Fighting Fenton Chemistry: A Highly Active Iron(III) Tetracarbene Complex in Epoxidation Catalysis

Article abstract of DOI:10.1002/cssc.201500930

Organometallic Fe complexes with exceptionally high activities in homogeneous epoxidation catalysis are reported. The compounds display Fe^II and Fe^III oxidation states and bear a tetracarbene ligand. The more active catalyst exhibits activities up to 183 000 turnovers per hour at room temperature and turnover numbers of up to 4300 at -30°C. For the Fe^III complex, a decreased Fenton-type reactivity is observed compared with Fe^II catalysts reported previously as indicated by a substantially lower H₂O₂ decomposition and higher (initial) turnover frequencies. The dependence of the catalyst performance on the catalyst loading, substrate, water addition, and the oxidant is investigated. Under all applied conditions, the advantageous nature of the use of the Fe^III complex is evident.

Full text of DOI:10.1002/cssc.201500930

DOI: 10.1002/cssc.201500930
Full Papers
Fighting Fenton Chemistry: A Highly Active Iron(III)
Tetracarbene Complex in Epoxidation Catalysis
[
a]
[a]
[a]
[b]
Jens W. Kück, Markus R. Anneser, Benjamin Hofmann, Alexander Pçthig,
[
c]
[a]
Mirza Cokoja, and Fritz E. Kühn*
II
Organometallic Fe complexes with exceptionally high activities
in homogeneous epoxidation catalysis are reported. The com-
Fe catalysts reported previously as indicated by a substantially
lower H O2 decomposition and higher (initial) turnover fre-
2
II
III
pounds display Fe and Fe oxidation states and bear a tetra-
carbene ligand. The more active catalyst exhibits activities up
to 183000 turnovers per hour at room temperature and turn-
quencies. The dependence of the catalyst performance on the
catalyst loading, substrate, water addition, and the oxidant is
investigated. Under all applied conditions, the advantageous
III
III
over numbers of up to 4300 at À308C. For the Fe complex,
nature of the use of the Fe complex is evident.
a decreased Fenton-type reactivity is observed compared with
Introduction
Fluctuating metal prices and declining resource stocks urge
the chemical industry towards more abundant (and therefore,
found that the performance of Fe cross-coupling catalysts ap-
parently depends on the commercial source of the Fe salts
[1]
[7]
cheaper) catalyst metals for all major applications. In contrast
to this, most organometallic catalysts applied currently for fine
used in catalysis. The most active catalysts had a higher con-
tent of trace metal impurities, which thus underlines the
higher catalytic activity of these trace metals. Beside cross-cou-
pling reactions, Fe-catalyzed hydrosilylation reactions that give
anti-Markovnikov products exclusively were reported by Chirik
[
1c,2]
chemical synthesis are based on expensive noble metals.
Complexes of first-row transition metals, especially Fe, present
a promising alternative. Fe is the most abundant transition
metal in the earth’s crust (4.7 wt%) and is usually considered
[
8]
et al. These results led to enantioselective hydrosilylation cat-
alysis and to industrial applications in cooperation with Mo-
[
1c,3]
nontoxic.
In the human body, the ferritin system regulates
[
9]
the Fe concentration, which renders the majority of Fe com-
mentive. Although this example supports the hope for more
[4]
pounds effectively harmless. In addition, Fe has a diverse
active Fe-based catalysts, other Fe-based reactions require
[
1a]
[1a,b,10]
redox reactivity and a tunable Lewis acidic character. Smart
ligand design can offer a plethora of Fe catalysts applicable to
high catalyst loadings and long reaction times.
Some ap-
plications that include Fe-catalyzed olefin metathesis have only
[1a,5]
[1a,11]
a broad range of organic transformations.
been predicted in computational studies.
Expensive, toxic, or rare catalyst metals have already been
replaced by Fe in a few homogeneous reactions Fe (e.g., cross-
For oxidation reactions, the goal to replace existing catalysts
by Fe complexes has proven to be very demanding. The main
reason for this is the oxophilicity of Fe compounds and their
[
1a,b,6]
coupling reactions).
For some of these systems it was
[
12]
lability under oxidative conditions.
Still, Fe-catalyzed CÀH
+
+
[
a] J. W. Kück, M. R. Anneser, B. Hofmann, Prof. Dr. F. E. Kühn
Molecular Catalysis/Inorganic Chemistry
Department of Chemistry
bond oxidations and epoxidations are feasible. Porphyrin li-
[
13]
gands were the first to be used, followed by nonheme cata-
lysts, which have been of increasing interest during the past
Catalysis Research Center
Technische Universität München (TUM)
Lichtenbergstr. 4
[
3b,14]
decade.
