Iron complex with ionic tag-catalyzed olefin reduction under oxidative conditionsa-A different reaction for iron

Article abstract of DOI:10.1002/cssc.201200344

An iron(III) complex with ionic tags was applied to the reduction of alkenes in imidazolium-based ionic liquids (ILs) under oxidative conditions. The catalyst is very efficient to promote reactions of biomass derivatives. At least ten recycling reactions were performed without any loss of catalytic activity. Some important mechanistic insights for this new reaction are also provided based mostly on electrospray ionization quadrupole-time of flight mass spectrometry (ESI-QTOF-MS).

Full text of DOI:10.1002/cssc.201200344

DOI: 10.1002/cssc.201200344
Iron Complex with Ionic Tag-Catalyzed Olefin Reduction
under Oxidative Conditions—A Different Reaction for Iron
Marcelo R. dos Santos,[a] Alexandre F. Gomes,[b] Fabio C. Gozzo,[b] Paulo A. Z. Suarez,[a] and
Brenno A. D. Neto*[a]
An iron(III) complex with ionic tags was applied to the reduc-
tion of alkenes in imidazolium-based ionic liquids (ILs) under
oxidative conditions. The catalyst is very efficient to promote
reactions of biomass derivatives. At least ten recycling reac-
tions were performed without any loss of catalytic activity.
Some important mechanistic insights for this new reaction are
also provided based mostly on electrospray ionization quadru-
pole-time of flight mass spectrometry (ESI-QTOF-MS).
Introduction
The search for new, sustainable, inexpensive, efficient, non-
toxic, recyclable, and eco-friendly catalysts is a global reality
owing to climate, safety, and security reasons.[1] In this sense,
both the development of new Fe catalysts and the discovery
of new reactions for this metal are fundamental goals in
modern catalysis. The state-of-the-art of Fe-catalyzed reactions
is found in many reports and literature surveys,[2,3] including
Fe-catalyzed reduction processes.[4] Nowadays, it is commonly
agreed upon that Fe is a very attractive transition metal for
catalytic processes as it is virtually non-toxic, abundant, and
relative easy to handle. All of the aforementioned features of
Fe-based catalysts boost ongoing efforts from many research
groups towards new alternatives to Pd, Ru, Pt, Rh, and other
metals in catalysts.
containing ionic tags and ILs are, without doubt, a powerful
and promising combination, which usually afford outstanding
results.[10] Recently, we described the synthesis, characterization
and application of a new Fe catalyst with ionic tags (Figure 2)
as the promoter of olefin epoxidation using air or H2O2 as the
oxidizing agent.[11] Owing to the high affinity of the ionophilic
ligand and the metal center, at least ten recycling reactions
could be performed with no loss of activity.
As a result of our interest in chemical transformations in
ILs[12] and our experience with MS to investigate reaction
mechanisms,[13] we decided to investigate olefin reductions in
IL media submitted to unusual conditions, that is, under oxida-
tive conditions. Herein, we report our findings on this new re-
action catalyzed by Fe catalyst 1. During the execution of our
previously reported work on olefin epoxidation,[11] we found
that under the best conditions to perform the oxidation reac-
tion—air (oxidizing agent, 1.5 MPa), 1 (1 mol%), 908C, BMI·NTf2
(1 mL, BMI=1-n-butyl-3-methylimidazolium, NTf2 =bis(trifluor-
omethylsulfonyl)imide), 24 h—it was possible to reduce the
olefin instead of oxidizing it just by adding methanol to the re-
action medium. To the best of our knowledge, this is the first
report of Fe catalysis for this type of reaction.[14]
Ionic liquids (ILs), especially those based on the imidazolium
cation (Figure 1), play an important role for environmental ac-
Figure 1. Imidazolium-based ILs commonly used in catalysis.
