Highly effective and recoverable Pd(II) catalyst immobilized on thermally stable Schiff base polymer containing phenol group: Production, characterization and application in Suzuki coupling reactions

Article abstract of DOI:10.1016/j.jorganchem.2018.04.022

In this study, a novel highly efficient, recoverable, sustainable, and thermally stable Schiff base polymer-Pd(II) catalyst was designed for Suzuki coupling reactions in synthesis of biaryl compounds for the first time. Firstly, a novel Schiff base polymer containing phenol, Poly(4-((pyridine-2ylimino)methyl)benzene-1,3-diol) (P (4-PMB)), which was used as a support material in catalyst design, was synthesized in aqueous alkaline medium via oxidative polycondensation of 4-((pyridine-2-ylimino)methyl)benzene-1,3-diol (4-PMB) with NaOCl oxidant. The effects of oxidant concentration, polymerization temperature, and time on yield and molecular weight of the polymer were investigated. The structures of the synthesized compounds were illuminated by UV–Vis, FTIR, ¹HNMR, XRD, TG/DTG, and SEM/EDAX methods. Then, Pd(II) complex of P (4-PMB) was produced, and its catalytic performance was tested in the synthesis of various biaryl compounds by Suzuki reactions using microwave irradiation technique which is a highly effective, fast, solvent-free and green method. The designed Pd(II) catalyst exhibited superior catalytic effect against syntheses of various biaryl compounds with high conversion yields (99%) and very low catalyst loading (0.01 mol%) for 5 min. Recyclability and reproducibility tests showed that the catalyst could be reused for seven successive runs without any great loss of its catalytic performance (84%).

Full text of DOI:10.1016/j.jorganchem.2018.04.022

Accepted Manuscript
Highly effective and recoverable Pd(II) catalyst immobilized on thermally stable Schiff
base polymer containing phenol group: Production, characterization and application in
Suzuki coupling reactions
Nuray Yılmaz Baran, Talat Baran, Ayfer Menteş, Meral Karakışla, Mehmet Saçak
PII:
S0022-328X(18)30265-1
10.1016/j.jorganchem.2018.04.022
JOM 20414
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 21 January 2018
Revised Date: 2 March 2018
Accepted Date: 13 April 2018
Please cite this article as: N. Yılmaz Baran, T. Baran, A. Menteş, M. Karakışla, M. Saçak, Highly
effective and recoverable Pd(II) catalyst immobilized on thermally stable Schiff base polymer containing
phenol group: Production, characterization and application in Suzuki coupling reactions, Journal of
Organometallic Chemistry (2018), doi: 10.1016/j.jorganchem.2018.04.022.
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Highly Effective and Recoverable Pd(II) Catalyst Immobilized on Thermally Stable
Schiff Base Polymer Containing Phenol Group: Production, Characterization and
Application in Suzuki Coupling Reactions
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Nuray Yılmaz Baran , Talat Baran , Ayfer Menteş , Meral Karakışla , Mehmet Saçak
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Aksaray University, Technical Vocational School, Department of Chemistry Technology,
Aksaray 68100, Turkey
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Aksaray University, Faculty of Arts and Sciences, Department of Chemistry, Aksaray 68100,
Turkey
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Ankara University, Faculty of Science, Department of Chemistry, Ankara 06100, Turkey
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Corresponding author:
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Address: Aksaray University, Technical Vocational School, Department of Chemistry
Technology, Aksaray 68100, Turkey
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Tel.: +90 382 2882135; fax: +90 382 2882028.
E-mail address: nybaran@aksaray.edu.tr (Nuray Yılmaz Baran)
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Abstract
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In this study, a novel highly efficient, recoverable, sustainable, and thermally stable Schiff
base polymer-Pd(II) catalyst was designed for Suzuki coupling reactions in synthesis of biaryl
compounds for the first time. Firstly, a novel Schiff base polymer containing phenol, Poly(4-
((pyridine-2ylimino)methyl)benzene-1,3-diol) (P(4-PMB)), which was used as a support
material in catalyst design, was synthesized in aqueous alkaline medium via oxidative
polycondensation of 4-((pyridine-2-ylimino)methyl)benzene-1,3-diol (4-PMB) with NaOCl
oxidant. The effects of oxidant concentration, polymerization temperature, and time on yield
and molecular weight of the polymer were investigated. The structures of the synthesized
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compounds were illuminated by UV-Vis, FTIR, HNMR, XRD, TG/DTG, and SEM/EDAX
methods. Then, Pd(II) complex of P(4-PMB) was produced, and its catalytic performance was
tested in the synthesis of various biaryl compounds by Suzuki reactions using microwave
irradiation technique which is a highly effective, fast, solvent-free and green method. The
designed Pd(II) catalyst exhibited superior catalytic effect against syntheses of various biaryl
compounds with high conversion yields (99%) and very low catalyst loading (0.01 mol%) for
5 min. Recyclability and reusability tests showed that the catalyst could be reused for seven
successive runs without any great loss of its catalytic performance (84%).
