Indian Journal of Chemistry
Vol. 52A, June 2013, pp. 749-752
bis(
Rajarshi Chatterjee a
, Dipak K Hazra b
, Monika Mukherjee b
M Nethaji c
& Mahammad Ali
a,
* a Department of Chemistry, Jadavpur University,
Kolkata 700 032, India
Email: mali@chemistry.jdvu.ac.in b
Department of Solid State Physics, Indian Association for the
Cultivation of Science, Jadavpur, Kolkata 700 032, India c
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
Received 11 July 2012; re-revised and accepted 8 May 2013
Hydrothermal synthesis and structural characterization of a new organic polyoxometalate, namely, bis( 4,4 ′ -bipyridinium) dodecatungstosilicate dihydrate, [4,4 ′ -bipyH
2
]
2
[SiW
12
O
40
].2H
2
O (1) is reported. The crystal structure of (1) consists of a [SiW
12
O
40
]
4-
Keggin anion situated on a two-fold axis, two unique
4,4 ′ -bipyridinium cations, one of which is situated on a
two-fold axis, and two independent lattice water molecules.
The cations and anions are linked by H-bonding.
Keywords: Hydrothermal synthesis, Organic polyoxometalates,
Polyoxometalates, Dodecatungstosilicate, Tungsten
Polyoxometalates (POMs), the discrete, anionic metal-oxide clusters are one of the most widely used inorganic components in building hybrid compounds with potential applications in catalysis, biochemical analysis, medicinal chemistry, and materials science
1-5
.
An interesting aspect of POM chemistry is its underlying pseudo-simple synthetic procedure
6
.
This is because majority of POM clusters isolated so far are synthesized by a range of surprisingly straightforward synthetic procedures, which are usually referred as “one-pot” synthesis. From a crystal engineering point of view, the fact that these POMs are poly-anions and cannot exist without the charge balancing cations, which often define the network into which the anion is “complexed” and electroneutrality is achieved, is the most important factor that affects the formation of a particular POM species. One of the well known POM is the Keggin anion [XM
12
O
40
] n−
, which is made up of a central XO
4
(X=Si, Co etc.) tetrahedron, whose vertices are occupied by four
M
3
O
13
(M=W, Mo, V, etc.) trimetallic clusters.
For X=Si and M=W, the POM is known as dodecatungstosilicate anion [SiW
12
O
40
]
4-
. This ion is structurally flexible as evidenced by its characterization with a variety of counter cations
7-18
. In the present work, we have hydrothermally synthesized a new
Keggin POM, viz., [4,4 ′ -bipyH
2
]
2
[SiW
12
O
40
].2H
2
O (1) by a one-pot reaction from simple components, the details of which are described in this report.
Experimental
All chemicals used for the synthesis were of analytical grade. Na
2
WO
4
(AR Loba, India),
Na
2
SiO
3
.5H
2
O (AR Loba, India) and 4,4 ′ -bipyridine
(Merck, India) were used as received without further purification. Deionized water was used as the solvent. Elemental analysis was carried out on a Perkin–Elmer 240 elemental analyzer. Fourier transform infrared spectra (4000–400 cm
-1
) of complexes (1) was recorded on a Perkin–Elmer RX I
FT-IR system using KBr pellets. TG analysis was performed on a Perkin–Elmer Pyris Diamond
TGA/DTA Analyser instrument in flowing N
2
with a heating rate of 10 o
C min
-1
.
An aqueous solution was made by dissolving 0.329 g
(1 mmol) of Na
2
WO
4
.2H
2
O and 0.212 g (1 mmol)
Na
2
SiO
3
5H
2
O in 25 mL of water. Then, 0.5 mL of 6 M
HNO
3
was added dropwise to maintain pH at 4, which was followed by the addition of 0.156 g (1 mmol) of solid 4,4 ′ -bipyridyl under vigorous stirring.
The resulting solution was stirred as well as heated up to 80 °C for another half an hour. The entire mixture was then transferred into a teflon jacket in PTTE-lined stainless steel pressure vessel and was kept in oven at 160 °C for five consecutive days under autogeneous pressure. The solution was cooled by decreasing the temperature at a regular interval of 5 °C for one day. Yellow rectangular crystals were obtained along with major crystalline powder. The crystals were picked out, washed with distilled water, and air-dried.
