Hydrothermal synthesis of two supramolecular inorganic-organic hybrid phosphomolybdates based on Ni(II) and Co(II) ions: Structural diversity and heterogeneous catalytic activities

Article abstract of DOI:10.1039/c6nj00440g

Two new hybrid compounds based on phosphomolybdates, namely (NHEPH₂)₅[Ni^II(P₄Mo₆O₃₁)₂]·6H₂O (1) and {(NHEPH₂)₂[Co^II(H₂O)

Full text of DOI:10.1039/c6nj00440g

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Hydrothermal synthesis of two supramolecular
inorganic–organic hybrid phosphomolybdates
based on Ni(II) and Co(II) ions: structural diversity
and heterogeneous catalytic activities†
Cite this:DOI: 10.1039/c6nj00440g
a
a
b
a
Luna Paul, Malay Dolai, Anangamohan Panja and Mahammad Ali*
II
Two new hybrid compounds based on phosphomolybdates, namely (NHEPH
2
)
5
[Ni (P
4
Mo
6
O
31
)
2
]Á6H
2
O (1) and
II
{
(NHEPH
2
)
2
[Co (H
2
O)
6
]}[P
2
Mo
5
O
23]Á2H
2
O (2) (NHEP = N-(2-hydroxyethyl)-piperazine), have been synthesized
under hydrothermal conditions and characterized by elemental analyses, IR, TGA, single crystal X-ray
6
À
diffraction and PXRD studies. In compound 1, Ni(II) is sandwiched between two {P
4
Mo
6
O
31
}
clusters and
2+
6À
the residual negative charges are neutralized by NHEPH
2
. In compound 2, the [P Mo
2
5
O
23
]
ion exists
2+
Received (in Montpellier, France)
2+
9th February 2016,
as a discrete moiety and the negative charges are compensated by both [Co(H
2
O)
6
]
and NHEPH
2
. The
Accepted 6th June 2016
extensive hydrogen bonding involving the organic cations and water molecules yielded three-dimensional
3D) open framework structures in these systems. These phosphomolybdate based compounds showed
good catalytic efficiency in heterogeneous oxidation of styrene in the presence of an environmentally
benign oxidant, H , under mild conditions.
(
DOI: 10.1039/c6nj00440g
www.rsc.org/njc
2 2
O
4
5
6
including discrete clusters, one-dimensional chains, and two-,
Introduction
7
and three-dimensional frameworks.
1
Polyoxometalates (POMs) of early transition metal oxide clusters
Among the different POM clusters, phosphorus containing
have attracted great interest in the solid-state materials chemi- POMs constitute a remarkable family with diverse structures
stry, due to their structural and compositional diversity, and and applications. In all phosphomolybdates, the compounds
4 6 2 5
great potential toward applications in catalysis, sorption, ion constructed from {P Mo } and {P Mo } as the basic building
2
exchange, optical, electro- and magnetic materials. Recently, an units are structurally very diverse and have extensive application
intriguing area in this field has been the construction of prospects. However, the design and synthesis of such compounds
organic–inorganic hybrid materials based on POMs. Several pose a challenge due to the following facts: (i) most of the syntheses
successful strategies have been developed to design such involve one-pot reactions of negatively charged polyoxometalate
3
materials. One of the promising methods is to connect POM clusters with positively charged cations or structure directing
building units with secondary transition metal complexes acting agents; and (ii) under such conditions difficulties arise as the
as inorganic bridging groups via covalent bonds under hydro- reaction proceeds in a complex pathway. However, the charge
thermal or solvothermal conditions. These reaction conditions balancing cations assume the major role in the assembly of the
help in increasing the solubility of inorganic and organic mate- building blocks leading to the formation of the bulk material,
rials to form hybrid compounds. The transition metal complexes which can be manipulated to tailor the assembly process.
can dramatically influence the inorganic oxide microstructure.
Lately, the concept of secondary building units (SBUs) has
Therefore, one can combine the merit of POMs and transition been widely used by Yaghi, Williams, Kitagawa, and co-workers
metal complexes to constitute some interesting compounds with for understanding and predicting topologies of structures and
unique properties. Many examples have been explored recently, it has been proved to be helpful in directing the construction of
8
–13
desired structures.
