Actinide Polyoxometalates: Incorporation of Uranyl-Peroxo in U-Shaped 36-Tungsto-8-Phosphate

Organoarsonates Enable Single-Site Condensation of Hexalacunary {P2W12} Polyoxotungstates

A systematic study of the condensation reactions of arylarsonic-functionalized [α-P2W12O48]14- units in acidic aqueous media identified that the specific presence of an amino group in the ortho position of the phenyl rings induces a dimerization process that allowed isolation of discrete dimeric polyanions [(o-H2N-C6H4-AsO3)4P4W24O85]14- (1) with an unprecedented polyoxometalate skeleton characterized by two seminal {P2W12} groups joined via a single W-O-W bridge. At the same time, addition of divalent transition metal ions (MnII, CoII, and NiII) in the reaction mixture directed a condensation process on a completely different pathway resulting in one-dimensional (1D) coordination polymers based on V-shaped [{M(H2O)4}P4W24O92(C6H6AsNO)2]14- polyanions (M = MnII (2), CoII (3), and NiII (4)). All polyanions were isolated as hydrated mixed potassium/dimethylammonium salts and thoroughly characterized in the solid state. 31P NMR studies showed that the discrete cluster 1 is comparatively stable in 1 M LiCl aqueous solution and thus represents a potential precursor for subsequent reactions.

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.

Hexalacunary [α-H2 P2 W12 O48 ]12- Wells-Dawson Anion: X-ray Crystal Structural Evidence and Oligomerization to WO(OH2 )4+ -Bridged Dimer and Trimers

We report the first single-crystal X-ray structural evidence of the potassium salt of the hexalacunary [α-H2 P2 W12 O48 ]12- (abbreviated as {P2 W12 }) anion after its discovery by Contant and Ciabrini in 1977. In addition, we observed oligomerization of {P2 W12 } into a {WO(OH2 )}4+ -bridged Pacman-shaped [{WO(OH2 )}(α-HP2 W12 O48 )2 ]22- ({P4 W25 }) dimer and a cyclic [{WO(OH2 )}3 (P2 W12 O48 )3 ]30- ({P6 W39 }) trimer. The three phosphotungstate anions were synthesized through recrystallization of (NH4 )12 [α-H2 P2 W12 O48 ] from slightly alkaline (HOCH2 )3 CNH2 /KCl, CH3 NH3 Cl/KCl, and CH3 NH3 Cl/NH4 Cl solutions. The structure of {P2 W12 } is derived from [α-P2 W18 O62 ]6- that has six tungsten atoms one from each polar group and four from the belt-removed, and the center of the lacunary site is capped by a potassium cation. Structures of {P4 W25 } and {P6 W39 } are constructed by connecting two and three {P2 W12 } units with {WO(OH2 )}4+ , respectively. The isolation of a pure {P6 W39 } phosphotungstate framework without coordination with transition metal cations is unprecedented. Powder X-ray diffraction confirmed the bulk purity of these compounds, indicating that selective crystallization was achieved through the selection of countercations and pH.

Synthesis and oxidation catalysis of a difluoride-incorporated polyoxovanadate and isolation of active vanadium alkylperoxo species

Haloperoxidase-mimetic peroxo-vanadium species on an inorganic support showed catalytic reactivity for the epoxidation and bromination of alkenes. The structures of both native and peroxo forms were determined via single-crystal X-ray analysis.

Sandwich-Type Uranyl Phosphate-Polyoxometalate Cluster Exhibiting Strong Luminescence

A pure inorganic uranyl phosphate-polyoxometalate of Na₁₇{Na@[(SbW₉O₃₃)₂(UO₂)₆(PO₃OH)₆]}·xH₂O (abbreviated as Na@U₆P₆, with x ≈ 46) featuring a sandwich-type structure was prepared using Keggin-type trilacunary [α-B-SbW₉O₃₃]^9- units as building blocks, which were formed in situ by SbCl₃ and Na₂WO₄·2H₂O. Crystal structural analysis showed that six UO₂²⁺ cations and six PO₃OH^2- anions generated a wheel-like cluster unit with a Na⁺ center ([Na@(UO₂)₆(PO₃OH)₆]⁺) that is stabilized by two [α-B-SbW₉O₃₃]^9- units. Na@U₆P₆ displayed a solid-state photoluminescence quantum yield of 33% at 300 K. The temperature-dependent fluorescence emission spectra showed that Na@U₆P₆ has temperature-sensitive fluorescence in which its emission intensity decreased by 77% as the temperature increased from 200 to 300 K. These results suggest that such uranyl phosphate-polyoxometalate clusters could serve as potential temperature-sensitive molecular materials.

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.

