Functionalization of multi-walled carbon nanotubes with pramipexole for immobilization of palladium nanoparticles and investigation of catalytic activity in the Sonogashira coupling reaction

Article abstract of DOI:10.1002/aoc.3600

Pramipexole drug was attached to the surface of multi-walled carbon nanotubes (MWCNTs) by reaction of acylated carbon nanotubes with pramipexole for the first time. The modified MWCNTs were characterized using Fourier transform infrared spectroscopy, tran

Full text of DOI:10.1002/aoc.3600

Received: 22 July 2016
DOI 10.1002/aoc.3600
Revised: 31 July 2016
Accepted: 2 August 2016
F U L L PA P E R
Functionalization of multi‐walled carbon nanotubes with
pramipexole for immobilization of palladium nanoparticles and
investigation of catalytic activity in the Sonogashira coupling
reaction
*
Sahar Abbasi | Malak Hekmati
Department of Pharmaceutical Chemistry, Faculty
of Pharmaceutical Chemistry, Pharmaceutical
Sciences Branch, Islamic Azad University,
(IAUPS), Tehran, Iran
Pramipexole drug was attached to the surface of multi‐walled carbon nanotubes
(MWCNTs) by reaction of acylated carbon nanotubes with pramipexole for the first
time. The modified MWCNTs were characterized using Fourier transform infrared
spectroscopy, transmission and scanning electron microscopies and CHNS analysis.
The prepared pramipexole–MWCNTs were used for immobilization of palladium
nanoparticles as a novel nanocatalyst. After characterization of the final
nanocomposite, the pramipexole–MWCNTs/Pd was applied as a novel phosphine‐
free recyclable heterogeneous catalyst for Sonogashira reactions. Interestingly, the
novel catalyst could be recovered and recycled five times without any significant
loss in activity.
Correspondence
Malak Hekmati, Department of Pharmaceutical
Chemistry, Faculty of Pharmaceutical Chemistry,
Pharmaceutical Sciences Branch, Islamic Azad
University, (IAUPS), Tehran, Iran.
Email: mhekmatik@iaups.ac.ir
KEYWORDS
multi‐walled carbon nanotubes, nanocatalyst, palladium, primapexole, Sonogashira
1
|
INTRODUCTION
attractions among CNTs. Without surface modification,
most CNTs lack sufficient binding sites for anchoring
precursor metal ions or metal nanoparticles, which usually
leads to poor dispersion and aggregation of metal
nanoparticles, especially at high loading conditions. There-
fore, functionalization of CNTs is generally a prerequisite
to further applications. According to the nature of the
interaction between CNTs and functional groups, the
functionalization of CNTs can be categorized into covalent
and non‐covalent functionalization.
The immobilization methods used to deposit palladium
into heterogeneous solid beds have been studied extensively,
and diverse supports such as clay,[31] carbon nanofibres,[32]
montmorillonite,[33] magnetic mesoporous silica,[34] zeo-
lite[35] and metal oxides[36] have been investigated. A current
challenge in this area is the development of efficient
immobilized systems that can simultaneously fulfil the usual
targets of achieving high turnover number values and facili-
tate recovery and reuse as well as the need for obtaining
palladium‐free final products,[37,38] meeting the strict purity
specifications for the pharmaceutical industry.[39,40]
Since their discovery by Iijima,[1] carbon nanotubes (CNTs)
have attracted great interest in many fields of science and
nanotechnology due to their fascinating and unique optical,
electrical, mechanical, chemical and thermal properties.[2,3]
The tubular structure of CNTs makes them unique among
different forms of carbon, and they can thus be exploited as
an alternative material for catalyst support in heteroge-
neous catalysis[4] and in fuel cells due to the large surface
area‐to‐volume ratio, excellent electronic conductivity and
high chemical stability.[5–14] Consequently, the exceptional
properties of CNTs made them ideal candidates for numerous
