Tuning Selectivity to f-Elements through Bonding and Solvation Effects of a Sulfur Donor Ligand.

Bis(2,4,4-trimethylpentyl)dithiophosphinic acid, commonly referred to as HBTMPDTP or Cyanex301, is a sulfur-donating ligand that shows considerable promise in the challenging task of separating trivalent actinides (An3+) from lanthanides (Ln3+). Although its effectiveness has been established, the specific molecular details about the preference of HBTMPDTP for americium over europium have remained a mystery, puzzling researchers for over two decades. This study presents a comprehensive, dual-driven separation mechanism for this complex system combining experimental and theoretical approaches. A critical finding is the increased covalency in An-S bonds compared to Ln-S bonds, which plays a significant role in HBTMPDTP's intrinsic selectivity for An3+ over Ln3+. This leads to the formation of distinct An3+ and Ln3+ species, enhancing the ligand's actinide selectivity. Additionally, it provides crucial insights into the coordination chemistry of f-elements with sulfur-donating ligands, thereby deepening our understanding of this intricate field.

Coordination of 2,2'-(Trifluoroazanediyl)bis(N,N'-dimethylacetamide) with U(VI), Nd(III), and Np(V): A Thermodynamic and Structural Study.

Thermodynamic properties of the complexation of 2,2'-(trifluoroazanediyl)bis(N,N'-dimethylacetamide) (CF3ABDMA) with U(VI), Nd(III), and Np(V) have been studied in 1.0 M NaNO3 at 25 °C. Equilibrium constants of the complexation were determined by potentiometry and spectrophotometry. In comparison with a series of structurally related amine-bridged diacetamide ligands, including 2,2'-(benzylazanediyl)bis(N,N'-dimethylacetamide) (BnABDMA), 2,2'-azanediylbis(N,N'-dimethylacetamide) (ABDMA), and 2,2'-(methylazanediyl)bis(N,N'-dimethylacetamide) (MABDMA), CF3ABDMA forms weaker complexes with U(VI), Nd(III), and Np(V) due to the lower basicity of the center N atom in CF3ABDMA resulting from the attachment of the strong electron-withdrawing CF3- moiety. The complexation strength of CF3ABDMA with the three metal ions follows the order: UO22+ > Nd3+ > NpO2+, consistent with the order of the "effective" charges of the metal ions. Structural information on the U(VI)/CF3ABDMA complexes in solution and in solid was obtained by theoretical computation, single crystal X-ray diffractometry, 19F NMR, and electrospray ionization mass spectrometry. The structural data indicate that, similar to the three previously studied amine-bridged diacetamide ligands (BnABDMA, ABDMA, and MABDMA), the CF3ABDMA ligand coordinates to UO22+ in a tridentate mode, through the center nitrogen and the two amide oxygen atoms.

Complexation of Light Trivalent Lanthanides with N-(2-Hydroxyethyl)ethylenediamine-N,N',N'-triacetic Acid in Aqueous Solutions: Thermodynamic Analysis and Coordination Modes.

N-(2-Hydroxyethyl)ethylenediamine-N,N',N'-triacetic acid (HEDTA, denoted as H3L) is a strong chelating ligand that is widely used in the separation of f elements as relevant to the nuclear fuel cycle. There is much to be known about the structure and composition of the coordination sphere of the complexes of HEDTA with lanthanides. The complexation of HEDTA with light lanthanides (La3+, Nd3+, and Eu3+) was investigated thermodynamically and structurally in aqueous solutions. Potentiometry and microcalorimetry were performed to acquire the complexation constants (25-70 °C) and enthalpies (25 °C), respectively, at I = 1.0 mol·L-1 NaClO4. Coordination modes of the complexes were analyzed by luminescence spectroscopy and NMR spectroscopy. The results indicate that there are two successive Ln3+/HEDTA complexes, LnLaq and Ln2(H-1L)22- (Ln3+ refers to La3+, Nd3+, and Eu3+; H-1L4- refers to deprotonation of the hydroxyl group) during titration. The hydroxyl group of HEDTA is coordinated in the Ln3+/HEDTA complex. The dinuclear Ln2(H-1L)22- complex is present as a carboxyl-bridged centrosymmetric dimer, and two carboxyl groups in bridging positions are coordinated to two adjacent Ln3+ cations. Complexation of NdLaq is exothermic, while formation of the hydrolytic complex Nd2(H-1L)22- is endothermic. Both NdLaq and Nd2(H-1L)22- complexes are driven by entropic force. These data will help to predict the behavior of lanthanides in the separation process, where HEDTA is used as the aqueous complexant.

