Methanol aided synthesis of PdNPs decorated on montmorillonite K 10 and its implication in Suzuki Miyaura type cross coupling reaction under base free condition

Article abstract of DOI:10.1002/aoc.4192

An efficient catalyst PdNPs decorated on Montmorillonite K 10 is prepared by simply stirring Pd(OAc)₂ in methanol at room temperature without using any external reducing agent. The catalyst shows excellent activity for Suzuki Miyaura type cross coupling reaction between aryl diazonium salt and arylboronic acid under ligand and base free reaction conditions within short reaction time. The main advantage of this methodology is the easy synthesis of heterogeneous PdNPs @ Mont K 10 catalyst in a mild condition without using any reducing agent or additive and the catalyst is very efficient for biaryl synthesis. The catalyst is well characterized by SEM, EDX, TEM, BET, Powder XRD, TGA, DSC etc. The reaction pathway is greener with aqueous reaction medium, base free reaction condition, room temperature and reusable heterogeneous catalyst.

Full text of DOI:10.1002/aoc.4192

Received: 8 June 2017
DOI: 10.1002/aoc.4192
Revised: 30 October 2017
Accepted: 1 November 2017
F U L L P A P E R
Methanol aided synthesis of PdNPs decorated on
montmorillonite K 10 and its implication in Suzuki Miyaura
type cross coupling reaction under base free condition
Abhijit Mahanta | Prasanta Kumar Raul | Sanjib Saikia | Utpal Bora
| Ashim Jyoti Thakur
Department of Chemical Sciences, Tezpur
An efficient catalyst PdNPs decorated on Montmorillonite K 10 is prepared by
University (A central University), Tezpur
Napaam, 784028 Assam, India
simply stirring Pd(OAc) in methanol at room temperature without using any
2
external reducing agent. The catalyst shows excellent activity for Suzuki Miyaura
type cross coupling reaction between aryl diazonium salt and arylboronic acid
under ligand and base free reaction conditions within short reaction time. The
main advantage of this methodology is the easy synthesis of heterogeneous
PdNPs @ Mont K 10 catalyst in a mild condition without using any reducing
agent or additive and the catalyst is very efficient for biaryl synthesis. The catalyst
is well characterized by SEM, EDX, TEM, BET, Powder XRD, TGA, DSC etc. The
reaction pathway is greener with aqueous reaction medium, base free reaction
condition, room temperature and reusable heterogeneous catalyst.
Correspondence
Dr. Ashim Jyoti Thakur and Dr. Utpal
Bora, Tezpur University, Department of
Chemical Sciences, Assam, India.
Email: ashim@tezu.ernet.in;
ubora@tezu.ernet.in
KEYWORDS
cross coupling, methanol, palladium NPs, room temperature
1
| INTRODUCTION
friendly route for the synthesis of PdNPs in a single step
with minimum use of chemicals or by using environmen-
For centuries, the pursuit of designing green protocol in
chemistry for the synthesis of efficient nano catalyst has
become an utmost goal in research community as the
chemistry of nano materials has gained much interest
because of their higher efficiency compared to bulk
tally acceptable solvents. Biological resources such as plant
[
6]
[7]
extracts, microorganisms etc. can be used for the syn-
thesis of NPs as they have the reduction potential suitable
for NPs synthesis. Additionally, considerable efforts have
been made to prepare and stabilize PdNPs by using envi-
ronmentally benign, robust, cheap and available support/
[1]
materials. They have high surface area and found wide
applications in diverse areas such as catalysis, electronics,
[
8,9]
stabilizer.
Reports have been found on the synthesis
[
2]
biochemical sensor etc. Consequently, nano materials
have emerged as sustainable alternative to conventional
catalyst for different chemical transformations such as C‐
C and C‐heteroatom bond forming reactions, C‐H
activation reaction etc. Among the metal nanoparticles
of PdNPs by using bio‐reducing agents like leaf extract of
[
9]
[10]
colocasia esculanta, tea and coffee extract,
soyabean
[
6]
[11]
leaf extract, starch
etc. but, in most of the cases, tem-
perature is an important parameter for synthesizing metal
NPs which is against the prospect of ‘green chemistry’. So,
considering the above factors, synthesis of PdNPs at room
temperature without using any external reducing agent is
a very fascinating development in the area of nano cataly-
sis. Alcoholic solvent has reducing properties itself and can
be engaged for the synthesis of metal NPs which is consid-
(NPs), PdNPs find widespread applications as catalyst in
aforementioned bond forming areas. Although, PdNPs
exhibit excellent catalytic activity, but in most of the cases
[3]
their synthetic route require high temperature, ultra
[4]
[5]
sonication, active reducing agent etc. As a result, there
[
12a,b]
is growing interest for the development of green and eco‐
ered as green synthetic process.
Currently, a variety of
Appl Organometal Chem. 2017;e4192.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
1 of 9
https://doi.org/10.1002/aoc.4192

