1,2-Bis[2-(2′-carboxyalkyloxy)phenoxy]ethanes (3-6), 1,2bis[2-(2′-carboxyheptyloxy)phenoxy]benzene (7), 1,2-bis-[2-(2′-carboxyheptyloxy)ethoxy]benzene (8), and 1,2-bis-[2-(2′-carboxyheptyloxy)ethoxy]ethane (9), which possess pseudo-18-crown-6 frameworks, are synthesized for use in selective Pb(II) extraction. For comparison, monocarboxylic acid analogues of 2-phenoxyoctanoic acid (1) and 2-(2′-methoxyphenyloxy)octanoic acid (2) are also prepared. Extraction of Pb(II) and Cu(II) from buffered aqueous solutions of varying pH into chloroform by these ligands is examined in relation to their molecular structure. Polyether dicarboxylic acids 3-9 exhibit higher Pb(II)/Cu(II) selectivities than the monocarboxylic acids 1 and 2. The alkyl chain length variation from hexyl to octyl to dodecyl to hexadecyl does not affect the Pb(II)/ Cu(II) selectivity. Due to steric stress of the binding site, tribenzo ligand 7 shows a low Pb(II)/Cu(II) selectivity. For 3, 8, and 9, no significant difference in Pb(II)/Cu(II) selectivity is noted, even though the flexibility of the binding site increases in the order of 9 > 8 > 3 > 7. Slope analysis reveals that ligands 3-6 mainly form 1:2 (metal/ ligand) complexes in both Pb(II) and Cu(II) extraction, whereas ligand 9, which possesses a flexible binding site, tends to form 1:1 complexes. The extraction selectivity for heavy metal ions increases in the order Pb(II) > Cu(II) > Zn(II), Cd(II) > Ni(II). On the basis of the induced-fit mechanism of the pseudo-18-crown-6 binding site with Pb(II), the polyether dicarboxylic acids 3-9 are shown to have an excellent extraction selectivity for Pb(II).
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