Converting homogeneous to heterogeneous in electrophilic catalysis using monodisperse metal nanoparticles

Article abstract of DOI:10.1038/nchem.468

A continuing goal in catalysis is to unite the advantages of homogeneous and heterogeneous catalytic processes. To this end, nanoparticles represent a new frontier in heterogeneous catalysis, where this unification can also be supplemented by the ability

Full text of DOI:10.1038/nchem.468

ARTICLES
PUBLISHED ONLINE: 29 NOVEMBER 2009 | DOI: 10.1038/NCHEM.468
Converting homogeneous to heterogeneous in
electrophilic catalysis using monodisperse
metal nanoparticles
Cole A. Witham, Wenyu Huang, Chia-Kuang Tsung, John N. Kuhn, Gabor A. Somorjai and
*
F. Dean Toste
*
A continuing goal in catalysis is to unite the advantages of homogeneous and heterogeneous catalytic processes. To this
end, nanoparticles represent a new frontier in heterogeneous catalysis, where this unification can also be supplemented by
the ability to obtain new or divergent reactivity and selectivity. We report a novel method for applying heterogeneous
catalysts to known homogeneous catalytic reactions through the design and synthesis of electrophilic platinum
nanoparticles. These nanoparticles are selectively oxidized by the hypervalent iodine species PhICl , and catalyse a range
2
of p-bond activation reactions previously only catalysed through homogeneous processes. Multiple experimental methods
are used to unambiguously verify the heterogeneity of the catalytic process. The discovery of treatments for nanoparticles
that induce the desired homogeneous catalytic activity should lead to the further development of reactions previously
inaccessible in heterogeneous catalysis. Furthermore, a size and capping agent study revealed that Pt PAMAM dendrimer-
capped nanoparticles demonstrate superior activity and recyclability compared with larger, polymer-capped analogues.
he field of homogeneous catalysis is a source of easily tuned, until proven heterogeneous2,16,17,25. Thus, we aimed to design and
selective catalysts with high activity. Heterogeneous catalysts synthesize truly heterogeneous NPs—which show no leaching—
T
also offer many advantages, some of which are not displayed capable of catalysing an alternative yet broad class of reactions for
by their homogeneous counterparts, including recyclability, ease functionalizing p-bonds, which, to date, has been achieved exclu-
of separation from the reaction mixture and their use in continuous sively with homogeneous catalysts.
flow processes. It is highly desirable to develop new systems that
To accomplish this goal, the NP catalysts would require signifi-
blend the many advantages of heterogeneous catalysis with the ver- cant electrophilic character, which could be imparted by an increase
satility of homogeneous catalysts1
–3
.
in metal oxidation state, similar to that found in electron-deficient,
Most efforts approach from the homogeneous side and use late-metal homogeneous catalysts. Through this, we would obtain
immobilized homogeneous catalyst species that promote reactions new activity from heterogeneous metal NP catalysts resulting in
1,4
already accessible in solution-state conditions . We sought to use selective, solution-phase carbon–carbon and carbon–heteroatom
an alternative method whereby heterogeneous catalysts are extended bond-forming reactivity not previously observed with known
to reactions previously only catalysed by homogeneous species. Metal heterogeneous catalysts.
nanoparticles (NPs) that serve as heterogeneous catalysts and whose
particle size and oxidation state can be characterized are well suited to Results and discussion
our study. It is known that the reactivity and selectivity of mono- Catalyst development and reactivity. Given that the aforementioned
disperse metal NPs can be altered by changes in their size and set of desired transformations are currently known to be catalysed
shape5 . Although Pd and Au NPs have been demonstrated to by Pt halides, we focused our study on Pt NPs. Two general types
catalyse a range of cross-coupling and oxidation/reduction reactions of NPs were prepared in a range of sizes that allow for thorough
in solution1 , studies have not yet yielded the control necessary to investigation of the effects of various parameters on reactivity.
develop new NP catalysts applicable to a wider variety of reactions. These Pt NPs were synthesized using two techniques: dendrimer
A further challenge involves the specific nature of the catalyst. In and polymer encapsulation (Fig. 1a).
