Factors that control catalytic two-versus four-electron reduction of dioxygen by copper complexes

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.