Although the activity of these catalysts has im-
proved significantly, the performance of the most active Rh
[
15]
85747 Garching bei München (Germany)
and Mo complexes is still unchallenged.
E-mail: fritz.kuehn@ch.tum.de
To find active species of nonheme Fe complexes has been
a prominent goal of investigations during the last two deca-
[
b] Dr. A. Pçthig
Catalysis Research Center
Technische Universität München (TUM)
Ernst-Otto-Fischer-Straße 1
[
14c,16]
des.
The predominant current opinion is that for the ma-
jority of catalyst systems the first intermediate in the formation
III
[14c,k,l]
85747 Garching bei München (Germany)
of an active catalyst is an Fe ÀOOH complex.
Although
[
c] Dr. M. Cokoja
some studies suggest this complex to be active in epoxidation
Faculty of Chemistry, TUM
Lichtenbergstr. 4
[14a,17]
catalysis,
Nam et al. were able to demonstrate that a
Fe ÀOOH moiety that bears a pyridine-based ligand is not
a potent oxidant for the epoxidation of olefins, which
III
85747 Garching bei München (Germany)
[
16b]
+
[
] These authors contributed equally to this work.
prompts the question as to the nature of the active oxidant.
Investigations were conducted in great detail for nonheme
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201500930.
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complexes of cis-type geometry that resulted in a detailed
mechanistic network for the formation of high-valent iron oxo
[
14j–l]
complexes.
If water is present in the reaction mixture,
a water-assisted pathway for the formation of an active iron(V)
oxo hydroxo complex could be proven for complexes that ex-
[
18]
hibit cis-labile coordination,
tained in the presence of carboxylic acid.
a non-water-assisted pathway via a transitory side-on h -OÀO
complex was proposed for these complexes. The nature of
and similar results were ob-
[14j,19]
Furthermore
2
Scheme 1. Synthesis of 2 by the reaction of thianthrenyl hexafluorophos-
phate as a one-electron oxidant with 1.
[14e]
the active oxidant for nonheme Fe complexes in trans-labile
geometry, however, seems to differ. The heterolytic or homo-
lytic cleavage of the OÀO bond has been proposed to lead to
The complex exhibits an almost ideal octahedral coordina-
tion around the Fe center in which the tetracarbene ligand co-
ordinates in the expected equatorial fashion and there are two
axial acetonitrile ligands (Figure 1). The +3 oxidation state of
[20]
iron(V) and iron(IV) oxo complexes, respectively. The latter is
associated with a lower selectivity because of the generation
of hydroxyl radicals by homolytic OÀO cleavage.
II
À
Interestingly, in most catalytic reactions Fe precatalysts are
the Fe atoms is confirmed by the number of PF6 anions in the
used, which mandates a preoxidation to form the iron(III) hy-
droperoxo species that typically involves a Fenton-type radical
unit cell.
[
3b,14f,16c,21]
step.
This first oxidation step can potentially reduce
activity and selectivity because of radical formation. To the
best of our knowledge, no comparative study of structurally
II
III
equivalent Fe /Fe nonheme epoxidation catalysts has been re-
ported, which leaves an essential gap for a better understand-
ing of effective catalyst design.
To achieve very active Fe-mediated epoxidation catalysis,
our group has focused on the development of Fe complexes
[
22]
with N-heterocyclic carbenes (NHCs).
plexes, which bears two carbene and two pyridine moieties
iron(II) di(o-imidazol-2-ylidenepyridine)methane hexafluoro-
One of these com-
(
Me
phosphate; FeNCCN ), is active in epoxidation and aromatic
[
23]
hydroxylation. In addition to the dicarbene systems, we have
reported the synthesis and reactivity of an iron(II) cyclic tetra-
[
24]
carbene complex.
Herein, the activity of this complex to-
wards epoxidation catalysis is compared with that of its oxi-
dized counterpart to test the hypothesized leap in activity for
III
the isostructural Fe complex.
