A series of two salts of isoniazid and two cocrystals of its Schiff base have been synthesized and well characterized. Further, the hydrogen bonding patterns have been studied and the correlation between the bond distances and corresponding pKa's have been deduced.

In the crystal structure of the title compound, C7H5NO4·C9H9N3O2S2, the sulfa­thia­zole and 4-nitro­benzoic acid mol­ecules are held together by short π–π contacts between the thia­zole and nitro­benzene rings, with a centroid–centroid distance of 3.8226 (7) Å. The sulfa­thia­zole mol­ecules form dimers via N—H⋯N hydrogen bonds involving the thia­zole and sulfonamide moieties, owing to the fact that sulfathizole exhibits amide–imide tautomerism. The N—H (amine) groups of two sulfathiazole molecules are linked to the two S=O groups of a sulfathiazole via N—H⋯O hydrogen bonds. Two mol­ecules of coformer are held together by O—H⋯O hydrogen bonds. These units self-assemble, forming a three-dimensional network stabilized by (acid)C—H⋯π(sulfa­thia­zole benzene ring) inter­actions.

2-Chloro-4-nitrobenzoic acid (2c4n) is an antiviral agent used for the treatment of HIV infection and to boost the immune response in immune deficiency diseases. In the present study, a series of eight molecular salts of 2c4n with pyridyl and benzoic acid derivatives have been synthesized by a crystal engineering approach and were characterized structurally by various spectroscopic, thermal, and X-ray diffraction techniques. Crystal structures of all synthesized molecular salts were determined by single-crystal X-ray diffraction techniques. In all synthesized molecular salts, the charge-assisted acid···pyridine/amine heterosynthon was found to be the primary supramolecular synthon. The synthesized salts, namely, 2c4n.g and 2c4n.h salts were found to be isostructural. Further, in the current work, the occurrence of weak halogen bonds in the presence of strong hydrogen bonds in the synthesized and in the reported molecular salts/cocrystals of 2c4n has been investigated. A detailed inspection of the crystal structures of salts/cocrystals of 2c4n was carried out to demonstrate the importance of halogen bonds in these crystal structures. It was found that 4 out of 8 synthesized molecular salts and 12 out of 24 reported molecular adducts of 2c4n were found to exhibit halogen bonds in their crystal structures. A similar kind of conformational change was observed for molecular salts exhibiting halogen bonds in their crystal structures; however, the conformations were found to be slightly different in other molecular salts. It was observed that two-point primary supramolecular synthon and stronger intramolecular Cl···O halogen bonds in the molecular adducts of 2c4n are found to be more susceptible to exhibit halogen bonds in their crystal structures. Halogen bond interactions played a vital role in the crystal stabilization of these molecular adducts.

Six new salts of an API intermediate 2-aminothiazole with different carboxylic acid coformers were synthesized and characterized by IR (Infrared spectroscopy), 1 H-NMR, DSC (Differential scanning calorimetry), XRPD (X-ray powder diffraction) and single crystal XRD. The crystal structure of the salts with benzoic acid, 2,3-, 2,4-, 2,5-, 2,6-dihydroxybenzoic acids and 2,4-dinitrobenzoic acid were determined. The thiazole moiety exhibited solvent (polarity) assisted tautomerism in all reported salts and proton transfer was noticed to the ring N of thiazole due to which two point supramolecular synthon N + −H(thiazole)· · · O − (acid), N−H(amine)· · · O − (acid) was observed. The crystal structures were studied with respect to the positional effect of the competing functional groups like hydroxyl (−OH) and nitro (−NO 2 ) as well as their donor and acceptor abilities for hydrogen bonding. The presence of the non-conventional hydrogen bond (C−H· · · O) has been found to play a critical role in the formation of secondary supramolecular architectures.

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