Porous magnesium carboxylate framework: Synthesis, X-ray crystal structure, gas adsorption property and heterogeneous catalytic aldol condensation reaction

Article abstract of DOI:10.1039/c2dt00057a

A new three-dimensional alkaline-earth metal-organic framework (MOF) compound, [Mg(Pdc)(H₂O)]_n (1) (H₂Pdc = pyridine-2,5-dicarboxylic acid), has been synthesized and structurally characterized by single crystal X-ray diffr

Full text of DOI:10.1039/c2dt00057a

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Volume 39 | Number 3 | 21 January 2010 | Pages 657–964
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Porous Magnesium Carboxylate Framework: Synthesis, X-ray
Crystal Structure, Gas Adsorption Property and
Heterogeneous Catalytic Aldol Condensation Reaction†
a
a,b
a
*, a
Debraj Saha, Rupam Sen, Tanmoy Maity, and Subratanath Koner
a
Department of Chemistry, Jadavpur University, Kolkata 700 032, India. E. Mail:
snkoner@chemistry.jdvu.ac.in; Tel.: +913324146666 Ext. 2778; Fax: +913324146414
b
Present address: Department of Chemistry, CICECO, University of Aveiro, 3810ꢀ193
Aveiro, Portugal
_
_______________________________________________________________________
Electronic supplementary information (ESI) available: CCDC references 817494.
†
ORTEP diagram of 1, results of catalytic reactions in specific condition in tabular form,
IR spectral results and PXRD patterns of virgin and recovered catalyst, graphical
presentation of aldol condensation reaction between pꢀnitrobenzaldehyde and ketons in
1
five successive runs, Xꢀray crystallographic file (CIF), H NMR of all products of known
compounds, elemental analysis study of the isolated products.

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For Table of contents entry
A new threeꢀdimensional alkalineꢀearth metalꢀorganic framework (MOF) compound,
Mg(Pdc)(H O)] (1) has been synthesized through hydrothermal route. Compound 1 is
[
2
n
porous and demonstrates systematic variation of BET surface area during dehydrationꢀ
rehydration process reversibly. The framework compound efficiently catalyzes aldol
condensation reactions with various aromatic aldehydes and displays remarkable yields
with high selectivity within short time under heterogeneous condition. Notably, the
catalyst can be recycled and reused several times without any loss of activity.

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Abstract
A new threeꢀdimensional alkalineꢀearth metalꢀorganic framework (MOF)
compound, [Mg(Pdc)(H O)] (1) (H Pdc = pyridineꢀ2,5ꢀdicarboxylic acid), has been
2
n
2
synthesized and structurally characterized by single crystal Xꢀray diffraction analysis.
Compound 1 features a 3D porous framework afforded by the Mg ꢀdiad centers through
2
formation of interconnected chair like structural motifs. Nitrogen adsorption study
2
confirms the microporosity of compound 1 with a BET surface area of 211± 12 m / g.
2
Upon dehydration BET surface area of 1 is enhanced to a value of 463± 36 m / g due to
removal of coordinated water molecule. After rehydration, the compound reverts to its
original form as evidenced by powder Xꢀray diffraction and IR spectroscopic analysis
and N sorption measurement. Compound 1 retains its pore structure with a variable BET
2
surface area in several cycles of dehydration and rehydration processes indicating
robustness of the framework in [Mg(Pdc)(H O)] (1). Compound 1 catalyzes the aldol
2
n
condensation reactions of various aromatic aldehydes with acetone and cyclohexanone in
heterogeneous condition. Notably, catalytic activity of the compound is enhanced upon
dehydration. The catalyst can be recycled and reused several times without significant
loss of activity.
Introduction
Last few decades metal–organic frameworks (MOFs) have emerged as promising
hybrid materials in the diverse disciplines of crystal engineering and designing of
advanced multiꢀfunctional materials. Fabricating metalꢀorganic frameworks (MOFs)
1
continues to be intriguing for their unique topology and tunable properties. The interest

