Nicotine functionalized-silica palladium (II) complex: a highly efficient, environmentally benign and recyclable nanocatalyst for C-C bond forming reactions under mild conditions

Article abstract of DOI:10.1002/aoc.3507

An environmentally friendly silica-grafted nicotine-based palladium(II) complex was successfully prepared and evaluated for the first time as novel and efficient nanocatalyst in C-C bond forming reactions. Grafted-nicotine in this catalytic system plays an important role, and as an effective ligand and a quaternary ammonium salt demonstrates an efficient stabilizing effect on the Pd(II) species by a synergistic effect of coordination and electrostatic interactions. The catalyst was well characterized by FT-IR, CHN, XRD, TEM, SEM–EDX, ICP and TG analysis, and demonstrated a highly efficient catalytic activity in the reaction system under phosphine-free and low Pd loading conditions, and the coupled products were produced in good to excellent yields. Furthermore, the catalyst can be easily recovered and reused without a significant loss of activity. Copyright

Full text of DOI:10.1002/aoc.3507

Full paper
Received: 13 March 2016
Revised: 10 April 2016
Accepted: 11 April 2016
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3507
Nicotine functionalized-silica palladium (II)
complex: a highly efficient, environmentally
benign and recyclable nanocatalyst for C-C
bond forming reactions under mild conditions
Abdol R. Hajipoura,b*, Nayereh S. Tadayonia and Fatemeh Mohammadsaleha
An environmentally friendly silica-grafted nicotine-based palladium(II) complex was successfully prepared and evaluated for the
first time as novel and efficient nanocatalyst in C-C bond forming reactions. Grafted-nicotine in this catalytic system plays an im-
portant role, and as an effective ligand and a quaternary ammonium salt demonstrates an efficient stabilizing effect on the Pd(II)
species by a synergistic effect of coordination and electrostatic interactions. The catalyst was well characterized by FT-IR, CHN,
XRD, TEM, SEM–EDX, ICP and TG analysis, and demonstrated a highly efficient catalytic activity in the reaction system under
phosphine-free and low Pd loading conditions, and the coupled products were produced in good to excellent yields. Furthermore,
the catalyst can be easily recovered and reused without a significant loss of activity. Copyright © 2016 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: mizoroki–heck reaction; suzuki–miyaura reaction; magnetic nanoparticles; n-heterocyclic carbenes
for the important reactions involving low catalyst loadings under
Introduction
mild and environmentally friendly conditions.[21,22] Nano-scale par-
Nowadays, a great deal of progress has been attained in improving
the reaction conditions and designing efficient and environmentally
benign catalytic systems for the metal-catalyzed aryl-nucleophile
bond forming reactions.[1–4] Combinations of transition-metal
salts and ligands have been described as highly efficient catalytic
systems for the synthesis of C-C, C-N, and also C-O bonds.[5,6] The
development of green chemistry in the synthesis routes which
reduce contamination and pollution in chemical synthesis has
attracted much attention in recent years.[7] There are diversity of
competitive methodologies for the synthesis of aryl-nucleophile
bonds with various sources of C-, O-, N-, P-, and S-nucleophiles.[8]
Heterogeneous catalysis is noteworthy for both academic and
industrial synthesis due to its advantages over homogeneous
catalysis.[9–13] The immobilization of catalyst on the solid-support
improves the catalyst stability and facilitates the separation and
recovery of catalyst from reaction media.
