Calcium and strontium thiobarbiturates with discrete and polymeric structures

Abstract

Three new alkaline earth metal complexes, [Ca2(H2O)8(μ2-HTBA-O,O′)2(HTBA-O)2] (1), [Ca(H2O)5(HTBA-O)2]·2H2O (2), and [Sr(H2O)4(μ2-HTBA-O,S)2]n (3) (H2TBA = 2-thiobarbituric acid, C4H4N2O2S), were synthesized and characterized by FT-IR spectroscopy, TG-DSC, and single-crystal and powder X-ray diffraction analysis. The single-crystal X-ray diffraction data revealed that 1 and 2 are discrete structures, whereas 3 is a polymer. In 1 and 2, Ca2+ is seven-coordinate and forms a monocapped trigonal prism. In 1, the prisms are pairwise connected with the assistance of two [μ2-HTBA-O,O′]− ligands. In 3, Sr2+ is coordinated by four monodentate HTBA− via S or O donors and four waters, with the formation of a distorted square antiprism. The antiprisms are connected by μ2-O,S bridging HTBA−. Hydrogen bonding involving coordinated water and π–π interactions plays an important role in construction of the supramolecular 3-D structures in 1–3. Infrared spectroscopic data supported the structural data. The thermal stability of 1–3 decreases in the order 1 > 2 > 3. Dehydration of 1–3 was a multi-step process, followed by exothermic oxidative degradation of the 2-thiobarbiturate moiety between 290 and 800 °C.