Uranium(VI) sequestration by polyacrylic and fulvic acids in aqueous solution

Stability data on the formation of dioxouranium(VI) species with polyacrylic (PAA) and fulvic acids (FA) are reported with the aim to define quantitatively the sequestering capacity of these high molecular weight synthetic and naturally occurring ligands toward uranium(VI), in aqueous solution. Investigations were carried out at t = 25 °C in NaCl medium at different ionic strengths and in absence of supporting electrolyte for uranyl–fulvate ( \textUO₂²⁺ {{\text{UO}}_{2}}^{2+} –FA) and uranyl–polyacrylate ( \textUO ₂ ²⁺ {{\text{UO}}_{ 2}}^{ 2+ } –PAA, PAA MW 2 kDa) systems, respectively. The experimental data are consistent with the following speciation models for the two systems investigated: (i) UO₂(FA₁), UO₂(FA₁)(FA₂), UO₂(FA₁)(FA₂)(H) for \textUO ₂ ²⁺ {{\text{UO}}_{ 2}}^{ 2+ } –fulvate (where FA₁ and FA₂ represent the carboxylic and phenolic fractions, respectively, both present in the structure of FA), and (ii) UO₂(PAA), UO₂(PAA)(OH), (UO₂)₂(PAA)(OH)₂ for \textUO ₂ ²⁺ {{\text{UO}}_{ 2}}^{ 2+ } –polyacrylate. By using the stability data obtained for all the complex species formed, the uranium(VI) sequestration by PAA and FA was expressed by the pL₅₀ parameter [i.e. the −log(total ligand concentration) necessary to bind 50% of uranyl ion] at different pH values. A comparison between pL₅₀ values of FA and PAA and some low molecular weight carboxylic ligands toward uranyl ion is also given.     

Preparation and characterization of uranyl complexes with phosphonate ligands

The preparation, spectroscopic characterization and thermal stability of neutral complexes of uranyl ion, UO₂ ²⁺, with phosphonate ligands, such as diphenylphosphonic acid (DPhP), diphenyl phosphate (DPhPO) and phenylphosphonic acid (PhP) are described. The complexes were prepared by a reaction of hydrated uranyl nitrate with appropriate ligands in methanolic solution. The ligands studied and their uranyl complexes were characterized using thermogravimetric and elemental analyses, ESI-MS, IR and UV–Vis absorption and luminescence spectroscopy as well as luminescence lifetime measurements. Compositions of the products obtained dependent on the ligands used: DPhP and DPhPO form UO₂L₂ type of complexes, whereas PhP forms UO₂L complex. Based on TG and DTG curves a thermal stability of the complexes was determined. The complexes UO₂PhP·2H₂O and UO₂(DPhPO)₂ undergo one-step decomposition, while UO₂PhP · 2H₂O is decomposed in a two-step process. The thermal stability of anhydrous uranyl complexes increases in the series: DPhPO < PhP < DPhP. Obtained IR spectra indicate bonding of P–OH groups with uranyl ion. The main fluorescence emission bands and the lifetimes of these complexes were determined. The complex of DPhP shows a green uranyl luminescence, while the uranyl emission of the UO₂PhP and UO₂(DPhPO)₂ complexes is considerably weaker.     

Modulation of the unpaired spin localization in Pentavalent Uranyl Complexes

The electronic structure of various complexes of pentavalent uranyl species, namely UO₂⁺, is described, using DFT methods, with the aim of understanding how the structure of the ligands may influence the localisation of the unpaired 5f electron of uranium (V) and, finally, the stability of such complexes towards oxidation. Six complexes have been inspected: [UO₂py₅]⁺ (1), [(UO₂py₅)KI₂] (2), [UO₂(salan-^tBu₂)(py)K] (3), [UO₂(salophen-^tBu₂)(thf)K] (4), [UO₂(salen-^tBu₂)(py)K] (5), [and UO₂-cyclo[6]pyrrole]^1? (6), chosen to explore various ligands. In the five first complexes, the UO₂⁺ species is well identified with the unpaired electron localized on the 5f uranium orbital. Additionally, for the salan, salen and salophen ligands, some covalent interactions have been observed, resulting from the presence of both donor and acceptor binding sites. In contrast, the last complex is best described by a UO₂²⁺ uranyl (VI) coordinated by the anionic radical cyclopyrrole, the highly delocalized π orbitals set stabilizing the radical behaviour of this ligand.     

