"Polysiloxane-Pd" nanocomposites as recyclable chemoselective hydrogenation catalysts

Article abstract of DOI:10.1021/ja049604j

Polysiloxane-encapsulated "Pd"-nanoclusters were generated by reduction of Pd(OAc)₂ with polymethylhydrosiloxane, which functions as a reducing agent as well as a capping material for production and stabilization of catalytically active "Pd"-nanoparticles. Chemoselective hydrogenation of functional conjugated alkenes was achieved by in-situ- or ex-situ-generated polysiloxane-stabilized "Pd"- nanoclusters under mild reaction conditions in high yields. Electron microscopy, UV-vis, and NMR studies of the reaction mixture during the catalytic transformation were performed and, in conjunction with catalyst poisoning experiments, demonstrated unequivocally the role of polysiloxane-encapsulated "Pd"-nanoclusters as the real catalytic species. The recyclability of the "Pd"-nanoclusters was established by reusing the solid left after the reaction.

Full text of DOI:10.1021/ja049604j

Published on Web 06/19/2004
“Polysiloxane-Pd” Nanocomposites as Recyclable
Chemoselective Hydrogenation Catalysts
Bhanu P. S. Chauhan,*,† Jitendra S. Rathore, and Tariq Bandoo
Contribution from the Polymers and Engineered Nanomaterials Laboratory, Department of
Chemistry and Graduate Center, City UniVersity of New York at The College of Staten Island,
2800 Victory BouleVard, Staten Island, New York 10314
Received January 22, 2004; E-mail: chauhan@postbox.csi.cuny.edu
Abstract: Polysiloxane-encapsulated “Pd”-nanoclusters were generated by reduction of Pd(OAc)2 with
polymethylhydrosiloxane, which functions as a reducing agent as well as a capping material for production
and stabilization of catalytically active “Pd”-nanoparticles. Chemoselective hydrogenation of functional
conjugated alkenes was achieved by in-situ- or ex-situ-generated polysiloxane-stabilized “Pd”-nanoclusters
under mild reaction conditions in high yields. Electron microscopy, UV-vis, and NMR studies of the reaction
mixture during the catalytic transformation were performed and, in conjunction with catalyst poisoning
experiments, demonstrated unequivocally the role of polysiloxane-encapsulated “Pd”-nanoclusters as the
real catalytic species. The recyclability of the “Pd”-nanoclusters was established by reusing the solid left
after the reaction.
geneous5 catalysts are employed to achieve selective hydrogena-
tion. In a recent report by our group, we disclosed a new strategy
Introduction
The search for more efficient catalytic systems that might
combine the advantages of both homogeneous (catalyst modula-
tion) and heterogeneous (catalyst recycling) catalysis is one of
the most exciting challenges of modern chemistry.1a More
recently, with the advances of nanochemistry, it has been
possible to prepare “soluble” analogues of heterogeneous
catalysts in the form of metal nanoclusters.1 Metal nanoclusters
show an unparalleled combination of reactivity, selectivity, and
recyclability as catalyst.2 These catalytic properties arise due
to the structural specificities of the nanoclusters. As an example,
a high degree of selectivity is exhibited by epitaxially grown
nanoclusters because different faces of the crystals manifest
different selectivity and activity for the same transformation.3
Chemoselective hydrogenation of conjugated alkenes is an
important class of reaction used extensively in the petroleum
and vegetable oil industries. Both heterogeneous4 and homo-
for the generation of polysiloxane-stabilized “Pd” colloids and
evidenced their utility as a potent catalyst with better selectivity,
activity, and recyclability for selective silaesterification of
siloxane polymers.6 While examining the scope of this strategy
to devise catalytic routes for grafting polysiloxanes with
functional moieties such as alkenes, we examined polysubsti-
tution reactions of poly(methylhydro)siloxane (PMHS) with
styrene.
To our surprise, in place of generating hydrosilylated
copolymers, hydrogenation of styrene to ethylbenzene was
obtained. During these studies, the following interesting obser-
vations led us to investigate this catalysis in detail. (i) When
polyhydrosilanes were added to the reaction mixture containing
catalytic amounts of Pd(OAc)2, the color of the solution turned
black accompanied by gas formation, presumably H2. (ii) After
the transformation was complete, a black precipitate was formed,
and the solution became colorless. (iii) The black precipitate
can be redispersed in the solution in the presence of polyhy-
drosiloxanes and was equally active for hydrogenation reactions,
thus allowing recyclability.
† Also affiliated with the Macromolecular Assembly Institute of CUNY.
(1) (a) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102, 3757. (b)
Finke, R. G. In Metal Nanoparticles: Synthesis, Characterization and
Applications; Feldheim, D. L., Foss, C. A., Jr., Eds.; Dekker: New York,
2002; Chapter 2, pp 17-54 and references therein. (c) Alivisatos, A. P.
Science 1996, 271, 933. (d) Schmid, G. Chem. ReV. 1992, 92, 1709. (e)
Herrmann, W. A.; Cornils, B. Angew. Chem., Int. Ed. Engl. 1997, 36, 1049.
(2) (a) For an excellent review, see: Widegren, J. A.; Finke, R. G. J. Mol.
