Pd(OAc)2 immobilized on imine-functionalized microporous covalent triazine polymer as efficient heterogeneous catalyst for Mizoroki–Heck cross-coupling reaction

Article abstract of DOI:10.1007/s11164-018-3265-8

A template consisting of Pd(OAc)₂ immobilized on imine-functionalized microporous covalent triazine polymer network (Pd/TATAE) was synthesized and found to be an efficient and recyclable heterogeneous solid catalyst for Mizoroki–Heck C–C coupling of styrene with substituted aryl halides under mild reaction conditions and without addition of any ligand or additives. Furthermore, Pd/TATAE could be recycled for nine consecutive cycles with minor loss of catalytic activity. Finally, the Pd/TATAE catalyst showed good catalytic activity with excellent yield of 96?% and high turnover number of 3349.

Full text of DOI:10.1007/s11164-018-3265-8

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3265-8
Pd(OAc)2 immobilized on imine-functionalized
microporous covalent triazine polymer as efficient
heterogeneous catalyst for Mizoroki–Heck
cross-coupling reaction
1
1
•
Velu Sadhasivam1 Mariyappan Mathappan Muniyasamy Harikrishnan
•
•
Chinnadurai Chithiraikumar1 Sepperumal Murugesan1
•
•
Ayyanar Siva1
Received: 24 October 2017 / Accepted: 8 January 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract A template consisting of Pd(OAc)2 immobilized on imine-functionalized
microporous covalent triazine polymer network (Pd/TATAE) was synthesized and
found to be an efficient and recyclable heterogeneous solid catalyst for Mizoroki–
Heck C–C coupling of styrene with substituted aryl halides under mild reaction
conditions and without addition of any ligand or additives. Furthermore, Pd/TATAE
could be recycled for nine consecutive cycles with minor loss of catalytic activity.
Finally, the Pd/TATAE catalyst showed good catalytic activity with excellent yield
of 96 % and high turnover number of 3349.
Keywords Covalent organic polymer Á 1,4-Diaminobenzene Á Mizoroki–Heck
coupling Á Palladium catalyst Á Shelton test Á Triazine
Introduction
Covalent organic polymers (COPs) are a new class of porous crystalline polymeric
materials with tailorable [one-dimensional (1D), two-dimensional (2D), and three-
dimensional (3D)] structure and different functionalities. This emerging field
represents a critical current research area [1]. COPs are micro/mesoporous solid
materials that can be synthesized by extending an organic structure via robust
covalent linkage of organic building units consisting of light elements such as C, Si,
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11164-
018-3265-8) contains supplementary material, which is available to authorized users.
&
Ayyanar Siva
drasiva@gmail.com
1
Department of Inorganic Chemistry, School of Chemistry, Supramolecular and Organometallic
Chemistry Lab, Madurai Kamaraj University, Madurai, Tamil Nadu 625021, India
123

