Recyclable palladium-graphene nanocomposite catalysts containing ionic polymers: efficient Suzuki coupling reactions

Article abstract of DOI:10.1039/c6ra26998b

Palladium nanoparticles on ionic polymer-doped graphene (Pd-IPG) nanocomposite catalysts have been investigated for efficient Suzuki coupling reactions. This combination effected highly accelerated Suzuki coupling reactions due to several advantageous features associated with the flanking ionic polymer part of the catalyst system. These include a high level of Pd incorporation, excellent dispersion stability, and increased accessibility and diffusion of the substrates onto the surface of Pd NPs. The enhanced availability of the Pd catalyst to the reacting substrates is believed to allow for ca. 16-fold higher catalytic activity than that of Pd-graphene without the ionic polymer. Moreover, high recycling capability of the catalyst (10 times) in combination with excellent product yields (>96%) and no significant leaching of the catalyst upon hot-filtration test suggest that the Pd-IPG nanocomposite catalysts have high reusability with significant retention (>95%) of the Pd species.

Full text of DOI:10.1039/c6ra26998b

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Recyclable palladium–graphene nanocomposite
catalysts containing ionic polymers: efficient
Suzuki coupling reactions†
Cite this: RSC Adv., 2017, 7, 11684
c
b
Tae Hui Kwon,‡a Kie Yong Cho,‡bc Kyung-Youl Baek, Ho Gyu Yoon
*
a
*
and B. Moon Kim
Palladium nanoparticles on ionic polymer-doped graphene (Pd–IPG) nanocomposite catalysts have been
investigated for efficient Suzuki coupling reactions. This combination effected highly accelerated Suzuki
coupling reactions due to several advantageous features associated with the flanking ionic polymer part
of the catalyst system. These include a high level of Pd incorporation, excellent dispersion stability, and
increased accessibility and diffusion of the substrates onto the surface of Pd NPs. The enhanced
availability of the Pd catalyst to the reacting substrates is believed to allow for ca. 16-fold higher catalytic
activity than that of Pd–graphene without the ionic polymer. Moreover, high recycling capability of the
catalyst (10 times) in combination with excellent product yields (>96%) and no significant leaching of the
catalyst upon hot-filtration test suggest that the Pd–IPG nanocomposite catalysts have high reusability
with significant retention (>95%) of the Pd species.
Received 18th November 2016
Accepted 10th February 2017
DOI: 10.1039/c6ra26998b
rsc.li/rsc-advances
the catalyst because of maximized active surface area and
enhanced anchoring ability.8–11
1. Introduction
For desirable catalytic performance of heterogeneous Pd
catalysts, various carbon supports of Pd NPs have been devel-
oped, such as functionalized carbon nanotubes (CNTs), gra-
phenes and their combinations.12–20 These approaches have
been somewhat successful in solving the issues associated with
the homogenous catalysis system, however, the limited func-
tional groups on the carbon supports oen caused detrimental
aggregation through long range p–p stacking and weak inter-
facial interaction with Pd NPs, leading to low quality incorpo-
ration, easy dissolution and loss of Pd NPs from the support,
and ambiguous dispersion stability.21–23
Recently efforts to overcome these limitations have emerged
through employment of non-covalently functionalized carbon
supports equipped with polymer compatibilizers.24–27 These
systems allowed for a great nanocomposite catalyst with
uniform and well-distributed Pd NPs on the carbon supports as
well as noticeably enhanced catalytic performance due to better
dispersion property, strong interfacial linking, and adequate
interaction with substrates.10,27–31 Specically, in our recent
report, the ionic polymer-doped graphene support was explored
to develop effective catalysts for hydrogenation reaction
through combination with ne-sized Pd NPs (1–3 nm), resulting
in the ultra-accelerated catalytic reduction of methylene blue.
