Visible light-driven aerobic oxidation catalyzed by a diiron(IV) μ-oxo biscorrole complex

Article abstract of DOI:10.1016/j.apcata.2013.05.025

Under visible light irradiation, the diiron(IV) μ-oxo-bis(5,10,15- tripentafluorophenylcorrole) complex catalyzes the aerobic oxidations of hydrocarbons in sequences employing photo-disproportionation reactions. The active oxidant, highly reactive corrole-iron(V)-oxo species, can be detected in real time via laser flash photolysis methods.

Full text of DOI:10.1016/j.apcata.2013.05.025

Applied Catalysis A: General 464–465 (2013) 95–100
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Visible light-driven aerobic oxidation catalyzed by a diiron(IV) ␮-oxo
biscorrole complex
Rui Zhang∗, Eric Vanover, Tse-Hong Chen, Helen Thompson
Department of Chemistry, Western Kentucky University, Bowling Green, KY 42101-1079, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Under visible light irradiation, the diiron(IV) ␮-oxo-bis(5,10,15-tripentafluorophenylcorrole) complex
catalyzes the aerobic oxidations of hydrocarbons in sequences employing photo-disproportionation reac-
tions. The active oxidant, highly reactive corrole-iron(V)-oxo species, can be detected in real time via laser
flash photolysis methods.
Received 1 March 2013
Received in revised form 10 May 2013
Accepted 19 May 2013
Available online 29 May 2013
© 2013 Elsevier B.V. All rights reserved.
Keywords:
Iron corrole
Aerobic oxidation
Visible light
Photo-disproportionation
1. Introduction
transition metal catalyst is oxidized to a high-valent metal-oxo
species [6] by a sacrificial oxidant, and then the activated transition
Selective oxidation of organic compounds represents a lead-
ing technology for commodity chemical production [1]. Oxidation
catalysis also plays an essential role in both energy production
and energy conservation, as over 90% of all chemical processes
are catalytic. However, oxidation is among the most problem-
atic processes. Many stoichiometric oxidants with heavy metals
are expensive and toxic, thus, economically and environmentally
inviable. As the demand for “greener” processing increases, the
ideal system for catalytic oxidation is the use of molecular oxy-
gen or hydrogen peroxide as the primary oxygen source together
with recyclable catalysts in nontoxic solvents [2,3]. In this con-
text, many transition metal catalysts have been synthesized to
mimic the predominant oxidation catalysts in Nature, namely
the cytochrome P450 enzymes [4]. Most biological oxidations
mediated by P450 enzymes are highly, often completely, stereose-
lective and ecologically sustainable. However, biological oxidations
involve sophisticated electron and proton transfer steps to acti-
vate molecular oxygen and are currently difficult to implement
with a synthetic catalyst [5]. In biomimetic catalytic oxidations, a
metal-oxo intermediate oxidizes the substrate [7].
Photochemical reactions are intrinsically advantageous because
activation is obtained by the absorption of a photon, which leaves
no residue; whereas most chemical methods involve the use of
toxic/polluting reagents [8]. To this end, we aim to use visible light
(sunlight) to induce reversible redox processes of the metal center,
avoiding all the disadvantages deriving from the use of chemi-
cal reagents. Notably, trans-dioxoruthenium(VI) porphyrins have
been shown to catalyze the clean aerobic epoxidation of olefins in
the absence of a reducing agent [9,10]. However, due to product
inhibition, these reactions afforded only up to 50 turnovers and
proceeded at relatively slow rates. The chemistry of cofacial bis-
porphyrins has also drawn increased attention owing to the ability
of these systems to utilize molecular oxygen and visible light for
organic oxidations [11,12]. One example is the catalytic aerobic oxi-
dation by high-valent iron(IV)-oxo species in a process that involves
photo-disproportionation of a diiron(III)-␮-oxo bisporphyrin com-
plex [13,14]. The low reactivity of the formed iron(IV)-oxo oxidant
and the poor quantum efficiency are serious obstacles that prohibit
the use of iron porphyrins as practical photocatalysts. Recently,
Nocera and co-workers have achieved a remarkable improvement
of the catalytic efficiency of diiron(III)-␮-oxo bisporphyrin systems
by employing “Pacman” ligand designs with organic spacer-hinges
to preorganize two iron centers in a cofacial arrangement [15–18].
