ZnII and CdII MOFs based on an amidoisophthalic acid ligand: Synthesis, structure and catalytic application in transesterification

Article abstract of DOI:10.1039/c6ra17518j

Solvothermal reaction of zinc(ii) and cadmium(ii) salts with 5-propionamidoisophthalic acid (H₂L1) and 5-benzamidoisophthalic acid (H₂L2) in the presence or absence of an auxiliary ligand gives rise to a series of 1D, 2D and 3D Zn(ii) or Cd(ii) self-assembled metal-organic frameworks as revealed by X-ray diffraction structural analyses: [Zn(μ-L1-μ-1κOO′:2κO′′)(formamide)₂]_n (1), [{Zn₂(μ-L1-1κO:2κO′)₂(4,4′-bipyridine)₂(H₂O)}·2(DMF)·5(H₂O)]_n (2), [Cd(μ-L1-μ-1κO²O′:2κO′′²O′′′)(DMF)]_n (3), [{Zn(μ-L2-1κO:2κO′)(4,4′-bipyridine)(H₂O)}·(H₂O)]_n (4) and [{Cd(μ-L2-μ-1κO²O′:2κO′′²O′′′)(formamide)(4,4′-bipyridine)}·(formamide)]_n (5), which are also characterized by elemental analysis, IR spectroscopy and thermogravimetric analysis. The different architectures found in 1-5 result from the coordination modes of the carboxylate groups, which can assume nonbridging monodentate (in 1, 2 and 4), bridging bidentate (also in 1), bridging chelate (in 3 and 5) and chelate bidentate (also in 5) coordination. While 1 and 5 possess one dimensional double chain type structures, 3 has a 1D cyclic type structure and 2 and 4 have 2D or 3D wave-like structures, respectively. Topological analysis has shown that 1 and 5 have a 3-connected uninodal net with a topological type SP1-periodic net, 2 has a 2,4-connected binodal net, 3 has a 4-connected uninodal net structure and 4 has a more complex 2,2,2,4-connected tetranodal net. Frameworks 1-5 act as heterogeneous catalysts for the transesterification reaction of different carboxylate esters, with 4 showing the highest activity. These heterogeneous catalysts can be recycled without losing activity.

Full text of DOI:10.1039/c6ra17518j

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ZnII and CdII MOFs based on Amidoisophthalic acid ligand: Synthesis, Structure
and Catalytic Application in Transesterification
Anirban Karmakar*, Guilherme M. D. M. Rúbio, M. Fátima C. Guedes da Silva*, Ana P. C.
Ribeiro 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; fatima.guedes@tecnico.ulisboa.pt;
pombeiro@tecnico.ulisboa.pt.
Abstract
Solvothermal reaction of zinc(II) and cadmium(II) salts with 5-propionamidoisophthalic acid (H2L1) and 5-
benzamidoisophthalic acid (H2L2) in presence or absence of an auxiliary ligand gives rise to a series of
1D, 2D and 3D Zn(II) or Cd(II) self-assembled metal organic frameworks as revealed by X-ray diffraction
structural
analyses:
[[Zn(µ-L1-µ-1κOO’:2κO’’)(formamide)2]n
(1),
[{Zn2(µ-L1-1κO:2κO’)2(4,4’-
(3), [{Zn(µ-L2-
bipyridine)2(H2O)}.2(DMF).5(H2O)]n
(2),
[Cd(µ-L1-µ-1κO2O’:2κO’’2O’’’)(DMF)]n
1κO:2κO’)(4,4’-Bipyridine)(H2O)}.(H2O)]n (4) and [{Cd(µ-L2-µ-1κO2O’:2κO’’2O’’’)(formamide)(4,4’-
Bipyridine)}.(formamide)]n (5), which are also characterized by elemental analysis, IR spectroscopy and
thermogravimetric analysis. The different architectures found in 1-5 result from the coordination modes
of carboxylate groups which can assume nonbridging monodentate (in 1, 2 and 4), bridging bidentate
(also in 1), bridging chelate (in 3 and 5) and chelate bidentate (also in 5). While 1 and 5 possess one
dimensional double chain type structures, 3 has a 1D cyclic type structure and 2 and 4 have a 2D or a 3D
wave like structures, respectively. Topological analysis have shown that 1 and 5 have 3-connected
uninodal net with a topological type SP1-periodic net, 2 has a 2,4-connected binodal net, 3 has a 4-
connected uninodal net structure and 4 has a more complex 2,2,2,4-connected tetranodal net.
Frameworks 1-5 act as heterogeneous catalysts for the transesterification reaction of different
carboxylate esters, with 4 showing the highest activity. These heterogeneous catalysts can be recycled
without losing activity.
