Mixed cobalt/nickel metal–organic framework, an efficient catalyst for one-pot synthesis of substituted imidazoles

Article abstract of DOI:10.1007/s00706-016-1776-9

Abstract: In the present work, a three-dimensional bimetallic metal–organic framework, [CoNi(μ₃-tp)₂(μ₂-pyz)₂] (tp?=?terephthalic acid, pyz?=?pyrazine) was applied as a highly efficient and recoverable heterogeneous catalyst. The catalyzed reaction was the one-pot three-component condensation reaction between benzil or benzoin with various substituted aromatic aldehydes and ammonium acetate to synthesize the corresponding imidazoles in solvent-free conditions and high to quantitative yields. The catalyst can be recycled at least five times without significant loss in the catalytic activity. To the best of our knowledge, 2,4,5-trisubstituted 1H-imidazoles were not previously synthesized using MOFs as heterogeneous catalyst. The structure of previously reported cobalt/nickel MOF has been confirmed by various techniques, such as X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, and Brunauer–Emmett–Teller. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00706-016-1776-9

Monatsh Chem
DOI 10.1007/s00706-016-1776-9
ORIGINAL PAPER
Mixed cobalt/nickel metal–organic framework, an efficient
catalyst for one-pot synthesis of substituted imidazoles
Hamed Ramezanalizadeh1 Faranak Manteghi1
•
Received: 28 February 2016 / Accepted: 10 May 2016
Ó Springer-Verlag Wien 2016
Abstract In the present work,
a
three-dimensional
Graphical abstract
bimetallic metal–organic framework, [CoNi(l3-tp)2(l2-
pyz)2] (tp = terephthalic acid, pyz = pyrazine) was
applied as a highly efficient and recoverable heterogeneous
catalyst. The catalyzed reaction was the one-pot three-
component condensation reaction between benzil or ben-
zoin with various substituted aromatic aldehydes and
ammonium acetate to synthesize the corresponding imi-
dazoles in solvent-free conditions and high to quantitative
yields. The catalyst can be recycled at least five times
without significant loss in the catalytic activity. To the best
of our knowledge, 2,4,5-trisubstituted 1H-imidazoles were
not previously synthesized using MOFs as heterogeneous
catalyst. The structure of previously reported cobalt/nickel
MOF has been confirmed by various techniques, such as
X-ray diffraction, thermogravimetric analysis, Fourier
transform infrared spectroscopy, field emission scanning
electron microscopy, energy dispersive X-ray spec-
troscopy, and Brunauer–Emmett–Teller.
Keywords Heterogeneous catalysis Á
Substituted imidazole derivatives Á MOF Á Cobalt Á
Nickel
Introduction
Metal organic frameworks (MOFs) or porous coordination
polymers (PCPs) are crystalline porous materials whose
structures are defined by metal ions or clusters containing
metals acting as lattice nodes and rigid bipodal or multi-
podal organic linkers as spacers [1–3]. Exceptionally high
porosity, crystallinity, compositional and structural vari-
ability, large surface area, and acceptable thermal and
mechanical stability of MOFs make them ideal materials to
satisfy the needs of various applications, such as gas and
vapor sorption, chemical separation, catalysis, and so forth
[4–9]. Heterogeneous catalysis, an important cornerstone
of chemistry, is commonly carried out by traditional porous
materials such as zeolites. Early catalytic applications of
MOFs were mainly motivated by their analogy to zeolites
[10]. MOFs are constituted of large density of active metal
sites with free of exchangeable coordination positions and
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-016-1776-9) contains supplementary
material, which is available to authorized users.
& Faranak Manteghi
f_manteghi@iust.ac.ir
1
Department of Chemistry, Iran University of Science and
Technology, Tehran, Iran
123

H. Ramezanalizadeh, F. Manteghi
show poor solubility in common solvents as well as tunable
and uniform pore sizes, which make them smart hetero-
geneous catalysts. However, with the ability to assemble
well-defined molecular building blocks into their struc-
tures, MOFs are particularly suited for immobilizing
molecular catalysts to lead to a new generation of solid
catalysts with uniform catalytic sites and open canal
structures for shape-, size-, chemo-, and stereoselective
reactions. The heterogeneous nature of these MOF-based
catalysts allows simple recovery and reuse, which is highly
desirable for reducing processing and waste disposal costs
in large-scale reactions [9]. The most intriguing but also
most unpredictable class of MOF catalysts is undoubtedly
that in which structural metal ions at the nodes serve
themselves as the catalytic centers. Some MOFs contain
metal ions that can directly coordinate to the substrates to
catalyze a chemical transformation, and these are what we
refer to MOFs with coordinatively unsaturated sites [11].