Catal. A: Chem. 2003, 198, 317. (b) Reetz, M. T.; Winter, M.; Breinbauer,
R.; Thurn-Albrecht, T.; Vogel, W. Chem.-Eur. J. 2001, 7, 1084. (c) Lewis,
L. N.; Lewis, N. J. Am. Chem. Soc. 1986, 108, 7228. (d) Bronstein, L. M.;
Sidorov, S. N.; Gourkova, A. Y.; Valetsky, P. M.; Hartmann, J.; Breulmann,
M.; Colfen, H.; Antonietti, M. Inorg. Chim. Acta 1998, 280, 348. (e) Schulz,
J.; Roucoux, A.; Patin, H. Chem.-Eur. J. 2000, 6, 618.
In this publication, we describe a new approach to “Pd-
polysiloxane” nanocomposite-catalyzed one-pot, highly efficient
chemoselective hydrogenation of functional conjugated alkenes
at room temperature (Scheme 2). This strategy involves poly-
(5) (a) James, B. R. Homogeneous Hydrogenation; Wiley: New York, 1973.
(b) Pan, C.; Pelzer, K.; Philippot, K.; Chaudret, B.; Dassenoy, F.; Lecante,
P.; Casanove, M. J. J. Am. Chem. Soc. 2001, 123, 7584 and references
therein. (c) Birch, A. D.; Williamson, D. H. Org. React. 1976, 24, 1.
(6) (a) Chauhan, B. P. S.; Rathore, J. S.; Chauhan, M.; Krawicz, A. J. Am.
Chem. Soc. 2003, 125, 2876. (b) Chauhan, B. P. S.; Rathore, J. S.; Sardar,
R.; Tewari, P.; Latif, U. J. Organomet. Chem. 2003, 686, 24. (c) Chauhan,
B. P. S.; Rathore, J. S.; Chauhan, M.; Krawicz, A. 36th Organosilicon
Symposium, Akron, OH, May 29-31, 2003, P-30.
(3) Ahmadi, T.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. Science
1996, 272, 1924.
(4) (a) Rylander, P. N. Hydrogenation Methods; Academic Press: Orlando,
FL, 1985. (b) Johnstone, R. A. W.; Wilby, A. H.; Entwistle, I. D. Chem.
ReV. 1985, 85, 129. (c) Weir, J. R.; Patel, B. A.; Heck, R. F. J. Org. Chem.
1980, 45, 4926. (d) Cortese, N. A.; Heck, R. F. J. Org. Chem. 1978, 43,
3985. (e) Tour, J. M.; Cooper, J. P.; Pendalwar, L. J. Org. Chem. 1990,
55, 3452.
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J. AM. CHEM. SOC. 2004, 126, 8493-8500
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Chauhan et al.
Scheme 1. Selective Reduction of Styrene
Scheme 2. Polysiloxane-Stabilized “Pd”-Nanoclusters-Catalyzed
Reduction of Alkenes
clability and reproducibility studies emphasizing the (i) regen-
eration of the active nanoclusters, (ii) independent generation
of the nanoclusters and their catalytic actions, and (iii) size and
solubility effects on the catalysis.
(1) UV-Vis Studies of the Precursors and Their Trans-
formation to Catalytically Active Nanoclusters. In a typical
experiment, Pd(OAc)2 (0.02 mmol, 0.005 g), poly(methylhydro)-
siloxane 1 (PMHS) (2.00 mmol, 0.12 mL), and styrene (1.00
mmol, 0.12 mL) were dissolved in 2.5 mL of benzene (Scheme
1). The color of the reaction mixture turned black after 15-20
min of induction period, indicating the conversion of Pd(OAc)2
to “Pd”-nanoclusters. The color change was corroborated by
UV-vis analysis of the reaction mixture. The peak at 397 nm,
which was indicative of Pd(OAc)2, disappeared after the color
change, and a featureless absorption associated with “Pd”-
nanoparticles was observed (see Supporting Information). The
color changes and absorption spectra correlate well with the
UV-vis spectra of “Pd”-nanoparticles simulated by Creighton
and co-workers.8
(2) In-Situ Characterization Studies, Emphasizing TEM
Studies During the Catalysis. Transmission electron micros-
copy (TEM) characterization of the crude reaction mixture was
also performed. Only samples that were taken after the induction
period contained “Pd”-nanoparticles. The samples were directly
deposited on a carbon grid without workup; the process can be
viewed as “in-situ” TEM analysis. As is evident by the TEM
image of the reaction mixture, “Pd”-nanoclusters encapsulated
by polysiloxane were present in the reaction mixture during the
catalysis (Figure 2). This image is taken from a randomly chosen
part of the substrate, and it is a good representation of the overall
sizes and shapes of the particles. The particle size analysis of
200 particles showed 60% of the particles in the 3.5 nm size
regime. The catalysis was continued for 4 h and led to complete
consumption of styrene to produce the corresponding reduced
product, ethylbenzene, in quantitative yield.
After the reaction was over, the mixture turned clear, and a
black solid precipitated out. Characterization of the solid was
carried out with electron microscopy, multinuclear NMR
techniques, FT-IR, and UV-vis spectroscopy. Scanning electron
microscopy (SEM) analysis of the solid affirmed the presence
of “Pd”-nanoclusters in the size regime of 40-60 nm. After
the product was removed, the recovered black solid was washed
with an excess of toluene and was reused as catalyst for
hydrogenation reactions. An almost similar reactivity was
observed as compared to in-situ-generated “Pd”-nanoclusters.
