An investigation of the catalytic activity in a series of isoreticular Zn(II)-based metal-organic frameworks

Article abstract of DOI:10.1016/j.inoche.2016.08.010

The catalyst operation of four metal-organic frameworks (MOFs), [Zn(oba)(4-bpmb)_0.5]_n·(DMF)_1.5 (TMU-6), [Zn(oba)(R-4-bpmb)_0.5]_n·(DMF)_1.5 (R-TMU-6), [Zn(oba)(4-bpmn)_0.5]_n

Full text of DOI:10.1016/j.inoche.2016.08.010

An Investigation of the Catalytic Activity in a Series of isoreticular Zn(II)-
based Metal-Organic Frameworks
Fahime Bigdeli, Sedigheh Abedi, Hassan Hosseini-Monfared, Ali Mor-
sali
PII:
DOI:
Reference:
S1387-7003(16)30266-0
doi: 10.1016/j.inoche.2016.08.010
INOCHE 6406
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
7 June 2016
13 July 2016
15 August 2016
Please cite this article as: Fahime Bigdeli, Sedigheh Abedi, Hassan Hosseini-Monfared,
Ali Morsali, An Investigation of the Catalytic Activity in a Series of isoreticular Zn(II)-
based Metal-Organic Frameworks, Inorganic Chemistry Communications (2016), doi:
10.1016/j.inoche.2016.08.010
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An Investigation of the Catalytic Activity in a Series of isoreticular
Zn(II)-based Metal-Organic Frameworks
Fahime Bigdeli,a Sedigheh Abedi,b Hassan Hosseini-Monfared,a* and Ali Morsali b*
a Department of Chemistry, University of Zanjan, Zanjan, 45195-313, Islamic Republic of Iran
b Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14155-
4838, Tehran, Islamic Republic of Iran
Abstract
The catalyst operation of four metal-organic frameworks (MOFs), [Zn(oba)(4-
bpmb)0.5]n.(DMF)1.5 (TMU-6), [Zn(oba)(R-4-bpmb)0.5]n.(DMF)1.5 (R-TMU-6), [Zn(oba)(4-
bpmn)0.5]n.(DMF)1.5 (TMU-21) and [Zn(oba)(R-4-bpmn)0.5]n.(DMF)1.5 (R-TMU-21) containing
phenyl and naphthyl derivatives, respectively, is discussed. These compounds were easily
prepared via mechanosynthesis and utilized for catalysis of the Knoevenagel reaction. The effect
of reduction and nature of N-donor pillar ligands resulting different characteristics are
investigated on the catalytic performance of the MOFs. Using various aromatic aldehydes, the
catalytic activity seems to be enhanced by increased N-donor ligands basicity. Results show that
among the four compounds, R-TMU-21 with the most basic center reaction bearing naphtyl-
containing pillar has the highest catalytic activity.
Keywords: Heterogeneous catalysis, MOFs, N-donor ligands, Knoevenagel reaction.
1. Introduction
Recently, the industrial requirements are developing an extensive research toward porous
materials with basic properties. As porous materials, metal–organic frameworks (MOFs) have

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been widely researched over a number of years due to their highly porous and modular nature
leading to promising properties [1-3]. Many characteristic features make them applicable for
numerous areas, such as gas storage [4-5], catalysis [6], sensing [7], separations [8], removal of
hazardous materials [9] and luminescence [10].
The economy of many countries is affected by the chemical industry. However, during the
industrial synthesis of chemical compounds, large amounts of hazardous waste flow into the
environment. Heterogeneous catalysis as an effective way to reduce the harms and adverse
effects of these systems is strongly developing. Among them, solid base catalysts compared to
the liquid forms show high efficiency for basic catalysis reactions and simplify the separation
process for organic preparations. In addition, reutilization of them is simple due to less leaching
problems unlike homogeneous catalysts [11].
It is generally acknowledged that metal-organic frameworks (MOFs) exhibit unique and
outstanding properties. The structural nanoporosity of MOF materials places them suitable for
catalysis. The channel surfaces provide more host-guest interactions. Organic guest substrates
are located in the pores easily by π–π staking interactions or hydrogen bonds. These characters
certainly benefit for the catalytic process. MOFs, therefore appear to be excellent candidates for
catalysis [1,12]. MOF structures are desirable as heterogeneous catalytic materials for a wide
range of reactions, from acid-base to redox catalysis. Because MOFs are composed of organic
linkers and secondary building units (SBU), the regulation of the two parts is important for
forming favorite structures that allows for accurate material design [13,14]. With the aid of the
carboxylate donating ligand, formation of neutral MOFs is feasible which is vital for catalysis
because the counter anions often block the guest-accessible sites [11].

