Facile synthesis of palladium nanoparticles supported on silica: An efficient phosphine-free heterogeneous catalyst for Suzuki coupling in aqueous media

Article abstract of DOI:10.1016/j.catcom.2016.08.005

A convenient one-step method for synthesizing highly dispersed palladium nanoparticles supported on silica, without taking assistance from any external reductant or stabilizer, has been developed. The supported nanoparticles were characterized by N₂-adsorption desorption, XRD, HRTEM, SEM-EDX, XPS, ICP analyses and applied as catalyst for Suzuki-Miyaura reactions of aryl halides. The reactions with aryl bromides were performed in neat water at room temperature; while the reactions with aryl chlorides were conducted in aqueous-ethanol at 90?°C. The catalyst could be reused at least three times without compromising with its activity, however from the fourth cycle a progressive decrease in yield was noticed. No aggregation of NPs was observed by the TEM analysis of the six-time used catalyst.

Full text of DOI:10.1016/j.catcom.2016.08.005

Facile synthesis of palladium nanoparticles supported on silica: An efficient
phosphine-free heterogeneous catalyst for Suzuki coupling in aqueous media
Debojeet Sahu, Ana R. Silva, Pankaj Das
PII:
DOI:
Reference:
S1566-7367(16)30263-1
doi: 10.1016/j.catcom.2016.08.005
CATCOM 4735
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
26 April 2016
14 July 2016
2 August 2016
Please cite this article as: Debojeet Sahu, Ana R. Silva, Pankaj Das, Facile synthesis of
palladium nanoparticles supported on silica: An efficient phosphine-free heterogeneous
catalyst for Suzuki coupling in aqueous media, Catalysis Communications (2016), doi:
10.1016/j.catcom.2016.08.005
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ACCEPTED MANUSCRIPT
Facile synthesis of palladium nanoparticles supported on silica: An efficient phosphine-free
heterogeneous catalyst for Suzuki coupling in aqueous media
Debojeet Sahu,1 Ana R. Silva2 and Pankaj Das1*
1 Department of Chemistry, Dibrugarh University, Dibrugarh, Assam, India.
2
Department of Chemistry, CICECO-Aveiro Institute of Materials, University of Aveiro,
Aveiro, Portugal.
Abstract A convenient one-step method for synthesizing highly dispersed palladium
nanoparticles supported on silica, without taking assistance from any external reductant or
stabilizer, has been developed. The supported nanoparticles were characterized by N2-adsorption
desorption, XRD, HRTEM, SEM-EDX, XPS, ICP analyses and applied as catalyst for Suzuki-
Miyaura reactions of aryl halides. The reactions with aryl bromides were performed in neat
water at room temperature; while the reactions with aryl chlorides were conducted in aqueous-
ethanol at 90 oC. The catalyst could be reused at least three times without compromising with its
activity, however from the fourth cycle a progressive decrease in yield was noticed. No
aggregation of NPs was observed by the TEM analysis of the six-time used catalyst.
Keywords Palladium nanoparticles; N-ligand; heterogeneous catalyst; Suzuki-coupling; aqueous
media; aryl chloride.
1 Introduction
Over the past few decades, remarkable developments have been accomplished in the
palladium (Pd) catalyzed Suzuki-Miyaura cross-coupling reaction, which is one of the most
popular methods for the construction of carbon–carbon bonds [1-4]. Usually, in most cases
homogeneous ligand-based molecular complexes are used as catalysts. Although such
homogeneous systems often display amazing activities particularly while dealing with substrates
like aryl chlorides, however, their tedious syntheses and high costs pose a serious limitation to
their industrial-scale applications [2,5]. Recent examples suggest that Pd nanoparticles (NPs),
either in their colloidal form [6,7] or supported on solids [8-11], are attractive alternatives to
these molecular catalysts. Compared to colloidal NPs, supported NPs are more advantageous
*Corresponding author. Tel: +91-373-2370210; Fax: +91-373-2370323.
E-mail address: pankajd29@yahoo.com (P. Das)

