Mechanistic Studies of Catalytic O2-to-H2O2 Conversion at a Single Cobalt Site

Understanding oxygen reduction reaction (ORR) mechanisms is of fundamental significance. Although metal hydrosuperoxos and metal hydroperoxos are considered to be key intermediates in ORR, these two species as well as their reaction natures are poorly understood because they are highly active with various reaction pathways. Herein, we report on the mechanistic studies of the ORR with Co^II porphyrin 1. Complex 1 is selective for catalytic O₂-to-H₂O₂ conversion with decamethylferrocene as the reductant and HClO₄ as the proton source. By employing the molecular pocket of 1 to stabilize the O₂-adducts, we characterized Co^III-hydrosuperoxo, [Co^III(O₂^•H)]⁺, and Co^III-hydroperoxo, [Co^III(O₂H)], studied the one-electron reduction of [Co^III(O₂^•H)]⁺ to generate [Co^III(O₂H)], and revealed a proton transfer-electron transfer (PTET) pathway for [Co^III(O₂H)] to generate Co^II and H₂O₂. This work is therefore significant to confirm the key role of [Co^III(O₂^•H)]⁺ and [Co^III(O₂H)] in the catalytic ORR cycle and to establish a PTET pathway of [Co^III(O₂H)] to give H₂O₂.