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Keywords: Catalyst; oxidative polycondensation; Schiff base polymers; polyphenols; Suzuki
coupling reactions; biphenyl compounds.
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1. Introduction
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Biaryls are widely preferred compounds in many industrial applications such as
pharmacology, agrochemicals, cosmetics, and medicine thanks to their outstanding properties
[1]. Thus, synthesis of biaryls have gained more interest in recent years, and various methods
such as Stille [2], Heck [3], Negishi [4], Kumada [5] and Suzuki [6] coupling reactions have
been developed for this process. Among the carbon–carbon formation reactions, Suzuki
coupling reactions are extensively preferred thanks to their superior properties such as i)
stability against temperature, air, and moisture; ii) tolerance to wide range of functional
groups and water; iii) high selectivity; as well as iv) ease of removal of by-products [7].
Suzuki coupling reactions have been usually executed in the presence of catalysts containing
palladium, and heterogeneous catalyst systems are generally preferred by researchers for these
processes [8]. It is because, heterogeneous catalysts have outstanding advantages such as ease
of removal from the reaction medium and applicability in consecutive reactions compared to
homogenous catalysts [9]. Also, the most important problems, which are encountered in
catalytic systems such as low recycle ratios originating from leaching, difficulty in recovery,
and high cost of the catalyst have been overcome via heterogeneous catalysts [10]. Thus, in
recent years, heterogeneous catalysts have been hopeful materials for industrial applications.
However, Suzuki coupling reactions also have some disadvantages which can be listed as I)
use of toxic solvents, II) high reaction temperature, III) long reaction time, IV) labor-intensity
[11]. To overcome these problems, researchers have improved microwave irradiation
technique for Suzuki coupling reactions [12]. The novel efficient method produces high
product yields in a very short reaction time, and thus high TON and TOF values are obtained.
Also, this simple and fast technique eliminates requirement of toxic organic solvents [13]. All
of them are important parameters for use in industrial applications.
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In Suzuki coupling reactions, it is desired that support materials for catalysts should be
thermally and mechanically stable to be industrially applicable. Schiff base polymers
containing phenol groups may be alternative support materials for catalysts to synthesize
biaryl compounds via Suzuki coupling reactions. Schiff base polymers containing phenol
groups have been known as promising materials because of their semiconductive [14, 15],
antimicrobial [15, 16], outstanding optic [17], fluorescent [18] and electrochemical [14, 17],
catalytic [19] as well as high thermal stability, and mechanical [20] properties. These
polymers are generally synthesized using oxidative polycondensation reactions. This method
is executed using NaOCl, H O , and O as mild oxidants [21]. The most important advantages
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of this synthesis method are the cheapness of the used oxidants, ease of separation of
oxidants from the reaction media, mild reaction conditions, production of polymers with high
solubility, high molecular weight, good thermal and mechanical stabilities as well as
formation of environmentally friendly byproducts (NaCl and H O) [20, 21].
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A thermally stable Schiff base polymer was used in this research as a support material for
Suzuki coupling reactions in synthesis of biaryl compounds for the first time in the literature.
Thus, a novel highly efficient, thermally resistant, easily recoverable Schiff base polymer-Pd
(II) heterogeneous catalyst was designed for Suzuki coupling reactions. Firstly, the synthesis
of P(4-PMB) Schiff base polymer was carried out by oxidative polycondensation of 4-PMB,
using NaOCl oxidant in an aqueous alkaline medium. The effects of oxidant concentration
and polymerization temperature and time were examined on yield and molecular weight of
P(4-PMB). Various characterization techniques were used to verify the structures. Secondly,
P(4-PMB)-Pd (II) catalyst was synthesized, and its catalytic performance was investigated in
Suzuki coupling reactions for synthesis of various biaryl compounds using a simple, fast,
solvent-free microwave irradiation technique. Recycle and recoverability performances of the
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catalyst were tested, and the results exhibited that the performance of the catalyst continues
without any significant change even after many catalytic cycles.
