Molecular salts of the isoniazid derivatives. Expanding the scope of sulfonate-pyridinium synthon to design materials

Highlights: • Pharmaceutical co-crystal. • Supramolecular synthon. • Hydrogen bond. • Crystal engineering. Understanding the intermolecular interactions is central to crystal engineering and is pivotal in designing of the new materials. Of recent we have reported sulfonate-pyridinium interaction as robust synthon and herein we utilize the interaction to develop molecular complexes: [(ISN–I3C–H)⁺(5-SSA-H)] (1), [(ISN-pV-H)⁺(5-SSA-H).MeOH·H₂O] (2), and [(ISN-oV-H)⁺(5-SSA-H).H₂O] (3). 1–3 have been obtained by reaction of isoniazid derivatives: ((1H-indol-3-yl)methylene)isonicotinohydrazide (ISN–I3C), isonicotinic acid (4-hydroxy-3-methoxy-benzylidene)-hydrazide (ISN-pV) and isonicotinic acid (2-hydroxy-3-methoxy-benzylidene)-hydrazide (ISN-oV) with 5-sulfosalicylic acid (5-SSA-2H). Molecular salts 1 and 2 manifest direct charge assisted sulfonate-pyridinium interaction, while, in 3, the synthon is masked by lattice water. The Schiff base cations in 1 and 2 stack together through strong π-interactions forming 2-dimensional layers and 1-dimensional tapes, respectively, which, in turn, are bridged by the organo-sulfonate anions into a 3-dimensional ionic solids. While the crystal formers in 3, the hydrogen bonds are utilized to form three molecule thick sheets, which aggregate through C–H⋯O and π-π interactions to form a 3-dimensional supramolecule. Hirshfeld studies corroborate with structural studies and validate that reciprocal O–H/H–O interactions are the chief intermolecular interactions in these oxygen rich compounds while weak dispersive H–H contacts are the second dominant interactions responsible for the molecular packing. Framework energy, as well as individual contribution of columbic, potential and dispersive forces towards molecular packing, are visualized through framework energy analyses of 1–3, which also substantiate that very robust sulfonate-pyridinium contact is not the strongest intermolecular contact. The resultant multi-component crystals exhibit enhanced optical properties, which are attributed to proton transfer and/or π-stacking interactions. The band gap values of 1–3 observed in the Tauc plots corroborate with charge transfer interactions. The products also exhibit enhanced solubility in methanol, higher than the isoniazid derivatives, and the thermal stability up to 240°C. The molecular salts have also been screened for preliminary biological studies.