7 Search Results

Here, we report the isolation of the first plutonium(IV) complex with a Keggin ion chelator: Cs₂₀[Pu(PW₁₁O₃₉)₂]₂·13H₂O. Single crystal XRD and solid-state UV–vis absorbance analysis demonstrate the stabilization of Pu⁴⁺ by the Keggin ligand. The unit cell contains two [Pu(PW₁₁O₃₉)₂]^10– complexes (Pu(PW₁₁)₂) bridged by Cs⁺. Raman and ³¹P NMR spectra of Pu(PW₁₁)₂ are consistent with the analogous Zr⁴⁺, Hf⁴⁺, Ce⁴⁺, and Th⁴⁺ complexes. The Pu–O bond distances at the two Pu sites are 2.35(3) and 2.34(3) Å, matching the value extrapolated from the bonding trend built with the other 8-coordinated tetravalent cations. However, the long-range arrangement of the Pu(PW₁₁)₂ complexes withinmore » the lattice is unique in the series of M^IV(PW₁₁)₂ compounds: pairs of Pu(PW₁₁)₂ are organized perpendicular to each other. Based on solution-state UV–visible absorbance, small-angle X-ray scattering (SAXS), and ³¹P NMR, the tetravalent cations quantitatively form the 1:2 species in solution ([Pu(PW₁₁O₃₉)₂]^10–_(aq)) and no 1:1 species ([Pu(PW₁₁O₃₉)(H₂O)_x]^3–_(aq)). Finally, a linear correlation exists between the metal–oxygen distances in the M^IV(PW₁₁)₂ compounds and the corresponding metal dioxides, allowing for extrapolation for Pa⁴⁺, Am⁴⁺, and Bk⁴⁺. The results indicate that our microscale POM approach represents a viable pathway to probe properties of rare actinide ions in discrete molecules, beyond the traditional oxide extended solids.« less

Neptunium exhibits truly unique chemistry as its speciation is dominated by the neptunyl(V) ion (NpO₂⁺). Here, in this study, we describe the spontaneous destabilization and reduction of neptunyl(V) via complexation to the Keggin-type polyoxometalate (POM) ligand PW₁₁O₃₉^7–. The POM-mediated reduction of NpO₂⁺ does not require any reducing agent and occurs within minutes, at room temperature, and in aqueous solution. The resulting [Np(PW₁₁O₃₉)₂]^10- complex (Np(PW₁₁)₂) remains soluble, water-stable, and air-stable for weeks and persists over an extended acidity range. Single-crystal structure, solid-state Raman and UV–visible absorbance characterization of Np(PW₁₁)₂ revealed a mixed alpha/beta isomery of the Keggin ion, forming an unprecedentedmore » 50:50 mixture of Np(α-PW₁₁)₂ and Np(α-PW₁₁)(β-PW₁₁). Experiments with other tetravalent ions (i.e., Zr⁴⁺, Hf⁴⁺, Ce⁴⁺, and Th⁴⁺) indicate that the occurrence of the beta isomer is specific to Np⁴⁺ and independent of the cation’s size. Solution-state characterization of the Np-PW₁₁ system via UV-visible-NIR absorbance, ³¹P NMR, VT NMR, and relaxometry further elucidated the speciation. Moreover, comparative experiments with uranium revealed that the two types of actinyl ions (UO₂²⁺ vs NpO₂⁺) undergo drastically different reactions in the presence of PW₁₁. UO₂²⁺ is not reduced but instead uses PW₁₁ as a phosphate reservoir and precipitates as uranyl phosphate.« less

The production of ceramics from uranium coordination compounds can be achieved through thermal processing if an excess amount of the desired atoms (i.e., C or N), or reactive gaseous products (e.g., methane or nitrogen oxide) is made available to the reactive uranium metal core via decomposition/fragmentation of the surrounding ligand groups. Here, computational thermodynamic approaches were utilized to identify the temperatures necessary to produce uranium metal from some starting compounds—UI₄(TMEDA)₂, UCl₄(TMEDA)₂, UCl₃(pyridine)_x, and UI₃(pyridine)₄. Experimentally, precursors were irradiated by a laser under various gaseous environments (argon, nitrogen, and methane) creating extreme reaction conditions (i.e., fast heating, high temperature profile >2000more » °C, and rapid cooling). Despite the fast dynamics associated with laser irradiation, the central uranium atom reacted with the thermal decomposition products of the ligands yielding uranium ceramics. Residual gas analysis identified vaporized products from the laser irradiation, and the final ceramic products were characterized by powder X-ray diffraction. The composition of the uranium precursor as well as the gaseous environment had a direct impact on the production of the final phases.« less

