Structurally Characterized μ-1,2-Peroxo/Superoxo Dicopper(II) Pair.

Superoxo complexes of copper are primary adducts in several O-activating Cu-containing metalloenzymes as well as in other Cu-mediated oxidation and oxygenation reactions. Because of their intrinsically high reactivity, however, isolation of Cu(O) species is challenging. Recent work ( , , 9831; , , 12682) established fundamental thermochemical data for the H atom abstraction reactivity of dicopper(II) superoxo complexes, but structural characterization of these important intermediates was so far lacking. Here we report the first crystallographic structure determination of a superoxo dicopper(II) species () together with the structure of its 1e reduced peroxo congener (; a rare -μ-1,2-peroxo dicopper(II) complex). Interconversion of and occurs at low potential (-0.58 V vs Fc/Fc) and is reversible both chemically and electrochemically. Comparison of metric parameters ((O-O) = 1.441(2) Å for vs 1.329(7) Å for ) and of spectroscopic signatures (ν̃(O-O) = 793 cm for vs 1073 cm for ) reflects that the redox process occurs at the bridging O-derived unit. The Cu-O-Cu complex has an = 1/2 spin ground state according to magnetic and EPR data, in agreement with density functional theory calculations. Computations further show that the potential associated with changes of the Cu-O-O-Cu dihedral angle is shallow for both and . These findings provide a structural basis for the low reorganization energy of the kinetically facile 1e interconversion of μ-1,2-superoxo/peroxo dicopper(II) couples, and they open the door for comprehensive studies of these key intermediates in Cu/O chemistry.