Imidazolate-bridged dicopper(II) and copper(II)–zinc(II) complexes of macrocyclic ligand with methylimidazol pendants: Model study of copper(II)–zinc(II) superoxide dismutase

Two new homo- and hetero-dinuclear complexes, [Cu₂L(im)](ClO₄)₃4H₂O (1) and [CuZnL(im)](ClO₄)₃4H₂O (2) (where Im=1H-1midazole and L = 3, 6, 9, 16, 19, 22-hexaaza-6, 19-bis(1H-imidazol-4-ylmethyl)tricycle[22, 2, 2, 2^11,14]triaconta-1, 11, 13, 24, 27, 29-hexaene) were synthesized and characterized as model compounds for the active site of copper(II)–zinc(II) superoxide dismutase (Cu₂Zn₂–SOD). X-ray crystal structure analysis revealed that the metal centers in both complexes exhibit distorted trigonal-bipyramid coordination geometry and the CuCu and CuZn distances are both 6.02 Å. Magnetic and ESR spectral measurements of 1 showed antiferromagnetic exchange interactions between the imidazolate-bridged Cu(II) ions. The ESR spectrum of 2 displays typical signals of mononuclear Cu(II) complex, demonstrating the formation of heterodinuclear complex 2 rather than a mixture of homodinuclear Cu(II)/Zn(II) complexes. pH-dependent ESR and UV–visible spectral measurements manifest that the imidazolate exists as a bridging ligand from pH 6 to 11 for both complexes. The IC₅₀ values of 1.96 and 1.57 μM [per Cu(II) ion] for 1 and 2 suggest that they are good models for the Cu₂Zn₂–SOD.