Keggin type inorganic-organic hybrid material containing Mn(II) monosubstituted phosphotungstate and S-(+)-sec-butyl amine: Synthesis and characterization

Article abstract of DOI:10.1016/j.materresbull.2011.10.026

A new inorganic-organic POM-based hybrid material comprising Keggin type mono manganese substituted phosphotungstate and enantiopure S-(+)-sec-butyl amine was synthesized in an aqueous media by simple ligand substitution method. The synthesized hybrid mat

Full text of DOI:10.1016/j.materresbull.2011.10.026

Materials Research Bulletin 47 (2012) 425–431
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Materials Research Bulletin
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Keggin type inorganic–organic hybrid material containing Mn(II)
monosubstituted phosphotungstate and S-(+)-sec-butyl amine:
Synthesis and characterization
*
Ketan Patel, Anjali Patel
Chemistry Department, Faculty of Science, M.S. University of Baroda, Vadodara 390 002, India
A R T I C L E I N F O
A B S T R A C T
Article history:
A new inorganic–organic POM-based hybrid material comprising Keggin type mono manganese
substituted phosphotungstate and enantiopure S-(+)-sec-butyl amine was synthesized in an aqueous
media by simple ligand substitution method. The synthesized hybrid material was systematically
characterized in solid as well as solution by various physicochemical techniques such as elemental
analysis, TGA, UV–vis, FT-IR, ESR and multinuclear solution NMR (31P, 1H, 13C). The presence of chirality
in the synthesized material was confirmed by CD spectroscopy and polarimeter. The above study reveals
the attachment of S-(+)-sec-butyl amine to Keggin type mono manganese substituted phosphotungstate
through N ! Mn bond. It also indicates the retainment of Keggin unit and presence of chirality in the
synthesized material. An attempt was made to use the synthesized material as a heterogeneous catalyst
for carrying out aerobic asymmetric oxidation of styrene using molecular oxygen. The catalyst shows the
potential of being used as a stable recyclable catalytic material after simple regeneration without
significant loss in conversion.
Received 29 April 2011
Received in revised form 23 August 2011
Accepted 26 October 2011
Available online 6 November 2011
Keywords:
A. Inorganic compounds
C. Nuclear magnetic resonance (NMR)
D. Catalytic properties
ß 2011 Elsevier Ltd. All rights reserved.
1. Introduction
coordination site on the metal is occupied by an aqua ligand.
Thus aqua ligand is labile and can be replaced by any organic group
The tailoring and synthesis of inorganic–organic hybrid
materials constructed from distinctive building blocks represents
an outstanding research area in material science, crystal engineer-
ing, magnetic materials, photosensitive material and catalysis [1].
All these reflect a current interest in combining different inorganic
and organic moieties by molecular assemblies to produce hybrid
materials, which combines the unique feature of both inorganic
and organic components. In this regards, polyoxometalates (POMs)
can act as excellent inorganic multidentate O-donor ligands [2] to
assemble various inorganic–organic hybrid compounds. Polyox-
ometalates (POMs) are metal oxygen anion clusters and their
properties may be modified depending on the elemental composi-
tion, structure as well as counteractions [3].
Transition metal substituted polyoxometalates (TMSPOMs) are
of excellent candidate in POMs chemistry due to their unique
electrochemical, magnetic, medicinal and catalytic properties [2–
4]. Further, they can also be rationally modified on the molecular
level including shape, size, charge density, redox states as well as
stability. In the TMSPOMs the transition metal is coordinated with
available five oxygen atoms of the POMs, while the sixth
or even by organometallic groups [5]. The obtained material is the
so-called inorganic–organic hybrid material based on POMs, which
have potential application in various fields from material science to
biology [6,7].
A number of strategies have been reported for the synthesis of
hybrid compounds based on POMs [8,9]. This includes transition
metal-substituted POMs [10], lacunary POMs [11], saturated POMs
and simple inorganic units as the starting materials [12]. Since the
POMs and organic moiety have different chemical and structural
properties, their synthesis remains a challenge. The reported
syntheses include hydrothermal conditions [13], self-assembly
reactions starting with the minimal building blocks [12] or the use
of organic solvent [14]. Recently Peng et al. has synthesized such
hybrid materials, containing functionalized manganese substitut-
ed silicotungstate and imidazol as well as cobalt substituted
silicotungstate and 4,40-bipyridine as an antenna ligand and fully
characterized spectroscopically as well as crystallographically
[9b,15]. They have carried out the synthesis in aqueous media
under normal bench conditions using coordination competition
approach.