V. Sadhasivam et al.
O, B, and N only [2]. These porous solid materials possess superior advantages for
use in potential applications such as gas storage [3], optical and semiconducting
devices [4], electrochemistry [5], drug delivery [6], photocatalysis [7], and sensing
[8, 9], among many others, and especially in heterogeneous catalysis [10–12]
because of their low density, eclipsed layered porous structure [13], large surface
area, tunable properties, thermal and chemical stability, and other functionalities
[14, 15]. In addition, porous COP materials are not soluble in any other organic or
aqueous solvents and are highly stable in high-boiling solvents.
In recent years, transition-metal catalysis has become an essential tool in organic
synthesis [16, 17]. Among transition metals, catalysts containing palladium play a
significant role in various cross-coupling reactions such as C–C, C–N, C–O, and C–
Se bond formation reactions [18], being widely used for synthetic organic
transformations in pharmaceutical and medicinal chemistry [19]. Over recent
decades, various homogeneous catalytic systems [20, 21], which always exhibit
better activity and high selectivity, have been developed for such organic
transformations [22, 23]. However, most homogeneous catalysts suffer from
drawbacks such as high-cost phosphine ligands, use of various additives, difficulty
in separation, metal leaching, recovery, recyclability, and the toxicity of phosphine
ligands. In this regard, Pd catalysts can be heterogenized to overcome such
difficulties and make such protocols green.
Recently, a number of studies have focused on incorporation of Pd nanoparticles
into various solid supporting materials, including zeolite [24], silica [25],
metalorganic frameworks (MOFs) [26], functionalized graphene oxide [27], and
COPs to generate heterogeneous catalysts. Moreover, coordination reactions
between imine-type (Schiff base) ligands incorporating a variety of metal ions
have effectively demonstrated that these are highly stable materials. Thus, over the
last few decades, many research groups have been motivated to explore the
possibility of using imine-based COFs as excellent platforms for heterogeneous
catalysis [16]. Yaghi et al. and Ding et al. first reported imine-based COFs and
investigated them for heterogeneous catalysis of C–C coupling reactions [28–30]. In
2011, Farhad Panahi et al. reported palladium nanoparticles supported on a silica-
starch substrate (PNP-SSS) as an efficient catalyst for Mizoroki–Heck and
Sonogashira coupling reactions [31]. Rahul Banerjee et al. reported synthesis of
Pd(II)- and Pd(0)-doped imine-functionalized COF materials and their multifunc-
tional character with catalytic activity in Mizoroki–Heck, Sonogashira, and
oxidative biaryl (C–H activation) coupling reactions [32].
Recently, our group successfully synthesized and reported triazine-based
microporous covalent imine-functionalized polymers as novel solid supports for
palladium (Pd/TATAE)-mediated Suzuki–Miyaura coupling reaction in aqueous
medium. In the work reported herein, COP materials were prepared according to our
previous literature report [11]. The present study represents an extension of previous
work towards the Mizoroki–Heck coupling reaction between styrene and various
aryl halides with the aim of achieving good catalytic activity with excellent yield,
and high stability in the Mizoroki–Heck coupling reaction, without losing the
crystalline structure of the Pd/TATAE catalyst.
123

Pd(OAc)2 immobilized on imine-functionalized microporous…
Experimental
Materials and instruments
All chemicals and reagents were purchased from commercial suppliers and used
without further purification as commercially available unless otherwise noted. All
solvents were obtained with laboratory reagent grade. Nitrostyrene, styrene,
4-methylstyrene, bromobenzene, 3-bromopyridine, 2-bromothiophene, and 4-bro-
mobenzaldehyde used in the present study were purchased from Sigma Aldrich. 1H
and 13C nuclear magnetic resonance (NMR) spectra were recorded on Bruker
(Avance) 300 and 400 MHz NMR instruments with tetramethylsilane (TMS) as
internal standard and CDCl3 as solvent. Chemical shifts are given in ppm (d-scale),
and coupling constants are given in Hz. Solution-state ultraviolet–visible (UV–Vis)
absorbance was recorded using a JASCO V-630, and solid-state UV diffuse
reflectance (DR) spectra were obtained using a JASCO V-670 UV–Vis–near infrared
(NIR) spectrometer. The morphology and particle size of the polymerized materials
were examined by scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) (Bruker). Fourier-transform infrared (FT-IR) spectra were
recorded from KBr discs on a JASCO FT/IR-410 spectrometer. Silica gel-G plates
(Merck) were used for thin-layer chromatography (TLC) analysis with a mixture of n-
hexane and ethyl acetate as eluent. Column chromatography was carried out on silica
gel (60–120 mesh) using petroleum ether and ethyl acetate as eluent (Scheme 1).
Synthesis of TATAE and Pd/TATAE
Partially crystalline samples of the synthesized compounds of imine-functionalized
triazine covalent organic frames such as TATAE and Pd/TATAE were synthesized
using our reported procedure (Fig. S1) [11].
Dioxane/Mesitylene
O
6M AcOH
1500C/ 3
TATAE
Days
NH2
N
N
N
O
O
NH2
DAB
TATA
Scheme 1 Schematic representation of synthesis of TATAE and Pd/TATAE (see Fig. S1)
123

