Selectivity in solvent extraction of metal ions by organic cation exchangers synergized by macrocycles: Factors relating to macrocycle size and structure

Crown ethers and related macrocycles strongly synergize the extraction (phase transfer) of certain metal ions from aqueous nitrate solutions to toluene solutions of organophilic sulfonic, phosphoric, and carboxylic acids. For alkali metals, the degree of synergism is related to the size correspondence between the macrocycle cavity and the metal ion if macrocycles of similar type and substitution are considered. Alkaline earth metals are similarly strongly synergized but the size-correspondence relationship is less pronounced. Other factors seem to affect selectivity of these elements. Terf.-butylbenzo-substituted crown ethers of the same size are much less effective synergists for the alkaline earths than cyclohexano — or tert.–butylcyclohexano—substituted compounds. However, with synergism in alkali metal extraction, the reverse is generally true; that is, terf-butylbenzo-substituted crown ethers are better synergists than cyclohexano-substituted crown ethers. Effects involving the electron-withdrawing influence of the aromatic substituents and the flexibility of the ring may explain this selectivity reversal. The extraction of the divalent first-row transition metals, iron, cobalt, and nickel, is not synergized by any of the macrocycles. However, manganese and zinc with half and completely filled d-shells are synergized suggesting a possible correlation of synergistic coordination effects with ligand field stabilization in the first—row transition elements. Studies to determine the stoichiometry of the organic-phase complexes indicate a single crown ether associated with each molecule of the metal salt of the organic-phase acid. In some cases neutral organic acid molecules may also be associated with the complex.