Thermal properties of N-alkyl-N-methylpyrrolidinium and N-butylpyridinium fluorometallates and physicochemical properties of their melts

A series of N-alkyl-N-methylpyrrolidinium (RMPyr+, where R = E: ethyl, B: butyl, and H: hexyl) and N-butylpyridinium (BPy+) salts based on the fluorocomplex anions, BF4−, PF6−, SbF6−, NbF6−, TaF6−, and WF7−, have been synthesized and their thermal behavior has been investigated. The melting points of the RMPyr+ salts are above room temperature with the trend; BMPyrAF6 < HMPyrAF6 < EMPyrAF6 for the hexafluorocomplex salts. Some of the salts containing BMPyr+ and HMPyr+ exhibit phase transitions in the solid states. Similar melting points of BPy+ salts of PF6−, SbF6−, NbF6−, TaF6−, and WF7− are observed at around 350 K. Ionic conductivity and viscosity for BMPyrNbF6 (3.0 mS cm−1 and 164 cP at 328 K) are similar to those for BMPyrTaF6 (3.0 mS cm−1 and 165 cP at 328 K), resulting from the similarity of the anions in size. The activation energies of ionic conductivity for the NbF6− and TaF6− salts are 18 and 20 kJ mol−1, and those for viscosity are 23 and 25 kJ mol−1, respectively calculated by Arrhenius equation in the temperature range between 328 and 348 K. Electrochemical windows of BMPyrNbF6, BMPyrTaF6, and BMPyrWF7 are about 4.0, 5.0 and 3.1 V, respectively.