Ultrasound-accelerated synthesis of biphenyl compounds using novel Pd(0) nanoparticles immobilized on bio-composite

Article abstract of DOI:10.1016/j.ultsonch.2018.03.017

This study describes (i) an eco-friendly approach for design of Pd(0) nanoparticles on a natural composite, which is composed of carboxymethyl cellulose/agar polysaccharides (CMC/AG), without using any toxic reducing agents and (ii) development of ultrasound assisted simple protocol for synthesis of biphenyl compounds. Chemical characterization studies of Pd(0) nanoparticles (Pd NPs@CMC/AG) revealed that size of the particles were in the range of 37–55 nm. Catalytic performance of Pd NPs@CMC/AG was evaluated in synthesis of various biphenyl compounds by using the ultrasound-assisted method that was developed in this study. Pd NPs@CMC/AG exhibited excellent catalytic performance by producing high reaction yields. In addition, Pd NPs@CMC/AG was successfully used up to six reaction cycles without losing its catalytic activity, indicating high reproducibility of Pd NPs@CMC/AG. Additionally, compared to conventional the methods, new ultrasound-assisted synthesis technique that was followed in this study exhibited some advantages such as shorter reaction time, greener reaction conditions, higher yields and easier work-up.

Full text of DOI:10.1016/j.ultsonch.2018.03.017

Accepted Manuscript
Ultrasound-accelerated synthesis of biphenyl compounds using novel Pd(0)
nanoparticles immobilized on bio-composite
Talat Baran
PII:
S1350-4177(18)30307-9
DOI:
https://doi.org/10.1016/j.ultsonch.2018.03.017
ULTSON 4130
Reference:
Ultrasonics Sonochemistry
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Revised Date:
Accepted Date:
25 February 2018
17 March 2018
20 March 2018
Please cite this article as: T. Baran, Ultrasound-accelerated synthesis of biphenyl compounds using novel Pd(0)
nanoparticles immobilized on bio-composite, Ultrasonics Sonochemistry (2018), doi: https://doi.org/10.1016/
j.ultsonch.2018.03.017
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Ultrasound-accelerated synthesis of biphenyl compounds using novel Pd(0)
nanoparticles immobilized on bio-composite
Talat Baran
Aksaray University, Faculty of Science and Letters, Department of Chemistry, 68100
Aksaray, Turkey
Corresponding author:
Address: Department of Chemistry, Faculty of Science and Letters, Aksaray University,
Aksaray, Turkey, 68100
Tel.: +90 382 2882162; fax: +90 382 2882299.
E-mail address: talatbaran_66@hotmail.com (T. Baran)
1

Abstract
This study describes (i) an eco-friendly approach for design of Pd(0) nanoparticles on
a natural composite, which is composed of carboxymethyl cellulose/agar polysaccharides
(CMC/AG), without using any toxic reducing agents and (ii) development of ultrasound
assisted simple protocol for synthesis of biphenyl compounds. Chemical characterization
studies of Pd(0) nanoparticles (Pd NPs@CMC/AG) revealed that size of the particles were in
the range of 37–55 nm. Catalytic performance of Pd NPs@CMC/AG was evaluated in
synthesis of various biphenyl compounds by using the ultrasound-assisted method that was
developed in this study. Pd NPs@CMC/AG exhibited excellent catalytic performance by
producing high reaction yields. In addition, Pd NPs@CMC/AG was successfully used up to
six reaction cycles without losing its catalytic activity, indicating high reproducibility of Pd
NPs@CMC/AG. Additionally, compared to conventional the methods, new ultrasound-
assisted synthesis technique that was followed in this study exhibited some advantages such
as shorter reaction time, greener reaction conditions, higher yields and easier work-up.
Keywords: carboxymethyl cellulose; agar; ultrasound; biphenyl
2