4 6
The basic building unit like {P Mo } can
also be further assembled via a transition metal cation which is
a
b
n+
(12Àn)À 14
6 2
Department of Chemistry Jadavpur University, Kolkata 700 032, India.
E-mail: mali@chemistry.jdvu.ac.in; Fax: +91-33-2414-6223
sandwiched between two P Mo units to form M [P Mo ]
.
4
6
4
Again, the construction of a simple metal–organic framework
system can also be achieved by using simple transition metal
cations and/or organic amine cations to balance the charge on
the POM units. In this paper, we report two inorganic–organic
hybrid materials formed from phosphomolybdates, namely
Postgraduate Department of Chemistry, Panskura Banamali College, Panskura RS,
Purba Medinipur, West Bengal 721 152, India
†
Electronic supplementary information (ESI) available. CCDC 1431341 and
431342. For ESI and crystallographic data in CIF or other electronic format
1
see DOI: 10.1039/c6nj00440g
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II
II
(
NHEPH
Mo
hydroxyethyl)-piperazine) derived under hydrothermal conditions. in the 2y range of 3–50 using a LynxEye detector (1D mode) with
2
)
5
[Ni (P
4
Mo
6
O
31
)
2
]Á6H
2
O (1) and {(NHEPH
2
)
2
-[Co (H
2
O)
6
]}- X-ray diffractometer using Cu-K
a
radiation (l = 1.5418 Å)
2
+
[P
2
5
O
23]Á2H O (2) (NHEPH
2
2
, doubly protonated N-(2- operating at 40 kV and 40 mA. The XRD patterns were recorded
2+
The structure directing role of NHEPH2 is quite noteworthy as a step size of 0.02 and a dwell time of 1 s per step.
the heteroatom (O and N) donor sites not only participate in the
charge compensation but are also involved in strong hydrogen
bonding interactions resulting in the construction of 3D (three-
dimensional) open network structures. Interestingly, compound 1
has a sandwiched structure in which the Ni(II) ion is trapped
Single-crystal X-ray diffractions
Single-crystal X-ray data of complexes 1 and 2 were collected at
293 K on a Bruker SMART APEX-II CCD diffractometer using
6
À
graphite monochromated Mo-K radiation (l = 0.71073 Å). Data
between two {P
4
Mo
6 31
O }
clusters, while in complex 2, the Co(II)
a
2+
collection, reduction, structure solution, and refinement were
performed using the Bruker APEX-II suite (v2.0-2) program. All
available reflections to 2ymax were harvested and corrected for
Lorentz and polarization factors using Bruker SAINT plus.
Reflections were then corrected for absorption; inter-frame
ion is present as a discrete [Co(H O) ] complex cation, compen-
sating the charge of phosphomolybdates, {P Mo O } , together
with the NHEPH2 cation. Efficient heterogeneous catalytic oxi-
dation of styrene in the presence of an environmentally benign
2
6
6À
2
5 23
2+
1
5
2 2
oxidant, H O , under mild conditions has also been investigated.
1
6
scaling, and other systematic errors using SADABS. The
structures were solved by direct methods and refined by means
2
Experimental section
Materials and reagents
of full-matrix least-squares technique based on F using
16
SHELX-97 software package. During the refinement of 1 and 2,
non-hydrogen atoms were anisotropically refined; however, for a
Chemicals Na
2
MoO
4
, Co(NO
3
)
2
Á6H
2
O and NiCl
2
Á6H
2
O, NaCl,
2+
2
disordered cation NHEPH
sitting on an inversion centre in
H
3
PO were obtained from Merck, India. N-(2-Hydroxyethyl)-
4
compound 1 the ADP restraint was applied due to its unreasonable
anisotropic Ueq parameters. All of the hydrogen atoms belonging to
carbon atoms were placed in their geometrically idealized positions
and were isotropically treated based on their respective parent
atoms. Although the H atoms attached to heteroatoms in the
piperazine was purchased from Riedel-de Haen. All these chemicals
were of reagent grade and used as received. De-ionized water was
used as the solvent.
