Ferrocene derivatives as electron carriers for selective oxidation and reduction reactions through a liquid membrane

Abstract

1-Hexadecanoyl- and 1-hexadecyl-ferrocene, which undergo one-electron oxidation to the corresponding ferrocenium cations at distinctly different potentials (0.25 and –0.05 V vs. ferrocenium–ferrocene, respectively, in dichloromethane solution, 0.1 mol dm^–3 in NBu₄ClO₄), have been used as carriers for the cross-transport of electrons and perchlorate ions across a bulk dichloromethane membrane. The liquid membrane interfaces an aqueous layer containing [Fe^II(CN)₆]^4–(the electron source phase, e.s.p) and an aqueous layer containing Ce^IV(the electron receiving phase, e.r.p.). The occurrence of the transport is accounted for on the basis of an empirical juxtaposition of the electrochemical scales (i) in water and (ii) in CH₂Cl₂, which gives the sequence: E°(Ce^IV/III) > E°[Fe(C₅H₅)(C₅H₄COC₁₅H₃₁)^+/0] > E°[Fe(C₅H₅)(C₅H₄C₁₆H₃₃)^+/0] > E°[Fe^III/II(CN)₆^3–/4–]. The special position of the potential of the aqueous Fe^III–Fe^II couple, which is intermediate between those of the two ferrocene derivatives, makes it possible to carry out selective electron-transport experiments: if the e.r.p. contains a mixture of Ce^IV and Fe^III, when [Fe(C₅H₅)(C₅H₄C₁₆H₃₃)] is used as a carrier, both the oxidizing cations are reduced; when [Fe(C₅H₅)(C₅H₄COC₁₅H₃₁)] is used, Ce^IV is reduced, but Fe^III is left intact. These examples clearly illustrate the novel principle of the selective oxidation and reduction reactions performed under three-phase conditions.