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

Non-heme diiron enzymes activate O 2 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 III X(μ-O)Fe III X} (X - = Cl - , OAc - , and OH - ) or an {Fe III (μ-O)(μ-OAc)Fe III } core. The complex-induced oxidation of EtOH with H 2 O 2 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.