Functionalization of multi-walled carbon nanotubes by the baclofen drug to immobilize palladium nanoparticles and catalyze Sonogashira coupling reactions

Article abstract of DOI:10.1002/aoc.4729

The baclofen-MWCNTs-Pd nanocatalyst was synthesized through covalent grafting of baclofen molecules onto surface-modified carbon nanotubes and immobilizing Pd nanoparticles by the baclofen ligands. The chemical structure of the produced nanocatalyst was s

Full text of DOI:10.1002/aoc.4729

Received: 6 August 2018
DOI: 10.1002/aoc.4729
Revised: 29 October 2018
Accepted: 30 October 2018
F U L L P A P E R
Functionalization of multi‐walled carbon nanotubes by the
baclofen drug to immobilize palladium nanoparticles and
catalyze Sonogashira coupling reactions
Pooneh Nazari1 | Malak Hekmati2
1 Department of Pharmaceutical
The baclofen‐MWCNTs‐Pd nanocatalyst was synthesized through covalent
Chemistry, Faculty of Pharmaceutical
Chemistry, Tehran Medical Sciences,
grafting of baclofen molecules onto surface‐modified carbon nanotubes and
Islamic Azad University, Tehran, Iran
2 Department of Organic Chemistry,
immobilizing Pd nanoparticles by the baclofen ligands. The chemical structure
of the produced nanocatalyst was studied by Raman spectroscopy, Fourier
Faculty of Pharmaceutical Chemistry,
Tehran Medical Sciences, Islamic Azad
University, Tehran, Iran
transform‐infrared spectroscopy, energy‐dispersive spectroscopy (EDS),
elemental mapping and inductively coupled plasma analysis. Also, its surface
morphology was determined using the scanning and transmission electron
microscopy techniques. Furthermore, the obtained baclofen‐MWCNTs‐Pd
nanocatalyst is demonstrated to exhibit very high activity as a heterogeneous
phosphine‐free catalyst in Sonogashira cross‐coupling of aryl halides by giving
good to excellent yields of different products. In addition, the nanocatalyst can
be reused four times without any significant leaching or loss of activity.
Correspondence
Malak Hekmati, Department of Organic
Chemistry, Faculty of Pharmaceutical
Chemistry, Tehran Medical Sciences,
Islamic Azad University, Tehran, Iran.
Email: mhekmatik@yahoo.com
KEYWORDS
baclofen, multi‐walled carbon nanotubes, nanocatalyst, palladium, Sonogashira
1 | INTRODUCTION
as
the
intermediates
required
to
synthesize
pharmaceutics, natural products and diverse organic
materials, for example, nanowires and nano‐sized molec-
ular architectures. The reaction of SCC has been most
commonly performed using the Pd‐based homogeneous
catalysts that are generated from PdCl2 or Pd (OAc)2
and organic ligands. Such catalysts are devised to present
high activity and selectivity towards definite reactions.
The point is that homogeneous Pd catalysts cannot be
easily separated from reaction mixtures. Therefore, they
might contaminate the reaction products.[4] This issue is
a great problem as the presence of metal contaminants
in pharmaceutical ingredients can negatively affect public
health. Consequently, pharmaceutical industries have to
pay additional costs to remove the residual metals via
post‐reaction treatments. Alternatively, the catalyst nano-
particles (NPs) can be immobilized on solid supports to
form heterogenous catalysts and facilitate their removal.
The commonly applied supports include active carbon,
Palladium (Pd) is a transition metal that has played
important roles in almost all areas of organic synthesis.
