Electrochemical and spectroscopic properties of dicobalt cofacial porphyrins in non-aqueous media: Influence of the interporphyrin distance on the redox, spectrophotometric and EPR properties of the cobalt centers

Abstract

The electrochemical and spectroscopic properties of various dicobalt cofacial porphyrin dimers have been studied in a non-aqueous medium (benzonitrile). Some of these compounds catalyze the direct reduction of dioxygen to water in aqueous acid, and their properties are of particular interest. The formal potentials for the cobalt(III)/(II) and cobalt (II)/(I) systems are reported. The behavior of the cobalt(III)/(II) redox couples in the dimers is shown to be a strong function of the distance between the two porphyrin units. The two cobalt(II) centers of the dimers with the largest interporphyrin separation are oxidized and reduced simultaneously. For all the other biscobalt cofacial dimers with shorter separations, one observes two discrete monoelectronic steps for the oxidation and for the reduction of the two metal centers. The peculiar redox properties of the latter group of cofacial porphyrins appears to originate from interporphyrin separation, a fact supported by EPR spectroscopy. The results suggest that the main factor in the redox systems splitting is the inter-ring separation rather than the metal—metal distance. The oxidized forms of these dimers have been characterized by UV—visible and EPR spectroscopy. The nature of these species, which could be written as either π-cation radicals Co(II)P.+ or pentacoordinated Co(III), is discussed.