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.