In catalytic macrocycle-iron-oxo chemistry, the transient with
iron in the formal +5 oxidation state exists mainly as an iron(IV)-oxo
complex with the ligand oxidized to a radical cation [19]. The well
known porphyrin-manganese(V)-oxo complexes showed higher
Abbreviations:
TPFC, 5,10,15-tripentafluorophenylcorrole trianion; TPC,
5,10,15-triphenylcorrole trianion; TMP, 5,10,15,20-tetramesitylporphyrin dianion;
TPP, 5,10,15,20-tetraphenylporphyrin dianion; LFP, laser flash photolysis; n.d., not
detected.
∗
Corresponding author at: Department of Chemistry, Western Kentucky Univer-
sity, 1906 College Heights Blvd. #11079, Bowling Green, KY 42101-1079, USA.
Tel.: +1 270 745 3803; fax: +1 270 745 5361.
E-mail address: rui.zhang@wku.edu (R. Zhang).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.05.025

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R. Zhang et al. / Applied Catalysis A: General 464–465 (2013) 95–100
reactivity than the well known iron(IV)-oxo radical cation in the
same ligand system [20–22]. A reported oxoiron(V)-oxo complex
supported by a tetraanionic ligand showed truly unprecedented
reactivity [23]. The porphyrin/corrole-iron(V)-oxo transients pro-
duced by laser flash photolysis (LFP) methods also displayed
the appropriate high level of reactivity [24,25]. Due to recent
progress in the methods for triarylcorrole synthesis [26,27], met-
allocorrole complexes have attracted considerable interest in view
of their similarities to metalloporphyrins [28,29]. Corrole-metal-
oxo complexes along with meso-N-substituted analogs, namely
corrolazines, are inherently more stable than the corresponding
porphyrin congeners [30]. The relatively stable manganese(V)-oxo
corrole/corrolazine complexes were readily isolated and character-
ized by oxidation of the manganese(III) salt with chemical oxidants
[31–33]. In a recent report, Goldberg and co-workers described the
generation of new manganese(V)-oxo ␲-cation radical corrolazine
that is more reactive oxidant than its one-electron-reduced analog
[34].
Recently, we have reported that ruthenium(IV) ␮-oxo bis-
porphyrins catalyze the aerobic oxidation of hydrocarbons through
a ruthenium(V)-oxo species in a photo-disproportionation mech-
anism [35]. Another way to investigate the scope of the
photo-disproportionation reactions could be the utilization of cor-
role complexes with iron metal. Our preliminary results have
shown that the photochemistry of a fluorinated iron(IV) ␮-oxo
biscorrole appears to present a photo-disproportionation mani-
fold similar to that of the iron porphyrin systems [36]. In this
work, we report a visible-light driven aerobic oxidation of hydro-
carbons catalyzed by a bis-corrole-iron(IV) ␮-oxo complex using
only molecular oxygen without the need for an external reducing
reagent. The proposed catalytic sequence involves the following:
(1) photo-disproportionation of the ␮-oxo iron(IV) biscorrole to
form iron(III) and iron(V)-oxo oxidizing species; (2) substrate oxi-
dation by resulting reactive iron(V)-oxo species to give oxidized
products and a second iron(III) species; (3) aerobic oxidation of
iron(III) complex to regenerate the ␮-oxo iron(IV) dimer. The
suggested transient iron(V)-oxo species that are highly reactive
oxidants for a wide range of oxidations can be detected and studied
in real time via laser flash photolysis methods.
