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Lewis versus Brønsted Acid Activation of a Mn(IV) Catalyst for Alkene Oxidation
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Lewis versus Brønsted Acid Activation of a Mn(IV) Catalyst for Alkene Oxidation
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  • Jorn D. Steen
    Jorn D. Steen
    Molecular Inorganic Chemistry, Stratingh Institute for Chemistry, Faculty of Science and Engineering, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
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  • Stepan Stepanovic
    Stepan Stepanovic
    Faculty of Chemistry, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia
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  • Mahsa Parvizian
    Mahsa Parvizian
    Molecular Inorganic Chemistry, Stratingh Institute for Chemistry, Faculty of Science and Engineering, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
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  • Johannes W. de Boer
    Johannes W. de Boer
    Catexel B.V., BioPartner Center Leiden, Galileiweg 8, 2333 BD Leiden, The Netherlands
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  • Ronald Hage
    Ronald Hage
    Molecular Inorganic Chemistry, Stratingh Institute for Chemistry, Faculty of Science and Engineering, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
    Catexel B.V., BioPartner Center Leiden, Galileiweg 8, 2333 BD Leiden, The Netherlands
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    Juan Chen
    Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, China
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  • Marcel Swart
    Marcel Swart
    IQCC & Departament de Química, Universitat de Girona, Campus Montilivi (Ciències), 17003 Girona, Spain
    ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain
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    Orcidhttp://orcid.org/0000-0002-8174-8488
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    Maja Gruden
    Faculty of Chemistry, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia
    *E-mail: [email protected]
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    Orcidhttp://orcid.org/0000-0002-0746-5754
  • Wesley R. Browne*
    Wesley R. Browne
    Molecular Inorganic Chemistry, Stratingh Institute for Chemistry, Faculty of Science and Engineering, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
    *E-mail: [email protected]
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Inorganic Chemistry

Cite this: Inorg. Chem. 2019, 58, 21, 14924–14930
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https://doi.org/10.1021/acs.inorgchem.9b02737
Published October 18, 2019

Copyright © 2019 American Chemical Society. This publication is licensed under CC-BY-NC-ND.

Abstract

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Lewis acid (LA) activation by coordination to metal oxido species has emerged as a new strategy in catalytic oxidations. Despite the many reports of enhancement of performance in oxidation catalysis, direct evidence for LA-catalyst interactions under catalytically relevant conditions is lacking. Here, we show, using the oxidation of alkenes with H2O2 and the catalyst [Mn2(μ-O)3(tmtacn)2](PF6)2 (1), that Lewis acids commonly used to enhance catalytic activity, e.g., Sc(OTf)3, in fact undergo hydrolysis with adventitious water to release a strong Brønsted acid. The formation of Brønsted acids in situ is demonstrated using a combination of resonance Raman, UV/vis absorption spectroscopy, cyclic voltammetry, isotope labeling, and DFT calculations. The involvement of Brønsted acids in LA enhanced systems shown here holds implications for the conclusions reached in regard to the relevance of direct LA-metal oxido interactions under catalytic conditions.

Copyright © 2019 American Chemical Society

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Synopsis

Lewis acid activation of oxidation catalysts is proposed to be through binding of the Lewis acids to metal-oxo species. The activity of the catalyst [Mn2(μ-O)3(tmtacn)2](PF6)2 in the oxidation of alkenes with H2O2 appears to correlate with the strength of the Lewis acid used for its activation. We show that the correlation arises from the relative propensity of the Lewis acids to generate Brønsted acids in situ.

Introduction

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The interaction of Lewis acids (LAs) with transition metal complexes and clusters can profoundly change their reactivity, which is most clearly manifested in the critical role of calcium ions in the oxygen evolving complex of photosystem (PS) II. (1,2) Recent reports have highlighted correlations between Lewis acidity and properties of transition metal complexes, such as redox potential, (3−5) and by extrapolation the enhancements in activity that they bring in oxidation catalysis, e.g., using iron (6−15) and manganese complexes. (16−29) However, the causal nature of the effects of LAs and indeed the actual interactions between them and transition metal complexes under catalytic conditions are unclear. In particular, their binding to reactive species, although postulated, has not been confirmed in solution.
For example, Watkinson and Nodzewska (30) and the group of Yin (31) have described the exceptional impact of Lewis acids on the oxidation of alkenes with H2O2 catalyzed by the complex [Mn2(μ-O)3(tmtacn)2](PF6)2 (1, where tmtacn is N,N′,N″-trimethyl-1,4,7-triazacyclononane, Scheme 1). The catalytic activity of 1 is dependent on the presence of Lewis acidic metal triflates such as Sc(OTf)3; alkene oxidation is not observed under the same conditions without a Lewis acid. This dependence was ascribed to binding of the Lewis acid to either 1 or the reactive intermediate responsible for substrate oxidation. Direct interaction between, e.g., Sc3+, and 1, was inferred from spectroscopic data and by analogy with known M-O-LA structures obtained in the solid state. (32−34) Definitive evidence for such binding in solution is not available, however, especially under reaction conditions with, e.g., H2O2, where water is added with the oxidant in excess.

Scheme 1

Scheme 1. Oxidation of Alkenes with H2O2 Catalyzed by [Mn2(μ-O)3(tmtacn)2]2+ (1) and Proposed Roles of Lewis Acids
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In the present contribution we show through a combination of spectroscopy and DFT calculations that the changes that follow addition of Lewis acids to 1 are not due to LA binding to an oxido unit of 1, as proposed for related FeIV═O complexes. (32−34) Instead, the effects observed are due to the release of a strong Brønsted acid upon hydrolysis of the metal triflates by adventitious water either present in the solvent or as water of crystallization in 1. The released Brønsted acid facilitates reduction of 1 by H2O2, and subsequent ligand exchange and redox reactions (35) provide for the observed increase in catalytic performance.

Results and Discussion

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As reported by Watkinson and Nodzewska and the group of Yin, we find here that the addition of metal triflates to 1 prior to the addition of H2O2 results in conversion of styrene to styrene oxide, albeit still with a substantial loss of H2O2 through disproportionation (Scheme 2 and Figures S1 and S2). The turnover numbers (TONs) achieved here in the presence of LAs, ca. 250, are consistent with the earlier reports (ca. 100). (30,31) In-line monitoring of the oxidation of styrene reveals that, in the presence of Lewis acids, the reaction proceeds through two distinct phases (Scheme 2, Figure S2). The addition of H2O2 is followed by an induction period, after which both alkene oxidation and disproportionation of the H2O2 begin concomitantly. The duration of the induction period and the ratio of styrene conversion to H2O2 disproportionation depends on the time, here referred to as standing time, between the addition of the Lewis acid to 1 in anhydrous acetonitrile and the subsequent addition of styrene and H2O2 (Scheme 2). Disproportionation of H2O2 is observed regardless of the standing time, whereas conversion of styrene is observed only when the standing time exceeds several minutes. During the reaction, a white precipitate forms that is a mixture of insoluble manganese and scandium salts (by ICP, see Experimental Section for details) of, most likely, acetate formed by hydrolysis of acetonitrile. (36,37) Although the addition of a second equivalent of H2O2 results in continued oxidation of styrene (Figure S3), the isolated precipitate is not catalytically active.

Scheme 2

Scheme 2. Key Stages in the Oxidation of Styrene (Blue) Catalyzed by 1 (1 mM) with Sc(OTf)3 (2 mM) Using H2O2 (Magenta) As Oxidanta
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aDifferent standing times were used: 20 s (empty); 60 min (filled).

Other Lewis acids (31) have similar effects to that of Sc(OTf)3, in terms of induction period and rate of oxidation. With Al(OTf)3 consistently higher, conversion of styrene was obtained, whereas with Y(OTf)3, the decomposition of H2O2 was slow, and negligible conversion of styrene was observed (Figure S2). Notably, however, with Y(CF3CO2)3, both rapid decomposition of H2O2 and significant conversion of styrene were observed. The relative performance of the Lewis acids correlates with their relative rates of hydrolysis; (38) however, the counterion plays a role in the outcome of the reaction also. These data prompted us to examine the interaction between the LAs, and especially Sc(OTf)3, and 1.

Effect of Lewis Acids on the Electronic and Vibrational Spectroscopy of 1

The UV/vis absorption spectrum of 1 in acetonitrile shows a broad visible absorption at 490 nm and several more intense bands below 400 nm. (39,40) The addition of 2 equiv of Sc(OTf)3 results in an increase in absorbance over the range 400 and 650 nm and the appearance of weak bands at ca. 750 and 850 nm (Figure 1a). The relative rate of change in absorbance is constant across the entire spectrum, indicative of a single step process. Notably, the changes are not immediate but take >30 s. The Raman spectrum at λexc 355 nm undergoes concomitant changes with the resonantly enhanced Mn–O–Mn symmetric stretching band (40,41) at 699 cm–1 decreasing in intensity and a band at 687 cm–1 appearing together with an increase in intensity of the band at 799 cm–1 (Figure 1b). DFT calculations and 18O labeling indicate that the band at 699 cm–1 is a vibrational mode of the Mn–(μ-O)3–Mn core, while the band at 799 cm–1 involves mostly the Mn–N bonds, with little displacement of the Mn–(μ-O)3–Mn core (Figure S4). Similar changes are observed at λexc 457 nm (Figure S5). Weaker bands appear also that correspond to modes of the tmtacn ligand observed under nonresonant conditions (λexc 785 nm, Figure S6). The addition of Al(OTf)3 resulted in identical changes to the UV/vis absorption and resonance Raman spectra of 1, whereas the addition of Y(OTf)3 and Y(CF3CO2)3 did not (Figure S7).

Figure 1

Figure 1. (a) UV/vis absorption spectroscopy of 1 in CH3CN before (black), after the addition of Sc(OTf)3 (2 equiv; green) or TfOH (6 eq.; blue), and after subsequent addition of excess H2O (292 equiv; magenta), inset: absorbance at 500 nm over time after the addition of Sc(OTf)3 (2 equiv; green) or TfOH (6 equiv; blue). (b) Raman spectra (λexc 355 nm) of 1 in CH3CN before (black), after the addition of Sc(OTf)3 (2 equiv; green) or TfOH (6 equiv; blue), and after subsequent addition of excess H2O (6200 equiv; magenta). See Figures S4 and S6 for measured and calculated shifts.

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The changes in the UV/vis absorption spectrum are similar to those reported by Lv et al., (31) who proposed the formation of a mononuclear manganese(IV) complex analogous to that reported earlier by Chin Quee-Smith et al. (i.e., [MnIV(tmtacn)(OMe)3](PF6)). (42) However, the final UV/vis absorption spectrum is identical to that reported earlier by Hage et al. for 1 in concentrated H2SO4. (40) The addition of excess water after the addition of Sc(OTf)3 resulted in an immediate recovery of the initial UV/vis absorption and resonance Raman spectra of 1 (Figure 1), and indeed even 0.2 vol % of water is sufficient for full recovery (see Figures S8a and S8b). Furthermore, using H218O did not result in incorporation of 18O into 1 (by Raman spectroscopy, Figures S4 and S8c). (40,43) Hence, the changes upon the addition of Sc(OTf)3 are unlikely to be due to “opening” of the Mn–O–Mn bridges. (35) Furthermore, DFT calculations indicate that although the formation of a ScIII–O–(MnIV)2 bond is thermodynamically feasible, the calculated frequencies of the relevant vibrational mode (symmetric) do not match the shifts observed experimentally by Raman spectroscopy (Figure S9). In contrast the shifts calculated for 1 and H1+ match well (Figure S4). These data indicate that, in solution, Lewis acidic metal ions (e.g., Sc3+) do not bind to a bridging oxygen of 1, but instead 1 is protonated by Brønsted acids, vide infra.

Effect of Lewis Acids on the Cyclic Voltammetry of 1

A key role of Brønsted acids in activating 1 in catalytic oxidations is to shift its reduction potential to more positive potentials. This shift facilitates reduction of 1 by H2O2 from a MnIV2 state to dinuclear MnII and MnIII species. (41) These latter species are catalytically active as established earlier where 1 was used in the presence of carboxylic acids. (35)
DFT calculations indicate that binding of Sc3+ to the Mn–(μ-O)3–Mn core is thermodynamically favorable and changes the Mn–O bond lengths substantially (see SI). The Sc–O bond is predicted to have a significant covalent bond character, close to that of the O–H bond in H1+. Hence, notwithstanding the discussion above, binding of Sc(OTf)3 could shift the reduction potential of 1 in a similar manner to that induced by protonation and thereby facilitate reduction by H2O2. Indeed, cyclic voltammetry shows that the reduction of 1 at −0.6 V vs SCE (40) moves to ca. 0.4 V upon the addition of Sc(OTf)3 (Figure 2). The increase in current indicates a multielectron process, and new oxidation waves at ca. 1.0 V on the return cycles are consistent with the formation of new species as shown earlier by de Boer et al. (35)

Figure 2

Figure 2. Comparison of the cyclic voltammetry of 1 (black) with that obtained after the addition of either Sc(OTf)3 (3 equiv; green) or TfOH (9 equiv; blue) in 0.1 M TBAPF6 in CH3CN. Current axis offset for clarity. Initial potential and scan direction indicated by ∗ and black arrows, respectively.

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Notably these changes are almost identical to those observed upon the addition of TfOH to 1 (Figure 2). As with Lewis acids, the addition of water results in only a minor shift of the redox waves back toward negative potentials, and essentially the same general shape of the redox wave is observed (Figure S10), despite that H1+ reverts to 1. It should be noted that even weak acids that are not able to fully protonate 1 can provide sufficient acidity to enable reduction due to the fast equilibriums involved (Figure S11). (35) After standing for several minutes, additional redox waves at 0.6 V are observed with both TfOH and Sc(OTf)3 (Figures S12 and S13). (35,40) The new redox waves at 0.65 and 1.05 V that appear over time in the presence of Sc(OTf)3 and of TfOH correspond to those of [MnIII2(μ-O)(μ-OAc)2(tmtacn)2]2+ in the presence of acid (Figure S14). (35)

Comparison of the Lewis and Brønsted Acids on the Spectroscopy of 1 and Its Catalytic Activity

As for the cyclic voltammetry, Sc(OTf)3 and TfOH have essentially identical effects on the UV/vis absorption and resonance Raman spectra of 1 (Figure 1). Indeed, these same spectroscopic changes are observed upon the addition of concentrated H2SO4 (or D2SO4, Figure S15) to 1 in acetonitrile, and the changes are consistent with formation of the monoprotonated complex H1+. (40,44) Notably the changes induced by Brønsted acids are instantaneous, in contrast to the gradual changes (>30 s) observed upon the addition of Sc(OTf)3. This difference is consistent with release of Brønsted acids by hydrolysis of Sc(OTf)3 (38,45) prior to protonation of 1. It should be noted that 1 supplies 1 equivalent of water as water of crystallization, in addition to residual water already present in acetonitrile.
Having confirmed the spectroscopic similarities between the addition of TfOH and Sc(OTf)3 to 1, the Brønsted acid assisted oxidation (18,28,46) of styrene was examined. Essentially identical catalytic behavior was observed when using TfOH or Sc(OTf)3, including a lag period followed by rapid onset of H2O2 decomposition and oxidation of styrene (Figure 3). Similar trends were observed with trifluoroacetic acid (Figure S16), reinforcing that triflic acid is not unique and other Brønsted acids are capable of activating 1 in the same way, i.e., by protonation assisted reduction from the MnIV,IV2 state. (47)

Figure 3

Figure 3. Comparison of kinetics of styrene conversion (blue) and H2O2 consumption (magenta) by 1 activated by either TfOH (6 equiv; filled) or Sc(OTf)3 (2 equiv; empty) for a 1 h standing time. In the absence of any acid, only disproportionation of H2O2 to O2 is observed (Figure S1).

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The release of Brønsted acids from metal triflates in ostensibly anhydrous solvents has been noted in the literature under various conditions. For example in chlorinated solvents, Hintermann et al. reported that the reaction of AgOTf with a chlorinated substrate, and subsequently solvent, releases TfOH, (48) and recently, Schlegel et al. reported the release of catalytically active triflic acid in the metal triflate catalyzed glycosylation reactions. (49) Gunnoe et al. have proposed the in situ generation of triflic acid from Al(OTf)3 in the hydroamination of nonactivated alkenylamines in solvents such as DMSO and nitrobenzene etc. (50)
In situ formation of TfOH, specifically in acetonitrile, has been proposed by Dumeunier and Markó in the acylation of alcohols catalyzed by metal triflates, which serve as reservoirs of the Brønsted acid. (51) Spencer et al. have identified Brønsted acids as the active catalysts in hetero-Michael additions to α,β-unsaturated ketones in the presence of various metal salts (52) and related the catalytic ability of a metal salt to the extent of hydrolysis—conversion was not observed with metal salts that do not undergo hydrolysis. Additionally, water (more than 2 equiv vs metal catalyst) retards the reaction due to its Brønsted basicity. The water in that case most likely originates from side reactions such as imine condensation and acetal/thioacetal formation, which are unavoidable under the nonbasic conditions used for the hetero-Michael addition.
In the present report, 1 equiv of water is present by default due to the fact that 1 is a monohydrate, but as discussed by Spencer et al. even if this is not the case water can form due to background reactions. Indeed, even when anhydrous, the water content is at a minimum 0.001–0.005 vol %, which corresponds to approximately 0.5–3 mM of H2O. This is in the same concentration range as the manganese complex (1 mM) and metal triflates (2 mM). Furthermore, in addition to water added with the oxidant H2O2, even when in 90 wt % concentration, the disproportionation of H2O2 generates H2O and O2, and during epoxidation 1 equiv of H2O is released also.
The pKa of 1 is lower than most strong acids and hence the leveling effect of water means that when present in excess of the TfOH formed, the strongest acid present is the hydronium ion, which is unable to protonate 1 to an extent detectable by spectroscopic methods. Neither yttrium(III) salts nor CF3CO2H induce changes in the UV/vis absorption and Raman spectra of 1, although they provide sufficient Brønsted acidity to facilitate reduction of 1 by H2O2, as observed with carboxylic acids earlier. (35,41) Indeed, there is no reason that the pKa of Sc(H2O)x species formed by hydrolysis should be lower than that of TfOH and hence the species responsible for protonation of 1 cannot be defined. It is of note, however, that cyclic voltammetry with TfOH is nearly identical to that with Sc(OTf)3. Hence, although we have characterized Brønsted acidity in the present study as being due to the formation of TfOH in situ, in reality the nature of the species that protonates 1 to form H1+ is ill-defined. Ultimately, the actual Brønsted acid responsible is of little concern in this case, but rather the effects observed are due to Brønsted rather than Lewis acidity. A point that is certain is that once water is added in molar excess, e.g., with H2O2, or formed by side reactions, the hydronium ion is the Brønsted acid involved. Notably the hydronium ion is a much weaker acid than H1+, and its addition, as shown above, results in a recovery of the original spectral features of 1. Nevertheless, the equilibrium position is sufficient (see cyclic voltammetry) to provide enough H1+ in solution for H2O2 to be able to initiate reduction. The initial reduction triggers an autocatalytic transformation of 1 into species in lower oxidation states as shown earlier. (35,41)

Conclusion

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In summary, we have shown here that Lewis acidic metal triflates undergo rapid hydrolysis to generate strong Brønsted acids in acetonitrile under the conditions used for catalytic oxidations with H2O2. Indeed, even in anhydrous acetonitrile, residual water (ca. 0.5 to 3 mM H2O) and water of crystallization (1 molecule per 1) can be sufficient for hydrolysis of the Lewis acid (Sc(OTf)3). In the case of oxidation of alkenes with H2O2 and 1, the hydrolysis occurs well before the onset of substrate conversion. Hence, the postulated binding of Lewis acids to 1, or a putative reactive species, does not occur and the changes in spectral properties and enhancements in catalytic activity observed are due to Brønsted acids formed in situ. Indeed, Brønsted acids, i.e., carboxylic acids, were shown earlier to suppress disproportionation (41) and allow for H2O2 to be used with complete efficiency in the oxidation of alkenes catalyzed by 1 with, e.g., CCl3CO2H, with turnover numbers (TONs) exceeding 3000. (35,47)
Although Sc3+-bound species have been observed crystallographically, (32−34) the reactivity changes induced by such Lewis acids in solution are highly likely to be due to the release of Brønsted acids. The role of LAs as a source of Brønsted acids shown here impacts more broadly, for example, in the study of Lewis acid activation of iron and other metal catalysts. Beyond this, however, in recognizing the possibility to introduce strong Brønsted acids into reactions via Sc(OTf)3, the use of often difficult to handle strong acids directly can be circumvented.

Experimental Section

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General Information

All reagents were of commercial grade (Sigma-Aldrich, TCI) and were used as received unless stated otherwise. H2O2: Sigma-Aldrich, 50 wt %. H218O: Rotem Industries Ltd., 98%. Anhydrous CH3CN (Sigma-Aldrich): 99.8%, <0.005% H2O (3 mM). The 1.5 M Na18OH (in H218O) was prepared by adding a piece of Na metal (33 mg, 0.29 mmol) to 1.0 mL of H218O. The mixture of 1 M H2O2 and 1.5 M Na18OH in H218O was prepared by mixing a solution of 50 wt % H2O2 (13 μL) and H218O (250 μL) with the previously prepared 1.5 M Na18OH (aq.; 200 μL). [Mn2(μ-O)3(tmtacn)2](PF6)2·H2O (1) was generously supplied by Catexel Ltd., and it was analyzed by elemental analysis (calcd for Mn2C18H44N6O4P2F12): C, 26.82% (26.74%); H, 5.39% (5.49%); N, 10.40% (10.40%).

Synthesis of 18O-1

The synthesis of [Mn2(μ-18O)3(tmtacn)2](PF6)2·H2O (18O-1) was carried out by adaptation of the method described by Hage et al. (53) A three-necked flask equipped with a small stirring egg was charged with tmtacn (37 mg, 0.22 mmol) and 1 mL of H218O. The mixture was purged with N2 and held under a N2 atmosphere. MnSO4·H2O (38 mg, 0.22 mmol) was added to the reaction mixture, after which the flask was cooled in an ice bath for 15 min. A premixed solution of 200 μL of 1.5 M Na18OH and 263 μL of 1 M H2O2 in H218O was added slowly to the brown reaction mixture, upon which it turned black. The mixture was stirred on ice for 40 min, during which it turned dark red, and was stirred at room temperature for another 45 min. Then, 1 M H2SO4 in H218O (150 μL) was added to the reaction mixture to reach pH 2. The dark red reaction mixture was subsequently filtered by gravity over paper into a 10 mL flask. KPF6 (45 mg, 0.24 mmol) was added to the filtrate, upon which a precipitate formed immediately. The flask was kept in the fridge overnight to form microcrystalline needles, after which the solution was removed from the vial by Pasteur pipet, and the solid was washed with diethyl ether. The crystalline product was dried in the vial with gentle heating to remove residual water. The 18O-labeled product 18O-1 was characterized by Raman spectroscopy (λexc 785 nm (Figure S6)) and UV–vis absorption spectroscopy in CH3CN.

Physical Measurements

Inductively Coupled Plasma Atomic absorption (ICP-AAS) spectra were recorded on a PerkinElmer Optima 7000 DV ICP. Electrochemical measurements were carried out on a model 760c Electrochemical Analyzer (CH Instruments). Analyte concentrations were typically 1.0 mM in anhydrous acetonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6), and a Teflon-shrouded glassy carbon (GC) working electrode (CH Instruments), a Pt wire auxiliary electrode, and a saturated calomel electrode (SCE) reference electrode were employed unless stated otherwise. Cyclic voltammograms were obtained at scan rates between 0.1 V/s and 1.0 V/s. UV/vis absorption spectra were recorded on an Analytik Jena Specord S300 or S600 spectrophotometer using 1 cm path length quartz cuvettes. Raman spectra at λexc 785 nm were recorded using either a PerkinElmer Raman Station 400F or RamanFlex or a home-built Raman spectrometer comprised of a Shamrock-300i spectrograph equipped with a iVac-A-DR-316B-LDC-DD-RES CCD camera (Andor Technology) fiber coupled to an integrated Raman probe (100 mW at source, Innovative Photonic Solutions). Raman spectra at λexc 355 nm (10 mW at source, Cobolt Lasers) were recorded in a 180° backscattering arrangement. Raman scattering was collected by a 2.5 cm diameter plano-convex lens (f = 7.5 cm), and the collimated Raman scattering was passed through an appropriate long pass edge filter (Semrock) after which it was focused by a second 2.5 cm diameter plano-convex lens (f = 15 cm) into a Shamrock500i spectrograph (Andor Technology) with a 2399 L/mm grating blazed at 300 nm, and it was acquired with an iDus-420-BU2 CCD camera (Andor Technology). The spectral slit width was set to 12 μm. Raman spectra at λexc 457 nm (50 mW at source, Cobolt Lasers) were recorded in a 180° backscattering arrangement. Raman scattering was collected by a 2.5 cm diameter plano-convex lens (f = 7.5 cm), and the collimated Raman scattering was passed through an appropriate long pass edge filter (Semrock) after which it was focused by a second 2.5 cm diameter plano-convex lens (f = 7.5 cm) into a Shamrock300i spectrograph (Andor Technology) with a 1200 L/mm grating blazed at 500 nm, and it was acquired with an iDus-420-BU CCD camera (Andor Technology).

