Encapsulated Gold Nanoparticles as a Reactive Quasi-Homogeneous Catalyst in Base-Free Aerobic Homocoupling Reactions

Article abstract of DOI:10.1002/cctc.201901710

Multiple gold nanoparticles (AuNPs) are physically encapsulated within a poly(N-isopropylacrylamide) particle at room temperature by an in situ reduction method using trisodium citrate (Cit) and sodium oxalate (Oxa) to serve as quasi-homogeneous catalysts for carbon?carbon bond-forming reactions under base-free aerobic conditions. The use of Oxa results in a more rapid formation and higher loading of AuNPs within the polymer particles compared to those prepared with conventional Cit. Upon dispersing both composite particles in EtOH-rich aqueous solvents, the polymer matrices maintain a fully swollen structure while still providing great stability to the AuNPs whose surfaces are analyzed to be nearly free from the reducing/stabilizing agents. As a catalyst in homocoupling reactions, the composite particles prepared with Oxa exhibit higher activity, selectivity, and recyclability with a much lower activation energy than those prepared with Cit. The preparation of the composite particles and their catalytic properties are optimized under environmentally-friendly reaction conditions, and the relationship between structural features and catalytic functions are examined to potentially develop robust, versatile, and green catalytic systems.

Full text of DOI:10.1002/cctc.201901710

Accepted Article
Title: Encapsulated gold nanoparticles as a reactive quasi-
homogeneous catalyst in base-free aerobic homocoupling
reactions
Authors: Wongi Jang, Hongsik Byun, and Jun-Hyun Kim
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To be cited as: ChemCatChem 10.1002/cctc.201901710
Link to VoR: http://dx.doi.org/10.1002/cctc.201901710
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Encapsulated gold nanoparticles as a reactive quasi-
homogeneous catalyst in base-free aerobic homocoupling
reactions
Wongi Jang,[a][b] Hongsik Byun,*[b] and Jun-Hyun Kim*[a]
Abstract
:
Multiple gold nanoparticles (AuNPs) are physically
some drawbacks associated with their reactivity, selectivity,
and recyclability.
encapsulated within a poly(N-isopropylacrylamide) particle at room
temperature by an in situ reduction method using trisodium citrate (Cit) Here we prepared
a quasi-homogenous catalyst to
and sodium oxalate (Oxa) to be employed as quasi-homogeneous
catalysts for carbon-carbon bond-forming reactions under base-free
aerobic conditions. The use of Oxa results in a more rapid formation
and higher loading of AuNPs within the polymer particles compared
to those prepared with conventional Cit. Upon dispersing both
composite particles in EtOH-rich aqueous solvents, the polymer
matrices maintain a fully swollen structure while still providing great
stability to the AuNPs whose surfaces are analyzed to be nearly free
from the reducing/stabilizing agents. As a catalyst in homocoupling
reactions, the composite particles prepared with Oxa exhibit higher
activity, selectivity, and recyclability with a much lower activation
energy than those prepared with Cit. The preparation of the
composite particles and their catalytic properties are optimized under
environmentally-friendly reaction conditions, and the relationship
between structural features and catalytic functions are examined to
potentially develop robust, versatile, and green catalytic systems.
overcome the limitations for both catalytic systems by
integrating gold nanoparticles (AuNPs) into a biocompatible
polymer particle. The formation of the composite particles
was precisely regulated to maximize the active sites of the
physically encapsulated AuNPs, which exhibited an
unexpected high catalytic activity in homocoupling
reactions.[4] In addition, these composite particles showed
an excellent compatibility with various environmentally-
friendly solvents (e.g., water and short-chain alcohols) by
imparting a long-term stability, enabling their utilization as
recyclable catalysts. Although most catalytic homocoupling
reactions are generally promoted under basic conditions,[5]
controlling the ratio of the water content in alcohol solvents
completely eliminates the need for inorganic bases, where
the composite particles, acting as a catalyst, only required
reactants to form products. Furthermore, these base-free
aerobic reaction conditions did not damage the structural
features of the composite particles, which allowed for the
examination of their recyclability and versatility in the
homocoupling of other arylboronic acid derivatives to
understand their atypical catalytic performance. Thus,
establishing effective modification strategies and reaction
conditions to enhance their catalytically active sites, long-term
stability, and applicability in green reaction environments is
central to the success of AuNP-based catalytic systems.
