Isolable C@Fe3O4 nanospheres supported cubical Pd nanoparticles as reusable catalysts for Stille and Mizoroki-Heck coupling reactions

Article abstract of DOI:10.1016/j.tetlet.2017.07.025

Cubical Pd nanoparticles incorporated magnetic nanospheres (Pd cNPs/C@Fe₃O₄) are found to be efficient catalysts for Stille and Mizoroki-Heck coupling reactions. A variety of aryl halides, including chlorides, are converted to biaryls and diphenylethenes with excellent yield and high TON. This immobilization of Pd cNPs on the surface C@Fe₃O₄ results in structurally stable catalytic sites. The observed enhanced catalytic activity is attributed to the high density of low-coordinated Pd {1?0?0} atoms present at the surface of the Pd cNPs/C@Fe₃O₄ catalyst. The advantages of the proposed catalytic system are its heterogeneity, high stability, absence of any toxic ligands, gram scale applicability, magnetic separability and consequent reusability.

Full text of DOI:10.1016/j.tetlet.2017.07.025

Accepted Manuscript
Isolable C@Fe3O4 Nanospheres Supported Cubical Pd Nanoparticles as Reus-
able Catalysts for Stille and Mizoroki-Heck Coupling Reactions
Basuvaraj Suresh Kumar, Rajagopal Anbarasan, Arlin Jose Amali, Kasi
Pitchumani
PII:
S0040-4039(17)30871-7
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.07.025
TETL 49107
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
7 June 2017
4 July 2017
6 July 2017
Please cite this article as: Kumar, B.S., Anbarasan, R., Amali, A.J., Pitchumani, K., Isolable C@Fe3O4 Nanospheres
Supported Cubical Pd Nanoparticles as Reusable Catalysts for Stille and Mizoroki-Heck Coupling Reactions,
Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.2017.07.025
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Isolable C@Fe3O4 Nanospheres Supported Cubical Pd Nanoparticles as Reusable
Catalysts for Stille and Mizoroki-Heck Coupling Reactions
Basuvaraj Suresh Kumar,a Rajagopal Anbarasan,a Arlin Jose Amali*a,b and
Kasi Pitchumani*a,b
aDepartment of Natural Products Chemistry, School of Chemistry, Madurai Kamaraj
University, Madurai – 625021, Tamilnadu, India.
bCentre for Green Chemistry Processes, School of Chemistry, Madurai Kamaraj University,
Madurai-21, Tamilnadu, India.
Email: pit12399@yahoo.com & jose.arlin@gmail.com
Phone: 91-0452-2456614(O), 91-0452-2383945(R); Fax: 91-0452-2459181, 2459105
Abstract
Cubical Pd nanoparticles incorporated magnetic nanospheres (Pd cNPs/C@Fe3O4) are
found to be efficient catalysts for Stille and Mizoroki-Heck coupling reactions. A variety of
aryl halides, including chlorides, are converted to biaryls and diphenylethenes with excellent
yield and high TON. This immobilization of Pd cNPs on the surface C@Fe3O4 results in
structurally stable catalytic sites. The observed enhanced catalytic activity is attributed to the
high density of low-coordinated Pd {100} atoms present at the surface of the Pd
cNPs/C@Fe3O4 catalyst. The advantages of the proposed catalytic system are its
heterogeneity, high stability, absence of any toxic ligands, gram scale applicability, magnetic
separability and consequent reusability.
Keywords: Cubical Pd nanoparticles; Stille; Mizoroki-Heck; magnetic separation;
heterogeneous catalyst.
1

