A New Dinuclear Heme-Copper Complex Derived from Functionalized Protoporphyrin IX.

A new biomimetic model for the heterodinuclear heme/copper center of respiratory oxidases is described. It is derived from iron(III) protoporphyrin IX by covalent attachment of a Gly-L-His-OMe residue to one propionic acid substituent and an amino-bis(benzimidazole) residue to the other propionic acid substituent of the porphyrin ring, yielding the FeIII complex 1, and subsequent addition of a copper(II) or copper(I) ion, according to needs. The fully oxidized FeIII/CuII complex, 2, binds azide more strongly than 1, and likely contains azide bound as a bridging ligand between FeIII and CuII. The two metal centers also cooperate in the reaction with hydrogen peroxide, as the peroxide adducts obtained at low temperature for 1 and 2 display different optical features. Support to this interpretation comes from the investigation of the peroxidase activity of the complexes, where the activation of hydrogen peroxide has been studied through the phenol coupling reaction of p-cresol. Here the presence of CuII improves the catalytic performance of complex 2 with respect to 1 at acidic pH, where the positive charge of the CuII ion is useful to promote O–O bond cleavage of the iron-bound hydroperoxide, but it depresses the activity at basic pH because it can stabilize an intramolecular hydroxo bridge between FeIII and CuII. The reactivity to dioxygen of the reduced complexes has been studied at low temperature starting from the carbonyl adducts of the FeII complex, 3, and FeII/CuI complex, 4. Also in this case the adducts derived from the FeII and FeII/CuI complexes, that we formulate as FeIII–superoxo and FeIII/CuII–peroxo exhibit slightly different spectral properties, showing that the copper center participates in a weak interaction with the dioxygen moiety.