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

The 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 pK_a. 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 Cu^II(O₂⋅⁻)(tmpa)]⁺ in acetonitrile. This results in higher catalytic rates for the reduction of hydrogen peroxide than for the ORR.