The homocoupling of arylboronic acids has been extensively
studied because the resulting biaryl products serve as
important building blocks to produce specialty chemicals for
pharmaceuticals,
polymers,
agrochemicals,
and
biomaterials.[1] These carbon-carbon (C-C) bond-forming
reactions are typically carried out by homogeneous and
heterogeneous reaction systems using transition-metal
catalysts under basic conditions. Although palladium has
performed as the leading catalyst in the coupling reaction,
relatively less toxic gold-based materials have shown the
capability to serve as effective catalysts in green solvent
environments.[2] As such, the modification of gold–derived
catalysts has been widely explored to design homogeneous
catalysts with ligands/complexes and heterogeneous
catalysts
with
supported/colloidal
Both of these catalysts have shown
nanoparticles,
2f, 3]
respectively.[1e,
Composite (Cit) > 45 °C
Composite (Oxa) > 45 °C
Composite (Cit) < 25 °C
Composite (Oxa) < 25 °C
PNIPAM
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Composite (Cit) < 25 °C
Composite (Oxa) < 25 °C
Composite (Cit) > 45 °C
Composite (Oxa) > 45 °C
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
[a]
[b]
W. Jang, Prof. J.-H. Kim
Department of Chemistry
Illinois State University
4160 Campus Box, Normal, IL 61790, U.S.A.
E-mail: jkim5@ilstu.edu
W. Jang, Prof. H. Byun
Department of Chemical Engineering
Keimyung University
reversible
in EtOH
400
in water
400
500
600
700
800
500
600
700
800
Wavelength (nm)
Wavelength (nm)
2800 Dalgubeol Daero, Daegu 42601, South Korea
E-mail: hsbyun@kmu.ac.kr
Figure 1. Encapsulated AuNPs with PNIPAM particles prepared with Cit and
Oxa under fluorescent light irradiation as well as their corresponding
absorption patterns in water and EtOH upon heating and cooling.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201901710
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(LCST),[6, 9] which were characterized by reversible SPR bands.
The detectable redshift of the SPR bands above the LCST was
presumably induced by the interparticle coupling of the
encapsulated AuNPs. The subsequent blueshift of the SPR
bands upon cooling below the LCST implied that the composite
particles returned to their swollen structure in water. The overall
SPR bands and their reversibility patterns also indicated the
successful formation of the comparable composite particles using
the two different reducing agents with the light-induced synthetic
method. To quantify the amount of the encapsulated AuNPs
within the polymer particles, thermogravimetric analysis (TGA)
and atomic absorption (AA) measurements were performed
(Figure 2 and Figure S4). The weight loss for the composite
particles prepared using Oxa was slightly less (~2 wt%) than
those with Cit whereas the bare PNIPAM particles exhibited a
notably greater weight loss (5-7 wt%). Similarly, the AA
spectroscopy also confirmed that the composite particles
prepared with Oxa embraced a slightly more amount of AuNPs
within the polymer particles (0.81 mg/mL) than those with Cit (0.77
mg/mL).
