Influence of amines on the size control of in situ synthesized ruthenium nanoparticles in imidazolium ionic liquids

Article abstract of DOI:10.1039/c0dt00596g

Very stable suspensions of small sized (c.a. 1.2 nm) and homogeneously dispersed ruthenium nanoparticles (RuNPs) were obtained by decomposition, under H₂, of (η⁴-1,5-cyclooctadiene)(η⁶-1,3, 5-cyclooctatriene)ruthenium(0), [Ru(COD)(COT)], in various imidazolium derived ionic liquids (ILs: [RMIm][NTf₂] (R = C_nH_2n+1 where n = 2; 4; 6; 8; 10) and in the presence of amines as ligands (1-octylamine, 1-hexadecylamine). These nanoparticles were compared to others stabilized either in pure ILs or by the same ligands in THF. NMR experiments (¹³C solution and DOSY) demonstrate that the amines are coordinated to the surface of the RuNPs. These RuNPs were investigated for the hydrogenation of aromatics and have shown a high level of recyclability (up to 10 cycles) with neither loss of activity nor significant agglomeration.

Full text of DOI:10.1039/c0dt00596g

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2011, 40, 4660
www.rsc.org/dalton
PAPER
Influence of amines on the size control of in situ synthesized ruthenium
nanoparticles in imidazolium ionic liquids
Gorka Salas, d Catherine C. Santini,* Karine Philippot,* Vincent Colli e` re,a,b Bruno Chaudret,*a,b,e
a,b,c
c
a,b
e,b
Bernard Fenet and Pier Francesco Fazzini
Received 3rd June 2010, Accepted 21st January 2011
DOI: 10.1039/c0dt00596g
Very stable suspensions of small sized (c.a. 1.2 nm) and homogeneously dispersed
ruthenium nanoparticles (RuNPs) were obtained by decomposition, under H
(
2
, of
h -1,5-cyclooctadiene)(h -1,3,5-cyclooctatriene)ruthenium(0), [Ru(COD)(COT)], in various
imidazolium derived ionic liquids (ILs: [RMIm][NTf ] (R = C 2n+1 where n = 2; 4; 6; 8; 10) and in the
presence of amines as ligands (1-octylamine, 1-hexadecylamine). These nanoparticles were compared to
4
6
2
n
H
1
3
others stabilized either in pure ILs or by the same ligands in THF. NMR experiments ( C solution and
DOSY) demonstrate that the amines are coordinated to the surface of the RuNPs. These RuNPs were
investigated for the hydrogenation of aromatics and have shown a high level of recyclability (up to
1
0 cycles) with neither loss of activity nor significant agglomeration.
1
. Introduction
stabilizers has been developed. This approach affords MNPs with
narrow size distributions, but the size of these particles can not
be predicted. Nevertheless, comparative studies have shown that
ligands can be classified into two main categories—weak and
The use of metal nanoparticles (MNPs) as catalysts is a topic
of growing interest at the frontier between homogeneous and
heterogeneous catalysis. While homogeneous catalysts are gen-
erally difficult to extract from the reaction products, but highly
efficient and selective, heterogeneous ones are easily separated
from the reaction mixtures and are generally not altered by
1,2
8
9,10
strong ligands. Strong ligands such as thiols and phosphines,
tend to favour the formation of small MNPs with very narrow
size-distributions, while weak ligands such as amines lead to
11
larger size distributions and allow the growth of the particles.
3
high reaction temperatures. This is probably the main reason
However, the presence of appropriate ligands can lead to selective
why heterogeneous catalysis has been very successful so far.
The development of nanocatalysts is expected to overcome the
disadvantages of both kinds of catalysts while keeping their
advantages. As the activity and selectivity of metal nanoparticles
are dependent on their size and shape, it is of primary importance
to control these parameters.
Several stabilization methods are known to be effective at
obtaining stable MNPs, such as the use of polymers, ligands and
organic or inorganic templates. In this area, a general procedure
for the synthesis of MNPs in organic solvents, by the reduction
of organometallic complexes under dihydrogen in the presence
of various ligands such as alcohols, phosphines or amines as
12
catalysts.
Besides classical modes of stabilization for MNPs, the use of
ionic liquids (ILs) is emerging as an environmentally friendly
13
alternative for NP synthesis. ILs play a double role, acting as
14,15
both the solvent and the stabilizer.
4–7
Currently, ILs are defined as “supramolecular” fluids due to
16
their extended hydrogen-bond networks. In addition to this
H-bond network, there is a continuous tridimensional network
of ionic channels, coexisting with the non-polar domains which
form, in some cases, dispersed microphases (n = 2) or, in others,
continuous ones (n = 12). From literature data, both experiments
and simulations have shown that the side-chain length has a strong
influence on the size of the non-polar domains of ILs. In addition,
X-ray diffraction (XRD) experiments proved that, in [RMIm][Cl],
1,6
a
CNRS; LCC (Laboratoire de Chimie de Coordination du CNRS), 205,
route de Narbonne, F-31077, -Toulouse cedex 04, France. E-mail: karine.
philipot@lcc-toulouse.fr; Fax: +33 561553003; Tel: +33 561333100
(
n
R = C H2n+1), the size of non-polar domains increases from 13
17–20
˚
to 27 A when n varies from 3 to 10.
