Unusual Li+ Ion Solvation Structure in Bis(fluorosulfonyl)amide Based Ionic Liquid

Raman spectra of 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide [C(2)mIm(+)] [FSA(-)] ionic liquid solutions dissolving LiFSA salt of various concentrations were measured at 298 K. FSA(-) ((FSO2)(2)N-) is an analogue anion of bis(trifluoromethanesulfonyl)amide ((CF3SO2)(2)N-; TFSA(-)). We found that a solvation number of the Li+ ion in [C(2)mIm(+)] [FSA(-)] is 3, though it has been well established that Li+ ion is solvated by two TFSA(-) anions in the corresponding ionic liquids below the Li+ ion mole fraction of x(Li+) < 0.2. To yield further insight into larger solvation numbers, Raman spectra were measured at higher temperatures up to 364 K. The Li+ ion solvation number in [C(2)mIm][FSA(-)] evidently decreased when the temperature was elevated. Temperature dependence of the Li+ ion salvation number was analyzed assuming an equilibrium between [Li(FSA)(2)](-) and [Li(FSA)(3)](2-), and the enthalpy Delta H degrees and the temperature multiplied entropy T Delta S degrees for one FSA(-) liberation toward a bulk ionic liquid were successfully evaluated to be 35(2) kJ mol(-1) and 29(2) kJ mol(-1), respectively. The Delta H degrees and Delta S degrees suggest that the Li+ ion is coordinated by one of bidentate and two of monodentate FSA(-) at 298 K, and that the more weakly solvated monodentate FSA(-) is liberated at higher temperatures. The high-energy X-ray diffraction (HEXRD) experiments of these systems were carried out and were analyzed with the aid of molecular dynamics (MD) simulations. In radial-distribution functions evaluated with HEXRD, a peak at about 1.94 angstrom appeared and was attributable to the Li+-O(FSA(-)) correlations. The longer Li+-O(FSA(-)) distance than that for the Li+-O(TFSA(-)) of 1.86 angstrom strongly supports the larger solvation number of the Li+ ions in the FSA(-) based ionic liquids. MD simulations at least qualitatively reproduced the Raman and HEXRD experiments.