Ultrafine Pt nanoparticles supported on a dendrimer containing thiol groups: An efficient catalyst for the synthesis of benzimidazoles and benzothiazoles from benzyl alcohol derivatives in water

Article abstract of DOI:10.1039/d0ra06471h

A novel and unique platform was prepared based on a dendrimer containing thiol groups supported on nanosilica (nSTDP), and ultrafine platinum nanoparticles were synthesized and immobilized on the thiol decorated branches of nSTPD. The new catalyst, (Ptnp?nSTDP), was characterized by different techniques such as FE-SEM, TEM, ICP, XPS and DR UV-vis. This heterogeneous catalyst presented an outstanding performance for the synthesis of benzimidazole and benzothiazole derivatives through a reaction between benzyl alcohol derivatives and 2-aminothiophenol or 1,2-phenylenediamine. No requirement for the pre-reduction of catalysts and using water as a green solvent make it an individual catalyst for these reactions. Furthermore, the catalyst can be easily recovered and reused five consecutive times in the production of benzimidazoles and benzothiazoles without significant leaching of Pt and loss of its activity which illustrated the chemical stability of the catalyst during the reaction.

Full text of DOI:10.1039/d0ra06471h

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Ultrafine Pt nanoparticles supported on
a dendrimer containing thiol groups: an efficient
catalyst for the synthesis of benzimidazoles and
benzothiazoles from benzyl alcohol derivatives in
water†
Cite this: RSC Adv., 2020, 10, 33137
Zahra Zamani Nori, Amir Landarani-Isfahani,
Mehrnaz Bahadori,
*
*
Majid Moghadam,
Valiollah Mirkhani, Shahram Tangestaninejad
and Iraj Mohammadpoor-Baltork
A novel and unique platform was prepared based on a dendrimer containing thiol groups supported on
nanosilica (nSTDP), and ultrafine platinum nanoparticles were synthesized and immobilized on the thiol
decorated branches of nSTPD. The new catalyst, (Ptnp@nSTDP), was characterized by different
techniques such as FE-SEM, TEM, ICP, XPS and DR UV-vis. This heterogeneous catalyst presented an
outstanding performance for the synthesis of benzimidazole and benzothiazole derivatives through
a reaction between benzyl alcohol derivatives and 2-aminothiophenol or 1,2-phenylenediamine. No
requirement for the pre-reduction of catalysts and using water as a green solvent make it an individual
catalyst for these reactions. Furthermore, the catalyst can be easily recovered and reused five
consecutive times in the production of benzimidazoles and benzothiazoles without significant leaching
of Pt and loss of its activity which illustrated the chemical stability of the catalyst during the reaction.
Received 25th July 2020
Accepted 28th August 2020
DOI: 10.1039/d0ra06471h
rsc.li/rsc-advances
efficiently in catalytic systems, (b) control the size of nano-
particles and production of homogeneous nano-metals with
Introduction
Noble metals have attracted more attention in catalytic systems1
and Pt-based catalysts like Pt nanoparticles are one of the most
useful and valuable catalysts in chemical reactions.2,3 Due to the
high cost and scarcity of platinum metal, its immobilizing with
preservation of the activity and homogeneous size distribution
of nanoparticles is one of the main topics in environmental,
economic and safety issues. Different supports such as poly-
mers,4 metal–organic frameworks,5 metal oxides6 and den-
drimers7 have been used for immobilizing the Pt nanoparticles.
But synthesis of structures with a proper surface which mini-
mize the size of particles (less than 5 nm) and maximize the
efficiency of catalyst by increasing the surface active sites, is still
a challenge.8–10 Consequently, for the employment of all of the
advantages of Pt-nanoparticles, it is important to nd a support
that can produce very small nanoparticles and prevents their
agglomeration. Dendrimer-encapsulated nanoparticles (DES)
could be considered as excellent candidates for hosting nano-
particles because of: (a) hyperbranched structure which leads to
adsorption of the substrates for performing of reaction
narrow size distribution, and (c) tunability of branches for
specic interaction with nano-metals and avoiding the
agglomeration of them aer synthesis.11,12
Chandler and co-workers synthesized polyamidoamine
(PAMAM) dendrimers for encapsulation of Pt nanoparticles and
used it as a precursor for the production of nanoparticle cata-
lysts.13 Crooks et. al. have immobilized dendrimer-encapsulated
Pt nanoparticles on glassy carbon electrodes and used it for the
Department of Chemistry, Catalysis Division, University of Isfahan, Isfahan
81746-73441, Iran. E-mail: moghadamm@sci.ui.ac.ir; mirkhani@sci.ui.ac.ir
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra06471h
Scheme 1 The structure of immobilized Pt-nano particles on sulfur
decorated dendrimer.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 33137–33147 | 33137

View Article Online
RSC Advances
Paper
alcohols or aldehydes under acceptor- and additive-free condi-
tions.25 But some problems such as pre-reduction of catalyst,
using an organic solvent or low yield of the product aer
recovery of catalyst should be considered.
On the other hands, utilizing water as a green solvent is the
most substantial issue in these reactions.
