Organic Ionic Crystals: Solid‐Phase Structure and Thermal Properties of Dicationic α, ω‐Bis[N, N′‐(4‐alkylpyridinium)]alkane Hexafluorophosphate Salts. Issue 9 (20th August 2019)

• Record Type: Journal Article
• Authors: Chae, Hyunho; Lee, A. Reum; Yoon, Minyoung; Choi, U Hyeok; Park, Gyungse; Lee, Minjae
• Is Part Of: Asian journal of organic chemistry. Volume 8:Issue 9(2019)
• Page Start: 1718
• Page End: 1725
• Publication Date: 2019-08-20
• Subjects: dicationic pyridinium ions -- thermal stability -- phase transitions -- liquid crystals -- X-ray structure; Chemistry, Organic -- Periodicals; Organic compounds -- Synthesis -- Periodicals; 547.005
• DOI: 10.1002/ajoc.201900341
• Languages: English
• ISSNs: 2193-5807

Abstract

Abstract: As organic ionic crystals, C2 ‐, C3 ‐, and C4 ‐alkylene α, ω‐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 2PF6 − (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 C2 ‐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 ( ΔSf =25.0 J mol −1 K −1 ). The C2 ‐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 C3 ‐ and C4 ‐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 C2 ‐bridged compounds and show only one melting phase transition. Abstract : Plastic flags : A series of bis( N, N′ ‐(4‐alkylpyridinium)]alkane 2PF6 − salts are synthesized. A solid‐phase structure was characterized by X‐ray crystallography. DSC, TGA and POM showed interesting phase behaviors of the new ionic salts: multiple solid‐solid phase transitions with high entropy changes. These (bis)pyridinium salts represent a family of soft crystalline materials, such as organic ionic plastic crystals.