B-α-[AsW9O33](9-) polyoxometalates incorporating hexanuclear uranium {U6O8}-like clusters bearing the U(IV) form or unprecedented mixed valence U(IV)/U(VI) involving direct U(VI)=O-U(IV) bonding

Leveraging a reduced polyoxomolybdate-alkoxide cluster for the formation of a stable U(v) sandwich complex

The synthesis and characterization of a series of (TBA)2[M{Mo5O13(OMe)4NO}2] (M = Zr, Hf, Th, and U) sandwich complexes is reported. A preformed lacunary, Lindqvist-type, polyoxomolybdate-alkoxide cluster provides access to first examples of actinide-polyoxomolybdate sandwich complexes isolated under non-aqueous conditions. Incorporation of metal(iv) cations into this framework was found to "switch on" reversible redox chemistry at the {Mo5} ligands, with the Zr and Hf containing complexes accepting up to two electrons, while the Th and U derivates accommodate as many as four additional electrons. The enhancement of the redox properties of the cluster upon actinide incorporation is an exciting observation, presenting actinide "doping" as a novel approach for accessing functional redox-active materials. Oxidation of the uranium containing sandwich complex (TBA)2[U{Mo5O13(OMe)4NO}2], chemically or electrochemically, allows access to the U(v) centered species, which was characterized both spectroscopically and by single crystal X-ray diffraction. This represents the first example of a U(v)-polyoxometalate sandwich complex to be isolated and structurally characterized.

Electrocatalytic Hydrogen Evolution by a Uranium(VI) Polyoxometalate: an Environmental Toxin for Sustainable Energy Generation

The uranyl ion (UO2)2+, a uranium nuclear waste, is one of the serious contaminants in our ecosystem because of its radioactivity, relevant human activities, and highly mobile and complex nature of living cells. In this article, we have reported the synthesis and structural characterization of an uranyl cation-incorporated polyoxometalate (POM) compound, K10[{K4(H2O)6}{UO2}2(α-PW9O34)2]·13H2O (1), in which the uranyl cations are complexed with an in situ generated [α-PW9O34]9- cluster. Single-crystal X-ray diffraction (SCXRD) analysis of compound 1 reveals that the uranyl-potassium complex cationic species, [{K4(H2O)6}{UO2}2]8+, is sandwiched by two [α-PW9O34]9- clusters resulting in a Dawson type of POM. Compound 1 was further characterized by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis and infrared (IR), Raman, electronic absorption, and solid-state photoluminescence spectral studies. IR stretching vibrations at 895 and 856 cm-1 and the Raman signature peak at 792 cm-1 in the IR and Raman spectra of compound 1 primarily confirm the presence of a trans-[O═U═O]2+ ion. The solid-state photoluminescence spectrum of 1 exhibits a typical vibronic structure, resulting from symmetrical vibrations of [O═U═O]2+ bands, corresponding to the electronic transitions of S11 → S10 and S10 → S0υ (υ = 0-3). Interestingly, title compound 1 shows efficient electrocatalytic hydrogen evolution by water reduction with low Tafel slope values of 186.59 and 114.83 mV dec-1 at 1 mA cm-2 along with optimal Faradaic efficiency values of 82 and 87% at neutral pH and in acidic pH 3, respectively. Detailed electrochemical analyses reveal that the catalytic hydrogen evolution reaction (HER) activity mediated by compound 1 is associated with the UVI/UV redox couple of the POM. The microscopic as well as routine spectral analyses of postelectrode samples and controlled experiments have confirmed that compound 1 behaves like a true molecular electrocatalyst for the HER. To our knowledge, this is the first paradigm of a uranium-containing polyoxometalate that exhibits electrocatalytic water reduction to molecular H2. In a nutshell, an environmental toxin (a uranium-oxo compound) has been demonstrated to be utilized as an efficient electrocatalyst for hydrogen generation from water, a green approach of sustainable energy production.

[α-AsW9O33]9- bridged hexagonal clusters of Ln(III) showing field induced SMM behavior: experimental and theoretical insight

Polyoxometalates (POM), as inorganic polydentate oxygen donors, provide binding opportunities for oxophilic lanthanide metal centers to construct novel Ln-substituted POM materials with exciting structures and attractive properties. Herein, we have reported four arsenotungstate [α-AsW₉O₃₃]^9- based lanthanide-containing polyoxometalates [Cs_xK_36-x{Ln₆(H₂O)₁₂(α-AsW₉O₃₃)₆}]·yH₂O (Ln = Er (1), Gd (2), Ho (3), and Tb (4)), which are synthesized in an alkaline medium. Complexes 1-3 are the dimeric structures of [Ln₃(H₂O)₆(α-AsW₉O₃₃)₃]^18- polyanions, whereas complex 4 is a hexamer of the polyanion [Tb (H₂O)₂(α-AsW₉O₃₃)]^6- as a building unit. In all the complexes, [α-AsW₉O₃₃]^9- units are staggered up and down and give rise to the chair conformation, where one [α-AsW₉O₃₃]^9- unit bridges two Ln(III) centers through four μ₂-oxygen and two terminal oxygen atoms, resulting in the hexagonal arrangement of lanthanides. The dynamic magnetic measurement indicates that only complex 1 exhibits slow relaxation of magnetization with an applied dc field (1500 Oe). To gain insight into the slow relaxation of magnetization in complex 1, the ligand-field parameters and the splitting of the ground-state multiplet of the Er(III) ions have been estimated. The ab initio calculation results confirm that the ground state wave function of these molecules (1, 3, and 4) is mainly composed of a mixture of m_J states, and the non-axial crystal field (CF) terms are more predominant than the axial CF term. The solid-state fluorescence spectra of 1-4 reveal that the photoexcitation O → M ligand-to-metal charge-transfer (LMCT) of arsenotungstate fragments is effectively quenched due to the spatial coordination environment around the Ln(III) ion.