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Knoevenagel condensation is a reaction which is performed between carbonyl compounds and
active methylene compounds. It is one of the most important carbon-carbon double-forming
reactions in organic synthesis, and widely used in the synthesis of chemically, biologically, and
pharmaceutically significant compounds. A number of organic chemists have continuously
improved this useful conversion, due to the multisubstituted alkenes that are produced can
further be used for a variety of molecular transformations, such as Michael additions and
Diels−Alder reactions [15, 16].
Several solid base catalyst systems have been investigated for the Knoevenagel condensation
reaction, such as mesoporous titanosilicate [17], modified MCM-41 [18], Lewis acid [19], ionic
liquids [20], functionalized solid supports [21], zeolites [22], LDH-F hydrotalcite [23], etc.
However, the use of such bases/acids and solvents in large scale Knoevenagel reactions are not
environmentally friendly because of the formation of undesirable side products resulting from
polymerization and the total dissolved salts formed during the neutralisation of the base or acid
catalysts. Also the usage of supports in the catalysts is generally tedious. Zeolites find limitations
in liquid phase reactions due to narrow pore sizes available in conventional microporous zeolites.
Nevertheless, most of these known methods have the other significant limitations, such as low
yields and long reaction times. It has been proposed that MOFs can overcome these limitations.
As MOFs have commonly low thermal stabilities, (200-500°C) they can be utilized for the
reactions that occur under mild conditions, such as knoevenagel condensation. Only a few
sustainable procedures and particularly with basic heterogeneous catalyst were reported [24-28].
Therefore, further development of appropriate and more efficient heterogeneous catalysts for the
Knoevenagel reactions is necessity.

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In this study, we have been focusing on the MOFs [Zn(oba)(4-bpmb)0.5]n.(DMF)1.5 (TMU-6)
[29], [Zn(oba)(R-4-bpmb)0.5]n.(DMF)1.5 (R-TMU-6), [Zn(oba)(4-bpmn)0.5]n.(DMF)1.5 (TMU-
21), and [Zn(oba)(R-4-bpmn)0.5]n.(DMF)1.5 (R-TMU-21)[30] (H2oba=4,4′-oxybisbenzoic acid,
4-bpmb=(N1E,N4E)-N1,N4-bis(pyridin-4-ylmethylene)benzen-1,4-diamine,
R-4-
bpmb=(N1E,N4E)-N1,N4-bis(pyridin-4-ylmethyl)benzen-1,4-diamine, 4-nbpy=(N1E,N5E)-N1,N5-
bis(pyridin-4-ylmethylene)naphthalene-1,5-diamine, R-4-nbpy=(N1E,N5E)-N1,N5-bis(pyridin-4-
ylmethyl)naphthalene-1,5-diamine, (Scheme 1), for catalysis of Knoevenagel condensation
reactions of Malononitrile with aromatic aldehydes. We applied these four isoreticular MOFs for
Knoevenagel condensation of different aromatic aldehyde to investigate how their structural
diversity of pillars can be affected and directed their catalytic activity.
2. Experimental Section
2.1 Materials and physical techniques
Zinc(II) acetate dihydrate and 4,4'-oxybis(benzoic acid) (H2OBA) were purchased from Merck
Company and used as received. Fourier transform infrared (FT-IR) spectra were recorded using a
Perkin-Elmer 597 spectrophotometer after making pellets with KBr powder. Powder X-ray
diffraction patterns were collected using the Bruker, D8ADVANCE, Germany (wavelength
1.5406 Å (Cu Kα), voltage: 40 kV, and current, 40 mA). The thermal behavior was measured
with a PL-STA 1500 apparatus with the rate of 10 °C.min-1 in a static atmosphere of argon. Gas
chromatography (GC) runs were performed on an HP Agilent 6890 gas chromatograph equipped
with an HP-5 capillary column (phenyl methyl siloxane 30 m × 320 mm × 0.25 mm) and a
flame-ionization detector.
2.2 General procedure for MOFs preparation

