Hydrocarbon oxidation by homogeneous and heterogeneous non-heme iron (III) catalysts with H2O2

Article abstract of DOI:10.1016/j.cattod.2010.03.063

Homogeneous (LFe^III) and heterogeneous (LFe ^III·SiO₂) [L = 3-{2-[2-(3-hydroxy-1,3-diphenyl- allylideneamino)-ethylamino]-ethylimino}-1,3-diphenyl-propen-1-ol] catalysts have been synthesized and evaluated catalytically. In CH₃CN, both the homogeneous and heterogeneous catalysts were efficient in alkene oxidations. Cyclohexane oxidation provides total yield of 12.1% and 7.3% with an alcohol/ketone (A/K) ratio of 1.75 and 1.60 by the LFe^III and the LFe^III·SiO₂ catalysts respectively. UV-vis kinetic data suggest formation of LFe^III-OOH species. EPR data show that in the presence of CH₃CN, low-spin Fe^III (S = 1/2) centers are formed, which are responsible for the catalytic activity. The heterogeneous LFe·SiO₂ catalyst, tested up to 5 re-uses, shows a total yield loss ～4% per use, providing the same distribution of oxidation products.

Full text of DOI:10.1016/j.cattod.2010.03.063

Catalysis Today 157 (2010) 101–106
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Catalysis Today
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Hydrocarbon oxidation by homogeneous and heterogeneous non-heme iron (III)
catalysts with H2O2
G. Bilisa, K.C. Christoforidisb, Y. Deligiannakisb, M. Louloudia,∗
a Department of Chemistry, University of Ioannina, Department of Chemisry, 45110 Ioannina, Greece
b University of Ioannina, Department of Environmental and Natural Resources Management, Laboratory of Physical Chemistry, Pyllinis 9, 30100 Agrinio, Greece
a r t i c l e i n f o
a b s t r a c t
Homogeneous (LFeIII
)
and heterogeneous (LFeIII·SiO2) [L = 3-{2-[2-(3-hydroxy-1,3-diphenyl-
Article history:
Available online 11 May 2010
allylideneamino)-ethylamino]-ethylimino}-1,3-diphenyl-propen-1-ol] catalysts have been synthesized
and evaluated catalytically. In CH3CN, both the homogeneous and heterogeneous catalysts were efficient
in alkene oxidations. Cyclohexane oxidation provides total yield of 12.1% and 7.3% with an alcohol/ketone
(A/K) ratio of 1.75 and 1.60 by the LFeIII and the LFeIII·SiO2 catalysts respectively. UV–vis kinetic data
suggest formation of LFeIII–OOH species. EPR data show that in the presence of CH3CN, low-spin FeIII
(S = 1/2) centers are formed, which are responsible for the catalytic activity. The heterogeneous LFe·SiO2
catalyst, tested up to 5 re-uses, shows a total yield loss ∼4% per use, providing the same distribution of
oxidation products.
Keywords:
Non-heme iron
Biomimetic catalysis
Hydrocarbon oxidation
H2O2
Fe–OOH
Low-spin Fe
© 2010 Elsevier B.V. All rights reserved.
FeIII–OOH intermediate is a result of H2O2 deprotonation and sub-
sequent ligation to the FeIII center. This FeIII–OOH species is the
1. Introduction
The development of new iron catalysts for hydrocarbon oxi-
dations is attracting synthetic chemists intending to explore new
industrial approaches. Nature has evolved iron-enzymes, like
non-heme iron oxygenases, capable to carry out hydrocarbon oxi-
dations with high selectivity, under mild conditions. For example,
methane monooxygenase [1–3] selectively oxidizes methane to
methanol, Rieske dioxygenases [4,5] are capable of stereospecific
dihydroxylation of arenes. The antitumor drug iron-bleomycin
– a metallo-glycopeptide – causes oxidative DNA cleavage [6,7]
and also oxidizes a wide variety of organic substrates [6,8]. Thus,
the synthesis of low molecular weight metal complexes aiming
to mimic key – structural and – functional properties of natural
enzymes is a topic of considerable interest. Of particular, demand
is to achieve catalytic efficiency using green oxidants e.g. like H2O2
or O2 [7,9–13]. In this context, stereoselective hydroxylation, epox-
idation and cis-dihydroxylation by synthetic biomimetic catalysts
have been reported [14–22].