applications in nanoelectronics,[15–18] catalysis,[19–25]
diagnostics[26,27] and drug delivery.[28,29] Wildgoose et al.
reviewed the recent developments in this area by exploring
the various techniques to functionalize CNTs with metals
and other nanoparticles and the diverse applications of the
resulting materials.[30] However, the applications of CNTs
have been impeded by their poor solubility in solvents and
polymers, which originates from strong van der Waals
Appl. Organometal. Chem. 2016; 1–8
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
ABBASI AND HEKMATI
The Sonogashira cross‐coupling of aryl halides with ter-
Sonogashira reactions were selected to investigate its cata-
lytic activity and recyclability (Scheme 1).
minal alkynes has been employed for the synthesis of several
compounds bearing an internal alkyne moiety, including bio-
logically active molecules and functional polymers.[41–43]
Toxic and expensive homogeneous palladium phosphine
complexes, which are rarely recoverable without elaborate
and wasteful procedures,[44] have been used as catalysts for
the Sonogashira reaction. Phosphines can be a more serious
economic burden than even the palladium itself in some
cases.[45] From economic and environmental standpoints, it
is desirable to use solid catalysts without the use of hazardous
phosphine additives as a greener alternative to conventional
2
| EXPERIMENTAL
2.1 | Materials
All reagents were purchased from Aldrich and Merck used
without any purification. SOCl2, HCl, H2SO4, HNO3, H2O2
(30 wt%, aqueous), deionized water, NaH (80%), anhydrous
dimethylformamide (DMF), CaH2 and pramipexole were
obtained from Sigma Aldrich and Merck.
homogeneous
processes.[46]
Indeed,
phosphine‐free
Sonogashira coupling processes under homogeneous as well
as heterogeneous conditions have been reported.[47–51] In this
paper, in continuation of our studies of the synthesis of novel
supported catalysts,[52] we report the design and synthesis of
novel pramipexole–multi‐walled CNTs (MWCNTs) with
which to immobilize palladium nanoparticles (pramipexole–
MWCNTs/Pd; Scheme 1). After characterization,
2.2 | Covalent grafting of Pramipexole to MWCNTs
Pristine MWCNTs were refluxed under stirring in a mixture
of 1:3 (v/v) HNO3 and H2SO4 at 70°C for 30 h, which was
followed by centrifugation and repeated washings with deion-
ized water. The dried carboxylated MWCNTs (MCNTs–
COOH) were suspended in a solution of thionyl chloride
(excess) and DMF (1 ml). The suspension was stirred at
65°C for 24 h. The solid was then separated by filtration and
washed with anhydrous tetrahydrofuran (30 ml), and dried
in vacuum to obtain MWCNTs–COCl. The final product
was then subjected to functionalization with pramipexole
(amine‐to‐MWCNTs weight ratio of 5:1). Pramipexole was
mixed with 10 ml of DMF and 1 ml of triethylamine (TEA)
and then stirred for 5 min. The obtained MWCNTs–COCl,
dispersed in DMF (20 ml) using an 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 0.15 mmol g−1
which was determined using CHNS analysis.
,
2.3 | Preparation of Pramipexole–MWCNTs/Pd
Pramipexole–MWCNTs (500 mg) were dispersed in CH3CN
(30 ml) using an ultrasonic bath for 30 min. Subsequently, a
yellow solution of PdCl2 (30 mg) in 30 ml of acetonitrile
was added to the dispersion of pramipexole–MWCNTs and
the mixture was stirred for 10 h at 25°C. Then, the
pramipexole–MWCNTs/Pd(II) was separated by centrifuga-
tion and washed successively with CH3CN, water and ace-
tone to remove the unattached substrates.
The reduction of pramipexole–MWCNTs/Pd(II) using
hydrazine hydrate was performed as follows: 50 mg of
pramipexole–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 95°C for
2 h. The final product, pramipexole–MWCNTs/Pd(0), was
washed with water and dried in vacuum at 40°C. Scheme 1
depicts the synthetic procedure of pramipexole–MWCNTs/
SCHEME 1 Fabrication of pramipexole–MWCNTs/Pd

