Solvent isotope effect in ion-pair extraction of aqueous lanthanoid(III) picrates with a crown ether

Abstract

Lanthanoid-specific, global extractability doubling was achieved without changing the original cation-selectivity sequence by using D₂O, instead of water, as an aqueous phase in the solvent extraction of aqueous light lanthanoid(III) picrates (La³⁺–Gd³⁺) with 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) in dichloromethane. Under comparable conditions, on appreciable effects were observed with alkali- and alkaline-earth-metal picrates. Quantitative solvent extraction studies using water, D₂O, and their 1:1 mixture revealed that the first extraction step, affording a 1:1 complex between partially hydrated lanthanoid and the ligand, is exclusively responsible for the solvent isotope effect, while the subsequent sandwich complexation of the 1:1 complex with another ligand molecule in the organic phase is not affected by the deuteriated aqueous phase. Thus, the lanthanoid-specific solvent isotope effect is attributed to hydrogen-bond breaking between hydrating water in the first solvation shell and bulk water upon the extraction of incompletely dehydrated lanthanoid ions into the dichloromethane phase.