precursor for a high-valent Fe-oxo species, which is responsible for
substrate oxidation [2]. Three main pathways can be considered
for the cleavage of the O–O bond [25]. (a) Homolytic cleavage leads
to a FeIV O species plus a short-lived highly reactive •OH radical,
which is immediately transferred to the substrate [25]. (b) Het-
erolytic cleavage of the O–O bond generates a Fev O (analogous to
heme-peroxidase) [26] and a OH− species. (c) Finally, the FeIII–OOH
intermediate itself could be involved in substrate oxidation pro-
viding an oxidant with moderate reactivity and higher selectivity
[27,28].
The heterogenization of catalysts, in contemporary environ-
mentally conscious days, provides important ‘green benefits’ e.g.
when solvent and catalyst losses during separation lead to unac-
ceptable waste levels [29]. Thus, replacement of a homogeneous
catalyst by an active heterogeneous one changes the synthetic
process towards a more desirable and clean one. This provides
advantages e.g. easy-handling, easy product separation, catalyst
recovery and waste minimization [30]. However, very few liter-
ature reports are available on active heterogeneous non-heme iron
catalysts functioning with H2O2 [31,32].
In several bioinspired iron-based oxidation catalysts Fe-peroxo
species have been invoked to be formed during the catalytic
reaction [23,24]. More particularly, a FeIII–OOH intermediate has
been suggested as key-specie in oxidation reactions of several
biomimetic non-heme iron complexes [9]. The formation of the
In the present work (a) we report the synthesis and
catalytic evaluation of (i)
a
homogeneous LFeIIICl and
(ii) the heterogeneous LFeIII·SiO2 catalyst.
L
stands for
3-{2-[2-(3-hydroxy-1,3-diphenyl-allylideneamino)-ethylamino]-
ethylimino}-1,3-diphenyl-propen-1-ol (Scheme 1). Catalytic
oxidations were evaluated using H2O2. (b) The stability of the
catalysts has been also studied. Significant recyclability was found
∗
Corresponding author.
E-mail addresses: ideligia@cc.uoi.gr (Y. Deligiannakis), mlouloud@uoi.gr
(M. Louloudi).
0920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.03.063

102
G. Bilis et al. / Catalysis Today 157 (2010) 101–106
Scheme 1. The ligand [3-{2-[2-(3-hydroxy-1,3-diphenyl-allylideneamino)-
ethylamino]-ethylimino}-1,3-diphenyl-propen-1-ol] (L) used herein to form the
LFeIII catalysts for hydrocarbon oxidation with H2O2.
for the heterogeneous LFeIII·SiO2 catalyst. (c) UV–vis kinetic data
suggest formation of LFeIII–OOH species and EPR data show that in
the presence of CH3CN, low-spin FeIII (S = 1/2) centers are formed,
which are responsible for the catalytic activity. The synthesis and
characterization of the ligand L has been reported in our recent
reports [33,34] (Scheme 1).
Fig. 1. Distribution of oxidation products catalyzed by LFeCl and LFe·SiO2 in CH3CN
in the presence of H2O2. See Table 1 for further details.