Catalytic reactions of MOFs include Lewis acid catalysis,
Lewis base catalysis, enantioselective catalysis, etc. MOFs
having coordinatively unsaturated metal centers can
potentially interact with the substrate and act as Lewis acid
catalyst [12–15]. As a consequence, Lewis acidic proper-
ties of metal centers and Bronsted basic sites in organic
linkers of MOFs can easily catalyze the organic transfor-
mations. Thus, occurrence of these two combined
properties in MOFs makes them good and highly efficient
candidates in the field of catalysis. Several MOFs have
been employed as solid catalyst or catalyst supports for a
variety of organic reactions. These include oxidation [16],
Friedel–Crafts [17], Biginelli [18], Sonogashira [19],
Knoevenagel condensation [20], Henry [21], Friedlander
[22], and cyclization reactions [23].
the importance of imidazoles in biological systems has
attracted considerable interests because of their chemical
and biochemical properties, and compounds with imidazole
ring system also have many pharmacological properties
and can play an important role in biochemical processes.
The heterocyclic scaffolds comprising 2,4,5-trisubstituted
and 1,2,4,5-tetrasubstituted imidazoles are present in
compounds possessing versatile pharmacological action,
such as being antibacterial agents, anti-inflammatory
agents, CSBP kinase inhibitor, antitumor agents, inhibitors
of mammalian 15-LOX, and inhibitors of B-Raf kinase
[29]. Thus, several methods for the construction of these
heterocyclic scaffolds have been developed. Among these
methods, the one-pot reaction of diketone is extensively
used for the synthesis of 2,4,5-trisubstituted imidazoles.
Various catalysts can be employed in this reaction, such as
nano SnCl4ÁSiO2 [30], NBS [31], NiCl2Á6H2O [32], sili-
casulfuric
acid
[33],
silica
chloride
[34],
H14[NaP5W30O110] [35], and so on. Nevertheless, many of
these approaches suffer from one or more drawbacks, such
as a long reaction time, unsatisfactory yields, strong acidic
conditions, difficult work-up and purification procedures,
generation of significant amount of waste materials,
occurrence of side reactions, low yields, use of moisture
sensitive reagents/catalysts, and excessive use of reagents
and catalyst. Therefore, the development of a new mild
method to overcome the disadvantages still remains a
challenge for organic chemists.
There are a few reports on the application of MOFs as a
heterogeneous catalyst in the one-pot synthesis of multi-
component reaction [36]. Also, channels of MOF materials
acting as nanoscale reaction vessels (nano reactors) have
been used in the last decades [37].
Heterometallic MOFs have been investigated for the
modulation of framework properties, such as the
enhancement of framework stability, gas sorption behavior,
catalytic activity, and the tuning of breathing behavior,
luminescence, and magnetic properties [24–26]. A general
strategy toward heterometallic MOFs, especially bimetallic
MOFs, is to use different metal ions as reactants during the
conventional solvothermal reaction process. The reaction
of a ligand with two different metal ions sometimes results
in a bimetallic MOF as a pure phase rather than a mixture
of two different homometallic MOFs [27, 28].
Surprisingly, interpenetrated mixed (Co/Ni) metal–or-
ganic framework acts as a highly efficient recoverable
heterogeneous catalyst for the one-pot synthesis of 2,4,5-
trisubstituted imidazoles under mild solvent-free conditions.
To the best of our knowledge, there are a few reports on
the application of bimetallic MOFs as a catalyst in MCRs.
We herein report the results for the application of
[CoNi(l3-tp)2(l2-pyz)2] as a suitable, efficient, and green
catalyst for the one-pot synthesis of substituted imidazoles
without any solvents, salts, and additives, with good to
excellent yields of 2,4,5-trisubstituted imidazole deriva-
tives due to biological and medicinal interest.
