One-electron reduction of dihexadecyl phosphate vesicle bound viologens by pentacyanocobaltate(II) ion

Abstract

N-Methyl-N'-alkyl-4,4;-bipyridinium (C/sub n/MV/sup 2 +/) ions bound to dihexadecyl phosphate (DHP) vesicles are rapidly reduced to radical cations by Co(CN)/sub 5//sup 3 -/ in alkaline solutions containing cyanide ion. For the short-chain viologens investigated (n less than or equal to 10), the predominant pathway obeys the rate law d(C/sub n/MV/sup +/)/dt = a(C/sub n/MV/sup 2 +/)/(b + c(C/sub n/MV/sup 2 +/)), where a is pseudo first order in (Co(CN)/sub 5//sup 3 -/) and (CN/sup -/) under the experimental conditions. Reaction rates for longer chain viologens (n greater than or equal to 14) are several-fold slower at comparable reactant concentrations; the kinetic traces are biphasic, with both components first order in (C/sub n/MV/sup 2 +/) and pseudo first order in (Co(CN)/sub 5//sup 3 -/) and (CN/sup -/). A mechanism consistent with the data comprises outer-sphere electron transfer preceded by Co(CN)/sub 4//sup 3 -/ ligation of a sixth CN/sup -/ ion. This mechanism was confirmed for C/sub 6/MV/sup 2 +/-DHP vesicles by quantitative isolation and spectrophotometric identification of Co(CN)/sub 6//sup 37/ ion as the Co(III) product. The biphasic reduction of the long-chain viologens was taken as evidence for two distinct binding environments within DHP vesicles. The significance of these findings is discussedmore » with the context of other studies on the dynamic properties of C/sub n/MV/sup 2 +/-DHP particles. 25 references, 4 figures, 1 table.« less