Temperature‐induced pseudopolymorphism of molecular salts from a pyridyl bis‐urea macrocycle and naphthalene‐1,5‐disulfonic acid

Molecular salts, often observed as cocrystals, play an important role in the fields of pharmaceutics and materials science, where salt formation is used to tune the properties of active pharmaceutical ingredients (APIs) and improve the stability of solid‐state materials. Salt formation via a proton‐transfer reaction typically alters hydrogen‐bonding motifs and influences supramolecular assembly patterns. We report here the molecular salts formed by the pyridyl bis‐urea macrocycle 3,5,13,15,21,22‐hexaazatricyclo15.3.1.1^7,11]docosa‐1(21),7(22),8,10,17,19‐hexaene‐4,14‐dione, ( 1 ), and naphthalene‐1,5‐disulfonic acid (H₂NDS) as two salt cocrystal solvates, namely 4,14‐dioxo‐3,5,13,15,21,22‐hexaazatricyclo15.3.1.1^7,11]docosa‐1(21),7(22),8,10,17,19‐hexaene‐21,22‐diium naphthalene‐1,5‐disulfonate dimethyl sulfoxide disolvate, C₁₆H₂₀N₆O₂²⁺·C₁₀H₆O₆S₂^2?·2C₂H₆OS, ( 2 ), and the corresponding monosolvate, C₁₆H₂₀N₆O₂²⁺·C₁₀H₆O₆S₂^2?·C₂H₆OS, ( 3 ). This follows the ΔpK_a rule such that there is a proton transfer from H₂NDS to ( 1 ), forming the reported molecular salts through hydrogen bonding. Prior to salt formation, ( 1 ) is relatively planar and assembles into columnar structures. The salt cocrystal solvates were obtained upon slow cooling of dimethyl sulfoxide–acetonitrile solutions of the molecular components from two temperatures (363 and 393 K). The proton transfer to ( 1 ) significantly alters the conformation of the macrocycle, changing the formerly planar macrocycle into a step‐shaped conformation with trans–cis urea groups in ( 2 ) or into a bowl‐shape conformation with trans–trans urea groups in ( 3 ).