Multicomponent reactions (MCRs) are one-pot reac-
tions, which involve reaction of three or more accessible
components to form a single product, where most or all the
atoms of starting materials are incorporated in the final
product. The rapid and easy access to biologically relevant
compounds by MCRs and the scaffold diversity of MCRs
have been recognized by the synthetic community in
industry and academia as a preferred method to design and
discover biologically active compounds. In recent years,
Results and discussion
The synthesized MOF was characterized by several tech-
niques including X-ray diffraction (XRD), thermogravimetric
analysis (TGA), Fourier transform infrared (FT-IR) spec-
troscopy, field emission scanning electron microscopy
123

Mixed cobalt/nickel metal–organic framework, an efficient catalyst for one-pot synthesis of…
(FESEM), energy dispersive X-ray spectroscopy (EDS), and
Brunauer-Emmett-Teller (BET). The results were in good
agreement with the reported analyses [38]. The comparison
analyses are given in SEI.
elemental analysis as Co, Ni, O, N, and C in weight% ratio
of 10.5, 10.8, 17.1, 20.9, and 40.7, respectively. The per-
centages are in agreement with the reported formula [38].
To put in evidence the role and effect of the catalyst in
rate of reaction, the model reaction was carried out in the
absence of any catalyst (Table 1, entry 1). The results
revealed that yield of reaction was very low, and the time
has not had significant impact on efficiency of reaction. To
evaluate the appropriate catalyst loading, the model reac-
tion was carried out using different amounts of catalyst. It
was found that the most effective amount of catalyst was
15 mg (Table 1, entry 8) and larger amounts of catalyst do
not increase the reaction yield.
The excess analyses including thermogravimetric/dif-
ferential thermal analysis (TG/DTA), energy dispersive
X-ray spectroscopy (EDX), and BET/BJH were performed.
The TG/DTA studies on polycrystalline sample under
oxygen-free nitrogen atmosphere with a heating rate of
10 °C/min in the temperature range of 0–800 °C, were
conducted to characterize the thermal stabilities of MOF
structure. As shown in Fig. 1, on heating the compound, no
obvious decomposition was observed until ca. 400 °C. The
TGA plot shows an obvious weight loss of 80 % above
410 °C, relating to the organic parts. The two interpene-
trated mixed (Co/Ni) metal–organic frameworks exhibited
a high thermal stability in comparison with similar struc-
tures that start to lose weight in the region of 150 °C [39].
It seems that the observed stability of MOF structure up
to 400 °C is due to the interpenetration of two highly
coordinated mixed metal as nodes and lack of any co-
crystals or solvent coordinated in the structure, and this
leads to one-step weight loss compared to analogous
structures that decompose in two or three steps because of
their water ligands or presence of co-crystallized species in
their framework [40].
Subsequently, to investigate the efficiency and applica-
bility of this catalyst in the synthesis of 2,4,5-trisubstituted
1H-imidazoles 4a-4n, the reaction was extended to other
substituted benzaldehydes 3 (Scheme 1) at 120 °C in sol-
vent-free conditions (Table 2). It is obvious from Table 2
that when benzoin (1) is used instead of benzil (2), the
reaction time is increased. All the aromatic aldehydes having
either electron-withdrawing substitutes or electron-donating
ones demonstrated excellent reactivity and yields for the
synthesis of 2,4,5-trisubstituted imidazole derivatives.
Lower yields obtained for electron-donating and ortho-
substituted aldehydes as compared with the electron-with-
drawing para groups may be attributed to the competitive
formation of less reactive corresponding imines. On the
other hand, electron-withdrawing para derivatives have
excellent yields relative to other derivatives, because para
derivatives have minimum steric hindrance, and electron-
deficient derivatives can stabilize the intermediate in
comparison to electron-rich substituents (Table 2).
In another point of view, interpenetrated motifs that
minimize the empty space could significantly enhance the
stability of frameworks, not only through filling of void
space but also in the formation of repulsive forces that
serve to prevent one net from collapsing in on itself [38].
