Kinetics and thermodynamics study on electron transfer in microheterogeneous media: a comparative study

The kinetics of outer sphere electron-transfer reaction of surfactant cobalt(III) complexes, cis-[Co(en)2(4CNP)(C12H25NH2)](ClO4)3 (1), cis-[Co(trien)(4CNP)(C12H25NH2)](ClO4)3 (2), and cis-[Co(trien)(4AMP)(C12H25NH2)](ClO4)3(3) (en: ethylenediamine, trien: triethylenetetramine, 4CNP: 4-cyanopyridine, 4AMP: 4-aminopyridine, C12H25NH2: dodecylamine) have been investigated by Fe2+ ion in micellar and β-cyclodextrin medium at different temperatures (298, 303, 308, 313, 318, and 323 K) under pseudo-first-order conditions using an excess of the reductant. Experimentally, the reactions were found to be second-order and the electron transfer postulated as outer sphere. The rate constant increases with the increase in the initial concentration of micelles in the media. By assuming outer sphere mechanism, the results have been explained based on the presence of aggregated structures containing cobalt(III) complexes at the surface of the self-micelles formed by the surfactant cobalt(III) complexes in the reaction medium. The effects of inclusion of the long aliphatic chain of the surfactant complex ions into β-cyclodextrin (β-CD) on these reactions have also been studied. But inclusion of the long aliphatic chain of the surfactant complex into β-CD has decreased the rate of the reaction. The results have been explained based on the hydrophobic effect and the presence of pyridine ligand containing 4-cyano and 4-amino substituents. Through isokinetic plots, we have established that the mechanism of the reaction does not alter during the micellar and β-cyclodextrin medium.