Uranyl complexes with diamide ligands: a quantum mechanics study of chelating properties in the gas phase

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.

Rationale for the reactivity differences between main group and d0 transition metal complexes toward olefin polymerisation

In contrast to early transition metal complexes of d0 electron configuration, their main group metal analogues are usually poor catalysts for ethylene polymerisation due to their diminished tendency to insert ethylene into an M-R bond. Interestingly, we found that ring strain in the transition structure of the insertion reaction is most likely responsible to set the ease of the process. Ethylene insertion into an M-R bond requires a four-membered ring transition structure. Strain in a four-membered ring was shown to be dependent on the metal identity (transition or main group/d or p block). For early transition metals, due to the presence of empty valence d orbitals, the strain is negligible but, for main group metals, the strain is significant and so destabilizes the corresponding transition structure. Our claim gains support from investigation of ethylene insertion into an M-allyl bond. In this case, the relevant insertion preferentially passes through a six-membered ring transition structure with an accessibly low activation barrier. In contrast to four-membered ring transition structures, six-membered ones do not suffer significantly from ring strain, causing the insertion activation barrier to become independent of the metal identity. It becomes obvious from our study that this previously undisclosed factor should play the pivotal role in determining the reactivity of many catalysts.

Synthesis and characterization of accessible metal surfaces in calixarene-bound gold nanoparticles

Use of organic ligands to partially passivate nanoparticles against sintering yet retain a degree of small molecule accessibility to the metal surface has been a lofty goal in functional materials synthesis, which in principle also enables the design of preferred electronic and steric environments on a nanoparticle surface. Catalysis using gold in particular requires donor ligands that facilitate an electron-rich metal surface and generalizable strategies for dealing with deactivation due to sintering. Here, synthesis and characterization of gold nanoparticles postsynthetically modified with the chelating ligand cone-5,11,17,23,29,35-hexa(tert-butyl)-37,39,41-tris(diphenylphosphinomethoxy)-38,40,42-trimethoxycalix[6]arene (1) is reported. In solution as well as when supported on the surface of TiO2, nanoparticles modified with tripodal calix[6]arene phosphine ligand 1 demonstrate enhanced protection against sintering relative to unmodified, tetraoctylammonium bromide-surfactant-stabilized gold nanoparticles. In between adsorbed calixarene ligands, there is accessible gold surface area in these nanoparticles, and this is measured quantitatively for the first time for a calixarene-modified nanoparticle, using a newly developed fluorescence methodology involving 2-naphthalenethiol as a relevant chemisorption probe molecule. Ligand steric bulk critically influences amount of accessible surface on the metal nanoparticle since the use of a smaller calix[4]arene ligand (MBC) results in a 7-fold lower accessible surface area relative to using 1 under otherwise similar conditions. In addition, surface coverage of 1 controls accessible surface area in an unintuitive fashion: a 4-fold increase in accessible metal surface area is observed upon increasing the surface coverage of 1 to be 1.5-fold higher than the minimum required for surface saturation. This is presumably the result of a more open ligand packing of 1 at higher surface coverages, which allows greater accessibility to 2-napthalenethiol.

DFT studies of structure and vibrational frequencies of isotopically substituted diamin uranyl nitrate using relativistic effective core potentials

The infrared and Raman spectra of UO(2)(NH(3))(2)(NO(3))(2) with (14)NH(3)/(15)NH(3) isotopic substitution were measured. The structure was optimized and the vibrational spectrum was calculated by DFT (B3LYP/6-31G(d)) methodology using relativistic effective core potential for U atom. The results for force constant and vibrational frequencies support the experimental assignments and the proposed model, mainly in the far-infrared region, where the metal-ligand bond and lattice vibrations are observed. Based on the theoretical findings and the observed spectra a structure of distorted D(2h) symmetry with the nitrate group acting like bidentate ligands for the UO(2)(NH(3))(2)(NO(3))(2) is proposed.

Coordination Mode of Nitrate in Uranyl(VI) Complexes: A First-Principles Molecular Dynamics Study

According to Car-Parrinello molecular dynamics simulations for [UO(2)(NO(3))(3)](-), [UO(2)(NO(3))(4)](2-), and [UO(2)(OH(2))(4-)(NO(3))](+) complexes in the gas phase and in aqueous solution, the nitrate coordination mode to uranyl depends on the interplay between ligand-metal attractions, interligand repulsions, and solvation. In the trinitrate, the eta(2)-coordination is clearly favored in water and in the gas phase, leading to a coordination number (CN) of 6. According to pointwise thermodynamic integration involving constrained molecular dynamics simulations, a change in free energy of +6 kcal/mol is predicted for eta(2)- to eta(1)-transition of one of the three nitrate ligands in the gas phase. In the gas phase, the mononitrate-hydrate complex also prefers a eta(2)-binding mode but with a CN of 5, one H(2)O molecule being in the second shell. This contrasts with the aqueous solution where the nitrate binds in a eta(1)-fashion and uranyl coordinates to four H2O ligands. A driving force of ca. -3 kcal/mol is predicted for the eta(2)- to eta(1)- transition in water. This structural preference is interpreted in terms of steric arguments and differential solvation of terminal vs uranyl-coordinated O atoms of the nitrate ligands. The [UO(2)(NO(3))(4)](2-) complex with two eta(2)- and two eta(1)- coordinated nitrates, observed in the solid state, is stable for 1-2 ps in the gas phase and in solution. In the studied series, the modulation of uranyl-ligand distances upon immersion of the complex in water is found to depend on the nature of the ligand and the composition of the complex.

