Electron-transfer studies of a peroxide dianion

A peroxide dianion (O₂^2-) can be isolated within the cavity of hexacarboxamide cryptand, [(O₂)∪mBDCA-5t-H ₆]^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 O ₂^2- 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 [(O ₂)∪mBDCA-5t-H₆]^2- as a chemical reagent.