Electrochemical and spectroscopic behavior of dicobalt cofacial diporphyrins. The redox sites revisited

Abstract

Abstract The electrochemical behavior of dicobalt cofacial porphyrins is reported and the EPR and UV-Vis spectra of their oxidized forms are described. The results are discussed in terms of a comparison with the properties of the same complexes bearing non-electroactive metals, which have been described recently. On the basis of UV-Vis spectrophotometry, EPR and electrochemical criteria, an original formal writing is offered for the oxidation pathway of the dicobalt porphyrins in PhCN. It is proposed that the first oxidation process concerns the π-rings. the second is the oxidation of the cobalt (II) into cobalt (III) and a third one the second oxidation of the π-rings. Based on the X-ray structure of the Co2FTF4 diporphyrin, extended Huckel MO calculations can provide a theoretical basis for the electrochemical and spectroscopic behavior. This arises from crossings of the π-ring ligand and (cobalt) metallic levels due to a combination of π−π and d-d electronic interactions between the two moieties of the dimers. A steric effect is also emphasized: the size of the interporphyrin cavity exerts a selectivity as regards internal ligation. The present findings afford a completion of the so-called ‘cofacial effect’ of intramolecular interactions in dimers proposed in a previous paper.