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1. Single-Electron Redox Chemistry on the [Cp*Rh] Platform Enabled by a Nitrated Bipyridyl Ligand

   https://doi.org/10.3390/molecules23112857  

   Moore, William; Henke, Wade; Lionetti, Davide; Day, Victor; Blakemore, James (November 2018, Molecules)

   [Cp*Rh] complexes (Cp* = pentamethylcyclopentadienyl) are attracting renewed interest in coordination chemistry and catalysis, but these useful compounds often undergo net two-electron redox cycling that precludes observation of individual one-electron reduction events. Here, we show that a [Cp*Rh] complex bearing the 4,4′-dinitro-2,2′-bipyridyl ligand (dnbpy) (3) can access a distinctive manifold of five oxidation states in organic electrolytes, contrasting with prior work that found no accessible reductions in aqueous electrolyte. These states are readily generated from a newly isolated and fully characterized rhodium(III) precursor complex 3, formulated as [Cp*Rh(dnbpy)Cl]PF6. Single-crystal X-ray diffraction (XRD) data, previously unavailable for the dnbpy ligand bound to the [Cp*Rh] platform, confirm the presence of both [η5-Cp*] and [κ2-dnbpy]. Four individual one-electron reductions of 3 are observed, contrasting sharply with the single two-electron reductions of other [Cp*Rh] complexes. Chemical preparation and the study of the singly reduced species with electronic absorption and electron paramagnetic resonance spectroscopies indicate that the first reduction is predominantly centered on the dnbpy ligand. Comparative cyclic voltammetry studies with [NBu4][PF6] and [NBu4][Cl] as supporting electrolytes indicate that the chloride ligand can be lost from 3 by ligand exchange upon reduction. Spectroelectrochemical studies with ultraviolet (UV)-visible detection reveal isosbestic behavior, confirming the clean interconversion of the reduced forms of 3 inferred from the voltammetry with [NBu4][PF6] as supporting electrolyte. Electrochemical reduction in the presence of triethylammonium results in an irreversible response, but does not give rise to catalytic H2 evolution, contrasting with the reactivity patterns observed in [Cp*Rh] complexes bearing bipyridyl ligands with less electron-withdrawing substituents. 

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2. Electrochemical studies of tris(cyclopentadienyl)thorium and uranium complexes in the +2, +3, and +4 oxidation states

   https://doi.org/10.1039/D1SC01906F  

   Wedal, Justin C.; Barlow, Jeffrey M.; Ziller, Joseph W.; Yang, Jenny Y.; Evans, William J. (June 2021, Chemical Science)

   Electrochemical measurements on tris(cyclopentadienyl)thorium and uranium compounds in the +2, +3, and +4 oxidation states are reported with C 5 H 3 (SiMe 3 ) 2 , C 5 H 4 SiMe 3 , and C 5 Me 4 H ligands. The reduction potentials for both U and Th complexes trend with the electron donating abilities of the cyclopentadienyl ligand. Thorium complexes have more negative An( iii )/An( ii ) reduction potentials than the uranium analogs. Electrochemical measurements of isolated Th( ii ) complexes indicated that the Th( iii )/Th( ii ) couple was surprisingly similar to the Th( iv )/Th( iii ) couple in Cp′′-ligated complexes. This suggested that Th( ii ) complexes could be prepared from Th( iv ) precursors and this was demonstrated synthetically by isolation of directly from UV-visible spectroelectrochemical measurements and reactions of with elemental barium indicated that the thorium system undergoes sequential one electron transformations. 

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3. C–H bond activation via concerted metalation–deprotonation at a palladium( iii ) center

   https://doi.org/10.1039/D3SC00034F  

   Bouley, Bailey S.; Tang, Fengzhi; Bae, Dae Young; Mirica, Liviu M. (April 2023, Chemical Science)