At the same time, synthesis using ligand substitution approach
was not used at all. Further, it was also found that no reports are
available on the derivatization of the Mn-substituted phospho-
tungstate with S-(+)-sec-butylamine (SBA). So, it was thought of
* Corresponding author. Tel.: +91 265 2795552; fax: +91 265 2795392.
E-mail address: aupatel_chem@yahoo.com (A. Patel).
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.10.026

426
K. Patel, A. Patel / Materials Research Bulletin 47 (2012) 425–431
interest to synthesize POMs based hybrid material containing SBA
using ligand substitution approach. Mn(II)-substituted phospho-
tungstate(PW11Mn) was selected as inorganic building block based
on two reasons. Recently we have reported the single crystal X-ray
analysis of PW11Mn [16] and expertise in using such inorganic
building block. As SBA having N atom as potential coordination
sites which may play important role to extend the structure with
chiral centre, it was select as an organic ligand.
3200 Gauss). Solution NMR (1H, 13C, 31P) was recorded in D2O as
well as in CDCl3 on Bruker ACF 300 MHz instrument. Optical activity
was carried out using JASCO (J-815 CD Spectrometer, model no.: J-
815-150L) and JASCO (P-2000 Digital Polarimeter). The optical
rotation was carried outfor neat S-SBA as well as forPW11Mn-SBA in
methanol and water respectively. The 5% solution was used to carry
out optical rotation.
The present paper consists of synthesis of a new inorganic–
organic hybrid material containing Keggin type manganese
substituted phosphotungstate and SBA by ligand substitution
method. The synthesized hybrid material was systematically
characterized by elemental analysis, TGA, UV–vis, ESR, FT-IR,
multinuclear NMR (1H, 13C, 31P), CD spectroscopy and polarimeter.
An attempt was made to use the synthesized hybrid material for
carrying out aerobic oxidation of styrene at ambient temperature
under mild reaction condition.
2.3. Catalytic activity
The catalytic activity was evaluated for the non-solvent
oxidation of styrene using molecular oxygen as an oxidant and
TBHP as a co-oxidant. Oxidation reaction was carried out in a batch
type reactor operated under atmospheric pressure. In a typical
reaction, a measured amount of catalyst was added to a three-
necked flask containing alkenes and initiator TBHP (0.014 g) at
80 8C (for styrene). The reaction was started by bubbling O2 into
the reaction medium and was continuously stirred on magnetic
hot plate.
2. Materials
In the oxidation of styrene, after completion of reaction the
reaction mixture was allowed to cool to room temperature and
then 10% aqueous solution Na2CO3 was added with constant
stirring. The resultant mixture (organic and aqueous) was allowed
to stand for 15–20 min in order to separate the two distinct layers.
The aqueous layer was collected and concentrated HCl was added
slowly with constant stirring. No precipitates of benzoic acid were
separated. The remained organic layer was extracted with
dichloromethane and analyzed by Agilent Technologies 6890N
gas chromatography (FID, 19091G-B213 chiral capillary column
All chemicals used were of A.R. grade. 12-Tungstophosphoric
acid, H3PW12O40ꢀnH2O (Loba Chemie, Mumbai), sodium hydroxide,
MnCl2ꢀ4H2O and cesium chloride were obtained from Merck and
used as received. S-(+)-sec-butyl amine was obtained from Sigma
and used as received.
2.1. Synthesis
The synthesis was carried out in two steps.
(30 m ꢁ 0.32 mm ꢁ 0.25
mm)) using nitrogen as a carrier gas with
2.1.1. Synthesis of mono manganese substituted phosphotungstate
(PW11Mn)
flow rate 30 ml/min. The injector temperature, detector tempera-
ture, and oven temperature were 250 8, 250 8, and 100 8C,
respectively. The authentic samples of benzaldehyde, R- and S-
configuration styrene epoxides were used as the standard product
to determine the yields by comparison of peak height and area.