ticles usually display distinctive properties in comparison with their
bulk materials and the isolated atoms in catalytic reactions.[23,24]
The novel supported palladium nano-catalysts can be used in a
variety of reactions, including Sonogashira and Suzuki carbon–
carbon coupling reactions.[25–28]
The construction of carbon–carbon bonds is important in the
production of pharmaceuticals, and natural and industrial
products.[29,30]
The pd.-catalyzed Suzuki–Miyaura cross-coupling reaction is a
considerable procedure for carbon–carbon bond formation,
especially for the synthesis of biaryl compounds, which are used
as the building blocks for most of the pharmaceuticals, natural
products and advanced materials.[31,32]
Introduction of a triple bond into aromatic systems via cross-
coupling of aryl/alkyl halides and terminal alkynes.[28,33]
Quaternary ammonium and phosphonium salts combined with
metals such as palladium and copper have been successfully
applied for chemical synthesis.[34] The efficiency of these systems
is due to the involvement of metal nanoparticles stabilized by qua-
ternary salts and/or the formation of a new catalytic systems.[35–37]
In heterogeneous catalysis research, recent attention has been
concerned with the stabilization of nano-catalysts on a solid
surface.[14–17] A number of important studies have been focused
on development of the supported nano-catalysts manufactured
by reacting a functionalized surface with available functional
groups on the catalyst atoms.
Palladium -based catalysts are powerful for the formation of
carbon-nucleophile bonds, which have excessive applications in
both laboratory and large-scale synthesis.[18–20] In view of modern
synthesis, the non-renewable use of expensive palladium-based
catalysts is less valuable, in particular, for industrial scale produc-
tion. In recent years, palladium-based nanocatalysts stabilized on
the solid surface have been widely applied as efficient catalysts
*
Correspondence to: Hajipour, Abdol, Chemistry, Isfahan Uni. Of Technol. E-mail:
haji@cc.iut.ac.ir
a Pharmaceutical Research Laboratory, Department of Chemistry, Isfahan
University of Technology, Isfahan 84156, IR, Iran
b Department of Pharmacology, University of Wisconsin, Medical School, 1300
University Avenue, Madison 53706-1532, WI, USA
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.

A. R. Hajipour, N. S. Tadayoni and F. Mohammadsaleh
The Sonogashira coupling reaction is a powerful synthetic
method for the formation of C(sp)-C(sp2) bonds by the Niu et al.[38]
have applied Multiwalled Carbon Nanotubes-containing quater-
nary salts to support Au nanoparticles. Wang et al.[39] reported a
click-imidazolium-modified copolymer as a solid support to
stabilize the pd.- NPs.
Recently, we have used benzyl nicotinium chloride as the pro-
moter in metal-catalyzed C-C and C-S coupling reactions.[40–42]
We have also reported the investigation of a series of nicotine-
based ammonium salts as catalysts for chemical fixation of carbon
dioxide into cyclic carbonates, and it was found that
monobenzylnicotinium bromide showed a much higher activity
than dibenzylnicotinium bromide.[43]
In continuation of our recent studies on the development of
eco-friendly catalytic protocols,[44,45] herein, we report the synthesis
and characterization of a nicotine-functionalized silica supported
Pd complex as a recyclable solid catalyst. This new catalyst system
having a synergistic effect of coordination and electrostatic interac-
tions was found to be efficient in palladium-catalyzed C-C bond
formation.
The products SiO2-NICOT and SiO2-NICOT Pd (II) complex were
separated and washed several times with ethanol and acetone
and were characterized by FT-IR (Figure. 1). In the FT-IR spectrum
depicted in Figure 1, the characteristic peaks of C = N stretching
(1640 cmÀ1), C = C stretching (1691 cmÀ1), C–H stretching
(3010–2900 cmÀ1), CH2 bending (1460 cmÀ1), Si–C stretching
(1212 cmÀ1), and Si–O stretching (1050–1110 cmÀ1) vibrations are
assignable to the SiO2-NICOT.
The presence of the organic phase in the SiO2-NICOT support
was also confirmed by elemental analysis (CHN). Accordingly, the
amount of grafted nicotine based on the nitrogen content was
found to be 0.43 mmol/g.
By the SEM–EDX analysis of SiO2-NICOT Pd (II) complex(Figure 2),
the presence of palladium phase was well confirmed.
The content of palladium in the case of SiO2-NICOT Pd (II) was
measured by ICP, which was 0.4%.