Particularly, the organometallic complexes containing
Pd have managed to catalyze many well‐known reactions,
such as the Heck, Sonogashira, Stille, Suzuki and
Buchwald–Hartwig cross‐couplings reactions, the Wacker
process,[1] and the reaction of Tsuji–Trost allylation.[2]
Furthermore, this transition metal has been successfully
applied to hydrogenolysis, hydrogenation, carbonylation,
cycloisomerization, formation of the C–C, C–N, C–O
and C–S bonds, and pericyclic reactions.[3]
In general, Pd‐based materials have the potential of
catalyzing many reactions, including the Sonogashira
cross‐coupling (SCC) reaction, which gives conjugated
acetylenic compounds through coupling unsaturated
organic halides with terminal acetylenes. The products
obtained from the SCC reaction can be further employed
Appl Organometal Chem. 2019;33:e4729.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4729

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NAZARI AND HEKMATI
zeolites and carbon nanotubes (CNTs).[5] Among various
solid supports, CNTs have attracted much interest due to
their outstanding electrical, thermal, sensor and mechan-
ical properties.[6]
pure MWCNTs without functional groups were
purchased from Petrol Co. (Tehran, Iran). HCl, H2SO4,
HNO3, deionized water, NaH (80%), ethylenediamine,
acetonitrile, PdCl2 and N,N‐dicyclohexylcarbodiimide
(DCC) were obtained from Sigma Aldrich and Merck.
The crystalline structures of the samples were evaluated
by X‐ray diffraction (XRD) analysis on a Bruker D8
Advance diffractometer with CuKα radiation at 40 kV
and 20 mA. FT‐IR spectra were recorded with a Perkin
Carbon nanotubes can be single‐ or multi‐walled. The
single‐walled CNTs (SWCNTs) are formed through
rolling of a single carbon sheet, while the multi‐walled
CNTs (MWCNTs) are obtained by rolling several carbon
sheets. The MWCNTs are more abundant and less expen-
sive, compared with the SWCNTs. The catalytic activity of
CNTs can be improved by introducing heteroatom‐
containing functional groups onto their surface. In this
respect, the boron, nitrogen, oxygen, sulfur and phospho-
rus atoms have been widely used. The negative aspect of
CNT application is that CNTs are water‐insoluble and
manipulation of CNTs is difficult in most solvents. As
an outcome, CNTs should be surface functionalized.[7]
One of the most feasible and commonly used
methods of CNT surface modification is its oxidation by
strong acids. This method introduces ‐COOH groups
onto the surface of MWCNTs and creates MWCNT‐
COOH. It should be noted the ‐COOH functionalized
CNTs can be further modified by reacting the ‐COOH
groups with various organic molecules including
biomolecules, for example, drugs, vitamins, amino acids,
carbohydrates and proteins.[8–11] A potential candidate
for modification of MWCNT‐COOH is baclofen. This
lipophilic drug is a gamma‐aminobutyric acid (GABA)
derivative that acts as a GABA‐B agonist, inhibits the
activity of presynaptic motor neurons, and results in an
antispastic response.[12]
Elmer 65 spectrometer in the range of 400–4000 cm−1
.
TEM images at the accelerating voltage of 80 kV were
taken with a Zeiss‐EM10C. Morphology and particle
dispersion was investigated by FE‐SEM (Cam scan
MV2300). The chemical composition of the prepared
nanostructures was measured by energy‐dispersive X‐
ray spectroscopy (EDS) performed in scanning electron
microscopy (SEM).
2.2 | Covalent grafting of baclofen to
MWCNTs
Pristine MWCNTs (p‐MWCNTs) were refluxed under stir-
ring in the mixture of 1:3 (v/v) HNO3 and H2SO4 at 70°C
for 30 hr, which was followed by centrifugation and
repeated washings with DI water. The dried carboxylated
MWCNTs (MCNTs‐COOH) were suspended in a solution
of thionyl chloride (excess) and DMF (1 mL). The suspen-
sion was stirred at 65°C for 24 hr. The solid was then
separated by filtration and washed with anhydrous THF
(30 mL), and dried in vacuum to obtain MWCNTs‐COCl.