Introduction
Metal nanoparticles (NPs) based catalysts have emerged as sustainable alternatives to
conventional catalysts. Higher activities of NPs in catalysis, owing to their high surface-to-
volume ratio, have been realized in various catalytic reactions.1 However, their use is often
complicated by issues surrounding the separation of the nanocatalyst from the desired product
and its stability to retain its activity and reusability without loss of its catalytic activity.
Incorporation of NPs on magnetic support provides an additional functionality to the catalyst
system which is amenable for magnetic separation from the reaction medium.2 The catalytic
activity of NPs not only depends on size, but also on their shape and the number of surface
(vertex, edge and plane) atoms,3 and this plays a crucial role in their catalytic performance as
different crystallographic facets in different shapes may result in different reactivities and
selectivities.4
Recent investigations on the shape-dependent effect of NPs have focused on
palladium (Pd) based nanocatalysts. For example, Pd nanocrystals with different shapes such
as cubic, octahedral, cuboctahedral, octahedral and nanowires morphologies were synthesized
and their catalytic activity was studied in alkynol hydrogenation,5 Suzuki-Miyaura reaction,6
formic acid oxidation,7 direct hydrogen peroxide synthesis,8 and hydrogenation of p-
nitrophenol to p-aminophenol.9 From an industrial perspective of reusability concern, studies
showed that Pd nanocrystals with different morphologies deposited on heterogeneous
supports such as silica, titania or alumina are found to be efficient in aerobic oxidation of 1-
phenylethanol10 and oxidation of carbon monoxide.11 Though this may result in recyclable
catalysts, the stability to metal leaching is not satisfactory and also it involves difficulty in
synthesis, tiresome recovery using centrifugation, or time-consuming filtration.
In organic synthesis, the ultimate testimony of value is utility. The Pd catalyzed Heck
reaction12 now stands as a remarkably robust and efficient method for carbon–carbon bond
formation, particularly in the generation of tertiary and quaternary stereocenters and
intramolecular ring formation, and in Merck Singulair process (a Heck coupling involving
isomerization of an allylic alcohol). In addition to being a powerful intermolecular carbon–
carbon bond-forming process, the Pd catalyzed Stille reaction13 has found widespread use in
the generation of cyclic structures.14 Because of the ready availability of organostannane
reagents, their air- and moisture-stability, Stille reaction finds wide application in synthesis of
small molecules, natural products, polymers, pharmaceuticals, sunscreen agents,
carbohydrate chemistry and biologically active compounds in both industrial and academic
2

researches.15 Based on this importance and also our continued research interest in Pd based
heterogeneous catalysts,16 we have explored the catalytic activity of cubical Pd naoparticles
(Pd cNPs) having {100} surface facets, incorporated in a magnetic support (C@Fe3O4)
namely Pd cNPs/C@Fe3O4 for Stille and Mizoroki-Heck coupling reactions. We have also
compared the shape- dependent catalytic activity of Pd cNPs with spherical palladium
nanoparticles (Pd sNPs) for both Stille and Mizoroki-Heck coupling reactions.
Results and Discussion
A three-step procedure (based on our previous report)17 was used to fabricate the
magnetic nanosphere incorporated with Pd catalyst (Pd/C@Fe3O4). It was characterized using
PXRD, XPS (Figures S1 and S2, Supporting Information), TEM and ICP-OES analyses. The
as synthesized material shows a core-shell feature (C@Fe3O4) with Pd uniformly deposited
on the surface, resulting in a magnetic nanoreactor (Pd cNPs/C@Fe3O4).
a)
b)
c)
20 nm
d)
e)
f)
Figure 1. TEM images of a) Pd cNPs, b) Pd sNPs, c) Fe3O4, d) C@Fe3O4, e) Pd
sNPs/C@Fe3O4 and f) Pd cNPs/C@Fe3O4.
Transmission Electron Microscopic (TEM) images of Pd cNPs indicate a cubic
structure with an edge length of 13 nm and surfaces covered by {100} facets (Figure 1a).
−4 nm in size (Figure 1
Similarly, TEM images of Pd sNPs show 3
b). TEM studies confirmed
the structure of Pd sNPs, Pd cNPs, Fe3O4, C@Fe3O4 and firmly incorporated Pd sNPs, Pd
cNPs on the surface of C@Fe3O4 nanospheres (Figures 1a-f). The development of carbon
shell on Fe3O4 core can protect the Fe3O4 particles from being corrupted and oxidised by
acids and oxygen; The presence of functional groups such as -COOH, -OH, and -C=O on
carbon shell can also be beneficially used to immobilize the catalytic active metal species to
3

