Selectivity of bis-triazinyl bipyridine ligands for americium(iii) in Am/Eu separation by solvent extraction. Part 1. Quantum mechanical study on the structures of BTBP complexes and on the energy of the separation

Abstract

Theoretical studies were carried out on two pairs of americium and europium complexes formed by tetra-N-dentate lipophilic BTBP ligands, neutral [ML(NO(3))(3)] and cationic [ML(2)](3+) where M = Am(III) or Eu(III), and L = 6,6'-bis-(5,6-diethyl-1,2,4-triazin-3-yl)-2,2'-bipyridine (C2-BTBP). Molecular structures of the complexes have been optimized at the B3LYP/6-31G(d) level and total energies of the complexes in various media were estimated using single point calculations performed at the B3LYP/6-311G(d,p) and MP2/6-311G(d,p) levels of theory. In the calculations americium and europium ions were treated using pseudo-relativistic Stuttgart-Dresden effective core potentials and the accompanying basis sets. Selectivity in solvent extraction separation of two metal ions is a co-operative function of contributions from all extractable metal complexes, which depend on physico-chemical properties of each individual complex and on its relative amount in the system. Semi-quantitative analysis of BTBP selectivity in the Am/Eu separation process, based on the contributions from the two pairs of Am(III) and Eu(III) complexes, has been carried out. To calculate the energy of Am/Eu separation, a model of the extraction process was used, consisting of complex formation in water and transfer of the formed complex to the organic phase. Under the assumptions discussed in the paper, this simple two-step model results in reliable values of the calculated differences in the energy changes for each pair of the Am/Eu complexes in both steps of the process. The greater thermodynamic stability (in water) of the Am-BTBP complexes, as compared with the analogous Eu species, caused by greater covalency of the Am-N than Eu-N bonds, is most likely the main reason for BTBP selectivity in the separation of the two metal ions. The other potential reason, i.e. differences in lipophilic properties of the analogous complexes of Am and Eu, is less important with regard to this selectivity.

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

“…The types of coordination polyhedra coincide. The Eu-O interatomic distances in 20, according to published data, [38] range from 2.489 to 2.544Ǻ at an average value of 2.505 Å, whereas the Am-O distances in 21 range from 2.528 to 2.585Ǻ at an average value of 2.554Ǻ. We found very similar values.…”

“…A very close similarity is observed when comparing the geometric parameters of cations [Eu(H 2 O) 9 ] 3+ (20) and [Am (H 2 O) 9 ] 3+ (21) with those obtained in reference [38]. The types of coordination polyhedra coincide.…”

“…The lacking of all necessary data in reference [38] does not allow one to calculate E and G 298 but makes it possible to calculate E Eu,Am , which is 9.6 kcal/mol according to the DFT calculation with the 6-31G(d) basis set and 12.8 kcal/mol when using the 6-311G(d,p) basis set. The correspondence with a value of 18.3 kcal/mol obtained by us can be considered quite appropriate.…”

Selective separation of Am(III) from Eu(III) by 2,9-Bis(dialkyl-1,2,4-triazin-3-yl)-1,10-phenanthrolines: a relativistic quantum chemistry study

“…2c and 2d, besides binding to one ligand, the metal cation can coordinate with another four water molecules at most in the inner sphere complexation [37][38][39], giving the metal cation C.N. 8 [37][38][39] and other reports for BTBPs [47], indicating there is no much difference in the structures between BTPhen and BTBP complexes.…”

“…In addition, the shorter bond length of Am-N 2 compared to that of Am-N 1 shows Am(III) might have stronger coordina- tion with N atoms in the auxiliary triazine moieties than those in the phenanthroline skeleton. The dihedral angle N 2 N 1 N 1 N 2 constituted by four N atoms can be used to evaluate the deformation of the planar conformation of the complexes [47]. Actually, the dihedral angles N 2 N 1 N 1 N 2 of the two complexes are very close to 0 ∘ , revealing that the preorganized phenanthroline_based ligand provides suitable cavity for matching the size of Am(III) and Eu(III) cations.…”