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

Abstract

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 1 ) and 4,4,4-trifluoro-1-phenyl-1,3-butanedione ( L 2 ), in an ionic liquid (C 6 mim·NTf 2 ). UV–vis spectral analysis of complexation using Nd 3+ revealed the predominance of ML 2 + and ML 4 – species. The stability constants for the PB complexes were higher (β 2 ∼ 10.45 ± 0.05, β 4 ∼ 15.51 ± 0.05) than those for the TPB (β 2 ∼ 7.56 ± 0.05, β 4 ∼ 13.19 ± 0.06). The photoluminescence titration using Eu 3+ corroborated the same observations with slightly higher stability constants, probably due to the higher ionic potential of Eu 3+ . The more asymmetric (A L2 ML4 ∼ 5.2) Eu- L 2 complex was found to contain one water molecule in the primary coordination sphere of Eu 3+ with more covalency of the Eu 3+ -O bond (Ω 2 L1 = 8.5 × 10 –20, Ω 4 L1 = 1.3 × 10 –20 ) compared to the less asymmetric Eu- L 1 complex (A L1 ML4 ∼ 3.5) with two water molecules having less Eu–O covalency (Judd-Offelt parameters: Ω 2 L1 = 7.3 × 10 –20, Ω 4 L1 = 1.0 × 10 –20 ). Liquid–liquid extraction studies involving Nd 3+ and Eu 3+ revealed the formation of the ML 4 – 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 Eu 3+ was confirmed from the FTIR spectra. 1 H NMR titration of the β-diketones with La(NTf 2 ) 3 evidenced the participation of α-H of the β-diketones and protons at C 2, C 4, and C 5 positions of the methylimidazolium ring. For the ML 2 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 4 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 3+ ion was higher by 9.7 kcal·mol –1 than that of the La 3+ 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.

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