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.