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2
3
of MOF compounds are focused on the catalysis, moleculeꢀbased magnetic materials,
4
artificial nucleases for the hydrolytic cleavage of DNA and RNA, contrast agents for
5
6
magnetic resonance imaging, and fixation media for atmospheric gases. Chemistry of
alkaline earth metals has remained largely an unexplored area. There have been a few
7
studies on the coordination behavior of these metals both in aqueous and nonaqueous
media. Fromm et. al have recently reviewed the chemistry of coordination polymers of sꢀ
7
a
block metal ions. In comparison to dꢀblock cations, reluctance in using the alkalineꢀ
earth cations as building blocks arises from their unpredictable coordination numbers and
8
geometries as no ligand field stabilization effects govern their bonding. The main issue
for framework assembly with alkaline earth ions is their high affinity for oxygen donors.
Further, lack of intrinsically useful properties (i.e. magnetism or variable oxidation states)
might appear to make alkaline earth metals less interesting. Nevertheless these metals do
have some advantages for application in materials science; they are nonꢀtoxic, cheap and
generally soluble in aqueous medium. For these reasons alkaline earth salts are the
preferred formulations for a host of commercial materials, including many common
7
a
pharmaceuticals, dyes and pigments. In this context some recent studies concerning
9
with alkalineꢀearth metal complexes deserve to be mentioned. Choudary et al. studied
aldolꢀcondensation reaction over nanocrystaline magnesium oxide which showed higher
activity in the direct asymmetric aldolꢀcondensation reaction in heterogeneous
10
condition. Baruah et al. studied aldolꢀcondensation reaction catalyzed by magnesium
1
1
carboxylate compounds in homogenous condition. Homogeneous catalysts, however,
suffer from the problems of recycling of the catalyst as the separation of catalyst from

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reaction mixture is troublesome. Recycling of catalysts is a task of great economic and
environmental advantages in chemical and pharmaceutical industry.
In the course of our continuing investigation on catalytic uses of MOFs we have
successfully employed layered metal carboxylates and vanadium(IV) hydrogenphosphate
and lanthanideꢀbased MOFs to catalyze olefin epoxidation reaction in heterogeneous
12
condition. Further exploration of alkalineꢀearth metal based MOFs afforded a new 3D
MOF involving magnesium and pyridineꢀ2,5ꢀdicarboxylic acid [H Pdc], namely
2
[
Mg(Pdc)(H O)] (1). Interestingly, compound 1 features a porous structure with variable
2 n
surface area and it efficiently catalyzes the aldolꢀcondensation reactions in heterogeneous
condition. To our knowledge compound 1 is the first example of alkalineꢀearth metal
based framework system which is capable of catalyzing aldol condensation reaction
under heterogeneous condition.
Experimental section
Materials. Magnesium nitrate hexahydrate, pyridineꢀ2,5ꢀdicarboxylic acid, and
substituted benzaldehydes, cyclohexanone and tertꢀbutylcyclohexanone were purchased
from Aldrich and were used as received and other chemicals were purchased from Merck
(
India). Benzaldehyde, acetone and tetrahydrofuran were dried and distilled before use.
Benzaldehyde was kept over NaA molecular sieves to trap possible traces of benzoic
acid.
Physical measurements. Elemental analysis was performed by using Perkinꢀ
Elmer 240C elemental analyzer. Fourier transformed infrared spectra of KBr pellets were
measured on a Perkin–Elmer RX I FTꢀIR spectrometer. The powder Xꢀray diffraction