Introduction
Metal–organic frameworks (MOFs) are crystalline coordination networks consisting of metal ions or
clusters and multidentate organic ligands1. This research field is undergoing a rapid growth not only
because of the intriguing architectures of MOFs but also due to potential applications as functional
materials in catalysis, magnetism, nonlinear optics, gas storage and separation, etc2. Their crystalline
nature, large surface area, high porosity, tuneable surface properties, adequate framework stability and
the presence of active sites either in the framework or hosted in the voids also make MOFs very
promising as heterogeneous catalysts for liquid phase reactions.3
1

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However, the controlled synthesis of MOFs is still a challenge on account of the many factors that
influence the product structures, such as the nature of the ligand and metal, solvent, reaction
temperature, time, metal to ligand ratio, pH and method of crystallization etc.4 In particular,
multidentate carboxylate ligands are good candidates for the construction of novel structures and
topologies since they have diverse coordination modes and can adopt different conformations. Rigid
benzene dicarboxylate5, tricarboxylate6, tetracarboxylate7 and azolate-based ligands8, as well as their
derivatives, are commonly used as organic building blocks, and this approach has effectively produced a
wide range of MOFs with interesting properties.
MOFs constructed from amide based linkers have attracted considerable attention due to their ability to
provide guest-accessible functional organic sites and to obtain controlled architectures with interesting
topologies.9 Recently, we have focused on the design of various types of coordination polymers and
discrete complexes using amido carboxylate ligands and different metal ions and on the study of their
catalytic activity in various organic transformations.10 The amide based ligands are chosen because they
can construct kinetically inert and thermodynamically stable metal complexes and the presence of the
amide group in the framework provides an extra opportunity for guest binding. Besides, the presence of
carboxylate groups allows slight conformational changes for the potential assembly of different network
architectures.
Heterogeneous catalysis concerns one of the earliest proposed applications for crystalline MOFs and
presently many organic reactions are effectively catalyzed by them11. Recently, our group has reported
various MOFs which are catalytically active for the oxidation12a-c, nitroaldol10, 12d, cyanosilylation12e and
Knoevenagel condensation reactions12f. Now we wish to extend our study to another type of reaction,
i.e., transesterification, which concerns an important transformation in organic synthesis, in industrial
and in academic laboratories13, with applications in polyester synthesis and biodiesel production.14 Many
homogenous catalysts have been developed for these reactions15, but usually they require high reaction
temperature and acidic reaction conditions. Lately, some MOFs have been used to catalyze such a type
of reactions16, but there is still a demand to develop new types of catalysts based on cheap and
environmentally tolerable compounds that could be easily recyclable and show a high efficiency under
mild conditions.
Thus, the main objectives of the present work are: (i) to synthesize Zn(II) and Cd(II)-MOFs by using 5-
propionamidoisophthalic acid (H2L1) and 5-benzamidoisophthalic acid (H2L2) linkers in the presence or
absence of auxiliary ligands, under various solvothermal conditions; (ii) to apply the synthesized MOFs
as heterogeneous catalysts for the transesterification reaction.
Hence, we report herein the solvothermal synthesis and characterization of five new self-assembled
zinc(II) and cadmium(II) MOFs, namely [[Zn(µ-L1-µ-1κOO’:2κO’’)(formamide)2]n (1), [{Zn2(µ-L1-
1κO:2κO’)2(4,4’-bipyridine)2(H2O)}.2(DMF).5(H2O)]n (2), [Cd(µ-L1-µ-1κO2O’:2κO’’2O’’’)(DMF)]n (3), [{Zn(µ-
L2-1κO:2κO’)(4,4’-Bipyridine)(H2O)}.(H2O)]n (4) and [{Cd(µ-L2-µ-1κO2O’:2κO’’2O’’’)(formamide)(4,4’-
Bipyridine)}.(formamide)]n (5) [L1 = 5-propionamidoisophthalate, L2 = 5-benzamidoisophthalate, DMF =
N,N-dimethylformamide]. These compounds have been characterized by elemental analysis, IR
2

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spectroscopy, thermogravimetric analysis and single-crystal X-ray diffraction. They act as heterogeneous
catalysts in the transesterification reactions of various aromatic esters.
Results and Discussion
Syntheses and Characterization
In general we have used solvothermal reaction procedures for the syntheses of these frameworks (1-5).
The solvothermal reaction of H2L1 with zinc(II) nitrate hexahydrate in the presence of formamide lead to
the formation of [[Zn(µ-L1-µ-1κOO’:2κO’’)(formamide)2]n (1), but when in presence of 4,4’-bipyridine as
auxiliary ligand and in a dimethylformamide (DMF) and methanol mixture, the two dimensional
framework [{Zn2(µ-L1-1κO:2κO’)2(4,4’-bipyridine)2(H2O)}.2(DMF).5(H2O)]n (2) is formed. The
solvothermal reaction of Cd(NO3)2.4H2O with H2L1 in DMF, methanol and water mixture produced a 1D
framework [Cd(µ-L1-µ-1κO2O’:2κO’’2O’’’)(DMF)]n (3) [Scheme 1]. The solvothermal reaction of H2L2 with
zinc(II) nitrate hexahydrate and 4,4’-bipyridine led to [{Zn(µ-L2-1κO:2κO’)(4,4’-Bipyridine)(H2O)}.(H2O)]n
(4), while the hydrothermal reaction of H2L2 and cadmium (II) nitrate in the presence of 4,4’-bipyridine
as auxiliary ligand and in formamide, led to the formation of the one dimensional metal organic
framework [{Cd(µ-L2-µ-1κO2O’:2κO’’2O’’’)(formamide)(4,4’-Bipyridine)}.(formamide)]n (5) (Scheme 2).
