Active hydrogenation Rh nanocatalysts protected by new self-assembled supramolecular complexes of cyclodextrins and surfactants in water

Article abstract of DOI:10.1039/C6RA21851B

The stability of inclusion complexes between randomly methylated β-cyclodextrin (RaMeCD) or its leucine-grafted analogue (RaMeCDLeu) with two hydroxylated ammonium surfactants was investigated. The binding isotherms and complexation constants were measured using the Isothermal Titration Calorimetry (ITC) technique. These host-guest inclusion complexes were used as protective agents during the formation of rhodium(0) nanoparticles by chemical reduction of rhodium trichloride in water. The amount of protective agent was adjusted in order to ensure both stability and reactivity of the rhodium nanocatalysts under the catalytic conditions. The size and dispersion of air-stable and water-soluble rhodium suspensions were determined by Transmission Electron Microscopy (TEM) analyses. These spherical nanoparticles, with sizes between 1.20 to 1.50 nm according to the nature of inclusion complexes, were evaluated in the biphasic hydrogenation of various reducible compounds (olefins, linear or aromatic ketones), showing promising results in terms of activity and selectivity.

Full text of DOI:10.1039/C6RA21851B

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Active hydrogenation Rh nanocatalysts protected
by new self-assembled supramolecular complexes
of cyclodextrins and surfactants in water†
Cite this: RSC Adv., 2016, 6, 108125
Nguyet Trang Thanh Chau,a Stephane Menuel,b Sophie Colombel-Rouen,a
´
a
Miguel Guerrero,cd Eric Monflier,b Karine Philippot,cd Audrey Denicourt-Nowicki
*
a
*
and Alain Roucoux
The stability of inclusion complexes between randomly methylated b-cyclodextrin (RaMeCD) or its leucine-
grafted analogue (RaMeCDLeu) with two hydroxylated ammonium surfactants was investigated. The
binding isotherms and complexation constants were measured using the Isothermal Titration
Calorimetry (ITC) technique. These host–guest inclusion complexes were used as protective agents
during the formation of rhodium(0) nanoparticles by chemical reduction of rhodium trichloride in water.
The amount of protective agent was adjusted in order to ensure both stability and reactivity of the
rhodium nanocatalysts under the catalytic conditions. The size and dispersion of air-stable and water-
soluble rhodium suspensions were determined by Transmission Electron Microscopy (TEM) analyses.
These spherical nanoparticles, with sizes between 1.20 to 1.50 nm according to the nature of inclusion
complexes, were evaluated in the biphasic hydrogenation of various reducible compounds (olefins, linear
or aromatic ketones), showing promising results in terms of activity and selectivity.
Received 31st August 2016
Accepted 2nd November 2016
DOI: 10.1039/c6ra21851b
www.rsc.org/advances
constitute sine qua non conditions for eco-responsible applica-
tions.11 Thus, the use of colloidal metallic particles nely
Introduction
dispersed in water constitutes one of the most effective and
sustainable approaches to achieve the targeted recycling objec-
tive, through a biphasic water-substrate catalytic process.12 For
this purpose, highly water-soluble protective agents, such as
surfactants,13 dendrimers14 or novel ligands such as N-
heterocyclic carbenes15,16 are necessary to maintain the nano-
species within the aqueous phase. More recently, as a class of
nontoxic oligosaccharides, cyclodextrins have been extensively
investigated in host–guest chemistry for construction of versatile
supramolecular complexes owing to their special hydrophobic
inner cavities.17 For instance, the inclusion complexation of
randomly methylated cyclodextrins (RaMeCD) with an ammo-
nium surfactant18 (HEA16Cl: N,N-dimethyl-N-hexadecyl-N-(2-
hydroxyethyl)-ammonium chloride) or a sulfonated phosphine19
as guest molecules has been used to efficiently stabilize nano-
sized ruthenium(0) hydrogenation catalysts in aqueous media
and thus tune the catalytic performances of the metallic centers.