The Effect of Metal Cations on the Aqueous Behavior of Dopamine. Thermodynamic Investigation of the Binary and Ternary Interactions with Cd2+, Cu2+ and UO22+ in NaCl at Different Ionic Strengths and Temperatures

The interactions of dopamine [2-(3,4-Dihydroxyphenyl)ethylamine, (Dop⁻)] with cadmium(II), copper(II) and uranyl(VI) were studied in NaCl(aq) at different ionic strengths (0 ≤ I/mol dm⁻³ ≤ 1.0) and temperatures (288.15 ≤ T/K ≤ 318.15). From the elaboration of the experimental data, it was found that the speciation models are featured by species of different stoichiometry and stability. In particular for cadmium, the formation of only MLH, ML and ML₂ (M = Cd²⁺; L = dopamine) species was obtained. For uranyl(VI) (UO₂²⁺), the speciation scheme is influenced by the use of UO₂(acetate)₂ salt as a chemical; in this case, the formation of ML₂, MLOH and the ternary MLAc (Ac = acetate) species in a wide pH range was observed. The most complex speciation model was obtained for the interaction of Cu²⁺ with dopamine; in this case we observed the formation of the following species: ML₂, M₂L, M₂L₂, M₂L₂(OH)₂, M₂LOH and ML₂OH. These speciation models were determined at each ionic strength and temperature investigated. As a further contribution to this kind of investigation, the ternary interactions of dopamine with UO₂²⁺/Cd²⁺ and UO₂²⁺/Cu²⁺ were investigated at I = 0.15 mol dm⁻³ and T = 298.15K. These systems have different speciation models, with the MM’L and M₂M’L₂OH [M = UO₂²⁺; M’ = Cd²⁺ or Cu²⁺, L = dopamine] common species; the species of the mixed Cd²⁺ containing system have a higher stability with respect the Cu²⁺ containing one. The dependence on the ionic strength of complex formation constants was modelled by using both an extended Debye–Hückel equation that included the Van’t Hoff term for the calculation of the formation enthalpy change values and the Specific Ion Interaction Theory (SIT). The results highlighted that, in general, the entropy is the driving force of the process. The quantification of the effective sequestering ability of dopamine towards the studied cations was evaluated by using a Boltzmann-type equation and the calculation of pL_0.5 parameter. The sequestering ability was quantified at different ionic strengths, temperatures and pHs, and this resulted, in general, that the pL_0.5 trend was always: UO₂²⁺ > Cu²⁺ > Cd²⁺.     

Phosphate-Rich Biomimetic Peptides Shed Light on High-Affinity Hyperphosphorylated Uranyl Binding Sites in Phosphoproteins

Some phosphoproteins such as osteopontin (OPN) have been identified as high-affinity uranyl targets. However, the binding sites required for interaction with uranyl and therefore involved in its toxicity have not been identified in the whole protein. The biomimetic approach proposed here aimed to decipher the nature of these sites and should help to understand the role of the multiple phosphorylations in UO₂²⁺ binding. Two hyperphosphorylated cyclic peptides, pS₁₆₈ and pS₁₃₆₈ containing up to four phosphoserine (pSer) residues over the ten amino acids present in the sequences, were synthesized with all reactions performed in the solid phase, including post-phosphorylation. These β-sheet-structured peptides present four coordinating residues from four amino acid side chains pointing to the metal ion, either three pSer and one glutamate in pS₁₆₈ or four pSer in pS₁₃₆₈ . Significantly, increasing the number of pSer residues up to four in the cyclodecapeptide scaffolds produced molecules with an affinity constant for UO₂²⁺ that is as large as that reported for osteopontin at physiological pH. The phosphate-rich pS₁₃₆₈ can thus be considered a relevant model of UO₂²⁺ coordination in this intrinsically disordered protein, which wraps around the metal ion to gather four phosphate groups in the UO₂²⁺ coordination sphere. These model hyperphosphorylated peptides are highly selective for UO₂²⁺ with respect to endogenous Ca²⁺, which makes them good starting structures for selective UO₂²⁺ complexation.     

