Facile synthesis of palladium nanoclusters and their catalytic activity in Sonogashira coupling reactions

Article abstract of DOI:10.1016/j.tetlet.2008.06.089

This work reports a facile synthesis of palladium nanoclusters (PdNCs) in MeCN/MeOH mixture without any stabilizer. The PdNCs were found to be effective catalysts for copper-free, amine-free and ligand-free Songashira coupling reactions under ambient cond

Full text of DOI:10.1016/j.tetlet.2008.06.089

Tetrahedron Letters 49 (2008) 5286–5288
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Facile synthesis of palladium nanoclusters and their catalytic activity
in Sonogashira coupling reactions
a
b
a,*
J. Athilakshmi , S. Ramanathan , Dillip Kumar Chand
a
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai 600 036, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
This work reports a facile synthesis of palladium nanoclusters (PdNCs) in MeCN/MeOH mixture without
any stabilizer. The PdNCs were found to be effective catalysts for copper-free, amine-free and ligand-free
Songashira coupling reactions under ambient conditions.
Received 17 April 2008
Revised 17 June 2008
Accepted 21 June 2008
Available online 25 June 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Palladium
Nanoclusters
Catalysis
Sonogashira coupling
The size and shape of materials at nanometer level modulates
the related physical and chemical properties greatly. Thus, con-
trolled synthesis of nanosized materials is of particular interest.
Solutions of palladium acetate in the solvent systems were pre-
pared and the concentration of the metal was maintained at
2 mM. Reduction of the palladium salt was facile in all the mixed
solvents within a period of a few hours at room temperature,
1
A plethora of metal nanoparticles have been prepared by several
2
4,5
groups aiming at new and exciting properties. Recently, palladium
where MeOH acted as the reducing agent. The PdNCs synthe-
nanoparticles (PdNPs) have been shown to be effective catalysts for
sized in the MeCN/MeOH mixture exhibited excellent stability,
and no precipitation was detected for at least two weeks at room
temperature and for months in a refrigerator. In all other cases,
the clusters precipitated out within a day. MeCN favors the disso-
3
carbon–carbon coupling reactions. Reduction of palladium ions by
4
5
methanol or methanol mixed with a co-solvent is a known strat-
egy for the preparation of PdNPs. However, the presence of a spe-
cial stabilizer is very crucial in this technique. The size and stability
8
ciation of trimeric palladium acetate with time as reported in the
4
,5
of these nanoparticles are controlled by the stabilizer employed
e.g., polymers, dendrimers, and bulky organic ligands). The draw-
literature. Further, MeCN also arguably prevented precipitation of
the clusters, and thereby provided stabilization.
(
9
back of the synthesis includes the efforts required for the prepara-
tion of stabilizers as well as possible contamination due to the
same. Such problems can be solved by utilizing a co-solvent having
weak coordination ability,6 which could provide stability to the
particles so that the use of stabilizers can be avoided.
It is known from the literature that disappearance of the peak
above 400 nm (d–d band) and the appearance of a broad shoulder
around 370–390 nm (surface plasmon peaks) in the UV–vis spectra
correspond to the reduction of Pd(II) to Pd(0) and formation of
nanoclusters, respectively. Since TEM, XRD, and EDX are ex situ
methods, it is necessary to rely on UV–vis spectrometry to study
the in situ behavior of the formation of palladium nanoclusters.
We observed a comparable pattern in the UV–vis spectra, and
hence infer the formation of nanoclusters (Fig. 1). Further, the ex
situ methods stated above were also employed to characterize
the material. The formation of Pd(0) was also confirmed from the
powder X-ray diffraction pattern of the sample (Fig. 2). XRD pat-
tern peaks were observed at 40°, 46°, 68°, and 82°. These peaks cor-
respond to the {111}, {200}, {220}, and {311} planes of a fcc
lattice, respectively, indicating that the material has a fcc structure.
TEM images demonstrated that the sizes of the PdNCs ranged from
ꢀ20 to 50 nm (Fig. 3a). For freshly prepared samples as well as
Herein, we report a facile synthesis of stable palladium nanocl-
7
usters (PdNCs) by dissolving Pd(II) in a mixed solvent system
(
MeCN/MeOH) in the absence of any stabilizer. We have also suc-
cessfully tested the resulting nanoclusters as a catalyst for copper-
free, amine-free and ligand-free Sonogashira coupling reactions at
room temperature without employing an inert atmosphere.
