Ruthenium nanoparticles stabilized by N-heterocyclic carbenes: Ligand location and influence on reactivity

Article abstract of DOI:10.1002/anie.201106348

NHCs go nano: Ruthenium nanoparticles were formed from (cyclooctadiene) (cyclooctatriene)ruthenium(0) and stabilized by N-heterocyclic carbenes (NHCs). Solid-state NMR spectroscopy revealed both the coordination of the NHC ligands on the surface of the particles and their surface reactivity. Copyright

Full text of DOI:10.1002/anie.201106348

Communications
DOI: 10.1002/anie.201106348
Surface Reactivity
Ruthenium Nanoparticles Stabilized by N-Heterocyclic Carbenes:
Ligand Location and Influence on Reactivity**
Patricia Lara, Orestes Rivada-Wheelaghan, Salvador Conejero, Romuald Poteau,
Karine Philippot, and Bruno Chaudret*
Despite the huge number of reports dealing with the prepa-
ration and use of metal nanoparticles (NPs), in particular for
catalysis, very few are dedicated to the understanding of the
surface chemistry and the influence of organic ligands, both on
the chemistry and on the physics of the nanoparticles.[1] The
most successful recent class of ligands in organometallic
chemistry is no doubt N-heterocyclic carbenes (NHCs).[2]
There are however to the best of our knowledge only three
reports on the use of NHC ligands for stabilizing or modifying
nanoparticles.[3–5] Among them, only that of Tilley and Vignolle
reports the use of NHCs to synthesize gold NPs.[3] The others
describe the addition of a chiral NHC modifier to Pd nano-
particles supported on iron oxide,[4] and the substitution of
ligands by NHCs but leading to unstable particles.[5] Further-
more, two reports propose the intermediacy of NHCs in the
stabilization of nanoparticles in ionic liquids.[6] None of these
reports deals with the characterization of the coordination
mode of the NHC ligands onto the nanoparticle surface.
Classical carbenes have been used successfully for the
stabilization of ruthenium nanoparticles.[7] The NHC ligands
have however many advantages, such as their strong electron-
donating properties, strong binding to transition metals,
absence of oxidation in contrast to for example phosphines,
and the fact that they contain only C, H, and N and no other
heteroatom, which makes them ideal candidates for stabiliz-
ing catalysts.[8] It is therefore astonishing not to find more
examples on the use of these ligands for the stabilization of
nanoparticles and is thus important to determine whether
these ligands are suitable or not in this field.
We have recently developed the use of various NMR
spectroscopy methods to understand the mode of coordina-
tion and the dynamics of ligands and adsorbates on the
surface of nanoparticles.[9] This is in our opinion the key to
control the growth and the surface state of nanoparticles, and
further to obtain new selective catalysts. Thus, many in situ
and operando techniques have been recently developed,[10]
but alternative simple methods, similar to the molecular
methods, such as NMR spectroscopy in particular, could bring
additional information on the reaction sites and possibly on
reaction intermediates.[9,11] For example, for ruthenium nano-
particles sterically stabilized by poly(vinylpyrrolidone)
(PVP), we have determined the location and dynamics of
hydrides as well as their reactivity towards olefins leading to a
facile splitting of a carbon–carbon bond.[9b]
Herein we present the use of NHC ligands, namely N,N-
di(tert-butyl)imidazol-2-ylidene (ItBu; L1)[12] and 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene (IPr; L2)[13] to stabilize
ruthenium nanoparticles (RuNPs) and the control of NHC
binding by the control of the substituents on nitrogen.
Furthermore, thanks to the synthesis of the analogous ligands
13C-labeled on the carbene carbon, we present NMR spec-
troscopic evidence for NHC binding to nanoparticles, and the
use of chemical reactivity (ligand exchange, CO binding, CO
oxidation, styrene hydrogenation) to characterize the surface
and also the location of the ligands.