2. Experimental
yield of 88.5% and 99.8% respectively, while the same reactions
catalyzed by the LFe·SiO2 gave combined yields of 82.5% and
64.5%. Cyclohexene undergoes mainly allylic oxidation forming 2-
cyclohexene-1-ol and 2-cyclohexene-1-one. However cyclohexene
epoxidation was also observed, with low epoxide yields. The major
products detected from limonene oxidation were (i) two epoxides
(cis and trans) originating from epoxidation of the electron-rich
double bond in 1,2-position, (ii) alcohols derived from hydroxyla-
tion of the double bond in 1- and 2-position and from hydroxylation
in 6-position close to 1,2-double bond. Oxidation products from the
more accessible, but less electron-rich, double bond at 8,9-position
were not observed. Additionally, considerable amounts of the cor-
responding ketone at 6-position were also formed (see details in
Table 1). Methyl-substituted alkenes were more reactive, in both
epoxidation and allylic oxidation. Thus, in the case of methyl-
cyclohexene, the detected oxidation products were cis-epoxide,
1-methyl-2-cyclohexen-1-ol, 3-methyl-2-cyclohexen-1-ol and 3-
methyl-2-cyclohexen-1-one. The total yield of methyl-cyclohexene
oxidation products was 96.4% and 91.9% by LFeCl and LFe·SiO2
respectively. Cis-cyclooctene as substrate afforded a single-product
reaction e.g. giving only the cis-cyclooctene epoxide with 37.0% and
19.8% yield catalyzed by LFeCl and LFe·SiO2 respectively (Fig. 1).
In the oxidation of cis-stilbene, the major product was benzalde-
hyde as oxidative cleavage product (36.0% and 30.0% by LFeCl and
LFe·SiO2 respectively). Considerable amounts of cis-stilbene epox-
ide (15.8% in homogeneous and 9.0% in heterogeneous medium)
have been also detected. Styrene oxidation provided also benzalde-
hyde as major product deriving from oxidative cleavage of the
exo-cyclic double bond. However, epoxide and phenyl acetalde-
hyde have been also formed e.g. by direct oxidation of the same
double bond. Overall, styrene was oxidized by LFeCl and LFe·SiO2
with total oxidation yields of 49.5% and 38.0% respectively. The
methyl-substituted styrene, trans-␤-methyl styrene is more reac-
tive providing total oxidation yields of 93.2% and 49.7% by LFeCl and
LFe·SiO2 respectively. The identified products were trans-epoxide,
methyl-benzyl-ketone and benzaldehyde as oxidation cleavage
adduct.
All substrates were purchased from Aldrich, in their highest
commercial purity, stored at 5 ◦C and purified by passage through
a column of basic alumina prior to use. Thirty percent aqueous
solution of hydrogen peroxide was used. Infrared spectra were
recorded on a spectrum GX PerkinElmer FT-IR System. UV–vis
spectra were recorded using a UV/VIS/NIR JASCO spectropho-
tometer and a PerkinElmer Lamda 35 with a diffuse reflectance
setup. Fe quantitation was done by Flame Atomic Absorption
Spectroscopy (FAAS) on a PerkinElmer AAS-700 spectrometer.
Mössbauer spectra were recorded with a constant acceleration
spectrometer using a 57Co(Rh) source at room temperature and
a variable temperature. X-Band Electron Paramagnetic Resonance
(EPR) spectra were recorded using a Brucker ER200D spectrom-
eter at liquid N2 temperatures, equipped with an Agilent 5310A
frequency counter running under a home-made software based on
LabView described earlier [35]. Mass spectra were measured on
an Agilent 1100 Series LC-MSD-Trap-SL spectrometer and solution.
GC analysis was performed using an 8000 Fisons chromatograph
with a flame ionization detector and a Shimadzu GC-17A gas
chromatograph coupled with a GCMS-QP5000 mass spectrome-
ter.
Details about catalyst’s preparation and characterization, cat-
alytic experiments, UV–vis and EPR sample preparation are given
in Supporting Information.
3. Results and discussion
3.1. Hydrocarbon oxidation by the homogeneous and
heterogeneous LFeIII catalysts
The catalytic properties of the homogeneous LFeCl and hetero-
geneous LFe·SiO2 catalysts have been evaluated for hydrocarbon
oxidation with H2O2. The oxidation reactions were carried out at
room temperature under Ar atmosphere. When the catalytic exper-
iments took place under air, O2 affected both yield and selectivity
of the oxidation products. Thus, in the data shown herein all the
catalytic reactions were studied under strict inert Ar atmosphere.