Although ruthenium(0) nanospecies have been successfully pro-
tected by cyclodextrins alone,20,21 the less oxophilic rhodium
metal proved to be more difficult to stabilize with the formation
of visual aggregates. Based on these results, we investigated the
use of original inclusion complexes formed between various
cyclodextrins and ammonium surfactants as pertinent supra-
molecular assemblies to protect rhodium(0) species. These
nanocatalysts were evaluated and compared in terms of stability,
catalytic activity, and selectivity in the hydrogenation of various
The development of highly selective and readily recyclable
catalysts,1 as well as the substitution of volatile organic solvents
with novel reaction media (e.g. ionic liquids, supercritical uids
or water),2,3 constitute crucial parameters in the quest towards
sustainable and more environmentally-friendly chemical
processes.4,5 Recently, nanometer-sized metallic particles have
been developed as relevant catalysts for performing green
organic transformation processes because of their outstanding
intrinsic properties, combining the advantages of homoge-
neous and heterogeneous catalysts.6–9 Besides their pertinent
activities owing to a high number of surface-exposed metal
atoms and their original selectivities regarding to their unique
surface structures and combined supramolecular assemblies,10
they could also provide improved recovery potentialities, which
a
´
´
Ecole Nationale Superieure de Chimie de Rennes, CNRS UMR 6226, 11 Allee de
Beaulieu, CS 50837, 35708 Rennes Cedex 7, France. E-mail: alain.roucoux@
ensc-rennes.fr; audrey.denicourt@ensc-rennes.fr; Fax: +33 02 2323 8199; Tel: +33
02 2323 8037
b
´
´
Universite d'Artois, CNRS UMR 8181, Faculte des Sciences Jean Perrin, Rue Jean
Souvraz, SP 18, F-62307 Lens Cedex, France
cCNRS, LCC (Laboratoire de Chimie de Coordination), 205 Route de Narbonne, F-
31077 Toulouse, France
d
´
Universite de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France
† Electronic supplementary information (ESI) available: ITC thermograms and
isotherms, T-ROESY NMR spectra of inclusion complexes and TEM pictures
have been reported. See DOI: 10.1039/c6ra21851b
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Scheme 1 Synthesis of rhodium(0) nanoparticles stabilized by meth-
ylated b-cyclodextrin/ammonium surfactant inclusion complexes.
Fig. 1 Structure of CDs and surfactants.
substrates: a functionalized olen (methyl-2-acetamidoacrylate),
an activated ketone (ethyl pyruvate) and acetophenone, a perti-
nent substrate in terms of chemoselectivity (aromatic ring vs.
carbonyl group). Among the classical RaMeCD cyclodextrin or
HEA16Cl ammonium salt, a methylated cyclodextrin graed with
groups of the surfactant's polar head and the CD.18 The
thermodynamic parameters determined from ITC experi-
ments also conrm that the surfactants have the same
structural motif included into the b-CD cavity. Indeed, the
inclusion processes are all mainly entropy driven, showing
similar positive DS (Table 1). These results conrmed the
formation of stable inclusion complexes between methylated
CDs and hydroxylated ammonium surfactants, which could
potentially be pertinent protective assemblies of rhodium(0)
nanoparticles (Scheme 1).
a
L-leucine moiety (RaMeCDLeu)22 and an optically active
ammonium salt (QCD16Br = (1S,2R,4S,5R)-(+)-N-hexadecyl-5-
vinyl-2-quinuclidinium-methanol bromide)23 were also used as
potential guest/host molecules to tune the selectivity, and
potentially the asymmetric induction as a more challenging
objective (Fig. 1).
Rhodium(0) nanoparticles protected by these inclusion
complexes were synthesized, using a two-step methodology as
previously reported for the preparation of Ru0@RaMeCD–
HEA16Cl NPs.18 The rhodium trichloride hydrate (RhCl3$3H2O)
was chemically reduced, using sodium borohydride as reducing
agent, in the presence of RaMeCD and HEA16Cl. As previously
described in the literature18,26 the metallic surface is surrounded
by a double layer of stabilizers (Scheme 2). The rst layer is
composed of surfactant molecules, whose polar head is directed
towards the metallic surface, according to the adsorption of
counter-ion to electron-decient metal surfaces.27–29 The second
layer is constituted of cyclodextrins interacting with the lipo-
philic tail of the surfactant.
