Synthesis and catalytic activities of a Zn(ii) based metallomacrocycle and a metal-organic framework towards one-pot deacetalization-Knoevenagel tandem reactions under different strategies: A comparative study

Article abstract of DOI:10.1039/d0dt01312a

Solvothermal reactions between a pyridine based amide functionalized dicarboxylic acid, 4,4′-{(pyridine-2,6-dicarbonyl)bis(azanediyl)}dibenzoic acid (H₂L), and zinc(ii) nitrate in the absence and presence of a base produced the binuclear metallomacrocyclic compound [Zn₂(L)₂(H₂O)₄]·2(H₂O)·6(DMF) (1) and the metallomacrocyclic based two dimensional MOF [Zn₅(L)₄(OH)₂(H₂O)₄]_n·8n(DMF)·4n(H₂O) (2), respectively. Compound 1 bears two tetrahedral Zn(ii) centres, whereas the 2D framework 2 includes a penta-nuclear Zn(ii) cluster as a secondary building block unit, with two of the metal cations assuming a tetrahedral type geometry and the remaining three an octahedral type geometry. The topological analyses reveal that compound 1 has a 2-connected uninodal net and framework 2 has a 2, 8-connected binodal net. These compounds heterogeneously catalyse the tandem deacetalization-Knoevenagel condensation reactions carried out under conventional heating, microwave irradiation or ultrasonic irradiation. Comparative studies show that ultrasonic irradiation (final product yield of 99percent after 2 h of reaction time) provides the most favourable method (e.g., microwave irradiation leads to a final product yield of 91percent after 3 h of reaction time). Moreover, the catalysts can be reused at least for five consecutive cycles without losing activity significantly.

Full text of DOI:10.1039/d0dt01312a

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10.1039/D0DT01312A.
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DOI: 10.1039/D0DT01312A
Synthesis and Catalytic Activities of a Zn(II) Based Metallomacrocycle
and a Metal-Organic Framework towards One-pot Deacetalization−
Knoevenagel Tandem Reactions under Different Strategies: A
Comparative Study
Anirban Karmakar*, Mohamed M.A. Soliman, Guilherme M. D. M. Rúbio, M. Fátima C. Guedes da
Silva and Armando J. L. Pombeiro*
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,
1049–001, Lisbon, Portugal. E-mail: anirbanchem@gmail.com; pombeiro@tecnico.ulisboa.pt.
Abstract
Solvothermal reactions between a pyridine based amide functionalized dicarboxylic acid, 4,4'-
{(pyridine-2,6-dicarbonyl)bis(azanediyl)}dibenzoic acid (H2L), and zinc(II) nitrate in absence and
presence
of
base
produced
the
binuclear
metallomacrocyclic
compound
[Zn2(L)2(H2O)4].2(H2O).6(DMF) (1) and the metallomacrocyclic based two dimensional MOF
[Zn5(L)4(OH)2(H2O)4]n.8n(DMF).4n(H2O) (2), respectively. Compound 1 bears two tetrahedral Zn(II)
centres, whereas the 2D framework 2 includes a penta-nuclear Zn(II) cluster as secondary building
block unit, with two of the metal cations assuming a tetrahedral type geometry and the remaining
three an octahedral type one. The topological analyses reveal that compound 1 has a 2-connectd
uninodal net and framework 2 has a 2, 8-connected binodal net. These compounds heterogeneously
catalysed the tandem deacetalization − Knoevenagel condensation reactions carried out under
conventional heating, microwave irradiation or ultrasonic irradiation. Comparative studies show that
ultrasonic irradiation (final product yield of 99% after 2h of reaction time) provides the most
favourable method (e.g., microwave irradiation leads to a final product yield of 91% after 3h of
reaction time). Moreover, the catalysts can be reused at least for five consecutive cycles without losing
activity significantly.
Introduction
In recent years metal-organic frameworks (MOFs) have drawn a significant interest due, e.g., to their
usefulness as gas storage materials, photocatalysts, heterogenous catalysts and drug delivery
materials, on account of their interesting properties such as high porosity, large surface area, often
flexible and tunable in nature.1-8 On the other hand, the design and synthesis of metallomacrocycles
using suitable multidentate ligands and metal ions are also attracting a high attention.9-11 Recently, a
noticeable improvement in the synthesis of MOFs and metallo-macrocyclic compounds with amide
1

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functionalized carboxylate ligands has been achieved,12-16 but the challenge for developing efficient
DOI: 10.1039/D0DT01312A
synthetic strategies to targeted structures and expected properties is still remaining.
Recently, various MOFs were examined as adjustable supramolecular platforms to develop
heterogeneous catalysts for diverse organic transformations, especially for liquid-phase reactions.17-
20 Compared with conventional homogeneous catalysts, heterogeneous ones have several advantages,
including their separation and recovery.21-22 On the other hand, polynuclear zinc compounds have
been extensively examined not only due to their structures23-25 but also for their biological26 and
catalytic27-28 properties. The study on Zn(II) MOFs as heterogeneous catalysts is a topic of significant
attention.29-31 In this context, recently we have reported several amide functionalized Zn(II) MOFs
which are quite effective for catalyzing various reactions, such as transesterification, Knoevenagel
condensation, cyanosilylation and Henry reaction, etc.32-34
Tandem reactions can provide cheap and efficient chemical processes and have attracted a significant
research interest recently.35-36 They concern two or multiple individual reactions carried out
consecutively in a single pot, which can decrease the energy consumption and reaction time, waste of
chemicals and solvents. One of the interesting examples of one-pot tandem reaction is
deacetalization-Knoevenagel condensation, where benzylidene malononitrile can be synthesized
directly by using benzaldehyde dimethyl acetal and malononitrile.37-38 Several MOFs and
metallomacrocycles functionalized with acidic and basic sites have been reported to be high-
performance catalysts for such reactions.39-42 In order to develop suitable catalysts the combination
of Lewis acidic and basic functional groups in the catalyst molecule can be relevant and in this study
we explore this possibility.