Procedure Employed for Catalysis Studies

The Lewis acid (30 μL of 100 mM solution in CH3CN, 3 μmol) or Brønsted acid (30 μL of 300 mM solution in CH3CN, 9 μmol) was added to 1.21 mL of a 1.24 mM solution of 1 (1.5 μmol) in anhydrous CH3CN, and this mixture was stirred for a certain standing time, after which styrene (172 μL, 1500 μmol) was added. The final concentrations in 1.5 mL reaction mixture: 1 (1 mM), Lewis acid (2 mM) or Brønsted acid (6 mM), styrene (1 M), H2O2 (1 M). Reaction progress was determined by Raman spectroscopy (λexc 785 nm) with the initial time (t = 0) defined as the point of addition of H2O2 (85 μL of 50 wt % in H2O, 1500 μmol). The conversion of styrene and consumption of H2O2 were monitored for approximately 30 min. Epoxide formation was confirmed by 1H NMR spectroscopy.
Caution! Complete disproportionation of H2O2 to oxygen and water can occur and hence the reactions should not be carried out in sealed vessels.
Note: Comparison of reaction progress data obtained in the present study with that in previous reports by Nodzewska and Watkinson showed the same reaction time (3–4 min). (30) Lv et al., (31) however, applied general reaction conditions to each tested substrate, and therefore a reaction time of 2 h was reported for styrene. It is of note that in the aforementioned studies 0.1 M styrene was used, in contrast to the present study with 1 M and hence the effect of standing time would not have manifested itself in a difference in conversion in those studies.
ICP analysis confirms that the white precipitate formed during catalysis contained 10–16 wt % of Mn and 5–20 wt % of Sc in the form of insoluble salts. The insolubility and %metal content is consistent with the anion being acetate. The FTIR spectrum indicates that the counterion is an organic compound which is affected by deuteration of the solvent (d3-acetonitrile) but does not contain a nitrile group (Figure S17). This precipitate is formed in the absence of substrate, and a preciptate is formed in the absence of the manganese complex also. Hence although the organic component could be due to a degradation product of the tmtacn ligand; more probably, hydrolysis of acetonitrile is responsible since the spectrum is solvent deuteration dependent. Comparison of these spectra with those of commercially available, and relatively anhydrous, scandium and manganese acetates is hampered by the effect of water (hydration state) on the spectrum, but the spectrum of the precipitate is close to that of NaOAc (Figure S18).

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  • Corresponding Authors
  • Authors
    • Jorn D. Steen - Molecular Inorganic Chemistry, Stratingh Institute for Chemistry, Faculty of Science and Engineering, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
    • Stepan Stepanovic - Faculty of Chemistry, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia;  Present Address: Center for Chemistry, ICTM, University of Belgrade, Njegoševa 12, 11001 Belgrade, Serbia
    • Mahsa Parvizian - Molecular Inorganic Chemistry, Stratingh Institute for Chemistry, Faculty of Science and Engineering, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
    • Johannes W. de Boer - Catexel B.V., BioPartner Center Leiden, Galileiweg 8, 2333 BD Leiden, The Netherlands
    • Ronald Hage - Molecular Inorganic Chemistry, Stratingh Institute for Chemistry, Faculty of Science and Engineering, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands;  Catexel B.V., BioPartner Center Leiden, Galileiweg 8, 2333 BD Leiden, The Netherlands
    • Juan Chen - Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, China
    • Marcel Swart - IQCC & Departament de Química, Universitat de Girona, Campus Montilivi (Ciències), 17003 Girona, Spain;  ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain;  Orcidhttp://orcid.org/0000-0002-8174-8488
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The COST association action CM1305 ECOSTBio (STSM grant 34080), the European Research Council (ERC 279549, W.R.B.), MINECO (CTQ2017-87392-P, M.S.), GenCat (2014SGR1202, M.S.), FEDER (UNGI10-4E-801, M.S.), the Chinese Scholarship Council (CSC), and The Netherlands Ministry of Education, Culture and Science (Gravity Program 024.001.035) are acknowledged for financial support. The Peregrine high performance computing cluster of the University of Groningen is acknowledged for computational resources.

References

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This article references 53 other publications.

