Kt. Moore et al., Mechanistic studies of (porphinato)iron-catalyzed isobutane oxidation. Comparative studies of three classes of electron-deficient porphyrin catalysts, INORG CHEM, 39(15), 2000, pp. 3125-3139
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.