In our previous work, we have reported two Zn(II) and Cd(II) MOFs based on amide functionalized
carboxylate ligands and explore their catalytic activity toward the one-pot cascade
deacetalization−Knoevenagel condensation reaction of benzaldehyde dimethyl acetal with
malononitrile under mild conditions and achieved a high catalytic activity,43 but without investigating
the effect of microwave and ultrasound irradiation on the yield of the reaction.
Hence, the main objectives of the current work are as follows: (i) to synthesize a Zn(II) based MOF
and/or a metallomacrocycle by using a pyridine amidocarboxylate based pro-ligand, 4,4'-{(pyridine-
2,6-dicarbonyl)bis(azanediyl)}dibenzoic acid (H2L), as the linker, under various solvothermal
conditions; (ii) to apply the synthesized compounds as heterogeneous catalysts for the one-pot
tandem deacetalization−Knoevenagel condensation reactions; (iii) to explore and compare the effect
of conventional heating, as well as microwave or ultrasonic irradiations on the yield of the reaction.
2

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In the present work, we have synthesized the pro-ligand 4,4'-{(DpOyIr:i1d0i.n10e3-92/D,60D- T01312A
dicarbonyl)bis(azanediyl)}dibenzoic acid (H2L) and employed it for the construction of the novel
dinuclear metallomacrocyclic complex [Zn2(L)2(H2O)4].2(H2O).6(DMF) (1) and 2D MOF
[Zn5(L)4(OH)2(H2O)4]n.8n(DMF).4n(H2O) (2). These compounds have been characterized by elemental
analysis, FT-IR, thermogravimetric analysis and single-crystal and powder X-ray diffraction analyses.
Moreover, they comprise both Lewis acid (metal ion) and basic centres (pyridyl and amide groups)
and are insoluble in common organic solvents, features that make them promising candidates for
bifunctional heterogeneous catalysis. Thus, we have studied their heterogeneous catalytic activity
towards the one-pot tandem deacetalization−Knoevenagel condensation reaction of various acetals
with malononitrile by using the abovementioned three different methods. This is the first example
where we have performed such a comparison on this tandem reaction and studied the effects of such
reaction methods on the final product yield. Moreover, we have also tested the recyclability and
heterogeneity of these catalysts.
Results and Discussion
Synthesis and Characterization
o
Both compounds were synthesized using a solvothermal method at 70 C. The metallomacrocyclic
compound [Zn2(L)2(H2O)4].2(H2O).6(DMF) (1) was synthesized by the reaction of H2L with
Zn(NO3)2.6H2O in dimethylformamide (DMF). However, performing the reaction under basic medium
led
to
polymerization
to
produce
metallomacrocyclic
based
2D
MOF
[Zn5(L)4(OH)2(H2O)4]n.8n(DMF).4n(H2O) (2) (Scheme 2).
O
N
O
NH
HN
HOOC
COOH
4,4'-((pyridine-2,6-dicarbonyl)bis(azanediyl))dibenzoic acid (H2L)
Zn(NO3)2.6H2O
DMF, NH4OH,
70oC, 48h
Zn(NO3)2.6H2O
DMF, 70oC, 48h
[Zn2(L)2(H2O)4].2(H2O).6(DMF) (1)
[Zn5(L)4(OH)2(H2O)4]n.8n(DMF).4n(H2O) (2)
Scheme 2
In the FT-IR spectra, the asymmetric and symmetric vibrations for the coordinated carboxylate groups
occur at 1521–1545 cm1 and 1357-1404 cm1, respectively. Moreover, the C=O stretching of amide
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groups appears at 1601-1620 cm-1 and the NH of the amide groups are observed in thDeO3I:2104.140-3392/D703DT01312A
cm-1 range.44 These frameworks were also characterized by CHN analysis, thermogravimetry and single
crystal X-ray diffraction analysis.
Thermogravimetric analyses were carried out under N2 from 30 oC to ca. 750 oC at a heating rate of 5
˚C min-1. The metallomacrocyclic compound 1 shows weight loss of 35.7% between 30˚C and 311˚C,
corresponding to the loss of six DMF and 6 water molecules (calcd: 35.9%). Upon further heating, it
o
continued to decompose until 750 C. MOF 2 exhibits a weight loss of 24.9% in the 81-261 ˚C
temperature range, which accounts for the total removal of eight water and six DMF molecules (calcd:
25.6%). Upon further heating, it remains stable upto 285oC and then the remaining material
decomposes gradually until 750 oC (Figure S1, supporting information).