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2. Experimental
2.1. Materials
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2,4-dihydroxybenzaldehyde,
2-aminopyridine, potassium hydroxide (KOH), disodium
tetrachloropalladate (Na PdCl ), sodium carbonate (Na CO ) cesium carbonate (Cs CO ),
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potassium carbonate (K CO ), magnesium sulfate (MgSO ), phenyl boronic acid, aryl halides
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hydrochloric acid (HCl, 37%), iodine, acetone, N,N-dimethylformamide (DMF), dimethyl
sulfoxide (DMSO), ethyl acetate, methanol, ethanol, n-methylpyrrolidone, 1,4-dioxane, n-
heptane, tetrahydrofuran (THF) and toluene were supplied by Merck Chem. Co. (Germany).
Sodium hypochlorite (NaOCl, 15% aqu.) was provided from Birpa Co. (Turkey). All
chemicals were used as received without further purification.
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2.2. Method
2.2.1. Synthesis of 4-PMB
4-PMB Schiff base monomer was synthesized by general condensation reaction of 2,4-
dihydroxybenzaldehyde and 2-aminopyridine (Scheme 1). A solution of (10 mmol, 1.38 g)
2,4-dihydroxybenzaldehyde in 15 mL methanol was added on 10 mmol (0.94 g) of 2-
˚
aminopyridine dissolved in 5 mL methanol. The mixture was continuously stirred at 40 C.
The completion of the reaction was intermittently checked by FTIR and TLC techniques.
After 24 h, the mixture which contained the precipitated reddish product was filtered out and
then washed with cold methanol to remove unreacted amine and aldehyde. Recrystallization
of 4-PMB was performed from heptane and the purified Schiff base monomer was dried in
vacuum desiccator (Yield: 91%).
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2.2.2. Synthesis of P(4-PMB)
P(4-PMB) was obtained by oxidative polycondensation reaction of 4-PMB using NaOCl
oxidant in aqueous alkaline medium (Scheme 1). 1 mmol (0.214 g) of 4-PMB was dissolved
in aqueous solution of KOH (10%, 1 mmol) by refluxing under nitrogen atmosphere. After
the dissolution was completed and determined reaction temperature was reached, 1 mmol of
NaOCl was added to the reaction medium dropwise for 10 min. As soon as the oxidant was
added, the reddish color of the monomer solution turned into dark brown. When the
determined polymerization time was completed, the obtained polymer solution was cooled to
room temperature and precipitated by neutralizing with HCl (37%). The precipitated product
was filtered and washed with hot water and heptane to separate from mineral salts and
unpolymerized monomer, respectively. The brown polymer was dried in an oven at 80 ˚C and
weighed. The yield of the polymer (Yield, %) was determined according to Equation 1:
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ꢃ
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Yieldꢀꢁ%ꢂ =ꢀꢃ × 100
(1)
ꢅ
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where w and w describe the obtained polymer and initial monomer weights, respectively.
p
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2.2.3. Synthesis of P(4-PMB)-Pd catalyst
0.2 g of P(4-PMB) was dissolved in 15 mL methanol at 70 C and Na PdCl (0.2g) solution
°
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which was prepared in 10mL water was added into the polymer solution. The reaction mixture
was continuously stirred under reflux for 24h. Then, the precipitated dark brown product was
filtered and rinsed with water and methanol to remove unreacted Na PdCl and polymer,
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respectively (Yield: 92%).
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Scheme 1. Synthesis of 4-PMB, P(4-PMB) and P(4-PMB)-Pd catalyst
2.2.4 General procedure for Suzuki coupling reactions
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Firstly, the coupling reaction of 4-bromoanisol and phenylboronic acid was
determined as the model reaction. Then, to detect ideal reaction conditions, the effects of base
system, reaction time, and catalyst amount on the yield of the biaryl compounds were
investigated for the model coupling reaction which occurred in the presence of the
synthesized P(4-PMB)-Pd (II) catalyst.
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As a result of the determined optimum conditions, several aryl halides (1.12 mmol),
phenyl boronic acid (1.87 mmol), potassium carbonate (3.75 mmol), and P(4-PMB)-Pd
catalyst (0.01 mol%) were put into a Schlenk tube and mixed. The reactions were actualized
by microwave irradiation technique using 400W power in solventless media for 5 min. At the
end of the coupling reactions, toluene (4 mL) was added on the obtained products and filtered.
Then, extraction of the supernatant was done with water (2 mL) for three times, and the
organic phase, which contained synthesized biaryl compound, was separated from water using
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separatory funnel. To remove the water completely, MgSO was added on the organic phase.
