Carboxymethyl chitosan Schiff base supported heterogeneous palladium(II) catalysts for Suzuki cross-coupling reaction

Article abstract of DOI:10.1016/j.molcata.2015.06.008

In this study, two new O-carboxymethyl chitosan Schiff bases supported Pd(II) catalyst were synthesized (OCMCS-3aPd and OCMCS-4aPd). The catalysts were characterized with FTIR, TG/DTG, SEM/EDAX, XRD, ICP-OES, UV-vis, magnetic moment and molar conductivity. The catalytic activities of these catalysts were tested in the synthesis of biaryl compounds by Suzuki cross-coupling reactions. Characterizations of the biaryls were performed with GC-MS and ¹H NMR. In synthesis of the biaryl compounds in the presence of the Pd(II) catalyst, high selectivity was observed; no homo-coupling byproducts were detected in the spectra. A reusability test demonstrated that the catalysts were highly efficient even after ten run. The mercury poisoning and leaching tests indicated that the catalysts have heterogeneous nature.

Full text of DOI:10.1016/j.molcata.2015.06.008

Journal of Molecular Catalysis A: Chemical 407 (2015) 47–52
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Carboxymethyl chitosan Schiff base supported heterogeneous
palladium(II) catalysts for Suzuki cross-coupling reaction
∗
Talat Baran, Eda A c¸ ıksöz, Ayfer Mente s¸
Department of Chemistry, Faculty of Science and Letters, Aksaray University, Aksaray 68100 Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, two new O-carboxymethyl chitosan Schiff bases supported Pd(II) catalyst were synthesized
Received 10 April 2015
Received in revised form 23 May 2015
Accepted 6 June 2015
(OCMCS-3aPd and OCMCS-4aPd). The catalysts were characterized with FTIR, TG/DTG, SEM/EDAX, XRD,
ICP-OES, UV–vis, magnetic moment and molar conductivity. The catalytic activities of these catalysts were
tested in the synthesis of biaryl compounds by Suzuki cross-coupling reactions. Characterizations of the
Available online 19 June 2015
1
biaryls were performed with GC–MS and H NMR. In synthesis of the biaryl compounds in the presence
of the Pd(II) catalyst, high selectivity was observed; no homo-coupling byproducts were detected in the
spectra. A reusability test demonstrated that the catalysts were highly efficient even after ten run. The
mercury poisoning and leaching tests indicated that the catalysts have heterogeneous nature.
Keywords:
Biopolymer-supported catalyst
O-carboxymethyl chitosan
Schiff base
©
2015 Elsevier B.V. All rights reserved.
Suzuki
Reusable catalyst
1
. Introduction
carboxymethylation) act as suitable ligands for transition metal
ions [19–21].
Suzuki cross-coupling reactions are one of the fundamental
This study aimed to synthesize two new O-carboxymethyl chi-
tosan Schiff base supported Pd(II) catalysts. The chemical nature of
the compounds were illuminated with FTIR, TG/DTG, SEM/EDAX,
XRD, ICP-OES, UV–vis, magnetic moment and molar conductiv-
ity. The catalytic activity tests were conducted in reactions of
aryl halides with phenyl boronic acid. Suzuki cross-coupling was
achieved with high product selectivity and yield.
methods employed in the synthesis of biaryls [1,2]. Palladium is
among the transition metals used in Suzuki cross-coupling reac-
tions [3,4]. Pd(II) catalysts have been used in homogenous and
heterogeneous systems [5]. Homogenous Pd(II) catalysts produce
high yields; however due to the difficulties in separation of the
products from the catalyst, researchers prefer heterogeneous cat-
alysts to homogenous ones [6,7].
There have been many recent studies on palladium catalysts
supported by solids, such as polystyrene, corn starch, silica and cel-
lulose [8–11]. Biopolymer supported catalysts have gained much
attention due to the unique characteristics of the biomateri-
als; which are nontoxic, biodegradable, renewable and abundant
2. Materials and methods
2.1. Material
Aryl halides, phenyl boronic, palladium chloride, potassium car-
bonate, sodium chloride, magnesium sulphate, toluene, methanol
and Hg(0) obtained from Sigma–Aldrich.
[
12–14]. Chitosan is a biomacromolecule that has these properties.
This cationic biopolymer is synthesized via alkaline deacetyla-
tion of chitin [15]. Additionally, chitosan has an environmentally
friendly nature and exhibits high affinity for metal ions due to
the NH2 and OH groups on its chains [12,16–18]. These functional
groups also make it possible to modified chitosan (Schiff base, O-
2.2. Physical measurements
The FTIR spectra of samples were performed on PerkinElmer
Spectrum 100 FTIR spectrophotometer. Biaryls compounds were
analyzed with Agilent GC-7890 A- MS 5975. 1H NMR spectra of
the biaryls were recorded on Bruker Avance III 400 MHz spec-
trometer using Aceton-d6 solvents. TG/DTG curves of catalysts
were obtained under a nitrogen atmosphere at a sample heat-
∗ Corresponding author. Fax: +903822882141.
E-mail addresses: talatbaran 66@hotmail.com (T. Baran), edaacksz@gmail.com
E. A c¸ ıksöz), ayfermentes@yahoo.com (A. Mente s¸ ).
(
http://dx.doi.org/10.1016/j.molcata.2015.06.008
381-1169/© 2015 Elsevier B.V. All rights reserved.
1