Effective interaction and diffusion of reacting species onto the
surface of Pd NPs along with its desirable dispersion property in
the aqueous medium were believed to be the main reason for
the successful catalytic activity.29 However, the ne-sized metal
NPs can be generally less stable under harsh reaction
Palladium-catalyzed Suzuki-coupling transformations consti-
tute a powerful methodology for the formation of carbon–
carbon (C–C) bonds. It features several benecial points such as
high convergence in synthetic strategy, tolerance to a broad
range of functional groups, insensitivity towards water, and
generally high regio- and stereo-selectivities.1–3 To date, the
Suzuki-coupling reactions have been conventionally performed
primarily through the use of a homogeneous Pd catalysts.4,5
However, Pd-catalyzed homogeneous reactions have suffered
from limitations such as the requirement of toxic and air-
sensitive phosphine ligands, safety issues of products due to
metal contamination and difficulties in catalyst recovery and
recyclability.6,7 Tireless efforts toward alleviating these short-
comings have continued and development of heterogeneous
catalytic systems employing various supports to Pd nano-
particles (NPs) came as one remedy favorably leading to effec-
tive retaining of catalytic activity and promoting durability of
aDepartment of Chemistry, Seoul National University, Seoul 08826, Republic of Korea.
E-mail: kimbm@snu.ac.kr
bDepartment of Materials Science and Engineering, Korea University, Seoul 02841,
Republic of Korea. E-mail: hgyoon@korea.ac.kr
cMaterials Architecturing Research Center, Korea Institute of Science and Technology,
Seoul 02792, Republic of Korea
† Electronic supplementary information (ESI) available: Detailed experimental
procedures and calculations, Tables S1 and S2, Schemes S1 and S2, Fig. S1–S5.
See DOI: 10.1039/c6ra26998b
‡ These authors contributed equally to this work.
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conditions, such as high temperature and the presence of reaction solution with ethyl acetate, the solution was ltered to
a strong base.32
separate the Pd–IPG. The acquired Pd–IPG was redispersed in
Herein, we demonstrate that the combination of multi- ethanol/water (1 : 1 v/v, 4.0 mL) for the next reaction and the
scaled Pd NPs with ionic polymer-doped reduced graphene organic part was used for GC analysis to calculate the reaction
oxide (rGO) supports (Pd–IPG) culminated in markedly yield aer extraction with ethyl acetate. The foregoing proce-
enhanced catalytic activity (TOF: 990 hꢀ1) and yield (>99%) in dure was repeated up to ten times to investigate the recyclability
the Suzuki coupling reaction between iodobenzene and phe- of Pd–IPG.
nylboronic acid in comparison to that of the catalyst without the
ionic polymer (Pd–rGO). The Pd–IPG nanocomposite catalysts
2.4. Hot-ltration test of the Pd–IPG nanocomposite
showed good recyclability during 10-time repeated cycles.
catalysts
For hot-ltration test, a Suzuki coupling reaction between
phenylboronic acid (0.24 mmol) and iodobenzene (0.20 mmol)
was conducted under optimized conditions. The reaction was
maintained for 30 min and then a half volume of the reaction
solution was taken and ltered while hot. Thereaer, the reac-
tion yields of the hot ltrate and an unltered half of the
solution were monitored through GC analysis aer sampling
small amounts of the reaction solutions at the predetermined
time.
2. Experimental section
2.1. Synthesis of Pd–IPG nanocomposite catalysts
Pyrene functionalized poly(dimethylaminoethyl methacrylate)-
b-poly[(ethylene glycol) methyl ether methacrylate] (Py-
PDMAEMA-b-PPEGMEMA), quaternary ammonium cation con-
taining ionic polymer-doped reduced graphene oxide (0.4 mg,
IPG), prepared following our previous report, was dispersed in
15 mL DMF by sonication for 10 min.31 Benzoic acid (7.3 mg,
Sigma-Aldrich, >99.5%) was sequentially added to the IPG
solution, and the mixture was gently sonicated for 5 min.
Thereaer, this solution was placed on an ice bath and agitated
for 5 min. Aer the foregoing procedure, aq. Na2PdCl4 solution
(2 mL, 20 mM in water, Sigma-Aldrich, 99.995%) was injected to
the prepared solution, and the mixture was strongly agitated in
an ice bath for 10 s. The reaction was retained at 130 ꢁC for
30 min. Thereaer, the reaction solution was stirred in an ice
bath for 10 min. The crude product was puried by centrifu-
gation 3 times using an ethanol and acetone (2 : 1) mixture. The
obtained precipitates were dispersed in 100 mL of THF by
sonication for 10 min and the resulting products were obtained
by ltration under reduced pressure. The ltered powder was
dried at room temperature under reduced pressure for 24 h.
