Acid-induced mechanism change and overpotential decrease in dioxygen reduction catalysis with a dinuclear copper complex.

Catalytic four-electron reduction of O2 by ferrocene (Fc) and 1,1'-dimethylferrocene (Me2Fc) occurs efficiently with a dinuclear copper(II) complex [Cu(II)2(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 [Cu(II)2(XYLO)(OH)](2+) (1) undergo protonation with HClO4 to produce [Cu(II)2(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 V; 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 on the basis of 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 [Cu(I)2(XYLOH)](2+) (3) results in the reversible formation of the hydroperoxo complex ([Cu(II)2(XYLO)(OOH)](2+)) (4), followed by proton-coupled electron-transfer (PCET) reduction to complete the overall O2-to-2H2O catalytic conversion.