V. Sadhasivam et al.
General procedure for Mizoroki–Heck C–C coupling reactions
The potential activity of the newly prepared Pd/TATAE catalyst was analyzed in the
Mizoroki–Heck cross-coupling reaction (Scheme 2). Aryl bromide (1 mmol),
styrene (1.2 mmol), K2CO3 (1.2 mmol), and Pd/TATAE (20 mg) were taken in
dimethylformamide (DMF)/water mixture (1:2 ratio, 2 mL), and the mixture was
stirred at 70 °C for about 3 h. After reaction completion (monitored by TLC), the
reaction mixture was diluted with 5 mL ethyl acetate, then the catalyst was removed
by simple filtration using Whatman-40 filter paper. The filtrate was extracted with
excess amount of ethyl acetate, then concentrated under reduced pressure. The crude
product was further purified by silica gel (60–120 mesh) column chromatography
(petroleum ether/ethyl acetate) to afford the corresponding products.
Results and discussion
Characterization of TATAE and Pd/TATAE [11]
The UV-DR spectra of the synthesized materials, viz. TATAE and Pd/TATAE, are
shown in Fig. 1, confirming presence of Pd(OAc)2 incorporated on TATAE.
R2
X
Pd/TATAE
H
2:1
3h
2O:DMF/
R1
R2
K
2CO3
R1
,
Scheme 2 Mizoroki–Heck C–C cross-coupling reactions
TATAE
0.45
Pd/TATA
After the 3rd cycle
(π−π∗)
*
0.30
0.15
0.00
(d-d)
*
375
450
525
600
675
750
Wavelength (nm)
Fig. 1 UV-DRS spectra of TATAE, Pd/TATAE, and after the third cycle
123

Pd(OAc)2 immobilized on imine-functionalized microporous…
TATAE showed an absorbance band at 362 nm, which can be attributed to p–p*
transition of C=C, C=N. In addition, Pd/TATAE showed a new absorbance band at
590 nm, attributed to d–d transition of Pd2? metal ions [33]. Thus, the UV-DRS
results clearly indicate successful incorporation of Pd(OAc)2 on imine-functional-
ized microporous covalent triazine polymer. The extended spectrum of the d–d band
is shown in Fig. S2.
The FT-IR spectra of TATAE and Pd/TATAE catalyst matched our previously
reported catalyst. The C=N and C=C stretching frequency appeared at 1635 and
1310 cm-1, clearly confirming formation of imine linkages. Furthermore, core
triazine ring C=C and C–N stretching appeared at 1514 and 1397 cm-1
,
respectively. It is suggested that all starting materials were consumed to form the
imine-based covalent triazine network, as confirmed by the FT-IR spectrum in
Fig. S3. The bands observed at 1709 and 3425 cm-1 for TATA and 1,4-
diaminobenzene (DAB) can be attributed to free C=O (aldehyde) and primary amine
stretching, respectively. Moreover, the free C=O (aldehyde) for TATA and primary
amine group stretching band were absent in case of TATAE, indicating that all
starting materials were successfully incorporated into the TATAE network. Thus, it
is evident from the FT-IR spectrum that both the aldehyde and amine groups were
converted into the Schiff base network. Pd/TATAE showed the same absorbance
pattern as TATAE, but the C=N stretching absorbance band was shifted by
27 cm-1, while the (t Pd–N) stretching frequency was observed at 562 cm-1 [34],
which may reflect possible coordination between Pd(OAc)2 and imine nitrogen
atoms. This was further confirmed by measuring the UV-DRS and FT-IR spectra of
Pd/TATAE catalyst after the third reuse, revealing no significant changes with
respect to the fresh catalyst (Figs. 1, 2). These data clearly suggest that the catalyst
retained its structural integrity and stability without any leaching during three cycles
under the experimental conditions applied.
After 3rd cycle
Pd/TATAE
TATAE
500
1000
1500
2000
2500
3000
3500
Wavenumber (cm-1)
Fig. 2 FT-IR spectra of TATAE, Pd/TATAE, and after the third cycle
123