[*] Dr. P. Lara, Dr. K. Philippot
CNRS; LCC (Laboratoire de Chimie de Coordination)
205 , Route de Narbonne, 31077 Toulouse (France)
and
Universitꢀ de Toulouse; UPS, INPT, LCC
31077 Toulouse (France)
O. Rivada-Wheelaghan, Dr. S. Conejero
Instituto de Investigaciones Quꢁmicas-Departamento de Quꢁmica
Inorgꢂnica, CSIC, Universidad de Sevilla
Ruthenium nanoparticles were synthesized by decompo-
sition of (1,5-cyclooctadiene)(1,3,5-cyclooctatriene)ruthe-
nium(0) [(Ru(cod)(cot)] in pentane at room temperature
under 3 bar H2 and in the presence of variable amounts of the
carbene ligands L1 or L2 (Scheme 1). The carbenes and not the
imidazolium precursors are used to avoid any pollution of the
nanoparticle surface. Different [ligand]/[Ru] molar ratios
were tested to find the best reaction conditions to obtain well-
controlled RuNPs.
Avda. Amꢀrico Vespucio, 49, 41092 Sevilla (Spain)
Dr. R. Poteau, Dr. B. Chaudret
LPCNO; Laboratoire de Physique et Chimie de Nano-Objets
135, Avenue de Rangueil, 31077 Toulouse (France)
E-mail: chaudret@insa-toulouse.fr
[**] We thank V. Colliꢃre and L. Datas (UPS-TEMSCAN) and P. Lecante
(CNRS-CEMES) for TEM/HR-TEM and WAXS facilities, respectively.
CNRS and ANR (Siderus project ANR-08-BLAN-0010-03) are also
thanked for financial support. P.L. is grateful to the Spanish
Ministerio de Educaciꢄn for a research contract. Financial support
from the Junta de Andalucꢁa (project no. FQM-3151) and the
Spanish Ministerio de Ciencia e Innovaciꢄn (projects CTQ2010-
17476 and CONSOLIDER-INGENIO 2010 CSD2007-00006, FEDER
support) is acknowledged. O.R.-W. thanks the Spanish Ministerio
de Ciencia e Innovaciꢄn for a research grant.
Reaction of [Ru(cod)(cot)] with 0.2 equiv L1 leads to a
black precipitate and an absence of stabilization of the
nanoparticles. However, using 0.5 equiv L1, a brown stable
colloidal solution is formed. Transmission electron microsco-
py (TEM) analysis revealed the presence of non-agglomer-
ated nanoparticles of (1.7 Æ 0.2) nm mean size, displaying a
narrow size distribution and adopting the hcp structure of
bulk ruthenium as revealed by fast Fourier transform analysis
(colloid 1, Figure 1).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106348.
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Angew. Chem. Int. Ed. 2011, 50, 12080 –12084

1, 2, and 3 respectively, in good agreement with the mean sizes
of the particles determined by HRTEM and hence with the
crystalline nature of the particles.
The nanoparticles in colloids 1–3 are stable in solution and
in the solid state in anaerobic conditions and can be handled
like molecular complexes. However, colloid 1 starts precip-
itating after one day in solution. The surface species were
studied by spectroscopic techniques as well as by titration for
the surface hydrides. The latter method is based on the
hydrogenation of olefins in the absence of added dihydrogen,
which can thus be only induced by the existing surface
hydrides (see the Supporting Information).[14] The resulting
values are 1.1, 1.3, and 2.5 H/Rus for colloids 1, 2, and 3,
respectively (Supporting Information, Table S2). They are in
agreement with those previously found for PVP- and dppb-
stabilized RuNPs.[9b,11d] The result for colloid 3 is however
quite high and may result from transfer dehydrogenation of
the alkyl group of L2 in excess, as suggested by NMR studies,
in a way similar to that described originally by Crabtree to
dehydrogenate alkanes.[15] This therefore leaves a surface
density of hydrides and demonstrates the high reactivity of
these RuNPs for hydrogen transfer.
Scheme 1. The N-heterocyclic carbenes and reaction conditions used
for the synthesis of ruthenium nanoparticles.