The segregation of
b
Universit e´ de Toulouse; UPS, INPT; LCC, F-31077, Toulouse, France
c
polar and non-polar domains in imidazolium-based ILs has a
strong influence on their solvation and their ability to interact
with different species. Indeed, polar substrates are preferentially
dissolved in polar domains and non-polar compounds in non-
polar ones. The two extreme situations are those such as hexane
and water solutions where the solute is concentrated either in the
Universit e´ de Lyon, Institut de Chimie de Lyon, UMR 5265 CNRS-
Universit e´ de Lyon 1-ESCPE Lyon, C2P2, Equipe Chimie Organom e´ tallique
de Surface, 43 bd du 11 Novembre 1918, 69626, Villeurbanne Cedex, France.
E-mail: santini@cpe.fr; Fax: +33 472431795; Tel: +33 472431794
d
Centre Commun de RMN, UCB Lyon, 1 - ESCPE, Lyon, France
e
Laboratoire de Physique et Chimie des Nano-Objet, Institut National des
Sciences Appliqu e´ es 135 avenue de Rangueil, F- 31077, Toulouse, France.
E-mail: chaudret@insa-toulouse.fr
18,21
non-polar domain or in the charged domain.
4
660 | Dalton Trans., 2011, 40, 4660–4668
This journal is © The Royal Society of Chemistry 2011

View Article Online
Recently, a relationship between the size of these non-
they have not been used in the in situ synthesis of nanoparticles in
ILs.
In a Fischer–Porter reactor, the metal source [Ru(COD)(COT)]
polar domains and the size of RuNPs prepared in situ
4
6
from the organometallic complex (h -1,5-cyclooctadiene)(h -
1
,3,5-cyclooctatriene)ruthenium(0), [Ru(COD)(COT)], has been
(0.21 mmol), and the chosen ligand ([RNH ]/[Ru(COD)(COT)] =
2
22
found.
This suggests that the phenomenon of crystal
0.2), were dispersed in the IL (5 mL). The reaction mixture
◦
growth is probably controlled by the local concentration of
Ru(COD)(COT)] and consequently, limited to the size of these
was then set to 30 C, stirred for 2 h, and then pressurized
[
with a H atmosphere (3 bar). After 20 h of vigorous stirring,
2
non-polar domains. Ionic liquids can be considered to be or-
ganized systems that are formed via a self-assembly process
and resemble nano-reactors in which the size of RuNPs can be
controlled.
the remaining hydrogen and cyclooctane (COA; a by-product
of the reaction resulting from the metal precursor reduction)
were eliminated under vacuum. The black colloidal suspensions
obtained were stored under argon at room temperature. These
colloidal suspensions remained stable for long periods of time,
as checked by transmission electron microscopy (TEM) in ILs,
showing neither precipitation nor aggregation after more than
9 months under argon. Selected TEM micrographs with corre-
sponding size histograms are presented in Fig. 1a–f. The different
colloidal systems obtained in this way are described in Table 1.
The RuNPs are well dispersed on the TEM grids and no
agglomeration was observed. In addition, they display an almost
spherical shape and are of similar sizes that are between the limits
of error, in the range 1.1–1.3 nm, whatever the IL or the ligand
used for their synthesis (Table 1).
However, under standard experimental catalytic conditions
(
temperature, stirring, or carbon monoxide atmosphere) the
RuNPs in ILs have not been found to be stable towards agglomer-
23
ation and coalescence. In order to develop new catalytic systems
based on stable and size-controlled NPs in ILs we combined
two approaches, namely the control of the local concentration
22
of [Ru(COD)(COT)] into the non-polar domains of ILs and the
8,24–27
use of ligands to increase NP stability.
In this paper, we describe the synthesis of RuNPs by decomposi-
tion, under H
in mild conditions (3 bar H
2
, of the organometallic precursor [Ru(COD)(COT)]
◦
2
, 30 C) and in several ILs
1
-alkyl-3-methyl-imidazoliumbis(trifluoromethanesulfonyl)imide
Im]][NTf ], (C = butyl, hexyl, octyl, decyl), in the presence
The nanoparticles obtained in the presence of octylamine have
also been characterized by high resolution electron microscopy
(HRTEM) in ILs (Fig. 2). The high resolution results have been
used to check the crystalline structure of the nano-particles.
Fourier (FFT) analyses performed on the high resolution images
[C
1
C
n
2
n
of amines as stabilizing ligands (1-octylamine, 1-hexadecylamine).
The resulting colloidal suspensions have been investigated for the
catalytic hydrogenation of toluene.
(see Fig. 2c) confirm that the particles have the same hexagonal
structure as bulk ruthenium if the experimental errors (a 5% error
on the calibration of the magnification has been considered) are
taken into account. In addition, the measured distances and angles
are compatible (within a 5% error) with the hexagonal structure
of bulk ruthenium showing that the particle is oriented in a [2, -1,
-1, 3] zone axis (see Fig. 3).
2
. Results and discussion
2.1 Synthesis and characterization of ruthenium nanoparticles
Decomposition of [Ru(COD)(COT)], by dihydrogen is a well-
known method for synthesizing RuNPs in organic solvents. The
same procedure has been successfully applied to the synthesis of
8
,27
The characteristics of these RuNPs can be compared with those
22,28–30
RuNPs in pure imidazolium ionic liquids.
Here we report the
of previously obtained RuNPs synthesized with the same ligands
8
synthesis of RuNPs in different ILs and in the presence of amines
added to the reaction medium as stabilizing ligands, as shown in
Scheme 1.
in an organic solvent (THF), as well as in the same-but-ligandless
22,28–30
ILs.
