Zirconium based porous coordination polymer (PCP) bearing organocatalytic ligand: A promising dual catalytic center for ultrasonic heterocycle synthesis

Article abstract of DOI:10.1016/j.ultsonch.2019.104653

Herein, the efficient role of ultrasonic irradiation both in synthesis of Zr based porous coordination polymer (Zr-PCP) nanoparticles and boosting its catalytic activity, towards the benzimidazoles synthesis is represented. We use an amine based ligand (amino-terephthalate) for PCP and we exhibit that it can have a synergistic catalytic activity. In this work, a unique nano-engineering of cooperative and synergistic catalytic activity of zirconium, as a Lewis acid, and aminophenylene, as an organocatalyst, in the synthesis of heterocycles is presented for the synthesis of benzimidazole from cascade reaction of phenylene diamine with aldehyde at ambient temperature. Zr and amine groups of the Zr-PCP are active catalytic sites which in combination with the ultrasonic irradiation leads to a high selectivity and rapid catalytic production of benzylimidazoles. N₂ adsorption-desorption along with BJH analyses confirm the microporosity of the catalyst and recyclability shows that the catalyst is green and sustainable heterogeneous microporous catalyst.

Full text of DOI:10.1016/j.ultsonch.2019.104653

Accepted Manuscript
Zirconium based porous coordination polymer (PCP) bearing organocatalytic
ligand: a promising dual catalytic center for ultrasonic heterocycle synthesis
Paria Panahi, Nasrin Nouruzi, Esmail Doustkhah, Hamed Mohtasham, Arefeh
Ahadi, Azra Ghiasi-Moaser, Sadegh Rostamnia, Ghodrat Mahmoudi, Alireza
Khataee
PII:
S1350-4177(19)30564-4
https://doi.org/10.1016/j.ultsonch.2019.104653
104653
DOI:
Article Number:
Reference:
ULTSON 104653
Ultrasonics Sonochemistry
To appear in:
Received Date:
Revised Date:
Accepted Date:
15 April 2019
11 June 2019
20 June 2019
Please cite this article as: P. Panahi, N. Nouruzi, E. Doustkhah, H. Mohtasham, A. Ahadi, A. Ghiasi-Moaser, S.
Rostamnia, G. Mahmoudi, A. Khataee, Zirconium based porous coordination polymer (PCP) bearing
organocatalytic ligand: a promising dual catalytic center for ultrasonic heterocycle synthesis, Ultrasonics
Sonochemistry (2019), doi: https://doi.org/10.1016/j.ultsonch.2019.104653
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Zirconium based porous coordination polymer (PCP) bearing organocatalytic
ligand: a promising dual catalytic center for ultrasonic heterocycle synthesis
a
a
a,b
a
a
Paria Panahi, Nasrin Nouruzi, Esmail Doustkhah,
Hamed Mohtasham, Arefeh Ahadi,
Azra Ghiasi-Moaser, a Sadegh Rostamnia, a,* Ghodrat Mahmoudi, c,* Alireza Khataee d,e
a
Organic and Nano Group (ONG), Department of Chemistry, Faculty of Science, University of
Maragheh, PO Box 55181-83111, Maragheh, Iran Tel: +98(421) 2276066; Fax: +98(421) 2276066;
Email: rostamnia@maragheh.ac.ir; srostamnia@gmail.com
b International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science,
1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
c
Department of Chemistry, Faculty of Science, University of Maragheh, PO Box 55181-83111,
Maragheh, Iran
d
Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied
Chemistry, Faculty of Chemistry, University of Tabriz, 51666-16471 Tabriz, Iran Email:
a_khataee@tabrizu.ac.ir
e
Department of Materials Science and Nanotechnology Engineering, Faculty of Engineering, Near East
University, 99138 Nicosia, Mersin 10, Turkey
1

