Experimental and Theoretical Insight into the Ionic Liquid-Mediated Complexation of Trivalent Lanthanides with β-Diketone and Its Fluorinated Analogue.

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 () and 4,4,4-trifluoro-1-phenyl-1,3-butanedione (), in an ionic liquid (Cmim·NTf). UV-vis spectral analysis of complexation using Nd revealed the predominance of ML and ML species. The stability constants for the PB complexes were higher (β ∼ 10.45 ± 0.05, β ∼ 15.51 ± 0.05) than those for the TPB (β ∼ 7.56 ± 0.05, β ∼ 13.19 ± 0.06). The photoluminescence titration using Eu corroborated the same observations with slightly higher stability constants, probably due to the higher ionic potential of Eu. The more asymmetric (A ∼ 5.2) Eu- complex was found to contain one water molecule in the primary coordination sphere of Eu with more covalency of the Eu-O bond (Ω = 8.5 × 10, Ω = 1.3 × 10) compared to the less asymmetric Eu- complex (A ∼ 3.5) with two water molecules having less Eu-O covalency (Judd-Offelt parameters: Ω = 7.3 × 10, Ω = 1.0 × 10). Liquid-liquid extraction studies involving Nd and Eu revealed the formation of the ML complex following an 'anion exchange' mechanism. The shift of the enol peak from 1176 to 1138 cm on the complexation of the β-diketones with Eu was confirmed from the FTIR spectra. H NMR titration of the β-diketones with La(NTf) evidenced the participation of α-H of the β-diketones and protons at C, C, and C positions of the methylimidazolium ring. For the ML 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 ML 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 Nd ion was higher by 9.7 kcal·mol than that of the La 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.