Exploring the Crystal Structure Landscape of Sulfasalazine through Various Multicomponent Crystals

Sulfasalazine is used as an anti-inflammatory drug to treat large intestine diseases and atrophic arthritis. In the solid state, two tautomers are known: an amide tautomer (triclinic polymorph) and an imide tautomer (monoclinic polymorph). Crystallization of six new multicomponent solids of sulfasalazine with three cocrystal formers and three salt formers has been achieved by slurry, liquid-assisted grinding and slow evaporation methods. All of the solid forms are characterized by X-ray diffraction techniques, thermal analysis, and Fourier transform infrared spectroscopy. The crystal structural analysis reveals that two sulfasalazine molecules or anions arrange in a head-to-head fashion involving their pyridyl, amide, and sulfonyl groups in an R₂²(7):R₂²(8):R₂²(7) motif. This is the key structural unit appearing in both sulfasalazine imide polymorph and all six multicomponent crystals. In addition, sulfasalazine exists in the amide form in all unsolvated multicomponent crystals obtained in this work and adopts the imide tautomer in the solvated cocrystals and salt. Hirshfeld surface analysis and the associated two-dimensional (2D) fingerprint plots demonstrate that sulfasalazine has significant hydrogen bond donor capability when cocrystallized and is a significant hydrogen bond acceptor in the salts. The frontier molecular orbital analysis indicates that sulfasalazine cocrystals are chemically more stable than the salts.