Oxidation of cobalt(II) tetrapyrroles in the presence of an electron acceptor

Abstract

The interaction of cobalt(II) porphyrins and chlorin with 4,6-dinitrobenzofurazan 1-oxide (dnbfo) has been studied by spectroscopic methods. Initially all the cobalt(II) porphyrins form molecular complexes with the electron acceptor dnbfo, but with time a complete electron transfer is observed. This results in the formation of cobalt(III) porphyrins and the anion radical of dnbfo. The electron transfer is facilitated in the presence of molecular oxygen. The rate of oxidation is dependent on the porphyrin ring substituents as well as on the solvent mixture used. In the case of cobalt(II) 5,10,15,20-tetraphenylchlorin first oxidation occurs to cobalt(II) 5,10,15,20-tetraphenylporphyrin and this is followed by metal oxidation to give the cobalt(III) porphyrin. The oxidation of the cobalt(II) porphyrins and formation of the anion radical of dnbfo has been monitored by ESR spectroscopy. The g value and the linewidth of the radical compares well with that of the anion formed by chemical reduction. Proton and ¹⁹F NMR data confirm the occurrence of the electron-transfer reaction.