Scheme 1
In the IR spectra of 1-5, the characteristic strong bands of coordinated carboxylate groups appear at
1585–1560 cm-1 or 1388-1345 cm-1 for the asymmetric or the symmetric stretching, respectively18. The
bands in the regions 1635–1613 cm-1 and 1438–1418 cm-1 are attributed to the C=C stretching
frequencies of the aromatic rings, and those in the range 1689-1653 cm-1 to the stretching frequency of
the amide C=O group. Due to insolubility in common NMR solvents, these frameworks were only
characterized by infrared, microanalysis and single crystal X-ray diffraction.
3

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Scheme 2
Crystal structures analyses
According to the X-ray diffraction analysis, frameworks 1 and 5 have one dimensional double chain type
structures, whereas framework 3 have 1D cyclic type structure, and 2 and 4 have very interesting two
dimensional and 3D wave like structures, respectively.
The asymmetric unit of 1 contains one deprotonated ligand (L12-), one Zn(II) cation and two coordinated
formamide molecules (Figures 1A and 1B). The metal cation has all-O trigonal bipyramidal geometry (τ5
= 0.66)19a, with a basal plane made the O-atoms from both nonbridging monodentate and bridging
bidentate carboxylate groups. While the former binding mode of the carboxylate groups leads to
sixteen-membered C10O4Zn2 metallacycles, the latter assembles into C2O4Zn2 cores, in a chair
conformation, which act as secondary building block units in the construction of the one dimensional
double chain polymeric assembly (Figure 1C). In this core, the Zn-OL1 assume values of 1.9882(17) and
1.9758(17) Å [Zn1–O1 and Zn1–O2, respectively], the coordination sphere of the metal being then
fulfilled by the sole coordination of the other O-carboxylate group [Zn1–O4, 1.9713(18) Å] and by the O-
atoms of formamide [Zn1-O6, 2.084(2) Å; Zn1-O7, 2.231(2) Å] molecules.
4

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O
O
NH
Zn1'
O
O
NH2
O
O
Zn1"
Zn
1
H2N
O
O
O
A
B
C
Figure 1: (A) Perspective view of framework 1 with partial atom labelling scheme and (B) its schematic
representation. (C) One dimensional structure of 1.
Framework 2 contains two Zn2+ ions bound to two deprotonated ligands (L12-), two 4,4’-bipyridine and
one water molecule, the asymmetric unit of this compound also comprising two non-coordinated
dimethylformamide and five water molecules (Figure 2A).
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A
B
C
Figure 2: (A) Perspective view of framework 2 with partial atom labelling scheme, and (B) its schematic
representation. (C) Two dimensional structure of framework 2.
Framework 2 contains two different types of Zn(II) atoms with respect to their coordination
environments [tetrahedral (τ4 = 0.88)18b for Zn1 and trigonal bipyramidal (τ5 = 0.38)18a for Zn2]. The Zn1
cation is coordinated to two of the O-atoms of two distinct L12- ligands [Zn1–O1, 1.9446(14) Å and Zn1–
O6, 1.9373(13) Å] and further coordinated by the two N-atom of a bridging bipyridine moieties [Zn1–N3,
2.0539(17) Å and Zn1–N5, 2.0264(15) Å], which fulfill the tetrahedral coordination environment around
the metal center (τ4 = 0.88)18b. The coordination sphere around the Zn2 center fulfill by the coordination
of two μ2-L12- ligands [Zn2–O4, 1.9871(13) Å and Zn2–O8, 1.9925(14) Å], two N-atom of a bridging
bipyridine [Zn2–N4, 2.2311(17) Å and Zn2–N6, 2.0970(15) Å] and the remaining site fulfill by the water
molecule [Zn2–O11, 2.2007(15) Å] (Figure 2B). The carboxylate groups in 2 bind the metal in just the
nonbridging monodentate fashion. Framework 2 features a stair type two-dimensional coordination
polymeric chain, two L12- ligands bind two the two different metal atoms and forming a 1D chains which
are further connected by two 4,4’-bipyridine molecules leading to the formation of 2D network (Figure
2C), giving rise to tetranuclear C22N4O4Zn4 metalacycles.
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A
B
C
D
Figure 3: (A) Perspective view of framework 3 with partial atom labelling scheme, and (B) schematic
representation of 3. (C) View of the one dimensional framework of 3 (arbitrary view) and (D) of the open
channels running along the crystallographic c-axis.