Crystal Structure Analysis
Single
crystal
X-ray
diffraction
analysis
revealed
that
the
metallomacrocycle
[Zn2(L)2(H2O)4]2(H2O)6(DMF) (1) crystallizes in the triclinic P-1 space group, its asymmetric unit
containing one L2- ligand and two water molecules coordinated to a Zn2+ ion, and also two non-
coordinated H2O and three DMF molecules. Symmetry expansion reveals a dimeric Zn(II) based
metallomacrocycle (Figure 1A) with the Zn(II) center sitting in a tetrahedral environment (τ4 = 0.93)45
with the four positions occupied with two Ocarboxylate atoms from two L2- units [Zn1–O1, 1.964(3) Å;
Zn1–O5, 1.963(3) Å] and the O-atoms from two water molecules [Zn1–O12, 2.014(4) Å; Zn1–O13,
2.005(5) Å]. The doubly deprotonated organic ligand (L2-) is virtually planar (highest deviation of 0.468
Å from the ligand least-square plan pertaining to O3) and bridges two zinc(II) cations through the two
monodentate carboxylate groups, the metals in the macrocyclic unit being 14.201 Å apart although
the shortest intermolecular ZnZn distance reaches only 5.327 Å.
The structure is stabilized by a number of hydrogen bonding interactions involving the non-
coordinated water and DMF molecules, the formers donating to the carboxylate oxygen O6 which thus
act as bifurcated acceptor. The amide NH is also involved in strong hydrogen bonding with non-
coordinated DMF oxygen via N1-H1A···O9 [dD–A 2.888(5) Å; <D–H····A 169˚] and N3-H3A···O9 [dD–A
2.993(6) Å; <D–H····A 158˚] interactions. Moreover, the H-atoms of water molecules are hydrogen
bonded with carboxylate oxygen atoms via O12-H12B···O2 [dD–A 3.009(6) Å; <D–H····A 157˚] and O13-
H13A···O2 [dD–A 2.995(6) Å; <D–H····A 154˚] interactions which helps its expansion to the uni dimension
(Figure 1C).
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DOI: 10.1039/D0DT01312A
C
Figure 1 (A) Crystal structure of 1, its schematic representation (B) and hydrogen bonded one
dimensional packing diagram (C).
The asymmetric unit of 2 contains 2.5 Zn2+ ions, two deprotonated L2- ligands, one hydroxo and two
water molecules, apart from two non-coordinated water and three DMF molecules. Symmetry
expansion generates a 2D polymer (Figure 2A) containing three different types of Zn(II) atoms with
respect to their coordination environments, Zn1 and Zn2 with an octahedral geometry and Zn3 with a
tetrahedral one (Figure 2B). The Zn1 cation is bound to two of the O-atoms of two distinct L2- ligands
[Zn1–O2, 2.080(4) Å] and further to two µ3-bridging hydroxo and two water molecules [Zn1–O13,
2.041(5) Å and Zn1–O14, 2.186(5) Å]. The coordination sphere around the Zn2 center is fulfilled with
the O-atoms from three L2- ligands [Zn2–O1, 2.051(4) Å, Zn2–O5, 2.059(5) Å and Zn2–O11, 2.085(4) Å],
one µ3-bridging hydroxo [Zn2–O13, 2.095(4) Å], one water molecule [Zn2–O15, 2.070(5) Å] and the
remaining site is engaged with a bridging water [Zn2–O14, 2.222(5) Å]. The tetrahedral coordination
scheme around the Zn3 center is occupied with three O-atoms from three different L2- ligands [Zn3–
O6, 1.995(5) Å, Zn3–O7, 1.937(5) Å and Zn3–O12, 1.958(5) Å], and the remaining site with a µ3-OH
group [Zn3–O13, 2.095(4) Å]. Therefore, two different types of L2- ligands are present in this
framework: one in which both carboxylate groups bridge-chelate to two different metal ions, and
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another where just one of the carboxylate groups bridge-chelates two Zn(II)cations whereas the other
DOI: 10.1039/D0DT01312A
is monodentate. The Zn-ions are arranged in pentanuclear clusters (Figure 2B). The minimum
Zn1····Zn2, Zn1····Zn3 and Zn2····Zn3 distances are of 3.1037 Å, 3.5878 Å and 3.183(1) Å, respectively.
Polymer 2 also has open channels with approximate dimension of 10 X 10 Å2 (Figure 2C) and a void
space of 26.8% per unit cell (PLATON),57 determined after removing the non-coordinated solvent
molecules.
A
B
C
D
E
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Figure 2 (A) The asymmetric unit of 2 with partial atom labeling scheme (non-coordinated molecules
DOI: 10.1039/D0DT01312A
where omitted for clarity), its penta-nuclear secondary building block (B), two-dimensional packing
diagram (C) also with encapsulated DMF molecules (which are represented in space-fill model) (D),
and node-and-linker-type representation (the metal nodes are represented in pink and the linker L2-
in cyan color) (E).
In this framework the non-coordinated DMF and water molecules are extensively hydrogen bonded
with amide oxygens via several N-H···O interactions. All the hydrogen bonded distance and angles are
listed in table S6. Moreover, several O-H···O and C-H···O interactions are present in this structure
which help to extend this framework to the third dimension.
The topological analyses of frameworks 1 and 2 were performed using TOPOS 4.0, by reducing their
multidimensional structures to simple node-and-linker nets.46 This analysis revealed that 1 has a 2-
connectd uninodal net (figure S2, supporting information). In 2 the pentanuclear Zn(II) unit was
considered as a single node connected with eight different L2- ligands and each ligand connected with
two Zn(II) clusters, which resulted in a 2, 8-connected binodal net.
