Design of a bio-inspired imidazole-based iron catalyst for epoxidation of olefins: Mechanistic insights

Article abstract of DOI:10.1016/j.cattod.2010.04.034

A novel defined iron catalyst for the epoxidation of aromatic and aliphatic olefins with hydrogen peroxide as the terminal oxidant is described. Our catalyst approach is based on bio-inspired both alkyl- and aryl-substituted imidazoles in combination with cheap and abundant iron trichloride hexahydrate. Heterocycles similar to imidazole can be used as ligands in this epoxidation system, too. The novel system is stable towards air and water. It is shown that the mechanism depends strongly on the used ligands and substrates. In the presence of radical scavengers no carbon-centered radical could be detected.

Full text of DOI:10.1016/j.cattod.2010.04.034

Catalysis Today 157 (2010) 364–370
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Catalysis Today
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Design of a bio-inspired imidazole-based iron catalyst for epoxidation of olefins:
Mechanistic insights
Kristin Schröder, Kathrin Junge, Anke Spannenberg, Matthias Beller∗
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Str. 29a, D-18059 Rostock, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 9 June 2010
A novel defined iron catalyst for the epoxidation of aromatic and aliphatic olefins with hydrogen per-
oxide as the terminal oxidant is described. Our catalyst approach is based on bio-inspired both alkyl-
and aryl-substituted imidazoles in combination with cheap and abundant iron trichloride hexahydrate.
Heterocycles similar to imidazole can be used as ligands in this epoxidation system, too. The novel system
is stable towards air and water. It is shown that the mechanism depends strongly on the used ligands
and substrates. In the presence of radical scavengers no carbon-centered radical could be detected.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Epoxidation
Iron
Homogeneous catalysis
Hydrogen peroxide
N-ligands
1. Introduction
on a large scale. Moreover, it is noteworthy that iron is involved in
manifold biological systems, for instance in metalloproteinase like
The catalytic epoxidation of olefins providing oxiranes consti-
tutes an important and challenging oxidation reaction. From an
industrial point of view aliphatic epoxides are of special inter-
est, namely 1,2-propylene oxide and ethylene oxide, which are
annually produced on a million ton-scale [1]. State-of-the-art
heterogeneous catalysts like Ti-substituted silicalite (TS-1) for the
epoxidation of propylene [2] or supported silver catalysts on Al2O3
for the production of ethylene oxide [3] make use of benign oxi-
dants such as hydrogen peroxide or molecular oxygen. Clearly,
these heterogeneous catalysts have advantages with respect to
separation and reuse. Nevertheless, in general they require
relatively harsh reaction conditions and are more difficult to mech-
anistically understand on a molecular basis [4]. Hence, there is a
continuing interest in active, selective and molecular-defined epox-
idation catalysts. In this respect transition metal complexes provide
powerful and tunable models for new type of oxidation catalysts.
Homogeneous metal complexes which provide active epoxidation
catalysts in the presence of hydrogen peroxide are mainly based on
ruthenium [5], rhenium [6], manganese [7], and in the recent past
iron [8]. Notably, until today the development of a general applica-
ble, active and selective catalyst system which is able to epoxidize
both aromatic and aliphatic olefins is still a challenging goal.
Iron-based catalysts offer significant advantages compared to
precious metals like abundance and low toxicity [9]. Obviously, a
variety of iron salts and iron complexes are commercially available
methane monooxygenases as diiron core for the metabolic aerobic
pathway of methane to methanol [10]. Based on these structurally
well-characterized enzymes, model complexes have been prepared
to explore mainly mechanistic issues and partially oxidation cata-
lysis. Unfortunately, most of the known systems have limitations
including tedious complex preparation, limited substrate scope,
low selectivity and/or the use of ‘non-green’ and expensive oxidants
such as hypervalent iodo compounds, peracids etc. [11].
