Immobilization of palladium nanoparticles on Metformin-functionalized graphene oxide as a heterogeneous and recyclable nanocatalyst for Suzuki coupling reactions and reduction of 4-nitrophenol

Article abstract of DOI:10.1016/j.poly.2018.11.038

In this study, Pd nanoparticles supported on Metformin modified graphene oxide (GO-Met-Pd) is proposed as a green heterogeneous catalyst. The structural properties of the as-synthesized nanocatalyst were determined by X-ray diffraction (XRD), Fourier transformed infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), wavelength-dispersive X-ray spectroscopy (WDX) and inductively coupled plasma (ICP). Activity of the catalyst was investigated by focusing on the Suzuki-Miyaura coupling reaction and room-temperature reduction of 4-nitrophenol (4-NP), as model reactions. The catalyzed reactions provided high yields of biphenyl compounds through the Suzuki-Miyaura reaction and outlined highly efficient conversion of 4-NP to its reduced form. Moreover, no significant Pd leaching was detected in the reaction solutions and the catalyst could be recovered, at least, 6 times without any considerable loss of its catalytic activity.

Full text of DOI:10.1016/j.poly.2018.11.038

Accepted Manuscript
Immobilization of palladium nanoparticles on Metformin-functionalized gra-
phene oxide as a heterogeneous and recyclable nanocatalyst for Suzuki coupling
reactions and reduction of 4-nitrophenol
Saba Hemmati, Lida Mehrazin, Mozhgan Pirhayati, Hojat Veisi
PII:
S0277-5387(18)30763-0
https://doi.org/10.1016/j.poly.2018.11.038
POLY 13585
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
7 June 2018
20 November 2018
Please cite this article as: S. Hemmati, L. Mehrazin, M. Pirhayati, H. Veisi, Immobilization of palladium
nanoparticles on Metformin-functionalized graphene oxide as a heterogeneous and recyclable nanocatalyst for
Suzuki coupling reactions and reduction of 4-nitrophenol, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.
2018.11.038
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Immobilization of palladium nanoparticles on Metformin-functionalized
graphene oxide as a heterogeneous and recyclable nanocatalyst for Suzuki
coupling reactions and reduction of 4-nitrophenol
Saba Hemmati,*,a Lida Mehrazin,b Mozhgan Pirhayati,c Hojat Veisi*,a
bDepartment of Chemistry, Payame Noor University (PNU), Tehran, Iran
bDepartment of Pharmaceutical Chemistry, Faculty of Pharmaceutical Chemistry, Pharmaceutical Sciences
Branch, Islamic Azad University, (IAUPS), Tehran, Iran
cDepartment of Applied Chemistry, Faculty of Science, Malayer University, Malayer, Iran
Email: s_organo2007@yahoo.com; hojatveisi@yahoo.com
Abstract. In this study, Pd nanoparticles supported on Metformin modified graphene oxide
(GO-Met-Pd) is proposed as a green heterogeneous catalyst. The structural properties of the
as-synthesized nanocatalyst were determined by X-ray diffraction (XRD), Fourier
transformed infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), field
emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy
(EDS), wavelength-dispersive X-ray spectroscopy (WDX) and inductively coupled plasma
(ICP). Activity of the catalyst was investigated by focusing on the Suzuki-Miyaura coupling
reaction and room-temperature reduction of 4-nitrophenol (4-NP), as model reactions. The
catalyzed reactions provided high yields of biphenyl compounds through the Suzuki-Miyaura
reaction and outlined highly efficient conversion of 4-NP to its reduced form. Moreover, no
significant Pd leaching was detected in the reaction solutions and the catalyst could be
recovered, at least, 6 times without any considerable loss of its catalytic activity.
Keywords. Graphene oxide, Metformin, Palladium, Nanocatalyst, Suzuki, 4-Nitrophenol
1. Introduction
Graphene is a one atom-thick material that is composed of a closely packed honeycomb two-
dimensional lattice. This material exhibits high thermal stability, excellent electronic
conductivity and good mechanical strength. Furthermore, it possesses a considerable
adsorption capacity due to its high specific surface area (2630 m2 g-1). All these features
represent graphene as an interesting option for supporting catalytic nanoparticles (NPs) [1-5].
Another advantageous characteristic of graphene sheets is that they can be decorated with
various functional groups or combined with different nanomaterials to give graphene
composites. Graphene composites provide exceptional properties that are beneficial for the
development of a new generation of catalysts [6]. In other words, the noticeable
characteristics of graphene can be combined with the properties of the NPs decorated on
graphene’s surface to promote performance of the designed catalyst. So that, supporting
metal NPs on graphene advances synergistic interactions between the metal and graphene
and, therefore, presents a detectable improvement in the electronic properties of the catalyst.
One of the metals that can be combined with graphene sheets is Palladium (Pd). Since the
discovery of Pd-catalyzed cross-coupling reactions in 1970s, they have been of strategic
importance in organic synthesis [7-11]. For example, they have been accepted as a general
catalyst for preparation of bioactive functionalized biphenyls, which are essential compounds
in pharmaceuticals and agricultural industries [7-11]. Due to the wide application of Pd-based
catalysts for formation of C–C bonds, they have continuously gained interest. However, the
focus of researchers has shifted towards developing highly efficient and recyclable catalysts
that can be used for ecofriendly industrial purposes.
1