Results and Discussion
Figure 1. ORTEP-style representation of one of the two tricationic crystallo-
graphically independent molecules of 2 in the unit cell. Ellipsoids are shown
Synthesis and characterization
at a 50% probability level. Hydrogen atoms, co-crystallized acetonitrile mole-
II
À
cules, and PF
6
anions are omitted for clarity. Selected bond lengths [Š] for
Smith and Long reported an Fe monocarbene tetrapyridine
III
the shown cationic unit (values for the second independent cation are given
in parentheses): Fe2ÀC1=1.937(3) (1.957(3)), Fe2ÀC5=1.944(2) (1.955(3)),
Fe2ÀC9=1.944(3) (1.957(3)), Fe2ÀC13=1.941(3) (1.955(3)), Fe2À
N10=1.914(3) (1.923(1)), Fe2ÀN9=1.929(2) (1.923(1)).
complex that could be oxidized selectively to its Fe derivative
[25]
using thianthrenyl hexafluorophosphate. We used this ap-
II
proach with the cyclic tetracarbene Fe complex reported pre-
III
viously to obtain the corresponding Fe complex (Scheme 1).
Complex 2 is formed by an outer-sphere one-electron oxida-
tion and isolated in 91% yield. The purple complex was char-
acterized by single-crystal XRD, UV/Vis spectroscopy, cyclic vol-
tammetry, ESI-MS, and elemental analysis, which confirmed its
purity and bulk composition. The electrochemical potentials
The overall geometry is not significantly different to that of
[
24]
the structure determined for 1. The most evident change is
the slight elongation of the FeÀcarbene bond after oxidation.
The lengths of the FeÀcarbene bonds in the crystal structure
of 1 are in the range of 1.902(3)–1.912(3) Š, whereas they are
1.937(3)–1.957(2) Š for 2. The FeÀN bond lengths are less af-
fected: 1.930(3)–1.933(3) Š for 1 and 1.914(2)–1.929(2) Š for 2.
These bond lengths to the axial acetonitrile ligands show a sig-
nificant overlap in the 3s confidence intervals for 1 and 2 and
are interpreted as not dependent on the oxidation state of the
II
match the data obtained for the Fe complex and show that
the chemical oxidation with thianthrenyl hexafluorophosphate
indeed yields the same product as the electrochemical oxida-
[24]
tion reported previously. Single crystals of 2 suitable for XRD
were obtained by the slow diffusion of diethyl ether into an
acetonitrile solution of 2.
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Fe atoms. These values are in good accord with bond lengths
reported in the literature for similar compounds in these oxida-
a relative catalyst concentration of 0.05 mol%. Here, the rate
behavior in the first minutes matches that of 1 with a reduced
rate at À108C and essentially unchanged rates for the three
other experiments (Figure 2).
[
5,22a,26]
tion states.
The bond angles around the Fe center are
very close to the ideal octahedral angle with a mean deviation
of 0.488 from the ideal 908 angles of adjacent coordination
sites.
Epoxidation catalysis
II
III
Catalytic experiments using both the Fe (1) and Fe (2) cCCCC
complexes were conducted to compare their performance as
epoxidation catalysts. We used the standard conditions applied
Me
[23a]
previously for FeNCCN (2 mol% catalyst, 5 min, 258C)
for
the epoxidation of cis-cyclooctene to achieve the complete
conversion of cis-cyclooctene for both 1 and 2 as catalysts
(
Table 1).
Table 1. Performance of iron carbene complexes in the epoxidation of
[a]
cis-cyclooctene.
Relative catalyst
concentration [mol%]
Epoxide yield (selectivity) [%]
Me [b]
[FeNCCN
]
1
2
2
1
0
0
0
.0
.0
.5
.25
.1
92 (>99)
66 (>99)
–
100 (>99)
100 (>99)
99 (>99)
82 (>99)
37 (>99)
100 (>99)
100 (>99)
100 (>99)
96 (>99)
55 (>99)
–
4 (>99)
[
a] Reaction conditions: cis-cyclooctene (269 mmol, 100 mol%), H
2 2
O (aq.
5
0%, 403 mmol, 150 mol%), solvent MeCN, t=5 min, T=258C; yields and
selectivities were determined by GC–FID; reactions without catalyst did
not yield any epoxide. [b] For reaction conditions refer to Ref. [23a].
Figure 2. Temperature-dependent yield data for the epoxidation of cis-cyclo-
octene using 1 and 2 as catalysts. a) 0.1 mol% 2, b) 0.05 mol% 2,
c) 0.1 mol% 1, d) relative activities of 1 and 2 as indicated by relative yields
after 30 s for experiments shown in a)–c), values are calculated as follows:
yield(30 s)/yield(final).