Platonic and Archimedean solids in discrete metal-containing clusters

Metal-containing clusters have attracted increasing attention over the past 2-3 decades. This intense interest can be attributed to the fact that these discrete metal aggregates, whose atomically precise structures are resolved by single-crystal X-ray diffraction (SCXRD), often possess intriguing geometrical features (high symmetry, aesthetically pleasing shapes and architectures) and fascinating physical properties, providing invaluable opportunities for the intersection of different disciplines including chemistry, physics, mathematical geometry and materials science. In this review, we attempt to reinterpret and connect these fascinating clusters from the perspective of Platonic and Archimedean solid characteristics, focusing on highly symmetrical and complex metal-containing (metal = Al, Ti, V, Mo, W, U, Mn, Fe, Co, Ni, Pd, Pt, Cu, Ag, Au, lanthanoids (Ln), and actinoids) high-nuclearity clusters, including metal-oxo/hydroxide/chalcogenide clusters and metal clusters (with metal-metal binding) protected by surface organic ligands, such as thiolate, phosphine, alkynyl, carbonyl and nitrogen/oxygen donor ligands. Furthermore, we present the symmetrical beauty of metal cluster structures and the geometrical similarity of different types of clusters and provide a large number of examples to show how to accurately describe the metal clusters from the perspective of highly symmetrical polyhedra. Finally, knowledge and further insights into the design and synthesis of unknown metal clusters are put forward by summarizing these "star" molecules.

Structure of the {U13} polyoxo cluster U13O8Cl x (MeO)38-x (x = 2.3, MeO = methoxide)

The structure of a new type of polyoxo cluster complex that contains thirteen uranium atoms, {U13}, is reported. The complex crystallized from methanol containing tetra-valent uranium (UIV) with a basic organic ligand, and was characterized as di-chloridoocta-cosa-μ2-methano-lato-octa-kis-(methano-lato)octa-μ4-oxido-trideca-uranium, [U13(CH3O)35.7Cl2.3O8] or [U13(μ4-Ooxo)8Cl x (MeO)38-x ] (x = 2.3, MeO = methoxide) (I), by single-crystal X-ray diffraction. The characterized {U13} polyoxo cluster complex (I) possesses a single cubic uranium polyhedron at the centre of the cluster core. To the best of our knowledge, this is the very first example of a polyoxo actinide complex that bears a single cubic polyhedron in its structure. The cubic polyhedron in I is well comparable in shape with those in bulk UO2. The U-O bonds in the cubic polyhedron of I are, however, significantly shorter than those not only in bulk UO2 but also in another analogue in the {U38} cluster. This shortening of U-O bonds, together with BVS calculations and the overall negative charge (2-) of I, suggests that the central uranium atom in I, which forms the single cubic coordination polyhedron, is presumably oxidized to the penta-valent state (UV) from the original tetra-valent state (UIV). Complex I is, hence, the first example of a polyoxo cluster possessing a single cubic coordination polyhedron of UV.

Tubular polyoxoanion [(SeMo6O21)2(C2O4)3]10− and its transformations

Tubular [(SeMo₆O₂₁)₂(C₂O₄)₃]¹⁰⁻ polyoxoanion can be assembled from Na₂MoO₄/SeO₂/H₂C₂O₄ system. TBABr precipitates complexes of [Mo₆O₁₉]²⁻ and [β-Mo₈O₂₆Na₂(NO₃)]³⁻ from its aqueous solution. Speciation of [β-Mo₈O₂₆]⁴⁻ and [Mo₆O₁₉]²⁻ has been studied.

Self-assembly of Keggin-type U(vi)-containing tungstophosphates with a sandwich structure: an efficient catalyst for the synthesis of sulfonyl pyrazoles

Two Keggin-type U(vi)-containing tungstophosphates with sandwich structure were synthesized and characterized. Compound 1 presents excellent catalytic activity towards the condensation cyclization of sulfonyl hydrazines with 1,3-diketones to synthesize sulfonyl pyrazoles.

Building [UIV 70 (OH)36 (O)64 ]4- Oxocluster Frameworks with Sulfate, Transition Metals, and UV

Uranium(IV) oxide clusters, colloids, and materials are designed and studied for 1) nuclear materials applications, 2) understanding the environmental fate and transport of actinides, and 3) exploring the complex bonding behavior of open-shell f-elements. U^IV -oxyhydroxsulfate clusters are particularly relevant in industrial processes and in nature. Recent studies have shown that counter-cations to these polynuclear anions differentiate rich structural topologies in the solid-state. Herein, we present nine different structures with wheel-shaped [U₇₀ (OH)₃₆ (O)₆₄ (SO₄ )₆₀ ]^4- (U₇₀ ) linked into one- and two-dimensional frameworks with sulfate, divalent transition metals (Cr^II , Fe^II , Co^II , Ni^II ) and U^V . Small-angle X-ray scattering of these phases dissolved in butylamine reveals differing supramolecular assembly of U₇₀ clusters, controlled primarily by sulfates. However, observed trends in transition metal linking guide future design of U₇₀ materials with different topologies. Finally, U₇₀ linking via U^IV -O-U^V -O-U^IV bridges presents a rare example of mixed-oxidation-state uranium oxides without disorder.

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.