Further characterization of SiO2-NICOT-Pd(II) complex was done
by transmission electron microscopy (TEM). As shown in Figure 3,
the TEM micrographs of catalyst show well-defined dispersion of
nano-sized palladium particles in the catalyst matrix. From
histogram, the nanoparticles size mostly are about 30 nm.
Sasaki et al.[46] have prepared and characterized a series of
immobilized metal ion-containing imidazolium based quaternary
salts (ImmM-QSs) on silica surfaces. They found that the
immobilized metal ion-imidazolium salts were more active than
Results and discussion
In this work, we have reported the synthesis and characterization of
a N-donor quaternary ammonium-based palladium complex and
evaluated its catalytic activity in the Suzuki and Sonogashira cross
coupling reactions.
As shown in Scheme 1, at first, activated SiO2 reacts with
3-chloropropyl trimethoxysilane in dry toluene. Then, the modified
silica treats with nicotine in acetonitrile solvent to obtain the
silica-nicotine (SiO2-NICOT) support. In the next step, this support
reacts with PdCl2 to achieve SiO2-NICOT Pd (II) complex.
Figure 1. FT-IR spectrum of: SiO2-NICOT (a) and SiO2-NICOT-Pd (b).
Figure 2. SEM–EDX spectra of catalyst.
Scheme 1. The nanocatalyst preparation.
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Nicotine functionalized-silica palladium (II) complex
Table 2. Optimization of Suzuki reaction of bromoacetophenone with
phenyl boronicacid in the presence of SiO2-NICOT-Pd(II)a
Entry
Solvent
T (°C)
Base
Catalyst (mol%) Yield b.%
1
2
3
4
5
6
EtOH/H2O
H2O
50
50
80
80
80
80
K2CO3
K2CO3
K2CO3
K2CO3
KOH
0.07
0.07
0.07
0.1
65
50
80
97
97
97
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
0.1
NaHCO3
0.1
aThe reaction conditions: bromoacetophenone (1.0 mmol),
phenylboronic acid (1.1 mmol) and solvent (2.0 mL) for 2 h.
bGC yield
Figure 3. TEM images and particle size distribution results for SiO2-NICOT
Pd (II) catalyst.
similar systems unsupported on silica, and in all of the studied
ImmM-QSs, no metal–metal bond was observed. They reported
that in the structures of immobilized metal ion-imidazolium salts
(M = Ni2+, Cu2+, Zn2+,Mn2+, Pd2+ and Co2) the metal ions as MCl4
species are held between two imidazolium ions on the surface of
SiO2. Based on this report, we have suggested the immobilization
of PdCl4 species combined with nicotinium ions on the silica
support in this work.
ascribed to the dispersity and size of the palladium species
embedded in the catalyst matrix. Therefore, we chose the catalyst
with 0.4% of Pd-content for further studies as optimized catalyst.
We explored the catalytic properties of SiO2-NICOT-Pd (II) in the
Suzuki reaction under various conditions. At first, base, solvent
and reaction temperature were examined in the coupling reaction
of 4-bromoacetophenone (1.0 mmol) and phenyl boronicacid
(1.2 mmol), as model reaction of Suzuki cross-coupling.
Two green solvent system such as H2O and EtOH /H2O, were
examined, and EtOH /H2O was chosen. It was also found that the
reaction temperature and the amount of catalyst have important
influence on this reaction. The kind of base has no effect on
reaction (Table 2 entries 4–6).
These optimized reaction conditions were used for Suzuki
reaction of different aryl halides. As it is demonstrated in Table 3,
aryl halides with electron withdrawing substituents and lower steric
hindrance, gave higher yields than the derivatives containing
electron-donating groups and higher steric hindrance.
The activity of this catalyst in the Sonogashira reaction was also
studied. In order to optimize reaction conditions, the reaction of
4-Iodoanisol with phenylacetylene was chosen as a model reaction
and the effect of different bases and solvents, was studied (Table 4).
