Synthesis and Structure of Uranyl Succinate Complex with Isonicotinic Acid and New Polymorph of Uranyl Succinate Monohydrate.

Synthesis, IR spectroscopic and X-ray structural studies of crystals [UO2(C4H4O4)(C6H5NO2)2] (I) and a new modification [UO2(C4H4O4)(H2O)] (II), where C4H4O42– are succinate ions, and C6H5NO2 is isonicotinic acid. Structure I is formed by chains in which succinate ions play the role of bridging ligands Q02 and B2, and isonicotinic acid molecules in the form of a zwitterion are monodentate ligands M1. Chains I correspond to the crystal chemical formula АQ020.5B20.5M12, where = UO22+, Q02 and B2 = C4H4O42–, and M1 = C6H5NO2. The reasons of the change in the coordination type of half of the succinate ions in I from characteristic Q02 to rare B2 are explained from the standpoint of the 18-electron rule in uranyl compounds. Structure II, like the two already known modifications, corresponds to the crystal chemical formula АQ4M1, where A = UO22+, Q4 = C4H4O42–, and M1 = H2O. By the example of uranyl succinate monohydrate polymorphs, it was found that, like silica polymorphs, chemically identical 3D uranyl-containing frameworks can differ both in topology (reconstructive isomers) and in the symmetry of the framework (deformation isomers). It was found that the new γ-form of II in the region of ≈180 K reversibly transforms into the already known low-temperature β-modification. It was shown using the Voronoi–Dirichlet polyhedra, that the available data for the fourth polymorph [UO2(C4H4O4)(H2O)] registered in the CSD as {SUCCUR02}, are erroneous. [ABSTRACT FROM AUTHOR]