Recently, we demonstrated the conversion of various olefins
to the corresponding epoxides using hydrogen peroxide in the
presence of an in situ generated catalyst composed of FeCl3·6H2O,
pyridine-2,6-dicarboxylic acid (H2pydic), and an organic base like
pyrrolidine, N-benzylamines or formamidines [12–14]. Unfortu-
nately, these in situ generated catalysts are difficult to study
mechanistically. Therefore, we designed
a bio-inspired two-
component-protocol consisting of imidazole derivatives as ligands
and iron as catalyst core [15]. Here, we report an account of our
work on iron–imidazole systems and additional novel mechanistic
insights.
2. Experimental
2.1. General remarks
The imidazole ligands 12 and 13 were synthesized according to
a known literature protocols [16]. All other reagents were used as
purchased from commercial suppliers (Aldrich, Fluka, Merck, etc.)
without further purification. “30%” aqueous H2O2 from Merck was
used as received. The peroxide content varied from 30% to 40%
∗
Corresponding author. Tel.: +49 381 1281113; fax: +49 381 128151113.
E-mail address: matthias.beller@catalysis.de (M. Beller).
0920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.04.034

K. Schröder et al. / Catalysis Today 157 (2010) 364–370
365
as determined by titration. GC analyses were performed with a
Hewlett Packard HP 6890. GC calibrations for alkenes and epoxides
were carried out with authentic samples and dodecane as an inter-
nal standard. Mass spectra were in general recorded on a HP 5989A
EI mass selective detector. NMR spectra were measured using a
Bruker ARX 300 or ARX 400 spectrometer.
2.2. General procedure for the epoxidation of olefins
In a test tube, FeCl3·6H2O (0.025 mmol), t-amyl alcohol (9 mL),
heterocyclic ligand (0.05 mmol), olefin (0.5 mmol) and dodecane
(GC internal standard, 100 ␮L) were added in sequence at room
temperature in air. To this stirred mixture a solution of 30%
hydrogen peroxide (aqueous, 170 ␮L, 1.5 mmol) in t-amyl alcohol
(830 ␮L) was added over a period of 1 h at room temperature by a
syringe pump. Conversion and yield were determined by GC ana-
lysis without further manipulations and compared with authentic
samples.
2.3. X-ray data
Fig. 1. Naphthalene dioxygenase with a mononuclear iron at the active site [20b].
Data were collected on a STOE IPDS II diffractometer using
graphite-monochromated Mo K␣ radiation. The structures were
solved by direct methods (SHELXS-97) [17] and refined by full-
matrix least-squares techniques on F2 (SHELXL-97) [18]. XP (Bruker
AXS) was used for graphical representations. CCDC 748745 contains
the supplementary crystallographic data for (mer-[FeIII(13)Cl3]) in
this paper. CCDC 714375–714377 contain additional data for com-
plexes trans-[FeCl2(2)4]Cl and [(12)4ClFeOFeCl3] [15b]. These data
can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data request/cif.
of importance for the stabilizing hydrogen bonding in the epoxida-
tion reaction as previously discussed [15b]. By changing the pyrrole
nitrogen of the imidazole towards oxygen or sulphur (Table 1,
entries 3, 6 and 7) a drop in activity was observed in each case.
The best performances were still shown with imidazoles 2 and 12;
an overview is presented in Scheme 1 [15].
Stilbenes and styrenes were epoxidized with ligand 2 in high
yields with good selectivity. Additional various aliphatic olefins
showed with imidazole 12 significant higher yields, e.g. cyclooctene
was oxidized in up to 65% yield with 84% selectivity.
3. Results and discussion
3.2. Mechanistic investigations
3.1. Catalysis results
In Scheme 2 we proposed a mechanism for the epoxidation of
olefins in the presence of a dimeric and a monomeric iron imidazole
complex, based on our previous results [15].
A competitive equilibrium between the ␮-oxo diiron complexes
A and the more active mononuclear Fe(III)-complex B by reaction
with water was adopted [21]. Apparently, A is easily converted to
the active species. After ligand solvent exchange C the resulting
hydroperoxo complex D is stabilized intramolecularly by hydrogen
bonding from the attached ligand.
