Catalytic two-electron reduction of dioxygen catalysed by metal-free triphyrin(2.1.1)†
The catalytic two-electron reduction of dioxygen (O2) by octamethylferrocene (Me8Fc) occurs with a metal-free triphyrin (HTrip) in the presence of perchloric acid (HClO4) in benzonitrile (PhCN) at 298 K to yield Me8Fc+ and H2O2. Detailed kinetic analysis has revealed that the catalytic two-electron reduction of O2 by Me8Fc with HTrip proceeds via proton-coupled electron transfer from Me8Fc to HTrip to produce H3Trip˙+, followed by a second electron transfer from Me8Fc to H3Trip˙+ to produce H3Trip, which is oxidized by O2via formation of the H3Trip/O2 complex to yield H2O2. The rate-determining step in the catalytic cycle is hydrogen atom transfer from H3Trip to O2 in the H3Trip/O2 complex to produce the radical pair (H3Trip˙+ HO2˙) 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 Å from the zero-field splitting constant (D).