Role of High-Spin Species and Pendant Amines in Electrocatalytic Alcohol Oxidation by a Nickel Phosphine Complex

The
development of active and efficient electrocatalysts for oxidation
of alcohols using earth-abundant metals will aid progress toward
a renewable energy economy. Here, we present a detailed mechanistic
study of electrocatalytic benzyl alcohol oxidation by a molecular
nickel complex containing pendant amines using a combination of kinetic
studies, nuclear magnetic resonance spectroscopy, and density functional
theory. The catalyst preferentially binds alcohol in a high-spin octahedral
geometry, but this complex is not readily deprotonated by an exogenous
base and inhibits catalysis. Dissociation of one or more solvent ligands
returns the complex to a low-spin state that can be deprotonated.
Kinetic modeling indicates that the off-cycle high-spin intermediate
lowers the catalytic turnover frequency by a factor of 8, suggesting
that substantial gains in activity can be attained by improvements
to the catalyst coordination geometry. In a second finding, we demonstrate
that the pendant amine of the catalyst only functions as a proton
relay for the potential-determining step, oxidation of a nickel hydride
intermediate, but does not have a substantial impact on the overall
rate for oxidation of benzyl alcohol to benzaldehyde. This result
contradicts the common expectation for pendant amines to participate
in rate-limiting proton transfer reactions.