Phenoxyl radicals iron complexes

Fig. 16.4 Interaction between quercetin (Quer) and iron and the balance between pro-oxidative and antioxidative effects. Quercetin may reduce Fe(H20) to yield Fe(H20) active in the Fenton region forming hydroxyl radicals ( OFI) or alkoxyl radicals ( OR), in effect being pro-oxidative. In contrast, quercetin may form a complex with iron(II), inactive in reducing FI2O2 to OFI, but rather oxidised in the quercetin ligand, in effect being antioxidative. Quer (-H) is the phenoxyl radical.

For example, in the Fe (d ) coordinated phenoxyl-radical complex (Fe -0 -Ph), the formal oxidation state of the metal is classed as +IV, since a closed shell phenolato anion would have to be removed. However, in many cases spectroscopic measurements, amongst others EPR, have proven the presence of a high-spin d electron configuration at the iron and a phenoxyl ligand in such complexes. In this case, the iron ion has a physical oxidation number of +III even though the formal oxidation state would be classed as -I-IV. As a result of these potential confusions, several research groups have prepared numerous examples of metal-coordinated ligand-radical complexes, particularly coordinated phenoxyl radicals, in order to examine the nature of the metal oxidation states and the extent of spin delocalisation in such complexes. 

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