NHC-stabilized ruthenium nanoparticles as new catalysts for the hydrogenation of aromatics

Article abstract of DOI:10.1039/c2cy20561k

The application of ruthenium nanoparticles (RuNPs) stabilized by the N-heterocyclic carbenes (NHC) N,N′-di(tert-butyl)imidazol-2-ylidene (I^tBu) and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene IPr as catalysts in the hydrogenation of several substrates is reported under various reaction conditions (solvent, substrate concentration, substrate/metal ratio, temperature). The RuNHC nanoparticles are active catalysts in the hydrogenation of aromatics and show an interesting ligand effect, RuIPr NPs being generally more active than RuI^tBu. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cy20561k

Catalysis
Science&Technology
www.rsc.org/catalysis
Volume 3 | Number 1 | January 2013 | Pages 1–252
ISSN 2044-4753
COVER ARTICLE
van Leeuwen et al.
NHC-stabilized ruthenium nanoparticles as new catalysts for the
hydrogenation of aromatics
2044-4753(2013)3:1;1-C

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NHC-stabilized ruthenium nanoparticles as new
catalysts for the hydrogenation of aromatics†
Cite this: Catal. Sci. Technol.,
2013, 3, 99
a
bc
David Gonzalez-Galvez,z Patricia Lara,z Orestes Rivada-Wheelaghan,d
Salvador Conejero,d Bruno Chaudret,e Karine Philippotbc and
Piet W. N. M. van Leeuwen*a
The application of ruthenium nanoparticles (RuNPs) stabilized by the N-heterocyclic carbenes (NHC) N,N0-di(tert-
butyl)imidazol-2-ylidene (ItBu) and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene IPr as catalysts in the
hydrogenation of several substrates is reported under various reaction conditions (solvent, substrate con-
centration, substrate/metal ratio, temperature). The RuNHC nanoparticles are active catalysts in the hydro-
genation of aromatics and show an interesting ligand effect, RuIPr NPs being generally more active than RuItBu.
Received 9th August 2012,
Accepted 11th October 2012
DOI: 10.1039/c2cy20561k
www.rsc.org/catalysis
efforts have been made in the synthesis of ligand-stabilized
nanoparticles to achieve control of their properties. 20 years
Introduction
Metal nanoparticles (MNPs) are widely used nowadays in very
different research domains, such as optoelectronics, sensing,
medicine, and catalysis.1–6 In particular the investigation of
nanoparticles in catalysis is receiving an ever-increasing interest
because MNPs present an efficient combination of the advantages
of heterogeneous and homogeneous catalysts such as their small
size and particular electronic configuration that make them
extremely active catalysts. In comparison with traditional mole-
cular complexes, they may catalyze reactions in which molecular
systems are less active, such as arene hydrogenation,7 and they are
extensively applied by the international scientific community for
C–C coupling8,9 and hydrogenation reactions.10–14
Metal nanoparticles may be stabilized by the addition of
polymers, surfactants or ligands which allows controlling of
their size, shape and dispersion and also their surface state.
Consequently, an adequate choice of the stabilizer should allow
the tuning of the surface properties and, therefore, the catalytic
reactivity (activity and/or selectivity). This ligand influence has
been recently examined by several research groups.15 Significant
ago, some of us started using a new strategy for the synthesis of
metal NPs that involves the decomposition of an organometallic
precursor under mild conditions of pressure and temperature.
This methodology provides well-controlled NPs regarding size,
dispersion, and shape, the control of which depends on the
ligands used for their stabilization.16,17
RuNPs are interesting candidates to be applied in catalytic
processes since (i) they can be characterized by different techni-
ques including spectroscopic ones, NMR (due to the absence of a
Knight shift) and IR that give information on their surface state
and (ii) they are very active catalysts in arene hydrogenation. We
have recently described for the first time the synthesis of N-hetero-
cyclic carbenes (NHC)-stabilized RuNPs using the organometallic
approach.18 The use of 13C labeled carbenes and also the addition
of 13CO to the prepared NPs allowed us to prove the strong binding
of the carbenes to the surface of the particles and also to
demonstrate the influence of the structure of the carbenes on
their location (mainly due to steric effects) and on their reactivity
towards styrene hydrogenation.
