Synthesis, characterization and catalytic reactivity of ruthenium nanoparticles stabilized by chiral N-donor ligands

Article abstract of DOI:10.1039/b509378c

The decomposition of the organometallic precursor [Ru(cod)(cot)] (cod = 1,5-cyclooctadiene; cot = 1,3,5-cyclooctatriene) under mild conditions (room temperature, 3 bars H₂) and in the presence of optically pure ligands, L*, namely (R)-2-aminobutanol 1, amino(oxazolines) (2, 3), hydroxy(oxazoline) (4) and bis(oxazolines) (5-8), leads to stable ruthenium nanoparticles exhibiting a mean diameter between 1.6-2.5 nm. These nanoparticles can be isolated and re-dispersed. They display different mean sizes, shapes and dispersions depending on the stabilizer nature. These new colloids (Ru1-Ru18) have been characterized by both solid state and molecular chemistry techniques, including TEM/HRTEM, WAXS, elemental analysis, and IR and NMR spectroscopy. To further characterize the surface state of these particles, their catalytic behaviour has been examined in the reduction of organic prochiral unsaturated substrates. Although the asymmetric induction obtained is modest, it reveals the influence of the asymmetric ligand coordinated at the surface of the particles. the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006.

Full text of DOI:10.1039/b509378c

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Synthesis, characterization and catalytic reactivity of ruthenium
nanoparticles stabilized by chiral N-donor ligands
ab
a
b
b
Susanna Jansat, David Picurelli, Katrin Pelzer, Karine Philippot,* Montserrat
a
a
c
b
Gomez,* Guillermo Muller, Pierre Lecante and Bruno Chaudret*
´
Received (in Montpellier, France) 4th July 2005, Accepted 15th November 2005
First published as an Advance Article on the web 7th December 2005
DOI: 10.1039/b509378c
The decomposition of the organometallic precursor [Ru(cod)(cot)] (cod = 1,5-cyclooctadiene;
2
cot = 1,3,5-cyclooctatriene) under mild conditions (room temperature, 3 bars H ) and in the
presence of optically pure ligands, L*, namely (R)-2-aminobutanol 1, amino(oxazolines) (2, 3),
hydroxy(oxazoline) (4) and bis(oxazolines) (5–8), leads to stable ruthenium nanoparticles
exhibiting a mean diameter between 1.6–2.5 nm. These nanoparticles can be isolated and re-
dispersed. They display different mean sizes, shapes and dispersions depending on the stabilizer
nature. These new colloids (Ru1–Ru18) have been characterized by both solid state and molecular
chemistry techniques, including TEM/HRTEM, WAXS, elemental analysis, and IR and NMR
spectroscopy. To further characterize the surface state of these particles, their catalytic behaviour
has been examined in the reduction of organic prochiral unsaturated substrates. Although the
asymmetric induction obtained is modest, it reveals the influence of the asymmetric ligand
coordinated at the surface of the particles.
proach aimed at characterizing the presence of surface ligands
Introduction
focuses on the reactivity of the resulting nanoparticles, the best
example being the selectivity often found in catalytic pro-
cesses. Despite impressive progress in asymmetric catalysis,
however, only a few colloidal systems have been shown to
display specific behaviour. The most relevant systems involve
Pt or Pd nanoparticles stabilized by cinchonidine, in the
The synthesis of well-defined metal nanoparticles has experi-
enced tremendous development over the past few years,
primarily due to their physical properties and their potential
1
for applications in the field of micro/nanoelectronics. Metal
nanoparticles have also attracted interest because of their
2
potential in catalysis. As such nanoparticles have proven to
9
hydrogenation of pyruvate derivatives.
be efficient and selective catalysts for reactions also catalyzed
by molecular complexes, such as olefin hydrogenation or C–C
3
coupling reactions, but moreover for reactions which are not,
We are therefore interested in metal nanoparticle reactivity,
not only with the aim of identifying reactions which are not
10
catalyzed by molecular complexes, but also with the goal of
or are poorly catalyzed by molecular species, such as aromatic
4
hydrocarbon hydrogenation. Unambiguous distinction bet-
characterizing those differences present in well-known reactions
catalyzed by either molecular complexes or metal nanoparticles.
In this respect, we have recently reported the use of palladium
species stabilized by a chiral diphosphite for asymmetric allylic
alkylation, and found that the use of nanoparticles leads to
slower catalysts but displaying an important discrimination
ween colloidal and true homogeneous catalysis is, however,
5
often very difficult to make.
