Triazole-Functionalized Silica Supported Palladium(II) Complex: A Novel and Highly Active Heterogeneous Nano-catalyst for C–C Coupling Reactions in Aqueous Media

Article abstract of DOI:10.1007/s10562-018-2316-5

Abstract: Two novel triazole-modified silica supports A and B were successfully prepared via “click” reaction of azide-functionalized SiO₂ with propargyl alcohol (A) and propargyl amine (B), in which the click-triazole as an important functional entity, in addition to a molecular linker, provides capabilities of metal coordination as an excellent chelator. Treatment of the resulting click-supports with Pd(OAc)₂ afforded the click-catalysts A and B, which were well characterized and evaluated in Suzuki–Miyaura coupling in terms of activity and recyclability in H₂O/EtOH solvent. The catalyst A showed more reasonable results and so, was applied as a highly efficient and recyclable catalyst in the coupling reactions of various aryl halides with phenylboronic acid under phosphine-free and low Pd loading conditions. Graphical Abstract: Two novel nano pd-catalysts were successfully prepared by immobilization of Pd(OAc)₂ onto the silica supports containing the nitrogen-rich triazole chelators and were investigated in Suzuki–Miyaura coupling in aqueous solvent.

Full text of DOI:10.1007/s10562-018-2316-5

Catalysis Letters
https://doi.org/10.1007/s10562-018-2316-5
Triazole-Functionalized Silica Supported Palladium(II) Complex:
A Novel and Highly Active Heterogeneous Nano-catalyst for C–C
Coupling Reactions in Aqueous Media
Abdol Reza Hajipour1,2 · Fatemeh Mohammadsaleh1
Received: 12 November 2017 / Accepted: 23 January 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Two novel triazole-modified silica supports A and B were successfully prepared via “click” reaction of azide-functionalized
SiO2 with propargyl alcohol (A) and propargyl amine (B), in which the click-triazole as an important functional entity, in
addition to a molecular linker, provides capabilities of metal coordination as an excellent chelator. Treatment of the result-
ing click-supports with Pd(OAc)2 afforded the click-catalysts A and B, which were well characterized and evaluated in
Suzuki–Miyaura coupling in terms of activity and recyclability in H2O/EtOH solvent. The catalyst A showed more reason-
able results and so, was applied as a highly efficient and recyclable catalyst in the coupling reactions of various aryl halides
with phenylboronic acid under phosphine-free and low Pd loading conditions.
Graphical Abstract
Two novel nano pd-catalysts were successfully prepared by immobilization of Pd(OAc)2 onto the silica supports containing
the nitrogen-rich triazole chelators and were investigated in Suzuki–Miyaura coupling in aqueous solvent.
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-018-2316-5) contains
supplementary material, which is available to authorized users.
Extended author information available on the last page of the article
Vol.:(0123456789)
1 3

A. R. Hajipour, F. Mohammadsaleh
Keywords Heterogeneous catalysis · Click chemistry · Triazoles · Coupling reaction
with peracetic acid. Astruc et al. designed 2-pyridyl-1,2,3-
triazole ligands 3 that were powerful stabilizing ligands for
palladium-catalyzed C–C coupling reactions [20]. Messerle
et al. prepared two new bidentate pyrazolyl–triazolyl donor
ligands 4 and investigated the catalytic activity of rhodium(I)
and iridium(I) complexes containing these N–N′ ligands for
the intramolecular cyclization of alkenamines and nonter-
minal alkynamines [21].
1 Introduction
The copper(I)-catalyzed azide–alkyne 1,3-dipolar cycload-
dition (CuAAC) reaction known as a highly useful example
of “click chemistry”, which has been proved to be a power-
ful synthetic tool in molecular conjugation chemistry [1–8].
1,2,3-Triazoles are nowadays an important class of N-het-
erocyclic compounds, which have shown numerous appli-
cations in all aspects of bioconjugation, drug discovery,
and materials science [9–11]. This nitrogen-rich unit has a
large dipole moment, and in the recent years, different stud-
ies have shown that 1,2,3-triazoles are able to coordinate a
metal center as the ligand [12, 13]. Such property make an
important role for triazoles in the catalysis field, especially
in metal-based catalysis. The clicked 1,2,3-triazole as the
N-donor ligand can form chelates with metal ions and stabi-
lize catalytic sites. Recently, some researchers have demon-
strated the coordination of metal ions with the 1,2,3-triazolyl
group via the nitrogen atoms in single-crystal structures [12,
14–19]. Figure 1 shows some examples using 1,2,3-triazole-
derived ligands reported by other groups.
Metal-catalyzed cross-coupling reactions are one of the
challenging areas of research in modern organic synthesis
[22, 23]. There are variety of competitive methods for the
synthesis of carbon–carbon and carbon–heteroatom bonds
N
N
N
N
N
N
N
N
N
N
N
N
N
N
r
A
r
A
2
1
Scrivanti et al. synthesized the triazole ligand 1 for the
palladium-catalyzed Suzuki–Miyaura coupling reactions
[15]. A variety of triazole-based N4 tetradenate ligands 2
was reported by Hao and Wang [16], and Mn(II) complexes
containing these ligands showed efficient catalytic activi-
ties in the epoxidation of various aliphatic terminal olefins
R
N
N
N
N
n
Ph
n=
N
N
N
0 1
,
=
4
R H H Ph
C
,
3
2
Fig. 1 Examples of 1,2,3-triazolyl ligands used in metal catalysis
1 3