The catalytic data are summarized in Table 1, and Fig. 1 provides a
histogram of them.
Based on Table 1, both the homogeneous and heterogeneous
catalysts were efficient in alkene oxidations providing significant
yields. More specifically, cyclohexene and limonene oxidation cat-
alyzed by the LFeCl provided oxidation products with a combined
Finally, cyclohexane oxidation gave cyclohexanol and cyclo-
hexanone with combined yields of 12.2% and 7.2% and an
alcohol/ketone (A/K) ratio of 1.75 and 1.60 for the LFeIIICl and the
LFeIII·SiO2 catalyst respectively. The A/K ratio in cyclohexane can be
used as a criterion for the presence and lifetime of free alkyl radi-
cal intermediates [24] as follows. (i) When A/K = 1, it assumes that
the alkyl radicals are long-lived with a strong tendency to interact

G. Bilis et al. / Catalysis Today 157 (2010) 101–106
103
Table 1
Hydrocarbon oxidation catalyzed by LFeCl and LFe·SiO2 with H2O2.
d
d
Substrate
Products
LFeClc yield (%)
LFe·SiO2 yield (%)
LFeClc total yield (%)
LFe·SiO2 total yield (%)
Cyclohexenea
cis-Epoxide
2-Cyclohexenol
2-Cyclohexenone
6.5
52.0
30.0
4.9
32.9
44.7
88.5
82.5
1-Methyl-cyclohexenea
cis-Epoxide
20.0
25.6
42.0
8.8
25.4
27.7
32.3
6.5
1-Methyl-2-cyclohexen-1-ol
3-Methyl-2-cyclohexen-1-ol
3-Methyl-2-cyclohexen-1-one
96.4
37.0
91.9
19.8
Cyclooctenea
Limonenea
cis- Epoxide
37.0
19.8
cis-1,2 Epoxide
21.0
12.3
54.0f
12.5
8.5
8.0
trans-1,2 Epoxide
Limonene alcohole
Limoneme ketoneh
41.9g
6.0
99.8
49.5
64.4
38.0
Styreneb
Epoxide
Phenyl acetaldehyde
Benzaldehyde
7.5
7.0
35.0
3.0
3.0
32.0
Methyl-styreneb
cis-Stylbeneb
trans-Epoxide
Methyl-benzyl-ketone
Benzaldehyde
41.9
11.3
40.0
10.9
8.8
30.0
93.2
51.8
12.2
49.7
39.0
7.2
cis-Epoxide
Benzaldehyde
15.8
36.0
9.0
30.0
Cyclohexanea
Cyclohexanol
Cyclohexanone
7.7
4.4
4.5
2.8
a
Conditions: ratio of catalyst:H2O2:substrate = 1:20:1000.
Conditions: ratio of catalyst:H2O2:substrate = 1:50:1000.
Reaction was completed within 4 h.
b
c
d
e
f
Reaction was completed within 24 h.
Limonene alcohols were found to be a mixture of 1-ol, 2-ol and 6-ol.
54% yield corresponds to 23% for 1-ol, 13.5% for 2-ol and 17.5% for 6-ol.
41.9% yield corresponds to 25.3% for 1-ol, 6.9% for 2-ol and 10.0% for 6-ol.