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owing to their ease in separations and better recycling efficiencies. As a result, during the past
few years, a number of materials such as silica [12-14], MCM-41(Mobil Composition of Matter
No. 41) [15], SBA-15 (Santa Barbara amorphous type silica No. 15) [16], carbon [17,18], Metal-
Organic Framework (MOF) [19], etc. have been employed as solid supports for PdNPs to design
efficient heterogeneous catalysts for Suzuki-Miyaura reaction. Typically, supported PdNPs are
synthesized using a PdII precursor, an organically modified support, a reducing agent and a
stabilizing or dispersing agent via chemical reduction techniques [20,21]. Recently, we have
demonstrated that PdNPs could also be synthesized without the assistance of an external
reducing and stabilizing agent using a commercially available phosphine ligand bound to silica
[22]. However, the high cost of the ligand (approx. $430/g) along with its inherent toxicity and
air sensitivity forms the bottleneck in the large-scale use of the afore-mentioned catalyst. Thus, it
is of great importance to develop new cost-effective methods for synthesizing PdNPs under
phosphine-free conditions.
In line with this, herein, we report synthesis of a new Pd-nanocatalyst using PdCl2 and a
supported nitrogen-based ligand without using an external reductant or stabilizer. The
effectiveness of the catalyst was tested for the Suzuki-Miyaura cross-coupling reactions of aryl
halides in aqueous media. It is worth noting that significant efforts have been devoted nowadays
to explore aqueous condition in Suzuki reactions, however most of the successful systems are
still homogeneous in nature [23,24]. To the best of our knowledge, very few heterogeneous Pd
catalysts are known to date that can promote the Suzuki reactions in aqueous media [16-19,25-
27]. Moreover, majority of those systems showed promising activity mostly with aryl bromides
and failed when aryl chlorides were used as substrates. It may be noted that from the cost and
accessibility point of view, aryl chlorides are the most desirable substrates in cross-coupling
reactions. Hence, the development of new heterogeneous Pd catalysts for efficient activations of
aryl halides including less reactive aryl chlorides in aqueous media is one of the most attractive
areas of research in Suzuki reaction.
2 Experimental
2.1. Synthesis of the supported Pd nanocatalyst
2.15 g of NMe2@SiO2 was added to 40 mL of acetonitrile solution of 1.5 mmol of PdCl2 under
o
continuous stirring at 70 C. After 6 h, the reaction mixture was filtered to get a black solid

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residue which was washed several times through soxhlet extraction using acetonitrile for 12 h in
each time. No coloration of the washed extract confirmed that the solid catalyst was free from
o
physically adsorbed PdCl2. The solid was then dried at 80 C in an oven in air for 12 h. The
material so obtained was designated as PdNP-NMe2@SiO2.
2.2. Suzuki-Miyaura cross-coupling reaction
For Suzuki-Miyaura reaction, appropriate amount of the catalyst, PdNP-NMe2@SiO2, was added
to a mixture of aryl halide (0.5 mmol), arylboronic acid (0.65 mmol), K2CO3 (1.5 mmol) in 6 mL
solvent. The reaction was then stirred under desired temperature for the required time. The initial
progress of the reaction was monitored by TLC using aluminum coated TLC plates (Merck)
under UV light and the product formation was determined using GC-MS. After completion, the
catalyst was collected by filtration and washed with isopropanol-water. The filtrate was diluted
with water and extracted with ether and then dried over Na2SO4. After evaporation of the solvent
under reduced pressure, the residue was chromatographed (silica gel, ethyl acetate–hexane, 1:9)
to obtain the desired product.
3 Results and discussion
3.1 Synthesis and characterization of the catalyst
The palladium nanoparticles (PdNPs) were synthesized by a simple one-step reaction
between PdCl2 and an amine-functionalized silica gel (NMe2@SiO2) (Scheme 1). No separate
reducing or stabilizing agent was used. The formation of highly dispersed NPs was clearly
visible from TEM picture (Fig. 1). The particles are usually spherical/elliptical in nature (ESI:
Fig. S1a) and have the sizes in the range of 2-7 nm with majority of the particles fall between 4
and 5 nm (Fig. 1a, inset). The crystalline nature of the NPs was evident from the selected area
electron diffraction (SAED) pattern which shows a fringe width of about 0.2 nm consistent with
the face centre cubic (fcc) arrangement of the nanocrystals (ESI: Fig. S1b) [28,29]. The SEM
micrograph (ESI: Fig. S2a) of the catalyst material shows silica particles are of different
dimensions and the EDX spectra shows the presence of Pd (ESI: Fig. S2a). The Pd content from
the ICP-AES analysis was found to be 0.39 mmol g-1 of silica gel. The N2-adsorption-desorption
isotherm (ESI: Fig. S3) of the catalyst shows a type IV isotherm with a specific surface area
(ABET) of 328 m2/g. Compared to the amine-supported silica material (438 m2/g) this value is

