Construction of new biopolymer (chitosan)-based pincer-type Pd(II) complex and its catalytic application in Suzuki cross coupling reactions

Article abstract of DOI:10.1016/j.molstruc.2017.01.005

In this paper we described the fabrication, characterization and application of a new biopolymer (chitosan)-based pincer-type Pd(II) catalyst in Suzuki cross coupling reactions using a non-toxic, cheap, eco-friendly and practical method. The catalytic activity tests showed remarkable product yields as well as TON (19800) and TOF (330000) values with a small catalyst loading. In addition, the catalyst indicated good recyclability in the Suzuki C-C reaction. This biopolymer supported catalyst can be used with various catalyst systems due to its unique properties, such as being inert, green in nature, low cost and chemically durable.

Full text of DOI:10.1016/j.molstruc.2017.01.005

Accepted Manuscript
Construction of new biopolymer (chitosan)-based pincer-type Pd(II) complex and its
catalytic application in Suzuki cross coupling reactions
Talat Baran, Ayfer Menteş
PII:
S0022-2860(17)30005-4
10.1016/j.molstruc.2017.01.005
MOLSTR 23310
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 22 October 2016
Revised Date: 1 January 2017
Accepted Date: 2 January 2017
Please cite this article as: T. Baran, A. Menteş, Construction of new biopolymer (chitosan)-based pincer-
type Pd(II) complex and its catalytic application in Suzuki cross coupling reactions, Journal of Molecular
Structure (2017), doi: 10.1016/j.molstruc.2017.01.005.
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ACCEPTED MANUSCRIPT
Construction of new biopolymer (chitosan)-based pincer-type Pd(II) complex and its
catalytic application in Suzuki cross coupling reactions
Talat Baran*, Ayfer Menteş
Department of Chemistry, Faculty of Science and Letters, Aksaray University, Aksaray,
68100 Aksaray, Turkey
Corresponding author:
Address: Department of Chemistry, Faculty of Science and Letters, Aksaray University,
Aksaray, Turkey, 68100
Tel.: +90 382 2882135; fax: +90 382 2882125.
E-mail address: talatbaran_66@hotmail.com (T. Baran)
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Abstract
In this paper we described the fabrication, characterization and application of a new
biopolymer (chitosan)-based pincer-type Pd(II) catalyst in Suzuki cross coupling reactions
using a non-toxic, cheap, eco-friendly and practical method. The catalytic activity tests
showed remarkable product yields as well as TON (19800) and TOF (330000) values with a
small catalyst loading. In addition, the catalyst indicated good recyclability in the Suzuki C-C
reaction. This biopolymer supported catalyst can be used with various catalyst systems due to
its unique properties, such as being inert, green in nature, low cost and chemically durable.
Keywords: bio-polymer, biaryls, Schiff base, microwave, reusability
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1. Introduction
The Suzuki-Miyaura reaction is known as a carbon-carbon bond-forming process that is
extensively used for the synthesis of biaryl compounds [1-8]. Biaryl compounds are used in
many fields, such as cosmetics, medicine and pharmacology, due to them having unique
properties [9]. Palladium is one of the most important transition metals for cross coupling
reactions [10-16]. Biopolymer based palladium catalysts play an important role in Suzuki
cross coupling reactions due to their green nature [17-19]. Especially, over the last few
decades, biopolymers of biological origin have drawn much attention [20]. Among them,
chitosan has a special place due to its high thermal stability, renewability and low cost [21,
2
22]. In addition, the -NH groups on the chitosan easily enable chemical modification, like the
Schiff base, which provides coordination sites for metal ions [23, 24].
Catalyst systems with high turnover numbers (TON) and turnover frequencies (TOF) are
desired for industrial applications [25]. To obtain a high reaction yield, high TON and TOF
values for the Suzuki cross-coupling reactions, inert atmosphere, long reaction times and high
reaction temperature are required. To overcome these limitations, in recent years, researchers
have focused on the simple, green and fast microwave irradiation technique [26]. Many
studies in the literature show the use of microwave heating systems with homogenous and
heterogeneous catalysts to improve the reaction yields, TON, TOF and selectivity [27]. More
importantly, this technique can decrease the reaction time so it is not as labor intensive.