2.5. Characterizations
Scanning transmission electron microscopy (STEM) was
applied for analysis of elemental mapping and energy disper-
sive X-ray (EDX) (FEI Talos F200X, maximum accelerating
voltage of 200 kV) analyses. X-ray photoelectron spectroscopy
(XPS) analysis was performed under reduced pressure using an
X-ray photoelectron spectrometer (X-TOOL, ULVAC-PHI) with
a monochromatic AlKa source. The phase was identied by
matching each characteristic peak with the JCPDS les.
Synthesized graphene supports were characterized using
Raman spectrometer (LabRam ARAMIS IR2, Horiba, Japan). X-
ray diffraction patterns were acquired on a Rigaku diffractom-
eter (Rigaku Smart Lab, Rigaku Co., Japan) operated at 45 kV
˚
and 40 mA with CuKa radiation (l ¼ 1.5406 A) using a diffracted
beam monochromator. Data were collected between 2q ¼ 5ꢁ and
100ꢁ at 0.01ꢁ intervals. Evaluation of yields and catalytic activity
was determined by gas chromatography (GC, Hewlett Packard
2.2. Evaluation of catalytic activity of the Pd–IPG
nanocomposite catalysts for Suzuki coupling reactions
Suzuki coupling reactions using the Pd–IPG were performed at
60 ꢁC aer preparing the reaction solution including arylbor-
onic acid (0.12 mmol), aryl iodide (0.1 mmol), NaOH (0.20
mmol), anisole (0.1 mmol), Pd–IPG (0.1 mol%) in ethanol/water
(1 : 1 v/v, 0.8 mL). The reaction yield was monitored by GC
analysis, where a small amount of the crude solution was taken
and applied to GC analysis at the predetermined time aer
simple ltration. The completed reaction mixture was cooled to
room temperature. Thereaer, Pd–IPG was separated from the
reaction by facile ltration and the ltrate was extracted with
ethyl acetate. The obtained organic part was used for GC anal-
ysis. The reaction yield was calculated by using anisole as an
internal standard.
1
5890) analysis. H and 13C NMR spectra were obtained on Agi-
lent MR DD2 (400 MHz) spectrophotometer.
3. Results and discussion
The multi-scaled Pd NPs loaded on the pyrene functionalized
poly(dimethylaminoethyl methacrylate)-b-poly[(ethylene glycol)
methyl ether methacrylate] ionic polymer-doped graphene
support (Pd–IPG) were synthesized by a facile in situ method
(Schemes S1 and S2†), which used sodium tetrachloropalladate
as a metal precursor, benzoic acid (BA) as a structure-directing
agent, N,N-dimethyl formamide (DMF) as a solvent and
a reducing agent. For comparison, Pd NPs loaded rGO without
the ionic polymer (Pd–rGO, Pd: 42.7 wt%) was synthesized using
the same conditions for the synthesis of the Pd–IPG and char-
2.3. Recyclability test of the Pd–IPG nanocomposite catalysts
Suzuki coupling reactions of phenylboronic acid (0.60 mmol) acterized as shown in the ESI (Fig. S1†).
with iodobenzene (0.50 mmol) were performed using Pd–IPG
The acquired Pd–IPG nanocomposite catalysts were charac-
(0.1 mol%), NaOH (1.0 mmol), and anisole (0.50 mmol) in terized through scanning transmission electron microscopy
ethanol/water (1 : 1 v/v, 4.0 mL). Aer dilution of the crude (STEM), which exhibited well-distributed multi-scaled Pd NPs
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on the IPG (Fig. 1A). STEM energy-dispersive X-ray (STEM-EDX), originated from the IPG carbon support (Fig. 2A).10,31 Moreover,
powder X-ray diffraction (XRD), and X-ray photoelectron spec- the average crystalline size of Pd NPs on IPG was evaluated to be
troscopy (XPS) analyses of Pd–IPG were performed for the 8.4 nm from the XRD pattern with the relation of Scherrer
investigation of the composition and structure of Pd NPs on formula, which was in good agreement with corresponding
IPG. The STEM-EDX elemental mapping of Pd–IPG showed that STEM analysis exhibiting the broad distribution of Pd NPs in
the observed NPs comprised Pd atoms, and the STEM-EDX a size (large numbers: 2–5 nm, small numbers: 15–30 nm), as
spectrum exhibited 49.7 wt% of Pd relative to C, O, and N described in the ESI (eqn (S1) and Table S1†).10 The deconvo-
contents originated from IPG (Fig. 1B and C, respectively).