Physical measurements
2+
NHEPH2 cation and water molecules were located in the differ-
Elemental analyses were carried out using a Perkin-Elmer 240 ence Fourier maps in compound 2, H atoms on water molecules
elemental analyzer. The FTIR spectra of the compounds were in 1 cannot be found from the residual peaks and were just
recorded using KBr pellets at ambient temperatures in the directly included into the final molecular formula, as the short
À1
range 4000–400 cm on a Nicolet Magna IR 750 series-II FTIR
O
waterÁÁÁOPOM and OwaterÁÁÁOwater distances (2.5–2.8 Å) implied the
spectrophotometer. Thermo gravimetric analyses (TGA) were presence of extensive H-bonding interactions in the solid state in 1.
performed on a Perkin-Elmer Pyris Diamond TGA/DTA analyzer The drawings of molecules were generated using the DIAMOND-3.0
À1
in flowing N at a heating rate of 12 1C min . The powder X-ray and Mercury 2.4 programs. The method of crystallographic data
2
diffraction (PXRD) data were registered on a Bruker D8 Advance collection and structural detail are listed in Table 1.
Table 1 Crystallographic data and details of the structure determination
Formula
Formula weight
Crystal system
Space group
a [Å]
b
c
C
30
H
64Mo12
N
10NiO73
P
8
12 5 4 35 2
C H52CoMo N O P
3190.64
Triclinic
P1% (no. 2)
1413.15
Monoclinic
C2/c (no. 15)
9.7440(11)
28.086(4)
18.685(3)
90
126.780(5)
90
4095.6(12)
4
13.244(5)
13.546(5)
14.691(5)
74.100(5)
69.748(5)
65.588(5)
2225.0(14)
1
a [1]
b
g
3
V [Å ]
Z
À3
D(calc.) [g cm
]
2.381
2.292
m(MoK
F(000)
a
) [mm]
2.104
1550
2.072
2796
Temperature (K)
Radiation [Å] MoK
y min–max [1]
Dataset
Tot., uniq. data, R(int)
Observed data [I 4 2.0 s(I)]
293
0.71073
1.5, 27.2
À16: 16; À17: 17; À18: 18
35 849, 9812, 0.028
9188
293
0.71073
1.8, 32.6
À42: 42; À14: 14; À27: 27
29 665, 6300, 0.075
4412
a
N
R, wR
ref, Npar
9812, 630
0.0318, 0.0912, 1.05
À1.89, 2.27
6300, 311
0.0619, 0.1545, 1.08
À1.81, 2.41
2
, S
À3
Min. and max. resd. dens. [e Å
]
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Syntheses of complexes
crystals were obtained, filtered, washed several times with
distilled water and dried in air. Yield, 32% (based on Mo).
Elemental analysis: anal. calc. value for molecular formula,
C H Mo N NiO P (MW = 3190.64): C, 11.29; H, 2.02; N,
II
(
NHEPH
2
)
5
[Ni (P
4
Mo
6
O
31
)
2
]Á6H
2 2 4
O (1). Na MoO (1.20 g,
5
mmol) was dissolved in 20 mL of 1.0 M NaCl solution. The
3
0
64
12 10
73 8
pH was adjusted to 3.5 by adding 4.0 M nitric acid. The
colourless solution was heated to 80 1C and stirred for 30 min.
N-(2-Hydroxyethyl)piperazine (NHEP) (0.703 g, 5.4 mmol) and
H PO (0.25 g, 2.55 mmol, 85%) were added slowly to the hot
4
.39; found C, 11.34; H, 2.05; N, 4.46.
II
{
(NHEPH
was prepared following a very similar procedure as described
in 1 using Co(NO O (0.308 g, 1.04 mmol) instead of
Á6H
NiCl O. Deep pink block shaped crystals were obtained
Á6H
2
)
2
[Co (H
2
O)
6
]}[P
2
Mo
5
O23]Á2H
2
O (2). Compound 2
3
4
3
)
2
2
solution while stirring and the mixture became blue in colour.
