Multicomponent Supramolecular Assemblies of Melamine and α‑Hydroxycarboxylic Acids: Understanding the Hydrogen Bonding Patterns and Their Physicochemical Consequences

The reaction of 1,3,5-triazine-2,4,6-triamine (melamine, me) with alpha-hydroxycarboxylic acids, namely, glycolic acid (H2ga) and mandelic acid (H2ma), provided three ionic multicomponent crystal structures (MCSs) of composition: [(Hme+)­(Hga–)]­·1/2H2O (I), [(Hme+)­(Hma–)]­·dioxane­·2H2O (II), and [(Hme+)­(Hma–)]­·(me)­·DMSO­·3H2O (III). Those MCSs revealed the typical propensity of “me”, and particularly of its monoprotonated form (Hme+), to form supramolecular aggregates such as linear and crinkled tapes and molecular ribbons. The energetic landscape of those assemblies has been explored with the use of density functional theory calculations of various models of Hme+ and “me” hydrogen-bonded aggregates. The stabilization energies of Hme+-based adducts are for the first time rationalized within the context of the anti-electrostatic hydrogen bonding concept. In addition, we discussed the supramolecular arrangement of I–III in terms of synthon formation and explored structural peculiarities imparted by hydration and solvation. The salt-to-cocrystal continuum is analyzed based on structural analysis and physicochemical properties such as vibrational characteristics, thermal stability, and solubility. Finally, the analysis of dnorm-mapped Hirshfeld surfaces of melamine at different protonation states allowed us to establish strongly characteristic protonation-dependent features of those surfaces, whose peculiar features are reflected in associated fingerprint plots (FPs). By comparison with literature precedents, we have found that our findings on FP plots of “me” and Hme+ species are indeed general