Complexation of Ln3+ with Pyridine-2,6-dicarboxamide: Formation of the 1:2 Complexes in Solution and Gas Phase

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.

Strong Periodic Tendency of Trivalent Lanthanides Coordinated with a Phenanthroline-Based Ligand: Cascade Countercurrent Extraction, Spectroscopy, and Crystallography

Phenanthroline-diamide ligands have been reported in the selective separation of actinides over Eu(III); on the contrary, relevant basic coordination chemistry studies are still limited, and extraction under actual application conditions is rarely involved. In this work, N,N'-diethyl-N,N'-ditolyl-2,9-diamide-1,10-phenanthroline [Et-Tol-DAPhen (L)] was applied to explore the coordination performance of lanthanides in simulative high-level liquid waste. For the first time, cascade countercurrent extraction was conducted with Et-Tol-DAPhen as the extractant, which reveals the periodic tendency of the extraction efficiency of lanthanides to decrease gradually as the atomic number increases. Comparison of elements with similar radii verifies the hypothesis that the increase in the atomic number leads to a decrease in the ionic radius, thus reducing the coordination and extraction capacity of ligands. Slope analysis, electrospray ionization mass spectrometry, and ultraviolet-visible titration results show that the ligand forms 1:1 and 1:2 complexes with lanthanides and the coordination ability follows the tendency of extraction efficiency, and the first crystal structures of Lns(III) with a phenanthroline-diamide ligand, i.e., [LaL(NO₃)₃(H₂O)] and [LaL₂(NO₃)₂][(NO₃)], were obtained, which confirms the conclusions described above. This work promises to enhance our comprehension of the chemical properties of Lns(III) and offer new clues for the design and synthesis of novel separation ligands.

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⁺.