No further products apart from the epoxide were detected
1
in all H NMR spectra and gas chromatograms. A reduction of
the catalyst concentration to 0.25 mol% leads to the incom-
plete oxidation of the olefin with a yield of 96%. The yield of
epoxide is 14% higher for 2 than 1 (82% yield). If the catalyst
concentration is further lowered to 0.1 mol%, the performance
difference between 2 and 1 is even more pronounced with
conversions of 55 and 37%, respectively, compared to only 4%
In combination, these experiments show the high speed of
the catalytic reactions that achieved a maximum yield within
30 min at À108C or even faster at higher temperatures. The in-
crease in yields at low temperatures matches previous results
and is a consequence of enhanced catalyst stability, which re-
Me
[23a]
for FeNCCN . Overall, these data show that under these con-
sults in a longer catalyst lifetime.
This stability effect is dom-
ditions 2 is the best catalyst and that both tetracarbene com-
plexes are significantly better catalysts than FeNCCN .
inant over the expected reaction rate decrease at lower tem-
peratures. Unfortunately, the interpretation of the time-depen-
dent yield data is not straightforward as the initial turnover fre-
quency (TOF) cannot be determined from the first data point
at 30 s. After 30 s, the yields already approach the final yields
closely and thus do not give sensible data for the initial linear
slopes. However, a comparison of the relative activities be-
tween this set of experiments is feasible. For this purpose, the
activity is expressed as the relative fraction of the yield that is
formed within the first 30 s compared to the final yield at the
respective temperature. At 258C these values are 84% for
0.1 mol% 1, 98% for 0.1 mol% 2, and 93% for 0.05 mol% 2. At
the lowest investigated temperature of À108C, the values are
reduced significantly to 13, 61, and 34%, respectively. Between
À10 and 258C the relative activity is highly dependent on the
reaction temperature and decreases strongly with decreasing
Me
Time-dependent yield studies with 0.1 mol% catalyst at
room temperature showed the completion of the reaction
within 30 s for both catalysts. To slow the reaction, the temper-
ature was reduced, which increased the overall yield from 55%
at 258C to 100% at À108C for 2 and from 37% at 258C to
9
7% at À108C for 1. However, the initial rate of product for-
mation is so high that it is difficult to determine initial slopes
for the first 30 s for reactions of 0.1 mol% of 1 at various tem-
peratures with the exception of reactions at À108C, which
show a slower initial rate. Experiments with 0.1 mol% 2 are
more difficult to interpret as the final yield of 100% is reached
within the first two minutes at 08C and À108C, albeit with
a slightly slower rate at À108C. To get a more interpretable set
of experiments, comparable data for 2 were recorded with
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temperature. Generally, the values are lower for 1 than for 2
under identical conditions. This indicates that the oxidation
state either has an immediate influence on the activity or that
a lag phase in the epoxidation of cis-cyclooctene occurs for 1.
As expected, the values for 0.05 mol% of 2 are lower than
those for 0.1 mol% of 2.
To further decelerate the reaction, the temperature was de-
creased to À308C. However, at this temperature cis-cyclooc-
tene crystallizes in acetonitrile, therefore, a change of solvent
is required to guarantee homogeneous reaction conditions.
A mixture of acetonitrile and methylene chloride (1:1) was
used instead. These experiments show that the reaction pro-
ceeds very similarly to the experiments conducted previously
in acetonitrile (Figure 3). The final yields increase significantly
at À20 and À308C, whereas the yields for experiments at À10
and 108C are reduced slightly compared to that of the experi-
Figure 4. Time-dependent yields of catalysts (0.025 mol%) 1 (red) and 2
(black) at À408C in MeCN/CH
2 2
Cl (1:1). For 1 an induction period is clearly
observable.
Notably, this phase is more pronounced at high dilution and
only visible at temperatures lower than À108C. Also the global
order of reaction, that is, the sum of all orders of reaction, was
determined to be 2 in both cases with significantly different
rate constants. The determined reaction rate after the initiation
À1 À1
phase is lower for 1 (k : 0.047 s m ) than for 2 (k :
rel
rel
À1 À1
0
.25 s m ; Figures S16–S18). Together these findings suggest
II
III
strongly a preoxidation of 1 from Fe to Fe that causes a lag
phase that is not necessary for 2.