The final product was then subjected to functionalization
with baclofen. Baclofen (drug‐to‐MWCNTs weight ratio
was 5:1) was mixed with a 10 mL solution of DMF and
TEA (1 mL), and then stirred for 5 min. The obtained acyl
chloride MWCNTs that dispersed in DMF (20 mL) by
ultrasonic bath for 15 min were subsequently added to
the suspension. The reaction mixture was kept at 120°C
for 2 days. The solid was then separated by filtration
and washed with CH2Cl2 and deionized water several
times and dried in vacuum. The concentration of drug
on MWCNTs was 0.13 mmol g−1, which was determined
by CHNS analysis.
According to our previous works,[8,13] acidic groups
and baclofen can be applied to modify the surface of
MWCNTs. In this respect, we decided to focus on the
amino group of baclofen to attach them onto MWCNT‐
COOH. The resultant baclofen‐MWCNTs (BMWCNTs)
can act as novel and efficient support for immobilization
of Pd NPs. The corresponding catalyst, i.e. BMWCNT/Pd,
is adopted as an active and reusable catalyst (Scheme 1)
to catalyze the SCC reaction after its characterization
using the field emission scanning electron microscopy
(FE‐SEM), energy‐dispersive X‐ray spectroscopy, trans-
mission electron microscopy (TEM), inductively coupled
plasma (ICP), CHNS and Fourier transform‐infrared
(FT‐IR) techniques.
2.3 | Preparation of the
baclofen‐MWCNTs/Pd
The baclofen‐MWCNTs (500 mg) were dispersed in
CH3CN (30 mL) by ultrasonic bath for 30 min. Subse-
quently, a yellow solution of PdCl2 (15 mg) in 30 mL ace-
tonitrile was added to the dispersion of baclofen‐
MWCNTs, and the mixture was stirred for 10 hr at
25ºC. Then, the baclofen‐MWCNTs/Pd (II) was separated
2 | EXPERIMENTAL
2.1 | Materials
All the reagents were purchased from Aldrich and
Merck, and were used without any purification. The

NAZARI AND HEKMATI
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SCHEME 1 Schematic diagram of
baclofen‐multi‐walled carbon nanotubes
(MWCNTs)/Pd fabrication
by centrifugation and washed by CH3CN, H2O and ace-
tone, respectively, to remove the unattached substrates.
The reduction of baclofen‐MWCNTs/Pd (II) by hydra-
zine hydrate was performed as follows: 50 mg of baclofen‐
MWCNTs/Pd (II) was dispersed in 60 mL of water, and
then 100 μL of hydrazine hydrate (80%) was added. The
pH of the mixture was adjusted to 10 with 25% ammo-
nium hydroxide and the reaction was carried out at
100°C for 2 hr. The final product baclofen‐MWCNTs/
Pd(0) was washed with water and dried in vacuum at
40°C. Scheme 1 depicted the synthetic procedure of
baclofen‐MWCNTs/Pd. The concentration of palladium
in the prepared catalyst was 0.13 mmol g−1, which was
determined by ICP‐AES.
2.4 | General procedure for the
Sonogashira coupling reactions
In a typical reaction, 10 mg of the baclofen‐MWCNTs/Pd
(17 mg = 0.2 mol% Pd) was placed in a 25‐mL Schlenk
tube, 1 mmol of the aryl halide in 3 mL of DMF was
added, and phenyl acetylene (1.5 mmol) and Et3N (2
mmol). The mixture was then stirred for the desired time
at 100°C. The progress of the reaction was monitored by
thin‐layer chromatography. After the reaction was over,
the catalyst was removed by centrifuge. The filtrate was
then extracted with ethyl acetate followed by washing
with brine solution. The organic part thus obtained was
dried by sodium sulfate and the solvent was then

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NAZARI AND HEKMATI
removed by rotary evaporation under reduced pressure.
