The Complexation Behavior Of Ln(Ⅲ) With Phosphine Oxide Ligands In Ionic Liquid

As a"green solvent",ionic liquid has shown great potential for application in the field of metal separation as compared to the traditional organic solvent.It is of great significance to understand the fundamental complexation behavior of metal ions in ionic liquids,thus helping further expand the application of ionic liquids in this field.This paper studied the complexation behavior of trivalent lanthanide ions,Ln（III）,with a series of phosphine oxide ligands in an ionic liquid（C₄minNTf₂）by UV-Vis spectroscopy,isothermal microcalorimetry,and Luminescence emission and lifetime measurements,etc.The complexation speciation were identified and the corresponding thermodynamic parameters（stability constants,enthalpy,and entropy）were obtained.The main results are as follows:（1）The complexation of Ln（III）with CMPO（Octyl（phenyl）-N,N-diisobutyl-carbamoylmethylphosphine oxide）in two ionic liquids containing different content of water（one is“dry”ionic liquid with a water content<250 ppm,the other is“wet”ionic liquid containing around 12500 ppm water）was studied.The thermodynamic parameters for the complexation were obtained by UV-Vis titrations and calorimetric titrations.In both ionic liquids,CMPO and Ln（III）formed four complexes（1:1 to 1:4Ln/CMPO）.The results show that water solvation has played important roles in the complexation process.In“wet”ionic liquid,the enthalpy of the complexation reaction（ΔH）is positive,which means the reaction is endothermic.The entropy change（ΔS）is also positive because a large amount of water molecules have been released from the inner coordination sphere of Ln（III）,leading to the increase of the degree of disorder.Comparisons between the fitted absorption spectra of Nd/CMPO complexes in“wet”IL and that of extractive samples from biphasic solvent extraction have clearly identified the extracted species as NdL₄³⁺during the extraction.Luminescence emission spectra and lifetime of Eu（III）provide further evidence to illustrate the solvation and complexation behavior of Ln（III）in ILs.The results shed light on how solvation affects the complexation of metal ions in ILs and how fundamental thermodynamic findings could help reveal the mechanism of biphasic extraction in real applications.（2）The complexation of Ln（III）with three other phosphine oxide ligands,TBP（Tributyl phosphate）,DBBP（Dibutyl butanephosphonate）,and TBPO（Tri-n-butylphosphine oxide）,in“wet”ionic liquid was also studied.Using UV-Vis spectroscopic and calorimetric titrations,the thermodynamic parameters（logb,ΔH,ΔS）for the complexations were obtained.The results show that Nd（III）could form up to 2,3,and 4 complexes with TBP,DBBP,and TBPO,respectively.Moreover,the complexation strength increases from TBP to TBPO.All the three complexation reactions’enthalpies（ΔH）are positive,indicating that they are all endothermic reactions.The thermodynamic parameters of these complexations show inner sphere coordination features（ΔH>0,ΔS>0）,which is consistent with the theoretical prediction results.Luminescence emission spectroscopic titrations of Eu³⁺with TBP,DBBP,TBPO show that the complexation ability of the ligands with Eu³⁺increases from TBP to TBPO.The results from luminescence lifetime measurements provide further information on the complexation process,showing that the water molecules in the primary coordination sphere of Eu³⁺were gradually replaced by the ligands.The results from this paper reveal the distinct complexation behavior of Ln（III）with a series of phosphine oxide ligands in ionic liquid.The thermodynamic results also help identified the extractive species during relevant solvent extractions.We believe the findings in this paper could provide important guidance for the development of ionic liquids based separation systems in the future.