Experimental and Theoretical Insight into the Ionic Liquid-Mediated Complexation of Trivalent Lanthanides with...

A multitechnique approach with theoretical insights has been employed to understand the complexation of trivalent lanthanides with two β-diketones, viz. 1-phenyl-1,3-butanedione (L) and 4,4,4-trifluoro-1-phenyl-1,3-butanedione (L), in an ionic liquid (C₆mim·NTf₂). UV-vis spectral analysis of complexation using Nd³⁺ revealed the predominance of ML₂⁺ and ML₄^- species. The stability constants for the PB complexes were higher (β₂ ∼ 10.45 ± 0.05, β₄ ∼ 15.51 ± 0.05) than those for the TPB (β₂ ∼ 7.56 ± 0.05, β₄ ∼ 13.19 ± 0.06). The photoluminescence titration using Eu³⁺ corroborated the same observations with slightly higher stability constants, probably due to the higher ionic potential of Eu³⁺. The more asymmetric (A_L2^ML4 ∼ 5.2) Eu-L complex was found to contain one water molecule in the primary coordination sphere of Eu³⁺ with more covalency of the Eu³⁺-O bond (Ω₂^L1 = 8.5 × 10^-20, Ω₄^L1 = 1.3 × 10^-20) compared to the less asymmetric Eu-L complex (A_L1^ML4 ∼ 3.5) with two water molecules having less Eu-O covalency (Judd-Offelt parameters: Ω₂^L1 = 7.3 × 10^-20, Ω₄^L1 = 1.0 × 10^-20). Liquid-liquid extraction studies involving Nd³⁺ and Eu³⁺ revealed the formation of the ML₄^- complex following an 'anion exchange' mechanism. The shift of the enol peak from 1176 to 1138 cm^-1 on the complexation of the β-diketones with Eu³⁺ was confirmed from the FTIR spectra. ¹H NMR titration of the β-diketones with La(NTf₂)₃ evidenced the participation of α-H of the β-diketones and protons at C₂, C₄, and C₅ positions of the methylimidazolium ring. For the ML₂ complex, 4 donor O atoms are suggested to coordinate to the trivalent lanthanides with bond distances of 2.3297-2.411 Å for La-O, 2.206-2.236 Å for Eu-O, and 2.217-2.268 Å for Nd-O, respectively, while for the ML₄ complex, 8 donor O atoms were coordinated with bond lengths of 2.506-2.559 Å for La-O, 2.367-2.447 Å for Eu-O, and 2.408-2.476 Å for Nd-O. The Nd³⁺ ion was higher by 9.7 kcal·mol^-1 than that of the La³⁺ ion for the 1:4 complex. The complexation energy with L1 was quite higher than that with L2 for both 1:2 and 1:4 complexes. Using cyclic voltammetry, the redox behavior of trivalent lanthanides Eu and Gd with β-diketonate in ionic liquid medium was probed and their redox energetic and kinetic parameters were determined.