Batch reactions at 258C with a maximum reaction time of
1
0 s were conducted to determine the activity of 1 and 2
more precisely. The reaction for 2 is nearly complete in 10 s as
can be seen by the yield of 51% epoxide, which is almost
equal to the final yield for 2 at room temperature. In contrast,
Figure 3. a) Time-dependent yield data at low temperatures for the epoxida-
tion of cis-cyclooctene using 0.05 mol% of 2 as catalyst in a solvent mixture
of MeCN/methylene chloride (1:1). b) Dependence of yield after 5 and
1
5
yields only 14% epoxide after 10 s compared to 37% after
À1
min. These values correspond to TOFs of 183600 h for 2
À1
60 min on the concentration of 2.
and 50400 h for 1. This reactivity is unprecedented and to
the best of our knowledge 2 and 1 are both more active than
[
15a]
the most active homogeneous epoxidation catalyst to date.
ments in acetonitrile. The activity is still very high with a reac-
tion time of 60 min at À208C. This increase in yield is caused
by an increased catalyst lifetime from less than 30 s to 60 min
upon lowering the temperature from 10 to À208C. At À308C
the complete conversion of olefin is reached before the cata-
lyst decomposes, which thus gives no information about cata-
lyst stability.
This leap in activity is even more pronounced in comparison
to all previous Fe catalysts. The highest TOF of an Fe epoxida-
2
+
tion catalyst ([(bpmen)Fe(MeCN) ] ; bpmen=N,N’-dimethyl-
2
N,N’-bis-(pyridin-2-ylmethyl)-1,2-diaminoethane) reported to
À1
date is 25200 h , which corresponds to 50% epoxide yield
[
14f]
with 0.5 mol% catalyst after 14 s.
As seen from the time-de-
pendent yields, 2 appears to be significantly more active than
1 at room temperature with a TOF three times as high as that
of 1. This can either be explained by a dominant lag phase for
1, which causes an underestimation of the TOF, or a significant-
ly higher activity for 2 than 1. Consequently, the performance
of 2 in epoxidation catalysis was investigated further (Table 2).
Several common peroxides were examined as oxidants in
the epoxidation of cis-cyclooctene. The yield of epoxide is re-
duced if peroxides other than H O are applied. The use of the
However, a significant decrease in activity was not detected.
In the course of this reduction in temperature the yield is in-
creased from 40 to 100%. With a catalyst concentration of
0
.03 mol%, a yield of 96% is reached after 60 min at À308C.
A further reduction of the catalyst loading lowers the yield lin-
early to 22% at a catalyst loading of 0.005 mol%. This corre-
sponds to a turnover number (TON) of more than 4300 at
[
14f]
À308C, which is a remarkable stability for an Fe catalyst,
2
2
albeit at low temperatures.
urea hydrogen peroxide adduct yields less epoxide because of
its lower solubility in acetonitrile. Alkyl hydroperoxides are also
less effective than H O . Interestingly, tert-butyl hydroperoxide
At À408C, the lowest reaction temperature monitored, the
activity of 1 and 2 are compared at different dilutions to inves-
tigate the initial behavior of the catalyst more clearly. For 1 an
initiation phase is observable in these kinetic experiments,
whereas for 2 no such initial lag phase can be detected
2
2
(TBHP) is more suitable in n-decane solution than in aqueous
conditions. As H O is the best oxidant for the catalytic epoxi-
2
2
dation, the influence of its relative concentration on the epox-
ide yield was investigated (Figure 5).
(
Figure 4).
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comparison to 2 with maximum yields of 39% at 200 mol%
Table 2. Performance of 2 in the epoxidation of cis-cyclooctene using
various hydroperoxides.
[a]
H O and 34% at 50 mol% H O , respectively. These reduced
2
2
2
2
yields at low oxidant concentrations again show a competitive
decomposition of H O . Even at higher catalyst concentrations
Oxidant
Oxidant concen-
tration [mol%]
Epoxide yield [%]
(amount of 2 [mol%])
2
2
of 0.25 mol% the decomposition of H O is more pronounced
2
2
H
H
H
H
H
2
2
2
2
2
O
O
O
O
O
2
2
2
2
(aq. 50%)
(aq. 50%)
(aq. 50%)
(aq. 50%)
(aq. 50%)
50
150
300
1000
150
150
150
150
150
47 (0.1)
56 (0.1)
55 (0.1)
31 (0.1)
96 (0.25)
65 (0.25)
71 (0.25)
93 (0.25)
74 (0.25)
for 1 than for 2, which again yields epoxide. Under these con-
ditions, the activity of 1 is no longer dependent on the catalyst
concentration as a result of a much faster decomposition of
H O by the catalyst. These findings show that the oxidation
2
[
2
2
b]
UHP
TBHP (aq.)
