A magnesium-based multifunctional metal-organic framework: Synthesis, thermally induced structural variation, selective gas adsorption, photoluminescence and heterogeneous catalytic study

Article abstract of DOI:10.1039/c3dt51509e

Three magnesium based carboxylate framework systems were prepared through a temperature-dependent synthesis. The compounds were synthesized by a hydrothermal method and characterized by single crystal X-ray diffraction analysis. A stepwise increase in the

Full text of DOI:10.1039/c3dt51509e

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A magnesium-based multifunctional metal–organic
framework: synthesis, thermally induced structural
variation, selective gas adsorption, photoluminescence
and heterogeneous catalytic study†
Cite this: Dalton Trans., 2013, 42, 13912
Debraj Saha, Tanmoy Maity, Soma Das and Subratanath Koner*
Three magnesium based carboxylate framework systems were prepared through a temperature-depen-
dent synthesis. The compounds were synthesized by a hydrothermal method and characterized by single
crystal X-ray diffraction analysis. A stepwise increase in the temperature of the medium resulted a step-
wise increase in the dimensionality of the network, ultimately leading to the formation of a new 2D
layered alkaline earth metal–organic framework (MOF) compound, {[Mg2(HL)2(H2O)4]·H2O}n (1) (H3L =
pyrazole-3,5-dicarboxylate). Compound 1 selectively adsorbs hydrogen (H2) (ca. 0.56 wt% at 77 K) over
nitrogen at 1 atm and demonstrates a strong blue fluorescent emission band at 480 nm (λmax) upon exci-
tation at 270 nm. Notably, the 2D framework compound efficiently catalyzes the aldol condensation reac-
tions of various aromatic aldehydes with ketones in a heterogeneous medium under environmentally
friendly conditions. The catalyst can be recycled and reused several times without any significant loss of
activity.
Received 7th June 2013,
Accepted 11th July 2013
DOI: 10.1039/c3dt51509e
www.rsc.org/dalton
porous networks containing metal ions throughout the perio-
dic system.6 The ligand H3L (H3L = 3,5-pyrazoledicarboxylic
Introduction
The last few decades have witnessed the proliferation of acid) has six potential donor sites involving the two nitrogen
diverse disciplines of crystal engineering and the design of atoms of the pyrazole ring and the four carboxylate oxygens
advanced multi-functional materials, exploiting the bridging from two carboxylate groups when it is fully deprotonated and
potential of different organic linkers and the geometry of the exhibits
a multiple coordination motif with the metal
metal ions.1 The research in the area of the fabrication of center.4,7 A variety of coordination compounds using this
metal–organic frameworks (MOFs) continues to be intriguing ligand have been synthesized and reported in the literature
by virtue of their unique topology and tunable properties.2 For and the majority of the compounds are concerned with tran-
the desired framework and functionality in a hybrid solid, it is sition metals and rare earth cations.7,8 In comparison to the
of importance to control and understand the external factors, reports on the coordination behavior of other metals, studies
such as temperature, pH and solvent, that govern the crystalli- on alkaline earth metals are scanty.9 Fromm et al. have
zation process and the overall stability of the crystal.3,4 Metal recently reviewed the chemistry of the coordination polymers
carboxylates are particularly interesting because they exhibit of the s-block metal ions.9a The avoidance of using the alka-
open-framework structures and their carboxylate groups act as line earth cations as building blocks arises from their unpre-
a linker between inorganic moieties.5 Dicarboxylates have dictable coordination numbers and geometries, as no ligand
found applications in the diverse field of designing the coordi- field stabilization effects govern their coordinative bonding,
nation polymer arrays, as they are useful building blocks for and their tendency to form solvated metal centers which leads
to the formation of typical alternating [M(H2O)6]L (M = alka-
line earth metal, L = ligand) organic–inorganic ionic layers.10
As a result, the chemistry of the alkaline earth metals has
Department of Chemistry, Jadavpur University, Kolkata 700 032, India.