The UV-vis spectra and TEM analysis of the reaction mixture
indicated that polysiloxane-encapsulated “Pd”-nanoclusters (2-3
nm) were regenerated after the addition of reactants. These
observations imply that the addition of 1 to bigger nanoclusters
hydrosiloxane as the hydrogen source for reduction of alkenes
as well as the stabilizing agent for catalytically active “Pd”-
nanoclusters. We also critically examine the role of “Pd” clusters
as recyclable catalysts, examine the mechanism of their
regeneration, and provide evidence that the hybrids of polysi-
loxane and Pd colloids can be regarded as molecular entities,
which inherit the profitable solution behavior of silicon polymers
and the catalytic activity of metal nanoparticles.
Results and Discussion
Investigations of the Nature of the True Catalyst. A
problem that has caused considerable consternation in the study
of nanocluster catalysis is the difficult task of distinguishing
homogeneous, single metal-complex catalysts from soluble
nanocluster or colloid (vide supra) catalysts. The literature in
this area dates back to about 1980 and includes contributions
from Maitlis,7a Whitesides,7b,c Crabtree,7d-f Collman,7g Lewis,7h,i
and Finke.7j-l Recently, Finke and co-workers7l outlined a
thorough and more general approach for distinguishing the
nanocluster catalysis from single metal-complex catalysis. In
the present catalysis, studies as outlined in Figure 1 were
performed to critically investigate the catalytic action of “Pd-
polysiloxane” nanoconjugates.
We performed the following experiments to delineate the real
catalysts in this process: (1) UV-vis studies of the precursors
and their transformation to catalytically active nanoclusters; (2)
in-situ characterization studies, emphasizing TEM studies during
the catalysis; (3) quantitative poisoning studies; and (4) recy-
(7) (a) Hamlin, J. E.; Hirai, K.; Millan, A.; Maitlis, P. M. J. Mol. Catal. 1980,
7, 543. (b) Whitesides, G. M.; Hackett, M.; Brainard, R. L.; Lavalleye, J.
P. P. M.; Sowinski, A. F.; Izumi, A. N.; Moore, S. S.; Brown, D. W.;
Staudt, E. M. Organometallics 1985, 4, 1819. (c) Foley, P.; DiCosimo, R.;
Whitesides, G. M. J. Am. Chem. Soc. 1980, 102, 6713. (d) Anton, D. R.;
Crabtree, R. H. Organometallics 1983, 2, 855. (e) Crabtree, R. H.; Mellea,
M. F.; Mihelcic, J. M.; Quirk, J. M. J. Am. Chem. Soc. 1982, 104, 107. (f)
Crabtree, R. H.; Mihelcic, J. M.; Quirk, J. M. J. Am. Chem. Soc. 1979,
101, 7738. (g) Collman, J. P.; Kosydar, K. M.; Bressan, M.; Lamanna,
W.; Garrett, T. J. Am. Chem. Soc. 1984, 106, 2569. (h) Lewis, L. N.; Lewis,
N. J. Am. Chem. Soc. 1986, 108, 7228. (i) Lewis, L. N. J. Am. Chem. Soc.
1990, 112, 5998. (j) Lin, Y.; Finke, R. G. Inorg. Chem. 1994, 33, 4891.
(k) Ozkar, S.; Finke, R. G. J. Am. Chem. Soc. 2002, 124, 5796. (l) Aiken,
J. D.; Finke, R. G. J. Mol. Catal. A: Chem. 1999, 145, 1.
(8) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87,
3881.
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“Polysiloxane-Pd” Nanocomposites
A R T I C L E S
Figure 1. Schematic drawing of investigations performed to establish “Pd-polysiloxane” nanocomposites as real recyclable catalysts.
Figure 2. TEM (bar 100 nm) and particle size analysis of the reaction mixture during the catalytic conversion of styrene to ethylbenzene. The inset shows
the high-resolution image of the nanoparticles (bar 20 nm).
leads to regeneration of stable small nanoclusters (2-4 nm) from
the solid, which contained “Pd”-nanoclusters in the size regime
of 40-60 nm. This unique aspect of the catalysis is systemati-
cally examined in the section titled recycling and reproducibility
studies.
Progress of the reaction was also followed by NMR. Only 13-
15% conversion to desired product was observed even after 24
h of reaction. The mixture was examined by TEM before and
after the addition of mercury. As is clear from the TEM images
of the reaction mixture (Figure 3a), “Pd”-nanoclusters present
in the solution before the addition of mercury disappeared after
the addition (Figure 3b). This experiment strongly suggests that
polysiloxane-stabilized “Pd”-nanoclusters were the real catalyst.