Results and discussion
Design and characterization of rhodium(0) nanoparticles
The Isothermal Titration Calorimetry (ITC) measurements were
achieved to check the stability and to determine equilibrium
constants of the novel inclusion complexes obtained from the
combination of methylated CDs with ammonium surfactants,
including RaMeCD–QCD16Br, RaMeCDLeu–HEA16Cl and
RaMeCDLeu–QCD16Br supramolecular assemblies (ESI-
Fig. S1†). The association constant values (Table 1) were
compared to the one of RaMeCD–HEA16Cl inclusion complex.
The association constants were found to be similar to that
of RaMeCD–HEA16Cl inclusion complex. In fact, T-ROESY
experiments indicated that the alkyl chain of ammonium
surfactants was included inside the CD cavity whatever the
inclusion complex (ESI-Fig. S2–S4†). According to the litera-
ture, dipolar contacts between the alkyl chain protons of the
hexadecyltrimethylammonium bromide and protons inside
the CD cavity provide a binding constant about 67 700 Mꢀ1, as
initially reported by Holzwarth24 and more recently revised to
60 733 ꢁ 11 484 Mꢀ1 by B. Tutaj and coworkers.25 Here, the
slightly higher values achieved with the surfactants bearing
a hydroxylated polar heads, could be attributed to the
formation of a hydrogen bond between the exible hydroxy
The zeta potential value z of the well-dispersed Rh0@Ra-
MeCD–HEA16Cl suspension, measured by electrophoretic light
scattering (DLS spectroscopy), was used to determine the
apparent charge of the nanoparticle in solution. A signicant
positive apparent charge of +50 mV for the rhodium species
coated with RaMeCD–HEA16Cl assemblies was obtained as
usually observed with ammonium stabilizers.30 This value,
which explains the good stability of the aqueous colloidal
suspensions, characterizes the difference of potential between
the solution far from the metal/solution interface (Stern layer
based on adsorbed ionic species) and the mobile part of the
disperse layer where salt concentration decreases when the
distance increases.
Table 1 Thermodynamic data obtained by ITC of various inclusion complexesa
Inclusion complex
K (Mꢀ1
)
DG (cal molꢀ1
)
DH (cal molꢀ1
)
ꢀTDS (cal molꢀ1
)
RaMeCD–HEA16Cl
84 500 ꢁ 14 700
75 200 ꢁ 12 800
96 100 ꢁ 26 800
91 300 ꢁ 25 400
ꢀ6714 ꢁ 104
ꢀ6645 ꢁ 102
ꢀ6790 ꢁ 170
ꢀ6760 ꢁ 169
856 ꢁ 17
664 ꢁ 18
237 ꢁ 63
212 ꢁ 65
ꢀ7570 ꢁ 121
ꢀ7309 ꢁ 120
ꢀ7027 ꢁ 233
ꢀ6972 ꢁ 234
RaMeCDLeu–HEA16Cl
RaMeCD–QCD16Br
RaMeCDLeu–QCD16Br
a
Determined by isothermal titration calorimetry at 298 K.
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Scheme 2 Proposed scheme for the supramolecular organization of inclusion complexes near the nanoparticle surface.
The RaMeCD/HEA16Cl ratio has been optimized since the reduction conditions was also checked. The conversion was
use of a stoichiometric ratio leads to the destabilization of the monitored by gas chromatography analysis. The specic activity
rhodium(0) particles in the 1-tetradecene hydrogenation under (SA) was calculated, considering an optimized reaction time for
standard conditions (1 bar H2, room temperature, 3 h). Two a complete conversion of the substrate and based on the total
equivalents of each protective agents proved to be efficient for introduced amount of metal and not on the exposed surface
the stabilization of Rh0 NPs, under atmospheric hydrogenation, metal. These specic activity values were clearly underestimated
as well as high pressure conditions (10 bar of H2). The particle but were suitable from an economic point of view.
size, morphology and dispersion were determined by Trans-
In a rst set of experiments, RaMeCD–QCD16Br, RaMeC-
mission Electron Microscopy (TEM) analyses (ESI-Fig. S5–S7†). DLeu–HEA16Cl and RaMeCDLeu–QCD16Br rhodium nano-
Whatever the stabilizer association, the rhodium particles catalysts were evaluated in the reduction of methyl-2-
were well-dispersed on the grid, with spherical morphology. The acetamidoacrylate (Table 2).