E-mail: snkoner@chemistry.jdvu.ac.in; Fax: +91 3324146414;
remained largely an unexplored area.
Tel: +91 3324146666 Ext. 2778
†Electronic supplementary information (ESI) available: a table for the intermole-
The lack of intrinsically useful properties (i.e. magnetism or
variable oxidation states) might appear to make alkaline earth
metals less interesting. Nevertheless, the alkaline earth metals
have some advantages for applications in materials science;
they are non-toxic, cheap and generally soluble to aqueous
cular contacts and the solvent effect in the aldol condensation reactions, a plot
for TGA and DTA, the CO2 adsorption isotherm at 273 K, solid state UV-VIS
spectra, 1H NMR of all products of known compounds and the elemental analy-
sis study of the isolated products. CCDC number 943160 for 1. For ESI and crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/c3dt51509e
13912 | Dalton Trans., 2013, 42, 13912–13922
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preparation. For these reasons alkaline earth salts are the pre- was kept over NaA molecular sieves to trap possible traces of
ferred formulations for a host of commercial materials, includ- benzoic acid.
ing many common pharmaceuticals, dyes and pigments.9 In
this context some recent studies concerning alkaline earth
Physical measurements
metal complexes deserve to be mentioned.3j,4,11–16 MOFs Elemental analysis was performed on a Perkin-Elmer 240C
formed by light main-group metals such as Mg2+ and Al3+ may elemental analyzer. The Fourier transformed infrared spectra
play an important role in exhibiting a promising hydrogen of the KBr pellets were measured using a Perkin-Elmer RX I
storage capacity, owing to their low framework density, high FT-IR spectrometer. The powder X-ray diffraction (XRD) pat-
specific surface area and tuneable surface structures on which terns of the sample were recorded with a Scintag XDS-2000
hydrogen molecules can be adsorbed.12 Platero-Prats et al. diffractometer using CuKα radiation at the desired tempera-
studied alkene hydrogenation and ketone hydrosilylation reac- ture. TG analysis was performed on a Perkin-Elmer (SINGA-
tions using alkaline earth sulfonate metal–organic compounds PORE) Pyris Diamond TGA unit. The heating rate was
under homogeneous conditions.13a The same reactions were programmed at 5 °C min−1 with a protecting stream of N2
studied in heterogeneous conditions over an alkaline earth flowing at a rate of 150 ml min−1. The metal content of the
carboxylate metal–organic framework.13b Lin et al. studied the sample was estimated on a Varian Techtron AA-ABQ atomic
Michael addition of α,β-unsaturated γ-butyrolactam using a absorption spectrometer. Gas sorption isotherms were
3,3′-Ph2-BINOL-Mg catalyst which showed a direct asymmetric measured in the pressure range of 0–1 bar using an Autosorb
vinylogous Michael addition with a high yield and an excellent iQ (Quantachorme Inc., USA) gas sorption system. For all iso-
enantioselectivity.14 Brinkmann et al. employed calcium and therms, warm and cold free space correction measurements
strontium amides as precatalysts for the intermolecular hydro- were performed using ultrahigh purity He gas (99.999%
amination of styrenes and dienes.15 Homogeneous catalysts, purity). N2 and H2 isotherms at 77 K were measured in a liquid
however, suffer from the problems of recycling as their sepa- nitrogen bath using UHP-grade gas sources.