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Then, toluene was removed at room temperature, and the yield of biaryl compound which was
completely dried was calculated. To verify the structures of the biphenyl compounds, they
were analyzed by GC/MS technique.
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2.3. Characterization
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To determine molecular weight distributions (M , M and PDI values) of the synthesized
n
w
polymers, Shimadzu Prominence Gel Permeation Chromatography was used as equipped with
a preparative Nucleogel GPC 103-5 VA300/7.7 column (eluent: dimethylformamide (DMF),
flow rate: 0.5 mL/min temperature: 40 °C), and monodisperse polystyrene standards was used
for the calibration. UV-Vis spectra of 4-PMB, P(4-PMB), and P(4-PMB)-Pd were obtained
from their solution prepared in DMSO in the wavelength range of 260-700 nm by Shimadzu
UV-1700 PharmaSpec UV-Visible Spectrophotometer. FTIR spectra of the synthesized
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products were taken in the wavenumber range of 4000-650 cm using Perkin Elmer FTIR
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Spectrometer. To identify the molecular structures of 4-PMB and P(4-PMB), their H-NMR
spectra were recorded from their solutions in DMSO by Bruker Avance 500 MHz NMR at 25
°
C. Thermal behaviors of 4-PMB, P(4-PMB), and P(4-PMB)-Pd were investigated by
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TG/DTG techniques taken under nitrogen atmosphere in the temperature range of 30-1200 C
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at heating rate of 10 C/min in a platinum crucible using EXSTAR S11 7300 thermal analyzer.
Powder X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku smart lab
system (at 40 kV, 30 mA, and 2θ with a scan angle of 10−70°). The surface morphologies of
the products were obtained by coating with platinum using QUANTA-FEG 250 ESEM. An
EDAX-Metek was used to determine the presence of palladium ion on polymer. Palladium
ion content of the Pd catalyst was also analyzed by Perkin Elmer Optima 2100 DV
Inductively Coupled Plasma (ICP) Optical Emission Spectrometer (OES). Verification of the
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structures of the synthesized biaryl compounds was done by GC-MS Agilent GC-7890 A- MS
5975. Catalytic tests were done with a domestic microwave oven.
3. Results and Discussion
3.1. Effect of Synthesis Conditions on Polymer Yield and Molecular Weight Distribution
of P(4-PMB)
3.1.1. Oxidant concentration
The changes of the yield and molecular weight distribution of the polymer with oxidant
concentration were investigated and the obtained data are presented in Table 1. As seen in the
table, the polymer yield increased to 0.1 mol/L NaOCl concentration but the yield did not
significantly change after this concentration [22].
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The results of GPC analysis of P(4-PMB) show that the molecular weight values increased to
0.1 mol/L NaOCl concentration but after this concentration, they decreased (Table 1). The
decrease which was determined at higher concentrations can be explained by the increase in
the number of active centers. Therefore, the growing chains remain short due to this increase
[23].Since the highest polymer yield and molecular weight values (M =36710, Mn=20890
w
g/mol and PDI=1.75) were obtained using 0.1 mol/L NaOCl, the subsequent experiments were
performed with 0.1 mol/L NaOCl concentration.
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Table 1. The effect of NaOCl concentration on the yield and the molecular weight
distribution of P(4-PMB)
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NaOCl
concentration
Yield
%)
Mw
(g/mol)
Mn
(g/mol)
PDI
(
(
mol/L)
0
.05
49.6
74.3
76.2
75.6
10798
36710
11641
7812
8870
20890
4353
3672
1.21
1.75
2.67
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([4-PMB] =[KOH] =0.1 mol/L, polymerization temperature: 80 °C, time: 6 h).
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3.1.2. Polymerization temperature and time
The effects of polymerization temperature and time on the polymer yield were investigated
and the molecular weight values and the obtained results were exhibited in Table 2. When the
change of polymer yield with polymerization temperature was examined, it was seen that the
polymer yield decreased with increasing temperature [24]. While 81.4% yield was obtained at
90 ˚C, the yield was determined to be 95.6% at 25 ˚C. This case can be explained by breaking
of the polymer chains due to increasing temperature. Short chains which form after breaking
of the chains may be removed by dissolving during washing of polymer, and thus the polymer
may be obtained with low yield.
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On the other hand, molecular weight values of the polymer increased with increase in
temperature (Table 2). M , M and PDI values of the polymer which was synthesized at 25 °C
w
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were 4882, 3552 g/mol and 1.37, respectively. The values for the polymer which was
synthesized at 90 °C were measured as 93199, 51700 g/mol and 1.80, respectively. This
increase is an expected phenomenon for polycondensation reactions in the literature [22, 25].
