Heterogeneous catalysis over a barium carboxylate framework compound: Synthesis, X-ray crystal structure and aldol condensation reaction

Article abstract of DOI:10.1016/j.poly.2012.05.043

A new three-dimensional alkaline-earth metal-organic framework (MOF) compound, [Ba(pdc)]_n (1) (H₂pdc = pyridine-2,5- dicarboxylate), has been hydrothermally synthesized and structurally characterized by single crystal X-ray diffracti

Full text of DOI:10.1016/j.poly.2012.05.043

Polyhedron 43 (2012) 63–70
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Heterogeneous catalysis over a barium carboxylate framework compound:
Synthesis, X-ray crystal structure and aldol condensation reaction
1
⇑
Debraj Saha, Tanmoy Maity, Rupam Sen , Subratanath Koner
Department of Chemistry, Jadavpur University, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new three-dimensional alkaline-earth metal–organic framework (MOF) compound, [Ba(pdc)]n (1)
pdc = pyridine-2,5-dicarboxylate), has been hydrothermally synthesized and structurally character-
ized by single crystal X-ray diffraction analysis. X-ray single-crystal analysis reveals that each metal
Received 28 March 2012
Accepted 30 May 2012
Available online 17 June 2012
(H
2
ion in compound 1 is coordinated by seven O atoms from seven different carboxylate groups. While each
2ꢀ
pyridine-2,5-dicarboxylato (pdc ) ligand coordinates to seven alkaline earth metal centers through two
Keywords:
2
2
2
1
carboxylate groups adopting
4 3
l -g :g -bridging and l -g :g -bridging coordination modes to afford a
Hydrothermal synthesis
Alkaline earth metal
Metal–organic framework
Heterogeneous catalysis
Aldol condensation
3D network. Thermogravimetric analysis reveals that compound 1 is thermally stable up to around
560 °C. Compound 1 efficiently catalyzes aldol condensation reactions of various aromatic aldehydes
with acetone and cyclohexanone under environment friendly heterogeneous condition, giving remark-
able yields (up to 96%) with high selectivity (100%) within a short reaction time (6 h). The catalyst was
recycled for five successive runs without significant loss of activity. There was no evidence of leaching
of metal ions or no loss of structural integrity of the catalyst during the catalytic reaction.
Ó 2012 Elsevier Ltd. All rights reserved.
1
. Introduction
metal and lanthanide ions [13–15]. In comparison to other pyridine
2
dicarboxylic acids, H pdc seems to easily coordinate to metal ions
Hybrid inorganic–organic compounds are of current interest, not
to form an infinite structure, because the two carboxylate groups
on the opposite sides of the aromatic moiety may give rise to more
stability in MOF structures.
only because of their fascinating variety of architectures and topol-
ogies [1], but also owing to their potential applications, particularly
in catalysis [2,3], molecule-based magnetic materials [4–6], artifi-
cial nucleases for the hydrolytic cleavage of DNA and RNA [7], con-
trast agents for magnetic resonance imaging [8] and fixation media
for atmospheric gases [9]. In such materials, functionality can be
introduced from either the inorganic species or the organic linkers.