In the present work, we aim to demonstrate the influence of
the nature of NHC ligands attached to the surface of NPs on
their catalytic activity in the hydrogenation of different sub-
strates, extending the work recently reported with roof-shaped
phosphine stabilized RuNPs.19
a
¨
Institute of Chemical Research of Catalonia (ICIQ), Av. Paısos Catalans 16,
43007-Tarragona, Spain. E-mail: pvanleeuwen@iciq.es
b Laboratoire de Chimie de Coordination, CNRS, LCC, 205 Route de Narbonne,
F-31077 Toulouse, France
c
´
Universite de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France
d
´
Instituto de Investigaciones Quımicas (IIQ), CSIC and Universidad de Sevilla,
´
Avenida Americo Vespucio 49, 41092 Isla de la Cartuja, Sevilla, Spain
Results and discussion
e LPCNO, Laboratoire de Physique et Chimie de Nano-Objets,
135 Avenue de Rangueil, F-31077 Toulouse, France
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c2cy20561k
The nanoparticles were prepared following a reported proce-
dure,18 using the organometallic complex [Ru(COD)(COT)] (1,5-
cyclooctadiene)(1,3,5-cyclooctatriene)ruthenium(0) as the metal
‡ These authors contributed equally to this work.
c
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Scheme 1 N-heterocyclic carbenes and reaction conditions used for the synthesis
of RuNHCs nanoparticles.
source and the carbenes ItBu20 and IPr21 as stabilizers (Scheme 1).
Under all reaction conditions, small and monodisperse nano-
particles are formed, with a mean size of 1.7 (0.2) nm for the
particles prepared using 0.5 molar equiv. of ItBu per introduced
ruthenium and 1.7 (0.2) and 1.5 (0.2) nm for the particles prepared
using 0.2 and 0.5 molar equiv. of IPr, respectively. These nano-
materials, respectively, named colloids RuItBu0.5, RuIPr0.2 and
RuIPr0.5 (Scheme 1 and Fig. S1 in ESI†), are extremely air-sensitive
and immediately auto-ignite in the presence of oxygen.
Fig. 1 Hydrogenation of o-methylanisole (0.5 M in pentane) under 40 bar of H2
and 298 K in the presence of 0.3% Ru: RuItBu0.5 (black), RuIPr0.2 (red), RuIPr0.5
(green).
‘‘active’’ materials, as was found in our study on phosphines;19
Initially, we investigated the hydrogenation of o-methylanisole or a strong electronic effect, as ItBu is a more basic carbene, it
(Scheme 2; 1o) using the different colloids prepared under various can make the cluster surface electronically richer and avoid rich
reaction conditions (solvent, concentration, substrate/metal ratio, arenes coordination. RuIPr0.2 was found to exhibit the highest
temperature). We chose first this substrate as a model because its activity. This is in agreement with the observed stabilization of
hydrogenation reaction has previously been studied in the the nanoparticles by the IPr ligand at low concentration and the
presence of RuNPs stabilized with N-donor ligands [aminoalcohol, fact that the faces are readily available for this colloid as
amino(oxazolines), hydroxy(oxazoline) and bis(oxazolines)],22 deduced from CO coordination reactions, while in RuIPr0.5
P-donor ligands (phosphines,19 and phosphites23,24), and surfac- both apexes and faces are covered.18
tants.25 It has also been performed in the presence of other
metal colloids such as Rh24,26,27 and Ir.24
o-Methylanisole hydrogenation was performed in various
solvents except those that can be hydrogenated (e.g. acetone,
As can be seen in Scheme 2, this reaction can lead to a large acetonitrile, toluene, etc.) to avoid undesired reactions, and a
range of products, from the desired hydrogenation products clear solvent effect can be observed (Fig. 2 and Table 1). The
(3 for total and 5 and 6 for partial hydrogenation) to products reaction did not work in protic solvents (such as methanol) and
derived from hydrogenolysis (2, 7 and 8). However, since chiral worked better in apolar solvents (such as pentane) than in polar
products are produced when complete and partial hydrogenation non-protic solvents (such as THF). A similar tendency has
occurs, this reaction represents a good target for studying asym- previously been reported with diphosphite-stabilized RuNPs
metric hydrogenation as well.