1. Introduction
Recently, metallic nanomaterials have attracted great attention because of their
potential applications in electronics [1], optics materials [2], sensors [3] and biomedicine [4].
Metallic nanoparticles are also becoming increasingly important for catalytic systems [5-7].
Among noble metal nanoparticles such as Ag, Au and Pt, palladium nanoparticles (Pd NPs)
are very promising materials and therefore they have been widely used in different areas such
as fuel cell, hydrogenation, oxidation and cross-coupling reactions [8-10]. Especially, Pd NPs
play a pivotal role in synthesis of biphenyl compounds [11, 12], which are commonly used in
cosmetics, medicine and pharmacology [13, 14]. Nevertheless, Suzuki-Miyaura reactions
have some drawbacks because they require use of excessive energy, longer reaction time and
toxic organic solvents. Recently, non-conventional methods such as microwave irradiation,
photo-activated processes, sunlight and ultrasonication have become very popular and they
have been used in Suzuki-Miyaura reactions to overcome the problems addressed above. [15].
Among the non-conventional energy sources, ultrasonication one of the most powerful
method and it has been widely utilized in catalytic systems since it offers high selectivity, low
reaction time, mild conditions and operational simplicity [16-18]. Therefore, ultrasonication
technique has great potential for synthesis of organic compounds in terms of lower use of
excessive energy and formation of chemical waste.
The material that serves as support for nanoparticles has great impact on size, shape,
and distribution immobilized metallic nanoparticles on the surface [19]. These physical
properties of metallic nanoparticles directly affect their catalytic activity in applications.
Recently, researchers have reported production of different noble metal nanoparticles
stabilized on different inorganic materials such as bentonite [20], graphene oxide [21], carbon
nanotube [22] magnetic-materials [23] and mesoporous materials [24]. On the other hand,
considering their high thermal durability, large surface area, biodegradability, eco-friendly
3

nature, cost effectiveness and abundance in nature, carbohydrate polymers are also noted as
promising support materials for metal nanoparticles [25, 26]. Therefore, these superior
materials or their composites should be used as stabilizer for immobilization different noble
metallic particles and their activity should be explored in different catalytic reactions.
In this study, novel palladium nanoparticles were immobilized on carboxymethyl
cellulose/agar composite (CMC/AG) without using any toxic reducing agents. Structural
characterization of Pd NPs@CMC/AG was performed with FT-IR, TGA, SEM-EDAX, XRD
and ICP-OES analyses. SEM images showed that palladium particles had spherical
morphology and the size of the particles varied in the range of 37–55 nm. Catalytic behavior
of Pd NPs@CMC/AG was investigated in the Suzuki-Miyaura reaction under ultrasonic
sonication in water at 60°C for 30 min. The tests showed that Pd NPs@CMC/AG had good
activity in the synthesis of various biphenyl compounds via Suzuki-Miyaura reaction.
Reproducibility of Pd NPs@CMC/AG was tested in coupling reaction and it was determined
that it could be reused up to six runs with good reaction yields. Catalytic activity of Pd
NPs@CMC/AG was also tested in conventional reflux-heating system as a comparison of
ultrasonic method. The test revealed that the ultrasonication technique dramatically
accelerated the Suzuki-Miyaura reactions and higher reaction yields were obtained compared
to conventional heating system.
2. Material and methods
2.1 Materials
Carboxymethyl cellulose (CMC) Agar (AG), PdCl2, Na2PdCl4, PdCl2(CH3CN)2,
K2CO3, NaOH, KOH, Cs2CO3, phenyl boronic acid, aryl halides, ethanol, DMF and toluene
were obtained from Sigma-Aldrich
2.2 Instrumentation
4

