PH dependent synthesis of Zn(ii) and Cd(ii) coordination polymers with dicarboxyl-functionalized arylhydrazone of barbituric acid: Photoluminescence properties and catalysts for Knoevenagel condensation

Article abstract of DOI:10.1039/c5nj02411k

Four novel Zn(ii) and Cd(ii) compounds based on a dicarboxyl-functionalized arylhydrazone of barbituric acid ligand, 5-[2-{2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene}hydrazinyl]isophthalic acid (H₅L), namely [Zn(1κO:2κO′:3κO′′-H₃L

Full text of DOI:10.1039/c5nj02411k

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pH dependent synthesis of Zn(II) and Cd(II) coordination polymers
with dicarboxyl-functionalized arylhydrazone of barbituric acid:
photoluminescence properties and catalysts for Knoevenagel
condensation
a,
a
a,b
a,
Anirban Karmakar, * Anup Paul, Kamran T. Mahmudov, M. Fátima C. Guedes da Silva *
a,
and Armando J. L. Pombeiro *
a
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,
1
049–001, Lisbon, Portugal, E-mail: anirbanchem@gmail.com; fatima.guedes@tecnico.ulisboa.pt;
pombeiro@tecnico.ulisboa.pt
b
Department of Chemistry, Baku State University, Z. Xalilov Str. 23, Az 1148 Baku, Azerbaijan.
Abstract
Four novel Zn(II) and Cd(II) compounds based on a dicarboxyl-functionalized arylhydrazone of
barbituric acid ligand, 5-[2-{2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene}hydrazinyl]isophthalic acid
(H
5
L), namely [Zn(1κO:2κO’:3κO’’-H
3
L)(H
2
O)
2
]
n
(1), [Cd(1κO,O’:2κO’:3κO’’,O’’’-H
3 2 2 n
L)(H O) ] (2), [Zn(κN-
H
3
L)(H O) ]∙3H O (3) and [Cd(1κO,O’:2κN-H
2
3
2
3
L)(H O) ∙4nH O (4), have been synthesized at different
2
3
]
n
2
pH under hydrothermal conditions and characterized by elemental and thermogravimetric, FT-IR,
and X-ray single-crystal diffraction analyses. Compounds 1 and 2 prepared in acidic medium are
coordination polymers exhibiting double chain type one-dimensional (1D) structures, whereas 3 and
4
are obtained in basic medium. Compound 3 is a mononuclear Zn(II) complex containing three
2
-
lattice water molecules, but in 4, Cd(II) cations are bridged by H L ligands to form an interesting zig-
3
zag type 1D coordination polymer. The crystal structures of these compounds are stabilized by
strong hydrogen bonding interactions, forming two and three-dimensional networks. The pH value of
the reaction system plays a crucial role in the ligand deprotonation and in the assembly of the metal-
organic polymers. These compounds act as effective heterogeneous catalysts, under mild conditions,
for the Knoevenagel condensation reaction of different aldehydes with malononitrile and can be
recycled at least five times without losing activity. Further, compounds 1-4 exhibit luminescent
properties in the solid state at room temperature.
Keywords: Zinc, Cadmium, X-ray diffraction, Arylhydrazone of barbituric acid, Catalysis, Knoevenagel
condensation
1
. Introduction
Metal organic frameworks or coordination polymers (CPs) have a recognized importance owing to
their wide range of potential applications in the growing fields of molecular storage, magnetism,
1
electrical conductivity, sensor capability and catalysis. The architectural diversities found in CPs
results not only from covalent metal–ligand bonds, but also from weak non-covalent interactions,
2
like hydrogen and halogen bonding, π-anion, π-cation and π-stacking interactions. However, their
synthesis in a truly rational manner continues to be a challenging task on account of the many
factors that influence the product structures, such as nature of the ligand and metal, solvent,
1

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3
reaction temperature, metal to ligand ratio, pH, etc. Particularly, the pH of the reaction medium can
4
play a very important role in the construction of CPs, and its alteration can significantly affect the
5
protonation level of the organic ligand resulting in architectural changes in the structures. The
6
selection of the organic ligand is also crucial, namely for the control of topologies.
In this context, Barbituric acid containing polydentate ligands are excellent candidates for the
construction of novel structures and topologies because they have multiple coordination site and
can adopt versatile conformations. Besides their pharmacological properties, barbiturates
(
pyrimidine-2,4,6(1H,3H,5H)-triones) are useful synthons for the non-covalent interactions assisted
7
construction of supramolecular and coordination compounds. The modification of the pyrimidine
moiety by introduction of different substituents to the C5 atom of the ring can change the nuclearity
7
c
and the topology of the formed CPs. Incorporation of a carboxyl-functionalized arylhydrazone
moiety to the pyrimidine ring increases the donor centres of the ligand and its coordination
versatility. Further, the presence of pyrimidine and carboxyl moieties in the ligand framework may
affects the self-assembly process at different pH.
Based on the above considerations, we have synthesized, as a potential linker for CPs, the barbituric
acid derivative 5-[2-{2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene}hydrazinyl] isophthalic acid (H
5
L)
(Scheme 1), which contains both amide and carboxylic acid groups. We have chosen such a type of
linker taking into account the following factors: (a) the polydentate coordination sites may generate
multidimensional structures; (b) this ligand contains an amide group which may be suitable to
5
produce hydrogen bonded networks; (c) the deprotonation of H L may be affected by pH which will
have an influence on the network topology.