make them stable and recyclable.18 Pd content in the Pd cNPs/C@Fe3O4 and Pd
sNPs/C@Fe3O4 is quantitatively determined by ICP-OES analysis and the values are 3.66 ×
10−6 mol and 3.10 × 10−6 mol respectively. The synthesized Pd cNPs/C@Fe3O4 is used for
the Stille and Mizoroki-Heck coupling reactions. (Scheme 1)
Stille Coupling
Scheme 1: Pd cNPs/C@Fe3O4 catalyzed Stille and Mizoroki-Heck coupling reactions.
Under the optimised reaction conditions (Table S1, Supporting Information), the
scope of the reaction was further studied in a wide variety of diversely substituted aryl
iodides, bromides and chlorides with tri-n-butylphenylstannane. It was observed that aryl
iodides containing electron-withdrawing and electron-donating groups gave excellent yields.
When compared to para-substituted reactants (Table 1, entries 2-3), meta-substituted aryl
iodides gave lower yields (Table 1, entry 4). Aryl bromides also reacted readily giving
excellent yields (Table 1, entries 5-11). In addition, heterocyclic compounds such as 2-acetyl-
5-bromothiophene, 3-bromopyridine (Table 1, entries 12-13) also reacted readily with tri-n-
butylphenylstannane to give the corresponding coupled products. Studies were also extended
to less reactive chloro substrates and very poor yields were obtained. While adding TBAB as
additive an increase in the amount of catalyst loading (1.5 mol %) increased the product yield
(Table 1, entries 15-20).
a
Table 1: Stille coupling reaction of substituted aryl halides with Pd cNPs/C@Fe3O4 as
catalyst.
Entry
Reactant
Product
Time
(h)
Yield TON
(%)
1
2
3
10
10
10
>99
92
135
126
120
88
4

4
10
18
82
95
112
130
5
6
18
99
135
7
8
18
18
18
18
>99
92
135
126
131
135
9
96
10
11
>99
18
18
90
82
123
112
12
13
18
18
85
92
116
126
14
15c
24
24
78
70
53
48
16c
17c
24
82
56
18c
19c
20c
24
24
24
80
68
55
55
46
37
aReaction conditions: Aryl halide (0.5 mmol), tri-n-butylphenylstannane (0.6 mmol),
b
K2CO3 (1.5 mmol), DMF (3 mL) and catalyst (20 mg, 0.73 mol % of Pd). Isolated
yield. cTBAB (0.5 mmol), catalyst (40 mg, 1.5 mol % of Pd) for chloro substrates.
Mizoroki-Heck coupling reaction
Scheme 2: Pd cNPs/C@Fe3O4 catalyzed Stille and Mizoroki-Heck coupling reactions.
5

After identifying the optimal conditions (Table S2, Supporting information) for the
Mizoroki-Heck coupling catalyzed by Pd cNPs/C@Fe3O4 (Scheme 2), the activity of a wide
range of aryl iodides and aryl bromides is evaluated and the results are summarized in table 2.
Aryl iodides containing electron-withdrawing groups gave excellent yields when compared to
aryl iodides containing electron-donating groups. For example, 4-COCH3 substituted aryl
iodide gave 99 % yield when compared to 3,4-dimethoxy substituted aryl iodides, which gave
85 % of yield (Table 2, entries 2 and 4). Similar results were also observed with aryl
bromides. Aryl bromides containing electron-withdrawing groups such as -NO2, -CHO, -CN,
-COCH3, -CF3 gave excellent yield when compared to electron-donating groups substituted
aryl bromide -CH3 (Table 2, entries 6-11). In addition, heterocyclic 3-bromopyridine (Table
2, entries 12) also reacted with styrene to give the corresponding ethene product in 70 %
yield. Studies are also extended to less reactive aryl chlorides which furnished very poor
yield. Upon addition of TBAB and increasing the amount of catalyst loading (1.5 mol %),
moderate yield was observed after 24 h (Table 2, entries 13-15).
a
Table 2: Substrate variation for Pd cNPs/C@Fe3O4 catalyzed Mizoroki-Heck Coupling
reaction.
Entry Halobenzene
Product
bYield(%)
TON
1
>99
135
2
3
>99
90
135
123
COCH3
OCH3
4
85
116
CH3
CH3
5
6
90
92
123
126
CF3
6