Quantitative Analysis of Surface Sites on Carbon Dots and Their Interaction with Metal Ions by a Potentiometric Titration Method.

Carbon dots (CDs) possess abundant functional groups on their surface which are related to their application in various fields such as sensing, imaging, and catalysis. Understanding the amount and properties of these functional groups and their interaction with metal ions is essential but has posed longstanding challenges because of the diverse and complex structures of CDs. In this work, potentiometric titration is demonstrated as an effective method to figure out the categories and amounts of functional groups. Surface complexation modeling with the FITEQL program was applied to the quantification of the surface sites on CDs with the titration data. Then with the obtained molar concentrations of the surface sites, the pKas of these surface sites were calculated with the Hyperquad program. Finally, titration experiments of CDs with and without Fe(III) were carried out and the stability constants of Fe(III) and ArgCDs were simulated on the Hyperquad program. By utilizing the stability constants, the distribution of Fe(III) species at different pHs and the concentrations of Fe(III) and CDs were also investigated. This potential method might be used for characterizing the surface sites on other CDs or even other soluble nanoparticles as well as for investigating the interactions of the surface sites with different metal ions.

Complexation of U(VI) with BiPDA, DmBiPDA, and PhenDA: Comparison on Structures and Binding Strengths in Aqueous and DMSO/20%(v)H2O Solutions.

The stability constants (log ß) of 1:1 uranyl complexes with three N,O-mixed donor ligands (L = 2,2'-dipyridyl-6,6'-dicarboxylate, 3,3'-dimethyl-2,2'-bipyridine-6,6'-dicarboxylate, and 1,10-phenanthroline-2,9-dicarboxylate, denoted as BiPDA, DmBiPDA, and PhenDA, respectively) in aqueous and DMSO/20%(v)H2O solutions were determined by spectrophotometry in 0.1 M tetraethylammonium perchlorate. The effects of ligand preorganization, steric hindrance, and solvation on the binding strength of U(VI) with the three ligands were discussed. In aqueous solution, PhenDA forms stronger complexes with U(VI) than BiPDA due to its well-preorganized structure. In DMSO/20%(v)H2O solution, in contrast, the strong solvation effect of DMSO on the ligands reduces the energy gap between the trans- and cis-conformations of BiPDA, resulting in log ß(UO2(BiPDA)) > log ß(UO2(PhenDA)). The steric hindrance of methyl groups on DmBiPDA makes the complex UO2(DmBiPDA) of the lowest stability in both aqueous and DMSO/20%(v)H2O solutions. Single-crystal structural data of U(VI) complexes with the three ligands indicate that the ligand coordinates with UO22+ via aromatic nitrogen atoms and carboxylate oxygen atoms. There is no clear correlation between the trend of the stability constants in solutions and the U-N/O bond lengths of the three crystal complexes. Nevertheless, DmBiPDA coordinates to UO22+ in a high-strain fashion as a result of the steric hindrance of methyl groups while BiPDA in a low-strain fashion, which is in accordance with the relative complexation strength of the two respective complexes. The results from this work help us understand the effect of ligand preorganization and solvation on the binding strength of actinides with multidentate N,O-mixed ligands in solid and solutions, which is of importance in designing ligands for the partitioning of actinides from nuclear wastes.

Siderophore-inspired chelator hijacks uranium from aqueous medium.

