Switching Off Electron Transfer Reactions in Confined Media: Reduction of [Co(dipic)<sub>2</sub>]<sup>−</sup> and [Co(edta)]<sup>−</sup> 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)<sub>2</sub>]<sup>−</sup> increased from 55 M<sup>–1</sup> s<sup>–1</sup>in aqueous media to 85 M<sup>–1</sup> s<sup>–1</sup> in a <i>w</i><sub>o</sub> = 20 RM. The one-dimensional (1-D) and two-dimensional (2-D) <sup>1</sup>H 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)]<sup>−</sup> is switched off, in contrast to [Co­(dipic)<sub>2</sub>]<sup>−</sup>, these observations are attributed to the penetration of the [Co­(edta)]<sup>−</sup> into the interfacial region of the RM whereas [Co­(dipic)<sub>2</sub>]<sup>−</sup> 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.