luted XPS Pd 3d spectra of Pd–IPG showed predominant peaks
The XRD patterns of Pd–IPG showed predominant peaks at at 334.7/340.0 eV and 336.0/340.9 eV corresponding to Pd0 and
25.1, 39.5, 45.9, 67.0ꢁ corresponding to the face-centered cubic Pd2+, respectively (Fig. 2B).16 The XPS result indicated that the
(FCC) structure, except for the broad peak at 25.1ꢁ, which can be Pd NPs in Pd–IPG mainly comprises the metallic state Pd.
The interfacial interaction of three components including
rGO, ionic polymer, and Pd NPs in Pd–IPG was characterized
through Raman spectroscopy (Fig. 2C). Due to the presence of
the ionic polymer to rGO, the predominant peaks of the rGO
Raman spectrum at 1339.1 and 1582.5 cmꢀ1 corresponding to D
and G bands were shied to higher wavenumbers to 1347.1 and
1585.4 cmꢀ1, respectively. Aer incorporation of IPG with Pd
NPs, these two bands were also slightly shied to 1348.2 and
1589.6 cmꢀ1. These positive Raman shis can be explained by
the inuence of intermolecular electron-transfer, indicating the
well-developed intercalation structure.29
To optimize the catalytic reaction conditions of Pd–IPG,
Suzuki coupling reactions using iodobenzene and phenyl-
boronic acid as model substrates were screened with regard to
various bases and solvents (Table 1). The reactions were rst
ꢁ
screened with various bases at 60 C with 0.3 mol% Pd–IPG in
ethanol/water (v/v ¼ 1 : 1) (Table 1, entries 1–6). As shown,
reactions using most inorganic bases proceeded furnishing
high yields (>98%) except for that with KHCO3 (81%), indicating
Fig. 1 (A) STEM images (inset (a): magnified image for small-sized Pd
NPs, size distribution 3–5 nm), (B) STEM-EDX elemental mapping Fig. 2 (A) Powder XRD patterns of IPG and Pd–IPG. (B) XPS Pd 3d
images for Pd, and (C) STEM-EDX spectrum of Pd–IPG.
spectrum of Pd–IPG. (C) Raman spectra of rGO, IPG, and Pd–IPG.
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Table 1 Optimization of the reaction conditions for Suzuki coupling
reactions between iodobenzene and phenylboronic acid catalyzed by
the Pd–IPG nanocomposite catalystsa
The optimized reaction protocol for the Suzuki coupling
reaction between various substituted aryl halides and aryl
boronic acids was applied to evaluate the catalytic activity of Pd–
IPG using Pd–rGO as a reference catalyst, and corresponding
yields and turnover frequency (TOF) values were summarized in
Table 2. The use of Pd–IPG (0.1 mol%) in the coupling reaction
between iodobenzene and phenylboronic acid showed high
catalytic activity, with 99% yield in 1 h and the TOF value of 990
hꢀ1, and the reactivity employing the Pd–IPG catalyst was
largely dictated by the electronic nature of substituents in aryl
iodides; aryl iodides with an electron withdrawing substituent
showed higher catalytic activity relative to those with electron-
donating substituents. Specically, the reaction with aryl
iodide having an acetyl substituent showed 2-fold higher TOF
value (2000 hꢀ1) for the coupling product than that of iodo-
benzene, and achieved 8-fold higher TOF value in comparison
Entry
Solvent
Base
Cat. (mol%)
Yieldb (%)
1
2
3
4
5
6
7
8
EtOH : H2O ¼ 1 : 1
EtOH : H2O ¼ 1 : 1
EtOH : H2O ¼ 1 : 1
EtOH : H2O ¼ 1 : 1
EtOH : H2O ¼ 1 : 1
EtOH : H2O ¼ 1 : 1
EtOH : H2O ¼ 1 : 1
EtOH : H2O ¼ 1 : 1
EtOH : H2O ¼ 1 : 1
H2O
TEA
0.3
0.3
0.3
0.3
0.3
0.3
0.2
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
51
81
98
98
99
>99
99
99
60
20
83
86
1
KHCO3
K2CO3
Cs2CO3
Na2CO3
NaOH
NaOH
NaOH
Na2CO3
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
to the corresponding hydroxyl-substituted aryl iodide (245 hꢀ1
Table 2, entries 1, 2 and 4, respectively).
