Supramolecular architectures in crystals of melamine and aromatic carboxylic acids

Abstract

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 N H···N hydrogen bonds into a planar dimer in ( 1) and one-dimensional polymer in ( 2). The N H···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.