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Neptunium Pyridine Dipyrrolide Complexes
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  • Leyla R. Valerio
    Leyla R. Valerio
    Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
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  • Andrew W. Mitchell
    Andrew W. Mitchell
    H. C. Brown Laboratory, James Tarpo Jr. and Margaret Tarpo Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
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  • Lauren M. Lopez
    Lauren M. Lopez
    H. C. Brown Laboratory, James Tarpo Jr. and Margaret Tarpo Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
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  • Matthias Zeller
    Matthias Zeller
    H. C. Brown Laboratory, James Tarpo Jr. and Margaret Tarpo Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
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    Orcidhttps://orcid.org/0000-0002-3305-852X
  • Suzanne C. Bart*
    Suzanne C. Bart
    H. C. Brown Laboratory, James Tarpo Jr. and Margaret Tarpo Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-8918-9051
  • Ellen M. Matson*
    Ellen M. Matson
    Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0003-3753-8288
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Organometallics

Cite this: Organometallics 2025, 44, 2, 439–446
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https://doi.org/10.1021/acs.organomet.4c00472
Published January 9, 2025

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Abstract

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Two pyridine dipyrrolide neptunium(IV) complexes, (MesPDPPh)NpCl2(THF) and Np(MesPDPPh)2, where (MesPDPPh)2– is the doubly deprotonated form of 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine, have been prepared. Characterization of the complexes has been performed through a combination of solid- and solution-state methods, including single-crystal X-ray diffraction and electronic absorption and nuclear magnetic resonance spectroscopies. Collectively, these data confirm the formation of the mono- and bis-ligated species. Electrochemistry of a series of bis-ligated actinide complexes, An(MesPDPPh)2 (An = Th, U, Np), is presented.

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Introduction

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The nonaqueous chemistry of actinide ions is motivated by the ability to elucidate fundamental bonding and electronic structure trends not possible in aqueous environments. (1−5) An advantage of this relatively nonpolar environment is that it facilitates the study of a variety of organic ligands to encapsulate actinide ions in a wide range of oxidation states. Understanding the chemistry of actinides in nonaqueous environments is important in separations chemistry for nuclear fuel reprocessing, where there are commonly aqueous/organic interfaces from which ions must be extracted. (1,2) The compositional complexity of spent nuclear fuel mixtures warrants a comparison between the electronic properties of isostructural actinide complexes. (6−8)
The majority of nonaqueous actinide studies have focused on U and Th, whereas far less progress has been made for transuranium elements due to their reduced availability and comparatively high specific activities. (1,6) The chemistry of neptunium (237Np) is a burgeoning area in recent years because of its intriguing solution-phase redox chemistry that can span oxidation states of +2 to +7. (9) Neptunium also lends itself to stabilization by a variety of ligand frameworks that can be modified to tune the electronic properties of the central ion. Early examples include “classic” organometallic ligands such as the cyclopentadienyl (Cp = (C5H5)−) anion and cyclooctatetraene dianion COT (C8H82–). (9,10) More recently, such studies have been expanded to include the dipyrrolide, dianionic macrocycle trans-calix[2]benzene[2]pyrrolide2–, which has been used for the complexation of Th(IV), U(IV), and Np(IV) cations, where the steric protection imparted by the bulky macrocycle has allowed for facile reduction of the uranium and neptunium ions. (11,12) Sessler and co-workers have additionally described the synthesis and structural characterization of a series of Th(IV), U(IV), and Np(IV) complexes featuring coordination to dipyriamethyrin. (13) Such series have also been developed in collaboration with Albrecht-Schönzart, where the synthesis of the dioxophenoxazine ligand, DOPO (DOPO = 2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazine-9-olate), was reported, including derivatives of Th(IV), U(IV), Np(IV), and Pu(IV), which featured ligand radicals due to the highly reducing actinides and redox-active nature of the dioxophenoxazine ligand. (14)
Of late, a subset of tridentate pincer ligands, pyridine dipyrrolides (PDPs), in the dianionic, doubly deprotonated form, have attracted attention as redox-active ligands for transition metal, main group, and actinide elements. (15−28) Indeed, the reported complexes have been demonstrated to show great promise in the fields of photochemistry and catalysis. For example, the group(IV)-derived photosensitizer Zr(MesPDPPh)2, where (MesPDPPh)2– is the doubly deprotonated form of 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine, was reported to have a long-lived triplet excited state that exhibited photoluminescence (ΦPL = 0.45) that could be utilized for photoredox catalysis. (25) Recent efforts have focused on the isolation of heavier element congeners of the Zr(MesPDPPh)2 complex, notably Hf(IV), (29) Sn(IV), (16) and Th(IV) (28) to investigate the effect of heavy atoms on the photophysical properties of the complexes.
Recently, some of us have reported the synthesis and characterization of high-valent uranyl and U(IV) and Th(IV) adducts of the pyridine dipyrrolide ligand class. (27,28) It was determined that the redox-active PDP ligand is capable of stabilizing various actinide ion oxidation states. An early report details the synthesis of uranyl adducts of the PDP ligand (MesPDPPh)2– and (Cl2PhPDPPh)2–. (27,28) Upon investigation of the electrochemical properties of (PDP)UO2(THF) complexes using cyclic voltammetry (CV), reversible UVI/UV reduction couples at modest reduction potentials (−1.22 to −1.15 V vs Fc+/0) were revealed, suggesting that reduction of the metal ion should be facile. Moreover, the PDP ligand was observed to coordinate midvalent U(IV) and Th(IV) cations, forming complexes with unique optical properties. (28) The absorption spectra of the uranium derivatives were dominated by ligand-to-metal charge transfer (LMCT) transitions, whereas the thorium complexes exhibited exclusively intraligand charge transfer (ILCT) and ligand-to-ligand charge transfer (LLCT) transitions. The thorium derivatives were also photoluminescent, with long-lived excited states (0.250–0.300 ms) and high quantum efficiencies between 40 and 45%.
Herein, we extend our studies with the PDP ligand from early actinides to neptunium by the synthesis and spectroscopic characterization of (MesPDPPh)NpCl2(THF) (1-Np) and Np(MesPDPPh)2 (2-Np). Electrochemical studies of 2-Np in comparison to the Th and U congeners reveal systematic shifts in the An(IV)/An(III) couples, where Np(III) derivatives of these molecules are more easily accessed due to the less negative overall reduction potential of Np ions (vs Th, U). (30) From these studies, we aim to develop a series of PDP-actinide compounds that allow us to probe the influence of increasing the f-electron count on their spectroscopic and electrochemical properties.

Experimental Section

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General Considerations

All air- and moisture-sensitive manipulations with Np were performed using standard Schlenk techniques or in an MBraun negative-pressure argon atmosphere drybox. The MBraun drybox was equipped with a cold well and a −35 °C freezer for cooling samples and crystallizations. All air- and moisture-sensitive manipulations with thorium or uranium were carried out using a standard high-vacuum line, Schlenk techniques, or an MBraun inert atmosphere drybox containing an atmosphere of purified dinitrogen. Solvents for sensitive manipulations were dried and deoxygenated using literature procedures with a Seca solvent purification system or a glass contour solvent purification system (Pure Process Technology, LLC) and stored over activated 4 Å molecular sieves (Fisher Scientific) prior to use. Benzene-d6 was purchased from Cambridge Isotope Laboratories, degassed by three freeze–pump–thaw cycles, and dried with molecular sieves. NpCl4(DME)2, (31) UCl4, (32) ThCl4(DME)2, (33) and H2(MesPDPPh) (19) were prepared according to literature procedures. All other chemicals were purchased from commercial sources and used without further purification.

Safety Considerations

Caution! 237Np represents a health risk due to its α and γ emission and its decay to the short-lived 233Pa isotope (t1/2 = 27.0 days), which is a strong β and γ emitter. All studies with Np were conducted in a laboratory equipped for radioactive materials. All studies were modeled on depleted uranium prior to working with 237Np. Depleted uranium (primary isotope 238U) is a weak α-emitter (4.197 MeV) with a half-life of 4.47 × 109 years, and 232Th is a weak α-emitter (4.082 MeV) with a half-life of 1.41 × 1010 years; manipulations and reactions should be carried out in monitored fume hoods or in an inert atmosphere drybox in a radiation laboratory equipped with α and β counting equipment. Due to the radiological hazards associated with working with 237Np and the subsequent need to recover neptunium material for recycling measures, elemental analyses of the neptunium complexes reported in this study were not obtained.

Synthesis of (MesPDPPh)NpCl2(THF) (1-Np)

In the glovebox, H2MesPDPPh (0.010 g, 0.017 mmol, 1 equiv) was dissolved in approximately 2 mL of toluene in a 20 mL scintillation vial equipped with a magnetic stirrer. In a separate vial, LiHMDS (0.006 g, 0.034 mmol, 2.05 equiv) was dissolved in 2 mL of toluene and added dropwise to the stirring H2MesPDPPh solution, to make Li2MesPDPPh. The resultant solution was stirred for 1 h. In a third 20 mL scintillation vial, NpCl4(DME)2 (0.010 g, 0.018 mmol, 1 equiv) was dissolved in ∼0.5 mL of THF, and to it was added the Li2MesPDPPh solution while stirring. The reaction was stirred at room temperature for 2 h, and the resultant red solution was filtered over Celite with a glass microfiber plug and dried in vacuo. The solid was triturated with pentane until washings ran clear and dried again, producing the title compound. Yield: 63%. Red single crystals were obtained by vapor diffusion of pentane into a saturated solution of the product in toluene at −30 °C. 1H NMR (500 MHz, C6D6) δ 4.48 (4H), 4.22 (4H), 0.09 (5H), −0.23 (5H), −0.55 (5H), −0.90 (8H), −2.04 (2H), −4.33 (12H), −9.25 (4H), −16.10 (6H), −25.00 (4H).

Synthesis of Np(MesPDPPh)2 (2-Np)

In the glovebox, H2MesPDPPh (0.021 g, 0.036 mmol, 1 equiv) was dissolved in approximately 2 mL of toluene in a 20 mL scintillation vial equipped with a magnetic stirrer. In a separate vial, LiHMDS (0.012 g, 0.072 mmol, 2.05 equiv) was dissolved in 1 mL of toluene and added dropwise to the stirring H2MesPDPPh solution, to make Li2MesPDPPh. The resultant solution was stirred for 1 h. In a third 20 mL scintillation vial, NpCl4(DME)2 (0.010 g, 0.018 mmol, 0.5 equiv) was suspended in ∼1 mL of toluene and to it was added the Li2MesPDPPh solution while stirring. The reaction was heated at ∼90 °C for 6 h. The product was cooled to room temperature and filtered over a bed of Celite with a glass microfiber plug. The resultant red solution was dried in vacuo, triturated with pentane until washings ran clear, and dried again, producing the title compound. Yield: 70%. 1H NMR (400 MHz, C6D6) δ 14.63 (t, J = 7.8 Hz, 2H), 13.94 (d, J = 8.1 Hz, 4H), 11.06 (s, 4H), 8.34 (d, J = 7.5 Hz, 8H), 7.97 (t, J = 7.5 Hz, 8H), 7.61 (t, J = 7.5 Hz, 4H), 7.06 (s, 8H), 3.28 (s, 12H), −3.40 (s, 24H).

Synthesis of Np(PhPDPPh)2 (3-Np)

In the glovebox, H2PhPDPPh (0.021 g, 0.036 mmol, 1 equiv) was dissolved in approximately 2 mL of toluene in a 20 mL scintillation vial equipped with a magnetic stirrer. In a separate vial, LiHMDS (0.012 g, 0.072 mmol, 2.05 equiv) was dissolved in 1 mL of toluene and added dropwise to the stirring H2PhPDPPh solution, to make Li2PhPDPPh. The resultant solution was stirred for 1 h. In a third 20 mL scintillation vial, NpCl4(DME)2 (0.010 g, 0.018 mmol, 0.5 equiv) was suspended in ∼1 mL of toluene and to it was added the Li2PhPDPPh solution while stirring. The reaction was stirred for 12 h at room temperature. The product was filtered over a bed of Celite with a glass microfiber plug. The resultant red solution was dried in vacuo, triturated with pentane until washings ran clear, and dried again, producing the title compound. Yield: 74%. 1H NMR (400 MHz, C6D6) δ 16.09 (d, J = 8.1 Hz, 4H), 15.43 (t, J = 7.8 Hz, 2H), 11.34 (s, 4H), 9.73 (d, J = 7.5 Hz, 8H), 8.48 (t, J = 7.5 Hz, 8H), 8.05 (dt, J = 19.3, 7.2 Hz, 8H), 7.02 (t, J = 6.8 Hz, 4H), 6.84 (d, J = 6.8 Hz, 8H), −14.52 (s, 8H).

Physical Measurements

1H NMR spectra for neptunium compounds were recorded at room temperature on a Bruker AV-III-HD-400 spectrometer operating at 400.13 MHz. All chemical shifts are reported with respect to residual solvent relative to the chosen deuterated solvent as a standard. 1H spectra were collected with 0.5 s acquisition time, 0 s delay time, and a sweep width of 200 ppm and for 128 scans. 1H NMR spectra for all other compounds (Th, U) were recorded at room temperature on a 400 MHz Bruker AVANCE spectrometer or a 500 MHz Bruker AVANCE spectrometer locked on the signal of deuterated solvents. All chemical shifts are reported relative to the chosen deuterated solvent as a standard. Cyclic voltammetry (CV) on neptunium complexes was performed using a three-electrode setup inside a negative-pressure argon glovebox (MBraun UniLab) using a CH Instruments 620E potentiostat. Cyclic voltammetry (CV) on thorium and uranium complexes was performed using a three-electrode setup inside a nitrogen-filled glovebox (MBraun UniLab) using a Bio-Logic SP 150 potentiostat/galvanostat and the EC-Lab software suite. The concentrations of the complexes and the supporting electrolyte (TBAPF6) were kept at 1 and 100 mM, respectively, throughout all measurements. CVs were recorded using a 3 mm diameter glassy carbon working electrode (CH Instruments), a Pt wire auxiliary electrode (CH Instruments), and a silver wire reference electrode with ferrocene used as an internal standard after completion of the measurements. Potentials were then referenced versus the Fc+/0 redox couple. Electronic absorption measurements for neptunium were recorded at room temperature in anhydrous dichloromethane or toluene in sealed 1 cm quartz cuvettes using a JASCO V-770 UV–vis–NIR spectrophotometer equipped with a fiber optic stage and sample holder. Electronic absorption measurements for thorium and uranium were recorded at room temperature in anhydrous dichloromethane solution in sealed 1 cm quartz cuvettes using an Agilent Cary 6000i UV–vis/NIR spectrophotometer.

X-ray Crystallography

Single crystals suitable for X-ray diffraction were coated with poly(isobutylene) oil in the glovebox and quickly transferred to the goniometer head of a Bruker Quest diffractometer with a fixed chi angle, a sealed tube fine focus X-ray tube, a single-crystal curved graphite incident beam monochromator, a Photon II area detector, and an Oxford Cryosystems low-temperature device. Examination and data collection were performed with Mo Kα radiation (λ = 0.71073 Å) at 150 K.

Results and Discussion

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Targeting a scaled-down synthesis of the monoligand U complex, (MesPDPPh)UCl2(THF) (1-U), two equivalents of LiN(SiMe3)2 were added to a solution of H2(MesPDPPh) in toluene, affording Li2(MesPDPPh) in situ. Addition of this solution to one equivalent of UCl4 dissolved in a minimal amount of THF resulted in a gradual color change to dark red after stirring at room temperature for about 2 h. Workup of the product and analysis by 1H NMR spectroscopy confirmed the formation of 1-U.
Following the successful formation of 1-U, the synthesis of the monoligand PDP Np complex was pursued. The addition of one equivalent of Li2(MesPDPPh) in toluene to a solution of NpCl4(DME)2 in THF results in a color change to dark red after stirring for 2 h at room temperature. Workup of the product afforded (MesPDPPh)NpCl2(THF) (1-Np) in 63% yield (Scheme 1; see the Experimental Section for additional details). The formation of the monoligated neptunium species was probed by 1H NMR spectroscopy. We note that 13C NMR spectroscopy was not pursued due to the paramagnetism of the Np ion. The 1H NMR spectrum revealed paramagnetically shifted and broadened resonances ranging from +5 to −25 ppm (Figure 1); the overall pattern of resonances is similar to that reported for 1-U. Analysis of the integrations in the 1H NMR spectrum revealed resonances at −4.33 and −16.10 ppm with relative integrations of 12H to 6H. These signals are assigned to the ortho-methyl and para-methyl groups of the mesityl substituent of the PDP ligand. Interestingly, these resonances are shifted significantly upfield in comparison to those reported for 1-U (9.75 and −5.59 ppm, respectively), indicating that the protons are shielded to a greater degree with the Np(IV), f3 ion coordinated. The resonance corresponding to the pyrrole protons, typically used as a spectroscopic handle for PDP compounds, was located at −2.04 ppm with a relative integration of 2H. The remaining resonances in the 1H NMR spectrum have relative integrations of 4H, but they are not baseline resolved or are overlapping with one another, making integration ineffective and precluding definitive assignments.

Figure 1

Figure 1. 1H NMR spectrum (400 MHz) of 1-Np stacked with 1-U for comparison collected in C6D6 at room temperature (∼21 °C). For full assignments, see the Supporting Information.

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Scheme 1

Scheme 1. Synthesis of (MesPDPPh)AnCl2(THF) Complexes (1-Th, 1-U, 1-Np)
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To further characterize the product, crystals of 1-Np suitable for single-crystal X-ray diffraction (SCXRD) were grown from slow diffusion of pentane into a concentrated toluene solution of the product in toluene at −30 °C. Refinement of the data confirmed the structural composition of 1-Np as the six-coordinate species (MesPDPPh)NpCl2(THF) (Figure 2 and Table 1). As observed in the crystal structure of 1-U, 1-Np crystallizes in the Pbcn space group and displays a distorted octahedral geometry at neptunium. The equatorial plane of 1-Np is occupied by the (MesPDPPh)2– ligand and a bound THF molecule, with two chloride atoms in axial positions, slightly distorted from linearity (Cl–Np–Cl bond angle = 173.14(4)o). The Np–Npyridine distance of 2.465(4) Å is similar to that of 1-U (U–Npyridine = 2.474(4) Å) but is significantly shortened in comparison to other reported Np–Npyridine bond distances. For example, the Np–Npyridine bond distances reported for NpCl4(pyr)4 are 2.686(14) Å and 2.688(14) Å and in NpCl4(tBuBipy)2 range from 2.610(3) to 2.650(3) Å. (34) Sessler and co-workers have also recently reported an Np–Npyridine bond distance of 2.635(8) Å for a Np(IV) expanded porphyrin complex, Np(IV)[dipyriamethyrin](OTMS)2. (13) Additionally, contraction of the Np–Npyrrolide bond distances in 1-Np is observed (Np–Npyrrolide = 2.305(4) and 2.304(5) Å) in comparison to the Np porphyrin complex (average Np–Npyrrolide = 2.687(8) Å). (13) These results indicate that, like the isostructural uranium complex, 1-Np binds more tightly to the (MesPDPPh)2– ligand compared to other complexes featuring neptunium–nitrogen bonds. The Np–Cl bond distances in 1-Np (2.568(13) and 2.574(14) Å) are similar to the distances of axially bound chloride atoms in NpCl4(THF)3 (2.568(2) and 2.575(2) Å) (35) and shorter than reports of equatorially bound chloride atoms (2.588(9)–2.628(9) Å). (31,34,35) Overall, the major structural differences between 1-Np, 1-U, and the isostructural thorium derivative, (MesPDPPh)ThCl2(THF) (1-Th), arise due to differences in the ionic radius of the actinide ions (Np < U < Th).

