Systematic Raman spectroscopic study of the complexation of uranyl with fluoride

A simple aqueous complexing system of UO22+ with F- is selected to systematically illustrate the application of Raman spectroscopy in exploring uranyl(VI) chemistry. Five successive complexes, UO2F+, UO2F2(aq), UO2F3-, UO2F42-, and UO2F53-, are identified, as well as the formation constants except for the 1 : 5 species UO2F53-, which was experimentally observed here for the first time. The standard relative molar Raman scattering intensity for each species is obtained by deconvolution of the spectra collected during titrations. The results of relativistic quantum chemical first-principles and ab initio calculations are presented for the complete set of [UO2(H2O)mFn]2-n complexes (n = 0-5), both for the gas phase as well as for aqueous solution modelling bulk water using the conductor-like screening model. Electronic structure calculations at the Møller-Plesset second-order perturbation theory level provide accurate geometrical parameters and in particular reveal that k water molecules in the second coordination sphere coordinating to the F- ligands in the resulting [UO2(H2O)mFn]2-n(H2O)k complexes need to be treated explicitly in order to obtain vibrational frequencies in very good agreement with experimental data. The thermodynamics and structural information obtained in this work and the developed methodology could be instructive for the future experimental and computational research on the complexation of the uranyl ion.

Influence of Aqueous Phase Acidity on Ln(III) Coordination by N,N,N',N'-Tetraoctyldiglycolamide

This study highlights the importance of combining distribution ratio measurements with multiple spectroscopic techniques to provide a more comprehensive understanding of organic phase Ln coordination chemistry. Solvent extraction investigations with N,N,N',N'-tetraoctyldiglycolamide (TODGA) in n-heptane reveal the sensitivity of Ln complexation to the HNO3 concentration. Distribution ratio measurements in tandem with UV-Vis demonstrated that increasing the concentration of HNO3 above 0.5 M with a constant NO3- of 1 M increases the number of coordinating TODGA molecules, from a 1:2 to a 1:3 Ln:TODGA complex. At each concentration of HNO3 considered herein (from 0.01 to 1 M), Eu lifetime analysis demonstrated no evidence of H2O coordination. Results from Fourier transform infrared investigations suggest the presence of inner-sphere NO3- under low concentrations of HNO3 when the 1:2 Ln:TODGA complex is present. Increasing the HNO3 concentration above 0.5 M increases the propensity for outer-sphere interactions by removing the coordinated NO3- and saturating the Ln coordination sphere with three TODGA molecules, resulting in the well-established cationic, trischelate homoleptic [Ln(TODGA)3]3+ complex. This work demonstrates the importance in considering the NO3- source for solvent extraction systems. In particular, for systems with an affinity for outer-sphere interactions with molar concentrations of HNO3, changing the NO3- source can change the inner-sphere coordination of the Ln complex, which, in turn, affects the separation efficacy.

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.

Influence of Electronic Modulation of Phenanthroline-Derived Ligands on Separation of Lanthanides and Actinides

The solvent extraction, complexing ability, and basicity of tetradentate N-donor 2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline (CyMe4-BT- Phen) and its derivatives functionalized by Br, hydroxyphenyl, nitryl were discussed and compared. It was demonstrated that four BTPhen ligands are able to selectively extract Am(lll) over Eu(lll). It was notable that the distribution ratio of 5-nitryl-CyMe4-BTPhen for Eu(lll) was suppressed under 0.02, which was much lower compared to DEu(lll) = 1 by CyMe4-BTPhen. The analysis of the effect of the substituent on the affinity to lanthanides was conducted by UV/vis and fluorescence spectroscopic titration. The stability constants of various ligands with Eu(lll) were obtained by fitting titration curve. Additionally, the basicity of various ligands was determined to be 3.1 ± 0.1, 2.3 ± 0.2, 0.9 ± 0.2, 0.5 ± 0.1 by NMR in the media of CD3OD with the addition of DClO4. The basicity of ligands follows the order of L1 > L2 > L3 > L4, indicating the tendency of protonation decreases with the electron-withdrawing ability increase.