An important issue about Pd-based catalysts refers to the fact that cross-coupling reactions
are usually performed under homogeneous conditions by adopting a ligand that can improve
the activity and selectivity of the catalyst for specific reactions [7-11]. As it is well-known,
homogeneous catalysis is challenging for pharmaceutical applications since homogenous
catalysts cannot be recycled and residual metals might contaminate the reaction products [12,
13]. This problem can be overcome by developing heterogeneous catalysts. That is why many
researchers have attempted to perform cross-coupling reactions using palladium particles
fixed on different solid supports including activated carbon [14,15], zeolites [16,17],
polymers [18,19] and NPs [20-24]. Though heterogeneous catalysts can be recycled
efficiently, reduced activity of immobilized catalysts is observed frequently [25, 26].
Consequently, it is necessary to develop heterogeneous Pd nanocatalysts that can provide
high activity, catalytic stability and recyclability, simultaneously. Otherwise, the catalyst
would not be appropriate for potential applications in the chemical and pharmaceutical
industries. To fabricate such ideal catalyst, the research activities of our research group have
been devoted to immobilization of palladium NPs on Metformin-modified materials. Our
proposed catalysts have been employed for efficient catalysis of C-C coupling reactions [27-
30]. In continuation of our work, herein, we fabricate a new Pd catalyst supported on
Metformin-modified graphene oxide sheets, for the first time, and investigate its performance
in catalysis of the Suzuki-Miyaura coupling reaction, as a model reaction. According to the
evaluations, this catalyst can demonstrate the high reactivity of homogeneous catalysts, in
addition to possessing the recyclability characteristic of heterogenous catalysts.
2. Experimental
2.1. Materials
The required chemicals were purchased from the Merck and Aldrich companies. Since the
materials were highly pure, they were used without any further purification. Reaction
completion was assessed by performing TLC analysis on silica gel plates. For this purpose,
the solvent of n-hexane-EtOAc (4:1) was used. A Bruker Tensor 27 spectrometer (Bruker,
Karlsrohe, Germany) was employed to record the FT-IR spectra of the pressed KBr discs.
Also, X-ray diffraction (XRD) spectroscopy was carried out by a Philips powder
diffractometer (PW 1373 model; Cu Ka = 1.5406 Å). The XRD patterns were recorded over
the 2θ range of 10 to 80º at a ... scanning rate. Moreover, field emission scanning electron
microscopy (FE-SEM) (Cam scan MV2300) was utilized to determine the morphology and
particle dispersion of the samples while an energy dispersive X-ray (EDS) detector was
coupled to the SEM system to investigate the chemical composition of the nanostructures.
Finally, TEM images of the samples were taken by a Philips-EM-2085 transmission electron
microscope using 100.0 kV accelerating voltage.
2.2. General procedure for preparation of graphene oxide (GO)
Natural graphite powder was used to prepare the nanosheets of graphene oxide (GO) via the
modified method of Hummer [16]. According to this method, 3 g graphite powder and 1 g
sodium nitrate (NaNO3) were gently mixed with a solution of 98% H2SO4 (46 mL).
Meanwhile, the solution was stirred for 15 min in an ice bath at 0 to 5 ºC. Then, 6 g
potassium permanganate (KMnO4) was slowly mixed with the suspension, under vigorous
stirring at 10 to 15 ºC. Stirring of the suspension continued for 2 h. The obtained reaction
mixture was continuously stirred at 35 ºC for another 30 min. After that, 138 mL distilled
water was gently added to the reaction suspension under vigorous stirring. Addition of water
2