FT-IR spectra of the CMC/AG composite and Pd NPs@CMC/AG were obtained on a
Perkin Elmer Spectrum 100 FT-IR spectrophotometer. Thermal and mechanical durability of
CMC/AG composite and Pd NPs@CMC/AG were studied on EXSTAR S11 7300 (nitrogen
atmosphere; 30–650°C heating range). The surface properties of CMC/AG composite and Pd
NPs@CMC/AG were obtained on QUANTA-FEG 250 ESEM by coating with platinum. The
analysis of Pd ion on the CMC/AG composite was determined using EDAX-Metek. X-ray
diagram of Pd NPs@CMC/AG was recorded on a Rigaku smart lab system (at 40 kV, 30 mA,
and 2θ with a scan angle of 10−80°). Palladium ion content of Pd NPs@CMC/AG was
investigated by using Perkin Elmer Optima 2100 DV Inductively Coupled Plasma (ICP)
Optical Emission Spectrometer (OES). The characterizations of biphenyl compounds were
done on GC–MS Agilent GC-7890 A- MS 5975. VWR Ultrasonic Cleaner USC-TH 50Hz
ultrasonic bath sonicator was used in the Suzuki-Miyaura cross coupling reactions.
2.3 Preparation of Pd NPs@CMC/AG
0.5 g of CMC was dissolved in 100 mL of water and then 0.5 g of AG was added to
the reaction mixture and stirred for overnight at room temperature (RT) to form CMC/AG
composite. Subsequently, solution of PdCl2 in water was added in the reaction media (0.2 g,
50 mL) and refluxed at 100°C for 5 h to ensure completely conversion of Pd(II) to Pd(0).
After this time period, the colorless reaction solution turned into dark gray, confirming
reduction Pd(II) to Pd(0). Then, the reaction mixture was cooled at RT and dark gray
palladium nanoparticles was obtained by evaporating the solvent (Scheme 1).
5

………………………………………..Scheme 1………………………………………..
2.4 General procedure of C-C coupling reactions
The mixture of 1.0 mmol aryl halides, 1.8 mmol phenyl boronic acid, 3.5 mmol base,
1x10-2 mmol Pd NPs@CMC/AG and 6 mL of water were sonicated at 60°C for 30 min.
Formation of biphenyl compounds was followed by thin layer chromatography. After the
coupling reactions, the reaction mixture was extracted with toluene. Organic phase, which
containing biphenyl compounds, was separated with separation funnel and desired coupling
products were obtained by evaporating of the solvent.
3. Results and discussion
3.1 Characterization studies
Characterization of CMC/AG composite and Pd NPs@CMC/AG were performed with
FT-IR, TGA, SEM-EDAX, XRD and ICP-OES analyses. Fig.1 demonstrates FT-IR spectra of
CMC/AG composite and Pd NPs@CMC/AG. The FT-IR spectrum of the synthesized
CMC/AG composite (Fig. 1a) showed prominent peaks at 3266, 2918, 1736, 1590, 1323 and
1039 cm−1, which were ascribed to the stretching of –OH group of AG, C–H stretching of AG,
the stretching of C=O in the substituent group of the CMC, the stretching of asymmetric
COO−, the stretching of symmetric COO−, –OH bending vibration and the stretching of C-O
bond of ether (C-O-C) and alcohol in glucose units of CMC and AG, respectively [27, 28].
6

FT-IR analysis of Pd NPs@CMC/AG showed that there were no dramatic changes in
functional groups of CMC/AG composite following the stabilization of Pd NPs on the
CMC/AG composite surface (Fig. 1b). However, it was observed that the functional group
peaks of Pd NPs@CMC/AG shifted to higher wavenumber and the intensity of the peaks
decreased compared to the spectrum of CMC/AG composite due to strongly bonding
interactions between the functional groups of CMC/AG composite and palladium ions [29].
…………………………….………Figure 1……………………………
Thermal durability of CMC/AG composite and Pd NPs@CMC/AG was explored with
TG/DTG and displayed in Fig 2. Maximum thermal degradation temperature (Tmax) of
CMC/AG composite was found as 231.8°C from the thermogram (Fig. 2a). On the other hand,
it was observed that Pd NPs@CMC/AG had slightly higher thermal stability (Tmax: 247.6°C)
than CMC/AG composite because palladium particles, which were stabilized on composite,
contributed to the thermal stability (Fig. 2b). This high thermal durability of Pd
7