Important applications of CPs concern their use in catalysis. Recently they been applied in
8
9
10a
heterogeneous catalysis in many organic reactions, e.g., alkylation of aromatics, carbonyl–ene
1
0b
10c
10d
and Diels–Alder reaction,
cyanotrimethylsilation,
isomerization of α-pinene oxide
and
1
0e
Mukaiyama–aldol reaction.
Acidic and basic polymers can exhibit high catalytic activity in
1
1
12
13
condensation reactions like the Pechmann, Knoevenagel and Prins reactions. Recently, our
group has reported various CPs which are catalytically active for the oxidation, nitroaldol and
1
4
cyanosilylation reactions, and now we wish to extend our study to another type of reaction, the
Knoevenagel condensation. This is one of the most useful C–C bond forming reactions, widely used
1
5
in the synthesis of fine chemicals and pharmaceuticals. However, only few CPs have been
1
6
reported which actively catalyse this reaction and there is still a great demand to develop new
types of catalysts based on cheap and environmentally tolerable metal complexes that could be
easily recyclable (hence forming an heterogeneous system) and could show high efficiency under
1
6a
16b
mild conditions. Recently, Kitagawa et al. and Zhou et al. reported that amide functionalized
MOFs are quite effective for the Knoevenagel condensation. Besides, some Zn-MOFs can also
1
6c-d
effectively catalyze this condensation in an heterogeneous way.
Thus, the development of
1
6e-
suitable MOF (or CP) based catalysts for the Knoevenagel condensation is a topic of great interest.
g
Hence, the two main objectives of the current work concern: (i) to synthesize Zn(II) and Cd(II)
complexes, namely CPs, by using the 5-[2-{2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene}hydrazinyl]
2

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5
isophthalic acid (H L) linker at different pHs, under hydrothermal reaction conditions; (ii) to apply
the synthesized complexes as heterogeneous catalysts for the Knoevenagel condensation reaction of
different aldehydes with an active methylene compound; (iii) to study the photoluminescence
properties of the complexes.
In this work, we have synthesized four different compounds, namely [Zn(1κO:2κO’:3κO’’-H
1), [Cd(1κO,O’:2κO’:3κO’’,O’’’-H L)(H O) (2), [Zn(κN-H L)(H O) ]∙3H O (3) and [Cd(1κO,O’:2κN-
L)(H O) ∙4nH O (4), under solvothermal conditions at different values of pH which play a key role
3 2 2 n
L)(H O) ]
(
3
2
2
]
n
3
2
3
2
H
3
2
3
]
n
2
in the structural arrangement process. To our knowledge, no previous study on Zn(II) and Cd(II)
coordination chemistry with 5-[2-{2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene}hydrazinyl]
isophthalic acid (H L) has been reported. The heterogeneous catalytic properties of the obtained
5
compounds (1-4) towards the Knoevenagel condensation reaction of malononitrile with various
aldehydes have been successfully tested. In addition, the solid state photoluminescence properties
of these compounds have also been investigated.
2
. Results and Discussion
2
.1 Syntheses and characterization
5
-[2-{2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene}hydrazinyl]isophthalic
acid
synthesized according to the procedure reported earlier. The solvothermal reaction of
Zn(NO .6H O or Cd(NO .4H O with H L in DMF and water in acidic medium led to the formation of
the double chain type 1D coordination polymer [Zn(1κO:2κO’:3κO’’-H L)(H O) (1) or
Cd(1κO,O’:2κO’:3κO’’,O’’’-H L)(H O) ] (2), respectively. But, when performed at a basic pH, the
(H
5
L)
was
1
7
3
)
2
2
3
)
2
2
5
3
2
2 n
]
[
3
2
2 n
reaction produced the mononuclear zinc(II) complex [Zn(κN-H
dimensional Cd(II) polymer [Cd(1κO,O’:2κN-H L)(H O) ∙4nH O (4) (Scheme 1). Several attempts to
obtain X-ray quality crystals at neutral pH medium failed.
3 2 3 2
L)(H O) ]∙3H O (3) or the zig-zag one
3
2
3
]
n
2
Scheme 1 Syntheses of compounds 1-4.
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In the IR spectra of 1-4, the characteristic strong bands of the coordinated carboxylate
-
1
-1
groups appear at 1585–1561 cm for the asymmetric stretching and 1384-1360 cm for the
1
8
-1
symmetric one. A medium broad band between 3418 and 3477 cm is due to ν(NH) of amide
-
1
groups, whereas ν(OH) of water molecules is in the 3377–3244 cm region. Due to their insolubility
in common solvents, the NMR spectra of these compounds could not be run, but they were
characterized by IR spectroscopy, single crystal X-ray diffraction, elemental (see experimental
section) and thermogravimetric analysis (supporting information, Figure S3).
2
.2 Crystal structure analyses
According to the crystal structure analyses, 1-4 crystallize in different space groups (1 and 2 in P-1, 3
in P2 /c and 4 in C2/c). Compounds 1 and 2 are similar double chain 1D CPs while 4 possesses a zig-
zag type 1D chain. Compound 3 has a mononuclear structure. The variation of pH affects the
1
2
-
3
formation of these frameworks. At acidic pH (2.0) the H L ligand coordinates via carboxylate groups
and the amide-N atoms remain protonated; but in basic medium the ligand coordinates via the
deprotonated amide-N (for compound 3) or via this atom and carboxylate-O atoms (for compound
4
).
