Acid-Induced Mechanism Change and Overpotential Decrease in Dioxygen Reduction Catalysis with a Dinuclear Copper Complex

Abstract

Catalytic four-electron reduction of O2 by ferrocene (Fc) and 1,1′-dimethylferrocene (Me2Fc) occurs efficiently with a dinuclear copper(II) complex [CuII2(XYLO)(OH)]2+ (1), where XYLO is a m-xylene-linked bis[(2-(2-pyridyl)ethyl)amine dinucleating ligand with copper-bridging phenolate moiety], in the presence of perchloric acid (HClO4) in acetone at 298 K. The hydroxide and phenoxo group in [CuII2(XYLO)(OH)]2+ (1) undergo protonation with HClO4 to produce [CuII2(XYLOH)]4+ (2) where the two copper centers become independent and the reduction potential shifts from −0.68 V vs SCE in the absence of HClO4 to 0.47 Vj this makes possible the use of relatively weak one-electron reductants such as Fc and Me2Fc, significantly reducing the effective overpotential in the catalytic O2-reduction reaction. The mechanism of the reaction has been clarified based on kinetic studies on the overall catalytic reaction as well as each step in the catalytic cycle and also by low-temperature detection of intermediates. The O2-binding to the fully reduced complex [CuI2(XYLOH)]2+ (3) results in the reversible formation of the hydroperoxo complex ([CuII2(XYLO)(OOH)]2+) (4), followed by proton-coupled electron-transfer (PCET) reduction to complete the overall O2-to-2H2O catalytic conversion.

Lewis Acid-Induced Change from Four- to Two-Electron Reduction of Dioxygen Catalyzed by Copper Complexes Using Scandium Triflate

“…The k cat value was determined to be (9.4 ± 0.5) × 10 4 M −1 s −1 at 298 K. This value is twice of the rate constant ( k et ) of electron transfer from Fc* to 1 (5.0 ± 0.1) × 10 5 M −1 s −1 in acetone at 298 K, 24a i.e., k cat = 2 k et , because the electron transfer should occur twice to reduce O 2 in the catalytic cycle when two equiv of Fc *+ are formed. This inidicates that electron transfer from Fc* to 1 is the rate-determining step in the catalytic two-electron reduction of O 2 by Fc* with 1 .…”

“…24a,26 When Sc(OTf) 3 is employed as a Lewis acid instead of HClO 4 or CF 3 COOH, 1 also efficiently catalyzes the reduction of O 2 where Fc* + is also produced (Figure 1). In this case, however, the stoichiometry of O 2 reduction is different and follows eq 2,…”

“…16–18 There have been many reports on the electrocatalytic and homogeneous four-electron reduction of O 2 with copper complexes. 16–24 In contrast, there has been only few examples for the catalytic two-electron reduction of O 2 using a copper complex. 25 Whether copper complexes are effective for two-or four-electron reduction of O 2 depends on a variety of factors, including the ligand type and resulting nature of copper-oxygen intermediates formed as reactive species in the O 2 reduction catalysis.…”

“…25 Whether copper complexes are effective for two-or four-electron reduction of O 2 depends on a variety of factors, including the ligand type and resulting nature of copper-oxygen intermediates formed as reactive species in the O 2 reduction catalysis. 24,25 The counter anions of proton sources employed also affects the O 2 -reduction catalytic reactivity with copper complexes, with-respect-to the observed stoichiometry and/or mechanism of reaction. 26,27 However, there has been no report on the change in the number of electrons to reduce O 2 (two-electron vs four-electron) in O 2 reduction catalysis induced by metal ions acting as Lewis acids.…”

Development of a Novel Cu(II) Complex Modified Electrode and a Portable Electrochemical Analyzer for the Determination of Dissolved Oxygen (DO) in Water

“…Copper complexes have recently attracted increasing attention as O2 reduction electrocatalysts [8,9]. Results clearly demonstrate electrocatalytic O2 reduction coupled with Cu(II)/Cu(I) conversion in aqueous solutions.…”