Direct and remote control of electronic structures and redox potentials in μ-oxo diferric complexes†

Non-heme diiron enzymes activate O₂ for the oxidation of substrates in the form of peroxo Fe^III₂ or high-valent Fe^IV₂ intermediates. We have developed a dinucleating bis(tetradentate) ligand system that stabilizes peroxo and hydroperoxo Fe^III₂ complexes with terminal 6-methylpyridine donors, while the peroxo Fe^III₂ intermediate is reactive with terminal pyridine donors presumably via conversion to a fluent high-valent Fe^IV₂ intermediate. We present here a derivative with electron-donating methoxy substituents at the pyridine donors and its diferric complexes with an {Fe^IIIX(μ-O)Fe^IIIX} (X⁻ = Cl⁻, OAc⁻, and OH⁻) or an {Fe^III(μ-O)(μ-OAc)Fe^III} core. The complex-induced oxidation of EtOH with H₂O₂ provides μ-OAc⁻, and in acetone, the complex with mixed OH⁻/OAc⁻ exogenous donors is obtained. Both reactivities indicate a reactive fluent peroxo Fe^III₂ intermediate. The coupling constant J and the LMCT transitions are insensitive to the nature of the directly bound ligands X⁻ and reflect mainly the electronic structure of the central {Fe^III(μ-O)Fe^III} core, while Mössbauer spectroscopy and d–d transitions probe the local Fe^III sites. The remote methoxy substituents decrease the potential for the oxidation to Fe^IV by ∼100 mV, while directly bound OH⁻ in {Fe^III(OH)(μ-O)Fe^III(OH)} with a short 1.91 Å Fe^III–O^OH bond decreases the potential by 590 mV compared to {Fe^III(OAc)(μ-O)Fe^III(OAc)} with a 2.01 Å Fe^III–O^OAc bond. Interestingly, this Fe^III–OH bond is even shorter (1.87 Å) in the mixed OH⁻/OAc⁻ complex but the potential is the mean value of the potentials of the OH⁻/OH⁻ and OAc⁻/OAc⁻ complexes, thus reflecting the electron density of the central {Fe^III(μ-O)Fe^III} core and not of the local Fe^III–OH unit.