pH-Dependent Assembly of Metal–Organic Frameworks
FULL PAPER
24.1, H 4.1, N 11.4. For [Mg(HL)ACHTNUGTRENNUNG
coverable by filtration, and could be reused several times
without any loss of catalytic activity. The reactants were
converted into their respective products in high yield and
with greater selectivity in a short time by virtue of the high
surface area of the catalyst. Compounds 1 and 2 also cata-
lyze aldol condensation reactions between aromatic alde-
hydes and acetone in tetrahydrofuran under homogeneous
conditions. The results of aldol condensation reactions using
compounds 1 and 2 are given in Table S3 (see the Support-
ing Information). In contrast, compounds 3 and 4 do not cat-
alyze the aldol condensation reaction.
À
disk): n˜ =1644, 1602 (nas; CO2À), 1407 (ns; CO2 ), 1346, 1182 (ns; C O),
À
3600–3200 cmÀ1 (br; O H); elemental analysis calcd (%): C 25.81, H
À
3.44, N 12.04; found: C 26.1, H 3.5, N 12.4. For [Mg
A
U
selected IR peaks (KBr disk): n˜ =1685, 1613 (nas; CO2À), 1479 (ns; CO2À),
À1
À
À
1366, 1275 (ns; C O), 3600–3200 cm (br; O H); elemental analysis
calcd (%): C 30.88, H 2.57, N 14.41; found: C 31.2, H 2.8, N 14.8. For
{[Mg3(L)2ACHTNUGTRENNUG(H2O)2] (H3L)·2HCATUNTGREN(NUGN H2O)}n (4): selected IR peaks (KBr disk): n˜ =
1658, 1592 (nas; CO2À), 1459 (ns; CO2 ), 1346, 1284 (ns; C O), 3600–
À
À
3200 cmÀ1 (br; O H); elemental analysis calcd (%): C 27.98, H 2.17, N
À
13.05; found: C 28.4, H 2.5, N 13.6. For {[Mg3(L)(OH)
3A
selected IR peaks (KBr disk): n˜ =1653, 1578 (nas; CO2À), 1490 (ns; CO2À),
À1
À
À
1350, 1230 (ns; C O), 3175–3423 cm (br; O H); elemental analysis
calcd (%): C 18.12, H 3.02, N 8.45; found: C 18.5, H 3.2, N 8.6.
X-ray crystallography: X-ray diffraction data for 1–5 were collected at
293(2) K on a Bruker SMART APEX CCD X-ray diffractometer using
graphite-monochromated MoKa radiation (l=0.71073 ꢁ). Integrated in-
tensities were determined and the cell was refined with the SAINT[25]
software package using a narrow-frame integration algorithm. An empiri-
cal absorption correction[26] (SADABS) was applied. All of the structures
were solved by direct methods and refined against F2 using full-matrix
least-squares techniques with anisotropic displacement parameters for
non-hydrogen atoms with the programs SHELXS97 and SHELXL97.[27]
Data for compound 2 were not of good quality and there was some disor-
der in the positions the water oxygen atoms, so the hydrogen atoms of
the water of crystallization were not included in the refinement for this
compound. For the other compounds, hydrogen atoms were placed in cal-
culated positions using suitable riding models with isotropic displacement
parameters derived from their carrier atoms. There were no remarkable
peaks in the final difference Fourier maps, except for ghost peaks sur-
rounding the metal centers in each of the compounds. A summary of
crystal data and relevant refinement parameters for complexes 1–5 is
given in Table S4 (see the Supporting Information). CCDC-821016 (1),
CCDC-821017 (2), CCDC-795755 (3), CCDC-821018 (4), and CCDC-
795756 (5) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Conclusion
We have successfully developed a new series of MOF sys-
tems, among which we tactically increased the dimensionali-
ty from 0D to 3D merely by tuning the pH of the medium.