The reduction of Pd(II) was performed at room temperature in
3
five different mixed solvent systems viz. CHCl /MeOH, DMSO/
MeOH, H O/MeOH, DMF/MeOH, and MeCN/MeOH in 1:1 ratios.
2
*
Corresponding author. Tel.: +91 44 2257 4224; fax: +91 44 2257 4202.
E-mail address: dillip@iitm.ac.in (D. K. Chand).
0
040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.06.089

J. Athilakshmi et al. / Tetrahedron Letters 49 (2008) 5286–5288
5287
4
2
0
1
min
3
6
9
0 min
0 min
0 min
120 min
50 min
1
3
00
400
500
600
700
800
wavelength (nm)
Figure 1. UV–vis spectra of the 2 mM solution obtained from a mixture of Pd(OAc)
2
in MeCN/MeOH (1:1) at 30 °C and monitored after 1, 30, 60, 90, 120, and 150 min
following preparation of the solution.
2
1
1
1
1
1
000
800
600
400
200
000
8
6
4
00
00
00
3
0
40
50
60
70
80
90
2
theta (degree)
Figure 2. Powder XRD pattern of palladium nanoclusters obtained from the
reduction of Pd(II) by standing a 2 mM solution of Pd(OAc)
mixture of MeCN/MeOH.
2
dissolved in a 1:1
those aged for a few days, the TEM pattern remained unchanged,
indicating the stability of the clusters. The lattice fringes are clearly
exposed in the high resolution TEM (Fig. 3b), suggesting a highly
crystalline nature of the material. The elemental composition of
the material was confirmed by energy dispersive spectroscopy.
Palladium compounds are well-known for catalyzing a variety
of organic reactions. Cross-coupling reactions of aryl halides with
terminal alkyne (Sonogashira coupling reactions) are important
in organic synthesis. Sonogashira reactions are generally co-cata-
Figure 3. (a) TEM and (b) HRTEM image of palladium nanoclusters obtained from a
2
mM solution of palladium acetate in a 1:1 ratio MeCN/MeOH mixture. (c)
Representative dark field TEM image of Pd nanoparticles obtained from 2 mM
palladium acetate in a 1:1 ratio of MeCN/MeOH during the catalytic reaction.
1
0
lyzed by Cu(I) and a phosphine-based ligand for palladium. The
presence of the copper co-catalyst favors oxidative homocoupling
(
Glaser coupling) of the terminal alkyne to the corresponding
diyne by-product, which is difficult to separate from the desired
product. Moreover, Sonogashira reactions are carried out in
Sonogashira reactions were carried between iodobenzenes and
terminal alkynes in the presence of PdNCs (5 mol % in Pd) prepared
1
1
high boiling solvents such as amines, toluene, THF, DMF, or diox-
2 3
in 1:1 MeCN/MeOH mixture as catalyst and K CO as base under
ane and at high temperature.1
2,13
The reactions also need a base,
ambient conditions. To our knowledge, this is the first report of
1
4
17
which is usually an amine such as triethylamine or piperidine.
Sonogashira coupling reactions using palladium nanoclusters in
However, the elimination of amines (generally used in large ex-
cess) from these reactions is of importance because of environ-
mental concern.
a mixed solvent system of MeCN/MeOH without using any other
stabilizing agent. It is worth noting that the reactions were suc-
cessful under an open atmosphere. Various reactions for the cou-
pling of representative substrates are listed in Table 1. Alkyl and
aryl alkynes were coupled with iodobenzene as well as with iodo-
benzenes containing electron releasing and withdrawing substitu-
ents. The desired products were isolated in high yields. The catalyst
obtained at the end of the reaction was washed with hexane and
Recently, instead of the palladium complexes, PdNPs have been
exploited for Sonogashira coupling reactions.1
5,16
We have used
the PdNCs prepared in this work for Sonogashira reactions under
modified reaction conditions which address all the concerns noted
above.

5
288
J. Athilakshmi et al. / Tetrahedron Letters 49 (2008) 5286–5288
2. (a) Sanders, A. W.; Routenberg, D. A.; Wiley, B. J.; Xia, Y.; Dufresne, E. R.; Reed,
Table 1
a
Palladium nanocluster-catalyzed coupling of terminal alkynes and aryl iodides
M. A. Nano Lett. 2006, 6, 1822; (b) Anyaogu, K. C.; Fedorov, A. V.; Neckers, D. C.
Langmuir 2008, 24, 4340; (c) Yoon, T.-J.; Kim, J. S.; Kim, B. G.; Yu, K. N.; Cho, M.-
H.; Lee, J.-K. Angew. Chem., Int. Ed. 2005, 44, 1068.
