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-hexaazatricyclo[15.3.1.17,11 ]docosa-1(21),7(22),8,10,17,19-hexaene-4,14-dione, (1), and naphthalene-1,5-disulfonic acid (H2 NDS) as two salt cocrystal solvates, namely 4,14-dioxo-3,5,13,15,21,22-hexaazatricyclo[15.3.1.17,11 ]docosa-1(21),7(22),8,10,17,19-hexaene-21,22-diium naphthalene-1,5-disulfonate dimethyl sulfoxide disolvate, C16 H20 N6 O2 2+ ·C10 H6 O6 S2 2- ·2C2 H6 OS, (2), and the corresponding monosolvate, C16 H20 N6 O2 2+ ·C10 H6 O6 S2 2- ·C2 H6 OS, (3). This follows the ΔpKa rule such that there is a proton transfer from H2 NDS 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).