Living nanocatalyst for effective synthesis of symmetrical biaryls

Article abstract of DOI:10.1016/j.ica.2011.01.056

A convenient procedure is described for the synthesis of symmetrical biaryls via Ullmann coupling reaction catalyzed by palladium nanoparticles stabilized in a polymer matrix. A wide range of aryl halide substrates is transformed into corresponding symmetrical biaryls at a mild reaction condition. Living nature of the nanocatalysts is revealed by in situ recycling methodology and transmission electron microscopy (TEM). Calculation based on density functional theory (DFT) was performed to have better insight on mechanistic cycle and the result was supported by experimental evidence. Preliminary investigation also reveals that these PdNPs act as efficient catalyst in Heck and copper free Sonogashira reactions.

Full text of DOI:10.1016/j.ica.2011.01.056

Inorganica Chimica Acta 372 (2011) 47–52
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Living nanocatalyst for effective synthesis of symmetrical biaryls
⇑
Subhankar Santra, Priyadarshi Ranjan, Swadhin K. Mandal , Pradip Kr. Ghorai
Department of Chemical Sciences, Indian Institute of Science Education and Research-Kolkata, Mohanpur-741252, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 3 February 2011
A convenient procedure is described for the synthesis of symmetrical biaryls via Ullmann coupling reac-
tion catalyzed by palladium nanoparticles stabilized in a polymer matrix. A wide range of aryl halide sub-
strates is transformed into corresponding symmetrical biaryls at a mild reaction condition. Living nature
of the nanocatalysts is revealed by in situ recycling methodology and transmission electron microscopy
Dedicated to Prof. S.S. Krishnamurthy
(
TEM). Calculation based on density functional theory (DFT) was performed to have better insight on
Keywords:
Palladium nanoparticles
Nanocatalysis
Carbon–carbon coupling reaction
Ullmann reaction
mechanistic cycle and the result was supported by experimental evidence. Preliminary investigation also
reveals that these PdNPs act as efficient catalyst in Heck and copper free Sonogashira reactions.
Ó 2011 Elsevier B.V. All rights reserved.
Density functional theory
1
. Introduction
The importance of the biaryl motif in the areas of organome-
making them rather expensive. As a result, there is a burgeoning
interest in recent years to develop ‘‘ligand-free’’ catalytic systems.
For example, development of catalytically active metals sup-
ported on charcoal [20], dendrimers [21,22], metal oxides [23],
clays [24], and silica [25] have been used over the last few years.
These catalytically active metals can be generated from the pre-
cursor metal salt and a reducing agent using a variety of polymers
such as polyvinylpyrrilidone, polyvinyl alcohol, and co-polymers
[26–30] for the stabilization of metal nanoparticles and in this
connection, only in recent years a few Ullmann reactions have
been reported using palladium nanoparticles of which most
reports describe catalysis using either solid support [31–34] or
using special reaction condition [35] such as ionic liquids
[36,37] or performed in microemulsion [38]. However, there is
only one report by Monopoli and co-workers which describes
the Ullmann coupling reaction in water with palladium nanopar-
ticles as catalysts without using any solid support [39]. Herein,
we demonstrate the palladium nanoparticles as catalysts for
Ullmann coupling reaction without using any solid support. We
prepared the palladium nanoparticles (PdNPs) of nearly uniform
tallic catalysis [1], material science [2,3], supramolecular chemis-
try [4], and its occurrence in several bioactive natural products
[
5–7], pharmaceuticals, agrochemicals [8,9] has stimulated the
development of numerous methods for its construction [6,7,10].
Several modern methods for the preparation of biaryls are avail-
able, such as Suzuki, Stille, Negishi, Kumada, and Hiyama cross-
coupling reactions involving the palladium (or nickel) catalyzed
coupling of aryl halides with an organometallic counterpart,
Ar–M (M = B, Sn, Zn, Mg or Si) [11–13]. However, most of these
methodologies require tedious preparation of the aryl metal
nucleophiles and their use in stoichiometric amounts. Alterna-
tively, the synthesis of symmetrical biaryls by the direct reduc-
tive homocoupling of aryl halides is a more convenient and
straightforward route as it bypasses the synthesis of the aryl
organometallic counterpart. In this context, the well known cop-
per catalyzed Ullmann reaction [14] is a powerful process, but it
generally requires an excess of copper and harsh reaction condi-
tions (T > 200 °C) [15]. Several attempts have been made in re-
cent years to establish a milder method by replacing copper
with other catalyst such as Ni(0) [11,16–18] or Pd(0) [11,19] in
the presence of a reducing agent.
w
size using polyethyleneimine (PEI, M = 25 000) as a stabilizing
agent and ethylene glycol (EG) as the chemical reductant. The
PdNPs obtained by this method exhibit good catalytic activity in
the transformation of a wide range of aryl halide substrates into
the symmetrical biaryls at relatively mild reaction condition.
Living nature of the nanocatalysts is revealed by in situ recycling
methodology [40] and transmission electron microscopy (TEM).
Additionally, we addressed the issue of commonly observed
hydrodehalogenation process in Ullmann coupling reaction using
calculation based on density functional theory (DFT) and experi-
mental evidence.
Among the approaches described in the literature, numerous
efforts have been devoted to the development of ligand-promoted
catalytic systems. However, many ligands are not only sensitive
to air and/or moisture but also comparatively difficult to prepare
⇑
Corresponding author. Tel.: +91 9903676563; fax: +91 33 25873031.
E-mail address: swadhin.mandal@iiserkol.ac.in (S.K. Mandal).
0
020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.01.056