Fig. 1. UV–visible spectra of [FeIV(TPFC)]2O (solid lines) and FeIII(TPFC)(OEt)2 (dot-
ted lines) in CH3CN.
heated at reflux for 60 min. Evaporation of solvent followed by
column chromatography on silica gel (diethyl ether), resulted in
isolation of the desired FeIII(TPFC)(OEt)2 in >90% yield. Following
the known procedure [39], the fluorinated diiron(IV) ␮-oxo bis-
corrole (3), formulated as [FeIV(TPFC)]2O, was prepared by aerobic
oxidation of FeIII(TPFC)(OEt)2 in a solution of acetonitrile and
cycloheptane. Complex 3 was characterized by UV–visible, IR and
1H NMR spectra that matched those previously reported [39]. The
UV–visible spectra of complex 3 and its corrole-iron(III) precursor
2 are shown in Fig. 1.
UV–vis spectra measurements were performed on an Agilent
8453 diode array spectrophotometer at room temperature. IR spec-
tra were obtained on a Bio-Rad FT-IR spectrometer. 1H NMR spectra
were recorded on a JEOL ECA-500 MHz spectrometer at 298 K
with tetramethylsilane (TMS) as internal standard. Chemical shifts
(ppm) are reported relative to TMS.
2.2. General procedure for photocatalytic aerobic oxidations
A Rayonet photoreactor (RPR-100) with a wavelength range
of 400–500 nm (ꢀmax = 420 nm) from 300 W mercury lamps
(RPR-4190×12) was used for the photocatalytic reactions. The
photochemical reactions typically consisted of 1.0–1.5 mg of
[Fe(TPFC)]2O (approximate 0.5–1 ␮mol) in 5 mL of acetonitrile con-
taining over 4 mmol of organic substrates. Dry oxygen gas was
bubbled through the solution as it was irradiated. Aliquots of
the reaction solution at constant time interval were analyzed
by GC/MS spectrometer (Agilent GC6890/MS5973) to determine
the formed products and yields with an internal standard (1,2,4-
trichlorobenzene). The trend in the product yields roughly parallels
irradiation time. Monitoring reaction by UV–vis spectroscopy indi-
cated that no significant degradation of catalyst 3 was found after
24 h photolysis
2. Experimental
2.1. Materials and instruments
Acetonitrile was obtained from Fisher Scientific and dis-
tilled over P2O5 prior to use. All reactive substrates for
photocatalytic studies were the best available purity from
Sigma–Aldrich Chemical Company and were purified by pass-
ing through a dry column of active alumina (Grade I) before
use. Pyrrole (98%) and benzaldehyde from Sigma–Aldrich were
distilled prior to use. Pentafluorobenzaldehyde, trifluoroacetic
acid (TFA) (99%) and N,N-dimethylformamide (DMF) were pur-
chased from Sigma–Aldrich and used as received. [FeIII(TPP)]2O
(TPP = meso-tetrakisphenylporphyrinato dianion) was obtained
from Strem Chemical Company and used as received. RuVI(TMP)O2
(TMP = meso-tetrakismesityl-porphyrinato dianion) was prepared
according to the known procedure [37].
Corrole free ligands (1) employed in this study,
5,10,15-tripentafluorphenylcorrole (H3TPFC) [27] and 5,10,15-
triphenylcorrole (H3TPC) [38], were prepared according to the
literature procedures, and their characterization data (1H NMR and
UV/vis) were consistent with reported values. The corrole-iron(III)
dietherate complexes (2) were prepared as previously described
[39]. In a typical procedure, a solution of 1 (50 mg, 68 mmol) and
a large excess of iron(II) chloride (127 mg, 680 mmol) in DMF was
2.3. Lash flash photolysis studies
Laser flash photolysis (LFP) experiments were conducted at
ambient temperature (22 1 ◦C) on an Applied Photophysics LKS-
60 kinetic spectrometer equipped with an SX-18MV stopped-flow
mixing unit. Oversampling (64:1) was employed in all cases to
improve the signal-to-noise. In experiments with the diiron(IV) ␮-
oxo biscorrole 3, 100 ␮L of a CH3CN solution of freshly prepared
dimer 3 (ca. 2.0 × 10−5 M) was mixed in a 2 mm × 10 mm optical
cell with an equal volume of acetonitrile (for self decay studies) or
acetonitrile containing a reactive substrate, and the solution was
irradiated with a ca. 5 mJ of 355 nm light from a Nd-YAG laser (ca.
