Salts of barbituric and 2-thiobarbituric acids with imidazole: polymorphism, supramolecular structure, thermal stability and water solubility

Abstract

Barbituric acids are a useful tool for the construction of various supramolecular compounds with intriguing physicochemical properties. Also, barbiturates play a significant role in biology and medicine. The same is true for imidazole derivatives. Herein, three novel salts of barbituric (H₂BA) and 2-thiobarbituric (H₂TBA) acids with imidazole (Im), namely imidazolium barbiturate in two polymorphic modifications, HIm(HBA) (1) (pale yellow) and (2) (pale orange), and imidazolium 2-thiobarbiturate, HIm(HTBA) (3), were synthesized and characterized by CHNS elemental analysis, single-crystal and powder X-ray diffraction, FT-IR and UV-Vis spectroscopy techniques. Crystal structure analysis revealed that the compounds feature three different supramolecular architectures with the formula unit count (Z′) varying from 1 for 3 to 4 for 1, which is governed by an unusual geometry of hydrogen bonding motifs. Intermolecular interactions in crystals 1–3 were analyzed using 2D-fingerprint plots derived from the Hirshfeld surfaces. According to the TG-DSC analysis, compounds 1–3 were thermally stable up to ∼100 °C in air. The solubility studies (solubility product constants and solubility at a fixed pH value were determined) showed that salts 2 and 3 were more soluble in water than H₂(T)BA. To the best of our knowledge, this is one of the first reports on the phenomenon of polymorphism among organic salts with barbiturate moieties. Thus, the present findings broaden our understanding of the supramolecular organization of barbiturates, complement the systematic studies of the correlation between their crystal structure and physicochemical properties and lay the foundation for the further development of novel materials based on imidazolium (2-thio)barbiturates.