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

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.