Green tea extract–modified silica gel decorated with palladium nanoparticles as a heterogeneous and recyclable nanocatalyst for Buchwald-Hartwig C–N cross-coupling reactions

Article abstract of DOI:10.1016/j.jpcs.2019.109256

A novel green tea extract–encapsulated silica gel decorated with in situ–generated Pd nanoparticles is reported as an efficient, green heterogeneous catalyst in the Buchwald-Hartwig C–N cross-coupling reaction. It was characterized by several analytical techniques. Thereafter, a wide range of aryl amines were synthesized in good to excellent yields by reaction of different substituted aryl halides and secondary amines over the catalyst. The material is sufficiently stable and could be used at least six times in a model Buchwald-Hartwig reaction without noticeable change in its catalytic activity. Heterogeneity of the catalyst was examined by a hot filtration test.

Full text of DOI:10.1016/j.jpcs.2019.109256

Journal Pre-proof
Green tea extract–modified silica gel decorated with palladium nanoparticles as
a heterogeneous and recyclable nanocatalyst for Buchwald-Hartwig C–N cross-
coupling reactions
Hojat Veisi, Taibeh Tamoradi, Bikash Karmakar, Saba Hemmati
PII:
S0022-3697(19)31958-4
https://doi.org/10.1016/j.jpcs.2019.109256
PCS 109256
DOI:
Reference:
To appear in: Journal of Physics and Chemistry of Solids
Received Date: 30 August 2019
Revised Date: 1 November 2019
Accepted Date: 2 November 2019
Please cite this article as: H. Veisi, T. Tamoradi, B. Karmakar, S. Hemmati, Green tea extract–modified
silica gel decorated with palladium nanoparticles as a heterogeneous and recyclable nanocatalyst for
Buchwald-Hartwig C–N cross-coupling reactions, Journal of Physics and Chemistry of Solids (2019), doi:
https://doi.org/10.1016/j.jpcs.2019.109256.
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Graphical Abstract
Decorated palladium nanoparticles over green tea extract-modified silica gel
as a heterogeneous and recyclable nanocatalyst for Buchwald-Hartwig C–N
cross coupling reactions
HojatVeisi,*TaibehTamoradi, BikashKarmakar,* Saba Hemmati
O
OH
HO
O
O
O
O
O
R1
OH
X
green tea extract
Coating
Na PdCl4
2
R2 R3
N
H
R1
SiO2
SiO @green tea
SiO @green tea/Pd
2
2
N
R3
=
Pd
R2

Green tea extract–modified silica gel decorated with palladium nanoparticles as a
heterogeneous and recyclable nanocatalyst for Buchwald-Hartwig C–N cross-coupling
reactions
,a
a
b
a
Hojat Veisi,* Taibeh Tamoradi, Bikash Karmakar, * Saba Hemmati
a
Department of Chemistry, Payame Noor University, Tehran, Iran
b
Department of Chemistry, Gobardanga Hindu College, 24-Parganas (North), India
Email: hojatveisi@yahoo.com; bikashkarm@gmail.com
Abstract. A novel green tea extract–encapsulated silica gel decorated with in situ–generated Pd
nanoparticles is reported as an efficient, green heterogeneous catalyst in the Buchwald-Hartwig
C–N cross-coupling reaction. It was characterized by several analytical techniques. Thereafter, a
wide range of aryl amines were synthesized in good to excellent yields by reaction of different
substituted aryl halides and secondary amines over the catalyst. The material is sufficiently stable
and could be used at least six times in a model Buchwald-Hartwig reaction without noticeable
change in its catalytic activity. Heterogeneity of the catalyst was examined by a hot filtration
test.
Keywords: Pd nanoparticles, Green tea extract, Heterogeneous nanocomposite, Buchwald-
Hartwig reaction, C–N coupling
1
. Introduction
Aromatic amines are ubiquitous in natural products and synthetic functional materials such as
drugs and pharmaceuticals, agrochemicals, dyes, flavors, and perfumes [1, 2]. Consequently, the
development of synthetic protocols for C–N bonds has achieved enormous importance in both
industry and academia. The Buchwald-Hartwig reaction represents a transition metal–promoted
versatile catalytic method for the formation of C–N bonds to obtained substituted aryl amines [3–
5
]. In the beginning, for these syntheses, aryl halides and organotin amides and amines were used
as reactants for the cross-coupling in the presence of palladium-based catalysts [6–8]. Later,
1

