Structural and vibrational properties of lanthanide Lindqvist polyoxometalate complexes

Molecular spin qubits have demonstrated immense potential in quantum information science research due to the addressability of electron spins using microwave frequencies, and the scalability and tunability of molecular systems. Exemplary in this regard is the holmium polyoxometalate, [Na9Ho(W5O18)2]·35H2O (HoW10), which features an accessible atomic clock transition at 9.4 GHz; however, the coherence time of this molecule is limited by spin-phonon coupling driven decoherence processes. To limit these decoherence pathways, materials need to be designed to reduce energy overlap between spin and phonon states, and this necessitates developing a better understanding on how structural modifications impact the vibrational landscape for classes of complexes. Herein we conducted a full investigation into the fundamental structural and vibrational properties of the lanthanide Lindqvist polyoxometalate series, [Na9Ln(W5O18)2]·xH2O (Ln = La(III)-Lu(III), except Pm(III)) (LnW10), to assess how structural changes effect vibrational characteristics and to elucidate pathways to improve the coherence properties of HoW10. Single crystal X-ray diffraction results revealed four distinct structural polymorphs in complexes 1-14 wherein first coordination spheres were identical, and differences manifested as changes in lattice packing. Interestingly, the subtle changes in packing exhibited by the four polymorphs were found to impact distortions away from ideal D4d symmetry for each of the LnW10 complexes. Raman and far-infrared (FIR) spectra of complexes 1-14 were collected to identify vibrational modes present in low energy regions and peak fitting assignments were made according to literature precedents. Qualitative and Partial least squares (PLS) analysis show correlations between complex structural parameters with the low energy Raman and FIR vibrational modes of interest. Overall, this investigation shows that the second coordination sphere plays an integral role in modulation of the structural and vibrational characteristics of LnW10 complexes, which makes it a viable route for tuning spin and vibrational manifolds of species within this series.

Ultrafiltration separation of Am(VI)-polyoxometalate from lanthanides

Partitioning of americium from lanthanides (Ln) present in used nuclear fuel plays a key role in the sustainable development of nuclear energy1-3. This task is extremely challenging because thermodynamically stable Am(III) and Ln(III) ions have nearly identical ionic radii and coordination chemistry. Oxidization of Am(III) to Am(VI) produces AmO22+ ions distinct with Ln(III) ions, which has the potential to facilitate separations in principle. However, the rapid reduction of Am(VI) back to Am(III) by radiolysis products and organic reagents required for the traditional separation protocols including solvent and solid extractions hampers practical redox-based separations. Herein, we report a nanoscale polyoxometalate (POM) cluster with a vacancy site compatible with the selective coordination of hexavalent actinides (238U, 237Np, 242Pu and 243Am) over trivalent lanthanides in nitric acid media. To our knowledge, this cluster is the most stable Am(VI) species in aqueous media observed so far. Ultrafiltration-based separation of nanoscale Am(VI)-POM clusters from hydrated lanthanide ions by commercially available, fine-pored membranes enables the development of a once-through americium/lanthanide separation strategy that is highly efficient and rapid, does not involve any organic components and requires minimal energy input.

Contrasting Trivalent Lanthanide and Actinide Complexation by Polyoxometalates via Solution-State NMR

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 and 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. 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.

Polyoxometalates as ligands to synthesize, isolate and characterize compounds of rare isotopes on the microgram scale

The synthesis and study of radioactive compounds are both inherently limited by their toxicity, cost and isotope scarcity. Traditional methods using small inorganic or organic complexes typically require milligrams of sample-per attempt-which for some isotopes is equivalent to the world's annual supply. Here we demonstrate that polyoxometalates (POMs) enable the facile formation, crystallization, handling and detailed characterization of metal-ligand complexes from microgram quantities owing to their high molecular weight and controllable solubility properties. Three curium-POM complexes were prepared, using just 1-10 μg per synthesis of the rare isotope 248Cm3+, and characterized by single-crystal X-ray diffraction, showing an eight-coordinated Cm3+ centre. Moreover, spectrophotometric, fluorescence, NMR and Raman analyses of several f-block element-POM complexes, including 243Am3+ and 248Cm3+, showed otherwise unnoticeable differences between their solution versus solid-state chemistry, and actinide versus lanthanide behaviour. This POM-driven strategy represents a viable path to isolate even rarer complexes, notably with actinium or transcalifornium elements.

Lanthanide-Containing 22-Tungsto-2-germanates [Ln(GeW11O39)2]13-: Synthesis, Structure, and Magnetic Properties

We report on the synthesis and structural characterization of two series of lanthanide-containing 22-tungsto-2-germanates. The first series corresponds to a family of polyanions with the formula [Ln(β₂-GeW₁₁O₃₉)₂]^13- (Ln^III = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Gd (6), Dy (7)), and the second series corresponds to a family with the formula [Ln(β₂-GeW₁₁O₃₉)(α-GeW₁₁O₃₉)]^13- (Ln^III = Ho (8), Er (9), Tm (10)). All compounds were characterized in the solid state by single-crystal and powder XRD, IR, TGA, and SQUID magnetometry. The polyanions were synthesized in aqueous medium by direct reaction of the monolacunary [β₂-GeW₁₁O₃₉]^8- POM precursor with the corresponding lanthanide salts. The structure of the polyanions consists of an 8-coordinated lanthanide ion in a square-antiprismatic geometry, which is sandwiched either between two [β₂-GeW₁₁O₃₉]^8- units for 1-7 or between a [β₂-GeW₁₁O₃₉]^8- and a [α-GeW₁₁O₃₉]^8- unit for 8-10. Furthermore, the effect of the central paramagnetic lanthanide ion on the magnetic behavior of the polyanions was investigated, with the erbium-derivative [Er(β₂-GeW₁₁O₃₉)(α-GeW₁₁O₃₉)]^13- (9) showing single-molecule magnet (SMM) behavior.

Polyoxometalate-based complexes as ligands for the study of actinide chemistry

The complexation of actinide cations by polyoxometalates (POMs) has been extensively studied over the past 50 years. In this perspective article, we present the rich structural diversity of actinide-POM complexes and their contribution to the extension of our knowledges of actinide chemistry, especially regarding aspect of their redox chemistry, as well as application for the capture and separation of these cations in the context of nuclear fuel remediation. These heterometallic assemblies have also proven highly valuable as model for heterogeneous systems based on actinides supported by metal oxide surfaces. In particular, activation of the An-O bond of actinyl fragments upon complexation with lacunary POMs has been reported, creating opportunities for future developments regarding the reactivity of these heterometallic assemblies.

Trends and new directions in the crystal chemistry of actinide oxo-clusters incorporated in polyoxometalates

This highlight article reports the synthetic methods, the structural topologies and the behaviour of actinide-polyoxometalate species in aqueous solution.

Complexation of tetravalent uranium cations by the As4W40O140 cryptand

The polyanionic cryptand {As₄W₄₀O₁₄₀} was successfully used to bind up to four tetravalent uranium cations leading to the formation of three new cryptates. The obtained species appears to be stable in solution and the cryptand was used for U^IV/Nd^III separation studies.