The Cd2+ metal cations in 3 present all-O distorted seven coordinate pentagonal bipyramidal
environments (Figures 3A and 3B), each one binding four L1 units by means of six O-atoms from
carboxylate groups working in bridging chelating fashions [Cd1-O3, 2.195(4) Å; Cd1-O4, 2.270(5) Å; Cd1-
O1, 2.277(4) Å; Cd1-O2, 2.346(4) Å; Cd1-O1’, 2.414(4) Å]. The remaining coordination site is engaged
with a formamide molecule [Cd1-O6, 2.3730(4) Å]. Compound 3 features a one-dimensional
coordination polymeric chain (Figure 3C) with open channels, with approximate dimension of 2.3 x 2.3
Å2, running along the crystallographic c-axis (Figure 3D). The L1 ligand binds the metal in such a way that
the flexible (amide) part, and the formamide moiety, is directed outwards such channels (Figure 3D).
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O
O
NH
O
O
O
OH2
N
Zn1'
Zn1
O
N
N
Zn1"
N
Zn1"'
A
B
C
Figure 4: (A) Perspective view of framework 4 with partial atom labelling scheme (only one component
of the disordered benzamido group is shown), and (B) its schematic representation. (C) Wave like three
dimensional organisation of 4.
Single-crystal X-ray diffraction studies revealed that 4 is a 3D network with an asymmetric unit made of
one Zn2+ ion bound to two L22- ligands, two halves of 4,4’-bipyridine and one water molecule, together
with one non-coordinated water molecule (Figure 4A). The Zn1 has trigonal bipyramidal coordination
geometry (τ5 = 0.53)19a, with the equatorial plane including two O atoms from two nonbridging
monodentate carboxylate groups from two L22- ligands [Zn1-O3, 1.934(7) Å; Zn1-O6, 1.936(5) Å] and the
N-atom from a 4, 4’-bipyridine molecule [Zn1-N2, 2.051(7) Å]; the axial sites are occupied by the water
molecule [Zn1-O1, 2.206(7) Å] and the N-atom of another 4, 4’-bipyridine ligand [Zn1-N1, 2.234(6) Å]
(Figure 4B). In this framework one of the bipyridine molecules is almost planar, as measured by the nil
angle between its two phenyl rings, but in the other that angle is of 38.03o. In addition, the phenyl
groups of the L2 ligand make an angle of 73.92 º. The bipyridine molecules and the 5-
benzamidoisophthalate ligands are coordinated to the metal centers in such a way that a three
dimensional wave like coordination networks is formed (Figure 4C).
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A
B
C
Figure 5: (A) A view of framework 5 with partial atom labelling scheme, and (B) schematic
representation of 5. (C) One dimensional double chain type structure of 5.
When we perform the reaction of 5-benzamidoisophthalic acid (H2L2) with cadmium(II) nitrate in the
presence of 4,4’-bipyridine in formamide, we have obtained another interesting one dimensional Cd(II)
compound, framework 5. The asymmetric unit of 5 contains one Cd2+ ion coordinated to one
deprotonated L2 ligand, one 4,4’-bipyridine and one formamide molecule, an additional non-
coordinated formamide molecule also being present in the structure. The Cd1 center presents a
distorted seven coordinated pentagonal bipyramidal environment (Figure 5A) and binds to five
carboxylate oxygen atoms of three neighboring L2 units, two of them in a bridging chelate [Cd1-O1
2.251(2) Å, Cd1-O1’ 2.551(2) Å, Cd1-O2 2.649(2) Å] and the remaining one in a chelate bidentate fashion
[Cd1-O5 2.352(2) Å, Cd1-O6 2.3711(19) Å], and to one 4,4’-bipyridine molecule [Cd1-N2 2.265(2) Å]
(Figure 5B). In 5 the L2 ligand is almost planar and the phenyl rings of 4,4’-bipyridine are relatively
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twisted by 39.82 º, features that conceivably prevented the formation of a structure with a higher
dimensionality (as in 4) and a 1D double chain type structure is formed instead (Figure 5C).
To improve the description of the crystal structures of 1-5 we performed their topological analysis by
reducing their multidimensional structures to simple node-and-linker nets where the metallic nodes and
the organic linkers represent secondary building units (SBUs)19. We have carried out these analyses by
using TOPOS 4.019b. frameworks 1 and 5 reveals that they have 3-connected uninodal nets with
topological SP1-periodic types (Figure 6A), 2 has a 2,4-connected binodal (Figure 6) and 3 a 4-connected
unimodal net, but 4 has a more complex 2,2,2,4-connected tetranodal structure.
A
B
C
D
Figure 6: Node-and-linker-type descriptions of the 1D coordination frameworks in 1 and 5 (A), 2D in 2
(B), 3D in 4 (C) and 1D in 3 (D).The metal nodes are represented in green (Zn) and cyano (Cd) color,
carboxylate linker in blue color and the bipyridine in pink color.
Thermogravimetric analyses
Thermogravimetric analyses were carried out under dinitrogen in the range from room temperature to
ca. 750 ˚C at a heating rate of 5 ˚C min-1. Features of the thermal stability of complex 1-5 are illustrated
in Figure 7.