NPs@CMC/AG reveals that it is a desired catalyst for catalytic reactions which require high
temperature.
…………………………….………Figure 2……………………………
SEM analysis was used to investigate CMC/AG composite and Pd NPs@CMC/AG
surface morphology and their SEM micrographs are presented in Fig. 3. As can be seen in
Fig. 3a, CMC/AG composite has very regular surface morphology like a film. In case of Pd
NPs@CMC/AG (Fig. 3b,c), the formation of small sized spherical palladium nanoparticles
are clearly seen. In addition, palladium nanoparticles had spherical morphology and the
average sizes ranged from 37 to 55 nm. Furthermore, palladium nanoparticles were
homogeneously dispersed on CMC/AG composite and no aggregation formation was
observed in the SEM image of Pd NPs@CMC/AG. These results revealed a good interaction
between designed CMC/AG composite and palladium ions. On the other hand, EDAX
analysis was performed to determine elemental composition of Pd NPs@CMC/AG (Fig. 4).
In the EDAX spectrum of Pd NPs@CMC/AG, palladium peaks were clearly observed,
indicating nanoparticles was successfully prepared. Composition of palladium on CMC/AG
composite was determined as 20.37 wt% using EDAX. Palladium content of nanoparticle was
also investigated with ICP-OES and it was found to be 19.89 wt%. Determined very close
8

palladium values showed that palladium nanoparticle was homogeneously dispersed on the
surface of the fabricated CMC/AG composite.
…………………………….………Figure 3……………………………
…………………………….………Figure 4……………………………
Crystalline nature of Pd NPs@CMC/AG was investigated with powder XRD analysis
and its pattern is given in Fig. 5. When XRD pattern was examined, three significant peaks
were observed at 40.06°, 46.74° and 68.16° which were ascribed to (111), (200) and (220)
crystallographic planes of palladium [30]. These peaks also showed the purity of palladium
nanoparticle on the surface of CMC/AG composite.
9

…………………………….………Figure 5……………………………
3.2 Catalytic activity experiments
3.2.1 Optimization studies
Before the evaluation of the catalytic performance of Pd NPs@CMC/AG, operating
conditions such as temperature, time, catalyst loading, base and solvent were investigated on
the model coupling reaction using ultrasonic assisted method. The coupling reaction of para-
iodide anisole and phenyl boronic acid was preferred as a model reaction. Influence of the
reaction temperature on ultrasound assisted Suzuki-Miyaura coupling reaction was studied in
the temperature range of 30-70°C. It was observed that the reaction yield increased with
elevating the reaction temperature up to 60°C, but higher reaction temperature did not much
contribute to the reaction yield; 30°C (45 %), 40°C (70%), 50°C (85%), 60°C (98%), 70°C
(98%). Therefore, the ideal reaction temperature was recorded as 60°C. Suzuki-Miyaura
reaction was also carried out at different reaction times under ultrasonic sonication to
determine best reaction time. The reaction yield increased from 15% to 98% with increasing
reaction times and the best reaction time was determined as 30 min according to the control
10

studies; 5 min (15 %), 10 min (23%), 15 min (41%), 20 min (65), 25 min (82), 30 min (98%).
The effect of catalyst loading was also explored on ultrasound assisted model Suzuki-Miyaura
coupling reaction using different amount of the catalyst. Catalyst loading of 1x10−2 mmol
produced the best result; 98%. Base system is one of the most important parameters affecting
the reaction yield in Suzuki-Miyaura coupling reactions. Four different inorganic bases
(K2CO3 (98%), Cs2CO3 (75%), NaOH (51%) and KOH (42%)) were tested and K2CO3 was
selected as base system because it worked efficiently compared to other bases. Additionally,
model coupling reaction was performed in various solvents such as ethanol (70%), water
(98%), DMF (40%) and toluene (35%) to determine the ideal solvent. The highest reaction
yield was reached with water. Catalytic activity of Pd NPs@CMC/AG was tested against
various Suzuki-Miyaura coupling reactions using determined operating conditions (K2CO3,
60°C, 30 min, 1×10−2 mmol catalyst, in water).
3.2.2 Catalytic behavior of Pd NPs@CMC/AG in Suzuki-Miyaura coupling reactions
Catalytic performance of Pd NPs@CMC/AG was tested in the synthesis of biphenyl
compounds which were obtained by coupling reactions of a series of aryl iodides, bromides
and chloride with phenyl boronic acid under ultrasonic sonication (Table 1). Catalytic cross
coupling reactions which containing aryl iodides as substrate showed excellent catalytic
performance in the presence of Pd NPs@CMC/AG (Table 1, entry 1-6). For example, aryl
bromide with para-substituted –OCH3 gave high reaction yield (%98). The coupling reaction
of phenyl boronic acid and meta-NO2 substituted aryl iodide (electron-attracting group)
produced reaction yield of 90%. In addition, it was observed that aryl iodides with electron-
repelling groups had lower efficiency than electron attracting groups (Table 1, entry 4-6).
Catalytic activity of Pd NPs@CMC/AG was also evaluated using different aryl bromides
containing both electron-rich and electron-poor substituents and the high reaction yields were
obtained. As seen in Table 1, OCH3-para-substituted aryl bromide gave excellent biphenyl
11