In this work, gold ions were effectively reduced by trisodium citrate
(Cit) and sodium oxalate (Oxa) under a fluorescent light source in
the
presence
of
functional-group-free
poly(N-
isopropylacrylamide), PNIPAM, particles (Figure 1 and S1). This
in situ process reliably resulted in the encapsulation of multiple
gold nanoparticles (AuNPs) within the biocompatible polymer
networks to form composite particles where the host polymer
particles encapsulated the guest AuNPs without any strong
6]
electrostatic and/or covalent interactions.[4a,
Unlike the
formation of bare AuNPs with different sizes in the absence of the
PNIPAM particles, the ability to control the size and shape of the
encapsulated AuNPs is somewhat limited (~20 nm in diameter)
in the presence of the PNIPAM particles using these reducing
agents (Figure 2S). The formation processes of these AuNPs
under various reaction conditions were thoroughly examined by
the distinctive peak position and width of the surface plasmon
resonance (SPR).[7] Under the optimized reaction conditions, the
use of Oxa as the reducing agent resulted in the more rapid and
controlled formation of AuNPs (e.g., narrower size distribution and
better loading efficiency) within the polymer particles compared to
those prepared with conventional Cit, as supported by the TEM
images (Figure 1 and Figure S3).
Subsequent purification and redispersion of the composite
particles in EtOH effectively removed the unreacted species and
reduced the amount of stabilizing agents from the surface of the
loaded AuNPs,[4d, 8] which was verified by the significant decrease
in the zeta potential (from -45 mV to -22 mV) regardless of the
reducing agents. Minimizing the role of the surface stabilizing
agents around the AuNPs could be advantageous for their use as
a catalyst because these stabilizers often serve as a physical
barrier to decrease the reactivity.[4b-d] In addition, the use of pure
EtOH and EtOH-rich aqueous solvents readily eliminated the
cononsolvency and thermosensitivity of the composite particles to
maintain the fully swollen structures, which was confirmed by the
negligible changes of the SPR bands in the temperature range
o
o
from 20 C to 65 C. However, the composite particles in water
promptly underwent temperature-induced swelling and deswelling
structural changes at the lower critical solution temperature
Figure 3. Base-free homocoupling of phenylboronic acid using the composite
particles in various EtOH-rich aqueous solvents at 80 oC (all reaction
produced less than 3% of phenol byproduct).
AA analysis of composite particle (mg of Au)
Cit (0.77 mg/mL) and Oxa (0.81 mg/mL)
Our previous work demonstrated the capability of improving the
aerobic homocoupling of arylboronic acid derivatives in alcohol-
rich aqueous solvents using inorganic bases and encapsulated
AuNPs within PNIPAM particles under ambient conditions.[4a] As
the use of Oxa allowed for the rapid preparation of the composite
particles with an improved loading efficiency and greater
uniformity of AuNPs, both the composite particles (~2.3 mol% of
the Au atoms based on the total amount of AuNPs) and ~0.13
mol% of the Au atoms based on the outer surface of the AuNPs
were employed in the homocoupling of phenylboronic acid to
compare their catalytic properties at 22 C and 50 C as a function
of time (Figure S5; calculations are shown in the supporting
information section). In contrast to the bare AuNPs prepared with
Cit and Oxa, which exhibited poor catalytic activities due to their
instability (the digital photos in Figure S5a), both composite
particles in pure EtOH easily yielded over 90% regardless of the
reaction temperature. Interestingly, the formation of the biphenyl
product was notably faster with the composite particles prepared
10
100.0
0
80.0
-10
weight loss%
60.0
76.8 wt%
79.9 wt%
88.8 wt%
-20
-30
-40
-50
40.0
20.0
0.0
Composite (Oxa)
Composite (Cit)
PNIPAM particles
80 180 280 380 480 580 680 780
Temperature (°C)
Figure 2. Thermogravimetric and differential scanning calorimetric analysis
(TGA/DSC) of bare PNIPAM and composite particles prepared with Cit and
Oxa, as well as the amount of loaded gold content examined by atomic
absorption spectrometer.
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10.1002/cctc.201901710
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with Oxa than those prepared with Cit (Figure S5b and S5c). This
fast conversion rate led us to revisit the homocoupling reactions
in the absence of K2CO3 base because of the limited solubility of
100
80
60
40
20
0
(a)
8h
4h
the inorganic base in pure EtOH.
