The effect of sterically restricting cyclopentane bridges on the electron-transfer behavior of copper(II/I) macrocyclic tetrathiaether complexes

Copper(II/I) redox couples play an important role in the electron-transfer kinetics of biological systems. Most redox-active copper proteins exchange electrons more rapidly than do inorganic copper complexes. This has been attributed to the fact that the coordination geometry preferred by Cu(II) (tetragonal) and by Cu(I) (tetrahedral) are significantly different so that the inorganic complexes may involve large changes in coordination geometry. The current investigation was aimed at testing this hypothesis by imposing geometric constraints on the ligand structure to determine the effect upon the rate of electron-transfer kinetics.; The electron-transfer kinetics of the copper(II/I) complexes formed with seven cyclopentanediyl-substituted derivatives of the macrocyclic tetrathiaether-[14]aneS ₄ were studied with several counter reagents to characterize the kinetic behavior of these complexes in water and in acetonitrile. The Marcus cross relation was used to calculate the self-exchange rate constants, k₁₁, of these complexes for both oxidation and reduction. All complexes were found to involve the dual pathway “square-scheme” mechanism which had been previously postulated in which the ligand conformational change and the electron-transfer step occur sequentially. Control of the electron-transfer process arising from kinetically-limiting conformational changes, known as “gated” behavior, was experimentally found for copper complexes formed with cis-cypt-[14]aneS₄, trans -cypt-[14]aneS₄ and cis,trans-dicypt-[14]aneS ₄. The observation of both reaction pathways as well as the demonstration of the “gated” behavior was observed for one system with a single counter reagent for the first time.; Molecular mechanical calculations were made on all four relevant conformers of the oxidized and reduced forms of the seven copper complexes in an attempt to estimate the relative strain induced by the cyclopentane rings upon the macrocyclic tetrathiaether copper complexes in the various conformers which are postulated to exist during the overall electron transfer reaction. The kinetic and thermodynamic results of this study were compared with the results previously obtained with a similar series of copper complexes formed with cyclohehanediyl-substituted-[14]aneS₄ ligands. The results showed similar behavior for the reaction pathway involving distorted Cu(I) intermediate species, but distinctly different behavior in the kinetics for the alternate reaction pathway involving distorted Cu(II) intermediates.