Physicochemical properties and plastic crystal structures of phosphonium fluorohydrogenate salts

Abstract

Fluorohydrogenate salts of quaternary phosphonium cations with alkyl and methoxy groups (tetraethylphosphonium (P₂₂₂₂⁺), triethyl-n-pentylphosphonium (P₂₂₂₅⁺), triethyl-n-octylphosphonium (P₂₂₂₈⁺), and triethylmethoxymethylphosphonium (P_222(101)⁺)) have been synthesized by the metatheses of anhydrous hydrogen fluoride and the corresponding phosphonium bromide or chloride precursors. The three salts with asymmetric cations, P_222m(FH)_2.1F (m = 5, 8, and 101), are room temperature ionic liquids (ILs) and are characterized by differential scanning calorimetry, density, viscosity, and conductivity measurements. Linear sweep voltammetry using a glassy carbon working electrode shows these phosphonium fluorohydrogenate ILs have wide electrochemical windows (>4.9 V) with the lowest viscosity and highest conductivity in the known phosphonium-based ILs. Thermogravimetry shows their thermal stabilities are also improved compared to previously reported alkylammonium cation-based fluorohydrogenate salts. Differential scanning calorimetry and X-ray diffraction revealed that tetraethylphosphonium fluorohydrogenate salt, P₂₂₂₂(FH)₂F, exhibits two plastic crystal phases. The high temperature phase has a hexagonal lattice, which is the first example of a plastic crystal phase with an inverse nickel arsenide-type structure, and the low-temperature phase has an orthorhombic lattice. The high-temperature plastic crystal phase exhibits a conductivity of 5 mS cm⁻¹ at 50 °C, which is the highest value for the neat plastic crystals.