4
8
S. Santra et al. / Inorganica Chimica Acta 372 (2011) 47–52
2
. Experimental
2.5. Computational details
2.1. Materials and instruments
We used the Beck’s three parameter hybrid exchange function
41], combined with the Lee–Yang–Parr non-local correlation func-
[
All aryl halides, anhydrous PdCl
Sigma–Aldrich). Ethylene glycol, ethyl alcohol and all other chem-
2
were used directly as received
tion [42] abbreviated as B3LYP. The split-valence basis set with dif-
fuse functions, namely 6-311++G, have been employed for all
atoms in GAUSSIAN 03 [43]. Vibrational frequencies were calcu-
lated for optimized molecular structures to verify that no negative
frequencies were present for minimum energy structures.
(
icals were purchased from Merck, India and used without further
1
13
purification. H NMR and C NMR spectra were recorded on a JEOL
00 MHz spectrometer and on a Bruker 500 MHz spectrometer.
4
Chemical shifts (d) downfield from the reference standard were as-
signed positive values. UV–Vis spectral studies were carried out on
a HITACHI UV4100 spectrophotometer. Transmission electron
microscopic (TEM) experiments were performed on a JEOL, JEM
3
. Results and discussion
3.1. Synthesis and characterization of PEI stabilized palladium
2
010/111. All products were isolated by short chromatography
nanoparticles
on a silica gel (100–200 mesh) column using the required ratio of
petroleum ether (60–80 °C) and ethyl acetate. Compounds de-
scribed in the literature were characterized by comparison of their
The palladium nanoparticles stabilized in a polymer matrix
(PEI) was synthesized by chemical reduction using ethylene glycol
at 90 °C. The yellow solution of H PdCl turned to dark brown col-
2 4
oration indicating reduction of Pd(II) to Pd(0). This was confirmed
by UV–Vis spectroscopy (Fig. 1). The absence of a peak at 350 nm
indicates complete reduction of Pd(II) to Pd(0) as documented in
previous studies [44]. The formation of palladium nanoparticles
was confirmed by recording the TEM image (Fig. 2a); collected
on a lacey carbon grid reveals that the particles are of nearly uni-
form size ranging approximately 3–5 nm (Fig. 2b).
1
13
H and C NMR spectra to the reported data. The diameter distri-
bution of the palladium nanoparticles was calculated manually
taking 50 numbers of nanoparticles. All catalytic reactions were
carried out under an atmosphere of dry nitrogen using standard
Schlenk techniques.
2.2. Synthesis of palladium nanoparticles (PdNPs)
2 4
H PdCl was first prepared by reacting 0.025 g of anhydrous
PdCl
2
(0.141 mmol) with 0.035 mL of concentrated HCl into a
5
1
0
0
0 mL Schlenk flask keeping it in closed condition for more than
5 min. In another portion, 5 mL ethylene glycol was added into
3.1.1. Catalytic Ullmann coupling reactions
We performed Ullmann coupling reactions on a number of aryl
halide substrates (Scheme 1) using the as prepared PdNPs as cata-
lyst. Initial experiments were performed with 4-bromotoluene to
establish the most productive reaction condition. Table 1 depicts
the different reaction conditions used for this particular conver-
sion. The EG–water combination (entries 1–3, Table 1) exhibited
poor substrate consumption rate due to the immiscibility of organ-
ic substrates in water. On the other hand, the ethylene glycol–eth-
anol combination resulted in the dramatic increase in substrate
consumption rate (entries 4–6, Table 1) as the use of ethanol as
co-solvent makes the whole process more homogeneous. The cou-
pling reactions were also performed varying the temperature (en-
tries 7–12, Table 1). Till 60 °C, the substrate consumption was
found to be quite high (ꢁ90%), but it slowed down drastically be-
low 60 °C and no transformation was observed at room
temperature.
.05 g PEI (2
w
lmol, M = 25 000) followed by the addition of
.28 g (7 mmol) NaOH in powder form (which serves as initiator
2+
for the chemical reduction of Pd by EG) and this mixture was
sonicated for about 10 min until a clear solution is formed. This
clear solution was then added to the as prepared H
and it was stirred first at room temperature for 15 min under the
positive flow of N and then at 90 °C until deep brown colloidal dis-
persion was obtained.
2 4
PdCl solution
2
2.3. Procedure for Ullmann coupling reactions
To the aliquot (2.5 mL) of the as prepared catalyst solution, 5 mL
of ethanol, 2 mmol of base and 1.42 mmol of aryl halide were
added at 25 °C. After purging the reaction mixture with N at
5 °C for at least 15–20 min, it was heated for appropriate time.
After completion, it was cooled, extracted with diethyl ether
2
2
(
3 ꢀ 30 mL). The combined organic layers were washed with dis-
tilled water (2 ꢀ 30 mL), brine solution (1 ꢀ 20 mL) and dried over
anhydrous sodium sulfate. After concentrating under reduced
pressure it was purified through a column chromatography (silica
gel 100–200 mesh) using petroleum ether (60–80 °C bp) and ethyl
acetate mixture as eluant.
2.4. Procedure for catalyst longevity experiment
To the aliquot (2.5 mL) of the as prepared catalyst solution, 5 mL
of ethanol, 2 mmol of potassium phosphate and 1.42 mmol of 4-
bromotoluene were added at 25 °C. After purging the reaction mix-
2
ture with N at 25 °C for at least 15–20 min, it was heated to 90 °C.
1
The reaction mixture was monitored by H NMR spectroscopy by
taking aliquots of the reaction mixture at certain interval and the
reaction was stopped when the substrate consumption was com-
plete. The reaction mixture was cooled to 25 °C under nitrogen
atmosphere, and the fresh substrate (4-bromotoluene, 1.42 mmol)
was added and the similar process was repeated for a total of four
consecutive catalytic runs.
Fig. 1. UV–Vis spectral change during the formation of PdNPs: (i) before reduction
and (ii) after reduction to Pd(0) nanoparticles.

