Post-modified anionic nano-porous metal-organic framework as a novel catalyst for solvent-free Michael addition reactions

Article abstract of DOI:10.1039/c4ra05226a

By the strategy developed here, a novel application of anionic metal-organic frameworks as heterogeneous catalysts has been introduced. We have been able to prepare a cation exchanged anionic MOF as a new heterogeneous catalytic system for solvent-free Michael addition of pyrrole to electron-deficient β-nitrostyrenes. This catalyst was easily recovered and reused for at least six reaction cycles without a significant degradation in activity. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra05226a

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Post-modified anionic nano-porous metal–organic
framework as a novel catalyst for solvent-free
Michael addition reactions†
Cite this: RSC Adv., 2014, 4, 37036
Saeideh Beheshti and Ali Morsali*
By the strategy developed here, a novel application of anionic metal–organic frameworks as heterogeneous
catalysts has been introduced. We have been able to prepare a cation exchanged anionic MOF as a new
heterogeneous catalytic system for solvent-free Michael addition of pyrrole to electron-deficient
b-nitrostyrenes. This catalyst was easily recovered and reused for at least six reaction cycles without a
significant degradation in activity.
Received 2nd June 2014
Accepted 30th July 2014
DOI: 10.1039/c4ra05226a
www.rsc.org/advances
harmful solvents in this environmentally friendly process is
particularly appealing.
1
. Introduction
The Michael reaction is a useful transformation that has been
widely employed for carbon–carbon bond formation in the
synthesis of several ne chemicals. Nitroolens are among the
1
2. Experimental
strongest Michael acceptors because the nitro moiety in them is 2.1. Materials and physical techniques
a strong electron-withdrawing group that can be readily trans-
formed into a range of different functionalities which lead to
All reagents for the synthesis and analysis were commercially
available and used as received. Microanalysis was carried out
using a Heraeus CHN–O– rapid analyzer. Melting points were
measured on an Electrothermal 9100 apparatus and are
uncorrected. IR spectra were recorded using Perkin-Elmer 597
important building blocks for agricultural and pharmaceutical
2–7
compounds. To date, a wide range of catalysts have been
investigated for this reaction. However, most of them were Lewis
8–10
acids.
Solid base catalysts have been scarcely explored for
1
and Nicolet 510P spectrophotometers. H-NMR spectra were
application in the Michael addition of pyrrole to nitroolens.
measured in d -DMSO with a BRUKER DRX-500 AVANCE
6
Because of rather modest basicity of these heterogeneous
spectrometer at 500 MHz and all chemical shis are reported in
d units downeld from Me Si. The molecular structure plot and
4
11
systems, chemoselectivity is more controllable in the reaction.
Metal–organic frameworks (MOFs), also known as porous coor-
dination polymers (PCPs), have attracted a great deal of interest
due to their large surface areas and high pore volumes. Their
potential applications in gas storage, separation and heteroge-
simulated XRD powder pattern based on single crystal data were
prepared using Mercury soware. X-ray powder diffraction
23
(XRD) measurements were performed using a Philips diffrac-
tometer of X'pert company with monochromated Cu-Ka radia-
tion. GC runs were performed on an Echrom GC A90 gas
chromatograph.
12–21
neous catalysis have been extensively studied.
Well-ordered
porous structures, exible and dynamic behaviours in
response to guest molecules and designable channel surface
functionalities are the most important advantages of MOFs as
22
2.2. Synthesis of [HDMA] [Zn (BDC) (DMA) ]$6DMF (1)
2
2
3
2
catalysts. Herein, we report for the rst time anionic MOFs as
efficient heterogeneous catalysts which effectively catalyze 60 mmol (9.960 g) 1,4-benzenedicarboxylic acid (H BDC), 60
2
Michael addition of pyrrole to b-nitrostyrenes under solvent-free mmol (17.820 g) Zn(NO ) $6H O and 30 mmol (3.840 g) ter-
3
2
2
condition to afford the corresponding Michael adducts in good ephthalodinitrile were dissolved in 100 mL DMF. The mixture
to excellent yields. In this case, by cation-exchange as post- was transferred to a large Teon-lined steel autoclave and
ꢀ
modication in the anionic MOF, [HDMA] [Zn (BDC) (DMA) ]$ heated at 125 C. Aer 48 hours, the Teon-lined steel autoclave
2
2
3
2
ꢀ
6
DMF, catalytic activity was improved. Avoiding the utilization of was slowly cooled to room temperature with the rate of 0.1 C
ꢁ
1
min . The crystalline product was washed with DMF and store
ꢀ
in it, d.p. ¼ above 300 C, yield: 5.345 g, 34% based on nal
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box
product, anal. calc. for C H N O Zn: C, 48.27; H, 6.82; N,
2
5
42 5 9
1
+
†
4115-175, Tehran, Islamic Republic of Iran. E-mail: morsali_a@modares.ac.ir; Tel:
98-21-82884416
1
1.26, found; C, 48.21; H, 6.85; N, 11.22%. The XRD pattern of
resulting product (Fig. S1†) approved that compound 1 was
synthesized successfully in large scale.
