Supramolecular architectures in crystals of melamine and aromatic carboxylic acids

Two crystals containing melamine (M), melaminium (MH+), and aromatic carboxylic acids are synthesized and characterized by single-crystal X-ray diffraction analysis: melamine–melaminium 3,4,5-trihydroxybenzoate (THB−) dihydrate (1), and melamine–melaminium 2-acetylbenzoate (AB−) dihydrate (2). In the crystal structures protonated (MH+) and non-protonated melamine (M) molecules are interconnected via NH···N hydrogen bonds into a planar dimer in (1) and one-dimensional polymer in (2). The NH···O hydrogen bonding interaction between the protonated nitrogen atom of MH+ and the carboxylate oxygen atom of the corresponding anion is observed in both structures. However, only in (1) it results in an almost planar ionic unit (THB−MH+) where additional π–π interactions between the anions appear, that are missing in (2). The water molecules act as donor and as acceptor of hydrogen bonds linking the substructures observed in the (1) and (2) crystals into 3D supramolecular networks.