One-pot synthesis of Au@SiO2 catalysts: A click chemistry approach

Article abstract of DOI:10.1021/co5000932

Using the copper-catalyzed azide-alkyne cycloaddition "click" reaction, a library of triazole amphiphiles with a variety of functional polar "heads" and hydrophobic or superhydrophobic "tails" was synthesized. The amphiphiles were evaluated for their abil

Full text of DOI:10.1021/co5000932

Letter
pubs.acs.org/acscombsci
One-Pot Synthesis of Au@SiO2 Catalysts: A Click Chemistry Approach
Vera A. Solovyeva,† Khanh B. Vu,† Zulkifli Merican,† Rachid Sougrat,‡ and Valentin O. Rodionov*,†
‡
†Division of Physical Sciences and Engineering and KAUST Catalysis Center and KAUST Advanced Nanofabrication Imaging and
Characterization Core Laboratory, King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900, Kingdom
of Saudi Arabia
S
* Supporting Information
ABSTRACT: Using the copper-catalyzed azide−alkyne cycloaddition “click”
reaction, a library of triazole amphiphiles with a variety of functional polar “heads”
and hydrophobic or superhydrophobic “tails” was synthesized. The amphiphiles were
evaluated for their ability to stabilize small Au nanoparticles, and, at the same time,
serve as templates for nanocasting porous SiO2. One of the Au@SiO2 materials thus
prepared was found to be a highly active catalyst for the Au nanoparticle-catalyzed
regioselective hydroamination of alkynes.
KEYWORDS: gold nanoparticles, click chemistry, fluorous surfactant, hydroamination
phile templates. Using the copper-catalyzed azide−alkyne
atalysis with gold nanoparticles (AuNPs) is a topic of
Cconsiderable current interest. That particles be small is a cycloaddition (CuAAC) “click” reaction,18 we synthesized a
1
library of triazole amphiphiles with a variety of functional polar
“heads” and hydrophobic or superhydrophobic “tails”. The
amphiphiles were evaluated for their ability to stabilize small
AuNPs,19 and, at the same time, to serve as templates for
nanocasting porous SiO2.20 One of the Au@SiO2 materials thus
prepared was found to be highly active catalyst for the AuNP-
catalyzed regioselective hydroamination of alkynes.6,21
We synthesized alkyne-bearing, polar surfactant “heads”
starting from cinchonidine (compounds 1a−b) and cinchonine
(compounds 2a−3b), two readily available alkaloids. These
alkaloids were chosen because we hypothesized that their
quinoline and tertiary amine moieties could catalyze the
hydrolysis of SiO2 precursors and/or bind to AuNP surfaces.
Two simpler alkyne “heads” were synthesized based on
oligo(ethylene glycol) (compound 4)22 and trimethylammo-
nium (compound 5). Two types of hydrophobic azide “tails”
were used, C10H21N3 linear hydrocarbon and perfluorinated
C8F17CH2CH2N3. The resulting library is presented in Chart 1.
The amphiphiles thus prepared were evaluated for their ability
to stabilize AuNPs, and, following that, used for one-pot
emulsion syntheses of Au@SiO2 materials. We found that the
morphologies of the AuNPs and the SiO2 support were
strongly influenced by the structure of the amphiphiles.
prerequisite for high catalytic activity in many of the gold-
catalyzed chemistries.2 Small AuNPs have been stabilized using
a variety of surface ligands,3,4 macromolecules,5−7 and solid
supports,8,9 such as SiO2, Al2O3, and mesoporous carbon. The
nature of the stabilizing ligands and supports can dramatically
influence both the stability and the catalytic competency of
AuNPs. Fine control over the structures of both the support
and the nanoparticles is highly desirable for the development of
practical catalysts.
Postsynthesis incorporation methods, such as wet impreg-
nation/calcining,10 immobilization of premade nanoparticles,11
and chemical vapor deposition12 allow full control over the
structure of solid support. However, the chosen support
necessarily determines the size of the nanoparticles, their spatial
distribution, and surface properties. Furthermore, nanoparticles
often localize close to the surface of the support, and may block
the channels/pores, limiting the catalyst’s accessibility.13
In situ incorporation methods, in which the solid support is
synthesized in the presence of premade nanoparticles or the
metal salt precursor, are experimentally simple and can yield
materials with a uniform distribution of confined nanoparticles.
