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-bridged heterodinuclear copper(II)-zinc(II) (2) complexes of a macrobicyclic ligand L (L = 1,4,12,15,18,26,31,39-octaazapentacyclo[13.13.13.1(6,10).1(20,24).1(33,37)]-tetratetracontane-6,8,10,20,22,24,33,35,37-nonaene) were synthesized as possible models for superoxide dismutase (SOD). Their crystal structures determined by X-ray diffraction methods have shown that the two ions are pentacoordinated in a coordination environment that is found to be a slightly distorted trigonal bipyramid for one 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 shorter than the value in the (Cu-Cu) SOD or in the (Cu-Zn) SOD. Magnetic measurements and ESR spectroscopy of the dicopper complex have shown an antiferromagnetic exchange interaction with a coupling constant of -2J = 88 cm(-1). From pH-dependent ESR and electronic spectroscopic studies, 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 indicated quasi-reversible behavior in dimethylacetamide solution for the 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 serum albumine. All the results indicate that complexes 1 and 2 act as good models for superoxide dismutase.