Multidimensional M–O–M frameworks have demonstrated cooper-
ative effects which can also lead to improved thermal stabilities
There have been quite a few studies [16] on the coordination
behavior of alkaline earth metals in both aqueous and non-aqueous
media. Barium based MOFs containing the organic linker 2,5-pyrid-
inedicarboxlic acid have been reported [17,18]. Fromm has recently
reviewed coordination polymer networks with s-block metal ions
[16]. Two reasons prevent the use of s-blocks in the metal–organic
frameworks design, viz. their unpredictable coordination numbers
and geometries as no ligand field stabilization effects govern their
coordinate bonding, and their tendency to form solvated metal cen-
[
10]. Metal carboxylates are particularly interesting because they
exhibit open-framework structures and the carboxylate group acts
as a linker between inorganic moieties [11]. Metal dicarboxylates
have a wide range of application in the construction of coordination
polymer arrays, especially with the more rigid ligands, as they act
as building blocks for porous networks with various metal ions
2 6
ters, and thus, the formation of the typical alternating [M(H O) ]L
(M = alkaline-earth metal, L = ligand) organic–inorganic ionic lay-
ers [19]. Besides, lack of intrinsically useful properties (i.e. magne-
tism or variable oxidation states) might appear to make alkaline
earth metals less interesting. Nevertheless these metals have some
advantages for the application in materials science; they are non-
toxic, cheap and generally soluble in aqueous preparation. For these
reasons alkaline earth salts are the preferred formulation for a host
of commercial materials, including many common pharmaceuti-
cals, dyes and pigments [16]. In this context some recent studies
concerning alkaline-earth metal complexes deserve to be
mentioned [20–23]. Platero-Prats et al. studied alkene hydrogena-
tions and ketone hydrosilylation reactions over alkaline-earth
[
12]. Pyridine dicarboxylic acid (H
gid ligand in building metal–organic framework compounds
MOFs). The O- and N-donor sites of H pdc allow it to produce dif-
ferent types of interesting 3D structures consisting of transition
2
pdc) seems to be a potential ri-
(
2
⇑
Corresponding author. Tel.: +91 33 2457 2778; fax: +91 33 2414 6414.
E-mail address: snkoner@chemistry.jdvu.ac.in (S. Koner).
Present address: Department of Chemistry, CICECO, University of Aveiro, 3810-
1
1
93 Aveiro, Portugal.
0
277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.05.043

6
4
D. Saha et al. / Polyhedron 43 (2012) 63–70
2ꢀ
s
2ꢀ
s
sulfonate metal–organic frameworks, which show 100% selectivity
towards ethylbenzene under homogeneous conditions [24]. Hydro-
genation of styrene catalyzed by a Ca based MOF has also been stud-
ied under heterogeneous conditions [25]. Choudary et al. studied
aldol-condensation reactions over nanocrystalline magnesium
oxide, which showed higher activity in direct asymmetric aldol-
condensation reactions under heterogeneous conditions [26]. Bar-
uah et al. studied aldol-condensation reactions catalyzed by mag-
nesium carboxylate compounds under homogeneous conditions
[
m
as (CO )], 1390 [
m
(CO )], 1366, 1177 [
m
(C–O)]. Anal. Calc. C,
27.80; H, 0.99; N, 4.63. Found: C, 27.8; H, 1.0; N, 4.5%.
2
.3. X-ray crystallography
X-ray diffraction data of 1 were collected at 293(2) K on a Bru-
ker SMART APEX CCD X-ray diffractometer using graphite-monochro-
mated Mo K radiation (k = 0.71073 Å). Integrated intensities were
determined and the unit cell was refined with the SAINT [32] soft-
a
[
27]. Homogeneous catalysts, however, suffer from the problems
ware package using a narrow-frame integration algorithm. An
of recycling of the catalyst as the separation of the catalyst from
the reaction mixture is troublesome. Recycling of a catalyst is a task
of great economic and environmental advantage in chemical and
pharmaceutical industries.
In the course of our continuing investigation on the catalytic
uses of MOFs we have successfully used layered metal carboxylates
and lanthanide-based MOFs to catalyze olefin epoxidation reac-
tions under heterogeneous condition [28–31]. Further exploration
of alkaline-earth metal based MOFs afforded a new 3D MOF involv-
empirical absorption correction (SADABS) [33] was applied. The
structure was solved by direct methods and refined against F2
using the full-matrix least-squares technique with anisotropic dis-
placement parameters for non-hydrogen atoms with the programs
SHELXS97 and SHELXL97 [34]. The carbon bound hydrogen atoms were
freely refined. There were no remarkable peaks in the final differ-
ence Fourier maps, except for ghost peaks surrounding the metal
center.
ing barium and pyridine-2,5-dicarboxylic acid [H
2 n
pdc], [Ba(pdc)]
2
.3.1. X-ray crystallography data of [Ba(pdc)]
n
(1)
(1). Interestingly, compound 1 features a 3D framework structure
C H
7
3
NO Ba, M = 302.43, orthorhombic, space group Pbca, a
4
and efficiently catalyzes the aldol-condensation reactions under
heterogeneous condition. Compound 1 is a rare example of a bar-
ium based framework compound which is capable of catalyzing al-
dol condensation reactions under heterogeneous condition.