various research groups introduced aryl triflates, sulfamates, tosylates, esters, ethers, and
carbamates as a substitute for aryl halides as the electrophilic counterpart [9–13]. Thereafter, the
protocol was developed exceptionally by catalytic means in view of the broad range of accessible
substrates. There are several reports of this reaction over Pd-complexed homogeneous catalysts
[14–20]. However, the complexing ligands, such as phosphine and amine derivatives, are
expensive and they make the catalysts moisture sensitive. Therefore, alternative means of
heterogenization are necessary.
Heterogeneous catalysts has achieved precedence over homogeneous catalysts because of their
easy handling, effortless isolation, and reusability [21, 22]. To increase the efficiency and
stability of homogeneous catalysts, several approaches have been proposed following
immobilization on different heterogeneous supports [23–25]. A number of Pd/heterogeneous
supported catalysts have been reported for the Buchwald-Hartwig reaction by several research
groups [26–29]. The catalytic protocol for the reaction was further developed when Pd
nanoparticles were introduced. There are a few reports in literature on Pd nanoparticle catalysis
in the Buchwald-Hartwig reaction [30–35]. Notably, different methods, such as chemical and
electrochemical reduction, ion exchange, vapor deposition, thermal decomposition, and a polyol
process, have been used in the synthesis of Pd nanoparticles. For stabilization, they usually
require a capping agent.
In the modern trend for green chemistry, a bio-inspired method for metal nanoparticle synthesis
is a landmark. The soft and mild biogenic synthesis, eco-friendliness of the procedure, use of
water as a green solvent, absence of toxic chemicals, simple workup, and cost-effectiveness have
attached attention in these approaches involving biomolecular surface functionalization [36, 37].
Furthermore, these unique strategies do not require high pressure, energy, and temperature [38].
However, despite having remarkable features and the advantages of biological plant extract–
mediated green synthesis of nanoparticles, their applications are still very limited. This prompted
us to use green tea extract, a biomolecule-rich floral medium that itself is a good reducing agent
to reduce metal salts to produce nanoparticles and a stabilizer too. However, because of the
nanometer size of the as-synthesized nanoparticles, isolation following standard procedures is
difficult and therefore immobilization on silica gel, a heterogeneous support, was used, which
favors their dispersion and stabilization [39, 40].
2

