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

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.