3
(
Å) = 7.762(10), b (Å) = 10.506(2), c (Å) = 18.3021(4), V (Å ) =
ꢀ3
ꢀ3
1
1
492.69(5), Z = 8, Rint = 0.0322, Dcalc (g/cm ) = 2.692, F(000) =
ꢀ1
120,
l
(mm ) = 5.296,
D
q
max
(e Å ) = 0.874,
Dq
min
ꢀ3
2
(
e Å ) = ꢀ0.979, Goodness-of-fit (GOF) on F = 1.270, T = 293(2).
2.5. Catalytic reactions
2
. Experimental
The catalytic reactions were carried out in a glass batch reactor
according to the following procedure. Acetone/cyclohexanone
10 mmol), triethylamine (2 mmol), tetrahydrofuran (2 ml) and
the catalyst (5 mg) were placed in a round-bottom flask. The flask
was then placed in an ice-bath, maintaining the temperature
–10 °C. The desired aldehyde (2 mmol) was then added to the
solution and the reaction mixture was stirred for 6 h (Scheme 1).
After the reaction, the catalyst was separated by filtration and
the product was purified by column chromatography on silica gel
2
.1. Materials and physical measurements
(
Barium nitrate, pyridine-2,5-dicarboxylic acid and the substi-
tuted benzaldehydes were purchased from Aldrich and were used
as received. Other chemicals were purchased from Merck (India).
Benzaldehyde, acetone and tetrahydrofuran were distilled before
use. Benzaldehyde was kept over NaA molecular sieves to trap pos-
sible traces of benzoic acid. Elemental analyses were performed
using a Perkin–Elmer 240C elemental analyzer. Fourier trans-
formed infrared spectra of samples in KBr pellets were measured
on a Perkin–Elmer RX I FT-IR spectrometer. Powder X-ray diffrac-
tion (XRD) patterns of the samples were recorded with a Scintag
5
(
60–120 mesh), eluting with n-hexane/ethyl acetate. The product
1
was analyzed by H NMR spectroscopy and elemental analysis,
and the data were compared with those of authentic samples.
XDS-2000 diffractometer using Cu K
performed on a Perkin–Elmer (SINGAPORE) Pyris Diamond TGA
a radiation. TG analysis was
3. Results and discussion
ꢀ
1
unit. The heating rate was programmed at 5 °C min with a pro-
tecting stream of N sorption
flowing at a rate of 150 ml minꢀ1. N
n
3.1. X-ray structure of [Ba(pdc)] (1)
2
2
measurements were performed on an Autosorb iQ (Quantachorme
Inc., USA) gas sorption system at 77 K. The sample was degassed at
A single-crystal X-ray crystallographic study reveals that com-
pound 1 crystallizes in the space group Pbca with Z = 8. The coordi-
nation geometry of the central metal atom in compound 1 is a
distorted pentagonal bipyramid. Selected bond distances and an-
gles are collated in Table 1. An ORTEP diagram of 1 with the atom
numbering scheme is shown in Fig. S1 (see Supplementary
material). In this compound each metal ion is coordinated by seven
O atoms from seven different carboxylate groups (Fig. 1a). The Ba–
O (carboxylate) bond distances range from 2.710(3) to 2.878(3) Å,
which are in agreement with the Ba–O bond lengths observed in
other alkaline earth metal carboxylate complexes [16]. Addition-
ally, there are two weak coordinations from carboxylate oxygen
ꢀ3
1
00 °C for 3 h under vacuum (10 Torr) prior to nitrogen sorption
measurements.
2.2. Synthesis of the compound
Compound 1 was synthesized through a hydrothermal route.
