Extremely short C–H⋯F contacts in the 1-methyl-3-propyl-imidazolium SiF 6 —the reason for ionic “liquid” unexpected high melting point

The synthesis and XRD investigation of hexafluorosilicate salt with 1-propyl-3-methyl imidazolium ([Pmim]+) cation is described. Analysis of crystal packing has revealed that an unexpectedly high melting point (mp, 210 °C) of salt resulted from the presence of extremely short interionic C(Im)H⋯F contacts in the crystal (1.94–2.42 Å). The absence of strong C–H⋯F interaction for alkyl radicals led to high mobility of substituents and resulted in phase transition of the order–disorder type. The total energy of the CH⋯F interactions in the hypothetical [SiF6(Pmim)6]+4 cluster according to DFT calculation and topological analysis of the electron density distribution attains ca. 33 kcal mol−1.