Electron Transfer Reaction of Ion Pairs: 1. Surfactant Cobalt(III) Complexes by Fe(CN)6 4− in Microheterogeneous Media

Abstract

Abstract The kinetic study of reduction of single chain surfactant octahedral cobalt(III) complexes, cis-[Co(en)2(4AMP)(DA)](ClO4)3 and cis-[Co(trien)(4CNP)(DA)](ClO4)3 (where en = ethylenediamine; trien = triethylenetetramine; 4AMP = 4-aminopyridine; 4CNP = 4-cyanopyridine; DA = dodecylamine) with [Fe(CN)6]4– has been carried out by spectrophotometric method under pseudo first order conditions using an excess of a reductant in different media namely, micelles, β-cyclodextrim (β-CD), liposome vesicles (DPPC) and ionic liquids ((BMIM)Br) at different temperatures in order to have deep insight into the molecular behavior during the electron transfer reaction. The analysis of kinetic results strongly suggests that the reduction reaction between the surfactant complex and [Fe(CN)6]4– occurs via a second order outer sphere electron transfer mechanism. The remarkable increase in the rate of electron transfer from the oxidant to reductant is observed in ionic liquid medium as compared to the rest of all other media studied in the present investigation. This can be ascribed due to the fact that the formation of aggregated and constrained geometry of the surfactant cobalt(III) complexes (oxidants) in ionic liquid medium renders the rate of one electron transfer from the oxidants to the reductant, [Fe(CN)6]4– to be higher. Furthermore, the rate constant value increases with increase in the concentration of micelles, however in the presence of β-cyclodextrin medium the rate of electron transfer substantially decreases due to the inclusion of long aliphatic chain of the surfactant cobalt(III) complexes into β-cyclodextrin. In liposome media, second order rate constant for this electron transfer reaction from both the oxidants was found to decrease with increase in the concentration of the liposome below the phase transfer temperature of DPPC. However, above the phase transition temperature the reaction was found to increase with increase in the concentration of the DPPC. The main driving force for this phenomenon is considered to be the intervesicular hydrophobic interaction between the surfactant complexes and vesicular surface. The activation parameters (ΔS‡ and ΔH‡) of this outer sphere electron transfer reaction have been calculated by varying temperatures and the analysis of the data corroborates the kinetics of the reaction.