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 (Kc) and free energy of the mixed-valent species (ΔGc) have been evaluated. Electrochemical results have been analysed in terms of the effect due to solvent, donor groups, and steric constraints. Values of Kc for all of the compounds are greater in MeCN than those in CH2Cl2, and the Kc 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 CuIICuIL+ species are discussed.