The use of ultrasmall iron(0) nanoparticles as catalysts for the selective hydrogenation of unsaturated C-C bonds

Article abstract of DOI:10.1039/c3cc00152k

The performance of well-defined ultrasmall iron(0) nanoparticles (NPs) as catalysts for the selective hydrogenation of unsaturated C-C and CX bonds is reported. Monodisperse iron nanoparticles of about 2 nm size are synthesized by the decomposition of {Fe(N[Si(CH₃)₃]₂) ₂}₂ under dihydrogen. They are found to be active for the hydrogenation of various alkenes and alkynes under mild conditions and weakly active for CO bond hydrogenation.

Full text of DOI:10.1039/c3cc00152k

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The use of ultrasmall iron(0) nanoparticles as catalysts
for the selective hydrogenation of unsaturated C–C
bonds†
Vinciane Kelsen,a Bianca Wendt,b Svenja Werkmeister,b Kathrin Junge,b
Matthias Bellerb and Bruno Chaudret*a
Cite this: Chem. Commun., 2013,
49, 3416
Received 7th January 2013,
Accepted 5th March 2013
DOI: 10.1039/c3cc00152k
www.rsc.org/chemcomm
The performance of well-defined ultrasmall iron(0) nanoparticles method involves the use of a Grignard reagent like EtMgCl to
(NPs) as catalysts for the selective hydrogenation of unsaturated reduce the starting iron complex FeCl3. Complementary to all
Q
C–C and C X bonds is reported. Monodisperse iron nanoparticles these interesting developments, in the present work, we introduce
of about 2 nm size are synthesized by the decomposition of the use of ultrasmall iron(0) nanoparticles synthesized by a
{Fe(N[Si(CH3)3]2)2}2 under dihydrogen. They are found to be active method described previously by some of us10 as catalysts for
for the hydrogenation of various alkenes and alkynes under mild various hydrogenation reactions. Our synthetic protocol leads to
Q
conditions and weakly active for C O bond hydrogenation.
well-defined soluble nanoparticles and allows in particular the
determination of the number of surface hydrides, following a
The development and application of environmentally benign non- method previously reported for ruthenium nanoparticles.11
noble metal complexes constitutes an actual and important topic
The synthesis of ultrasmall iron(0) nanoparticles proceeds via
in catalysis. In this respect, iron-based catalysts are of large decomposition of {Fe(N[Si(CH3)3]2)2}2 under 3 bar of dihydrogen
interest because of their low toxicity and the abundance of iron at 150 1C for 12 h. In this process HN[Si(CH3)3]2 (HMDS) is
on Earth. Moreover, nanometric-sized materials show promising released and may further interact with the particles. This method
potential in catalysis because of the high ratio of atoms present at provides monodisperse iron nanoparticles of 1.5 Æ 0.2 nm size
the surface and accessible for the catalytic reaction. Although (Fig. 1). The formation of air-sensitive metallic iron nanoparticles
10
¨
iron-based NPs have been used for different applications, their was ascertained by WAXS, SQUID and Mossbauer analysis.
catalytic potential has not yet been fully exploited.1 More specifi- HR-TEM analysis evidenced the amorphous nature of these
cally, it has been shown that they display significant activity and nanoparticles. In addition, we performed the titration of hydrides
selectivity in Fischer–Tropsch synthesis.2 Iron nanoparticles are present at the surface of the nanoparticles in order to study their
also widely used in the synthesis of carbon nanotubes3 and in surface state and evaluate whether they could be interesting for
biomedical applications such as hyperthermia agents4 and con- applications in catalysis. The general procedure for the quantifi-
trast agents for magnetic resonance imaging.5 Recently, the use of cation of hydrogen atoms adsorbed onto the surface of iron
iron nanoparticles as catalysts for hydrogenation reactions has nanoparticles consists of the titration of surface hydrides through
also emerged. Hence, iron–iron oxide core–shell nanoparticles the hydrogenation of 2-norbornene.11,12 With the percentage of
have been described for olefin and alkyne hydrogenation.6 Iron conversion of olefin into alkane and the estimated percentage of
nanoparticles supported on chemically-derived graphene dis- surface atoms on the nanoparticles (almost 60% for nanoparticles
played activity also in alkene hydrogenation.7 Notably, Morris of 1.5 nm), the number of hydrides per surface atom iron was
and co-workers described the application of iron nanoparticles,
supposed to be formed during the catalysis, in the asymmetric
transfer hydrogenation of ketones.8 Furthermore, a general
method for the synthesis of soluble iron nanoparticles active in
alkene and alkyne hydrogenation has been reported.9 This latter
a
´
Laboratoire de Physique et Chimie des Nano Objets, INSA, Universite de Toulouse, 135,
avenue de Rangueil, F-31077 Toulouse, France. E-mail: chaudret@insa-toulouse.fr
b
¨
¨
Leibniz-Institut fur Katalyse an der Universitat Rostock e. V., Albert-Einstein-Str. 29a,
18059 Rostock, Germany. E-mail: matthias.beller@catalysis.de
† Electronic supplementary information (ESI) available: Experimental procedure
for the hydride titration, WAXS analysis and GC chromatograms. See DOI:
10.1039/c3cc00152k
Fig. 1 Low magnification TEM image of iron nanoparticles.