The initial base-free
homocoupling of phenylboronic acid using both types of
composite particles in EtOH only showed a trace level of biphenyl
formation, implying the important role of the base in the reaction.
As the alcohol-rich aqueous solvents readily eliminated the
cononsolvency and temperature-responsiveness of PNIPAM-
based particles,[10] introducing a small quantity of water into EtOH
only disturbed the pH of the solvent (i.e., possibly creating a
slightly basic environment) where the composite particles
exhibited atypical catalytic activity in the homocoupling reaction.
Figure 3 shows the unexpectedly high catalytic property of both
composite particles upon the addition of varying amounts of water
at 80 C. The use of 20-30 vol% water greatly improved the yields
of biphenyl, particularly with the composite particles formed with
Oxa (presumably due to the slightly greater amount of loaded
AuNPs and their uniform size distribution). The presence of a
small amount of water evidently played an important role in
increasing the catalytic function of the composite particles. It is
worth mentioning that the desired biphenyl product has a good
solubility in the EtOH-rich solvents and so can avoid an additional
extraction step, which enables the composite particles to serve as
a quasi-homogeneous catalyst after a brief recovery step by
centrifugation.
An additional interesting finding was the trace amount of phenol
formation in the base-free reaction conditions. To examine this
atypical catalytic selectivity, the same reaction was performed
with and without the K2CO3 base at 50 C and 80 C. The use of
the base greatly accelerated the coupling reaction to yield 75% of
biphenyl in 1 h, but rapidly formed the phenol by-product up to
25% as well, obviously limiting the selectivity (Figure S6). Both
composite particles in the base-free reaction conditions resulted
in the modest yields of biphenyl with the undetectable formation
of phenol at 50 C in 8 h. Raising the reaction temperature to 80
C promptly resulted in a biphenyl yield of over 80% with the
negligible formation of phenol (≤ 3%) in 4 h, evidently suggesting
a high selectivity in the absence of a base. As the temperature-
dependent reactivity is often explained by the collision theory, the
activation energies of the reaction were calculated from the
Composite (Oxa)
Composite (Cit)
0
1
2
3
4
5
6
7
Recycled batch
Initial composite (Cit)
Initial composite (Oxa)
After 6th batch (Cit)
After 6th batch (Oxa)
Collected top soln (Cit)
Collected top soln (Oxa)
(b)
2.0
1.5
1.0
0.5
0.0
300 400 500 600 700 800 900 1000
Wavelength (nm)
Figure 4. Recyclability of the composite particles under base-free aerobic
homocoupling of phenylboronic acid at 80 oC for
4 h (a) and their
representative absorption patterns before and after the 6th batch (b).
Cit and 3.39 ± 0.11 ppm for composite by Oxa in each cycle) was
also observed by atomic emission analysis (Figure S8). By simply
extending the reaction time, recycling the decreased amount of
the composite particles (i.e., the amount recovered after the 6th
cycle) significantly improved the conversion yield of biphenyl,
particularly for the composite particles prepared with Oxa,
strongly supporting the idea that the reduced catalytic activity was
somewhat caused by the loss of the composite particles rather
than their complete deactivation. As such, the greatly improved
stability and easily accessible surfaces of the encapsulated
AuNPs synergistically influenced the overall catalytic
performance of the composite particles.
Given their atypical catalytic activity, selectivity, and recyclability
in the base-free reaction conditions, we took one further step to
screen the homocoupling reactions of the arylboronic acid
derivatives (Table 1). The coupling of 4-methylphenylboronic acid
easily reached the over 90% yield in
methylphenylboronic acid generally exhibited
4
h, whereas 2-
low yield,
Arrhenius plot (Figure S7).
