New potassium- and cesium-selective ionophoric bis(crown ether)s derived from xanthene-4,5-dicarboxylic acid

A series of bis(crown ether)s based-upon a xanthene-4,5-dicarboxylic acid skeleton was prepared and their ionophoric properties toward alkali metal cations were investigated. Bis(crown ether)s bearing 15-crown-5 and 18-crown-6 moieties exhibited pronounced extraction efficiencies toward K+ and Cs+ ions, respectively, and the extraction constant estimated by solvent extraction studies was as high as 10(9) for the 2-K+ and 3-Cs+ systems. Using UV titration of potassium picrate with 2 in THF, the complex was found to have a structure of a completely encapsulated guest in the host. In transport experiments, the bis(crown ether)s showed no significant selectivity pattern compared with extraction results, again implying the strong complexation of bis(crown ether)s. Ion-selective electrode studies also demonstrated that the selective ionophoric properties of 2 toward K+ were reminiscent of the natural antibiotic valinomycin except for a somewhat slow response.