Additional poisoning tests were performed with dodecanethiol
(DDT). Thiols are strong coordinating ligands and are known
to passivate the active sites present on the surface of the metal
nanoclusters, making them inactive catalysts. The catalytic
activity of the nanoparticles was periodically examined in the
presence of varying amounts of DDT after validating the
presence of nanoclusters in the solution. The results are
summarized in Table 1. In presence of 0.5 equiv of DDT (with
respect to Pd), 40% conversion to reduction product was
observed, indicating comparatively less opportunities for reactant
to catalyst interactions. On the other hand, when enough DDT
(1 to 2 equiv) required to passivate the total surface of
(3) Catalyst Poisoning Studies. To further probe “Pd”-
nanocluster catalysis, poisoning tests were carried out. Mercury
is known to poison metal colloids due to physioabsorption of
the metal colloids on the mercury surface and also because of
the facile formation of amalgam with metals such as Pt, Pd,
and Ni. Mercury poisoning experiments were carried out after
ensuring that “Pd”-nanoclusters were formed and catalytic
transformation of alkene to alkanes had ensued. In a standard
reaction mixture, styrene (1 mmol), PMHS (2 mmol), and
Pd(OAc)2 (0.05 mmol) were stirred at room temperature for
30 min to ensure the formation of “Pd”-nanoclusters. NMR
analysis of the reaction mixture indicated 10% transformation
of styrene to ethylbenzene. At this juncture, mercury (∼2 equiv
with respect to Pd) was added to reaction mixture. Just after
the mercury addition, the reaction mixture turned colorless.
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A R T I C L E S
Chauhan et al.
Figure 3. TEM images obtained by depositing the crude reaction solution before (3a) and after (3b) the addition of the Hg.
Figure 4. 29Si CPMAS NMR (4a) signatures and SEM (4b, inset shows the high-resolution image) of the polysiloxane-stabilized isolable “Pd”-nanoparticles.
Table 1. Poisoning Studies of “Pd”-Nanocluster-Catalyzed
Reduction of Styrene in the Presence of DDA
moieties was further documented by FT-IR spectra, which
showed characteristic signals associated with Si-H and Si-
O-Si bonds. SEM analysis (Figure 4b) of this solid was also
time (h) % conv (0.5 equiv of DDT) % conv (1 equiv of DDT) % conv (2 equiv of DDT)
4
8
24
10
25
40
8
10
10
0
0
0
undertaken. Particles were found to be in the nanometer size
regime (40-50 nm) and were stabilized by the polymeric
network.
(ii) Recyclability Studies. The regeneration of “Pd”-nano-
clusters is of crucial importance for maintaining catalytic activity
after several recyclings. During the recycling experiments, we
observed that as soon as 1 was added to the reaction mixture,
the color of the solution changed to black after 15-20 min of
stirring the reaction mixture at room temperature. This color
change also triggers the reduction of alkene (NMR monitoring
of the reaction mixture indicated only traces of reduction product
before the color change). This observation led us to hypothesize
that addition of 1 (i.e., PMHS) leads to regeneration of small
soluble nanoclusters (2-3 nm) from the solid, which contained
“Pd”-nanoclusters in the size regime of 40-60 nm. To gain
more insight and to elucidate the polysiloxane-induced regen-
eration (decrease in size) and stabilization process, the following
experiments were performed: (a) the reduction reaction was
carried out using H2 as a reductant in place of polysiloxane 1
under identical reaction conditions and molar ratios;9 (b) the
reduction reaction was carried out using H2 as a reductant in
the presence of an analogous polymeric structure such as poly-
(dimethylsiloxane) PDMS {O-(SiMe2-O-)n-}, which does
not contain Si-H bonds; (c) an experiment was performed in
nanoparticles was employed, no further reaction took place.
During and after the poisoning studies, the reaction mixture was
also assayed by TEM, which showed the presence of nanoclus-
ters in the solution after the addition of dodecanethiol (see
Supporting Information). The presence of nanoclusters indicated
that dodecanethiol coordinates to soluble clusters and renders
them inactive as catalyst.
(4) Recyclability and Reproducibility Studies. (i) Inde-
pendent Synthesis of Siloxane-Stabilized “Pd”-Nanoclusters
and Their Characterization. Pd(OAc)2 (0.09 g, 0.4 mmol) was
dissolved in 20 mL of benzene, and PMHS (0.72 mL, 12.0
mmol) was added at room temperature. The reaction was carried
out in air. The color of the solution changed from yellow to
black within 5 min of stirring. Stirring was stopped after 2-3
h. The reaction mixture was filtered under vacuum, and the
residue was collected after washing with an excess of toluene.
The solid obtained was characterized with various techniques.
29Si CPMAS NMR (Figure 4a) of the obtained solid displayed
peaks at δ -36.15, -64.74, and -75.74 ppm corresponding to
a polymeric network containing Si-H, SiOCOR, and Si-OH
moieties, respectively. The signals at δ 102 and -111 ppm
indicate a fully condensed silica network. The presence of these
(9) We are thankful to one of the reviewer’s who suggested an experiment in
the presence of exogenous hydrogen.
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“Polysiloxane-Pd” Nanocomposites
A R T I C L E S
Figure 5. NMR analysis of the reduction reactions carried out with or without PMHS in the presence of hydrogen (1 atm, balloon).
the presence of 1 (0.20 equiv to styrene), but H2 was still used
as a primary reducing agents; and (d) finally, the reaction was
carried out in the presence of both 1 and H2.
of “Pd”-nanoparticles and subsequent color change occur only
when Si-H-containing polysiloxanes are added to the reaction
mixture.
