Mössbauer and spectroscopic studies on substituted tetraphenylporphyrinato iron(III) complexes in aqueous solutions and the formation of the μ-oxo-bridged species

Abstract

Electronic and 57Fe Mössbauer spectra are reported for two new water-soluble porphyrinato iron(III) complexes. Equilibrium constants for μ-oxo bishaem formation are calculated assuming two protons are released. Comparisons are made of the data with other porphyrinato iron(III) systems and it is shown that, in the absence of well-defined fifth ligands, the mononuclear species in acidic solution probably contain two axial water ligands. The μ-oxo bishaems do not contain water or hydroxide coordinated to iron but may hold water by hydrogen-bonding to the oxygen bridge or possibly by aquation of the porphyrin ligands. μ-Oxo bridge formation is controlled by the acid strength of the water coordinated to the iron in the mononuclear species, low p K a values assisting oxo-bridge formation. Such low p K a values are assisted by electron-attracting substituents on the porphyrin periphery. It is noted that this same property assists the stabilisation of iron(II) complexes. Steric inhibition of oxo-bridge formation requires large substituents, unsubstituted phenyl groups being apparently not large enough.