Triazole-Functionalized Silica Supported Palladium(II) Complex: A Novel and Highly Active…
including combinations of transition metals and ligands as
the efficient catalytic systems [24]. The palladium-catalyzed
coupling reactions are ranked nowadays among the most
general transformations in organic synthesis, which have
great applications in both academic and industrial settings
[25]. In traditional protocols, catalysts used in these reac-
tions are based on homogeneous palladium complexes [26].
Although these homogeneous catalysts have many advan-
tages, they have some significant limitations in terms of the
separation and recycling. In view of modern organic syn-
thesis, the non-renewable use of expensive palladium cata-
lysts is less constructive, in particular, for industrial scale
synthesis. Additionally, leaching the metal to the products
and contamination of product with ligands is always one of
the most important problems of these catalysts. In contrast,
Pd catalyst immobilized on a support can be easily separated
from reaction media through centrifuge or simple filtration
and successfully reused [27–29]. The solid-supported cata-
lysts produce a heterogeneous catalyst and improve catalyst
stability, which have newly attracted much interest owing to
the increasing environmental concern [30]. Supported cata-
lysts are manufactured by reacting a functionalized support
with available and suitable functional groups on the catalyst
atoms. Numerous methodologies have been reported for the
modification of various supports surface [31, 32].
capabilities of hydrogen bonding as both a donor and an
acceptor, metal coordination, and π–π interactions in this
click-modified supports [13]. This nitrogen-rich heterocyclic
compound, has drawn considerable attention for the catalyst
immobilization as a highly stable linker or/and a chelator to
graft catalysts onto the supports. Ding and co-workers have
reported a SBA-15-supported palladium catalyst containing
a triazole framework via the click reaction, and its applica-
tion for the aerobic oxidation of alcohols [33]. Luo et al.
have successfully prepared a magnetic nanoparticle-sup-
ported nano-palladium catalyst via a “click” route [34]. They
evaluated the efficiency of this catalyst in Suzuki–Miyaura
coupling in terms of activity and recyclability in aqueous
ethanol solvent.
Recently, Shen et al. have reported the single crys-
tal structure of a Pd click-catalyst, in which the glycosyl
pyridyl-triazole coordinates with palladium in a bidentate
(N,N) coordination mode [37]. They applied this click cata-
lyst in palladium-catalyzed C–C coupling reactions such as
Suzuki–Miyaura coupling, Heck and Sonogashira reaction.
Sangtrirutnugul et al. have also prepared the Pd(II) com-
plexes containing 1,2,3-triazol ligands and described their
crystal structures [38]. Catalytic studies of these click cata-
lysts exhibited moderate to high activity for Suzuki–Miyaura
coupling.
“Click” chemistry-based surface modification has
recently received much attention, especially in the design
of metal based heterogeneous catalysts [20, 33–35]. Some
researcher groups have successively modified the surface
of various solid supports via the “click” rout [36]. The
1,2,3-triazole grafted on this supports, as an important func-
tional unit, in addition to a molecular linker, could provide
The palladium-catalyzed Suzuki cross-coupling is a pow-
erful synthetic procedure for C–C bond formation via cou-
pling of aryl halides with arylboronic acids to provide biaryl
derivatives, which are used as the building blocks in the
synthesis of natural products, pharmaceuticals and advanced
materials [39–45].
Scheme 1 Synthesis of silica-
supported nano-palladium
click-catalyst A and B. Reaction
conditions (a) CH3CN, NaN3,
TBAB, 80 °C 24 h; (b) toluene,
reflux, 24 h; (c) Cu2O, H2O/
MeOH, r.t., 3 days; (d) toluene,
Pd(OAc)2, r.t., 24 h
1 3