The starting point of our work was inspired by a closer look
to known oxidatively active enzymes. In the majority of these
enzymes with an iron core a similar structural motif of ligands sur-
rounding the metal inside the enzyme is found [19]. Next to amino
acid residues like aspartate, glutarate or cysteine and co-factors
like ␣-ketoglutarate in non-heme enzymes, histidine ligands are
involved in nearly all enzymes whereas histidine displaces one of
the essential amino acids. The imidazole scaffold binds mainly to
the iron center in non-heme enzymes (Fig. 1) [20].
In Table 1 the reactivities of different imidazoles and additional
heterocycles are shown. As a typical benchmark reaction the epoxi-
dation of trans-stilbene in the presence of FeCl3·6H2O and 30% H2O2
as terminal oxidant was investigated.
In order to elucidate the mechanism in more detail, here we
report on the influence of ligands on the reactivity of the iron cen-
The previously best performing co-ligand pyrrolidine (Table 1,
entry 1) did not show any activity without H2pydic. FeCl3·6H2O
itself without any ligands showed a certain activity in the presence
of hydrogen peroxide (blank sample: 5% yield with 31% selectivity
for trans-stilbene). Simple imidazole 3 gave the corresponding oxi-
rane in 38% yield with a selectivity of 90% (Table 1, entry 3). Other
basic heterocyclic ligands such as pyrazole 4 gave similar yields
but slightly higher conversion (Table 1, entry 4). Both methylated
heterocycles showed diminished yield at high selectivity compared
to their non-methylated counterparts (Table 1, entries 8 and 9),
whereas imidazole 8 gave significantly higher yield than pyra-
zole 9. Aryl-substituted derivatives show even more pronounced
effects: while phenyl-imidazole 10 provided good yields phenyl-
pyrazole 11 showed nearly no yield. Even though imidazole and
pyrazole are similar, in general electronically there are some dif-
ferences. Besides, the free 2-position at the imidazole scaffold is
Scheme 1. Selection of obtained yields of various olefins.

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K. Schröder et al. / Catalysis Today 157 (2010) 364–370
Table 1
Reactivity of different heterocycles in the benchmark reactiona
.
No
1
Ligand (L)
Conv. [%]b
4
Yield [%]b
2
Sel. [%]c
40
2
3
83
43
80
38
97
90
4
5
53
36
42
30
79
83
6
7
34
30
18
29
54
98
8
9
19
7
18
6
91
82
10
74
66
89
11
12
8
2
32
92
72
66

K. Schröder et al. / Catalysis Today 157 (2010) 364–370
367
Table 1 (Continued )
No
Ligand (L)
Conv. [%]b
Yield [%]b
Sel. [%]c
13
76
70
92
a
Reaction conditions: in a test tube, FeCl3·6H2O (0.025 mmol), heterocylic derivative (0.05–0.075 mmol), tert-amyl alcohol (9 mL), trans-stilbene (0.5 mmol) and dodecane
(GC internal standard, 100 ␮L) were added in sequence at r.t. in air. To this mixture, a solution of 30% H2O2 (170 ␮L, 1.5 mmol) in tert-amyl alcohol (830 ␮L) was added over
a period of 1 h at r.t. by a syringe pump.
b
Conversion and yield were determined by GC analysis.
Selectivity refers to the ratio of yield to conversion in percentage.
c
Scheme 2. Proposed mechanism of the epoxidation reaction with the iron/imidazole system [15b].
ter. In addition, the formation of radicals and the influence of air
and water were studied. In order to get an indication of the occur-
rence of radicals, ␤-pinene was applied as substrate in the presence
of the best two ligands 2 (from the alkyl-substituted imidazoles)
and 12 (from the 1-aryl-substituted imidazoles). It should be noted
that ␤-pinene points to the formation of radical intermediates via
rearrangement of the tensed four-membered carbon cycle (Fig. 2)
[22].
