Dioxygen Reduction to Hydrogen Peroxide by a Molecular Mn Complex: Mechanistic Divergence between Homogeneous and Heterogeneous Reductants

The selective electrocatalytic reduction of dioxygen (O-2) to hydrogen peroxide (H2O2) could be an alternative to the anthraquinone process used industrially, as well as enable the on-demand production of a useful chemical oxidant, obviating the need for long-term storage. There are challenges associated with this, since the two-proton/two-electron reduction of H2O2 to two equivalents of water (H2O) or disproportionation to O-2 and H2O can be competing reactions. Recently, we reported a Mn(III) Schiff base-type complex, Mn((tbu)dhbpy)Cl, where 6,6'-di(3,5-di-tert-butyl-2-phenolate)-2,2'-bipyridine = [(tbu)dhbpy](2-), which is active for the electrocatalytic reduction of O-2 to H2O2 (ca. 80% selectivity). The less-than-quantitative selectivity could be attributed in part to a thermal disproportionation reaction of H2O2 to O-2 and H2O. To understand the mechanism in greater detail, spectrochemical stopped-flow and electrochemical techniques were employed to examine the catalytic rate law and kinetic reaction parameters. Under electrochemical conditions, the catalyst produces H2O2 by an ECCEC mechanism with appreciable rates down to overpotentials of 20 mV and exhibits a catalytic response with a strong dependence on proton donor pK(a). Mechanistic studies suggest that under spectrochemical conditions, where the homogeneous reductant decamethylferrocene (Cp*Fe-2) is used, H2O2 is instead produced via a disproportionation pathway, which does not show a strong acid dependence. These results demonstrate that differences in mechanistic pathways can occur for homogeneous catalysts in redox processes, dependent on whether an electrode or homogeneous reductant is used.