Photochemical and Photocatalytic Properties Iron-Tetra-Aryl-Porphyrins

We examine here the photoredox properties of a series of iron-tetra-aryl-porphyrins to test their capability of working as biomimetic catalysts in photoinduced oxygen and electron transfer processes. Photochemical excitation of these complexes in ethanol solution induces an intramolecular electron transfer from the ethanolate axial ligand to Fe (III) . As a consequence, the axial ligand is oxidized and Fe(III) is reduced to Fe(II) . The electronic properties of the meso-aryl substituents do not affect the energy and the reactivity of the excited state responsible for the primary photoredox process, which presents quantum yields of the order of 10-2. On the other hand, an ESR investigation indicates that the oxidation pathway of the ethanolate axial ligand depends on the steric hindrance of the ortho substituents on the phenyl groups, indicating the presence of important effects caused by the microheterogeneity of the system. The porphyrins containing bulky meso-aryl substituents are very resistant against degradation under continuous irradiation, so that they appear the most suitable for catalytic purposes. Sterically hindered iron-tetra-aryl porphyrins are very good catalysts in the photoreduction of CCl4 by ethanol. In the proposed mechanism the primary photoproducts Fe (II) porphyrin and CH3CH2O are able to react with CCl4, initiating a catalytic cycle, which leads to the formation of CHCl3, CI-, CH3CHO, H+. These complexes can also photoassist reactions of dioxygen with cyclohexene, with formation of the corresponding alcohols and ketones as main products. A comparison with the product distribution obtained with analogous systems, by chemical and photochemical methods, suggests a peculiar role of the axially bound ethanol in the oxygenation process.