Peroxouranyl-Containing W48 Wheel: Synthesis, Structure, and Detailed Infrared and Raman Spectroscopy Study

We report on the first example of a peroxouranium-containing {P₈W₄₈} wheel, [{(UO₂)₄(O₂)₄}₂(P₈W₄₈O₁₈₄)]^40- (1), which was synthesized by a one-pot reaction of UO₂(NO₃)₂·6H₂O with the 48-tungsto-8-phosphate wheel [H₇P₈W₄₈O₁₈₄]^33- and aqueous hydrogen peroxide in a pH 6 lithium acetate solution at 50 °C. Polyanion 1 comprises two tetrauranyl squares with side-on peroxo bridging ligands in the cavity of the {P₈W₄₈} wheel, and was isolated as the hydrated potassium-lithium salt K₁₈Li₂₂[{(UO₂)₄(O₂)₄}₂(P₈W₄₈O₁₈₄)]·133H₂O (KLi-1), which was characterized in the solid state by single-crystal X-ray diffraction, as well as thermogravimetric and elemental analyses. A detailed Fourier transform infrared and Raman spectroscopy study was also performed.

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.

The homometallic polyoxotungstate archetype {P4W24}

Integration of phenylphosphonate and phenylarsonate groups render the {P₂W₁₂}-based polyoxotungstate {P₄W₂₄} accessible, yet still sufficiently reactive for consecutive functionalization steps.

Dimethyltin-functionalized cyclic selenotungstates based on {Se2W12} units

Two dimethyltin-functionalized cyclic selenotungstates based on {Se₂W₁₂} units were obtained, where (CH₃)₂Sn²⁺ moieties were first introduced into “pure” POMs wheels.

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.

Microwave-assisted synthesis of organically functionalized hexa-molybdovanadates

The microwave-assisted reaction of (TBA)₄[β-Mo₈O₂₄] and (TBA)₃[H₃V₁₀O₂₈] with pentaerythritol or tris(hydroxymethyl)aminomethane yields polyanions with the general formula (TBA)₂[V₃Mo₃O₁₆(O₃–R)] (R: C₅H₈OH – 1; R: C₄H₆NH₂ – 3). Post-synthetic esterification of 1 yields the acylated derivative 2, with all compounds being characterized in the solid and solution state.

Discovery of Heteropolytantalate: Synthesis and Structure of Two 6-Peroxotantalo-4-phosphate Clusters

Polyoxometalates (POMs) of Nb and Ta are greatly different from those of Mo, W, and V that have been studied extensively and developed well. The latter can be formed simply by acidification of their aqueous monomeric oxoanions and has found application areas from catalysis to magnetism, materials science, medicine, and nanotechnology. Even now the polyoxoniobate (PONb) chemistry has accelerated dramatically over the last 15 years, and a vast expansion of available PONbs has been reported. However, after nearly 200 years of POM research, Ta-based POM chemistry is still at its infant stage and only dominated by the isopolyoxotantalate ions (Ta₆ and Ta₁₀) and transition-metal-capped Ta₆ species, along with two Ti-substituted polyoxotantalates [Ti₂Ta₈O₂₈]^8- and [Ti₁₂Ta₆O₄₄]^10- reported very recently. In this study, we discover two novel peroxotantalophosphate clusters [P₄(TaO₂)₆O₂₅]^12- (1) and [P₄(TaO₂)₆O₂₄]^10- (2) by incorporating phosphorus heteroatom into Ta-oxo framework, which represent the first two examples of heteropolytantalate. Interestingly, two P₂Ta₃ half-units are cis- and trans-condensed in 1 and 2, leading to "open" and "closed" configurations, respectively. These two chemically and structurally related clusters can be isolated in a controlled manner, and the yields are relatively high. Both compounds were characterized in the solid state by single-crystal X-ray diffraction, ³¹P MAS NMR, FT-IR, TGA, and elemental analysis as well as by ³¹P NMR in solution. The results presented here provide a strategy to be applicable to other heteroatom-incorporated polyoxotantalates and further expand the phase space for polyoxotantalate chemistry.

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.

Structure and Solution Speciation of U(IV) Linked Phosphomolybdate (Mo(V)) Clusters

Crystals of [NaU(Mo6P4O31H7)2]·5Na·(H2O)n (NaUMo6) have been synthesized by slow evaporation of an aqueous mixture containing uranyl nitrate, sodium molybdate, phosphoric acid, and sodium dithionate. Single crystal diffraction of NaUMo6 reveals the assembly of {Mo6P4} clusters linked into one-dimensional chains with alternating Na(+) and U(4+) cations. To our knowledge, NaUMo6 is a unique example of Mo(5+) based polyoxometalate associated with actinides. With the use of similar synthesis conditions but without uranium in the aqueous solution, [Na(Mo6P4O31H10)2]·5Na·(H2PO4)·(H2O)n (NaMo6) is obtained. NaMo6 is a sandwich type cluster which is built on the assemblage of two {Mo6P4} units linked by one sodium cation. Using small-angle X-ray scattering techniques and aqueous electrolyte based dissolution strategies, we can accurately observe chains of [UNa(Mo6P4O31H7)2]n(5n), where n = 7 is the dominant soluble specie. Likewise, the dimeric form of [Na(Mo6P4O31H10)2](5-) dominates the aqueous solution, revealing the structural units observed in the crystal structure are also stable in solution, under appropriate dissolution conditions.