During the last few years, we have studied the synthesis of
6
metal nanoparticles using an organometallic approach. This
methodology has led to nearly monodisperse particles of very
small size that display remarkable surface coordination chem-
istry. The use of spectroscopic techniques (IR, NMR in
solution or in the solid state) has permitted the study of the
coordination chemistry of amines at the surface of ruthenium
nanoparticles, as well as their dynamic behaviour in solu-
11
between the two enantiomers of racemic substrates.
In the present study, we examine another well-known
process catalyzed by molecular complexes, namely, asym-
metric reduction of unsaturated substrates using dihydrogen
or isopropanol in a basic medium as a hydrogen source. Ru
nanoparticles were synthesized using nitrogen donor ligands
such as b-amino alcohols (1) and chiral oxazolines (2–8), easily
prepared from commercially available amino acids and nitrile
or carboxylic acid derivatives (Fig. 1). A comparative catalytic
study between colloidal and conventional molecular systems is
also described.
7
,8
tion. Furthermore, such studies have evidenced the presence
8
of surface hydrides on these same particles. Another ap-
a
Departament de Quı´mica Inorga`nica, Universitat de Barcelona,
Martı´ i Franque`s 1-11, 08028 Barcelona, Spain
E-mail: gomez@chimie.ups-tlse.fr
Laboratoire de Chimie de Coordination du CNRS, 205, route de
b
The coordination chemistry of these chiral ligands, as well
as their applications in several enantioselective catalytic pro-
cesses, has been previously studied: specifically, the reduction
Narbonne, 31077 Toulouse Ce´dex 04, France. E-mail: chaudret@
lcc-toulouse.fr. E-mail: phil@lcc-toulouse.fr
Centre d’Elaboration des Mate´riaux et d’Etudes Structurales du
c
1
2
of unsaturated substrates (Ru, Rh, Ir), allylic substitutions
1
CNRS, 29, rue Jeanne Marvig, BP 4347, 31055 Toulouse Ce´dex 04,
France
3
14
(Pd) and olefin epoxidations (Mo). b-Amino alcohols,
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brown powder consisting of nanoparticles which could then be
re-dispersed in THF or isopropanol. In all cases, the particles
were found to be stable over time, and did not show any sign
of decomposition under an argon atmosphere. The average
yield of these syntheses after work-up was approximately
4
0%, based on ruthenium.
Characterization of the nanoparticles
The sizes of the nanoparticles were determined by TEM
(
transmission electron microscopy) and their structures by
WAXS (wide angle X-ray scattering), and in selected cases
by HRTEM (high resolution transmission electron micro-
scopy). In all instances, well-crystallized hcp nanoparticles of
small size and narrow size distribution were obtained.
Fig. 1 Chiral N-donor ligands: b-amino alcohol 1, mono(oxazolines)
–4, and bis(oxazolines) 5–8.
2
By using (R)-2-aminobutanol, (R)-1, nearly spherical mono-
disperse nanoparticles of isotropic shape and a mean size near
2
.5 nm, were obtained (Ru1). These particles exhibited a strong
previously shown to be excellent chiral auxiliaries in Ru-
1
5
tendency to self-assemble, as shown in Fig. 2. The coherence
length measured on the WAXS spectrum was shorter than the
size measured by TEM, a finding consistent with a lack of
crystalline order throughout the nanoparticles (Fig. 3).
When amino(oxazolines) were used, particle sizes remained
near 2.5 nm (Ru2, Ru3). These particles were well separated
but showed a tendency to agglomerate on the microscopy grid
catalyzed asymmetric hydrogen transfer processes, were also
examined. In this paper, we present the synthesis of new Ru
nanoparticles, stabilized by both chiral b-amino alcohols and
oxazolines, and their catalytic behaviour in the reduction of
unsaturated substrates.
Results and discussion
(
Fig. 4). In addition they also displayed a slightly anisotropic
shape, which is more visible in Ru3. This, or a lack of crystal-
linity, may account for the shortening of coherence length, as
measured by WAXS, in Ru3 (Fig. 5).
Syntheses of ligands and stabilized nanoparticles
Four types of N-donor ligands (1–8) were used as stabilizers
for the synthesis of Ru nanoparticles (Ru1–Ru8): b-amino
alcohol (1), amino(oxazolines) (2, 3), hydroxy(oxazoline) (4)
and bis(oxazolines) (5–8) (Fig. 1). The oxazoline derivatives
were synthesized following our previously described proce-
1
2b,13
dures,
and obtained in a diastereomerically pure form,
with the exception of 8, which was separated as a mixture of
1
3b
two diastereomers due to its axial chirality.
Phenyl-mono
(
oxazolines), 2–4, contain alcohols or amines as functional
groups in an ortho-position with respect to the heterocycle,
capable of interacting with the nanoparticle surfaces.
