Density functional theoretical analysis of structure, bonding, interaction and thermodynamic selectivity of hexavalent uranium (UO2 2+) and tetravalent plutonium (Pu4+) ion complexes of tetramethyl diglycolamide (TMDGA)

Understanding the Interaction of Uranyl Cation with Two C-Pivot Tripodal Amides: Synthesis, Complexation, Microcalorimetry, and DFT Studies

Complexation of uranyl ions with two structurally related C-pivotal tripodal amides with varying spacer lengths, synthesized for the first time, was studied by optical spectroscopy. In the tripodal amides, the coordination was through the carbonyl O atoms where the carbonyl groups were away from the central C-atom by three spacer atoms (LI) and four spacer atoms (LII), respectively. Increasing the spacer atoms going from LI to LII favors the complexation with the linear uranyl cations and results in stronger complex formation. The complexation heat between the uranyl cations and the two amide ligands was directly measured by microcalorimetric titrations. The complexation with both the ligands was driven by exothermic enthalpy and positive entropy changes. Formation of the complex proceeded by the replacement of water molecules from the primary coordination sphere of the uranyl cation. Both ligands formed bisolvated (ML2-type) complexes in which one unit of the ligand binds in a monodentate manner and the other in a bidentate mode. Density functional theory calculations further supported our experimental observations.

Experimental and Theoretical Insight into the Ionic Liquid-Mediated Complexation of Trivalent Lanthanides with β-Diketone and Its Fluorinated Analogue

A multitechnique approach with theoretical insights has been employed to understand the complexation of trivalent lanthanides with two β-diketones, viz. 1-phenyl-1,3-butanedione (L1) and 4,4,4-trifluoro-1-phenyl-1,3-butanedione (L2), in an ionic liquid (C6mim·NTf2). UV-vis spectral analysis of complexation using Nd3+ revealed the predominance of ML2+ and ML4- species. The stability constants for the PB complexes were higher (β2 ∼ 10.45 ± 0.05, β4 ∼ 15.51 ± 0.05) than those for the TPB (β2 ∼ 7.56 ± 0.05, β4 ∼ 13.19 ± 0.06). The photoluminescence titration using Eu3+ corroborated the same observations with slightly higher stability constants, probably due to the higher ionic potential of Eu3+. The more asymmetric (AL2ML4 ∼ 5.2) Eu-L2 complex was found to contain one water molecule in the primary coordination sphere of Eu3+ with more covalency of the Eu3+-O bond (Ω2L1 = 8.5 × 10-20, Ω4L1 = 1.3 × 10-20) compared to the less asymmetric Eu-L1 complex (AL1ML4 ∼ 3.5) with two water molecules having less Eu-O covalency (Judd-Offelt parameters: Ω2L1 = 7.3 × 10-20, Ω4L1 = 1.0 × 10-20). Liquid-liquid extraction studies involving Nd3+ and Eu3+ revealed the formation of the ML4- complex following an 'anion exchange' mechanism. The shift of the enol peak from 1176 to 1138 cm-1 on the complexation of the β-diketones with Eu3+ was confirmed from the FTIR spectra. 1H NMR titration of the β-diketones with La(NTf2)3 evidenced the participation of α-H of the β-diketones and protons at C2, C4, and C5 positions of the methylimidazolium ring. For the ML2 complex, 4 donor O atoms are suggested to coordinate to the trivalent lanthanides with bond distances of 2.3297-2.411 Å for La-O, 2.206-2.236 Å for Eu-O, and 2.217-2.268 Å for Nd-O, respectively, while for the ML4 complex, 8 donor O atoms were coordinated with bond lengths of 2.506-2.559 Å for La-O, 2.367-2.447 Å for Eu-O, and 2.408-2.476 Å for Nd-O. The Nd3+ ion was higher by 9.7 kcal·mol-1 than that of the La3+ ion for the 1:4 complex. The complexation energy with L1 was quite higher than that with L2 for both 1:2 and 1:4 complexes. Using cyclic voltammetry, the redox behavior of trivalent lanthanides Eu and Gd with β-diketonate in ionic liquid medium was probed and their redox energetic and kinetic parameters were determined.

