Complexation and solvent extraction of lithium salts with 2,3,6,7,9,10-hexahydro-5H-1,4,8,11-benzotetraoxacyclotridecin (benzo-13-crown-4)

Abstract

Solubility measurements were conducted in order to compare the affinity towards lithium salts of several 12- to 16-membered ring ‘crown’ ethers. The 13-membered ring benzo-13-crown-4 (L¹) was found to be the most effective complexing agent for lithium among the investigated macrocyclic ethers. The effect of counter ions on the solubility was found to be OH^– < Cl^– < ClO₄^–≈ SCN^– < picrate. The benzene and the methylene chloride solutions of L¹ extract lithium salts selectively from aqueous solutions. In the latter solvent the selectivity factor γ(Li⁺/Mⁿ⁺) was found to be 2.5, 44, 216, and 355 for Na⁺, K⁺, Mg²⁺, and Ca²⁺ respectively. Hydrogen-1 n.m.r. measurements for the solutions of the lithium complexes of L¹ indicate that a planar conformation of the quadridentate macrocyclic ring seems to be favoured in the complexed L¹. The 1 : 1 complexes have been found to be dominant in all investigated solvent systems. However, in methylene chloride and in nitromethane the 2 : 1 ‘sandwich’ type complexes could also be detected. The stability constants of the complexes were found to increase in order of decreasing donicity of the solvent media.