FeIII(PMA)]2+: A Mononuclear Non-Heme Iron Complex That Catalyzes Alkane Oxidation

Abstract

The iron(III) complex [FeIII(PMA)](ClO4)2 (3) of a designed ligand PMAH (1; H is a dissociable amide H), which mimics the metal-binding domain of the antitumor antibiotic bleomycin, promotes alkane oxidation in conjunction with TBHP or H2O2. In anhydrous acetonitrile, reaction of 3 + TBHP with cyclohexane affords high yields of cyclohexanol and cyclohexanone (1:1 ratio, 1700% yield on the basis of catalyst concentration) along with the mixed peroxide tBu−OO−Cy. The presence of water lowers the yields of oxidized products. Product ratios obtained in oxidations of adamantane and 3-methylpentane with 3 + TBHP confirm that reaction occurs at more-substituted carbon centers. No oxidation of an aromatic C−H bond is observed with 3 + TBHP. EPR spectra at low temperatures indicate that the low-spin Fe(III)−peroxo intermediate [(PMA)FeIII−O−OR]+ (R = H, tBu) is formed in the reaction mixture. Since the PMA- ligand framework is incapable of stabilizing the high-valent iron center and oxidation of both cyclohexene and norbornene with 3 + TBHP yields only the allylic oxidation products and the exo-epoxide, respectively, involvement of a perferryl [(PMA)FeVO]2+ intermediate in the oxidation reactions appears unlikely. Product distributions of the oxidation reactions strongly suggest that homolysis of the O−O bond in [(PMA)FeIII−O−OtBu]+ occurs in the reaction mixtures. Abstraction of an H atom from cyclohexane by tBuO•, a product of the homolytic cleavage of the O−O bond in [(PMA)FeIII−O−OtBu]+, results in the production of the cyclohexyl radical. A kH/kD value of 6.5 ± 0.5 for the cyclohexane oxidation by the 3 + TBHP system indicates that C−H bond cleavage is an important step in the overall oxidation mechanism. Reaction of the cyclohexyl radical with O2 (generated in reactions of tBuO• and tBuOO•) produces CyOO• in the reaction mixture, which undergoes a Russell-type termination to afford cyclohexanol and cyclohexanone in a 1:1 ratio. Combination of tBuOO• and the cyclohexyl radical results in the formation of the mixed peroxide tBu−OO−Cy. The second product from the homolytic cleavage of the O−O bond in [(PMA)FeIII−O−OtBu]+, namely [(PMA)FeIII−O•]+, also takes part in the catalytic process by abstracting an H atom from TBHP and generating tBuOO•, a species that provides both O2 and tBuO• in the reaction mixture. Since catalysis is noted only in the presence of iron complexes that give rise to the [(ligand)nFeIII−O−OR]+ intermediates (EPR results) and the yields of the oxidation products vary depending on the nature of the ligands in the starting iron complexes, it is evident that the iron complexes do take part in the oxidation reactions. Results of the present work also suggest that low-spin Fe(III) peroxides are viable intermediates in peroxide-mediated alkane oxidations by certain non-heme iron complexes.