The percentage-conversion of the substrate and the percentage-
selectivity of the products in the epoxidation reaction are
calculated as:
PW11MnO39 was synthesized by reported method by us
recently [16]. 2.88 g of H3PW12O40ꢀnH2O was dissolved in 10 ml
of water and the pH of the solution adjusted to 4.8 using NaOH
solution. The solution was heated to 90 8C with stirring. To this hot
solution was added 0.197 g of MnCl2ꢀ4H2O solution dissolved in
10 ml water. The final pH of the solution was 4.8. The solution was
heated at 90 8C with stirring for 1 h and filtered hot. 10 ml
saturated solution of CsCl was added to the hot filtrate. The
resultant mixture was allowed to stand overnight at room
temperature. The solution was filtered and the orange colored
crystals (87.9%) were dried at 50 8C. The isolated solid was
designated as PW11Mn.
ðinitial mol%Þ ꢂ ðfinal mol%Þ
Conversion ð%Þ ¼
Selectivity ð%Þ ¼
ꢁ 100
ꢁ 100
initial mol%
product formed ðmolesÞ
substrate converted ðmolesÞ
2.1.2. Synthesis of POMs based hybrid material (PW11Mn-SBA)
0.8985 g of PW11Mn was dissolved in 10 ml of distilled water.
0.1314 g of SBA was dissolved in 15 ml of ethanol. The ethanolic
solution of SBA was added drop wise to the aqueous solution of
PW11Mn. The pH of the solution was found to be 6.4. This resulted
mixture was refluxed for 10 h and then allowed to cool, filtered and
washed with water:ethanol (1:1) mixture. The obtain powder was
designed as PW11Mn-SBA.
3. Results and discussion
The observed values for the elemental analysis in the isolated
complex (PW11Mn) are in good agreement with the theoretical
values. Anal calc: Cs, 19.1; W, 57.9; P, 0.89; Mn, 1.57; O, 20.2.
Found: Cs, 19.8; W, 56.68; P, 0.86; Mn, 1.53; O, 20.06. The
number of water molecules calculated from TGA curve, based on
total weight loss (2.5%), corresponds to loss of
5 water
molecules. From the elemental as well as the thermal analysis
the chemical formula of the isolated complex is proposed as:
Cs5[PW11O39Mn(H2O)]ꢀ4H2O (PW11Mn).
2.2. Characterization
Elemental analysis was carried out using JSM 5610 LV combined
with INCA instrument for EDX-SEM analyzer for the quantitative
identificationofmetalions. Thermogravimetricanalysiswascarried
out on the Mettler Toledo Star SW 7.01 upto 600 8C in air with the
heating rate of 5 8C/min. FT-IR spectra of the samples were recorded
as the KBr pellet on the Perkin Elmer instrument. The UV–vis
spectrum was recorded at ambient temperature on Perkin Elmer 35
LAMDA instrument using the 1 cm quartz cell. The ESR spectra were
recorded on a Varian E-line Century series X-band ESR spectrometer
(liquid nitrogen temperature and scanned from 2000 to
The observed values for the elemental analysis in the isolated
complex (PW11Mn-SBA) are in good agreement with the theoreti-
cal values. Anal calc: Cs, 18.11; W, 55.13; P, 0.84; Mn, 1.49; O,
21.81; C, 1.30; H, 0.89; N, 0.38. Found: Cs, 17.92; W, 54.84; P, 0.83;
Mn, 1.47; O, 21.49; C, 1.20; H, 0.80; N, 0.35. The number of water
molecules calculated from TGA curve, based on total weight loss
(4.3%), corresponds to loss of 11 water molecules. From elemental
as well as thermal analysis the chemical formula of the isolated
complex is proposed as: Cs5[PW11O39Mn(C4H11N)]ꢀ11H2O
(PW11Mn-SBA).

K. Patel, A. Patel / Materials Research Bulletin 47 (2012) 425–431
427
Table 1
FT-IR frequency data.