Electronic supplementary information (ESI) available: Other gures, full
12
synthetic and analytical details. See DOI: 10.1039/c4ra05226a
37036 | RSC Adv., 2014, 4, 37036–37040
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.3. Post-synthetic cation exchange process of 1 with
RSC Advances
ꢀ
2
was stirred at 70 C for 4 h. The course of the reaction was
followed by GC analysis. Reaction mixture diluted with 5 mL of
CH Cl . On completion of the reaction, the catalyst was recov-
2 2
+
tetraethylammonium (TEA ) and tetrapropylammonium
+
(
TPA )
+
ered by centrifugation and the supernatant liquid was collected
and evaporated to dryness. Pure products were obtained by
short column chromatography on silicagel (ethyl acetate–
petroleum ether). The catalyst was washed repeatedly with
EtOAc, dried and reused when required.
Dimethylammonium cations (HDMA ) in 1 were replaced with
+
either tetraethylammonium (TEA ) or tetrapropylammonium
TPA ) cations via cation exchange to yield [TEA]
+
(
(
2
[Zn
]$xDMF (3),
2 3
(BDC) -
DMA)
2
]$xDMF (2) and [TPA]
2
[Zn
2
(BDC)
3
(DMA)
2
respectively. Crystals of as-synthesized 1 were immersed in a
saturated solution of TEA bromide and TPA iodide in DMF at
+
+
room temperature. The crystals were soaked for ten days, during 3. Results and discussion
this time the saturated solution was refreshed three times daily.
3.1. Characterization the post-synthetic MOF materials of 2
Unfortunately the single crystallinity of these samples was not
maintained during these cation exchange processes and we
could not determine the structures of these cation exchanged
samples by single crystal X-ray crystallography. Aer decanting
the metal ion saturated solutions, the cation-exchanged
samples were rinsed and soaked in DMF for up to three days
and 3
Fig. S3a† shows the IR spectrum of 1. The typical bands that are
2
ꢁ
related to the BDC ligands, dimethylammonium ions, DMF
and coordinated amine molecules can be observed in the
vibrational spectra of 1. The stretching vibration bands of
methyl groups between 2850 and 2950 cm belong to the DMF
molecules, dimethylamine (DMA) and to the ammonium ions
ꢁ
1
+
+
to remove residual TEA bromide and TPA iodide from the
frameworks. Anal. calc. for [TEA] [Zn (BDC) (DMA) (2)
: C, 54.27; H, 6.83; N, 5.75, found; C, 54.31; H,
.79; N, 5.81%. Anal. calc. for [TPA] [Zn (BDC) (DMA) ]$xDMF
3) C52 : C, 57.51; H, 7.61; N, 5.16, found; C, 57.48;
H, 7.65; N, 5.21%.
2
2
3
2
]
+
ꢁ1
ꢁ1
(HDMA ). Vibrational bands at 1602 cm and 1386 cm indi-
44 66 4 2
C H N O12Zn
cate the asymmetric and symmetric stretching vibration of the
carboxylic groups of the BDC coordinated to a metal center.