Scheme 2 Synthesis of benzimidazoles and benzothiazoles from
alcohols catalyzed by Ptnp@nSTDP.
In this project, a sulfur decorated dendrimer based on nano
silica was designed, synthesized and applied as a convenient
support for the immobilization of Pt-nanoparticles. The activity
of the obtained catalyst (Ptnp@nSTDP) was checked for the
synthesis of benzimidazoles or benzothiazoles with benzyl
alcohol derivatives in water under mild conditions.
O2 reduction reaction (ORR).14 Based on the reports on the
design of dendrimers with different functional groups such as
nitrogen,15 oxygen16 and sulfur17 and the good interaction
between platinum and sulfur,18,19 the rst step of this study is
decoration of a dendrimer with sulfur functional group for
stabilizing of Pt-nanoparticles. The selection of appropriate
central core was the next step in the architecture of the den-
drimer structure due to the difficulty of dendrimers separation
and the importance of recovery and reusability of DES.20
Regarding the ability to be functionalized and benets of nano-
size structures, nano-silica sounds a very intelligently choice for
central core of designed dendrimer (Scheme 1).
Results and discussion
Characterization
As illustrated in Scheme 3, the synthesis of Ptnp@nSTDP is
started with functionalization of nano-silica and continued by
To conrm the catalytic activity of immobilized Pt-
nanoparticles on designed dendrimer, the synthesis of benz-
imidazoles and benzothiazoles was investigated (Scheme 2).
Benzimidazoles and benzothiazoles specially 2-substituted
ones, are a valuable class of heterocycles that have many
applications in biological activities including antitumor, anti-
cancer, and antibacterial,21 agricultural chemicals,22 and
organic functional materials.23 Shiraishi and co-workers re-
ported an efficient dehydrogenative coupling reaction for the
synthesis of benzimidazole compounds from primary alco-
hols.24 Shimizu et al. reported a new Pt based catalyst for
synthesis of 2-substituted benzothiazoles and benzimidazoles
from 2-aminothiophenol and 1,2-phenylenediamine with
Fig. 1 (a) TEM image of Ptnp@nSTDP and (b) particle size distribution
histogram of Ptnp@nSTDP.
Scheme 3 Synthesis route of Ptnp@nSTPD.
33138 | RSC Adv., 2020, 10, 33137–33147
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Fig. 2 FE-SEM images of Ptnp@nSTDP.
growing the thiol dendrimer (divergent growth strategy). Then,
the Pt nanoparticles were trapped by the thiol groups of den-
drimer via reduction of K2PtCl4 with NaBH4 to obtain the
Ptnp@nSTDP.
The step by step synthesis of the Ptnp@nSTDP was moni-
tored by FT-IR spectroscopy. The FTIR spectra for PT-nSiO2,
BCC1-nSiO2, G1, BCC2-nSiO2, and G2 are illustrated in Fig. S1.†
The bands at 500, 850, 1150 and 3000–3700 cmꢀ1 are attributed
to Si–O asymmetric bending, Si–O symmetric stretching, Si–O–
Si asymmetric stretching and O–H stretching in nano-silica or
adsorbed water, respectively, which conrmed the presence of
nano-silica in all structures. The C–H bending band at 1400 and
C–H stretching bands at 2800–3000 cmꢀ1 display the growth of
dendrimer branches in the structures. The characteristic bands
at 1700 and 1750 cmꢀ1 correspond to carbonyl groups.
The amount of platinum nanoparticles supported on the
dendritic polymer was measured by ICP which showed a value
of about 0.06 mmol per gram of the heterogeneous catalyst. One
of the main targets of this research is achieving of Pt nano-
particle with size less than 5 nm. Based on TEM images (Fig. 1),
we successfully synthesized very small Pt nanoparticle with an
excellent size distribution around 1–2 nm.
Fig. 4 Elemental mapping (S, C, O and Pt) of Ptnp@nSTDP.
By considering the 1.5 nm for average diameter of nano-
particles and density of platinum (21.45 g cmꢀ3), we can use NPt
¼ d(p/6)D3 equation for calculation of the number of Pt nano-
particles in one cluster.26–28 Where d is the density of Pt nano-
particles and D is the average diameter of nanoparticles. Based
on this formula, the number of Pt atoms in the one cluster was
calculated as 117 atoms and considering the result of ICP
Fig.
nSTDP.
5
Diffuse reflectance UV-vis spectrum of Ptnp@nSTDP and
RSC Adv., 2020, 10, 33137–33147 | 33139
Fig. 3 SEM-EDX spectrum of Ptnp@nSTDP.
This journal is © The Royal Society of Chemistry 2020

View Article Online
RSC Advances
Paper
absorption spectrum of thiol dendrimer shows peaks at about 225
and 285 nm (Fig. 5). The UV-vis spectrum of Ptnp@nSTDP illus-
trated two peaks, which attributed to thiol dendrimer and two
absorption peaks at 216 and 264 nm which are assigned to Pt
nanoparticles,29 are vanished the beneath of dendrimer absorption.
