Metal complexes that transport electrons across liquid membranes: the [FeII,IIIL3]2+,3+ redox system (L = 4,4′-di-tert-butyl-2,2′-bipyridine)

Abstract

The lipophilic complex [Fe^IIL₃][ClO₄]₂(L = 4,4′-di-tert-butyl-2,2′-bipyridine) has been prepared to be used as a carrier for the transport of electrons from an aqueous oxidizing phase to an aqueous reducing phase, separated by a CH₂Cl₂ bulk liquid membrane. Two-phase redox experiments indicated that the CH₂Cl₂-dissolved [Fe^IIL₃][ClO₄]₂ complex is oxidized by aqueous Ce^IV to give [Fe^IIIL₃][ClO₄]₃. On the other hand, [Fe^IIIL₃][ClO₄]₃ can be reduced, under two-phase conditions, by a series of aqueous reducing agents according to the rate sequence: NO₂^– > [Fe^II(CN)₆]^4– > Fe^II > SO₃^2–. Three-phase experiments have been carried out in which electrons are transported by [Fe^IIL₃][ClO₄]₂ from the aqueous reducing phase to the aqueous phase containing Ce^IV, across the bulk liquid membrane, and ClO₄^– ions are transported by [Fe^IIIL₃][ClO₄]₃ in the opposite direction. The rate of the electron transport is controlled by the rate of the slowest of the two redox processes at the two sides of the membrane. By varying the concentrations of the aqueous reactants, it is possible to determine the two-phase rate-determining step of the overall three-phase process and to perform selective oxidation by Ce^IV of the investigated reducing agents.