidn. of benzyl alc. by the electrogenerated [Ru(IV)(terpy)(bpz)(O)]2+ was evaluated by cyclic voltammetry. At pH = 11 in phosphate buffer, kcat is 23.0(7) M-1 s-1. An electrocatalytic rate const., kcat = 36.1(15) M-1 s-1, was measured in 0.1M NaOH for the oxidn. of benzyl alc. by a related compd., [Ru(IV)(terpy)(bpy)(O)]2+, where bpy = 2,2'-bipyridine.
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Seok, W. K.; Meyer, T. J. Mechanism of Oxidation of Benzaldehyde by Polypyridyl Oxo Complexes of Ru(IV). Inorg. Chem. 2005, 44, 3931– 3941, DOI: 10.1021/ic040119z
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65
Mechanism of Oxidation of Benzaldehyde by Polypyridyl Oxo Complexes of Ru(IV)
Seok, Won K.; Meyer, Thomas J.
Inorganic Chemistry (2005), 44 (11), 3931-3941CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
The oxidn. of benzaldehyde and several of its derivs. to their carboxylic acids by cis-[RuIV(bpy)2(py)(O)]2+ (RuIV:O2+; bpy is 2,2'-bipyridine, py is pyridine), cis-[RuIII(bpy)2(py)(OH)]2+ (RuIII-OH2+), and [RuIV(tpy)(bpy)(O)]2+ (tpy is 2,2':6',2''-terpyridine) in acetonitrile and water has been investigated using a variety of techniques. Several lines of evidence support a one-electron hydrogen-atom transfer (HAT) mechanism for the redox step in the oxidn. of benzaldehyde. They include (i) moderate kC-H/kC-D kinetic isotope effects of 8.1 ± 0.3 in CH3CN, 9.4 ± 0.4 in H2O, and 7.2 ± 0.8 in D2O; (ii) a low kH2O/D2O kinetic isotope effect of 1.2 ± 0.1; (iii) a decrease in rate const. by a factor of only ∼5 in CH3CN and ∼8 in H2O for the oxidn. of benzaldehyde by cis-[RuIII(bpy)2(py)(OH)]2+ compared to cis-[RuIV(bpy)2(py)(O)]2+; (iv) the appearance of cis-[RuIII(bpy)2(py)(OH)]2+ rather than cis-[RuII(bpy)2(py)(OH2)]2+ as the initial product; and (v) the small ρ value of -0.65 ± 0.03 in a Hammett plot of log k vs σ in the oxidn. of a series of aldehydes. A mechanism is proposed for the process occurring in the absence of O2 involving (i) preassocn. of the reactants, (ii) H-atom transfer to RuIV:O2+ to give RuIII-OH2+ and PḣCO, (iii) capture of PḣCO by RuIII-OH2+ to give RuII-OC(O)Ph+ and H+, and (iv) solvolysis to give cis-[RuII(bpy)2(py)(NCCH3)]2+ or the aqua complex and the carboxylic acid as products.
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Shimoyama, Y.; Ishizuka, T.; Kotani, H.; Kojima, T. Catalytic Oxidative Cracking of Benzene Rings in Water. ACS Catal. 2019, 9, 671– 678, DOI: 10.1021/acscatal.8b04004
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66
Catalytic Oxidative Cracking of Benzene Rings in Water
Shimoyama, Yoshihiro; Ishizuka, Tomoya; Kotani, Hiroaki; Kojima, Takahiko
ACS Catalysis (2019), 9 (1), 671-678CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
Efficient degrdn. of harmful benzene rings in water is indispensable for achieving a clean water environment. We report herein unprecedented catalytic oxidative benzene cracking (OBC) in water using a ruthenium(II)-aqua complex having an N-heterocyclic carbene ligand as a catalyst and a cerium(IV) salt as a sacrificial oxidant under mild conditions. The OBC reactions produced carboxylic acids such as formic acid, which can be converted to dihydrogen directly from the OBC soln. using a rhodium(III) catalyst with adjustment of the soln. pH to 3.3. The OBC reactions can be applied to monosubstituted benzene derivs. such as ethylbenzene, chlorobenzene, and benzoic acid. Initial rates of the OBC reactions showed a linear relationship in the Hammett plot with a neg. slope, indicating the electrophilicity of a Ru(III)-oxyl complex as the reactive species in the catalytic OBC reaction. Also, we discuss a plausible mechanism of the catalytic OBC reactions based on the kinetic anal. and the product stoichiometry for the OBC reaction of nonvolatile sodium m-xylene sulfonate. The addn. of an electrophilic radical to the arom. ring to form arene oxide/oxepin is proposed as the initial step of the OBC reaction.
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Fukuzumi, S.; Kobayashi, T.; Suenobu, T. Efficient Catalytic Decomposition of Formic Acid for the Selective Generation of H₂ and H/D Exchange with a Water-Soluble Rhodium Complex in Aqueous Solution. ChemSusChem 2008, 1, 827– 834, DOI: 10.1002/cssc.200800147
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67
Efficient catalytic decomposition of formic acid for the selective generation of H2 and H/D exchange with a water-soluble rhodium complex in aqueous solution
Fukuzumi, Shunichi; Kobayashi, Takeshi; Suenobu, Tomoyoshi
ChemSusChem (2008), 1 (10), 827-834CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
Formic acid (HCOOH) decomps. efficiently to afford H2 and CO2 selectively in the presence of a catalytic amt. of a water-sol. rhodium aqua complex, [RhIII(Cp*)(bpy)(H2O)]2+ (Cp* = pentamethylcyclopentadienyl, bpy = 2,2'-bipyridine) in aq. soln. at 298 K. No CO was produced in this catalytic decompn. of HCOOH. The decompn. rate reached a max. value at pH 3.8. No deterioration of the catalyst was obsd. during the catalytic decompn. of HCOOH, and the catalytic activity remained the same for the repeated addn. of HCOOH. The rhodium-hydride complex was detected as the catalytic active species that undergoes efficient H/D exchange with water. When the catalytic decompn. of HCOOH was performed in D2O, D2 was produced selectively. Such an efficient H/D exchange and the observation of a deuterium kinetic isotope effect in the catalytic decompn. of DCOOH in H2O provide valuable mechanistic insight into this efficient and selective decompn. process.
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(a) Chen, X.; Choing, S. N.; Aschaffenburg, D. J.; Pemmaraju, C. D.; Prendergast, D.; Cuk, T. J. Am. Chem. Soc. 2017, 139, 1830– 1841, DOI: 10.1021/jacs.6b09550
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68a
The Formation Time of Ti-O• and Ti-O•-Ti Radicals at the n-SrTiO3/Aqueous Interface during Photocatalytic Water Oxidation
Chen, Xihan; Choing, Stephanie N.; Aschaffenburg, Daniel J.; Pemmaraju, C. D.; Prendergast, David; Cuk, Tanja
Journal of the American Chemical Society (2017), 139 (5), 1830-1841CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The initial step of photocatalytic water oxidn. reaction at the metal oxide/aq. interface involves intermediates formed by trapping photogenerated, valence band holes on different reactive sites of the oxide surface. In SrTiO3, these one-electron intermediates are radicals located in Ti-O• (oxyl) and Ti-O•-Ti (bridge) groups arranged perpendicular and parallel to the surface resp., and form electronic states in the band gap of SrTiO3. Using an ultrafast sub band gap probe of 400 nm and white light, we excited transitions between these radical states and the conduction band. By measuring the time evolution of surface reflectivity following the pump pulse of 266 nm light, we detd. an initial radical formation time of 1.3 ± 0.2 ps, which is identical to the time to populate the surface with titanium oxyl (Ti-O•) radicals. The oxyl was sep. obsd. by a subsurface vibration near 800 cm-1 from Ti-O located in the plane right below Ti-O•. Second, a polarized transition optical dipole allows us to assign the 1.3 ps time const. to the prodn. of both O-site radicals. After a 4.5 ps delay, another distinct surface species forms with a time const. of 36 ± 10 ps with a yet undetd. structure. As would be expected, the radicals' decay, specifically probed by the oxyl's subsurface vibration, parallels that of the photocurrent. These results led us to propose a nonadiabatic kinetic mechanism for generating radicals of the type Ti-O• and Ti-O•-Ti from valence band holes based on their solvation at aq. interfaces.
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(b) Herlihy, D. M.; Waegele, M. M.; Chen, X.; Pemmaraju, C. D.; Prendergast, D.; Cuk, T. Detecting the Oxyl Radical of Photocatalytic Water Oxidation at an n-SrTiO₃/Aqueous Interface through Its Subsurface Vibration. Nat. Chem. 2016, 8, 549– 555, DOI: 10.1038/nchem.2497
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68b
Detecting the oxyl radical of photocatalytic water oxidation at an n-SrTiO3/aqueous interface through its subsurface vibration
Herlihy, David M.; Waegele, Matthias M.; Chen, Xihan; Pemmaraju, C. D.; Prendergast, David; Cuk, Tanja
Nature Chemistry (2016), 8 (6), 549-555CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
A subsurface vibration of the oxygen directly below, and uniquely generated by, the oxyl radical (Ti-O•), has been detected using theor. calcns. and ultrafast in situ IR spectra of photocatalysis at an n-SrTiO3/aq. interface. This interfacial Ti-O stretch vibration, once decoupled from the lattice, couples to reactant dynamics (water librations). These expts. demonstrate subsurface vibrations and their coupling to solvent and electron dynamics to detect nascent catalytic intermediates at the solid-liq. interface at the mol. level. One can envision using the subsurface vibrations and their coupling across the interface to track and control catalysis dynamically.
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Corona, T.; Pfaff, F. F.; Acuña-Parés, F.; Draksharapu, A.; Whiteoak, C. J.; Martin-Diaconescu, V.; Lloret-Fillol, J.; Browne, W. R.; Ray, K.; Company, A. Reactivity of a Nickel(II) Bis(amidate) Complex with meta-Chloroperbenzoic Acid: Formation of a Potent Oxidizing Species. Chem. - Eur. J. 2015, 21, 15029– 15038, DOI: 10.1002/chem.201501841
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69
Reactivity of a Nickel(II) Bis(amidate) Complex with meta-Chloroperbenzoic Acid: Formation of a Potent Oxidizing Species
Corona, Teresa; Pfaff, Florian F.; Acuna-Pares, Ferran; Draksharapu, Apparao; Whiteoak, Christopher J.; Martin-Diaconescu, Vlad; Lloret-Fillol, Julio; Browne, Wesley R.; Ray, Kallol; Company, Anna
Chemistry - A European Journal (2015), 21 (42), 15029-15038CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
Herein, we report the formation of a highly reactive nickel-oxygen species that has been trapped following reaction of a NiII precursor bearing a macrocyclic bis(amidate) ligand with meta-chloroperbenzoic acid (HmCPBA). This compd. is only detectable at temps. below 250 K and is much more reactive toward org. substrates (i.e., C-H bonds, C-C bonds, and sulfides) than previously reported well-defined nickel-oxygen species. Remarkably, this species is formed by heterolytic O-O bond cleavage of a Ni-HmCPBA precursor, which is concluded from exptl. and computational data. On the basis of spectroscopy and DFT calcns., this reactive species is proposed to be a NiIII-oxyl compd.
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Srnec, M.; Navrátil, R.; Andris, E.; Jašík, J.; Roithová, J. Experimentally Calibrated Analysis of the Electronic Structure of CuO⁺: Implications for Reactivity. Angew. Chem., Int. Ed. 2018, 57, 17053– 17057, DOI: 10.1002/anie.201811362
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70
Experimentally Calibrated Analysis of the Electronic Structure of CuO+: Implications for Reactivity
Srnec, Martin; Navratil, Rafael; Andris, Erik; Jasik, Juraj; Roithova, Jana
Angewandte Chemie, International Edition (2018), 57 (52), 17053-17057CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
The CuO+ core is a central motif of reactive intermediates in copper-catalyzed oxidns. occurring in nature. The high reactivity of CuO+ stems from a weak bonding between the atoms, which cannot be described by a simple classical model. To obtain the correct picture, we have investigated the acetonitrile-ligated CuO+ ion using neon-tagging photodissocn. spectroscopy at 5 K. The spectra feature complex vibronic absorption progressions in NIR and visible regions. Employing Franck-Condon analyses, we derived low-lying triplet potential energy surfaces that were further correlated with multireference calcns. This provided insight into the ground and low-lying excited electronic states of the CuO+ unit and elucidated how these states are perturbed by the change in ligation. Thus, we show that the bare CuO+ ion has prevailingly a copper(I)-biradical oxygen character. Increasing the no. of ligands coordinated to copper changes the CuO+ character towards the copper(II)-oxyl radical structure.
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Kojima, T.; Hayashi, K.; Iizuka, S.; Tani, F.; Naruta, Y.; Kawano, M.; Ohashi, Y.; Hirai, Y.; Ohkubo, K.; Matsuda, Y.; Fukuzumi, S. Synthesis and Characterization of Mononuclear Ruthenium(III) Pyridylamine Complexes and Mechanistic Insights into Their Catalytic Alkane Functionalization with m-Chloroperbenzoic Acid. Chem. - Eur. J. 2007, 13, 8212– 8222, DOI: 10.1002/chem.200700190
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71
Synthesis and characterization of mononuclear ruthenium(III) pyridylamine complexes and mechanistic insights into their catalytic alkane functionalization with m-chloroperbenzoic acid
Kojima, Takahiko; Hayashi, Ken-ichi; Iizuka, Shin-ya; Tani, Fumito; Naruta, Yoshinori; Kawano, Masaki; Ohashi, Yuji; Hirai, Yuichirou; Ohkubo, Kei; Matsuda, Yoshihisa; Fukuzumi, Shunichi
Chemistry - A European Journal (2007), 13 (29), 8212-8222CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
Mononuclear RuIII complexes [RuCl2(L)]+, where L is tris(2-pyridylmethyl)amine (TPA) or one of four TPA derivs. as tetradentate ligand, were prepd. and characterized by spectroscopic methods, x-ray crystallog., and electrochem. measurements. The geometry of a RuIII complex having a nonthree-fold-sym. TPA ligand bearing one dimethylnicotinamide moiety was detd. to show that the nicotine moiety resides trans to a pyridine group, but not to the chloride ligand. The substituents of the TPA ligands regulate the redox potential of the Ru center, as indicated by a linear Hammett plot in the range of 200 mV for RuIII/RuIV couples with a relatively large ρ value (+0.150). These complexes act as effective catalysts for alkane functionalization in MeCN with m-chloroperbenzoic acid (mCPBA) as terminal oxidant at room temp. They exhibited fairly good reactivity for oxidn. of cyclohexane (C-H bond energy 94 kcal mol-1), and the reactivity can be altered significantly by the electronic effects of substituents on TPA ligands in terms of initial rates and turnover nos. Catalytic oxygenation of cyclohexane by a RuIII complex with 16O-mCPBA in the presence of H218O gave 18O-labeled cyclohexanol with 100% inclusion of the 18O atom from the H2O mol. Resonance Raman spectra under catalytic conditions without the substrate indicate formation of a RuIV = O intermediate with lower bonding energy. Kinetic isotope effects (KIEs) in the oxidn. of cyclohexane suggest that H abstraction is the rate-detg. step and the KIE values depend on the substituents of the TPA ligands. Thus, the reaction mechanism of catalytic cyclohexane oxygenation depends on the electronic effects of the ligands.
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Kojima, T.; Matsuo, H.; Matsuda, Y. A Novel and Highly Effective Halogenation of Alkanes with Halides on Oxidation with m-Chloroperbenzoic Acid: Looks Old, but New Reaction. Chem. Lett. 1998, 27, 1085– 1086, DOI: 10.1246/cl.1998.1085
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73
Fokin, A. A.; Schreiner, P. R. Selective Alkane Transformations via Radicals and Radical Cations: Insights into the Activation Step from Experiment and Theory. Chem. Rev. 2002, 102, 1551– 1594, DOI: 10.1021/cr000453m
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73
Selective Alkane Transformations via Radicals and Radical Cations: Insights into the Activation Step from Experiment and Theory
Fokin, Andrey A.; Schreiner, Peter R.
Chemical Reviews (Washington, D. C.) (2002), 102 (5), 1551-1593CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review. The activation of alkanes, which are commonly known to be not very reactive, with radicals is reviewed. Radical as well as single-electron-transfer chem. are discussed because the authors feel that these are at different ends of the same mechanistic spectrum. The review covers first radical chem., moving from traditional reagents to electrophilic radical-like species. The structures of σ-radical cations generated from different sources follow next, including the reactions of SET-oxidizers of low electrophilicity.
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Moonshiram, D.; Alperovich, I.; Concepcion, J. J.; Meyer, T. J.; Pushkar, Y. Experimental Demonstration of Radicaloid Character in a Ru^V═O Intermediate in Catalytic Water Oxidation. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 3765– 3770, DOI: 10.1073/pnas.1222102110
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74
Experimental demonstration of radicaloid character in a RuV=O intermediate in catalytic water oxidation
Moonshiram, Dooshaye; Alperovich, Igor; Concepcion, Javier J.; Meyer, Thomas J.; Pushkar, Yulia
Proceedings of the National Academy of Sciences of the United States of America (2013), 110 (10), 3765-3770, S3765/1-S3765/13CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
Water oxidn. is the key half reaction in artificial photosynthesis. An absence of detailed mechanistic insight impedes design of new catalysts that are more reactive and more robust. A proposed paradigm leading to enhanced reactivity is the existence of oxyl radical intermediates capable of rapid water activation, but there is a dearth of exptl. validation. Here, we show the radicaloid nature of an intermediate reactive toward formation of the O-O bond by assessing the spin d. on the oxyl group by ESR. In the study, an 17O-labeled form of a highly oxidized, short-lived intermediate in the catalytic cycle of the water oxidn. catalyst cis,cis-[(2,2-bipyridine)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+ was investigated. It contains Ru centers in oxidn. states [4,5], has at least one RuV = O unit, and shows |Axx| = 60G 17O hyperfine splittings (hfs) consistent with the high spin d. of a radicaloid. Destabilization of pi-bonding in the d3 RuV = O fragment is responsible for the high spin d. on the oxygen and its high reactivity.
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Gersten, S. W.; Samuels, G. J.; Meyer, T. J. Catalytic Oxidation of Water by an Oxo-Bridged Ruthenium Dimer. J. Am. Chem. Soc. 1982, 104, 4029– 4030, DOI: 10.1021/ja00378a053
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75
Catalytic oxidation of water by an oxo-bridged ruthenium dimer
Gersten, Susan W.; Samuels, George J.; Meyer, Thomas J.
Journal of the American Chemical Society (1982), 104 (14), 4029-30CODEN: JACSAT; ISSN:0002-7863.
When oxidized by ≥4 equiv of Ce(IV), the oxo-bridged dimer, (bpy)2(H2O)RuORu(H2O)(bpy)24+ (bpy = 2,2'-bipyridine) catalytically oxidizes water to O.
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Yamaguchi, K.; Shoji, M.; Saito, T.; Isobe, H.; Nishihara, S.; Koizumi, K.; Yamada, S.; Kawakami, T.; Kitagawa, Y.; Yamanaka, S.; Okumura, M. Theory of Chemical Bonds in Metalloenzymes. XV. Local Singlet and Triplet Diradical Mechanisms for Radical Coupling Reactions in the Oxygen Evolution Complex. Int. J. Quantum Chem. 2010, 110, 3101– 3128, DOI: 10.1002/qua.22914
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76
Theory of chemical bonds in metalloenzymes. XV. Local singlet and triplet diradical mechanisms for radical coupling reactions in the oxygen evolution complex
Yamaguchi, Kizashi; Shoji, Mitsuo; Saito, Toru; Isobe, Hiroshi; Nishihara, Satomichi; Koizumi, Kenichi; Yamada, Satoru; Kawakami, Takashi; Kitagawa, Yasutaka; Yamanaka, Shusuke; Okumura, Mitsutaka
International Journal of Quantum Chemistry (2010), 110 (15), 3101-3128CODEN: IJQCB2; ISSN:0020-7608. (John Wiley & Sons, Inc.)
Reaction mechanisms of oxygen evolution in native and artificial photosynthesis II (PSII) systems have been investigated on the theor. grounds, together with exptl. results. First of all, our previous broken-symmetry (BS) MOs (MO) calcns. are reviewed to elucidate the instability of the dπ-pπ bond in high-valent (HV) Mn(X)=O systems and the dπ-pπ-dπ bond in HV Mn=O=Mn systems. The triplet instability of these bonds entails strong or intermediate diradical characters: ·Mn(IV)=O·and ·n-O-Mn· the BS MO resulted from strong electron correlation, leading to the concept of electron localizations and local spins. The BS computations have furthermore revealed guiding principles for derivation of selection rules for radical reactions of local spins. As a continuation of these theor. results, the BS MO interaction diagrams for oxygen-radical coupling reactions in the oxygen evolution complex (OEC) in the PSII have been depicted to reveal scope and applicability of local singlet diradical (LSD) and local triplet diradical (LTD) mechanisms that have been successfully utilized for theor. understanding of oxygenation reactions mechanisms by P 450 and methane monooxygenase (MMO). The manganese-oxide cluster models examd. are London, Berlin, and Berkeley models of CaMn4O4 and related clusters Mn4O4 and Mn3Ca. The BS MO interaction diagrams have revealed the LSD and/or LTD mechanisms for generation of mol. oxygen in the total low-, intermediate and high-spin states of these clusters. The spin alignments are found directly corresponding to the spin-coupling mechanisms of oxygen-radical sites in these clusters. The BS UB3LYP calcns. of the clusters have been performed to confirm the comprehensive guiding principles for oxygen evolution; charge and spin densities by BS UB3LYP are utilized for elucidation and confirmation of the LSD and LTD mechanisms. Applicability of the proposed selection rules are examd. in comparison with a lot of accumulated exptl. and theor. results for oxygen evolution reactions in native and artificial PSII systems.
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(a) Li, X.; Siegbahn, P. E. M. Alternative Mechanisms for O₂ Release and O–O Bond Formation in the Oxygen Evolving Complex of Photosystem II. Phys. Chem. Chem. Phys. 2015, 17, 12168– 12174, DOI: 10.1039/C5CP00138B
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77a
Alternative mechanisms for O2 release and O-O bond formation in the oxygen evolving complex of photosystem II
Li, Xichen; Siegbahn, Per E. M.
Physical Chemistry Chemical Physics (2015), 17 (18), 12168-12174CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
In a previous detailed study of all the steps of water oxidn. in photosystem II, it was surprisingly found that O2 release is as crit. for the rate as O-O bond formation. A new mechanism for O2 release has now been found, which can be described as an opening followed by a closing of the interior of the oxygen evolving complex. A transition state for peroxide rotation forming a superoxide radical, missed in the previous study, and a structural change around the outside manganese are two key steps in the new mechanism. However, O2 release may still remain rate-limiting. Addnl., for the step forming the O-O bond, an alternative, exptl. suggested, mechanism was investigated. The new model calcns. can rule out the precise use of that mechanism. However, a variant with a rotation of the ligands around the outer manganese by about 30° will give a low barrier, competitive with the old DFT mechanism. Both these mechanisms use an oxyl-oxo mechanism for O-O bond formation involving the same two manganese atoms and the central oxo group (O5).
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(b) Siegbahn, P. E. M. Nucleophilic Water Attack is Not a Possible Mechanism for O-O Bond Formation in Photosystem II. Proc. Natl. Acad. Sci. U. S. A. 2017, 114, 4966– 4968, DOI: 10.1073/pnas.1617843114
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77b
Nucleophilic water attack is not a possible mechanism for O-O bond formation in photosystem II
Siegbahn, Per E. M.
Proceedings of the National Academy of Sciences of the United States of America (2017), 114 (19), 4966-4968CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
Two different types of mechanisms are at present suggested for the O-O bond-formation step in photosystem II. The first one is a coupling between an oxyl radical and a bridging oxo. The second one is a nucleophilic water attack on a terminal oxo (or oxyl) group. In the present short paper, the six most reasonable versions of the latter mechanism have been studied and compared with the oxo-oxyl mechanism. The barriers are found to be much too high for the water attack, and that mechanism can therefore safely be ruled out. The reason is that the protonated peroxide product is always very high in energy.
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Sproviero, E. M.; Gascón, J. A.; McEvoy, J. P.; Brudvig, G. W.; Batista, V. S. Quantum Mechanics/Molecular Mechanics Study of the Catalytic Cycle of Water Splitting in Photosystem II. J. Am. Chem. Soc. 2008, 130, 3428– 3442, DOI: 10.1021/ja076130q
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78
Quantum Mechanics/Molecular Mechanics Study of the Catalytic Cycle of Water Splitting in Photosystem II
Sproviero, Eduardo M.; Gascon, Jose A.; McEvoy, James P.; Brudvig, Gary W.; Batista, Victor S.
Journal of the American Chemical Society (2008), 130 (11), 3428-3442CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
This paper investigates the mechanism of water splitting in photosystem II (PSII) as described by chem. sensible models of the oxygen-evolving complex (OEC) in the S0-S4 states. The reaction is the paradigm for engineering direct solar fuel prodn. systems since it is driven by solar light and the catalyst involves inexpensive and abundant metals (calcium and manganese). Mol. models of the OEC Mn3CaO4Mn catalytic cluster are constructed by explicitly considering the perturbational influence of the surrounding protein environment according to state-of-the-art quantum mechanics/mol. mechanics (QM/MM) hybrid methods, in conjunction with the X-ray diffraction (XRD) structure of PSII from the cyanobacterium Thermosynechococcus elongatus. The resulting models are validated through direct comparisons with high-resoln. extended X-ray absorption fine structure spectroscopic data. Structures of the S3, S4, and S0 states include an addnl. μ-oxo bridge between Mn(3) and Mn(4), not present in XRD structures, found to be essential for the deprotonation of substrate water mols. The structures of reaction intermediates suggest a detailed mechanism of dioxygen evolution based on changes in oxidization and protonation states and structural rearrangements of the oxomanganese cluster and surrounding water mols. The catalytic reaction is consistent with substrate water mols. coordinated as terminal ligands to Mn(4) and calcium and requires the formation of an oxyl radical by deprotonation of the substrate water mol. ligated to Mn(4) and the accumulation of four oxidizing equiv. The oxyl radical is susceptible to nucleophilic attack by a substrate water mol. initially coordinated to calcium and activated by two basic species, including CP43-R357 and the μ-oxo bridge between Mn(3) and Mn(4). The reaction is concerted with water ligand exchange, swapping the activated water by a water mol. in the second coordination shell of calcium.
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Crandell, D. W.; Xu, S.; Smith, J. M.; Baik, M.-H. Intramolecular Oxyl Radical Coupling Promotes O-O Bond Formation in a Homogeneous Mononuclear Mn-based Water Oxidation Catalyst: A Computational Mechanistic Investigation. Inorg. Chem. 2017, 56, 4435– 4445, DOI: 10.1021/acs.inorgchem.6b03144
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79
Intramolecular Oxyl Radical Coupling Promotes O-O Bond Formation in a Homogeneous Mononuclear Mn-based Water Oxidation Catalyst: A Computational Mechanistic Investigation
Crandell, Douglas W.; Xu, Song; Smith, Jeremy M.; Baik, Mu-Hyun
Inorganic Chemistry (2017), 56 (8), 4435-4445CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
The mechanism of water oxidn. performed by a recently discovered manganese pyridinophane catalyst [Mn(Py2NtBu2)(H2O)2]2+ is studied using d. functional theory methods. A complete catalytic cycle is constructed and the catalytically active species is identified to consist of a MnV-bis(oxo) moiety that is generated from the resting state by a series of proton-coupled electron transfer reactions. Whereas the electronic ground state of this key intermediate is found to be a triplet, the most favorable pathway for O-O bond formation is found on the quintet potential energy surface and involves an intramol. coupling of two oxyl radicals with opposite spins bound to the Mn-center that adopts an electronic structure most consistent formally with a high-spin MnIII ion. Therefore, the thermally accessible high-spin quintet state that constitutes a typical and innate property of a first-row transition metal center plays a crit. role for catalysis. It enables facile electron transfer between the oxo moieties and the Mn-center and promotes O-O bond formation via a radical coupling reaction with a calcd. reaction barrier of only 14.7 kcal mol-1. This mechanism of O-O coupling is unprecedented and provides a novel possible pathway to coupling two oxygen atoms bound to a single metal site.
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Ashley, D. C.; Baik, M.-H. The Electronic Structure of [Mn(V)═O]: What is the Connection Between Oxyl Radical Character, Physical Oxidation State, and Reactivity?. ACS Catal. 2016, 6, 7202– 7216, DOI: 10.1021/acscatal.6b01793
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80
The Electronic Structure of [Mn(V)=O]: What is the Connection between Oxyl Radical Character, Physical Oxidation State, and Reactivity?
Ashley, Daniel Charles; Baik, Mu-Hyun
ACS Catalysis (2016), 6 (10), 7202-7216CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
MnV=O functionalities are important in synthetic and bioinorg. chem., being relevant to both C-H activation and the O-O bond formation steps in enzymic water oxidn., for example. The triplet and quintet spin states are believed to be active in these reactions, but they have only been sparingly characterized exptl. D. functional theory (DFT) gives varying results, depending on the exchange-correlation functional employed, leading to ambiguity about whether the triplet MnV=O is better represented as MnIV-O•. While recent CASPT2 studies confirmed that the MnIV-O• character is exaggerated by hybrid functionals, questions still remain about the nature of this bonding. Using high-level wave function methods, the authors studied the fundamental relation between the spin polarization, diradical character, and the phys. oxidn. state assignments. In terms of formal oxidn. assignment, these species are best described as being between the MnV=O and MnIV-O• extremes. While the extent of the oxyl radical character is exaggerated in B3LYP, it is significantly underestimated by local functionals. The authors also exploited the DFT-functional dependence of the oxyl radical character to examine its effect on O-O bond formation barrier heights and concluded that, although, for radical combination reactions, the oxyl character is a significant effect, for nucleophilic water attack reactions, the effect is much smaller and is likely not a requisite feature.
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81
Leto, D. F.; Massie, A. A.; Rice, D. B.; Jackson, T. A. Spectroscopic and Computational Investigations of a Mononuclear Manganese(IV)-Oxo Complex Reveal Electronic Structure Contributions to Reactivity. J. Am. Chem. Soc. 2016, 138, 15413– 15424, DOI: 10.1021/jacs.6b08661
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81
Spectroscopic and Computational Investigations of a Mononuclear Manganese(IV)-Oxo Complex Reveal Electronic Structure Contributions to Reactivity
Leto, Domenick F.; Massie, Allyssa A.; Rice, Derek B.; Jackson, Timothy A.
Journal of the American Chemical Society (2016), 138 (47), 15413-15424CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The mononuclear Mn(IV)-oxo complex [MnIV(O)(N4py)]2+, where N4py is the pentadentate ligand N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine, was proposed to attack C-H bonds by an excited-state reactivity pattern. In this model, a 4E excited state is used to provide a lower-energy barrier for hydrogen-atom transfer. This proposal is intriguing, as it offers both a rationale for the relatively high hydrogen-atom-transfer reactivity of [MnIV(O)(N4py)]2+ and a guideline for creating more reactive complexes through ligand modification. Here the authors employ a combination of electronic absorption and variable-temp. MCD spectroscopy to exptl. evaluate this excited-state reactivity model. Using these spectroscopic methods, in conjunction with time-dependent d. functional theory (TD-DFT) and complete-active space SCF calcns. (CASSCF), the authors define the ligand-field and charge-transfer excited states of [MnIV(O)(N4py)]2+. Through a graphical anal. of the signs of the exptl. C-term MCD signals, the authors unambiguously assign a low-energy MCD feature of [MnIV(O)(N4py)]2+ as the 4E excited state predicted to be involved in hydrogen-atom-transfer reactivity. The CASSCF calcns. predict enhanced MnIII-oxyl character on the excited-state 4E surface, consistent with previous DFT calcns. Potential-energy surfaces, developed using the CASSCF methods, are used to det. how the energies and wave functions of the ground and excited states evolved as a function of Mn=O distance. The unique insights into ground- and excited-state electronic structure offered by these spectroscopic and computational studies are harmonized with a thermodn. model of hydrogen-atom-transfer reactivity, which predicts a correlation between transition-state barriers and driving force.
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82
(a) Cook, S. A.; Borovik, A. S. Molecular Designs for Controlling the Local Environments around Metal Ions. Acc. Chem. Res. 2015, 48, 2407– 2414, DOI: 10.1021/acs.accounts.5b00212
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82a
Molecular Designs for Controlling the Local Environments around Metal Ions
Cook, Sarah A.; Borovik, A. S.
Accounts of Chemical Research (2015), 48 (8), 2407-2414CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
A review. The functions of metal complexes are directly linked to the local environment in which they are housed; modifications to the local environment (or secondary coordination sphere) are known to produce changes in key properties of the metal centers that can affect reactivity. Noncovalent interactions are the most common and influential forces that regulate the properties of secondary coordination spheres, which leads to complexities in structure that are often difficult to achieve in synthetic systems. Using key architectural features from the active sites of metalloproteins as inspiration, the authors have developed mol. systems that enforce intramol. H bonds (H-bonds) around a metal center via incorporation of H-bond donors and acceptors into rigid ligand scaffolds. The authors used these mol. species to probe mechanistic aspects of biol. dioxygen activation and H2O oxidn. This Account describes the stabilization and characterization of unusual M-oxo and heterobimetallic complexes. These types of species were implicated in a range of oxidative processes in biol. but are often difficult to study because of their inherent reactivity. The authors' H-bonding ligand systems allowed the authors to prep. an FeIII-oxo species directly from the activation of O2 that was subsequently oxidized to form a monomeric FeIV-oxo species with an S = 2 spin state, similar to those species proposed as key intermediates in nonheme monooxygenases. Also a single MnIII-oxo center that was prepd. from H2O could be converted to a high-spin MnV-oxo species via stepwise oxidn., a process that mimics the oxidative charging of the O-evolving complex (OEC) of photosystem II. Current mechanisms for photosynthetic O-O bond formation invoke a MnIV-oxyl species rather than the isoelectronic MnV-oxo system as the key oxidant based on computational studies. However, there is no exptl. information to support the existence of a Mn-oxyl radical. The authors therefore probed the amt. of spin d. on the oxido ligand of the authors' complexes using EPR spectroscopy in conjunction with O-17 labeling. The authors' findings showed that there is a significant amt. of spin on the oxido ligand, yet the M-oxo bonds are best described as highly covalent and there is no indication that an oxyl radical is formed. These results offer the intriguing possibility that high-spin M-oxo complexes are involved in O-O bond formation in biol. Ligand redesign to incorporate H-bond accepting units (sulfonamido groups) simultaneously provided a metal ion binding pocket, adjacent H-bond acceptors, and an auxiliary binding site for a 2nd metal ion. These properties allowed the authors to isolate heterobimetallic complexes of FeIII and MnIII in which a Group II metal ion were coordinated within the secondary coordination sphere. Examn. of the influence of the 2nd metal ion on the electron transfer properties of the primary metal center revealed unexpected similarities between CaII and SrII ions, a result with relevance to the OEC. The presence of a 2nd metal ion was found to prevent intramol. oxidn. of the ligand with an O atom transfer reagent.
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(b) Gupta, R.; Taguchi, T.; Lassalle-Kaiser, B.; Bominaar, E. L.; Yano, J.; Hendrich, M. P.; Borovik, A. S. High-Spin Mn-Oxo Complexes and their Relevance to the Oxygen-Evolving Complex within Photosystem II. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 5319– 5324, DOI: 10.1073/pnas.1422800112
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82b
High-spin Mn-oxo complexes and their relevance to the oxygen-evolving complex within photosystem II
Gupta, Rupal; Taguchi, Taketo; Lassalle-Kaiser, Benedikt; Bominaar, Emile L.; Yano, Junko; Hendrich, Michael P.; Borovik, A. S.
Proceedings of the National Academy of Sciences of the United States of America (2015), 112 (17), 5319-5324CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
The structural and electronic properties of a series of manganese complexes with terminal oxido ligands are described. The complexes span three different oxidn. states at the manganese center (III-V), have similar mol. structures, and contain intramol. hydrogen-bonding networks surrounding the Mn-oxo unit. Structural studies using X-ray absorption methods indicated that each complex is mononuclear and that oxidn. occurs at the manganese centers, which is also supported by ESR (EPR) studies. This gives a high-spin MnV-oxo complex and not a MnIV-oxy radical as the most oxidized species. In addn., the EPR findings demonstrated that the Fermi contact term could exptl. substantiate the oxidn. states at the manganese centers and the covalency in the metal-ligand bonding. Oxygen-17-labeled samples were used to det. spin d. within the Mn-oxo unit, with the greatest delocalization occurring within the MnV-oxo species (0.45 spins on the oxido ligand). The exptl. results coupled with d. functional theory studies show a large amt. of covalency within the Mn-oxo bonds. Finally, these results are examd. within the context of possible mechanisms assocd. with photosynthetic water oxidn.; specifically, the possible identity of the proposed high valent Mn-oxo species that is postulated to form during turnover is discussed.
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83
Srnec, M.; Wong, S. D.; Matthews, M. L.; Krebs, C.; Bollinger, J. M., Jr.; Solomon, E. I. Electronic Structure of the Ferryl Intermediate in the α-Ketoglutarate Dependent Non-Heme Iron Halogenatse SyrB2; Contributions to H Atom Abstraction Reactivity. J. Am. Chem. Soc. 2016, 138, 5110– 5122, DOI: 10.1021/jacs.6b01151
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83
Electronic Structure of the Ferryl Intermediate in the α-Ketoglutarate Dependent Non-Heme Iron Halogenase SyrB2: Contributions to H Atom Abstraction Reactivity
Srnec, Martin; Wong, Shaun D.; Matthews, Megan L.; Krebs, Carsten; Bollinger, J. Martin; Solomon, Edward I.
Journal of the American Chemical Society (2016), 138 (15), 5110-5122CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Low temp. magnetic CD (LT MCD) spectroscopy in combination with quantum-chem. calcns. are used to define the electronic structure assocd. with the geometric structure of the FeIV=O intermediate in SyrB2 that was previously detd. by nuclear resonance vibrational spectroscopy. These studies elucidate key frontier MOs (FMOs) and their contribution to H atom abstraction reactivity. The VT MCD spectra of the enzymic S = 2 FeIV=O intermediate with Br- ligation contain information-rich features that largely parallel the corresponding spectra of the S = 2 model complex (TMG3tren)FeIV=O. However, quant. differences are obsd. that correlate with π-anisotropy and oxo donor strength that perturb FMOs and affect reactivity. Due to π-anisotropy, the FeIV=O active site exhibits enhanced reactivity in the direction of the substrate cavity that proceeds through a π-channel that is controlled by perpendicular orientation of the substrate C-H bond relative to the halide-FeIV=O plane. Also, the increased intrinsic reactivity of the SyrB2 intermediate relative to the ferryl model complex is correlated to a higher oxyl character of the FeIV=O at the transition states resulting from the weaker ligand field of the halogenase.
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84
Bollinger, J. M., Jr.; Price, J. C.; Hoffart, L. M.; Barr, E. W.; Krebs, C. Mechanism of Taurine: α-Ketoglutarate Dioxygenase (TauD) from Escherichia coli. Eur. J. Inorg. Chem. 2005, 2005, 4245– 4254, DOI: 10.1002/ejic.200500476
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85
Ye, S.; Neese, F. Nonheme Oxo-Iron(IV) Intermediates Form an Oxyl Radical upon Approaching the C-H Bond Activation Transition State. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 1228– 1233, DOI: 10.1073/pnas.1008411108
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85
Nonheme oxo-iron(IV) intermediates form an oxyl radical upon approaching the C-H bond activation transition state
Ye, Shengfa; Neese, Frank
Proceedings of the National Academy of Sciences of the United States of America (2011), 108 (4), 1228-1233, S1228/1-S1228/2CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
Oxo-iron(IV) species are implicated as key intermediates in the catalytic cycles of heme and nonheme oxygen activating iron enzymes that selectively functionalize aliph. C-H bonds. Ferryl complexes can exist in either quintet or triplet ground states. D. functional theory calcns. predict that the quintet oxo-iron(IV) species is more reactive toward C-H bond activation than its corresponding triplet partner, however; the available exptl. data on model complexes suggests that both spin multiplicities display comparable reactivities. To clarify this ambiguity, a detailed electronic structure anal. of alkane hydroxylation by an oxo-iron(IV) species on different spin-state potential energy surfaces is performed. According to our results, the lengthening of the Fe-oxo bond in ferryl reactants, which is the part of the reaction coordinate for H-atom abstraction, leads to the formation of oxyl-iron(III) species that then perform actual C-H bond activation. The differential reactivity stems from the fact that the two spin states have different requirements for the optimal angle at which the substrate should approach the (FeO)2+ core because distinct electron acceptor orbitals are employed on the two surfaces. The H-atom abstraction on the quintet surface favors the "σ-pathway" that requires an essentially linear attack; by contrast a "π-channel" is operative on the triplet surface that leads to an ideal attack angle near 90°. However, the latter is not possible due to steric crowding; thus, the attenuated orbital interaction and the unavoidably increased Pauli repulsion result in the lower reactivity of the triplet oxo-iron(IV) complexes.
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86
Pardue, D. B.; Mei, J.; Cundari, T. R.; Gunnoe, T. B. Density Functional Theory Study of Oxygen-Atom Insertion into Metal-Methyl Bonds of Iron(II), Ruthenium(II), and Osmium(II) Complexes: Study of Metal-Mediated C-O Bond Formation. Inorg. Chem. 2014, 53, 2968– 2975, DOI: 10.1021/ic402759w
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86
Density Functional Theory Study of Oxygen-Atom Insertion into Metal-Methyl Bonds of Iron(II), Ruthenium(II), and Osmium(II) Complexes: Study of Metal-Mediated C-O Bond Formation
Pardue, Daniel B.; Mei, Jiajun; Cundari, Thomas R.; Gunnoe, T. Brent
Inorganic Chemistry (2014), 53 (6), 2968-2975CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
Metal-mediated C-O bond formation is a key step in hydrocarbon oxygenation catalytic cycles; however, few examples of this reaction have been reported for low-oxidn.-state complexes. Oxygen insertion into a metal-carbon bond of Cp*M(CO)(OPy)R (Cp* = η5-pentamethylcyclopentadienyl; R = Me, Ph; OPy = pyridine-N-oxide; M = Fe, Ru, Os) was analyzed via d. functional theory calcns. Oxygen-atom insertions through a concerted single-step organometallic Baeyer-Villiger pathway and a two-step pathway via a metal-oxo intermediate were studied; calcns. predict that the former pathway was lower in energy. The results indicated that functionalization of M-R to M-OR (R = Me, Ph) is plausible using iron(II) complexes. Starting from Cp*Fe(CO)(OPy)Ph, the intermediate Fe-oxo showed oxyl character and, thus, is best considered an FeIIIO•- complex. Oxidn. of the π-acid ancillary ligand CO was facile. Substitutions of CO with dimethylamide and NH3 were calcd. to lower the activation barrier by ∼1-2 kcal/mol for formation of the FeIIIO•- intermediate, whereas a chloride ligand raised the activation barrier to 26 kcal/mol from 22.9 kcal/mol.
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87
Gupta, R.; Lacy, D. C.; Bominaar, E. L.; Borovik, A. S.; Hendrich, M. P. Electron Paramagnetic Resonance and Mössbauer Spectroscopy and Density Functional Theory Analysis of a High-Spin Fe^IV-Oxo Complex. J. Am. Chem. Soc. 2012, 134, 9775– 9784, DOI: 10.1021/ja303224p
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87
Electron Paramagnetic Resonance and Moessbauer Spectroscopy and Density Functional Theory Analysis of a High-Spin FeIV-Oxo Complex
Gupta, Rupal; Lacy, David C.; Bominaar, Emile L.; Borovik, A. S.; Hendrich, Michael P.
Journal of the American Chemical Society (2012), 134 (23), 9775-9784CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
High-spin FeIV-oxo species are known to be kinetically competent oxidants in nonheme iron enzymes. The properties of these oxidants are not as well understood as the corresponding intermediate-spin oxidants of heme complexes. The present work gives a detailed characterization of the structurally similar complexes [FeIVH3buea(O)]-, [FeIIIH3buea(O)]2-, and [FeIIIH3buea(OH)]- (H3buea = tris[(N'-tert-butylureaylato)-N-ethylene]aminato) using Mossbauer and dual-frequency/dual-mode EPR spectroscopies. The [FeIVH3buea(O)]- complex has a high-spin (S = 2) configuration imposed by the C3-sym. ligand. The EPR spectra of the [FeIVH3buea(O)]- complex presented here represent the 1st documented examples of an EPR signal from an FeIV-oxo complex, demonstrating the ability to detect and quantify FeIV species with EPR spectroscopy. Quant. simulations allowed the detn. of the zero-field parameter, D = +4.7 cm-1, and the species concn. D. functional theory (DFT) calcns. of the zero-field parameter are in agreement with the exptl. value and indicated that the major contribution to the D value is from spin-orbit coupling of the ground state with an excited S = 1 electronic configuration at 1.2 eV 17O isotope enrichment expts. allowed the detn. of the hyperfine consts. 17OAz = 10 MHz for [FeIVH3buea(O)]- and 17OAy = 8 MHz, 17OAz = 12 MHz for [FeIIIH3buea(OH)]-. The isotropic hyperfine const. (17OAiso = -16.8 MHz) was derived from the exptl. value to allow a quant. detn. of the spin polarization (ρp = 0.56) of the oxo p orbitals of the Fe-oxo bond in [FeIVH3buea(O)]-. This is the 1st exptl. detn. for nonheme complexes and indicates significant covalency in the Fe-oxo bond. High-field Mossbauer spectroscopy gave an 57Fe Adip tensor of (+5.6, +5.3, -10.9) MHz and Aiso = -25.9 MHz for the [FeIVH3buea(O)]- complex, and the results of DFT calcns. were in agreement with the nuclear parameters of the complex.
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88
Sun, X.; Geng, C.; Huo, R.; Ryde, U.; Bu, Y.; Li, J. Large Equatorial Ligand Effects on C-H Bond Activation by Nonheme Iron(IV)-oxo Complexes. J. Phys. Chem. B 2014, 118, 1493– 1500, DOI: 10.1021/jp410727r
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88
Large Equatorial Ligand Effects on C-H Bond Activation by Nonheme Iron(IV)-oxo Complexes
Sun, Xiaoli; Geng, Caiyun; Huo, Ruiping; Ryde, Ulf; Bu, Yuxiang; Li, Jilai
Journal of Physical Chemistry B (2014), 118 (6), 1493-1500CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
In this article, we present d. functional theory (DFT) calcns. on the iron(IV)-oxo catalyzed methane C-H activation reactions for complexes in which the FeIVO core is surrounded by five neg. charged ligands. We found that it follows a hybrid pathway that mixes features of the classical σ- and π-pathways in quintet surfaces. These calcns. show that the Fe-O-H arrangement in this hybrid pathway is bent in sharp contrast to the collinear character as obsd. for the classical quintet σ-pathways before. The calcns. have also shown that it is the equatorial ligands that play key roles in tuning the reactivity of FeIVO complexes. The strong π-donating equatorial ligands employed in the current study cause a weak π(FeO) bond and thereby shift the electronic accepting orbitals (EAO) from the vertically oriented O pz orbital to the horizontally oriented O px. In addn., all the equatorial ligands are small in size and would therefore be expected have small steric effects upon substrate horizontal approaching. Therefore, for the small and strong π-donating equatorial ligands, the collinear Fe-O-H arrangement is not the best choice for the quintet reactivity. This study adds new element to iron(IV)-oxo catalyzed C-H bond activation reactions.
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Geng, C.; Ye, S.; Neese, F. Analysis of Reaction Channels for Alkane Hydroxylation by Nonheme Iron(IV)-Oxo Complexes. Angew. Chem., Int. Ed. 2010, 49, 5717– 5720, DOI: 10.1002/anie.201001850
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89
Analysis of Reaction Channels for Alkane Hydroxylation by Nonheme Iron(IV)-Oxo Complexes
Geng, Caiyun; Ye, Shengfa; Neese, Frank
Angewandte Chemie, International Edition (2010), 49 (33), 5717-5720, S5717/1-S5717/13CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
Geometric parameters of the transition states of the [FeIV(O)(NH3)5]2+ system calcd. at the B3LYP/TZVP level of theory. This is the first time that all viable pathways have been identified in the same system, which allows us to compare their relative reactivities.
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90
(a) Ishizuka, T.; Watanabe, A.; Kotani, H.; Hong, D.; Satonaka, K.; Wada, T.; Shiota, Y.; Yoshizawa, K.; Ohara, K.; Yamaguchi, S.; Kato, S.; Fukuzumi, S.; Kojima, T. Homogeneous Photocatalytic Water Oxidation with a Dinuclear Co^III-Pyridylmethyl Amine Complex. Inorg. Chem. 2016, 55, 1154– 1164, DOI: 10.1021/acs.inorgchem.5b02336
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90a
Homogeneous Photocatalytic Water Oxidation with a Dinuclear CoIII-Pyridylmethylamine Complex
Ishizuka, Tomoya; Watanabe, Atsuko; Kotani, Hiroaki; Hong, Dachao; Satonaka, Kenta; Wada, Tohru; Shiota, Yoshihito; Yoshizawa, Kazunari; Ohara, Kazuaki; Yamaguchi, Kentaro; Kato, Satoshi; Fukuzumi, Shunichi; Kojima, Takahiko
Inorganic Chemistry (2016), 55 (3), 1154-1164CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
A bis-hydroxo-bridged dinuclear CoIII-pyridylmethylamine complex (1) was synthesized and the crystal structure was detd. by X-ray crystallog. Complex 1 acts as a homogeneous catalyst for visible-light-driven water oxidn. by persulfate (S2O82-) as an oxidant with [RuII(bpy)3]2+ (bpy = 2,2'-bipyridine) as a photosensitizer affording a high quantum yield (44%) with a large turnover no. (TON = 742) for O2 formation without forming catalytically active Co-oxide (CoOx) nanoparticles. In the water-oxidn. process, complex 1 undergoes proton-coupled electron-transfer (PCET) oxidn. as a rate-detg. step to form a putative dinuclear bis-μ-oxyl CoIII complex (2), which has been suggested by DFT calcns. Catalytic water oxidn. by 1 using [RuIII(bpy)3]3+ as an oxidant in a H216O and H218O mixt. was examd. to reveal an intramol. O-O bond formation in the two-electron-oxidized bis-μ-oxyl intermediate, prior to the O2 evolution.
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(b) Kotani, H.; Hong, D.; Satonaka, K.; Ishizuka, T.; Kojima, T. Mechanistic Insight into Dioxygen Evolution from Diastereomeric μ-Peroxo Dinuclear Co(III) Complexes Based on Stoichiometric Electron-Transfer Oxidation. Inorg. Chem. 2019, 58, 3676– 3682, DOI: 10.1021/acs.inorgchem.8b03245
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90b
Mechanistic Insight into Dioxygen Evolution from Diastereomeric μ-Peroxo Dinuclear Co(III) Complexes Based on Stoichiometric Electron-Transfer Oxidation
Kotani, Hiroaki; Hong, Dachao; Satonaka, Kenta; Ishizuka, Tomoya; Kojima, Takahiko
Inorganic Chemistry (2019), 58 (6), 3676-3682CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
Stoichiometric electron-transfer (ET) oxidn. of two diastereomeric μ-peroxo-μ-hydroxo dinuclear Co(III) complexes with tris(2-pyridylmethyl)amine (TPA) was examd. to scrutinize the reaction mechanism of O2 evolution from the peroxo complexes, as seen in the final step in H2O oxidn. by a Co(III)-TPA complex. The two isomeric Co(III)-peroxo complexes were synthesized and selectively isolated by recrystn. under different conditions. Although cyclic voltammograms of the two isomers in aq. solns. showed one reversible wave at 1.1 V vs. normal H electrode at pH 2.0, two oxidn. waves were obsd. at 1.0 and 1.4 V at pH 7.0 in the aq. solns., the latter of which is responsible for the O2-releasing process. At pH 7, one diastereomer showed higher reactivity than the other in O2 evolution, indicating the importance of structures of the μ-peroxo complexes in the reaction. To clarify the O2-evolving mechanism, the authors performed EPR and resonance Raman (RR) measurements for characterizing 1-electron oxidized species: The obsd. EPR and RR signals supported the formation of μ-superoxo-μ-hydroxo dinuclear Co(III) complexes; however, no characteristic difference was obsd. between two isomers in the EPR parameters including g values and superhyperfine coupling consts. ET-oxidn. rate consts. of the isomers are much faster than the O2-evolving rate consts., indicating that the O2-releasing step is the rate-detg. step in the O2 evolution through the stoichiometric ET oxidn. of the dinuclear Co(III)-μ-peroxo complexes. Therefore, the difference of reactivity in the O2 evolution for the two isomers should be derived from the thermodn. stability of two-electron oxidized species of the dinuclear Co(III)-μ-peroxo complexes, μ-dioxygen-μ-hydroxo dinuclear Co(III) intermediates.
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91
Surendranath, Y.; Kanan, M. W.; Nocera, D. G. Mechanistic Studies of the Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH. J. Am. Chem. Soc. 2010, 132, 16501– 16509, DOI: 10.1021/ja106102b
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91
Mechanistic Studies of the Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH
Surendranath, Yogesh; Kanan, Matthew W.; Nocera, Daniel G.
Journal of the American Chemical Society (2010), 132 (46), 16501-16509CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The mechanism of the O evolution reaction (OER) by catalysts prepd. by electrodepositions from Co2+ solns. in phosphate electrolytes (Co-Pi) was studied at neutral pH by electrokinetic and 18O isotope expts. Low-potential electrodepositions enabled the controlled prepn. of ultrathin Co-Pi catalyst films (<100 nm) that could be studied kinetically in the absence of mass transport and charge transport limitations to the OER. The Co-Pi catalysts exhibit a Tafel slope approx. equal to 2.3 × RT/F for the prodn. of O from H2O in neutral solns. The electrochem. rate law exhibits an inverse 1st order dependence on proton activity and a zeroth order dependence on phosphate for [Pi] ≥ 0.03 M. In the absence of phosphate buffer, the Tafel slope is increased ∼3-fold and the overall activity is greatly diminished. Together, these electrokinetic studies suggest a mechanism involving a rapid, one electron, one proton equil. between CoIII-OH and CoIV-O in which a phosphate species is the proton acceptor, followed by a chem. turnover-limiting process involving O-O bond coupling.
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92
McCool, N. S.; Robinson, D. M.; Sheats, J. E.; Dismukes, G. C. A Co₄O₄ “Cubane” Water Oxidation Catalyst Inspired by Photosynthesis. J. Am. Chem. Soc. 2011, 133, 11446– 11449, DOI: 10.1021/ja203877y
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92
A Co4O4 "Cubane" Water Oxidation Catalyst Inspired by Photosynthesis
McCool, Nicholas S.; Robinson, David M.; Sheats, John E.; Dismukes, G. Charles
Journal of the American Chemical Society (2011), 133 (30), 11446-11449CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Herein we describe the mol. Co4O4 cubane complex Co4O4(OAc)4(py)4 (1), which catalyzes efficient water oxidizing activity when powered by a std. photochem. oxidn. source or electrochem. oxidn. The pH dependence of catalysis, the turnover frequency, and in situ monitoring of catalytic species have revealed the intrinsic capabilities of this core type. The catalytic activity of complex 1 and analogous Mn4O4 cubane complexes is attributed to the cubical core topol., which is analogous to that of nature's water oxidn. catalyst, a cubical CaMn4O5 cluster.
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93
Egan, J. W., Jr.; Haggerty, B. S.; Rheingold, A. L.; Sendlinger, S. C.; Theopold, K. H. Crystal Structure of a Side-On Superoxo Complex of Cobalt and Hydrogen Abstraction by a Reactive Terminal Oxo Ligand. J. Am. Chem. Soc. 1990, 112, 2445– 2446, DOI: 10.1021/ja00162a069
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93
Crystal structure of a side-on superoxo complex of cobalt and hydrogen abstraction by a reactive terminal oxo ligand
Egan, James W., Jr.; Haggerty, Brian S.; Rheingold, Arnold L.; Sendlinger, Shawn C.; Theopold, Klaus H.
Journal of the American Chemical Society (1990), 112 (6), 2445-6CODEN: JACSAT; ISSN:0002-7863.
Mg redn. of Tp'CoIIX (Tp' = hydridotris(3-tert-butyl-5-methylpyrazolyl)borate, X = Cl, I) in a N atm. yielded Tp'CoI(N2) (I). Exposure of I to an excess of O2 yielded Tp'CoII(O2) (II), which has been structurally characterized by x-ray diffraction. II crystd. in the monoclinic space group P21/n with a 9.615(4), b 30.260(12), c 9.577(4) Å, β 102.14(4)°, and Z = 4. II features the first example of a side-on bound superoxide ligand (dO-O = 1.262(8) Å). II has been further characterized by vibrational spectroscopy (ν16O-16O = 961 cm-1, ν16O-18O = 937 cm-1, ν18O-18O = 908 cm-1) and magnetic susceptibility measurements (μeff(298 K) = 3.88 μB). II reacted with I to yield Tp'CoIIOH. The mechanism of this reaction is thought to involve formation of a reactive Co oxo complex, which decomps. by H abstraction.
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94
Reinaud, O. M.; Theopold, K. H. Hydrogen Tunneling in the Activation of Dioxygen by a Tris(pyrazolyl)borate Cobalt Complex. J. Am. Chem. Soc. 1994, 116, 6979– 6980, DOI: 10.1021/ja00094a080
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94
Hydrogen tunneling in the activation of dioxygen by a tris(pyrazolyl)borate cobalt complex
Reinaud, Olivia M.; Theopold, Klaus H.
Journal of the American Chemical Society (1994), 116 (15), 6979-80CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The dioxygen complex, [Tp''Co(O2)] (5; Tp'' = hydridotris(3-isopropyl-5-methylpyrazolyl)borate), was prepd. and found to be stable in the solid state, but decomps. in soln. to give [(Tp''Co)2(μ-OH)2] (6). Upon warming, 5 decomps. with the formation of a transient intermediate [(Tp''Co)2(μ-O2)] (7), which could also be prepd. by the reaction of [(Tp''Co)2(μ-N2)] with oxygen. The kinetics of the thermal decompn. of 7 was studied and isotope effects measured and evidence is given for a tunneling contribution to the hydrogen atom abstraction from the ligand.
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95
Nurdin, L.; Spasyuk, D. M.; Fairburn, L.; Piers, W. E.; Maron, L. Oxygen-Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O₂ Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical. J. Am. Chem. Soc. 2018, 140, 16094– 16105, DOI: 10.1021/jacs.8b07726
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95
Oxygen-Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O2 Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical
Nurdin, Lucie; Spasyuk, Denis M.; Fairburn, Laura; Piers, Warren E.; Maron, Laurent
Journal of the American Chemical Society (2018), 140 (47), 16094-16105CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidn.-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen redn. catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and exptl. implicated through chem. interception and isotope labeling expts. In the absence of competing chem. reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidn. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric redn. of O2 to H2O with 2 equiv of Co(II) and suggests a new pathway for selective redn. of O2 to water via Co(III)-O-O-Co(III) peroxo intermediates.
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96
Matsunaga, P. T.; Hillhouse, G. L.; Rheingold, A. L. Oxygen-Atom Transfer from Nitrous Oxide to a Nickel Metallacycle. Synthesis, Structure, and Reactions of (2,2′-Bipydine)Ni(OCH₂CH₂CH₂CH₂). J. Am. Chem. Soc. 1993, 115, 2075– 2077, DOI: 10.1021/ja00058a085
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96
Oxygen-atom transfer from nitrous oxide to a nickel metallacycle. Synthesis, structure, and reactions of [cyclic] (2,2'-bipyridine)Ni(OCH2CH2CH2CH2)
Matsunaga, Phillip T.; Hillhouse, Gregory L.; Rheingold, Arnold L.
Journal of the American Chemical Society (1993), 115 (5), 2075-7CODEN: JACSAT; ISSN:0002-7863.
N2O oxidized metallacyclopentane I to 55% II (1 atm, 50°, 48h).
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Figg, T. M.; Cundari, T. R. Mechanistic Study of Oxy Insertion into Nickel-Carbon Bonds with Nitrous Oxide. Organometallics 2012, 31, 4998– 5004, DOI: 10.1021/om300270x
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97
Mechanistic Study of Oxy Insertion into Nickel-Carbon Bonds with Nitrous Oxide
Figg, Travis M.; Cundari, Thomas R.
Organometallics (2012), 31 (14), 4998-5004CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
Transition-metal-mediated oxy insertion into metal-C bonds is useful for the development of catalytic cycles for selective hydrocarbon oxidn. However, there are few bona fide examples of net oxy insertion with transition-metal complexes. An extremely rare example of a 3d metal mediating oxy insertion into metal-C bonds is NiII alkyl complexes reacting with nitrous oxide (N2O) reported by Hillhouse and coworkers; however, the mechanism was never fully elucidated. A computational study was performed on bipyridyl Ni metallacycles that form Ni alkoxides upon reaction with N2O to attain insight into future catalyst design for O atom transfer reactions. Two possible mechanisms are explored. Of the two pathways, the computations suggest that the preferred mechanism proceeds through a Ni-oxyl intermediate followed by alkyl migration of the Ni-C bond to form an alkoxide. Oxyl formation is the rate-detg. step, with a free energy barrier of 29.4 kcal/mol for bpyNiII(cyclo-(CH2)4). Complexes that contain sp2-hybridized mols. at the β-C site within the metallacycle ring do not undergo oxy insertion due to elevated barriers. While exploring insertion with another oxidant, pyridine N-oxide, the authors found that N2O is crit. for net oxy insertion with this complex due to the substantial thermodn. advantage of N2 expulsion. Reaction with pyridine N-oxide necessitated expulsion of a worse leaving group, resulting in much higher barriers (ΔG⧧ = 49.7 kcal/mol) for the oxyl formation step.
