Model compounds for microbial iron-transport compounds. Part 1. Solution chemistry and Mössbauer study of iron(II) and iron(III) complexes from phenolic and catecholic systems

Abstract

The iron complexes found in the systems FeCl₃–phenol, FeCl₃–catechol, FeCl₃-2,3-dihydroxybenzaldehyde, FeCl₃–2,3-dihydroxybenzoic acid, and FeCl₃–salicylic acid have been investigated by pH and conductance titrations together with Mössbauer spectroscopy. On the basis of further studies using ascorbic acid and salicylaldehyde, a mechanism for the reduction of Fe^III to Fe^II and the reverse oxidation via a phenolic or catecholic radical is discussed. The properties of enterobactin are explored in the light of the chemistry of the model systems. The Mössbauer spectra of frozen aqueous solutions of hydroxybenzene compounds containing iron cations show that those complexes that are green or blue at acid pH contain Fe^II, whereas those that are purple or red contain Fe^III. It is proposed that these results make the colour of the complex a good indication of the oxidation state of the iron in phenolic and catecholic systems.