Dinuclear metal complexes. Part 2. Synthesis, characterisation, and electrochemical studies of macrocyclic dicopper(II) complexes

Abstract

The synthesis, characterisation, and electrochemical studies of dicopper(II) complexes [Cu2L1][ClO4]2·nH2O of the macrocycle 7,11;19,23-dimetheno-9,21-dimethyl[1,5,13,17]-tetra-azacycloicosa-5,7,9,12,17,19,21,24-octaene-25,26-diol (H2L1), and of some 6,12,18,24-substituted (Me4; Prn4; Ph4; Ph, Me, Ph, Me) derivatives have been carried out. These compounds undergo sequential one-electron transfers at two different potentials. For all of these compounds, except for [Cu2L1][ClO4]2·2H2O, two reversible or almost reversible reduction steps have been observed in acetonitrile medium using a hanging mercury drop electrode. In the case of [Cu2L1][ClO4]2·2H2O, the second couple became obscured due to the presence of an adsorption phenomenon or secondary electrode reaction. However, in NN-dimethylformamide (dmf) medium, satisfactory voltammograms have been obtained only with [Cu2L1][ClO4]2·2H2O. The mixed-valent complexes are considerably more stable in acetonitrile than is [CuIICuIL1]+ in dmf. The potentials of the first reduction step (E1) remain practically invariant throughout the series and are independent of the extent of magnetic interactions in the dicopper(II) complexes. The potentials of the second reduction step (E2) vary with the alkyl and aryl groups present, and a linear plot has been obtained for E2vs. the Hammett function σm. On the basis of previously reported observations and the present study it is inferred that in acetonitrile medium the unpaired electron in the mixed-valent complexes remains localised on one of the copper atoms.