Hydrosilylation of 1-hexyne promoted by acetone solvated gold atoms derived catalysts

Article abstract of DOI:10.1016/j.jorganchem.2004.10.051

Supported gold nanoparticles, prepared by deposition of acetone solvated Au atoms on supports as carbon and γ-Al₂O₃, behave as valuable catalysts for the regioselective hydrosilylation of 1-hexyne with different silanes. The catalytic behaviour of gold-based systems is compared with the activity of supported platinum catalysts and a different affinity between the metals and the silanes is observed.

Full text of DOI:10.1016/j.jorganchem.2004.10.051

Journal of Organometallic Chemistry 690 (2005) 1063–1066
www.elsevier.com/locate/jorganchem
Hydrosilylation of 1-hexyne promoted by acetone solvated
gold atoms derived catalysts
a,
Anna Maria Caporusso , Laura Antonella Aronica , Eleonora Schiavi ,
a
a
*
b
a
Gianmario Martra , Giovanni Vitulli , Piero Salvadori
a
a
Dipartimento di Chimica e Chimica Industriale, Istituto del CNR di Chimica dei Composti Organo Metallici (ICCOM), sezione di Pisa,
Universit a` di Pisa, Via Risorgimento 35, 56126 Pisa, Italy
b
Dipartimento di Chimica IFM, Universit a` di Torino, Via P. Giuria 7-10125, Torino, Italy
Received 21 July 2004; accepted 4 October 2004
Abstract
Supported gold nanoparticles, prepared by deposition of acetone solvated Au atoms on supports as carbon and c-Al
2 3
O , behave
as valuable catalysts for the regioselective hydrosilylation of 1-hexyne with different silanes. The catalytic behaviour of gold-based
systems is compared with the activity of supported platinum catalysts and a different affinity between the metals and the silanes is
observed.
ꢀ
2004 Elsevier B.V. All rights reserved.
Keywords: Gold nanoparticles; Hydrosilylation; Metal vapour synthesis; Supported catalysts
1
. Introduction
Considering the high affinity of Au for carbon–car-
bon triple bonds [6], we investigated the catalytic prop-
erties of supported gold nanoparticles in the
hydrosilylation of acetylenes. Here we report our preli-
minary results on the hydrosilylation of terminal alkynes
using Au/C and Au/c-Al O as catalysts, and their com-
The hydrosilylation of carbon–carbon multiple bonds
provides a very convenient route to the preparation of
organo-silicon compounds which have found industrial
application to photo resistors, semiconductors, adhe-
sives, binders and represent an important class of re-
agents for organic synthesis [1]. A considerable
number of organometallic complexes containing Pt,
Pd, Ir, Ru, Rh, Ni [2] and also supported metals [3,4]
have been employed as catalysts for the hydrosilylation
of organic molecules, while gold has been thought to be
chemically inert for long times. Only in 2000 Hosomi
and co-workers [5] described the first example of silanes
addition to aldehydes using a catalytic amount of a
AuCl(PPh3).
2
3
parison with the performances of highly regioselective
platinum based catalysts.
2. Results and discussion
Gold nanoparticles were prepared using the metal va-
pour synthesis (MVS) technique [7]. According to the
procedure early described by Klabunde [8], gold was
vaporised under vacuum and deposited on the inside
walls (cooled to ꢀ196 ꢁC) of a glass reactor simulta-
neously with vapours of acetone. A deep purple solid
matrix was obtained which gave, on melting, ‘‘gold
atoms’’ solvated by CH COCH molecules (0.3 mg
*
Corresponding author. Tel.: +39 050 2219298; fax: +39 050
2219260.
E-mail address: capored@dcci.unipi.it (A.M. Caporusso).
3
3
0
022-328X/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.10.051

