Solvent extraction of lanthanoid(III) picrates with a bis(crown ether): enhanced extractability through double-sandwich complexation

Abstract

Solvent extractions of aqueous light lanthanoid(III) picrates (La–Gd) with 1,8-dioxooctamethylenebis(4′-benzo-15-crown-5)4 were conducted at low ionic strength in the absence of dense background salts. Possessing two crown ether units, the bis(crown ether) gave much higher extractabilities for the lanthanoids than did the reference ligand, 4′-acetylbenzo-15-crown-5 3, or related ligands 15-crown-5 1 and benzo-15-crown-5 2, although the profile of the relative cation-selectrivity sequence became flat. Unexpectedly, quantitative solvent extraction studies revealed that the cation: ligand stoichiometry of the complex is not 1:1 but 1:2. This indicates that the enhanced extractabilities and the decreased cation selectivity arise not from the conventional intramolecular sandwich complexation but from the unique bimolecular double-sandwich complexation incorporating four crown ether units wrapping around a partially hydrated lanthanoid ion that has to be shielded effectively from the surrounding organic solvent molecules.