It seems that KOH has acted better than other bases and EtOH/H2O
(1:1) showed better result as solvent. The reactions at different
temperatures were also examined and 50 C was selected as the
optimized conditions. Also the amount of catalyst was studied.
Finally, 0.07 mol% of catalyst was chosen as the optimized catalyst
loading.
The resulted catalyst was stable and its catalytic activity as a
solid-catalyst was investigated in Suzuki and Sonogashira cross
coupling reactions.
First, we optimized the catalyst preparation by investigation of
activity of the resulted catalyst. For this, we prepared two catalysts
with 4% and 0.4% of palladium content (the amount of palladium
was determined by ICP) and the activity of both catalyst was exam-
ined in Suzuki reaction. A comparison of the reaction results
showed that the catalytic performance is strongly influenced by
the palladium content of catalyst. It was found that by the reaction
of bromoacetophenone with phenyl boronicacid in the present of
K2CO3 and EtOH/H2O at 80 °C, the use of catalyst with a lower Pd
loading gave 80% of coupled product, whereas the catalyst with a
higher Pd loading (0. 4%) resulted in 50% conversion, at the same
conditions (Table 1). Hence, the effect of palladium content on
the catalytic activity of catalyst was examined, and it was found that
lower content showed much higher activity, probably being
Table 1. Optimization of catalyst preparation in Suzuki reaction of
bromoacetophenone with phenylboronicacid in the presence of SiO2-
NICOT-Pd (II) a
Under the optimized reaction conditions (Table 4, entry 7),
various aryl bromides reacted with phenyl acetylene and produced
Sonogashira products (Table 5). In this reaction system , similar to
the Suzuki coupling, aryl halides with electron withdrawing
substituents and lower steric hindrance gave higher yields than
the derivatives contain electron-donating groups and higher steric
hindrance. Also, the reaction condition is mild.
The reusability of catalyst is an essential subject in the field of
catalysis from economic and environmental points of view that is
commonly attributed to its stability. We surveyed the possibility
of recovery and reuse of the SiO2-NICOT-Pd(II) catalyst using the
Suzuki reaction of bromoacetophenone and phenyl boronicacid
as the substrates under the same conditions reported in Table 3.
Entry
Catalyst (mol%)
Yield b (%)
1
2
0.07 (4% Pd-content)
53
0.07 (0.4% Pd-content)
80
aThe reaction conditions: bromoacetophenone (1.0 mmol), phenyl
boronicacid(1.2 mmol), base (2 equiv.) solvent (2.0 mL) for 2 h.
bGC yield
Appl. Organometal. Chem. (2016)
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A. R. Hajipour, N. S. Tadayoni and F. Mohammadsaleh
Table 3. Suzuki–Miyaura cross coupling of aryl halides and phenyl
Table 5. Sonogashira cross coupling of aryl halides and
boronic acid in the presence of SiO2-NICOT-Pd a
phenylacetylene in the presence of SiO2-NICOT-Pda
Yield(%)b
Entry
X
R1C6H4X
Time (h)
Yield(%)b
Entry
R
Time(h)
1
2
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
I
4-NO2
2-NO2
4-Cl
2.5
4.5
4
93
84
75
60
83
88
97
92
80
95
94
65
80
65
1
2
3
4
5
6
4-NO2
2-NO2
4-COCH3
4-CN
1.5
3
93
70
90
94
85
65
3
3
4
2-Cl
4.5
2.5
2
1
5
H
4-OMe
H
2
6
4-OMe
4-OCH3
3-OCH3
2-OCH3
4-CN
2
7
2
aThe reaction was carried out with aryl halide (1.0 mmol),
Phenylacetylene (1.1 mmol) using KOH (2 eq) and EOH: H2O (1:1) 2 mL
in the presence of 0.7 mol % of Pd-catalyst at 50 C.