–15
6–25
particular, the distinction between homogeneous and hetero-
geneous catalysis is often difficult to determine. This is due to the (PAMAM) dendrimers (G4OH) were used as capping agents
possibility that metal leaches from a heterogeneous catalyst into sol- for small 1.0 nm, 40 atom Pt (Pt ) NPs. Alternatively, polyvinyl-
Fourth-generation
hydroxyl-terminated
polyamidoamine
4
,17,26
40
ution and acts as the catalytically active homogeneous species. Thus, pyrrolidone (PVP) was used to generate 1.5, 2.9 and 5.0 nm-sized
2
7
it was important for us to consider in our studies the fact that for NPs by existing procedures . Fabrication of the NPs was followed
many of the NPs used in carbon–carbon bond-forming reactions, by deposition onto SBA-15, or in the case of the larger 5.0 nm
formation of a catalytically active homogeneous species from the NPs, MCF-17 mesoporous silica. Notably, NPs capped with
NP precursor cannot be excluded. For example, it is generally PAMAM dendrimers are active for a variety of reactions when sup-
accepted that Pd nanoparticles, when used in solution, degrade ported on SBA-15 silica, and calcinations or other similar manipu-
into various homogeneous species and provide a strong case for lations of the NPs are not necessary, unlike NPs capped with PVP or
2
6,28
the assumption that a nanoparticle catalyst is leaching in solution other agents . In addition to allowing for recycling of the catalyst,
Department of Chemistry, University of California, Berkeley, California 94720 and Chemical and Materials Sciences Divisions, Lawrence Berkeley National
Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA. *e-mail: somorjai@berkeley.edu; fdtoste@berkeley.edu
36
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.468
ARTICLES
a
deposition of the PAMAM dendrimer NPs on SBA-15 imparts
4
,26,28
additional thermal stability against aggregation
. Furthermore,
+
as PAMAM dendrimers are normally soluble only in protic liquids,
loading the NPs on SBA-15 allows for their use in organic solvents.
Our initial studies focused on a hydroalkoxylation reaction in
which an electrophilic Pt catalyst activates an alkyne towards
nucleophilic attack by an oxygen functionality . When 1.5 nm
Pt/PVP/SBA-15 NPs were used, no appreciable reaction was
observed. Given that the homogeneous Pt catalysts for this type of
reaction exist in the þ2 or þ4 oxidation state, we hypothesized
that the failure of the NPs was due to an overabundance of Pt in
the 0 oxidation state. In fact, X-ray photoelectron spectroscopy
2
9
+
OH
b
[
Pt]
O
Ph
(
XPS) studies on these Pt/PVP NPs have confirmed that .90%
Ar (1 atm), toluene
00 °C, 15 h
1
of the metal exists in a metallic Pt(0) oxidation state. Thus, we
Ph
2
1
turned to the Pt /G4OH NPs, which are .70% oxidized as deter-
4
0
3
0–32
mined by XPS
. However, exposure of alkyne 1 to 1.0 nm
Catalyst
Yield
Pt /G4OH NPs generated only a 10% yield of product. This
40
2
.9 nm Pt/PVP/SBA-15 (2.5 mol%)
9%
10%
result is most likely due to the presence of catalytically inactive Pt
Pt40/G4OH/SBA-15 (4 mol%)
2
Pt40/G4OH/SBA-15 (4 mol%), PhICl2 (12 mol%)
PtCl2 (5 mol%) (2 h)29
oxide species on the Pt /G4OH NP surface.
40
.9 nm Pt/PVP/SBA-15 (2.5 mol%), PhICl2 (7.5 mol%) >99%
To address this lack of activity, we postulated that known oxidiz-
ing agents for metals might be used to selectively modify the NP
surface to produce a catalyst species with the requisite activity.
98%
97%
2
Recent work has shown the formation of oxidized [PtCl ] layers
4
3
3
Figure 1 | Depiction of the two nanoparticle synthesis techniques used and
the initial reactivity results for electrophilic catalysis. a, In the top scheme,
Pt ions are loaded onto a PAMAM dendrimer and reduced to form a
on Pt surfaces with Cl etching , as well as oxidation of Pd catalysts
2
to form Pd–Cl bonds with the hypervalent iodine species iodoso-
3
4
benzene dichloride (PhICl ) . These precedents serve to reinforce
2
dendrimer-encapsulated NP. Sonication deposits the NPs on the mesoporous the ability of PhICl to act as a mild oxidant to transform the Pt
2
silica SBA-15 to generate the NP catalysts. In the bottom scheme,
NP surface into a catalytically active state. We found that PhICl2
successfully generated the desired electrophilic catalyst species.