Apparently, the base-free
a
homocoupling reaction with the composite (Oxa) (~31.8 kJ/mol)
is easier than that with the composite (Cit) (~41.4 kJ/mol), which
were within the range of reported values.[2c, 5b, 11]
presumably due to the steric hindrance. Slightly hydrophilic 4-
cyano and 4-methoxyphenylboronic acids resulted in moderate
yields, possibly due to the dissimilar solubility in the EtOH-rich
aqueous solvent conditions. The coupling of hydrophilic 4-
hydroxyphenylboronic acid led to a relatively low yield, which was
possibly caused by the deprotonation of the slightly acidic alcohol
in the phenol group. This process might compete against the
The recyclability of both composite particles was then tested
under the base-free aerobic homocoupling reactions (Figure 4).
The catalytic activity of the composite particles gradually
diminished from third batch, possibly due to the loss of catalytic
activity and/or amount of the composite particles. The latter
speculation was withdrawn by our extensive washing of the
composite particles (e.g., minimum of three times for precipitation
and resuspension) in this recycling test in order to completely
remove remaining reactants and products although the recovery
of the particles was simply achieved by centrifugation. The
detectable reduction of SPR band intensities without changing
their shapes after being recycled six times evidently indicated the
loss of the composite particles. In addition, a detectable leaching
of the encapsulated AuNPs (1.67 ± 0.11 ppm for composite by
‾
formation of 4-hydroxyphenyl-B(OH)3 species in the base-free
coupling reactions.[3d, 11] Extending the reaction time (e.g., 8 h)
appreciably increased the yields of all products to moderately
overcome the steric and miscibility barriers under the same
reaction conditions.
Table 1. Base-free homocoupling of phenylboronic acid derivatives in a
mixed solvent of water and EtOH (30:70 v/v%) at 80 oC for 4 h using the
composite particles prepared with Oxa.
Reactant
Product
Yield (%)
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remove any free AuNPs and reducing agents. The purified composite
particles were then resuspended in various volume ratios of water and
EtOH (0 - 40 v/v% in ethanol) prior to use in the catalytic reactions.
The bare AuNPs were also prepared under the same process in the
absence of PNIPAM particles. After purification, the bare AuNPs were
resuspended in water for the UV-vis measurements and in EtOH for
the homocoupling reactions (Figure S3).
30a) (38)
≥ 90[a]
65[b] (75)
50[c] (57)
30[d] (56)
Homocoupling of arylboronic acid derivatives
[a] GC; [b] GC extraction with acetonitrile; [c] GC extraction with toluene;
[d] HPLC, all parentheses indicate the yields of the same reactions for 8 h
*Trace level of byproduct formation (≤ 3%)
The homocoupling of phenylboronic acid was carried out with the
composite particles and bare AuNPs in pure EtOH and a mixture of
water-EtOH solvents (Figure S5a). For example, an aliquot (2.0 mL)
of the composite particles in EtOH was mixed with phenylboronic acid
(21 mg, 0.17 mmol) and K2CO3 (67.0 mg, 0.48 mmol) in a capped
glass vial. After stirring at ambient conditions for 24 h, the resulting
solutions were centrifuged at 10,000 rpm for 5 min (Sorvall Legend
Micro17 centrifuge, Thermo Scientific) to separate the composite
particles, and the top liquid layer (1.0 mL) was directly subjected to
GC analysis. Similarly, the homocoupling reaction was performed
under various conditions, including the temperature, solvents, and
time, in the presence and absence of the K2CO3 base. For the
recyclability tests, the composite particles were suspended in a pre-
prepared EtOH-rich aqueous solvent (30 v/v% of water and EtOH)
solution for the homocoupling reaction at 80 °C for 4 h. After the
reaction, the composite particles were quantitatively transferred to an
Eppendorf tube and purified three times by repeating the
centrifugation (10,000 rpm for 30 min) and redispersion of the
composite particles in the solvent to completely remove the organic
reactants and products. The recovered composite particles were then
redispersed in 2 mL of the solvent prior to the next cycle. The base-
free homocoupling of the arylboronic acid derivatives containing the
methyl, cyano, methoxy, and hydroxyl groups was performed under
the same reaction conditions. In the case of the products exhibiting a
poor solubility in the mother liquor, the reaction mixture was
completely dried by a rotary evaporator and redispersed in the proper
solvent (e.g., 4,4’-dicyanobiphenyl in acetonitrile and 4,4’-
dimethoxybiphenyl in toluene). After a brief sonication, the mixture
was centrifuged to collect the top solution for the GC and HPLC
analyses.
