Two-dimensional rhombus grid coordination polymer showing heterogeneous catalytic activities

Article abstract of DOI:10.1016/j.molcata.2011.04.016

A coordination polymer, {[Cd(L)₂(DMF)₂] ·(ClO₄)₂·(2DMF)}_n (1) (L = bis-(4-imidazol-1-yl-phenyl)-diazene) has been synthesized at room temperature. The polymer forms a two-dimensional (2D) rhombus-grid sheet structure. These 2D sheets are stacked in ABAB... mode with an inter-planar spacing of 3.729 . Each Cd(II) ion is equatorially bonded to four imidazole N atoms belong to four different ligand units while O atoms from two DMF molecules occupy the axial sites. Two ClO₄^- counter anions as well as two DMF molecules occupy the void space in each rhombus grid. This compound catalyzes cyanosilylation reactions of aromatic aldehydes in dichloromethane under mild conditions in heterogeneous fashion. On heating 1 at 120 °C for 4 h, all DMF molecules are lost and the ClO₄^- anions move out of the void space to occupy the axial sites on the metal to yield, de-solvated {[Cd(L)₂(ClO₄)₂]}_n (1) without losing crystallinity. De-solvated 1 exhibits heterogeneous catalytic activity in several Knoevenagel condensation reactions at RT.

Full text of DOI:10.1016/j.molcata.2011.04.016

Journal of Molecular Catalysis A: Chemical 342–343 (2011) 6–10
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Editor’s choice paper
Two-dimensional rhombus grid coordination polymer showing heterogeneous
catalytic activities
∗
Manish K. Sharma, Prabal P. Singh, Parimal K. Bharadwaj
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A
coordination polymer, {[Cd(L) (DMF) ]·(ClO ) ·(2DMF)} (1) (L = bis-(4-imidazol-1-yl-phenyl)-
2
2
4
2
n
Received 14 March 2011
Received in revised form 22 April 2011
Accepted 23 April 2011
diazene) has been synthesized at room temperature. The polymer forms a two-dimensional (2D)
rhombus-grid sheet structure. These 2D sheets are stacked in ABAB. . . mode with an inter-planar spac-
ing of 3.729 A˚ . Each Cd(II) ion is equatorially bonded to four imidazole N atoms belong to four different
Available online 3 May 2011
−
ligand units while O atoms from two DMF molecules occupy the axial sites. Two ClO4 counter anions
as well as two DMF molecules occupy the void space in each rhombus grid. This compound catalyzes
cyanosilylation reactions of aromatic aldehydes in dichloromethane under mild conditions in heteroge-
neous fashion. On heating 1 at 120 C for 4 h, all DMF molecules are lost and the ClO4 anions move out
of the void space to occupy the axial sites on the metal to yield, de-solvated {[Cd(L)2(ClO4)2]}n (1) with-
out losing crystallinity. De-solvated 1 exhibits heterogeneous catalytic activity in several Knoevenagel
condensation reactions at RT.
Keywords:
Coordination polymers
Heterogeneous catalysis
Cyanosilylation reaction
Knoevenagel condensation
◦
−
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
by heating. However, a robust structure can withstand the heat
treatment with minimum damage to afford a porous material that
may act as a catalyst.
Porous coordination polymers can be synthesized from metal
ions and organic multi-dentate ligands under mild conditions.
These materials have certain desirable properties such as large
surface area, different pore volumes and their distribution, size
and shape of pores, etc., that can be controlled via ligand design
as well as choice of metal ion(s) [1–5]. A few coordination poly-
mers have been used as heterogeneous catalysts [6–9]. For use
in heterogeneous catalysis, the organic groups can be designed
to have guest-accessible functional sites [10] where reactants can
bind via various non-covalent interactions leading to their activa-
tion. Alternatively, labile solvent molecules bound to metal ions,
can be removed by heating to make the metal ions as Lewis acid
sites [11]. Heterogeneous catalysis is playing an increasingly impor-
tant role [12,13] in small-scale chemical manufacturing, often with
the result of a major reduction in waste. While enhancement in
catalytic performance, be it in terms of activity and/or selectiv-
ity, is clearly desirable, the principal motive for heterogenization
is to facilitate separation, recovery and re-use of the catalyst. It is
often found that catalytic activity of metal complexes immobilized
on a solid support such as oxide or zeolite, deteriorate [14,15] on
exposure to moisture or heat leading to loss of catalytic activity. A
number of coordination polymers also suffer from degradation of
the porous structure upon removal of the lattice solvent molecules
Cyanosilylation reactions provide a convenient route to
cyanohydrins. Cyanohydrins are versatile intermediates in
organic synthesis [16]. While cyanosilylation reactions have
been extensively studied in homogeneous media, only few stud-
ies have employed heterogeneous catalysts [17,18]. Recently,
Long et al. have used the coordination polymer, Mn [(Mn Cl)
3
4
3
(btt) (CH OH)10]2 as a catalyst for cyanosilylation reactions. It
8
3
is proposed that the Mn(II) ions exposed on the surface of the
framework act as Lewis acid sites [11]. Besides, Cu(II)- [19], Cd(II)-
[20] and Sm(III)-based [21] MOFs have also been used as active
catalysts for cyanosilylation of aldehydes.
