Influence of Asymmetry on the Molecular and Electronic Structures of μ‐Oxo Diferric Complexes

μ‐Oxo‐bridged diferric complexes serve as models for nonheme diiron proteins to elucidate their molecular and electronic structures and to gain insight into their reactivities. A bis(tetradentate) dinucleating ligand system with two tert ‐amine and two pyridine donors per ligand compartment has been developed. The introduction of a 6‐methyl substituent at the pyridines lengthens the MN py bonds by steric repulsion, resulting in less electron donation to the metal ions. Herein, μ‐oxo diferric complex with an asymmetric dinucleating ligand that has 6‐methyl substituents only in one of the ligand‐compartments is presented. The molecular structure exhibits different FeN py bond lengths. The two different Fe III sites in the asymmetric complex show only one quadrupole doublet while the quadrupole doublets for the two symmetric complexes differ significantly. There are two 6 A → 4 T 1 transitions, both of which are shifted to a higher energy compared to symmetric complexes, demonstrating the influence of asymmetry on electronic structures. This is supported by electrochemical investigations. The oxidation of Fe III to Fe IV is 80 mV more difficult, and the reduction of Fe III to Fe II is 230 mV more difficult, compared to the symmetric complexes. The origin of this effect is discussed, which is probably caused by an overcompensating charge donation of the μ‐oxo ligand.