1064
A.M. Caporusso et al. / Journal of Organometallic Chemistry 690 (2005) 1063–1066
Au/ml acetone). This solution was added to a proper
amount of the support (carbon or c-Al O ) and stirred
(Table 1, entries 2–4). Moreover, the catalytic properties
(expressed as specific activity) of the carbon supported
gold nanoparticles were strictly related to the structure
of the hydrosilane. Et SiH and Me PhSiH resulted the
2
3
at room temperature until the liquid phase became col-
ourless. The organic layer was then removed via syringe
and the powdered catalyst was dried under vacuum at
room temperature. A Au/C 0.1% w/w and a Au/c-
Al O 0.1% w/w species were thus obtained. HRTEM
3
2
most reactive compounds, while very polarised silanes
such as (EtO) SiH showed low conversion (34%) and
3
chloro functionalised reagents (Cl MeSiH) yielded no
2
2
3
indicated that in both catalysts metal particles are well
dispersed onto the support (representative images in
Fig. 1), and exhibit a narrow size distribution with a
mean diameter of 4.0 nm (histograms in Fig. 1).
products.
These data are in contrast with the well known affin-
ity between platinum catalysts and chlorosilane [1,2]. In-
deed using the Pt/C MVS catalyst (0.1% w/w) [4] a
reverse order of specific activity was observed and
As reference systems, Pt/C and Pt/c-Al O MVS cat-
2
3
alysts, having the same metal loading (0.1% w/w), and
the metal particle size narrowly distributed around 2.0
nm, were used. We had previously reported [4] that these
MVS platinum species were very good catalysts for a
selective syn addition of hydrosilanes to unsaturated
substrates.
Cl MeSiH was completely consumed after 2 h at room
2
temperature (Table 1, entry 5). On the other hand, in
the presence of aliphatic and aromatic silanes (Table 1,
entries 7,8) longer reaction times were necessary to im-
prove the conversion of the reagents and a considerable
loss of regioselectivity was detected, i.e., the yield of the
a-isomer increased until around a 20% of the product
mixture.
1
-Hexyne was chosen as model compound for the
reaction of hydrosilylation and treated with several sil-
anes having different steric and electronic properties.
In a typical experiment, an excess of the alkyne was re-
acted with the hydrosilane at 70 ꢁC in the presence of a
catalytic amount of Au/C (0.1% w/w). The obtained re-
sults are described in Table 1 and compared with the
data previously obtained using the MVS Pt/C catalyst
It is of interest to notice that the observed difference
in specific activity between the Au/C catalyst and the
Pt/C catalyst towards Et SiH and Me PhSiH should
3
2
be even larger by taking into consideration the surface
metal atoms, as Au particles (mean size 4.0 nm) have
a lower surface/volume ratio than the Pt ones (mean
size: 2.0 nm).
[
4].
As evident from the table, the gold promoted reaction
The peculiar catalytic behaviour of the Au catalyst
appeared independent on the type of support. As re-
ported in Table 2, a Au/c-Al O catalyst exhibited ex-
resulted highly regio- and stereoselective. Indeed, the
hydrosilylation of a terminal acetylene such as 1-hexyne
can yield three isomeric products: b-(Z), b-(E) and a
2
3
actly the same order of reactivity detected for Au/C,
i.e., the best specific activities (6440–8000) were obtained
using Me PhSiH and Et SiH (Table 2, entries 3,4). As
(
Table 1, upper part) and their distribution markedly de-
pends upon the nature of the catalyst. The MVS Au/C
specie clearly favours the formation of the trans iso-
mer b-(E) which is almost the only product obtained
2
3
already observed for the carbon supported gold nano-
particles (Table 1), the reaction proceeds with high
2 3
Fig. 1. Electron micrographs (original magnification 200,000·) and histograms of particle size distribution of Au/C 0.1% w/w and Au/c-Al O 0.1%
w/w.

