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

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 binucleating 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(center dot-))](2+) (2). The formation kinetics provide that k(on) = 9 X 10(-2) M-1 s(-1) (-80 degrees C), Delta H-double dagger = 31.1 kJ mol(-1) and Delta S-double dagger = -99.4 J K-1 mol(-1) (from -60 to -90 degrees C 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 degrees C); a standard reduction potential for the superoxo-peroxo pair is calculated to be E degrees = +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 mu-1,2- (2(1,2)) and mu-1,1- (2(1,1)) isomers, which are in rapid equilibrium. The electron transfer process involves only the mu-1,2-superoxo complex [Cu-2(II)(UN-O-)(mu-1,2-O-2(center dot-))](2+) (2(1,2)) and mu-1,2-peroxo structures [Cu-2(II)(UN-O-)(O-2(2-))(+) (3) having a small bond reorganization energy of 0.4 eV (lambda(in)). A stopped-flow kinetic study results reveal an outer-sphere electron transfer process with a total reorganization energy (lambda) of 1.1 eV between 2(1,2) and 3 calculated in the context of Marcus theory.