Green tea extract contains many phenolic group [41] and carbonyl [42] moieties, which in turn
can be used in the surface modification of SiO (Scheme 1). The feasibility of complexation of
2
these organic oxo functionalities with polyvalent cations in aqueous solution improves the
2
surface properties of the green tea extract–coated silica gel (SiO @green tea extract). Moreover,
the nanobiocomposite (SiO @green tea) has the potential to act as both a reducing agent and a
2
stabilizing agent for the in situ generation and immobilization of Pd nanoparticles.
Finally, in this manner, we demonstrate a procedure herein for the preparation of a
nanocomposite of Pd nanoparticles stabilized and supported by green tea extract–coated silica
gel (Scheme 1). The catalyst was used in the efficient synthesis of arylamines by C–N
heterocoupling between aryl halides and different amines in the presence of Cs CO as a
2
3
base in dimethylformamide (DMF) following the Buchwald-Hartwig protocol (Scheme
). The novel bio-inspired procedure, high surface area material, cheap and stable reagents, high
yields and easy long-term reusability of catalyst are the key features of our protocol.
1
2
. Experimental
2
.1. Preparation of green tea extract
Leaves of green tea were collected in the hills of north Iran. The fresh green tea leaves were
washed thoroughly three times with double-distilled water before use. Ten grams of the leaves
was added to 100 mL of deionized water, and the mixture was boiled for 15 min in a water bath.
The mixture was then cooled and filtered through Whatman filter paper no. 1 to obtain a clear
aqueous extract. It was stored in refrigerator at 4 °C for further use.
2
.2. Preparation of the SiO @green tea nanocomposite
2
Silica gel (230–400 mesh, 500 mg) was dispersed in 50 mL water and sonicated for 20 min. The
green tea extract was then added to the mixture, and the resulting mixture was stirred for 24 h at
room temperature. The precipitate so obtained was separated by centrifugation and washed
several times with deionized water. Finally, it was dried in a vacuum oven at 40 °C for 12 h to
produce the SiO @green tea nanocomposite.
2
2
2
.3. Preparation of SiO @green tea/Pd
3

2
SiO @green tea (500 mg) was dispersed in deionized water (200 mL) in an ultrasonic bath for 30
min. Subsequently, a solution of Na PdCl (50 mg) in 20 mL H O was added to the dispersion,
2
4
2
and the mixture was stirred for 6 h at 100 °C to ensure complete reduction of Pd(II) ions in the
precursor solution. The SiO @green tea/Pd nanocomposite was separated by centrifugation and
2
washed with H O followed by acetone to remove the attached organic substrates. The final
2
nanocomposite was dried in a vacuum at 40 °C.
2
.4. General procedure for the Buchwald-Hartwig C–N cross-coupling reaction using the
SiO @green tea/Pd nanocatalyst
2
A solution containing aryl halide (1.0 mmol), amine (1.1 mmol), and Cs CO (2.0 mmol) in the
2
3
2
presence of the SiO @green tea/Pd catalyst (0.020 g, 0.5 mol%) in 5 mL DMF was agitated for
1
2 h at 100 °C under a N atmosphere. After completion of the reaction (checked by thin-layer
2
chromatography), the catalyst was separated by simple filtration. To the filtrate, 10 mL water
was added, followed by 10 mL ethyl acetate to extract organic compounds. Concentration of the
organic layers and chromatographic purification afforded the products. All the products were
known and confirmed by comparison with authentic samples.
3
. Results and discussion
3
.1. Catalyst characterization
As continuation of our studies on the synthesis, development, and application of functionalized
nanomaterials [43], we report green tea extract–encapsulated silica gel supported with Pd(0)
nanoparticles for use in the Buchwald-Hartwig reaction between aryl halides and secondary
amines. After successful preparation of the nanocomposite material, it was well characterized by
sophisticated analytical techniques, such as scanning electron microscopy (SEM), transmission
electron microscopy, energy-dispersive X-ray spectroscopy, powder X-ray diffraction, X-
ray photoelectron spectroscopy (XPS), and inductively coupled plasma atomic emission
spectroscopy (ICP-AES). Structural identity of the material was ascertained from the
SEM image, as shown in Fig 1. It depicts a porous flake-like morphology. The surface of
the flakes is somewhat homogeneous. It seems the material is partially agglomerated.
However, the presence of Pd nanoparticles was not detected from the SEM image.
4

