Complexation of Trivalent Lanthanides with Hexaalkyl Nitrilotriacetamides into Methylimidazolium-Based Ionic Liquids: Spectroscopic, Electrochemical, Calorimetric, and Theoretical Insights

Exothermic, spontaneous inner-sphere complexation of trivalent lanthanides with N,N,N′,N′,N″,N″-hexaalkyl-substituted nitrilotriacetamides (HRNTAs) into 1-hexyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide (C₆mim NTf₂) is reported with a predominant presence of ML and ML₂ complexes having complexation constants, β₁^Eu 3.10 ± 0.02 ,β₁^Nd 2.67 ± 0.03 and β₂^Eu 5.22 ± 0.04, β₂^Nd 4.95 ± 0.02, respectively, for the n-butyl-substituted HRNTA (HBNTA); while those for the n-hexyl derivative (HHNTA) are β₁^Eu 4.27 ± 0.03, β₁^Nd 3.71 ± 0.03 and β₂^Eu 7.70 ± 0.03, β₂^Nd 7.18 ± 0.04, respectively. HHNTA shows better complexing ability; whereas the stronger complexation of the Nd³⁺ over Eu³⁺ is attributed to the lanthanide contraction. Furthermore, the nature of the ionic liquid also influences the extent of complexation with the trend: C₄mim NTf₂ > C₆mim NTf₂ > C₈mim NTf₂, which follows the order of their dielectric constants. Judd-Ofelt parameters were calculated from photoluminescence data to get an idea about the symmetry of the Eu³⁺ complexes. Electrochemical investigations give diffusion coefficient values of 1.17 × 10^-7 and 8.26 × 10^-8 cm²/s for the Eu³⁺ complexes of HBNTA and HHNTA, respectively. Changes in the spectral characteristics and peak positions are evidenced in the FTIR spectra on the complexation of Eu³⁺ with the HRNTA ligands in C₆mim NTf₂. Structure optimization for the complexes was performed by DFT computations.