Phosphine-stabilized Pd nanoparticles supported on silica as a highly active catalyst for the Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1039/c4ra11186a

We have prepared a silica supported palladium nanocatalyst (PdNP@PPh₂-SiO₂) with high dispersion via a one-step reaction between commercially available 2-diphenylphosphinoethyl-functionalized silica gel (PPh₂-SiO₂) and PdCl₂ without the assistance of an external reductant or stabilizer. The catalyst was well characterized by N₂-adsorption, XRD, HRTEM, SEM-EDX and ICP analyses. The supported catalyst exhibited excellent activity for the Suzuki-Miyaura cross-coupling reactions of aryl halides (including aryl chlorides) with arylboronic acids. The facile synthesis of the catalyst combined with high product yields at relatively low catalyst loading (<0.5 mol%) represents one of the most efficient silica supported heterogeneous systems for the Suzuki coupling of aryl chlorides. The catalyst could be reused several times without compromising its activity.

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Page 1 of 21
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Phosphine-stabilized Pd nanoparticles supported on silica as highly active catalyst for
Suzuki-Miyaura cross-coupling reaction
Debojeet Sahu and Pankaj Das
We have prepared a silica supported palladium nanocatalyst (PdNP@PPh2ꢀSiO2) with
high dispersion via oneꢀstep reaction between commercially available 2ꢀ
diphenylphosphinoethylꢀfunctionalized silica gel (PPh2ꢀSiO2) and PdCl2 without taking
assistance of an external reductant or stabilizer. The catalyst was well characterized by N2ꢀ
adsorption, XRD, HRTEM, SEMꢀEDX and ICP analyses. The supported catalyst exhibited
excellent activity for the SuzukiꢀMiyaura crossꢀcoupling reactions of aryl halides (including
aryl chlorides) with arylboronic acids. The facile synthesis of the catalyst combined with high
product yields at relatively low catalyst loading (<0.5 mol%) represents one of the most
efficient silica supported heterogeneous systems for the Suzuki coupling of aryl chlorides.
The catalyst could be reused several times without compromising with its activity.
Key words: supported catalyst; palladium nanoparticle; silica gel; SuzukiꢀMiyaura reaction;
aryl chlorides;
Introduction
Remarkable developments have been achieved in the palladium (Pd) mediated organic
synthesis during the past two decades.1 Among various Pd catalyzed reactions, the Suzukiꢀ
Miyaura reaction of aryl halides with arylboronic acids forming biaryls has emerged as one of
the most intense areas of research both in industry and in the laboratory.2 These biaryl
compounds have a wide range of applications in pharmaceutical, material and agrochemical
industries.2b,c Over the last two decades, significant advancements have been made in
developing efficient Pd catalysts for this application.3 Unfortunately, majority of the
successful catalysts for Suzuki reactions are still homogeneous in nature where catalyst
separation and reusability are often problematic. Compared to homogeneous protocols,
_________________________________________________________________________
Department of Chemistry, Dibrugarh University, Dibrugarh, Assam, India. Fax: +91-373-
2370323; Tel: +91-373-2370210; E-mail: pankajd29@yahoo.com.
†Electronic Supplementary Information (ESI) available: Textural data of BET studies, XRD
patterns, catalyst optimization study, reusability diagram, 1H NMR and mass spectral data of
crossꢀcoupling products.
1

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heterogeneous systems possess several advantages which include easy separation, better
recyclability, costꢀeffectiveness, etc. As a result, during the past few years, numerous efforts
have been made to design novel heterogeneous catalysts4 by immobilizing Pd salts or
complexes into/onto various supports such as silica,5 MCMꢀ41,6 SBAꢀ15/SBAꢀ16,7 alumina,8
Zeolite,9 etc. Among the supports, silica remains one of the most preferred choices because of
its multiple advantages that include low cost, wide accessibility, high thermal stability,
excellent porosity and high surface reactivity which facilitate grafting of various ligand
functionalities (such as –NH2, ꢀPPh2, ꢀSH, etc.) onto its surfaces.4b,10 Although a significant
progress has already been achieved in terms of development of stable and reusable silica
supported Pd catalysts of high efficiency,5 majority of such catalysts showed good activities
only with aryl bromides / iodides and failed when aryl chlorides were used as substrates. As
an example, Costa et. al., few years back, developed a silica supported Pd catalyst that
showed moderate to good activities with aryl bromides but completely failed when aryl
chlorides were used as coupling partner with arylboronic acids.5e In a similar way, Chen et.