and maintenance of the solution at 95 to 98 ºC for 1 h resulted in a color change and provided
a yellowish solution. The resultant solution was further diluted with 200 mL warm distilled
water (40 ºC) and treated with 18 mL hydrogen peroxide (H2O2, 30%). Again, the solution
color changed and converted to brown. The brown precipitate was centrifuged and washed by
a solution of hydrochloric acid (HCl, 10%) several times. Finally, the brown GO particles
were filtered and dried for 24 h under vacuum at 60 ºC.
To obtain GO nanosheets, 100 mg GO was dispersed in 100 mL distilled water and sonicated
to exfoliate the GO particles. Sonication was conducted in an ultrasonic bath with 100 W
power. The sonication process was stopped when the solution became transparent. Then, the
transparent solution was centrifuged for 10 min to separate any unexfoliated GO particle. At
the end, the solution was dried for 24 h in a vacuum oven at 80 ºC to acquire the GO
nanosheets. Therefore, in summary, aqueous GO nanosheets were produced through
oxidization and, subsequently, exfoliation of graphite powder.
2.3. Synthesis of GO-Met-Pd(II) and GO-Met-Pd(0)
Metformin modified GO sheets (GO-Met) were prepared via condensation of Metformin and
GO nanosheets. In this way, 50 mg GO nanosheets was added to a DMF solution (50 mL)
and ultrasonicated for 1 h. Then, the sonicated solution was cooled down to 0 °C and 6 mmol
NHS and 6 mmol EDC were added. The reaction mixture was stirred for 2 h at 0 °C. After
that, 10 mmol Metformin was added, the temperature was raised to 100 ºC and the solution
was stirred at that temperature for 24 h. This process gave GO-Met. The GO-Met sheets were
filtered, washed with acetone and water, respectively, and dried in vacuum at 40 °C. In the
next step, 50 mg GO-Met was dispersed in 100 mL water to synthesize GO-Met-Pd(II) and
GO-Met-Pd(0). After sonicating the suspension for 15 min, 5 mg PdCl2 was poured in the
solution and the suspension was again sonicated for 10 min. The sonicated solution was kept
on a working magnetic stirrer overnight. Then, the resultant GO-Met-Pd(II) particles were
filtered, washed subsequently with water and acetone and dried at 40 °C. To obtain GO-Met-
Pd(0) from GO-Met-Pd(II), 30 mg GO-Met-Pd(II) was dispersed in 50 mL water. After that,
90 μL hydrazine hydrate (80%) was mixed with the suspension and 25% ammonium
hydroxide was added to adjust pH of the solution on 10. The Pd(II) reduction reaction was
performed at 95 °C for 2 h to obtain the final product of GO-Met-Pd(0). Then, the reduced
particles were washed using water and dried at 50 °C. Scheme 1 illustrates the synthetic
procedure conducted for preparation of GO-Met-Pd(II) and GO-Met-Pd(0). Based on the
ICP-AES measurements, the concentration of palladium in GO-Met-Pd(II) and GO-Met-
Pd(0) was 4.3 and 4.2 wt%, respectively.
2.4. General procedure for Suzuki-Miyaura reactions
1.0 mmol aryl halide was dissolved in a 3-mL mixture of H2O-EtOH (1:1, v/v), in a 25-mL
round bottom flask. After that, 1.0 mmol aryl boronic acid, 2 mmol K2CO3 and 10 mg
nanocatalyst (0.1 mol %) were added to the flask. The reaction mixture was stirred for the
desired time at 60 °C. The reaction times are reported in Table 2. After confirming
completion of the reaction by TLC, the reaction solution was diluted with 10 mL distilled
water and the products were extracted using ethyl acetate (3 × 10 mL). The solvent was
evaporated under vacuum and the mixture of the organic compounds was dried over
anhydrous Na2SO4. The products were purified by flash chromatography using
hexane:ethylacetate (4:1) as mobile phase.
2.5. General procedure for reduction of 4-NP
3

