HEMOCYANIN AND TYROSINASE MODELS - SYNTHESIS, AZIDE BINDING, AND ELECTROCHEMISTRY OF DINUCLEAR COPPER(II) COMPLEXES WITH POLY(BENZIMIDAZOLE) LIGANDS MODELING THE MET FORMS OF THE PROTEINS

The new poly(benzimidazole) ligands alpha,alpha'-bis[[bis(1-methyl-2-benzimidazolyl)methyl]amino]-m-xylene (L-5,5) and alpha,alpha'-bis[bis[2-(1-methyl-2-benzimidazolyl)ethyl]amino]-m-xylene(L-6,6) and their dicopper(I) and bis(aquo)-dicopper(II) complexes are reported. The ligands provide one tertiary amino and two benzimidazole nitrogen donors to each metal center; each of the two ''arms'' of L-5,5 binds the metal with two adjacent five-membered chelate rings, while with L-6,6 these chelate rings are six-membered. The dicopper(I) complexes react with dioxygen to produce the bis(hydroxo)dicopper(II) complexes. The bis(aquo)- and bis(hydroxo)dicopper(II) complexes can be interconverted in a single step by addition of base and acid, respectively. The electrochemical behavior of the bis(aquo)dicopper(II) complex of L-6,6 shows reversible reduction to the corresponding dicopper(I) complex whereas the analogous complex of L-5,5 is irreversibly reduced. The bis(hydroxo)dicopper(II) complexes of both ligands also undergo irreversible reduction. Azide adducts of the dicopper(II) complexes have been isolated; the anion bridges the two coppers in mu-1,1 fashion in the L-5,5 derivative and in mu-1,3 fashion in the L-6,6 derivative. The spectral properties of the two complexes are significantly different. Binding studies performed in solution for the bis(aquo)- And bis(hydroxo)dicopper(II) complexes show that up to four azide molecules can bind to the complexes and the affinity of azide decreases with the charge of the complex. Electrochemistry shows that, upon increasing the number of bound azide groups, the successive reductions of the two copper(II) centers tend to coalesce, thus indicating progressive lowering of the electronic communication between the metal centers. The relevance of the spectroscopic and binding data of these azide complexes to hemocyanin and tyrosinase is discussed.