Over millennia, nature has evolved an ability to selectively recognize and sequester specific metal ions by employing a wide variety of supramolecular chelators. Iron-specific molecular carriers-siderophores-are noteworthy for their structural elegance, while exhibiting some of the strongest and most selective binding towards a specific metal ion. Development of simple uranyl (UO22+) recognition motifs possessing siderophore-like selectivity, however, presents a challenge. Herein we report a comprehensive theoretical, crystallographic and spectroscopic studies on the UO22+ binding with a non-toxic siderophore-inspired chelator, 2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5-triazine (H2BHT). The optimal pKa values and structural preorganization endow H2BHT with one of the highest uranyl binding affinity and selectivity among molecular chelators. The results of small-molecule standards are validated by a proof-of-principle development of the H2BHT-functionalized polymeric adsorbent material that affords high uranium uptake capacity even in the presence of competing vanadium (V) ions in aqueous medium.

Complexation of Uranium(VI) with N-(2-Hydroxyethyl)ethylenediamine- N, N', N'-triacetic Acid in Aqueous Solution: Thermodynamic Studies and Coordination Analyses.

N-(2-Hydroxyethyl)ethylenediamine- N, N', N'-triacetic acid (HEDTA, denoted as H3L in this work, and the three dissociable protons represent those of the three carboxylic groups) is a strong chelating ligand and plays an important role in the treatment and disposal of nuclear wastes as well as separation sciences of f-elements. In this work, the complexation of HEDTA with U(VI) was studied thermodynamically and structurally in aqueous solutions. Potentiometry and microcalorimetry were used to measure the complexation constants (298-343 K) and enthalpies (298 K), respectively, at I = 1.0 mol·L-1 NaClO4. Thermodynamic studies identified three 1:1 U(VI)/HEDTA complexes with different degrees of deprotonation, namely, UO2(HL)(aq), UO2L-, and UO2(H-1L)2-, where H-1 represents the deprotonation of the hydroxyl group. The results indicated that all three complexation reactions are endothermic and driven by entropy only. Coordination modes of the three complexes were investigated by NMR and extended X-ray absorption fine structure spectroscopies. In the UO2(HL)(aq) complex, HEDTA holds a tridentate mode, and the coordination occurs to the end of the ethylenediamine backbone. Two oxygens of the two carboxylic groups and one nitrogen of the amine group participate in the coordination. In both UO2L- and UO2(H-1L)2-, HEDTA holds a tetradentate mode and coordinates to U(VI) along the side of the ethylenediamine backbone. The difference is that in the UO2(H-1L)2- complex, the alkoxide form of the HEDTA hydroxyl group directly binds to the U(VI) atom, forming a highly strong chelation.

Complexation-assisted reduction: complexes of glutaroimide-dioxime with tetravalent actinides (Np(iv) and Th(iv)).

Glutaroimide-dioxime forms strong complexes with tetravalent Th(iv) and Np(iv) in aqueous solution. In conjunction with literature data on the complexation of glutaroimide-dioxime with other metal ions, it was found that the complexes become weaker as the effective charge density on the metal ions decreases: V5+ > Th4+ ≈ Fe3+ > UO22+ > Eu3+/Nd3+ > Cu2+ > Pb2+ > NpO2+ > Ca2+/Mg2+. In the glutaroimide-dioxime complexes with Th(iv) and Np(iv), deprotonation of the imide group and relocation of the two hydrogen atoms from oxygen to nitrogen of the oxime groups result in a large conjugated system (-O-N-C-N-C-N-O-) that coordinates strongly to the metal center in a tridentate mode via the central imide nitrogen atom and the two oxime oxygen atoms. Because the stability of glutaroimide-dioxime complexes with Np(iv) is much higher than those with Np(v), the redox potential of the Np(v)/Np(iv) couple is expected to be shifted significantly. As a result, crystals of glutaroimide-dioxime complexes with Np(iv) were readily obtained from initial solutions containing Np(v). A mechanism of complexation-assisted reduction integrating the thermodynamic and structural data from this work is discussed.

Complexation of NpO2+ with Amine-Functionalized Diacetamide Ligands in Aqueous Solution: Thermodynamic, Structural, and Computational Studies.