Gas Adsorption Studies
The gas adsorption studies of framework 2 were performed using N2 and CO2. The sample of
compound 2 was prepared in acetone for 3 days, followed by activation under vacuum at 100 °C for
12 h. The N2 adsorption experiment was undertaken at the temperature of 77 K and the CO2
measurements were performed at 273K and 293K. The nitrogen adsorption-desorption isotherms of
our compound are presented in Figure 3A, showing a reversible type II isotherm with a saturated
uptake of 333 cm3 g−1. The Brunauer-Emmett-Teller (BET) surface area and avarage pore diameter
(dBJH) was estimated to be 137 m2g−1 with a saturated uptake of 70.9ꢀcm3ꢀg−1 and 12.8 nm, repectively.
Since, various amide functionalized MOFs show high and selective CO2 adsoption properties,47 we
have also tested the carbon dioxide adsoption porteries of our framework 2 at different temperatures.
The CO2 adsorption reached 32.3 and 22.6 cm3 g−1 at 273 and 298 K, respectively, under 1 atm (Figure
3B) and these values are lower than those reported for other MOFs.48
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DOI: 10.1039/D0DT01312A
Figure 3 (A) N2 sorption isotherm of framework 2 at 77 K. (B) CO2 sorption isotherm of 2 at 273 K and
298 K.
Catalytic activity
We have explored the catalytic activities of 1 and 2 as heterogeneous catalysts for the one-pot tandem
deacetalization-Knoevenagel condensation reactions (Scheme 3 for the particular case of forming
benzylidene malononitrile). The first step of the reaction is deacetalization of benzaldehyde dimethyl
acetal (A, as a model compound), to benzaldehyde (B) which is catalyzed by a Lewis acid. In the next
step, the reaction is the base-catalyzed Knoevenagel condensation to produce benzylidene
malononitrile (C) by the reaction of the intermediate benzaldehyde (B) with malononitrile. Our newly
synthesized compounds 1 and 2 possess both a Lewis acid metal site (Zn2+) and basic centres (free
pyridyl-N and amide functional groups of the ligand) which can catalyze the first and the second steps
of the tandem reaction, respectively. Moreover, these compounds serve as heterogenous catalysts
due to their high thermal stability and insolubility in common solvents.
CN
CH3 CH3
H
O
O
O
H
NC
NC
CN
Catalyst 1 or 2
A
80 oC, 2-3 h, Solvent
B
C
Scheme 3 One-pot tandem deacetalization-Knoevenagel condensation reactions to give
benzylidene malononitrile.
By using benzaldehyde dimethyl acetal (A) as a test compound, we found that 1 led to a higher product
yield as compared to 2, after the same reaction time and temperature. The experimental conditions
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were thus optimized for catalyst 1 by changing the solvents, temperature (21-80 °C), catDaOlyI:s1t0.l1o0a39d/iDn0gDT01312A
(0.5-5 mol %) and reaction time (up to 3 h). Moreover, we have optimized the reaction conditions for
every reaction method [conventional heating (HT), microwave irradiation (MW) and ultrasonic
irradiation (US)] and the optimized results are presented in Tables 1, S1, S2 and S3, where the blank
tests results are also indicated. For instance, DMF was found to be the best solvent (highest product
yields were achieved) among the tested ones (DMF, CH3CN, THF, MeOH and DMSO), whereas the
increase of temperature above 80˚C did not result in better yields.
Under optimal conditions, a mixture of benzaldehyde dimethyl acetal (0.152 g, 1.0 mmol),
malononitrile (0.132 g, 2.0 mmol) and catalyst (15.2 mg for 1 and 27.0 mg for 2; 1.0 mol %) in 0.5 mL
of DMF was confined in a capped glass vessel and was stirred and heated at 80˚C using conventional
heating or microwave irradiation (15 W) or ultrasonic irradiation (50/60 Hz) for the desired time. The
reaction mixture was then centrifuged to remove the solid catalyst and the organic products were
extracted with dichloromethane. Evaporation of the solvent from the extract gave the crude products
which were assessed by 1H NMR (figure S6, supporting information).
Table 1: One-pot deacetalization−Knoevenagel condensation reactions between benzaldehyde dimethyl
acetal and malononitrile with 1 and 2 as the catalysts under different conditionsa,b and c
Amount of
catalyst
(mol%)
Yield of
unreacted A
(%)d
Yield of C
Entry Catalyst Time (min)
T (˚C)
Solvent
Yield of B (%)d
(%)d
Conventional heating
1
2
3
4
5
1
1
1
1
1
15
30
60
120
180
1
1
1
1
1
80
80
80
80
80
DMF
DMF
DMF
DMF
DMF
85
74
54.5
31.5
6.5
13
16
4.5
0
2
10
41
68.5
93.5
0
6
7
8
9
10
2
2
2
2
2
15
30
60
120
180
1
1
1
1
1
80
80
80
80
80
DMF
DMF
DMF
DMF
DMF
87
68
42
31
7
12
27
44
28
0
1
5
14
41
93
Microwave irradiation
11
12
13
14
15
1
1
1
1
1
15
30
60
120
180
1
1
1
1
1
80
80
80
80
80
DMF
DMF
DMF
DMF
DMF
92
76
51
30
16
8
0
9
13
49
84
15
36
21
0
16
17
18
19
2
2
2
2
15
30
60
1
1
1
1
80
80
80
80
DMF
DMF
DMF
DMF
90
81
47
42
10
13
15
2
0
6
38
56
120
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DOI: 10.1039/D0DT01312A
20
2
180
1
80
DMF
9
0
91
Ultrasonic irradiation
21
22
23
24
25
1
1
1
1
1
15
30
60
90
120
1
1
1
1
1
80
80
80
80
80
DMF
DMF
DMF
DMF
DMF
15
1
0
0
0
47
24
6
2
0.5
38
75
94
98
99
26
27
28
29
30
2
2
2
2
2
15
30
60
90
120
1
1
1
1
1
80
80
80
80
80
DMF
DMF
DMF
DMF
DMF
30
14
6
1
0
49
16
5
2
2
21
70
91
97
98
aReaction conditions in conventional heating: 1 mol% of catalyst 1 or 2, DMF (0.5 mL), benzaldehyde
dimethyl acetal (152 mg, 1.0 mmol) and malononitrile (132 mg, 2.0 mmol), 80 oC.
