Polyoxometalate-molybdenylacetylacetonate hybrid complex: A reusable and efficient catalyst for oxidation of alkenes with tert-butylhydroperoxide

Article abstract of DOI:10.1016/j.inoche.2009.11.022

The hybrid complex consist of molybdenylacetylacetonate complex covalently linked to a lacunary Keggin-type polyoxometalate, K₈[SiW₁₁O₃₉] (POM), was synthesized and characterized by elemental analysis, SEM, XRD, diffuse re

Full text of DOI:10.1016/j.inoche.2009.11.022

Inorganic Chemistry Communications 13 (2010) 244–249
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Polyoxometalate–molybdenylacetylacetonate hybrid complex: A reusable
and efficient catalyst for oxidation of alkenes with tert-butylhydroperoxide
*
*
Majid Moghadam , Valiollah Mirkhani , Shahram Tangestaninejad, Iraj Mohammadpoor-Baltork,
Maedeh Moshref Javadi
Chemistry Department, Catalysis Division, University of Isfahan, Isfahan 81746-73441, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 July 2009
Accepted 23 November 2009
Available online 6 December 2009
The hybrid complex consist of molybdenylacetylacetonate complex covalently linked to a lacunary
Keggin-type polyoxometalate, K
analysis, SEM, XRD, diffuse reflectance UV–Vis and FT-IR spectroscopic methods. The hybrid complex,
MoO (acac)–POM] (1), was used for alkene epoxidation with tert-BuOOH in 1,2-dichloroethane as sol-
8
[SiW11O39] (POM), was synthesized and characterized by elemental
[
2
vent. The complex (1) can catalyze epoxidation of various olefins including non-activated terminal ole-
fins. The effect of reaction parameters such as oxidant, solvent, and temperature on the epoxidation of
cyclooctene was also investigated. This heterogeneous catalyst was reused several times in the oxidation
of cyclooctene.
Keywords:
MoO (acac)
2 2
Polyoxometalate
Olefin oxidation
tert-BuOOH
Ó 2009 Elsevier B.V. All rights reserved.
Hybrid compound
Applying and using heterogeneous catalysts offers numerous
advantages in contrast to corresponding homogeneous com-
pounds. For instance, better stability, separation, and simple recov-
ery and recycling. Distinct methodologies have been developed for
the immobilization of homogeneous catalysts or the creation of
heterogeneous catalysts [1]. Heterogeneous catalysts have played
a vast role in many reactions such as oxidation. Academic and
industrial interest is drawn to epoxidation of alkenes and reaction
products based on epoxides [2]. Surfactants, detergents, antistatic
agents and corrosion protection agents, lubricating oils, textiles
and cosmetics are technically vital products obtained from the
reaction of epoxides [3]. Above all, valuable organic intermediates
such as cyclooctene and cyclohexene oxides are employed in the
synthesis of pesticides, chiral pharmaceuticals, rubber promoters,
epoxy paints and dyestuff products [4].
plexes have also been synthesized [25–27] and have been good
catalyst precursors for olefin epoxidation [28,29].
Many catalysts show good results when hybridized. Among
some capable hybrid compounds are metal–organic–polyoxometa-
lates [30] and metal Schiff bases hybridized with polyoxometalates
[31]. Several of these metal complexes have been synthesized and
studied towards various reactions [32–36]. A major drawback in
these catalysts is that they are homogeneous and have difficulties
in being separated. Different approaches have been used to turn
homogenous catalysts into heterogeneous counterparts by hybrid-
izing different materials.
In this article, a hybrid compound, by combining the properties
of molybdenylacetylacetonate and a Keggin-type polyoxometalate,
was synthesized and used as a heterogeneous and efficient catalyst
for the epoxidation of olefins with tert-BuOOH. The effect of differ-
ent solvents, oxidants, and temperatures on the activity and selec-
tivity of the catalyst was also studied (Scheme 1).