In an earlier report, we have discussed [22] room
temperature
synthesis
of
the
coordination
polymer,
{[CdL (DMF) ]·(ClO ) ·2DMF}n (1) by reacting Cd(ClO4)2 with
2
2
4 2
the ligand, bis-(4-imidazol-1-yl-phenyl)-diazene (L). Compound
1 catalyzes a number of cyanosilylation reactions while the
fully de-solvated 1 obtained upon heating of 1 catalyzes several
Knoevenagel condensation reactions at RT (Scheme 1).
2
. Experimental
2
.1. Materials
The metal salts were obtained from Aldrich and used as received.
All other chemicals were procured from Aldrich and S. D. Fine
Chemicals, India. All solvents were purified prior to use.
∗ Corresponding author. Tel.: +91 512 2597034/2597340.
E-mail address: pkb@iitk.ac.in (P.K. Bharadwaj).
1
381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.04.016

M.K. Sharma et al. / Journal of Molecular Catalysis A: Chemical 342–343 (2011) 6–10
7
A
B
O
O
1 (5 wt% )
HO
CN
H
+
(Me) SiCN
3
DCM, 0 °C
R1
R1
H
14 h, 2M HCl
Desolvated 1(5 wt %)
R2
R2
+
R1
R1
H
CN
C6H6
CN
Scheme 1. Cyanosilylation (A) and Knoevenagel (B) reactions of aromatic aldehy-
des.
2.2. Preparation of the ligand
Fig. 1. A perspective view of metal coordination environment in 1.
The ligand bis-(4-imidazol-1-yl-phenyl)-diazene was prepared
in two steps as described earlier [22].
◦
(
5 C/min heating rate under a nitrogen atmosphere) Mettler
1
Toledo Star System; H NMR (500 MHz, CDCl3 and DMSO d , TMS)
6
JEOL JNM-LA500 FT instrument. Microanalyses data for the com-
pounds were obtained from CDRI, Lucknow.
2
.3. Preparation of {[Cd(L) (DMF) ]·(ClO ) ·(2DMF)}n (1)
2
2
4 2
A hot DMF solution (3 ml) of ligand L (50 mg, 0.16 mmol) was
added to MeOH/H O mixed solution (1:1, 2 ml) of Cd(ClO ) ·xH O
2.8. X-ray structural studies
2
4
2
2
(
49 mg, 0.16 mmol). On slow evaporation of the filtrate at RT, orange
crystals were obtained in ∼80% yield. Calc. for C48H56CdCl N16O12
:
Single-crystal X-ray data were collected at 100 K on a Bruker
SMART APEX CCD diffractometer using graphite-monochromatized
MoK␣ radiation (ꢀ, 0.71069 A˚ ) on 1 and de-solvated 1 as described
2
C, 46.78; H, 4.58; N, 18.18%. Found: C, 46.82; H, 4.62; N, 18.12%.
earlier [22]. Crystals of de-solvated 1 were not of good quality as
several cracks developed on heating. Nevertheless, a data could be
collected and an approximate structure could be obtained.
2
.4. Preparation of de-solvated 1 {[Cd(L) (ClO ) ]}n
2
4 2
◦
◦
Crystals of 1 was heated at 120 C (heating rate of 5 C/min.)
for 4 h under vaccum to obtain de-solvated 1. Calc. for
C36H28CdCl N O : C, 46.78; H, 4.58; N, 18.18%. Found: C, 46.82;
3. Results and discussion
2
12
8
H, 4.62; N, 18.12%.
