A DFT study of uranyl hydroxyl complexes: structure and stability of trimers and tetramers

A DFT study of U(VI) hydroxy complexes was performed with special attention paid to the [(UO₂)₃(OH)₅(H₂O)_4–7]⁺ and [(UO₂)₄(OH)₇(H₂O)_5–8]⁺ species. It was established that the ionicity of the U=O bond increased when moving from [(UO₂)(H₂O)₅]²⁺, [(UO₂)₂(OH)(H₂O)₈]³⁺, [(UO₂)₂(OH)₂(H₂O)₆]²⁺, [(UO₂)₃(OH)₅(H₂O)_4–6]⁺ to [(UO₂)₄(OH)₇(H₂O)_5–8]⁺ species. In both [(UO₂)₃(OH)₅(H₂O)_4–6]⁺ and [(UO₂)₄(OH)₇(H₂O)_5–8]⁺ complexes, the U=O bond was observed to have a range of different lengths which depended on the composition of the first coordination sphere of UO₂ ²⁺. The cyclic structures of trimeric complexes were somewhat more stable than their linear structures, which was probably due to the steric effect.

     

3D Uranyl Organic Frameworks Supported by Rigid Octadentate Carboxylate Ligand: Synthesis,Structure Diversity,and Luminescence Properties

Seven three dimensional (3D) uranyl organic frameworks (UOFs), formulated as [NH₄][(UO₂)₃(HTTDS)(H₂O)] ( 1 ), [(UO₂)₄(HTTDS)₂](HIM)₆ ( 2 , IM=imidazole), [(UO₂)₄(TTDS)(H₂O)₂(Phen)₂] ( 3 , Phen=1,10-phenanthroline), [Zn(H₂O)₄]_0.5[(UO₂)₃(HTTDS)(H₂O)₄] ( 4 ), and {(UO₂)₂[Zn(H₂O)₃]₂(TTDS)} ( 5 ), {Zn(UO₂)₂(H₂O)(Dib)_0.5(HDib)(HTTDS)} ( 6 , Dib=1,4-di(1H-imidazol-1-yl)benzene) and [Na]{(UO₂)₄[Cu₃(u₃-OH)(H₂O)₇](TTDS)₂} ( 7 ) have been hydrothermally prepared using a rigid octadentate carboxylate ligand, tetrakis(3,5-dicarboxyphenyl)silicon(H₈TTDS). These UOFs have different 3D self-assembled structures as a function of co-ligands, structure-directing agents and transition metals. The structure of 1 has an infinite ribbon formed by the UO₇ pentagonal bipyramid bridged by carboxylate groups. With further introduction of auxiliary N-donor ligands, different structure of 2 and 3 are formed, in 2 the imidazole serves as space filler, while in 3 the Phen are bound to [UO₂]²⁺ units as co-ligands. The second metal centers were introduced in the syntheses of 4–7 , and in all cases, they are part of the final structures, either as a counterion ( 4 ) or as a component of framework ( 5 − 7 ). Interesting, in 7 , a rare polyoxometalate [Cu₃(μ₃-OH)O₇(O₂CR)₄] cluster was found in the structure. It acts as an inorganic building unit together with the dimer [(UO₂)₂(O₂CR)₄] unit. Those uranyl carboxylates were sufficiently determined by single crystal X-ray diffraction, and their topological structures and luminescence properties were analyzed in detail.     

Visible‐Light‐Induced Water Oxidation Mediated by a Mononuclear‐Cobalt(II)‐Substituted Silicotungstate