c
3416 Chem. Commun., 2013, 49, 3416--3418
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Table 1 Investigation of the reactivity of ultrasmall iron(0) nanoparticles as
catalysts for the hydrogenation of different functional groupsa
Table 2 Iron nanoparticle-catalyzed reduction of alkenesa
Conv.b (%)
Yieldb (%)
Entry Substrate
Conv.b (%) Yieldb (%)
Entry
Substrate
1
2
3
4
5
6
1-Octene
>99
>99
>99
1
2
3
4
5
6
1-Octene
>99 (>99)c
0
>99
>99
>99
>99 (>99)c
Phenylacetylene
Acetophenone
2-Butanone
Benzaldehyde
N-(1-Phenylethylidene)aniline
89 (ethylbenzene)
Trans-4-octene
1,7-Octadiene
2-Methyl-1-heptene
Trans-2-pentene
Styrene
0
>99
>99
95
4 (4)c
2 (2)c
7 (31)c
0 (0)c
7 (31)c
0 (0)c
0 (16)c
0 (11)c
>99
87
7
8
9
10
11
12
3-Chlorostyrene
3-(Trifluoromethyl)styrene
Trans-stilbene
Cyclohexene
Cyclopentene
2-Norbornene
>99
>99
>99
>99
98
97
a
Reaction conditions: 1 mmol substrate, 2.4 mol% iron(0) nano-
b
particles, 1 mL mesitylene, 10 bar H2, r.t., 20 h. Determined by GC
using n-hexadecane as internal standard. Values in brackets are
obtained by increasing the temperature to 80 1C.
>99
>99
>99
>99 (>99)c
c
>99
>99 (>99)c
a
Reaction conditions: 1 mmol substrate, 2.4 mol% iron(0) nano-
estimated to be between 0.4 and 0.6. Notably, this is the first
direct evidence for the presence of surface hydrides on iron
nanoparticles and hence demonstrates the absence of surface
oxidation in the present case.12
b
particles, 1 mL mesitylene, 10 bar H2, r.t., 20 h. Determined by GC
using n-hexadecane as internal standard. Values in brackets are
obtained by using 0.5 mol% of catalyst.
c
Next, the catalytic reactivity of the iron nanoparticles was
investigated for the hydrogenation of several functional groups.
As shown in Table 1, alkenes, alkynes, ketones, aldehydes and
imines were explored. In general in this study, 1 mmol of
substrate, 2.4 mol% of iron nanoparticles and 1 mL of mesity-
lene were mixed together in an autoclave under an inert atmos-
phere. Then, the autoclave was pressurized with 10 bar of H2 and
stirred at room temperature for 20 hours. Under these condi-
tions excellent yields for the hydrogenation of 1-octene and
phenylacetylene (Table 1, entries 1 and 2) were obtained. Iso-
merization does not occur in the case of 1-octene. Phenyl-
acetylene was totally converted into ethylbenzene, while no
hydrogenation of the phenyl ring was observed. In addition,
the iron nanoparticles displayed a weak activity for the hydro-
genation of ketones (Table 1, entries 3 and 4). Yields below 10%
were obtained for acetophenone and 2-butanone at room
temperature. However, the yield of 2-butanone increased to
31% at a higher temperature (80 1C), while keeping the same
reaction time. Ketone hydrogenation reactions have recently
Table 3 Reduction of alkynes catalyzed by ultrasmall iron(0) nanoparticlesa
Entry Substrate
Conv.b (%) Yieldb (%)
1
2
3
4
5
6
7
8
1-Octyne
3-Octyne
Phenylacetylene
4-Phenyl-1-butyne
4-Fluorophenylacetylene
4-Methoxyphenylacetylene >99
4-Acetylphenylacetylene
3-Hydroxyphenylacetylene
>99
0
>99
>99
>99
>99
0
89
>99
>99
>99
90
>99
>99
12 (3-ethylphenol)
88 (3-vinylphenol)
>99
9
Diphenylacetylene
>99
a
Reaction conditions: 1 mmol substrate, 2.4 mol% iron(0) nano-
b
particles, 1 mL mesitylene, 10 bar H2, r.t., 20 h. Determined by GC
using n-hexadecane as internal standard.
been found to be catalyzed by iron nanoparticles8 but the occurred in the case of 4-octene. We explain this difference in
mechanism is proposed to involve a hydrogen transfer in this reactivity by the increased steric hindrance of 4-octene, which
case. However, our nanoparticles were inactive in the hydro- displays a trans configuration. In the case of 2-pentene, a mixture
genation of aldehydes and imines (Table 1, entries 5 and 6). Only of cis/trans olefins was used, displaying a lower steric hindrance
a small amount of the imine was converted to amine at 80 1C.
