Coordination Structure and Fragmentation Chemistry of the Tripositive Lanthanide-Thio-Diglycolamide Complexes

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.

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.

HMNTA Complexes of Tetravalent Metal Ions: On the Roles of Carbonyl Oxygen and Amine Nitrogen in the Stabilization of Gas-Phase M(HMNTA)24+ Complexes

Gas-phase tetrapositively charged M(HMNTA)₂⁴⁺ (M = Zr, Hf, Th, and U) ions were generated via electrospray ionization of the M(ClO₄)₄ and N,N,N',N',N″,N″-hexamethylnitrilotriacetamide (HMNTA) mixtures in acetonitrile. In these complexes, the Zr⁴⁺, Hf⁴⁺, Th⁴⁺, and U⁴⁺ metal centers are coordinated by two neutral HMNTA ligands forming antitriangular prism geometry on the basis of DFT calculations. Bonding analysis reveals that the M⁴⁺ center is stabilized by six carbonyl oxygen atoms, while the interactions between M⁴⁺ and two central amine nitrogen atoms are negligible. This is further confirmed by the calculation results of two tetrapositive model complexes without either central amine nitrogen or carbonyl oxygen atoms, indicating the central nitrogen atom of HMNTA is not necessary in forming tetrapositive metal complexes that can be stabilized in gas phase. Collision-induced dissociation of Zr(HMNTA)₂⁴⁺, Hf(HMNTA)₂⁴⁺, and Th(HMNTA)₂⁴⁺ shows the formation of similar charge reducing products with the oxidation state of metal retaining IV whereas ions with other oxidation states were observed for the fragmentation products of U(HMNTA)₂⁴⁺.

Tetrapositive Hafnium-Diamide Complexes in the Gas Phase: Formation, Structure and Reaction

Tetrapositive hafnium complexes in the form of Hf(TMPDA)₃⁴⁺ and Hf(TMOGA)₃⁴⁺ were produced by ESI of acetonitrile solutions of Hf(ClO₄)₄/TMPDA and Hf(ClO₄)₄/TMOGA respectively. Analogous Hf(TMGA)₃⁴⁺ and Hf(TMTDA)₃⁴⁺ were not observed when the Hf(ClO₄)₄/TMGA and Hf(ClO₄)₄/TMTDA solutions were subjected to ESI under similar conditions. Geometry optimizations on these four tetrapositive complexes revealed that the Hf(TMPDA)₃⁴⁺ and Hf(TMOGA)₃⁴⁺ complexes possess C₃ and D₃ geometries respectively with the Hf⁴⁺ center coordinated by nine atoms. Similar geometries were found for Hf(TMGA)₃⁴⁺ and Hf(TMTDA)₃⁴⁺, but both are six-coordinate complexes, which should account for their absence in the gas phase. In addition, no tetrapositive hafnium ion was observed when methanol was used as a solvent instead of acetonitrile. The much stronger affinity of Cl^- toward Hf⁴⁺ than ClO₄^- should be the reason why tetrapositive hafnium ions were not observed when HfCl₄ was used as the hafnium source. CID of the Hf(TMPDA)₃⁴⁺ and Hf(TMOGA)₃⁴⁺ complexes resulted in the formation of Hf(TMPDA)(TMPDA-H)³⁺ and Hf(TMOGA)(TMOGA-H)³⁺ respectively as the major products. The most stable structures of both tripositive hafnium products arise from the deprotonation of CH₃ cis to O_carbonyl, and the Hf(IV) center in both cases is six coordinate. Compared with the loss of protonated ligand observed in the experiments, it is much higher in energy for either Hf(TMPDA)₃⁴⁺ or Hf(TMOGA)₃⁴⁺ to lose neutral or cationic ligand on the basis of DFT calculations.

Formation and Characterization of Zr4+ Stabilized by Neutral Tridentate Ligands in the Gas Phase

Ligated tetrapositive metal ions are rare gas-phase species which tend to form complexes with lower charges due to the high 4th ionization energies of metals. We report the observation of tetrapositive Zr(TMPDA)₃⁴⁺ and Zr(TMOGA)₃⁴⁺ complexes in the gas phase by electrospray ionization of Zr(ClO₄)₄/TMPDA and Zr(ClO₄)₄/TMOGA mixtures. The Zr⁴⁺ center in both complexes is coordinated by nine atoms from three neutral diamide ligands forming nine-coordinate twisted tricapped trigonal prismatic geometry on the basis of DFT calculations. Collision-induced dissociation of both complexes resulted in the loss of protonated ligands to form tripositive Zr(TMPDA)(TMPDA-H)³⁺ and Zr(TMOGA)(TMOGA-H)³⁺ products which retain the IV oxidation state of zirconium at the cost of charge reduction from 4+ to 3+ of the whole complexes. The very high 4th ionization energy of zirconium (34.34 eV) makes tetrapositive zirconium complex the most challenging tetracation to be stabilized against charge reduction in the gas phase to date. Graphical abstract ᅟ.