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    A review. The authors summarize the current understanding of the structure of the Mn4CaO5 cluster of the photosynthetic oxygen-evolving complex, as well as the water oxidn. reaction based on insights learned primarily from x-ray techniques.
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    Within photosynthetic organisms, the oxygen-evolving complex (OEC) of photosystem II generates dioxygen from water using a catalytic Mn4CaOn cluster (n varies with the mechanism and nature of the intermediate). The authors report here the rational synthesis of a [Mn3CaO4]6+ cubane that structurally models the tri-manganese-calcium-cubane subsite of the OEC. Structural and electrochem. comparison between Mn3CaO4 and a related Mn4O4 cubane alongside characterization of an intermediate calcium-manganese multinuclear complex reveals potential roles of calcium in facilitating high oxidn. states at manganese and in the assembly of the biol. cluster.
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    Tsui, E. Y.; Agapie, T. Reduction Potentials of Heterometallic Manganese-Oxido Cubane Complexes Modulated by Redox-Inactive Metals. Proc. Natl. Acad. Sci. U. S. A. 2013, 110 (25), 10084– 10088,  DOI: 10.1073/pnas.1302677110
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    Understanding the effect of redox-inactive metals on the properties of biol. and heterogeneous H2O oxidn. catalysts is important both fundamentally and for improvement of future catalyst designs. Heterometallic Mn-oxido cubane clusters [MMn3O4] (M = Sr2+, Zn2+, Sc3+, Y3+) with polypyridyl and acetate ligands structurally relevant to the O-evolving complex (OEC) of photosystem II were prepd. and characterized. The redn. potentials of these clusters and other related mixed metal Mn-tetraoxido complexes are correlated with the Lewis acidity of the apical redox-inactive metal in a manner similar to a related series of heterometallic Mn-dioxido clusters. The redox potentials of the [SrMn3O4] and [CaMn3O4] clusters are close, which is consistent with the observation that the OEC is functional only with one of these two metals. Considering the authors' previous studies of [MMn3O2] moieties, the present results with more structurally accurate models of the OEC ([MMn3O4]) suggest a general relation between the redn. potentials of heterometallic oxido clusters and the Lewis acidities of incorporated cations that applies to diverse structural motifs. These findings support proposals that one function of Ca in the OEC is to modulate the redn. potential of the cluster to allow electron transfer.
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    Redox-inactive metals are found in biol. and heterogeneous water oxidn. catalysts, but, at present, their roles in catalysis are not well understood. Here, we report a series of high-oxidn.-state tetranuclear-dioxido clusters comprising three manganese centers and a redox-inactive metal (M). Crystallog. studies show an unprecedented Mn3M(μ4-O)(μ2-O) core that remains intact on changing M or the manganese oxidn. state. Electrochem. studies reveal that the redn. potentials span a window of 700 mV and are dependent on the Lewis acidity of the second metal. With the pKa of the redox-inactive metal-aqua complex as a measure of Lewis acidity, these compds. demonstrate a linear dependence between redn. potential and acidity with a slope of ∼100 mV per pKa unit. The Sr2+ and Ca2+ compds. show similar potentials, an observation that correlates with the behavior of the oxygen-evolving complex of photosystem II, which is active only if one of these two metals is present.
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    Morimoto, Y.; Kotani, H.; Park, J.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. Metal Ion-Coupled Electron Transfer of a Nonheme Oxoiron(IV) Complex: Remarkable Enhancement of Electron-Transfer Rates by Sc3+. J. Am. Chem. Soc. 2011, 133 (3), 403– 405,  DOI: 10.1021/ja109056x
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    Park, J.; Morimoto, Y.; Lee, Y.-M.; You, Y.; Nam, W.; Fukuzumi, S. Scandium Ion-Enhanced Oxidative Dimerization and N-Demethylation of N,N-Dimethylanilines by a Non-Heme Iron(IV)-Oxo Complex. Inorg. Chem. 2011, 50 (22), 11612– 11622,  DOI: 10.1021/ic201545a
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    Inorganic Chemistry (2011), 50 (22), 11612-11622CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Oxidative dimerization of N,N-dimethylaniline (DMA) occurs with a nonheme iron(IV)-oxo complex, [FeIV(O)(N4Py)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), to yield the corresponding dimer, tetramethylbenzidine (TMB), in acetonitrile. The rate of the oxidative dimerization of DMA by [FeIV(O)(N4Py)]2+ is markedly enhanced by the presence of scandium triflate, Sc(OTf)3 (OTf = CF3SO3-), when TMB is further oxidized to the radical cation (TMB·+). In contrast, we have obsd. the oxidative N-demethylation with para-substituted DMA substrates, since the position of the C-C bond formation to yield the dimer is blocked. The rate of the oxidative N-demethylation of para-substituted DMA by [FeIV(O)(N4Py)]2+ is also markedly enhanced by the presence of Sc(OTf)3. In the case of para-substituted DMA derivs. with electron-donating substituents, radical cations of DMA derivs. are initially formed by Sc3+ ion-coupled electron transfer from DMA derivs. to [FeIV(O)(N4Py)]2+, giving demethylated products. Binding of Sc3+ to [FeIV(O)(N4Py)]2+ enhances the Sc3+ ion-coupled electron transfer from DMA derivs. to [FeIV(O)(N4Py)]2+, whereas binding of Sc3+ to DMA derivs. retards the electron-transfer reaction. The complicated kinetics of the Sc3+ ion-coupled electron transfer from DMA derivs. to [FeIV(O)(N4Py)]2+ are analyzed by competition between binding of Sc3+ to DMA derivs. and to [FeIV(O)(N4Py)]2+. The binding consts. of Sc3+ to DMA derivs. increase with the increase of the electron-donating ability of the para-substituent. The rate consts. of Sc3+ ion-coupled electron transfer from DMA derivs. to [FeIV(O)(N4Py)]2+, which are estd. from the binding consts. of Sc3+ to DMA derivs., agree well with those predicted from the driving force dependence of the rate consts. of Sc3+ ion-coupled electron transfer from one-electron reductants to [FeIV(O)(N4Py)]2+. Thus, oxidative dimerization of DMA and N-demethylation of p-substituted DMA derivs. proceed via Sc3+ ion-coupled electron transfer from DMA derivs. to [FeIV(O)(N4Py)]2+.
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    Park, J.; Morimoto, Y.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. Metal Ion Effect on the Switch of Mechanism from Direct Oxygen Transfer to Metal Ion-Coupled Electron Transfer in the Sulfoxidation of Thioanisoles by a Non-Heme Iron(IV)–Oxo Complex. J. Am. Chem. Soc. 2011, 133 (14), 5236– 5239,  DOI: 10.1021/ja200901n
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    The mechanism of sulfoxidn. of thioaniosoles by a nonheme iron(IV)-oxo complex is switched from direct oxygen transfer to metal ion-coupled electron transfer by the presence of Sc3+. The switch in the sulfoxidn. mechanism is dependent on the 1-electron oxidn. potentials of thioanisoles. The rate of sulfoxidn. is accelerated ≤102-fold by the addn. of Sc3+.
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    Morimoto, Y.; Park, J.; Suenobu, T.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. Mechanistic Borderline of One-Step Hydrogen Atom Transfer versus Stepwise Sc3+-Coupled Electron Transfer from Benzyl Alcohol Derivatives to a Non-Heme Iron(IV)-Oxo Complex. Inorg. Chem. 2012, 51 (18), 10025– 10036,  DOI: 10.1021/ic3016723
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    Inorganic Chemistry (2012), 51 (18), 10025-10036CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The rate of oxidn. of 2,5-dimethoxybenzyl alc. (2,5-(MeO)2C6H3CH2OH) by [FeIV(O)(N4Py)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) was enhanced significantly in the presence of Sc(OTf)3 (OTf- = trifluoromethanesulfonate) in acetonitrile (e.g., 120-fold acceleration in the presence of Sc3+). Such a remarkable enhancement of the reactivity of [FeIV(O)(N4Py)]2+ in the presence of Sc3+ was accompanied by the disappearance of a kinetic deuterium isotope effect. The radical cation of 2,5-(MeO)2C6H3CH2OH was detected in the course of the reaction in the presence of Sc3+. The dimerized alc. and aldehyde were also produced in addn. to the monomer aldehyde in the presence of Sc3+. These results indicate that the reaction mechanism is changed from one-step hydrogen atom transfer (HAT) from 2,5-(MeO)2C6H3CH2OH to [FeIV(O)(N4Py)]2+ in the absence of Sc3+ to stepwise Sc3+-coupled electron transfer, followed by proton transfer in the presence of Sc3+. In contrast, neither acceleration of the rate nor the disappearance of the kinetic deuterium isotope effect was obsd. in the oxidn. of benzyl alc. (C6H5CH2OH) by [FeIV(O)(N4Py)]2+ in the presence of Sc(OTf)3. Moreover, the rate consts. detd. in the oxidn. of various benzyl alc. derivs. by [FeIV(O)(N4Py)]2+ in the presence of Sc(OTf)3 (10 mM) were compared with those of Sc3+-coupled electron transfer from one-electron reductants to [FeIV(O)(N4Py)]2+ at the same driving force of electron transfer. This comparison revealed that the borderline of the change in the mechanism from HAT to stepwise Sc3+-coupled electron transfer and proton transfer is dependent on the one-electron oxidn. potential of benzyl alc. derivs. (ca. 1.7 V vs SCE).
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    Redox-inactive metal ions that function as Lewis acids play pivotal roles in modulating the reactivity of oxygen-contg. metal complexes and metalloenzymes, such as the oxygen-evolving complex in photosystem II and its small-mol. mimics. Here, the authors report the synthesis and characterization of nonhaem Fe(III)-peroxo complexes that bind redox-inactive metal ions, (TMC)FeIII-(μ,η2:η2-O2)-Mn+ (Mn+ = Sr2+, Ca2+, Zn2+, Lu3+, Y3+ and Sc3+; TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). The Ca2+ and Sr2+ complexes showed similar electrochem. properties and reactivities in 1-electron oxidn. or redn. reactions. However, the properties and reactivities of complexes formed with stronger Lewis acidities are markedly different. Complexes that contain Ca2+ or Sr2+ ions were oxidized by an electron acceptor to release O2, whereas the release of O2 did not occur for complexes that bind stronger Lewis acids. These results in the light of the functional role of the Ca2+ ion in the oxidn. of H2O to dioxygen by the oxygen-evolving complex are discussed.
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    Kal, S.; Draksharapu, A.; Que, L. Sc3+ (or HClO4) Activation of a Nonheme FeIII–OOH Intermediate for the Rapid Hydroxylation of Cyclohexane and Benzene. J. Am. Chem. Soc. 2018, 140 (17), 5798– 5804,  DOI: 10.1021/jacs.8b01435
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    Journal of the American Chemical Society (2018), 140 (17), 5798-5804CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    [Fe(β-BPMCN)(CH3CN)2]2+ (1, BPMCN = N,N'-bis(pyridyl-2-methyl)-N,N'-dimethyl-trans-1,2-diaminocyclo-hexane) is a relatively poor catalyst for cyclohexane oxidn. by H2O2 and cannot perform benzene hydroxylation. However, addn. of Sc3+ activates the 1/H2O2 reaction mixt. to be able to hydroxylate cyclohexane and benzene within seconds at -40 °C. A metastable S = 1/2 FeIII-(η1-OOH) intermediate 2 is trapped at -40 °C, which undergoes rapid decay upon addn. of Sc3+ at rates independent of [substrate] but linearly dependent on [Sc3+]. HClO4 elicits comparable reactivity as Sc3+ at the same concn. We thus postulate that these additives both facilitate O-O bond heterolysis of 2 to form a common highly electrophilic FeV=O oxidant that is comparably reactive to the fastest nonheme high-valent iron-oxo oxidants found to date. Safety: 90% H2O2 is potentially explosive.
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  15. 15
    Kal, S.; Que, L. Activation of a Non-Heme FeIII-OOH by a Second FeIII to Hydroxylate Strong C–H Bonds: Possible Implications for Soluble Methane Monooxygenase. Angew. Chem., Int. Ed. 2019, 58 (25), 8484– 8488,  DOI: 10.1002/anie.201903465
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    15
    Activation of a non-heme FeIII-OOH by a second FeIII to hydroxylate strong C-H Bonds: Possible implications for soluble methane monooxygenase
    Kal, Subhasree; Que, Lawrence Jr.
    Angewandte Chemie, International Edition (2019), 58 (25), 8484-8488CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Non-heme iron oxygenases contain either monoiron or diiron active sites, and the role of the second iron in the latter enzymes is a topic of particular interest, esp. for sol. methane monooxygenase (sMMO). Herein we report the activation of a non-heme FeIII-OOH intermediate in a synthetic monoiron system using FeIII(OTf)3 to form a high-valent oxidant capable of effecting cyclohexane and benzene hydroxylation within seconds at -40°C. Our results show that the second iron acts as a Lewis acid to activate the iron-hydroperoxo intermediate, leading to the formation of a powerful FeV=O oxidant-a possible role for the second iron in sMMO.
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  16. 16
    Park, Y. J.; Ziller, J. W.; Borovik, A. S. The Effects of Redox-Inactive Metal Ions on the Activation of Dioxygen: Isolation and Characterization of a Heterobimetallic Complex Containing a MnIII-(μ-OH)-CaII Core. J. Am. Chem. Soc. 2011, 133 (24), 9258– 9261,  DOI: 10.1021/ja203458d
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    16
    The Effects of Redox-Inactive Metal Ions on the Activation of Dioxygen: Isolation and Characterization of a Heterobimetallic Complex Containing a MnIII-(μ-OH)-CaII Core
    Park, Young Jun; Ziller, Joseph W.; Borovik, A. S.
    Journal of the American Chemical Society (2011), 133 (24), 9258-9261CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Rate enhancements for the redn. of dioxygen by a MnII complex were obsd. in the presence of redox-inactive group 2 metal ions. The rate changes were correlated with an increase in the Lewis acidity of the group 2 metal ions. These studies led to the isolation of heterobimetallic complexes contg. MnIII-(μ-OH)-MII cores (MII = CaII, BaII) in which the hydroxo oxygen atom is derived from O2. This type of core structure has relevance to the oxygen-evolving complex within photosystem II.
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  17. 17
    Leeladee, P.; Baglia, R. A.; Prokop, K. A.; Latifi, R.; De Visser, S. P.; Goldberg, D. P. Valence Tautomerism in a High-Valent Manganese-Oxo Porphyrinoid Complex Induced by a Lewis Acid. J. Am. Chem. Soc. 2012, 134 (25), 10397– 10400,  DOI: 10.1021/ja304609n
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    17
    Valence Tautomerism in a High-Valent Manganese-Oxo Porphyrinoid Complex Induced by a Lewis Acid
    Leeladee, Pannee; Baglia, Regina A.; Prokop, Katharine A.; Latifi, Reza; de Visser, Sam P.; Goldberg, David P.
    Journal of the American Chemical Society (2012), 134 (25), 10397-10400CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Addn. of the Lewis acid Zn2+ to (TBP8Cz)MnV(O) induces valence tautomerization, resulting in the formation of [(TBP8Cz+·)MnIV(O)-Zn2+]. This new species was characterized by UV-vis, EPR, the Evans method, and 1H NMR and supported by DFT calcns. Removal of Zn2+ quant. restores the starting material. Electron-transfer and hydrogen-atom-transfer reactions are strongly influenced by the presence of Zn2+.
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  18. 18
    Baglia, R. A.; Krest, C. M.; Yang, T.; Leeladee, P.; Goldberg, D. P. High-Valent Manganese-Oxo Valence Tautomers and the Influence of Lewis/Brönsted Acids on C-H Bond Cleavage. Inorg. Chem. 2016, 55 (20), 10800– 10809,  DOI: 10.1021/acs.inorgchem.6b02109
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    18
    High-Valent Manganese-Oxo Valence Tautomers and the Influence of Lewis/Bronsted Acids on C-H Bond Cleavage
    Baglia, Regina A.; Krest, Courtney M.; Yang, Tzuhsiung; Leeladee, Pannee; Goldberg, David P.
    Inorganic Chemistry (2016), 55 (20), 10800-10809CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The addn. of Lewis or Bronsted acids (LA = Zn(OTf)2, B(C6F5)3, HBArF, TFA) to the high-valent manganese-oxo complex MnV(O)(TBP8Cz) results in the stabilization of a valence tautomer MnIV(O-LA)(TBP8Cz•+). The ZnII and B(C6F5)3 complexes were characterized by manganese K-edge X-ray absorption spectroscopy (XAS). The position of the edge energies and the intensities of the pre-edge (1s to 3d) peaks confirm that the Mn ion is in the +4 oxidn. state. Fitting of the extended X-ray absorption fine structure (EXAFS) region reveals 4 N/O ligands at Mn-Nave = 1.89 Å and a fifth N/O ligand at 1.61 Å, corresponding to the terminal oxo ligand. This Mn-O bond length is elongated compared to the MnV(O) starting material (Mn-O = 1.55 Å). The reactivity of MnIV(O-LA)(TBP8Cz•+) toward C-H substrates was examd., and it was found that H• abstraction from C-H bonds occurs in a 1:1 stoichiometry, giving a MnIV complex and the dehydrogenated org. product. The rates of C-H cleavage are accelerated for the MnIV(O-LA)(TBP8Cz•+) valence tautomer as compared to the MnV(O) valence tautomer when LA = ZnII, B(C6F5)3, and HBArF, whereas for LA = TFA, the C-H cleavage rate is slightly slower than when compared to MnV(O). A large, nonclassical kinetic isotope effect of kH/kD = 25-27 was obsd. for LA = B(C6F5)3 and HBArF, indicating that H-atom transfer (HAT) is the rate-limiting step in the C-H cleavage reaction and implicating a potential tunneling mechanism for HAT. The reactivity of MnIV(O-LA)(TBP8Cz•+) toward C-H bonds depends on the strength of the Lewis acid. The HAT reactivity is compared with the analogous corrole complex MnIV(O-H)(tpfc•+) recently reported (J. Am. Chem. Soc.2015, 137, 14481-14487).
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  19. 19
    Choe, C.; Lv, Z.; Wu, Y.; Chen, Z.; Sun, T.; Wang, H.; Li, G.; Yin, G. Promoting a Non-Heme Manganese Complex Catalyzed Oxygen Transfer Reaction by Both Lewis Acid and Brønsted Acid: Similarities and Distinctions. Mol. Catal. 2017, 438, 230– 238,  DOI: 10.1016/j.mcat.2017.05.030
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    19
    Promoting a non-heme manganese complex catalyzed oxygen transfer reaction by both Lewis acid and Bronsted acid: Similarities and distinctions
    Choe, Cholho; Lv, Zhanao; Wu, Yunfeng; Chen, Zhuqi; Sun, Tingting; Wang, Haibin; Li, Guangxing; Yin, Guochuan
    Molecular Catalysis (2017), 438 (), 230-238CODEN: MCOADH ISSN:. (Elsevier B.V.)
    This work demonstrates that certain Lewis and Bronsted acids can sharply improve the oxygen transfer efficiency of a manganese(II) catalyst bearing non-heme ligand. In the absence of Lewis and Bronsted acids, oxidn. of manganese(II) complex will generate di-μ-oxo-bridged dinuclear Mn2(III,IV) core which is very sluggish for olefin epoxidn. Adding non-redox metal ions as Lewis acid or Bronsted acid will both improve the catalytic epoxidn. of olefin, and this improvement is dependent on the pKa of Bronsted acid, or the net charge of non-redox metals of Lewis acid. Mechanism study revealed that similar promotional effect by either Lewis or Bronsted acids was originated from a similar reaction pathway by dissocg. aforementioned sluggish di-μ-oxo core. However, distinctions of reactive intermediate were also demonstrated for Lewis or Bronsted acids.
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  20. 20
    Sharma, N.; Jung, J.; Ohkubo, K.; Lee, Y.-M.; El-Khouly, M. E.; Nam, W.; Fukuzumi, S. Long-Lived Photoexcited State of a Mn(IV)-Oxo Complex Binding Scandium Ions That Is Capable of Hydroxylating Benzene. J. Am. Chem. Soc. 2018, 140 (27), 8405– 8409,  DOI: 10.1021/jacs.8b04904
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    20
    Long-Lived Photoexcited State of a Mn(IV)-Oxo Complex Binding Scandium Ions That is Capable of Hydroxylating Benzene
    Sharma, Namita; Jung, Jieun; Ohkubo, Kei; Lee, Yong-Min; El-Khouly, Mohamed E.; Nam, Wonwoo; Fukuzumi, Shunichi
    Journal of the American Chemical Society (2018), 140 (27), 8405-8409CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Photoexcitation of a MnIV-oxo complex binding scandium ions ([(Bn-TPEN)MnIV(O)]2+-(Sc(OTf)3)2) in a solvent mixt. of trifluoroethanol and acetonitrile (vol./vol. = 1:1) resulted in formation of the long-lived photoexcited state, which can hydroxylate benzene to phenol. The photohydroxylation of benzene by [(Bn-TPEN)MnIV(O)]2+-(Sc(OTf)3)2 was made possible by electron transfer from benzene to the long-lived 2E excited state of [(Bn-TPEN)MnIV(O)]2+-(Sc(OTf)3)2 to produce a benzene radical cation, which reacted with water as revealed by laser-induced transient absorption measurements.
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  21. 21
    Sankaralingam, M.; Lee, Y.-M.; Pineda-Galvan, Y.; Karmalkar, D. G.; Seo, M. S.; Jeon, S. H.; Pushkar, Y.; Fukuzumi, S.; Nam, W. Redox Reactivity of a Mononuclear Manganese-Oxo Complex Binding Calcium Ion and Other Redox-Inactive Metal Ions. J. Am. Chem. Soc. 2019, 141 (3), 1324– 1336,  DOI: 10.1021/jacs.8b11492
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    21
    Redox Reactivity of a Mononuclear Manganese-Oxo Complex Binding Calcium Ion and Other Redox-Inactive Metal Ions
    Sankaralingam, Muniyandi; Lee, Yong-Min; Pineda-Galvan, Yuliana; Karmalkar, Deepika G.; Seo, Mi Sook; Jeon, So Hyun; Pushkar, Yulia; Fukuzumi, Shunichi; Nam, Wonwoo
    Journal of the American Chemical Society (2019), 141 (3), 1324-1336CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Mononuclear nonheme manganese(IV)-oxo complexes binding calcium ion and other redox-inactive metal ions, [(dpaq)MnIV(O)]+-Mn+ (1-Mn+, Mn+ = Ca2+, Mg2+, Zn2+, Lu3+, Y3+, Al3+, and Sc3+) (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), were synthesized by reacting a hydroxomanganese(III) complex, [(dpaq)MnIII(OH)]+, with iodosylbenzene (PhIO) in the presence of redox-inactive metal ions (Mn+). The Mn(IV)-oxo complexes were characterized using various spectroscopic techniques. In reactivity studies, the authors obsd. contrasting effects of Mn+ on the reactivity of 1-Mn+ in redox reactions such as electron-transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. In the OAT and ET reactions, the reactivity order of 1-Mn+, such as 1-Sc3+ ≈ 1-Al3+ > 1-Y3+ > 1-Lu3+ > 1-Zn2+ > 1-Mg2+ > 1-Ca2+, follows the Lewis acidity of Mn+ bound to the Mn-O moiety; i.e., the stronger the Lewis acidity of Mn+, the higher the reactivity of 1-Mn+ becomes. In sharp contrast, the reactivity of 1-Mn+ in the HAT reaction was reversed, giving the reactivity order 1-Ca2+ > 1-Mg2+ > 1-Zn2+ > 1-Lu3+> 1-Y3+> 1-Al3+ ≈ 1-Sc3+; i.e., the higher is Lewis acidity of Mn+, the lower the reactivity of 1-Mn+ in the HAT reaction. The latter result implies that the Lewis acidity of Mn+ bound to the Mn-O moiety can modulate the basicity of the metal-oxo moiety, thus influencing the HAT reactivity of 1-Mn+; cytochrome P 450 uses the axial thiolate ligand to increase the basicity of the iron-oxo moiety, which enhances the reactivity of compd. I in C-H bond activation reactions.
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  22. 22
    Yoon, H.; Lee, Y.-M.; Wu, X.; Cho, K.-B.; Sarangi, R.; Nam, W.; Fukuzumi, S. Enhanced Electron-Transfer Reactivity of Nonheme Manganese(IV)–Oxo Complexes by Binding Scandium Ions. J. Am. Chem. Soc. 2013, 135 (24), 9186– 9194,  DOI: 10.1021/ja403965h
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    22
    Enhanced Electron-Transfer Reactivity of Nonheme Manganese(IV)-Oxo Complexes by Binding Scandium Ions
    Yoon, Heejung; Lee, Yong-Min; Wu, Xiujuan; Cho, Kyung-Bin; Sarangi, Ritimukta; Nam, Wonwoo; Fukuzumi, Shunichi
    Journal of the American Chemical Society (2013), 135 (24), 9186-9194CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    One and two scandium ions (Sc3+) are bound strongly to nonheme manganese(IV)-oxo complexes, [(N4Py)MnIV(O)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) and [(Bn-TPEN)MnIV(O)]2+ (Bn-TPEN = N-benzyl-N,N',N'-tris(2-pyridylmethyl)-1,2-diaminoethane), to form MnIV(O)-(Sc3+)1 and MnIV(O)-(Sc3+)2 complexes, resp. The binding of Sc3+ ions to the MnIV(O) complexes was examd. by spectroscopic methods as well as by DFT calcns. The one-electron redn. potentials of the MnIV(O) complexes were markedly shifted to a pos. direction by binding of Sc3+ ions. Accordingly, rates of the electron transfer reactions of the MnIV(O) complexes were enhanced as much as 107-fold by binding of two Sc3+ ions. The driving force dependence of electron transfer from various electron donors to the MnIV(O) and MnIV(O)-(Sc3+)2 complexes was examd. and analyzed in light of the Marcus theory of electron transfer to det. the reorganization energies of electron transfer. The smaller reorganization energies and much more pos. redn. potentials of the MnIV(O)-(Sc3+)2 complexes resulted in remarkable enhancement of the electron-transfer reactivity of the MnIV(O) complexes. Such a dramatic enhancement of the electron-transfer reactivity of the MnIV(O) complexes by binding of Sc3+ ions resulted in the change of mechanism in the sulfoxidn. of thioanisoles by MnIV(O) complexes from a direct oxygen atom transfer pathway without metal ion binding to an electron-transfer pathway with binding of Sc3+ ions.
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  23. 23
    Chen, J.; Lee, Y.-M.; Davis, K. M.; Wu, X.; Seo, M. S.; Cho, K.-B.; Yoon, H.; Park, Y. J.; Fukuzumi, S.; Pushkar, Y. N. A Mononuclear Non-Heme Manganese(IV)–Oxo Complex Binding Redox-Inactive Metal Ions. J. Am. Chem. Soc. 2013, 135 (17), 6388– 6391,  DOI: 10.1021/ja312113p
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    23
    A Mononuclear Non-Heme Manganese(IV)-Oxo Complex Binding Redox-Inactive Metal Ions
    Chen, Junying; Lee, Yong-Min; Davis, Katherine M.; Wu, Xiujuan; Seo, Mi Sook; Cho, Kyung-Bin; Yoon, Heejung; Park, Young Jun; Fukuzumi, Shunichi; Pushkar, Yulia N.; Nam, Wonwoo
    Journal of the American Chemical Society (2013), 135 (17), 6388-6391CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Redox-inactive metal ions play pivotal roles in regulating the reactivities of high-valent metal-oxo species in a variety of enzymic and chem. reactions. A mononuclear nonheme Mn(IV)-oxo complex bearing a pentadentate N5 ligand was synthesized and used in the synthesis of a Mn(IV)-oxo complex binding scandium ions. The Mn(IV)-oxo complexes were characterized with various spectroscopic methods. The reactivities of the Mn(IV)-oxo complex are markedly influenced by binding of Sc3+ ions in oxidn. reactions, such as a ∼ 2200-fold increase in the rate of oxidn. of thioanisole (i.e., oxygen atom transfer) but a ∼ 180-fold decrease in the rate of C-H bond activation of 1,4-cyclohexadiene (i.e., hydrogen atom transfer). The present results provide the first example of a nonheme Mn(IV)-oxo complex binding redox-inactive metal ions that shows a contrasting effect of the redox-inactive metal ions on the reactivities of metal-oxo species in the oxygen atom transfer and hydrogen atom transfer reactions.
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  24. 24
    Dong, L.; Wang, Y.; Lv, Y.; Chen, Z.; Mei, F.; Xiong, H.; Yin, G. Lewis-Acid-Promoted Stoichiometric and Catalytic Oxidations by Manganese Complexes Having Cross-Bridged Cyclam Ligand: A Comprehensive Study. Inorg. Chem. 2013, 52 (9), 5418– 5427,  DOI: 10.1021/ic400361s
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    24
    Lewis-Acid-Promoted Stoichiometric and Catalytic Oxidations by Manganese Complexes Having Cross-Bridged Cyclam Ligand: A Comprehensive Study
    Dong, Lei; Wang, Yujuan; Lv, Yanzong; Chen, Zhuqi; Mei, Fuming; Xiong, Hui; Yin, Guochuan
    Inorganic Chemistry (2013), 52 (9), 5418-5427CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Redox-inactive metal ions have been recognized to be able to participate in redox metal-ion-mediated biol. and chem. oxidative events; however, their roles are still elusive. This work presents how the redox-inactive metal ions affect the oxidative reactivity of a well-investigated manganese(II) with its corresponding manganese(IV) complexes having cross-bridged cyclam ligand. In dry acetone, the presence of these metal ions can greatly accelerate stoichiometric oxidns. of triphenylphosphine and sulfides by the manganese(IV) complexes through electron transfer or catalytic sulfoxidns. by the corresponding manganese(II) complexes with PhIO. Significantly, the rate enhancements are highly Lewis-acid strength dependent on added metal ions. These metal ions like Al3+ can also promote the thermodn. driving force of the MnIV-OH moiety to facilitate its hydrogen abstraction from ethylbenzene having a BDECH value of 85 kcal/mol, while it is exptl. limited to 80 kcal/mol for MnIV-OH alone. Adding Al3+ may also improve the manganese(II)-catalyzed olefin epoxidn. with PhIO. However, compared with those in electron transfer, improvements in hydrogen abstraction and electron transfer are minor. The existence of the interaction between Lewis acid and the manganese(IV) species was evidenced by the blue shift of the characteristic absorbance of the manganese(IV) species from 554 to 537 nm and by converting its EPR signal at g = 2.01 into a hyperfine 6-line signal upon adding Al3+ (I = 5/2). Cyclic voltammograms of the manganese(IV) complexes reveal that adding Lewis acid would substantially shift its potential to the pos. direction, thus enhancing its oxidizing capability.
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  25. 25
    Zhang, Z.; Coats, K. L.; Chen, Z.; Hubin, T. J.; Yin, G. Influence of Calcium(II) and Chloride on the Oxidative Reactivity of a Manganese(II) Complex of a Cross-Bridged Cyclen Ligand. Inorg. Chem. 2014, 53 (22), 11937– 11947,  DOI: 10.1021/ic501342c
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    25
    Influence of Calcium(II) and Chloride on the Oxidative Reactivity of a Manganese(II) Complex of a Cross-Bridged Cyclen Ligand
    Zhang, Zhan; Coats, Katherine L.; Chen, Zhuqi; Hubin, Timothy J.; Yin, Guochuan
    Inorganic Chemistry (2014), 53 (22), 11937-11947CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Available data from different labs. confirmed that both Ca2+ and Cl- are crucial for H2O oxidn. in Photosystem II. However, their roles are still elusive. Using a Mn(II) complex having a cross-bridged cyclen ligand as a model, the influence of Ca2+ on the oxidative reactivity of the Mn(II) complex and its corresponding Mn(IV) analog were studied. Adding Ca2+ can significantly improve the oxygenation efficiency of the Mn(II) complex in sulfide oxidn. and further accelerate the oxidn. of sulfoxide to sulfone. Similar improvements also were obsd. for Mg2+, Sr2+, and Ba2+. A new monomeric Mn(IV) complex having two cis-hydroxide ligands also was isolated through oxidn. of the corresponding Mn(II) complex with H2O2 in the presence of NH4PF6. This rare cis-dihydroxomanganese(IV) species was well characterized by x-ray crystallog., electrochem., ESR, and UV-visible spectroscopy. Notably, using the Mn(IV) complex as a catalyst demonstrates higher activity than the corresponding Mn(II) complex, and adding Ca2+ further improves its catalytic efficiency. However, adding Cl- decreases its catalytic activity. In electrochem. studies of Mn(IV) complexes with no chloride ligand present, adding Ca2+ pos. shifted the redox potential of the MnIV/MnIII couple but neg. shifted its MnV/MnIV couple. In the Mn(II) complex having a chloride ligand, adding Ca2+ shifted both the MnIV/MnIII and MnV/MnIV couples in the neg. direction. The revealed oxidative reactivity and redox properties of the Mn species affected by Ca2+ and Cl- may provide new clues to understanding their roles in the H2O oxidn. process of Photosystem II.
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  26. 26
    Choe, C.; Yang, L.; Lv, Z.; Mo, W.; Chen, Z.; Li, G.; Yin, G. Redox-Inactive Metal Ions Promoted the Catalytic Reactivity of Non-Heme Manganese Complexes towards Oxygen Atom Transfer. Dalt. Trans. 2015, 44 (19), 9182– 9192,  DOI: 10.1039/C4DT03993A
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    26
    Redox-inactive metal ions promoted the catalytic reactivity of non-heme manganese complexes towards oxygen atom transfer
    Choe, Cholho; Yang, Ling; Lv, Zhanao; Mo, Wanling; Chen, Zhuqi; Li, Guangxin; Yin, Guochuan
    Dalton Transactions (2015), 44 (19), 9182-9192CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    Redox-inactive metal ions can modulate the reactivity of redox-active metal ions in a variety of biol. and chem. oxidns. Many synthetic models have been developed to help address the elusive roles of these redox-inactive metal ions. Using a non-heme manganese(II) complex as the model, the influence of redox-inactive metal ions as a Lewis acid on its catalytic efficiency in oxygen atom transfer was investigated. In the absence of redox-inactive metal ions, the manganese(II) catalyst is very sluggish, for example, in cyclooctene epoxidn., providing only 9.9% conversion with 4.1% yield of epoxide. However, addn. of 2 equiv. of Al3+ to the manganese(II) catalyst sharply improves the epoxidn., providing up to 97.8% conversion with 91.4% yield of epoxide. EPR studies of the manganese(II) catalyst in the presence of an oxidant reveal a 16-line hyperfine structure centered at g = 2.0, clearly indicating the formation of a mixed valent di-μ-oxo-bridged diamond core, MnIII-(μ-O)2-MnIV. The presence of a Lewis acid like Al3+ causes the dissocn. of this diamond MnIII-(μ-O)2-MnIV core to form monomeric manganese(IV) species which is responsible for improved epoxidn. efficiency. This promotional effect has also been obsd. in other manganese complexes bearing various non-heme ligands. The findings presented here have provided a promising strategy to explore the catalytic reactivity of some di-μ-oxo-bridged complexes by adding non-redox metal ions to in situ dissoc. those dimeric cores and may also provide clues to understand the mechanism of methane monooxygenase which has a similar diiron diamond core as the intermediate.
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    Chen, Z.; Yang, L.; Choe, C.; Lv, Z.; Yin, G. Non-Redox Metal Ion Promoted Oxygen Transfer by a Non-Heme Manganese Catalyst. Chem. Commun. 2015, 51 (10), 1874– 1877,  DOI: 10.1039/C4CC07981G
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    27
    Non-redox metal ion promoted oxygen transfer by a non-heme manganese catalyst
    Chen, Zhuqi; Yang, Ling; Choe, Cholho; Lv, Zhanao; Yin, Guochuan