Rh0 NPs displayed a mean size in the range 1.2–1.5 nm,
Whatever the inclusion complexes (RaMeCD–QCD16Br,
depending on the cyclodextrin/surfactant association. Based on RaMeCDLeu–HEA16Cl or RaMeCDLeu–QCD16Br), the reduc-
the same surfactant (QCD16Br), the inclusion complex con- tion of methyl 2-acetamidoacrylate occurred exclusively on the
taining the more sterically hindered L-leucine-graed cyclo- activated C]C double bond with complete conversion in an
dextrin (Fig. 2a vs. Fig. 2c) led to the formation of larger NPs optimized 1.5 h time. It is noteworthy to report that the
than the RaMeCD (1.5 nm vs. 1.2 nm, respectively). Similarly, reduction of this substrate by Rh0 suspensions, solely stabilized
compared to Rh0@RaMeCDLeu/QCD16Br system, the colloids by cyclodextrins (RaMeCD or RaMeCDLeu), could not be ach-
stabilized by the RaMeCD combined with the less sterically ieved in these experimental conditions due to the destabiliza-
hindered surfactant (HEA16Cl) exhibited slightly smaller tion of the nanocatalysts (Table 2, Entries 4 and 5). Finally, the
diameters (1.3 nm vs. 1.5 nm) (Fig. 2b vs. Fig. 2c). In all cases, use of the sole ammonium surfactant (HEA16Cl or QCD16Br) as
the metal core size of these original Rh0 NPs were smaller than protective agents leads to well stable colloidal suspensions with
the host–guest inclusion complex-capped Ru0 NPs with an pertinent catalytic results in the reduction of methyl 2-acet-
average diameter of about 4 nm.18 This better control of the NPs amidoacrylate (Table 2, Entries 6 and 7). According to previous
growth could be easily explained by the presence of higher results based on similar investigations performed in our
quantity of protective supramolecular assemblies (inclusion group,23,30 these efficient activities could be explained by the
complex/Rh ratio ¼ 2/1) which provide a better control during dynamic behaviour of the double layer which facilitates the
the growth step of the nanoparticles.
access of the substrate at the particle's surface.
The reduction of the carbonyl function in ethylpyruvate as
prochiral model substrate was also performed in similar reac-
tion conditions (Table 3).
The transformation of ethylpyruvate into the corresponding
alcohol was totally achieved, with various specic activities
values according to the supramolecular assemblies in the
Catalytic hydrogenation using rhodium(0) nanoparticles
The catalytic performances of Rh0@RaMeCD–QCD16Br,
Rh0@RaMeCDLeu–HEA16Cl and Rh0@RaMeCDLeu–QCD16Br
NPs were evaluated in the hydrogenation of various model
substrates, such as an activated olen (methyl 2-acet-
amidoacrylate), an activated a-ketoester (ethylpyruvate) and an
aromatic ketone (acetophenone).
following order: RaMeCDLeu–HEA16Cl
>
RaMeCDLeu–
QCD16Br > RaMeCD–QCD16Br (SA of 67, 17 and 6 hꢀ1
,
respectively). These results could be justied by a more favor-
able displacement during the catalysis of free RaMeCDLeu vs.