Ionic Liquid Assisted Exothermic Complexation of Trivalent Lanthanides with Fluorinated β Diketone: Multitechnique Approach with Theoretical Insight

The complexation of the betadiketone,1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedione (HFOD) was studied with trivalent lanthanide ions, viz. Nd3+, La3+, and Eu3+ in several methylimidazolium-based ionic liquids (Cnmim•NTf2, where, n = 4,6,8). In C6mim•NTf2, predominant formation of ML2+ and ML4- species was evidenced from the UV-vis absorption (Nd3+) as well as luminescence (Eu3+) spectral studies with log β2 ≈ 5.88 ± 0.04, log β4 ≈ 10.95 ± 0.06. The formation constants followed the trend C4mim•NTf2 > C6mim•NTf2 > C8mim•NTf2. The asymmetry factors for the ML2+ and ML4- species were found to be 1.2 and 1.59, respectively. The ML4- complex was found to have one primary coordination sphere water molecule with enhanced covalency between Eu3+ and O from HFOD (Judd Offelt constants Ω2 and Ω4 ≈ 17.2 and 2.35) compared to Eu3+aq, yet comparable to other β diketones. Complexation-induced temperature increase was confirmed by calorimetric measurements, indicating the exothermic complexation reaction (ΔHcomplexation ≈ -13.7 kJ mol-1), which is also spontaneous in nature (ΔG ≈ -68.1 kJ mol-1), with an enhancement in the entropy values. Due to complexation, the shifts in the peak positions (1686.66 cm-1, 1633.53 cm-1) associated with β diketone/ketone functional groups were evidenced. Density functional theory (DFT) calculation was performed to optimize the structural parameters including bond distance, bond angles, and energetics associated with the complexation.

A computational study on the coordination modes and electron absorption spectra of the complexes U(iv) with N,N,N',N'-tetramethyl-diglycolamide and anions

Lipophilic N,N,N',N'-tetraalkyl-diglycolamides (TRDGAs) are promising extractants for actinides separation in spent nuclear fuel reprocessing. Usually, in the extracted complexes of actinide and lanthanide ions of various oxidation states, the metal ions are completely surrounded by 2 or 3 TRDGA molecules, and the counter anions do not directly coordinate with them. In contrast, the extracted complexes of U(iv) from different media presenting different absorption spectra indicate that the anions (Cl- and NO3-) are directly involved in the coordination with U(iv) in the first inner sphere. Based on this exceptional observation in solvent extraction, taking the coordination of U(iv) with N,N,N',N'-tetramethyl-diglycolamide (TMDGA, the smallest analogue of TRDGA) as the research object, we mimic the behaviours of counterions (Cl- and NO3-) and the water molecule during coordination of TMDGA with U(iv), especially combining with the simulation of the absorption spectra. We demonstrate that during the complexing of TMDGA to U(iv), the counterion Cl- will occupy one coordination number in the inner coordination sphere, and NO3- will occupy two by bidentate type; however, the ubiquitous water cannot squeeze in the inner coordination sphere. In addition, the coordination of Cl- and NO3- is proved to favour the extraction with the lower binding energy. Moreover, the simulation of absorption spectra is in good agreement with the observation from experiments, further verifying the aforementioned conclusion. This work in some way will provide guidance to improve the computation methods in research of actinides by mimicking the absorption spectra of actinide ions in different complexes.