Entry Aryl iodide
Alkyne
Product
Time (h) Yield (%)
3
.
.
(a) Astruc, D. Inorg. Chem. 2007, 46, 1884; (b) Tsuji, Y.; Fujihara, T. Inorg. Chem.
2007, 46, 1895.
(a) Naka, K.; Itoh, H.; Chujo, Y. Nano Lett. 2002, 2, 1183; (b) Teranishi, T.;
Miyake, M. Chem. Mater. 1998, 10, 594; (c) Moreno-Mañas, M.; Pleixats, R.;
Villarroya, S. Chem. Commun. 2002, 60.
1
2
3
4
5
I
3
6
3
9
4
90
80
95
90
80
4
MeO
I
MeO
5
.
(a) Arul Dhas, N.; Gedanken, A. J. Mater. Chem. 1998, 8, 445; (b) Tanaka,
H.; Koizumi, S.; Hashimoto, T.; Itoh, H.; Satoh, M.; Naka, K.; Chujo, Y.
Macromolecules 2007, 40, 4327; (c) Fukuoka, A.; Araki, H.; Sakamoto, Y.;
Inagaki, S.; Fukushima, Y.; Ichikawa, M. Inorg. Chim. Acta 2003, 350, 371;
2
O N
I
2
O N
I
(
4
d) Ornelas, C.; Salmon, L.; Aranzaes, J. R.; Astruc, D. Chem. Commun. 2007,
946.
H
6. (a) Maddanimath, T.; Kumar, A.; D’Arcy-Gall, J.; Ganesan, P. G.; Vijayamohanan,
K.; Ramanath, G. Chem. Commun. 2005, 1435; (b) Vidoni, O.; Philippot, K.;
Amiens, C.; Chaudret, B.; Balmes, O.; Malm, J.-O.; Bovin, J.-O.; Senocq, F.;
Casanove, M.-J. Angew. Chem., Int. Ed. 1999, 38, 3736.
I
OH
OH
a
Reaction conditions: Terminal alkyne (1 mmol), aryl iodide (1 mmol), K
2 3
CO
(
2 mmol), PdNCs (5 mol % in Pd), 25 mL of 1:1 MeCN/MeOH mixture, air, room
7
.
Procedure for the synthesis of palladium nanoclusters: Palladium(II) acetate
0.0112 g, 0.05 mmol) was added to a 1:1 MeCN/MeOH mixture (25 mL) to
prepare a 2 mM solution. This solution was stirred at room temperature
temperature (see Ref. 17).
(
(
ꢀ30 °C) for about 3 h until the yellow color of the solution was changed to
methanol, and re-dispersed in the solvent mixture and could be re-
used for Sonogashira coupling without loss of the activity. The
brownish-black due to reduction of Pd(II) to Pd(0). It is also possible to perform
the reaction by stirring the solution at 60 °C for only 10 min.
(a) Yatsimirsky, A. K.; Ryabov, A. D.; Zagorodnikov, V. P.; Sakodinskaya, I. K.;
Kavetskaya, O. I.; Berezin, I. V. Inorg. Chim. Acta 1981, 48, 163; (b) Bakhmutov,
V. I.; Berry, J. F.; Cotton, F. A.; Ibragimov, S.; Murillo, C. A. Dalton Trans. 2005,
1989.
8
.
products obtained were purified via column chromatography and
characterized by spectroscopic methods such as 1H NMR and
13
C
NMR. The analytical data were in good agreement with reported
1
6,18
data.
9. (a) Gaikwad, A. V.; Rothenberg, G. Phys. Chem. Chem. Phys. 2006, 8, 3669; (b)
Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881; (c)
Tromp, M.; Sietsma, J. R. A.; van Bokhoven, J. A.; van Strijdonck, G. P. F.; van
Haaren, R. J.; van der Eerden, A. M. J.; van Leeuwen, P. W. N. M.; Koningsberger,
D. C. Chem. Commun. 2003, 128.
0. (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 16, 4467; (b)
Sonogashira, K. J. Organomet. Chem. 2002, 653, 46; (c) Tykwinski, R. R. Angew.
Chem., Int. Ed. 2003, 42, 1566.
It is already demonstrated13,19 that leached Pd(0) species from
the catalyst surface are true catalytic species in various cross cou-
pling reactions catalyzed by PdNCs. It is also claimed that leaching
processes are favored by the aryl halide and the coordinating base
1
1
9a
used in the reactions.
