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

• Published in: Journal of the American Chemical Society Vol. 135; no. 35; pp. 13168 - 13184
• Main Authors: Yu, Renyuan Pony, Darmon, Jonathan M, Milsmann, Carsten, Margulieux, Grant W, Stieber, S. Chantal E, DeBeer, Serena, Chirik, Paul J
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 04.09.2013
• Subjects: Alkenes - chemistry; Catalysis; Cobalt - chemistry; Electrons; Hydrogen - chemistry; Hydrogenation; Models, Molecular; Molecular Structure; Organometallic Compounds - chemical synthesis; Organometallic Compounds - chemistry; Pyridines - chemistry; Quantum Theory; Water - chemistry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja406608u

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.