Theoretical Insights on the Complexation of Americium(III) and Europium(III) with Diglycolamide- and Dimethylacetamide-Functionalized Calix[4]arenes

Separation of minor actinides from lanthanides is one of the biggest challenges in spent fuel reprocessing due to the similar physicochemical properties of trivalent lanthanides (Ln(III)) and actinides (An(III)). Therefore, developing ligands with excellent extraction and separation performance is essential at present. As an excellent pre-organization platform, calixarene has received more attention on Ln(III)/An(III) separation. In this work, we systematically explored the complexation behaviors of the diglycolamide (DGA)/dimethylacetamide (DMA)-functionalized calix[4]arene extractants for Eu(III) and Am(III) using relativistic density functional theory (DFT). These calix[4]arene-derived ligands were obtained by functionalization with two or four binding units at the narrow edge of the calix[4]arene platform. All bonding nature analyses suggested that the Eu-L complexes possess stronger interaction compared to Am-L analogues, resulting in the higher extraction capacity of the these calix[4]arene ligands toward Eu(III). Thermodynamic analysis demonstrates that these pre-organized ligands on the calix[4]arene platform with four binding units yield better extraction abilities than the single ligands. Although DMA-functionalized ligands show stronger complexation stability for metal ions, in acidic solutions, the calix[4]arene ligands with DGA binding units have better extraction performance for Eu(III) and Am(III) due to the basicity of the DMA ligand. This work enabled us to gain a deeper understanding of the bonding properties between supramolecular ligands and lanthanides/actinides and afford useful insights into designing efficient supramolecular ligands for separating Ln(III)/An(III).

Theoretical Insights into the Selectivity of Hydrophilic Sulfonated and Phosphorylated Ligands to Am(III) and Eu(III) Ions

The separation of lanthanides and actinides has attracted great attention in spent nuclear fuel reprocessing up to date. In addition, liquid-liquid extraction is a feasible and useful way to separate An(III) from Ln(III) based on their relative solubilities in two different immiscible liquids. The hydrophilic bipyridine- and phenanthroline-based nitrogen-chelating ligands show excellent performance in separation of Am(III) and Eu(III) as reported previously. To profoundly explore the separation mechanism, herein, we first of all designed four hydrophilic sulfonated and phosphorylated ligands L¹, L², L³, and L⁴ based on the bipyridine and phenanthroline backbones. In addition, we studied the structures of these ligands and their neutral complexes [ML(NO₃)₃] (M = Am, Eu) as well as the thermodynamic properties of complexing reactions through the scalar relativistic density functional theory. According to the changes of the Gibbs free energy for the back-extraction reactions, the phenanthroline-based ligands L² and L⁴ have stronger complexing capacity for both Am(III) and Eu(III) ions while the phosphorylated ligand L³ with the bipyridine framework has the highest Am(III)/Eu(III) selectivity. In addition, the charge decomposition analysis revealed a higher degree of charge transfer from the ligand to Am(III), suggesting stronger donor-acceptor interactions in the Am(III) complexes. This study can provide theoretical insights into the separation of actinide(III)/lanthanide(III) using hydrophilic sulfonated and phosphorylated N-donor ligands.

Aggregation Behavior of Nitrilotriacetamide (NTAmide) Ligands in Thorium(IV) Extraction from Acidic Medium: Small-Angle Neutron Scattering, Fourier Transform Infrared, and Theoretical Studies

Two tripodal amides obtained from nitrilotriacetic acid with n-butyl and n-octyl alkyl chains (HBNTA(L_I) and HONTA(L_II), respectively) were studied for the extraction of Th(IV) ions from nitric acid medium. The effect of the diluent medium, i.e., n-dodecane alone and a mixture of n-dodecane and 1-decanol, onto aggregate formation were investigated using small angle neutron scattering (SANS) studies. In addition, the influence of the ligand structure, nitric acid, and Th(IV) loading onto ligand aggregation and third-phase formation tendency was discussed.The L_I/L_II exist as monomers (aggregarte radius for L_I: 6.0 Å; L_II:7.4 Å) in the presence of 1-decanol, whereas L_II forms dimers (aggregarte radius for L_II:9.3 Å; L_I does not dissolve in n-dodecane) in the absence of 1-decanol. The aggregation number increases for both the ligands after HNO₃ and Th(IV) loading. The maximum organic concentration (0.050 ± 0.004 M) of Th(IV) was reached without third-phase formation for 0.1 M L_I/L_II dissolved in 20% isodecanol +80% n-dodecane. The interaction of 1-decanol with L_II and HNO₃/Th(IV) with amidic oxygens of L_I/L_II results in shift of carbonyl stretching frequency, as shown by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) studies. The structural and bonding information of the Th-L_I/L_II complex were derived from the density functional theoretical (DFT) studies. The molecular dynamics (MD) simulations suggested that the aggregation behavior of the ligand in the present system is governed by the population of hydrogen bonds by phase modifier around the ligand molecules. Although the theoretical studies suggested higher Gibbs free energy of complexation for Th⁴⁺ ions with L_I than L_II, the extraction was found to be higher with the latter, possibly due to the higher lipophilicity and solubility of the Th-L_II aggregate in the nonpolar media.

