Understanding the Mechanisms of Cobalt-Catalyzed Hydrogenation and Dehydrogenation Reactions

Cobalt­(II)
alkyl complexes of aliphatic PNP pincer ligands have
been synthesized and characterized. The cationic cobalt­(II) alkyl
complex [(PNHPCy)­Co­(CH2SiMe3)]­BArF4 (4) (PNHPCy = bis­[(2-dicyclohexylphosphino)­ethyl]­amine)
is an active precatalyst for the hydrogenation of olefins and ketones
and the acceptorless dehydrogenation of alcohols. To elucidate the
possible involvement of the N–H group on the pincer ligand
in the catalysis via a metal–ligand cooperative interaction,
the reactivities of 4 and [(PNMePCy)­Co­(CH2SiMe3)]­BArF4 (7) were compared. Complex 7 was found to be an active
precatalyst for the hydrogenation of olefins. In contrast, no catalytic
activity was observed using 7 as a precatalyst for the
hydrogenation of acetophenone under mild conditions. For the acceptorless
dehydrogenation of 1-phenylethanol, complex 7 displayed
similar activity to complex 4, affording acetophenone
in high yield. When the acceptorless dehydrogenation of 1-phenylethanol
with precatalyst 4 was monitored by NMR spectroscopy,
the formation of the cobalt­(III) acetylphenyl hydride complex [(PNHPCy)­CoIII(κ2-O,C-C6H4C­(O)­CH3)­(H)]­BArF4 (13) was detected. Isolated complex 13 was found
to be an effective catalyst for the acceptorless dehydrogenation of
alcohols, implicating 13 as a catalyst resting state
during the alcohol dehydrogenation reaction. Complex 13 catalyzed the hydrogenation of styrene but showed no catalytic activity
for the room temperature hydrogenation of acetophenone. These results
support the involvement of metal–ligand cooperativity in the
room temperature hydrogenation of ketones but not the hydrogenation
of olefins or the acceptorless dehydrogenation of alcohols. Mechanisms
consistent with these observations are presented for the cobalt-catalyzed
hydrogenation of olefins and ketones and the acceptorless dehydrogenation
of alcohols.