2
2
After 15 min, 5 mL aqueous solution of NiCl Á6H O (0.427 g,
2
2
on cooling which were then filtered, washed several times with
distilled water, and dried in air. Yield, 25% based on Mo.
Elemental analysis: anal. calc. value for molecular formula,
C H CoMo N O P (M = 1413.15): C, 10.20; H, 3.71; N, 3.96;
1.8 mmol) was added dropwise to obtain a brown heavy pre-
cipitate. Then, the entire mixture was transferred into a Teflon
jacket in a PTTE-lined stainless steel pressure vessel and was
kept in an oven at 160 1C for consecutive five days under
autogenous pressure. The solution was cooled by decreasing
the temperature at a regular interval of 5 1C for one day. After
cooling the autoclave to room temperature, brown block shaped
1
2
52
5
4
35
2
W
found C, 10.30; H, 3.41; N, 3.96.
Results and discussion
Complexes 1 and 2 were synthesized by the reaction between
II
Na
2
II
MoO
4
and appropriate metal ions (Ni for complex 1 and
3 4
Co for complex 2) in the presence of H PO and a structure
directing agent like N-(2-hydroxyethyl)piperazine (NHEP) at
pH B 3.5 under hydrothermal conditions (Scheme 1).
Structural description of complexes 1 and 2
The single crystal X-ray diffraction analysis shows that complex
II
(
NHEPH
2
)
5
[Ni (P
4
Mo
6
O
31
)
2
]Á6H
2
O (1) crystallized in the triclinic
system with space group P1%. The anionic part of complex 1 is
composed of two {P
6
À
4
Mo
6
O
31
}
clusters linked by the hexa-
coordinated Ni(II) ion in which the Ni centre is sandwiched
6
À
between two {P Mo O } units (Fig. 1a and d). Two wheel-like
4
6 31
6
À
frameworks of {P Mo O } units are connected by one Ni
4
6 31
centre (Fig. 1b and e). The charge on the polyoxo anion is
2
+
balanced by the presence of doubly protonated NHEPH
cations in the asymmetric unit.
2
Scheme 1 The preparation scheme of complexes 1 and 2.
Fig. 1 The molecular view of complex 1 along the different crystallographic axes: c axis (a and d), a axis (b and e) and b axis (c and f), respectively.
All counter ions and water molecules are omitted for clarity.
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Fig. 2 (a) Type of H-bonding interactions involved in complex 1; (b) the two dimensional (2D) supramolecular framework in the crystallographic ac
plane.
As shown in Fig. 1, the [P Mo ] cluster is constructed by six
4
6
6 4
edge-sharing MoO octahedra and four PO tetrahedra with
alternating three long MoÁ Á ÁMo contacts (avg. 3.485 Å) and
three short Mo–Mo single bonds (avg. 2.594 Å). These three
shorter MoÁ Á ÁMo contacts are supported by one oxygen atom
externally and one oxygen atom internally which are directly
connected to the Ni1 centre. Therefore, the central Ni(II) centre,
located at a crystallographic inversion center, is octahedrally
coordinated by six m O atoms from the two neighboring [P Mo ]
3
4
6
clusters. The Ni–O bond lengths lie in the range 2.141(3)–
2
8
.165(3) Å and O–Ni–O bond angles vary in the range
3.78(11)–180.001, showing a distorted octahedron. Selected
bond distances and bond angles are given in Table S1 (ESI†).
Out of four tetrahedral PO units, the central PO unit shares
three m -O atoms with the six molybdenum atoms and three
other PO groups share the corners with two MoO octahedra in
Fig. 3 The supra-molecular H-bonded 3D network along the crystallo-
graphic b axis.
4
4
3
4
6
which the central phosphate ion bridges the ring internally and are responsible for the extension of the structure of complex 1
each one of the other three phosphate groups bridges one long into a 2D framework (Fig. 2) which is further extended to form a
MoÁ Á ÁMo contact externally. Each P atom is in a tetrahedral 3D network along the crystallographic b axis (Fig. 3). Now, some
environment surrounded by four oxygen atoms. Now, a closer of the related H-bonding interactions formed in complex 1 by
inspection of the structure of complex 1 reveals the presence of the non-coordinated water molecules (O35 and O34) in the
a number of H-bonding interactions between the non-coordinated asymmetric unit are given in Table S2 (ESI†).