Materials
FT-IR frequencies
P–O
W5O
W–O–W
Mn–N
–
C–N
N–H
C–H
PW11Mn
SBA
1078
1053
955
884
813
–
–
–
–
–
952
953
–
–
672
670
1290
1259
1148
1598
1541
1541
2928
2850
1450
PW11Mn-SBA
1075
1054
880
812
1200
1173
2922
2852
1448
R1-PW11Mn-SBA
1078
1055
882
815
1201
1171
2920
2848
1451
The frequencies of FT-IR bands for PW11Mn; SBA and PW11Mn-
SBA are shown in Table 1. PW11Mn displays a characteristic IR
fingerprint [16] in the region of 1000–700 cmꢂ1, attributed to the
stretching vibration of the tetrahedral P–O bonds (1078 and
1053 cmꢂ1), terminal W55O bonds (955 cmꢂ1) and the two types of
bridging W–O–W bonds of the cluster (884 and 813 cmꢂ1). SBA
gives characteristic IR fingerprint region for C–N vibration (1290,
1259 and 1148 cmꢂ1), symmetric and asymmetric stretching
vibrations of aliphatic C–H bonds (2928, 2850 and 1450 cmꢂ1) and
N–H stretching vibration (1598 cmꢂ1).
PW11Mn-SBA complex shows IR vibration bands correspond to
PW11Mn as well as SBA. It displays a characteristic IR fingerprint in
the region of 1000–700 cmꢂ1, attributed to the stretching vibration
of the tetrahedral P–O bonds (1075 and 1054 cmꢂ1), terminal
W55O bonds (952 cmꢂ1) and the two types of bridging W–O–W
bonds of the cluster (880 and 812 cmꢂ1). These characteristic IR
data indicate the retainment of Keggin structure even after
introduction of SBA. However, little shift was observed which
may due to the introduction of SBA in the coordination sphere of
PW11Mn.
It is interesting to note that after introduction of an organic
moiety (i.e. for PW11Mn-SBA), the vibration bands that originated
from C–N bond gives two broad bands at 1200 and 1273 cmꢂ1
instead of three characteristic bands. Further a very significant
shift, from 1598 cmꢂ1 to 1541 cmꢂ1 for N–H stretching is observed.
This indicates the formation of dative bond from N ! Mn. As a
result of decreases in electron density on N atom, N–H bond length
decreases which leads to the significant shift. In addition, the
formation of new band at 672 cmꢂ1 also supports the formation of
N ! Mn bond. Further, Raman spectra of PW11Mn-SBA indicate the
single line at 374, which may indicate the formation of N ! Mn
bond in the synthesized material.
The UV–vis spectrum of PW11Mn and in water, shows a W ! O
charge-transfer peak at 291 nm accompanied by another lmax at
398 nm (Fig. 1a), which can be attributed to the d–d electronic
transition of the Mn center in the [PW11O39Mn(H2O)] anion. The
observed values are in good agreement with the reported one [16].
The UV–vis spectrum of PW11Mn-SBA in water shows two peaks:
one at 289 nm corresponding to the W ! O charge-transfer and
another
electronic transition of the Mn center in the [PW11O39Mn(SBA)]
anion. The shift in max from 398 nm to 407 nm may be due to the
lmax at 407 nm (Fig. 1b) which can be attributed to the d–d
In addition, from the FT-IR spectra we can obtain the structural
information about the organic ligand. A pair of weak peaks at 2922
and 2852 cmꢂ1 for the synthesized material is attributed to the
symmetric and asymmetric stretching vibrations of aliphatic C–H
bonds. These results ensure the successful formation of POMs
based hybrid material with PW11Mn and SBA.
l
replacement of the labile aquo ligand by SBA. The ability of the
manganesepolyoxometalatetocoordinatewiththeorganicligandis
well documented in the literature [9b] and it was reported that,
changes in the position or shape or increase in intensity of ligand
Fig. 1. UV–vis spectra of (a) PW11Mn, and (b) PW11Mn-SBA.

428
K. Patel, A. Patel / Materials Research Bulletin 47 (2012) 425–431
Fig. 2. ESR spectra of (a) PW11Mn, and (b) PW11Mn-SBA.
field band was observed when aquo ligand was replaced by any
ligand. The change of the ligand field band was observed for number
of ligand in case of Co(II) undecatungstocobalto(II) by Weakly [17].
The obtained result is as expected.
at room temperature but NMR spectrum can be obtained at room
temperature.
