Hydrogen-bonded supramolecular ribbons in the antifolate drug pyrimethamine

In the crystal structure of pyrimethamine [2,4-diamino-5-(p-chlorophenyl)-6-ethylpyrimidine], C12H13ClN4, the asymmetric unit contains two crystallographically independent pyrimethamine molecules (A and B), differing in their conformations. The dihedral angle between the pyrimidine and the substituted phenyl plane is 74.4 (1)degrees in molecule A and 82.4 (1)degrees in molecule B. Molecule A is linked with molecule B through a pair of N-H...N hydrogen bonds on one side and through another pair of N-H...N hydrogen bonds on the other side. Thus, there is a supramolecular ribbon, consisting of A and B molecules arranged in an alternating manner. Two such ribbons are connected centrosymmetrically through a pair of N-H...Cl bonds. These ABAB pairs and BABA interconnections lead to a supramolecular sheet-like structure.