Among the various metals used as catalysts, molybdenum is
one of the most important. It is the only second row transition me-
tal essential for life. Many homogeneous and heterogeneous cata-
lysts of molybdenum have been synthesized and used up to date
The MoO
[37]. The lacunary Keggin-type polyoxometalate, K
(POM) was prepared and characterized as reported by Tézé and
Hervé [38], and the tetrahexylammmonium salt of {SiW11
2
(acac)
2
was prepared as reported in the literature
8
[SiW11
39
O ]
[
5–11], especially in the oxidation of alkenes [12–16], sulfoxidation
and hydrosilylation of carbonyl groups [17], hydrodesulfurization
18], synthesis of metal containing polymers [19], optical and elec-
O
39
4À
[
[O(SiR)
ported by Neumann and coworkers [31].
The [MoO (acac)–POM] complex was synthesized using the
{SiW11 39[O(SiR) ]} precursor and modified procedure
Scheme 2) [39]. Several attempts to prepare its single crystal were
2
]} in which R is CH
2 2
CH NH
2
ÁHCl was synthesized as re-
trochemical sensors to enhance selectivity and sensitivity [20],
DNA interactions [21], nitrate reduction [22–24], and lots of other
reactions. Several heterogeneous dioxomolybdenum(VI) com-
2
Q
4
O
2
a
(
failed. Therefore, hybrid complex was characterized by elemental
analysis, SEM, diffuse reflectance UV–Vis and FT-IR spectroscopic
methods and powder X-ray diffraction.
*
Corresponding authors. Tel.: +98 311 7932712; fax: +98 311 6689732.
E-mail addresses: moghadamm@sci.ui.ac.ir (M. Moghadam), mirkhani@sci.
ui.ac.ir (V. Mirkhani).
1
387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.11.022

M. Moghadam et al. / Inorganic Chemistry Communications 13 (2010) 244–249
245
Si
Si
H N
2
O
POM
MoO (acac)
2
H N
2
+
1,2-DCE, tert-BuOOH
Oxidation products
Scheme 1.
Primary evidence of the electronic effect of the polyoxometalate
on the MoO (acac) was observed in the UV–Vis spectra. As seen in
Fig. 1, the hybrid MoO (acac)–POM, has a different UV–Vis spec-
trum from the un-hybrided MoO (acac) . The MoO (acac) has a
max at 410 nm which is recognized as the O(p ) ? Mo(d
charge-transfer transition [40]. This is shifted to 362 nm in the
MoO (acac)–POM complex. This shift may be deducted to an intra-
molecular effect of the POM on the MoO (acac) and/or strong sig-
2
2
2
2
2
2
2
k
p
p
)
2
2
2
ma donor nature of the amine ligands, when one of the acac ligands
is replaced with two strong sigma donors ligands such as amine, it
is expected that the energy of the LMCT band increases.
The SEM images show different morphological surfaces for pure
POM and the referring MoO (acac)–POM (Fig. 2). The amount of
2
2
Fig. 1. DR UV–Vis spectrum of [MoO (acac)–POM].
Mo loading in the supported catalyst was determined by ICP,
which showed a value of 0.198 mmol per gram of catalyst. The
À1
FT-IR spectra shows two distinct bands at 918 and 973 cm which
are attributed to Mo@O vibrations (Fig. 3).
The XRD pattern of POM and [MoO
Fig. 4. As can be seen, it is clear that MoO
fully supported on POM.
2
(acac)–POM] are shown in
(acac) has been success-
2
2
The catalytic activity of the prepared catalyst was tested using
cyclooctene as reference alkene. Oxidation was carried out with
tert-BuOOH as an oxidant and in the presence of catalytic amounts
2
of [MoO (acac)–POM]. The optimum conditions used for the oxida-
tion of cyclooctene by this catalytic system was catalyst, oxidant,
and substrate in a molar ratio of 1:150:63, respectively. To further
optimize the conditions, different solvents were also used in the
oxidation of cyclooctene with tert-BuOOH. The results showed that
the highest yield obtained in 1,2-dichloroethane and even though
dichloromethane and chloroform also had high yields (Table 1).
2 2 4
The effect of different oxidants such as H O , NaIO , tert-BuOOH
and H /Urea (UHP) was also investigated in the oxidation of
2 2
O
cyclooctene. The results showed that tert-BuOOH is the best oxy-
gen source (Table 2).
