Thermodynamic Study of the Complexation of Trivalent Actinide and Lanthanide Cations by ADPTZ, a Tridentate N-Donor Ligand

Abstract

To better understand the bonding in complexes of f-elements by polydentate N-donor ligands, the complexation of americium(III) and lanthanide(III) cations by 2-amino-4,6-di-(pyridin-2-yl)-1,3,5-triazine (ADPTZ) was studied using a thermodynamic approach. The stability constants of the 1:1 complexes in a methanol/water mixture (75/25 vol %) were determined by UV-visible spectrophotometry for every lanthanide(III) ion (except promethium), and yttrium(III) and americium(III) cations. The thermodynamic parameters (DeltaH degrees , DeltaS degrees) of complexation were determined from the temperature dependence of the stability constants and by microcalorimetry. The trends of the variations of DeltaG degrees , DeltaH degrees , and DeltaS degrees across the lanthanide series are compared with published results for other tridentate ligands and confirm strongly ionic bonding in the lanthanide-ADPTZ complexes. Comparison of the thermodynamic properties between the Am- and Ln-ADPTZ complexes highlights an increase in stability of the complexes by a factor of 20 in favor of the americium cation. This difference arises from a more exothermic reaction enthalpy in the case of Am, which is correlated with a greater degree of covalency in the americium-nitrogen bonds. Quantum chemistry calculations performed on a series of trivalent actinide and lanthanide-ADPTZ complexes support the experimental results, showing a slightly greater covalence in the actinide-ligand bonds that originates from a charge transfer from the ligand sigma orbitals to the 5f and 6d orbitals of the actinide ion.

Effect of long-range interactions on ion equilibria in liquid–liquid extraction

“…molar volumes and polarisabilities) used in the model vary by typically 5%, and the insert of Figure 4 has been plotted by considering this 5% difference between two lanthanides species L1 and L2. As can be seen, with an identical complexation force G 0 for the two lanthanides, selectivities L1/L2 can vary by a factor of two, the order of magnitude observed [27,28]. Selectivity is not only driven by differences in complexation with first neighbors atoms in the ligands [2], but includes terms at colloidal scale as much as 90% of the balance of free energy.…”

DFT modeling of the relative affinity of nitrogen ligands for trivalent f elements: an energetic point of view

“…Such studies, usually based on structural comparisons between actinide and lanthanide complexes, have been enriched by several thermodynamic works. [17][18][19][20][21][22][23][24] The exploited techniques-e.g. microcalorimetry, spectrophotometry, NMR and mass spectroscopies-have reached sufficient accuracy to differentiate americium and curium behavior from that of lanthanides.…”

“…Enthalpy terms have also been determined and proved to be strengthened for minor actinides (americium and curium), which was assumed to be in line with a greater amount of covalency within the metal-ligand bond. 17,25 In this regard, we have recently discussed the reasons of BTP efficiency on the basis of quantum chemistry calculations, 26 showing that donation on curium d and f orbitals may act as a determining factor in its selective complexation to BTP. Theoretical calculations can indeed help in interpreting experimental findings by providing tools for a detailed description of the metal-ligand bond.…”

Extraction Chromatographic Separation of Trivalent Minor Actinides Using iHex-BTP/SiO2-P Resin

“…In 1999 Kolarik et al firstly reported that BTP is one of the strongest candidates (5,6). The BTP containing nitrogen atom, which is a soft donor, has a greater extraction selectivity for 5f elements compared to 4f elements through the covalent coordination (7)(8)(9)(10)(11). Recent studies have been performed using 2,6-bis-(5,6-dialkyl-1,2,4-triazine-3-yl)-pyridine (R-BTP) derivatives like 2,6-bis-(5,6-dibutyl-1,2,4-triazine-3-yl)-pyridine (nBu-BTP) (12) and its isomer i-Bu-BTP (13).…”