Photochemical generation of a highly reactive iron-oxo intermediate. A true iron(v)-oxo species?

Article abstract of DOI:10.1021/ja0542439

Laser flash photolysis of 5,10,15-tris(pentafluorophenyl)corrole-iron(IV) chlorate or nitrate, prepared from the corresponding chloride, gave a highly reactive iron-oxo transient identified as an iron(V)-oxo species on the basis of its UV-visible spectrum and high reactivity as well as by analogy to photochemical ligand cleavage reactions of related manganese species. The transient was shown to be an oxo transfer agent in a preparative reaction with cis-cyclooctene. Representative rate constants for oxidation reactions by the new transient at ambient temperature were k = 5900 M^-1 s^-1 for cyclooctene and k = 570 M^-1 s^-1 for ethylbenzene. The new transient is more than 6 orders of magnitude more reactive with typical organic reductants than expected for an iron(IV)-oxo corrole radical cation and 100 times more reactive than an analogous positively charged iron(IV)-oxo porphyrin radical cation. Slow electron transfer isomerization of ligand iron(V)-oxo species to iron(IV)-oxo ligand radical cations might be important in reactions of porphyrin-iron catalysts in the laboratory and in nature. Copyright

Full text of DOI:10.1021/ja0542439

Published on Web 09/17/2005
Photochemical Generation of a Highly Reactive Iron-Oxo Intermediate. A
True Iron(V)-Oxo Species?
Dilusha N. Harischandra, Rui Zhang,* and Martin Newcomb*
Department of Chemistry, UniVersity of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607
Received June 27, 2005; E-mail: men@uic.edu
High-valent metal-oxo transients are active intermediates in
many catalytic oxidation processes in nature and in the laboratory.1
Iron is one of the most common metals in the catalysts, especially
the catalysts in nature, where it is found in the form of porphyrin-
iron, mononuclear non-heme iron, and diiron assemblies.2 The
readily accessible high oxidation state of iron is +4. Porphyrin-
iron(IV)-oxo species are well-known, and two-electron oxo transfer
oxidation of porphyrin-iron(III) salts gives observable iron(IV)-
oxo porphyrin radical cations in enzymes (Compound I) and in
models.3 Species with more highly oxidized iron atoms are rare.4
Figure 1. (A) Time-resolved spectrum for transient 4 (22 °C, CH3CN) at
0.5, 2, 4, 8, 12, and 20 ms after formation. (B) Time-resolved decay spectrum
from reaction of neutral iron(III)-corrole with mCPBA (22 °C, CH3CN)
over 20 ms. (C) Observed rate constants for reactions of 4.
The active oxidizing species from an iron catalyst is usually ascribed
to an iron(IV)-oxo ligand radical cation, but paradoxically,
independently generated iron(IV)-oxo porphyrin radical cations
do not exhibit the high reactivity necessary for hydrocarbon
oxidation reactions.5
We recently introduced laser flash photolysis (LFP)-induced
ligand cleavage reactions that generate ligand-manganese(V)-oxo
derivatives that can be studied in real time.6 Herein, we report
extension of the methods to production of a highly reactive high-
valent iron-oxo transient that we tentatively identify as a corrole-
iron(V)-oxo species.7 The results indicate that isomerization of a
ligand-iron(V)-oxo species to the isomeric iron(IV)-oxo ligand
radical cation can be slow, permitting substrate oxidation by the
highly reactive iron(V)-oxo species.
Air oxidation of 5,10,15-tris(pentafluorophenyl)corrole-iron(III)
in the presence of HCl gave the known corrole-iron(IV) chloride
salt 1,8 which was allowed to react with Ag(ClO3) or with Ag(NO3)
in acetonitrile (Scheme 1). The reaction of 1 with Ag(ClO3) gave
an intermediate (2a) that reacted rapidly (τ ) 1 s) to give a stable
product (3a). The reaction of 1 with Ag(NO3) gave a species (2b)
that was spectroscopically similar to 2a and considerably more
stable with τ > 100 s. From the UV-visible and ESR spectra, we
assign the structures of compounds 2 as corrole-iron(IV) chlorate
and nitrate, respectively, and compound 3a as an iron(III) corrole
radical cation.9
LFP of 2a in CH3CN with 355 nm light gave a new reactive
transient (4) with a strong, sharp Soret band that decayed rapidly
(Figure 1A). LFP of nitrate 2b gave the same transient, albeit with
lower efficiency. The same species apparently also was formed as
a short-lived transient when the neutral corrole-iron(III) species
was mixed with excess mCPBA (Figure 1B); the rate constants for
decay of these species were the same (τ ≈ 0.005 s; see Supporting
Information). The photochemical reaction was specific for 2; 355
nm photolysis of 3a did not result in a noticeable reaction. We
tentatively assign the structure of 4 as the corrole-iron(V)-oxo
species shown in Scheme 1 on the basis of its UV-visible spectrum,
its high reactivity (see below), and analogy to the photochemical
reactions of corrole-manganese(IV) chlorates, which give corrole-
manganese(V)-oxo species (e.g., 5) upon 355 nm irradiation.6c
Transient 4 was shown to be a reactive iron-oxo intermediate
in a preparative reaction. Nitrate precursor 2b was prepared by
mixing 1 with 1 equiv of Ag(NO3) in CH3CN; under these
conditions, the yield of 2b determined by UV-visible spectroscopy
was ca. 90%. An excess of cis-cyclooctene was added, and the
mixture was photolyzed with 350 nm light. Following product
workup, quantitative GC analysis showed the presence of cis-
cyclooctene oxide in 50% yield based on precursor 2b. No reaction
of 2b with cyclooctene was observed without light.
Scheme 1
Species 4 reacted rapidly when produced in CH3CN solutions.