[a] Dr. M. R. d. . Santos, Prof. Dr. P. A. Z. Suarez, Prof. Dr. B. A. D. Neto
Laboratory of Medicinal & Technological Chemistry
University of Brasꢀlia, Chemistry Institute (IQ-UnB)
Campus Universitꢁrio Darcy Ribeiro
ceptability.[5] For some time, these salts were regarded as
promising candidates for green and sustainable processes,[6]
and nowadays they are successfully used in many industrial
processes.[7] Their unique physicochemical properties render
these salts very attractive media to efficiently support a ple-
thora of catalysts. And, as one can expect, ILs are of special in-
terest in the use of catalysts that bear ionic tags.[8]
CEP 70904-970, P.O.Box 4478, Brasꢀlia, DF (Brazil)
Fax: (+55)61-32734149
E-mail: brenno.ipi@gmail.com
[b] A. F. Gomes, Prof. Dr. F. C. Gozzo
Institute of Chemistry
University of Campinas
CP 6154, Campinas, SP,13083-970 (Brazil)
In this context, catalysts with ionic tags are outstanding al-
ternatives to traditional systems.[9] The combination of catalyst-
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201200344.
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B. A. D. Neto et al.
under the tested conditions, which has been recently reported
elsewhere with Ru or Rh as catalysts[17a] and for some commer-
cially available Fe derivatives.[17b] The conjugated olefins proba-
bly hydrogenate faster than the isolated olefin, which may ex-
plain the quantitative yield observed for this reaction.
Recycling reactions that used methyl oleate as the model
substrate were performed, and the results are displayed in
Figure 3. After the reaction, the product could be easily sepa-
rated by decantation. If necessary, hexane was used to extract
any remaining product.
Figure 2. Fe complex 1 with ionic tags and the ionophilic ligand 2.[11]
Results and Discussion
First, we chose to investigate methyl oleate as the model com-
pound to perform the reduction[15] as the importance of hydro-
genated oils, fats, and biodiesel derivatives is well known.[12a,16]
By using 1, we were able to perform the reduction of methyl
oleate (under an oxidative atmosphere) by adding methanol to
the reaction media (Scheme 1). Other reaction conditions re-
mained the same as those previously optimized for air epoxi-
dation,[11] that is, 1.5 MPa of synthetic air at 908C with 1 mol%
of 1 for 24 h.
Methyl stearate was obtained in 83–85% yield independent
of the IL used. However, BMI·PF6 and BMI·BF4 turned dark,
which is an indication of anion degradation (HF formation).
Thus, we decided to keep BMI·NTf2 as the ionic media for this
study.
Figure 3. Recycling of 1. No loss of activity was noted after 10 runs.
Inductively coupled plasma (ICP) analysis revealed that the
use of 1 (with three ionic tags) greatly prevents catalyst leach-
ing, and as little as 2 ppm of Fe was detected in the oil phase.
This indicates the efficient anchorage of 1 in BMI·NTf2. More-
over, the activity of 1 remains unchanged even after ten runs.
It is worth remembering that 1 is used at low concentrations
(65 mm, 1 mol%) for all reactions, whereas Fe catalysts are com-
monly used up to 10 mol%. Some exceptions are also found.
For example, Chirick et al. used 0.3 mol% of Fe0 to perform hy-
drogenation reactions.[4e]
To verify the efficiency of the system in the performance of
the reduction reaction, biodiesel derived from soybean oil was
also used in a mixture of methyl esters of oleic acid (18:1), lino-
leic acid (18:2), and linolenic acid (18:3) in the presence of
1 with BMI·NTf2 as the reaction media for 24 h. The reduction
occurred almost quantitatively. It is likely that olefin isomeriza-
tion in linolenic and linoleic methyl ester derivatives occurs
To gain insight into the generality of this catalytic system, it
was applied to reduction reactions of several other substrates
from biomass, and the results are summarized in Table 1.
Scheme 1. Methyl oleate reduction catalyzed by the Fe complex 1 with imidazolium tags.
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Iron-Complex-Catalyzed Olefin Reduction
Table 1. Reduction reactions[a] under oxidative conditions by using 1.
Entry
Substrate
Product yield[b]
[%]
1
2
3
4
5
6
7
8
soybean oil
biodiesel (from soybean oil)
rapeseed oil
corn oil
sunflower oil
castor oil
oleic acid
1-hexadecene
1-decene
89
99
87
85
89
81[c]
84
81
78
9
10
11
[d]
styrene
cyclohexene
–
–
[a] Complex 1 supported in BMI·NTf2, methanol (2 mL), synthetic air
(1.5 MPa), 24 h, 908C. [b] Yields of completely hydrogenated products
only. [c] Also 11% of transesterification reaction and a partial oxidation of
the OH group in the side chain were observed. [d] Only polymerized
product.
