An Ionic Liquid State Composed of Superoxide Radical Anions and Crownether-Coordinated Potassium Cations

Abstract

© The Author(s) 2017. Published by ECS. All rights reserved. We report an “ionic liquid state” substance composed of superoxide radical anions (O 2 – ) and crownether-coordinated potassium cations. A binary mixture of 18-crown-6-ether (18C6) and potassium superoxide (KO2) becomes liquid above ca. 38 ◦ C. The coordination of 18C6 to K + and the presence of O 2 – were confirmed in the liquid state by Raman spectroscopy measurements. Below 38 ◦ C, however, the 18C6-KO 2 mixture does not form a complex but undergoes phase separation to parent phases of solid 18C6 and KO 2 , because the ion-dipole interaction between K + and 18C6 is overwhelmed by the ion-ion interaction between K + and O 2 – . In other words, the pair of O 2 – and 18C6-coordinated K + only appears above the melting point of pure 18C6, to form an “ionic liquid state”. Among radical-containing ionic liquids, the liquid 18C6-KO 2 mixture has high fluidity and comparable conductivity. The findings show stabilization of superoxide radical anions at liquid state with high concentration, which could be used for many radical reactions.