Deciphering the solution chemistry and speciation of actinides is inherently difficult due to radioactivity, rarity, and cost constraints, especially for transplutonium elements. In this context, the development of new chelating platforms for actinides and associated spectroscopic techniques is particularly important. In this study, we investigate a relatively overlooked class of chelators for actinide binding, namely, polyoxometalates (POMs). We provide the first NMR measurements on americium–POM and curium–POM complexes, using one-dimensional (1D) ³¹P NMR, variable-temperature NMR, and spin-lattice relaxation time (T₁) experiments. The proposed POM–NMR approach allows for the study of trivalent f-elements even when only microgram amounts are available andmore » in phosphate-containing solutions where f-elements are typically insoluble. The solution-state speciation of trivalent americium, curium, plus multiple lanthanide ions (La³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Yb³⁺, and Lu³⁺), in the presence of the model POM ligand PW₁₁O₃₉^7– was elucidated and revealed the concurrent formation of two stable complexes, [M^III(PW₁₁O₃₉)(H₂O)_x]^4– and [M^III(PW₁₁O₃₉)₂]^11–. Interconversion reaction constants, reaction enthalpies, and reaction entropies were derived from the NMR data. The NMR results also provide experimental evidence of the weakly paramagnetic nature of the Am³⁺ and Cm³⁺ ions in solution. Furthermore, the study reveals a previously unnoticed periodicity break along the f-element series with the reversal of T₁ relaxation times of the 1:1 and 1:2 complexes and the preferential formation of the long T₁ species for the early lanthanides versus the short T₁ species for the late lanthanides, americium, and curium. Furthermore, given the broad variety of POM ligands that exist, with many of them containing NMR-active nuclei, the combined POM–NMR approach reported here opens a new avenue to investigate difficult-to-study elements such as heavy actinides and other radionuclides.« less

A sharp signal at 72 ppm was recently observed in ²⁷Al-NMR spectra during the isomerization of Keggin-Al₁₃ ions in the presence of calcium and glycine [1]. It has been proposed that this signal corresponds to one of the Keggin isomers. Conversion of the most common ε-isomer of the Keggin series of molecules, having the stoichiometry [AlO₄Al₁₂(OH)₂₄(OH₂)₁₂]⁷⁺, is enhanced by the addition of calcium and glycine [2,3]. Here we show that a 72 ppm signal can also be observed in the absence of calcium or glycine; the magnitude of which depends on aluminum concentration and temperature. Additionally, isomerization of the ε-Keggin-Al₁₃more » to the γ-Keggin-Al₁₃ isomer was observed in the absence of calcium and glycine at elevated temperatures after several days. Furthermore, calcium and glycine were previously shown to enhance rates of γ-Keggin-Al₁₃ formation.^2.« less

The oxygen-isotope fractionations between brucite and water, portlandite and water, and brucite and portlandite have been calculated over the temperature range of 0 to 450 °C using quantum-chemical methods and several basis sets and functionals. The calculations also employ embedded clusters that are chosen using the Pauling-bond-strength-conserving termination method that maintains a neutral cluster with fractional charges assigned to terminal atoms. These calculations improve upon the previous semiempirical methods for predicting mineral-mineral fractionations. These semiempirical methods fail to accurately predict the relative enrichment and depletion of oxygen isotopes for the brucite-portlandite pair. The quantum calculations presented here also fail tomore » predict at the absolute values for enrichment of oxygen isotopes between minerals and water, and a simple correction must be employed to achieve agreement with experiments if water is in the reaction. No such correction is needed to predict fractionation between minerals. The trends derived from the calculations are robust to changes in basis sets and functionals.« less