Surprisingly, the obtained results varied significantly depending
on the ligand. With 91% conversion the main product of the epoxi-
dation of ␤-pinene with ligand 12 was the corresponding epoxide
14a. Next to allylic oxidation product 14b which was detected in
small amounts, also traces of cleavage product nopinone 14c were
detected by GC–MS analysis (pathway A). Perillyl alcohol 14e and
hydroxylated derivatives 14d (and analogues), which indicated the
formation of OH• radicals (pathway B) were only observed in small
traces similar to 14c. Ligand 2 gave entirely different results. Instead
of any epoxide formation, the oxidation gave no main product with
54% conversion of the olefin. Next to 14b and traces of 14c also the
rearrangement products 14d and 14e were observed both in similar
small quantities than 14b. In contrast to the epoxidation reaction
with ligand 12 application of ligand 2 leads to a less active and also
OH• radical-based reaction. However, aromatic olefins gave 40%
yield or more of the corresponding oxiranes and high selectivities
(>85%) using ligand 2 [15a]. These results clearly demonstrate the
strong influence of the ligand depending on the substrate.
Next, three radical scavengers were applied to see further
effects (Fig. 3). Applying our benchmark system with ligands 2, 12
and also 4 in the presence of duroquinone (2,3,5,6-tetramethyl-
p-benzoquinone) (1 equiv. and 5 mol%) no change in yield or
conversion was observed. This shows that duroquinone neither
plays a role as ligand [23] nor is involved in radical inhibition [24].
A
different behavior was observed in the presence of
TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and BPN (N-tert-
Fig. 3. Radical scavengers applied in the iron/imidazole catalyzed epoxidation reac-
tion.
Fig. 2. Products during the oxidation of ␤-pinene.
Fig. 4. Proposed oxo-hydroxo tautomerism.

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K. Schröder et al. / Catalysis Today 157 (2010) 364–370
Table 2
Behavior of radical traps in the epoxidation with the iron/imidazole systema
.
No.
Radical scavenger [mmol]
Ligand
Conv. [%]b
Yield [%]b
Sel. [%]c
1
2
3
4
5
6
7
8
9
–
–
12
2
12
12
2
72
66
92
97
83
80
BPN [0.5]
BPN [0.025]
BPN [0.5]
BPN [0.025]
TEMPO [0.5]
TEMPO [0.025]
TEMPO [0.5]
TEMPO [0.025]
55 (61)d
44 (51)d
80(83)d
93
67
62
64 (64)d
52 (55)d
82(87)d
88
2
84
1
23
6
74
1
18
5
12
12
2
94
78
84
73
10
2
62
45
a
Reaction conditions: in a test tube, FeCl3·6H2O (0.025 mmol), imidazole derivative (0.05 mmol), tert-amyl alcohol (9 mL), trans-stilbene (0.5 mmol) and dodecane (GC
internal standard, 100 ␮L) were added in sequence at r.t. in air. The radical scavenger (1 equiv. or 5 mol%) was added directly before the H2O2 addition or with a delay of
15 min. To this mixture, a solution of 30% H2O2 (170 ␮L, 1.5 mmol) in tert-amyl alcohol (830 ␮L) was added over a period of 1 h at r.t. by a syringe pump.
b
Conversion and yield were determined by GC analysis.
Selectivity refers to the ratio of yield to conversion in percentage.
The radical trap was added to the solution with delay of 15 min after the addition of hydrogen peroxide.