Treatment of either Pt /G4OH/SBA-15 (further reduced under
H2 atmosphere at 100 8C for 24 h before reaction) or
Pt/PVP/SBA-15 NPs with three equivalents (relative to catalyst
polyvinylpyrrolidone encapsulates the NP. Deposition on SBA-15 follows to
produce the catalyst. In both cases, the NPs are synthesized before loading
onto SBA-15. b, Hydroalkoxylation of 1 with Pt NPs. To obtain electrophilic
4
0
activity from the Pt NPs, treatment with the mild oxidant PhICl is required.
2
Pt40/G4OH/SBA-15 NPs must be further reduced under H atmosphere at
loading) of PhICl resulted in an excellent yield of .95% for benzo-
2
2
1
00 8C for 24 h before reaction. This treatment generates catalytically active
furan 2 (Fig. 1b). Further evidence for the oxidation of the Pt NPs
was obtained from an XPS spectrum of 1.5 nm Pt/PVP NPs
NPs that activate the p-bond in 1 resulting in hydroalkoxylation to
benzofuran 2. Yields were determined by NMR versus internal standard.
treated with PhICl showing that .25% of the Pt was oxidized.
2
a 100
b
100
90
80
70
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
60
50
40
30
20
10
0
0
10
20
30
40
50
60
0
10
20
30
Time (h)
40
50
60
Time (h)
c
100
100
90
80
70
60
50
40
30
20
10
d
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
0
0
10
20
30
40
50
60
0
20
40
60
Time (h)
80
100
Time (h)
Figure 2 | Monitoring the Pt nanoparticle catalysed reaction to show size and capping-agent effects as well as recyclability. Graphs display the reaction
yield of 2 from 1 as a function of time. All catalysts were treated with PhICl (three catalytic equivalents). a,b, Initial run of Pt40/G4OH/SBA-15 (3 mol%)
2
(a), and recycled run (b). c,d, Initial run of 1.5 nm Pt/PVP/SBA-15 (1 mol%) (c), and recycled run (80 h reaction time) (d). Note the excellent recyclability of
the Pt40/G4OH catalyst as opposed to the significant deactivation of the Pt/PVP NPs.
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2010 Macmillan Publishers Limited. All rights reserved.

NATURE CHEMISTRY DOI: 10.1038/NCHEM.468
ARTICLES
a
H
N
CO2Me
The stability of the Pt40 catalyst was additionally tested by isolation
of the SBA-15-supported catalyst and immediate resubmission to a
new solution of substrate. In this case, an overall decrease in activity
was observed, but the catalyst remained consistently active until all
substrate was consumed with no further deactivation.
CO2Me
Catalyst
N
Ph
Ar (1 atm), toluene
1
00 °C, 15 h
3
Ph
4
A: 96% yield
B: 95% yield
C: 99% yield
The Pt /G4OH PAMAM NPs, when supported on SBA-15,
40
showed excellent recyclability over multiple cycles after simple fil-
tration, reduction and retreatment with PhICl2 (Fig. 2b). A
sample of Pt /G4OH/SBA-15 NPs has been recycled four times
b
O
O
40
t-Bu
Catalyst
with a consistent yield of .90% under the reported reaction con-
ditions. In addition, a single batch of catalyst was active for a
month, and a turnover number of 400 (per metal basis) was
obtained. Monitoring of the reaction indicated no loss of activity
after recycling compared to the initial catalyst use, and simple fil-
tration of the catalyst through a glass microfibre filter was sufficient
to separate the metal species from the product solution.
Much like their use in the gas phase, PAMAM dendrimers show
significant advantages as NP capping agents in solution. This tem-
plating strategy allows for the generation of monodisperse NPs as
small as 1 nm and is complemented by the resulting performance
enhancements and potential for further catalyst development in
combination with the oxidative modification.