The compound was obtained as colorless block crystals after heat-
ing in a 25 ml Teflon-lined Parr acid digestion bomb at 150 °C for
3
days, followed by slow cooling at a rate of 5 °C/h to room temper-
ature. The initial reaction mixture was prepared by mixing barium
nitrate (0.522 g, 2 mmol) and pyridine-2,5-dicarboxylic acid
#
3
and pyridine nitrogen atoms with longer Ba1–O4 (#3: ꢀx, ꢀ1/
#
4
(
0.167 g, 1 mmol) in 8 ml of milliQ water. The pH of the reaction
2 + y, 3/2 ꢀ z) and Ba1–N1 (#4: ꢀx, 2 ꢀ y, 1 ꢀ z) bond lengths
of 3.010(3) and 3.117(3) Å, respectively [35]. All the carboxylate
oxygen atoms in compound 1 are doubly bridged, taking the weak-
mixture was adjusted to 8 by adding dilute NaOH solution. The
crystals obtained were filtered off by suction and washed several
times with distilled water and then with acetone to remove impu-
rities, and finally dried in air. The bulk compound was character-
ized by elemental analysis and an IR spectroscopic study. For
2ꢀ
er Ba1–O4 bond into account (Fig. 1). Each pdc ligand coordi-
nates to seven alkaline earth metal centers through the
2
2
carboxylato oxygen atoms, adopting fascinating
l
4
-g
:g
-bridging
ꢀ
1
2
1
[
Ba(Pdc)]
n
(1). Selected IR peaks (KBr disk,
m
, cm ): 1607, 1685
and l -g :g -bridging coordination modes to afford a 3D network
3

D. Saha et al. / Polyhedron 43 (2012) 63–70
65
Scheme 1. Aldol condensation reaction catalyzed by compound 1.
Table 1
Selected bond lengths ( AÅ ) and angles (°) for the compound 1.
by the edge sharing of the Ba1 polyhedron to afford two-dimen-
sional layers. The interlayer separation was measured to be about
5.75 Å. Further, a dicarboxylate ligand bridges the 2D layers to gen-
erate a three-dimensional framework structure (Fig. 2).
0
Bond lengths
Bond angles
O1–Ba1–O4#
1
Ba1–O1
2.721(3)
2.825(3)
2.762(2)
3.010(3)
2.878(3)
2.710(3)
3.117(3)
2.780(3)
2.711(3)
72.54(8)
137.82(8)
97.03(8)
72.41(9)
73.46(9)
147.00(9)
136.41(9)
143.11(8)
67.66(9)
107.83(9)
93.28(9)
70.80(8)
73.91(9)
131.42(9)
69.43(9)
108.79(9)
68.80(9)
72.53(8)
145.17(9)
140.49(9)
73.58(9)
Ba1–O4#
1
O1–Ba1–O1
O1–Ba1–O5
O1–Ba1–O2
O1–Ba1–O2
#2
#3
#4
#5
#6
#
2
3
3
4
4
5
Ba1–O1
Ba1–O4#
3.2. Thermogravimetric analysis
#
Ba1–O5
Ba1–O2#
O1–Ba1–O5
Thermogravimetric analysis of 1 was performed using a pow-
dered sample under a nitrogen atmosphere. TG measurement con-
firmed that compound 1 is thermally stable up to approximately
560 °C, indicating good thermal stability of this MOF compound
(see Supplementary material Fig. S2). The TG curve indicates that
compound 1 starts to decompose at approximately 560 °C and com-
pletes decomposition at around 700 °C. There are only a few MOF
compounds that are reported to be thermally stable above 500 °C
[36,37]. The thermal stability of MOFs is usually explained in terms
of the rigidity of the linkers, absence of solvent molecules in the net-
work of the MOFs, strong bonds between the metal ions and the or-
ganic linkers, etc. Compound 1 satisfies all these requirements.