c
d
butylphenylnitrone) as radical scavengers, which should trap
possible existing carbon-centered radicals. Employing BPN in the
benchmark reaction for ligands 2 and 12 slightly decreased yields
and conversions were observed (Table 2, entries 3–6). However,
no carbon-centered radical was noticed by direct addition. Slow
addition of the radical trap BPN gave a high yield of benzalde-
hyde (69% for entry 3), which mainly arose by the decomposition
of the nitrone trap. Therefore the possible amount of benzaldehyde
Fig. 5. Three differenttypes ofinvolved iron complexes (hydrogen atoms are omitted for clarity): (a) crystals of thecomplex Fe(12)4Cl–O–FeCl3. Selectedbonddistances (Å) and
angles (◦): Cl1–Fe1 2.4548(8), Cl2–Fe2 2.2309(9), Fe1–O1 1.8184(19), Fe1–N7 2.128(2), Fe1–N5 2.136(2), Fe1–N1 2.138(2), Fe1–N3 2.151(2), Fe2–O1 1.7651(18); O1–Fe1–N7
92.64(8), N7–Fe1–N5 89.67(8), N5–Fe1–N1 174.17(9), N7–Fe1–N3 175.56(9), O1–Fe1–Cl1 178.20(6), O1–Fe2–Cl3 110.86(7), Cl3–Fe2–Cl4 107.45(5). (b) ORTEP diagram of the
corresponding crystal structure. Selected bond distances (Å) and angles (◦): Cl1–Fe1 2.3777(6), Cl2–Fe1 2.3096(5), Cl3–Fe1 2.3071(6), Fe1–N5 2.1319(15), Fe1–N1 2.1524(15),
Fe1–N3 2.1684(16); N5–Fe1–N1 174.18(6), N5–Fe1–N3 87.86(6), N5–Fe1–Cl3 91.61(5), N1–Fe1–Cl3 90.62(5), N3–Fe1–Cl2 175.13(5), Cl3–Fe1–Cl2 96.10(2), Cl3–Fe1–Cl1
173.44(2). (c) ORTEP diagram of Fe(13)3Cl3, and (d) ORTEP diagram of trans-[FeCl2(2)4]Cl. The thermal ellipsoids correspond to 30% probability. Selected bond distances
(Å) and angles (◦): N1–Fe1 2.123(2), N3–Fe1 2.1315(19), N5–Fe1 2.125(2), N7–Fe1 2.1518(19), Cl1–Fe1 2.3115(6), Cl2–Fe1 2.3036(7); N1–Fe1–N5 177.58(8), N1–Fe1–N3
90.57(8), N1–Fe1–Cl2 89.99(6), Cl2–Fe1–Cl1 176.77(3).

K. Schröder et al. / Catalysis Today 157 (2010) 364–370
369
Scheme 3. Comparison of the molecular-defined catalyst with the in situ system under our reaction conditions.
in these reactions is enhanced; it displays not only the byproduct
but also the decomposition product. Furthermore, cis-cyclooctene
was used as substrate. As expected ligand 12 gave significantly
higher yield (37% yield with 76% selectivity) compared to ligand
2 (13% yield with 58% selectivity) in the presence of 0.5 mmol BPN.
Although ligand 2 yielded less epoxide the decomposition of the
nitrone trap to benzaldehyde was almost complete. In the case
of ligand 12 the nitrone trap as well as 12 was still observable in
GC–MS.
When TEMPO was applied as an additive it mainly inhibited the
epoxide formation, whereas no trapped species could be detected.
Contrary to known alcohol oxidations the persistent TEMPO radical
does not favor the oxidation process [25]. In no case carbon-
centered radical could be observed.
Next, the role of water was confirmed. This is of special interest
due to the possible exchange between the oxygen of the hydroper-
oxide and of the water via tautomeric iron–oxo species (Fig. 4) [26].
A competitive equilibrium was proposed between less active ␮-
oxo diiron complexes (Scheme 2, A) and the active mononuclear
Fe(III)-complex (Scheme 2, B) by reaction with water. Preliminary
investigations of the epoxidation of trans-stilbene with ligands 2
and 12 in the presence of 10 equiv. of water and 50% H2O2 or the
urea–H2O2 adduct as oxidant again showed the striking influence
of the ligand. Low conversion and yield were observed with ligand
12. However, in the presence of ligand 2 and 50% hydrogen perox-
ide as oxidant 36% conversion with 35% yield was obtained after
1 h addition. The yield was even increased to 50% with 55% con-
version after stirring for 20 h. Similarly, applying the urea–H2O2
adduct improved results were obtained with ligand 2. Hence, we
propose that ligand 12 favor the production of inactive ␮-oxo com-
plexes (Fig. 5, proposed and observed in [15b]), more than ligand
2, which is consistent with the obtained and catalytic active X-ray
structures.