yield (94 %) in presence of Pd NPs@CMC/AG (entry 7-16). On the other hand, aryl bromides
with -OCH3 at the meta and ortho-position showed slightly lower catalytic performance
compared to the para-position due to steric hindrance effect (92% and 85%, respectively).
Following the coupling reaction of para-substituted -NO2 and -CN aryl bromides produced
also good reaction yields; 96%, 95%, respectively. Considering obtained results, the
substrates versatility of Pd NPs@CMC/AG was also investigated using various aryl chlorides
under the optimum conditions. When aryl chlorides are used as substrate, low reaction yields
are generally obtained due to their poor reactivity in Suzuki-Miyaura coupling reactions.
However, in this study, good results were obtained with aryl chlorides in presence of Pd
NPs@CMC/AG (Table 1, entry 17-23). For example, the reaction yield reached to 77% when
para-substituted -CN aryl chloride was used as substrate. On the other hand, low biphenyl
yields were obtained with aryl chlorides bearing electron donor groups. For example, after the
ultrasound assisted coupling reactions, yields of 43% and 54 % were produced with -meta and
para-substituted-CH3 aryl chlorides. Characterizations of the biphenyl compounds were
performed by GC/MS.
Furthermore, model coupling reaction was carried out under conventional heating
system (60°C, 5 h) in water to show performance of ultrasonication technique and the result is
given in Table 1 (entry 24). The catalyst gave higher reaction yield with shorter reaction time
using ultrasonic sonication than conventional heating system. This test revealed that
ultrasonication technique is more suitable for Pd NPs@CMC/AG. In addition, the model
coupling reaction was performed with all reactants except Pd NPs@CMC/AG under
ultrasonic sonication and no biphenyl product was obtained. This result revealed that Suzuki-
Miyaura reaction could not be executed without Pd NPs@CMC/AG.
Catalytic performance of different commercial palladium compounds such as PdCl2,
Na2PdCl4 and PdCl2(CH3CN)2 were investigated on the model reaction under the optimum
12

conditions to evaluate catalytic activity of Pd NPs@CMC/AG. It is clearly seen from Table 1,
Pd NPs@CMC/AG had far better catalytic performance over commercial palladium
compounds (Table 1, entry 25-27).
13