TBHP (n-decane)
CHP
state of the catalyst is the main factor that determines the de-
composition of H O . Consequently, it can be assumed that
2
2
[
c]
a Fenton step is at least partly responsible for the significantly
II
III
lower yields of the Fe complex compared to that of the Fe
complex. This effect of the oxidation state has been known for
[
a] Reaction conditions: cis-cyclooctene (269 mmol, 100 mol%), solvent
MeCN, t=5 min, T=258C; yields and selectivities were determined by
GC–FID. [b] Urea hydrogen peroxide adduct. [c] Cumene hydroperoxide.
[
21]
classical Fe salts used as Fenton reagents. These findings, in
combination with kinetic studies at À408C, suggest strongly
II
III
that a first oxidation from Fe to Fe is required for the reac-
tion to proceed, which indicates that 2 is the first step of 1 to-
wards the formation of an active species in epoxidation reac-
tions.
Stability is one of the biggest challenges for the applicability
[
16b,f,18a,23a]
of Fe epoxidation catalysts.
Many Fe complexes de-
compose rather quickly in aqueous H O solution and remain
2
2
[
14e,f,23a]
active only for a short period of time.
Although this
problem has been known for a while, other factors also impact
stability of Fe catalysts. To better understand the influence of
the reaction conditions on catalyst stability, the influence of
water in the reaction mixture was investigated. As a result of
the use of aqueous H O , the catalytic system contains water
Figure 5. Influence of H
oxidation at room temperature after 5 min. Black: 0.1 mol% of 2 used; Red:
.1 mol% of 1 used; Green: 0.25 mol% of 1 used (50 mol% H only).
2 2
O concentration on the yield of cis-cyclooctene ep-
2
2
necessarily. However, additional equivalents of water result in
noticeable effects (Figure 6). The epoxide formation decreases
from 53 to 42% if 10 equivalents water relative to cis-cyclooc-
tene are added. This influence is quite unexpected as 2 is re-
markably stable in undried acetonitrile, yet under oxidative
conditions the reaction appears to be somewhat sensitive to
water. The experiments with TBHP presented in Table 2 may
be rationalized by the effect of water. To verify that the notice-
able decrease in yields is in fact a stability effect rather than an
activity effect, time-dependent yield data were collected with
0.1 mol% 2 at À108C. In the case of the addition of
0
2 2
O
An increase in the H O concentration up to 250 mol% re-
2
2
sults in slightly higher epoxide yields. The addition of further
H O decreases the epoxide yield strongly. A reduced catalyst
2
2
stability emerges in the presence of large excesses of oxidant,
which thus lowers the overall epoxide yield. The effect is no-
ticeable for both 1 and 2, albeit slightly more pronounced for
1
because of the overall lower product yields. However, if the
amount of H O is reduced below 150 mol% the yield decreas-
2
2
es, so that at low H O concentrations the overall yield does
2
2
not reach the maximum possible yield achieved for approxi-
mately 150–300 mol% H O . This effect is probably caused by
2
2
the catalytic decomposition of H O , which has been known
2
2
II
III
for Fe and Fe salts for decades and is also indicated by gas
[21,27]
formation at the very beginning of the reaction.
Conse-
quently, the yield is limited by the H O concentration, which
2
2
does not allow for the complete conversion of the olefin. How-
ever, with 47% yield for 2 at 50 mol% H O , the yield is quite
2
2
close to the possible maximum of 50%. If a second portion of
H O is added, the overall yield increases to 95%. This shows
2
2
that the catalyst is still active and the lowered yield is not an
effect of reduced activity or stability. Compared to the
II
Me
Fe NCCN system reported previously, which shows a more
pronounced H O decomposition, 2 does not exhibit a strong
2
2
Figure 6. Influence of added water on the yield of cis-cyclooctene epoxida-
tion at room temperature using 0.1 mol% 2 and 150 mol% H O (50% in
H O) after 5 min.
2
Fenton reactivity, indicated by its lower H O decomposi-
2
2
2
2
[
23a]
tion.
As noted previously, the yields are decreased for 1 in
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1
000 mol% water, the product formation is unaffected initially.
