Properties of fluorosulfate-based ionic liquids and geometries of (FO2SOH)OSO2F− and (FO2SOH)2O2SOF−

Abstract

A room temperature ionic liquid (IL) based on the fluorosulfate anion (SO₃F⁻) has been synthesized by the reaction of 1-ethyl-3-methylimidazolium (EMIm⁺) chloride and fluorosulfuric acid (HOSO₂F). The viscosity, ionic conductivity, and electrochemical window of EMImSO₃F at 25 °C are 46.6 mPa s, 10.8 mS cm⁻¹, and 4.3 V, respectively. According to a solvatochromic measurement using ILs, there is a trend in the donor ability of fluoro- and oxofluoroanions, PF₆⁻ < BF₄⁻ < N(SO₂CF₃)₂⁻ < SO₃CF₃⁻ < SO₃F⁻ < PO₂F₂⁻, which is explained by the atomic charges obtained from quantum mechanical calculations. The 1 : 2 and 1 : 3 stoichiometric reactions of EMImCl and HOSO₂F give EMIm(FO₂SOH)OSO₂F and EMIm(FO₂SOH)₂O₂SOF, respectively. Both the salts are liquid at room temperature without a HOSO₂F dissociation pressure (< 1 Pa) and have low viscosity and high ionic conductivity (9.2 mPa s and 30.8 mS cm⁻¹ for EMIm(FO₂SOH)OSO₂F and 5.1 mPa s and 43.2 mS cm⁻¹ for EMIm(FO₂SOH)₂O₂SOF). The vibrational modes and bonding properties of these anionic species are discussed with the aid of quantum mechanical calculations. The (FO₂SOH)OSO₂F⁻ anion in EMIm(FO₂SOH)OSO₂F does not have an inversion centre, which stands in contrast to the one with an inversion centre (e.g. observed in solid Cs(FO₂SOH)OSO₂F). The (FO₂SOH)₂O₂SOF⁻ anion in EMIm(FO₂SOH)₂O₂SOF is characterized by vibrational spectroscopy under C_s symmetry.