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

<div><p>On the basis of extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Eu<sup>3+</sup>(aq) + 3A<sup>−</sup>(aq) + <b>1</b>(nb) ⇆ <b>1</b><b>·</b>Eu<sup>3+</sup>(nb) + 3A<sup>‑</sup>(nb) occurring in the two-phase water–nitrobenzene system (A<sup>−</sup> = CF<sub>3</sub>SO<sup>−</sup><sub>3</sub>; <b>1</b> = lithium ionophore VIII; aq = aqueous phase, nb = nitrobenzene phase) was determined as log <i>K</i><sub>ex</sub> (<b>1</b><b>·</b>Eu<sup>3+</sup>, 3A<sup>‑</sup>) = 2.5 ± 0.1. Furthermore, the extremely high stability constant of the <b>1</b><b>·</b>Eu<sup>3+</sup> complex in nitrobenzene saturated with water was calculated: log β<sub>nb</sub> (<b>1</b><b>·</b>Eu<sup>3+</sup>) = 15.6 ± 0.1. Finally, by using DFT calculations, the most probable structure of the cationic complex species <b>1</b><b>·</b>Eu<sup>3+</sup> was derived. In the resulting complex, the ‘central’ cation Eu<sup>3+</sup> 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 Eu<sup>3+</sup> cation could be considered as a potential extraction agent for nuclear waste treatment.</p></div>