Catalytically active nanoparticles stabilized by host-guest inclusion complexes in water

Article abstract of DOI:10.1039/b818786j

Hydrogenation of arene derivatives can be successfully performed in water by using ruthenium(0) nanoparticles stabilized by 1: 1 inclusion complexes formed between methylated cyclodextrins and an ammonium salt bearing a long alkyl chain. The Royal Society of Chemistry.

Full text of DOI:10.1039/b818786j

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Catalytically active nanoparticles stabilized by host–guest inclusion
complexes in waterw
Claudie Hubert,a Audrey Denicourt-Nowicki,a Alain Roucoux,a David Landy,b Bastien Leger,c
Gregory Crowync and Eric Monflier*c
Received (in Cambridge, UK) 22nd October 2008, Accepted 3rd December 2008
First published as an Advance Article on the web 14th January 2009
DOI: 10.1039/b818786j
Hydrogenation of arene derivatives can be successfully
performed in water by using ruthenium(0) nanoparticles stabi-
lized by 1 : 1 inclusion complexes formed between methylated
cyclodextrins and an ammonium salt bearing a long alkyl chain.
surface of the gold NPs.8 Finally, the self assembly of gold
NPs produced by sputter deposition onto microcrystal faces of
a a-CD–dodecanethiol inclusion compound has been recently
reported by Jara et al.9 Surprisingly, no study has been
performed to evaluate the potential of inclusion complexes
as stabilizer agents of catalytically active metallic NPS.
In this paper, we report the preparation of ruthenium(0)
NPs stabilized by inclusion complexes formed between the
chloride salt of N,N-dimethyl,N-hexadecyl,N-(2-hydroxyethyl)-
ammonium (HEA16Cl) and randomly methylated cyclo-
dextrins (RAME-b-CD)z and the ability of these stabilized
Ru(0) NPs to perform hydrogenation of various aromatic
derivatives. Our attention was focused on inclusion complexes
formed between CD and ammonium salts bearing a long
alkyl chain as it is well known that these compounds can
strongly interact with b-CD derivatives.10 Furthermore, we
have demonstrated that these compounds and methylated
CDs could be used separately to stabilize metal NPs in
water.11
The synthesis of well-controlled nanometric size metal species
has received a great deal of interest over the past few years due
to their physical and chemical properties.1 Based on their high
surface specific area and their small size providing numerous
potential active sites, their use in catalysis is associated to the
development of fine chemistry in an economical and environ-
mental context.2 Several methods have been developed for the
preparation of metal nanoparticles according to the ‘‘organic’’
or ‘‘aqueous’’ reaction media and the nature of the metal
precursor or protecting agents.3
In this context, we have explored the possibility to stabilize
catalytically active metallic nanoparticles (NPs) in water by
inclusion complexes formed between b-cyclodextrin (CD)
derivatives and organic compounds as guests. In fact, a careful
examination of the literature indicate that CD inclusion
compounds can be adsorbed on metallic NPs. For instance,
the strong affinity of ferrocene derivatives for the b-CD has
been used to modify the surface of CD capped NPs.4 Thus,
Kaifer et al. showed that hydrophilic b-CD capped NPs can be
solubilized in chloroform by adding cationic ferrocene
moieties that have long aliphatic chains.5 Flocculation of gold
NPs has also been achieved by the same group using a dimeric
ferrocene guest.6 Similar work was also performed by
Reinhoudt and co-workers. They probed the influence of
multivalency and cooperativity on the assembly of network
aggregates from b-CD-capped gold NPs and various
adamantyl-containing guest molecules.7 Highly photo-
responsive monolayer-protected gold NPs can also be
achieved by self-assembling of an inclusion complex between
a-CD and an azobenzene-terminated alkanethiol on the
First, 1D and 2D NMR spectroscopic studies have been
performed to prove the formation of inclusion complexes
between the HEA16Cl surfactant and RAME-b-CD in
aqueous medium. For comparison, the interactions between
HEA16Cl and native b-CD were also examined. The existence
of inclusion processes and the stoichiometry of the inclusion
complexes were provided by the continuous variation techni-
que (Job’s method).12 Indeed, the 1H NMR spectra of aqueous
solutions containing variable ratios of CD and HEA16Cl
exhibited chemical shift variations for the HEA16Cl protons
and for most of the CD protons (see ESIw). Furthermore, all
1
Job’s plots derived from the corresponding H NMR spectra
show a maximum at r = 0.5 and symmetrical shapes,
indicating that the stoichiometry of inclusion complexes is
1 : 1 (see ESIw). The formation of inclusion complexes was
definitively proved by two-dimensional T-ROESY experi-
ments. Indeed, strong dipolar contacts were observed between
the alkyl chain protons of the HEA16Cl and protons situated
inside the hydrophobic CD cavity (see ESIw). The association
constant of each inclusion complex was evaluated at 298 K
from UV-Vis spectroscopic data. The association constant for
a
´
Ecole Nationale Superieure de Chimie de Rennes, Equipe Chimie
Organique et Supramoleculaire, UMR CNRS 6226 Sciences
´
Chimiques de Rennes, Avenue du Gal Leclerc, 35700 Rennes, France
´
Universite du Littoral, LSOE, EA 2599145 Avenue Maurice
Schumann, 59140 Dunkerque, France
b
c
the b-CD/HEA16Cl inclusion complex (Ka = 181 000 Mꢀ1
)
´
´
Universite d’Artois, Unite de Catalyse et de Chimie du Solide,
UMR CNRS 8181, Rue Jean Souvraz, SP 18, 62307 Lens Ce´dex,
France. E-mail: eric.monflier@univ-artois.fr;
was found to be higher than that of the RAME-b-CD/
HEA16Cl inclusion complex (Ka = 97 000 Mꢀ1), indicating
that methylation of the CD decreases the recognition ability of
the CD (see ESIw).
