J. Am. Chem. Soc. 1997, 119, 1791-1792
1791
oxidations utilized two archetypal electron deficient PFe oxida-
tion catalysts; one is based on the well-studied 5,10,15,20-
tetrakis(pentafluorophenyl)porphyrin [(C6F5)4PH2] ligand sys-
tem, while the other features the recently developed, significantly
more electron poor, 5,10,15,20-tetrakis(heptafluoropropyl)-
porphyrin [(C3F7)4PH2] macrocycle.11-12
High-Pressure NMR Studies of
(Porphinato)iron-Catalyzed Isobutane Oxidation
Utilizing Dioxygen as the Stoichiometric Oxidant
Kevin T. Moore,† Istva´n T. Horva´th,*,‡ and
Michael J. Therien*,†
Because the FeII oxidation state is postulated to play a primary
role in the generation of a dioxygen-derived active alkane-
hydroxylating species,2,4 the autoclave reactors and the sapphire
NMR tubes were initially charged with PFeII‚(ligand)2 com-
plexes. 3-Fluoropyridine (3-F-py) was used as the axial ligand
for this study, since it has moderate Lewis basicity13 and
provides a convenient marker band for 19F NMR.
Variable temperature 19F NMR spectra show reversible
dissociation of axial 3-F-py over a moderate temperature domain
for both complexes in benzene-d6 in the absence of isobutane
and oxygen. Consistent with the electronic properties of the
macrocycle,11 the (C3F7)4PFeII‚(3-F-py)2 complex (Figure 1A)
exists primarily as six-coordinate iron(II) at 26 °C in benzene-
d6, (δ ) -127.1 ppm for free 3-F-py; δ ) -126.1 ppm for
3-F-py in (C3F7)4PFeII‚(3-F-py)2 at 26 °C); in contrast, (C6F5)4-
PFeII‚(3-F-py)2 (Figure 1E) exhibits an equally sharp 3-F-py
resonance at -40 °C, but shows a significantly broadened ligand
peak at 26 °C, congruent with reversible 3-F-py dissociation at
ambient temperature. At 80 °C, both complexes display broad
19F NMR chemical shifts for 3-F-py.14
Department of Chemistry, UniVersity of PennsylVania
Philadelphia, PennsylVania 19104-6323
Corporate Research Laboratories, Exxon Research and
Engineering Company, Annandale, New Jersey 08801
ReceiVed May 7, 1996
We report the first high-pressure NMR1 study of any metal-
catalyzed oxidation reaction, enabling us to identify the
predominant species present in solution during a (porphinato)-
iron [PFe]-catalyzed oxidation of isobutane in which the
hydrocarbon oxidizing equivalents are derived from dioxygen.2-7
This is particularly important given (i) the potential commercial
importance of these catalytic reactions and (ii) the fact that in
contrast to the wealth of information regarding both mechanism
and reactivity for catalytic oxidations of hydrocarbons involving
PFeIII complexes and O atom donors, such as iodosylbenzene,8
comparatively little is known regarding analogous catalytic
oxidations that utilize O2. Our work demonstrates that the nature
of the porphyrinic species under catalytic conditions differs from
what has previously been surmised.2,4
Within 15 min of pressurizing the sapphire NMR tubes
containing the PFeII species in benzene-d6 with isobutane and
O2, the spectra shown in Figure 1C,G are manifest. The
porphyrinic 19F NMR signals of Figure 1C correspond to the
electron poor µ-oxo dimer [(C3F7)4PFeIII]2O, while those of
Figure 1G verify that a mixture of [(C6F5)4PFeIII]2O and [(C6F5)4-
PFeIII‚OH] is present for the more sterically encumbered and
electron rich (C6F5)4PH2-based catalyst.12b,15,16 Because the free
ligand peaks integrate 2:1 relative to the porphyrin NMR peaks,
we conclude that PFeIII species are produced stoichiometrically
from these PFeII(L)2 precursors; thus, within the detection limits
of the experiment, no other porphyrinic species other than
PFeIII‚OH and (PFeIII)2O are observable in either system prior
to the onset of catalysis. 13C NMR spectroscopy shows at this
time that no oxidation of isobutane has taken place in either
sample; in fact, significant induction periods (∼hours) are
required before any oxidation products are observable.17 Analo-
gous NMR experiments show a similar distribution of PFeIII
species in the absence of isobutane; furthermore, the nature of
these products does not change as the catalyst concentration is
increased to 0.1 M.
We have performed catalytic isobutane oxidation reactions
at 80 °C in sapphire high-pressure NMR tubes.1b,9 In situ 19F
NMR experiments are particularly useful to establish the nature
of PFe species, since the magnitude of the 19F chemical shifts
for fluorine-containing porphyrins depends intimately on metal-
centered electronic properties.10 Parallel experiments in glass-
lined autoclaves9 enable us to monitor the formation of the
oxidation products by GC and the fate of the catalysts by
electronic absorption spectroscopy. These catalytic isobutane
† University of Pennsylvania.
‡ Exxon Research and Engineering Company.
(1) (a) Horva´th, I. T.; Millar, J. M. Chem. ReV. 1991, 91, 1339-1351.
(b) Horva´th, I. T.; Ponce, E. C. ReV. Sci. Instrum. 1991, 62, 1104-1105.
(2) (a) Ellis, P. E., Jr.; Lyons, J. E. Coord. Chem. ReV. 1990, 105, 181-
193 and references therein. (b) Lyons, J. E.; Ellis, P. E., Jr. Catal. Lett.
