Pendent Relay Enhances H2O2 Selectivity during Dioxygen Reduction Mediated by Bipyridine-Based Co-N2O2 Complexes.

Abstract

Generally, cobalt-N2O2 complexes show selectivity for hydrogen peroxide during electrochemical dioxygen (O2) reduction. We recently reported a Co(III)-N2O2 complex with a 2,2'-bipyridine-based ligand backbone which showed alternative selectivity: H2O was observed as the primary reduction product from O2 (71 ± 5%) with decamethylferrocene as a chemical reductant and acetic acid as a proton donor in methanol solution. We hypothesized that the key selectivity difference in this case arises in part from increased favorability of protonation at the distal O position of the key intermediate Co(III)-hydroperoxide species. To interrogate this hypothesis, we have prepared a new Co(III) compound that contains pendent -OMe groups poised to direct protonation toward the proximal O atom of this hydroperoxo intermediate. Mechanistic studies in acetonitrile (MeCN) solution reveal two regimes are possible in the catalytic response, dependent on added acid strength and the presence of the pendent proton donor relay. In the presence of stronger acids, the activity of the complex containing pendent relays becomes O2 dependent, implying a shift to Co(III)-superoxide protonation as the rate-determining step. Interestingly, the inclusion of the relay results in primarily H2O2 production in MeCN, despite minimal difference between the standard reduction potentials of the three complexes tested. EPR spectroscopic studies indicate the formation of Co(III)-superoxide species in the presence of exogenous base, with greater O2 reactivity observed in the presence of the pendent -OMe groups.