SYNTHESIS OF A NOVEL 16-MEMBERED TETRAPEPTIDE MACROCYCLE, DIBENZO[C,K] 6,9,14]TETRA-AZACYCLOHEXADECANE[2,5,10,13]TETRAONE - ELECTROCHEMICALEVIDENCE FOR THE STABILITY OF -OXIDATION STATE OF COBALT AND COPPER(1)

The title ligand, a 16-membered tetrapeptide macrocycle (L), has been obtained from the reaction of 1,2-diaminoethane and phthalic anhydride in dioxane, and characterized by physicochemical methods. Complexes w ith the stoichiometry [CoLCl]Cl and [CuLCl2] were obtained from the in teraction of the macrocyclic ligand with the corresponding metal salts . The [CoLCl]Cl complex is a low-spin square pyramid, with possible ex istence of spin cross-over point at room temperature, whereas [CuLCl2] contains octahedral geometry. The cyclic voltammetric studies are con sistent with the formation of a reversible redox couple (Cu(II) half a rrow right over half arrow left Cu(I); E0 = -0.48 V) for [CuLCl2], whe reas irreversible reduction (Co(II)-->Co(I)) is indicated for the [CoL Cl]Cl complex in DMSO solution. These facts are rationalized in terms of the large cavity size of the macrocyclic ligand.