al. reported a silicaꢀbound Pd phosphine catalyst that showed excellent result with aryl
bromides under mild condition but very poor activity with aryl chlorides.5g Very recently, we
have developed two silicaꢀsupported Schiffꢀbase derived Pd catalysts that showed excellent
activities for the Suzuki reactions of aryl bromides and iodides under mild condition but they
miserably failed when aryl chlorides were used as substrates.11 It needs to mention that aryl
chlorides are the most desirable substrates in Suzuki reaction as these chlorides are easily
accessible and less expensive compared to corresponding bromides or iodides. Hence, the
development of new silicaꢀsupported Pd catalysts for efficient activation of aryl chlorides,
preferably with low Pd loading, still considered as one of the most challenging works in
Suzuki reaction.
Recent examples show that Pd nanoparticles (NPs) either in their colloidal form12 or
immobilized on solid supports13 are highly active catalysts for activation of aryl halides in
Suzuki reaction. As a normal practice, such NPs are synthesized via reduction of a Pd(II)
species with an external reducing agent (such as hydrazine hydrate, NaBH4), in presence of a
surfactant or ligandꢀbased stabilizer as capping agent.13c,14 Compared to surfactant, the
ligandꢀbased stabilizers are preferable as removal of residual surfactants from the reaction
mixture is a troublesome affair and thus making the system more expensive and industrially
unacceptable.
Herein, we report facile synthesis of a new silicaꢀsupported Pd nanocatalyst
(PdNP@PPh2ꢀSiO2) stabilized by an anchored phosphine and its application in the Suzukiꢀ
2

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Miyaura reactions of aryl halides (including chlorides). To the best of our knowledge, this is
the first example for preparation of highly dispersed PdNPs on silica gel without using any
external reductant or stabilizer. Noteworthy to mention that although, there are some reports
on the use of silicaꢀsupported PdNPs as active catalysts for SuzukiꢀMiyaura reaction, until
now their activities were limited only with aryl bromides and iodides5e,13b-e and very little is
known about their performance in activating aryl chlorides.
Experimental
General information
2ꢀDiphenylphosphinoethylꢀfunctionalized silica gel (PPh2ꢀSiO2: PPh2 loading, 1.05
mmol/g of silica gel; 200ꢀ400 mesh) was purchased from SigmaꢀAldrich. PdCl2 was
purchased from Acros Organics. The substrates used in the SuzukiꢀMiyaura reactions were
purchased from SigmaꢀAldrich, Spectrochem and Rankem. All other chemicals and solvents
were purchased from different Indian firms. The solvents were distilled and dried prior to use.
Powder Xꢀray diffractions (XRD) were performed on a Bruker AXS D8 Advance
Diffractometer with CuꢀKα (1.541 Ao) radiation. The SEM images were recorded in JEOL
Model JSM – 6390 LV and the EDX peaks were obtained from JEOL Model JED – 2300.
TEM images were recorded on a JEMꢀ2100 operating at 200 kV. The palladium content was
determined using ICPꢀAES of model Thermo Electron IRIS INTREPID II XSP DUO.
Adsorption of nitrogen was carried out at 77K using a Quantacrome, ASIQA 2010 analyzer
for analyzing surface area and poreꢀsize distributions of the materials. Prior to analysis the
samples were degassed by heating at 200 oC for 4 h under vacuum. Specific surface area was
calculated following the BrunauerꢀEmmettꢀTeller (BET) calculation and poreꢀsize
distribution was obtained by using the BarrettꢀJoynerꢀHalenda (BJH) pore analysis method
applied to the desorption branch of the nitrogen adsorption/desorption isotherms. The Gas
chromatographic studies of the products were carried out using Agilent 7820A GC system.