A mononuclear‐cobalt(II)‐substituted silicotungstate, K₁₀[Co(H₂O)₂(γ‐SiW₁₀O₃₅)₂] ? 23 H₂O (POM‐ 1 ), has been evaluated as a light‐driven water‐oxidation catalyst. With in situ photogenerated [Ru(bpy)₃]³⁺ (bpy=2,2′‐bipyridine) as the oxidant, quite high catalytic turnover number (TON; 313), turnover frequency (TOF; 3.2 s^?1), and quantum yield (Φ_QY; 27 %) for oxygen evolution at pH 9.0 were acquired. Comparison experiments with its structural analogues, namely [Ni(H₂O)₂(γ‐SiW₁₀O₃₅)₂]^10? (POM‐ 2 ) and [Mn(H₂O)₂(γ‐SiW₁₀O₃₅)₂]^10? (POM‐ 3 ), gave the conclusion that the cobalt center in POM‐ 1 is the active site. The hydrolytic stability of the title polyoxometalate (POM) was confirmed by extensive experiments, including UV/Vis spectroscopy, linear sweep voltammetry (LSV), and cathodic adsorption stripping analysis (CASA). As the [Ru(bpy)₃]²⁺/visible light/sodium persulfate system was introduced, a POM–photosensitizer complex formed within minutes before visible‐light irradiation. It was demonstrated that this complex functioned as the active species, which remained intact after the oxygen‐evolution reaction. Multiple experimental parameters were investigated and the catalytic activity was also compared with the well‐studied POM‐based water‐oxidation catalysts (i.e., [Co₄(H₂O)₂(α‐PW₉O₃₄)₂]^10? (Co₄‐POM) and [Co^IIICo^II(H₂O)W₁₁O₃₉]^7? (Co₂‐POM)) under optimum conditions.     

Coordination Chemistry of [Nb2(S2)2]4+ Clusters: Preparation and Crystal Structures of K4[Nb2(S2)2(C2O4)4] · 6 H2O and (NH4)6[Nb2(S2)2(C2O4)4](C2O4)

Bis(disulfido)bridged Nb^IV cluster oxalate complexes [Nb₂(S₂)₂(C₂O₄)₄]^4– were prepared by ligand substitution reaction from the aqua ion [Nb₂(μ‐S₂)₂(H₂O)₈]⁴⁺ and isolated as K₄[Nb₂(S₂)₂(C₂O₄)₄] · 6 H₂O ( 1 ), (NH₄)₆[Nb₂(S₂)₂(C₂O₄)₄](C₂O₄) ( 2 ) and Cs₄[Nb₂(S₂)₂(C₂O₄)₄] · 4 H₂O ( 3 ). The crystal structures of 1 and 2 were determined. The crystals of 1 belong to the space group P1, a = 720.94(7) pm, b = 983.64(10) pm, c = 1071.45(10) pm, α = 109.812(1)°, β = 91.586(2)°, γ = 105.257(2)°. The crystals of 2 are monoclinic, space group C2/c, a = 1567.9(2) pm, b = 1906.6(3) pm, c = 3000.9(4) pm, β = 95.502(2)°. The packing in 2 shows alternating layers of cluster anions and of ammonium/uncoordinated oxalates perpendicular to the [1 0 1] direction. Vibration spectra, electrochemistry and thermogravimetric properties of the complexes are also discussed.     

Hydrogen‐bonded network structures in dipyridinium,bis(2‐methylpyridinium), bis(3‐methylpyridinium) and bis(4‐methylpyridinium) dioxidobis(oxydiacetato)uranate(VI)

Four complexes containing the [UO₂(oda)₂]²⁻ anion (oda is oxydiacetate) are reported, namely dipyridinium dioxidobis(oxydiacetato)uranate(VI), (C₅H₆N)₂[U(C₄H₄O₅)₂O₂], (I), bis(2‐methylpyridinium) dioxidobis(oxydiacetato)uranate(VI), (C₈H₈N)₂[U(C₄H₄O₅)₂O₂], (II), bis(3‐methylpyridinium) dioxidobis(oxydiacetato)uranate(VI), (C₈H₈N)₂[U(C₄H₄O₅)₂O₂], (III), and bis(4‐methylpyridinium) dioxidobis(oxydiacetato)uranate(VI), (C₈H₈N)₂[U(C₄H₄O₅)₂O₂], (IV). The anions are achiral and are located on a mirror plane in (I) and on inversion centres in (II)–(IV). The four complexes are assembled into three‐dimensional structures via N—H...O and C—H...O interactions. Compounds (III) and (IV) are isomorphous; the [UO₂(oda)₂]²⁻ anions form a porous matrix which is nearly identical in the two structures, and the cations are located in channels formed in this matrix. Compounds (I) and (II) are very different from (III) and (IV): (I) forms a layered structure, while (II) forms ribbons.     

