Synthetic Strategies for the Synthesis of Ternary Uranium(IV) and Thorium(IV) Fluorides

A series of new U(IV) and Th(IV) fluorides, Na₇U₆F₃₁ (1), NaUF₅ (2), NaU₂F₉ (3), KTh₂F₉ (4), NaTh₂F₉ (5), (H₃O)Th₃F₁₃ (6), and (H₃O)U₃F₁₃ (7), was obtained using hydrothermal and low-temperature flux methods. Mild hydrothermal reactions with uranyl acetate as a precursor yielded 1, 7, and the monoclinic polymorph of NaU₂F₉, whereas direct reactions between UF₄ and NaF led to the formation of 2 and orthorhombic NaU₂F₉ (3). This highlights an unexpected difference in reaction products when different starting uranium sources are used. All seven compounds were characterized by single-crystal X-ray diffraction, and their structures are compared on the basis of cation topology, revealing a close topological resemblance between fluorides on the basis of the layers observed in NaUF₅(H₂O). Phase-pure samples of 1, 2, and both polymorphs of NaU₂F₉ were obtained, and their spectroscopic and magnetic properties were measured. The UV-vis data are dominated by the presence of U⁴⁺ cations and agree well with the electronic transitions. Effective magnetic moments of the studied compounds were found to range from 3.08 to 3.59 μ_B.