Catalytic two-electron reduction of dioxygen catalysed by metal-free [14]triphyrin(2.1.1)

Abstract

The catalytic two-electron reduction of dioxygen (O-2) by octamethylferrocene (Me(8)Fc) occurs with a metal-free triphyrin (HTrip) in the presence of perchloric acid (HClO4) in benzonitrile (PhCN) at 298 K to yield Me(8)Fc(+) and H2O2. Detailed kinetic analysis has revealed that the catalytic two-electron reduction of O-2 by Me(8)Fc with HTrip proceeds via proton-coupled electron transfer from Me(8)Fc to HTrip to produce H(3)Tripc(center dot+), followed by a second electron transfer from Me(8)Fc to H(3)Tripc(center dot+) to produce H(3)Trip, which is oxidized by O-2 via formation of the H(3)Trip/O-2 complex to yield H2O2. The rate-determining step in the catalytic cycle is hydrogen atom transfer from H(3)Trip to O-2 in the H(3)Trip/O-2 complex to produce the radical pair (H(3)Tripc(center dot+) HO(2)c) as an intermediate, which was detected as a triplet EPR signal with fine-structure by the EPR measurements at low temperature. The distance between the two unpaired electrons in the radical pair was determined to be 4.9 angstrom from the zero-field splitting constant (D).