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

ABSTRACTThis paper presents a general approach to solving urgent practical problem of separation of very similar in properties 4f-(lanthanides, Ln3+) and 5f-elements (actinides, An3+) 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 (SFAm/Eu) in extraction experiments on semi-quantitative level.The applicability of the method and the approach demonstrated by DFT-modeling (nonempirical РВЕ 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 new model proposed that describes the mechanism of Ln3+ and An3+ extraction in two-phase system highly acidic water solution - organic solvent, which adequately explains the experimentally observed influence of acidity and bulk hydrophobic anions on the extraction equilibrium. According to this model, 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”). It is shown that the reason for this effect is a smaller “ligand pre-organization energy” for the first, calculated as the difference in the energies of unfolded conformations of free ligands and syn–syn-conformations of bound ligands. Based on simulation results the structure of modified ligand L suggested, which should ensure maximum An3+/Ln3separation selectivity in the series of dipicolinic acid diamides.