Abstract
Herein, the efficient role of ultrasonic irradiation both in synthesis of Zr based porous
coordination polymer (Zr-PCP) nanoparticles and boosting its catalytic activity, towards the
benzimidazoles synthesis is represented. We use an amine based ligand (amino-terephthalate) for
PCP and we exhibit that it can have a synergistic catalytic activity. In this work, a unique nano-
engineering of cooperative and synergistic catalytic activity of zirconium, as a Lewis acid, and
aminophenylene, as an organocatalyst, in the synthesis of heterocycles is presented for the
synthesis of benzimidazole from cascade reaction of phenylene diamine with aldehyde at ambient
temperature. The catalyst shows Zr and amine groups of the Zr-PCP are active catalytic sites
which in combination with the ultrasonic irradiation leads to a high selectivity and rapid catalytic
production of benzylimidazoles. N2 adsorption-desorption along with BJH analyses confirm the
microporosity of the catalyst and recyclability shows that the catalyst is green and sustainable
heterogeneous microporous catalyst.
Keywords: Ultrasound Irradiation; Zr-PCP-NH2; Benzimidazole; Sonochemical Synthesis,
Green Catalyst.
2

1. Introduction
Nanoporous coordination polymers (PCP), nowadays, are important class of porous
nanoarchitechtures providing a large opportunities to design advanced set of nanomaterials such
as catalysts [1, 2], adsorbents [3], and sensors [4], drug carrier [5]. Incorporation of
organocatalytic ligands is an interesting approach to design a hybrid bifunctional catalyst (metal
as Lewis acid and ligand functional group such as organobase or organocatalyst) with porous and
crystalline structures [6]. By varying metal and ligand, a wide variety of structures with different
pore shapes, sizes, and functionalities [7] can be achievable. Synthetic procedure of these
materials include different methods such as ball-milling [8], sonication [9] and solvothermal
approaches [10-15]. Among PCPs, we selected a 3D octahedral Zr-based PCP (Zr-PCP), owing
to its high chemical stability and interesting chemistry for our further catalytic investigations in
this project. One of the convenient properties of Zr-PCP is the aqueous stability of this PCP
which convert it to a an interesting candidate for catalytic heterocyclic organic molecules
synthesis [16].
Benzimidazoles as N-rich heterocycles are important class of heterocyclic compounds [17]
that show various biological activities including antineoplastic [18], antiprotozoal [19],
antiproliferative [20], antimicrobial, anthelmintic [21], and antiulcer [22] activities. Besides, they
can be used to synthesis of ionic liquid [23], liquid crystals [24] and carbine based organometallic
catalysts [25]. Two procedures are popular for the 2-substituted benzimidazoles synthesis; (1)
phenylenediamines coupling with carboxylic acids [26] and (2) a two-step method including
oxidative dehydrogenation of aniline manufactured in situ from the phenylenediamines
3

condensation with aldehydes [27]. The first method involves acidic conditions and high
temperatures. But the last method is mainly used owing to the availability of the reagents and
aldehydes. Another protocol is the one-pot tandem preparation from alcohols accelerated by
bifunctional catalysts, in which alcohol is in situ oxidized to aldehyde and then cyclocondensated
with amines [28, 29]. But this process suffers from drastic conditions, hard work-up, low yield
and using homogeneous catalysts that cannot be recovered. In this regard, using the
heterogeneous catalysts for selective benzimidazole synthesis can be greatly favorable [30].
Ultrasonic technology has been proving to be a safe and energy saving procedure for organic
synthesis, because of the physical and chemical changes through acoustic cavitation phenomena
i.e. quick formation, development and crumble of unsteady bubbles in liquids. The resulting local
temperature, pressure and very high cooling rates derive extraordinary properties into the
sonicated solutions [31-35]. Dhopte et al. applied graphene oxide as a reusable heterogeneous
catalyst for the nenzimidazole synthesis, under ultrasonication for 60 minutes. They reached the
2-substituted benzimidazole with the yield of 89%. They also found that 60 minutes of
ultrasonication at 35 °C is much more effective than 3 hour of stirring at 60 °C [36]. Similarly,
Ziarati et al. synthesized benzimidazole derivatives, under ultrasonication. They used
nanostructured NiFe-xEuxO4 for accelerating reaction, that reused for five cycles [37].
Herein, we designed a bifunctional catalyst with environmentally benign, stable and affordable
and reusable features for green benzimidazole synthesis. We exploited both metal and organosites
for the sustainable synthesis of benzimidazoles. These agents not only have disruptive effect on
the performance of each other, but also synergizes each other in the catalysis. Ultrasonic
irradiation is the third function that acts as a synergistic agent in this catalytic process (Figure 1).
4

