Cadmium Naphthalenedisulfonate Complex as Heterogeneous Catalyst in the Knoevenagel Condensation

Article abstract of DOI:10.1134/S1070363219010213

The reaction of naphthalene-1,5-disulfonate (1,5-nds), 2,4,5-tris(pyridin-4-yl)imidazole (tpim), and cadmium(II) acetate dihydrate Cd(OAc)₂·2H₂O afforded a 2D coordination polymer, [Cd(tpim)(1,5-nds)]_n (1). Crystal structu

Full text of DOI:10.1134/S1070363219010213

ISSN 1070-3632, Russian Journal of General Chemistry, 2019, Vol. 89, No. 1, pp. 117–121. © Pleiades Publishing, Ltd., 2019.
Cadmium Naphthalenedisulfonate Complex as Heterogeneous
Catalyst in the Knoevenagel Condensation
a
a
a
D.-S. Deng , H. Guo *, X. He, and B. Ji *
a
Henan Key Laboratory of Function-Oriented Porous Materials, College of Chemistry and Chemical Engineering,
Luoyang Normal University, Luoyang, 471934 China
*
e-mail: zsudds168@126.com
Received April 19, 2018; revised January 22, 2019; accepted January 22, 2019
Abstract—The reaction of naphthalene-1,5-disulfonate (1,5-nds), 2,4,5-tris(pyridin-4-yl)imidazole (tpim), and
cadmium(II) acetate dihydrate Cd(OAc) ·2H O afforded a 2D coordination polymer, [Cd(tpim)(1,5-nds)] (1).
2 2 n
Crystal structure analysis of the obtained complex showed the presence of its molecule of uncoordinated
pyridyl groups as active organic sites. The complex was found to act as efficient and reusable heterogeneous
catalyst in the Knoevenagel condensation. Its stability under the reaction conditions was confirmed by
comparison of the powder X-ray diffraction patterns of the fresh and reused catalysts.
Keywords: organosulfonate, heterogeneous catalysis, Knoevenagel reaction, basic catalyst
DOI: 10.1134/S1070363219010213
Over the past decade, metal–organic frameworks
MOFs) have gained much attention due to their
synthesis [13]. In this sense, the solvent-free strategy is
simple with fascinating versatility. It reduces the use of
organic solvents and minimizes negative environmental
impact. The reactions occur under mild conditions and
usually require easier workup procedures and simpler
equipment. These merits promoted us to evaluate the
possibility of performing some organic transformations
under solvent-free conditions, so as to develop
environmentally benign reactions. As is well known,
the Knoevenagel reaction is not only regarded as a
weak base-catalyzed model reaction [14–18], but also
is an extremely important and reliable method for the
formation of C–C bonds. Our groups have recently
demonstrated that the complex [Cd(tpim)(1,5-nds)]n
(1) can be used as a promising heterogeneous catalyst
for epoxide ring-opening reaction of epoxides and
amines [19]. In continuation of our ongoing studies on
the development of new catalytic systems, herein we
report an efficient protocol applicable to the Knoevenagel
condensation under solvent-free conditions with complex
1 as a heterogeneous catalyst.
(
potential applications in molecular recognition, gas
storage, luminescence, magnetism, heterogeneous
catalysis, and so on [1–4]. Metal–organic frameworks
can act as heterogeneous catalysts through either
immobilization of coordinatively unsaturated (open)
metal sites (OMS) [5] or introduction of organic
groups to provide functional organic sites (FOS) [6]. In
the former case, some MOFs can be used as Lewis
acid catalysts due to the presence of OMS which show
Lewis acidity; such MOFs have already been
successfully applied to catalyze organic reactions, e.g.,
cyanosilylation of aldehydes [7], Mukaiyama aldol
reaction [8], ring-opening reaction of styrene [9], etc.
In the latter case, a variety of FOS possessing Lewis
basicity (e.g., pyridine, amide, or amine fragments) can
serve as active basic sites to promote Knoevenagel
condensation [10], acetylation of phenols [11], and aza-
Michael reaction [12]. However, there is great interest
in the use of OMS for Lewis acid catalysis, but less
attention has been devoted to the study of FOS, despite
their importance. Thus, the design and synthesis of
functionalized MOFs with base-type FOS
demonstrating base catalytic properties in
heterogeneous reactions remain a major challenge.
The crystal structure analysis showed that complex
1
possesses guest-accessible functional pyridyl groups
capable of acting as active base sites (Fig. 1). To
evaluate possible base-type catalytic behavior of
complex 1, a Knoevenagel reaction catalyzed by 1 was
performed. We chose the reaction of benzaldehyde
with malononitrile as a model reaction for optimization
of the reaction conditions. In this sense, we checked
On the other hand, the central objective in chemical
industry has recently been to develop greener and more
economically competitive processes for multifaceted
1
17