To investigate the catalytic performance of the GO-Met-Pd nanocomposite, the reactant
solutions of 4-NP (3 mM) and NaBH4 (0.3 M) were prepared, freshly. Then, the reactants (1
mL each) were mixed by magnetic stirring and transferred to a quartz cuvette. 2 mg of the
catalyst was added to the cuvette to initiate the reaction. To monitor reduction of 4-NP to 4-
AP, absorption intensity of 4-NP was recorded at 400 nm excitation wavelength by an
ultraviolet-visible (UV-vis) spectrophotometer.
3. Results and Discussion
Scheme 1 depicts the synthesis steps that were carried out to prepare the catalyst. In the first
step, the Hummer’s method helped to oxidize graphite powder to GO and provide hydrophilic
oxygen containing functional groups, e.g. epoxy, carboxyl and hydroxyl, on the surfaces of
the GO nanosheets. These hydrophilic groups stabilized dispersion of GO nanosheets in
aqueous solutions [31]. In the next step, GO was treated with Metformin to functionalize the
GO and obtain Metformin modified GO sheets. In the last step, Pd NPs were immobilized on
the Metformin functionalized GO sheets. With respect to ICP analysis, 0.39 mmol Pd/g of the
catalyst was loaded on the GO-Met nanosheets.
H3C
CH3
N
OH
HN
HN
NH
NH
O
HO
CH3
N
O
H
HO
H2N
N
O
CH3
O
NH NH
EDC, NHS, DMF
CH3
O
HO
HN
OH
O
HN
N
H
N
N
CH3
N
H
N
H
H
N
H3C
O
NH
HO
NH
H3C
O
GO
HO
H3C
HN
CH3
l 2
GO-Met
C
N
d
P
Cl
Cl
NH
Pd
Cl
Pd
Cl
NH
HN
H3C
CH3
O
N
HO
Cl
Pd
Cl
Cl
NH
Pd
HN
Cl
Pd
Pd
CH3
O
HN
NH
HN
O
HN
H
N
N
CH3
N
H
H
NH2NH2
N
O
N
H3C
N
HO
H
NH
NH
Pd
H3C
Pd
O
Cl
Pd
Pd
CH3
O
Pd
HO
GO-Met-Pd(II)
Cl
Cl
HN
Pd
O
Cl
HN
H
N
CH3
N
H
N
N
H
N
H3C
N
H
NH
NH
H3C
O
Pd
HO
GO-Met-Pd(0)
Scheme 1. Schematic diagram of GO-Met-Pd fabrications.
Fig. 1a shows the XRD patterns of the samples. The XRD pattern of GO-Met-Pd (Fig. 1a)
contains the well-defined peaks of 39.8°, 46.3°, 67.2° and 80.7°, which correspond to the
4