31P NMR spectra for PW11Mn and PW11Mn-SBA are shown in
Fig. 3a and b. 31P NMR spectroscopy is a useful method to identify
the change in the environment around the transition metal centre
incorporated into the POMs. Further, it also showed the purity of
the formed product. However many a time it is well documented
that in case of preparing such kind hybrid material there is lack of
getting pure product in aqueous media resulting into the two to
three line in 31P NMR spectrum [18]. Such kind of material
exhibiting spectrum corresponds to the formed product along with
presence of other species (i.e. monolacunary POMs and TMSPOMs).
In the present case only single peak is observed confirming the
formation of single pure products. The significant upfield shift in
PW11Mn-SBA as compared to PW11Mn may be due to the change in
the environment around the Mn(II)-centre. This also changes the
central P environment, i.e. PO4–Mn–H2O becomes PO4–Mn–SBA.
Thus 31P NMR indicates the successful replacement of aquo ligand
by SBA.
Thus the UV–vis spectrum supports the formation of N ! Mn.
Paramagnetic compounds have different relaxation times. If the
electronic relaxation is slow, good ESR spectra can be obtained at
room temperature while, if the electronic relaxation is fast, good
ESR spectra can be obtained only at low temperature. The low
temperature ESR for PW11Mn and PW11Mn-SBA were recorded in
the range of 3200–2000 G. The low temperature ESR shows (Fig. 2a
and b) a well resolved six lines ESR spectrum with g ꢃ 2.3 and 2.1
which is as expected and confirms the presence of Mn(II) in the
synthesized material. The slight shift in g value may be due to the
change in the environment around the Mn(II) centre.
Thus the FT-IR, UV–vis and ESR spectrum support the formation
of inorganic–organic hybrid material through N ! Mn bond.
4. Nuclear magnetic resonance spectra
Fig. 4 shows the structure of SBA. The chemical shift of 13C NMR
spectrum for SBA as well as PW11Mn-SBA is presented in Table 2.
a
It is known that the presence of paramagnetic metal centre
generally results in a poorly defined NMR spectrum. In the present
case, well resolved NMR spectra (13C, 31P) are obtained. This may be
due to the twofacts: the concentrationofparamagneticmetalcentre
(present case 1.53% Mn(II)); and the electronic relaxation for
paramagnetic compound is fast. So ESR spectra cannot be obtained
As shown in Table 2, no significant shift in CH3 carbon indicates
that the amine remains intact in the synthesized material. The
considerable upfield shift was observed for bCH3 and cCH2 and
downfield shift was observed for dCH. This may be due to the bond
formation between N atom of SBA and Mn of PW11Mn. Once, dative
bond forms between Mn and N, the electron density on N atom
Fig. 3. 31P NMR of (a) PW11Mn, (b) PW11Mn-SBA, and (c) R1-PW11Mn-SBA.

K. Patel, A. Patel / Materials Research Bulletin 47 (2012) 425–431
429
Fig. 4. S-(+)-sec-butyl amine.
Table 2
Chemical shift for 13C NMR.
Chemical shift
SBA
PW11Mn-SBA
aCH3
bCH3
cCH2
dC–H
10.8
23.6
33.8
48.6
10.6
Fig. 6. CD spectra of PW11Mn-SBA.
18.83
28.76
50.84
4.1. Catalytic activity
A neat reaction (without catalyst) was carried out and it showed
no conversion for the substrate indicating that there is no
autooxidation taking place. In order to study the role of TBHP the
same sets of reactions were carried under two different conditions:
(i) alkene + oxidant + TBHP and (ii) alkene + oxidant + PW11Mn-
SBA. In both the cases the reaction did not progress significantly.
These observations indicate that the liberation of O2 from TBHP was
not sufficient to induce the reaction as well as the activation of
manganese. Hence it may be concluded that in the present study
TBHP acts as an initiator only.
decreases, which leads to the downfield shift for dCH carbon. The
result is as expected. The NMR study confirms the formation of
N ! Mn bond.
1H NMR spectra of the PW11Mn-SBA (Fig. 5) show broadened
peaks or very poorly defined spectra corresponding to the CH3 and
CH2 protons. This indicates the presence of SBA moiety into the
coordination sphere of PW11Mn. Presence of paramagnetic centre
will be responsible to give broadened peaks or very poorly defined
spectra.
