Mn-MOF@Pi composite: synthesis, characterisation and an efficient catalyst for the Knoevenagel condensation reaction

Article abstract of DOI:10.1016/j.tetlet.2017.11.004

We have synthesised new Mn-MOF@Pi composite by encapsulation of piperidine in desolvated Mn-MOF and investigated its catalytic performance in Knoevenagel condensation reaction. The newly developed composite is compatible with various aromatic aldehydes an

Full text of DOI:10.1016/j.tetlet.2017.11.004

Tetrahedron Letters xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Mn-MOF@Pi composite: synthesis, characterisation and an efficient
catalyst for the Knoevenagel condensation reaction
b,
Darsi Rambabu b, Mohammad Ashraf b, Pooja c, Ankush Gupta a, , Abhimanew Dhir
⇑
⇑
a Department of Chemistry, DAV University, Jalandhar, Punjab, India
b Indian Institute of Technology (IIT), Mandi 175005, Himachal Pradesh, India
c DAV College, Jalandhar, Punjab, India
a r t i c l e i n f o
a b s t r a c t
Article history:
We have synthesised new Mn-MOF@Pi composite by encapsulation of piperidine in desolvated Mn-MOF
and investigated its catalytic performance in Knoevenagel condensation reaction. The newly developed
composite is compatible with various aromatic aldehydes and converting them to the desired
Knoevenagel condensation products in good yields and selectivity. Furthermore, composite Mn-
MOF@Pi is reusable and shows good catalytic activity than native MOF.
Received 9 September 2017
Revised 31 October 2017
Accepted 2 November 2017
Available online xxxx
Ó 2017 Elsevier Ltd. All rights reserved.
Keywords:
MOF-composites
Knoevenagel condensation reaction
Mn-MOF@Pi composite
Catalytic activity
Introduction
advantages of both MOFs (ultrahigh porosity, low density and crys-
talline pores) and functional materials (having unique optical, elec-
Metal-organic frameworks (MOFs) are the compounds having
ultrahigh porosity and incredibly large internal surface areas up
to 10000 m2 per gram1 which play an important role in functional
applications, typically in gas storage and separation2 sensing3 and
drug delivery.4 Therefore, many research groups worldwide are
involved to develop new MOF structures and exploring their vari-
ous applications. However, poor chemical stability of MOFs
prompted researchers to further enhance the properties by intro-
ducing new functionalities. In view of this, MOF composites are
composed of MOF and other distinct constituent materials, includ-
ing other MOFs, with properties different from those of the individ-
ual components.5 In recent years, MOF composites have attracted
great attention in various research fields due to their potential
applications in gas adsorption and storage, liquid adsorption, mole-
cule separation, biomedical applications, drug delivery and cataly-
sis.6 Therefore, enhanced performance of MOF composites
resulting from the cooperative of both MOF and other active com-
ponents make them good candidate for a wide range of applica-
tions. To date various MOF composites have been reported with
metal nanoparticles, quantum dots, polymers, graphene, carbon
nanotubes, biomolecules and so on. In composite materials, the
trical, magnetic and catalytic properties) can be combined
effectively, which results into new physical and chemical proper-
ties as well as enhanced performance as compared to individual
constituents.7 In this context, we aimed at the synthesis of new
MOF-composite for Knoevenagel condensation reaction. This reac-
tion is traditionally catalyzed by solid bases like alkali metal
hydroxide or weak bases like primary, secondary and tertiary ami-
nes under homogeneous conditions with the limitation of catalyst
recycling and recovery.8 Over the past few years, a wide range of
catalysts such as Lewis acids9 amine-functionalized solid sup-
ports10 cation-exchanged zeolites11 ionic liquids12 and
organometallic catalysts13 have been investigated for this reaction.
However, number of these methods has limitations of using haz-
ardous solvents, high catalyst loading and non-recoverable cata-
lysts. In light of this, there is
a great demand for the
development of new catalytic systems that is free from these prob-
lems. Keeping in view of all the considerations, we have synthe-
sized new Mn-MOF@Pi composite by encapsulation of piperidine
in desolvated Mn-MOF and investigated its catalytic performance
in Knoevenagel condensation reaction. To our pleasure, the newly
developed composite is compatible with various aromatic aldehy-
des and converting them to the desired Knoevenagel condensation
products in good yields and selectivity. Furthermore, composite
Mn-MOF@Pi is reusable and shows good catalytic activity than
native MOF. To the best of our knowledge, this is the first report
⇑
Corresponding authors.
