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(+))-(TpA(2-))(0.5)center dot H2O (2), and (MAH(+))-(H(2)TMA(-))center dot 3H(2)O (3). Meanwhile, crystallization of MA and trimellitic acid (H(3)BTA) in a 1:1 ratio in hot methanol solution led to esterification of one carboxylic group in H3BTA, resulting in cocrystals of (MAH(+))center dot(HBTA-M-)-H2O (4). Extensive N-H center dot center dot center dot O/N-H center dot center dot center dot 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 center dot center dot center dot O/N-H center dot center dot center dot N hydrogen bonds and weak C-H center dot center dot center dot 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.