that showed a better reactivity in pentane than in THF in the
All colloids were tested in the hydrogenation of o-methylanisole, hydrogenation of methyl anisole (o and m).23 Especially remark-
1o (Fig. 1 and Table 1), and all show a different behavior. As can be able is the activity when assaying the reaction under neat condi-
seen in the graph, the sample containing RuItBu NPs, as a tions; despite the fact that the reactant polarity is similar to that of
stabilizing agent, shows a very low rate compared to samples THF, high rates are obtained. This could be due to a concentration
containing RuIPr NPs. The low activity of RuItBu0.5 compared to effect and substrate vs. solvent competitive coordination on the
RuIPr NPs is surprising. Since excess ligand (0.5 equiv.) is necessary surface.
to stabilize these particles, it may arise from a high surface coverage
Pressure and temperature effects were also studied. Hydrogen
when ItBu is used as the ligand. This does not prevent CO from pressure affects the rate of the reaction, but not the selectivity
accessing the particle as previously described18 but may hinder the (Table 1, entries 7 and 13). On the other hand, temperature has a
coordination of arenes on the surface. Alternative explanations strong effect on rates and selectivity (Fig. 3 and Table 1). The rate
could be that aryl groups are needed in the ligands to obtain increases with temperature, achieving a maximum at approximately
Scheme 2 Methylanisoles, 1o and 1m, and o-cresol, 2o, hydrogenation.
c
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Table 1 Hydrogenation of o-methylanisole (in the presence of solvent: 0.5 M and 0.3% of Ru; neat: 0.1% of Ru)
Yield (%)
Catalyst
Solvent
t/h
T/K
Conv. (%)
3o (de)a
5o + 6o (ratio)
TONb
TOFc
1
2
3
4
5
6
7
8
RuItBu0.5
RuIPr0.2
RuIPr0.5
RuIPr0.2
RuIPr0.2
RuIPr0.2
RuIPr0.2
RuIPr0.2
RuIPr0.2
RuIPr0.2
RuIPr0.2
RuIPr0.2
RuIPr0.2 d
Pentane
Pentane
Pentane
THF
THF
MeOH
—
—
—
—
—
16
16
16
16
16
16
298
298
298
298
353
298
298
323
353
373
393
393–353
298
12
72
61
25
84
0
42
79
98
92
89
43
13
9 (90)
69 (88)
57 (88)
20 (93)
75 (86)
0
40 (89)
75 (88)
95 (85)
85 (87)
79 (87)
24 (90)
10 (90)
2 (57 : 43)
2 (57 : 43)
2 (55 : 45)
5 (61 : 39)
7 (57 : 43)
0
2 (56 : 44)
3 (54 : 46)
1 (56 : 44)
6 (56 : 44)
8 (56 : 44)
6 (57 : 43)
2 (57 : 43)
46
579
613
188
714
3 (9)
36 (86)
38 (73)
12 (38)
45 (163)
0
0
2.15
1163
2176
2771
2458
2303
717
541 (611–616)
1012 (1696)
1289 (5052)
1143 (4052)
1071 (4084)
333 (732)
135 (140)
2.15
2.15
2.15
2.15
2.15
2.15
9
10
11
12
13
—
—
310
a
b
Diastereomeric excess between the cis and trans product, cis is always the major one. Turnover calculated considering mol of H2 consumed per
mol of total Ru. TOF = mol H2 mol Ruꢀ1 ꢀ1. In brackets, initial TOF calculated after 15 min of reaction. p(H2) = 8 bar.
h
c
d
353 K. Nevertheless, the reaction slows at temperatures above
373 K. To check the stability of the colloids with temperature,
the RuNPs were heated at 393 K for one hour, cooled to 353 K
and then pressurized with dihydrogen. Under these conditions,
a much lower rate was observed. These results are consistent
with deactivation of the colloids at temperatures higher than
373 K. Interestingly, although the rate is lower at high tem-
peratures, the selectivity towards partial hydrogenation is
higher. At 393 K and 20% conversion, over 60% selectivity
towards partially hydrogenated products (5o and 6o) was
obtained. When performing the reaction at 353 K and using
THF as the solvent, also an increased selectivity towards
alkenes was observed (Table 1, entries 4 and 5).
o-Cresol, 2o, o-methylanisole 1o and m-methylanisole 1m
hydrogenations were studied under several conditions (Table 2).
The following conclusions can be extracted from the obtained
results: (i) the activity of NHC-stabilized RuNPs is very similar to
that reported for phosphine-stabilized ones.19 (ii) These materials
hydrogenate phenol derivatives with very good activity. (iii) THF is
Fig. 2 Hydrogenation of o-methylanisole (0.5 M in solvents) under 40 bar of H2
and 298 K in the presence of RuIPr0.2 (0.3% Ru): pentane (black), THF (red),
methanol (blue), no-solvent (green, 0.1% Ru).