Aqueous solution chemistry of alkyltin(IV) compounds for speciation studies in biological fluids and natural waters

Organotin(IV) cations behave as Lewis acids of different strength depending on the charge, according to the following acidity scale: RSn³⁺ > R₂Sn²⁺ > R₃Sn⁺. For this reason they can react with Lewis bases containing –O, –N, –S donor groups to form complex species of different stability. Complex formation of organotin(IV) moieties with a great number of inorganic and organic ligands in aqueous solution is reviewed here in the light of their environmental and biological impact. To this end, complex species formation was considered in different ionic media and at different ionic strengths, with reference to the composition of natural waters and biological fluids. In particular, the interaction of alkyltin(IV) compounds with the following ligands was taken into account: hydroxo, chloride, sulfate, fluoride, carbonate and phosphate; carboxylates, amines, amino-carboxylates, nucleotides, saccharides, S-containing ligands and antibiotics. Moreover, the interaction of organotin(IV) cations with synthetic (polyacrylate) and natural occurring (fulvic and alginic acids) polyelectrolytes was also considered. The strength of interaction is reported in terms of stability constants of complex species formed and of other thermodynamic parameters, such as formation enthalpy. The stability trend of the complexes is alkyltin(IV)-S > alkyltin(IV)-N > alkyltin(IV)-O-donor ligands. On the basis of data in the literature, empirical relationships are provided to predict the stability of alkyltin(IV) species with some classes of ligands. The complexation models proposed by the different authors for the species formation of mono-, di- and tri-alkyltin(IV) in the presence of various ligands were considered in the light of defining the speciation picture of this class of compounds in aquatic systems.     

Complex formation of peroxouranyl (and uranyl) with polyaminocarboxylate ligands

Complexation of the uranyl ion (UO₂²⁺) and of the peroxouranyl species (UO₄) by some polyaminocarboxylate ligands has been investigated in solution (3M NaClO₄) at 25°C. The logarithms of the cumulative formation constants of the UO₂²⁺ chelates formed are: UO₂edta^2? (15.65), UO₂Hedta^? (18.59), (UO₂)₂edta (20.24); UO₂edda (16.02); UO₂Hnta (12.19); UO₂ida (9.63), UO₂H₂(ida)₂ (23.80). The equilibrium UO₂²⁺ + H₂O₂ ? UO₄ + 2H⁺ has a stability log K = ?3.99. The peroxocomplexes formed are UO₄Hedda^? (14.81, expressed from UO₂²⁺ and H₂O₂) and UO₄Hnta^2? (8.50). Solution structures of the chelates are proposed.     

Polarographic Behaviour and Determination of Stability Constants of Uranyl-Ascorbate Complex

The complex species of UO₂(HA)(H₂A)⁺ and UO₂(HA)₂ were identified in the ascorbic acid solution of uranyl ion at pH<2.1 and pH>2.1, respectively. Polarographic wave was proved to be the simultaneous reduction of UO₂⁺² and UO₂(HA)(H₂A)⁺ at pH <2.1. However, at pH>2.1, the wave is due to the reduction of U0₂(HA)₂ The stability constants of the two complex species were found to be 5.1×10⁺ and 1.0×10⁵, respectively. The hydrolysis constant of uranyl ion in the solution of ascorbic acid was determined.     

Relationship between the Structure and Nonlinear Optical Properties of R[UO<Subscript>2</Subscript>L<Subscript>3</Subscript>] and R<Subscript>3</Subscript>[UO<Subscript>2</Subscript>L<Subscript>3</Subscript>]<Subscript>4</Subscript> Crystals (L—Carboxylate Ion)

The nonlinear optical activity (Q) of uranyl carboxylates containing [UO₂(L)₃]^– complexes in the crystal structure, where L is the anion of aliphatic or unsaturated monocarboxylic acids, has been characterized for the first time by the second harmonic generation method. It has been shown using molecular Voronoi–Dirichlet polyhedra that specific features of the cationic sublattice of U and R atoms in carboxylate structures depend on noncovalent interactions between outer-sphere R⁺ cations and [UO₂(L)₃]^– complex anions. The existence of a relationship between Q and the magnitude of the vector characterizing the displacement of the uranium atom nucleus from the centroid of its Voronoi–Dirichlet polyhedron in the cation sublattice of U and R atoms is revealed.     

Cathodic Action of Uranyl-5-Sulfosalicylate Complexes at Dropping Mercury Electrode

The uranyl ion in 5-sulfosalicylic acid (5-SSA) solution has been investigated by polarography. The constitution of the chelate species was confirmed by conductometric titration, under the experimental conditions: temperature 25±0.1C. pH = 2.0-9.0. ligand concentration 10-200 mM. At pH < 4.8, the chelate species, UO₂(HA)2^2?, is predominant. At pH 6.0. the complex species. UO₂(A)₂⁴ mainly exists. The chelate species. UO₂(HA)₂^2?, and UO₂(A)₂⁴, co-exist between pH 4.8-6.0. The diffusion coefficients and stability constants have been determined by equations derived from Ilkovic equation.     

