On polymorphism of 2-(4-fluorophenylamino)-5-(2,4-dihydroxybenzeno)-1,3,4-thiadiazole (FABT)


grafika4        Structural and computational studies of two polymorphic (triclinic P[1 with combining macron] and monoclinic P21/n) DMSO solvates of the biologically active molecule 2-(4-fluorophenylamino)-5-(2,4-dihydroxybenzeno)-1,3,4-thiadiazole (FABT) show that their structures are stabilized mainly by hydrogen bonds between the FABT and DMSO molecules. The geometry of both polymorphic molecules is very similar with a few significant differences in the corresponding valence angles. The only exceptions are the valence angles associated with the terminal para-hydroxyl group in both polymorphs. This group is differently H-bonded to the neighbouring solvent molecules. Additionally, the molecule in the second polymorph is slightly bent compared to the molecule in the first one. Both polymorphs also have very similar packing with layers of FABT molecules separated by DMSO moieties. The Hirshfeld surface analysis shows the most significant differences in the relative contributions of intermolecular interactions to the total Hirshfeld surface area for the FABT polymorphs. They are found for the C⋯H, C⋯C and H⋯H interactions. The triclinic polymorph crystallises as the first one and is thermodynamically less stable, while the monoclinic one is thermodynamically more stable but occurs in the crystallization mixture after a much longer time. According to the computational results, the monoclinic polymorph is ca. −9.93 kJ mol−1 more stable than the triclinic one, and dispersive interactions are dominant in these polymorphic crystals. It appears that the FABT⋯FABT interactions in the crystal lattice are stronger (ca. −75 kJ mol−1) for both polymorphs than the interactions of the central FABT moiety with the neighbouring DMSO molecules (ca. −52 kJ mol−1). Interlayer interaction energies calculated for the most characteristic slabs defined in the crystal lattices of the both polymorphs can be related to the stability of crystals.