Lithium ionophore VIII as an extraordinarily strong receptor for the trivalent europium cation

On the basis of extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Eu³⁺(aq) + 3A⁻(aq) + 1(nb) ⇆ 1·Eu³⁺(nb) + 3A^‑(nb) occurring in the two-phase water–nitrobenzene system (A⁻ = CF₃SO⁻₃; 1 = lithium ionophore VIII; aq = aqueous phase, nb = nitrobenzene phase) was determined as log K_ex (1·Eu³⁺, 3A^‑) = 2.5 ± 0.1. Furthermore, the extremely high stability constant of the 1·Eu³⁺ complex in nitrobenzene saturated with water was calculated: log β_nb (1·Eu³⁺) = 15.6 ± 0.1. Finally, by using DFT calculations, the most probable structure of the cationic complex species 1·Eu³⁺ was derived. In the resulting complex, the ‘central’ cation Eu³⁺ is bound by six very strong bond interactions to the corresponding six oxygen atoms of the parent ligand 1. It is evident that this exceptionally effective receptor 1 for the Eu³⁺ cation could be considered as a potential extraction agent for nuclear waste treatment.