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 8Fc) 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 2O 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 IICu 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. © 2012 American Chemical Society.