First of all, the synthesis was carried out in THF in the presence
The ILs used in this work are: 1-alkyl-3-methyl-imidazolium
of octylamine in the same molar ratio as reported above, i.e.
bis(trifluoromethanesulfonyl)imide [C
hexyl, octyl, decyl) and the ligands: 1-octylamine (C
-hexadecylamine (C16 ). Octylamine and hexadecylamine
1
C
n
ImNTf
2
], (C
n
= butyl,
[C
8
H
17NH ]/[Ru(COD)(COT)] = 0.2, which gives rise to spherical
2
H
17NH
2
) and
RuNPs with a mean size of 2.3 nm which are agglomerated
into spherical superstructures of approximately 100 nm. In the
same reaction conditions, but with hexadecylamine as stabilizer,
elongated RuNPs of a mean size of 1.9 nm were observed, both
of which are larger than the RuNPs formed by the new method
which display a mean diameter of around 1.2 nm. In addition,
8
1
H
33NH
2
have previously been described as effective agents for the stabiliza-
8
tion of RuNPs in tetrahydrofuran (THF) as solvent. The increase
in the stability of metal NPs in ILs, by addition of an extra ligand
15
has already been reported, however to the best of our knowledge,
Scheme 1 In situ synthesis of RuNPs in an IL and in the presence of amines as stabilizers.
This journal is © The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 4660–4668 | 4661

View Article Online
1 n 2
Table 1 Mean sizes of Ru NPs, depending on the nature of the ligand and of the ILs [C C ImNTf ]
a
b
Entry
IL
Ligand
Average diameter (nm)
[L]/[Ru
S
]
Colloid
1
2
3
4
5
6
C
C
C
C
C
C
1
1
1
1
1
1
C
C
C
C
C
C
4
4
4
6
8
ImNTf
ImNTf
ImNTf
ImNTf
ImNTf
2
2
2
2
2
hexadecylamine
octylamine, no stirring
octylamine
octylamine
octylamine
1.3 (0.4)
1.2 (0.4)
1.1 (0.3)
1.2 (0.3)
1.2 (0.3)
1.3 (0.4)
0.27
0.26
0.25
0.26
0.26
0.27
C4·HDA
C4·OA·NS
C4·OA
C6·OA
C8·OA
10ImNTf
2
octylamine
C10·OA
a
b
ratio [RNH
2
]/[Ru(COD)(COT)] = 0.2 used for the synthesis. ratio between the total concentration of the ligand and the concentration of Ru atoms at
the surface.
Fig. 1 a–f Left to right and up to down: colloids in ILs of C4·OA, C4·HDA, C4·OA·NS, C6·OA, C8·OA and C10·OA (see Table 1).
4
662 | Dalton Trans., 2011, 40, 4660–4668
This journal is © The Royal Society of Chemistry 2011

View Article Online
Fig. 2 a) general HRTEM image of C4·OA-stabilized RuNPs in C
1
C
4
ImNTf
2
; b) HRTEM image of an isolated and well-crystallised C4·OA-stabilized
Ru NP with c) FFT.
nanoparticles produced in ILs follows the trend C
1
2
C10ImNTf >
C
1
C
8
ImNTf > C ImNTf > C ImNTf . This phenomenon is
2
1
C
6
2
1
C
4
2
explained by the properties of imidazolium ILs, which show some
degree of self-organization, leading to a separation between the
17–19
polar (cation and anion) and non-polar domains (alkyl chains).
Since [Ru(COD)(COT)] is preferentially dissolved in non-polar
regions, its local concentration is controlled by the size of the
non-polar domains. This means that the number of Ru nuclei
available for nanoparticle growth is dependent on the length of
the alkyl chain attached to the imidazolium ring. In contrast,
such a phenomenon is not observed for nanoparticles synthesized
in the same ILs (C
1
C
4
ImNTf
2
, C
1
C
6
ImNTf
2
, C
1
C
8
ImNTf and
2
C
1
C
10ImNTf
2
) but in the presence of an added amine. In fact,
Fig. 3 High resolution image showing the fringe spacing and direction in
a Ru particle. The measured distances and angles are compatible (within
a 5% error) with the hexagonal structure of bulk ruthenium, showing that
the particle is oriented in a [2, -1, -1, 3] zone axis.
when using octylamine, the average size of the nanoparticles is
the same (between error limits) independently of the length of the
alkyl chain of the IL (Table 1, entries 3–6). Furthermore, it is worth
commenting that no aggregation or formation of superstructures is
observed for the colloids synthesized in the presence of octylamine
they are spherical and do not exhibit any agglomeration. These
facts indicate a better stabilization effect and a better size control
when a combination of an IL and an extra ligand is used for the
RuNPs synthesis.
in C
1
C
10ImNTf , in contrast to previous observations when no
2
22
ligands were added in the reaction medium.
◦
In summary of this part, RuNPs synthesized at 30 C in ILs
in the presence of ligands do not present any agglomeration. In
addition, their size is significantly smaller than that of both NPs
prepared in the same ILs but in the absence of ligand, and NPs
prepared in THF with the same ligand. This suggests a combined
effect of both the IL and the amine on the NPs. Furthermore,
no correlation was observed between the size of the alkyl chain
of the IL and the size of the nanoparticles. This suggests that,
while the IL is still preventing particle agglomeration, the ligand
is playing the main role in controlling their size and shape. It
appears therefore that the method of preparation of nanoparticles
described in this paper is suitable for the controlled synthesis of
small size, homogeneously dispersed and stable nanoparticles. A
point of interest is that in organic solutions, the nanoparticles
stabilized by amine ligands display an elongated shape and give
Secondly, RuNPs synthesized in C
1
C
4
ImNTf in the same
2
reaction conditions but without any added ligand displayed a
29
mean size of 2.3 nm, whereas in the present work, RuNPs of
.1 nm average diameter are obtained in C ImNTf , whatever
1
1
C
4
2
the amine, octylamine or hexadecylamine. Only RuNPs obtained
◦
in C
1
C
4
ImNTf
2
at 0 C without stirring display a similar size. This
evidences the beneficial effect afforded by the combination of an
IL and a ligand to produce small and stable RuNPs.