RaMeCD in the aqueous phase starting from inclusion
complexes, as mentioned in Scheme 2, thus providing an effi-
cient solubilisation of the organic substrate inside the
RaMeCDLeu's cavity and a better diffusion towards the metal
surface. Moreover, based on the observed conversions at 1.5 h
(Table 3) and the comparison of the head group inside QCD16Br
The reduction of these prochiral compounds was carried out
under pure biphasic conditions (water/substrate), at room
temperature and under 10 bar of H2 with 1 mol% of the nano-
catalysts (cyclodextrins/surfactant/rhodium ¼ 2/2/1) as opti-
mized in 1-tetradecene hydrogenation (3 h, 10 bar of H2 for
100% conv. vs. 20 h under 1 bar of H2). These conditions were
chosen in order to provide good yields in reduced products.
Moreover, the stability of the triazole ring under the catalytic
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Table
2
Hydrogenation of methyl-2-acetamidoacrylate using
rhodium(0) nanoparticlesa
Entry
Stabilizing agent
Conv.b (%)
SAc (hꢀ1
)
1
2
3
4
5
6
7
RaMeCD–QCD16Br
RaMeCDLeu–HEA16Cl
RaMeCDLeu–QCD16Br
RaMeCDd
100
100
100
n.de
n.de
100f
100f
66.7
66.7
66.7
n.de
n.de
66.7
66.7
RaMeCDLeud
HEA16Cl
QCD16Br
a
Conditions: [substrate]/[Rh0] molar ratio ¼ 100/1, 10 mL of aqueous
b
suspension, 10 bar of H2, rt, 1.5 h. Determined by GC analysis.
c
Specic activity (SA) dened as number of mol of transformed
d
substrate per mol of Rh per hour. Formation of metal aggregates.
e
Non determined. f Not optimized time.
Table
3 Hydrogenation of ethylpyruvate using rhodium(0)
nanoparticlesa
Entry Stabilizing agent
Conv. at 1.5 h (%) Timeb (h) SAc (hꢀ1
)
1
2
3
RaMeCD–QCD16Br
RaMeCDLeu–HEA16Cl 100
RaMeCDLeu–QCD16Br 45
30
17d
1.5
6
6
66.7
17
a
Conditions: [substrate]/[Rh0] molar ratio ¼ 100/1, 10 mL of aqueous
b
suspension, 10 bar of H2, rt. Time for a complete conversion
determined by GC analysis. Specic activity (SA) dened as number
of mol of transformed substrate per mol of Rh per hour. Checked
c
d
twice.
From kinetics investigations, two reaction pathways could be
involved for the formation of the totally hydrogenated product
(1-cyclohexylethanol 3), either via 1-cyclohexylethanone 1 from
the reduction of aromatic ring, or either via 1-phenylethanol 2
from the reduction of carbonyl group. The higher ratio of 2 over
1 suggests that the carbonyl group is more easily reduced than
the aromatic ring. However, the signicant quantity of 1 (up to
24%) proves that the RaMeCD or RaMeCDLeu could not effi-
ciently wrap the aromatic ring and avoid its hydrogenation. In
addition, aer 1.5 h of reaction, complete conversions were
observed in the case of Rh0 NPs stabilized by the less sterically
hindered inclusion complexes (RaMeCD–QCD16Br and
RaMeCDLeu–HEA16Cl), compared to 60% conversion with
Rh0@RaMeCDLeu–QCD16Br catalytic system. Increasing reac-
tion time up to 3 h improved signicantly the conversion by
a factor of 2.5, however with a slightly destabilization of the Rh0
suspension with formation of aggregates. These results could be
Fig. 2 TEM pictures and size distributions of (a) Rh0@RaMeCD–
QCD16Br, (b) Rh0@RaMeCDLeu–HEA16Cl and (c) Rh0@RaMeCDLeu–
QCD16Br NPs.
and HEA16Cl, it seems that kinetics is slower when the
QCD16Br surfactant is located at the vicinity of the particle's
surface, due to an increased steric hindrance, which probably
limits the access of the substrate.
Finally, the chemoselectivity of the catalytic system was
investigated towards the hydrogenation of acetophenone
(reduction of the ketone vs. the aromatic ring (Table 4)).
