Resonance Raman investigation of pH-dependent equilibria of water-soluble iron porphyrins

Abstract

Resonance Raman spectroscopy has been used to probe the structures of; tetrakis(1-methylpyridinium-4-yl)porphinatoiron(III), Fe^III(T4MPyP); tetrakis(1-methylpyridium-2-yl)porphinatoiron(III), Fe^III(T2MPyP); tetrakis(4-sulphonatophenyl)porphinatoiron(III), Fe^III(TSPP); and tetrakis(4-carboxylatophenyl)porphinatoiron(III), Fe^III(TCPP), over a wide pH range. The anionic complexes Fe^III(TSPP) and Fe^III(TCPP) contain high-spin iron(III) at all pHs. Both these complexes exhibit marked spectral changes at ca. pH 6 which correspond to conversion from the diaquo species, in acid solution, to hydroxy-or µ-oxo dimer, transitions at ca. pH 6. However, at pH > 11.5 for Fe^III(T4MPyP) and pH > 9 for Fe^III(T2MPyP) a second equilibrium process is observed, leading to two new species. One of these is readily assigned as the low-spin iron(III) dihydroxy complex by analogy with spectra of the dicyano complex. The second species is assigned to the hydroxy iron(II) complex by comparison with photochemically generated Fe^II(T4MPyP)(OH). The formation of iron(II) species in alkaline solutions of Fe^III(T4MPyP) and Fe^III(T2MPyP) is entirely unexpected and the significance of the observation to previous investigations of the pH-dependent behaviour of these complexes is discussed.