Complexation of Np(V) with three structurally related amine-functionalized diacetamide ligands, including 2,2'-azanediylbis( N, N'-dimethylacetamide) (ABDMA), 2,2'-(methylazanediyl)bis( N, N'-dimethylacetamide) (MABDMA), and 2,2'-(benzylazanediyl)bis( N, N'-dimethylacetamide) (BnABDMA), in aqueous solutions was investigated. The stability constants of two successive complexes, namely, NpO2L+ and NpO2L2+, where L stands for the ligands, were determined by absorption spectrophotometry. The results suggest that the stability constants of corresponding Np(V) complexes follow the trend: MABDMA > ABDMA ≈ BnABDMA. The data are discussed in terms of the basicity of the ligands and compared with those for the complexation of Np(V) with an ether oxygen-linked diacetamide ligand. Extended X-ray absorption fine structure data indicate that, similar to the complexation with Nd3+ and UO22+, the ligands coordinate to NpO2+ in a tridentate mode through the amine nitrogen and two oxygen atoms of the amide groups. Computational results, in conjunction with spectrophotometric data, verify that the 1:2 complexes (NpO2(L)2+) in aqueous solutions are highly symmetric with Np at the inversion center, so that the f-f transition of Np(V) is forbidden and NpO2(L)2+ does not display significant absorption in the near-IR region.

Thermodynamic, Structural, and Computational Investigation on the Complexation between UO22+ and Amine-Functionalized Diacetamide Ligands in Aqueous Solution.

The stability constants (log ß), enthalpies of complexation (ΔH), and entropies of complexation (ΔS) for the complexes of uranium(VI) with a series of amine-functionalized diaetamide ligands, 2,2'-benzylazanediylbis(N,N'-dimethylacetamide) (BnABDMA), 2,2'-azanediylbis(N,N'-dimethylacetamide) (ABDMA), and 2,2'-methylazanediylbis(N,N'-dimethylacetamide) (MABDMA), in aqueous solution were determined by potentiometry and calorimetry. Electronspray ionization mass spectrometry was used to verify the presence of uranium(VI) complexes in solution. The thermodynamic data indicate that the binding strengths of the three ligands with UO22+ follow the order BnABDMA < ABDMA < MABDMA, parallel to the order of the protonation constants as well as the order of the stability of the Nd3+ complexes, suggesting that the complexation of UO22+ with the ligands consist predominantly of electrostatic interactions. Denisty functional theory calculations were conducted to reveal the structures, electronic charge distribution, and energetics of the uranium(VI) complexes, providing insight into the thermodynamic trends of the complexation. Extended X-ray absorption fine structure spectroscopy was used to identify the structures of the uranium(VI) complexes in aqueous solution.

Origin of the unusually strong and selective binding of vanadium by polyamidoximes in seawater.

Amidoxime-functionalized polymeric adsorbents are the current state-of-the-art materials for collecting uranium (U) from seawater. However, marine tests show that vanadium (V) is preferentially extracted over U and many other cations. Herein, we report a complementary and comprehensive investigation integrating ab initio simulations with thermochemical titrations and XAFS spectroscopy to understand the unusually strong and selective binding of V by polyamidoximes. While the open-chain amidoxime functionalities do not bind V, the cyclic imide-dioxime group of the adsorbent forms a peculiar non-oxido V5+ complex, exhibiting the highest stability constant value ever observed for the V5+ species. XAFS analysis of adsorbents following deployment in environmental seawater confirms V binding solely by the imide-dioximes. Our fundamental findings offer not only guidance for future optimization of selectivity in amidoxime-based sorbent materials, but may also afford insight to understanding the extensive accumulation of V in some marine organisms.

Kinetics of complexation of V(v), U(vi), and Fe(iii) with glutaroimide-dioxime: studies by stopped-flow and conventional absorption spectroscopy.

