The double-armed diazacrown ethers, which have a base diazacrown ether ring with two diamide-type side chains, were designed and synthesized on the basis of the proposed molecular model for the novel neutral Ca2+ and Mg2+ ionophores. The potentiometric ion-selective electrodes were prepared with over 20 kinds of systematically synthesized diazacrown ether derivatives. The relationship between the molecular structures of the ionophores and the ion selectivities was fully discussed. The electrodes based on the 21- and 18-membered diazacrown ether derivatives possessing a glycolic diamide and malonic diamide in their side chains (K23E1 and K22B5) exhibited excellent Ca2+ and Mg2+ selectivities, respectively. The ion-selectivity features of the novel Ca2+ and Mg2+ ionophores supply important structural information about the design of host molecules for alkaline earth metal cations.
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