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
[CuII2(UN-O–)­(O2•–)]2+ (2). The
formation kinetics provide that kon =
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 [CuII2(UN-O–)­(O22–)]+ (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 Ket = 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- (21,2) and μ-1,1- (21,1) isomers, which are in rapid equilibrium.
The electron transfer process involves only the μ-1,2-superoxo
complex [CuII2(UN-O–)­(μ-1,2-O2•–)]2+ (21,2) and μ-1,2-peroxo structures [CuII2(UN-O–)­(O22–)]+ (3) having a small bond reorganization
energy of 0.4 eV (λin). A stopped-flow kinetic study
results reveal an outer-sphere electron transfer process with a total
reorganization energy (λ) of 1.1 eV between 21,2 and 3 calculated in the context
of Marcus theory.