The EDS analysis given in Fig. S1 of ESI indicates surface
Fig. 1 TGA/DTA diagram of [CoNi(l3-tp)2(l2-pyz)2]
123

H. Ramezanalizadeh, F. Manteghi
Table 1 Optimization of reaction conditions catalyzed by mixed (Co/Ni) metal–organic framework
Entry
Catalyst weight/mg
Condition
Temperature/°C
Time/mina
Yield/%b
1
0
1
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
EtOH
80
30
30
30
30
30
30
60
10
35
35
60
60
60
0
10
10
50
70
75
30
95
87
75
86
30
5
2
100
3
5
120
4
8
120
5
10
12
15
15
15
15
15
15
15
120
6
120
7
25
8
120
9
130
10
11
12
13
150
Reflux
Reflux
Reflux
H2O
CH3CN
Experimental condition: benzil (1 mmol), 3-nitrobenzaldehyde (1 mmol), ammonium acetate (2.5 mmol), solvent-free at 120 °C, and 15 mg of
catalyst
a
Reaction progress monitored by TLC
b
Isolated yield
Scheme 1
CHO
(Co/Ni)-MOF
N
O
O
O
NH
4OAc
or
+
N
H
R
Solvent-free
120 °C
OH
R
1
2
3
4a 4n
-
A proposed mechanism for the synthesis of 2,4,5-
trisubstituted 1H-imidazoles is shown in Scheme 2.
According to the mechanism, ammonia as a nitrogen
source exists in the reaction environment from the
decomposition of NH4OAc. It can be proposed that the
hydrogen atoms in ammonia and free-orbitals of the metal
centers of MOF having the Lewis acidic properties, are
responsible for the activation of carbonyl groups, and thus
increase the rate of imine production through coordination
bonding for nucleophilic attack of amines (Scheme 2) [41].
In more details, in the initial step, the flexible Lewis acid
Co(II) and Ni(II) ions in the MOF might be preferentially
coordinated by one oxygen atom from the benzaldehyde
according to the soft and hard acids and bases theory.
Electron flow from the Co(II) and Ni(II) atoms with
unsaturated coordination environments accelerates the rate
of imine production. It seems that Lewis acidic sites of the
(Co/Ni)-MOF catalyst synergistically affect the reactive
substrates and intermediates during the progress of reac-
tion. Presence of unsaturated Co(II) and Ni(II) metal nodes
in the MOF structure is the driving force for the activation
of carbonyl groups and causes the quick conversion of
initial precursors to desirable products. We have also cal-
culated the turnover number (TON) and turnover frequency
(TOF) in the synthesis of 2,4,5-trisubstituted 1H-imida-
zoles (Table 3). According to the obtained results, TON
has greater values from the TOF due to a very short
reaction time.
We compared the catalytic performance of our system
with the previously reported results for the synthesis of
2,4,5-trisubstituted 1H-imidazole derivatives. The results
are summarized in Table 4.
In the XRD pattern of MOF shown in Fig. 2, all the
diffraction peaks are indexed as the orthorhombic (Co/Ni)-
MOF structure. According to JCPDS card number (36-
1451), the peaks at 2h values of 9.98°, 11.1°, 14.1°, 14.5°,
18.9°, 20.7°, 22.4°, 25.1°, 25.6°, 27°, 28°, 30°, 32°, 34°,
38°, 40°, and 45° can be indexed to 020, 021, 022, 111,
032, 040, 132, 042, 133, 200, 124, 044, 231, 026, 302, 321,
and 175 crystal planes, respectively. No other impurity
123

Mixed cobalt/nickel metal–organic framework, an efficient catalyst for one-pot synthesis of…
Table 2 Synthesis of 2-aryl-4,5-diphenyl-1H-imidazoles using Co/Ni-MOF in solvent-free conditions (Scheme 1)