distinctive crystalline planes of (111), (200), (220), and (311), respectively. These crystal
planes represent the face centered cubic lattice of Pd(0) [32]. In addition, there is a peak at
10°, in Fig. 1a. This peak refers to the (002) plane of GO nanosheets and indicates restoration
of the well-ordered lamellar structure of GO nanosheets in GO-Met-Pd.
The FT-IR spectra of GO, GO-Met and GO-Met-Pd(II) are presented in Fig. 1b. In the FT-IR
spectrum of GO-Met, the band located around 2950 cm-1 is related to C-H stretching while
the bands observed at 3350 and 1580 cm-1 correspond to N-H stretching. Also, the peak
positioned at 1658 cm-1 can be ascribed to the stretching mode of C=NH in the metformin
functional groups. Therefore, these vibrational bands verify that Metformin is successfully
bound to the surface of the GO sheets through the amidation reaction. Curve b of Fig. 1b
displays the FT-IR spectra of GO-Met-Pd(II). In this spectrum, the characteristic peak of
C=NH stretching is shifted from 1658 to 1652 cm-1 due to the coordination of Pd to the
Metformin groups. This shift is evident from comparison of the spectra and confirms
coordination of Pd2+ ions to the Metformin ligands.
Fig. 1. a) XRD pattern of GO-Met-Pd; b) FT-IR spectra for (a) GO, (b) GO-Met, and (c) GO-
Met-Pd(II).
Figs. 2a and 2b illustrate morphology of the GO-Met sheets and the GO-Met-Pd particles,
respectively. According to the FE-SEM images of Fig. 2, a noticeable quantity of Pd NPs has
dispersed on the surface of the GO-Met sheets. Furthermore, observation of a strong EDX
peak at 3 keV (Fig. 2c) approves the presence of elemental Pd NPs. Also, the EDX analysis
unravels the existence of carbon, oxygen and nitrogen, as well as Au, in the GO-Met-Pd
particles. This elemental composition indicates that the presence of Metformin is responsible
for capping and stabilization of Pd NPs on the surface of the GO sheets. In consistent with the
SEM results, the TEM images (Fig. 2d) exhibit a homogenous distribution of Pd NPs on the
surface of the Metformin modified GO sheets. In general, the characterization results
demonstrate the important role of Metformin in enhancing dispersibility of Pd. Furthermore,
the histogram clearly defined the average hydrodynamic size of Pd NPs to be in the around of
5.24 nm (Fig. 3).
5

Fig. 2. (a) FESEM image of GO, (b) FESEM image of GO-Met-Pd, (c) EDX of GO-Met-Pd
and (d) TEM image of GO-Met-Pd.
Fig. 3. The particle size distribution of Pd NPs.
In addition to XRD, FT-IR, SEM and TEM analysis, the samples were studied by
wavelength-dispersive X-ray spectroscopy (WDX) coupled with quantified FESEM mapping
(Fig. 4). This research tool can present qualitative information about distribution of various
6

chemical elements in the context of different catalysts. As the compositional maps of C, N
and Pd show, the Pd NPs have well dispersed throughout the GO-Met-Pd composite.
Fig. 4. FESEM image of GO-Met-Pd and elemental maps of C, N and Pd atoms.
X-ray photoelectron spectroscopy (XPS) analysis of GO-Met and GO-Met-Pd in the N1s
region as evidence of coordination has also been investigated (Figure 5). To investigate the
sitting position of the Pd NPs in GO-Met-Pd, curve fittings of N1s region in the XPS spectra
of GO-Met and GO-Met-Pd are produced. tree binding energies of N1s at approximately 400
eV are observed in both cases of GO-Met and GO-Met-Pd (Figure 5 a,b). The comparison
shows that there is small shift of binding energy of N1s (400.02 eV to 401.05 eV) with or
without Pd NPs, and the proportion of binding energy at 401 eV increases from 35% to 65%
after the deposition of Pd NPs. This change in N1s spectra confirm the interaction or
proximity between the Pd atoms and the N atoms.
7

Fig. 5. XPS spectra of a) curve fitting of N1s region in the XPS spectrum of GO-Met; b)
curve fitting of N1s region in the XPS spectrum of GO-Met-Pd.
3.1. Catalytic activity of GO-Met-Pd in the Suzuki-Miyaura reaction
The composite catalyst, i.e. GO-Met-Pd, was applied to C-C bond formation between aryl
halides and phenyl boronic acid to evaluate its catalytic performance. Such reaction is usually
known as the Suzuki-Miyaura cross coupling reaction. At first, the catalytic conditions were
optimized by focusing on the Suzuki coupling reaction of 4-methylbromobenzene with
phenyl boronic acid, as a model reaction. The optimized parameters included reaction
solvent, temperature, base, and catalyst dosage. The optimization results are summarized in
Table 1. As expected, no target product was produced in the absence of the catalyst (Table 1,
Entry 11) whereas addition of the GO-Met-Pd particles to the reaction mixture prompted
high-yield synthesis of the desired product. Based on the optimization trials, H2O/EtOH (1:1)
was selected as the best solvent. Also, among the examined bases, K2CO3 was identified as
8