The reaction temperature was also optimized by repeating the
reaction in various temperatures. At room temperature (25 °C),
the product yields were low and with increasing the reaction tem-
Si
Si
Si
Si
NH2
NH2
(
EtO) SiCH CH CH NH
3
2
2
2
2
POM
POM
DMF
MoO (acac)2
2
Si
H N
2
POM
MoO (acac)
2
Fig. 2. SEM image of: (a) Keggin-type polyoxometalate, K
8
[SiW11
O
39] (POM), and
Si
H N
(b) [MoO (acac)–POM].
2
2
1
perature to 75 °C (refluxing 1,2-DCE), both the conversion and
Scheme 2.
selectivity increased (Table 3).

2
46
M. Moghadam et al. / Inorganic Chemistry Communications 13 (2010) 244–249
2
Fig. 3. FT-IR spectrum of [MoO (acac)–POM].
8 2
Fig. 4. XRD pattern of: (A) Keggin-type polyoxometalate, K [SiW11O39] (POM), and (B) [MoO (acac)–POM].

M. Moghadam et al. / Inorganic Chemistry Communications 13 (2010) 244–249
247
Table 1
Table 3
Effect of solvent on the oxidation of cyclooctene catalyzed by [MoO
2
(acac)–POM]
Effect of temperatures on the oxidation of cyclooctene catalyzed by [MoO
POM].
2
(acac)–
under reflux conditions.a
a
Row
Solvent
Conversion (%)b after 8 h
Row
Temperature (°C)
Conversion (%)b after 8 h
1
2
3
4
5
6
1,2-Dichloroethane
100
90
44
59
50
95
1
2
3
4
25
40
60
6
CH
CH
2
Cl
COCH
2
40
43
100
3
3
CCl
CH
CHCl
4
75 (reflux)
3
CN
a
Reaction conditions: cyclooctene (0.5 mmol), tert-BuOOH (1.5 mmol), catalyst
0.008 mmol), 1,2- DCE (2 ml).
GC yield.
3
(
a
b
Reaction conditions: cyclooctene (0.5 mmol), tert-BuOOH (1.5 mmol), catalyst
(
0.008 mmol), solvent (2 ml).
GC yield.
b
1
00% yield. Oxidation of cyclohexene gave an allylic oxidation
product, 2-cyclohexene-1-one, which was also the major product,
with 92% yield. In this case the alcohol and enone can arise via
allylic oxidation or via Lewis acid catalyzed rearrangement of the
epoxide, and subsequent elimination and secondary oxidation. To
check this, a base such as pyridine was added to the reaction mix-
ture. The results showed that the distribution of products were as
observed in the absence of pyridine. These observations indicated
that the products were aroused via allylic oxidation. With styrene
Table 2
Effect of different oxidants on the oxidation of cyclooctene catalyzed by [MoO
cac) –POM] under reflux conditions.
2
2
(a-
a
Conversion (%)b after 8 h
Row
Oxidant
Solvent
1,2-DCE
1
2
3
4
5
H
2
O
2
8
36
30
100
trace
NaIO
4
2
H O/CH
3
CN
H
2
O
2
/Urea (UHP)
1,2-DCE
1,2-DCE
1,2-DCE
tert-BuOOH
No oxidant
and a-methylstyrene the major products were benzaldehyde and
acetophenone, respectively. These products are resulted from the
attack of t-BuOOH to epoxide and cleavage of the respected inter-
mediated as shown in the proposed mechanism (Scheme 3). These
results show that the conjugation of aromatic ring with carbonyl
group stabilizes the products. The great feature of the catalytic
a
Reaction conditions: cyclooctene (0.5 mmol), oxidant (1.5 mmol), catalyst
(
0.008 mmol).