#
#1
#3
#1
#1
#1
#6
#3
#4
#5
#6
#3
#3
#6
#5
#6
#6
Ba1–N1
O4 –Ba1–O5
Ba1–O2#
O1 –Ba1–O4
#2
Ba1–O5#6
O2 –Ba1–O4
#5
#4
O2 –Ba1–O4
#1
O4 –Ba1–O5
#2
O1 –Ba1–O5
#2
O1 –Ba1–O2
#2
O1 –Ba1–O2
#2
O1 –Ba1–O5
#5
O2 –Ba1–O5
#7
O2 –Ba1–O5
#3
O5 –Ba1–O5
#4
O2 –Ba1–O2
#4
O2 –Ba1–O5
#5
O2 –Ba1–O5
3
.3. Nitrogen sorption study
Symmetry codes: #1: 1/2 ꢀ x, 2 ꢀ y, ꢀ1/2 + z; #2: ꢀ1/2 + x, 3/2 ꢀ y, 1 ꢀ z; #3: ꢀx,
ꢀ
1/2 + y, 3/2 ꢀ z; #4: ꢀx, 2 ꢀ y, 1 ꢀ z; #5: 1/2 ꢀ x, ꢀ1/2 + y, z; #6: x, 3/2 ꢀ y, ꢀ1/
+ z.
2
The nitrogen sorption measurement was undertaken to verify
the porous nature of compound 1 as well as to calculate the surface
area of the framework. The BET (Brunauer–Emmett–Teller) [38]
compound. Ba1 and its symmetry related congeners form one-
dimensional zig-zag chains through edge sharing of their coordina-
tion polyhedra. The 1D chains are further connected to each other
surface area of 1 was calculated from the N adsorption isotherm.
The adsorption and desorption isotherms of 1 are shown in Fig. 3.
The adsorption of N2 follows a type II isotherm, and indicates a
2
Fig. 1. A segment of compound 1, showing the local coordination environment of the barium(II) ion (yellow dotted lines for weak interaction) (a); coordination mode of the
pdc2 ligand in compound 1 (b). (Color online.)
ꢀ

6
6
D. Saha et al. / Polyhedron 43 (2012) 63–70
Fig. 2. 3D framework of compound 1.
bond formation reactions [41]. Magnesium oxide nano-particles
have also been studied for their efficacy in catalyzing aldol conden-
sation reactions [26]. Magnesium carboxylate compounds have at-
tracted little attention with regards to C–C bond formation
reactions due to their limited synthetic accessibility and hygro-
scopic nature [27]. Recently barium containing compounds have
been used to catalyze C–C bond formation reactions [42–46] and
some of them catalyze aldol condensation reactions [45,46]. We re-
port here the catalytic activity of compound 1 in aldol condensa-
tion reactions under heterogeneous conditions (Scheme 1).
The catalytic reactions were performed with a large excess of
ketone to prevent self-condensation and to effectively use the alde-
hyde [47]. During the catalytic reactions, the temperature of the
reaction medium was maintained in the range 5–10 °C. On increas-
ing the reaction temperature, benzylideneketone (the condensed
product) was obtained from the b-aldol. The reactions were per-
formed in THF medium in the presence of triethylamine, and the
results obtained in the reactions are shown in Table 2. The catalytic
reactions were also performed in a THF–water medium and under
solvent-less conditions using p-nitrobenzaldehyde. The best result
was obtained in THF medium (Table 3). In THF medium, as well as
in solvent-less conditions, the aldehydes were converted to their
respective b-aldols as the sole product. We noticed that the b-aldol
products did not undergo further transformation to form unsatu-
rated carbonyl compounds, which has been generally observed in
previous studies [48,49]. The efficiency of the catalyst followed
the order: THF > THF–water > solvent-less condition. No conver-
sion was observed in the absence of catalyst. The catalytic
reactions also did not occur in the absence of base (i.e. triethyl-
amine). The yield of the b-aldol product decreased in the sequence
p- > o- > m-nitrobenzaldehyde. A nitro substituent at the ortho and
para position may give rise to both a negative inductive effect and a
negative mesomeric effect, thereby increasing the electrophilicity
of the C@O groups of these nitrobenzaldehydes, whereas substitu-
tion at the meta position affords only the negative inductive effect.