In the reaction with ligand 2, 50% hydrogen peroxide and
10 equiv. of labeled water no incorporation of labeled oxygen into
the epoxide (50% yield with 52% conversion) was noticed. Only
traces of the side product benzaldehyde (∼1% yield) showed incor-
poration of 18O in an amount of approximately 1/3. Hence, for this
reaction the oxo-hydroxo tautomerism is to slow or no high-valent
iron–oxo species does exist in our system. Furthermore, the role of
air, which is known to support autooxidation processes in many
non-heme model systems was examined [27]. Thus, the model
epoxidation reaction was performed under inert conditions. In the
presence of ligand 2 no variation of yield was obtained compared
to the epoxidation on air. These findings demonstrate the stability
of our catalyst system which works smoothly in the presence of air
and suppresses autooxidation processes, which is different from
most non-heme model systems.
our applied epoxidation system (Fig. 5c). In addition to the ␮-oxo-
diiron complexes (Fig. 5b) and the mononuclear iron complexes
with four imidazole ligands (Fig. 5d), we were able to discover
a neutral iron complex consisting of three imidazole ligands and
three chloride ions. Fig. 5 shows the structurally different com-
plexes found in our reaction system.
The novel octahedral mer-[FeIII(13)Cl3] complex is structurally
similar to mer-[Fe (N-methylimidazole)3 Cl3] reported by Cotton et
al. [28]. However, the iron chloride distances in mer-[FeIII(13)Cl3]
trans- to nitrogen(N3) (Fe–Cl2 = 2.3096(5) Å) and trans- to Cl1
(Fe–Cl3 = 2.3071(6) Å) are significantly shorter than the Fe–Cl1
distance with 2.3777(6) Å. Compared to trans-[FeCl2(2)4]Cl with
distances (Fe–Cl1 = 2.3115(6) Å) and (Fe–Cl2 = 2.3036(7) Å) the long
Fe–Cl1 distance in mer-[FeIII(13)Cl3] is also unexpected. Probably,
this is a hint for a facilitated exchange with an imidazole ligand
to give the other catalytically active trans-[FeCl2(imidazole)4]Cl
species. To prove these findings we tested the molecular-defined
complexes under our reaction conditions. The found catalytic reac-
tivity is consistent with our assumption (Scheme 3).
Although the complex is structurally different from trans-
[FeCl2(2)4]Cl, it is clearly shown that the novel mononuclear iron
complex [FeCl3(13)3] is an active, highly selective and defined
epoxidation catalyst.
4. Conclusion
The iron/imidazole catalyst system allows for a biomimetic and
convenient epoxidation of various aromatic and aliphatic olefins
using environmentally benign hydrogen peroxide as oxidant. Both
the in situ catalysts as well as the molecular-defined iron species
are cheap and easy-to-use without any special precautions. With
respect to the mechanism of this reaction important insights have
been obtained. While most of the known non-heme iron-based
epoxidation systems require oxygen free atmosphere, the stability
towards air and water of our system is remarkable. Notably, both
the ligand and the used substrate show a strong influence on the
reactivity and the different reaction pathways. It is shown that
modification of the ligand structure alters also the structure of the
resulting mono- or binuclear iron complexes. It is likely, that under
reaction conditions there is equilibrium between the different char-
acterized complexes.
Acknowledgements
We thank the State of Mecklenburg-Western Pommerania,
the Federal Ministry of Education and Research (BMBF) and the
Deutsche Forschungsgemeinschaft (SPP 1118 and Leibniz-prize)
for financial support. K. Schröder appreciates the financial sup-
port provided by the Graduiertenkolleg 1213 “Neue Methoden für
Nachhaltigkeit in Katalyse und Technik” and the Max-Buchner-
While studying the effects of different ligands, a novel crys-
tal structure was obtained directly from the reaction mixture of

370
K. Schröder et al. / Catalysis Today 157 (2010) 364–370
Forschungsstiftung (DECHEMA). Dr. habil. M. K. Tse and Dr. B. Join
are acknowledged for fruitful discussions and M. Heyken for tech-
nical assistance.
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