Table 2 Hydrogenation of benzene derivatives (0.5 M in THF) under 40 bar of H2
and 298 K in the presence of RuIPr0.2 after 16 h
Yield (%)
Ru
Conv.
3 +
3/4 (de) 4/8 TONa hꢀ1
TOFb/
Substrate Solvent (%) (%)
1
2
3
4
5
6
7
8
9
1o
1m
2o
1o
1m
2o
1o
1m
2o
Pentane 0.3
Pentane 0.3
Pentane 0.3
Pentane 0.03
Pentane 0.03
Pentane 0.03
100
100
100
1
9
28
100
100
100
18
22
16
100 (87)
100 (60)
99 (29)
0
0
1
1024
1001
1024
79
64
63
64
5
1 (—) o1
8 (65)
1
822
51
15 (35) 13 2410 151
100 (90)
100 (59)
THF
THF
THF
THF
THF
THF
0.3
0.3
0.3
0.03
0.03
0.03
0
0
1024
1001
64
63
63
99 (31) o1 1005
10 1o
11 1m
12 2o
15 (92)
21 (60)
14 (35)
3
1
2
1746 109
2163 135
1584
99
a
Fig. 3 Hydrogenation of o-methylanisole under 40 bar of H2 in the presence of
RuIPr0.2 (0.1% Ru): 298 K (black), 323 K (red), 353 K (green), 373 K (dark-blue),
393 K (pale-blue), 353 K-pretreated at 393 K (pink).
Turnover calculated considering mol of H2 consumed per mol of total
Ru. TOF = mol H2 mol Ruꢀ1 hꢀ1 calculated at the end of the
experiment.
b
c
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Table 3 Hydrogenation of benzene under 40 bar of H2 and 298 K in the presence
of 0.3% Ru
Catalyst
Solvent
Conv. (%)
TONa
TOFb
1
2
3
4
RuItBu0.5
RuIPr0.2
RuIPr0.5
RuIPr0.2
—
—
66
100
93
703
954
973
44 (90)
60 (421)
61 (353)
66 (372)
—
THFc
100
1061
a
Turnover calculated considering mols of H2 consumed per mol of
total Ru after 16 h. TOF = mol H2 mol Ruꢀ1
TOF calculated after 15 min of reaction. 50% v/v mixture.
h
ꢀ1. In brackets, initial
b
c
Scheme 4 Acetophenone hydrogenation.
a better solvent for methylanisole hydrogenation, while cresol is
hydrogenated faster in pentane. (iv) m-Methylanisole, 1m, is
hydrogenated faster and with lower diastereomeric excess than
1o. (v) During hydrogenation of o-cresol, 46% of chiral ketone
8o is produced at low conversions, a result similar to the
previous work by Roucoux and co-workers with RhNPs,28 but
far from selectivity of over 99% obtained with Pd—NPs.29–31
The RuNHC colloids were also investigated in benzene
hydrogenation, and the results were similar to those obtained
in o-methylanisole hydrogenation (Table 3), i.e. RuIPr0.2 presented
the highest activity and the reaction was faster when performed
neat. The most active nanomaterial, RuIPr0.2, was also tested in
the hydrogenation of benzenes monosubstituted with electron-
withdrawing or donating groups (Scheme 3). As previously
found,32–35 substrates with electron donating groups are hydro-
genated much faster than those with electron withdrawing groups
(Table 4). Bromobenzene, 13, and benzonitrile, 15, were not
hydrogenated at all, while nitrobenzene, 16, was selectively
hydrogenated at the nitro group yielding aniline without alter-
ing the aromatic ring (no further hydrogenation is observed).
Anisole, 11, shows the highest conversion. The good conversion
of dimethylaniline, 10, demonstrates that these RuNPs are
stable, active materials in the presence of coordinating groups.
However, in the presence of 2% didodecyldisulfide (S/Ru =
14 : 1) benzene was not hydrogenated by any of these materials,
which shows the sensitivity towards certain poisons.