Another important difference can be observed when considering
the length of the alkyl chain attached to the imidazolium ring. As
22
previously described, there is a correlation between the size of the
alkyl chain of the IL and the mean diameter of the nanoparticles.
More precisely, in the absence of added ligand, the size of the
This journal is © The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 4660–4668 | 4663

View Article Online
rise to worm-shaped structures. This has not been observed in
the case of Ru NPs stabilized by amines in ILs. This suggests a
confinement of the amine inside the apolar domains of the ILs and
perhaps an increased pressure leading to an increased association
constant of the ligands in the ILs. This will be discussed in the
NMR section.
Table 2 Diffusion coefficients (D) of octylamine and imidazolium cations
in different samples in neat ILs
2
-1
2
-1
Sample
D
OA (m s )
DIm (m s )
-
11
-11
-11
Octylamine in C C ImNTf
C4·OA
1
4
2
5.740 ¥ 10-
3.631 ¥ 10
3.043 ¥ 10
2.970 ¥ 10
11
2
.2 Study of the role of the amine in the stabilization of the
shifted. This indicates the absence of mobility for the ligand which
can result either from a strong interaction between the alkyl chain
and the ruthenium surface or from a confinement in the non-polar
domains. When a slight excess of amine was added to the NMR
tube containing the colloid, the resulting spectrum evidenced the
superposition of the signals corresponding to the free amine and
those corresponding to the amine coordinated to nanoparticles
(t) (Fig. 4c) and therefore indicates the lack of exchange between
the free ligand in the organic solvent and that which is most likely
confined in the IL. This result differs from the reactions in THF
for which a fast exchange between free and coordinated ligand was
observed.
ruthenium nanoparticles
As the amine ligands appeared crucial for the size control
of the RuNPs, we tried to determine their exact role by gas
chromatography (GC) and NMR.
First of all, to ascertain the presence of the ligand in the colloidal
solutions after their synthesis, a sample of C4·OA was diluted in
acetonitrile and analyzed by GC. The chromatogram showed a
peak corresponding to the presence of octylamine in the colloidal
solutions.
The colloidal solution of C4·OA was further studied by NMR
spectroscopy and compared to a solution of the stabilising amine
in the IL. The H NMR spectrum of an octylamine solution in
C
1
To try and shed more light on the coordination of the amine,
1
1
C
4
ImNTf
2
shows a triplet (J = 7 Hz) at 2.60 ppm, corresponding
H DOSY NMR experiments were performed to measure the
to the protons attached to the Ca carbon close to the nitrogen
atom of the octylamine. The other signals are not observable
because of overlap with the more intense imidazolium signals. In
colloidal C4·OA, only a broad signal from octylamine is observed
at 3.15 ppm.
Fig. 4 presents C NMR spectra registered for the free ligand in
IL (Fig. 4a), for colloidal C4·OA (Fig. 4b), and for a mixture
of the colloidal C4·OA with a ligand excess (Fig. 4c). In the
spectrum of colloidal C4·OA (Fig. 4b), we can observe some
differences in comparison with the spectrum of the free amine
in IL (Fig. 4a). First of all, most of the signals corresponding to
the free ligand (*) are missing. No signal is observed for Ca near
diffusion coefficients (D) of octylamine and the imidazolium
cations.
Indeed, it is described for palladium NPs in imidazolium ionic
liquids in the presence of methanol, that a larger decrease in the
diffusion coefficient of the cation proves that the NPs interact with
1
3
31
the imidazolium cations and not with methanol.
Two samples were thus prepared for a comparison by DOSY
NMR: one containing a solution of octylamine in the neat IL (with
the same concentration as the one employed for the synthesis of
NPs), and the other one corresponding to the colloidal C4·OA.
Table 2 reports the values of the diffusion coefficient (D) of the
amine and the imidazolium cations in both samples.
4
2 ppm as expected, but all of the other signals are very broad,
The results show that there is almost no change in the diffusion
coefficient of the imidazolium cations between the two samples,
whereas the D value of octylamine is significantly lower for the
including the signals corresponding to the alkyl chain near 13,
2
for RuNPs stabilized by alkylamines, for which the coordination
of the ligands at the particle surface led to the absence of the
signal for Ca while the signals for the carbons located further
away from nitrogen displayed a lower intensity and were slightly
2 and 29 ppm. Similar NMR results were previously observed
8
colloidal C4·OA compared to the amine in C
1
C
4
ImNTf . The fact
2
that the diffusion of the amine is decreased in the presence of the
RuNPs can be explained by an interaction of the amine with the
RuNP surface.
1
3
Fig. 4
C NMR spectra of (a) octylamine in neat C
1
C
4
ImNTf
2
, (b) the colloidal C4·OA, (c) a mixture of the colloidal C4·OA and octylamine (excess).
2 2
Most intense peaks are from the IL. The other peaks were assigned to free (*) and coordinated amine (t). The multiplet at ca. 53 ppm. is CD Cl (coaxial
capillary tube). The sharp peaks (᭜) were probably resulting from decomposition of the amine.