Transmission electron microscopy images are shown in Fig. 2. The green tea–fabricated silica
gel moiety assumes a cotton-like morphology (Fig. 2a). Pd(0) nanoparticles decorating the
matrix are evident from the image. The black dots signifying Pd(0) nanoparticles are well
dispersed over the whole surface. A closer look at the Pd(0) nanoparticles (Fig. 2b) reveals their
size is around 18–20 nm. The lattice planes inside the nanoparticles are clearly visible, which
2
indicates high crystallinity. The elemental identity of the SiO @green tea/Pd nanocomposite was
obtained from the energy-dispersive X-ray spectroscopy pattern (Fig. 3). The evolved peaks
represent the Si, C, N, O, and Pd atoms. The presence of C, N, and O atoms confirmed the
2
substituted polyphenolic attachment from green tea extract to the SiO surface. The exact Pd
content in the catalyst was found to be 0.25 mmol/g as determined from ICP-AES analysis.
The crystalline texture of the nanocomposite material was confirmed by an X-ray diffraction
study (Fig. 4). There was no phase separation in the X-ray diffraction pattern, which indicates
the formation of a firm nanocomposite. The broad peak at 2
θ
= 24° is due to the amorphous state
= 40° (111) and 2 = 47° (200), are
of typical silica. Two significant sharp reflection peaks, at 2
attributed to crystalline Pd(0) nanoparticles.
θ
θ
XPS was used to determine the chemical composition and chemical valence states of the
components present on the surface of the nanocomposite (Fig. 5). The XPS survey spectrum of
2
the SiO @green tea/Pd nanocomposite confirmed the presence of elemental Pd, Si, O, N, and C.
The binding energy peaks observed at 335.8 and 341.2 eV are attributed to metallic Pd(0) and
relate to the 3d5/2 and 3d3/2 states of Pd. This demonstrated that Pd was present in the metallic
Pd(0) state, confirming the successful reduction of Pd(II).
3
.2. Evaluation of catalytic activity
After the synthesis and meticulous characterization of the catalyst, its catalytic performance was
investigated in classic Buchwald-Hartwig C–N coupling reactions. A very straightforward
protocol was followed in the reaction. A mixture of substituted haloarenes, a base, and secondary
amines was heated in a specified solvent over the catalyst. The progress of the reaction was
checked by thin-layer chromatography, and after completion, the reaction mixture was poured
into water and extracted with ethyl acetate. The organic layer was concentrated and purified with
use of a silica gel column. To determine the optimum conditions, the reaction between
bromobenzene and morpholine was chosen as a model reaction in the presence of the
5

2
SiO @green tea/Pd catalyst. The effects of different factors, such as the solvent, base used,
catalyst load, and temperature, on the outcome of the reaction were examined. The results are
documented in Table 1. No product was found in the absence of the catalyst. There was only a
trace of product in the absence of a base. The reaction was then screened for a range of solvents,
such as toluene, EtOH, DMF, and H O in K CO base, with use of 0.5 mol% Pd–loaded catalyst,
2
2
3
and DMF gave the best result. With DMF as the best solvent, we varied the nature of the base.
Among different bases, Cs CO was the most productive for reaction at 100 °C. On increase of
2
3
the temperature, no further increase in yield was observed. When the temperature and catalyst
load were lowered, the yield decreased proportionally. Therefore, the reaction responded best in
DMF at 100 °C with Cs CO as a base in the presence of 0.5 mol% Pd–loaded catalyst.
2
3
After optimization of the reaction conditions, we studied substrate variants. A wide range of
arylamine derivatives were prepared by the coupling of substituted aryl halides and secondary
amines under stabilized conditions. Different substituent, such as CH , OCH , and CN, on
3
3
bromoarenes and iodoarenes were very compatible with the procedure, indicating there was not
any marked difference in the electron-withdrawing or electron-donating effects of the
substituents. However, the chloroarenes were found to be less reactive than their bromo or iodo
analogs, which was evident from their yields. The results are documented in Table 2. The
isolated purified products were authenticated by comparison with standard samples.
3
.3. Test of reusability and heterogeneity
In view of the green chemical implications of the protocol, a test of reusability of the catalyst is
of utmost importance. The reaction of bromobenzene and morpholine was chosen as a model
reaction with a larger batch size (2.0 mmol). After reaction the catalyst was separated by
centrifugation, washed with ethanol, dried, and reused in the next run. It was used six successive
times without any noticeable change in catalytic activity. The output is shown in Fig. 6. The ICP-
AES analysis of the catalyst after the sixth run further confirmed that the Pd load of the catalyst
(0.23 mmol/g) did not change considerably as there was no leaching of Pd, indicating its
robustness.
To determine the heterogeneity of the catalyst, a hot filtration test was conducted for the same
reaction under the optimized conditions. After a half time of the reaction, the catalyst was
6