In this paper, a new biopolymer (chitosan)-based pincer-type Pd(II) catalyst was prepared for
the synthesis of biaryl compounds via Suzuki coupling reactions. Spectroscopic methods,
1
13
such as FTIR, XRD H-NMR, TGA,ICP-OES, C-NMR, SEM-EDAX, magnetic moment and
molar conductivity were employed for the structural characterization of the support material
and palladium (II) catalyst. The catalytic performance of the catalyst was studied in the
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synthesis of biaryls via Suzuki C-C cross coupling reactions (12 different coupling reactions)
under employing the microwave heating technique in a solvent-free medium. The products
were identified on a GC-MS. The catalyst exhibited high activity and selectivity with low
catalyst loading by giving higher TON and TOF; 19800 and 330000, respectively. The
catalyst retained its activity after eight cycles.
2. Materials and Methods
2.1. Materials
Chitosan, 1-naphthylamine, 2,6-diacetylpyridine, K
PdCl , Na PdCl , aryl halides, phenyl boronic acid, toluene, ethanol and methanol were
purchased from Sigma-Aldrich.
2 3 2 3 4
CO , NaOH, Na CO , KOH, MgSO ,
2
2
4
2.2 Instrumentation
The FT-IR spectra of the samples were recorded on a Perkin Elmer Spectrum 100 FTIR
1
13
spectrophotometer. H-NMR and C-NMR spectra were recorded on an Agilent 600 MHz
spectrometer. X-ray spectrums were recorded on a Rigaku D max 2000 system (at 40 kV, 30
mA and 2θ with a scan angle: 5−50°). Thermal analysis was done on an EXSTAR S11 7300
(nitrogen atmosphere; heating range: 30-650°C). The surface morphology of the samples were
determined on a QUANTA-FEG 250 ESEM. The analyses of palladium and chloride ions on
the bio-based Pd(II) catalyst were done using an EDAX-Metek. The palladium ion content of
the catalyst was analyzed using Perkin Elmer Optima 2100 DV Inductively Coupled Plasma
(ICP) Optical Emission Spectrometer (OES). Chemical analysis of biaryl compounds were
done on GC-MS Agilent GC-7890 A- MS 5975. In the catalytic activity performance a
domestic microwave oven was used. The magnetic susceptibility of the new biopolymer
(chitosan)-based pincer-type Pd(II) catalyst was measured with a Sherwood magnetic
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susceptibility balance at 25 °C. The molar conductivity was determined using a tabletop-
digital conductivity-meter CD-2005 at 25 °C.
2.3. Experimental Studies
2.3.1 Synthesis of 1-(6-(1-naphthalen-1-ylimino)ethyl)pyridin-2-yl)ethanone (NS)
2,6-diacetylpyridine (1 g, 6.12 mmol) and 1-naphthylamine (0.87 g, 6.12 mmol) were
dissolved in ethanol (35 mL) and stirred at room temperature for 48 h. The Schiff base
reaction was monitored by TLC and FT-IR analyses. At the end of this period, a yellowish
product was filtered and rinsed with water and acetone (Scheme 1).
2.3.2 Design of support material (OCS- NS)
OCS-NS was prepared according to the method from our previous study [28]. Briefly, the
Schiff base reaction of chitosan and NS compound was performed by refluxing for 72 h. Then
monochloroacetic acid (2.5 g) was added to the reaction mixture and stirred at 60 °C for 4 d.
Following this period, a light brown precipitate was filtered and washed with hot methanol to
remove any unreacted NS compound (Scheme 1). Then the OCS-NS was dried at room
temperature.