Effect of Hydration on Coordination Properties of Uranyl(VI) Complexes. A First-Principles Molecular Dynamics Study

Results from Car-Parrinello molecular dynamics simulations are reported for [UO2(OH2)5]2+, UO2(NO3)2(OH2)2, and UO2(NO3)2(eta2-tmma) (tmma = tetramethylmalonamide) in the gas phase and in aqueous solution. The distances between uranyl and neutral ligands such as water and tmma are decreased by up to 0.2 angstroms upon hydration, whereas those between uranyl and the nitrate ion are increased by up to 0.08 angstroms. According to pointwise thermodynamic integration involving constrained molecular dynamics simulations, solvation facilitates the transition of the chelating nitrate ligand to a eta1-bonding mode: the free energy of UO2(eta2-NO3)(eta1-NO3)(OH2)2 relative to the bis-chelating minimum drops from 3.9 kcal/mol in vacuo to 1.4 kcal/mol in water. Optimizations in a polarizable continuum (specifically, the conductor-like screening model in conjunction with the zero-order regular approximation and triple-zeta Slater basis sets) can qualitatively reproduce the geometrical changes from explicit hydration.

Uranyl Extraction by β-Diketonate Ligands to SC−CO2: Theoretical Studies on the Effect of Ligand Fluorination and on the Synergistic Effect of TBP

We report theoretical investigations on the effect of H --> F substitution in acetylacetonate ligands in order to understand why fluorination promotes the extraction of uranyl to supercritical CO(2) with a marked synergistic effect of tri-n-butyl phosphate "TBP". The neutral LH and deprotonated L(-) forms of the ligand, and the uranyl complexes UO(2)L(2) and UO(2)L(2)S (S = H(2)O versus trimethyl phosphate "TMP" which mimics TBP) are studied by quantum mechanics (QM) in the gas phase, whereas the ligands LH and their UO(2)L(2) and UO(2)L(2)S complexes are studied by molecular dynamics (MD) in SC-CO(2) solution as well as at a CO(2)-water interface. Several effects are found to favor F ligands over the H ligands. (i) First, intrinsically (in the gas phase), the complexation reaction 2 LH + UO(2)(2+) --> UO(2)L(2) is more exothermic for the F ligands, mainly due to their higher acidity, compared to the H ligands. (ii) The unsaturated UO(2)L(2) complexes with F ligands bind more strongly TMP than H(2)O, thus preferentially leading to the UO(2)L(2)(TMP) complex, more hydrophobic than UO(2)L(2)(H(2)O). (iii) Molecular dynamics simulations of SC-CO(2) solutions show that the F ligands and their UO(2)L(2) and UO(2)L(2)S complexes are better solvated than their H analogues, and that the UO(2)L(2)(TBP) complex with F ligands is the most CO(2)-philic. (iv) Concentrated solutions of UO(2)L(2)(TBP) complexes at the CO(2)-water interface display an equilibrium between adsorbed and extracted species, and the proportion of extracted species is larger with F- than with H- ligands, in agreement with experimental observations. Thus, TBP plays a dual synergistic role: its co-complexation by UO(2)L(2) yields a hydrophobic and CO(2)-philic complex suitable for extraction, whereas TBP in excess at the interface facilitates the migration of the complex to the supercritical phase.

DFT studies of uranyl acetate, carbonate, and malonate, complexes in solution

The aim of this work is to demonstrate that theoretical chemistry can be used as a complementary tool in determining geometric parameters of a number of uranyl complexes in solution, which are not observable by experimental methods. In addition, we propose plausible structures with partial geometric data from experimental results. A gradient corrected DFT methodology with relativistic effects is used employing a COSMO solvation model. The theoretical calculations show good agreement with experimental X-ray and EXAFS data for the triacetato-dioxo-uranium(VI) and tricarbonato-dioxo-uranium(VI) complexes and are used to assign possible geometries for dicalcium-tricarbonato-dioxo-uranium(VI) and malonato-dioxo-uranium(VI) complexes. The results of this exercise indicate that carbonate bonding in these complexes is mainly bidentate and that hydroxo bridging plays a critical role in the stabilization of the polynuclear uranyl complexes.