Linear Free Energy Relationships in the Hydrogen Evolution Reaction: Kinetic Analysis of a Cobaloxime Catalyst

Abstract

Kinetic analysis of hydrogen production catalyzed by Co(dmgBF2)2(CH3CN)2 (dmgBF2 = difluoroboryl-dimethylglyoxime) was performed in acetonitrile with a series of para-substituted anilinium acids. It was determined that the mechanism of hydrogen evolution is governed by three elementary steps; two are acid concentration and pKa dependent, whereas the third was shown to be intrinsic to the catalyst, likely reflecting either H–H bond formation or H2 release. The kinetics of the first proton transfer step, the protonation of the singly reduced catalyst, were evaluated using foot-of-the-wave analysis, as well as current–potential analysis for voltammograms displaying total catalysis behavior. Analysis of the total catalysis peak shift required the empirical determination of a new equation for the ECEC′ catalytic mechanism using digital simulations. The kinetics of the second proton transfer step—assigned to protonation of the doubly reduced, singly protonated species—and the acid-independent step were determined by analyzing the plateau current of the catalytic wave over a range of acid concentrations. Both proton transfer steps follow linear free energy relationships of log(k) vs acid pKa. These linear relationships give slopes of −0.94 and −0.77 for the first and second proton transfers, respectively, indicating that both steps become faster with increasing acid strength.