All of the experiments (a-d) were monitored by NMR to
probe the product formation, and by UV-vis and TEM analysis
to authenticate the presence of soluble nanoparticles. The
following results were obtained: (a) TEM and UV analysis of
the reaction mixtures resulting from experiments a and b showed
that nanoparticles were not present in the reaction mixture, and
NMR analysis indicated only traces of the reduction product
(Figure 5a and 5b) were formed even after 5 h of reaction. This
experiment indicated that suspended bigger “Pd”-particles do
not redissolve to produce catalytically active soluble nanopar-
ticles. It should be pointed out that under identical reaction
conditions, complete reduction of styrene was achieved when
Pd(OAc)2 was used as catalyst. In experiments c and d, the color
of the reaction mixtures changed after addition of 1, and
complete conversion to the corresponding reduced product was
obtained (Figure 5c and 5d). TEM and UV analysis of the
reaction mixtures showed the presence of ∼2-4 nm size “Pd”-
nanoparticles during the course of the reaction. It should be
pointed out that in experiment c, the abundance of nanoparticles
was low (TEM analysis) in comparison to experiment d. The
results of experiments c and d strongly suggest that redispersion
On the basis of these results, we can roughly elucidate the
polysiloxane-induced regeneration (decrease in size) and sta-
bilization process as follows: Addition of PMHS (i.e., Si-H
bonds) to the reaction mixture containing insoluble bigger “Pd”-
nanoparticles triggers the oxidative addition and reductive
elimination sequence with Si-H bonds of PMHS (Scheme 3).
During this process, large particles are broken into small
particles, which, due to the steric protection provided by
chemoabsorbed PMHS, become stable and soluble. These
polysiloxane-stabilized 2-3 nm particles remain active, stable,
and soluble until substantial amounts of Si-H bonds are present
in the reaction mixture. When Si-H bonds are consumed,
smaller nanoparticles aggregate to become larger particles
(Oswald ripening) and precipitate out from the reaction mixture.
Although extracting preformed nanoparticles into polymer
matrices is a known process and an effective strategy for
dispersing the nanoparticles in polymer composites, in-situ
regeneration, decomposition, and growth of catalytically active
nanoparticles is unprecedented.
Scope and Limitations of “Pd”-Polysiloxane-Catalyzed
Reductions. The chemoselective reduction of aromatic group-
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Table 2. Selective Reduction of Conjugated Aromatic Alkenes
Scheme 3. Proposal for the Regeneration of Smaller
“Pd Nanoparticles” in the Presence of PMHS
Catalyzed by “Pd-PMHS” Nanocomposite
substituted conjugated alkenes was performed in benzene with
2 equiv of polymethyhydrosiloxane (PMHS), 1, in the presence
of “Pd”-nanoclusters at room temperature. In most cases,
complete conversion to the corresponding reduction product was
achieved in 3-6 h of reaction. Although Pd-based catalysts are
known to catalyze hydrosilylation and rearrangement reactions,
to our surprise, these side reactions were not observed. As is
evident from the list in Table 1, excellent chemoselectivity
toward hydrogenation of external CdC was achieved in the
present system. The products were isolated simply by filtration
from the siloxane-supported palladium nanoclusters followed
by drying and removal of the solvent in vacuo. No further
purification was necessary to afford pure material. The scope
and limitations of the “Pd”-nanocluster-catalyzed reductions of
alkenes containing functionalities such as aromatic CdC are
summarized in Table 2. The limitations of the process were
examined by investigating the hydrogenation of vinylic moieties
containing functionalities such as aromatic CdC (entries 1-7,
Table 2), OMe (entry 2, Table 2), OH (entry 3, Table 2), CN
(entry 4, Table 2), and Cl (entry 5, Table 2). In all of the cases,
selective reduction of vinylic functionality was observed without
any detectable amounts of side products. It should be pointed
out that functional groups such as alcohols (entry 3, Table 2)
are particularly susceptible toward facile side reactions such as
alcoholysis of silanes. The bulky substituent also does not hinder
the reactivity appreciably, because alkenes such as vinyl
naphthalene (entry 7, Table 2) and vinyl anthracene (entry 8,
Table 2) underwent reduction reactions at room temperature.
The presence of functional groups after the reduction reactions
a All of the reactions were performed with 2:1 molar equivalents of
1:alkene and were stirred under argon or nitrogen atmosphere. Reaction
progress was monitored by 1H NMR and/or FT-IR spectroscopy. b Isolated
yields.
To our surprise, facile reduction of vinyl ferrocene was
cleanly achieved at room temperature (Scheme 4). The corre-
sponding reduction product was isolated in 85% yield. This
transformation is noteworthy, because vinylferrocene has been
reported to undergo hydrosilylation reactions with linear and
cyclic polysiloxanes in the presence of single-metal-complex
catalysts.10
Effect of the Solvents. To gain more insight into the catalysis,
the course of a representative hydrogenation reaction was
examined in common organic solvents under identical reaction
conditions and molar ratios (Figure 6). The factors such as
reaction medium, solvent coordination ability, and solvent
polarity can drastically alter the reaction outcome via influencing
the stability and durability of the catalyst (nanoclusters). The
hydrogenation reaction of the styrene was conducted in common
organic solvents under identical reaction conditions and molar
ratios. In all of the solvents studied, the reaction begins after
an induction period of 10-15 min. The graphs were plotted
after the induction period. In THF and benzene, quantitative
conversion to reduction product was achieved approximately
1
was confirmed by FT-IR and H and 13C NMR spectroscopy
and was compared to authentic samples.
