Co-crystals of polyhalogenated diaminobenzonitriles with 18-crown-6: effect of fluorine on the stoichiometry and supramolecular structure

A series of polyhalogenated diaminobenzonitriles and 18-crown-6 ether were used to reveal the dependence of the stoichiometry and supramolecular structure of co-crystals on various factors. 2,6-Diamino-3,5-difluoro-, 2,6-diamino-4-chloro-3,5-difluoro-, and 2,6-diamino-4-chloro-3-fluorobenzonitriles, as well as 2,4-diamino-3,5-difluorobenzonitrile give 1 : 1 co-crystals regardless of the diamine : crown ratio in the crystallization solution (from 2.5 : 1 to 1 : 2.5) and the nature of the solvent. According to X-ray diffraction data, the supramolecular structures of these co-crystals can be considered as 1D assemblies with the only structure-forming N-H⋯O_crhydrogen bond. 2,4-Diamino-3,5,6-triflurobenzonitrile forms no 1 : 1 co-crystal, but yields co-crystals of 4 : 3 (under most of the studied conditions) and 2 : 1 (in CCl₄solution) stoichiometry. The N-H⋯NC hydrogen bond and p⋯π electron interactions, along with the N-H⋯O_crH-bond, participate in the formation of the 3D supramolecular structures of these co-crystals. The effect of the number of F atoms on the co-crystallization behaviour of difluoro- and trifluoro-2,4-diaminobenzonitriles was rationalized using quantum-chemical computations of the interaction energies of N-H⋯NC bonded pairs of these diamines in the experimental crystals and DFT simulated models. The DSC curves of each co-crystal contain a single peak corresponding to a crystal-to-liquid phase transition (melting), which does not change in melting-crystallization cycles, indicating that the stoichiometry and crystal structure are reproducible.