Figure 1: (A) The asymmetric unit of framework 1 with partial atom labelling scheme. (B) One
dimensional structure of framework 1. (C) Topological representation of frameworks 1 and 2.
In all cases extensive H-bond interactions could be found that play a role towards the extension to
3
D supramolecular frameworks. A list of selected hydrogen bonding interactions is presented in
Table S2 (supporting information). Selected bond distances and angles are listed in Table S3
supporting information).
(
2
-
2+
The asymmetric unit of 1 contains one deprotonated ligand (H L ) and one Zn ion along with two
3
2
+
19
coordinated water molecules (Figure 1A). The Zn ion has a trigonal bipyramidal (τ
5
= 0.76)
geometry and is involved in the dinuclear Zn } core by means of two syn-syn-type bridging
{
C
2
O
4
2
4

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10 4 2
bidentate carboxylate groups [Zn1–O3, 1.964(2) Å and Zn1–O4, 1.964(2) Å], and in {C O Zn } cores
also with the involvement of non-bridging monodentate [Zn1–O1, 1.968(2) Å] carboxylate moieties
Figure 1B). Coordination sites are further occupied by the O-atoms of two coordinated water [Zn1-
(
O8, 2.215(3) Å; Zn1-O9, 2.095(3) Å] molecules. The minimum Zn∙∙∙Zn intra-chain distance is 3.8242(7)
Å in 1.
Figure 2: (A) The asymmetric unit of framework 2 with partial atom labelling scheme. (B) One
dimensional double chain type structure of framework 2.
The CP 2 has a similar structural arrangement to that of 1. The asymmetric unit of 2 also contains
2
L
-
2+
one H
3
ligand, one Cd ion and two water molecules. The Cd(II) ion presents a distorted
pentagonal bipyramidal environment which results from the chelating bidentate mode of two
2
-
carboxylate groups from two H
3
L ligands and a bridging monodentate carboxylate moiety from
another of such ligands [Cd1-O1; 2.525(2) Å, Cd1-O2, 2.231(2) Å; Cd1-O3, 2.244(2) Å and Cd1-O4,
2
2
.209(2) Å], the coordination sphere being accomplished with two water molecules [Cd1-O8,
.347(2) Å and Cd1-O9, 2.356(2) Å] (Figure 2A). The outcome of such organization is that the metal
in CP 2 is involved in {Cd
2 2 2 4
O } and in {C10Cd O } dimetallic cores (Figure 2B). In this framework the
minimum Cd∙∙∙Cd distance is 3.8415(4) Å.
Topological analysis of frameworks 1 and 2 reveals that they have a 3-connected uninodal net with a
topological type SP1-periodic net (Figure 1C).
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Figure 3: (A) The structure of compound 3 with partial atom labelling scheme. (B) Hydrogen bonded
network of compound 3.
The asymmetric units of 3 contains the zinc(II) ion coordinated by a deprotonated N-amide of the
2
-
H
3
L ligand [Zn1-N3, 2.011(3) Å] and three water molecules [Zn1-O10 2.017(3) Å; Zn1-O8 2.021(3) Å;
Zn1-O9 2.042(4) Å], as well as three crystallization water molecules (Figure 3A). The coordination
1
9
2-
geometry is tetrahedral (τ
4
3
= 0.94). In the H L ligand the carboxylate and carboxylic acid groups
remain uncoordinated.
The single crystal analysis of 4 reveals a zig-zag type 1D framework constructed by Cd(II) ions and
2
-
2+
2-
3 3
H L ligands (Figure 4A). The asymmetric unit contains a Cd ion, one deprotonated ligand (H L ),
along with three coordinated water and one non-coordinated water molecule (Figure 4B). The Cd(II)
centre displays a distorted octahedral geometry, the equatorial positions being occupied by the two
2
-
oxygen atoms of a carboxylate from one H L ligand [Cd1–O1, 2.446(5) Å and Cd1–O2, 2.415(5) Å],
3
one amide nitrogen [Cd1-N3, 2.247(4) Å] and one water molecule [Cd1-O10, 2.296(6) Å]. The
remaining two water molecules occupy the axial positions [Cd1–O8, 2.349(6) Å, and Cd1–O9,
2
.338(6) Å].
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Figure 4: (A) One dimensional structure of framework 4. (B) The structure of compound 4 with
partial atom labelling scheme. (C) Node-and-linker-type description of the 1D coordination
framework in 4.
Topological analysis of framework 4 revealed that it has a 2-connected uninodal net with topological
type 2C1 (Figure 4C).
In compounds 1-4 the organic ligand is almost plane, as well as the polymeric assemblies in 1, 2 and
2
‒
4
. The co-planarity of H
3
L can be expressed by the angle between the least-square planes of the
hetero- and the aromatic rings: 5.59º (1), 3.98º (2), 10.66º (3) and 5.22º (4).
O
N
O
N
O
N
O
N
M
M
HN
NH
HN
NH
HN
N
HN
N
O
O
O
O
O
O
O
O
NH
NH
NH
NH
O
O
O
O
O
O
M
O
O
O
O
M
O
M
O
M
OH
O
M
OH
O
M
M
1
2
3
4
2
-
Scheme 2 Coordination modes of H
3
L in 1-4.