,
9
10
11
12
13
14
15
16
17
MeOH
EtOH
CH3CN
DMF
It is intriguing to note that for the production of 4-methoxy-
1,10-biphenyl the reaction between 4-methoxyphenylboronic
acid and phenyl iodide (entry 5) is a much better choice with 3-
fold higher TOF value (970 hꢀ1) compared to that of 4-
methoxyphenyl iodide and phenylboronic acid (entry 3).
However, the reaction of 4-hydroxyphenylboronic acid led to
a considerable decrease in the reactivity with iodobenzene
(Table 2, entry 6). These results indicate that the rates of
coupling reactions using Pd–IPG are quite sensitive to the
NR
NR
50
48
THF
DMF : H2O ¼ 1 : 1
THF : H2O ¼ 1 : 1
a
Reaction conditions: iodobenzene (0.1 mmol), phenylboronic acid
(0.12 mmol), base (0.2 mmol), and solvent (0.8 mL) at 60 ꢁC under air.
b
Yields were calculated from GC analysis with anisole as an internal
standard.
Table
2 Evaluation of catalytic activity of Pd–IPG and Pd–rGO
that strong bases are more effective than weak ones. Whereas,
TEA, an organic base, was much less effective in this system. To
identify the best base for this system, the content of Pd–IPG was
reduced from 0.3 mol% to 0.1 mol%, and NaOH still showed
sustained reactivity with high yield (99%). However, Na2CO3
showed noticeably reduced yield from 99% to 60% aer the
catalyst content was decreased to 0.1 mol%. From the base
screening tests, the most optimal Suzuki coupling reaction with
Pd–IPG was achieved through the use of NaOH.
With the NaOH in hand as an optimal base for this system,
representative protic (H2O, MeOH, EtOH) and aprotic solvents
(CH3CN, THF) were screened for the reaction with Pd–IPG.
Reactions in protic solvents, except for water, proved to proceed
with much better reactivity in comparison to those in aprotic
ones (Table 1, entries 11 and 12 vs. 13–15, respectively). This
result could be attributed to the poor solubility of NaOH in
aprotic solvents. To conrm the solubility issue of NaOH in
aprotic solvents, aqueous solutions of either THF or DMF were
applied for the Suzuki coupling reaction and the results clearly
supported the proposed reason for low reactivity in the cases of
the aprotic solvents (Table 1, entries 16 and 17). Meanwhile, the
use of water only as a solvent produced only 20% yield despite
good solubility of NaOH, presumably due to the poor solubility
of the organic substrates in water. As such, the Suzuki coupling
reaction at 60 ꢁC using NaOH as a base in EtOH/H2O (v/v ¼ 1 : 1)
as a solvent was selected as an optimized condition for the
reaction employing 0.1 mol% of Pd–IPG.
according to the various substrates under optimized conditionsa
Entry Catalyst
X
R1
R2
Time Yieldb TOFc (hꢀ1
)
1
2
3
4
5
6
7
8
Pd–IPGd
I
I
I
I
H
H
H
H
H
1 h
99%
990
4-COMe
4-OMe
4-OH
H
0.5 h 100% 2000
3 h
4 h
91%
98%
97%
303.3
245
970
I
4-OMe 1 h
I
Br
H
H
4-OH
H
24 h 42%
24 h 24%
17.5
10.0
Br 4-COMe
H
H
H
H
24 h 100% 41.7
9
Pd–rGOe
I
H
3 h
2 h
6 h
3 h
92%
95%
91%
93%
61.3
95
30.3
62
10
11
12
13
14
15
16
I
I
I
I
4-COMe
4-OMe
4-OH
H
H
4-OMe 2 h
100% 100
I
Br
H
H
4-OH
H
H
24 h 76% 6.3
24 h 5%
24 h 56%
0.4
4.7
Br 4-COMe
a
Reaction conditions: aryl halide (0.1 mmol), arylboronic acid (0.12
mmol), NaOH (0.2 mmol), and EtOH/H2O (0.8 mL) at 60 ꢁC under air.
b
GC yield were calculated by using anisole as an internal standard.
