Isomorphism and Crystal Engineering: Organic Ionic Ladders Formed by Supramolecular Motifs in Pyrimethamine Salts

Pyrimethamine [2,4-diamino-5-(p-chlorophenyl)-6-ethyl-pyrimidine] (PMN) is an antifolate drug. Four new organic salts, namely, PMN hydrogen maleate (1), PMN hydrogen succinate (2), PMN hydrogen phthalate (3), and PMN fumarate (4), have been synthesized and their hydrogen bonding patterns have been analyzed. As expected, in all the structures, the PMN moieties are protonated at one of the nitrogen atoms of the pyrimidine rings. The carboxylate group of the respective anions interacts with the protonated pyrimidine moiety in near linear fashion through a pair of N−H···O hydrogen bonds to form a cyclic hydrogen-bonded motif. The crystal structures of compounds 1 and 2 are isomorphous as revealed by the very good agreement among their cell parameters. Interestingly, the hydrogen succinate mimics the hydrogen maleate here. In all three crystal structures (1−3), the hydrogen bonding motifs and their supramolecular patterns are the same, which is significant from a crystal engineering point of view in the building up of organic ionic ladders. In compound 4, the cyclic hydrogen-bonded motifs are self-organized through N−H···O, N−H···Cl, and C−H···N hydrogen bonds to the formation of different types of hydrogen bonding patterns.