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Elwell, C. E.; Gagnon, N. L.; Neisen, B. D.; Dhar, D.; Spaeth, A. D.; Yee, G. M.; Tolman, W. B. Copper-Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity. Chem. Rev. 2017, 117, 2059– 2107, DOI: 10.1021/acs.chemrev.6b00636
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98
Copper-Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity
Elwell, Courtney E.; Gagnon, Nicole L.; Neisen, Benjamin D.; Dhar, Debanjan; Spaeth, Andrew D.; Yee, Gereon M.; Tolman, William B.
Chemical Reviews (Washington, DC, United States) (2017), 117 (3), 2059-2107CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A longstanding research goal has been to understand the nature and role of copper-oxygen intermediates within copper-contg. enzymes and abiol. catalysts. Synthetic chem. has played a pivotal role in highlighting the viability of proposed intermediates and expanding the library of known copper-oxygen cores. In addn. to the no. of new complexes that have been synthesized since the previous reviews on this topic in this journal (Mirica, L.M.; Ottenwaelder, X.; Stack, T.D.P.Chem.Rev.2004, 104, 1013-1046 and Lewis, E.A.; Tolman, W.B.Chem.Rev.2004, 104, 1047-1076), the field has seen significant expansion in the (1) range of cores synthesized and characterized, (2) amt. of mechanistic work performed, particularly in the area of org. substrate oxidn., and (3) use of computational methods for both the corroboration and prediction of proposed intermediates. The scope of this review has been limited to well-characterized examples of copper-oxygen species but seeks to provide a thorough picture of the spectroscopic characteristics and reactivity trends of the copper-oxygen cores discussed.
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99
Garcia-Bosch, I.; Company, A.; Frisch, J. R.; Torrent-Sucarrat, M.; Cardellach, M.; Gamba, I.; Güell, M.; Casella, L.; Que, L., Jr.; Ribas, X.; Luis, J. M.; Costas, M. O₂ Activation and Selective Phenolate ortho Hydroxylation by an Usymmetric Dicopper μ-η¹:η¹-Peroxido Complex. Angew. Chem., Int. Ed. 2010, 49, 2406– 2409, DOI: 10.1002/anie.200906749
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99
O2 Activation and Selective Phenolate ortho Hydroxylation by an Unsymmetric Dicopper μ-η1:η1-Peroxido Complex
Garcia-Bosch, Isaac; Company, Anna; Frisch, Jonathan R.; Torrent-Sucarrat, Miquel; Cardellach, Mar; Gamba, Ilaria; Gueell, Mireia; Casella, Luigi; Que, Lawrence, Jr.; Ribas, Xavi; Luis, Josep M.; Costas, Miquel
Angewandte Chemie, International Edition (2010), 49 (13), 2406-2409, S2406/1-S2406/41CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
Reaction of Unsym. Dicopper μ-η1:η1-Peroxido Complex (I) with the sodium salt of para-substituted phenolates were studied. Hammett plot (σ+) affords a linear correlation which gives a ρ value of -0.6 (R2 = 0.98) consistent with an electrophilic oxidizing species that attacks the arom. ring in the rate-detg. step of the reactions.
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100
(a) Mirica, L. M.; Vance, M.; Rudd, D. J.; Hedman, B.; Hodgson, K. O.; Solomon, E. I.; Stack, T. D. P. Tyrosinase Reactivity in a Model Complex: An Alternative Hydroxylation Mechanism. Science 2005, 308, 1890– 1892, DOI: 10.1126/science.1112081
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100a
Tyrosinase Reactivity in a Model Complex: An Alternative Hydroxylation Mechanism
Mirica, Liviu M.; Vance, Michael; Rudd, Deanne Jackson; Hedman, Britt; Hodgson, Keith O.; Solomon, Edward I.; Stack, T. Daniel P.
Science (Washington, DC, United States) (2005), 308 (5730), 1890-1892CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
The binuclear copper enzyme tyrosinase activates O2 to form a μ-η2:η2-peroxodicopper(II) complex, which oxidizes phenols to catechols. Here, a synthetic μ-η2:η2-peroxodicopper(II) complex, with an absorption spectrum similar to that of the enzymic active oxidant, is reported to rapidly hydroxylate phenolates at -80°C. Upon phenolate addn. at extreme temp. in soln. (-120°C), a reactive intermediate consistent with a bis-μ-oxodicopper(III)-phenolate complex, with the O-O bond fully cleaved, is obsd. exptl. The subsequent hydroxylation step has the hallmarks of an electrophilic arom. substitution mechanism, similar to tyrosinase. Overall, the evidence for sequential O-O bond cleavage and C-O bond formation in this synthetic complex suggests an alternative intimate mechanism to the concerted or late stage O-O bond scission generally accepted for the phenol hydroxylation reaction performed by tyrosinase.
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(b) Mirica, L. M.; Rudd, D. J.; Vance, M. A.; Solomon, E. I.; Hodgson, K. O.; Hedman, B.; Stack, T. D. P. μ-η²:η²-Peroxodicopper(II) Complex with a Secondary Diamine Ligand: A Functional Model of Tyrosinase. J. Am. Chem. Soc. 2006, 128, 2654– 2665, DOI: 10.1021/ja056740v
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100b
μ-η2:η2-Peroxodicopper(II) Complex with a Secondary Diamine Ligand: A Functional Model of Tyrosinase
Mirica, Liviu M.; Rudd, Deanne Jackson; Vance, Michael A.; Solomon, Edward I.; Hodgson, Keith O.; Hedman, Britt; Stack, T. Daniel P.
Journal of the American Chemical Society (2006), 128 (8), 2654-2665CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The activation of dioxygen (O2) by Cu(I) complexes is an important process in biol. systems and industrial applications. In tyrosinase, a binuclear copper enzyme, a μ-η2:η2-peroxodicopper(II) species is accepted generally to be the active oxidant. Reported here is the characterization and reactivity of a μ-η2:η2-peroxodicopper(II) complex synthesized by reacting the Cu(I) complex of the secondary diamine ligand N,N'-di-tert-butyl-ethylenediamine (DBED), [(DBED)Cu(MeCN)](X) (1·X, X = CF3SO3-, CH3SO3-, SbF6-, BF4-), with O2 at 193 K to give [{Cu(DBED)}2(O2)](X)2 (2·X2). The UV-vis and resonance Raman spectroscopic features of 2 vary with the counteranion employed yet are invariant with change of solvent. These results implicate an intimate interaction of the counteranions with the Cu2O2 core. Such interactions are supported further by extended X-ray absorption fine structure (EXAFS) analyses of solns. that reveal weak copper-counteranion interactions. The accessibility of the Cu2O2 core to exogenous ligands such as these counteranions is manifest further in the reactivity of 2 with externally added substrates. Most notable is the hydroxylation reactivity with phenolates to give catechol and quinone products. Thus the strategy of using simple bidentate ligands at low temps. provides not only spectroscopic models of tyrosinase but also functional models.
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101
Kamachi, T.; Lee, Y.-M.; Nishimi, T.; Cho, J.; Yoshizawa, K.; Nam, W. Combined Experimental and Theoretical Approach To Understand the Reactivity of a Mononuclear Cu(II)–Hydroperoxo Complex in Oxygenation Reactions. J. Phys. Chem. A 2008, 112, 13102– 13108, DOI: 10.1021/jp804804j
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101
Combined Experimental and Theoretical Approach To Understand the Reactivity of a Mononuclear Cu(II)-Hydroperoxo Complex in Oxygenation Reactions
Kamachi, Takashi; Lee, Yong-Min; Nishimi, Tomonori; Cho, Jaeheung; Yoshizawa, Kazunari; Nam, Wonwoo
Journal of Physical Chemistry A (2008), 112 (50), 13102-13108CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
A copper(II) complex bearing a pentadentate ligand, [CuII(N4Py)(MeCN)(CF3SO3)2] (1, N4Py = N,N-bis(2-pyridylmethyl)bis(2-pyridyl)methylamine), was synthesized and characterized with various spectroscopic techniques and x-ray crystallog. A mononuclear CuII-hydroperoxo complex, [CuII(N4Py)(OOH)]+ (2), was then generated in the reaction of 1 and H2O2 in the presence of base, and the reactivity of the intermediate was studied in the oxidn. of various substrates at -40°. In the reactivity studies, 2 showed a low oxidizing power such that 2 reacted only with triethylphosphine but not with other substrates such as thioanisole, benzyl alc., 1,4-cyclohexadiene, cyclohexene, and cyclohexane. In theor. work, the authors have conducted d. functional theory (DFT) calcns. on the epoxidn. of ethylene by 2 and a [CuIII(N4Py)(O)]+ intermediate (3) at the B3LYP level. The activation barrier is 39.7 and 26.3 kcal/mol for distal and proximal oxygen attacks by 2, resp. The direct ethylene epoxidn. by 2 is not a plausible pathway, as the authors obsd. in the exptl. work. In contrast, the ethylene epoxidn. by 3 is a downhill and low-barrier process. Also 2 cannot be a precursor to 3, since the homolytic cleavage of the O-O bond of 2 is very endothermic (i.e., 42 kcal/mol). From the exptl. and theor. results, a mononuclear CuII-hydroperoxo species bearing a pentadentate N5 ligand is a sluggish oxidant in oxygenation reactions.
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102
(a) Wada, A.; Harata, M.; Hasegawa, K.; Jitsukawa, K.; Masuda, H.; Mukai, M.; Kitagawa, T.; Einaga, H. Structural and Spectroscopic Characterization of a Mononuclear Hydroperoxo-Copper(II) Complex with Tripodal Pyridylamine Ligands. Angew. Chem., Int. Ed. 1998, 37, 798– 799, DOI: 10.1002/(SICI)1521-3773(19980403)37:6<798::AID-ANIE798>3.0.CO;2-3
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102a
Structural and spectroscopic characterization of a mononuclear hydroperoxo - copper(II) complex with tripodal pyridylamine ligands
Wada, Akira; Harata, Manabu; Hasegawa, Koji; Jitsukawa, Koichiro; Masuda, Hideki; Mukai, Masahiro; Kitagawa, Teizo; Einaga, Hisahiko
Angewandte Chemie, International Edition (1998), 37 (6), 798-799CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH)
The reaction of hydrogen peroxide with either [CuII(bppa-)]ClO4 or [CuII(bppa)(CH3CO2)]ClO4 (bppa = bis(6-pivalamido-2-pyridylmethyl)(2-pyridylmethyl)amine) led to [CuII(bppa)(OOH)]ClO4, which was characterized by x-ray crystallog. (monoclinic, space group P21/a, R = 0.062). The hydroperoxo complex was further examd. by absorption, ESR, resonance Raman and ESI mass spectroscopy. The mononuclear hydroperoxo complex has an axially compressed trigonal bipyramidal geometry with the pyridyl nitrogens of the tripodal ligand located in the equatorial plane and the tertiary amine nitrogen and hydroperoxo oxygen in the axial positions. Hydrogen bonding from the amido nitrogens to the hydroperoxo oxygen help stabilize the complex, which could serve as an enzyme model.
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(b) Osako, T.; Nagatomo, S.; Tachi, Y.; Kitagawa, T.; Itoh, S. Low-Temperature Stopped-Flow Studies on the Reactions of Copper(II) Complexes and H₂O₂: The First Detection of a Mononuclear Copper(II)-Peroxo Intermediate. Angew. Chem., Int. Ed. 2002, 41, 4325– 4328, DOI: 10.1002/1521-3773(20021115)41:22<4325::AID-ANIE4325>3.0.CO;2-Y
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102b
Low-temperature stopped-flow studies on the reactions of copper(II) complexes and H2O2: The first detection of a mononuclear copper(II)-peroxo intermediate
Osako, Takao; Nagatomo, Shigenori; Tachi, Yoshimitsu; Kitagawa, Teizo; Itoh, Shinobu
Angewandte Chemie, International Edition (2002), 41 (22), 4325-4328CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
The low-temp. stopped-flow technique was used to evaluate the reactions between copper(II) complexes of tridentate and tetradentate (pyridylethyl)amine ligands and H2O2. The copper complexes include the newly prepd. [Cu(L1)(ClO4)2] (L1 = I) and [Cu(TEPA)(ClO4)]ClO4 (TEPA = tris[2-(2-pyridyl)ethyl]amine) and the known copper(II) complex [Cu(L2)(ClO4)2] (L2 = II). The results show that mononuclear CuII-peroxo complexes are generated from initially formed CuII-hydroperoxo intermediates. The reaction of [Cu(TEPA)(ClO4)](ClO4), contg. the tetradentate TEPA ligand, and H2O2 under the same conditions yielded an intermediate exhibiting a similar absorption spectrum, and also the same kinetic behavior as the reactions for the complex with the L1 ligand. The ligand denticity and the steric effects of the N-alkyl substituents in the pyridylethylamine derivs. significantly altered the reactivity of the initially formed CuII-hydroperoxo intermediates. These findings demonstrate significant information on the dioxygen-activation mechanism in biol. and industrial systems.
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(c) Fujii, T.; Naito, A.; Yamaguchi, S.; Wada, A.; Funahashi, Y.; Jitsukawa, K.; Nagatomo, S.; Kitagawa, T.; Masuda, H. Construction of a Square-Planar Hydroperoxo-Copper(II) Complex Inducing a Higher Catalytic Reactivity. Chem. Commun. 2003, 2700– 2701, DOI: 10.1039/b308073k
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102c
Construction of a square-planar hydroperoxo-copper(II) complex inducing a higher catalytic reactivity
Fujii, Tatsuya; Naito, Asako; Yamaguchi, Syuhei; Wada, Akira; Funahashi, Yasuhiro; Jitsukawa, Koichiro; Nagatomo, Shigenori; Kitagawa, Teizo; Masuda, Hideki
Chemical Communications (Cambridge, United Kingdom) (2003), (21), 2700-2701CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
The complex [Cu(BPBA)(MeOH)](ClO4)2 (1, BPBA = bis(2-pyridylmethyl)tert-butylamine) was prepd. and characterized by x-ray crystallog. and spectroscopic methods. A novel hydroperoxo-copper(II) complex with a square-planar geometry, [Cu(BPBA)(OOH)]+ (2), was prepd. from 1. 2 Exhibited a higher selectivity and catalytic reactivity for oxidn. of di-Me sulfide, in contrast to that with the trigonal-bipyramidal complex [Cu(TPA)(OOH)]+ (3, TPA = tris(2-pyridylmethyl)amine).
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103
Klinman, J. P. The Copper-Enzyme Family of Dopamine β-Monooxygenase and Peptidylglycine α-Hydroxylating Monooxygenase: Resolving the Chemical Pathway for Substrate Hydroxylation. J. Biol. Chem. 2006, 281, 3013– 3016, DOI: 10.1074/jbc.R500011200
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103
The copper-enzyme family of dopamine β-monooxygenase and peptidylglycine α-hydroxylating monooxygenase: Resolving the chemical pathway for substrate hydroxylation
Klinman, Judith P.
Journal of Biological Chemistry (2006), 281 (6), 3013-3016CODEN: JBCHA3; ISSN:0021-9258. (American Society for Biochemistry and Molecular Biology)
A review. Dopamine β-monooxygenase (I)and peptidylglycine α-hydroxylating monooxygenase (II) belong to a small class of Cu-proteins found exclusively in higher eukaryotes. These physiol. important enzymes resp. catalyze the transformation of dopamine to norepinephrine and C-terminal glycine-extended peptides to α-hydroxylated products. Although their substrate specificities are markedly different, these 2 enzymes greatly resemble each other in many other respects. Both enzymes are localized in subcellular compartments: I in chromaffin vesicles of the adrenal gland or synaptic vesicles of the sympathetic nervous system and II in secretory vesicles of the pituitary gland. Although I and II exist in sol. and membrane-bound forms within the vesicular compartments, the majority of studies have been conducted with the more tractable sol. enzymes. The physiol. role played by the sol. and membrane-bound forms may be different, but the chem. mechanisms are almost certain to be the same. Comparison of the primary sequence of the II catalytic core with the larger I indicates a central core of ∼300 amino acids from I that is 27% identical and 40% homologous to II. In addn., I contains ∼200 amino acids toward its N-terminus and ∼100 amino acids toward the C-terminus that bear no relation to II. Of particular note is the conservation of the ligands to the 2 Cu atoms per enzyme subunit, designated CuH and CuM. Although I and II belong to a multi-Cu family of proteins, the Cu atoms appear to perform different functions, that of substrate hydroxylation (CuH) and electron storage/transfer (CuM). Perhaps the most startling feature to emerge from x-ray crystallog. studies is the fully solvent-exposed nature of the Cu sites, raising the questions of (1) how I and II carry out regio- and stereospecific hydroxylations, and (2) how they carry out controlled electron transfer from CuH to CuM through bulk solvent. The formation of a Cu-superoxo intermediate appears to provide a working mechanism that is capable of rationalizing the voluminous data available for I and II. Many exptl. challenges remain, which include the precise tuning of the active site for H-transfer and the possible participation of regions distal from the active site in this process. The vexing question of the exact mechanism of long-range electron transfer between the CuH and CuM sites also awaits elaboration.
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104
Chufán, E. E.; Prigge, S. T.; Siebert, X.; Eipper, B. A.; Mains, R. E.; Amzel, L. M. Differential Reactivity between Two Copper Sites in Peptidylglycine α-Hydroxylating Monooxygenase. J. Am. Chem. Soc. 2010, 132, 15565– 15572, DOI: 10.1021/ja103117r
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104
Differential Reactivity between Two Copper Sites in Peptidylglycine α-Hydroxylating Monooxygenase
Chufan, Eduardo E.; Prigge, Sean T.; Siebert, Xavier; Eipper, Betty A.; Mains, Richard E.; Amzel, L. Mario
Journal of the American Chemical Society (2010), 132 (44), 15565-15572CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Peptidylglycine α-hydroxylating monooxygenase (PHM) catalyzes the stereospecific hydroxylation of the Cα of C-terminal glycine-extended peptides and proteins, the first step in the activation of many peptide hormones, growth factors, and neurotransmitters. The crystal structure of the enzyme revealed two nonequivalent Cu sites (CuM and CuH) sepd. by ∼11 Å. In the resting state of the enzyme, CuM is coordinated in a distorted tetrahedral geometry by one methionine, two histidines, and a water mol. The coordination site of the water mol. is the position where external ligands bind. The CuH has a planar T-shaped geometry with three histidines residues and a vacant position that could potentially be occupied by a fourth ligand. Although the catalytic mechanism of PHM and the role of the metals are still being debated, CuM is identified as the metal involved in catalysis, while CuH is assocd. with electron transfer. To further probe the role of the metals, we studied how small mols. such as nitrite (NO2-), azide (N3-), and carbon monoxide (CO) interact with the PHM copper ions. The crystal structure of an oxidized nitrite-soaked PHMcc, obtained by soaking for 20 h in mother liquor supplemented with 300 mM NaNO2, shows that nitrite anion coordinates CuM in an asym. bidentate fashion. Surprisingly, nitrite does not bind CuH, despite the high concn. used in the expts. (nitrite/protein > 1000). Similarly, azide and carbon monoxide coordinate CuM but not CuH in the PHMcc crystal structures obtained by cocrystn. with 40 mM NaN3 and by soaking CO under 3 atm of pressure for 30 min. This lack of reactivity at the CuH is also obsd. in the reduced form of the enzyme: CO binds CuM but not CuH in the structure of PHMcc obtained by exposure of a crystal to 3 atm CO for 15 min in the presence of 5 mM ascorbic acid (reductant). The necessity of CuH to maintain its redox potential in a narrow range compatible with its role as an electron-transfer site seems to explain the lack of coordination of small mols. to CuH; coordination of any external ligand will certainly modify its redox potential.
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105
Kim, S.; Ståhlberg, J.; Sandgren, M.; Paton, R. S.; Beckham, G. T. Quantum Mechanical Calculations Suggest that Lytic Polysaccharide Monooxygenases Use a Copper-Oxyl, Oxygen-Rebound Mechanism. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 149– 154, DOI: 10.1073/pnas.1316609111
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105
Quantum mechanical calculations suggest that lytic polysaccharide monooxygenases use a copper-oxyl, oxygen-rebound mechanism
Kim, Seonah; Stahlberg, Jerry; Sandgren, Mats; Paton, Robert S.; Beckham, Gregg T.
Proceedings of the National Academy of Sciences of the United States of America (2014), 111 (1), 149-154CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
Lytic polysaccharide monooxygenases (LPMOs) exhibit a mononuclear Cu-contg. active site and use O2 and a reducing agent to oxidatively cleave glycosidic linkages in polysaccharides. LPMOs represent a unique paradigm in carbohydrate turnover and exhibit synergy with hydrolytic enzymes in biomass depolymn. To date, several features of Cu binding to LPMOs have been elucidated, but the identity of the reactive O species (ROS) and the key steps in the oxidative mechanism have not been elucidated. Here, DFT calcns. were used with an enzyme active site model to identify the ROS and compare 2 hypothesized reaction pathways in LPMOs for H atom abstraction and polysaccharide hydroxylation; namely, (1) a mechanism that employs a η1-superoxo intermediate, which abstrs. a substrate H atom and a hydroperoxo species is responsible for substrate hydroxylation, and (2) a mechanism wherein a copper-oxyl radical abstrs. a H atom and subsequently hydroxylates the substrate via an oxygen-rebound mechanism. The results predicted that O binds end-on (η1) to Cu, and that a copper-oxyl-mediated, O-rebound mechanism is energetically preferred. N-terminal His methylation was also examd., which is thought to modify the structure and reactivity of the enzyme. DFT calcns. suggested that this post-translational modification had only a minor effect on the LPMO active site structure or reactivity for the examd. steps. Overall, this study suggests the steps in the LPMO mechanism for oxidative cleavage of glycosidic bonds.
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106
Dietl, N.; van der Linde, C.; Schlangen, M.; Beyer, M. K.; Schwarz, H. Diatomic [CuO]⁺ and Its Role in the Spin-Selective Hydrogen- and Oxygen-Atom Transfers in the Thermal Activation of Methane. Angew. Chem., Int. Ed. 2011, 50, 4966– 4969, DOI: 10.1002/anie.201100606
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106
Diatomic [CuO]+ and Its Role in the Spin-Selective Hydrogen- and Oxygen-Atom Transfers in the Thermal Activation of Methane
Dietl, Nicolas; van der Linde, Christian; Schlangen, Maria; Beyer, Martin K.; Schwarz, Helmut
Angewandte Chemie, International Edition (2011), 50 (21), 4966-4969CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
More than ten years after its theor. prediction to serve as a powerful converter of methane to methanol the bare [CuO]+ cation has been successfully generated in the gas phase. A combination of mass spectrometry and DFT calcns. revealed the crucial role of two-state reactivity and oxygen-centered radicals in the selectivity in the oxidn. of methane.
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107
Rodgers, M. T.; Walker, B.; Armentrout, P. B. Reactions of Cu⁺ (¹S and ³D) with O₂, CO, CO₂, N₂, NO, N₂O and NO₂ studied by guided ion beam mass spectrometry. Int. J. Mass Spectrom. 1999, 182-183, 99– 120, DOI: 10.1016/S1387-3806(98)14228-8
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107
Reactions of Cu+(1S and 3D) with O2, CO, CO2, N2, NO, N2O, and NO2 studied by guided ion beam mass spectrometry
Rodgers, M. T.; Walker, Ben; Armentrout, P. B.
International Journal of Mass Spectrometry (1999), 182/183 (), 99-120CODEN: IMSPF8; ISSN:1387-3806. (Elsevier Science B.V.)
Reactions of Cu+(1S and 3D) with O2, CO, CO2, N2, NO, N2O, and NO2 are studied using guided ion beam mass spectrometry. Cross sections as a function of kinetic energy are measured for each system to over 17 eV. In all cases, the obsd. reactions of Cu+(1S) are endothermic. Because of the closed shell character of ground state Cu+ (1S, 3d10), most of these systems exhibit cross sections with onsets and peaks at much higher energies than expected from the known thermochem. Such behavior indicates that the reactions occur on relatively repulsive potential energy surfaces and by impulsive processes. Reliable thermodn. information is obtained primarily from the NO2 system where an anal. of the kinetic energy dependence of the reaction cross sections is used to obtain D0(Cu+-O) = 1.35 ± 0.12 eV and D0(Cu-O) = 2.94 ± 0.12 eV. Although speculative, the threshold for an excited state product asymptote in the N2O system also allows the derivation of D0(Cu+-N2) = 0.92 ± 0.31 eV. Reactions of the Cu+(3D, 4s1 3d9) excited state are generally more efficient than those of the ground state and are exothermic in several cases.
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108
Rezabal, E.; Gauss, J.; Matxain, J. M.; Berger, R.; Diefenbach, M.; Holthausen, M. C. Quantum Chemical Assessment of the Binding Energy of CuO⁺. J. Chem. Phys. 2011, 134, 064304, DOI: 10.1063/1.3537797
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108
Quantum chemical assessment of the binding energy of CuO+
Rezabal, Elixabete; Gauss, Juergen; Matxain, Jon M.; Berger, Robert; Diefenbach, Martin; Holthausen, Max C.
Journal of Chemical Physics (2011), 134 (6), 064304/1-064304/13CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
We present a detailed theor. investigation on the dissocn. energy of CuO+, carried out by means of coupled cluster theory, the multireference averaged coupled pair functional (MR-ACPF) approach, diffusion quantum Monte Carlo (DMC), and d. functional theory (DFT). At the resp. extrapolated basis set limits, most post-Hartree-Fock approaches agree within a narrow error margin on a De value of 26.0 kcal mol-1 coupled-cluster singles and doubles level augmented by perturbative triples corrections, CCSD(T), 25.8 kcal mol-1 (CCSDTQ via the high accuracy extrapolated ab initio thermochem. protocol), and 25.6 kcal mol-1 (DMC), which is encouraging in view of the disaccording data published thus far. The configuration-interaction based MR-ACPF expansion, which includes single and double excitations only, gives a slightly lower value of 24.1 kcal mol-1, indicating that large basis sets and triple excitation patterns are necessary ingredients for a quant. assessment. Our best est. for D0 at the CCSD(T) level is 25.3 kcal mol-1, which is somewhat lower than the latest exptl. value (D0 = 31.1 ± 2.8 kcal mol-1; reported by the Armentrout group). These highly correlated methods are, however, computationally very demanding, and the results are therefore supplemented with those of more affordable DFT calcns. If used in combination with moderately-sized basis sets, the M05 and M06 hybrid functionals turn out to be promising candidates for studies on much larger systems contg. a CuO+ core. (c) 2011 American Institute of Physics.
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109
Wang, B.; Johnston, E. M.; Li, P.; Shaik, S.; Davies, G. J.; Walton, P. H.; Rovira, C. QM/MM Studies into the H₂O₂-Dependent Activity of Lytic Polysaccaride Monooxygenases: Evidence for the Formation of a Caged Hydroxyl Radical Intermediate. ACS Catal. 2018, 8, 1346– 1351, DOI: 10.1021/acscatal.7b03888
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109
QM/MM Studies into the H2O2-Dependent Activity of Lytic Polysaccharide Monooxygenases: Evidence for the Formation of a Caged Hydroxyl Radical Intermediate
Wang, Binju; Johnston, Esther M.; Li, Pengfei; Shaik, Sason; Davies, Gideon J.; Walton, Paul H.; Rovira, Carme
ACS Catalysis (2018), 8 (2), 1346-1351CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
Lytic polysaccharide monooxygenases (LPMOs) are promising enzymes for the conversion of lignocellulosic biomass into biofuels and biomaterials. Classically considered oxygenases, recent work suggests that H2O2 can, under certain circumstances, also be a potential substrate. Here we present a detailed mechanism of the activation of H2O2 by a C4-acting LPMO using small-model DFT and QM/MM calcns. We show that there is an efficient mechanism to break the O-O bond of H2O2, with a low barrier of 5.8 kcal/mol, via a one-electron transfer from the LPMO-Cu(I) site to form an HO• radical, stabilized by hydrogen bonding interactions. Our QM/MM calcns. further show that the H-bonding machinery of the enzyme directs the HO• radical to abstr. a hydrogen atom from the Cu(II)-OH unit rather than from the substrate in what is essentially a caged-radical reaction, thereby forming a Cu(II)-oxyl species. The Cu(II)-oxyl species then exclusively oxidizes the C4-H bond due to the suitable position of the substrate. Our calcns. also suggest that the C4-hydroxylated intermediate can be efficiently hydrolyzed in water, and this process does not require enzymic catalysis.
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110
(a) Hong, S.; Huber, S. M.; Gagliardi, L.; Cramer, C. C.; Tolman, W. B. Copper(I)-α-Ketocarboxylate Complexes: Characterization and O₂ Reactions That Yield Copper-Oxygen Intermediates Capable of Hydroxylating Arenes. J. Am. Chem. Soc. 2007, 129, 14190– 14192, DOI: 10.1021/ja0760426
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110a
Copper(I)-α-Ketocarboxylate Complexes: Characterization and O2 Reactions That Yield Copper-Oxygen Intermediates Capable of Hydroxylating Arenes
Hong, Sungjun; Huber, Stefan M.; Gagliardi, Laura; Cramer, Christopher C.; Tolman, William B.
Journal of the American Chemical Society (2007), 129 (46), 14190-14192CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Cu(I)-α-ketocarboxylate complexes with aryl substituted iminoethylpyridines were prepd. and shown to exhibit variable coordination modes of the α-ketocarboxylate ligand. Reaction with O2 induces decarboxylation of the α-ketocarboxylate, and the derived Cu-O intermediate(s) was intercepted, resulting in hydroxylation of an arene substituent on the supporting N-donor ligand. Theor. calcns. provided intriguing mechanistic notions for the process, notably implicating hydroxylation pathways that involve novel [CuI-OOC(O)R] and [CuII-O-• ↔ CuIII:O2-]+ species.
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.
(b) Huber, S. M.; Ertem, M. Z.; Aquilante, F.; Gagliardi, L.; Tolman, W. B.; Cramer, C. J. Generating Cu^II-Oxyl/Cu^III-Oxo Species from Copper(I)-α-Ketocarboxylate Complexes and O₂: In Silico Studies on Ligand Effects and C–H-Activation Reactivity. Chem. - Eur. J. 2009, 15, 4886– 4895, DOI: 10.1002/chem.200802338
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110b
Generating CuII-Oxyl/CuIII-Oxo Species from CuI-α-Ketocarboxylate Complexes and O2: In Silico Studies on Ligand Effects and C-H-Activation Reactivity
Huber, Stefan M.; Ertem, Mehmed Z.; Aquilante, Francesco; Gagliardi, Laura; Tolman, William B.; Cramer, Christopher J.
Chemistry - A European Journal (2009), 15 (19), 4886-4895, S4886/1-S4886/334CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
Theor. speaking: The mechanistic details assocd. with the generation and reaction of [CuO]+ species from CuI-α-ketocarboxylate complexes, esp. with respect to modifications of the ligand supporting the copper center, were investigated. Theor. models were used to characterize the electronic structures of different [CuO]+ species and their reactivity in C-H activation and O-atom transfer reactions. A mechanism for the oxygenation of CuI complexes with α-ketocarboxylate ligands that is based on a combination of d. functional theory and multireference second-order perturbation theory (CASSCF/CASPT2) calcns. is elaborated. The reaction proceeds in a manner largely analogous to those of similar FeII-α-ketocarboxylate systems, i.e., by initial attack of a coordinated oxygen mol. on a ketocarboxylate ligand with concomitant decarboxylation. Subsequently, two reactive intermediates may be generated, a Cu-peracid structure and a [CuO]+ species, both of which are capable of oxidizing a Ph ring component of the supporting ligand. Hydroxylation by the [CuO]+ species is predicted to proceed with a smaller activation free energy. The effects of electronic and steric variations on the oxygenation mechanisms were studied by introducing substituents at several positions of the ligand backbone and by investigating various N-donor ligands. In general, more electron donation by the N-donor ligand leads to increased stabilization of the more CuII/CuIII-like intermediates (oxygen adducts and [CuO]+ species) relative to the more CuI-like peracid intermediate. For all ligands investigated, the [CuO]+ intermediates are best described as CuII-O·- species with triplet ground states. The reactivity of these compds. in C-H abstraction reactions decreases with more electron-donating N-donor ligands, which also increase the Cu-O bond strength, although the Cu-O bond is generally predicted to be rather weak (with a bond order of about 0.5). A comparison of several methods to obtain singlet energies for the reaction intermediates indicates that multireference second-order perturbation theory is likely more accurate for the initial oxygen adducts, but not necessarily for subsequent reaction intermediates.
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111
Tsuji, T.; Zaoputra, A. A.; Hitomi, Y.; Mieda, K.; Ogura, T.; Shiota, Y.; Yoshizawa, K.; Sato, H.; Kodera, M. Specific Enhancement of Catalytic Activity by a Dicopper Core: Selective Hydroxylation of Benzene to Phenol with Hydrogen Peroxide. Angew. Chem. 2017, 129, 7887– 7890, DOI: 10.1002/ange.201702291
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112
Augusti, R.; Dias, A. O.; Rocha, L. L.; Lago, R. M. Kinetics and Mechanism of Benzene Derivative Degradation with Fenton’s Reagent in Aqueous Medium Studies by MIMS. J. Phys. Chem. A 1998, 102, 10723– 10727, DOI: 10.1021/jp983256o
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112
Kinetics and Mechanism of Benzene Derivative Degradation with Fenton's Reagent in Aqueous Medium Studied by MIMS
Augusti, Rodinei; Dias, Adelson O.; Rocha, Lilian L.; Lago, Rochel M.
Journal of Physical Chemistry A (1998), 102 (52), 10723-10727CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
Membrane introduction mass spectrometry (MIMS) was used to investigate kinetic and mechanistic aspects of the reaction of benzene derivs. with Fenton's reagent (Fe2+/H2O2) in water. Under the conditions employed, the reaction rate showed a first-order dependence on the arom. compd. concn. The order of reactivity obsd. was C6H5Cl > C6H5Br > C6H6 > C6H5CH3 > C6H5OCH3 > C6H5NO2 > C6H5OH, and, with the exception of C6H5NO2, a linear Hammett relationship (log kX/kH vs. σp) was obsd. This fact suggests that electronic factors significantly influence reactivity with the Fenton's reagent. Expts. with C6H6 and C6D6 showed the presence of an isotopic effect of kH/kD = 1.7, suggesting that cleavage of the benzene C-H bond occurs in the reaction rate controlling step. Mechanistic studies with chlorobenzene showed that mineralization to CO2 and chloride proceeds via hydroxylation steps producing phenolic, hydroquinonic, and quinonic intermediates.
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113
Maiti, D.; Lucas, H. R.; Sarjeant, A. A. N.; Karlin, K. D. Aryl Hydroxylation from a Mononuclear Copper-Hydroperoxo Species. J. Am. Chem. Soc. 2007, 129, 6998– 6999, DOI: 10.1021/ja071704c
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113
Aryl Hydroxylation from a Mononuclear Copper-Hydroperoxo Species
Maiti, Debabrata; Lucas, Heather R.; Narducci Sarjeant, Amy A.; Karlin, Kenneth D.
Journal of the American Chemical Society (2007), 129 (22), 6998-6999CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Mononuclear CuII--OOH entities or derived species were seriously considered as important in chem. or biochem. oxidns. Yet, synthetic chem. studies have thus far revealed they have very limited substrate reactivity. Here, a significant aryl hydroxylation reaction occurs from a hydroperoxocopper(II) complex possessing a tripodal tetradentate ligand with appended aryl substituent. A phenolate-Cu(II) complex is detected following a proposed O-O cleavage event, that is suggested from precedent from nonheme Fe chem. A bis-μ-oxo-dicopper(III) complex as active oxygenating agent is ruled out.
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114
Jacobson, R. R.; Tyeklar, Z.; Farooq, A.; Karlin, K. D.; Liu, S.; Zubieta, J. A Copper-Oxygen (Cu₂-O₂) Complex. Crystal Structure and Characterization of a Reversible Dioxygen Binding System. J. Am. Chem. Soc. 1988, 110, 3690– 3692, DOI: 10.1021/ja00219a071
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114
A copper-oxygen (Cu2-O2) complex. Crystal structure and characterization of a reversible dioxygen binding system
Jacobson, Richard R.; Tyeklar, Zoltan; Farooq, Amjad; Karlin, Kenneth D.; Liu, Shuncheng; Zubieta, Jon
Journal of the American Chemical Society (1988), 110 (11), 3690-2CODEN: JACSAT; ISSN:0002-7863.
The 1st x-ray structural characterization of a Cu-O2 complex is reported. The tripodal tetradentate ligand tris[(2-pyridyl)methyl]amine (L) was used to prep. [CuLL1]+ (L1 = RCN, PPh3). Oxygenation of CuL(NCR)+ in EtCN or CH2Cl2 at -80° provides [{LCu}2(O2)]2+ (I; Cu:O2 = 2:1) which has characteristic UV-visible absorptions at 525 and 590 nm. The binding of O2 (and CO) to [CuL(NCR)]+ to form I is reversible; reaction of I with either CO or PPh3 affords CuL(CO)+ or CuL(PPh3)+, resp. Protonation of I gives H2O2 and CuL(NCR)2+. The properties of I indicate that it is best described as a peroxo dicopper(II) complex. The Cu(II) ions (e.g. d-d band at 1035 nm) appear to be strongly magnetically coupled, based on the ESR silence and sharp 1H NMR spectrum exhibited by I. A single crystal x-ray structural characterization of [{LCu}2(O2)](PF6)2.5Et2O (-90°; P‾1, a 11.062(3), b 12.758(4), c 13.280(5) Å, α 96.72(3), β 110.57(3), γ 103.73(3)°, Z = 1, R = 0.0581, Rw = 0.0580) shows that it has a trans μ-1,2-peroxo ligation to Cu(II) ions in a trigonal bipyramidal coordination.
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115
Vilella, L.; Conde, A.; Balcells, D.; Díaz-Requejo, M. M.; Lledós, A.; Pérez, P. J. A Competing, Dual Mechanism for Catalytic Direct Benzene Hydroxylation from Combined Experimental-DFT Studies. Chem. Sci. 2017, 8, 8373– 8383, DOI: 10.1039/C7SC02898A
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115
A competing, dual mechanism for catalytic direct benzene hydroxylation from combined experimental-DFT studies
Vilella, Laia; Conde, Ana; Balcells, David; Diaz-Requejo, M. Mar; Lledos, Agusti; Perez, Pedro J.
Chemical Science (2017), 8 (12), 8373-8383CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
A dual mechanism for direct benzene catalytic hydroxylation is described. Exptl. studies and DFT calcns. have provided a mechanistic explanation for the acid-free, TpxCu-catalyzed hydroxylation of benzene with hydrogen peroxide (Tpx = hydrotrispyrazolylborate ligand). In contrast with other catalytic systems that promote this transformation through Fenton-like pathways, this system operates through a copper-oxyl intermediate that may interact with the arene ring following two different, competitive routes: (a) electrophilic arom. substitution, with the copper-oxyl species acting as the formal electrophile, and (b) the so-called rebound mechanism, in which the hydrogen is abstracted by the Cu-O moiety prior to the C-O bond formation. Both pathways contribute to the global transformation albeit to different extents, the electrophilic substitution route seeming to be largely favored.
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116
Zhang, M.-T.; Chen, Z.; Kang, P.; Meyer, T. J. Electrocatalytic Water Oxidation with a Copper(II) Polypeptide Complex. J. Am. Chem. Soc. 2013, 135, 2048– 2051, DOI: 10.1021/ja3097515
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116
Electrocatalytic Water Oxidation with a Copper(II) Polypeptide Complex
Zhang, Ming-Tian; Chen, Zuofeng; Kang, Peng; Meyer, Thomas J.
Journal of the American Chemical Society (2013), 135 (6), 2048-2051CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
A self-assembly-formed triglycylglycine macrocyclic ligand (TGG4-) complex of Cu(II), [(TGG4-)CuII-OH2]2-, efficiently catalyzes water oxidn. in a phosphate buffer at pH 11 at room temp. by a well-defined mechanism. In the mechanism, initial oxidn. to Cu(III) is followed by further oxidn. to a formal "Cu(IV)" with formation of a peroxide intermediate, which undergoes further oxidn. to release oxygen and close the catalytic cycle. The catalyst exhibits high stability and activity toward water oxidn. under these conditions with a high turnover frequency of 33 s-1.
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117
(a) Fisher, K. J.; Materna, K. L.; Mercado, B. Q.; Crabtree, R. H.; Brudvig, G. W. Electrocatalytic Water oxidation by a Copper(II) Complex of an Oxidatio-Resistant Ligand. ACS Catal. 2017, 7, 3384– 3387, DOI: 10.1021/acscatal.7b00494
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117a
Electrocatalytic Water Oxidation by a Copper(II) Complex of an Oxidation-Resistant Ligand
Fisher, Katherine J.; Materna, Kelly L.; Mercado, Brandon Q.; Crabtree, Robert H.; Brudvig, Gary W.
ACS Catalysis (2017), 7 (5), 3384-3387CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
The Cu(II) complex Cu(pyalk)2 (pyalk = 2-pyridyl-2-propanoate) is a robust homogeneous H2O-oxidn. electrocatalyst under basic conditions (pH > 10.4). H2O oxidn. occurs at a relatively low overpotential for Cu of 520-580 mV with a turnover frequency of ∼0.7 s-1. Controlled potential electrolysis expts. over 12 h at 1.1 V vs. normal H electrode gave >30 catalytic turnovers of O2 with only ∼20% catalyst degrdn. The robustness of the catalyst under fairly harsh conditions and the low overpotential further highlight the oxidn. resistance and strong donor character of pyalk.
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(b) Rudshteyn, B.; Fisher, K. J.; Lant, H. M. C.; Yang, K. R.; Mercado, B. Q.; Crabtree, R. H.; Brudvig, G. W.; Batista, V. S. Water-Nucleophilic Attack Mechanism for the Cu^II(pyalk)₂ Water-Oxidation Catalyst. ACS Catal. 2018, 8, 7952– 7960, DOI: 10.1021/acscatal.8b02466
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117b
Water-Nucleophilic Attack Mechanism for the CuII(pyalk)2 Water-Oxidation Catalyst
Rudshteyn, Benjamin; Fisher, Katherine J.; Lant, Hannah M. C.; Yang, Ke R.; Mercado, Brandon Q.; Brudvig, Gary W.; Crabtree, Robert H.; Batista, Victor S.
ACS Catalysis (2018), 8 (9), 7952-7960CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
We investigate the mechanism of water oxidn. catalyzed by the CuII(pyalk)2 complex, combining d. functional theory with exptl. measurements of turnover frequencies, UV-visible spectra, H/D kinetic isotope effects (KIEs), electrochem. anal., and synthesis of a deriv. complex. We find that only in the cis form does CuII(pyalk)2 convert water to dioxygen. In a series of alternating chem. and electrochem. steps, the catalyst is activated to form a metal oxyl radical species that undergoes a water-nucleophilic attack defining the rate-limiting step of the reaction. The exptl. H/D KIE (3.4) is in agreement with the calcd. value (3.7), shown to be detd. by deprotonation of the substrate nucleophile upon O-O bond formation. The reported mechanistic findings are particularly valuable for rational design of complexes inspired by CuII(pyalk)2.
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118
Yang, X.; Baik, M.-H. cis,cis-[(bpy)₂Ru^VO]₂O⁴⁺ Catalyzes Water Oxidation Formally via in Situ Generation of Radicaloid Ru^IV–O•. J. Am. Chem. Soc. 2006, 128, 7476– 7485, DOI: 10.1021/ja053710j
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118
cis,cis-[(bpy)2RuVO]2O4+ Catalyzes Water Oxidation Formally via in Situ Generation of Radicaloid RuIV-O•
Yang, Xiaofan; Baik, Mu-Hyun
Journal of the American Chemical Society (2006), 128 (23), 7476-7485CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The mechanism of the catalytic oxidn. of water by cis,cis-[(bpy)2Ru(OH2)]2O4+ to give mol. dioxygen was investigated using D. Functional Theory (DFT). A series of four oxidn. and four deprotonation events generate the catalytically competent species cis,cis-[(bpy)2RuVO]2O4+, which breaks the H-OH bond homolytically at the rate detg. transition state to give a hydroperoxo intermediate. Our calcns. predict a rate detg. activation barrier of 25.9 kcal/mol in soln. phase, which is in reasonable agreement with the previously reported exptl. est. of 18.7-23.3 kcal/mol. A no. of plausible coupling schemes of the two metal sites including strong coupling, weak ferromagnetic and weak antiferromagnetic coupling have been considered. In addn., both high-spin and low-spin states at each of the Ru(V)-d3 centers were explored and we found that the high-spin states play an important mechanistic role. Our calcns. suggest that cis,cis-[(bpy)2RuVO]2O4+ performs formally an intramol. ligand-to-metal charge transfer when reacting with water to formally give a cis,cis-[(bpy)2RuIVO•]2O4+ complex. We propose that the key characteristic of the diruthenium catalyst that allows it to accomplish the most difficult first two oxidns. of the overall four-electron redox reaction is directly assocd. with this in situ generation of two radicaloid oxo moieties that promote the water splitting reaction. A proton coupled metal-to-metal charge transfer follows to yield a Ru(V)/Ru(III) peroxo/aqua mixed valence complex, which performs the third redox reaction to give the superoxo/aqua complex. Finally, intersystem crossing to a ferromagnetically coupled Ru(IV)/Ru(III) superoxo/aqua species is predicted, which will then promote the last redox event to release triplet dioxygen as the final product. A no. of key features of the computed mechanism are explored in detail to derive a conceptual understanding of the catalytic mechanism.
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119
Rüttinger, W.; Dismukes, G. C. Synthetic Water-Oxidation Catalysts for Artificial Photosynthetic Water Oxidation. Chem. Rev. 1997, 97, 1– 24, DOI: 10.1021/cr950201z
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119
Synthetic Water-Oxidation Catalysts for Artificial Photosynthetic Water Oxidation
Ruettinger, Wolfgang; Dismukes, G. Charles
Chemical Reviews (Washington, D. C.) (1997), 97 (1), 1-24CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review with 132 refs. in which homogeneous and some heterogeneous catalysts for oxidn. of water are described. Current views are presented of the photosynthetic water-oxidizing complex (WOC) and its functionality, followed by anal. of the thermodn. and kinetic constraints for water oxidn. that have to be overcome by any catalyst. Since manganese is the metal that performs this reaction in the WOC, manganese photocatalysts are discussed, also other transition metals, particularly ruthenium are discussed. Principles of reactivity learned from theory and existing models are summarized, which can lead to synthesis of better catalysts in the future.
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120
(a) Balcells, D.; Raynaud, C.; Crabtree, R. H.; Eisenstein, O. The Rebound Mechanism in Catalytic C-H Oxidation by MnO(tpp)Cl from DFT Studies: Electronic Nature of the Active Species. Chem. Commun. 2008, 744– 766, DOI: 10.1039/B715939K
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120a
The rebound mechanism in catalytic C-H oxidation by MnO(tpp)Cl from DFT studies: electronic nature of the active species
Balcells, David; Raynaud, Christophe; Crabtree, Robert H.; Eisenstein, Odile
Chemical Communications (Cambridge, United Kingdom) (2008), (6), 744-746CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
DFT studies show that the rebound mechanism for MnO(tpp)(Cl)-catalyzed C-H hydroxylation is favored for spin states with oxyl character.
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(b) Mayer, J. M. Hydrogen Atom Abstraction by Metal-Oxo Complexes: Understanding the Analogy with Organic Radical Reactions. Acc. Chem. Res. 1998, 31, 441– 450, DOI: 10.1021/ar970171h
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120b
Hydrogen Atom Abstraction by Metal-Oxo Complexes: Understanding the Analogy with Organic Radical Reactions
Mayer, James M.
Accounts of Chemical Research (1998), 31 (8), 441-450CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
A review with 56 refs. This Account describes our progression from thinking about radicals and spin d. to an approach based on the thermochem. of the hydrogen atom transfer step. The spin-d. intuition was based on an analogy with org. radical chem., but as described below, a better analogy is based on bond strengths. This approach provides both new qual. insight and quant. predictions.
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121
Lippert, C. A.; Hardcastle, K. I.; Soper, J. D. Harnessing Redox-Active Ligands for Low-Barrier Radical Addition at Oxorhenium Complexes. Inorg. Chem. 2011, 50, 9864– 9878, DOI: 10.1021/ic200923q
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121
Harnessing Redox-Active Ligands for Low-Barrier Radical Addition at Oxorhenium Complexes
Lippert, Cameron A.; Hardcastle, Kenneth I.; Soper, Jake D.
Inorganic Chemistry (2011), 50 (20), 9864-9878CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
The addn. of an [X]+ electrophile to the five-coordinate oxorhenium(V) anion [ReV(O)(apPh)2]- {[apPh]2- = 2,4-di-tert-butyl-6-(phenylamido)phenolate} gives new products contg. Re-X bonds. The Re-X bond-forming reaction is analogous to oxo transfer to [ReV(O)(apPh)2]- in that both are 2e- redox processes, but the electronic structures of the products are different. Whereas oxo addn. to [ReV(O)(apPh)2]- yields a closed-shell [ReVII(O)2(apPh)2]- product of 2e- metal oxidn., [Cl]+ addn. gives a diradical ReVI(O)(apPh)(isqPh)Cl product ([isqPh]·- = 2,4-di-tert-butyl-6-(phenylimino)semiquinonate) with 1e- in a Re d orbital and 1e- on a redox-active ligand. The differences in electronic structure are ascribed to differences in the π basicity of [O]2- and Cl- ligands. The observation of ligand radicals in ReVI(O)(apPh)(isqPh)X provides exptl. support for the capacity of redox-active ligands to deliver electrons in other bond-forming reactions at [ReV(O)(apPh)2]-, including radical addns. of O2 or TEMPO· to make Re-O bonds. Attempts to prep. the electron-transfer series monomers between ReVI(O)(apPh)(isqPh)X and [ReV(O)(apPh)2]- yielded a sym. bis(μ-oxo)dirhenium complex. Formation of this dimer suggested that ReVI(O)(apPh)(isqPh)Cl may be a source of an oxyl metal fragment. The ability of ReVI(O)(apPh)(isqPh)Cl to undergo radical coupling at oxo was revealed in its reaction with Ph3C·, which affords Ph3COH and deoxygenated metal products. This reactivity is surprising because ReVI(O)(apPh)(isqPh)Cl is not a strong outer-sphere oxidant or oxo-transfer reagent. The authors postulate that the unique ability of ReVI(O)(apPh)(isqPh)Cl to effect oxo transfer to Ph3C· arises from symmetry-allowed mixing of a populated Re[n.58876]O π bond with a ligand-centered [isqPh]·- ligand radical, which gives oxyl radical character to the oxo ligand. This allows the closed-shell oxo ligand to undergo a net 2e- oxo-transfer reaction to Ph3C· via kinetically facile redox-active ligand-mediated radical steps. Harnessing intraligand charge transfer for radical reactions at closed-shell oxo ligands is a new strategy to exploit redox-active ligands for small-mol. activation and functionalization. The implications for the design of new oxidants that use low-barrier radical steps for selective multielectron transformations are discussed.
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122
Conry, R. R.; Mayer, J. M. Oxygen Atom Transfer Reactions of Cationic Rhenium(III), Rhenium(V), and Rhenium(VII) Triazacyclononane Complexes. Inorg. Chem. 1990, 29, 4862– 4867, DOI: 10.1021/ic00349a010
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122
Oxygen atom transfer reactions of cationic rhenium(III), rhenium(V), and rhenium(VII) triazacyclononane complexes
Conry, Rebecca R.; Mayer, James M.
Inorganic Chemistry (1990), 29 (24), 4862-7CODEN: INOCAJ; ISSN:0020-1669.
Re(O)Cl3(Me2S)(OPPh3) reacts readily with 1,4,7-trimethyltriazacyclononane (Me3tacn) to form [ReV(O)Cl2(Me3tacn)]+ (I) in good yield. With the unsubstituted triazacyclononane (tacn), however, both [ReV(O)Cl2(tacn)]+ (II) and [ReVII(O)3(tacn)]+ (III) are formed, even under anaerobic conditions. Oxidn. of II to III [Re(V) → Re(VII)] can be easily accomplished with a variety of mild oxidizing agents such as Me2SO and I2, but the oxidn. of I requires over a month at 80° in aq. nitric acid. I is reduced [Re(V) → Re(III)] by oxygen atom transfer to phosphines, forming [ReIII(OPR3)Cl2(Me3tacn)]+ (IV; R = Ph, Me). The OPPh3 ligand in IV is easily displaced by other neutral ligands such as MeCN or Me2CO. [Re(OCMe2)Cl2(Me3tacn)]+ is readily oxidized back to I [Re(III) → Re(V)] by the O atom donors BuNCO, OAsPh3, Me2SO, ethylene oxide, pyridine N-oxide, and N2O. These reactions require an open coordination site at the Re(III) center. Surprisingly, it is not substantially easier to oxidize IV than II. On the basis of these reactions, simple thermochem. cycles are used to est. the Re-oxo bond strength in I to be 141 ± 9 kcal/mol.
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123
Verat, A. Y.; Fan, H.; Pink, M.; Chen, Y.-S.; Caulton, K. G. Spin State, Structure, and Reactivity of Terminal Oxo and Dioxygen Complexes of the (PNP)Rh Moiety. Chem. - Eur. J. 2008, 14, 7680– 7686, DOI: 10.1002/chem.200800573
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123
Spin state, structure, and reactivity of terminal oxo and dioxygen complexes of the (PNP)Rh moiety
Verat, Alexander Y.; Fan, Hongjun; Pink, Maren; Chen, Y.-S.; Caulton, Kenneth G.
Chemistry - A European Journal (2008), 14 (25), 7680-7686CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
[RhIIIH{(tBu2PCH2SiMe2NSiMe2CH2PtBu(CMe2CH2))}], ([RhH(PNP*)]), reacts with O2 in the time taken to mix the reagents to form a 1:1 η2-O2 adduct, for which O-O bond length is discussed with ref. to the reducing power of [RhH(PNP*)]. DFT calcns. faithfully replicate the obsd. O-O distance, and were used to understand the oxidn. state of this coordinated O2. The reactivity of [Rh(O2)(PNP)] towards H2, CO, N2, and O2 is tested and compared to the assocd. DFT reaction energies. Three different reagents effect single O atom transfer to [RhH(PNP*)]. The resulting [RhO(PNP)], characterized at ≥ -60° and by DFT calcns., is a ground-state triplet, is nonplanar, and reacts, ⪆+15°, with its own tBu C-H bond, to cleanly form a diamagnetic complex, [Rh(OH){N(SiMe2CH2PtBu2)(SiMe2CHPtBu{CMe2CH2})}].
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124
Streb, C. New Trends in Polyoxometalate Photoredox Chemistry: From Photosensitization to Water Oxidation Catalysis. Dalton Trans. 2012, 41, 1651– 1659, DOI: 10.1039/C1DT11220A
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124
New trends in polyoxometalate photoredox chemistry: From photosensitisation to water oxidation catalysis
Streb, Carsten
Dalton Transactions (2012), 41 (6), 1651-1659CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
Mol. metal oxide clusters, so-called polyoxometalates (POM) have been extensively used as homogeneous photocatalysts in various photoredox reactions such as the oxidn. of alkanes, alkenes and alcs. as well as the light-induced mineralization of various org. and inorg. pollutants. The more general application of POMs as photoactive compds., in particular in solar energy harnessing, has been hampered as the clusters typically absorb light in the UV-region only. Over the past decade, concepts have been put forward on how the reactivity of this class of compds. can be optimized to improve their overall photoactivity, and a particular focus has been on the design of photocatalytic processes which allow the conversion of solar light into useful chem. reactivity. This perspective gives a brief overview of general aspects of POM photochem. and critically discusses the advantages and challenges of a range of POM-based systems for photooxidns. and photoredns. with a focus on the development of sustainable solar light conversion systems.
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125
(a) Papaconstantinou, E. Photocatalytic Oxidation of Organic Compounds Using Heteropoly Electrolytes of Molybdenum and Tungsten. J. Chem. Soc., Chem. Commun. 1982, 12– 13, DOI: 10.1039/c39820000012
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125a
Photocatalytic oxidation of organic compounds using heteropoly electrolytes of molybdenum and tungsten
Papaconstantinou, E.
Journal of the Chemical Society, Chemical Communications (1982), (1), 12-13CODEN: JCCCAT; ISSN:0022-4936.
Org. compds. are photochem. oxidized in the presence of heteropoly compds., e.g., [PW12O40]3- (I); the heteropoly compds. may be reoxidized, forming the basis of a photocatalytic oxidn. process. E.g., Me2CHOH was oxidized to Me2CO by I in the presence of sunlight; the corresponding redn. of I was reversible on exclusion of light in the presence of O.
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(b) Zhang, Z.; Lin, Q.; Kurunthu, D.; Wu, T.; Zuo, F.; Zheng, S.-T.; Bardeen, C. J.; Bu, X.; Feng, P. Synthesis and Photocatalytic Properties of a New Heteropolyoxoniobate Compound: K₁₀[Nb₂O₂(H₂O)₂][SiNb₁₂O₄₀]·12H₂O. J. Am. Chem. Soc. 2011, 133, 6934– 6937, DOI: 10.1021/ja201670x
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125b
Synthesis and Photocatalytic Properties of a New Heteropolyoxoniobate Compound: K10[Nb2O2(H2O)2][SiNb12O40]·12H2O
Zhang, Zhenyu; Lin, Qipu; Kurunthu, Dharmalingam; Wu, Tao; Zuo, Fan; Zheng, Shou-Tian; Bardeen, Christopher J.; Bu, Xianhui; Feng, Pingyun
Journal of the American Chemical Society (2011), 133 (18), 6934-6937CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The synthesis and photocatalytic properties of a heteropolyoxoniobate, K10[Nb2O2(H2O)2][SiNb12O40]·12H2O (1), are reported, revealing an important role of Zr4+ additives in the crystn. Compd. 1 exhibits overall photocatalytic water splitting activity, and its photocatalytic activity is significantly higher than that of Na10[Nb2O2][SiNb12O40]·xH2O (2). Fluorescence lifetime measurements suggest that the enhanced photocatalytic activity of 1 likely results from a larger yield of longer-lived charge trapping states in 1 due to the coordination of one water mol. to the bridging Nb5+, leading to highly unsym. seven-coordinated Nb5+ sites.
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126
(a) Renneke, R. F.; Hill, C. L. Selective Photochemical Dehydrogenation of Saturated Hydrocarbons with Quantum Yields Approaching Unity. Angew. Chem., Int. Ed. Engl. 1988, 27, 1526– 1527, DOI: 10.1002/anie.198815261
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(b) Hill, C. L.; Bouchard, D. A. Catalytic Photochemical Dehydrogenation of Organic Substrates by Polyoxometalates. J. Am. Chem. Soc. 1985, 107, 5148– 5157, DOI: 10.1021/ja00304a019
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126b
Catalytic photochemical dehydrogenation of organic substrates by polyoxometalates
Hill, Craig L.; Bouchard, Donald A.
Journal of the American Chemical Society (1985), 107 (18), 5148-57CODEN: JACSAT; ISSN:0002-7863.
The photochem. behavior of polyoxometalates (POM) based on W, Mo, V, Nb and Ta in the presence of H2O or 1 of a variety of org. substrates (including alcs., amides, ethers, aldehydes, carboxylic acids, nitriles, ketones and ureas) is examd. Irradn. of the charge-transfer bands of POM dissolved in org. media at 25° leads in most cases to oxidn. of the org. substrate and redn. of the POM. The POM fall into 3 categories defined by their thermal and photochem. redox chem. in the presence of org. substrates. Type I complexes, exemplified by those of Nb and Ta, do not photooxidize any org. substrate upon irradn. Type II complexes, exemplified by decavanadate and most heteropoly- and isopolymolybdates, and Type III complexes, exemplified by most heteropoly- and isopolytungstates, do not oxidize a wide range of org. substrates upon irradn. Reoxidn. of the reduced forms of the Type II complexes, either by reaction with O2 or by evolution of H2, is kinetically or thermodynamically unfavorable; analogous reoxidn. of the reduced forms of the Type III complexes is not. Several factors affecting the quantum yields for prodn. of reduced POM are outlined, and the energetic features regarding H2 evolution are discussed. The IR, UV, and 31P, 183W and 17O NMR spectral properties of α-H3PW12O40.6H2O (I) and other POM remain the same before and after catalytic photochem. dehydrogenation of representative alc., ether or amide substrates. Little if any POM decompn. occurs during the photoredox chem. Interactions between org. substrates and POM have profound effects on the electronic structure of the POM. The charge-transfer transitions of I display different sensitivities to medium in the low-energy (λ >300 nm) vs. high-energy region of the UV-visible spectral range. The highest quantum yields for photoredox chem. involving org. substrates and I are obsd. in the low-energy or absorption-tail region. One possible model explaining the wavelength dependence of the absorption and photochem. action spectra is discussed. A general mechanism in agreement with all the exptl. data is proposed for org. substrate oxidn. and the effective capture of light energy in these POM-org. substrate systems.
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127
Duncan, D. C.; Netzel, T. L.; Hill, C. L. Early-Time Dynamics and Reactivity of Polyoxometalate Excited States. Identification of a Short-Lived LMCT Excited State and a Reactive Long-Lived Charge-Transfer Intermediate following Picosecond Flash Excitation of [W₁₀O₃₂]^4– in Acetonitrile. Inorg. Chem. 1995, 34, 4640– 4646, DOI: 10.1021/ic00122a021
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127
Early-Time Dynamics and Reactivity of Polyoxometalate Excited States. Identification of a Short-Lived LMCT Excited State and a Reactive Long-Lived Charge-Transfer Intermediate following Picosecond Flash Excitation of [W10O32]4- in Acetonitrile
Duncan, Dean C.; Netzel, Thomas L.; Hill, Craig L.
Inorganic Chemistry (1995), 34 (18), 4640-6CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
The authors report picosecond flash excitation results on [W10O32]4-, which demonstrate that the initially prepd. ligand-to-metal charge-transfer (LMCT) excited state decays within ∼30 ps to a single intermediate that persists for >15 ns. Little or no substrate reaction is derived from the short-lived LMCT excited state. Furthermore, the long-lived intermediate is not the 1-electron-reduced species [W10O32]5- or one of its protonated derivs. This long-lived intermediate is the primary photoreactant and has substantial charge-transfer character itself. Addnl. the intermediate and [W10O32]5- are likely to have similar W-orbital electron d.; the principal differences in electronic structure derive from the presence of an oxidized oxygen site in the intermediate which is lacking in [W10O32]5-.
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Hiroto Fujisaki, Tomoya Ishizuka, Yoshihiro Shimoyama, Hiroaki Kotani, Yoshihito Shiota, Kazunari Yoshizawa, Takahiko Kojima. Selective catalytic 2e − -oxidation of organic substrates by an Fe II complex having an N-heterocyclic carbene ligand in water. Chemical Communications 2020, 56 (68) , 9783-9786. https://doi.org/10.1039/D0CC03289A
Erick Ramírez, Md. Kamal Hossain, Marcos Flores-Alamo, Matti Haukka, Ebbe Nordlander, Ivan Castillo. Oxygen Transfer from Trimethylamine N -Oxide to Cu I Complexes Supported by Pentanitrogen Ligands. European Journal of Inorganic Chemistry 2020, 2020 (29) , 2798-2808. https://doi.org/10.1002/ejic.202000488
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Namita Sharma, Yong‐Min Lee, Wonwoo Nam, Shunichi Fukuzumi. Photoinduced Generation of Superoxidants for the Oxidation of Substrates with High C−H Bond Dissociation Energies. ChemPhotoChem 2020, 4 (4) , 271-281. https://doi.org/10.1002/cptc.201900219
Anuvab Das, Gerard Pierre Van Trieste, David C. Powers. . Comments on Inorganic Chemistry 2020,,, 116. https://doi.org/10.1080/02603594.2020.1747054