The X-ray photoelectron spectroscopy (XPS) was carried out
for investigation of the detailed elemental composition and
chemical state of the Pt nanoparticles (Fig. 6). The XPS survey
spectrum shows the peaks attributed to C 1s, Si 2p, O 1s, S 2p
and Pt 4f in the nano catalyst which is in accordance with EDX
results. The doublet peak at 76.0 and 72.8 eV with D ¼ 3.2
corresponds to Pt 4f7/2 and 4f5/2, respectively and proved the
reduction of Pt(II) to metallic Pt in the dendrimer branches.
Furthermore, the binding energy for Pt shied to higher value
compared with Pt bulk while it is lower than the binding energy
for Pt(II).30,31 This shi in binding energy has been corresponded
to two factors: particle size and ligand effect.32 Regarding to this
shi in ultrane Pt nanoparticle33,34 and the ligand effect in
same-size Pt nanoparticles,31 this shi can be attributed to
effect of ligand and interaction between Pt on the surface of
nanoparticles and sulfurs in the dendrimer structure.35–37
Thermal gravimeter analysis (TGA) curves for PT-nSiO2,
BCC1-nSiO2, G1, BCC2-nSiO2 and G2 (nSTDP) were displayed in
Fig. S2.† The PT-nSiO2, BCC1-nSiO2 and G1 decomposed
between 350–450 ꢂC and BCC2-nSiO2 showed two distinguished
steps related to the decomposition of different functional
Fig. 6 The XPS spectra of catalyst: (a) the elemental survey scan, (b) Pt
4f7/2 and Pt 4f5/2 binding energies.
analysis (0.06 mmol Pt per g), there are 31 ꢁ 1016 Pt atom in one
gram of catalyst.
The overall morphology of Ptnp@nSTDP nano-catalyst was
evaluated by eld emission scanning electron microscopy (FE-
SEM) (Fig. 2). As can be seen the catalyst particles are spher-
ical with an average size of about 50 nm. The presence of Si, S,
and Pt in the texture of the Ptnp@nSTDP was proved using the
energy dispersive X-ray (EDX) results, collected from SEM
analysis (Fig. 3). Also, elemental mapping of Ptnp@nSTDP
displays the uniform dispersion of Pt nanoparticles in the
nSTDP (Fig. 4).
The UV-vis spectrum provided informative evidence for immo-
bilization of the Pt nanoparticles on thiol dendrimer. The
Table 1 Optimization of the conditions for synthesis of benzimidazoles and benzothiazoles
Ion liquid
(1 mmol)
Yield (%)
(X ¼ NH)
Yield (%)
(X ¼ S)
Entry
Catalyst (mg, mmol Pt)
Solvent (2 mL)
T (ꢂC)
Time (min)
1
2
3
4
5
6
7
8
—
TBPB
TBPB
TBPB
TBPB
TBPB
TBPB
TBPB
TBPB
TBPB
TBPB
TBPB
TBPB
TBPB
TBPB
TBPB
TBPB
H2O
H2O
H2O
H2O
H2O
H2O
H2O
—
DMF
CH3CN
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
80
80
80
80
80
80
80
80
120
70
80
80
80
60
90
100
80
80
80
80
80
90
90
90
90
90
30
120
90
90
90
90
90
90
90
90
90
90
90
90
90
90
—
3
5
—
4
5
nSiO2 (30)
nSTDP (30)
Ptnp@nSiO2 (30)
75
97
70
97
—
75
30
78
97
97
70
97
97
8
65
93
73
94
—
75
35
78
95
97
70
97
96
8
Ptnp@nSTDP (30, 0.0018)
Ptnp@nSTDP (30, 0.0018)
Ptnp@nSTDP (30, 0.0018)
Ptnp@nSTDP (30, 0.0018)
Ptnp@nSTDP (30, 0.0018)
Ptnp@nSTDP (30, 0.0018)
Ptnp@nSTDP (10, 0.0006)
Ptnp@nSTDP (50, 0.003)
Ptnp@nSTDP (100, 0.006)
Ptnp@nSTDP (30, 0.0018)
Ptnp@nSTDP (30, 0.0018)
Ptnp@nSTDP (30, 0.0018)
Ptnp@nSTDP (30, 0.0018)
Ptnp@nSTDP (30, 0.0018)
Ptnp@nSTDP (30, 0.0018)
Ptnp@nSTDP (30, 0.0018)
Ptnp@nSTDP (30, 0.0018)
9
10
11
12
13
14
15
16
17
18
19
20
21
—
TBAB
[BMIM]Br
[BMIM]Cl
[BMIM]NO3
77
60
45
42
80
65
45
40
33140 | RSC Adv., 2020, 10, 33137–33147
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Table 2 Synthesis of benzimidazoles catalyzed by Ptnp@nSTDP
Entry
1
G
Alcohol
Product
Time (min)
95
Yield (%)
94
TOF (hꢀ1
329.8
)
H
2
3
H
H
90
97
97
359.2
269.4
120
4
5
4-Me
H
180
100
89
96
164.8
300.6
6
7
4-Cl
190
200
90
84
145.1
127.2
4-Me
8
H
170
90
160.7
9
H
210
210
89
90
127.1
128.5
10
4-Me
11
H
95
91
303.3
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 33137–33147 | 33141

View Article Online
RSC Advances
Paper
Table 3 Synthesis of benzothiazoles catalyzed by Ptnp@nSTDP
Entry
1
Alcohol
Product
Time (min)
95
Yield (%)
94
TOF (hꢀ1
329.8
)
2
3
90
97
97
359.2
269.4
120
4
5
6
125
95
91
94
96
227.5
313.3
240.0
120
7
8
9
190
150
120
83
90
92
133.8
180
230
10
100
90
281.2
groups. The decomposition of G2 started before 300 ꢂC is benzimidazoles and benzothiazoles from different benzyl
probably due to the presence of different organic moieties in the alcohols. In this regard, the reaction between 1,2-phenylenedi-
structure which is not distinct because of variety of functional amine (1 mmol) and 4-chlorobenzylalcohol (1 mmol) was
groups. The weight of residue was decreased as expected with chosen as a model reaction. The effect of different reaction
the growth of dendrimer on the nano silica step by step.
