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Catalytic Hydrogenation Activity and Electronic Structure Determination of Bis(arylimidazol-2-ylidene)pyridine Cobalt Alkyl and Hydride Complexes

Yu, Renyuan Pony, Darmon, Jonathan M., Milsmann, Carsten, Margulieux, Grant W., Stieber, S. Chantal E., DeBeer, Serena, Chirik, Paul J.
Journal of the American Chemical Society 2013 v.135 no.35 pp. 13168-13184
alkenes, catalysts, catalytic activity, cobalt, free radicals, hydrides, hydrogen, hydrogenation, pyridines, spectroscopy
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.