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 [Cu^IICu^IL¹]⁺ 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 σ_m. 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.