bReaction conditions in microwave: 1 mol% of catalyst 1 or 2, DMF (0.5 mL), benzaldehyde dimethyl acetal
(152 mg, 1.0 mmol) and malononitrile (132 mg, 2.0 mmol), MW (initial power (25 W), 80 oC (15 W).
cReaction conditions in ultrasonic: 1 mol% of catalyst 1 or 2, DMF (0.5 mL), benzaldehyde dimethyl acetal
o
d
1
(152 mg, 1.0 mmol) and malononitrile (132 mg, 2.0 mmol), US (80 C, 50/60 Hz). Calculated by H NMR
analysis. A = benzaldehyde dimethyl acetal; B = benzaldehyde; C = benzylidene malononitrile.
Figure 3 Final product yield (2-benzylidenemalononitrile) from the one-pot tandem deacetalization-
Knoevenagel reactions catalyzed by 1 (green pillar) and 2 (purple pillar) using conventional heating (3
h), microwave (3 h) and ultrasonic (2 h) methods.
We have observed that under optimized heating conditions compounds 1 and 2 give 93.5% and 93%
of final product (C) yields after 3 h under conventional heating (entries 5 and 10, Table 1). However,
almost quantitative product yields [99% (for 1) and 98% (for 2)] were obtained using ultrasonic
method only after 2 h (entries 25 and 30, Table 1) (Figure 3). On the other hand, the use of microwave
(MW) irradiation in catalysis has emerged in last few years due to eventual advantages over traditional
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heating,49-50 e.g., a shorter reaction time and higher product yield and selectivity. Thus, recently
DOI: 10.1039/D0DT01312A
several research groups (including ours) used MW irradiation for various catalytic transformations.51-
53
Accordingly, we have also used MW irradiation in the current study of the catalysed tandem
deacetalization-Knoevenagel condensation reactions and obtained 84-91% product yields after 3 h of
reaction time (entries 15 and 20, Table 1). Thus, for our catalytic systems, the ultrasonic irradiation
process is more effective possibly due to a higher dispersion of catalyst into the reaction mixture
(decrease the particle size and increase the overall surface area).54
The kinetic plots of 2-benzylidenemalononitrile (C) and benzaldehyde (B) yields (and also of the
relative amount of benzaldehyde dimethyl acetal, A) vs. time using the US method are shown in Figure
4A for 1 and in Figure 4B for 2. Those concerning the conventional heating and the microwave method
are presented in Figures S4 and S5, respectively (supporting information). The final product (C) yield
increases along 2 h, beyond which only a rather slow yield increase occurs. Moreover, the yield of the
benzaldehyde intermediate (B) increases with time until reaching a maximum at ca. 0.25 h. The
relative amount of the substrate (A, benzaldehyde dimethyl acetal) continuously decreases with time.
These behaviours are typical for an A→B→C reaction sequence, as expected.
A
B
Figure 4: Plots of yield vs. time for the one-pot tandem deacetalization-Knoevenagel condensation
reactions catalysed by compounds 1 (A) and 2 (B) under ultrasonic irradiation.
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DOI: 10.1039/D0DT01312A
100
80
60
40
20
0
1
2
3
4
5
A
B
Figure 5: (A) Plot of yield vs. time for the one-pot tandem deacetalization-Knoevenagel condensation
reactions under ultrasonic method catalysed by 1 [black line: yield of 2-benzylidenemalononitrile (C);
dotted green line: yield of 2-benzylidenemalononitrile (C) after removing the catalyst after 15 min of
reaction time] and 2 [red line: yield of 2-benzylidenemalononitrile (C); dotted blue line: yield of 2-
benzylidenemalononitrile (C) after removing the catalyst after 15 min of reaction time]. (B) Effect of
the catalyst recycling on the yield of 2-benzylidenemalononitrile from the one-pot tandem
deacetalization-Knoevenagel reactions catalyzed by 1 (red pillar) and 2 (yellow pillar).
One of the key aspects of a heterogeneous catalyst concerns its reusability. We have recycled our
catalysts for at least six times and no significant loss of catalytic activity was observed (Figure 5B).
Moreover, we have also performed a leaching test by removing the catalyst 1 or 2 by centrifugation
after 15 min and keeping the catalyst-free reaction mixture under the same irradiation for additional
1.5 h and no significant increase in the product yield was observed for both cases (Figure 5A). Besides
that, we have also estimated the amount of Zn(II) ions in the solution after removing the catalyst from
the reaction mixture, and obtained only a negligible value (0.011-0.015% of the amount used in the
reaction), which shows that no leaching occurs through the catalytic process. The structural integrity
of our catalysts after the tandem reaction was also checked by PXRD analyses done before and after
their use (Figure S7, supporting information) and no significant differences were observed, indicating
the structural stability of both catalysts along the catalytic process.