V. Sadhasivam et al.
Fig. 3 SEM images and EDS spectrum of a, b TATAE, c, d Pd/TATAE, and after the third cycle
The structural morphology of TATAE and Pd/TATAE was confirmed by SEM
(Fig. 3), clearly indicating that the TATAE material consisted of porous polymer
with crystalline ordered structure. This observation confirms that TATA and 1,4-
diaminobenzene led to ordered uniform morphology. After incorporation of
Pd(OAc)2 into the TATAE network, Pd/TATAE showed sheet-like morphology.
Moreover, the morphological structure changed from porous crystalline ordered to
sheet like, indicating that Pd(OAc)2 stacked with each layer of the TATAE network
[11]. In addition, the stability of the catalyst under experimental condition was
investigated based on SEM images of Pd/TATAE catalyst in fresh state and after
three cycles of reuse (Fig. 3), clearly revealing that the morphology of Pd/TATAE
retained the sheet-like structure after the Mizoroki–Heck coupling reaction.
123

Pd(OAc)2 immobilized on imine-functionalized microporous…
The elemental composition of both materials was confirmed by EDS analysis
(Fig. 4). The metal content of the Pd/TATAE material was investigated using EDS
spectra, confirming presence of C, N, and O elements and Pd2? ions at different
points in the microporous imine-functionalized triazine covalent organic frame-
work. The atomic percentage of each element according to the EDS spectrum is
presented in Table S1.
Catalytic studies of Pd/TATAE in Mizoroki–Heck C–C coupling reaction
Initially, to check the catalytic activity of the heterogeneous Pd/TATAE in the
Mizoroki–Heck coupling reaction (Scheme 2), styrene and bromobenzene were
taken as model substrate in presence of Pd/TATAE catalyst for optimization of the
reaction condition; the results are summarized in Table 1. Recently, most such
coupling reactions have been conducted in aqueous media, attracting a great deal of
attention, because water is benign, cheap, nontoxic, more abundant, and environ-
mentally friendly. The Mizoroki–Heck coupling reaction was performed in presence
of K2CO3 as base in mixture of H2O/DMF (2:1 ratio, 2 mL) as solvent for 3 h in air
atmosphere at 70 °C with yield of 96 % (Table 1, entry 13); only the target product
stilbene was observed, while the homocoupled byproduct was not obtained. In
addition, no product was obtained (Table 1, entries 1, 2) either in absence of
catalyst and base or in presence of TATAE alone. These results clearly demonstrate
that Pd/TATAE played a crucial role in the Mizoroki–Heck coupling reaction. The
influence of various factors on the reactivity of styrene and aryl halides is discussed
below.
When the reaction was conducted with Pd(OAc)2 and PdCl2 instead of Pd/
TATAE, yield of 46 and 47 % was observed (Table 1, entries 15, 16). The reaction
was optimized with various solvents such as DMF, dimethylsulfoxide (DMSO),
ethanol, acetonitrile, H2O, and DMF/H2O mixture (Table 1, entries 10–14).
Excellent product yield of 96 % was observed in DMF/H2O (1:2) mixture solvent
even at 70 °C. Among these solvents, the DMF/H2O mixture has high dielectric
constant and so is highly soluble and stabilized with the substrate and reactive
intermediates compared with other solvents. When the temperature was below
Fig. 4 EDS spectrum of a Pd/TATAE, b TATAE
123

