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

Fluoroacid - base reactions of a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium fluorohydrogenate (EMIm(HF)(2.3)F, EMIm = 1-ethyl-3-methylimidazolium cation), and Lewis fluoroacids (BF3, PF5, AsF5, NbF5, TaF5 and WF6) 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)(2.3)F 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 KSbF6. 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 (PF6- < AsF6- < SbF6- < NbF6- approximate to TaF6-).