O
O
Ar (1 atm), toluene
1
00 °C, 15 h
Ph
5
Ph
6
A: 80% yield
B: 82% yield
C: 78% yield
c
NHTs
NTs
Catalyst
Ar (1 atm), toluene
1
00 °C, 15 h
·
7
8
A: 60% yield
B: 64% yield
C: 75% yield
For larger NP sizes, PVP capping was required. However, the 1.5,
2.9 and 5.0 nm Pt/PVP/SBA-15 NPs had distinctly different activity
from the smaller, dendrimer-capped NPs. In addition to a decrease in
activity as NP size increased, these larger NPs demonstrated a
marked initial spike in activity, followed by rapid deactivation and
prolonged time to reach completion (Fig. 2c). For example, when
only 1 mol% of 1.5 nm Pt/PVP/SBA-15 NPs treated with PhICl2
were used, approximately 70% yield was reached in the first
d
Catalyst
Ar (1 atm), toluene
1
00 °C, 15 h
MeO
9
10
MeO
1
.5 hours. However, nearly 60 hours were required to reach complete
A: 73% yield, 75:25 para:ortho (–OMe)
B: 73% yield, 75:25 para:ortho (–OMe)
C: 70% yield, 80:20 para:ortho (–OMe)43
conversion to product. Immediate resubmission of the PVP NPs to a
new batch of substrate did indeed prove the catalyst was deactivated
as the reaction was extremely slow and reached a lower level of com-
pletion in significantly more time than initially required. Even after
reduction and PhICl retreatment of the recycled catalyst, substantial
deactivation of the catalyst was observed (Fig. 2d). In comparison,
Figure 3 | Cyclization reactions using Pt nanoparticles. a–d, Using either
PVP- or dendrimer-encapsulated NPs results in good to excellent yields of
nitrogen- and oxygen-containing heterocycles through p-bond activation by
electrophilic Pt. Catalyst systems: A: Pt40/G4OH/SBA-15 (4 mol%), PhICl2
2
2 mol% of PhICl -oxidized 5.0 nm Pt/PVP/SBA-15 NPs generated
2
(
12 mol%); B: 2.9 nm Pt/PVP/SBA-15 (2.5 mol%), PhICl (7.5 mol%); C:
only ꢀ14% yield in the initial activity spike, and after 70 hours, only
2
PtCl (5 mol%). Pt40/G4OH/SBA-15 NPs must be further reduced under H2
60% yield was obtained, demonstrating the decrease in activity result-
2
atmosphere at 100 8C for 24 h before reaction. Yields were determined by NMR ing from larger NP sizes. It is important to note that although the
43
versus internal standard. In d, the catalyst system C reacted at 80 8C for 17 h .
PVP-capped catalyst did show a more rapid deactivation, a batch
of 2.9 nm Pt/PVP/SBA-15 NPs treated with PhICl2 remained
As predicted, the oxidized NP catalysts demonstrated a size and active for one month. A turnover number of 415 (per metal basis)
capping-agent effect on catalytic activity. Although multiple studies was obtained. For these larger NP sizes, we believe that the NP has
conducted on various reactions in both the gas and solution phase more metallic character and tends to revert to the Pt(0) oxidation
3
8–40
have demonstrated the importance of NP parameters for reactivity state
. With smaller sizes, the NP more readily retains its oxidized
and selectivity, fewer discuss supported NPs in solution –15,35. state as the electronic valency of the surface atoms is less saturated
Under our oxidative modification conditions, additional size and and more atomic character is present. It is also likely that the
capping-agent effects were found for the PVP- and dendrimer- increased activity of smaller NPs is due to the larger number of Pt
capped, SBA-15-supported Pt NPs. Results from detailed reaction atoms, and therefore active sites after treatment with PhICl2,
monitoring indicate that smaller dendrimer-capped NPs are more
5
stable in higher oxidation states than larger NPs and remain in
this catalytically active state for prolonged times. Based on previous
OH
O
Pd catalyst
Ph
observations, this trend may be due to stability imparted by the
Ar (1 atm), toluene
100 °C, 15 h
PAMAM dendrimers26,28,30–32.
Ph
2
It is also possible that, under the reaction conditions, the dendri-
mer could decompose. Any decomposition, however, does not seem
to affect the catalytic activity as shown by the similar activity of the
Pt /G4OH/SBA-15 NP catalyst after recycling and its consistent
1
Pd40/G4OH/SBA-15 (4 mol%)
Pd40/G4OH/SBA-15 (4 mol%), PhICl2 (12 mol%)
10% yield
48% yield
50% yield
PdCl2 (4 mol%)
40
difference from the PVP-capped NPs. Furthermore, recent reports
suggest more harsh conditions are required for complete dendrimer Figure 4 | Cyclization reaction using Pd nanoparticles. Reaction with
removal3
1,36,37
. Regardless, Pt40 proved to be the most robust catalyst Pd NPs demonstrates generality for the oxidative modification.
and, after oxidative modification with PhICl , a steady rate of Pd40/G4OH/SBA-15 NPs must be further reduced under H atmosphere at
2
2
reaction was maintained, which only decreased as the reaction 100 8C for 24 h before reaction. Yields were determined by NMR versus
neared complete consumption of the starting material (Fig. 2a).
internal standard.
