Electron-Transfer Studies of a Peroxide Dianion

Abstract

A peroxide dianion (O₂^2–) can be isolated within the cavity of hexacarboxamide cryptand, [(O₂)⊂mBDCA-5t-H₆]^2–, stabilized by hydrogen bonding but otherwise free of proton or metal-ion association. This feature has allowed the electron-transfer (ET) kinetics of isolated peroxide to be examined chemically and electrochemically. The ET of [(O₂)⊂mBDCA-5t-H₆]^2– with a series of seven quinones, with reduction potentials spanning 1 V, has been examined by stopped-flow spectroscopy. The kinetics of the homogeneous ET reaction has been correlated to heterogeneous ET kinetics as measured electrochemically to provide a unified description of ET between the Butler–Volmer and Marcus models. The chemical and electrochemical oxidation kinetics together indicate that the oxidative ET of O₂^2– occurs by an outer-sphere mechanism that exhibits significant nonadiabatic character, suggesting that the highest occupied molecular orbital of O₂^2– within the cryptand is sterically shielded from the oxidizing species. An understanding of the ET chemistry of a free peroxide dianion will be useful in studies of metal–air batteries and the use of [(O₂)⊂mBDCA-5t-H₆]^2– as a chemical reagent