Study on the extraction of lanthanides by isomeric diglycolamide extractants: an experimental and theoretical study

Two isomeric diglycolamide (DGA) extractants, N,N′-dimethyl-N,N′-dioctyl diglycolamide (L^I) and N,N-dimethyl-N′,N′-dioctyl diglycolamide (L^II), were used to perform a comparative study on the extraction performances towards several lanthanides by extraction experiments and theoretical calculations. The experimental results show that both L^I and L^II show a positive sequence on the extraction of lanthanides, and L^I exhibits the higher complex ability with these lanthanides than L^II, except for La and Ce. Slope analysis shows that 1:2 or 1:3 complexes are formed between the two ligands and the metal ions. The geometrical structures of the complexes were optimized in the gas phase by density functional theory (DFT) on the basis of complex compositions. The results of bond lengths, MBOs and topological analysis indicated that the electrostatic interaction between metal ions and two amide O atoms in the L^II ligand is not as homogeneous as in L^I, and this inhomogeneity is likely to be related to the poor extraction performance of L^II.

The ‘imbalance’ of the coordination ability of the two O_amide affects the extraction performance by experimental and theoretical study.

Gas-phase structure, bonding, and fragmentation chemistry of the An (IV)-TMPDCAM complexes studied using mass spectrometry and theoretical calculation (An = Th and U)

RATIONALE

Pyridine-2,6-dicarboxamides (PDCAMs) exhibit a certain extraction ability for tetravalent actinide ions, but quite limited information regarding the structures and reactivities of the corresponding An⁴⁺ -PDCAMs complexes is available. Neutral diamides can form multiply charged complexes with lanthanide and actinide cations, which are well suited for gas-phase investigations using electrospray ionization (ESI) mass spectrometry in conjunction with theoretical calculation.

METHODS

Binary Th (TMPDCAM)₃ ⁴⁺ /U (TMPDCAM)₃ ⁴⁺ (TMPDCAM = N,N,N',N'-tetramethylpyridine-2,6-dicarboxamide) complexes were generated in the gas phase via ES) of Th (ClO₄ )₄ /U (ClO₄ )₄ and TMPDCAM mixtures in acetonitrile; collision-induced dissociation (CID) was employed to reveal their fragmentation behaviors; the structure and bonding were investigated by density functional theory (DFT) calculation.

RESULTS

An (TMPDCAM)₃ ⁴⁺ (An = Th and U) tetracations dominated the ESI mass spectra of An (ClO₄ )₄ and TMPDCAM mixtures in acetonitrile. DFT calculations indicate that the two An (TMPDCAM)₃ ⁴⁺ complexes have C₃ geometry, and the bonding analyses demonstrate that the thorium or uranium center interacts with both O_carbonyl and N_pyridine , but the latter is weaker. CID of Th (TMPDCAM)₃ ⁴⁺ generated a series of multiply charged thorium-containing products via bond cleavages of the TMPDCAM ligand, whereas U (TMPDCAM)₃ ⁴⁺ yielded only oxygen extraction product UO (TMPDCAM)²⁺ and hydrolysis product UO (OH)⁺ .

CONCLUSION

An⁴⁺ (An = Th and U) can form stable tetrapositive complexes in the gas phase on coordination of three neutral TMPDCAM ligands. The structure and bonding analyses indicate that the two An (TMPDCAM)₃ ⁴⁺ complexes possess twisted tricapped trigonal prismatic geometry and the An⁴⁺ centers are coordinated by six O_carbonyl and three N_pyridine atoms while the interactions between An⁴⁺ and O_carbonyl are stronger. The fragmentation chemistry of Th (TMPDCAM)₃ ⁴⁺ and U (TMPDCAM)₃ ⁴⁺ is quite different from each other, which reveals that the gas-phase chemistry of quadruply charged actinide-diamide complexes is affected by the metal centers with distinct properties.

A comparative study on the coordination of diglycolamide isomers with Nd(iii): extraction, third phase formation, structure, and computational studies

A novel asymmetric diglycolamide N,N-dimethyl-N′,N′-dioctyl diglycolamide (L^II) was synthesized. The Nd(iii) extraction behavior from HNO₃ and loading capability of the solution of L^II in 40/60 (v/v)% n-octanol/kerosene were studied. Analyses by the slope method, ESI-MS, and FT-IR indicated that, similar to the previously studied isomer ligand N,N′-dimethyl-N,N′-dioctyl diglycolamide (L^I), 1 : 3 Nd(iii)/L^II complexes formed. Under the same experimental conditions, the distribution ratio and limiting organic concentration of L^II towards Nd(iii) were smaller than those of L^I, but the critical aqueous concentration of L^II was larger, which implies that L^II exhibited poorer extraction and loading capabilities towards Nd(iii) than L^I, and L^II has a tendency to be less likely to form the third phase. The quasi-relativistic density functional theory (DFT) calculation was performed to provide some explanations for the differences in their extraction behaviors. The electrostatic potential of the ligands indicated that the electron-donating ability of the amide O atoms in L^II displayed certain differences compared with L^I. This inhomogeneity in L^II affected the interaction between L^II and Nd(iii), as supported by QTAIM and bonding nature analysis, and it seemed to reflect in the extraction performance towards Nd(iii).