Entry
Aryl
Comp
Time/min
Benzil
Yield/%a
M.p./°C
Lit. M.p./°C
References
Benzoin
Benzil
Benzoin
1
2
C6H5
4a
4b
10
10
14
15
95
94
93
90
270–271
190–191
270
[42]
[43]
[44]
[43]
[43]
[43]
[45]
[46]
[47]
[48]
[49]
[45]
[43]
[42]
[44]
[41]
[42]
4-CH3–C6H4
226–227
158–161
227–228
203–205
260–261
187–188
230–232
230–231
297–299
199–201
173–174
261–263
261–262
260–261
213–216
215–219
3
4
5
6
7
4-CH3O–C6H4
2-OH–C6H4
4-OH–C6H4
2-Cl–C6H4
4c
4d
4e
4f
15
12
15
20
15
20
16
20
25
25
90
92
95
94
87
86
85
92
91
85
227–229
203–205
260–262
190–192
230–231
2-NO2–C6H4
4g
8
3-NO2–C6H4
4h
4i
7
8
15
10
15
20
20
30
25
95
94
95
94
95
89
95
93
91
93
92
91
84
92
290–291
199–201
174–175
261–263
262–263
260–261
216–218
9
4-NO2–C6H4
10
11
12
13
14
2,4-Cl2–C6H3
4j
10
12
15
15
20
4-Br–C6H4
4k
4l
3,4,5-(CH3O)3–C6H2
4-(CH3)2N–C6H4
3,4-(CH3O)2–C6H3
4m
4n
a
Yields refer to isolated pure products. The known products were characterized and compared by their physical properties with authentic
samples
Scheme 2
Table 3 Calculated values of turnover number (TON) and turnover
frequency (TOF) for 2-aryl-4,5-diphenyl-1H-imidazoles
MOF
CHO
NH3
TONa
Benzil
TOF/h-1
Benzil
Ph
Ph
Entry
Comp
O
O
Ph
Ph
O
Benzoin
Benzoin
NH
1
4a
4b
4c
4d
4e
4f
8.7
8.5
54.38
53.87
33
36.95
33
B
NH
H
2
8.62
8.25
8.44
8.7
8.25
7.88
7.8
NH3
A
Ph
3
23.87
30
4
42.2
34.8
26.12
31.92
79
HO
5
8.44
8.34
7.79
8.53
8.34
8.5
25.57
20.34
19
Ph
Ph
N
H
MOF
Ph
C
6
8.62
7.98
8.7
NH
Ph
NH
7
4g
4h
4i
Ph
8
34.12
52.12
34
Ph
N
9
8.62
8.7
66.3
54.37
43.1
34.8
32.64
26.36
Ph
OH
10
11
12
13
14
4j
Ph
Ph
N
NH
4k
4l
8.62
8.7
8.44
8.34
7.7
25.57
25.57
15.4
21
H
Ph
Ph
H
Ph
N
H
N
H
4m
4n
8.16
8.7
8.4
H
2O
with the previously reported MOF [36]. The general
scheme of placing the reactants in the pores of MOF crystal
lattice is suggested in Fig. 3.
peaks were detected, indicating the highly crystalline
structure of MOF structure. XRD pattern indicates the high
purity of the synthesized MOF and is also in accordance
Moreover, to study the pore structural and the specific
surface area of the title MOF, Brunauer-Emmett-Teller
123

H. Ramezanalizadeh, F. Manteghi
Table 4 Comparison of efficiency of various catalysts with (Co/Ni)-MOF in the synthesis of trisubstituted imidazoles
Entry
Catalyst
Reaction condition
Time/min
Yield/%
References
1
2
3
4
5
6
7
8
9
10
Nano SnCl4 SiO2
NBS
Solvent-free, 130 °C
Solvent-free, 120 °C
EtOH, reflux
120
45
95
92
94
81
85
80
80
95
92
90
[30]
[31]
NiCl2 6H2O
Silica sulfuric acid
Silica chloride
Zeolite
25
[32]
Water, reflux
480
30
[33]
Solvent-free, 80 °C
EtOH, reflux
[34]
60
[50]
Yb(OPf)3
C10F18/80 °C
360
10
[51]
(Co/Ni)-MOF
Ni-MOF
Solvent-free, 120 °C
Solvent-free, 120 °C
Solvent-free, 120 °C
Present work
Present work
Present work
15
Co-MOF
20
XRD pattern
19900
17900
15900
13900
11900
9900
(a)simulated
(b)synthesized
7900
5900
3900
1900
-100
0
5
10
15
20
25
30
35
40
45
50
55
2 /degree
Fig. 2 Powder XRD patterns of simulated (bottom) and synthesized (up) [CoNi(l3-tp)2(l2-pyz)2]
(BET) gas sorption measurement is carried out. Nitrogen
adsorption–desorption isotherm of the (Co/Ni)-MOF elec-
trochemical tests is shown in Fig. 2S (provided in ESI), and
the corresponding Barrett–Joyner–Halenda (BJH) pore size
distribution plot is illustrated in Fig. 3S (provided in ESI).