2
.3.3 Synthesis of new biopolymer (chitosan)-based pincer-type Pd(II), (OCS-NS -Pd(II))
OCS-NS (0.3 g) and Na PdCl (0.4 g) were dissolved in water (30 mL) and stirred for 5 h at
0°C. At the end of the reaction, a yellowish-green OCS-NS-Pd(II) catalyst was filtrated out,
2
4
5
washed with water and then dried in an oven at 60 °C (Scheme 1).
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…
……………………………………….Scheme 1……………………………………….
2.3.4 General procedure for synthesis of biaryl compounds
A mixture of phenyl boronic acid (1.87 mmol), the new biopolymer (chitosan)-based pincer-
−
3
type Pd(II) catalyst (5x10 % mol), K CO (3.75 mmol) and aryl halide (1.12 mmol) was
2
3
irradiated in a microwave oven at 50 °C, 400 W for 4 min in a solvent-free media. Following
the reaction procedure, the reaction media was cooled and extracted with toluene:water (2:1).
4
The organic phase was separated and dried with MgSO .
3. Results and Discussion
3.1 Characterization studies
The FT-IR spectra of the chitosan, NS, OCS-NS and OCS-NS-Pd(II) catalyst are present in
-1
Fig.1. In the spectrum of the NS compound the following peaks were observed: 1697 cm
-1
(
stretching of free carbonyl group, -C=O), 1642 cm (imine stretching, -C=N), and 1574,
-1
1
507 cm (stretching of aromatic ring, -C=C-) (Fig. 1b). These significant peaks showed that
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the Schiff base reaction occurred at a 1:1 mole ratio. When examining the spectrum of the
OCS-NS (Fig. 1c), the following changes were observed: (i) in the free carbonyl group of NS
-1
and NH
2
the stretching of chitosan (at 1590 cm ) had disappeared, (ii) a new imine starching
-1
band at 1634 cm was observed, (iii) stretching of the carbonyl group of carboxymethyl
-1
chitosan was observed at 1726 cm and (iv) stretching of the asymmetric and symmetric
-1
-1
carbonyl groups of carboxymethyl chitosan was indicated at 1615 cm and 1514 cm ,
respectively. All these important changes indicated that a chemical modification of the
chitosan was accomplished [29]. On the other hand, in the case of the OCS-NS-Pd(II), the
-1
stretching of the -C=N groups was shifted from 1635 to 1622 cm (Fig. 1d). This change may
correspond to the coordination of the palladium ions to the nitrogen atoms of the azomethine
groups of the OCS-NS [30].
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…
……………………………………Figure 1…………………………………..
1
3
In the C-NMR spectrum of the NS compound, two important signals were observed at
167.70 and 199.21 ppm that were attributed to the carbon atom of the imine (-C=N) and
carbonyl (-C=O) groups (please refer to supplementary data). These signals proved that Schiff
base formation of the NS compound was achieved at a 1:1 mole ratio. In addition, other
1
13
significant H-NMR and C-NMR signals of the NS compound are given below:
1
13
H-NMR, δ: 2.32 (s, 3H); 2.70 (s, 3H); 6.90-8.61 (m, Ar-H, 10H). C-NMR, δ: 16.60; 25.81;
1
1
13.77; 123.08; 123.52; 124.10; 125.02; 125.19; 126.22; 126.44; 126.79; 128.41; 134.17;
38.79; 139.14; 147.18; 152.69; 155.54; 167.70; 199.21
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13
The C-NMR spectrum of the OCS-NS shows the presence of a new carbon atom at 163.04
ppm, which indicates the condensation of NS with chitosan [31]. In addition, following
carboxymethylation, a characteristic signals at 199.21 and 40.83 ppm was observed (please
refer to supplementary data). This signal confirms that carboxymethylation was achieved.