We wanted to check the occurrence of
leaching during the Sonogashira coupling reactions in our system.
A dark field TEM image recorded during the catalysis is shown in
Figure 3c, which indicates leaching of the PdNCs. This shows that
the material existed as leached PdNPs during the reactions, which
could be the true catalytic species.
In conclusion, we have synthesized palladium nanoclusters by a
simple method in a 1:1 mixed solvent system of MeCN/MeOH
without any stabilizer. The PdNCs were found to catalyze the cop-
per-free, amine-free and ligand-free Sonogashira coupling reac-
tions and ambient conditions and an air atmosphere.
11. Siemsen, P.; Livingston, R. C.; Diederich, F. Angew. Chem., Int. Ed. 2000, 39, 2632.
1
2. (a) Djakovitch, L.; Rollet, P. Adv. Synth. Catal. 2004, 346, 1782; (b) Yoon, H.; Ko,
S.; Jang, J. Chem. Commun. 2007, 1468; (c) Urgaonkar, S.; Verkade, J. G. J. Org.
Chem. 2004, 69, 5428; (d) Cheng, J.; Sun, Y.; Wang, F.; Guo, M.; Xu, J.-H.; Pan, Y.;
Zhang, Z. J. Org. Chem. 2004, 69, 5428.
1
3. (a) Thathagar, M. B.; Kooyman, P. J.; Boerleider, R.; Jansen, E.; Elsevier, C. J.;
Rothenberg, G. Adv. Synth. Catal. 2005, 347, 1965; (b) Thathagar, M. B.; ten
Elshof, J. E.; Rothenberg, G. Angew. Chem., Int. Ed. 2006, 45, 2886.
14. (a) Heuzé, K.; Méry, D.; Gauss, D.; Astruc, D. Chem. Commun. 2003, 2274; (b)
Park, S. B.; Alper, H. Chem. Commun. 2004, 1306.
1
5. (a) Li, P.; Wang, L.; Li, H. Tetrahedron 2005, 61, 8633; (b) Xue, C.; Palaniappan,
K.; Arumugam, G.; Hackney, S. A.; Liu, J.; Liu, H. Catal. Lett. 2007, 116, 94; (c)
Soler, R.; Cacchi, S.; Fabrizi, G.; Forte, G.; Mart ´ı n, L.; Mart ´ı nez, S.; Molins, E.;
Moreno-Mañas, M.; Petrucci, F.; Roig, A.; Sebastián, R. M.; Vallribera, A.
Synthesis 2007, 19, 3068.
6. Gholap, A. R.; Venkatesan, K.; Pasricha, R.; Daniel, T.; Lahoti, R. J.; Srinivasan, K.
V. J. Org. Chem. 2005, 70, 4869.
Acknowledgments
1
D.K.C. thanks IIT Madras for financial support. J.A. thanks IIT
Madras for a fellowship.
17. Procedure for the representative Sonogashira reaction: K
was added to 25 mL of a freshly prepared solution of palladium nanoclusters
as described in Ref. 7), followed by phenyl acetylene (0.102 g, 1 mmol) and 1-
iodo-4-nitrobenzene (0.249 g, 1 mmol). The solution so obtained contains
mol % of Pd(0). The solution was allowed to stir at room temperature
2 3
CO (0.276 g, 2 mmol)
(
5
Supplementary data
(
ꢀ30 °C) for 3 h under ambient condition. The reaction was monitored by TLC.
After completion, the solvent was evaporated and the crude mixture was
extracted with 1:9 ethyl acetate/hexane mixture. The organic layer was
evaporated, and the residue was purified by column chromatography on silica
gel using hexane as an eluent to obtain (4-nitrophenyl)phenylacetylene as a
yellow solid (0.212 g, 95%).
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2008.06.089.
References and notes
18. (a) Yao, X.; Li, C.-J. Org. Lett. 2005, 7, 4395; (b) Cai, M.; Sha, J.; Xu, Q. Tetrahedron
2007, 63, 4642.
1
.
(a) Roduner, E. Chem. Soc. Rev. 2006, 35, 583; (b) El-Sayed, M. A. Acc. Chem. Res.
001, 34, 257; (c) Wiley, B.; Sun, Y.; Xia, Y. Acc. Chem. Res. 2007, 40, 1067.
19. (a) Biffis, A.; Zecca, M.; Basato, M. Eur. J. Inorg. Chem. 2001, 1131; (b) Zhao, F.;
Bhanage, B. M.; Shirai, M.; Arai, M. Chem. Eur. J. 2000, 6, 843.
2