ABBASI AND HEKMATI
3
FIGURE 3 TEM images of pramipexole–MWCNTs
FIGURE 1 FT‐IR spectra of (a) MWCNTs, (b) MWCNTs–COOH and (c)
pramipexole–MWCNTs
FIGURE 4 TEM image of pramipexole–MWCNTs/Pd
FIGURE
2
SEM images of MWCNTs (a) before and (b) after
functionalization with pramipexole
FIGURE 5 SEM image of pramipexole–MWCNTs/Pd

4
ABBASI AND HEKMATI
Pd. The concentration of palladium in the prepared catalyst
was 1.9 wt% (0.2 mmol g−1), which was determined using
inductively coupled plasma atomic emission spectroscopy
(ICP‐AES) and energy‐dispersive X‐ray spectroscopy (EDS).
Figure 1(b), the band at 1705 cm−1 corresponds to carbonyl
stretch of the carboxylic acid group. Moreover, appearance
of a broad peak at 2400–3450 cm−1 implies the presence of
OH groups, which means that oxidation of the MWCNTs
has induced functional groups such as carboxyl and hydroxyl
on the surface of nanoparticles. Figure 1(c) shows the FT‐IR
2.4 | General procedure for Sonogashira coupling
reactions
In a typical reaction, 10 mg of pramipexole–MWCNTs/Pd
(10 mg = 0.002 mmol Pd) was placed in a 25 ml Schlenk
tube, to which were added 1 mmol of aryl halide in 3 ml of
DMF, phenylacetylene (1.5 mmol) and Et3N (2 mmol). The
mixture was then stirred for the desired time at 80°C. The
progress of the reaction was monitored by TLC. After the
reaction was over, the catalyst was removed by centrifuga-
tion. The filtrate was then extracted with ethyl acetate
followed by washing with brine solution. The organic part
thus obtained was dried using sodium sulfate and the solvent
was then removed by rotary evaporation under reduced pres-
sure. The product thus obtained was further purified before
spectroscopic analysis.
2.5 | Procedure for reusing catalyst
After the reaction time, 5 ml of ethyl acetate was added to the
reaction mixture and stirred for 5 min. After this time, the cata-
lyst was separated by centrifugation. In the next step, the recov-
ered solid was washed using 70% ethanol (5 ml) and dried under
vacuum. Then, the recovered catalyst was used for another run.
3
| RESULTS AND DISCUSSION
The pramipexole‐functionalized MWCNTs were conveniently
synthesized from commercially available and cheap materials
via reaction of acylated CNTs with pramipexole. The route of
pramipexole–MWCNTs fabrication is shown in Scheme 1.
The surface structure of the materials was confirmed
using Fourier transform infrared (FT‐IR) spectroscopy.
Figure 1 show the FT‐IR spectra obtained for MWCNTs,
MWCNTs–COOH and pramipexole–MWCNTs. As seen in
FIGURE 7 FESEM image of pramipexole–MWCNTs/Pd and elemental
FIGURE 6 EDS analysis of pramipexole–MWCNTs/Pd
maps of C and Pd atoms

