Fundamental understanding of tetravalent actinide chemistry is essential to optimize many steps of the nuclear fuel cycle, as well as to predict actinide speciation in the environment. Herein, we report synthesis and structure of three open inorganic frameworks composed of Th4+ -oligomers and sulfate anions. The compounds [Th10 O4 (OH)8 (SO4 )12 (H2 O)18 ]⋅28 H2 O (1), [Th9 O4 (OH)5 (SO4 )12 (H2 O)18 ]⋅ 1 TMA⋅18 H2 O (2), and [Th8.5 O4 (OH)4 (SO4 )12 (H2 O)18 ]⋅2 TMA⋅n H2 O (3) were obtained by slow evaporation of aqueous Th4+ -SO42- solutions, with variable SO42- :Th4+ ratios (SO42- :Th4+ =1:1, 1.5:1 and 2:1 for 1, 2, and 3; TMA=tetramethylammonium). The structure of 1 features a neutral open framework architecture with two interpenetrating networks, built from Th6 O4 (OH)412+ hexamers and Th2 (OH)26+ dimers, linked by sulfate anions. The complex anionic framework of 2 is built from hexamers, dimers, and monomers, with charge-balancing TMA cations. Compound 3 is very similar to 2, but it is constructed only from hexamers and monomers. Small angle X-ray scattering (SAXS) of the reaction solutions reveals pre-assembled hexamer building units in the presence of sulfate, and underlines the key role of the sulfate-oxoanion ligand on thorium polymerization processes.
Keywords: SAXS; X-ray diffraction; metal-oxo clusters; open inorganic frameworks; sulfate; thorium.
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