First Example of Fluorinated Phenanthroline Diamides: Synthesis, Structural Study, and Complexation with Lanthanoids

4-Oxo-7-fluoro-1,10-phenanthroline-2,9-diamides: Synthesis, Structural Features, Lanthanide Complexes, and Am(III)/Ln(III) Solvent Extraction

A highly efficient synthetic approach was developed for the synthesis of unsymmetrical 1,10-phenanthroline-2,9-diamides with two different substituents in the fourth and seventh positions of the phenanthroline core. The structures of these ligands were confirmed using various spectral methods including 2D-NMR and X-ray analysis. Quantum chemical calculations supported the presence of tautomeric forms of these ligands. Furthermore, it was discovered that these compounds exhibit polydentate ligand behavior toward lanthanide nitrates. The structural characteristics of the complexes formed between these ligands and lanthanide nitrates were investigated both in the solid state and in solution. To further understand the binding properties of these novel unsymmetrical ligands, the binding constants for potential complexes were quantitatively measured by using UV-vis spectrophotometric titration. This allowed for a comprehensive analysis of the binding affinity and stability of these complexes. Extraction experiments of f-elements were performed for symmetrical and unsymmetrical diamides. Overall, this study presents significant advancement in the synthesis and characterization of unsymmetrical 1,10-phenanthroline-2,9-diamides and provides valuable insights into their potential applications as polydentate ligands for lanthanide nitrates.

First 24-Membered Macrocyclic 1,10-Phenanthroline-2,9-Diamides-An Efficient Switch from Acidic to Alkaline Extraction of f-Elements

A reaction of acyl chlorides derived from 1,10-phenanthroline-2,9-dicarboxylic acids with piperazine allows the preparation of the corresponding 24-membered macrocycles in good yield. The structural and spectral properties of these new macrocyclic ligands were thoroughly investigated, revealing promising coordination properties towards f-elements (Am, Eu). It was shown that the prepared ligands can be used for selective extraction of Am(III) from alkaline-carbonate media in presence of Eu(III) with an SF_Am/Eu up to 40. Their extraction efficiency is higher than calixarene-type extraction of the Am(III) and Eu(III) pair. Composition of macrocycle-metal complex with Eu(III) was investigated by luminescence and UV-vis spectroscopy. The possibility of such ligands to form complexes of L:Eu = 1:2 stoichiometry is revealed.

Pyrrolidine-Derived Phenanthroline Diamides: An Influence of Fluorine Atoms on the Coordination of Lu(III) and Some Other f-Elements and Their Solvent Extraction

Three pyrrolidine-derived phenanthroline diamides were studied as ligands for lutetium trinitrate. The structural features of the complexes have been studied using various spectral methods and X-ray. The presence of halogen atoms in the structure of phenanthroline ligands has a significant impact on both the coordination number of lutetium and the number of solvate water molecules in the internal coordination sphere. The stability constants of complexes with La(NO3)3, Nd(NO3)3, Eu(NO3)3, and Lu(NO3)3 were measured to demonstrate higher efficiency of fluorinated ligands. NMR titration was performed for this ligand, and it was found that complexation with lutetium leads to an approximately 13 ppm shift of the corresponding signal in the 19F NMR spectrum. The possibility of formation of a polymeric oxo-complex of this ligand with lutetium nitrate was demonstrated. Experiments on the liquid–liquid extraction of Am(III) and Ln(III) nitrates were carried out to demonstrate advantageous features of chlorinated and fluorinated pyrrolidine diamides.

Complexation of a Nitrilotriacetate-Derived Triamide Ligand with Trivalent Lanthanides: A Thermodynamic and Crystallographic Study

Non-heterocyclic N-donor nitrilotriacetate-derived triamide ligands are one of the most promising extractants for the selective extraction separation of trivalent actinides over lanthanides, but the thermodynamics and mechanism of the complexation of this kind of ligand with actinides and lanthanides are still not clear. In this work, the complexation behaviors of N,N,N',N',N″,N″-hexaethylnitrilotriacetamide (NTAamide(Et)) with four representative trivalent lanthanides (La³⁺, Nd³⁺, Eu³⁺, and Lu³⁺) were systematically investigated by using ¹H nuclear magnetic resonance (¹H NMR), ultraviolet-visible (UV-vis) and fluorescence spectrophotometry, microcalorimetry, and single-crystal X-ray diffractometry. ¹H NMR spectroscopic titration of La³⁺ and Lu³⁺ indicates that two species of 1:2 and 1:1 metal-ligand complexes were formed in NO₃^- and ClO₄^- media. The stability constants of NTAamide(Et) with Nd³⁺ and Eu³⁺ obtained by UV-vis and fluorescence titration show that the complexing strength of NTAamide(Et) with Nd³⁺ is lower than that with Eu³⁺ in the same anionic medium, while that of the same lanthanide complex is higher in ClO₄^- medium than in NO₃^- medium. Meanwhile, the formation reactions for all metal-ligand complexes are driven by both enthalpy and entropy. The structures of lanthanide complexes in the single ClO₄^- and NO₃^- medium and the mixed one were determined to be [LnL₂(MeOH)](ClO₄)₃ (Ln = La, Nd, Eu, and Lu), [LaL₂(EtOH)₂][La(NO₃)₆], and [LaL₂(NO₃)](ClO₄)₂, separately. The average bond lengths of lanthanide complexes decrease gradually with the decrease in ionic radii of Ln³⁺, indicating that heavier lanthanides form stronger complexes due to the lanthanide contraction effect, which coincides with the trend of the complexing strength obtained by spectroscopic titration. This work not only reveals the thermodynamics and mechanism of the complexation between NTAamide ligands and lanthanides but also obtains the periodic tendency of complexation between them, which may facilitate the separation of trivalent lanthanides from actinides.

Solvation-Anionic Exchange Mechanism of Solvent Extraction: Enhanced U(VI) Uptake by Tetradentate Phenanthroline Ligands

Phenanthroline diamides (L) demonstrated a unique ability to extract uranium from nitric acid solutions into a polar organic solvent forming complexes of 1:2 stoichiometry as tight ion pairs {[UO₂LNO₃]⁺[UO₂(NO₃)₃]^-} by a novel extraction mechanism, which is a combination of two already well-known mechanisms: solvation and ion-pair anion exchange. A UV-vis study was used to confirm the formation of such complexes directly in the organic phase. Moreover, chemical synthesis and single crystal growth were performed to confirm unambiguously the structure of the complexes in the solid state.