7
8
9
82
90
88
112
123
120
10
11
12
>99
75
135
102
96
70
13c
14c
15c
55
58
48
38
40
33
aReaction conditions: Styrene (0.5 mmol), arylhalide (0.5 mmol), K2CO3 (1.5
b
mmol), DMF (3 mL) and catalyst (20 mg, 0.73 mol % of Pd); Isolated yield;
cTBAB (0.5 mmol), catalyst (40 mg, 1.5 mol % of Pd) was used for chloro
substrates.
Figure 2: Pictorial representation for the gram scale reaction of Mizoroki-Heck coupling
reaction using Pd cNPs/C@Fe3O4 as catalyst.
7

Mizoroki-Heck reaction was found to be very clean. The pure products were obtained
by a simple workup involving magnetic separation followed by column chromatography. In a
scaled-up gram scale reaction, Mizoroki-Heck reaction between iodobenzene (15 mmol, 1.65
mL), styrene (15 mmol, 0.86 mL), DMF (50 mL), K2CO3 (45 mmol, 6.2 gms) and Pd
cNPs/C@Fe3O4 (600 mg, 0.73 mol % of Pd) provided as isolated yield of 85 % (TON = 116).
This result clearly indicates that Pd cNPs/C@Fe3O4 constitutes a practical catalyst for
Mizoroki-Heck coupling reaction amenable to gram scale operations, which can be readily
extendable to the synthesis of various diphenylethene derivatives. Hence, it can be used as
versatile catalysts for industrial applications.
We have also studied the shape-dependent catalytic activity of Pd nanocatalysts in
Stille and Mizoroki-Heck coupling reactions by comparing the reactivity of Pd cNPs with
excess of {100} surface facets, and Pd sNPs with mixed surface facets. Interestingly, the
enhanced catalytic activity of the {100} surface facets of Pd cNPs is apparent with the yield
of biphenyl from Stille coupling between iodobenzene with tri-n-butylphenylstannane
increasing from 62 % for Pd sNPs having mixed surface facets to 99 % for Pd cNPs consist
of {100} surface facets. This represents more than 1.3 fold increase in the TON for Pd cNPs
compared to Pd sNPs (Table 3). Similarly, the yield of diphenylethene in Mizoroki-Heck
coupling reaction between iodobenzene and styrene has increased from 43 % for Pd sNPs to
99 % for Pd cNPs, representing nearly 2 fold increase in the TON for Pd cNPs compared to
Pd sNPs (Table 3).
To rationalize that shape effect is more important than the size effect, Pd sNPs with
the size of 10-15 nm which is almost of similar size to Pd cNPs were prepared and deposited
on the surface of C@Fe3O4. The catalytic activity of both Pd cNPs and Pd sNPs having
almost similar size of 10-15 nm (Figure S3, Supporting Information) are also studied in both
Stille and Mizoroki-Heck coupling reaction. The enhanced catalytic activity of the {100}
surface facets of Pd cNPs is apparent with the yield of biphenyl from Stille coupling between
iodobenzene with tri-n-butylphenylstannane increasing from 50 % for Pd sNPs having mixed
surface facets to 99 % for Pd cNPs consisting of {100} surface facets. This represents more
than 1.7 fold increase in the TON for Pd cNPs compared to Pd sNPs (Table 3). Similarly, the
yield of diphenylethene in Mizoroki-Heck coupling reaction between iodobenzene and
styrene has increased from 35 % for Pd sNPs to 99 % for Pd cNPs, representing nearly 2.5
fold increase in the TON for Pd cNPs compared to Pd sNPs (Table 3).
The origin of the enhanced reactivity of Pd cNPs is attributed to the high density of
low-coordinated {100} surface atoms present at the surface of the catalyst.6 Cubical Pd
8