Uranium extracted from seawater is a promising source of uranium for nuclear energy, and the extraction technology using polymer sorbents has been shown to be feasible. However, improving selectivity for uranium over other metals, notably vanadium and iron, is essential to increase efficiency and reduce costs. In the present work, the kinetics of the binding of these three metals with glutaroimide-dioxime as a molecular analogue of polymer sorbents has been studied using stopped-flow and conventional UV Visible absorption spectroscopy to monitor the reactions over a range of time scales. Qualitatively, vanadium reacts the slowest of the three metals despite being able to form a very strong complex, with the 1 : 2 vanadium/ligand complex forming over weeks, likely due to the slow hydrolysis of the strong oxido ligands, while iron reacts fast and uranyl faster still, despite the presence of carbonate in the uranyl species. Conditional rate constants were determined for the formation of 1 : 1 glutaroimide-dioxime complexes with the three metal ions. Besides, in a narrow and near neutral pH region, a rate equation for the formation of the 1 : 1 vanadium/glutaroimide-dioxime complex was developed, showing the reaction is the first order with respect to [V], [ligand], and [H+]. These observations, some being qualitative and others quantitative, are consistent with previous marine tests of polymer adsorbents, and give mechanistic insight into how glutaroimide-dioxime forms complexes with uranium, iron, and vanadium.

A Uranyl Peroxide Dimer in the Gas Phase.

The gas-phase uranyl peroxide dimer, [(UO2)2(O2)(L)2]2+ where L = 2,2'-trifluoroethylazanediyl)bis(N,N'-dimethylacetamide), was synthesized by electrospray ionization of a solution of UO22+ and L. Collision-induced dissociation of this dimer resulted in endothermic O atom elimination to give [(UO2)2(O)(L)2]2+, which was found to spontaneously react with water via exothermic hydrolytic chemisorption to yield [(UO2)2(OH)2(L)2]2+. Density functional theory computations of the energies for the gas-phase reactions are in accord with observations. The structures of the observed uranyl dimer were computed, with that of the peroxide of particular interest, as a basis to evaluate the formation of condensed phase uranyl peroxides with bent structures. The computed dihedral angle in [(UO2)2(O2)(L)2]2+ is 145°, indicating a substantial deviation from the planar structure with a dihedral angle of 180°. Energies needed to induce bending in the most elementary gas-phase uranyl peroxide complex, [(UO2)2(O2)]2+, were computed. It was found that bending from the lowest-energy planar structure to dihedral angles up to 140° required energies of <10 kJ/mol. The gas-phase results demonstrate the inherent stability of the uranyl peroxide moiety and support the notion that the uranyl-peroxide-uranyl structural unit is intrinsically planar, with only minor energy perturbations needed to form the bent structures found in studtite and uranyl peroxide nanostructures.

Interactions of Bis(2,4,4-trimethylpentyl)dithiophosphinate with Trivalent Lanthanides in a Homogeneous Medium: Thermodynamics and Coordination Modes.

Complexation of trivalent lanthanides with a sulfur-bearing ligand, bis(2,4,4-trimethylpentyl) dithiophosphinate, was studied in ethanol under identical conditions by optical spectroscopy, microcalorimetry, luminescence lifetime measurement, and extended X-ray absorption fine structure (EXAFS). Three successive complexes, LnL2+, LnL2+, and LnL3, where Ln and L denote the trivalent lanthanide and the dithiophosphinate ligand, respectively, formed in the solution. In contrast to the general findings that heavier lanthanides form stronger complexes due to the lanthanide contraction effect, the complexation strength between Ln(III) and dithiophosphinate first increases from La(III) to Nd(III) and then decreases gradually toward heavier Ln(III) across the lanthanide series. This trend agrees well with the results of solvent extraction using the same ligand as an extractant. The complexation is driven by highly positive entropies and opposed by endothermic enthalpies. The enthalpies of complexation become less endothermic from lighter to heavier Ln(III), suggesting that less energy is required for desolvation for the complexation of heavier Ln(III). EXAFS study shows that, from lighter to heavier Ln(III), the number of sulfur atoms in the primary coordination sphere decreases while the number of oxygen atoms increases, which confirms that fewer solvent molecules are desolvated from heavier Ln(III) during the complexation process. A correlation between the thermodynamics trends and the coordination modes has thereby been well established.

Thermodynamics of biphasic lanthanide extraction by tripodal diglycolamide: a solution calorimetry study.