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(XRD) patterns of the samples were recorded with a Scintag XDSꢀ2000 diffractometer
using MoK radiation at desired temperature. Thermogravimetric analysis was performed
α
on a PerkinElmer (SINGAPORE) Pyris Diamond TGA unit. The heating rate was
ꢀ1
ꢀ
programmed at 5°C min with a protecting stream of N flowing at a rate of 150 ml min
2
1
.
Gas sorption isotherms for pressures in the range of 0ꢀ1 bar were measured using an
Autosorb iQ (Quantachorme Inc., USA) gas sorption system. A sample of ca. 50 mg of
asꢀsynthesized material was introduced into a preweighed analysis tube (6 mm diameter,
3
6
cm bulb), capped with a gasꢀtight transeal to prevent leakage of air and moisture
ꢀ3
during transfer and weighing. The samples were evacuated under dynamic vacuum (10
torr) at desired temperature, until a constant weight was achieved. The analysis tube was
then weighed again to determine the mass of the evacuated sample. For all isotherms,
warm and cold free space correction measurements were performed using ultrahigh pure
He gas (99.999% purity). N isotherm at 77 K was measured in a liquid nitrogen bath
2
using ultrahigh pure (99.999% purity) gas source.
Synthesis of the compound 1. [Mg(Pdc)(H O)] (1) was synthesized through
2
n
hydrothermal route. The initial reaction mixture was prepared as follows: Pyridineꢀ2,5ꢀ
dicarboxylic acid (0.167 g, 1 mmol) first dissolved in 5 ml of milliQ water and pH of the
solution was adjusted to 8 by mixing a dilute NaOH solution, to this aqueous solution of
magnesium nitrate hexahydrate (0.513 g, 2 mmol) was added and the final mixture was
stirred for half an hour. Compound 1 was obtained as colorless block crystals in a 20 ml
capacity teflonꢀlined acid digestion bomb, at 175 ºC for 3 days followed by slow cooling
at the rate of 5ºC/h to room temperature. Yield was ca. 80% based on the metal. For
characterization of the bulk compound, elemental analysis and IR spectroscopic study

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were undertaken. Anal. Calcd. for [Mg(Pdc)(H O)] 1; C = 40.54, H = 2.43, N= 6.75,
2
n
ꢀ1
found C = 40.7, H = 2.5, N= 6.8. Selected IR peaks (KBr disk, ν, cm ): 1647, 1602 [υ
as
ꢀ
ꢀ
(
CO )], 1495 [υ (CO )], 1365, 1260 [υ (CꢀO)], and 3606ꢀ3370 s.br [υ (OꢀH)].
2 s 2 s
X-ray crystallography. Xꢀray diffraction data for 1 were collected at 293(2) K
on a Bruker SMART APEX CCD Xꢀray diffractometer using graphiteꢀmonochromated
MoKα radiation (λ = 0.71073Å). Determination of integrated intensities and cell
1
3
refinement were performed with the SAINT software package using a narrowꢀframe
1
4
integration algorithm. An empirical absorption correction (SADABS) was applied. All
the structures were solved by direct methods and refined using fullꢀmatrix leastꢀsquares
2
technique against F with anisotropic displacement parameters for nonꢀhydrogen atoms
1
5
with the programs SHELXS97 and SHELXL97. Hydrogen atoms were placed at
calculated positions using suitable riding models with isotropic displacement parameters
derived from their carrier atoms. In the final difference Fourier maps there were no
remarkable peaks except for the ghost peaks surrounding the metal centers. A summary
of crystal data and relevant refinement parameters for compound 1 are given in Table 1.
Catalytic reaction. The catalytic reactions were carried out in a glass batch
reactor according to the following procedure. Acetone/cyclohexanone/substituted
cyclohexanone (10 mmol), triethylamine (2 mmol), tetrahydrofuran (2 ml) and catalysts
(5 mg) were taken in a round bottom flask. It was than placed in an iceꢀbath maintaining
temperature 5–10 °C. To this solution aldehyde (2 mmol) was added and the reaction
mixture was stirred for 6 h (Scheme 1). Compound 1 was calcined at 140 °C under
vacuum for 3 h prior to using it in the catalytic reactions. The catalytic reactions were
performed in dry nitrogen atmosphere. After completion of reaction catalysts were