Ruthenium colloids (Ru1–Ru8) were prepared following our
previously reported procedure, but using the chiral ligands
7
described above as stabilizers (Scheme 1). In a typical experi-
1
6
ment, the precursor [Ru(cod)(cot)] is dissolved at 193 K in a
THF or THF–methanol solution containing 0.2 equivalents
of the appropriate ligand, L*, in a Fischer–Porter bottle
(
Scheme 1).
The resulting yellow solution is then exposed to a dihydro-
gen atmosphere (3 bar), allowed to warm to room temperature
and stirred vigorously for 24 hours. Ligand stability was tested
under the same conditions and no imine bond reduction was
observed. The colloids obtained (Ru1–Ru8) were purified by
precipitation upon the addition of pentane, followed by filtra-
tion and drying under reduced pressure, giving rise to a dark
Scheme 1 Synthesis of Ru colloids Ru1–Ru8 stabilized by the corre-
sponding ligands, 1–8.
Fig. 2 TEM micrograph and size histogram of Ru1.
1
16 | New J. Chem., 2006, 30, 115–122
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Fig. 3 Solid line: experimental RDF (radial distribution function) of
Ru1; dashed line: RDF computed from a spherical model according to
bulk ruthenium (1.6 nm in diameter; distances in the model
were increased by 1.5% for better agreement with the experimental
function).
Fig. 5 Solid line: experimental RDF of Ru3; dashed line: RDF
computed from a spherical model according to bulk ruthenium
(1.6 nm in diameter).
All of these structural observations denote a good argument
for the coordination of the ligands used at the surface of the
ruthenium particles.
Hydroxy(oxazoline) 4 led to well-dispersed and slightly
elongated nanoparticles of 2.7 nm mean size (Fig. 6).
Concerning the bis(oxazoline) ligands, well-defined nano-
particles of very small size (nearly 1.6 nm) and of isotropic
shape (Ru5–Ru8) were obtained in all cases. TEM micro-
graphs of Ru5 and Ru6 are shown as representative examples
in Fig. 7. These particles are crystalline, as determined by
WAXS, with a coherence length close to 1.6 nm, and exhibit-
ing a strong tendency to aggregate into large and regular
mesoscopic super-structures.
IR spectra were recorded for all samples. However, due to
the low overall content in the ligand, the bands displayed a
weak intensity (recording more than 100 scans) and conse-
quently, we were unable to reach any structural conclusions.
We have previously noted that platinum nanoparticles
stabilized by oxazoline ligands self-assemble into one-dimen-
sional super-structures (‘‘hairs’’), due to the formation of
1
7
hydrogen bond networks, particularly for ligand 4. Although
in the present case, formation of a hydrogen bond network
may have occurred, no extended super-structures were ob-
served. Both bis(oxazolines) 5 and 6, containing, respectively,
two and four methylenic groups within their backbone, gave
large spherical super-structures (bowls). This organization
may result from the coordination of both ends of the ligands
to different nanoparticles, in agreement with the very short
interparticle distance observed in these objects (ca. 1 nm).
Such super-structures would then result from the statistical
connection between nanoparticles, consistent with the appar-
ent lack of organization in these ‘‘bowls’’.
Fig. 4 TEM micrographs of Ru2 (a) and Ru3 (b).
Fig. 6 TEM micrograph and size histogram of Ru4.
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Table 1 Ru colloid-catalyzed hydrogen transfer of acetophenone by
chiral N-donor ligands (Scheme 2)
a
Time
(h)
Conversion
(%)
Ee
II (%)
b
c
c
Entry
RuL*
Ru : L*total
1
2
3
4
5
6
7
8
9
Ru1
Ru2
Ru3
Ru4
Ru5
Ru6
Ru7
Ru8
Ru1
Ru1
Ru3
Ru3
Ru8
Ru8
1 : 0.2
1 : 0.2
1 : 0.2
1 : 0.2
1 : 0.2
1 : 0.2
1 : 0.2
1 : 0.2
1 : 1.2
1 : 2.2
1 : 1.2
1 : 2.2
1 : 1.2
1 : 2.2
12
12
12
12
72
12
48
12
12
12
12
72
12
12
77
58
65
39
14
53
12
98
31
5
0
0
10 (S)
0
0
0
0
0
0
—
10 (S)
10 (S)
—
Fig. 7 TEM micrographs of Ru5 (a) and Ru6 (b).
