Extraction Properties of p-Methyloxacalix[3]arene Methyl Ether for Alkali Metal Ions

Abstract

A neutral macrocyclic ligand, p-methyloxacalix[3]arene methyl ether (MOC3A-Me), was synthesized and its extraction properties for alkali metal picrates was investigated in a dichloromethane/water system at 25ºC. The alkali metal ions were extracted by MOC3A-Me with a selectivity sequence Li⁺ < Na⁺ < K⁺ ≤ Rb⁺ ≤ Cs⁺, whereas they were little extracted by the parent oxacalix[3]arene which contains free hydroxyl groups. From extraction equilibrium analysis, MOC3A-Me (L) was found to extract each alkali metal picrate (M⁺A⁻) by the reaction M⁺_w + L_o + A⁻_w⇄ ML⁺A⁻_o, where subscripts ‘o’ and ‘w’ denote the organic and aqueous phases, respectively. The logarithmic values of the extraction equilibrium constants were determined to be 1.73, 2.12, 2.89, 3.01, and 3.07 for Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺, respectively. The structures and thermodynamic stabilities of the ML⁺ complexes in the gas phase were evaluated by DFT calculations to discuss factors determining the cation selectivity of MOC3A-Me.