A new chitosan Schiff base supported Pd(II) complex for microwave-assisted synthesis of biaryls compounds

Article abstract of DOI:10.1016/j.molstruc.2017.03.122

In this study, a new heterogeneous palladium (II) catalyst that contains O-carboxymethyl chitosan Schiff base has been designed for Suzuki coupling reactions. The chemical structures of the synthesized catalyst were characterized with the FTIR, TG/DTG, ICP-OES, SEM/EDAX, ¹H NMR, ¹³C NMR, GC/MS, XRD, and magnetic moment techniques. The reusability and catalytic behavior of heterogeneous catalyst was tested towards Suzuki reactions. As a result of the tests, excellent selectivity was obtained, and by-products of homo coupling were not seen in the spectra. The biaryls products were identified on a GC/MS. In addition, it was determined in the reusability tests that the catalysts could be used several times (seven runs). More importantly, with very low catalyst loading (6?×?10^?3?mol %) in very short reaction time (5?min), chitosan Schiff base supported Pd(II) complex gave high TON and TOF values. These findings showed that Schiff base supported Pd(II) catalyst is suitable for Suzuki cross coupling reactions.

Full text of DOI:10.1016/j.molstruc.2017.03.122

Accepted Manuscript
A new chitosan Schiff base supported Pd(II) complex for microwave-assisted
synthesis of biaryls compounds
Talat Baran
PII:
S0022-2860(17)30420-9
10.1016/j.molstruc.2017.03.122
MOLSTR 23630
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 18 February 2017
Revised Date: 29 March 2017
Accepted Date: 30 March 2017
Please cite this article as: T. Baran, A new chitosan Schiff base supported Pd(II) complex for microwave-
assisted synthesis of biaryls compounds, Journal of Molecular Structure (2017), doi: 10.1016/
j.molstruc.2017.03.122.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
A new Chitosan Schiff base supported Pd(II) complex for microwave-assisted synthesis
of biaryls compounds
Talat Baran
Department of Chemistry, Faculty of Science and Letters, Aksaray University, Aksaray,
6
8100 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
1

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Abstract
In this study, a new heterogeneous palladium (II) catalyst that contains O-carboxymethyl
chitosan Schiff base has been designed for Suzuki coupling reactions. The chemical structures
of the synthesized catalyst were characterized with the FTIR, TG/DTG, ICP-OES,
1
13
SEM/EDAX, H-NMR, C-NMR, GC/MS, XRD, and magnetic moment techniques. The
reusability and catalytic behavior of heterogeneous catalyst was tested towards Suzuki
reactions. As a result of the tests, excellent selectivity was obtained, and by-products of homo
coupling were not seen in the spectra. The biaryls products were identified on a GC/MS. In
addition, it was determined in the reusability tests that the catalysts could be used several
-3
times (seven runs). More importantly, with very low catalyst loading (6x10 mol %) in very
short reaction time (5 min), chitosan Schiff base supported Pd(II) complex gave high TON
and TOF values. These findings showed that Schiff base supported Pd(II) catalyst is suitable
for Suzuki cross coupling reactions
Keywords: Biaryls; Schiff base; reusability; TOF; chitosan; Pd(II) catalyst
2

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1
. Introduction
Catalysts are promising materials for industrial applications due to their excellent properties
such as recoverability, reusability, and cost reduction on production. To be industrially
applicable, a catalyst should have important properties such as high reaction yields, long life
time, and high thermal stability and durability to air or moisture [1, 2]. Especially, solid
supported heterogeneous catalysts have been widely used because they are easily recovered
from the reaction media and easy to work up [3, 4]. Following this, natural biopolymers such
as cellulose, chitosan, and starch have recently gained significant attention as support
materials for transition metal atoms in catalytic systems [5, 6]. Amongst the natural
biopolymers is chitosan, which is obtained from the deacetylation of chitin in an alkali media
and the second most abundant material after cellulose in world [7]. In addition, chitosan has
high thermal stability, which is a desired property for catalyst support materials because
catalytic reactions require a high reaction temperature. Therefore, chitosan is a suitable
support material for catalytic systems, and many researchers have investigated the catalytic
performance of catalysts prepared by using different chitosan modifications in different
reactions [8]. On the other hand, chitosan has attractive functions, such as -NH and -OH
2
groups, and these reactive groups can provide easy modification to improve the physical and
chemical properties of chitosan [9-11]. Schiff bases can be easily obtained by using the free
NH groups of chitosan under mild reaction conditions, and they have been widely used in
2
coordination chemistry due to their azomethine nitrogen donor atoms [12, 13]. Schiff base
ligands are electron-rich, so they are used in metal catalyzed catalytic reactions such as
Suzuki, Heck, and oxidation reactions.