GC yield.
b
2
epoxidation with MoO (acac)–POM is that non-activated terminal
Under the optimized conditions which obtained for oxidation of
olefins such as 1-heptene and 1-octene, could be proficiently trans-
formed to the corresponding epoxides in high yields.
cyclooctene, different alkenes such as cyclohexene, styrene,
a-
methylstyrene, 1-heptene, 1-octene and 1-dodecene were oxi-
dized. The results are shown in Table 4 [41]. Clearly, electron rich
olefins are more reactive than electron poor ones. In the case of
cyclooctene, the cyclooctene oxide was the selective product with
It was found that the alkenes were not oxidized in the absence
of catalyst or oxidant. Mizuno and coworkers have already exam-
ined the epoxidation of olefins with H
2 2
O catalyzed by mono-
8
À
and tri-vacant lacunary compounds such as [
a
-SiW11
O
39
]
and
Table 4
Oxidation of alkenes with TBHP catalyzed by [MoO
2
(acac)–POM].a
Producta
Entry
1
Alkene
Conversion (%)b
100
Selectivity (%)b
100
Time (h)
8
TOF (h
7.81
À1
)
O
2
OH
96
(1) 4
6
10.00
O
(
(
2) 95
3) 1
O
(
1)
(
2)
(
3)
3
4
O
O
94
98
((171
((2 29
8
8
7.35
7.66
O
H
(
1)
1)
(
2)
(1) 95
(
(25
O
CH3
(
(
2)
5
100
100
8
7.81
O
6
7
91
17
91
17
8
8
7.11
1.38
O
O
a
Reaction conditions: alkene (0.5 mmol), tert-BuOOH (1.5 mmol), catalyst (0.008 mmol), 1,2-dichloroethane (2 ml), T = 75 °C.
GLC yield based on starting alkene.
b

2
48
M. Moghadam et al. / Inorganic Chemistry Communications 13 (2010) 244–249
Mo
(L)n - 2
Reflux
+
Mo(L)n
O
O
Ph
t-BuOOH
t-Bu
Ph
O
+
H
O
Mo(L)n - 2
H
H
O
H
C
O
+
Ph
CH2
t-BuOH
Ph
t-Bu
+
O
O
O
OH
O
Ph
H
t-Bu
t-Bu
Scheme 3.
Acknowledgment
Table 5
Investigation of [MoO
2
(acac)–POM] reusability in the epoxidation of cyclooctene.a
We acknowledge the support of this work by Centre of Excel-
lence of Chemistry of University of Isfahan (CECUI).
Run
Conversion (%)b after 8 h
Amount of Mo leached (%)c
1
2
3
4
100
88
88
2.5
0
0
References
88
0
a
Reaction conditions: cyclooctene (0.5 mmol), tert-BuOOH (1.5 mmol), catalyst
[1] G. Simonneaux, P. Le Maux, Y. Ferrand, J. Rault-Berthelot, Coord. Chem. Rev.
250 (2006) 2212.
[2] T. Luts, R. Frank, W. Suprun, S. Fritzsche, E. Hey-Hawkins, H. Papp, J. Mol. Catal.
A: Chem. 273 (2007) 250.
(
0.008 mmol), 1,2-DCE (2 ml), T = 75 °C.
b
GC yield.
Determined by ICP.
c
[
[
3] G. Sienel, R. Rieth, K.T. Rowbottom, Epoxides, VCH Publishers, New York, 1985.
4] A.K. Yudin, Aziridines and Epoxides in Organic Synthesis, Wiley-VCH,
Weinheim, 2006.
[
5] S. Tangestaninejad, M. Moghadam, V. Mirkhani, I. Mohammadpoor-Baltork, K.
Ghani, J. Iran. Chem. Soc. 5 (2008) S71.
6] G. Grivani, S. Tangestaninejad, A. Halili, Inorg. Chem. Commun. 10 (2007) 914.
7] D.V. Deubel, J. Sundermeyer, G. Frenking, Inorg. Chem. 39 (2000) 2314.
8] M. Jia, A. Seifert, W.R. Thiel, Chem. Mater. 15 (2003) 2174.
34]1 and showed that these compounds were inactive
0À
[
[
a
9
-SiW O
[
[
[
42]. Here, we used tert-BuOOH towards oxidation of cyclooctene
in the presence of only polyoxometalate and the results were the
same, indicating that the polyoxometalate was almost inactive.
The most advantage of MoO
geneous nature in the oxidation reactions. Therefore, this hetero-
geneous catalyst can be easily recovered and reused.
[9] J.M. Mitchell, N.S. Finney, J. Am. Chem. Soc. 123 (2001) 862.
