Effects of HF content in the (FH)ₙF⁻anion on the formation of ionic plastic crystal phases of N-ethyl-N-methylpyrrolidinium and N,N-dimethylpyrrolidinium fluorohydrogenate salts

Abstract

Fluorohydrogenate salts based on N-ethyl-N-methylpyrrolidinium (EMPyr(FH)ₙF) and N, N-dimethylpyrrolidinium (DMPyr(FH)ₙF) cations were synthesized, and the effects of the HF content n in EMPyr(FH)ₙF (1.0 ≤ n ≤ 2.3) and DMPyr(FH)ₙF (1.0 ≤ n ≤ 2.0) on their thermal and structural properties were discussed, focusing on the characterization of ionic plastic crystal (IPC) phases. Several solid phases (IPC (I) and IPC (II) phases, and crystal phases of EMPyr(FH)₁F, EMPyr(FH)₂F, and EMPyr(FH)₃F) are observed in the EMPyr(FH)ₙF system. The IPC (I) phase has an NaCl-type structure and is composed of EMPyr⁺ cations and (FH)ₙF⁻ (n = 1, 2, and 3) anions randomly occupying the anion positions in the lattice over a wide range of n values in (FH)ₙF⁻. The melting point of EMPyr(FH)ₙF in the range 1.8 ≤ n ≤ 2.3 is maximal at n = 2.0, whereas it increases with a decrease in n in the range 1.0 ≤ n ≤ 1.2. Furthermore, in the range 1.3 ≤ n ≤ 1.7, the solid phase is regarded as the IPC phase (IPC (II)), and their melting points are nearly constant (260–270 K). In the DMPyr(FH)ₙF system, the IPC (I′) phase and crystal phases of DMPyr(FH)1F and DMPyr(FH)₂F were observed. Although the IPC (I′) phase has an NaCl-type structure, similar to the IPC (I) phase of EMPyr(FH)ₙF, it has higher ordering compared to the IPC (I) phase. The melting point of DMPyr(FH)ₙF increases monotonously with decreasing n but disappears in the small n region where the salt decomposes below the melting point.