isolated from the reaction mixture by filtration and the reaction was allowed to continue. There
was no further increase in the yield, verifying the heterogeneity of the catalyst.
4
. Conclusion
In summary, a SiO @green tea/Pd composite nanomaterial was successfully prepared by in situ
2
biogenetic reduction of metal salts immobilized on green tea extract–coated silica gel. Green tea
extract functions as a natural reductant as well as a stabilizer of Pd(0) nanoparticles. The
catalytic application of the material was investigated in Buchwald–Hartwig C–N coupling
reactions by our reacting substituted aryl halides with different secondary amines. The green
protocol is attractive in terms of its simplicity, cost-effectiveness, convenient operation, easy
purification of products, reusability of the catalyst, and excellent yields.
Acknowledgments
The authors are thankful to Payame Noor University for financial support and Gobardanga
Hindu College for providing research facilities.
Conflict of interest
The authors report no conflicts of interest in this work.
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[
2
[43] (a) H. Veisi, S. Razeghi, P. Mohammadi, S. Hemmati, Silver nanoparticles decorated on
thiol-modified magnetite nanoparticles (Fe O /SiO -Pr-S-Ag) as a recyclable nanocatalyst for
3
4
2
degradation of organic dyes, Mater. Sci. Eng.
https://doi.org/10.1016/j.msec.2018.12.076.
C
97 (2019) 624-631,
(b) M. Shahriary, H. Veisi, M. Hekmati, S. Hemmati, In situ green synthesis of Ag nanoparticles
on herbal tea extract (Stachys lavandulifolia)-modified magnetic iron oxide nanoparticles as
12

antibacterial agent and their 4-nitrophenol catalytic reduction activity, Mater. Sci. Eng. C 90
(2018) 57-66, https://doi.org/10.1016/j.msec.2018.04.044
(c) H. Veisi, M. Pirhayati, A. Kakanejadifard, P. Mohammadi, M.R. Abdi, J. Gholami, S.
Hemmati, In situ green synthesis of Pd nanoparticles on tannic acid‐modified magnetite
nanoparticles as a green reductant and stabilizer agent: its application as a recyclable
nanocatalyst (Fe
3
O
4
@TA/Pd) for reduction of 4‐nitrophenol and Suzuki reactions, ChemSelect 3
(2018) 1820-1826, https://doi.org/10.1002/slct.201702869
(d) H. Veisi, M. Pirhayati, A. Kakanejadifard, Immobilization of palladium nanoparticles on
ionic liquid-triethylammonium chloride functionalized magnetic nanoparticles: as a magnetically
separable, stable and recyclable catalyst for Suzuki-Miyaura cross-coupling reactions,
Tetrahedron Lett. 58 (2017) 4269-4276, https://doi.org/10.1016/j.tetlet.2017.09.078
(
e) H. Veisi, S.A. Mirshokraie, H. Ahmadian, Synthesis of biaryls using palladium nanoparticles
immobilized on metformine-functionalized polystyrene resin as a reusable and efficient
nanocatalyst, Int. J. Biol. Macromol. 108 (2018) 419-425,
https://doi.org/10.1016/j.ijbiomac.2017.12.036
(
f) H. Veisi, S. Najafi, S. Hemmati, Pd(II)/Pd(0) anchored to magnetic nanoparticles (Fe O )
3 4
modified with biguanidine-chitosan polymer as a novel nanocatalyst for Suzuki-Miyaura
coupling reactions, Int. J. Biol. Macromol. 113 (2018) 186-194,
https://doi.org/10.1016/j.ijbiomac.2018.02.120
(
g) H. Veisi, M. Ghorbani, S. Hemmati, Sonochemical in situ immobilization of Pd nanoparticles
on green tea extract coated Fe nanoparticles: an efficient and magnetically recyclable
3
O
4
nanocatalyst for synthesis of biphenyl compounds under ultrasound irradiations, Mater. Sci. Eng.
C 98 (2019) 584-593, https://doi.org/10.1016/j.msec.2019.01.009
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modified multi walled carbon nanotubes as a recyclable nanocatalyst for Ullmann-type C–O and
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Scheme 1. Green synthesis of the SiO @green tea/Pd nanocomposite and its catalytic application
2
for Buchwald-Hartwig C–N coupling.
Fig. 1. Scanning electron microscopy image of the SiO @green tea/Pd nanocomposite.
2
2
Fig. 2. Transmission electron microscopy images of the SiO @green tea/Pd nanocomposite at
different magnifications.
2
Fig. 3. Energy-dispersive X-ray spectroscopy pattern of the SiO @green tea/Pd nanocomposite.
Fig. 4. X-ray diffraction pattern of the SiO @green tea/Pd nanocomposite.
2
Fig. 5. X-ray photoelectron spectroscopy spectrum related to the elemental survey scan of the
SiO @green tea/Pd nanocomposite and peaks in the Pd 3d region (inset).
2
Fig. 6. Recycling of the catalyst for the Buchwald-Hartwig reaction between bromobenzene and
morpholine.
Table 1. Optimization of the reaction parameters for the Buchwald‐Hartwig amination reaction
between bromobenzene with morpholine over the SiO @green tea/Pd nanocomposite.
2
14