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separated by filtration and products were purified by column chromatography over silica
gel (mesh 60ꢀ120) using nꢀhexene/ethyl acetate mixture as eluent to give the desired
1
product. Products were analyzed by H NMR, elemental analysis and comparing them
with authentic samples.
Results and discussion
X-ray structure of compound 1. Singleꢀcrystal Xꢀray crystallographic study
reveals that compound 1 crystallizes in the space group P2 /c with Z value of 4. Selected
1
bond distances and angles are collated in Table 2. The basic unit consists of one
magnesium(II) center with distorted octahedron geometry (Fig. 1). ORTEP diagram is
II
given in the Fig. S1 (see ESI). The basal plane of coordination environment around Mg
is afforded by one chelated carboxylate ligand (N1 and O4) and oxygen atoms (O2 and
O3) from two other carboxylate ligands. One apical position of octahedron is occupied by
the oxygen atom (O1) of a carboxylate ligand while another one by oxygen atom (O5) of
water molecule. Two adjacent magnesium atoms are held together by two synꢀsyn
bridging carboxylate ligand to form a Mg ꢀdiad (Mg C O ), which acts as SBU
2
2
2
4
(
secondary building unit) of the framework (Fig. 2a). The Mg ꢀdiad, Mg C O SBUs
2 2 2 4
2
ꢀ
further bridged by the organic Pdc linkers to afford a binodal 3,6ꢀ connected threeꢀ
dimensional network, consisting of one kind of threeꢀcoordinated nodes (shown as blue
triangles in Fig. 2) and a sixꢀcoordinated node of the Mg ꢀdiad (Mg C O ), (shown as a
2
2
2
4
magenta hexagon in Fig. 2). Three such Mg ꢀdiad centers afford chair like motifs which
2
are connected with each other to form the 3D porous framework where the coordinated
water molecules are pointing towards the interior of the cavity (Fig. 3) which are, upon

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dehydration, capable of creating open metal sites. Now the compound 1 features a
rectangle type channel [12.2 Å × (3.6→5.7) Å] parallel to crystallographic aꢀaxis (Fig. 4),
with the open metal sites exposed to the pores for the better catalytic activity. Such open
1
6
metal sites also play significant roles on gas storage and separation. Additional
reinforcement in the networked structure is achieved by intramolecular hydrogen bonding
O5—H5ꢁꢁꢁO4(1+X,Y,Z).
Thermogravimetric analysis. Thermogravimetric analysis of 1 was performed
using powdered sample under nitrogen atmosphere. TG measurement confirms that the
compound 1 is thermally stable up to ~150 °C (Fig. 5). The TG curve indicates that
compound 1 starts to loose water molecules at ~150 °C and completes dehydration at
around 160 °C. The mass loss of ~8.6 % shown in the range 150–160°C corresponds to
the loss of one molecule of water. Thereafter TG curve remains flat up to ~200 °C
indicating no mass loss in this temperature range. On further heating TG curve shows
continuous mass loss up to 500°C due to decomposition of the compound. Crystal
structure analysis showed compound 1 has one molecule of coordinated water per
magnesium site. TG analysis clearly indicates that upon heating compound 1 loses its
coordinated water to produce dehydrated product which is stable up to ~200 °C. Upon
exposure to humid atmosphere (relative humidity ~90%) dehydrated species absorbs
water and revert to its original hydrated form. TG curve of rehydrated compound
corresponds well with that of the virgin compound 1 (Fig. 5), thus thermogravimetric
analyses showed dehydrationꢀrehydration processes are reversible.
Powder X-ray diffraction measurements. To verify the structural integrity of
the rehydrated species powder Xꢀray diffraction patterns were measured. Required

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amount of dehydrated species of the compound 1 has been collected from TG analyzer in
several batches. The solid sample thus collected was then exposed to humid atmosphere
(relative humidity ~ 90%) for about 10 days. Powder Xꢀray diffraction patterns of
compound 1 and rehydrated species were measured on a Scintag XDSꢀ2000
diffractometer using MoKα radiation. Xꢀray diffraction patterns of compound 1,
rehydrated species as well as the powder patterns simulated using single crystal data were
shown in Fig. 6. Comparison of Xꢀray diffraction patterns of the rehydrated species and
[
Mg(Pdc)(H O)] (1) indicates that the structure of the compound is remained unaltered
2 n
after rehydration. Xꢀray diffraction patterns of 1 and the rehydrated species corresponds
well with the powder patterns simulated from single crystal Xꢀray data (Fig. 6). Besides,
Xꢀray powder diffraction patterns of the dehydrated species of 1 have also been measured
at 140 ºC (Fig. 6). Compound 1 was slowly heated to 140 °C and outgassed under
ꢀ3
vacuum (10 Torr) for 3 h prior to Xꢀray measurement to ensure complete dehydration of
the sample. Comparison of Xꢀray diffraction patterns of the [Mg(Pdc)(H O)] (1) and
2
n
dehydrated species indicates that the framework structure of the compound is remained
unaltered after dehydration.
Nitrogen sorption study. 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 compound. Prior to sorption experiments, samples were outgassed at desired
ꢀ3
17
temperature (80ºC) for 3 h under vacuum (10 Torr). BET (BrunauerꢀEmmettꢀTeller)
surface area of 1 was calculated from the N adsorption isotherms. The adsorption and
2
desorption isotherms of 1 is shown in Fig. 7 (1A, 1D). The adsorption of N follows a
2
2
ꢀ1
type II isotherm with a surface area (SBET) 211 ± 12 m g . Thermogravimetric analysis