Figure 2

Figure 2. Molecular structure of (MesPDPPh)NpCl2(THF) (1-Np) is shown with 30% probability ellipsoids. Hydrogen atoms have been omitted for clarity.

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Table 1. Selected Bond Distances and Angles for (MesPDPPh)NpCl2(THF) (1-Np); Distances and Angles for (MesPDPPh)UCl2(THF) (1-U) and (MesPDPPh)ThCl2(THF) (1-Th) Are Included for Comparison (28)
complex(MesPDPPh)NpCl2(THF) (1-Np)(MesPDPPh)UCl2(THF) (1-U)(MesPDPPh)ThCl2(THF) (1-Th)
An-Cl2.5684(13), 2.5741(14) Å2.5844(13), 2.5927(14) Å2.6528(10), 2.6566(11) Å
Cl-An-Cl173.14(4)°174.04(4)°172.33(3)°
An-Npyr2.465(4) Å2.474(4) Å2.548(3) Å
An-Npyrrolide2.304(5), 2.306(4) Å2.309(5), 2.301(5) Å2.362(3), 2.358(4) Å
Previously, we found that the redox-active nature of the (MesPDPPh)2– ligand imparted interesting optical properties on the isostructural (MesPDPPh)MCl2(THF) (M = Th(IV), U(IV)) family. (28) For 1-U, evidence for LMCT was observed in the electronic absorption spectrum, signaled by low-energy transitions ranging from 400 to 550 nm that extended out to the NIR region, which was corroborated with time-dependent density-functional theory (TD-DFT) calculations in our original report. (28) In contrast, 1-Th exhibited exclusively ILCT and LLCT transitions, consistent with the very negative reduction potential and high energy of the 5f orbitals of Th(IV) ions. (30) These differences in the UV–vis spectra, in combination with the observed color change to dark red upon metalation of the PDP ligand with NpCl4(DME)2, prompted our interest in the analysis of the optical properties of 1-Np via electronic absorption spectroscopy (Figure 3). The absorption profile of (MesPDPPh)NpCl2(THF) features a relatively intense band at 450 nm (ε = 2750 M–1 cm–1), which is tentatively assigned to a ligand-to-metal charge transfer (LMCT) transition. The tail of the absorption extends far into the near-infrared region, causing the broadening of the band and resulting in the dark-red color of 1-Np. Analysis of the near-infrared region of the spectrum of 1-Np reveals f-f transitions, consistent with the assignment of a +4 oxidation state (5f3 valence electron configuration) of neptunium (Figure S6). (36,37)

Figure 3

Figure 3. Electronic absorption spectra for (MesPDPPh)NpCl2(THF) (1-Np), with 1-Th and 1-U included for comparison. Spectra were collected at room temperature in dichloromethane.

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The isostructural actinide complexes were synthesized in part to probe the influence of increasing f-electron count on the spectroscopic properties of the target complexes. As referenced previously, 1-Th contains only LLCT and ILCT transitions, whereas the energetically accessible 5f orbitals in 1-U and 1-Np (and the reducibility of these metals) result in operative LMCT transitions. The broadening of the absorption bands (and extension into the NIR region) for 1-U and 1-Np give rise to their red color, whereas the absence of these features in 1-Th results in the observed yellow color of the product. There is no discernible trend in the band position in the electronic absorption spectra as the 5f-electron count of the An(IV) cation increases (1-Th = 458 nm, 1-U = 470 nm, 1-Np = 450 nm). The red shift of the absorption maximum for 1-U compared to 1-Th is expected due to the lowering in energy of the valence 5f orbitals of uranium compared to thorium. (38) However, the blue shift (∼20 nm) of 1-Np is surprising, given that it would be expected for the 5f orbitals of neptunium to be lower in energy in comparison to that of the uranium(IV) derivative. This suggests that there is likely a larger gap between the 5f orbitals of 1-Np and the LUMO of the PDP ligand, resulting in the observed increase in the energy of the charge transfer band in the electronic absorption spectrum. Computational analysis of the electronic structure of 1-Np is required to firmly establish the nature of the electronic transitions within the complex but is beyond the scope of this initial report.
Next, the formation of a bis-ligated neptunium PDP complex was targeted, as other M(IV) analogues have displayed intriguing electrochemical and spectroscopic profiles. (16,24,25) Previously, the bis-ligand actinide compounds, M(MesPDPPh)2 (M = Th, U), were synthesized. (28) In this study, it was hypothesized that alkylation of (MesPDPPh)MCl2(THF) (M = Th(IV), U(IV)) using benzyl potassium (KCH2Ph) would generate basic actinide–carbon bonds that would drive the formation of M(MesPDPPh)2 by deprotonation of an additional equivalent of H2(MesPDPPh). To avoid having to access the organoneptunium intermediate (which can often have a negative impact on reaction yields), the synthesis of the bis-ligated PDP complex through salt metathesis was pursued, using depleted uranium as a model metal center (see the Experimental Section for details). With a new route to access the bis-ligated actinide complex in hand, an extension of this chemistry to neptunium(IV) was pursued. Addition of one equivalent of Li2(MesPDPPh) in toluene to a suspension of half an equivalent of NpCl4(DME)2 in toluene results in a gradual color change to dark red after heating to ∼90 °C for 6 h. Workup of this product afforded Np(MesPDPPh)2 (2-Np) in a 70% yield (Scheme 2).

Scheme 2

Scheme 2. Synthesis of An(MesPDPPh)2 Complexes (1-Th, 1-U, 1-Np)
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The formation of 2-Np was initially confirmed by 1H NMR spectroscopy. Nine paramagnetically shifted resonances ranging from +15 to −4 ppm were observed, consistent with the formation of a D2d-symmetric product in solution (Figure 4). Analysis of the relative integrations in the 1H NMR spectrum revealed two prominent resonances at 3.28 and −3.40 ppm with integrations of 12H and 24H, respectively. These signals are assigned to the para-methyl and ortho-methyl groups of the mesityl substituent of the PDP ligand. The 4-pyridyl proton was located at 14.63 ppm as a well-defined triplet resonance, integrating to two protons. In addition, the signal for the pyrrole protons was located as a singlet at 11.06 ppm. To our surprise, the 1H NMR spectrum of 2-Np (Np(IV), f3) is significantly different compared to that of 2-U (U(IV), f2). In 2-U, significant deshielding of the ortho-methyl protons was observed and attributed to their distance from the paramagnetic 5f2 ion, where the steric bulk imparted by the PDP ligand forces the mesityl groups closer to uranium. For 2-Np, the opposite behavior is observed, where the ortho-methyl protons are more shielded. Similarly, the para-methyl protons for 2-U are visible at −2.83 ppm in the 1H NMR spectrum versus 3.29 ppm for 2-Np.

Figure 4

Figure 4. 1H NMR spectrum (400 MHz) of 2-Np stacked with 2-U for comparison was collected in C6D6 at room temperature (∼21 °C). For full assignments, see the Supporting Information.

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To probe this phenomenon, a Np(IV) derivative utilizing a PDP ligand with phenyl substituents in place of mesityl groups, H2(PhPDPPh), was targeted in order to investigate the possibility of similar magnetic effects. Salt metathesis between Li2(PhPDPPh) and NpCl4(DME)2 afforded what is hypothesized to be Np(PhPDPPh)2 (3-Np). The 1H NMR spectrum of 3-Np (Figure S5) displays a similar resonance pattern to 2-Np, suggesting that the observed paramagnetic shifting is characteristic for these bis(ligand) neptunium complexes and not unique to 2-Np. Interestingly, it appears that the change in valence f-electrons at the actinide ion within these systems has a significant impact on the local environment of the protons. This suggests different bonding contributions of the 5f orbitals within these systems, though theoretical calculations are necessary to definitively confirm this hypothesis. Though multiple attempts were made to characterize both Np(MesPDPPh)2 (2-Np) and Np(PhPDPPh)2 (3-Np) by SCXRD, severe disorder in all suitable crystals precluded the characterization of the bis-ligated product through this method.
Analysis of the optical properties of red 2-Np was performed by electronic absorption spectroscopy (Figure 5). The near-infrared region of the spectrum reveals weak and sharp f-f transitions, consistent with retention of a + 4 oxidation state of neptunium upon coordination of two PDP ligands (Figure S7). These values are similar to reports of other Np(IV) organometallic complexes. (36,37) In the visible region, the absorption profile of 2-Np in DCM displays an intense band at 473 nm (ε = 3250 M–1 cm–1). There are notable differences upon comparison of the electronic absorption spectra of 2-Th, 2-U, and 2-Np in DCM. Like 1-Th, 2-Th displays a single intense band at 452 nm (ε = 5088 M–1 cm–1) that is assigned as a ligand-based transition, with computational analysis confirming that all filled frontier molecular orbitals are exclusively ligand-centered, with negligible contributions from the metal ion (<3%). (28) Interestingly, the position of the absorption band for 2-Np is red-shifted ∼20 nm compared to 2-Th, and it displays a similar intensity. The band positions of 2-Th and 2-Np closely resemble each other, with the most notable difference being the increase in bandwidth in 2-Np that results in an extension of absorption to ∼600 nm, responsible for the red color. The observed similarities between the thorium and neptunium complexes suggest that the electronic transition for 2-Np may contain some ligand-based charge transfer character. However, the extension of the band to lower energies, ascribed to ligand-to-metal charge transfer character in our original report, indicates that the band in 2-Np may also contain some LMCT character and that the electronic transitions within 2-Np are complicated.

Figure 5

Figure 5. Electronic absorption spectra in the visible region for Np(MesPDPPh)2 (2-Np), with 2-Th and 2-U included for comparison. Spectra were collected at room temperature in dichloromethane.

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There are significant differences when analyzing the electronic absorption spectrum of 2-U compared to 2-Np. Two bands are observed at 466 nm (ε = 1322 M–1 cm–1) and 545 nm (ε = 887 M–1 cm–1), consistent with low-energy LMCT transitions calculated with TD-DFT in our original report for the U(IV) complex. (28) It is noted that low-energy transitions in the visible region are not observed experimentally for 2-Np apart from the band at 473 nm, although it was originally hypothesized that these transitions should be more readily facile to access, given the likelihood of the increased orbital overlap between the π orbitals of the PDP ligand and the lower energy 5f orbitals of the Np ion. This would theoretically result in a lower energy LMCT state and a red shift in the absorption spectrum for 2-Np. As such, it is possible that the band at 473 nm corresponds to a ligand-to-metal charge transfer transition that is more favorable (more “allowed”) compared to the low-intensity LMCT transitions in 2-U. The results herein demonstrate that the electronic transitions of 2-Np are convoluted due to the 5f3 electronic configuration of the metal ion and likely require computational analysis to fully unravel the complicated electronic transitions.
The photophysical properties of bis-ligand PDP complexes (e.g., SnIV(MePDPPh)2 (5s05p0), (16) ZrIV(MesPDPPh)2 (4d0), (25) and HfIV(MesPDPPh)2 (5d0) (29)) have garnered attention due to their long-lived triplet excited states and high quantum efficiencies. More recently, this series was extended to 2-Th (5f0) and 2-U (5f2) to investigate the role of f-electrons in emission processes. (28) 2-Th possessed strong room-temperature photoluminescence with a high quantum yield (ΦPL = 42%) and long-lived excited state (τ = 0.304 ms), whereas 2-U was not luminescent. The emission decay pathway in 2-U was hypothesized to proceed through the 5f orbital manifold upon excitation into the LMCT band, resulting in a quenching of emission. Due to the lowering in energy of the 5f orbitals for neptunium compared to thorium and uranium, it was hypothesized that 2-Np (5f3) would also be nonluminescent like 2-U. Analysis of a sample of 2-Np under ultraviolet irradiation showed no emission from the complex, which affirms this hypothesis.
Having established a thorough understanding of the solution-phase and solid-state structures of both 2-U and 2-Np, the electrochemical properties of these compounds using cyclic voltammetry (CV) were investigated (Figure 6). It is noted that due to the electrochemical instability of 1-U under oxidizing conditions (Figure S8), the poorly resolved cyclic voltammogram precluded electrochemical analysis of 1-Np. Thus, the electrochemistry of 1-U and 1-Np is excluded from this study. However, it was hypothesized that the steric bulk imparted by the coordination of two PDP ligands to the actinide would increase the stability of the complexes, allowing for analysis of their electrochemical properties. Accordingly, CV measurements of U(MesPDPPh)2 and Np(MesPDPPh)2 were conducted on 1 mM solutions of the compounds in DCM with 0.1 M TBA(PF6) and referenced to Fc+/0. For both 2-U and 2-Np, scanning anodically revealed a reversible feature at 0.317 V (2-U) and 0.298 V (2-Np). Due to the similarity in potential, these redox events are assigned as oxidation of the PDP ligand. This is further supported by the presence of a similar redox event in the CV of MesPDPPhUO2(THF) (0.30 V vs Fc+/0), (27) where uranium is already in its highest possible oxidation state, supporting ligand-based oxidation. The oxidative event in 2-Np is shifted 20 mV relative to that of 2-U, suggesting that upon coordination of Np(IV), the PDP chelate is more readily oxidized. The cyclic voltammogram of 2-Th, in comparison, contains a pseudoreversible oxidative event at 0.521 V vs Fc+/0 (Figure 5). This oxidation is expected to be exclusively ligand-based due to the similarity in potential to 2-U and 2-Np, coupled with the lack of accessible 5f-electrons in Th(IV) (5f0).

Figure 6

Figure 6. Cyclic voltammograms of Th(MesPDPPh)2, U(MesPDPPh)2, and Np(MesPDPPh)2 recorded in DCM (1 mM analyte, 0.1 M TBAPF6, scan rate = 200 mV s–1).

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When scanning cathodically, a reversible reduction event is observed in 2-U centered at −2.06 V (vs Fc+/0) and at −1.41 V (vs Fc+/0) for 2-Np. These events are assigned as U(IV) → U(III) and Np(IV) → Np(III) reductions due to these potentials being in the expected range for these processes. (9) Notably, the reduction is shifted anodically by 0.65 V for 2-Np relative to 2-U, consistent with other reports of An(IV)/An(III) redox couples for isostructural uranium and neptunium complexes. For example, the redox potentials of An(IV)/An(III) couples in a series of CpR- complexes, Cp4An, Cp3AnCl, and Cp*2AnCl2, were measured and showed more modest reduction potentials in the neptunium derivatives compared to the uranium complexes. (39) This indicates that 2-Np can be readily reduced and the Np(III) compound should be accessible chemically with mild reducing agents.

Conclusions

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In summary, we have reported the synthesis of two new neptunium(IV) pyridine dipyrrolide complexes, MesPDPPhNpCl2(THF) (1-Np) and Np(MesPDPPh)2 (2-Np). These derivatives were made in moderate yields and fully characterized using a variety of spectroscopic techniques and, where possible, X-ray crystallography. The steric bulk imparted by the PDP chelate results in a distorted octahedral geometry around the neptunium center in the solid-state structure of 1-Np. Neptunium oxidation states were confirmed using electronic absorption spectroscopy, with data showing both species in the +4 oxidation state. The electronic absorption spectra of these compounds feature intense charge transfer bands that are hypothesized to be a mixture of ligand-to-metal charge transfer and ligand-based transitions. In DCM solution, U(MesPDPPh)2 and Np(MesPDPPh)2 display rich electrochemistry with reversible ligand-based oxidative chemistry and An(IV)/(III) reduction couples, suggesting that reduced analogues of the bis-ligated complexes may be chemically isolable. Notably, systematic shifting toward anodic potentials is observed for the neptunium complex compared to the uranium complex, indicative of the overall ease of reducibility of the Np(IV) ion compared to U(IV). With the thorium and uranium analogues being previously synthesized, we have begun to develop a series of PDP-actinide compounds that allow us to probe the influence of increasing f-electron count on their spectroscopic and electrochemical properties: Th(IV), f0, U(IV), f2, and Np(IV), f3. Future studies will focus on the reactivity of these species as well as isolation of low-valent derivatives.

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.organomet.4c00472.

  • Additional spectroscopic, crystallographic, and voltammetric data (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Leyla R. Valerio - Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
    • Andrew W. Mitchell - H. C. Brown Laboratory, James Tarpo Jr. and Margaret Tarpo Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
    • Lauren M. Lopez - H. C. Brown Laboratory, James Tarpo Jr. and Margaret Tarpo Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
    • Matthias Zeller - H. C. Brown Laboratory, James Tarpo Jr. and Margaret Tarpo Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States;  Orcidhttps://orcid.org/0000-0002-3305-852X
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Heavy Element Chemistry Program under Award Number DE-SC0020436 (E.M.M.) and Award Number DE-SC0008479 (S.C.B.). L.R.V. and A.W.M. acknowledge support from the National Science Foundation Graduate Research Fellowship Program.

References

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This article references 39 other publications.

