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

The phases, ion crystal packing and thermal properties of the N -alkyl- N -methylpyrrolidinium and piperidinium bis(trifluoromethanesulfonyl)imide (PYR 1 R TFSI and PIP 1 R TFSI (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 PIP 12 TFSI has been determined at 123 K. The salt crystallizes in the triclinic space group with Z = 8. Structural data are also reported for PYR 11 TFSI at 153 K and PIP 12 TFSI at 223 K. PIP 11 TFSI has identical ion crystal packing to the analogous pyrrolidinium salt PYR 11 TFSI. 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.