Scheme 2. Alcohol oxidation experiments using synthetic air as the oxidant
(1.5 MPa).
Based on the obtained results, we propose the catalytic
cycle in Scheme 3. High-resolution electrospray ionization
quadrupole-time of flight mass spectrometry (ESI-QTOF-MS, in
W mode for higher accuracy and resolution) measurements
were performed to allow a better understanding of the catalyt-
ic cycle for this transformation.
Notably, the reaction occurred in the presence of multiple
double bonds, which were found in different proportions in
the tested substrates (Table S1, Supporting Information).
Reduction reactions occurred in good to excellent yields (81–
99%). The use of castor oil as the substrate resulted in a biodie-
sel derivative as a consequence of a partial transesterification
reaction, and a partial oxidation of the side-chain OH group
from ricinoleic acid was observed (Table 1, Entry 6). We believe
that the difference in solubility of the tested substrates in ILs
(biphasic catalysis) may be why almost quantitative yields were
not observed for all reactions. It is important to highlight that
we have recently shown that the difference in the solubility of
reagents and products in ILs can modulate the selectivity of
the hydrogenation of polyunsaturated substrates.[12a,d] The use
of styrene (Table 1, entry 10) resulted only in the polymerized
product (almost quantitative) as observed without methanol.[11]
The use of volatile olefins (cyclohexane) produced no result
(Table 1, entry 11). However, heavier olefins (Table 1, entries 8
and 9) gave good results.
First, oxygen oxidizes the Fe center from FeIII to a high-
valent state. In the presence of methanol, the oxidized com-
plex dissociates and an alcohol molecule coordinates to form I
(Scheme 3). At this stage, the hydrogen slated for abstraction
is transferred to the metal center and the oxidation of the alco-
hol occurs to result in the metal hydride II. This step, I!II,
may be a result of the rearrangement of an initially formed
iron-oxo complex from alcohol coordination, which is similar
to a previously proposed transition state with hydrogen migra-
tion in catalytic asymmetric epoxidations with chiral Fe por-
phyrins.[20] After that, olefin coordination followed by hydride
transfer leads to III. Then, another methanol molecule coordi-
nates. Species IV then undergoes substrate protonation and
eliminates the reduced substrate, which restores the catalyti-
cally active species I.
To gain insight into the mechanism of this new reaction,
some experiments were performed in the presence and ab-
sence and of the olefin (methyl oleate). Initially, two alcohols
were tested: methanol and isopropanol. In the absence of
olefin, the catalytic system oxidized methanol to a mixture of
formaldehyde and traces of formic acid, which reduced the
metal center, as shown by the MS experiments. Isopropanol
was also oxidized to acetone, but in insignificant amounts
(Scheme 2).
Intentionally, we have briefly discussed the metal oxidation
state in the proposed catalytic cycle. Nevertheless, some im-
portant insights must be highlighted:
(i) Methanol seems to be the best alcohol. Primary alcohols
such as ethanol or n-butanol can also be used (lower
yields were observed), but not as efficiently as methanol.
(ii) The reaction does not occur without synthetic air in the
reactor (1.5 MPa). This indicates that it is necessary to
force the oxidation (by O2) of the metal center (high-
valent Fe) to form the metal hydride species through the
oxidation of the primary alcohol.
The use of methanol in the presence of methyl oleate af-
forded the reduced compound and formaldehyde (detected by
GC). Isopropanol gave poor results (Scheme 2). The obtained
results were expected; it has been described that steric hin-
drance, owing to the presence of different ligands, prevents
the coordination of branched alcohols (such as isopropanol) to
the cationic Fe center[18] and that less hindered alcohols such
as methanol are capable of coordination. Structurally different
ligands have shown a similar steric hindrance effect.[18]
(iii) As a consequence, a high-valent Fe center is necessary to
facilitate the formation of both I and II as it is an electron-
deficient Fe center.[21] Probably, a mixture of FeIV and FeV
species are involved in the cycle. Additionally, it has been
proposed that high-valent FeIV cations with electron-defi-
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B. A. D. Neto et al.
