Palladium on ionic liquid derived nanofibrillated mesoporous carbon: A recyclable catalyst for the ullmann homocoupling reactions of aryl halides in water

Article abstract of DOI:10.1002/cctc.201300893

Palladium nanoparticles supported on nanofibrillated mesoporous carbon (Pd@IFMC) were found to be highly active in heterogeneous catalysis for the Ullmann homocoupling of a broad range of aryl chlorides, bromides and iodides in aqueous medium without the need to any chemical co-reducing agents. The catalysts could be recovered and reused several times without significant loss of activity. Giving Ullmann fibrillations: Palladium nanoparticles supported on an ionic-liquid-derived nanofibrillated mesoporous carbon are highly active in catalyzing the Ullmann homocoupling of a range of aryl chlorides, bromides, iodides, and heteroaryl bromides in aqueous medium without the need for chemical co-reducing agents. The catalysts are recovered and reused several times without significant loss of activity.

Full text of DOI:10.1002/cctc.201300893

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DOI: 10.1002/cctc.201300893
Palladium on Ionic Liquid Derived Nanofibrillated
Mesoporous Carbon: A Recyclable Catalyst for the
Ullmann Homocoupling Reactions of Aryl Halides in Water
Babak Karimi,*[a] Hesam Behzadnia,[a] and Hojatollah Vali[b]
Palladium nanoparticles supported on nanofibrillated mesopo-
rous carbon (Pd@IFMC) were found to be highly active in het-
erogeneous catalysis for the Ullmann homocoupling of
a broad range of aryl chlorides, bromides and iodides in aque-
ous medium without the need to any chemical co-reducing
agents. The catalysts could be recovered and reused several
times without significant loss of activity.
mild reaction conditions in aqueous media. From practical and
sustainable standpoints, the use of high performance recovera-
ble catalysts, which can be applied in low concentrations for
the activation of aryl chlorides and heteroaryl halides is pref-
erable. Therefore, the quest for alternative heterogeneous cata-
lysts for efficient Ullmann homocoupling of aryl and heteroaryl
halides in water as the most environmentally benign solvent
remains a great challenge. We have recently shown that
a novel ionic liquid derived nanofibrillated mesoporous carbon
(IFMC) displays excellent electrochemical[9] and chemical prop-
erties and can be used as a support for the activation and sta-
bilization of Pd nanoclusters (Pd@IFMC) in the aerobic oxida-
tion of a wide range of alcohols in water at low temperature.[10]
Based on X-ray photoelectron spectroscopy (XPS) and electron
tomography analyses, it was also evident that the high catalyt-
ic performance and stability of Pd@IFMC owed most likely to
the presence of nitrogen functionalities on the surface of pris-
tine IFMC and also the unique fibrous structure of the materi-
als (Figure 1, inset), which not only resulted in a good distribu-
tion of small Pd nanoclusters but also suppressed their dissolu-
tion and agglomeration.[10]
The aryl–aryl coupling reaction has become arguably one of
the most important synthetic transformations for constructing
biaryl units, which constitute a wide range of pharmaceuticals,
natural products, and conducting polymers.[1] Of the aryl–aryl
bond forming reactions, the Ullmann homocoupling of aryl
halides has been widely used for the preparation of symmetri-
cal biaryls over the past century.[2] However, this reaction has
typically been performed at very high reaction temperatures
(over 2008C) with stoichiometric copper, which renders these
methods impractical in the synthesis of biaryls bearing thermo-
sensitive functional groups. Although considerable improve-
ments have been made in recent years with varied Pd, Cu, and
Ni-based catalysts,[1a,2b,3] these are mostly homogeneous proce-
dures and require an excess of co-reductant, thereby suffering
from limited utility in practical processes because of challenges
associated with their separation, recovery, and product con-
tamination. As a result, the use of heterogeneous catalysis is
often favored for many chemical reactions to minimize the
problems associated with homogeneous systems and improve
the economic and environmental features of the processes.