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much lower indicating immobilization of Pd onto silica (ESI: Table S1). The powder XRD
patterns (ESI: Fig. S4) of the Pd-based material shows a broad peak for silica at 2θ = 22.40, along
with four other low intensity peaks at (111), (200), (220) and (311) planes corresponding to
Pd(0) nanocrystals (JCPDS card No. 001-1201). The XPS survey spectrum of the Pd-
nanocatalyst, PdNP-NMe2@SiO2, confirms the presence of Pd, C, N, O and Si (ESI: Fig. S5a).
The N 1s high resolution spectrum (Fig. S5b) can be deconvoluted into two peaks centered at
402.3 and 400.3eV respectively. The higher energy peak is very close to the value (401.9 eV)
usually obtained for an iminium ion [29], while the lower binding energy peak is typical of an
amine group [30,31]. Although the chemical pathway for formation of this iminium ion is not
very clear; there are literature evidences where tertiary amines in the presence of transition
metals like Pd are converted to iminium ion [32,33], which possibly isomerized to enamine and /
or hydrolyzed to aldehyde. In fact, the weak binding energy peaks in the XPS spectrum for C1S
at 287.6 eV and O1S at 534.1 (ESI Fig. S6) might also suggest the presence of a carbonyl group.
The binding energies of the Pd 3d5/2 and 3d3/2 electrons in the high resolution spectrum (Fig. 2)
appear at 335.7 eV and 340.8 eV respectively and these values are consistent with the formation
of Pd(0) NPs on silica matrix [34].
3.2 Catalytic studies
The Suzuki-Miyaura activity of PdNP-NMe2@SiO2 was first tested with aryl bromides
employing p-bromoanisole and phenylboronic acid as model substrates using water as solvent
and K2CO3 as base with 0.5 mol% Pd. We were delighted to see that the coupling reaction was
completed smoothly in water at room temperature and, almost quantitative conversion was
obtained (ESI: Table S2, entry 1). In a controlled experiment, when the same reaction was
performed with homogeneous PdCl2 as catalyst without using the supported ligand, only 44%
yield was obtained (Table S2, entry 2). Interestingly, under similar experimental condition, when
a water-soluble Pd salt such as, Pd(NO3)2, was used as catalyst only slight improvement in
product yield was observed (entry 11). Hence, it can be concluded that the cooperative influence
between Pd and supported amine is responsible for the improved activity of PdNP-NMe2@SiO2
over homogeneous Pd species.
Besides K2CO3, our system can also tolerate other bases like Na2CO3, Cs2CO3 and NaOH.
Optimization of catalyst quantity revealed that the use of 0.1 mol% metal loading resulted in

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completion of the reaction within 4 h (Table S2, entry 10). Motivated by our initial optimization
study, we extended the scope of the catalyst for a range of sterically and electronically different
aryl bromides (Table 1). Good-to-excellent yields of cross-coupling products were obtained in
most of the cases. Indeed, the efficient activities were also observed in case of hindered
substrates like, 2-bromobenzaldehyde (entry 5) or 1-bromo-2,5-dimethoxybenzene (entry 6).
Interestingly, biaryls containing heterocyclic groups were also accessible by our catalytic system
under slightly warm conditions (entries 8, 9 and 11). It may also be noted that changing the
boronic acid from phenyl to tolyl (entry 13) / chloro (entry 12) had very little influence in our
catalytic system. However the heterocyclic boronic acid viz. 2-thienylboronic acid gave
relatively low yield (entry 14).
With high yielding results with aryl bromides, the potentiality of our catalyst was tested for
aryl chlorides. The initial reaction in water was conducted between p-chloronitrobenzene and
phenylboronic acid by following the same experimental conditions used for aryl bromide (ESI:
Table S3, entry 1). Unfortunately, the catalytic system was found to be almost inactive at room
temperature. Increasing the temperature to 90 oC and catalyst loading to 1.0 mol%, no significant
improvement in yield was observed (Table S3, entry 2). Interestingly, changing the solvent
system had a dramatic effect on the catalytic system. For instance, when water was replaced with
EtOH as solvent, yield of 4-nitrobiphenyl was increased from 12 to 80% (Table S3, entries 2 and
5). Similarly when DMF was used as solvent almost quantitative product formation was
achieved. DMF and H2O-EtOH (1:1) was found to be the best solvent systems in our study
(Table S3, entries 6 and 8). Although the activity in H2O-EtOH was slightly lower compared to
DMF (88 and 96% respectively), considering economic and environmental benefits, we chose to
use H2O-EtOH system for further studies (Table 2). It is noteworthy that both electron-poor
(Table 2; entries 1, 2 and 5) and electron-rich (entries 6 and 7) aryl chlorides underwent smooth
coupling with phenylboronic acid and produced the corresponding biaryls in good-to-excellent
yields. This high yield was also maintained when phenylboronic was replaced by p-tolylboronic
acid (entry 11) or p-chloroboronic acid (entry 10), while a significant drop in yield was seen
when thienylboronic acid was used (entry 12). The ortho- substituted aryl (entry 3) or heteroaryl
chlorides (entries 8 and 9) also provided reasonably good yields with our catalyst.
3.3 Reusability and heterogeneity test