All systems were characterized by infrared spectroscopy,
for which no salient feature has been found but which
confirms the presence of carbenes, as well as by solid-state
and solution NMR spectroscopy (after dissolution of the
particles into [D8]toluene for the latter). The 1H NMR
spectrum of colloid 1 only shows a broad signal around
1.4 ppm attributed to the tBu groups but no signals in the 6–
7 ppm region where the CH protons of the imidazole
backbone are expected. The absence of signals for groups
immobilized close to nanoparticle surfaces has previously
been observed by solution NMR spectroscopy.[11b,16,17] It is
characteristic of the coordination of the ligands at the surface
of the particles and results both from the slow tumbling of the
particles in solution leading to fast T2 relaxation and from
surface heterogeneities.[18] Such a phenomenon has also been
observed by Tilley and Vignolle in the characterization of
NHC-stabilized gold nanoparticles.[3] In the case of colloid 2,
no signals corresponding to the aromatic protons of the
phenyl rings, the CH imidazole protons, and the isopropyl CH
proton (CH(CH3)2) groups are visible. A very broad signal is
only discernable in the 1.0–1.8 ppm region for the methyl
groups together with some hydrogenated carbene, as in
Figure 1. a) TEM image (scale bar: 50 nm) and b) HREM image (scale
bar: 2 nm) with corresponding size histogram (c) of colloid 1 ([ItBu]/
[Ru]=0.5).
The analogous reaction carried out in the presence of 0.2
or 0.5 equiv L2 leads to stable colloidal solutions in both cases
(colloids 2 and 3, respectively). TEM and HRTEM analyses
revealed the presence of non-agglomerated, spherical, mono-
disperse, and highly crystalline nanoparticles (Supporting
Information, Figures S2 and S3) with mean sizes slightly
decreasing from 1.7(0.2) nm in the case of the [L2]/[Ru] = 0.2
system, to 1.5(0.2) for [L2]/[Ru] = 0.5. Higher ligand concen-
trations thus lead to smaller particles, as previously observed
in other systems.[11b] Upon further addition of ligands
(1 equiv), even smaller particles were observed (1.2 nm),
but these were associated with molecular species as revealed
by 1H NMR spectroscopy.
1
colloid 1. The analysis of the H NMR spectrum of colloid 3
leads to the same conclusion. These observations demonstrate
the rigidity of the coordination of both L1 and L2 carbene
ligands on the ruthenium surface. Furthermore, in the case of
colloid 2, it suggests a close proximity of the phenyl rings to
the surface of the particles and possibly their participation in
the stabilization of the nanoparticles through a p or an agostic
interaction. We previously observed a similar NMR behavior
in the case of RuNPs stabilized with the 4-(3-phenylpropyl)-
pyridine for which p coordination of the phenyl ring was
demonstrated.[17]
An HRTEM study of colloid 3 ([L2]/[Ru] = 0.5) (Support-
ing Information, Figure S4) shows unambiguously the highly
crystalline character of the particles and their hexagonal close
packed structure for which fast Fourier transformation (FFT)
¯
¯
shows reflections corresponding to the (0111), (1101), and
¯
(1010) atomic planes. The hcp structure of the different
colloids is also shown by wide-angle X-ray scattering (WAXS)
measurements performed on powder samples after evapora-
tion of the solvent. Moreover, the coherence lengths of the
different samples are found to be 2.0, 1.7, and 1.7 for colloids
To evaluate the strength of the interaction between the
carbene ligands and the surface of the nanoparticles, ligand
substitution was attempted by adding an excess of good s-
donor ligands, such as diphenylphosphinobutane or dodeca-
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nethiol directly in a [D8]toluene solution of colloids 1 and 2.
This reaction was followed by 1H NMR. The addition of
diphenylphosphinobutane to colloids 1 and 2 and of dodec-
anethiol to colloid 1 only showed the signals for the free
added ligands (diphosphine or thiol) and no evidence either
for the coordination of such ligands or for free L1 or L2 in
solution. This result confirms that the carbenes are firmly
coordinated to the ruthenium surface, in good agreement with
the known strong bonds that NHCs form with metal
complexes.[19]
Figure 3. 13C MAS (bottom) and CP-MAS (top) NMR spectra of colloid
3* (13C-labeled [IPr]/[Ru]=0.5 NPs).