1
1
0
1
15
20
79
56
The same held true for our attempts to characterize colloids by
1
12
13
14
a
13
H or C NMR spectroscopy. In the case of Ru3, broad bands
1
—
by H NMR were observed, which may be attributable to the
equilibrium found at the particle surface between free and
coordinated ligand molecules, characteristic of the Ru–hexa-
0
.12 mmol acetophenone and 0.024 mmol t-BuOK in 4 mL of
ꢂ3
isopropanol, and 6 ꢁ 10 mmol colloid. Results from duplicated
b
experiments. Ruthenium–ligand ratio taking into account the total
8
decylamine system. In this instance, some dihydrogen, re-
amount of ligand present in the reaction, from the synthesis of colloid
c
with the extra ligand added to the catalytic reaction. Substrate
leased in the solution, was detected, which was similarly
1
observed for ruthenium colloids stabilized with heptanol or
8
conversion and ee determined by GC using a chiral column.
8
hexadecylamine (HDA). The release of dihydrogen suggests
that nanoparticles may additionally contain mobile hydrides
at their surface.
ruthenium) were added (entry 1 vs. 9 and 10; entry 3 vs. 11
and 12; entry 8 vs. 13 and 14), in contrast to the behaviour of
1
2,19
the molecular ruthenium catalysts.
more significant when larger amounts of ligand were added
entry 9 vs. 10; entry 13 vs. 14). In only the enantioselective
This decrease was even
Catalytic reactivity
(
To demonstrate the coordination chemistry found at the sur-
face of these particles, we decided to evaluate the catalytic
reactivity of these new ruthenium colloids. In this way, the
selective metal-catalyzed processes offer us a useful tool. An
intrinsic problem in the application of colloids as catalysts is
the necessity to distinguish between a true colloidal and a
molecular catalyst. We therefore chose reactions well-known
to be catalyzed by ruthenium complexes, namely, asymmetric
hydrogen transfer and hydrogenation.
system Ru3, the total ligand amount in the catalytic medium
did not induce changes in the enantiomeric excess of II (entries
3
, 11 and 12, Table 1).
Analyzing the catalytic behaviour of these Ru colloids,
based on the nature of the ligand, we found that those colloids
containing amino functional groups (ligands 1–3) were more
active than hydroxy(oxazoline) (4) and bis(oxazolines) (5–8).
This trend agrees with results published in the literature on
molecular Ru catalysts, for which at least one primary or
secondary amine moiety must be present in the ligand in order
We will first discuss the results obtained for the asymmetric
hydrogen transfer of acetophenone (I), using isopropanol as a
hydrogen source under basic conditions, and the ruthenium
colloids Ru1–Ru8 as catalysts (Table 1). The reactions were
carried out at room temperature and monitored by gas
chromatography (Scheme 2). The substrate–metal ratio was
2
0
to obtain high yields. For bis(oxazoline) systems (entries 5–8,
Table 1), an important backbone effect was observed: those
bis(oxazolines) containing shorter spacers (5 and 7) were less
active than those with longer ones (6 and 8), Ru8 proving to be
the most active colloidal catalytic system. Ligands 5, 6, and 8
induced the formation of super-structures, probably by using
their two oxazoline rings to link to different nanoparticles, and
in the case of 8, to thioether moieties. In contrast, ligands 5
and 7 may have been too firmly fixed on their respective
nanoparticles, rendering any approach by the reactants to
the metallic surface difficult.
2
0 : 1, whereby the ruthenium amount accounts for all metal
atoms included in the nanoparticles (based on the organome-
tallic precursor, [Ru(cod)(cot)]).
When no further free ligand was added in the catalytic
medium (entries 1–8, Table 1), the systems were not selective
(
except Ru3 which afforded 10% (S) for II), but did exhibit
moderate to high activities (up to 98% conversion after 12
hours, entry 8, Table 1). While certain tests were conducted at
low temperature (273 K), enantioselectivity did not improve.
Optimum activities corresponded with those ruthenium sys-
tems stabilized by amino alcohol 1 (entry 1), amino(oxazoline)
In order to compare the catalytic behaviour between mole-
cular and colloidal ruthenium species, the data corresponding
3
(entry 3) and bis(oxazoline) 8 (entry 8). In order to observe
the influence of ligand concentration, supplemental free ligand
was added to the catalytic medium for the more active systems,
Ru1, Ru3, and Ru8 (entries 9–14, Table 1). In the three
catalytic systems, a remarkable decrease in activity was ob-
served when 1 or 2 equivalents of free ligand (relative to
Scheme 2 Hydrogen transfer of acetophenone catalyzed by ruthe-
nium systems containing N-donor chiral ligands (L* = 1–8).