Figure 1: Schematic illustration for the Zr-PCP-NH2 catalyzed benzimidazole synthesis.
2. Experimental section
2.1. Materials and instruments
Zirconium tetrachloride tetrahydrofuran complex (ZrCl4. 4H2O), 2-aminoterephthalic acid (NH2-
H2BDC), terephthalic acid (H2BDC), aldehydes and o-phenylenediamine (OPD) were obtained from
Merck. The other materials were purchased from Sigma and Merck (Germany) and Fluka (Switzerland),
which were used without further purification.
Fourier transform Infrared (FT-IR) spectra were obtained by a Shimadzu IR-460 spectrometer
(Japan). Scanning Electron Microscopy (SEM) images were recorded by a Zeiss-DSM 960A microscope
(Germany). The crystalline structure of the nanoparticles was examined by X-Ray Diffraction (XRD)
instrument (Philips-PW1800 diffractometer (Germany)). The ultrasound irradiation was applied using a
FAPAN 1200 UPS (Iran) with a wave of frequency 28 ± 1 kHz and power 100 W. Adsorption-desorption
isotherms were obtained by BEL sorp-mini autosorb (Japan). NMR spectroscopy was performed on a
Varian (300 MHz) Brucker (Germany). DT/TGA of PCP-NH2 was recorded on a Netzsch STA449 under
nitrogen atmosphere from 50 to 800 °C (Figure S3).
2.1. Synthesis of Zr-PCP-H
5

To synthesize Zr-PCPs with open metal centers, a mixture of zirconium chloride (1 mmol),
hydrochloric acid (2 mL, 12 mol.L-1) in 20 mL of dimethyl formamide (DMF) was sonicated for
10 min to obtain a transparent solution. Then, 2.1 mmol of terephthalic acid (for Zr-PCP-H
synthesis) or aminoterephtalic acid (for Zr-PCP-NH2 synthesis) were dissolved in 40 mL of DMF
and added to the first solution. The mixture was sonicated for 30 min and it was transferred to a
Teflon autoclave. The autoclave was placed in an oven at 80 °C for 24 h. After cooling down the
mixture at ambient temperature, it was centrifuged and washed with anhydrous DMF and
acetone, several times. The white solid was dried under vacuum.
2.2. Synthesis of benzimidazole derivatives
Benzaldehyde (2 mmol), OPD (1 mmol), the catalyst (0.02 g) were added to a 3 mL ethanol at
room temperature and stirred for 4 h. After completion of the reaction (monitored by TLC,
hexane:ethylacetate, 1:4, respectively), the reaction temperature was cooled downed to 0 °C to
precipitate. Then, the precipitate was washed with water and dried. The crude product was
recrystallized by ethanol to afford the final product. In another pot, the same reaction was done
by ultrasound irradiation for 30 min.
For investigating the reusability of the catalyst after completion the reaction, the mixture was
filtered and the Zr-PCP-NH2 nanocatalyst was rinsed twice with ethyl acetate; the filtrate was
dried and the solvent was vaporized under vacuum and was reused for the next run.
2.3. Spectral data of synthesized compounds
All compounds are known and are previously reported in the literatures [38, 39]. 1H NMR and
13C NMR spectra data of the synthesized products were given in the supporting information and
one case of the 1H NMR and 13C NMR spectra was given in Figures S1 and S2, respectively.
6