the most effective base. The effect of catalyst dosage was evaluated by adopting several
catalyst quantities ranging from 0.050 mol% to 0.200 mol% (Table 1, Entries 7 to 9). The
best product yield was achieved using 0.010 g (0.1 mol%) catalyst (Table 1, entry 8). Also,
the optimal reaction temperature was found to be 60 °C (Table 1, Entry 8 vs. Entries 12 to
14). So that, at 50 °C, the product yield was lower than the yield obtained at 60 °C and
increasing the reaction temperature to 100 °C did not result in any significant improvement in
the yield of the desired product.
Table 1. The optimization of reaction parameters for the Suzuki reaction of 4-
methylbromobenzene with phenyl boronic acid.a
Br
various conditions
CH3
+
H3C
B(OH)2
Entry Pd (mol%)
Solvent
DMF
Base
T (°C) Time (h) Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0.1
0.1
0.1
0.1
0.1
0.1
0.05
0.1
0.2
0.1
0.0
0.1
0.1
0.1
K2CO3
K2CO3
K2CO3
K2CO3
80
80
60
90
60
90
60
60
60
60
60
25
50
100
2
2
2
5
2
4
2
1
1
12
12
6
2
1
80
78
77
55
65
80
75
98
96
Trace
0.0
65
80
98
Toluene
EtOH
H2O
EtOH/H2Oc NaOAc
EtOH/H2Oc
Et3N
EtOH/H2Oc K2CO3
EtOH/H2Oc K2CO3
EtOH/H2Oc K2CO3
EtOH/H2Oc No base
EtOH/H2Oc K2CO3
EtOH/H2Oc K2CO3
EtOH/H2Oc K2CO3
EtOH/H2Oc K2CO3
aReaction conditions: 4-methylbromobenzene (1.0 mmol), phenylboronic acid (1.0 mmol), catalyst, base (2
mmol) and solvent (3 mL).
bIsolated yield.
c3 mL (1:1)
After determination of the optimal conditions, several coupling reactions between different
aryl halides (I, Br and Cl) and phenyl boronic acid were performed under the optimal
conditions. The results are reported in Table 2. Based on Table 2, 0.1 mol GO-Met-Pd can
efficiently catalyze the reaction of phenyl iodides, bromides and chlorides with phenylboronic
acid (Entries 1 to 17). So that, aryl halides with electron-donating or withdrawing groups were
observed to react with phenylboronic acid to provide high yields of the associated products.
Notably, the reaction of the 2-bromothiophene and 2-iodothiophene heteroaryl halides with
phenylboronic acid gave the related coupled products in 96% and 90% yields, respectively
(Table 2, Entries 16 and 17). It was noticed that the coupling reaction of aryl chlorides with
phenylboronic acid requires a longer reaction time and leads to a moderate reaction yield, with
respect to aryl iodides and bromides (Table 2, Entries 3, 6 and 9).
Table 2. Suzuki-Miyaura coupling reaction of different aryl halides with
phenylboronic acid.a
X
+
GO-Met-Pd (10 mg)
R
H2O-EtOH, K2CO3, 60oC
R
B(OH)2
Entry RC6H4X
R
H
X
I
Time (h) Yield (%)b
0.25 98
1
H
9