2
+
counter cation NHEPH
namely O5 and O30) with the Mo2 centre of the sandwich complex 2 crystallizes in a triclinic system with space group
complex in a bifurcated fashion with N4–H4DÁÁÁO5 = 2.42(9) Å C2/c. Compound 2 is based on the {P Mo } cluster. As it is
2
and the coordinated oxygen atoms
The single crystal X-ray diffraction analysis reveals that
(
2
5
and N4–H4DÁÁÁO30 = 2.27(7) Å.
Again, the hydroxyl group (–OH) of this non-coordinated viewed as two PO
counter cation is involved in bifurcated H-bonding with the irregular ring of five distorted MoO6 octahedra, linked by one
2 5
usually observed that the {P Mo } cluster (see Fig. 4) can be
4
tetrahedra that cap, from either sides, an
oxygen atom of one phosphate group (O20) (O32AÁ Á ÁO20 = corner-shared and four edge-shared oxygen atoms. Each PO
4
2
.634 Å) and a water molecule (O37) (O32AÁ Á ÁO37 = 2.845 Å). tetrahedron shares three oxo-groups with the MO
6
octahedron.
Likewise, the other non-coordinated counter cation (O36) is One of these oxo-groups adopts a m
2
-bridging mode, linking
bifurcated and hydrogen bonded with the oxygen atom of one molybdenum site and the phosphorus atom and the other
another phosphate group (O19) (O36Á Á ÁO19 = 2.727 Å) and a two adopt a m -bridging mode, linking two molybdenum and
3
water solvent (O37) (O36Á Á ÁO37 = 2.732 Å) in the outer coordina- one phosphorus atoms and the other remain free from coordina-
2+
2 6
tion sphere. As a consequence, O37 acts as a connector between tion. The two [Co(H O) ] ions, two water molecules (O32 and O33)
2
+
two hydroxyl groups (–OH) of two counter cations through and two NHEPH
2
cations, are also present in the asymmetric unit
6À
H-bonding interactions. Hence, these H-bonding interactions which balances the overall charge on {P
2
Mo O } . The organic
5 23
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Fig. 6 (a) Thermo-gravimetric analysis of complex 1 and (b) of complex 2.
Fig. 4 The molecular view of complex 2 with a polyhedral representation.
À1
17
3
212 cm is associated with the water of crystallization. The IR
spectra are shown in Fig. S2(a) and (b) (ESI†).
Thermo-gravimetric analysis
Thermal stability for compounds 1 and 2 were investigated by
TGA. For both compounds, thermal decomposition proceeds via
three steps with very similar profiles (Fig. 6a and b respectively).
The initial endothermic mass loss below 100 1C for 1 corre-
2
sponds to the release of six lattice H O molecules (calc. 9.66%;
found 9.69%). Again, in the temperature range 100–480 1C mass
2
2
+
loss corresponds to the decomposition of three NHEPH
molecules (calc. 8.42%; found 8.49%). The final mass loss is
obtained above 900 1C for 1 may be due to complete rupture of
the POM moiety into P O and NiMoO fragments. Again for
2
5
4
complex 2 endothermic mass loss below 200 1C corresponds to
the release of two lattice H O and six coordinated H O molecules
2
2
(
calc. 8.98%; found 9.30%). The resulting phase is thermally
II
2+
2+
Fig. 5 (a) The [Co (H
2
O)
6
]
involved in H-bonding with NHEPH
2
and stable up to 480 1C. Again in the region 486–800 1C mass loss
water molecules; (b) the H-bonding pattern in complex 2; (c) the corresponds to the decomposition of two NHEPH
H-bonded supramolecular 2D framework in the crystallographic ac plane.
molecules
2
(
calc. 7.12%; found 7.13%) from complex 2. The final mass loss
is obtained above 800 1C for 2 probably due to complete rupture
2+
2
of the POM moiety into P O and MoO fragments.