A. R. Hajipour, F. Mohammadsaleh
In continuation of our efforts to design and discover
new catalytic systems [46–48], we have herein focused on
click-modification of silica support and immobilization of
palladium catalyst to prepare an effective and recyclable
heterogeneous nano-palladium catalyst for cross coupling
reactions. Initially, the surface of silica was functionalized
with azide group and then reacted via a “click” route with
propargyl amine or propargyl alcohol as the alkyne sec-
tion to construct the click- modified silica support. Then,
immobilization of palladium(II) acetate into the resulting
click-supports gave palladium-based solid catalysts A and
B, which were comparatively investigated in terms of their
catalytic activity and recycling for Suzuki–Miyaura coupling
reactions. The detailed route is shown in Scheme 1.
2.2 Synthesis of Click‑Supports A and B
Azide- functionalized silica (0.5 g) and Cu2O (0.02 g) were
mixed in 30 mL H2O/MeOH (1:1), and then an excess
amount of propargyl alcohol (for the support A) or propargyl
amine (for the support B) was added. The reaction mixture
was stirred at room temperature for 3 days. After comple-
tion, the solid particles were filtered, washed several times
with methanol, water, HCl solution (5% solution in water),
water and acetone, and finally dried at 60 °C under vacuum.
The solid was obtained as a withe powder for the click-sup-
port A and a light brown powder for the click-support B.
2.3 Synthesis of Click‑Catalysts A and B
A mixture of the click-support (A or B, 0.3 g) and palla-
dium acetate (0.2 mmol, 0.045 g) were stirred in dry toluene
(10 mL) at room temperature for 30 h. After the complexa-
tion, the solid phase was separated by centrifugation, washed
thoroughly with toluene and acetone several times in order
to remove any adsorbed palladium on the surface, and finally
allowed to dry at room temperature.
2 Experimental
2.1 Synthesis of Azide‑ Functionalized Silica
Support
First, 3-chloropropyltrimethoxysilane was converted to the
3-azidopropyltrimethoxysilane according to the method
reported by Ding et al. [33], 3-chloropropyltrimethoxysi-
lane (0.92 mL, 5 mmol) was added to a solution of NaN3
(0.46 g, 7 mmol) and tetrabutylammonium bromide (TBAB,
0.32 g, 1 mmol) in dry acetonitrile (30.0 mL) under nitrogen
atmosphere, and the mixture was heated under reflux for
24 h. After cooling, the solvent was removed under reduced
pressure. The residue was then diluted in Et2O (20 mL) and
the mixture was filtered and washed with Et2O. The com-
bined solvent was removed to give 3-azidopropyltrimeth-
oxysilane, and the flask charged with 50 mL dry toluene
for further reaction with SiO2 gel. Activated silica was
prepared by immersing SiO2-gel (150–230 mesh; 5 g) in
concentrated HCl for 24 h to hydrolyze the surface Si–O–Si
bonds to Si–OH, and then washed with deionized water sev-
eral times until the pH of the filtrate was higher than 6 and
dried at 150 °C for 8 h. The fresh activated silica (2 g) was
suspended in the above solution of 3-azidopropyltrimeth-
oxysilane in toluene, and the suspension was refluxed under
nitrogen for 24 h. After cooling, the silica particles were
collected by filtration and washed thoroughly with toluene,
methanol and finally with acetone. The resulting solid was
dried under vacuum at 60 °C to give azide- functionalized
silica support. The formation of 3-azidopropyltrimethoxysi-
lane and the successful attachment of azide group onto the
surface of silica was confirmed according to FT-IR spectra
(Figs. S1 and S2).
2.4 General Procedure for Suzuki–Miyaura
Coupling Reaction
Aryl halide (1 mmol), K2CO3 (2.0 mmol), phenylboronic
acid (1.2 mmol), click-catalyst A (0.2 mol%) and 3 mL
H2O/EtOH (1:1, V/V) were mixed in a round-bottom flask
equipped with condenser and under an air atmosphere. The
mixture was heated in an oil bath at 70 °C (for the aryl bro-
mides) and stirred continuously during the reaction and
monitored by thin-layer chromatography (TLC) and gas
chromatography (GC). After the reaction was complete, the
mixture was diluted with EtOH. The catalyst was separated
by centrifugation, washed with EtOH/H2O and then reused
for recycling experiments without further pretreatment.
Then, the coupled product was extracted with EtOAc and
the organic phase was dried over MgSO4, filtered and con-
centrated. The residue was purified by recrystallization or
silica gel column chromatography (n-hexane:EtOAc), and
was characterized.
3 Results and Discussion
Silica-supported nano-palladium click-catalysts A and
B were prepared following the route shown in Scheme 1.
The all processes of click modification and catalyst immo-
bilization were investigated by FTIR analysis. At first, the
silica was functionalized with azide group through a simple
1 3