    Chemical Communications (Cambridge, United Kingdom) (2015), 51 (10), 1874-1877CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    This work demonstrates that non-redox metal ions as Lewis acids can sharply improve the oxygen transfer efficiency of a manganese(II) catalyst having a non-heme ligand. In the absence of Lewis acid, oxidn. of a manganese(II) complex will generate the known di-μ-oxo-bridged dinuclear Mn2(III,IV) core which is very sluggish for olefin epoxidn. Adding non-redox metal ions causes the dissocn. of the dinuclear core, leading to sharp improvement in its oxygen transfer efficiency.
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  28. 28
    Kim, S.; Cho, K.-B.; Lee, Y.-M.; Chen, J.; Fukuzumi, S.; Nam, W. Factors Controlling the Chemoselectivity in the Oxidation of Olefins by Nonheme Manganese(IV)-Oxo Complexes. J. Am. Chem. Soc. 2016, 138 (33), 10654– 10663,  DOI: 10.1021/jacs.6b06252
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    Factors Controlling the Chemoselectivity in the Oxidation of Olefins by Nonheme Manganese(IV)-Oxo Complexes
    Kim, Surin; Cho, Kyung-Bin; Lee, Yong-Min; Chen, Junying; Fukuzumi, Shunichi; Nam, Wonwoo
    Journal of the American Chemical Society (2016), 138 (33), 10654-10663CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We report the oxidn. of cyclic olefins, such as cyclohexene, cyclohexene-d10, and cyclooctene, by mononuclear nonheme manganese(IV)-oxo (MnIVO) and triflic acid (HOTf)-bound MnIVO complexes. In the oxidn. of cyclohexene, the MnIVO complexes prefer the C-H bond activation to the C=C double bond epoxidn., whereas the C=C double bond epoxidn. becomes a preferred reaction pathway in the cyclohexene oxidn. by HOTf-bound MnIVO complexes. In contrast, the oxidn. of cyclohexene-d10 and cyclooctene by the MnIVO complexes occurs predominantly via the C=C double bond epoxidn. This conclusion is drawn from the product anal. and kinetic studies of the olefin oxidn. reactions, such as the epoxide vs. allylic oxidn. products, the formation of Mn(II) vs. Mn(III) products, and the kinetic analyses. Overall, the exptl. results suggest that the energy barrier of the C=C double bond epoxidn. is very close to that of the allylic C-H bond activation in the oxidn. of cyclic olefins by high-valent metal-oxo complexes. Thus, the preference of the reaction pathways is subject to changes upon small manipulation of the reaction environments, such as the supporting ligands and metal ions in metal-oxo species, the presence of HOTf (i.e., HOTf-bound MnIVO species), and the allylic C-H(D) bond dissocn. energies of olefins. This is confirmed by DFT calcns. in the oxidn. of cyclohexene and cyclooctene, which show multiple pathways with similar rate-limiting energy barriers and depending on the allylic C-H bond dissocn. energies. In addn., the possibility of excited state reactivity in the current system is confirmed for epoxidn. reactions.
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  29. 29
    Hong, S.; Lee, Y.-M.; Sankaralingam, M.; Vardhaman, A. K.; Park, Y. J.; Cho, K.-B.; Ogura, T.; Sarangi, R.; Fukuzumi, S.; Nam, W. A Manganese(V)–Oxo Complex: Synthesis by Dioxygen Activation and Enhancement of Its Oxidizing Power by Binding Scandium Ion. J. Am. Chem. Soc. 2016, 138 (27), 8523– 8532,  DOI: 10.1021/jacs.6b03874
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    29
    A Manganese(V)-Oxo Complex: Synthesis by Dioxygen Activation and Enhancement of Its Oxidizing Power by Binding Scandium Ion
    Hong, Seungwoo; Lee, Yong-Min; Sankaralingam, Muniyandi; Vardhaman, Anil Kumar; Park, Young Jun; Cho, Kyung-Bin; Ogura, Takashi; Sarangi, Ritimukta; Fukuzumi, Shunichi; Nam, Wonwoo
    Journal of the American Chemical Society (2016), 138 (27), 8523-8532CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A mononuclear nonheme Mn(V)-oxo complex, [MnV(O)(TAML)]- (1, H4TAML = tetraamino macrocyclic ligand 3,4,8,9-tetrahydro-3,3,6,6,9,9-hexamethyl-1H-1,4,8,11-benzotetraazacyclotridecane-2,5,7,10(6H,11H)tetrone), was synthesized by activating dioxygen in the presence of olefins with weak allylic C-H bonds and characterized structurally and spectroscopically. In mechanistic studies, the formation rate of 1 depends on the allylic C-H bond dissocn. energies (BDEs) of olefins, and a kinetic isotope effect (KIE) value of 16 was obtained in the reactions of cyclohexene and cyclohexene-d10. Probably a H atom abstraction from the allylic C-H bonds of olefins by a putative MnIV-superoxo species, which is formed by binding O2 by a high-spin (S = 2) [MnIII(TAML)]- complex, is the rate-detg. step. A Mn(V)-oxo complex binding Sc3+ ion, [MnV(O)(TAML)]--(Sc3+) (2), was also synthesized in the reaction of 1 with Sc3+ ion and then characterized using various spectroscopic techniques. The binding site of the Sc3+ ion probably is the TAML ligand, not the Mn-O moiety, probably due to the low basicity of the oxo group compared to the basicity of the amide carbonyl group in the TAML ligand. Reactivity studies of the Mn(V)-oxo intermediates, 1 and 2, in O atom transfer and electron-transfer reactions revealed that the binding of Sc3+ ion at the TAML ligand of Mn(V)-oxo enhanced its oxidizing power with a pos. shifted 1-electron redn. potential (ΔEred = 0.70 V). This study reports the first example of tuning the second coordination sphere of high-valent metal-oxo species by binding a redox-inactive metal ion at the supporting ligand site, thereby modulating their electron-transfer properties as well as their reactivities in oxidn. reactions.
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    Nodzewska, A.; Watkinson, M. Remarkable Increase in the Rate of the Catalytic Epoxidation of Electron Deficient Styrenes through the Addition of Sc(OTf)3 to the MnTMTACN Catalyst. Chem. Commun. 2018, 54 (12), 1461– 1464,  DOI: 10.1039/C7CC09698D
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    Remarkable increase in the rate of the catalytic epoxidation of electron deficient styrenes through the addition of Sc(OTf)3 to the MnTMTACN catalyst
    Nodzewska, Aneta; Watkinson, Michael
    Chemical Communications (Cambridge, United Kingdom) (2018), 54 (12), 1461-1464CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    The effect of Lewis acids on the catalytic activity of [Mn2(μ-O)3(TMTACN)2](PF6)2 in the epoxidn. of styrenes using hydrogen peroxide as the oxidant has shown that the addn. of Sc(OTf)3 at low catalytic loading results in a very significant increase in the efficiency of the catalyst and a redn. of the reaction time to only 3 min in most cases.
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  31. 31
    Lv, Z.; Choe, C.; Wu, Y.; Wang, H.; Chen, Z.; Li, G.; Yin, G. Non-Redox Metal Ions Accelerated Oxygen Atom Transfer by Mn-Me3tacn Complex with H2O2 as Oxygen Resource. Mol. Catal. 2018, 448, 46– 52,  DOI: 10.1016/j.mcat.2018.01.022
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    31
    Non-redox metal ions accelerated oxygen atom transfer by Mn-Me3tacn complex with H2O2 as oxygen resource
    Lv, Zhanao; Choe, Cholho; Wu, Yunfeng; Wang, Haibin; Chen, Zhuqi; Li, Guangxing; Yin, Guochuan
    Molecular Catalysis (2018), 448 (), 46-52CODEN: MCOADH ISSN:. (Elsevier B.V.)
    This work demonstrates a novel strategy that the introduction of non-redox metal ions as Lewis acids to the classic dinuclear manganese complex [MnIV2(μ-O)3(Me3tacn)2](PF6)2 can greatly promote the alkene epoxidn. efficiency under mild conditions with H2O2 as the solely terminal oxidant because of its economic and environmental advantages. When [MnIV2(μ-O)3(Me3tacn)2](PF6)2 was used as the catalyst in the absence of Lewis acids, only 16.4% conversion of cyclooctene with 6.2% yield of epoxide was obtained and the obvious decompn. of H2O2 was obsd. However, the oxygen transfer efficiency of the catalyst was sharply improved with 100% conversion and 90.2% yield of epoxide under identical conditions when the non-redox metal ion, such as Sc3+, was introduced to the catalytic system. The novel strategy was successfully applied to the epoxidn. reactions of different types of alkenes. Through UV-vis, FT-IR, EPR and CV characterizations, it was evidenced that the non-redox metal ions with high pos. charge as Lewis acids could dissoc. the sluggish dinuclear Mn-(μ-O)3-Mn core and the open-loop dinuclear manganese complex, HO-MnIII-(μ-O)-MnIV = O or O = MnIV-(μ-O)-MnIV = O, was proposed as the active species, which was capable of the alkene epoxidn. process. This work illustrated an alternative protocol to manipulate the reactivity of those sluggish catalysts by the introduction of non-redox metal ions and provided clues to understand the role of non-redox metal ions in metalloenzymes and heterogeneous catalysts.
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  32. 32
    Fukuzumi, S.; Morimoto, Y.; Kotani, H.; Naumov, P.; Lee, Y.-M.; Nam, W. Crystal Structure of a Metal Ion-Bound Oxoiron(IV) Complex and Implications for Biological Electron Transfer. Nat. Chem. 2010, 2 (9), 756– 759,  DOI: 10.1038/nchem.731
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    Crystal structure of a metal ion-bound oxoiron(IV) complex and implications for biological electron transfer
    Fukuzumi, Shunichi; Morimoto, Yuma; Kotani, Hiroaki; Naumov, Pance; Lee, Yong-Min; Nam, Wonwoo
    Nature Chemistry (2010), 2 (9), 756-759CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
    Crit. biol. electron-transfer processes involving high-valent oxometal chem. occur widely, for example in haem proteins [oxoiron(IV); FeIV(O)] and in photosystem II. Photosystem II involves Ca2+ as well as high-valent oxomanganese cluster species. However, there is no example of an interaction between metal ions and oxoiron(IV) complexes. Here, the authors report new findings concerning the binding of the redox-inactive metal ions Ca2+ and Sc3+ to a nonhaem oxoiron(IV) complex, [(TMC)FeIV(O)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). As detd. by x-ray diffraction anal., an oxo-Sc3+ interaction leads to a structural distortion of the oxoiron(IV) moiety. More importantly, this interaction facilitates a two-electron redn. by ferrocene, whereas only a 1-electron redn. process occurs without the metal ions. This control of redox behavior provides valuable mechanistic insights into oxometal redox chem., and suggests a possible key role that an auxiliary Lewis acid metal ion could play in nature, as in photosystem II.
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    Swart, M. A Change in the Oxidation State of Iron: Scandium Is Not Innocent. Chem. Commun. 2013, 49 (59), 6650– 6652,  DOI: 10.1039/c3cc42200c
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    A change in the oxidation state of iron: scandium is not innocent
    Swart, Marcel
    Chemical Communications (Cambridge, United Kingdom) (2013), 49 (59), 6650-6652CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Through extensive mol. modeling of FeIII/IV-oxo and FeIII-hydroxo complexes it is shown here unambiguously that the assignment of the iron oxidn. state of a Sc3+-capped iron-oxygen species should be revised. Iron in this Lewis-acid capped metal-bound oxygen system is FeIII, coinciding with water as a secondary axial ligand to scandium.
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  34. 34
    Prakash, J.; Rohde, G. T.; Meier, K. K.; Jasniewski, A. J.; Van Heuvelen, K. M.; Münck, E.; Que, L. Spectroscopic Identification of an FeIII Center, Not FeIV, in the Crystalline Sc–O–Fe Adduct Derived from [FeIV(O)(TMC)]2+. J. Am. Chem. Soc. 2015, 137 (10), 3478– 3481,  DOI: 10.1021/jacs.5b00535
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    Spectroscopic Identification of an FeIII Center, not FeIV, in the Crystalline Sc-O-Fe Adduct Derived from [FeIV(O)(TMC)]2+
    Prakash, Jai; Rohde, Gregory T.; Meier, Katlyn K.; Jasniewski, Andrew J.; Van Heuvelen, Katherine M.; Munck, Eckard; Que, Lawrence, Jr.
    Journal of the American Chemical Society (2015), 137 (10), 3478-3481CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The apparent Sc3+ adduct of [FeIV(O)(TMC)]2+ (1, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) was synthesized in amts. sufficient to allow its characterization by various spectroscopic techniques. Contrary to the earlier assignment of a +4 oxidn. state for the iron center of 1, 1 has a high-spin iron(III) center based on its Mossbauer and EPR spectra and its quant. redn. by 1 equiv of ferrocene to [FeII(TMC)]2+. Thus, 1 is best described as a ScIII-O-FeIII complex, in agreement with previous DFT calcns. (Swart, M. Chem. Commun. 2013, 49, 6650.). These results shed light on the interaction of Lewis acids with high-valent metal-oxo species.
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  35. 35
    de Boer, J. W.; Browne, W. R.; Brinksma, J.; Alsters, P. L.; Hage, R.; Feringa, B. L. Mechanism of Cis-Dihydroxylation and Epoxidation of Alkenes by Highly H2O2 Efficient Dinuclear Manganese Catalysts. Inorg. Chem. 2007, 46 (16), 6353– 6372,  DOI: 10.1021/ic7003613
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    Mechanism of Cis-Dihydroxylation and Epoxidation of Alkenes by Highly H2O2 Efficient Dinuclear Manganese Catalysts
    de Boer, Johannes W.; Browne, Wesley R.; Brinksma, Jelle; Alsters, Paul L.; Hage, Ronald; Feringa, Ben L.
    Inorganic Chemistry (2007), 46 (16), 6353-6372CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    In the presence of carboxylic acids [MnIV2(μ-O)3(tmtacn)2]2+ (1, where tmtacn = N,N',N''-trimethyl-1,4,7-triazacyclononane) is highly efficient in catalyzing the oxidn. of alkenes to the corresponding cis-diol and epoxide with H2O2 as terminal oxidant. The selectivity of the catalytic system with respect to (w.r.t.) either cis-dihydroxylation or epoxidn. of alkenes is dependent on the carboxylic acid employed. High turnover nos. (t.o.n. > 2000) can be achieved esp. w.r.t. Cis-dihydroxylation for which the use of 2,6-dichlorobenzoic acid allows for the highest t.o.n. Reported thus far for cis-dihydroxylation of alkenes catalyzed by a first-row transition metal and high efficiency w.r.t. The terminal oxidant (H2O2). The high activity and selectivity is due to the in situ formation of bis(μ-carboxylato)-bridged dinuclear manganese(III) complexes. Tuning of the activity of the catalyst by variation in the carboxylate ligands is dependent on both the electron-withdrawing nature of the ligand and on steric effects. By contrast, the cis-diol/epoxide selectivity is dominated by steric factors. The role of solvent, catalyst oxidn. state, H2O, and carboxylic acid concn. and the nature of the carboxylic acid employed on both the activity and the selectivity of the catalysis are explored together with speciation anal. and isotope labeling studies. [Mn2(μ-O)(μ-R-CO2)2(tmtacn)2]2+, which show remarkable redox and solvent-dependent coordination chem., are the resting state of the catalytic system and they retain a dinuclear structure throughout the catalytic cycle. The mechanistic understanding obtained from these studies holds considerable implications for both homogeneous manganese oxidn. catalysis and in understanding related biol. systems such as dinuclear catalase and arginase enzymes.
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    Krieble, V. K.; Noll, C. I. The Hydrolysis of Nitriles with Acids. J. Am. Chem. Soc. 1939, 61 (3), 560– 563,  DOI: 10.1021/ja01872a005
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    Hydrolysis of nitriles with acids
    Krieble, Vernon K.; Noll, Clarence I.
    Journal of the American Chemical Society (1939), 61 (), 560-3CODEN: JACSAT; ISSN:0002-7863.
    The rates of hydrolysis of aceto-, propio-, and α- and β-hydroxypropio-nitriles, and CNCH2CO2H were studied. In presence of HCl the rate increases approx. as the square of the mean ion activity increases. H2SO4 is a less efficient catalyst than HCl. No relation between the acidity of the solns. and velocity of hydrolysis was observed.
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    Lei, X. R.; Gong, C.; Zhang, Y. L.; Xu, X. Influence of the Acetamide from Acetonitrile Hydrolysis in Acid-Contained Mobile Phase on the Ultraviolet Detection in High Performance Liquid Chromatography. Chromatographia 2016, 79 (19–20), 1257– 1262,  DOI: 10.1007/s10337-016-3145-6
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    Influence of the Acetamide from Acetonitrile Hydrolysis in Acid-Contained Mobile Phase on the Ultraviolet Detection in High Performance Liquid Chromatography
    Lei, Xiao-Rui; Gong, Can; Zhang, Yao-Li; Xu, Xu
    Chromatographia (2016), 79 (19-20), 1257-1262CODEN: CHRGB7; ISSN:0009-5893. (Springer)
    The paper presents the influence of acetamide on UV detection after generation of acetonitrile by hydrolysis in the presence of trifluoroacetic acid in the HPLC mobile phase. The acetonitrile, with added varying contents of trifluoroacetic acid and water, was detd. by GC-MS at various times. The concn. of acetamide increased approx. linearly with time. Using a mixed std. sample soln. and a model mobile phase of acetonitrile-water-trifluoroacetic acid (25:75:0.3, vol./vol.) after 0, 24 and 48 h, the influence of the hydrolyzate acetamide on HPLC detection was studied at the wavelength of 205-220 nm. The limit of detection (LOD) of std. sample lost ∼30% after the mobile phase was placed 48 h. It is suggested that the acid-contg. mobile phase was placed ≤24 h for HPLC trace anal. at the wavelength of 205-220 nm.
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    Kobayashi, S.; Nagayama, S.; Busujima, T. Lewis Acid Catalysts Stable in Water. Correlation between Catalytic Activity in Water and Hydrolysis Constants and Exchange Rate Constants for Substitution of Inner-Sphere Water Ligands. J. Am. Chem. Soc. 1998, 120 (32), 8287– 8288,  DOI: 10.1021/ja980715q
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    38
    Lewis Acid Catalysts Stable in Water. Correlation between Catalytic Activity in Water and Hydrolysis Constants and Exchange Rate Constants for Substitution of Inner-Sphere Water Ligands
    Kobayashi, Shu; Nagayama, Satoshi; Busujima, Tsuyoshi
    Journal of the American Chemical Society (1998), 120 (32), 8287-8288CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The catalytic activity of water-stable Lewis acids of group 1-15 metal chlorides was studied in a model reaction of benzaldehyde with (Z)-1-phenyl-1-(trimethylsiloxy)propene (the Mukaiyama aldol reaction) in aq. media. The chloride salts of Fe(II), Cu(II), Zn(II), Cd(II), In(III), and Pb(II) as well as the rare earths (Sc(III), Y(III), Ln(III)) gave promising yields. When the chloride salts of B(III), Si(IV), P(III), P(IV), Ti(IV), V(III), Ge(IV), Zr(IV), Nb(V), Mo(V), Sn(IV), Sb(V), Hf(IV), Ta(V), W(VI), Re(V), and Tl(III) were used, decompn. of the silyl enol ether occurred rapidly and no aldol adduct was obtained. Some of these salts are stable in water, but have low catalytic ability. The same aldol reaction was carried out with the corresponding metal perchlorates or trifluoromethanesulfonates (triflates); Lewis acids based on Fe(II), Cu(II), Zn(II), Cd(II), and Pb(II) as well as the rare earths (Sc(III), Y(III), Ln(III)) were both stable and active in water. The mechanism of Lewis acid catalysis in water is assumed to be as follows: when metal compds. are added to water, dissocn. and hydration occur immediately; intramol. and intermol. exchange reactions of water mols. frequently occur. If an aldehyde exists in the system, there is a chance for it to coordinate to metal cations instead of water mols. and the aldehyde is then activated; a silyl enol ether attacks this activated aldehyde to produce the aldol adduct.
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    Wieghardt, K.; Bossek, U.; Nuber, B.; Weiss, J.; Bonvoisin, J.; Corbella, M.; Vitols, S. E.; Girerd, J. J. Synthesis, Crystal Structures, Reactivity, and Magnetochemistry of a Series of Binuclear Complexes of Manganese(II), -(III), and -(IV) of Biological Relevance. The Crystal Structure of [L′MnIV(μ-O)3MnIVL′](PF6)2.H2O Containing an Unprecedented Short Mn. J. Am. Chem. Soc. 1988, 110 (22), 7398– 7411,  DOI: 10.1021/ja00230a021
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    Synthesis, crystal structures, reactivity, and magnetochemistry of a series of binuclear complexes of manganese(II), -(III), and -(IV) of biological relevance. The crystal structure of [L'MnIV(μ-O)3MnIVL'](PF6)2.H2O containing an unprecedented short Mn···Mn distance of 2.296 Å
    Wieghardt, Karl; Bossek, Ursula; Nuber, Bernhard; Weiss, Johannes; Bonvoisin, J.; Corbella, M.; Vitols, S. E.; Girerd, J. J.
    Journal of the American Chemical Society (1988), 110 (22), 7398-411CODEN: JACSAT; ISSN:0002-7863.
    The disproportionation reactions of Mn2O(OAc)2Q2 [I; Q = 1,4,7-triazacyclononane (L); N,N',N''-trimethyl-1,4,7-triazacyclononane (L1)], in which Q are capping ligands, in aq. soln. under anaerobic conditions lead to a variety of novel binuclear MnIIIMnIV and MnIV2 dimers. [L2MnIIIMnIV(μ-O)2(μ-OAc)][BPh4]2.CH3CN, [L12MnIIIMnIV(μ-O)(μ-OAc)2](ClO4)3, [L2MnIV2(OH)(μ-O)2][MnII3(C2O4)4(OH2)2].6H2O (II), and [L12MnIV2(μ-O)3](PF6)2.H2O (III) were formed. [L4MnIV4O6]Br4.5.5H2O (IV) is generated as a thermodynamically very stable product from a MnII contg. aq. soln. of L in the presence of O. In the absence of O MeOH solns. of Mn(ClO4)2.2H2O or Mn(OAc)2 react with L1 to form [L12MnII2(μ-OH)(μ-OAc)2](ClO4) and [L12MnII2(μ-OAc)3][BPh4] (V). The oxo- and acetato-bridges in I are labile; addn. of anions X- (X = Cl, Br, NCS, N3) to MeCN solns. of I yields the monomers LMnX3 and L1MnX3. The electrochem. of all compds. was studied; e.g., I (Q = L1) is reversibly oxidized by 2 1-electron processes to generate MnIIIMnIV and MnIV2 dimers in liq. SO2. The crystal structures of II, III, IV and V were detd. by x-ray crystallog. V is orthorhombic Pcab; II is monoclinic C2/c, IV is monoclinic P21/c, and III is orthorhombic Pnma. VII consists of the cofacial bioctahedral cation [L1MnIV(μ-O)3MnIVL1]2+ and PF6- anions. The Mn...Mn distance is unusually short (2.296(2) Å). Bulk magnetic properties of all compds. were studied at 100-298 K, and in some instances 4-298 K. In I (Q = L1) the Mn(III) ions are ferromagnetically coupled, J = +18 (1) cm-1; whereas the MnII centers in V are weakly antiferromagnetically coupled, J = -3.5(2) cm-1. Very strong intramol. antiferromagnetic coupling is obsd. in III (J = -780 cm-1).
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    Hage, R.; Krijnen, B.; Warnaar, J. B.; Hartl, F.; Stufkens, D. J.; Snoeck, T. L. Proton-Coupled Electron-Transfer Reactions in [MnIV2(μ-O)3L′2]2+ (L′ = 1,4,7-Trimethyl-1,4,7-Triazacyclononane). Inorg. Chem. 1995, 34 (20), 4973– 4978,  DOI: 10.1021/ic00124a010
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    Proton-Coupled Electron-Transfer Reactions in [MnIV2(μ-O)3L'2]2+ (L' = 1,4,7-Trimethyl- 1,4,7-triazacyclononane)
    Hage, Ronald; Krijnen, Bert; Warnaar, Johann B.; Hartl, Frantisek; Stufkens, Derk J.; Snoeck, Theo L.
    Inorganic Chemistry (1995), 34 (20), 4973-8CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The pKa value of [MnIV2(μ-O)3L'2]2+ (L' = 1,4,7-trimethyl-1,4,7-triazacyclononane) was detd. spectrophotometrically by carrying out titrn. expts. with concd. sulfuric acid. The extremely low pKa value of -2.0 suggests that the electron d. on the bridging oxygen atoms is very small. The asym. Mn-O-Mn vibration is obsd. at 670 cm-1, while the sym. Mn-O-Mn vibration is present at 702 cm-1. The unusually high frequencies of these vibrations are due to the small Mn-O-Mn angle of 78°. Protonation of an oxygen bridge shifts both the asym. and sym. vibrations to 683 cm-1. Electrochem. expts. in acetonitrile showed that 1-electron redn. of the complex is chem. irreversible. IR, EPR, and UV-visible studies of the reduced species suggest a MnIIIMnIV(μ-O)2(μ-OH) core. PH-dependent differential pulse voltammetry expts. in aq. solns. revealed an apparent pKa value of ∼4.0 for the reduced mixed-valence species in various buffer systems. The redn. wave at pH > 4 is obsd. at ∼-0.10 V vs. SCE. Cyclic voltammetry revealed that the reduced species is prone to reaction with carboxylate groups. A bis(carboxylate)mono-oxo-bridged Mn(III)-Mn(III) species is formed in citric acid buffer which exhibits an anodic peak around +0.6 V vs. SCE, and a UV-visible spectrum that is typical of such a species.
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    Angelone, D.; Abdolahzadeh, S.; de Boer, J. W.; Browne, W. R. Mechanistic Links in the In-Situ Formation of Dinuclear Manganese Catalysts, H2O2 Disproportionation, and Alkene Oxidation. Eur. J. Inorg. Chem. 2015, 2015 (21), 3532– 3542,  DOI: 10.1002/ejic.201500195
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    Mechanistic Links in the in-situ Formation of Dinuclear Manganese Catalysts, H2O2 Disproportionation, and Alkene Oxidation
    Angelone, Davide; Abdolahzadeh, Shaghayegh; de Boer, Johannes W.; Browne, Wesley R.
    European Journal of Inorganic Chemistry (2015), 2015 (21), 3532-3542CODEN: EJICFO; ISSN:1434-1948. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The oxidn. of substrates, such as alkenes, with H2O2 and the catalyst [MnIV2(μ-O)3(tmtacn)2]2+ (1; tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) is promoted by the addn. of carboxylic acids through the in situ formation of bis(carboxylato) complexes of the type [MnIII2(μ-O)(μ-RCO2)2(tmtacn)2]2+. The conversion of 1 to these complexes requires a complex series of redox reactions coupled with the overall exchange of μ-oxido ligands for μ-carboxylato ligands. Here, we show that the mechanism by which this conversion occurs holds implications with regard to the species that is directly engaged in the catalytic oxidn. of alkenes. Through a combination of UV/Vis absorption, Raman, resonance Raman and ESR spectroscopy, it is shown that the conversion proceeds by an autocatalytic mechanism and that the species that engages in the oxidn. of org. substrates also catalyzes H2O2 decompn., and the former process is faster.
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    Chin Quee-Smith, V.; DelPizzo, L.; Jureller, S. H.; Kerschner, J. L.; Hage, R. Synthesis, Structure, and Characterization of a Novel Manganese(IV) Monomer, [MnIV(Me3TACN)(OMe)3](PF6) (Me3TACN = N,N ′,N ″-Trimethyl-1,4,7-Triazacyclononane), and Its Activity toward Olefin Oxidation with Hydrogen Peroxide. Inorg. Chem. 1996, 35 (22), 6461– 6465,  DOI: 10.1021/ic951522w
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    Synthesis, Structure, and Characterization of a Novel Manganese(IV) Monomer, [Mn(IV)(Me(3)TACN)(OMe)(3)](PF(6)) (Me(3)TACN = N,N',N"-Trimethyl-1,4,7-triazacyclononane), and Its Activity toward Olefin Oxidation with Hydrogen Peroxide
    Chin Quee-Smith Vikki; DelPizzo Lisa; Jureller Sharon H.; Kerschner Judith L.; Hage Ronald
    Inorganic chemistry (1996), 35 (22), 6461-6465 ISSN:.
    A novel manganese(IV) monomer, [Mn(IV)(Me(3)TACN)(OMe)(3)](PF(6)), has been synthesized in methanol by the reaction of MnCl(2) with the ligand, N,N',N"-trimethyl-1,4,7-triazacyclononane (Me(3)TACN), in the presence of Na(2)O(2). The resulting product was isolated as the red/brown crystalline hexafluorophosphate salt. The compound crystallizes in the space group P2/c with the cell dimensions a = 15.652(2) ÅA, b = 8.740(1) ÅA, c = 15.208(2) ÅA, beta = 108.81(1) degrees, V = 1969.4(4) ÅA(3), and Z = 4. The structure was solved by the heavy-atom method and was refined by full-matrix least-squares techniques to a final value of R = 0.067 (R(w) = 0.097) based upon 3087 observations. The manganese atom in the molecule is six-coordinate in an N(3)O(3) ligand environment with the triazacyclononane facially coordinated. Pertinent average bond distances and angles are as follows: Mn-O, 1.797(5) ÅA; Mn-N, 2.116(5) ÅA; O-Mn-O, 97.8(2) degrees; N-Mn-N, 81.4(2) degrees; O-Mn-N, 167.8 degrees (2); O-Mn-N, 86.8(2) degrees; O-Mn-N, 92.8(2) degrees. The complex was further characterized by UV-vis and EPR spectroscopies, solution magnetic susceptibility measurements, FAB-MS, and electrochemistry. [Mn(IV)(Me(3)TACN)(OMe)(3)](PF(6)) was found to catalyze the oxidation of water-soluble olefins using hydrogen peroxide as the oxidant in an aqueous medium. The catalyzed rates of oxidation of these olefins indicate at least a 12-fold rate enhancement over oxidant alone. The unusual stability of the catalytic species was demonstrated by the repeated additions of substrate and oxidant while maintaining a constant catalytic rate of oxidation.
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    Journal of the American Chemical Society (2017), 139 (25), 8718-8724CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A reactive high-valent dinuclear Ni(IV) oxido-bridged complex is reported that can be formed at room temp. by reaction of [(L)2Ni(II)2(μ-X)3]X (X = Cl or Br) with NaOCl in MeOH or MeCN (L = 1,4,7-trimethyl-1,4,7-triazacyclononane). The unusual Ni(IV) oxido species is stabilized within a dinuclear tris-μ-oxido-bridged structure as [(L)2Ni(IV)2(μ-O)3]2+. Its structure and its reactivity with org. substrates are demonstrated through a combination of UV-visible absorption, resonance Raman, 1H NMR, EPR, and x-ray absorption (near-edge) spectroscopy, ESI mass spectrometry, and DFT methods. The identification of a Ni(IV)-O species opens opportunities to control the reactivity of NaOCl for selective oxidns.
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    Miao, C.; Wang, B.; Wang, Y.; Xia, C.; Lee, Y.-M. M.; Nam, W.; Sun, W. Proton-Promoted and Anion-Enhanced Epoxidation of Olefins by Hydrogen Peroxide in the Presence of Nonheme Manganese Catalysts. J. Am. Chem. Soc. 2016, 138 (3), 936– 943,  DOI: 10.1021/jacs.5b11579
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    Journal of the American Chemical Society (2016), 138 (3), 936-943CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    In the presence of nonracemic nonheme manganese complex I and H2SO4, alkenes, particularly trans-chalcones such as (E)-PhCOCH:CHPh and chromenes, underwent chemo-, diastereo-, and enantioselective epoxidn. with H2O2 or other oxygen sources to yield nonracemic epoxides such as II. The yields of epoxides as well as the chemo- and enantioselectivities increase dramatically in the presence of H2SO4; no formation of epoxides is obsd. in the absence of H2SO4. Epoxide yields and enantioselectivities depended strongly on the manganese catalysts and Bronsted acids used. The catalytic epoxidn. of olefins by other oxidants, such as peracids, alkyl hydroperoxides, and iodosylbenzene, was also affected by the presence of H2SO4; product yields and enantioselectivities are high and similar irresp. of the oxidants in the presence of H2SO4, suggesting that a common epoxidizing intermediate is generated in the reactions of I. Mechanistic studies, performed with 18O-labeled water (H218O) and cumyl hydroperoxide, reveal that a high-valent manganese-oxo species is formed as an epoxidizing intermediate via O-O bond heterolysis of manganese peroxide species. The role of H2SO4 is proposed to facilitate the formation of a high-valent Mn-oxo species and to increase the oxidizing power and enantioselectivity of the Mn-oxo oxidant in olefin epoxidn. reactions. D. functional theory (DFT) calcns. support exptl. results such as the formation of a Mn(V)-oxo species as an epoxidizing intermediate. The structure of I was detd. by X-ray crystallog.
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    Journal of the American Chemical Society (2005), 127 (22), 7990-7991CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The carboxylic acid promoted cis-dihydroxylation and epoxidn. of alkenes catalyzed by [MnIV2O3(tmtacn)2]2+ 1 employing H2O2 as oxidant is described. The use of carboxylic acids at cocatalytic levels not only is effective in suppressing the inherent catalase activity of 1, but also enables the tuning of the catalyst's selectivity. Spectroscopic studies and X-ray anal. confirm that the control arises from the in situ formation of carboxylate-bridged dinuclear complexes, for example, 2 {[MnIII2O(CCl3CO2)2(tmtacn)2]2+} and 3 {[MnII2(OH)(CCl3CO2)2(tmtacn)2]+}, during catalysis. For the first time, the possibility to tune, through the carboxylate ligands employed, both the selectivity and activity of dinuclear Mn-based catalysts is demonstrated. To our knowledge, the system 1/2,6-dichlorobenzoic acid (up to 2000 turnover nos. for cis-cyclooctanediol) is the most active Os-free cis-dihydroxylation catalyst reported to date.
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    Are Bronsted Acids the True Promoter of Metal-Triflate-Catalyzed Glycosylations? A Mechanistic Probe into 1,2-cis-Aminoglycoside Formation by Nickel Triflate
    Sletten, Eric T.; Tu, Yi-Jung; Schlegel, H. Bernhard; Nguyen, Hien M.
    ACS Catalysis (2019), 9 (3), 2110-2123CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Metal triflates have been utilized to catalytically facilitate numerous glycosylation reactions under mild conditions. In some methods, the metal triflate system provides stereocontrol during the glycosylation, rather than the nature of protecting groups on the substrate. Despite these advances, the true activating nature of metal triflates remains unclear. Our findings indicated that the in situ generation of trace amts. of triflic acid from metal triflates can be the active catalyst species in the glycosylation. This fact has been mentioned previously in metal-triflate-catalyzed glycosylation reactions; however, a thorough study on the subject and its implications on stereoselectivity has yet to be performed. Exptl. evidence from control reactions and 19F NMR spectroscopy have been obtained to confirm and quantify the triflic acid released from nickel triflate, for which it is of paramount importance in achieving a stereoselective 1,2-cis-2-amino glycosidic bond formation via a transient anomeric triflate. A putative intermediate resembling that of a glycosyl triflate has been detected using variable temp. NMR (1H and 13C) expts. These observations, together with d. functional theory calcns. and a kinetic study, corroborate a mechanism involving triflic-acid-catalyzed stereoselective glycosylation with N-substituted trifluoromethylbenzylideneamino-protected electrophiles. Specifically, triflic acid facilitates formation of a glycosyl triflate intermediate which then undergoes isomerization from the stable α-anomer to the more reactive β-anomer. Subsequent SN2-like displacement of the reactive anomer by a nucleophile is highly favorable for the prodn. of 1,2-cis-2-aminoglycosides. Although there is a previously reported work regarding glycosyl triflates, none of these reports have been confirmed to come from the counterion of the metal center. Our work provides supporting evidence for the induction of a glycosyl triflate through the role of triflic acid in metal-triflate-catalyzed glycosylation reactions.
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Cite this: Inorg. Chem. 2019, 58, 21, 14924–14930
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https://doi.org/10.1021/acs.inorgchem.9b02737
Published October 18, 2019