To further investigate the utility of this mild, high-yielding
process, reduction reactions of conjugated enones were exam-
ined. Table 3 lists the reductions of conjugated enones with 1
and catalytic amounts of “Pd”-polysiloxane nanocomposite at
ambient temperature. All runs demonstrated complete 1,4-
selectivity to provide the corresponding ketones. No significant
differences were observed in the reactivity of cyclic and acyclic
systems. Commonly observed possible byproducts such as allylic
alcohols were detected in the reaction mixture.1a
(10) For an excellent review, see: Casado, C. M.; Cuadrado, I.; Moran, M.;
Alonso, B.; Barranco, M.; Losada, J. Appl. Organomet. Chem. 1999, 13,
245 and references therein.
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Table 3. Selective Reductions of Conjugated Enones Catalyzed
by “Pd-PMHS” Nanocomposite
Figure 6. Plot of yield versus reaction time for reduction of styrene at
room temperature.
atmosphere using standard Schlenk-tube techniques. Solvents were
purchased from EM science (Merck) and distilled over sodium/
benzophenone. PMHS (Mw ≈ 2000, 33-35 Si-H units) and Pd(OAc)2
1
were obtained from Aldrich and used as received. H NMR and 13C
NMR spectra were recorded in CDCl3 on a Varian Unity NMR
instrument (200 MHz). A Varian Unity NMR instrument (300 MHz)
was used to carry out the CP/MAS 29Si NMR spectroscopy experiments.
Proton spectra were referenced internally to protonated solvent shifts.
All of the peaks in the NMR spectra were reported in ppm. UV-vis
spectra were recorded on a Varian Cary model 50 UV spectrophotom-
eter. Infrared spectra were measured on the neat compounds with an
FT Infrared Nicolet Mx-1 spectrophotometer. Scanning electron
microscope Amray 1910 (SEM) was used to conduct the morphological
studies of the solid materials. The solid was mounted on the carbon
tab and coated with carbon vapors before analyzing the sample. Philips
CM 100-transmission electron microscope (TEM) was applied to
examine the liquid samples for the presence of “Pd”-nanoclusters during
the reaction. Typically, TEM pictures of each sample were taken at
three different magnifications to obtain information about the sample
in general, plus a closer visualization of the clusters. A few drops of
the reaction mixture were taken out with a syringe and were directly
deposited on a carbon-Formvar-coated copper grid at room temperature.
The solvent was allowed to evaporate from the grid under normal
temperature and pressure for 1 h before TEM examination of the grid.
A number of control experiments were done which provided good
evidence that results are truly representative of the sample (i.e., save
any crystallization in the electron beam) and that the sample is not
otherwise perturbed by application of the TEM beam [e.g., controls
showing that varying the sample spraying method (in air or under N2)
or depositing the sample as a drop and letting it dry did not change the
results; controls showing that changing the beam voltage or changing
the exposure time did not change the images]. The particle size analysis
of the “Pd”-nanoclusters was carried out manually on the TEM images,
using “Scion Image Software”. Around 200-250 particles were
measured manually using the software on each TEM image before
plotting the “particle size analysis” graph.
a All of the reactions were carried out with 2:1 molar equivalents of
1:enones and were stirred under argon or nitrogen atmosphere. Reaction
progress was monitored by 1H NMR and/or FT-IR spectroscopy. b Isolated
yields.
Scheme 4. “Pd”-Nanoparticle-Catalyzed Reduction of
Vinylferrocene
after 4 h of reaction. However, in acetonitrile and dichlo-
romethane, total conversion to desired product was not observed
even after 24 h of reaction.
Conclusions
We have demonstrated the first example of recyclable highly
active “Pd”-polysiloxane nanocomposites as selective catalysts
for hydrogenation reactions. Controlled poisoning experiments
in conjunction with electron microscopy studies confirm “Pd”-
nanoclusters as the real catalytic species. We also established
that, in addition to their role as reducing agents, in this catalysis
polysiloxane serves multiple functions, for example, as a
stabilizer for keeping the nanoparticles from aggregating;
provides desired functional interfaces between the nanoparticles
and the bulk of the materials; and facilitates the self-assembling
and processing of the composites. In conclusion, we have shown
that “Pd”-polysiloxane nanocomposites offer the selectivity of
homogeneous single-metal-complex catalysts, and activity as
well as recyclability of traditional heterogeneous catalysts.
Experimental Procedure for Investigations of the Nature of the
True Catalyst. Pd(OAc)2 (0.02 mmol, 0.005 g) or Pd colloids (0.010
g), PMHS (2.00 mmol, 0.12 mmol), and styrene (1.00 mmol, 0.12
mmol) were added together at room temperature in 2.5 mL of freshly
dried and distilled benzene. Evolution of a gas (presumably hydrogen)
was observed while the mixture was stirred at room temperature. The
progress of the reactions was monitored by 1H, 13C NMR, FT-IR, and
UV spectroscopy. Stirring was stopped after 4 h. The reaction mixture
was centrifuged, and the filtrate was passed through a silica gel column
to obtain pure product. In all of the cases, formation of corresponding
reduction products was achieved within 3-8 h, unless noted otherwise.
