Abstract
Resonance Raman spectroscopy has been used to probe the structures of; tetrakis(1-methylpyridinium-4-yl)porphinatoiron(III), FeIII(T4MPyP); tetrakis(1-methylpyridium-2-yl)porphinatoiron(III), FeIII(T2MPyP); tetrakis(4-sulphonatophenyl)porphinatoiron(III), FeIII(TSPP); and tetrakis(4-carboxylatophenyl)porphinatoiron(III), FeIII(TCPP), over a wide pH range. The anionic complexes FeIII(TSPP) and FeIII(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 FeIII(T4MPyP) and pH > 9 for FeIII(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 FeII(T4MPyP)(OH). The formation of iron(II) species in alkaline solutions of FeIII(T4MPyP) and FeIII(T2MPyP) is entirely unexpected and the significance of the observation to previous investigations of the pH-dependent behaviour of these complexes is discussed.
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