This one-pot approach has been used to prepare SiO2-
supported Pd, Pt and Au nanoparticle catalysts, using
polymers,14 such as Pluronics F1279 and P123,15,16 chitosan,15
or cross-linked polystyrene-co-polystyrenesulfonate,17 as both
nanoparticle stabilizing agents and structure-directing agents for
the SiO2 solid support. Here, we present a combinatorial
approach that provides easy access to a range of SiO2-confined
AuNP materials (Au@SiO2) based on small-molecule amphi-
Neither of the hydrocarbon-tail cinchona surfactants 1b or
2b were effective stabilizers for gold nanoparticles, as extensive
Received: June 18, 2014
Revised: September 8, 2014
© XXXX American Chemical Society
A
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ACS Combinatorial Science
Letter
synthesis of SiO2, markedly different morphologies of silica
resulted (Figure S15, SI). Amphiphile 1a templated highly
porous, spherical, uniformly sized SiO2 nanoparticles, with a
tendency to form hollow shell structures.23 Rod-shaped SiO2
particles templated by 2a were significantly smaller and more
disordered. Using mixtures of 1a and 2a provided access to a
range of spherical to rod-shaped SiO2 morphologies. We
attribute these changes in morphology to differences in the
long-range order that exists in micellar assemblies of 1a, 2a, and
their mixtures. We chose to use amphiphile 1a for the
remainder of the study due to the high porosity (Figure S7 and
Table S1, SI) and regular structure of SiO2 templated by it.
The UV−vis spectra of the 1a-stabilized AuNPs and of the
corresponding Au@SiO2 material prepared in situ were found
to be very similar to each other (Figure S2, SI), and did not
show a prominent plasmonic peak. This indicates that the
AuNPs growth was not significant in the presence of SiO2 or
Si(OEt)4 precursor, and is supported by the size distributions of
the 1a-stabilized AuNP (Figure S3, SI) and the as-made Au@
SiO2 (Figure S4, SI). The zeta potential of the 1a-stabilized
AuNPs (Figure S6, SI) was +33.9 ( 0.5) mV at pH 9.3,
suggesting that the interaction between amphiphile-embedded
AuNPs and SiO2 and its precursor (negative zeta potential at
high pH) came from the direct co-condensation of anionic
inorganic species with the cationic surfactant.24 After the
reaction, the zeta potential of the Au@SiO2 material was −19.6
( 1.0) mV at pH 8.7, indicating that the surfactant-stabilized
AuNPs were covered by silica.
Chart 1. Library of fluorous 1,2,3-triazole surfactants
This Au@SiO2 material retained a significant amount of
amphiphile 1a (Figure S8, SI). The as-made material is easily
dispersible in perfluorodecalin and other fluorous solvents. In
biphasic fluorous-hydrocarbon mixtures, the 1a-Au@SiO2
material partitions into the fluorous phase (Figure S16, SI).
The removal of encapsulated surfactant by calcining the sample
in static air at 500 °C for 4 h resulted in AuNPs with a bimodal
particle size distribution (Figure 1 F and Figure S14, SI). This
suggests that the remnant 1a plays a role in the stabilization of
AuNPs in the SiO2 matrix. The calcined Au@SiO2 completely
lost its dispersibility in fluorous solvents.
Additional insight into the aggregation behavior of AuNPs
and growth of SiO2 in the presence of amphiphile 1a was
gained from cryo-transmission electron microscopy (cryo-
TEM) images of the 1a-stabilized AuNPs and corresponding
Au@SiO2 materials.
Images of AuNPs@1a with different electron doses are
shown in Figure 1A and B. With an electron dose of 100 e−/Å2
(Figure 1A), we observed raspberry-like assemblies of low- and
high-density objects, corresponding to aggregates of 1a and
AuNPs, respectively. It can be seen that 1a forms stable and
well-defined aggregates in water. The assemblies are ∼20 nm in
diameter, and have a positive zeta potential (Figure S6, SI).
These assemblies are remarkable in their ability to stop Ostwald
ripening of small AuNPs (Figure S3, SI). To verify that the low-
density objects are indeed amphiphile aggregates, we increased
the electron dose from 100 to 400 e−/Å2. The low-density
objects were damaged by the electron beam, as can be seen in
Figure 1 B, while the high-density objects remained stable.
Cryo-TEM images of the corresponding Au@SiO2 material
with low and high electron doses are shown in Figure 1C and
D. The images suggest that SiO2 shells are growing around the
AuNP@1a aggregates. This interaction between the SiO2 shells
and 1a-stabilized AuNPs was confirmed by increasing the
electron dose from 100 to 400 e−/Å2. The soft/organic parts of
aggregation and precipitation was observed (Figure S1 D-E,
Supporting Information (SI)). The neutral oligo(ethylene
glycol)-based surfactant 4 was neither able to stabilize the
gold nanoparticles, nor to serve as a structure-directing agent
for SiO2. The average diameter of the gold nanoparticles
obtained in the presence of 4 was 12.3 nm, and there was no
obvious interaction between the gold and silica (Figure S10,
SI). We found that amphiphile 5 could serve as a template for
structured SiO2, although the diameters of the gold nano-
particles were far from uniform and hopelessly too big for any
catalytic application (11.2 nm, Figure S11, SI).
The surfactant 3a bearing two fluorous “tails” was capable of
stabilizing somewhat smaller AuNPs, with an average diameter
of 4.5 nm (Figure S9, SI). However, the structure of SiO2
directed by 3a was highly irregular, and low solubility of this
amphiphile precluded its use in larger scale syntheses of Au@
SiO2. Likewise, the hybrid fluorous-hydrocarbon amphiphile 3b
stabilized relatively large AuNPs, as indicated by the obvious
plasmonic band at ∼500 nm in the UV−vis spectrum (Figure
S13, SI).
