Catalytic four-electron reduction of O2 by ferrocene (Fc) and 1 1

Catalytic four-electron reduction of O2 by ferrocene (Fc) and 1 1 Rabbit polyclonal to ABCG1. (Me2Fc) occurs efficiently with a dinuclear copper(II) complex [CuII2(XYLO)(OH)]2+ (1) where XYLO is usually a this Delamanid makes possible the use of relatively poor one-electron reductants such as Fc and Me2Fc significantly reducing the effective overpotential in the catalytic O2-reduction reaction. it was confirmed that no H2O2 had formed after completion of the reaction (Physique S2 in SI). Thus the four-electron reduction of O2 by Me2Fc occurs efficiently with a catalytic amount of 1 1 in the presence of HClO4 (eq 2). Physique 1 UV-vis spectral changes observed in the four-electron reduction of O2(1.0 mM) by Me2Fc (6.0 mM) with HClO4 (40 mm) catalyzed by 1 (0.20 mM) in acetone at 298 K. Inset shows the time profile of the absorbance at 650 nm due to Me2Fc+. = ?31 kJ mol?1 and Δ= ?86 J K?1 mol?1. The equilibrium constant at 298 K was estimated to be 11 M?1 from the extrapolation of Delamanid the van’t Hoff plot. The equilibrium lies to the reactant side at 298 K when only a small portion of [CuI2(XYLOH)]2+ is usually converted to [CuII2(XYLO)(OOH)]2+ (~10%). Physique 7 Formation of the Delamanid hydroperoxo complex [CuII2(XYLO)(OOH)]2+ (is the equilibrium constant of formation Delamanid of 4 with O2 from 3. The derived kinetic equation (eq 5) agrees with the experimental observations in eqs 3 and 4. If the protonation of 4 to produce H2O2 was the major pathway at 298 K the rate would not be dependent of Me2Fc because electron transfer from Me2Fc to 2 was shown to be too fast to be involved in the rate-determining step. Because the rate is usually proportional to [Me2Fc] the rate-determining step must be the PCET reduction of 4 by Me2Fc. It should be noted that this protonation of 1 1 is usually completed in the presence of HClO4 (> 10 mM) as shown in Physique 2 when the linear dependence of the rate on concentration of HClO4 (> 10 mM) in Physique 2c results from the rate-determining PCET reduction of 4. Conclusion A dinuclear copper(II) complex ([CuII2(XYLO)(OH)]2+) acts as an efficient catalyst for the four-electron reduction of O2 by Me2Fc and Fc with HClO4 in acetone as shown in Scheme 7. The hydroxide group as well as the phenoxo group of [CuII2(XYLO)(OH)]2+ (1) were protonated with HClO4 to produce [CuII2(XYLOH)]4+ (2) which can be reduced by Me2Fc and Fc to produce [CuI2(XYLOH)]2+ (3). The dinuclear Cu(I) complex [CuI2(XYLOH)]2+ (3) reacts with O2 to produce the hydroperoxo complex ([CuII2(XYLO)(OOH)]+ (4)) and this is usually followed by PCET reduction leading to the catalytic four-electron reduction of O2 by Fc and Me2Fc. It is instructive to compare and contrast the chemistry described here with that previously reported 38 both with exactly the same catalyst [CuII2(XYLO)(OH)]2+ (1) (Schemes 1 & 8) but having very differing behaviors. As indicated in the summary in Scheme 8 1 is quite difficult to reduce but in the presence of HClO4 the bridging hydroxide Delamanid ligand is usually displaced (as H2O) and now reduction to a dicopper(I) (or a mixed-valent form [CuIICuI(XYLO)]2+)38 is possible; it is this/these forms which are required for O2-binding and initial reduction to the peroxide level. With HOTF however the phenoxo O-atom still bridges the Cu(II) ions leaving the redox potential unfavorable enough to require stronger reductants such as Me8Fc or Fc*. A key coordination chemistry aspect is usually that HClO4 as proton source is usually strong enough to break the phenoxide bridge between copper ions allowing facile reduction of the Cu(II) ions Delamanid with Me2Fc or even Fc itself;the complex produced [CuII2(XYLOH)]4+ (2) now has Cu(II) ions possessing only N3 bis[(2-(2-pyridyl)ethyl)amine chelation (Scheme 8). Thus the change to perchloric acid facilitates a drop in effective overpotential of ~ 0.30 V or more (Scheme 8). Scheme 8 Perchloric acid effects another dramatic change; the reaction mechanism switches from the catalytic two-electron two-proton reduction of O2 to H2O2 with HOTF to the catalytic four-electron four-proton reduction of O2 to water with HClO4. Firstly for the HOTF case dicopper(II) reduction is usually rate limiting but with HClO4 PCET reduction/protonation of [CuII2(XYLO)-(OOH)]2+ (4) is the rate-determining step. Secondly note that in both systems the hydroperoxo complex 4 is the key oxygen-intermediate which is usually formed. HOTF readily protonates off the bound -OOH ligand giving H2O2 but it is not strong enough to allow PCET hydroperoxide reduction to water. Perchloric acid does facilitate the latter hydroperoxide reductive cleavage to water accounting for the differing stoichiometries of catalytic O2-reduction chemistry. Although the mechanism of the PCET reduction of 4 by Me2Fc has yet to be clarified the chemistry described.