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showed compound 1 undergoes dehydration in the temperature range 150ꢀ160 °C to
produce a thermally stable dehydrated product of 1. It is of interest, therefore, to see if the
dehydrated species has different characteristic of porosity in comparison to the compound
1
. We performed another set of nitrogen sorption experiments where compound 1 was
ꢀ
3
outgassed at ~140 ºC for 3 h under vacuum (10 Torr). The weight loss was measured
after degassing the sample was of ~8.6 % which corresponds to the loss of one molecule
of coordinated water. The adsorption and desorption isotherm of dehydrated species of 1
is shown in Fig. 7 (2A, 2D). Interestingly, the dehydrated product showed a higher BET
2
ꢀ1
surface area (SBET = 463 ± 36 m g ) than that of the compound 1. This indicates surface
area of the compound 1 vary widely on dehydration. After this experiment the sample
was kept in humid atmosphere (relative humidity ~90%) for 10 days. During this period
the dehydrated sample absorbed water from air and reverted to its initial weight. We have
undertaken nitrogen sorption measurement of rehydrated species and subsequently
nd
dehydrated (2 . time) species. To do this, rehydrated sample was again outgassed at 80
ꢀ3
ºC for 3 h under vacuum (10 Torr) and N adsorption isotherms were measured. The
2
adsorption and desorption isotherms of the rehydrated species of 1 is shown in Fig. 7
2
ꢀ1
(
3A, 3D). BET surface area calculated from isotherm was (SBET) 243 ± 13 m g which is
close to the value of the virgin compound 1. Finally the rehydrated species is subjected to
ꢀ3
outgassing at 140 ºC for 3 h under vacuum (10 Torr) to remove coordinated water again.
nd
Nitrogen sorption measurement of this dehydrated (2 . time) species showed the BET
2
ꢀ1
surface area 453 ± 40 m g . The adsorption and desorption isotherms of this dehydrated
nd
st
(
2 . time) species are comparable with the previous experiment (1 . time dehydrated)
Fig. 7; 4A, 4D). Retention of pore structure along with reproducible surface area in
(

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several cycles of dehydration and rehydration processes indicating the robustness of
framework in [Mg(Pdc)(H O)] (1). Further IR spectra and Xꢀray powder diffraction
2
n
patterns (see above) of compound 1 and after its rehydration were measured. Comparison
of IR spectra of compound 1 and rehydrated species convincingly demonstrate that the
structural integrity of the compound is remained unaltered after rehydration (Fig. 8).
Catalytic aldol condensation reactions. Environment friendly heterogeneous
catalytic processes are increasingly receiving attention in the chemical industry.
Designing of highly active solid Brønstedꢀtype basic catalysts which are capable to
catalyze C–C bond formation reactions is a challenge. The main drawbacks of using
NaOH or KOH as homogeneous catalysts are separation difficulties, corrosion in the
equipments and generation of large amount of wastes. In order to overcome these
disadvantages several efforts have been made to arrange new catalytic systems with
controlled basic properties in order to increase efficacy of the process. Among various
magnesium compounds which are used as mediators or catalysts for several organic
1
8
reactions, magnesium oxide is a versatile catalyst for carbon–carbon bond formation
1
9
reactions. The magnesium oxide nanoꢀparticles also have been employed in catalytic
1
0
2ꢀ
aldol condensation reactions. It is proposed that the surface ꢀOH and O of these oxide
1
8
crystals are expected to trigger the carbon–carbon bond formation reactions.
Magnesium carboxylate compounds have attracted little attention in CꢀC bond formation
1
1
reaction due to their limited synthetic procedure and hygroscopic nature. In this study
catalytic activity of compound 1 has been investigated in aldol condensation reaction
under heterogeneous condition.