Raman spectra of zeolites exchanged with uranyl(VI) cations—II. Zeolite X

The formation of hydrolysed uranyl(VI) species in UO₂X zeolites prepared by various methods has been investigated by Raman spectroscopy. Ion-exchange in aqueous (pH>3) and non-aqueous (anhydrous methanol and uranyl nitrate melts) media resulted in the formation of hydroxy-bridged complexes such as [(UO₂)₃(OH)₄]²⁺, [(UO₂)₃(OH)₅]⁺, and [(UO₂)₄(OH)₇]⁺. Ion-exchange in more acidic media (initial pH < 3) was accompanied by the formation of a disordered phase incorporating UO₃, following extensive collapse of the zeolite framework structure. Cation speciation in the UO₂X system is compared to that in UO₂Y zeolites.     

Crystal Structure of the Mixed-Valence Neptunium Compound Na6[(NpVO2)2(NpVIO2)(MoO4)5] · 13H2O

The crystal structure of the mixed-valence Np(V) and Np(VI) compound Na₆[(Np^VO₂)₂(Np^VIO₂)(MoO₄)₅] · 13H₂O was determined. The structure is built of the anionic layers [(Np^VO₂)₂(Np^VIO₂)(MoO₄)₅] ^6n- _n with the Na⁺ cations and crystal water molecules between them. The Np(V) and Np(VI) atoms in the anionic layers are ordered. The motif of the anionic layer is close to that found in Mg₂[(UO₂)₃(SeO₄)₅] · 16H₂O. The isostructural mixed-valence Np(V) and U(VI) compound was also synthesized.     

Acid‐Stable Peroxoniobophosphate Clusters To Make Patterned Films

Two new peroxoniobophosphate clusters were isolated as tetramethylammonium (TMA) salts having the stoichiometries: TMA₅[HNb₄P₂O₁₄(O₂)₄]?9 H₂O and TMA₃[H₇Nb₆P₄O₂₄(O₂)₆]?7 H₂O. The former is stable over the pH range: 3<pH<12 and the latter is stable only below pH 3. These two molecules interconvert as a function of solution pH. The [H₇Nb₆P₄O₂₄(O₂)₆]^3? cluster can be used to fabricate patterned niobium phosphate films by electron‐beam lithography after solution deposition.     

Hydrothermal synthesis and structure of K6[(UO2)4(CrO4)7]·6H2O: A layered uranyl chromate with a new uranyl sheet topology

The layered uranyl chromate, K₆[(UO₂)₄(CrO₄)₇]·6H₂O (1), has been synthesized by reacting UO₃ with K₂Cr₂O₇ under mild hydrothermal conditions. The structure of 1 is formed from UO₂²⁺ cations that are bound by chromate anions to yield a pentagonal bipyramidal geometry around the uranium centers. These polyhedra are bridged by chromate anions to yield one-dimensional _∞¹[UO₂(CrO₄)₂]²⁻ chains. The chains are similar to the _∞¹[UO₂(CrO₄)₂(H₂O)]²⁻ chains found in the previously reported one-dimensional uranyl chromate, K₂[UO₂(CrO₄)₂(H₂O)]·3H₂O, a phase that forms concomitantly with 1. These chains are condensed with the loss of the coordinated water molecules into two-dimensional _∞²[(UO₂)₄(CrO₄)₇]⁶⁻ layers with additional one-dimensional _∞¹[(UO₂)₂(CrO₄)₃]²⁻ chains. These anionic layers form a new type of anionic sheet topology and are charge balanced by both intra- and interlayer K⁺ cations. Crystallographic data (193 K): 1, orthorhombic, space group P2₁2₁2, a=10.9583(5) Å, b=22.582(1) Å, c=7.9552(4) Å, Z=2, MoKα, λ=0.71073, R(F)=1.77% for 268 parameters with 4657 reflections with I>2σ(I).     

Two transition-metal-modified Nb/W mixed-addendum polyoxometalates for visible-light-mediated aerobic benzylic C–H oxidations

The visible-light-induced selective oxidation of ubiquitous C–H bonds into valuable C=O bonds under aerobic conditions is one of the most attractive approaches for the construction of carbonyl-containing molecules. In this work, two transition metal-containing Nb/W mixed-addendum POMs dimers with the formula of K₂Na₂H₅[(Fe(H₂O)₄)₃(P₂W₁₅Nb₃O₆₂)₂]·24H₂O (POM[Fe]) and K₂Na₃H₄[(Cr(H₂O)₄)₃(P₂W₁₅Nb₃O₆₂)₂]·32H₂O (POM[Cr]) have been synthesized and characterized by various analytical and spectral techniques. POM[Fe] was proved to be an efficient photocatalyst for benzylic C–H oxidation under visible light and using oxygen as an oxidant to produce the corresponding carbonyl complex in good yields. A plausible mechanism involving superoxide radical was proposed for the catalytic reaction. POM[Fe] showed good reusability in the recycling experiments. IR spectroscopy and XRD analysis indicate that POM[Fe] can retain its integrity after catalysis.     

