Complexation of actinides(III) and lanthanides(III) cations by tridentate nitrogen ligands

To understand the properties of some systems able to extract actinides (III) from lanthanides(III) selectively, the solution chemistry of lanthanide(III) and actinide(III) cations with poly-hetero-aromatic nitrogen-containing ligands was studied by Time-Resolved Laser Induced Fluorimetry (TRLIF) and UV-visible spectrophotometry, combined with chemo-metric methods. Three soft donor ligands (L) were selected for the study: 2,2':6;2-ter-pyridine (Tpy),4,6-tri-(pyridine-2-yl)-1,3,5-triazine (Tptz) and 2,6-bis-(5,6-dimethyl-1,2,4-triazine-3-yl)-pyridine (MeBtp). Tpy and Tptz exhibit moderate affinity (distribution ratio) and selectivity when used in the synergistic liquid-liquid extraction of americium(III) (with a lipophilic carboxylic acid). MeBtp is also very efficient, and extracts Am(III) with high selectivity; The TRLIF study analyzed the Eu(III) fluorescence emission spectrum. By analyzing the respective changes in the band intensities, and the lifetimes of the Eu(III) excited states, when the ligands were added in homogeneous phase, the following conclusions were drawn: - for Tpy and Tptz, only one EuL3+ complex species was detected, with a low symmetry in the first coordination sphere, and the Eu(III) hydration number (number of water molecules in the Eu(III) first sphere of coordination) in these complexes was found to be around 5-6; - for MeBtp, two species were detected, one with a low symmetry and a hydration number close to 5-6, the other with a high symmetry and almost completely dehydrated. This is indicative of the formation of the complexes: EuL3+ for L =Tpy and Tptz, and Eu(MeBtp)3+ and Eu(MeBtp)33+ in the case of MeBtp. The formation of these complexes, as well as the protonated ligands, was quantitatively studied using UV-visible spectrophotometry. In each case, the variation in the absorption spectrum of one species was monitored, while the concentration of the other was varied. The complex formation constants were then evaluated by mathematical processing of the whole spectrum. The following conclusions were drawn: - the protonation constants for MeBtp are far lower than for Tptz or Toy. This can be correlated to the fact that MeBtp can extract M(III) nitrates from solutions up to 2 molar nitric acid, whereas Tpy and Tptz are limited to acidities below 0,1 mol/l and require the use of a lipophilic anion; - all three ligands behave differently towards lanthanides(III) and americium(III): they all are quite stable AmL23+ complexes, and MeBtp forms a very stable Am(MeBtp)33+ complex. This is consistent with the fact that both types of ligand selectively extract Am(III) cation, extractants of the Btp family demonstrating singular behavior. The number, stoichiometries and stability constants of the complexes formed in aqueous homogeneous conditions between lanthanide(III) and americium(III) cations and poly-hetero-aromatic nitrogen-containing ligands were determined through the combined use of Time-Resolved Laser Induced Fluorimetry and UV-visible spectrophotometry. Data obtained in homogeneous conditions confirmed the ability of nitrogen-containing ligands to discriminate between 4f and 5f cations, an ability already observed in solvent extraction studies. Evidence for the very high selectivity of 2,6-bis-(1,2,4-triazine-3-yl)-pyridine derivatives (Btp's) for americium(III) vs lanthanide(III) cations could also be obtained. This constitutes the first clue for a correlation, at least qualitative, between homogeneous and solvent extraction results. (authors)