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Catalytic reactions were performed in presence of large excess of ketone to
2
0
prevent self condensation of ketone as well as for effective use of aldehyde. Reactions
were performed in THF medium in presence of triethylamine maintaining the
temperature 5–10 °C using calcined catalyst under heterogeneous condition. With
increasing the reaction temperature βꢀaldol product transformed to benzylidene ketone
(condense product). The results of the catalytic aldol condensation reaction of different
substrates are summarized in Table 3. The catalytic reaction was also performed in THFꢀ
water medium and solventꢀless condition using pꢀnitrobenzaldehyde. The best result was
obtained in THF medium (See ESI; Table S1). In THF medium as well as in solventꢀless
condition aldehyde substrates are converted to their respective βꢀaldols as the sole
product. In this study we noticed βꢀaldol products do not undergo further transformation
to form unsaturated carbonyl compounds. Generally βꢀaldol product undergoes
2
1
dehydration to give a conjugated enone in aldol condensation reaction. The efficiency
of catalyst follows the order: THF > THFꢀwater > solventꢀless condition. Under identical
reaction conditions no conversion was observed in absence of catalyst. Catalytic reaction
also does not occur in absence of base (i.e. triethylamine). Aldolꢀcondensation reaction
catalyzed by nonporous crystaline magnesium oxide demonstrates 75% conversion for pꢀ
1
0
nitrobenzaldehyde and acetone in 24h in heterogeneous condition. Yield of the βꢀaldol
product decreases from pꢀnitrobenzaldehyde to mꢀnitrobenzaldehyde through oꢀ
nitrobenzaldehyde. It may be realized that nitro substituent at ortho and para position
affords both negative inductive effect and negative mesomeric effect which increases
electrophilicity of >C=O group of nitrobenzaldehydes but substitution at meta position
affords only the negative inductive effect. Between pꢀnitrobenzaldehyde and oꢀ

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nitrobenzaldehyde, more steric crowding at ortho position may leads to low conversion
for ortho variety. On the other hand presence of electron donating group in the ring in
case of pꢀmethoxyꢀbenzaldehyde yield decreases further and demonstrates the lowest
conversion. In case of cyclohexanone a similar trend in the yield of βꢀaldol products was
observed. In all the catalytic reactions calcined species of compound 1 has been used as
catalyst. However, when [Mg(Pdc)(H O)] (1) was used as catalyst without calcination
2
n
the yield of the catalytic reactions decreases remarkably. Aldolꢀcondensation of pꢀ
nitrobenzaldehyde and acetone catalyzed by calcined species goes smoothly, showing a
yield of 90% (Table 3), whereas the yield of the same reaction decreases to 68% if
[
Mg(Pdc)(H O)] (1) is used as catalyst (Table 3). Crystal structure analysis reveals that
2 n
in compound 1 coordinated water molecule occupy the space of the void channel that
exist parallel to crystallographic aꢀaxis (see above) in the lattice. It may be realized that
upon heating [Mg(Pdc)(H O)] (1) loses the water molecule, as a result, channels of the
2
n
framework becomes more spacious and hence, relatively less encumbrance to the guest
molecules to enter into the channel. Consequently, the BET surface area of the
dehydrated species (calcined catalyst) increases remarkably. Therefore, demonstration of
increased catalytic reactivity of dehydrated species is not inconsistent. Aldol reaction
with bulkier substrate, tertꢀbutylcyclohexanone demonstrates least yield of the product in
comparison to the other two smaller substrates acetone and cyclohexanone (Table 3).
The size of the reactants and products, except for tertꢀbutylcyclohexanone, indicate that
the pore windows (12.2 Å × (3.6→5.7) Å) are large enough for diffusion of molecules
through pores. However, in case of aldol reaction of tertꢀbutylcyclohexanone and pꢀ
nitrobenzaldehyde, the reactant (tertꢀbutylcyclohexanone) and product are larger than the

