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1. Structural and chemical properties of half-sandwich rhodium complexes supported by the bis(2-pyridyl)methane ligand

   https://doi.org/10.1039/C9DT01821B  

   Lionetti, Davide ; Day, Victor W. ; Blakemore, James D. ( August 2019 , Dalton Transactions)

   [Cp*Rh] complexes (Cp* = pentamethylcyclopentadienyl) supported by bidentate chelating ligands are a useful class of compounds for studies of redox chemistry and catalysis. Here, we show that the bis(2-pyridyl)methane ligand, also known as dipyridylmethane or dpma, can support [Cp*Rh] complexes in the formally + iii and + ii rhodium oxidation states. Specifically, two new rhodium complexes ([Cp*Rh(dpma)(L)] n+ , L = Cl − , CH 3 CN) have been isolated and structurally characterized, and the properties of the complexes have been compared with those of [Cp*Rh] complexes bearing the related dimethyldipyridylmethane (Me 2 dpma) ligand. Complex [Cp*Rh(dpma)(NCCH 3 )] 2+more »displays a quasireversible rhodium( iii / ii ) reduction by cyclic voltammetry; related electron paramagnetic resonance (EPR) spectroscopic studies confirm access to the unusual rhodium( ii ) oxidation state. Further reduction to the formally rhodium( i ) oxidation state, however, is followed by deprotonation of dpma, as observed in electrochemical studies and chemical reduction experiments. This reactivity can be understood to occur as a consequence of the presence of doubly benzylic protons in the dpma ligand, since use of the analogous Me 2 dpma enables reduction to rhodium( i ) without involvement of ligand deprotonation. These findings highlight the important role of the ligand backbone substitution pattern in influencing the stability of highly-reduced complexes, a key class of metal species for study of electron and proton management in catalysis.« less
2. Semiquinone radical-bridged M ₂ (M = Fe, Co, Ni) complexes with strong magnetic exchange giving rise to slow magnetic relaxation

   https://doi.org/10.1039/d0sc03078c  

   Chakarawet, Khetpakorn ; Harris, T. David ; Long, Jeffrey R. ( August 2020 , Chemical Science)

   The use of radical bridging ligands to facilitate strong magnetic exchange between paramagnetic metal centers represents a key step toward the realization of single-molecule magnets with high operating temperatures. Moreover, bridging ligands that allow the incorporation of high-anisotropy metal ions are particularly advantageous. Toward these ends, we report the synthesis and detailed characterization of the dinuclear hydroquinone-bridged complexes [(Me 6 tren) 2 MII2(C 6 H 4 O 2 2− )] 2+ (Me 6 tren = tris(2-dimethylaminoethyl)amine; M = Fe, Co, Ni) and their one-electron-oxidized, semiquinone-bridged analogues [(Me 6 tren) 2 MII2(C 6 H 4 O 2 − ˙)] 3+ .more »Single-crystal X-ray diffraction shows that the Me 6 tren ligand restrains the metal centers in a trigonal bipyramidal geometry, and coordination of the bridging hydro- or semiquinone ligand results in a parallel alignment of the three-fold axes. We quantify the p -benzosemiquinone–transition metal magnetic exchange coupling for the first time and find that the nickel( ii ) complex exhibits a substantial J < −600 cm −1 , resulting in a well-isolated S = 3/2 ground state even as high as 300 K. The iron and cobalt complexes feature metal–semiquinone exchange constants of J = −144(1) and −252(2) cm −1 , respectively, which are substantially larger in magnitude than those reported for related bis(bidentate) semiquinoid complexes. Finally, the semiquinone-bridged cobalt and nickel complexes exhibit field-induced slow magnetic relaxation, with relaxation barriers of U eff = 22 and 46 cm −1 , respectively. Remarkably, the Orbach relaxation observed for the Ni complex is in stark contrast to the fast processes that dominate relaxation in related mononuclear Ni II complexes, thus demonstrating that strong magnetic coupling can engender slow magnetic relaxation.« less
3. Physicochemical implications of surface alkylation of high-valent, Lindqvist-type polyoxovanadate-alkoxide clusters

   https://doi.org/10.1039/D0NR09201K  

   Fertig, Alex A. ; Rabbani, S. M. ; Koch, Melissa D. ; Brennessel, William W. ; Miró, Pere ; Matson, Ellen M. ( April 2021 , Nanoscale)