2 5 3
cation NHEPH
has terminal N, O (N1, N2 and O30, O31)
sites which make interaction with the water molecule of the
2+
Catalysis
[
Co(H O) ] ion and with the O-atom (O13) of the POM cluster.
2 6
The free solvent water molecules also take part in hydrogen A mixture of complex 1 (catalyst) (20 mg, 0.0063 mmol) and
2+
bonding with [Co(H
2
O)
6
]
and the POM which are responsible styrene (substrate) (0.5 g, 4.8 mmol) was dissolved in 10 mL
for generating a 2D supramolecular structure (Fig. 5). This is MeCN solvent and the resulting solution was taken into a 50 mL
further extended to form a 3D framework along the crystallo- two-necked round bottom flask, of which one neck was closed
graphic b axis (Fig. S1, ESI†). The H-bonding interactions are with a rubber septum and the other was fitted with a condenser.
given in Table S3 (ESI†).
2 2
To the above solution was then added 1 mL of 30% H O . The
resulting solution did not show any change in the colour. The
solution was then heated in an oil bath to reflux for 24 h at 60 1C.
IR analysis
Complex 1 exhibits complex patterns of the IR spectrum with No further addition of H O was done. The reaction was mon-
2
2
À1
bands at 506–961 cm ascribed to n(Mo–O) and n(Mo–O–Mo). itored thoroughly and it was found that the complex was not
The absorption bands at 1009 cm are assigned to n(P–O). A soluble in acetonitrile medium. So, the overall reaction mixture
series of bands in the region 1460–1751 cm are characteristic was heterogeneous and no ether extraction was needed for GC
À1
À1
2
+
À1
of NHEPH
2
. The broad band at 3435 cm is ascribed to analysis. An aliquot (0.1 mL) of the reaction solution was with-
H
2
O molecules. These results are in accordance with the drawn with the help of a long needle 10 mL syringe and injected
structural findings. For complex 2, two strong bands in the to the GC port. The same catalytic protocol was followed with
À1
range 690–920 cm
are associated with nsym(MoQO) and complex 2 (20 mg, 0.0144 mmol) and it was also found to be
nasym(MoQO) and the medium to strong intensity band in heterogeneous for oxidation of styrene in acetonitrile medium.
À1
the 560–590 cm region is attributed to n(Mo–O–Mo). A series The results of oxidation are presented in Tables 2 and 3.
À1
of medium intensity bands in the 1070–1747 cm range are
In the case of Ni–POM (catalyst 1), the total conversion of styrene
2
+
associated with NHEPH
2
cations, and the broad band at about after 24 h was 81.14% with benzaldehyde selectivity of 84.47%.
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Table 2 Heterogeneous catalytic oxidation of styrene catalysed by In the case of Co–POM (catalyst 2), after 24 h reaction the total
complex 1 in MeCN (temperature = 60 1C)
conversion was 54.85% with high benzaldehyde selectivity, i.e.,
8
9.15%. A plot of conversion of styrene (in mmol) as a function of
Total conversion
of styrene
Styrene
oxide
Benzoic
acid
Time (h)
Benzaldehyde
time (in h) is shown in Fig. 7. Linear fits of these time based
conversions up to 6 h give kobs(1) = 0.31 mmol h and kobs(2) =
0.15 mmol h for complexes 1 and 2, respectively.