Theoretical Probing of Size-Selective Crown Ether Macrocycle Ligands for Transplutonium Element Separation

Effective separation and recovery of chemically similar transplutonium elements from adjacent actinides is extremely challenging in spent fuel reprocessing. Deep comprehension of the complexation of transplutonium elements and ligands is significant for the design and development of ligands for the in-group separation of transplutonium elements. Because of experimental difficulties of transplutonium elements, theoretical calculation has become an effective means of exploring transplutonium complexes. In this work, we systematically investigated the coordination mechanism between transplutonium elements (An = Am, Cm, Bk, Cf) and two crown ether macrocyclic ligands [N,N'- bis[(6-carboxy-2-pyridyl)methyl]-1,10-diaza-18-crown-6 (H₂bp18c6) and N,N'-bis[(6-methylphosphinic-2-pyridyl)methyl]-1,10-diaza-18-crown-6 (H₂bpp18c6)] through quasi-relativistic density functional theory. The extraction complexes of [Anbp18c6]⁺ and [Anbpp18c6]⁺ possess similar geometrical structures with actinide atoms located in the cavity of the ligands. Bonding nature analysis indicates that the coordination ability of the coordinating atoms in pendent arms is stronger than that in the crown ether macrocycle because of the limitation of the macrocycle. Most of the coordination atoms of the H₂bp18c6 ligand have a stronger ability to coordinate with metal ions than those of the H₂bpp18c6 ligand. In addition, the bonding strength between the metal ions and ligands gradually weakens from Am to Cf, which is mainly attributed to the size selectivity of the ligands. Thermodynamic analysis shows that the H₂bp18c6 ligand has a stronger extraction capacity than the H₂bpp18c6 ligand, while the H₂bpp18c6 ligand is superior in terms of the in-group separation ability. The extraction capacity of the two ligands for metal ions gradually decreases across the actinide series, indicating that these crown ether macrocycle ligands have size selectivity for these actinide cations as a result of steric constraint of the crown ether ring. We hope that these results offer theoretical clues for the development of macrocycle ligands for in-group transplutonium separation.

Two-Dimensional Imprinting Strategy to Create Specific Nanotrap for Selective Uranium Adsorption with Ultrahigh Capacity

Uranium extraction is highly challenging because of low uranium concentration, high salinity, and a large number of competing ions in different environments. The template strategy is developed to address the defect of poor selectivity, but the adsorption capacity is limited by cavity blocking during the preparation of materials. Herein, a two-dimensional (2D) imprinting strategy is adopted to design 2D imprinted networks with specific nanotraps for effective uranium capture. The imprinted networks are established through the condensation polymerization of uranyl complexes, which are formed by aromatic building units coordinating with uranyl ions on the equatorial plane. Different from traditional imprinting materials that contain many invalid cavities (buried cavities or unreleased cavities), the as-prepared adsorbents possess tailored 2D nanotraps, which are open and specific to uranyl. Thus, the optimized networks not only show excellent selectivity for uranium (K_d = 964,500 mL/g in multi-ion solution) and slight disturbance of high salinity but also possess an ultrahigh adsorption capacity of 1365.7 mg/g. In addition, this adsorbent shows a high extraction efficiency for uranium under a wide range of pH conditions and exhibits good regeneration performance. This work proposes a pioneering strategy of 2D imprinting networks to capture uranium specifically with high capacity and can be applied to material design in many other fields.