38
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.468
ARTICLES
n-Pr
n-Pr
n-Pr
n-Pr
Me
Me
O
Pt catalyst
N
N
0.5
+
N
Ar (1 atm), toluene
N
N
110°C, 48h
N
O
Me
OMe
11
12
13
Me
Catalyst
PtCl2 (5 mol%)
PtCl4 (5 mol%)
PtCl2 (5 mol%), SBA-15
Pt40/G4OH/SBA-15 (4 mol%)
.9 nm Pt/PVP/SBA-15 (2.5 mol%)
Yield (12, 13)
8%, 25%
8%, 13%
16%, 5%
43%, 6%
48%, 7%
2
Figure 5 | Oxidatively modified Pt nanoparticle catalysed cyclization of phenylurea 11. Whereas homogeneous catalysts provided low yields of tetracycle 13,
the NPs result in an unprecedented reaction to the methylene-bridged bisindole compound 12.
present on the surface of the NP. Beyond size considerations, the of its heterogeneity, we wanted further verification that the NPs
increased rate of deactivation and limited recyclability for the PVP were not leaching to form a homogeneous active catalyst. The first
capping agent renders the oxidatively modified PAMAM dendri- example is the striking improvement in long-term activity of the
mer-capped NPs as the optimal catalyst system.
smaller-sized PAMAM dendrimer-capped NPs over the larger
Having developed two highly active Pt NP catalysts, we sought to PVP-capped NPs, which is complemented by the differing
test them for a range of p-bond activation reactions previously recycling ability based on capping agent. Both results illustrate the
known to occur only with electrophilic homogeneous catalysts. effect of size and capping agent, which would not be possible if
The NPs were found to catalyse several reactions in the solution homogeneous catalysts were generated as the active species even if
phase under relatively mild conditions with minimal or no side reac- a ‘release and capture’ dynamic were present. In a more rigorous
tions. Figure 3 shows the results of these reactions with both G4OH test for the formation of a homogeneous species, a three-phase
PAMAM dendrimer and PVP capping agents. Five- and six-mem- test45,46 was used with Wang resin-bound substrate 14 (Fig. 6a). In
bered-ring heterocycles were formed by addition of nitrogen and the presence of homogeneous PtCl , 26% conversion of resin-
2
oxygen nucleophiles to alkynes and allenes (Fig. 3a–c)4 , activated bound substrate to product was observed. However, both 2.9 nm
1,42
by the Pt nanoparticle catalysts. Carbon–carbon bond formation has Pt/PVP/SBA-15 and Pt /G4OH/SBA-15 NPs oxidized with
40
4
3
also been accomplished with a hydroarylation reaction (Fig. 3d) . It PhICl2 resulted in ,2% conversion. In addition, transmission
is important to note that all reactions are catalysed by Pt NPs in yields electron microscopy images of supported Pt /G4OH/SBA-
40
comparable to, if not slightly better than those obtained with homo- 15 NPs before and after reaction do not show any appreciable
geneous PtCl . Furthermore, in the case of the cyclization of com- aggregation or leaching (Fig. 6b).
2
pound 9, our NP catalysts show selectivity favouring the para-
Furthermore, an experiment was conducted in which a
isomer 10 over the ortho-isomer that is nearly equivalent to that Pt /G4OH/mesoporous silica pellet was used to allow for facile
40
observed in homogeneous PtCl systems. This highlights the poten- removal of the reaction solution from the NP catalyst under the
2
tial of unique heterogeneous catalysts to offer similar selectivities reaction conditions and inert atmosphere. In this case, detailed in
relative to homogeneous species. Generality for the oxidative treat- Fig. 6c, a solution of starting material, PhICl and solvent are
2
ment was shown when Pd /G4OH/SBA-15 NPs were used for added to the catalyst pellet in a reactor (A) and begin to generate
40
the formation of 2 from 1 (Fig. 4). Just as was found for Pt, the Pd product. After 42% yield is achieved, the reaction solution in A is
NPs required further reduction (H (1 atm), 100 8C, 24 h) followed transferred to a new vessel with no catalyst (B). The oxidized catalyst
2
by in situ treatment with PhICl to generate electrophilic catalytic pellet remains in A. A fresh solution of starting material and solvent
2
activity. The yield obtained with the Pd NPs (48%) was comparable is then added into A. Now, the solution in A begins to convert to
to that obtained with PdCl homogeneous catalyst.
product while the solution in B does not react and remains at
2
Additionally, we found that when urea 11 was treated with either 42% yield. This further indicates the active catalyst is heterogeneous.