ration from the reaction mixture is troublesome. Recycling the
catalyst is a task of great economic and environmental advan- (1) was synthesized through a hydrothermal route. The initial
tage in the chemical and pharmaceutical industries. reaction mixture was prepared as follows: pyrazole-3,5-dicar-
The synthesis of the compound 1. {[Mg2(HL)2(H2O)4]·H2O}n
In the course of our continuing investigation on the cataly- boxylic acid monohydrate (0.087 g, 0.5 mmol) first dissolved in
tic uses of MOFs,4,17 we have successfully employed alkaline 5 ml of milliQ water and the pH of the solution was adjusted
earth metal-based MOFs to catalyze the aldol condensation to 4 by mixing a weak base, sodium azide. To this, magnesium
reaction under heterogeneous conditions in presence of nitrate hexahydrate (0.256 g, 1 mmol) was added and the final
triethylamine.4,17e Recently, we have developed a barium-based mixture was stirred for half an hour. Compound 1 was
MOF which successfully catalyzed the aldol condensation reac- obtained as colorless block crystals in a 20 ml capacity teflon-
tion under heterogeneous base-free conditions.17f The further lined acid digestion bomb, at 210 °C for 3 days followed by
exploration of alkaline earth metal based MOFs afforded a new slow cooling to room temperature. The yield was ca. 63%
2D MOF involving magnesium and 3,5-pyrazoledicarboxylic based on the metal. The mononuclear complex [Mg(HL)-
acid [H3L], namely {[Mg2(HL)2(H2O)4]·H2O}n (1). Herein, we (H2O)4] and its dinuclear congener [Mg(HL)(H2O)3]2 were
wish to report a new approach for the temperature-controlled obtained at 170 °C and 190 °C, respectively. Notably, these two
synthesis of MOFs involving magnesium and H3L. Interest- were converted to 1 when they were treated hydrothermally in
ingly, compound 1 features a layered 2D framework structure a 25 ml teflon-lined Parr acid digestion bomb at 210 °C for 3
which is photoluminescent and capable of adsorbing hydro- days in water. For characterization of the bulk compound,
gen selectively. To best of our knowledge compound 1 is the elemental analyses and IR spectroscopic studies were under-
first example of a magnesium based framework compound taken. Anal. calcd for {[Mg2(HL)2(H2O)4]·H2O}n 1; C = 26.85, H
that catalyzes the aldol condensation reaction under hetero- = 3.15, N = 12.54, found C = 26.8, H = 3.1, N = 12.5. Selected IR
geneous base-free conditions. Notably, 2D layered alkaline peaks (KBr disk, ν, cm−1): 1632, 1556 [νas (CO2−)], 1497, 1460,
earth metal frameworks rarely adsorb hydrogen selectively over 1431 [νs (CO2−)], 1358, 1234 [νs (C–O)], and 3163–3430 s br
nitrogen gas.
[ν (O–H)].
X-ray crystallography
The X-ray diffraction data for 1 were collected at 293(2) K on a
Bruker SMART APEX CCD X-ray diffractometer using graphite-
monochromated MoKα radiation (λ = 0.71073 Å). The determi-
nation of the integrated intensities and cell refinement were
Experimental
Materials
Magnesium nitrate hexahydrate, 3,5-pyrazoledicarboxylic acid performed with the SAINT18 software package using a narrow-
monohydrate, cyclopentanone, acetophenone and substituted frame integration algorithm. An empirical absorption correc-
benzaldehydes were purchased from Aldrich and were used as tion19 (SADABS) was applied. The structure was solved by
received. Other chemicals were purchased from Merck (India). direct methods and refined using the full-matrix least-squares
The liquid aldehydes were distilled before use. Benzaldehyde technique against F2 with anisotropic displacement
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eluent to get the desired product. The products were analyzed
by H NMR spectroscopy and elemental analysis, and the data
were compared with those of the authentic samples.