The only observed ketone is the 6-one.
g
h
with O2 to form alkyl-peroxy-radicals [25]. At the end, following
a Russell-type terminal stage [36], recombination of these radi-
cals results in the formation of equimolar amounts of alcohol and
ketone [14,24]. (ii) When the A/K > 1, the radical •OH is formed by
a metal-based system and the metal center reacts directly to form
the corresponding alcohol.
an almost linear increase of the tons, up to the fourth dose of H2O2
(Fig. 2). Noticeably, turnover numbers approached a value of 40.2
for 80 equiv. of H2O2. Remarkably, the LFe·SiO2 was active after the
addition of a fifth amount of 20 equiv. of H2O2. Even a sixth one
provides new amounts of oxidation products (Fig. 2). No catalyst
degradation has been detected by MS. Overall the data in Fig. 2 and
the MS-data demonstrate that under the catalytic conditions used,
the LFe·SiO2 was much more stable than the homogeneous LFe cat-
alyst. These findings encouraged us to investigate in more detail the
recyclability of the LFe·SiO2 catalyst.
3.2. Catalysts stability
The efficiency and stability of FeIII catalysts under operating con-
ditions were studied by progressive addition of H2O2 to cyclohex-
ene in CH3CN. The starting molar ratio [catalyst:H2O2:substrate]
was [1:20:1000]. After the end of each oxidation reaction an addi-
tional amount of 20 equiv. of H2O2 was added. The results are
presented in Fig. 2.
3.2.1. Homogeneous catalyst
According to Fig. 2, in the oxidation reaction catalyzed by the
LFeCl, the first 20 equiv. of H2O2 resulted in 17.7 tons. The addi-
tion of a second dose of 20 equiv. of H2O2 caused a yield increase
to 23.7 tons. Third addition of oxidant improved slightly the total
tons to 28.3. Overall, the data in Fig. 2 (solid symbols) indicate that
the evolution of the catalytic reaction catalyzed by the LFeCl has
practically stopped during the second run. Probably this was due
to catalyst destruction. This is strongly evidenced by discoloration
of the orange reaction mixture and MS-detection of molecular-
fragments derived from LFeCl.
3.2.2. Heterogeneous catalyst
In the oxidation reaction catalyzed by the LFe·SiO2, the first
20 equiv. of H2O2 resulted in 16.5 tons (Fig. 2 open symbols). In con-
trast to LFeCl, in the case of the LFe·SiO2 addition of H2O2 caused
Fig. 2. Cyclohexene oxidation upon progressive addition of H2O2 catalyzed by
homogeneous LFeCl (solid symbols) and heterogeneous LFe·SiO2 (open symbols)
catalysts.

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G. Bilis et al. / Catalysis Today 157 (2010) 101–106
reaction. (b) A progressive loss by ∼4% of activity was observed
per use: from 80% (in 1st use) the total yield was 76% (in 2nd use)
and progressively was decreasing in a quasilinear way e.g. after
the 5th re-use the total yield was ∼60%. (c) Importantly in all the
uses the same distribution of oxidation products was observed.
The DR-UV–vis spectrum of the 5-times used LFe·SiO2 was com-
parable with the spectrum of the non-used LFe·SiO2 catalyst. In
addition, no catalyst degradation has been detected by MS. Over-
all, we consider that the 4% activity loss per use shows that the
heterogeneous LFe·SiO2 catalyst presents good recyclability behav-
ior. An analogous stability enhancement has been reported by us
[33,34] and other researchers [37,38] for other catalysts supported
on SiO2. Thus we consider that the silica support probably protects
the active catalyst centers against oxidation destruction.
Fig. 3. Bar chart representation of cyclohexene oxidation catalyzed by recycled
LFe·SiO2.
3.4. Changes in UV–vis spectrum of LFeIII in the presence of H2O2
The progress of cyclohexene oxidation with H2O2 catalyzed
by LFeCl was monitored by UV–vis spectroscopy at 0 ◦C, for
[catalyst:H2O2:cyclohexene] = [1:20:1000]. Final catalyst concen-
tration was 0.33 mM in CH3CN. At 0 ◦C after completion of H2O2
addition, the bands at 413 and 512 nm – attributed to LMCT – were
progressively increased in intensity and slightly blue shifted (solid
line). This was continued for 30 min (upward-arrows in Fig. 4). After
this period, a decrease in the bands at 413 and 512 nm was observed
(downward-arrows in Fig. 4). Finally, after 200 min, these bands
were no longer detectable (dashed-dotted line). The observed spec-
tral changes in Fig. 4 parallel the observation reported previously
[16,22,39,40]. According to reference [39], spectral changes such as
in Fig. 4, confirm the interaction between the Fe complex and the
H2O2 involved in the catalytic process [39]. Thus, on the basis of the
UV–vis kinetic data after oxidant addition, formation and concomi-
tant consumption of an LFeIII–OOH species might be concluded
[16,22,39,40] (Fig. 4).
