Redox reactions of osmium porphyrins

Abstract

The metal centre in osmium porphyrinates (OsP) can exist in a wide range of oxidation states, each being stabilised by axial complexation with a particular ligand. Thus, Os^IIP is stabilised by ligation with a CO molecule. Oxidation occurs readily to form the corresponding Os^IIIP which retains the CO ligand and can be reduced quantitatively to the original Os^II complex. Further oxidation gives Os^IVP which is stabilised by axial ligation of two alcohol or water molecules. These Os^IV complexes are stable to both oxidation and reduction. Treating the Os^IIP with an organic peroxide results in formation of an Os^VIP complex which is stabilised by two oxo ligands but can be reduced to the Os^IVP species. Photolysis of Os^IIP and Os^IIIP causes loss of the CO ligand, the naked porphyrin species so formed undergoes oxidative reactions with the solvent, O₂, or trace impurities, in most cases forming an Os^IVP complex. The [Os^VIP(O)₂] species does not oxidise alkanes or alkenes in the dark at room temperature but does oxidise cyclohexene under visible light irradiation.