To our delight, the single-tail fluorous amphiphile 1a and its
pseudoenantiomer 2a could stabilize small, sub-2 nm AuNPs, as
indicated by the color of the colloidal solutions and the absence
of plasmonic bands in the range of 500−550 nm in their UV−
vis spectra7 (Figure S1, S2 and S12, SI). The AuNP solutions
stabilized by 1a and 2a were found to be stable for at least 10
months (Figure S3, SI).
The stereochemistry of the surfactant “head” in 1a and 2a
did not significantly affect the ability of the amphiphiles to
stabilize AuNPs. However, when the pseudoenantiomeric
amphiphiles were used as structure-directing agents for the
B
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ACS Combinatorial Science
Letter
mined by XPS and ICP-OES was 1.1 and 1.6 wt %, respectively.
These results confirm that the material is indeed AuNPs
encapsulated in SiO2, since the apparent gold content from the
surface analysis (XPS) is lower than that from the bulk analysis
(ICP-OES).25 Since Au 4f7/2 and 4f5/2 of the Au 4f spectrum of
Au@SiO2 were not well separated and broad compared with
those of the Au 4f spectrum of the Au0 film, more than one
valence state of Au should be expected. We attributed the Au
4f7/2 at 83.87 eV and that at 84.60 eV to Au0 and Auδ+,
respectively.26 The assignment of the partially charged Auδ+
(∼40% of total Au present) in this case was neither Au+ (85.6
eV) nor Au3+ (87.5 eV), for which the binding energies are well
established.27 A possible explanation for this observation is that
the low and high binding energy represents the bulk (core) and
surface (the shell interacting with the fluorinated surfactant) of
the AuNPs, respectively. The same observation was made for
thiolate-protected AuNPs.4,28
The Au@SiO2 material, prepared in the presence of 1a,
exhibited excellent activity and 100% selectivity for the imine
products when used in hydroamination of alkynes with anilines
(Table 1). It is important to note that for each of the reactions
a
Table 1. Hydroamination of Alkynes with Anilines
time
(h)
alkyne conv.
b
entry
R1
R2
(%)
TON
1
2
3
4
5
H
H
H
H
H
H
16
16
16
16
16
97
100
93
1605
1640
1361
1640
1605
p-Et
Figure 1. Cryo-TEM and TEM images of 1a-stabilized AuNPs and
Au@SiO2 materials. (A and B) Cryo-TEM images of 1a-stabilized
AuNPs, total doses of 100 and 400 e−/Å2; (C and D) Cryo-TEM
images of Au@SiO2, total doses of 100 and 400 e−/Å2; (E and F):
High resolution TEM images of as-made Au@SiO2 and calcined Au@
SiO2.
p-PhO
p-C5H11
100
98
1-ethynyl-
naphthalene
6
H
p-MeO
p-MeO
p-MeO
p-MeO
p-MeO
16
16
16
16
16
100
100
100
95
1640
1640
1640
1560
1619
7
p-Et
8
p-PhO
p-C5H11
the material (aggregates of 1a) were damaged, while the SiO2
shells remained intact.
We further characterized the Au@SiO2 material prepared in
the presence of 1a using X-ray photoelectron spectroscopy
(XPS) (Figure 2) and inductively coupled plasma optical
emission spectroscopy (ICP-OES). The gold content deter-
9
10
1-ethynyl-
naphthalene
99
11
12
13
14
H
p-Me
p-Me
p-Me
p-Me
16
16
16
16
88
92
94
99
1441
1514
1534
1626
p-Et
p-C5H11
1-ethynyl-
naphthalene
15
16
2,4-di(CF3)
H
H
36
36
60
77
984
2,4-
di(CF3)
1263
c
c
17
18
19
H
H
H
H
16
16
16
97
1605
c
c
p-Et
p-PhO
83
1361
c
c
77
1256
a
Reaction conditions: 1 mmol of acetylene, 1.5 mmol of aniline, 7.5
mg of as-made catalyst (0.61 mmol, 0.61 × 10−3 eq Au), under Ar, 135
b
°C. Determined by GC/MS with chlorobenzene as internal standard;
c
With calcined catalyst (3.2 mg, 0.61 mmol, 0.61 × 10−3 equiv of Au).
in the Table, the corresponding blank reactions did not lead to
the formation of any imine product. The optimal reaction
conditions were achieved by using a neat mixture of 1 equiv of
alkyne and 1.5 equiv of aniline under Ar atmosphere at 135 °C
for 16 h. These conditions led to an almost quantitative
conversion of the alkyne to an imine for most of the substrates,
Figure 2. XPS spectra of the Au 4f level.
C
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ACS Combinatorial Science
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with only a trace amount of a hydrolysis byproduct
(corresponding acetophenone). The presence of electron-
donating substituents in both the alkyne and aniline substrates
activated them for hydroamination (Table 1, entries 2−14).
The reactions were markedly slowed by electron-withdrawing
substituents (Table 1, entries 15−16).
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