Synthesis of the palladium catalyst
1.90 g of PPh2ꢀSiO2 was added to a solution of 1.00 mmol of PdCl2 in 40 mL of
o
acetonitrile and the reaction mixture was stirred at 70 C for 6 h under nitrogen atmosphere.
The resulting solution was filtered off and the yellow solids were washed several times
through Soxhlet extraction with acetonitrile (until the extracted solvent become colorless).
The material was designated as PdNP@PPh2ꢀSiO2.
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General procedure for the Suzuki Miyaura reactions of aryl halides using Pd-catalyst
A 50 mL round bottomed flask was charged with a mixture of aryl halide (0.5 mmol),
arylboronic acid (0.75 mmol), K2CO3 (1.5 mmol), PdNP@PPh2ꢀSiO2 (0.1ꢀ0.5 mol%), solvent
(6 mL). The mixture was heated under stirring condition at appropriate temperature for the
required time. The reaction progress was monitored by thin layer chromatography using
aluminum coated TLC plates (Merck) under UV light or in some cases by GC. After
completion, the catalyst was separated by filtration and washed the residual solid with the
same reaction solvent. The filtrate was diluted with water (20 mL) and extracted with ether (3
x 20 mL). The combined extract was washed with brine (3 x 20 mL) and 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. For recycling experiments,
after each cycle of utilization, the catalyst was washed several times under refluxing
condition with iPrOH/H2O followed by DCM to remove any occluded reactants and products
o
physically attached with the catalyst surface. After drying the recovered catalyst at 80 C
overnight, transferred it to a reaction vessel containing substrates, base and solvent and
performed the reusability experiments under identical condition. The various products
separated were characterized 1H NMR and mass spectra.
Results and Discussion
Synthesis and characterization of the catalyst
The silica supported Pd catalyst was prepared by reacting commercially available 2ꢀ
diphenylphosphinoethylꢀfunctionalized silica gel (PPh2ꢀSiO2) with PdCl2 in acetonitrile at 70
oC (Scheme 1) without taking the assistance of any external reductant or stabilizer. The TEM
image of the fresh catalyst is shown in Fig. 1a and reveals formation of discrete spherical
palladium NPs without any signs of aggregations. The particle size histogram shows that the
particles sizes vary from 5ꢀ25 nm with majority of the particles are within the range of 10ꢀ15
nm (Fig. 1a). Since we have not used any reducing or capping agent, it is reasonable to
conclude that the phosphine ligand bound to silica would perform the two important roles:
firstly, reduction of Pd+2→Pd0; secondly, stabilization of NPs to promote good dispersions.
The roles of phosphines in stabilizing metal NPs are well documented in literature, however
synthesizing such NPs are always assisted by an external reducing agent.15 On the other hand,
4

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reducing properties of phosphines are also well known,16 but the utilities of phosphines to act
as reducing and stabilizing agent simultaneously in a supported system are not common. The
selected area electron diffraction (SAED) pattern of a single NP (Fig. 1b inset) shows the
reticular plan extended throughout the nanocrystal without any fault or defect. The spacing
between two crystal planes is found to be about 0.21 nm (Fig. 1b), which corresponds to the
face centre cubic (fcc) arrangement of the metal nanocrystals.17 The SEM image (Fig 2a) of
the catalyst material shows silica particles are of different dimensions. The SEMꢀEDX
spectra of the material (Fig. 2b) show the presence of Pd. From ICPꢀAES analysis, the Pd
content was found to be 0.072 mmol/g of silica gel.
The N2 adsorptionꢀdesorption isotherm measured at 77K, of the PdNP@PPh2ꢀSiO2
shows a type IV pattern with a sharp hysteresis loop, which is characteristics of mesoporous
materials18 (Fig. 3b). A specific surface area (ABET) of 100 m2/g and pore volume of 0.19
cm3/g has been obtained for the catalyst material. Compared to the supported ligand these
values are smaller indicating immobilization of Pd onto silica matrix (ESI: Table S1). The
BJH pore size distribution of PdNP@PPh2ꢀSiO2 shows that the pore diameter slightly shifts
towards the lower value in comparison with the parent supported ligand indicating generation
of NPs inside the pores resulting in decrease of pore volume and pore sizes (Fig. 3b inset).