664 | Dalton Trans., 2011, 40, 4660–4668
4
This journal is © The Royal Society of Chemistry 2011

View Article Online
Thus, in conclusion of this part, these NMR experiments
unambiguously demonstrate the coordination of the amine to the
surface of the ruthenium nanoparticles. The amine ligands present
at the surface of the particles are unable to exchange with excess
amine as in organic solvents, probably resulting from the location
of the nanoparticles inside apolar domains which isolate them
from one another, preventing ligand diffusion and thus allowing
them to maintain a small size and spherical shape.
differences in the viscosity of the reaction mixtures or by a higher
steric hindrance of hexadecylamine once coordinated on a RuNP
surface.
We also studied the influence of the IL on the catalytic activity.
For that purpose, the colloidal systems prepared in the presence
of octylamine, and the C
1
C
4
ImNTf
2
, C
1
C
6
ImNTf
2
, C
1
C
8
ImNTf
2
and C ILs were investigated at different reaction times
1
C
10ImNTf
2
(30, 60, 90, 180 and 300 min), as reported in Fig. 5. Small
differences are observed when the length of the alkyl chain
in the imidazolium cation increases. These variations could be
due to different solubilities of toluene, methylcyclohexane and
dihydrogen in the different ionic liquids, and also the differences
2.3. Catalytic hydrogenation of toluene using ruthenium
nanoparticles as the catalyst
36,37
in the viscosity of the reaction mixtures.
To probe the surface state of the synthesized RuNPs, they were
investigated as catalysts. In the literature, RuNPs in ionic liquids
are reported as efficient catalysts for the hydrogenation of alkenes
and aromatic compounds in mild reaction conditions (4 atm,
◦
28
7
5 C), while it is known that molecular Ru compounds do
2,32–35
not catalyse the hydrogenation of aromatic compounds.
We
therefore studied the catalytic activity of the RuNPs synthesized
in this work for the hydrogenation of toluene (PhMe) into
methylcyclohexane (CyMe).
The catalytic experiments were carried out using a preformed
colloidal suspension on which toluene was added, leading to
biphasic toluene/IL system. More precisely, in a Schlenck tube,
toluene (0.80 mmol, 0.085 ml) was added over 0.5 mL of the
chosen colloid. The reaction system was stirred, kept at the
desired temperature and finally filled with H
2
(1.2 bar). The
H
2
pressure was kept constant for 5 h. Then the reaction was
◦
stopped, cooled down to 25 C and quenched with acetonitrile to
yield a homogeneous mixture, which was further analyzed by gas
chromatography. Consequently, each point of the catalytic graph
corresponds to an independent experiment.
Fig. 5 Reaction profiles of the hydrogenation of toluene into methylcy-
clohexane, employing as catalysts colloids of RuNPs in ionic liquids with
different alkyl chain lengths.
Catalytic tests were first performed with the RuNPs synthesized
in C
1
C
4
ImNTf
2
in the presence of octylamine, at different tem-
◦
peratures (30, 50, 75 and 100 C), under 1.2 bar H
2
and with a
2
.4. Recycling of the catalyst.
reaction time of 5 h (Table 3). The conversion obtained is low but
◦
◦
◦
reproducible, with a value of 4% at 30 C, 9% at 50 C, 17% at 75 C
Thanks to the low vapour pressure of ILs as well as to their
thermal stability, NPs dispersed in ILs are interesting systems for
recycling purposes. After catalysis, the unreacted toluene (PhMe)
and the hydrogenated product, methyl-cyclohexane (CyMe), can
be evacuated under vacuum, allowing their separation from
the colloid. Then, the relative composition of the PhMe-CyMe
mixture can be determined by gas chromatography.
◦
◦
and no improvement obtained at 100 C. A temperature of 75 C
was chosen for studying the reaction with the other colloids. The
results are also collected in Table 3. A slightly lower conversion
is obtained with hexadecylamine instead of octylamine although
the NPs have a similar size and shape, and the same amount of
ligand. The observed difference can therefore be explained by the
Table 3 Hydrogenation of toluene with the RuNPs synthesized in C
1
C
4
ImNTf
2
in the presence of amine
◦
a
k
initial (10-2 mol L-1 h-1)b
IL/ligand
T ( C)
PhMe/Ru
T
PhMe/Ru
S
S
Ru /Ru
T
% conversion
TON
C4·OA
C4·OA
C4·OA
C4·OA
C4·HDA
30
50
75
100
75
38/1
38/1
38/1
38/1
38/1
46/1
46/1
46/1
46/1
52/1
0.82
0.82
0.82
0.82
0.73
4
9
17
17
14
2
4.5
8
8
6.5
12 ± 1
C4·OA
C6·OA
C8·OA
C10·OA
75
75
75
75
38/1
38/1
38/1
38/1
46/1
49/1
49/1
52/1
0.82
0.77
0.77
0.73
17
16
11
14
8
8
5.5
7
12 ± 1
8 ± 1
5 ± 1
7 ± 1
Reaction conditions: constant pressure of 1.2 bar of H
atoms. Turn over number (moles of product converted per mol of Ru
2
; reaction time = 5 h; Ru
T
= total amount of ruthenium atoms; Ru
S
= amount of surface ruthenium
a
b
S
). Estimated initial rates.
