Complexes of Uranyl Nitrate with 2,6-Pyridinedicarboxamides: Synthesis, Crystal Structure, and DFT Study

Utility of Interchangeable Coordination Modes of N,N'-Dialkyl-2,6-pyridinediamide Tridentate Pincer Ligands for Solvent Extraction of Pd(II) and Zr(IV) from High-Level Radioactive Liquid Waste

A new class of ligands, N,N'-dialkyl-2,6-pyridinediamide (DRPDA), has been designed with the specific intention of exhibiting interchangeable diversity in coordination modes, including organometallic interactions, for the purpose of solvent extraction of elements relevant to the proper treatment of high-level radioactive liquid waste (HLLW) generated after nuclear fuel reprocessing. Consequently, DRPDA has been observed to extract Pd(II) and Zr(IV) from HNO3(aq) to 1-octanol in nearly quantitative yields when the selected ligand is sufficiently hydrophobic. However, concomitance of some of other HLLW components were also found. The extraction selectivity toward Pd(II) and Zr(IV) was markedly enhanced by employing n-dodecane instead of 1-octanol as evidenced by good distribution ratios (DM) of Pd(II) (DPd = 72.5) and Zr(IV) (DZr = 12.9), which is several orders of magnitude greater than DM's of other HLLW components (10-3-10-2), where addition of 20 vol % 1-octanol is still required to accelerate the extraction kinetics. Despite direct contact with the highly acidic aqueous phase, deprotonation from one of the amide NH moieties of DRPDA proceeds to form [Pd(DRPDA-)(NO3)] as a good extractables in the current biphasic system. This Pd(II) complex with a rather unique asymmetric N-^N^O tridentate coordination was characterized by SCXRD, elemental analysis and 1H NMR, and theoretically corroborated by DFT calculations and NBO analysis. In contrast, DRPDA also interacts with Zr4+ in different tridentate O^N^O mode without any deprotonation. Based on mechanistic differences in the extraction chemistry we clarified, Pd(II) and Zr(IV) coextracted to the organic phase were recovered stepwise by using appropriate stripping agents such as 1.0 M HCl(aq) and 0.10 M HNO3(aq), respectively.

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.

Structural Insight into Complexation Ability and Coordination of Uranyl Nitrate by 1,10-Phenanthroline-2,9-diamides

Reprocessing of spent nuclear fuel (SNF) is an important task in a frame of ecology and rational use of natural resources. Uranium, as the main component of SNF (>95%), can be recovered for further use as fresh nuclear fuel. To minimize an amount of solid radioactive waste generated during SNF reprocessing, new extractants are under investigation. Diamides of 1,10-phenanthroline-2,9-dicarboxylic acid are perspective tetradentate N-donor ligands that form strong complexes with f-elements, which are soluble in polar organic solvents. As an example of three ligands of this class, we conducted a comparative study and showed how the substituent in the amide functional group affects the extraction ability toward uranyl nitrate from nitric acid media. We have performed a careful study (NMR, FT-IR, XRD, RMC-EXAFS) of the structures of synthesized complexes of new ligands with uranyl nitrate and used quantum mechanical calculations to explain the discovered regularities through.

Quantitative Precipitation of Uranyl or Plutonyl Nitrate with N-(1-Adamantyl)acetamide in Nitric Acid Aqueous Solution

The reactivity of the N-(1-adamantyl)acetamide ligand (L = adam) has been evaluated as precipitating agent for the hexavalent uranyl cation ([U] = 20-60 g L^-1) in concentrated nitric acid aqueous solution (0.5-5 M). It results in the formation of a crystalline complex (UO₂)(adam)₂(NO₃)₂·2(adam) (1), in which the uranyl center is 8-fold coordinated to two chelating nitrate groups and two N-(1-adamantyl)acetamide (= adam) ligands through the oxygen atom of the amide function. Two other noncoordinating adam moieties are also observed in the crystal structure packing and interact through a hydrogen-bond scheme with the uranyl-centered species. A similar molecular assembly has been obtained with the plutonyl(VI) cation, in the complex (PuO₂)(adam)₂(NO₃)₂·2(adam) (2). Precipitation studies indicate high (UO₂)(adam)₂(NO₃)₂·2(adam) formation yields (up to 99%_U for an L/U molecular ratio of 5/1) for HNO₃ concentration in the 0.5-5 M range. However, the precipitation kinetics is rather slow and the reaction is completed after several hours (3-4 h). The calcination of the resulting solid under an air atmosphere led to the formation of the U₃O₈ oxide from 400 °C through a transient phase UO₂ fluorite-type (from 200 °C).

Ion recognition properties of new pyridine-2,6-dicarboxamide bearing propeller-like pendant residues: multi-spectroscopic approach

Abstract

The synthesis and ion binding properties of new amide derived from propeller-like tris(2-pyridyl)amine and 2,6-pyridinedicarboxylic acid chloride were described. Amide binds divalent metal cations: copper(II), nickel(II), zinc(II), and lead(II) in acetonitrile. In acetonitrile:water mixture (9:1 v/v) amide interacts only with copper(II) and nickel(II) cations forming complexes of 1:1 stoichiometry. It was found that the introduction of bulky, nitrogen donor atom bearing pendant groups can influence coordination mode of pyridine-2,6-dicarboxamides. The probable model of ligand-ion interactions is proposed on the basis of 1H NMR and FT-IR spectroscopy.

Graphic abstract

A novel highly selective ligand for separation of actinides and lanthanides in the nuclear fuel cycle. Experimental verification of the theoretical prediction

Predicted by DFT simulation dilactams (B) are selective and efficient extractants for the separation of Eu³⁺ and Am³⁺.