3. Results and discussion
First, PCPs bearing different ligands including terephthalate and aminoterephtalate were
synthesized. Zr-PCP-H and Zr-PCP-NH2 [40-42]. The synthetic strategy is illustrated in Scheme
1. In this process, the synthetic details and hydrothermally factors have played a leading role.
According to reports [43], the nature of the ligand and metal cluster contributes to the formation
of porous structure and the chemical properties results in the porous structure.
Scheme 1. Schematic preparation of Zr-PCP-H and Zr-PCP-NH2.
The chemical structure of the Zr-PCP-NH2 and Zr-PCP-NH2 were studied by Fourier
transform infrared (FT-IR) spectrum (Figure 2). A broad band appeared at ~3000-3600 cm-1 can
be ascribed to stretching vibration of O-H bond of the carboxylic acids and amine moieties (3447
and 3366) of Zr-PCP-NH2. The FT-IR spectra of the Zr-PCP-NH2 and Zr-PCP-NH2 also show
Zr-O stretches at approximately 478 and 489 cm-1. For Zr-PCP-NH2, the strong peaks at 1386
cm-1 symmetric and 1652 cm-1 asymmetric vibration are corresponding to carboxylate presence in
the PCP. The peaks at 1577 cm-1 and 2923 cm-1 are due to the stretching and bending vibrations
7

of the aromatic C=C bonds of aromatic ring and aliphatic C-H of coordinated DMF and C-H of
aromatic rings.
Figure 2. FT-IR spectra of synthesized Zr-PCP-H (red) and Zr-PCP-NH2 (blue).
Energy dispersive X-ray spectroscopy (EDX) analysis was carried out to determine the
existing elemental composition of Zr-PCP-NH2. It can be observed that Zr-PCP-NH2 is
composed of C, O, N and Zr elements and Z-PCP-H is devoid of any N atom (Figure 3a,b),
which is in consistent with its structure where the Zr-cluster coordinates with 2-
aminoterephthalate ligand [6]. According to the elemental analysis, the molar ratio of C:Zr:O is
5:0.6:1.4, respectively. Judging from the X-ray diffraction (XRD) patterns of Zr-PCP-NH2
(Figure 3c) and Zr-PCP-H (Figure 3d), these PCPs have pseudo-crystalline structure, which are
close to reported literatures [44].
8

Figure 3. EDX analyses of Zr-PCP-NH2 and Zr-PCP-H, respectively (a, b), and XRD patterns of Zr-PCP-
NH2 and Zr-PCP-H, respectively (c, d).
Scanning electron microscopy SEM images of the Zr-PCP-NH2 and Zr-PCP-H revealed that
the synthesized PCP are in nanoparticle shapes (Figure 4a,b). Line and surface profiles of the
planar area from the upside surface of Zr-PCP-NH2 and Zr-NH are shown in Figure 4c-f,
presenting the correspondence topography quantitatively.
9

10

Figure 4. SEM images of (a) Zr-PCP-NH2 and (b) Zr-PCP-H. (c, e) Plot and surface profile of Zr-PCP-
NH2 and Zr-PCP-H. (d, f) Plot, and surface profile of Zr-PCP-NH2 and Zr-PCP-H.
The BET specific surface areas of the prepared Zr-PCP-NH2 and Zr-PCP-H were recorded by
nitrogen adsorption-desorption isotherms at 77 K (Figure 5). The N2 adsorption-desorption
isotherms showing the surface areas of Zr-PCP-NH2 and Zr-PCP-H as 396 and 415 m2/g,
respectively, exhibiting that NH2 induced an insignificant decrease on the surface area. Given the
magnified shape, we can conclude that both isotherms follow type I, suggesting a microporous
structure [45, 46]. The calculated pore volumes from t-plot for Zr-PCP-NH2 and Zr-PCP-H were
11

