Synthesis, characterization and thermal behavior of nine new -type quaternary ammonium tetrafluoroborate or hexafluorophosphate salts prepared by metathesis from analogous halide salts

Nine new quaternary ammonium tetrafluoroborate or hexafluoroborate salts were prepared from analogous bromide or chloride salts using anion exchange reaction in which the corresponding bromide or chloride salt was treated with HBF4 or HPF6 acid in aqueous solutions. The characterizations were performed by 1H NMR and 13C NMR spectroscopy as well as by elemental analysis. The single crystals of three tetrafluoroborate and two hexafluorophosphate salts were obtained by slow evaporation from a methanol/ethyl acetate solution and the crystal structures were determined by X-ray single crystal diffraction. Four of the compounds crystallized in the orthorhombic and one in the monoclinic crystal system. In all cases, the packing is mainly affected by Coulombic interactions and weak CH⋯F hydrogen bonds. The thermal behaviour of the compounds was studied using TGA and DSC methods. The anion exchange had no clear lowering effect on the observed melting points of the compounds. However, thermal stability of the compounds enhanced significantly compared to analogous halide salts as in most cases notably larger liquid ranges and sharper melting transitions were observed for tetrafluoroborate/hexafluorophosphate salts than for previously reported halide salts which melted either on decomposing and/or a melting was not observed at all.