Ionic Liquids for the Selective Solvent Extraction of Lithium from Aqueous Solutions: A Theoretical Selection Using COSMO-RS

In this study, the theoretical design of ionic liquids (ILs) for predicting selective extraction of lithium from brines has been conducted using COSMO-RS. A theoretical model for the solvent extraction (SX) of the metal species present in brines was established considering extraction stoichiometry, the distribution of the extractants between aqueous and IL phases, and IL dissociation in the aqueous phase. Theoretical results were validated using experimental extraction percentages from previous works. Results indicate that, in general, the theoretical results for lithium extraction follow experimental trends, except from magnesium extraction. Finally, based on the model, an IL was proposed that was based on the phosphonium cation as the extractant, along with the phase modifier tributylphosphate (TBP) in an organic diluent in order to improve selectivity for lithium extraction over sodium. These results provide an insight for the application of ILs in lithium processing, avoiding the long purification times reported in the conventional process.

Role of ligand spacer length of a tripodal amide on uranium(vi) and plutonium(iv) complexation: synthesis, solvent extraction, liquid membrane transport and theoretical studies

Liquid–liquid extraction of several actinide and lanthanide ions, viz., U(vi), Pu(iv), Am(iii) and Eu(iii) was investigated from nitric acid feed solutions employing N,N,N′,N′,N′′,N′′-hexa-n-octyl nitrilotripropamide (HONTP) in 90% n-dodecane + 10% isodecanol.

Theoretical Insights into the Separation of Am(III)/Eu(III) by Hydrophilic Sulfonated Ligands

In this work, we focused on the separation of Am(III)/Eu(III) with four hydrophilic sulfonated ligands (L) based on the framework of phenanthroline and bipyridine through scalar relativistic density functional theory. We studied the electronic structures of [ML(NO₃)₃] (M = Am, Eu) complexes and the bonding nature between metal and ligands as well as evaluated the separation selectivity of Am(III)/Eu(III). The tetrasulfonated ligand L² with a bipyridine framework has the strongest complexing ability for metal ions probably because of the better solubility and flexible skeleton. The disulfonated ligand L¹ has the highest Am(III)/Eu(III) selectivity, which is attributed to the covalent difference between the Am-N and Eu-N bonds based on the quantum theory of atoms in the molecule analysis. Thermodynamic analysis shows that the four hydrophilic sulfonated ligands are more selective toward Am(III) over Eu(III). In addition, these hydrophilic sulfonated ligands show better complexing ability and Am(III)/Eu(III) selectivity compared to the corresponding hydrophobic nonsulfonated ones. This work provides theoretical support for the separation of Am(III)/Eu(III) using hydrophilic sulfonated ligands.

Considering Density Functional Approaches for Actinide Species: The An66 Molecule Set

The importance of spin-orbit effects on the predictions of energetic properties of actinide compounds has been considered for 18 different density functionals, comparing the spin-orbit and non-spin-orbit ("standard") forms of density functional theory (DFT). A set of enthalpies of formation for 66 small actinide (Th-Am) compounds-the An66 set, for which experimental data are available-have been investigated. The set includes actinide halides, oxides, and oxohalides in the general form AnO_mX_n, where n = 0-6, m = 0-3, and X = F, Cl, Br, or I. The impact of basis set choice was investigated, and to help account for the impact of relativity, the Stuttgart general and segmented contracted atomic natural orbital (ANO) basis sets paired with small core relativistic effective core potentials (RECP) as well as all-electron calculations utilizing the third-order Douglas-Kroll-Hess were considered.

