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

Oxygenation of [Cu₂(UN-O^-)(DMF)]²⁺ (1), a structurally characterized dicopper Robin-Day class I mixed-valent Cu(II)Cu(I) complex, with UN-O^- as a binucleating ligand and where dimethylformamide (DMF) binds to the Cu(II) ion, leads to a superoxo-dicopper(II) species [Cu^II₂(UN-O^-)(O₂^•-)]²⁺ (2). The formation kinetics provide that k_on = 9 × 10^-2 M^-1 s^-1 (-80 °C), δH^‡ = 31.1 kJ mol^-1 and δS^‡ = -99.4 J K^-1 mol^-1 (from -60 to -90 °C data). Complex 2 can be reversibly reduced to the peroxide species [Cu^II₂(UN-O^-)(O₂^2-)]⁺ (3), using varying outer-sphere ferrocene or ferrocenium redox reagents. A Nernstian analysis could be performed by utilizing a monodiphenylamine substituted ferrocenium salt to oxidize 3, leading to an equilibrium mixture with K_et = 5.3 (-80 °C); a standard reduction potential for the superoxo-peroxo pair is calculated to be E° = +130 mV vs SCE. A literature survey shows that this value falls into the range of biologically relevant redox reagents, e.g., cytochrome c and an organic solvent solubilized ascorbate anion. Using mixed-isotope resonance Raman (rRaman) spectroscopic characterization, accompanied by DFT calculations, it is shown that the superoxo complex consists of a mixture of μ-1,2- (2^1,2) and μ-1,1- (2^1,1) isomers, which are in rapid equilibrium. The electron transfer process involves only the μ-1,2-superoxo complex [Cu^II₂(UN-O^-)(μ-1,2-O₂^•-)]²⁺ (2^1,2) and μ-1,2-peroxo structures [Cu^II₂(UN-O^-)(O₂^2-)]⁺ (3) having a small bond reorganization energy of 0.4 eV (λ_in). A stopped-flow kinetic study results reveal an outer-sphere electron transfer process with a total reorganization energy (λ) of 1.1 eV between 2^1,2 and 3 calculated in the context of Marcus theory.