Fax: (+33)3-2179-1755
w Electronic supplementary information (ESI) available: Experimental
details, 1H and T-ROESY NMR spectra of inclusion complexes,
determination of association constants, procedure for the nanoparticle
synthesis and catalytic experiments. See DOI: 10.1039/b818786j
As the above results provided evidences of the formation
of very stable inclusion complex between HEA16Cl and
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comparison, catalytic experiments with HEA16Cl stabilized
Ru(0) NPs were also performed in the same conditions. It
should be noted that experiments with the RAME-b-CD
stabilized Ru(0) NPs cannot be performed in these
experimental conditions as the catalytic system was found
unstable. Indeed, stable RAME-b-CD stabilized Ru(0) NPs
can only be obtained for a RAME-b-CD : Ru ratio higher
than 3 : 1.11c The catalytic results are summarized in Table 1.
Whatever the substrate, the best catalytic activities have
been obtained with RAME-b-CD/HEA16Cl stabilized Ru(0)
NPs. In the case of anisole, the conversion performed with
inclusion complex stabilized Ru(0) NPs is three times higher
than that obtained with the HEA16Cl stabilized Ru(0) NPs
(compare entry 2 with 1). In the case of toluene, an increase by
a factor of 4.5 was observed (compare entry 4 with 3). In the
same way, a complete conversion of styrene into ethylbenzene
required a shorter reaction time in the case of supramolecular
complex stabilization than in the case of surfactant stabiliza-
tion (compare entry 7 with 5). Interestingly, hydrogenation of
the aromatic ring was also observed for longer reaction times
in the case of inclusion complex stabilized Ru(0) NPs. Indeed,
29% of the ethylbenzene was hydrogenated into ethylcyclo-
hexane after 7 h reaction time vs. 1% with the HEA16Cl/Ru(0)
catalytic system (compare entry 8 with 6). Finally, it should be
noted that the colloidal suspensions were stable at the end of
the catalytic reaction, demonstrating that the RAME-b-CD/
HEA16Cl complex is a very efficient stabilizing agent.
Scheme 1 Chemical preparation of RAME-b-CD/HEA16Cl inclu-
sion complex stabilized Ru(0) NPs
RAME-b-CD, the possibility to stabilize ruthenium NPs with
this inclusion complex was investigated (Scheme 1).
Adapted from procedures previously reported for the
synthesis of HEA16Cl11a or RAME-b-CD11b stabilized
ruthenium(0) NPs, NPs stabilized by the inclusion complex
were synthesized in two steps. First, an aqueous solution
containing a stoichiometric amount of RAME-b-CD and
HEA16Cl was prepared and maintained under vigorous
stirring during 15 min at room temperature. Then sodium
borohydride (2.5 eq.) was added to the above solution and the
resulting solution was immediately added to a ruthenium
trichloride salt solution (RAME-b-CD : Ru = 1 : 1). The
solution was stirred overnight before catalytic experiments.
The obtained colloidal suspensions are stable without visual
agglomeration during several weeks. Compared to usual
catalytic systems composed of only RAME-b-CD or
HEA16Cl, efficient NPs stability is clearly observed. Indeed,
control experiments performed by using only RAME-b-CD or
HEA16Cl as stabilizer indicate that colloidal suspensions are
unstable for a [HEA16Cl] : [Ru] ratio of 1 : 1 and for a
[RAME-b-CD] : [Ru] ratio of 2 : 1 or less. The particle size of
the RAME-b-CD/HEA16Cl-stabilized Ru(0) NPs has been
determined by transmission electron microscopy (Fig. 1).
