Diphosphite ligands derived from carbohydrates as stabilizers for ruthenium nanoparticles: Promising catalytic systems in arene hydrogenation

Article abstract of DOI:10.1039/b802316f

Ruthenium nanoparticles (RuNPs) were prepared through the hydrogenation of [Ru(COD)(COT)] (COD = 1,5-cyclooctadiene, COT = 1,3,5-cyclooctatriene) in the presence of diphosphites derived from carbohydrates as stabilizing agents, and interestingly, structur

Full text of DOI:10.1039/b802316f

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Diphosphite ligands derived from carbohydrates as stabilizers for
ruthenium nanoparticles: promising catalytic systems in arene
hydrogenationw
Aitor Gual,a M. Rosa Axet,b Karine Philippot,*b Bruno Chaudret,b Audrey Denicourt-Nowicki,c
Alain Roucoux,*c Sergio Castillond and Carmen Claver*a
Received (in Cambridge, UK) 11th February 2008, Accepted 17th March 2008
First published as an Advance Article on the web 9th April 2008
DOI: 10.1039/b802316f
Ruthenium nanoparticles (RuNPs) were prepared through the
hydrogenation of [Ru(COD)(COT)] (COD = 1,5-cycloocta-
diene, COT = 1,3,5-cyclooctatriene) in the presence of diphos-
phites derived from carbohydrates as stabilizing agents, and
interestingly, structural modifications of the diphosphite back-
bone were found to influence nanoparticle size and dispersity, as
well as their catalytic activity in arene hydrogenation.
The use of chiral ligands as stabilizing agents for RuNPs
offers the possibility of modulating the environment at the
surface of the NPs through electronic or steric modifications.
These variations can thus influence the catalytic properties of
the NPs. For this purpose, we have used modified furanose
ligands of C1-symmetry, such as 1 and its derivatives, in
homogeneous asymmetric catalysis.7 More recently, diphos-
phite 1a was used as a stabilizing agent for PdNPs, and
furthermore, these PdNPs were applied with success in asym-
metric allylic alkylation.8
Arene hydrogenation represents an important chemical pro-
cess for the synthesis of fine chemicals,1 with a wide range of
applications from laboratory to industrial scale processes. The
reaction is usually performed using heterogeneous catalysts,
which are easily separated from the reaction products and
recycled. Although rhodium catalysts have proved to be more
active than other metals (such as Pd and Ni), less expensive
ruthenium catalysts have also been successfully used in
industrial processes.2
Here, we report the stabilization of new RuNPs by di-
phosphite ligands derived from carbohydrates 1 and 2, and
their preliminary use in the asymmetric hydrogenation of
prochiral methylanisoles (Scheme 1). These ligands were se-
lected because of their successful enantioselective discrimina-
tion in several catalytic processes and easy modification.7,8 We
also describe the influence of the ligand backbone structure on
the size and dispersion of the RuNPs, as well as on their
catalytic activity in hydrogenation of prochiral arenes, such as
3-methylanisole (3) and 2-methylanisole (5) (Scheme 2).
To the best of our knowledge, only a few papers have
reported on this area, but no significant enantiomeric excesses
were achieved.6b,9
Homogeneous catalysts, for which the activity and the
selectivity of the reactions are easily tunable by choosing
suitable ligands, have also been reported for this process.