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    Dutkiewicz, Michal S.; Farnaby, Joy H.; Apostolidis, Christos; Colineau, Eric; Walter, Olaf; Magnani, Nicola; Gardiner, Michael G.; Love, Jason B.; Kaltsoyannis, Nikolas; Caciuffo, Roberto; Arnold, Polly L.
    Nature Chemistry (2016), 8 (8), 797-802CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
    Studies of transuranic organometallic complexes provide a particularly valuable insight into covalent contributions to the metal-ligand bonding, in which the subtle differences between the transuranium actinide ions and their lighter lanthanide counterparts are of fundamental importance for the effective remediation of nuclear waste. Unlike the organometallic chem. of uranium, which has focused strongly on U(iii) and has seen some spectacular advances, that of the transuranics is significantly tech. more challenging and has remained dormant. In the case of neptunium, it is limited mainly to Np(iv). Here we report the synthesis of three new Np(iii) organometallic compds. and the characterization of their mol. and electronic structures. These studies suggest that Np(iii) complexes could act as single-mol. magnets, and that the lower oxidn. state of Np(ii) is chem. accessible. In comparison with lanthanide analogs, significant d- and f-electron contributions to key Np(iii) orbitals are obsd., which shows that fundamental neptunium organometallic chem. can provide new insights into the behavior of f-elements.
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  12. 12
    Arnold, P. L.; Farnaby, J. H.; White, R. C.; Kaltsoyannis, N.; Gardiner, M. G.; Love, J. B. Switchable π-coordination and C–H metallation in small-cavity macrocyclic uranium and thorium complexes. Chem. Sci. 2014, 5, 756– 765,  DOI: 10.1039/C3SC52072B
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    Switchable π-coordination and C-H metallation in small-cavity macrocyclic uranium and thorium complexes
    Arnold, Polly L.; Farnaby, Joy H.; White, Rebecca C.; Kaltsoyannis, Nikolas; Gardiner, Michael G.; Love, Jason B.
    Chemical Science (2014), 5 (2), 756-765CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    New, conformationally restricted ThIV and UIV complexes, [ThCl2(L)] and [UI2(L)], of the small-cavity, dipyrrolide, dianionic macrocycle trans-calix[2]benzene[2]pyrrolide (L)2- are reported and are shown to have unusual κ5:κ5 binding in a bent metallocene-type structure. Single-electron redn. of [UI2(L)] affords [UI(THF)(L)] and results in a switch in ligand binding from κ5-pyrrolide to η6-arene sandwich coordination, demonstrating the preference for arene binding by the electron-rich UIII ion. Facile loss of THF from [UI(THF)(L)] further increases the amt. of U-arene back donation. [UI(L)] can incorporate a further UIII equiv., UI3, to form the very unusual dinuclear complex [U2I4(L)] in which the single macrocycle adopts both κ5:κ5 and η6:κ1:η6:κ1 binding modes in the same complex. Hybrid d. functional theory calcns. carried out to compare the electronic structures and bonding of [UIIII(L)] and [UIII2I4(L)] indicate increased contributions to the covalent bonding in [U2I4(L)] than in [UI(L)], and similar U-arene interactions in both. MO anal. and QTAIM calcns. find minimal U-U interaction in [U2I4(L)]. In contrast to the reducible U complex, treatment of [ThCl2(L)] with either a reductant or non-nucleophilic base results in metalation of the aryl rings of the macrocycle to form the (L-2H)4- tetraanion and two new and robust Th-C bonds in the -ate complexes [K(THF)2ThIV(μ-Cl)(L-2H)]2 and K[ThIV{N(SiMe3)2}(L-2H)].
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  13. 13
    Brewster, J. T., II; Mangel, D. N.; Gaunt, A. J.; Saunders, D. P.; Zafar, H.; Lynch, V. M.; Boreen, M. A.; Garner, M. E.; Goodwin, C. A. P.; Settineri, N. S.; Arnold, J.; Sessler, J. L. In-Plane Thorium(IV), Uranium(IV), and Neptunium(IV) Expanded Porphyrin Complexes. J. Am. Chem. Soc. 2019, 141, 17867– 17874,  DOI: 10.1021/jacs.9b09123
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    13
    In-Plane Thorium(IV), Uranium(IV), and Neptunium(IV) Expanded Porphyrin Complexes
    Brewster, James T.; Mangel, Daniel N.; Gaunt, Andrew J.; Saunders, Douglas P.; Zafar, Hadiqa; Lynch, Vincent M.; Boreen, Michael A.; Garner, Mary E.; Goodwin, Conrad A. P.; Settineri, Nicholas S.; Arnold, John; Sessler, Jonathan L.
    Journal of the American Chemical Society (2019), 141 (44), 17867-17874CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Here authors report the first series of in-plane thorium(IV), uranium(IV), and neptunium(IV) expanded porphyrin complexes. These actinide (An) complexes were synthesized using a hexa-aza porphyrin analog, termed dipyriamethyrin, and the nonaq. An(IV) precursors, ThCl4(DME)2, UCl4, and NpCl4(DME)2. The mol. and electronic structures of the ligand, each An(IV) complex, and a corresponding uranyl(VI) complex were characterized using NMR and UV-vis spectroscopies as well as single-crystal x-ray diffraction anal. Computational analyses of these complexes, coupled to their structural features, provide support for the conclusion that a greater degree of covalency in the ligand-cation orbital interactions arises as the early actinide series is traversed from Th(IV) to U(IV) and Np(IV). The axial ligands in the present An(IV) complexes proved labile, allowing for the electronic features of these complexes to be further modified.
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  14. 14
    Galley, S. S.; Pattenaude, S. A.; Ray, D.; Gaggioli, C. A.; Whitefoot, M. A.; Qiao, Y.; Higgins, R. F.; Nelson, W. L.; Baumbach, R.; Sperling, J. M.; Zeller, M.; Collins, T. S.; Schelter, E. J.; Gagliardi, L.; Albrecht-Schönzart, T. E.; Bart, S. C. Using Redox-Active Ligands to Generate Actinide Ligand Radical Species. Inorg. Chem. 2021, 60, 15242– 15252,  DOI: 10.1021/acs.inorgchem.1c01766
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    14
    Using Redox-Active Ligands to Generate Actinide Ligand Radical Species
    Galley, Shane S.; Pattenaude, Scott A.; Ray, Debmalya; Gaggioli, Carlo Alberto; Whitefoot, Megan A.; Qiao, Yusen; Higgins, Robert F.; Nelson, W. L.; Baumbach, Ryan; Sperling, Joseph M.; Zeller, Matthias; Collins, Tyler S.; Schelter, Eric J.; Gagliardi, Laura; Albrecht-Schonzart, Thomas E.; Bart, Suzanne C.
    Inorganic Chemistry (2021), 60 (20), 15242-15252CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Using a redox-active dioxophenoxazine ligand, DOPO (DOPO = 2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazine-9-olate), a family of actinide (U, Th, Np, Pu) and Hf tris(ligand) coordination compds. were synthesized. Full characterization of these species using 1H NMR spectroscopy, electronic absorption spectroscopy, SQUID magnetometry, and x-ray crystallog. showed these compds. are analogous, existing in the form M(DOPOq)2(DOPOsq), where two ligands are of the oxidized, quinone form (DOPOq), and the third is of the reduced, semiquinone (DOPOsq) form. The electronic structures of these complexes were further investigated using CASSCF calcns., which revealed electronic structures consistent with metals in the +4 formal oxidn. state and one unpaired electron localized on one ligand in each complex. Furthermore, f-orbitals of the early actinides show a sizable bonding overlap with the ligand 2p orbitals. Notably, this is the first example of a plutonium-ligand radical species and a rare example of magnetic data recorded for a homogeneous plutonium coordination complex.
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    Yadav, S.; Dash, C. One-pot Tandem Heck alkynylation/cyclization reactions catalyzed by Bis(Pyrrolyl)pyridine based palladium pincer complexes. Tetrahedron 2020, 76, 131350  DOI: 10.1016/j.tet.2020.131350
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    15
    One-pot Tandem Heck alkynylation/cyclization reactions catalyzed by Bis(Pyrrolyl)pyridine based palladium pincer complexes
    Yadav, Seema; Dash, Chandrakanta
    Tetrahedron (2020), 76 (30), 131350CODEN: TETRAB; ISSN:0040-4020. (Elsevier Ltd.)
    Ligand assisted palladium catalyzed one-pot tandem Heck alkynation/cyclization reactions for the synthesis of benzofurans I (Y = CH, N; R = H, 4-Me, 3-NH2, 4-Et, etc.; R1 = H, OMe; R2 = H, CHO) were reported in this paper. Well-defined palladium-pincer complexes II (R3 = H, OMe; R4 = Me, OMe-Ph) exhibited excellent catalytic activities for the one-pot tandem Heck alkynation/cyclization reactions yielding benzofuran derivs. I using 0.1 mol% catalyst. All the catalytic reactions are performed in air. The effects of variables such as solvents, the temp. on the catalytic activity are also reported. High product conversion was obtained for differently substituted 2-iodophenols such as 2-iodophenol, 2-iodopyridin-3-ol, 4-hydroxy-3-iodo-5-methoxybenzaldehyde at 120°C in 10 h.
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  16. 16
    Gowda, A. S.; Lee, T. S.; Rosko, M. C.; Petersen, J. L.; Castellano, F. N.; Milsmann, C. Long-Lived Photoluminescence of Molecular Group 14 Compounds through Thermally Activated Delayed Fluorescence. Inorg. Chem. 2022, 61, 7338– 7348,  DOI: 10.1021/acs.inorgchem.2c00182
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    16
    Long-Lived Photoluminescence of Molecular Group 14 Compounds through Thermally Activated Delayed Fluorescence
    Gowda, Anitha S.; Lee, Tia S.; Rosko, Michael C.; Petersen, Jeffrey L.; Castellano, Felix N.; Milsmann, Carsten
    Inorganic Chemistry (2022), 61 (19), 7338-7348CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Photoluminescent mols. exploiting the sizable spin-orbit coupling consts. of main group metals and metalloids to access long-lived triplet excited states are relatively rare compared to phosphorescent transition metal complexes. Here we report the synthesis of three air- and moisture-stable group 14 compds. E(MePDPPh)2, where E = Si, Ge, or Sn and [MePDPPh]2- is the doubly deprotonated form of 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine. In soln., all three mols. exhibit exceptionally long-lived triplet excited states with lifetimes in the millisecond range and show highly efficient photoluminescence (Φ ≤ 0.49) due to competing prompt fluorescence and thermally activated delayed fluorescence at and around room temp. Temp.-dependent steady-state emission spectra and photoluminescent lifetime measurements provided conclusive evidence for the two distinct emission pathways. Picosecond transient absorption spectroscopy allowed further anal. of the intersystem crossing (ISC) between singlet and triplet manifolds (τISC = 0.25-3.1 ns) and confirmed the expected trend of increased ISC rates for the heavier elements in otherwise isostructural compds.
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  17. 17
    Gowda, A. S.; Petersen, J. L.; Milsmann, C. Redox Chemistry of Bis(pyrrolyl)pyridine Chromium and Molybdenum Complexes: An Experimental and Density Functional Theoretical Study. Inorg. Chem. 2018, 57, 1919– 1934,  DOI: 10.1021/acs.inorgchem.7b02809
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    17
    Redox Chemistry of Bis(pyrrolyl)pyridine Chromium and Molybdenum Complexes: An Experimental and Density Functional Theoretical Study
    Gowda, Anitha S.; Petersen, Jeffrey L.; Milsmann, Carsten
    Inorganic Chemistry (2018), 57 (4), 1919-1934CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The three- and four-membered redox series [Cr(MePDP)2]z (z = 1-, 2-, 3-) and [Mo(MePDP)2]z (z = 0, 1-, 2-, 3-) were synthesized to study the redox properties of the pincer ligand MePDP2- (H2MePDP = 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine). The monoanionic complexes were characterized by x-ray crystallog., UV/visible/NIR spectroscopy, and magnetic susceptibility measurements. Exptl. and d. functional theory (DFT) studies are consistent with closed-shell MePDP2- ligands and +III oxidn. states (d3, S = 3/2) for the central metal ions. Cyclic voltammetry established multiple reversible redox processes for [M(MePDP)2]1- (M = Cr, Mo), which were further investigated via chem. oxidn. and redn. For molybdenum, one-electron oxidn. yielded Mo(MePDP)2 which was characterized by x-ray crystallog., UV/visible/NIR, and magnetic susceptibility measurements. The exptl. and computational data indicate metal-centered oxidn. to a MoIV complex (d2, S = 1) with two MePDP2- ligands. In contrast, one- and two-electron redns. are ligand centered giving MePDP•3- radicals, in which the unpaired electron is predominantly located on the central pyridine ring of the ligand. The presence of ligand radicals was established exptl. by observation of ligand-to-ligand intervalence charge transfer (LLIVCT) bands in the UV/visible/NIR spectra of the dianionic and trianionic complexes and further supported by broken-symmetry DFT calcns. X-ray crystallog. analyses of the one-electron-reduced species [M(MePDP)2]2- (S = 1, M = Cr, Mo) established structural indicators for pincer redn. and showed localization of the radical on one of the two pincer ligands. The two-electron-reduced, trianionic complexes (S = 1/2) were characterized by UV/visible/NIR spectroscopy, magnetic susceptibility measurements, and EPR spectroscopy. The electronic structures of the reduced complexes are best described as contg. +III metal ions (d3) antiferromagnetically coupled to one and two radical ligands for the dianionic and trianionic species, resp.
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  18. 18
    Hakey, B. M.; Darmon, J. M.; Akhmedov, N. G.; Petersen, J. L.; Milsmann, C. Reactivity of Pyridine Dipyrrolide Iron(II) Complexes with Organic Azides: C–H Amination and Iron Tetrazene Formation. Inorg. Chem. 2019, 58, 11028– 11042,  DOI: 10.1021/acs.inorgchem.9b01560
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    18
    Reactivity of Pyridine Dipyrrolide Iron(II) Complexes with Organic Azides: C-H Amination and Iron Tetrazene Formation
    Hakey, Brett M.; Darmon, Jonathan M.; Akhmedov, Novruz G.; Petersen, Jeffrey L.; Milsmann, Carsten
    Inorganic Chemistry (2019), 58 (16), 11028-11042CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Reaction of (MesPDPPh)Fe(THF) (H2MesPDPPh = 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine) with org. azides was studied. The identity of the azide substituent had a profound impact on the transformation type and nature of the obsd. products. Reaction with arom. p-tolyl azide, N3Tol, resulted in exclusive formation of the corresponding iron tetrazene complex (MesPDPPh)Fe(N4Tol2). In contrast, the use of bulky 1-adamantyl azide led to clean intramol. C-H amination of one of the benzylic C-H bonds of a mesityl substituent on the pyridine dipyrrolide, PDP, supporting ligand. The smaller aliph. substituent in benzyl azide allowed for the isolation of two different compds. from distinct reaction pathways. One product is the result of double C-H amination of the PDP ligand via nitrene transfer, while the second one contains a dibenzyltetrazene and a benzaldimine ligand. All isolated complexes were characterized using a combination of x-ray crystallog., solid state magnetic susceptibility measurements, 1H NMR and 57Fe Mossbauer spectroscopy, and d. functional theory (DFT), and their electronic structures were elucidated. Potential electronic structures for putative iron(IV) imido or iron(III) imidyl radical complexes were explored via DFT calcns.
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  19. 19
    Hakey, B. M.; Darmon, J. M.; Zhang, Y.; Petersen, J. L.; Milsmann, C. Synthesis and Electronic Structure of Neutral Square-Planar High-Spin Iron(II) Complexes Supported by a Dianionic Pincer Ligand. Inorg. Chem. 2019, 58, 1252– 1266,  DOI: 10.1021/acs.inorgchem.8b02730
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    Synthesis and Electronic Structure of Neutral Square-Planar High-Spin Iron(II) Complexes Supported by a Dianionic Pincer Ligand
    Hakey, Brett M.; Darmon, Jonathan M.; Zhang, Yu; Petersen, Jeffrey L.; Milsmann, Carsten
    Inorganic Chemistry (2019), 58 (2), 1252-1266CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Two square-planar high-spin FeII complexes bearing a dianionic pyridine dipyrrolate pincer ligand and a di-Et ether or THF ligand were synthesized and structurally characterized, and their electronic structures were elucidated by a combined spectroscopic and computational approach. In contrast to previous examples, the S = 2 ground states of these square-planar FeII complexes do not require an overall anionic charge of the compds. or incorporation of alkali metal cations. The THF complex exhibits an equil. between four- and five-coordinate species in soln., which was supported by 1H NMR and 57Fe Mossbauer spectroscopy and comparison to a structurally characterized five-coordinate pyridine dipyrrolate iron bis-pyridine adduct. A detailed computational anal. of the electronic structures of the four- and five-coordinate species via d. functional theory provides insight into the origins of the unusual ground state configurations for FeII in a square-planar ligand field and explains the assocd. characteristic spectroscopic parameters.
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  20. 20
    Hakey, B. M.; Leary, D. C.; Rodriguez, J. G.; Martinez, J. C.; Vaughan, N. B.; Darmon, J. M.; Akhmedov, N. G.; Petersen, J. L.; Dolinar, B. S.; Milsmann, C. Effects of 2,6-Dichlorophenyl Substituents on the Coordination Chemistry of Pyridine Dipyrrolide Iron Complexes. Z. Anorg. Allg. Chem. 2021, 647, 1503– 1517,  DOI: 10.1002/zaac.202100117
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    Effects of 2,6-Dichlorophenyl Substituents on the Coordination Chemistry of Pyridine Dipyrrolide Iron Complexes
    Hakey, Brett M.; Leary, Dylan C.; Rodriguez, Jose G.; Martinez, Jordan C.; Vaughan, Nicholas B.; Darmon, Jonathan M.; Akhmedov, Novruz G.; Petersen, Jeffrey L.; Dolinar, Brian S.; Milsmann, Carsten
    Zeitschrift fuer Anorganische und Allgemeine Chemie (2021), 647 (14), 1503-1517CODEN: ZAACAB; ISSN:1521-3749. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A series of iron complexes featuring the pyridine dipyrrolide (PDP) pincer ligand [Cl2PhPDPPh]2-, obtained via deprotonation of 2,6-bis(5-(2,6-dichlorophenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine, H2Cl2PhPDPPh, is reported and structurally and spectroscopically characterized. While the bis-pyridine adduct (Cl2PhPDPPh)Fe(py)2 exhibits nearly identical features as previously reported (MesPDPPh)Fe(py)2 (H2MesPDPPh=2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine), the di-Et ether and THF adducts (Cl2PhPDPPh)Fe(OEt2) and (Cl2PhPDPPh)Fe(thf) show addnl. weak Fe-Cl interactions that impact the overall coordination geometries and result in strong deviations from planar coordination environments. The reaction of (Cl2PhPDPPh)Fe(thf) with 1-adamantyl azide provided the isolable iron imido complex (Cl2PhPDPPh)Fe(N1Ad), highlighting the improved stability of [Cl2PhPDPPh]2- towards intramol. nitrene group transfer from the high-valent iron-imido unit. The electronic structure of (Cl2PhPDPPh)Fe(N1Ad) was investigated by d. functional theory (DFT) and complete active space SCF (CASSCF) calcns. These computational studies suggest energetically close-lying diamagnetic and paramagnetic states and help to conceptualize the unusual magnetic properties of the complex obsd. by variable-temp. 1H NMR spectroscopy.
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  21. 21
    Hakey, B. M.; Leary, D. C.; Xiong, J.; Harris, C. F.; Darmon, J. M.; Petersen, J. L.; Berry, J. F.; Guo, Y.; Milsmann, C. High Magnetic Anisotropy of a Square-Planar Iron-Carbene Complex. Inorg. Chem. 2021, 60, 18575– 18588,  DOI: 10.1021/acs.inorgchem.1c01860
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    High Magnetic Anisotropy of a Square-Planar Iron-Carbene Complex
    Hakey, Brett M.; Leary, Dylan C.; Xiong, Jin; Harris, Caleb F.; Darmon, Jonathan M.; Petersen, Jeffrey L.; Berry, John F.; Guo, Yisong; Milsmann, Carsten
    Inorganic Chemistry (2021), 60 (24), 18575-18588CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Among Earth-abundant catalyst systems, iron-carbene intermediates that perform C-C bond forming reactions such as cyclopropanation of olefins and C-H functionalization via carbene insertion are rare. Detailed descriptions of the possible electronic structures for iron-carbene bonds are imperative to obtain better mechanistic insights and enable rational catalyst design. Here, the authors report the first square-planar iron-carbene complex (MesPDPPh)Fe(CPh2), where [MesPDPPh]2- is the doubly deprotonated form of [2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine]. The compd. was prepd. via reaction of the disubstituted diazoalkane N2CPh2 with (MesPDPPh)Fe(thf) and represents a rare example of a structurally characterized, paramagnetic iron-carbene complex. Temp.-dependent magnetic susceptibility measurements and applied-field Mossbauer spectroscopic studies revealed an orbitally near-degenerate S = 1 ground state with large unquenched orbital angular momentum resulting in high magnetic anisotropy. Spin-Hamiltonian anal. indicated that this S = 1 spin system has uniaxial magnetic properties arising from a ground MS = ±1 non-Kramers doublet that is well-sepd. from the MS = 0 sublevel due to very large axial zero-field splitting (D = -195 cm-1, E/D = 0.02 estd. from magnetic susceptibility data). This remarkable electronic structure gives rise to a very large, pos. magnetic hyperfine field of more than +60 T for the 57Fe nucleus along the easy magnetization axis obsd. by Mossbauer spectroscopy. Computational anal. with complete active space SCF (CASSCF) calcns. provides a detailed electronic structure anal. and confirms that (MesPDPPh)Fe(CPh2) exhibits a multiconfigurational ground state. The majority contribution originates from a configuration best described as a singlet carbene coordinated to an intermediate-spin FeII center with a (dxy)2{(dxz),(dz2)}3(dyz)1(dx2-y2)0 configuration featuring near-degenerate dxz and dz2 orbitals.
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  22. 22
    Sorsche, D.; Miehlich, M. E.; Searles, K.; Gouget, G.; Zolnhofer, E. M.; Fortier, S.; Chen, C.-H.; Gau, M.; Carroll, P. J.; Murray, C. B.; Caulton, K. G.; Khusniyarov, M. M.; Meyer, K.; Mindiola, D. J. Unusual Dinitrogen Binding and Electron Storage in Dinuclear Iron Complexes. J. Am. Chem. Soc. 2020, 142, 8147– 8159,  DOI: 10.1021/jacs.0c01488
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    Unusual Dinitrogen Binding and Electron Storage in Dinuclear Iron Complexes
    Sorsche, Dieter; Miehlich, Matthias E.; Searles, Keith; Gouget, Guillaume; Zolnhofer, Eva M.; Fortier, Skye; Chen, Chun-Hsing; Gau, Michael; Carroll, Patrick J.; Murray, Christopher B.; Caulton, Kenneth G.; Khusniyarov, Marat M.; Meyer, Karsten; Mindiola, Daniel J.