Between p-nitrobenzaldehyde and o-nitrobenzaldehyde, more ste-
ric crowding at the ortho position may leads to a low conversion for
the ortho variety. Additionally, the presence of an electron donat-
ing group in the ring, for example p-methoxy-benzaldehyde,
decreases the yield further and demonstrates the lowest conver-
sion. In case of cyclohexanone a similar trend in the yield of the
b-aldol products was observed. Turn over frequencies (TOF) of
2
Fig. 3. Plot of the N adsorption–desorption isotherm of compound 1 at 77 K.
2
ꢀ1
2
ꢀ1
surface area (SBET) of 108 m g (SLangmuir = 218 m g ). [Ba(pdc)]
1) has a very densely packed structure, which is supported by the
n
(
PLATON [39] calculations. Dense packing can be easily seen in a space
filling representation of the structure of 1. Careful examination of
the crystal structure ensures that there are no pores or cavities of
n
appreciable size in compound 1. [Ba(pdc)] (1) does not contain
either guest or coordinated solvent molecules which could give
rise to empty voids in the network after thermal activation. Fur-
ther, the value of the surface area is low in comparison to that of
microporous metal–organic frameworks. The surface area calcu-
lated from the N sorption measurement is, therefore, presumably
2
coming mainly from the external surface of the powdered MOF
compound.
3
.4. Catalytic aldol condensation reaction
Environmental friendly heterogeneous catalytic processes are
attracting increased attention in chemical industries. The identifi-
cation of highly active solid Brønsted-type basic catalysts capable
of performing C–C bond formation remains a challenge. The main
drawbacks of using NaOH or KOH as homogeneous catalysts are
corrosion problems in the equipment, separation difficulties and
generation of large amounts of waste. To overcome these disadvan-
tages, several efforts have been made to design new catalytic sys-
tems with controlled basic properties in order to increase the
efficiency of the process. Among the various magnesium com-
pounds which are used as mediators or catalysts for several organic
reactions [40], magnesium oxide is a versatile catalyst for C–C
n
the aldol reactions catalyzed by [Ba(pdc)] (1) are given in Table
2. Shibasaki et al. studied similar aldol condensation reactions over
a barium catalyst under homogeneous conditions, which showed
impressive conversion (up to 99%), however the turnover frequen-
ꢀ1
cies of these reactions were very low (0.32–1 h ) [50]. Aldol con-
densation reactions catalyzed by magnesium carboxylate under
homogeneous conditions also demonstrate lower TOF values (2–
ꢀ1
5 h ) than that of compound 1 [27].

D. Saha et al. / Polyhedron 43 (2012) 63–70
67
Table 2
Reactions of various aromatic aldehydes and acetone/cyclohexanone catalyzed by compound 1.a
Entry
1
Ketone
Aldehyde
Major product
Isolated yield (wt.%)
96, 94b
TOFc (hꢀ1
19, 19b
)
Acetone
2
Acetone
88
18
3
Acetone
82
16
70
14
4
Acetone
5
Acetone
62
12
6
Cyclohexanone
90, 87b
18, 17b
(
continued on next page)

6
8
D. Saha et al. / Polyhedron 43 (2012) 63–70
Table 2 (continued)
Entry
7
Ketone
Cyclohexanone
Aldehyde
Major product
Isolated yield (wt.%)
80
TOFc (hꢀ1
16
)
8
Cyclohexanone
74
15
9
Cyclohexanone
62
12
1
0
Cyclohexanone
55
11
a
Reaction conditions: aldehyde (2 mmol), acetone/cyclohexanone (10 mmol), triethylamine (2 mmol), solvent (2 ml) and catalysts (5 mg); temperature = 5–10 °C. Yields
were isolated after 6 h of reaction.
b
Fifth cycle.
c
Turn over frequency (TOF) = moles converted per moles of active site per unit time.
Table 3
a
Solvent effect on the aldol condensation of p-nitrobenzaldehyde and acetone/cyclohexanone catalyzed by 1.
