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

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 P2(1)/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 I. 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. (C) 2015 Elsevier B.V. All rights reserved.