1
18
DENG et al.
condensation. In all cases, the reaction mixture was clean
as only the starting materials and the products were ob-
served by thin layer chromatography during the reaction.
Having determined the optimal reaction conditions
established, we next examined the substrate scope of
this transformation. As shown in Table 3, all reactions
of aromatic aldehydes with malononitrile proceeded
smoothly in the presence of complex 1, affording the
desired products in good to excellent yields. It is seen
that the nature of the substituent on the aromatic ring
has no dramatic effect on the reaction yield; the lowest
yield was obtained for ortho-substituted substrate like
o-chlorobenzaldehyde (2h). All the products were
isolated pure just by washing with ethanol and
subsequent recrystallization from aqueous ethanol; no
tedious chromatographic purification was needed in
most cases. The isolated products were fully charac-
terized on the basis of analytical data and detailed
Active organic
site
Cd
S
O
N
C
Fig. 1. Active organic base sites in the crystal structure of
complex 1.
the effect of changing solvents and the catalyst loading.
Under solvent-free conditions, the reaction proceeded
smoothly in the presence of 4 mol % of 1 at room
temperature for 24 h, resulting in a 99% conversion of
benzaldehyde. Excellent yield was only obtained under
solvent-free conditions, whereas lower yields were
observed in common organic solvents such as aceto-
nitrile, dichloromethane, benzene, tetrahydrofuran, and
methanol (Table 1). Furthermore, a lower catalyst
loading (<4%) can be used, but prolonged reaction
time was required (Table 2). Thus, the use of 4 mol %
of complex 1 under solvent-free conditions at room tem-
perature is the optimal conditions for the Knoevenagel
1
spectral studies including H NMR spectroscopy. All
the known compounds had physical and spectroscopic
data identical to the literature values.
After each Knoevenagel condensation reaction
finished, the catalyst was isolated from the reaction
mixture, dried at 100°C for 8 h, and then reused
directly without further purification for the second run.
The initial activity of the catalyst was also obtained in
the second run, showing that the framework of the
catalyst may remain intact during the catalytic cycles.
The catalyst remained effective for up to 4 cycles for
the Knoevenagel reaction (Fig. 2). The PXRD patterns
Table 1. Effect of various organic solvents on the Knoevenagel
condensation of benzaldehyde (2a) with malononitrile
Table 2. Effect of catalyst amount on the Knoevenagel of
benzaldehyde (2a) with malononitrile
a
a
1
(4 mol %)
solvent
O
O
CN
CN
CN
CN
1, no solvent
Ph
+
Ph
+
Ph
H
Ph
H
CN
CN
3a
CN
CN
3a
2a
2a
b
Solvent
Yield, %
b
Amount of catalyst, mol %
Yield, %
Acetonitrile
Dichloromethane
Benzene
92
93
70
85
93
99
1
2
3
4
5
60
75
92
99
99
Tetrahydrofuran
Methanol
No solvent
a
a
Reaction conditions: benzaldehyde, 0.1 mmol; malononitrile,
.3 mmol; catalyst, 4 mol %; room temperature, reaction time 24 h.
According to the GLC data.
Reaction conditions: benzaldehyde, 0.1 mmol; malononitrile,
0.3 mmol; solvent-free; room temperature, reaction time 24 h.
According to the GLC data.
0
b
b
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 1 2019

CADMIUM NAPHTHALENEDISULFONATE COMPLEX AS HETEROGENEOUS CATALYST
119
Reaction time, h
2
θ, deg
Fig. 3. PXRD patterns of complex 1: (1) the simulated XRD
pattern, (2) the experimental one, (3) after the first run, (4)
after the second run, (5) after the third run, and (6) after the
fourth run.
Fig. 2. Recyclability test of complex 1 in the Knoevenagel
condensation: (1) first run, (2) second run, (3) third run, and
(
4) fourth run.
of catalysts 1 measured after the reactions show the
same profiles as the as-prepared samples, indicating
unchanged coordination frameworks although the
crystallinity seems to be weakened due to broadening
of the diffraction peaks, as shown in Fig. 3. Further
proof was obtained with a separate experiment. After
removal of the catalyst by filtration, no further reac-
tivity was observed in the filtrate, indicating that the
nature of reaction process was heterogeneous and there
was no reaction in the homogeneous phase.
2,4,5-tri(pyridin-4yl)imidazole (tpim) (0.15 g, 0.5 mmol),
Cd(OAc) ·2H O (0.118 g, 0.5 mmol), and H O
2
2
2
(10 mL) was sealed in a 23-mL Teflon-lined stainless
steel container and heated at 180°C for 3 days. The
mixture was then cooled to room temperature at a rate
Table 3. Knoevenagel condensation of various aromatic
a
aldehydes and malononitrile
1 (4 mol %)
no solvent
O
CN
CN
Ar
+
Ar
2a–2j
Initial aldehyde no.
H
In summary, we have successfully shown the utility
of complex 1 as effective catalyst for the Knoevenagel
condensation under solvent-free conditions. The solid
catalyst can be easily reused maintaining the particle
integrity with some gradual decay in activity. The
availability of MOFs and simple product isolation
procedure make this system quite attractive for both
academic and industrial applications.
CN
CN
3a–3j
Product no. Yield, %
b
Ar
2
2
a
Ph
4-MeC
4-O NC
4-ClC
3a
3b
3c
3d
3e
3f
99
99
99
99
99
99
98
96
97
99
b
6
H
4
2
c
2
6
H
4
2
d
6 4
H
EXPERIMENTAL
2
e
6
4-BrC H
4
All reagents were commercially available and used
as received. Powder X-ray diffraction (PXRD) data
were recorded on a Bruker D8-ADVANCE X-ray
2
f
6
4-NCC H
4
2
g
4-MeOC
6
H
4
3g
3h
3i
diffractometer with CuK radiation (λ = 1.5418 Å). The
2h
2-ClC H4
α
6
1
H NMR spectra were recorded at room temperature on a
2
i
6 4
4-PhC H
Varian UNITY/NOVA 400 NMR spectrometer using
CDCl as solvent; the chemical shifts are given relative
2j
3-O NC
2
6
H
4
3j
3
a
to tetramethylsilane.
Reaction conditions: aldehyde 2a–2j, 0.1 mmol; malononitrile,
.3 mmol; solvent-free; catalyst 1, 4 mol %; room temperature,
0
Synthesis of complex 1 [19]. A mixture of di-
sodium naphthalene-1,5-disulfonate (0.165 g, 0.5 mmol),
reaction time 24 h.
According to the GLC data.
b
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 1 2019

