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 FeIII2 or high-valent FeIV2 intermediates. We have developed a dinucleating bis(tetradentate) ligand system that stabilizes peroxo and hydroperoxo FeIII2 complexes with terminal 6-methylpyridine donors, while the peroxo FeIII2 intermediate is reactive with terminal pyridine donors presumably via conversion to a fluent high-valent FeIV2 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 FeIII2 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.