Fig. 4. Evolution of catalytic cyclohexene oxidation in CH3CN with H2O2.
3.5. Catalysis monitored by EPR spectroscopy
3.3. Recyclability of the heterogeneous LFe·SiO2 catalyst
A Mössbauer spectrum for LFeIIICl powder is shown in Fig. 5A.
The presence of only one doublet in conjunction with Fe-analysis is
clear evidence for the formation of a mononuclear FeIII-complex.
At 77 K, the Mössbauer parameters were ı = 0.50 mm/s and
ꢀEQ = 0.54 mm/s indicating an octahedral high-spin FeIII (S = 5/2)
center, with the iron bound to N- and O-atom donors [41]. The
EPR spectrum of solid LFeIIICl was characterized by two peaks at
g = 4.3 and 9.2 characteristic of a high-spin (HS) FeIII (S = 5/2) cen-
ter in a rhombic ligand-field characterized by E/D∼0.33 [42]. Fig. 6
shows low temperature X-Band EPR spectra of LFeCl in different
solvents. The EPR spectra of all samples contained a signal cen-
The recycling and regenerating ability of the LFe·SiO2
were tested using cyclohexene as substrate, in CH3CN, for a
[catalyst:H2O2:substrate] = [1:20:1000]. After the end of each oxi-
dation reaction, the solid catalyst was isolated from the reaction
mixture by filtration, washed with CH3CN and dried under pressure
(80 ◦C, 2 h). Subsequently the recovered solid catalyst was reused
for a new oxidation reaction under the same catalytic conditions.
The catalytic data obtained by this set of experiments are displayed
in Fig. 3. The data in Fig. 3 shows that (a) after repetitive uses the
recycled catalyst was able to activate H2O2 in the new catalytic
Fig. 5. Mössbauer (A) and EPR (B) spectrum for LFeCl. Temperature 77 K.

G. Bilis et al. / Catalysis Today 157 (2010) 101–106
105
data also [16,43]. Overall, it is concluded that for the formation
of the LS state, CH3CN must be used as solvent, which occupies
the sixth coordination site. Based on the g-values of the low-spin
FeIII specie the rhombicity (V/ꢀ) parameters are 1.3. The tetrago-
nality (ꢀ/ꢁ) parameter – which depends primarily on the detailed
balance of the charge of the iron atom – is 6.2. Similar EPR spec-
tra of low-spin ferric complex with rhombic character have been
observed [44]. Fig. 7A shows the time dependence of the EPR inten-
sity of the LS FeIII signal in the presence of 5 and 20 equiv. of H2O2
with respect to the catalyst and in the absence or presence of sub-
strate. In the presence of substrate and 20 equiv. H2O2, the LS FeIII
signal decreased rapidly, disappearing within 2 min. On the other
hand, with 5 equiv. of H2O2 a 70% decrease of the initial LS signal
was observed within 10 min incubation time. In both cases, beside
the decrease of the EPR intensity no other changes were detected
in the EPR spectra. Oxidation of the iron and formation of an EPR
silent species could take place. A third sample was also prepared
containing 1000 equiv. of the substrate with respect to the cata-
lyst incubated for 10 min, and 20 equiv. of H2O2 (Fig. 7A (ꢀ)). In
this case, a variation of the EPR intensity of the LS FeIII signal was
observed. However, the signal was constantly present contrary to
the case where the substrate was added. Fig. 7B shows the effect
of H2O2 and substrate on the time dependence of the HS FeIII sig-
nal’s intensity. With 5 equiv. of H2O2 an increase of the HS FeIII
was observed. In contrast, a gradual decrease of the LS EPR sig-
nal was observed (Fig. 7A). In the presence of 20 equiv. of H2O2
with respect to the catalyst an increase was also observed at short
incubation times (i.e. t < 0.5 min), followed by constant decrease.