Application of capillary electrophoresis to the uranyl/citrate system in moderately acidic solutions

The capillary electrophoresis method has been applied to the speciation study of uranium(VI) at room temperature, in 0.02M citrate buffer solutions, at pH values between 2.5 and 5.5 and at citrate/U ratios between 20 and 40. No negatively charged species have been pointed out at pH values lower than 3. For a pH value higher than 5, the electropherograms are ill-defined and the signals cannot be analyzed simply (owing to a high and rough baseline). In the pH range 3–5, up to 4 peaks can be attributed to U(VI) species. Two of them are likely due to the expected monomer [(UO₂)(Cit)]⁻and dimer [(UO₂)₂(Cit)₂]²⁻ complex species and these species are shown to be in quasi-equilibrium with two other species possessing slightly lower migration velocities, [(UO₂)H(Cit)(OH)]⁻ and [(UO₂)₂H₂(Cit)₂(OH)₂]²⁻, respectively. Speciation diagrams calculated by an exact analytical approach are proposed in order to explain the experimental results. A complete agreement between theoretical and experimental results needs to take into account kinetic and hydrolysis effects.     

Low‐Dimensional Structural Units in Amine‐Templated Uranyl Oxoselenates(VI): Synthesis and Crystal Structures of [C3H12N2][(UO2)(SeO4)2(H2O)2](H2O), [C5H16N2]2[(UO2)(SeO4)2(H2O)](NO3)2, [C4H12N][(UO2)(SeO4)(NO3)], and [C4H14N2][(UO2)(SeO4)2(H2O)]

The crystals of four amine‐templated uranyl oxoselenates(VI), [C₃H₁₂N₂][(UO₂)(SeO₄)₂(H₂O)₂](H₂O) ( 1 ), [C₅H₁₆N₂]₂[(UO₂)(SeO₄)₂(H₂O)](NO₃)₂ ( 2 ), [C₄H₁₂N][(UO₂)(SeO₄)(NO₃)] ( 3 ), and [C₄H₁₄N₂][(UO₂)(SeO₄)₂(H₂O)] ( 4 ) were prepared by evaporation from aqueous solution of uranyl nitrate, selenic acid and the respective amine. The crystal structures of all four compounds have been solved by direct methods from X‐ray diffraction data. The structure of 1 (triclinic, , a = 7.5611(16), b = 7.7650(17), c = 12.925(3) Å, α = 94.605(18), β = 94.405(17), γ = 96.470(17)°, V = 748.8(3) ų, R₁ = 0.029 for 2769 unique observed reflections) is based upon 0D‐units of the composition [(UO₂)₂(SeO₄)₄(H₂O)₄]^4?. These discrete units are composed from two pentagonal [UO₇]^8? bipyramids linked via [SeO₄]^2? tetrahedra and are unknown in structural chemistry of uranium so far. The structure of 2 (monoclinic, C2/c, a = 28.916(5), b = 8.0836(10), c = 11.9856(16) Å, β = 110.909(11)°, V = 2617.1(6) ų, R₁ = 0.035 for 2578 unique observed reflections) contains [(UO₂)(SeO₄)₂(H₂O)]^2? chains of corner‐sharing pentagonal [UO₇]^8? bipyramids and [SeO₄]^2? tetrahedra. The chains run parallel to the c axis and are arranged into layers parallel to (100). In the structure of 3 (monoclinic, C2/m, a = 21.244(5), b = 7.1092(11), c = 8.6581(18) Å, β = 97.693(17)°, V = 1295.8(4) ų, R₁ = 0.027 for 1386 unique observed reflections), pentagonal [UO₇]^8? bipyramids share corners with three [SeO₄]^2? tetrahedra each and an edge with a [NO₃]^? anion to form [(UO₂)(SeO₄)(NO₃)]^? chains parallel to the b axis. In the structure of 4 (triclinic, , a = 6.853(2), b = 10.537(3), c = 10.574(3) Å, α = 99.62(3), β = 94.45(3), γ = 100.52(3)°, V = 735.6(4) ų, R₁ = 0.045 for 2713 unique observed reflections), one symmetrically independent pentagonal [UO₇]^8? bipyramid shares corners with four [SeO₄]^2? tetrahedra to form the [(UO₂)(SeO₄)₂(H₂O)]^2? chains parallel to the a axis. A comparison to related uranyl compounds is given.     