SiO @green tea/Pd
2
Br + HN
O
N
O
Pd (mol%)
Solvent
Base
T (°C)
Isolated
b
yield (%)
70
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
.5
.5
.5
.5
.5
.5
.5
.5
.5
.5
.5
.5
.5
.4
.3
.0
DMF
Toluene
EtOH
K CO3
100
100
80
2
K
2
CO
3
30
K CO3
2
50
H O
2
K CO3
2
100
100
100
100
100
100
80
30
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
NaOAc
65
Et N
65
3
Cs
2
CO
3
95
NaOH
–
60
10
Cs
2
CO
3
70
Cs CO3
60
35
2
Cs
2
CO
3
25
25
Cs CO3
120
100
100
100
95
2
Cs CO3
80
2
Cs
2
CO
3
50
Cs CO3
–
2
The reaction conditions were as follows: 1.0 mmol bromobenzene, 1.1 mmol morpholine,
catalyst, 2 mmol base, and 3 mL solvent for 12 h.
DMF, dimethylformamide.
Table 2. N-arylation of aryl halides using the Buchwald‐Hartwig reaction in the presence of the
SiO @green tea/Pd nanocomposite.
2
Aryl halide
Amine
Isolated
Reference
15

yield (%)
96
C
6
H
5
I
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
[31]
[31]
[31]
[30]
[30]
[30]
[31]
[31]
[44]
[44]
[45]
[45]
[45]
[46]
[46]
C H Br
95
6
6
5
5
C
H
Cl
60
p-NCC H I
96
6
4
p-NCC H Br
90
6
4
p-NCC
6
H
4
Cl
60
p-CH C H I
96
3
6
4
p-CH
3
C
6
H
4
Br
90
p-CH OC H I
90
3
6
4
p-CH OC H Br Morpholine
85
3
6
4
C
6
H
5
I
Piperidine
Piperidine
Piperidine
Imidazole
Imidazole
96
C H Br
90
6
5
C
6
H
5
Cl
70
C H I
90
6
5
C H Br
80
6
5
The reaction conditions were as follows: 1.0 mmol aryl halide, 1.2 mmol amine, 0.5 mol%
SiO @green tea/Pd catalyst, 2.0 mmol Cs CO , and 3.0 mL dimethylformamide at 100 °C for 12
2
2
3
h.
Table 3. Comparison of efficiency of SiO @green tea/Pd and some previously reported catalysts
2
for the Buchwald
Hartwig amination reaction of bromobenzene and morpholine.
Catalyst
Conditions
Tim Yield Referen
e (h)
24
(%)
94
ce
Pd-PFMN
K
2
CO
3
, 120 °C
[35]
[44]
RD Pd NCs
1,4-Dioxane, t-BuOK,
00 °C
K CO , DMF, 120°°C
24
92
1
PNP-SSS
12
24
89
80
[30]
[47]
2
3
[Pd(pp3S4)(dba)]
2-Propanol, Cs CO ,
2 3
8
0 °C
16

Pd(II) thiosemicarbazone
complex
2-BuOH, K CO ,
24
89.2
[45]
2
3
100 °C
PFG-Pd
t-BuONa, 110 °C
t-BuOK, DMF,
12
12
12
88
90
95
[48]
[31]
