Influence of bulky and halogen substituents on crystal packing of pyrazolo[1,5-a]pyrimidines

The crystalline structure of five 3-(R3) and/or 5-(R5)-substituted 7-trichloromethyl-2-methylpyrazolo[1,5-a]pyrimidines [where R3, R5 = H, Pr (1); Br, H (2); Br, Me (3); H, H (4); and H, Me (5)] were studied from X-ray diffractometry data. It was found that compounds presented themselves as main intermolecular interactions: Cl⋯Cl (1), Br⋯Cl (2,3); and Cl⋯N-pyrimidine (4,5). In compounds 1–3, other interactions reinforce the stabilization of the crystal structure, such as C–H⋯N-pyrimidine (1,2) Cl⋯π-pyrazole (1); C–Br⋯π-pyrimidine (2); and pyrazole-π⋯π-pyrimidine, pyrimidine-π⋯π-pyrimidine (3). This study shows that the Cl⋯N-pyrimidine interactions present in compounds 4 and 5 are a typical halogen⋯base Lewis interaction. Geometric information of Cl⋯Br and Cl⋯Cl interactions could be explained by the σ-hole concept. In compound 2, the chlorine atom participates with its σ-hole, while in compound 3, the σ-hole of the interaction comes from the bromine atom. The Cl⋯π-pyrazole and Br⋯π-pyrimidine interactions have a different nature; while the Br⋯π-pyrimidine interaction is a strong lone pair⋯π interaction, the Cl⋯π-pyrazole is a new interaction: σ-hole⋯π.