Unusual Chemical Ratio, Z″ Values, and Polymorphism in Three New N-Methyl Aminopyridine–4-Nitrophenol Adducts

Cocrystallization of 4-nitrophenol (I) with N-methyl substituted aminopyridines, 4-N-methylaminopyridine 1, 2-N-methylaminopyridine 2, and 2-N,N-dimethylaminopyridine 3, resulted in three novel adducts 1·2­(I), 2·3­(I), and 3·3­(I), one of which, 2·3­(I), was found in three polymorphic forms, A, B, and C. The single crystals were grown by slow evaporation from ethanol. The proton transfer from the phenoxy to the pyridine moieties was registered in all compounds. The adducts comprise pyridinium cations, 4-nitrophenolate anions, and varying in number neutral 4-nitrophenol molecules. Though the asymmetric hydrogen-bonded network involving the −N⁺H groups of pyridinium cations and the −C–O^– and −C–OH groups of 4-nitrophenol moieties is registered in the adducts, the delicate balance of noncovalent interactions that include CH···O hydrogen bonds and face-to-face stacking interactions between the extended antiparallel arrays of components controls the centrosymmetric packing. Although three polymorphs of 2·3­(I) share several structural common features, they reveal significant differences in the conformation of the pyridinium cation, and the hydrogen-bonding patterns.