PtCl or PtCl homogeneous catalyst in toluene at 110 8C, both 13 If any homogeneous leached species were present, it would have also
2
4
(ref. 44) and a previously unreported product, the bis-indole 12, been transferred to B and an increase in yield would have been
were observed, although in low yields. However, addition of observed for the solution after it was removed from the heterogeneous
SBA-15 resulted in a suppression of the formation of 13 and a sub- catalyst pellet in A. Moreover, the new solution added to A begins to
sequent increase in the yield of bis-indole 12 (Fig. 5). Interestingly, react, showing that the active catalyst has, in fact, remained in A.
the use of either Pt/PVP/SBA-15 or Pt /G4OH/SBA-15 NPs oxida-
Finally, elemental analysis by inductively coupled plasma of acen-
40
tively modified with PhICl resulted in a much higher 43–48% yield trifuged solution (1 with Pt /G4OH/SBA-15 and PhICl in
2
40
2
after 48h for the bis-indole 12. Although a small amount of toluene) after reaction was unable to detect any significant amount
tetracycle 13 was still formed by the NP catalysts, the majority of (,1 ppm) of Pt above the instrument’s detection limits. By the
the consumed starting material went towards this alternative same analytical method, no loss of Pt was observed from the
catalytic reaction pathway. In this case, the NPs selectively catalyse Pt /G4OH/SBA-15 catalyst when it was isolated after the reaction.
40
a reaction cascade that generates several new carbon–carbon
All together, this collection of experiments strongly indicates that
bonds and combines aspects of two starting material molecules to no homogeneous catalytically active species had leached from the
afford 12. Furthermore, by utilizing supported NPs for this treated/oxidized Pt /G4OH/SBA-15 NPs during the reaction,
40
process, we have developed a recyclable catalyst system that can and that the catalytically active species is the heterogeneous NP.
access this reactive pathway in yields greater than those obtained
Conclusion
by simply mixing homogeneous catalyst with SBA-15.
We have contributed to the larger goal of bridging the gap between
Leaching tests. Although the remarkably consistent recyclability of homo- and heterogeneous catalysis with a strategy for developing
the PhICl -treated Pt /G4OH/SBA-15 NPs is a strong indication catalytically active NPs capable of performing reactions previously
2
40
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2010 Macmillan Publishers Limited. All rights reserved.
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.468
ARTICLES
a
O
most notably the oxidative modification, illustrate the ability to
obtain new reactivity from existing NP systems, and, in some
cases, divergent reaction pathways are accessible.
O
Catalyst
O
O
Ar (1 atm), toluene
In a broader sense, this concept represents a significant advance-
ment in the solution-phase applications of supported NPs, and
further application with other metals and treatment methods may
facilitate the development of heterogeneous catalysts with novel
activity or selectivity in an even larger array of chemical reactions.
Efforts are underway to better understand the nature of the oxi-
dation in order to further increase as well as maintain catalyst
activity to allow for the development of a continuous flow system.
Future studies can also move beyond traditional homogeneous
ligand control and use NP shape or dendrimer composition to
provide opportunities for selective solution-phase reactions.
100 °C, 15 h
Ph
Ph
1
4
6
Catalyst
Yield
26%
<2%
<2%
PtCl2 (4 mol%)
.9 nm Pt/PVP/SBA-15 (2.5 mol%), PhICl2 (7.5 mol%)
Pt40/G4OH/SBA-15 (4 mol%), PhICl2 (12 mol%)
2
b
Methods
Synthesis of dendrimer-templated NPs. Fourth-generation dendrimers (G4OH)
were purchased from Dendritech as 10.2% (mass) methanol solutions. A dendrimer
stock solution (250 mM) was prepared by adding water to the dendrimer methanol
solution. The dendrimer stock solution was mixed with 15–40 mole equivalents of an
aqueous solution of 0.01 M K PtCl in a 20 ml vial. The vial was purged with Ar for
2
0nm
20nm
2
4
c
30 min, tightly sealed with a septum and left for 66 h for complexation. Then a 20-fold
60
50
40
30
20
10
0
excess of freshly prepared 0.5 M NaBH (stored at 0 8C before use) was injected
4
dropwise into the vial with vigorous stirring. The reaction solution was stirred for an
additional 8 h after which the reaction solution (10 ml) was purified by dialysis against
i
ii
2
1
l of deionized water in cellulose dialysis sacks with a molecular weight cutoff of
2,000 (Sigma-Aldrich). Dialysis occurred over 24 h with the water changed four times.