The N2 adsorption and desorption experiment of the sample
shows a typical type IV isotherm, and proposes the
microporous structure. The BET specific surface area of the
sample is measured to be 1054 m2 g-1. Also, the average
according to the Barrett–Joyner–Halenda (BJH) plot. The
data are given in ESI.
To understand the location of the catalytic performance
in the MOF structure, we have calculated the kinetic
diameter of reactants and reaction product molecules with
the aid of molecular mechanics (MM?) method [52] and
compared the results with the textural properties (pore
sizes, pore windows) of (Co/Ni)-MOF structure. The cal-
culated kinetic diameters for reactants and products
participated in the synthesis of imidazole derivatives are
pore size was 2.1 nm, and pore volume was 0.43 cm3 g-1
,
123

Mixed cobalt/nickel metal–organic framework, an efficient catalyst for one-pot synthesis of…
Fig. 3 Crystal packing of the title MOF along the a axis, and a
scheme of entering reactants in its pores and exit of imidazole as
product. The coordination polyhedron around the two interpenetrated
mtallic sites, with labled atoms are shown in the circle. The crystal
packing is drawn using cif of the title MOF [38]
given in Table 5. According to the obtained results, it
seems that the catalytic reaction can be done on the surface
and in the pores of (Co/Ni)-MOF catalyst.
carried out on a Shimadzu FTIR-8400S spectrophotometer
1
using KBr pallet for sample preparation. H and 13C NMR
spectra were recorded on a Bruker DRX-400 Avance
spectrometer at 400 and 100 MHz, respectively. X-ray
powder diffraction (XRD) patterns were recorded using a
Cu radiation (k = 1.5406) source on a STOE Powder
diffraction system. A Netzsch Thermoanalyzer STA 504
was used for the thermogravimetric analysis (TGA) with a
heating rate of 10 °C/min under air atmosphere. Field
emission scanning electron microscopy (FESEM) was
carried out on a Zeiss Supra ATM 55 at an acceleration of
15 kV. The surface area of the MOF structure was deter-
mined using the Brunauer–Emmett–Teller (BET) method
from N2 adsorption and desorption isotherms which were
measured on a Micrometritics ASAP 2020 system. The
pore size distribution was calculated from desorption
branches of the nitrogen isotherms applying the Barret–
Joyner–Halenda (BJH) method.
Conclusion
In summary, a bimetallic MOF consisting of Co and Ni as
metal nodes with the mixed O- and N-donor ligands was
used as heterogeneous and highly efficient catalyst for the
synthesis of 2,4,5-trisubstituted 1H-imidazoles under sol-
vent-free conditions. The high catalytic activity of the (Co/
Ni)-MOF is further highlighted when compared with the
other catalysts in this reaction. The results show that the
MOF performed its catalytic activity on both outer and
inner surfaces. Remarkable advantages of our work are
environmentally benign reaction conditions, improved
efficiency of reaction using solvent-free system, reduced
reaction time using recyclable catalyst, and enhanced
reaction rates.
