Complexation of 2-thenoyltrifluoroacetone (HTTA) with trivalent f-cations in an ionic liquid: solvent extraction and spectroscopy studies

Abstract

The complexation of 2-thenoyltrifluoroacetone (HTTA) with trivalent lanthanides was studied in a room temperature ionic liquid, viz. 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (C₄mim·Tf₂N) by liquid–liquid extraction, absorption spectroscopy and time-resolved fluorescence spectroscopy. The metal ion distribution data indicated the extraction of anionic species of the type Ln(TTA)₄⁻, where the charge balance of the C₄mim·Tf₂N phase is maintained by the transfer of one Tf₂N⁻ anion into the aqueous phase. Spectrophotometric titration of Nd³⁺ with HTTA indicated that the latter existed predominantly as the keto form in C₄mim·Tf₂N and did not form a complex with the metal ion. On the other hand, it formed complexes when TTA⁻ salt was used where it predominantly existed in the enol form. Titration of Nd³⁺ with TTA⁻ confirmed the formation of Nd(TTA)²⁺, Nd(TTA)₂⁺, Nd(TTA)₃° and Nd(TTA)₄⁻ species with the stability constant (log β) values of 5.82, 10.6, 14.5 and 17.8, respectively. Fluorescence measurements confirmed that the fully hydrated aqua Eu³⁺ cation in C₄mim·Tf₂N was completely dehydrated upon complexation with TTA⁻. Based on spectroscopic evidence, it was deduced that TTA⁻ formed an 8 coordinated [Eu·(TTA)₄]⁻ anionic complex in C₄mim·Tf₂N.