[10] J. Artner, H. Bautz, F. Fan, W. Habicht, O. Walter, M. Döring, U. Arnold, J. Catal.
2
(acac)–POM catalyst is its hetero-
255 (2008) 180.
[
11] S. Tangestaninejad, M. Moghadam, V. Mirkhani, I. Mohammadpoor-Baltork, K.
Ghani, Inorg. Chem. Commun. 11 (2008) 270.
In order to show the reusability of the catalyst, cyclooctene was
used as model substrate. The reactions were performed as de-
scribed above. At the end of the reaction, the catalyst was filtered
and reused in the next run. The results showed that after the first
run, the amount of epoxides was 88%. This is due to the leaching
phenomenon, in which about 2.5% of initial Mo is leached in the
first run. In the next runs, no Mo was detected in the reaction mix-
ture (Table 5). To check that whether released Mo is being ab-
sorbed on the POM surface or bonded to POM, the catalyst has
been washed thoroughly with water, methanol and diethyl ether,
and used for epoxidation of cyclooctene with tert-BuOOH. The re-
sults showed that the same amount of leaching was observed. This
may due to the trapped Mo which is not been washed with solvent.
[12] A.O. Chong, K.B. Sharpless, J. Org. Chem. 42 (1977) 1587.
[
[
13] L. Feng, E. Urnezius, R.L. Luck, J. Organomet. Chem. 693 (2008) 1564.
14] J.A. Brito, H. Teruel, G. Muller, S. Massou, M. Gomez, Inorg. Chim. Acta 361
(
2008) 2740.
[15] R. Mbeleck, K. Ambroziak, B. Saha, D.C. Sherrington, React. Funct. Polym. 67
2007) 1448.
(
[
16] K. Ambroziak, R. Pelech, E. Milchert, T. Dziembowska, Z. Rozwadowski, J. Mol.
Catal. A: Chem. 211 (2004) 9.
[17] K.V.R. Chary, H. Ramakrishna, G.M. Dhar, J. Mol. Catal. 68 (1991) L25.
[
[
[
18] R.J. Angelici, Acc. Chem. Res. 21 (1988) 387.
19] N. Chantarasiri, T. Tuntulani, N. Chanma, Eur. Polym. J. 36 (2000) 889.
20] N. Ibrahim, S. Sharif, Eur. J. Chem. 4 (2007) 531.
[21] S. Battacharjee, J.A. Anderson, Chem. Commun. (2004) 554.
[
[
22] P.J. Gonzalez, C. Correia, I. Moura, C.D. Brondino, J.J.G. Moura, J. Inorg. Biochem.
00 (2006) 1015.
23] B.M. Barney, H.-I. Lee, P.C. Dos Santos, B.M. Hoffman, D.R. Dean, L.C. Seefeldt,
Dalton Trans. (2006) 2277.
1
In conclusion, we observed that the catalytic activity of MoO
2
(a-
[24] M.C. Durrant, Inorg. Chem. Commun. 4 (2001) 60.
[
[
25] P.C.H. Mitchell, Quarterly Rev., p. 103.
26] S.M. Bruno, J.A. Fernandes, L.S. Martins, I.S. Goncalves, M. Pillinger, P. Ribeiro-
Claro, J. Rocha, A.A. Valente, Catal. Today 114 (2006) 263.
cac) can be modified by a Keggin-type polyoxometalate through
2
covalent bonding. A strong electronic effect was observed upon
attachment of polyoxometalate, leading to an intramolecular
charge separation and description of (1) as a charge-transfer com-
plex [31,43,44]. The complex (1) exhibits a good catalytic activity
in the oxidation of various olefins including non-reactive terminal
[27] N.R. Pramanik, S. Ghosh, T.K. Raychaudhuri, S. Ray, R.J. Butcher, S.S. Manda,
Polyhedron 23 (2004) 1595.
[
28] S.M. Bruno, S.S. Balula, A.A. Valente, F.A. Almeida Paz, M. Pillinger, C. Sousa, J.
Klinowski, C. Freire, P. Ribeiro-Claro, I.S. Goncalves, J. Mol. Catal. A: Chem. 270
(2007) 185.