4
8
T. Baran et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 47–52
Fig. 1. Structure of OCMCS-3aPd (a), OCMCS-4aPd (b).
Fig. 2. FTIR spectra of OCMCS-3aPd (a), OCMCS-4aPd (b).
Scheme 1. Chitosan Schiff base supported Pd(II) catalyzed Suzuki coupling reac-
tions.
PdCl2 (1.0 g, 5.63 mmol) and NaCl (0.66 g, 11.28 mmol) were dis-
solved in 100 mL water and stirred over night at room temperature.
Upon achieving complete dissolution, water in the medium was
◦
−1
using EXSTAR S11 7300. XRD diagrams
ing rate of 10 C min
of the Pd(II) complexes were carried out Rigaku Dmax 2000 sys-
evaporated and the product, Na PdCl was collected.
2
4,
◦
◦
The catalysts (OCMCS-3aPd and OCMCS-4aPd) were synthesized
following the procedure given below. 0.2 g of the O-carboxymethyl
tem at 40 kV, 30 mA and 2ꢀ with a scan angle from 5 to 50 .
The surface morphologies were examined using QUANTA-FEG 250
ESEM. The EDAX spectra of the Pd(II) catalysts were determined
by EDAX-Metek. The metal contents of the catalysts were deter-
mined with PerkinElmer Optima 2100 DV inductively coupled
plasma (ICP) optical emission spectrometer (OES). Electronic spec-
tra of the complexes were obtained with Genesys 10S UV–vis
Spectrophotometer. The molar conductivities of the catalysts were
chitosan Schiff bases ([OCMCS-3a]·2H O and [OCMCS-4a]·2H O)
2
2
were synthesized following the method in our previous study [22]
and was dissolved in 10 mL of water at room temperature. Then
0
.35 g of Na PdCl4 in 10 mL water was added into the reaction
2
◦
media and stirred for 5 h at 50 C. After which the mixture was
cooled to room temperature, and the yellow–green product was
collected. Finally, the product was rinsed with water to remove
measured using tablet op-Digital conductivity meter CD-2005 at
◦
unreacted reactants. The products (OCMCS-3aPd and OCMCS-4aPd)
2
5 C. Magnetic susceptibilities of the catalysts were performed
◦
(
Fig. 1) were dried at 50 C in an oven.
using Sherwood magnetic susceptibility balance at room tempera-
ture.
2.4. General procedure for Suzuki coupling
2.3. Synthesis of Pd(II) complexes (OCMCS-3aPd and
OCMCS-4aPd)
The catalyst, (OCMCS-3aPd or OCMCS-4aPd), 0.04 (% mol),
.2 mmol aryl halides, 1.87 mmol phenyl boronic acid, 3.75 mmol
1
First, water soluble of Na PdCl4 was prepared as follows:
K CO and 6 mL toluene were put into a Schlenk tube and stirred
2 3
2
Table 1
Effect of complexes on Suzuki cross-coupling reaction.
Entry
X
Y
Yield (%)
TON
OCMCS-3aPd
OCMCS-4aPd
OCMCS-3aPd
OCMCS-4aPd
1
2
3
4
5
6
7
8
9
Br
Br
Br
Br
Br
Br
Br
I
3-OCH3
4-OCH3
3-CH3
3-NH2
4-NH2
4-NO2
4-CN
4-CH3
3-OCH3
4-CH3
4-OCH3
94
97
37
75
70
98
94
51
6
95
98
29
92
90
88
84
39
5
2350
2425
925
1875
1458
2450
2350
1275
150
2375
2450
725
2300
2250
1833
2100
975
Cl
Cl
Br
125
75
1450
1
0
1
2
55
3
58
50
1375
a
1
◦
Reaction conditions: 1.12 mmol of aryl halide, 1.87 mmol of phenyl boronic acid, 3.75 mmol of K2CO3, 0.04 mol% catalyst, 6 mL of toluene, 100 C, 48 h.
TON: (turnover number, yield of product/per mole of Pd).
a
In the presence of excess Hg (Hg:Pd = 300:1).

