IMIDAZOLATE-BRIDGED DICOPPER(II) AND COPPER-ZINC COMPLEXES OF A MACROBICYCLIC LIGAND (CRYPTAND) - A POSSIBLE MODEL FOR THE CHEMISTRY OF CU-ZN SUPEROXIDE-DISMUTASE

The imidazolate-bridged dinuclear copper(II) (1) and the imidazolate-b ridged heterodinuclear copper(II)-zinc(II) (2) complexes of a macrobic yclic ligand L (L = etratetracontane-6,8,10,20,22,24,33,35,37-nonaene) were synthesized as possible models for superoxide dismutase (SOD). T heir crystal structures determined by X-ray diffraction methods have s hown that the two ions are pentacoordinated in a coordination environm ent that is found to be a slightly distorted trigonal bipyramid for on e metal center and a distorted square pyramid for the other. The metal -N bond lengths range from 1.91 to 2.33 Angstrom, and the distances Cu -Cu and Cu-Zn are 5.95 and 5.93 Angstrom, respectively, slightly short er than the value in the (Cu-Cu) SOD or in the (Cu-Zn) SOD. Magnetic m easurements and ESR spectroscopy of the dicopper complex have shown an antiferromagnetic exchange interaction with a coupling constant of -2 J = 88 cm(-1). From pH-dependent ESR and electronic spectroscopic stud ies, the two complexes have been found to be stable over very large pH ranges 4.5-12 for 1 and 6-10.5 for 2. Electrochemical studies have in dicated quasi-reversible behavior in dimethylacetamide solution for th e first step of the reduction: E(1/2) = -0.27 V/SCE for 1 (Cu-2(II)/Cu -2(I) redox process) and E(1/2) = -0.31 V/SCE for 2 (Cu-II/Cu-I redox process). The two complexes catalyze the dismutation of superoxide at biological pH, and the activity survives in the presence of bovine ser um albumine. All the results indicate that complexes 1 and 2 act as go od models for superoxide dismutase.