Properties of fluorosulfate-based ionic liquids and geometries of (FO_2SOH)OSO_2F^- and (FO_2SOH)_2O_2SOF^-.

Abstract

A room temperature ionic liquid (IL) based on the fluorosulfate anion (SO_3F^-) has been synthesized by the reaction of 1-ethyl-3-methylimidazolium (EMIm^+) chloride and fluorosulfuric acid (HOSO_2F). The viscosity, ionic conductivity, and electrochemical window of EMImSO_3F 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, PF_6^- < BF_4^- < N(SO_2CF_3)_2^- < SO_3CF_3^- < SO_3F^- < PO_2F_2^-, which is explained by the atomic charges obtained from quantum mechanical calculations. The 1 : 2 and 1 : 3 stoichiometric reactions of EMImCl and HOSO_2F give EMIm(FO_2SOH)OSO_2F and EMIm(FO_2SOH)_2O_2SOF, respectively. Both the salts are liquid at room temperature without a HOSO_2F dissociation pressure (< 1 Pa) and have low viscosity and high ionic conductivity (9.2 mPa s and 30.8 mS cm^(-1) for EMIm(FO_2SOH)OSO_2F and 5.1 mPa s and 43.2 mS cm^(-1) for EMIm(FO_2SOH)_2O_2SOF). The vibrational modes and bonding properties of these anionic species are discussed with the aid of quantum mechanical calculations. The (FO_2SOH)OSO_2F^- anion in EMIm(FO_2SOH)OSO_2F does not have an inversion centre, which stands in contrast to the one with an inversion centre (e.g. observed in solid Cs(FO_2SOH)OSO_2F). The (FO_2SOH)_2O_2SOF^- anion in EMIm(FO_2SOH)_2O_2SOF is characterized by vibrational spectroscopy under Cs symmetry.