    Journal of the American Chemical Society (2020), 142 (18), 8147-8159CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A rare example of a dinuclear iron core with a non-linearly bridged dinitrogen ligand is reported in this work. One-electron redn. of [(tBupyrr2py)Fe(OEt2)] (1) (tBupyrr2py2- = 2,6-bis((3,5-di-tert-butyl)pyrrol-2-yl)pyridine) with KC8 yields the complex [K]2[(tBupyrr2py)Fe]2(μ2-η1:η1-N2) (2) where the unusual cis-divacant octahedral coordination geometry about each iron and the η5-cation-π coordination of two potassium ions with four pyrrolyl units of the ligand cause distortion of the bridging end-on μ-N2 about the FeN2Fe core. Attempts to generate an Et2O free version of 1 resulted instead in a dinuclear helical dimer [(tBupyrr2py)Fe]2 (3) via bridging of the pyridine moieties of the ligand. Redn. of 3 by two-electrons under N2 does not break up the dimer nor does it result in formation of 2, but instead formation of the ate-complex [K(OEt2)]2[(tBupyrr2py)Fe]2 (4). Redn. of 1 by two-electrons and in the presence of crown-ether forms the tetraanionic N2 complex [K2][K(18-crown-6)]2(tBupyrr2py)Fe2(μ2-η1:η1-N2) (5), also having a distorted FeN2Fe moiety akin to 2. Complex 2 is thermally unstable and loses N2, disproportionating to Fe nanoparticles among other products. A combination of single-crystal X-ray diffraction studies, soln. and solid state magnetic studies, and 57Fe Mossbauer spectroscopy were applied to characterize complexes 2-5, whereas DFT studies were used to help explain the bonding and electronic structure in these unique diiron-N2 complexes 2 and 5.
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  23. 23
    Yang, M.; Sheykhi, S.; Zhang, Y.; Milsmann, C.; Castellano, F. N. Low power threshold photochemical upconversion using a zirconium(iv) LMCT photosensitizer. Chem. Sci. 2021, 12, 9069– 9077,  DOI: 10.1039/D1SC01662H
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    23
    Low power threshold photochemical upconversion using a zirconium(IV) LMCT photosensitizer
    Yang, Mo; Sheykhi, Sara; Zhang, Yu; Milsmann, Carsten; Castellano, Felix N.
    Chemical Science (2021), 12 (26), 9069-9077CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    The current investigation demonstrates highly efficient photochem. upconversion (UC) where a long-lived Zr(IV) ligand-to-metal charge transfer (LMCT) complex serves as a triplet photosensitizer in concert with well-established 9,10-diphenylanthracene (DPA) along with newly conceived DPA-carbazole based acceptors/annihilators in THF solns. The initial dynamic triplet-triplet energy transfer (TTET) processes (ΔG ∼ -0.19 eV) featured very large Stern-Volmer quenching consts. (KSV) approaching or achieving 105 M-1 with bimol. rate consts. between 2 and 3 x 108 M-1 s-1 as ascertained using static and transient spectroscopic techniques. Both the TTET and subsequent triplet-triplet annihilation (TTA) processes were verified and throughly investigated using transient absorption spectroscopy. The Stern-Volmer metrics support 95% quenching of the Zr(IV) photosensitizer using modest concns. (0.25 mM) of the various acceptor/annihilators, where no aggregation took place between any of the chromophores in THF. Each of the upconverting formulations operated with continuous-wave linear incident power dependence (λex = 514.5 nm) down to ultralow excitation power densities under optimized exptl. conditions. Impressive record-setting ηUC values ranging from 31.7% to 42.7% were achieved under excitation conditions (13 mW cm-2) below that of solar flux integrated across the Zr(IV) photosensitizer's absorption band (26.7 mW cm-2). This study illustrates the importance of supporting the continued development and discovery of mol.-based triplet photosensitizers based on earth-abundant metals.
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  24. 24
    Zhang, Y.; Leary, D. C.; Belldina, A. M.; Petersen, J. L.; Milsmann, C. Effects of Ligand Substitution on the Optical and Electrochemical Properties of (Pyridinedipyrrolide)zirconium Photosensitizers. Inorg. Chem. 2020, 59, 14716– 14730,  DOI: 10.1021/acs.inorgchem.0c02343
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    24
    Effects of Ligand Substitution on the Optical and Electrochemical Properties of (Pyridinedipyrrolide)zirconium Photosensitizers
    Zhang, Yu; Leary, Dylan C.; Belldina, Anne M.; Petersen, Jeffrey L.; Milsmann, Carsten
    Inorganic Chemistry (2020), 59 (20), 14716-14730CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Seven bis(pyridinedipyrrolide)zirconium complexes, Zr(R1PDPR2)2, where [R1PDPR2]2- is the doubly deprotonated form of [2,6-bis(5-R1-3-R2-1H-pyrrol-2-yl)pyridine], were prepd. and characterized in soln. by NMR, UV/visible absorption, and emission spectroscopy and cyclic voltammetry. The mol. structures were detd. by single-crystal x-ray crystallog. All complexes exhibit remarkably long emission lifetimes (τ = 190-576μs) with high quantum efficiencies (ΦPL = 0.10-0.38) upon excitation with visible light in a benzene soln. The substituents on the pyrrolide rings have significant effects on the photoluminescence and electrochem. properties of these compds. The R2 substituents (R2 = H, Me, Ph, or C6F5) show only limited effects on the absorption and emission profiles of the complexes but allow systematic tuning of the ground- and excited-state redox potentials over a range of almost 600 mV. The R1 substituents (R1 = H, Me, Ph, or 2,4,6-Me3Ph) influence both the optical and electrochem. properties through electronic effects. Addnl., the R1 substituents have profound consequences for the structural flexibility and overall stability of the compds. Distortions of the Zr(PDP)2 core from idealized D2d symmetry in the solid state can be traced to the steric profiles of the R1 substituents and correlate with the obsd. Stokes shifts for each compd. The complex with the smallest ligand system, Zr(HPDPH)2, coordinates two addnl. solvent mols. in a THF soln., which gave photoluminescent, eight-coordinate Zr(HPDPH)2(THF)2. The photoredox catalytic dehalogenation of aryl iodides and aryl chlorides using the most reducing deriv., Zr(MePDPMe)2, highlights the potential of Zr(PDP)2 photosensitizers to promote challenging reductive transformations under mild conditions upon excitation with green light. The effects of pyrrolide substitution on the optical and electrochem. properties of photoluminescent bis(pyridinedipyrrolide)zirconium complexes, Zr(PDP)2, are reported. The steric effects of the substituents heavily influence the structural rigidity of the complexes, translating into pronounced changes in the optical properties. The electronic characteristics of the substituents have remarkably limited consequences for the optical properties but allow systematic tuning of the ground- and excited-state redox potentials.
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  25. 25
    Zhang, Y.; Lee, T. S.; Favale, J. M.; Leary, D. C.; Petersen, J. L.; Scholes, G. D.; Castellano, F. N.; Milsmann, C. Delayed fluorescence from a zirconium(iv) photosensitizer with ligand-to-metal charge-transfer excited states. Nat. Chem. 2020, 12, 345– 352,  DOI: 10.1038/s41557-020-0430-7
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    25
    Delayed fluorescence from a zirconium(IV) photosensitizer with ligand-to-metal charge-transfer excited states
    Zhang, Yu; Lee, Tia S.; Favale, Joseph M.; Leary, Dylan C.; Petersen, Jeffrey L.; Scholes, Gregory D.; Castellano, Felix N.; Milsmann, Carsten
    Nature Chemistry (2020), 12 (4), 345-352CODEN: NCAHBB; ISSN:1755-4330. (Nature Research)
    Advances in chem. control of the photophys. properties of transition-metal complexes are revolutionizing a wide range of technologies, particularly photocatalysis and light-emitting diodes, but they rely heavily on mols. contg. precious metals such as ruthenium and iridium. Although the application of earth-abundant 'early' transition metals in photosensitizers is clearly advantageous, a detailed understanding of excited states with ligand-to-metal charge transfer (LMCT) character is paramount to account for their distinct electron configurations. Here we report an air- and moisture-stable, visible light-absorbing Zr(IV) photosensitizer, Zr(MesPDPPh)2, where [MesPDPPh]2- is the doubly deprotonated form of [2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine]. This mol. has an exceptionally long-lived triplet LMCT excited state (τ = 350μs), featuring highly efficient photoluminescence emission (Φ = 0.45) due to thermally activated delayed fluorescence emanating from the higher-lying singlet configuration with significant LMCT contributions. Zr(MesPDPPh)2 engages in numerous photoredox catalytic processes and triplet energy transfer. Our investigation provides a blueprint for future photosensitizer development featuring early transition metals and excited states with significant LMCT contributions. [graphic not available: see fulltext].
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  26. 26
    Zhang, Y.; Petersen, J. L.; Milsmann, C. A Luminescent Zirconium(IV) Complex as a Molecular Photosensitizer for Visible Light Photoredox Catalysis. J. Am. Chem. Soc. 2016, 138, 13115– 13118,  DOI: 10.1021/jacs.6b05934
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    26
    A Luminescent Zirconium(IV) Complex as a Molecular Photosensitizer for Visible Light Photoredox Catalysis
    Zhang, Yu; Petersen, Jeffrey L.; Milsmann, Carsten
    Journal of the American Chemical Society (2016), 138 (40), 13115-13118CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Titanium and zirconium complexes carrying two 2,6-bis(pyrrolyl)pyridine ligands have been synthesized and characterized. The neutral complexes Ti(MePDP)2 and Zr(MePDP)2 (MePDP = 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine) show intense ligand-to-metal charge-transfer bands in the visible region and undergo multiple reversible redox events under highly reducing conditions. Zr(MePDP)2 exhibits photoluminescent behavior and its excited state can be quenched by mild reductants to generate a powerful electron transfer reagent with a ground state potential of -2.16 V vs Fc+/0. This reactivity was utilized to facilitate dehalogenation reactions, the redn. of electron-poor olefins, and the reductive coupling of benzyl bromide via photoredox catalysis. In these reactions, the earth-abundant metal complex Zr(MePDP)2 acts as a substitute for the precious metal photosensitizer [Ru(bpy)3]2+.
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  27. 27
    Hakey, B. M.; Leary, D. C.; Lopez, L. M.; Valerio, L. R.; Brennessel, W. W.; Milsmann, C.; Matson, E. M. Synthesis and Characterization of Pyridine Dipyrrolide Uranyl Complexes. Inorg. Chem. 2022, 61, 6182– 6192,  DOI: 10.1021/acs.inorgchem.2c00348
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    27
    Synthesis and Characterization of Pyridine Dipyrrolide Uranyl Complexes
    Hakey, Brett M.; Leary, Dylan C.; Lopez, Lauren M.; Valerio, Leyla R.; Brennessel, William W.; Milsmann, Carsten; Matson, Ellen M.
    Inorganic Chemistry (2022), 61 (16), 6182-6192CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The first actinide complexes of the pyridine dipyrrolide (PDP) ligand class, (MesPDPPh)UO2(THF) and (Cl2PhPDPPh)UO2(THF), are reported as the UVI uranyl adducts of the bulky aryl substituted pincers (MesPDPPh)2- and (Cl2PhPDPPh)2- (derived from 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine (H2MesPDPPh, Mes = 2,4,6-trimethylphenyl), and 2,6-bis(5-(2,6-dichlorophenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine (H2Cl2PhPDPPh, Cl2Ph = 2,6-dichlorophenyl), resp.). Following the in situ deprotonation of the proligand with lithium hexamethyldisilazide to generate the corresponding dilithium salts (e.g., Li2ArPDPPh, Ar = Mes of Cl2Ph), salt metathesis with [UO2Cl2(THF)2]2 afforded both compds. in moderate yields. The characterization of each species has been undertaken by a combination of solid- and soln.-state methods, including combustion anal., IR, electronic absorption, and NMR spectroscopies. In both complexes, single-crystal x-ray diffraction has revealed a distorted octahedral geometry in the solid state, enforced by the bite angle of the rigid meridional (ArPDPPh)2- pincer ligand. The electrochem. anal. of both compds. by cyclic voltammetry in THF (THF) reveals rich redox profiles, including events assigned as UVI/UV redox couples. A time-dependent d. functional theory study has been performed on (MesPDPPh)UO2(THF) and provides insight into the nature of the transitions that comprise its electronic absorption spectrum.
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  28. 28
    Valerio, L. R.; Hakey, B. M.; Leary, D. C.; Stockdale, E.; Brennessel, W. W.; Milsmann, C.; Matson, E. M. Synthesis and Characterization of Isostructural Th(IV) and U(IV) Pyridine Dipyrrolide Complexes. Inorg. Chem. 2024, 63, 9610– 9623,  DOI: 10.1021/acs.inorgchem.3c04391
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    Leary, D. C.; Zhang, Y.; Rodriguez, J. G.; Akhmedov, N. G.; Petersen, J. L.; Dolinar, B. S.; Milsmann, C. Organometallic Intermediates in the Synthesis of Photoluminescent Zirconium and Hafnium Complexes with Pyridine Dipyrrolide Ligands. Organometallics 2023, 42, 1220– 1231,  DOI: 10.1021/acs.organomet.3c00058
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    29
    Organometallic Intermediates in the Synthesis of Photoluminescent Zirconium and Hafnium Complexes with Pyridine Dipyrrolide Ligands
    Leary, Dylan C.; Zhang, Yu; Rodriguez, Jose G.; Akhmedov, Novruz G.; Petersen, Jeffrey L.; Dolinar, Brian S.; Milsmann, Carsten
    Organometallics (2023), 42 (11), 1220-1231CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
    The two com. available Zr complexes tetrakis(dimethylamido)zirconium, Zr(NMe2)4, and tetrabenzylzirconium, ZrBn4, were studied for their utility as starting materials in the synthesis of bis(pyridine dipyrrolide)zirconium photosensitizers, Zr(PDP)2. Reaction with one equiv. of the ligand precursor 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine, H2MePDPPh, resulted in the isolation and structural characterization of the complexes (MePDPPh)Zr(NMe2)2THF and (MePDPPh)ZrBn2, which could be converted to the desired photosensitizer Zr(MePDPPh)2 upon addn. of a 2nd equiv. of H2MePDPPh. Using the more sterically encumbered ligand precursor 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine, H2MesPDPPh, only ZrBn4 yielded the desired bis-ligand complex Zr(MesPDPPh)2. Careful monitoring of the reaction at different temps. revealed the importance of the organometallic intermediate (cyclo-MesPDPPh)ZrBn, which was characterized by x-ray diffraction anal. and 1H NMR spectroscopy and shown to contain a cyclometalated MesPDPPh unit. Taking inspiration from the results for Zr, syntheses for two Hf photosensitizers, Hf(MePDPPh)2 and Hf(MesPDPPh)2, were established and shown to proceed through similar intermediates starting from tetrabenzylhafnium, HfBn4. Initial studies of the photophys. properties of the photoluminescent Hf complexes indicate similar optical properties compared to their Zr analogs.
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    Cotton, S. Introduction to the Actinides. In Lanthanide and Actinide Chemistry ; 2006; pp 145– 153.
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    Reilly, S. D.; Brown, J. L.; Scott, B. L.; Gaunt, A. J. Synthesis and characterization of NpCl4(DME)2 and PuCl4(DME)2 neutral transuranic An(IV) starting materials. Dalton Trans. 2014, 43, 1498– 1501,  DOI: 10.1039/C3DT53058B
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    Synthesis and characterization of NpCl4(DME)2 and PuCl4(DME)2 neutral transuranic An(iv) starting materials
    Reilly, Sean D.; Brown, Jessie L.; Scott, Brian L.; Gaunt, Andrew J.
    Dalton Transactions (2014), 43 (4), 1498-1501CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    The 1,2-dimethoxyethane (DME) solvento adducts of Np(iv) and Pu(iv) tetrachloride were prepd. and isolated in good and moderate yields, resp., along with single-crystal structural detns. These neutral mols. are expected to provide alternative synthetic pathways in the pursuit of nonaq. and organometallic complexes.
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    Hermann, J. A.; Suttle, J. F.; Hoekstra, H. R. Uranium(IV) Chloride. Inorg. Synth. 1957, 5, 143– 145,  DOI: 10.1002/9780470132364.ch39
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    Uranium(IV) chloride
    Hermann, John A.; Suttle, John F.
    (1957), 5 (), 143-5 ISSN:.
    cf. C.A. 50, 6239i.
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    Cantat, T.; Scott, B. L.; Kiplinger, J. L. Convenient access to the anhydrous thorium tetrachloride complexes ThCl4(DME)2, ThCl4(1,4-dioxane)2 and ThCl4(THF)3.5 using commercially available and inexpensive starting materials. Chem. Commun. 2010, 46, 919– 921,  DOI: 10.1039/b923558b
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    Convenient access to the anhydrous thorium tetrachloride complexes ThCl4(DME)2, ThCl4(1,4-dioxane)2 and ThCl4(THF)3.5 using commercially available and inexpensive starting materials
    Cantat, Thibault; Scott, Brian L.; Kiplinger, Jaqueline L.
    Chemical Communications (Cambridge, United Kingdom) (2010), 46 (6), 919-921CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Anhyd. thorium tetrachloride complexes ThCl4(DME)2, ThCl4(1,4-dioxane)2, and ThCl4(THF)3.5 have been easily accessed from inexpensive, com. available reagents under mild conditions and serve as excellent precursors to a variety of thorium(iv) halide, alkoxide, amide and organometallic compds.
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    Lopez, L. M.; Uible, M. C.; Zeller, M.; Bart, S. C. Lewis base adducts of NpCl4. Chem. Commun. 2024, 60, 5956– 5959,  DOI: 10.1039/D4CC01560F
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    Pattenaude, S. A.; Anderson, N. H.; Bart, S. C.; Gaunt, A. J.; Scott, B. L. Non-aqueous neptunium and plutonium redox behaviour in THF─access to a rare Np(III) synthetic precursor. Chem. Commun. 2018, 54, 6113– 6116,  DOI: 10.1039/C8CC02611D
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    Staun, S. L.; Stevens, L. M.; Smiles, D. E.; Goodwin, C. A. P.; Billow, B. S.; Scott, B. L.; Wu, G.; Tondreau, A. M.; Gaunt, A. J.; Hayton, T. W. Expanding the Nonaqueous Chemistry of Neptunium: Synthesis and Structural Characterization of [Np(NR2)3Cl], [Np(NR2)3Cl]−, and [Np{N(R)(SiMe2CH2)}2(NR2)]− (R = SiMe3). Inorg. Chem. 2021, 60, 2740– 2748,  DOI: 10.1021/acs.inorgchem.0c03616
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    Expanding the Nonaqueous Chemistry of Neptunium: Synthesis and Structural Characterization of [Np(NR2)3Cl], [Np(NR2)3Cl]-, and [Np{N(R)(SiMe2CH2)}2(NR2)]- (R = SiMe3)
    Staun, Selena L.; Stevens, Lauren M.; Smiles, Danil E.; Goodwin, Conrad A. P.; Billow, Brennan S.; Scott, Brian L.; Wu, Guang; Tondreau, Aaron M.; Gaunt, Andrew J.; Hayton, Trevor W.
    Inorganic Chemistry (2021), 60 (4), 2740-2748CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Reaction of 3 equiv of NaNR2 (R = SiMe3) with NpCl4(DME)2 in THF afforded the Np(IV) silylamide complex, [Np(NR2)3Cl] (1), in good yield. Reaction of 1 with 1.5 equiv of KC8 in THF, in the presence of 1 equiv of dibenzo-18-crown-6, gave [{K(DB-18-C-6)(THF)}3(μ3-Cl)][Np(NR2)3Cl]2 (4), also in good yield. Complex 4 represents the first structurally characterized Np(III) amide. Finally, reaction of NpCl4(DME)2 with 5 equiv of NaNR2 and 1 equiv of dibenzo-18-crown-6 afforded the Np(IV) bis(metallacycle), [{Na(DB-18-C-6)(Et2O)0.62(κ1-DME)0.38}2(μ-DME)][Np{N(R)(SiMe2CH2)}2(NR2)]2 (8), in moderate yield. Complex 8 was characterized by 1H NMR spectroscopy and x-ray crystallog. and represents a rare example of a structurally characterized neptunium-hydrocarbyl complex. To support these studies, the authors also synthesized the uranium analogs of 4 and 8, namely, [K(2,2,2-cryptand)][U(NR2)3Cl] (2), [K(DB-18-C-6)(THF)2][U(NR2)3Cl] (3), [Na(DME)3][U{N(R)(SiMe2CH2)}2(NR2)] (6), and [{Na(DB-18-C-6)(Et2O)0.5(κ1-DME)0.5}2(μ-DME)][U{N(R)(SiMe2CH2)}2(NR2)]2 (7). Complexes 2, 3, 6, and 7 were characterized by a no. of techniques, including NMR spectroscopy and x-ray crystallog.
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    Grödler, D.; Sperling, J. M.; Rotermund, B. M.; Scheibe, B.; Beck, N. B.; Mathur, S.; Albrecht-Schönzart, T. E. Neptunium Alkoxide Chemistry: Expanding Alkoxides to the Transuranium Elements. Inorg. Chem. 2023, 62, 2513– 2517,  DOI: 10.1021/acs.inorgchem.2c04338
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    Su, J.; Batista, E. R.; Boland, K. S.; Bone, S. E.; Bradley, J. A.; Cary, S. K.; Clark, D. L.; Conradson, S. D.; Ditter, A. S.; Kaltsoyannis, N. Energy-Degeneracy-Driven Covalency in Actinide Bonding. J. Am. Chem. Soc. 2018, 140, 17977– 17984,  DOI: 10.1021/jacs.8b09436
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    Energy-Degeneracy-Driven Covalency in Actinide Bonding
    Su, Jing; Batista, Enrique R.; Boland, Kevin S.; Bone, Sharon E.; Bradley, Joseph A.; Cary, Samantha K.; Clark, David L.; Conradson, Steven D.; Ditter, Alex S.; Kaltsoyannis, Nikolas; Keith, Jason M.; Kerridge, Andrew; Kozimor, Stosh A.; Loble, Matthias W.; Martin, Richard L.; Minasian, Stefan G.; Mocko, Veronika; La Pierre, Henry S.; Seidler, Gerald T.; Shuh, David K.; Wilkerson, Marianne P.; Wolfsberg, Laura E.; Yang, Ping
    Journal of the American Chemical Society (2018), 140 (51), 17977-17984CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Evaluating the nature of chem. bonding for actinide elements represents one of the most important and long-standing problems in actinide science. We directly address this challenge and contribute a Cl K-edge X-ray absorption spectroscopy and relativistic d. functional theory study that quant. evaluates An-Cl covalency in AnCl62- (AnIV = Th, U, Np, Pu). The results showed significant mixing between Cl 3p- and AnIV 5f- and 6d-orbitals (t1u*/t2u* and t2g*/eg*), with the 6d-orbitals showing more pronounced covalent bonding than the 5f-orbitals. Moving from Th to U, Np, and Pu markedly changed the amt. of M-Cl orbital mixing, such that AnIV 6d- and Cl 3p-mixing decreased and metal 5f- and Cl 3p-orbital mixing increased across this series.
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    Sonnenberger, D. C.; Gaudiello, J. G. Cyclic voltammetric study of organoactinide compounds of uranium(IV) and neptunium(IV) Ligand effects on the M(IV)/M(III) couple. Inorg. Chem. 1988, 27, 2747– 2748,  DOI: 10.1021/ic00288a036
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    Cyclic voltammetric study of organoactinide compounds of uranium(IV) and neptunium(IV). Ligand effects on the M(IV)/M(III) couple
    Sonnenberger, David C.; Gaudiello, John G.
    Inorganic Chemistry (1988), 27 (15), 2747-8CODEN: INOCAJ; ISSN:0020-1669.
    The electrochem. redn. of a series of organoactinide complexes [Cp4M (Cp = η5-C5H5; M = U, Np], Cp3MCl (M = U, Np), and Cp2*MCl2 [Cp* = η5-C5Me5; M = U, Np]) were investigated by cyclic voltammetry. The ease with which these complexes are reduced varies with the nature of the ligand environment. The redox potential of the U(IV)/U(III) couple is more sensitive to ligand environment than the Np(IV)/Np(III) couple.
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Published January 9, 2025

