Catalytically active μ-oxodiiron(IV) oxidants from iron(III) and dioxygen

The reaction between an Fe-III complex and O-2 to afford a stable catalytically active diiron(IV)-mu-oxo compound is described. Phosphonium salts of orange five-coordinated Fe-III-TAML complexes with an axial aqua ligand ([PPh4]1-H2O, tetraamidato macrocyclic Fe-III 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 O-2 in CH2Cl2 or other weakly coordinating solvents to produce black mu-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, Mossbauer, and H-1 NMR spectroscopies. Mossbauer data show that the diamagnetic Fe-O-Fe unit contains antiferromagnetically coupled S = 1 Fe-IV sites; diamagnetic H-1 NMR spectra are observed. The oxidation of PPh3 to OPPh3 by 2 was confirmed by UV/Vis and GC-MS. Labeling experiments with O-18(2) and (H2O)-O-18 established that the bridging oxygen atom of 2 derives from O-2. 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 O-2 is an abundant, clean, and inexpensive oxidizing agent.