parameters such as type of solvent, temperature, amount of
Ptnp@nSTDP catalyst and the inuence of ionic liquids was
investigated on the reaction yield (Table 1). For the investigation
of the efficiency of the catalyst some model reactions were
designed and performed in the absence of catalyst and in the
Catalytic activity of Ptnp@nSTDP
Aer the Ptnp@nSTDP catalyst was characterized by different
technical analyses, its ability was explored in the synthesis of
33142 | RSC Adv., 2020, 10, 33137–33147
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
presence of nSiO2, nSTDP, Ptnp@nSiO2, and Ptnp@nSTDP (Table 1, entry 5) was used as the best conditions for synthesis
(Table 1, entries 2–5). The results showed that Ptnp@nSTDP was of different benzothiazoles.
the most effective catalyst for this reaction. The yield in the
Aer determination of ideal reaction conditions, various
absence of catalyst and in the presence of nSiO2 and nSTDP was aromatic benzylalcohols were reacted efficiently with 1,2-phe-
negligible which illustrated that nSiO2 and thiol containing nylenediamine in the presence of catalytic amounts of Ptnp@-
dendritic polymer cannot catalyze the reaction.
nSTDP at 80 ꢂC to afford the conforming benzimidazoles in
On the other hand, the Ptnp@nSiO2 and Ptnp@nSTDP high yields (89–97%) (Table 2). Furthermore, this catalytic
promoted the reaction yield, and Ptnp@nSTDP showed higher system was used for the reaction of different benzylalcohols
activity. This is due to the well dispersion and small size of Pt with 2-aminothiophenol for the synthesis of benzothiazoles.
nanoparticles on the nSTDP which provides more active sites Based on the results in Table 3, the high yield of benzothiazoles
for catalyzing the reaction. Investigation on reaction time (more than 90%) was obtained for all of the substrates. Two sets
showed that the best time for gaining the high yield of benz- of reactions illustrated very excellent performance in the water
imidazole is 90 min (Table 1, entries 5–7).
for both electron-donating and electron-withdrawing alcohols.
Based on the literature,25 the plausible mechanism is pre-
For evaluation of the effect of solvent on the conversion, the
reaction was carried out under solvent-free conditions and no sented in Fig. 7. In the rst step, Pt is converted to PtH2 and
product was produced. DMF, chloroform and water as solvent catalyzes dehydrogenation of benzyl alcohol to give the corre-
displayed similar yields for DMF and water, and regarding the sponding benzaldehyde. o-Phenylenediamine (or 2-amino-
green chemistry aspects, water was selected as the best solvent. thiophenol) and benzaldehyde produce the
C through
Furthermore, the amount of catalyst was studied, and 30 mg intermediate A and B. In this step, water accelerates the
was chosen as the desired amount. The model reaction was conversion to C through 5-exo-tet cyclization via a tetrahedral
carried out at 60, 80, 90 and 100 C and the same results were intermediate (Baldwins rules).38,39 In the last step, the
ꢂ
ꢂ
achieved at 80, 90 and 100 C. As expected, the reaction in the
absence of ionic liquid was less effective and for more investi-
gation, different ionic liquids were tested in the model reaction
and TBPB was selected as phase transfer catalyst (Table 1,
entries 17–21). Due to the bigger size of cation, TBPB is more
useful than ammonium and imadazolium-based ionic liquids.
The optimization of the parameters continued with reaction
of 2-aminothiophenol (1 mmol) with 4-chlorobenzylalcohol (1
mmol) as a model reaction for the optimization of the reaction
conditions in the synthesis of benzothiazoles. The data, which
are summarized in Table 1, showed similar results obtained in
the synthesis of benzimidazoles. Finally, 30 mg of catalyst
(containing 0.0018 mmol Ptnp) in H2O and 1 mg TBPB at 80 ꢂC
Fig. 8 (a) The recycling of Ptnp@nSTDP in the synthesis of benz-
Fig. 7 Purposed mechanism for synthesis of benzimidazole and imidazoles between benzylalcohols and phenylenediamine under
benzothiazole in the presence of Pt nanoparticle as catalyst (X ¼ NH, optimized conditions. (b) UV-vis spectra for fresh Ptnp@nSTDP and
S).