1991, 8, 45-52. (c) Lyons, J. E.; Ellis, P. E., Jr. Appl. Catalysis A. General
1992, 84, L1-L6. (d) Lyons, J. E.; Ellis, P. E., Jr.; Myers, H. K., Jr.;
Wagner, R. W. J. Catalysis 1993, 141, 311-315. (e) Lyons, J. E.; Ellis, P.
E., Jr.; Myers, H. K., Jr. J. Catalysis, 1995, 155, 59-73. (f) Wijesekera, T.
P.; Lyons, J. E.; Ellis, P. E., Jr. Catal. Lett. 1996, 36, 69-73.
(3) (a) Ellis, P. E.; Lyons, J. E. U.S. Patent 4 898 989, 1990. (b) Ellis,
P. E.; Lyons, J. E. U.S. Patent 5 118 886, 1992. (c) Ellis, P. E.; Lyons, J.
E.; Myers, H. K. Eur. Patent 471 561, 1992. (d) Ellis, P. E.; Lyons, J. E.
U.S. Patent 5 120 882, 1992. (e) Ellis, P. E.; Lyons, J. E. U.S. Patent 5 120
886, 1992.
After the induction period, 13C NMR shows the formation
of isobutanol, acetone, and tert-butyl peroxide. 19F NMR shows
the presence of several new resonances that correspond to
(4) (a) Grinstaff, M. W.; Hill, M. G.; Labinger, J. A.; Gray, H. B. Science
1994, 264, 1311-1313. (b) Labinger, J. A. Catal. Lett. 1994, 26, 95-99.
(5) (a) Balch, A. L.; Olmstead, M. M.; Safari, N.; St. Claire, T. N. Inorg.
Chem. 1994, 33, 2815-2822. (b) Chin, D.-H.; La Mar, G. N.; Balch, A. L.
J. Am. Chem. Soc. 1980, 102, 4344-4350.
(6) Rodgers, K. R.; Arafa, I. M.; Goff, H. M. J. Chem. Soc., Chem.
Commun. 1990, 1323-1324.
(11) (a) DiMagno, S. G.; Williams, R. A.; Therien, M. J. J. Org. Chem.
1994, 59, 6943-6948. (b) Goll, J. G.; Moore, K. T.; Ghosh, A.; Therien,
M. J. J. Am. Chem. Soc. 1996, 118, 8344-8354.
(12) (a) The FeII redox state for (C3F7)4PFeII.(ligand)2 complexes is
stabilized with respect to that observed for (C6F5)4PFeII‚(ligand)2 species,
consistent with the superior electron-withdrawing properties of the former
macrocycle. (b) Moore, K. T.; Therien, M. J. Manuscript in preparation.
(13) (a) Henderson, W. A.; Streuli, C. A. J. Am. Chem. Soc. 1960, 82,
5791-5794. (b) Gordon, A. J.; Ford, R. A. In Chemist’s Companion;
Wiley: New York, 1972; p 145.
(7) Momenteau, M.; Reed, C. A. Chem. ReV. 1994, 94, 659-698.
(8) For example, see: (a) Groves, J. T.; Nemo, T. E. J. Am. Chem. Soc.
1983, 105, 5786-5791. (b) Nappa, M. J.; Tolman, C. A. Inorg. Chem. 1985,
24, 4711-4719. (c) Traylor, T. G.; Tsuchiya, S. Inorg. Chem. 1987, 26,
1338-1339. (d) Castellino, A. J.; Bruice, T. C. J. Am. Chem. Soc. 1988,
110, 158-162. (e) Collman, J. P.; Zhang, X.; Lee, V. J.; Uffelman, E. S.;
Brauman, J. I. Science 1993, 261, 1404-1411. (f) Sorokin, A.; Robert, A.;
Meunier, B. J. Am. Chem. Soc. 1993, 115, 7293-7299. (g) Mansuy, D.
Coord. Chem. ReV. 1993, 125, 129-142.
(9) Reaction conditions for the autoclave (Table 1) and high-pressure
NMR tube experiments were identical with respect to [catalyst], [isobutane],
reaction time, and temperature. While a continuous O2 feed maintained a
constant pressure in the autoclave experiments, pressure decreased simul-
taneously with oxygen consumption in our NMR experiments.
(10) (a) Birnbaum, E. R.; Hodge, J. A.; Grinstaff, M. W.; Schaefer, W.
P.; Henling, L.; Labinger, J. A.; Bercaw, J. E.; Gray, H. B. Inorg. Chem.
1995, 34, 3625-3632. (b) Toi, H.; Homma, M.; Suzuki, A.; Ogoshi, H. J.
Chem. Soc., Chem. Commun. 1985, 1791-1792.
(14) For (C3F7)4PFeII‚(3-F-py)2, the 19F NMR chemical shift at -126.5
ppm progressively broadens over the 26-80 °C temperature domain. For
(C6F5)4PFeII‚(3-F-py)2, we initially observed broadening of the signal at
-126.5 ppm, followed by a progressive shift of this resonance to -122.2
ppm on warming the sample from 26 to 80 °C.
(15) Spectroscopic assignments were made via independently synthesized
PFeIII complexes (see Supporting Information). The nature of the PFeIII
species present along with their chemical shifts have been shown to be
independent of isobutane concentration.16
(16) Moore, K. T.; Horva´th, I. T.; Therien M. J. In preparation.
(17) Induction periods could be as high as several days and do not
correlate with the electronic structure of the porphyrin ligand.
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