Supramolecular Architectures and Helical Water Chains in Cocrystals of Melamine and Aromatic Carboxylic Acids

Melamine (MA) base was crystallized with different aromatic carboxylic acids trimesicacid (TMA),
terephthalic acid (TPA), and 3,5-dinitrobenzoic acid (DNBA), affording cocrystals of (MAH+)·(DNBA-) (1),
(MAH+)·(TPA2-)0.5·H2O (2), and (MAH+)·(H2TMA-)·3H2O (3). Meanwhile, crystallization of MA and trimellitic acid
(H3BTA) in a 1:1 ratio in hot methanol solution led to esterification of one carboxylic group in H3BTA, resulting in
cocrystals of (MAH+)·(HBTA-M-)·H2O (4). Extensive N−H···O/N−H···N hydrogen bonds are found in 1−4, featuring
different hydrogen-bonding motifs. Compounds 1 and 2 have two-dimensional (2D) layers stabilized by strong
N−H···O/N−H···N hydrogen bonds and weak C−H···O hydrogen bonds. Compound 3 exhibits a thermally stable
three-dimensional (3D) architecture formed by the stacking of 2D layers, featuring one-dimensional (1D) pores
enclosing unusual helical water chains. In compound 4, three kinds of 1D chains are intersected to form a 3D structure
consolidated by extensive hydrogen bonds including unusual triple hydrogen bonds between an HMA+ ion and a
carboxylate group.