(yield: 30%). The product isolated in crystalline form was a minor product. The composition of the product with respect to the atom% of W and
Si were estimated by SEM-EDX (JEOL, model
JSM-6700F) and found to be 12.6:1 which satisfies the required value of 12:1. Anal. (%): Found: C, 7.46;

750 INDIAN J CHEM, SEC A, JUNE 2013
H, 0.64; N, 1.53; Si, 0.81; W, 68.28. Calc. based on
C
20
H
24
N
4
SiW
12
O
42
: C, 7.43; H, 0.74; N, 1.73; Si, 0.86;
W, 68.38.
X-ray diffraction data for (1) were collected at 293(2) K on a Bruker Smart Apex CCD X-ray diffractometer using graphite-monochromated Mo-K
α radiation (
SAINT
19
λ correction (SADABS)
19
The oxygen atoms (O1, O11, O1W and O2W) and some atoms (C9, C11, C14, N2 and N4) of the organic ligands were restrained using the ISOR option in
SHELXL during anisotropic refinement. All hydrogen
=1.5418Å). Integrated intensities were determined and cell refinement was performed the
software package using a narrow-frame integration algorithm. An empirical absorption
was applied. The structure was solved by direct methods and refined using fullmatrix least-squares technique on F
2
with anisotropic displacement parameters for non-hydrogen atoms by the programs SHELXS97 and SHELXL97
20
.
Results and discussion
The IR spectrum of compound (1) shows absorption peaks between 1100 and 780 cm
-1
, which are characteristic of Keggin units (Supplementary data,
Fig. S1). The characteristic peaks at 922, 973,
888 and 787 cm
-1
are attributed to ν (Si–O a
), ν (W–O t
),
ν (W–O b
–W), ν (W–O c
–W) of the [SiW
12
O
40
]
4polyanion, respectively. Compared to those of
α -[H
4
SiW
12
O
40
]. nH
2
O
21
, all the peaks experience a slight shift to higher wave numbers. This may be due to the interaction between the polyanion and the organo-nitrogen coordination cations. In addition, bands at 1596, 1517, 1490 and 1414 cm
-1
are assigned to the characteristic vibrations of 4,4'-bipyridine.
In addition, peaks observed above 3000 cm
-1
are assigned to ν (−OH) absorption along with the hydrogen bonds which proves the presence of lattice water. atoms were placed at the calculated positions using suitable riding models with isotropic displacement parameters derived from their carrier atoms. In the final difference Fourier maps, there
For determining the thermal stability, the samples were placed in the sample container and heated from 30-70 °C at 10 °C/min while measuring the weight loss as a function of temperature in a nitrogen were no remarkable peaks except the ghost peaks surrounding the metal atoms. A summary of crystal data and relevant refinement parameters for complex (1) is provided in Table 1.
atmosphere. The TG curve (Supplementary data,
Fig. S2) exhibits three steps of weight losses. The first weight loss is 0.99% in the temperature range of
200-250 o
C, corresponding to the release of the two non-coordinated water molecules (calcd 1.1%). o
The second weight loss is 9.5% from 250-520 C,
Table 1 – Crystal data and structure refinement for [4,4 ′ -bipyH
2
]
2
[SiW
12
O
40
].2H
2
O (1)
Empirical formula
Formula wt.
C
20
H
24
N
4
O
42
SiW
12
3226.72 corresponding to the release of two free 4,4'-bipy molecules (calcd 9.7%). This high temperature range reflects the strong electrostatic interaction between the bipyridinium cation and silicotungstate anion.
Temp. (K)
Wavelength (Å)
Crystal system
Space group
293(2)
1.5418
Monoclinic
C2/c
The third weight loss is due to decomposition of dodecatungstosilicate ion.