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  • Abstract

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    Scheme 1

    Scheme 1. Oxidation of Alkenes with H2O2 Catalyzed by [Mn2(μ-O)3(tmtacn)2]2+ (1) and Proposed Roles of Lewis Acids
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    Scheme 2

    Scheme 2. Key Stages in the Oxidation of Styrene (Blue) Catalyzed by 1 (1 mM) with Sc(OTf)3 (2 mM) Using H2O2 (Magenta) As Oxidanta
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    aDifferent standing times were used: 20 s (empty); 60 min (filled).

    Figure 1

    Figure 1. (a) UV/vis absorption spectroscopy of 1 in CH3CN before (black), after the addition of Sc(OTf)3 (2 equiv; green) or TfOH (6 eq.; blue), and after subsequent addition of excess H2O (292 equiv; magenta), inset: absorbance at 500 nm over time after the addition of Sc(OTf)3 (2 equiv; green) or TfOH (6 equiv; blue). (b) Raman spectra (λexc 355 nm) of 1 in CH3CN before (black), after the addition of Sc(OTf)3 (2 equiv; green) or TfOH (6 equiv; blue), and after subsequent addition of excess H2O (6200 equiv; magenta). See Figures S4 and S6 for measured and calculated shifts.

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    Figure 2

    Figure 2. Comparison of the cyclic voltammetry of 1 (black) with that obtained after the addition of either Sc(OTf)3 (3 equiv; green) or TfOH (9 equiv; blue) in 0.1 M TBAPF6 in CH3CN. Current axis offset for clarity. Initial potential and scan direction indicated by ∗ and black arrows, respectively.

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    Figure 3

    Figure 3. Comparison of kinetics of styrene conversion (blue) and H2O2 consumption (magenta) by 1 activated by either TfOH (6 equiv; filled) or Sc(OTf)3 (2 equiv; empty) for a 1 h standing time. In the absence of any acid, only disproportionation of H2O2 to O2 is observed (Figure S1).

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  • References

    This article references 53 other publications.