[H4AsW18O62]7−, A novel Dawson heteropolyanion and two of its sandwich-type derivatives [Zn4(H2O)2(H4AsW15O56)2]18−, [Cu4(H2O)2(H4AsW15O56)2]18−: cyclic voltammetry and electrocatalytic properties towards nitrite and nitrate

The cyclic voltammetry behaviours of the new Dawson heteropolyanion (HPA) octadecatungstomonoarsenate [H₄AsW₁₈O₆₂]⁷⁻ were studied as a function of pH. The electrochemistry of the fully symmetrical octadecatungstodiarsenate [As₂W₁₈O₆₂]⁶⁻ in a pH 0.3 medium is constituted mainly by four reduction steps, the first two featuring reversible one-electron processes. The comparison of the cyclic voltammetry behaviours of two complexes reveals, interestingly, that the merging of the first two waves of the monoarsenate occurs in a less acidic pH medium than for the diarsenate. The voltammograms of the two corresponding trivacant species, [H₄AsW₁₅□₃O₅₆]¹³⁻ and [As₂W₁₅□₃O₅₆]¹²⁻, in which the symbol □ represents the vacancy, were also studied and compared. The comparison was extended to the Zn- and Cu-sandwich-type derivatives, with particular emphasis on the compounds of the monoarsenate series, [Zn₄(H₂O)₂(H₄AsW₁₅O₅₆)₂]¹⁸⁻ and [Cu₄(H₂O)₂(H₄AsW₁₅O₅₆)₂]¹⁸⁻. All these species prove efficient in the electrocatalytic reduction of nitrite, but only copper-substituted derivatives are active for the electrocatalytic reduction of nitrate. To our knowledge, this observation constitutes the first example of electrocatalysis of nitrate reduction by a sandwich complex of the polyoxometalate family.     

[Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3], crystal structure and comparison with uranium minerals with U3O8-type sheets

The new U(VI) compound, [Ni(H₂O)₄]₃[U(OH,H₂O)(UO₂)₈O₁₂(OH)₃], was synthesized by mild hydrothermal reaction of uranyl and nickel nitrates. The crystal-structure was solved in the P-1 space group, a=8.627(2), b=10.566(2), c=12.091(4) Å and α=110.59(1), β=102.96(2), γ=105.50(1)°, R=0.0539 and wR=0.0464 from 3441 unique observed reflections and 151 parameters. The structure of the title compound is built from sheets of uranium polyhedra closely related to that in β-U₃O₈. Within the sheets [(UO₂)(OH)O₄] pentagonal bipyramids share equatorial edges to form chains, which are cross-linked by [(UO₂)O₄] and [UO₄(H₂O)(OH)] square bipyramids and through hydroxyl groups shared between [(UO₂)(OH)O₄] pentagonal bipyramids. The sheets are pillared by sharing the apical oxygen atoms of the [(UO₂)(OH)O₄] pentagonal bipyramids with the oxygen atoms of [NiO₂(H₂O)₄] octahedral units. That builds a three-dimensional framework with water molecules pointing towards the channels. On heating [Ni(H₂O)₄]₃[U(OH,H₂O)(UO₂)₈O₁₂(OH)₃] decomposes into NiU₃O₁₀.     