E-mail addresses: chemankush10315@davuniversity.org (A. Gupta), abhimane-
w@iitmandi.ac.in (A. Dhir).
https://doi.org/10.1016/j.tetlet.2017.11.004
0040-4039/Ó 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Rambabu D., et al. Tetrahedron Lett. (2017), https://doi.org/10.1016/j.tetlet.2017.11.004

2
D. Rambabu et al. / Tetrahedron Letters xxx (2017) xxx–xxx
of this type of Mn-MOF-Pi composite prepared by deposition of
piperidine in Mn-MOF.
Results and discussion
Mn-MOF(Mn3(NDC)3(DMF)4) was synthesized according to
slightly modified previously reported procedure.14 The reaction
of 2,6-napthlene dicarboxylic acid (H2NDC) with Mn(NO3)2 4H2O
in solvent DMF gave yellow colour crystals of Mn-MOF in 69.9%
yield (Scheme 1).
Scheme 2. Synthesis of Mn-MOF@Pi composite.
In addition to FTIR study, immobilization of piperidine in
D_Mn-MOF was also confirmed by TGA analysis. For this, we
recorded the TGA of Mn-MOF@Pi composite which shows a grad-
ual weight loss up to 140 °C corresponding to piperidine loss. How-
ever, such kind of loss was not observed in D_Mn-MOF confirms
the formation of MOF composite Mn-MOF@Pi (Fig. S3).
In addition, sharp peaks observed in powder XRD spectrum
(Fig. S4) of MOF composite Mn-MOF@Pi as compared to desolvated
Mn-MOF (D_Mn-MOF) showed revival of partial crystalline nature
which might be due to the entrapment of piperidine molecules.
Further, the desolvated MOF was prepared by heating Mn-MOF
at 195 °C for 24 h.14 It was observed that on desolvation the colour
of Mn-MOF changes from yellow to grey (Scheme 1 and Fig. S1)
and named as D_Mn-MOF which was characterized by FTIR, pow-
der XRD and TGA studies.
The presence of solvent molecule in Mn-MOF and desolvation of
Mn-MOF were confirmed by FTIR spectroscopy. The FTIR spectrum
of Mn-MOF showed asymmetric stretching band at 2933 cmÀ1 cor-
responding to aliphatic CAH group of DMF molecule. In addition to
this, a strong stretching band of the carbonyl group of DMF was
observed at 1658 cmÀ1 thus, confirms the presence of coordinated
DMF solvent molecule in Mn-MOF (Fig. S2). Further, in the IR spec-
trum of D_Mn-MOF, absence of bands corresponding to C @ O and
CAH indicated the release of coordinated DMF molecule from Mn-
MOF, thus, shows the desolvation of Mn-MOF (Fig. S2).
Removal of DMF molecule was also investigated by TGA analy-
sis where TGA curve of Mn-MOF showed a weight loss from 140 °C
to 210 °C corresponding to the loss of DMF molecules (Fig. S3),
however, such kind of loss was not observed in case of D_Mn-
MOF from 140 °C to 210 °C, further confirms the formation of
D_Mn-MOF.
Optimization of the reaction conditions for Knoevenagel condensation
reaction
Presence of piperidine molecule and metal-carboxylate in the
Mn-MOF@Pi composite prompted us to investigate its behavior
as a catalyst for Knoevenagel condensation reaction. It was
observed that condensation of aldehydes with compounds contain-
ing activated methylene groups often used reaction to test the cat-
alytic activities of basic catalysts.15 Typically, Knoevenagel
condensation reactions can be catalyzed by bases, acids or materi-
als containing both acidic and basic sites.16–25 For this, Knoeve-
nagel condensation of para-methoxy benzaldehyde and
malononitrile in the presence of Mn-MOF@Pi was monitored in dif-
ferent solvents at room temperature (RT) (Table 1). We found that,
there was no product formation in the absence of Mn-MOF@Pi at
RT (Table 1, entry 1). In addition to this, we found that, toluene,
chloroform, acetonitrile, and THF are poor solvents used for Kno-
evenagel condensation. In DMF, the condensation of aldehyde
reached only to 65% conversion. The highest activity of Mn-
MOF@Pi composite as catalyst found when water was used as sol-
vent. The highest yield of 98% was obtained using water as a sol-
vent under same condition. Therefore, the optimized conditions
were found to be 10 mg of catalyst Mn-MOF@Pi in water, (98%,
Table 1 entry 9) with reaction time of 2 h at room temperature.
