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

The selective electrocatalytic reduction of dioxygen (O₂) 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 O₂ and H₂O can be competing reactions. Recently, we reported a Mn(III) Schiff base-type complex, Mn(^tbudhbpy)Cl, where 6,6'-di(3,5-di- tert-butyl-2-phenolate)-2,2'-bipyridine = [^tbudhbpy]^2-, which is active for the electrocatalytic reduction of O₂ to H₂O₂ (ca. 80% selectivity). The less-than-quantitative selectivity could be attributed in part to a thermal disproportionation reaction of H₂O₂ to O₂ 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 p K_a. Mechanistic studies suggest that under spectrochemical conditions, where the homogeneous reductant decamethylferrocene (Cp*₂Fe) 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.