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The potential for recyclability of the catalyst was tested using p-bromoanisole and
phenylboronic acid (ESI: Fig S7). The catalyst was found to exhibit recyclability up to three
cycles without compromising with its activity, however from the fourth cycle a gradual decrease
in product yield was noticed, and after the sixth cycle the conversion dropped to 92%. The ICP-
AES analysis of the 6th time used catalyst shows only negligible loss in Pd content (< 2%). SEM
image of the used catalyst shows a significant decrease in the silica particle size compared to that
of fresh catalyst (ESI: Fig. S2b). No aggregation of Pd NPs was observed by the TEM analysis
of the used catalyst (Fig. 1b), although a slight decrease in sizes of the Pd-NPs was observed for
the used catalyst. A similar type of decrease in Pd particle size has also been reported earlier
[35]. Hot filtration test (ESI: Fig. S8) together with solid phase poisoning test (ESI: Sec. 3.4)
confirm that the catalyst is truly heterogeneous in nature.
4 Conclusion
In conclusion we have disclosed a convenient protocol for synthesizing a silica-supported Pd
nanocatalyst without involving phosphine, and also without using any external reducing or
stabilizing agent. The Pd nanocatalyst could efficiently catalyze the Suzuki-Miyaura cross-
coupling reactions of aryl bromides in water at room temperature and aryl chlorides in aqueous-
ethanol under relatively mild condition.
Acknowledgements The CSIR), New Delhi, is gratefully acknowledged for financial support
(Grant no: 02/0081/12). The UGC New Delhi was also acknowledged for SAP-DRS programme
and for providing fellowship to D.S under BSR-RFSMS scheme.
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Legends for scheme, figures and tables
Scheme 1: Synthesis of silica supported Pd nanocatalyst, PdNP-NMe2@SiO2.
Fig. 1: TEM spectra of PdNP-NMe2@SiO2 along with particle size distribution histogram; (a)
Before catalysis and (b) After catalysis.
Fig. 2: High resolution XPS (Pd 3d) spectrum of the silica supported catalyst PdNP-
NMe2@SiO2.
Table 1: Suzuki-Miyaura cross coupling reactions of aryl/heteroaryl bromides with arylboronic
acids with PdNP-NMe2@SiO2 as catalyst.
Table 2: Suzuki-Miyaura cross coupling reactions of aryl chlorides with arylboronic acids with
PdNP-NMe2@SiO2 as catalyst.

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Pd
Scheme 1
(b)
(a)
35
30
25
20
15
10
5
40
35
30
25
20
15
10
5
0
0
2
3
4
5
6
7
2
3
4
5
6
7
Size (nm)
Size (nm)
Fig. 1

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Pd 3d
Pd 3d5/2
335.7
Pd 3d3/2
341.1
350 348 346 344 342 340 338 336 334 332 330
binding energy (eV)
Fig. 2
Table 1
Entry
1
R-Br
R'-B(OH)2
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
Time (h) Yield (%)b
4
4
4
4
6
6
6
8
8
8
8
7
7
8
98
97
85
96
80
87
83
82c
80c
86c
76c
88
4-OCH3-C6H4
C6H5
4-CHO-C6H4
4-CH3-C6H4
2-CHO-C6H4
2,5-OCH3-C6H3
4-COPh-C6H4
5-Bromopyrimidine
2-Bromo-3-methylthiophene
Methyl 2-bromobenzoate
5-Bromo-2-furaldehyde
2
3
4
5
6
7
8
9
10
11
12
13
14
4-OCH3-C6H4
4-OCH3-C6H4
C6H5
4-Cl-C6H4
4-CH3-C6H4
2-Thenylboronic acid
92
63c
aReaction conditions: 0.5 mmol aryl bromide, 0.65 mmol arylboronic acid, 1.5 mmol K2CO3,
6mL H2O;
bIsolated Yield;
cReactions are conducted at 50 oC.

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Table 2
Entry
1
2
3
4
5
6
7
8
R-Cl
R'-B(OH)2
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
Time (h)
Yield (%)b
4-NO2-C6H4
3-NO2-C6H4
2-NO2-C6H4
C6H5
4-CHO-C6H4
4-COCH3-C6H4
4-OCH3-C6H4
3-Chloropyridine
2-Chlorothiophene
4-OCH3-C6H4
4-OCH3-C6H4
C6H5
6
7
7
6
6
6
6
8
8
6
6
8
88
80
52
94
85
88
95
70
65
88
82
56
C6H5
C6H5
9
10
11
12
4-Cl-C6H4
4-CH3-C6H4
2-Thenylboronic acid
aReaction conditions: 0.5 mmol aryl chloride, 0.65 mmol arylboronic acid, 1.5 mmol K2CO3,
6mL H2O-EtOH (1:1);
bIsolated Yield.

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Graphical Abstract

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Highlights
 A simple one step method of synthesizing silica-supported palladium nanoparticles.
 No external reducing or stabilizing agent is necessary.
 The Suzuki reactions of aryl bromides and chlorides are performed in aqueous media.
 The catalyst could be recycled up to six times without loss in activity.