Kinetics and Mechanism of Photochemical Formation of a Pyrazine Bridged Fe(II) Protoporphyrin Polymeric Compound

Abstract

Polymeric, electron rich ligand bridged Fe(II) macrocyclic compounds are expected to give rise to directional electron transfer through the iron atoms. In light of this peculiarity, they have been studied in view of their use as catalysts of redox pro- cesses of biological interest (Wang, 1960), as monodimensional electrical conductors (Schneider, 1983), as models in investigations concerning the light absorption prop- erties of chlorophyll (Kats, 1973). The photochemical production of this type of compounds appears to be of some interest because it is possible to induce ‘in situ’ redox reactions in chemical as well as in biological systems, so ruling out undesir- able secondary reactions. In the last years we have conducted investigations on the photoredox reactions of Fe(III) porphyrin complexes (Bartocci, 1980, 1983; Maldotti, 1983, 1985). We have observed that Fe(III) porphyrin compounds might be photoreduced, in the presence of nitrogenous bases, that are able to axially coordinate to Fe(II), to give Fe(II) porphyrin hexa-coordinated complexes (hemochromes). On the basis of this experience, we started an investigation in which the photoreduction of Fe(III) porphyrins were carried out in the presence of bifunctional nitrogenous ligands (e.g. pyrazine) to photochemically obtain ligand bridged Fe(II) porphyrin polynuclear com- pounds (Bartocci, 1986).