The effect of anion fluorination in ionic liquids—physical properties of a range of bis(methanesulfonyl)amide salts

Abstract

The bis(trifluoromethanesulfonyl)amide, TFSA, anion is widely used in the genesis of room temperature ionic liquids as it is non-spherical, fluorinated and has a particularly diffuse charge. However, the extent to which each of these structural features is responsible for the low melting point, fluidity and excellent stability of the resultant ionic liquids has yet to be described. We present the synthesis and analysis of a range of analogous, non-fluorinated species containing the bis(methanesulfonyl)amide, NMes₂⁻, ligand. Utilisation of this anion produces ionic liquids that are hydrophilic and extremely low melting, but with decreased thermal and electrical stability and significantly increased viscosity. The crystal structures of the dimethyl pyrrolidinium bis(methanesulfonyl)amide and TFSA species are compared, and the number of close contacts within each is assessed. Comparison of these structural and physical properties provides new insight into the effect of anion fluorination on these ionic liquids.