Electrochemical and spectral studies of a di-iron complex and its mixed-valence form

The complex Fe2(TIED)4+ (compound I) [see PDF] has a characteristic intense low energy electronic absorption band (λ = 874 nm, ε = 24,600 M-1cm-1). This band is assigned as a metal to ligand charge transfer, MLCT, and it exhibits a very large solvent dependence which is attributed to replacement of the axial ligands on the iron atoms by solvent. The complex has two reversible oxidation and two reversible reduction steps in the cyclic voltammogram in dry CH3CN. The oxidation involves the iron atoms while the reduction places electrons on the conjugated part of the ligand. The cyclic voltammogram shows a single, apparently, two-electron reduction process in DMF and in H2O. A mixed-valence FeII/FeIII (Fe2(TIED)5+) compound has been obtained by electrochemical oxidation of Fe2(TIED)4+. The mixed-valence compound has an intense near-infrared band (λ = 940 nm, ε = 28,000 M-1cm-1). Analysts of this band using the Hush equations for an intervalence transition indicates that the system is delocalized. The difference in oxidation potentials, ΔE1/2 = ΔE1/2 (FeIII/FeIII) - ΔE1/2 (FeII/FeIII), is 500 mV and this corresponds to a comproportionation constant of 2.8 x 108. This large value indicates the mixed-valence species is stabilized by delocalization. The Mössbauer spectrum of the mixed-valence compound consists of a single quadrupole-split doublet at 90 K and 4.2 K. This establishes that the iron atoms are equivalent of the Mössbauer time scale, and the mixed-valence compound is classified as valence-average, ((Fe2.5/Fe2.5)