   Herein we report the direct observation of C–H bond activation at an isolated mononuclear Pd( iii ) center. The oxidation of the Pd( ii ) complex ( Me N4)Pd II (neophyl)Cl (neophyl = –CH 2 C(CH 3 ) 2 Ph; Me N4 = N , N ′-dimethyl-2,11-diaza[3.3](2,6)pyridinophane) using the mild oxidant ferrocenium hexafluorophosphate (FcPF 6 ) yields the stable Pd( iii ) complex [( Me N4)Pd III (neophyl)Cl]PF 6 . Upon the addition of an acetate source, [( Me N4)Pd III (neophyl)Cl]PF 6 undergoes Csp 2 –H bond activation to yield the cyclometalated product [( Me N4)Pd III (cycloneophyl)]PF 6 . This metalacycle can be independently prepared, allowing for a complete characterization of both the starting and final Pd( iii ) complexes. The C–H activation step can be monitored directly by EPR and UV-Vis spectroscopies, and kinetic isotope effect (KIE) studies suggest that either a pre-association step such as an agostic interaction may be rate limiting, or that the C–H activation is partially rate-limiting in conjunction with ligand rearrangement. Density functional theory calculations support that the reaction proceeds through a κ 3 ligand coordination and that the flexible ligand structure is important for this transformation. Overall, this study represents the first example of discrete C–H bond activation occurring at a Pd( iii ) center through a concerted metalation–deprotonation mechanism, akin to that observed for Pd( ii ) and Pd( iv ) centers. 

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4. In search of tris(trimethylsilylcyclopentadienyl) thorium

   https://doi.org/10.1039/C9DT03674A  

   Wedal, Justin C.; Bekoe, Samuel; Ziller, Joseph W.; Furche, Filipp; Evans, William J. (November 2019, Dalton Transactions)

   Reduction of Cp′ 3 ThCl, Cp′ 3 ThBr, and Cp′ 3 ThI (Cp′ = C 5 H 4 SiMe 3 ) with potassium graphite generates dark blue solutions with reactivity and spectroscopic properties consistent with the formation of Cp′ 3 Th. The EPR and UV-visible spectra of the solutions are similar to those of crystallographically-characterized tris(cyclopentadienyl) Th( iii ) complexes: [C 5 H 3 (SiMe 3 ) 2 ] 3 Th, (C 5 Me 4 H) 3 Th, (C 5 t Bu 2 H 3 ) 3 Th, and (C 5 Me 5 ) 3 Th. Density functional theory (DFT) analysis indicates that the UV-visible spectrum is consistent with Cp′ 3 Th and not [Cp′ 3 ThBr] 1− . Although single crystals of Cp′ 3 Th have not been isolated, the blue solution reacts with Me 3 SiCl, I 2 , and HCCPh to afford products expected from Cp′ 3 Th, namely, Cp′ 3 ThCl, Cp′ 3 ThI, and Cp′ 3 Th(CCPh), respectively. Reactions with MeI give mixtures of Cp′ 3 ThI and Cp′ 3 ThMe. Evidence for further reduction of the blue solutions to a Cp′-ligated Th( ii ) complex has not been observed. The crystal structures of Cp′ 3 ThMe and (Cp′ 3 Th) 2 (μ-O) were also determined as part of these studies. 

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5. Electronic structure of (MePh ₃ P) ₂ [Ni ^II (bdtCl ₂ ) ₂ ]·2(CH ₃ ) ₂ SO and (MePh ₃ P)[Ni ^III (bdtCl ₂ ) ₂ ] (bdtCl ₂ = 3,6-dichlorobenzene-1,2-dithiolate)

   https://doi.org/10.1107/S2052520621011495  

   Adamko Koziskova, Julia; Chen, Yu-Sheng; Grass, Su-Yin; Chuang, Yu-Chun; Hsu, I-Jui; Wang, Yu; Lutz, Martin; Volkov, Anatoliy; Herich, Peter; Vénosová, Barbora; et al (December 2021, Acta Crystallographica Section B Structural Science, Crystal Engineering and Materials)

   High-resolution X-ray diffraction experiments, theoretical calculations and atom-specific X-ray absorption experiments were used to investigate two nickel complexes, (MePh 3 P) 2 [Ni II (bdtCl 2 ) 2 ]·2(CH 3 ) 2 SO [complex (1)] and (MePh 3 P)[Ni III (bdtCl 2 ) 2 ] [complex (2)]. Combining the techniques of nickel K - and sulfur K -edge X-ray absorption spectroscopy with high-resolution X-ray charge density modeling, together with theoretical calculations, the actual oxidation states of the central Ni atoms in these two complexes are investigated. Ni ions in two complexes are clearly in different oxidation states: the Ni ion of complex (1) is formally Ni II ; that of complex (2) should be formally Ni III , yet it is best described as a combination of Ni 2+ and Ni 3+ , due to the involvement of the non-innocent ligand in the Ni— L bond. A detailed description of Ni—S bond character (σ,π) is presented. 

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