GO-Chit-Pd
SiO @green tea/Pd
Cs CO , DMF, 100 °C
This
work
2
2
3
DMF, dimethylformamide.
PFMN, phosphine‐functionalized magnetic nanoparticles.
RD Pd NCs, rhombic dodecahedra palladium nano clusters.
PNP‐SSS, Pd nanoparticles on silica‐starch substrate.
[
Pd(pp3S4)(dba)], pp3S4is tris[2‐(diphenylphosphino)ethyl]phosphine tetrasulfide and dba is
dibenzylideneacetone.
PFG, phosphine‐functionalized graphene.
GO‐Chit, Graphene‐Chitosan.
t‐BuOK, Potassium tert‐butoxide.
t‐BuONa, Sodium t‐Butoxide.
17

O
OH
HO
O
O
O
O
O
R1
OH
X
green tea extract
Coating
Na PdCl
2
4
R2 R3
N
H
R1
SiO2
SiO @green tea
2
SiO @green tea/Pd
2
N
R3
=
Pd
R2
Scheme 1. Green metric synthesis SiO @green tea/Pd nanocomposite and its catalytic
2
application for Buchwald-Hartwig C–N coupling.
Fig. 1. SEM image of the SiO @green tea/Pd nanocomposite
2
18

2
Fig. 2. a) TEM images of the SiO @green tea/Pd nanocomposite at different magnifications.
Fig. 3. EDX pattern of SiO @green tea/Pd nanocomposite.
2
19

Fig. 4. X-ray diffraction study of the SiO @green tea/Pd nanocomposite
2
2
0

2
Fig. 5. XPS spectrum related to the elemental survey scan of SiO @green tea/Pd nanocomposite
and peaks of the Pd 3d region (inset).
Fig.6. Recycling of catalyst for the B-H reaction between bromobenzene and morpholine.
Graphical Abstract
Decorated palladium nanoparticles over green tea extract-modified silica gel
as a heterogeneous and recyclable nanocatalyst for Buchwald-Hartwig C–N
cross coupling reactions
HojatVeisi,*TaibehTamoradi, BikashKarmakar,* Saba Hemmati
21

O
OH
HO
O
O
O
O
O
R1
OH
X
green tea extract
Coating
Na PdCl4
2
R2 R3
N
H
R1
SiO2
SiO @green tea
SiO @green tea/Pd
2
2
N
R3
=
Pd
R2
22

Highlights
•
In situ–generated palladium nanoparticles anchored to green tea extract–coated silica
gel
•
Excellent performance as a reusable catalyst in Buchwald-Hartwig C–N cross-
coupling reactions
•
•
Excellent yield of the products
Catalyst used up to six times
1

Conflict of Interest
The authors report no conflicts of interest in this work.