Shahnaz et al. [14] reported that a bidentate Schiff-base derived palladium catalyst had high
catalytic activity in Suzuki coupling reactions. In another study, Dolaz et al.[15] prepared a
Schiff base and its metal complex. Then, the catalytic performances of Cu(II), Co(II), and
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Ni(II) complexes were tested in a cyclohexane oxidation reaction. Liu et al. [16] synthesized
an air-stable symmetric Schiff base and palladium complex, and its catalytic activity was
studied in Suzuki coupling reactions.
Palladium is an important transition metal and has been the subject of great attention for cross
coupling reactions, such as Kumada [17], Stille [18], Sonogashira[19], and Negishi [20]
2
2
reactions. Suzuki C(sp )-C(sp ) reaction is one of the most important reactions which can be
efficiently used for the construction of C–C bonds. [21, 22]. In addition, this significant
reaction has some advantages, such as excellent functional group tolerance, easy accessibility,
and that the products obtained from the reaction have biological properties that can be widely
used in material chemistry, pharmacology, and cosmetics [23, 24]. However, Suzuki C-C
reactions require a high reaction temperature, long reaction time, and inert reaction
atmosphere [25]. In recent years, to overcome these disadvantages, a simple and rapid
microwave heating technique has been extensively used in catalyst systems [26]. This
technique provides advantages such as clean synthesis, higher reaction yield, and operational
simplicity.
In this study, a new O-carboxymethyl chitosan Schiff base (CS-NNSB) was synthesized as a
support material for palladium ions. Then Pd(II) catalyst (CS-NNSB-Pd(II)) was synthesized
in water. The chemical structures of CS-NNSB and CS-NNSB-Pd(II) were characterized with
1
13
FT-IR, H-NMR, C-NMR, TG/DTG, XRD, SEM-EDAX, ICP-OES, and magnetic moment
analyses. Then the catalytic activity of CS-NNSB-Pd(II) was examined in Suzuki cross
coupling reactions under mild reaction conditions (50 °C) using a simple microwave heating
technique (5 min) and solvent free reaction media. The recycling of CS-NNSB-Pd(II) was
done under optimum conditions. The Suzuki coupling reactions test showed that CS-NNSB-
Pd(II) catalyst has high catalytic performance and selectivity.
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2
. Material and Method
2
.1. Materials
Chitosan, 2,2′-pyridil, 4-isopropyl aniline, Na PdCl , phenyl boronic acid, aryl halides,
2
4
NaOH, KOH, K CO , MgSO , toluene, and ethanol were obtained from Sigma-Aldrich and
2
3
4
used as supplied without any purification.
2
.2. Instrumentation
FT-IR spectra were recorded on a Perkin Elmer Spectrum 100 FTIR spectrophotometer. The
1
13
H-NMR and C-NMR spectra of NSB were recorded on an Agilent 600 MHz spectrometer
1
3
using DMSO-d solvent. The C-NMR spectrum of CS-NNSB was measured with an Agilent
6
6
00 MHz spectrometer using D O solvent. Thermal stability analysis was performed on an
2
EXSTAR S11 7300 (nitrogen atmosphere; heating range: 30-650°C). X-ray diffractograms
were recorded on a Rigaku D max 2000 system (at 40 kV, 30 mA, and 2θ with a scan angle of
5
−50°). The surface morphology examination was conducted on a QUANTA-FEG 250
ESEM. The analyses of palladium and chloride ions on the catalyst were done using an
EDAX-Metek. Palladium ion content of the catalyst was analyzed by using Perkin Elmer
Optima 2100 DV Inductively Coupled Plasma (ICP) Optical Emission Spectrometer (OES).
