Communications
DOI: 10.1002/anie.200702032
Gold Catalysts
Gold Nanoparticles and Gold(III) Complexes as General and Selective
Hydrosilylation Catalysts**
Avelino Corma,* Camino Gonzµlez-Arellano, Marta Iglesias, and FØlix Sµnchez
[
10]
Gold salts and soluble complexes have provided access to
reactions that could not be reached before with more
active for hydrosilylation until Hosomi and co-workers
showed the possibility to hydrosilylate aldehydes with
[(PPh )AuCl]. Unfortunately, low conversions are observed
with this and similar catalysts as a result of precipitation of
inactive metallic gold, unless a large excess (20 mol%) of
[
1]
traditional catalysts. Moreover, when Au is prepared in
the form of nanoparticles, it becomes an active and selective
3
[2]
catalyst for reactions such as CO oxidation, chemoselective
[
3]
[19–20]
reduction of substituted nitroaromatics by H , selective
tributylphosphine is added.
2
[
4]
oxidation of alcohols, and some CÀC bond-forming reac-
To date, hydrosilylation with gold has been restricted to
aldehydes and alkynes, but the process is still far from ideal
[
5]
[21]
tions. Together with solids, the use of Au complexes in
homogeneous catalysis has undergone a renaissance and
as it is hampered by the use of phosphine derivatives and
owing to the lack of results for the hydrosilylation of olefins
and imines. It is therefore of interest to synthesize new
phosphine-free homogeneous gold catalysts and, even better,
to design recyclable solid gold catalysts for hydrosilylation of
a variety of groups.
Herein we show that nanoparticles of gold, on carriers
which are able to stabilize cationic forms of gold, can be a
general recyclable catalyst for the hydrosilylation of a large
variety of functionalities (aldehydes, ketones, olefins, imines,
and alkynes). To discuss the nature of the active gold species
numerous publications have recently emphasized the benefi-
I [6–10]
cial role of Au .
A broad range of transformations
III
catalyzed by inorganic Au salts have also been reported;
[
11]
examples include hydroamination and functionalization of
[
12]
aromatic CÀH bonds. Most often, AuX (X = Cl, Br) salts
3
are used directly and only a limited number of examples are
known of well-defined organogold(III) complexes acting as
[
6h,13]
III
catalysts.
Recently, we have shown that Au phenolic
Schiff base complexes catalyze the homocoupling of aryl
boronic acids but are unable to catalyze the Suzuki cross-
[
14]
I
10
I
III
coupling. However, Au , which has the same d electronic
on solid catalysts, Au and Au phosphine-free complexes
0
[22]
configuration as Pd , is an active catalyst for performing
both homogeneous and heterogenized on MCM-41
Suzuki couplings and the copper-free Sonogashira cross-
(Figure 1) have been prepared and tested. We show that
solid gold catalysts are not only active catalysts for a large
variety of hydrosilylation reactions but are also chemo-
selective for the hydrosilylation of aldehydes in the presence
of olefins.
[
15]
coupling. We have also reported that stable unsymmetrical
I
N-heterocyclic carbene (NHC) Au complexes (soluble and
heterogenized) are effective catalysts for the Suzuki cross-
[
16]
coupling and enantioselective hydrogenations.
Hydrosilylation reactions are a very important route to
Recently, we demonstrated that nanoparticles of CeO
2
[
17]
I
III [23]
silicon polymers. In general, organosilicon compounds have
found industrial applications as photoresistors, semiconduc-
tors, adhesives, binders, and for preparative organic synthesis.
Hydrosilylation catalysts normally involve Pt, Pd, Ir, Ru, Rh,
can stabilize cationic forms of gold (Au and Au ). If this is
so, a solid catalyst formed by gold on nanoparticulated CeO2
could act as a catalyst for hydrosilylation reactions. Indeed,
results show that Au/CeO with an average gold particle size
2
[
18]
Co, and Ni complexes, while gold was believed not to be
of 4 nm is active and selective for the hydrosilylation of
olefins, aldehydes, ketones, alkynes, and imines (Table S3a
and S3b in the Supporting Information). The catalyst can be
filtered and recycled at least four times without loss of activity
or selectivity.
[*] Prof. A. Corma
Instituto de Tecnología Química, UPV-CSIC,
Avda. de los Naranjos s/n
I
III
0
46022 Valencia (Spain)
Three gold species, that is, Au , Au , and Au were
identified by different spectroscopic techniques on the Au/
Fax: (+34)96-3877809
E-mail: acorma@itq.upv.es
CeO catalyst used in this work, with relative abundances of
2
0
III
I [23]
Dr. C. Gonzµlez-Arellano, Dr. M. Iglesias
Instituto de Ciencia de Materiales de Madrid, CSIC
C/Sor Juana InØs de la Cruz 3, Cantoblanco
Au > Au > Au . To find which, among those species, are
the active sites for hydrosilylation, we tested a series of Au
I
(
[(PPh )AuCl], [(carbene)AuCl] (3Au), and [(tht)AuCl];
3
28049 Madrid (Spain)
III
III
tht = tetrahydrothiophene) and Au complexes (1,2Au )
and salts (KAuCl ), as well as colloidal Au with a mean
Dr. F. Sµnchez
Instituto de Química Orgµnica, CSIC
C/Juan de la Cierva 3, 28006 Madrid (Spain)
0
4
diameter of 5 nm, which is very similar to that observed for
gold nanoparticles supported on the nanocrystalline ceria.
[
**] The authors thank the Ministerio de Educación y Ciencia (Projects
MAT2006-14274-C02-01 and 02) for financialsupport. C.G.-A.
thanks the I3P Program for financialsupport.
When [(PPh )AuCl] is used as catalyst, it shows no activity
3
toward the hydrosilylation of either carbonyl or olefinic
compounds as a result of the formation of inactive gold
metallic particles that agglomerate. However, when the Schiff
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
7820
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 7820 –7822