Despite these premises, very few heterogeneous and recycla-
ble catalyst systems have been explored for the Ullmann ho-
mocoupling of aryl halides[4–8] and most of them are applicable
only to the coupling of aryl iodides and bromides. Moreover,
to the best of our knowledge, there have been no reports of
Ullmann homocoupling of extremely challenging heteroaryl
substrates by using heterogeneous palladium catalysts under
[a] Prof. Dr. B. Karimi, Dr. H. Behzadnia
Department of Chemistry
Figure 1. TEM micrograph of Pd@IFMC (scale bar=100 nm). A good distribu-
tion of Pd nanoparticles can be clearly observed. Inset: Electron tomogram
accompanied with a 3D-reconstructed tomogram of IFMC.
Institute for Advanced Studies in Basic Sciences (IASBS)
P.O.Box 45137-6731, Gava Zang, Zanjan (Iran)
Fax: (+98)241 4153125
E-mail: karimi@iasbs.ac.ir
In addition, the catalyst exhibited much improved catalytic
performance in aqueous solution compared to Pd@CMK-3 and
the commercial Pd@C catalyst in the aerobic oxidation of alco-
hols. Developing on this, we herein demonstrate that the syn-
ergism between the nitrogen functionalities and supported Pd
nanoclusters in Pd@IFMC with appropriate palladium content
[b] Prof. H. Vali
Anatomy and Cell Biology and Facility for Electron Microscopy Research
McGill University
Montreal, Quebec, H3A 2 A7 (Canada)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300893.
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Figure 3. XPS of Pd@IFMC in the Pd region.
340.3Æ0.2 eV respectively, which is attributed to Pd in the
metallic (Pd0) state (Figure 3 and S5, Supporting Information).
On the other hand, the high-resolution N1s scan indicated
the presence of both pyrrolic (401.0Æ0.2 eV) and pyridinic
(399.2Æ0.2 eV) nitrogen in the materials (Figure S6, Supporting
Information).[12] First, we began our investigation on the Ull-
mann coupling reaction of 4-methoxyl iodobenzene by using
Pd@IFMC (0.1 mmol palladium on 1 g of IFMC) in the presence
of Et3N (3 equiv.) in water at 808C. Under these conditions, we
found that the coupling reaction proceeded well, affording the
promising moderate yields of 63% of the corresponding biaryl
with a high selectivity of >99% (Table 1, entry 1). This result
directed our attention towards applying the interesting physi-
cochemical properties of IFMC and well-distributed Pd NPs in
its pore structure to enhance the Ullmann coupling reaction of
varied aryl halides, without the need for co-reductant through
Figure 2. a) TEM micrograph of a single nanofiber of Pd@IFMC, b) 3D TEM
reconstruction of a cross-section of Pd@IFMC to show exact Pd NPs distribu-
tions, and c) 3D TEM reconstruction of Pd@IFMC.
(Figures 1 and 2) can be utilized as an efficient and recyclable
catalyst for the Ullmann homocoupling of a variety of aryl hal-
ides without need for a co-reducing agent.
IFMC was synthesized through a two-stage carbonization of
1-methyl-3-phenethyl-1H-imidazolium
hydrogen
sulfate
(MPIHS) in the presence of SBA-15 as hard template, following
our previously reported procedure.[10,11] The supported Pd cata-
lyst (Pd@IFMC) was then prepared by simple impregnation of
well-dispersed IFMC with Pd(OAc)2 as the Pd precursor in THF,
followed by treatment with a solution of hydrazine hydrate.
