Kinetics and thermodynamics of formation and electron-transfer reactions of Cu–O2 and Cu2–O2 complexes

The kinetics and thermodynamics of formation of Cu(II)-superoxo (Cu–O2) complexes by the reaction of Cu(I) complexes with dioxygen (O2) and the reduction of Cu(II)-superoxo complexes to dinuclear Cu-peroxo complexes are discussed. In the former case, electron transfer from a Cu(I) complex to O2 occurs concomitantly with binding of O2−to the corresponding Cu(II) species. This is defined as an inner-sphere Cu(II) ion-coupled electron transfer process. Electron transfer from another Cu(I) complex to preformed Cu(II)-superoxo complexes also occurs concomitantly with binding of the Cu(II)-peroxo species with the Cu(II) species to produce the dinuclear Cu-peroxo (Cu2–O2) complexes. The kinetics and thermodynamics of outer-sphere electron-transfer reduction of Cu2–O2 complexes are also been discussed in light of the Marcus theory of outer-sphere electron transfer.

In terms of Marcus theory of electron-transfer, the kinetics and thermodynamics of O2-reaction with copper(I) complexes are overviewed and evaluated, including for the reactions leading to the formation of copper(II)-superoxo products. Dinuclear species of three common types may then form from copper(II)-superoxo complex reaction with another ligand-copper(I) reductant. All of these reactions occur via an inner-sphere Cu(II) ion-coupled electron transfer process. A case of copper(II) superoxo complex reduction by hydride reagents is also presented. Of interest in catalytic dioxygen reduction and other chemistries, the kinetics and thermodynamics of outer-sphere electron-transfer reduction of peroxodicopper(II) or bis-μ-oxodicopper(III) adducts are also discussed.Figure optionsDownload high-quality image (166 K)Download as PowerPoint slideHighlights
► Dioxygen binding to copper(I) complexes gives superoxo-copper(II), peroxodicopper(II) or bis-μ-oxodicopper(III) adducts.
► Electron-transfer from copper(I) occurs concomitant with superoxide anion binding to copper(II) rather than by stepwise electron-transfer followed by ligation.
► The kinetics and thermodynamics of electron-transfer reduction of various Cu2–O2 species are discussed in terms of Marcus theory.
► Electron-transfer reduction of Cu2–O2 complexes is the fastest for bis-μ-oxodicopper(III) complexes and for at least one case, electron-transfer reduction of a μ–η2:η2-peroxodicopper(II) complex occurs directly and without prior conversion to an isomeric bis-μ-oxodicopper(III) species.