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

Abstract

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 (BF₃, PF₅, AsF₅, NbF₅, TaF₅ and WF₆) give EMIm salts of the corresponding fluorocomplex anions, EMImBF₄, EMImPF₆, EMImAsF₆, EMImNbF₆, EMImTaF₆ and EMImWF₇, respectively. Attempts to prepare EMImVF₆ 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 EMImSbF₆ was successfully prepared only by the metathesis of EMImCl and KSbF₆. EMImBF₄, EMImSbF₆, EMImNbF₆, EMImTaF₆ and EMImWF₇ are liquids at room temperature whereas EMImPF₆ and EMImAsF₆ 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. EMImAsF₆ (mp 326 K) and EMImSbF₆ (mp 283 K) are isostructural with the previously reported EMImPF₆. The melting point of the hexafluorocomplex EMIm salt decreases with the increase of the size of the anion (PF₆⁻ < AsF₆⁻ < SbF₆⁻ <NbF₆⁻ ≈ TaF₆⁻).