Synthesis of polysiloxane stabilized palladium colloids and evidence of their participation in silaesterification reactions

Article abstract of DOI:10.1021/ja028957k

The role of "Pd" colloids in Pd(OAc)₂ -catalyzed silaesterification reactions was investigated. The first example of the generation and utilizations of recyclable polysiloxane network stabilized palladium nanoparticles is described. We also provide the evidence that the silicon polymers play the role of stabilizing agents, preventing the generation of bulk palladium without compromising the activity of the catalyst. Copyright

Full text of DOI:10.1021/ja028957k

Published on Web 02/13/2003
Synthesis of Polysiloxane Stabilized Palladium Colloids and Evidence of
Their Participation in Silaesterification Reactions
Bhanu P. S. Chauhan,*,† Jitendra S. Rathore, Moni Chauhan, and Alexandra Krawicz
†
‡
†
Department of Chemistry, City UniVersity of New York at the College of Staten Island, 2800, Victory BouleVard,
Staten Island, New York 10314, and Department of Chemistry, City UniVersity of New York at the Queensborough
Community College, Bayside, New York 11364
Received October 15, 2002 ; E-mail: chauhan@postbox.csi.cuny.edu
Interest in the synthesis and properties of colloidal metal particles
and metal clusters has grown constantly because of their unique
properties. For noble metal colloids, especially catalytic applications
are considered, because a unique combination of reactivity, stability,
and selectivity is expected.1
-3
Various noble metal colloids stabilized by surfactants and
solvents have been used for hydrogenation,4 Heck, and Suzuki
-6
7
8
coupling reactions. Even enantioselective hydrogenation reactions
on noble metal colloids with chiral ligands have been performed.9
The controlled generation of Pd colloids is also promising for a
number of other reactions, such as oxidative conversions and
cyclizations.1
0-12
On the other hand, in the case of catalytic
transformation involving silicon, only platinum-catalyzed hydro-
silylation reactions are known to undergo the Si-C bond formation
reaction via Pt colloid formation.13
Recently, we have described a novel route to biodegradable
polysilylesters, via esterification of polyhydrosiloxanes in the
presence of Pd(OAc)
2
(Scheme 1).1
4a
Scheme 1. Pd-Catalyzed Silylester Synthesis
Figure 1. (a) UV-visible spectra of the reaction mixture; (b) TEM image
and the particle size analysis of the Pd colloids obtained after the addition
of starting materials (no staining was required).
During these studies, the following observations led us to
investigate this catalysis in detail: (i) When hydrosilanes were
added to the reaction mixture containing catalytic amounts of
Pd(OAc)
gas formation, presumably H
complete, a black precipitate was formed, and the solution became
colorless. (iii) The black precipitate can be redispersed in the
solution and was found catalytically active for silylesterification
reactions, thus allowing recyclability.
2
, the color of the solution turned black accompanied by
. (ii) After the transformation was
One drop was deposited on Formvar/carbon coated grid and
analyzed by transmission electron microscopy (TEM) (Figure 1b).
As is evident from Figure 1b, Pd colloids were formed after the
addition of PMHS and were stabilized by siloxane polymer network.
Particle size analysis showed the average particle size of 6 nm with
a standard deviation of 1 nm.
2
On the basis of these elucidations, we decided to investigate the
possibility that Pd colloids were formed during the catalysis and
these particles were the active catalysts. In this communication,
we present the first example of soluble polysiloxane network
stabilized Pd colloids and unequivocal evidence of their participation
After completion of the catalysis, the precipitated black sticky
solid was analyzed by scanning electron microscopy (SEM), IR,
and multinuclear NMR techniques. Silicon moieties were present
29
in the solid as evidenced by Si NMR. The peaks at δ -36.15,
-64.74, and -75.74 ppm correspond to a polymeric network
in Pd(OAc)
In a Schlenk tube, Pd(OAc)
2
-catalyzed Si-H to Si-OCOR bond transformations.
(0.004 g, 0.02 mmol) and acetic
containing Si-H, SiOCOR, and Si-OH moieties, respectively. The
signal at δ -110.94 ppm indicates fully condensed network. FT-
IR spectra also displayed characteristic signals associated with SiH
and Si-O-Si bonds. SEM analysis (Figure 2) of this solid was
also undertaken. Particles were found to be in the nanometer size
regime (40-50 nm) and were stabilized by the polymeric network.
After product isolation, the remaining sticky solid was washed
with benzene, and the same catalysis was repeated again. A black
solution was obtained after addition of the reactants. Catalysis was
continued until total conversion to silylester, indicating that the
presence of silicon network stabilization facilitates the redispersion
of catalytically active Pd particles.
2
acid (0.06 mL, 1.00 mmol) were dissolved in 2.5 mL of benzene,
and the mixture was examined by UV-visible spectroscopy. A peak
at 400 nm indicative of Pd(OAc)
Polymethylhydrosiloxane 1 (PMHS) (0.06 mL, 1 mmol, mw ≈
000, 33-35 Si-H units) was added, which was accompanied by
generation of a gas, presumably H . After 5 min of addition of 1,
the Pd(OAc) peak (400 nm) disappeared completely (Figure 1a),
2
was observed (Figure 1a).
2
2
2
indicating the formation of a new catalytically active species.
†
City University of New York at the College of Staten Island.
City University of New York at the Queensborough Community College.
‡
2876
9
J. AM. CHEM. SOC. 2003, 125, 2876-2877
10.1021/ja028957k CCC: $25.00 © 2003 American Chemical Society