Triazole-Functionalized Silica Supported Palladium(II) Complex: A Novel and Highly Active…
of Cu2O catalyst in aqueous methanol. During the “click”
reaction, azide groups react with alkyne sections to obtain
click-supports A and B containing the 1,2,3-triazole units.
The nearly complete conversion of all azide groups was con-
firmed by FTIR spectra, which showed disappearance of the
absorption band at 2100 cm−1 attributed to the N3 (Fig. 2).
The amount of click-ligand grafted onto the surface of silica
was estimated by elemental analysis of the nitrogen content
using click-supports A and B, and were found to be 1.0 and
0.95 mmolg−1, respectively.
The resulting click-supports were further treated with
Pd(OAc)2 in toluene at room temperature to afford the
desired solid click-catalysts A and B, meanwhile, their pal-
ladium loading was respectively measured to be 0.38 and
0.31 mmolg−1 by inductively coupled plasma (ICP).
Field emission scanning electron microscopy (FE-SEM)
images in Fig. 3 demonstrate a comparison of the sur-
face morphology and the particles distribution in virginal
silica, click-catalysts A and B. As shown in the FE-SEM
micrographs, the surface morphology of virginal silica
was clearly changed after click-modification and catalyst
immobilization.
Fig. 2 FTIR spectra of azide-functionalized silica (a), click-support A
(b) and click-support B (c)
procedure to achieve click-capability, and the presence of
azide group was confirmed by the appearance of the N3 band
at 2100 cm−1 in FT-IR spectra (Figs. S1 and S2). The result-
ing azide-functionalized silica was treated with an excess
of propargyl alcohol or propargyl amine in the presence
Fig. 3 FE-SEM images of
virginal silica (a), click-catalysts
A (b) and B (c)
1 3

A. R. Hajipour, F. Mohammadsaleh
Fig. 4 TEM images (a) and EDX spectrum (b) of the click-catalyst A
In addition to the conventional SEM images, the mean-
ingful pictures of the element distribution of the surface of
click-catalyst A were provided by EDX mapping (Fig. S3).
Chelation of the click-ligand grafted on the silica surface
with palladium(II) ions prevent the aggregation of particles
in the catalyst system. Transmission Electron Microscopy
(TEM) image of the click-catalyst B in the Fig. 4 shows the
distribution of particles and confirm that the nano-sized par-
ticles have been well distributed throughout the catalyst sys-
tem, and the nanoparticle average size was found to be about
5–6 nm. The presence of palladium complexes is appearing
as black spots. By the SEM-EDX analysis the presence of
palladium in the catalyst was approved (Fig. 4b).
To further investigation of the catalysts A and
B, efficiency of these catalysts was evaluated in the
Suzuki–Miyaura cross-coupling reactions.
At first, the coupling of 4-bromonitrobenzene and phe-
nylboronic acid was chosen as the model reaction to begin
this investigation. It was found that by employing click-
catalysts A or B (0.2 mol%), K2CO3 and mixed solvent
EtOH/H2O (50:50) at 70 °C, the corresponding biaryl was
obtained in 100% yield (GC-yield) after 30 min. This result
demonstrates that both catalysts are effective in this reaction
system.
Despite the excellent catalytic efficiency of the click-cat-
alysts in the first use, we carried out further investigations
1 3