Table 1. Catalytic activity of Pd NPs@CMC/AG in Suzuki-Miyaura coupling reaction
Entry
1
X
I
I
I
I
R
Yield
98
90
84
56
59
75
85
92
94
95
91
96
78
80
59
70
51
54
69
74
77
43
54
75
45
51
47
4-OCH3
3-NO2
4-NH2
2-CH3
3-CH3
4-CH3
2-OCH3
3-OCH3
4-OCH3
4-CN
3-NO2
4-NO2
3-NH2
4-NH2
3-CH3
4-CH3
2-OCH3
3-OCH3
4-OCH3
3-NO2
4-CN
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24a
25b
26c
27d
I
I
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
I
3-CH3
4-CH3
4-OCH3
4-OCH3
4-OCH3
4-OCH3
I
I
I
Reaction conditions: 1.0 mmol aryl halides, 1.2 mmol phenyl boronic acid, 3.5 mmol K2CO3 and 1x10-2 mmol
Pd NPs@CMC/AG, 60°C, 30 min under ultrasonic sonication
a: 1.0 mmol aryl halides, 1.2 mmol phenyl boronic acid, 3.5 mmol K2CO3 and 1x10-2 mmol Pd NPs@CMC/AG
under conventional heating (60°C, 5h)
b: 1.0 mmol aryl halides, 1.2 mmol phenyl boronic acid, 3.5 mmol K2CO3 and 1x10-2 mmol PdCl2, 60°C, 30 min
under ultrasonic sonication
c: 1.0 mmol aryl halides, 1.2 mmol phenyl boronic acid, 3.5 mmol K2CO3 and 1x10-2 mmol Na2PdCl4, 60°C, 30
min under ultrasonic sonication
d: 1.0 mmol aryl halides, 1.2 mmol phenyl boronic acid, 3.5 mmol K2CO3 and 1x10-2 mmol PdCl2(CH3CN)2,
60°C, 30 min under ultrasonic sonication
14

3.2.3 Reproducibility of Pd NPs@CMC/AG
Reproducibility of catalysts is an attractive property as it decreases the production
cost, and this feature makes it useful for industrial applications [31]. At this point, reusability
of the catalyst should be investigated in the catalytic reactions. In this study, reproducibility of
Pd NPs@CMC/AG was explored as follows; following the ultrasound assisted coupling
reaction, used Pd NPs@CMC/AG was recovered with centrifugation and reactivated by
washing with hot ethanol. This regenerated process was repeated for each cycle and the same
Pd NPs@CMC/AG was used in all reproducibility studies. Reproducibility of Pd
NPs@CMC/AG was studied on specified model Suzuki-Miyaura reactions. This test showed
that Pd NPs@CMC/AG could be reused even after 6 runs (Fig. 6). This result clearly
indicated that Pd NPs@CMC/AG catalyst has practical recyclability.
…………………………….………Figure 6……………………………
15

4. Conclusions
In conclusion, Pd NPs have been successfully stabilized on prepared CMC/AG
composite by using green and simple method. Structural characterization of Pd
NPs@CMC/AG was performed with various analytic tools. These analyses showed that Pd
NPs@CMC/AG has been successfully prepared. Catalytic behavior of Pd NPs@CMC/AG
was tested in ultrasound-assisted Suzuki-Miyaura coupling reactions. It was found that aryl
halides were converted to the desired biphenyl compounds with excellent reaction yields in
the presence of Pd NPs@CMC/AG at only 30 min in water. Additionally, reproducibility test
of Pd NPs@CMC/AG indicated that it could be reused for at least six successive reactions. It
was observed that the ultrasonication technique dramatically accelerated the Suzuki-Miyaura
coupling reaction in the presence of Pd NPs@CMC/AG compared to the conventional heating
system. Consequently, both the designed catalyst and the developed ultrasound-assisted
synthesis method provided some advantages such as highly efficiency, practicability,
simplicity, inexpensiveness, and green approach for Suzuki-Miyaura coupling reactions.
16

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Figure and Scheme Captions
Fig.1 FT-IR spectra of a) CMC/AG composite b) Pd NPs@CMC/AG
Fig.2 TG/DTG curves of a) CMC/AG composite b) Pd NPs@CMC/AG
Fig.3 SEM images of a) CMC/AG composite b-c) Pd NPs@CMC/AG
Fig.4 EDAX spectra of Pd NPs@CMC/AG
Fig.5 XRD diagram of Pd NPs@CMC/AG
Fig.6 Reusability of Pd NPs@CMC/AG
Scheme 1. Synthesis pathway of Pd NPs@CMC/AG
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Highlights
Pd NPs was immobilized on CMC/AG composite using without any reduction agents
Suzuki reactions were performed under ultrasonic sonication
Pd NPs@CMC/AG showed good catalytic performance in water
Pd NPs@CMC/AG reused for many times
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