Pertechnetate/Perrhenate-Linked Frameworks of U^IV, U^IV₆-Hexamer and Uranyl (VI) Building Units

Uranium-TcO₄^- structures enhance understanding of comobility, solid-phase formation, and solution speciation in nuclear materials including spent fuel and wastes. Different solubility and coordination of U(IV) and U(VI) drive their behavior in these complex matrices and in the environment. The present study provides bonding information from 11 U(VI) and U(IV) pertechnetate/perrhenate compounds (ReO₄^-, nonradioactive surrogate). Reported phases are synthesized from U^IV/U^VIO₂²⁺ acetate salts and perrhenic/pertechnic acid at room temperature. Single-crystal X-ray diffraction reveals the assembly of U^IV and U^VIO₂²⁺ into low-dimensional frameworks, linked by pertechnetate/perrhenate. In the series of compounds, nuclearity is dictated by oxoanion to uranium ratio in the reaction solution, as well as corresponding acidity introduced by the pertechnic/perrhenic acid. Higher MO₄^-:U^VIO₂²⁺ (M = Tc, Re) ratios (4:1, 3:1, 2:1) yielded predominantly 1D and 2D networks with no connectivity between uranium polyhedra. Lower ratios (i.e., 1:1) crystallized compounds featuring uranium clusters linked into frameworks by pertechnetate/perrhenate including a tetrameric uranyl clusters for both ReO₄^- and TcO₄^-, and the well-known U^IV₆O₄(OH)₄¹²⁺-hexamer (U^IV₆) linked by ReO₄^-. Small-angle X-ray scattering identified assembly of U^IV₆ in reaction solutions prior to crystallization. Because U^IV₆ is a molecular analogue of UO₂, these structures provide insight into pertechnetate-uranium oxide interactions in nuclear materials.