Isothermal titration calorimetry was employed for the direct measurement of the enthalpy of extraction (ΔHextr) of Eu(NO3)3 by using a tripodal diglycolamide (T-DGA) ligand dissolved in n-dodecane containing 5% (v/v) 2-decanol. The enthalpy of extraction obtained by titration calorimetry was in good agreement with the enthalpy of extraction calculated from the temperature dependence of the distribution coefficients by using the van't Hoff equation. The Gibbs free energy and the entropy of extraction (ΔGextr and ΔSextr) for the extraction of Eu(NO3)3 by T-DGA were also obtained by solvent extraction experiments. The complexation of Eu3+ with T-DGA in a mixture of acetonitrile/nitric acid was also studied by spectrophotometry and calorimetry to determine the stability constants and the enthalpy of complexation (ΔHcomp) for the Eu3+/T-DGA complexes in a single phase. The enthalpy of complexation, though obtained in a solvent different from that in the solvent extraction, allows a rough estimate of the enthalpy of phase transfer of the Eu3+/T-DGA complexes from the aqueous phase to the organic phase.

Thermodynamic study of the complexation between Nd(3+) and functionalized diacetamide ligands in solution.

A series of amine functionalized ligands, including 2,2'-(benzylazanediyl)bis(N,N'-dimethylacetamide) (BnABDMA), 2,2'-azanediylbis(N,N'-dimethylacetamide) (ABDMA), and 2,2'-(methylazanediyl)bis(N,N'-dimethylacetamide) (MABDMA), are synthesized for the thermodynamic study of their complexation with Nd(3+) ions. Their complexation in solution is investigated using potentiometry, spectrophotometry, calorimetry, and electrospray ionization mass spectrometry. The results suggest that these ligands act as tridentate ligands. Furthermore, direct comparison between ABDMA and an analogous ether-functionalized ligand, 2,2'-oxybis(N,N'-dimethylacetamide) (TMDGA), showed that the amine functionalized ligand forms thermodynamically stronger complexes with Nd(3+) ions than the ether-functionalized ligand. In addition, the amine functionalized ligand can allow the fine-tuning of the binding strength with metal ions via substitution on the central amine N atom with different functional groups, which is not possible for ether functionalized ligands such as TMDGA.

Complexation of Lanthanides with Glutaroimide-dioxime: Binding Strength and Coordination Modes.

The complexation of lanthanides (Nd(3+) and Eu(3+)) with glutaroimide-dioxime (H2L), a cyclic imide dioxime ligand that has been found to form stable complexes with actinides (UO2(2+) and NpO2(+)) and transition metal ions (Fe(3+), Cu(2+), etc.), was studied by potentiometry, absorption spectrophotometry, luminescence spectroscopy, and microcalorimetry. Lanthanides form three successive complexes, M(HL)(2+), M(HL)L, and M(HL)2(+) (where M stands for Nd(3+)/Eu(3+) and HL(-) stands for the singly deprotonated ligand). The enthalpies of complexation, determined by microcalorimetry, show that the formation of these complexes is exothermic. The stability constants of Ln(3+)/H2L complexes are several orders of magnitude lower than that of the corresponding Fe(3+)/H2L complexes but are comparable with that of UO2(2+)/H2L complexes. A structure of Eu(3+)/H2L complex, identified by single-crystal X-ray diffractometry, shows that the ligand coordinates to Eu(3+) in a tridentate mode, via the two oxygen atoms of the oxime group and the nitrogen atom of the imide group. The relocation of protons of the oxime groups (-CH═N-OH) from the oxygen to the nitrogen atom, and the deprotonation of the imide group (-CH-NH-CH-) result in a conjugated system with delocalized electron density on the ligand (-O-N-C-N-C-N-O-) that forms strong complexes with the lanthanide ions.

Complexation of U(VI) with benzoic acid at variable temperatures (298-353 K): thermodynamics and crystal structures of U(VI)/benzoate complexes.