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2
.3 Comparison of the structures of 1–4 and pH-dependence
The structural differences of 1-4 reflect the extent and type of deprotonation of the 5-[2-{2,4,6-
trioxotetrahydropyrimidin-5(2H)-ylidene}hydrazinyl]isophthalic acid (H L) ligand precursor. The
5
linking fashions are illustrated in Scheme 2; the carboxylate groups adopt the unidentate, chelate,
bridging and bridging chelating modes, whereas the deprotonated amide group shows the
monodentate coordination. At acidic pH (1 and 2), the amide-N atoms of the ligand remain
protonated, restricting themselves to coordination. The carboxylate groups coordinate to the metal
ions via monodentate and bridging bidentate fashions (in case of 1), or chelating plus bridging
chelating bidentate modes (in case of 2). Thus, 1D double chain type structures are formed in case of
1
and 2.
With the increase of the pH value (10.0), compounds 3 and 4 are the obtained ones. In both of them
one amide-N of the barbituric ring is deprotonated and coordinates in the monodentate fashion,
while the other amide remains protonated. For compound 3, both carboxylic acid and carboxylate
groups of the ligand are uncoordinated and a mononuclear complex is obtained. But in 4 the
carboxylate group coordinates to another Cd(II) ion in the chelating coordination mode, while the
carboxylic group remains uncoordinated, generating a zig-zag type 1D structure. Therefore, taking
into account the properties of H
5
L, it is possible to construct derived CPs of different structural types
based on pH-control, which plays a fundamental role in the formation of the resulting structure.
2
.4 Photoluminescence studies
Luminescent compounds have gained a great deal of attention over the years and many CPs have
2
0
been investigated and emerged as a new type of promising luminescent materials. Therefore the
luminescent properties of the free ligand precursor H L and complexes 1-4 have been investigated in
the solid state at room temperature (Figure 5). The H L exhibits a weak emission at 522 nm (λex = 370
5
5
2
1
nm), which could be attributed to the π*-π or π*-n transitions. Complexes 1 and 2 exhibit emission
characteristics similar to that of the free ligand. The emission peaks at 511 nm (λ = 370 nm) in 1
ex
and 514 nm (λex = 370 nm) in 2 show a blue shift of ca. 8-10 nm, compared to that of the free ligand,
2
2
which can be ascribed to the intra-ligand transition of the coordinated ligands. An analogous
spectral pattern is observed for complex 3 (λem = 492, λ = 370 nm) and 4 (λ = 543 nm, λ = 370
ex
em
ex
nm), where the former shows a blue shift of ca. 30 nm and the latter exhibits a red shift of ca. 20 nm
compared to that of the free ligand.
Figure 5 Solid state emission spectra of H
5
L and complexes 1-4 at room temperature (λex = 370 nm).
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2
.5 Catalytic application
1
5
The Knoevenagel condensation is one of the most useful C–C bond forming reactions. The
condensation is usually catalysed by bases or acids and requires high reaction temperatures or
2
3
microwave irradiation. While various types of heterogeneous catalysts have been developed for
1
5,16
such organic transformation,
there is still a considerable interest in developing new ones, on
account of their easy separation and reusability.
Therefore, we have tested the possible catalytic behaviour of our synthesised compounds 1-4 for
such a condensation reaction. Tests performed with benzaldehyde and malononitrile in different
3 2
solvents (THF, CH CN, EtOH, MeOH and H O) (Table 1, entries 7, 13-16) show that THF is the best
one, leading to the highest yield (94%). The reactions under solvent-free conditions are much
slower. Catalyst 3 is the most effective one, the catalytic performance following the order of
3
>1>2>4 under the same experimental conditions (Table 1, entries 7, 21-23). Although the
relationship between structure and catalytic activity is not clearly understood, the highest
conversation shown by the compound 3 may be due its higher number of labile H O ligands and the
2
easier accessibility of the zinc(II) centre to the substrate. In the other cases (1, 2 and 4), the tight
packing is unfavouring.
Consequently, the optimization of the reaction conditions (temperature, reaction time, amount of
catalyst) was carried out in a model malononitrile–benzaldehyde system with 3 as the catalyst
(Scheme 3, Table 1).
CN
CHO
NC
Catalyst
+
NC CN
+
H O
2
o
THF, 50 C, 4h
Scheme 3 Catalysed Knoevenagel condensation reaction of benzaldehyde with malononitrile.
The plot of yield versus time for the Knoevenagel condensation reaction of benzaldehyde and
malononitrile with compound 3 is presented in Fig. 6A, showing that 4 h is the best reaction time.
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Fig. 6 (A) Plots of yield vs. time for the Knoevenagel condensation reaction of benzaldehyde and
malononitrile in presence of catalyst 3 (black curve) and after filtering off the catalyst 3 (red curve).
(B) Yield for five consecutive cycles of Knoevenagel condensation of benzaldehyde and malononitrile
catalysed by 3.
The reaction between benzaldehyde and malononitrile was also monitored in blank, as well as using
free ligand precursor H
catalysts (entries 24-27, Table 1). The reaction did not proceed at all in blank or in the presence of
L. Only 26 or 30% of product yield was obtained using Zn(NO .6H O or Cd(NO .4H O as catalyst,
L and Zn(II) or Cd(II) salt
2 3 2 2 3 2 2
L and the metal salts Zn(NO ) .6H O and Cd(NO ) .4H O, instead of the
H
5
3
)
2
2
3
)
2
2
respectively. Moreover, the reaction performed by using 1:1 mixtures of H
led to only to yields of 25 or 31% (entries 28-29, Table 1).