The analyses of biaryl compounds were done on GC/MS Agilent GC-7890 A- MS 5975. In
the catalytic activity tests, a domestic microwave oven was used. The magnetic moment of the
palladium complex was determined with a Sherwood magnetic susceptibility balance at 25°C.
2
.3. Synthesis of (E)-2-((4-isopropylphenyl)imino)-1,2-di(pyridin-2-yl)ethanone (NSB)
NSB was prepared by stirring a solution containing 2,2′-pyridil (1g, 4.71 mmol) and 4-
isopropyl aniline (0.64 mL, 4.71 mmol) in ethanol (30 ml) at 50°C for 24 h. Following the
reaction time, the mixture was allowed to cool to room temperature, and a yellow product was
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1
obtained (90 % yield). H-NMR, δ: 1.09 (s, 6H); 2.75 (m, H); 6.77 (d, 2H); 7.05 (d, 2H); 7.90
13C
(d, H); 8.25 (d, H); 8.41 (d, H); 8.60 (d, H); 7.97-8.00 (t, 2H).
NMR, δ: 24.20; 33.19;
1
1
20.21; 121.16; 121.61; 126.14; 127.06; 128.64; 137.88; 138.36; 145.52; 146.89; 149.22;
50.13; 152.68; 154.84; 167.43; 198.55
2
.4. Synthesis of support material (CS-NNSB)
CS-NNSB was synthesized as reported in our previous study [27]. In summary, O-
carboxymethyl chitosan Schiff base derivative was synthesized by refluxing chitosan and
NSB for 72 h, and then ethanol solution of monochloroacetic (2.5g, 15mL) was added into the
reaction mixture and stirred at 60°C. After five days, the support material was obtained and
1
3
dried at room temperature (70 % yield) (Scheme 1). C-NMR, δ: 24.79; 33.74; 43.14; 55.80;
5
9.93; 70.06; 74.74; 76.35; 97.56; 120.24; 121.14; 121.92; 126.27; 127.39; 128.54; 137.02;
38.76; 145.35; 146.21; 149.64; 150.82; 152.20; 153.09; 162.25; 167.51; 174.04
1
…
..………………………………………Scheme 1……………………………………..
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2
.5. Preparation of CS-NNSB-Pd(II) catalyst
0
.2 g of CS-NNSB was added into water solution of Na PdCl (15 mL, 0.35 g), and the
2 4
reaction mixture was stirred at 50 °C for 6h. Subsequently, the reaction was cooled, and the
precipitate which was formed (CS-NNSB-Pd(II)) was filtered from the reaction media and
dried (55 % yield) at room temperature.
2
.6. Typical procedure for cross coupling reactions
.6.1. Suzuki C-C reaction
2
A mixture of phenyl boronic acid (1.87 mmol), aril halides (1.12 mmol), potassium carbonate
-
3
(
3.75 mmol), and CS-NNSB-Pd(II) catalyst (6x10 % mol) were put into a Schlenk tube and
irradiated in a microwave oven at 50 °C (400 W) for 5 min. Following of this period,
monitored by thin layer chromatography), the mixture was cooled and extracted with a
(
tolune:water (2:1) mixture. The organic phase was removed from the mixture and dried over
anhydrous magnesium sulfate. Then, the solution was evaporated, and the products were
isolated.
2
.6.2. Recycle tests
The reusability performance of the CS-NNSB-Pd(II) catalyst in Suzuki C-C reaction was
carried out on model reaction under optimum conditions. After the Suzuki coupling reactions
were completed, CS-NNSB-Pd(II) catalyst was filtered from the mixture and rinsed with hot
water, methanol, and ethanol, respectively, and dried at 100 °C before the next run. This
regeneration process for CS-NNSB-Pd(II) catalyst was repeated after every run.