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Table
4 Hydrogenation of acetophenone using rhodium(0)
According to optically active skeletons graed on randomly
methylated b-cyclodextrin (RaMeCDLeu) or surfactant
(QCD16Br), these assemblies alone or associated produced
a chiral environment onto the nanoparticle surfaces. Finally
Rh0@RaMeCD–QCD16Br, Rh0@RaMeCDLeu–HEA16Cl and
Rh0@RaMeCDLeu–QCD16Br were evaluated in the biphasic
liquid–liquid hydrogenation of various functional groups (C]
O, C]C) and demonstrated efficiencies in terms of stability and
catalytic performances. During acetophenon hydrogenation,
various differences in terms of conversions and selectivities
relative to the competitive hydrogenation of the C]O bond
versus the aromatic ring were observed, evidencing a direct
impact of the steric hindrance of the supramolecular associa-
tion. Nevertheless, in all cases, no signicant induction was
observed as expected. Finally, taking advantage of the supra-
molecular properties of assemblies to modulate the surface
reactivity of nanoparticles, this original work may open new
opportunities in the eld of nanocatalysis and desirably in
asymmetric reaction.
nanoparticlesa
Entry
Stabilizing agent
Time (h)
1 : 2 : 3b (%)
1
2
3
RaMeCD–QCD16Br
1.5
3
24 : 30 : 46
12 : 0 : 88
13 : 48 : 39
2 : 2 : 96
16 : 28 : 16
11 : 41 : 48
RaMeCDLeu–HEA16Cl
RaMeCDLeu–QCD16Br
1.5
3c
1.5
3c
a
Conditions: [substrate]/[Rh0] molar ratio ¼ 100/1, 10 mL of aqueous
b
suspension, 10 bar of H2, rt. Selectivity determined by GC analysis.
c
Catalytic system started to be destabilized.
attributed to the competitive affinity of the substrate (aceto-
phenone) and the surfactants inside the RaMeCDLeu's cavity.
During the catalytic process, a partial release of the CDs in the
bulk aqueous media allows the solubilisation of organic
substrate by some free CDs (Scheme 2) and, as a consequence,
enables a better diffusion of the substrates towards the metallic
Experimental
Reagents and chemicals
N,N-Dimethyl-N-hexadecyl-N-(2-hydroxyethyl)ammonium chlo-
ride (HEA16Cl) and (1S,2R,4S,5R)-(+)-N-hexadecyl-5-vinyl-2-
quinuclidinium-methanol bromide (QCD16Br) were synthe-
sized from an already described procedure.23,30 Randomly
methylated b-cyclodextrin (RaMeCD (1.8)) was purchased from
Sigma-Aldrich. This cyclodextrin was partially methylated at
positions 2, 3 or 6 and 1.8 hydroxyl groups per glucopyranose
unit were statistically modied. L-Leucine-N-[(1-random-
lymethylated-b-cyclodextrinyl-1H-1,2,3-triazol-4-yl)
amide] (RaMeCDLeu) was prepared according to the procedure
described in the literature.31
Rhodium(III) chloride hydrate was obtained from Strem
Chemicals. Sodium borohydride and all substrates were
purchased from Sigma-Aldrich, Acros Organics or Alfa Aesar
and were used without further purication. Water was puried
using Millipore Elix 5 (type MSP 100) system.
surface. Nevertheless,
a strong affinity between organic
substrate and CDs could disturb the dynamic organization of
the protective agents around the NPs.
Unfortunately, no selectivities could be achieved with the use
of chiral molecules as QCD16Br, RaMeCDLeu alone or associ-
ated, whatever the investigated prochiral substrate is (Tables 2–
4). Based on previous investigations in our group on chiral
ammonium-capped rhodium(0) nanocatalysts,23 the lack of
enantiodiscrimination during these asymmetric catalytic reac-
tions could be attributed to a signicant mobility of the inclu-
sion complex at the metal surface in water. Moreover, in
contrast with suitable coordinating ligands, the interaction of
the HEA16Cl and QCD16Br surfactants within the particle's core
seems to be too weak to provide asymmetric induction.