7 ns pulse). The signal was monitored at fixed wavelengths with
3–20 nm steps to produce a time resolved spectrum.

R. Zhang et al. / Applied Catalysis A: General 464–465 (2013) 95–100
97
Table 1
Photocatalytic aerobic oxidation of cis-cyclooctene by a diiron(IV) ␮-oxo biscorrole.a
Entry
1
Catalyst
Solvent
CH3CN
T (day)
TONb,c
[FeIV(TPFC)]2O
1
2
3
4
1
1
1
1
1
1
1
1
1
1
1
90 10
200 12
320 30
540 40
2
3
4
CHCl3
C6H6
THF
34
5
60 10
70
9
5d
6e
7f
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
<10
140 16
81 10
8
[FeIII(TPFC)]·Et2O
85
5
9e
10
11g
12
154 26
n.d.
[FeIII(TPC)]·Et2O
[FeIII(TPP)]2O
Ru(TMP)O2
40
18
5
2
a
The reaction was carried out in a Rayonet reactor, with ca. 0.5 ␮mol of catalyst
in 5 mL of solvent containing 4 mmol of cis-cyclooctene. Oxygen-saturated solutions
were irradiated with visible light (ꢀmax = 420 nm) or otherwise noted. Products were
analyzed on an HP 6890 GC with a DB-5 capillary column employing an internal
standard (1,2,4-trichlorobenzene).
Fig. 2. UV–visible spectral change of [FeIV(TPFC)]2O (3) in the presence of Ph3P
(4 mM) upon irradiation with a visible lamp (ꢀmax = 420 nm) in anaerobic CH3CN
solution over a span of 15 min at 2 min intervals.
b
TON represents the total number of moles of product produced per mole of
catalyst. All reactions were run 2–3 times, and the data reported are the averages
with standard deviation (1ꢂ). Note the product yields in our catalytic studies are not
given because we typically have large excesses of substrates to catalyst (>8000:1).
3. Results and discussions
c
The major product was cis-cyclooctene oxide, detected in >95% yield.
UV light (ꢀmax = 350 nm).
5 mg of anthracene was added.
5 mg of N-tert-butyl-phenylnitrone (radical inhibitor) was added.
3.1. Synthesis and photochemical behavior of the diiron(IV)
ꢁ-oxo biscorrole
d
e
f
g
Enone formation (∼20%) was detected.
Our study started with the preparation of corrole-iron(III)
complexes which were obtained by metalation of free cor-
role ligand (H3TPFC or H3TPC) in hot DMF with FeCl2 as
the metal source (Scheme 1). According to the procedure
described by Gross et al. [39] diiron(IV)-␮-oxo-bis(5,10,15-
tripentafluorocorrole) catalyst (3) was readily prepared by aerobic
oxidation of the monomeric precursor FeIII(TPFC) (2) in oxygen-
saturated acetonitrile and heptanes solution. The diiron(IV)
␮-oxo-bis(5,10,15-tripentafluorocorrole) precursor (3) with ꢀmax
at 380 nm was characterized by UV–vis, 1H NMR and IR spec-
tra, matching those reported [39]. It is worth mentioning, that
electron-withdrawing substituents such as F on corrole ligand are
necessary for ␮-oxo dimer formation and using the same pro-
cedure by reacting the iron(III) complex with a non-halogenated
5,10,15-triphenylcorrole; i.e. FeIII(TPC) in air did not give the corre-
sponding ␮-oxo products. Presumably, the electron-withdrawing
subsitutents would stabilize the iron(IV) complex in a dimeric form
by reducing the electron density of metal atoms. In our recent
study, we also found the marked dependence of formation of
ruthenium(IV) ␮-oxo bisporphyrins on the electron-withdrawing
substituents on phenyl ring [40].
