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

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 (HClO(4)) in benzonitrile (PhCN) at 298 K to yield Me(8)Fc(+) and H(2)O(2). 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)Trip˙(+), followed by a second electron transfer from Me(8)Fc to H(3)Trip˙(+) to produce H(3)Trip, which is oxidized by O(2)via formation of the H(3)Trip/O(2) complex to yield H(2)O(2). 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)Trip˙(+) HO(2)˙) 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).