OXOIRON(IV) PORPHYRINS DERIVED FROM CHARGED IRON(III) TETRAARYLPORPHYRINS AND CHEMICAL OXIDANTS IN AQUEOUS AND METHANOLIC SOLUTIONS

The oxidation of six charged iron(III) tetraarylporphyrins with chemical oxidants has been investigated. In aqueous solution each can be converted by tert-butyl hydroperoxide or monopersulphate into its corresponding oxoiron(IV) porphyrin, whereas in methanol only the iron(III) tetra(N-methylpyridyl)porphyrins form detectable ferryl porphyrins at ambient temperatures. On standing, the iron species revert to the parent porphyrin with a small loss due to non-reversible oxidative destruction. That the oxidised porphyrin intermediates are oxoiron(IV) species has been determined using UV-VIS, resonance Raman, H1 NMR and EPR spectroscopy. The mechanism of formation of the ferryl porphyrins and the factors that influence their stability are examined and discussed. In particular steric effects and to a lesser extent electron-withdrawing substituents are shown to be important in stabilising the iron(IV) species. These effects are explained in terms of their influence on the disproportionation of two ferryl species into an iron(III) porphyrin and an iron(IV) porphyrin pi radical cation.