However, the decay of the HS state was much slower than the LS
state. Differences on the time dependence of the LS and HS FeIII
signal’s intensity were also observed in the presence of the sub-
strate. Contrary to the case of the LS state, the HS FeIII intensity
remained approximately unchanged with 20 equiv. of H2O2 (Fig. 7B
(ꢀ)).
Fig. 6. EPR spectra of LFe complex (a) in CH3CN, (b) in MeOH, (c) in CH2Cl2.
Experimental conditions: modulation amplitude 10 G, microwave power 32 mW,
modulation frequency 100 Kh, temperature 77 K.
tered at g = 4.3. This signal is characteristic of a typical non-heme
high-spin FeIII (S = 5/2). A close inspection of Fig. 6 reveals small
differences between the HS FeIII signals indicating the formation
of HS FeIII species with different ligand-field E/D rhombicity ratios
[42]. According to the g-values, in the presence of CH3CN the E/D
rhombicity ratio is 0.25 [42] while in MeOH or CH2Cl2 E/D∼0.33
[42] (Fig. 6). In CH3CN, a strong EPR signal centered at 3345 G was
also detected (Fig. 6, spectrum (a)) at the expense of the g∼4.3
signal. The new FeIII state is characterized by g-values: g1 = 2.052,
g2 = 2.005 and g3 = 1.80 and is assigned to low-spin (LS) FeIII (S = 1/2)
complexes. Based on the HS and LS EPR signals in Fig. 6 we estimate
that 40% of the Fe centers are in the LS state while 60% remained
in the HS state. The LS signal was absent at the EPR spectra of the
LFeCl in MeOH or CH2Cl2. Formation of LS FeIII (S = 1/2) has been pre-
viously reported for non-heme iron complexes in CH3CN [16,43].
Coordination of CH3CN to non-heme iron complexes leading to the
formation of LS states has been documented by crystallographic
The differences of the time depended intensity between the LS
and HS states of FeIII with respect to 5 and 20 equiv. indicate that the
HS state has higher redox potential than the LS state. Furthermore,
the variation of the LS and the unchanged intensity of the HS state
under the conditions of the catalytic experiments, indicate that the
catalytic performance most likely depends on the LS state of the
LFeCl rather than the HS. This hypothesis is supported by the higher
Fig. 7. Relative EPR intensity of (A) the low-spin FeIII and (B) the high-spin FeIII. Experimental condition: 0.2 mM FeL1 in CH3CN in the presence of 1 mM H2O2 (solid line (ꢀ))
4 mM H2O2 (dashed line (ꢁ)) and 200 mM substrate followed by the addition of 4 mM H2O2 (dotted line (ꢀ)).

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G. Bilis et al. / Catalysis Today 157 (2010) 101–106
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compared with the cases of MeOH and CH2Cl2.
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In CH3CN both the homogeneous and heterogeneous catalysts
were efficient in alkene oxidations providing significant yields. In
cyclohexane oxidation an alcohol/ketone (A/K) ratio of 1.75 and
1.60 was found by LFeIIICl and LFeIII·SiO2 catalysts respectively.
UV–vis kinetic data suggest formation and concomitant consump-
tion of a LFeIII–OOH species. EPR data show that in CH3CN, low-spin
FeIII (S = 1/2) centers are formed, which are responsible for the cat-
alytic activity. Under the catalytic conditions used, the LFe·SiO2 was
much more stable than the homogeneous LFe catalyst. The recycled
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yield loss ∼4% per use, providing the same distribution of oxidation
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Appendix A. Supplementary data
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