The product thus obtained was further purified before
spectroscopic analysis.
carbonyl in the carboxylic acid groups. In addition, this
CNT sample has outlined a broad peak of OH vibration
from 2400 to 3450 cm−1. These two peaks confirm the
presence of the hydroxyl and carboxyl functional groups
on the surface of the acid‐treated MWCNTs. In the
case of the BMWCNTs (curve c of Figure 1), there are
three prominent vibrational bands at 1668, 2933 and
3433 cm−1 that refer to the ‐C=O stretching of the amide
groups (‐CONH‐), the bending vibration of CH2 and
stretching of NH, respectively. These results declare
bonding of baclofen to the surface functionalized
MWCNTs through amidation.
The success of the functionalization reactions was
also assessed by performing Raman spectroscopy. The
recorded Raman spectra are exhibited in Figure 2. As
can be seen, the Raman spectra of the samples include
two main Raman bands at 1338 and 1596 cm−1, which
are indicative of type D and G bonds, respectively. The
D band corresponds to the disordered MWCNT atoms
with sp3 hybridization, while the G band represents the
sp2‐hybridized carbons of graphene sheets. The extent of
defects in the MWCNTs can be determined with respect
to the area ratio of the D to G bands (ID/IG). Based on
the Raman spectra, the ID/IG ratio of the BMWCNTs
(ID/IG = 1.36) is more than that of the bare MWCNTs.
This finding verifies successful modification of the
MWCNTs with the baclofen molecules. Furthermore,
3 | RESULTS AND DISCUSSION
To synthesize the catalyst, the steps described in Scheme 1
were followed. As illustrated in Scheme 1, first the
MWCNTS were oxidized by adding a HNO3/H2SO4 mix-
ture. Then, the oxidized CNTs were activated with SOCl2
to graft the baclofen molecules onto the acylated CNTs.
After that, the Pd NPs were deposited onto the surface
of the BMWCNTs. The deposited Pd NPs were quantified
by ICP analysis, which indicated that the BMWCNT/Pd
particles contain 0.13 mmol g−1 palladium. The synthe-
sized nanocatalyst was characterized using powder
XRD, EDS, FT‐IR spectroscopy, elemental mapping,
Raman spectroscopy, CHN analysis, and the FESEM
and TEM techniques.
The chemical structure of the bare and surface‐
modified MWCNTs was analyzed by FT‐IR spectroscopy.
The FT‐IR spectra recorded for: (a) the MWCNTs; (b) the
MWCNTs‐COOH; and (c) the BMWCNTs are shown in
Figure 1. As curve b of Figure 1 displays, the oxidized
MWCNTs have given rise to a vibrational band at 1705
cm−1. This band is related to the stretching vibration of
FIGURE 1 Fourier transform‐infrared
(FT‐IR) spectra of (a) multi‐walled carbon
nanotubes (MWCNTs), (b) MWCNTs‐
COOH, (c) baclofen‐MWCNTs

NAZARI AND HEKMATI
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EDS. The resultant FESEM image is depicted in
Figure 4. Based on the FESEM image, the surface of the
MWCNTs is uniformly coated with the applied functional
groups and Pd NPs.
The catalyst was analyzed by EDS to determine its
chemical composition. The obtained spectrum (Figure 5)
demonstrates the well‐resolved peaks of C, O, N and Pd.
Therefore, the EDS spectrum approves successful
immobilization of the Pd NPs onto the surface of the
BMWCNTs, in agreement with the FESEM results.
Powder XRD was employed to reveal the crystal struc-
ture of the BMWCNT/Pd particles. The obtained XRD
pattern is presented in Figure 6. In this figure, the XRD
peak observed at 26.5° represents the (002) crystal plane
of hexagonal graphite, and its appearance after surface
modification of the MWCNTs indicates that the
functionalization process has not altered or destroyed
FIGURE 2 Raman spectra of the multi‐walled carbon nanotubes
(MWCNTs), and the baclofen‐MWCNTs
the observed increase of ID can be attributed to the
increased number of sp3‐hybridized carbon atoms as
there is no amorphous carbon in the samples. This
observation can be accepted as evidence for successful
functionalization of the MWCNTs.
