The structures and dielectric properties of high temperature phase transition compounds with 4-(dimethylamino)pyridinium

•High temperature phase transitions•DSC and dielectric analyses of 1 and 2 confirm the phase transitions at 360.0 K and 381.0 K.•The order–disorder FeCl4− and the orientational motions of organic cations are the driving forces.

The reactions of 4-(dimethylamino)pyridine (DMAP) and metal chloride in dilute hydrochloric acid yield two four-coordinate complexes, 1 (DMAP+)(FeCl4−) and 2 (DMAP+)2(CoCl42 −). The differential scanning calorimetry (DSC) measurements, temperature-dependent dielectric properties and variable-temperature single-crystal X-ray diffraction analyses of compound 1 confirmed the reversible isostructural phase transition with the same space group P21/n (No. 14) at 298 K and 373 K. The temperature-dependent single crystal structural analyses of compound 1 reveal the distinct and synchronous molecular motions of both the cationic and anionic moieties, where the orientational changes of organic cations and the order–disorder transformations of FeCl4− anions are mainly responsible for dielectric properties of compound 1. The variable-temperature structure analyses and the variable-temperature PXRD measurements reveal that compound 2 undergoes a reversible structural phase transition at around 381 K. The space groups of compound 2 change from triclinic P-1 in room temperature phase (RTP) to monoclinic C2/c in high temperature phase (HTP), accompanied by thermal and dielectric anomalies.

Graphical abstractThe two four-coordinate complexes, 1 (DMAP+)(FeCl4−) and 2 (DMAP+)2(CoCl42 −), and their temperature-dependent crystal structures and dielectric properties have been synthesized and investigated. The orientational motions of organic cations and order–disorder transformations of FeCl4− anions result in the significant changes of the hydrogen bonds, however, with no change in the space group. In addition, the piezoelectric and SHG measurements were performed on compound 1 to further confirm the same centrosymmetric space group in both room and high temperature phases. Compound 2 undergoes a reversible structural phase transition at around 381.0 K. Their structural changes are reflected in the reversible dielectric transitions.DSC and dielectric analyses of 1 and 2 confirm the phase transitions at 360.0 K and 381.0 K, respectively.Figure optionsDownload full-size imageDownload as PowerPoint slide