New room temperature ionic liquids prepared from an old phosphine: Using branching and diastereomeric mixtures to inherently benefit physical properties

Abstract

First reported in 1966–1967, the compound tri(2-ethylhexyl)phosphine appears to be a hidden gem for ionic liquids. Although this compound offers many potential intriguing properties that are inherent due to its branching and diastereomeric nature, there is only a single report employing this material as an ionic liquid constituent. Through quaternization and anion exchange, a total of 24 new room temperature ionic liquids were prepared containing bromide (Br−), bis(trifluoromethanesulfonyl)imide (Tf2N−), hexafluorophosphate (PF6−), and tetrafluoroborate (BF4−) anions. In addition to the usual analytical methods, the compounds’ physical properties such as density, solubility, viscosity, and surface tension were probed to ascertain their suitability for aerospace applications. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were employed to study the thermal behavior of the materials focusing on phase transitions and degradation behavior. Remarkably, all ionic liquids prepared in this study are liquids at room temperature with relatively low viscosities and are soluble in many common organic solvents. Structure-property relationships were assessed, particularly focusing on the effect that the chain length appendage and/or anion type has on a given physical property. An updated synthetic procedure and characterization of the tri(2-ethylhexyl)phosphine starting material are provided. A new synthesis procedure for the starting material silver(I) tetrafluoroborate is also reported.