SPECTROSCOPIC AND ELECTROCHEMICAL CHARACTERIZATION OF A BIS-MACROCYCLIC DIIRON COMPOUND

The properties of the complex [Fe2(C20H36N8)(CH3CN)4](ClO4)4 have been extensively investigated by optical and Mossbauer spectroscopy as well as by electrochemistry. The binucleating ligand TIED (tetraiminoethylene dimacrocycle) is an exceptionally good pi acceptor as indicated by the Mossbauer spectrum, which shows a low value for the center shift and a high value for the quadrupole splitting parameter. This behavior is consistent with the unusually high value for the first oxidation potential (1.18 V vs NHE) since Fe2+ is a much better pi donor than Fe3+. Also, extensive back-bonding from iron to an unoccupied pi* orbital of the ligand may help produce the very short Fe-N(imine) distance of 1.89 angstrom that was found in the complex. Electrochemical oxidation apparently occurs at the metal centers, since the potential shows a large solvent dependence, which we attribute to axial ligand exchange by coordinating solvent molecules. Electrochemical reduction, while formally occurring at the ligand, shows indications of extensive mixing of ligand and metal orbitals. There is a change from two, well-separated one-electron reductions in CH3CN to closely overlapping reductions in DMF and H2O. Another indication of orbital mixing is the fact that substitution of Ni for Fe in the complex causes a large shift of 780 mV in the first ligand reduction potential.