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  • Abstract

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    Figure 1

    Figure 1. 1H NMR spectrum (400 MHz) of 1-Np stacked with 1-U for comparison collected in C6D6 at room temperature (∼21 °C). For full assignments, see the Supporting Information.

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    Scheme 1

    Scheme 1. Synthesis of (MesPDPPh)AnCl2(THF) Complexes (1-Th, 1-U, 1-Np)
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    Figure 2

    Figure 2. Molecular structure of (MesPDPPh)NpCl2(THF) (1-Np) is shown with 30% probability ellipsoids. Hydrogen atoms have been omitted for clarity.

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    Figure 3

    Figure 3. Electronic absorption spectra for (MesPDPPh)NpCl2(THF) (1-Np), with 1-Th and 1-U included for comparison. Spectra were collected at room temperature in dichloromethane.

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    Scheme 2

    Scheme 2. Synthesis of An(MesPDPPh)2 Complexes (1-Th, 1-U, 1-Np)
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    Figure 4

    Figure 4. 1H NMR spectrum (400 MHz) of 2-Np stacked with 2-U for comparison was collected in C6D6 at room temperature (∼21 °C). For full assignments, see the Supporting Information.

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    Figure 5

    Figure 5. Electronic absorption spectra in the visible region for Np(MesPDPPh)2 (2-Np), with 2-Th and 2-U included for comparison. Spectra were collected at room temperature in dichloromethane.

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    Figure 6

    Figure 6. Cyclic voltammograms of Th(MesPDPPh)2, U(MesPDPPh)2, and Np(MesPDPPh)2 recorded in DCM (1 mM analyte, 0.1 M TBAPF6, scan rate = 200 mV s–1).

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  • References

    This article references 39 other publications.