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      Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9 Å
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      Nature (London, United Kingdom) (2011), 473 (7345), 55-60CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Photosystem II is the site of photosynthetic water oxidn. and contains 20 subunits with a total mol. mass of 350 kDa. The structure of photosystem II has been reported at resolns. from 3.8 to 2.9 Å. These resolns. have provided much information on the arrangement of protein subunits and cofactors but are insufficient to reveal the detailed structure of the catalytic center of water splitting. Here we report the crystal structure of photosystem II at a resoln. of 1.9 Å. From our electron d. map, we located all of the metal atoms of the Mn4CaO5 cluster, together with all of their ligands. We found that five oxygen atoms served as oxo bridges linking the five metal atoms, and that four water mols. were bound to the Mn4CaO5 cluster; some of them may therefore serve as substrates for dioxygen formation. We identified more than 1,300 water mols. in each photosystem II monomer. Some of them formed extensive hydrogen-bonding networks that may serve as channels for protons, water or oxygen mols. The detn. of the high-resoln. structure of photosystem II will allow us to analyze and understand its functions in great detail.
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      A review. The authors summarize the current understanding of the structure of the Mn4CaO5 cluster of the photosynthetic oxygen-evolving complex, as well as the water oxidn. reaction based on insights learned primarily from x-ray techniques.
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      Kanady, J. S.; Tsui, E. Y.; Day, M. W.; Agapie, T. A Synthetic Model of the Mn3Ca Subsite of the Oxygen-Evolving Complex in Photosystem II. Science (Washington, DC, U. S.) 2011, 333 (6043), 733– 736,  DOI: 10.1126/science.1206036
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      A synthetic model of the Mn3Ca subsite of the oxygen-evolving complex in photosystem II
      Kanady, Jacob S.; Tsui, Emily Y.; Day, Michael W.; Agapie, Theodor
      Science (Washington, DC, United States) (2011), 333 (6043), 733-736CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Within photosynthetic organisms, the oxygen-evolving complex (OEC) of photosystem II generates dioxygen from water using a catalytic Mn4CaOn cluster (n varies with the mechanism and nature of the intermediate). The authors report here the rational synthesis of a [Mn3CaO4]6+ cubane that structurally models the tri-manganese-calcium-cubane subsite of the OEC. Structural and electrochem. comparison between Mn3CaO4 and a related Mn4O4 cubane alongside characterization of an intermediate calcium-manganese multinuclear complex reveals potential roles of calcium in facilitating high oxidn. states at manganese and in the assembly of the biol. cluster.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXps1Sgtrk%253D&md5=39e9a84d10b59dddbedcddaa2b829271
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      Tsui, E. Y.; Agapie, T. Reduction Potentials of Heterometallic Manganese-Oxido Cubane Complexes Modulated by Redox-Inactive Metals. Proc. Natl. Acad. Sci. U. S. A. 2013, 110 (25), 10084– 10088,  DOI: 10.1073/pnas.1302677110
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      Reduction potentials of heterometallic manganese-oxido cubane complexes modulated by redox-inactive metals
      Tsui, Emily Y.; Agapie, Theodor
      Proceedings of the National Academy of Sciences of the United States of America (2013), 110 (25), 10084-10088, S10084/1-S10084/6CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Understanding the effect of redox-inactive metals on the properties of biol. and heterogeneous H2O oxidn. catalysts is important both fundamentally and for improvement of future catalyst designs. Heterometallic Mn-oxido cubane clusters [MMn3O4] (M = Sr2+, Zn2+, Sc3+, Y3+) with polypyridyl and acetate ligands structurally relevant to the O-evolving complex (OEC) of photosystem II were prepd. and characterized. The redn. potentials of these clusters and other related mixed metal Mn-tetraoxido complexes are correlated with the Lewis acidity of the apical redox-inactive metal in a manner similar to a related series of heterometallic Mn-dioxido clusters. The redox potentials of the [SrMn3O4] and [CaMn3O4] clusters are close, which is consistent with the observation that the OEC is functional only with one of these two metals. Considering the authors' previous studies of [MMn3O2] moieties, the present results with more structurally accurate models of the OEC ([MMn3O4]) suggest a general relation between the redn. potentials of heterometallic oxido clusters and the Lewis acidities of incorporated cations that applies to diverse structural motifs. These findings support proposals that one function of Ca in the OEC is to modulate the redn. potential of the cluster to allow electron transfer.
      >> More from SciFinder ®
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      Tsui, E. Y.; Tran, R.; Yano, J.; Agapie, T. Redox-Inactive Metals Modulate the Reduction Potential in Heterometallic Manganese-Oxido Clusters. Nat. Chem. 2013, 5 (4), 293– 299,  DOI: 10.1038/nchem.1578
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      Redox-inactive metals modulate the reduction potential in heterometallic manganese-oxido clusters
      Tsui, Emily Y.; Tran, Rosalie; Yano, Junko; Agapie, Theodor
      Nature Chemistry (2013), 5 (4), 293-299CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
      Redox-inactive metals are found in biol. and heterogeneous water oxidn. catalysts, but, at present, their roles in catalysis are not well understood. Here, we report a series of high-oxidn.-state tetranuclear-dioxido clusters comprising three manganese centers and a redox-inactive metal (M). Crystallog. studies show an unprecedented Mn3M(μ4-O)(μ2-O) core that remains intact on changing M or the manganese oxidn. state. Electrochem. studies reveal that the redn. potentials span a window of 700 mV and are dependent on the Lewis acidity of the second metal. With the pKa of the redox-inactive metal-aqua complex as a measure of Lewis acidity, these compds. demonstrate a linear dependence between redn. potential and acidity with a slope of ∼100 mV per pKa unit. The Sr2+ and Ca2+ compds. show similar potentials, an observation that correlates with the behavior of the oxygen-evolving complex of photosystem II, which is active only if one of these two metals is present.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXjtlKru70%253D&md5=3c4cddc8495dc3bde19cceb2b07b7408
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      Morimoto, Y.; Kotani, H.; Park, J.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. Metal Ion-Coupled Electron Transfer of a Nonheme Oxoiron(IV) Complex: Remarkable Enhancement of Electron-Transfer Rates by Sc3+. J. Am. Chem. Soc. 2011, 133 (3), 403– 405,  DOI: 10.1021/ja109056x
      6
      Metal Ion-Coupled Electron Transfer of a Nonheme Oxoiron(IV) Complex: Remarkable Enhancement of Electron-Transfer Rates by Sc3+
      Morimoto, Yuma; Kotani, Hiroaki; Park, Ji-Yun; Lee, Yong-Min; Nam, Won-Woo; Fukuzumi, Shun-Ichi
      Journal of the American Chemical Society (2011), 133 (3), 403-405CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Rates of electron transfer from a series of one-electron reductants to a nonheme oxoiron(IV) complex, [(N4Py)FeIV(O)]2+, are enhanced as much as 108-fold by addn. of metal ions such as Sc3+, Zn2+, Mg2+, and Ca2+; the metal ion effect follows the Lewis acidity of metal ions. The one-electron redn. potential of [(N4Py)FeIV(O)]2+ is shifted to a pos. direction by 0.84 V in the presence of Sc3+ ion (0.20 M).
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      Park, J.; Morimoto, Y.; Lee, Y.-M.; You, Y.; Nam, W.; Fukuzumi, S. Scandium Ion-Enhanced Oxidative Dimerization and N-Demethylation of N,N-Dimethylanilines by a Non-Heme Iron(IV)-Oxo Complex. Inorg. Chem. 2011, 50 (22), 11612– 11622,  DOI: 10.1021/ic201545a
      7
      Scandium Ion-Enhanced Oxidative Dimerization and N-Demethylation of N,N-Dimethylanilines by a Non-Heme Iron(IV)-Oxo Complex
      Park, Jiyun; Morimoto, Yuma; Lee, Yong-Min; You, Youngmin; Nam, Wonwoo; Fukuzumi, Shunichi
      Inorganic Chemistry (2011), 50 (22), 11612-11622CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Oxidative dimerization of N,N-dimethylaniline (DMA) occurs with a nonheme iron(IV)-oxo complex, [FeIV(O)(N4Py)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), to yield the corresponding dimer, tetramethylbenzidine (TMB), in acetonitrile. The rate of the oxidative dimerization of DMA by [FeIV(O)(N4Py)]2+ is markedly enhanced by the presence of scandium triflate, Sc(OTf)3 (OTf = CF3SO3-), when TMB is further oxidized to the radical cation (TMB·+). In contrast, we have obsd. the oxidative N-demethylation with para-substituted DMA substrates, since the position of the C-C bond formation to yield the dimer is blocked. The rate of the oxidative N-demethylation of para-substituted DMA by [FeIV(O)(N4Py)]2+ is also markedly enhanced by the presence of Sc(OTf)3. In the case of para-substituted DMA derivs. with electron-donating substituents, radical cations of DMA derivs. are initially formed by Sc3+ ion-coupled electron transfer from DMA derivs. to [FeIV(O)(N4Py)]2+, giving demethylated products. Binding of Sc3+ to [FeIV(O)(N4Py)]2+ enhances the Sc3+ ion-coupled electron transfer from DMA derivs. to [FeIV(O)(N4Py)]2+, whereas binding of Sc3+ to DMA derivs. retards the electron-transfer reaction. The complicated kinetics of the Sc3+ ion-coupled electron transfer from DMA derivs. to [FeIV(O)(N4Py)]2+ are analyzed by competition between binding of Sc3+ to DMA derivs. and to [FeIV(O)(N4Py)]2+. The binding consts. of Sc3+ to DMA derivs. increase with the increase of the electron-donating ability of the para-substituent. The rate consts. of Sc3+ ion-coupled electron transfer from DMA derivs. to [FeIV(O)(N4Py)]2+, which are estd. from the binding consts. of Sc3+ to DMA derivs., agree well with those predicted from the driving force dependence of the rate consts. of Sc3+ ion-coupled electron transfer from one-electron reductants to [FeIV(O)(N4Py)]2+. Thus, oxidative dimerization of DMA and N-demethylation of p-substituted DMA derivs. proceed via Sc3+ ion-coupled electron transfer from DMA derivs. to [FeIV(O)(N4Py)]2+.
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    8. 8
      Park, J.; Morimoto, Y.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. Metal Ion Effect on the Switch of Mechanism from Direct Oxygen Transfer to Metal Ion-Coupled Electron Transfer in the Sulfoxidation of Thioanisoles by a Non-Heme Iron(IV)–Oxo Complex. J. Am. Chem. Soc. 2011, 133 (14), 5236– 5239,  DOI: 10.1021/ja200901n
      8
      Metal ion effect on the switch of mechanism from direct oxygen transfer to metal ion-coupled electron transfer in the sulfoxidation of thioanisoles by a non-Heme iron(IV)-oxo complex
      Park, Jiyun; Morimoto, Yuma; Lee, Yong-Min; Nam, Wonwoo; Fukuzumi, Shunichi
      Journal of the American Chemical Society (2011), 133 (14), 5236-5239CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The mechanism of sulfoxidn. of thioaniosoles by a nonheme iron(IV)-oxo complex is switched from direct oxygen transfer to metal ion-coupled electron transfer by the presence of Sc3+. The switch in the sulfoxidn. mechanism is dependent on the 1-electron oxidn. potentials of thioanisoles. The rate of sulfoxidn. is accelerated ≤102-fold by the addn. of Sc3+.
      >> More from SciFinder ®
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    9. 9
      Morimoto, Y.; Park, J.; Suenobu, T.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. Mechanistic Borderline of One-Step Hydrogen Atom Transfer versus Stepwise Sc3+-Coupled Electron Transfer from Benzyl Alcohol Derivatives to a Non-Heme Iron(IV)-Oxo Complex. Inorg. Chem. 2012, 51 (18), 10025– 10036,  DOI: 10.1021/ic3016723
      9
      Mechanistic Borderline of One-Step Hydrogen Atom Transfer versus Stepwise Sc3+-Coupled Electron Transfer from Benzyl Alcohol Derivatives to a Non-Heme Iron(IV)-Oxo Complex
      Morimoto, Yuma; Park, Jiyun; Suenobu, Tomoyoshi; Lee, Yong-Min; Nam, Wonwoo; Fukuzumi, Shunichi
      Inorganic Chemistry (2012), 51 (18), 10025-10036CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The rate of oxidn. of 2,5-dimethoxybenzyl alc. (2,5-(MeO)2C6H3CH2OH) by [FeIV(O)(N4Py)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) was enhanced significantly in the presence of Sc(OTf)3 (OTf- = trifluoromethanesulfonate) in acetonitrile (e.g., 120-fold acceleration in the presence of Sc3+). Such a remarkable enhancement of the reactivity of [FeIV(O)(N4Py)]2+ in the presence of Sc3+ was accompanied by the disappearance of a kinetic deuterium isotope effect. The radical cation of 2,5-(MeO)2C6H3CH2OH was detected in the course of the reaction in the presence of Sc3+. The dimerized alc. and aldehyde were also produced in addn. to the monomer aldehyde in the presence of Sc3+. These results indicate that the reaction mechanism is changed from one-step hydrogen atom transfer (HAT) from 2,5-(MeO)2C6H3CH2OH to [FeIV(O)(N4Py)]2+ in the absence of Sc3+ to stepwise Sc3+-coupled electron transfer, followed by proton transfer in the presence of Sc3+. In contrast, neither acceleration of the rate nor the disappearance of the kinetic deuterium isotope effect was obsd. in the oxidn. of benzyl alc. (C6H5CH2OH) by [FeIV(O)(N4Py)]2+ in the presence of Sc(OTf)3. Moreover, the rate consts. detd. in the oxidn. of various benzyl alc. derivs. by [FeIV(O)(N4Py)]2+ in the presence of Sc(OTf)3 (10 mM) were compared with those of Sc3+-coupled electron transfer from one-electron reductants to [FeIV(O)(N4Py)]2+ at the same driving force of electron transfer. This comparison revealed that the borderline of the change in the mechanism from HAT to stepwise Sc3+-coupled electron transfer and proton transfer is dependent on the one-electron oxidn. potential of benzyl alc. derivs. (ca. 1.7 V vs SCE).
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    10. 10
      Bang, S.; Lee, Y.-M.; Hong, S.; Cho, K.-B.; Nishida, Y.; Seo, M. S.; Sarangi, R.; Fukuzumi, S.; Nam, W. Redox-Inactive Metal Ions Modulate the Reactivity and Oxygen Release of Mononuclear Non-Haem Iron(III)–Peroxo Complexes. Nat. Chem. 2014, 6 (10), 934– 940,  DOI: 10.1038/nchem.2055
      10
      Redox-inactive metal ions modulate the reactivity and oxygen release of mononuclear non-haem iron(III)-peroxo complexes
      Bang, Suhee; Lee, Yong-Min; Hong, Seungwoo; Cho, Kyung-Bin; Nishida, Yusuke; Seo, Mi Sook; Sarangi, Ritimukta; Fukuzumi, Shunichi; Nam, Wonwoo
      Nature Chemistry (2014), 6 (10), 934-940CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
      Redox-inactive metal ions that function as Lewis acids play pivotal roles in modulating the reactivity of oxygen-contg. metal complexes and metalloenzymes, such as the oxygen-evolving complex in photosystem II and its small-mol. mimics. Here, the authors report the synthesis and characterization of nonhaem Fe(III)-peroxo complexes that bind redox-inactive metal ions, (TMC)FeIII-(μ,η2:η2-O2)-Mn+ (Mn+ = Sr2+, Ca2+, Zn2+, Lu3+, Y3+ and Sc3+; TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). The Ca2+ and Sr2+ complexes showed similar electrochem. properties and reactivities in 1-electron oxidn. or redn. reactions. However, the properties and reactivities of complexes formed with stronger Lewis acidities are markedly different. Complexes that contain Ca2+ or Sr2+ ions were oxidized by an electron acceptor to release O2, whereas the release of O2 did not occur for complexes that bind stronger Lewis acids. These results in the light of the functional role of the Ca2+ ion in the oxidn. of H2O to dioxygen by the oxygen-evolving complex are discussed.
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      Zhang, J.; Wang, Y.; Luo, N.; Chen, Z.; Wu, K.; Yin, G. Redox Inactive Metal Ion Triggered N-Dealkylation by an Iron Catalyst with Dioxygen Activation: A Lesson from Lipoxygenases. Dalt. Trans. 2015, 44 (21), 9847– 9859,  DOI: 10.1039/C5DT00804B
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    12. 12
      Prakash, J.; Que, L. Formation of the Syn Isomer of [FeIV(Oanti)(TMC)(NCMe)]2+ in the Reaction of Lewis Acids with the Side-on Bound Peroxo Ligand in [FeIII(H2-O2)(TMC)]+. Chem. Commun. 2016, 52 (52), 8146– 8148,  DOI: 10.1039/C6CC01660J
      12
      Formation of the syn isomer of [FeIV(Oanti)(TMC)(NCMe)]2+ in the reaction of Lewis acids with the side-on bound peroxo ligand in [FeIII(η2-O2)(TMC)]+
      Prakash, Jai; Que, Lawrence
      Chemical Communications (Cambridge, United Kingdom) (2016), 52 (52), 8146-8148CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      The reactions of [FeIII(η2-O2)(TMC)]+ (TMC = tetramethylcyclam) with Lewis acids (H+ and NO+) afford the recently described syn isomer of [FeIV(O)(TMC)(NCMe)]2+ (and not the anti isomer as had been tacitly assumed). This outcome is a logical consequence of the fact that the side-on peroxo ligand is bound to the syn face of the Fe(TMC) unit in the precursor.
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    13. 13
      Zhang, J.; Wei, W. J.; Lu, X.; Yang, H.; Chen, Z.; Liao, R. Z.; Yin, G. Nonredox Metal Ions Promoted Olefin Epoxidation by Iron(II) Complexes with H2O2: DFT Calculations Reveal Multiple Channels for Oxygen Transfer. Inorg. Chem. 2017, 56 (24), 15138– 15149,  DOI: 10.1021/acs.inorgchem.7b02463
      13
      Nonredox Metal Ions Promoted Olefin Epoxidation by Iron(II) Complexes with H2O2: DFT Calculations Reveal Multiple Channels for Oxygen Transfer
      Zhang, Jisheng; Wei, Wen-Jie; Lu, Xiaoyan; Yang, Hang; Chen, Zhuqi; Liao, Rong-Zhen; Yin, Guochuan
      Inorganic Chemistry (2017), 56 (24), 15138-15149CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Nonredox metal ions play significant roles in a wide range of biol. and chem. oxidns. in which they can modulate the oxidative reactivity of those redox metal ions. With environmentally benign H2O2 as oxidant, the influence of nonredox metal ions on an iron(II) complex mediated olefin epoxidn. was investigated through exptl. studies and theor. calcns. It was found that adding nonredox metal ions like Sc3+ can substantially improve the oxygen transfer efficiency of the iron(II) complex toward cyclooctene epoxidn. even in the presence of certain amt. of water. In 18O-labeling expts. with 18O water, the presence of Sc3+ provided a higher 18O incorporation in epoxide. In UV-vis studies, it was found that the presence of Sc3+ makes both FeIII-OOH and FeIV=O species unstable. D. functional theory calcns. further disclosed that, in the presence of Sc(OTf)3, the Sc3+ adducts of FeIII-OOH and FeIV=O species are capable of epoxidizing olefin as well as FeV=O species, thus opening multiple channels for oxygenation. In particular, in the pathway of cyclooctene epoxidn., the FeIV=O/Sc3+ adduct-mediated epoxidn. is more energetically favorable than that of the FeV=O species (12.2 vs 17.2 kcal/mol). This information may implicate that the presence of certain nonredox metal ions can facilitate these redox metal ions mediating biol. and chem. oxidns. happening at a relatively low oxidn. state, which is more energetically accessible.
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    14. 14
      Kal, S.; Draksharapu, A.; Que, L. Sc3+ (or HClO4) Activation of a Nonheme FeIII–OOH Intermediate for the Rapid Hydroxylation of Cyclohexane and Benzene. J. Am. Chem. Soc. 2018, 140 (17), 5798– 5804,  DOI: 10.1021/jacs.8b01435
      14
      Sc3+ (or HClO4) Activation of a Nonheme FeIII-OOH Intermediate for the Rapid Hydroxylation of Cyclohexane and Benzene
      Kal, Subhasree; Draksharapu, Apparao; Que, Lawrence
      Journal of the American Chemical Society (2018), 140 (17), 5798-5804CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      [Fe(β-BPMCN)(CH3CN)2]2+ (1, BPMCN = N,N'-bis(pyridyl-2-methyl)-N,N'-dimethyl-trans-1,2-diaminocyclo-hexane) is a relatively poor catalyst for cyclohexane oxidn. by H2O2 and cannot perform benzene hydroxylation. However, addn. of Sc3+ activates the 1/H2O2 reaction mixt. to be able to hydroxylate cyclohexane and benzene within seconds at -40 °C. A metastable S = 1/2 FeIII-(η1-OOH) intermediate 2 is trapped at -40 °C, which undergoes rapid decay upon addn. of Sc3+ at rates independent of [substrate] but linearly dependent on [Sc3+]. HClO4 elicits comparable reactivity as Sc3+ at the same concn. We thus postulate that these additives both facilitate O-O bond heterolysis of 2 to form a common highly electrophilic FeV=O oxidant that is comparably reactive to the fastest nonheme high-valent iron-oxo oxidants found to date. Safety: 90% H2O2 is potentially explosive.
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    15. 15
      Kal, S.; Que, L. Activation of a Non-Heme FeIII-OOH by a Second FeIII to Hydroxylate Strong C–H Bonds: Possible Implications for Soluble Methane Monooxygenase. Angew. Chem., Int. Ed. 2019, 58 (25), 8484– 8488,  DOI: 10.1002/anie.201903465
      15
      Activation of a non-heme FeIII-OOH by a second FeIII to hydroxylate strong C-H Bonds: Possible implications for soluble methane monooxygenase
      Kal, Subhasree; Que, Lawrence Jr.
      Angewandte Chemie, International Edition (2019), 58 (25), 8484-8488CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Non-heme iron oxygenases contain either monoiron or diiron active sites, and the role of the second iron in the latter enzymes is a topic of particular interest, esp. for sol. methane monooxygenase (sMMO). Herein we report the activation of a non-heme FeIII-OOH intermediate in a synthetic monoiron system using FeIII(OTf)3 to form a high-valent oxidant capable of effecting cyclohexane and benzene hydroxylation within seconds at -40°C. Our results show that the second iron acts as a Lewis acid to activate the iron-hydroperoxo intermediate, leading to the formation of a powerful FeV=O oxidant-a possible role for the second iron in sMMO.
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    16. 16
      Park, Y. J.; Ziller, J. W.; Borovik, A. S. The Effects of Redox-Inactive Metal Ions on the Activation of Dioxygen: Isolation and Characterization of a Heterobimetallic Complex Containing a MnIII-(μ-OH)-CaII Core. J. Am. Chem. Soc. 2011, 133 (24), 9258– 9261,  DOI: 10.1021/ja203458d
      16
      The Effects of Redox-Inactive Metal Ions on the Activation of Dioxygen: Isolation and Characterization of a Heterobimetallic Complex Containing a MnIII-(μ-OH)-CaII Core
      Park, Young Jun; Ziller, Joseph W.; Borovik, A. S.
      Journal of the American Chemical Society (2011), 133 (24), 9258-9261CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Rate enhancements for the redn. of dioxygen by a MnII complex were obsd. in the presence of redox-inactive group 2 metal ions. The rate changes were correlated with an increase in the Lewis acidity of the group 2 metal ions. These studies led to the isolation of heterobimetallic complexes contg. MnIII-(μ-OH)-MII cores (MII = CaII, BaII) in which the hydroxo oxygen atom is derived from O2. This type of core structure has relevance to the oxygen-evolving complex within photosystem II.
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    17. 17
      Leeladee, P.; Baglia, R. A.; Prokop, K. A.; Latifi, R.; De Visser, S. P.; Goldberg, D. P. Valence Tautomerism in a High-Valent Manganese-Oxo Porphyrinoid Complex Induced by a Lewis Acid. J. Am. Chem. Soc. 2012, 134 (25), 10397– 10400,  DOI: 10.1021/ja304609n
      17
      Valence Tautomerism in a High-Valent Manganese-Oxo Porphyrinoid Complex Induced by a Lewis Acid
      Leeladee, Pannee; Baglia, Regina A.; Prokop, Katharine A.; Latifi, Reza; de Visser, Sam P.; Goldberg, David P.
      Journal of the American Chemical Society (2012), 134 (25), 10397-10400CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Addn. of the Lewis acid Zn2+ to (TBP8Cz)MnV(O) induces valence tautomerization, resulting in the formation of [(TBP8Cz+·)MnIV(O)-Zn2+]. This new species was characterized by UV-vis, EPR, the Evans method, and 1H NMR and supported by DFT calcns. Removal of Zn2+ quant. restores the starting material. Electron-transfer and hydrogen-atom-transfer reactions are strongly influenced by the presence of Zn2+.
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    18. 18
      Baglia, R. A.; Krest, C. M.; Yang, T.; Leeladee, P.; Goldberg, D. P. High-Valent Manganese-Oxo Valence Tautomers and the Influence of Lewis/Brönsted Acids on C-H Bond Cleavage. Inorg. Chem. 2016, 55 (20), 10800– 10809,  DOI: 10.1021/acs.inorgchem.6b02109
      18
      High-Valent Manganese-Oxo Valence Tautomers and the Influence of Lewis/Bronsted Acids on C-H Bond Cleavage
      Baglia, Regina A.; Krest, Courtney M.; Yang, Tzuhsiung; Leeladee, Pannee; Goldberg, David P.
      Inorganic Chemistry (2016), 55 (20), 10800-10809CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The addn. of Lewis or Bronsted acids (LA = Zn(OTf)2, B(C6F5)3, HBArF, TFA) to the high-valent manganese-oxo complex MnV(O)(TBP8Cz) results in the stabilization of a valence tautomer MnIV(O-LA)(TBP8Cz•+). The ZnII and B(C6F5)3 complexes were characterized by manganese K-edge X-ray absorption spectroscopy (XAS). The position of the edge energies and the intensities of the pre-edge (1s to 3d) peaks confirm that the Mn ion is in the +4 oxidn. state. Fitting of the extended X-ray absorption fine structure (EXAFS) region reveals 4 N/O ligands at Mn-Nave = 1.89 Å and a fifth N/O ligand at 1.61 Å, corresponding to the terminal oxo ligand. This Mn-O bond length is elongated compared to the MnV(O) starting material (Mn-O = 1.55 Å). The reactivity of MnIV(O-LA)(TBP8Cz•+) toward C-H substrates was examd., and it was found that H• abstraction from C-H bonds occurs in a 1:1 stoichiometry, giving a MnIV complex and the dehydrogenated org. product. The rates of C-H cleavage are accelerated for the MnIV(O-LA)(TBP8Cz•+) valence tautomer as compared to the MnV(O) valence tautomer when LA = ZnII, B(C6F5)3, and HBArF, whereas for LA = TFA, the C-H cleavage rate is slightly slower than when compared to MnV(O). A large, nonclassical kinetic isotope effect of kH/kD = 25-27 was obsd. for LA = B(C6F5)3 and HBArF, indicating that H-atom transfer (HAT) is the rate-limiting step in the C-H cleavage reaction and implicating a potential tunneling mechanism for HAT. The reactivity of MnIV(O-LA)(TBP8Cz•+) toward C-H bonds depends on the strength of the Lewis acid. The HAT reactivity is compared with the analogous corrole complex MnIV(O-H)(tpfc•+) recently reported (J. Am. Chem. Soc.2015, 137, 14481-14487).
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    19. 19
      Choe, C.; Lv, Z.; Wu, Y.; Chen, Z.; Sun, T.; Wang, H.; Li, G.; Yin, G. Promoting a Non-Heme Manganese Complex Catalyzed Oxygen Transfer Reaction by Both Lewis Acid and Brønsted Acid: Similarities and Distinctions. Mol. Catal. 2017, 438, 230– 238,  DOI: 10.1016/j.mcat.2017.05.030
      19
      Promoting a non-heme manganese complex catalyzed oxygen transfer reaction by both Lewis acid and Bronsted acid: Similarities and distinctions
      Choe, Cholho; Lv, Zhanao; Wu, Yunfeng; Chen, Zhuqi; Sun, Tingting; Wang, Haibin; Li, Guangxing; Yin, Guochuan
      Molecular Catalysis (2017), 438 (), 230-238CODEN: MCOADH ISSN:. (Elsevier B.V.)
      This work demonstrates that certain Lewis and Bronsted acids can sharply improve the oxygen transfer efficiency of a manganese(II) catalyst bearing non-heme ligand. In the absence of Lewis and Bronsted acids, oxidn. of manganese(II) complex will generate di-μ-oxo-bridged dinuclear Mn2(III,IV) core which is very sluggish for olefin epoxidn. Adding non-redox metal ions as Lewis acid or Bronsted acid will both improve the catalytic epoxidn. of olefin, and this improvement is dependent on the pKa of Bronsted acid, or the net charge of non-redox metals of Lewis acid. Mechanism study revealed that similar promotional effect by either Lewis or Bronsted acids was originated from a similar reaction pathway by dissocg. aforementioned sluggish di-μ-oxo core. However, distinctions of reactive intermediate were also demonstrated for Lewis or Bronsted acids.