T. Baran et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 47–52
49
Fig. 4. Scanning electron microscopy pictures of OCMCS-3aPd (a), OCMCS-4aPd (b).
Fig. 3. TG/DTG spectra of OCMCS-3aPd (a), OCMCS-4aPd (b).
3. Results and discussion
3
.1. FTIR spectra
◦
at 100 C for 48 h (Scheme 1). Reaction was followed by TLC. Then
the reaction media was allowed to cool at room temperature. The
mixture was extracted with 10 mL of water and the aqueous and
organic phases were separated in a separatory funnel. The extrac-
tion was repeated three times. MgSO4 was added into the organic
phase to ensure complete water removal. The products were ana-
lyzed and characterized with GC–MS and 1H NMR.
The imine stretching bands of [OCMCS-3a]·2H O and [OCMCS-
2
−1
4
a]·2H O were observed at 1625 and 1635 cm in our previous
2
study [22]. In the FTIR spectra of the catalysts, these bands were
shifted to lower wavenumbers (OCMCS-3aPd: 1614 cm
OCMCS-4aPd: 1618 cm ) (Fig. 2), which indicated that palladium
ions were coordinated with the ligands via nitrogen atoms [23]. Fur-
−1
and
−1
−
1
−1
thermore, the bands at 1729 cm (OCMCS-3aPd) and 1728 cm
OCMCS-4aPd) can be attributed to the C O stretching of car-
(
2
.5. Mercury poisoning experiments
boxymethyl groups [22]. The other bands observed in the spectra
of OCMCS-3aPd and OCMCS-4aPd were as follows; NH and OH
stretching: 3349, 3351 cm
−
1
−1
CH stretching: 2935, 2929 cm ,
Mercury poisoning test were conducted with the model reaction
,
−
1
in the optimum conditions (Table 1, entry 2). The model reaction (4-
bromoanisole with phenyl boronic acid) was done for 24 h without
adding mercury. Yields for catalysts OCMCS-4aPd and OCMCS-3aPd
are 60, 56%, respectively. Then, 300 molar equivalents of mercury,
relative to the Pd catalyst, was added in to the reaction medium
and stirred for another 24 h. After the allowed time the procedure
as in Section 2.4 was followed. The expected products (4-methoxy
biphenyl) were obtained with lower yields after 48 h. Yields for
OCMCS-4aPd and OCMCS-3aPd are 58, 55%, respectively.
C
N stretching: 1364, 1371 cm (Fig. 2).
3.2. TG-DTG analysis
The maximum thermal decomposition temperature (DTGmax)
of the modified chitosan is supposed to be lower than that of pris-
tine chitosan due to the deformation of hydrogen bonds and the
degree of the crystallinity index of chitosan [24,25]. The DTGmax
◦
values of OCMCS-3aPd and OCMCS-4aPd were recorded to be 244 C