5
Table 1: Optimization of the parameters of the Knoevenagel condensation reaction between benzaldehyde and
a
malononitrile with 3 as the catalyst
b
c
Entry
Catalyst
Time (h)
Amount of
Catalyst (mol%)
3.0
T (˚C)
Solvent
Yield (%)
TON
1
2
3
4
5
6
7
8
9
3
3
3
3
3
3
3
3
3
0.5
1.0
2.0
2.5
3
50
50
50
50
50
50
50
50
50
THF
THF
THF
THF
THF
THF
THF
THF
THF
25
40
61
69
79
88
94
94
94
8
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
13
20
23
26
29
31
31
31
3.5
4.0
4.5
5.0
10
11
12
3
3
3
4.0
4.0
4.0
1.0
5.0
7.0
50
50
50
THF
THF
THF
65
93
94
22
31
31
13
14
15
16
3
3
3
3
4.0
4.0
4.0
4.0
3.0
3.0
3.0
3.0
50
50
50
50
CH
3
CN
82
70
75
12
27
23
25
4
EtOH
MeOH
2
H O
o
17
18
19
20
3
3
3
3
4.0
8.0
3.0
3.0
3.0
3.0
RT (22 C)
THF
THF
THF
THF
32
64
89
93
13
21
29
31
o
RT (22 C)
o
12.0
4.0
RT (22 C)
o
70 C
21
22
23
1
2
4
4.0
4.0
4.0
3.0
3.0
3.0
50
50
50
THF
THF
THF
84
76
80
28
25
27
24
25
26
27
28
Blank
H5L
4.0
4.0
4.0
4.0
4.0
-
50
50
50
50
50
THF
THF
THF
THF
THF
No Reaction
-
-
3.0
3.0
3.0
-
No Reaction
Zn(NO
Cd(NO
H L+
3
)
2
.6H
2
O
26
30
25
9
10
-
3
)
2
.6H
2
O
5
Zn(NO ) .6H O
3
2
2
29
H
5
L+
4.0
-
50
THF
31
-
1
0

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DOI: 10.1039/C5NJ02411K
Cd(NO
3 2 2
) .6H O
a
Reaction conditions unless stated otherwise: 3.0 mol% of catalyst 3, solvent (THF) 1 mL, malononitrile (66 mg, 1.0 mmol)
b
1
c
and benzaldehyde (0.50 mmol). Calculated by H NMR. Number of moles of product per mole of catalyst.
The effects of temperature and catalyst amount were also monitored. At room temperature the
reaction proceeds very slowly producing 89% yield after 12 h (entries 17-19, Table 1). However,
increasing the temperature has a positive influence and the maximum yield (94%) was reached at 50
°C (entry 7, Table 1). Further temperature increase did not have any significant effect on the yield
(entry 20, Table 1). Catalyst loading had a similar effect i.e., its increase from 1.0 to 3.0 mol%
increases the product yield from 65 to 94% (entries 7 and 10, Table 1), but further increase in the
former had no significant effect on the latter (entries 11–12, Table 1).
We have also investigated the catalytic activity of 3 with different substituted aromatic, aliphatic,
alicyclic and heterocyclic aldehydes in the reaction with malononitrile (Table 2). The highest yield
was obtained by using p-nitrobenzaldehyde (100%), whilst the lowest one was for p-
methoxybenzaldehyde (26%), suggesting that an electron-withdrawing substituent promotes the
reactivity, relative to an electron-donor moiety, what can be related to the increase of the
electrophilicity of the substrate in the former case. The yields for aliphatic, alicyclic and heterocyclic
aldehydes are also good (66–79%).
a
Table 2: Knoevenagel condensation reaction of various aldehydes with malononitrile with catalyst 3
b
c
+
Entry
Compound
Yield (%)
TON
ESI-MS (m/z, M )
1
100
42
33
200.24
185.26
2
14
3
4
5
89
35
68
30
12
23
189.85
169.78
171.65
6
60
20
181.56
7
8
9
Acetaldehyde
72
66
75
24
22
25
93.87
107.54
156.89
Propionaldehyde
1
1
0
1
71
79
24
26
161.67
161.32
a
Reaction conditions: 3.0 mol% of catalyst 3, solvent (THF) 1 mL, malononitrile (66 mg, 1.0 mmol) and
b
1
c
benzaldehyde (0.50 mmol). Calculated by H NMR. Number of moles of product per mole of catalyst.
Leaching in general is a common phenomenon observed in the catalytic systems, leading to loss of
activity. In order to test the possibility of leaching and the heterogeneity of our catalytic system, an
2
4
experiment was performed following the procedure described by Sheldon et al. After the reaction
mixture was stirred for 1 h at 50 °C (only ca. 40 % conversion of benzaldehyde was then observed)
1
1

New Journal of Chemistry
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the catalyst was removed by filtration. The removal of the solid catalyst ceased the reaction
completely (Fig. 6A), and no further conversion was observed even upon an extended time,
supporting the heterogeneity of the catalyst with no appreciable leaching. Additionally, the filtrated
solution, after the separation of the catalyst 3, was evaporated to dryness and the amount of zinc
determined was only 0.01% of that used in the reaction, thus ruling out any significant leaching of
the catalyst.