Acetophenone (17) was used as the substrate in order to
compare the selectivity of the hydrogenation process when
other functional groups (carbonyls) are present. As shown in
Scheme 4 three possible reaction products can be formed: if the
aromatic ring is first hydrogenated, cyclohexylmethylketone,
18, is obtained, while chiral (racemic) phenylethanol, 19, would
be formed if the carbonyl group is hydrogenated first. Further
hydrogenation of either 18 or 19 would finally produce cyclo-
hexylethanol, 20. In addition, hydrogenolysis products can be
obtained, such as ethylbenzene and ethylcyclohexane.
The chemoselective reduction of aromatic ketones to benzyl-
alcohols can be carried out effectively by direct hydrogenation and
hydrogen transfer reactions, both employing homogeneous cata-
lysts,36 but the selective hydrogenation of the aromatic ring is more
complicated. This reaction has been studied in detail,28,32–34,37–51
usually producing a mixture of the three products when hetero-
geneous catalysts are used and the reaction is not complete.
Scheme 3 Hydrogenation of benzene derivatives.
Table 4 Hydrogenation of benzene derivatives (0.5 M in THF) under 40 bar of H2
and 298 K in the presence of 1% Ru after 1.5 h
Substrate
Xa (%)
Xb (%)
TONa
TOFb
1
2
3
4
5
6
7
10
11
12
13
14
15
16c
20
62
9
0
3.4
0
0
2
0
0
0.4
0
0
63
175
35
0
12
0
42
114
24
0
8
0
0
0
0
a
Turnover calculated considering mols of H2 consumed per mol of
total Ru. TOF = mol H2 mol Ruꢀ1 hꢀ1 calculated at the end of the
experiment. 15% of aniline as the product.
b
Fig. 4 Hydrogenation of acetophenone at 298 K under 40 bar of H2 in the
presence of RuIPr0.2 (0.1% Ru) in pentane (blue), THF (red), and MeOH (green).
c
c
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Fig. 5 Hydrogenation of acetophenone (0.5 M in THF) at 298 K under 40 bar of H2 in the presence of RuIPr0.2. Left: activity, 1% Ru (brown), 0.3% (green), 0.1% (blue),
0.03% (red). Right: product evolution of 0.3% Ru, 17 (black), 18 (red), 19 (blue), 20 (green).
The effect of several parameters on acetophenone hydro-
The effect of substrate/catalyst ratio on the selectivity was
genation activity and selectivity was studied. The rate of hydro- also studied (Fig. 5 and ESI†). The activity was higher when
genation is much higher in THF than that in pentane or MeOH lower quantities of Ru were used. However, the conversion was
(see Fig. 4). In MeOH, hydrogenation of the aromatic ring much lower, and, independent of the ratio, the catalyst
stopped after 1 h of reaction time, and ketone hydrogenation degraded with reaction time and favored ketone hydrogenation
after 6 h; very similar results were obtained in EtOH and iPrOH, over arene hydrogenation. As we can observe in the selectivity
which shows that the material is not active in alcohols with graphs, selectivities of up to 80% were obtained using only
protons in the a position (2o hydrogenation means that it is 0.03% of total Ru, but with conversions under 50%. On the
stable in front of alcohols), this may be due to decomposition of other hand, at higher Ru loading (0.1–0.3%), over 60% of 18
such alcohols and deactivation of the catalyst by CO poisoning. was obtained at almost full conversion of acetophenone.
In pentane, hydrogenation of both the aromatic ring and
Pressure did not affect the ketone hydrogenation rate, but it
ketone proceeded slowly perhaps in that case because the did affect the arene hydrogenation rate, with a low selectivity
product is anchored to the surface and pentane does not for 18 obtained at lower pressures (compare Fig. 5 (left) and
remove it properly. On the other hand, in THF, 18 was imme- Fig. 6 (left)). Increasing the temperature had a positive effect on
diately produced together with some 19, and both disappeared the activity, but not on selectivity as nanomaterial degradation
slowly to produce fully hydrogenated 20 (see ESI† for detailed in THF occurs much faster. When the reaction was performed
conversions and activities).
neat, the activity increased (TOF = 4000 hꢀ1 at 353 K), but the
Fig. 6 Left: hydrogenation of acetophenone (0.5 M in THF) at 298 K in the presence of RuIPr0.2: 0.1% Ru-40 bar (red), 0.1% Ru-10 bar (blue), 0.3% Ru-40 bar (green),
0.3% Ru-10 bar (brown). Right: product evolution of 0.3% Ru-10 bar, 17 (black), 18 (red), 19 (blue), 20 (green).
