Pentafluoroethyl(fluoro)phosphate ionic liquids: Synthesis and properties

Abstract

Ionic liquids (ILs) comprised of the 1-ethyl-3-methylimidazolium [EMIm]+, 1-butyl-3-methylimidazolium [BMIm]+, or butyl¬methyl-pyrrolidinium [Pyr14]+ cation and fluorophosphate anions with three, two, or one pentafluorethyl substituent bonded to phosphorus [mer-(C2F5)3PF3]– (FAP3), [trans-(C2F5)2PF4]– (FAP2), and [(C2F5)PF5]– (FAP1) were synthesized from tris(pentafluorethyl)¬-difluorophosphorane. The FAP ILs were characterized by NMR, IR, and Raman spectroscopy, and elemental analysis and the single crystal structures of the [EMIm]+ ILs with the anions FAP3, FAP2, and FAP1 and of [BMIm]FAP1 were elucidated. The thermal, physicochemical, and electrochemical properties of the ionic liquids were investigated. The interpretation of the data was aided by results from density functional calculations (DFT). Evaluation of structural, spectroscopic, and theoretical data showed the weakly coordinating nature of the series of phosphate anions to increase in the order [PF6]– < FAP1 < FAP2 < FAP3. The influence of the size, shape, and flexibility and of the coordinative behavior of the three FAP anions and the hexafluorophosphate anion on the ionic liquid properties was assessed.