C O MMU N I C A T I O N S
Scheme 2. Mechanistic Proposal for the Pd Colloid-Catalyzed
Silaesterification Reactions
Figure 2. SEM image of the gummy solid obtained after the catalysis.
Table 1. Comparison of the Catalytic Activity
acid
silane
Pd colloids
2% Pd(OAc)
2
yield
CH3COOH
CH3COOH
C6H5COOH
C6H5COOH
2
1
2
1
rt, 5 h
rt, 8 h
95%
95%
95%
85%
70 °C, 12 h
70 °C, 6 h
70 °C, 24 h
70 °C, 3.5 h
70 °C, 6 h
70 °C, 24 h
After total consumption of Si-H and COOH bonds, these
particles precipitate out along with silicon matrix. When these
particles are recharged with Si-H and acid containing moieties,
they redisperse and again undergo the oxidative-addition, reductive-
elimination sequence. The presence of the soluble polysiloxane
matrix (which is not fully condensed in the form of silica) allows
for particle redispersion, hence making them recyclable.
In conclusion, we have successfully demonstrated Pd particle
stabilization by polysiloxane network and their active role as a
recyclable catalyst in the silaesterification reactions. We are further
examining the catalytic efficiency of polysiloxane stabilized Pd
colloids for industrially important conversions.
To further probe the identity of the “real catalyst”, poisoning
3
experiments using mercury, as well as added ligand (PPh ), were
performed.15 A standard reaction mixture was prepared, and after
addition of the polysiloxane (i.e., after generation of Pd colloids),
2
.5 equiv of Hg was added to the reaction mixture. After 30 min,
the reaction mixture became completely clear and was examined
by NMR and TEM. Only 15-20% of the expected product was
formed. If the catalysis was continued for an even further 24 h,
there was no increase in the silylester formation. Under identical
reaction conditions, the quantitative conversion to corresponding
silylester takes place after 3.5-4 h.
Acknowledgment. We acknowledge the NIST-research grant,
NSF-instrumentation grant, CSI-CUNY startup grant, and PSC-
CUNY grant. We also thank “Deans Summer Scholarships Pro-
gram” for providing research support for A.K.
When the silaesterification was carried out in the presence of
PPh
3
(3:1 PPh
3
:Pd(OAc)
2
), no reaction took place under standard
was added after the generation
conditions even after 18 h. If PPh
of the Pd colloids, only 25% conversion of reactants to product
was obtained after 24 h of reaction.
3
Supporting Information Available: Experimental details, TEM
images (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
Catalyst poisoning tests along with UV, TEM, and catalyst
isolation (evacuation to dryness) and then reuse experiments firmly
establish that Pd colloids are the real catalysts in the present system.
On the basis of this evidence, it seems plausible that the silicones
play the role of intermediate host stabilizing agents.
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(
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4b
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