ABBASI AND HEKMATI
5
spectrum of pramipexole–MWCNTs in which the absorption
band at 1642 cm−1 is attributed to the carbonyl stretching of
the amide groups (─CONH─). Also, the bands observed
around 2925 cm−1 are assigned to the bending vibration of
CH2. The prominent bands at 3411 and 3547 cm−1 are
ascribed to NH stretching vibrations. These results indicate
that pramipexole is bonded to the surface of MWCNTs
through amidation reaction.
Scanning electron microscopy (SEM) images of pristine
MWCNTs and pramipexole–MWCNTs are shown in
Figure 2. The image of pristine MWCNTs shows that the tube
surfaces are clear and smooth (Figure 2a). However, under
the same magnification, the surface of pramipexole–
MWCNTs (Figure 2b) is distinctly different from that of the
untreated MWCNTs (Figure 2a). It is observed that for the
pramipexole–MWCNTs, the surface of the CNTs is uni-
formly coated with drug moieties, thus clearly indicative that
pramipexole units are attached to MWCNTs.
IR spectroscopy, TEM, SEM and CHNS analysis, the
prepared pramipexole–MWCNTs were used for the immobi-
lization of palladium nanoparticles (Scheme 1). The resulting
material was then characterized using TEM, field‐emission
SEM (FESEM), ICP‐AES, EDS and wavelength‐dispersive
X‐ray spectroscopy (WDX).
TEM investigations are carried out to observe the morphol-
ogy and distribution of palladium particles supported on
pramipexole–MWCNTs. The existence of palladium
nanoparticles deposited on functionalized MWCNTs is clearly
distinguishable as dark spots in Figure 4. From Figure 4, we
can see that palladium is well dispersed on the surface of
pramipexole‐modified MWCNTs. The results indicate that
pramipexole plays an important role in improving the
dispersibility of palladium. The mean diameter of palladium
nanoparticles immobilized on pramipexole–MWCNTs is found
to be around 5–8 nm. Also, SEM analysis of pramipexole–
MWCNTs/Pd (Figure 5) confirms the successful
functionalization of MWCNTs and deposition of palladium
nanoparticles on the surface of the modified MWCNTs.
Transmission electron microscopy (TEM) investigations
were carried out to observe the morphology and possible
grafting of pramipexole groups supported on MWCNTs.
The existence of pramipexole groups, covalently attached to
MWCNTs, is clearly distinguishable as identified by spots in
Figure 3. This result indicates that the MWCNTs are cova-
lently grafted with pramipexole.
The elemental composition was determined using EDS
analysis, and the results, shown in Figure 6, indicate C, N,
O, S and Pd signals that are provided by pramipexole–
MWCNTs/Pd. For further characterization of the sample,
WDX‐coupled quantified FESEM mapping of the sample
was also conducted (Figure 7). WDX can provide qualitative
information about the distribution of different chemical
After successful modification of MWCNTs, and charac-
terization of the resulting pramipexole–MWCNTs using FT‐
TABLE 1 Optimization of reaction conditions for reaction of iodobenzene with phenylacetylenea
Entry
Solvent
DMF
Base
Pd (mol%)
T (°C)
t (h)
Yield (%)b
1
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Et3N
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.05
0.15
0.0
80
80
80
80
80
80
80
80
80
80
80
80
80
50
100
80
80
80
12
12
12
12
2
75
55
2
H2O
3
EtOH
60
4
CH3CN
H2O–EtOH (1:1)
H2O–EtOH (1:2)
H2O–EtOH (2:1)
Solvent‐free
DMF
60
5
50
6
2
50
7
2
65
8
12
12
2
60
9
70
10
11
12
13
14
15
16
17
18
DMF
96
DMF
NaHCO3
K3PO4
No base
Et3N
5
70
DMF
5
65
DMF
12
5
Trace
80
DMF
DMF
Et3N
2
96
DMF
Et3N
5
80
DMF
Et3N
2
92
DMF
Et3N
24
0
aReaction conditions: iodobenzene (1.0 mmol), phenylacetylene (1.2 mmol), catalyst, base (2 mmol) and solvent (3 ml).
bIsolated yield.