Abstract
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Figure 1
Figure 1. (a) Thermochemical PCET square scheme for Y and X–H. The horizontal arrows represent electron-transfer (ET) processes, and the vertical ones represent proton-transfer (PT) processes. (b) Schematic representation of genuine HAT from H–X to Y^•. (c) Schematic representation of PCET from H–X to M–L.
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Figure 2
Figure 2. Schematic descriptions of the resonance structures of metal–oxo and metal–oxyl species with their corresponding Lewis structures. A red circle in part a represents a hole on the oxygen ligand.(57)
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Figure 3
Figure 3. Schematic description of the formation of a Zn^II–oxyl species from a Zn^II–η²-O₃^•– species.(58a)
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Figure 4
Figure 4. UV–vis–NIR vibronic absorption spectra of the Zn^II–O^• complex formed in the MFI-type zeolite framework. (a) NIR regions for the Zn–¹⁶O^• (top) and Zn–¹⁸O^• species (bottom). (b) Schematic description of vibronic transitions of the Zn^II–O^• species. The dotted lines in part a represent the individual Gaussian contribution of the corresponding transition. This figure has been provided through a courtesy of Dr. A. Oda [PREST (JST)/Okayama University].
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Figure 5
Figure 5. Optimized structures of Zn^II–oxyl species in the MFI models: in the [Si₉₁Al₁O₁₅₁H₆₆]⁻ (a) and [Si₂Al₁O₄H₈]⁻ (b) frameworks. This figure has been provided through a courtesy of Dr. A. Oda [PREST (JST)/Okayama University].
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Figure 6
Figure 6. Deprotonation of the Ru^III–OH₂ complexes to generate the corresponding Ru^II–O^• species.(59)
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Figure 7
Figure 7. ORTEP drawing of the Ru^II–O^• species Ru^II_DBSQ–O^•.(59a) All hydrogen atoms are omitted for clarity. This figure has been provided through a courtesy of Prof. K. Tanaka and Dr. K. Kobayashi [Kyoto University].
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Figure 8
Figure 8. Plausible mechanism of O–O bond formation in Ru₂SQ proposed by Tanaka and co-workers.(60e)
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Figure 9
Figure 9. Oxidation of hydrocarbons by di- and mononuclear Ru^III–hydroxo–quinone complexes (Ru₂Q and RuQ) in the presence of AgClO₄ and ^tBuOK.(61)
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Figure 10
Figure 10. PCET oxidation of the Ru^II–OH₂ complex to afford a Ru^III–O^• species.(62)
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Figure 11
Figure 11. Oxidation of benzaldehydes by the Ru^II(NHC)–aqua complex using CAN as an oxidant.
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Figure 12
Figure 12. Hammett plots for the oxidation of benzaldehyde derivatives: the Ru^III–O^• complex in water (blue line)(62) and [Ru^IV(O)(bpy)₂(py)]²⁺ in CH₃CN (black line).(65)
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Figure 13
Figure 13. Catalytic oxidative benzene cracking in water using the Ru^II(NHC)–aqua complex (Figure 10) as a catalyst and CAN as an oxidant at 283 K.
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Figure 14
Figure 14. Change of the selectivity in benzene derivatives by the Ru^II–OH₂ complex in Figure 10 in water using CAN as an oxidant.(66)
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Figure 15
Figure 15. Proposed mechanism of oxidative cracking of benzene by the Ru^III–O^• complex.(66)
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Figure 16
Figure 16. (a) Water-adsorbed n-SrTiO₃ surface (O_w, oxygen of adsorbed water; O_h, hydroxide). (b) Surface after photoexcitation. Changes of the electron density are described in yellow for a decrease and in cyan for an increase. Reprinted with permission from (68b). Copyright 2016 Springer Nature Publishing.
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Figure 17
Figure 17. Reaction of a nickel(II) complex with mCPBA to form a Ni^III–O^• complex.(69)
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Figure 18
Figure 18. Ru^III–O^• complex in a larger contribution in the resonance structures.
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Figure 19
Figure 19. Structures of [4,5]⁴⁺ (left), [3,4]⁴⁺ (center), and [3,4]⁴⁺-prime (right).
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Figure 20
Figure 20. Mn^IV–oxyl intermediate in water oxidation by [Mn^II(Py₂NR₂)(H₂O)₂]²⁺.(79)
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Figure 21
Figure 21. Schematic description of a Mn^V–oxo complex reported by Borovik and co-workers.(82)
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Figure 22
Figure 22. Water oxidation by a Co^III–TPA complex via the formation of a dinuclear Co^III–bis(μ-oxyl) intermediate.(90a)
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Figure 23
Figure 23. Proposed mechanism of intramolecular HAT by the transient Co^II–oxyl (Co^II–O^•) species itself.(93)
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Figure 24
Figure 24. Formation of a Co^IV–O^• complex having pentadentate B₂Pz₄Py^2– as a ligand [Ar = p-methylphenyl (p-tolyl)].(95)
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Figure 25
Figure 25. Oxygen-atom insertion into a nickel(II) metallacycle complex to form a Ni^II–alkoxo metallacycle.(96,97)
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Figure 26
Figure 26. (a) Hydrogen-bonding network around the HO^• radical in an intermediate (1IC1 in part a) formed in the LPMO active site. (b) Relative energies calculated by the QM/MM methods (UB3LYP/B2, kcal/mol) for the reaction profile of the Cu^I–H₂O₂ intermediate formed in LPMO in the presence of polysaccharide. Reprinted with permission from (109). Copyright 2018 American Chemical Society.
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Figure 27
Figure 27. Calculated mechanism for arene substituent hydroxylation of the model.(110b)
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Figure 28
Figure 28. Proposed mechanism of H₂O₂ activation and benzene hydroxylation by Cu₂(6-hpa).(111)
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Figure 29
Figure 29. Schematic representation of benzene oxidation to phenol catalyzed by Tp^XCu^I(NCMe).(115)
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Figure 30
Figure 30. Putative Cu^III–oxyl intermediates formed in electrocatalytic water oxidation: (a) [Cu^III(O^•)(TGG)]^2–;(116) (b) [Cu^III(O^•)(pyalk)₂].(117b)
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Figure 31
Figure 31. Diruthenium complexes [3,3]⁴⁺ and [5,5]⁴⁺ (top) and two possible mechanisms of O–O bond formation (bottom).
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Figure 32
Figure 32. Optimized structures of diruthenium complex [5,5]⁴⁺ (antiferromagnetically coupled spin state) in staggered (a) and eclipsed (b) geometry.(118) (c) Formation of a precursor complex having two Ru^IV–O^• moieties triggered by a water molecule.
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Figure 33
Figure 33. Reaction between Re^VI–oxo species Re^VI(O) and a trityl radical.(121)
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Figure 34
Figure 34. Qualitative π-orbital interactions in Re^VI(O). Reprinted with permission from (121). Copyright 2011 American Chemical Society.
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Figure 35
Figure 35. Reaction of a Rh^III–H (Rh–H) species with N₂O in toluene-d₈.(123)
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Figure 36
Figure 36. Water (a) or alcohol (b) oxidation by W^V–O^• species generated from W^VI═O under photoirradiation.
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Figure 37
Figure 37. Selected MOs of [Ru^III(O^•)(BPIm)(bpy)]²⁺ (Figure 10) corresponding to the bonding and antibonding orbitals of the Ru–O bond. Values in parentheses are the MBO contribution from the corresponding MOs. Reprinted with permission from (62). Copyright 2016 Wiley-VCH.
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Figure 38
Figure 38. Electron flow through MO formation to stabilize a metal–oxyl species: (a) strong σ donation from a ligand binding at the trans position to the oxyl ligand; (b) strong π-back bonding to a π-accepting ligand.
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Figure 39
Figure 39. Spectroscopic and structural features and experimental methodologies to prove metal–oxyl species. Some methodologies in parentheses are applicable to the cases where they are effective.
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Figure 40
Figure 40. Proposed and determined structures of spectroscopically and crystallographically characterized metal–oxyl complexes.
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Figure 41
Figure 41. Proposed structures of partially characterized metal–oxyl species.
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Yoshihiro Shimoyama
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Dr. Yoshihiro Shimoyama received his Ph.D. degree in 2019 from the Department of Chemistry at University of Tsukuba, Ibaraki, Japan, under the supervision of Prof. Kojima. He is currently a postdoctorial researcher at the Interdisciplinary Research Center for Catalytic Chemistry, AIST. His current research interests lie in the development of innovative catalytic substrate oxidation and reduction systems to afford useful materials in water.
Takahiko Kojima
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Prof. Takahiko Kojima graduated from the Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, in 1986 and obtained his doctor degree in engineering from Graduate School of Engineering, The University of Tokyo, in 1991 under the supervision of Prof. Masanobu Hidai. After working as a postdoctoral associate in the group of Prof. Lawrence Que, Jr., at University of Minnesota, he joined the Department of Chemistry, Kyushu University, as an assistant professor in 1994. In 2005, he moved to the Department of Materials and Life Sciences, Osaka University, as an associate professor in the group led by Prof. Shunichi Fukuzumi. Since 2008, he has been a professor in the Department of Chemistry, University of Tsukuba. He obtained the Award for Creative Work from Japan Society of Coordination Chemistry in 2018. His research interests include the development of functionality of transition-metal complexes and porphyrin derivatives (especially nonplanar porphyrins) based on redox and photochemical reactions, including PCET and artificial photosynthesis.
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  Brazeau, Brian J.; Austin, Rachel N.; Tarr, Carly; Groves, John T.; Lipscomb, John D.
  Journal of the American Chemical Society (2001), 123 (48), 11831-11837CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Norcarane is a valuable mechanistic probe for enzyme-catalyzed hydrocarbon oxidn. reactions because different products or product distributions result from concerted, radical, and cation based reactions. Sol. methane monooxygenase (sMMO) from Methylosinus trichosporium OB3b catalyzes the oxidn. of norcarane to afford 3-hydroxymethylcyclohexene and 3-cycloheptenol, compds. characteristic of radical and cationic intermediates, resp., in addn. to 2- and 3-norcaranols. Past single turnover transient kinetic studies have identified several optically distinct intermediates from the catalytic cycle of the hydroxylase component of sMMO. Thus, the reaction between norcarane and key reaction intermediates can be directly monitored. The presence of norcarane increases the rate of decay of only one intermediate, the high-valent bis-μ-oxo Fe(IV)2 cluster-contg. species compd. Q, showing that it is responsible for the majority of the oxidn. chem. The observation of products from both radical and cationic intermediates from norcarane oxidn. catalyzed by sMMO is consistent with a mechanism in which an initial substrate radical intermediate is formed by hydrogen atom abstraction. This intermediate then undergoes either oxygen rebound, intramol. rearrangement followed by oxygen rebound, or loss of a second electron to yield a cationic intermediate to which OH- is transferred. The estd. lower limit of 20 ps for the lifetime of the putative radical intermediate is in accord with values detd. from previous studies of sterically hindered sMMO probes.
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4. 4
  (a) Burzlaff, N. I.; Rutledge, P. J.; Clifton, I. J.; Hensgens, C. M. H.; Pickford, M.; Adlington, R. M.; Roach, P. L.; Baldwin, J. E. The Reaction Cycle of Isopenicillin N Synthase Observed by X-ray Diffraction. Nature 1999, 401, 721– 724, DOI: 10.1038/44400
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  4a
  The reaction cycle of isopenicillin N synthase observed by X-ray diffraction
  Burzlaff, Nicolai I.; Rutledge, Peter J.; Clifton, Lan J.; Hensgens, Charles M. H.; Pickford, Michael; Adlington, Robert M.; Roach, Peter L.; Baldwin, Jack E.
  Nature (London) (1999), 401 (6754), 721-724CODEN: NATUAS; ISSN:0028-0836. (Macmillan Magazines)
  Isopenicillin N synthase (IPNS), a non-heme iron-dependent oxidase, catalyzes the biosynthesis of isopenicillin N (IPN), the precursor of all penicillins and cephalosporins. The key steps in this reaction are the two iron-dioxygen-mediated ring closures of the tripeptide δ-(L-α-aminoadipoyl)-L-cysteinyl-D-valine (ACV). It has been proposed that the four-membered β-lactam ring forms initially, assocd. with a highly oxidized iron(IV)-oxo (ferryl) moiety, which subsequently mediates closure of the five-membered thiazolidine ring. Here we describe observation of the IPNS reaction in crystals by X-ray crystallog. IPNS·Fe2+ substrate crystals were grown anaerobically, exposed to high pressures of oxygen to promote reaction and frozen, and their structures were elucidated by X-ray diffraction. Using the natural substrate ACV, this resulted in the IPNS·Fe2+·IPN product complex. With the substrate analog, δ-(L-α-aminoadipoyl)-L-cysteinyl-L-S-methyl-cysteine (ACmC) in the crystal, the reaction cycle was interrupted at the monocyclic stage. These mono- and bicyclic structures support our hypothesis of a two-stage reaction sequence leading to penicillin. Furthermore, the formation of a monocyclic sulfoxide product from ACmC is most simply explained by the interception of a high-valency iron-oxo species.
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  .
  (b) Ge, W.; Clifton, I. J.; Stok, J. E.; Adlington, R. M.; Baldwin, J. E.; Rutledge, P. J. Isopenicillin N Synthase Mediates Thiolate Oxidation to Sulfenate in a Depsipeptide Substrate Analogue: Implications for Oxygen Binding and a Link to Nitrile Hydratase?. J. Am. Chem. Soc. 2008, 130, 10096– 10102, DOI: 10.1021/ja8005397
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  4b
  Isopenicillin N Synthase Mediates Thiolate Oxidation to Sulfenate in a Depsipeptide Substrate Analogue: Implications for Oxygen Binding and a Link to Nitrile Hydratase?
  Ge, Wei; Clifton, Ian J.; Stok, Jeanette E.; Adlington, Robert M.; Baldwin, Jack E.; Rutledge, Peter J.
  Journal of the American Chemical Society (2008), 130 (31), 10096-10102CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Isopenicillin N synthase (IPNS) is a nonheme iron oxidase that catalyzes the central step in the biosynthesis of β-lactam antibiotics: oxidative cyclization of the linear tripeptide δ-L-α-aminoadipoyl-L-cysteinyl-D-valine (ACV) to isopenicillin N (IPN). The ACV analog δ-L-α-aminoadipoyl-L-cysteine (1-(S)-carboxy-2-thiomethyl)ethyl ester (ACOmC) has been synthesized as a mechanistic probe of IPNS catalysis and crystd. with the enzyme. The crystal structure of the anaerobic IPNS/Fe(II)/ACOmC complex was detd. to 1.80 Å resoln., revealing a highly congested active site region. By exposing these anaerobically grown crystals to high-pressure oxygen gas, an unexpected sulfenate product has been obsd., complexed to iron within the IPNS active site. A mechanism is proposed for formation of the sulfenate-iron complex, and it appears that ACOmC follows a different reaction pathway at the earliest stages of its reaction with IPNS. Thus it seems that oxygen (the cosubstrate) binds in a different site to that obsd. in previous studies with IPNS, displacing a water ligand from iron in the process. The iron-mediated conversion of metal-bound thiolate to sulfenate has not previously been obsd. in crystallog. studies with IPNS. This mode of reactivity is of particular interest when considered in the context of another family of nonheme iron enzymes, the nitrile hydratases, in which post-translational oxidn. of two cysteine thiolates to sulfenic and sulfinic acids is essential for enzyme activity.
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5. 5
  Riggs-Gelasco, P. J.; Price, J. C.; Guyer, R. B.; Brehm, J. H.; Barr, E. W.; Bollinger, J. M., Jr.; Krebs, C. EXAFS Spectroscopic Evidence for an Fe═O Unit in the Fe(IV) Intermediate Observed during Oxygen Activation by Taurine: α-Ketoglutarate Dioxygenase. J. Am. Chem. Soc. 2004, 126, 8108– 8109, DOI: 10.1021/ja048255q
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  5
  EXAFS Spectroscopic Evidence for an Fe:O Unit in the Fe(IV) Intermediate Observed during Oxygen Activation by Taurine:α-Ketoglutarate Dioxygenase
  Riggs-Gelasco, Pamela J.; Price, John C.; Guyer, Robert B.; Brehm, Jessica H.; Barr, Eric W.; Bollinger, J. Martin, Jr.; Krebs, Carsten
  Journal of the American Chemical Society (2004), 126 (26), 8108-8109CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The Fe(II)- and α-ketoglutarate-dependent dioxygenases catalyze hydroxylation reactions of considerable biomedical and environmental significance. Recently, the first oxidized iron intermediate in the reaction of a member of this family, taurine:α-ketoglutarate dioxygenase (TauD), was detected and shown to be a high-spin Fe(IV) complex. In this study the authors have used x-ray absorption spectroscopy to demonstrate the presence of a short (1.62 Å) interaction between the iron and one of its ligands in the Fe(IV) intermediate but not in the Fe(II) starting complex. The detection of this interaction strongly corroborates the hypothesis that the intermediate contains an Fe:O structural motif.
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6. 6
  Blasiak, L. C.; Vaillancourt, F. H.; Walsh, C. T.; Drennan, C. L. Crystal Structure of the Non-Haem Iron Halogenase SyrB2 in Syringomycin Biosynthesis. Nature 2006, 440, 368– 371, DOI: 10.1038/nature04544
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  6
  Crystal structure of the non-heme iron halogenase SyrB2 in syringomycin biosynthesis
  Blasiak, Leah C.; Vaillancourt, Frederic H.; Walsh, Christopher T.; Drennan, Catherine L.
  Nature (London, United Kingdom) (2006), 440 (7082), 368-371CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
  Non-heme Fe(II)/α-ketoglutarate (αKG)-dependent enzymes harness the reducing power of αKG to catalyze oxidative reactions, usually the hydroxylation of unactivated C atoms, and are involved in processes such as natural product biosynthesis, the mammalian hypoxic response, and DNA repair. These enzymes couple the decarboxylation of αKG with the formation of a high-energy ferryl-oxo intermediate that acts as a H-abstracting species. All previously structurally characterized mononuclear Fe-enzymes contain a 2-His, 1-carboxylate motif that coordinates the Fe. The 2 His residues and 1 carboxylate moiety, known as the 'facial triad', form one triangular side of an octahedral Fe coordination geometry. A subclass of mononuclear Fe-enzymes has been shown to catalyze halogenation reactions, rather than the more typical hydroxylation reaction. SyrB2, a member of this subclass, is a non-heme Fe(II)/αKG-dependent halogenase that catalyzes the chlorination of threonine in syringomycin E biosynthesis by Pseudomonas syringae pv. syringae B301D. Here, the authors report the crystal structure of SyrB2 with both a Cl- ion and αKG coordinated to Fe at 1.6 Å resoln. This structure reveals a previously unknown coordination of Fe, in which the carboxylate ligand of the facial triad is replaced by a Cl- ion.
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7. 7
  (a) Que, L., Jr. The Road to Non-Heme Oxoferryls and Beyond. Acc. Chem. Res. 2007, 40, 493– 500, DOI: 10.1021/ar700024g
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  7a
  The road to non-heme oxoferryls and beyond
  Que, Lawrence
  Accounts of Chemical Research (2007), 40 (7), 493-500CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
  A review. Oxoiron(IV) species are often implicated in the catalytic cycles of O2-activating non-heme iron enzymes. The paucity of suitable model complexes has stimulated the authors to fill this void, and their synthetic efforts have afforded a no. of oxoiron(IV) complexes. Here, the authors provide a chronol. perspective of the observations that contributed to the generation of the 1st non-heme iron(IV)-oxo complexes in high yield and summarizes their salient properties to date.
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  .
  (b) Nam, W. High-Valent Iron(IV)-Oxo Complexes of Heme and Non-Heme Ligands in Oxygenation Reactions. Acc. Chem. Res. 2007, 40, 522– 531, DOI: 10.1021/ar700027f
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  7b
  High-Valent Iron(IV)-Oxo Complexes of Heme and Non-Heme Ligands in Oxygenation Reactions
  Nam, Wonwoo
  Accounts of Chemical Research (2007), 40 (7), 522-531CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
  A review. High-valent iron(IV)-oxo species have been implicated as the key reactive intermediates in the catalytic cycles of dioxygen activation by heme and non-heme iron enzymes. Our understanding of the enzymic reactions has improved greatly via investigation of spectroscopic and chem. properties of heme and non-heme iron(IV)-oxo complexes. In this Account, reactivities of synthetic iron(IV)-oxo porphyrin π-cation radicals and mononuclear non-heme iron(IV)-oxo complexes in oxygenation reactions have been discussed as chem. models of cytochrome P 450 and non-heme iron enzymes. These results demonstrate how mechanistic developments in biomimetic research can help our understanding of dioxygen activation and oxygen atom transfer reactions in nature.
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  .
  (c) Nam, W.; Lee, Y.-M.; Fukuzumi, S. Tuning Reactivity and Mechanism in Oxidation Reactions by Mononuclear Nonheme Iron(IV)-Oxo Complexes. Acc. Chem. Res. 2014, 47, 1146– 1154, DOI: 10.1021/ar400258p
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  7c
  Tuning reactivity and mechanism in oxidation reactions by mononuclear nonheme iron(IV)-oxo complexes
  Nam, Wonwoo; Lee, Yong-Min; Fukuzumi, Shunichi
  Accounts of Chemical Research (2014), 47 (4), 1146-1154CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
  A review. Mononuclear nonheme iron enzymes generate high-valent Fe(IV)-oxo intermediates that effect metabolically important oxidative transformations in the catalytic cycle of O2 activation. In 2003, researchers 1st spectroscopically characterized a mononuclear nonheme Fe(IV)-oxo intermediate in the reaction of taurine-α-ketoglutarate dioxygenase (TauD). This nonheme Fe-contg. enzyme with a Fe active center was coordinated to a 2-His-1-carboxylate facial triad motif. In the same year, researchers obtained the 1st crystal structure of a mononuclear nonheme Fe(IV)-oxo complex bearing a macrocyclic supporting ligand, [(TMC)FeIV(O)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecene), in studies that mimicked the biol. enzymes. With these breakthrough results, many other studies have examd. mononuclear nonheme Fe(IV)-oxo intermediates trapped in enzymic reactions or synthesized in biomimetic reactions. Over the past decade, researchers in the fields of biol., bioinorg., and oxidn. chem. have extensively investigated the structure, spectroscopy, and reactivity of nonheme Fe(IV)-oxo species, leading to a wealth of information from these enzymic and biomimetic studies. Here, the authors summarize the reactivity and mechanisms of synthetic mononuclear nonheme Fe(IV)-oxo complexes in oxidn. reactions and examines factors that modulate their reactivities and change their reaction mechanisms. The authors focus on several reactions including the oxidn. of org. and inorg. compds., electron transfer, and O atom exchange with water by synthetic mononuclear nonheme Fe(IV)-oxo complexes. In addn., the authors recently obsd. that C-H bond activation by nonheme Fe(IV)-oxo and other nonheme metal(IV)-oxo complexes does not follow the H-atom abstraction/oxygen-rebound mechanism, which has been well-established in heme systems. The structural and electronic effects of supporting ligands on the oxidizing power of Fe(IV)-oxo complexes are significant in these reactions. However, the difference in spin states between nonheme Fe(IV)-oxo complexes with an octahedral geometry (with an S = 1 intermediate-spin state) or a trigonal bipyramidal (TBP) geometry (with an S = 2 high-spin state) does not lead to a significant change in reactivity in biomimetic systems. Thus, the importance of the high-spin state of Fe(IV)-oxo species in nonheme Fe-contg. enzymes remains unexplained. The authors also discuss how the axial and equatorial ligands and binding of redox-inactive metal ions and protons to the Fe-oxo moiety influence the reactivities of the nonheme Fe(IV)-oxo complexes. The authors emphasize how these changes can enhance the oxidizing power of nonheme metal(IV)-oxo complexes in O atom transfer and electron-transfer reactions remarkably. The authors demonstrate great advancements in the understanding of the chem. of mononuclear nonheme Fe(IV)-oxo intermediates within the last 10 yr.
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  .
  (d) Ray, K.; Pfaff, F. F.; Wang, B.; Nam, W. Status of Reactive Non-Heme Metal-Oxygen Intermediates in Chemical and Enzymatic Reactions. J. Am. Chem. Soc. 2014, 136, 13942– 13958, DOI: 10.1021/ja507807v
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  7d
  Status of reactive non-heme metal-oxygen intermediates in chemical and enzymatic reactions
  Ray, Kallol; Pfaff, Florian Felix; Wang, Bin; Nam, Wonwoo
  Journal of the American Chemical Society (2014), 136 (40), 13942-13958CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  A review. Selective functionalization of unactivated C-H bonds, water oxidn., and dioxygen redn. are extremely important reactions in the context of finding energy carriers and conversion processes that are alternatives to the current fossil-based oil for energy. A range of metalloenzymes achieve these challenging tasks in biol. by using cheap and abundant transition metals, such as Fe, Cu, and Mn. High-valent metal-oxo and metal-O2 (superoxo, peroxo, and hydroperoxo) cores act as active intermediates in many of these processes. The generation of well-described model compds. can provide vital insights into the mechanisms of such enzymic reactions. Here, the authors provide a focused rather than comprehensive review of recent advances in the chem. of biomimetic high-valent metal-oxo and metal-O2 complexes, which can be related to an understanding of the biol. systems.
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  .
  (e) Oloo, W. N.; Que, L., Jr. Bioinspired Nonheme Iron Catalysts for C-H and C═C Bond Oxidation: Insights into the Nature of the Metal-Based Oxidants. Acc. Chem. Res. 2015, 48, 2612– 2621, DOI: 10.1021/acs.accounts.5b00053
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  7e
  Bioinspired Nonheme Iron Catalysts for C-H and C=C Bond Oxidation: Insights into the Nature of the Metal-Based Oxidants
  Oloo, Williamson N.; Que, Lawrence, Jr.
  Accounts of Chemical Research (2015), 48 (9), 2612-2621CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
  A review. Recent efforts to design synthetic iron catalysts for the selective and efficient oxidn. of C-H and C=C bonds have been inspired by a versatile family of nonheme iron oxygenases. These bioinspired nonheme (N4)FeII catalysts use H2O2 to oxidize substrates with high regio- and stereoselectivity, unlike in Fenton chem. where highly reactive but unselective hydroxyl radicals are produced. In this Account, we highlight our efforts to shed light on the nature of metastable peroxo intermediates, which we have trapped at -40 °C, in the reactions of the iron catalyst with H2O2 under various conditions and the high-valent species derived therefrom. Under the reaction conditions that originally led to the discovery of this family of catalysts, we have characterized spectroscopically an FeIII-OOH intermediate (EPR gmax = 2.19) that leads to the hydroxylation of substrate C-H bonds or the epoxidn. and cis-dihydroxylation of C=C bonds. Surprisingly, these org. products show incorporation of 18O from H218O, thereby excluding the possibility of a direct attack of the FeIII-OOH intermediate on the substrate. Instead, a water-assisted mechanism is implicated in which water binding to the iron(III) center at a site adjacent to the hydroperoxo ligand promotes heterolytic cleavage of the O-O bond to generate an FeV(O)(OH) oxidant. This mechanism is supported by recent kinetic studies showing that the FeIII-OOH intermediate undergoes exponential decay at a rate enhanced by the addn. of water and retarded by replacement of H2O with D2O, as well as mass spectral evidence for the FeV(O)(OH) species obtained by the Costas group. The nature of the peroxo intermediate changes significantly when the reactions are carried out in the presence of carboxylic acids. Under these conditions, spectroscopic studies support the formation of a (κ2-acylperoxo)iron(III) species (EPR gmax = 2.58) that decays at -40 °C in the absence of substrate to form an oxoiron(IV) byproduct, along with a carboxyl radical that readily loses CO2. The alkyl radical thus formed either reacts with O2 to form benzaldehyde (as in the case of PhCH2COOH) or rebounds with the incipient FeIV(O) moiety to form phenol (as in the case of C6F5COOH). Substrate addn. leads to its 2-e- oxidn. and inhibits these side reactions. The emerging mechanistic picture, supported by DFT calcns. of Wang and Shaik, describes a rather flat reaction landscape in which the (κ2-acylperoxo)iron(III) intermediate undergoes O-O bond homolysis reversibly to form an FeIV(O)(•OC(O)R) species that decays to FeIV(O) and RCO2• or isomerizes to its FeV(O)(O2CR) electromer, which effects substrate oxidn. Another short-lived S = 1/2 species just discovered by Talsi that has much less g-anisotropy (EPR gmax = 2.07) may represent either of these postulated high-valent intermediates.
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8. 8
  (a) Groves, J. T.; Haushalter, R. C.; Nakamura, M.; Nemo, T. E.; Evans, B. J. High-Valent Iron-Porphyrin Complexes Related to Peroxidase and Cytochrome P-450. J. Am. Chem. Soc. 1981, 103, 2884– 2886, DOI: 10.1021/ja00400a075
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  8a
  High-valent iron-porphyrin complexes related to peroxidase and cytochrome P-450
  Groves, John T.; Haushalter, Robert C.; Nakamura, Mikio; Nemo, Thomas E.; Evans, B. J.
  Journal of the American Chemical Society (1981), 103 (10), 2884-6CODEN: JACSAT; ISSN:0002-7863.
  The oxidn. of chloro-5,10,15,20-tetramesitylporphinatoiron(III) (TMPFeCl) with m-chloroperoxybenzoic acid at -78° produced a green intermediate (I). The 1H NMR and Moessbauer spectra of I were consistent with an oxoiron(IV)-porphyrin radical cation structure for I. Treatment of I with Me4NOH or oxidn. of TMPFeCl with iodosylbenzene produced a red compd. (II). The 1H NMR spectrum of II showed a resonance which was assigned to the β-pyrrole H atoms. The magnetic susceptibility and the Moessbauer spectrum of II suggested an Fe(IV) or Fe(V) structure for II. Both I and II reacted with I- to produce TMPFe hydroxide and with olefins to regenerate TMPFeCl and to produce epoxides. O transfer to olefins in the presence of H218O produced epoxide with 99% incorporation of the label.
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  .
  (b) Nam, W.; Choi, S. K.; Lim, M. H.; Rohde, J.-U.; Kim, I.; Kim, J.; Kim, C.; Que, L., Jr. Reversible Formation of Iodosylbenzene-Iron Porphyrin Intermediates in the Reaction of Oxoiron(IV) Porphyrin π-Cation Radicals and Iodobenzene. Angew. Chem., Int. Ed. 2003, 42, 109– 111, DOI: 10.1002/anie.200390036
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  8b
  Reversible formation of iodosylbenzene-iron porphyrin intermediates in the reaction of oxoiron(IV) porphyrin π-cation radicals and iodobenzene
  Nam, Wonwoo; Choi, Sun Kyung; Lim, Mi Hee; Rohde, Jan-Uwe; Kim, Inwoo; Kim, Jinheung; Kim, Cheal; Que, Lawrence, Jr.
  Angewandte Chemie, International Edition (2003), 42 (1), 109-111CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  New [(porph)FeIIIOIPh]+ intermediates are generated in the reaction of oxoiron(IV) porphyrin π-cation radicals [(porph)FeIV:O]+ with PhI, and the electronic nature of iron porphyrin complexes and iodobenzene derivs. markedly influences the equil. between these two forms. These intermediates are converted back to the starting Fe(III) complexes ((porph)FeIII(CF3SO3)) upon addn. of olefins, which are epoxidized.
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  .
  (c) Groves, J. T.; Watanabe, Y. Oxygen Activation by Metalloporphyrins Related to Peroxidase and Cytochrome P-450. Direct Observation of the Oxygen-Oxygen Bond Cleavage Step. J. Am. Chem. Soc. 1986, 108, 7834– 7836, DOI: 10.1021/ja00284a058
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  8c
  Oxygen activation by metalloporphyrins related to peroxidase and cytochrome P-450. Direct observation of the oxygen-oxygen bond cleavage step
  Groves, John T.; Watanabe, Yoshihito
  Journal of the American Chemical Society (1986), 108 (24), 7834-6CODEN: JACSAT; ISSN:0002-7863.
  In this abstr. TMP is 5,10,15,20-tetramesitylporphyrinato ligand. Treatment of Fe(III)TMP(OH) with RC6H4CO3H (I; R = p-NO2) at -46° gave the corresponding RC(O)OOFe(III)TMP (II; R = p-O2NC6H4) which, upon standing in soln., smoothly decompd. to the corresponding oxoiron(IV) porphyrin radical cation [RCO2Fe(O)TMP]+• (III). The 0.5-order dependence of the rate on the concn. of excess I indicated acid-catalyzed O-O bond cleavage. I contg. electron withdrawing groups facilitated the conversion of II to III and the relative rates, at const. acidity, had on LFER in σ with ρ 0.5. The temp. dependence for the conversion of II (R = m-ClC6H4) to III in the presence of excess I (R = m-Cl) showed that the conversion had a very low activation enthalpy and a large neg. activation entropy. The conversion of II to III involves acid catalyzed heterolytic O-O bond cleavage.
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  (d) Yamaguchi, K.; Watanabe, Y.; Morishima, I. Direct Observation of the Push Effect on the Oxygen-Oxygen Bond Cleavage of Acylperoxoiron(III) Porphyrin Complexes. J. Am. Chem. Soc. 1993, 115, 4058– 4065, DOI: 10.1021/ja00063a026
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  8d
  Direct observation of the push effect on the oxygen-oxygen bond cleavage of acylperoxoiron(III) porphyrin complexes
  Yamaguchi, Kazuya; Watanabe, Yoshihito; Morishima, Isao
  Journal of the American Chemical Society (1993), 115 (10), 4058-65CODEN: JACSAT; ISSN:0002-7863.
  The 1st direct observation of the push effect on heterolytic and homolytic O-O bond cleavage steps is reported for ligand dissocn. reactions in acylperoxoiron(III) meso-substituted porphyrin complexes (5). In transformation of 5 to the corresponding oxoferryl (O:FeIV) porphyrin cation radicals (6) in CH2Cl2 at -80°, heterolytic O-O bond cleavage is 1st order. Introduction of electron-donating substituents at the meso-positions of the porphyrin ring facilitates the O-O bond cleavage in 5. Addn. of 1 equiv of imidazole derivs. to a CH2Cl2 soln. of 5 immediately gave an acylperoxoiron(III) porphyrin-imidazole adduct (9). Heterolytic bond cleavage in 9 6 also is 1st order in [9], and was accelerated by the coordination of electron-rich imidazole derivs. However, the push effect on the homolytic O-O bond cleavage reaction was examd. in toluene at -6° to ∼-40°. The homolytic O-O bond cleavage of 9 afforded the imidazole adduct of oxoferryl porphyrin complex when phenylperacetic acid was employed. Homolysis of the O-O bond is enhanced by the imidazole ligation; however, the push effect on homolysis is much less than that on heterolysis. These results explain the biol. use of strong electron-donor ligands in heme enzymes such as peroxidase, cytochrome P 450, and catalase.
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  .
  (e) Fujii, H. Effects of the Electron-Withdrawing Power of Substituents on the Electronic Structure and Reactivity in Oxoiron(IV) Porphyrin π-Cation Radical Complexes. J. Am. Chem. Soc. 1993, 115, 4641– 4648, DOI: 10.1021/ja00064a027
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  8e
  Effects of the electron-withdrawing power of substituents on the electronic structure and reactivity in oxoiron(IV) porphyrin π-cation radical complexes
  Fujii, Hiroshi
  Journal of the American Chemical Society (1993), 115 (11), 4641-8CODEN: JACSAT; ISSN:0002-7863.
  The effects of the electron-withdrawing power of the substituents bound to a porphyrin ring on the electronic structures and the reactivities of oxoiron(IV) porphyrin π-cation radical complexes were studied by using 2,7,12,17-tetramethyl-3,8,13,18-tetraarylporphyrins (I; aryl = mesityl, 2-chloro-6-methylphenyl, 2,6-dichlorophenyl, or 2,4,6-trichlorophenyl) and tetrakis-5,10,15,20-tetraarylporphyrins (II). The electronic structures of oxoiron(IV) porphyrin π-cation radicals were investigated by low-temp. UV-visible absorption spectra and 1H NMR measurements. The absorption spectra features of oxoiron(IV) porphyrin π-cation radicals of I changed with an increase of the electron-withdrawing power of ring substituents, while those of II did not. 1H NMR measurements demonstrated that oxoiron(IV) porphyrin radicals of I have an a1u radical character and that those of II are better described as an a2u radical species. The reactivities of oxygen atoms of oxoiron(IV) porphyrin π-cation radicals were examd. by competitive epoxidn. of cyclohexene by two oxoiron(IV) porphyrin π-cation radicals with different radical orbital occupancies or oxidn. potentials. The reactivity of the O atom of the oxoiron(IV) porphyrin π-cation radical depends on its oxidn. potential and is not affected by the a1u/a2u orbital occupancy.
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9. 9
  (a) Cussó, O.; Ribas, X.; Costas, M. Biologically Inspired Non-Heme Iron-Catalysts for Asymmetric Epoxidation; Design Principles and Perspectives. Chem. Commun. 2015, 51, 14285– 14298, DOI: 10.1039/C5CC05576H
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  9a
  Biologically inspired non-heme iron-catalysts for asymmetric epoxidation; design principles and perspectives
  Cusso, Olaf; Ribas, Xavi; Costas, Miquel
  Chemical Communications (Cambridge, United Kingdom) (2015), 51 (76), 14285-14298CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
  Iron coordination complexes with nitrogen and oxygen donor ligands have long since been known to react with peroxides producing powerful oxidizing species. These compds. can be regarded as simple structural and functional models of the active sites of non-heme iron dependent oxygenases. Research efforts during the last decade have uncovered basic principles and structural coordination chem. motifs that permit us to control the chem. that evolves when these iron complexes react with peroxides, in order to provide powerful metal-based, but at the same time selective, oxidising agents. Oxidn. methodologies with synthetic value are currently emerging from this approach. The current review focuses on asym. epoxidn., a reaction which has large value in synthesis, and where iron/H2O2 based methodologies may represent not only a sustainable choice, but may also expand the scope of state-of-the-art oxidn. methods. Basic principles that underlay catalyst design as well as H2O2 activation are discussed, while limitations and future perspectives are also reviewed.
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  .
  (b) Kleespies, S. T.; Oloo, W. N.; Mukherjee, A.; Que, L., Jr. C-H Bond Cleavage by Bioinspired Nonheme Oxoiron(IV) Complexes, Including Hydroxylation of n-Butane. Inorg. Chem. 2015, 54, 5053– 5064, DOI: 10.1021/ic502786y
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  9b
  C-H Bond Cleavage by Bioinspired Nonheme Oxoiron(IV) Complexes, Including Hydroxylation of n-Butane
  Kleespies, Scott T.; Oloo, Williamson N.; Mukherjee, Anusree; Que, Lawrence, Jr.
  Inorganic Chemistry (2015), 54 (11), 5053-5064CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  The development of efficient and selective hydrocarbon oxidn. processes with low environmental impact remains a major challenge of the 21st century because of the strong and apolar nature of the C-H bond. Naturally occurring iron-contg. metalloenzymes can, however, selectively functionalize strong C-H bonds on substrates under mild and environmentally benign conditions. The key oxidant in a no. of these transformations is postulated to possess an S = 2 FeIV=O unit in a nonheme ligand environment. This oxidant has been trapped and spectroscopically characterized and its reactivity toward C-H bonds demonstrated for several nonheme iron enzyme classes. In order to obtain insight into the structure-activity relationships of these reactive intermediates, over 60 synthetic nonheme FeIV(O) complexes have been prepd. in various labs. and their reactivities investigated. This Forum Article summarizes the current status of efforts in the characterization of the C-H bond cleavage reactivity of synthetic FeIV(O) complexes and provides a snapshot of the current understanding of factors that control this reactivity, such as the properties of the supporting ligands and the spin state of the iron center. In addn., new results on the oxidn. of strong C-H bonds such as those of cyclohexane and n-butane by a putative S = 2 synthetic FeIV(O) species that is generated in situ using dioxygen at ambient conditions are presented.
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10. 10
  (a) Chan, S. L.-F.; Kan, Y.-H.; Yip, K.-L.; Huang, J.-S.; Che, C.-M. Ruthenium Complexes of 1,4,7-Trimethyl-1,4,7-triazacyclononane for Atom and Group Transfer Reations. Coord. Chem. Rev. 2011, 255, 899– 919, DOI: 10.1016/j.ccr.2010.11.026
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  10a
  Ruthenium complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane for atom and group transfer reactions
  Chan, Sharon Lai-Fung; Kan, Yu-He; Yip, Ka-Lai; Huang, Jie-Sheng; Che, Chi-Ming
  Coordination Chemistry Reviews (2011), 255 (7-8), 899-919CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
  A review. With support by macrocyclic tertiary amine ligand 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3tacn), a no. of mononuclear metal-ligand multiple bonded complexes were isolated. Starting with a brief summary of these complexes, the present review focuses on ruthenium-oxo and -imido complexes of Me3tacn. A family of monooxoruthenium(IV) complexes [RuIV(Me3tacn)O(N-N)]2+ (N-N = 2,2'-bipyridines) and a cis-dioxoruthenium(VI) complex cis-[RuVI(Me3tacn)O2(CF3CO2)]+ were isolated, and the structures of [RuIV(Me3tacn)O(bpy)](ClO4)2 (bpy = 2,2'-bipyridine) and cis-[RuVI(Me3tacn)O2(CF3CO2)]ClO4 were detd. by x-ray crystallog. Oxidn. of [RuIII(Me3tacn)(NHTs)2(OH)] (Ts = p-toluenesulfonyl) with Ag+ and electrochem. oxidn. of [RuIII(Me3tacn)(H2L)](ClO4)2 (H3L = α-(1-amino-1-methylethyl)-2-pyridinemethanol) probably generate ruthenium-imido complexes supported by Me3tacn. DFT calcns. on cis-[RuVI(Me3tacn)O2(CF3CO2)]+ and proposed ruthenium-imido complexes were performed. [RuIV(Me3tacn)O(N-N)]2+ are reactive toward alkene epoxidn., and cis-[RuVI(Me3tacn)O2(CF3CO2)]+ efficiently oxidizes various org. substrates including concerted [3 + 2] cycloaddn. reactions with alkynes and alkenes to selectively afford α,β-diketones, cis-diols, or C=C bond cleavage products. Related oxidn. reactions catalyzed by ruthenium Me3tacn complexes include epoxidn. of alkenes, cis-dihydroxylation of alkenes, oxidn. of alkanes, alcs., aldehydes, and arenes, and oxidative cleavage of C≡C, C=C, and C-C bonds, all of which exhibit high selectivity. Ruthenium Me3tacn complexes are also active catalysts for amination of satd. C-H bonds.
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  .
  (b) Yip, W.-P.; Ho, C.-M.; Zhu, N.; Lau, T.-C.; Che, C.-M. Homogeneous [Ru^III(Me₃tacn)Cl₃]-Catalyzed Alkene cis-Dihydroxylation with Aqueous Hydrogen Peroxide. Chem. - Asian J. 2008, 3, 70– 77, DOI: 10.1002/asia.200700237
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  10b
  Homogeneous [RuIII(Me3tacn)Cl3]-catalyzed alkene cis-dihydroxylation with aqueous hydrogen peroxide
  Yip, Wing-Ping; Ho, Chi-Ming; Zhu, Nianyong; Lau, Tai-Chu; Che, Chi-Ming
  Chemistry - An Asian Journal (2008), 3 (1), 70-77CODEN: CAAJBI; ISSN:1861-4728. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A simple and green method that uses [Ru(Me3tacn)Cl3] (1; Me3tacn = N,N',N''-trimethyl-1,4,7-triazacyclononane) as catalyst, aq. H2O2 as the terminal oxidant, and Al2O3 and NaCl as additives is effective in the cis-dihydroxylation of alkenes in aq. tert-butanol. Unfunctionalized alkenes, including cycloalkenes, aliph. alkenes, and styrenes (14 examples) were selectively oxidized to their corresponding cis-diols in 70-96% yields based on substrate conversions of up to 100%. The prepn. of cis-1,2-cycloheptanediol (119 g, 91% yield) and cis-1,2-cyclooctanediol (128 g, 92% yield) from cycloheptene and cyclooctene, resp., on the 1-mol scale can be achieved by scaling up the reaction without modification. Results from Hammett correlation studies on the competitive oxidn. of para-substituted styrenes (ρ = -0.97, R = 0.988) and the detection of the cycloadduct [(Me3tacn)ClRuHO2(C8H14)]+ by ESI-MS for the 1-catalyzed oxidn. of cyclooctene to cis-1,2-cyclooctanediol are similar to those of the stoichiometric oxidn. of alkenes by cis-[(Me3tacn)-(CF3CO2)RuVIO2]+ through [3+2] cycloaddn.
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  .
  (c) Kojima, T.; Matsuo, H.; Matsuda, Y. Catalytic Hydrocarbon Oxygenation by Ruthenium-Pyridylamine Complexes with Alkyl Hydroperoxides: A Mechanistic Insight. Inorg. Chim. Acta 2000, 300–302, 661– 667, DOI: 10.1016/S0020-1693(99)00571-X
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  10c
  Catalytic hydrocarbon oxygenation by ruthenium-pyridylamine complexes with alkyl hydroperoxides: a mechanistic insight
  Kojima, T.; Matsuo, H.; Matsuda, Y.
  Inorganica Chimica Acta (2000), 300-302 (), 661-667CODEN: ICHAA3; ISSN:0020-1693. (Elsevier Science S.A.)
  The use of TBHP (t-Bu hydroperoxide) or CHP (cumene hydroperoxide) with bis-μ-chloro Ru(II) dimers, [RuIICl(L)]2(ClO4)2 (L=tris(2-pyridylmethyl)amine and tris(5-methyl-2-pyridylmethyl)amine), gave catalytic alkane and alkene oxygenation at 40°C. These reactions were found to proceed via a Haber-Weiss-type electron-transfer reaction to generate alkoxo (RO√) and alkylperoxo (ROO√) radicals as reactive species. This electron transfer was mainly governed by the redox potentials of a complex employed as a catalyst. In addn., the environment around a ruthenium center(s) involving steric hindrance due to substituents on the pyridine rings and an intramol. π-π interaction, should be also significant to regulate the reactivity of the catalyst; probably in terms of shielding of the ruthenium center(s) against the approach of peroxides to undergo the electron transfer. For those reactions with alkyl hydroperoxides, O2 generated from peroxide decompn. plays an important role in promoting reactions as a radical chain carrier.
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11. 11
  Huynh, M. H. V.; Meyer, T. J. Proton-Coupled Electron Transfer. Chem. Rev. 2007, 107, 5004– 5064, DOI: 10.1021/cr0500030
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  11
  Proton-Coupled Electron Transfer
  Huynh, My Hang V.; Meyer, Thomas J.
  Chemical Reviews (Washington, DC, United States) (2007), 107 (11), 5004-5064CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review. Proton-Coupled Electron Transfer (PCET) describes reactions in which there is a change in both electron and proton content between reactants and products. It originates from the influence of changes in electron content on acid-base properties and provides a mol.-level basis for energy transduction between proton transfer and electron transfer. A review with 855 refs.
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12. 12
  (a) England, J.; Guo, Y.; Farquhar, E. R.; Young, V. G., Jr.; Münck, E.; Que, L., Jr. The Crystal Structure of a High-Spin Oxoiron(IV) Complex and Characterization of Its Self-Decay Pathway. J. Am. Chem. Soc. 2010, 132, 8635– 8644, DOI: 10.1021/ja100366c
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  12a
  The Crystal Structure of a High-Spin Oxoiron(IV) Complex and Characterization of Its Self-Decay Pathway
  England, Jason; Guo, Yisong; Farquhar, Erik R.; Young, Victor G., Jr.; Munck, Eckard; Que, Lawrence, Jr.
  Journal of the American Chemical Society (2010), 132 (25), 8635-8644CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  [FeIV(O)(TMG3tren)]2+ (1; TMG3tren = 1,1,1-tris{2-[N2-(1,1,3,3-tetramethylguanidino)]ethyl}amine) is a unique example of an isolable synthetic S = 2 oxoiron(IV) complex, which serves as a model for the high-valent oxoiron(IV) intermediates obsd. in nonheme iron enzymes. Congruent with DFT calcns. predicting a more reactive S = 2 oxoiron(IV) center, 1 has a lifetime significantly shorter than those of related S = 1 oxoiron(IV) complexes. The self-decay of 1 exhibits strictly first-order kinetic behavior and is unaffected by solvent deuteration, suggesting an intramol. process. This hypothesis was supported by ESI-MS anal. of the iron products and a significant retardation of self-decay upon use of a perdeuteromethyl TMG3tren isotopomer, d36-1 (KIE = 24 at 25°C). The greatly enhanced thermal stability of d36-1 allowed growth of diffraction quality crystals for which a high-resoln. crystal structure was obtained. This structure showed an Fe=O unit (r = 1.661(2) Å) in the intended trigonal bipyramidal geometry enforced by the sterically bulky tetramethylguanidinyl donors of the tetradentate tripodal TMG3tren ligand. The close proximity of the Me substituents to the oxoiron unit yielded three sym. oriented short C-D···O nonbonded contacts (2.38-2.49 Å), an arrangement that facilitated self-decay by rate-detg. intramol. hydrogen atom abstraction and subsequent formation of a ligand-hydroxylated iron(III) product. EPR and Mossbauer quantification of the various iron products, referenced against those obtained from reaction of 1 with 1,4-cyclohexadiene, allowed formulation of a detailed mechanism for the self-decay process. The soln. of this first crystal structure of a high-spin (S = 2) oxoiron(IV) center represents a fundamental step on the path toward a full understanding of these pivotal biol. intermediates.
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  .
  (b) Klinker, E. J.; Kaizer, J.; Brennessel, W. W.; Woodrum, N. L.; Cramer, C. J.; Que, L., Jr. Structures of Nonheme Oxoiron(IV) Complexes from X-ray Crystallography, NMR Spectroscopy, and DFT Calculations. Angew. Chem., Int. Ed. 2005, 44, 3690– 3694, DOI: 10.1002/anie.200500485
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  12b
  Structures of nonheme oxoiron(IV) complexes from X-ray crystallography, NMR spectroscopy, and DFT calculations
  Klinker, Eric J.; Kaizer, Jozsef; Brennessel, William W.; Woodrum, Nathaniel L.; Cramer, Christopher J.; Que, Lawrence, Jr.
  Angewandte Chemie, International Edition (2005), 44 (24), 3690-3694CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  From a combination of x-ray crystallog., NMR spectroscopy, and DFT calcns., the relative thermal stabilities of two oxoiron(IV) complexes with pentaaza ligands, [FeIV(O)(N4Py)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) and [FeIV(O)(Bn-TPEN)]2+ (Bn-TPEN = N-benzyl-N,N',N'-tris(2-pyridylmethyl)-1,2-diaminoethane) can be ascribed to the no. of pyridine rings that are oriented parallel to the Fe:O bond.
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13. 13
  (a) Mandimutsira, B. S.; Ramdhanie, B.; Todd, R. C.; Wang, H.; Zareba, A. A.; Czernuszewicz, R. S.; Goldberg, D. P. A Stable Manganese(V)-Oxo Corrolazine Complex. J. Am. Chem. Soc. 2002, 124, 15170– 15171, DOI: 10.1021/ja028651d
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  13a
  A Stable Manganese(V)-Oxo Corrolazine Complex
  Mandimutsira, Beaven S.; Ramdhanie, Bobby; Todd, Ryan C.; Wang, Hailin; Zareba, Adelajda A.; Czernuszewicz, Roman S.; Goldberg, David P.
  Journal of the American Chemical Society (2002), 124 (51), 15170-15171CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  I (R = p-tBuC6H4) reacted with Mn(acac)3 to give MnL (H3L = I) which was oxidized to MnO(L). Stable MnO(L) was characterized by resonance Raman spectra. The oxidn. of PPh3 or Me2S by MnO(L) was obsd. with the formation of MnL.
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  .
  (b) Zaragoza, J. P. T.; Siegler, M. A.; Goldberg, D. P. A Reactive Manganese(IV)-Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds. J. Am. Chem. Soc. 2018, 140, 4380– 4390, DOI: 10.1021/jacs.8b00350
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  13b
  A Reactive Manganese(IV)-Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds
  Zaragoza, Jan Paulo T.; Siegler, Maxime A.; Goldberg, David P.
  Journal of the American Chemical Society (2018), 140 (12), 4380-4390CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  High-valent metal-hydroxide species are invoked as crit. intermediates in both catalytic, metal-mediated O2 activation (e.g., by Fe porphyrin in Cytochrome P 450) and O2 prodn. (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear MIV(OH) complexes remain a rarity. Herein the authors describe the synthesis of MnIV(OH)(ttppc) (3) (ttppc = tris(2,4,6-triphenylphenyl) corrole), which was characterized by XRD. The large steric encumbrance of the ttppc ligand allowed for isolation of 3. The complexes MnV(O)(ttppc) (4) and MnIII(H2O)(ttppc) (1·H2O) were also synthesized and structurally characterized, providing Mn complexes related only by the transfer of H atoms. Both 3 and 4 abstr. an H atom from the O-H bond of 2,4-di-tert-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (3 = 90(2)%, 4 = 91(5)%). Complex 3 reacts with 2,4-DTBP with a rate const. of k2 = 2.73(12) × 104 M-1 s-1, which is ∼3 orders of magnitude larger than 4 (k2 = 17.4(1) M-1 s-1). Reaction of 3 with para-substituted 2,6-di-tert-butylphenol derivs. (4-X-2,6-DTBP; X = OMe, Me, tBu, H) gives rate consts. in the range k2 = 510(10)-36(1.4) M-1 s-1 and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, O-H cleavage occurs by a concerted H atom transfer (HAT) mechanism and the MnIV(OH) complex is a much more powerful H atom abstractor than the higher-valent MnV(O) complex, or even some FeIV(O) complexes.
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  .
  (c) Halbach, R. L.; Gygi, D.; Bloch, E.; Anderson, B. L.; Nocera, D. G. Structurally Characterized Terminal Manganese(IV) Oxo Tris(alkoxide) Complex. Chem. Sci. 2018, 9, 4524– 4528, DOI: 10.1039/C8SC01164H
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  13c
  Structurally characterized terminal manganese(IV) oxo tris(alkoxide) complex
  Halbach, Robert L.; Gygi, David; Bloch, Eric D.; Anderson, Bryce L.; Nocera, Daniel G.
  Chemical Science (2018), 9 (19), 4524-4528CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
  A Mn(IV) complex featuring a terminal oxo ligand, [MnIV(O)(ditox)3][K(15-C-5)2] (3; ditox = tBu2MeCO-, 15-C-5 = 15-crown-5-ether) has been isolated and structurally characterized. Treatment of the colorless precursor [MnII(ditox)3][K(15-C-5)2] (2) with iodosobenzene affords 3 as a green free-flowing powder in high yields. The X-ray crystal structure of 3 reveals a pseudotetrahedral geometry about the central Mn, which features a terminal oxo (d(Mn-Oterm = 1.628(2) Å)). EPR spectroscopy, SQUID magnetometry, and Evans method magnetic susceptibility indicate that 3 consists of a high-spin S = 3/2 Mn(IV) metal center. 3 promotes C-H bond activation by a hydrogen atom abstraction. The [MnIV(O)(ditox)3]- furnishes a model for the proposed terminal oxo of the unique manganese of the oxygen evolving complex of photosystem II.
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14. 14
  (a) Qin, K.; Incarvito, C. D.; Rheingold, A. L.; Theopold, K. H. Hydrogen Atom Abstraction by a Chromium(IV) Oxo Complex Derived from O₂. J. Am. Chem. Soc. 2002, 124, 14008– 14009, DOI: 10.1021/ja028382r
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  14a
  Hydrogen Atom Abstraction by a Chromium(IV) Oxo Complex Derived from O2
  Qin, Kun; Incarvito, Christopher D.; Rheingold, Arnold L.; Theopold, Klaus H.
  Journal of the American Chemical Society (2002), 124 (47), 14008-14009CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The Cr(III) hydroxide [TptBu,MeCr(OH)(pz'H)]BARF (1, TptBu,Me = hydrotris(3-tert-butyl-5-methylpyrazolyl)borate, pz'H = 3-tert-butyl-5-methylpyrazole, BARF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) is produced by reaction of [TptBu,MeCr(pz'H)]BARF with [TptBu,MeCr(O2)(pz'H)]BARF or O atom donors ONMe3 or PhIO in Et2O. However, reaction of [TptBu,MeCr(pz'H)]BARF with PhIO in pure CH2Cl2 yields the Cr(IV) oxo complex [TptBu,MeCr(O)(pz'H)]BARF (2). 2 Abstrs. H atoms from org. mols. with weak C-H bonds to form 1. Both 1 and 2 were structurally characterized by x-ray crystallog.
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  .
  (b) Collins, T. J.; Slebodnick, C.; Uffelman, E. S. Chromium(V)-Oxo Complexes of Macrocyclic Tetraamido-N-Ligands Tailored for Highly Oxidized Middle Transition Metal Complexes: A New ¹⁸O-Labeling Reagent and a Structure with Four Nonplanar Amides. Inorg. Chem. 1990, 29, 3433– 3436, DOI: 10.1021/ic00343a030
  [ACS Full Text ], [CAS], Google Scholar
  14b
  Chromium(V)-oxo complexes of macrocyclic tetraamido-N ligands tailored for highly oxidized middle transition metal complexes: a new oxygen-18-labeling reagent and a structure with four nonplanar amides
  Collins, Terrence J.; Slebodnick, Carla; Uffelman, Erich S.
  Inorganic Chemistry (1990), 29 (18), 3433-6CODEN: INOCAJ; ISSN:0020-1669.
  Me4N[CrOL] (LH4 = I) and Me4N[CrOL1] (L1H4 = II) were prepd. and characterized by x-ray crystallog. and IR and EPR spectroscopies. Because exchange of the oxo ligand with H2O is slow, the easily synthesized, stable, cryst. Me2(H18O18O)CCH2CH2C(18O18OH)Me2 was prepd. and used to conveniently synthesized 18O-labeled oxo complexes in high yields. The bonding of the 2 unique oxidn.-resistant macrocyclic tetraamides to Cr is compared. The structural and EPR properties are consistent with a Cr-centered radical in each case and suggest that a Cr(V) oxidn. state assignment is equally appropriate whether the ancillary ligand is the innocent [η4-L]4- or the potentially noninnocent [η4-L1]4-. Both oxo complexes contain nonplanar amide groups. The distortions of [Cr(O)(η4-L)]- are more marked, and it is a unique species in contg. 4 distinctly nonplanar amides. The discovery of these unusual structural parameters expands the class of nonplanar amides arising from ring constraint. Me4N[CrOL] is orthorhombic, space group P212121, Z = 4 whereas Me4N[CrOL1] is monoclinic, space group P21/c, Z = 4.
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  .
  (c) Srinivasan, K.; Kochi, J. K. Synthesis and Molecular Structure of Oxochromium(V) Cations. Coordination with Donor Ligands. Inorg. Chem. 1985, 24, 4671– 4679, DOI: 10.1021/ic00220a049
  [ACS Full Text ], [CAS], Google Scholar
  14c
  Synthesis and molecular structure of oxochromium(V) cations. Coordination with donor ligands
  Srinivasan, K.; Kochi, J. K.
  Inorganic Chemistry (1985), 24 (26), 4671-9CODEN: INOCAJ; ISSN:0020-1669.
  CrOL+ (H2L = bis(salicylidene)ethylenediamine and its 5,5'-dichloro, 7,7'-dimethyl, 7,7'-diphenyl, 8,8,8',8'-tetramethyl, 5,5'-dichloro-8,8,8',8'-tetramethyl-, and 8,8'-benzo derivs.) were prepd. by O-transfer reactions of [CrL(H2O)2]X (X = triflate) with isodosylbenzene or m-ClC6H4CO2OH. X-ray crystallog. anal. of [CrOL]X (H2L = 7,7'-dimethyl deriv.) indicates that the 5-coordinate Cr atom is situated 0.53 Å above the Schiff base (mean) plane and describes a square-pyramidal configuration with the oxo ligand occupying the apical position. Isotopic 18O-substitution leads to a shift in the O:Cr stretching frequency from 1004 to 965 cm-1 in accord with theor. predictions. Similarly the magnetic susceptibility and the well-resolved isotropic ESR spectra reliably reflect the d1 electron configuration of the oxochromium(V) species in CH3CN solns. CrOL+ and various donor ligands such as pyridine N-oxide (Q), Ph3PO, and H2O form 1:1 assocn. complexes, the formation consts. K of which vary from 10-2 to 103 M-1, depending on the donor ligand and the substituent groups located on the Schiff base periphery. X-ray crystallog. detn. of [CrOLQ]X (H2L = 5,5'-dichloro-8,8,8'8'-tetramethyl deriv.) indicates that the donor ligand fills the apical position in CrOL+ to complete the octahedral coordination about Cr. Isotopic 18O-tracer studies of the formation of oxochromium(V) by O atom transfer to the Cr(III) complex are described. [CrOL]X is monoclinic, space group P21/n, with a 16.233(2), b 6.439(1), c 19.523(4) Å, β 94.44(1)°, Z = 4. [CrOLQ]X is tetragonal, space group P43212, with a 11.938(1), c 43.366(9) Å, Z = 8. The cyclic voltammetry of the oxochromium(V) cations is described.
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15. 15
  (a) Cundari, T. R.; Saunders, L.; Sisterhen, L. L. Molecular Modeling of Vanadium-Oxo Complexes. A Comparison of Quantum and Classical Methods. J. Phys. Chem. A 1998, 102, 997– 1004, DOI: 10.1021/jp972827u
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  15a
  Molecular Modeling of Vanadium-Oxo Complexes. A Comparison of Quantum and Classical Methods
  Cundari, Thomas R.; Saunders, Leah; Sisterhen, Laura L.
  Journal of Physical Chemistry A (1998), 102 (6), 997-1004CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  A force field for vanadium-oxos was developed and tested with a variety of complexes with coordination nos. of 5 or 6 and formal oxidns. states of +4 or +5 on the metal. Similarly, a semiempirical quantum mech. method for transition metals was extended to vanadium. In this research soft and hard ligands were studied, as were ligands coordinated through single, multiple, and dative bonds. Despite the diversity of vanadium coordination chem., generally good modeling is achieved in a fraction of the time with less computational resources using mol. mechanics and semiempirical quantum mechanics. The L4V4+O and L5V5+O groups were emphasized given their prevalence and importance. In general, the predictive ability was superior for the former structural motif. The combination of mol. mechanics and semiempirical quantum calcns. provide an effective and efficient tool for anal. of the steric and electronic energy differences between isomers.
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  .
  (b) Mchiri, C.; Amiri, N.; Jabli, S.; Roisnel, T.; Nasri, H. The (oxo)[2,3,7,8,12,13,17,18-octachloro-5,10,15,20-tetrakis(4-tolylporphyrinato)]vanadium(IV): Synthesis, UV-Visible, Cyclic Voltammetry and X-ray Crystal Structure. J. Mol. Struct. 2018, 1154, 51– 58, DOI: 10.1016/j.molstruc.2017.10.032
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  15b
  The (oxo)[(2,3,7,8,12,13,17,18-octachloro)-5,10,15,20-tetrakis(4-tolylporphyrinato)]vanadium(IV): Synthesis, UV-visible, Cyclic voltammetry and X-ray crystal structure
  Mchiri, Chadlia; Amiri, Nesrine; Jabli, Souhir; Roisnel, Thierry; Nasri, Habib
  Journal of Molecular Structure (2018), 1154 (), 51-58CODEN: JMOSB4; ISSN:0022-2860. (Elsevier B.V.)
  The present work is concerned with the oxo V(IV) complex of 2,3,7,8,12,13,17,18-octachloro-5,10,15,20-tetrakis(4-tolylporphyrin) [V(Cl8TTP)O] (1), which was prepd. by reacting the (oxo)[5,10,15,20-tetrakis(4-tolylporphyrinato)]vanadium(IV) complex ([V(TTP)O]), under aerobic atm., with a large excess of thionyl chloride (SOCl2). The title compd. was characterized by UV-visible spectroscopy, cyclic voltammetry and x-ray crystal structure. The electron-withdrawing Cl substituents at the pyrrole carbons in the vanadyl-Cl8TTP deriv. produce remarkable red shifts of the Soret and Q absorption bands and an important anodic shift of the porphyrin ring oxidn. and redn. potentials. This is an indication that the porphyrin core of 1 is severely nonplanar in soln. The mol. structure of the vanadyl deriv. shows a very high saddle distortion and an important ruffled deformation of the porphyrin macrocycle. The crystal structure of 1 consists of 1-dimensional chains parallel to the c axis where channels are located between these chains.
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  .
  (c) Abernethy, C. D.; Codd, G. M.; Spicer, M. D.; Taylor, M. K. A Highly Stable N-Heterocyclic Carbene Complex of Trichloro-oxo-vanadium(V) Displaying Novel Cl-C_carbene Bonding Interactions. J. Am. Chem. Soc. 2003, 125, 1128– 1129, DOI: 10.1021/ja0276321
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  15c
  A Highly Stable N-Heterocyclic Carbene Complex of Trichloro-oxo-vanadium(V) Displaying Novel Cl-Ccarbene Bonding Interactions
  Abernethy, Colin D.; Codd, Gareth M.; Spicer, Mark D.; Taylor, Michelle K.
  Journal of the American Chemical Society (2003), 125 (5), 1128-1129CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Reaction of 1,3-dimesitylimidazol-2-ylidene and trichloro-oxo-vanadium(V) yields an air stable 1:1 adduct, which demonstrates the utility of N-heterocyclic carbenes to stabilize metal complexes in high oxidn. states. The stabilizing influence of the carbene ligand was further demonstrated electrochem. The mol. structure of this compd. reveals that the chloride ligands cis to the carbene are oriented toward the Ccarbene atom. D. functional theory calcns. on a hypothetical dimethyl- deriv. show that a bonding interaction occurs between lone pairs of these chlorides and the formally unoccupied p-orbital of the carbene. Previous studies indicated that this orbital was not involved in the bonding of N-heterocyclic carbenes to transition metals. The obsd. interaction therefore represents a new bonding mode for these widely used ligands.