A
B
A
ii
B
i
Synthesis of PVP-capped NPs. Chloroplatinic acid (H PtCl .6H O, 99.9% pure on
2 6 2
metals basis) and polyvinylpyrrolidone (PVP) with a molecular weight of 29,000
were purchased from Sigma-Aldrich.
Solution in A to B
Fresh solution
added to A
For the synthesis of 1.5 nm Pt particles, NaOH was dissolved in ethylene glycol
(12.5 ml, 0.5 M). This solution was added to an ethylene glycol solution (12.5 ml)
.
containing H PtCl 6H O (0.25 g, 0.48 mmol). During N purging, this combined
2
6
2
2
A
A
B
A
B
0
20 40 60
Time (h)
80 100
solution was heated to 160 8C and held for 3 h. The resulting NPs were precipitated
with 2 M HCl and dispersed in an ethanol/PVP mixture.
Solvent +
reactant
= Pellet catalyst
Product
=
=
For the synthesis of 2.9 nm Pt particles, aqueous H PtCl 6H O (20 ml of
.
2
6
2
6.0 mM) was added into 180 ml of methanol. PVP (133 mg) was then dissolved in
this mixture and refluxed for 3 h.
Figure 6 | Results from catalyst heterogeneity tests. a, A three-phase test
to determine the presence of a homogeneous catalyst by leaching or release
and capture. The reaction rate between the heterogeneous catalyst and
polymer-bound substrate should be negligible. Yields were determined by
NMR versus internal standard. b, Transmission electron microscope (TEM)
images of Pt40/G4OH/SBA-15 before reaction (left), and after treatment with
For the synthesis of 5.0 nm Pt particles, freshly prepared 2.9 nm Pt particles were
mixed in a 90% methanol/10% water solution (100 ml). Methanol (90 ml) and a
solution of H PtCl .6H O (36.9 mg) in water (10 ml) were added and the combined
2
6
2
mixture refluxed for 3 h.
Synthesis of mesoporous SBA-15 silica. Pluronic P123 (6.0 g, BASF) was dissolved
in deionized water (45 g) and 2 M HCl (180 g) while stirring at 35 8C for 1 h.
Tetraethylorthosilicate (12.8 g, Sigma Aldrich, 98%) was added to the solution and
allowed to stir for 20 h. The mixture was then aged at 100 8C for 24 h. The mixture
was filtered to give a white powder, which was further purified by washing with
ethanol and deionized water. This purified product was dried in air at 100 8C and
then calcined at 550 8C for 12 h. The white powder was stored in a dessicator.
PhICl and reaction with 1 (right). No leaching or aggregation is observed.
2
Further TEM data including average diameters and standard deviations can
be found in the Supplementary Information. c, A filtrate transfer test.
A reaction vessel (A) was charged with a substrate. Into this vessel was
added a pellet of Pt40/G4OH/mesoporous silica catalyst and PhICl oxidant
2
(
blue section). The mixture was stirred and the yield monitored (i). When the
Preparation of Pt/SBA-15 catalysts. Pt NPs were loaded onto the mesoporous
SBA-15 silica before the catalytic studies. SBA-15 was added to a colloidal solution of
the Pt NPs and the resulting slurry was sonicated for 3 h at room temperature. The
NP supported SBA-15 was separated from the solution by centrifuge at 4,200 rpm
for 6 min. After centrifugation, the solution was clear. The solution was then
decanted and the catalyst was dried under ambient conditions and then at 100 8C.
PVP-encapsulated NPs were loaded to 1.0 wt% Pt. SBA-15 was used for 1.5 and
2.9 nm NPs. MCF-17 was used for 5.0 nm NPs. The larger size mesoporous silica
was required for successful loading of the larger NPs.
reaction in vessel A reached 42% yield, the solution was removed and placed
into a new vessel (B). A fresh solution of starting material was added to
vessel A where the catalyst pellet remained (yellow section). Both vessel A
and B were then stirred and monitored (ii). The yield in B remained constant,
whereas product was generated in A (red section). The yield was determined
by gas chromotography versus internal standard. All transfers and reactions
were conducted under Ar at 100 8C. The catalyst pellet was pre-reduced
Dendrimer-encapsulated NPs were loaded to 0.7 wt% Pt. SBA-15 was used as
the support.
under H atmosphere at 100 8C for 36 h before reaction.
