Thermal phase behaviour of N-alkyl-N-methylpyrrolidinium and piperidinium bis(trifluoromethanesulfonyl)imide salts

The phases, ion crystal packing and thermal properties of the N-aikyl-N-methylpyrrolidinium and piperidinium bis(trifluoromethanesulfonyl)imide (PYRiRTFSI and PIPirTFSI (subscript R = 1 for methyl and 2 for ethyl), respectively) salts are compared using powder and single-crystal x-ray diffraction (XRD) and differential scanning calorimetry (DSC). The crystal structure of PIPi2TFSI has been determined at 123 K. The salt crystallizes in the triclinic space group Pi with Z = 8. Structural data are also reported for PYRUTFSI at 153 K and PIP12TFSI at 223 K. PIPaTFSI has identical ion crystal packing to the analogous pyrrolidinium salt PYRnTFSI. Since increasing the cation alkyl chain length to propyl or butyl (R = 3 or 4) reduces the melting point of the salts below room temperature, this study may provide valuable insight into why these pyrrolidinium and piperidinium salts form room-temperature ionic liquids.