reused Ptnp@nSTDP.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 33137–33147 | 33143

View Article Online
RSC Advances
Paper
benzimidazole (or benzothiazole) is produced by dehydroge- nanoparticles have been presented in Table 4. Pt nanoparticle
nation of PtH2 and regeneration of Pt.
on the TiO2 was used for the synthesis of benzimidazole that
aer 24 h, the yield was 60% and under photoirradiation
conditions aer 12 h, 82% of benzimidazole was produced
(Table 4, entries 1 and 3). In this work, Ptnp@nSTDP can
produce 97% of the corresponding benzimidazole aer 90 min
in H2O which is an excellent result compared to the previous
catalysts. Furthermore, the comparison of this catalyst and Pt/
Al2O3 (Table 4, entry 2) shows a very individual performance of
Ptnp@nSTDP for the synthesis of benzothiazole. Very ultrane
Pt nanoparticles and high stability of catalyst during the reac-
tion are two important factors for this excellent operation.
Catalyst recycling and reuse
Since the catalyst recycling is the most important issue for
a heterogeneous catalyst from economic, environmental and
industrial points of view, we investigated the recycling of the
catalyst for the synthesis of benzimidazoles by the reaction
between 4-chlorobenzyl alcohol with 1,2-phenylenediamine
under the optimized conditions.
Aer completion of each reaction the catalyst was washed
with ethyl acetate and was separated by centrifugation; dried in
a vacuum oven at 80 ꢂC and then reused. As shown in Fig. 8, DR
UV-vis spectrum of the catalyst aer ve catalytic cycles showed
no signicant change compared to UV-vis spectrum of the fresh
catalyst. The amount of PtnpnSTDP catalyst leached was
Experimental
Methods
measured by ICP. The result showed that the amount of plat- Melting points were determined using a Stuart Scientic SMP2
inum in the recovered PtnpnSTDP catalyst aer the ve cycles apparatus. 1H and 13C (400 and 100 MHz) spectra were recorded
was 0.0564 mmol gꢀ1 to produce benzimidazole (about 9% of in CDCl3 solvent on a Bruker-Avance 400 spectrometer.
the initial Pt is leached).
Elemental analyses were done on a LECO, CHNS-932 analyzer.
The role of alkanethiols in the stabilization of Pt nano- Thermogravimetric analyses (TGA) were carried out on a Mettler
particles was investigated aer rst prepared gold alkane- TG50 instrument under air ow at a uniform heating rate of
thiolated nanoparticles40 and proved in previous litera-
5
ꢂC minꢀ1 in the range of 30–600 ꢂC. The UV-vis diffuse
tures.41–43 The Pt nanoparticles have also stabilized through reectance spectra of the samples were recorded by a JASCO V-
encapsulation in dendrimers by protecting them from 670 spectrophotometer. The X-ray photoelectron spectroscopy
agglomeration.13,44,45 The combination of these factors has been (XPS) measurements were performed using a Gammadata-
seen in Ptnp@nSTDP and makes the Pt nanoparticles very stable Scienta ESCA200 hemispherical analyzer equipped with an Al
during reaction via encapsulation it in dendrimer and interac- (K ¼ 1486.6 eV) X-ray source. The Pt content of the catalyst was
tion between nanoparticles and thiol groups.
measured by an inductively coupled plasma optical emission
For comparison of catalytic performance of Ptnp@nSTDP, spectrometry (ICP-OES), using a Jarrell-Ash 1100 ICP analyzer.
the catalytic activity of similar systems containing Pt Scanning electron microscopy measurements were performed
Table 4 Comparison of catalytic performance of Ptnp@nSTDP with similar systems
Entry
Catalyst
Sub.
Sub.
Time
12 h
Yield (%)
82a
1 (ref. 24)
Pt@TiO2
2 (ref. 25)
3 (ref. 25)
4
Pt/Al2O3
24 h
24 h
1.5 h
55b
60c
97d
Pt/TiO2
PtnpnSTDP (this work)
5
PtnpnSTDP (this work)
1.5 h
94d
Reaction conditions: catalyst (5 mg), alcohol (750 mmol), MeCN (5 mL), under N2 (1 atm), l > 300 nm. b Reaction conditions: catalyst (1 mol%),
mesitylene (1.2 mL), reux, under N2. c Reaction conditions: catalyst (1 mol%), mesitylene (1.2 mL) reux, under N2. d Reaction conditions: catalyst
(30 mg, containing 0.0018 mmol Ptnp), alcohol (1 mmol) and 1 mmol TBPB in H2O at 80 ꢂC.
a
33144 | RSC Adv., 2020, 10, 33137–33147
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
on a Hitachi S-4700 eld emission-scanning electron micro-
scope (FE-SEM). Transmission electron microscopy (TEM) was
carried out on a Philips CM10 analyzer operating at 100 kV.
2 K. Na, N. Musselwhite, X. Cai, S. Alayoglu and G. A. Somorjai,
Promotional effects of mesoporous zeolites with pt
nanoparticle
catalysts
during
reforming
of
methylcyclopentane, J. Phys. Chem. A, 2014, 118, 8446–8452.
