Supramolecular heterosynthon assemblies of ortho‐phenylenediamine with substituted aromatic carboxylic acids

Co-crystallization experiments conducted between ortho-phenyl­enedi­amine (OPDA) and five substituted aromatic acids (phthalic acid, salicylic acid, 4-hy­droxy­benzoic acid, 4-nitro­benzoic acid and 3,5-di­nitro­benzoic acid) reveal the formation of supramolecular networks constructed from acid–base heterosynthons of ortho-phenyl­enedi­ammonium cations with respective aromatic anions. All of these coformers are generally regarded as safe (GRAS) molecules. The five reported crystal structures are sustained predominantly by intermolecular N+−H⋯O−, N—H⋯O− and N—H⋯O hydrogen-bonding interactions; in addition intramolecular O—H⋯O and intermolecular O—H⋯O, O—H⋯O− and C—H⋯O interactions contribute to the formation of various networks. Five 1:1 salts [NH2C6H4NH3]+·[COOHC6H4COO]− (1); [NH2C6H4NH3]+·[OHC6H4COO]− (2); [{NH2C6H4NH2}2·{OHC6H4COOH}2·{NH2C6H4NH3}+2·{OHC6H4COO}−2] (OPDPHB) (3); [NH2C6H4NH3]+·[NO2C6H4COO]− (4) and [NH2C6H4NH3]+·[(NO2)2C6H4COO]− (5) were isolated as single crystals by the slow evaporation method and were characterized using spectroscopic and X-ray crystallographic techniques. X-ray diffraction studies confirmed the formation of salts. The pKa difference between the amine and respective acid favours the transfer of a proton from the acid to the amine, which leads to the formation of the anion and the cation. The interactions between these ions resulted in a stable heterosynthon in each case. The asymmetric units of salts (1), (2), (4) and (5) contain one anion and one cation each, but salt (3) consists of two anions, two cations and two neutral species in its asymmetric unit. A polymorph of salt (3) was also isolated from the crystallization of the ground material from liquid-assisted grinding [{NH2C6H4NH2}·{NH2C6H4NH3}+·{OHC6H4COO}−] (OPDPHB 3P). The polymorph crystallized in the monoclinic non-centrosymmetric space group P21. The liquid-assisted grinding experiments using a 1:1 ratio also revealed the formation of the expected salts, except salt (3), where this product matches with polymorph (OPDPHB 3P).