Moreover, thermal stabilities of the polymers which were synthesized at 25 °C and 90 °C
were compared by TG/DTG analysis. It was determined that while the polymer which was
obtained at 25 °C completely degraded at 600 °C, the weight loss of the polymer obtained at
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90 °C continued to 1200 °C. Thus, when the polymer was evaluated in terms of thermal
stability and molecular weight values, the subsequent experiments were performed at 90 °C.
When Table 2 is examined in terms of the change of polymerization time, it is seen that the
polymer yield increased up to 3h and reached 96.8%. After this polymerization time, the yield
decreased with the increase in polymerization time. These obtained results are in accord with
the literature [22, 26, 27]. This may be explained by breaking of the polymer chains. The
polymeric chains which were synthesized at high temperature for higher durations may
remain short due to the breaking and the short chains may move away by dissolving during
washing of the polymer.
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Moreover, the change of the molecular weight values of P(4-PMB) with polymerization time
was analyzed (Table 2) and a gradually decrease in molecular weight values of P(4-PMB) was
determined with the increase in polymerization time. The highest molecular weight values
(M =223362Mn=139350g/mol and PDI=1.60) were obtained at the end of 1 h. This can be
w
explained by breaking of the polymer chains synthesized at high temperature during higher
durations [22].
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Table 2. The effect of polymerization temperature and time on the yield and the molecular
weight distribution of P(4-PMB)
.
Temperature
°C
Time
(h)
Yield
(%)
Mw
(g/mol)
Mn
(g/mol)
PDI
(
)
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95.6
88.2
86.2
84.3
81.4
50.2
79.8
96.8
84.3
81.4
68.6
4882
15841
17797
36710
93199
223362
157940
125380
105790
93199
9071
3552
9511
1.37
1.66
1.57
1.75
1.80
1.60
1.79
1.82
1.83
1.80
1.29
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11320
20890
51700
139350
88160
68640
57828
51700
7004
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15
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([4-PMB] =[KOH] =[NaOCl] = 0.1 mol/L)
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3.2. Structures of 4-PMB, P(4-PMB), and P(4-PMB)-Pd catalyst
3.2.1. UV-Vis spectra
UV-Vis spectra of 4-PMB, P(4-PMB) and its palladium (II) catalyst were recorded by
dissolving in DMSO. In UV-Vis spectrum of 4-PMB, two bands were observed at the
wavelengths of 290 and 342 nm. The presence of these bands can be referred to π-π∗
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transitions belonging to benzene ring (-C=C) and π-π transitions of the imine functional
group. The same transitions were detected at 295 and 386 nm in the spectrum of P(4-PMB).
In the spectrum of P(4-PMB)-Pd, a new band was seen at 537 nm which originates from d-d
∗
transitions of palladium ion in addition to the bands of π-π transitions at 297 and 392 nm
[28]. This new band confirms that the complexation was achieved. When the spectra of the
monomer and the polymer were compared to each other, it was determined that the spectrum
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of the polymer broadened to 700 nm while the monomer spectrum terminated at 500 nm.
Moreover, all the bands of the polymer shifted to red and broadened too much. All these
differences originate from the increasing conjugation and verify that the polymerization was
accomplished [15, 19, 29, 30].
3.2.2. FTIR Spectra
FTIR analysis was used to support the structures of the synthesized samples, and the obtained
spectra for 4-PMB, P(4-PMB), and P(4-PMB)-Pd are presented in Supplementary Figure S1.
-1
When the spectra are investigated, the peaks at 3038, 3075, and 3094 cm are due to the
vibrations of phenolic OH groups for 4-PMB, P(4-PMB) and P(4-PMB)-Pd, respectively.
While CH=N- vibration, which is characteristic for Schiff bases and their polymers, is seen at
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1610 cm in the monomer spectrum, the peaks which belong to this vibration are observed at
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1613 and 1602 cm in the polymer and its Pd (II) catalyst spectra, respectively. The presence
of azomethine peak in the polymer spectrum also demonstrate that these characteristic groups
are protected without degradation during polymerization [15]. The other peaks for 4-PMB,
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P(4-PMB), and P(4-PMB)-Pd are observed at 1584-1432, 1474-1436, and 1434 cm (aromatic
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C=C vibrations); 1205, 1229, and 1233 cm (C-O stretching vibrations); 1143, 1170, and
1081 (C-N vibrations of pyridine ring), respectively. When FTIR spectrum of the polymer is
compared to that of the monomer, (i) shifting of –CH=N– vibration to higher wavenumber,
(ii) broadening of the peaks, and (iii) the decrease in the peak number are detected in the
polymer spectrum. All of these findings originate from the increase in the conjugation length
and prove that polymeric structure is achieved [15, 19, 22, 29, 30]. Moreover, shifting of –
-1
-1
CH=N– vibration of Pd(II) catalyst from 1613 cm to 1603 cm compared to that of the
polymer, shows that the palladium ion was coordinated to the polymeric structure through the
nitrogens of azomethine groups.