We also studied the effect of temperature on yield of the reaction
which shows that increase in temperature lead to increase in the
rate of reaction with decrease in reaction time (Fig. S5). The yield
was calculatedusing an internal standard method by NMR where
The powder XRD of Mn-MOF exhibit sharp peaks because of its
crystalline nature whereas the PXRD of D_Mn-MOF showed broad
peaks indicated its amorphous nature after the removal of DMF
molecules (Fig. S4).
Synthesis and characterization of Mn-MOF@Pi composite
For immobilization of piperidine in D_Mn-MOF, it was incu-
bated in piperidine at room temperature for 72 h (Scheme 2). After
incubation, the gray colour of compound D_Mn-MOF became dis-
tinctly brown which might be due to the translocation of piperi-
dine into the MOF (Fig. S1). We named this composite as Mn-
MOF@Pi. The presence of the piperidine into the pores of D_Mn-
MOF was further confirmed by FTIR, TGA and powder XRD studies.
The FTIR spectrum of composite Mn-MOF@Pi showed a stretch-
ing band at 2947 cmÀ1 which is characteristic CAH bond stretching
frequency of piperidine, thus, indicated the presence of piperidine
in D_Mn-MOF (Fig. S2).
Scheme 1. Synthesis and desolvation of Mn-MOF.
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D. Rambabu et al. / Tetrahedron Letters xxx (2017) xxx–xxx
3
Table 1
reaction time (entries 2, 3, 4, 6 and 9 in Table S1) as compared to
the electron-donating groups (entry 5 and 10 in Table S1). In addi-
tion, the catalytic activity was extended to various aldehydes with
ethyl cyanoacetate and showed high activity (entries 7–11). Elec-
tron-withdrawing derivatives, such as 4-nitrobenzaldehyde (entry
9) along with the electron-donating derivatives, such as 4-hydrox-
ybenzaldehyde (entry 7), showed satisfactory yields of 95–98%. To
our pleasure, the electron-withdrawing group gave the products in
shorter time as compared to the electron-donating groups. These
results suggest that Mn-MOF@Pi composite successfully promoted
the Knoevenagel condensation reactions of various aldehydes hav-
ing either electron-withdrawing groups or electron-donating
groups.
Optimization conditions for the Knoevenagel condensation reaction.a
Entry
Solvent
Time(h)
Yield (%)b
c
1
2
3
4
5
6
7
8
9
Water
3
3
3
3
3
3
3
3
2
–
Methanol
Ethanol
DMF
Acetonitrile
Chloroform
THF
95
75
65
35
30
25
15
98
Reusability of Mn-MOF@Pi
Toluene
Water
In addition to above studies we also investigated the reusability
of Mn-MOF@Pi composite. Interestingly, Mn-MOF@Pi also showed
excellent reusability as catalyst without significant decrease in its
activity. For reusability of the Mn-MOF@Pi catalyst, we reused the
same catalyst in four cycles of para-methoxy benzaldehyde and
malononitrile condensation reaction and even after fourth cycle
the time taken to complete the reaction was 2 h which was equiv-
alent to the time taken by the catalyst during first cycle (Fig. 1),
thus, indicated not any significant loss in the activity of the catalyst
and the catalyst can be reused for the Knoevenagel condensation
reaction. The stability of the reused catalyst after 4th cycle was
confirmed by FTIR (Fig. S6), powder XRD (Fig. S7) and SEM
(Fig. 2) analysis, which showed that the chemical nature of the cat-
alyst remains similar, even after the 4th cycle.
a
Reaction conditions: Mn-MOF@Pi (10 mg), Aldehyde (1 mmol), Active methy-
lene compound (1.5 mmol) in 2 ml solvent at room temperature (RT).
b
Isolated yield.
No catalyst was used.
c
NMR spectra of reaction mixture was taken after fixed interval of
time and dibromomethane was used as an internal standard.
