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Kinetic analysis of hydrogen production catalyzed by Co(dmgBF 2 ) 2 (CH 3 CN) 2 (dmgBF 2 = 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 p K a dependent, whereas the third was shown to be intrinsic to the catalyst, likely reflecting either H–H bond formation or H 2 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 p K a . 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.
Rountree et al. (Fri,) studied this question.