nanoparticles (Pd cNPs) have higher catalytic activity than that of their spherical counterpart
(Pd sNPs) since the cubic one has a larger particle size (13 nm), than that of the spherical one
(3-4 nm), suggesting that the {100} facet on the surface accounts for the higher catalytic
activity. Also, despite their larger particle size (13 nm), than the spherical one (3-4 nm) with
the same Pd metal content, Pd cNPs show much higher activity than the Pd sNPs, indicating
that the shape effect is more important than the size effect in this case. This observation that
the {100} facet of the Pd is highly active for the Stille and Mizoroki-Heck coupling reaction,
clearly demonstrates that controlling the shape of the nanoparticles to expose more facets of
high activity is indeed a very effective strategy to optimize the performance of the catalyst.
Table 3: Shape-dependent catalytic activity of Pd (cubical and spherical) nanoparticles in
Stille and Mizoroki-Heck coupling reaction.
Catalyst (mol of Pd)
Stille coupling reactiona
Yield.
(%)c
Time TON
(h)
Pd cNPs/C@ Fe3O4 (3.66 x 10-6)
Pd sNPs/C@ Fe3O4 (3.10 x 10-6)
Pd sNPs/C@ Fe3O4 -10-15 nm size of Pd (3.30 x 10-6)
Mizoroki-Heck coupling reactionb
99
10
10
10
135
100
76
62
50
Pd cNPs/C@ Fe3O4 (3.66 x 10-6)
Pd sNPs/C@ Fe3O4 (3.10 x 10-6)
99
43
12
12
135
70
Pd sNPs/C@ Fe3O4 -10-15 nm size of Pd (3.30 x 10-6)
35
12
53
aReaction conditions: Iodobenzene (0.5 mmol), tri-n-butylphenylstannane (0.6 mmol), catalyst
b
(20 mg), DMF (3 mL), 10 h, 100oC. Reaction conditions: Iodobenzene (0.5 mmol), styrene
(0.5mmol), catalyst (20 mg), DMF (3 mL), 12 h, 100 oC. cIsolated yield.
a
Table 4: Results of Sheldon test for the Pd cNPs/C@Fe3O4 catalyzed Mizoroki-Heck
coupling reaction.
Catalyst
bYield (%)
(6+6) h
48
6 h
46
Reused catalyst
98
Pd cNPs/C@Fe3O4
aReaction Conditions: Iodobenzene (0.5 mmol), styrene (0.5 mmol), catalyst (0.73 mol %
of Pd, 20 mg), K2CO3 (1.5 mmol), 100 oC, DMF (3 mL); bisolated yield.
The heterogeneity of the Pd cNPs/C@Fe3O4 catalyst was checked by carrying out a
hot filtration test (Table 4) with iodobenzene and styrene as substrates, to find out whether Pd
is leaching out from the solid support to the solution or whether the catalyst is truly
heterogeneous in nature. After continuing the reaction under optimized conditions for about 6
9

h, the catalyst was filtered under hot conditions from the reaction mixture with (46 %)
formation of 1,2-diphenylethene. With the remaining filtrate, a fresh reaction under the same
conditions without the catalyst was performed. After heating for an additional 6 h, the
mixture was cooled and then extracted with ethyl acetate and 48 % of product was observed.
This marginal increase in conversion is attributed to negligible amount of Pd {100} surface
atoms, which were leached from the surface of Pd cNPs under hot conditions. Even the
leached Pd atoms, if any can be easily reabsorbed on cooling in a “boomerang”-like process
onto the support.19 These results clearly demonstrate that Pd cNPs/C@Fe3O4 is truly
heterogeneous in nature.
a)
99
99
96
94
85
90
Fresh Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5
b)
c)
Figure 3: a) Recycling of Pd cNPs/C@Fe3O4 catalyst and b,c) TEM images of reused
catalyst (Pd cNPs/C@Fe3O4)
Reaction condition for Mizoroki-Heck coupling reaction for reusability tests: Iodobenzene (0.5
mmol), styrene (0.5 mmol), catalyst PdcNPs/C@Fe3O4 (20 mg, 0.73 mol %), K2CO3 (1.5 mmol),
DMF (3 mL), time (12 h) and temp. (100 oC).
One of the main advantages of using the present heterogeneous catalyst (Pd
cNPs/C@Fe3O4) from an industrial perspective is that it can be recovered and reused
efficiently. After completion of the reaction, the catalyst was separated by an external magnet
and washed with water and EtOH to dry at room temperature in a vacuum desiccator for 2 h.
The dried catalyst was reused upto five consecutive cycles to afford good yield (Figure 3a).
After five cycles, Pd ions are still present on the surface of C@Fe3O4 (Figure 3b,c). ICP-OES
analyses of reused (after 5 times) catalyst shows 2.9×10−6 mol of Pd which shows that a small
yet significant leaching of Pd was observed in the reused catalyst. Since cubical nanoparticles
10

