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

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Abstract
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), n-propyl (DnPTP), and iso-propyl (DiPTP), have been investigated in acidic solutions to check the aqueous-phase stability of Eu(L)33+ ions identified previously in the solid state. The data obtained show, first, the presence of stable Eu(L)33+ ions with DnPTP (log β3app = 12.0 ± 0.5) and DiPTP (log β3app = 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)33+ ions. Other complexes such as Eu(L)23+ ions are only observed for DnPTP (log β2app = 6.7 ± 0.5) and DMTP (log β2app = 6.3 ± 0.1) and Eu(L)3+ only for DMTP (log β1app = 2.9 ± 0.2). The log βnapp values for the Eu(L)n3+ (n = 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.