Cocrystals of 6-chlorouracil and 6-chloro-3-methyluracil: exploring their hydrogen-bond-based synthon motifs with several triazine and pyrimidine derivatives.

Abstract

In order to obtain complexes held together by hydrogen as well as halogen bonds, 6-chlorouracil [6-chloropyrimidin-2,4(1H,3H)-dione; 6CU] and its 3-methyl derivative [6-chloro-3-methylpyrimidin-2,4(1H,3H)-dione; M6CU] were cocrystallized with 2,4,6-triaminopyrimidine and the three triazine derivatives 2,4,6-triamino-1,3,5-triazine (melamine), 2,4-diamino-6-methyl-1,3,5-triazine and 2-chloro-4,6-diamino-1,3,5-triazine, which all offer complementary hydrogen-bonding sites. Three of these compounds form cocrystals with 6CU; however, melamine yielded only a new pseudopolymorph with 6CU, but formed a cocrystal with M6CU. All six cocrystals contain solvent molecules (N,N-dimethylformamide, N,N-dimethylacetamide or N-methylpyrrolidin-2-one), whose intermolecular interactions contribute significantly to the stabilization of the crystal packing. Each of these structures comprises chains, which are primarily formed by strong hydrogen bonds with a basic framework built by R2(2)(8) hydrogen bonds of either pure N-H...N or mixed patterns. Solvent molecules are aligned to the border of these chains via N-H...O hydrogen bonds. Two of the reported crystal structures containing 6CU show additional Cl...O halogen bonds, which connect the chains to two-dimensional layers, while one weak and one strong Cl...Cl interaction are observed in the two structures in which molecules of M6CU are present.