Adsorption of uranyl species onto the rutile (110) surface: a periodic DFT study

To model the structures of dissolved uranium contaminants adsorbed on mineral surfaces and further understand their interaction with geological surfaces in nature, we have performed periodic density funtional theory (DFT) calculations on the sorption of uranyl species onto the TiO₂ rutile (110) surface. Two kinds of surfaces, an ideal dry surface and a partially hydrated surface, were considered in this study. The uranyl dication was simulated as penta‐ or hexa‐coordinated in the equatorial plane. Two bonds are contributed by surface bridging oxygen atoms and the remaining equatorial coordination is satisfied by H₂O, OH^?, and CO₃^2? ligands; this is known to be the most stable sorption structure. Experimental structural parameters of the surface–[UO₂(H₂O)₃]²⁺ system were well reproduced by our calculations. With respect to adsorbates, [UO₂(L1)_x(L2)_y(L3)_z]ⁿ (L1=H₂O, L2=OH^?, L3=CO₃^2?, x≤3, y≤3, z≤2, x+y+2z≤4), on the ideal surface, the variation of ligands from H₂O to OH^? and CO₃^2? lengthens the U? O_surf and U? Ti distances. As a result, the uranyl–surface interaction decreases, as is evident from the calculated sorption energies. Our calculations support the experimental observation that the sorptive capacity of TiO₂ decreases in the presence of carbonate ions. The stronger equatorial hydroxide and carbonate ligands around uranyl also result in U?O distances that are longer than those of aquouranyl species by 0.1–0.3 Å. Compared with the ideal surface, the hydrated surface introduces greater hydrogen bonding. This results in longer U?O bond lengths, shorter uranyl–surface separations in most cases, and stronger sorption interactions.     

Polarography of Uranium Chelate with 2,3-Cresotic Acid

The polarography of uranyl ion in 2,3-cresotic acid solution has been studied at 25°C under varying conditions of ligand concentration and pH. The ligands species were proved to be a 2,3-cresotate anion. The half-wave potential vs. pH value interpreted on the basis of pK value for the acid ionization, and resulted in agreement with the deduction. The mole ratio of metal to ligand was found to be 1:1 and 1:2 by conductometric titration. At pH < pK₁, the complex species of UO₂(H₂A)²⁺ and UO₂(HA)⁺ was identified. At pK₁ < pH < pK₂, the co-existence of UO₂(HA)⁺, UO₂(OH)(HA)₂^? and UO₂(A)₂^2– was confirmed. At pH > pK₂, the complex species of UO₂(OH) (A)₂^3– was formed.     

The Energy Landscape of Uranyl–Peroxide Species

Nanoscale uranyl peroxide clusters containing UO₂²⁺ groups bonded through peroxide bridges to form polynuclear molecular species (polyoxometalates) exist both in solution and in the solid state. There is an extensive family of clusters containing 28 uranium atoms (U₂₈ clusters), with an encapsulated anion in the center, for example, [UO₂(O₂)_3?x(OH)_x^4?], [Nb(O₂)₄^3?], or [Ta(O₂)₄^3?]. The negative charge of these clusters is balanced by alkali ions, both encapsulated, and located exterior to the cluster. The present study reports measurement of enthalpy of formation for two such U₂₈ compounds, one of which is uranyl centered and the other is peroxotantalate centered. The [(Ta(O₂)₄]‐centered U₂₈ capsule is energetically more stable than the [(UO₂)(O₂)₃]‐centered capsule. These data, along with our prior studies on other uranyl–peroxide solids, are used to explore the energy landscape and define thermochemical trends in alkali–uranyl–peroxide systems. It was suggested that the energetic role of charge‐balancing alkali ions and their electrostatic interactions with the negatively charged uranyl–peroxide species is the dominant factor in defining energetic stability. These experimental data were supported by DFT calculations, which agree that the [(Ta(O₂)₄]‐centered U₂₈ capsule is more stable than the uranyl‐centered capsule. Moreover, the relative stability is controlled by the interactions of the encapsulated alkalis with the encapsulated anion. Thus, the role of alkali‐anion interactions was shown to be important at all length scales of uranyl–peroxide species: in both comparing clusters to clusters; and clusters to monomers or extended solids.     