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16. 16
  (a) Kojima, T.; Nakayama, K.; Ikemura, K.; Ogura, T.; Fukuzumi, S. Formation of a Ruthenium(IV)-Oxo Complex by Electron-Transfer Oxidation of a Coordinatively Saturated Ruthenium(II) Complex and Detection of Oxygen-Rebound Intermediates in C-H Bond Oxygenation. J. Am. Chem. Soc. 2011, 133, 11692– 11700, DOI: 10.1021/ja2037645
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  16a
  Formation of a Ruthenium(IV)-Oxo Complex by Electron-Transfer Oxidation of a Coordinatively Saturated Ruthenium(II) Complex and Detection of Oxygen-Rebound Intermediates in C-H Bond Oxygenation
  Kojima, Takahiko; Nakayama, Kazuya; Ikemura, Ken-Ichiro; Ogura, Takashi; Fukuzumi, Shun-Ichi
  Journal of the American Chemical Society (2011), 133 (30), 11692-11700CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  A coordinatively satd. ruthenium(II) complex having tetradentate tris(2-pyridylmethyl)amine (TPA) and bidentate 2,2'-bipyridine (bpy), [Ru(TPA)(bpy)]2+ (1), was oxidized by a Ce(IV) ion in H2O to afford a Ru(IV)-oxo complex, [Ru(O)(H+TPA)(bpy)]3+ (2). The crystal structure of the Ru(IV)-oxo complex 2 was detd. by X-ray crystallog. In 2, the TPA ligand partially dissocs. to be in a facial tridentate fashion and the uncoordinated pyridine moiety is protonated. The spin state of 2, which showed paramagnetically shifted NMR signals in the range of 60 to -20 ppm, was detd. to be an intermediate spin (S = 1) by the Evans' method with 1H NMR spectroscopy in acetone-d6. The reaction of 2 with various org. substrates in acetonitrile at room temp. afforded oxidized and oxygenated products and a solvent-bound complex, [Ru(H+TPA)(bpy)(CH3CN)], which is intact in the presence of alcs. The oxygenation reaction of satd. C-H bonds with 2 proceeds by two-step processes: the hydrogen abstraction with 2, followed by the dissocn. of the alc. products from the oxygen-rebound complexes, Ru(III)-alkoxo complexes, which were successfully detected by ESI-MS spectrometry. The kinetic isotope effects in the first step for the reaction of dihydroanthrathene (DHA) and cumene with 2 were detd. to be 49 and 12, resp. The second-order rate consts. of C-H oxygenation in the first step exhibited a linear correlation with bond dissocn. energies of the C-H bond cleavage.
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  .
  (b) Fackler, N. L. P.; Zhang, S.; O’Halloran, T. V. Stabilization of High-Valent Terminal-Oxo Complexes: Interplay of d-Orbital Occupancy and Coordination Geometry. J. Am. Chem. Soc. 1996, 118, 481– 482, DOI: 10.1021/ja953051i
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  16b
  Stabilization of High-Valent Oxo-Terminal Complexes: Interplay of d-Orbital Occupancy and Coordination Geometry
  Fackler, Nathanael L. P.; Zhang, Songsheng; O'Halloran, Thomas V.
  Journal of the American Chemical Society (1996), 118 (2), 481-2CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Pr4N[RuO(PHAB)] (I) (H4PHAB = 1,2-bis(2,2-diphenyl-2-hydroxyethanamido)benzene) was prepd. from RuO4- and H4PHAB and was oxidized by CeIV to [RuO(PHAB)] (II). The structures of I and II·Me2CO are reported. II represents the 1st structurally characterized mono-oxo RuVI complex. Comparison of I and II show that the preferred coordination geometry depends strongly on the formal occupancy of the metal-oxo π* orbitals. The stability of these complexes is estd. from reactivity comparisons. I and II facilitate C-H bond activation and O atom transfer reactions, I catalyzing the air oxidn. of PPh3. These results have important implications for the design of ligands that stabilize specific intermediates in catalytic reactions.
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  .
  (c) Che, C.-M.; Wong, K.-Y.; Mak, T. C. W. Oxo-Ruthenium(V) Complexes of Macrocyclic Tetradentate Tertiary Amines That Function as Active Electrochemical Oxidative Catalysts, and X-ray Crystal Structure of trans-[Ru^IV(tmc)O(Cl)]ClO₄ (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane). J. Chem. Soc., Chem. Commun. 1985, 988– 990, DOI: 10.1039/c39850000988
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  16c
  Oxoruthenium(V) complexes of macrocyclic tetradentate tertiary amines that function as active electrochemical oxidative catalysts, and x-ray crystal structure of trans-[Ru(IV)(tmc)O(Cl)]ClO4 (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane)
  Che, Chi Ming; Wong, Kwok Yin; Mak, Thomas C. W.
  Journal of the Chemical Society, Chemical Communications (1985), (14), 988-90CODEN: JCCCAT; ISSN:0022-4936.
  trans-[Ru(IV)LO(Cl)]ClO4 (L = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (tmc), 1,4,8,12-tetramethyl-1,4,8,12-tetraazacyclopentadecane) were prepd. from trans-[Ru(VI)LO2](ClO4)2 suspended in acetone and an excess of PPh3. trans-Ru(V)LO(Cl)]2+, generated electrochem. from the corresponding trans-[Ru(IV)LO(Cl)]+ in MeCN contg. 1% C6H5CH2OH, catalyzed the in situ oxidn. of C6H5CH2OH to C6H5CHO. The structure of trans-[Ru(IV)(tmc)O(Cl)]ClO4 was detd. by x-ray crystallog.; crystals are orthorhombic, space group Pna21, with a 12.254(4), b 15.470(4), c 10.821(2) Å, d.(exptl.) = 1.63, d.(calcd.) = 1.646 g cm-3, and R = 0.077 for 1697 reflections.
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17. 17
  (a) Visentin, R.; Rossin, R.; Giron, M. C.; Dolmella, A.; Bandoli, G.; Mazzi, U. Synthesis and Characterization of Rhenium(V) Oxo Complexes with N-[N-(3-Diphenylphosphinopropionyl)glycyl]cysteine Methyl Ester. X-ray Crystal Structure of {ReO[Ph₂P(CH₂)₂C(O)-Gly-Cys-OMe(P, N, N, S)]}. Inorg. Chem. 2003, 42, 950– 959, DOI: 10.1021/ic025859r
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  17a
  Synthesis and Characterization of Rhenium(V) Oxo Complexes with N-[N-(3-Diphenylphosphinopropionyl)glycyl]cysteine Methyl Ester. X-ray Crystal Structure of {ReO[Ph2P(CH2)2C(O)-Gly-Cys-OMe(P,N,N,S)]}
  Visentin, Roberta; Rossin, Raffaella; Giron, Maria Cecilia; Dolmella, Alessandro; Bandoli, Giuliano; Mazzi, Ulderico
  Inorganic Chemistry (2003), 42 (4), 950-959CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  The PN2S chelate N-[N-(3-diphenylphosphinopropionyl)glycyl]-S-tritylcysteine Me ester [PN2S(Trt)-OMe] was synthesized and reacted with ReOCl3(PPh3)2 and Ph4P[ReOCl4]. The reactions of both tritylated and detritylated ligands with ReVO precursors gave two positional isomers, 9a and 9b, of the ReO(PN2S-OMe) complex. The two isomers, produced in a 1:1 molar ratio, are stable and do not interconvert. They were sepd. by reverse-phase HPLC and characterized by NMR, FTIR, and UV-visible spectroscopy and electrospray mass spectrometry. X-ray anal. established for 9a the presence in the solid of the syn isomer. Compd. 9a, C21H23N2O5PSRe, crystd. from warm MeCN in the triclinic space group P‾1, a 9.828(2), b 11.163(2), c 11.641(2) Å, α 106.48(3), β 109.06(3), γ 102.81(3)°, Z = 2. The PN2S coordination set is in the equatorial plane, and the complex shows a distorted square pyramidal coordination. The anti configuration assigned to 9b is consistent with all the available physicochem. data. Follow-up of the reaction of the detritylated ligand with Ph4P[ReOCl4] in EtOH or MeCN indicated that the P atom of the chelate binds first to the metal and that this bond acts as the driving force for coordination.
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  .
  (b) Most, K.; Köpke, S.; Dall’Antonia, F.; Mösch-Zanetti, N. C. The First Molybdenum Dioxo Compounds with η²-Pyrazolate Ligands: Crystal Structure and Oxo Transfer Properties. Chem. Commun. 2002, 1676– 1677, DOI: 10.1039/B205420E
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  17b
  The first molybdenum dioxo compounds with η2-pyrazolate ligands: crystal structure and oxo transfer properties
  Most, Kerstin; Koepke, Sinje; Dall'Antonia, Fabio; Moesch-Zanetti, Nadia C.
  Chemical Communications (Cambridge, United Kingdom) (2002), (16), 1676-1677CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
  Mo dioxo compds. [MoO2Cl(η2-pz)] and [MoO2(η2-pz)2] with pz = η2-3,5-di-tert-butylpyrazolate were synthesized; crystallog. data, catalytic activity, and oxo transfer properties are described.
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  .
  (b1) Karunadasa, H. I.; Chang, C. J.; Long, J. R. A Molecular Molybdenum-Oxo Catalyst for Generating Hydrogen from Water. Nature 2010, 464, 1329– 1333, DOI: 10.1038/nature08969
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  17b1
  A molecular molybdenum-oxo catalyst for generating hydrogen from water
  Karunadasa, Hemamala I.; Chang, Christopher J.; Long, Jeffrey R.
  Nature (London, United Kingdom) (2010), 464 (7293), 1329-1333CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
  A growing awareness of issues related to anthropogenic climate change and an increase in global energy demand have made the search for viable C-neutral sources of renewable energy one of the most important challenges in science today. The chem. community is therefore seeking efficient and inexpensive catalysts that can produce large quantities of hydrogen gas from H2O. Here the authors identify a Mo-oxo complex that can catalytically generate gaseous hydrogen either from H2O at neutral pH or from sea water. High-valency metal-oxo species can be used to create redn. catalysts that are robust and functional in H2O, a concept that has broad implications for the design of green' and sustainable chem. cycles.
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  .
  (c) Rayati, S.; Rafiee, N.; Wojtczak, A. cis-Dioxo-molybdenum(VI) Schiff Base Complexes: Synthesis, Crystal Structure and Catalytic Performance for Homogeneous Oxidation of Olefins. Inorg. Chim. Acta 2012, 386, 27– 35, DOI: 10.1016/j.ica.2012.02.005
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  17c
  cis-Dioxo-molybdenum(VI) Schiff base complexes: Synthesis, crystal structure and catalytic performance for homogeneous oxidation of olefins
  Rayati, Saeed; Rafiee, Nasim; Wojtczak, Andrzej
  Inorganica Chimica Acta (2012), 386 (), 27-35CODEN: ICHAA3; ISSN:0020-1693. (Elsevier B.V.)
  The synthesis of two Mo(VI) tetradentate Schiff base complexes derived from 2,2'-dimethylpropylenediamine and 2-hydroxy-1-naphthaldehyde (hnaphnptnH2 = (2-HOC10H6CH:NCH2)2CMe2) or 3-methoxysalicylaldehyde (salnptn(3-OMe)2H2 = (2-HO-3-MeOC6H3CH:NCH2)2CMe2), (MoO2{hnaphnptn} (1) and MoO2{salnptn(3-OMe)2} (2)) is reported. Full characterization of these complexes was accomplished with elemental analyses, spectroscopic studies (1H NMR, IR, and UV-visible) and x-ray structure anal. X-ray crystallog. studies reveal that these complexes adopt a distorted octahedral six-coordinate configuration formed by tetradentate Schiff base ligand and two O atoms. Catalytic performance of the prepd. Mo complexes for oxidn. of different olefins with tert-Bu hydroperoxide was evaluated. These complexes are efficient and selective catalysts for the homogeneous oxidn. of various olefins. MoO2{salnptn(3-OMe)2} (2) with a methoxy groups on the salicylidene ring of the ligand promotes the effectiveness of the catalyst.
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18. 18
  (a) Dinda, S.; Drew, M. G. B.; Bhattacharyya, R. Oxo-Rhenium(V) Complexes with Bidentate Phosphine Ligands: Synthesis, Crystal Structure and Catalytic Potentiality in Epoxidation of Olefins Using Hydrogen Peroxide Activated Bicarbonate as Oxidant. Catal. Commun. 2009, 10, 720– 724, DOI: 10.1016/j.catcom.2008.11.028
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  18a
  Oxo-rhenium(V) complexes with bidentate phosphine ligands: Synthesis, crystal structure and catalytic potentiality in epoxidation of olefins using hydrogen peroxide activated bicarbonate as oxidant
  Dinda, Subhajit; Drew, Michael G. B.; Bhattacharyya, Ramgopal
  Catalysis Communications (2009), 10 (5), 720-724CODEN: CCAOAC; ISSN:1566-7367. (Elsevier B.V.)
  Two oxorhenium(V) complexes with bidentate phosphine ligands were synthesized and isolated as [ReOCl3(dppm)] and [ReOCl3(dppp)] [dppm = 1,1-bis(diphenylphosphino)methane; dppp = 1,3-bis(diphenylphosphino)propane]. The dppp complex was structurally characterized. Both the complexes were used as catalysts in the epoxidn. of olefins using NaHCO3 as co-catalyst and H2O2 as terminal oxidant.
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19. 19
  (a) Galas, A. M. R.; Hursthouse, M. B.; Behrman, E. J.; Midden, W. R.; Green, G.; Griffith, W. P. The X-ray Crystal Structures of the Oxo-Osmium Complexes, OsO₂(OH)₂phen (1) and Os₂O₆py₄ (2). Transition Met. Chem. 1981, 6, 194– 195, DOI: 10.1007/BF00624344
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  19a
  The x-ray crystal structures of the oxoosmium complexes, OsO2(OH)2phen (1) and Os2O6py4 (2)
  Galas, Anita M. R.; Hursthouse, Michael B.; Behrman, E. J.; Midden, W. R.; Green, G.; Griffith, William P.
  Transition Metal Chemistry (Dordrecht, Netherlands) (1981), 6 (3), 194-5CODEN: TMCHDN; ISSN:0340-4285.
  Os O2(OH)2 phen.5H2O (I) is monoclinic, space group P21/n, with a 8.125(2), b 18.259(2), c 11.197(1) Å, and β 90.18(1)°; d.(calcd.) = 2.10 for Z = 4. Os2O6Py4.6H2O (II) is triclinic, space group P‾1, with a 8.238(10), b 11.701(6), c 8.499(5) Å, α 109.91(5), β 110.54(8), and γ 67.74(7)°; d.(calcd.) = 2.28 for Z = 1. The structures were refined to final R's = 0.032 and 0.081, resp. The Os atoms in both I and II have octahedral coordination. In II the bridging O's occupy equatorial positions with an Os-Os bond of 3.018(2) Å. Raman spectral data are given and discussed based on the structure.
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  .
  (b) Bailey, A. J.; Bhowon, M. G.; Griffith, W. P.; Shoair, A. G.; White, A. J. P.; Williams, D. J. Oxo Osmium(VIII) Complexes in Oxidation: Crystal Structures of OsO₄·nmo (nmo = N-Methylmorpholine N-Oxode) and OsO₄·nmm (nmm = N-Methylmorpholine), and Use of cis-[OsO₄(OH)₂]^2– as an Oxidation Catalyst. J. Chem. Soc., Dalton Trans. 1997, 3245– 3250, DOI: 10.1039/a702965i
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  19b
  Oxo osmium(VIII) complexes in oxidation: crystal structures of OsO4·nmo (nmo = N-methylmorpholine N-oxide) and OsO4·nmm (nmm = N-methylmorpholine), and use of cis-[OsO4(OH)2]2- as an oxidation catalyst
  Bailey, Alan J.; Bhowon, Minu G.; Griffith, William P.; Shoair, Abdel G. F.; White, Andrew J. P.; Williams, David J.
  Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1997), (18), 3245-3250CODEN: JCDTBI; ISSN:0300-9246. (Royal Society of Chemistry)
  The new complexes OsO4·nmo (nmo = N-methylmorpholine N-oxide) and OsO4·nmm (nmm = N-methylmorpholine) were made, their crystal structures detd., and their possible involvement in the catalyzed dihydroxylation of alkenes considered. The use of cis-[OsO4(OH)2]2- as a catalyst for the oxidn. of alcs., aldehydes and alkyl halides to carboxylic acids with [Fe(CN)6]3- and other cooxidants and also for the cleavage and dihydroxylation of alkenes with [Fe(CN)6]3- was investigated.
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  .
  (c) Barthazy, P.; Wörle, M.; Rüegger, H.; Mezzetti, A. Oxo Complexes of Osmium(IV) Formed via Dioxygen Activation. X-ray Structures of [OsX(dcpe)₂]PF₆ (X = Cl, Br), [OsCl(η-O₂)(dcpe)₂]BPh₄, and [OsCl(O)(dcpe)₂]BPh₄ (dcpe = 1,2-Bis(dicyclohexylphosphino)ethane). Inorg. Chem. 2000, 39, 4903– 4912, DOI: 10.1021/ic0002420
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  19c
  Oxo Complexes of Osmium(IV) Formed via Dioxygen Activation. X-ray Structures of [OsX(dcpe)2]PF6 (X = Cl, Br), [OsCl(η2-O2)(dcpe)2]BPh4, and [OsCl(O)(dcpe)2]BPh4 (dcpe = 1,2-Bis(dicyclohexylphosphino)ethane)
  Barthazy, Peter; Woerle, Michael; Rueegger, Heinz; Mezzetti, Antonio
  Inorganic Chemistry (2000), 39 (21), 4903-4912CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  Dioxygen addn. to the 16-electron complexes [OsX(P-P)2]+ gives the dioxygen adducts [OsCl(η2-O2)(P-P)2]+ (4), which in turn react with HCl gas to give the novel Os(IV) oxo complexes trans-[OsX(O)(P-P)2]+ (5) (X = Cl, Br; P-P = 1,2-bis(dicyclohexylphosphino)ethane (dcpe), 1,2-bis(diethylphosphino)ethane (depe), 1,2-bis((2R,5R)-2,5-dimethylphospholano)benzene (Me-duphos)). [OsX(dcpe)2]+ (X = Cl, Br) were studied by x-ray crystallog. and have a Y-shaped coordination geometry in the equatorial plane. The x-ray structural anal. of [OsCl(η2-O2)(dcpe)2]+ (4a) reveals an exceptionally short O-O bond (1.315(5) Å). Trans-[OsCl(O)(dcpe)2]+ (5a), the 1st oxo complex of Os(IV) studied crystallog., exhibits a long Os-O distance of 1.834(3) Å. The reactivity of 4 and 5 as oxidants is described. The dioxygen complex 4a transfers one O atom to PPh3 (to give Ph3PO) or oxidizes iodide ions to triiodide ions in the presence of anhyd. HCl. In both reactions, the corresponding oxo species 5a is quant. formed as the only metal-contg. product. Oxo complexes 5 are surprisingly stable and unreactive toward std. reducing agents such as phosphines.
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  .
  (d) Liu, Y.; Ng, S.-M.; Lam, W. W. Y.; Yiu, S.-M.; Lau, T.-C. A Highly Reactive Seven-Coordinate Osmium(V) Oxo Complex: [Os^V(O)(qpy)(pic)Cl]²⁺. Angew. Chem., Int. Ed. 2016, 55, 288– 291, DOI: 10.1002/anie.201507933
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  19d
  A Highly Reactive Seven-Coordinate Osmium(V) Oxo Complex: [OsV(O)(qpy)(pic)Cl]2+
  Liu, Yingying; Ng, Siu-Mui; Lam, William W. Y.; Yiu, Shek-Man; Lau, Tai-Chu
  Angewandte Chemie, International Edition (2016), 55 (1), 288-291CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Seven-coordinate ruthenium oxo species are proposed as active intermediates in catalytic water oxidn. by a no. of highly active ruthenium catalysts, however such species have yet to be isolated. Reported herein is the 1st example of a seven-coordinate group 8 metal-oxo species, [OsV(O)(qpy)(pic)Cl]2+ (qpy = 2,2':6',2'':6'',2'''-quaterpyridine, pic = 4-picoline). The x-ray crystal structure of this complex shows that it has a distorted pentagonal bipyramidal geometry with an Os=O distance of 1.7375 Å. This oxo species undergoes facile O-atom and H-atom-transfer reactions with various org. substrates. Notably it can abstr. H atoms from alkylaroms. with C-H bond dissocn. energy ≤90 kcal mol-1. Probably highly active oxidants are designed based on Group 8 seven-coordinate metal oxo species.
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20. 20
  (a) Mondal, B.; Neese, F.; Bill, E.; Ye, S. Electronic Structure Contributions of Non-Heme Oxo-Iron(V) Complexes to the Reactivity. J. Am. Chem. Soc. 2018, 140, 9531– 9544, DOI: 10.1021/jacs.8b04275
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  20a
  Electronic Structure Contributions of Non-Heme Oxo-Iron(V) Complexes to the Reactivity
  Mondal, Bhaskar; Neese, Frank; Bill, Eckhard; Ye, Shengfa
  Journal of the American Chemical Society (2018), 140 (30), 9531-9544CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Oxo-iron(V) species have been implicated in the catalytic cycle of the Rieske dioxygenase. Its synthetic analog, [FeV(O)(OC(O)CH3)(PyNMe3)]2+ (1, PyNMe3 = 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3,6,9- trimethyl), derived from the O-O bond cleavage of its acetylperoxo iron(III) precursor, has been shown exptl. to perform regio- and stereo-selective C-H and C=C bond functionalization. However, its structure-activity relation is poorly understood. Herein we present a detailed electronic-structure and spectroscopic anal. of complex 1 along with well-characterized oxo-iron(V) complexes, [FeV(O)(TAML)]- (2, TAML = tetraamido macrocyclic ligand), [FeV(O)(TMC)(NC(O)CH3)]+ (4, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and [FeV(O)(TMC)(NC(OH)CH3)]2+ (4-H+) using wavefunction-based multireference complete active-space SCF calcns. Our results reveal that the x/y anisotropy of the 57Fe A-matrix is not a reliable spectroscopic marker to identify oxo-iron(V) species, and that the drastically different Ax and Ay values detd. for complexes 1, 4 and 4-H+ have distinctive origins compared to complex 2, a genuine oxo-iron(V) species. Complex 1, in fact, has a dominant character of [FeIV(O•••OC(O)CH3)2-•]2+, i.e. an SFe = 1 iron(IV) center antiferromagnetically coupled to an O-O σ* radical, where the O-O bond has not been completely broken. Complex 4 is best described as a triplet ferryl unit that strongly interacts with the trans acetylimidyl radical in an antiferromagnetic fashion, [FeIV(O)(•N=C(O-)CH3)]+. Complex 4-H+ features a similar electronic structure, [FeIV(O)(•N=C(OH)CH3)]2+. Owing to the remaining approx. half σ-bond in the O-O moiety, complex 1 can arrange two electron-accepting orbitals (α σ* O-O and β Fe-dxz) in such a way that both orbitals can simultaneously interact with the doubly occupied electron-donating orbitals (σC-H or πC-C). Hence, complex 1 can promote a concerted yet asynchronous two-electron oxidn. of the C-H and C=C bonds, which nicely explains the stereospecificity obsd. for complex 1 and the related species.
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  (b) Ye, S.; Geng, C.-Y.; Shaik, S.; Neese, F. Electronic Structure Analysis of Multistate Reactivity in Transition Metal Catalyzed Reactions: The Case of C-H Bond Activation by Non-Heme Iron(IV)-Oxo Cores. Phys. Chem. Chem. Phys. 2013, 15, 8017– 8030, DOI: 10.1039/c3cp00080j
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  20b
  Electronic structure analysis of multistate reactivity in transition metal catalyzed reactions: the case of C-H bond activation by non-heme iron(iv)-oxo cores
  Ye, Shengfa; Geng, Cai-Yun; Shaik, Sason; Neese, Frank
  Physical Chemistry Chemical Physics (2013), 15 (21), 8017-8030CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
  A review. This perspective discusses the principles of the multistate scenario often encountered in transition metal catalyzed reactions, and is organized as follows. First, several important theor. concepts (phys. vs. formal oxidn. states, orbital interactions, use of (spin) natural and corresponding orbitals, exchange enhanced reactivity and the connection between valence bond and MO based electronic structure anal.) are presented. These concepts are then used to analyze the electronic structure changes occurring in the reaction of C-H bond oxidn. by FeIVoxo species. The anal. reveals that the energy sepn. and the overlap between the electron donating orbitals and electron accepting orbitals of the FeIVoxo complexes dictate the reaction stereochem., and that the manner in which the exchange interaction changes depends on the identity of these orbitals. The electronic reorganization of the FeIVoxo species during the reaction is thoroughly analyzed and it is shown that the FeIVoxo reactant develops oxyl radical character, which interacts effectively with the σCH orbital of the alkane. The factors that det. the energy barrier for the reaction are discussed in terms of MO and valence bond concepts.
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  (c) Ogliaro, F.; de Visser, S. P.; Groves, J. T.; Shaik, S. Chameleon States: High-Valent Metal-Oxo Species of Cytochrome P450 and Its Ruthenium Analogue. Angew. Chem., Int. Ed. 2001, 40, 2874– 2878, DOI: 10.1002/1521-3773(20010803)40:15<2874::AID-ANIE2874>3.0.CO;2-9
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  20c
  Chameleon states. High-valent metal-oxo species of cytochrome P450 and its ruthenium analogue
  Ogliaro, Francois; De Visser, Samuel P.; Groves, John T.; Shaik, Sason
  Angewandte Chemie, International Edition (2001), 40 (15), 2874-2878CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH)
  D.-functional (DFT) calcns. were used to compare the electromeric states of the [(L)PorMzO] (M = Fe, Ru; Z = III-V, L = SH and SMe) complexes. The states, orbital occupancies, and relative energies are listed for the isolated mol. and for the mol. in a polarizing medium as well as the spin densities, resonance structures in the A2u and Πs states, and the π-π* orbital energy gaps. The catalytic activity of these complexes in monooxygenation reactions is discussed in relation to their electronic structure. The electronic structure of the Ru(V)O complex is distinctly different from that of the Fe complexes. The vacant π* orbital in the Ru(V)O catalyst heightened the electrophilicity of this compd. Such a heightened electrophilic nature was indeed found in the oxidn. reactions of substituted toluenes using a Ru porphyrin catalyst (Groves, 2000).
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  (d) Decker, A.; Rohde, J.-U.; Que, L.; Solomon, E. I. Spectroscopic and Quantum Chemical Characterization of the Electronic Structure and Bonding in a Non-Heme Fe^IV═O Complex. J. Am. Chem. Soc. 2004, 126, 5378– 5379, DOI: 10.1021/ja0498033
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  20d
  Spectroscopic and Quantum Chemical Characterization of the Electronic Structure and Bonding in a Non-Heme FeIV:O Complex
  Decker, Andrea; Rohde, Jan-Uwe; Que, Lawrence, Jr.; Solomon, Edward I.
  Journal of the American Chemical Society (2004), 126 (17), 5378-5379CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  High valent FeIV:O species are key intermediates in the catalytic cycles of many mononuclear non-heme iron enzymes involving the binding and activation of dioxygen. Using variable temp. magnetic CD (VT MCD) spectroscopy and exptl. calibrated d. functional calcns., we are able to present the first detailed description of the electronic structure of a non-heme FeIV:O S = 1 complex. These studies define the nature of the FeIV:O bond and present the basis for understanding high-valent oxygen intermediates in non-heme iron enzymes.
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21. 21
  Ray, K.; Heims, F.; Pfaff, F. F. Terminal Oxo and Imido Transition-Metal Complexes of Groups 9–11. Eur. J. Inorg. Chem. 2013, 2013, 3784– 3807, DOI: 10.1002/ejic.201300223
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  21
  Terminal Oxo and Imido Transition-Metal Complexes of Groups 9-11
  Ray, Kallol; Heims, Florian; Pfaff, Florian Felix
  European Journal of Inorganic Chemistry (2013), 2013 (22-23), 3784-3807CODEN: EJICFO; ISSN:1434-1948. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review. This review summarizes the properties of group 9-11 metal-oxo and metal-imido complexes, which have been either isolated or proposed as reactive intermediates in metal-catalyzed org. transformations. We begin with a general description of the bonding of transition-metal-oxo and -imido complexes in various geometries, followed by a discussion of complexes from groups 9-11. The focus of this review is to provide a clear picture of the state of the art as well as insight towards potential future synthetic endeavors.
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22. 22
  Carter, E. A.; Goddard, W. A., III Early- versus Late-Transition-Metal-Oxo Bonds: The Electronic Structure of VO⁺ and RuO⁺. J. Phys. Chem. 1988, 92, 2109– 2115, DOI: 10.1021/j100319a005
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  22
  Early- versus late-transition-metal-oxo bonds: the electronic structure of oxovanadium(1+) and oxoruthenium(1+)
  Carter, Emily A.; Goddard, William A., III
  Journal of Physical Chemistry (1988), 92 (8), 2109-15CODEN: JPCHAX; ISSN:0022-3654.
  From all-electron ab initio generalized valence bond calcns. (GVBCI-SCF) on VO+ and RuO+, an accurate description of the bonding is obtained only when important resonance configurations are included self-consistently in the wave function. The ground state of VO+(3Σ-) has a triple bond similar to that of CO, with Decalcd(V-O) = 128.3 kcal/mol [Deexptl(V-O) = 131 ± 5 kcal/mol], while the ground state of RuO+(4Δ) has a double bond similar to that of O2, with Decalcd(Ru-O) = 67.1 kcal/mol. Vertical excitation energies for a no. of low-lying electronic states of VO+ and RuO+ are also reported. These results indicate fundamental differences in the nature of the metal-oxo bond in early and late metal-oxo complexes that explain the obsd. trends in reactivity (e.g., early metal oxides are thermodynamically stable whereas late metal oxo complexes are highly reactive oxidants). These results were used to predict the ground states of MO+ for other first-row transition-metal oxides.
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23. 23
  (a) Koizumi, K.; Shoji, M.; Nishiyama, Y.; Maruno, Y.; Kitagawa, Y.; Soda, K.; Yamanaka, S.; Okumura, M.; Yamaguchi, K. The Electronic Structure and Magnetic Property of Metal-Oxo, Porphyrin Manganese-Oxo, and μ-Oxo-Bridged Manganese Porphyrin Dimer. Int. J. Quantum Chem. 2004, 100, 943– 956, DOI: 10.1002/qua.20152
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  23a
  The electronic structure and magnetic property of metal-Oxo, porphyrin manganese-Oxo, and μ-Oxo-bridged manganese porphyrin dimer
  Koizumi, K.; Shoji, M.; Nishiyama, Y.; Maruno, Y.; Kitagawa, Y.; Soda, K.; Yamanaka, S.; Okumura, M.; Yamaguchi, K.
  International Journal of Quantum Chemistry (2004), 100 (6), 943-956CODEN: IJQCB2; ISSN:0020-7608. (John Wiley & Sons, Inc.)
  Hybrid d. functional theory (HDFT) and post Hartree-Fock CCSD(T) methods are applied to elucidate the binding energies and the optimized M-O distances of transition metal oxides: MO (M = Cr, Mn, Fe, Co, Ni, Cu). The HDFT method can reproduce the CCSD(T) results, in agreement with the exptl. ones. The nature of the manganese-oxygen bonds in the Mn(VI)-O, Mn(IV)-O porphyrin (PP), and Mn(V)-O PP systems are examd. in relation to possible mechanisms of oxygen evolution from H2O2 and H2O in native and non-native manganese complexes. It is found that the radical character of the high-valent (PP)Mn(V)-O bond is remarkable, showing the strong potential to generate mol. oxygen because of its high reactivity. The electronic structure and magnetic property of μ-oxo-bridged manganese porphyrin dimer (PPMn(III)OMn(III)PP) are investigated for further discussion of structure and reactivity of PPMn(X)O (X = II-IV). The potential curve for release of mol. oxygen from PPMn(II)O2 is also examd. to show weak affinity of O2 in the Mn complex where the oxidn. no. (X) of Mn is low. Implications of the computational results are also discussed in relation to oxygen evolution reactions.
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  (b) Yamaguchi, K.; Takahara, Y.; Fueno, T. Ab-Initio Molecular Orbital Studies of Structure and Reactivity of Transition Metal-OXO Compounds. Appl. Quant. Chem. 1986, 155– 184, DOI: 10.1007/978-94-009-4746-7_11
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  Ab-initio molecular orbital studies of structure and reactivity of transition metal-oxo compounds
  Yamaguchi, K.; Takahara, Y.; Fueno, T.
  (1986), (), 155-84CODEN: 55IFAJ ISSN:. (Reidel)
  A review with 49 refs. deals with electronic properties, structure, and reactivity of transition metal oxo compds.
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24. 24
  Solomon, E. I. Geometric and Electronic Structure Contributions to Function in Bioinorganic Chemistry: Active Sites in Non-Heme Iron Enzymes. Inorg. Chem. 2001, 40, 3656– 3669, DOI: 10.1021/ic010348a
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  24
  Geometric and Electronic Structure Contributions to Function in Bioinorganic Chemistry: Active Sites in Non-Heme Iron Enzymes
  Solomon, Edward I.
  Inorganic Chemistry (2001), 40 (15), 3656-3669CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  A review with 75 refs. Spectroscopy has played a major role in the definition of structure/function correlations in bioinorg. chem. The importance of spectroscopy combined with electronic structure calcns. is clearly demonstrated by the non-heme iron enzymes. Many members of this large class of enzymes activate dioxygen using a ferrous active site that has generally been difficult to study with most spectroscopic methods. A new spectroscopic methodol. has been developed utilizing variable temp., variable field magnetic CD, which enables one to obtain detailed insight into the geometric and electronic structure of the non-heme ferrous active site and probe its reaction mechanism on a mol. level. This spectroscopic methodol. is presented and applied to a no. of key mononuclear non-heme iron enzymes leading to a general mechanistic strategy for O2 activation. These studies are then extended to consider the new features present in the binuclear non-heme iron enzymes and applied to understand (1) the mechanism of the two electron/coupled proton transfer to dioxygen binding to a single iron center in hemerythrin and (2) structure/function correlations over the oxygen-activating enzymes stearoyl-ACP Δ9-desaturase, ribonucleotide reductase, and methane monooxygenase. Electronic structure/reactivity correlations for O2 activation by non-heme relative to heme iron enzymes will also be developed.
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25. 25
  Yang, X.; Baik, M.-H. Electronic Structure of the Water-Oxidation Catalyst [(bpy)₂(OH_x)RuORu(OH_y)(bpy)₂]^z+: Weak Coupling between the Metal Centers is Preferred over Strong Coupling. J. Am. Chem. Soc. 2004, 126, 13222– 13223, DOI: 10.1021/ja0462427
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  25
  Electronic Structure of the Water-Oxidation Catalyst [(bpy)2(OHx)RuORu(OHy)(bpy)2]z+: Weak Coupling between the Metal Centers Is Preferred over Strong Coupling
  Yang, Xiaofan; Baik, Mu-Hyun
  Journal of the American Chemical Society (2004), 126 (41), 13222-13223CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  High-level DFT calcns. indicate that the singlet ground state of the water-oxidizing blue Ru dimer [(bpy)2(OH2)RuIIIORuIII(OH2)(bpy)2]4+ is not due to a strong coupling of the excess electrons from each of the low-spin d5 RuIII centers across the Ru-O-Ru moiety, as has been assumed to date. Instead, broken symmetry orbital calcns. suggest that a weak antiferromagnetically (AF) coupled singlet state is energetically more favorable by 10-35 kcal/mol. Exptl. obsd. redox potentials can only be reproduced if antiferromagnetic coupling is invoked.
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26. 26
  (a) Nam, W. High-Valent Iron(IV)-Oxo Complexes of Heme and Non-Heme Ligands in Oxygenation Reactions. Acc. Chem. Res. 2007, 40, 522– 531, DOI: 10.1021/ar700027f
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  26a
  High-Valent Iron(IV)-Oxo Complexes of Heme and Non-Heme Ligands in Oxygenation Reactions
  Nam, Wonwoo
  Accounts of Chemical Research (2007), 40 (7), 522-531CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
  A review. High-valent iron(IV)-oxo species have been implicated as the key reactive intermediates in the catalytic cycles of dioxygen activation by heme and non-heme iron enzymes. Our understanding of the enzymic reactions has improved greatly via investigation of spectroscopic and chem. properties of heme and non-heme iron(IV)-oxo complexes. In this Account, reactivities of synthetic iron(IV)-oxo porphyrin π-cation radicals and mononuclear non-heme iron(IV)-oxo complexes in oxygenation reactions have been discussed as chem. models of cytochrome P 450 and non-heme iron enzymes. These results demonstrate how mechanistic developments in biomimetic research can help our understanding of dioxygen activation and oxygen atom transfer reactions in nature.
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  .
  (b) Nam, W.; Lee, Y.-M.; Fukuzumi, S. Tuning Reactivity and Mechanism in Oxidation Reactions by Mononuclear Nonheme Iron(IV)-Oxo Complexes. Acc. Chem. Res. 2014, 47, 1146– 1154, DOI: 10.1021/ar400258p
  [ACS Full Text ], [CAS], Google Scholar
  26b
  Tuning reactivity and mechanism in oxidation reactions by mononuclear nonheme iron(IV)-oxo complexes
  Nam, Wonwoo; Lee, Yong-Min; Fukuzumi, Shunichi
  Accounts of Chemical Research (2014), 47 (4), 1146-1154CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
  A review. Mononuclear nonheme iron enzymes generate high-valent Fe(IV)-oxo intermediates that effect metabolically important oxidative transformations in the catalytic cycle of O2 activation. In 2003, researchers 1st spectroscopically characterized a mononuclear nonheme Fe(IV)-oxo intermediate in the reaction of taurine-α-ketoglutarate dioxygenase (TauD). This nonheme Fe-contg. enzyme with a Fe active center was coordinated to a 2-His-1-carboxylate facial triad motif. In the same year, researchers obtained the 1st crystal structure of a mononuclear nonheme Fe(IV)-oxo complex bearing a macrocyclic supporting ligand, [(TMC)FeIV(O)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecene), in studies that mimicked the biol. enzymes. With these breakthrough results, many other studies have examd. mononuclear nonheme Fe(IV)-oxo intermediates trapped in enzymic reactions or synthesized in biomimetic reactions. Over the past decade, researchers in the fields of biol., bioinorg., and oxidn. chem. have extensively investigated the structure, spectroscopy, and reactivity of nonheme Fe(IV)-oxo species, leading to a wealth of information from these enzymic and biomimetic studies. Here, the authors summarize the reactivity and mechanisms of synthetic mononuclear nonheme Fe(IV)-oxo complexes in oxidn. reactions and examines factors that modulate their reactivities and change their reaction mechanisms. The authors focus on several reactions including the oxidn. of org. and inorg. compds., electron transfer, and O atom exchange with water by synthetic mononuclear nonheme Fe(IV)-oxo complexes. In addn., the authors recently obsd. that C-H bond activation by nonheme Fe(IV)-oxo and other nonheme metal(IV)-oxo complexes does not follow the H-atom abstraction/oxygen-rebound mechanism, which has been well-established in heme systems. The structural and electronic effects of supporting ligands on the oxidizing power of Fe(IV)-oxo complexes are significant in these reactions. However, the difference in spin states between nonheme Fe(IV)-oxo complexes with an octahedral geometry (with an S = 1 intermediate-spin state) or a trigonal bipyramidal (TBP) geometry (with an S = 2 high-spin state) does not lead to a significant change in reactivity in biomimetic systems. Thus, the importance of the high-spin state of Fe(IV)-oxo species in nonheme Fe-contg. enzymes remains unexplained. The authors also discuss how the axial and equatorial ligands and binding of redox-inactive metal ions and protons to the Fe-oxo moiety influence the reactivities of the nonheme Fe(IV)-oxo complexes. The authors emphasize how these changes can enhance the oxidizing power of nonheme metal(IV)-oxo complexes in O atom transfer and electron-transfer reactions remarkably. The authors demonstrate great advancements in the understanding of the chem. of mononuclear nonheme Fe(IV)-oxo intermediates within the last 10 yr.
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27. 27
  (a) Bryant, J. R.; Mayer, J. M. Oxidation of C-H Bonds by [(bpy)₂(py)Ru^IVO]²⁺ Occurs by Hydrogen Atom Abstraction. J. Am. Chem. Soc. 2003, 125, 10351– 10361, DOI: 10.1021/ja035276w
  [ACS Full Text ], [CAS], Google Scholar
  27a
  Oxidation of C-H Bonds by [(bpy)2(py)RuIVO]2+ Occurs by Hydrogen Atom Abstraction
  Bryant, Jasmine R.; Mayer, James M.
  Journal of the American Chemical Society (2003), 125 (34), 10351-10361CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Anaerobic oxidns. of 9,10-dihydroanthracene (DHA), xanthene, and fluorene by [(bpy)2(py)RuIVO]2+ in acetonitrile soln. give mixts. of products including oxygenated and non-oxygenated compds. The products include those formed by org. radical dimerization, such as 9,9'-bixanthene, as well as by oxygen-atom transfer (e.g., xanthone). The kinetics of these reactions have been measured. The kinetic isotope effect for oxidn. of DHA vs DHA-d4 gives kH/kD ≥ 35 ± 1. The data indicate a mechanism of initial hydrogen-atom abstraction forming radicals that dimerize, disproportionate and are trapped by the oxidant. This mechanism also appears to apply to the oxidns. of toluene, ethylbenzene, cumene, indene, and cyclohexene. The rate consts. for H-atom abstraction from these substrates correlate well with the strength of the C-H bond that is cleaved. Rate consts. for abstraction from DHA and toluene also correlate with those for oxygen radicals and other oxidants. The rate const. for H-atom transfer from toluene to [(bpy)2(py)RuIVO]2+ appears to be close to that predicted by the Marcus cross relation, using a tentative rate const. for hydrogen atom self-exchange between [(bpy)2(py)RuIIIOH]2+ and [(bpy)2(py)RuIVO]2+.
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  .
  (b) Saouma, C. T.; Mayer, J. M. Do Spin State and Spin Density Affect Hydrogen Atom Transfer Reativity?. Chem. Sci. 2014, 5, 21– 31, DOI: 10.1039/C3SC52664J
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  27b
  Do spin state and spin density affect hydrogen atom transfer reactivity?
  Saouma, Caroline T.; Mayer, James M.
  Chemical Science (2014), 5 (1), 21-31CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
  A review. The prevalence of hydrogen atom transfer (HAT) reactions in chem. and biol. systems has prompted much interest in establishing and understanding the underlying factors that enable this reactivity. Arguments have been advanced that the electronic spin state of the abstractor and/or the spin-d. at the abstracting atom are crit. for HAT reactivity. This is consistent with the intuition derived from introductory org. chem. courses. Alternative view on the role of spin state and spin d. in HAT reactions. After a brief introduction, the second section introduces a new and simple fundamental kinetic anal., which shows that unpaired spin cannot be the dominant effect. The third section examines published computational studies of HAT reactions, which indicates that the spin state affects these reactions indirectly, primarily via changes in driving force. The essay concludes with a broader view of HAT reactivity, including indirect effects of spin and other properties. It is suggested that some of the controversy in this area may arise from the diversity of HAT reactions and their overlap with proton-coupled electron transfer (PCET) reactions.
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28. 28
  Wang, B.; Lee, Y.-M.; Tcho, W.-Y.; Tussupbayev, S.; Kim, S.-T.; Kim, Y.; Seo, M. S.; Cho, K.-B.; Dede, Y.; Keegan, B. C.; Ogura, T.; Kim, S. H.; Ohta, T.; Baik, M.-H.; Ray, K.; Shearer, J.; Nam, W. Synthesis and Reactivity of a Mononuclear Non-Haem Cobalt(IV)-Oxo Complex. Nat. Commun. 2017, 8, 14839, DOI: 10.1038/ncomms14839
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  28
  Synthesis and reactivity of a mononuclear non-haem cobalt(IV)-oxo complex
  Wang, Bin; Lee, Yong-Min; Tcho, Woon-Young; Tussupbayev, Samat; Kim, Seoung-Tae; Kim, Yujeong; Seo, Mi Sook; Cho, Kyung-Bin; Dede, Yavuz; Keegan, Brenna C.; Ogura, Takashi; Kim, Sun Hee; Ohta, Takehiro; Baik, Mu-Hyun; Ray, Kallol; Shearer, Jason; Nam, Wonwoo
  Nature Communications (2017), 8 (), 14839CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
  Terminal cobalt(IV)-oxo (CoIV-O) species have been implicated as key intermediates in various cobalt-mediated oxidn. reactions. Herein we report the photocatalytic generation of a mononuclear non-haem [(13-TMC)CoIV(O)]2+ (2) by irradiating [CoII(13-TMC)(CF3SO3)]+ (1) in the presence of [RuII(bpy)3]2+, Na2S2O8, and water as an oxygen source. The intermediate 2 was also obtained by reacting 1 with an artificial oxidant (i.e., iodosylbenzene) and characterized by various spectroscopic techniques. In particular, the resonance Raman spectrum of 2 reveals a diat. Co-O vibration band at 770 cm-1, which provides the conclusive evidence for the presence of a terminal Co-O bond. In reactivity studies, 2 was shown to be a competent oxidant in an intermetal oxygen atom transfer, C-H bond activation and olefin epoxidn. reactions. The present results lend strong credence to the intermediacy of CoIV-O species in cobalt-catalyzed oxidn. of org. substrates as well as in the catalytic oxidn. of water that evolves mol. oxygen.
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29. 29
  (a) Poverenov, E.; Efremenko, I.; Frenkel, A. I.; Ben-David, Y.; Shimon, L. J. W.; Leitus, G.; Konstantinovski, L.; Martin, J. M. L.; Milstein, D. Evidence for a Terminal Pt(IV)-Oxo Complex Exhibiting Diverse Reactivity. Nature 2008, 455, 1093– 1096, DOI: 10.1038/nature07356
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  29a
  Evidence for a terminal Pt(IV)-oxo complex exhibiting diverse reactivity
  Poverenov, Elena; Efremenko, Irena; Frenkel, Anatoly I.; Ben-David, Yehoshoa; Shimon, Linda J. W.; Leitus, Gregory; Konstantinovski, Leonid; Martin, Jan M. L.; Milstein, David
  Nature (London, United Kingdom) (2008), 455 (7216), 1093-1096CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
  Terminal oxo complexes of transition metals have crit. roles in various biol. and chem. processes. For example, the catalytic oxidn. of org. mols., some oxidative enzymic transformations, and the activation of dioxygen on metal surfaces are all thought to involve oxo complexes. Moreover, they are believed to be key intermediates in the photocatalytic oxidn. of water to give mol. oxygen, a topic of intensive global research aimed at artificial photosynthesis and water splitting. The terminal oxo ligand is a strong π-electron donor, so it readily forms stable complexes with high-valent early transition metals. As the d orbitals are filled up with valence electrons, the terminal oxo ligand becomes destabilized. Here we present evidence for a dn (n > 5) terminal oxo complex that is not stabilized by an electron withdrawing ligand framework. This d6 Pt(IV) complex exhibits reactivity as an inter- and intramol. oxygen donor and as an electrophile. In addn., it undergoes a water activation process leading to a terminal dihydroxo complex, which may be relevant to the mechanism of catalytic reactions such as water oxidn.
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  .
  (b) Efremenko, I.; Poverenov, E.; Martin, J. M. L.; Milstein, D. DFT Study of the Structure and Reactivity of the Terminal Pt(IV)-Oxo Complex Bearing No Electron-Withdrawing Ligands. J. Am. Chem. Soc. 2010, 132, 14886– 14900, DOI: 10.1021/ja105197x
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  29b
  DFT Study of the structure and reactivity of the terminal Pt(IV)-oxo complex bearing no electron-withdrawing ligands
  Efremenko, Irena; Poverenov, Elena; Martin, Jan M. L.; Milstein, David
  Journal of the American Chemical Society (2010), 132 (42), 14886-14900CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The recently published [(PCN)Pt:O]+ complex is interesting as a unique example of a stable d6 terminal transition metal oxo complex not stabilized by electron withdrawing ligands and as a model of oxo complexes frequently implicated as key intermediates in various processes of oxygen transfer. In the present work, we report an extensive DFT study of its geometric and electronic structure, compn. in soln., and reactivity. The thermodn. data and calcd. 195Pt NMR chem. shifts reveal that one solvent mol. is weakly coordinated to the complex in acetone soln. This ancillary ligand is responsible for the diamagnetic state of the complex, retards intramol. oxygen transfer, and facilitates CO oxidn. Chem. transformations of the coordinated acetone mol., coordination of other ancillary ligands present in the reaction mixt., and protonation of the Pt-oxo group in nonacidic media are excluded based on thermodn. or kinetic considerations. Bonding of the terminal oxo ligand with strong electrophiles presents the key interaction in the mechanisms of intramol. oxygen insertion into the Pt-P bond, in CO oxidn. and in water activation mediated by microsolvation. Low affinity of the terminal oxo ligand toward "soft" covalent interactions brings about intermediate formation of agostic hydrido and hydroxo complexes along the reaction pathway of dihydrogen oxidn. Stabilization of the Pt-oxo bonding is attributed to bending of the terminal oxo ligand out of the plane of the complex and to significant transfer of electron d. from compact low lying Pt 5d orbitals to more diffuse 6s and 6p orbitals.
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30. 30
  (a) Zhou, M.; Schley, N. D.; Crabtree, R. H. Cp* Iridium Complexes Give Catalytic Alkane Hydroxylation with Retention of Stereochemistry. J. Am. Chem. Soc. 2010, 132, 12550– 12551, DOI: 10.1021/ja1058247
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  30a
  Cp* Iridium Complexes Give Catalytic Alkane Hydroxylation with Retention of Stereochemistry
  Zhou, Meng; Schley, Nathan D.; Crabtree, Robert H.
  Journal of the American Chemical Society (2010), 132 (36), 12550-12551CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  A series of Cp*Ir complexes can catalyze C-H oxidn., with ceric ammonium nitrate as the terminal oxidant and water as the source of oxygen. Remarkably the hydroxylation of cis-decalin and 1,4-dimethylcyclohexane proceeds with retention of stereochem. With H2O18, cis-decalin oxidn. gave 18O incorporation into the product cis-decalol.
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  .
  (b) Zhou, M.; Balcells, D.; Parent, A. R.; Crabtree, R. H.; Eisenstein, O. Cp* Iridium Precatalysts for Selective C-H Oxidation via Direct Oxygen Insertion: A Joint Experimental/Computational Study. ACS Catal. 2012, 2, 208– 218, DOI: 10.1021/cs2005899
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  30b
  Cp* iridium precatalysts for selective C-H oxidation via direct oxygen insertion: A joint experimental/computational study
  Zhou, Meng; Balcells, David; Parent, Alexander R.; Crabtree, Robert H.; Eisenstein, Odile
  ACS Catalysis (2012), 2 (2), 208-218CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  A series of Cp*Ir complexes are active precatalysts in C-H oxidn. of cis-decalin, cyclooctane, 1-acetylpyrrolidine, tetrahydrofurans, and γ-lactones. Moderate to high yields were achieved, and surprisingly, high selectivity for mono-oxidn. of cyclooctane to cyclooctanone was obsd. Kinetic isotope effect expts. in the C-H oxidn. of ethylbenezene to acetophenone yield kH/kD = 15.4 ± 0.8 at 23 °C and 17.8 ± 1.2 at 0 °C, which are consistent with C-H oxidn. being the rate-limiting step with a significant tunneling contribution. The nature of the active species was investigated by TEM, UV-vis, microfiltration, and control expts. DFT calcns. showed that the C-H oxidn. of cis-decalin by Cp*Ir(ppy)(Cl) (ppy = o-phenylpyridine) follows a direct oxygen insertion mechanism on the singlet potential energy surface, rather than the radical rebound route that would be seen for the triplet, in good agreement with the retention of stereochem. obsd. in this reaction.
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31. 31
  (a) Wu, X.; Yang, X.; Lee, Y.-M.; Nam, W.; Sun, L. A Nonheme Manganese(IV)-Oxo Species Generated in Photocatalytic Reaction Using Water as an Oxygen Source. Chem. Commun. 2015, 51, 4013– 4016, DOI: 10.1039/C4CC10411K
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  31a
  A nonheme manganese(IV)-oxo species generated in photocatalytic reaction using water as an oxygen source
  Wu, Xiujuan; Yang, Xiaonan; Lee, Yong-Min; Nam, Wonwoo; Sun, Licheng
  Chemical Communications (Cambridge, United Kingdom) (2015), 51 (19), 4013-4016CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
  A nonheme manganese(IV)-oxo complex, [MnIV(O)(BQCN)(H2O)]2+ (where BQCN = N,N'-dimethyl-N,N'-bis(8-quinolyl)cyclohexanediamine), was generated in the photochem. and chem. oxidn. of [MnII(BQCN)(OTf)2] with water as an oxygen source, resp. The photocatalytic oxidn. of org. substrates, such as alc. and sulfide, by [MnII(BQCN)]2+ has been demonstrated in both neutral and acidic media.
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  .
  (b) Wu, X.; Seo, M. S.; Davis, K. M.; Lee, Y.-M.; Chen, J.; Cho, K.-B.; Pushkar, Y. N.; Nam, W. A Highly Reactive Mononuclear Non-Heme Manganese(IV)-Oxo Complex That Can Activate the Strong C-H Bonds of Alkanes. J. Am. Chem. Soc. 2011, 133, 20088– 20091, DOI: 10.1021/ja208523u
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  31b
  A Highly Reactive Mononuclear Non-Heme Manganese(IV)-Oxo Complex That Can Activate the Strong C-H Bonds of Alkanes
  Wu, Xiujuan; Seo, Mi Sook; Davis, Katherine M.; Lee, Yong-Min; Chen, Junying; Cho, Kyung-Bin; Pushkar, Yulia N.; Nam, Wonwoo
  Journal of the American Chemical Society (2011), 133 (50), 20088-20091CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  A mononuclear nonheme manganese(IV)-oxo complex was synthesized and characterized using various spectroscopic methods. The Mn(IV)-oxo complex shows high reactivity in oxidn. reactions, such as C-H bond activation, oxidns. of olefins, alcs., sulfides, and arom. compds., and N-dealkylation. In C-H bond activation, the Mn(IV)-oxo complex can activate C-H bonds as strong as those in cyclohexane. Probably C-H bond activation by the nonheme Mn(IV)-oxo complex does not occur via an oxygen-rebound mechanism. The electrophilic character of the nonheme Mn(IV)-oxo complex is demonstrated by a large neg. ρ value of -4.4 in the oxidn. of para-substituted thioanisoles.
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32. 32
  (a) Conte, V.; Coletti, A.; Floris, B.; Licini, G.; Zonta, C. Mechanistic Aspects of Vanadium Catalysed Oxidations with Peroxides. Coord. Chem. Rev. 2011, 255, 2165– 2177, DOI: 10.1016/j.ccr.2011.03.006
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  32a
  Mechanistic aspects of vanadium catalyzed oxidations with peroxides
  Conte, Valeria; Coletti, Alessia; Floris, Barbara; Licini, Giulia; Zonta, Cristiano
  Coordination Chemistry Reviews (2011), 255 (19-20), 2165-2177CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
  A review. The enhancement of the reactivity of peroxides, particularly H2O2 and alkylhydroperoxides, in the presence of V catalysis is a very known process. The catalytic effect is detd. by the formation of an intermediate whose nature depends on the peroxides used and on its interaction with the metal precursor, high-valent peroxo V species being usually the reactive oxidants. During the last decades the mechanistic details for several types of oxidn. reactions have been elucidated. In a no. of cases theor. calcns. offered support to the proposed reaction pathways. In general, V(V) peroxo species behave as electrophilic O transfer reagents thus reacting preferentially with the more nucleophilic functional group present in the mol. In several instances the chemoselectivity obsd. in such processes is very high when not abs. As far as V peroxides are concerned, a radical oxidative reactivity toward alkanes and aroms. was also obsd.; also for this latter chem., diverse research groups studied in detail the mechanism. However, no clear-cut evidence of nucleophilic reactivity of V peroxo complexes was obtained. Here the authors collect a selection of recent achievements concerning the reaction mechanisms in the V catalyzed oxidn. and bromination reactions with peroxides.
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  (b) Waidmann, C. R.; DiPasquale, A. G.; Mayer, J. M. Synthesis and Reactivity of Oxo-Peroxo-Vanadium(V) Bipyridine Compounds. Inorg. Chem. 2010, 49, 2383– 2391, DOI: 10.1021/ic9022618
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  32b
  Synthesis and Reactivity of Oxo-Peroxo-Vanadium(V) Bipyridine Compounds
  Waidmann, Christopher R.; Di Pasquale, Antonio G.; Mayer, James M.
  Inorganic Chemistry (2010), 49 (5), 2383-2391CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  The V(IV) compd. [VIVO(OH)(tBu2bpy)2]BF4 (VIVO(OH)) (tBu2bpy = 4,4'-di-tert-butylbipyridine) is slowly oxidized by O2 in ethereal solvents to give the oxo-peroxo compd. [VVO(O2)(tBu2bpy)2]BF4 (VVO(O2)) in excellent yield. This and related compds. were fully characterized by NMR, IR, and optical spectroscopies; mass spectrometry; elemental analyses; and an x-ray crystal structure of the 4,4'-dimethylbipyridine analog, [VVO(O2)(Me2bpy)2]BF4. Monitoring the reaction of VIVO(OH) with O2 in THF/MeCN mixts. by 1H NMR and optical spectroscopies surprisingly shows that the initial product is the cis-dioxo compd. [VV(O)2(tBu2bpy)2]BF4 (VVO2), which then converts to VVO(O2). Reaction of VIVO(OH) with 18O2 gives ∼60% triply 18O labeled VVO(O2). The mechanism of formation of VVO(O2) is complex and may occur via initial redn. of O2 at V(IV) to give a superoxo-V(V) intermediate, autoxidn. of the THF solvent, or both. That VVO2 is generated 1st appears to be due to the ability of VIVO(OH) to act as a hydrogen atom donor. For instance, VIVO(OH) reacts with VVO(O2) to give VVO2. VVO(O2) is also slowly reduced to VIVO(OH) by the org. hydrogen atom donors hydroquinone and TEMPOH (2,2,6,6-tetramethylpiperidin-1-ol) as well as by PPh3. Notably, the peroxo complex VVO(O2) is much less reactive with these substrates than the analogous dioxo compd. VVO2.
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33. 33
  (a) Liu, S.; Mase, K.; Bougher, C.; Hicks, S. D.; Abu-Omar, M. M.; Fukuzumi, S. High-Valent Chromoium-Oxo Complex Acting as an Efficient Catalyst Precursor for Selective Two-Electron Reduction of Dioxygen by a Ferrocene Derivative. Inorg. Chem. 2014, 53, 7780– 7788, DOI: 10.1021/ic5013457
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  33a
  High-valent chromium-oxo complex acting as an efficient catalyst precursor for selective two-electron reduction of dioxygen by a ferrocene derivative
  Liu, Shuo; Mase, Kentaro; Bougher, Curt; Hicks, Scott D.; Abu-Omar, Mahdi M.; Fukuzumi, Shunichi
  Inorganic Chemistry (2014), 53 (14), 7780-7788CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  Efficient catalytic two-electron redn. of dioxygen (O2) by octamethylferrocene (Me8Fc) produced hydrogen peroxide (H2O2) using a high-valent chromium(V)-oxo corrole complex, [(tpfc)CrV(O)] (tpfc = tris(pentafluorophenyl)corrole) as a catalyst precursor in the presence of trifluoroacetic acid (TFA) in acetonitrile (MeCN). The facile two-electron redn. of [(tpfc)CrV(O)] by 2 equiv of Me8Fc in the presence of excess TFA produced the corresponding chromium(III) corrole [(tpfc)CrIII(OH2)] via fast electron transfer from Me8Fc to [(tpfc)CrV(O)] followed by double protonation of [(tpfc)CrIV(O)]- and facile second-electron transfer from Me8Fc. The rate-detg. step in the catalytic two-electron redn. of O2 by Me8Fc in the presence of excess TFA is inner-sphere electron transfer from [(tpfc)CrIII(OH2)] to O2 to produce the chromium(IV) superoxo species [(tpfc)CrIV(O2•-)], followed by fast proton-coupled electron transfer redn. of [(tpfc)CrIV(O2•-)] by Me8Fc to yield H2O2, accompanied by regeneration of [(tpfc)CrIII(OH2)]. Thus, although the catalytic two-electron redn. of O2 by Me8Fc was started by [(tpfc)CrV(O)], no regeneration of [(tpfc)CrV(O)] was obsd. in the presence of excess TFA, regardless of the tetragonal chromium complex being to the left of the oxo wall. In the presence of a stoichiometric amt. of TFA, however, disproportionation of [(tfpc)CrIV(O)]- occurred via the protonated species [(tpfc)CrIV(OH)] to produce [(tpfc)CrIII(OH2)] and [(tpfc)CrV(O)].
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  .
  (b) Premsingh, S.; Venkataramanan, N. S.; Rajagopal, S.; Mirza, S. P.; Vairamani, M.; Rao, P. S.; Velavan, K. Electron Transfer Reaction of Oxo(salen)chromium(V) Ion with Anilines. Inorg. Chem. 2004, 43, 5744– 5353, DOI: 10.1021/ic049482w
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  33b
  Electron Transfer Reaction of Oxo(salen)chromium(V) Ion with Anilines
  Premsingh, Sundarsingh; Venkataramanan, Natarajan Sathiyamoorthy; Rajagopal, Seenivasan; Mirza, Shama. P.; Vairamani, Mariappanadar; Rao, P. Sambasiva; Velavan, K.
  Inorganic Chemistry (2004), 43 (18), 5744-5753CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  The kinetics of oxidn. of 16 meta-, ortho-, and para-substituted anilines with nine oxo(salen)chromium(V) ions have been studied by spectrophotometric, ESIMS, and EPR techniques. During the course of the reaction, two new peaks with λmax at 470 and 730 nm appear in the absorption spectrum, and these peaks are due to the formation of emeraldine forms of oligomers of aniline supported by the ESIMS peaks with m/z values 274 and 365 (for the trimer and tetramer of aniline). The rate of the reaction is highly sensitive to the change of substituents in the aryl moiety of aniline and in the salen ligand of chromium(V) complexes. Application of the Hammett equation to analyze kinetic data yields a ρ value of -3.8 for the substituent variation in aniline and +2.2 for the substituent variation in the salen ligand of the metal complex. On the basis of the spectral, kinetic, and product anal. studies, a mechanism involving an electron transfer from the nitrogen of aniline to the metal complex in the rate controlling step has been proposed. The Marcus equation has been successfully applied to this system, and the calcd. values are compliant with the measured values.
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34. 34
  Blakemore, J. D.; Crabtree, R. H.; Brudvig, G. W. Molecular Catalytic for Water Oxidation. Chem. Rev. 2015, 115, 12974– 13005, DOI: 10.1021/acs.chemrev.5b00122
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  Molecular Catalysts for Water Oxidation
  Blakemore, James D.; Crabtree, Robert H.; Brudvig, Gary W.
  Chemical Reviews (Washington, DC, United States) (2015), 115 (23), 12974-13005CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review; mol. catalysts for water oxidn. are discussed.
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35. 35
  (a) Gilbert, J. A.; Eggleston, D. S.; Murphy, W. R., Jr.; Geselowitz, D. A.; Gersten, S. W.; Hodgson, D. J.; Meyer, T. J. Structure and Redox Properties of the Water-Oxidation Catalyst [(bpy)₂(OH₂)RuORu(OH₂)(bpy)₂]⁴⁺. J. Am. Chem. Soc. 1985, 107, 3855– 3864, DOI: 10.1021/ja00299a017
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  35a
  Structure and redox properties of the water-oxidation catalyst [(bpy)2(OH2)RuORu(OH2)(bpy)2]4+
  Gilbert, John A.; Eggleston, Drake S.; Murphy, Wyatt R., Jr.; Geselowitz, Daniel A.; Gersten, Susan W.; Hodgson, Derek J.; Meyer, Thomas J.
  Journal of the American Chemical Society (1985), 107 (13), 3855-64CODEN: JACSAT; ISSN:0002-7863.
  The crystal and mol. structure of the water-oxidn. catalyst μ-oxobis[aquabis(2,2'-bipyridine)ruthenium(III)] perchlorate dihydrate, [(bpy)2(OH2)RuORu(OH2)(bpy)2](ClO4)4.2H2O [where bpy is C10H8N2] was detd. from 3-dimensional x-ray counterdata. The complex crystallizes in the monoclinic space group C2/c with 4 mols. in a cell with a 22.712(9), b 13.189(4), and c 20.084(5) Å, β = 122.08 (3)°. The structure was refined to a weighted R factor of 0.052 based on 2887 independent intensities with I ≥ 3σ(I). The structure shows that the bridging Ru-O-Ru angle is 165.4°, the Ru-O bond lengths being 1.869 (1) Å. Electrochem. studies show that the RuIII-RuIII dimer undergoes an initial 1-electron oxidn. to RuIII-RuIV and that the potential of the couple has a complex pH dependence because of the acid-base properties of the 2 redox states. Above pH 2.2, oxidn. to RuIII-RuIV is followed by a 2-electron oxidn. to [(bpy)2(O)RuIVORuV(O)(bpy)2]3+ followed by a pH-independent, 1-electron oxidn. to [(bpy)2(O)RuVORuV(O)(bpy)2]4+. In solns. with pH < 2.2, RuIV-RuV is unstable with respect to disproportionation, and oxidn. of the RuIII-RuIV dimer to [(bpy)2(O)RuVORuV(O)(bpy)2]4+ occurs via a 3-electron step.
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36. 36
  Tagore, R.; Chen, H.; Zhang, H.; Crabtree, R. H.; Brudvig, G. W. Homogeneous Water Oxidation by a Di-μ-Oxo Dimanganese Complex in the Presence of Ce⁴⁺. Inorg. Chim. Acta 2007, 360, 2983– 2989, DOI: 10.1016/j.ica.2007.02.020
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  Homogeneous water oxidation by a di-μ-oxo dimanganese complex in the presence of Ce4+
  Tagore, Ranitendranath; Chen, Hongyu; Zhang, Hong; Crabtree, Robert H.; Brudvig, Gary W.
  Inorganica Chimica Acta (2007), 360 (9), 2983-2989CODEN: ICHAA3; ISSN:0020-1693. (Elsevier B.V.)
  O2 evolution was obsd. upon mixing aq. [(terpy)(H2O)Mn(O)2Mn(H2O)(terpy)](NO3)3 (1, terpy = 2,2':6',6''-terpyridine) with aq. solns. of Ce4+. However, when the soln. of 1 was incubated at pH 1 (by dissolving in dil. HNO3) before mixing with Ce4+, very small amts. of O2 were obsd. This observation of acid-induced deactivation suggests an explanation, both for the previously reported lack of O2 evolution from aq. solns. of 1 with Ce4+ as oxidant, and the present observation of low amts. of O2 prodn. with the very acidic Ce4+ reagent. Evidence is provided for water being the source of evolved O2, and for the requirement of a high valent multinuclear Mn species for O2 evolution. We test the possibility of complications in the use of ceric ammonium nitrate (CAN) in oxidn. chem. due to the presence of the oxidizable NH4+ ion.
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37. 37
  Ellis, W. C.; McDaniel, N. D.; Bernhard, S.; Collins, T. J. Fast Water Oxidation Using Iron. J. Am. Chem. Soc. 2010, 132, 10990– 10991, DOI: 10.1021/ja104766z
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  Fast Water Oxidation Using Iron
  Ellis, W. Chadwick; McDaniel, Neal D.; Bernhard, Stefan; Collins, Terrence J.
  Journal of the American Chemical Society (2010), 132 (32), 10990-10991CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Photolysis of water, a long-studied strategy for storing solar energy, involves two half-reactions: the redn. of protons to dihydrogen and the oxidn. of water to dioxygen. Proton redn. is well-understood, with catalysts achieving quantum yields of 34% when driven by visible light. Water oxidn., on the other hand, is much less advanced, typically involving expensive metal centers and rarely working in conjunction with a photochem. powered system. Before further progress can be made in the field of water splitting, significant developments in the catalysis of oxygen evolution are needed. Herein the authors present an iron-centered tetraamido macrocyclic ligand (Fe-TAML) that efficiently catalyzes the oxidative conversion of water to dioxygen. When the catalyst is combined in unbuffered soln. with ceric ammonium nitrate, its turnover frequency exceeds 1.3 s-1. Real-time UV-vis and oxygen monitoring of the active complex give insights into the reaction and decay kinetics.
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38. 38