S. Santra et al. / Inorganica Chimica Acta 372 (2011) 47–52
49
Fig. 2. (a) TEM image of the PdNPs and (b) diameter distribution of the PEI encapsulated PdNPs.
withdrawing group at the para position exhibit conversion into
the corresponding biphenyl derivatives (entries 8–9, Table 2).
One of the major problems in the catalysis involving the metal
nanoparticles without solid support is their ready tendency to
aggregate into bulk materials on heating. As a result, the recyclabil-
ity (or longevity) of the active catalyst is greatly hampered. To
check the longevity of the catalyst we have carried out the catalytic
runs consecutively with same amount of catalyst by charging fresh
substrate (4-bromotoluene) after each catalytic run in the same
reaction pot. The living nature of the catalyst was tested by moni-
toring the time required for complete consumption of substrate by
Scheme 1. Ullmann coupling reaction catalyzed by palladium nanoparticles.
A range of substituted aryl halides was used as substrates and
the result is presented in Table 2. The result clearly indicates that
the PdNPs stabilized with PEI act as active catalyst for Ullmann
coupling reaction under relatively milder condition (at 90 °C) than
the traditional copper catalyst (at T > 200 °C) [15]. The yield of the
isolated product varies from 24% to 67%, although the substrate
consumption from the reaction mixture reveals almost quantita-
tive conversion in a number of catalytic runs (entries 1–6, 9–10,
Table 2) as determined by 1H NMR spectroscopy of reaction
mixture. The relatively low isolated yield may be explained by an
alternative conversion of the substrate into volatile hydrodehalo-
genated product (see later). The more challenging substrate aryl
chloride does not exhibit any conversion (entry 7, Table 2) at the
reaction condition supporting the fact that the activation of aryl
chloride is more difficult than their bromide counterpart. Never-
theless, the activated aryl chloride substrates bearing electron
1
H NMR spectroscopy in four consecutive catalytic cycles. The re-
sult reveals that the nanoparticles remain catalytically active upto
four consecutive runs (Fig. 3a). The first two runs survive with al-
most equal activity indicating complete substrate consumption in
5 h time; however, from third run steady drop in the catalytic
activity is observed. Fig. 3b shows that the PdNPs remain almost
same in size after and before (Fig. 2a) the first catalytic run per-
formed at 90 °C indicating thermal stability of these nanoparticles.
The plausible mechanistic cycle is schematically shown in
Scheme 2 which involves a number of key intermediates as re-
vealed by previous studies [37,45]. The first step is the oxidative
addition of aryl halide with the Pd(0) to the haloaryl intermediate
Table 1
a
Ullmann coupling reaction of 4-bromotoluene.
Pd nanoparticles
Base, Solvents, Temp., N atm.
Me
Br
Me
Me
2
Entry
Solvent ratio
Base
(–)c
Temperature (°C)
Time (h)
% Substrate consumptionb
1
2
3
4
5
6
7
8
9
EG:Water (1:2)
EG:Water (1:2)
EG:Water (1:2)
EG:EtOH (1:2)
EG:EtOH (1:2)
EG:EtOH (1:2)
EG:EtOH (1:2)
EG:EtOH (1:2)
EG:EtOH (1:2)
EG:EtOH (1:2)
EG:EtOH (1:2)
EG:EtOH (1:2)
90
90
90
90
90
90
80
70
60
50
40
25
5
5
5
5
5
5
5
5
5
5
5
12
31
24
14
83
87
98
98
96
92
30
15
K
K
(–)
2
CO
PO
3
3
4
c
K
K
K
K
K
K
K
K
2
3
3
3
3
3
3
3
CO
3
4
4
4
4
4
4
4
PO
PO
PO
PO
PO
PO
PO
1
1
1
0
1
2
d
(–)
a
b
c
4
2
-Bromotoluene (1.42 mmol), ethanol (5 mL), anhydrous potassium phosphate or potassium carbonate (2 mmol), and catalyst (2.5 mL, 5 mol%) under N atm.
1
The reactions were monitored by checking the consumption of the substrate by H NMR spectroscopy of the reaction mixture after usual work up.
No extra base was added.
No product was formed.
d

