Redox-switched crown ethers. Part 2. Redox-mediated monocrown-biscrown interconversion and its application to membrane transport

To give a redox-switch function to crown ethers, 4′-mercaptomonobenzo-15-crown-5 (CrSH), 4′-mercaptomethylmonobenzo-15-crown-5 (CrCH₂SH), and the corresponding oxidised forms (CrSSCr and CrCH₂SSCH₂Cr, respectively) were synthesized. The affinity of these crown ethers for metal, together with that of monobenzo-15-crown-5 (Cr), was evaluated by the solvent-extraction method. The results proved that (i) the affinity of CrSSCr for alkali-metal cations is almost equal to that of Cr, whereas CrSH has an affinity greater than CrSSCr probably because of the electron-donating effect of the 4′-mercapto group toward the metal-binding crown centre, and (ii) CrCH₂SSCH₂Cr has an affinity for large alkali-metal cations greater than CrCH₂SH because of the co-operative action of the two crown rings to form 1:2 cation-crown sandwich-type complexes. The formation of 1:2 complexes in CrCH₂SSCH₂Cr was also supported by the concentration dependence of the extraction equilibrium and spectral analysis of alkali picrates in tetrahydrofuran. The difference between CrSSCr and CrCH₂SSCH₂Cr was accounted for by a difference in conformational preference, i.e., the cis-conformation of diphenyl disulphide is very unfavourable and the distance between the two crown rings is too short to sandwich a metal ion even though it adopts the cis-conformation, whereas the cis-conformer of CrCH₂SSCH₂Cr can provide a moderate cavity consisting of the two crown rings due to the methylene groups. The redox function between CrCH₂SH and CrCH₂SSCH₂Cr was applied to ion transport across a liquid membrane. It was shown that in K⁺ transport, (i) CrCH₂SSCH₂Cr is a more efficient carrier than CrCH₂SH, and (ii) when CrCH₂SH is oxidised to CrCH₂SSCH₂Cr by iodine added to the membrane phase, the rate of the K⁺ transport is efficiently accelerated.