The TG-DTG spectra of the OCS-NS and palladium catalyst are depicted in Fig. 2. From the
TG-DTG analyzes, the main thermal degradation temperature of the chitosan was recorded as
3
02 °C. Following the carboxymethylation and Schiff base modifications, the thermal
stability of the OCS-NS was determined as 187°C (Fig. 2a). This may be explained by the
decrease in the number of -NH and –OH groups on the chitosan chain after the chemical
2
processes [32]. This result is consistent with previous studies [33]. In addition, we recorded
higher chemical stability for the catalyst (222°C) (Fig. 2b). These results demonstrate that the
palladium catalyst is suitable for coupling reactions. In addition, we determined the thermal
and chemical stability of the samples followed the order of chitosan˃ OCS-NS-Pd(II) ˃ OCS-
NS.
…
……………………………………Figure 2…………………………………..
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The XRD diagram of chitosan, OCS-NS and palladium catalyst are presented in Fig.3. From
Fig. 3, the chitosan has two sharp peaks at ~10 and 20° (Fig. 3a.). After the chemical
modification, the peak at ~10° disappeared and the intensity of the peak at 20° for the
chitosan was decreased (Fig. 3b).
This indicates that Schiff base formation and
carboxymethylation were accomplished [33]. On the other hand, the XRD diagram of the
OCS-NS-Pd(II) catalyst indicated lower crystallinity than its ligand (Fig. 3c). All these result
showed that the catalyst had a more amorphous nature than the OCS-NS and chitosan.
…
……………………………………Figure 3…………………………………..
The SEM images of the chitosan, OCS-NS and OCS-NS-Pd(II) catalyst are displayed in Fig.
Chitosan had a smooth and regular surface (Fig. 4a), but following the chemical
4.
modification, the OCS-NS displayed a rougher and more irregular surface structure (Fig. 4b).
These changes in the surface properties of the chitosan can be attributed to the reduction in
the number of -OH and NH
2
groups on the chitosan polymer after the chemical processes
[
33]. However, the OCS-NS-Pd(II) catalyst exhibited a more regular surface image in
comparison to the OCS-NS (Fig. 4c). In addition, to verify the coordination of the palladium
with the imine groups of the OCS-NS, the EDAX spectrum of the catalyst was recorded and it
is given in Fig. 5. The presence of palladium and chloride ions in the EDAX spectrum showed
that the catalyst was successfully prepared.
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…
……………………………………Figure 4…………………………………..
…
……………………………………Figure 5…………………………………..
The magnetic properties and molar conductivity of the OCS-NS-Pd(II) catalyst were explored
at room temperature. The magnetic measurement of the catalyst showed that the catalyst had a
diamagnetic property. The molar conductivity value of the chitosan based on the palladium
−1
2
−1
catalyst was found to be 60 ꢀ cm mol , which indicated that the catalyst possessed a 1:1
1
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electrolyte nature [34]. These analyses indicated that the palladium catalyst had a square
planar geometry.
3.2 Catalytic studies
The product yield and selectivity of the Suzuki cross coupling reactions are dependent upon
different parameters, such as the reaction temperature, catalyst loading and base systems.