3 S. Bong, B. Jang, D. Han and Y. Piao, Effective
electrochemical activation of oleate-residue-fouled pt
nanoparticle catalysts for methanol and formic acid
oxidation, ACS Omega, 2019, 4, 20330–20334.
4 S. Mei, M. Staub and C. Y. Li, Directed nanoparticle assembly
through polymer crystallization, Chem.–Eur. J., 2020, 26,
349–361.
Preparation of Pt nanoparticles immobilized on nano-silica
thiolated dendritic polymer (Ptnp@nSTDP)
The nano-silica thiolated dendritic polymer, nSTDP, was
prepared according to our previously reported work (see ESI†).46
K2PtCl4 (0.01 g) was added to a mixture of nSTDP (G2) (0.1 g)
in H2O (10 mL) and stirred at room temperature for 12 h. A
solution of NaBH4 (0.5 g in 20 mL cold deionized water) was
added to this mixture and stirred at room temperature for 12 h.
At the end of the reaction, the solid material was ltered,
washed sequentially with H2O (25 mL) and then dried in
¨
5 S. Proch, J. Herrmannsdorfer, R. Kempe, C. Kern, A. Jess,
L. Seyfarth and J. Senker, Pt@MOF-177: synthesis,
room-temperature hydrogen storage and oxidation
catalysis, Chem.–Eur. J., 2008, 14, 8204–8212.
6 K. An, Q. Zhang, S. Alayoglu, N. Musselwhite, J.-Y. Shin and
G. A. Somorjai, High-temperature catalytic reforming of n-
hexane over supported and core–shell Pt nanoparticle
catalysts: role of oxide–metal interface and thermal
stability, Nano Lett., 2014, 14, 4907–4912.
ꢂ
a vacuum oven at 80 C to obtain the Ptnp@nSTDP catalyst as
a yellow solid.
General procedure for synthesis of benzimidazoles or
benzothiazoles in the presence of Ptnp@nSTDP catalyst
In a round-bottomed ask equipped with a condenser and
a magnetic stirrer, a mixture of alcohol (1 mmol), 1,2-phenyl-
enediamine or 2-aminothiophenol (1 mmol), Ptnp@nSTDP
(30 mg, containing 0.0018 mmol Ptnp) and (1 mmol) tetra-n-
butylphosphonium bromide (TBPB, 1 mg) in H2O (2 mL) was
stirred at 80 ꢂC. The progress of the reaction was monitored by
TLC (eluent: petroleum ether/EtOAc, 2 : 1 for benzimidazoles
and petroleum ether/EtOAc, 5 : 1 for benzothiazoles). The
catalyst was separated by ltration and washed with EtOAc (10
mL). The ltrate was evaporated, and the crude product was
puried by recrystallization from EtOAc or EtOH to afford the
pure benzimidazole. The benzothiazoles was obtained by
recrystallization from n-hexane/EtOAc (10 : 1).
7 L. Xu, Y. Zhu, L. Tang, X. Yang and C. Li, Biosensor based on
self-assembling
glucose
oxidase
and
dendrimer-encapsulated pt nanoparticles on carbon
nanotubes for glucose detection, Electroanalysis, 2007, 19,
717–722.
8 A. Boubnov, S. Dahl, E. Johnson, A. P. Molina,
S. B. Simonsen, F. M. Cano, S. Helveg, L. J. Lemus-Yegres
and J.-D. Grunwaldt, Structure–activity relationships of Pt/
Al2O3 catalysts for CO and NO oxidation at diesel exhaust
conditions, Appl. Catal., B, 2012, 126, 315–325.
9 D. E. Doronkin, A. B. Kuriganova, I. N. Leontyev, S. Baier,
H. Lichtenberg, N. V. Smirnova and J.-D. Grunwaldt,
Electrochemically synthesized Pt/Al2O3 oxidation catalysts,
Catal. Lett., 2016, 146, 452–463.
10 A. D. Allian, K. Takanabe, K. L. Fujdala, X. Hao, T. J. Truex,
J. Cai, C. Buda, M. Neurock and E. Iglesia, Chemisorption
of CO and mechanism of CO oxidation on supported
platinum nanoclusters, J. Am. Chem. Soc., 2011, 133, 4498–
4517.
Conclusions
In conclusion, we synthesized a dendritic polymer, based on
nano silica, containing thiol groups for immobilization of Pt
nanoparticles with 1–2 nm size on the branches of dendrimer
(Ptnp@nSTDP). This material was used as a unique heteroge-
neous catalyst for synthesis of benzimidazoles and benzothia-
zoles from benzyl alcohol derivatives in the water as a green
solvent and high yields were obtained for a variety of substrates.
Furthermore, with easy separation, the catalyst was reused ve
times at two reactions without loss of activity which conrms
the stability of catalyst during the reaction.
11 M. R. Knecht, M. G. Weir, V. S. Myers, W. D. Pyrz, H. Ye,
V. Petkov, D. J. Buttrey, A. I. Frenkel and R. M. Crooks,
Synthesis and characterization of Pt dendrimer-
encapsulated nanoparticles: effect of the template on
nanoparticle formation, Chem. Mater., 2008, 20, 5218–5228.