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3.2.3. H-NMR Spectra
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To illuminate the structure of 4-PMB and P(4-PMB), their H-NMR spectra were recorded
and are displayed in Supplementary Figure S2. The signals of the protons of –OH and -
1
CH=N- functional groups in H-NMR spectra of 4-PMB are observed at 9.90 (s, 1H) and 9.32
(s, 1H) ppm, respectively. The same signals which belong to the polymer are seen at 11.65 (s,
1H) and 9.81 (s, 1H) ppm, respectively. The other signals which verify the aromatic structure
of 4-PMB are determined at δ= 8.47 (s, 1H); 7.87(d, 1H); 7.54 (d, 1H); 7.36 (d, 1H); 7.08 (t,
1H); 7.33 (d, 1H,); and 7.27 (d, 1H) ppm. Additonally, the signals which prove the aromatic
structure of P(4-PMB) are found at δ= 7.95 (d, 1H); 7.85 (t, 1H); 7.55 (d, 1H); 6.95 (d, 1H);
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6.83 (t, 1H); and 6.67 (d, 1H) ppm. When H-NMR spectrum of polymer is compared to that
of the monomer, broadening in aromatic signals of the polymer occurs because several
polymer chains with different molecular weights have different chemical environments.
Moreover, (I) the absence of the aromatic singlet signal of the monomer, in the polymer
spectrum and (II) the observation of OH signal of the polymer with 1H integration compared
to OH signal of the monomer with 2H integration, express that the polymerization process
continues from C-O-C and C-C couplings which occur from OH groups and ortho positions of
OH groups during polymerization, respectively [14, 15, 22, 30].
3.2.4. Thermal Degradation Process
TG and DTG curves were recorded to determine thermal behaviors of 4-PMB, P(4-PMB), and
P(4-PMB)-Pd catalyst (Figure 7). As seen in TG curve of 4-PMB, the monomer began to
°
degrade at 99 C and degraded in three steps. While 37.2% weight loss was seen in the first
°
step in the temperature range of 99-300 C, 22.8% weight loss was determined for the second
°
step in the temperature range of 300-452 C, and 40% weight loss was defined in the third
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step (452-600 C). The maximum degradation temperatures (Tmax) belonging to these
°
degradation steps were determined from DTG curves at 232, 354 and 635 C, respectively.
°
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Additionally, TG curve of P(4-PMB) was recorded up to 1200 C, and the degradation of the
polymer actualized in three steps. Firstly, the weight loss of 3.37% in the temperature range of
°
30-122 C can be ascribed to the removal of adsorbed water. The other weight losses in the
°
temperature ranges of 122-518, 518-852, and 52-1200 C were determined as 37.31%,
17.87%, and 14.2%, respectively. Tmax values which belong to these degradation steps were
°
also found as 260, 585 and, 1036 C, respectively.
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As seen in TG curve of P(4-PMB)-Pd catalyst, the catalyst degraded in three steps up to 1200
°
°
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C. The weight loss in the temperature range of 30-142 C (4.5%) can be attributed to the
removal of adsorbed water. The weight losses which belong to degradation steps of the
catalyst were determined as 19.2%, 8.0%, 13.7%, and 10.2% in the temperatures ranges of
°
142-378, 378-520, 520-873, and 873-1200 C, respectively.
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When the thermal behaviors of the monomer, polymer and Pd(II) catalyst were compared to
°
each other, 30% weight loss took place at 260, 360, and 480 C, and 50% weight loss occurred
°
at 369, 622, and 1018 C, respectively. These findings indicate that the thermal stability of the
polymer is higher than that of the monomer due to the increasing conjugation in the polymer
structure. Moreover, while 4-PMB did not leave any carbon residue, P(4-PMB) and P(4-
°
PMB)-Pd catalyst had carbon residues at 1200 C which were found as 27.5% and 44.4%,
respectively. The reason why the catalyst has the highest residue also originates from metal
oxide residue [15, 19, 30].
