Catalytic Hydrogenation Activity and Electronic Structure Determination of Bis(arylimidazol-2-ylidene)pyridine Cobalt Alkyl and Hydride Complexes
- Supporting Info ->
- Figures
- Reference QuickView
- Add to ACS ChemWorx
† Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
‡ Max-Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany
§ Department of Chemistry and Chemical Biology,Cornell University, Ithaca, New York 14853, United States
J. Am. Chem. Soc., Article ASAP
DOI: 10.1021/ja406608u
Publication Date (Web): August 5, 2013
Copyright © 2013 American Chemical Society
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.
コメント