Caution! Perchlorate salts can form explosive mixture when
added to organic compounds and hence should be handled with
extreme caution and in small (<100 mg) quantities.
3.1. Structural description
Compound {[CdL (DMF) ]·(ClO ) ·(2DMF)} (1) once isolated,
n
2
2
4
2
is found to be air-stable and slightly soluble in DMSO but insoluble
in other common organic solvents or water.
2.5. General procedure for cyanosilylation reactions
The X-ray crystallographic analysis of 1 reveals that each Cd(II)
ion is octahedrally coordinated where equatorial coordination
comes from imidazole N from four different ligand units. Two O
from two DMF molecules occupy the axial positions. All Cd–N and
Cd–O bond distances are found to be normal compared to literature
values [23]. The overall structure looks like a 2D layer consisting
A solution of an aromatic aldehyde (1.32 mmol) and trimethylsi-
lyl cyanide (2.64 mmol) in DCM (10 mL) is stirred for 5 min and then
◦
catalyst 1 (5 wt%) is added at 0 C. The suspension is stirred under
ice-cold condition for 45 min and progress of the reaction is moni-
tored by TLC. Once the reaction is complete, the catalyst is removed
by filtration, washed with DCM and recovered for reuse. After evap-
oration of solvent from the filtrate in a rotary evaporator, the crude
solid obtained is recrystallized with EtOAc/hexane mixture.
−
of rhombus grids with two DMF and two ClO4 ions occupy the
void spaces (Fig. 1). These 2D layers are stacked in ABAB. . . fashion
(Fig. 2) with an inter-planar spacing of ∼3.729 A˚ . A Cd(II) ion in one
grid is situated approximately in the middle of the void of the grid
2
.6. General procedure for Knoevenagel condensation
below/above with the Cd· · ·Cd separation of ∼9.233 A˚ .
An intricate array of C–H· · ·O and C–H· · ·␲ interactions involving
−
The catalytic reactions are carried out in benzene. In a typical
DMF, phenyl and/or imidazole rings and ClO4 anions stabilize the
reaction, a solution (10 ml) containing an aromatic aldehyde and
malononitrile (1:1 molar ratio) is stirred for 5 min and then 5 wt%
of de-solvated 1 is added. The suspension is allowed to stir at room
temperature under nitrogen atmosphere. The progress of reaction
is monitored by TLC. On completion of the reaction, the catalyst
is removed by filtration, washed with MeOH followed by benzene
and air-dried. The filtrate is evaporated to dryness under reduced
pressure and the residue is recrystallized from EtOAc:hexane (1:10)
mixture to get the pure products.
overall 3D structure of 1. It has been shown earlier [22] that careful
heating of crystals of 1 affords de-solvated 1 in single-crystal to
single-crystal fashion. The structure of de-solvated 1 shows that all
DMF molecules are lost and the ClO4 anions have moved to occupy
the axial positions on the Cd(II) ion creating a large void space in
the structure (Fig. 3).
−
In the IR spectrum [24] of 1, C O stretching vibration of DMF
−
1
appears at 1670 cm while N N stretching vibrations appear in
−
1
−
gives a broad
4
the range, 1602–1515 cm
and the ionic ClO
−
1
band centering around 1120 cm . In de-solvated 1, the peak at
1670 cm disappears suggesting absence of DMF molecules. The
splits into two due to coordination of
ClO4 anions to Cd(II) centers [25].
−
1
2.7. Physical measurements
−
1
broad band at 1120 cm
−
Spectroscopic data were collected as follows: IR spectra (KBr
−1
disk, 400–4000 cm ) Perkin Elmer Model 1320 spectrometer; X-
Thermal analysis of 1 shows ∼23.4% of weight loss in the tem-
◦
◦
ray powder patterns (Cu K␣ radiation, 3 /min scan rate, 293 K)
perature range, 70–150 C corresponding to loss of both lattice and
Philips PW100 diffractometer; thermogravimetric analysis (TGA)
coordinated DMF molecules (calc. 23.7%). The de-solvated frame-

8
M.K. Sharma et al. / Journal of Molecular Catalysis A: Chemical 342–343 (2011) 6–10
Fig. 4. Time conversion plot for the cyanosilylation reactions of different aromatic
aldehydes catalyzed by 1.
