Accessing Square Planar Cobalt Nitrenoid Redox Isomers across Three Oxidation States

The bonding structures of cobalt nitrenoid complexes are fundamental to their reactivity across various transformations. However, unraveling the electronic configuration of these highly covalent Co-N interactions remains a significant challenge. In this study, we present the synthesis and electronic structure elucidation of a cobalt nitrenoid series in the underexplored square planar ligand field. Using a structurally rigid pincer-type platform and arylazides as the nitrene source, a suite of terminal metal nitrenoids featuring formal Co(III/IV/V) centers and electronically distinct aryl substituents was isolated and structurally characterized. Cyclic voltammetry analysis establishes a linear free energy relationship between their reduction potentials and the electronic properties of nitrene substituents. A combination of spectroscopic and computational methods was employed to probe the electronic structures of the cobalt nitrenoid series, including electron paramagnetic resonance (EPR) spectroscopy, X-ray absorption spectroscopy (XAS), and density functional theory (DFT) calculations. The result suggests that on the continuum of metal nitrenoid redox isomerism─ranging from metal imide (NR^2-) to imidyl radical (²NR^-) to nitrene adduct (¹NR/³NR) formulations─the isolated formal Co(III/IV/V) complexes feature predominantly [Co(II)-²NR^-], [Co(III)-²NR^-]⁺, and [Co(III)-¹NR]²⁺ characters, respectively.