Triazole-Functionalized Silica Supported Palladium(II) Complex: A Novel and Highly Active…
X-Ray reflective diffraction (XRD) of catalysts was
carried out before and after use in reaction system to get
detailed information about the architecture of the novel
click-catalysts A and B (Fig. 6). The XRD patterns of both
catalysts before using show only characteristic peaks of
amorphous SiO2 at 2θ=22°, and no characteristic peaks for
Pd (0) nanoparticles are observed. The XRD pattern of cata-
lyst A after five times use and recycling from the reaction
media displays the slight appearance of new peaks attributed
to Pd (0) species, which verifies in situ formation of par-
tially metallic palladium nanoparticles during the reaction.
However, in the XRD pattern of catalyst B after using and
recovery there was no appearance of new peaks. It indicates
that the Pd(II) complexes on the surface of solid catalysts
could be still maintained after use in the reaction.
Based on these results two catalysts A and B exhibit dif-
ferent behaviors and properties, which can probably be due
to the difference of their structures.
1,2,3-Triazoles having several donor sites are potentially
powerful chelator for metal coordination [49]. In the nitro-
gen-coordination mode of triazoles, DFT calculations have
shown that N3 is a better donor compared to N2 [50]. How-
ever, as shown in Scheme 2, when there are other donor sites
nearby, synergetic coordination through N3 and additional
donor site is possible to form a bi- or poly-dentate chelator
[49, 51–53]. Tale et al. have recently reported 1, 2, 3-triazol-
4-yl methanol-based ligands as the accelerator in copper cat-
alyzed azide–alkyne cycloaddition (CuAAC) reaction [54].
They found that, the presence of hydroxymethyl (–CH2OH)
moiety along with electron releasing substituent on the ter-
minal benzene ring of 1, 2, 3-triazole in these click-ligands,
has favorable effect on the rate enhancement of the CuAAC.
Based on these reports, we assumed synergetic coordina-
tion of N-atom of triazolyl ligand and OH or NH2 groups
present in the structure of studied catalysts to palladium that
leads to the formation of Pd(II) complexes on the support
surface.
Fig. 5 The reusability results of the click-catalysts A and B. Reaction
conditions 4-nitrobromobenzene (1 equiv.), phenylboronic acid (1.2
equiv.), H2O/EtOH (1:1), K2CO3 (2 equiv.), 70 °C. GC conversion
focusing on their reusability and the catalyst recycling. The
recovery of catalyst becomes an important factor due to eco-
nomic and environmental concerns, especially for expensive
metal catalysts such as Pd. Therefore, the recyclability of
these click-catalysts was examined by using the above model
reaction by simple filtration and washing with EtOH/H2O.
These recycling studies have shown that the catalyst A could
be readily recovered and reused several times without sig-
nificant loss of activity. However, the catalyst B could be
also recycled, although a slight reduction in the yield of the
coupled product and an increase in the reaction time were
observed (Fig. 5).
Further characterization of the click-catalysts A and B
was performed by UV–Vis diffuse reflectance spectroscopy,
as shown in Fig. 7. We studied the UV–Vis spectra of click-
supports and the resulting catalysts after complexation in the
range of 200–800 nm. The UV–Vis spectra of Pd-catalysts
show a slight red shift compared with their corresponding
supports, which could confirm palladium complexation
and suggests that palladium existing on the solid supports
with a coordinated mode. Before the metallation reactions,
the click-supports A and B showed a distinct π–π* transi-
tion around 250–300 nm. The wide band in the range of
350–400 nm after coordination is probably due to the n→π*
transition or to the ligand-to-metal charge-transfer transition
(LMCT).
As a result, click-catalyst A showed the better perfor-
mance and the more reasonable reusability than those of
the click-catalyst B, and could successfully reused up to five
cycles.
Next, we checked the Pd leaching for the click-cata-
lysts. Importantly, no significant leaching of the Pd could
be detected during the experiments as ascertained by ICP
analysis of click-catalysts after recycling from reaction mix-
ture, indicating that the reactions are truly heterogeneous in
nature. Hence, metal leaching does not seem to be the reason
of the activity shortcoming of click-catalyst B during the
recycling experiments.
1 3

