Lewis Acid-Induced Change from Four- to Two-Electron Reduction of Dioxygen Catalyzed by Copper Complexes Using Scandium Triflate

Mononuclear copper complexes, [(tmpa)­CuII(CH3CN)]­(ClO4)2 (1, tmpa = tris­(2-pyridylmethyl)­amine) and [(BzQ)­CuII(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)­CuII(O2)­CuII(tmpa)]2+, the two-electron reduced product of O2 with Sc3+ is observed to be scandium peroxide ([ScIII(O22–)]+). In the presence of 3 equiv of hexamethylphosphoric triamide (HMPA), [ScIII(O22–)]+ was oxidized by [Fe­(bpy)3]3+ (bpy = 2,2-bipyridine) to the known superoxide species [(HMPA)3ScIII(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.