Iso-structural phase transitions in urea 4-carboxyanilinium nitrate crystal having disordered-nitrate ions trapped in a caged structure

• Published in: Journal of molecular structure 2024-12, Vol.1317, p.139026, Article 139026
• Main Authors: Panicker, Lata, Mestry, Mayuresh, Agarwal, Neeraj, Sakuntala, T.
• Format: Article
• Language: English
• Subjects: Disorder nitrate anion; DSC; Hydrogen bond; interaction topology; Single crystal; Urea 4-carboxyanilinium nitrate; X-ray diffraction
• ISSN: 0022-2860
• DOI: 10.1016/j.molstruc.2024.139026

•Synthesis of a new compound urea 4-carboxyanilinium nitrate (U4-CAN) and determination of its crystal structure.•Typical hydrogen bond pattern in the crystal structure of U4-CAN shows presence of two caged structure with disordered-nitrate ions trapped in it.•U4-CAN undergo isostructural transition at 359.1 K with no change in cell parameters.•U4-CAN also undergo isostructural transition when cooled to 100 K with change in cell parameters.•Of the two trapped disordered nitrate ions, one remains disorder even when cooled to 100 K. Urea 4-carboxyanilinium nitrate (U4-CAN) a new organo-nitrate adduct was synthesized and investigated using, differential scanning calorimetry (DSC), variable temperature single crystal X-ray diffraction and computational analysis. DSC heating scans of U4-CAN shows presence of a reversible first-order structural phase transition at 359.1 K. The single crystal X-ray diffraction of U4-CAN was collected at temperatures both above and below the structural phase transition (293 K and 368 K). In both phases the crystal structure was found to be monoclinic, space group C2/c, with one molecule each of 4-carboxyanilinium cation, urea and two nitrate anions in an asymmetric unit. The U4-CAN crystal structure in both phases has organic and inorganic layered structure which are connected and stabilised by the three dimensional network of strong charge-assisted NH…O, and COH…O hydrogen bonds. The typical hydrogen bond pattern in U4-CAN lead to the formation of a characteristic caged structure in which highly disorder nitrate ions is trapped. There are two types of caged structures in the crystal structure, which differ from each other in the hydrogen bond interaction pattern. Both caged structures have disorder nitrate ions trapped in it. Each of the trapped nitrate ions also differ in the hydrogen bond interaction patterns. The structural phase transition involve breaking and making of the few hydrogen bonds without affecting the caged structure. This constrained transition lead to an isostructural transition with no change in space group. Cooling U4-CAN crystal to 100 K lead to one more isostructural phase transition, having space group C2/c with different cell parameters. Interestingly one of the caged disorder nitrate ions remains disorder even when cooled to 100 K, while the other disorder nitrate ions become normal i.e. ordered when cooled to 100 K. Thus the unusual characteristic hydrogen bond patterns in U4-CAN plays an important role