Pyridinedicarboxylic Acid Diamides as Selective Ligands for Extraction and Separation of Trivalent Lanthanides and Actinides: DFT Study

Abstract

This article presents a general approach to solving the urgent practical problem of separation of 4f-(lanthanides, Ln³⁺) and 5f-elements (actinides, An³⁺) very similar in properties based on the DFT quantum-chemical supercomputer simulation of Ln³⁺ and An³⁺ complexes with polydentate nitrogen-containing heterocyclic ligands. The method allows to calculate the geometry parameters of ligands and complexes and the metal to ligand binding energies with accuracy, permitting a direct comparison of calculation results with the experimental data, and estimate selectivity factors for separation of Eu³⁺/Am³⁺ model pair cations (SF_Am/Eu) in extraction experiments on a semi-quantitative level.

The applicability of the method and the approach demonstrated by DFT-modeling (nonempirical PBE functional, extended relativistic full-electron basis set) of a large series of diamides of pyridine-2,6-dicarboxylic (dipicolinic) acid (L) with different substituents at the amide nitrogen atoms and in the pyridine cycle, as well as their complexes [LM]³⁺, (H₂O)_nM(NO₃)₃ (n = 3, 4), and LM(NO₃)₃ (M = Eu, Am).

Based on the theoretical analysis a new model is proposed that describes the mechanism of Ln³⁺ and An³⁺ extraction in two-phase system highly acidic water solution-organic solvent, according to which the formation of An³⁺ and Ln³⁺ complexes occurs at the water/organic interface as a substitution reaction of hydroxonium ion in a cavity of a protonated ligand for the metal cation.

Calculation results confirm the experimentally established higher extraction ability of dipicolinic acid diamides containing one aryl and one alkyl substituent at the amide nitrogen atoms compared to the N,N,N′,N′-tetraalkyl diamides (“effect of anomalous aryl strengthening”). Based on the simulation results the structure of the modified ligand L suggested that it should ensure maximum An³⁺/Ln³separation selectivity in the series of dipicolinic acid diamides.

Keywords:

• actinides/lanthanides separation
• DFT modelling
• polydental N,O-donor ligands

ACKNOWLEDGMENTS

The authors are grateful to Dr. D. N. Laikov and to Dr. N. E. Borisova for fruitful advice and discussions.

FUNDING

This work was financially supported by the Federal target program “Research and Development on Priority Directions of the Scientific Technological Complex of Russia for 2007-2012” (state contract no. 16.516.11.6075 and no. 14.513.11.0090)

SUPPLEMENTAL MATERIAL

Table S1: Hirschfeld charges on atoms and the energies of frontier orbitals for a series of pyridinedicarboxylic acid diamides (V).

Table S2: Energies (E) and free energies (G) of ions and Eu(III) and Am(III) complexes according to the calculation data.

Tables S3-S16: Cartesian coordinates of the optimized structures of complexes 14–29.

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