Switching Off Electron Transfer Reactions in Confined Media: Reduction of [Co(dipic)₂]⁻ and [Co(edta)]⁻ by Hexacyanoferrate(II)

The kinetics of reduction of two cobalt­(III) complexes with similar redox potentials by hexacyanoferrate­(II) were investigated in water and in reverse micelle (RM) microemulsions. The RMs were composed of water, surfactant [(sodium­(bis­(2-ethylhexylsulfosuccinate)), NaAOT], and isooctane. Compared to the reaction in water, the reduction rates of (ethylenediaminetetraacetato)­cobaltate­(III) by hexacyanoferrate­(II) were dramatically suppressed in RM microemulsions whereas a slight rate increase was observed for reduction of bis-(2,6-dipicolinato)­cobaltate­(III). For example, the ferrocyanide reduction of [Co­(dipic)₂]⁻ increased from 55 M^–1 s^–1in aqueous media to 85 M^–1 s^–1 in a <i>w</i>_o = 20 RM. The one-dimensional (1-D) and two-dimensional (2-D) ¹H NMR and FT-IR studies are consistent with the reduction rate constants of these two complexes being affected by their location within the RM. Since reduction of [Co­(edta)]⁻ is switched off, in contrast to [Co­(dipic)₂]⁻, these observations are attributed to the penetration of the [Co­(edta)]⁻ into the interfacial region of the RM whereas [Co­(dipic)₂]⁻ is in a region highly accessible to the water pool and thus hexacyanoferrate­(II). These results demonstrated that compartmentalization completely turns off a redox reaction in a dynamic microemulsion system by either reactant separation or alteration of the redox potentials of the reactants