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Compounds [Zn(oba)(4-bpmb)0.5]n.(DMF)1.5 (TMU-6), [Zn(oba)(R-4-bpmb)0.5]n.(DMF)1.5 (R-
TMU-6), [Zn(oba)(4-nbpy)0.5]n.(DMF)1.5 (TMU-21) and [Zn(oba)(R-4-nbpy)0.5]n.(DMF)1.5 (R-
TMU-21) were prepared according to our previous reports by mechanochemical reaction
(grinding by hand) of a mixture of Zn(OAc)2.2H2O, 4,4′-oxydibenzoic acid (H2oba) and N-donor
ligand (4-bpmb, R-4-bpmb, 4-nbpy and R-4-nbpy) for 15 minutes [29, 30]. The resulting powder
was washed with small amounts of DMF in order to remove any unreacted substrate. The
product was heated at 100 °C for 8h after washing with DMF.
All of the structures are confirmed by IR spectrum and PXRD analyses. FT-IR (KBr pellet)
selected bands for TMU-6 and R-TMU-6: 660(m), 777(m), 882(m), 1010(m), 1160(s), 1240(vs),
1400(vs), 1500(m), 1620(vs) and 3430(w-br). FT-IR (KBr pellet) selected bands for TMU-21
and R-TMU-21: 661(w), 775(w), 880(w), 1160(s), 1240(vs), 1400(vs), 1610(vs) and 3418(w-br).
(Fig. S1-S4). The PXRD patterns of MOFs are similar to those of derived from the single crystal
structure of MOFs, also good structural matching between compounds and their reduced
analogues is observed (Fig. 1 and Fig. S5-S6).
3. General procedure for Knoevenagel condensation reactions
To evaluate the activity of MOFs, the Knoevenagel condensation reaction was performed in the
presence of these MOFs. A solution of an aromatic aldehyde; benzaldehyde, 4-
Methylbenzaldehyde or 4-Chlorobenzaldehyde (1 mmol), malononitrile (1.1 mmol) and catalyst
(2 wt%) in different solvents MeOH, EtOH or H2O (3 mL) were stirred for 30 min at 25 °C. At
the end of the reaction, the obtained suspension was centrifuged and the amount of products was
determined using a gas chromatograph apparatus (carrier gas-nitrogen; flow: 0.7 ml/min, oven:
190°C, injector: 250°C, detector: 250 °C). The results were summarized in (Tables 1-5).

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4. Result and Discussion
In the present study, the catalytic activity of compounds [Zn(oba)(4-bpmb)0.5]n.(DMF)1.5 (TMU-
6), [Zn(oba)(R-4-bpmb)0.5]n.(DMF)1.5 (R-TMU-6), [Zn(oba)(4-nbpy)0.5]n.(DMF)1.5 (TMU-21)
and [Zn(oba)(R-4-nbpy)0.5]n.(DMF)1.5 (R-TMU-21) was investigated during the Knoevenogel
condensation reaction known as probe for catalytic activity of these kinds of MOFs. As
previously mentioned, all MOFs were prepared by the mechanochemical method [29]. The
product was heated at 100 °C for 8h to afford the de-solvated one that used as activated catalysts.
After activation process, IR spectra displayed that the band of 1670 cm-1 related to DMF solvent
has been eliminated (Figs. S1-S4). The PXRD patterns of MOFs indicated that all
mechanosynthesized prepared MOFs were structurally identical to those prepared by
solvothermal synthesis (Fig. 1 and Fig. S5-S6). Also, the IR spectra indicated that there was a
good agreement between the spectra for TMU-21 and R-TMU-21 before and after catalytic
reactions (Fig. S7-S8).
The C–C bond coupling reaction of Knoevenagel condensation occurring between a C=O group
and an activated methylene group, is useful within the drug industry where important
intermediates are prepared. In addition, it is employed as a classic test reaction to analyze the
activity of a variety of solid base catalysts [31]. We chose four aromatic aldehydes:
benzaldehyde, 4-methylbenzaldehyde, 4-chlorobenzaldehyde and 4-methoxybenzaldehyde which
were allowed to react individually with malononitrile as the active methylene compound. The
catalytic activity of different catalysts on this condensation reaction was tested and summarized
in Tables 1-3. As indicated in Scheme 1, the whole MOFs comprising N-donor ligands in which
two uncoordinated nitrogen atoms are decorated on the wall of the channels, providing the active
base sites for the catalytic reaction. During the optimization of the catalytic conditions, it was