V. Sadhasivam et al.
Table 1 Optimization of Mizoroki–Heck coupling reaction conditions
Entry
Catalyst
Solvent
Base
Temp. (°C)
Yield (%)a
Conv.f
1
–
DMF–H2O
DMF–H2O
DMF–H2O
DMF–H2O
DMF–H2O
DMF–H2O
DMF–H2O
DMF–H2O
DMF–H2O
DMSO
K2CO3
K2CO3
Na2CO3
NaOH
TEA
70
70
70
70
70
70
70
70
70
90
70
70
70
90
70
70
70
90
60
70
70
70
70
70
70
–
NRe
NRe
70
2
TATAE
–
3
4c
Pd/TATAE
Pd/TATAE
Pd/TATAE
Pd/TATAE
Pd/TATAE
Pd/TATAE
Pd/TATA
Pd/TATAE
Pd/TATAE
Pd/TATAE
Pd/TATAE
Pd/TATAE
PdCl2
70
82
88
45
73
78
84
90
64
65
96
48
47
45
92
96
92
96
73
31
73
61
60, 77
84
86
5
88
6
KOAc
K3PO4
Cs2CO3
Pyridine
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
45
7
73
8
78
9
84
10
11
12
13
14c
15
16b,c
17d
18
19
20g
21
22
23
24
25h
90
EtOH
64
ACN
65
DMF–H2O
H2O
96
50
DMF–H2O
DMF–H2O
DMF–H2O
DMF–H2O
DMF–H2O
DMF–H2O
DMF–H2O
DMF–H2O
DMF–H2O
DMF–H2O
DMF–H2O
47
Pd(OAc)2
Pd/TATAE
Pd/TATAE
Pd/TATAE
Pd/TATAE
Pd/C
48
92
96
92
96, 92
76
Pd/Al2O3
Pd(PPh3)4
PdCl2(PPh3)2
Pd/TATAE
37
81
72
68, 83, 88
Reaction condition: bromobenzene (1 mmol), styrene (1.2 mmol), base (1.2 mmol), H2O–DMF solvent
(2:1 ratio, 2 mL), catalyst (20 mg Pd/TATAE with Pd content 0.14 wt%, 0.13 mol%), 3 h
aIsolated yield based on bromobenzene
b3 mol% Pd(OAc)2
c3 % phenol byproduct formed
dYield of reaction carried out with 1.0 mmol base
eNR = no reaction
f
Determined by GC analysis
gYield of reaction carried out with 20, 15 Pd/TATAE catalyst
hReaction performed with 5, 10, 15 mg catalyst
123

Pd(OAc)2 immobilized on imine-functionalized microporous…
70 °C (Table 1, entry 19), the product yield decreased to 92 %, whereas increasing
the temperature to 90 °C (Table 1, entry 18) had no significant effect on the overall
yield. Among different organic and inorganic bases such as K2CO3, CH3COOK,
NEt3, Na2CO3, pyridine, K3PO4, and NaOH (Table 1, entries 3–9) used for this
Mizoroki–Heck coupling reaction, the highest conversion yield of Mizoroki–Heck
coupled product was observed for K2CO3 (Table 1, entry 13) compared with other
bases. In presence of NaOH base, a small amount (3 %) of phenol byproduct formed
(Table 1, entry 4). When the amount of base was reduced to 1 mmol, the yield also
decreased to 92 %. These results show that the optimum condition for this
palladium-catalyzed Mizoroki–Heck coupling was Pd/TATAE (20 mg) catalyst in
DMF–H2O (1:2 ratio, 2 mL) at 70 °C with K2CO3 (1.2 mmol) as base for 3 h.
Furthermore, the Mizoroki–Heck coupling reaction was also carried out in presence
of some commercially available palladium catalysts such as Pd(PPh3)4, PdCl2(-
PPh3)2, Pd/C, and Pd/Al2O3 instead of Pd/TATAE, providing moderate yield of 73,
73, 31, and 61 % (Table 1, entries 21–24). The amount of catalyst was also varied
to 5, 10, 15 and 20 mg, resulting in conversions of bromobenzene of 63, 83, 88, and
96 % (Table 1, entry 25).
The reactivity of various substituted aryl halides (X = I, Br) and styrene,
including heterocyclic system, was also studied, and the results are presented in
Table 2 (entries 1–20) for a wide range of substrates containing both electron-
withdrawing and electron-donating groups. In addition, we carried out the
Mizoroki–Heck coupling reaction for less reactive aryl chlorides such as
chlorobenzene and 4-chlorobenzaldehyde; the reaction proceeded successfully,
but with greatly decreased yield (32 %). Moreover, for chloro-substituted deriva-
tives, less amount of starting materials was consumed (Table 2, entries 19, 20).
Among the various substrates, the bromobenzene-mediated reaction gave the Heck
coupled products in excellent yield of up to 96 % with high turnover number up to
3349 (Table 2, entry 3).
Shelton test
To investigate the heterogeneity of the Pd/TATAE catalyst in our system, we
applied the Shelton test for the Mizoroki–Heck coupling reaction of bromobenzene
and styrene under the optimized reaction condition (Fig. 5a). During this catalytic
reaction, the solid catalyst was filtered from the reaction mixture by simple
centrifugation after 30 min (conversion 10.4 %), and the reaction was performed for
a further 2.5 h. Thereafter, the reaction mixture was analyzed by gas chromatog-
raphy, revealing no improvement in product conversion. This result confirms no
leaching of Pd into the filtrate solution, suggesting that the Pd/TATAE catalyst was
truly heterogeneous in nature and that Pd(OAc)2 was strongly supported on the
TATAE network (Table 3).
Reusability test
Recovery and recycling of solid heterogeneous catalysts is one of the major
concerns in metal-supported catalysis. Related results are shown in Fig. 5b. Some
123

V. Sadhasivam et al.
Table 2 Water/DMF (2:1)-mediated Mizoroki–Heck coupling reaction catalyzed by Pd/TATAE with
various substituents
Entry
R1
R2
X
Product
Yield (%)a
TONsb
1
H
CN
Br
Br
Br
Br
I
3a
3b
3c
3d
3e
3f
91
85
96
82
93
88
82
85
90
82
85
93
91
93
90
89
88
84
46
32
2788
2373
3349
2155
2995
2523
2768
2949
2721
1408
2724
2980
2931
2781
2691
2518
2313
2344
1605
966
2
H
NO2
3
H
H
4
NO2
H
Me
5
NH2
6
H
CH2CN
3-Pyridine
2-Thiophene
CH3
Br
Br
Br
Br
Br
Br
Br
Br
I
7
H
3g
3h
3i
8
H
9
CH3
CH2Br
H
10
11
12
13
14
15
16
17
18
19
20
CH2Br
CHO
OH
3j
3k
3l
H
H
NH2
3m
3n
3o
3p
3q
3r
3s
H
OMe
OMe
COMe
NO2
H
Br
Br
Br
Br
Cl
Cl
H
4-Me
NO2
H
H
H
H
CHO
3t
Aryl bromides (1mmol), styrene (1.2 mmol), K2CO3 (1.2 equiv), H2O–DMF solvent (2:1 ratio, 2 mL), Pd/
TATAE (20 mg Pd/TATAE with Pd content 0.14 wt%, 0.13 mol%), 70 °C, 3h
aIsolated yield based on aryl bromide
bTON = mol products/per mol Pd
supported catalysts are not reusable and require a tedious work-up procedure, and
after recovery a remarkable decrease in catalyst efficiency is observable. In our
catalytic system, reusability was tested using the Mizoroki–Heck coupling reaction
of bromobenzene and styrene in presence of (20 mg) Pd/TATAE and K2CO3 at
70 °C for 3 h, obtaining yield of 96 %. Afterwards, the catalyst was filtered using
Whatman-40 filter paper and washed with water, methanol, ethyl acetate, and excess
acetone to remove base and other impurities. Finally, the recovered catalyst was
dried in an oven for about 1 h. The catalyst reused in another eight successive cycles
under the same reaction condition gave yield of 94, 94, 92, 90, 88, 85, 81, and 77 %
123