In order to examine the reusability and stability of 3 during the catalytic process, it was recycled in
five consecutive experiments and its catalytic activity was monitored. It remained almost the same
(Figure 6B). FT-IR and powder X-ray diffraction of catalyst 3 taken before and after the reaction
indicated that the structure of the solid was retained (Figure S1).
The efficiency of our catalyst in comparison with various CP catalysts for this kind of reaction is
presented in Table 3. The obtained yield in our system is higher than that reported for a zinc(II)
framework with a terephthalate derivative in which the reaction of benzaldehyde and malononitrile
o
leads to an overall yield of 77% after 24 h reaction time at 70 C (i.e., for a longer time and a higher
2
5a
temperature than in our case) (Table 3, entry 2). Similarly, a 3D MOF built from nickel(II) ions and
o
25b
methanetetrabenzoate ligands produced 78% yield after 6 h at 130 C (Table 3, entry 3). Moreover,
the reaction catalyzed by [Tb(BTATB)(DMF) (H O)] [BTATB 4,4′,4″-(benzene-1,3,5-
triyltris(azanediyl))tribenzoate] produced 99% yield by using 4 mol% of catalyst at 60 C for 24 h, i.e.,
=
2
2
n
o
2
5c
longer time, higher temperature and higher catalyst amount than in our case (Table 3, entry 4). At
room temperature, our catalyst 3 led to a higher reaction yield than that reported for a Cd(II) MOF
2
5d
(
Table 3, entries 5 and 6). In comparison with other reported catalysts, ours have the advantages
of being easy-to-prepare, rather cheap and highly active. Moreover, they are recyclable without
appreciable loss of catalytic activity.
Table 3: A comparison of catalytic activity of various coordination polymers in the Knoevenagel condensation
reaction of benzaldehyde and malononitrile
Entry Catalyst
Amount of catalyst Solvent/Temp/Time
used (mol%)
Yield (%)
Ref.
1
[Zn(κN-H
3
L)(H
2
O)
3
]∙3H
2
O (3)
3
THF/ 50 °C/ 5 h
94
This study
2
3
[Zn
2
dobdc]
n
1
4
Toluene/ 70 °C/ 24 h
p-xylene/ 130 °C/ 6 h
77
78
25a
25b
{[Ni (μ
4
6
-MTB) (μ -
2 2
H O) (H O) ]∙10DMF∙11H O}
2
4
2
4
2
n
4
[Tb(BTATB)(DMF)
2
(H
2
O)]
n
4
CH
3
CN/ 60 °C/ 24 h
99
25c
5
6
[Zn(κN-H
3
L)(H
2
O)
3
]∙3H
2
O (3)
3
4
THF/ RT/ 12 h
89
80
This study
25d
{[Cd(TPCT) (SO ) ]DMF.4H O}
CHCl /RT/12 h
2
4 2
2
n
3
dobdc
=
2,5-dioxidoterephthalate; MTB
=
methanetetrabenzoate; BTATB
=
4,4′,4″-(benzene-1,3,5-
triyltris(azanediyl))tribenzoate; TPCT = tri(pyridin-4-yl)cyclohexane-1,3,5-tricarboxamide
Among the four compounds (1-4), the zinc compounds (3 and 1) show higher catalytic activities than
the cadmium complexes, which can be due to a higher Lewis acidic nature of Zn(II) ion. A possible
mechanism for the Knoevenagel condensation catalyzed by 1-4 is presented in Scheme 4, on the
2
6,16g
basis of reported proposals.
1
2

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New Journal of Chemistry
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DOI: 10.1039/C5NJ02411K
2
6,16g
In the catalytic process it is assumed
that the zinc or cadmium centre (Lewis acid site) interacts
with the carbonyl group of the benzaldehyde, increasing the electrophilic character of the carbonylic
carbon atom, thus favouring the attack from the malononitrile. Similarly, the interaction of a cyano
group of malononitrile with the Lewis acid metal site increases the acidity of its methylene moiety.
The basic sites (carboxylate-O, amide-N or hydrazone-N) of our catalyst can abstract the proton from
the methylenic compound to generate the corresponding nucleophilic species, which attacks the
carbonyl group of benzaldehyde, with C–C bond formation and dehydration.
Scheme 4: Proposed catalytic cycle for the Knoevenagel condensation reaction catalyzed by 1-4 (M =
Zn(II) or Cd(II) centre; X = O or N of a coordinated group).
3
. Conclusion
In conclusion, four different zinc(II) and cadmium(II) compounds containing dicarboxyl-
functionalized arylhydrazone of barbituric acid have been synthesized upon variation of pH under
hydrothermal conditions, and structurally characterized. Compounds 1 and 2 have been obtained at
acidic pH and they have one dimensional double chain-like structures. The metal ions are
coordinated via ligand carboxylate groups and the amide functions remain protonated. However,
compounds 3 and 4, obtained under alkaline conditions, exhibit mononuclear and one dimensional
zig-zag structures, respectively. In 3, the Zn(II) ion is coordinated by a deprotonated amide-N
whereas for 4 the Cd(II) centres are coordinated via both amide-N and carboxylate groups. These
compounds show interesting zero dimensional and one dimensional structures with various
topologies. The pH of the reaction system and the coordination modes of the arylhydrazone based
ligand play pivotal roles in the formation of these structures. In addition, complexes 1-4 exhibit
photoluminescence properties in the solid state at room temperature.
