Structural analysis of mono-substituted N-butyl-pyridinium salts: in search of ionic liquids
Four mono-substituted N-butylpyridinium salts, 1-butyl-4-dimethylaminopyridinium chloride [b4dmapy]Cl, 1-butyl-4-methylpyridinium bromide [b4mpy]Br, 1-butyl-4-methylpyridinium hexafluorophosphate [b4mpy][PF6], and 1-butyl-3-methylpyridinium hexafluorophosphate [b3mpy][PF6] were synthesized and characterized using single crystal X-ray diffraction. The crystal structures were examined with the intent of identifying ion interactions leading to higher melting points of the halide salts with respect to the PF6 salts. The changes in hydrogen bonding, C-H...pi, and van der Waals interactions have been analyzed with respect to anion, functional groups, and the symmetry of the cation to establish interdependence with the compound's physicochemical properties. It has been observed that the cation-anion interactions are represented by highly directional hydrogen bonds and show strong preference to positions of interaction depending on the anion. The cations of the halide salts show strong tendency towards higher dimensional formations, while those of the PF6 salts prefer low dimensional assemblies both being based mainly on the weaker van der Waals interactions. These interactions depend on the shape of the cation but may offer certain structure-ordering rigidity accommodating variable anions.