The TEM image of the BMWCNT/Pd catalyst is
shown in Figure 3. This image unravels that the
MWCNTs are covered with a homogenous layer of Pd
NPs. Notably, the Pd NPs are not aggregated onto the
CNTs. Consequently, the baclofen molecules should
have played a key role in promoting dispersibility of the
Pd NPs.
The surface morphology and elemental composition
of the catalyst were determined by FESEM coupled with
FIGURE 4 Field emission scanning electron microscopy (FE‐
SEM) image with its elemental mapping of baclofen‐multi‐walled
carbon nanotubes (MWCNTs)/Pd
FIGURE 3 Transmission electron microscopy (TEM) image of
baclofen‐multi‐walled carbon nanotubes (MWCNTs)/Pd
FIGURE 5 Energy‐dispersive spectroscopy (EDS) data of the
baclofen‐multi‐walled carbon nanotubes (MWCNTs)/Pd

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NAZARI AND HEKMATI
the DMF solvent, the Et3N base and 0.2 mol% catalyst at
a temperature of 100°C (Table 1, Entry 7) are expected to
provide the maximal product yield via the BMWCNT/Pd‐
catalyzed SCC reactions.
After optimizing the reaction conditions, the effi-
ciency of BMWCNT/Pd was evaluated by applying it to
the SCC reaction of different aryl halides. The results
are outlined in Table 2. According to this table, the pre-
pared catalyst can efficiently catalyze the reaction of the
examined aryl iodides (Entries 1–6) and aryl bromides
(Entries 7–13) with phenylacetylene to give good to excel-
lent yields of their SCC‐associated products. However, the
activity of the catalysts towards SCC of the studied aryl
chlorides is insignificant.
FIGURE 6 X‐ray diffraction (XRD) pattern of the baclofen‐multi‐
walled carbon nanotubes (MWCNTs)/Pd
the original structure of the MWCNTs. On the other
hand, the diffraction peaks positioned at about 39, 47
and 67° are related to the (111), (200) and (220) planes
of face‐centered cubic (fcc) Pd particles.[14] Therefore,
the XRD pattern illustrates efficient immobilization of
fcc Pd nanostructures on the BMWCNTs.
The BMWCNT/Pd catalyst was applied to the SCC
reaction to investigate its catalytic performance. The reac-
tion conditions were optimized by concentrating on the
reaction of iodobenzene with phenylacetylene, as a model
reaction (Table 1). The optimized parameters included
base type, solvent type, catalyst dose and reaction temper-
ature. Based on the results reported in Table 1, the use of
To assure about heterogeneity of the catalyst, the SCC
reaction of iodobenzene and phenylacetylene was
monitored through two different experiments. In the first
experiment, the cross‐coupling reaction was terminated
after 60 min. At this point, the BMWCNT/Pd particles
were centrifuged and the reaction was followed for
another 60 min using the centrifugation supernatant. In
the second experiment, the same reaction was conducted
but the terminated reaction was not continued. It was
observed that both experiments lead to the same product
yield (60%). Therefore, the catalyst is heterogeneous.
Inductively coupled plasma analysis was also used to
determine the leaching of Pd NPs into the reaction
TABLE 1 Optimization of the reaction conditions for the reaction of iodobenzene with phenylacetylenea
Entry
Solvent
Base
Pd (mol%)
T (°C)
t (hr)
Yield (%)b
1
DMF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Et3N
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.1
0.3
0.0
100
100
80
10
10
10
10
10
10
2
70
50
60
65
65
70
96
75
60
Trace
80
96
75
96
0
2
H2O
3
EtOH
CH3CN
Solvent‐free
DMF
4
70
5
100
100
100
100
100
100
50
6
7
DMF
8
DMF
NaHCO3
K3PO4
No base
Et3N
4
9
DMF
4
10
11
12
13
14
15
DMF
12
6
DMF
DMF
Et3N
120
100
100
100
2
DMF
Et3N
6
DMF
Et3N
2
DMF
Et3N
24
aReaction conditions: iodobenzene (1.0 mmol), phenylacetylene (1.2 mmol), catalyst, base (2 mmol) and solvent (3 mL).
bIsolated yields.

