Fast Oxygen Reduction Catalyzed by a Copper(II) Tris(2-pyridylmethyl)amine Complex through a Stepwise Mechanism

Catalytic pathways for the reduction of dioxygen can either lead to the formation of water or peroxide as the reaction product. We demonstrate that the electrocatalytic reduction of O₂ by the pyridylalkylamine copper complex [Cu(tmpa)(L)]²⁺ in a neutral aqueous solution follows a stepwise 4 e^- /4 H⁺ pathway, in which H₂ O₂ is formed as a detectable intermediate and subsequently reduced to H₂ O in two separate catalytic reactions. These homogeneous catalytic reactions are shown to be first order in catalyst. Coordination of O₂ to Cu^I was found to be the rate-determining step in the formation of the peroxide intermediate. Furthermore, electrochemical studies of the reaction kinetics revealed a high turnover frequency of 1.5×10⁵  s^-1 , the highest reported for any molecular copper catalyst.