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pore window. This indicates pore of the compound 1 has a significant role in enhanced
efficiency of the catalyst. Magnesium present on the pore mouth of catalyst, however,
might catalyze the reaction in case of bulkier substrate. It is noteworthy that condensation
reaction of tertꢀbutylcyclohexanone and pꢀnitrobenzaldehyde catalyzed by simple
magnesium diꢀcarboxylate compound (magnesium pyrazoleꢀ3,5ꢀdicarboxylate) goes
smoothly to yield 72% product.
Separation, catalyst reuse and heterogeneity test. To ascertain the catalysis is
indeed heterogeneous we have performed hot filtration test. To test if metal is leached out
from the solid catalyst during reaction, the liquid phase of the reaction mixture is
collected by filtration after ~40% of the reaction is completed and the residual activity of
the supernatant solution after separation of the catalysts was studied. The supernatant
solution kept in reaction condition for another 8 h and the composition of the solution
was analyzed time to time. No progress of reaction was observed during this period,
which excludes the presence of active species in solution. These experiments clearly
demonstrate that metal is not being leached out from the solid catalyst during the
reactions.
In order to check the stability of the catalysts, we have characterized the
recovered catalyst. After the catalytic reactions are over, solid catalyst was recovered by
centrifugation and washed thoroughly with tetrahydrofuran and dried. The recovered
catalyst was then subjected to Xꢀray powder diffraction and IR spectral analysis.
Comparison of IR spectra (See ESI; Fig. S2) and Xꢀray diffraction patterns (See ESI; Fig.
S3) of the pristine compound and recovered catalyst convincingly demonstrate that the

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structural integrity of the compound is remained unaltered after the aldol condensation
reactions.
For the recycling study, aldol condensation reaction was performed using pꢀ
nitrobenzaldehyde, acetone and cyclohexanone. After first cycle of reaction the catalyst
was recovered by centrifugation and then washed as mentioned above. The recovered
catalyst was dried under vacuum overnight. The performance of the recycled catalyst in
C–C coupling reactions up to five successive runs has been studied (Table 3) (See also
ESI; Fig. S4 and Fig. S5). The catalytic efficacy of the recovered catalyst remains almost
unaltered in each run. The recovered catalyst was calcined, as described previously, in
every cycle before using it in catalytic aldol condensation reactions.
Conclusion
In summary we have synthesized a new threeꢀdimensional alkalineꢀearth metalꢀ
organic framework (MOF) compound, [Mg(Pdc)(H O)] (1) through hydrothermal route.
2
n
Compound 1 is porous and demonstrates systematic variation of BET surface area by
dehydrationꢀrehydration process reversibly. The framework compound demonstrates
excellent catalytic activities in aldol condensation reactions towards various aromatic
aldehydes and displays remarkable yields with high selectivity in a short reaction time
under heterogeneous condition. Notably, the catalyst can be recycled and reused several
times without any loss of activity. Further investigations of the application of alkalineꢀ
earth metal based framework catalysts in other organic reactions are currently in progress
in our laboratory.

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Acknowledgment
We acknowledge the Department of Science and Technology (DST), Government of
India for funding a project (to SK) (SR/S1/ICꢀ01/2009). We also thank DST for funding
Department of Chemistry, Jadavpur University to procure a single crystal Xꢀray facility
under DSTꢀFIST programme.
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1

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Legends to the Figures
Fig. 1. The segment of compound 1, showing the local coordination environment of
magnesium (II) ion (colour code for the atoms; cyan magnesium, blue nitrogen, red
oxygen and white carbon).
Fig. 2. 3,6ꢀ connected network in compound 1. Hydrogen atoms have been omitted for
clarity.
Fig. 3. 3D network of compound 1
Fig. 4. Packing view of the 3D pore with exposed active metal centers (coordinated water
molecules have been omitted).
Fig. 5. TGA curve of compound 1 and rehydrated species
Fig. 6. Xꢀray powder diffraction patterns of compound 1, rehydrated species, dehydrated
sample and the simulated patterns using single crystal data of 1.
Fig. 7. N adsorptionꢀdesorption isotherm of compound 1 at 77K (A denotes adsorption,
2
D denotes desorption; 1 stands for pristine compound, 2 for compound after dehydration
(first time), 3 for compound after rehydration, 4 for compound after dehydration (second
time))
Fig. 8. IR spectra of compound 1 before dehydration and after rehydration

