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 Kex (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 βnb (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.