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      Jones, M. B.; Gaunt, A. J. Recent Developments in Synthesis and Structural Chemistry of Nonaqueous Actinide Complexes. Chem. Rev. 2013, 113, 1137– 1198,  DOI: 10.1021/cr300198m
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      Recent Developments in Synthesis and Structural Chemistry of Nonaqueous Actinide Complexes
      Jones, Matthew B.; Gaunt, Andrew J.
      Chemical Reviews (Washington, DC, United States) (2013), 113 (2), 1137-1198CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. The actinide chem. literature has been particularly vibrant over recent years, esp. for uranium and thorium, with 1821 actinide structures added to the Cambridge Structural Database since the beginning of 2006, a no. that accounts for 44% of the total actinide entries to date. Given this swelling vol. of research, there is a clear need for review articles to assist in providing a concise location for tracking progress in the different areas of actinide chem. The authors focus on the topic of nonaq. actinide coordination chem.
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      Neidig, M. L.; Clark, D. L.; Martin, R. L. Covalency in f-element complexes. Coord. Chem. Rev. 2013, 257, 394– 406,  DOI: 10.1016/j.ccr.2012.04.029
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      Covalency in f-element complexes
      Neidig, Michael L.; Clark, David L.; Martin, Richard L.
      Coordination Chemistry Reviews (2013), 257 (2), 394-406CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      A review. The presence of covalency in complexes of the 4f and 5f elements has been a source of intense research and controversy. In addn. to academic interest in this debate, there is an industrial motivation for better understanding of bonding in f-element complexes due to the need to sep. trivalent trans-plutonium elements from trivalent lanthanide fission products in advanced nuclear fuel cycles. This review discusses the key evidence for covalency in f-element bonds derived from structural, spectroscopic and theor. studies of some selected classes of mols., including octahedral hexahalides, linear actinyl and organometallic sandwich complexes. This evidence is supplemented by a discussion of covalency, including the possibility of both overlap and near-degeneracy driven covalency and the need to quantify their relative contributions in actinide metal-ligand bonds.
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      Kozimor, S. A.; Yang, P.; Batista, E. R.; Boland, K. S.; Burns, C. J.; Clark, D. L.; Conradson, S. D.; Martin, R. L.; Wilkerson, M. P.; Wolfsberg, L. E. Trends in Covalency for d- and f-Element Metallocene Dichlorides Identified Using Chlorine K-Edge X-ray Absorption Spectroscopy and Time-Dependent Density Functional Theory. J. Am. Chem. Soc. 2009, 131, 12125– 12136,  DOI: 10.1021/ja9015759
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      Trends in Covalency for d- and f-Element Metallocene Dichlorides Identified Using Chlorine K-Edge X-ray Absorption Spectroscopy and Time-Dependent Density Functional Theory
      Kozimor, Stosh A.; Yang, Ping; Batista, Enrique R.; Boland, Kevin S.; Burns, Carol J.; Clark, David L.; Conradson, Steven D.; Martin, Richard L.; Wilkerson, Marianne P.; Wolfsberg, Laura E.
      Journal of the American Chemical Society (2009), 131 (34), 12125-12136CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The use of Cl K-edge x-ray absorption spectroscopy (XAS) and both ground-state and time-dependent hybrid d. functional theory (DFT) are used to probe the electronic structure and det. the degree of orbital mixing in M-Cl bonds for (C5Me5)2MCl2 (M = Ti, 1; Zr, 2; Hf, 3; Th, 4; U, 5), where direct comparison is made to a class of structurally similar compds. for d- and f-elements. Pre-edge features in the Cl K-edge XAS data for the Group IV transition-metals 1-3 provide direct evidence of covalent M-Cl orbital mixing. The amt. of Cl 3p character was exptl. detd. to be 25%, 23%, and 22% per M-Cl bond for 1-3, resp. For actinides, a pre-edge shoulder for 4 (Th) and distinct and weak pre-edge features for U, 5 were found. The amt. of Cl 3p character is 9% for 5. Using hybrid DFT calcns. with relativistic effective core potentials, the electronic structures of 1-5 were calcd. and used as a guide to interpret the exptl. Cl K-edge XAS data. For transition-metal compds. 1-3, the pre-edge features arise due to transitions from Cl 1s electrons into the 3d-, 4d-, and 5d-orbitals, with assignments provided in the text. For Th, 4, 5f- and 6d-orbitals are nearly degenerate and give rise to a single pre-edge shoulder in the XAS. For U, 5, the 5f- and 6d-orbitals fall into two distinct energy groupings, and Cl K-edge XAS data are interpreted in terms of Cl 1s transitions into both 5f- and 6d-orbitals were found. Time-dependent DFT was used to calc. the energies and intensities of Cl 1s transitions into empty metal-based orbitals contg. Cl 3p character and provide simulated Cl K-edge XAS spectra for 1-4. For 5, which has two unpaired 5f electrons, simulated spectra were obtained from transition dipole calcns. using ground-state Kohn-Sham orbitals. This represents the 1st application of Cl K-edge XAS to actinide systems. Overall, this study allows trends in orbital mixing within a well-characterized structural motif to be identified and compared between transition-metals and actinide elements. The orbital mixing for the d-block compds. slightly decreases in covalency with increasing principal quantum no., in the order Ti > Zr ≈ Hf, and U displays approx. half the covalent orbital mixing of transition elements.
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      Tsipis, A. C.; Kefalidis, C. E.; Tsipis, C. A. The Role of the 5f Orbitals in Bonding, Aromaticity, and Reactivity of Planar Isocyclic and Heterocyclic Uranium Clusters. J. Am. Chem. Soc. 2008, 130, 9144– 9155,  DOI: 10.1021/ja802344z
      4
      The role of the 5f orbitals in bonding, aromaticity, and reactivity of planar isocyclic and heterocyclic uranium clusters
      Tsipis, Athanassios C.; Kefalidis, Christos E.; Tsipis, Constantinos A.
      Journal of the American Chemical Society (2008), 130 (28), 9144-9155CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The mol. and electronic structures, stabilities, bonding features and magnetic properties of prototypical planar isocyclic cyclo-UnXn (n = 3, 4; X = O, NH) and heterocyclic cyclo-Un(μ2-X)n (n = 3, 4; X = C, CH, NH) clusters as well as the E@[c-U4(μ2-C)4], (E = H+, C, Si, Ge), and U@[c-U5(μ2-C)5] mols. including a planar tetracoordinate element E (ptE) and pentacoordinate U (ppU) at the ring centers, resp., have been thoroughly investigated by means of electronic structure calcn. methods at the DFT level. It was shown that 5f orbitals play a key role in the bonding of these f-block metal systems significantly contributing to the cyclic electron delocalization and the assocd. magnetic diatropic (magnetic aromaticity) response. The aromaticity of the perfectly planar cyclo-UnXn (n = 3, 4; X = O, NH), cyclo-Un(μ2-X)n (n = 3, 4; X = C, CH, NH), E@[c-U4(μ2-C)4], (E = H+, C, Si, Ge), and U@[c-U5(μ2-C)5] clusters was verified by an efficient and simple criterion in probing the aromaticity/antiaromaticity of a mol., that of the nucleus-independent chem. shift, NICS(0), NICS(1), NICSzz(0), and the most refined NICSzz(1) index in conjunction with the NICS scan profiles. Natural bond orbital analyses provided a clear picture of the bonding pattern in the planar isocyclic and heterocyclic uranium clusters and revealed the features that stabilize the ptE's inside the six- and eight-member uranacycle rings. The ptE's benefit from a considerable electron transfer from the surrounding uranium atoms in the E@[c-U4(μ2-C)4], (E = H+, C, Si, Ge) and U@[c-U5(μ2-C)5] clusters justifying the high occupancy of the np orbitals of the central atom E.
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    5. 5
      Vitova, T.; Pidchenko, I.; Fellhauer, D.; Bagus, P. S.; Joly, Y.; Pruessmann, T.; Bahl, S.; Gonzalez-Robles, E.; Rothe, J.; Altmaier, M. The role of the 5f valence orbitals of early actinides in chemical bonding. Nat. Commun. 2017, 8, 16053  DOI: 10.1038/ncomms16053
      5
      The role of the 5f valence orbitals of early actinides in chemical bonding
      Vitova, T.; Pidchenko, I.; Fellhauer, D.; Bagus, P. S.; Joly, Y.; Pruessmann, T.; Bahl, S.; Gonzalez-Robles, E.; Rothe, J.; Altmaier, M.; Denecke, M. A.; Geckeis, H.
      Nature Communications (2017), 8 (), 16053CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      One of the long standing debates in actinide chem. is the level of localization and participation of the actinide 5f valence orbitals in covalent bonds across the actinide series. Here we illuminate the role of the 5f valence orbitals of uranium, neptunium and plutonium in chem. bonding using advanced spectroscopies: actinide M4,5 HR-XANES and 3d4f RIXS. Results reveal that the 5f orbitals are active in the chem. bonding for uranium and neptunium, shown by significant variations in the level of their localization evidenced in the spectra. In contrast, the 5f orbitals of plutonium appear localized and surprisingly insensitive to different bonding environments. We envisage that this report of using relative energy differences between the 5fδ/φ and 5fπ*/5fσ* orbitals as a qual. measure of overlap-driven actinyl bond covalency will spark activity, and extend to numerous applications of RIXS and HR-XANES to gain new insights into the electronic structures of the actinide elements.
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    6. 6
      Silver, M. A.; Cary, S. K.; Johnson, J. A.; Baumbach, R. E.; Arico, A. A.; Luckey, M.; Urban, M.; Wang, J. C.; Polinski, M. J.; Chemey, A.; Liu, G.; Chen, K.-W.; Van Cleve, S. M.; Marsh, M. L.; Eaton, T. M.; Lambertus, J. V.d.B.; Gray, A. L.; Hobart, D. E.; Hanson, K.; Maron, L.; Gendron, F.; Autschbach, J.; Speldrich, M.; Kögerler, P.; Yang, P.; Braley, J.; Albrecht-Schmitt, T. E. Characterization of berkelium(III) dipicolinate and borate compounds in solution and the solid state. Science 2016, 353, aaf3762  DOI: 10.1126/science.aaf3762
      There is no corresponding record for this reference.
    7. 7
      Galley, S. S.; Pattenaude, S. A.; Gaggioli, C. A.; Qiao, Y.; Sperling, J. M.; Zeller, M.; Pakhira, S.; Mendoza-Cortes, J. L.; Schelter, E. J.; Albrecht-Schmitt, T. E.; Gagliardi, L.; Bart, S. C. Synthesis and Characterization of Tris-chelate Complexes for Understanding f-Orbital Bonding in Later Actinides. J. Am. Chem. Soc. 2019, 141, 2356– 2366,  DOI: 10.1021/jacs.8b10251
      7
      Synthesis and Characterization of Tris-chelate Complexes for Understanding f-Orbital Bonding in Later Actinides
      Galley, Shane S.; Pattenaude, Scott A.; Gaggioli, Carlo Alberto; Qiao, Yusen; Sperling, Joseph M.; Zeller, Matthias; Pakhira, Srimanta; Mendoza-Cortes, Jose L.; Schelter, Eric J.; Albrecht-Schmitt, Thomas E.; Gagliardi, Laura; Bart, Suzanne C.
      Journal of the American Chemical Society (2019), 141 (6), 2356-2366CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      An isostructural family of f-element compds. (Ce, Nd, Sm, Gd; Am, Bk, Cf) of the redox-active dioxophenoxazine ligand (DOPOq; DOPO = 2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazin-9-olate) was prepd. This family, of the form M(DOPOq)3, represents the first nonaq. isostructural series, including the later actinides berkelium and californium. The lanthanide derivs. were fully characterized using 1H NMR spectroscopy and SQUID magnetometry, while all species were structurally characterized by x-ray crystallog. and electronic absorption spectroscopy. In order to probe the electronic structure of this new family, CASSCF calcns. were performed and revealed these systems to be largely ionic in contrast to previous studies, where berkelium and californium typically have a small degree of covalent character. To validate the zeroth order regular approxn. (ZORA) method, the same CASSCF anal. using exptl. structures vs. UDFT-ZORA optimized structures does not exhibit sizable changes in bonding patterns. UDFT-ZORA combined with CASSCF could be a useful first approxn. to predict and investigate the structure and electronic properties of actinides and lanthanides that are difficult to synthesize or characterize.
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    8. 8
      Taylor, R.; Mathers, G.; Banford, A. The development of future options for aqueous recycling of spent nuclear fuels. Prog. Nucl. Energy 2023, 164, 104837  DOI: 10.1016/j.pnucene.2023.104837
      There is no corresponding record for this reference.
    9. 9
      Arnold, P. L.; Dutkiewicz, M. S.; Walter, O. Organometallic Neptunium Chemistry. Chem. Rev. 2017, 117, 11460– 11475,  DOI: 10.1021/acs.chemrev.7b00192
      9
      Organometallic Neptunium Chemistry
      Arnold, Polly L.; Dutkiewicz, Michal S.; Walter, Olaf
      Chemical Reviews (Washington, DC, United States) (2017), 117 (17), 11460-11475CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Fifty years have passed since the foundation of organometallic neptunium chem., and yet only a handful of complexes have been reported, and even fewer have been fully characterized. Yet, increasingly, combined synthetic/spectroscopic/computational studies are demonstrating how covalently bonding, soft, carbocyclic organometallic ligands provide an excellent platform for advancing the fundamental understanding of the differences in orbital contributions and covalency in f-block metal-ligand bonding. Understanding the subtleties is the key to the safe handling and sepns. of the highly radioactive nuclei. This review describes the complexes that have been synthesized to date and presents a crit. assessment of the successes and difficulties in their anal. and the bonding information they have provided. Because of increasing recent efforts to start new Np-capable air-sensitive inorg. chem. labs., the importance of radioactivity, the basics of Np decay and its ramifications (including the radiochem. synthesis of one organometallic compd.), and the available anhyd. starting materials are also surveyed. The review also highlights a range of instances in which important differences in the chem. behavior between Np and its closest neighbors, uranium and plutonium, are found.
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    10. 10
      Baumgärtner, F.; Fischer, E. O.; Kanellakopulos, B.; Laubereau, P. Tetrakis(cyclopentadienyl)neptunium(IV). Angew. Chem., Int. Ed. 1968, 7, 634,  DOI: 10.1002/anie.196806341
      There is no corresponding record for this reference.
    11. 11
      Dutkiewicz, M. S.; Farnaby, J. H.; Apostolidis, C.; Colineau, E.; Walter, O.; Magnani, N.; Gardiner, M. G.; Love, J. B.; Kaltsoyannis, N.; Caciuffo, R.; Arnold, P. L. Organometallic neptunium(III) complexes. Nat. Chem. 2016, 8, 797– 802,  DOI: 10.1038/nchem.2520
      11
      Organometallic neptunium(iii) complexes
      Dutkiewicz, Michal S.; Farnaby, Joy H.; Apostolidis, Christos; Colineau, Eric; Walter, Olaf; Magnani, Nicola; Gardiner, Michael G.; Love, Jason B.; Kaltsoyannis, Nikolas; Caciuffo, Roberto; Arnold, Polly L.
      Nature Chemistry (2016), 8 (8), 797-802CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
      Studies of transuranic organometallic complexes provide a particularly valuable insight into covalent contributions to the metal-ligand bonding, in which the subtle differences between the transuranium actinide ions and their lighter lanthanide counterparts are of fundamental importance for the effective remediation of nuclear waste. Unlike the organometallic chem. of uranium, which has focused strongly on U(iii) and has seen some spectacular advances, that of the transuranics is significantly tech. more challenging and has remained dormant. In the case of neptunium, it is limited mainly to Np(iv). Here we report the synthesis of three new Np(iii) organometallic compds. and the characterization of their mol. and electronic structures. These studies suggest that Np(iii) complexes could act as single-mol. magnets, and that the lower oxidn. state of Np(ii) is chem. accessible. In comparison with lanthanide analogs, significant d- and f-electron contributions to key Np(iii) orbitals are obsd., which shows that fundamental neptunium organometallic chem. can provide new insights into the behavior of f-elements.
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    12. 12
      Arnold, P. L.; Farnaby, J. H.; White, R. C.; Kaltsoyannis, N.; Gardiner, M. G.; Love, J. B. Switchable π-coordination and C–H metallation in small-cavity macrocyclic uranium and thorium complexes. Chem. Sci. 2014, 5, 756– 765,  DOI: 10.1039/C3SC52072B
      12
      Switchable π-coordination and C-H metallation in small-cavity macrocyclic uranium and thorium complexes
      Arnold, Polly L.; Farnaby, Joy H.; White, Rebecca C.; Kaltsoyannis, Nikolas; Gardiner, Michael G.; Love, Jason B.
      Chemical Science (2014), 5 (2), 756-765CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      New, conformationally restricted ThIV and UIV complexes, [ThCl2(L)] and [UI2(L)], of the small-cavity, dipyrrolide, dianionic macrocycle trans-calix[2]benzene[2]pyrrolide (L)2- are reported and are shown to have unusual κ5:κ5 binding in a bent metallocene-type structure. Single-electron redn. of [UI2(L)] affords [UI(THF)(L)] and results in a switch in ligand binding from κ5-pyrrolide to η6-arene sandwich coordination, demonstrating the preference for arene binding by the electron-rich UIII ion. Facile loss of THF from [UI(THF)(L)] further increases the amt. of U-arene back donation. [UI(L)] can incorporate a further UIII equiv., UI3, to form the very unusual dinuclear complex [U2I4(L)] in which the single macrocycle adopts both κ5:κ5 and η6:κ1:η6:κ1 binding modes in the same complex. Hybrid d. functional theory calcns. carried out to compare the electronic structures and bonding of [UIIII(L)] and [UIII2I4(L)] indicate increased contributions to the covalent bonding in [U2I4(L)] than in [UI(L)], and similar U-arene interactions in both. MO anal. and QTAIM calcns. find minimal U-U interaction in [U2I4(L)]. In contrast to the reducible U complex, treatment of [ThCl2(L)] with either a reductant or non-nucleophilic base results in metalation of the aryl rings of the macrocycle to form the (L-2H)4- tetraanion and two new and robust Th-C bonds in the -ate complexes [K(THF)2ThIV(μ-Cl)(L-2H)]2 and K[ThIV{N(SiMe3)2}(L-2H)].
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    13. 13
      Brewster, J. T., II; Mangel, D. N.; Gaunt, A. J.; Saunders, D. P.; Zafar, H.; Lynch, V. M.; Boreen, M. A.; Garner, M. E.; Goodwin, C. A. P.; Settineri, N. S.; Arnold, J.; Sessler, J. L. In-Plane Thorium(IV), Uranium(IV), and Neptunium(IV) Expanded Porphyrin Complexes. J. Am. Chem. Soc. 2019, 141, 17867– 17874,  DOI: 10.1021/jacs.9b09123
      13
      In-Plane Thorium(IV), Uranium(IV), and Neptunium(IV) Expanded Porphyrin Complexes
      Brewster, James T.; Mangel, Daniel N.; Gaunt, Andrew J.; Saunders, Douglas P.; Zafar, Hadiqa; Lynch, Vincent M.; Boreen, Michael A.; Garner, Mary E.; Goodwin, Conrad A. P.; Settineri, Nicholas S.; Arnold, John; Sessler, Jonathan L.
      Journal of the American Chemical Society (2019), 141 (44), 17867-17874CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Here authors report the first series of in-plane thorium(IV), uranium(IV), and neptunium(IV) expanded porphyrin complexes. These actinide (An) complexes were synthesized using a hexa-aza porphyrin analog, termed dipyriamethyrin, and the nonaq. An(IV) precursors, ThCl4(DME)2, UCl4, and NpCl4(DME)2. The mol. and electronic structures of the ligand, each An(IV) complex, and a corresponding uranyl(VI) complex were characterized using NMR and UV-vis spectroscopies as well as single-crystal x-ray diffraction anal. Computational analyses of these complexes, coupled to their structural features, provide support for the conclusion that a greater degree of covalency in the ligand-cation orbital interactions arises as the early actinide series is traversed from Th(IV) to U(IV) and Np(IV). The axial ligands in the present An(IV) complexes proved labile, allowing for the electronic features of these complexes to be further modified.
