Dinuclear metal complexes. Part 4. Electrochemical studies of macrocyclic dicopper(II) complexes. Investigation of the effect of solvents, donor groups, and steric constraints on the stability of mixed-valence copper(II)–copper(I) complexes

Abstract

Two series of macrocyclic dicopper(II) complexes of stoicheiometry [Cu₂L][ClO₄]₂·2H₂O have been synthesized and characterized. Compounds belonging to series (1) are derived from the 6,12,18,24-substituted (H,H,Me,Me; H,H,Me,Ph; Me,Me,Me,Ph; Me,Me,Ph,Ph) macrocycie 7,11;19,23-dimetheno-9,21-dimethyl-1,5,13,17-tetra-azacyclotetracosa-5,7,9,12,17,19,21,24-octaene-25,26-diol (H₂L). Complexes belonging to series (2) are related to those of series (1) by replacement of two of the –CHN– linkages by –CH₂–NH– groups and the segment containing two azomethine linkages having the 6,12-substituents H,H; Me,Me; Me,Ph; Ph,Ph. The electrochemistry of these compounds has been investigated in MeCN and CH₂Cl₂ solvents using platinum and glassy carbon electrodes. In all cases cyclic voltammograms show sequential one-electron transfers at two different potentials, and from these data the conproportionation constant (K_c) and free energy of the mixed-valent species (ΔG_c) have been evaluated. Electrochemical results have been analysed in terms of the effect due to solvent, donor groups, and steric constraints. Values of K_c for all of the compounds are greater in MeCN than those in CH₂Cl₂, and the K_c value for any compound belonging to series (1) is greater than its analogue in series (2). Factors affecting stability and intramolecular electron-transfer rates of Cu^IICu^IL⁺ species are discussed.