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

Abstract

<div><p>On the basis of extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Eu³⁺(aq) + 3A⁻(aq) + <b>1</b>(nb) ⇆ <b>1</b><b>·</b>Eu³⁺(nb) + 3A^‑(nb) occurring in the two-phase water–nitrobenzene system (A⁻ = CF₃SO⁻₃; <b>1</b> = lithium ionophore VIII; aq = aqueous phase, nb = nitrobenzene phase) was determined as log <i>K</i>_ex (<b>1</b><b>·</b>Eu³⁺, 3A^‑) = 2.5 ± 0.1. Furthermore, the extremely high stability constant of the <b>1</b><b>·</b>Eu³⁺ complex in nitrobenzene saturated with water was calculated: log β_nb (<b>1</b><b>·</b>Eu³⁺) = 15.6 ± 0.1. Finally, by using DFT calculations, the most probable structure of the cationic complex species <b>1</b><b>·</b>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 <b>1</b>. It is evident that this exceptionally effective receptor <b>1</b> for the Eu³⁺ cation could be considered as a potential extraction agent for nuclear waste treatment.</p></div