Ionic Liquid Assisted Exothermic Complexation of Trivalent Lanthanides with Fluorinated β Diketone: Multitechnique Approach with Theoretical Insight.

Abstract

The complexation of the betadiketone,1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedione (HFOD) was studied with trivalent lanthanide ions, viz. Nd3+, La3+, and Eu3+ in several methylimidazolium-based ionic liquids (Cnmim•NTf2, where, n = 4,6,8). In C6mim•NTf2, predominant formation of ML2+ and ML4- species was evidenced from the UV-vis absorption (Nd3+) as well as luminescence (Eu3+) spectral studies with log β2 ≈ 5.88 ± 0.04, log β4 ≈ 10.95 ± 0.06. The formation constants followed the trend C4mim•NTf2 > C6mim•NTf2 > C8mim•NTf2. The asymmetry factors for the ML2+ and ML4- species were found to be 1.2 and 1.59, respectively. The ML4- complex was found to have one primary coordination sphere water molecule with enhanced covalency between Eu3+ and O from HFOD (Judd Offelt constants Ω2 and Ω4 ≈ 17.2 and 2.35) compared to Eu3+aq, yet comparable to other β diketones. Complexation-induced temperature increase was confirmed by calorimetric measurements, indicating the exothermic complexation reaction (ΔHcomplexation ≈ -13.7 kJ mol-1), which is also spontaneous in nature (ΔG ≈ -68.1 kJ mol-1), with an enhancement in the entropy values. Due to complexation, the shifts in the peak positions (1686.66 cm-1, 1633.53 cm-1) associated with β diketone/ketone functional groups were evidenced. Density functional theory (DFT) calculation was performed to optimize the structural parameters including bond distance, bond angles, and energetics associated with the complexation.