These compounds act as heterogeneous catalysts for the Knoevenagel condensation of aldehydes
with malononitrile, at moderate temperature. Complex 3 is the most effective catalyst for this
reaction. It effectively catalyzes the reaction of various aldehydes and malononitrile producing the
corresponding benzylidenemalononitriles in high yields, which depend on the electrophilicity of the
substrate. Moreover, the catalyst is stable and recyclable. This study provides further evidence that
zinc(II) and Cd(II) coordination polymers containing arylhydrazone based ligand can be utilized as
effective heterogeneous catalysts, under mild conditions, in this important organic reaction. Further
explorations into the uses of this catalyst family in other organic transformations, as well as
mechanistic investigations, are ongoing.
1
3

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4
. Experimental
The synthetic work was performed in air and at room temperature. All the chemicals were obtained
-
1
from commercial sources and used as received. The infrared spectra (4000–400 cm ) 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. Thermal properties
-
1
were analyzed with a Perkin-Elmer Instrument system (STA6000) at a heating rate of 5˚C min under
a dinitrogen atmosphere. 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˚. Emission spectra in solid
state at room temperature were recorded on a Perkin Elmer Fluorescence Spectrometer (LS-55).
4
.1 Syntheses
[
Zn(1κO:2κO’:3κO’’-H L)(H O) ] (1)
3
2
2 n
3 2 2 5 5
A mixture of Zn(NO ) .6H O (29 mg, 0.10 mmol) and H L (32 mg, 0.10 mmol) [H L = 5-[2-{2,4,6-
trioxotetrahydropyrimidin-5(2H)-ylidene}hydrazinyl]isophthalic acid] was dissolved in 2 mL of DMF
and water (10 : 1). The pH of the solution was adjusted to 2.0 by the addition of HCl solution (1.0 M).
The resulting mixture was sealed in an 8 mL glass vessel and heated at 75˚C for 48 h. Subsequent
-
1
gradual cooling to room temperature (0.2 ˚C min ) afforded light yellow crystals of 1. Yield: 67%
based on Zn). Anal. Calcd. for C12 Zn (M = 419.61): C, 34.35; H, 2.40; N, 13.35; Found: C,
4.22; H, 2.45; N, 13.02. FT-IR (KBr, cm ): 3477 (bs), 3377 (bs), 3029 (bm), 1719 (s), 1675 (s), 1611
(
10 4 9
H N O
-
1
3
(
s), 1567 (s), 1525 (s), 1450 (s), 1401 (s), 1283 (m), 1261 (m), 1111 (w), 865(m), 799 (w), 779 (m), 522
m), 495 (m).
(
[
Cd(1κO,O’:2κO’:3κO’’,O’’’-H L)(H O) ] (2)
3 2 2 n
3 2 2 5 5
Cd(NO ) .4H O (31 mg, 0.10 mmol) and H L (32 mg, 0.10 mmol) [H L = 5-[2-{2,4,6-
trioxotetrahydropyrimidin-5(2H)-ylidene}hydrazinyl]isophthalic acid] were dissolved in 2 mL of DMF
and water (10 : 1). The pH of the solution was adjusted to 2.0 by the addition of HCl solution (1.0 M).
The resulting mixture was sealed in a capped glass vessel and heated to 75˚C for 48 h. Subsequent
-
1
gradual cooling to room temperature (0.2 ˚C min ) afforded yellow crystals of 2 in ca. 62% yield
based on Cd). Anal. Calcd. for C12 (M = 466.64): C, 30.89; H, 2.16; N, 12.01; Found: C,
0.41; H, 2.01; N, 12.05. FT-IR (KBr, cm ): 3438 (bs), 3367 (bs), 3049 (bs), 2955 (bs), 1738 (s), 1691
s), 1655 (s), 1633 (m), 1585 (m), 1528 (s), 1449 (m), 1386 (s), 1254 (s), 1139 (w), 1026 (w), 858 (s),
79 (m), 703 (s), 513 (s), 482 (s).
(
4 9
H10CdN O
-
1
3
(
7
[
Zn(κN-H
3
L)(H
2
O)
3
]∙3H
2
O (3)
A mixture of Zn(NO
3
)
2
2 5 5
.6H O (29 mg, 0.10 mmol) and H L (32 mg, 0.10 mmol) [H L = 5-[2-{2,4,6-
trioxotetrahydropyrimidin-5(2H)-ylidene}hydrazinyl]isophthalic acid] was dissolved in 2 mL of DMF
and water (10 : 1). The pH of the solution was adjusted to 10.0 by the addition of 30% ammonium
hydroxide solution. The resulting mixture was sealed in an 8 mL glass vessel and heated at 75 ˚C for
-
1
4
8 h. It was subsequently cooled to room temperature (0.2 ˚C min ), affording plate-like yellow
crystals of 3. Yield: 71% (based on Zn). Anal. Calcd. for C H N O Zn (M = 491.67): C, 29.31; H, 3.69;
1
2
18
4
13
1
4

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DOI: 10.1039/C5NJ02411K
-
1
N, 11.40; Found: C, 29.23; H, 3.52; N, 11.21. FT-IR (KBr, cm ): 3427 (bs), 3351 (bs), 3232 (bs), 3150
bs), 1681 (s), 1623 (m), 1561 (s), 1510 (s), 1459 (s), 1393 (s), 1360 (s), 1287 (m), 1259 (s), 1100 (w),
85 (m), 821 (m), 785 (s), 548 (w), 490 (m).