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    20. 20
      Sharma, N.; Jung, J.; Ohkubo, K.; Lee, Y.-M.; El-Khouly, M. E.; Nam, W.; Fukuzumi, S. Long-Lived Photoexcited State of a Mn(IV)-Oxo Complex Binding Scandium Ions That Is Capable of Hydroxylating Benzene. J. Am. Chem. Soc. 2018, 140 (27), 8405– 8409,  DOI: 10.1021/jacs.8b04904
      20
      Long-Lived Photoexcited State of a Mn(IV)-Oxo Complex Binding Scandium Ions That is Capable of Hydroxylating Benzene
      Sharma, Namita; Jung, Jieun; Ohkubo, Kei; Lee, Yong-Min; El-Khouly, Mohamed E.; Nam, Wonwoo; Fukuzumi, Shunichi
      Journal of the American Chemical Society (2018), 140 (27), 8405-8409CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Photoexcitation of a MnIV-oxo complex binding scandium ions ([(Bn-TPEN)MnIV(O)]2+-(Sc(OTf)3)2) in a solvent mixt. of trifluoroethanol and acetonitrile (vol./vol. = 1:1) resulted in formation of the long-lived photoexcited state, which can hydroxylate benzene to phenol. The photohydroxylation of benzene by [(Bn-TPEN)MnIV(O)]2+-(Sc(OTf)3)2 was made possible by electron transfer from benzene to the long-lived 2E excited state of [(Bn-TPEN)MnIV(O)]2+-(Sc(OTf)3)2 to produce a benzene radical cation, which reacted with water as revealed by laser-induced transient absorption measurements.
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    21. 21
      Sankaralingam, M.; Lee, Y.-M.; Pineda-Galvan, Y.; Karmalkar, D. G.; Seo, M. S.; Jeon, S. H.; Pushkar, Y.; Fukuzumi, S.; Nam, W. Redox Reactivity of a Mononuclear Manganese-Oxo Complex Binding Calcium Ion and Other Redox-Inactive Metal Ions. J. Am. Chem. Soc. 2019, 141 (3), 1324– 1336,  DOI: 10.1021/jacs.8b11492
      21
      Redox Reactivity of a Mononuclear Manganese-Oxo Complex Binding Calcium Ion and Other Redox-Inactive Metal Ions
      Sankaralingam, Muniyandi; Lee, Yong-Min; Pineda-Galvan, Yuliana; Karmalkar, Deepika G.; Seo, Mi Sook; Jeon, So Hyun; Pushkar, Yulia; Fukuzumi, Shunichi; Nam, Wonwoo
      Journal of the American Chemical Society (2019), 141 (3), 1324-1336CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Mononuclear nonheme manganese(IV)-oxo complexes binding calcium ion and other redox-inactive metal ions, [(dpaq)MnIV(O)]+-Mn+ (1-Mn+, Mn+ = Ca2+, Mg2+, Zn2+, Lu3+, Y3+, Al3+, and Sc3+) (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), were synthesized by reacting a hydroxomanganese(III) complex, [(dpaq)MnIII(OH)]+, with iodosylbenzene (PhIO) in the presence of redox-inactive metal ions (Mn+). The Mn(IV)-oxo complexes were characterized using various spectroscopic techniques. In reactivity studies, the authors obsd. contrasting effects of Mn+ on the reactivity of 1-Mn+ in redox reactions such as electron-transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. In the OAT and ET reactions, the reactivity order of 1-Mn+, such as 1-Sc3+ ≈ 1-Al3+ > 1-Y3+ > 1-Lu3+ > 1-Zn2+ > 1-Mg2+ > 1-Ca2+, follows the Lewis acidity of Mn+ bound to the Mn-O moiety; i.e., the stronger the Lewis acidity of Mn+, the higher the reactivity of 1-Mn+ becomes. In sharp contrast, the reactivity of 1-Mn+ in the HAT reaction was reversed, giving the reactivity order 1-Ca2+ > 1-Mg2+ > 1-Zn2+ > 1-Lu3+> 1-Y3+> 1-Al3+ ≈ 1-Sc3+; i.e., the higher is Lewis acidity of Mn+, the lower the reactivity of 1-Mn+ in the HAT reaction. The latter result implies that the Lewis acidity of Mn+ bound to the Mn-O moiety can modulate the basicity of the metal-oxo moiety, thus influencing the HAT reactivity of 1-Mn+; cytochrome P 450 uses the axial thiolate ligand to increase the basicity of the iron-oxo moiety, which enhances the reactivity of compd. I in C-H bond activation reactions.
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    22. 22
      Yoon, H.; Lee, Y.-M.; Wu, X.; Cho, K.-B.; Sarangi, R.; Nam, W.; Fukuzumi, S. Enhanced Electron-Transfer Reactivity of Nonheme Manganese(IV)–Oxo Complexes by Binding Scandium Ions. J. Am. Chem. Soc. 2013, 135 (24), 9186– 9194,  DOI: 10.1021/ja403965h
      22
      Enhanced Electron-Transfer Reactivity of Nonheme Manganese(IV)-Oxo Complexes by Binding Scandium Ions
      Yoon, Heejung; Lee, Yong-Min; Wu, Xiujuan; Cho, Kyung-Bin; Sarangi, Ritimukta; Nam, Wonwoo; Fukuzumi, Shunichi
      Journal of the American Chemical Society (2013), 135 (24), 9186-9194CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      One and two scandium ions (Sc3+) are bound strongly to nonheme manganese(IV)-oxo complexes, [(N4Py)MnIV(O)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) and [(Bn-TPEN)MnIV(O)]2+ (Bn-TPEN = N-benzyl-N,N',N'-tris(2-pyridylmethyl)-1,2-diaminoethane), to form MnIV(O)-(Sc3+)1 and MnIV(O)-(Sc3+)2 complexes, resp. The binding of Sc3+ ions to the MnIV(O) complexes was examd. by spectroscopic methods as well as by DFT calcns. The one-electron redn. potentials of the MnIV(O) complexes were markedly shifted to a pos. direction by binding of Sc3+ ions. Accordingly, rates of the electron transfer reactions of the MnIV(O) complexes were enhanced as much as 107-fold by binding of two Sc3+ ions. The driving force dependence of electron transfer from various electron donors to the MnIV(O) and MnIV(O)-(Sc3+)2 complexes was examd. and analyzed in light of the Marcus theory of electron transfer to det. the reorganization energies of electron transfer. The smaller reorganization energies and much more pos. redn. potentials of the MnIV(O)-(Sc3+)2 complexes resulted in remarkable enhancement of the electron-transfer reactivity of the MnIV(O) complexes. Such a dramatic enhancement of the electron-transfer reactivity of the MnIV(O) complexes by binding of Sc3+ ions resulted in the change of mechanism in the sulfoxidn. of thioanisoles by MnIV(O) complexes from a direct oxygen atom transfer pathway without metal ion binding to an electron-transfer pathway with binding of Sc3+ ions.
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    23. 23
      Chen, J.; Lee, Y.-M.; Davis, K. M.; Wu, X.; Seo, M. S.; Cho, K.-B.; Yoon, H.; Park, Y. J.; Fukuzumi, S.; Pushkar, Y. N. A Mononuclear Non-Heme Manganese(IV)–Oxo Complex Binding Redox-Inactive Metal Ions. J. Am. Chem. Soc. 2013, 135 (17), 6388– 6391,  DOI: 10.1021/ja312113p
      23
      A Mononuclear Non-Heme Manganese(IV)-Oxo Complex Binding Redox-Inactive Metal Ions
      Chen, Junying; Lee, Yong-Min; Davis, Katherine M.; Wu, Xiujuan; Seo, Mi Sook; Cho, Kyung-Bin; Yoon, Heejung; Park, Young Jun; Fukuzumi, Shunichi; Pushkar, Yulia N.; Nam, Wonwoo
      Journal of the American Chemical Society (2013), 135 (17), 6388-6391CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Redox-inactive metal ions play pivotal roles in regulating the reactivities of high-valent metal-oxo species in a variety of enzymic and chem. reactions. A mononuclear nonheme Mn(IV)-oxo complex bearing a pentadentate N5 ligand was synthesized and used in the synthesis of a Mn(IV)-oxo complex binding scandium ions. The Mn(IV)-oxo complexes were characterized with various spectroscopic methods. The reactivities of the Mn(IV)-oxo complex are markedly influenced by binding of Sc3+ ions in oxidn. reactions, such as a ∼ 2200-fold increase in the rate of oxidn. of thioanisole (i.e., oxygen atom transfer) but a ∼ 180-fold decrease in the rate of C-H bond activation of 1,4-cyclohexadiene (i.e., hydrogen atom transfer). The present results provide the first example of a nonheme Mn(IV)-oxo complex binding redox-inactive metal ions that shows a contrasting effect of the redox-inactive metal ions on the reactivities of metal-oxo species in the oxygen atom transfer and hydrogen atom transfer reactions.
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    24. 24
      Dong, L.; Wang, Y.; Lv, Y.; Chen, Z.; Mei, F.; Xiong, H.; Yin, G. Lewis-Acid-Promoted Stoichiometric and Catalytic Oxidations by Manganese Complexes Having Cross-Bridged Cyclam Ligand: A Comprehensive Study. Inorg. Chem. 2013, 52 (9), 5418– 5427,  DOI: 10.1021/ic400361s
      24
      Lewis-Acid-Promoted Stoichiometric and Catalytic Oxidations by Manganese Complexes Having Cross-Bridged Cyclam Ligand: A Comprehensive Study
      Dong, Lei; Wang, Yujuan; Lv, Yanzong; Chen, Zhuqi; Mei, Fuming; Xiong, Hui; Yin, Guochuan
      Inorganic Chemistry (2013), 52 (9), 5418-5427CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Redox-inactive metal ions have been recognized to be able to participate in redox metal-ion-mediated biol. and chem. oxidative events; however, their roles are still elusive. This work presents how the redox-inactive metal ions affect the oxidative reactivity of a well-investigated manganese(II) with its corresponding manganese(IV) complexes having cross-bridged cyclam ligand. In dry acetone, the presence of these metal ions can greatly accelerate stoichiometric oxidns. of triphenylphosphine and sulfides by the manganese(IV) complexes through electron transfer or catalytic sulfoxidns. by the corresponding manganese(II) complexes with PhIO. Significantly, the rate enhancements are highly Lewis-acid strength dependent on added metal ions. These metal ions like Al3+ can also promote the thermodn. driving force of the MnIV-OH moiety to facilitate its hydrogen abstraction from ethylbenzene having a BDECH value of 85 kcal/mol, while it is exptl. limited to 80 kcal/mol for MnIV-OH alone. Adding Al3+ may also improve the manganese(II)-catalyzed olefin epoxidn. with PhIO. However, compared with those in electron transfer, improvements in hydrogen abstraction and electron transfer are minor. The existence of the interaction between Lewis acid and the manganese(IV) species was evidenced by the blue shift of the characteristic absorbance of the manganese(IV) species from 554 to 537 nm and by converting its EPR signal at g = 2.01 into a hyperfine 6-line signal upon adding Al3+ (I = 5/2). Cyclic voltammograms of the manganese(IV) complexes reveal that adding Lewis acid would substantially shift its potential to the pos. direction, thus enhancing its oxidizing capability.
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    25. 25
      Zhang, Z.; Coats, K. L.; Chen, Z.; Hubin, T. J.; Yin, G. Influence of Calcium(II) and Chloride on the Oxidative Reactivity of a Manganese(II) Complex of a Cross-Bridged Cyclen Ligand. Inorg. Chem. 2014, 53 (22), 11937– 11947,  DOI: 10.1021/ic501342c
      25
      Influence of Calcium(II) and Chloride on the Oxidative Reactivity of a Manganese(II) Complex of a Cross-Bridged Cyclen Ligand
      Zhang, Zhan; Coats, Katherine L.; Chen, Zhuqi; Hubin, Timothy J.; Yin, Guochuan
      Inorganic Chemistry (2014), 53 (22), 11937-11947CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Available data from different labs. confirmed that both Ca2+ and Cl- are crucial for H2O oxidn. in Photosystem II. However, their roles are still elusive. Using a Mn(II) complex having a cross-bridged cyclen ligand as a model, the influence of Ca2+ on the oxidative reactivity of the Mn(II) complex and its corresponding Mn(IV) analog were studied. Adding Ca2+ can significantly improve the oxygenation efficiency of the Mn(II) complex in sulfide oxidn. and further accelerate the oxidn. of sulfoxide to sulfone. Similar improvements also were obsd. for Mg2+, Sr2+, and Ba2+. A new monomeric Mn(IV) complex having two cis-hydroxide ligands also was isolated through oxidn. of the corresponding Mn(II) complex with H2O2 in the presence of NH4PF6. This rare cis-dihydroxomanganese(IV) species was well characterized by x-ray crystallog., electrochem., ESR, and UV-visible spectroscopy. Notably, using the Mn(IV) complex as a catalyst demonstrates higher activity than the corresponding Mn(II) complex, and adding Ca2+ further improves its catalytic efficiency. However, adding Cl- decreases its catalytic activity. In electrochem. studies of Mn(IV) complexes with no chloride ligand present, adding Ca2+ pos. shifted the redox potential of the MnIV/MnIII couple but neg. shifted its MnV/MnIV couple. In the Mn(II) complex having a chloride ligand, adding Ca2+ shifted both the MnIV/MnIII and MnV/MnIV couples in the neg. direction. The revealed oxidative reactivity and redox properties of the Mn species affected by Ca2+ and Cl- may provide new clues to understanding their roles in the H2O oxidn. process of Photosystem II.
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    26. 26
      Choe, C.; Yang, L.; Lv, Z.; Mo, W.; Chen, Z.; Li, G.; Yin, G. Redox-Inactive Metal Ions Promoted the Catalytic Reactivity of Non-Heme Manganese Complexes towards Oxygen Atom Transfer. Dalt. Trans. 2015, 44 (19), 9182– 9192,  DOI: 10.1039/C4DT03993A
      26
      Redox-inactive metal ions promoted the catalytic reactivity of non-heme manganese complexes towards oxygen atom transfer
      Choe, Cholho; Yang, Ling; Lv, Zhanao; Mo, Wanling; Chen, Zhuqi; Li, Guangxin; Yin, Guochuan
      Dalton Transactions (2015), 44 (19), 9182-9192CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      Redox-inactive metal ions can modulate the reactivity of redox-active metal ions in a variety of biol. and chem. oxidns. Many synthetic models have been developed to help address the elusive roles of these redox-inactive metal ions. Using a non-heme manganese(II) complex as the model, the influence of redox-inactive metal ions as a Lewis acid on its catalytic efficiency in oxygen atom transfer was investigated. In the absence of redox-inactive metal ions, the manganese(II) catalyst is very sluggish, for example, in cyclooctene epoxidn., providing only 9.9% conversion with 4.1% yield of epoxide. However, addn. of 2 equiv. of Al3+ to the manganese(II) catalyst sharply improves the epoxidn., providing up to 97.8% conversion with 91.4% yield of epoxide. EPR studies of the manganese(II) catalyst in the presence of an oxidant reveal a 16-line hyperfine structure centered at g = 2.0, clearly indicating the formation of a mixed valent di-μ-oxo-bridged diamond core, MnIII-(μ-O)2-MnIV. The presence of a Lewis acid like Al3+ causes the dissocn. of this diamond MnIII-(μ-O)2-MnIV core to form monomeric manganese(IV) species which is responsible for improved epoxidn. efficiency. This promotional effect has also been obsd. in other manganese complexes bearing various non-heme ligands. The findings presented here have provided a promising strategy to explore the catalytic reactivity of some di-μ-oxo-bridged complexes by adding non-redox metal ions to in situ dissoc. those dimeric cores and may also provide clues to understand the mechanism of methane monooxygenase which has a similar diiron diamond core as the intermediate.
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    27. 27
      Chen, Z.; Yang, L.; Choe, C.; Lv, Z.; Yin, G. Non-Redox Metal Ion Promoted Oxygen Transfer by a Non-Heme Manganese Catalyst. Chem. Commun. 2015, 51 (10), 1874– 1877,  DOI: 10.1039/C4CC07981G
      27
      Non-redox metal ion promoted oxygen transfer by a non-heme manganese catalyst
      Chen, Zhuqi; Yang, Ling; Choe, Cholho; Lv, Zhanao; Yin, Guochuan
      Chemical Communications (Cambridge, United Kingdom) (2015), 51 (10), 1874-1877CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      This work demonstrates that non-redox metal ions as Lewis acids can sharply improve the oxygen transfer efficiency of a manganese(II) catalyst having a non-heme ligand. In the absence of Lewis acid, oxidn. of a manganese(II) complex will generate the known di-μ-oxo-bridged dinuclear Mn2(III,IV) core which is very sluggish for olefin epoxidn. Adding non-redox metal ions causes the dissocn. of the dinuclear core, leading to sharp improvement in its oxygen transfer efficiency.
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    28. 28
      Kim, S.; Cho, K.-B.; Lee, Y.-M.; Chen, J.; Fukuzumi, S.; Nam, W. Factors Controlling the Chemoselectivity in the Oxidation of Olefins by Nonheme Manganese(IV)-Oxo Complexes. J. Am. Chem. Soc. 2016, 138 (33), 10654– 10663,  DOI: 10.1021/jacs.6b06252
      28
      Factors Controlling the Chemoselectivity in the Oxidation of Olefins by Nonheme Manganese(IV)-Oxo Complexes
      Kim, Surin; Cho, Kyung-Bin; Lee, Yong-Min; Chen, Junying; Fukuzumi, Shunichi; Nam, Wonwoo
      Journal of the American Chemical Society (2016), 138 (33), 10654-10663CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      We report the oxidn. of cyclic olefins, such as cyclohexene, cyclohexene-d10, and cyclooctene, by mononuclear nonheme manganese(IV)-oxo (MnIVO) and triflic acid (HOTf)-bound MnIVO complexes. In the oxidn. of cyclohexene, the MnIVO complexes prefer the C-H bond activation to the C=C double bond epoxidn., whereas the C=C double bond epoxidn. becomes a preferred reaction pathway in the cyclohexene oxidn. by HOTf-bound MnIVO complexes. In contrast, the oxidn. of cyclohexene-d10 and cyclooctene by the MnIVO complexes occurs predominantly via the C=C double bond epoxidn. This conclusion is drawn from the product anal. and kinetic studies of the olefin oxidn. reactions, such as the epoxide vs. allylic oxidn. products, the formation of Mn(II) vs. Mn(III) products, and the kinetic analyses. Overall, the exptl. results suggest that the energy barrier of the C=C double bond epoxidn. is very close to that of the allylic C-H bond activation in the oxidn. of cyclic olefins by high-valent metal-oxo complexes. Thus, the preference of the reaction pathways is subject to changes upon small manipulation of the reaction environments, such as the supporting ligands and metal ions in metal-oxo species, the presence of HOTf (i.e., HOTf-bound MnIVO species), and the allylic C-H(D) bond dissocn. energies of olefins. This is confirmed by DFT calcns. in the oxidn. of cyclohexene and cyclooctene, which show multiple pathways with similar rate-limiting energy barriers and depending on the allylic C-H bond dissocn. energies. In addn., the possibility of excited state reactivity in the current system is confirmed for epoxidn. reactions.
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    29. 29
      Hong, S.; Lee, Y.-M.; Sankaralingam, M.; Vardhaman, A. K.; Park, Y. J.; Cho, K.-B.; Ogura, T.; Sarangi, R.; Fukuzumi, S.; Nam, W. A Manganese(V)–Oxo Complex: Synthesis by Dioxygen Activation and Enhancement of Its Oxidizing Power by Binding Scandium Ion. J. Am. Chem. Soc. 2016, 138 (27), 8523– 8532,  DOI: 10.1021/jacs.6b03874
      29
      A Manganese(V)-Oxo Complex: Synthesis by Dioxygen Activation and Enhancement of Its Oxidizing Power by Binding Scandium Ion
      Hong, Seungwoo; Lee, Yong-Min; Sankaralingam, Muniyandi; Vardhaman, Anil Kumar; Park, Young Jun; Cho, Kyung-Bin; Ogura, Takashi; Sarangi, Ritimukta; Fukuzumi, Shunichi; Nam, Wonwoo
      Journal of the American Chemical Society (2016), 138 (27), 8523-8532CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A mononuclear nonheme Mn(V)-oxo complex, [MnV(O)(TAML)]- (1, H4TAML = tetraamino macrocyclic ligand 3,4,8,9-tetrahydro-3,3,6,6,9,9-hexamethyl-1H-1,4,8,11-benzotetraazacyclotridecane-2,5,7,10(6H,11H)tetrone), was synthesized by activating dioxygen in the presence of olefins with weak allylic C-H bonds and characterized structurally and spectroscopically. In mechanistic studies, the formation rate of 1 depends on the allylic C-H bond dissocn. energies (BDEs) of olefins, and a kinetic isotope effect (KIE) value of 16 was obtained in the reactions of cyclohexene and cyclohexene-d10. Probably a H atom abstraction from the allylic C-H bonds of olefins by a putative MnIV-superoxo species, which is formed by binding O2 by a high-spin (S = 2) [MnIII(TAML)]- complex, is the rate-detg. step. A Mn(V)-oxo complex binding Sc3+ ion, [MnV(O)(TAML)]--(Sc3+) (2), was also synthesized in the reaction of 1 with Sc3+ ion and then characterized using various spectroscopic techniques. The binding site of the Sc3+ ion probably is the TAML ligand, not the Mn-O moiety, probably due to the low basicity of the oxo group compared to the basicity of the amide carbonyl group in the TAML ligand. Reactivity studies of the Mn(V)-oxo intermediates, 1 and 2, in O atom transfer and electron-transfer reactions revealed that the binding of Sc3+ ion at the TAML ligand of Mn(V)-oxo enhanced its oxidizing power with a pos. shifted 1-electron redn. potential (ΔEred = 0.70 V). This study reports the first example of tuning the second coordination sphere of high-valent metal-oxo species by binding a redox-inactive metal ion at the supporting ligand site, thereby modulating their electron-transfer properties as well as their reactivities in oxidn. reactions.
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    30. 30
      Nodzewska, A.; Watkinson, M. Remarkable Increase in the Rate of the Catalytic Epoxidation of Electron Deficient Styrenes through the Addition of Sc(OTf)3 to the MnTMTACN Catalyst. Chem. Commun. 2018, 54 (12), 1461– 1464,  DOI: 10.1039/C7CC09698D
      30
      Remarkable increase in the rate of the catalytic epoxidation of electron deficient styrenes through the addition of Sc(OTf)3 to the MnTMTACN catalyst
      Nodzewska, Aneta; Watkinson, Michael
      Chemical Communications (Cambridge, United Kingdom) (2018), 54 (12), 1461-1464CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      The effect of Lewis acids on the catalytic activity of [Mn2(μ-O)3(TMTACN)2](PF6)2 in the epoxidn. of styrenes using hydrogen peroxide as the oxidant has shown that the addn. of Sc(OTf)3 at low catalytic loading results in a very significant increase in the efficiency of the catalyst and a redn. of the reaction time to only 3 min in most cases.
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    31. 31
      Lv, Z.; Choe, C.; Wu, Y.; Wang, H.; Chen, Z.; Li, G.; Yin, G. Non-Redox Metal Ions Accelerated Oxygen Atom Transfer by Mn-Me3tacn Complex with H2O2 as Oxygen Resource. Mol. Catal. 2018, 448, 46– 52,  DOI: 10.1016/j.mcat.2018.01.022
      31
      Non-redox metal ions accelerated oxygen atom transfer by Mn-Me3tacn complex with H2O2 as oxygen resource
      Lv, Zhanao; Choe, Cholho; Wu, Yunfeng; Wang, Haibin; Chen, Zhuqi; Li, Guangxing; Yin, Guochuan
      Molecular Catalysis (2018), 448 (), 46-52CODEN: MCOADH ISSN:. (Elsevier B.V.)
      This work demonstrates a novel strategy that the introduction of non-redox metal ions as Lewis acids to the classic dinuclear manganese complex [MnIV2(μ-O)3(Me3tacn)2](PF6)2 can greatly promote the alkene epoxidn. efficiency under mild conditions with H2O2 as the solely terminal oxidant because of its economic and environmental advantages. When [MnIV2(μ-O)3(Me3tacn)2](PF6)2 was used as the catalyst in the absence of Lewis acids, only 16.4% conversion of cyclooctene with 6.2% yield of epoxide was obtained and the obvious decompn. of H2O2 was obsd. However, the oxygen transfer efficiency of the catalyst was sharply improved with 100% conversion and 90.2% yield of epoxide under identical conditions when the non-redox metal ion, such as Sc3+, was introduced to the catalytic system. The novel strategy was successfully applied to the epoxidn. reactions of different types of alkenes. Through UV-vis, FT-IR, EPR and CV characterizations, it was evidenced that the non-redox metal ions with high pos. charge as Lewis acids could dissoc. the sluggish dinuclear Mn-(μ-O)3-Mn core and the open-loop dinuclear manganese complex, HO-MnIII-(μ-O)-MnIV = O or O = MnIV-(μ-O)-MnIV = O, was proposed as the active species, which was capable of the alkene epoxidn. process. This work illustrated an alternative protocol to manipulate the reactivity of those sluggish catalysts by the introduction of non-redox metal ions and provided clues to understand the role of non-redox metal ions in metalloenzymes and heterogeneous catalysts.
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    32. 32
      Fukuzumi, S.; Morimoto, Y.; Kotani, H.; Naumov, P.; Lee, Y.-M.; Nam, W. Crystal Structure of a Metal Ion-Bound Oxoiron(IV) Complex and Implications for Biological Electron Transfer. Nat. Chem. 2010, 2 (9), 756– 759,  DOI: 10.1038/nchem.731
      32
      Crystal structure of a metal ion-bound oxoiron(IV) complex and implications for biological electron transfer
      Fukuzumi, Shunichi; Morimoto, Yuma; Kotani, Hiroaki; Naumov, Pance; Lee, Yong-Min; Nam, Wonwoo
      Nature Chemistry (2010), 2 (9), 756-759CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
      Crit. biol. electron-transfer processes involving high-valent oxometal chem. occur widely, for example in haem proteins [oxoiron(IV); FeIV(O)] and in photosystem II. Photosystem II involves Ca2+ as well as high-valent oxomanganese cluster species. However, there is no example of an interaction between metal ions and oxoiron(IV) complexes. Here, the authors report new findings concerning the binding of the redox-inactive metal ions Ca2+ and Sc3+ to a nonhaem oxoiron(IV) complex, [(TMC)FeIV(O)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). As detd. by x-ray diffraction anal., an oxo-Sc3+ interaction leads to a structural distortion of the oxoiron(IV) moiety. More importantly, this interaction facilitates a two-electron redn. by ferrocene, whereas only a 1-electron redn. process occurs without the metal ions. This control of redox behavior provides valuable mechanistic insights into oxometal redox chem., and suggests a possible key role that an auxiliary Lewis acid metal ion could play in nature, as in photosystem II.
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    33. 33
      Swart, M. A Change in the Oxidation State of Iron: Scandium Is Not Innocent. Chem. Commun. 2013, 49 (59), 6650– 6652,  DOI: 10.1039/c3cc42200c
      33
      A change in the oxidation state of iron: scandium is not innocent
      Swart, Marcel
      Chemical Communications (Cambridge, United Kingdom) (2013), 49 (59), 6650-6652CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Through extensive mol. modeling of FeIII/IV-oxo and FeIII-hydroxo complexes it is shown here unambiguously that the assignment of the iron oxidn. state of a Sc3+-capped iron-oxygen species should be revised. Iron in this Lewis-acid capped metal-bound oxygen system is FeIII, coinciding with water as a secondary axial ligand to scandium.
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    34. 34
      Prakash, J.; Rohde, G. T.; Meier, K. K.; Jasniewski, A. J.; Van Heuvelen, K. M.; Münck, E.; Que, L. Spectroscopic Identification of an FeIII Center, Not FeIV, in the Crystalline Sc–O–Fe Adduct Derived from [FeIV(O)(TMC)]2+. J. Am. Chem. Soc. 2015, 137 (10), 3478– 3481,  DOI: 10.1021/jacs.5b00535
      34
      Spectroscopic Identification of an FeIII Center, not FeIV, in the Crystalline Sc-O-Fe Adduct Derived from [FeIV(O)(TMC)]2+
      Prakash, Jai; Rohde, Gregory T.; Meier, Katlyn K.; Jasniewski, Andrew J.; Van Heuvelen, Katherine M.; Munck, Eckard; Que, Lawrence, Jr.
      Journal of the American Chemical Society (2015), 137 (10), 3478-3481CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The apparent Sc3+ adduct of [FeIV(O)(TMC)]2+ (1, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) was synthesized in amts. sufficient to allow its characterization by various spectroscopic techniques. Contrary to the earlier assignment of a +4 oxidn. state for the iron center of 1, 1 has a high-spin iron(III) center based on its Mossbauer and EPR spectra and its quant. redn. by 1 equiv of ferrocene to [FeII(TMC)]2+. Thus, 1 is best described as a ScIII-O-FeIII complex, in agreement with previous DFT calcns. (Swart, M. Chem. Commun. 2013, 49, 6650.). These results shed light on the interaction of Lewis acids with high-valent metal-oxo species.
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    35. 35
      de Boer, J. W.; Browne, W. R.; Brinksma, J.; Alsters, P. L.; Hage, R.; Feringa, B. L. Mechanism of Cis-Dihydroxylation and Epoxidation of Alkenes by Highly H2O2 Efficient Dinuclear Manganese Catalysts. Inorg. Chem. 2007, 46 (16), 6353– 6372,  DOI: 10.1021/ic7003613
      35
      Mechanism of Cis-Dihydroxylation and Epoxidation of Alkenes by Highly H2O2 Efficient Dinuclear Manganese Catalysts
      de Boer, Johannes W.; Browne, Wesley R.; Brinksma, Jelle; Alsters, Paul L.; Hage, Ronald; Feringa, Ben L.