Table 1 Crystal data and structure refinement parameters of compound 1
1
Compound
1
Formula
Formula weight
C
10H14N4O13Mg2
446.87
Temperature (K)
Crystal system
293(2)
Monoclinic
Results and discussion
Space group
C2/c
The synthesis and the temperature dependent structural
transformation
a (Å)
b (Å)
c (Å)
12.5972(3)
10.0305(3)
13.8663(4)
β (°)
94.483(10)
1746.73(8)
4
1.699
0.219
920
−16 ≤ h ≥ 15, −13 ≤ k ≥ 9,
−16 ≤ l ≥ 18
7243
1886
2025
R1 = 0.0407, wR2 = 0.1086
R1 = 0.0425, wR2 = 0.1105
0.0199
1.143
0.425, −0.673
With much promise for versatility, the H3L ligand contains
three different types of hydrogen atom, one heterocyclic amine
hydrogen atom and two carboxylic protons. The hydrogen of
the carboxylate group adjacent to the imine group is easier to
remove as it forms a stable chelating complex with the metal
center. The hydrogen atom attached to the imine nitrogen dis-
sociates less readily than the carboxylate hydrogen atoms. As
the ligand possesses multidonor coordination sites involving
both nitrogen atoms and carboxylate oxygen atoms, it exhibits
a variety of coordination modes to the bridge metal ions
(Scheme 2). Three different types of coordination mode of the
carboxylate oxygen atoms of H3L can be achieved at pH 4
(keeping other conditions unaltered) by merely varying the
temperature (see ESI; Fig. S1†). A detailed description of the
temperature dependent structural transformation is given
in the ESI.†
Volume (Å3)
Z
Calculated density (Mg m−3
)
Absorption coefficient (mm−1
F(000)
)
Intervals of reflection indices
Measured reflections
Reflections with [I > 2σ(I)]
Independent reflections
Final R indices [I > 2σ(I)]
R indices (all data)
Rint
Goodness of fit on F2
Largest diff. peak and hole (e Å−3
)
parameters for non-hydrogen atoms with the programs
SHELXS97 and SHELXL97.20 The hydrogen atoms were placed
at calculated positions using suitable riding models with iso-
tropic displacement parameters derived from their carrier
atoms. In the final difference Fourier maps there were no
remarkable peaks except the ghost peaks surrounding the
metal centers. A summary of the crystal data and the relevant
refinement parameters for compound 1 is given in Table 1.
X-ray structure of compound 1
Upon the hydrothermal treatment of the mixture of pyrazole-
3,5-dicarboxylic acid monohydrate and magnesium nitrate hexa-
hydrate at 210 °C, the new kind of 2D framework compound
crystallized in the space group C2/c with Z = 4, with one mole-
cule of water of crystallization. The ORTEP diagram with the
atom-numbering scheme is shown in Fig. 1. Selected bond
Catalytic reaction
The catalytic reactions were carried out in a glass batch reactor
according to the following procedure. Ketone (4 mmol), water
(1 ml), tetrahydrofuran (3 ml) and catalyst (5 mg) were placed
in a round bottom flask. This was then placed in an ice-bath
and the temperature was maintained at 5–10 °C. To this solu-
tion, aldehyde (2 mmol) was added and the reaction mixture
was stirred for 6 h (Scheme 1). To isolate the products at the
end of the catalytic reaction, the catalyst was first separated
out by filtration and then the filtrate was extracted four times
with dichloromethane (5 ml). The organic layers thus collected
were first treated with brine solution, dried over anhydrous
Na2SO4, filtered and finally concentrated in a vacuum. The
residue was purified by column chromatography over silica gel
(mesh 60–120) using a n-hexane–ethyl acetate mixture as the
Scheme 1 The aldol condensation reaction catalyzed by compound 1.
13914 | Dalton Trans., 2013, 42, 13912–13922
Scheme 2 The metal coordination modes of pyrazole-3,5-dicarboxylic acid.
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Fig. 1 The ORTEP diagram of compound 1 with 40% ellipsoid probability.
Table 2 Selected bond lengths (Å) and angles (°) for compound 1
Bond lengths
Bond angles
Fig. 2 The segment of compound 1, showing the local coordination environ-
ment of the magnesium(II) ion.