Similar to the well established photo-disproportionation of
diiron(III)-␮-oxo bisporphyrin complexes [13], irradiation of the
thermally stable complex 3 with visible light in the presence
of excess triphenylphosphine (4 mM) converted to iron(III) cor-
role monomer 2 with well-anchored isosbestic points (Fig. 2).
The reactants and products of the overall photoreaction (depicted
in the inset) are matched to their respective absorption pro-
files [41]. This photochemical observation of a clean reduction of
[FeIV(TPFC)]2O to FeIII(TPFC) supports a photo-disproportionation
pathway that allows for the generation of the iron(V)-oxo oxidant.
Similar spectral changes were also observed upon the photolyses
of diruthenium(IV)-␮-oxo bisporphyrin complexes in the presence
of excess Ph3P [35].
O2. First the efficacy of photo-disproportionation of ␮-oxo iron(IV)
biscorrole 3 for a light-driven oxidation catalysis was evaluated in
the aerobic oxidation of cis-cyclooctene (Table 1). Our immediate
goal in this section was to identify the most efficient systems and
optimal conditions from the screening reactions. After 24 h of pho-
tolysis with visible light (ꢀmax = 420 nm), cis-cyclooctene oxide was
obtained as the only identifiable oxidation product (>95% by GC)
with ca. 90 turnovers (abbreviated as TON representing moles of
product/moles of catalyst) of catalyst 3 (entry 1). The trend in the
TONs roughly paralleled irradiation times and up to 540 TONs was
obtained after four day irradiation. Control experiments showed
that no epoxide was formed in the absence of either the catalyst or
light. The results cannot be ascribed to the chemistry of singlet oxy-
gen, which is characterized by efficient “ene” reactions of alkenes
[42]. The use of other solvents instead of CH3CN resulted in reduced
TONs (entries 2–4), presumably due to a slower formation of the
dimeric precursor 3. It is known that the transformation of 2 to its
␮-oxo dimer 3 has a very large solvent effect [41]. Catalyst degra-
dation was a serious problem with higher-energy light and the use
of UV irradiation (ꢀmax = 350 nm) shut down the catalytic activity
completely (entry 5). Monitoring reaction by UV–vis spectroscopy
indicated that the corrole catalyst was completely degraded only
after 30 min photolysis of UV light.
In order to gain insight into the possible contribution of
the radical autooxidation to the observed photocatalytic process,
the experiment was also carried out in the presence of radical
scavenger. Of note is the well known radical scavenger N-tert-
butyl-phenylnitrone had almost no effect on cyclooctene expoxide
formation (entry 7), which implies a non-radical oxidation process.
It is interesting to note that the catalytic activity was enhanced
by adding small amount of anthracene (entries 6 and 9), which
could act as a photosensitizer. Similar enhancement effect was
observed in the ruthenium(IV) ␮-oxo bisporphyrins system [35].