A.M. Caporusso et al. / Journal of Organometallic Chemistry 690 (2005) 1063–1066
1065
Table 1
Hydrosilylation of 1-hexyne promoted by carbon supported catalysts
a
n-Bu
SiR3 n-Bu
H
n-Bu
H
H
n-BuC
CH + R3SiH
+
+
H
β (Z)
b
H
H
β (E) SiR3 R Si
3
α
c
d
Entry
3
R SiH
t
Conv (%)
Product distribution (%)
Specific activity
b-(Z)
b-(E)
a
Au/C (MVS) 0.1% w/w
1
2
3
4
Cl
(EtO)
Et SiH
Me PhSiH
2
MeSiH
1 h
2 h
1 h
0
–
–
–
–
–
–
–
3
SiH
34
83
71
98
97
98
2
341
1662
7012
3
3
2
0
2
15
Pt/C (MVS) 0.1% w/w
e,f
5
6
7
8
Cl
(EtO)
Et SiH
Me PhSiH
2
MeSiH
2 h
2 h
7 h
4 h
100
100
85
–
–
–
–
95
78
86
79
5
22
14
21
4166
3571
867
f
f
f
3
SiH
3
2
68
2267
a
ꢀ4
ꢀ3
Experimental conditions: 1-hexyne = 8 mmol; silane = 2 mmol; catalyst = 1.5 · 10 –3 · 10 mg-atom Au or Pt; 70 ꢁC; all reactions carried out
without solvents.
b
GC conversion of the silane.
Isomeric ratio derived from GC areas and H NMR integrals of the olefinic protons.
Specific activity calculated as moles of silane converted/g-atom of metal per hour.
c
d
e
f
1
Reaction performed at 25 ꢁC.
See [4].
Table 2
a
Hydrosilylation of 1-hexyne catalysed by c-Al
2
O
3
supported catalysts
b
c
d
Entry
R
3
SiH
t
Conv (%)
Product distribution (%)
Specific activity
b-(Z)
b-(E)
a
Au/c-Al
O
2 3
MVS (0.1% w/w)
Cl MeSiH
(EtO) SiH
Et SiH
Me PhSiH
1
2
3
4
2
1 h
1 h
0
–
–
3
–
–
–
8
4
1
–
3
88.5
80.5
100
92
93
99
1770
6440
8000
0
3
15
0
2
15
Pt/c-Al
5
O
2 3
MVS (0.1% w/w)
Et SiH
e
0
3
15
52
–
88
12
4208
a
ꢀ3
Experimental conditions: 1-hexyne = 8 mmol; silane = 2 mmol; catalyst = 10 mg-atom Au or Pt; 90 ꢁC; all reactions carried out without
solvents.
b
GC conversion of the silane.
c
1
Isomeric ratio derived from GC areas and H NMR integrals of the olefinic protons.
Specific activity calculated as moles of silane converted/g-atom of metal per hour.
See [4].
d
e
regioselectivity, the b-(E) isomer being formed in major
amount.
silylation of terminal acetylenes. High yields and almost
100% selectivity towards the b-(E) isomer formation
make these gold systems a very good alternative to more
expensive Pt-catalysts in the hydrosilylation reaction
and a quite attractive specie for developing new syn-
thetic processes.
Once again, in the 1-hexyne silylation with Et SiH
3
lower catalytic efficiency and regioselectivity were ob-
tained by using the Pt/c-Al O catalyst (Table 2, entry
2
3
5
vs. entry 3).
In conclusion, we have found that solvated gold
atoms derived catalysts are very efficient for the hydro-
Moreover, taking into account the opposite affinity of
the gold and platinum derived catalysts towards the

1066
A.M. Caporusso et al. / Journal of Organometallic Chemistry 690 (2005) 1063–1066
different silanes, it is possible to control the chemoselec-
tivity of the hydrosilylation process. Indeed if Me Ph-
perature. The colourless surnatant solution was re-
moved and the solid was washed with pentane and
dried under reduced pressure.
2
SiH or Et SiH are to be reacted with a carbon–carbon
3
triple bond a gold catalytic system is requested, while
a platinum catalyst must be used in the presence of
Cl MeSiH.
3.2. General procedure for hydrosilylation of 1-hexyne
2
Catalytic runs have been carried out in Pyrex Carius
tubes fitted with rotaflo taps. To a certain amount of
Au/support were added via syringe 8 mmol (0.91 ml)
of 1-hexyne and 2 mmol of the silane. The suspension
was stirred for a chosen time at the reported tempera-
ture, filtered on celite and the filtrate evaporated under
vacuum to remove the excess 1-hexyne. The crude prod-
3
. Experimental
All the operation concerning the Metal Vapour Syn-
thesis technique and the catalytic reactions were per-
formed under argon atmosphere. Hydrosilanes and
1
ucts were characterised by GC analysis and H NMR
proton signals.
1
-hexyne were distilled and degassed before use. GLC
analyses were performed with a DB1 capillary column
30 m · 0.52 mm, 5 lm) using He as the carrier gas
(
1
and a flame ionisation detector (FID). H NMR spectra
were recorded with a Gemini 200 MHz instrument, in
CDCl solution using CHCl as internal standard. The
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3
3
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[
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2
3
(
30 ml). The mixture was stirred for 24 h at room tem-