Coordination of Nd(iii) and Eu(iii) with monodentate organophosphorus ligands in ionic liquids: spectroscopy and thermodynamics

Abstract

The complexation of two representative trivalent lanthanides, Nd(III) and Eu(III), with three monodentate organophosphorus ligands, tributyl phosphate (TBP), dibutyl butyl phosphonates (DBBP) and tributylphosphine oxide (TBPO) in the ionic liquid (IL) C₄mimNTf₂ was investigated by absorption spectroscopy, luminescence spectroscopy, and microcalorimetry. The Nd(III) complexes formed in the IL were successfully identified and their corresponding thermodynamic parameters were determined. As the alkoxyl group is gradually replaced by an alkyl group from TBP to TBPO, the stability of the Nd(III) complexes increases significantly. Moreover, the nature of the complexes was further elucidated on the basis of thermodynamic analysis as well as luminescence characterization. The overall endothermic feature of the complexation process indicates that Nd(III) and the ligands form inner-sphere coordination complexes in the ionic liquid, which is further supported by luminescence studies with Eu(III) as the surrogate. Upon introduction of organophosphorus ligands, the symmetry of Eu(III) complexes becomes lower and water molecules in the primary coordination sphere of Eu(III) were gradually replaced by the organophosphorus ligands. The results from this work provide valuable thermodynamic and structural insights into the coordination chemistry of lanthanides in ionic liquids.