Catalytic hydrogenation activity and electronic structure determination of bis(arylimidazol-2-ylidene)pyridine Cobalt Alkyl and Hydride Complexes

The bis(arylimidazol-2-ylidene)pyridine cobalt methyl complex, ( ^iPrCNC)CoCH₃, was evaluated for the catalytic hydrogenation of alkenes. At 22 C and 4 atm of H₂ pressure, ( ^iPrCNC)CoCH₃ is an effective precatalyst for the hydrogenation of sterically hindered, unactivated alkenes such as trans-methylstilbene, 1-methyl-1-cyclohexene, and 2,3-dimethyl-2-butene, representing one of the most active cobalt hydrogenation catalysts reported to date. Preparation of the cobalt hydride complex, (^iPrCNC)CoH, was accomplished by hydrogenation of (^iPrCNC)CoCH₃. Over the course of 3 h at 22 C, migration of the metal hydride to the 4-position of the pyridine ring yielded (4-H₂-^iPrCNC)CoN₂. Similar alkyl migration was observed upon treatment of (^iPrCNC)CoH with 1,1-diphenylethylene. This reactivity raised the question as to whether this class of chelate is redox-active, engaging in radical chemistry with the cobalt center. A combination of structural, spectroscopic, and computational studies was conducted and provided definitive evidence for bis(arylimidazol-2- ylidene)pyridine radicals in reduced cobalt chemistry. Spin density calculations established that the radicals were localized on the pyridine ring, accounting for the observed reactivity, and suggest that a wide family of pyridine-based pincers may also be redox-active.