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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.