Complexation thermodynamics of lanthanides with 2-thenoyltrifluoroacetone in a room temperature ionic liquid. Part-II: calorimetry and MD simulation studies

Abstract

The complexation thermodynamics of 2-thenoyltrifluoroacetone (HTTA) with lanthanide cations was investigated in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (Bumim·Tf₂N) medium by liquid–liquid extraction and microcalorimetry. The Eu³⁺ ion distribution ratio measurement from aqueous pH 3.5 to 5.2 confirmed the extraction of anionic species of the type Ln(TTA)₄⁻, and this species did not change with pH. Measurement of direct heat of complexation between Nd³⁺ and HTTA confirmed endothermic complexation reactions with ΔH° values of 4.81, 17.4, 23.3 and 37.9 kJ mol⁻¹ for Nd(TTA)²⁺, Nd(TTA)₂⁺, Nd(TTA)₃ and Nd(TTA)₄⁻ species, respectively. The complexation was favoured by positive stepwise entropy changes of 128, 134, 94 and 112 J mol⁻¹ K⁻¹ for ML₁, ML₂ ML₃ and ML₄ species, respectively. The overall extraction enthalpy of the Ln(TTA)₄⁻ complex into Bumim·Tf₂N was also endothermic with a ΔH° value of 42.5 kJ mol⁻¹. The enthalpy change for mass transfer of the Ln(TTA)₄⁻ complex into Bumim·Tf₂N was estimated to be 4.6 kJ mol⁻¹, indicating that the overall extraction process was governed by the complexation enthalpy with the phase transfer enthalpy playing only a minor role. Efforts were devoted to investigate the structural features of the Ln(TTA)₄⁻ complex with the support of molecular dynamics simulations.