Electron-transfer reduction of dinuclear copper peroxo and bis-μ-oxo complexes leading to the catalytic four-electron reduction of dioxygen to water.

The four-electron reduction of dioxygen by decamethylferrocene (Fc*) to water is efficiently catalyzed by a binuclear copper(II) complex (1) and a mononuclear copper(II) complex (2) in the presence of trifluoroacetic acid in acetone at 298 K. Fast electron transfer from Fc* to 1 and 2 affords the corresponding Cu(I) complexes, which react at low temperature (193 K) with dioxygen to afford the η(2):η(2)-peroxo dicopper(II) (3) and bis-μ-oxo dicopper(III) (4) intermediates, respectively. The rate constants for electron transfer from Fc* and octamethylferrocene (Me(8)Fc) to 1 as well as electron transfer from Fc* and Me(8)Fc to 3 were determined at various temperatures, leading to activation enthalpies and entropies. The activation entropies of electron transfer from Fc* and Me(8)Fc to 1 were determined to be close to zero, as expected for outer-sphere electron-transfer reactions without formation of any intermediates. For electron transfer from Fc* and Me(8)Fc to 3, the activation entropies were also found to be close to zero. Such agreement indicates that the η(2):η(2)-peroxo complex (3) is directly reduced by Fc* rather than via the conversion to the corresponding bis-μ-oxo complex, followed by the electron-transfer reduction by Fc* leading to the four-electron reduction of dioxygen to water. The bis-μ-oxo species (4) is reduced by Fc* with a much faster rate than the η(2):η(2)-peroxo complex (3), but this also leads to the four-electron reduction of dioxygen to water.