To examine the chiroptical and stable activities of synthesized
material in the solution state, the CD spectra were carried out in
water (Fig. 6). The spectrum of the synthesized material exhibits
Cotton effect at 250 nm. This is region for characteristic of the
oxygen-to-tungsten charge-transfer bands of Keggin polyoxoanions
[19]. The induced circular dichroism in the POM clusters can be
clearly seen in the CD spectra. Optical rotation of SBA and PW11Mn-
4.2. Effect of amount of catalyst
The effect of concentration of the catalyst amount on the
conversion and selectivity is shown in Table 3. In the present study
epoxidation of styrene gives styrene oxide as well as benzalde-
hyde. However with an increase in the amount of catalyst, %
conversion also increases. This suggests that the manganese centre
functions as active sites for oxidation. It is very interesting to
observe the difference in the selectivity of the products with an
increase in the amount of the catalyst. As shown in Table 3, up to
25 mg of catalyst, epoxide was observed. On further increasing the
amount of the catalyst, the product selectivity shifts from the less
stable intermediate (epoxide) to the more stable product
(benzaldehyde). This may be due to the fact that with increase
in the amount of the active species the reaction becomes very fast
which favours the conversion of the formed styrene oxide to
benzaldehyde. Due to the known importance of epoxide, the
amount of the catalyst was optimized at 25 mg.
SBA was found to be [a]20/D + 1.49 and [a]20/D + 0.4 respectively.
This result ensures that the formed hybrid material is chiral.
4.3. Effect of reaction time
It is seen from the Table 4 that the distribution of the product
changes with increase in the reaction time. Initially, after the
completion of 2 h the 21% of styrene oxide is observed. As the
Table 3
Effect of amount of catalyst.
Catalyst
Amount
(mg)
Conversion
(%)
Selectivity (%)
Benzaldehyde
ee (S)
(%)
Styrene
oxide
PW11Mn-SBA
5
10
15
20
25
30
6
19
32
41
52
58
75
78
25
22
17
14
11
6
8
83
7
86
9
89
10
>99
Substrate; styrene (100 mmol); oxidant, O2 (4 ml/min); TBHP, 0.15 mmol; reaction
Fig. 5. 1H NMR spectra of PW11Mn-SBA.
time, 4 h; temperature, 80 8C.

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K. Patel, A. Patel / Materials Research Bulletin 47 (2012) 425–431
Table 4
Effect of reaction time.
Catalyst
Reaction time (h)
Conversion (%)
Selectivity (%)
Benzaldehyde
ee (S) (%)
Styrene oxide
PW11Mn-SBA
2
4
6
29
52
56
79
89
21
11
9
10
>99
Substrate; styrene (100 mmol); oxidant, O2 (4 ml/min); TBHP, 0.15 mmol; catalyst, 25 mg; temperature, 80 8C.
Table 5
Oxidation of styrene using PW11Mn and PW11Mn-SBA (under optimized condition).
cAlkenes
Conversion (%)
a52/61b
Products
Selectivity (%)
a11
ee (S)
10
dTON
a7251/8507b
O
O
a89/>99b
a
Epoxidation of alkenes catalyzed by PW11Mn-SBA.
b
c
Epoxidation of alkenes catalyzed by Cs5[PMn(H2O)W11O39]ꢀ4H2O.
Conversion based on substrate; styrene, 100 mmol; oxidant, O2 (4 ml/min); TBHP, catalyst, 25 mg; reaction time, 4 h.
Turnover number based on conversion.
d
reaction time increases the product selectivity shifts towards BA.
With increase in the reaction time the unstable intermediate,
epoxide, is converted to the more stable product BA. Due to the
known industrial importance of styrene oxide, the reaction time
was optimized at 4 h.
the organic ligand, which facilitates the formation of stabilization of
the formed styrene oxide.
4.5. Leaching as well as heterogeneity test
Any leaching of the active species makes the catalyst
unattractive and hence it is necessary to study the stability as
well as leaching of PW11Mn-SBA. The catalyst was filtered after
completion of reaction and the filtrate was characterized for UV–
vis spectroscopy. For comparison, UV–vis spectrum of PW11Mn-
SBA was recorded (Fig. 7). The absence of characteristic peaks in
filtrate indicates that there is no leaching of PW11Mn-SBA and the
catalyst remains completely insoluble under reaction condition
and could be reused.
For rigorous proof of heterogeneity, a test was carried out by
filtering catalyst from the reaction mixture at 80 8C after 2 h and
the filtrate was allowed to react up to completion of reaction (4 h).
