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

Non-heme diiron enzymes activate O 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 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 {FeX(μ-O)FeX} (X = Cl, OAc, and OH) or an {Fe(μ-O)(μ-OAc)Fe} core. The complex-induced oxidation of EtOH with HO 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 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(μ-O)Fe} core, while Mössbauer spectroscopy and d-d transitions probe the local Fe sites. The remote methoxy substituents decrease the potential for the oxidation to Fe by ∼100 mV, while directly bound OH in {Fe(OH)(μ-O)Fe(OH)} with a short 1.91 Å Fe-O bond decreases the potential by 590 mV compared to {Fe(OAc)(μ-O)Fe(OAc)} with a 2.01 Å Fe-O bond. Interestingly, this Fe-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(μ-O)Fe} core and not of the local Fe-OH unit.