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3.2.5. XRD Analysis
To learn about the crystalline structures of 4-PMB, P(4-PMB) and P(4-PMB)-Pd catalyst,
XRD patterns were recorded in the region of 2θ= 5−60° and are illustrated in Figure 1. A
°
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broad diffraction peak which is centered at about 22-25 (2θ) is observed for all synthesized
samples. It means that the prepared samples have amorphous nature. As seen in the figure, the
°
diffraction peak of the polymer is broader than the monomer peak, and it shifted to 25 (2θ)
°
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from 22 (2θ). This broadening expresses a decrease of crystallinity and an increase of
amorphous nature which verify that the polymerization occurred successfully. Moreover, the
peak of Pd(II) catalyst is also broader than that of the polymer. This phenomenon is also seen
in literature and explained by the coordination of palladium ion with the polymer [31]. On the
other hand, two new weak peaks were observed at 40.12 (2θ) and 46.78 (2θ) in the diffraction
pattern of Pd(II) catalyst. These diffraction peaks can be attributed to the presence of
palladium species [32].
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Figure 1. XRD patterns of 4-PMB, P(4-PMB), and P(4-PMB)-Pd catalyst.
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3.2.6. SEM/EDAX and ICP-OES analysis
SEM micrographs of 4-PMB, P(4-PMB), and P(4-PMB)-Pd catalyst were obtained to show
their surface morphologies and these images are displayed in Figure 2. while the monomer
surface is determined to consist uniform spheres, it is seen that the polymer surface has a
layered structure. These differences emphasize that the surface changed following
condensation polymerization. When P(4-PMB)-Pd catalyst surface is analyzed, it is seen that
the particles of palladium ions homogenously covered the polymer surface had layered
structure.
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Figure 2. SEM of micrographs of 4-PMB, P(4-PMB) and P(4-PMB)-Pd catalyst.
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Moreover, EDAX analysis of Pd(II) catalyst was carried out to confirm the presence of
palladium ions coordinated to the polymeric structure (Figure 3). In EDAX spectrum,
palladium signals are apparently seen and verify that palladium ions were successfully
coordinated. Additionally, the amount of the palladium ions was also found by ICP-OES and
as 35% by mass.
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Figure 3. EDAX spectrum of Pd(II) catalyst.
3.3 Catalytic performance of P(4-PMB)-Pd catalyst
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3.3.1 Optimal reaction conditions for Suzuki cross coupling reactions
All Suzuki coupling reactions which were carried out for the synthesis of various biaryl
compounds were performed with microwave irradiation technique which is simple, fast,
solvent-free, and highly efficient. The reaction of p-bromoanisole and phenyl boronic acid
was determined as model Suzuki coupling reaction to detect optimum reaction conditions.
The effects of catalyst amount, base type, and reaction time on the yield of synthesized biaryl
compound (4-methoxybiphenyl) were investigated and the obtained results are presented in
Figure 4.a, b, and c, respectively. As seen in Figure 4.a, it was determined that the product
yield increased up to 0.01 mol% catalyst amount, and after this catalyst amount, the yield
gradually decreased with the increasing catalyst amount. Since the highest product yield was
obtained with 0.01 mol% catalyst amount, the subsequent experiments was continued with
this catalyst loading. To determine the effect of base type, the coupling reactions were tested
in the medium of NaOH, KOH, Na CO , Cs CO and K CO (Figure 4.b). Since the highest
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biphenyl yield (99%) was obtained with K CO base, the following experiments were carried
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determined as 5 min. (Figure 4.c). Thus, 5 min was used as the optimal reaction time for all
Suzuki coupling reactions.
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Figure 4. The effects of catalyst loading (a), reaction time (b) and base type (c) on catalyst
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activity of P(4-PMB)-Pd.
3.3.2 Catalytic activity of P(4-PMB)-Pd catalyst in the Suzuki-Miyaura reactions
The catalytic performance of the prepared P(4-PMB)-Pd catalyst was studied in Suzuki cross
coupling reactions under the aforementioned optimization conditions by using microwave
irradiation technique, and the results are given in Table 3. The catalyst gave excellent
biphenyl yields with aryl bromides containing electron-withdrawing group (Table 3). For
example, when p-bromonitrobenzene and p-bromobenzonitrile were used as the substrate,
91% and 89% conversion yields, respectively, were reached (Table 3, entry 4,5). In addition,
good biphenyl yields were obtained with aryl bromides bearing -meta and –para substituted
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electron-donating groups (-CH ) (Table 3, entry 7,8). The catalytic activity of the catalyst was
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also tested with electronically different aryl iodides, and the perfect reaction yields were
reached in the presence of the catalyst (Table 3, entry 9,10). For example, the cross coupling
reaction of p-iodoanisole with phenyl boronic acid produced excellent yield (99%). Aryl
iodide with meta-substituted-NO gave an isolated yield of 90%. We also investigated the
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aryl bromides and iodides, as expected (Table 3, entry 11,12). For example, the coupling
reaction gave 55% yield with p-chloroanisole and 65% yield with 1-chloro-3-nitrobenzene.