These particles are monodispersed in size with an average
diameter of about 4 nm. In comparison with HEA16Cl or
RAME-b-CD stabilized Ru(0) colloids, NPs stabilized by
inclusion complex have larger dimension. Indeed, the average
size for HEA16Cl or RAME-b-CD stabilized Ru(0) NPs were
found to be 3 and 2.5 nm, respectively. Fig. 1 shows also
that NPs stabilized by inclusion complexes are organized in
superstructures which are similar to those observed with
RAME-b-CD-stabilized Ru(0) NPs.
The difference of catalytic behaviour between the
HEA16Cl/Ru(0) catalytic system and the RAME-b-CD/
HEA16Cl/Ru(0) catalytic system could be explained by the
fact that the organization of the protective agents around the
nanoparticle is greatly different. As previously characterized
by Chen13 and El Sayed14 with ammonium salts stabilized Cu
and Ni NPs or Au nanorods, HEA16Cl stabilized ruthenium
NPs in aqueous solution are protected by a surfactant double-
layer as schematically represented in Fig. 2(a).
In the case of ruthenium NPs stabilized by inclusion
complex, we assume that the RAME-b-CD/HEA16Cl inclu-
sion complex is adsorbed onto the surface metallic species, as
schematically displayed in Fig. 2(b). According to the
literature and the well-known adsorption of chloride anion
to electron deficient metal surfaces,15 the polar head of the
surfactant is directed towards the metallic surface and the
RAME-b-CD interacting with the alkyl chain of the surfactant
is located close to the bulk aqueous phase. This adsorption
mode is preferred to an adsorption of the RAME-b-CD on the
The catalytic properties of Ru(0) NPs stabilized by inclusion
complex were evaluated in the hydrogenation reaction of
aromatic compounds by using a stabilizer to ruthenium ratio
of 2 : 1 (RAME-b-CD–HEA16Cl–Ru = 1 : 1 : 1). For
Fig. 1 TEM micrograph and size distribution of RAME-b-CD/
HEA16Cl stabilized Ru(0) NPs
Fig. 2 Model proposed for the stabilization of (a) HEA16Cl/Ru NPs
and (b) RAME-b-CD/HEA16Cl/Ru NPs
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Table 1 Hydrogenation of aromatic compounds with Ru(0) NPsa
Entry
Stabilizer
t/h
Substrate
TOFb/hꢀ1
Productc (%)
1
2
3
4
5
6
7
8
HEA16Cl
RAME-b-CD/HEA16Cl
HEA16Cl
RAME-b-CD/HEA16Cl
HEA16Cl
HEA16Cl
24
24
24
24
3.2
7
Anisole
Anisole
Toluene
Toluene
Styrene
Styrene
Styrene
Styrene
3.4
10.2
2.2
10.1
31.2
14.3
83.3
26.7
Methoxycyclohexane (27)
Methoxycyclohexane (82)
Methylcyclohexane (18)
Methylcyclohexane (81)
Ethylbenzene (100)
Ethylbenzene (99), ethylcyclohexane (1)
Ethylbenzene (100)
Ethylbenzene (71), ethylcyclohexane (29)
RAME-b-CD/HEA16Cl
RAME-b-CD/HEA16Cl
1.2
7
a
Reaction conditions: Ru(0) (3.8 ꢂ 10ꢀ5 mol), HEA16Cl (7.6 ꢂ 10ꢀ5 mol) or HEA16Cl (3.8 ꢂ 10ꢀ5 mol) + RAME-b-CD (3.8 ꢂ 10ꢀ5 mol),
b
c
Substrate (mol/mol) = 100, 1 bar H2, 20 1C, H2O (10 mL), stirred at 1500 rpm. Based on hydrogen consumption. Determined by GC analysis.
assumed that the cyclodextrin can interact with the surfactant
adsorbed onto the surface but also with organic substrate.
Work is currently underway in our laboratories to confirm this
hypothesis.
Notes and references
z The RAME-b-CD was a native b-CD partially O methylated with
statistically 1.8 OH groups modified per glucopyranose unit.
Fig. 3 Dynamic organisation of the stabilizers around the Ru(0) NPs
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Taken together, these results demonstrated that catalytically
active metallic NPs can be stabilized by a CD inclusion
complex in water. These NPs appear more active than the
NPs stabilized by classical surfactants. The origin of this better
efficiency is probably in connection with a dynamic organiza-
tion of the protective agents around the NPs. Indeed, it is
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1230 | Chem. Commun., 2009, 1228–1230