Factors That Control Catalytic Two- versus Four-Electron Reduction of Dioxygen by Copper Complexes

Abstract

The selective two-electron reduction of O2 by one-electron reductants such as decamethylferrocene (Fc*) and octamethylferrocene (Me8Fc) is efficiently catalyzed by a binuclear Cu(II) complex ([CuII2(LO)(OH)]2+ (D1) {LO is a binucleating ligand with copper-bridging phenolate moiety} in the presence of trifluoroacetic acid (HOTF) in acetone. The protonation of the hydroxide group of [CuII2(LO)(OH)]2+ with HOTF to produce [CuII2(LO)(OTF)]2+ (D1-OTF) makes it possible for this to be reduced by two equiv of Fc* via a two-step electron transfer sequence. Reactions of the fully reduced complex [CuI2(LO)]+ (D3) with O2 in the presence of HOTF led to the low-temperature detection of the absorption spectra due to the peroxo complex ([CuII2(LO)(OO)]) (D) and the protonated hydroperoxo complex ([CuII2(LO)(OOH)]2+ (D4). No further Fc* reduction of D4 occurs, and it is instead further protonated by HOTF to yield H2O2 accompanied by regeneration of [CuII2(LO)(OTF)]2+ (D1-OTF) thus completing the catalytic cycle for the two-electron reduction of O2 by Fc*. Kinetic studies on the formation of Fc*+ under catalytic conditions as well as for separate examination of the electron transfer from Fc* to D1-OTF reveal there are two important reaction pathways operating. One is a rate-determining second reduction of D1-OTF, thus electron transfer from Fc* to a mixed-valent intermediate [CuIICuI(LO)]2+ (D2) which leads to [CuI2(LO)]+ which is coupled with O2 binding to produce [CuII2(LO)(OO)]+ (D). The other involves direct reaction of O2 with the mixed-valent compound D2 followed by rapid Fc* reduction of a putative superoxo-dicopper(II) species thus formed, producing D.

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

“…The kinetic formulation in eqs 3 and 4 obtained in this study is quite unique because the rate is proportional to concentrations of not only 1 but also O 2 , HClO 4 and Me 2 Fc, in sharp contrast to the previously reported cases of Cu catalysis for O 2 reduction in which the rate was rather independent of O 2 or H + . 36-38 In such a case, the rate-determining step in the catalytic cycle should involve the reactions of 1 with Me 2 Fc, O 2 and H + . In order to elucidate the catalytic mechanism that can explain such a unique kinetic formulation, we decided to examine each portion of the catalytic cycle, step by step.…”

“…The absorption band at 378 nm due to [Cu II 2 (XYLO)(OH)] 2+ decreased with increasing concentration of HClO 4 , and this was completely different from the spectral behavior of complex 1 with CF 3 COOH (HOTF) where the absorption band was shifted to 420 nm and a clean isosbestic point was observed at 430 nm. 38 This spectral changes in the presence of HClO 4 indicate that not only the hydroxide group but also the phenoxo group of [Cu II 2 (XYLO)(OH)] 2+ ( 1 ) is protonated with HClO 4 , a much stronger acid than CF 3 COOH, to produce [Cu II 2 (XYLOH)] 4+ ( 2 ) (Scheme 2). …”

“…However, the dinuclear copper(II) complex [Cu II 2 (XYLO)-(OH)] 2+ ( 1 ) (Scheme 1) {where XYLO is a m -xylene-linked bis[(2-(2-pyridyl)ethyl)amine dinucleating ligand with copper-bridging phenolate moiety] catalyzes the two-electron reduction of O 2 to hydrogen peroxide in the presence of trifluoroacetic acid (HOTF) via the hydroperoxo intermediate ( D1 ). 38 In all cases, however, only a strong one-electron reductant such as decamethylferrocene (Fc*) could be used to reduce O 2 with the copper complexes. There has so far been no example for the catalytic reduction of O 2 by weaker one-electron reductants than Fc* with Cu complexes.…”

Di- and Trinuclear Mixed-Valence Copper Amidinate Complexes from Reduction of Iodine

“…Finally, the Karlin group has many examples of O 2 activation with dinuclear Cu(I) complexes for modeling multinuclear copper clusters that catalyze oxidase reactions. 39 …”

High-Valent Chromium–Oxo Complex Acting as an Efficient Catalyst Precursor for Selective Two-Electron Reduction of Dioxygen by a Ferrocene Derivative

“…32,33 Thus, metal complexes composed of metals to the right of the Fe−Ru−Os group catalyze selective two-electron reduction of O 2 to produce H 2 O 2 via M−O bond cleavage of hydroperoxo intermediate, as shown in In contrast, even mononuclear complexes composed of metals to the right of the Fe−Ru−Os group can catalyze the four-electron reduction of O 2 via the formation of dinuclear peroxo complexes, which act as key intermediates followed by homolytic O−O bond cleavage to produce H 2 O (Scheme 1c). 24−28 Except for the dinuclear copper complex reported by Karlin and co-workers, 31 it is well-known that Cu I reacts with O 2 to afford the superoxo species, which reacts rapidly with a second equivalent of Cu I to form dinuclear peroxo species, followed by further reduction in the presence of acid to facilitate the four-electron reduction of O 2 to produce H 2 O. 24 Consequently, metal complexes composed of early transition metals to the left of Fe−Ru−Os group are thought to catalyze selective four-electron reduction of O 2 via the formation of stable metal oxo complexes, as shown in Scheme 1b.…”