Ionic liquids and plastic crystals with a symmetrical pyrrolidinium cation

Abstract

Solid and liquid salts utilising the N-methyl-N-alkyl pyrrolidinium cation, [C_nmpyr]⁺, and their use in electrochemical devices, are well established. However, new materials with enhanced properties, such as higher conductivity, lower viscosity or more favourable thermal phase behaviour, are still required. Here we report the synthesis and characterisation of new ionic liquids and plastic crystals using the N,N-diethylpyrrolidinium cation ([C₂epyr]⁺) with six different anions. With the fluorosulfonyl(trifluoromethanesulfonyl)imide ([FTFSI]⁻) and dicyanamide ([DCA]⁻) anions, room temperature ionic liquids with low viscosity are formed. With the bis(trifluoromethanesulfonyl)imide ([NTf₂]⁻), bis(fluorosulfonyl)imide ([FSI]⁻), hexafluorophosphate ([PF₆]⁻) and tetrafluoroborate ([BF₄]⁻) anions, organic ionic plastic crystals are produced. Of the new solid salts, [C₂epyr][FSI] has the highest conductivity, higher than the well-established methyl-substituted analogue, giving 1.9 × 10⁻⁵ S cm⁻¹ at 30 °C. Thermal analysis, conductivity and the Walden relationship are used to compare the new N,N-diethylpyrrolidinium salts across the different anions and with some previously reported N-methyl-N-alkylpyrrolidinium salts.