The inhomogeneous interactions of M–O_amide in the L^II ligand result in differences between the metal-ion extraction performances of two isomeric ligands.

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.

Novel diamide ligands with a central carbonyl group and their comparative evaluation with the diglycolamide ligand: synthesis, extraction, DFT and chromatographic studies

The extraction behaviour of U(VI), Th(IV) and Nd(III) was investigated as a function of nitric acid concentration for diamide based extractants, namely, N,N,N′,N′-tetraoctyl-3-carbonylpentanediamide (TOCPDA) and 4-carbonyl-heptanedioic acid bis-dioctylamide (CHADA). In addition, the distribution ratio was also measured for Pu(IV) and Sr(II) with 1.1 M CHADA in n-dodecane. These extractants were synthesized by adopting simple acid, amine coupling reaction with DCC (dicyclohexylcarbodiimide) and DMAP (N,N′-dimethylaminopyridine) as the coupling agent. The newly synthesized extractants were characterized by FT-IR, NMR, Mass, CHNS and HPLC. The extraction results indicated that CHADA shown has better extraction behavior for U(VI) compared to TOCPDA. In addition, CHADA coated HPLC column was examined for the retention behaviour of U(VI), Th(IV), and Nd(III). Computation studies based on density functional theory (DFT) were carried out to understand the complexing behaviour of U(VI), Pu(IV) and Sr(II) with CHADMA and TMCPDA.

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.

A diglycolamide-functionalized TREN-based dendrimer with a ‘crab-like’ grip for the complexation of actinides and lanthanides

TREN-G1-DGA, a multiple DGA ligand, was synthesized and evaluated for the extraction of actinides and fission product ions.

Insight into the Complexation of Actinides and Lanthanides with Diglycolamide Derivatives: Experimental and Density Functional Theoretical Studies

Extraction of actinide (Pu⁴⁺, UO₂²⁺, Am³⁺) and lanthanide (Eu³⁺) ions was carried out using different diglycolamide (DGA) ligands with systematic increase in the alkyl chain length from n-pentyl to n-dodecyl. The results show a monotonous reduction in the metal ion extraction efficiency with increasing alkyl chain length and this reduction becomes even more prominent in case of the branched alkyl (2-ethylhexyl) substituted DGA (T2EHDGA) for all the metal ions studied. Steric hindrance provided by the alkyl groups has a strong influence in controlling the extraction behavior of the DGAs. The distribution ratio reduction factor, defined as the ratio of the distribution ratio values of different DGAs to that of T2EHDGA, in n-dodecane follows the order UO₂²⁺ > Pu⁴⁺ > Eu³⁺ > Am³⁺. Complexation of Nd³⁺ was carried out with the DGAs in methanol by carrying out UV-vis spectrophotometric titrations. The results indicate a significant enhancement in the complexation constants upon going from methyl to n-pentyl substituted DGAs. They decreased significantly for DGAs containing alkyl substituents beyond the n-pentyl group, which corresponds to the observed trend from the solvent extraction studies. DFT-based calculations were performed on the free and the Nd³⁺ complexes of the DGAs both in the gas and the solvent (methanol) phase and the results were compared the experimental observations. Luminescence spectroscopic investigations were carried out to understand the complexation of Eu³⁺ with the DGA ligands and to correlate the nature of the alkyl substituents on the photophysical properties of the Eu(III)-DGA complexes. The monoexponential nature of the decay profiles of the complex revealed the predominant presence of single species, while no water molecules were present in the inner coordination sphere of the Eu³⁺ ion.

Complexation of tetraalkyl diglycolamides with trivalent f-cations in a room temperature ionic liquid: extraction and spectroscopic investigations

This paper reports the complexation of a series of tetraalkyl diglycolamides (TRDGA) with trivalent f-cations in a room temperature ionic liquid,viz. 3-octyl-1-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C₈mim][Tf₂N]).