Kinetic and Mechanistic Characterization of Low-Overpotential, HO-Selective Reduction of O Catalyzed by NO-Ligated Cobalt Complexes.

A soluble, bis-ketiminate-ligated Co complex [Co(NO)] was recently shown to catalyze selective reduction of O to HO with an overpotential as low as 90 mV. Here we report experimental and computational mechanistic studies of the Co(NO)-catalyzed O reduction reaction (ORR) with decamethylferrocene (Fc*) as the reductant in the presence of AcOH in MeOH. Analysis of the Co/O binding stoichiometry and kinetic studies support an O reduction pathway involving a mononuclear cobalt species. The catalytic rate exhibits a first-order kinetic dependence on [Co(NO)] and [AcOH], but no dependence on [Fc*] or [O]. Differential pulse voltammetry and computational studies support Co-hydroperoxide as the catalyst resting state and protonation of this species as the rate-limiting step of the catalytic reaction. These results contrast previous mechanisms proposed for other Co-catalyzed ORR systems, which commonly feature rate-limiting protonation of a Co-superoxide adduct earlier in the catalytic cycle. Computational studies show that protonation is strongly favored at the proximal oxygen of the Co(OOH) species, accounting for the high selectivity for formation of hydrogen peroxide. Further analysis shows that a weak dependence of the ORR rate on the p K values of the protonated Co(OOH) species across a series of Co(NO) catalysts provides a rationale for the unusually low overpotential observed for O reduction to HO.