In addition, the monomeric iron corrole FeIII(TPFC) was tested as
photocatalyst, which is capable of in situ formation of the ␮-oxo
dimer, giving comparable activity (entries 8 and 9). Instead, non-
halogenated FeIII(TPC) did not show appreciable catalytic activity
3.2. Screening catalytic oxidations of cis-cyclooctene
As shown in the Fig. 2, the photo-disproportionation reaction of
diiron(IV)-␮-oxo biscorrole (3) becomes catalytic in the presence of

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R. Zhang et al. / Applied Catalysis A: General 464–465 (2013) 95–100
Scheme 1. Synthesis of the corrole-iron(III) and diiron(IV) ␮-oxo complexes.
under identical conditions (entry 10). Clearly, the formation of
␮-oxo dimer by molecular oxygen is essential for reactivity in
the photocatalytic epoxidation of cis-cyclooctene. For the pur-
pose of comparison, the well-known diiron(III) ␮-oxo bisporphyrin
complex [FeIII(TPP)]2O and trans-dioxoruthenium(VI) tetramesity-
porphyrin, i.e. RuVI(TMP)O2, were also evaluated under identical
conditions, giving comparatively low activities (entries 12 and 13).
substrates were examined under visible light and optimized con-
ditions. Table 2 lists the oxidized products and corresponding
TONs using the 3 as the photocatalyst. The trend in the TONs
roughly parallels the substrate reactivity and significant activity
was observed with up to 1580 TON. Similar to cis-cyclooctene,
norborbene was oxidized to corresponding epoxide (exo mainly)
with 72 TON after 24 h photolysis (entry 1). Cyclohexene, in con-
trast, which is susceptible to the allylic oxidation, gave primarily
2-cyclohexenol and 2-cyclohexenone along with minor expox-
ide (entry 2). This product distribution is no different than those
typically obtained from cofacial iron and ruthenium porphyrin
photocatalysts [17]. Activated hydrocarbons including triphenyl-
methane, diphenylmethane, ethylbenzene and xanthenes were
3.3. Photocatalytic aerobic oxidation of alkenes and activated
hydrocarbons
To further explore the substrate scope of the diiron(IV) ␮-oxo
corrole catalyst, the photocatalytic oxidations of a variety of organic
Table 2
Turnover numbers of photocatalyst 3 for alkenes and benzylic C-H oxidations.a
Entry
Substrate
Product
TONb
1c
2
72
40
230 24
102 16
380 40
6
4
Cyclohexene oxide
2-Cyclohexenol
2-Cyclohexenone
Ph3COH
3d
4
Ph3CH
Ph2CH2
706 60
220 20
5
PhCH2CH3
80
10
5
2
6
7
1235 120
1300 140
8
1580 175
a
Typically with 0.5 ␮mol of 3 in 5 mL of CH3CN containing 4–8 mmol of substrate and 5 mg anthracene.
Determined for a 24 h photolysis (ꢀmax = 420 nm). The values (1ꢂ) reported are the averages of 2–3 runs.
>90% exo isomer.
b
c
d
A minor product was detected by GC but not identified.

R. Zhang et al. / Applied Catalysis A: General 464–465 (2013) 95–100
99
Fig. 3. (A) Time-resolved difference spectrum at 1, 2, 3, 4, 5, and 10 ms of the irradiation of 3 after the laser pulse in CH3CN, difference spectrum = spectrum (t) − spectrum
(0). (B) Time-resolved spectrum over 10 ms following 355 nm irradiation of FeIV(ClO3)TPFC (5) in CH3CN at 25 ◦C.
oxidized to the corresponding alcohols and/or ketones from over-
oxidation with total TONs ranging from 80 to 1200 (entries 3–6). As
is evident from Table 2, the oxidation of secondary benzylic alcohols
gave the highest catalytic activities (entries 7 and 8).