bGC yield
8
2
9
3
10
11
12
13c
14c
1
4-CHO
3-Cl
3
4.5
1.5
1
4-OMe
3-NO2
Table 6. Recyclability of catalyst in the Suzuki reaction a
I
Run
Yield (%)
Run
Yield(%)b
aThe reaction was carried out with aryl halide (1.0 mmol), phenyl boronic
acid (1.1 mmol) using K2CO3 (2 eq) and EOH: H2O (1:1) 2 mL in the
presence of catalyst (0.1 mol % of Pd) at 80 °C.
bGC yield
cRoom temperature
1
2
3
97
95
91
4
5
6
91
88
87
aThe reaction was carried out with bromoacetophenone (1.0 mmol),
phenyl boronicacid (1.2 mmol), base (2 equiv.) solvent (2.0 mL) for
20 min.
bGC yield
Table 4. Optimization of Sonogashira reaction of 4-Iodoanisol with
phenyl acetylene in the presence of SiO2-NICOT-Pd a
Figure 4 shows the XRD patterns of fresh and recovered catalyst.
As seen in the Figure 4a, no characteristic peaks of metallic
palladium was observed in the XRD pattern of fresh catalyst,
confirming the presence of palladium species as the Pd(II)
complexes in the catalyst system. In this pattern, the characteristic
peak of amorphous SiO2 at 2θ = 22° is clear.
After using the catalyst in the reaction, the catalyst color turned
to black, which shows the formation of metallic palladium in the
catalyst during the reaction. Sharp peaks at 2θ = 40.1 and 46 in
the XRD pattern of recovered catalyst (Figure 4b) confirmed the
formation of metallic palladium (Pd (0)) in the catalyst after using
in the reaction.
Entry
Solvent.
Base
T ( C)
Catalyst (mol%)
Yield(%)b
1
2
3
4
5
6
7
8
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
H2O
K2CO3
K2CO3
K3PO4
KOH
r.t
50
50
50
50
80
50
50
0.1
0.1
32
55
70
90
40
53
88
45
0.1
0.1
KOH
0.1
H2O
KOH
0.1
EtOH/H2O
EtOH/H2O
KOH
0.07
0.04
KOH
To realize that the leaching is occurred or not, catalyst remove
from mixture of reaction during the reaction. With removing
catalyst during reaction, reaction doesn’t proceed more over and
this observation confirmed that no leaching has occurred in during
reaction.
aThe reaction was carried out with 4-Iodoanisol (1.0 mmol), phenyl
acetylene (1.1 mmol) and solvent (2.0 mL) for 3 h.
bisolated yield
After each reaction-run, the catalyst was separated by centrifuga-
tion, washed with ethanol and acetone and dried. Then, the
resulting catalyst was reused directly for the next run without
further purification. As showed in Table 6, the catalyst was reused
six times without significant loss of activity.
The recovered catalyst was invesigated using FT-IR and XRD anal-
ysis. In IR spectrum, no remarkable changes were found in the
catalyst structure after six times reused (supporting information F1).
Experimental
General
All chemicals were purchased from Merck Chemical Co. (Germany)
and Sigma-Aldrich and were used without any purifications. The
FT-IR spectra were recorded on a Jasco-680 (Japan) spectropho-
tometer and reported in cmÀ1 (diluted with KBr). The X-ray
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Appl. Organometal. Chem. (2016)

Nicotine functionalized-silica palladium (II) complex
the reaction, the mixture was cooled to room temperature
CH2Cl2 (15 mL) was added and the catalyst was separated by
centrifuge. The organic phase was washed with H2O
(3 × 10 mL), dried over CaCl2, and the solvent was evaporated
by rotary. The product was isolated by column chromatogra-
phy (n-hexane / ethyl acetate, 5:1. The substituted biphenyls
synthesized in this work are known compounds and were char-
acterized by their IR, 1H and 13C NMR spectra, which have been
reported previously in the literature.[47–54]
General procedure for Sonogashira cross-coupling reaction by
SiO2-NICOT-Pd(II) nanocatalyst
In a round-bottomed flask equipped with a magnetic stirrer, phenyl
acetylene (1.2 mmol), aryl halide (1.0 mmol), catalyst (0.7 mol % Pd)
and KOH (2 eq.) in EtOH-H2O (1:1, 2 mL) was stirred under air atmo-
sphere at 50 °C. The progress of the reaction was monitored by TLC
(n-hexan/ethyl acetate, 5:1) and GC. After completion of the reac-
tion, CH2Cl2 (15 mL) was added and the catalyst was separated by
centrifuge. The organic layer was washed with water (3 × 10 mL)
and dried over CaCl2. The product was isolated by column chroma-
tography (n-hexane / ethyl acetate, 5:1) to obtain the corresponding
products in to 65–93% yields. The arylalkyne products were
Figure 4. XRD pattern of the fresh catalyst (a) and recovered catalyst (b).
