Intercalation of copper salt to montmorillonite K-10 and its application as a reusable catalyst for Chan–Lam cross-coupling reaction

Article abstract of DOI:10.1002/aoc.5554

A simple and efficient catalytic system has been developed by adsorption of copper salt in the interlayers of montmorillonite K-10. The catalytic system impressively exercises the green chemistry perspective leading to effortless recovery and recyclabilit

Full text of DOI:10.1002/aoc.5554

Received: 3 April 2019
DOI: 10.1002/aoc.5554
Revised: 26 December 2019
Accepted: 13 January 2020
F U L L P A P E R
Intercalation of copper salt to montmorillonite K-10 and its
application as a reusable catalyst for Chan–Lam cross-
coupling reaction
1
1
2
2
Manashi Sarmah | Anindita Dewan | Purna K. Boruah | Manash R. Das
|
1
Utpal Bora
1
Department of Chemical Sciences,
A simple and efficient catalytic system has been developed by adsorption of
copper salt in the interlayers of montmorillonite K-10. The catalytic system
impressively exercises the green chemistry perspective leading to effortless
recovery and recyclability for the cross-coupling of arylboronic acids and N-
nucleophiles. The effect of catalytic activity was studied for electronically
diverse arylboronic acids and imidazoles relying on various optimizations
under optimum thermal condition. The catalytic system was characterized and
the morphology was determined.
Tezpur University, Tezpur, 784028,
Assam, India
2
Advanced Materials Group, Materials
Sciences and Technology Division, CSIR-
North East Institute of Science and
Technology, Jorhat, 785006, Assam, India
Correspondence
Utpal Bora, Department of Chemical
Sciences, Tezpur University, Tezpur
784028, Assam, India.
Email: utbora@yahoo.co.in; ubora@tezu.
ernet.in
K E Y W O R D S
arylboronic acid, copper sulfate, imidazole, montmorillonite
1
| INTRODUCTION
2:1) structure which can form nanocomposites with vari-
ous organic compounds. Along with both Brønsted and
Transition metal-based catalysts serve as a valuable syn-
thetic tool in the preparation of numerous industrially
Lewis acid catalytic sites that allow alteration of the acidic
nature of a material by a simple ion exchange method.
[6]
[1–3]
and medicinally important organic compounds.
The
The applications of transition metals such as Ir, Ru, Fe
and Au incorporated on nanoporous montmorillonite
high cost of transition metals along with complex or diffi-
cult recovery on use as homogeneous catalysts has led to
an interest in immobilizing transition metal catalyst onto
supports. This heterogeneity can facilitate remarkable
areas of opportunity in green chemistry, for example as
robust support/stabilizer for synthesis, both the isolation
and reusability of the catalyst providing environmentally
suitable protocols. The stabilizer/support plays an impor-
tant role in controlling the particle size, morphology,
[
7–10]
have been reported.
The incorporation of transition
metals in solid supports has been intensively pursued
for various organic transformations. Dutta and co-
workers have reported the stabilization of Cu(0)-based
nanoparticles into the nanopores of montmorillonite
using acidic pretreatment followed by reduction with
[
11]
NaBH4.
Previously we have reported the impregnation
of Pd in montmorillonite pores and its catalytic potential
[4,5]
[12,13]
dispersion and activity of the catalyst.
As per global
was investigated in C–C cross-coupling reactions.
needs, industries demand the most practical, economi-
cally and environmentally viable manufacturing pro-
cesses. In this respect, the application of natural and
modified clays has emerged as novel supports for transi-
tion metal immobilization and are well documented in
organic synthesis due to their versatile properties.
Montmorillonite is the most important smectic (TOT or
Similarly, in the work reported here, we studied the
simple cation exchange properties of montmorillonite
K-10 and subsequent adsorption of copper salt in the
interlayer of its TOT smectic structure. We adopted a
simple pretreatment of the clay which was subsequently
interacted with CuSO avoiding any vigorous or acid-
4
treatment methodology.
Appl Organometal Chem. 2020;e5554.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5554