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MAHANTA ET AL.
reports have been published where PdNPs are embedded
available, non‐toxic and inexpensive alternatives for aryl
halides as a coupling partner for concerned coupling
reaction. In last few decades, aryl diazonium salts come
[13]
on various metal solid supports such as zeolite,
micro
[14a,b]
[15a,b,c]
porous polymer,
mesoporous silica,
aluminium
charcoal,
[16]
[17]
[18]
[19a,b,c]
hydroxide,
agarose,
BaCO3,
out to be an extremely productive coupling partner
[20]
[21a–c]
[28a,b]
clay,
metal oxides
etc. and they show excellent
for Suzuki‐Miyaura cross‐coupling reaction
as they
catalytic activity.
In this manuscript, we wish to report a mild methodol-
ogy for synthesizing PdNPs decorated on Montmorillonite
can be easily prepared from anilines which are more
economical than aryl halides. The other advantages of
diazonium salts are that they are more reactive than
corresponding halides; so, reaction takes place within short
reaction time and is feasible under mild and aqueous condi-
(K 10) by simple stirring at room temperature using meth-
anol as solvent without using any external reducing agent.
We have chosen Mont. K 10 because the same is considered
as one of the most demanding catalytic materials in hetero-
geneous catalysis due to its low cost and eco‐friendliness. It
[
29]
tion as diazonium salts are better electrophile
than the
aryl halides. Eventually, the chemistry of a ‘base free’
synthetic route for Suzuki‐Miyaura reaction is also highly
appreciable because racemization of optically active com-
pounds takes place in presence of base. Moreover, accord-
ing to green chemistry prospect, lesser the chemical used
in the reactant side higher will be the atom economy and
atom efficiency and so more greener the reaction pathway
as both are the chief parameters of green chemistry.
So, as a part of our ongoing programme to develop
simplistic protocol for Suzuki Miyaura type cross coupling
C‐C bond forming reaction herein, we wish to report
PdNPs@K 10 as an efficient heterogeneous catalyst for
base free Suzuki Miyaura type cross‐coupling reaction in
aqueous medium at room temperature.
[
22]
also possesses high surface area and swelling property.
This clay is mainly comprised of aluminosilicate layers,
where one octahedral alumina sheet is sandwiched
between twice of the tetrahedral silica sheets. Again, in
the layers of aluminosilicates, the partial isomorphous sub-
4+
3+
stitution of Si ions by trivalent metal cations and Al
ions by divalent metal cations origins a charge deficiency.
To balance this charge deficiency, hydrated alkali and
alkaline earth metal cations reside in the interlayer spaces
of montmorillonite. On the other hand these cations in the
interlayers are found to be highly exchangeable, as a result,
making montmorillonite able to immobilize various guest
molecules like metal nanoparticles or metal complexes in
[23]
its interlayers.
To note, the immobilization of metal
2
2
| EXPERIMENTAL
NPs in this type support like in mont. K 10 prevents
agglomeration by maintaining them in smaller sizes. These
type of supports act like small molecular containers which
modify the characteristics of free catalysts by escalating the
steric constraints and enhancing the local concentration of
reagents near the surface where the reaction takes place
This type of heterogeneous catalytic system is able to
overcome the drawbacks of homogeneous catalysts in case
.1 | Preparation of the catalyst
In a round bottom flask containing methanol as solvent,
0.075 g of Palladium acetate and 0.750 g of K 10 (Montmo-
rillonite) were taken and the solution was stirred for two
days at room temperature. The colour of the reaction mix-
ture turned to black from brown. It was filtered by suction
funnel and washed with MeOH for several times, the
residue was collected and dried. The collecting residue
was kept under a flow of air overnight and under vacuum
for 24 h which gave a black powder of heterogeneous K 10
supported palladium catalyst.
[
24a,b]
of recoverability issue.
Over the last few decades, Palladium catalyzed
Suzuki‐Miyaura cross‐coupling reaction of aryl halide
and organoboron derivatives has become one of the most
widely used methods for construction of C‐C bond which
have made an enormous contribution to research as their
cross‐coupled products present as subunits of many
2
.2 | General Experimental procedure for
[
25a,b]
[26]
marketed drugs
and agrochemicals.
The reaction
Suzuki Miyaura type cross coupling
reaction
has main advantage of having organoboron starting mate-
rials which are air stable, easily available, large functional
group tolerance and moisture compatible.
mide and aryl iodides are the common coupling partners
in Suzuki‐Miyaura reaction. However, it is a challenging
task to perform Suzuki‐Miyaura reaction using aryl
chlorides as coupling partner under mild condition as
they react sluggishly and are considered as pollutants.
Furthermore, aryl bromides and iodides are found to be
costly, so it is distinctively an endeavour to find readily
[27a,b]
Aryl bro-
A 50 ml round‐bottom flask was charged with a mixture
of 10 wt % PdNPs @K 10 catalyst, 1.2 mmol of arylboronic
acid and 1 mmol of aryldiazonium salt and stirred at
room temperature in 3 ml distilled water for the required
time. The reaction was monitored by TLC and after
completion of the reaction the solution was extracted
three times with ethylacetate (3 × 10 ml). The products
were purified by column chromatography and confirmed