Other characteristic bands of the BDC ligands due to the C]C
stretching vibration of the p-disubstituted aromatic ring are
found at 1507 cm . The absorption at 1680 cm
assigned to the C]O stretching of the amide (DMF). It is also
possible to assign some bands to the dimethylammonium ion:
6
2
2
3
2
2ꢁ
(
82 4 2
H N O12Zn
2
ꢁ
ꢁ
1
ꢁ1
can be
2
.4. Synthesis of [HDMA] [Zn (BDC) ]$DMF$H O (4) and
2 3 4 2
preparation of its single crystals
24
This anionic MOF was reported by Burrows et al. Colourless the broad N–H stretching bands in the region between 2850 and
ꢁ1
crystals of 4 were prepared by dissolving 10 mmol (1.660 g) 1,4- 2700 cm
are typical for the hydrogen bonds of the two
ꢁ1
2
benzenedicarboxylic acid (H BDC), 10 mmol (2.970 g) hydrogen atoms of the ammonium ion. The band at 3068 cm
+
Zn(NO $6H
3
)
2
2
O and 5 mmol (1.050 g) 4-bpdb (1,4-bis(4-pyridyl)- belongs to the N–H stretching vibrations. IR spectra of the post
+
+
2,3-diaza-1,3-butadiene) in 8 mL DMF. Aer preparing a colloidal synthesized MOF materials with TEA (2) and TPA (3) are
solution by stirring at room temperature, it was transferred to the illustrated in Fig. S3b and c,† respectively. They are not signi-
ꢀ
Teon-lined steel autoclave and heated at 125 C. Aer 48 hours, cantly different from 1 (Fig. S3a†), but the broad N–H stretching
ꢁ1
the Teon-lined steel autoclave was slowly cooled to room bands in the region between 2850 and 2700 cm and the band
ꢀ
ꢁ1
ꢁ1
+
temperature with the rate of 0.01 C min . The resulting at 3068 cm that is related to existence of HDMA ions in 1 were
ꢁ
1
product was washed with DMF and kept in it, d.p. ¼ above 300 disappeared and new bands at 2970 cm were appeared that are
ꢀ
+
+
+
C, yield: 2.072 g, 80% based on nal product. Anal. calc. for related to replacement of HDMA ions with TEA and TPA .
: C, 45.21; H, 4.00; N, 4.05, found; C, 45.23; H,
The post-synthetic MOF materials of 2 and 3 maintain their
.95; N, 3.99%. The XRD pattern of resulting product (Fig. S7†) crystallinity during and aer cation exchange, as evidenced by
approved that compound 4 was synthesized successfully.
X-ray powder diffraction (Fig. S4†). Compound 1 has essential
39 41 3 3
C H N O18Zn
3
ꢀ
ꢀ
peaks at 2q ¼ 5.33 and 2q ¼ 8–12 . If these essential peaks
remain aer cation exchange process, we could conclude that
the structure of this MOF does not change approximately during
cation exchange. It should be also mentioned that the
compound 3 is stable in benzene, toluene, dichloromethane
and ethanol but unstable in tetrahydrofuran (THF) (Fig. S6†).
2
.5. Activation of anionic MOF materials and preparation of
their apohost materials
In order to activate anionic MOF materials of 1–4 and prepara-
tion of their apohost materials, we exchanged guest DMF
molecules with MeCN. Selection of MeCN was due to linear
structure of it and probably its easy removal from channels of 1–
1
Fig. S5a† shows the H-NMR spectrum of compound 1
crystalline sample aer washing several times with acetone in
order to remove surface adsorbed DMF molecules. Since this
MOF due to its especial 3D framework is insoluble, molecules of
4
. These guest exchanges were done by immersing each samples
of 1–4 in MeCN for 10 days, as the MeCN was refreshed each day.
ꢀ
Since the boiling point of MeCN is about 81 C, the exchanged
6
d -DMSO as solvent exchange with guest DMF molecules and
ꢀ
sample with MeCN was heated in furnace at 100 C for 8 hours.
results in appearance of DMF peaks at 2.71 and 2.87 ppm.
Fig. S5b† is attributed to crystalline sample of 1 in the presence
of excess ammonium acetate. Existence of excess ammonium
2
.6. Michael addition
To a mixture of pyrrole (1.1 mmol) and b-nitrostyrenes (NH
1 mmol), was added 10 mol% of catalyst. The resulting mixture with NH
+
4
+
) ions in NMR tube resulted in replacement of HDMA
+
+
(
4
ions. Thus HDMA organic cations removed from
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one-dimensional channels of 1 and entered to the solution samples of [TEA]
2
[Zn
2
(BDC)
3
(DMA)
2
]
(2) and [TPA]
2
[Zn
2
-
+
phase. Now HDMA peak can be observed at 2.45 ppm. The (BDC) (DMA) ] (3), were used aer post-synthetic cation
3
2
observed peak in region of 1.74–1.80 ppm is attributed to exis- exchange of 1 with larger organic cations of tetraethylammo-
ꢁ
+
+
tence of excess acetate (CH COO ) ions in solution. Fig. S5c† nium (TEA ) and tetrapropylammonium (TPA ), respectively. By
3
1
+
+
shows the H-NMR spectrum of 2 aer washing several times using larger organic cations of TEA (in 2) and TPA (in 3), it was
with acetone. As the HDMA peak only appears in the presence expected to decrease the acidic properties of them and as a
of excess ammonium acetate, we added 5 mg of NH COO to result, basicity of the anionic framework increased. The results
4
the NMR tube of 2. Fig. S5c† shows that HDMA peak at 2.45 of applying the apohost frameworks of 2 and 3 in Michael
+
+
ꢁ
+
ppm was disappeared and new peaks at 1.2 and 3.2 ppm appear reaction approved this hypothesis and indicate that the catalytic
+
which indicate that completely replacement of HDMA ions activity decreased in the order of 3 > 2 > 1 (Table 1).
