Crystal structures and supramolecular and Hirshfeld surface analysis of three novel 2,4,6-triaminopyrimidine salts

Three new hydrous molecular salts, namely, 2,4,6-triaminopyrimidin-1-ium anthranilate (2-aminobenzoate) monohydrate, C₄H₈N₅⁺·C₇H₆NO₂^-·H₂O (I), 2,4,6-triaminopyrimidin-1-ium 5-aminoisophthalate monohydrate, C₄H₈N₅⁺·C₈H₆NO₄^-·H₂O (II), and bis(2,4,6-triaminopyrimidin-1-ium) 4-aminophthalate trihydrate, 2C₄H₈N₅⁺·C₈H₅NO₄^2-·3H₂O (III), have been synthesized and characterized by single-crystal X-ray diffraction techniques. In all three crystal structures, protonation of the pyrimidine moiety occurs at the N1 position and is reflected in a widening of the C-N-C bond angle. In all three cases, protonation of the triaminopyrimidine (TAP) molecule leads to the formation of the robust charge-assisted R₂²(8) TAP-carboxylate heterosynthon, which acts as the primary supramolecular building unit. In salt I, TAP base pairing and water-mediated contacts produce tetrameric aggregates and a DDDA (D is donor and A is acceptor) supramolecular array that propagates into a 3D hydrogen-bonded sheet structure. Salt II exhibits a combination of R₂²(8), R₃²(8) and R₂⁴(8) motifs that fuse to form a continuous hydrogen-bonded chain characterized by a DDDAAA supramolecular array. These chains further assemble into extended sheets which are interconnected by higher-order ring motifs and are reinforced by π-π stacking interactions between the pyrimidine ring and the aromatic anion. The structure of salt III displays the most complex architecture. Multiple hydrogen-bonded arrays, including DADA-type quadruple motifs and large fused-ring motifs, arise through co-operative N-H...O, N-H...N and O-H...O interactions involving the cations, anion and water molecules, resulting in a densely interconnected 3D framework. Hirshfeld surface analysis reveals that H...H contacts dominate the crystal packing, while O...H/H...O and N...H/H...N interactions play a crucial role in directing the supramolecular assembly. The study highlights how anion topology and hydration influence hydrogen-bond hierarchy and supramolecular dimensionality in TAP-based salts.