1
20
DENG et al.
of 2 deg/h, and the colorless block crystals of 1 were
ppm: 7.78 d (2H, J = 8.0 Hz), 7.71 d (2H, J = 8.0 Hz),
filtered, washed with DMF, and dried in air. Yield 348 mg
7.68 s (1H).
(
70% based on Cd).
2
-(4-Cyanobenzylidene)malononitrile (3f). Yield
1
Typical procedure for the Knoevenagel condensa-
177 mg (99%), white solid. H NMR spectrum, δ,
ppm: 7.97 d (2H, J = 8.0 Hz), 7.81 d (2H, J = 8.0 Hz),
7.78 s (1H).
tion. A 10 mL round-bottomed flask was charged with
aldehyde 2a–2j (0.1 mmol), malononitrile (0.3 mmol),
and catalyst 1 (0.004 mmol). The reaction mixture was
stirred for a required time at room temperature under
solvent-free conditions. The progress of the reaction
was monitored by the thin layer chromatography. After
the reaction finished, 10 mL of ethanol was added, the
mixture was stirred for 5 min, the catalyst was filtered
off, and the filtrate was concentrated under reduced
pressure. The crude product was purified by recrys-
tallization from aqueous ethanol. The products were
2
-(4-Methoxybenzylidene)malononitrile (3g).
1
Yield 180 mg (98%), white solid. H NMR spectrum,
δ, ppm: 7.85 d (2H, J = 8.0 Hz), 7.58 s (1H), 6.95 d
2H, J = 8.0 Hz), 3.85 s (3H).
(
2-(2-Chlorobenzylidene)malononitrile (3h). Yield
1
180 mg (96%), white solid. H NMR spectrum, δ,
ppm: 8.27 s (1H), 8.19 d (1H, J = 8.0 Hz), 7.56 d (2H,
J = 4.0 Hz), 7.47–7.43 m (1H).
1
fully characterized by H NMR or otherwise compared
2
-[Biphenyl-4-yl)methylidene]malononitrile (3i).
with authentic samples. The recovered catalyst was
washed with ethanol, dried, and reused without further
purification. The recovered catalysts were characterized
by X-ray powder diffraction which showed identical
results to those of a fresh sample.
1
Yield 223 mg (97%). white solid. H NMR spectrum,
δ, ppm: 8.00 d (2H, J = 8.0 Hz), 7.78 t (3H, J =
.0 Hz), 7.65 d (2H, J = 8.0 Hz), 7.51 m (2H), 7.44 s
1H).
4
(
2
-(3-Nitrobenzylidene)malononitrile (3j). Yield
Experimental procedure for reusability test. The
reusability of complex 1 was tested for the
Knoevenagel condensation reaction. When the reaction
finished, the mixture was centrifuged, the liquid phase
was decanted, and the catalyst was dried at 100°C for
1
1
97 mg (99%), yellow solid. H NMR spectrum, δ,
ppm: 8.65 s (1H), 8.48 d (1H, J = 8.0 Hz), 8.33 d (1H,
J = 8.0 Hz), 7.87 s (1H), 7.80 d (1H, J = 8.0 Hz).
FUNDING
8
h, then reused directly without further purification
for the second run with fresh aldehyde and
malononitrile. It was used for four consecutive runs
with minor loss in activity.
We gratefully acknowledge the financial support
from the National Natural Science Foundation of
China (grant nos. 21 272 109, 21 372 112) and from
the Foundation of the Education Department of Henan
Province (no. 2011 B 150 021).
2
-Benzylidenemalononitrile (3a). Yield 153 mg
1
(
(
99%), white solid. H NMR spectrum, δ, ppm: 7.85 d
2H, J = 4.0 Hz), 7.72 s (1H), 7.58 t (1H, J = 12.0 Hz),
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