The process of preparing Pd@IFMC was monitored by N2 ad-
sorption–desorption XPS and TEM analysis. N2 sorption analysis
showed that the isotherm shape of the pristine IFMC was pre-
served after modification, evidencing that the supported Pd
nanoparticles (NPs) did not block or alter the mesopore struc-
ture. However, as expected, the pore diameter, surface area,
and pore volume of Pd@IFMC decreased slightly, indicating
that Pd NPs were incorporated into the interior nanospaces of
the pristine IFMC. The (high-resolution) TEM, electron tomogra-
phy, and 3D-reconstructed images of the materials revealed an
almost monomodal distribution of Pd NPs, with particle sizes
typically of 4–5 nm (Figures 1 and 2, and S5 Supporting Infor-
mation).
Table 1. Optimization of the Ullmann reaction.
Entry
Catalyst
Base
t [h]
Yield [%]
1
2
3
4
5
6
7
8
Pd@IFMC
Pd@IFMC
Pd@IFMC
Pd@IFMC
Pd@IFMC
Pd@IFMC
Pd@IFMC
Pd@IFMC
Pd/CMK-3
Pd/C
Et3N
18
18
18
18
18
18
18
18
24
24
18
24
24
18
63
75
90
83
99
52
70[a]
89[b]
37[c]
38[c]
n.d.
n.d.
n.d.
25[d]
K2CO3
Et3N:K2CO3
NaOAc
Cs2CO3
KOH
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
–
As shown in Figure 2a and c, the Pd NPs in Pd@IFMC are
supported not only on the external surface of the carbon ma-
terials, they are also well-distributed inside the mesopores. Al-
though a relatively high Pd concentration of 2.5% was used,
no evidence of Pd NPs agglomeration was observed. This ex-
cellent Pd NPs distribution is believed to be related to the
high porosity and surface area as well as the fibrillar structure
of the support. Pd XPS of the catalyst revealed a doublet peak
corresponding to Pd 3d5/2 and Pd 3d3/2 at 335.2Æ0.2 and
9
10
11
12
13
14
Pd@IFMC
IFMC
CMK-3
Cs2CO3
Cs2CO3
Cs2CO3
Pd@IFMC
[a] 0.2 mol% catalyst. [b] T=708C. [c] Considerable amounts (ꢀ10–15%)
of deiodination byproduct (anisole) were formed. [d] Under oxygen
atmosphere.
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a cooperative pathway. Various bases such as K2CO3, K2CO3/
Et3N, NaOAc, Cs2CO3, and KOH, were also screened for their
effect on the reaction. The best results in term of biaryl yields
and product selectivity were obtained in the presence of
Cs2CO3 at 808C in water (entries 2–6). Furthermore, well-known
Pd@CMK-3 and commercial Pd/C, a widely employed catalyst
in CÀC coupling reactions, were also tested under the opti-
mized reaction conditions (entry 5) for comparative purposes.
The observed conversions of 4-methoxy iodobenzene in both
cases were remarkably inferior, with considerable formation of
deiodination byproduct (>10%) (entries 9 and 10). Notably,
base-free reactions and those using either the pristine IFMC or
CMK-3 gave no conversion in the present system under the
same reaction conditions (entries 11–13). These data confirmed
that the high catalytic performance of Pd@IFMC (regarding
both selectivity and activity) most likely originated from the
synergism between the chemical functionalities on the IFMC
and Pd NPs, which highlighted the decisive role of both metal
and IFMC in catalyzing Ullmann coupling reaction efficiently.
To gain better insight into the activity of the present catalyst
system in the Ullmann coupling reaction, we then examined
the catalytic properties of Pd@IFMC in an O2-saturated reaction
system. Interestingly, the coupling reaction of 4-methoxy iodo-
benzene declined substantially under otherwise identical reac-
tion conditions, as compared to the same reaction under an
inert (Ar) atmosphere (entries 14 vs. 5).
