Time-controlled synthesis of the 3D coordination polymer U(1,2,3-Hbtc)2 followed by the formation of molecular poly-oxo cluster {U14} containing hemimellitate uranium(iv)

Two coordination compounds bearing tetravalent uranium were synthesized in the presence of tritopic hemimellitic acid in acetonitrile with a controlled amount of water (H2O/U ≈ 8) and structurally characterized. Compound 1, [U(1,2,3-Hbtc)2]·0.5CH3CN is constructed around an eight-fold coordinated uranium cationic unit [UO8] linked by the poly-carboxylate ligands to form dimeric subunits, which are further connected to form infinite corrugated ribbons and a three-dimensional framework. Compound 2, [U14O8(OH)4Cl8(H2O)16(1,2,3-Hbtc)8(ox)4(ac)4] ({U14}) exhibits an unprecedented polynuclear {U14} poly-oxo uranium cluster surrounded by O-donor and chloride ligands. It is based on a central core of [U6O8] type surrounded by four dinuclear uranium-subunits {U2}. Compound 1 was synthesized by a direct reaction of hemimellitic acid with uranium tetrachloride in acetonitrile (+H2O), while the molecular species ({U14} (2)) crystallized from the supernatant solution after one month. The slow hydrolysis reaction together with the partial decomposition of the starting organic reactants into oxalate and acetate molecules induces the generation of such a large poly-oxo cluster with fourteen uranium centers. Structural comparisons with other closely related uranium-containing clusters, such as the {U12} cluster based on the association of inner core [U6O8] with three dinuclear sub-units {U2}, were performed.

An Ultrastable Heterobimetallic Uranium(IV)/Vanadium(III) Solid Compound Protected by a Redox-Active Phosphite Ligand: Crystal Structure, Oxidative Dissolution, and First-Principles Simulation

The first heterobimetallic uranium(IV)/vanadium(III) phosphite compound, Na₂UV₂(HPO₃)₆ (denoted as UVP), was synthesized via an in situ redox-active hydrothermal reaction. It exhibits superior hydrolytic and antioxidant stability compared to the majority of structures containing low-valent uranium or vanadium, further elucidated by first-principles simulations, and therefore shows potential applications in nuclear waste management.

U(IV) Aqueous Speciation from the Monomer to UO2 Nanoparticles: Two Levels of Control from Zwitterionic Glycine Ligands

The fate of U(IV)O₂ in the environment in a colloidal form and its dissolution and growth in controlled environments is influenced by organic ligation and redox processes, where both affect solubility, speciation, and transport. Here we investigate U(IV) aqueous speciation from pH 0 to 3 with the glycine (Gly) ligand, the smallest amino acid. We document evolution of the monomeric to the hexameric form from pH 0 to 3 via UV-vis spectroscopy and small-angle X-ray scattering (SAXS). Crystals of the hexamer [U₆O₄(OH)₄(H₂O)₆(HGly)₁₂]·12Cl^-·12(H₂O) (U₆) were isolated at pH 2.15. The structure of U₆ is a hexanuclear oxo/hydroxo cluster U₆O₄(OH)₄ decorated by 12 glycine ligands and 6 water molecules. The effect of pH and temperature on U₆ conversion to UO₂ nanoparticles, or simply reversible aggregation, is detailed by transmission electron microscopy imaging, in addition to SAXS and UV-spectroscopy. Because of the zwitterion behavior of glycine, pH and temperature control over U(IV) speciation is complex. Unexpectedly, stability of the polynuclear cluster actually increases with increased pH. Speciation is sensitive to not only metal-oxo hydrolysis but also ligand lability and hydrophobic ligand-ligand interactions.