Electron-Transfer Studies of a Peroxide Dianion

A peroxide dianion (O22–) 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 O22– occurs by an outer-sphere mechanism that exhibits significant nonadiabatic character, suggesting that the highest occupied molecular orbital of O22– 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.