Synthesis and characterization of novel hexafluorophosphate salts with tropine-type cations

Abstract

In this study, a series of new ionic liquids (ILs) with tropine as cationic nucleus and hexafluorophosphate as anion were synthesized. As two types of functional groups, aliphatic chains and benzyl with different substituents have been linked on the tropine nucleus in order to modulate the physicochemical properties of the ILs. The whole family of ILs was comprehensively identified by FT-IR, NMR and MS. Furthermore, the thermal properties of these ILs were evaluated by different scanning calorimetry (DSC) together with their melting points and enthalpy of fusion. Then the polarities of these tropine-type ILs were investigated and discussed. Resultantly, it was proved that their polarity was comparable with that of short-chain alcohols. Moreover, the Hammett function values of these involved ILs were determined to describe their Brønsted acidities. Finally, molar conductivities (Λ) of the tropine-type ILs in dilute solutions at 298.15K were determined. The data were correlated with the Arrhenius–Ostwald model to obtain limiting molar conductivities (Λ∞) and association constants (KA). The influences of the branch chain of ILs on the ion transfer and association were also discussed. It was found that the size of cation was the main factor influencing the ion transfer. And the weak ion association of tropine-type ILs in water was proved by KA, which was less than 7L·mol−1.