c
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a
Scheme 5 Hydrogenation of compounds containing an arene and a ketone (results at 100% conversion, except 24). 353K, solvent free and 26% of conversion.
selectivity was much lower (45% yield of product 18). This is surface and allow access of the arene substrates. There is a clear
probably due to a marked change in ‘‘solvent’’ polarity while thermodynamic preference for arene coordination over ketone
the reaction proceeds, as alcohols are produced.
coordination which leads to an interesting selectivity in the
Subsequently RuIPr0.2 was tested as a catalyst in the hydro- case of arylketones. The case of acetophenone is particular
genation of other aromatic compounds containing ketones in perhaps because of its lack of flexibility and the possibility for
the best solvent so far, THF (Scheme 5). Interestingly, we acetophenone to have both phenyl and carbonyl groups
observed that as the length of the alkyl chain increases the reduced upon coordination to the nanoparticle which would
selectivity toward hydrogenation of the arene fragment is lower the selectivity. The solvent has a major effect on the
enhanced. The selectivity was found over 98% at full conversion activity and the most suitable solvent varies with the substrate,
for 4-phenyl-2-butanone, 22, and 5-phenyl-2-pentanone, 23. probably as a function of the competition between solvent,
These remarkable results show that nanoparticles may hydro- substrate and product adsorption on the nanoparticle surface.
genate very selectively the arene groups in arylketones as a During hydrogenation of ketones (e.g. acetophenone) the medium
probable consequence of the thermodynamic preference for changes as alcohols are produced, which apparently changes the
arene coordination over ketone coordination on ruthenium catalysts thus affecting catalysis.
surfaces. In the case of acetophenone, it seems likely that the
The above results show that ligands used in the preparation
approach through the phenyl ring is also preferred as deduced of RuNPs and solvents used in catalysis play a major role in
from the faster production of 18 over 19. The lower selectivity determining the properties in hydrogenation reactions, as do
could result from the possibility of concomitant hydrogenation ligands and solvents in homogeneous catalysts. A lot has to be
of both the phenyl ring and the CO bond upon coordination via learnt yet about how we can control the catalyst properties, but
the phenyl ring.
several trends are emerging. First, the presence of aromatic
3-Acetylpyridine, 24, was also hydrogenated (Fig. S10, ESI†). groups in the ligands leads to more stable catalysts. Second, the
The pyridine derivative reacted more slowly than the benzene more electron-donating is the ligand the faster is the hydrogena-
derivative, but still with a TOF >800 hꢀ1 at 353 K and 40 bar of tion of aromatic compounds: NHC > PAlk2Ar >> PAr3, but the
hydrogen (acetophenone TOF > 4000 hꢀ1). In this case, a faster catalysts are also the least stable. The variety of selectivities
complex mixture of compounds was obtained, in which at and rates observed in our studies holds a promise for selective
26% conversion, the major compounds were the ones derived hydrogenation of substituted aromatics to valuable products.
from ketone hydrogenation (15%) and the one with a,b-saturated
ketone (8%), with only 3% of products in which the pyridine ring
Acknowledgements
was fully hydrogenated. This result is in concordance with those
found in the literature.52,53
The authors thank UPS-TEMSCAN for electron microscopy
facilities. The authors also acknowledge financial support from
CNRS and the European Union for ERC Advanced Grant
Conclusions
(NANOSONWINGS 2009-246763). The Spanish Ministerio de Ciencia
´
e Innovacion (projects CTQ2010-17476 and CONSOLIDER-INGENIO
We have shown that RuNPs stabilized by the NHC ligands ItBu
and IPr are active nanocatalysts for arene hydrogenation, the
latter giving much more active catalysts. RuNPs-ItBu nano-
particles need a higher content of ligand for their stabilization.
The presence of this excess ligand coordinated on the surface
may lead to a slower catalytic system. The aromatic IPr ligand
may be coordinated either on the corner, edges and defects18 at
low concentration or additionally on the faces at higher concen-
tration. The excess ligand is removed in both cases and the
surface of the particle is accessible. It is likely that the large
substituted arene groups interact weakly with the ruthenium
2010 CSD2007-00006, FEDER support) is acknowledged. O. R.-W.
´
thanks the Spanish Ministerio de Ciencia e Innovacion for a
research grant.
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