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.