5
0
S. Santra et al. / Inorganica Chimica Acta 372 (2011) 47–52
Table 2
Ullmann coupling reactions catalyzed by palladium nanoparticles.a
Pd nanoparticles
EtOH, K PO , 90 °C, N
R
R
X
3
4
2
atm.
R
X = Cl, Br, I
Entry
1
Substrate
Product
Time (h)
5
% Yieldb
63
Br
2
3
5
5
61
67
Me
Me
Br
Me
Me
Me
Br
Br
Me
4
5
5
5
51
31
F3C
F3C
Br
F3C
F3C
CF3
CF3
F
3
C
F
3
C
CF
3
6
5
33
Br
N
N
N
7
8
9
5
5
8
5
5
5
5
(–)c
47
49
24
38
46
48
Cl
NC
Cl
Cl
I
NC
CN
F3C
F3C
MeO
F3C
F3C
MeO
CF3
CF3
OMe
OMe
1
1
1
1
0
1
2
3
I
MeO
MeO
Br
MeO
MeO
Br
Br
OMe
1
4
5
(–)d
N
N
a
b
c
Aryl bromide (1.42 mmol), ethanol (5 mL), potassium phosphate (2 mmol), and catalyst (2.5 mL, 5 mol%) under N
Isolated yield of the purified product.
No product was formed.
2
atm. at 90 °C.
d
Hydrodehalogenated product was isolated.
ꢂ1 ꢂ1
ꢂ1
I. The intermediate I can undergo a side reaction by forming the
arene II via a proton abstraction (hydrodehalogenation pathway).
Alternatively, the reaction of I with Ar–X with loss of X would
produce a diarylpalladium species III [45]. This particular step
brings the two aryl moieties in close proximity and subsequent
reductive elimination leads to the formation of symmetrical biaryl.
To understand the competitive hydrodehalogenation and bisaryl-
palladium intermediate formation pathways, a DFT level calcula-
18.73 cal mol
K
and ꢂ93.43 kcal mol , respectively. The simi-
S and H values for
lar trend was also observed for iodobenzene (
D
D
ꢂ
ꢂ1 ꢂ1
hydrodehalogenation process are 21.23 cal mol
K
and
ꢂ1
ꢂ85.56 kcal mol , respectively; for bisarylpalladium formation
ꢂ1 ꢂ1
pathway
D
S
and
D
H
values are 18.23 cal mol
K
and
ꢂ1
ꢂ90.92 kcal mol , respectively) and bromoanisole (
D
S and DH
ꢂ1 ꢂ1
values for hydrodehalogenation process are 15.04 cal mol
K
ꢂ1
and ꢂ81.37 kcal mol , respectively; for bisarylpalladium forma-
ꢂ1 ꢂ1
tion was performed.
palladium atoms was considered for performing these calculations.
The change of entropy ( S) and enthalpy ( H) of these parallel
processes were calculated using bromobenzene as substrate. The
A
small palladium cluster of seven
tion pathway
D
S and
D
H values are 16.47 cal mol
K
and
ꢂ1
ꢂ87.2 kcal mol , respectively) as substrates. These results clearly
demonstrate that energetically both the processes are feasible. This
type of hydrodehalogenation in Ullmann coupling reaction has also
been observed previously [31,38]. In the present study, the hyd-
rodehalogenated product of type II was isolated and spectroscopi-
cally characterized with 3-bromoquinoline (entry 14, Table 2). In
D
D
D
S and
D
H values for hydrodehalogenation pathway are calculated
ꢂ1 ꢂ1
ꢂ1
as 22.91 cal mol
K
and ꢂ89.82 kcal mol , respectively; while
for bisarylpalladium formation pathway, those values are