Fig. 2. A view of 1 (top) showing ABAB. . ..stacking mode of grid layers (CPK model).
2
D rhombus grid networks of 1 (bottom) with cell edges (guest molecules are
in DCM for 10 h at RT, the solid is collected by filtration, dried in
air and is subjected to IR study. The IR spectrum shows absence of
removed for clarity).
C
O stretching vibrations corresponding to DMF and C O stretch-
Table 1
−1
ing vibration of benzaldehyde undergoes a red-shift by ∼12 cm
Cyanosilylation reactions catalyzed by 1.
suggesting that the benzaldehyde interacts with the Cd(II) ion
Entry
R1
Time (h)
Yield (%)
−1
(Figs. S7 and 8). Also, the broad band around 1100–1080 cm sug-
1
2
3
4
5
Ph
14
14
14
14
14
92
80
72
40
6
gests the presence of ionic perchlorate in the lattice.
4-NO2Ph
4-BrPh
4-MePh
Naphthyl
When one uses a solid catalyst, a crucial issue is the possibility
that some of active sites can migrate to the liquid phase and that
these leached species become responsible for the catalytic activity
to a significant extent. In order to determine if leaching is a problem
for the cyanosilylation reaction, the whole suspension is filtered
after 1 h (∼16% conversion) to remove the catalyst 1. The filtrate
does not afford further conversion (Fig. 5, dotted line). This proce-
dure is repeated with another batch and the suspension is filtered
after 5 h (∼60% conversion) and obtained the same result with the
filtrate (Fig. 5, dashed line). Thus, there is no active species leached
into the liquid phase and the conversion is only possible with the
solid catalyst (1).
◦
work is stable up to ∼300 C [24]. Thermal analysis of de-solvated 1
◦
indicates that the framework starts to decompose only after 350 C.
Variable temperature PXRD data show that upon heating, the crys-
talline nature of framework is maintained even at high temperature
Fig. S13).
(
3
.2. Cyanosilylation reactions
Compound 1 is assessed for its activity in the cyanosilyla-
3.3. Knoevenagel condensation
tion reactions of different aromatic aldehydes with trimethylsilyl
cyanide to form cyanohydrins as the principal product (Table 1)
after work-up. The reactions are carried out using 5 wt% of 1 in
DCM. With a substrate like benzaldehyde, 4-nitrobenzaldehyde,
Knoevenagel condensation reactions are well known organic
reactions largely employed in order to form C–C bonds for the
synthesis of important derivatives for various applications [26,27].
Base catalyzed Knoevenagel condensation reactions have been
extensively studied in homogeneous systems [28] but very few
studies have employed heterogeneous catalyst [10,29–32]. To date,
heterogeneous catalytic studies have shown either low activity or
have been prepared in a complex way. Since all DMF molecules
4
-bromobenzaldehyde or p-tolualdehyde, the yield is moderate
(
Fig. 4). When 1-napthaldehyde is used as the substrate, the yield is
very poor suggesting that the aldehyde here is not accommodated
as per the transition-state geometry requirement for driving the
reaction to the product. Compound 1 is stirred with benzaldehyde
Fig. 3. A representation of the formation of de-solvated 1 from 1.

M.K. Sharma et al. / Journal of Molecular Catalysis A: Chemical 342–343 (2011) 6–10
9
Fig. 5. Evidence of heterogeneous nature of catalysis in cyanosilylation reaction
of benzaldehyde. (ꢀ) Continuous reaction; (᭹) catalyst was removed after 1 h; (ꢁ)
catalyst was removed after 5 h.
Fig. 6. X-ray powder diffraction (PXRD) patterns of (a) simulated pattern from crys-
tal data of 1 (black) (b) de-solvated 1 and (c) de-solvated 1 after 5 catalytic runs.
