Comparative study of donor atom effects on the thermodynamic and electron-transfer kinetic properties of copper(ii/i) complexes with sexadentate macrocyclic ligands. [CuII/I([18]aneS4N2)] and [CuII/I([18]aneS4O2)]

Abstract

Studies have been conducted on the copper complexes formed with two sexadentate macrocyclic ligands containing four thioether sulfur donor atoms plus either two nitrogen or two oxygen donor atoms on opposing sides of the ring. The resulting two ligands, L, designated as [18]aneS₄N₂ and [18]aneS₄O₂, respectively, represent homologues of the previously studied Cu(II/I) system with a macrocycle having six sulfur donor atoms, [18]aneS₆. Crystal structures of [Cu^II([18]aneS₄O₂)](ClO₄)₂ and [Cu^I([18]aneS₄O₂)]ClO₄ have been determined in this work. Comparison of the structures of all three systems reveals that the oxidized complexes are six coordinate with two coordinate bonds undergoing rupture upon reduction. However, the geometric changes accompanying electron transfer appear to differ for the three systems. The stability constants and electrochemical properties of both of the heteromacrocyclic complexes have been determined in acetonitrile and the Cu^II/IL electron-transfer kinetics have been studied in the same solvent using six different counter reagents for each system. The electron self-exchange rate constants have then been calculated using the Marcus cross relationship. The results are compared to other Cu^II/IL systems in terms of the effect of ligand geometric changes upon the overall kinetic behavior.