An ionic liquid state composed of superoxide radical anions and crownether-coordinated potassium cations

Abstract

We report an “ionic liquid state” substance composed of superoxide radical anions (O₂⁻) 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 O₂⁻ were confirmed in the liquid state by Raman spectroscopy measurements. Below 38°C, however, the 18C6-KO₂ mixture does not form a complex but undergoes phase separation to parent phases of solid 18C6 and KO₂, because the ion-dipole interaction between K⁺ and 18C6 is overwhelmed by the ion-ion interaction between K⁺ and O₂⁻. In other words, the pair of O₂⁻ 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-KO₂ 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.