A comparison of the activities of catalysts 1 and 2 in the reactions of a variety of substituted
benzaldehyde dimethyl and diethyl acetals, producing the corresponding benzylidenemalononitrile,
was also performed, with the obtained yields ranging from 67 to 99% (Table 2). The reaction yields
using the para-substituted bromo (−Br) and methoxy (−OCH3) dimethyl acetals for 1 and 2 are
comparable and in the range of 98-99% after 120 min of reaction time. However, for 3-bromo
12

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benzaldehyde diethyl acetal, only 67-83% of the final product were obtained under tDhOeI: i1n0.d10ic3a9/tDe0dDT01312A
reaction conditions.
Table 2: Deacetalization-Knoevenagel condensation reaction of various benzaldehyde dimethyl or
diethyl acetals and malononitrile under ultrasonic irradiationa
Yieldb(%) with
catalyst 1
97
Yieldb(%) with
catalyst 2
>99
Entry
Compound
Time (min)
OMe
60
120
60
MeO
Br
1
>99
>99
94
OMe
OMe
>99
>99
2
3
120
60
98
OMe
OEt
OEt
4
15
120
83
67
Br
aReaction conditions: 1 mol% of catalyst 1 or 2, DMF (0.5 mL), benzaldehyde dimethyl acetal (152 mg,
1.0 mmol) and malononitrile (132 mg, 2.0 mmol), US (80 oC, 50/60 Hz). bCalculated by 1H NMR analysis.
A comparison of the activity of our catalysts 1 and 2 with other reported MOFs towards the one-pot
deacetalization-Knoevenagel condensation reactions is shown in Table S4 (supporting information),
demonstrating that our catalysts (leading to product yields of 98-99%) are possibly the most active
ones under ultrasonic conditions. For example, by using [Zn4(TBCB).(H2O)6]n.5n(DMAc) (where TBCB =
2,2’,6,6’-tetrakis[3,5-bis-3,5-benzenedicarboxylate]benzidine, DMAc = N,N-dimethylacetamide) as
catalyst, a 99% yield is reached after 4 h at 90 oC (entry 5, Table S4), i.e., for higher reaction time and
temperature than ours.42 Moreover, another 2D Zn(II) MOF, [Zn2(L’)(H2O)4]n.4n(H2O), constructed
from 5,5'-{(pyridine-2,6-dicarbonyl)bis(azanediyl)}diisophthalate (L’) produced also 99% yield after 3
o
h at 75 C under heating conditions (entry 6, Table S4).43 The use of various Cd(II) MOFs leads to an
yield of 57-94% (entries 7 and 8, Table S) after 3-5h of reaction time under conventional heating
conditions.43,38 Different Cu(II) frameworks, such as Cu-PCN-124 and Cu-HNUST-8, also lead to 99%
yields but require a longer time, i.e., 12 h and 48 h, respectively (entries 9 and 11, Table S4).40,37 For
MIL-101(Al)-NH2, an yield of 94% is achieved at 90oC after 3h (entry 10, Table S4), a higher reaction
time than ours.41 With our catalysts 1 and 2, under conventional heating, 93-93.5% yields are reached
after 3 h at 80 oC. However, under ultrasonic irradiation, we have obtained 98-99% of yields only after
2 h at 80oC. According to the above comparisons, under conventional heating conditions our catalysts
show comparable activity with other reported catalysts, but, our catalysts are usually more active
under ultrasonic conditions.
Conclusions
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We successfully synthesized and characterized the binuclear metallomacrocyclic compound
DOI: 10.1039/D0DT01312A
[Zn2(L)2(H2O)4]2(H2O)6(DMF) (1) and the metallomacrocyclic based two dimensional MOF
[Zn5(L)4(OH)2(H2O)4]n8n(DMF)4n(H2O) (2) from the reaction between 4,4'-{(pyridine-2,6-
dicarbonyl)bis(azanediyl)}dibenzoic acid (H2L) and zinc(II) nitrate under different reaction conditions.
Their structures were established by single crystal X-ray diffraction analysis revealing two tetrahedral
Zn(II) centres in 1, whereas 2 contains a penta-nuclear Zn(II) cluster as a secondary building block unit,
with the metal cations in tetrahedral and octahedral environments. The topological analysis of
compounds 1 and 2 revealed a 2-connectd uninodal net and a 2,8-connected binodal net, respectively.
The compounds are insoluble in common solvents and in view of the presence of both Lewis acid (the
metal cation) and Lewis basic (the amide and pyridyl groups), they were thought to be appropriate
candidates for bifunctional catalysis. Indeed, they act as heterogeneous catalysts towards the one-pot
tandem deacetalization−Knoevenagel reactions of benzaldehyde dimethyl acetal and malononitrile
either with conventional heating, or under microwave or ultrasonic irradiations. The activity of both
compounds is enhanced with ultrasounds (99% yield with 1 and 98% with 2 within 2 h), what may be
due to the high dispersion of the catalysts in the reaction mixture (small particle size and high surface
area), therefore increasing the accessibility of the active sites and promoting the reaction rate. Under
microwave irradiation the reaction was less effective (final product yields of 84% with catalyst 1 and
91% with 2), as well as with conventional heating (final product yields of 93.5% for 1 and 93% for 2).
