Extraction of uranyl from spent nuclear fuel wastewater via complexation-a local vibrational mode study

CONTEXT

The efficient extraction of uranyl from spent nuclear fuel wastewater for subsequent reprocessing and reuse is an essential effort toward minimization of long-lived radioactive waste. N-substituted amides and Schiff base ligands are propitious candidates, where extraction occurs via complexation with the uranyl moiety. In this study, we extensively probed chemical bonding in various uranyl complexes, utilizing the local vibrational modes theory alongside QTAIM and NBO analyses. We focused on (i) the assessment of the equatorial O-U and N-U bonding, including the question of chelation, and (ii) how the strength of the axial U = O bonds of the uranyl moiety changes upon complexation. Our results reveal that the strength of the equatorial uranium-ligand interactions correlates with their covalent character and with charge donation from O and N lone pairs into the vacant uranium orbitals. We also found an inverse relationship between the covalent character of the equatorial ligand bonds and the strength of the axial uranium-oxygen bond. In summary, our study provides valuable data for a strategic modulation of N-substituted amide and Schiff base ligands towards the maximization of uranyl extraction.

METHOD

Quantum chemistry calculations were performed under the PBE0 level of theory, paired with the relativistic NESCau Hamiltonian, currently implemented in Cologne2020 (interfaced with Gaussian16). Wave functions were expanded in the cc-pwCVTZ-X2C basis set for uranium and Dunning's cc-pVTZ for the remaining atoms. For the bonding properties, we utilized the package LModeA in the local modes analyses, AIMALL in the QTAIM calculations, and NBO 7.0 for the NBO analyses.

Effect of Oxygen-Donor Charge on Adjacent Nitrogen-Donor Interactions in Eu3+ Complexes of Mixed N,O-Donor Ligands Demonstrated on a 10-Fold [Eu(TPAMEN)]3+ Chelate Complex

To reduce high-level radiotoxic waste generated by nuclear power plants, highly selective separation agents for minor actinides are mandatory. The mixed N,O-donor ligand N,N,N',N'-tetrakis[(6-carboxypyridin-2-yl)methyl]ethylenediamine (H₄TPAEN; 1) has shown good performance as a masking agent in Am³⁺/Eu³⁺ separation studies. Adjustments on the pyridyl backbone to raise the hydrophilicity led to a decrease in selectivity and a decrease in M³⁺-N^am interactions. An enhanced basicity of the pyridyl N-donors was given as a cause. In this work, we examine whether a decrease in O-donor basicity can promote the M³⁺-N^am interactions. Therefore, we replace the deprotonated "charged" carboxylic acid groups of TPAEN^4- by neutral amide groups and introduce N,N,N',N'-tetrakis[(6-N″,N''-diethylcarbamoylpyridin-2-yl)methyl]ethylenediamine (TPAMEN; 2) as a new ligand. TPAMEN was crystallized with Eu(OTf)₃ and Eu(NO₃)₃·6H₂O to form positively charged 1:1 [Eu(TPAMEN)]³⁺ complexes in the solid state. Alterations in the M-O/N bond distances are compared to [Eu(TPAEN)]^- and investigated by DFT calculations to expose the differences in charge/energy density distributions at europium(III) and the donor functionalities of the TPAEN^4- and TPAMEN. On the basis of estimations of the bond orders, atomic charges spin populations, and density of states in the Eu and potential Am and Cm complexes, the specific contributions of the donor-metal interaction are analyzed. The prediction of complex formation energy differences for the [M(TPAEN)]^- and [M(TPAMEN)]³⁺ (M³⁺ = Eu³⁺, Am³⁺) complexes provide an outlook on the potential performance of TPAMEN in Am³⁺/Eu³⁺ separation.

The unique molecular behavior of water at the chloroform-water interface

The molecular bonding and orientation of water at the chloroform-water interface has been examined in this study using vibrational sum-frequency spectroscopy (VSFS). The results provide a key puzzle piece towards our understanding of the systematic changes in the interfacial bonding and orientation of water that occur with variations in the polarity of the organic phase, especially when compared with previous studies of different liquid-liquid interfacial systems. In these VSFS studies the OH spectral responses of interfacial water molecules are used to characterize the interactions between water and the organic phase. The spectral analysis, aided by isotopic dilution studies, shows that the moderate polarity of the chloroform phase results in a mixed interfacial region with stronger organic-water bonding and fewer bonding interactions between adjacent water molecules than was previously found for studies of non-polar organic liquid-water interfaces. Even with the more mixed interfacial region and stronger organic-water interactions, interfacial water retains a significant amount of orientational ordering. These results are compared with recent predictions from molecular dynamics simulations about how molecules behave at the chloroform-water interface.

Synthesis and biological evaluation of purine derivatives incorporating metal chelating ligands as HIV integrase inhibitors

Because of its essential role in HIV replication and lack of human counterpart, HIV integrase is an attractive target for the development of novel anti-AIDS agents. Among the recently developed integrase inhibitors, only the alpha,gamma-diketo acid (DKA) compounds were biologically validated as potent and selective integrase inhibitors. The general structure of DKAs contains a diketo acid moiety as the Mg(2+) chelating pharmacophore, and an adjacent aryl group to provide selectivity. Numerous structure-activity relationship (SAR) studies on DKAs have been conducted, which generally involved substituting the carboxylate group or the aryl group. Our objective was to investigate the SARs of the DKA molecule by incorporating a purine ring in the aryl moiety and replacing the labile diketo acid moiety with other divalent metal (Me(2+)) chelating ligands. A series of amide substituted purine derivatives were synthesized via palladium-catalyzed amidation reactions, and their biological activities against HIV integrase were evaluated. These purine derivatives showed anti-integrase activity at low micromolar range. The biological results indicated that the type of Me(2+) ligands, two-point ligand picolinamide or three-point ligand 8-hydroxy-quinoline-7-carboxamide, affected inhibitory potency depending on the substitution position of the para-fluorobenzyl group. The C(6)-,C(8)-dipicolinamide substituted purine (32) exhibited the best potency among this series.