The reaction mixture of 2 h and filtrate was analyzed on gas
chromatogram. The results are presented in Table 6. No change in %
4.4. Effect of SBA
Toexaminetheeffect organic ligand, the oxidation ofstyrene was
carried out using PW11Mn as well as PW11Mn-SBA in optimized
condition and the distribution of products are reported in Table 5.
PW11Mn showed 61% conversion for styrene with >99% selectivity
towards benzaldehyde whereas PW11Mn-SBA showed 52% conver-
sion with 11% selectivity towards 89% selectivity towards benzal-
dehyde. The difference in the catalytic activity can be explained on
the basis of the structural difference between PW11Mn and
PW11Mn-SBA. This could be explained on the basis of the nature
of the catalyst. The excellent catalytic performances of the PW11Mn-
SBA catalysts are mainly due to the easy electron-donating ability of
Fig. 7. UV–vis spectra of (a) PW11Mn-SBA, and (b) filtrate.

K. Patel, A. Patel / Materials Research Bulletin 47 (2012) 425–431
431
even after the introduction of SBA. The multinuclear NMR (13C and
31P) studies indicate the formation of N ! Mn bond. The presence
of chirality in the synthesized material was confirmed by CD
spectroscopy and polarimeter. The above studies reveal the
attachment of SBA to the PW11Mn without any distortion of
structure as well as with retainment of chirality. The synthesized
hybrid material was successfully used as heterogeneous catalyst.
Our future aspect would be to increase the % selectivity and % ee for
the styrene oxide.
Table 6
% conversion and % selectivity for oxidation of styrene (with and without catalyst).
Catalyst
Reaction Conversion Selectivity (%)
time (h) (%)
ee (S)
(%)
Benzaldehyde Styrene
oxide
PW11Mn-SBA (2 h)
Filtrate (4 h)
2
4
27
27
81
81
19
19
10
10
Substrate; styrene (100 mmol); oxidant, O2 (4 ml/min); TBHP, 25 mg catalyst;
reaction time, 4 h; temperature, 80 8C.
Acknowledgement
Table 7
Mr. Ketan Patel is thankful to Department of Science and
Technology (SR/S1/IC-13/2007), New Delhi for financial support.
Recycling of the catalyst.
Catalyst
Reaction Conversion Selectivity (%)
time (h) (%)
ee (S)
(%)
References
Benzaldehyde Styrene
oxide
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(c) D. Loy, K. Shea, Chem. Rev. 95 (1995) 1431;
PW11Mn-SBA
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4
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89
90
11
10
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R1-PW11Mn-SBA
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Substrate; styrene (100 mmol); oxidant, O2 (4 ml/min); TBHP, 25 mg catalyst;
reaction time, 4 h; temperature, 80 8C.
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4.6. Regeneration and recycling of the catalyst
In order to investigate the stability of the catalyst during
oxidation reaction, the catalyst was separated by simple filtration
after completion of reaction. The separated catalyst was washed
with dichloromethane and dried at 100 8C (designated as R1-
PW11Mn-SBA). Oxidation of styrene was then carried out with the
recycled catalyst, under the optimized conditions (R1-PW11Mn-
SBA). The obtained results are presented in Table 7. As seen from
Table 7, the recycled catalyst did not show any appreciable change
in the activity, indicating that the catalyst is stable and can be
regenerated for repeated use.
Further, the recycled catalyst (R1-PW11Mn-SBA) was charac-
terized by FT-IR (Table 1) and 31P NMR (Fig. 3c) spectroscopy.
The FT-IR spectra of the R1-PW11Mn-SBA display all the
characteristic bands for the C–N, N–H stretching and N ! Mn
bond.
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Further, the 31P NMR spectra of R1-PW11Mn-SBA show the
chemical shift at ꢂ11.56 ppm (Fig. 3c). No shift in FT-IR bands and
the chemical shift indicate that the PW11Mn-SBA remains stable in
the present reaction condition.
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5. Conclusion
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In conclusion, we have developed chiral POM-based materials
utilizing enantiopure SBA ligand and Keggin-type mono-manga-
nese substituted phosphotungstate via ligand substituted method.
Spectral studies show that the Keggin unit retains its structure