In Situ Preconcentration during the Di-(2-ethylhexyl) Phosphoric Acid-Assisted Dissolution of Uranium Trioxide in an Ionic Liquid: Spectroscopic, Electrochemical, and Theoretical Studies

Dissolution of uranium oxide was carried out using a solution of HD2EHP in C₈mim·NTf₂, which was apparently facilitated by the in situ generation of water during the complex formation reaction. The dissolved complex in the ionic liquid phase led to splitting of the latter into a light phase and a heavy phase where the former contained predominantly the UO₂(HL₂)₂ complex (HL = HD2EHP), while the latter contained the ionic liquid as supported by FTIR and UV-Visible spectral analyses. The complexation of the uranyl ion was suggested to take place in the equatorial plane where two dimeric units of the H-bonded HD2EHP molecules took part in complexation. An increase in temperature facilitated the dissolution rate with an activation energy of 31.0 ± 2.8 kJ/mol. The cyclic voltammetry studies indicated potential chances of recovery of the dissolved uranium by electrodeposition at the cathode. The proposed dimeric structure of HD2EHP in the complexation with U(VI) was supported by DFT studies also.

Insights from quantum chemical calculations into inner and outer-sphere complexation of plutonium(IV) by monoamide and carbamide extractants

The strong influence of the structure of amide derivatives on their extraction properties has been demonstrated in several studies in the literature. To investigate and rationalize the influence of the nature and length of the monoamide alkyl chains on Pu(iv) extraction/complexation, a theoretical study was performed using the Density Functional Theory (DFT) method in the scalar relativistic framework. For that, the geometries for the inner/outer-sphere complexes and interaction energies of [Pu(NO3)4] and [Pu(NO3)6]2- with different ligands have been calculated. For both inner and outer-sphere complexes, it is found that the introduction of a bulky alkyl group on the carbonyl side strongly diminishes the complexation energy. This is fully consistent with monamide extraction properties. The influence of the bulkiness of the alkyl group is as or even more important for outer than for inner-sphere interactions. This result was unexpected when considering that there are less flexibility and stronger steric constraints in the inner sphere compared to the outer one. However, this can be attributed to specific electrostatic interactions between the two outer-sphere amide ligands and two nitrate ions of [Pu(NO3)6]2-. By increasing the polarity of the solution, such interactions diminish and the outer-sphere ligands move away from [Pu(NO3)6]2-. Consequently, the solvent effects were found to be very significant for outer-sphere complexation while rather small for inner-sphere complexation. This gives the key possibility to tune the substituent effect by changing the polarity of the solution. As for carbamide ligands, it was found that the weak interactions (dispersion) have remarkable effects on both inner and outer-sphere complexations.

Highly efficient diglycolamide-functionalized dendrimers for the sequestration of tetravalent actinides: solvent extraction and theoretical studies

Two DGA-functionalized dendrimers were used for the extraction and bonding computations of tetravalent actinide ions Np⁴⁺ and Pu⁴⁺ from HNO₃ medium.

Prediction of Binding Stability of Pu(IV) and PuO2(VI) by Nitrogen Tridentate Ligands in Aqueous Solution

Plutonium has potential applications in energy production in well-controlled nuclear reactors. Since nuclear power plants have great merit as environmentally friendly energy sources with a recyclable system, a recycling system for extracting Pu from spent fuels using suitable extractants has been proposed. Pu leakage is a potential environmental hazard, hence the need for chemical sensor development. Both extractants and chemical sensors involve metal–ligand interactions and to develop efficient extractants and chemical sensors, structural information about Pu ligands must be obtained by quantum calculations. Herein, six representative nitrogen tridentate ligands were introduced, and their binding stabilities were evaluated. The tridentate L6, which contains tri-pyridine chelate with benzene connectors, showed the highest binding energies for Pu(IV) and PuO₂(VI) in water. Analysis based on the quantum theory of atoms in molecular analysis, including natural population analysis and electron density studies, provided insight into the bonding characteristics for each structure. We propose that differences in ionic bonding characteristics account for the Pu-ligand stability differences. These results form a basis for designing novel extractants and organic Pu sensors.

Exploring functionalized titania for task specific application of efficient separation of trivalent f-block elements

Functionalized titania, obtained by grafting the dipicolinic acid functionality, was explored for task specific application of highly efficient separation of trivalent f-block elements.