Proposed mechanism for Knoevenagel condensation over Mn-MOF@Pi
Knoevenagel condensation reactions can be catalyzed by bases,
acids or materials containing both acidic and basic sites.16–25
Metal-carboxylate based MOFs possess ‘‘Mn+–O2– Lewis acid–base’’
pair and participate in the Knoevenagel catalysis through deproto-
nation, aldol and dehydration steps.23,24 We propose the following
mechanism (Scheme S1) for the Knoevenagel condensation with
Mn-MOF@Pi. The activation of the carbonyl group of benzaldehyde
proceeds at the Lewis acid site (stage 1).23,24 Simultaneously, the
basic site of the Mn-MOF@Pi leads to removal of acidic proton of
methylene group of malononitrile with generation of a carbanion.
The carbanion reacts with the carbonyl of benzaldehyde to give
intermediate (I).23,24 Afterwards, in intermediate (I) there occurs
a rearrangement with removal of water molecule (stage 3), leading
to formation of condensation product. Simultaneously, In addition
to metal-carboxylate of Mn-MOF@Pi, piperidine (Pi) is also act as
catalyst which is present in the Mn-MOF@Pi composite and it is
well known base catalyst for the Knoevenagel condensation reac-
tion26 (Scheme S1). Due to presence of piperidine, the Knoevenagel
condensation catalytic activity of catalyst Mn-MOF@Pi further
increases.
Comparison with different catalysts
We have compared the catalytic activity of Mn-MOF@Pi com-
posite with Mn-MOF and desolvated Mn-MOF. The result showed
that the rate of the reaction was fast in case of Mn-MOF@Pi com-
posite compared to native Mn-MOF (Fig. 3). However the catalytic
activity was moderate in the case of D_Mn-MOF due to the
increase of pore size after the removal of solvent. Therefore, the
improved catalytic activity of Mn-MOF@Pi (over D-Mn-MOF) could
also be due to the combined catalytic effects of Mn-MOF and piper-
idine incorporated in it. We also compared the catalytic efficiency
of the Mn-MOF@Pi with various MOFs that have been utilized for
condensation of benzaldehyde and malononitrile and the results
are shown in Table S2. In comparison to reported MOF systems,
Scope of the catalytic Knoevenagel condensation activity with Mn-
MOF@Pi
After optimizing the reaction conditions in hand, Mn-MOF@Pi
used as catalyst on variety of different aldehydes and active
methylene compounds listed in Table S1. It was found that Mn-
MOF@Pi exhibits good catalytic activity and desired products were
obtained in good yields (Table S1). In addition to this, we observed
that aldehydes readily undergo condensation with malononitrile
compared with ethyl cyanoacetate (Table S1). This may be due to
the fact that abstraction of a proton from the active methylene
group of ethyl cyanoacetate is difficult due to lower acidity com-
pared to malononitrile. The catalytic activity of Mn-MOF@Pi was
also affected by substituent group present on the aromatic ring
of aldehyde.
The electron-withdrawing groups present on aromatic aldehy-
des promoted this condensation efficiently with reduction in the
Fig. 1. Recyclability of the catalyst Mn-MOF@Pi was tested for the condensation of
para-methoxy benzaldehyde and malononitrile in water at RT (2 h time duration).
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D. Rambabu et al. / Tetrahedron Letters xxx (2017) xxx–xxx
Fig. 2. SEM images of (A) Mn-MOF@Pi and (B) Mn-MOF@Pi after 4th cycle.
tute of Technology, Mandi, Himachal Pradesh, India for facilities.
DRB thankful to Council of Scientific and Industrial Research (CSIR)
India for research fellowship and DRB acknowledges Mr. Prateep
Singh Sagara and for the helping in NMR analysis.
A. Supplementary data
Supplementary material that may be helpful in the review pro-
cess should be prepared and provided as a separate electronic file.
That file can then be transformed into PDF format and submitted
along with the manuscript and graphic files to the appropriate edi-
torial office.
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.tetlet.2017.11.004.
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Acknowledgments
A.G. is highly thankful to DAV University, Jalandhar, India for
providing the research facilities. A.D. is thankful to Department
of Science and Technology, New Delhi, India for INSPIRE faculty
award and Advance Material Research Centre (AMRC), Indian Insti-
Please cite this article in press as: Rambabu D., et al. Tetrahedron Lett. (2017), https://doi.org/10.1016/j.tetlet.2017.11.004