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SARMAH ET AL.
Nitrogen-containing organic compounds such as
was dried under vacuum for 24 hours and stored at room
temperature. The catalyst thus prepared was designated
as Cu(II)@MMT.
N-aryl species are important intermediates of biologically
active compounds with immense application in academia
and industry, such as material sciences, agrochemicals,
[14–19]
pharmaceuticals and synthetic chemistry.
Tradi-
tionally, various strategies have been adopted, and the
most convenient and proven synthetic routes for
installing N-aryl functionality are copper-catalyzed
2.2 | General procedure for N-arylation
coupling reaction
[20]
Ullman reaction
Hartwig amination.
and palladium-promoted Buchwald–
To a 50 ml round-bottom flask, a mixture of imidazole
(1 mmol), arylboronic acid (1.2 mmol), Cu(II)@MMT
(20 wt%) and K CO (1 mmol) in methanol (4 ml) was
[
21–25]
Besides these, significant
improvements for accessing these N-arylated derivatives
have been made by Chan and co-workers and Collman
and co-workers highlighting the wide applicability of
2
3
ꢀ
added and stirred at 60 C. The progress of the reaction
was monitored via TLC. After completion, the reaction
mixture was extracted with ethyl acetate (3 × 10 ml) and
the combined organic layer was washed with brine
(20 ml), dried over Na SO and concentrated in vacuo
.
[26–29]
copper source as catalyst.
Moreover, subsequent
research efforts over the decades led to significant
improvements in C–N coupling with various nucleophilic
derivatives such as amides, oximes, sulfoximes and thiols
co-catalyzed by additives such as TEMPO, molecular oxy-
2
4
The residue was purified by column chromatography on
silica gel (eluent: EtOAc–hexane, 1:1) to afford the
corresponding N-arylated compound. The desired prod-
[30–38]
gen, pyridine-N-oxide, etc.
Considering the signifi-
1
13
cance of N-aryl derivatives, we investigated the effect of
copper sulfate-adsorbed montmorillonite in the C–N
cross-coupling reactions of arylboronic acids and imidaz-
oles. Moreover, the catalytic system was analyzed using
inductively coupled plasma optical emission (ICP-OES),
Fourier transform infrared (FT-IR) and solid-state UV
spectroscopies, scanning electron microscopy (SEM),
energy-dispersive X-ray (EDX) analysis, energy-dispersive
spectroscopy (EDS) mapping and Brunauer–Emmett–
Teller (BET) surface analysis. The cross-coupling prod-
ucts were characterized by comparing H NMR and
C
C
1
13
NMR data with those of authentic samples ( H and
spectra are available in the supporting information).
2.3 | General procedure for recycling the
catalytic system
The reusability of the catalytic system was investigated
using imidazole (2 mmol) and phenylboronic acid
(2.4 mmol) as coupling partners. After the first cycle, the
reaction mixture was extracted with ethyl acetate
followed by centrifugation. The clearly separated organic
fraction was removed from the system and evaporated to
afford the crude product. The remaining catalyst was
washed with acetone and dried under vacuum and used
for the next consecutive reaction cycle followed by addi-
tion of fresh reactants and solvent.
1
ucts were isolated and characterized using FT-IR, H
13
NMR and C NMR spectroscopies.
2
2
| EXPERIMENTAL
.1 | General experimental procedure for
preparation of copper catalyst
In a 100 ml round-bottom flask, 1 g of montmorillonite
K-10 was mixed with 20 ml of tetrahydrofuran (THF).
The mixture was then subjected to ultrasonication for
3 | RESULTS AND DISCUSSION
2
0 minutes and thereafter, THF was evaporated under
reduced pressure. After complete evaporation of THF,
0 ml of methanol was added. The mixture was allowed
3.1 | Characterization of supported
copper catalyst
5
to stir for 3 hours and then 0.05 g of CuSO Á5H O was
ICP-OES analysis revealed that 5.9 wt% (0.0009 mol%)
of copper sulfate was adsorbed in the interlayers of
montmorillonite K-10 (0.1 g). Further, the structural/
morphological features of Cu(II)@MMT were elucidated
using SEM–EDX, powder X-ray diffraction (XRD),
solid-state UV spectroscopy and BET surface analysis.
The FTIR spectrum of the parent montmorillonite
(Figure 1a) shows an intense absorption band at ca
1044 cm− for Si O stretching vibrations of a tetrahedral
4
2
added to the solution. The resulting dispersion was then
stirred under reflux conditions for 24 hours until the
blue-colored solution became colorless. To ensure the
maximum copper metal ion exchange, we additionally
[39]
added 0.05 g of CuSO Á5H O to the solution.
The solid
4
2
obtained was washed with water and ethanol for several
times and then centrifuged, after which the aqueous por-
tion was subjected to ICP-OES analysis and the catalyst
1

