Temperature Independent Catalytic Two-Electron Reduction of Dioxygen by Ferrocenes with a Tris2-(2-pyridyl)ethylamine-Copper(II) Catalyst in the Presence of Perchloric Acid

摘要

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⁺ {tepa = tris[2-(2-pyridyl)ethyl]amine} (>1) in acetone. The E1/2 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 presence of HClO4 to afford the hydroperoxo complex, [Cu^II(tepa)(OOH)]²⁺ (>3). The 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 an 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)]²⁺ (>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.