Kinetic advantage of inner sphere electron transfer reactions of copper(III,II) peptide complexes with cyano complexes of iron, molybdenum and tungsten

Abstract

An inner sphere electron transfer process via a cyanide bridge is proposed for the reactions of Cu(III,II) peptide complexes with Fe(CN)^3−,4−₆, Mo(CN)^3−,4−₈ and W(CN)^3−,4−₈. Cu(III) peptide complexes were generated electrolytically from the Cu(II) precursors. The direction of spontaneous reactions is such that Fe(CN)⁴⁻₆ and W(CN)⁴⁻₈ reduce Cu(III) peptide complexes and Mo(CN)³⁻₈ oxidizes Cu(II) peptide complexes at pH ca. 10. However, since all reactions are equilibrium processes, by the very fast continuous decomposition of the reduced Cu(II) product in a slightly acidic reaction medium (pH 5), the Mo(CN)⁴⁻₈ reduction of Cu(III) peptide complexes could be driven to completion and studied kinetically. Kinetically determined equilibrium constants and electrochemically calculated equilibrium constants are mutually consistent. The experimentally observed inner sphere rate constants, k_is, for these reactions are significantly larger than the corresponding outer sphere rate constant, k_os, for the outer sphere electron transfer processes calculated with the Marcus theory, with or without work terms. It is concluded that if the kinetic advantage k_is/k_os is substantially larger than 1, it provides evidence for an inner sphere reaction pathway. The magnitude of the kinetic advantage of the present redox reactions varies from 1 to 63 and is dependent on the metal-to-metal distance in the cyanide-bridged intermediates.