Ultrasonic irradiation was thus proved of interest to enhance such a type of tandem reaction. The
catalysts remained stable under the catalytic conditions and could be recycled at least up to five times
without losing their activity.
The present work reveals that amide functionalized compounds can behave as heterogeneous
bifunctional catalysts under mild conditions for the deacetalization−Knoevenagel tandem reactions,
also providing a better understanding on the applicability of MOFs as platforms for the design of new
acid−base bifunctional heterogenous catalysts, a matter that deserves to be further explored.
Experimental
The synthetic work was performed in air and at relatively high temperature. All the chemicals were
obtained from commercial sources and used as received. The infrared spectra (4000–400 cm-1) were
recorded on a Bruker Vertex 70 instrument in KBr pellets; abbreviations: s = strong, m = medium, w =
weak, bs = broad and strong, mb = medium and broad. The 1H NMR spectra were recorded at ambient
temperature on a Bruker Avance II + 300 (UltraShieldTMMagnet) spectrometer operating at 300.130
MHz. The chemical shifts are reported in ppm using tetramethylsilane as the internal reference;
14

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abbreviations: s = singlet, d = doublet, t = triplet, q = quartet. Carbon, hydrogen and nitrogen elemental
DOI: 10.1039/D0DT01312A
analyses were carried out by the Microanalytical Service of the Instituto Superior Técnico. Thermal
properties were analyzed with a Perkin-Elmer Instrument system (STA6000) at a heating rate of 5˚C
min-1 under a dinitrogen atmosphere. Powder X-ray diffraction (PXRD) was conducted in a D8 Advance
Bruker AXS (Bragg Brentano geometry) theta-2-theta diffractometer, with copper radiation (Cu Kα, λ
= 1.5406 Å) and a secondary monochromator, operated at 40 kV and 40 mA. Flat plate configuration
was used and the typical data collection range was between 5˚ and 40˚.
Synthesis of 4,4’-[(pyridine-2,6-dicarbonyl)bis(azanediyl)] dibenzoic acid (H2L)
This compound was synthesized by following a two-step procedure (Scheme 1).
In the first step, methyl 4-aminobenzoate (1.51 g, 10 mmol) and NEt3 (1.51 g, 15 mmol) were placed
in a round bottom flask and then dry CH2Cl2 (25 mL) was added, followed by dropwise addition of 2,6-
pyridinedicarboxylic acid dichloride (1.01 g, 5 mmol) to the reaction mixture which was stirred
overnight at room temperature. Upon removal of the solvent under reduced pressure a white solid
was obtained. Water (30 mL) was then added into the flask and the dichloromethane-based organic
phase was decanted and collected over anhydrous sodium sulphate; consequent removal of the
solvent gave the methyl ester H2L1.
O
O
O
O
N
NEt3, dry CH2Cl2
16 h, RT
Cl
Cl
N
NH
HN
+
O
O
NH2
MeOOC
COOMe
H2L1
THF:H2O
NaOH, 90oC, 1h
O
O
N
NH
HN
HOOC
COOH
4,4'-[(pyridine-2,6-dicarbonyl)bis(azanediyl)]
dibenzoic acid (H2L)
Scheme 1 Synthesis of H2L
In the second step, this isolated ester (0.86 g, 2 mmol) and NaOH (0.8 g, 20 mmol) were dissolved in
25 mL of a THF:water (4:1) solution. The reaction mixture was refluxed for 1 h at 90 oC, and then stirred
overnight at room temperature, after which it was acidified to pH = 2 with a diluted solution of HCl.
The obtained white solid product H2L was isolated by filtration and thoroughly washed with water.
Yield: 73%. Anal. Calcd. for C21H15N3O6 (M = 405.36) C, 62.22; H, 3.73; N, 10.37. Found: C, 62.35; H,
3.67; N, 10.42. 1H-NMR (DMSO-d6): 13.14 (2H, bs, -COOH), 11.27 (2H, s, -NH), 8.43 (2H, d, Ar-H), 8.34
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(1H, d, Ar-H), 8.05 (8H, m, Ar-H). FT-IR (KBr, cm-1): 3447 (bs), 1689 (s), 1609 (s), 1537 (s), 1D4O1I0: 1(0s.1)0,3193/D200DT01312A
(s), 1300 (s), 1242 (s), 1179 (s), 1120 (m), 1085 (m), 1000 (m), 932 (m), 854 (s), 770 (s), 748 (s), 714
(m), 552 (m).
Synthesis of compound 1
H2L (10.1 mg; 0.025mmol) and zinc(II) nitrate hexahydrate (14.9 mg; 0.050 mmol) were dissolved in 2
o
mL of DMF, transferred to a glass vessel and heated in a hydrothermal reactor at 70 C for 48 h.
Subsequent cooling of the solution to room temperature afforded small plate like colorless crystals of
1. Yield 72 % (based on H2L). FT-IR (KBr, selected peaks)[cm-1]: 3425 (bs), 3264 (bs), 2931 (w), 1649 (s),
1601 (s), 1527 (w), 1438 (w), 1384 (s), 1253 (s), 1177 (w), 1098 (s), 1061 (w), 827 (w), 782 (m), 660 (s).
Anal. Calcd. for C69H105N15O29Zn2 (M = 1737.58): C, 47.64; H, 6.08; N, 12.08. Found: C, 47.53; H, 6.45;
N, 12.65.