The coordination of the carbene ligands L1 and L2 on the
surface was studied by solid-state NMR at magic-angle
1
spinning (MAS-NMR) with and without H–13C cross-polar-
ization (CP) with the aim of determining their location and
possible dynamics. The CP MAS spectrum of colloid 1 shows
two broad signals near 120 and 55 ppm attributed to the
(CHN) and (-C(CH3)3) groups of L1, respectively, as well as a
larger signal near 30 ppm for the methyl of the tBu moieties
(Supporting Information, Figure S5). When using L1 13C
labeled on the carbene carbon (L1*), the spectrum of the
corresponding colloid, 1*, shows a broad peak centered at
190 ppm that is attributed to the coordinated carbene
function. Colloids 2 and 3 only show a broad signal near
25 ppm for the iPr carbon atoms and peaks near 125 ppm and
135–150 ppm for the aromatic carbon atoms. However, when
using a labeled ligand (L2*, colloid 2*), the coordination of
the carbene carbon was observed at 205 ppm (Figure 2).
Using the CP-MAS technique, the intensity of the terminal
CO signal increases (Figure 4). This indicates their location in
the vicinity of hydrogen carriers, namely the L1 carbene
Figure 4. 13C MAS (bottom) and CP-MAS (top) NMR spectra of colloid
1 after exposure to a 13CO atmosphere (0.6 bar, 20 h, RT).
ligands. Addition of 13CO was also performed on colloids 2
and 3 (or colloids 2* and 3*) prepared in the presence of 0.2 or
0.5 equiv of ligand L2 (or labeled L2 *), respectively. For
colloid 2, a spectrum similar to that of colloid 1 is observed,
namely a broad bridging CO signal centered at 240 ppm and
rigid terminal CO groups located near hydrogen carriers and
overlapping with the carbene signal in the case of colloid 2*
(Supporting Information, Figure S13). However, for colloids
3 (Figure 5) or 3*, only static terminal CO signals are
observed and no (or very little) bridging CO (Supporting
Information, Figures S7 and S12). Furthermore, at room
temperature, Colloid 1 + CO burns in air, Colloid 2 + CO is
degraded in air leading to CO oxidation, and colloid 3 + CO
reacts very slowly with dioxygen in air. This provides evidence
for the ability of ruthenium nanoparticles to oxidize CO
under mild conditions and also suggests in the case of colloid 3
the deactivation of the reactive faces after coordination of the
Figure 2. 13C MAS (bottom) and CP-MAS (top) NMR spectra of colloid
2* (13C-labeled [IPr]/[Ru]=0.2 NPs).
Lower chemical shifts of the carbene carbon atom in the
13C NMR are consistent with ligand coordination to transition
metals.[20] Interestingly, for colloid 3* which contains excess
L2*, besides the signal at 205 ppm, another one appears at
195 ppm which does not correspond to the free ligand
(216 ppm). The presence of these two signals suggests differ-
ent chemical environments for the carbene ligand L2*, which
could result from coordination at two different ruthenium
sites (Figure 3).
To probe the free sites present on the particle surfaces,
13CO was added to solid samples of colloids 1–3 under mild
conditions (room temperature, 0.5 or 0.6 atm). In the case of
colloid 1, the 13C MAS NMR spectrum shows an intense and
very broad signal for bridging carbonyl groups centered at
240 ppm, with no apparent spinning sideband and a weaker
but sharper signal near 200 ppm with spinning sidebands
corresponding to nondiffusing terminal carbonyl groups.
Figure 5. 13C MAS (bottom) and CP-MAS (top) NMR spectra of colloid
3 after exposure to a 13CO atmosphere (0.5 bar, 20 h, RT).
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Angew. Chem. Int. Ed. 2011, 50, 12080 –12084

carbene ligand. The terminal CO groups on the edges are not
oxidized.