[
[
[
29] M. Masteri-Farahani, F. Farzaneh, M. Ghandi, Catal. Commun. 8 (2007) 6.
30] I. Bar-Nahum, R. Neumann, Chem. Commun. (2003) 2690.
31] I. Bar-Nahum, H. Cohen, R. Neumann, Inorg. Chem. 42 (2003) 3677.
2
olefins. The MoO (acac)–POM catalyst is a heterogeneous catalyst
and can be easily separated and reused.

M. Moghadam et al. / Inorganic Chemistry Communications 13 (2010) 244–249
249
[
[
[
[
[
32] V. Mirkhani, M. Moghadam, S. Tangestaninejad, I. Mohammadpoor-Baltork, N.
Rasouli, Catal. Commun. 9 (2008) 2171.
33] V. Mirkhani, M. Moghadam, S. Tangestaninejad, I. Mohammadpoor-Baltork, E.
Shams, N. Rasouli, Appl. Catal. A: Gen. 334 (2008) 106.
34] V. Mirkhani, M. Moghadam, S. Tangestaninejad, I. Mohammadpoor-Baltork, N.
Rasouli, Catal. Commun. 9 (2008) 2411.
35] V. Mirkhani, M. Moghadam, S. Tangestaninejad, I. Mohammadpoor-Baltork, N.
Rasouli, Inorg. Chem. Commun. 10 (2007) 1537.
36] V. Mirkhani, M. Moghadam, S. Tangestaninejad, I. Mohammadpoor-Baltork, N.
Rasouli, Catal. Commun. 9 (2008) 219.
37] G.J.J. Chen, J.W. Mc Donald, W.E. Newton, Inorg. Chem. 15 (1976) 2612.
38] A. Tézé, G. Hervé, J. Inorg. Nucl. Chem. 39 (1977) 999.
or Carbowax 20 m. Diffuse reflectance UV–Vis (DR UV–Vis) spectra were
obtained on a Shimadzu UV 265 spectrometer using BaSO as reference
compound. {SiW11 39[O(SiR) ]} (1.5 g, 0.32 mmol) was dissolved in
degassed DMF (5 ml), and heated to 75 °C. solution of MoO (acac)
2
(0.106 g, 0.32 mmol) was added and the reactants were maintained for an
additional 2 h before being cooled. Methanol (30 ml) was added to attain the
precipitate (pale yellow color), which was carefully washed with methanol
and ether before vacuum drying.
4
Q
4
O
2
A
2
[40] L.S. Gonzalez, K.S. Nagaraja, Polyhedron 6 (1987) 1635.
[41] General procedure for oxidation reactions. All reactions were carried out in a
25 ml glass reactor which was equipped with a reflux condenser and a gas
inlet. In a typical procedure, a mixture of catalyst (1) (40 mg, 0.008 mmol),
alkene (0.5 mmol), tert-BuOOH (1.5 mmol) in 1,2- dichloroethane (2 ml) was
prepared. The reaction progress was monitored by GC. At the end of the
reaction, the catalyst was filtered. Then, the solvent was evaporated and the
crude product was purified on a silica-gel column. The identities of the
products were confirmed by 1H NMR and IR spectral data.
[
[
[
39] Experimental. All materials were of commercial reagent grade and prepared
from Merck, Aldrich or Fluka. Alkenes were obtained from Merck or Fluka and
were passed through a column containing active alumina to remove peroxide
impurities. Scanning electron microscopy (SEM) images were taken using a
Phillips XL 30 model. FT Infrared (FT-IR) spectra were obtained as potassium
À1
bromide pellets in the range of 400–4000 cm with a Nicolet Impact 400D
[42] N. Mizuno, K. Yamaguchi, K. Kamata, Coord. Chem. Rev. 249 (2005) 1944.
[43] Y. Tsuda, K. Takahashi, T. Yamaguchi, J. Mol. Catal. A: Chem. 130 (1998) 285.
[44] I. Bar-Nahum, K.V. Narasimhulu, R. Neumann, Inorg. Chem. 44 (2005) 4900.
instrument. Gas chromatography experiments (GC) were performed on a
Shimadzu GC-16A instrument using a 2 m column packed with silicon DC-200