consist mostly of {100} surface atoms, and from a careful look into the TEM images (Figure
3b,c), it seems that the shape of Pd cNPs may have changed after reuse, due to removal of
small number of {100} surface atoms. This is also evident from the observation that the size
of the cubical NPs decreases from 13 nm to 8-10 nm. However, the powder XRD pattern of
reused (after 5 times) catalyst shows that it still retains {100} surface facets on the Pd cNPs
(Figure S4, Supporting information). We believe that this observation coupled with the very
high reactivity of Pd cNPs is responsible for the higher activity of the present catalytic system
even after 5th cycle with only a small decrease in the overall yield.
Our catalytic system was also compared with reported Pd containing catalytic systems
for Stille coupling aryl halides with tri-n-butylphenylstannane (supporting information, table
S1) and Mizoroki-Heck coupling in aryl halides with vinyl derivatives (supporting
information, table S2). The other reported catalytic systems have higher Pd content, absence
of gram scale synthesis and some of them are not suitable for less reactive chloro substrates.
On the other hand, our catalytic system is more efficient in terms of facile synthesis of
catalyst, higher catalytic activity with lower Pd content (0.73 mol %), wide substrate scope,
heterogeneous nature, reusability involving using an external magnet (figure 2) and
applicability to gram scale synthesis.
Conclusions
In summary, we have developed efficient C@Fe3O4 magnetic core-shell nanospheres
as a good support for cubical Pd NPs with excessive {100} surface facets (Pd
cNPs/C@Fe3O4). This is used as a heterogeneous catalyst in Stille coupling reaction between
aryl halides with tri-n-butylphenylstannane and in Mizoroki-Heck coupling reaction of aryl
halides with styrene. A variety of aryl iodides, bromides and chlorides reacted with tri-n-
butylphenylstannane and styrene to afford the corresponding biphenyl and diphenylethene
derivatives in good to excellent yields. The origin of the enhanced catalytic activity is
attributed to presence of excess of {100} surface facets on the catalyst. The as prepared
catalyst is better in terms of facile synthesis of catalyst, higher catalytic efficiency with lower
Pd content, heterogeneous nature, wide substrate scope, reusability involving applying an
external magnet, gram scale applicability and ligand free-methodogy.
11

Acknowledgements
BSK gratefully acknowledges financial support from UGC, New Delhi, for UGC-BSR-SRF.
AJA thanks to DST, New Delhi for DST-INSPIRE Faculty Fellowship. KP acknowledges
financial support provided by CSIR, New Delhi.
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Highlights
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An efficient heterogeneous catalyst Pd cNPs/C@Fe3O4 was developed.
Stille and Mizoroki-Heck coupling reactions were performed with good yield.
Facet dependent catalytic activity of Pd cNPs and Pd sNPs were studied.
Wide substrate scope, magnetically recyclable and gram scale operation.
Alternative synthetic strategies for construction of biaryl and diphenylethene.
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Isolable C@Fe3O4 Nanospheres Supported Cubical Pd Nanoparticles as Reusable
Catalysts for Stille and Mizoroki-Heck Coupling Reactions
Graphical abstract
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