MAHANTA ET AL.
3 of 9
1
13
by H, C NMR spectroscopy (Supporting Information)
and mass spectrometry.
surface area of the clay and the catalyst was found to be
2
‐1
209 and 154 m g , respectively. The decrease in the value
for the catalyst indicates the incorporation of the metal to
the clay. Furthermore, from ICP‐OES analysis it was
found that 0.056 g of Pd was loaded on montmorillonite
i.e., 0.33 mg/g.
2
.3 | Characterizations of the catalyst:
The catalyst was characterised by BET surface area
measurement, Energy Dispersive X‐Ray analysis (EDX),
Powder XRD, Thermo Gravimetric Analysis (TGA), Dif-
ferential Scanning Calorimetry (DSC), Scanning electron
microscopy (SEM) and Transmission electron microscopy
2.3.2 | SEM and EDX:
EDX analysis of the catalyst clearly depicts the presence of
Pd in the catalyst (Figure 2b). EDX spectrum also shows
the presence of metal Si, Fe, K, Mg, Cl, O etc which are
the basic constituents of the clay Montmorillonite. The
scanning electron microscopic image for catalyst shows
the surface morphology of the catalyst (Figure 2a)
(
TEM) etc.
2
.3.1 | BET surface area
Figure 1 depicts the N adsorption–desorption isotherm
2
and pore size distribution of the prepared catalyst under
study. It can be predicted from the figure that the
compounds are mesoporous material with highly uniform
pore size distribution (Type IV isotherm). The BET
2
.3.3 | TEM
The particle size and surface morphology of the synthesized
catalyst is studied by transmission electron microscopy
(
TEM) and high resolution transmission electron micros-
copy (HRTEM). The TEM image (Figure 3) of the NPs
clearly shows that uniformly dispersed Pd(0)NPs on the sur-
face of montmorillonite and most of the particles are spher-
ical in nature. The crystal lattice of PdNPs is clearly resolved
in the HRTEM image (Figure 3f) and it is found to be
FIGURE 1 (a) N
2
adsorption–desorption isotherm (b) pore size
distribution curve of the catalyst
FIGURE 2 SEM and EDX analysis spectra of the catalyst

