Cytochrome c oxidase models: synthesis and reactivity of iron(III)–copper(II) complexes of deuterohaemin–polybenzimidazole dinucleating ligands

Abstract

Deuterohaemin complexes modified through covalent linkage of bis-(1) or tirs-(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 (II) 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^III–Cu^I complex is obtained, while the stronger reductant sodium dithionite produces the fully reduced Fe^II–Cu^I 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^III–Cu^II complex. The EPR spectra (–150 °C) of the iron–copper complexes show a decrease in intensity of the Cu^II 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^III–OH–Cu^II bridge.