Use of Electrospray Mass Spectrometry (ESI-MS) for the Study of Europium(III) Complexation with Bis(dialkyltriazinyl)pyridines and Its Implications in the Design of New Extracting Agents

ESI mass spectrometry was used to investigate the europium complexation by tridentate ligands L ≡ 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)-pyridines (DATP) that have shown unique separation properties of actinides(III) from lanthanides(III) in nitric acid solutions. Complexes of three ligands, namely methyl (DMTP), <i>n</i>-propyl (DnPTP), and <i>iso</i>-propyl (DiPTP), have been investigated in acidic solutions to check the aqueous-phase stability of Eu(L)<sub>3</sub><sup>3+</sup> ions identified previously in the solid state. The data obtained show, first, the presence of stable Eu(L)<sub>3</sub><sup>3+</sup> ions with DnPTP (log β<sub>3</sub><sup>app</sup> = 12.0 ± 0.5) and DiPTP (log β<sub>3</sub><sup>app</sup> = 14.0 ± 0.6) in methanol/water (1:1 v/v) solutions under pH range 2.8−4.6 and, second, a mechanism whereby alkyl moieties contribute to a self-assembling process leading to the formation of Eu(L)<sub>3</sub><sup>3+</sup> ions. Other complexes such as Eu(L)<sub>2</sub><sup>3+</sup> ions are only observed for DnPTP (log β<sub>2</sub><sup>app</sup> = 6.7 ± 0.5) and DMTP (log β<sub>2</sub><sup>app</sup> = 6.3 ± 0.1) and Eu(L)<sup>3+</sup> only for DMTP (log β<sub>1</sub><sup>app</sup> = 2.9 ± 0.2). The log β<i><sub>n</sub></i><sup>app</sup> values for the Eu(L)<i><sub>n</sub></i><sup>3+</sup> (<i>n</i> = 1−3) complexes were determined at pH 2.8. Better insight was given in this study concerning the role of the hydrophobic exterior of the ligands for the design of a new range of extracting agents.