      Inorganic Chemistry (2007), 46 (16), 6353-6372CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      In the presence of carboxylic acids [MnIV2(μ-O)3(tmtacn)2]2+ (1, where tmtacn = N,N',N''-trimethyl-1,4,7-triazacyclononane) is highly efficient in catalyzing the oxidn. of alkenes to the corresponding cis-diol and epoxide with H2O2 as terminal oxidant. The selectivity of the catalytic system with respect to (w.r.t.) either cis-dihydroxylation or epoxidn. of alkenes is dependent on the carboxylic acid employed. High turnover nos. (t.o.n. > 2000) can be achieved esp. w.r.t. Cis-dihydroxylation for which the use of 2,6-dichlorobenzoic acid allows for the highest t.o.n. Reported thus far for cis-dihydroxylation of alkenes catalyzed by a first-row transition metal and high efficiency w.r.t. The terminal oxidant (H2O2). The high activity and selectivity is due to the in situ formation of bis(μ-carboxylato)-bridged dinuclear manganese(III) complexes. Tuning of the activity of the catalyst by variation in the carboxylate ligands is dependent on both the electron-withdrawing nature of the ligand and on steric effects. By contrast, the cis-diol/epoxide selectivity is dominated by steric factors. The role of solvent, catalyst oxidn. state, H2O, and carboxylic acid concn. and the nature of the carboxylic acid employed on both the activity and the selectivity of the catalysis are explored together with speciation anal. and isotope labeling studies. [Mn2(μ-O)(μ-R-CO2)2(tmtacn)2]2+, which show remarkable redox and solvent-dependent coordination chem., are the resting state of the catalytic system and they retain a dinuclear structure throughout the catalytic cycle. The mechanistic understanding obtained from these studies holds considerable implications for both homogeneous manganese oxidn. catalysis and in understanding related biol. systems such as dinuclear catalase and arginase enzymes.
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    36. 36
      Krieble, V. K.; Noll, C. I. The Hydrolysis of Nitriles with Acids. J. Am. Chem. Soc. 1939, 61 (3), 560– 563,  DOI: 10.1021/ja01872a005
      36
      Hydrolysis of nitriles with acids
      Krieble, Vernon K.; Noll, Clarence I.
      Journal of the American Chemical Society (1939), 61 (), 560-3CODEN: JACSAT; ISSN:0002-7863.
      The rates of hydrolysis of aceto-, propio-, and α- and β-hydroxypropio-nitriles, and CNCH2CO2H were studied. In presence of HCl the rate increases approx. as the square of the mean ion activity increases. H2SO4 is a less efficient catalyst than HCl. No relation between the acidity of the solns. and velocity of hydrolysis was observed.
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    37. 37
      Lei, X. R.; Gong, C.; Zhang, Y. L.; Xu, X. Influence of the Acetamide from Acetonitrile Hydrolysis in Acid-Contained Mobile Phase on the Ultraviolet Detection in High Performance Liquid Chromatography. Chromatographia 2016, 79 (19–20), 1257– 1262,  DOI: 10.1007/s10337-016-3145-6
      37
      Influence of the Acetamide from Acetonitrile Hydrolysis in Acid-Contained Mobile Phase on the Ultraviolet Detection in High Performance Liquid Chromatography
      Lei, Xiao-Rui; Gong, Can; Zhang, Yao-Li; Xu, Xu
      Chromatographia (2016), 79 (19-20), 1257-1262CODEN: CHRGB7; ISSN:0009-5893. (Springer)
      The paper presents the influence of acetamide on UV detection after generation of acetonitrile by hydrolysis in the presence of trifluoroacetic acid in the HPLC mobile phase. The acetonitrile, with added varying contents of trifluoroacetic acid and water, was detd. by GC-MS at various times. The concn. of acetamide increased approx. linearly with time. Using a mixed std. sample soln. and a model mobile phase of acetonitrile-water-trifluoroacetic acid (25:75:0.3, vol./vol.) after 0, 24 and 48 h, the influence of the hydrolyzate acetamide on HPLC detection was studied at the wavelength of 205-220 nm. The limit of detection (LOD) of std. sample lost ∼30% after the mobile phase was placed 48 h. It is suggested that the acid-contg. mobile phase was placed ≤24 h for HPLC trace anal. at the wavelength of 205-220 nm.
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    38. 38
      Kobayashi, S.; Nagayama, S.; Busujima, T. Lewis Acid Catalysts Stable in Water. Correlation between Catalytic Activity in Water and Hydrolysis Constants and Exchange Rate Constants for Substitution of Inner-Sphere Water Ligands. J. Am. Chem. Soc. 1998, 120 (32), 8287– 8288,  DOI: 10.1021/ja980715q
      38
      Lewis Acid Catalysts Stable in Water. Correlation between Catalytic Activity in Water and Hydrolysis Constants and Exchange Rate Constants for Substitution of Inner-Sphere Water Ligands
      Kobayashi, Shu; Nagayama, Satoshi; Busujima, Tsuyoshi
      Journal of the American Chemical Society (1998), 120 (32), 8287-8288CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The catalytic activity of water-stable Lewis acids of group 1-15 metal chlorides was studied in a model reaction of benzaldehyde with (Z)-1-phenyl-1-(trimethylsiloxy)propene (the Mukaiyama aldol reaction) in aq. media. The chloride salts of Fe(II), Cu(II), Zn(II), Cd(II), In(III), and Pb(II) as well as the rare earths (Sc(III), Y(III), Ln(III)) gave promising yields. When the chloride salts of B(III), Si(IV), P(III), P(IV), Ti(IV), V(III), Ge(IV), Zr(IV), Nb(V), Mo(V), Sn(IV), Sb(V), Hf(IV), Ta(V), W(VI), Re(V), and Tl(III) were used, decompn. of the silyl enol ether occurred rapidly and no aldol adduct was obtained. Some of these salts are stable in water, but have low catalytic ability. The same aldol reaction was carried out with the corresponding metal perchlorates or trifluoromethanesulfonates (triflates); Lewis acids based on Fe(II), Cu(II), Zn(II), Cd(II), and Pb(II) as well as the rare earths (Sc(III), Y(III), Ln(III)) were both stable and active in water. The mechanism of Lewis acid catalysis in water is assumed to be as follows: when metal compds. are added to water, dissocn. and hydration occur immediately; intramol. and intermol. exchange reactions of water mols. frequently occur. If an aldehyde exists in the system, there is a chance for it to coordinate to metal cations instead of water mols. and the aldehyde is then activated; a silyl enol ether attacks this activated aldehyde to produce the aldol adduct.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK1cXkvF2ns7Y%253D&md5=6655b11a8bc51a2425408de645ab8d6c
    39. 39
      Wieghardt, K.; Bossek, U.; Nuber, B.; Weiss, J.; Bonvoisin, J.; Corbella, M.; Vitols, S. E.; Girerd, J. J. Synthesis, Crystal Structures, Reactivity, and Magnetochemistry of a Series of Binuclear Complexes of Manganese(II), -(III), and -(IV) of Biological Relevance. The Crystal Structure of [L′MnIV(μ-O)3MnIVL′](PF6)2.H2O Containing an Unprecedented Short Mn. J. Am. Chem. Soc. 1988, 110 (22), 7398– 7411,  DOI: 10.1021/ja00230a021
      39
      Synthesis, crystal structures, reactivity, and magnetochemistry of a series of binuclear complexes of manganese(II), -(III), and -(IV) of biological relevance. The crystal structure of [L'MnIV(μ-O)3MnIVL'](PF6)2.H2O containing an unprecedented short Mn···Mn distance of 2.296 Å
      Wieghardt, Karl; Bossek, Ursula; Nuber, Bernhard; Weiss, Johannes; Bonvoisin, J.; Corbella, M.; Vitols, S. E.; Girerd, J. J.
      Journal of the American Chemical Society (1988), 110 (22), 7398-411CODEN: JACSAT; ISSN:0002-7863.
      The disproportionation reactions of Mn2O(OAc)2Q2 [I; Q = 1,4,7-triazacyclononane (L); N,N',N''-trimethyl-1,4,7-triazacyclononane (L1)], in which Q are capping ligands, in aq. soln. under anaerobic conditions lead to a variety of novel binuclear MnIIIMnIV and MnIV2 dimers. [L2MnIIIMnIV(μ-O)2(μ-OAc)][BPh4]2.CH3CN, [L12MnIIIMnIV(μ-O)(μ-OAc)2](ClO4)3, [L2MnIV2(OH)(μ-O)2][MnII3(C2O4)4(OH2)2].6H2O (II), and [L12MnIV2(μ-O)3](PF6)2.H2O (III) were formed. [L4MnIV4O6]Br4.5.5H2O (IV) is generated as a thermodynamically very stable product from a MnII contg. aq. soln. of L in the presence of O. In the absence of O MeOH solns. of Mn(ClO4)2.2H2O or Mn(OAc)2 react with L1 to form [L12MnII2(μ-OH)(μ-OAc)2](ClO4) and [L12MnII2(μ-OAc)3][BPh4] (V). The oxo- and acetato-bridges in I are labile; addn. of anions X- (X = Cl, Br, NCS, N3) to MeCN solns. of I yields the monomers LMnX3 and L1MnX3. The electrochem. of all compds. was studied; e.g., I (Q = L1) is reversibly oxidized by 2 1-electron processes to generate MnIIIMnIV and MnIV2 dimers in liq. SO2. The crystal structures of II, III, IV and V were detd. by x-ray crystallog. V is orthorhombic Pcab; II is monoclinic C2/c, IV is monoclinic P21/c, and III is orthorhombic Pnma. VII consists of the cofacial bioctahedral cation [L1MnIV(μ-O)3MnIVL1]2+ and PF6- anions. The Mn...Mn distance is unusually short (2.296(2) Å). Bulk magnetic properties of all compds. were studied at 100-298 K, and in some instances 4-298 K. In I (Q = L1) the Mn(III) ions are ferromagnetically coupled, J = +18 (1) cm-1; whereas the MnII centers in V are weakly antiferromagnetically coupled, J = -3.5(2) cm-1. Very strong intramol. antiferromagnetic coupling is obsd. in III (J = -780 cm-1).
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    40. 40
      Hage, R.; Krijnen, B.; Warnaar, J. B.; Hartl, F.; Stufkens, D. J.; Snoeck, T. L. Proton-Coupled Electron-Transfer Reactions in [MnIV2(μ-O)3L′2]2+ (L′ = 1,4,7-Trimethyl-1,4,7-Triazacyclononane). Inorg. Chem. 1995, 34 (20), 4973– 4978,  DOI: 10.1021/ic00124a010
      40
      Proton-Coupled Electron-Transfer Reactions in [MnIV2(μ-O)3L'2]2+ (L' = 1,4,7-Trimethyl- 1,4,7-triazacyclononane)
      Hage, Ronald; Krijnen, Bert; Warnaar, Johann B.; Hartl, Frantisek; Stufkens, Derk J.; Snoeck, Theo L.
      Inorganic Chemistry (1995), 34 (20), 4973-8CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The pKa value of [MnIV2(μ-O)3L'2]2+ (L' = 1,4,7-trimethyl-1,4,7-triazacyclononane) was detd. spectrophotometrically by carrying out titrn. expts. with concd. sulfuric acid. The extremely low pKa value of -2.0 suggests that the electron d. on the bridging oxygen atoms is very small. The asym. Mn-O-Mn vibration is obsd. at 670 cm-1, while the sym. Mn-O-Mn vibration is present at 702 cm-1. The unusually high frequencies of these vibrations are due to the small Mn-O-Mn angle of 78°. Protonation of an oxygen bridge shifts both the asym. and sym. vibrations to 683 cm-1. Electrochem. expts. in acetonitrile showed that 1-electron redn. of the complex is chem. irreversible. IR, EPR, and UV-visible studies of the reduced species suggest a MnIIIMnIV(μ-O)2(μ-OH) core. PH-dependent differential pulse voltammetry expts. in aq. solns. revealed an apparent pKa value of ∼4.0 for the reduced mixed-valence species in various buffer systems. The redn. wave at pH > 4 is obsd. at ∼-0.10 V vs. SCE. Cyclic voltammetry revealed that the reduced species is prone to reaction with carboxylate groups. A bis(carboxylate)mono-oxo-bridged Mn(III)-Mn(III) species is formed in citric acid buffer which exhibits an anodic peak around +0.6 V vs. SCE, and a UV-visible spectrum that is typical of such a species.
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    41. 41
      Angelone, D.; Abdolahzadeh, S.; de Boer, J. W.; Browne, W. R. Mechanistic Links in the In-Situ Formation of Dinuclear Manganese Catalysts, H2O2 Disproportionation, and Alkene Oxidation. Eur. J. Inorg. Chem. 2015, 2015 (21), 3532– 3542,  DOI: 10.1002/ejic.201500195
      41
      Mechanistic Links in the in-situ Formation of Dinuclear Manganese Catalysts, H2O2 Disproportionation, and Alkene Oxidation
      Angelone, Davide; Abdolahzadeh, Shaghayegh; de Boer, Johannes W.; Browne, Wesley R.
      European Journal of Inorganic Chemistry (2015), 2015 (21), 3532-3542CODEN: EJICFO; ISSN:1434-1948. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The oxidn. of substrates, such as alkenes, with H2O2 and the catalyst [MnIV2(μ-O)3(tmtacn)2]2+ (1; tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) is promoted by the addn. of carboxylic acids through the in situ formation of bis(carboxylato) complexes of the type [MnIII2(μ-O)(μ-RCO2)2(tmtacn)2]2+. The conversion of 1 to these complexes requires a complex series of redox reactions coupled with the overall exchange of μ-oxido ligands for μ-carboxylato ligands. Here, we show that the mechanism by which this conversion occurs holds implications with regard to the species that is directly engaged in the catalytic oxidn. of alkenes. Through a combination of UV/Vis absorption, Raman, resonance Raman and ESR spectroscopy, it is shown that the conversion proceeds by an autocatalytic mechanism and that the species that engages in the oxidn. of org. substrates also catalyzes H2O2 decompn., and the former process is faster.
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    42. 42
      Chin Quee-Smith, V.; DelPizzo, L.; Jureller, S. H.; Kerschner, J. L.; Hage, R. Synthesis, Structure, and Characterization of a Novel Manganese(IV) Monomer, [MnIV(Me3TACN)(OMe)3](PF6) (Me3TACN = N,N ′,N ″-Trimethyl-1,4,7-Triazacyclononane), and Its Activity toward Olefin Oxidation with Hydrogen Peroxide. Inorg. Chem. 1996, 35 (22), 6461– 6465,  DOI: 10.1021/ic951522w
      42
      Synthesis, Structure, and Characterization of a Novel Manganese(IV) Monomer, [Mn(IV)(Me(3)TACN)(OMe)(3)](PF(6)) (Me(3)TACN = N,N',N"-Trimethyl-1,4,7-triazacyclononane), and Its Activity toward Olefin Oxidation with Hydrogen Peroxide
      Chin Quee-Smith Vikki; DelPizzo Lisa; Jureller Sharon H.; Kerschner Judith L.; Hage Ronald
      Inorganic chemistry (1996), 35 (22), 6461-6465 ISSN:.
      A novel manganese(IV) monomer, [Mn(IV)(Me(3)TACN)(OMe)(3)](PF(6)), has been synthesized in methanol by the reaction of MnCl(2) with the ligand, N,N',N"-trimethyl-1,4,7-triazacyclononane (Me(3)TACN), in the presence of Na(2)O(2). The resulting product was isolated as the red/brown crystalline hexafluorophosphate salt. The compound crystallizes in the space group P2/c with the cell dimensions a = 15.652(2) ÅA, b = 8.740(1) ÅA, c = 15.208(2) ÅA, beta = 108.81(1) degrees, V = 1969.4(4) ÅA(3), and Z = 4. The structure was solved by the heavy-atom method and was refined by full-matrix least-squares techniques to a final value of R = 0.067 (R(w) = 0.097) based upon 3087 observations. The manganese atom in the molecule is six-coordinate in an N(3)O(3) ligand environment with the triazacyclononane facially coordinated. Pertinent average bond distances and angles are as follows: Mn-O, 1.797(5) ÅA; Mn-N, 2.116(5) ÅA; O-Mn-O, 97.8(2) degrees; N-Mn-N, 81.4(2) degrees; O-Mn-N, 167.8 degrees (2); O-Mn-N, 86.8(2) degrees; O-Mn-N, 92.8(2) degrees. The complex was further characterized by UV-vis and EPR spectroscopies, solution magnetic susceptibility measurements, FAB-MS, and electrochemistry. [Mn(IV)(Me(3)TACN)(OMe)(3)](PF(6)) was found to catalyze the oxidation of water-soluble olefins using hydrogen peroxide as the oxidant in an aqueous medium. The catalyzed rates of oxidation of these olefins indicate at least a 12-fold rate enhancement over oxidant alone. The unusual stability of the catalytic species was demonstrated by the repeated additions of substrate and oxidant while maintaining a constant catalytic rate of oxidation.
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    43. 43
      Padamati, S. K.; Angelone, D.; Draksharapu, A.; Primi, G.; Martin, D. J.; Tromp, M.; Swart, M.; Browne, W. R. Transient Formation and Reactivity of a High-Valent Nickel(IV) Oxido Complex. J. Am. Chem. Soc. 2017, 139 (25), 8718– 8724,  DOI: 10.1021/jacs.7b04158
      43
      Transient Formation and Reactivity of a High-Valent Nickel(IV) Oxido Complex
      Padamati, Sandeep K.; Angelone, Davide; Draksharapu, Apparao; Primi, Gloria; Martin, David J.; Tromp, Moniek; Swart, Marcel; Browne, Wesley R.
      Journal of the American Chemical Society (2017), 139 (25), 8718-8724CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A reactive high-valent dinuclear Ni(IV) oxido-bridged complex is reported that can be formed at room temp. by reaction of [(L)2Ni(II)2(μ-X)3]X (X = Cl or Br) with NaOCl in MeOH or MeCN (L = 1,4,7-trimethyl-1,4,7-triazacyclononane). The unusual Ni(IV) oxido species is stabilized within a dinuclear tris-μ-oxido-bridged structure as [(L)2Ni(IV)2(μ-O)3]2+. Its structure and its reactivity with org. substrates are demonstrated through a combination of UV-visible absorption, resonance Raman, 1H NMR, EPR, and x-ray absorption (near-edge) spectroscopy, ESI mass spectrometry, and DFT methods. The identification of a Ni(IV)-O species opens opportunities to control the reactivity of NaOCl for selective oxidns.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXpt1Kmsrk%253D&md5=0ecf8402272f05f8b1df4dcca4267fae
    44. 44
      Niemann, A.; Bossek, U.; Wieghardt, K.; Butzlaff, C.; Trautwein, A. X.; Nuber, B. A New Structure–Magnetism Relationship for Face-Sharing Transition-Metal Complexes with D3–D3 Electronic Configuration. Angew. Chem., Int. Ed. Engl. 1992, 31 (3), 311– 313,  DOI: 10.1002/anie.199203111
      There is no corresponding record for this reference.
    45. 45
      Brown, P. L.; Ellis, J.; Sylva, R. N. The Hydrolysis of Metal Ions. Part 6. Scandium(III). J. Chem. Soc., Dalton Trans. 1983, 35– 36,  DOI: 10.1039/dt9830000035
      45
      The hydrolysis of metal ions. Part 6. Scandium(III)
      Brown, Paul L.; Ellis, John; Sylva, Ronald N.
      Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1972-1999) (1983), (1), 35-6CODEN: JCDTBI; ISSN:0300-9246.
      A potentiometric titrn. technique was used to study the hydrolysis of Sc3+ in 0.10 M KNO3 at 25°. Anal. of the results indicates the presence of Sc(OH)2+, Sc2(OH)24+, and Sc3(OH)54+; the resp. formation consts. are given.
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    46. 46
      Miao, C.; Wang, B.; Wang, Y.; Xia, C.; Lee, Y.-M. M.; Nam, W.; Sun, W. Proton-Promoted and Anion-Enhanced Epoxidation of Olefins by Hydrogen Peroxide in the Presence of Nonheme Manganese Catalysts. J. Am. Chem. Soc. 2016, 138 (3), 936– 943,  DOI: 10.1021/jacs.5b11579
      46
      Proton-Promoted and Anion-Enhanced Epoxidation of Olefins by Hydrogen Peroxide in the Presence of Nonheme Manganese Catalysts
      Miao, Chengxia; Wang, Bin; Wang, Yong; Xia, Chungu; Lee, Yong-Min; Nam, Wonwoo; Sun, Wei
      Journal of the American Chemical Society (2016), 138 (3), 936-943CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      In the presence of nonracemic nonheme manganese complex I and H2SO4, alkenes, particularly trans-chalcones such as (E)-PhCOCH:CHPh and chromenes, underwent chemo-, diastereo-, and enantioselective epoxidn. with H2O2 or other oxygen sources to yield nonracemic epoxides such as II. The yields of epoxides as well as the chemo- and enantioselectivities increase dramatically in the presence of H2SO4; no formation of epoxides is obsd. in the absence of H2SO4. Epoxide yields and enantioselectivities depended strongly on the manganese catalysts and Bronsted acids used. The catalytic epoxidn. of olefins by other oxidants, such as peracids, alkyl hydroperoxides, and iodosylbenzene, was also affected by the presence of H2SO4; product yields and enantioselectivities are high and similar irresp. of the oxidants in the presence of H2SO4, suggesting that a common epoxidizing intermediate is generated in the reactions of I. Mechanistic studies, performed with 18O-labeled water (H218O) and cumyl hydroperoxide, reveal that a high-valent manganese-oxo species is formed as an epoxidizing intermediate via O-O bond heterolysis of manganese peroxide species. The role of H2SO4 is proposed to facilitate the formation of a high-valent Mn-oxo species and to increase the oxidizing power and enantioselectivity of the Mn-oxo oxidant in olefin epoxidn. reactions. D. functional theory (DFT) calcns. support exptl. results such as the formation of a Mn(V)-oxo species as an epoxidizing intermediate. The structure of I was detd. by X-ray crystallog.
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    47. 47
      de Boer, J. W.; Brinksma, J.; Browne, W. R.; Meetsma, A.; Alsters, P. L.; Hage, R.; Feringa, B. L. Cis-Dihydroxylation and Epoxidation of Alkenes by [Mn2O(RCO2)2(Tmtacn)2]: Tailoring the Selectivity of a Highly H2O2-Efficient Catalyst. J. Am. Chem. Soc. 2005, 127 (22), 7990– 7991,  DOI: 10.1021/ja050990u
      47
      cis-Dihydroxylation and Epoxidation of Alkenes by [Mn2O(RCO2)2(tmtacn)2]: Tailoring the Selectivity of a Highly H2O2-Efficient Catalyst
      De Boer, Johannes W.; Brinksma, Jelle; Browne, Wesley R.; Meetsma, Auke; Alsters, Paul L.; Hage, Ronald; Feringa, Ben L.
      Journal of the American Chemical Society (2005), 127 (22), 7990-7991CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The carboxylic acid promoted cis-dihydroxylation and epoxidn. of alkenes catalyzed by [MnIV2O3(tmtacn)2]2+ 1 employing H2O2 as oxidant is described. The use of carboxylic acids at cocatalytic levels not only is effective in suppressing the inherent catalase activity of 1, but also enables the tuning of the catalyst's selectivity. Spectroscopic studies and X-ray anal. confirm that the control arises from the in situ formation of carboxylate-bridged dinuclear complexes, for example, 2 {[MnIII2O(CCl3CO2)2(tmtacn)2]2+} and 3 {[MnII2(OH)(CCl3CO2)2(tmtacn)2]+}, during catalysis. For the first time, the possibility to tune, through the carboxylate ligands employed, both the selectivity and activity of dinuclear Mn-based catalysts is demonstrated. To our knowledge, the system 1/2,6-dichlorobenzoic acid (up to 2000 turnover nos. for cis-cyclooctanediol) is the most active Os-free cis-dihydroxylation catalyst reported to date.
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    48. 48
      Dang, T. T.; Boeck, F.; Hintermann, L. Hidden Brønsted Acid Catalysis: Pathways of Accidental or Deliberate Generation of Triflic Acid from Metal Triflates. J. Org. Chem. 2011, 76 (22), 9353– 9361,  DOI: 10.1021/jo201631x
      48
      Hidden Bronsted Acid Catalysis: Pathways of Accidental or Deliberate Generation of Triflic Acid from Metal Triflates
      Dang, Tuan Thanh; Boeck, Florian; Hintermann, Lukas
      Journal of Organic Chemistry (2011), 76 (22), 9353-9361CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
      The generation of a hidden Bronsted acid as a true catalytic species in hydroalkoxylation reactions from metal precatalysts has been clarified in case studies. The mechanism of triflic acid (CF3SO3H or HOTf) generation starting either from AgOTf in 1,2-dichloroethane (DCE) or from a Cp*RuCl2/AgOTf/phosphane combination in toluene has been elucidated. The deliberate and controlled generation of HOTf from AgOTf and cocatalytic amts. of tert-Bu chloride in the cold or from AgOTf in DCE at elevated temps. results in a hidden Bronsted acid catalyst useful for mechanistic control expts. or for synthetic applications.
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    49. 49
      Sletten, E. T.; Tu, Y. J.; Schlegel, H. B.; Nguyen, H. M. Are Brønsted Acids the True Promoter of Metal-Triflate-Catalyzed Glycosylations? A Mechanistic Probe into 1,2- Cis-Aminoglycoside Formation by Nickel Triflate. ACS Catal. 2019, 9 (3), 2110– 2123,  DOI: 10.1021/acscatal.8b04444
      49
      Are Bronsted Acids the True Promoter of Metal-Triflate-Catalyzed Glycosylations? A Mechanistic Probe into 1,2-cis-Aminoglycoside Formation by Nickel Triflate
      Sletten, Eric T.; Tu, Yi-Jung; Schlegel, H. Bernhard; Nguyen, Hien M.
      ACS Catalysis (2019), 9 (3), 2110-2123CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Metal triflates have been utilized to catalytically facilitate numerous glycosylation reactions under mild conditions. In some methods, the metal triflate system provides stereocontrol during the glycosylation, rather than the nature of protecting groups on the substrate. Despite these advances, the true activating nature of metal triflates remains unclear. Our findings indicated that the in situ generation of trace amts. of triflic acid from metal triflates can be the active catalyst species in the glycosylation. This fact has been mentioned previously in metal-triflate-catalyzed glycosylation reactions; however, a thorough study on the subject and its implications on stereoselectivity has yet to be performed. Exptl. evidence from control reactions and 19F NMR spectroscopy have been obtained to confirm and quantify the triflic acid released from nickel triflate, for which it is of paramount importance in achieving a stereoselective 1,2-cis-2-amino glycosidic bond formation via a transient anomeric triflate. A putative intermediate resembling that of a glycosyl triflate has been detected using variable temp. NMR (1H and 13C) expts. These observations, together with d. functional theory calcns. and a kinetic study, corroborate a mechanism involving triflic-acid-catalyzed stereoselective glycosylation with N-substituted trifluoromethylbenzylideneamino-protected electrophiles. Specifically, triflic acid facilitates formation of a glycosyl triflate intermediate which then undergoes isomerization from the stable α-anomer to the more reactive β-anomer. Subsequent SN2-like displacement of the reactive anomer by a nucleophile is highly favorable for the prodn. of 1,2-cis-2-aminoglycosides. Although there is a previously reported work regarding glycosyl triflates, none of these reports have been confirmed to come from the counterion of the metal center. Our work provides supporting evidence for the induction of a glycosyl triflate through the role of triflic acid in metal-triflate-catalyzed glycosylation reactions.
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    50. 50
      Chen, J.; Goforth, S. K.; McKeown, B. A.; Gunnoe, T. B. Brønsted Acid-Catalysed Intramolecular Hydroamination of Unactivated Alkenes: Metal Triflates as an in Situ Source of Triflic Acid. Dalt. Trans. 2017, 46 (9), 2884– 2891,  DOI: 10.1039/C6DT04710F
      There is no corresponding record for this reference.
    51. 51
      Dumeunier, R.; Markó, I. E. On the Role of Triflic Acid in the Metal Triflate-Catalysed Acylation of Alcohols. Tetrahedron Lett. 2004, 45 (4), 825– 829,  DOI: 10.1016/j.tetlet.2003.11.034
      51
      On the role of triflic acid in the metal triflate-catalysed acylation of alcohols
      Dumeunier, Raphael; Marko, Istvan E.
      Tetrahedron Letters (2004), 45 (4), 825-829CODEN: TELEAY; ISSN:0040-4039. (Elsevier Science B.V.)
      The acylation of alcs. by anhydrides, catalyzed by a wide range of metal triflates, is a powerful and mild method for the prepn. of a variety of esters. Mechanistic insights demonstrate that triflic acid is generated under these reaction conditions and that, at least, two competing catalytic cycles are operating at the same time: a rapid one involving triflic acid and a slower one involving the metal triflate.
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    52. 52
      Wabnitz, T. C.; Yu, J.-Q.; Spencer, J. B. Evidence That Protons Can Be the Active Catalysts in Lewis Acid Mediated Hetero-Michael Addition Reactions. Chem. - Eur. J. 2004, 10 (2), 484– 493,  DOI: 10.1002/chem.200305407
      52
      Evidence that protons can be the active catalysts in Lewis acid mediated hetero-Michael addition reactions
      Wabnitz, Tobias C.; Yu, Jin-Quan; Spencer, Jonathan B.
      Chemistry - A European Journal (2004), 10 (2), 484-493CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The mechanism of Lewis acid catalyzed hetero-Michael addn. reactions of weakly basic nucleophiles to α,β-unsatd. ketones was investigated. Protons, rather than metal ions, were identified as the active catalysts. Other mechanisms have been ruled out by analyses of side products and of stoichiometric enone-catalyst mixts. and by the use of radical inhibitors. No evidence for the involvement of π-olefin-metal complexes or for carbonyl-metal-ion interactions was obtained. The reactions did not proceed in the presence of the non-coordinating base 2,6-di-tert-butylpyridine. An excellent correlation of catalytic activities with cation hydrolysis consts. was obtained. Different reactivities of mono- and dicarbonyl substrates have been rationalized. A 1H NMR probe for the assessment of proton generation was established and Lewis acids have been classified according to their propensity to hydrolyze in org. solvents. Bronsted acid-catalyzed conjugate addn. reactions of nitrogen, oxygen, sulfur and carbon nucleophiles are developed and implications for asym. Lewis acid catalysis are discussed.
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    53. 53
      Kemp, R. W.; Hage, R.; Zhao, W.; Zhang, J.; Jiang, Y.; Xie, H. Catalysts. WO2013/033864, 2009.
      There is no corresponding record for this reference.

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