Synthesis, conformational studies, vibrational spectra and thermal properties, of new 1,4-(phenylenebis(methylene) bis(methyl-imidazolium) ionic liquids

Three new substituted dicationic ionic liquids (DILs) based on biphenylenedimethylene linked bis-1-methylimidazolium ionic liquids [bis-(PhCH2MIm)+], combined with three anions, namely dichloride [Cl]2, di (hexafluorophosphate) [PF6-]2 and di(bis(trifluoromethylsulfonyl)Imide) [(CF3SO2)2N-]2 were synthesized. The three synthesized dicationic ionic liquids were characterized by Nuclear Magnetic Resonance and Fourier transform infrared (FTIR) and Raman spectroscopy in the region 4000-600 cm-1 and 4000-45 cm-1, respectively. A complete vibrational assignments have been performed; the biphenylene linkage spacer is confirmed by the presence of strong Raman band at 1616 cm-1 related to the C=C vibration in all our investigated DILs. Moreover, a conformational investigation of the cation was performed by means of DFT calculations using the 6 31G** basis set and the B3LYP theory. The comparison of the computed and experimental Raman spectra further corroborate the structure of the cation. Finally, the thermal stability of the DILs was investigated by TGA and DTA measurements. The DILs with Cl-, PF6- and [(CF3SO2)2N-] anions decompose at 252, 323 and 420 °C, respectively.