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Compound 1 shows two step decompositions and loses 23.7 % of its weight within 102-204 ˚C, due to
loss of two coordinated formamide molecules (calcd: 23.0%). The remaining material then started to
decompose from 409 ˚C until 538 ˚C. Compound 2 shows a weight loss of 21.5 % within 41-248 ˚C,
corresponding to the loss of six molecules of water and two dimethylformamide molecules (calcd: 21.7
%). Upon further heating, it is stable up to 362 ˚C and then started to decompose. Compound 3 exhibits
a weight loss of 17.1 % in the 99-333 ˚C temperature range, which accounts for the total removal of one
dimethylformamide molecule (calcd: 17.3%). The remaining material then started to decompose from
380 ˚C until 523 ˚C. Compound 4 shows a weight loss of 7.1% in the 97-158 ˚C range, corresponding to
the loss of two molecules of water (calcd: 6.6%). Compound 5 shows a continuous decomposition and
loses 15.1 % of its weight within 153-298˚C, most likely due to loss of two formamide molecules (calcd:
14.0%). The remaining material continues decomposing until 468˚C.
Figure 7 Thermogravimetric curves for 1-5.
Transesterification
Due to the very low solubility of 1-5 in alcohols, they are potentially good candidates for heterogeneous
catalytic transesterification reactions. We have tested their catalytic activities for this reaction by using a
mixture of methyl-4-nitrobenzoate in 2 mL EtOH with the catalyst (1-5) (Scheme 1) contained in a
o
capped glass vessel and stirred at 75 C, typically for 30 h. The catalyst was then removed by
centrifugation and the solvent was evaporated in vacuum, leading to a crude mixture of products which
was analyzed by 1H NMR (Table 1).
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Scheme 3 Transesterification reaction
We have optimized the transesterification reaction conditions (temperature, reaction time, amount of
catalyst and solvent) using 4 as the catalyst since we found that this framework showed a higher activity
than the others (1, 2, 3 and 5) after the same reaction time and at the same temperature.
Table 1: Transesterification reaction for methyl-4-nitrobenzoate in ethanol with 1-5 as catalyst and
optimization of the parameters for 4a
Entry
Catalyst
Time (h)
Amount of
catalyst (mol%)b
2.0
T (˚C)
Alcohol
Yield (%)c
1
2
3
4
5
6
7
4
4
4
4
4
4
4
75
75
75
75
75
75
75
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
1
2
12
21
48
72
85
91
92
2.0
2.0
2.0
2.0
2.0
2.0
4
8
12
30
48
8
9
4
4
75
75
EtOH
EtOH
30
30
1.0
5.0
65
92
10
11
12
4
4
4
2.0
2.0
2.0
EtOH
EtOH
EtOH
30
30
30
RT
50
5
21
89
100
13
14
4
4
2.0
2.0
75
75
1-PrOH
2-PrOH
30
30
56
38
15
16
17
18
1
2
3
5
2.0
2.0
2.0
2.0
75
75
75
75
EtOH
EtOH
EtOH
EtOH
30
30
30
30
78
81
65
54
19
20
21
22
23
Blank
Zn(NO3)2.6H2O
Cd(NO3)2.4H2O
H2L1
-
75
75
75
75
75
EtOH
EtOH
EtOH
EtOH
EtOH
30
30
30
30
30
-
11
7
2.0
2.0
2.0
2.0
-
H2L2
-
a
Reaction conditions, unless stated otherwise: 2.0 mol% of catalyst, methyl-4-nitrobenzoate (1.0
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mmol) and alcohol (2.0 mL) at 75 oC for 30 h. bRelative to substrate. cCalculated by 1H NMR.
The increase of the catalyst amount from 1.0 and 2.0 mol% improves the product yield from 65% to 91%
(30 h reaction time), but a further increase in the catalyst amount to 5 mol% did not increase the yield
significantly (entries 6, 8, 9, Table 1). The highest yield was achieved with ethanol (91% in 30 h, entry 6,
Table 1), while yields of 56 % and 38 % were obtained with 1-propanol and 2-propanol, respectively
o
(entries 13 and 14, Table1). Increasing the reaction temperature from room temperature to 75 C
increased the yield, e.g., from 5% to 91% for ethanol but a further rise in the temperature led to a slight
yield decrease (entries 6 and 10-12, Table 1). The product yield vs. time plot is shown in Figure 8A.
The reaction between methyl-4-nitrobenzoate and ethanol was monitored in blank, also using the free
ligand precursors H2L1 and H2L2 as well as the metal salts Zn(NO3)2.6H2O and Cd(NO3)2.4H2O, instead of
the catalysts (entries 19-23, Table 1). The reaction did not progress at all in the absence of any metal
compound, and only 11% or 7% of product yield was obtained when using Zn(NO3)2.6H2O or
Cd(NO3)2.4H2O as catalyst, respectively.