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    14. 14
      Galley, S. S.; Pattenaude, S. A.; Ray, D.; Gaggioli, C. A.; Whitefoot, M. A.; Qiao, Y.; Higgins, R. F.; Nelson, W. L.; Baumbach, R.; Sperling, J. M.; Zeller, M.; Collins, T. S.; Schelter, E. J.; Gagliardi, L.; Albrecht-Schönzart, T. E.; Bart, S. C. Using Redox-Active Ligands to Generate Actinide Ligand Radical Species. Inorg. Chem. 2021, 60, 15242– 15252,  DOI: 10.1021/acs.inorgchem.1c01766
      14
      Using Redox-Active Ligands to Generate Actinide Ligand Radical Species
      Galley, Shane S.; Pattenaude, Scott A.; Ray, Debmalya; Gaggioli, Carlo Alberto; Whitefoot, Megan A.; Qiao, Yusen; Higgins, Robert F.; Nelson, W. L.; Baumbach, Ryan; Sperling, Joseph M.; Zeller, Matthias; Collins, Tyler S.; Schelter, Eric J.; Gagliardi, Laura; Albrecht-Schonzart, Thomas E.; Bart, Suzanne C.
      Inorganic Chemistry (2021), 60 (20), 15242-15252CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Using a redox-active dioxophenoxazine ligand, DOPO (DOPO = 2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazine-9-olate), a family of actinide (U, Th, Np, Pu) and Hf tris(ligand) coordination compds. were synthesized. Full characterization of these species using 1H NMR spectroscopy, electronic absorption spectroscopy, SQUID magnetometry, and x-ray crystallog. showed these compds. are analogous, existing in the form M(DOPOq)2(DOPOsq), where two ligands are of the oxidized, quinone form (DOPOq), and the third is of the reduced, semiquinone (DOPOsq) form. The electronic structures of these complexes were further investigated using CASSCF calcns., which revealed electronic structures consistent with metals in the +4 formal oxidn. state and one unpaired electron localized on one ligand in each complex. Furthermore, f-orbitals of the early actinides show a sizable bonding overlap with the ligand 2p orbitals. Notably, this is the first example of a plutonium-ligand radical species and a rare example of magnetic data recorded for a homogeneous plutonium coordination complex.
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    15. 15
      Yadav, S.; Dash, C. One-pot Tandem Heck alkynylation/cyclization reactions catalyzed by Bis(Pyrrolyl)pyridine based palladium pincer complexes. Tetrahedron 2020, 76, 131350  DOI: 10.1016/j.tet.2020.131350
      15
      One-pot Tandem Heck alkynylation/cyclization reactions catalyzed by Bis(Pyrrolyl)pyridine based palladium pincer complexes
      Yadav, Seema; Dash, Chandrakanta
      Tetrahedron (2020), 76 (30), 131350CODEN: TETRAB; ISSN:0040-4020. (Elsevier Ltd.)
      Ligand assisted palladium catalyzed one-pot tandem Heck alkynation/cyclization reactions for the synthesis of benzofurans I (Y = CH, N; R = H, 4-Me, 3-NH2, 4-Et, etc.; R1 = H, OMe; R2 = H, CHO) were reported in this paper. Well-defined palladium-pincer complexes II (R3 = H, OMe; R4 = Me, OMe-Ph) exhibited excellent catalytic activities for the one-pot tandem Heck alkynation/cyclization reactions yielding benzofuran derivs. I using 0.1 mol% catalyst. All the catalytic reactions are performed in air. The effects of variables such as solvents, the temp. on the catalytic activity are also reported. High product conversion was obtained for differently substituted 2-iodophenols such as 2-iodophenol, 2-iodopyridin-3-ol, 4-hydroxy-3-iodo-5-methoxybenzaldehyde at 120°C in 10 h.
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    16. 16
      Gowda, A. S.; Lee, T. S.; Rosko, M. C.; Petersen, J. L.; Castellano, F. N.; Milsmann, C. Long-Lived Photoluminescence of Molecular Group 14 Compounds through Thermally Activated Delayed Fluorescence. Inorg. Chem. 2022, 61, 7338– 7348,  DOI: 10.1021/acs.inorgchem.2c00182
      16
      Long-Lived Photoluminescence of Molecular Group 14 Compounds through Thermally Activated Delayed Fluorescence
      Gowda, Anitha S.; Lee, Tia S.; Rosko, Michael C.; Petersen, Jeffrey L.; Castellano, Felix N.; Milsmann, Carsten
      Inorganic Chemistry (2022), 61 (19), 7338-7348CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Photoluminescent mols. exploiting the sizable spin-orbit coupling consts. of main group metals and metalloids to access long-lived triplet excited states are relatively rare compared to phosphorescent transition metal complexes. Here we report the synthesis of three air- and moisture-stable group 14 compds. E(MePDPPh)2, where E = Si, Ge, or Sn and [MePDPPh]2- is the doubly deprotonated form of 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine. In soln., all three mols. exhibit exceptionally long-lived triplet excited states with lifetimes in the millisecond range and show highly efficient photoluminescence (Φ ≤ 0.49) due to competing prompt fluorescence and thermally activated delayed fluorescence at and around room temp. Temp.-dependent steady-state emission spectra and photoluminescent lifetime measurements provided conclusive evidence for the two distinct emission pathways. Picosecond transient absorption spectroscopy allowed further anal. of the intersystem crossing (ISC) between singlet and triplet manifolds (τISC = 0.25-3.1 ns) and confirmed the expected trend of increased ISC rates for the heavier elements in otherwise isostructural compds.
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    17. 17
      Gowda, A. S.; Petersen, J. L.; Milsmann, C. Redox Chemistry of Bis(pyrrolyl)pyridine Chromium and Molybdenum Complexes: An Experimental and Density Functional Theoretical Study. Inorg. Chem. 2018, 57, 1919– 1934,  DOI: 10.1021/acs.inorgchem.7b02809
      17
      Redox Chemistry of Bis(pyrrolyl)pyridine Chromium and Molybdenum Complexes: An Experimental and Density Functional Theoretical Study
      Gowda, Anitha S.; Petersen, Jeffrey L.; Milsmann, Carsten
      Inorganic Chemistry (2018), 57 (4), 1919-1934CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The three- and four-membered redox series [Cr(MePDP)2]z (z = 1-, 2-, 3-) and [Mo(MePDP)2]z (z = 0, 1-, 2-, 3-) were synthesized to study the redox properties of the pincer ligand MePDP2- (H2MePDP = 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine). The monoanionic complexes were characterized by x-ray crystallog., UV/visible/NIR spectroscopy, and magnetic susceptibility measurements. Exptl. and d. functional theory (DFT) studies are consistent with closed-shell MePDP2- ligands and +III oxidn. states (d3, S = 3/2) for the central metal ions. Cyclic voltammetry established multiple reversible redox processes for [M(MePDP)2]1- (M = Cr, Mo), which were further investigated via chem. oxidn. and redn. For molybdenum, one-electron oxidn. yielded Mo(MePDP)2 which was characterized by x-ray crystallog., UV/visible/NIR, and magnetic susceptibility measurements. The exptl. and computational data indicate metal-centered oxidn. to a MoIV complex (d2, S = 1) with two MePDP2- ligands. In contrast, one- and two-electron redns. are ligand centered giving MePDP•3- radicals, in which the unpaired electron is predominantly located on the central pyridine ring of the ligand. The presence of ligand radicals was established exptl. by observation of ligand-to-ligand intervalence charge transfer (LLIVCT) bands in the UV/visible/NIR spectra of the dianionic and trianionic complexes and further supported by broken-symmetry DFT calcns. X-ray crystallog. analyses of the one-electron-reduced species [M(MePDP)2]2- (S = 1, M = Cr, Mo) established structural indicators for pincer redn. and showed localization of the radical on one of the two pincer ligands. The two-electron-reduced, trianionic complexes (S = 1/2) were characterized by UV/visible/NIR spectroscopy, magnetic susceptibility measurements, and EPR spectroscopy. The electronic structures of the reduced complexes are best described as contg. +III metal ions (d3) antiferromagnetically coupled to one and two radical ligands for the dianionic and trianionic species, resp.
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    18. 18
      Hakey, B. M.; Darmon, J. M.; Akhmedov, N. G.; Petersen, J. L.; Milsmann, C. Reactivity of Pyridine Dipyrrolide Iron(II) Complexes with Organic Azides: C–H Amination and Iron Tetrazene Formation. Inorg. Chem. 2019, 58, 11028– 11042,  DOI: 10.1021/acs.inorgchem.9b01560
      18
      Reactivity of Pyridine Dipyrrolide Iron(II) Complexes with Organic Azides: C-H Amination and Iron Tetrazene Formation
      Hakey, Brett M.; Darmon, Jonathan M.; Akhmedov, Novruz G.; Petersen, Jeffrey L.; Milsmann, Carsten
      Inorganic Chemistry (2019), 58 (16), 11028-11042CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Reaction of (MesPDPPh)Fe(THF) (H2MesPDPPh = 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine) with org. azides was studied. The identity of the azide substituent had a profound impact on the transformation type and nature of the obsd. products. Reaction with arom. p-tolyl azide, N3Tol, resulted in exclusive formation of the corresponding iron tetrazene complex (MesPDPPh)Fe(N4Tol2). In contrast, the use of bulky 1-adamantyl azide led to clean intramol. C-H amination of one of the benzylic C-H bonds of a mesityl substituent on the pyridine dipyrrolide, PDP, supporting ligand. The smaller aliph. substituent in benzyl azide allowed for the isolation of two different compds. from distinct reaction pathways. One product is the result of double C-H amination of the PDP ligand via nitrene transfer, while the second one contains a dibenzyltetrazene and a benzaldimine ligand. All isolated complexes were characterized using a combination of x-ray crystallog., solid state magnetic susceptibility measurements, 1H NMR and 57Fe Mossbauer spectroscopy, and d. functional theory (DFT), and their electronic structures were elucidated. Potential electronic structures for putative iron(IV) imido or iron(III) imidyl radical complexes were explored via DFT calcns.
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    19. 19
      Hakey, B. M.; Darmon, J. M.; Zhang, Y.; Petersen, J. L.; Milsmann, C. Synthesis and Electronic Structure of Neutral Square-Planar High-Spin Iron(II) Complexes Supported by a Dianionic Pincer Ligand. Inorg. Chem. 2019, 58, 1252– 1266,  DOI: 10.1021/acs.inorgchem.8b02730
      19
      Synthesis and Electronic Structure of Neutral Square-Planar High-Spin Iron(II) Complexes Supported by a Dianionic Pincer Ligand
      Hakey, Brett M.; Darmon, Jonathan M.; Zhang, Yu; Petersen, Jeffrey L.; Milsmann, Carsten
      Inorganic Chemistry (2019), 58 (2), 1252-1266CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Two square-planar high-spin FeII complexes bearing a dianionic pyridine dipyrrolate pincer ligand and a di-Et ether or THF ligand were synthesized and structurally characterized, and their electronic structures were elucidated by a combined spectroscopic and computational approach. In contrast to previous examples, the S = 2 ground states of these square-planar FeII complexes do not require an overall anionic charge of the compds. or incorporation of alkali metal cations. The THF complex exhibits an equil. between four- and five-coordinate species in soln., which was supported by 1H NMR and 57Fe Mossbauer spectroscopy and comparison to a structurally characterized five-coordinate pyridine dipyrrolate iron bis-pyridine adduct. A detailed computational anal. of the electronic structures of the four- and five-coordinate species via d. functional theory provides insight into the origins of the unusual ground state configurations for FeII in a square-planar ligand field and explains the assocd. characteristic spectroscopic parameters.
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    20. 20
      Hakey, B. M.; Leary, D. C.; Rodriguez, J. G.; Martinez, J. C.; Vaughan, N. B.; Darmon, J. M.; Akhmedov, N. G.; Petersen, J. L.; Dolinar, B. S.; Milsmann, C. Effects of 2,6-Dichlorophenyl Substituents on the Coordination Chemistry of Pyridine Dipyrrolide Iron Complexes. Z. Anorg. Allg. Chem. 2021, 647, 1503– 1517,  DOI: 10.1002/zaac.202100117
      20
      Effects of 2,6-Dichlorophenyl Substituents on the Coordination Chemistry of Pyridine Dipyrrolide Iron Complexes
      Hakey, Brett M.; Leary, Dylan C.; Rodriguez, Jose G.; Martinez, Jordan C.; Vaughan, Nicholas B.; Darmon, Jonathan M.; Akhmedov, Novruz G.; Petersen, Jeffrey L.; Dolinar, Brian S.; Milsmann, Carsten
      Zeitschrift fuer Anorganische und Allgemeine Chemie (2021), 647 (14), 1503-1517CODEN: ZAACAB; ISSN:1521-3749. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A series of iron complexes featuring the pyridine dipyrrolide (PDP) pincer ligand [Cl2PhPDPPh]2-, obtained via deprotonation of 2,6-bis(5-(2,6-dichlorophenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine, H2Cl2PhPDPPh, is reported and structurally and spectroscopically characterized. While the bis-pyridine adduct (Cl2PhPDPPh)Fe(py)2 exhibits nearly identical features as previously reported (MesPDPPh)Fe(py)2 (H2MesPDPPh=2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine), the di-Et ether and THF adducts (Cl2PhPDPPh)Fe(OEt2) and (Cl2PhPDPPh)Fe(thf) show addnl. weak Fe-Cl interactions that impact the overall coordination geometries and result in strong deviations from planar coordination environments. The reaction of (Cl2PhPDPPh)Fe(thf) with 1-adamantyl azide provided the isolable iron imido complex (Cl2PhPDPPh)Fe(N1Ad), highlighting the improved stability of [Cl2PhPDPPh]2- towards intramol. nitrene group transfer from the high-valent iron-imido unit. The electronic structure of (Cl2PhPDPPh)Fe(N1Ad) was investigated by d. functional theory (DFT) and complete active space SCF (CASSCF) calcns. These computational studies suggest energetically close-lying diamagnetic and paramagnetic states and help to conceptualize the unusual magnetic properties of the complex obsd. by variable-temp. 1H NMR spectroscopy.
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    21. 21
      Hakey, B. M.; Leary, D. C.; Xiong, J.; Harris, C. F.; Darmon, J. M.; Petersen, J. L.; Berry, J. F.; Guo, Y.; Milsmann, C. High Magnetic Anisotropy of a Square-Planar Iron-Carbene Complex. Inorg. Chem. 2021, 60, 18575– 18588,  DOI: 10.1021/acs.inorgchem.1c01860
      21
      High Magnetic Anisotropy of a Square-Planar Iron-Carbene Complex
      Hakey, Brett M.; Leary, Dylan C.; Xiong, Jin; Harris, Caleb F.; Darmon, Jonathan M.; Petersen, Jeffrey L.; Berry, John F.; Guo, Yisong; Milsmann, Carsten
      Inorganic Chemistry (2021), 60 (24), 18575-18588CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Among Earth-abundant catalyst systems, iron-carbene intermediates that perform C-C bond forming reactions such as cyclopropanation of olefins and C-H functionalization via carbene insertion are rare. Detailed descriptions of the possible electronic structures for iron-carbene bonds are imperative to obtain better mechanistic insights and enable rational catalyst design. Here, the authors report the first square-planar iron-carbene complex (MesPDPPh)Fe(CPh2), where [MesPDPPh]2- is the doubly deprotonated form of [2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine]. The compd. was prepd. via reaction of the disubstituted diazoalkane N2CPh2 with (MesPDPPh)Fe(thf) and represents a rare example of a structurally characterized, paramagnetic iron-carbene complex. Temp.-dependent magnetic susceptibility measurements and applied-field Mossbauer spectroscopic studies revealed an orbitally near-degenerate S = 1 ground state with large unquenched orbital angular momentum resulting in high magnetic anisotropy. Spin-Hamiltonian anal. indicated that this S = 1 spin system has uniaxial magnetic properties arising from a ground MS = ±1 non-Kramers doublet that is well-sepd. from the MS = 0 sublevel due to very large axial zero-field splitting (D = -195 cm-1, E/D = 0.02 estd. from magnetic susceptibility data). This remarkable electronic structure gives rise to a very large, pos. magnetic hyperfine field of more than +60 T for the 57Fe nucleus along the easy magnetization axis obsd. by Mossbauer spectroscopy. Computational anal. with complete active space SCF (CASSCF) calcns. provides a detailed electronic structure anal. and confirms that (MesPDPPh)Fe(CPh2) exhibits a multiconfigurational ground state. The majority contribution originates from a configuration best described as a singlet carbene coordinated to an intermediate-spin FeII center with a (dxy)2{(dxz),(dz2)}3(dyz)1(dx2-y2)0 configuration featuring near-degenerate dxz and dz2 orbitals.
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    22. 22
      Sorsche, D.; Miehlich, M. E.; Searles, K.; Gouget, G.; Zolnhofer, E. M.; Fortier, S.; Chen, C.-H.; Gau, M.; Carroll, P. J.; Murray, C. B.; Caulton, K. G.; Khusniyarov, M. M.; Meyer, K.; Mindiola, D. J. Unusual Dinitrogen Binding and Electron Storage in Dinuclear Iron Complexes. J. Am. Chem. Soc. 2020, 142, 8147– 8159,  DOI: 10.1021/jacs.0c01488
      22
      Unusual Dinitrogen Binding and Electron Storage in Dinuclear Iron Complexes
      Sorsche, Dieter; Miehlich, Matthias E.; Searles, Keith; Gouget, Guillaume; Zolnhofer, Eva M.; Fortier, Skye; Chen, Chun-Hsing; Gau, Michael; Carroll, Patrick J.; Murray, Christopher B.; Caulton, Kenneth G.; Khusniyarov, Marat M.; Meyer, Karsten; Mindiola, Daniel J.
      Journal of the American Chemical Society (2020), 142 (18), 8147-8159CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A rare example of a dinuclear iron core with a non-linearly bridged dinitrogen ligand is reported in this work. One-electron redn. of [(tBupyrr2py)Fe(OEt2)] (1) (tBupyrr2py2- = 2,6-bis((3,5-di-tert-butyl)pyrrol-2-yl)pyridine) with KC8 yields the complex [K]2[(tBupyrr2py)Fe]2(μ2-η1:η1-N2) (2) where the unusual cis-divacant octahedral coordination geometry about each iron and the η5-cation-π coordination of two potassium ions with four pyrrolyl units of the ligand cause distortion of the bridging end-on μ-N2 about the FeN2Fe core. Attempts to generate an Et2O free version of 1 resulted instead in a dinuclear helical dimer [(tBupyrr2py)Fe]2 (3) via bridging of the pyridine moieties of the ligand. Redn. of 3 by two-electrons under N2 does not break up the dimer nor does it result in formation of 2, but instead formation of the ate-complex [K(OEt2)]2[(tBupyrr2py)Fe]2 (4). Redn. of 1 by two-electrons and in the presence of crown-ether forms the tetraanionic N2 complex [K2][K(18-crown-6)]2(tBupyrr2py)Fe2(μ2-η1:η1-N2) (5), also having a distorted FeN2Fe moiety akin to 2. Complex 2 is thermally unstable and loses N2, disproportionating to Fe nanoparticles among other products. A combination of single-crystal X-ray diffraction studies, soln. and solid state magnetic studies, and 57Fe Mossbauer spectroscopy were applied to characterize complexes 2-5, whereas DFT studies were used to help explain the bonding and electronic structure in these unique diiron-N2 complexes 2 and 5.
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    23. 23
      Yang, M.; Sheykhi, S.; Zhang, Y.; Milsmann, C.; Castellano, F. N. Low power threshold photochemical upconversion using a zirconium(iv) LMCT photosensitizer. Chem. Sci. 2021, 12, 9069– 9077,  DOI: 10.1039/D1SC01662H
      23
      Low power threshold photochemical upconversion using a zirconium(IV) LMCT photosensitizer
      Yang, Mo; Sheykhi, Sara; Zhang, Yu; Milsmann, Carsten; Castellano, Felix N.
      Chemical Science (2021), 12 (26), 9069-9077CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      The current investigation demonstrates highly efficient photochem. upconversion (UC) where a long-lived Zr(IV) ligand-to-metal charge transfer (LMCT) complex serves as a triplet photosensitizer in concert with well-established 9,10-diphenylanthracene (DPA) along with newly conceived DPA-carbazole based acceptors/annihilators in THF solns. The initial dynamic triplet-triplet energy transfer (TTET) processes (ΔG ∼ -0.19 eV) featured very large Stern-Volmer quenching consts. (KSV) approaching or achieving 105 M-1 with bimol. rate consts. between 2 and 3 x 108 M-1 s-1 as ascertained using static and transient spectroscopic techniques. Both the TTET and subsequent triplet-triplet annihilation (TTA) processes were verified and throughly investigated using transient absorption spectroscopy. The Stern-Volmer metrics support 95% quenching of the Zr(IV) photosensitizer using modest concns. (0.25 mM) of the various acceptor/annihilators, where no aggregation took place between any of the chromophores in THF. Each of the upconverting formulations operated with continuous-wave linear incident power dependence (λex = 514.5 nm) down to ultralow excitation power densities under optimized exptl. conditions. Impressive record-setting ηUC values ranging from 31.7% to 42.7% were achieved under excitation conditions (13 mW cm-2) below that of solar flux integrated across the Zr(IV) photosensitizer's absorption band (26.7 mW cm-2). This study illustrates the importance of supporting the continued development and discovery of mol.-based triplet photosensitizers based on earth-abundant metals.
