Oxidizing Intermediates from the Sterically Hindered Iron Salen Complexes Related to the Oxygen Activation by Nonheme Iron Enzymes

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Abstract

Oxidizing intermediates are generated from nonheme iron(III) complexes to investigate the electronic structure and the reactivity, in comparison with the oxoiron(IV) porphyrin pi-cation radical (compound I) as a heme enzyme model. Sterically hindered iron salen complexes, bearing a fifth ligand Cl (1), OH(2) (2), OEt (3), and OH (4), are oxidized both electrochemically and chemically. Stepwise one-electron oxidation of 1 and 2 generates iron(III)-mono- and diphenoxyl radicals, as revealed by detailed spectroscopic investigations, including UV-vis, EPR, Mössbauer, resonance Raman, and ESIMS spectroscopies. In contrast to the oxoiron(IV) formation from the hydroxoiron(III) porphyrin upon one-electron oxidation, the hydroxo complex 4 does not generate oxoiron(IV) species. Reaction of 2 with mCPBA also results in the formation of the iron(III)-phenoxyl radical. One-electron oxidation of 3 leads to oxidative degradation of the fifth EtO ligand to liberate acetaldehyde even at 203 K. The iron(III)-phenoxyl radical shows high reactivity for alcoxide on iron(III) but exhibits virtually no reactivity for alcohols including even benzyl alcohol without a base to remove an alcohol proton. This study explains unique properties of mononuclear nonheme enzymes with Tyr residues and also the poor epoxidation activity of Fe salen compared to Mn and Cr salen compounds.

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