0.33 and 0.38 cm3/g, respectively. These show that the substitution of amine groups has a minor
influence on the pore organization. According to the BJH, both structures contain miscroporosity
and they did not show any mesoporosity in the network (Figure S4).
Figure 5. N2 adsorption-desorption isotherms of Zr-PCP-NH2 (a) and its magnified (inset) and Zr-PCP-H
(b and its magnified as (inset).
More recently, one-pot green and sustainable synthesis of biologically interest nitrogen-rich
heterocyclic compounds with easy and commercially available materials has drawn increasing
attention [47, 48]. After synthesis and characterization of Zr-PCP-H and Zr-PCP-NH2, we
applied them as catalysts for the ultrasonic assisted one-pot synthesis of benzimidazole
heterocycle was investigated using mentioned catalysts as the bifunctional porous (Figure 6a). By
applying each ratio of OPD and benzaldehyde, both mono- and di-substituted products are
produced, but by applying different reaction conditions the products yields can be varied. To
obtain the best reaction conditions, we used benzaldehyde (2 mmol), OPD (1 mmol) in ethanol,
as the model reactants, by using 0.02 g of different catalysts i.e. Zr-PCP-H and Zr-PCP-NH2 at
12

room temperature (Figure 6b). It was observed that the Zr-PCP-NH2 yielded the highest
conversation (Figure 6a). Therefore, the mixed ligands Zr-PCP-NH2 was chosen as the best
catalyst for the following steps.
Figure 6. Benzimidazole synthesis in the presence of Zr-PCP-H, Zr-PCP-NH2 and Zr-PCP-NH2-H (a).
Ultrasonic assisted synthesis of mono- and di-substituted benzimidazoles by different OPD/benzimidazole
ratios (b).
In the next step and in continuous of obtaining the best reaction conditions, the reaction was
performed by using 0.02 g Zr-PCP-NH2 catalyst. Two products with one and two substituents
were obtained. It should be noticed that the reaction was performed once under high speed
stirring (HSS) for 4 h (after 4 h, no significant change in the yield) and once again under
ultrasound irradiation (US) for 30 min (after 30 min, no significant change in the yield). In each
section, the results of these two conditions were given, separately. As shown in Table 1, the
reaction conversion under the ultrasonication was higher than that under the high speed stirring.
Another evidence of the improvement in reaction yield is seen from the change in the sonication
length. By increasing the reaction time up to 30 min, the yield increased but further increase did
not significantly affect the progression. The amount of catalyst influenced the reaction
13