5
0
T. Baran et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 47–52
Fig. 5. SEM/EDAX spectra of OCMCS-3aPd (a), OCMCS-4aPd (b).
◦
◦
and 242 C (Fig. 3), respectively. When compared to chitosan, the
reduction in the thermal stability of the catalysts demonstrated
the coordination of the metal ions with the ligands. Since C–C cou-
pling reactions are carried out at high temperatures, catalysts are
required to remain stable in the range of the reaction tempera-
ture to ensure catalyst activity and recyclability [26]. We observed
that the catalysts were thermally stable at high temperature, which
indicates that the catalysts are suitable for C–C coupling reactions.
I110 is the maximum intensity at ∼20 Iam is the intensity of amor-
◦
phous diffraction at 16 .
3.5. Electronic spectra (UV–vis)
In an earlier study [22], benzene ꢁ–ꢁ* and (C N) n–ꢁ*
transitions of the ligands ([OCMCS-3a]·2H O: 286 and 313 nm;
2
[
OCMCS-4a]·2H O: 266 and 305 nm) were reported. With com-
2
plexation of the palladium ions, these transitions were shifted to
higher wavelengths. The shifting confirmed the coordination of
palladium ions via nitrogen atoms of the ligands [29]. In addi-
tion, we observed two new weak d–d transitions at 360–450 nm
3
.3. SEM–EDAX
SEM images of OCMCS-3aPd and OCMCS-4aPd exhibited that
(
OCMCS-3aPd) and 365–470 nm (OCMCS-4aPd). These bands were
the surface morphologies of the catalysts were completely differ-
ent from those of the ligands and chitosan; the catalysts had more
regular stacked structure (Fig. 4). These observations demonstrated
that palladium ions coordinated with the ligands [27]. Addition-
ally, to confirm the presence of the metal ions, EDAX spectra of the
catalysts were recorded. The spectra proved the presence of the
palladium and chloride ions that had already been coordinated to
the palladium (Fig. 5).
1
1
1
1
attributed to A1 → E , A → B transitions. These transitions
g
1g
1g
1g
demonstrated that the geometry of the complexes was square pla-
nar [31,32].
3
.6. Molar conductivity, magnetic properties and ICP-OES
To define the geometry of the complexes, molar conductivity
and magnetic moment measurements were performed. The molar
conductivities were determined to be 19 ꢂ cm mol (OCMCS-
3aPd) and 15 ꢂ cm mol (OCMCS-4aPd). These low values
−
1
2
−1
−
1
2
−1
3
.4. XRD
showed the nonconductivity of the complexes. Magnetic moment
measurements revealed that the complexes had diamagnetic prop-
erties. These findings supported the square planar geometry of the
complexes that had d8 system [31,33]. The metal content of the
complexes (% mass) were determined with ICP-OES (OCMCS-3aPd:
13.02% and OCMCS-4aPd: 15.63%).
When chitosan is modified, its thermal stability and crystallinity
are subject to reduce because of the deformation of hydrogen bonds
28,29]. When compared to the chitosan, in the XRD patterns of the
catalysts, decreases in the crystallinity values peak intensity and
peak broadening were observed (Fig. 6). These findings demon-
strated that palladium ions were coordinated with the ligands. The
crystallinity values of the catalysts were determined to be [OCMCS-
[
3.7. Catalysis of the Suzuki reaction
3
aPd > OCMCS-4aPd] using the following equation[30].
In the palladium catalyzed Suzuki C–C coupling reactions, the
type and amount of the catalyst, reaction time, base system and
temperature are important parameters. To optimize the reaction
(
I110 − Iam)
Crystallineindex (%) = [
] × 100
(1)
I1
10

