title: Catalytic hydrogenation activity and electronic structure determination of bis(arylimidazol-2-ylidene)pyridine cobalt alkyl and hydride complexes.
abstract: The bis(arylimidazol-2-ylidene)pyridine cobalt methyl complex, ((iPr)CNC)CoCH3, was evaluated for the catalytic hydrogenation of alkenes. At 22 °C and 4 atm of H2 pressure, ((iPr)CNC)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, ((iPr)CNC)CoH, was accomplished by hydrogenation of ((iPr)CNC)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-(iPr)CNC)CoN2. Similar alkyl migration was observed upon treatment of ((iPr)CNC)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.
authors: Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States.
authors: Renyuan Pony
authors: RP
authors: Yu
authors: Jonathan M
authors: JM
authors: Darmon
authors: Carsten
authors: C
authors: Milsmann
authors: Grant W
authors: GW
authors: Margulieux
authors: S Chantal E
authors: SC
authors: Stieber
authors: Serena
authors: S
authors: DeBeer
authors: Paul J
authors: PJ
authors: Chirik
mesh_terms: D000475:Alkenes / Q000737:chemistry; D002384:Catalysis; D003035:Cobalt / Q000737:chemistry*; D004583:Electrons; D006859:Hydrogen / Q000737:chemistry*; D006865:Hydrogenation; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D011725:Pyridines / Q000737:chemistry*; D011789:Quantum Theory; D014867:Water / Q000737:chemistry
chemical_list: D000475:Alkenes; D009942:Organometallic Compounds; D011725:Pyridines; D014867:Water; D003035:Cobalt; D006859:Hydrogen
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