and bisporphyrin diruthenium(IV)-␮-oxo complexes [35]. When
biscorrole-diiron(IV)-␮-oxo complex 3 was dissolved in acetoni-
trile solution and irradiated with 355 nm laser light, a short-lived
transient 4 was formed. Fig. 3A shows a time-resolved UV–visible
spectrum of transient 4 acquired at ambient temperature over
10 ms following 355 nm laser irradiation of 3. Species 4 with a
Soret band at ꢀmax ≈ 415 nm decays in about 10 ms to give a prod-
uct with a Soret band at ꢀmax ≈ 380 nm. In view of its unique
transient absorption spectrum and very high reactivity, transient
4 was tentatively assigned the corrole-iron(V)-oxo structure. We
3.4. Mechanistic studies of transient oxidizing species
To probe the nature of the transient oxidizing species,
we conducted laser flash photolysis (LFP) studies similar to
those previously described for iron, manganese complexes [43]
Scheme 2. A proposed catalytic cycle for the photocatalytic aerobic oxidations by the diiron(IV) ␮-oxo biscorrole. The scheme also shows formation of the oxidizing
corrole-iron(V)-oxo species (4) by photolysis of iron(IV) chlorate monomer (5).

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R. Zhang et al. / Applied Catalysis A: General 464–465 (2013) 95–100
have found that the oxidizing species 4 generated from hemolytic
photocleavage of the Fe-O bond of 3 is spectroscopically and kinet-
ically indistinguishable from the species formed by photolysis
of the corresponding corrole-iron(IV) chlorate complex (Fig. 3B)
[24]. Regardless of the fact that iron(V)-oxo species are generally
rare and elusive, Goldberg and coworkers have recently reported
the spectroscopic evidence for a high-valnet corrolazine-iron-oxo
intermediate at the iron(V) oxidation level [44,45].
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On the basis of the present experimental facts, a catalytic cycle
is proposed in Scheme 2. First, photo-disproportionation of ␮-oxo 3
gives one molecule of a corrole-iron(III) species 2 in addition to the
highly reactive iron(V)-oxo transient 4. Subsequently, oxidation of
an organic substrate by the resulting 4 gives a second molecule of
2. After the oxygen transfer, autooxidation of the corrole-iron(III)
complex 2 then regenerates the bis-corrole-diiron(IV) ␮-oxo com-
plex 3 (Scheme 2). The pivotal role of the ␮-oxo dimer reformation
was unequivocally confirmed when the non-halogenated FeIII(TPC)
was employed as catalyst. The notable absence of catalytic activity
in the case of FeIII(TPC) compared to FeIII(TPFC) for the epoxidation
of cis-cyclooctene illustrates that the formation of ␮-oxo dimer 3 by
aerobic oxidation of 2 is essential to effect the photocatalytic oxi-
dation. Although the oxidation of 2 by O2 to form 3 has been known
for some time, the detailed mechanism has hitherto apparently not
been fully disclosed. In this process, four electrons are needed to
reduce O2 and produce 3. We speculate that the reaction could start
with diiron(IV) peroxo-bridged complex to give two iron(V)-oxo
intermediates, then dimerize to generate the diiron(IV)-␮-oxo bis-
corrole 3 with loss of a H2O, which is similar to the well-known
process described by Bach and co-workers for diiron(III)-␮-oxo
bisporphyrin complexes [46].
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4. Conclusions
In conclusion, we have shown that a fluorinated diiron(IV)-␮-
oxo biscorrole catalyzed aerobic oxidation of alkenes and activated
hydrocarbons under visible light irradiation. The observed pho-
tocatalytic oxidation is ascribed to a photo-disproportionation
mechanism to afford a highly reactive corrole-iron(V)-oxo species
that can be directly observed by laser flash photolysis methods. It
is noteworthy that the use of visible light (solar light) for activa-
tion of atmospheric oxygen without the consumption of a reducing
agent in aforementioned photocatalysis is particularly relevant
to realizing innovative and economically advantageous processes
for conversion of hydrocarbons into oxygenates. Further studies
to define synthetic applications and characterize the proposed
iron(V)-oxo transients more fully are ongoing in our laboratory.
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Acknowledgments
This work was supported by the NSF grant (CHE-1213971) and
an internal grant from WKU Office of Research (RCAP). We thank
Dr. Martin Newcomb at UIC (University of Illinois at Chicago) for
help with the LFP experiments and Dr. Eric Conte at WKU for the
assistance of GC analysis.
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