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Catalytically Active μ-Oxodiiron(IV) Oxidants from Iron(III) and Dioxygen

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journal contribution
posted on 02.03.2005, 00:00 by Anindya Ghosh, Filipe Tiago de Oliveira, Toshihiro Yano, Takanori Nishioka, Evan S. Beach, Isamu Kinoshita, Eckard Münck, Alexander D. Ryabov, Colin P. Horwitz, Terrence J. Collins
The reaction between an FeIII complex and O2 to afford a stable catalytically active diiron(IV)-μ-oxo compound is described. Phosphonium salts of orange five-coordinated FeIII−TAML complexes with an axial aqua ligand ([PPh4]1−H2O, tetraamidato macrocyclic FeIII species derived from 3,3,6,6,9,9-hexamethyl-3,4,8,9-tetrahydro-1H-1,4,8,11-benzotetraazacyclotridecine-2,5,7,10(6H,11H)-tetraone) react rapidly with O2 in CH2Cl2 or other weakly coordinating solvents to produce black μ-oxo-bridged diiron(IV) complexes, 2, in high yields. Complexes 2 have been characterized by X-ray crystallography (2 cases), microanalytical data, mass spectrometry, UV/Vis, Mössbauer, and 1H NMR spectroscopies. Mössbauer data show that the diamagnetic Fe−O−Fe unit contains antiferromagnetically coupled S = 1 FeIV sites; diamagnetic 1H NMR spectra are observed. The oxidation of PPh3 to OPPh3 by 2 was confirmed by UV/Vis and GC−MS. Labeling experiments with 18O2 and H218O established that the bridging oxygen atom of 2 derives from O2. Complexes 2 catalyze the selective oxidation of benzylic alcohols into the corresponding aldehydes and bleach rapidly organic dyes, such as Orange II in MeCN−H2O mixtures; reactivity evidence suggests that free radical autoxidation is not involved. This work highlights a promising development for the advancement of green oxidation technology, as O2 is an abundant, clean, and inexpensive oxidizing agent.