Crystal Structure and C-H Bond-Cleaving Reactivity of a Mononuclear Co^IV-Dinitrate Complex

High-valent Fe^IV═O intermediates with a terminal metal-oxo moiety are key oxidants in many enzymatic and synthetic C-H bond oxidation reactions. While generating stable metal-oxo species for late transition metals remains synthetically challenging, notably, a number of high-valent non-oxo-metal species of late transition metals have been recently described as strong oxidants that activate C-H bonds. In this work, we obtained an unprecedented mononuclear Co^IV-dinitrate complex (2) upon one-electron oxidation of its Co(III) precursor supported by a tridentate dianionic N3 ligand. 2 was structurally characterized by X-ray crystallography, showing a square pyramidal geometry with two coordinated nitrate anions. Furthermore, characterization of 2 using combined spectroscopic and computational methods revealed that 2 is a low-spin (S = 1/2) Co(IV) species with the unpaired electron located on the cobalt d orbital, which is well positioned for substrate oxidations. Indeed, while having a high thermal stability, 2 is able to cleave sp³ C-H bonds up to 87 kcal/mol to afford rate constants and kinetic isotope effects (KIEs) of 2-6 that are comparable to other high-valent metal oxidants. The ability to oxidize strong C-H bonds has yet to be observed for Co^IV-O and Co^III═O species previously reported. Therefore, 2 represents the first high-valent Co(IV) species that is both structurally characterized by X-ray crystallography and capable of activating strong C-H bonds.