Complexation of indole-3-acetic acid with iron(III): Influence of coordination on the pi-electronic system of the ligand

The iron(III) complex of indole-3-acetic acid (1) was prepared, and its phy sicochemical properties, mode of iron(III) coordination, and electronic str ucture were studied using UV/Vis, diffuse reflectance infrared Fourier tran sform I:DRIFT), and transmission Fe-57 Mossbauer spectroscopic techniques. The data obtained provide evidence that iron(III) is not only coordinated b y the carboxylic O-donor atom, but also via the conjugated rr-electronic sy stem of the pyrrole moiety involving both the non-shared electronic pair of the heteroatom and the C(2)-(:(3) double bond. Considering the well-known increased sensitivity of the pyrrole residue in indole derivatives to oxida tion as compared to the benzene ring, as well as the formation of a triple complex (peroxidase-1-O-2) proposed for the enzymatic 1 oxidative degradati on mechanism involving as a key step the Fe3+ --> Fe2+ transition in the en zyme form as discussed in literature, it is concluded that iron(III) coordi nation with 1 can influence the redox properties of the pyrrole ring by aff ecting its pi -electronic system.