These factors should be studied before investigating the catalytic efficiency of the chitosan
2
2
based palladium catalyst. Therefore, the Suzuki C(sp )-C(sp ) reaction of 4-bromoanisole and
phenyl boronic acid was chosen as the representative reaction, and optimization studies were
conducted under microwave heating and solvent free media. To examine the effect of the base
systems, a coupling reaction was performed using various base systems and the results are
given in Table 1. As depicted in Table 1, the highest product yield (99%) was reached with
potassium carbonate. In addition, the effect of the reaction time on the C-C reactions was
investigated with the model reaction, and the most effective reaction time was found to be 4
min (Fig. 6). The base system and reaction time parameters were kept constant, and the model
Suzuki coupling reaction was carried out using various amounts of the catalyst to determine
the influence of the catalyst loading (Fig. 7). As shown in Fig. 8, the highest reaction yield
-3
was achieved with 5x10 mol % catalyst. Following the optimum reaction conditions, the
catalytic efficiency of the OCS-NS-Pd(II) catalyst was tested with different substrates, and the
results are summarized in Table 2. According to Table 2, the catalyst showed remarkable
catalytic activity with aryl bromide and aryl iodide. However, aryl chloride gave a lower
product yield compared to aryl bromide and aryl iodide. This low reaction efficiency can be
explained due to the poor activity of the aryl chloride in the Suzuki reactions [35]. On the
other hand, we investigated the effect of the electron withdrawing and electron donor groups
on the Suzuki C-C reactions (Table 2). As expected, the coupling reactions with the electron
2 2
withdrawing groups (p-NO , m-NO , p-CN, etc.) gave a higher product yield than the
1
2

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substrates bearing the electron donor groups (p-CH , m-CH , etc.). In addition, the effect of
3
3
the position of the substitution groups on the aromatic rings on the reaction yield was studied
in the C-C coupling reactions with all the aryl halides. We found that the –para substitute
groups were more reactive than the -meta and -ortho substituents. The TON and TOF values
of a catalyst are very important parameters, particularly for industrial applications [25]. Thus,
the TON and TOF values of the OCS-NS-Pd(II) catalyst were calculated and the results are
presented in Table 2. As shown in Table 2, the catalyst gave high TON and TOF values with a
very short reaction time. These values show that the OCS-NS-Pd(II) catalyst can be suitable,
both practically and economically, for industrially applications.
In addition, under the optimum experimental conditions, the catalytic performance of the
OCS-NS-Pd(II) catalyst was tested using a conventional reflux-heating system (Table 2). The
test showed that a microwave heating system is more suitable for the coupling reactions, in
terms of the labor force, reaction time and yield. In addition, the catalytic performance of the
OCS-NS-Pd(II) catalyst with commercial palladium salts was compared with a representative
reaction under the optimum experimental conditions, and the findings are given in Table. 2.
As illustrated in Table 2, the OCS-NS-Pd(II) catalyst had higher catalytic performance,
selectivity and TON and TOF values than the commercial catalysts.
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Table 1. Optimization of base for Suzuki reaction
Base System
NaOH
Reaction Yield (%) TON
TOF
55
35
11000
7000
183333
116667
330000
273333
KOH
K
2
CO
3
99
82
19800
16400
Na CO
2
3
-3
Reaction conditions: 1.12 mmol 4-bromoanisole, 1.87 mmol phenyl boronic acid, 3.75 mmol base, 5x10 mol % new
biopolymer (chitosan)-based pincer-type Pd(II), 50 °C, 4 min. (∼0.06 h) under MW
…
……………………………………Figure 6…………………………………..
…
……………………………………Figure 7…………………………………..
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Longevity is one of the most significant parameters for catalysts for different industrial
applications [36]. Therefore, the reusability performance of the OCS-NS-Pd(II) catalyst was
investigated under the optimum experimental conditions, and the results are listed in Table 3.
At the end of each cycle, the OCS-NS-Pd(II) catalyst was easily removed by simple filtration
2
from the reaction media and washed with hot H O and MeOH and then dried. This procedure
was repeated for each subsequent cycle of the Suzuki reactions. The recycle tests showed that
the catalyst has a long life time.
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Table 2. Effect of new biopolymer (chitosan)-based pincer-type Pd(II) catalyst on Suzuki C-C reactions
Entry Boronic acid
Substrate
Product
Reaction Yield (%) TON TOF
1
95
19000 316667
2
97
19400 323333
3
4
99
90
19800 330000
18000 300000
5
6
90
91
18000 300000
18200 303333
1
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7
8
55
58
11000 183333
11600 193333
9
1
97
86
19400 323333
17200 286667
NO2
0
1
2
3
I
1
1
1
50
60
66
10000 166667
12000 200000
13200 220000
1
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1
4
44
8800
9800
146667
163333
1
1
5
6
49
59
11800 196667
17
15
3000
50000
a
b
c
1
1
2
8
9
0
15
39
49
3000
7800
9800
50000
130000
408
-3
Reaction conditions: 1.12 mmol 4-bromoanisole, 1.87 mmol phenyl boronic acid, 3.75 mmol K
∼0.06 h) under MW
2 3
CO , 5x10 mol % new biopolymer (chitosan)-based pincer-type Pd(II) catalyst, 50 °C, 4 min.