(
8
3 2 3 n 2
[Cd(1κO,O’:2κN-H L)(H O) ] ∙4nH O (4)
A mixture of Cd(NO ) .4H O (31 mg, 0.10 mmol) and H L (32 mg, 0.10 mmol) [H L = 5-[2-{2,4,6-
3
2
2
5
5
trioxotetrahydropyrimidin-5(2H)-ylidene}hydrazinyl]isophthalic acid] were dissolved in 2 mL of DMF
and water (10 : 1). The pH of the solution was adjusted to 10.0 by the addition of 30% ammonium
hydroxide solution. The resulting mixture was sealed in an 8 mL glass vessel and heated at 75 ˚C for
-
1
4
8 h. It was subsequently cooled to room temperature (0.2 ˚C min ), affording yellow crystals of 4.
Yield: 58% (based on Cd). Anal. Calcd. for C12 14 (M = 556.72): C, 25.89; H, 3.62; N, 10.06;
Found: C, 25.73; H, 3.41; N, 10.34. FT-IR (KBr, cm ): 3418 (bs), 3244 (bs), 1667 (s), 1614 (s), 1566 (s),
H
4
20CdN O
-
1
1
4
506 (s), 1457 (m), 1384 (s), 1286 (m), 1259 (s), 1160 (m), 1103 (w), 812 (s), 777 (s), 731 (w), 531 (w),
91 (m).
4
.2 Procedure for Knoevenagel condensation reaction catalyzed by the Zn(II) and Cd(II)
coordination compounds
A reaction mixture of benzaldehyde (51 μL, 0.50 mmol), malononitrile (66 mg, 1.0 mmol) and
catalyst (6.2 mg of 1, 7.0 mg of 2, 7.3 mg of 3 or 8.2 mg of 4, 3 mol%) was placed in a capped glass
vessel, and then 1 mL THF was added into it. The mixture was heated at 50 ˚C for 4 h, and
subsequently quenched by centrifugation and filtration at room temperature. The filtrate was
evaporated in vacuum to give the crude product [2-(Phenylmethylene) Malononitrile]. The residue
1
1
3
was dissolved in CDCl and analyzed by H NMR. The H-NMR spectra and the calculation of the yield
for compound 3 in the Knoevenagel reaction is presented in the supporting information (Figure S2).
+
The obtained m/z for 2-(phenylmethylene) malononitrile is 155.46 (M ).
In order to perform the catalyst recycling experiments, the used catalyst (separated by
centrifugation of the supernatant solution) was washed with THF and dried in air. It was then reused
for the Knoevenagel condensation reaction as described above.
4
.3 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 (1-4). 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
2
7
27a
parameters were retrieved using Bruker SMART software and refined using Bruker SAINT on all
2
7a
the observed reflections. Absorption corrections were applied using SADABS . Structures were
2
7b
27b
solved by direct methods by using the SHELXS-97 package
and refined with SHELXL-97 .
27c
Calculations were performed using the WinGX System–Version 1.80.03 . The hydrogen atoms
attached to carbon atoms and to the 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 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
1
5

New Journal of Chemistry
Page 16 of 20
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DOI: 10.1039/C5NJ02411K
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. 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 to achieve a reasonable
result. Least square refinements with anisotropic thermal motion parameters for all the non-
2
7e
hydrogen atoms and isotropic ones for the remaining atoms were employed. PLATON/SQUEEZE
3
was used to correct the data and in compound 4 the potential volume of 1690 Å was found with
42 electrons per unit cell worth of scattering. The electron count suggests the presence of ca. 3
water molecules per asymmetric unit. Crystallographic data are summarized in Table S1
Supplementary Information file) and selected bond distances and angles are presented in Table S2.
2
(
CCDC 1422595-1422598 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.
5
. Acknowledgements
This work has been supported by the Foundation for Science and Technology (FCT), Portugal (project
UID/QUI/00100/2013). Author A. Karmakar and Anup Paul express their gratitude to the FCT for
post-doctoral fellowships (Ref. No. SFRH/BPD/76192/2011 and SFRH/BPD/88450/2012).
6
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pH dependent synthesis of Zn(II) and Cd(II) coordination polymers
with dicarboxyl-functionalized arylhydrazone of barbituric acid:
photoluminescence properties and catalysts for Knoevenagel
condensation
a,
a
a,b
a,
Anirban Karmakar, * Anup Paul, Kamran T. Mahmudov, M. Fátima C. Guedes da Silva *
a,
and Armando J. L. Pombeiro *
a
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,
1
049–001, Lisbon, Portugal, E-mail: anirbanchem@gmail.com; fatima.guedes@tecnico.ulisboa.pt;
pombeiro@tecnico.ulisboa.pt
b
Department of Chemistry, Baku State University, Z. Xalilov Str. 23, Az 1148 Baku, Azerbaijan.
Coordination polymers of Zn(II) and Cd(II) act as recyclable heterogeneous catalysts for Knoevenagel
condensation reaction of aldehydes with malononitrile.