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found that using 4 mol% of the catalysts at 25 °C within 30 minutes, (Tables 1-3), the yields of
all reactions are relatively high. Hence in these reaction conditions, catalytic performance of
different catalysts is relatively similar and comparison of their capabilities is not applicable.
Consequently, the catalytic investigations were followed in the presence of lower amount of the
catalysts, 2 mol% (Table 1, entries 1,2).
Different solvents including water, methanol, ethanol and acetonitrile were used for the
Knoevenagel condensation. The results indicated that the highest yield of 100% was obtained
using water as a solvent in the presence of TMU-21 and R-TMU-21 as catalysts (Table 1, entries
1 and 2). When acetonitrile was used, no detectable products were obtained with the whole
catalysts and in the reactions of all types activated and deactivated substrates (Tables 1-3).
Generally, in electrophilic reactions like Knoevenagel, the substrates containing electron-
withdrawing groups that operate the reaction more efficiently called “activated” and the
substrates including electron-donating groups that are intrinsically less active in electrophilic
reactions called “deactivated” substrates. For nucleophilic reactions the definitions become
completely vice versa.
According to the data in Tables 1-3, polar solvents MeOH and EtOH have relatively good
operations, but MeOH proved to be better than EtOH. These results proved the importance of
solvent nature on Knoevenagel condensation reaction. It can be observed that the yield of
reactions increases with increasing the polarity of the solvents. This clearly indicates that the
reaction intermediates and products are more stabilized when polarity of solvent increases. Table
4 indicates the best results observed in the presence of TMU-21 and R-TMU-21 in H2O. The
results of the reactions without the presence of catalysts are also summarized in Table 5. These

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data clearly confirmed significant increasing the whole reaction rates with the catalysts (Tables
1-4).
The results of Knoevenagel condensation of different aldehydes showed that the aromatic
aldehydes, having different substituents including methyl and chloro lead to various yields
(Tables 1-3). The reaction of 4-Chlorobenzaldehyde, with electron-withdrawing group, happened
faster than those with electron-donating groups such as methoxy and methyl. The Knoevenagel
reaction of 4-Methoxybenzaldehyde bearing a power deactivating functional group, in all
solvents was not proceeded. The presence of electron donor group, methoxy, on the aromatic
ring reduces the electrophilic character of aldehyde center and makes them very deactive toward
nucleophilic addition.
The difference of catalytic activity was investigated by different catalysts and the results are
summarized in Tables 1-3. It is observed that R-TMU-21 shows higher catalytic activity than
TMU-21, (Tables 1-3, entries 1-2). Also R-TMU-6 is more efficient than TMU-6, (Tables 1-3,
entries 3-4). Generally, TMU-21 and R-TMU-21 show improved catalytic performance
compared to TMU-6 and R-TMU-6 (Table 1-3). The results indicate that the MOFs containing
amine-based ligands are more reactive than their analogues structures containing less basic
imine-ligands. Imine groups with sp2-hybridization of nitrogen atom have less basicity than
amine groups, in which the nitrogen is sp3-hybridized. This phenomenon is explained using
orbital theory and the inductive effect; sp2-hybridization atoms have about 67% s character. The
sp3 orbitals of amine nitrogen are only 25% s character. More electronegative atoms absorb
electron density more readily, and contain more acidic character. So, our imine containing pillar
ligands, 4-bpmb and 4-nbpy are less basic than their amine analogous (Scheme 1). Also
compounds TMU-21 and R-TMU-21 having naphthyl pillar derivatives, play stronger base role

