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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
- 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
- Das D.; Lee Y.-M.; Ohkubo K.; Nam W.; Karlin K.D.; Fukuzumi S.
- Ewha Authors
- 남원우; Shunichi Fukuzumi; 이용민; Kenneth D. Karlin
- SCOPUS Author ID
- 남원우; Shunichi Fukuzumi; 이용민
- Issue Date
- Journal Title
- Journal of the American Chemical Society
- vol. 135, no. 7, pp. 2825 - 2834
- SCI; SCIE; SCOPUS
- 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 E1/2 value for [CuII(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 CuI 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 [CuII(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 [CuII(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. © 2013 American Chemical Society.
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