Experimental Section
General Information. All of the experiments and manipulations
were performed under a positive pressure of argon or nitrogen
9
J. AM. CHEM. SOC. VOL. 126, NO. 27, 2004 8499

A R T I C L E S
Chauhan et al.
(i) Mercury Poisoning Experiment. Pd (OAc)2 (0.02 mmol, 0.005
g), styrene (1.00 mmol, 0.12 mL), and PMHS (2.00 mmol, 0.12 mL)
were dissolved in 2.5 mL if benzene in a Schlenk tube. The reaction
was allowed to stir until the presence of “Pd”-nanoclusters in the
reaction mixture was confirmed by TEM and UV-vis spectroscopy.
The reaction mixture turned colorless after the addition of mercury
[2:1, Hg:Pd(OAc)2] at room temperature. Characterization of the
solution was performed by TEM and NMR spectroscopy. No nano-
clusters were found during the TEM examination of the grid prepared,
after the addition of mercury.
NMR (CDCl3, 200 MHz) δ 14.10, 23.04, 32.11, 115.27, 120.80, 127.81,
130.39.
1
3-Phenylpropionitrile (Entry 4; Table 2): H NMR (CDCl3, 200
MHz) δ 2.09(t), 2.54(t), 6.99(m), 7.15(m), 7.62(m); 13C NMR (CDCl3,
200 MHz) δ 18.73, 31.22, 118.78, 129.55, 130.76,137.96, 149.93; FT-
IR (CHCl3) 1280 (-CN), 2962 (C-H).
(2-Chloroethyl)benzene (Entry 5; Table 2): 1H NMR (CDCl3, 200
MHz) δ 2.98(t), 3.89(t), 7.09(m), 7.16(m), 7.37(m); 13C NMR (CDCl3,
200 MHz) δ 30.52, 52.32, 124.5, 127.32, 129.39, 144.23, 173.24; FT-
IR (CHCl3) 1760 (CdO), 2982 (C-H).
1,2-Diphenylethane (Entry 6; Table 2): 1H NMR (CDCl3, 200
MHz) δ 2.73(t), 6.90(m), 7.20(m), 7.35(m); 13C NMR (CDCl3, 200
MHz) δ 38.45, 126.12, 127.45, 129.36.
(ii) Thiol Poisoning Experiments. Pd (OAc)2 (0.02 mmol, 0.005
g), styrene (1.00 mmol, 0.12 mL), and PMHS (2.00 mmol, 0.12 mL)
were dissolved in 2.5 mL of benzene in a Schlenk tube. The reaction
mixture was examined by TEM to verify the presence of “Pd”-
nanoclusters. Dodecanethiol (0.04 mmol, 0.01 mL) was added to the
reaction mixture, and the reaction mixture was allowed to stir for up
1
2-Ethylnaphthalene (Entry 7; Table 2): H NMR (CDCl3, 200
MHz) δ 1.13(t), 2.62(q), 7.20-7.25(m), 7.42(1H, m), 7.58(m); 13C
NMR (CDCl3, 200 MHz) δ 15.49, 29.01, 124.97, 125.51, 125.78,
127.04, 131.93, 133.70, 141.71.
1
to 24 h. H NMR spectroscopy and transmission electron microscopy
(TEM) were employed to monitor the progress and the presence of
nanoclusters in the reaction mixture, respectively, after a regular period
of 2 h.
9-Ethylanthracene (Entry 8; Table 2): 1H NMR (CDCl3, 200 MHz)
δ 1.50(t), 3.65(q), 6.82(m), 7.30(m), 7.64(m), 7.75(m), 8.34(m),
8.39(m); 13C NMR (CDCl3, 200 MHz) δ 16.8, 25.2, 124.4, 124.9, 125.0,
126.0, 128.2, 131.5, 132.2.
Procedure for Enone Hydrogenation. As outlined above for other
alkenes, identical reaction conditions, molar ratios, and product isolation
procedures were used. In all of the cases, reduction reactions were
complete within 4-6 h at room temperature.
Propionic Acid Methyl Ester (Entry 1; Table 3): 1H NMR (CDCl3,
200 MHz) δ 1.02(t), 1.72, 3.92(q); 13C NMR (CDCl3, 200 MHz) δ
9.1, 26.1, 50.4, 172.0.
(iii) Generation of Polysiloxane-Stabilized “Pd”-Nanoclusters.
Pd(OAc)2 (0.09 g, 0.4 mmol) was dissolved in 20 mL of benzene in a
50-mL round-bottom flask at room temperature. PMHS (0.72 mL, 12.00
mmol) was added to this solution. A color change from yellow to black
was observed within 5-10 min of stirring with vigorous evolution of
gas (H2, presumably). Black solid was observed after stirring the
solution for 2-3 h, and the reaction mixture turned colorless. The
stirring was stopped at this point, the reaction mixture was filtered under
vacuum, and black residue was collected after washing with an excess
of toluene. The black solid obtained was characterized by CP/MAS
29Si NMR spectroscopy, FT-IR spectroscopy, and scanning electron
microscopy (SEM). 29Si (CPMAS NMR) δ -36.15 (Me-Si-H-con-
taining silicons), -64.74 (Me-Si-OCOR), -75.74 (Me-Si-OH),
-110.94 (SiO3). FT-IR spectra (KBr) 2162(m), 1021(b), 900.77(s),
2964.86 (m).