1
18 | New J. Chem., 2006, 30, 115–122
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Table 2 Ru colloids, RuL*, and the related molecular species,
RuML*, for L* = 1–8, as catalysts in the hydrogen transfer of
acetophenone (Scheme 2)
a
Time Conversion Ee
(h)
b
c
c
Entry L* Catalyst Ru : L* total
(%)
II (%)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
1
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
Ru1
RuM1
Ru2
RuM2
Ru3
RuM3
Ru4
RuM4
Ru5
RuM5
Ru6
RuM6
Ru7
RuM7
Ru8
RuM8
1 : 0.2
1 : 1
1 : 0.2
1 : 1
1 : 0.2
1 : 1
1 : 0.2
1 : 1
1 : 0.2
1 : 1
1 : 0.2
1 : 1
1 : 0.2
1 : 1
1 : 0.2
1 : 1
12
1.5
77
93
58
15
65
55
39
0
14
88
53
90
12
42
98
62
0
12 (R)
0
43 (R)
10 (S)
45 (S)
0
12
12
12
12
12
72
72
20
12
12
48
20
12
12
0
0
25 (S)
0
11 (R)
0
29 (S)
0
0
1
1
1
1
1
1
1
a
Scheme 3 Hydrogenation of ortho- and para-methylanisole catalyzed
by the Ru3 colloidal system [relative percentage following catalysis
(determined by GC) is indicated below each chemical].
the hydrogenation of prochiral olefins such as anisole deriva-
tives (III and V) and dimethyl itaconate (VII).
0
.12 mmol acetophenone and 0.024 mmol t-BuOK in 4 mL of
ꢂ3
Concerning the disubstituted arenes, ortho- and para-
methylanisole, the molecular system, generated in situ from
ꢂ3
isopropanol; 3 ꢁ 10 mmol [Ru(p-cymene)Cl
2
]
2
with 6 ꢁ 10 mmol
ꢂ3
L* for molecular catalysts generated in situ, RuML*; or 6 ꢁ 10
[Ru(cod)(cot)] and ligand 3 (see Experimental section for
mmol of colloid catalyst, RuL*. Results from duplicated experi-
b
ments. Ruthenium–ligand ratio, taking into account the total
details), was not found active, in agreement with the well-
c
amount of ligand present in the reaction. Substrate conversion and
known trend for catalytic homogenous systems in the reduc-
,22
3
ee determined by GC using a chiral column.
tion of arenes.
However, under the same conditions (40 bar
hydrogen pressure and 50 1C for 6 h), the colloidal system Ru3
led to a more than 50% conversion of the substrate, with the
cyclohexane derivatives (IV and VI) being the main products:
1
2
to the molecular systems (RuM1–RuM8) and the nanoclus-
ters (Ru1–Ru8), which are stabilized by the same kind of
ligands, were collected in Table 2. We observed that for amino
alcohol 1 (entries 1 and 2, Table 2) and bis(oxazolines) 5–7
5
7% for substrate III and 87% for V (percentages based on
the total amount of detected products). In both cases, the
trans-isomer was favoured, up to a trans : cis ratio of 19 : 1 for
23
VI (Scheme 3), in contrast to the heterogeneous catalytic
24
systems which favour the cis-derivatives. It is important to
(
entries 9–14, Table 2), the colloidal systems were less active
than their corresponding molecular ones. This trend changed,
however, for amino(oxazolines) 2 and 3, as well as for the
thioether bis(oxazoline) 8 (entries 3–6 and 15–16, Table 2).
In addition, the RuM4 molecular system was inactive, while
the corresponding colloidal one was active (entry 8 vs. 7, Table
emphasize that the addition of free ligand (Ru : 3 ratio up to
1
: 0.5) rendered the system completely inactive.
With regard to dimethyl itaconate, Ru3, as well as the
homogenous analogous system RuM3, were found to be active
for the hydrogenation of VII (Scheme 4). In this particular
2
). This suggests a reaction of the molecular precursor result-
ing from the presence of the phenoxo group (formation of a
s- or p-bonded phenoxide complex), which would render the
molecular system inactive, an outcome not seen in ruthenium
2
1
nanoclusters.
It is also noteworthy that Ru8 is the fastest colloidal
catalyst, even more so than its molecular counterpart (entry
1
5 vs. 16, Table 2). This ligand is quite bulky and contains an
Scheme 4 Hydrogenation of dimethyl itaconate catalyzed by ruthe-
nium systems containing ligand 3.
aromatic biphenyl backbone capable of interacting with the
nanoparticle surface and thioether substituents, which are
good electron releasing groups.
Table 3 Ru systems containing ligand 3 (colloidal, Ru3 and mole-
cular, RuM3) as catalysts in the hydrogenation of dimethyl itaconate
a
The activity behaviour of these systems indicates a specific
coordination chemistry of ligands at the metallic surface,
depending on the nature of the ligand.
b
c
c
Entry
Catalyst
Ru : L*total
Conversion (%)
Ee VIII (%)
1
2
3
a
RuM3
Ru3
Ru3
1 : 2
1 : 0.2
1 : 0.4
100
100
80
18 (R)
0
0
With regard to the selectivity, only the colloidal system
stabilized by amino(oxazoline) 3 gives asymmetric induction,
lower than that of its corresponding molecular system (entry
2
mmol dimethyl itaconate, 2 mL n-butanol (internal standard) and
ꢂ2
5
vs. 6, Table 2).