The powder Xꢀray diffraction patterns (ESI: Fig. S1) of phosphine functionalized
silica gel, PPh2ꢀSiO2, exhibits a broad peak at 2θ = 22.403o which upon grafting of Pd shows
almost no significant change indicating that silica maintains its mesoporous structure even
after palladium loading. The PdNP@PPh2ꢀSiO2 material also shows three additional low
intensity diffraction peaks corresponds to (111), (200) and (220) planes of the Pd(0)
nanoparticles.19
Catalytic activity
The SuzukiꢀMiyaura activity of the silicaꢀbased material, PdNP@PPh2ꢀSiO2, was first
evaluated for aryl bromides. The screening reactions were performed using pꢀbromoanisole
o
and phenylboronic acid as model substrates in presence of 0.5 mol% catalyst at 100 C, as
this condition was found to give best results with other reported heterogeneous catalysts.5e,6b
Initial solvent and base optimization study (Fig. 3) with selected bases and solvents revealed
i
that PrOHꢀH2O (1:1) was the best solvent and K2CO3 was the best base. In fact, under the
condition defined above, quantitative conversion of pꢀmethoxybiphenyl was obtained by GC
after 2 h of reaction time (ESI: Table S2, entry 1). Interestingly, this excellent yield was
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maintained even after reducing the catalyst loading up to 0.1 mol% (entry 6). Further, the
temperature optimization study revealed that the catalyst was virtually inactive at room
temperature producing only 16% yield after 6 h (entry 4). The optimum temperature for the
model reaction was found to be 60 oC (ESI: Table S2, entry 3). With the optimized condition
(iPrOHꢀH2O, K2CO3, 0.1 mol% Pd, 60 oC), we examined the scope of our catalyst for several
representative coupling reactions with a range of aryl bromides and the results are
summarized in Table 1. Our result shows that steric and electronic natures of the substrates
have a little influence on our system. Aryl bromides bearing electronꢀdeficient (pꢀCHO),
electronꢀneutral and electronꢀrich (pꢀOCH3) groups reacted efficiently with phenylboronic
acid producing corresponding biaryls in excellent yields (Table 1, entries 1ꢀ3). This high
yields were also maintained with different arylboronic acids (Table 1, entries 10 and 11).
Under slightly extended reaction time, our catalyst also tolerates sterically demanding
substrates such as oꢀbromobenzaldehyde and 1ꢀbromoꢀ2,5ꢀdimethoxybenzene and gave good
yields (Table 1, entries 5 and 6). It is interesting to note that the heteroaromatic halides are
considered to be difficult substrates to activate in SuzukiꢀMiyaura reaction and often they
gave low yields.20 However, in our case, heteroaryl bromides such as 5ꢀbromopyrimidine and
2ꢀbromoꢀ3ꢀmethylthiophene gave moderate yields at the present condition. Nevertheless,
o
increasing the reaction temperature (80 C) and catalyst loading (0.2 mol%) substantial
increase in yields were achieved (Table 1, entries 8 and 9).
Encouraged by the results of our catalyst with aryl bromides, we turned our attention
to the use of aryl chlorides as substrates. It needs to mention that there are several reports of
the use of silicaꢀsupported Pd catalysts for SuzukiꢀMiyaura reaction but, only very few of
them are successful for promoting aryl chlorides.5i,5j Thus, in order to expand the scope of our
catalyst for aryl chlorides, initially we have conducted the screening reaction with pꢀ
chloronitrobenzene and phenylboronic acid as coupling partners using the same optimum
condition used for aryl bromides. Our result suggests that this experimental condition is
virtually ineffective for the coupling reaction of aryl chlorides as only 17% biaryl was
achieved after 6 h of reaction time (ESI: Table S3, entry 1). However, search for an
alternative condition revealed that changing the solvent to DMF, increasing the temperature
o
to 100 C and increasing the catalyst loading to 0.5 mol% almost quantitative conversion of
biaryl has been achieved (ESI: Table S3, entry 5). Our study reveals that 0.5 mol% catalyst
loading is the lower limit of our catalyst for completion of the reaction. On decreasing the
catalyst loading to 0.2 mol%, only 63% (ESI: Table S3, entry 9) nitrobiophenyl was formed.
