Synthesis, thermal stability, vibrational spectra and conformational studies of novel dicationic meta-xylyl linked bis-1-methylimidazolium ionic liquids

Abstract

In recent years, dicationic ionic liquids have been subject of a strongly growing research effort for their interesting and important properties that suggest them to be potential candidate for many applications. Therefore, the knowledge about the synthesis, thermal stability, vibrational spectra and conformational behavior of these compounds is an important subject of study. The objective of the work presented in this paper is to investigate the influence of the substituent relative meta-position of dicationic ionic liquids (ILs), namely meta-xylyl linked bis-1-methylimidazolium ILs, on thermal stability, vibrational spectra and conformational studies. In the first step, four new meta-xylyl linked bis-imidazolium [m-C6H4(CH2ImMe)2+] ionic liquid (ILs) containing the dichloride [Cl−]2, di(bis(trifluoromethylsulfonyl) Imide) [(CF3SO2)2N−]2, bis(tetrafluoroborate) [BF4−]2 and bis (hexafluorophosphate) [PF6−]2 anions were synthesized. The obtained dicationic ILs were characterized by 1H NMR, 13C NMR, 19F NMR, 31P NMR and FT-IR spectroscopy. Also, their thermal properties were determined and compared. The thermal behavior confirmed that [m-C6H4(CH2ImMe)2+] dicationic ILs containing [(CF3SO2)2N−]2 and [PF6−]2 are more stable that the dichloride [Cl−]2, and bis(tetrafluoroborate) [BF4−]2 dicationic ILs, and they show a good thermal stability up to 340 °C which makes them suitable for thermal application. In the next step, a computational study of the conformers of [m-C6H4(CH2ImMe)2], by means of DFT calculations with the 6-31G** basis set and the B3LYP theory was performed. Additionally, vibrational spectroscopy studies were conducted by infrared (FTIR) and Raman (FT-Raman) spectroscopy on the four dicationic ILs, differing for their anions. Finally a computational investigation of the interactions between anions and cation is presented.