Quest for Ether-Coordinated Superoxide Ionic Liquids

Superoxide anion O₂^– is obtained by one electron reduction reaction of the oxygen molecule (O₂ + e → O₂^–)[1]. Since O₂^– has one unpaired electron it is also used for the radical polymerization initiator for organic synthesis[1]. Generally O₂^– is dissolved in aprotic solvents because it decomposes in the presence of H₂O or protic solvents (e.g. 2O₂⁻ + H₂O → O₂ + HO₂⁻ + OH⁻ and 2HO₂⁻ → O₂ + 2OH⁻). It is known that  potassium superoxide (KO₂) has little solubility even in dimethylsulfoxide, a highly polar aprotic organic solvent. Researcher has demonstrated that the solubility of KO₂ in aprotic solvents  increases dramatically by adding 18-crown-6-ether[1]. Nevertheless, the maximum solubility of superoxide is only about 20 mmol dm^–3 in aprotic solvents. Spectacularly improving the concentration of superoxide anion, it could be one method that we synthesize novel superoxide compounds in the liquid state at room temperature while the reported quaternary alkylammonium superoxides are all solid at room temperature[4]. In this work, we tried to synthesize superoxide ionic liquids by complexation of K⁺ cation to decrease charge density of cations substantially.

   KO₂, a yellow solid at room temperature, was chosen as an O₂⁻ source and mixed with two kinds of chain ethers i.e. diethylene glycol dimethyl ether (G2) and tetraethylene glycol dimethyl ether (G4), and a macrocyclic ether i.e. 18-crown-6-ether. Mixture of KO₂ and each ether was stirred at 500 rpm and heated under several situations in a glove box of Ar atmosphere (H₂O, O₂ < 1 ppm). Molar ratio of KO₂ : ether was 2 : 1 for G2, 1 : 1 for G4, and 1 : 1 for 18-crown-6-ether. It was visually judged from the absence of discoloration and precipitation of the mixture if these two substances chemically reacted without substantial decomposition of O₂^–. The coordination of 18-crown-6-ether to K⁺cation was checked with Raman spectrometry.

   Figure 1 schematically illustrates possible states of K⁺ cation complxed with three kinds of ethers : (a) G2, (b) G4, and (c) 18-crown-6-ether (K⁺ : G2 = 1 : 2, K⁺ : G4 = 1 : 1, K⁺ : 18-crown-6-ether = 1 : 1 in molar). KO₂ and G2 or G4 did not result in a liquid state even at 60 °C. By contrast, the mixture of KO₂ and 18-crown-6-ether became clear yellow liquid at 50 °C (see Figure 2 (b)). The melting point was 41 ~ 42 °C. We speculate that the strong coordination ability of 18-crown-6-ether to K⁺cation contributed to the sizable decrease in charge density of cations and thereby the melting point of the superoxide became near room temperature.

   Figure 3 demonstrates the Raman spectra for (a) uncomplexed liquid 18-crown-6-ether and (b) liquid [(18-crown-6)K]O₂ measured at 50 °C. For the latter case we observed evolution of sharp peaks between 860 and 900 cm^–1 in comparison of the former case. It is attributed to the conformation change of 18-crown-6-ether from C_i conformation to D_3d conformation[5]. This strongly suggersts that 18-crown-6-ether coordinates K⁺to form much weaker Lewis acidic cation.

[1] D. T. Sawyer et al., Acc. Chem. Res., 14, 393 (1981).

[2] K. Yamaguchi et al., Inorg. Chem., 25, 1289 (1986).

[3] K. Fukuhara et al., J. Mol. Struct., 224, 203 (1990).