Mechanistic studies of (porphinato)iron-catalyzed isobutane oxidation. Comparative studies of three classes of electron-deficient porphyrin catalysts

We report herein a comprehensive study of (porphinato)iron [PFe]-catalyzed isobutane oxidation in which molecular oxygen is utilized as the sole oxida nt; these catalytic reactions were carried out and monitored in both autocl ave reactors and sapphire NMR tubes. In situ F-19 and C-13 NMR experiments, coupled with GC analyses and optical spectra obtained from the autoclave r eactions have enabled the identification of the predominant porphyrinic spe cies present during PFe-catalyzed oxidation of isobutane. Electron-deficien t PFe catalysts based on 5,10,15, 20-tetrakis(pentafluorophenyl)porphyrin [ (C6F5)(4)PH2], 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(pentafluor ophenyl)porphyrin [Br-8(C6F5)(4)PH2], and 5,10,15,20-tetrakis(heptafluoropr opyl)por [(C3F7)(4)PH2] macrocycles were examined. The nature and distribut ion of hydrocarbon oxidation products show that an autoxidation reaction pa thway dominates the reaction kinetics, consistent with a radical chain proc ess. For each catalytic system examined, PFeII species were shown not to be stable under moderate O-2 pressure at 80 degrees C; in every case, the PFe II catalyst precursor was converted quantitatively to high-spin PFeIII comp lexes prior to the observation of any hydrocarbon oxidation products. Once catalytic isobutane oxidation is initiated, all reactions are marked by con comitant decomposition of the porphyrin-based catalyst. In situ O-17 NMR sp ectroscopic studies confirm the incorporation of O-17 from labeled water in to the oxidation products, implicating the involvement of PFe-OH in the cat alytic cycle. Importantly, Br-8(C6F5)(4)PFe-based catalysts, which lack mac rocycle C-H bonds, do not exhibit augmented stability with respect to analo gous catalysts based on (C6F5)(4)PFe and (C3F7)(4)PFe species. The data pre sented are consistent with a hydrocarbon oxidation process in which PFe com plexes play dual roles of radical chain initiator, and the species responsi ble for the catalytic decomposition of organic peroxides. This modified Hab er-Weiss reaction scheme provides for the decomposition of tert-butyl hydro peroxide intermediates via reaction with PFe-OH complexes; the PFeIII speci es responsible for hydroperoxide decomposition are regenerated by reaction of PFeII with dioxygen under these experimental conditions.