4
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MAHANTA ET AL.
FIGURE 3 TEM images of PdNPs@K 10 before reaction (a) SAED pattern of PdNPs, (b‐e) TEM images along with particle size distribution,
f) HRTEM image along with fring spacing
(
th
0
.20 nm. The size of the NPs is found to be around in a range
we have chosen the recovered catalyst after 4 cycle of the
th
of 10‐15 nm. The bright spot in the SAED pattern [Figure 3
reaction. TEM images of the catalyst after 4 cycle shows
(
a)] depicts the crystalline state of the NPs.
(Figure 4) that the immobilized PdNPs on montmorillon-
ite also have particle size in the range of approx. 10‐15 nm
which is also found in the TEM image of the catalyst
before the reaction. Therefore, from the above results, it
We have investigated also the surface morphology and
size of the PdNPs after performing reaction by character-
izing the catalyst by TEM analysis after reaction. For that,
FIGURE 4 TEM images (i‐iv) of the recovered catalyst after 4th cycle

MAHANTA ET AL.
5 of 9
is evident that the catalyst does not undergo any change
in size as well as morphology, which in turn does not
show any significant decrease in the catalytic activity.
2.3.5 | TGA and DSC:
To know the thermal stability of the catalyst it is character-
ized by TGA analysis. The analysis shows [Figure 6 (a)]
that the catalyst is stable up to 600°C. The weight loss at
temperature range 250‐300°C is due to the evaporation of
the physically absorbed water molecules. Like that DSC
analysis [Figure 6 (b)] is also performed for the catalyst.
From the curve it can be concluded that there are two
endothermic peaks within 80‐100°C regions due to
the volatilization of water and low molecular weight
compounds.
2.3.4 | Powder XRD
Powder XRD analysis for the parent montmorillonite and
the fresh catalyst has been done before performing the
reaction. In the [Figure 5 (a)] three peaks are observed
for the catalyst at 2θ=39.57°, 46.17° and 67.90°
corresponding to the reflections of (111), (200), and
(220) planes of PdNPs, (JCPDS no. 88‐2335) respectively.
The broadening of the peaks is a sign of formation of nano
particles. Again we have done powder XRD analysis for
the recovered catalyst after 4 cycle of the reaction. The
XRD pattern [Figure 5 (b)] shows the prominent peaks
at 2θ values of about 39.94, 45.96, 67.43 are assigned to
the (111), (200), (220) crystallographic planes of PdNPs
which clearly confirms that PdNPs remain in zero
oxidation state as before performing the reaction.
3
3
| RESULT AND DISCUSSION
th
.1 | Optimization of reaction condition
To explore the catalytic activity of the newly synthesized
catalyst for Suzuki Miyaura type cross coupling reaction,
FIGURE 5 (a) Powder XRD pattern of the catalyst and the parent
clay, (b) powder XRD pattern of the recovered catalyst after 4 cycle
FIGURE 6 (a) TGA curve for the catalyst and (b) DSC curve for
the catalyst
th