diffraction (XRD) patterns were measured by a Philips X’pert X-ray
powder diffractometer. The metal content of the complex was
assessed by inductively coupled plasma (ICP). TGA curve was ob-
tained with a heating rate of 10 °C/min under nitrogen atmosphere.
The transmission electron microscopy (TEM) images were obtained
using Philips CM10 microscope.
1
known compounds and were characterized by their IR, H and
General procedure for synthesis of SiO2-NICOT-Pd(II)
nanocatalyst
13C-NMR spectra which have been reported previously in the
literature.[54–56]
Scheme 1 summarizes the synthetic route to the SiO2-NICOT-Pd(II)
nanocatalyst. Activated SiO2 was prepared by adding SiO2 gel
(150–230 mesh; 5 g) in HCl 37% (20 mL) for one day to obtained
active silica gel, and then washed with deionized water several
times until the pH of the filtrate achieve about 7. And the obtained
product was dried at 150 °C overnight in vacuum oven. In a round
bottom flask equipped with magnetic stirrer and condenser, a
mixture of active silica gel (1 g) and 3-chloropropyltrimethoxysilane
(0.4 mmol, 0.74 mL) was refluxed in dry toluene (100 mL) at 110 °C
for 72 hours. After that, the solids were separated from the reaction
mixture by centrifuge, washed with ethanol and methanol several
times and dried under vacuum at 70 °C to obtain chloropropyl-
SiO2 gel.
Conclusions
In this work we have reported synthesis and characterization of
SiO2-NICOT-Pd(II) nanocatalyst and investigated its application
in Suzuki and Sonogashira cross coupling reaction in mild con-
dition. Excellent yields in short time have been archived using
this catalyst. In addition, the catalyst could be reused in a facile
manner from the reaction mixture and recycled for the several
times.
Acknowledgements
The chloropropyl-SiO2 gel and nicotine (NICOT, 0.8 ml, 5 mmol)
were added in a round bottom flask equipped with magnetite
stirrer and condenser, and the mixture was refluxed in acetonitrile
(30 ml) at 80 °C for 40 hours. The mixture was filtered and washed
with ethanol and acetone, then dried in room temperature to
obtained SiO2-NICOT.
We gratefully acknowledge the funding support received for this
project from the Isfahan University of Technology (IUT), IR of Iran.
Further financial support from the Center of Excellence in Sensor
and Green Chemistry Research, Isfahan University of Technology,
is gratefully acknowledged.
The final catalyst SiO2-NICOT-Pd were obtained as light brown
solid by addition of Na2PdCl4 (5 mg PdCl2 and 2 mg NaCl in
methanol) to SiO2-NICOT (0.7 g) in methanol solvent at room
temperature for 24 h. Palladium content was determined 0.4% by
ICP. Another catalyst with higher Pd content (4%) was prepared
similarly by addition of Na2PdCl4 (17 mg PdCl2 and 6.6 mg NaCl in
methanol) to 0.24 g of SiO2-NICOT support in methanol solvent.
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Appl. Organometal. Chem. (2016)