  Brunschwig, B. S.; Chou, M. H.; Creutz, C.; Ghosh, P.; Sutin, N. Mechanisms of Water Oxidation to Oxygen: Cobalt(IV) as an Intermediate in the Aquocobalt(II)-Catalyzed Reaction. J. Am. Chem. Soc. 1983, 105, 4832– 4833, DOI: 10.1021/ja00352a050
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  38
  Mechanisms of water oxidation to oxygen: cobalt(IV) as an intermediate in the aquocobalt(II)-catalyzed reaction
  Brunschwig, Bruce S.; Chou, Mei H.; Creutz, Carol; Ghosh, Pushpito; Sutin, Norman
  Journal of the American Chemical Society (1983), 105 (14), 4832-3CODEN: JACSAT; ISSN:0002-7863.
  The kinetics and product distribution of the Co(II)-catalyzed Ru(bpy)33+ (bpy = 2,2'-bipyridine) oxidn. of H2O were studied at pH 7. In accord with previous work, the O2 yield is stoichiometric (0.25 O2 per Ru(III)) when the initial [Ru(III)] is ∼10 [Co(II)]. With [Ru(III)]0 = (1-10) × 10-5 M, excess Ru(II), [Co(II)] = (1-10) × 10-6 M, the rate law is d[Ru(III)]/dt = a[Ru(III)]2[Co(II)]/[Ru(II)][H+]2 with a = (4 ± 1) × 10-10 M s-1 at 25°, pH 6.5-7.2 (0.025 M phosphate, 0.06 ionic strength). The results indicate rate-detg. formation of Co(IV) which then reacts with H2O (or OH-) to give H2O or O2 and regenerate Co(II). Depending on the [Co(II)] and the [Ru(III)]/[Co(II)] ratio, catalyst deactivation (via scavenging of Co(IV) and Co(II) or oligomerization of Co(III)) may deplete the active Co(II) pool and yield hydroxocobalt(III) solid, slow rates, and low O2 yields.
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39. 39
  Thomsen, J. M.; Huang, D. L.; Crabtree, R. H.; Brudvig, G. W. Iridium-Based Complexes for Water Oxidation. Dalton Trans. 2015, 44, 12452– 12472, DOI: 10.1039/C5DT00863H
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  Iridium-based complexes for water oxidation
  Thomsen, Julianne M.; Huang, Daria L.; Crabtree, Robert H.; Brudvig, Gary W.
  Dalton Transactions (2015), 44 (28), 12452-12472CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
  Organometallic Ir precatalysts have been found to yield homogeneous Ir-based water-oxidn. catalysts (WOCs) with very high activity. The Cp*Ir catalyst series can operate under a variety of regimes: it can either act as a homogeneous or a heterogeneous catalyst; it can be driven by chem., photochem., or electrochem. methods; and the mol. catalyst can either act in soln. or supported as a mol. unit on a variety of solid oxides. In addn. to optimizing the various reaction conditions, work has continued to elucidate the catalyst activation mechanism and identify water-oxidn. intermediates. This perspective describes the development of the Cp*Ir series, their many forms as WOCs, and their ongoing characterization.
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40. 40
  (a) Winkler, J. R.; Gray, H. B. Electronic Structures of Oxo-Metal Ions. Struct. Bonding (Berlin, Ger.) 2011, 142, 17– 28, DOI: 10.1007/430_2011_55
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  (b) Winkler, J. R.; Gray, H. B. Living with Oxygen. Acc. Chem. Res. 2018, 51, 1850– 1857, DOI: 10.1021/acs.accounts.8b00245
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  Living with Oxygen
  Gray, Harry B.; Winkler, Jay R.
  Accounts of Chemical Research (2018), 51 (8), 1850-1857CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
  A review. Work on the electronic structures of metal-oxo complexes began in Copenhagen over fifty years ago. This work led to the prediction that tetragonal multiply bonded transition metal-oxos would not be stable beyond the iron-ruthenium-osmium oxo wall in the periodic table, and that triply bonded metal-oxos could not be protonated, even in the strongest Bronsted acids. In this theory, only doubled bonded metal-oxos could attract protons, with basicities a function of the electron donating ability of ancillary ligands. Such correlations of electronic structure with reactivity have gained importance in recent years, most notably owing to the widespread recognition that high-valent iron-oxos are intermediates in biol. reactions crit. to life on Earth. In this Account we focus attention on the oxygenations of inert org. substrates by cytochromes P 450, as these reactions involve multiply bonded iron-oxos. We emphasize that P 450 iron-oxos are strong oxidants, so strong that they would destroy nearby amino acids if substrates are not oxygenated rapidly; it is our view that these high valent iron oxos are such dangerous reactive oxygen species that Nature surely found ways to disable them. Looking more deeply into this matter, mainly by examg. many thousands of structures in the protein data bank, we have found that P450s and other enzymes that require oxygen for function have chains of tyrosines and tryptophans that extend from active-site regions to protein surfaces. Tyrosines are near the heme active sites in bacterial P450s, whereas tryptophan is closest in most human enzymes. High-valent iron-oxo survival times taken from hole hopping maps range from a few nanoseconds to milliseconds, depending on the distance of the closest Trp or Tyr residue to the heme. In our proposed mechanism, multistep hole tunneling (hopping) through Tyr/Trp chains guides the damaging oxidizing hole to the protein surface, where it can be quenched by sol. protein or small mol. reductants. As the Earth's oxygenic atm. is believed to have developed about 2.5 billion years ago, the increase in occurrence frequency of tyrosine and tryptophan since the last universal evolutionary ancestor may be in part a consequence of enzyme protective functions that developed to cope with the environmental toxin - O2.
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41. 41
  Holm, R. H. Metal-Centered Oxygen Atom Transfer Reactions. Chem. Rev. 1987, 87, 1401– 1449, DOI: 10.1021/cr00082a005
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  Metal-centered oxygen atom transfer reactions
  Holm, R. H.
  Chemical Reviews (Washington, DC, United States) (1987), 87 (6), 1401-49CODEN: CHREAY; ISSN:0009-2665.
  A review with 545 refs. of O atom transfer reactions of transition metal complexes.
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42. 42
  O’Halloran, K. P.; Zhao, C.; Ando, N. S.; Schultz, A. J.; Koetzle, T. F.; Piccoli, P. M. B.; Hedman, B.; Hodgson, K. O.; Bobyr, E.; Kirk, M. L.; Knottenbelt, S.; Depperman, E. C.; Stein, B.; Anderson, T. M.; Cao, R.; Geletii, Y. V.; Hardcastle, K. I.; Musaev, D. G.; Neiwert, W. A.; Fang, X.; Morokuma, K.; Wu, S.; Kögerler, P.; Hill, C. L. Revisiting the Polyoxometalate-Based Late-Transition-Metal-Oxo Complexes: The “Oxo Wall” Stands. Inorg. Chem. 2012, 51, 7025– 7031, DOI: 10.1021/ic2008914
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  Revisiting the Polyoxometalate-Based Late-Transition-Metal-Oxo Complexes: The "Oxo Wall" Stands
  O'Halloran, Kevin P.; Zhao, Chongchao; Ando, Nicole S.; Schultz, Arthur J.; Koetzle, Thomas F.; Piccoli, Paula M. B.; Hedman, Britt; Hodgson, Keith O.; Bobyr, Elena; Kirk, Martin L.; Knottenbelt, Sushilla; Depperman, Ezra C.; Stein, Benjamin; Anderson, Travis M.; Cao, Rui; Geletii, Yurii V.; Hardcastle, Kenneth I.; Musaev, Djamaladdin G.; Neiwert, Wade A.; Fang, Xikui; Morokuma, Keiji; Wu, Shaoxiong; Kogerler, Paul; Hill, Craig L.
  Inorganic Chemistry (2012), 51 (13), 7025-7031CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  Terminal oxo complexes of the late transition metals Pt, Pd, and Au are reported by the authors in Science and Journal of the American Chem. Society. Despite thoroughness in characterizing these complexes (multiple independent structural methods and up to 17 anal. methods in one case), the authors have continued to study these structures. Initial work on these systems was motivated by structural data from x-ray crystallog. and neutron diffraction and 17O and 31P NMR signatures which all indicated differences from all previously published compds. With significant new data, the authors now revisit these studies. New x-ray crystal structures of previously reported complexes K14[P2W19O69(OH2)] and K10Na3[PdIV(O)(OH)WO(OH2)(PW9O34)2] and a closer examn. of these structures are provided. Also presented are the 17O NMR spectrum of an 17O-enriched sample of [PW11O39]7- and a careful combined 31P NMR-titrn. study of the previously reported K7H2[Au(O)(OH2)P2W20O70(OH2)2]. These and considerable other data collectively indicate that previously assigned terminal Pt-oxo and Au-oxo complexes are in fact cocrystals of the all-tungsten structural analogs with noble metal cations, while the Pd-oxo complex is a disordered Pd(II)-substituted polyoxometalate. The neutron diffraction data were re-analyzed, and new refinements are fully consistent with the all-tungsten formulations of the Pt-oxo and Au-oxo polyoxometalate species.
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43. 43
  Stull, J. A.; Stich, T. A.; Hurst, J. K.; Britt, R. D. Electron Paramagnetic Resonance Analysis of a Transient Species Formed During Water Oxidation Catalyzed by the Ion [(bpy)₂Ru(OH₂)]₂O⁴⁺. Inorg. Chem. 2013, 52, 4578– 4586, DOI: 10.1021/ic4001158
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  43
  Electron Paramagnetic Resonance Analysis of a Transient Species Formed During Water Oxidation Catalyzed by the Complex Ion [(bpy)2Ru(OH2)]2O4+
  Stull, Jamie A.; Stich, Troy A.; Hurst, James K.; Britt, R. David
  Inorganic Chemistry (2013), 52 (8), 4578-4586CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  The Ru blue dimer [(bpy)2Ru(OH2)]2O4+-the 1st well-defined mol. complex able to catalyze H2O oxidn. at low overpotentials-was the subject of numerous exptl. and computational studies. However, elements of the reaction mechanism remain controversial. Of particular interest is the nature of the O-O bond-forming step. Herein, the authors report the 1st advanced EPR spectroscopic studies of a high-valent intermediate that appears under conditions in which the catalyst is actively turning over. Results from previous studies suggested that this intermediate is derived from [(bpy)2RuV(O)]2O4+, denoted {5,5}. Under photooxidizing conditions, the corresponding EPR signal disappears at a rate comparable to the turnover rate of the catalyst once the illumination source is removed. The electronic and geometric structures of this species were explored using a variety of EPR techniques. Continuous wave (CW) EPR spectroscopy was used to probe the hyperfine coupling of the Ru ions, while corresponding ligand 14N hyperfine couplings were characterized with electron spin echo envelope modulation (ESEEM) and hyperfine sublevel correlation spectroscopy (HYSCORE) methods. Finally, 1H/2H ENDOR was performed to monitor any exchangeable protons. The authors' studies strongly suggest that the accumulating transient is an S = 1/2 species. This spin state formulation of the so-called {5,5} species is consistent with only a limited no. of electronic structures, each of which is discussed. Notably, the obsd. large metal hyperfine coupling indicates that the orbital carrying the unpaired spin has significant ruthenyl-oxyl character, contrary to an earlier electronic structure description that had tentatively assigned the signal to formation of a bipyridine ligand radical.
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44. 44
  (a) Gajhede, M.; Schuller, D. J.; Henriksen, A.; Smith, A. T.; Poulos, T. L. Crystal Structure of Horseradish Peroxidase C at 2.15 Å Resolution. Nat. Struct. Biol. 1997, 4, 1032– 1038, DOI: 10.1038/nsb1297-1032
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  Crystal structure of horseradish peroxidase C at 2.15 Å resolution
  Gajhede, Michael; Schuller, David J.; Henriksen, Anette; Smith, Andrew T.; Poulos, Thomas L.
  Nature Structural Biology (1997), 4 (12), 1032-1038CODEN: NSBIEW; ISSN:1072-8368. (Nature America)
  The crystal structure of horseradish peroxidase isoenzyme C (I) was solved to 2.15 Å resoln. An important feature unique to the class III peroxidases is a long insertion, 34 residues in I, between helixes F and G. This region, which defines part of the substrate access channel, is not present in the core conserved fold typical of peroxidases from classes I and II. Comparison of I and peanut peroxidase (II), the only other class III (higher plant) peroxidase for which an x-ray structure has been completed, reveals that the structure in this region is highly variable even within class III. For peroxidases of the I type, characterized by a larger FG insertion (7 residues relative to II) and a shorter F' helix, the authors identified the key residue involved in direct interactions with arom. donor mols. I is unique in having a ring of 3 peripheral Phe residues, 142, 68, and 179. These guard the entrance to the exposed heme edge. The authors predict that this arom. region is important for the ability of I to bind arom. substrates.
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  (b) Rodríguez-López, J. N.; Lowe, D. J.; Hernández-Ruiz, J.; Hiner, A. N. P.; García-Cánovas, F.; Thorneley, R. N. F. Mechanism of Reaction of Hydrogen Peroxide with Horseradish Peroxidase: Identification of Intermediates in the Catalytic Cycle. J. Am. Chem. Soc. 2001, 123, 11838– 11847, DOI: 10.1021/ja011853+
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  Mechanism of reaction of hydrogen peroxide with horseradish peroxidase: Identification of intermediates in the catalytic cycle
  Rodriguez-Lopez, Jose Neptuno; Lowe, David J.; Hernandez-Ruiz, Josefa; Hiner, Alexander N. P.; Garcia-Canovas, Francisco; Thorneley, Roger N. F.
  Journal of the American Chemical Society (2001), 123 (48), 11838-11847CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The mechanism of the reaction of horseradish peroxidase isoenzyme C (HRPC) with H2O2 to form reactive enzyme intermediate compd. I was studied using electronic absorbance, rapid-scan stopped-flow, and ESR spectroscopies at both acid and basic pH. The roles of active site residues His-42 and Arg-38 in controlling heterolytic cleavage of the H2O2 O-O bond were probed with site-directed mutant enzymes H42L, R38L, and R38G. The biphasic reaction kinetics of H42L with H2O2 suggested the presence of an intermediate species and, at acid pH, a reversible 2nd step, probably due to a neutral enzyme-H2O2 complex and the ferric-peroxoanion-contg. compd. 0. ESR also indicated the formation of a protein radical situated more than ∼10 Å from the heme Fe. The stoichiometry of the reaction of the H42L/H2O2 reaction product and 2,2'-azinobis(3-ethylbenzothiazolinesulfonic acid) (ABTS) was concn.-dependent and fell from a value of 2 to 1 above 0.7 mM ABTS. These data could be explained if H2O2 underwent homolytic cleavage in H42L. The apparent rate of compd. I formation by H42L, while low, was pH-independent in contrast to wild-type HRPC where the rate fell at acid pH, indicating the involvement of an ionizable group with pKa of ∼4. In R38L and R38G, the apparent pKa was shifted to ∼8, but there was no evidence that homolytic cleavage of H2O2 occurred. These data suggest that His-42 acts initially as a proton acceptor (base catalyst) and then as a donor (acid catalyst) at neutral pH and predict the obsd. slower rate and lower efficiency of heterolytic cleavage obsd. at acid pH. Arg-38 was influential in lowering the pKa of His-42 and addnl. in aligning H2O2 in the active site, but it did not play a direct role in proton transfer.
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45. 45
  (a) Dubey, K. D.; Shaik, S. Cytochrome P450–The Wonderful Nanomachine Revealed through Dynamic Simulations of the Catalytic Cycle. Acc. Chem. Res. 2019, 52, 389– 399, DOI: 10.1021/acs.accounts.8b00467
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  45a
  Cytochrome P450-The Wonderful Nanomachine Revealed through Dynamic Simulations of the Catalytic Cycle
  Dubey Kshatresh Dutta; Shaik Sason
  Accounts of chemical research (2019), 52 (2), 389-399 ISSN:.
  This Account addresses the catalytic cycle of the enzyme cytochrome P450 (CYP450) as a prototypical biological machine with automatic features. CYP450 is a nanomachine that uses dioxygen and two reducing and two proton equivalents to oxidize a plethora of molecules (so-called substrates) as a means of supplying bio-organisms with essential molecules (e.g., brain neurotransmitters, sex hormones, etc.) and protecting biosystems against poisoning. An enticing property of CYP450s is that entrance of an oxidizable substrate into the active site initiates a series of events that constitute the catalytic cycle, which functions "automatically" in a regulated sequence of events culminating in the production of the oxidized substrates (e.g., hydroxylated, epoxidized, etc.), oftentimes with remarkable stereo- and regioselectivities. It is timely to demonstrate how theory uses molecular dynamics (MD) simulations and quantum-mechanical/molecular-mechanical (QM/MM) calculations to complement experiments and elucidate the choreography by which the protein regulates the catalytic cycle. CYP450 is a heme enzyme that contains a ferric ion (Fe(III)) coordinated by a porphyrin ligand, a water molecule, and a cysteinate ligand that is provided by a strategic residue of the encapsulating protein. While many of the individual steps are sufficiently well-understood, we shall provide here an overview of the factors that cause all of the steps to be sequentially coordinated. To this end, we use examples from three different CYP450 enzymes: the bacterial ones CYP450BM3 and CYP450CAM and the mammalian enzyme CYP4503A4. The treatment is limited to the catalytic cycle, as aspects of two-state reactivity were reviewed previously (e.g., Shaik , S. ; et al. Chem. Rev. 2005 , 105 , 2279 ). What are the principles that govern the seeming automatic feature? For example, how do substrate entrance and binding gate the enzyme? How does the reductase attachment to the enzyme affect the next steps? What triggers the attachment of the reductase? How does the electron transfer (ET) that converts Fe(III) to Fe(II) occur? Is the ET coordinated with the entrance of O2 into the active site? What is the mechanism of the latter step? Since the entrance of the substrate expels the water molecules from the active site, how do water molecules re-enter to form a proton channel, which is necessary for creating the ultimate oxidant Compound I? How do mutations that disrupt the water channel nevertheless create a competent oxidant? By what means does the enzyme produce regio- and stereoselective oxidation products? What triggers the departure of the oxidized product, and how does the exit occur in a manner that generates the resting state ready for the next cycle? This Account shows that the entrance of the substrate triggers all of the ensuing events.
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  (b) Ortiz de Montellano, P. R. Hydrocarbon Hydroxylation by Cytochrome P450 Enzymes. Chem. Rev. 2010, 110, 932– 948, DOI: 10.1021/cr9002193
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  45b
  Hydrocarbon hydroxylation by cytochrome P 450 enzymes
  Ortiz de Montellano, Paul R.
  Chemical Reviews (Washington, DC, United States) (2010), 110 (2), 932-948CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review. Here, the author focuses on cytochrome P 450-catalyzed hydrocarbon hydroxylation, the reaction that is most characteristic of P 450 isoforms. However, the principles that apply in these reactions also apply to other hydroxylation reactions, including those that occur on C atoms adjacent to N, S, or O atoms.
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46. 46
  Price, J. C.; Barr, E. W.; Tirupati, B.; Bollinger, J. M.; Krebs, C. The First Direct Characterization of a High-Valent Iron Intermediate in the Reaction of an α-Ketoglutarate-Dependent Dioxygenase: A High-Spin Fe(IV) Complex in Taurine/α-Ketoglutarate Dioxygenase (TauD) from Escherichia coli. Biochemistry 2003, 42, 7497– 7508, DOI: 10.1021/bi030011f
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  46
  The first direct characterization of a high-valent iron intermediate in the reaction of an α-ketoglutarate-dependent dioxygenase: A high-spin Fe(IV) complex in taurine/α-ketoglutarate dioxygenase (TauD) from Escherichia coli
  Price, John C.; Barr, Eric W.; Tirupati, Bhramara; Bollinger, J. Martin, Jr.; Krebs, Carsten
  Biochemistry (2003), 42 (24), 7497-7508CODEN: BICHAW; ISSN:0006-2960. (American Chemical Society)
  The Fe(II)- and α-ketoglutarate (αKG)-dependent dioxygenases have roles in synthesis of collagen and sensing of oxygen in mammals, in acquisition of nutrients and synthesis of antibiotics in microbes, and in repair of alkylated DNA in both. A consensus mechanism for these enzymes, involving (i) addn. of O2 to a five-coordinate, (His)2(Asp)-facially coordinated Fe(II) center to which αKG is also bound via its C-1 carboxylate and ketone oxygen; (ii) attack of the uncoordinated oxygen of the bound O2 on the ketone carbonyl of αKG to form a bicyclic Fe(IV)-peroxyhemiketal complex; (iii) decarboxylation of this complex concomitantly with formation of an oxo-ferryl (Fe(IV):O2-) intermediate; and (iv) hydroxylation of the substrate by the Fe(IV):O2- complex via a substrate radical intermediate, has repeatedly been proposed, but none of the postulated intermediates occurring after addn. of O2 has ever been detected. An oxidized Fe intermediate in the reaction of one of these enzymes, taurine/α-ketoglutarate dioxygenase (TauD) from Escherichia coli, has been directly demonstrated by rapid kinetic and spectroscopic methods. Characterization of the intermediate and its one-electron-reduced form (obtained by low-temp. γ-radiolysis of the trapped intermediate) by Moessbauer and ESR spectroscopies establishes that it is a high-spin, formally Fe(IV) complex. Its Moessbauer isomer shift is, however, significantly greater than those of other known Fe(IV) complexes, suggesting that the iron ligands in the TauD intermediate confer significant Fe(III) character to the high-valent site by strong electron donation. The properties of the complex and previous results on related αKG-dependent dioxygenases and other nonheme-Fe(II)-dependent, O2-activating enzymes suggest that the TauD intermediate is most probably either the Fe(IV)-peroxyhemiketal complex or the taurine-hydroxylating Fe(IV):O2- species. The detection of this intermediate sets the stage for a more detailed dissection of the TauD reaction mechanism than has previously been reported for any other member of this important enzyme family.
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47. 47
  Rohde, J.-U.; In, J.-H.; Lim, M. H.; Brennessel, W. W.; Bukowski, M. R.; Stubna, A.; Münck, E.; Nam, W.; Que, L., Jr. Crystallographic and Spectroscopic Characterization of a Nonheme Fe(IV)═O Complex. Science 2003, 299, 1037– 1039, DOI: 10.1126/science.299.5609.1037
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  Crystallographic and Spectroscopic Characterization of a Nonheme Fe(IV)=O complex
  Rohde, Jan-Uwe; In, Jun-Hee; Lim, Mi Hee; Brennessel, William W.; Bukowski, Michael R.; Stubna, Audria; Muenck, Eckard; Nam, Wonwoo; Que, Lawrence, Jr.
  Science (Washington, DC, United States) (2003), 299 (5609), 1037-1039CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
  Following the heme paradigm, it is often proposed that dioxygen activation by nonheme monoiron enzymes involves an iron(IV)=oxo intermediate that is responsible for the substrate oxidn. step. Such a transient species has now been obtained from a synthetic complex with a nonheme macrocyclic ligand and characterized spectroscopically. Its high-resoln. crystal structure reveals an iron-oxygen bond length of 1.646(3) angstroms, demonstrating that a terminal iron(IV)=oxo unit can exist in a nonporphyrin ligand environment and lending credence to proposed mechanisms of nonheme iron catalysis.
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48. 48
  (a) Kryatov, S. V.; Rybak-Akimova, E. V.; Schindler, S. Kinetics and Mechanisms of Formation and Reactivity of Non-Heme Iron Oxygen Intermediates. Chem. Rev. 2005, 105, 2175– 2226, DOI: 10.1021/cr030709z
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  Kinetics and Mechanisms of Formation and Reactivity of Non-heme Iron Oxygen Intermediates
  Kryatov, Sergey V.; Rybak-Akimova, Elena V.; Schindler, Siegfried
  Chemical Reviews (Washington, DC, United States) (2005), 105 (6), 2175-2226CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review. Understanding the mechanisms of dioxygen activation at the metal centers is important for unraveling the mechanisms of metal-contg. oxidases and oxygenases, synthesizing new selective oxidn. catalysts and new drugs analogous to bleomycin, and suppressing free radical pathways of oxidative damage in biol. systems. Dioxygen-binding and -activating biomols. with nonheme iron centers include a unique glycopeptide antibiotic bleomycin and numerous proteins, which are generally grouped into two large families: mononuclear (having only one iron at the active site) and dinuclear (having two proximate irons connected by bridging ligands at the active site). Bleomycin and nonheme iron enzymes are briefly introduced. The only nonheme iron dioxygen carrier, hemerythrin, is considered in some detail. General aspects of model chem. are introduced, followed by detailed sections on the kinetics and mechanisms of dioxygen binding and activation with mono- and dinuclear nonheme iron complexes and related reactions. The last section of this review is devoted to issues of kinetic methodol. specific for dioxygen-binding studies.
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  (b) Abu-Omar, M. M.; Loaiza, A.; Hontzeas, N. Reaction Mechanism of Mononuclear Non-Heme Iron Oxygenases. Chem. Rev. 2005, 105, 2227– 2252, DOI: 10.1021/cr040653o
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  48b
  Reaction mechanisms of mononuclear non-heme iron oxygenases
  Abu-Omar, Mahdi M.; Loaiza, Aristobulo; Hontzeas, Nikos
  Chemical Reviews (Washington, DC, United States) (2005), 105 (6), 2227-2252CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review. A large no. of non-heme iron enzymes are known to catalyze a wide range of reactions with O2. The major focus of this review is on reaction mechanisms of pterin-dependent arom. amino acid hydroxylases, followed by comparisons with 3 substrate hydroxylases, and ending with a discussion of intramol. dioxygenases.
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  (c) McDonald, A. R.; Que, L., Jr. High-Valent Nonheme Iron-Oxo Complexes: Synthesis, Structure, and Spectroscopy. Coord. Chem. Rev. 2013, 257, 414– 428, DOI: 10.1016/j.ccr.2012.08.002
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  High-valent nonheme iron-oxo complexes: Synthesis, structure, and spectroscopy
  McDonald, Aidan R.; Que, Lawrence
  Coordination Chemistry Reviews (2013), 257 (2), 414-428CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
  A review. High-valent iron-oxo intermediates have often been implicated, and in some cases identified, as the active oxidant in oxygen activating nonheme iron enzymes. Recent synthetic efforts have yielded pivotal insights into the generation of oxoiron(IV and V) complexes, and allowed thorough study of their spectroscopic, structural, and electronic properties. Furthermore, insight into the mechanisms by which nonheme iron sites activate dioxygen to yield high valent iron-oxo intermediates was obtained. This review covers the great successes in iron-oxo chem. over the past decade, detailing various efforts to obtain iron-oxo complexes in high yield, and to delve into their diverse structural and spectroscopic properties.
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  (d) Fujii, H. Electronic Structure and Reactivity of High-Valent Oxo Iron Porphyrins. Coord. Chem. Rev. 2002, 226, 51– 60, DOI: 10.1016/S0010-8545(01)00441-6
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  Electronic structure and reactivity of high-valent oxo iron porphyrins
  Fujii, Hiroshi
  Coordination Chemistry Reviews (2002), 226 (1-2), 51-60CODEN: CCHRAM; ISSN:0010-8545. (Elsevier Science B.V.)
  A review of high valent oxo Fe porphyrin complexes, their electronic structure and their reactivity as models for compds.-I and compds.-II in heme enzymes.
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49. 49
  (a) Kotani, H.; Kaida, S.; Ishizuka, T.; Mieda, K.; Sakaguchi, M.; Ogura, T.; Shiota, Y.; Yoshizawa, K.; Kojima, T. Importance of the Reactant-State Potentials of Chromium(V)-Oxo Complexes to Determine the Reactivity in Hydrogen-Atom Transfer Reactions. Inorg. Chem. 2018, 57, 13929, DOI: 10.1021/acs.inorgchem.8b02453
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  49a
  Importance of the Reactant-State Potentials of Chromium(V)-Oxo Complexes to Determine the Reactivity in Hydrogen-Atom Transfer Reactions
  Kotani, Hiroaki; Kaida, Suzue; Ishizuka, Tomoya; Mieda, Kaoru; Sakaguchi, Miyuki; Ogura, Takashi; Shiota, Yoshihito; Yoshizawa, Kazunari; Kojima, Takahiko
  Inorganic Chemistry (2018), 57 (21), 13929-13936CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  A new chromium(V)-oxo complex, [CrV(O)(6-COO--py-tacn)]2+ (1; 6-COO--py-tacn = 1-(6-carboxylato-2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane), was synthesized and characterized to evaluate the reactivity of CrV(O) complexes in a hydrogen-atom transfer (HAT) reaction by comparing it with that of a previously reported CrV(O) complex, [CrV(O)(6-COO--tpa)]2+ (2; 6-COO--tpa = N,N-bis(2-pyridylmethyl)-N-(6-carboxylato-2-pyridylmethyl)amine). Definitive differences of these two CrV(O) complexes were obsd. in resonance Raman scatterings of the Cr-O bond (ν = 911 cm-1 for 1 and 951 cm-1 for 2) and the redn. potential (0.73 V vs SCE for 1 and 1.23 V for 2); this difference should be derived from that of the ligand bound at the trans position to the oxo ligand, a tertiary amino group in 1, and a pyridine nitrogen in 2. When we employed 9,10-dihydroanthracene as a substrate, the second-order rate const. (k) of 1 was 4000 times smaller than that of 2. Plots of normalized k values for both complexes relative to bond dissocn. energies (BDEs) of C-H bonds to be cleaved in several substrates showed a pair of parallel lines with slopes of -0.91 for 1 and -0.62 for 2, indicating that the HAT reactions by the two complexes proceed via almost the same transition states. Judging from estd. BDEs of CrIV(OH)/CrV(O) (85-87 kcal mol-1 for 1 and 92-94 kcal mol-1 for 2) and the activation barrier in the HAT reaction of DHA (Ea = 7.9 kcal mol-1 for 1 and Ea = 4.8 kcal mol-1 for 2), the reactivity of CrV(O) complexes in HAT reactions depends on the energy level of the reactant state rather than the product state.
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  (b) Kotani, H.; Kaida, S.; Ishizuka, T.; Sakaguchi, M.; Ogura, T.; Shiota, Y.; Yoshizawa, K.; Kojima, T. Formation and Characterization of a Reactive Chromium(V)-Oxo Complex: A Mechanistic Insight into Hydrogen-Atom Transfer Reactions. Chem. Sci. 2015, 6, 945– 955, DOI: 10.1039/C4SC02285H
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  49b
  Formation and characterization of a reactive chromium(V)-oxo complex: mechanistic insight into hydrogen-atom transfer reactions
  Kotani, Hiroaki; Kaida, Suzue; Ishizuka, Tomoya; Sakaguchi, Miyuki; Ogura, Takashi; Shiota, Yoshihito; Yoshizawa, Kazunari; Kojima, Takahiko
  Chemical Science (2015), 6 (2), 945-955CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
  A mononuclear Cr(V)-oxo complex, [CrV(O)(6-COO--tpa)](BF4)2 (1; 6-COO--tpa = N,N-bis(2-pyridylmethyl)-N-(6-carboxylato-2-pyridylmethyl)amine) was prepd. through the reaction of a Cr(III) precursor complex with iodosylbenzene as an oxidant. Characterization of 1 was achieved using ESI-MS spectrometry, ESR, UV-visible, and resonance Raman spectroscopies. The redn. potential (Ered) of 1 is 1.23 V vs. SCE in acetonitrile based on anal. of the electron-transfer (ET) equil. between 1 and a one-electron donor, [RuII(bpy)3]2+ (bpy = 2,2'-bipyridine). The reorganization energy (λ) of 1 also is 1.03 eV in ET reactions from phenol derivs. to 1 on the basis of the Marcus theory of ET. The smaller λ value in comparison with that of an Fe(IV)-oxo complex (2.37 eV) is caused by the small structural change during ET due to the dπ character of the electron-accepting LUMO of 1. When benzyl alc. derivs. (R-BA) with different oxidn. potentials were employed as substrates, corresponding aldehydes were obtained as the 2e--oxidized products in moderate yields as detd. from 1H NMR and GC-MS measurements. One-step UV-visible spectral changes were obsd. in the oxidn. reactions of BA derivs. by 1 and a kinetic isotope effect (KIE) was obsd. in the oxidn. reactions for deuterated BA derivs. at the benzylic position as substrates. The rate-limiting step is a concerted proton-coupled electron transfer (PCET) from substrate to 1. In sharp contrast, in the oxidn. of trimethoxy-BA (Eox = 1.22 V) by 1, trimethoxy-BA radical cation was obsd. by UV-visible spectroscopy. Thus, the mechanism of the oxidn. reaction changed from one-step PCET to stepwise ET-proton transfer (ET/PT), depending on the redox potentials of R-BA.
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  (c) Cho, J.; Woo, J.; Eun Han, J.; Kubo, M.; Ogura, T.; Nam, W. Chromium(V)-Oxo and Chromium(III)-Superoxo Complexes Bearing a Macrocyclic TMC Ligand in Hydrogen Atom Abstraction Reactions. Chem. Sci. 2011, 2, 2057– 2062, DOI: 10.1039/c1sc00386k
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  Chromium(V)-oxo and chromium(III)-superoxo complexes bearing a macrocyclic TMC ligand in hydrogen atom abstraction reactions
  Cho, Jaeheung; Woo, Jaeyoung; Han, Jung Eun; Kubo, Minoru; Ogura, Takashi; Nam, Wonwoo
  Chemical Science (2011), 2 (10), 2057-2062CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
  A Cr(V)-oxo complex bearing a macrocyclic TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) ligand, [CrV(TMC)(O)(OCH3)]2+, was synthesized, isolated, and characterized by various physicochem. methods, including UV-visible, ESI-MS, resonance Raman, EPR and x-ray anal. The reactivity of the Cr(V)-oxo complex was studied in C-H and O-H bond activation reactions. The reactivity of a Cr(III)-superoxo complex, [CrIII(TMC)(O2)(Cl)]+, was studied in O-H bond activation reactions as well. By comparing reactivities of the Cr(III)-superoxo and Cr(V)-oxo complexes under the identical reaction conditions, the authors were able to demonstrate that the Cr(III)-superoxo complex is more reactive than the Cr(V)-oxo complex in the activation of C-H and O-H bonds. The present results provide strong evidence that under certain circumstances, metal-superoxo species can be an alternative oxidant for high-valent metal-oxo complexes in oxygenation reactions.
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50. 50
  (a) Mandimutsira, B. S.; Ramdhanie, B.; Todd, R. C.; Wang, H.; Zareba, A. A.; Czernuszewicz, R. S.; Goldberg, D. P. A Stable Manganese(V)-Oxo Corrolazine Complex. J. Am. Chem. Soc. 2002, 124, 15170– 15171, DOI: 10.1021/ja028651d
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  A Stable Manganese(V)-Oxo Corrolazine Complex
  Mandimutsira, Beaven S.; Ramdhanie, Bobby; Todd, Ryan C.; Wang, Hailin; Zareba, Adelajda A.; Czernuszewicz, Roman S.; Goldberg, David P.
  Journal of the American Chemical Society (2002), 124 (51), 15170-15171CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  I (R = p-tBuC6H4) reacted with Mn(acac)3 to give MnL (H3L = I) which was oxidized to MnO(L). Stable MnO(L) was characterized by resonance Raman spectra. The oxidn. of PPh3 or Me2S by MnO(L) was obsd. with the formation of MnL.
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  Ishizuka, T.; Kotani, H.; Kojima, T. Characteristics and Reactivity of Ruthenium-Oxo Complexes. Dalton Trans. 2016, 45, 16727– 16750, DOI: 10.1039/C6DT03024F
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  Characteristics and reactivity of ruthenium-oxo complexes
  Ishizuka, Tomoya; Kotani, Hiroaki; Kojima, Takahiko
  Dalton Transactions (2016), 45 (42), 16727-16750CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
  A review. In this perspective, the authors have surveyed the synthetic procedure, characteristics, and reactivity of high-valent ruthenium-oxo complexes. The ruthenium-oxo complexes have served as ideal species to elucidate the characteristics of metal-oxo complexes in terms of not only geometrical and electronic structures but also oxidn. reactivity and mechanisms of oxidn. reactions. Due to the high stability and excellent reversibility of redox processes, ruthenium-oxo complexes provided significant mechanistic insights into the oxidn. of org. compds. including alcs., alkenes, and alkanes and also water from detailed kinetic anal.
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  Moyer, B. A.; Meyer, T. J. Oxobis(2,2′-bipyridine)pyridineruthenium(IV) Ion, [(bpy)₂(py)Ru═O]²⁺. J. Am. Chem. Soc. 1978, 100, 3601– 3603, DOI: 10.1021/ja00479a054
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  Oxobis(2,2'-bipyridine)pyridineruthenium(IV) ion, [(bpy)2(py)Ru:O]2+
  Moyer, Bruce A.; Meyer, Thomas J.
  Journal of the American Chemical Society (1978), 100 (11), 3601-3CODEN: JACSAT; ISSN:0002-7863.
  The novel Ru(IV) complex [(bpy)2(py)Ru:O](ClO4)2 (bpy = 2,2'-bipyridine) contg. a single terminal oxo ligand was prepd. by the 2-electron oxidn. of [(bpy)2(py)RuOH2]2+ using Ce(IV). The complex was characterized by cond. studies in aq. soln., magnetic susceptibility, and IR and electronic spectroscopy. Spectrophotometric and electrochem. techniques were used to show that the conversion in 1 M HClO4. [(Bpy)2(py)Ru:O]2+-2H+-e- → [(bpy)2(py)RuOH2]3+ - e- → [(bpy)2(py)RuOH2]3+ is chem. reversible where the Ru(IV)/Ru(III) and Ru(III)/Ru(II) redn. potentials in 1 M HClO4 at 25° are +0.994V and +0.781V, resp., vs. the SCE. The reversible acid-base behaviors of the Ru(III), [(bpy)2(py)RuOH2]3+ .dblharw. [(bpy)2(py)RuOH]2+ + H+, and Ru(II) complexes, [(bpy)2(py)RuOH2]2+ .dblharw. [(bpy)2(py)RuOH]+ + H+, were studied by spectrophotometric titrns. The resp. pKa values were 0.83 and 10.8. As an oxidant, the [(bpy)2(py)Ru:O]2+ ion reacts rapidly with PPh3 in MeCN by a net O atom transfer reaction which gives the phosphine oxide complex [(bpy)2(py)Ru(OPPh3)]2+. The phosphine oxide complex undergoes a slow 1st-order solvolysis reaction (t1/2 = 100 min. at 25°) to give free OPPh3 and [(bpy)2(py)Ru(MeCN)]2+.
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53. 53
  Dietl, N.; Schlangen, M.; Schwarz, H. Thermal Hydrogen-Atom Transfer from Methane: The Role of Radicals and Spin States in Oxo-Cluster Chemistry. Angew. Chem., Int. Ed. 2012, 51, 5544– 5555, DOI: 10.1002/anie.201108363
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  53
  Thermal Hydrogen-Atom Transfer from Methane: The Role of Radicals and Spin States in Oxo-Cluster Chemistry
  Dietl, Nicolas; Schlangen, Maria; Schwarz, Helmut
  Angewandte Chemie, International Edition (2012), 51 (23), 5544-5555CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review. Hydrogen-atom transfer (HAT), as one of the fundamental reactions in chem., is investigated with state-of-the-art gas-phase expts. in conjunction with computational studies. The focus of this Minireview concerns the role that the intrinsic properties of gaseous oxo-clusters play to permit HAT reactivity from satd. hydrocarbons at ambient conditions. In addn., mechanistic implications are discussed which pertain to heterogeneous catalysis. From these combined exptl./computational studies, the crucial role of unpaired spin d. at the abstracting atom becomes clear, in distinct contrast to recent conclusions derived from soln.-phase expts.
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54. 54
  Usharani, D.; Lacy, D. C.; Borovik, A. S.; Shaik, S. Dichotomous Hydrogen Atom Transfer vs Proton-Coupled Electron Transfer During Activation of X-H Bonds (X = C, N, O) by Nonheme Iron-Oxo Complexes of Variable Basicity. J. Am. Chem. Soc. 2013, 135, 17090– 17104, DOI: 10.1021/ja408073m
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  54
  Dichotomous Hydrogen Atom Transfer vs Proton-Coupled Electron Transfer During Activation of X-H Bonds (X = C, N, O) by Nonheme Iron-Oxo Complexes of Variable Basicity
  Usharani, Dandamudi; Lacy, David C.; Borovik, A. S.; Shaik, Sason
  Journal of the American Chemical Society (2013), 135 (45), 17090-17104CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  We describe herein the hydrogen-atom transfer (HAT)/proton-coupled electron-transfer (PCET) reactivity for FeIV-oxo and FeIII-oxo complexes (1-4) that activate C-H, N-H, and O-H bonds in 9,10-dihydroanthracene (S1), DMF (S2), 1,2-diphenylhydrazine (S3), p-methoxyphenol (S4), and 1,4-cyclohexadiene (S5). In 1-3, the iron is pentacoordinated by tris-[N'-tert-butylureaylato-N-ethylene]-aminato ([H3buea]3) or its derivs. These complexes are basic, in the order 3 » 1 > 2. Oxidant 4, [FeIVN4Py-(O)]2+ (N4Py: N,N-bis-(2-pyridylmethyl)-bis-(2-pyridyl)-methylamine), is the least basic oxidant. The DFT results match exptl. trends and exhibit a mechanistic spectrum ranging from concerted HAT and PCET reactions to concerted-asynchronous proton transfer (PT)/electron transfer (ET) mechanisms, all the way to PT. The singly occupied orbital along the O···H···X (X = C, N, O) moiety in the TS shows clearly that in the PCET cases, the electron is transferred sep. from the proton. The Bell-Evans-Polanyi principle does not account for the obsd. reactivity pattern, as evidenced by the scatter in the plot of calcd. barrier vs reactions driving forces. However, a plot of the deformation energy in the TS vs the resp. barrier provides a clear signature of the HAT/PCET dichotomy. Thus, in all C-H bond activations, the barrier derives from the deformation energy required to create the TS, whereas in N-H/O-H bond activations, the deformation energy is much larger than the corresponding barrier, indicating the presence of a stabilizing interaction between the TS fragments. A valence bond model is used to link the obsd. results with the basicity/acidity of the reactants.
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55. 55
  (a) Ohzu, S.; Ishizuka, T.; Hirai, Y.; Jiang, H.; Sakaguchi, M.; Ogura, T.; Fukuzumi, S.; Kojima, T. Mechanistic Insight into Catalytic Oxidations of Organic Compounds by Ruthenium(IV)-Oxo Complexes with Pyridylamine Ligands. Chem. Sci. 2012, 3, 3421– 3431, DOI: 10.1039/c2sc21195e
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  55a
  Mechanistic insight into catalytic oxidations of organic compounds by ruthenium(IV)-oxo complexes with pyridylamine ligands
  Ohzu, Shingo; Ishizuka, Tomoya; Hirai, Yuichirou; Jiang, Hua; Sakaguchi, Miyuki; Ogura, Takashi; Fukuzumi, Shunichi; Kojima, Takahiko
  Chemical Science (2012), 3 (12), 3421-3431CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
  A series of Ru(IV)-oxo complexes with tris(2-pyridylmethyl)amine derivs. [N(CH2C5H4)(CH2C5H3R)RuO(OH2)n]n+ (4-6, R = H, CO2; n = 0, 1) were synthesized from the corresponding Ru(II)-aqua complexes [N(CH2C5H4)(CH2C5H3R)Ru(OH2)m]n+ (1-3, same R, m = 1, 2) and fully characterized by 1H NMR and resonance Raman spectroscopies, and ESI-MS spectrometry. Based on the diamagnetic character confirmed by the 1H NMR spectroscopy in D2O, the spin states of 5 and 6 were detd. to be S = 0 in the d4 configuration, in sharp contrast to that of 4 being in the S = 1 spin state. The aqua-complexes 1-3 catalyzed oxidn. of alcs. and olefins using (NH4)2[CeIV(NO3)6] (CAN) as an electron-transfer oxidant in acidic aq. solns. Comparison of the reactivity of electrochem. generated oxo-complexes 4-6 was made in the light of kinetic analyses for oxidn. of 1-propanol and a water-sol. ethylbenzene deriv. The oxo complexes 4-6 exhibited no significant difference in the reactivity for the oxidn. reactions, judging from the similar catalytic rates and the activation parameters. The slight difference obsd. in the reaction rates can be accounted for by the difference in the redn. potentials of the oxo-complexes, but the spin states of the oxo-complexes have hardly affected the reactivity. The activation parameters and the kinetic isotope effects (KIE) obsd. for the oxidn. reactions of methanol indicate that the oxidn. reactions of alcs. with the RuIV:O complexes proceed via a concerted proton-coupled electron transfer mechanism.
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  .
  (b) Kojima, T.; Hirai, Y.; Ishizuka, T.; Shiota, Y.; Yoshizawa, K.; Ikemura, K.; Ogura, T.; Fukuzumi, S. A Low-Spin Ruthenium(IV)-Oxo Complex: Does the Spin State Have an Impact on the Reactivity?. Angew. Chem., Int. Ed. 2010, 49, 8449– 8453, DOI: 10.1002/anie.201002733
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  55b
  A Low-Spin Ruthenium(IV)-Oxo Complex: does the Spin State Have an Impact on the Reactivity?
  Kojima, Takahiko; Hirai, Yuichirou; Ishizuka, Tomoya; Shiota, Yoshihito; Yoshizawa, Kazunari; Ikemura, Kenichiro; Ogura, Takashi; Fukuzumi, Shunichi
  Angewandte Chemie, International Edition (2010), 49 (45), 8449-8453, S8449/1-S8449/23CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A low-spin ruthenium(IV)-oxo complex and does the spin state have an impact on the reactivity are discussed.
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56. 56
  Schröder, D.; Roithová, J.; Alikhani, E.; Kwapien, K.; Sauer, J. Preferential Activation of Primary C-H Bonds in the Reactions of Small Alkanes with the Diatomic MgO^•+ Cation. Chem. - Eur. J. 2010, 16, 4110– 4119, DOI: 10.1002/chem.200902373
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57. 57
  Siegbahn, P. E. M. O-O Bond Formation in the S4 State of the Oxygen-Evolving Complex in Photosystem II. Chem. - Eur. J. 2006, 12, 9217– 9227, DOI: 10.1002/chem.200600774
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  57
  O-O bond formation in the S4 state of the oxygen-evolving complex in photosystem II
  Siegbahn, Per E. M.
  Chemistry - A European Journal (2006), 12 (36), 9217-9227CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Based on recent X-ray structures of the oxygen-evolving complex in photosystem II, quantum chem. geometry optimizations of several thousand structures have been performed in order to elucidate the mechanism for dioxygen formation. Many of the results of these calcns. have been presented previously. The energetically most stable structure of the S4 state has been used in the present study to investigate essentially all the possible ways the O-O bond can be formed in this structure. A key feature, emphasized previously, of the S4 state is that an oxygen radical ligand is present rather than an MnV state. Previous studies have indicated that this oxygen radical can form an O-O bond by an attack from a water mol. in the second coordination shell. The present systematic investigation has led to a new type of mechanism that is significantly favored over the previous one. A calcd. transition-state barrier of 12.5 kcal mol-1 was found for this mechanism, whereas the best previous results gave 18-20 kcal mol-1. A requirement on the spin alignment for a low barrier is formulated.
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58. 58
  (a) Oda, A.; Ohkubo, T.; Yumura, T.; Kobayashi, H.; Kuroda, Y. Identification of a Stable Zn^II-Oxyl Species Produced in an MFI Zeolite and Its Reversible Reactivity with O₂ at Room Temperature. Angew. Chem., Int. Ed. 2017, 56, 9715– 9718, DOI: 10.1002/anie.201702570
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  58a
  Identification of a Stable ZnII-Oxyl Species Produced in an MFI Zeolite and Its Reversible Reactivity with O2 at Room Temperature
  Oda, Akira; Ohkubo, Takahiro; Yumura, Takashi; Kobayashi, Hisayoshi; Kuroda, Yasushige
  Angewandte Chemie, International Edition (2017), 56 (33), 9715-9718CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Although a terminal oxyl species bound to certain metal ions is believed to be the intermediate for various oxidn. reactions, such as O-O bond generation in photosystem II (PSII), such systems have not been characterized. Herein, we report a stable ZnII-oxyl species induced by an MFI-type zeolite lattice and its reversible reactivity with O2 at room temp. Its intriguing characteristics were confirmed by in situ spectroscopic studies in combination with quantum-chem. calcns., namely analyses of the vibronic Franck-Condon progressions and the ESR signal features of both ZnII-oxyl and ZnII-ozonide species formed during this reversible process. MO analyses revealed that the reversible reaction between a ZnII-oxyl species and an O2 mol. proceeds via a radical O-O coupling-decoupling mechanism; the unpaired electron of the oxyl species plays a pivotal role in the O-O bond generation process.
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  .
  (b) Oda, A.; Ohkubo, T.; Yumura, T.; Kobayashi, H.; Kuroda, Y. Room-Temperature Activation of the C-H Bond in Methane over Terminal Zn^II-Oxyl Species in an MFI Zeolite: A Combined Spectroscopic and Computational Study of the Reactive Frontier Molecular Orbitals and Their Origins. Inorg. Chem. 2019, 58, 327– 338, DOI: 10.1021/acs.inorgchem.8b02425
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  58b
  Room-Temperature Activation of the C-H Bond in Methane over Terminal ZnII-Oxyl Species in an MFI Zeolite: A Combined Spectroscopic and Computational Study of the Reactive Frontier Molecular Orbitals and Their Origins
  Oda, Akira; Ohkubo, Takahiro; Yumura, Takashi; Kobayashi, Hisayoshi; Kuroda, Yasushige
  Inorganic Chemistry (2019), 58 (1), 327-338CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  Oxygenase reactivity toward selective partial oxidn. of CH4 to CH3OH requires an at. oxygen-radical bound to metal (M-O•: oxyl intermediate) that is capable of abstracting an H atom from the significantly strong C-H bond in CH4. Because such a reaction is frequently obsd. in metal-doped zeolites, it has been recognized that the zeolite provides an environment that stabilizes the M-O• intermediate. However, no exptl. data of M-O• have so far been discovered in the zeolite; thus, little is known about the correlation among the state of M-O•, its reactivity for CH4, and the nature of the zeolite environment. Here, we report a combined spectroscopic and computational study of the room-temp. activation of CH4 over ZnII-O• in the MFI zeolite. One ZnII-O• species does perform H-abstraction from CH4 at room temp. The resultant CH•3 species reacts with the other ZnII-O• site to form the ZnII-OCH3 species. The H2O-assisted extn. of surface methoxide yields 29 μmol g-1 of CH3OH with a 94% selectivity. The quantum mechanics (QM)/mol. mechanics (MM) calcn. detd. the central step as the oxyl-mediated hydrogen atom transfer which requires an activation energy of only 10 kJ mol-1. On the basis of the findings in gas-phase expts. regarding the CH4 activation by the free [M-O•]+ species, the remarkable H-abstraction reactivity of the ZnII-O• species in zeolites was totally rationalized. Addnl., the exptl. validated QM/MM calcn. revealed that the zeolite lattice has potential as the ligand to enhance the polarization of the M-O• bond and thereby enables to create effectively the highly reactive M-O• bond required for low-temp. activation of CH4. The present study proposes that tuning of the polarization effect of the anchoring site over heterogeneous catalysts is the valuable way to create the oxyl-based functionality on the heterogeneous catalyst.
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59. 59
  (a) Kobayashi, K.; Ohtsu, H.; Wada, T.; Kato, T.; Tanaka, K. Characterization of a Stable Ruthenium Complex with an Oxyl Radical. J. Am. Chem. Soc. 2003, 125, 6729– 6739, DOI: 10.1021/ja0211510
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  59a
  Characterization of a Stable Ruthenium Complex with an Oxyl Radical
  Kobayashi, Katsuaki; Ohtsu, Hideki; Wada, Tohru; Kato, Tatsuhisa; Tanaka, Koji
  Journal of the American Chemical Society (2003), 125 (22), 6729-6739CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The ruthenium oxyl radical complex, [RuII(trpy)(Bu2SQ)O•-] (trpy = 2,2':6',2''-terpyridine, Bu2SQ = 3,5-di-tert-butyl-1,2-benzosemiquinone) was prepd. for the first time by the double deprotonation of the aqua ligand of [RuIII(trpy)(Bu2SQ)(OH2)](ClO4)2. [RuIII(trpy)(Bu2SQ)(OH2)](ClO4)2 is reversibly converted to [RuIII(trpy)(Bu2SQ)(OH-)]+ upon dissocn. of the aqua proton (pKa 5.5). Deprotonation of the hydroxo proton gave rise to intramol. electron transfer from the resultant O2- to Ru-dioxolene. The resultant [RuII(trpy)(Bu2SQ)O•-] showed antiferromagnetic behavior with a RuII-semiquinone moiety and oxyl radical, the latter of which was characterized by a spin trapping technique. The most characteristic structural feature of [RuII(trpy)(Bu2SQ)O•-] is a long Ru-O bond length (2.042(6) Å) as the first terminal metal-O bond with a single bond length. To elucidate the substituent effect of a quinone ligand, [RuIII(trpy)(4ClSQ)(OH2)](ClO4)2 (4ClSQ = 4-chloro-1,2-benzosemiquinone) was prepd. and the authors compared the deprotonation behavior of the aqua ligand with that of [RuIII(trpy)(Bu2SQ)(OH2)](ClO4)2. Deprotonation of the aqua ligand of [RuIII(trpy)(4ClSQ)(OH2)](ClO4)2 induced intramol. electron transfer from OH- to the [RuIII(4ClSQ)] moiety affording [RuII(trpy)(4ClSQ)(OH•)]+, which then probably changed to [RuII(trpy)(4ClSQ)O•-]. The antiferromagnetic interactions (J values) between RuII-semiquinone and the oxyl radical for [RuII(trpy)(Bu2SQ)O•-] and for [RuII(trpy)(4ClSQ)O•-] were 2J = -0.67 cm-1 and -1.97 cm-1, resp.
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  .
  (b) Kobayashi, K.; Ohtsu, H.; Wada, T.; Tanaka, K. Ruthenium Oxyl Radical Complex Containing o-Quinone Ligand Detected by ESR Measurements of Spin Trapping Technique. Chem. Lett. 2002, 31, 868– 869, DOI: 10.1246/cl.2002.868
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60. 60
  (a) Wada, T.; Tsuge, K.; Tanaka, K. Electrochemical Oxidation of Water to Dioxygen Catalyzed by the Oxidized Form of the Bis(ruthenium-hydroxo) Complex in H₂O. Angew. Chem., Int. Ed. 2000, 39, 1479– 1482, DOI: 10.1002/(SICI)1521-3773(20000417)39:8<1479::AID-ANIE1479>3.0.CO;2-4
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  60a
  Electrochemical oxidation of water to dioxygen catalyzed by the oxidized form of the bis(ruthenium-hydroxo) complex in H2O
  Wada, Tohru; Tsuge, Kiyoshi; Tanaka, Koji
  Angewandte Chemie, International Edition (2000), 39 (8), 1479-1482CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH)
  Electrochem. oxidn. of water is catalyzed by an ITO electrode modified with a bis(ruthenium-hydroxo) complex. This ruthenium complex is prepd. by reacting RuCl3 with 1,8-bis(terpyridyl)anthracene (btpyan) in MeOH, treating [Ru2Cl6(btpyan)] with 3,6-di-tert-butyl-1,2-benzenediol in the presence of KOAc in MeOH. This [RuII2(OAc)(3,6-tBu2sq)2(btpyan)]+ (3,6-tBu2sq = 3,6-di(tert-butyl)-1,2-semiquinone) was treated with triflic acid in MeOH contg. water and then sodium hexafluoroantimonate to give [Ru2(OH)2(3,6-tBu2qui)2(btpyan)](SbF6)2. When a controlled-potential electrolysis for this complex on ITO electrode was conducted at +1.7 V (vs. Ag/AgCl) in water, 1.1 mL of O2 evolved after 20.2 C had passed in the electrolysis. The current efficiency for O2 evolution was 95%.
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  .
  (b) Wada, T.; Tsuge, K.; Tanaka, K. Syntheses and Redox Properties of Bis(hydroxoruthenium) Complexes with Quinone and Bipyridine Ligands. Water-Oxidation Catalysis. Inorg. Chem. 2001, 40, 329– 337, DOI: 10.1021/ic000552i
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  60b
  Syntheses and Redox Properties of Bis(hydroxoruthenium) Complexes with Quinone and Bipyridine Ligands. Water-Oxidation Catalysis
  Wada, Tohru; Tsuge, Kiyoshi; Tanaka, Koji
  Inorganic Chemistry (2001), 40 (2), 329-337CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  The novel bridging ligand 1,8-bis(2,2':6',2''-terpyridyl)anthracene (btpyan) was synthesized by three reactions from 1,8-diformylanthracene to connect two [Ru(L)(OH)]+ units (L = 3,6-di-tert-butyl-1,2-benzoquinone (3,6-tBu2qui) and 2,2'-bipyridine (bpy)). An addn. of tBuOK (2.0 equiv) to a methanolic soln. of [RuII2(OH)2(3,6-tBu2qui)2(btpyan)](SbF6)2 ([1](SbF6)2) gave [RuII2(O)2(3,6-tBu2sq)2(btpyan)]0 (3,6-tBu2sq = 3,6-di-tert-butyl-1,2-semiquinone) due to the redn. of quinone coupled with the dissocn. of the hydroxo protons. The resultant complex [RuII2(O)2(3,6-tBu2sq)2(btpyan)]0 undergoes ligand-localized oxidn. at E1/2 = +0.40 V (vs. Ag/AgCl) to give [RuII2(O)2(3,6-tBu2qui)2(btpyan)]2+ in MeOH soln. Also, metal-localized oxidn. of [RuII2(O)2(3,6-tBu2qui)2(btpyan)]2+ at Ep = +1.2 V in CF3CH2OH/ether or H2O gives [RuIII2(O)2(3,6-tBu2qui)2(btpyan)]4+, which catalyzes H2O oxidn. Controlled-potential electrolysis of [1](SbF6)2 at +1.70 V in the presence of H2O in CF3CH2OH evolves dioxygen with a current efficiency of 91% (21 turnovers). The turnover no. of O2 evolution increases to 33,500 when the electrolysis is conducted in H2O (pH 4.0) by using a [1](SbF6)2-modified ITO electrode. However, the analogous complex [RuII2(OH)2(bpy)2(btpyan)](SbF6)2 ([2](SbF6)2) shows neither dissocn. of the hydroxo protons, even in the presence of a large excess of tBuOK, nor activity for the oxidn. of H2O under similar conditions. The structure of btpyan ligand was detd. by x-ray crystallog.
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  .
  (c) Muckerman, J. T.; Polyansky, D. E.; Wada, T.; Tanaka, K.; Fujita, E. Water Oxidation by a Ruthenium Complex with Noninnocent Quinone Ligands: Possible Formation of an O-O Bond at a Low Oxidation State of the Metal. Inorg. Chem. 2008, 47, 1787– 1802, DOI: 10.1021/ic701892v
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  60c
  Water Oxidation by a Ruthenium Complex with Noninnocent Quinone Ligands: Possible Formation of an O-O Bond at a Low Oxidation State of the Metal
  Muckerman, James T.; Polyansky, Dmitry E.; Wada, Tohru; Tanaka, Koji; Fujita, Etsuko
  Inorganic Chemistry (2008), 47 (6), 1787-1802CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  Tanaka and co-workers reported a novel dinuclear Ru complex, [Ru2(OH)2(3,6-Bu2Q)2(btpyan)](SbF6)2 (3,6-Bu2Q = 3,6-ditert-butyl-1,2-benzoquinone, btpyan = 1,8-bis(2,2':6',2''-terpyrid-4'-yl)anthracene), that contains redox active quinone ligands and has an excellent electrocatalytic activity for H2O oxidn. when immobilized on an In-Sn-oxide electrode (Inorg. Chem., 2001, 40, 329-337). The novel features of the dinuclear and related mononuclear Ru species with quinone ligands, and comparison of their properties to those of the Ru analogs with the bpy ligand (bpy = 2,2'-bipyridine) replacing quinone, are summarized here together with new theor. and exptl. results that show striking features for both the dinuclear and mononuclear species. The identity and oxidn. state of key mononuclear species, including the previously reported oxyl radical, were reassigned. The gas-phase theor. calcns. indicate that the Tanaka Ru-dinuclear catalyst seems to maintain predominantly Ru(II) centers while the quinone ligands and H2O moiety are involved in redox reactions throughout the entire catalytic cycle for H2O oxidn. The theor. study identifies [Ru2(O2-)(Q-1.5)2(btpyan)] as a key intermediate and the most reduced catalyst species that is formed by removal of all 4 protons before 4-electron oxidn. takes place. While the study toward understanding the complicated electronic and geometric structures of possible intermediates in the catalytic cycle is still in progress, the current status and new directions for kinetic and mechanistic studies, and key issues and challenges in H2O oxidn. with the Tanaka catalyst (and its analogs with Cl- or NO2-substituted quinones and a species with a xanthene bridge instead an anthracene) are discussed.
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  .
  (d) Wada, T.; Muckerman, J. T.; Fujita, E.; Tanaka, K. Substituents Dependent Capability of Bis(ruthenium-dioxolene-terpyridine) Complexes toward Water Oxidation. Dalton Trans. 2011, 40, 2225– 2233, DOI: 10.1039/C0DT00977F
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  60d
  Substituents dependent capability of bis(ruthenium-dioxolene-terpyridine) complexes toward water oxidation
  Wada, Tohru; Muckerman, James T.; Fujita, Etsuko; Tanaka, Koji
  Dalton Transactions (2011), 40 (10), 2225-2233CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
  The bridging ligand, 1,8-bis(2,2':6',2''-terpyrid-4'-yl)anthracene (btpyan) was synthesized by the Miyaura-Suzuki cross coupling reaction of anthracenyl-1,8-diboronic acid and 4'-triflyl-2,2':6'-2''-terpyridine in the presence of Pd(PPh3)4 (5 mol%) with 68% in yield. Three ruthenium-dioxolene dimers, [Ru2(OH)2(dioxolene)2(btpyan)]0 (dioxolene = 3,6-di-tert-butyl-1,2-benzosemiquinone ([1]0), 3,5-dichloro-1,2-benzosemiquinone ([2]0) and 4-nitro-1,2-benzosemiquinone ([3]0)) were prepd. by the reaction of [Ru2Cl6(btpyan)]0 with the corresponding catechol. The electronic structure of [1]0 is approximated by [RuII2(OH)2(sq)2(btpyan)]0 (sq = semiquinonato). On the other hand, the electronic states of [2]0 and [3]0 are close to [RuIII2(OH)2 (cat)2(btpyan)]0 (cat = catecholato), indicating that a dioxolene having electron-withdrawing groups stabilizes [RuIII2(OH)2(cat)2(btpyan)]0 rather than [RuII2(OH)2(sq)2(btpyan)]0 as resonance isomers. No sign was found of deprotonation of the hydroxo groups of [1]0, whereas [2]0 and [3]0 showed an acid-base equil. in treatments with t-BuOLi followed by HClO4. Furthermore, controlled potential electrolysis of [1]0 deposited on an ITO (indium-tin oxide) electrode catalyzed the four-electron oxidn. of H2O to evolve O2 at potentials more pos. than +1.6 V (vs. SCE) at pH 4.0. On the other hand, the electrolysis of [2]0 and [3]0 deposited on ITO electrodes did not show catalytic activity for water oxidn. under similar conditions. Such a difference in the reactivity among [1]0, [2]0 and [3]0 is ascribed to the shift of the resonance equil. between [RuII2(OH)2(sq)2(btpyan)]0 and [RuIII2(OH)2(cat)2(btpyan)]0.
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  (e) Tanaka, K.; Isobe, H.; Yamanaka, S.; Yamaguchi, K. Similarities of Artificial Photosystems by Ruthenium Oxo Complexes and Native Water Splitting Systems. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15600– 15605, DOI: 10.1073/pnas.1120705109
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  60e
  Similarities of artificial photosystems by ruthenium oxo complexes and native water splitting systems
  Tanaka, Koji; Isobe, Hiroshi; Yamanaka, Shusuke; Yamaguchi, Kizashi
  Proceedings of the National Academy of Sciences of the United States of America (2012), 109 (39), 15600-15605, S15600/1-S15600/11CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
  The nature of chem. bonds of ruthenium(Ru)-quinine(Q) complexes, mononuclear [Ru(trpy)(3,5-t-Bu2Q)(OH2)](ClO4)2 (trpy = 2,2':6',2''-terpyridine, 3,5-di-tert-butyl-1,2-benzoquinone) (1), and binuclear [Ru2(bt pyan)(3,6-di-Bu2Q)2(OH2)]2+ (btpyan = 1,8- bis(2,2':6',2''-terpyrid-4'-yl)anthracene, 3,6-t-Bu2Q = 3,6-di-tert-butyl-1,2-benzoquinone) (2), has been investigated by broken-symmetry (BS) hybrid d. functional (DFT) methods. BS DFT computations for the Ru complexes have elucidated that the closed-shell structure (2b) Ru(II)-Q complex is less stable than the open-shell structure (2bb) consisting of Ru(III) and semiquinone (SQ) radical fragments. These computations have also elucidated eight different electronic and spin structures of tetraradical intermediates that may be generated in the course of water splitting reaction. The Heisenberg spin Hamiltonian model for these species has been derived to elucidate six different effective exchange interactions (J) for four spin systems. Six J values have been detd. using total energies of the eight (or seven) BS solns. for different spin configurations. The natural orbital analyses of these BS DFT solns. have also been performed in order to obtain natural orbitals and their occupation nos., which are useful for the lucid understanding of the nature of chem. bonds of the Ru complexes. Implications of the computational results are discussed in relation to the proposed reaction mechanisms of water splitting reaction in artificial photosynthesis systems and the similarity between artificial and native water splitting systems.
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  (f) Kikuchi, T.; Tanaka, K. Mechanistic Approaches to Molecular Catalysts for Water Oxidation. Eur. J. Inorg. Chem. 2014, 2014, 607– 618, DOI: 10.1002/ejic.201300716
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  Mechanistic Approaches to Molecular Catalysts for Water Oxidation
  Kikuchi, Takashi; Tanaka, Koji
  European Journal of Inorganic Chemistry (2014), 2014 (4), 607-618CODEN: EJICFO; ISSN:1434-1948. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review. Water oxidn., in which two water mols. undergo the coupled loss of four electrons and four protons to form an O-O bond, is one of the most appealing target reactions for mol. catalysts in view of hydrogen prodn. by electrolytic or photolytic water splitting to cope with urgent energy and environmental problems. Inspired by a natural oxygen-evolving multinuclear manganese cluster, a no. of water oxidn. catalysts based on multinuclear transition-metal complexes have been developed over the last three decades. In recent years, in parallel with the discovery of mononuclear oxygen-evolving complexes, both exptl. and theor. studies have yielded important insight into O-O bond-formation pathways in these water-oxidizing complexes. In this microreview, we will present an updated view of selected current literature focusing on the working mechanism of ruthenium-based water oxidn. catalysts and on the development of rationally designed ruthenium complexes that activate water at mild potentials.
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  Wada, T.; Tsuge, K.; Tanaka, K. Oxidation of Hydrocarbons by Mono- and Dinuclear Ruthenium Quinone Complexes via Hydrogen Atom Abstraction. Chem. Lett. 2000, 29, 910, DOI: 10.1246/cl.2000.910
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  Shimoyama, Y.; Ishizuka, T.; Kotani, H.; Shiota, Y.; Yoshizawa, K.; Mieda, K.; Ogura, T.; Okajima, T.; Nozawa, S.; Kojima, T. A Ruthenium(III)-Oxyl Complex Bearing Strong Radical Character. Angew. Chem., Int. Ed. 2016, 55, 14041– 14045, DOI: 10.1002/anie.201607861
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  A Ruthenium(III)-Oxyl Complex Bearing Strong Radical Character
  Shimoyama, Yoshihiro; Ishizuka, Tomoya; Kotani, Hiroaki; Shiota, Yoshihito; Yoshizawa, Kazunari; Mieda, Kaoru; Ogura, Takashi; Okajima, Toshihiro; Nozawa, Shunsuke; Kojima, Takahiko