A. R. Hajipour, F. Mohammadsaleh
Fig. 6 XRD pattern of the fresh
(a) and recovered click-catalysts
A and B (b)
Scheme 2 The coordination mode of chelating triazole with transi-
tion metal (M)
Although both studied catalysts showed relatively good
performance, but we chose click-catalyst A as the superior
catalyst and hence, continued further studies using this cata-
lyst. Figure 8 shows the FTIR spectrum of the catalyst A
compared with the FTIR spectra of click-support A and pure
Pd(OAc)2. The spectrum of catalyst A showed the charac-
teristic bands of (COO) at 1562 and 1410 cm−1, evidencing
the presence of OAc groups.
Fig. 7 UV–Vis spectra of click-catalyst A (a), support A (b), catalyst
B (c) and support B (d)
1 3

Triazole-Functionalized Silica Supported Palladium(II) Complex: A Novel and Highly Active…
Thermogravimetric (TG) analysis was used to study
the thermal behaviour and stability of the supported click-
catalyst A (Fig. 9). The measurement was carried out in
the temperature range of 60–800 °C with a heating rate of
20 °C min−1 under nitrogen atmosphere. As shown in the
TG curve, an initial weight loss of 2.7% is observed up to
150 °C, which is due to the elimination of water adsorbed
onto the silica. Thermal degradation of the catalyst attrib-
uted to the decomposition of the organic groups on the silica
surface occurred after 300 °C, which revealed the excellent
thermal stability. Hence, the click-catalyst A found to be
very stable and could be used within a broad temperature
range.
As already observed for the catalyst A, this showed
excellent catalytic activity and recyclability in Suzuki
cross-coupling reaction of 4-bromonitrobenzene and phe-
nylboronic acid as the model reaction in green solvent H2O/
EtOH (50:50) at 70 °C. Being encouraged we attempted to
optimize the reaction conditions, and examined the effect of
various reaction parameters such as base and temperature
on the yields and reaction times of a series of screening
experiments carried out by using the above model reaction
as summarized in Table 1.
Fig. 8 FTIR spectra of click-support A, catalyst A and Pd(OAc)2
Due to green chemistry and environmental concerns,
only water and alcohol was chosen as solvent in all experi-
ments. The advantage of the aqueous mixed-solvents might
be attributed to the good solubility of the organic reactants
and the inorganic base. Among the different bases tested
such as K2CO3, KOH and K3PO4, K2CO3 and KOH were
found to act as the effective bases (Table 1, entries 1–3), that
we continue our work with k2CO3. The reaction temperature
and the catalyst loadings were also examined for the title
reaction. The yield of the model reaction increased when
the reaction was conducted at 70 °C (Table 1, entries 2, 5).
A catalyst loading of 0.2 mol% was found to be optimal
Fig. 9 TGA/DTA analysis of click-catalyst A
Table 1 Optimization of the reaction conditions for the Suzuki–Miyaura coupling
Entry
Base
Catalyst (mol%)
T (°C)
t (min)
Yield (%)a
1
2
3
4
5
6
7
K3PO4
K2CO3
KOH
0.2
0.2
0.2
0.2
0.35
none
0.1
70
70
70
70
RT
70
70
30
30
30
30
300
60
20
100
100
Trace
Trace
–
None
K2CO3
K2CO3
K2CO3
30
82
Reaction conditions 4-nitrobromobenzene (0.5 mmol), Phenylboronic acid (0.52 mmol), H2O/EtOH (1:1) 3 mL, base (2 equiv.)
aGC yield
1 3