T. Baran et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 47–52
51
Table 2
Recycling of OCMCS-3aPd and OCMCS-4aPd.
Yield (%)
The number of cycle
OCMCS-3aPd
OCMCS-4aPd
1
2
3
4
5
6
7
8
9
1
st
97
98
96
94
92
91
86
82
79
73
98
98
95
94
91
90
89
87
80
75
nd
rd
th
th
th
th
th
th
0th
Reaction conditions: 1.12 mmol of 4-bromoanisole, 1.87 mmol of phenyl boronic
◦
acid, 3.75 mmol of K2CO3, 0.04 mol% catalyst, 6 mL of toluene, 100 C, 48 h.
higher yields for aryl bromide in the C–C coupling reactions of chi-
tosan Schiff bases supported palladium catalyst. On the other hand,
lower yields were recorded for the reactions with aryl chlorides
(entries 9 and 10). This can be attributed to the poor reactivity of
aryl chlorides [34]. Furthermore, the catalysts were more efficient
in the reactions of compounds with electron withdrawing groups
(
entries 6 and 7). Turnover numbers (TON) were calculated [35] for
1
all the products and listed in Table 1. H NMR and GC–MS spectra of
synthesis of the biaryl compounds are given in the Supplementary
section (Fig. S4-S17).
3.8. Reusability test of catalyst
The reusability of the catalysts was tested and the findings are
given in Table 2. Upon achieving C–C coupling reactions, the cat-
alysts were removed from the reaction media with filtration and
the filtrate was rinsed with water and hot methanol to ensure
the catalytic activity of the catalysts. The recycling reactions were
carried out under the optimum conditions; it was observed that
the catalyst retained their activity even after ten run. After 10th
run, we observed that yield decrease to 75% (OCMCS-4aPd) and
7
3% (OCMCS-3aPd). Also at the end of the 10th run, hot filtrate
was analyzed with ICP-OES and very low palladium leaching was
observed (<1%). The leaching test indicated that the catalysts have
heterogeneous nature.
3.9. Mercury poisoning tests
It is already known that if addition of mercury stops the catalytic
activity, then the reaction mechanism is said to be heterogeneous. If
not, it is concluded that the reaction mechanism has a homogenous
nature [36–38]. The mercury poisoning tests showed that the reac-
tion mechanism follows a heterogeneous pathway (Table 1, entry
Fig. 6. XRD patterns of OCMCS-3aPd (a), OCMCS-4aPd (b).
1
1).
conditions (see Fig. S1-S3 and Table S1, S2 in Supplementary data),
the reaction of 4-bromoanisole with phenyl boronic acid was pre-
ferred as a model reaction. The studies revealed the optimum
4. Conclusion
conditions in the presence of the catalysts; amount of catalyst:
◦
0
.04% mol, temperature: 100 C, base system: K CO and reaction
In this study, two new O-carboxymethyl chitosan Schiff base
supported heterogeneous Pd(II) catalysts were synthesized. The
mode of bonding (FTIR), thermal stability (TG/DTG), surface mor-
phology (SEM/EDAX), crystallinity index (XRD) and metal content
(ICP-OES) of the catalysts were investigated. Moreover, to define
the geometry of the catalysts, UV–vis, magnetic moment and molar
conductivity measurements were performed. The catalytic activity
was tested for the first time in the synthesis of biaryls and the prod-
2
3
time: 48 h (see Fig. S1-S3 and Table S1, S2 in Supplementary data).
Under the optimum conditions, the catalytic activity was tested
in Suzuki coupling reaction of aryl halides with phenyl boronic acid.
These findings are presented in Table 1. An evaluation of the Suzuki
coupling reactions demonstrated that the reactions with aryl bro-
mide (entries 1–7) had a high yield with the exception of the entry
3
. Also, when aryl iodides were compared with aryl chlorides, it was
1
observed that the aryl iodides had higher yields (entry 8). The dif-
ferentiation observed in the reaction yield can be accounted for by
the bond dissociation energies and the size of the halide atoms [14].
In an earlier study by Makhubela et al. (2011), the authors reported
ucts were characterized with GC–MS and H NMR. The selectivity of
the catalysts was high in Suzuki coupling reactions. There reusabil-
ity studies showed that the catalysts retained their activity after ten
run. Leaching studies observed that very low leaching of Pd (<%1)

5
2
T. Baran et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 47–52
and mercury test indicated that these catalysts have heterogeneous
nature. In conclusion, the advantages of these catalysts include (1)
their ease of separation from the reaction medium, (2) having oxy-
gen insensitivity, (3) stability at high temperatures, (4) long life
time and (5) achieving high TON values with small amount of cata-
lysts. These catalysts can be used in both industrial operations and
pharmaceutical applications.
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This work was financial support by the Scientific and Tech-
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