Alkoxymethyl-Substituted 18-Crown-6 and 21-Crown-7 Ligands: Synthesis, Complexation Properties, and Metal Ion Membrane Separations

Three novel alkoxymethyl-substituted 18-crown-6 (8–10) and one new alkoxymethyl-substituted 21-crown-7 (11) ligands have been prepared. First, 1-O-tritylpropanetriol (1) and penta- or hexaethylene glycol ditosylate were cyclized in the presence of an alkali metal hydride. The resulting trityloxymethyl-substituted crowns were deprotected to give the hydroxymethyl-substituted 18-crown-6 (6) and hydroxymethyl-substituted 21-crown-7 (7). The latter compounds were alkylated using sodium hydride and the appropriate l-bromoalkane to form 8–11. Complexation properties of these new ligands with the alkali metal cations were studied by a calorimetric titration technique. Compared with the parent 18-crown-6 and 21-crown-7 ligands, the substituted 18-crown-6 ligands demonstrate superior selectivity for K+, while the substituted 21-crown-7 ligand demonstrated less selectivity for Cs+ over other alkali cations. Solvent extraction and membrane separation experiments using these new ligands were also carried out. The results demonstrated that the structural features of these new ligands have a significant influence on cation complexation selectivity and separations potential.