REUSABLE CATALYST FOR CHAN-LAM CROSS COUPLING REACTION
3 of 8
sheet. The bands at 797, 527 and 467 cm−1 were due to
amorphous silica, Si O Al and Si O Si bending vibra-
tions, respectively. And the absorption band at 3435 cm−
is due to stretching vibrations of the OH groups. The FT-
IR spectrum of the supported catalyst shows slight shift
in absorption peak of the parent montmorillonite
1
−1
(Figure 1a).The absorption peak at 618.2 cm is due to
−
1
Cu O vibration which is shifted from 616.8 cm for
FIGURE 1 (a) FT-IR spectra of parent montmorillonite, Cu(II)@MMT and CuSO
MMT [Colour figure can be viewed at wileyonlinelibrary.com]
4
Á5H
2 4
O. (b) UV–visible spectra of CuSO and Cu(II)
@
FIGURE 2 (a) Powder XRD pattern of Cu(II)@MMT. (b) Nitrogen adsorption–desorption isotherm and pore size distribution (inset) of
Cu(II)@MMT [Colour figure can be viewed at wileyonlinelibrary.com]
FIGURE 3 (a) EDX analysis of Cu(II)@MMT. (b) SEM image and EDS mapping of Cu(II)@MMT [Colour figure can be viewed at
wileyonlinelibrary.com]

4
of 8
SARMAH ET AL.
CuSO Á5H O. The shifting of the band might possibly be
indicating the adsorption of metal salt in the pores of
montmorillonite.
4
2
due to the adsorption of CuSO in the interlayers of
4
[12,40]
montmorillonite.
Additionally, UV–visible absorp-
EDX analysis of the catalyst shows the presence
of Cu on the surface of modified montmorillonite
along with the other elements of clay (Figure 3a).
SEM study furnishes the surface topology and morpho-
logical evidence of the montmorillonite-embedded
copper cations (Figure 3b). Furthermore, the precursor
compound was assessed via EDS mapping which
confirms the corresponding elemental distribution of
the sample. The respective elemental mappings for
(S + Fe + Na + K + Si + Cu) are presented in Figure 3b.
tion spectroscopy shows the difference in absorption shift
of CuSO and Cu(II)@MMT, thereby revealing the Cu O
4
interaction in the montmorillonite interlayer spacing
[41]
(Figure 1b).
The powder XRD pattern of the sample was recorded
ꢀ
over a 2θ range of 10–70 (Figure 2a). The diffraction
peaks resemble those of the crystal planes corresponding
to CuSO (JCPDS 802209). The pore characteristics and
4
surface area of the prepared catalyst were examined
using nitrogen adsorption and desorption analysis (BET).
ꢀ
Prior to BET analysis the sample was degassed at 250 C
for 3 hours. The corresponding adsorption–desorption
isotherm and pore size distribution curve (inset) of the
prepared catalyst are shown in Figure 2b. The pore size
distribution curve was derived via the Barrett–Joyner–
Halenda method. The isotherm belongs to type IV with
3.2 | Catalytic activity of Cu(II)@MMT
in C–N cross-coupling reaction
The catalytic performance of Cu(II)@MMT was investi-
gated for the coupling of imidazole (1 mmol) and
arylboronic acid (1.2 mmol). Firstly, the activity of the
an H3 hysteresis loop at P/P of 0.4–0.99 which defines
0
the mesoporosity of the material. The specific surface
2
−1
area of the catalyst was found to be 65.584 m g , with
an average pore diameter of 62.49 Å and pore volume of
TABLE 2 Effect of various solvents and bases on C–N
coupling reaction
3
−1
0
.102 cm g , which is less than that of the parent mont-
a
2
−1
morillonite K10 (surface area of 220–270 m g ), thereby
TABLE 1 Effect of Cu source and its amount on coupling
a
reaction
b
Entry
Solvent
Base
Time (h)
Yield (%)
71
1
H
2
O
K
2
K
2
K
2
K
2
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
4
2
2
MeOH
EtOH
93
3
3
87
4
EtOH–H
2
O
4
75
b
Entry
Copper source (mol%)
Time (h)
Yield (%)
5
MeOH–H O
2
K CO
4
80
2
1
2
3
4
5
CuSO
4
4
Á5H
Á5H
2
2
O (5)
5
5
5
5
4
4
2
5
5
2
55
60
60
65
69
75
93
50
70
94
6
i-PrOH
K
2
K
2
K
2
K
2
CO
CO
CO
CO
5
64
CuSO
O (10)
7
THF
6
<10
30
Cu (NO
Cu (OAc)
CuSO
Á5H
3
)
2
Á3H
2
O (10)
8
DMF
2
2
ÁH O (10)
2
9
DMSO
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
2
30
4
2
O (5) + MMT
10
11
12
13
—
10
6
20
d
c
6
Cu(II)@MMT (20 )
NaOH
Et
Cs
30
c
7
8
9
Cu(II)@MMT (20 )
3
N
6
35
c
Cu(II)@MMT (5 )
2
CO
3
2
93
c
c
Cu(II)@MMT (15 )
14
K
2
2
CO
CO
3
3
2
50
c
d
1
0
Cu(II)@MMT (25 )
15
K
2
50
a)
a)
Reaction conditions: imidazole (1 mmol), phenylboronic acid (1.2 mmol),
Reaction conditions: imidazole (1 mmol), phenylboronic acid (1.2 mmol),
ꢀ
ꢀ
K
b)
2
CO
3
(1 mmol), MeOH (4 ml), 60 C.
base (1 mmol), solvent (4 ml), Cu(II)@MMT (20 wt%), 60 C.
b)
Isolated yield.
Isolated yield.
c)
c)
Weight percent of catalyst.
0.5 mmol of base.
d)
d)
1
mmol of phenylboronic acid.
Room temperature.