3
. Results and Discussion
3
.1 Characterization studies
3
.1.1. XRD
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The XRD diagrams of chitosan, CS-NNSB, and CS-NNSB-Pd(II) are presented Fig. 1. As
shown in Fig. 1a, the chitosan has two sharp crystalline peaks at ~10 and 20°. In the XRD
pattern of CS-NNSB, the peak at ~10° disappeared, and the peak at ~20° widened, and so the
crystallinity of the polymer decreased after the Schiff base formation (Fig. 1b). The CrI values
of the chitosan and CS-NNSB were calculated as 85 and 46 % using the equation bellow [28].
This result shows that CS-NNSB has a poor crystalline nature compared to the chitosan. This
lower crystallinity arises from the disruption of the hydrogen bonds of the chitosan due to the
condensation of NH groups on the chitosan backbone with the free carbonyl groups of NSB
2
[
29]. After the complexation of the palladium ion to CS-NNSB, weak diffraction peaks and
peak broadening appeared in the spectrum of the catalyst (Fig. 1c). The CrI value of the
catalyst was determined to be 5 %. In addition, new peaks at 40.8° and 46.6 were observed in
the spectrum. These new peaks confirmed the coordination of the palladium atoms to the CS-
NNSB [30].
Crystalline index (%) = [(I − I )/I ] × 100
(1)
1
10
am 110
where I110 denotes the maximum intensity at ~20°, and Iam is the intensity of the amorphous
diffraction at 16°.
…..………………………………………Figure 1……………………………………..
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3
.1.2 FT-IR analysis
Following the condensation reaction of 2,2′-pyridil and 4-isopropyl aniline, there was a peak
-1
in the FTIR spectrum of the NSB belonging to the imine group observed at 1640 cm , and the
−
1
NH stretching bands (at ~3433–3353 cm ) of 4-isopropyl aniline disappeared (see
2
Supplementary material). In addition, the stretching band of the free carbonyl groups of 2,2′-
-1
pyridil was seen at 1694 cm . All these significant changes prove that condensation was
accomplished at a 1:1 ratio. In the FTIR spectrum of CS-NNSB, NH stretching of chitosan
2
and C=O stretching of 2,2′-pyridil disappeared, and a new azomethine imine band was
-1
observed at 1635 cm . This new band concludes the Schiff base condensation of chitosan
with NSB [28]. After the carboxymethylation procedure, three characteristics peaks for the
-1
carbonyl groups of carboxymethyl were also observed at 1726, 1587, and 1521 cm in the
spectrum of CS-NNSB .These observations indicate that the carboxymethylation procedure
was achieved. On the other hand, the azomethine vibration band of CS-NNSB-Pd(II) shifted
-1
from 1635 to 1622 cm in the spectrum of the catalyst. This important change can be
explained with the coordination of the palladium atom to nitrogen atoms of the azomethine
groups of CS-NNSB [31].
1
13
3
.1.3 H-NMR and C-NMR analyses
1
3
In the C-NMR spectrum of NSB, two important signals were observed at 167.43 and 198.55
ppm, which are attributed to the imine (-C=N) and the free carbonyl (-C=O) signals,
respectively (see Supplementary material). These important data indicate the condensation of
2
,2′-pyridil with 4-isopropyl aniline (1:1 moles). After the chemical modification of chitosan
(Schiff base and carboxymethylation), the free carbonyl signal of NSB disappeared, and a
new azomethine signal appeared at 162.25 ppm in the spectrum of CS-NNSB. This signal
shows the condensation of chitosan with NSB was accomplished. In addition, the
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characteristic carbonyl signal of carboxymethyl was exhibited at 174.04 ppm in the spectrum
[
32].
3
.1.4 TD/DTG analysis
TG/DTG thermograms were recorded to examine the thermal behaviors of the chitosan, CS-
NNSB, and CS-NNSB-Pd(II). The maximum degradation temperature (Tmax) of the chitosan
and the CS-NNSB were determined to be 302 and 200 °C, respectively (see Supplementary
material). As expected, CS-NNSB had a lower thermal satiability than the chitosan due to the
decreasing crystallinity value of the chitosan following the condensation reaction. In addition,
this result is in accordance with the calculated CrI values for CS-NNSB. When examining the
TG/DTG spectrum of CS-NNSB-Pd(II), Tmax value was found to be 235 °C (see
Supplementary material). This indicates that CS-NNSB-Pd(II) is more stable than CS-NNSB.