Quantum chemical studies of selective back-extraction of Am(III) from Eu(III) and Cm(III) with two hydrophilic 1,10-phenanthroline-2,9-bis-triazolyl ligands

We theoretically investigated the selective back-extraction towards Am(III) over Eu(III) and Cm(III) with two water-soluble 2,9-bis-triazolyl-1,10-phenanthroline derivatives BTrzPhen1 (with two ethanol side chains) and BTrz-Phen2 (with two 1,2-butanediol side chains) by density functional theory (DFT). The molecular geometries and formation reactions of the metal-ligand complexes were modeled by using M(BTrzPhen)(NO3)3 and [M(BTrzPhen)2(NO3)]2+. Am(III) selectivity over Eu(III) and Cm(III) with BTrzPhen2 was successfully reproduced by back-extraction reaction free energy analysis. Moreover, bonding properties between the metal cations and coordinated ligands (model complexes) were studied in terms of Mayer bond order and quantum theory of atoms in molecule (QTAIM). The difference in covalency between An–N and Eu–N bonds were found to be the key factors for Am(III)/Eu(III) separation, while the Am(III) selectivity over Cm(III) of BTrzPhen2 might be attributed to the competition of donor atoms for cation binding preference toward Am(III) and Cm(III).

Highly Efficient N-Pivot Tripodal Diglycolamide Ligands for Trivalent f-Cations: Synthesis, Extraction, Spectroscopy, and Density Functional Theory Studies

A series of four N-pivot tripodal diglycolamide (DGA) ligands, where three DGA moieties are attached to the central N atom via spacers of different lengths and with varying alkyl substituents on the amidic nitrogen of DGA (L_I-L_IV), were studied for their extraction and complexation ability toward trivalent lanthanide/actinide ions, including solvent extraction, complexation using spectrophotometric titrations, and luminescence spectroscopic studies. Introduction of a methyl group on the amidic nitrogen atom gives rise to a 400 fold increase of the Eu distribution ( D) value [L_III (NMe) vs L_II (NH)] at 1 M HNO₃. Enlargement of the spacer length between the pivotal N atom and the DGA moieties with one carbon atom results in a 14 times higher D_Eu value [L_I (C3) vs L_II (C2)]. Slope analyses showed that Eu³⁺ was extracted as a bis-solvated species with all four ligands. The compositions of the Eu³⁺/L complexes were further confirmed by spectroscopic measurements, its formation constants following the order: L_III > L_IV > L_I > L_II. Luminescence spectroscopy and electrospray ionization mass spectrometry revealed that all four ligands form [Eu(L)₂(NO₃)₃] complexes. Density functional theory and thermodynamic parameters corroborated the existence of [Eu(L)₂(NO₃)₃] complexes.

Role of Ligand Straining in Complexation of Eu3+-Am3+ Ions by TPEN and PPDEN, Scalar Relativistic DFT Exploration in Conjunction with COSMO-RS

To search for new ligands suitable for the separation of minor actinides (MA) from lanthanides (Ln) in nuclear waste reprocessing, theoretical (density functional theory) studies were carried out on the complexation (structures, bonding, and thermodynamics) of La³⁺, Sm³⁺, Eu³⁺, and Am³⁺ complexes with moderately soft donor ligands TPEN [N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine] and PPDEN [N,N,N',N″,N″-pentakis(2-pyridylmethyl) diethylenetriamine] in aqueous and nitrobenzene solutions. B3LYP level of theory was used in conjunction with the conductor-like screening model for real systems (COSMO-RS). The metal ions in [M(NO₃)₂(TPEN)]NO₃ and [M(NO₃)(PPDEN)](NO₃)₂ complexes were deca-coordinated with both TPEN and PPDEN. The enthalpy of the complexation with TPEN in an aqueous solution was found to be negative, indicating the exothermic nature of the reaction as observed in the experiments. The calculated values of free energy of complexation follow the experimental trend: Am³⁺ > Sm³⁺ > La³⁺. Furthermore, the calculated free energy with PPDEN is reduced compared to that with TPEN, which may be attributed to the ligand straining during complex formation, which is also reflected in greater residual charges on both the Eu³⁺ and Am³⁺ central ions in the complexes of octadentate PPDEN compared to hexadentate TPEN. The experimental complexation selectivity of Am³⁺ over Eu³⁺ with TPEN is established by employing COSMO-RS. Furthermore, TPEN is Am³⁺-selective, whereas PPDEN is Eu³⁺-selective, which could be exploited for the efficient separation of MA from Ln.

