Cocrystals of 6-propyl-2-thiouracil: N-H•••O versus N-H•••S hydrogen bonds

In order to investigate the relative stability of N-H center dot center dot center dot O and N-H center dot center dot center dot S hydrogen bonds, we cocrystallized the antithyroid drug 6-propyl-2-thiouracil with two complementary heterocycles. In the cocrystal pyrimidin-2-amine-6-propyl-2-thiouracil (1/2), C4H5N3 center dot 2C(7)H(10)N(2)OS, (I), the 'base pair' is connected by one N-H center dot center dot center dot S and one N-H center dot center dot center dot N hydrogen bond. Homodimers of 6-propyl-2-thiouracil linked by two N-H center dot center dot center dot S hydrogen bonds are observed in the cocrystal N-(6-acetamidopyridin-2-yl)acetamide-6-propyl-2-thiouracil (1/2), C9H11N3O2 center dot 2C(7)H(10)N(2)OS, (II). The crystal structure of 6-propyl-2-thiouracil itself, C7H10N2OS, (III), is stabilized by pairwise N-H center dot center dot center dot O and N-H center dot center dot center dot S hydrogen bonds. In all three structures, N-H center dot center dot center dot S hydrogen bonds occur only within R-2(2)(8) patterns, whereas N-H center dot center dot center dot O hydrogen bonds tend to connect the homo-and heterodimers into extended networks. In agreement with related structures, the hydrogen-bonding capability of C=O and C=S groups seems to be comparable.