2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
H
H
4-CH3
4-CH3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Br
Cl
I
Br
Cl
I
Br
Cl
I
Br
I
1.0
12
0.25
1.5
12
0.5
2
12
0.5
1.5
1.0
2.0
3.0
5.0
1.5
5.0
98
70
98
98
75
98
96
65
98
96
96
88
92
85
96
90
4-CH3
4-COCH3
4-COCH3
4-COCH3
4-CH3O
4-CH3O
4-NH2
4-NH2
4-OH
Br
I
Br
I
4-OH
2-Thienyl
2-Thienyl
Br
aReactions were carried out under aerobic conditions in 3 mL of H2O/EtOH (1:1), 1.0 mmol arylhalide,1.0 mmol
phenylboronic acid and 2 mmol K2CO3 in the presence of catalyst (0.010 g, 0.1 mol% Pd) at 60 ºC.
bThe yield is detected by 1H NMR.
To investigate reusability of the catalyst, the GO-Met-Pd particles were recovered
conveniently through centrifugation and reused for five more catalytic cycles. It was
observed that application of the catalyst to the Suzuki coupling of phenylboronic acid with 4-
methylbromobenzene for 6 consecutive catalytic runs can give 98%, 98%, 96%, 95%, 90%
and 83% yield, respectively. Moreover, the reaction mixture was centrifuged after completion
of the reaction to separate the catalyst. The separated particles were washed with deionized
water and ethanol several times and, then, dried in an oven at 50 °C. The recovered catalyst
was saved for the next reaction cycle. To indicate any Pd leaching from the nanocomposite
catalyst during the reactions, several samples of the filtrates, in addition to the washings
obtained from the conducted standard reactions, were analyzed by ICP. The concentration of
Pd detected in each sample was below 3.0 ppm. Furthermore, the model reaction was re-
performed using the recovered filtrate. However, no reaction product could be isolated. In
this way, the results declared that the GO-Met-Pd nanocomposite, rather than the leached Pd
particles, are responsible for catalysis of the reaction and provision of high catalytic activity.
According to these observations, a plausible reaction mechanism was proposed for catalysis
of the Suzuki coupling reaction by the synthesized particles (Scheme 2).
Ar1
X
Oxidative addition
Reductive elimination
Ar1 Ar2
GO
Ar1
X
Ar1
Ar2
Ar2
Suzuki Reaction
X
Ar1
Ar1
GO
GO
Transmetalation
Ar2 B(OH)2
K2CO3
XB(OH)2
K2CO3
10

Scheme 2. Possible mechanism of Suzuki coupling reaction catalyzed by GO-Met-Pd.
3.2. Catalytic activity of GO-Met-Pd for reduction of 4-NP
Catalysis of the reduction of 4-NP to 4-AP by the prepared GO-Met-Pd particles was
carried out at room temperature, in the presence of NaBH4 as a hydrogen donor. As the
reaction progressed, the yellow color of the reaction solution turned into colorless.
Therefore, the reaction progress was monitored by recording the UV-Vis spectra of the
solution at short intervals. During the reaction, the characteristic peak of 4-NP attenuated
at 400 nm and the indicative absorption peak of 4-AP appeared and intensified at 304 nm
(Fig. 6a). The rate of the reduction reaction was assumed to be independent of NaBH4
concentration since an excess amount of this reagent was used. With respect to this
assumption, the catalyzed reduction of 4-NP was considered as a pseudo-first-order
reaction and the absorption data were fitted into the first-order rate law. As Fig. 6b
illustrates, a linear relationship was obtained between ln(A/A0) and reaction time.
According to the plotted relationship, the rate constant of the reaction, k, was calculated as
0.069 s-1. Also, the parameter of catalyst activity (Ka= k/m) was calculated as 69 s-1g-1 for
the GO-Met-Pd nanocomposite by dividing the determined rate constant by the amount of
catalyst utilized for the reaction. This parameter was estimated to compare the catalytic
activity of GO-Met-Pd with the performance of the other catalysts that have been used for
catalysis of 4-NP reduction (see Table 3). As Table 3 demonstrates, the catalytic activity
of GO-Met-Pd is considerably higher than activities of the outlined nanocatalysts.
Therefore, it can be stated that GO-Met-Pd can present superior catalytic activity for
reduction reactions.
Table 3. The comparison of catalytic activities for the reduction of 4-NP with catalysts
reported in literatures.
Catalyst
K (s-1)
Ka (s-1g-1)
1.03
0.019
0.04
0.172
69
Ref
[33]
[34]
[35]
[36]
This work
TAC-Ag-10
P(AMPS)-Ni
P(AMPS)-Co
P(AMPS)-Cu
GO-Met-Pd
5.19 10-3
9.38 10-4
2 10-3
1.72 10-3
69 10-3
Fig. 6. (a) UV-visible spectra for catalytic reduction of 4-NP to 4-AP, (b) Plot of the
Ln(At/A0) against the reaction different times.
11