TOF value was calculated by moles of product with per mole of Pd
c
d
e
catalyst per hour. Pd–IPG (0.1 mol% to aryl halide). Pd–rGO (0.5
mol% to aryl halide).
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electronic nature of the substrates and the selection of appro- the reason for low catalytic activity in the Suzuki coupling
priate substrates is necessary for more effective coupling reactions with bromide substrates should be addressed and we
reactions.
found that the catalytic activity is considerably governed by an
To explore the ionic polymer doping effect, the Suzuki active surface area, which is mainly related to the metal NP size.
¨
coupling reactions with the same substrates employing Pd–rGO The report by Mulhaupt et al. provides important information
as a reference catalyst were examined (Table 2, entries 9–16). to address the effects of the NP size on the reactivity.12 Based on
¨
Since the reactions employing 0.1 mol% Pd–rGO required the Mulhaupt's study, we realized that the size effect of Pd NPs
signicantly long reaction time (over 24 h), 0.5 mol% Pd–rGO is more important than the effect of functionalized supports for
¨
was employed for the comparison reaction. The activity prole enhanced catalytic activity. Furthermore, Mulhaupt's report
of Pd–rGO with respect to the electronic nature of the substrates also expressed that the lager content of oxygenated carbons in
including aryl bromides was fully identical to that of the cor- the graphene support can promote Suzuki-coupling reactions
responding Pd–IPG. However, the catalytic activities of Pd–rGO and this effect corroborated the advantages of the ionic polymer
were signicantly lower for all substrates than that of Pd–IPG, doping in the Pd–rGO catalysts.
exhibiting 21-fold maximum TOF reduction in the case of entry
10 compared to entry 2 (Table 2).
Comparative evaluation of the Pd–IPG catalyst was per-
formed in contrast with previously reported, state-of-the-art
The Suzuki coupling reactions with bromide substrates were heterogeneous catalysts for the Suzuki coupling reaction.
performed to evaluate the comparative reactivity of the Pd–IPG Table S2† lists the results of the coupling reactions between
to the Pd–rGO as a reference catalyst (Table 2, entries 7, 8, 15, iodobenzene and phenylboronic acid employing various
and 16). The improved reactivity of Pd–IPG based on the ionic heterogeneous catalyst systems. Reactions with most of the
polymer doping effect was also prominent in the case of reac- listed catalysts proceeded with high yields (>90%). However, in
tions with bromide substrates, specically furnishing almost 9- many cases rather harsh reaction conditions were used in
fold maximum enhancement in the reaction of acetyl- comparison to that of Pd–IPG, for example use of toxic solvents
substituted aryl bromide with phenylboronic acid (Table 2, (DMF or DME), higher temperature, and longer reaction time.
entries 8 and 16). However, the TOF values in the reaction with The Pd–IPG exhibited higher catalytic activity (TOF: 990 hꢀ1),
bromide substrates suggest that the absolute reactivity was than the carbon supported Pd catalysts including Pd/SiC–CNT,
relatively degraded in comparison to the reaction with iodide Pd/CNT–graphene hydrogel, and Pd–Fe3O4@C, which exhibited
substrates (Table 2).
350, 799, and 324 hꢀ1 TOF values, respectively (Table S2,†
The observed highly enhanced catalytic activities of Pd–IPG entries 6, 7, and 8).13,33,34 These results indicate that the Pd–IPG
relative to Pd–rGO can be explained by two possible reasons: (i) is an effective catalyst with high catalytic activity requiring
the doped ionic polymers on rGO render the system to exhibit environmentally clean reaction conditions for Suzuki coupling
good affinity in the aqueous medium, leading to the better reactions.