3
.1.5 SEM/EDAX analysis
We have reported in a previous study that chitosan has a smooth surface [27]. The SEM
image shows that CS-NNSB does not have the same surface morphology as the chitosan (Fig.
2
b-c). This difference in the surface images can be attributed to the chemical modification of
chitosan [29]. In addition, images at different magnifications of the catalyst were recorded
and are presented in Fig. 2 d-e. The surface images of CS-NNSB-Pd(II) shows completely
different surface properties, such as particle size and form, when compared to its ligand. This
observation also proved the coordination of the palladium atom to its ligand [27].
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…..………………………………………Figure 2……………………………………..
In addition, EDAX analysis of CS-NNSB-Pd(II) was done to confirm the complexation, and
palladium and chloride ion peaks were detected in the spectrum (Fig. 3). This result revealed
that Pd(II) catalyst was obtained.
…..………………………………………Figure 3……………………………………..
3
.1.6. Magnetic moment analysis
8
It is known that Pd(II) complexes with square planar geometry (low spin d system) have
diamagnetic properties [33]. In this paper, magnetic moment analysis was done for the
determination of the geometry of CS-NNSB-Pd(II) at room temperature, and the catalyst was
1
1

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determined to have the diamagnetic property. This result suggests that CS-NNSB-Pd(II) has a
square planar geometry [28]. In addition, ICP analysis of CS-NNSB-Pd(II) was carried out to
determine palladium content of the catalyst, and it was found as 23.5 % (mass %).
3
.2 Catalytic activity of CS-NNSB-Pd(II) in Suzuki coupling reactions
The amount of catalyst loading, reaction time, reaction temperature, and base system are
important factors in the cross coupling reactions, and these parameters influence the isolated
reaction yield, percentage conversion, and selectivity. To determine the optimum reaction
conditions, the Suzuki C-C reaction of phenyl boronic acid with 4-bromoanisol was utilized
as the model reaction, and control experiments were studied. The influence of the catalyst
loading on the reaction yield was explored by using different amounts of the catalyst, and the
-3
optimum values was determined to be 6x10 mol % (see Supplementary material). The affect
of the reaction time on the coupling reaction was also studied, and the best reaction time was
found to be 5 min (see Supplementary material). To design the best base system, the
influences of the various base systems were explored on the model reaction, and the results
are given Table 1. K CO gave a high reaction yield, so it was chosen as the base system. In
2
3
summary, the optimum reaction conditions for the Suzuki C-C reactions were determined to
-3
be as follows: base system K CO ; amount of the catalyst 6x10 mol %; and reaction time 5
2
3
min under microwave heating. The catalytic activity of CS-NNSB-Pd(II) was investigated for
the synthesis of various biaryls compounds.
1
2

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Table 1. Optimization of base for Suzuki reaction
Base System
NaOH
Reaction Yield (%)
TON
6333
TOF
79162
38
50
97
KOH
8333
104162
202087
K CO3
2
16167
-
3
Reaction conditions: 1.12 mmol 4-bromoanisole, 1.87 mmol phenyl boronic acid, 3.75 mmol base, 6x10 mol % CS-NNSB-
Pd(II) catalyst, 50 °C, 5 min, under MW.
Following the optimization of the reaction conditions, the effect of the substrate on the Suzuki
coupling reactions was explored under the optimum conditions, and the results are presented
Table 2. The coupling reactions were done with a range of aryl halides (substrates), which
have withdrawing groups or electron donating functional groups. As seen in Table 2, the
coupling reactions of the electron-withdrawing or electron-donor aryl bromides delivered the
desired biaryls products with excellent reaction yields. As expected, aryl iodides as the
substrates also gave considerably higher selectivity towards the Suzuki C-C reactions. On the
other hand, relatively lower reaction yields were obtained with the aryl halides due to the poor
reaction activity. This result is consistent with previous studies [4, 34]. In addition, the orders
of reactivity of the substitute groups in all Suzuki coupling reaction were found to be para ˃
meta ˃ ortho. The TON and TOF values were calculated for the Suzuki C-C reaction and are
presented Table 2. The higher TON and TOF values (up to 16167 and 202087) were obtained
with very small catalyst loadings and very short reaction times. These results mean that both
CS-NNSB-Pd(II) catalyst and the microwave heating system work effectively in the Suzuki
C-C reactions. The characterization of the synthesis of the biaryls compounds was conducted
with GC/MS, and the results are given in the supplementary data.
