Cytochrome c Oxidase Models - Synthesis and Reactivity of Iron(III)/Copper(II) Complexes of Deuterohaemin-Polybenzimidazole Dinucleating Ligands

Deuterohaemin complexes modified through covalent linkage of bis- (1 ) or tris-(benzimidazole) (2)
residues to one of the propionic acid side chains have been obtained and spectrally characterised. While in
1 the benzimidazole groups cannot bind intramolecularly to the iron centre, in 2 such an arrangement
is possible because the carbon chain connecting the aromatic donor group to the propionic carbonyl
group is slightly longer. As a consequence, while complex 1 binds two molecules of exogenous
donor bases in two steps for 2 simultaneous binding affinity for two donor bases is very low. The
intramolecular binding of benzimidazole markedly increases the reactivity of 2 in catalytic peroxidative
reactions. Both complexes bond one copper(1i) ion at the polybenzimidazole site. For 2 some
competition between iron(iii) and copper(ii) for one of the ligand donor bases is evident from EPR
measurements. Reduction experiments show that upon treatment with sodium ascorbate the partially
reduced Fe"'-Cul complex is obtained, while the stronger reductant sodium dithionite produces the
fully reduced FeIl-Cu' complex. Oxidation of the latter with dioxygen at low temperature (about
-40 "C) occurs without any destruction of the porphyrin and gives the fully oxidised Fe"'-Cu" complex.
The EPR spectra (-1 50 "C) of the ironxopper complexes show a decrease in intensity of the Cu"
signal upon addition of small amounts of OH-, but the marked tendency towards intermolecular
association prevents a simple interpretation in terms of formation of an intramolecular Fe"'-OH--Cu"
bridge