electron-withdrawing groups, affording the corresponding
biaryl units in excellent yields and selectivities (Table 2, en-
tries 2–12). The coupling reactions of chloroarenes with an
electron-withdrawing group are often accompanied with the
production of byproduct arising from hydrodechlorination.[13]
However, it was proved that Pd@IFMC exhibited excellent cata-
lytic activity and selectivity for the Ullmann coupling of these
substrates and no hydrodechlorination products were ob-
served under the investigated conditions (Table 2, entries 10
and 11). In addition, to the best of our knowledge, there has
been only one example of employing homogeneous Pd NPs in
the Ullmann coupling reaction of heteroaryl bromides in the
presence of stoichiometric amounts of co-reductant (propio-
naldehyde) and excess of tetrabutylammonium acetate.[14] No-
tably, by employing Pd@IFMC, 2-bromothiophene and 3-bro-
mopyridine underwent the Ullmann coupling with high selec-
tivities, although somewhat lower yields of the corresponding
biaryls were obtained by using these extremely challenging
substrates (Table 2, entries 13, 14). Therefore, the present cata-
lytic system could be considered as the first example of Ull-
mann homocoupling of heteroaryl halides in water in the ab-
sence of co-reductant.
The stability, durability, and reusability of the heterogeneous
catalyst are important. Therefore, the recycling performance of
Pd@IFMC was studied in the Ullmann reaction of 4-iodoanisole
in water. Once the reaction was completed in the first run, the
catalyst was recovered by simple filtration, followed by wash-
ing with excess of EtOAc and dried under vacuum. The recov-
ered catalyst was then reused directly for the next run. As
shown in Figure 4, the catalytic activity remained almost con-
To illustrate the general applicability of Pd@IFMC, the proto-
col was extended to the Ullmann coupling of a series of substi-
tuted bromoarenes and chloroarenes. The results of this inves-
tigation are summarized in Table 2. The protocol was applica-
ble to the homocoupling of various types of substituted
bromo- and chloroarenes bearing either electron-donating or
Table 2. Ullmann coupling of various aryl halides using Pd@IFMC.
Entry
R
X
Y [mol%]
t [h]
Yield [%][a]
1
2
3
4
5
6
7
8
OMe
H
Et
CN
Cl
NO2
MeO
H
CH3
NO2
CN
MeO
I
0.3
0.7
1
0.7
0.7
0.7
1
1.5
1.5
1.5
1.5
1.5
18
20
24
15
24
15
24
24
24
24
24
24
99
96
97
98
90
97
96
92
90
97
96
85
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Figure 4. Recyclability of the Ullmann reaction of 4-iodoanisole in water
with Pd@IFMC.
9
10
11
12
stant over 5 reaction cycles in water. After the 5th reaction
cycle, the total Pd loading in the recovered Pd@IFMC was
0.08 mmolgÀ1, approximately 80% of the original value in the
pristine Pd@IFMC. As the amount of Pd residue in the final
product was very low at less than 5 ppb, as measured by in-
ductively coupled plasma atomic emission spectroscopy, the
possibility that trace Pd was lost from the catalyst during the
13
1
24
58
14
1
24
78
[a] Isolated yields.
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cated for 30 min. Pd(OAc)2 (0.018 mmol) was then dissolved in THF
(15 mL) and added to the IFMC suspension. To load Pd NPs onto
IFMC, hydrazine hydrate (10 mL) was added to the suspension
under vigorous stirring. After vigorous stirring at RT for 1 h, one
drop of 30% hydrogen peroxide was added. Pd@IFMC was afford-
ed after filtration, washed several times with THF, and dried at
408C under vacuum overnight. The total palladium content in
Pd@IFMC was estimated to be 0.1 mmolgÀ1 through the Pd mea-
surement of an acid-washed (HNO3, 65%, 6 mL) solution obtained
from a 100 mg sample of the catalysts by inductively coupled
plasma atomic emission spectroscopy.
recovery and recycling (e.g., during the filtration and washing
stages) could not be completely excluded.
