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

A multitechnique approach with theoretical insights has been employed to understand the complexation of trivalent lanthanides with two beta-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)mimNTf2). 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 (beta(2) similar to 10.45 +/- 0.05, beta(4) similar to 15.51 +/- 0.05) than those for the TPB (beta(2) similar to 7.56 +/- 0.05, beta(4) similar to 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) similar to 5.2) Eu-L-2 complex was found to contain one water molecule in the primary coordination sphere of Eu3+ with more covalency of the Eu3+-O bond (Omega(L1)(2) = 8.5 x 10(-20), Omega(L1)(4) = 1.3 x 10(-20)) compared to the less asymmetric Eu-L-1 complex (A(L1)(ML4) similar to 3.5) with two water molecules having less Eu-O covalency (Judd-Offelt parameters: Omega(L1)(2) = 7.3 x 10(-20), Omega(L1)(4) = 1.0 x 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 beta-diketones with Eu3+ was confirmed from the FTIR spectra. H-1 NMR titration of the beta-diketones with La(NTf2)(3) evidenced the participation of alpha-H of the beta-diketones and protons at C-2, C-4, and C-5 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 & Aring; for La-O, 2.206-2.236 & Aring; for Eu-O, and 2.217-2.268 & Aring; for Nd-O, respectively, while for the ML4 complex, 8 donor O atoms were coordinated with bond lengths of 2.506-2.559 & Aring; for La-O, 2.367-2.447 & Aring; for Eu-O, and 2.408-2.476 & Aring; for Nd-O. The Nd3+ ion was higher by 9.7 kcalmol(-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 beta-diketonate in ionic liquid medium was probed and their redox energetic and kinetic parameters were determined.