in comparison with 4-octene. On the other hand, the more
Due to the activity in the hydrogenation of 1-octene and reactive trans-stilbene was totally converted into the corres-
phenylacetylene various other alkenes and alkynes were tested ponding diphenyl ethane (Table 2, entry 9). In the case of iron
in the presence of the ultrasmall iron particles. The scope of the nanoparticles synthesized with the Grignard reagent,9b a tem-
catalyst is presented for both alkenes and alkynes in Tables 2 perature of 100 1C was necessary to reach a full conversion. In
and 3. For this study, we generally used 2.4 mol% of catalyst, our case, the reduction proceeded smoothly at room tempera-
although even 0.5 mol% of catalyst was sufficient for the ture. We explain the increased reactivity of this internal olefin by
1-octene and 2-norbornene hydrogenation (vide infra).
the coordination of the aromatic rings of stilbene to the easily
Concerning the hydrogenation of alkenes, terminal aliphatic accessible surface of the small nanoparticles. Similar to stilbene,
alkenes (Table 2, entries 1, 3 and 4) were completely hydro- styrene showed good reactivity and is converted to ethylbenzene
genated at room temperature. In the case of 1,7-octadiene both in excellent yield (Table 2, entry 6). The hydrogenation of the
double bonds are reduced under these conditions. For internal double bond occurred even with halide-substituted styrenes
aliphatic olefins (Table 2, entries 2 and 5), a peculiar result was (Table 2, entries 7 and 8). Here, no reduction of the C–X bond
observed: while 2-pentene was fully hydrogenated, no reaction or the phenyl ring was observed. Concerning cyclic olefins
c
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(Table 2, entries 10–12), the hydrogenation of the C–C double hydrogenation of some internal C–C double and triple bonds
bond was complete giving cyclic alkanes in high yields without whereas our nanoparticles are not good candidates for this
increasing the temperature of the reaction. The reactivity of our reaction. However, the ultrasmall nanoparticles can completely
iron nanoparticles was further tested by decreasing the loading hydrogenate cyclic olefins and trans-stilbene at room temperature
of catalyst to 0.5 mol% (Table 2, entries 1 and 12). Gratifyingly, while in the works of De Vries significantly higher temperature of
full conversion of 1-octene and 2-norbornene was achieved 100 1C is needed.9b,c Concerning the hydrogenation of styrene and
under these conditions proving the efficiency of iron nano- its derivatives, in the work of Jacobi von Wangelin, a H2 pressure
particles as catalysts for hydrogenation reactions. Lowering the of 30 bar is needed to avoid the competitive styrene polymeriza-
pressure to 3 bars H2, almost full conversion of 1-octene was tion reaction.9d Under our experimental conditions, i.e. at 10 bar
observed after 20 h at 2.4% Fe loading but only 5% conversion at of H2, no polymerization occurred and styrenes are selectively
1% Fe loading. This result nevertheless demonstrates the ability converted into the corresponding ethylbenzenes.
of this catalyst to work under very mild conditions.
Partial hydrogenation of 3-hydroxyphenylacetylene proved
The hydrogenation of different alkynes was achieved by apply- the possibility of modulating the hydrogenation properties of
ing the same reaction conditions (1 mmol substrate, 2.4 mol% of the nanoparticles presumably through the coordination of the
catalyst, 1 mL mesitylene, 10 bar H2, r.t., 20 h; Table 3). Except hydroxo group. Interestingly, our nanoparticles are very selective
for 3-hydroxyphenylacetylene, in all the tested substrates the for the hydrogenation of terminal alkenes and alkynes. They are
C–C triple bond was selectively reduced to give the corres- inactive for internal olefins except for aryl substituted ones. This
ponding alkanes. Similarly to octenes, the terminal aliphatic may result from the steric bulk at the surface of the nano-
alkyne (1-octyne) (Table 3, entry 1) was fully converted into particles arising from the presence of amido groups or from
octane with high yield, whereas no hydrogenation was observed HMDS. Phenyl rings may compete for coordination and allow
for the internal one (3-octyne) (Table 3, entry 2). Diphenyl- the hydrogenation of aryl substituted species. Finally, the nano-
acetylene (Table 3, entry 9) was also successfully hydrogenated as particles prepared by our method also display a weak activity in
well as stilbene in the case of alkenes. Moreover, different sub- the direct hydrogenation of CQO bonds which further demon-
stituted phenylacetylenes were tested (Table 3, entries 3 and 5–8). In strates the catalytic potential of these nanoparticles.
each case up to 99% conversion was obtained. High yields
It should be noted that our ultrasmall iron(0) nanoparticles
of alkanes were achieved with phenylacetylenes bearing halide, are good models for the intrinsic reactivity of iron because they
methoxy- or carbonyl groups (Table 3, entries 5–7). However, in the are well-defined and do not present oxides at their surface.
case of 3-hydroxyphenylacetylene, partial hydrogenation was Further work is in progress to enlarge the scope of catalytic
observed yielding 12% of 3-ethylphenol and 88% of vinylphenol reactions and to dope iron nanoparticles with other metals.
(Table 3, entry 8). Interestingly, De Vries et al. reported the need for
Notes and references
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´
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c
3418 Chem. Commun., 2013, 49, 3416--3418
This journal is The Royal Society of Chemistry 2013