REUSABLE CATALYST FOR CHAN-LAM CROSS COUPLING REACTION
5 of 8
a
TABLE 3 Coupling reaction of various arylboronic acids and imidazoles
a)
ꢀ
2 3
CO (1 mmol), MeOH (4 ml), Cu(II)@MMT (20 wt%), 60 C.
Reaction conditions: imidazole (1 mmol), phenylboronic acid (1.2 mmol), K
Isolated yield.
b)
catalyst was compared with that of homogeneous coun-
terparts considering imidazole and phenylboronic acid as
the model substrates; the results are presented in Table 1.
We observed that the effectiveness of homogeneous cop-
per salts towards the coupling reaction was significantly
low (Table 1, entries 1–4). However, the reaction was
slightly accelerated with the addition of montmorillonite
with 5 mol% of CuSO Á5H O (Table 1, entry 5). A consid-
4
2
erable increase in reaction efficiency was observed using
Cu(II)@MMT (Table 1, entries 6–10). On varying the
amount of catalyst loading, appreciable alteration in reac-
tion time and yield was noticed. Notably, there was excel-
lent catalytic activity using 20 wt% of Cu(II)@MMT
(Table 1, entry 7). On increasing the catalyst amount to
2
5 wt%, no significant difference in reaction yield was
observed (Table 1, entry 10). Furthermore, varying the
ratio of phenylboronic acid, we observed that the reaction
performed efficiently with a 1:1.2 ratio of imidazole to
arylboronic acid (Table 1, entries 6 and 7).
FIGURE 4 Reusability of the catalyst for coupling reaction
[Colour figure can be viewed at wileyonlinelibrary.com]
It is well known that, in order to achieve the smooth
progress of a reaction, solvent plays a key role. Various
Chan–Lam reactions have been reported where methanol
present catalytic system and the results are summarized
in Table 2. On performing the reaction using 20 wt%
Cu(II)@MMT in water, only 71% of the isolated product
was obtained (Table 2, entry 1). On widening the solvent
selection, we noticed that the reaction affords appreciable
exhibits
solvents.
a
superior activity over many organic
Based on these considerations, we next
[
42–45]
opted to study the effect of various solvents on the