6
of 9
MAHANTA ET AL.
a model reaction between 1.2 mmol of phenylboronic acid
and 1 mmol of 4‐nitrobenzenediazonium tetrafluroborate
was investigated in presence of 10 wt% of the catalyst
under open air condition using methanol at room temper-
ature (Table 1, entry 1). It has been observed that no base
is required when aryl diazonium tetraflouroborate salt
has used as a substrate for Suzuki Miyaura type cross‐cou-
ethanol, iso‐propanol, isopropanol: water (1:1) and water
in both 10 wt% and 5 wt% of the catalyst (Table 1, entries
3‐10). From the experiments it was found the reaction
deliver slight lesser yield and need almost double times
for its completion when performed with 5 wt% of the
catalyst than with 10 wt% of the same in all of the solvents
mentioned. But, interestingly, the reaction showed
excellent progress in water with 93% isolated yield with
10 wt% of the catalyst (Table 1, entry 9). Though benzene
diazomium tetraflouroborate salt is sparingly soluble in
water, but here, in this case we believe that the heteroge-
neous catalyst provides a better surface for smooth
advancement of the reaction and it seems to progress
through ‘on water’ reaction route. The reaction under
aqueous system completed within the comparable time
as the alcoholic reaction system. For the next assessment,
an attempt was made with 15 wt% of catalyst loading
keeping other reaction conditions same using water as
reaction medium. Interestingly, we found no significant
variation in yield and time for 15 wt% of the catalyst
(Table 1, entry 11). The reaction was also performed in
non‐polar solvents like DCM and toluene, reaction in
both of the solvents showed excellent results as it was
completed in shorter reaction time with excellent yield
pling reaction. It may be due to the in‐situ generation of
‐
conjugate base BF ion in the system which can activate
4
the boronic acid moiety. We first checked the reaction in
methanol as it was reported as most apposite solvent for
[28]
this coupling reaction.
Encouraging us, the reaction
was found to be completed with excellent yield of desired
product within 45 min (Table 1, entry 1). Though various
reports are found on base free Suzuki Miyaura type cross
coupling
reaction
with
benzene
diazonium
tetraflouroborate, but still literature reveals only a few
methodologies for the concerned reaction which have
been performed at room temperature. In the next experi-
ment checked the reaction with 5 wt% of the catalyst
which also showed an almost good yield of 90% of the
coupling product (Table 1 entry 2).
To advance the progress of the aforementioned
reaction, we optimized the effect of different solvents like
TABLE 1 Optimization of reaction condition for catalyst, base, solvent
Entry
Catalyst
Solvent (3 ml)
Time(min)
Yield (%)
1
10 wt%
Methanol
45
93
2
3
4
5
6
7
8
9
5 wt%
Methanol
Ethanol
45
90
10 wt%
5 wt%
45
92
Ethanol
90
88
10 wt%
5 wt%
Iso‐propanol
Iso‐propanol
Iso‐propanol:Water
Iso‐propanol:Water
Water
45
92
90
86
10 wt%
5 wt%
45
92
90
88
10 wt%
5 wt%
45
93
10
11
12
13
14
15
Water
90
86
15 wt%
10 wt%
10 wt%
1 mol%
10 wt%
Water
90
93
DCM
30 min
35 min
120
120
95
Toluene
92
a
b
Water
Not completed
Not completed
Water
Reaction condition: Phenylboronic acid (1.2 mmol), 4‐nitrobenzediazoniumtetrafluroborate (1 mmol)
a
homogeneous Pd(OAc)
2
was used
b
heterogeneous Pd(0)/C was used.

MAHANTA ET AL.
7 of 9
than in water or other polar solvents (Table 1, entries 12
and 13). Although results were better in non‐polar
solvent, yet we preferred the optimized reaction condition
taking water as solvent because a reaction in aqueous sol-
vent and at room temperature is highly appreciable as it
refers to an exciting area of research from green chemistry
point of view and has good application in both academia
and industries due to availability, cost and environmental
acceptability of water. Therefore, we fixed the reaction
condition with 10 wt% of the catalyst in water (Table 1,
entry 9). We also investigated the activity of homogeneous
Pd(OAc)2 and heterogeneous Pd(0)/C were not fully
converted towards the desired product till 2 hours. The
higher reactivity of PdNPs@K 10 may be due to the
presence of hydroxyl groups of aluminium, silica etc. on
montmorillonite surface which might activate the
arylboronic acid moiety as shown in Figure 7.
Moreover, such an enhancement and increase in
activity of the PdNPs@ K 10 catalyst may be due to the
formation of Pd‐O interaction at the particle/support
interface between PdNPs and metal oxides present in
[
30]
the montmorillonite.
Pd(OAc) and heterogeneous Pd(0)/C catalysts towards
2
the coupling reaction (Table 1, entries 14 and 15). Both
of the catalysts failed to show better result than
PdNPs@K 10. The reactions catalysed by homogeneous
3.2 | Generalization of reaction condition
Being inspired by the above studies, next, we focused on
the substrate scope of the reaction with different electron
donating and withdrawing arylboronic acids and aryl
diazonium salts and found satisfactory result with excel-
lent yield in most of the cases within 3 hours. Only the
reaction with heteroarylboronic acid, 6‐methoxy pyridine
3‐yl boronic acid needed a longer time of 8 h for its
completion (Table 2, entry 14). We found no effect of
electron donating and withdrawing groups on the rate of
the reaction as illustrated in Table 2:
FIGURE 7 Activation of the boronic acid moiety by the hydroxyl
groups of montmorillonite
TABLE 2 Cross‐coupling of aryl diazonium salt and arylboronic acid
a
Entry
Arylboronic acid (R
1
)
Aryl diazonium salt(R
2
)
Time
Yield (%)
1
‐H
4‐OMe
1.5 h
92
2
3
4
5
6
7
8
9
4‐OMe
4‐F
4‐OMe
55 min
1 h
98
89
95
95
93
87
88
92
96
89
95
92
81
84
4‐NO
2
2
4‐Cl
4‐CHO
‐H
4‐NO
1.5 h
50 min
45 min
75 min
1.5 h
2.5 h
45 min
1 h
4‐OMe
4‐NO
2
4‐F
4‐OMe
4‐OMe
4‐OMe
4‐CH
3‐Me
3
10
11
12
13
14
15
4‐OMe
4‐NO
4‐OMe
4‐NO
4‐OMe
2
2‐Me
4‐tertbutyl
4‐tertbutyl
2
2 h
2 h
6‐methoxy 3 pyridine yl
‐H
4‐NO
2
8 h
3‐me
3 h
Reaction condition: Phenylboronic acid (1.2 mmol), 4‐bromonitro benzene (1 mmol), PdNPs@ K 10 (10 wt%), water (3 ml) at room temperature
a
isolated yield