This journal is © The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 4660–4668 | 4665

View Article Online
We have checked the reusability of our RuNPs colloidal suspen-
sion obtained in C ImNTf and in the presence of octylamine.
in IL was analyzed in situ using TEM by depositing a droplet
on a holey carbon film supported by a copper grid. The TEM
analyses were performed at the “Service Commun de Microscopie
Electronique de l’Universit e´ Paul Sabatier” (TEMSCAN), using
a JEOL 1011 electron microscope operating at 100 kV with
1
C
4
2
Ten consecutive experiments of toluene hydrogenation (1.2 bar H
2
,
◦
75 C, 5h) were carried out with the same colloid (Fig. 6). After
each catalytic cycle, PhMe and CyMe were vacuum-separated
from the colloid. Samples of RuNPs taken after one and after
ten catalytic experiments respectively were analyzed by TEM. The
TEM analysis after ten catalytic cycles shows a larger mean size
of the nanoparticles and a broader size distribution (1.6 ± 0.7
nm) than those observed before catalysis (1.1 ± 0.3 nm) and after
one catalytic test (1.2 ± 0.6 nm). Although there is a smooth
trend to lower conversions, it is worthwhile to note that after
ten consecutive experiments with the same batch of nanoparticles,
they remain still active.
˚
resolution point of 4.5 A, and at the “Centre Technologique
des Microstructures”, UCBL1, Villeurbanne, France, using a
Philips 120 CX electron microscope with acceleration voltage
of 120 kV. High-resolution TEM images were obtained at the
“Service Commun de Microscopie Electronique de l’Universit e´ Paul
Sabatier” (TEMSCAN) using a JEOL JEM 2100 F at 200 kV with
˚
a resolution point of 2.3 A. The size distributions were determined
through a manual analysis of ensembles of over 300 particles
found in randomly selected areas of the enlarged micrographs,
with Imagetool software to obtain a statistical size distribution
and a mean diameter.
3
. Experimental
Solution NMR spectra were recorded in a Bruker AV-300
1
13
spectrometer. H and C spectra were referred internally to
deuterated solvents; chemical shifts are relative to Me Si.
3.1. Materials and characterization methods
4
1
-Methylimidazole (>99%) was purchased from Sigma-Aldrich
2D DOSY (Diffusion Order Spectroscopy) experiments were
performed on a Bruker Avance III 500 spectrometer, using
the standard Bruker sequence dstebpgp3s in order to minimize
convection effects when measuring short diffusion coefficients,
trapezoidal gradients for shorter pulses, and the “baseopt” option
for acquisition mode to get a very good baseline. The diffusion
evolution time D was set to 100 ms and gradient pulses to 3 ms
leading to d = 6 ms. The diffusion space was sampled by 32 linearly
spaced gradients. Processing used the DOSY module of TOPSPIN
and distilled prior to use. Ruthenium trichloride hydrate
and anhydrous 1-chlorobutane (>99.5%) were also received
from Sigma-Aldrich and used without further purification.
Bis(trifluoromethanesulfonyl)imide lithium salt (>99%) was pro-
duced by Solvionic company and Zinc (A. R. Grade) by Merck
and both were used as received. 1,5-cyclooctadiene (>99%)
was purchased from Sigma-Aldrich and purified through a
neutral alumina column. The ionic liquids were prepared ac-
38
◦
cording to the known method
dried overnight under high
2.1. Temperature was set to 25 C, and 32 dummy scans were used
vacuum and stored in a glovebox. (1,5-cyclooctadiene)(1,3,5-
cyclooctatriene)ruthenium, [Ru(COD)(COT)], was synthesized
using the procedure reported by Pertici and Vitulli Toluene was
dried and purified in a MBraun SPS-800 purification system, and
degassed prior to use.
for temperature equilibration in the sample.
Gas chromatography analyses were performed in a HP5890
chromatograph equipped with a polymethylsiloxane column. The
injector and detector temperature was set at 250 C. For the
analysis the column temperature was initially fixed at 100 C,
39
◦
◦
◦
Transmission electronic microscopy (TEM) experiments were
performed directly in the IL media. Each solution of RuNPs
followed by a temperature ramp of 10 C until reaching
◦
150 C.
◦
Fig. 6 Conversion of toluene into methylcyclohexane obtained for ten consecutive experiments with the same colloid C4·OA (P = 1.2 bar H
2
; 75 C; 5 h.
Inside histograms show normalized size distributions of RuNPs observed after the first catalytic experiment and the tenth one, respectively. The black
line represents the mean conversion (9%) calculated for all recycling experiments.
4
666 | Dalton Trans., 2011, 40, 4660–4668
This journal is © The Royal Society of Chemistry 2011

View Article Online
3
.2. Synthesis of Ru NPs in the presence of octylamine
liquid nitrogen and connected to the Schlenk flask). The RuNP
suspension is thus ready for a new catalytic experiment. Ten
consecutive catalytic experiments were performed with the same
batch of colloid.
The mixture of toluene/methylcyclohexane was diluted in
acetonitrile and examined by gas chromatography. A copper grid
of RuNPs in suspension was prepared for TEM analysis after ten
consecutive experiments of catalysis. The copper grid for TEM
analysis after only one catalysis experiment was prepared from an
independent experiment.
In a typical experiment, [Ru(COD)(COT)] (66.2 mg, 0.21 mmol)
was placed in a Fischer–Porter bottle, on which the ionic liquid 1-
butyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide
-
2
(
C
1
C
4
ImNTf , 5 mL) and the octylamine (6.9 mL, 4.2 ¥ 10 mmol)
2
were added. The obtained mixture was a yellow suspension that
was stirred for 2 h. The mixture was then frozen in a liquid nitrogen
bath, and a vacuum was made. The system was fixed to 30 C, filled
◦
with H (3 bar), and stirred for 20 h. After that time, the remaining
2
hydrogen and volatile alkanes were evacuated under vacuum. The
final mixture was a black colloidal suspension that remained stable
for months.
