Catalytic hydrogenation activity and electronic structure determination of bis(arylimidazol-2-ylidene)pyridine Cobalt Alkyl and Hydride Complexes

Renyuan Pony Yu, Jonathan M. Darmon, Carsten Milsmann, Grant W. Margulieux, S. Chantal E. Stieber, Serena Debeer, Paul J. Chirik

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Abstract

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.

Original languageEnglish (US)
Pages (from-to)13168-13184
Number of pages17
JournalJournal of the American Chemical Society
Volume135
Issue number35
DOIs
StatePublished - Sep 4 2013

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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