Trends in the spectral and redox potential data of mononuclear iron(III)(S=5/2) phenolate complexes

Abstract

A series of high-spin octahedral iron(III) complexes of Schiff bases derived from salicyladehyde and aromatic amines has been synthesised. The ligands were selected to encompass various co-ordination spheres FeN₂O₄, FeN₂O₂O′₂, FeN₃O₃ and FeN₄O₂(where O represents a phenolic oxygen, N an aliphatic or aromatic nitrogen and O′ a carboxylate oxygen) to provide generalizations regarding the overall co-ordination environment of the iron centre in a closely related group of complexes. They reveal the effect of stereochemical and/or donor atom variations on the UV/VIS and EPR spectra and Fe^III–Fe^II redox potentials. Information on the iron(III) site symmetry has been obtained by EPR measurements. The optical spectra are largely determined by transitions originating in the iron–salicylaldiminate chromophore. The ligand-to-metal charge-transfer bands systematically shift to higher energy as the number of phenolate-containing donor sites increases. This blue shift is also reflected in more negative Fe^III–Fe^II redox potentials. The order of increasing cathodic potential shift with respect to the co-ordination sphere is N₂O₄ > N₂O₂O′₂ > N₃O₃ > N₄O₂. This is a reflection of the decreased Lewis acidity of the iron(III) centre due to the increase in basicity of the donor atom. A linear spectroelectrochemical correlation has been obtained between the phenolate-to-iron(III) charge-transfer band energy and the Fe^III–Fe^II redox potential. Based on this correlation, trends in the redox potentials of iron tyrosinate proteins are discussed.