Validation of the supramolecular synthon preference through DFT and physicochemical property investigations of pyridyl salts of organo-sulfonates

Abstract

Synthon preference is perceived in the pyridyl salts of multi-functional organo-sulfonates and validated through density functional theory (DFT) studies. The salt 1 of 5-sulfosalicylic acid (5-SSA-3H) as well as previously reported pyridyl salts of 5-SSA-3H indicates preferential formation of sulfonate-pyridinium interaction I over common and robust carboxylic-pyridine/carboxylate-pyridinium II as well as hydroxyl-pyridine/hydroxylate-pyridinium III interactions. Pyridyl salts of further functional organo-sulfonates 8-hydroxyquinoline sulfonic acid (8-HQSA-2H) 2 and Schiff bases of sulfanilic acid and syringaldehyde (4-ABSA-SA-2H) 3 show preference for bifurcated hydroxyl assisted pyridinium-pyridine IV and methoxy assisted pyridinium-hydroxylate V synthons. The observed synthon preference is also supported by previously reported organic salts. The overall synthon preference IV ∼ V > I ≫ II ≫ III is further validated through DFT studies, whereas QTAIM studies substantiate the electrostatic nature of interactions I–V with some covalent character. Hirshfeld studies of the organic salts indicate that reciprocal O–H⋯H–O and H⋯H interactions are the major interactions responsible for packing, whereas 3 exhibits higher C⋯C interactions than 1 and 2, which is substantiated by significant π-stacking in 3 and its absence in 1 and 2. Furthermore, the organic salts exhibit augmented thermal and chemical stability, plausibly due to the ionic nature of the crystal lattice. The optical behaviour of the salts corroborates with crystallographic packing and red shift in the absorption of 3 is attributed to π–π interactions between crystal formers. Organic salts 2 and 3 exhibit augmented aqueous solubility compared to pristine precursors, and enhancement in the solubility of 3 is striking with a value of 1175% compared to the organo-sulfonate precursor.