New developments in the chemistry of heteropolytungstates of rhodium and cerium

The heteropolytungstates [PW₁₁O₃₉RhCl]⁵⁻ (I), [PW₁₁O₃₉RhCH₂COOH]⁵⁻ (III), and [PW₁₁O₃₉Rh₂(OAc)₂]⁵⁻ (IV) are prepared by hydrothermal reaction of [PW₁₁O₃₉]⁷⁻ with RhCl₃ or Rh₂(OAc)₄ and characterized by NMR and X-ray structure determination. Electrolytic reduction of I yields the dimeric species [(PW₁₁O₃₉Rh₂)]¹⁰⁻ (II) which reacts with C₆H₅CH₂Br under photochemical conditions to yield dibenzyl and [PW₁₁O₃₉RhBr]⁵⁻ in a process analogous to Rh-porphyrins. Anion III and the corresponding Sicentered anion are rare examples of complexes with Rh-C bonds prepared in aqueous solution. Reaction of B-α [As^IIIW₉O₃₃]⁹⁻ with Ln^III yields new, massive heteropolytungstates [As₂Ln₂(H₂O)W₂₉O₁₀₃]¹⁷⁻ Ln = La, Ce(V), [Ce₄(H₂O)₄ − 4x(AsW₉O₃₃)₄As(WO₃)_{2 + x}(WO₅)]²⁵⁻, x < 0.5 (VI), and As₁₂Ln₁₆(H₂O)₃₆W₁₈₄O₃₂₄]⁷⁶⁻; Ln La,Ce (VII) with approximate relative molecular masses of 7 600, 10 500, and 40 000 respectively. Anion VII has a folded cyclic structure with a radius of 4 nm and is the largest heteropolytungstate so far reported. It is synthesized in yields of ca 30 % under mild conditions.     

A new adamantanecarboxylate coordination polymer: poly[[(μ3‐adamantane‐1,3‐dicarboxylato)aquadioxidouranium(VI)] monohydrate]

The title compound, {[U(C₁₂H₁₄O₄)O₂(H₂O)]·H₂O}_n, is the first actinide complex featuring adamantanecarboxylate ligands. The metal ion possesses a pentagonal–bipyramidal UO₇ coordination involving two axial oxide ligands [U—O = 1.732 (5) and 1.764 (5) Å] and five equatorial O atoms [U—O = 2.259 (5)–2.494 (4) Å] of aqua and carboxylate ligands. The latter display pseudo‐chelating and bridging coordination modes of the carboxylate groups that are responsible for the generation of the centrosymmetric discrete uranium–carboxylate [UO₂(μ‐RCOO)₂UO₂] dimers [U...U = 5.5130 (5) Å] and their connection into one‐dimensional chains. Hydrogen bonding involving two coordinated and two solvent water molecules [O...O = 2.719 (7)–2.872 (7) Å] yields centrosymmetric (H₂O)₄ ensembles and provides noncovalent linkage between the coordination chains to generate a three‐dimensional network structure.     

Solubility of (UO2)3(PO4)2?4H2O in H+-Na+-OH?-H2PO?4-HPO2?4-PO3?4-H2O and Its Comparison to the Analogous PuO2 +2 System

The objectives of this study were to address uncertainties in the solubility product of (UO₂)₃(PO₄)₂⋅4H₂O(c) and in the phosphate complexes of U(VI), and more importantly to develop needed thermodynamic data for the Pu(VI)-phosphate system in order to ascertain the extent to which U(VI) and Pu(VI) behave in an analogous fashion. Thus studies were conducted on (UO₂)₃(PO₄)₂⋅4H₂O(c) and (PuO₂)₃(PO₄)₂⋅4H₂O(am) solubilities for long-equilibration periods (up to 870 days) in a wide range of pH values (2.5 to 10.5) at fixed phosphate concentrations of 0.001 and 0.01 M, and in a range of phosphate concentrations (0.0001–1.0 M) at fixed pH values of about 3.5. A combination of techniques (XRD, DTA/TG, XAS, and thermodynamic analyses) was used to characterize the reaction products. The U(VI)-phosphate data for the most part agree closely with thermodynamic data presented in Guillaumont et al.,⁽¹⁾ although we cannot verify the existence of several U(VI) hydrolyses and phosphate species and we find the reported value for formation constant of UO₂PO⁻₄ is in error by more than two orders of magnitude. A comprehensive thermodynamic model for (PuO₂)₃(PO₄)₂⋅4H₂O(am) solubility in the H⁺-Na⁺-OH⁻-Cl⁻-H₂PO⁻₄-HPO²⁻₄-PO³⁻₄-H₂O system, previously unavailable, is presented and the data shows that the U(VI)-phosphate system is an excellent analog for the Pu(VI)-phosphate system.     

