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)](ClO₄)₂ (1, tmpa = tris(2-pyridylmethyl)amine) and [(BzQ)Cu^II(H₂O)₂](ClO₄)₂ (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)]²⁺, the two-electron reduced product of O₂ with Sc³⁺ 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]³⁺ (bpy = 2,2-bipyridine) to the known superoxide species [(HMPA)₃Sc^III(O₂^-)]²⁺ 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 Sc³⁺ 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 Sc³⁺ on copper-oxygen intermediates.