However, after 5 min the reaction terminates and reaches only
4% epoxide yield (see Supporting Information). Experiments
clohexene and 1-phenyl cyclohexene yield 92 and 93%, re-
spectively, which shows that the latter is much more easily ep-
oxidized compared to styrene. These general trends for aryl-
substituted alkenes are in agreement with the literature, with
9
without the addition of water show higher yields and longer
periods of activity (Figure 2). This behavior might be consid-
ered quite unusual on first glance as most nonheme Fe cata-
lysts exhibit enhanced activity if water is added to the reaction
mixture as a water-assisted pathway in the formation of the
II
Me
[23a]
a slight improvement compared to the Fe NCCN catalyst.
Of the substrates examined for epoxidation, acyclic terminal
olefins are the most difficult to epoxidize, with comparable
yields for 1-hexene, 1-octene, and 1-decene. However, the in-
ternal olefin 2-octene leads to the second highest yield of the
eight substrates tested with a distinctively different result for
the two geometric isomers. In this particular case, epoxidation
of the Z isomer is favored over that of the E isomer with epox-
ide yields of 93 and 67%, respectively. This selectivity is typical
for Fe epoxidation catalysts and has been reported previous-
[
14c,e]
active species is generally accepted for these catalysts.
However, Que and co-workers showed that this only holds true
for catalysts that exhibit cis-labile coordination. These com-
plexes are able to undergo a water-assisted OÀO cleavage in
V
the intermediate iron hydroperoxo complex to form a Fe oxo
hydroxo complex, which is the active species in the epoxida-
tion. Thus the product yield increases upon the addition of
water because of the more facile formation of the active cata-
[
14e]
ly.
The overall preference of more highly substituted alkenes
over terminal alkenes and the decreased yields for aryl alkenes
as well as the high selectivities indicate the electrophilic nature
[14c,e]
18
lyst.
Through the addition of H2 O, it was found that one
[
28]
of the oxygen atoms originates from water in solution, that is,
of the active species.
18
18
O, in cis complexes. Through a tautomerization, this O is
subsequently partially incorporated into the epoxide. In con-
trast, 2 exhibits trans coordination, in which case a decrease in
product yield is observed. This shows that the water-assisted
pathway does not apply to 2, which is expected from previous
Conclusions
Iron tetracarbene systems are applied as homogeneous epoxi-
III
dation catalysts with H O as the oxidant. In particular, the Fe
2
2
[14e]
studies on complexes of this coordination geometry.
This is
derivative has an exceptionally high activity at ambient tem-
1
8
also corroborated by the lack of incorporation of O upon the
II
peratures and below. The less active Fe compound is still on
1
8
addition of 1000 mol% H2 O, which again shows that a water-
assisted pathway does not apply.
par with the most active homogeneous organometallic cata-
lysts described previously that usually contain much more ex-
pensive metals (e.g., Re, Mo, etc.). Oxidant decomposition by
Various cyclic and acyclic alkyl and aryl alkenes were used as
substrates in epoxidation reactions catalyzed by 2 (Table 3).
For all substrates no diol formation is observed. The highest
epoxide yields are obtained for cyclic olefins, of which cis-cy-
clooctene yields the most epoxide. The other cyclic olefins cy-
III
radical pathways is reduced significantly if the Fe -based cata-
lyst is used. This difference in behavior can be attributed to
II
the difference in oxidation state. An initiation phase for the Fe
III
complex indicates a need for a first oxidation to form an Fe
complex.
These encouraging results support the view that Fe-based
organometallic catalysts are able to compete successfully with
other catalysts based on (much) more expensive metals. The
metal oxidation state, the ligand sphere, temperature, and sol-
vent apparently allow fine-tuning to reach optimal reaction
conditions. To further improve this system, catalyst immobiliza-
tion through the ligand sphere (to allow easy catalyst recy-
cling) and the reduction of catalyst decomposition (to further
increase the turnover number) have to be the focus of re-
search efforts.
[
a]
2 2
Table 3. Catalytic epoxidation of alkenes with 2 in MeCN with H O .
Alkene
Epoxide yield (selectivity) [%]
00 (>99)
1
[
b]
53 (>99)
91 (>99)
3
3
3
6
5 (>99)
9 (>99)
6 (>99)
7 (>99)
Experimental Section
9
9
3 (>99)
General remarks
3 (95)
CAUTION: H O and organic peroxides are potentially explosive if
2
2
highly concentrated and exposed to heat or mechanical impact. All
chemicals were purchased from commercial suppliers and were
used without further purification. Complex 1 and thianthrenyl hex-
afluorophosphate were synthesized according to the literature pro-
4
5
6 (93)
9 (>99)
[24,29] 1
cedures.
