Quest for Ether-Coordinated Superoxide Ionic Liquids
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 Ci conformation to D3d conformation. This strongly suggersts that 18-crown-6-ether coordinates K+to form much weaker Lewis acidic cation.
 D. T. Sawyer et al., Acc. Chem. Res., 14, 393 (1981).
 K. Yamaguchi et al., Inorg. Chem., 25, 1289 (1986).