+
1
were occurred by TEA organic cations. Fig. S5d† shows the H-
As it is obvious, the ionization potential decreases in the
+
+
+
NMR spectrum of 3 aer washing several times with acetone. following order HDMA > TEA > TPA . Indeed by increasing in
Since the HDMA peak only appears in the presence of excess ionization potential of attributed organic cation in the 1D
ammonium acetate, we added 5 mg of NH
tube of 3. Fig. S5d† shows that HDMA peak at 2.45 ppm was nally, basic property of the anionic framework decreased.
+
+
ꢁ
4
COO to the NMR channels of 1, the acidic properties of ions increased and
+
25
disappeared and new peaks at 0.9, 1.6 and 3.2 ppm appear Similar process has also been observed in zeolites. Thus we
+
which indicate that completely replacement of HDMA ions could tune and increase the basic property of the anionic
+
+
were occurred by TPA organic cations.
framework of 1 by cation exchange process of HDMA with
+
+
[
HDMA] [Zn (BDC) (DMA) ]$6DMF (1), (HDMA : dimethy- larger organic cation of TPA (in 3). As shown in Table 1, best
2
2
3
2
2
ꢁ
lammonium, BDC : 1,4-benzenedicarboxylate, DMA: dimethyl- result obtained when [TPA] [Zn (BDC) (DMA) ] was used as
2
2
3
2
amine and DMF: N,N-dimethylformamide) is an anionic nano- catalyst (entry 3). In the absence of catalyst, no product was
porous MOF with 1D channels along the crystallographic b observed even aer 24 h, suggesting the potential catalytic
12
axis (Fig. 1). These channels with rectangular shapes (with activity of anionic MOFs in this reaction (Table 1). Compared
˚
width and length of about 7.7 and 12.5 A, respectively) are with other catalytic systems, attractive features of our approach
+
occupied with guest DMF molecules and HDMA cations include (1) avoiding environmentally unfriendly solvents such
2
(
Fig. S2a in the ESI†). BDC in the primary basic unit of 1 shows as aromatic hydrocarbons and halogenated solvents, (2) shorter
2
ꢁ
monodentate coordination behavior and each BDC ligand reaction time, (3) lower amount of the used catalyst and (4) high
bridges between two Zn ions (Fig. S2b in the ESI†). Only one yields.
type of Zn ion is present in 1 and in addition to three oxygen aromatic b-nitrostyrenes were employed under similar reaction
atoms from three carboxylate groups of three BDC , a nitrogen conditions to evaluate the scope of this reaction. The results are
atom from dimethylamine (DMA) molecule, which creates shown in Table 2. These reactions proceeded smoothly and no
II
26–28
Aer optimizing the reaction conditions, a variety of
II
2
ꢁ
II
from DMF hydrolysis, coordinates to each Zn ion (Fig. S2b in undesirable side reactions were observed. Both electron
the ESI†).
donating and -withdrawing groups on the phenyl ring were well
Catalytic activity of compound 1 apohost framework was tolerated affording the expected products in good yields but
studied in Michael addition of pyrrole to b-nitrostyrene. We using substrates with electron withdrawing groups led to higher
thought that catalytic activity of 1 is as a result of its anionic yields.
framework. Thus In order to approve of this idea, activated
2 2 3 2
In addition, [TPA] [Zn (BDC) (DMA) ] could be reused at
least 6 runs without any loss in its activity (Fig. 2) so that powder
XRD showed no changes in its structure (Fig. S10 in the ESI†).
To probe whether the catalytic reaction occurred inside the
channels of 1 (and other cation exchanged samples of 2 and 3)
Table 1 Michael addition of pyrrole to b-nitrostyrene catalysed by
anionic MOFs
Entry
Catalyst
[HDMA]
Time (h)
Yield (%)
1
2
3
4
5
2
[Zn
(BDC)
(BDC)
[Zn (BDC)
Without catalyst
2
(BDC)
(DMA)
(DMA)
3
(DMA)
2
]
4
4
4
4
54
70
100
21
0
[TEA]
[TPA]
[HDMA]
2
[Zn
2
3
2
]
Fig. 1 Showing the existence of one-dimensional channels along the
crystallographic b axis in the anionic nano-porous MOF of 1, guest
2
[Zn
2
3
2
]
2
4
3
]
+
DMF molecules and organic HDMA cations are omitted for clarity.
