Redox-switched crown ethers. 1. Redox-coupled control of metal-ionophore interactions and their application to membrane transport.

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Polyether ionophores with redox-functional thiol groups were synthesized: they are a diaza-18-crown-6 with a disulfide cap that exhibits redox-responsive interconversion between crown(1red) and cryptand(1ox) and tri- and tetra(oxyethylene)s having a mercapto and an 8-quinolyl group as their terminal groups that feature redox-responsive interconversion between monopodands (2red) and dipodands (2ox). For Na+ 1red and 1ox showed a similar ion affinity, but 1ox bound K+, Rb+, and Cs+ more efficiently than 1red because of the coordination of the cap oxygens to the complexed metal cations. The difference was rationalized by such that "K+ Rb+ and Cs+ also) perches on the crown ring whereas Na+ nests in it," 2ox showed, in most cases, the ion affinity significantly higher than corresponding 2red, indicating the cooperative action of the two poly(oxyethylene) chains on the ion-binding. The rate of K+ transport with 2red across a liquid membrane was very slow but 2ox carried K+ more efficiently. It was found that the rate of K+ transport started with is efficiently accelerated by the addition of I2. This is due to the redox-switch of the carrier from 2red to 2ox. These results suggest that the redox-functionalized ionophores provide novel applications of ion-extraction and ion-transport.
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