6
ABBASI AND HEKMATI
elements in the catalyst matrix. Looking at the compositional
maps C and Pd, the presence of palladium nanoparticles with
good dispersion is clearly distinguished in the composite.
The
catalytic
activity
of
prepared
catalyst
(pramipexole–MWCNTs/Pd) was evaluated in Sonogashira
coupling reactions. In order to find the optimized reaction
conditions, the reaction between iodobenzene and
phenylacetylene was chosen as model reaction (Table 1).
The effects of various factors such as solvent, base, amount
of catalyst and temperature were studied (Table 1). Results
indicate that using Et3N as a base, DMF as a solvent and
0.1 mol% catalyst at 80°C affords the highest yield of
the product (Table 1, entry 10).
TABLE 2 Sonogashira reaction of various aryl halides using pramipexole–
MWCNTs/Pd as a catalysta
Using these optimized reaction conditions, the efficiency
of the catalyst was studied in the Sonogashira reaction of var-
ious aryl halides and the results are summarized in Table 2.
Aryl iodide (entries 1–6) and aryl bromide (entries 7–13)
with various functional groups efficiently react with
phenylacetylene to produce Sonogashira products in good
to excellent yields. This catalytic system was also applied
for the reaction of aryl chlorides (Table 2, entries 14–16). It
is clear from Table 2 that aryl chlorides couple with
phenylacetylene in moderate yields within reaction times of
12–15 h.
The heterogeneity of the catalyst was evaluated to
study whether the reaction using solid palladium catalysts
occurred on the pramipexole–MWCNTs/Pd surface or was
catalysed by palladium species leached in the liquid phase.
To address this issue, two separate experiments were
conducted with iodobenzene and phenylacetylene. In the
first experiment, the reaction was terminated after
60 min; at this time, the catalyst was separated from the
reaction mixture by centrifugation and the reaction was
continued with the filtrate for an additional 60 min. In
the second experiment, the reaction was terminated after
60 min. In both cases, the desired product was obtained
in the same yield (55%). The result confirms the heteroge-
neous nature of the catalyst.
Entry
R
X
Time (h)
Yield (%)b
1
H
I
2
2
96
96
96
88
95
92
96
92
90
85
92
88
95
85
75
65
2
4‐CH3O
4‐CH3
2‐CH3
4‐Cl
I
3
I
2
4
I
3
5
I
2
6
3‐NO2
H
I
2
7
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
3
8
4‐CH3O
4‐CH3
2‐CH3
4‐Cl
5
9
5
10
11
12
13
14
15
16
8
5
3‐NO2
4‐COCH3
H
6
6
12
12
15
4‐CH3
2‐CH3O
aReaction conditions: aryl halide (1 mmol), phenylacetylene (1.2 mmol), Et3N
(2 mmol), DMF (3 ml), catalyst (0.1 mol%) at 80°C.
bIsolated yield.
In view of industrial application and green chemistry, the
reusability of the catalyst was tested by carrying out repeated
runs of the reaction with the same batch of catalyst in the case
of the model reaction (Table 1). In order to regenerate the cat-
alyst, after each cycle, it was separated by centrifugation, and
washed several times with deionized water and ethanol.
Then, it was dried in oven at 50°C and used in the next run.
The results show that this catalyst can be reused five times
without any significant loss in activity (Figure 8). This high
catalytic activity can be possibly related to the strong interac-
tion of palladium nanoparticles with active sites of
pramipexole groups.
FIGURE 8 Recycling of pramipexole–MWCNTs/Pd for Sonogashira cou-
pling reaction under similar conditions
TABLE 3 Comparison of activity of various catalysts in the Sonogashira coupling reaction of 4‐methylbromobenzene with phenylacetylene
Catalyst
Reaction conditions
Yield (%)
Ref.
Pramipexole–MWCNTs/Pd
SiO2‐pA‐Cyan‐Cys‐Pd
CoFe2O4/Pd
Et3N, DMF, 80°C, 5 h
90
84
13
40
This study
[53]
NaOAc, DMF–H2O, 80°C, 7 h
K3PO4, DMF, 80°C, 6 h
Et3N, NMP, 90 °C, 72 h
[54]
[55]
NS‐MCM‐41‐Pd

ABBASI AND HEKMATI
7
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Finally, to assess the present protocol with respect to
other reported methods for the Sonogashira coupling reaction
between 4‐methylbromobenzene and phenylacetylene, the
presented procedure using pramipexole–MWCNTs/Pd as cat-
alyst was compared with other systems. From Table 3, it is
evident that the present system gives higher conversions
and yields compared to other reported systems.
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4
| CONCLUSIONS
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In this study, pramipexole drug was successfully added to the
surface of MWCNTs by reaction of acylated carbon
nanotubes with pramipexole. The procedure was fast and
simple, and resulted in a high degree of functionalization.
The results of FT‐IR, SEM, TEM and CHN analyses con-
firmed the functionalization of MWCNTs with pramipexole.
The prepared pramipexole–MWCNTs were used for immobi-
lization of palladium nanoparticles as novel nanocatalyst. The
catalyst showed high activity in Sonogashira coupling reac-
tions of aryl halides (iodides, bromides, chlorides). The cata-
lyst showed good stability and was recycled several times
with a small decrease in its catalytic activity. These advan-
tages make the process highly valuable from the synthetic
and environmental points of view.
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