Mg1–O1
Mg1–O2
Mg1–O3
Mg1–O4
Mg1–aO2
Mg1–N6
2.0094(11)
2.0556(10)
2.0308(12)
2.0776(13)
2.1542(11)
2.2094(12)
O1–Mg1–O2
O1–Mg1–O3
O1–Mg1–O4
O1–Mg1–aO2
O1–Mg1–N6
O2–Mg1–O3
O2–Mg1–O4
O2–Mg1–aO2
O2–Mg1–N6
O3–Mg1–O4
O3–Mg1–aO2
O3–Mg1–N6
O4–Mg1–aO2
O4–Mg1–N6
aO2–Mg1–N6
111.42(5)
92.46(5)
83.26(5)
167.16(5)
99.03(5)
88.72(5)
93.98(5)
76.22(4)
149.64(4)
175.54(5)
98.27(4)
92.45(5)
85.84(4)
87.02(5)
73.59(4)
Symmetry codes: (a) 3/2 − x, 1/2 − y, 1 − z.
distances and bond angles are collated in Table 2. The basic
unit of compound 1 consists of an Mg1 center with distorted
octahedral geometry. The basal plane of 1 is generated by a
chelating bidentate pyrazole carboxylate ligand (O2 and N6)
and two oxygen atoms from the other two pyrazole carboxylate
ligands (O1 and O2*, * = 3/2 − x, 1/2 − y, 1 − z), while the
oxygen atom O2 and its symmetry-related O2* bridge two MgII
centers in a μ2-bridging mode leading to the formation of Mg2-
diad (Mg2O2). The remaining two apical positions are occupied
by water molecules (O3 and O4) (Fig. 2). The Mg–O (carboxy-
late) bond distances range from 2.009(11) to 2.154(11) Å which
are in agreement with the Mg–O bond lengths observed in
other magnesium carboxylate complexes.3j,4 Each pyrazole-3,5-
dicarboxylato ligand coordinates to three alkaline earth metal
centers through two carboxylato oxygen atoms and one pyrazo-
late nitrogen atom (Fig. 2) to give rise to a 2D net (Fig. 3). The
Mg2O2 SBUs formed in situ are bridged by organic HL2−
linkers. A topological analysis of compound 1 revealed that
this compound is a 5-nodal 3,4-c 2D net with the Schläfli
Fig. 3 The 2D network of compound 1.
Mg(II) centers with the point symbol {42.6.142.15} (Fig. 4).21 The
uncoordinated water molecules are entrapped within the layer
of the two-dimensional network by hydrogen bonding and
non-bonding attractions (Fig. 5). Additional reinforcements to
the network structure are achieved by intramolecular and
intermolecular hydrogen bonding among the carboxylate
oxygen atoms, pyrazolate imine protons and water molecules
(Fig. 5 and ESI; Table S1†).
Thermogravimetric analysis
symbol {3.142}{3.4.5}2{42.6.142.15}{42.6}, consisting of four The thermogravimetric analysis of 1 was performed using a
different kinds of three-coordinated nodes of HL2− ligands powdered sample in a nitrogen atmosphere. The TG measure-
with the point symbol {3.4.5}, {3.142}, {3.4.5}, {42.6} for C1, C2, ment confirms that compound 1 was thermally stable up to
N1, and O1 respectively and a four-coordinated node of the ∼130 °C ( ESI; Fig. S2†). The TG curve indicates that 1 starts to
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Fig. 4 The topological view of compound 1.
Fig. 6 The H2 and N2 adsorption–desorption isotherm of compound 1 at 77 K.
3 h under vacuum (10−3 Torr). The adsorption and desorption
isotherms of 1 are shown in Fig. 6. The sorption studies con-
firmed that compound 1 demonstrates very interesting selec-
tive adsorption of H2 over N2. The hydrogen uptake of
compound 1 reached about 0.6 wt% at the adsorbate pressure
of 1 atm. This is comparable to the corresponding value of
hydrogen uptake in other similar Mg-based MOFs.21 The
adsorption of these small molecule gases follows the type I iso-
therms. On approaching a sorbate pressure close to 1 atm.