0.024 mmol t-BuOK in 6 mL of methanol; 2 ꢁ 10 mmol Ru3 or 2 ꢁ
ꢂ2
ꢂ2
In addition, TEM analysis performed before and after
10 mmol [Ru(cod)(cot)] and 4 ꢁ 10 mmol of 3 for molecular
catalyst generated in situ, RuM3. Results from duplicated experi-
b
ments. Ruthenium–ligand ratio, taking into account the total
catalysis revealed that the size of ruthenium particles remained
unchanged, although some agglomeration was visible.
We also analyzed the behaviour of the sole enantioselective
colloidal catalyst, Ru3, for the hydrogen transfer reaction, in
c
amount of ligand present in the reaction. Substrate conversion and
ee determined by GC using a chiral column.
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6
case, reactions were carried out at room temperature under 40
bar H pressure for 7 hours, using a substrate–metal ratio of
00 : 1 (Table 3). When the catalyst was in situ generated from
Ru(cod)(cot)] and the free ligand 3 (Ru : L* ratio = 1 : 2),
[Ru(Z -p-cymene)Cl(m-Cl)]
2
(Strem), t-BuOK (Fluka), (R)-
2
(þ)-2-aminobutanol (Fluka) and L-valinol (Aldrich) were pur-
chased and used without prior purification. HPLC grade
isopropanol was purchased from Fluka. The other reagents
were purchased from Aldrich and most of the solvents from
SDS. The solvents were distilled under a nitrogen atmosphere.
THF was refluxed over sodium benzophenone, pentane over
calcium hydride, and methanol over magnesium after activa-
tion with iodine. All reagents and solvents were degassed
under vacuum at liquid nitrogen temperature by 3 vacuum–
argon cycles.
1
[
total conversion was achieved, giving a low ee, 18% (R) for
product VIII (entry 1, Table 3). When a colloidal catalyst was
used, activity was also high, but without inducing any enan-
tiomeric excess (entries 2 and 3, Table 3). Analogous with the
behaviour observed in acetophenone hydrogen transfer, the
addition of free ligand in the catalytic medium effected a
decrease in activity (entry 2 vs. 3, Table 3).
[
Ru(cod)(cot)] was prepared according to a published pro-
1
6
cedure. It was purified by recrystallization in pentane and the
resulting highly sensitive yellow crystals were stored under
argon at ꢂ30 1C. Their purity was checked by elemental
Conclusions
Ruthenium nanoparticles stabilized by chiral ligands were
prepared in an easy and reproducible way. Depending on
the stabilizer employed, different shapes and sizes were ob-
served.
1
analysis and H-NMR spectroscopy. Mono- and bis(oxazo-
1
2b,13a,13b
line) ligands were prepared as previously described.
Specimens for TEM analysis were prepared by slow eva-
poration of a drop of colloidal solution deposited under argon
onto holey carbon-covered copper grids. The TEM experi-
ments were performed at the ‘‘Service Commun de Micro-
Surface coordination of ligands not only led to an excellent
stabilization of metal particles in solution, but also strongly
influenced their catalytic behaviour. Mean diameters ca. 2.5
nm were found using mono(oxazoline) ligands, and ca. 1.6 nm
with bis(oxazolines). These trends are consistent with the size
decrease observed when ligand concentration increases, as well
as with the coordinating behaviour of bis(oxazolines), chelate
and bridge effects.
scopie Electronique de l’Universite Paul Sabatier’’ using a
´
JEOL 200 CX-T electron microscope operating at 200 kV
and a Philips CM12 electron microscope operating at 120 kV,
˚
with respective point resolutions of 4.5 and 5 A. HRTEM
observations were carried out with a JEOL JEM 2010 electron
microscope working at 200 kV with a resolution point of 2.5
One of the goals of this study was to find evidences for
differentiating colloidal from molecular catalysts. In the pre-
sent work, four clues agree with this objective. First, ligand 5
poisons the molecular system, probably through formation of
a P-bonded phenoxo complex, which was not observed in the
colloids. Second, ligand 8, which offers multiple possibilities
for interaction with the colloidal surface (nitrogen, sulfur,
P-system), is a better promoter of colloidal than molecular
catalysis. Third, in the three studied processes, the addition of
excess ligand decreased the activity of the catalytic system
without any effect on the selectivity; whereas an increase of ee
would be expected in the case of metal leaching to form a
molecular catalyst. And fourth, the ruthenium colloidal sys-
tem stabilized by ligand 3, was active in the reduction of arene
derivatives, in manifest contrast to the analogous mononuc-
lear systems. The very low asymmetric induction encountered
when colloidal catalysts are used, compared with molecular
systems, suggests a fluxional behaviour of the ligands at the
surface of ruthenium nanoparticles, as recently described for
Ru–amine colloids.
