Controlling Reactivity through Spin Manipulation: Steric Bulkiness of Peroxocobalt(III) Complexes

The intrinsic relationship between spin states and reactivity in peroxocobalt(III) complexes was investigated, specifically focusing on the influence of steric modulation on supporting ligands. Together with the previously reported [Co^III(TBDAP)(O₂)]⁺ (2), which exhibits spin crossover characteristics, two peroxocobalt(III) complexes, [Co^III(MDAP)(O₂)]⁺ (2) and [Co^III(ADDAP)(O₂)]⁺ (2), bearing pyridinophane ligands with distinct N-substituents such as methyl and adamantyl groups, were synthesized and characterized. By manipulating the steric bulkiness of the N-substituents, control of spin states in peroxocobalt(III) complexes was demonstrated through various physicochemical analyses. Notably, 2 oxidized the nitriles to generate hydroximatocobalt(III) complexes, while 2 displayed an inability for such oxidation reactions. Furthermore, both 2 and 2 exhibited similarities in spectroscopic and geometric features, demonstrating spin crossover behavior between S = 0 and S = 1. The steric bulkiness of the adamantyl and tert-butyl group on the axial amines was attributed to inducing a weak ligand field on the cobalt(III) center. Thus, 2 and 2 are an S = 1 state under the reaction conditions. In contrast, the less bulky methyl group on the amines of 2 resulted in an S = 0 state. The redox potential of the peroxocobalt(III) complexes was also influenced by the ligand field arising from the steric bulkiness of the N-substituents in the order of 2 (-0.01 V) < 2 (0.29 V) = 2 (0.29 V). Theoretical calculations using DFT supported the experimental observations, providing insights into the electronic structure and emphasizing the importance of the spin state of peroxocobalt(III) complexes in nitrile activation.