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compare to TMU-6 and R-TMU-6. In the case of compounds TMU-21 and R-TMU-21, as a
result of the more favorably п-п stacking interactions, substrates enclathration possibility can be
happened effectively and it leading to higher catalyst activity versus two other MOFs bearing
phenyl containing pillars. It was also found that compounds TMU-21 and R-TMU-21 could be
reused several times without a significant degradation in catalytic activity. (Fig. 2-3). More
investigations are in progress in our lab for systematic studies of these four isoreticular MOFs.
Since the knoevenagel reaction of these substrates were performed at room temperature,
heterogeneous character of the MOF catalysts was tested by ICP analysis of the reaction
solutions after filtration of the catalysts. In all reactions of benzaldehyde with these four MOFs,
no distinguishable leaching of the metal (zinc) was detected. Moreover, comparison of the
PXRD patterns of the MOF after reaction proved the heterogeneous behavior of this catalytic
system.
5. Conclusion
Four isoreticular metal–organic frameworks (MOFs), [Zn(oba)(4-bpmb)0.5]n.(DMF)1.5 (TMU-6),
[Zn(oba)(R-4-bpmb)0.5]n.(DMF)1.5 (R-TMU-6), [Zn(oba)(4-nbpy)0.5]n .(DMF)1..5 (TMU-21),
[Zn(oba)(R-4-nbpy)0.5]n.(DMF)1.5 (R-TMU-21) were synthesized mechanochemically. The
synthesized MOFs contain N-donor ligands with various basicity characters. The Knoevenagel
condensation reaction performed with aromatic aldehydes including benzaldehyde, 4-
Methylbenzaldehyde, 4-Chlorobenzaldehyde and Malononitrile in the presence of these MOFs. It
has been indicated that ligands basicity, plays a prominent role in the catalytic activity. R-TMU-
21 has the highest catalytic activity as a result of higher basicity of its N-donor ligand.
AUTHOR INFORMATION

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Corresponding Authors
*Phone: +98-21-82884416. Fax:+98-21-82884416. E-mail: Morsali_a@modares.ac.ir
*Phone: +98-24-33052576. Fax: +98-24-32283203. E-mail address: monfared@znu.ac.ir
Acknowledgements
This work was supported by University of Zanjan. The authors thank Tarbiat Modares University
for experimental equipment supports.
Supporting Information
Electronic supplementary information (ESI) available: IR spectra of all MOFs before and after
activation. PXRD patterns of the mechanochemical synthesized MOFs and their patterns after
coupling reaction in water.
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Scheme 1. Preparation of MOFs

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Fig. 1 Comparison of XRPD patterns for: a) simulated (derived from the single crystal structure of MOFs), b) TMU-21
(solvothermal synthesis), c) TMU-21 (Mechanosynthesized), d) R-TMU-21 (Mechanosynthesized), e) TMU-21 (after reaction),
(f) R-TMU-21 (after reaction), g) TMU-6 (Mechanosynthesized), (h) R-TMU-6 (Mechanosynthesized), i) TMU-6 (after
reaction) and (j) R-TMU-6 (after reaction).