The pH of the medium plays a vital role in the construction
of framework materials of the desired dimensionality. The
porous three-dimensional compound {[Mg3(L)(OH)3-
ACHTUNGTRENNUNG(H2O)2]·H2O}n displays excellent catalytic activity in aldol
condensation reactions of various aromatic aldehydes, giving
remarkable yields with high selectivities within short reac-
tion times under heterogeneous conditions. Notably, the cat-
alyst can be recycled and reused several times without any
loss of activity.
Experimental Section
Catalytic reactions: The catalytic reactions were carried out in a glass
batch reactor according to the following procedure. Acetone (580 mg,
10 mmol), triethylamine (2 mmol), tetrahydrofuran (2 mL), and catalyst
(5 mg) were placed in a round-bottomed flask. The flask was then placed
in an ice-bath, maintaining the temperature at 5–108C. The requisite al-
dehyde (2 mmol) was then added to the solution and the reaction mix-
ture was stirred for 6 h. After the reaction, the catalyst was separated by
filtration and the product was purified by column chromatography on
silica gel (60–120 mesh) eluting with n-hexane/ethyl acetate. The product
was analyzed by 1H NMR spectroscopy and elemental analysis, and the
data were compared with those of authentic samples.
Materials and physical measurements: Magnesium nitrate hexahydrate,
pyrazole-3,5-dicarboxylic acid, sodium azide, and substituted benzalde-
hydes 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 possible traces of benzoic acid. Ele-
mental analysis was performed on a Perkin–Elmer 240C elemental ana-
lyzer. Fourier-transform infrared spectra of samples in KBr pellets were
measured on a Perkin–Elmer RX I FTIR spectrometer. Powder X-ray
diffraction (XRD) patterns of the samples were recorded with a Scintag
XDS-2000 diffractometer using CuKa radiation. TG/DT analyses were
performed on a Perkin–Elmer (Singapore) Pyris Diamond TG/DTA unit.
The heating rate was programmed at 58CminÀ1 with a protecting stream
of N2 flowing at a rate of 150 mLminÀ1. N2 sorption measurements were
performed on an Autosorb iQ (Quantachrome Inc., USA) gas sorption
system at 77 K. Samples were degassed at 708C for 3 h under vacuum
(10À3 Torr) prior to nitrogen sorption measurements.
Acknowledgements
We acknowledge the Department of Science and Technology (DST) of
the Government of India for funding a project (to S.K.) (SR/S1/IC-01/
2009). We also thank the DST for funding the Department of Chemistry,
Jadavpur University, to procure a single-crystal X-ray facility under the
DST-FIST programme.
Synthesis of the compounds: All of the compounds were synthesized by
the hydrothermal route. The compounds were obtained as colorless
block-shaped crystals after heating in a Teflon-lined Parr acid digestion
bomb at 1708C for 3 days followed by slow cooling at a rate of 58ChÀ1 to
room temperature. The initial reaction mixture was prepared by mixing
magnesium nitrate and pyrazole-3,5-dicarboxylic acid in a 2:1 ratio in
water. The pH of the reaction mixture was adjusted by changing the
molar proportion of the weak base sodium azide. The pH of the medium
was thereby adjusted to 4, 5, 6, 7, and 8 to obtain monomer, dimer, 1D,
2D, and 3D framework compounds, respectively. The bulk compounds
were characterized by elemental analysis and IR spectroscopic studies.
[1] a) J.-M. Lehn, Supramolecular Chemistry, VCH, Weinheim, 1995;
Metals in Supramolecular Chemistry, ASI Kluwer Academic Publish-
ers, Dordrecht, 1994; d) F. Vogtle, Supramolecular Chemistry, Wiley,
1991; e) P. R. Ashton, D. Philip, N. Spencer, J. F. Stoddart, J. Chem.
For [Mg(HL)ACHTUNGTRENNUNG(H2O)4] (1): selected IR peaks (KBr disk): n˜ =1674, 1612
À
À1
(nas; CO2À), 1479 (ns; CO2 ), 1356, 1274 (ns; C O), 3600–3200 cm (br;
À
À
O H); elemental analysis calcd (%): C 23.96, H 3.99, N 11.18; found: C
Chem. Eur. J. 2012, 18, 5979 – 5986
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5985