A. R. Hajipour, F. Mohammadsaleh
Table 2 Scope of aryl halides in Suzuki–Miyaura coupling reaction with phenylboronic acid
Entry
R (X)
T (°C)
t (min)
Yield (%)a
TONb
TOF (min−1 c
)
1
4-NO2 (Br)
70
30
30
30
90
90
20
20
20
20
20
20
50
45
30
30
30
120
50
60
97
97
94
90
81
97
95
95
96
91
97
95
90
93
95
93
–
485
485
470
450
405
485
475
475
480
455
485
475
450
465
475
465
–
16.2
16.2
15.7
5
Run2
Run3
Run4
Run5
2
3
4
5
6
7
8
4.5
4-NO2 (I)
4-OCH3 (I)
4-COCH3 (I)
3-NO2 (I)
H (I)
4-COCH3 (Br)
4-OCH3 (Br)
H (Br)
4-COH (Br)
4-CN (Br)
4-Cl (Br)
4-NO2 (Cl)
2-Cl (Br)
RT
RT
RT
RT
RT
70
70
70
70
70
70
70
70
70
24.25
23.75
23.75
24
22.75
24.25
9.5
9
10
10
11
12
13
14
15
15.5
15.8
15.5
–
9.3
6.7
90
80
450
400
Reaction conditions aryl halide (1 mmol), phenylboronic acid (1.2 mmol), K2CO3 (2 mmol), solvent 3 mL
aIsolated yield
bTurnover number (TON) calculated as moles of product formed/moles of catalyst used
cTurnover frequency (TOF) calculated as moles of product formed/moles of catalyst used per min
(Table 1, entries 2, 6, 7) by employing various amounts
under optimized conditions. A lower catalyst concentration
usually led to slow reactions, increasing the amount of cata-
lyst shortened the reaction time.
Additionally, 4-chloronitrobenzene was chosen as the
challenging substrates, however, the catalytic system was
less effective even when prolonged reaction time (Table 2,
entry 13).
After optimization, catalyst A was applied to the Suzuki
coupling of aryl halides with phenylboronic acid, and a vari-
ous aryl iodides and bromides reacted efficiently to give the
corresponding biaryl derivatives in good to excellent yields.
As shown in Table 2, electron-poor as well as elec-
tron-rich aryl halides proceeded effectively to afford the
coupled products. This catalyst was compatible with a
wide range of functional groups such as nitro, methoxy,
cyano and carbonyl on the aryl halides. All aryl iodides
were rapidly converted to the corresponding Suzuki prod-
ucts at room temperature in excellent yields. However,
Aryl bromides required heat to react with phenylboronic
acid and afforded the corresponding products at 70 °C.
As previously reported by other groups, different cata-
lytic systems based on palladium catalysts combined
with the triazole ligand promote the Pd-catalyzed reac-
tions such as coupling or oxidation reactions [20, 33, 34].
Reaction temperature, reaction rate and selectivity are
important factors that must be considered in this transfor-
mations, especially for heterogeneous catalysts in addi-
tion to their stability and reusability. The present catalytic
method was compared with some procedures reported for
the Suzuki–Miyaura reaction of 4-iodoanisole with phe-
nylboronic acid in published literatures, and the results
have been shown in Table 3. Of course, the reaction condi-
tions are different, but the catalyst used in this study might
1 3

Triazole-Functionalized Silica Supported Palladium(II) Complex: A Novel and Highly Active…
Table 3 Comparison of catalytic activities with literature examples for the Suzuki reaction between 4-iodoanisole and phenylboronic acid
Entry
Catalyst (mol%)
Reaction conditions
Yield (%) Ref.
TOFa
1
2
3
4
5
6
7
8
PS-btsu-Pd(II) complex (0.5)
2-Pyridyl-1,2,3-triazole-Pd(II) complex (0.5)
[Pd(OAc)2]n@dendrimer, 113 (0.5)
Pd(0)-Montmorillonite clay (0.07)
Hollow Fe3O4-NH2-Pd (1.0)
Cellulose acetate (CA)/Pd(0) (0.5)
Fe3O4@PUNP-Pd (0.1)
H2O+MeOH (1:1)/RT/LiOH·H2O/4 h
EtOH:H2O (7.5 mL:5 mL)/RT/K2CO3/48 h
EtOH:H2O (7.5 mL:5 mL)/50 °C/K2CO3/48 h
H2O/60 °C/K2CO3/1 h
EtOH/80 °C/K2CO3/30 min
H2O/100 °C/K2CO3/3 h
90 [55]
77 [20]
96 [20]
94 [56]
96 [57]
97 [58]
96 [59]
95
0.75
0.038
0.067
22.3
3.2
1.1
16
H2O/90 °C/K2CO3/1 h/Argon atmosphere
EtOH:H2O (1:1)/RT/K2CO3/20 min
Present work
23.75
aTurnover frequency (TOF) calculated as moles of product formed/moles of catalyst used per min
be one of the best catalysts with regards to lower reaction
time and temperature.
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Affiliations
Abdol Reza Hajipour1,2 · Fatemeh Mohammadsaleh1
2
* Abdol Reza Hajipour
Department of Pharmacology, Medical School, University
haji@cc.iut.ac.ir
of Wisconsin, 1300 University Avenue, Madison,
WI 53706-1532, USA
1
Pharmaceutical Research Laboratory, Department
of Chemistry, Isfahan University of Technology,
84156 Isfahan, Islamic Republic of Iran
1 3