(
-3
a: 1.12 mmol 4-bromoanisole, 1.87 mmol phenyl boronic acid, 3.75 mmol K CO , 5x10 mol % PdCl , 50 °C, 4 min. (∼0.06 h) under MW
2
3
2
-3
b: 1.12 mmol 4-bromoanisole, 1.87 mmol phenyl boronic acid, 3.75 mmol K
2
CO
3
, 5x10 mol % Na
2 4
PdCl 50 °C, 4 min. (∼0.06 h) under MW
1
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-3
c: Conventional heating method: 1.12 4-bromoanisole, 1.87 mmol phenyl boronic acid, 3.75 mmol K CO , 5x10 mol % new biopolymer (chitosan)-based pincer-type Pd(II) catalyst, 100 °C,
2
3
24 h, 6 mL toluene.
TON: (turnover number, yield of product/ per mol of Pd).
TOF: (turn over frequency, TON/time of reaction)
1
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Table 3. Longevity of new biopolymer (chitosan)-based pincer-type Pd(II) catalyst
Suzuki C-C reaction
Cycle number
Yield (%)
TON
19800
19400
17000
15600
14800
14400
14200
13600
TOF
st
1
99
97
85
78
74
72
71
68
330000
323333
283333
260000
246667
240000
236667
226667
nd
2
rd
3
th
4
5
6
7
th
th
th
th
8
-3
Reaction conditions: 1.12 mmol 4-bromoanisole, 1.87 mmol phenyl boronic acid, 3.75 mmol base, 5x10 mol % new
biopolymer (chitosan)-based pincer-type Pd(II) catalyst, 50 °C, 4 min (~0.06 h) under MW
2
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4. Conclusions
In the current study, we devolved a new biopolymer (chitosan)-based pincer-type Pd(II)
catalyst for Suzuki cross coupling reactions. The chemical characterization of the catalyst was
done with different analytical tools. The catalytic performance of the eco-friendly catalyst was
evaluated for the synthesis of biaryls using a clean, fast and simple work-up microwave
technique. The catalytic activity tests showed that the palladium (II) catalyst had good
catalytic performance and selectivity. The catalyst yielded an excellent TON of up to 19800
and a TOF of up to 330000 with mild reaction conditions and very short reaction time. The
longevity test showed that the catalyst could be reused many times without a significant loss
of activity. In further studies, the catalytic performance of the biopolymer (chitosan)-based
pincer-type Pd(II) catalyst should be investigated using different catalyst systems due to its
outstanding properties.
Acknowledgments
The authors thank Aksaray University Scientific Research Projects Coordination for their
financial support (Project Number: 2016-021)
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[
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Figure and Scheme Captions
Fig.1. FT-IR spectra of a) chitosan b) NS c) OCS-NS d) Pd(II) catalyst
Fig.2. TG/DTG spectra of a) OCS-NS b) Pd(II) catalyst
Fig.3. XRD diagram of a) chitosan b) OCS-NS c) Pd(II) catalyst
Fig.4. SEM images of a) chitosan b) OCS-NS c) Pd(II) catalyst
Fig. 5. EDAX spectra of Pd(II) catalyst
Fig. 6. The effect of reaction time on Suzuki coupling reaction
Fig. 7. The effect of catalyst loading on Suzuki coupling reaction
Scheme 1. Design of the new biopolymer (chitosan)-based pincer-type Pd(II) catalyst
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Highlights
A new pincer-type Pd(II) catalyst was obtained using chitosan biopolymer
The catalyst gave outstanding TONs of up to 19800 and TOFs of up to 330000
This chitosan-based pincer-type Pd(II) catalyst showed a better recycle performance