However, when the nature of these homogeneous arene hydro-
genation catalysts was later reinvestigated, it was shown that
the active species were colloidal.3
The furanose derivative diphosphites 1 and 2 were used here
to stabilize RuNPs. The synthesis of diphosphite 1a has been
previously reported,7,8 and diphosphite 1b was synthesized
through similar procedures. Diphosphite 2b can be easily
prepared from 1,2-O-isopropylidene-D-xylose (see ESIw). Of
particular interest, the selective reduction of the 1,2-O-isopro-
pylidene acetal provides a hydroxyl function. A lipophilic
chain was introduced by etherification of the hydroxyl func-
tion in a position away from the coordinating P atoms. Thus,
the design of the diphosphite 2b, containing a tetradecyl chain,
was inspired by previous results, demonstrating the effect of
such chains in non-coordinating stabilizers on the size and
dispersion of the nanoparticles.10
Nowadays, soluble metal nanocatalysts are considered as
references in the hydrogenation of monocyclic arenes. Several
different types of rhodium and ruthenium nanoparticles (NPs)
have been reported as efficient catalysts under mild condi-
tions.4 The stabilizing agent used is chosen according to the
‘‘organic’’ or ‘‘aqueous’’ nature of reaction media, which
generally depends on the metal precursor. Among the methods
used to prepare metal NPs,5 the decomposition of organome-
tallic precursors in the presence of ligands as stabilizing agents
provides a clean and efficient route for the synthesis of NPs
under mild conditions.6
a
´
Departament de Quımica Fısica i Inorganica, Universitat Rovira i
Virgili, Marcel.lı Domingo, s/n 43007 Tarragona, Spain.
´
`
´
E-mail: carmen.claver@urv.net; Fax: +34 977 559563;
Tel: +34 977 559575
b Laboratoire de Chimie de Coordination, UPR CNRS 8241, route de
Narbonne, 31077 Toulouse, France
c Equipe Chimie Organique et Supramole´culaire, UMR CNRS 6226
´
)Sciences Chimiques de Rennes*, Ecole Nationale Superieure de
´
Chimie de Rennes, Avenue du General Leclerc, 35700 Rennes, France
Departament de Quımica Analıtica i Quımica Organica, Universitat
´
d
´
´
´
`
´
Rovira i Virgili, Marcel.lı Domingo, s/n 43007 Tarragona, Spain
w Electronic supplementary information (ESI) available: Synthesis
and characterisation of compounds. See DOI: 10.1039/b802316f
Scheme 1 Diphosphite ligands derived from carbohydrates.
Chem. Commun., 2008, 2759–2761 | 2759
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Scheme 2 Hydrogenation of prochiral methylanisoles catalyzed by
RuNPs.
Scheme 3 Synthesis of RuNPs by the decomposition of Ru(COD)-
(COT) in the presence of ligand (L).
The RuNPs reported here were synthesised according to
previously reported procedures.10b,11 [Ru(COD)(COT)] (7)
was decomposed in THF at room temperature in the presence
of the appropriate ligand (1 or 2; Ru/L = 10 : 1) under 3 bar of
H2 (Scheme 3). The RuNPs were isolated as black powders
after pentane precipitation.
Transmission electron microscopy (TEM) revealed the pre-
sence of small particles of 1 to 4 nm in size, as seen in Fig. 1,
with a hexagonal compact (hcp) structure for the ruthenium
bulk (WAXS). The mean size of the nanoparticles (dmean) is
clearly influenced by the diphosphite structure, being smaller
for Ru/2b (dmean =1.8 Æ 1.0 nm) than for Ru/1b (dmean
=
2.9 Æ 1.5 nm) and Ru/1a (dmean = 4.0 Æ 2.0 nm). Thus, the use
of diphosphite moiety b (Ru/1a vs. Ru/1b), and the introduc-
tion of a lipophilic chain in the diphosphite backbone (Ru/1b
vs. Ru/2b) enables RuNPs of smaller size and good dispersity
to be achieved.
Fig. 1 TEM micrographs and the corresponding size histograms of
nanoparticles Ru/1a, Ru/1b and Ru/2b.
related RuNPs stabilized by oxazoline, the diphos-
phite-stabilized RuNPs reported here show the opposite se-
lectivity and higher activity.11
These RuNPs were tested in the hydrogenation of 3 and 5 at
room temperature, with a substrate/Ru ratio of 100 : 1, for
15 h. The conversion and the selectivity were determined by
chiral gas chromatography (GC) analysis. The influence of the
ligand, and parameters such as solvent and pressure, on the
hydrogenation of prochiral 3 is presented in Table 1.
The influence of the ligand structure on the hydrogenation
of 5 was then investigated, and the results are reported in
Table 2. A similar trend to that shown in Table 1 was
observed. Interestingly, the Ru/2b system showed the most
promising performances in terms of catalytic activity when
compared to Ru/1b and Ru/1a nanocatalysts (runs 1, 2 and 3
vs. 4 and 5, Table 2). Moreover, the sole product exhibited a
cis-conformation, indicating 100% diastereoselectivity.