Study of solvent extraction of <Superscript>114m</Superscript>In(III) using quinaldic acid into isoamyl alcohol

A complementary study of hydroxyl radical formation in the depleted uranium (DU)-hydrogen peroxide (H₂O₂) system and the effect of biosubstances on the system were examined using the spin-trapping method. Hydroxyl radical was formed in the uranyl ion (UO₂ ²⁺), 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), and hydrogen peroxide (H₂O₂) mixture solution. The pseudo first order rate constants of DMPO-OH formation were estimated to be 0.033 s⁻¹ for UO₂ ²⁺-H₂O₂-DMPO solution and 0.153 s⁻¹ for UO₂₊-H₂O₂-DMPO solution. The obtained results indicated that the hydroxyl radical formation in the UO₂ ²⁺-H₂O₂ solution could be described as a stepwise reaction process including the reduction of UO₂ ²⁺ to UO₂ ²⁺ by H₂O₂ and the Fenton-type reaction of UO₂ ⁺ with H₂O₂. Biosubstances, such as proteins, amino acids and saccharides, decreased the DMPO-OH formation, which was caused by the direct hydroxyl radical scavenging and the suppression of hydroxyl radical formation by coupling with uranyl ion.     

U(VI) mono-hydroxo humate complexation

The complexation of the uranyl ion with humic acid is investigated. The humic acid ligand concentration is described as the concentration of reactive humic acid molecules based on the number of humic acid molecules, taking protonation of functional groups into account. Excess amounts of U(VI) are used and the concentration of the humic acid complex is determined by the solubility enhancement over the solid phase. pH is varied between 7.5 to 7.9 in 0.1M NaClO₄ under normal atmosphere and room temperature. The solubility of U(VI) in absence of humic acid is determined over amorphous solid phase between pH 4.45 and 8.62. With humic acid, only a limited range of data can be used for the determination of the complexation constant because of flocculation or sorption of the humic acid upon progressive complexation. Analysis of the complex formation dependency with pH shows that the dominant uranyl species in the concerned pH range are UO₂(OH)⁺ and (UO₂)₃(OH)₅ ⁺. The complexation constant is evaluated for the humate interaction with the to UO₂(OH)⁺ ion. The stability constant is found to be logβ = 6.94±0.3 l/mol. The humate complexation constant of the uranyl mono-hydroxo species thus is significantly higher than that of the nonhydrolyzed uranyl ion (6.2 l/mol). Published data on the Cm³⁺, CmOH²⁺ and Cm(OH)₂ ⁺ humate complexation are reevaluated by the present approach. The higher stability of the hydrolysis complex is also found for Cm(III) humate complexation.     

Polarographic studies of uranyl complexes with trans-and cis-butenedioic acids

The complexation of uranyl ion with fumaric and maleic acids was investigated by polarography and conductometry. The uranyl complexes of the two isomers differ: with fumaric acid, UO₂(HFum)₂ and UO₂Fum₂^2- were observed whereas with maleic acid, only one chelate, UO₂Mal₂^2-, was obtained. The dissociation constants obtained from the half-wave potential vs. pH plots were pK₁=3.05 and pK₂=4.55 for fumaric acid and pK₁=1.90 and pK₂=5.60 for maleic acid.     

Etude des complexes sulfosalicylate de cuivre(III), nickel(II), cobalt(II) et uranyle(II) a l'aide d'une resine echangeuse de cations: Study of the sulphosalicylate complexes ofcopper(II), nickel(II), cobalt(II) and uranyl(II) by means of cation-exchange resins.

Study of the sulphosalicylate complexes of copper(II), nickel(II), cobalt(II) and uranyl(II) by means of cation-exchange resins.The conditional stability constants of the 1:1 complexes of the sulphosalicylate ions (L^3-) with copper(II), nickel(II), cobalt(II) and uranyl ions have been determined in a sodium perchlorate solution (0.1 M) and at various pH values by a cation-exchange method based on Schubert's procedure. The limits of application of the method are discussed. The variation with pH of the conditional stability constants can be explained by the existence of the complexes: CuH₂L, CuHL, CuL^-; NiH₂L⁺, NiHL, NiL^-; CoHL, CoL^-; UO₂H₂L⁺, UO₂HL, UO₂L^-, UO₂LOH^2-. The stability constants of these complexes are reported. Distribution diagrams of the various complexes of each element with pH and total concentration of sulphosalicylate parameters are given.     

