title: Lewis acid-induced change from four- to two-electron reduction of dioxygen catalyzed by copper complexes using scandium triflate.
abstract: 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 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](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 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.
authors: Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, Osaka University , ALCA (JST), Suita, Osaka 565-0871, Japan.
authors: Saya
authors: S
authors: Kakuda
authors: Clarence J
authors: CJ
authors: Rolle
authors: Kei
authors: K
authors: Ohkubo
authors: Maxime A
authors: MA
authors: Siegler
authors: Kenneth D
authors: KD
authors: Karlin
authors: Shunichi
authors: S
authors: Fukuzumi
mesh_terms: D015082:2,2'-Dipyridyl / Q000737:chemistry; D002384:Catalysis; D056831:Coordination Complexes / Q000737:chemistry; D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D005296:Ferrous Compounds / Q000737:chemistry; D058116:Lewis Acids / Q000737:chemistry*; D008698:Mesylates / Q000737:chemistry*; D000075163:Metallocenes; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D011725:Pyridines / Q000737:chemistry; D012538:Scandium / Q000737:chemistry*
chemical_list: D056831:Coordination Complexes; D005296:Ferrous Compounds; D058116:Lewis Acids; D008698:Mesylates; D000075163:Metallocenes; D011725:Pyridines; C436603:scandium triflate; C431843:tris(2-pyridylmethyl)amine; D015082:2,2'-Dipyridyl; D003300:Copper; C012077:trifluoromethanesulfonic acid; D010100:Oxygen; C004998:ferrocene; D012538:Scandium