A
B
Figure 8 (A) Plot of product yield vs. time for the transesterification reaction of methyl-4-nitrobenzoate
and ethanol with 4 at 75oC (Dotted line: removal of catalyst after 4 h, see text). (B) Effect of the catalyst
recycling on the yield of ethyl-4-nitrobenzoate.
Under the optimized reaction conditions, when 2 mol% of compound 4 is used as catalyst, a conversion
of 91% of methyl-4-nitrobenzoate into ethyl-4-nitrobenzoate is reached after 30 h at 75oC (entry 6,
Table 1). Lengthening the reaction time to 48 h did not improve the yield of the reaction significantly.
Under these optimized conditions but using frameworks 1, 2, 3 and 5 as catalysts, conversions of 78%,
81%, 65% and 54% (entries 15 - 18, Table 1) were obtained, respectively. Catalyst 4 is the most effective
one, the catalytic performance following the order of 4>2>1>3>5 under the same experimental
conditions (Table 1, entries 7, 15-18). Hence the Zn-MOFs are better catalysts than the Cd ones.
Although a relationship between the catalysts structure and their activity towards the transesterification
reaction cannot be ascertained, it is clear that the Zn-frameworks 1, 2 and 4 have better performances
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as compared to the Cd 3 and 5, and among the former, compound 4 is the best. Apart from the central
metal cation, the three dimensional packing structure in 4 may also play a role.
Once having established that compound 4 is the best catalyst for the transesterification of methyl-4-
nitrobenzoate, we investigated the reaction of other methyl benzoates in ethanol (Table 2). The
substrates with the strongest electron-withdrawing nitro substituent show the highest activity (entry 2,
Table 2 and entry 6, Table 1) while that with the electron-donating methoxy group exhibits the lowest
one (entry 6, Table 2).
Table 2: Transesterification reaction of various methyl esters with 4 as catalysta
Entry
Substrate
Yield (%)b
1
2
3
4
5
6
Methyl Benzoate
68
89
71
65
62
52
Methyl-3-nitrobenzoate
Methyl-4-aminobenzoate
Methyl-3-hydroxybenzoate
Methyl-4-hydroxybenzoate
Methyl-4-methoxybenzoate
a Reaction conditions: 2.0 mol% of catalyst 4, methyl ester (1.0 mmol) and ethanol (2.0 mL)
at 75 oC for 30 h. b Calculated by 1H NMR.
Leaching in general is a common phenomenon observed in the heterogeneous catalytic systems. In
order to test the possibility of leaching and the heterogeneity of our catalytic system, an experiment
was performed following the procedure described by Sheldon et al.20 After the reaction mixture was
stirred for 4 h at 75°C (only ca. 48 % conversion of methyl-4-nitrobenzoate was then observed), the
catalyst was removed by filtration and no further conversion was observed (Fig. 8A). These results
support the heterogeneity of the catalyst with no appreciable leaching. Additionally, we have
determined the amount of zinc in the filtrated solution after the separation of the catalyst 4. The result
shows only 0.03% of zinc is present in the solution, thus ruling out any significant leaching of the
catalyst.
In order to examine the recyclability of catalyst 4, we removed it by filtration after the
transesterification reaction of methyl-4-nitrobenzoate in ethanol, dried it and reused it as catalyst in a
subsequent transesterification process. The catalytic system maintained constant activity for five
consecutive cycles (Fig. 8B). FT-IR and PXRD spectra of 4 taken before and after the reaction (Supporting
Information, Figure S2) did not indicate any important changes. This suggests that the integrity of
catalyst 4 was retained after the reaction.
A possible mechanism for the transesterification catalyzed by 1-5 is presented in Scheme 4, on the basis
of reported proposals. 10a,16c Coordination of the benzoate ester R1COOMe to the Lewis acid metal centre
leads to the increase of the electrophilicity of the carbonyl carbon which is thus activated towards
nucleophilic attack by the alcohol R2OH.
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Scheme 4: Proposed catalytic cycle for the transesterification reaction of methyl benzoates catalyzed by
1-5 ([M] = Zn(II) or Cd(II) centre; R1= benzoyl or substituted benzoyl; R2= Et or Pr).
Conclusion
We successfully synthesized five different metal organic frameworks (1 - 5) of zinc(II) or cadmium(II)
from 5-propionamidoisophthalic acid (H2L1) or 5-benzamidoisophthalic acid (H2L2) in presence or
absence of an auxiliary ligand under various synthetic conditions. They were structurally characterized
by single crystal X-ray diffraction analyses which reveal that they exhibit 1D, 2D or 3D structures with
interesting topologies. Compounds 1 and 5 possess one dimensional double chain type structures,
whereas framework 3 has a 1D cyclic type structure. Compounds 2 and 4 have two dimensional and
three dimensional wave like structures, respectively.