Synthesis and vibrational spectroscopy of halotrichite and bilinite

The new uranyl complexes with tetradentate unsymmetrical N₂O₂ Schiff base ligands were synthesized and characterized by IR, UV–vis, NMR and elemental analysis. The DMF solvent is coordinated to uranyl complexes. The thermogravimetry (TG) and differential thermoanalysis (DTA) of the uranyl complexes were carried out in the range of 20–700 °C. The UO₂L¹ complex was decomposed in two and the others were decomposed in three stages. Up to 100 °C, the coordinated solvent was released then the Schiff base ligands were decomposed in one or two steps. Decomposition of synthesized complexes is related to the Schiff base characteristics. The thermal decomposition reaction is first order for the studied complexes.     

Quantum relativistic investigation about the coordination and bonding effects of different ligands on uranyl complexes

The coordination and bonding effects of equatorial ligands such as fluoride (F⁻), chloride (Cl⁻), cyanide (CN⁻), isocyanide (NC⁻), and carbonate (CO₃⁻²) on uranyl dication (UO₂²⁺) has been studied using relativistic density functional theory. The ZORA Hamiltonian was applied for the inclusion of relativistic effects taking into account all the electrons for the optimization and the explicit inclusion of spin–orbit coupling effects. Geometry optimizations including the counterions and frequencies analysis were carried out with PW91 and PBE functional. Solvents effects were considered by using the conductor like screening model (COSMO) for water and acetonitrile. The Time-Dependent Density Functional Theory (TDDFT) was used to calculate the excitation energies with GGA SAOP functional and the electronic transitions were analyzed using double group irreducible representations. The theoretical results are in a good agreement with experimental IR, Raman and EXAFS spectra and previous theoretical results. New information about the effect of different (donor and acceptors) ligands on the bonding of uranyl ion and on the electronic transitions involved in these complexes is provided with a possible impact on the understanding of the uranyl coordination chemistry.     

Structural Observations of Heterometallic Uranyl Copper(II) Carboxylates and Their Solid‐State Topotactic Transformation upon Dehydration

The hydrothermal reactions of uranyl nitrate and metallic copper with aromatic polycarboxylic acids gave rise to the formation of five heterometallic UO₂²⁺? Cu²⁺ coordination polymers: (UO₂)Cu(H₂O)₂(1,2‐bdc)₂ ( 1 ; 1,2‐bdc=phthalate), (UO₂)Cu(H₂O)₂(btec) ? 4 H₂O ( 2 ) and (UO₂)Cu(btec) ( 2′ ; btec=pyromellitate), (UO₂)₂Cu(H₂O)₄(mel) ( 3 ; mel=mellitate), and (UO₂)₂O(OH)₂Cu(H₂O)₂(1,3‐bdc) ? H₂O ( 4 ; 1,3‐bdc=isophthlalate). Single‐crystal X‐ray diffraction (XRD) analysis of compound 1 revealed 2D layers of chains of UO₈ and CuO₄(H₂O)₂ units that were connected through the phthalate ligands. In compound 2 , these sheets were connected to each other through the two additional carboxylate arms of the pyromellitate, thus resulting in a 3D open‐framework with 1D channels that trapped water molecules. Upon heating, free and bonded water species (from Cu? OH₂) were evacuated from the structure. This thermal transition was followed by in situ XRD and IR spectroscopy. Heating induced a solid‐state topotactic transformation with the formation of a new set of Cu? O interactions in the crystalline anhydrous structure ( 2′ ), in order to keep the square‐planar environment around the copper centers. The structure of compound 3 was built up from trinuclear motifs, in which one copper center, CuO₄(OH₂)₂, was linked to two uranium units, UO₅(H₂O)₂. The assembly of this trimer, “U₂Cu”, with the mellitate generated a 3D network. Complex 4 contained a tetranuclear uranyl core of UO₅(OH)₂ and UO₆(OH) units that were linked to two copper centers, CuO(OH)₂(H₂O)₂, which were then connected to each other through isophthalate ligands and U?O? Cu interactions to create a 3D structure. The common structural feature of these different compounds is a bridging oxo group of U?O? Cu type, which is reflected by apical Cu? O distances in the range 2.350(3)–2.745(5) Å. In the case of a shorter Cu? O distance, a slight lengthening of the uranyl bond (U?O) is observed (e.g., 1.805(3) Å in complex 4 ).