24
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Table 2 Michael addition of pyrrole to b-nitrostyrene derivatives Table 3 Effect of solvents on Michael addition of pyrrole to b-nitro-
catalysed by 3
styrene catalysed by 3
Entry
R
Yield (%)
Entry
Solvent
Yield (%)
1
2
3
4
5
6
Ph
4-NO
3-NO
4-Cl–C
4-MeO–C
4-Me–C
100
100
100
98
94
91
b
1
CH
Ethanol
CH CN
DMF
Toluene
2
Cl
2
28
43
25
30
23
31
100
2
–C
–C
H
6 4
6
H
H
4
2
3
4
5
2
6
4
3
6
H
4
6
H
4
6
b
CHCl
3
—
7
a
b
For all runs, the solvent volume was 3 mL. Reaction was carried out
at room temparature.
or on the surface of them, similar anionic MOF 4 with smaller
pore sizes was used.
2ꢁ
Furthermore, another zinc based anionic MOF with BDC
linker was used as catalyst in this reaction. {(HDMA) [Zn (BDC) ]$
2
2ꢁ
3
4
on the external surface of 4. On the other hand this result
approved that the mentioned Michael addition was catalyzed in
the 1D channels of 1–3. Therefore, in anionic MOFs of 1–3, we
could benet from internal anionic surfaces of 1D channels in
performing Michael addition.
For Michael addition of pyrrole to b-nitrostyrenes, different
solvents such as dichloromethane, ethanol, acetonitrile, DMF,
toluene and chloroform were studied to optimize the reaction
conditions. The results are shown in Table 3. Among these
solvents, the reaction under solvent-free condition was much
faster and occurred in 4 h with 100% yield due to high
concentration and more availability of reactants. The occur-
rence of efficient solid-state reactions shows that the molecules
reacting are able to move freely in the solid state.
+
2
DMF$H O} (4) (HDMA : dimethylammonium, BDC : 1,4-benze-
nedicarboxylate and DMF: N,N-dimethylformamide), as an
24
anionic MOF was previously reported by Burrows et al. Two types
II
of Zn ion are present in 4 (Fig. S2c in the ESI†). Indeed the
framework of 4 is made up from a trinuclear Zn
Fig. S2c in the ESI†). Zn(2) ion is located between two Zn(1) ions
and this Zn cluster unit is bridged by four carboxylate groups
3
cluster unit
(
3
ꢁ
2
2ꢁ
from BDC with m and two carboxylate groups from BDC with
4
m
4
coordination modes (Fig. S8 in the ESI†). The trinuclear Zn
3
2ꢁ
cluster units and the two types of BDC with m
modes form a 2D layer in the crystallographic bc plane. BDC
ligand with m coordination mode acts similar to a pillar ligand
4
coordination
2ꢁ
2
and links these layers to each other (Fig. S8 in the ESI†). Thus we
can consider compound 4 as an anionic layer-pillared MOF which
˚
has 2D channels with smaller aperture size of ꢂ5.5 ꢃ 8.0 A and
˚
4. Conclusions
6
.8 ꢃ 8.3 A along the crystallographic b and c axis, respectively
(
Fig. S9 in the ESI†). The anionic apohost framework of 4, showed
the lowest catalytic activity in comparison with 1 and other cation
exchanged samples of 2 and 3. 10 mol% of [HDMA] [Zn (BDC)
4) apohost framework as catalyst at room temperature leads to
In summary, we have developed a novel, highly efficient and
environmentally green methodology for Michael addition of
pyrrole to b-nitrostyrenes. Post-synthetic cation exchanged
MOFs of 1–3 with anionic frameworks were used as new solid
base catalysts in Michael addition reaction. Cation exchange by
2
3
4
]
(
only a 21% yield aer 4 h (Table 1). This low catalytic activity is
due to smaller apertures of 2D channels in 4. Thus the reactants
could not entrance to its channels and the reaction only catalyzed
+
cations with lower ionization potential than HDMA , i.e. bulky
cations, leads to higher basic property of the framework,
thereby increasing yield of the reaction. Best results obtained
when [TPA] [Zn (BDC) (DMA) ] (3) was used as catalyst. Further
2
2
3
2
studies focusing on expanding the application of this catalytic
system in other reactions are currently underway and will be
reported in due course.
Acknowledgements
This work was supported by Tarbiat Modares University.
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