  Angewandte Chemie, International Edition (2016), 55 (45), 14041-14045CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Proton-coupled electron-transfer oxidn. of a RuII-OH2 complex, having an N-heterocyclic carbene ligand, gives a RuIII-O. species, which has an electronically equiv. structure of the RuIV=O species, in an acidic aq. soln. The RuIII-O. complex was characterized by spectroscopic methods and DFT calcns. The oxidn. state of the Ru center was shown to be close to +3; the Ru-O bond showed a lower-energy Raman scattering at 732 cm-1 and the Ru-O bond length was estd. to be 1.77(1) Å. The RuIII-O. complex exhibits high reactivity in substrate oxidn. under catalytic conditions; particularly, benzaldehyde and the derivs. are oxidized to the corresponding benzoic acid through C-H abstraction from the formyl group by the RuIII-O. complex bearing a strong radical character as the active species.
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  Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. An Overview of N-Heterocyclic Carbenes. Nature 2014, 510, 485– 496, DOI: 10.1038/nature13384
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  An overview of N-heterocyclic carbenes
  Hopkinson, Matthew N.; Richter, Christian; Schedler, Michael; Glorius, Frank
  Nature (London, United Kingdom) (2014), 510 (7506), 485-496CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
  A review. The successful isolation and characterization of an N-heterocyclic carbene in 1991 opened up a new class of org. compds. for investigation. From these beginnings as academic curiosities, N-heterocyclic carbenes today rank among the most powerful tools in org. chem., with numerous applications in com. important processes. Here we provide a concise overview of N-heterocyclic carbenes in modern chem., summarizing their general properties and uses and highlighting how these features are being exploited in a selection of pioneering recent studies.
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64. 64
  (a) Hirai, Y.; Kojima, T.; Mizutani, Y.; Shiota, Y.; Yoshizawa, K.; Fukuzumi, S. Ruthenium-Catalyzed Selective and Efficient Oxygenation of Hydrocarbons with Water an an Oxygen Source. Angew. Chem., Int. Ed. 2008, 47, 5772– 5776, DOI: 10.1002/anie.200801170
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  64a
  Ruthenium-catalyzed selective and efficient oxygenation of hydrocarbons with water as an oxygen source
  Hirai, Yuichirou; Kojima, Takahiko; Mizutani, Yasuhisa; Shiota, Yoshihito; Yoshizawa, Kazunari; Fukuzumi, Shunichi
  Angewandte Chemie, International Edition (2008), 47 (31), 5772-5776CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Water is not only the solvent but also the sole oxygen source in the smooth and efficient oxidn. of org. compds. catalyzed by a RuII-pyridylamine-aqua complex with CeIV as the oxidant. An intermediate-spin RuIV-oxo complex is formed as the reactive species. This catalytic system is durable and able to gain high turnover nos. for various substrates.
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  (b) Gerli, A.; Reedijk, J.; Lakin, M. T.; Spek, A. L. Redox Properties and Electrocatalytic Activity of the Oxo/Aqua System [Ru(terpy)(bpz)(O)]²⁺/[Ru(terpy)(bpz)(H₂O)]²⁺. X-ray Crystal Structure of [Ru(terpy)(bpz)Cl]PF₆·MeCN (terpy = 2,2′,2″-Terpyridine; bpz = 2,2′-Bipyradine). Inorg. Chem. 1995, 34, 1836– 1843, DOI: 10.1021/ic00111a035
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  64b
  Redox Properties and Electrocatalytic Activity of the Oxo/Aqua System [Ru(terpy)(bpz)(O)]2+/[Ru(terpy)(bpz)(H2O)]2+. X-ray Crystal Structure of [Ru(terpy)(bpz)Cl]PF6·MeCN (terpy = 2,2',2''-Terpyridine; bpz = 2,2'-Bipyrazine)
  Gerli, Alessandra; Reedijk, Jan; Lakin, Miles T.; Spek, Anthony L.
  Inorganic Chemistry (1995), 34 (7), 1836-43CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  [Ru(terpy)(bpz)X]n+, where terpy = 2,2',2''-terpyridine, bpz = 2,2'-bipyrazine, and X = Cl- (1), H2O (2), were prepd. and characterized by UV-visible and 1H NMR spectroscopies, and for the chloride deriv. also by x-ray diffraction. [Ru(terpy)(bpz)Cl]PF6·MeCN crystallizes in the triclinic space group P‾1 with the following crystallog. parameters: a 8.9173(4), b 12.6018(8), c 13.1743(8) Å, α 70.392(5), β 81.005(4), γ 76.954(5)°, Z = 2, R1 = 0.024 [for 5531 reflections Fo > 4σ(Fo)], and ωR2 = 0.061 for 6187 unique reflections. The redox properties of 2 were studied by electrochem. techniques over the pH range 0-12 in water. Only one reversible voltammetric wave (E1/2 = +0.66 V vs. SCE at pH = 7) is obsd. for 2 in the pH range 0-11, which was assigned to the Ru(II)/Ru(IV) couple. The two-electron nature of the redox process was confirmed by a spectrophotometric titrn. of 2 with Ce(IV). The 2nd-order rate const., kcat, for the oxidn. of benzyl alc. by the electrogenerated [Ru(IV)(terpy)(bpz)(O)]2+ was evaluated by cyclic voltammetry. At pH = 11 in phosphate buffer, kcat is 23.0(7) M-1 s-1. An electrocatalytic rate const., kcat = 36.1(15) M-1 s-1, was measured in 0.1M NaOH for the oxidn. of benzyl alc. by a related compd., [Ru(IV)(terpy)(bpy)(O)]2+, where bpy = 2,2'-bipyridine.
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65. 65
  Seok, W. K.; Meyer, T. J. Mechanism of Oxidation of Benzaldehyde by Polypyridyl Oxo Complexes of Ru(IV). Inorg. Chem. 2005, 44, 3931– 3941, DOI: 10.1021/ic040119z
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  65
  Mechanism of Oxidation of Benzaldehyde by Polypyridyl Oxo Complexes of Ru(IV)
  Seok, Won K.; Meyer, Thomas J.
  Inorganic Chemistry (2005), 44 (11), 3931-3941CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  The oxidn. of benzaldehyde and several of its derivs. to their carboxylic acids by cis-[RuIV(bpy)2(py)(O)]2+ (RuIV:O2+; bpy is 2,2'-bipyridine, py is pyridine), cis-[RuIII(bpy)2(py)(OH)]2+ (RuIII-OH2+), and [RuIV(tpy)(bpy)(O)]2+ (tpy is 2,2':6',2''-terpyridine) in acetonitrile and water has been investigated using a variety of techniques. Several lines of evidence support a one-electron hydrogen-atom transfer (HAT) mechanism for the redox step in the oxidn. of benzaldehyde. They include (i) moderate kC-H/kC-D kinetic isotope effects of 8.1 ± 0.3 in CH3CN, 9.4 ± 0.4 in H2O, and 7.2 ± 0.8 in D2O; (ii) a low kH2O/D2O kinetic isotope effect of 1.2 ± 0.1; (iii) a decrease in rate const. by a factor of only ∼5 in CH3CN and ∼8 in H2O for the oxidn. of benzaldehyde by cis-[RuIII(bpy)2(py)(OH)]2+ compared to cis-[RuIV(bpy)2(py)(O)]2+; (iv) the appearance of cis-[RuIII(bpy)2(py)(OH)]2+ rather than cis-[RuII(bpy)2(py)(OH2)]2+ as the initial product; and (v) the small ρ value of -0.65 ± 0.03 in a Hammett plot of log k vs σ in the oxidn. of a series of aldehydes. A mechanism is proposed for the process occurring in the absence of O2 involving (i) preassocn. of the reactants, (ii) H-atom transfer to RuIV:O2+ to give RuIII-OH2+ and PḣCO, (iii) capture of PḣCO by RuIII-OH2+ to give RuII-OC(O)Ph+ and H+, and (iv) solvolysis to give cis-[RuII(bpy)2(py)(NCCH3)]2+ or the aqua complex and the carboxylic acid as products.
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66. 66
  Shimoyama, Y.; Ishizuka, T.; Kotani, H.; Kojima, T. Catalytic Oxidative Cracking of Benzene Rings in Water. ACS Catal. 2019, 9, 671– 678, DOI: 10.1021/acscatal.8b04004
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  66
  Catalytic Oxidative Cracking of Benzene Rings in Water
  Shimoyama, Yoshihiro; Ishizuka, Tomoya; Kotani, Hiroaki; Kojima, Takahiko
  ACS Catalysis (2019), 9 (1), 671-678CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  Efficient degrdn. of harmful benzene rings in water is indispensable for achieving a clean water environment. We report herein unprecedented catalytic oxidative benzene cracking (OBC) in water using a ruthenium(II)-aqua complex having an N-heterocyclic carbene ligand as a catalyst and a cerium(IV) salt as a sacrificial oxidant under mild conditions. The OBC reactions produced carboxylic acids such as formic acid, which can be converted to dihydrogen directly from the OBC soln. using a rhodium(III) catalyst with adjustment of the soln. pH to 3.3. The OBC reactions can be applied to monosubstituted benzene derivs. such as ethylbenzene, chlorobenzene, and benzoic acid. Initial rates of the OBC reactions showed a linear relationship in the Hammett plot with a neg. slope, indicating the electrophilicity of a Ru(III)-oxyl complex as the reactive species in the catalytic OBC reaction. Also, we discuss a plausible mechanism of the catalytic OBC reactions based on the kinetic anal. and the product stoichiometry for the OBC reaction of nonvolatile sodium m-xylene sulfonate. The addn. of an electrophilic radical to the arom. ring to form arene oxide/oxepin is proposed as the initial step of the OBC reaction.
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67. 67
  Fukuzumi, S.; Kobayashi, T.; Suenobu, T. Efficient Catalytic Decomposition of Formic Acid for the Selective Generation of H₂ and H/D Exchange with a Water-Soluble Rhodium Complex in Aqueous Solution. ChemSusChem 2008, 1, 827– 834, DOI: 10.1002/cssc.200800147
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  67
  Efficient catalytic decomposition of formic acid for the selective generation of H2 and H/D exchange with a water-soluble rhodium complex in aqueous solution
  Fukuzumi, Shunichi; Kobayashi, Takeshi; Suenobu, Tomoyoshi
  ChemSusChem (2008), 1 (10), 827-834CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Formic acid (HCOOH) decomps. efficiently to afford H2 and CO2 selectively in the presence of a catalytic amt. of a water-sol. rhodium aqua complex, [RhIII(Cp*)(bpy)(H2O)]2+ (Cp* = pentamethylcyclopentadienyl, bpy = 2,2'-bipyridine) in aq. soln. at 298 K. No CO was produced in this catalytic decompn. of HCOOH. The decompn. rate reached a max. value at pH 3.8. No deterioration of the catalyst was obsd. during the catalytic decompn. of HCOOH, and the catalytic activity remained the same for the repeated addn. of HCOOH. The rhodium-hydride complex was detected as the catalytic active species that undergoes efficient H/D exchange with water. When the catalytic decompn. of HCOOH was performed in D2O, D2 was produced selectively. Such an efficient H/D exchange and the observation of a deuterium kinetic isotope effect in the catalytic decompn. of DCOOH in H2O provide valuable mechanistic insight into this efficient and selective decompn. process.
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68. 68
  (a) Chen, X.; Choing, S. N.; Aschaffenburg, D. J.; Pemmaraju, C. D.; Prendergast, D.; Cuk, T. J. Am. Chem. Soc. 2017, 139, 1830– 1841, DOI: 10.1021/jacs.6b09550
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  68a
  The Formation Time of Ti-O• and Ti-O•-Ti Radicals at the n-SrTiO3/Aqueous Interface during Photocatalytic Water Oxidation
  Chen, Xihan; Choing, Stephanie N.; Aschaffenburg, Daniel J.; Pemmaraju, C. D.; Prendergast, David; Cuk, Tanja
  Journal of the American Chemical Society (2017), 139 (5), 1830-1841CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The initial step of photocatalytic water oxidn. reaction at the metal oxide/aq. interface involves intermediates formed by trapping photogenerated, valence band holes on different reactive sites of the oxide surface. In SrTiO3, these one-electron intermediates are radicals located in Ti-O• (oxyl) and Ti-O•-Ti (bridge) groups arranged perpendicular and parallel to the surface resp., and form electronic states in the band gap of SrTiO3. Using an ultrafast sub band gap probe of 400 nm and white light, we excited transitions between these radical states and the conduction band. By measuring the time evolution of surface reflectivity following the pump pulse of 266 nm light, we detd. an initial radical formation time of 1.3 ± 0.2 ps, which is identical to the time to populate the surface with titanium oxyl (Ti-O•) radicals. The oxyl was sep. obsd. by a subsurface vibration near 800 cm-1 from Ti-O located in the plane right below Ti-O•. Second, a polarized transition optical dipole allows us to assign the 1.3 ps time const. to the prodn. of both O-site radicals. After a 4.5 ps delay, another distinct surface species forms with a time const. of 36 ± 10 ps with a yet undetd. structure. As would be expected, the radicals' decay, specifically probed by the oxyl's subsurface vibration, parallels that of the photocurrent. These results led us to propose a nonadiabatic kinetic mechanism for generating radicals of the type Ti-O• and Ti-O•-Ti from valence band holes based on their solvation at aq. interfaces.
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  (b) Herlihy, D. M.; Waegele, M. M.; Chen, X.; Pemmaraju, C. D.; Prendergast, D.; Cuk, T. Detecting the Oxyl Radical of Photocatalytic Water Oxidation at an n-SrTiO₃/Aqueous Interface through Its Subsurface Vibration. Nat. Chem. 2016, 8, 549– 555, DOI: 10.1038/nchem.2497
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  68b
  Detecting the oxyl radical of photocatalytic water oxidation at an n-SrTiO3/aqueous interface through its subsurface vibration
  Herlihy, David M.; Waegele, Matthias M.; Chen, Xihan; Pemmaraju, C. D.; Prendergast, David; Cuk, Tanja
  Nature Chemistry (2016), 8 (6), 549-555CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
  A subsurface vibration of the oxygen directly below, and uniquely generated by, the oxyl radical (Ti-O•), has been detected using theor. calcns. and ultrafast in situ IR spectra of photocatalysis at an n-SrTiO3/aq. interface. This interfacial Ti-O stretch vibration, once decoupled from the lattice, couples to reactant dynamics (water librations). These expts. demonstrate subsurface vibrations and their coupling to solvent and electron dynamics to detect nascent catalytic intermediates at the solid-liq. interface at the mol. level. One can envision using the subsurface vibrations and their coupling across the interface to track and control catalysis dynamically.
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69. 69
  Corona, T.; Pfaff, F. F.; Acuña-Parés, F.; Draksharapu, A.; Whiteoak, C. J.; Martin-Diaconescu, V.; Lloret-Fillol, J.; Browne, W. R.; Ray, K.; Company, A. Reactivity of a Nickel(II) Bis(amidate) Complex with meta-Chloroperbenzoic Acid: Formation of a Potent Oxidizing Species. Chem. - Eur. J. 2015, 21, 15029– 15038, DOI: 10.1002/chem.201501841
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  69
  Reactivity of a Nickel(II) Bis(amidate) Complex with meta-Chloroperbenzoic Acid: Formation of a Potent Oxidizing Species
  Corona, Teresa; Pfaff, Florian F.; Acuna-Pares, Ferran; Draksharapu, Apparao; Whiteoak, Christopher J.; Martin-Diaconescu, Vlad; Lloret-Fillol, Julio; Browne, Wesley R.; Ray, Kallol; Company, Anna
  Chemistry - A European Journal (2015), 21 (42), 15029-15038CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Herein, we report the formation of a highly reactive nickel-oxygen species that has been trapped following reaction of a NiII precursor bearing a macrocyclic bis(amidate) ligand with meta-chloroperbenzoic acid (HmCPBA). This compd. is only detectable at temps. below 250 K and is much more reactive toward org. substrates (i.e., C-H bonds, C-C bonds, and sulfides) than previously reported well-defined nickel-oxygen species. Remarkably, this species is formed by heterolytic O-O bond cleavage of a Ni-HmCPBA precursor, which is concluded from exptl. and computational data. On the basis of spectroscopy and DFT calcns., this reactive species is proposed to be a NiIII-oxyl compd.
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70. 70
  Srnec, M.; Navrátil, R.; Andris, E.; Jašík, J.; Roithová, J. Experimentally Calibrated Analysis of the Electronic Structure of CuO⁺: Implications for Reactivity. Angew. Chem., Int. Ed. 2018, 57, 17053– 17057, DOI: 10.1002/anie.201811362
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  70
  Experimentally Calibrated Analysis of the Electronic Structure of CuO+: Implications for Reactivity
  Srnec, Martin; Navratil, Rafael; Andris, Erik; Jasik, Juraj; Roithova, Jana
  Angewandte Chemie, International Edition (2018), 57 (52), 17053-17057CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The CuO+ core is a central motif of reactive intermediates in copper-catalyzed oxidns. occurring in nature. The high reactivity of CuO+ stems from a weak bonding between the atoms, which cannot be described by a simple classical model. To obtain the correct picture, we have investigated the acetonitrile-ligated CuO+ ion using neon-tagging photodissocn. spectroscopy at 5 K. The spectra feature complex vibronic absorption progressions in NIR and visible regions. Employing Franck-Condon analyses, we derived low-lying triplet potential energy surfaces that were further correlated with multireference calcns. This provided insight into the ground and low-lying excited electronic states of the CuO+ unit and elucidated how these states are perturbed by the change in ligation. Thus, we show that the bare CuO+ ion has prevailingly a copper(I)-biradical oxygen character. Increasing the no. of ligands coordinated to copper changes the CuO+ character towards the copper(II)-oxyl radical structure.
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71. 71
  Kojima, T.; Hayashi, K.; Iizuka, S.; Tani, F.; Naruta, Y.; Kawano, M.; Ohashi, Y.; Hirai, Y.; Ohkubo, K.; Matsuda, Y.; Fukuzumi, S. Synthesis and Characterization of Mononuclear Ruthenium(III) Pyridylamine Complexes and Mechanistic Insights into Their Catalytic Alkane Functionalization with m-Chloroperbenzoic Acid. Chem. - Eur. J. 2007, 13, 8212– 8222, DOI: 10.1002/chem.200700190
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  71
  Synthesis and characterization of mononuclear ruthenium(III) pyridylamine complexes and mechanistic insights into their catalytic alkane functionalization with m-chloroperbenzoic acid
  Kojima, Takahiko; Hayashi, Ken-ichi; Iizuka, Shin-ya; Tani, Fumito; Naruta, Yoshinori; Kawano, Masaki; Ohashi, Yuji; Hirai, Yuichirou; Ohkubo, Kei; Matsuda, Yoshihisa; Fukuzumi, Shunichi
  Chemistry - A European Journal (2007), 13 (29), 8212-8222CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Mononuclear RuIII complexes [RuCl2(L)]+, where L is tris(2-pyridylmethyl)amine (TPA) or one of four TPA derivs. as tetradentate ligand, were prepd. and characterized by spectroscopic methods, x-ray crystallog., and electrochem. measurements. The geometry of a RuIII complex having a nonthree-fold-sym. TPA ligand bearing one dimethylnicotinamide moiety was detd. to show that the nicotine moiety resides trans to a pyridine group, but not to the chloride ligand. The substituents of the TPA ligands regulate the redox potential of the Ru center, as indicated by a linear Hammett plot in the range of 200 mV for RuIII/RuIV couples with a relatively large ρ value (+0.150). These complexes act as effective catalysts for alkane functionalization in MeCN with m-chloroperbenzoic acid (mCPBA) as terminal oxidant at room temp. They exhibited fairly good reactivity for oxidn. of cyclohexane (C-H bond energy 94 kcal mol-1), and the reactivity can be altered significantly by the electronic effects of substituents on TPA ligands in terms of initial rates and turnover nos. Catalytic oxygenation of cyclohexane by a RuIII complex with 16O-mCPBA in the presence of H218O gave 18O-labeled cyclohexanol with 100% inclusion of the 18O atom from the H2O mol. Resonance Raman spectra under catalytic conditions without the substrate indicate formation of a RuIV = O intermediate with lower bonding energy. Kinetic isotope effects (KIEs) in the oxidn. of cyclohexane suggest that H abstraction is the rate-detg. step and the KIE values depend on the substituents of the TPA ligands. Thus, the reaction mechanism of catalytic cyclohexane oxygenation depends on the electronic effects of the ligands.
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72. 72
  Kojima, T.; Matsuo, H.; Matsuda, Y. A Novel and Highly Effective Halogenation of Alkanes with Halides on Oxidation with m-Chloroperbenzoic Acid: Looks Old, but New Reaction. Chem. Lett. 1998, 27, 1085– 1086, DOI: 10.1246/cl.1998.1085
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73. 73
  Fokin, A. A.; Schreiner, P. R. Selective Alkane Transformations via Radicals and Radical Cations: Insights into the Activation Step from Experiment and Theory. Chem. Rev. 2002, 102, 1551– 1594, DOI: 10.1021/cr000453m
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  73
  Selective Alkane Transformations via Radicals and Radical Cations: Insights into the Activation Step from Experiment and Theory
  Fokin, Andrey A.; Schreiner, Peter R.
  Chemical Reviews (Washington, D. C.) (2002), 102 (5), 1551-1593CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review. The activation of alkanes, which are commonly known to be not very reactive, with radicals is reviewed. Radical as well as single-electron-transfer chem. are discussed because the authors feel that these are at different ends of the same mechanistic spectrum. The review covers first radical chem., moving from traditional reagents to electrophilic radical-like species. The structures of σ-radical cations generated from different sources follow next, including the reactions of SET-oxidizers of low electrophilicity.
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74. 74
  Moonshiram, D.; Alperovich, I.; Concepcion, J. J.; Meyer, T. J.; Pushkar, Y. Experimental Demonstration of Radicaloid Character in a Ru^V═O Intermediate in Catalytic Water Oxidation. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 3765– 3770, DOI: 10.1073/pnas.1222102110
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  74
  Experimental demonstration of radicaloid character in a RuV=O intermediate in catalytic water oxidation
  Moonshiram, Dooshaye; Alperovich, Igor; Concepcion, Javier J.; Meyer, Thomas J.; Pushkar, Yulia
  Proceedings of the National Academy of Sciences of the United States of America (2013), 110 (10), 3765-3770, S3765/1-S3765/13CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
  Water oxidn. is the key half reaction in artificial photosynthesis. An absence of detailed mechanistic insight impedes design of new catalysts that are more reactive and more robust. A proposed paradigm leading to enhanced reactivity is the existence of oxyl radical intermediates capable of rapid water activation, but there is a dearth of exptl. validation. Here, we show the radicaloid nature of an intermediate reactive toward formation of the O-O bond by assessing the spin d. on the oxyl group by ESR. In the study, an 17O-labeled form of a highly oxidized, short-lived intermediate in the catalytic cycle of the water oxidn. catalyst cis,cis-[(2,2-bipyridine)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+ was investigated. It contains Ru centers in oxidn. states [4,5], has at least one RuV = O unit, and shows |Axx| = 60G 17O hyperfine splittings (hfs) consistent with the high spin d. of a radicaloid. Destabilization of pi-bonding in the d3 RuV = O fragment is responsible for the high spin d. on the oxygen and its high reactivity.
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75. 75
  Gersten, S. W.; Samuels, G. J.; Meyer, T. J. Catalytic Oxidation of Water by an Oxo-Bridged Ruthenium Dimer. J. Am. Chem. Soc. 1982, 104, 4029– 4030, DOI: 10.1021/ja00378a053
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  75
  Catalytic oxidation of water by an oxo-bridged ruthenium dimer
  Gersten, Susan W.; Samuels, George J.; Meyer, Thomas J.
  Journal of the American Chemical Society (1982), 104 (14), 4029-30CODEN: JACSAT; ISSN:0002-7863.
  When oxidized by ≥4 equiv of Ce(IV), the oxo-bridged dimer, (bpy)2(H2O)RuORu(H2O)(bpy)24+ (bpy = 2,2'-bipyridine) catalytically oxidizes water to O.
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76. 76
  Yamaguchi, K.; Shoji, M.; Saito, T.; Isobe, H.; Nishihara, S.; Koizumi, K.; Yamada, S.; Kawakami, T.; Kitagawa, Y.; Yamanaka, S.; Okumura, M. Theory of Chemical Bonds in Metalloenzymes. XV. Local Singlet and Triplet Diradical Mechanisms for Radical Coupling Reactions in the Oxygen Evolution Complex. Int. J. Quantum Chem. 2010, 110, 3101– 3128, DOI: 10.1002/qua.22914
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  76
  Theory of chemical bonds in metalloenzymes. XV. Local singlet and triplet diradical mechanisms for radical coupling reactions in the oxygen evolution complex
  Yamaguchi, Kizashi; Shoji, Mitsuo; Saito, Toru; Isobe, Hiroshi; Nishihara, Satomichi; Koizumi, Kenichi; Yamada, Satoru; Kawakami, Takashi; Kitagawa, Yasutaka; Yamanaka, Shusuke; Okumura, Mitsutaka
  International Journal of Quantum Chemistry (2010), 110 (15), 3101-3128CODEN: IJQCB2; ISSN:0020-7608. (John Wiley & Sons, Inc.)
  Reaction mechanisms of oxygen evolution in native and artificial photosynthesis II (PSII) systems have been investigated on the theor. grounds, together with exptl. results. First of all, our previous broken-symmetry (BS) MOs (MO) calcns. are reviewed to elucidate the instability of the dπ-pπ bond in high-valent (HV) Mn(X)=O systems and the dπ-pπ-dπ bond in HV Mn=O=Mn systems. The triplet instability of these bonds entails strong or intermediate diradical characters: ·Mn(IV)=O·and ·n-O-Mn· the BS MO resulted from strong electron correlation, leading to the concept of electron localizations and local spins. The BS computations have furthermore revealed guiding principles for derivation of selection rules for radical reactions of local spins. As a continuation of these theor. results, the BS MO interaction diagrams for oxygen-radical coupling reactions in the oxygen evolution complex (OEC) in the PSII have been depicted to reveal scope and applicability of local singlet diradical (LSD) and local triplet diradical (LTD) mechanisms that have been successfully utilized for theor. understanding of oxygenation reactions mechanisms by P 450 and methane monooxygenase (MMO). The manganese-oxide cluster models examd. are London, Berlin, and Berkeley models of CaMn4O4 and related clusters Mn4O4 and Mn3Ca. The BS MO interaction diagrams have revealed the LSD and/or LTD mechanisms for generation of mol. oxygen in the total low-, intermediate and high-spin states of these clusters. The spin alignments are found directly corresponding to the spin-coupling mechanisms of oxygen-radical sites in these clusters. The BS UB3LYP calcns. of the clusters have been performed to confirm the comprehensive guiding principles for oxygen evolution; charge and spin densities by BS UB3LYP are utilized for elucidation and confirmation of the LSD and LTD mechanisms. Applicability of the proposed selection rules are examd. in comparison with a lot of accumulated exptl. and theor. results for oxygen evolution reactions in native and artificial PSII systems.
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77. 77
  (a) Li, X.; Siegbahn, P. E. M. Alternative Mechanisms for O₂ Release and O–O Bond Formation in the Oxygen Evolving Complex of Photosystem II. Phys. Chem. Chem. Phys. 2015, 17, 12168– 12174, DOI: 10.1039/C5CP00138B
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  77a
  Alternative mechanisms for O2 release and O-O bond formation in the oxygen evolving complex of photosystem II
  Li, Xichen; Siegbahn, Per E. M.
  Physical Chemistry Chemical Physics (2015), 17 (18), 12168-12174CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
  In a previous detailed study of all the steps of water oxidn. in photosystem II, it was surprisingly found that O2 release is as crit. for the rate as O-O bond formation. A new mechanism for O2 release has now been found, which can be described as an opening followed by a closing of the interior of the oxygen evolving complex. A transition state for peroxide rotation forming a superoxide radical, missed in the previous study, and a structural change around the outside manganese are two key steps in the new mechanism. However, O2 release may still remain rate-limiting. Addnl., for the step forming the O-O bond, an alternative, exptl. suggested, mechanism was investigated. The new model calcns. can rule out the precise use of that mechanism. However, a variant with a rotation of the ligands around the outer manganese by about 30° will give a low barrier, competitive with the old DFT mechanism. Both these mechanisms use an oxyl-oxo mechanism for O-O bond formation involving the same two manganese atoms and the central oxo group (O5).
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  .
  (b) Siegbahn, P. E. M. Nucleophilic Water Attack is Not a Possible Mechanism for O-O Bond Formation in Photosystem II. Proc. Natl. Acad. Sci. U. S. A. 2017, 114, 4966– 4968, DOI: 10.1073/pnas.1617843114
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  77b
  Nucleophilic water attack is not a possible mechanism for O-O bond formation in photosystem II
  Siegbahn, Per E. M.
  Proceedings of the National Academy of Sciences of the United States of America (2017), 114 (19), 4966-4968CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
  Two different types of mechanisms are at present suggested for the O-O bond-formation step in photosystem II. The first one is a coupling between an oxyl radical and a bridging oxo. The second one is a nucleophilic water attack on a terminal oxo (or oxyl) group. In the present short paper, the six most reasonable versions of the latter mechanism have been studied and compared with the oxo-oxyl mechanism. The barriers are found to be much too high for the water attack, and that mechanism can therefore safely be ruled out. The reason is that the protonated peroxide product is always very high in energy.
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78. 78
  Sproviero, E. M.; Gascón, J. A.; McEvoy, J. P.; Brudvig, G. W.; Batista, V. S. Quantum Mechanics/Molecular Mechanics Study of the Catalytic Cycle of Water Splitting in Photosystem II. J. Am. Chem. Soc. 2008, 130, 3428– 3442, DOI: 10.1021/ja076130q
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  78
  Quantum Mechanics/Molecular Mechanics Study of the Catalytic Cycle of Water Splitting in Photosystem II
  Sproviero, Eduardo M.; Gascon, Jose A.; McEvoy, James P.; Brudvig, Gary W.; Batista, Victor S.
  Journal of the American Chemical Society (2008), 130 (11), 3428-3442CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  This paper investigates the mechanism of water splitting in photosystem II (PSII) as described by chem. sensible models of the oxygen-evolving complex (OEC) in the S0-S4 states. The reaction is the paradigm for engineering direct solar fuel prodn. systems since it is driven by solar light and the catalyst involves inexpensive and abundant metals (calcium and manganese). Mol. models of the OEC Mn3CaO4Mn catalytic cluster are constructed by explicitly considering the perturbational influence of the surrounding protein environment according to state-of-the-art quantum mechanics/mol. mechanics (QM/MM) hybrid methods, in conjunction with the X-ray diffraction (XRD) structure of PSII from the cyanobacterium Thermosynechococcus elongatus. The resulting models are validated through direct comparisons with high-resoln. extended X-ray absorption fine structure spectroscopic data. Structures of the S3, S4, and S0 states include an addnl. μ-oxo bridge between Mn(3) and Mn(4), not present in XRD structures, found to be essential for the deprotonation of substrate water mols. The structures of reaction intermediates suggest a detailed mechanism of dioxygen evolution based on changes in oxidization and protonation states and structural rearrangements of the oxomanganese cluster and surrounding water mols. The catalytic reaction is consistent with substrate water mols. coordinated as terminal ligands to Mn(4) and calcium and requires the formation of an oxyl radical by deprotonation of the substrate water mol. ligated to Mn(4) and the accumulation of four oxidizing equiv. The oxyl radical is susceptible to nucleophilic attack by a substrate water mol. initially coordinated to calcium and activated by two basic species, including CP43-R357 and the μ-oxo bridge between Mn(3) and Mn(4). The reaction is concerted with water ligand exchange, swapping the activated water by a water mol. in the second coordination shell of calcium.
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79. 79
  Crandell, D. W.; Xu, S.; Smith, J. M.; Baik, M.-H. Intramolecular Oxyl Radical Coupling Promotes O-O Bond Formation in a Homogeneous Mononuclear Mn-based Water Oxidation Catalyst: A Computational Mechanistic Investigation. Inorg. Chem. 2017, 56, 4435– 4445, DOI: 10.1021/acs.inorgchem.6b03144
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  79
  Intramolecular Oxyl Radical Coupling Promotes O-O Bond Formation in a Homogeneous Mononuclear Mn-based Water Oxidation Catalyst: A Computational Mechanistic Investigation
  Crandell, Douglas W.; Xu, Song; Smith, Jeremy M.; Baik, Mu-Hyun
  Inorganic Chemistry (2017), 56 (8), 4435-4445CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  The mechanism of water oxidn. performed by a recently discovered manganese pyridinophane catalyst [Mn(Py2NtBu2)(H2O)2]2+ is studied using d. functional theory methods. A complete catalytic cycle is constructed and the catalytically active species is identified to consist of a MnV-bis(oxo) moiety that is generated from the resting state by a series of proton-coupled electron transfer reactions. Whereas the electronic ground state of this key intermediate is found to be a triplet, the most favorable pathway for O-O bond formation is found on the quintet potential energy surface and involves an intramol. coupling of two oxyl radicals with opposite spins bound to the Mn-center that adopts an electronic structure most consistent formally with a high-spin MnIII ion. Therefore, the thermally accessible high-spin quintet state that constitutes a typical and innate property of a first-row transition metal center plays a crit. role for catalysis. It enables facile electron transfer between the oxo moieties and the Mn-center and promotes O-O bond formation via a radical coupling reaction with a calcd. reaction barrier of only 14.7 kcal mol-1. This mechanism of O-O coupling is unprecedented and provides a novel possible pathway to coupling two oxygen atoms bound to a single metal site.
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80. 80
  Ashley, D. C.; Baik, M.-H. The Electronic Structure of [Mn(V)═O]: What is the Connection Between Oxyl Radical Character, Physical Oxidation State, and Reactivity?. ACS Catal. 2016, 6, 7202– 7216, DOI: 10.1021/acscatal.6b01793
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  80
  The Electronic Structure of [Mn(V)=O]: What is the Connection between Oxyl Radical Character, Physical Oxidation State, and Reactivity?
  Ashley, Daniel Charles; Baik, Mu-Hyun
  ACS Catalysis (2016), 6 (10), 7202-7216CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  MnV=O functionalities are important in synthetic and bioinorg. chem., being relevant to both C-H activation and the O-O bond formation steps in enzymic water oxidn., for example. The triplet and quintet spin states are believed to be active in these reactions, but they have only been sparingly characterized exptl. D. functional theory (DFT) gives varying results, depending on the exchange-correlation functional employed, leading to ambiguity about whether the triplet MnV=O is better represented as MnIV-O•. While recent CASPT2 studies confirmed that the MnIV-O• character is exaggerated by hybrid functionals, questions still remain about the nature of this bonding. Using high-level wave function methods, the authors studied the fundamental relation between the spin polarization, diradical character, and the phys. oxidn. state assignments. In terms of formal oxidn. assignment, these species are best described as being between the MnV=O and MnIV-O• extremes. While the extent of the oxyl radical character is exaggerated in B3LYP, it is significantly underestimated by local functionals. The authors also exploited the DFT-functional dependence of the oxyl radical character to examine its effect on O-O bond formation barrier heights and concluded that, although, for radical combination reactions, the oxyl character is a significant effect, for nucleophilic water attack reactions, the effect is much smaller and is likely not a requisite feature.
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81. 81
  Leto, D. F.; Massie, A. A.; Rice, D. B.; Jackson, T. A. Spectroscopic and Computational Investigations of a Mononuclear Manganese(IV)-Oxo Complex Reveal Electronic Structure Contributions to Reactivity. J. Am. Chem. Soc. 2016, 138, 15413– 15424, DOI: 10.1021/jacs.6b08661
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  81
  Spectroscopic and Computational Investigations of a Mononuclear Manganese(IV)-Oxo Complex Reveal Electronic Structure Contributions to Reactivity
  Leto, Domenick F.; Massie, Allyssa A.; Rice, Derek B.; Jackson, Timothy A.
  Journal of the American Chemical Society (2016), 138 (47), 15413-15424CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The mononuclear Mn(IV)-oxo complex [MnIV(O)(N4py)]2+, where N4py is the pentadentate ligand N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine, was proposed to attack C-H bonds by an excited-state reactivity pattern. In this model, a 4E excited state is used to provide a lower-energy barrier for hydrogen-atom transfer. This proposal is intriguing, as it offers both a rationale for the relatively high hydrogen-atom-transfer reactivity of [MnIV(O)(N4py)]2+ and a guideline for creating more reactive complexes through ligand modification. Here the authors employ a combination of electronic absorption and variable-temp. MCD spectroscopy to exptl. evaluate this excited-state reactivity model. Using these spectroscopic methods, in conjunction with time-dependent d. functional theory (TD-DFT) and complete-active space SCF calcns. (CASSCF), the authors define the ligand-field and charge-transfer excited states of [MnIV(O)(N4py)]2+. Through a graphical anal. of the signs of the exptl. C-term MCD signals, the authors unambiguously assign a low-energy MCD feature of [MnIV(O)(N4py)]2+ as the 4E excited state predicted to be involved in hydrogen-atom-transfer reactivity. The CASSCF calcns. predict enhanced MnIII-oxyl character on the excited-state 4E surface, consistent with previous DFT calcns. Potential-energy surfaces, developed using the CASSCF methods, are used to det. how the energies and wave functions of the ground and excited states evolved as a function of Mn=O distance. The unique insights into ground- and excited-state electronic structure offered by these spectroscopic and computational studies are harmonized with a thermodn. model of hydrogen-atom-transfer reactivity, which predicts a correlation between transition-state barriers and driving force.
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82. 82
  (a) Cook, S. A.; Borovik, A. S. Molecular Designs for Controlling the Local Environments around Metal Ions. Acc. Chem. Res. 2015, 48, 2407– 2414, DOI: 10.1021/acs.accounts.5b00212
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  82a
  Molecular Designs for Controlling the Local Environments around Metal Ions
  Cook, Sarah A.; Borovik, A. S.
  Accounts of Chemical Research (2015), 48 (8), 2407-2414CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
  A review. The functions of metal complexes are directly linked to the local environment in which they are housed; modifications to the local environment (or secondary coordination sphere) are known to produce changes in key properties of the metal centers that can affect reactivity. Noncovalent interactions are the most common and influential forces that regulate the properties of secondary coordination spheres, which leads to complexities in structure that are often difficult to achieve in synthetic systems. Using key architectural features from the active sites of metalloproteins as inspiration, the authors have developed mol. systems that enforce intramol. H bonds (H-bonds) around a metal center via incorporation of H-bond donors and acceptors into rigid ligand scaffolds. The authors used these mol. species to probe mechanistic aspects of biol. dioxygen activation and H2O oxidn. This Account describes the stabilization and characterization of unusual M-oxo and heterobimetallic complexes. These types of species were implicated in a range of oxidative processes in biol. but are often difficult to study because of their inherent reactivity. The authors' H-bonding ligand systems allowed the authors to prep. an FeIII-oxo species directly from the activation of O2 that was subsequently oxidized to form a monomeric FeIV-oxo species with an S = 2 spin state, similar to those species proposed as key intermediates in nonheme monooxygenases. Also a single MnIII-oxo center that was prepd. from H2O could be converted to a high-spin MnV-oxo species via stepwise oxidn., a process that mimics the oxidative charging of the O-evolving complex (OEC) of photosystem II. Current mechanisms for photosynthetic O-O bond formation invoke a MnIV-oxyl species rather than the isoelectronic MnV-oxo system as the key oxidant based on computational studies. However, there is no exptl. information to support the existence of a Mn-oxyl radical. The authors therefore probed the amt. of spin d. on the oxido ligand of the authors' complexes using EPR spectroscopy in conjunction with O-17 labeling. The authors' findings showed that there is a significant amt. of spin on the oxido ligand, yet the M-oxo bonds are best described as highly covalent and there is no indication that an oxyl radical is formed. These results offer the intriguing possibility that high-spin M-oxo complexes are involved in O-O bond formation in biol. Ligand redesign to incorporate H-bond accepting units (sulfonamido groups) simultaneously provided a metal ion binding pocket, adjacent H-bond acceptors, and an auxiliary binding site for a 2nd metal ion. These properties allowed the authors to isolate heterobimetallic complexes of FeIII and MnIII in which a Group II metal ion were coordinated within the secondary coordination sphere. Examn. of the influence of the 2nd metal ion on the electron transfer properties of the primary metal center revealed unexpected similarities between CaII and SrII ions, a result with relevance to the OEC. The presence of a 2nd metal ion was found to prevent intramol. oxidn. of the ligand with an O atom transfer reagent.
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  .
  (b) Gupta, R.; Taguchi, T.; Lassalle-Kaiser, B.; Bominaar, E. L.; Yano, J.; Hendrich, M. P.; Borovik, A. S. High-Spin Mn-Oxo Complexes and their Relevance to the Oxygen-Evolving Complex within Photosystem II. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 5319– 5324, DOI: 10.1073/pnas.1422800112
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  82b
  High-spin Mn-oxo complexes and their relevance to the oxygen-evolving complex within photosystem II
  Gupta, Rupal; Taguchi, Taketo; Lassalle-Kaiser, Benedikt; Bominaar, Emile L.; Yano, Junko; Hendrich, Michael P.; Borovik, A. S.
  Proceedings of the National Academy of Sciences of the United States of America (2015), 112 (17), 5319-5324CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
  The structural and electronic properties of a series of manganese complexes with terminal oxido ligands are described. The complexes span three different oxidn. states at the manganese center (III-V), have similar mol. structures, and contain intramol. hydrogen-bonding networks surrounding the Mn-oxo unit. Structural studies using X-ray absorption methods indicated that each complex is mononuclear and that oxidn. occurs at the manganese centers, which is also supported by ESR (EPR) studies. This gives a high-spin MnV-oxo complex and not a MnIV-oxy radical as the most oxidized species. In addn., the EPR findings demonstrated that the Fermi contact term could exptl. substantiate the oxidn. states at the manganese centers and the covalency in the metal-ligand bonding. Oxygen-17-labeled samples were used to det. spin d. within the Mn-oxo unit, with the greatest delocalization occurring within the MnV-oxo species (0.45 spins on the oxido ligand). The exptl. results coupled with d. functional theory studies show a large amt. of covalency within the Mn-oxo bonds. Finally, these results are examd. within the context of possible mechanisms assocd. with photosynthetic water oxidn.; specifically, the possible identity of the proposed high valent Mn-oxo species that is postulated to form during turnover is discussed.
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83. 83
  Srnec, M.; Wong, S. D.; Matthews, M. L.; Krebs, C.; Bollinger, J. M., Jr.; Solomon, E. I. Electronic Structure of the Ferryl Intermediate in the α-Ketoglutarate Dependent Non-Heme Iron Halogenatse SyrB2; Contributions to H Atom Abstraction Reactivity. J. Am. Chem. Soc. 2016, 138, 5110– 5122, DOI: 10.1021/jacs.6b01151
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  83
  Electronic Structure of the Ferryl Intermediate in the α-Ketoglutarate Dependent Non-Heme Iron Halogenase SyrB2: Contributions to H Atom Abstraction Reactivity
  Srnec, Martin; Wong, Shaun D.; Matthews, Megan L.; Krebs, Carsten; Bollinger, J. Martin; Solomon, Edward I.
  Journal of the American Chemical Society (2016), 138 (15), 5110-5122CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Low temp. magnetic CD (LT MCD) spectroscopy in combination with quantum-chem. calcns. are used to define the electronic structure assocd. with the geometric structure of the FeIV=O intermediate in SyrB2 that was previously detd. by nuclear resonance vibrational spectroscopy. These studies elucidate key frontier MOs (FMOs) and their contribution to H atom abstraction reactivity. The VT MCD spectra of the enzymic S = 2 FeIV=O intermediate with Br- ligation contain information-rich features that largely parallel the corresponding spectra of the S = 2 model complex (TMG3tren)FeIV=O. However, quant. differences are obsd. that correlate with π-anisotropy and oxo donor strength that perturb FMOs and affect reactivity. Due to π-anisotropy, the FeIV=O active site exhibits enhanced reactivity in the direction of the substrate cavity that proceeds through a π-channel that is controlled by perpendicular orientation of the substrate C-H bond relative to the halide-FeIV=O plane. Also, the increased intrinsic reactivity of the SyrB2 intermediate relative to the ferryl model complex is correlated to a higher oxyl character of the FeIV=O at the transition states resulting from the weaker ligand field of the halogenase.
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84. 84
  Bollinger, J. M., Jr.; Price, J. C.; Hoffart, L. M.; Barr, E. W.; Krebs, C. Mechanism of Taurine: α-Ketoglutarate Dioxygenase (TauD) from Escherichia coli. Eur. J. Inorg. Chem. 2005, 2005, 4245– 4254, DOI: 10.1002/ejic.200500476
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85. 85
  Ye, S.; Neese, F. Nonheme Oxo-Iron(IV) Intermediates Form an Oxyl Radical upon Approaching the C-H Bond Activation Transition State. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 1228– 1233, DOI: 10.1073/pnas.1008411108
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  85
  Nonheme oxo-iron(IV) intermediates form an oxyl radical upon approaching the C-H bond activation transition state
  Ye, Shengfa; Neese, Frank
  Proceedings of the National Academy of Sciences of the United States of America (2011), 108 (4), 1228-1233, S1228/1-S1228/2CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
  Oxo-iron(IV) species are implicated as key intermediates in the catalytic cycles of heme and nonheme oxygen activating iron enzymes that selectively functionalize aliph. C-H bonds. Ferryl complexes can exist in either quintet or triplet ground states. D. functional theory calcns. predict that the quintet oxo-iron(IV) species is more reactive toward C-H bond activation than its corresponding triplet partner, however; the available exptl. data on model complexes suggests that both spin multiplicities display comparable reactivities. To clarify this ambiguity, a detailed electronic structure anal. of alkane hydroxylation by an oxo-iron(IV) species on different spin-state potential energy surfaces is performed. According to our results, the lengthening of the Fe-oxo bond in ferryl reactants, which is the part of the reaction coordinate for H-atom abstraction, leads to the formation of oxyl-iron(III) species that then perform actual C-H bond activation. The differential reactivity stems from the fact that the two spin states have different requirements for the optimal angle at which the substrate should approach the (FeO)2+ core because distinct electron acceptor orbitals are employed on the two surfaces. The H-atom abstraction on the quintet surface favors the "σ-pathway" that requires an essentially linear attack; by contrast a "π-channel" is operative on the triplet surface that leads to an ideal attack angle near 90°. However, the latter is not possible due to steric crowding; thus, the attenuated orbital interaction and the unavoidably increased Pauli repulsion result in the lower reactivity of the triplet oxo-iron(IV) complexes.
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86. 86
  Pardue, D. B.; Mei, J.; Cundari, T. R.; Gunnoe, T. B. Density Functional Theory Study of Oxygen-Atom Insertion into Metal-Methyl Bonds of Iron(II), Ruthenium(II), and Osmium(II) Complexes: Study of Metal-Mediated C-O Bond Formation. Inorg. Chem. 2014, 53, 2968– 2975, DOI: 10.1021/ic402759w
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  86
  Density Functional Theory Study of Oxygen-Atom Insertion into Metal-Methyl Bonds of Iron(II), Ruthenium(II), and Osmium(II) Complexes: Study of Metal-Mediated C-O Bond Formation
  Pardue, Daniel B.; Mei, Jiajun; Cundari, Thomas R.; Gunnoe, T. Brent
  Inorganic Chemistry (2014), 53 (6), 2968-2975CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  Metal-mediated C-O bond formation is a key step in hydrocarbon oxygenation catalytic cycles; however, few examples of this reaction have been reported for low-oxidn.-state complexes. Oxygen insertion into a metal-carbon bond of Cp*M(CO)(OPy)R (Cp* = η5-pentamethylcyclopentadienyl; R = Me, Ph; OPy = pyridine-N-oxide; M = Fe, Ru, Os) was analyzed via d. functional theory calcns. Oxygen-atom insertions through a concerted single-step organometallic Baeyer-Villiger pathway and a two-step pathway via a metal-oxo intermediate were studied; calcns. predict that the former pathway was lower in energy. The results indicated that functionalization of M-R to M-OR (R = Me, Ph) is plausible using iron(II) complexes. Starting from Cp*Fe(CO)(OPy)Ph, the intermediate Fe-oxo showed oxyl character and, thus, is best considered an FeIIIO•- complex. Oxidn. of the π-acid ancillary ligand CO was facile. Substitutions of CO with dimethylamide and NH3 were calcd. to lower the activation barrier by ∼1-2 kcal/mol for formation of the FeIIIO•- intermediate, whereas a chloride ligand raised the activation barrier to 26 kcal/mol from 22.9 kcal/mol.
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87. 87
  Gupta, R.; Lacy, D. C.; Bominaar, E. L.; Borovik, A. S.; Hendrich, M. P. Electron Paramagnetic Resonance and Mössbauer Spectroscopy and Density Functional Theory Analysis of a High-Spin Fe^IV-Oxo Complex. J. Am. Chem. Soc. 2012, 134, 9775– 9784, DOI: 10.1021/ja303224p
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  87
  Electron Paramagnetic Resonance and Moessbauer Spectroscopy and Density Functional Theory Analysis of a High-Spin FeIV-Oxo Complex
  Gupta, Rupal; Lacy, David C.; Bominaar, Emile L.; Borovik, A. S.; Hendrich, Michael P.
  Journal of the American Chemical Society (2012), 134 (23), 9775-9784CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  High-spin FeIV-oxo species are known to be kinetically competent oxidants in nonheme iron enzymes. The properties of these oxidants are not as well understood as the corresponding intermediate-spin oxidants of heme complexes. The present work gives a detailed characterization of the structurally similar complexes [FeIVH3buea(O)]-, [FeIIIH3buea(O)]2-, and [FeIIIH3buea(OH)]- (H3buea = tris[(N'-tert-butylureaylato)-N-ethylene]aminato) using Mossbauer and dual-frequency/dual-mode EPR spectroscopies. The [FeIVH3buea(O)]- complex has a high-spin (S = 2) configuration imposed by the C3-sym. ligand. The EPR spectra of the [FeIVH3buea(O)]- complex presented here represent the 1st documented examples of an EPR signal from an FeIV-oxo complex, demonstrating the ability to detect and quantify FeIV species with EPR spectroscopy. Quant. simulations allowed the detn. of the zero-field parameter, D = +4.7 cm-1, and the species concn. D. functional theory (DFT) calcns. of the zero-field parameter are in agreement with the exptl. value and indicated that the major contribution to the D value is from spin-orbit coupling of the ground state with an excited S = 1 electronic configuration at 1.2 eV 17O isotope enrichment expts. allowed the detn. of the hyperfine consts. 17OAz = 10 MHz for [FeIVH3buea(O)]- and 17OAy = 8 MHz, 17OAz = 12 MHz for [FeIIIH3buea(OH)]-. The isotropic hyperfine const. (17OAiso = -16.8 MHz) was derived from the exptl. value to allow a quant. detn. of the spin polarization (ρp = 0.56) of the oxo p orbitals of the Fe-oxo bond in [FeIVH3buea(O)]-. This is the 1st exptl. detn. for nonheme complexes and indicates significant covalency in the Fe-oxo bond. High-field Mossbauer spectroscopy gave an 57Fe Adip tensor of (+5.6, +5.3, -10.9) MHz and Aiso = -25.9 MHz for the [FeIVH3buea(O)]- complex, and the results of DFT calcns. were in agreement with the nuclear parameters of the complex.
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88. 88
  Sun, X.; Geng, C.; Huo, R.; Ryde, U.; Bu, Y.; Li, J. Large Equatorial Ligand Effects on C-H Bond Activation by Nonheme Iron(IV)-oxo Complexes. J. Phys. Chem. B 2014, 118, 1493– 1500, DOI: 10.1021/jp410727r
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  88
  Large Equatorial Ligand Effects on C-H Bond Activation by Nonheme Iron(IV)-oxo Complexes
  Sun, Xiaoli; Geng, Caiyun; Huo, Ruiping; Ryde, Ulf; Bu, Yuxiang; Li, Jilai
  Journal of Physical Chemistry B (2014), 118 (6), 1493-1500CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
  In this article, we present d. functional theory (DFT) calcns. on the iron(IV)-oxo catalyzed methane C-H activation reactions for complexes in which the FeIVO core is surrounded by five neg. charged ligands. We found that it follows a hybrid pathway that mixes features of the classical σ- and π-pathways in quintet surfaces. These calcns. show that the Fe-O-H arrangement in this hybrid pathway is bent in sharp contrast to the collinear character as obsd. for the classical quintet σ-pathways before. The calcns. have also shown that it is the equatorial ligands that play key roles in tuning the reactivity of FeIVO complexes. The strong π-donating equatorial ligands employed in the current study cause a weak π(FeO) bond and thereby shift the electronic accepting orbitals (EAO) from the vertically oriented O pz orbital to the horizontally oriented O px. In addn., all the equatorial ligands are small in size and would therefore be expected have small steric effects upon substrate horizontal approaching. Therefore, for the small and strong π-donating equatorial ligands, the collinear Fe-O-H arrangement is not the best choice for the quintet reactivity. This study adds new element to iron(IV)-oxo catalyzed C-H bond activation reactions.
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89. 89
  Geng, C.; Ye, S.; Neese, F. Analysis of Reaction Channels for Alkane Hydroxylation by Nonheme Iron(IV)-Oxo Complexes. Angew. Chem., Int. Ed. 2010, 49, 5717– 5720, DOI: 10.1002/anie.201001850
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  89
  Analysis of Reaction Channels for Alkane Hydroxylation by Nonheme Iron(IV)-Oxo Complexes
  Geng, Caiyun; Ye, Shengfa; Neese, Frank
  Angewandte Chemie, International Edition (2010), 49 (33), 5717-5720, S5717/1-S5717/13CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Geometric parameters of the transition states of the [FeIV(O)(NH3)5]2+ system calcd. at the B3LYP/TZVP level of theory. This is the first time that all viable pathways have been identified in the same system, which allows us to compare their relative reactivities.
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90. 90
  (a) Ishizuka, T.; Watanabe, A.; Kotani, H.; Hong, D.; Satonaka, K.; Wada, T.; Shiota, Y.; Yoshizawa, K.; Ohara, K.; Yamaguchi, S.; Kato, S.; Fukuzumi, S.; Kojima, T. Homogeneous Photocatalytic Water Oxidation with a Dinuclear Co^III-Pyridylmethyl Amine Complex. Inorg. Chem. 2016, 55, 1154– 1164, DOI: 10.1021/acs.inorgchem.5b02336
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  90a
  Homogeneous Photocatalytic Water Oxidation with a Dinuclear CoIII-Pyridylmethylamine Complex
  Ishizuka, Tomoya; Watanabe, Atsuko; Kotani, Hiroaki; Hong, Dachao; Satonaka, Kenta; Wada, Tohru; Shiota, Yoshihito; Yoshizawa, Kazunari; Ohara, Kazuaki; Yamaguchi, Kentaro; Kato, Satoshi; Fukuzumi, Shunichi; Kojima, Takahiko
  Inorganic Chemistry (2016), 55 (3), 1154-1164CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  A bis-hydroxo-bridged dinuclear CoIII-pyridylmethylamine complex (1) was synthesized and the crystal structure was detd. by X-ray crystallog. Complex 1 acts as a homogeneous catalyst for visible-light-driven water oxidn. by persulfate (S2O82-) as an oxidant with [RuII(bpy)3]2+ (bpy = 2,2'-bipyridine) as a photosensitizer affording a high quantum yield (44%) with a large turnover no. (TON = 742) for O2 formation without forming catalytically active Co-oxide (CoOx) nanoparticles. In the water-oxidn. process, complex 1 undergoes proton-coupled electron-transfer (PCET) oxidn. as a rate-detg. step to form a putative dinuclear bis-μ-oxyl CoIII complex (2), which has been suggested by DFT calcns. Catalytic water oxidn. by 1 using [RuIII(bpy)3]3+ as an oxidant in a H216O and H218O mixt. was examd. to reveal an intramol. O-O bond formation in the two-electron-oxidized bis-μ-oxyl intermediate, prior to the O2 evolution.
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  .
  (b) Kotani, H.; Hong, D.; Satonaka, K.; Ishizuka, T.; Kojima, T. Mechanistic Insight into Dioxygen Evolution from Diastereomeric μ-Peroxo Dinuclear Co(III) Complexes Based on Stoichiometric Electron-Transfer Oxidation. Inorg. Chem. 2019, 58, 3676– 3682, DOI: 10.1021/acs.inorgchem.8b03245
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  90b
  Mechanistic Insight into Dioxygen Evolution from Diastereomeric μ-Peroxo Dinuclear Co(III) Complexes Based on Stoichiometric Electron-Transfer Oxidation
  Kotani, Hiroaki; Hong, Dachao; Satonaka, Kenta; Ishizuka, Tomoya; Kojima, Takahiko
  Inorganic Chemistry (2019), 58 (6), 3676-3682CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  Stoichiometric electron-transfer (ET) oxidn. of two diastereomeric μ-peroxo-μ-hydroxo dinuclear Co(III) complexes with tris(2-pyridylmethyl)amine (TPA) was examd. to scrutinize the reaction mechanism of O2 evolution from the peroxo complexes, as seen in the final step in H2O oxidn. by a Co(III)-TPA complex. The two isomeric Co(III)-peroxo complexes were synthesized and selectively isolated by recrystn. under different conditions. Although cyclic voltammograms of the two isomers in aq. solns. showed one reversible wave at 1.1 V vs. normal H electrode at pH 2.0, two oxidn. waves were obsd. at 1.0 and 1.4 V at pH 7.0 in the aq. solns., the latter of which is responsible for the O2-releasing process. At pH 7, one diastereomer showed higher reactivity than the other in O2 evolution, indicating the importance of structures of the μ-peroxo complexes in the reaction. To clarify the O2-evolving mechanism, the authors performed EPR and resonance Raman (RR) measurements for characterizing 1-electron oxidized species: The obsd. EPR and RR signals supported the formation of μ-superoxo-μ-hydroxo dinuclear Co(III) complexes; however, no characteristic difference was obsd. between two isomers in the EPR parameters including g values and superhyperfine coupling consts. ET-oxidn. rate consts. of the isomers are much faster than the O2-evolving rate consts., indicating that the O2-releasing step is the rate-detg. step in the O2 evolution through the stoichiometric ET oxidn. of the dinuclear Co(III)-μ-peroxo complexes. Therefore, the difference of reactivity in the O2 evolution for the two isomers should be derived from the thermodn. stability of two-electron oxidized species of the dinuclear Co(III)-μ-peroxo complexes, μ-dioxygen-μ-hydroxo dinuclear Co(III) intermediates.
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91. 91
  Surendranath, Y.; Kanan, M. W.; Nocera, D. G. Mechanistic Studies of the Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH. J. Am. Chem. Soc. 2010, 132, 16501– 16509, DOI: 10.1021/ja106102b
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  91
  Mechanistic Studies of the Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH
  Surendranath, Yogesh; Kanan, Matthew W.; Nocera, Daniel G.
  Journal of the American Chemical Society (2010), 132 (46), 16501-16509CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The mechanism of the O evolution reaction (OER) by catalysts prepd. by electrodepositions from Co2+ solns. in phosphate electrolytes (Co-Pi) was studied at neutral pH by electrokinetic and 18O isotope expts. Low-potential electrodepositions enabled the controlled prepn. of ultrathin Co-Pi catalyst films (<100 nm) that could be studied kinetically in the absence of mass transport and charge transport limitations to the OER. The Co-Pi catalysts exhibit a Tafel slope approx. equal to 2.3 × RT/F for the prodn. of O from H2O in neutral solns. The electrochem. rate law exhibits an inverse 1st order dependence on proton activity and a zeroth order dependence on phosphate for [Pi] ≥ 0.03 M. In the absence of phosphate buffer, the Tafel slope is increased ∼3-fold and the overall activity is greatly diminished. Together, these electrokinetic studies suggest a mechanism involving a rapid, one electron, one proton equil. between CoIII-OH and CoIV-O in which a phosphate species is the proton acceptor, followed by a chem. turnover-limiting process involving O-O bond coupling.
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92. 92
  McCool, N. S.; Robinson, D. M.; Sheats, J. E.; Dismukes, G. C. A Co₄O₄ “Cubane” Water Oxidation Catalyst Inspired by Photosynthesis. J. Am. Chem. Soc. 2011, 133, 11446– 11449, DOI: 10.1021/ja203877y
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  92
  A Co4O4 "Cubane" Water Oxidation Catalyst Inspired by Photosynthesis
  McCool, Nicholas S.; Robinson, David M.; Sheats, John E.; Dismukes, G. Charles
  Journal of the American Chemical Society (2011), 133 (30), 11446-11449CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Herein we describe the mol. Co4O4 cubane complex Co4O4(OAc)4(py)4 (1), which catalyzes efficient water oxidizing activity when powered by a std. photochem. oxidn. source or electrochem. oxidn. The pH dependence of catalysis, the turnover frequency, and in situ monitoring of catalytic species have revealed the intrinsic capabilities of this core type. The catalytic activity of complex 1 and analogous Mn4O4 cubane complexes is attributed to the cubical core topol., which is analogous to that of nature's water oxidn. catalyst, a cubical CaMn4O5 cluster.
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93. 93
  Egan, J. W., Jr.; Haggerty, B. S.; Rheingold, A. L.; Sendlinger, S. C.; Theopold, K. H. Crystal Structure of a Side-On Superoxo Complex of Cobalt and Hydrogen Abstraction by a Reactive Terminal Oxo Ligand. J. Am. Chem. Soc. 1990, 112, 2445– 2446, DOI: 10.1021/ja00162a069
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  93
  Crystal structure of a side-on superoxo complex of cobalt and hydrogen abstraction by a reactive terminal oxo ligand
  Egan, James W., Jr.; Haggerty, Brian S.; Rheingold, Arnold L.; Sendlinger, Shawn C.; Theopold, Klaus H.
  Journal of the American Chemical Society (1990), 112 (6), 2445-6CODEN: JACSAT; ISSN:0002-7863.
  Mg redn. of Tp'CoIIX (Tp' = hydridotris(3-tert-butyl-5-methylpyrazolyl)borate, X = Cl, I) in a N atm. yielded Tp'CoI(N2) (I). Exposure of I to an excess of O2 yielded Tp'CoII(O2) (II), which has been structurally characterized by x-ray diffraction. II crystd. in the monoclinic space group P21/n with a 9.615(4), b 30.260(12), c 9.577(4) Å, β 102.14(4)°, and Z = 4. II features the first example of a side-on bound superoxide ligand (dO-O = 1.262(8) Å). II has been further characterized by vibrational spectroscopy (ν16O-16O = 961 cm-1, ν16O-18O = 937 cm-1, ν18O-18O = 908 cm-1) and magnetic susceptibility measurements (μeff(298 K) = 3.88 μB). II reacted with I to yield Tp'CoIIOH. The mechanism of this reaction is thought to involve formation of a reactive Co oxo complex, which decomps. by H abstraction.
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94. 94
  Reinaud, O. M.; Theopold, K. H. Hydrogen Tunneling in the Activation of Dioxygen by a Tris(pyrazolyl)borate Cobalt Complex. J. Am. Chem. Soc. 1994, 116, 6979– 6980, DOI: 10.1021/ja00094a080
  [ACS Full Text ], [CAS], Google Scholar
  94
  Hydrogen tunneling in the activation of dioxygen by a tris(pyrazolyl)borate cobalt complex
  Reinaud, Olivia M.; Theopold, Klaus H.
  Journal of the American Chemical Society (1994), 116 (15), 6979-80CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The dioxygen complex, [Tp''Co(O2)] (5; Tp'' = hydridotris(3-isopropyl-5-methylpyrazolyl)borate), was prepd. and found to be stable in the solid state, but decomps. in soln. to give [(Tp''Co)2(μ-OH)2] (6). Upon warming, 5 decomps. with the formation of a transient intermediate [(Tp''Co)2(μ-O2)] (7), which could also be prepd. by the reaction of [(Tp''Co)2(μ-N2)] with oxygen. The kinetics of the thermal decompn. of 7 was studied and isotope effects measured and evidence is given for a tunneling contribution to the hydrogen atom abstraction from the ligand.
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95. 95
  Nurdin, L.; Spasyuk, D. M.; Fairburn, L.; Piers, W. E.; Maron, L. Oxygen-Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O₂ Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical. J. Am. Chem. Soc. 2018, 140, 16094– 16105, DOI: 10.1021/jacs.8b07726
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  95
  Oxygen-Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O2 Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical
