Redox-Active Bis(phenolate) N-Heterocyclic Carbene [OCO] Pincer Ligands Support Cobalt Electron Transfer Series Spanning Four Oxidation States

A new family of low-coordinate Co complexes supported by three redox-noninnocent tridentate [OCO] pincer-type bis(phenolate) N-heterocyclic carbene (NHC) ligands are described. Combined experimental and computational data suggest that the charge-neutral four-coordinate complexes are best formulated as Co(II) centers bound to closed-shell [OCO]^2- dianions, of the general formula [(OCO)Co^IIL] (where L is a solvent-derived MeCN or THF). Cyclic voltammograms of the [(OCO)Co^IIL] complexes reveal three oxidations accessible at potentials below 1.2 V vs Fc⁺/Fc, corresponding to generation of formally Co(V) species, but the true physical/spectroscopic oxidation states are much lower. Chemical oxidations afford the mono- and dications of the imidazoline NHC-derived complex, which were examined by computational and magnetic and spectroscopic methods, including single-crystal X-ray diffraction. The metal and ligand oxidation states of the monocationic complex are ambiguous; data are consistent with formulation as either [(^SOCO)Co^III(THF)₂]⁺ containing a closed-shell [^SOCO]^2- diphenolate ligand bound to a S = 1 Co(III) center, or [(^SOCO^•)Co^II(THF)₂]⁺ with a low-spin Co(II) ion ferromagnetically coupled to monoanionic [^SOCO^•]^- containing a single unpaired electron distributed across the [OCO] framework. The dication is best described as [(^SOCO⁰)Co^II(THF)₃]²⁺, with a single unpaired electron localized on the d⁷ Co(II) center and a doubly oxidized, charge-neutral, closed-shell ^SOCO⁰ ligand. The combined data provide for the first time unequivocal and structural evidence for [OCO] ligand redox activity. Notably, varying the degree of unsaturation in the NHC backbone shifts the ligand-based oxidation potentials by up to 400 mV. The possible chemical origins of this unexpected shift, along with the potential utility of the [OCO] pincer ligands for base-metal-mediated organometallic coupling catalysis, are discussed.