By the reaction of urea or thiourea, acetylacetone and hydrogen halide (HF, HBr or HI), we have obtained seven new 4,6-dimethyl-2-pyrimido(thio)nium salts, which were characterized by single-crystal X-ray diffraction, namely, 4,6-dimethyl-2-oxo-2,3-dihydropyrimidin-1-ium bifluoride, C6H9N2O+·HF2- or (dmpH)F2H, 4,6-dimethyl-2-oxo-2,3-dihydropyrimidin-1-ium bromide, C6H9N2O+·Br- or (dmpH)Br, 4,6-dimethyl-2-oxo-2,3-dihydropyrimidin-1-ium iodide, C6H9N2O+·I- or (dmpH)I, 4,6-dimethyl-2-oxo-2,3-dihydropyrimidin-1-ium iodide-urea (1/1), C6H9N2O+·I-·CH4N2O or (dmpH)I·ur, 4,6-dimethyl-2-sulfanylidene-2,3-dihydropyrimidin-1-ium bifluoride-thiourea (1/1), C6H9N2S+·HF2-·CH4N2S or (dmptH)F2H·tu, 4,6-dimethyl-2-sulfanylidene-2,3-dihydropyrimidin-1-ium bromide, C6H9N2S+·Br- or (dmptH)Br, and 4,6-dimethyl-2-sulfanylidene-2,3-dihydropyrimidin-1-ium iodide, C6H9N2S+·I- or (dmptH)I. Three HCl derivatives were described previously in the literature, namely, 4,6-dimethyl-2-oxo-2,3-dihydropyrimidin-1-ium chloride, (dmpH)Cl, 4,6-dimethyl-2-sulfanylidene-2,3-dihydropyrimidin-1-ium chloride monohydrate, (dmptH)Cl·H2O, and 4,6-dimethyl-2-sulfanylidene-2,3-dihydropyrimidin-1-ium chloride-thiourea (1/1), (dmptH)Cl·tu. Structural analysis shows that in 9 out of 10 of these compounds, the ions form one-dimensional chains or ribbons stabilized by hydrogen bonds. Only in one compound are parallel planes present. In all the structures, there are charge-assisted N+-H...X- hydrogen bonds, as well as weaker CAr+-H...X- and π+...X- interactions. The structures can be divided into five types according to their hydrogen-bond patterns. All the compounds undergo thermal decomposition at relatively high temperatures (150-300 °C) without melting. Four oxopyrimidinium salts containing a π+...X-...π+ sandwich-like structural motif exhibit luminescent properties.
Keywords: anion–π interactions; chalcogen interactions; crystal structure; halides; pyrimidinethione; pyrimidone.