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Table 1. Crystal Data and Structure Refinement Parameters of Compound 1
Compound
Formula
1
C H NO Mg
7
5
5
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
207.41
293(2)
monoclinic
P2 /c
1
8.7856(2)
b (Å)
10.2316(2)
c (Å)
8.55530(10)
β
(°)
97.1070(10)
3
Volume (Å )
Z
763.13(2)
4
3
Calculated density (Mg/m )
1.805
ꢀ1
Absorption coefficient (mm )
0.226
F(000)
416
Intervals of reflection indices
Measured reflections
Reflections with [I > 2σ (I)]
Independent reflections
Final R indices [I>2σ(I)]
R indices (all data)
ꢀ11≤h≥10, ꢀ13≤k≥13, ꢀ11≤l≥11
11652
1641
1794
R =0.0285, wR =0.0814
1
2
R =0.0313, wR =0.0842
1
2
Rint
0.0253
2
Goodness of fit on F
1.097
ꢀ3
Largest diff. peak and hole (e. Å )
0.402, ꢀ0.253

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Table 2. Selected Bond Lengths (Ǻ) and Angles (˚) for the Compound 1
bond lengths
bond angles
O1ꢀMg1ꢀO2
Mg1ꢀO1
Mg1ꢀO2
Mg1ꢀO3
Mg1ꢀO4
Mg1ꢀO5
2.0848(11)
2.0132(10)
2.0071(10)
2.0976(9)
87.65(4)
97.48(4)
88.05(4)
170.40(4)
82.60(4)
105.61(4)
166.94(4)
92.25(5)
92.14(4)
87.18(4)
91.79(5)
162.24(4)
89.94(4)
75.07(4)
87.81(4)
O1ꢀMg1ꢀO3
O1ꢀMg1ꢀO4
O1ꢀMg1ꢀO5
a
2.0778(11)
2.2383(10)
O1ꢀMg1ꢀ N3
a
Mg1ꢀ N3
O2ꢀMg1ꢀO3
O2ꢀMg1ꢀO4
O2ꢀMg1ꢀO5
a
O2ꢀMg1ꢀ N3
O3ꢀMg1ꢀO4
O3ꢀMg1ꢀO5
a
O3ꢀMg1ꢀ N3
O4ꢀMg1ꢀO5
a
O4ꢀMg1ꢀ N3
a
O5ꢀMg1ꢀ N3
Symmetry codes: (a) ꢀ1+x, y, z
Table 3. Reactions of Various Aromatic Aldehydes and Ketons Catalyzed by 1 and
i
Calcined Catalyst
entry
ketone
aldehyde
product
isolated yield
wt %)
(
d
d
1
acetone
a) 68, 66
b) 90, 87

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a) 60
b) 85
2
acetone
3
4
acetone
acetone
a) 56
b) 80
a) 48
b) 64
5
acetone
a) 42
b) 58
d
d
6
7
cyclohexanone
cyclohexanone
a) 58, 54
b) 84, 82
a) 53
b) 76

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8
9
cyclohexanone
a) 48
c) 70
cyclohexanone
cyclohexanone
a) 40
b) 55
10
a) 34
b) 46
tertꢀ
a) 18
b) 23
11
butylcyclohexanone
(a), (b), corresponds to the catalytic performance of 1 and calcined catalyst, respectively.
i
Reaction conditions: Aldehyde (2 mmol), acetone/cyclohexanone (10 mmol),
triethylamine (2 mmol), tetrahydrofuran (2 ml) and catalysts (5 mg); temperature = 5ꢀ10
°
C; for dehydrated compound reaction was performed in nitrogen atmosphere. Yields
d
were isolated after 6 h of reaction. Fifth cycle.

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Scheme 1
Fig. 1

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Fig. 2