Redox-active liquid membranes: transport of electrons across a dichloromethane layer mediated by the [CoIIIL1X2]+–[CoIIL1X2] redox system (L1= 1-hexadecyl-1,4,8,11-tetra-azacyclotetradecane, X = Cl or ClO4)

Abstract

The lipophilic complex [Co^IIIL¹Cl₂]Cl (L¹= 1-hexadecyl-1,4,8,11-tetra-azacyclotetradecane) can transport electrons, through the Co^III–Co^II redox change, from an aqueous phase containing Cr^II to an aqueous phase containing the mild oxidizing agents Fe^III or [Ni^IIIL³]³⁺(L³= 1,4,8,11-tetra-azacyclotetradecane), in aqueous HCl, across a bulk CH₂Cl₂ membrane. The transport of electrons, to which a counter transport of Cl^– ions is coupled, implies that the potential associated with the [Co^IIIL¹Cl₂]Cl + e^–→[Co^IIL¹Cl₂]+ Cl^– half-reaction (–0.60 V vs. ferrocenium–ferrocene, measured in a CH₂Cl₂ solution, 0.1 mol dm^–3 in NBu₄Cl) should lie between that associated with the aqueous Cr^III–Cr^II couple and that associated with the aqueous ox–ox^– couple (ox = Fe^III or [Ni^IIIL³]³⁺). Replacement of Cl^– by ClO₄^– stops the electron transport from Cr^II to Fe^III or [Ni^IIIL³]³⁺. This is ascribed to the especially high potential associated with the [Co^IIIL¹(ClO₄)₂]ClO₄+ e^–→[Co^IIL¹(ClO₄)₂]+ ClO₄^– half-reaction. In particular, the [Co^IIL¹(ClO₄)₂] complex in a CH₂Cl₂ solution, covered by a layer of aqueous HClO₄, is not oxidized even by bubbling dioxygen. This complex can transport electrons, across the bulk CH₂Cl₂ membrane, from aqueous Cr^II only to the very strong oxidizing agent Ce^IV in aqueous HClO₄.