˚
A. Transmission electron microscopy was used as a standard
tool of analysis for determining the mean size of ruthenium
particles. Size distribution of the particles was determined by
manual analysis of enlarged images by measuring at least 200
particles on a given grid to obtain a statistical size distribution
as well as a mean diameter.
Data collection for the wide-angle X-ray scattering was
performed on small amounts of powder at the CEMES/
CNRS, Toulouse.
NMR spectra of particles were recorded on Bruker DRX
2
50 or 400 MHz (standard SiMe ) spectrometers, using THF-
4
d8 as solvent. Chemical shifts were reported downfield from
standards. IR spectra were recorded on a Perkin Elmer
spectrometer GX (FT-IR system). Colloid samples were pre-
pared as KBr pellets, unless otherwise stated.
Elemental analyses were performed in the ‘‘Services d’Ana-
lyses du CNRS’’ at the LCC in Toulouse for carbon, nitrogen
and hydrogen determinations, and at the ‘‘Service Central
d’Analyse’’ in Lyon for ruthenium, sulfur and oxygen deter-
minations.
To conclude, this study demonstrates the need to design
appropriate ligands, capable of binding to nanoparticles, as
well as of performing target reactions. This may well be the
case with ligand 8 which displays several sites of coordination.
Future work in this field should include this goal in order to
better exploit the adaptability of these colloidal systems.
GC analyses were performed on a Hewlett-Packard 5890
Series II gas chromatograph (50 m Ultra 2 capillary column)
with an FID detector. Enantiomeric excesses were determined
by GC on FS-cyclodex-b-I/P and FS-cyclodex-a-I/P columns.
General synthesis of Ru particles stabilized by asymmetric
organic ligands
Experimental
In a typical experiment, 150 mg of [Ru(cod)(cot)] (0.476
mmol) was introduced in a Fisher–Porter bottle and left in a
vacuum for 30 minutes. 125 mL of solvent (neat THF or THF–
methanol mixture), degassed by freeze–pump cycles, was then
General
All operations were carried out using Schlenk tube or Fisher–
Porter bottle techniques under argon. RuCl ꢃ H O (Strem),
3
2
1
20 | New J. Chem., 2006, 30, 115–122
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added. The resulting yellow solution was cooled to 193 K after
which a 25 mL solution of THF containing 0.2 equivalents of
the ligand was placed in the flask. The bottle was pressurized
under 3 bar of dihydrogen and the solution allowed to slowly
warm to room temperature under vigorous stirring. After 24
hours a homogenous brown solution was obtained. After
elimination of excess dihydrogen, approximately 3 mL of the
solution was passed under argon over a small alumina column.
The absence of filtrate colour indicated full decomposition of
the precursor. The volume of the solution was then reduced to
approximately 15 mL. 50 mL of deoxygenated pentane was
then added and the resulting mixture cooled to 193 K at which
temperature a brown precipitate formed after several hours.
Following filtration, the precipitate was washed with pentane
Catalytic reactions
Blank tests were carried out in order to evaluate the stability of
the ligands under the catalytic conditions. No hydrogenation
reactions were detected in any case.
Asymmetric hydrogen transfer of acetophenone. 0.12 mmol of
acetophenone (2 mL of solution 0.06 M in isopropanol) and
0
.024 mmol of t-BuOK (2 mL of solution 0.012 M in iso-
ꢂ3
propanol) were mixed at room temperature. 6 ꢁ 10 mmol of
colloid catalyst RuL (based on starting [Ru(cod)(cot)] com-
plex) in the presence of varying amounts of ligand L* (0.2, 1.0
or 2.0 equivalents, relative to the ruthenium; for molecular
ꢂ3
catalysts generated in situ: 3 ꢁ 10 mmol [Ru(p-cymene)Cl
2 2
]
ꢂ3
with 6 ꢁ 10 mmol L*) were then added. The reactions were
monitored by GC. Enantiomeric excesses and conversions
were determined by GC on a chiral column.
(
2 ꢁ 50 mL of deoxygenated pentane) and dried under reduced
pressure. The resulting particles were obtained as dark brown
powders. In all cases, ruthenium colloids were found to remain
stable over time and did not exhibit any signs of decomposi-
tion. They were characterized by IR spectroscopy, TEM, and
WAXS analysis.