conversion, and the higher conversion (98%) could be obtained with 0.02 g of Zr-PCP-NH2
(entry 7). The solvent nature is also crucial for the BI synthesis. When the reaction was
performed in water under HSS, insignificant amount of the products was obtained. By
ultrasonication, the conversion increased by only 19% (entry 10). But, by applying toluene,
dimethyl formamide (DMF), acetonitrile and ethanol as the solvent, the conversion increased that
in ethanol, the highest conversion (~98%) was obtained (entry 13). The reaction conversion was
reduced to 84%, when the temperature was increased to 70 °C (entry 17), therefore room
temperature was chosen as the optimal temperature.
Table 1. Optimization of benzimidazole synthesis reaction conditions. [a]
NH2
NH2
CHO
Zr-PCP-NH2
Conditions
N
N
N
C
+
C
+ 2
N
H
F1
F2
Ph
Yield (%)[d]
Sonication
(min)
Catal.
(g)
Conversion
Entry
Solvent
EtOH
Temp. (°C)
F1
F2
(US/HSS)
34/33
38/33
38/33
39/33
36/28
38/30
38/33
37/32
36/30
<15/Trace
26/18
25/20
38/33
23/16
38/33
35/30
33/27
(US[b]/HSS[c])%
(US/HSS)
58/52
60/52
60/52
57/52
54/42
57/47
60/52
59/50
59/50
33/29
47/37
67/54
60/52
45/30
60/52
52/46
51/41
1
2
3
4
5
6
7
8
15
30
45
60
30
30
30
30
30
30
30
30
30
30
30
30
30
0.02
R. T.
R. T.
R. T.
R. T.
R. T.
R. T.
R. T.
R. T.
R. T.
R. T.
R. T.
R. T.
R. T.
R. T.
R. T.
50
92/85
98/85
98/85
96/85
90/70
95/77
98/85
96/82
95/80
< 48/29
73/55
92/79
98/85
68/46
98/85
87/76
84/68
0.02
0.02
0.02
0.005
0.01
0.02
0.04
0.08
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
H2O
DMF
CH3CN
EtOH
Toluene
EtOH
EtOH
EtOH
9
10
11
12
13
14
15
16
17
70
[a] Reaction conditions: Benzaldehyde (2 mmol), OPD (1 mmol), solvent: 3 mL in the presence of Zr-PCP-NH2.
[b] US: Under ultra-sonication (US).
[c] HSS: Under high speed stirring (HSS) for 4 h.
[d] Isolated yield.
With the optimal conditions in hand, then the scope of the reaction was investigated by using
OPD and various benzaldehydes to yield the benzimidazole derivatives (Table 2). For all the
14

reaction, both mono- and di-substituted benzimidazole derivatives were obtained that can be due
to the catalyst effect. With respect to Table 2, it was concluded that the electron-releasing
substituents induce the di-substituted products and the electron-withdrawing substituents lead to
the mono-substituted product. Also, steric hindrance in benzaldehyde derivatives has led to the
reduction in the reaction conversion that can be due the nanoporous structure of the catalyst that
hinders the bulky reactants entry.
Table 2. Synthesis of various benzimidazoles under HSS as well as US.[a]
Zr-PCP-NH2
(0.02 g)
NH2
NH2
CHO
N
N
N
C
C
+ 2
+
EtOH, r.t.
N
H
R
R
R
30 min/US
4h/HSS
F2
F1
Ar
[b]
Entry
1
Aldehyde
Conv.%
(US/HSS)
Product
Yield(%)
F1
F2
F1
F2
(US/HSS)
(US/HSS)
N
N
N
O
H
92/85
78/65
63/59
29/26
F
F
N
H
F
F
O
N
N
2
3
20/15
58/48
58/50
31/24
OCH3
OCH3
H
N
N
H
H3CO
OCH3
H3C
H3C
N
O
88/72
90/82
N
N
H
N
H
CH3
H3C
Cl
Cl
N
O
4
64/60
26/22
N
H
N
H
N
Cl
Cl
N
O
N
N
5
6
85/72
80/64
63/54
20/17
22/18
60/47
NO2
NO2
H
N
H
O2N
NO2
OCH3
O
OCH3
N
N
N
H
N
H
OCH3
OCH3
15

[a] Reaction condition: Benzaldehyde (2 mmol), OPD (1 mmol), EtOH (3 mL) in the presence of Zr-PCP-NH2 (0.02
g) and under US for 30 min or HSS for 4 hours.
[b] Isolated yield.
The proposed mechanism for synergistic impact of catalyst in the synthesis of mono-
substituted 2-benzimidazole is followed by (1) the coordination of OPD with zirconium of the Zr-
PCP-NH2 and simultaneous nucleophilic attack of the amine to the benzaldehyde, resulted in
imine formation based on organocatalytic properties of -NH2 group, (2) amine groups exchange.
The coordinating the intermediate nitrogen of the OPD to the zirconium can stabilize it prevent it
from reverse reaction. Continuing this process in the next step leads to cyclization of the
intermediate and discarding the product from catalytic site by removing the linkage between
amine of PCP-NH2 and the reactant. After cyclization, the oxidation of produced ring in the
presence of air O2 leads to final product. Also, the second -NH2 group of the OPD can react with
the benzaldehyde that leads to the di-substituted product. The detailed mechanism was drawn in
Scheme 2.
Scheme 2. Proposed mechanism for Zr-PCP-NH2 catalyzed synthesis of benzimidazoles through
bifunctional organocatalytic behavior.
16