The crystal structure of (1) is composed of a
Keggin-type [SiW
12
O
40
]
4-
anion, two 4,4' bipyridinium
a, b, c(Å)
α ,
β
, γ (
°
)
Vol. (Å
3
)
Z
Calc. density (m g
-3
)
15.1051(6), 18.2013(7), 21.3445(7)
90.0, 103.05(1), 90.00
5716.8(4)
4
3.749 counter cations and two water molecules of crystallization (Fig. 1). This is the α form of Keggin POM, of T d symmetry, in which all the metal centres are equivalent. The oxygen atoms of the solvent
F(000)
θ range for data collection (°) 1.78—25.00
Limiting indices
5640
-17 ≤ h ≤ 12, -21 ≤ k ≤ 19, -250 ≤ l ≤ 24 water molecules are disordered between the two centro-symmetrically related positions and modeled over two positions with site occupancy 0.5. The dodecatungstosilicate [SiW
12
O
40
]
4-
consists of four
Reflections collected/unique 21378 / 5009 [R(int) = 0.0769
Completeness to 2 θ (%)
Refinement method
99.7
Full-matrix least-squares on F
2
Data/restraints/parameters 5009/60/423
Goodness-of-fit on F 2 1.023 corner-sharing trinuclear W
3
O
13
units, each of which is made up of three edge-sharing WO
6 octahedra around the central SiO
4
tetrahedron (Supplementary data, Fig. S3). The oxygen atoms are in different coordination environment, i.e., the unshared or
Final R indices [I>2 σ (I)] R
1
= 0.0490, wR
2
= 0.1134
R indices (all data) R
1
= 0.0628, wR
2
= 0.1212 terminal O atoms ‘O t
’, the bridging O atoms (O b
) connecting two W atoms, and the O atoms (O c
) of the

W1 - O1
W1 - O18
W2 - O8
W3 - O2
W3 - O7
W4 - O5
W5 - O1
W5 - O20
W6 - O12
Si1 - O1
O1-Si1-O2
*-x, y, -z+3/2
NOTES
Table 2 – Selected bond lengths (Å) and angles (º) with esd in parentheses for [4,4 ′ -bipyH
2
]
2
[SiW
12
O
40
].2H
2
O (1)
2.352(9)
1.906(10)
1.928(10)
2.327(9)
1.687(11)
1.684(11)
2.350(9)
1.697(11)
1.896(10)
1.629(10)
109.5(4)
W1 - O15
W1 - O19
W2 - O9
W3 - O3
W3 - O14
W4 - O8
W5 - O10
W5 - O19*
W6 - O13
Si1 - O2
1.683(12)
1.906(9)
1.675(10)
1.904(9)
1.901(10)
1.906(9)
1.889(10)
1.908(9)
1.701(11)
1.614(9)
W1 - O16
W2 - O2
W2 - O10
W3 - O4
W4 - O2
W4 - O12
W5 - O11
W6 - O1
W6 - O17
Si1 - O1*
O2-Si1-O1*
1.934(10)
2.359(9)
1.909(10)
1.940(10)
2.346(9)
1.903(10)
1.927(10)
2.317(9)
1.904(10)
1.629(10)
109.1(5)
W1 - O17
W2 - O3
W2 - O18*
W3 - O6
W4 - O4
W4 - O6*
W5 - O16
W6 - O11
W6 - O14*
Si1 - O2*
751
1.916(10)
1.903(9)
1.901(10)
1.908(10)
1.915(10)
1.925(10)
1.907(10)
1.892(10)
1.918(10)
1.614(9)
Fig. 1 – Crystal structure of [4,4 ′ -bipyH
2
]
2
[SiW
12
O
40
].2H
2
O (1) showing the atom labeling scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as circles of arbitrary radius. For clarity the disordered water molecules are not shown. [Symmetry code: (i) -x, y, 1.5-z;
(ii) 1-x, y, 1.5-z]. central SiO
4
moiety. The W-O t
bond lengths vary between 1.676(10)-1.702(10)Å, while the W-O b
,
W-O c
bond distances lie in the range 1.890(10)–
1.940(10)Å and 2.318(9)-2.359(9)Å, respectively.
In the SiO
4
tetrahedron, the Si-O bond lengths are in the range 1.615(9)-1.629(9) Å and the O-Si-O angles range from 109.1(5) to 109.8(7)º (Table 2).
This is consistent with the structural features of polyoxotungstates exhibiting Keggin structures
22,23
.
The crystal packing in (1) is stabilized by a combination of intermolecular N−H···O and C−H···O hydrogen bonds (Supplementary data, Table S1).
The organic cations and POM anions are linked by several H-bonding interactions (Supplementary data, Fig. S4). In summary, an organic Keggin POM has been synthesized through one pot reaction from simple components and structurally characterized.
Supplementary data
Crystallographic data for the structures reported in this paper have been deposited with the Cambridge
Crystallographic Data center under the deposition number CCDC 717892. Copies of the information may be obtained free of charge from the
Director, CCDC, 12 Union Road, Cambridge,
CB2 IEZ, UK. Fax: +44-1223-336033; Email: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
Other Supplementary data associated with this article, viz.; Figs S1-S4, and Table S1, are available in the electronic form at http://www.niscair.res.in/jinfo/ ijca/IJCA_52A(06)749-752_SupplData.pdf.
Acknowledgement
Financial supports from UGC [Project ref. no. F.39-
735/2010(SR)], New Delhi is gratefully acknowledged.
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