NAZARI AND HEKMATI
7 of 8
TABLE 2 Sonogashira reaction of various aryl halides using
baclofen‐MWCNTs/Pd as a catalysta
TABLE 3 Comparison of the activity of different processes in the
Sonogashira coupling reaction of 4‐methylbromobenzene with
phenylacetylene
Catalyst
Reaction conditions Yield (%) Ref.
Baclofen‐
MWCNTs/Pd
Et3N, DMF, 100°C,
5 hr
90
84
13
40
this study
Entry
R
X
Time (hr)
Yield (%)b
[15]
SiO2‐pA‐Cyan‐
Cys‐Pd
NaOAc, DMF‐H2O,
80°C, 7 hr
1
H
I
2
2
2
3
2
2
3
4
5
6
5
5
6
96
95
95
85
95
90
96
92
90
80
90
85
92
[16]
[17]
CoFe2O4/Pd
K3PO4, DMF, 80°C,
6 hr
2
4‐CH3O
4‐CH3
2‐CH3
4‐Cl
I
3
I
NS‐MCM‐41‐Pd
Et3N, NMP, 90°C,
72 hr
4
I
5
I
6
3‐NO2
H
I
the same batch system. As Figure 7 shows, the synthe-
sized catalyst can be reused four times with negligible
loss of efficiency.
Finally, the performance of BMWCNT/Pd in catalyz-
ing the SCC reaction of 4‐methylbromobenzene and
phenylacetylene was compared with the activity of the
other catalysts that have been reported in the literature.
With respect to Table 3, BMWCNT/Pd can lead to higher
conversion rates and yields, relative to the other catalysts.
7
Br
Br
Br
Br
Br
Br
Br
8
4‐CH3O
4‐CH3
2‐CH3
4‐Cl
9
10
11
12
13
3‐NO2
4‐COCH3
aReaction conditions: arylhalide (1 mmol), phenylacetylene (1.2 mmol), Et3N
(2 mmol), DMF (3 mL), catalyst (0.2 mol%) and 100°C.
bIsolated yield.
4 | CONCLUSIONS
In this research, the baclofen drug was successfully
grafted onto acylated MWCNTs to immobilize Pd NPs.
The structure of the resultant MWCNTs was confirmed
by the FT‐IR and Raman spectroscopies, and TEM,
FESEM, EDS, XRD, elemental mapping and ICP analysis.
The obtained particles were then applied as a novel
nanocatalyst to catalyze the Sonogashira coupling reac-
tion. The catalyst demonstrated high catalytic efficiency
for cross‐coupling of aryl bromides and iodides, in
addition to presenting high structural stability and
recyclability without any significant loss of activity.
solution following 5 cycles, which was shown to be 1.14%,
and showed the constancy of the catalyst over the
reaction.
Reusability of the catalyst was assessed to ensure
its industrial applicability. In this respect, the
BMWCNT/Pd particles were used to catalyze the model
reaction (Table 1). After that, the utilized catalyst was
centrifuged, washed several times with deionized water
and ethanol, dried at 40°C in an oven and reused in
another cycle of the reaction. This test was performed in
ACKNOWLEDGEMENTS
The authors are grateful to the Pharmaceutical Sciences
Branch, (IAUPS), Tehran, Iran, for their partial support
on this project.
ORCID
Malak Hekmati https://orcid.org/0000-0003-0790-569X
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FIGURE 7 The recycling of the baclofen‐multi‐walled carbon
nanotubes (MWCNTs)/Pd for the Sonogashira coupling reaction
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