No conversion was observed when the reaction was carried
out in THF or CH3CN (runs 1 and 2, Table 1), but an efficient
activity was measured in pentane (run 3, Table 1). This
difference may be explained by competitive adsorption be-
tween the substrate and the solvent (THF and CH3CN) at the
NP surface that prevents the reaction. The influence of the
ligand structure was thus explored in pentane. The Ru/1b
system displayed a higher catalytic activity than the Ru/1a
system (runs 3 and 4, Table 1). This result indicates that small
changes in the diol moiety influence the activity of the colloid.
Furthermore, and more significantly, the Ru/2b system dis-
plays a higher catalytic activity than Ru/1b (runs 5 and 6,
Table 1). This result could be explained by a more highly
active surface due to the smaller size of the NPs with diphos-
phite 2b, and by a better dispersion of the particles in solution,
which would lead to easier access to the active sites. Finally, a
decrease in H2 pressure from 10 to 2 bar with the most
promising system, Ru/2b, resulted in a decrease in activity.
However, under these conditions, the conversion is still sig-
nificant (runs 3 and 7, Table 1). In all cases, formation of the
cis-product was favoured. In contrast with previously reported
Table 1 Hydrogenation of 3 with RuNPsa
Run
RuNPs/
Solvent
PH
bar
/
Conversion
(%)b
Selectivity
(%)b,c
TONd
2
1
2
3
4
5
6
7
Ru/1a/THF
40
40
40
40
10
10
2
0
0
—
—
78
79
81
79
73
—
—
46
98
13
87
42
Ru/1a/CH3CN
Ru/1a/Pentane
Ru/1b/Pentane
Ru/1b/Pentane
Ru/2b/Pentane
Ru/2b/Pentane
39
100
13
100
48
a
Substrate/Ru = 100 : 1, 3 = 1.24 mmol, 10 mL solvent, T =
b
20 1C. Substrate conversion and selectivity determined by GC using
c
d
a chiral column: Chirasil-Dex CB. cis-Product 4. TON = mol of
hydrogenated product/mol of Ru metal in the NPs based on elemental
analysis.
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Table 2 Hydrogenation of 5 with RuNPsa
Notes and references
1. R. J. Farrauto and C. H. Bartholomew, Fundamentals of Industrial
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2. R. L. Augustine, Heterogeneous Catalysis for the Synthetic
Chemist, Marcel Dekker, New York, 1996, ch. 18, pp. 439.
Run
RuNPs/
Solvent
PH
bar
/
Conversion
(%)b
Selectivity
(%)b,c
TONd
2
1
2
3
4
5
Ru/1a/Pentane
Ru/1b/Pentane
Ru/1b/Pentane
Ru/2b/Pentane
Ru/2b/Pentane
40
40
10
10
2
14
100
65
77
39
100
100
100
100
100
17
98
64
67
34
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dispersity of RuNPs. Thus, the modification of the diol
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lipophilic chain, stabilizes smaller and better dispersed NPs
that are more soluble in organic media. Moreover, the Ru/2b
nanocatalyst provides good results in terms of activity and
diastereoselectivity (cis/trans ratio). Nevertheless, no signifi-
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are ongoing to design new carbohydrate ligands and improve
the properties of such colloidal catalytic systems.
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We are grateful to the Spanish Ministerio de Educacion y
´
Ciencia (CTQ2007-62288/BQU, Consolider Ingenio 2010,
CSD2006-0003) and the Generalitat de Catalunya
(2005SGR007777 Distinction for Research Promotion, 2003
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Ministerio de Educacion y Ciencia for awarding a research
´
grant to A. G. (FPU program/AP2005-1263). R. A., K. P. and
B. C. thank the Service Commun de Microscopie Electronique
de l’Universite Paul Sabatier (TEMSCAN) for TEM analysis,
´
and Egide (PAI Picasso no. 07165RE) and the CNRS (PICS
no. 2428,) for financial support.
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Chem. Commun., 2008, 2759–2761 | 2761