The impact of the anion size on the crystal packing in 2-mercaptopyrimidine halides; isostructurality and polymorphism

Abstract

The crystal structures of 2-mercaptopyrimidine (pyrimidine-2-thione) and its halides (Cl, Br and I) have been determined and the interactions important to the crystal packing are studied. The bromide complex crystallizes in two polymorphic forms, one of which is isostructural with the iodide compound, while the other shows some similarity with the chloride compound. This may suggest that the crystal packing is influenced by the anion size. Although we found that the size of the anion does not affect the type of interactions of the halogen atom, it significantly changes their geometry. The increase of the anion size causes jostling of the surrounding molecules and therefore the larger the size of the anion, the less regular the geometry of the halogen atom contacts. Moreover, the weakest interactions (pseudo anion–π and CH⋯halogen) are the first to be pushed out by larger halide anions. Pseudo anion–π interactions between the aromatic charged ring and the different anions were studied using the NICS method and a positive correlation between the distance and a possible interaction was confirmed.