Electron-transfer reactions: reductions of various double-chain surfactant cobalt(III) complexes by hexacyanoferrate(II) in microheterogeneous media

Abstract

The kinetics of outer sphere electron-transfer reaction of surfactant cobalt(III) complex ions, cis-[Co(en)2(C12H25NH2)2]3+, cis-[Co(dp)2(C12H25NH2)2]3+, cis-[Co(trien)(C12H25NH2)2]3+, cis-[Co(bpy)2(C12H25NH2)2]3+, and cis-[Co(phen)2(C12H25NH2)2]3+ (en, ethylenediamine; dp, diaminopropane; trien, triethylenetetramine; bpy, 2,2′-bipyridyl; phen, 1,10-phenanthroline; C12H25NH2, dodecylamine) investigated by [Fe(CN)6]4− ion in liposome vesicles (DPPC) media were studied at different temperatures (298, 303, 308, 313, 318, and 323 K) by spectrophotometry method under pseudo-first-order conditions using an excess of the reductant. Below the phase transition temperature of DPPC, the rate decreased with increasing concentration of DPPC, while above the phase transition temperature the rate increased with increasing concentration of DPPC. It is concluded that below the phase transition temperature there is an accumulation of surfactant cobalt(III) complex at the interior of the vesicle membrane through hydrophobic effects, and above the phase transition temperature the surfactant cobalt(III) complex is released from the interior to the exterior surface of the vesicle. By assuming outer-sphere mechanism, the results have been explained based on the presence of hydrophobic effect and increase in the size of amine ligand from aliphatic (ethylenediamine, 1,3-diaminopropane, triethylenetetramine) to aromatic (2,2′-bipyridyl, 1,10-phenanthroline) ligand and the reactants with opposite charge. The activation parameters (ΔS ≠ and ΔH ≠) of the reaction have been calculated which corroborate the kinetics of the reaction. Through isokinetic plots we have established that the mechanism of the reaction does not alter during the phase transition of DPPC.