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 ,,',',″,″-hexaalkyl-substituted nitrilotriacetamides (HRNTAs) into 1-hexyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide (Cmim NTf) is reported with a predominant presence of ML and ML complexes having complexation constants, β 3.10 ± 0.02 ,β 2.67 ± 0.03 and β 5.22 ± 0.04, β 4.95 ± 0.02, respectively, for the -butyl-substituted HRNTA (HBNTA); while those for the -hexyl derivative (HHNTA) are β 4.27 ± 0.03, β 3.71 ± 0.03 and β 7.70 ± 0.03, β 7.18 ± 0.04, respectively. HHNTA shows better complexing ability; whereas the stronger complexation of the Nd over Eu is attributed to the lanthanide contraction. Furthermore, the nature of the ionic liquid also influences the extent of complexation with the trend: Cmim NTf > Cmim NTf > Cmim NTf, 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 Eu complexes. Electrochemical investigations give diffusion coefficient values of 1.17 × 10 and 8.26 × 10 cm/s for the Eu complexes of HBNTA and HHNTA, respectively. Changes in the spectral characteristics and peak positions are evidenced in the FTIR spectra on the complexation of Eu with the HRNTA ligands in Cmim NTf. Structure optimization for the complexes was performed by DFT computations.