H NMR spectra were recorded by using a Bruker
[
2
a] Reaction conditions: alkene (134 mmol, 100 mol%), H
2
O
2
(aq. 50%,
Avance DPX 400. Chemical shifts are given in parts per million
02 mmol, 150 mol%), 0.375 mol% of 2, solvent MeCN-d , t=30 min, T=
3
(
ppm), and the spectra were referenced by using the residual sol-
1
1
À108C; yields and selectivities were determined by H NMR spectroscopy
using external standards. [b] 0.1 mol% of 2, t=5 min, T=258C.
H
vent shifts as internal standards ([D ]MeCN,
d=1.94 ppm).
3
A Thermo Scientific LCQ/Fleet spectrometer by Thermo Fisher Sci-
ChemSusChem 2015, 8, 4056 – 4063
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4061
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Full Papers
entific was used to collect ESI-MS data, and elemental analysis was
obtained from the microanalytical laboratory of TUM. GC with
flame ionization detection (FID) measurements were performed by
using a Varian CP-3800 equipped with an Optima 5-Amin column
dent experiment, reference reactions without the use of catalyst
were conducted.
1
H NMR experiments were conducted in [D ]MeCN with the same
3
stoichiometry in doubled absolute concentrations. The total reac-
(
FID; 1.50 mm; 30 m”0.32 mm) with p-xylene and indane as exter-
1
8
18
tion volume was 1 mL with the use of a stock solution of catalyst
nal standards. H2 O (97% O) was used as purchased, and reac-
tion results were quantified by GC–MS by using a HP 5890A and
a mass-selective detector HP 5970 (Hewlett–Packard) equipped
with a DB225-MS column (30 m”0.250 mm; 0.25 mm film, Agilent
Technologies). Temperature-controlled experiments were cooled by
using Julabo FP 40 and FP 50 cryostats with an external tempera-
ture probe.
À1
of 2.4 mgmL in [D ]MeCN. This solution was cooled to À108C
3
and added to a preformed solution of 0.1343 mmol of olefin
(
100 mol%) and 0.202 mmol H O2 (22.91 mL, 50% in H O,
2 2
1
50 mol%) at À108C. After 30 min the reaction was aborted by
the addition of activated MnO , and the external standards ben-
2
zene or pyridine were added. After filtration over neutral activated
1
alumina, H NMR spectra of the samples were recorded. The prod-
ucts were quantified by the integral ratios of the respective olefin,
diol, and epoxide protons.
Single-crystal XRD
À1
Single crystals of 2 (C H F FeN P , MW: 893.23 gmol ) suitable
20
22 18
10 3
for XRD were obtained by the slow diffusion of diethyl ether into Supporting Information
an acetonitrile solution of 2. The intense red crystal exhibits a tri-
clinic crystal system in the space group P 1¯ (No. 2) with the cell pa-
rameters a=11.9510(3) Š, b=12.2706(3) Š, c=16.6742(4) Š, a=
UV/Vis data, CV data, and XRD data for 2 in CIF format as well
as supplementary catalysis data that includes H NMR spectra
1
8
6.321(1)8, b=73.341(1)8, and g=84.714(1)8 (Z=3). Diffraction ex-
can be found online. CCDC-963849 contains the supplementa-
ry crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
periments were conducted at 123 K. The final quality factors of re-
finement were R1=0.0394, wR2=0.0977, and GOF=1.021.
Synthesis of trans-diacetonitrile[calix[4]imidazolyl]iron(III)
hexafluorophosphate (2)
Acknowledgements
Thianthrenyl hexafluorophosphate (144 mg, 0.40 mmol) and
J.W.K. gratefully acknowledges support from the TUM-GS.
1
(300 mg, 0.40 mmol) were dissolved in acetonitrile (10 mL). The
resulting purple solution was stirred at RT for 1 h. The first fraction
of sequential precipitation with diethyl ether yielded a purple
solid. The precipitate was subsequently washed twice with diethyl
Keywords: carbene ligands · epoxidation · homogeneous
catalysis · iron · peroxides
+
ether and dried in vacuo. Yield: 91%, 325 mg. ESI-MS ([M] ): m/z
À
À
À +
(
[
(
(
%): 439.88 [2À2MeCNÀ3PF +F +HCOO ]
(100), 565.53
6
À
À +
À
À +
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6
À
À 2+
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À1
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ChemSusChem 2015, 8, 4056 – 4063
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Published online on November 17, 2015
1
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim