Cu(OAc)2 entrapped on ethylene glycol-modified melamine–formaldehyde polymer as an efficient heterogeneous catalyst for Suzuki–Miyaura coupling reactions

Article abstract of DOI:10.1007/s11164-019-03984-0

This work is described as an environmental friendly approach for Cu(OAc)₂ entrapped on ethylene glycol-modified melamine–formaldehyde-based polymeric material (Cu@MCOP) which has been successfully synthesized by simple approaches using commercially available starting materials via solvothermal techniques and without using any toxic reagents and chemicals. The structural, morphological, physicochemical characteristics and catalytic activity of the heterogeneous catalyst (Cu@MCOP) were analyzed by various instrumental methods including powder X-ray diffraction, FT-IR, UV-DRS, X-ray photoelectron spectroscopy, SEM and elemental mapping which have been used to authenticate the polymeric materials Cu@MCOP. The catalytic performance of Cu@MCOP as solid heterogeneous catalyst was evaluated in synthesis of various biphenyl derivatives through Suzuki–Miyaura cross-coupling reactions of various aryl halides with substituted organoboranes under normal reaction conditions. Furthermore, the copper catalyst was easily available, low cost, cheap and best instead of palladium, which shows good catalytic activity and excellent yield (up to 86%); the catalyst can be separated easily and recycled for more than five times. Graphic abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s11164-019-03984-0

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03984-0
Cu(OAc)2 entrapped on ethylene glycol‑modifed
melamine–formaldehyde polymer as an efcient
heterogeneous catalyst for Suzuki–Miyaura coupling
reactions
Velu Sadhasivam1 · Balakrishnan Sankar1 · Ganesan Elamathi1 ·
Mathappan Mariyappan1 · Ayyanar Siva1
Received: 6 June 2019 / Accepted: 13 September 2019
© Springer Nature B.V. 2019
Abstract
This work is described as an environmental friendly approach for Cu(OAc)2
entrapped on ethylene glycol-modifed melamine–formaldehyde-based poly-
meric material (Cu@MCOP) which has been successfully synthesized by simple
approaches using commercially available starting materials via solvothermal tech-
niques and without using any toxic reagents and chemicals. The structural, morpho-
logical, physicochemical characteristics and catalytic activity of the heterogeneous
catalyst (Cu@MCOP) were analyzed by various instrumental methods including
powder X-ray difraction, FT-IR, UV-DRS, X-ray photoelectron spectroscopy, SEM
and elemental mapping which have been used to authenticate the polymeric materi-
als Cu@MCOP. The catalytic performance of Cu@MCOP as solid heterogeneous
catalyst was evaluated in synthesis of various biphenyl derivatives through Suzuki–
Miyaura cross-coupling reactions of various aryl halides with substituted organobo-
ranes under normal reaction conditions. Furthermore, the copper catalyst was easily
available, low cost, cheap and best instead of palladium, which shows good catalytic
activity and excellent yield (up to 86%); the catalyst can be separated easily and
recycled for more than fve times.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-019-03984-0) contains supplementary material, which is available to authorized users.
*
Ayyanar Siva
drasiva@gmail.com; siva.chem@mkuniversity.org
1
Supramolecular and Organometallic Chemistry Lab, Department of Inorganic Chemistry,
School of Chemistry, Madurai Kamaraj University, Madurai, Tamil Nadu 625 021, India
Vol.:(0123456789)
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V. Sadhasivam et al.
Graphic abstract
Keywords Suzuki–Miyaura · Melamine · Heterogeneous · Shelton test · Covalent
organic polymer
Introduction
Melamine–formaldehyde polymers (MFPs) are a new class of novel solid supports
mesoporous polymeric materials with highly cross-linked covalent organic polymers
(COPs) and various functionalities [1–3]. This emerging feld represents a critical
current research area. MFPs are micro/mesoporous solid materials; it can be easily
synthesized by polycondensation reaction to develop extending an organic structure
via robust irreversible covalent bond linkages. In MFP type of materials having dop-
ing with some light weight elements such as carbon, sulfde, phosphor, boron, and
nitrogen [4–6], these types of doped materials are attractive candidates for large sur-
face areas and superior mass transfer, good thermal insulator and mechanical prop-
erties. Melamine–formaldehyde-based polymeric materials have fascinate intensive
research interest in the feld of gas storage [7], heavy metal removal [8], electro-
chemistry, energy storage/conversion device [9–11], photocatalysis [12, 13], dye
sensitized solar cells [14], sensing [15, 16], water treatment [17], biosensors, and
DNA extraction. Recently, melamine–formaldehyde polymers (MFPs) have been
extensively employed as a solid supports for heterogeneous catalyst, because of their
low density, porous structure, large surface area, tunable properties, thermal and
chemical stability, and other functionalities. In addition to that, porous MFPs mate-
rials are not soluble in any other organic or aqueous solvents and these are highly
stable in high-boiling solvents. Furthermore, enhance the chemistry of these porous
materials particularly applicable in heterogeneous catalysis, because of the pres-
ence of heteroatoms with diferent electron confgurations of metal and neighboring
carbon atoms would attract or repel electrons and high content of nitrogen atoms,
1 3

Cu(OAc)2 entrapped on ethylene glycol‑modifed melamine–…
which is benefcial to incorporate a large number of metal binding sites and generat-
ing active sites for many kinds of organic reactions.
Transition metal-catalyzed cross-coupling reactions are played a vital role in syn-
thetic organic chemistry. These types of cross-coupling reactions are most impor-
tant methods to preparing biphenyl derivatives, which are key intermediates for the
synthesis of natural products [18], pharmaceuticals [19], polymers [20] and agro-
chemical industries [21, 22]. Hence, the Suzuki–Miyaura (S–M) cross-coupling
reaction involving an organoboron reagents and aryl halides in the presence of pal-
ladium catalyst is one of the most extensively studied for C–C bond formation reac-
tions [23–33]. Generally, during the last few decades cross-coupling reaction was
carried out by palladium catalyst [34–36]. In this context, the more number of pro-
tocol has been succeeded for S-M coupling with frst-row transition metal such as
Ni, Fe, Mn, Co and Cu which are also used instead of palladium in cross-coupling
reactions [37–40], due to air-sensitive, toxic and expensive precious metals such as
palladium in cross-coupling reactions. The alternative uses of copper catalyst have
been investigated in more cross-coupling reactions such as Suzuki–Miyaura [41],
Sonagashira, Ullmann and Chan–Lam cross-coupling. It is less expensive and eas-
ily available than palladium and produces a low residual metal content in the reac-
tion mass, non-toxicity, environmental friendly. During the past few decades, the
solid-supported materials stabilized by metal ions have presumably been the most
frequently investigated in modern organic synthesis. Melamine- and triazine-based
organic frames mainly used for solid supports for heterogeneous catalyst showed no
detectable degradation, minimal metal leaching and no change in structural regular-
ity even after use to several consecutive cycles [42, 43]. In this regard, our group
was reported various types of cross-coupling reactions such as C–C, C-Se and C-N
using covalent triazine-based covalent organic frameworks as solid hetero-generous
catalyst. Recently reported, the triazine-based covalent organic polymers as novel
solid supports for incorporated palladium NPs as a heterogeneous catalyst for sele-
nium insertion of aryl halide (Csp2-Se) coupling reactions and nitro to amine reduc-
tion reaction were also studied [44–46]. Herein, we have reported a novel strategy
for the synthesis of Cu(OAc)2 supported on nitrogen-rich melamine–formaldehyde-
based polymers as an efcient heterogeneous catalyst for S-M coupling reactions
and good catalytic activity with excellent yield under eco-friendly conditions.
Result and discussion
Characterization of MCOP and Cu@MCOP
We have designed a new series of triazine containing ethylene glycol-modifed mel-
amine-based polymer MCOP which was synthesized under solvothermal conditions
by condensation reaction between formaldehyde, ethylene glycol and melamine as
a starting materials and water/glycol as a solvent in the presence of acidic condition
(pH 4) at 100 °C for about 24 h to obtain melamine-based polymer-supported mate-
rials (MCOP). After that, Cu(OAc)2 was successfully incorporated into the MCOP
polymeric materials (Cu@MCOP). Furthermore, the Cu@MCOP chemical structure
1 3

V. Sadhasivam et al.
and environment were fully characterized by various instrumental techniques like
FT-IR, UV-DRS, powder XRD, TGA, SEM, TEM, and XPS analysis. Finally, the
synthesized polymeric materials were applied to check the catalytic application for
Suzuki–Miyaura coupling reaction under normal conditions.
In order to ascertain the coordination between the solid supports and dispersion of
Cu species in ethylene glycol-modifed melamine-based polymer, Cu@MCOP acts
as heterogeneous catalytic system which was studied by UV–Vis difuse-refectance
spectroscopy (DRS) and the results are shown in Fig. 1. The compounds MCOP
and Cu@MCOP display nearly identical features, two main characteristic absorption
bands were appeared at 250 and 325 nm for π–π* and n–π* transitions of C=C and
C=N in the UV region, and the Cu@MCOP represents weak and broad absorbance
band in the visible region at 450 and 550–750 nm, which indicated the presence of
surface bound Cu(II) sites; it is assigned to a LMCT band and d–d transition in the
Cu-supported nitrogen-rich containing melamine-based polymer Cu@MCOP [47,
48]. In addition, the third cycle reused catalyst display all types of transition bands
similar with Cu@MCOP fresh catalyst, which indicates the synthesized catalyst was
well maintained for structural regularity and truly heterogeneous in nature during
the course of reactions. Finally, the solid-state UV-DRS clearly stipulate that all the
Cu species are successfully incorporated on nitrogen-rich containing ethylene gly-
col-modifed melamine-based polymers.
The FT-IR spectra of compounds melamine starting material, MCOP and
Cu(OAc)2 incorporated on ethylene glycol-modifed melamine-based polymers
(Cu@MCOP), and 1st and 3rd recycled catalysts were investigated and are shown
in Fig. 2. The broad stretching bands at around 3350–3370 cm−1 are identical for all
these prepared compounds; it is attributed to the N–H stretching vibrations of sec-
ondary amino groups being present in the polymeric host materials. In this spectrum
(Fig. 2a) of raw materials, the core triazine ring –C–N, –C=N stretching vibration
band and the primary amine group (–NH2 stretching) of melamine were appeared
MCOP
250 nm
Cu@MCOP
1.5
1.0
0.5
0.0
3rd recycle Cu@MCOP
325 nm
LMCT
450 nm
200
400
600
800
Wavelength (nm)
Fig. 1 UV–Vis difuse-refectance spectra of compound MCOP, Cu@MCOP and 3rd reusable catalyst
1 3

Cu(OAc)2 entrapped on ethylene glycol‑modifed melamine–…
(a)
(b)
100
Melamine
MCOP
100
90
80
70
60
50
80
60
40
20
MCOP
Cu@MCOP
1 st Cycle
3rd Cycle
500
1000 1500 2000 2500 3000 3500 4000
500
1000
1500
2000
2500
3000
3500
4000
Wavenumber (cm-1)
Wavenumber (cm-1)
Fig. 2 FT-IR spectra of compound a starting materials b MCOP, Cu@MCOP and 3rd recycled catalyst
at 1196, 1545 and 3491 cm−1, respectively. Further, the synthesized MCOP shows
a alkoxy –C–O, aliphatic –C–H and –C–N stretching vibration band appeared at
1095, 2915 and 1335 cm−1. Meanwhile, the melamine band (3491 cm−1) had com-
pletely disappeared, which clearly speculated the formation of nitrogen-rich mela-
mine-based polymeric material through ether linkage between melamine and for-
maldehyde with ethylene glycol unit. Furthermore, the Cu@MCOP and frst and
third recycle catalysts represent similar weak vibration band observed at around 442,
567 cm−1 for (Cu–N) complexation between metal (Cu((OAc)2) and solid supports,
which clearly indicates that Cu species successfully incorporated into the nitrogen-
rich containing melamine-based polymeric resins (MCOP) [49, 50].
The structural regularity, integrity and crystallinity of synthesized materials
MCOP and Cu@MCOP were thoroughly analyzed by powder PXRD analysis and
are shown in Fig. 3. The MCOP and Cu@MCOP both materials have obtained broad
2500
MCOP
Cu@MCOP
3rd recycle Cu@MCOP
2000
1500
Cu(OAc)
2
polymer resin
1000
500
10
20
30
40
50
60
2 Theta (degree)
Fig. 3 Powder X-ray difraction of MCOP, Cu@MCOP and 3rd recycle catalyst
1 3

V. Sadhasivam et al.
difraction peak at around 7.26 (5.76–9.72) and 22.56 (17.58–30.01) in the range
of 2θ values, which suggested that the ethylene glycol-modifed melamine-based
polymer has a partial crystallinity nature and certain degree of ordered structures.
After that, the Cu metal precursor-supported catalyst Cu@MCOP shows sharp and
intense difraction peak which was observed at 12.72, 14.22, 16.50, 20.57, 25.34 and
39.24 for 2θ values, which corresponds to the Cu(OAc)2 successfully incorporated
on nitrogen-rich containing melamine-based polymeric resins, which is consistency
with previous reported literature [51, 52]. Moreover, the 3rd recycle catalyst shows
similar difraction pattern compared to Cu@MCOP, it is demonstrated that MCOP
has a good solid supports, and the successful loading of Cu(OAc)2 with retention of
the Cu@MCOP structural integrity was well maintained during the course of the
reaction conditions.
The oxidation state of the Cu species, chemical loading of Cu for Cu@MCOP
heterogeneous catalyst and bond nature of the supported material MCOP were
measured by using X-ray photoelectron spectroscopy (XPS) analysis. As shown in
Fig. 4b, the Cu@MCOP catalyst exhibited a binding energy values at 934.3 eV and
954.33 eV corresponding to Cu 2p3/2 and Cu 2p1/2, respectively. When the valence
electron interacted with outgoing electron excited it (shook it up) to a higher energy
level, the energy of the core electron was reduced and a satellite structure was
appeared at around 943.6 eV and 962.9 eV in the Cu 2p3/2 and Cu 2p1/2 region due
to the open 3d9 valence shell of Cu2+ species, which indicating that the copper has
bivalent oxidation state [53–55].
The XPS spectra of the C1s spectra show three binding energy values appeared at
284.7, 286.1 and 287.2 eV (Fig. 4a), which is corresponding to C–C, C=N and C-O
nature of chemical bonds, respectively. Moreover, the third binding energy value
at 287.2 eV was attributed to oxygen present in ethylene glycol unit coupled with
melamine formaldehyde intermediate to form ether linkages [56, 57], Further, we
analysis to determine the nitrogen-rich melamine-based solid supports (MCOP) by
N1s and O1s core level spectra shown in Fig. 5a, b.
In addition to that, the N1s spectra (Fig. 5a) of binding energy values show
400.4 eV and 401.7 eV correspond to core triazine ring nitrogen C=N–C and
N–H secondary amine group of solid supports (MCOP). The triazine nitrogen
1900
1800
1700
1600
1500
1400
1300
1200
3000
2500
2000
1500
1000
500
(a)
(b)
Cu@MCOP
Cu@MCOP
280
282
284
286
288
290
292
294
930
935
940
945
950
955
960
965
970
Binding Energy (eV)
Binding Energy (eV)
Fig. 4 XPS spectra of compound Cu@MCOP a C1s spectrum b Cu 2p spectrum
1 3

Cu(OAc)2 entrapped on ethylene glycol‑modifed melamine–…
4000
3500
3000
2500
2000
1500
1000
(a)
(b)
3500
3000
2500
2000
1500
1000
396
398
400
402
404
406
408
410
525
530
535
540
545
Binding Energy (eV)
Binding Energy (eV)
20000
15000
10000
5000
0
(c)
Cu@MCOP
200 300 400 500 600 700 800 900 1000 1100
Binding Energy (eV)
Fig. 5 XPS spectra of compound Cu@MCOP a N1s spectra b O1s spectra, c survey scan spectra of com-
pound Cu@MCOP
unit of N1s core level binding energy value was observed at 398.8 eV for pre-
viously reported literature [52]. In our case, the observed triazine nitrogen N1s
binding energy at 400.4 eV, which is slightly positive shifted to the Cu@MCOP.
These results clearly showed that Cu(OAc)2 is strongly interacted with core tria-
zine nitrogen unit. An O1s binding energy value of 532.5 eV (Fig. 5b), which
exhibited to C–O–C nature of bonds present in the synthesized materials, is con-
sist with previous reported literature. In this regard, our observed results show
binding energy value at 533.04 0.5 eV, it is attributed to formation of ether link-
ages, and positive shift clearly demonstrated that copper acetate strongly binds to
oxygen as well as nitrogen unit for MCOP solid-supported materials [4].
In order to determine the morphological structure and elemental analysis of
prepared melamine-based polymeric materials, MCOP and Cu@MCOP were
confrmed by SEM (scanning electron microscopy) and EDAX (energy-dis-
persive X-rays analysis) which are shown in Figs. 6 and 7. The SEM images of
both materials represent the direct visualization of porous cross-linked structure
with partially crystalline nature. The metal content of Cu@MCOP material was
investigated, elemental mapping was confrmed in the presence of C (green), O
(orange), N (pink) and Cu (blue), and the percentage of each element for both
materials are listed inside the table (see SI Table 1 and 2) at diferent points in the
nitrogen-rich containing melamine-based polymer resin.
1 3

V. Sadhasivam et al.
Fig. 6 SEM images of a MCOP, b Cu@MCOP catalyst
Catalytic activity studies of Cu@MCOP in Suzuki–Miyaura coupling reaction
To evaluate the catalytic ability of Cu@MCOP, polymeric material for
Suzuki–Miyaura cross-coupling reaction is summarized in Table 1. Initially, this
Suzuki–Miyaura cross-coupling reaction is well-established one for aryl halides,
and arylboronic acid provides biphenyl-based derivatives, its efcient route to form
C–C bonds under relatively mild conditions. We choose a model substrate phenyl
boronic acids, and bromobenzene was investigated using 15 mg of Cu@MCOP cata-
lyst in a DMF as solvent at 70 °C afording the desired biphenyl within 6 h in an
excellent isolated yield (86%) (Table 1, entry 5). The blank reaction was carried out
with bromobenzene and phenylboronic acid in the absence of Cu@MCOP under
normal condition no reaction was observed (Table 1, entry 1). No biphenyl product
was obtained in the presence of MCOP instead of Cu@MCOP (Table 1, entry 2); we
have performed a sequence of diferent experiments such as various bases, difer-
ent solvents and varied temperatures to optimize the reaction conditions under inert
free conditions. Thus, the reaction negligible amount of conversion was observed in
the absence of any base conditions (Table 1, entry 3). Hence, we confrmed that the
Cu@MCOP and base play a crucial role in the S-M cross-coupling catalytic reac-
tions. Further, we evaluated the reaction was performed in room temperature; no
signifcant conversion was observed (Table 1, entry 4). Furthermore, the reaction
was screening with various inorganic and organic bases such as NH4OH, K2CO3,
1 3

Cu(OAc)2 entrapped on ethylene glycol‑modifed melamine–…
Fig. 7 Elemental mapping images of Cu@MCOP catalyst
CH3COOK, NEt3, Na2CO3, Cs2CO3, triethylamine and pyridine (Table 1, entries
3–9) used for this cross-coupling reactions; only moderate amount of yield and
some base that gave very less amount of substrate conversion were observed except
Cs2CO3, and the Cs2CO3 obtained excellent yield and 86% conversions (Table 1,
entry 5). Other bases such as Na2CO3, K2CO3, CH3COOK, triethyl amine gave a
moderate amount of conversion 32%, 57%, 21% and 47% were observed (Table 1,
entries 6, 7, 9, 10) due to Cs2CO3 which is highly soluble in polar protic and aprotic
solvents, compared to other carbonates like lithium, potassium and sodium carbon-
ates depend upon the size and bond nature of cation and anion species, (Cs2CO3
is ten times more soluble in aprotic organic solvents than K2CO3) [58, 59], Hence,
these salts are largely dissociated than the corresponding carbonates salts like lith-
ium, sodium and potassium, so Cs2CO3 acts as a good base for the S-M cross-cou-
pling reaction catalyzed by Cu@MCOP. In addition to that, no product conversion
was observed in the presence of pyridine and NaOH and NH4OH base, due to the
formation of complex and Cu(OH)2 in basic medium, so pyridine strongly binds
with copper acetate; then the catalyst is deactivated in this reaction medium; it is
confrmed by change in color light green to blue color [60]. Moreover, we extended
to optimization of various solvent systems; we have chosen diferent polar/non
polar solvents, such as DMF, DMSO, toluene, acetonitrile, THF, DCM and EtOH
(Table 1, entries 12–17). Among these solvents, DMF only gave an excellent amount
of yield 86% of the product conversion observed at 70 °C temperature (Table 1,
entry 5); the remaining solvents are very poor, soluble and not so much dissociated
1 3

V. Sadhasivam et al.
Table 1 Optimization reaction condition for Suzuki–Miyaura cross-coupling reaction
Entry
Catalyst
Solvent
Base
Temp.
°C
Conversiona (%)
1
2
3
4
5
6
7
8
–
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Toluene
DMSO
THF
DCM
EtOH
Acetonitrile
DMF
DMF
DMF
DMF
Cs2CO3
Cs2CO3
_
Cs2CO3
Cs2CO3
K2CO3
CH3COOK
NaOH
70
70
70
rt
NR
NR
NR
NR
86
57
21
NR
32
47
NR
NR
56
20
36
7
65
56
62
MCOP
Cu@MCOP
Cu@MCOP
Cu@MCOP
Cu@MCOP
Cu@MCOP
Cu@MCOP
Cu@MCOP
Cu@MCOP
Cu@MCOP
Cu@MCOP
Cu@MCOP
Cu@MCOP
Cu@MCOP
Cu@MCOP
Cu@MCOP
Cu@MCOP
CuCl2
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
60
80, 90
9
Na2CO3
TEA
10
11
12
13
14
15
16
17
18
19
20
21
22
Pyridine
NH4OH
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cu(NO3)2
Cu@MCOP
Cu@MCOP
57
82
86
Reaction condition: bromobenzene (1 mmol), phenyl boronic acid (1.2 mmol), base (2.5 mmol), solvent
(2 mL), Cu@MCOP catalyst (15 mg) present in Cu(OAc)2 content 0.0225 mg, 0.000354 mmol, 6 h
aGC conversion based on bromobenzene
such as DCM, THF, toluene and acetonitrile. Moreover, the S-M coupling reaction
was compared with some commercially available copper metal salts like CuCl2 and
Cu(NO3)2 instead of Cu@MCOP to provide moderate amount of yield 62% and 57%
(Table 1, entries 21, 22). Finally, the reaction optimized with various temperatures
like 60, 80, and 90 °C to aford 82%, 86%, and 88% conversion.
With the above optimized reaction conditions, we further investigated to
check the catalytic activity of Cu@MCOP for Suzuki–Miyaura cross-coupling
reaction with various aryl halides and substituted boronic acid derivatives using
Cs2CO3 as a base at 70 °C. As shown in Table 2, the reaction was proceed-
ing with a wide range of substrates scopes containing both electron-donating
and withdrawing groups in para- and ortho-position that were coupled very
well to obtain excellent yield up to 86% and high turnover number up to 1153
(Table 2, entry 1). The substituent’s electron-donating and electron-withdraw-
ing groups did not show any signifcant infuence during the course of the reac-
tions, which suggests that electronic character has no signifcant efect on the
1 3

Cu(OAc)2 entrapped on ethylene glycol‑modifed melamine–…
Table 2 Cu@MCOP catalyzed by Suzuki–Miyaura cross-coupling reaction with diferent substituentsa
Entr
y
R
R1
X
Product
Yieldb
%
TONc
1
H
H
H
I
86
62
1153
1082
2
NH2
Br
90
92
42
76
1232
1285
735
3
4
5
H
H
H
H
Me
I
OMe
NO2
CHO
I
Br
1059
6
7
I
I
CHO CHO
71
1141
1 3

V. Sadhasivam et al.
Table 2 (continued)
Entr
y
R
R1
X
Product
Yieldb
%
TONc
81
70
52
64
32
1411
1457
909
8
H
H
H
Br
9
OMe
H
Br
10
11
12
CH2CN Br
1296
644
CN
H
Br
Br
CHO OMe
84
25
32
21
1151
436
559
367
13
14
15
16
H
H
H
H
NH2
I
CHO
CN
Br
Br
Br
2-NH2
1 3

Cu(OAc)2 entrapped on ethylene glycol‑modifed melamine–…
Table 2 (continued)
Entr
y
R
R1
X
Product
Yieldb
%
TONc
62
1142
17
18
OMe
Me
Me
Br
Br
58
48
980
840
CHO
19
20
H
Py
Br
I
64
1194
Me
Me
21
22
Me
H
NH2
I
I
51
42
894
735
2-Thiop
hene
aAryl bromides (1mmol), arylboronic acid (1.2 mmol), Cs2CO3 (2 equiv), DMF (2 mL),
Cu@MCOP (15 mg) present in Cu(OAc)2 content 0.0225 mg, 0.000354 mmol, 70 °C, 6h.
bIsolated yield based on aryl bromide, cTON = mol products/per mol Pd
catalytic reactions. On the other hand, the sterically hindered substrates 2-amino
bromobenzene, pyridine 3-boronic acid, 2-iodo aniline and 3-bromopyridine
some heterocycle system gave the corresponding products in moderate amount
of yields. In control experiment, 4-bromotoluene was used instead of bromoben-
zene to confrm the existence of 4-methyl-1,1′ biphenyl which was cross-coupled
by aryl halides and phenyl boronic acid rather than by phenylboronic self-homo-
coupling and Ullmann coupling reactions of aryl halides. The obtained S-M
cross-coupling products were thoroughly characterized by NMR spectroscopy,
and the corresponding spectral data presented in the Supporting Information.
1 3

V. Sadhasivam et al.
Sheldon test
The Sheldon test was determined to confrm heterogeneity of the synthesis cataly-
sis by Cu@MCOP melamine-based polymer that has truly heterogeneous nature, or
some amount of Cu species leaching to the fltrate solutions is shown in Fig. 8a. The
S-M cross-coupling reaction was performed under optimized reaction conditions,
and then reaction mixture was divided into half of the two portions separated into a
diferent reaction vial after 2 h; one portion of the solution present solid catalyst was
fltered from the reaction mixture by simple centrifugation under hot solutions (con-
version 40%); the reaction was performed for a further 4 h (fltrate solution). After
that, the reaction mixture was analyzed by gas chromatographic analysis, which
does not show any improvements in the product conversion. This result suggested no
leaching of copper into the fltrate solution and confrmed that our synthesis catalyst
was truly heterogeneous in nature.
Reusability test
The reusability of the heterogeneous catalyst was also studied; it is an important
one for large-scale industrial applications and to minimize environmental haz-
ards. In this context, the reusability of Cu@MCOP was evaluated in S-M cross-
coupling reaction of phenyl boronic acid and bromobenzene under the optimized
reaction conditions (Fig. 8b). After that completion of the frst cycle (fresh one),
the catalyst was recovered by normal fltration using Whatman-40 flter paper.
The fltered residual solid catalyst was washed with ethyl acetate, methanol and
water to remove starting material and followed by vacuum drying at 85 °C for
1 h, to activate the residual solid catalyst. The recycled catalyst does not show
any noticeable changes in their activity, which suggested that the catalyst was
stable and can be reused for next consecutive cycles. In addition to that, further
we confrmed the heterogeneity and leaching test for Cu@MCOP catalyst were
(a)
(b)
100
80
60
40
20
0
100
86%
with catalyst Cu@MCOP
84%
83%
82%
79%
without catalyst Cu@MCOP
80
60
40
20
0
71%
0
1
2
3
4
5
6
1
2
3
4
5
6
7
Time (h)
Number of Runs
Fig. 8 a Efect of reaction time on the percentage conversion of bromobenzene catalyzed by Cu@MCOP
for Suzuki–Miyaura coupling reactions and leaching experiment for Cu@MCOP heterogeneous catalyst,
b Reusability experiment for Cu@MCOP heterogeneous catalyst
1 3

Cu(OAc)2 entrapped on ethylene glycol‑modifed melamine–…
determine by UV–Vis spectroscopy (Figure S5 see SI); the reaction was suc-
cessfully completed, fltered, and then fltrate solution was analyzed by UV–vis-
ible spectroscopy. The spectra show no absorption peak was observed in the frst
four cycles, suggesting that Cu species does not occurred in the fltrate solution,
which corresponds to no metal leaching observed up to four cycles. However,
after fourth to sixth cycles, slightly new broad absorbance band was appeared at
250 nm and 440 nm; this may be due to the fact that small amount of Cu species
(only minor loss) was leaching into the fltrate solutions (Table 3).
Conclusions
In summary, we have developed the copper acetate incorporated ethylene glycol-
modifed melamine-based polymer supported by the facile condensation reaction.
The Cu@MCOP was a highly efcient heterogeneous catalyst for promoting the
Suzuki–Miyaura coupling reaction with a variety of aryl halides and arylboronic
acid in a DMF solvent medium, to give an excellent yield (up to 95%). In addi-
tion to that, the reaction was carried without any additional stabilizing ligands
and additives and also we got very good to excellent yield over a very short reac-
tion time. The reaction system shows tolerance toward numerous substituted aryl
halides to obtain the corresponding products. In best of our knowledge, there are
no reports describing for Cu(OAc)2 incorporated melamine-based polymer-sup-
ported (Cu@MCOP) performed Suzuki–Miyaura coupling reactions. The catalyst
was easily synthesized, low cost, thermally and chemically stable, shows negli-
gible metal leaching and could be reused under optimal conditions with only a
minor loss of its catalytic activity.
Table 3 Previously reported catalytic performance of diferent transition metal based catalyst for
Suzuki–Miyaura coupling reaction between bromobenzene with organoboranes
S.no. Catalyst
solvents
Base
Time (h) Temp (°C) Yield (%) References
1
2
3
4
5
6
7
8
9
Pd/COF-LZU1
P-Xylene
H2O/EtOH
K2CO3
Na2CO3
3
4
150
80
80
100
40
130
110
110
130
70
97
95
95
97
84
95
95
99
95
86
[61]
[62]
[63]
[64]
[65]
[66]
[67]
[68]
Pd2+-GO
PVP-Pd
40% Ethanol Na2PO4 12
Cs2CO3 16
LiOt-Bu
CS2CO3 20
Pd(OAc)2/CuCl DMF
NiBr2-Diglyme
CuI/DABCO
Benzene
DMF
4
Cu-Nano cluster DMF
K2CO3
NaI
–
2–8
CuI/X-Phos
CuI/1,10-Phen
Cu@MCOP
Dioxane
Diglyme
DMF
24
24
6
[69]
Present work
10
Cs2CO3
1 3

V. Sadhasivam et al.
Experimental section
Materials and methods
All the chemicals and reagents were purchased from commercial reagent suppli-
ers, used without further purifcation as commercially available unless otherwise
1
noted. All the solvents were obtained from laboratory reagent grade. The H and
13CNMR spectra were recorded on a Bruker (Avance) 300 and 400 MHz NMR
instrument using TMS as an internal standard and CDCl3 as a solvent. Chem-
ical shifts are given in ppm (δ-scale), and the coupling constants are given in
Hz. UV-difused refectance spectra were recorded using JASCO-500. The FT-IR
spectra were recorded from a JASCO FT/IR-410 spectrometer. Powder X-ray dif-
fraction (PXRD) patterns were obtained on a Rigaku difractometer using CuKα
(λ = 1.5404 Å) source with a scan rate of 0.5° min−1. The morphology and parti-
cle size of the polymerized materials were examined by microscopic techniques
SEM and TEM analysis. The scanning electron microscopy (SEM) measurement
was taken by VEGA3 TESCAN, CZECH REPUBLIC. The energy-dispersive
X-ray spectroscopy (EDX) was carried out using Bruker Nano, Germany. Silica
gel-G plates (Merck) were used for TLC analysis with a mixture of n-hexanes and
ethyl acetate as an eluent. Column chromatography was carried out on silica gel
(60–120 mesh) using n-hexanes and ethyl acetate as an eluent.
Preparation of MCOP polymer
In a typical synthetic procedure, ethylene glycol-modifed melamine–formalde-
hyde polymer was prepared according to the previous reported literature [34];
2.4 g of melamine and formaldehyde 2.5 mL were dissolved in 5 mL deion-
ized water in a Schlenk tube (Scheme 1); the pH of the solution was adjusted to
8–9 by 0.2 M NaOH basic solution. Further the reaction mixture was allowed
to stir at 60 °C for 1 h, and a clear viscous solution was obtained. After cool-
ing the solution to the room temperature, 2.5 mL of ethylene glycol was added
into the reaction mixture and then pH of the solution was adjusted to pH 4 by
using 0.3 M HCl solution. Finally, the reaction mixture was kept into oil bath,
heated at 100 °C for about 24 h to aford that colorless solid. In this solid doesn’t
soluble in water and any other common organic solvents. Further, it was fltered
and washed with acetone, THF, DCM, more than fve times. After the solid was
immersed in acetone, THF for 1 day, then the fltered product was dried at 90 °C
under the vacuum oven for 8 h to yield (3.2 g) MCOP as a colorless powder.
FT-IR (KBr powder with polymer, Wavenumber cm−1) 3418, 2921, 1555, 1489,
1346, 1196, 1104, 1026, 868, 817, 575, 470. Elemental analysis calculated by the
Energy Dispersive X-Ray Spectroscopy (EDS) analysis (Atomic %) for C 40.63,
N 22.35, O 37.02.
1 3

Cu(OAc)2 entrapped on ethylene glycol‑modifed melamine–…
Scheme 1 Synthesis of MCOP and Cu(OAc)2 supported on melamine-based polymer networks (Cu@
MCOP)
Preparation of Cu@MCOP polymer
The synthesized material 0.5 g of MCOP was dispersed 5 mL of MeOH in a
Schlenk tube, and 75.0 mg of Cu(OAc)2 metal precursor was added into the reac-
tion mixture. It was allowed to stir at room temperature for about 5 h until the
colorless changed into blue colored solid was obtained. Further, the solid was
fltered, washed with diferent solvent methanol (3 × 20 mL), acetone (3 × 20 mL)
and deionised water to remove the unreacted Cu(OAc)2, then the solid was dried
in a vacuum oven at 75 °C for about 6 h to aforded Cu@MCOP (0.56 g) as a
light blue colored powder. FT-IR (KBr powder with Cu@MCOP polymer, Wave-
number (cm−1)) 3425, 2915, 1705, 1608, 1444, 1391, 1156, 1084, 1019, 862,
698, 594, 516, 463. Elemental analysis calculated by the EDS analysis (Atomic %
of MCOP polymer) for C 43.54, N 22.75, O 32.92, Cu@MCOP (15 mg) present
in Cu(OAc)2 content 0.0225 mg, 0.000354 mmol of copper.
1 3

V. Sadhasivam et al.
Scheme 2 Suzuki–Miyaura cross-coupling reactions catalyzed by Cu@MCOP
General reaction procedure for Suzuki–Miyaura cross‑coupling reactions
Aryl halides (1 mmol) and phenyl boronic acid (1.2 mmol) were dissolved in a 2 mL
DMF solvent, 15 mg of Cu@MCOP and followed by Cs2CO3 (2.5 mmol) base sub-
sequently added into the reaction mixture and allowed to stirred at 70 °C for about
6 h. After completion of the reaction was monitored by TLC analysis, the reaction
mixture was diluted with 10 mL of ethyl acetate, after the Cu@MCOP heteroge-
neous catalyst was recovered by simple fltration through using Whatman-40 flter
paper, and the fltered catalyst was used in next cycle. Furthermore, the fltrate solu-
tion was thoroughly washed with brine solution and extracted with an excess amount
of ethyl acetate and then concentrated under reduced pressure. The crude product
was further purifed by column chromatography using (petroleum ether/ethyl ace-
tate) to obtain the corresponding biphenyl derivatives (Scheme 2).
Acknowledgements AS and VS acknowledge the fnancial support of the Department of Science and
Technology, SERB, Extramural Major Research Project (Grant No. EMR/2015/000969), Council of Sci-
entifc and Industrial Research (CSIR), HRDG, File No. 01(2901)/17/EMR-II, New Delhi, Department
of Science and Technology DST/TM/CERI/C130(G), New Delhi, India. Further, VS acknowledges the
fnancial support of the CSIR, New Delhi, for providing SRF fle No. 124065/2k17/1, 09/201(0420)/18-
EMR-I, New Delhi. We also acknowledge to UPE, DST-FIST for providing instrumental support.
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