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

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• 作者：Renyuan Pony Yu ; Jonathan M. Darmon ; Carsten Milsmann ; Grant W. Margulieux ; S. Chantal E. Stieber ; Serena DeBeer ; Paul J. Chirik
• 刊名：Journal of the American Chemical Society
• 出版年：2013
• 出版时间：September 4, 2013
• 年：2013
• 卷：135
• 期：35
• 页码：13168-13184
• 全文大小：830K
• 年卷期：v.135,no.35(September 4, 2013)
• ISSN：1520-5126

文摘

The bis(arylimidazol-2-ylidene)pyridine cobalt methyl complex, (^iPrCNC)CoCH₃, was evaluated for the catalytic hydrogenation of alkenes. At 22 掳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 掳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.