At this point, one critical question arose: how could
Pd@IFMC catalyze Ullmann coupling reactions of aryl halides in
the absence of a co-reductant? One explanation could be that
IFMC acts as both the catalyst support and the electron source
(reducing agent), a feature demonstrated previously only for
polyaniline nanofiber.[8] Another explanation could be the pres-
ence of heteroatom dopants in IFMC, particularly as other car-
bonaceous materials such as CMK-3 and activated carbon did
not display similar features.
This hypothesis is confirmed to some extent by the signifi-
cant suppression of the yield of coupled product by employing
a sample of the catalyst with approximately half of the IFMC
content with respect to the catalyst used in this study.[10] How-
ever, it remains unclear which type of functional groups at the
surface of IFMC and also which nitrogen-bonding configura-
tion play the key role in the present Ullmann coupling proto-
col. Work focused on more detailed studies of the exact reac-
tion mechanism of this process to clarify the synergism be-
tween IFMC and supported Pd NPs in the Ullmann homocou-
pling reaction is currently ongoing in our laboratory.
General procedure for the Ullmann coupling reaction in
water
Aryl halides (1 mmol), catalyst (0.3–1.5 mol%), Cs2CO3 (3 mmol),
and water (3 mL) were placed in a glass flask. The catalytic reaction
was performed at 808C with vigorous stirring (see Table 2 for reac-
tion times). The reaction was monitored by TLC. After completion
of the reaction, the products were separated by organic extraction
and were then purified by recrystallization.
Acknowledgements
In conclusion, we demonstrated that palladium nanoparti-
cles supported on ionic liquid derived nanofibrillated mesopo-
rous carbon (Pd@IFMC) are highly active in catalyzing the Ull-
mann homocoupling of a range of aryl chlorides, bromides,
and iodides and, in particular, heteroaryl bromides in aqueous
medium (water) without need for any chemical and stoichio-
metric co-reducing agents. The catalysts could be recovered
and reused several times without significant loss of activity.
The authors gratefully acknowledge the IASBS Research Council
and Iran National Science Foundation (INSF) for supporting this
work.
Keywords: CÀC coupling · carbon · heterogeneous catalyst ·
mesoporous materials · palladium nanoparticles
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Experimental Section
Preparation and analysis of IFMC
IFMC was prepared through the carbonization of MPIHS by using
ordered mesoporous silica SBA-15 as a template according to the
reported procedure.[10] At first, to deposit MPIHS into the surfac-
tant-free SBA-15 channels, a dry acetonitrile solution of the ionic
liquid (1 mL of MPIHS in 25 mL of acetonitrile) was added dropwise
to a suspension of SBA-15 in acetonitrile. After 24 h of stirring at
RT, the solvent was removed under reduced pressure. The resulting
powdery material was impregnated with an aqueous solution of
sulfuric acid (4 g H2O, 0.14 g H2SO4) and placed at 1008C in
a vacuum-drying oven for 6 h. Consequently, the oven temperature
was increased to 1608C and maintained for 6 h at that tempera-
ture to afford a dark brown powder. MPIHS 1 (0.4 mL) was incorpo-
rated again into mesoporous silica hosts by the same procedure.
After that, the ionic liquid was carbonized at 9008C for 3 h under
a flow of Ar. IFMC was then obtained by dissolving the silica frame-
work in NaOH aqueous solution for 24 h. The resulting carbon ma-
terial was filtered, washed several times with deionized water and
ethanol, and dried under vacuum.
[14] V. Calꢃ, A. Nacci, A. Monopoli, P. Cotugno, Chem. Eur. J. 2009, 15, 1272.
Preparation and analysis of Pd@IFMC
Synthesis of the Pd@IFMC was performed by reduction of Pd(OAc)2
with hydrazine hydrate in absolute THF (Acros Organics). A suspen-
sion of IFMC (50 mg) in absolute degassed THF (40 mL) was soni-
Received: October 21, 2013
Revised: December 16, 2013
Published online on February 3, 2014
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