{[Mg2(HL)2(H2O)4]·H2O}n
1 exhibits a significantly higher
Fig. 5 The hydrogen bonded 3D network in compound 1.
amount of hydrogen uptake (ca. 63 cm3 g−1) in comparison to
nitrogen (ca. 6 cm3 g−1). The big difference in the gas uptake
of compound 1 (more than ten folds higher for H2 than N2 at
77 K with the adsorbate pressure of 1 atm) underscores the
potential of using Mg-based MOFs for the selective separation
of H2 over N2. Such a preferential hydrogen uptake over nitro-
gen might be attributed to the small inter layer separations
(2.71→3.60 Å) which permit only H2 molecules (kinetic dia-
meters of 2.89 Å) to enter into the layers, blocking the entrance
for the N2 molecules (kinetic diameters of 3.64 Å). The selec-
tive adsorption of H2 over N2 for microporous MOFs including
alkaline earth metal frameworks has been reported previously
which looks promising for the utilization of these MOFs in gas
separation.22,23 Morris and coworkers evaluated the kinetic
separation of O2 and N2 on Cu(mcbdc) (mcbdc = 5-methoxycar-
bonyl-benzene-1,3-dicarboxylate) that features a 2D structure
containing two different channels with hydrophilic and hydro-
phobic surfaces, respectively.24 It was found that the adsorp-
tion of N2 in this MOF is significantly slower than that of O2,
which was attributed to a kinetic effect. The selective adsorp-
tion of H2 over N2 is another important concern because of its
lose water molecules at ∼130 °C and dehydrates completely at
around 285 °C. The mass loss of ∼20.0% shown in the range
130–285 °C is in agreement with the theoretical value for mass
loss of 20.0%, which corresponds to the loss of one crystalline
and four coordinated water molecules. No clear steps were
observed between the loss of the crystalline and coordinated
water molecules. The corresponding DTA curve of compound 1
shows three endothermic peaks ∼165 °C, ∼227 °C and
∼280 °C. The first endothermic peak seems to correspond to
the loss of the crystalline water molecules, as the coordinated
water molecules bind the metal center more tightly. Thereafter
the TG curve shows a small plateau up to ∼330 °C indicating
that no mass loss occurred in this temperature range. Finally
the compound decomposes steadily showing no clear steps in
the TG curve; the corresponding DTA curve exhibits a small
exothermic peak in this region.
Gas sorption study
Gas sorption measurements were undertaken to understand potential application for H2 enrichment from N2/H2 exhaust
the porous nature of compound 1 and to verify if the com- mixtures in ammonia synthesis.23c,f {[Mg2(HL)2(H2O)4]·H2O}n 1
pound exhibits any preference in adsorption amongst the adsorbs no hydrogen at 298 K and its carbon dioxide uptake is
small molecule gasses. Prior to the sorption experiments, the only ∼8 cm3 g−1 at standard temperature and pressure (STP)
samples were outgassed at the desired temperature (80 °C) for (see ESI; Fig. S3†).
13916 | Dalton Trans., 2013, 42, 13912–13922
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carbon–carbon bond formation reactions.30 Magnesium oxide
nano-particles also have been employed in the catalytic aldol
condensation reaction.18 It was proposed that the surface –OH
and O2− of these oxide crystals are expected to trigger the
carbon–carbon bond formation reactions.31 However, little
attentions have been paid in using magnesium carboxylates in
C–C bond formation reactions due to their limited synthetic
procedure and hygroscopic nature.16 Magnesium pyrazole-3,5-
dicarboxylate with a variety of structures (0D monomer to 3D
polymer) catalyzed the aldol condensation reaction under
homogeneous and heterogeneous conditions in presence of
triethylamine.4 Triethylamine is also to some extent corrosive
which should preferably be avoided. In contrast the catalytic
activity of compound 1 has been studied in the aldol conden-
sation reaction under heterogeneous base-free conditions
(Scheme 1).
Fig. 7 The solid state photoluminescence spectra of H3L and compound 1.
The catalytic reactions were performed in the presence of a
large excess of ketone to prevent the self-condensation of the
aldehyde as well as for the effective use of the aldehyde.32
Reactions were performed in a THF–water mixture medium.