4
. Conclusions
Stable, small and homogeneously dispersed ruthenium nanopar-
ticles were synthesized in imidazolium type ILs in the presence
of alkylamines as ligands. Whatever the alkylchain length of
the IL, the mean size of the obtained RuNPs is found to be
approximately 1.2 nm. No correlation between the length of the
alkyl chain of the imidazolium ring of the ILs and the size of
the RuNPs was observed, in contrast to the previous results
3
.3. Synthesis of RuNPs in the presence of octylamine without
stirring
[
Ru(COD)(COT)] (66.2 mg, 0.21 mmol) was placed in a Fischer–
Porter bottle, and the ionic liquid 1-butyl-3-methyl-imidazolium
bis(triflruoromethanelsulfonyl)imide (C ImNTf , 5 mL) and the
1
C
4
2
-
2
octylamine (6.9 mL, 4.2 ¥ 10 mmol) were added. The mixture
1
3
1
reported in ILs in the absence of ligands. C NMR and H
DOSY NMR experiments evidence the coordination of the added
amine to the NPs which explains the influence of the amine in
controlling the size of the in situ prepared RuNPs. The obtained
colloidal suspensions of RuNPs are stable for months, the ILs
preventing their aggregation. Despite a moderate activity, these
nanoparticles are able to catalyze the hydrogenation of toluene.
Furthermore, recycling of the nanocatalyst was possible for ten
consecutive times without either dramatic loss of activity or
agglomeration. Thus this study demonstrates that it is possible
to combine the properties of ionic liquids to confine nanoparticles
in the non-polar domains (“nanoreactors”) with the presence of a
weak ligand which stabilizes the particles at a very small size but
does not totally impede their catalytic activity.
was a yellow suspension that was stirred for 3 h. The mixture was
frozen in a liquid nitrogen bath, and placed in vacuo. The system
◦
was thermostated to 30 C, filled with H
2
(3 bar), and left without
stirring for 3 days. After that time, the remaining hydrogen and
volatile alkanes were evacuated under vacuum. The mixture was
a black colloidal suspension that remained stable for months.
3
.4. Synthesis of Ru NPs in the presence of hexadecylamine
In a typical experiment, [Ru(COD)(COT)] (66.2 mg, 0.21 mmol)
was placed in a Fischer–Porter bottle, and the ionic liquid
1
-butyl-3-methyl-imidazolium bis(triflruoromethylsulfonyl)imide
ImNTf , 5 mL) and the hexadecylamine (10.1 mg, 4.2 ¥
(
C
1
C
4
2
-
2
10
mmol) were added. The mixture was a yellow suspension that
◦
was thermostated to 30 C and stirred for two hours. The system
was evacuated, then was filled with H
2
(3 bar), and stirred for
Acknowledgements
2
0 h. After that time, the remaining hydrogen and volatile alkanes
were evacuated under vacuum. The mixture was a black colloidal
suspension that remained stable for months.
This work has been funded by CNRS and ANR (ANR project
CALIST, ANR-07-CP2D-02-03).
3
.5. Catalytic experiments
References
In a typical experiment, 0.5 mL of one of the colloidal suspensions
was placed in a Schlenk flask and toluene was added (85 mL,
1
D. Astruc, Nanoparticles and Catalysis, Wiley-VCH, Weinheim, 2008.
2 D. Astruc, F. Lu and J. R. Aranzaes, Angew. Chem., Int. Ed., 2005, 44,
852.
D. J. Cole-Hamilton, and R. P. Tooze, Catalyst separation, recovery and
recycling: Chemistry and Process Design, Springer, Dordrecht, 2006.
7
0
.8 mmol). The mixture was frozen in a liquid nitrogen bath, and
3
◦
a vacuum was made. Then, the system was thermostated to 75 C,
filled with H
hydrogen pressure. The reaction was quenched with acetonitrile
2 mL) and analyzed by gas chromatography.
2
(1.2 bar), and stirred for 5 h, keeping constant the
4 K. Lee, M. Kim and H. Kim, J. Mater. Chem., 2010, 20, 3791–3798.
5
6
A. Z. Moshfegh, J. Phys. D: Appl. Phys., 2009, 42, 233001–233032.
H. Boennemann and K. S. Nagabhushana, Metal Nanoclusters in
Catalysis and Materials Science: The Issue of Size Control, Elsevier
B.V, Amsterdam, eds; B. Corain, G. Schmid and N. Toshima, 2008, pp.
21–48.
(
3.6. Recycling experiments
7
G. A. Somorjai and J. Y. Park, Top. Catal., 2008, 49, 126–135.
The colloidal suspension C4·OA (0.5 mL) was placed in a Schlenk
flask and toluene was added (85 mL, 0.8 mmol). The mixture
8 C. Pan, K. Pelzer, K. Philippot, B. Chaudret, F. Dassenoy, P. Lecante
and M.-J. Casanove, J. Am. Chem. Soc., 2001, 123, 7584–7593.
9
F. Dassenoy, K. Philippot, T. Ould Ely, C. Amiens, P. Lecante, E.
Snoeck, A. Mosset, M.-J. Casanove and B. Chaudret, New J. Chem.,
1998, 22, 703–711.
was frozen in a liquid nitrogen bath, and a vacuum was made.
◦
The system was then thermostated to 75 C, filled with H
2
(
1.2 bar), and stirred for 5 h, keeping the hydrogen pressure
10 J. Garcia-Anton, M. R. Axet, S. Jansat, K. Philippot, B. Chaudret,
T. Pery, G. Buntkowsky and H.-H. Limbach, Angew. Chem., Int. Ed.,
constant. After that time, hydrogen was evacuated by creating
a vacuum in the system, so the hydrogen was evacuated and the
toluene and methylcyclohexane was collected in a trap (frozen in
2
008, 47, 2074–2078.