dispersion of the Pd–IPG in the reaction medium. As such, the
The Pd–IPG catalyst system was then evaluated for repeated
accessibility of substrates and the availability of the catalyst can use since recyclability of the catalysts is one of the most
be substantially enhanced in comparison to those of Pd–rGO. important issues in viewpoint of cost, and recycling of the Pd–
(ii) Upon the basis of the reaction mechanism, the strong IPG was investigated for ten repetitions of Suzuki coupling
cohesion of arylboronic acid and possibly some soluble Pd reactions with iodobenzene and phenylboronic acid as model
species detached from the NPs with the long ethylene oxide substrates under the optimized conditions. As shown in Table 3
groups in the ionic polymers can facilitate the effective diffu- all reactions showed consistently high yields (>96%). Aer ten
sion of reacting species together in the vicinity of the Pd NPs, reaction cycles, the recovered Pd–IPG catalyst was characterized
leading to the accelerated coupling reaction (Fig. 3). However, by STEM and STEM-EDX, which exhibited same morphology
and a high level of retention in the Pd composition (Pd:
47.4 wt%) (Fig. S3A and B†). For its reliability, further recycla-
bility tests up to 20 cycles were performed. The large retention
in the reactivity was observed up to the 14th cycle. However, the
reactivity of the Pd–IPG started to decline at the 15th cycle (89%
yield) and did not recover back to high yield (>95%) up to the
17th cycle (Table S3†). When we increased the reaction time
Table 3 Recyclability tests of Pd–IPG for the Suzuki coupling reaction
of iodobenzene with phenylboronic acid under optimized conditionsa
Runs
1st 2nd 3th 4th 5th 6th 7th 8th 9th 10th
Yield (%) 99 96
98
97
97
97
98
97
99
98
a
Reaction conditions: Pd–IPG (0.1 mol%), iodobenzene (0.1 mmol),
phenylboronic acid (0.12 mmol), base (0.2 mmol), and solvent (0.8
mL) at 60 ꢁC under air for 1 h.
Fig. 3 Schematic illustration of the Pd–IPG nanocomposite catalyst's
role on the reaction between aryl iodides and arylboronic acids.
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from 1 h to 1.5 h, the reaction yield went up to >95%, as shown Pd species and the strong cohesion with the reacting species.
in Table S3.† The reason why it went up to 99% is not clear at However, the reactivity was relatively degraded in the case of the
this point, but the yields at the 18th to 20th recycling were reaction with bromide-mediated substrates. The Pd–IPG cata-
equally high. The fact that there is an ‘induction period’ at the lyst exhibited excellent recyclability with consistently high
beginning of each reaction should also be considered. On the yields (>96%) and almost unchanged morphology and Pd
basis of STEM studies, the primary reason for the degradation content of the catalyst even aer 10 repeated use. Plausible
of the catalytic activity was the detrimental change of the Pd– reasons for the retention of Pd content without noticeable
IPG morphology (Fig. S4†). These results indicate that the Pd– leaching may include the capture of Pd species by nearby ionic
IPG's performance is dependable up to the 14th cycle, presum- polymer groups. These ndings suggest that Pd NPs-based
ably owing to the strong interfacial linking ability of the ionic catalysts with ionic polymer-doped graphene could be an
polymers. Furthermore, it should be mentioned that we excellent breakthrough for solving the problems of homoge-
observed insoluble white salts in the reaction mixture and the neous catalysts as well as to signicantly enhance the catalytic
content gradually increased with increased repetition activity of catalysts based on Pd NPs and carbon supports.
(Fig. S3C(ii)†). This precipitate was analyzed to be sodium
iodide (NaI) through STEM-EDX (Fig. S3†). The NaI by-product
was readily removed through ltration aer dilution with
Acknowledgements
water and the isolated Pd–IPG powder was redispersed in EtOH/
This research was supported by the Mid-Career Researcher
H2O (v/v ¼ 1 : 1) for further use.
Program from the National Research Fund (NRF) grant funded
To explore the Suzuki coupling reaction mechanism
by MSIP, Korea (Project No. NRF-2015R1A2A2A01005965) and
employing the Pd–IPG catalyst, a hot-ltration test was per-
partially supported by the Industrial Strategic Technology
formed from the reaction of iodobenzene with phenylboronic
Development Program funded by the Ministry of Trade,
acid under the optimized conditions (Fig. S5†).16,34 Aer 30 min,
Industry & Energy (MI), Korea (Project No. 10041850).
when the reaction proceeded to give 35% yield, the reaction
mixture was ltered to a preheated vial. The reaction from the
ltrate aer hot-ltration exhibited no increase in conversion
up to 12 h, while the reaction with Pd–IPG continued to achieve
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