Solvent
Ketone
Major product
Isolated yield (wt.%)
Selectivity (wt.%)
THF
THF
Acetone
Cyclohexanone
Acetone
Cyclohexanone
Acetone
Cyclohexanone
b-Aldol product
b-Aldol product
b-Aldol product
b-Aldol product
b-Aldol product
b-Aldol product
96
90
64
58
32
28
100
100
62
60
100
100
THF–water (1:1)
THF–water (1:1)
No solvent
No solvent
a
Reaction conditions are the same as those given in the footnote of Table 2.
3
.4.1. Separation, catalyst reuse and heterogeneity test
To ascertain that the catalysis was indeed heterogeneous, we
solid catalyst was recovered by centrifugation, washed thoroughly
with dichloromethane, and dried. The recovered catalyst was then
subjected to X-ray powder diffraction analysis and IR spectral anal-
ysis. A comparison of the IR spectra (Fig. 4) and X-ray diffraction
patterns (Fig. 5) of the pristine compound and recovered catalyst
convincingly demonstrates that the structural integrity of the com-
pound is retained after the aldol condensation reaction.
For the recycling study, the aldol condensation reaction was
performed using p-nitrobenzaldehyde with acetone and cyclohex-
anone. After the first cycle of the reaction, the catalyst was recov-
ered by centrifugation and then washed as mentioned above. The
recovered catalyst was dried under vacuum (ꢁ100 °C, 12 h) prior
performed a hot filtration test. The solid catalyst was filtered off
when the catalytic reaction was 30–40% complete and the liquid
phase of the reaction mixture was kept under the reaction condi-
tions for a further 8 h. Analysis of the products of the catalytic reac-
tion confirmed that there was no further increase of the product.
No reaction occurred in the absence of the solid catalyst. This result
suggests that there is no leaching of Ba from the solid catalyst dur-
ing reactions.
To check the stability of the catalyst, we characterized the
recovered material. After completion of a catalytic reaction, the

D. Saha et al. / Polyhedron 43 (2012) 63–70
69
to reuse as a catalyst. The performance of the recycled catalyst
after five successive runs is given in Table 2. The catalytic efficacy
of the recovered catalyst remained almost the same in each run
(Fig. 6).
4
. Conclusion
In essence, we have synthesized a new MOF compound,
[
Ba(pdc)]
n
(1), through a hydrothermal route, and this compound
(1) displays
is thermally stable up to around 560 °C. [Ba(pdc)]
n
excellent catalytic activity in aldol condensation reactions of vari-
ous aromatic aldehydes, giving remarkable yields with high selec-
tivity within a short reaction time under heterogeneous condition.
Evidently, the advantage of our system is that the catalyst is not air
or moisture sensitive; hence, there is no need to carry out the reac-
tion under an inert atmosphere. The catalyst can be recycled and
reused several times without any loss of activity. Our future work
will be focused on constructing alkaline-earth metal based
frameworks for higher catalytic activity in aldol condensation
reactions.
Fig. 4. Comparison of the IR spectra of pure and recovered catalyst for 1.
Acknowledgments
We acknowledge the Department of Science and Technology
(
DST), Government of India for funding a project (to S.K.) (SR/S1/
IC-01/2009). We also thank DST for funding the Department of
Chemistry, Jadavpur University to procure a single crystal X-ray
facility under the DST-FIST program. Financial assistance from
UGC under the CAS program is also gratefully acknowledged.
Appendix A. Supplementary data
CCDC 835092 contains the supplementary crystallographic data
for 1. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
ORTEP diagram of 1 (Fig. S1), TG analysis curve (Fig. S2), 1H NMR
of all products of known compounds and elemental analysis data
of the isolated products. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
Fig. 5. X-ray powder pattern of the virgin catalyst and recovered catalyst for 1.
1
0.1016/j.poly.2012.05.043.
100
90
80
70
60
50
40
30
20
10
0
acetone
cyclohexanone
cycle 1
cycle 2
cycle 3
cycle 4
cycle 5
Fig. 6. Aldol condensation of p-nitrobenzaldehyde and ketones catalyzed by 1 in five successive runs.

7
0
D. Saha et al. / Polyhedron 43 (2012) 63–70
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