Design and synthesis of highly selective ionophores for lithium ion based on 14-crown-4 derivatives for an ion-selective electrode

Highly Li⁺-selective ionophores based on 14-crown-4 derivatives were designed and synthesized, and the relationship of their molecular structures and ion selectivities was investigated in order to obtain a highly Li⁺-selective electrode. The ion selectivities were examined with 11 kinds of 14-crown-4 derivatives, which were prepared according to the proposed two types of Li⁺ ionophore models. Excellent Li⁺-selective ionophores were obtained by introducing a bulky "block" subunit into the ethano-bridge section of the base crown ring, which effectively prevents the formation of a 2:1 or 3:1 sandwich-type complex consisting of the crown ether and cations larger than Li⁺. The best Li⁺/Na⁺ selectivity (more than 1000) was obtained for the electrode using the 14-crown-4 derivative with a bulky decalin subunit. These results offer the general ion selectivity features of 14-crown-4-based compounds.