8
of 9
MAHANTA ET AL.
3.3 | Mechanism study
A plausible mechanism is designed and schematic
mechanistic pathway (Figure 8) is discussed below based
[31]
on the literature report.
In this mechanism, the palladium catalyst first inserts
into the C‐N bond to form an intermediate II. In the next
2
step, a fluoride from tetraflouroborate counter ion
displaces the diazonium and an intermediate III is formed
then the Palladium‐flouride complex III undergoes
transmetallation with arylboronic acid through the
formation of a strong fluorine boron bond. Finally, step
IV undergoes reductive elimination to form the biaryls.
3.4 | Reusability study
FIGURE 9 Reusability plot of the catalyst
From the green chemistry perspective, the reusability of
catalysts makes them more attractive. For the investigation
of the reusability of our catalyst, a reaction taking 2.4 mmol
of phenylboronic acid and 2 mmol of 4‐nitrodiazonium salt
as model substrates, 20 wt% of PdNPs@ K 10 catalyst and
6
ml of water at room temperature was carried out. For
easy recovery issue we have taken two times scale of model
reaction. As indicated in the Figure 9 the catalyst is
th
reusable up to 6 run without significant loss of the yield.
3.5 | Sheldon test
To check the heterogeneous nature of the catalyst, hot fil-
tration test was done. To do this, the reaction was carried
out at different time intervals taking the model reaction
condition and after half completion of the reaction, i.e.,
after 15 minutes the catalyst PdNPs@K10 was filtered
off from the reaction mixture. After removal of the
catalyst, the reaction was monitored for an additional
FIGURE 10 Hot filtration test
4
| CONCLUSION
6
h and found no further product formation (Figure 10).
This proves the heterogeneous nature of the catalyst
In conclusion, a highly efficient heterogeneous catalyst
PdNPs@K 10 has been developed for the cross‐coupling
reaction between aryl diazonium salt and arylboronic
acids with a wide range of both of the reactants in base
free condition. The reaction proceeds in easily available
green solvent water and at room temperature. The
catalyst we have synthesized showed excellent catalytic
activity and reused up to six times with slight decrease
of the yield of the reaction. We have done all essential
characterization in support of the catalyst.
ACKNOWLEDGEMENT
We gratefully thank BIT, Bangalore for BET surface area
analysis, SAIF‐NEHU, Shillong for TEM analysis. A. Mahanta
thanks Tezpur University for institutional fellowship.
FIGURE
heterogeneous catalyst
8
Mechanistic route for the reaction over the

MAHANTA ET AL.
9 of 9
ORCID
2679. (c) H. Bonin, D. Delbrayelle, P. Demonchaux, E. Gras,
Chem.Commun. 2010, 46, 2677.
Utpal Bora http://orcid.org/0000-0002-7403-0152
Ashim Jyoti Thakur http://orcid.org/0000-0003-0923-6725
[
[
20] G. B. B. Varadwaj, S. Rana, K. Parida, J. Phys. Chem. 2014, 118,
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21] (a) G. O. D. Estrada, A. L. P. Blanco, J. F. M. da Silva, C. G.
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