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

Abstract

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.