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

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 (C(n)mim center dot NTf2, where, n = 4,6,8). In C(6)mim center dot 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 beta(2) approximate to 5.88 +/- 0.04, log beta(4) approximate to 10.95 +/- 0.06. The formation constants followed the trend C(4)mim center dot NTf2 > C(6)mim center dot NTf2 > C(8)mim center dot 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 Omega(2) and Omega(4) approximate to 17.2 and 2.35) compared to Eu-aq(3+), yet comparable to other beta diketones. Complexation-induced temperature increase was confirmed by calorimetric measurements, indicating the exothermic complexation reaction (Delta H-complexation approximate to -13.7 kJ mol(-1)), which is also spontaneous in nature (Delta G approximate to -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 beta 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.