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A further decrease in catalyst loading to 0.1 mol%, only 43% product formation was achieved
(ESI: Table S3, entry 10). Under the same set of experimental condition, we have examined
the crossꢀcoupling reactions of a range of electronically and sterically diverse aryl chlorides
with phenylboronic acid and the results are depicted in Table 2. Our result establishes that
aryl chlorides bearing electronꢀwithdrawing group such as pꢀchloronitrobenzene / pꢀ
chloroacetophenone (entries 1 and 6), electronꢀdonating group such as pꢀchloroanisole (entry
7) and electronꢀneutral group such as chlorobenzene (entry 4) underwent smooth coupling
with phenylboronic acid affording the desired biphenyls in goodꢀexcellent yields.
Interestingly, our catalyst can also tolerate sterically bulky orthoꢀsubstituted chloride such as
oꢀchloronitrobenzene (entry 3) and gave the desired product in reasonably good yields. No
major differences in yields were noticed by replacing phenylboronic acid with pꢀtolylboronic
acid (entry 10) and pꢀchloroboronic acid (entry 11).
Reusability and heterogeneity test
Reusability is one of the most important properties of any heterogeneous catalyst to
determine its sustainability and to check this property, the catalyst material in the model
reaction between 4ꢀbromoanisole and phenylboronic acid was separated from the reaction
i
mixture by centrifugation and subsequent decantation, washed thoroughly with PrOHꢀH2O
(1:1), dried and reused for subsequent runs. The percentage conversions are diagrammatically
represented in ESI: Fig. S2, which shows that the catalyst could be reused at least for six
times without comprising with its activity. In order to check the extent of Pd leaching, if any,
into the solution we have performed ICPꢀAES analysis of the filtrate after 1st run, and only
negligible amount (<1ppm) of palladium was detected in the solution. To confirm this fact
further, we have also performed the hot filtration test by using pꢀbromoanisole and
phenylboronic acid as coupling partners. The reaction was stopped after 20 min. and the yield
was determined to be about 30% (analyzed by GC). The solid catalyst was then separated
from the solution by filtration under hot condition and the filtrate was then allowed to
continue stirring to react for another 3 h. The product concentration in the filtrate was found
to be almost constant (Fig. 5), indicating that no Pd species was leached into the solution and
the catalyst was truly heterogeneous in nature. A slight change in surface morphology of the
silicaꢀbased material before and after catalysis has been observed from the SEM pictures. The
after catalysis material shows a little decrease in the silica particle size (Fig. 2c) however, this
does not affect the reusability of the catalyst. The SEMꢀEDX (Fig. 2d) spectra and also the
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TEM image of the six times used catalyst clearly shows the presence of Pd on silica matrix
which suggests that the catalyst could be used for further runs.
Conclusion
In summary, we have developed a silicaꢀsupported Pd nanocatalyst stabilized by an anchored
phosphine without taking assistance of an external reductant or stabilizer. The supported
catalyst exhibited excellent activity for the SuzukiꢀMiyaura crossꢀcoupling reactions of aryl
bromides and chlorides with a low catalyst loading (<0.5 mol %). The catalyst could be
reused several times without compromising with its activity.
Acknowledgement
The Council of Scientific and Industrial Research (CSIR), New Delhi, is gratefully
acknowledged for financial support (Grant no: 02/0081/12). D. S. thanks the University
Grants Commission (UGC), New Delhi for BSRꢀRFSMS fellowship. DST, New Delhi has
also been acknowledged for providing Surface Area Analyzer to the Department under DSTꢀ
FIST programme.