These lower reaction yields can be explained by poor activities of aryl chlorides in Suzuki
coupling reactions [31]. Performed tests revealed that the designed catalyst is highly effective
towards Suzuki cross coupling reactions.
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Table 3. Suzuki coupling reactions of several aryl halides with phenylboronic acid using P(4-
PMB)-Pd catalyst.
Entry Phenyl boronic
acid
Aryl halide
Product
Yield (%)
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Reaction conditions: 1.12 mmol aryl halides, 1.87 mmol phenyl boronic acid, 3.75 mmol K CO , 0.01 mol%
2 3
Pd catalyst, 50 °C, 0.08 h under MW
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3.3.3 Reproducibility capacity of P(4-PMB)-Pd catalyst
Reproducibility performance for catalysts is one of the most important parameters from the
view point of industrial applicability [33]. Thus, the reusability performance of the designed
catalyst was examined on the model reaction under the determined optimum reaction
conditions. After the model reaction was completed, the filtered catalyst was washed with hot
water and methanol to renew the catalyst and the regenerated catalyst was reused in Suzuki
coupling reaction. This process was repeated for each cycle. The catalyst provided 84%
product conversion even after seven recycles. This result shows that the designed reproducible
and reusable catalyst is ideal for industrial applications to reduce operational cost. FTIR
spectra of the recycled catalyst and the fresh catalyst were compared to determine whether
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there was any change in the catalyst structure after recycle tests, and no significant change
was observed. It indicates that the recovered and reused catalyst preserved its chemical
structure without any changes. This reproducibility of the designed catalyst makes it a
promising material for industrial applications.
4. Conclusions
In conclusion, a novel Schiff base polymer-Pd(II) catalyst, which is highly efficient, thermally
stable, renewable, and sustainable, was fabricated for Suzuki coupling reactions for the first
time in the literature. A novel Schiff base polymer containing phenol group which was used
as support material was produced by oxidative polycondensation reaction of 4-PMB in an
aqueous alkaline medium in the presence of NaOCl oxidant. The effect of the change of
polymerization conditions on molecular weight monitoring of P(4-PMB) was monitored since
the molecular weight is one of the most important parameters for polymers in terms of
industrial applications. Thermal analysis data revealed that the polymer is thermally more
stable than its monomer which means that the polymer can be used in industrial materials that
require high thermal stability. Moreover, the fabricated Pd(II) catalyst was investigated in
terms of its catalytic activity for Suzuki coupling reactions in the synthesis of various biaryls
compounds. The catalytic performance experiments were carried out using microwave
irradiation technique which is easy, rapid, and solvent free, and require low workforce and
power. The designed catalyst gave outstanding catalytic activities by producing high biphenyl
yields. Additionally, the reproducibility tests revealed that the catalyst is renewable and can
be reused seven times without any great loss from its catalytic ability. The reproducibility
performance of the catalyst plays a key role for industrial applications in terms of cost
reduction.
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Acknowledgments
The authors would like to thank Aksaray University Research Funds for the financial support
of this work (Project No. 2016-022).
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Figure Captions
Figure 1. XRD patterns of 4-PMB, P(4-PMB), and P(4-PMB)-Pd catalyst.
Figure 2. SEM of micrographs of 4-PMB, P(4-PMB), and P(4-PMB)-Pd catalyst.
Figure 3. EDAX spectrum of Pd (II) catalyst.
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Figure 4. The effects of (a) catalyst loading, (b) reaction time and (c) base type on catalyst
activity of P(4-PMB)-Pd.
Scheme 1. Synthesis of 4-PMB, P(4-PMB), and P(4-PMB)-Pd catalyst.
8
Table Captions
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Table 1. The effect of NaOCl concentration on the yield and the molecular weight
distribution of P(4-PMB).
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Table 2. The effect of polymerization temperature and time on the yield and the molecular
weight distribution of P(4-PMB).
Table 3. Suzuki coupling reactions of several aryl halides with phenylboronic acid using P(4-
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PMB)-Pd catalyst.
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