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

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 (Cmim•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 (ΔH_complexation ≈ -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.