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Table 1 SuzukiꢀMiyaura cross coupling reactions of aryl/heteroaryl bromides with
arylboronic acidsa with PdNP@PPh2ꢀSiO2 as catalyst
Entry
R-Br
R'-B(OH)2
Time (h) Yield (%)b,c
TONd
1
3
3
98
94
980
2
940
3
4
5
3
3
4
96
98
88
960
980
880
6
7
4
4
92
91
920
910
4
58 (92)e
580 (460)
8
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4
4
66 (95)e
660 (475)
9
98
980
10
11
4
92
920
aReaction conditions: 0.5 mmol aryl bromide, 0.75 mmol arylboronic acid, 1.5 mmol K2CO3,
6mL iPrOH/H2O (1:1).
bIsolated Yield.
cYields are average of two runs.
dTON, product mole /Pd mole
eValues in parenthesis are yields for reaction with catalyst: 0.2 mol% ; T = 80 oC.
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Table 2 SuzukiꢀMiyaura cross coupling reactions of aryl chlorides with arylboronic acidsa
with PdNP@PPh2ꢀSiO2 as catalyst
Entry
R-Cl
R'-B(OH)2
Time (h) Yield (%)b.c
TONd
1
5
5
93
72
186
2
3
146
132
5
66
4
5
6
5
6
6
95
77
85
190
154
170
7
8
5
5
86
88
172
176
9
5
78
156
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10
11
5
5
86
89
172
178
aReaction conditions: 0.5 mmol aryl chloride, 0.75 mmol arylboronic acid, 1.5 mmol K2CO3,
6mL DMF.
bIsolated Yield.
cYields are average of two runs.
dTON, product mole /Pd mole.
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Legends: Scheme, Tables and Figures
Scheme 1: Synthesis of the silica supported palladium catalyst PdNP@SiO2ꢀPPh2.
Fig. 1 (a) TEM micrographs of PdNP@SiO2ꢀPPh2 and particle size histograms with a Gaussian
curve fitting (inset); (b) HRTEM image and SAED pattern (inset) of PdNP@SiO2ꢀPPh2
Fig. 2a SEMꢀEDX spectra of PdNP@SiO2ꢀPPh2 (before catalysis).
Fig. 2b SEMꢀEDX spectra of PdNP@SiO2ꢀPPh2 (after 6th use).
.
Fig. 3 N2 adsorptionꢀdesorption isotherms and BJH pore size distribution curves (inset) of (a)
PPh2ꢀSiO2 (b) PdNP@PPh2ꢀSiO2
Fig. 4 Comparison of yields obtained for solvents using different types of bases.
Fig. 5 Hot filtration test for the coupling reaction between pꢀbromoanisole and phenylboronic
acid catalyzed by PdNP@PPh2ꢀSiO2.
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Scheme 1
16

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40
35
30
25
20
15
10
5
(a)
(b)
0.21
0
0
5
10
20
25
30
Size1(5nm)
Fig. 1
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Fig. 2a
Fig. 2
Fig. 2b
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3.5
3.0
2.5
2.0
1.5
1.0
0.5
1.0
0.8
0.6
0.4
0.2
0.0
3.0
1.0
0.8
0.6
0.4
0.2
0.0
(a)
(b)
2.5
2.0
1.5
1.0
0.5
6
2
4
10
8
Pore Diameter (nm)
2
4
8
10
6
Pore Diameter (nm)
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
RelativePressureP/P
RelativePressureP/P
0
0
Fig. 3
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Fig. 4
100
80
60
40
20
0
Without Filtration
Hot Filtered
0
1
2
3
Time (h)
Fig. 5
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Phosphine-stabilized Pd nanoparticles supported on silica as highly active catalyst for
Suzuki-Miyaura cross-coupling reaction
Debojeet Sahu and Pankaj Das
Textual abstract
Highly dispersed silica-supported Pd nanoparticles have been synthesized via a simple
strategy without taking assistance of an external reductant or stabilizer. The nano material
showed excellent activity as catalyst for the Suzuki-Miyaura cross-coupling reactions of aryl
bromides and chlorides. The nanocatalyst could be recovered by simple filtration and reused
several times without loss of its activity.