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

Abstract

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(-1)in aqueous media to 85 M(-1) s(-1) in a w(o) = 20 RM. The one-dimensional (1-D) and two-dimensional (2-D) (1)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)](-) 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.

Carbonate Linkage Bearing Naphthalenediimides: Self‐Assembly and Photophysical Properties

“…The effect of confinement on the rates of electron transfer between metal centers was previously demonstrated to be dramatic in an electron‐transfer reaction between ferrocyanide and cobalt(III) complexes compartmentalized within a reverse micelle (RM) 7. For the ferrocyanide reduction of bis‐2,6‐dipicolinatocobaltate(III), a negligible effect was observed going from bulk water to the RM environments.…”

Electron‐Transfer Rate Enhancements in Nanosized Waterpools

“…Electrochemical studies on the redox chemistry of the ferri‐/ferrocyanide couple in RMs demonstrated that there are only small changes in the reduction potentials of this iron(II/III) couple 14. In our study of the ferrocyanide reduction of [Co(dipic) 2 ] – , we showed that there is only a twofold increase in the rate constant upon confinement within an RM, not two orders of magnitude 7. We also did not observe a significant rate enhancement in the inner‐sphere reduction of vanadium(V) by coordinated ascorbate ions if performed in RMs of various sizes 8.…”

“…Consequently, their redox reactions occur through an outer‐sphere pathway 11. In addition, previous studies showed that [Co(dipic) 2 ] – is located in the waterpool of RMs and, as such, the system is well defined 7…”

Confinement Effects on Chemical Equilibria: Pentacyano(Pyrazine)Ferrate(II) Stability Changes within Nanosized Droplets of Water

“…We have initiated a series of studies of how chemical reactivity of transition metal complexes is affected when placed into RMs, where the probe of interest is confined within a nano-sized water pool or near its interface [ 26 , 27 , 28 ]. These studies have shown both rate enhancement and diminution, depending on the system under investigation.…”