Thermodynamics of the U(VI) complexation with benzoic acid (HL) was studied by spectrophotometry at varied temperatures (298-353 K) with constant ionic strength (1.05 mol kg(-1) NaClO4). Two U(VI) benzoate complexes, UO2L(+) and UO2(OH)L(aq), were identified and their formation constants determined. The formation of both complexes is endothermic and driven exclusively by entropy. Two types of U(VI)/benzoate complex crystals were synthesized from aqueous solutions at different pH and ligand/metal ratios. Their structures were determined by single-crystal X-ray diffractometry. One structure is a 1 : 3 U(VI) benzoate complex (Na[UO2(C7H5O2)3]·2H2O), each benzoate holding a bidentate coordination mode to U(VI) in the equatorial plane of UO2(2+). The other is a U(VI) hydroxobenzoate complex with unique µ3-OH bridging ([(UO2)2(C7H5O2)2(µ3-OH)2]·4H2O). In the structure, each UO2(2+) ion holds five coordination oxygens in its equatorial plane, two carboxylate oxygens from two benzoate ligands and three oxygens from three µ3-OH groups.

Complexation of Neptunium(V) with Glutaroimide Dioxime: A Study by Absorption Spectroscopy, Microcalorimetry, and Density Functional Theory Calculations.

Complexation of NpO2(+) ions with glutaroimide dioxime (H2L), a cyclic imide dioxime ligand that has been shown to form strong complexes with UO2(2+) in aqueous solutions, was studied by absorption spectroscopy and microcalorimetry in 1.0 M NaClO4 aqueous solutions. NpO2(+) forms two successive complexes, NpO2(HL)(aq) and NpO2(HL)2(-) (where HL(-) stands for the partially deprotonated glutaroimide dioxime ligand), with stability constants of log ß111 = 17.8 ± 0.1 and log ß122 = 33.0 ± 0.2, respectively. The complexation is both enthalpy- and entropy-driven, with negative enthalpies (ΔH111 = -52.3 ± 1.0 kJ/mol and ΔH122 = -96.1 ± 1.4 kJ/mol) and positive entropies (ΔS111 = 164 ± 3 J/mol/K and ΔS122 = 310 ± 4 J/mol/K). The thermodynamic parameters suggest that, similar to complexation of UO2(2+), the ligand coordinates with NpO2(+) in a tridentate mode, via the two oxygen atoms of the oxime groups and the nitrogen atom of the imide group. Density functional theory calculations have helped to interpret the optical absorption properties of the NpO2(HL)2(-) complex, by showing that the cis and trans configurations of the complex have very similar energies so that both configurations could be present in the aqueous solutions. It is the noncentrosymmetric cis configuration that makes the 5f → 5f transition allowable so that the NpO2(HL)2(-) complex absorbs in the near-IR region.

Complexation of uranium(VI) with glutarimidoxioxime: thermodynamic and computational studies.

The complex formation between a cyclic ligand glutarimidoxioxime (denoted as HL(III) in this paper) and UO2(2+) is studied by potentiometry and microcalorimetry. Glutarimidoxioxime (HL(III)), together with glutarimidedioxime (H2L(I)) and glutardiamidoxime (H2L(II)), belongs to a family of amidoxime derivatives with prospective applications as binding agents for the recovery of uranium from seawater. An optimized procedure of synthesis that leads to the preparation of glutarimidoxioxime in the absence of other amidoxime byproducts is described in this paper. Speciation models based on the thermodynamic results from this study indicate that, compared with H2L(I) and H2L(II), HL(III) forms a much weaker complex with UO2(2+), UO2(L(III))(+), and cannot effectively compete with the hydrolysis equilibria of UO2(2+) under neutral or alkaline conditions. DFT computations, taking into account the solvation by including discrete hydration water molecules and bulk solvent effects, were performed to evaluate the structures and energies of the possible isomers of UO2(L(III))(+). Differing from the tridentate or η(2)-coordination modes previously found in the U(vi) complexes with amidoxime-related ligands, a bidentate mode, involving the oxygen of the oxime group and the nitrogen of the imino group, is found to be the most probable mode in UO2(L(III))(+). The bidentate coordination mode seems to be stabilized by the formation of a hydrogen bond between the carbonyl group of HL(III) and a water molecule in the hydration sphere of UO2(2+).