The temperature of the medium was maintained in the range
of 5–10 °C throughout the reaction. With the increasing reac-
tion temperature, the β-aldol product transformed to benzyli-
dene ketone (the condensed product). The catalytic reactions
were performed in different ratios of THF to H2O (see ESI;
Table S2†). The best result was obtained in a 3 : 1 THF–H2O
mixture and the results of the aldol condensation reactions
are summarized in Table 3. In all of the above conditions, the
aldehydes were converted to their respective β-aldols as the
sole product. In this study we noticed that the β-aldol pro-
ducts did not undergo further transformation to form unsatu-
rated carbonyl compounds. Generally, the β-aldol product
undergoes a dehydration to give a conjugated enone in the
aldol condensation reaction.33 The aldol condensation reac-
tion catalyzed by nonporous crystalline magnesium oxide
Photoluminescence study
It is established that the photoluminescence properties of a
luminescent ligand significantly enhance upon complex for-
mation with alkaline earth metals.25 The electronic and
photoluminescence spectra (Fig. 7) of the H3L ligand and
compound 1 are measured in the solid state as well as in
DMSO (see also ESI; Fig. S4–S6†). The ligand displayed a very
strong blue fluorescent emission (λmax = 425 nm in the solid
state and λmax = 350 nm in DMSO) in the solid state as well
as in DMSO upon excitation at 270 nm which is probably
due to the π* → π electronic transition. The shift of the fluo-
rescent emission of H3L in the solid state in comparison to
that in DMSO may be due to the solvent effect. In the case of
compound 1, the emission band was found at 480 nm (λmax
)
with a higher intensity and is red-shifted by ∼55 nm. The
red-shift of the emission spectra observed in compound 1 in
comparison with the corresponding free ligand, may be
attributed to the deprotonation of the ligand in the
complex.26 The photoluminescence properties of the alkaline
earth metals could have potential applications in fluorescent
materials.27
showed
a 75% conversion for p-nitrobenzaldehyde and
acetone in 24 h under heterogeneous conditions.31 The yield
of the β-aldol product decreased from p-nitrobenzaldehyde to
m-nitrobenzaldehyde through o-nitrobenzaldehyde. It may be
realized that the nitro substituent at the ortho and para posi-
tion affords both a negative inductive effect and a negative
Catalytic aldol condensation reactions
Environmentally friendly heterogeneous catalytic processes are mesomeric effect which increases the electrophilicity of the
increasingly receiving attention in the chemical industry. >CvO group of the nitrobenzaldehyde. The substitution at
Designing highly active solid Brønsted-type basic catalysts the meta position affords only the negative inductive effect.
which are capable of catalyzing C–C bond formation reactions Between p-nitrobenzaldehyde and o-nitrobenzaldehyde, more
is a challenge. The main drawbacks of using NaOH or KOH as steric crowding at the ortho position may lead to a lower con-
homogeneous catalysts are separation difficulties, corrosion in version for the ortho variety. In the case of the chloro substi-
the equipment and the generation of a large amount of waste. tuted benzaldehydes, the yield of the β-aldol product
In order to overcome these disadvantages several efforts have decreased from o-chlorobenzaldehyde to p-chlorobenzalde-
been made to arrange new catalytic systems with controlled hyde through m-chlorobenzaldehyde. On the other hand in
basic properties in order to increase the efficacy of the process. the presence of an electron donating group in the ring, such
Garcia et al. have recently reviewed the condensation reactions as methyl, the yield decreases significantly and the lowest
of carbonyl groups when using metal–organic frameworks as conversion was demonstrated. Interestingly in case of
the solid catalyst.28 Among the various magnesium com- p-methoxy-benzaldehyde, the yield was still very good. Presum-
pounds which are used as mediators or catalysts for several ably the methoxy group may interact with Mg2+ and facilitate
organic reactions,29 magnesium oxide is a versatile catalyst for the reaction.34 For acetophenone and cyclopentanone
a
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Table 3 The reactions of various aromatic aldehydes and ketones catalyzed by compound 1a
Entry
1
Ketone
Aldehyde
Major product
Isolated yield (wt%)
TON
Acetone
82, 80b
120, 117b
2
3
4
5
Acetone
Acetone
Acetone
Acetone
77
70
80
74
112
100
109
102
6
Acetone
69
92
7
8
Acetone
Acetone
58
52
84
79
9
Acetone
46
65
10
Acetophenone
78, 75b
116, 114b
11
12
13
14
Acetophenone
Acetophenone
Acetophenone
Acetophenone
71
65
75
70
110
97
106
96
15
Acetophenone
66
87
16
17
Acetophenone
Acetophenone
50
44
79
72
13918 | Dalton Trans., 2013, 42, 13912–13922
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Table 3 (Contd.)