−
are lost upon heating with concomitant movement of the ClO4
Heterogeneous nature of the catalyst is probed for the
Knoevenagel condensation reaction between benzaldehyde and
malononitrile as the rate is moderate and the progress of the reac-
tion can be easily monitored by GC–MS and 1H NMR. The result is
shown graphically in Fig. 7. Here, the solid line shows the progress
of the reaction, which is completed in about 30 min. In a control
experiment, the whole suspension is filtered after 10 min (62%
conversion) to remove de-solvated 1. The filtrate does not afford
further conversion (Fig. 7, dotted line). We repeated the above pro-
cedure with another batch and filtered the suspension after 5 min
(32% conversion) and obtained the same result with the filtrate
(Fig. 7, dashed line). These results clearly show that the reaction
is heterogeneous by nature and catalytically active species are not
eluted at all under the reaction conditions.
anions, which now occupy the axial positions on the Cd(II) cen-
ter, the MOF becomes more accessible by reagents. As a first batch
of catalytic reactions, several Knoevenagel condensation reactions
have been carried out with de-solvated 1 (Table 2). We have synthe-
sized compounds derived from benzylidene malononitrile (BMN)
as they have been found to be inhibitors for the protein tyrosine
kinase [33,34].
In each case, 5% by weight of de-solvated 1 is used as the catalyst.
The reactions proceed at various speeds depending upon the struc-
ture of the substrate as well as nature of the solvent (Table 2). Once
a reaction is over as monitored by TLC, the reaction mixture is fil-
tered to recover the catalyst and the filtrate is evaporated to dryness
under reduced pressure and finally the resulting residue recrys-
tallized with ethylacetate:hexane mixture (1:10) to get the pure
product. The recovered catalyst obtained is washed with methanol
followed by benzene, air-dried and re-used without observing any
loss of catalytic activity. We have carried out each reaction at least
five times to probe the reusability of the catalyst. The X-ray powder
pattern of the dried catalyst, recovered from the reaction mixture
after it is used for five times, is found to be almost same with respect
to the pattern (Fig. 6) obtained with de-solvated 1 taken before the
first run.
To probe possible mechanism for substrate activation, de-
solvated 1 is stirred with malononitrile in benzene for 8 h at room
temperature and the solid collected by filtration, washed with ben-
zene and air dried. The IR spectrum of the solid shows the C
N
−
1
−1
stretching frequency at 2237 cm that is red-shifted by ∼35 cm
−
1
and also a doublet at 2110 cm , which may be attributed to
the hydrogen bonding interaction between methylinic H atom of
−
malononitrile and O atom of metal bound ClO4 anion. Similar
observations are reported in the literature [10]. The Cl–O stretching
We have carried out the catalytic reactions in different solvents
such as MeOH, MeCN, benzene and MeOH:H O (1:1). Each reaction
2
is faster in benzene compared to other solvents. Also, it is found that
Cd(ClO ) ·xH O dissolved in MeOH/MeCN or suspended in benzene
4
2
2
can only poorly promote some Knoevenagel condensation reac-
tions (yield ∼20%) in 24 h. Under the same condition, the ligand
as well as CdO does not promote the reaction at all, even after 24 h.
De-solvated 1 is stirred separately in MeOH/MeCN/benzene for 2
days and the filtrate collected. In none of the cases, the filtrate can
catalyze any of the above reactions suggesting no leaching of Cd(II)
from de-solvated 1.
Table 2
Knoevenagel condensation reactions catalyzed by de-solvated 1.
Entry
R1
R2
Time (min)
Yield (%)
1
2
3
4
5
6
7
C6H5
CN
CN
CN
CN
CN
CN
CN
30
20
60
60
120
60
120
93
92
78
86
62
70
6
4-NO2–C6H4
4-CH3–C6H4
4-Br–C6H4
4-OCH3–C6H4
4-F–C6H4
Fig. 7. Evidence of heterogeneous nature of catalysis in the Knoevenagel condensa-
tion of benzaldehyde with malononitrile. (ꢀ) Continuous reaction; (᭹) catalyst was
removed after 5 min; (ꢁ) catalyst was removed after 10 min.
1-Naphthyl

1
0
M.K. Sharma et al. / Journal of Molecular Catalysis A: Chemical 342–343 (2011) 6–10
−1
−
of ClO4 anions shows a sharp doublet at ∼1100 cm . No change
in N N stretching frequency is observed. It is, therefore, proposed
that malononitrile interacts with the O atom(s) of metal bound
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. Conclusion
[
[
[
In conclusion, we have shown that the ligand, bis-(4-imidazol-
-yl-phenyl)-diazene (L) having two imidazole units at two ends,
1
[
forms a non-interpenetrated rhombus grid network with two DMF
−
molecules and two ClO4 anions occupying the void space. Two of
[
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without losing activity.
[
[
[
[
[
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