  Nurdin, Lucie; Spasyuk, Denis M.; Fairburn, Laura; Piers, Warren E.; Maron, Laurent
  Journal of the American Chemical Society (2018), 140 (47), 16094-16105CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidn.-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen redn. catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and exptl. implicated through chem. interception and isotope labeling expts. In the absence of competing chem. reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidn. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric redn. of O2 to H2O with 2 equiv of Co(II) and suggests a new pathway for selective redn. of O2 to water via Co(III)-O-O-Co(III) peroxo intermediates.
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96. 96
  Matsunaga, P. T.; Hillhouse, G. L.; Rheingold, A. L. Oxygen-Atom Transfer from Nitrous Oxide to a Nickel Metallacycle. Synthesis, Structure, and Reactions of (2,2′-Bipydine)Ni(OCH₂CH₂CH₂CH₂). J. Am. Chem. Soc. 1993, 115, 2075– 2077, DOI: 10.1021/ja00058a085
  [ACS Full Text ], [CAS], Google Scholar
  96
  Oxygen-atom transfer from nitrous oxide to a nickel metallacycle. Synthesis, structure, and reactions of [cyclic] (2,2'-bipyridine)Ni(OCH2CH2CH2CH2)
  Matsunaga, Phillip T.; Hillhouse, Gregory L.; Rheingold, Arnold L.
  Journal of the American Chemical Society (1993), 115 (5), 2075-7CODEN: JACSAT; ISSN:0002-7863.
  N2O oxidized metallacyclopentane I to 55% II (1 atm, 50°, 48h).
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97. 97
  Figg, T. M.; Cundari, T. R. Mechanistic Study of Oxy Insertion into Nickel-Carbon Bonds with Nitrous Oxide. Organometallics 2012, 31, 4998– 5004, DOI: 10.1021/om300270x
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  97
  Mechanistic Study of Oxy Insertion into Nickel-Carbon Bonds with Nitrous Oxide
  Figg, Travis M.; Cundari, Thomas R.
  Organometallics (2012), 31 (14), 4998-5004CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
  Transition-metal-mediated oxy insertion into metal-C bonds is useful for the development of catalytic cycles for selective hydrocarbon oxidn. However, there are few bona fide examples of net oxy insertion with transition-metal complexes. An extremely rare example of a 3d metal mediating oxy insertion into metal-C bonds is NiII alkyl complexes reacting with nitrous oxide (N2O) reported by Hillhouse and coworkers; however, the mechanism was never fully elucidated. A computational study was performed on bipyridyl Ni metallacycles that form Ni alkoxides upon reaction with N2O to attain insight into future catalyst design for O atom transfer reactions. Two possible mechanisms are explored. Of the two pathways, the computations suggest that the preferred mechanism proceeds through a Ni-oxyl intermediate followed by alkyl migration of the Ni-C bond to form an alkoxide. Oxyl formation is the rate-detg. step, with a free energy barrier of 29.4 kcal/mol for bpyNiII(cyclo-(CH2)4). Complexes that contain sp2-hybridized mols. at the β-C site within the metallacycle ring do not undergo oxy insertion due to elevated barriers. While exploring insertion with another oxidant, pyridine N-oxide, the authors found that N2O is crit. for net oxy insertion with this complex due to the substantial thermodn. advantage of N2 expulsion. Reaction with pyridine N-oxide necessitated expulsion of a worse leaving group, resulting in much higher barriers (ΔG⧧ = 49.7 kcal/mol) for the oxyl formation step.
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98. 98
  Elwell, C. E.; Gagnon, N. L.; Neisen, B. D.; Dhar, D.; Spaeth, A. D.; Yee, G. M.; Tolman, W. B. Copper-Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity. Chem. Rev. 2017, 117, 2059– 2107, DOI: 10.1021/acs.chemrev.6b00636
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  98
  Copper-Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity
  Elwell, Courtney E.; Gagnon, Nicole L.; Neisen, Benjamin D.; Dhar, Debanjan; Spaeth, Andrew D.; Yee, Gereon M.; Tolman, William B.
  Chemical Reviews (Washington, DC, United States) (2017), 117 (3), 2059-2107CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A longstanding research goal has been to understand the nature and role of copper-oxygen intermediates within copper-contg. enzymes and abiol. catalysts. Synthetic chem. has played a pivotal role in highlighting the viability of proposed intermediates and expanding the library of known copper-oxygen cores. In addn. to the no. of new complexes that have been synthesized since the previous reviews on this topic in this journal (Mirica, L.M.; Ottenwaelder, X.; Stack, T.D.P.Chem.Rev.2004, 104, 1013-1046 and Lewis, E.A.; Tolman, W.B.Chem.Rev.2004, 104, 1047-1076), the field has seen significant expansion in the (1) range of cores synthesized and characterized, (2) amt. of mechanistic work performed, particularly in the area of org. substrate oxidn., and (3) use of computational methods for both the corroboration and prediction of proposed intermediates. The scope of this review has been limited to well-characterized examples of copper-oxygen species but seeks to provide a thorough picture of the spectroscopic characteristics and reactivity trends of the copper-oxygen cores discussed.
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99. 99
  Garcia-Bosch, I.; Company, A.; Frisch, J. R.; Torrent-Sucarrat, M.; Cardellach, M.; Gamba, I.; Güell, M.; Casella, L.; Que, L., Jr.; Ribas, X.; Luis, J. M.; Costas, M. O₂ Activation and Selective Phenolate ortho Hydroxylation by an Usymmetric Dicopper μ-η¹:η¹-Peroxido Complex. Angew. Chem., Int. Ed. 2010, 49, 2406– 2409, DOI: 10.1002/anie.200906749
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  99
  O2 Activation and Selective Phenolate ortho Hydroxylation by an Unsymmetric Dicopper μ-η1:η1-Peroxido Complex
  Garcia-Bosch, Isaac; Company, Anna; Frisch, Jonathan R.; Torrent-Sucarrat, Miquel; Cardellach, Mar; Gamba, Ilaria; Gueell, Mireia; Casella, Luigi; Que, Lawrence, Jr.; Ribas, Xavi; Luis, Josep M.; Costas, Miquel
  Angewandte Chemie, International Edition (2010), 49 (13), 2406-2409, S2406/1-S2406/41CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Reaction of Unsym. Dicopper μ-η1:η1-Peroxido Complex (I) with the sodium salt of para-substituted phenolates were studied. Hammett plot (σ+) affords a linear correlation which gives a ρ value of -0.6 (R2 = 0.98) consistent with an electrophilic oxidizing species that attacks the arom. ring in the rate-detg. step of the reactions.
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100. 100
  (a) Mirica, L. M.; Vance, M.; Rudd, D. J.; Hedman, B.; Hodgson, K. O.; Solomon, E. I.; Stack, T. D. P. Tyrosinase Reactivity in a Model Complex: An Alternative Hydroxylation Mechanism. Science 2005, 308, 1890– 1892, DOI: 10.1126/science.1112081
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  100a
  Tyrosinase Reactivity in a Model Complex: An Alternative Hydroxylation Mechanism
  Mirica, Liviu M.; Vance, Michael; Rudd, Deanne Jackson; Hedman, Britt; Hodgson, Keith O.; Solomon, Edward I.; Stack, T. Daniel P.
  Science (Washington, DC, United States) (2005), 308 (5730), 1890-1892CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
  The binuclear copper enzyme tyrosinase activates O2 to form a μ-η2:η2-peroxodicopper(II) complex, which oxidizes phenols to catechols. Here, a synthetic μ-η2:η2-peroxodicopper(II) complex, with an absorption spectrum similar to that of the enzymic active oxidant, is reported to rapidly hydroxylate phenolates at -80°C. Upon phenolate addn. at extreme temp. in soln. (-120°C), a reactive intermediate consistent with a bis-μ-oxodicopper(III)-phenolate complex, with the O-O bond fully cleaved, is obsd. exptl. The subsequent hydroxylation step has the hallmarks of an electrophilic arom. substitution mechanism, similar to tyrosinase. Overall, the evidence for sequential O-O bond cleavage and C-O bond formation in this synthetic complex suggests an alternative intimate mechanism to the concerted or late stage O-O bond scission generally accepted for the phenol hydroxylation reaction performed by tyrosinase.
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  .
  (b) Mirica, L. M.; Rudd, D. J.; Vance, M. A.; Solomon, E. I.; Hodgson, K. O.; Hedman, B.; Stack, T. D. P. μ-η²:η²-Peroxodicopper(II) Complex with a Secondary Diamine Ligand: A Functional Model of Tyrosinase. J. Am. Chem. Soc. 2006, 128, 2654– 2665, DOI: 10.1021/ja056740v
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  100b
  μ-η2:η2-Peroxodicopper(II) Complex with a Secondary Diamine Ligand: A Functional Model of Tyrosinase
  Mirica, Liviu M.; Rudd, Deanne Jackson; Vance, Michael A.; Solomon, Edward I.; Hodgson, Keith O.; Hedman, Britt; Stack, T. Daniel P.
  Journal of the American Chemical Society (2006), 128 (8), 2654-2665CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The activation of dioxygen (O2) by Cu(I) complexes is an important process in biol. systems and industrial applications. In tyrosinase, a binuclear copper enzyme, a μ-η2:η2-peroxodicopper(II) species is accepted generally to be the active oxidant. Reported here is the characterization and reactivity of a μ-η2:η2-peroxodicopper(II) complex synthesized by reacting the Cu(I) complex of the secondary diamine ligand N,N'-di-tert-butyl-ethylenediamine (DBED), [(DBED)Cu(MeCN)](X) (1·X, X = CF3SO3-, CH3SO3-, SbF6-, BF4-), with O2 at 193 K to give [{Cu(DBED)}2(O2)](X)2 (2·X2). The UV-vis and resonance Raman spectroscopic features of 2 vary with the counteranion employed yet are invariant with change of solvent. These results implicate an intimate interaction of the counteranions with the Cu2O2 core. Such interactions are supported further by extended X-ray absorption fine structure (EXAFS) analyses of solns. that reveal weak copper-counteranion interactions. The accessibility of the Cu2O2 core to exogenous ligands such as these counteranions is manifest further in the reactivity of 2 with externally added substrates. Most notable is the hydroxylation reactivity with phenolates to give catechol and quinone products. Thus the strategy of using simple bidentate ligands at low temps. provides not only spectroscopic models of tyrosinase but also functional models.
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101. 101
  Kamachi, T.; Lee, Y.-M.; Nishimi, T.; Cho, J.; Yoshizawa, K.; Nam, W. Combined Experimental and Theoretical Approach To Understand the Reactivity of a Mononuclear Cu(II)–Hydroperoxo Complex in Oxygenation Reactions. J. Phys. Chem. A 2008, 112, 13102– 13108, DOI: 10.1021/jp804804j
  [ACS Full Text ], [CAS], Google Scholar
  101
  Combined Experimental and Theoretical Approach To Understand the Reactivity of a Mononuclear Cu(II)-Hydroperoxo Complex in Oxygenation Reactions
  Kamachi, Takashi; Lee, Yong-Min; Nishimi, Tomonori; Cho, Jaeheung; Yoshizawa, Kazunari; Nam, Wonwoo
  Journal of Physical Chemistry A (2008), 112 (50), 13102-13108CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  A copper(II) complex bearing a pentadentate ligand, [CuII(N4Py)(MeCN)(CF3SO3)2] (1, N4Py = N,N-bis(2-pyridylmethyl)bis(2-pyridyl)methylamine), was synthesized and characterized with various spectroscopic techniques and x-ray crystallog. A mononuclear CuII-hydroperoxo complex, [CuII(N4Py)(OOH)]+ (2), was then generated in the reaction of 1 and H2O2 in the presence of base, and the reactivity of the intermediate was studied in the oxidn. of various substrates at -40°. In the reactivity studies, 2 showed a low oxidizing power such that 2 reacted only with triethylphosphine but not with other substrates such as thioanisole, benzyl alc., 1,4-cyclohexadiene, cyclohexene, and cyclohexane. In theor. work, the authors have conducted d. functional theory (DFT) calcns. on the epoxidn. of ethylene by 2 and a [CuIII(N4Py)(O)]+ intermediate (3) at the B3LYP level. The activation barrier is 39.7 and 26.3 kcal/mol for distal and proximal oxygen attacks by 2, resp. The direct ethylene epoxidn. by 2 is not a plausible pathway, as the authors obsd. in the exptl. work. In contrast, the ethylene epoxidn. by 3 is a downhill and low-barrier process. Also 2 cannot be a precursor to 3, since the homolytic cleavage of the O-O bond of 2 is very endothermic (i.e., 42 kcal/mol). From the exptl. and theor. results, a mononuclear CuII-hydroperoxo species bearing a pentadentate N5 ligand is a sluggish oxidant in oxygenation reactions.
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102. 102
  (a) Wada, A.; Harata, M.; Hasegawa, K.; Jitsukawa, K.; Masuda, H.; Mukai, M.; Kitagawa, T.; Einaga, H. Structural and Spectroscopic Characterization of a Mononuclear Hydroperoxo-Copper(II) Complex with Tripodal Pyridylamine Ligands. Angew. Chem., Int. Ed. 1998, 37, 798– 799, DOI: 10.1002/(SICI)1521-3773(19980403)37:6<798::AID-ANIE798>3.0.CO;2-3
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  102a
  Structural and spectroscopic characterization of a mononuclear hydroperoxo - copper(II) complex with tripodal pyridylamine ligands
  Wada, Akira; Harata, Manabu; Hasegawa, Koji; Jitsukawa, Koichiro; Masuda, Hideki; Mukai, Masahiro; Kitagawa, Teizo; Einaga, Hisahiko
  Angewandte Chemie, International Edition (1998), 37 (6), 798-799CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH)
  The reaction of hydrogen peroxide with either [CuII(bppa-)]ClO4 or [CuII(bppa)(CH3CO2)]ClO4 (bppa = bis(6-pivalamido-2-pyridylmethyl)(2-pyridylmethyl)amine) led to [CuII(bppa)(OOH)]ClO4, which was characterized by x-ray crystallog. (monoclinic, space group P21/a, R = 0.062). The hydroperoxo complex was further examd. by absorption, ESR, resonance Raman and ESI mass spectroscopy. The mononuclear hydroperoxo complex has an axially compressed trigonal bipyramidal geometry with the pyridyl nitrogens of the tripodal ligand located in the equatorial plane and the tertiary amine nitrogen and hydroperoxo oxygen in the axial positions. Hydrogen bonding from the amido nitrogens to the hydroperoxo oxygen help stabilize the complex, which could serve as an enzyme model.
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  .
  (b) Osako, T.; Nagatomo, S.; Tachi, Y.; Kitagawa, T.; Itoh, S. Low-Temperature Stopped-Flow Studies on the Reactions of Copper(II) Complexes and H₂O₂: The First Detection of a Mononuclear Copper(II)-Peroxo Intermediate. Angew. Chem., Int. Ed. 2002, 41, 4325– 4328, DOI: 10.1002/1521-3773(20021115)41:22<4325::AID-ANIE4325>3.0.CO;2-Y
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  102b
  Low-temperature stopped-flow studies on the reactions of copper(II) complexes and H2O2: The first detection of a mononuclear copper(II)-peroxo intermediate
  Osako, Takao; Nagatomo, Shigenori; Tachi, Yoshimitsu; Kitagawa, Teizo; Itoh, Shinobu
  Angewandte Chemie, International Edition (2002), 41 (22), 4325-4328CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The low-temp. stopped-flow technique was used to evaluate the reactions between copper(II) complexes of tridentate and tetradentate (pyridylethyl)amine ligands and H2O2. The copper complexes include the newly prepd. [Cu(L1)(ClO4)2] (L1 = I) and [Cu(TEPA)(ClO4)]ClO4 (TEPA = tris[2-(2-pyridyl)ethyl]amine) and the known copper(II) complex [Cu(L2)(ClO4)2] (L2 = II). The results show that mononuclear CuII-peroxo complexes are generated from initially formed CuII-hydroperoxo intermediates. The reaction of [Cu(TEPA)(ClO4)](ClO4), contg. the tetradentate TEPA ligand, and H2O2 under the same conditions yielded an intermediate exhibiting a similar absorption spectrum, and also the same kinetic behavior as the reactions for the complex with the L1 ligand. The ligand denticity and the steric effects of the N-alkyl substituents in the pyridylethylamine derivs. significantly altered the reactivity of the initially formed CuII-hydroperoxo intermediates. These findings demonstrate significant information on the dioxygen-activation mechanism in biol. and industrial systems.
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  .
  (c) Fujii, T.; Naito, A.; Yamaguchi, S.; Wada, A.; Funahashi, Y.; Jitsukawa, K.; Nagatomo, S.; Kitagawa, T.; Masuda, H. Construction of a Square-Planar Hydroperoxo-Copper(II) Complex Inducing a Higher Catalytic Reactivity. Chem. Commun. 2003, 2700– 2701, DOI: 10.1039/b308073k
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  102c
  Construction of a square-planar hydroperoxo-copper(II) complex inducing a higher catalytic reactivity
  Fujii, Tatsuya; Naito, Asako; Yamaguchi, Syuhei; Wada, Akira; Funahashi, Yasuhiro; Jitsukawa, Koichiro; Nagatomo, Shigenori; Kitagawa, Teizo; Masuda, Hideki
  Chemical Communications (Cambridge, United Kingdom) (2003), (21), 2700-2701CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
  The complex [Cu(BPBA)(MeOH)](ClO4)2 (1, BPBA = bis(2-pyridylmethyl)tert-butylamine) was prepd. and characterized by x-ray crystallog. and spectroscopic methods. A novel hydroperoxo-copper(II) complex with a square-planar geometry, [Cu(BPBA)(OOH)]+ (2), was prepd. from 1. 2 Exhibited a higher selectivity and catalytic reactivity for oxidn. of di-Me sulfide, in contrast to that with the trigonal-bipyramidal complex [Cu(TPA)(OOH)]+ (3, TPA = tris(2-pyridylmethyl)amine).
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103. 103
  Klinman, J. P. The Copper-Enzyme Family of Dopamine β-Monooxygenase and Peptidylglycine α-Hydroxylating Monooxygenase: Resolving the Chemical Pathway for Substrate Hydroxylation. J. Biol. Chem. 2006, 281, 3013– 3016, DOI: 10.1074/jbc.R500011200
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  103
  The copper-enzyme family of dopamine β-monooxygenase and peptidylglycine α-hydroxylating monooxygenase: Resolving the chemical pathway for substrate hydroxylation
  Klinman, Judith P.
  Journal of Biological Chemistry (2006), 281 (6), 3013-3016CODEN: JBCHA3; ISSN:0021-9258. (American Society for Biochemistry and Molecular Biology)
  A review. Dopamine β-monooxygenase (I)and peptidylglycine α-hydroxylating monooxygenase (II) belong to a small class of Cu-proteins found exclusively in higher eukaryotes. These physiol. important enzymes resp. catalyze the transformation of dopamine to norepinephrine and C-terminal glycine-extended peptides to α-hydroxylated products. Although their substrate specificities are markedly different, these 2 enzymes greatly resemble each other in many other respects. Both enzymes are localized in subcellular compartments: I in chromaffin vesicles of the adrenal gland or synaptic vesicles of the sympathetic nervous system and II in secretory vesicles of the pituitary gland. Although I and II exist in sol. and membrane-bound forms within the vesicular compartments, the majority of studies have been conducted with the more tractable sol. enzymes. The physiol. role played by the sol. and membrane-bound forms may be different, but the chem. mechanisms are almost certain to be the same. Comparison of the primary sequence of the II catalytic core with the larger I indicates a central core of ∼300 amino acids from I that is 27% identical and 40% homologous to II. In addn., I contains ∼200 amino acids toward its N-terminus and ∼100 amino acids toward the C-terminus that bear no relation to II. Of particular note is the conservation of the ligands to the 2 Cu atoms per enzyme subunit, designated CuH and CuM. Although I and II belong to a multi-Cu family of proteins, the Cu atoms appear to perform different functions, that of substrate hydroxylation (CuH) and electron storage/transfer (CuM). Perhaps the most startling feature to emerge from x-ray crystallog. studies is the fully solvent-exposed nature of the Cu sites, raising the questions of (1) how I and II carry out regio- and stereospecific hydroxylations, and (2) how they carry out controlled electron transfer from CuH to CuM through bulk solvent. The formation of a Cu-superoxo intermediate appears to provide a working mechanism that is capable of rationalizing the voluminous data available for I and II. Many exptl. challenges remain, which include the precise tuning of the active site for H-transfer and the possible participation of regions distal from the active site in this process. The vexing question of the exact mechanism of long-range electron transfer between the CuH and CuM sites also awaits elaboration.
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104. 104
  Chufán, E. E.; Prigge, S. T.; Siebert, X.; Eipper, B. A.; Mains, R. E.; Amzel, L. M. Differential Reactivity between Two Copper Sites in Peptidylglycine α-Hydroxylating Monooxygenase. J. Am. Chem. Soc. 2010, 132, 15565– 15572, DOI: 10.1021/ja103117r
  [ACS Full Text ], [CAS], Google Scholar
  104
  Differential Reactivity between Two Copper Sites in Peptidylglycine α-Hydroxylating Monooxygenase
  Chufan, Eduardo E.; Prigge, Sean T.; Siebert, Xavier; Eipper, Betty A.; Mains, Richard E.; Amzel, L. Mario
  Journal of the American Chemical Society (2010), 132 (44), 15565-15572CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Peptidylglycine α-hydroxylating monooxygenase (PHM) catalyzes the stereospecific hydroxylation of the Cα of C-terminal glycine-extended peptides and proteins, the first step in the activation of many peptide hormones, growth factors, and neurotransmitters. The crystal structure of the enzyme revealed two nonequivalent Cu sites (CuM and CuH) sepd. by ∼11 Å. In the resting state of the enzyme, CuM is coordinated in a distorted tetrahedral geometry by one methionine, two histidines, and a water mol. The coordination site of the water mol. is the position where external ligands bind. The CuH has a planar T-shaped geometry with three histidines residues and a vacant position that could potentially be occupied by a fourth ligand. Although the catalytic mechanism of PHM and the role of the metals are still being debated, CuM is identified as the metal involved in catalysis, while CuH is assocd. with electron transfer. To further probe the role of the metals, we studied how small mols. such as nitrite (NO2-), azide (N3-), and carbon monoxide (CO) interact with the PHM copper ions. The crystal structure of an oxidized nitrite-soaked PHMcc, obtained by soaking for 20 h in mother liquor supplemented with 300 mM NaNO2, shows that nitrite anion coordinates CuM in an asym. bidentate fashion. Surprisingly, nitrite does not bind CuH, despite the high concn. used in the expts. (nitrite/protein > 1000). Similarly, azide and carbon monoxide coordinate CuM but not CuH in the PHMcc crystal structures obtained by cocrystn. with 40 mM NaN3 and by soaking CO under 3 atm of pressure for 30 min. This lack of reactivity at the CuH is also obsd. in the reduced form of the enzyme: CO binds CuM but not CuH in the structure of PHMcc obtained by exposure of a crystal to 3 atm CO for 15 min in the presence of 5 mM ascorbic acid (reductant). The necessity of CuH to maintain its redox potential in a narrow range compatible with its role as an electron-transfer site seems to explain the lack of coordination of small mols. to CuH; coordination of any external ligand will certainly modify its redox potential.
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105. 105
  Kim, S.; Ståhlberg, J.; Sandgren, M.; Paton, R. S.; Beckham, G. T. Quantum Mechanical Calculations Suggest that Lytic Polysaccharide Monooxygenases Use a Copper-Oxyl, Oxygen-Rebound Mechanism. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 149– 154, DOI: 10.1073/pnas.1316609111
  [Crossref], [PubMed], [CAS], Google Scholar
  105
  Quantum mechanical calculations suggest that lytic polysaccharide monooxygenases use a copper-oxyl, oxygen-rebound mechanism
  Kim, Seonah; Stahlberg, Jerry; Sandgren, Mats; Paton, Robert S.; Beckham, Gregg T.
  Proceedings of the National Academy of Sciences of the United States of America (2014), 111 (1), 149-154CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
  Lytic polysaccharide monooxygenases (LPMOs) exhibit a mononuclear Cu-contg. active site and use O2 and a reducing agent to oxidatively cleave glycosidic linkages in polysaccharides. LPMOs represent a unique paradigm in carbohydrate turnover and exhibit synergy with hydrolytic enzymes in biomass depolymn. To date, several features of Cu binding to LPMOs have been elucidated, but the identity of the reactive O species (ROS) and the key steps in the oxidative mechanism have not been elucidated. Here, DFT calcns. were used with an enzyme active site model to identify the ROS and compare 2 hypothesized reaction pathways in LPMOs for H atom abstraction and polysaccharide hydroxylation; namely, (1) a mechanism that employs a η1-superoxo intermediate, which abstrs. a substrate H atom and a hydroperoxo species is responsible for substrate hydroxylation, and (2) a mechanism wherein a copper-oxyl radical abstrs. a H atom and subsequently hydroxylates the substrate via an oxygen-rebound mechanism. The results predicted that O binds end-on (η1) to Cu, and that a copper-oxyl-mediated, O-rebound mechanism is energetically preferred. N-terminal His methylation was also examd., which is thought to modify the structure and reactivity of the enzyme. DFT calcns. suggested that this post-translational modification had only a minor effect on the LPMO active site structure or reactivity for the examd. steps. Overall, this study suggests the steps in the LPMO mechanism for oxidative cleavage of glycosidic bonds.
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106. 106
  Dietl, N.; van der Linde, C.; Schlangen, M.; Beyer, M. K.; Schwarz, H. Diatomic [CuO]⁺ and Its Role in the Spin-Selective Hydrogen- and Oxygen-Atom Transfers in the Thermal Activation of Methane. Angew. Chem., Int. Ed. 2011, 50, 4966– 4969, DOI: 10.1002/anie.201100606
  [Crossref], [CAS], Google Scholar
  106
  Diatomic [CuO]+ and Its Role in the Spin-Selective Hydrogen- and Oxygen-Atom Transfers in the Thermal Activation of Methane
  Dietl, Nicolas; van der Linde, Christian; Schlangen, Maria; Beyer, Martin K.; Schwarz, Helmut
  Angewandte Chemie, International Edition (2011), 50 (21), 4966-4969CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  More than ten years after its theor. prediction to serve as a powerful converter of methane to methanol the bare [CuO]+ cation has been successfully generated in the gas phase. A combination of mass spectrometry and DFT calcns. revealed the crucial role of two-state reactivity and oxygen-centered radicals in the selectivity in the oxidn. of methane.
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107. 107
  Rodgers, M. T.; Walker, B.; Armentrout, P. B. Reactions of Cu⁺ (¹S and ³D) with O₂, CO, CO₂, N₂, NO, N₂O and NO₂ studied by guided ion beam mass spectrometry. Int. J. Mass Spectrom. 1999, 182-183, 99– 120, DOI: 10.1016/S1387-3806(98)14228-8
  [Crossref], [CAS], Google Scholar
  107
  Reactions of Cu+(1S and 3D) with O2, CO, CO2, N2, NO, N2O, and NO2 studied by guided ion beam mass spectrometry
  Rodgers, M. T.; Walker, Ben; Armentrout, P. B.
  International Journal of Mass Spectrometry (1999), 182/183 (), 99-120CODEN: IMSPF8; ISSN:1387-3806. (Elsevier Science B.V.)
  Reactions of Cu+(1S and 3D) with O2, CO, CO2, N2, NO, N2O, and NO2 are studied using guided ion beam mass spectrometry. Cross sections as a function of kinetic energy are measured for each system to over 17 eV. In all cases, the obsd. reactions of Cu+(1S) are endothermic. Because of the closed shell character of ground state Cu+ (1S, 3d10), most of these systems exhibit cross sections with onsets and peaks at much higher energies than expected from the known thermochem. Such behavior indicates that the reactions occur on relatively repulsive potential energy surfaces and by impulsive processes. Reliable thermodn. information is obtained primarily from the NO2 system where an anal. of the kinetic energy dependence of the reaction cross sections is used to obtain D0(Cu+-O) = 1.35 ± 0.12 eV and D0(Cu-O) = 2.94 ± 0.12 eV. Although speculative, the threshold for an excited state product asymptote in the N2O system also allows the derivation of D0(Cu+-N2) = 0.92 ± 0.31 eV. Reactions of the Cu+(3D, 4s1 3d9) excited state are generally more efficient than those of the ground state and are exothermic in several cases.
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108. 108
  Rezabal, E.; Gauss, J.; Matxain, J. M.; Berger, R.; Diefenbach, M.; Holthausen, M. C. Quantum Chemical Assessment of the Binding Energy of CuO⁺. J. Chem. Phys. 2011, 134, 064304, DOI: 10.1063/1.3537797
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  108
  Quantum chemical assessment of the binding energy of CuO+
  Rezabal, Elixabete; Gauss, Juergen; Matxain, Jon M.; Berger, Robert; Diefenbach, Martin; Holthausen, Max C.
  Journal of Chemical Physics (2011), 134 (6), 064304/1-064304/13CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
  We present a detailed theor. investigation on the dissocn. energy of CuO+, carried out by means of coupled cluster theory, the multireference averaged coupled pair functional (MR-ACPF) approach, diffusion quantum Monte Carlo (DMC), and d. functional theory (DFT). At the resp. extrapolated basis set limits, most post-Hartree-Fock approaches agree within a narrow error margin on a De value of 26.0 kcal mol-1 coupled-cluster singles and doubles level augmented by perturbative triples corrections, CCSD(T), 25.8 kcal mol-1 (CCSDTQ via the high accuracy extrapolated ab initio thermochem. protocol), and 25.6 kcal mol-1 (DMC), which is encouraging in view of the disaccording data published thus far. The configuration-interaction based MR-ACPF expansion, which includes single and double excitations only, gives a slightly lower value of 24.1 kcal mol-1, indicating that large basis sets and triple excitation patterns are necessary ingredients for a quant. assessment. Our best est. for D0 at the CCSD(T) level is 25.3 kcal mol-1, which is somewhat lower than the latest exptl. value (D0 = 31.1 ± 2.8 kcal mol-1; reported by the Armentrout group). These highly correlated methods are, however, computationally very demanding, and the results are therefore supplemented with those of more affordable DFT calcns. If used in combination with moderately-sized basis sets, the M05 and M06 hybrid functionals turn out to be promising candidates for studies on much larger systems contg. a CuO+ core. (c) 2011 American Institute of Physics.
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109. 109
  Wang, B.; Johnston, E. M.; Li, P.; Shaik, S.; Davies, G. J.; Walton, P. H.; Rovira, C. QM/MM Studies into the H₂O₂-Dependent Activity of Lytic Polysaccaride Monooxygenases: Evidence for the Formation of a Caged Hydroxyl Radical Intermediate. ACS Catal. 2018, 8, 1346– 1351, DOI: 10.1021/acscatal.7b03888
  [ACS Full Text ], [CAS], Google Scholar
  109
  QM/MM Studies into the H2O2-Dependent Activity of Lytic Polysaccharide Monooxygenases: Evidence for the Formation of a Caged Hydroxyl Radical Intermediate
  Wang, Binju; Johnston, Esther M.; Li, Pengfei; Shaik, Sason; Davies, Gideon J.; Walton, Paul H.; Rovira, Carme
  ACS Catalysis (2018), 8 (2), 1346-1351CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  Lytic polysaccharide monooxygenases (LPMOs) are promising enzymes for the conversion of lignocellulosic biomass into biofuels and biomaterials. Classically considered oxygenases, recent work suggests that H2O2 can, under certain circumstances, also be a potential substrate. Here we present a detailed mechanism of the activation of H2O2 by a C4-acting LPMO using small-model DFT and QM/MM calcns. We show that there is an efficient mechanism to break the O-O bond of H2O2, with a low barrier of 5.8 kcal/mol, via a one-electron transfer from the LPMO-Cu(I) site to form an HO• radical, stabilized by hydrogen bonding interactions. Our QM/MM calcns. further show that the H-bonding machinery of the enzyme directs the HO• radical to abstr. a hydrogen atom from the Cu(II)-OH unit rather than from the substrate in what is essentially a caged-radical reaction, thereby forming a Cu(II)-oxyl species. The Cu(II)-oxyl species then exclusively oxidizes the C4-H bond due to the suitable position of the substrate. Our calcns. also suggest that the C4-hydroxylated intermediate can be efficiently hydrolyzed in water, and this process does not require enzymic catalysis.
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110. 110
  (a) Hong, S.; Huber, S. M.; Gagliardi, L.; Cramer, C. C.; Tolman, W. B. Copper(I)-α-Ketocarboxylate Complexes: Characterization and O₂ Reactions That Yield Copper-Oxygen Intermediates Capable of Hydroxylating Arenes. J. Am. Chem. Soc. 2007, 129, 14190– 14192, DOI: 10.1021/ja0760426
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  110a
  Copper(I)-α-Ketocarboxylate Complexes: Characterization and O2 Reactions That Yield Copper-Oxygen Intermediates Capable of Hydroxylating Arenes
  Hong, Sungjun; Huber, Stefan M.; Gagliardi, Laura; Cramer, Christopher C.; Tolman, William B.
  Journal of the American Chemical Society (2007), 129 (46), 14190-14192CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Cu(I)-α-ketocarboxylate complexes with aryl substituted iminoethylpyridines were prepd. and shown to exhibit variable coordination modes of the α-ketocarboxylate ligand. Reaction with O2 induces decarboxylation of the α-ketocarboxylate, and the derived Cu-O intermediate(s) was intercepted, resulting in hydroxylation of an arene substituent on the supporting N-donor ligand. Theor. calcns. provided intriguing mechanistic notions for the process, notably implicating hydroxylation pathways that involve novel [CuI-OOC(O)R] and [CuII-O-• ↔ CuIII:O2-]+ species.
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  .
  (b) Huber, S. M.; Ertem, M. Z.; Aquilante, F.; Gagliardi, L.; Tolman, W. B.; Cramer, C. J. Generating Cu^II-Oxyl/Cu^III-Oxo Species from Copper(I)-α-Ketocarboxylate Complexes and O₂: In Silico Studies on Ligand Effects and C–H-Activation Reactivity. Chem. - Eur. J. 2009, 15, 4886– 4895, DOI: 10.1002/chem.200802338
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  110b
  Generating CuII-Oxyl/CuIII-Oxo Species from CuI-α-Ketocarboxylate Complexes and O2: In Silico Studies on Ligand Effects and C-H-Activation Reactivity
  Huber, Stefan M.; Ertem, Mehmed Z.; Aquilante, Francesco; Gagliardi, Laura; Tolman, William B.; Cramer, Christopher J.
  Chemistry - A European Journal (2009), 15 (19), 4886-4895, S4886/1-S4886/334CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Theor. speaking: The mechanistic details assocd. with the generation and reaction of [CuO]+ species from CuI-α-ketocarboxylate complexes, esp. with respect to modifications of the ligand supporting the copper center, were investigated. Theor. models were used to characterize the electronic structures of different [CuO]+ species and their reactivity in C-H activation and O-atom transfer reactions. A mechanism for the oxygenation of CuI complexes with α-ketocarboxylate ligands that is based on a combination of d. functional theory and multireference second-order perturbation theory (CASSCF/CASPT2) calcns. is elaborated. The reaction proceeds in a manner largely analogous to those of similar FeII-α-ketocarboxylate systems, i.e., by initial attack of a coordinated oxygen mol. on a ketocarboxylate ligand with concomitant decarboxylation. Subsequently, two reactive intermediates may be generated, a Cu-peracid structure and a [CuO]+ species, both of which are capable of oxidizing a Ph ring component of the supporting ligand. Hydroxylation by the [CuO]+ species is predicted to proceed with a smaller activation free energy. The effects of electronic and steric variations on the oxygenation mechanisms were studied by introducing substituents at several positions of the ligand backbone and by investigating various N-donor ligands. In general, more electron donation by the N-donor ligand leads to increased stabilization of the more CuII/CuIII-like intermediates (oxygen adducts and [CuO]+ species) relative to the more CuI-like peracid intermediate. For all ligands investigated, the [CuO]+ intermediates are best described as CuII-O·- species with triplet ground states. The reactivity of these compds. in C-H abstraction reactions decreases with more electron-donating N-donor ligands, which also increase the Cu-O bond strength, although the Cu-O bond is generally predicted to be rather weak (with a bond order of about 0.5). A comparison of several methods to obtain singlet energies for the reaction intermediates indicates that multireference second-order perturbation theory is likely more accurate for the initial oxygen adducts, but not necessarily for subsequent reaction intermediates.
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111. 111
  Tsuji, T.; Zaoputra, A. A.; Hitomi, Y.; Mieda, K.; Ogura, T.; Shiota, Y.; Yoshizawa, K.; Sato, H.; Kodera, M. Specific Enhancement of Catalytic Activity by a Dicopper Core: Selective Hydroxylation of Benzene to Phenol with Hydrogen Peroxide. Angew. Chem. 2017, 129, 7887– 7890, DOI: 10.1002/ange.201702291
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  There is no corresponding record for this reference.
112. 112
  Augusti, R.; Dias, A. O.; Rocha, L. L.; Lago, R. M. Kinetics and Mechanism of Benzene Derivative Degradation with Fenton’s Reagent in Aqueous Medium Studies by MIMS. J. Phys. Chem. A 1998, 102, 10723– 10727, DOI: 10.1021/jp983256o
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  112
  Kinetics and Mechanism of Benzene Derivative Degradation with Fenton's Reagent in Aqueous Medium Studied by MIMS
  Augusti, Rodinei; Dias, Adelson O.; Rocha, Lilian L.; Lago, Rochel M.
  Journal of Physical Chemistry A (1998), 102 (52), 10723-10727CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  Membrane introduction mass spectrometry (MIMS) was used to investigate kinetic and mechanistic aspects of the reaction of benzene derivs. with Fenton's reagent (Fe2+/H2O2) in water. Under the conditions employed, the reaction rate showed a first-order dependence on the arom. compd. concn. The order of reactivity obsd. was C6H5Cl > C6H5Br > C6H6 > C6H5CH3 > C6H5OCH3 > C6H5NO2 > C6H5OH, and, with the exception of C6H5NO2, a linear Hammett relationship (log kX/kH vs. σp) was obsd. This fact suggests that electronic factors significantly influence reactivity with the Fenton's reagent. Expts. with C6H6 and C6D6 showed the presence of an isotopic effect of kH/kD = 1.7, suggesting that cleavage of the benzene C-H bond occurs in the reaction rate controlling step. Mechanistic studies with chlorobenzene showed that mineralization to CO2 and chloride proceeds via hydroxylation steps producing phenolic, hydroquinonic, and quinonic intermediates.
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113. 113
  Maiti, D.; Lucas, H. R.; Sarjeant, A. A. N.; Karlin, K. D. Aryl Hydroxylation from a Mononuclear Copper-Hydroperoxo Species. J. Am. Chem. Soc. 2007, 129, 6998– 6999, DOI: 10.1021/ja071704c
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  113
  Aryl Hydroxylation from a Mononuclear Copper-Hydroperoxo Species
  Maiti, Debabrata; Lucas, Heather R.; Narducci Sarjeant, Amy A.; Karlin, Kenneth D.
  Journal of the American Chemical Society (2007), 129 (22), 6998-6999CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Mononuclear CuII--OOH entities or derived species were seriously considered as important in chem. or biochem. oxidns. Yet, synthetic chem. studies have thus far revealed they have very limited substrate reactivity. Here, a significant aryl hydroxylation reaction occurs from a hydroperoxocopper(II) complex possessing a tripodal tetradentate ligand with appended aryl substituent. A phenolate-Cu(II) complex is detected following a proposed O-O cleavage event, that is suggested from precedent from nonheme Fe chem. A bis-μ-oxo-dicopper(III) complex as active oxygenating agent is ruled out.
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114. 114
  Jacobson, R. R.; Tyeklar, Z.; Farooq, A.; Karlin, K. D.; Liu, S.; Zubieta, J. A Copper-Oxygen (Cu₂-O₂) Complex. Crystal Structure and Characterization of a Reversible Dioxygen Binding System. J. Am. Chem. Soc. 1988, 110, 3690– 3692, DOI: 10.1021/ja00219a071
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  114
  A copper-oxygen (Cu2-O2) complex. Crystal structure and characterization of a reversible dioxygen binding system
  Jacobson, Richard R.; Tyeklar, Zoltan; Farooq, Amjad; Karlin, Kenneth D.; Liu, Shuncheng; Zubieta, Jon
  Journal of the American Chemical Society (1988), 110 (11), 3690-2CODEN: JACSAT; ISSN:0002-7863.
  The 1st x-ray structural characterization of a Cu-O2 complex is reported. The tripodal tetradentate ligand tris[(2-pyridyl)methyl]amine (L) was used to prep. [CuLL1]+ (L1 = RCN, PPh3). Oxygenation of CuL(NCR)+ in EtCN or CH2Cl2 at -80° provides [{LCu}2(O2)]2+ (I; Cu:O2 = 2:1) which has characteristic UV-visible absorptions at 525 and 590 nm. The binding of O2 (and CO) to [CuL(NCR)]+ to form I is reversible; reaction of I with either CO or PPh3 affords CuL(CO)+ or CuL(PPh3)+, resp. Protonation of I gives H2O2 and CuL(NCR)2+. The properties of I indicate that it is best described as a peroxo dicopper(II) complex. The Cu(II) ions (e.g. d-d band at 1035 nm) appear to be strongly magnetically coupled, based on the ESR silence and sharp 1H NMR spectrum exhibited by I. A single crystal x-ray structural characterization of [{LCu}2(O2)](PF6)2.5Et2O (-90°; P‾1, a 11.062(3), b 12.758(4), c 13.280(5) Å, α 96.72(3), β 110.57(3), γ 103.73(3)°, Z = 1, R = 0.0581, Rw = 0.0580) shows that it has a trans μ-1,2-peroxo ligation to Cu(II) ions in a trigonal bipyramidal coordination.
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115. 115
  Vilella, L.; Conde, A.; Balcells, D.; Díaz-Requejo, M. M.; Lledós, A.; Pérez, P. J. A Competing, Dual Mechanism for Catalytic Direct Benzene Hydroxylation from Combined Experimental-DFT Studies. Chem. Sci. 2017, 8, 8373– 8383, DOI: 10.1039/C7SC02898A
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  115
  A competing, dual mechanism for catalytic direct benzene hydroxylation from combined experimental-DFT studies
  Vilella, Laia; Conde, Ana; Balcells, David; Diaz-Requejo, M. Mar; Lledos, Agusti; Perez, Pedro J.
  Chemical Science (2017), 8 (12), 8373-8383CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
  A dual mechanism for direct benzene catalytic hydroxylation is described. Exptl. studies and DFT calcns. have provided a mechanistic explanation for the acid-free, TpxCu-catalyzed hydroxylation of benzene with hydrogen peroxide (Tpx = hydrotrispyrazolylborate ligand). In contrast with other catalytic systems that promote this transformation through Fenton-like pathways, this system operates through a copper-oxyl intermediate that may interact with the arene ring following two different, competitive routes: (a) electrophilic arom. substitution, with the copper-oxyl species acting as the formal electrophile, and (b) the so-called rebound mechanism, in which the hydrogen is abstracted by the Cu-O moiety prior to the C-O bond formation. Both pathways contribute to the global transformation albeit to different extents, the electrophilic substitution route seeming to be largely favored.
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116. 116
  Zhang, M.-T.; Chen, Z.; Kang, P.; Meyer, T. J. Electrocatalytic Water Oxidation with a Copper(II) Polypeptide Complex. J. Am. Chem. Soc. 2013, 135, 2048– 2051, DOI: 10.1021/ja3097515
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  116
  Electrocatalytic Water Oxidation with a Copper(II) Polypeptide Complex
  Zhang, Ming-Tian; Chen, Zuofeng; Kang, Peng; Meyer, Thomas J.
  Journal of the American Chemical Society (2013), 135 (6), 2048-2051CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  A self-assembly-formed triglycylglycine macrocyclic ligand (TGG4-) complex of Cu(II), [(TGG4-)CuII-OH2]2-, efficiently catalyzes water oxidn. in a phosphate buffer at pH 11 at room temp. by a well-defined mechanism. In the mechanism, initial oxidn. to Cu(III) is followed by further oxidn. to a formal "Cu(IV)" with formation of a peroxide intermediate, which undergoes further oxidn. to release oxygen and close the catalytic cycle. The catalyst exhibits high stability and activity toward water oxidn. under these conditions with a high turnover frequency of 33 s-1.
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117. 117
  (a) Fisher, K. J.; Materna, K. L.; Mercado, B. Q.; Crabtree, R. H.; Brudvig, G. W. Electrocatalytic Water oxidation by a Copper(II) Complex of an Oxidatio-Resistant Ligand. ACS Catal. 2017, 7, 3384– 3387, DOI: 10.1021/acscatal.7b00494
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  117a
  Electrocatalytic Water Oxidation by a Copper(II) Complex of an Oxidation-Resistant Ligand
  Fisher, Katherine J.; Materna, Kelly L.; Mercado, Brandon Q.; Crabtree, Robert H.; Brudvig, Gary W.
  ACS Catalysis (2017), 7 (5), 3384-3387CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  The Cu(II) complex Cu(pyalk)2 (pyalk = 2-pyridyl-2-propanoate) is a robust homogeneous H2O-oxidn. electrocatalyst under basic conditions (pH > 10.4). H2O oxidn. occurs at a relatively low overpotential for Cu of 520-580 mV with a turnover frequency of ∼0.7 s-1. Controlled potential electrolysis expts. over 12 h at 1.1 V vs. normal H electrode gave >30 catalytic turnovers of O2 with only ∼20% catalyst degrdn. The robustness of the catalyst under fairly harsh conditions and the low overpotential further highlight the oxidn. resistance and strong donor character of pyalk.
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  .
  (b) Rudshteyn, B.; Fisher, K. J.; Lant, H. M. C.; Yang, K. R.; Mercado, B. Q.; Crabtree, R. H.; Brudvig, G. W.; Batista, V. S. Water-Nucleophilic Attack Mechanism for the Cu^II(pyalk)₂ Water-Oxidation Catalyst. ACS Catal. 2018, 8, 7952– 7960, DOI: 10.1021/acscatal.8b02466
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  117b
  Water-Nucleophilic Attack Mechanism for the CuII(pyalk)2 Water-Oxidation Catalyst
  Rudshteyn, Benjamin; Fisher, Katherine J.; Lant, Hannah M. C.; Yang, Ke R.; Mercado, Brandon Q.; Brudvig, Gary W.; Crabtree, Robert H.; Batista, Victor S.
  ACS Catalysis (2018), 8 (9), 7952-7960CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  We investigate the mechanism of water oxidn. catalyzed by the CuII(pyalk)2 complex, combining d. functional theory with exptl. measurements of turnover frequencies, UV-visible spectra, H/D kinetic isotope effects (KIEs), electrochem. anal., and synthesis of a deriv. complex. We find that only in the cis form does CuII(pyalk)2 convert water to dioxygen. In a series of alternating chem. and electrochem. steps, the catalyst is activated to form a metal oxyl radical species that undergoes a water-nucleophilic attack defining the rate-limiting step of the reaction. The exptl. H/D KIE (3.4) is in agreement with the calcd. value (3.7), shown to be detd. by deprotonation of the substrate nucleophile upon O-O bond formation. The reported mechanistic findings are particularly valuable for rational design of complexes inspired by CuII(pyalk)2.
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118. 118
  Yang, X.; Baik, M.-H. cis,cis-[(bpy)₂Ru^VO]₂O⁴⁺ Catalyzes Water Oxidation Formally via in Situ Generation of Radicaloid Ru^IV–O•. J. Am. Chem. Soc. 2006, 128, 7476– 7485, DOI: 10.1021/ja053710j
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  118
  cis,cis-[(bpy)2RuVO]2O4+ Catalyzes Water Oxidation Formally via in Situ Generation of Radicaloid RuIV-O•
  Yang, Xiaofan; Baik, Mu-Hyun
  Journal of the American Chemical Society (2006), 128 (23), 7476-7485CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The mechanism of the catalytic oxidn. of water by cis,cis-[(bpy)2Ru(OH2)]2O4+ to give mol. dioxygen was investigated using D. Functional Theory (DFT). A series of four oxidn. and four deprotonation events generate the catalytically competent species cis,cis-[(bpy)2RuVO]2O4+, which breaks the H-OH bond homolytically at the rate detg. transition state to give a hydroperoxo intermediate. Our calcns. predict a rate detg. activation barrier of 25.9 kcal/mol in soln. phase, which is in reasonable agreement with the previously reported exptl. est. of 18.7-23.3 kcal/mol. A no. of plausible coupling schemes of the two metal sites including strong coupling, weak ferromagnetic and weak antiferromagnetic coupling have been considered. In addn., both high-spin and low-spin states at each of the Ru(V)-d3 centers were explored and we found that the high-spin states play an important mechanistic role. Our calcns. suggest that cis,cis-[(bpy)2RuVO]2O4+ performs formally an intramol. ligand-to-metal charge transfer when reacting with water to formally give a cis,cis-[(bpy)2RuIVO•]2O4+ complex. We propose that the key characteristic of the diruthenium catalyst that allows it to accomplish the most difficult first two oxidns. of the overall four-electron redox reaction is directly assocd. with this in situ generation of two radicaloid oxo moieties that promote the water splitting reaction. A proton coupled metal-to-metal charge transfer follows to yield a Ru(V)/Ru(III) peroxo/aqua mixed valence complex, which performs the third redox reaction to give the superoxo/aqua complex. Finally, intersystem crossing to a ferromagnetically coupled Ru(IV)/Ru(III) superoxo/aqua species is predicted, which will then promote the last redox event to release triplet dioxygen as the final product. A no. of key features of the computed mechanism are explored in detail to derive a conceptual understanding of the catalytic mechanism.
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119. 119
  Rüttinger, W.; Dismukes, G. C. Synthetic Water-Oxidation Catalysts for Artificial Photosynthetic Water Oxidation. Chem. Rev. 1997, 97, 1– 24, DOI: 10.1021/cr950201z
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  119
  Synthetic Water-Oxidation Catalysts for Artificial Photosynthetic Water Oxidation
  Ruettinger, Wolfgang; Dismukes, G. Charles
  Chemical Reviews (Washington, D. C.) (1997), 97 (1), 1-24CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review with 132 refs. in which homogeneous and some heterogeneous catalysts for oxidn. of water are described. Current views are presented of the photosynthetic water-oxidizing complex (WOC) and its functionality, followed by anal. of the thermodn. and kinetic constraints for water oxidn. that have to be overcome by any catalyst. Since manganese is the metal that performs this reaction in the WOC, manganese photocatalysts are discussed, also other transition metals, particularly ruthenium are discussed. Principles of reactivity learned from theory and existing models are summarized, which can lead to synthesis of better catalysts in the future.
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120. 120
  (a) Balcells, D.; Raynaud, C.; Crabtree, R. H.; Eisenstein, O. The Rebound Mechanism in Catalytic C-H Oxidation by MnO(tpp)Cl from DFT Studies: Electronic Nature of the Active Species. Chem. Commun. 2008, 744– 766, DOI: 10.1039/B715939K
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  The rebound mechanism in catalytic C-H oxidation by MnO(tpp)Cl from DFT studies: electronic nature of the active species
  Balcells, David; Raynaud, Christophe; Crabtree, Robert H.; Eisenstein, Odile
  Chemical Communications (Cambridge, United Kingdom) (2008), (6), 744-746CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
  DFT studies show that the rebound mechanism for MnO(tpp)(Cl)-catalyzed C-H hydroxylation is favored for spin states with oxyl character.
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  (b) Mayer, J. M. Hydrogen Atom Abstraction by Metal-Oxo Complexes: Understanding the Analogy with Organic Radical Reactions. Acc. Chem. Res. 1998, 31, 441– 450, DOI: 10.1021/ar970171h
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  120b
  Hydrogen Atom Abstraction by Metal-Oxo Complexes: Understanding the Analogy with Organic Radical Reactions
  Mayer, James M.
  Accounts of Chemical Research (1998), 31 (8), 441-450CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
  A review with 56 refs. This Account describes our progression from thinking about radicals and spin d. to an approach based on the thermochem. of the hydrogen atom transfer step. The spin-d. intuition was based on an analogy with org. radical chem., but as described below, a better analogy is based on bond strengths. This approach provides both new qual. insight and quant. predictions.
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121. 121
  Lippert, C. A.; Hardcastle, K. I.; Soper, J. D. Harnessing Redox-Active Ligands for Low-Barrier Radical Addition at Oxorhenium Complexes. Inorg. Chem. 2011, 50, 9864– 9878, DOI: 10.1021/ic200923q
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  121
  Harnessing Redox-Active Ligands for Low-Barrier Radical Addition at Oxorhenium Complexes
  Lippert, Cameron A.; Hardcastle, Kenneth I.; Soper, Jake D.
  Inorganic Chemistry (2011), 50 (20), 9864-9878CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  The addn. of an [X]+ electrophile to the five-coordinate oxorhenium(V) anion [ReV(O)(apPh)2]- {[apPh]2- = 2,4-di-tert-butyl-6-(phenylamido)phenolate} gives new products contg. Re-X bonds. The Re-X bond-forming reaction is analogous to oxo transfer to [ReV(O)(apPh)2]- in that both are 2e- redox processes, but the electronic structures of the products are different. Whereas oxo addn. to [ReV(O)(apPh)2]- yields a closed-shell [ReVII(O)2(apPh)2]- product of 2e- metal oxidn., [Cl]+ addn. gives a diradical ReVI(O)(apPh)(isqPh)Cl product ([isqPh]·- = 2,4-di-tert-butyl-6-(phenylimino)semiquinonate) with 1e- in a Re d orbital and 1e- on a redox-active ligand. The differences in electronic structure are ascribed to differences in the π basicity of [O]2- and Cl- ligands. The observation of ligand radicals in ReVI(O)(apPh)(isqPh)X provides exptl. support for the capacity of redox-active ligands to deliver electrons in other bond-forming reactions at [ReV(O)(apPh)2]-, including radical addns. of O2 or TEMPO· to make Re-O bonds. Attempts to prep. the electron-transfer series monomers between ReVI(O)(apPh)(isqPh)X and [ReV(O)(apPh)2]- yielded a sym. bis(μ-oxo)dirhenium complex. Formation of this dimer suggested that ReVI(O)(apPh)(isqPh)Cl may be a source of an oxyl metal fragment. The ability of ReVI(O)(apPh)(isqPh)Cl to undergo radical coupling at oxo was revealed in its reaction with Ph3C·, which affords Ph3COH and deoxygenated metal products. This reactivity is surprising because ReVI(O)(apPh)(isqPh)Cl is not a strong outer-sphere oxidant or oxo-transfer reagent. The authors postulate that the unique ability of ReVI(O)(apPh)(isqPh)Cl to effect oxo transfer to Ph3C· arises from symmetry-allowed mixing of a populated Re[n.58876]O π bond with a ligand-centered [isqPh]·- ligand radical, which gives oxyl radical character to the oxo ligand. This allows the closed-shell oxo ligand to undergo a net 2e- oxo-transfer reaction to Ph3C· via kinetically facile redox-active ligand-mediated radical steps. Harnessing intraligand charge transfer for radical reactions at closed-shell oxo ligands is a new strategy to exploit redox-active ligands for small-mol. activation and functionalization. The implications for the design of new oxidants that use low-barrier radical steps for selective multielectron transformations are discussed.
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122. 122
  Conry, R. R.; Mayer, J. M. Oxygen Atom Transfer Reactions of Cationic Rhenium(III), Rhenium(V), and Rhenium(VII) Triazacyclononane Complexes. Inorg. Chem. 1990, 29, 4862– 4867, DOI: 10.1021/ic00349a010
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  122
  Oxygen atom transfer reactions of cationic rhenium(III), rhenium(V), and rhenium(VII) triazacyclononane complexes
  Conry, Rebecca R.; Mayer, James M.
  Inorganic Chemistry (1990), 29 (24), 4862-7CODEN: INOCAJ; ISSN:0020-1669.
  Re(O)Cl3(Me2S)(OPPh3) reacts readily with 1,4,7-trimethyltriazacyclononane (Me3tacn) to form [ReV(O)Cl2(Me3tacn)]+ (I) in good yield. With the unsubstituted triazacyclononane (tacn), however, both [ReV(O)Cl2(tacn)]+ (II) and [ReVII(O)3(tacn)]+ (III) are formed, even under anaerobic conditions. Oxidn. of II to III [Re(V) → Re(VII)] can be easily accomplished with a variety of mild oxidizing agents such as Me2SO and I2, but the oxidn. of I requires over a month at 80° in aq. nitric acid. I is reduced [Re(V) → Re(III)] by oxygen atom transfer to phosphines, forming [ReIII(OPR3)Cl2(Me3tacn)]+ (IV; R = Ph, Me). The OPPh3 ligand in IV is easily displaced by other neutral ligands such as MeCN or Me2CO. [Re(OCMe2)Cl2(Me3tacn)]+ is readily oxidized back to I [Re(III) → Re(V)] by the O atom donors BuNCO, OAsPh3, Me2SO, ethylene oxide, pyridine N-oxide, and N2O. These reactions require an open coordination site at the Re(III) center. Surprisingly, it is not substantially easier to oxidize IV than II. On the basis of these reactions, simple thermochem. cycles are used to est. the Re-oxo bond strength in I to be 141 ± 9 kcal/mol.
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123. 123
  Verat, A. Y.; Fan, H.; Pink, M.; Chen, Y.-S.; Caulton, K. G. Spin State, Structure, and Reactivity of Terminal Oxo and Dioxygen Complexes of the (PNP)Rh Moiety. Chem. - Eur. J. 2008, 14, 7680– 7686, DOI: 10.1002/chem.200800573
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  Spin state, structure, and reactivity of terminal oxo and dioxygen complexes of the (PNP)Rh moiety
  Verat, Alexander Y.; Fan, Hongjun; Pink, Maren; Chen, Y.-S.; Caulton, Kenneth G.
  Chemistry - A European Journal (2008), 14 (25), 7680-7686CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  [RhIIIH{(tBu2PCH2SiMe2NSiMe2CH2PtBu(CMe2CH2))}], ([RhH(PNP*)]), reacts with O2 in the time taken to mix the reagents to form a 1:1 η2-O2 adduct, for which O-O bond length is discussed with ref. to the reducing power of [RhH(PNP*)]. DFT calcns. faithfully replicate the obsd. O-O distance, and were used to understand the oxidn. state of this coordinated O2. The reactivity of [Rh(O2)(PNP)] towards H2, CO, N2, and O2 is tested and compared to the assocd. DFT reaction energies. Three different reagents effect single O atom transfer to [RhH(PNP*)]. The resulting [RhO(PNP)], characterized at ≥ -60° and by DFT calcns., is a ground-state triplet, is nonplanar, and reacts, ⪆+15°, with its own tBu C-H bond, to cleanly form a diamagnetic complex, [Rh(OH){N(SiMe2CH2PtBu2)(SiMe2CHPtBu{CMe2CH2})}].
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124. 124
  Streb, C. New Trends in Polyoxometalate Photoredox Chemistry: From Photosensitization to Water Oxidation Catalysis. Dalton Trans. 2012, 41, 1651– 1659, DOI: 10.1039/C1DT11220A
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  New trends in polyoxometalate photoredox chemistry: From photosensitisation to water oxidation catalysis
  Streb, Carsten
  Dalton Transactions (2012), 41 (6), 1651-1659CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
  Mol. metal oxide clusters, so-called polyoxometalates (POM) have been extensively used as homogeneous photocatalysts in various photoredox reactions such as the oxidn. of alkanes, alkenes and alcs. as well as the light-induced mineralization of various org. and inorg. pollutants. The more general application of POMs as photoactive compds., in particular in solar energy harnessing, has been hampered as the clusters typically absorb light in the UV-region only. Over the past decade, concepts have been put forward on how the reactivity of this class of compds. can be optimized to improve their overall photoactivity, and a particular focus has been on the design of photocatalytic processes which allow the conversion of solar light into useful chem. reactivity. This perspective gives a brief overview of general aspects of POM photochem. and critically discusses the advantages and challenges of a range of POM-based systems for photooxidns. and photoredns. with a focus on the development of sustainable solar light conversion systems.
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125. 125
  (a) Papaconstantinou, E. Photocatalytic Oxidation of Organic Compounds Using Heteropoly Electrolytes of Molybdenum and Tungsten. J. Chem. Soc., Chem. Commun. 1982, 12– 13, DOI: 10.1039/c39820000012
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  Photocatalytic oxidation of organic compounds using heteropoly electrolytes of molybdenum and tungsten
  Papaconstantinou, E.
  Journal of the Chemical Society, Chemical Communications (1982), (1), 12-13CODEN: JCCCAT; ISSN:0022-4936.
  Org. compds. are photochem. oxidized in the presence of heteropoly compds., e.g., [PW12O40]3- (I); the heteropoly compds. may be reoxidized, forming the basis of a photocatalytic oxidn. process. E.g., Me2CHOH was oxidized to Me2CO by I in the presence of sunlight; the corresponding redn. of I was reversible on exclusion of light in the presence of O.
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  (b) Zhang, Z.; Lin, Q.; Kurunthu, D.; Wu, T.; Zuo, F.; Zheng, S.-T.; Bardeen, C. J.; Bu, X.; Feng, P. Synthesis and Photocatalytic Properties of a New Heteropolyoxoniobate Compound: K₁₀[Nb₂O₂(H₂O)₂][SiNb₁₂O₄₀]·12H₂O. J. Am. Chem. Soc. 2011, 133, 6934– 6937, DOI: 10.1021/ja201670x
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  125b
  Synthesis and Photocatalytic Properties of a New Heteropolyoxoniobate Compound: K10[Nb2O2(H2O)2][SiNb12O40]·12H2O
  Zhang, Zhenyu; Lin, Qipu; Kurunthu, Dharmalingam; Wu, Tao; Zuo, Fan; Zheng, Shou-Tian; Bardeen, Christopher J.; Bu, Xianhui; Feng, Pingyun
  Journal of the American Chemical Society (2011), 133 (18), 6934-6937CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The synthesis and photocatalytic properties of a heteropolyoxoniobate, K10[Nb2O2(H2O)2][SiNb12O40]·12H2O (1), are reported, revealing an important role of Zr4+ additives in the crystn. Compd. 1 exhibits overall photocatalytic water splitting activity, and its photocatalytic activity is significantly higher than that of Na10[Nb2O2][SiNb12O40]·xH2O (2). Fluorescence lifetime measurements suggest that the enhanced photocatalytic activity of 1 likely results from a larger yield of longer-lived charge trapping states in 1 due to the coordination of one water mol. to the bridging Nb5+, leading to highly unsym. seven-coordinated Nb5+ sites.
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126. 126
  (a) Renneke, R. F.; Hill, C. L. Selective Photochemical Dehydrogenation of Saturated Hydrocarbons with Quantum Yields Approaching Unity. Angew. Chem., Int. Ed. Engl. 1988, 27, 1526– 1527, DOI: 10.1002/anie.198815261
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  Catalytic photochemical dehydrogenation of organic substrates by polyoxometalates
  Hill, Craig L.; Bouchard, Donald A.
  Journal of the American Chemical Society (1985), 107 (18), 5148-57CODEN: JACSAT; ISSN:0002-7863.
  The photochem. behavior of polyoxometalates (POM) based on W, Mo, V, Nb and Ta in the presence of H2O or 1 of a variety of org. substrates (including alcs., amides, ethers, aldehydes, carboxylic acids, nitriles, ketones and ureas) is examd. Irradn. of the charge-transfer bands of POM dissolved in org. media at 25° leads in most cases to oxidn. of the org. substrate and redn. of the POM. The POM fall into 3 categories defined by their thermal and photochem. redox chem. in the presence of org. substrates. Type I complexes, exemplified by those of Nb and Ta, do not photooxidize any org. substrate upon irradn. Type II complexes, exemplified by decavanadate and most heteropoly- and isopolymolybdates, and Type III complexes, exemplified by most heteropoly- and isopolytungstates, do not oxidize a wide range of org. substrates upon irradn. Reoxidn. of the reduced forms of the Type II complexes, either by reaction with O2 or by evolution of H2, is kinetically or thermodynamically unfavorable; analogous reoxidn. of the reduced forms of the Type III complexes is not. Several factors affecting the quantum yields for prodn. of reduced POM are outlined, and the energetic features regarding H2 evolution are discussed. The IR, UV, and 31P, 183W and 17O NMR spectral properties of α-H3PW12O40.6H2O (I) and other POM remain the same before and after catalytic photochem. dehydrogenation of representative alc., ether or amide substrates. Little if any POM decompn. occurs during the photoredox chem. Interactions between org. substrates and POM have profound effects on the electronic structure of the POM. The charge-transfer transitions of I display different sensitivities to medium in the low-energy (λ >300 nm) vs. high-energy region of the UV-visible spectral range. The highest quantum yields for photoredox chem. involving org. substrates and I are obsd. in the low-energy or absorption-tail region. One possible model explaining the wavelength dependence of the absorption and photochem. action spectra is discussed. A general mechanism in agreement with all the exptl. data is proposed for org. substrate oxidn. and the effective capture of light energy in these POM-org. substrate systems.
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127. 127
  Duncan, D. C.; Netzel, T. L.; Hill, C. L. Early-Time Dynamics and Reactivity of Polyoxometalate Excited States. Identification of a Short-Lived LMCT Excited State and a Reactive Long-Lived Charge-Transfer Intermediate following Picosecond Flash Excitation of [W₁₀O₃₂]^4– in Acetonitrile. Inorg. Chem. 1995, 34, 4640– 4646, DOI: 10.1021/ic00122a021
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  127
  Early-Time Dynamics and Reactivity of Polyoxometalate Excited States. Identification of a Short-Lived LMCT Excited State and a Reactive Long-Lived Charge-Transfer Intermediate following Picosecond Flash Excitation of [W10O32]4- in Acetonitrile
  Duncan, Dean C.; Netzel, Thomas L.; Hill, Craig L.
  Inorganic Chemistry (1995), 34 (18), 4640-6CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  The authors report picosecond flash excitation results on [W10O32]4-, which demonstrate that the initially prepd. ligand-to-metal charge-transfer (LMCT) excited state decays within ∼30 ps to a single intermediate that persists for >15 ns. Little or no substrate reaction is derived from the short-lived LMCT excited state. Furthermore, the long-lived intermediate is not the 1-electron-reduced species [W10O32]5- or one of its protonated derivs. This long-lived intermediate is the primary photoreactant and has substantial charge-transfer character itself. Addnl. the intermediate and [W10O32]5- are likely to have similar W-orbital electron d.; the principal differences in electronic structure derive from the presence of an oxidized oxygen site in the intermediate which is lacking in [W10O32]5-.
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Metal–Oxyl Species and Their Possible Roles in Chemical Oxidations