Synthesis and Crystal Structures of α‐ and β‐Mg2[(UO2)3(SeO4)5](H2O)16

Single crystals of α‐ and β‐Mg₂[(UO₂)₃(SeO₄)₅](H₂O)₁₆ have been synthesized by evaporation from an aqueous solution of the ionic components. The structure of α‐Mg₂[(UO₂)₃(SeO₄)₅](H₂O)₁₆ (monoclinic, C2/c, a = 19.544(3), b = 10.4783(11), c = 18.020(3) Å, β = 91.352(12)°, V = 3689.3(9) ų) has been solved by direct methods and refined to R₁ = 0.048 on the basis of 4338 unique observed reflections. The structure of β‐Mg₂[(UO₂)₃(SeO₄)₅](H₂O)₁₆ (orthorhombic, Pbcm, a = 10.3807(7), b = 22.2341(19), c = 33.739(5) Å, V = 7787.2(14) ų) has been solved by direct methods and refined to R₁ = 0.107 on the basis of 3621 unique observed reflections. The structures of α‐ and β‐Mg₂[(UO₂)₃(SeO₄)₅](H₂O)₁₆ are based upon sheets with the chemical composition [(UO₂)₃(SeO₄)₅]^4‐. The sheets are formed by corner sharing between pentagonal bipyramids [UO₇]^8‐ and SeO₄^2‐ tetrahedra. In the α‐modification, the [(UO₂)₃(SeO₄)₅]^4‐ sheets are more or less planar and run parallel to (001). In the structure of the β‐modification, the uranyl selenate sheets are strongly corrugated and oriented parallel to (010). The [Mg(H₂O)₆]²⁺ polyhedra reside in the interlayers and provide three‐dimensional linkage of the uranyl selenate sheets via hydrogen bonding. In addition to H₂O groups attached to Mg²⁺ cations, both structures also contain H₂O molecules that are not bonded to any cation. The [(UO₂)₃(SeO₄)₅]^4‐ sheets in the structures of α‐ and β‐Mg₂[(UO₂)₃(SeO₄)₅](H₂O)₁₆ represent two different structural isomers. The sequences of the orientations of the tetrahedra within the sheets can be described by their orientational matrices with their shortened forms ( ddudd □ /uu □ uud ) and ( dd □ dd □ uu □ uu □ /uuduumdduddm ) for α‐ and β‐Mg₂[(UO₂)₃(SeO₄)₅](H₂O)₁₆, respectively. A short review on the isomerism of [(UO₂)₃(TO₄)₅]^4‐ sheets (T = S, Cr, Se, Mo) is given.     

Two 2D Polyoxometalate-Based Metal-Organic Complexes Constructed from a New Flexible N-Donor Ligand with High Capacitance Performances and Low Detection Limits of Cr(VI)

By designing and using a new flexible bis(pyrimidine)-bis(amide) ligand H₂L [H₂L=N,N′-bis(4-pyrimidinecarboxamido)-1,3-propane], two new polyoxometalate (POM)-based metal-organic complexes (MOCs), H₃[Cu₂(L)₂(PMo₁₂O₄₀)] ( 1 ) and [Cu₂(H₂L)₂(β-Mo₈O₂₆)] ( 2 ), were synthesized under solvothermal and hydrothermal conditions, respectively. In complexes 1 and 2 , metal-organic units and POM anions are linked together to form two distinct 2D structures. The [PMo₁₂O₄₀]³⁻ (PMo₁₂) anions were used as μ₄-bridging ligands in complex 1 and linked the 1D [Cu(L)]_n metal-organic chains to form a 2D layered structure. The [β-Mo₈O₂₆]⁴⁻ (Mo₈) anions adopted two diverse coordination modes in complex 2 and connected the 1D [Cu(H₂L)]_n²ⁿ⁺ metal-organic chains to generate a 2D grid structure. Complexes 1 – 2 can serve as electrode materials of supercapacitor and show large specific capacitances, up to 1065 and 956 F g⁻¹ at current density of 2 A g⁻¹, respectively, which surpass the parent POM and most of the previous reported POM-based electrode materials, thus demonstrating the important role of introducing metal-organic units in improving capacitive performances. Besides, the electrocatalytic redox activities of complexes 1 – 2 were also studied, both of them can be used as electrochemical sensors to detect Cr(VI) ions. They possess high sensitivity of 0.537 and 0.455 μA μM⁻¹ and low detection limits of 0.16 and 0.33 μM, which are below the maximum content of Cr(VI) in groundwater required by the WHO.