6
of 8
SARMAH ET AL.
improvements in yield using organic and biphasic media
Table 2, entries 2–6). Excellent catalytic activity was
achieved using methanol as sole medium (Table 2, entry
). On the other hand, a markedly low yield of desired
product was obtained in isopropyl alcohol (Table 2, entry
). Reactivity of other protic and aprotic solvents was
investigated but no appreciable result was obtained
3.3 | Recyclability of catalyst
(
On pursuing the principle of green chemistry, the
major challenge of a catalytic system is its recyclability.
So in an endeavor to expand the scope of this proto-
col, the reusability and efficiency of the catalytic
system was investigated using imidazole and phe-
nylboronic acid as coupling partners. Interestingly, the
catalytic activity remains up to the fourth reuse which
signifies the stability and efficient encapsulation of
Cu(II) in montmorillonite interlayers (Figure 4). How-
ever, a decrease in reaction yield was observed in the
fourth cycle which might be due to loss of catalyst
during recovery.
2
4
(Table 2, entries 7–9).
In addition to the above optimization, the role of base
in the C–N coupling reaction is noteworthy. Base plays a
significant role in the most crucial stage of the mecha-
nism, the transmetalation stage, the transfer of the
organoboron species to the metal cation. The reaction
was performed in the absence of base but lower conver-
sion to cross-coupling product was obtained (Table 2,
entry 10). Thereafter, the reaction was screened using
various bases. Among the bases examined, considerable
low yield of cross-coupled product was obtained for
To confirm the heterogeneous nature of Cu(II)
@MMT in the C–N cross-coupling reaction, we per-
formed the hot filtration test considering the standard
reaction conditions using imidazole and phenylboronic
acid. The hot reaction mixture was filtered to remove the
Cu(II)@MMT catalyst, the filtrate was allowed to further
react and the progress of the reaction was monitored by
TLC. Product formation was observed in the filtrate,
which suggests a quasi-homogeneous process with a
reversible dissolution–precipitation of Cu(II) species
NaOH and Et N (Table 2, entries 11 and 12). On the
3
contrary, K CO and Cs CO provided excellent yield of
2
3
2
3
N-arylated product (Table 2, entries 2 and 13). On lower-
ing the amount of base and conducting the reaction at
room temperature, a decrease in the product yield was
obtained (Table 2, entries 14 and 15).
Next, the generality of this coupling of arylboronic
acids with imidazoles was investigated using a variety of
electronically diverse arylboronic acids and a wide range
of imidazoles as substrates considering the optimized
reaction conditions (Table 3). The catalytic system
delivers excellent reactivity for both electron-
withdrawing and electron-donating arylboronic acids
[
46,47]
during the course of the reaction.
To further confirm the structural stability and
reusability of Cu(II)@MMT in the C–N coupling reac-
tion, the recovered catalyst was characterized using pow-
der XRD and field emission SEM. The powder XRD
pattern of the sample was recorded over a 2θ range of
ꢀ
10–80 (Figure 5a). The diffraction peaks resemble those
(Table 3, 3a–3o). Imidazole with substituted group at
of the parent Cu(II)@MMT corresponding to crystal
varying positions reacts well (Table 3, 3h–3n). However
benzimidazole directs the catalytic activity to consider-
ably low conversion of isolated product along with longer
reaction time (Table 3, 3o). From the aforementioned
results, the catalytic system exhibits excellent perfor-
mance for the N-arylation reaction.
planes of CuSO . This was further confirmed using a
4
morphological study: the SEM image of the reused cata-
lyst in Figure 5b clearly shows the distribution of CuSO4
crystals in the montmorillonite surface and interlayers.
Additionally, EDX analysis confirms the presence of Cu
in the catalyst after its reuse (Figure 5c).
FIGURE 5 (a) Powder XRD pattern, (b) SEM image and (c) EDX analysis of recovered Cu(II)@MMT after the fourth cycle [Colour
figure can be viewed at wileyonlinelibrary.com]

REUSABLE CATALYST FOR CHAN-LAM CROSS COUPLING REACTION
7 of 8
4
| CONCLUSIONS
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Montmorillonite, which is an inexpensive and environ-
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for the adsorption of copper sulfate. And this resourceful
heterogeneousness actively surmounts the selective use
of homogeneous catalysts. Moreover, the Cu(II)@MMT
catalytic system shows excellent result with easy recovery
and reuse. The system does not require environmentally
unfavorable organic ligands, is less expensive and avoids
protodeboronation of arylboronic acid. The robustness
and simplicity of this heterogeneous catalyst makes it
attractive for industrial application and also promising
for researchers in the exploration of new synthetic
strategies.
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ACKNOWLEDGMENTS
The authors thank the Department of Science and
Technology, New Delhi for providing financial support
[
[
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How to cite this article: Sarmah M, Dewan A,
Boruah PK, Das MR, Bora U. Intercalation of
copper salt to montmorillonite K-10 and its
application as a reusable catalyst for Chan–Lam
cross-coupling reaction. Appl Organometal Chem.
2020;e5554. https://doi.org/10.1002/aoc.5554
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