Metal ion complexation, extraction and transport by crown and lariat ether compounds
A wide variety of crown ethers have been employed for alkali and alkaline earth metal cation separations due to their superior binding ability for these metal ions. Lariat ethers are crown ethers with one or more donor-groupbearing arms. Attachment of one or more side arms with potential metal ion coordination sites to a crown ether framework may enhance metal selectivity.
To investigate the influence of nonionizable side arms upon the complexation of alkali metal cations by lariat ethers, solvent polymeric membrane electrodes containing the lariat ethers as ionophores were utilized. For dibenzo-16-crown-5 lariat ethers, Na* selectivity over other alkali and alkaline earth metal ions was enhanced by increasing the basicity of an oxygen atom in the side arm and by preorganizing the metal ion binding site through attachment of a hydrocarbon group geminal to the oxygen-containing side arm.
The effect of structural variation within N-(R) sulfonyl lariat ether carboxamides upon the extraction efficiency and selectivity was explored in solvent competitive extraction. Very high Na* selectivity and extraction efficiency were achieved for N-(R)sulfonyl sym-(alkyl)dibenzo-16-crown-5-oxyacetamides.
N-(R)sulfonyl lariat ether carboxamides were used also as metal ion carriers for transport across polymeric inclusion membranes. For N-(R)-sulfonyl syA77-(alkyl)dibenzo-16-crown-5-oxyacetamides, high Na* transport selectivity was noted. The flux was influenced by R variation. Also the flux was proportional to the diffusion coefficient of the complexed lariat ether in the membrane.