Organic Ionic Crystals: Solid-Phase Structure and Thermal Properties of Dicationic alpha,omega-Bis[N,N '-(4-alkylpyridinium)]alkane Hexafluorophosphate Salts

Abstract

As organic ionic crystals, C-2-, C-3-, and C-4-alkylene alpha,omega-bis[N,N '-(4-alkylpyridinium)] PF6- salts are synthesized to investigate their thermal properties and phase behaviors along with chemical structure changes. The X-ray crystal structure of 1,2-bis[N,N '-(4-methylpyridinium)]ethane 2PF(6)(-) (2-1) showed that two pyridinium rings in one cation molecule and two pyridinium ring planes of neighboring molecules were exactly parallel. Multiple C-H...F hydrogen bonds were observed for all protons of the bis-pyridinium with PF6- anions. Interestingly, most of the prepared C-2-bridged dicationic bis-pyridinium PF6- salts showed solid-solid phase transitions, which are typical features of soft crystalline materials, such as plastic and liquid crystals. Colorfully aligned crystalline textures of the polarized optical microscope images are formed with strong birefringence for the soft crystalline phase of 1,2-bis[N,N '-(4-n-propylpyridinium)]ethane PF6-, showing a low entropy of fusion (Delta S-f=25.0 J mol(-1) K-1). The C-2-bridged n-undecyl (2-11) and n-dodecyl (2-12) salts exhibit a very soft solid phase, showing large heat absorptions and entropy changes during their multiple solid-solid phase transitions. Additionally, the C-3- and C-4-bridged dicationic (bis)pyridinium PF6- salts are prepared to determine the property changes with different bridge lengths. The longer alkylene connected salts are thermally more stable than the C-2-bridged compounds and show only one melting phase transition.