Complexation thermodynamics of tetraalkyl diglycolamides with trivalent f-elements in ionic liquids: spectroscopic, microcalorimetric and computational studies

The thermodynamics of lanthanide complexation with DGA ligands was studied in an ionic liquid.

Understanding Am3+/Cm3+ separation with H4TPAEN and its hydrophilic derivatives: a quantum chemical study

Quantum chemical calculations have been used to help understand the back-extraction and separation of Am³⁺/Cm³⁺ with H₄TPAEN and its two hydrophilic derivatives.

Enhanced free energy of extraction of Eu3+ and Am3+ ions towards diglycolamide appended calix[4]arene: insights from DFT-D3 and COSMO-RS solvation models

Density functional theory in conjunction with COSMO and COSMO-RS solvation models employing dispersion correction (DFT-D3) has been applied to gain an insight into the complexation of Eu³⁺/Am³⁺ with diglycolamide (DGA) and calix[4]arene appended diglycolamide (CAL4DGA) in ionic liquids by studying structures, energetics, thermodynamics and population analysis. The calculated Gibbs free energy for both Eu³⁺ and Am³⁺ ions with DGA was found to be smaller than that with CAL4DGA. The entropy of complexation was also found to be reduced to a large extent with DGA compared to complexation with CAL4DGA. The solution phase free energy was found to be negative and was higher for Eu³⁺ ion. The entropy of complexation was not only found to be further reduced but also became negative in the case of DGA alone. Though the entropy was found to be negative it could not outweigh the high negative enthalpic contribution. The same trend was observed in the solution where the free energy of extraction, ΔG, for Eu³⁺ ions was shown to be higher than that for Am³⁺ ions towards free DGA. But the values of ΔG and ΔΔG(= ΔG_Eu-ΔG_Am) were found to be much higher with CAL4DGA (-12.58 kcal mol^-1) in the presence of nitrate ions compared to DGA (-1.69 kcal mol^-1) due to enhanced electronic interaction and positive entropic contribution. Furthermore, both the COSMO and COSMO-RS models predict very close values of ΔΔΔG (= ΔΔG_CAL4DGA - ΔΔG_nDGA), indicating that both solvation models could be applied for evaluating the metal ion selectivity. The value of the reaction free energy was found to be higher after dispersion correction. The charge on the Eu and Am atoms for the complexes with DGA and CAL4DGA indicates the charge-dipole type interaction leading to strong binding energy. The present theoretical results support the experimental findings and thus might be of importance in the design of functionalized ligands.

Unusual extraction of trivalent f-cations using diglycolamide dendrimers in a room temperature ionic liquid: extraction, spectroscopic and DFT studies

The complexation of Am³⁺ and Eu³⁺ was studied with three generations of DGA-Den ligands in a room temperature ionic liquid.

Diglycolamic acid-functionalized multiwalled carbon nanotubes as a highly efficient sorbent for f-block elements: experimental and theoretical investigations

Diglycolamic acid-functionalized carbon nanotubes were employed for the efficient and selective separation of Pu⁴⁺, PuO₂²⁺ and Am³⁺.

Sorption behaviour of Pu4+ and PuO22+ on amido amine-functionalized carbon nanotubes: experimental and computational study

Amido amine-functionalized multi-walled carbon nanotubes (MWCNT-AA) were used for efficient and selective solid phase separation of plutonium(iv) and plutonium(vi).