Table 4 represents a comparison of the catalytic activities
of our catalysts with other POM-based catalysts reported for
styrene oxidation. It is found that phosphomolybdate contain-
ing metal–salen complexes under homogeneous catalytic
À1
1
2
3
4
5
6
7.521
13.4036
19.96
24.187
33.25
39.45
81.141
89.009
77.35
79.96
79.46
79.99
81.58
84.47
5.02
6.57
5.46
4.702
4.73
3.97
1.813
1.02
8.9
8.743
9.56
9.138
3.67
7.39
À1
2
4
1
8–20
conditions in the presence of 30% H O
as oxidants led
2
2
to B80% conversion of styrene with 80–90% benzaldehyde
selectivity, whereas Co-POM complexes showed very poor
conversion (4–17%). Also, M-PMo11 based POMs (M = Co
Table 3 Heterogeneous catalytic oxidation of styrene catalysed by
complex 2 in MeCN (temperature= 60 1C)
Total conversion
of styrene
Styrene
oxide
Benzoic and Ni) also showed poor percentage of conversion of styrene
Time (h)
Benzaldehyde
acid
21
under solvent- free heterogeneous conditions. On the other
1
2
3
4
5
6
5.14
9.34
12.24
14.498
18.075
22.23
54.85
92.46
84.91
83.53
85.97
85.58
85.89
89.148
6.423
10.82
9.24
7.56
7.714
5.26
1.14
5.86
7.24
6.44
6.69
8.8
hand, our complexes showed better conversion and compar-
able benzaldehyde selectivity when compared with other POM
based heterogeneous catalysts as described in Table 4. To
check the catalytic efficacy of our POM complexes we have run
the blank reactions taking only (H O + styrene + MeCN),
2
2
2
4
3.249
7.58
where only 0.26% conversion was noted in 6 h. Again, when
the above reaction was carried out in the presence of either
3 2 2
Co(NO ) or NiCl , the % conversion of styrene was found to
be only B5% thereby clearly indicating the catalytic properties
of our Co-POM and Ni-POM complexes. A brief summation of
Fig. 7 A plot of conversion of styrene (mmol) as a function of time (h) for
complexes 1 and 2.
Fig. 8 Catalytic conversion of styrene to benzaldehyde/styrene oxide
formation with respect to time for complex 1.
Table 4 Comparison of the catalytic activities of different POM-based catalysts for styrene oxidation
Catalyst
Solvent
Temp (1C)
Oxidant
% Conversion
% Selectivity benzaldehyde
Catalytic nature
Ref.
Ni(salen)–POM
Co(salen)–POM
CH
CH
CH
CH
CH
CH
3
3
3
3
3
3
CN
CN
CN
CN
CN
CN
60
60
60
60
60
60
80
80
60
60
H
H
H
H
H
H
TBHP
TBHP
H
H
2
2
2
2
2
2
O
O
O
O
O
O
2
2
2
2
2
2
79
85
14.1
16.7
2.2
97.53
81
Homogeneous
Homogeneous
Homogeneous
Homogeneous
Homogeneous
Homogeneous
Heterogeneous
Heterogeneous
Heterogeneous
Heterogeneous
18
19
20
20
20
20
21
21
a
Co–POMb
86.2
81.4
72.0
76.2
92.2
87.2
84.47
89.15
Co–POM
c
Co–POM
d
Co–POM
3.8
PMo11Co
PMo11Ni
Ni–POM
Co–POM
Solvent free
Solvent free
41.1
39.9
81.14
54.85
CH
CH
3
CN
CN
2
O
O
2
Present work
Present work
3
2
2
a
II
II
V
b
II
V
(
H
4
TETA)
2
[(Co TREN)
2
Co Mo 12
P
8
O
52(OH)10]Á4H
2
O (TREN = tris(2-aminoethyl)amine). (H
4
TETA)
2
(C
6
H
4
18
N
TETA]
3
)
2
-[Co Mo 12
2
P
8
O
P
8
54-(OH)
8
]Á5H
2
O (C
2
O.
6 15 3
H N =
c
II
V
d
II
V
N-(2-aminoethyl)piperazine), TETA = triethylenetetramine. [H
4
TREN]
4
[Co Mo 12-P
8
O
56(OH)
6
]Á4H
2
O. [H
-[Co Mo 12
O48(OH)14]Á4H
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catalytic oxidation of styrene in the presence of an environmentally
benign oxidant H O under mild conditions leading to benz-
2 2
aldehyde with more than 80% selectivity.
Acknowledgements
Financial support from DST, West Bengal, is gratefully acknowl-
edged. L. P. and M. D. thank UGC (NET), New Delhi, for SRF
fellowship.
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