These frameworks (1 – 5) act as good heterogeneous catalysts in the transesterification reactions of
methyl carboxylate esters. The Zn-MOFs display a higher catalytic activity than the Cd ones and 4, with a
3D wave like structure, is the most active one. It effectively catalyzes the reaction of various methyl
benzoate esters with ethanol producing the corresponding ethyl benzoate esters (and methanol) in high
yields which depend on the electrophilicity of the benzoate substituent. The stability and recyclability of
the heterogeneous catalysts were proved. Hence, MOFs containing Zn(II) ions can be utilized as effective
heterogeneous catalysts for the transesterification reaction, with “green” features that deserve to be
further explored for other reactions.
Experimental
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. Carbon, hydrogen, and
nitrogen elemental analyses were carried out by the Microanalytical Service of the Instituto Superior
Técnico. Thermogravimetric analyses were carried out with a Perkin-Elmer Instrument system (STA6000)
at a heating rate of 10˚C min-1 under a dinitrogen atmosphere, in the range of room temperature up to
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ca. 750˚C. Powder X-ray diffraction (PXRD) was conducted in a D8 Advance Bruker AXS (Bragg Brentano
geometry) theta-2theta 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˚. The synthetic work was performed in air and at elevated
temperature.
Synthesis of 1
A mixture of Zn(NO3)2.6H2O (25.0 mg, 0.084 mmol) and H2L1 (20.0 mg, 0.084 mmol) was dissolved in 1
mL of formamide. The resulting mixture was sealed in an 8 mL glass vessel and heated at 75˚C for 48 h.
Subsequent gradual cooling to room temperature (0.2 ˚C min-1) afforded colorless crystals. Yield: 73%
(based on Zn). Anal. Calcd. for C13H15N3O7Zn (M = 390.65): C, 39.97; H, 3.87; N, 10.76; Found: C, 38.82; H,
3.65; N, 9.82. FT-IR (KBr, cm-1): 3393 (bs), 3320 (mb), 2986 (w), 2946 (w), 1674 (s), 1635 (s), 1578 (s),
1438 (s), 1388 (s), 1318 (m), 1220 (s), 1115 (w), 789 (s), 723 (s), 653 (w), 626 (s), 458 (m).
Synthesis of 2
A mixture of Zn(NO3)2.6H2O (25.0 mg, 0.084 mmol), H2L1 (20.0 mg, 0.084 mmol) and 4,4’-bipyridine
(13.1 mg, 0.084 mmol) was dissolved in 1 mL of DMF and methanol (1 : 1). A white precipitate was
obtained when 0.5 mL of 28% aqueous ammonia solution was added to this reaction mixture. The
precipitate was dissolved upon the addition of additional 0.5 mL of 28% aqueous ammonia solution.
Then, the resulting mixture was sealed in an 8 mL glass vessel and heated at 75 ˚C for 48 h. It was
subsequently cooled to room temperature (0.2 ˚C min-1), affording block-like colorless crystals. Yield:
61% (based on Zn). Anal. Calcd. for C48H60N8O18Zn2 (M = 1167.79): C, 49.37; H, 5.18; N, 9.60; Found: C,
49.11; H, 5.01; N, 9.95. FT-IR (KBr, cm-1) 3409 (bs), 3279 (bs), 2970 (w), 2933 (w), 1689 (s), 1664 (s), 1613
(s), 1585 (s), 1492 (m), 1421 (s), 1345 (s), 1224 (w), 1103 (w), 1073 (m), 829 (w), 780 (s), 729 (s), 639 (m).
Synthesis of 3
A mixture of Cd(NO3)2.4H2O (25.9 mg, 0.084 mmol) and H2L1 (20 mg, 0.084 mmol) was dissolved in 2 mL
of DMF, methanol and water (0.5 : 1: 0.5) mixture. The resulting mixture was sealed in an 8 mL glass
vessel and heated at 75 ˚C for 48 h. It was subsequently cooled to room temperature (0.2 ˚C min-1),
affording colorless crystals. Yield: 68% (based on Cd). Anal. Calcd. for C14H16CdN2O6 (M = 420.70): C,
39.97; H, 3.83; N, 6.66; Found: C, 39.88; H, 3.11; N, 6.10. FT-IR (KBr, cm-1): 3323 (bs), 3083 (b), 2983 (w),
2929 (w), 1659 (s), 1616 (s), 1567 (s), 1435 (s), 1366 (s), 1322 (m), 1244 (s), 1103 (s), 920 (w), 811 (m),
783 (s), 730 (s), 671 (w), 456 (m).
Synthesis of 4
A mixture of Zn(NO3)2.6H2O (20.7 mg. 0.07 mmol), H2L2 (20.0 mg. 0.07 mmol) and 4,4’-bipyridine (11.1
mg, 0.07 mmol) was dissolved in 2 mL of DMF and methanol (1 : 1). A white precipitate was obtained
when the mixture was stirred at room temperature. The precipitate was dissolved upon the addition of
0.5 mL of 28% aqueous ammonia solution. Then, the resulting mixture was sealed in an 8 mL glass vessel
and heated at 75 ˚C for 48 h. It was subsequently cooled to room temperature (0.2 ˚C min-1), affording
block-like colorless crystals. Yield: 49% (based on Zn). Anal. Calcd. for C25H21N3O7Zn (M = 540.83): C,
55.52; H, 3.91; N, 7.77; Found: C, 55.21; H, 3.92; N, 7.10. FT-IR (KBr, cm-1): 3404 (bs), 326 (bs), 1683 (s),
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1617 (s), 1562 (s), 1454 (m), 1418 (s), 1382 (s), 1232 (m), 1107 (w), 1054 (w), 771 (s), 732 (s), 627(m),
460 (w).