Scheme 3. Proposed catalytic cycle for olefin reduction catalyzed by 1 in the presence of methanol. Major ions detected in ESI(+)-QTOF-MS experiments are
indicated by their m/z values, and propositions for their origins from intermediates of the catalytic cycle are shown.
cient porphyrin ligands may exhibit a greater propensity
for electron transfer to an olefin.[22]
ences. Overall, we observed very similar results, but the intensi-
ties of the signals in the MS spectra showed slight changes,
which allowed us to select the most intense signals to perform
structural characterization by ESI-QTOF-MS/MS.
(iv) It was proposed for thiolate-ligated enzymes (cytochrome
family) that electron donation from the ligand to the
metal center (Fe) increases the basicity of the iron-oxo
group, which thereby allows a hydrogen atom to be ab-
stracted from CꢀH bonds.[23] This hypothesis is very impor-
tant, mostly because it helps to explain the high reactivity
of cytochrome P-450 in CꢀH bond activation reactions.[24]
In our case, supposedly, this logical increase in the Lewis
acidic character of the high-valent Fe species, facilitates
the hydride transfer to form II.
We were able to detect a high-valent FeV species with three
ionophilic ligands that also contains a metalꢀhydride bond (m/
z=780) formed by air oxidation. Representative ESI(+)-QTOF
mass spectra are shown in Figure 4. Common losses such as of
HCl and the ligand could be observed in the product ion spec-
trum (Figure 4a). The characteristic isotope pattern and exact
mass measurement (Figure 4b and c) point firmly to the inter-
ception of a derivative of intermediate II (Scheme 3). Thus, it is
expected that II can form as shown in the equilibrium de-
scribed in Scheme 3.
MS experiments produced outstanding results to support
most of these aforementioned insights and the proposed cata-
lytic cycle. By monitoring the reaction in a methanolic solution
by on-line direct infusion ESI(+)-QTOF-MS, we were able to
detect some interesting ions, which were then structurally
characterized by collision-induced dissociation (CID) product
ion spectrum experiments (ESI-QTOF-MS/MS). It is important to
remember that it is necessary to force the oxidation of the FeIII
to form a high-valent metal center (FeIV or FeV) and to allow
the olefin reduction to take place. Otherwise, the reaction
does not occur. In many reports, the oxidation of the metal is
forced with air (or pure O2) or with H2O2.[2–4] In this sense, we
have studied these two oxidizing agents to evaluate the differ-
Other important species have been detected and character-
ized by product ion spectrum experiments. We have also de-
tected a FeIII species (Scheme 3, m/z=328, Figure S1) from the
alcohol oxidation as discussed above. Naturally, the presence
of O2 in the reaction mixture (or other oxidizing agents) forces
the Fe to oxidize again. The species that gives rise to the peak
at m/z=328 is similar to I as proposed and shown in
Scheme 1. Features such as the isotopic pattern of the signal,
its low m/z error and its fragmentation (Figure S1) all corrobo-
rate the proposed structure. A neutral loss of a radical methoxy
C
group (CH3O , 31 Da) occurs to form an ion of m/z=297.
A fragment ion of m/z=206 (Scheme 3) is attributed as charac-
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Iron-Complex-Catalyzed Olefin Reduction
Figure 4. a) ESI(+)-QTOF product ion spectrum of m/z=780 from a reaction mixture of 1 dissolved in methanol and heated (open system). b) Simulated iso-
tope pattern for [C30H31Cl3FeN6O9]+. c) Expansion between m/z=777–788 of the spectrum (calcd. m/z=780.0536; found m/z=780.0543, error +0.9 ppm).
teristic of the ionophilic ligand and detected as
a cation radical species in accordance with a radical
loss. Notably, without the presence of an oxidizing
agent, such as H2O2 (or by heating the alcoholic solu-
tion in air to force the oxidation), no metal hydride
signal has been found. Nevertheless, in the presence
of any of those oxidants, we detected signals that
corroborate the presence of a FeꢀH bond and
a high-valent Fe center (Figures 4 and 5).