For studying the catalyst recyclability, the synthesis reaction was performed by recovered Zr-
PCP-NH2 under the same condition as the first run. The catalyst was filtered, washed with ethyl
acetate and methanol, dried under vacuum and reused for the next run. The reused catalyst
yielded over 80% conversion after six runs (Figure 7a). The SEM image of the Zr-PCP-NH2 after
six times of recycling, illustrated that its surface morphology was almost retained (Figure 7b).
Figure 7. Reusability of the Zr-PCP-NH2 for the benzimidazole synthesis and (a), its SEM image after six
cycle (b).
We confirmed the bifunctional synergistic effect of catalyst by comparing the Zr-PCP-NH2
with amine-less Zr-PCP-H and Zn-based metal organic framework of MOF-5. According to this
comparison (Table 3), when we eliminate amine group from the catalyst structure, the catalytic
activity has a remarkable decrease in the product yield, indicating the effective role of amine in
the catalysis. On the other hand, when we replaced Zn2+ with Zr species, the catalyst’s activity
dropped. This can be ascribed to the prevailing catalytic activity of Zr to Zn metal ions. Besides,
it conveys the catalytic role of Zr in synergizing the catalytic activity of Zr-PCP-NH2.
Table 3. Comparison of the Zr-PCP-NH2 with other catalysts for the BI synthesis. [a]
Zr-PCP-NH2
EtOH, r.t.
NH2
NH2
CHO
N
N
N
C
+
C
+
2
N
H
30 min/ US
4h/HSS
F1
F2
Ph
Entry Catalyst
Conv.%
17
Yield%

(US/HSS) F1(US/HSS)
F2(US/HSS)
1
2
3
4
5
Zn-BDC
NH2-(CO2Me)2BDC
Aniline
Zr-PCP-NH2
Zr-PCP-H
55/40
72/60
40/37
98/85
75/60
32/25
48/40
30/23
60/52
55/43
23/15
24/20
20/14
38/33
33/25
[a]
Reaction condition: Benzaldehyde (2 mmol), OPD (1 mmol), EtOH: 3 mL in the
presence of catalyst (0.02 g).
4. Conclusions
It showed that in porous coordination polymer, zirconium as an active Lewis acid site and
aminophenylene, as an organobase, owing to their discrete localizations are compatible with each
other and cause to synergistic catalytic activity in synthesis of heterocycles. On the other hand,
due to the diffusion limitations, ultrasonic irradiation boosts the mass transfer and increase the
reaction rate. Therefore, we achieved the benzimidazoles synthesis in a short reaction time, high
yields and selectivity. Zr-PCP-NH2 was efficiently recovered and reused at least for 6 times
without significant decrease in reaction conversion exhibiting the reusability of the catalyst.
Acknowledgment
The authors appreciate the support of the University of Maragheh and University of Tabriz for
this research.
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Graphical abstract
Zirconium based porous coordination polymer (PCP) bearing organocatalytic ligand, a
promising catalyst for heterocycle synthesis
24

Highlights
 Zr-PCP-NH2 was fabricated via ultrasonic irradiation as a fast and green method.
 Zr-PCP with different functional moieties was used in BIM synthesis.
 Zr-PCP-NH2 showed higher catalytic activity than pristine Zr-PCP-H.
 The catalyst is stable under the reaction conditions.
 Zr-PCP-NH2 can be reused multiple times without significant loss in its efficiency.
25