1
1 E. Ramirez, L. Erad e` s, K. Philippot, P. Lecante and B. Chaudret, Adv.
Funct. Mater., 2007, 17, 2219–2228.
This journal is © The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 4660–4668 | 4667

View Article Online
1
1
2 A. Roucoux, A. Nowicki and K. Philippot, in Nanoparticles, Ed.
D. Astruc, Wiley-VCH, Weinheim, Editon edn, 2007, p. 349–388.
3 J. Dupont, in Nanoparticles and catalysis, Wiley-VCH, Weinheim,
Editon edn, 2008.
27 K. Pelzer, O. Vidoni, K. Philippot, B. Chaudret and V. Colli e` re, Adv.
Funct. Mater., 2003, 13, 118–126.
28 E. T. Silveira, A. P. Umpierre, L. M. Rossi, G. Machado, J. Morais,
G. V. Soares, I. J. R. Baumvol, S. R Teixeira, P. F. P. Fichtner and J.
Dupont, Chem.–Eur. J., 2004, 10, 3734–3740.
29 T. Gutel, J. Garcia-Anton, K. Pelzer, K. Philippot, C. C. Santini, Y.
Chauvin, B. Chaudret and J.-M. Basset, J. Mater. Chem., 2007, 17,
3290–3292.
1
1
1
4 T. Welton, Chem. Rev., 1999, 99, 2071–2083.
5 J. Dupont and J. D. Scholten, Chem. Soc. Rev., 2010, 39, 1780–1804.
6 P. Wasserscheid and T. Welton, Ionic liquids in synthesis, Wiley-VCH,
Weinheim, 2008.
1
1
1
2
2
2
2
7 J. N. A. Canongia Lopes and A. A. H. Padua, J. Phys. Chem. B, 2006,
30 M. H. G. Prechtl, J. D. Scholten and J. Dupont, J. Mol. Catal. A: Chem.,
1
10, 3330–3335.
2009, 313, 74–78.
8 J. N. Canongia Lopes, M. F. Costa Gomes and A. A. H. Padua, J. Phys.
Chem. B, 2006, 110, 16816–16818.
31 J. Durand, F. Fernandez, C. Barriere, E. Teuma, K. Gomez, G.
Gonzalez and M. Gomez, Magn. Reson. Chem., 2008, 46, 739–
743.
32 C. M. Hagen, L. Vieille-Petit, G. Laurencky, G. S u¨ ss-Fink and R. G.
Finke, Organometallics, 2005, 24, 1819–1831.
33 P. Dyson, Dalton Trans., 2003, 2964–2974.
9 A. A. H. Padua, M. F. Costa Gomes and J. N. A. Canongia Lopes,
Acc. Chem. Res., 2007, 40, 1087–1096.
0 A. Triolo, O. Russina, H.-J. Bleif and E. Di Cola, J. Phys. Chem. B,
2
007, 111, 4641–4644.
1 M. G. Del Popolo, C. L. Mullan, J. D. Holbrey, C. Hardacre and P.
Ballone, J. Am. Chem. Soc., 2008, 130, 7032–7041.
34 J. A. Widegren, M. A. Bennett and R. G. Finke, J. Am. Chem. Soc.,
2003, 125, 10301–10310.
2 T. Gutel, C. C. Santini, K. Philippot, A. Padua, K. Pelzer, B. Chaudret,
Y. Chauvin and J.-M. Basset, J. Mater. Chem., 2009, 19, 3624–3631.
3 P. S. Campbell, C. C. Santini, D. Bouchu, B. Fenet, K. Philippot, B.
Chaudret, A. A. H. Padua and Y. Chauvin, Phys Chem Chem Phys 12,
35 J. A. Widegren and R. G. Finke, J. Mol. Catal. A: Chem., 2003, 198,
317–341.
36 Y. Cui, I. Biondi, M. Chaubey, X. Yang, Z. Fei, R. Scopelliti, C. G.
Hartinger, Y. Li, C. Chiappe and P. J. Dyson, Phys. Chem. Chem. Phys.,
2010, 12, 1834–1841.
4
217–4223, 2010, 12, 4217–4223.
2
2
2
4 S. Jansat, D. Picurelli, K. Pelzer, K. Philippot, M. Gomez, G. Muller,
P. Lecante and B. Chaudret, New J. Chem., 2006, 30, 115–122.
5 K. Pelzer, B. Laleu, F. Lefebvre, K. Philippot, B. Chaudret, J. P. Candy
and J. M. Basset, Chem. Mater., 2004, 16, 4937–4941.
37 P. Campbell, A. Podgor sˇ ek, T. Gutel, C. C. Santini, A. A. H. Padua,
M. F. Costa Gomes, F. Bayard, B. Fenet and Y. Chauvin, J. Phys. Chem.
B, 2010, 114, 8156–8165.
38 L. Magna, Y. Chauvin, G. P. Niccolai and J.-M. Basset,
Organometallics, 2003, 22, 4418–4425.
39 P. Pertici and G. Vitulli, Inorg. Synth., 1983, 22, 176–181.
6 K. Pelzer, K. Philippot and B. Chaudret, Z. Phys. Chem., 2003, 217,
1
539–1547.
4
668 | Dalton Trans., 2011, 40, 4660–4668
This journal is © The Royal Society of Chemistry 2011