Dioxygen Reduction in Acetonitrile with Copper Pyridylalkylamine Complexes: The Influence of Acid Strength on the Catalytic Performance

Abstract

AbstractThe pyridylalkylamine copper complex [Cu(tmpa)(L)]2+ has previously been proposed to reduce dioxygen via a dinuclear resting state, based on experiments in organic aprotic solvents using chemical reductants. Conversely, a mononuclear reaction mechanism was observed under electrochemical conditions in a neutral aqueous solution. We have investigated the electrochemical oxygen and hydrogen peroxide reduction reaction catalyzed by [Cu(tmpa)(L)]2+ in acetonitrile, using several different acids over a range of pKa. We demonstrate that strong acids lead to the loss of redox reversibility and to the destabilization of the copper complex under non‐catalytic conditions. Under milder conditions, the electrochemical oxygen reduction reaction (ORR) was shown to proceed via a mononuclear catalytic intermediate, similar to what we have previously observed in water. However, in acetonitrile the catalytic rate constants of the ORR are dramatically lower by a factor 105, which is caused by the unfavorable equilibrium of formation of [CuII(O2⋅−)(tmpa)]+ in acetonitrile. This results in higher catalytic rates for the reduction of hydrogen peroxide than for the ORR.