12 R. M. Anderson, D. F. Yancey, J. A. Loussaert and
R. M. Crooks, Multistep galvanic exchange synthesis
yielding fully reduced Pt dendrimer-encapsulated
nanoparticles, Langmuir, 2014, 30, 15009–15015.
13 H. Lang, R. A. May, B. L. Iversen and B. D. Chandler,
Dendrimer-encapsulated nanoparticle precursors to
supported platinum catalysts, J. Am. Chem. Soc., 2003, 125,
14832–14836.
Conflicts of interest
There are no conicts to declare.
Notes and references
14 H. Ye and R. M. Crooks, Electrocatalytic O2 reduction at
glassy carbon electrodes modied with dendrimer-
encapsulated Pt nanoparticles, J. Am. Chem. Soc., 2005,
127, 4930–4934.
1 Y. Deng, Y. Ge, M. Xu, Q. Yu, D. Xiao, S. Yao and D. Ma,
Molybdenum carbide: controlling the geometric and
electronic structure of noble metals for the activation of
O–H and C–H bonds, Acc. Chem. Res., 2019, 52, 3372–3383.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 33137–33147 | 33145

View Article Online
RSC Advances
Paper
15 A. Landarani Isfahani, I. Mohammadpoor-Baltork,
tip induced patterning on the clusters/Au (111) surface,
V. Mirkhani, A. R. Khosropour, M. Moghadam and
Langmuir, 2002, 18, 4124–4130.
S. Tangestaninejad, Pd nanoparticles immobilized on 29 E. Gharibshahi and E. Saion, Inuence of dose on particle
nanosilica triazine dendritic polymer: a reusable catalyst
size and optical properties of colloidal platinum
for the synthesis of mono-, di-, and trialkynylaromatics by
nanoparticles, Int. J. Mol. Sci., 2012, 13, 14723–14741.
sonogashira cross-coupling in water, Eur. J. Org. Chem., 30 W. Moddeman, J. Blackburn, G. Kumar, K. Morgan,
2014, 5603–5609.
16 M. Liu and J. M. Frechet, Designing dendrimers for drug
delivery, Pharm. Sci. Technol. Today, 1999, 2, 393–401.
17 G. Chen, J. Kumar, A. Gregory and M. H. Stenzel, Efficient
R. G. Albridge and M. Jones, Photoelectron spectroscopy of
coordination compounds. III. Comparison of platinum (II)
and platinum (IV) compounds, Inorg. Chem., 1972, 11,
1715–1717.
´
synthesis of dendrimers via a thiol-yne and esterication 31 X. Fu, Y. Wang, N. Wu, L. Gui and Y. Tang, Surface
process and their potential application in the delivery of
platinum anti-cancer drugs, Chem. Commun., 2009, 6291–
6293.
modication of small platinum nanoclusters with
alkylamine and alkylthiol: an XPS study on the inuence of
organic ligands on the Pt 4f binding energies of small
platinum nanoclusters, J. Colloid Interface Sci., 2001, 243,
326–330.
´
18 J. Alvarez, J. Liu, E. Roman and A. E. Kaifer, Water-soluble
platinum and palladium nanoparticles modied with
thiolated b-cyclodextrin, Chem. Commun., 2000, 1151–1152. 32 Y.-T. Kim and T. Mitani, Surface thiolation of carbon
˙
¨
19 O. Akyıldırım, H. Yuksek, H. Saral, I. Ermi¸s, T. Eren and
nanotubes as supports: a promising route for the high
dispersion of Pt nanoparticles for electrocatalysts, J. Catal.,
2006, 238, 394–401.
M. L. Yola, Platinum nanoparticles supported on nitrogen
and sulfur-doped reduced graphene oxide nanomaterial as
highly active electrocatalysts for methanol oxidation, J. 33 C. Yang, Q. Jiang, W. Li, H. He, L. Yang, Z. Lu and H. Huang,
Mater. Sci.: Mater. Electron., 2016, 27, 8559–8566.
20 C. Wang, G. Zhu, J. Li, X. Cai, Y. Wei, D. Zhang and S. Qiu,
Rigid nanoscopic containers for highly dispersed, stable
Ultrane Pt nanoparticle-decorated 3D hybrid architectures
built from reduced graphene oxide and MXene nanosheets
for methanol oxidation, Chem. Mater., 2019, 31, 9277–9287.
metal and bimetal nanoparticles with both size and site 34 Z. Xing, C. Han, D. Wang, Q. Li and X. Yang, Ultrane Pt
control, Chem.–Eur. J., 2005, 11, 4975–4982.
nanoparticle-decorated Co(OH)2 nanosheet arrays with
enhanced catalytic activity toward hydrogen evolution, ACS
Catal., 2017, 7, 7131–7135.
21 Y. Kumar and H. Ila, Domino synthesis of 2-substituted
benzothiazoles by base-promoted intramolecular C–S bond
formation, Org. Lett., 2019, 21, 7863–7867.
22 W.-X. Xu, X.-Q. Dai and J.-Q. Weng, K2S2O8-mediated
hydroxyalkylation of benzothiazoles with alcohols in
aqueous solution, ACS Omega, 2019, 4, 11285–11292.
