Temperature-Independent Catalytic Two-Electron Reduction of Dioxygen by Ferrocenes with a Copper(II) Tris[2-(2-pyridyl)ethyl]amine Catalyst in the Presence of Perchloric Acid

Abstract

Selective two-electron plus two-proton (2e^–/2H⁺) reduction of O₂ to hydrogen peroxide by ferrocene (Fc) or 1,1′-dimethylferrocene (Me₂Fc) in the presence of perchloric acid is catalyzed efficiently by a mononuclear copper(II) complex, [Cu^II(tepa)]²⁺ (1; tepa = tris[2-(2-pyridyl)ethyl]amine) in acetone. The E_1/2 value for [Cu^II(tepa)]²⁺ as measured by cyclic voltammetry is 0.07 V vs Fc/Fc⁺ in acetone, being significantly positive, which makes it possible to use relatively weak one-electron reductants such as Fc and Me₂Fc for the overall two-electron reduction of O₂. Fast electron transfer from Fc or Me₂Fc to 1 affords the corresponding Cu^I complex [Cu^I(tepa)]⁺ (2), which reacts at low temperature (193 K) with O₂, however only in the presence of HClO₄, to afford the hydroperoxo complex [Cu^II(tepa)(OOH)]⁺ (3). A detailed kinetic study on the homogeneous catalytic system reveals the rate-determining step to be the O₂-binding process in the presence of HClO₄ at lower temperature as well as at room temperature. The O₂-binding kinetics in the presence of HClO₄ were studied, demonstrating that the rate of formation of the hydroperoxo complex 3 as well as the overall catalytic reaction remained virtually the same with changing temperature. The apparent lack of activation energy for the catalytic two-electron reduction of O₂ is shown to result from the existence of a pre-equilibrium between 2 and O₂ prior to the formation of the hydroperoxo complex 3. No further reduction of [Cu^II(tepa)(OOH)]⁺ (3) by Fc or Me₂Fc occurred, and instead 3 is protonated by HClO₄ to yield H₂O₂ accompanied by regeneration of 1, thus completing the catalytic cycle for the two-electron reduction of O₂ by Fc or Me₂Fc.