Entry
18
Ketone
Aldehyde
Major product
Isolated yield (wt%)
37
TON
61
Acetophenone
19
Cyclopentanone
75, 74b
112, 109b
20
21
22
23
Cyclopentanone
Cyclopentanone
Cyclopentanone
Cyclopentanone
70
64
74
70
107
96
102
94
24
Cyclopentanone
65
84
25
26
Cyclopentanone
Cyclopentanone
51
45
74
69
27
Cyclopentanone
36
59
a Reaction conditions: aldehyde (2 mmol), ketone (4 mmol), tetrahydrofuran (3 ml), water (1 ml) and catalyst (5 mg); temperature = 5–10 °C.
Yields were isolated after 6 h of reaction. b Fifth cycle.
similar trend in the yield of the β-aldol products were of the reaction mixture thus collected by filtration confirms
observed but overall, the yield decreased from acetone to the absence of magnesium ions in the liquid phase.
cyclopentanone through acetophenone.
To check the stability of the catalyst, we characterized the
To ascertain that the catalysis was indeed heterogeneous, recovered material. After the completion of the catalytic reac-
we performed a hot filtration test. To test if the metal was tion, the solid catalyst was recovered by centrifugation, washed
leached out from the solid catalyst during the reaction, the thoroughly with dichloromethane and dried. The recovered
liquid phase of the reaction mixture was collected by filtration catalyst was then subjected to X-ray powder diffraction analy-
after ∼40% completion of reaction and the residual activity of sis. A comparison of the X-ray diffraction patterns (Fig. 8) of
the supernatant solution after separation from the catalyst was the pristine compound and the recovered catalyst convincingly
studied. The supernatant solution was kept under the reaction demonstrates that the structural integrity of the compound
conditions for another 8 h and the composition of the solution was retained after the aldol condensation reaction.
was analyzed from time to time. No progress of reaction was
For the recycling study, the aldol condensation reactions
observed during this period, which excludes the presence of were performed using p-nitrobenzaldehyde. After the first cycle
the active species in the solution. This result suggests that of reactions, the catalyst was recovered by centrifugation. The
there was no leaching of Mg from the solid catalyst during the recovered catalyst was then washed several times with dichloro-
reaction. In addition, an atomic absorption spectrometric ana- methane and dried under vacuum. The performance of the
lysis (sensitivity up to 0.001 ppm) of the supernatant solution recycled catalyst in the C–C coupling reactions in up to five
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Acknowledgements
Financial support from CSIR, New Delhi by a grant (grant no.
01(2542)/11/EMR-II) (to S. K.) is gratefully acknowledged. D. S.
thanks CSIR (Ref. no. 09/096(782)2013-EMR-I) for a Senior
Research Fellowship. We also thank DST for funding the
Department of Chemistry, Jadavpur University to procure
single crystal X-ray and NMR facility under the DST-FIST
programme.
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Conclusion
In summary we have synthesized a new two-dimensional alka-
line earth metal–organic framework (MOF) compound,
{[Mg2(HL)2(H2O)4]·H2O}n (1) through a hydrothermal route.
Synthetically we are able to correlate this framework with two
low dimensional compounds by tuning the temperature of the
medium. 1 exhibits very good solid state photoluminescence
properties at room temperature and a selective hydrogen-sorp-
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significant loss of activity. Further investigations regarding the
application of the alkaline earth metal based frameworks
towards gas sorption and catalytic reactions are currently
being investigated in our laboratory.
13920 | Dalton Trans., 2013, 42, 13912–13922
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