S. Santra et al. / Inorganica Chimica Acta 372 (2011) 47–52
51
Fig. 3. (a) Longevity of the catalyst was tested by monitoring the time required for complete consumption of substrate by 1H NMR spectroscopy in four consecutive catalytic
cycles and (b) TEM image of the PdNPs after first catalytic run revealing no appreciable change in the size of palladium nanoparticles.
Scheme 2. Plausible mechanistic pathway for Ullmann coupling reaction.
the mechanistic cycle, the intermediate III brings two aryl groups
in close proximity and upon subsequent reductive elimination it
produces the biaryl. To establish the formation of the intermediate
III, we carried out the catalytic reaction using a mixture of two dif-
ferent substrates namely bromobenzene and 4-bromotoluene. The
NMR analysis of the reaction mixture reveals the formation of
three different products. These are two symmetrical biaryls, biphe-
homocoupling of bromobenzene and 4-bromotoluene, respectively
and one unsymmetrical biaryl (4-methylbiphenyl, b, Scheme 3).
The formation of the unsymmetrical biaryl in this reaction clearly
supports the involvement of intermediate III in the catalytic cycle.
3.1.2. Catalytic Heck and Sonogashira coupling reactions
After successful catalytic reactions towards Ullmann coupling
reactions, we became interested to check the usefulness of these
0
nyl and 4,4 -dimethylbiphenyl (a and c, Scheme 3) arising from
Scheme 3. Ullmann coupling reaction using mixture of substrates.

5
2
S. Santra et al. / Inorganica Chimica Acta 372 (2011) 47–52
Scheme 4. Heck coupling reaction catalyzed by PdNPs.
Scheme 5. Copper free Sonogashira coupling reaction catalyzed by PdNPs.
nanoparticles in other type of C–C coupling processes. Preliminary
testing was performed on Heck (Scheme 4) and Sonogashira
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(
Scheme 5) reactions using these PdNPs as catalysts. A variety of
solvents (EtOH, dimethylacetamide, water) and bases (K PO
CO , KOAc) were applied to carry out these two coupling reac-
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,
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2
3
tions. trans-Stilbene was obtained in best yield at 90 °C (Scheme 4)
from iodobenzene and styrene while diphenylacetylene was ob-
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In summary, we have demonstrated an easy synthesis of poly-
mer coated palladium nanoparticles of nearly uniform size sus-
pended in organic solvent. These PdNPs exhibit good catalytic
activity in Ullmann coupling reactions transforming a range of aryl
halide substrates into their corresponding symmetrical biaryls at a
mild reaction condition. More importantly, these nanocatalysts
stay live for a number of successive catalytic cycles emphasizing
that the steric stabilization provided by the polyethyleneimine is
quite efficient against the usual problem of nanoparticle’s coagula-
tion into bulk materials on heating. Additionally, a DFT level calcu-
lation supported the feasibility of two parallel pathways namely
the hydrodehalogenation and the bisarylpalladium formation
pathway. Preliminary investigation also reveals that these PdNPs
act as efficient catalyst in Heck and Sonogashira reactions.
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We are thankful to DST, India for financial support. S.S. thanks
CSIR, India for a research fellowship. Sandip Niyogi and Arunim
Pal assisted with acquiring the TEM data at the DST Unit of Nano-
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[
Appendix A. Supplementary material
[
[
[
[
[
[
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