Synthesis of 5
A mixture of Cd(NO3)2.4H2O (31.0 mg, 0.10 mmol), H2L1 (28.5 mg, 0.10 mmol) and 4,4’-bipyridine (15.6
mg, 0.10 mmol) was dissolved in 2 mL of formamide. Then, the resulting mixture was sealed in an 8 mL
glass vessel and heated at 75 ˚C for 48 h. It was subsequently cooled to room temperature (0.2 ˚C min-1),
affording block-like colorless crystals. Yield: 79% (based on Cd). Anal. Calcd. for C27H23CdN5O7 (M =
641.91): C, 50.52; H, 3.61; N, 10.91; Found: C, 50.83; H, 3.92; N, 10.09. FT-IR (KBr, cm-1): 3392 (bs), 3281
(bs), 1653 (s), 1560 (s), 1491 (m), 1429 (s), 1367 (s), 1285 (m), 1228 (m), 1105 (w), 1069 (w), 777 (s), 733
(s), 625 (m).
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 (1-5). 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 SMART21 software and refined using Bruker SAINT21a on all the observed
reflections. Absorption corrections were applied using SADABS21a. Structures were solved by direct
methods by using the SHELXS-97 package21b and refined with SHELXL-2014/721b. Calculations were
performed using the WinGX System–Version 1.80.0321c. The hydrogen atoms attached to carbon 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 nitrogen atoms or the carbon
atoms for phenyl and methylene residues, and 1.5Ueq of the parent carbon atoms for the methyl
groups. The hydrogen atoms of coordinated water molecules were located from the final difference
Fourier map and the isotropic thermal parameters were set at 1.5 times the average thermal
parameters of the belonging oxygen atoms and their distances were fixed using DFIX and DANG. The
hydrogen atoms of the non-coordinated water were inserted in calculated positions by means of the
CALC-OH routine of WinGX but their distances were fixed using DFIX and DANG to achieve a reasonable
result. In compound 4, the banzamido group is highly disordered between two different positions: the
phenyl group and the N-amide atom were modelled by means of PART 1 and PART 2 instructions leading
to occupancies of 45.8 and 54.2 % for the disordered components; the strategy for the O-amide atom
was different, with the s.o.f. of both components set to 10.5000, one of them flanked by PART -1 and
PART 0 to exclude bonds to symmetry related atoms. Least square refinements with anisotropic thermal
motion parameters for all the non-hydrogen atoms and isotropic ones for the remaining atoms were
employed. Crystallographic data are summarized in Table S1 (Supplementary Information file) and
selected bond distances and angles are presented in Table S2. CCDC 1490728-1490732 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.
Procedure for the transesterification reaction catalyzed by MOFs
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A mixture of methyl-4-nitrobenzoate (180 mg, 1 mmol) and MOF-catalyst (8 mg for 1, 23 mg of 2, 8 mg
of 3, 11 mg of 4 and 13 mg of 5, 2 mol%) in 2 mL EtOH was added into a capped glass vessel. The mixture
was heated at 75˚C for the desired time (typically 30 h). The solution was then centrifuged to remove
the catalyst. The solvent was evaporated in vacuum, leading to a crude product. The product mixture
1
was analyzed by H NMR in CDCl3. The yield of the ethyl ester product (relatively to the methyl ester)
was established typically by taking into consideration the relative amounts of these compounds, as given
by 1H NMR. The 1H-NMR spectra are presented in Figure S1 (Supporting Information file). The
centrifuged catalyst was washed several time with methanol and dried at room temperature. After that
the recycling experiment was performed under the conditions mentioned above.
Acknowledgements
This work has been supported by the Foundation for Science and Technology (FCT) (project
UID/QUI/00100/2013), Portugal. Authors A. Karmakar and A. P. C. Ribeiro express his gratitude to the
FCT for post-doctoral fellowship (Ref. No. SFRH/BPD/76192/2011 and SFRH/BPD/90883/2012).
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Graphical Abstract
ZnII and CdII MOFs based on Amidoisophthalic acid ligand: Synthesis,
Structure and Catalytic Application in Transesterification
Anirban Karmakar*, Guilherme M. D. M. Rúbio, M. Fátima C. Guedes da Silva*, Ana P. C.
Ribeiro 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; fatima.guedes@tecnico.ulisboa.pt;
pombeiro@tecnico.ulisboa.pt.
Metal organic frameworks of Zn(II) and Cd(II) act as recyclable heterogeneous catalysts for
transesterification reaction of carboxylate esters.