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Abstract

Synopsis

1. Introduction

Figure 1

2. Definition of a Metal–Oxyl Species

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3. Experimentally Confirmed Metal–Oxyl Species

3.1. Zinc–Oxyl Species

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3.2. Ruthenium–Oxyl Complexes

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4. Partially Characterized Metal–Oxyl Species

4.1. Titanium–Oxyl Species

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4.2. Nickel–Oxyl Complexes

Figure 17

4.3. Copper–Oxyl Complex

4.4. Ruthenium–Oxyl Complexes

Figure 18

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5. Proposed Metal–Oxyl Species Based on DFT Calculations

5.1. Manganese–Oxyl Complexes

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5.2. Iron–Oxyl Complexes

5.3. Cobalt–Oxyl Complexes

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5.4. Nickel–Oxyl Complex

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5.5. Copper–Oxyl Complexes

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5.6. Ruthenium–Oxyl Complexes

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5.7. Rhenium–Oxyl Complexes

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5.8. Rhodium–Oxyl Complexes

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5.9. Tungsten–Oxyl Complexes

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6. How To Stabilize Metal–Oxyl Species

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7. Spectroscopic Criteria for Experimental Characterization of Metal–Oxyl Species

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8. Summary

Author Information

Biographies

Yoshihiro Shimoyama

Takahiko Kojima

Acknowledgments

References

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Abstract

Figure 1