Experimental Details of Redispersion Studies. Independently
synthesized polysiloxane-encapsulated “Pd”-nanoclusters (0.01 g),
styrene (1.0 mmol, 0.12 mL), and PMHS (0.25 mmol, 0.015 mL) were
mixed together in 2.5 mL of benzene in a 25 mL Schlenk tube. All of
the reactions were performed at room temperature. A hydrogen balloon
was used to maintain a constant positive pressure of hydrogen (H2,
gas) in the Schlenk tube during the reaction. The reaction mixture was
analyzed with TEM after being stirred for 30 min at room temperature.
Stirring was stopped after 5 h, and NMR spectroscopy examination of
the reaction mixture was performed.
Propionic Acid Butyl Ester (Entry 2, Table 3): 1H NMR (CDCl3,
200 MHz) δ 0.810(t), 1.02(t), 1.26(q), 1.50(m), 2.23(q), 3.99(m); 13C
NMR (CDCl3, 200 MHz) δ 9.15, 13.70, 19.14, 27.62, 30.71, 64.19,
174.60; FT-IR (CHCl3) 1727(CdO), 3073(C-H).
Butane-2-one (Entry 3; Table 3): 1H NMR (CDCl3, 200 MHz) δ
1.09(t), 1.85(q), 3.55; 13C NMR (CDCl3, 200 MHz) δ 18.24, 33.85,
51.51, 167.65; FT-IR (CHCl3) 1698(CdO).
1
1-Cyclohexylethanone (Entry 4, Table 3): H NMR (CDCl3, 200
MHz) δ 0.80(m), 1.22(m), 1.56(m), 1.70(m), 1.96(m), 2.11(m); 13C
NMR (CDCl3, 200 MHz) δ 14.28, 22.59, 25.86, 27.98, 28.65, 34.38,
51.63, 212.27; FT-IR (CHCl3) 1680(CdO).
1, 3-Diphenyl Propan-1-one (Entry 5, Table 3): 1H NMR (CDCl3,
200 MHz) δ 3.11(t), 3.33(t), 7.24(m), 7.28(m), 7.34(m), 7.47(m),
7.59(m), 8.01(m); 13C NMR (CDCl3, 200 MHz) δ 30.05, 40.34, 126.05,
127.96, 128.25, 132.92, 136.80, 141.22, 199.03; FT-IR (CHCl3)
1680(CdO), 2958(C-H).
Ethylferrocene (Scheme 4): 1H NMR (CDCl3, 200 MHz) δ 1.09(t),
2.27(q), 4.00(m), 4.73, 6.84(m), 7.43(m), 7.64(m); 13C NMR (CDCl3,
200 MHz) δ 8.25, 27.26, 62.03, 63.83, 65.07, 105.65.
Typical Procedure for Hydrogenation of Alkenes Conjugated
with Aromatic Functional Groups. Pd(OAc)2 (0.02 mmol, 0.005 g)
or Pd colloids (0.010 g), PMHS (2.00 mmol, 0.12 mmol), and styrene
(1.00 mmol, 0.12 mL) were added together at room temperature in 2.5
mL of freshly dried and distilled benzene. Evolution of gas (presumably
hydrogen) was observed during the stirring at room temperature. The
progress of the reactions was monitored by 1H, 13C NMR, FT-IR, and
UV spectroscopy. Stirring was stopped after 4 h. The reaction mixture
was centrifuged, and the filtrate was passed through a silica gel column
to obtain pure product. In all of the cases, the formation of the
corresponding reduction products was achieved within 3-8 h, unless
noted otherwise.
Acknowledgment. B.P.S.C. acknowledges support from a
NIST-research grant, a NSF-instrumentation grant, a GRTI (The
Graduate Research and Technology Initiative) grant, a Merck-
AAA grant, a PSC-CUNY grant, and a CSI-CUNY startup grant.
We also thank the “Deans Summer Scholarships Program”
(T.B.). Our thanks are also due to Professor Bill L’Amoreaux
for helping with the EM analysis.
1
Supporting Information Available: UV-vis spectra of the
reaction mixture indicating the conversion of Pd(OAc)2 to “Pd”-
nanoclusters and a TEM image of dodecanethiol-passivated,
catalytically inactive, yet soluble “Pd”-nanoparticles. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Ethylbenzene (Entry 1; Table 2): H NMR (CDCl3, 200 MHz) δ
1.12(t), 2.53(q), 7.12(m), 7.21(m), 6.98(m); 13C NMR (CDCl3, 200
MHz) δ 16.26, 28.75, 140.32, 127.5, 129.32, 125.45.
1-Ethyl-4-methoxybenzene (Entry 2; Table 2): 1H NMR (CDCl3,
200 MHz) δ 1.24 (t), 2.59(q), 3.73(s), 6.72(m), 7.01(m); 13C NMR
(CDCl3, 200 MHz) δ 16.1, 28.6, 56, 114, 128.9, 132.5, 159.2.
1
2-Propylphenol (Entry 3; Table 2): H NMR (CDCl3, 200 MHz)
δ 0.88(t), 1.57(q), 2.47(t), 4.07, 6.5(m), 6.7(m), 6.91(m), 7.01(m); 13
C
JA049604J
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8500 J. AM. CHEM. SOC. VOL. 126, NO. 27, 2004