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    24. 24
      Zhang, Y.; Leary, D. C.; Belldina, A. M.; Petersen, J. L.; Milsmann, C. Effects of Ligand Substitution on the Optical and Electrochemical Properties of (Pyridinedipyrrolide)zirconium Photosensitizers. Inorg. Chem. 2020, 59, 14716– 14730,  DOI: 10.1021/acs.inorgchem.0c02343
      24
      Effects of Ligand Substitution on the Optical and Electrochemical Properties of (Pyridinedipyrrolide)zirconium Photosensitizers
      Zhang, Yu; Leary, Dylan C.; Belldina, Anne M.; Petersen, Jeffrey L.; Milsmann, Carsten
      Inorganic Chemistry (2020), 59 (20), 14716-14730CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Seven bis(pyridinedipyrrolide)zirconium complexes, Zr(R1PDPR2)2, where [R1PDPR2]2- is the doubly deprotonated form of [2,6-bis(5-R1-3-R2-1H-pyrrol-2-yl)pyridine], were prepd. and characterized in soln. by NMR, UV/visible absorption, and emission spectroscopy and cyclic voltammetry. The mol. structures were detd. by single-crystal x-ray crystallog. All complexes exhibit remarkably long emission lifetimes (τ = 190-576μs) with high quantum efficiencies (ΦPL = 0.10-0.38) upon excitation with visible light in a benzene soln. The substituents on the pyrrolide rings have significant effects on the photoluminescence and electrochem. properties of these compds. The R2 substituents (R2 = H, Me, Ph, or C6F5) show only limited effects on the absorption and emission profiles of the complexes but allow systematic tuning of the ground- and excited-state redox potentials over a range of almost 600 mV. The R1 substituents (R1 = H, Me, Ph, or 2,4,6-Me3Ph) influence both the optical and electrochem. properties through electronic effects. Addnl., the R1 substituents have profound consequences for the structural flexibility and overall stability of the compds. Distortions of the Zr(PDP)2 core from idealized D2d symmetry in the solid state can be traced to the steric profiles of the R1 substituents and correlate with the obsd. Stokes shifts for each compd. The complex with the smallest ligand system, Zr(HPDPH)2, coordinates two addnl. solvent mols. in a THF soln., which gave photoluminescent, eight-coordinate Zr(HPDPH)2(THF)2. The photoredox catalytic dehalogenation of aryl iodides and aryl chlorides using the most reducing deriv., Zr(MePDPMe)2, highlights the potential of Zr(PDP)2 photosensitizers to promote challenging reductive transformations under mild conditions upon excitation with green light. The effects of pyrrolide substitution on the optical and electrochem. properties of photoluminescent bis(pyridinedipyrrolide)zirconium complexes, Zr(PDP)2, are reported. The steric effects of the substituents heavily influence the structural rigidity of the complexes, translating into pronounced changes in the optical properties. The electronic characteristics of the substituents have remarkably limited consequences for the optical properties but allow systematic tuning of the ground- and excited-state redox potentials.
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    25. 25
      Zhang, Y.; Lee, T. S.; Favale, J. M.; Leary, D. C.; Petersen, J. L.; Scholes, G. D.; Castellano, F. N.; Milsmann, C. Delayed fluorescence from a zirconium(iv) photosensitizer with ligand-to-metal charge-transfer excited states. Nat. Chem. 2020, 12, 345– 352,  DOI: 10.1038/s41557-020-0430-7
      25
      Delayed fluorescence from a zirconium(IV) photosensitizer with ligand-to-metal charge-transfer excited states
      Zhang, Yu; Lee, Tia S.; Favale, Joseph M.; Leary, Dylan C.; Petersen, Jeffrey L.; Scholes, Gregory D.; Castellano, Felix N.; Milsmann, Carsten
      Nature Chemistry (2020), 12 (4), 345-352CODEN: NCAHBB; ISSN:1755-4330. (Nature Research)
      Advances in chem. control of the photophys. properties of transition-metal complexes are revolutionizing a wide range of technologies, particularly photocatalysis and light-emitting diodes, but they rely heavily on mols. contg. precious metals such as ruthenium and iridium. Although the application of earth-abundant 'early' transition metals in photosensitizers is clearly advantageous, a detailed understanding of excited states with ligand-to-metal charge transfer (LMCT) character is paramount to account for their distinct electron configurations. Here we report an air- and moisture-stable, visible light-absorbing Zr(IV) photosensitizer, Zr(MesPDPPh)2, where [MesPDPPh]2- is the doubly deprotonated form of [2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine]. This mol. has an exceptionally long-lived triplet LMCT excited state (τ = 350μs), featuring highly efficient photoluminescence emission (Φ = 0.45) due to thermally activated delayed fluorescence emanating from the higher-lying singlet configuration with significant LMCT contributions. Zr(MesPDPPh)2 engages in numerous photoredox catalytic processes and triplet energy transfer. Our investigation provides a blueprint for future photosensitizer development featuring early transition metals and excited states with significant LMCT contributions. [graphic not available: see fulltext].
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    26. 26
      Zhang, Y.; Petersen, J. L.; Milsmann, C. A Luminescent Zirconium(IV) Complex as a Molecular Photosensitizer for Visible Light Photoredox Catalysis. J. Am. Chem. Soc. 2016, 138, 13115– 13118,  DOI: 10.1021/jacs.6b05934
      26
      A Luminescent Zirconium(IV) Complex as a Molecular Photosensitizer for Visible Light Photoredox Catalysis
      Zhang, Yu; Petersen, Jeffrey L.; Milsmann, Carsten
      Journal of the American Chemical Society (2016), 138 (40), 13115-13118CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Titanium and zirconium complexes carrying two 2,6-bis(pyrrolyl)pyridine ligands have been synthesized and characterized. The neutral complexes Ti(MePDP)2 and Zr(MePDP)2 (MePDP = 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine) show intense ligand-to-metal charge-transfer bands in the visible region and undergo multiple reversible redox events under highly reducing conditions. Zr(MePDP)2 exhibits photoluminescent behavior and its excited state can be quenched by mild reductants to generate a powerful electron transfer reagent with a ground state potential of -2.16 V vs Fc+/0. This reactivity was utilized to facilitate dehalogenation reactions, the redn. of electron-poor olefins, and the reductive coupling of benzyl bromide via photoredox catalysis. In these reactions, the earth-abundant metal complex Zr(MePDP)2 acts as a substitute for the precious metal photosensitizer [Ru(bpy)3]2+.
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    27. 27
      Hakey, B. M.; Leary, D. C.; Lopez, L. M.; Valerio, L. R.; Brennessel, W. W.; Milsmann, C.; Matson, E. M. Synthesis and Characterization of Pyridine Dipyrrolide Uranyl Complexes. Inorg. Chem. 2022, 61, 6182– 6192,  DOI: 10.1021/acs.inorgchem.2c00348
      27
      Synthesis and Characterization of Pyridine Dipyrrolide Uranyl Complexes
      Hakey, Brett M.; Leary, Dylan C.; Lopez, Lauren M.; Valerio, Leyla R.; Brennessel, William W.; Milsmann, Carsten; Matson, Ellen M.
      Inorganic Chemistry (2022), 61 (16), 6182-6192CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The first actinide complexes of the pyridine dipyrrolide (PDP) ligand class, (MesPDPPh)UO2(THF) and (Cl2PhPDPPh)UO2(THF), are reported as the UVI uranyl adducts of the bulky aryl substituted pincers (MesPDPPh)2- and (Cl2PhPDPPh)2- (derived from 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine (H2MesPDPPh, Mes = 2,4,6-trimethylphenyl), and 2,6-bis(5-(2,6-dichlorophenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine (H2Cl2PhPDPPh, Cl2Ph = 2,6-dichlorophenyl), resp.). Following the in situ deprotonation of the proligand with lithium hexamethyldisilazide to generate the corresponding dilithium salts (e.g., Li2ArPDPPh, Ar = Mes of Cl2Ph), salt metathesis with [UO2Cl2(THF)2]2 afforded both compds. in moderate yields. The characterization of each species has been undertaken by a combination of solid- and soln.-state methods, including combustion anal., IR, electronic absorption, and NMR spectroscopies. In both complexes, single-crystal x-ray diffraction has revealed a distorted octahedral geometry in the solid state, enforced by the bite angle of the rigid meridional (ArPDPPh)2- pincer ligand. The electrochem. anal. of both compds. by cyclic voltammetry in THF (THF) reveals rich redox profiles, including events assigned as UVI/UV redox couples. A time-dependent d. functional theory study has been performed on (MesPDPPh)UO2(THF) and provides insight into the nature of the transitions that comprise its electronic absorption spectrum.
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    28. 28
      Valerio, L. R.; Hakey, B. M.; Leary, D. C.; Stockdale, E.; Brennessel, W. W.; Milsmann, C.; Matson, E. M. Synthesis and Characterization of Isostructural Th(IV) and U(IV) Pyridine Dipyrrolide Complexes. Inorg. Chem. 2024, 63, 9610– 9623,  DOI: 10.1021/acs.inorgchem.3c04391
      There is no corresponding record for this reference.
    29. 29
      Leary, D. C.; Zhang, Y.; Rodriguez, J. G.; Akhmedov, N. G.; Petersen, J. L.; Dolinar, B. S.; Milsmann, C. Organometallic Intermediates in the Synthesis of Photoluminescent Zirconium and Hafnium Complexes with Pyridine Dipyrrolide Ligands. Organometallics 2023, 42, 1220– 1231,  DOI: 10.1021/acs.organomet.3c00058
      29
      Organometallic Intermediates in the Synthesis of Photoluminescent Zirconium and Hafnium Complexes with Pyridine Dipyrrolide Ligands
      Leary, Dylan C.; Zhang, Yu; Rodriguez, Jose G.; Akhmedov, Novruz G.; Petersen, Jeffrey L.; Dolinar, Brian S.; Milsmann, Carsten
      Organometallics (2023), 42 (11), 1220-1231CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      The two com. available Zr complexes tetrakis(dimethylamido)zirconium, Zr(NMe2)4, and tetrabenzylzirconium, ZrBn4, were studied for their utility as starting materials in the synthesis of bis(pyridine dipyrrolide)zirconium photosensitizers, Zr(PDP)2. Reaction with one equiv. of the ligand precursor 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine, H2MePDPPh, resulted in the isolation and structural characterization of the complexes (MePDPPh)Zr(NMe2)2THF and (MePDPPh)ZrBn2, which could be converted to the desired photosensitizer Zr(MePDPPh)2 upon addn. of a 2nd equiv. of H2MePDPPh. Using the more sterically encumbered ligand precursor 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine, H2MesPDPPh, only ZrBn4 yielded the desired bis-ligand complex Zr(MesPDPPh)2. Careful monitoring of the reaction at different temps. revealed the importance of the organometallic intermediate (cyclo-MesPDPPh)ZrBn, which was characterized by x-ray diffraction anal. and 1H NMR spectroscopy and shown to contain a cyclometalated MesPDPPh unit. Taking inspiration from the results for Zr, syntheses for two Hf photosensitizers, Hf(MePDPPh)2 and Hf(MesPDPPh)2, were established and shown to proceed through similar intermediates starting from tetrabenzylhafnium, HfBn4. Initial studies of the photophys. properties of the photoluminescent Hf complexes indicate similar optical properties compared to their Zr analogs.
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    30. 30
      Cotton, S. Introduction to the Actinides. In Lanthanide and Actinide Chemistry ; 2006; pp 145– 153.
      There is no corresponding record for this reference.
    31. 31
      Reilly, S. D.; Brown, J. L.; Scott, B. L.; Gaunt, A. J. Synthesis and characterization of NpCl4(DME)2 and PuCl4(DME)2 neutral transuranic An(IV) starting materials. Dalton Trans. 2014, 43, 1498– 1501,  DOI: 10.1039/C3DT53058B
      31
      Synthesis and characterization of NpCl4(DME)2 and PuCl4(DME)2 neutral transuranic An(iv) starting materials
      Reilly, Sean D.; Brown, Jessie L.; Scott, Brian L.; Gaunt, Andrew J.
      Dalton Transactions (2014), 43 (4), 1498-1501CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      The 1,2-dimethoxyethane (DME) solvento adducts of Np(iv) and Pu(iv) tetrachloride were prepd. and isolated in good and moderate yields, resp., along with single-crystal structural detns. These neutral mols. are expected to provide alternative synthetic pathways in the pursuit of nonaq. and organometallic complexes.
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    32. 32
      Hermann, J. A.; Suttle, J. F.; Hoekstra, H. R. Uranium(IV) Chloride. Inorg. Synth. 1957, 5, 143– 145,  DOI: 10.1002/9780470132364.ch39
      32
      Uranium(IV) chloride
      Hermann, John A.; Suttle, John F.
      (1957), 5 (), 143-5 ISSN:.
      cf. C.A. 50, 6239i.
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    33. 33
      Cantat, T.; Scott, B. L.; Kiplinger, J. L. Convenient access to the anhydrous thorium tetrachloride complexes ThCl4(DME)2, ThCl4(1,4-dioxane)2 and ThCl4(THF)3.5 using commercially available and inexpensive starting materials. Chem. Commun. 2010, 46, 919– 921,  DOI: 10.1039/b923558b
      33
      Convenient access to the anhydrous thorium tetrachloride complexes ThCl4(DME)2, ThCl4(1,4-dioxane)2 and ThCl4(THF)3.5 using commercially available and inexpensive starting materials
      Cantat, Thibault; Scott, Brian L.; Kiplinger, Jaqueline L.
      Chemical Communications (Cambridge, United Kingdom) (2010), 46 (6), 919-921CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Anhyd. thorium tetrachloride complexes ThCl4(DME)2, ThCl4(1,4-dioxane)2, and ThCl4(THF)3.5 have been easily accessed from inexpensive, com. available reagents under mild conditions and serve as excellent precursors to a variety of thorium(iv) halide, alkoxide, amide and organometallic compds.
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    34. 34
      Lopez, L. M.; Uible, M. C.; Zeller, M.; Bart, S. C. Lewis base adducts of NpCl4. Chem. Commun. 2024, 60, 5956– 5959,  DOI: 10.1039/D4CC01560F
      There is no corresponding record for this reference.
    35. 35
      Pattenaude, S. A.; Anderson, N. H.; Bart, S. C.; Gaunt, A. J.; Scott, B. L. Non-aqueous neptunium and plutonium redox behaviour in THF─access to a rare Np(III) synthetic precursor. Chem. Commun. 2018, 54, 6113– 6116,  DOI: 10.1039/C8CC02611D
      There is no corresponding record for this reference.
    36. 36
      Staun, S. L.; Stevens, L. M.; Smiles, D. E.; Goodwin, C. A. P.; Billow, B. S.; Scott, B. L.; Wu, G.; Tondreau, A. M.; Gaunt, A. J.; Hayton, T. W. Expanding the Nonaqueous Chemistry of Neptunium: Synthesis and Structural Characterization of [Np(NR2)3Cl], [Np(NR2)3Cl]−, and [Np{N(R)(SiMe2CH2)}2(NR2)]− (R = SiMe3). Inorg. Chem. 2021, 60, 2740– 2748,  DOI: 10.1021/acs.inorgchem.0c03616
      36
      Expanding the Nonaqueous Chemistry of Neptunium: Synthesis and Structural Characterization of [Np(NR2)3Cl], [Np(NR2)3Cl]-, and [Np{N(R)(SiMe2CH2)}2(NR2)]- (R = SiMe3)
      Staun, Selena L.; Stevens, Lauren M.; Smiles, Danil E.; Goodwin, Conrad A. P.; Billow, Brennan S.; Scott, Brian L.; Wu, Guang; Tondreau, Aaron M.; Gaunt, Andrew J.; Hayton, Trevor W.
      Inorganic Chemistry (2021), 60 (4), 2740-2748CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Reaction of 3 equiv of NaNR2 (R = SiMe3) with NpCl4(DME)2 in THF afforded the Np(IV) silylamide complex, [Np(NR2)3Cl] (1), in good yield. Reaction of 1 with 1.5 equiv of KC8 in THF, in the presence of 1 equiv of dibenzo-18-crown-6, gave [{K(DB-18-C-6)(THF)}3(μ3-Cl)][Np(NR2)3Cl]2 (4), also in good yield. Complex 4 represents the first structurally characterized Np(III) amide. Finally, reaction of NpCl4(DME)2 with 5 equiv of NaNR2 and 1 equiv of dibenzo-18-crown-6 afforded the Np(IV) bis(metallacycle), [{Na(DB-18-C-6)(Et2O)0.62(κ1-DME)0.38}2(μ-DME)][Np{N(R)(SiMe2CH2)}2(NR2)]2 (8), in moderate yield. Complex 8 was characterized by 1H NMR spectroscopy and x-ray crystallog. and represents a rare example of a structurally characterized neptunium-hydrocarbyl complex. To support these studies, the authors also synthesized the uranium analogs of 4 and 8, namely, [K(2,2,2-cryptand)][U(NR2)3Cl] (2), [K(DB-18-C-6)(THF)2][U(NR2)3Cl] (3), [Na(DME)3][U{N(R)(SiMe2CH2)}2(NR2)] (6), and [{Na(DB-18-C-6)(Et2O)0.5(κ1-DME)0.5}2(μ-DME)][U{N(R)(SiMe2CH2)}2(NR2)]2 (7). Complexes 2, 3, 6, and 7 were characterized by a no. of techniques, including NMR spectroscopy and x-ray crystallog.
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    37. 37
      Grödler, D.; Sperling, J. M.; Rotermund, B. M.; Scheibe, B.; Beck, N. B.; Mathur, S.; Albrecht-Schönzart, T. E. Neptunium Alkoxide Chemistry: Expanding Alkoxides to the Transuranium Elements. Inorg. Chem. 2023, 62, 2513– 2517,  DOI: 10.1021/acs.inorgchem.2c04338
      There is no corresponding record for this reference.
    38. 38
      Su, J.; Batista, E. R.; Boland, K. S.; Bone, S. E.; Bradley, J. A.; Cary, S. K.; Clark, D. L.; Conradson, S. D.; Ditter, A. S.; Kaltsoyannis, N. Energy-Degeneracy-Driven Covalency in Actinide Bonding. J. Am. Chem. Soc. 2018, 140, 17977– 17984,  DOI: 10.1021/jacs.8b09436
      38
      Energy-Degeneracy-Driven Covalency in Actinide Bonding
      Su, Jing; Batista, Enrique R.; Boland, Kevin S.; Bone, Sharon E.; Bradley, Joseph A.; Cary, Samantha K.; Clark, David L.; Conradson, Steven D.; Ditter, Alex S.; Kaltsoyannis, Nikolas; Keith, Jason M.; Kerridge, Andrew; Kozimor, Stosh A.; Loble, Matthias W.; Martin, Richard L.; Minasian, Stefan G.; Mocko, Veronika; La Pierre, Henry S.; Seidler, Gerald T.; Shuh, David K.; Wilkerson, Marianne P.; Wolfsberg, Laura E.; Yang, Ping
      Journal of the American Chemical Society (2018), 140 (51), 17977-17984CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Evaluating the nature of chem. bonding for actinide elements represents one of the most important and long-standing problems in actinide science. We directly address this challenge and contribute a Cl K-edge X-ray absorption spectroscopy and relativistic d. functional theory study that quant. evaluates An-Cl covalency in AnCl62- (AnIV = Th, U, Np, Pu). The results showed significant mixing between Cl 3p- and AnIV 5f- and 6d-orbitals (t1u*/t2u* and t2g*/eg*), with the 6d-orbitals showing more pronounced covalent bonding than the 5f-orbitals. Moving from Th to U, Np, and Pu markedly changed the amt. of M-Cl orbital mixing, such that AnIV 6d- and Cl 3p-mixing decreased and metal 5f- and Cl 3p-orbital mixing increased across this series.
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    39. 39
      Sonnenberger, D. C.; Gaudiello, J. G. Cyclic voltammetric study of organoactinide compounds of uranium(IV) and neptunium(IV) Ligand effects on the M(IV)/M(III) couple. Inorg. Chem. 1988, 27, 2747– 2748,  DOI: 10.1021/ic00288a036
      39
      Cyclic voltammetric study of organoactinide compounds of uranium(IV) and neptunium(IV). Ligand effects on the M(IV)/M(III) couple
      Sonnenberger, David C.; Gaudiello, John G.
      Inorganic Chemistry (1988), 27 (15), 2747-8CODEN: INOCAJ; ISSN:0020-1669.
      The electrochem. redn. of a series of organoactinide complexes [Cp4M (Cp = η5-C5H5; M = U, Np], Cp3MCl (M = U, Np), and Cp2*MCl2 [Cp* = η5-C5Me5; M = U, Np]) were investigated by cyclic voltammetry. The ease with which these complexes are reduced varies with the nature of the ligand environment. The redox potential of the U(IV)/U(III) couple is more sensitive to ligand environment than the Np(IV)/Np(III) couple.
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  • Supporting Information

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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.organomet.4c00472.

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