Lewis acid-induced change from four- to two-electron reduction of dioxygen catalyzed by copper complexes using scandium triflate.

Mononuclear copper complexes, (tmpa)Cu(II)(CH3CN)2 (1, tmpa = tris(2-pyridylmethyl)amine) and (BzQ)Cu(II)(H2O)22 (2, BzQ = bis(2-quinolinylmethyl)benzylamine)], act as efficient catalysts for the selective two-electron reduction of O2 by ferrocene derivatives in the presence of scandium triflate (Sc(OTf)3) in acetone, whereas 1 catalyzes the four-electron reduction of O2 by the same reductant in the presence of Brønsted acids such as triflic acid. Following formation of the peroxo-bridged dicopper(II) complex (tmpa)Cu(II)(O2)Cu(II)(tmpa), the two-electron reduced product of O2 with Sc(3+) is observed to be scandium peroxide (Sc(III)(O2(2-))). In the presence of 3 equiv of hexamethylphosphoric triamide (HMPA), Sc(III)(O2(2-)) was oxidized by Fe(bpy)3 (bpy = 2,2-bipyridine) to the known superoxide species (HMPA)3Sc(III)(O2(•-)) as detected by EPR spectroscopy. A kinetic study revealed that the rate-determining step of the catalytic cycle for the two-electron reduction of O2 with 1 is electron transfer from Fc* to 1 to give a cuprous complex which is highly reactive toward O2, whereas the rate-determining step with 2 is changed to the reaction of the cuprous complex with O2 following electron transfer from ferrocene derivatives to 2. The explanation for the change in catalytic O2-reaction stoichiometry from four-electron with Brønsted acids to two-electron reduction in the presence of Sc(3+) and also for the change in the rate-determining step is clarified based on a kinetics interrogation of the overall catalytic cycle as well as each step of the catalytic cycle with study of the observed effects of Sc(3+) on copper-oxygen intermediates.