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Table 1. The experimental data for Knoevenagel reaction of benzaldehyde in the presence of four
isoreticular MOFs.
Entry
1
Catalyst
TMU-21
Solvent
MeOH
EtOH
H2O
CH3CN
MeOH
EtOH
H2O
CH3CN
MeOH
EtOH
H2O
CH3CN
MeOH
EtOH
H2O
Yield (%)b
63%
58%
100%
—
85%
84%
100%
—
47%
44%
40%
—
60%
58%
45%
—
Yield (%)c
92%
88%
100%
—
96%
91%
100%
—
98%
82%
80%
—
98%
87%
80%
—
2
3
4
R-TMU-21
TMU-6
R-TMU-6
CH3CN
a GC yield, Reaction conditions: malononitrile (0.3mmol), benzaldehyde (0.2mmol), 25°C, 30 min., 3
mL of solvent.
b The product was obtained by 2mol% of catalyst
c The product was obtained by 4mol% of catalyst

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Table 2. The experimental data for the reaction of 4-methylbenzaldehyde
Entry
1
Catalyst
TMU-21
Solvent
MeOH
EtOH
H2O
CH3CN
MeOH
EtOH
H2O
CH3CN
MeOH
EtOH
H2O
CH3CN
MeOH
EtOH
H2O
Yield (%)b
70%
68%
93%
—
87%
77%
92%
—
47%
43%
40%
—
58%
57%
45%
—
Yield (%)c
99%
95%
100%
—
99%
90%
100%
—
52%
44%
80%
—
60%
46%
85%
—
2
3
4
R-TMU-21
TMU-6
R-TMU-6
CH3CN
a
GC yield, Reaction conditions: malononitrile (0.3mmol), 4-methylbenzaldehyde (0.2mmol),25°C,
30 min., 3 mL of solvent.
b The product was obtained by 2mol% of catalyst
c The product was obtained by 4mol% of catalyst

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Table 3.The experimental data for the reaction of 4-clorobenzaldehyde
Entry
1
Catalyst
TMU-21
Solvent
Yield (%)b
Yield (%)c
MeOH
EtOH
H2O
CH3CN
MeOH
EtOH
H2O
CH3CN
MeOH
EtOH
H2O
CH3CN
MeOH
EtOH
H2O
94%
92%
100%
—
98%
99%
100%
—
63%
64%
58%
—
89%
75%
62%
—
91%
89%
100%
—
99%
94%
100%
—
85%
85%
91%
—
92%
86%
90%
—
2
3
4
R-TMU-21
TMU-6
R-TMU-6
CH3CN
a GC yield, Reaction conditions: malononitrile (0.3 mmol), 4-clorobenzaldehyde (0.2mmol), 25°C, 30
min.,3 mL of solvent.
b The product was obtained by 2 mol% of catalyst
c The product was obtained by 4 mol% of catalyst
Table 4. Optimized reaction conditions for the knoevenagel reaction with more reactive catalysts.
Entry
X
H
CH3
Cl
Catalyst
mol(%)
2%
2%
Solvent
H2O
H2O
Yield(%)
100%
100%
1
2
3
TMU-21 or R-TMU-21
TMU-21 or R-TMU-21
TMU-21 or R-TMU-21
2%
H2O
100%
a
GC yield, Reaction conditions: malononitrile (0.3 mmol), 4-clorobenzaldehyde (0.2mmol), 25°C, 30
min.,3 mL of solvent
Table 5.The experimental data for the control reaction.
Entry
substrate
Solvent
MeOH
MeOH
MeOH
Yield(%)
40%
40%
1
2
3
benzaldehyde
4-methylbenzaldehyde
4-chlorobenzaldehyde
60%
a GC yield, Reaction conditions: malononitrile (0.3 mmol), aldehyde (0.2 mmol),
25°C, 30 min.,3 mL of solvent

ACCEPTED MANUSCRIPT
Fig. 2 Recyclability of TMU-21 in consecutive runs for the reaction of malononitrile with benzaldehyde in H2O solvent.
Fig. 3 Recyclability of R-TMU-21 in consecutive runs for the reaction of malononitrile with benzaldehyde in H2O solvent.

ACCEPTED MANUSCRIPT
Graphical abstract

ACCEPTED MANUSCRIPT
Highlights


Four isoreticular MOFs were easily prepared via facile mechanosynthesis method.
The effect of N-donor pillar ligands resulting different characteristics are investigated.

The catalyst reactivity was evaluated in Knovenagel reaction.