Asymmetric hydrogenation of dimethyl itaconate. 2 mmol of
ꢂ2
dimethyl itaconate, 2 mL n-butanol and 2 ꢁ 10 mmol of Ru3
ꢂ2
(for the molecular catalysts generated in situ: 2 ꢁ 10 mmol
ꢂ2
of [Ru(cod)(cot)] with 4 ꢁ 10 mmol of 3) were dissolved in 6
ꢂ1
Ru1. Solvent used in the synthesis: THF. IR (cm ): 3431,
mL of methanol at room temperature in an autoclave. Mole-
cular hydrogen was then introduced until 40 bar of pressure
was attained. The reaction was stirred for 7 h. The solution
was filtered over celite and purified by column chromatogra-
2
4
959, 2915, 2846, 1926, 1627. Elemental analysis (%): Ru =
8.9, C = 7.6, H = 4.1, N = 1.3, O = 1.2. Mean diameter
(
TEM, nm) = 2.5.
phy (SiO ; ethyl acetate). Enantiomeric excesses and conver-
2
ꢂ1
Ru2. Solvent used in the synthesis: THF. IR (cm ): 3392,
sions were determined by GC on a chiral column.
2
963, 2927, 2877, 1610. Elemental analysis (%): Ru = 49.3,
Asymmetric hydrogenation of para- and ortho-methylanisole.
ꢂ2
C = 12.7, H = 6.1, N = 1.1, O = 0.9. Mean diameter (TEM,
nm) = 2.5.
6
mmol of methylanisole (724.4 mg) and 6 ꢁ 10 mmol of
Ru3 (6 mg, based on [Ru(cod)(cot)]) were dissolved in 10 mL
ꢂ1
of methanol at 50 1C in an autoclave (for the molecular
Ru3. Solvent used in the synthesis: THF. IR (cm ): 3409,
ꢂ2
catalysts generated in situ: 2 ꢁ 10 mmol of [Ru(cod)(cot)]
2
959, 2922, 2856, 1610. Elemental analysis (%): Ru = 49.6,
ꢂ2
with 4 ꢁ 10 mmol of 3). Molecular hydrogen was then
C = 12.1, H = 5.3, N = 1.2, O = 1.3. Mean diameter (TEM,
nm) = 2.0.
introduced until 40 bar of pressure was attained. The reaction
was stirred for 6 h. The solution was filtered over celite and the
solution analyzed by GC. Catalyses were also carried out in
ꢂ1
Ru4. Solvent used in the synthesis: THF. IR (cm ): 3411,
ꢂ2
the presence of free ligand, using a mixture of 6 ꢁ 10 mmol
2
950, 2921, 2858, 1606. Elemental analysis (%): Ru = 46.1,
ꢂ3
of Ru3 and 6 ꢁ 10 mmol of 3 as a catalytic precursor.
C = 13.2, H = 6.1, N = 0.3, O = 2.3. Mean diameter (TEM,
nm) = 2.7.
Acknowledgements
ꢂ1
Ru5. Solvent used in the synthesis: THF. IR (cm ): 2907,
The authors are very grateful to V. Collie
tion to TEM/HRTEM analysis and the TEMSCAN (Service
Commun de Microscopie Electronique de l’Universite Paul
`
re for his contribu-
2
841, 1638. Elemental analysis (%): Ru = 44.2, C = 12.7,
H = 5.3, N = 0.9, O = 1.8. Mean diameter (TEM, nm) = 1.6.
´
ꢂ1
Sabatier). We would also like to thank CNRS, Egide (PAI
Picasso No 04230PL), the Ministerio de Educacio´n y Ciencia
Ru6. Solvent used in the synthesis: THF–MeOH. IR (cm ):
2
1
900, 2844, 1623. Elemental analysis (%): Ru = 44.3, C =
6.4, H = 0.9, N = 0.7, O = 0.9. Mean diameter (TEM,
(
CTQ2004-01546/BQU; Accio´n integrada HF2001-0024) and
the Generalitat de Catalunya for financial support. M. G.
would like to thank the Ministerio de Educacio´n, Cultura y
Deporte (PR2000-0178 0077107784) for a sabbatical grant.
nm) = 1.5.
ꢂ1
Ru7. Solvent used in the synthesis: THF. IR (cm ): 3411,
2
950, 2921, 2858, 1606. Elemental analysis (%): Ru = 45.0,
C = 14.2, H = 7.1, N = 0.8, O = 1.7. Mean diameter
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22 | New J. Chem., 2006, 30, 115–122
This journal is ꢀc the Royal Society of Chemistry the Centre National de la Recherche Scientifique 2006