Electron‐Transfer Reduction of Dinuclear Copper Peroxo and Bis‐μ‐oxo Complexes Leading to the Catalytic Four‐Electron Reduction of Dioxygen to Water

Abstract

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 CuI complexes, which react at low temperature (193 K) with dioxygen to afford the η2:η2-peroxo dicopper(II) (3) and bis-μ-oxo dicopper(III) (4) intermediates, respectively. The rate constants for electron transfer from Fc* and octamethylferrocene (Me8Fc) to 1 as well as electron transfer from Fc* and Me8Fc to 3 were determined at various temperatures, leading to activation enthalpies and entropies. The activation entropies of electron transfer from Fc* and Me8Fc 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 Me8Fc to 3, the activation entropies were also found to be close to zero. Such agreement indicates that the η2:η2-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 η2:η2-peroxo complex (3), but this also leads to the four-electron reduction of dioxygen to water.

Peroxo and Superoxo Moieties Bound to Copper Ion: Electron-Transfer Equilibrium with a Small Reorganization Energy

“…Another value for comparison is λ = 2.2 eV found for the electron transfer from Me 10 Fc and Me 8 Fc to the peroxo complex [Cu II 2 (N3)(O 2 2− )] 2+ 44 . This very large λ value has its origin in the fact that O–O bond cleavage occurs upon electron-transfer reduction.…”

Dioxygen Activation and Catalytic Reduction to Hydrogen Peroxide by a Thiolate-Bridged Dimanganese(II) Complex with a Pendant Thiol

“…It has, however, been shown that H 2 O 2 can spontaneously decompose, partially or completely, in the presence of ferrocene derivatives. 19 38 and their biomimetic models, is often proposed (and in some cases established) to be an influential factor in their reactivity. In these systems, metalbound thiols can act as proton relay during the catalytic process, but are also capable of tuning the redox potential of the active metal centers.…”

One-step selective hydroxylation of benzene to phenol with hydrogen peroxide catalysed by copper complexes incorporated into mesoporous silica–alumina

“…8–23 However, selective hydroxylation of benzene to phenol with H 2 O 2 has been difficult because phenol is normally easier to be oxidised to yield over-oxidised products such as p -benzoquinone. 8–21 Although selective hydroxylation of benzene to phenol without over-oxidation has been achieved in some cases, the catalytic mechanism of hydroxylation of benzene to phenol has yet to be clarified, 24–30 except for some cases of photocatalytic hydroxylation of benzene to phenol. 28–31 Extensive efforts have been devoted to enhance the product selectivity and catalytic activity with heterogeneous catalysts including metal complex catalysts incorporated into a mesoporous material.…”