2
in the exclusive purview of homogeneous chemistry. To this end, we Representative procedure for catalytic reactions. To a dry 10 ml glass reaction
have successfully identified novel electrophilic Pt NPs that catalyse a tube with stirbar and Teflon screwvalve cap under Ar was added
2
-(phenylethynyl)phenol (1) (20 mg, 0.1025 mmol), Pt(2.9 nm)/PVP/SBA-15
range of p-bond activation reactions with equivalent or superior
yields and selectivities relative to reported homogeneous variants.
Instrumental to this success were NP structural analyses and
(
50 mg, 0.0026 mmol, 2.5 mol%), PhICl (2.1 mg, 0.0077 mmol, 7.5 mol%),
2
mesitylene (15 ml, internal standard, Aldrich) and toluene-d (2 ml, Cambridge
Isotopes). The reaction mixture was degassed (freeze/pump method) three times
8
mechanistic insights that uncovered the importance of treatment and placed under 1 atm of Ar. The reaction tube was sealed and the mixture heated
with the hypervalent iodine oxidizing agent PhICl . Moreover, reac- with stirring to 100 8C for 15 hours. The mixture was then cooled to room
2
temperature and the solid catalyst filtered by glass microfibre filter. The filtrate was
transferred to a NMR tube for analysis.
tion kinetic analysis confirmed the capability of tuning nanoparticle
size and capping agent to improve catalytic ability. Multiple
experimental results indicate the heterogeneity of the dendrimer-
materials, the catalyst was added to the dry 10 ml glass reaction tube and placed
encapsulated NP catalyst supported on SBA-15. These discoveries, under 1 atm of H . The catalyst was then heated to 100 8C for 24 hours. After
For the Pt40/G4OH/SBA-15 catalyst, before addition of all other reaction
2
4
0
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.468
ARTICLES
cooling to room temperature and replacing the H atmosphere with Ar, the reaction
setup was continued as discussed above.
26. Crooks, R. M., Mingqi, Z., Sun, L., Checkhik, V. & Yeung, L. K. Dendrimer-
encapsulated metal nanoparticles: synthesis, characterization, and applications
to catalysis. Acc. Chem. Res. 34, 181–190 (2001).
2
2
-Phenylbenzofuran (2) can be isolated in the following manner. The toluene
solution containing the product from the catalytic reaction was concentrated and
purified by flash chromatography (5% ethyl acetate/hexanes, Fisher ACS grade).
27. Rioux, R. M., Song, H., Hoefelmeyer, J. D., Yang, P. & Somorjai, G. A.
High-surface-area catalyst design: synthesis, characterization, and reaction
studies of platinum nanoparticles in mesoporous SBA-15 silica. J. Phys. Chem. B
109, 2192–2202 (2005).
Received 7 July 2009; accepted 26 October 2009;
published online 29 November 2009
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8. Huang, W. et al. Dendrimer templated synthesis of one nanometer Rh and Pt
particles supported on mesoporous silica: catalytic activity for ethylene and
pyrrole hydrogenation. Nano Lett. 8, 2027–2034 (2008).
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We acknowledge support from the Director, Office of Science, Office of Basic Energy
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Author contributions
(ed. Astruc, D.) Ch. 10, 303–348 (Wiley-VCH, 2008).
C.A.W, W.H., C.-K.T. and J.N.K. performed the experiments and synthesized materials,
substrates and catalysts. F.D.T. and G.A.S. supervised the research. All authors contributed
to the conception of the experiments, discussed the results and commented on
the manuscript.
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catalysis by transition-metal nanoparticles. Acc. Chem. Res. 36, 638–643 (2003).
3. Durand, J., Teuma, E. & Gomez, M. An overview of palladium nanocatalysts:
surface and molecular reactivity. Eur. J. Inorg. Chem. 23, 3577–3586 (2008).
4. Astruc, D. Palladium nanoparticles as efficient green homogeneous and
heterogeneous carbon-carbon coupling precatalysts: a unifying view. Inorg.
Chem. 46, 1884–1894 (2007).
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Additional information
The authors declare no competing financial interests. Supplementary information and
chemical compound information accompany this paper at www.nature.com/
naturechemistry. Reprints and permission information is available online at http://npg.nature.
com/reprintsandpermissions/. Correspondence and requests for materials should be
addressed to G.A.S. and F.D.T.
2
886–2890 (2006).
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2010 Macmillan Publishers Limited. All rights reserved.
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