Two new members of the niobium-substituted polytungstophosphate family based on hexalacunary [H2P2W12O48]12− building blocks

Herein we present two structurally novel nanoscale clusters [{Nb₆(O₂)₄P₂W₁₂O₅₇}₂]²⁰⁻ (1) and [{P₂W₁₂Nb₇O₆₃(H₂O)₂}₄{Nb₄O₄(OH)₆}]³⁰⁻ (2) formed by controlling the reaction parameters.

Assembly of tetrameric dimethyltin-functionalized selenotungstates: from nanoclusters to one-dimensional chains

The first dimethyltin-functionalized selenotungstates have been isolated: nanocluster {Sn₇Se₈W₅₄} exhibiting photocatalytic H₂ evolution activity and chain {Sn₁₆Se₁₁W₅₆} contain {β-Se₂W₁₅} and {γ-Se₂W₁₄} building blocks, respectively.

Synthesis and oxidation catalysis of a Ti-substituted phosphotungstate, and identification of the active oxygen species

A Ti-substituted phosphotungstate (I) showed high catalytic performance for several oxidation reactions. The truly catalytically active species was successfully isolated, and its anion structure was determined.

Synthesis, crystal structure and photocatalytic properties of an unprecedented arsenic-disubstituted Lindqvist-type peroxopolyoxoniobate ion: {As2Nb4(O2)4O14H1.5}4.5−

An unprecedented arsenic-disubstituted Lindqvist-type ion {As₂Nb₄(O₂)₄O₁₄H_1.5}^4.5− which shows a certain photocatalytic water splitting activity has been successfully synthesized and characterized.

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.

Uranyl peroxide pyrophosphate cage clusters with oxalate and nitrate bridges

Two complex cage clusters built from uranyl hexagonal bipyramids and multiple types of bridges between uranyl ions, U(30)Py(10)Ox(5) and U(38)Py(10)Nt(4), were crystallized from aqueous solution under ambient conditions. These are built from 30 uranyl hexagonal bipyramids, 10 pyrophosphate groups, and five oxalate bridges in one case, and 38 uranyl hexagonal bipyramids, 10 pyrophosphate groups, and four nitrate groups in the other. The crystal compositions are (H(3)O)(10)Li(18)K(22)[(UO(2))(30)(O(2))(30)(P(2)O(7))(10)(C(2)O(4))(5)](H(2)O)(22) and Li(24)K(36)[(UO(2))(38)(O(2))(40)(OH)(8)(P(2)O(7))(10)(NO(3))(4)](NO(3))(4)(H(2)O)(n) for U(30)Py(10)Ox(5) and U(38)Py(10)Nt(4), respectively. Cluster U(30)Py(10)Ox(5) crystallizes over a narrow range of solution pH that encourages incorporation of both oxalate and pyrophosphate, with incorporation of oxalate only being favored under more acidic conditions, and pyrophosphate only under more alkaline conditions. Cluster U(38)Py(10)Nt(4) contains two identical lobes consisting of uranyl polyhedra and pyrophosphate groups, with these lobes linked into the larger cluster through four nitrate groups. The synthesis conditions appear to have prevented closure of these lobes, and a relatively high nitrate concentration in solution favored formation of the larger cluster.

A comprehensive comparison of transition-metal and actinyl polyoxometalates

While the d(0) transition-metal POMs of Group V (V(5+), Nb(5+), Ta(5+)) and Group VI (Mo(6+), W(6+)) have been known for more than a century, the actinyl peroxide POMs, specifically those built of uranyl triperoxide or uranyl dihydroxidediperoxide polyhedra, were only realized within the last decade. While virtually every metal on the Periodic Table can form discrete clusters of some type, the actinyls are the only-in addition to the transition-metal POMs- whose chemistry is dictated by the prevalence of the 'yl' oxygen ligand. Thus this emerging structural, solution, and computational chemistry of actinide POMs warrants comparison to the mature chemistry of transition-metal POMs. This assessment between the transition-metal POMs and actinyl POMs (uranyl peroxide POMs, specifically) has provided much insight to the similarities and differences between these two chemistries. We further break down the comparison between the alkaline POMs of Nb and Ta; and the acidic POMs of V, Mo and W. This more indepth literature review and discussion reveals that while an initial evaluation suggests the actinyl POMs are more akin to the alkaline transition-metal POMs, they actually share characteristics unique to the acidic POMs as well. This tutorial review is meant to provide fodder for deriving new POM chemistries of both the familiar transition-metals and the emerging actinides, as well as fostering communication and collaboration between the two scientific communities.