In the absence of an organic reductant, the pseudo-first-order decay
rate constant was k ) 200 s-1. In the presence of organic reductants,
the pseudo-first-order decay rate constants increased linearly with
substrate concentration (Figure 1C). The kinetic data were solved
via eq 1, where kobs is the observed rate constant, k0 is the
background rate constant, kox is the second-order rate constant for
oxidation of substrate, and [Sub] is the concentration of substrate.
The second-order rate constants were kox ) (7.4 ( 0.3) × 103 M-1
s-1 (cyclohexene), kox ) (5.9 ( 0.2) × 103 M-1 s-1 (cis-
cyclooctene), kox ) (5.7 ( 0.3) × 102 M-1 s-1 (ethylbenzene), and
kox ) (1.6 ( 0.2) × 102 M-1 s-1 (ethylbenzene-d10).
kobs ) k0 + kox[Sub]
(1)
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C O M M U N I C A T I O N S
even in low-temperature studies.13 The putative oxidants in both
cases are iron(IV)-oxo porphyrin radical cations, that is, Compound
I analogues. Authentic Compound I analogues do not hydroxylate
unactivated C-H bonds rapidly, however.5 Both in nature and in
the laboratory, oxidation of the porphyrin-iron(III) species will
give a porphyrin-iron(V)-oxo derivative as the first-formed
intermediate, and it is possible that undetected iron(V)-oxo species
hydroxylate unactivated C-H bonds in substrates faster than they
isomerize to Compound I analogues.
Table 1. Second-Order Rate Constants at Ambient Temperature
for Oxidations of cis-Cyclooctene and Ethylbenzenea
4
5
6
7
cyclooctene
PhCH2CH3
5900
570
4
n.d.
600000
130000
50
4
Acknowledgment. This work was supported by a grant from
the National Institutes of Health (GM48722). We thank Prof. L.
W. Fung for assistance in the acquisition of ESR spectra.
a In units of M-1 s-1. Ar ) (C6F5). Data for 5, 6, and 7 are from refs 5
and 6; n.d. ) not determined.
Supporting Information Available: UV-visible spectra, ESR
spectra, and experimental details. This material is available free of
charge via the Internet at http://pubs.acs.org.
Scheme 2
References
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The assignment of an iron(V)-oxo structure to transient 4, as
opposed to an iron(IV)-oxo corrole radical cation, should be
considered tentative until the species can be prepared in high
conversion under conditions that permit more complete character-
ization. Nonetheless, the UV-visible spectrum and especially the
reactivity of 4 suggest the iron(V)-oxo structure. For example,
consider the rate constants shown in Table 1. Transient 4 is about
3 orders of magnitude more reactive than the analogous corrole-
manganese(V)-oxo species 5. In the related series of porphyrins,
the porphyrin-manganese(V)-oxo cation 6 reacts more than 4
orders of magnitude faster than the iron(IV)-oxo porphyrin radical
cation 7. Remarkably, the neutral transient 4 is 100 times more
reactiVe than the positively charged iron(IV)-oxo porphyrin radical
cation 7. If the new transient were the iron(IV)-oxo corrole radical
cation (8 in Scheme 2), one would expect the reactivity at ambient
temperature to be at least 6 orders of magnitude smaller than
observed.10
(7) Corrole-iron complexes are known to be oxidation catalysts with
sacrificial oxidants, but no iron-oxo transients have been detected in such
studies; see: Gross, Z.; Simkhovich, L.; Galili, N. Chem. Commun. 1999,
599.
The corrole-iron(V)-oxo structure 4 is not necessarily the low-
energy electronic configuration of the system. By analogy to
porphyrin-iron-oxo species,11 the isomeric iron(IV)-oxo corrole
radical cation 8 (Scheme 2) should be a lower-energy species than
4. If an intrinsic barrier for the electron transfer isomerization exists,
structures 4 and 8 will be distinct isomers and not resonance forms.
Such a barrier might result from a large degree of structural
reorganization in the macrocycle upon oxidation and the attendant
loss of delocalization energy. A barrier for electron transfer
isomerization of 4 to 8 can function to trap high reactivity in the
iron(V)-oxo species 4, which would be expected to oxidize
substrates rapidly, whereas the lower-energy isomer 8 would be
expected to react sluggishly.
The possibility that a ligand-iron(V)-oxo species can convert
relatively slowly to its iron(IV)-oxo ligand radical cation isomer
might be an important feature for understanding catalytic oxidations
by porphyrin-iron complexes in the laboratory and in nature. Under
catalytic turnover conditions, a porphyrin-iron catalyst will oxidize
cyclohexane,12 and cytochrome P450cam hydroxylates an unactivated
C-H bond in camphor so fast that the oxidant cannot be detected
(8) Simkhovich, L.; Goldberg, I.; Gross, Z. Inorg. Chem. 2002, 41, 5433.
(9) The UV-vis spectra of compounds 2 had sharp Soret bands and no
obvious absorbance in the Q-band region, whereas the spectrum of 3a
had a weak Soret absorbance distinct from that of 2 and a strong
absorbance in the Q-band region (λmax ) 650 nm) consistent with a ligand
radical cation. Compounds 2 were ESR silent, as expected for Fe(IV)
species with broad absorbances, and the ESR spectrum of 3a had a signal
at g ) 4.5. The spectra allow us to rule out iron(IV) corrole radical cations
and the known neutral iron(III) species as structures of 2 and 3,
respectively. Spectra are shown in the Supporting Information. For an
example of a metal corrole radical cation complex, see: Simkhovich, L.;
Mahammed, A.; Goldberg, I.; Gross, Z. Chem.sEur. J. 2001, 7, 1041.
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manganese(V)-oxo species, see: (a) Gross, Z.; Gray, H. B. AdV. Synth.
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