title: Factors that control catalytic two- versus four-electron reduction of dioxygen by copper complexes.
abstract: The selective two-electron reduction of O(2) by one-electron reductants such as decamethylferrocene (Fc*) and octamethylferrocene (Me(8)Fc) is efficiently catalyzed by a binuclear Cu(II) complex [Cu(II)(2)(LO)(OH)](2+) (D1) {LO is a binucleating ligand with copper-bridging phenolate moiety} in the presence of trifluoroacetic acid (HOTF) in acetone. The protonation of the hydroxide group of [Cu(II)(2)(LO)(OH)](2+) with HOTF to produce [Cu(II)(2)(LO)(OTF)](2+) (D1-OTF) makes it possible for this to be reduced by 2 equiv of Fc* via a two-step electron-transfer sequence. Reactions of the fully reduced complex [Cu(I)(2)(LO)](+) (D3) with O(2) in the presence of HOTF led to the low-temperature detection of the absorption spectra due to the peroxo complex [Cu(II)(2)(LO)(OO)] (D) and the protonated hydroperoxo complex [Cu(II)(2)(LO)(OOH)](2+) (D4). No further Fc* reduction of D4 occurs, and it is instead further protonated by HOTF to yield H(2)O(2) accompanied by regeneration of [Cu(II)(2)(LO)(OTF)](2+) (D1-OTF), thus completing the catalytic cycle for the two-electron reduction of O(2) by Fc*. Kinetic studies on the formation of Fc*(+) under catalytic conditions as well as for separate examination of the electron transfer from Fc* to D1-OTF reveal there are two important reaction pathways operating. One is a rate-determining second reduction of D1-OTF, thus electron transfer from Fc* to a mixed-valent intermediate [Cu(II)Cu(I)(LO)](2+) (D2), which leads to [Cu(I)(2)(LO)](+) that is coupled with O(2) binding to produce [Cu(II)(2)(LO)(OO)](+) (D). The other involves direct reaction of O(2) with the mixed-valent compound D2 followed by rapid Fc* reduction of a putative superoxo-dicopper(II) species thus formed, producing D.
authors.affiliation: Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea. fukuzumi@chem.eng.osaka-u.ac.jp
authors.forename: Shunichi
authors.initials: S
authors.lastname: Fukuzumi
authors.forename: Laleh
authors.initials: L
authors.lastname: Tahsini
authors.forename: Yong-Min
authors.initials: YM
authors.lastname: Lee
authors.forename: Kei
authors.initials: K
authors.lastname: Ohkubo
authors.forename: Wonwoo
authors.initials: W
authors.lastname: Nam
authors.forename: Kenneth D
authors.initials: KD
authors.lastname: Karlin
mesh_terms: D002384:Catalysis; D003300:Copper / Q000737:chemistry*; D004583:Electrons; D006861:Hydrogen Peroxide / Q000138:chemical synthesis / Q000737:chemistry; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: D009942:Organometallic Compounds; D003300:Copper; D006861:Hydrogen Peroxide; D010100:Oxygen