Pd(OAc)2 immobilized on imine-functionalized microporous…
100
(a)100
(b)
80
60
40
20
0
75
50
25
0
with catalyst
without catalyst
0.0
0.5
1.0
1.5
2.0
2.5
3.0
1
2
3
4
5
6
7
8
9
10
Time (hrs)
Number of runs
Fig. 5 a Effect of reaction time and percentage bromobenzene conversion in Pd/TATAE-catalyzed C–C
coupling reactions and hot filtration test for Pd/TATAE catalyst. b Reusability of Pd/TATAE catalyst
Table 3 Previously reported catalytic performance of different Pd-incorporated solid supported catalysts
in coupling reaction of bromobenzene and styrene
S.
no.
Catalyst
Solvent
Base
Temp
(°C)
Time
(h)
Yield
(%)
Ref.
Mizoroki–Heck coupling reactions
1
2
3
Pd-MPTA-1
Water
DMA
K2CO3
NEt3
Reflux
90
6
9
6
92
97
95
[35]
[36]
[37]
Pd/CoBDCNH2
CPS-MNPs-NNN-Pd
H2O–DMF
(2:1)
K2CO3
90
4
5
6
PNP-SSS
Pd@HSQ
Pd@MCOP
Water
DMF
K2CO3
K2CO3
K2CO3
95
12
2
94
99
96
[38]
[39]
[40]
130
90
DMSO–H2O
(1:1)
10
7
MPCS-TI/Pd
Pd(0)-CA
DMF–H2O
CH3CN
DMF
NEt3
110
90
5
91
94
99
82
88
67
70
[41]
[42]
[43]
[44]
[45]
[46]
[47]
8
NEt3
3
9
C
60-IL-PdNPs
NEt3
120
80
4
10
11
12
13
Silica-Pd
DMF
K2CO3
K2CO3
5
PEG-Pd(0)
DMF
120
6
Fe2O3/SiO2/HPG-Pd
DMF
NaOAc 140
12
5
Polymer
iminophosphine-Pd
Dioxane
NEt3
Reflux
14
15
16
Size-controlled Pd NPs H2O
K2CO3
100
12
90
75
96
[48]
[49]
Self-assembled Pd-cat Acetonitrile
Pyridine 100
K2CO3 70
MW
3
Pd/TATAE
DMF–H2O
Present
work
(Fig. 5b), clearly revealing a decrease in catalytic activity and slight loss of product
yield after six cycles. In addition, UV–Vis spectroscopy (Fig. S4) of the filtrate
solution further confirmed that no absorption peak was observed in the first six
cycles, suggesting absence of Pd(OAc)2 in the filtrate solution and that no leaching
occurred up to six cycles. However, from the 6th to 10th cycle, new absorbance
123

V. Sadhasivam et al.
X
N
N
N
N
OAc
Pd
OAc
Oxidative Addition
-OAc
I
-OAc
Reductive
Elimination
N
N
N
N
N
N
N
Pd
X
H
N
Pd
II
VI
+
K
H
2O
2CO3
KX
-H abstraction
β
N
N
N
Pd
CO3
N
N
N
N
N
H
III
Pd
H
V
CO3
Insertion
N
N
N
N
Pd
IV
Scheme 3 Plausible mechanism for Pd(OAc)2-catalyzed C–C coupling reactions in the aqueous system
bands appeared at 365 and 310 nm, suggesting minor loss of Pd(OAc)2 by leaching
into the filtrate solution.
On the basis of the results presented above, and also in accordance with previous
literature reports, a plausible mechanism for the Mizoroki–Heck coupling reaction
with the Pd/TATAE catalyst is proposed in Scheme 3. In the first step, oxidative
addition of aryl halide with Pd(II) acetate forms Pd complex II. In the third step,
alkene undergoes insertion reaction, leading to Pd complex IV, followed by b-
hydrogen abstraction, which results in palladium hydrogen complex V. This
complex undergoes acetate anion displacement followed by reductive elimination to
give the Mizoroki–Heck coupled product.
Conclusions
We designed and synthesized triazine-containing covalent organic frameworks
using imine-forming condensation reactions. The covalent triazine framework acted
as a good solid support for immobilization of palladium acetate to form Pd/TATAE
123

Pd(OAc)2 immobilized on imine-functionalized microporous…
as a heterogeneous catalyst with efficient catalytic activity in Mizoroki–Heck C–C
coupling reactions in mild condition. In addition, the reaction could be performed in
absence of any additional stabilizing ligand in very short reaction time. The reaction
system showed tolerance toward numerous substituted aryl halides and styrene,
giving corresponding products in good to excellent yield. The reaction employs
environmentally friendly solvent and additives, inert atmosphere, and ligand-free
protocol under mild conditions.
Acknowledgments A.S. and V.S. acknowledge financial support from the Department of Science and
Technology, SERB, Extramural Major Research Project (grant no. EMR/2015/000969), Council of
Scientific and Industrial Research (CSIR), HRDG (no. 01(2901)/17/EMR-II), New Delhi, and the
Department of Science and Technology DST/TM/CERI/C130(G), New Delhi, India.
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