1
3

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Table 2. Effect of CS-NNSB-Pd(II) catalyst on Suzuki C-C reactions
Entry
Boronic acid
Substrate
Product
Reaction Yield (%) TON
TOF
1
78
13000 162500
2
96
16000 200000
3
4
97
90
16167 202087
15000 187500
5
6
91
38
15167 189597
6333
79162
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7
8
9
55
83
73
9167 114587
13833 172912
12167 152087
1
0
38
6333
79162
I
CH3
1
1
1
2
55
60
9167 114587
10000 125000
1
3
25
4167
52087
-
3
Reaction conditions: 1.12 mmol 4-bromoanisole, 1.87 mmol phenyl boronic acid, 3.75 mmol K CO , 6x10 mol % of CS-NNSB-Pd(II) catalyst, 50 °C, 5 min, under MW.
2
3
TON: (turnover number, yield of product/ per mol of Pd).
TOF: (turn over frequency, TON/time of reaction)
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3
.3 Longevity performance of CS-NNSB-Pd(II) catalyst
To determine the lifetime of CS-NNSB-Pd(II) catalyst, reusability tests were done under
optimum reaction conditions by using microwave heating. As can be seen in Table 3, the
catalyst was used for at least seven repeated cycles for Suzuki C-C reactions. This longevity
performance test also shows that the catalyst has excellent activity towards the Suzuki C-C
reactions.
Table 3. Reusability of CS-NNSB-Pd(II) catalyst
Suzuki C-C reaction
The number of cycle
Yield (%)
TON
16167
15333
15000
14500
13833
13667
12167
TOF
st
1
97
92
90
87
83
82
73
202087
191662
187500
181250
172912
170837
152087
nd
2
rd
3
th
4
5
6
th
th
th
7
-
3
Reaction conditions: 1.12 mmol 4-bromoanisole, 1.87 mmol phenyl boronic acid, 3.75 mmol base, 6x10 mol % CS-NNSB-
Pd(II) catalyst, 50 °C, 5 min, under MW.
1
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4
. Conclusion
In this study, a new biopolymer supported heterogeneous Pd(II) catalyst was prepared,
characterized, and its catalytic behavior against Suzuki coupling reactions was evaluated
under mild conditions in a free solvent media. In addition, reusability of CS-NNSB-Pd(II)
catalyst was tested. CS-NNSB-Pd(II) showed a great catalytic performance and remarkably
high TONs and TOFs with very small catalytic loading for the synthesis of biaryls in Suzuki
reactions. The recycle test also shows that the catalyst was recyclable and recoverable several
times. In summary, a green CS-NNSB-Pd(II) catalyst worked efficiently in the synthesis of
biaryls compounds. The catalyst can be transferred to different industrial applications due to
its high TON and TOF values as well as its longevity and chemical and mechanical stabilities.
1
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Figure and Scheme Captions
Fig.1. XRD diagram a) NSB b) CS-NNSB c) CS-NNSB-Pd(II)
Fig. 2. SEM images of chitosan (a), CS-NNSB (b-c), CS-NNSB-Pd(II) (d- e)
Fig. 3. EDAX spectra of CS-NNSB-Pd(II)
Scheme 1. Design of CS-NNSB-Pd(II) catalyst
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Highlights
The catalyst yielded outstanding TONs of up to 16167 and TOFs of up to 202087
This green catalyst showed a better performance against Suzuki C-C reactions
O-carboxymethyl chitosan Schiff base was used as support for palladium catalyst
O-carboxymethyl chitosan Schiff base supported Pd(II) catalyst was synthesised and
characterised