Syntheses, structures and properties of 1-ethyl-3-methylimidazolium salts of fluorocomplex anions

摘要

Fluoroacid-base reactions of a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium fluorohydrogenate (EMIm(HF)2.3F, EMIm = 1-ethyl-3-methylimidazolium cation), and Lewis fluoroacids (BF_3, PF_5, AsF_5, NbF_5, TaF_5 and WF_6) give EMIm salts of the corresponding fluorocomplex anions, EMImBF_4, EMImPF_6, EMImAsF_6, EMImNbF_6, EMImTaF_6 and EMImWF_7, respectively. Attempts to prepare EMImVF_6 by both the acid-base reaction of EMIm(HF)2.3F with VF5 and the metathesis of EMImCl with KVF6 failed due to the strong oxidizing power of the pentavalent vanadium, whereas EMImSbF_6 was successfully prepared only by the metathesis of EMImCl and KSbF_6. EMImBF_4, EMImSbF_6, EMImNbF_6, EMImTaF_6 and EMImWF_7 are liquids at room temperature whereas EMImPF_6 and EMImAsF_6 melts at around 330 K. Raman spectra of the obtained salts showed the existence of the EMIm cation and corresponding fluorocomplex anions. IR spectroscopy revealed that strong hydrogen bonds are not observed in these salts. EMImAsF_6(mp 326 K) and EMImSbF_6(mp 283 K) are isostructural with the previously reported EMImPF_6. The melting point of the hexafluorocomplex EMIm salt decreases with the increase of the size of the anion (PF_6~- < AsF_6~- < SbF_6~-