   We report a rare example of the direct alkylation of the surface of a plenary polyoxometalate cluster by leveraging the increased nucleophilicity of vanadium oxide assemblies. Addition of methyl trifluoromethylsulfonate (MeOTf) to the parent polyoxovanadate cluster, [V 6 O 13 (TRIOL R ) 2 ] 2− (TRIOL = tris(hydroxymethyl)methane; R = Me, NO 2 ) results in functionalisation of one or two bridging oxide ligands of the cluster core to generate [V 6 O 12 (OMe)(TRIOL R ) 2 ] 1− and [V 6 O 11 (OMe) 2 (TRIOL R ) 2 ] 2− , respectively. Comparison of the electronicmore »absorption spectra of the functionalised and unfunctionalised derivatives indicates the decreased overall charge of the complex results in a decrease in the energy required for ligand to metal charge transfer events to occur, while simultaneously mitigating the inductive effects imposed by the capping TRIOL ligand. Electrochemical analysis of the family of organofunctionalised polyoxovanadate clusters reveals the relationship of ligand environment and the redox properties of the cluster core: increased organofunctionalisation of the surface of the vanadium oxide assembly translates to anodic shifts in the reduction events of the Lindqvist ion. Overall, this work provides insight into the electronic effects induced upon atomically precise modifications to the surface structure of nanoscopic, redox-active metal oxide assemblies.« less
4. Isolation of the elusive bisbenzimidazole Bbim ³⁻ ˙ radical anion and its employment in a metal complex

   https://doi.org/10.1039/d1sc07245e  

   Benner, Florian ; Demir, Selvan ( May 2022 , Chemical Science)

   The discovery of singular organic radical ligands is a formidable challenge due to high reactivity arising from the unpaired electron. Matching radical ligands with metal ions to engender magnetic coupling is crucial for eliciting preeminent physical properties such as conductivity and magnetism that are crucial for future technologies. The metal-radical approach is especially important for the lanthanide ions exhibiting deeply buried 4f-orbitals. The radicals must possess a high spin density on the donor atoms to promote strong coupling. Combining diamagnetic 89 Y ( I = 1/2) with organic radicals allows for invaluable insight into the electronic structure and spin-density distribution.more »This approach is hitherto underutilized, possibly owing to the challenging synthesis and purification of such molecules. Herein, evidence of an unprecedented bisbenzimidazole radical anion (Bbim 3− ˙) along with its metalation in the form of an yttrium complex, [K(crypt-222)][(Cp* 2 Y) 2 (μ-Bbim˙)] is provided. Access of Bbim 3− ˙ was feasible through double-coordination to the Lewis acidic metal ion and subsequent one-electron reduction, which is remarkable as Bbim 2− was explicitly stated to be redox-inactive in closed-shell complexes. Two molecules containing Bbim 2− (1) and Bbim 3− ˙ (2), respectively, were thoroughly investigated by X-ray crystallography, NMR and UV/Vis spectroscopy. Electrochemical studies unfolded a quasi-reversible feature and emphasize the role of the metal centre for the Bbim redox-activity as neither the free ligand nor the Bbim 2− complex led to analogous CV results. Excitingly, a strong delocalization of the electron density through the Bbim 3− ˙ ligand was revealed via temperature-dependent EPR spectroscopy and confirmed through DFT calculations and magnetometry, rendering Bbim 3− ˙ an ideal candidate for single-molecule magnet design.« less
5. 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 boundmore »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.« less