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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.

Journal of the American Chemical Society (2013-02-12)
Dipanwita Das, Yong-Min Lee, Kei Ohkubo, Wonwoo Nam, Kenneth D Karlin, Shunichi Fukuzumi
ABSTRACT

Selective two-electron plus two-proton (2e(-)/2H(+)) reduction of O(2) to hydrogen peroxide by ferrocene (Fc) or 1,1'-dimethylferrocene (Me(2)Fc) 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 Me(2)Fc for the overall two-electron reduction of O(2). Fast electron transfer from Fc or Me(2)Fc to 1 affords the corresponding Cu(I) complex [Cu(I)(tepa)](+) (2), which reacts at low temperature (193 K) with O(2), however only in the presence of HClO(4), 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 O(2)-binding process in the presence of HClO(4) at lower temperature as well as at room temperature. The O(2)-binding kinetics in the presence of HClO(4) 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 O(2) is shown to result from the existence of a pre-equilibrium between 2 and O(2) prior to the formation of the hydroperoxo complex 3. No further reduction of [Cu(II)(tepa)(OOH)](+) (3) by Fc or Me(2)Fc occurred, and instead 3 is protonated by HClO(4) to yield H(2)O(2) accompanied by regeneration of 1, thus completing the catalytic cycle for the two-electron reduction of O(2) by Fc or Me(2)Fc.

MATERIALS
Product Number
Brand
Product Description

Supelco
Perchloric acid concentrate, 0.01 M HClO4 in water (0.01N), eluent concentrate for IC
Sigma-Aldrich
Perchloric acid, ACS reagent, 60%
Sigma-Aldrich
Perchloric acid, puriss. p.a., ACS reagent, packed in coated, shock- and leak-protected glass bottle, ≥60% (T)
Sigma-Aldrich
Perchloric acid, ACS reagent, 70%
Sigma-Aldrich
Perchloric acid, puriss. p.a., ACS reagent, reag. ISO, reag. Ph. Eur., 70.0-72.0%
Sigma-Aldrich
Perchloric acid, 70%, 99.999% trace metals basis
Sigma-Aldrich
Ferrocene, 98%