Uranyl Heteropolyoxometalate: Synthesis, Structure, and Spectroscopic Properties

Ionothermal Synthesis of Uranyl Vanadate Nanoshell Heteropolyoxometalates

Two uranyl vanadate heteropolyoxometalates (h-POMs) have been synthesized by ionothermal methods using the ionic liquid 1-ethyl-3-methylimidazolium diethyl phosphate (EMIm-Et₂PO₄). The hybrid actinide-transition metal shell structures have cores of (UO₂)₈(V₆O₂₂) and (UO₂)₆(V₃O₁₂), which we designate as {U₈V₆} and {U₆V₃}, respectively. The diethyl phosphate anions of the ionic liquids in some cases terminate the core structures to form actinyl oxide clusters, and in other cases the diethyl phosphate oxyanions link these cluster cores into extended structures. Three compounds exist for the {U₈V₆} cluster core: {U₈V₆}-monomer, {U₈V₆}-dimer, and {U₈V₆}-chain. Tungsten atoms can partially substitute for vanadium in the {U₆V₃} cluster, which results in a chain-based structure designated as {U₆V₃}-W. Each of these compounds contains charge-balancing EMIm cations from the ionic liquid. These compounds were characterized crystallographically, spectroscopically, and by mass spectrometry.

Hybrid Uranyl-Phosphonate Coordination Nanocage

We report herein a general synthetic approach for designing uranyl coordination cages. Compounds 1 and 2 are constructed through a temperature-dependent and solvent-driven self-assembly. In both cases, the synthetic strategy involves in situ phosphonate ligand condensation into a flexible pyrophosphonate ligand. This pyrophosphonate ligand formation is essential for the introduction of curvature into these compounds. In the presence of PF₆^- ions that are derived from hydrofluoric acid, a macrocyclic uranyl-phosphonate discrete compound, 1, whose cavity contains PF₆^- ions, hydronium ions, and water molecules, is obtained. When Cs⁺ cations are used in the synthesis, a remarkable uranyl coordination nanocage, 2, resulted. The macrocycle (1) is approximately 10.9 × 10.9 Ų in diameter while the nanocage (2) is approximately 15.0 × 11.3 Ų in diameter, as measured from the outer oxygen atoms of the uranyl centers. Both compounds are constructed from a UO₂²⁺ moiety, coordinated by an additional four oxygen atoms from the phosphonate group to form pentagonal bipyramidal geometry. All the compounds fluoresce at room temperature, showing characteristic vibronically coupled charge-transfer based emission.

A first principles study of energetics and electronic structural responses of uranium-based coordination polymers to Np incorporation

Recently developed coordination polymers (CPs) and metal organic frameworks (MOFs) may find applications in areas such as catalysis, hydrogen storage, and heavy metal immobilization. Research on the potential application of actinide-based CPs (An-CP/MOFs) is not as advanced as transition metal-based MOFs. In order to modify their structures necessary for optimizing thermodynamic and electronic properties, here, we described how a specific topology of a particular actinide-based CP or MOF responds to the incorporation of other actinides considering their diverse coordination chemistry associated with the multiple valence states and charge-balancing mechanisms. In this study, we apply a recently developed DFT-based method to determine the relative stability of transuranium incorporated CPs in comparison to their uranium counterpart considering both solid and aqueous state sources and sinks to understand the mechanism and energetics of charge-balanced Np5+ incorporation into three uranium-based CPs. The calculated Np5++H+ incorporation energies for these CPs range from 0.33 to 0.52 eV, depending on the organic linker, when using the solid oxide Np source Np2O5 and U sink UO3. Incorporation energies of these CPs using aqueous sources and sinks increase to 2.85–3.14 eV. The thermodynamic and structural analysis in this study aides in determining, why certain MOF topologies and ligands are selective for some actinides and not for others. This means that once this method is extended across a variety of CPs with their respective linker molecules and different actinides, it can be used to identify certain CPs with certain organic ligands being specific for certain actinides. This information can be used to construct CPs for actinide separation. This is the first determination of the electronic structure (band structure, density of states) of these uranium- and transuranium-based CPs which may eventually lead to design CPs with certain optical or catalytic properties. While the reduction of the DFT-determined-bandgap goes from 3.1 eV to 2.4 eV when going from CP1 to CP3, showing the influence of the linker, Np6+ incorporation reduces the bandgap for CP1 and CP3, while increasing it for CP2. The coupled substitution of U6+→Np5++H+ reduces the bandgap significantly, but only for CP3.

Recent advances in structural studies of heterometallic uranyl-containing coordination polymers and polynuclear closed species

A survey is given of recent original structural results on heterometallic species incorporating uranyl ions, particularly with carboxylate ligands.

Hybrid uranyl arsonate coordination nanocages

Nanoscopic uranyl coordination cages have been prepared by a facile route involving self-assembly via temperature and solvent-driven, in situ ligand synthesis. The synthesis of hydrogen arsenate and pyroarsonate ligands in situ enhances flexibility, which is an important factor in producing these compounds.

Synthesis and structural characterization of heterometallic thorium aluminum polynuclear molecular clusters

Aluminum can undergo hydrolysis in aqueous solutions leading to the formation of soluble molecular clusters, including polynuclear species that range from 1 to 2 nm in diameter. While the behavior of aluminum has been extensively investigated, much less is known about the hydrolysis of more complex mixed-metal systems. This study focuses on the structural characteristics of heterometallic thorium-aluminum molecular species that may have important implications for the speciation of tetravalent actinides in radioactive waste streams and environmental systems. Two mixed metal (Th(4+)/Al(3+)) polynuclear species have been synthesized under ambient conditions and structurally characterized by single-crystal X-ray diffraction. [Th(2)Al(6)(OH)(14)(H(2)O)(12)(hedta)(2)](NO(3))(6)(H(2)O)(12) (ThAl1) crystallizes in space group P2(1)/c with unit cell parameters of a = 11.198(1) Å, b = 14.210(2) Å, c = 23.115(3) Å, and β = 96.375° and [Th(2)Al(8)(OH)(12)(H(2)O)(10)(hdpta)(4)](H(2)O)(21) (ThAl2) was modeled in P1 with a = 13.136(4) Å, b = 14.481(4) Å, c = 15.819(4) Å, α = 78.480(9)°, β = 65.666(8)°, γ = 78.272(8)°. Infrared spectra were collected on both compounds, confirming complexation of the ligand to the metal center, and thermogravimetric analysis indicated that the thermal degradation of these compounds resulted in the formation of an amorphous product at high temperatures. These mixed metal species have topological relationships to previously characterized aluminum-based polynuclear species and may provide insights into the adsorption of tetravalent actinides on colloidal or mineral surfaces.