In summary, the light-induced synthetic method allowed for the
rapid and controlled formation of AuNPs within the PNIPAM
particles. Simply controlling the solvent conditions not only
decreased the role of the stabilizing agents around the
encapsulated AuNPs, but also eliminated the cononsolvency
effect and thermo-responsiveness of the composite particles to
render a greater stability. Given these features, the resulting
composite particles exhibited atypical catalytic reactivity,
selectivity, versatility, and recyclability in the base-free aerobic
homocoupling reactions in green solvent environments, which
can support the development of industrially practical quasi-
homogeneous catalysts.
Experimental Section
Encapsulation of multiple gold nanoparticles within a polymer
particle
Cross-linked poly(N-isopropylacrylamide), PNIPAM, particles were
synthesized via the radical polymerization of NIPAM (1.00 g) and BIS
(0.10 g) using the APS initiator (0.12 g) in a 500 mL round bottom
flask containing 200 mL water. The solution was purged with argon
gas for 1 h and then heated to 70 °C in an oil bath for 4 h. The
resulting milky white polymer solution was cooled to room
temperature and filtered through a filter paper (Whatman #40) prior to
use. An aliquot of the PNIPAM particle solution (10 mL) was then
mixed with 1.0 ml of the HAuCl4.3H2O solution (1.0 wt% in water) for
at least 45 min. Varying amounts of trisodium citrate solution (Cit, 1.0
wt% in water) and sodium oxalate solution (Oxa, 1.0 wt% in water)
were then added into the mixture, which was placed in a water-
jacketed beaker and exposed to a fluorescent light (~80 mW/cm2)
equipped with a UV filter. The light intensity was estimated by a
handheld optical power meter (1916-C Power Meter, Newport Inc.) in
the range of 520 to 550 nm. This in situ reduction method allowed for
the formation of multiple gold nanoparticles (AuNPs) within the
polymer particle template under light irradiation while keeping the
temperature of the reaction vial below 25 oC. The formation of AuNPs
was easily confirmed by the solution color change from yellow to red
in 1 h with Cit and 30 min with Oxa. The composite particles prepared
with Cit were additionally stirred overnight after 3 h of light irradiation,
while the composite particles prepared with Oxa were stirred for 2 h
after 2 h of light irradiation. The controlled formation of the composite
particles with their kinetics was monitored by the absorption
measurements to optimize the reaction conditions (Figure S1 and S2).
The final solution was centrifuged at 5000 rpm for 25 min twice
(Sorvall Legend X1 Centrifuge Series, Thermo Scientific) in EtOH to
Acknowledgements
This work is supported by the National Research Foundation of
Korea Grant funded by the Korean Government (NRF-
2018R1A2B6008854). This work is also supported by Illinois
State University’s Department of Chemistry.
Conflict of interest
The authors declare no conflict of interest.
Keywords: poly(N-isopropylacrylamide) • encapsulated gold
nanoparticles • base-free homocoupling • sodium citrate •
sodium oxalate
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10.1002/cctc.201901710
ChemCatChem
COMMUNICATION
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COMMUNICATION
Encapsulated gold nanoparticles
within a polymer particle exhibit a
high reactivity, selectivity, and
recyclability in base-free aerobic
homocoupling reactions to serve as
a green catalyst.
Wongi Jang,[a][b] Hongsik Byun,*[b]
and Jun-Hyun Kim*[a]
Page No. – Page No.
Encapsulated gold nanoparticles as
a reactive quasi-homogeneous
catalyst in base-free aerobic
homocoupling reactions
This article is protected by copyright. All rights reserved.