23 A. Ekbote, S. M. Mobin and R. Misra, Structure–property
35 H. I. Lee, S. H. Joo, J. H. Kim, D. J. You, J. M. Kim, J.-N. Park,
H. Chang and C. Pak, Ultrastable Pt nanoparticles supported
on sulfur-containing ordered mesoporous carbon via strong
metal-support interaction, J. Mater. Chem., 2009, 19, 5934–
5939.
relationship
in
multi-stimuli
responsive
D–A–A0 36 R. Ahmadi and M. K. Amini, Synthesis and characterization
benzothiazole functionalized isomers, J. Mater. Chem. C,
2018, 6, 10888–10901.
24 Y. Shiraishi, Y. Sugano, S. Tanaka and T. Hirai, One-Pot
of Pt nanoparticles on sulfur-modied carbon nanotubes for
methanol oxidation, Int. J. Hydrogen Energy, 2011, 36, 7275–
7283.
Synthesis
of
Benzimidazoles
by
Simultaneous 37 K. Kwon, S.-a. Jin, C. Pak, H. Chang, S. H. Joo, H. I. Lee,
Photocatalytic and Catalytic Reactions on Pt@TiO2
Nanoparticles, Angew. Chem., 2010, 122, 1700–1704.
25 C. Chaudhari, S. H. Siddiki and K.-i. Shimizu, Acceptorless
dehydrogenative synthesis of benzothiazoles and
J. H. Kim and J. M. Kim, Enhancement of electrochemical
stability and catalytic activity of Pt nanoparticles via strong
metal-support interaction with sulfur-containing ordered
mesoporous carbon, Catal. Today, 2011, 164, 186–189.
benzimidazoles from alcohols or aldehydes by 38 H. Hikawa, R. Ichinose, S. Kikkawa and I. Azumaya,
heterogeneous Pt catalysts under neutral conditions,
Tetrahedron Lett., 2015, 56, 4885–4888.
26 A. Daneshvar, M. Moghadam, S. Tangestaninejad,
Palladium-catalyzed dehydrogenation of benzyl alcohols
for construction of 2-arylbenzimidazoles “on water”, Asian
J. Org. Chem., 2018, 7, 416–423.
V. Mirkhani, I. Mohammadpoor-Baltork and A. Khalili, 39 Y.-S. Lee, Y.-H. Cho, S. Lee, J.-K. Bin, J. Yang, G. Chae and
Ruthenium hydride complex supported on gold
nanoparticle cored triazine dendrimers for C–C coupling
reactions, Organometallics, 2016, 35, 1747–1755.
C.-H. Cheon, Signicant facilitation of metal-free aerobic
oxidative cyclization of imines with water in synthesis of
benzimidazoles, Tetrahedron, 2015, 71, 532–538.
27 K. R. Gopidas, J. K. Whitesell and M. A. Fox, Synthesis, 40 M. Brust, M. Walker, D. Bethell, D. J. Schiffrin and
characterization, and catalytic applications of a palladium-
nanoparticle-cored dendrimer, Nano Lett., 2003, 3, 1757–
1760.
R. Whyman, Synthesis of thiol-derivatised gold
nanoparticles in a two-phase liquid–liquid system, J. Chem.
Soc., Chem. Commun., 1994, 801–802.
28 W. Yang, M. Chen, W. Knoll and H. Deng, Synthesis of 41 J. Yang, J. Y. Lee, T. Deivaraj and H.-P. Too, An improved
hexanedithiolate/decanethiolate
mixed
monolayer
procedure for preparing smaller and nearly monodispersed
protected gold clusters and scanning tunneling microscope
33146 | RSC Adv., 2020, 10, 33137–33147
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
thiol-stabilized platinum nanoparticles, Langmuir, 2003, 19, 45 R. Crooks and M. Zhao, Dendrimer-encapsulated Pt
10361–10365.
nanoparticles: synthesis, characterization, and applications
42 W. Tu, K. Takai, K.-i. Fukui, A. Miyazaki and T. Enoki,
to catalysis, Adv. Mater., 1999, 11, 217–220.
Interface effect on the electronic structure of alkanethiol- 46 M. Zakeri, M. Moghadam, V. Mirkhani, S. Tangestaninejad,
coated platinum nanoparticles, J. Phys. Chem. B, 2003, 107,
10134–10140.
I. Mohammadpoor-Baltork and Z. Pahlevanneshan,
Synthesis and characterization of a host (a new thiol based
43 C. Yee, M. Scotti, A. Ulman, H. White, M. Rafailovich and
J. Sokolov, One-phase synthesis of thiol-functionalized
platinum nanoparticles, Langmuir, 1999, 15, 4314–4316.
44 R. W. Scott, A. K. Datye and R. M. Crooks, Bimetallic
palladium–platinum dendrimer-encapsulated catalysts, J.
Am. Chem. Soc., 2003, 125, 3708–3709.
dendritic
polymer)–guest
(Pd
nanoparticles)
nanocomposite material: an efficient and reusable catalyst
for C–C coupling reactions, RSC Adv., 2016, 6, 104608–
104619.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 33137–33147 | 33147