we checked that the products of hydrogenation do not
stabilize nanoparticles. However, no hydrogenation (or a
very limited amount) is observed with L2. It is not clear why,
but it may result either from an electronic effects (less-basic
substituents on nitrogen and stabilization by conjugation in
the case of L2) or from a stabilization of the structure by ring–
metal interactions. The second point concerns styrene hydro-
genation, which is achieved at comparable rates for colloids 1
and 3. This reaction requires the p coordination of the
aromatic rings on the nanoparticle faces, which, in the case
of colloid 3, are not accessible to CO in a gas–solid reaction. It
is however likely that in solution styrene may displace the
possible p interactions developed by the phenyl rings of L2.[22]
In conclusion, we have presented the synthesis of stable
ruthenium nanoparticles using carbene ligands. Thanks to
NMR spectroscopy studies on the 13C labeling of the carbenes
and to the addition of 13CO, we observed the strong binding of
the NHC ligands to the surface and were able to propose a
location for them on the particles. Furthermore, we provide
evidence for the influence of the substituents of the carbenes
on the reactivity of the nanoparticles, which is mainly due to
steric effects. This study therefore describes new organome-
tallic objects covered by different species: hydrides, CO,
carbenes that can be located, the dynamics of which can be
studied, and which behave differently as a function of their
location on the particles (Figure 6). It is therefore an attempt
at the full molecular characterization of the surface of
catalytically active nanoparticles.
A typical test reaction for ruthenium nanoparticles is
arene hydrogenation. The reaction was carried out at room
temperature in THF in the presence of 3 bar H2 and for a
[substrate]/[catalyst] molar ratio of 315 using colloid 1 or
colloid 3 as catalysts. In both cases, the reaction proceeds as
expected, with full hydrogenation of the vinyl bond initially,
followed by hydrogenation of ethylbenzene into ethylcyclo-
hexane. The rates are similar for both colloids but signifi-
cantly slower than for a simple Ru/PVP system, thus
suggesting a reactivity limitation owing to ruthenium surface
covering by the carbene ligands (see the Supporting Infor-
mation).
This set of reactions demonstrates the high affinity of
carbene ligands for ruthenium nanoparticles. The nanopar-
ticles are stable in solution and can be used for catalytic
reactions. They do not show any sign of fluxionality or ligand
dissociation in THF by 1H NMR spectroscopy, and the
carbene ligands are not displaced in solution by diphosphines
or thiols. As expected, the carbenes first coordinate on the
most exposed atoms of the particles (apexes, edges). In the
case of L1, only the exposed ruthenium atoms can be linked to
the carbene, as the bulky tBu groups prevent L1 from
coordinating to a flat surface. In contrast, it is possible to
coordinate L2 first on the exposed atoms and on the faces. The
availability of coordination sites has been tested by adding
CO gas to solid samples of the different colloids. In the case of
colloid 1, the presence of almost only bridging CO reveals the
absence of carbene ligand on the faces whereas the edges are
encumbered by the bulky ligand. In the case of colloid 3, in
contrast, the absence of bridging CO is in agreement with the
presence of carbene ligands all over the particle, although
some free coordination sites for terminal CO could be found
in the vicinity of the carbene ligands. Furthermore, the
absence of fluxionality of the coordinated CO groups
indicates that their mobility is limited by the presence of
the carbene ligands. It is however possible to add sequentially
the carbene ligands in the case of L2 leading to a selective
complexation of the edges for a low L2 concentration and
leaving the faces accessible for CO coordination. These
studies therefore allow us to attain a good knowledge of the
nanoparticle surface coordination chemistry. However, two
facts raise questions. First, it is necessary to have an excess of
ligand L1 to stabilize colloid 1, which is not the case for ligand
L2 in colloid 2, although once coordinated, L1 is found only on
exposed atoms and is not displaced by diphosphine or thiols.
To answer this question, we have studied the hydrogenation
of the carbene ligands in the presence of catalytic amount of
[Ru(cod)(cot)] or of preformed colloids. We found that L1 is
easily hydrogenated at room temperature under 3 bar H2, first
into the unsaturated substituted 1,3-diazacyclopentene and
then into the corresponding 1,3-diazacyclopentane. This
reaction transforming the carbene carbon into a hydrogen
saturated aliphatic carbon has been observed previously
during hydrogenation of a bis(carbene)palladium(0) complex,
although the role of palladium black formed in this process
has not been clarified.[21] In addition, this reaction could
explain the necessity of excess L1 to obtain stable RuNPs, as
Figure 6. Space-filling model of a 1.8 nm hcp Ru nanoparticle stabi-
lized by 8L2 NHC ligands and accommodating 1.5 hydrides per surface
Ru.
Received: September 7, 2011
Published online: October 28, 2011
Keywords: nanoparticles · N-heterocyclic carbenes · ruthenium ·
solid-state NMR spectroscopy · surface reactivity
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Communications
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