methyl-
Analytical procedures
ITC analysis. Formation constants of methylated CD–am-
monium surfactant complexes were determined by an
Conclusions
To conclude, the formation of supramolecular inclusion isothermal calorimeter (ITC200, MicroCal Inc., USA), in
complexes between ammonium surfactant and cyclodextrin- a phosphate buffer (pH 6.5) at 298 K. In the case of RaMeCD,
based chiral molecules was investigated. The existence of titrations of a 0.1 mM surfactant solution (QCD16Br or
a stable interaction between the surfactant and cyclodextrin was HEA16Cl) were realized with a 5 mM solution of RaMeCD. The
clearly evidenced by NMR spectroscopy and isothermal titration inclusion ability of RaMeCDLeu was then investigated by
calorimetry measurements showed a strong affinity between the competitive experiments with RaMeCD (injection of a 5 mM
lipophilic chain of the surfactant and the hydrophobic cavity of RaMeCD solution into a 0.1 mM surfactant–0.1 mM RaMeC-
cyclodextrin compounds, which was corroborated with their DLeu solution). Each experiment was performed three times
signicant association constant values. These host–guest and consisted in the addition of an initial aliquot of 0.4 mL,
inclusion complexes were successfully used as pertinent followed by ten injections of 3.7 mL, delivered over 7.4 s, within
supramolecular assemblies to protect rhodium(0) nanospecies. a 204.5 mL cell. Time interval between two consecutive injec-
TEM analyses showed very small (1.2–1.5 nm) and well- tions was 60 s and agitation speed was 1000 rpm. The corre-
dispersed spherical nanoparticles without aggregates. sponding heat ow was recorded as a function of time, and the
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heat effects due to dilution were taken into account by per-
forming blank titrations. Binding constants were determined by
nonlinear regression analysis of binding isotherms (obtained by
integration of the raw signal) using a dedicated homemade
program.32
General procedure for high pressure hydrogenation reactions
The stainless steel autoclave was charged with the aqueous
colloidal rhodium(0) suspension (10 mL, 1.9 ꢃ 10ꢀ2 mmol) and
appropriate substrate ([substrate]/[Rh0] ratio ¼ 100/1). The
autoclave was degassed three times and hydrogen gas was
admitted to the system at a constant pressure (10 bar H2). The
mixture was stirred vigorously at room temperature. Samples
were removed from time to time to monitor the reaction by gas
chromatography in previously mentioned conditions.
TEM analysis. Transmission electron microscopy (TEM)
images were performed at the “Service Commun de Microscopie
´
Electronique de l’Universite Paul Sabatier” (UPS-TEMSCAN,
Toulouse) and recorded with a JEOL 1011 electron microscope
˚
operating at 100 kV with resolution point of 4.5 A. A drop of
rhodium nanoparticles in water was deposited on a carbon-
coated copper grid and dried in air. The size distributions
were determined through a manual analysis of enlarged
micrographs with Image J soware using Microso Excel to
generate histograms of the statistical size distribution and
a mean diameter.
Dynamic light scattering spectroscopy. The zeta potential z
of the nanoparticle aggregates were measured by dynamic light
scattering (DLS) using a DelsaNano C instrument (Beckman
Coulter). The aqueous suspensions of rhodium nanoparticles
were analysed at 25 ꢂC and measurements were started 10 min
aer the cell was placed in the DLS apparatus to allow the
temperature to equilibrate.
Acknowledgements
The authors are grateful to CNRS and the Agence Nationale de la
Recherche for the nancial support of the SUPRANANO
program (ANR-09-BLAN-0194). We also thank David Landy and
´
´
ˆ
Eleonore Bertaut (Universite du Littoral Cote Opale) for
Isothermal Titration Calorimetry measurements and Gregory
´
Crowyn (Universite d’Artois) for NMR measurements. We thank
Buchler GmbH for Quincoridine precursor gi.
References
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conversion and the selectivity were determined by gas chro-
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detector equipped with a chiral Varian Chiralsil-Dex CB capil-
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isotherm program with oven temperature, 90 ꢂC (ethylpyruvate)
or 130 ꢂC (acetophenone and 2-acetamido acrylate); carrier gas
pressure, 50 kPa.
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