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

摘要

Oxygenation of [Cu_2(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 binudeating ligand and where dimethylformamide (DMF) binds to the Cu(Ⅱ) ion, leads to a superoxo-dicopper(Ⅱ) species [Cu_2~Ⅱ(UN-O~-)(O_2~(•-))]~(2+) (2). The formation kinetics provide that k_(on) = 9 × 10~(-2) M~(-1) s~(-1) (-80 ℃), ΔH = 31.1 kJ mol~(-1) and ΔS = -99.4 J K~(-1) mol~(-1) (from -60 to -90 ℃ data). Complex 2 can be reversibly reduced to the peroxide species [Cu_2~Ⅱ(UN-O~-)(O_2~(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 ℃); 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_2~Ⅱ(UN-O~-)(μ-1,2-O_2~(•-))]~(2+) (2~(1,2)) and μ-1,2-peroxo structures [Cu_2~Ⅱ(UN-O~-)(O_2~(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.