Synthesis of compound 2
A 2 mL DMF solution containing H2L (10.1 mg; 0.025mmol) and Zn(NO3)2.6H2O (14.9mg; 0.050 mmol)
was prepared, whereupon 0.5 mL of a 30 % aqueous solution of NH4OH was added. This mixture was
then transferred to an 8 mL glass vessel, sealed and heated (hydrothermal reactor) at 70 oC for 48 h,
after which it was cooled to room temperature giving colorless block like crystals of 2. Yield 59 %
(based on H2L). FT-IR (KBr, cm-1): 3377(bs), 3273(bs), 1658 (s), 1603 (s), 1583 (s), 1521 (s), 1404 (s),
1316 (m), 1278 (m), 1106 (s), 1075 (w), 857 (s), 782 (s), 767 (s), 636 (s). Anal. Calcd. for
C108H126N20O42Zn5 (M = 2703.13): C, 47.99; H, 4.70; N, 10.36. Found: C, 47.43; H, 4.41; N, 9.92.
Procedure for the one-pot tandem deacetalization−Knoevenagel condensation reactions
A mixture of benzaldehyde dimethyl acetal (152 μL, 1.0 mmol), malononitrile (132 mg, 2.0 mmol) and
the catalyst (7.6 mg of 1, 19.0 mg of 2, 1 mol%) was placed in a capped glass vessel, and then 0.5 mL
DMF was added. The mixture was either heated at 80 oC in an oil bath or subjected to microwave (25
o
o
W, 80 C) or ultrasonic (50/60 Hz, 80 C) irradiation for the desired time. Then the mixture was
centrifuged to remove the catalyst and the separated solution was extracted with CH2Cl2. The extracts
were dried over anhydrous Na2SO4, whereupon evaporation of the solvent gave the crude product [2-
1
1
(phenylmethylene)malononitrile] which was analyzed by H NMR in CDCl3. An example of H-NMR
spectra is presented in Figure S6 (supporting information) and the reaction yield was calculated
according to a reported method.38
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In order to perform the catalyst recycling experiments, the used catalyst (separated by centrifugation
DOI: 10.1039/D0DT01312A
of the supernatant solution) was washed with methanol and dried in air. It was then reused for the
cascade reaction as described above.
Crystal structure determinations
X-ray quality single crystals of the compounds were immersed in cryo-oil, mounted in a nylon loop and
measured at room temperature (1-2). Intensity data were collected using a Bruker APEX-II PHOTON
100 diffractometer with graphite monochromated Mo-Kα (λ 0.71069) radiation. Data were collected
using phi and omega scans of 0.5° per frame and a full sphere of data was obtained. Cell parameters
were retrieved using Bruker SMART53 software and refined using Bruker SAINT55 on all the observed
reflections. Absorption corrections were applied using SADABS.56 Structures were solved by direct
methods by using the SHELXS-2014 package57 and refined with SHELXL-2014/6.57 Calculations were
performed using the WinGX System-Version 2014.1.58 The hydrogen atoms attached to carbon and
nitrogen atoms were inserted at geometrically calculated positions and included in the refinement
using the riding-model approximation; Uiso(H) were defined as 1.2Ueq of the parent atoms for phenyl
and 1.5Ueq of the parent atoms for the methyl groups and nitrogen atoms. Least square refinements
with anisotropic thermal motion parameters for all the non-hydrogen atoms and isotropic ones for
the remaining atoms were employed. Compound 1 contained disordered dimethyl formamide (DMF)
molecules which could not be modelled reliably. PLATON/SQUEEZE59 was used to correct the data and
potential volumes of 466 Å3 was found with 114 electrons per unit cell worth of scattering. The
electron count suggests the presence of three DMF per asymmetric unit. Elemental and
thermogravimetric analysis data also support this result. These were removed from the model and
included in the empirical formula. Crystallographic data are summarized in Table S5 (Supplementary
Information file) and selected bond distances and angles are presented in Table S7. CCDC 1983776-
1983777 contain the supplementary crystallographic data for this paper. These data can be obtained
free
of
charge
from
The
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
This work was supported by the Fundação para a Ciência e Tecnologia (FCT), Portugal, project
UIDB/00100/2020 of Centro de Química Estrutural. A. Karmakar expresses his gratitude to Instituto
Superior Técnico and FCT for Scientific Employment contract (Contrato No: IST-ID/107/2018) under
Decree-Law no. 57/2016, of August 29. M. Soliman expresses his gratitude to FCT for PhD fellowship
(PD/BD/114400/2016).
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DOI: 10.1039/D0DT01312A
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DOI: 10.1039/D0DT01312A
Graphical Abstract
Synthesis and Catalytic Activities of
a
Zn(II) Based
Metallomacrocycle and a Metal-Organic Framework towards One-
pot Deacetalization − Knoevenagel Tandem Reactions under
Different Strategies: A Comparative Study
Anirban Karmakar*, Mohamed M.A. Soliman, Guilherme M. D. M. Rúbio, M. Fátima C. Guedes da
Silva and Armando J. L. Pombeiro*
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,
1049–001, Lisbon, Portugal. E-mail: anirbanchem@gmail.com; pombeiro@tecnico.ulisboa.pt.
4,4'-{(pyridine-2,6-dicarbonyl)bis(azanediyl)}dibenzoic acid (H2L) is used to synthesize two new Zn(II)
based metal organic macrocycle and framework which act as heterogeneous catalysts for the
tandem deacetalization − Knoevenagel condensation reactions carried out under conventional
heating, microwave irradiation or ultrasonic irradiation.