After monitoring the reaction of 4-NP reduction, the catalyst was removed by
centrifugation, washed with water and ethanol several times and reused to evaluate
reusability of the catalyst for practical applications. It was observed that the performance
of the catalyst declines negligibly even after 8 catalytic cycles (Fig. 7). No significant Pd
leaching was detected in any of the evaluated reactions, which demonstrates one of the
advantageous features of this catalyst.
Fig. 7. Recyclability of GO-Met-Pd.
After the 5th catalytic run, ICP analysis was conducted to determine Pd leaching into the
reaction solution. 1.65% Pd was detected, which signifies stability of GO-Met-Pb under the
reaction conditions. Moreover, the TEM and SEM images of the catalyst after the 5th catalytic
cycle (Fig. 8) depicted conservation of the catalyst’s nanostructure. Also, the TEM image of
the recovered catalyst showed that the Pd NPs remain well dispersed without aggregation
over the GO nano-sheets even after 5 catalytic runs. Also, the histogram clearly defined that
the average size of Pd NPs did not significant change (Fig. 9).
Fig. 8. a) TEM and b) SEM images of reused catalyst after the 5 runs.
12

Fig. 9. The particle size distribution of Pd NPs in the reused catalyst after the 5 runs.
To investigate heterogeneity of the catalyst, a hot filtration test was performed on the GO-
Met-Pd-catalyzed Suzuki reaction of 4-bromotoluene with phenylboronic, under the optimal
conditions. In this test, after 30 min reaction time, 60% yield was obtained. Then, the catalyst
was removed and the reaction was sustained for another 30 min, in the absence of the
catalyst. No increase in the yield of the desired product was observed, which confirms
heterogeneity of the GO-Met-Pd catalyst.
4. Conclusions
This study introduced a recyclable palladium catalyst supported on Metformin modified
graphene oxide (GO) nanosheets. Characterization of the synthesized catalyst by the
techniques of XRD, FT-IR, FESEM, TEM, ICP, WDX and EDX verified successful
functionalization of the GO sheets and homogeneous dispersion of Pd nanoparticles on the
modified sheets. Findings of this research unraveled that immobolization of Pd particles on
the Metformin functionalized sheets can enhance stability of the active Pd species and
provide an efficient and reusable catalyst for Suzuki-Miyaura cross-coupling reactions. The
proposed catalyst was found to be a promising tool for preparation of biphenyl compounds.
Furthermore, it could be conveniently recycled and reused without any noticeable loss of its
catalytic activity. In addition to cross-coupling reactions, the catalyst was applied to reduction
of 4-NP to 4-AP in water, at room temperature, and the catalyst was recycled by an external
magnet and reused several times without any significant activity loss. No significant Pd
leaching was detected in any of the evaluated reactions, which demonstrates one of the
advantageous features of this catalyst. In general, the devised catalyst was declared as a green
approach towards Pd-based catalysis and elimination of toxic contaminants, e.g. nitro-
aromatics, from the environment.
Acknowledgments. We are thankful to Payame Noor University for partial support of this
work.
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13

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15

Graphical Abstract
OH
O2N
OH
H3C
CH3
N
NaBH4
H2N
HN NH
Pd
HN NH
O
HO
Pd
CH3
O
HN
O
HN
H
N
N
CH3
H
N
N
N
H3C
N
H
H
NH
Pd
NH
H3C
O
HO
o
H2O-EtOH, K2CO3, 60 C
GO-Met-Pd(0)
R
X
B(OH)2
+
R
Pd nanoparticles supported on Metformin modified graphene oxide (GO-Met-Pd) is proposed
as a green heterogeneous catalyst for Suzuki coupling reactions and reduction of 4-
nitrophenol.
16