In the presence of any of the oxidizing agents, an
ion of m/z=538 was detected and structurally char-
acterized by ESI(+)-QTOF-MS/MS (Figure S2). The ion
was attributed to a FeIII species bound to two li-
gands. This ion can be formed by the loss of metha-
nol from I (Scheme 3). The isotope pattern for the
ion of m/z=538 is in excellent agreement with that
of the simulated spectrum for [C20H20Cl2FeN4O6]+
(Figure S2). The product ion of m/z=538 shows
a loss of HCl to form a carbene ion of m/z=502
(Scheme 3, Figure S3). As already described, carbene
formation can be almost completely inhibited in the
presence of protic solvents such as methanol.[25] This
ion was not found in solution, but only as a result of
direct HCl loss in the collision-induced dissociation in
the gas phase, which has been previously demon-
strated for imidazolium derivatives.[26]
Figure 5. ESI(+)-QTOF mass spectra demonstrating an isotope pattern shift resulting
from the incorporation of deuterium atoms to ions of m/z=328 and 780, which result
from reactions performed in CD3OD compared to those in CH3OH. Expansion around m/
z=328 for reactions performed in a) CH3OH and b) CD3OD. Inset in (b) shows an expan-
sion around the m/z=331 ion (C11H10D3ClFeN2O4+). Expansion around m/z=780 for reac-
tions performed in c) CH3OH and d) CD3OD.
All of these results indicate that some parallel re-
actions occur simultaneously, such as: (i) direct alco-
hol oxidation (to form FeII) and (ii) the reaction pro-
posed in Scheme 3; both of which include alcohol
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B. A. D. Neto et al.
oxidation and seem to be favored under the developed condi-
tions, otherwise we would not observe good yields of the re-
duced olefin.
Keywords: ionic liquids · isotopic labeling · iron · mass
spectrometry · reduction
As observed by all ESI(+)-QTOF product ion spectra, the pro-
posed mechanism is acceptable but may be more complex
than those five steps suggested. Envisaging that the detected
transient intermediates of m/z=328 (Scheme 3, Figure 5) and
m/z=780 (Scheme 3, Figure 4) contain atoms that are indeed
derived from the methanol present in the reaction mixture, we
decided to perform the same on-line monitoring with CD3OD
(99.96%) instead of CH3OH to confirm the proposed species
and to guarantee these species are not artifacts. Isotopic label-
ing proved to be a powerful and elegant tool in MS.[27]
Thus, by repeating the reaction with CD3OD instead of
CH3OH, isotopomeric ions of m/z=331 (three deuterium
atoms) were analogously monitored and characterized (Fig-
ure 5a and b). This elegant experiment demonstrated that the
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·
C
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Conclusions
We have described a new reduction reaction catalyzed by a Fe
complex with ionic tags, which acts as a pre-catalyst to form
the active species in situ. The optimized reaction resulted in
good to excellent yields of the reduced products. Ten recycling
reactions could be conducted with no loss of activity. Based on
experiments and MS measurements, a catalytic cycle that in-
cludes a high-valent Fe was proposed. All results point towards
the proposed catalytic cycle, but they also indicate that the re-
action may be more complex than shown here. Nevertheless,
the isotopic labeling experiments are in agreement with the
proposed species, which indicate that the proposed catalytic
cycle is appropriate. Finally, this new Fe-catalyzed reaction has
the potential to open a new avenue of possibilities towards
greener and sustainable processes, and at the same time
boosting the development of Fe chemistry.
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Experimental Section
The Supporting Information contains full experimental details and
product characterizations.
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Acknowledgements
We acknowledge CAPES, CNPq, FAPDF, DPP-UnB, FAPESP, and
INCT-Catalysis for partial financial support.
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ChemSusChem 2012, 5, 2383 – 2389

Iron-Complex-Catalyzed Olefin Reduction
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and the substrate was recovered with almost no loss.
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Received: May 17, 2012
Revised: June 15, 2012
Published online on November 8, 2012
ChemSusChem 2012, 5, 2383 – 2389
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
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