Synthesis of [CoNi(l3-tp)2(l2-pyz)2]
[CoNi(l3-tp)2(l2-pyz)2] was synthesized using a recently
reported procedure which was slightly modified [38]. In a
Experimental
typical preparation approach,
a
solid mixture of
In this study, all reagents and starting materials were
obtained commercially from Sigma-Aldrich and Merck,
and were used as received without any further purification
unless otherwise noted. Melting points were measured on
Electrothermal 9100 apparatus. FT-IR analyses were
Co(NO3)2Á6H2O, Ni(NO3)2Á6H2O, terephthalic acid, and
pyrazine was dissolved in water. The resulting suspension
was stirred and then transferred into Teflon-lined stainless
steel autoclave, which was sealed and maintained at
200 °C for 48 h. After gradually cooling in oven to RT, the
123

H. Ramezanalizadeh, F. Manteghi
100
90
80
70
60
50
40
30
20
10
0
Table 5 Calculated kinetic diameters of reactants and products in
imidazole synthesis
Compound name
Kinetic diame-
ter/nm
Benzaldehyde
0.445
0.436
0.527
0.501
0.564
0.564
0.437
0.437
0.499
0.769
0.971
95
93
4-Chlorobenzaldehyde
3-Nitrobenzaldehyde
2,4-Dichlorobenzaldehyde
Benzil
90
86
4
84
5
82
6
Benzoin
Dimethylaminobenzaldehyde
Cyanobenzaldehyde
2-Chlorobenzaldehyde
3,4,5-Trimethoxybenzaldehyde
1
2
3
Run
Fig. 4 Reusability of [CoNi(l3-tp)2(l2-pyz)2] as a catalyst for the
synthesis of 2,4,5-triphenyl-1H-imidazole
2-(2,4-Dichlorophenyl)-4,5-diphenyl-1H-
imidazole
mixture was dissolved in appropriate amount of EtOH and
filtered to separate the catalyst. The filtered solution con-
taining the product was placed in the refrigerator to obtain
pure crystalline products in good to high yields.
2-(4-Methoxyphenyl)-4,5-bis(4-methoxyphenyl)- 1.01
1H-imidazole
2-(4-Chlorophenyl)-4,5-diphenyl-1H-imidazole
0.925
0.911
2-(3,4-Dimethoxyphenyl)-4,5-diphenyl-1H-
imidazole
Catalyst recycling procedure
2-(4-Nitrophenyl)-4,5-diphenyl-1H-imidazole
2-(4-Bromophenyl)-4,5-diphenyl-1H-imidazole
2-(4-Methylphenyl)-4,5-diphenylimidazole
4-Methylbenzaldehyde
0.924
0.924
0.924
0.504
0.503
0.468
0.437
0.499
0.584
One of the most important parameters in heterogeneous
catalysis is the reusability of the catalyst. Hence, we
examined the heterogeneity and recyclability of the MOF
catalyst. The supernatant from synthesis of trisubstituted
imidazoles after filtration through a regular filter did not
afford any additional product. To evaluate the stability of
the solid catalyst, we investigated recycling and reusing the
(Co/Ni)-MOF in the mentioned synthesis reactions. Upon
completion of the reaction, the catalyst could be recovered
in nearly quantitative yield and used repeatedly with neg-
ligible loss of activity for the following five runs.
According to the obtained results, the MOF behaves as a
truly heterogeneous catalyst. The conversions versus
reaction time of the fresh and recycled catalysts are given
in Fig. 4. Furthermore, as an evidence to recyclability of
the catalyst after five cycles, the MOF catalyst remained
highly crystalline and still performing the catalytic action,
indicating maintenance of its porous structure.
4-Methoxybenzaldehyde
2-Hydroxybenzaldehyde
4-Hydroxybenzaldehyde
2-Chlorobenzaldehyde
2-Nitrobenzaldehyde
2-(3-Nitrophenyl)-4,5-bis(4-methoxyphenyl)-1H- 1.02
imidazole
solid product was obtained by decanting the mother liquor
and washed with water (4 9 20 cm3) [36]. Solvent
exchange was then carried out with DMF (4 9 20 cm3) at
room temperature. The product was activated in vacuum at
80 °C for 12 h to remove residual DMF molecules that
coordinated to the metal sites and stayed under nitrogen
environment.
Acknowledgments The authors gratefully acknowledge the financial
support from the Research Council of Iran University of Science and
Technology.
General procedure for the preparation of 2,4,5-
trisubstituted imidazole derivatives
[CoNi(l3-tp)2(l2-pyz)2] was used as a solid catalyst in the
synthesis of 2,4,5-trisubstituted imidazoles in solvent-free
conditions. In a typical approach illustrated in Scheme 1,
the mixture of benzoin (1) or benzil (2) (1 mmol), ben-
zaldehyde 3 (1 mmol), and ammonium acetate (2.5 mmol)
was heated to 120 °C in the presence of 0.015 g catalyst in
solvent-free conditions. After completion of the reaction
which was monitored by TLC, the resulting reaction
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