Heterogeneous catalytic oxidation of styrene by an oxo bridged divanadium(V) complex of an acetohydrazide-Schiff base

Article abstract of DOI:10.1016/j.poly.2013.11.007

We have synthesized a μ-oxido divanadium compound [(VOL) ₂(μ-O)] with an aliphatic hydrazone ligand LH₂ = (E)-N'-(1-(2-hydroxyphenyl)ethylidene)acetohydrazide. The complex was characterized by elemental analysis, IR and UV-Vis spectroscopy and the molecular structure was established by single crystal X-ray diffraction technique. The complex has been infused over alumina to prepare a heterogeneous catalyst which was characterized by IR spectroscopy, thermogravimetric and powder XRD analyses. The catalyst has been studied for the oxidation of styrene in presence of H₂O₂ and appears to be easily recyclable. A conversion of 99.7% and selectivity of 88.1% for benzaldehyde formation and a turnover number of 1354 was detected under the most favorable reaction conditions.

Full text of DOI:10.1016/j.poly.2013.11.007

Polyhedron 69 (2014) 1–9
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Heterogeneous catalytic oxidation of styrene by an oxo bridged
divanadium(V) complex of an acetohydrazide-Schiff base
c,
Dipali Sadhukhan a, Monami Maiti a, Ennio Zangrando b, Soyeb Pathan c, Samiran Mitra a, , Anjali Patel
⇑
⇑
a Department of Chemistry, Jadavpur University, Raja S. C. Mullick Road, Kolkata 700032, West Bengal, India
b Department of Chemical and Pharmaceutical Sciences, Via Licio Giorgieri 1, 34127 Trieste, Italy
c Polyoxometalates and Catalysis Laboratory, Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 June 2013
Accepted 6 November 2013
Available online 15 November 2013
We have synthesized a l-oxido divanadium compound [(VOL)2(l-O)] with an aliphatic hydrazone ligand
LH2 = (E)-N0-(1-(2-hydroxyphenyl)ethylidene)acetohydrazide. The complex was characterized by ele-
mental analysis, IR and UV–Vis spectroscopy and the molecular structure was established by single crys-
tal X-ray diffraction technique. The complex has been infused over alumina to prepare a heterogeneous
catalyst which was characterized by IR spectroscopy, thermogravimetric and powder XRD analyses. The
catalyst has been studied for the oxidation of styrene in presence of H2O2 and appears to be easily recy-
clable. A conversion of 99.7% and selectivity of 88.1% for benzaldehyde formation and a turnover number
of 1354 was detected under the most favorable reaction conditions.
Keywords:
l-Oxido divanadium compound
Crystal structure
Catalytic oxidation of styrene
Al2O3 support
Ó 2013 Elsevier Ltd. All rights reserved.
Catalyst recycling
1. Introduction
Acid hydrazides R–CO–NH–NH2 and their corresponding hydra-
zones with aromatic carbonyl compounds R–CO–NH–N = CR1R2 are
Vanadium is a versatile bio-essential element. Its presence in
vanadate-dependent haloperoxidases [1,2] and vanadium-nitro-
genases [3], its therapeutic application as insulin-enhancing agent
in treatment of diabetes [4–10] and anticancer activity [11–14]
have stimulated a considerable amount of research. Possible phar-
macological activities of vanadate compounds are supported by the
coordinating ligands. Chelation improves absorption, transport-
ability and uptake of the metal ions into cells, thus reducing the
necessary dose of efficacy as well as the metal ion toxicity [15].
Vanadium ion in coordination complexes is capable of existing in
a range of oxidation states from 3+ to 5+ and possesses unusual re-
dox ability. In 4+ and 5+ oxidation states, vanadium is highly redox
active and acts as strong Lewis acid due to low radius to charge ra-
tio, which can stimulate various organic transformation reactions
[16]. The role of vanadium-based compounds as efficient catalysts
attests the applicability of this metal ion in bulk industrial produc-
tions, viz., the use of oxovanadium complexes in asymmetric syn-
thesis [17–21], in C–C bond formation as well as C–C, C–O and C–H
bond cleavages [22–26], involvement as an intermediate in hydro-
sulfurization of crude oils [27], oxidative halogenation and selec-
tive epoxidation of unsaturated hydrocarbons and allyl alcohols
[22,28].
the N/O donor ligands that can act as neutral as well as anionic
chelator towards metal ions and have remarkable biological impor-
tance [29]. Additionally, hydrazone metal complexes have also at-
tracted research interest for their possible pharmacological
applications [30] antimicrobial, antitumor activities [31] and also
in catalysis [32]. The usefulness of both vanadium and hydrazone
Schiff bases prompted us to investigate the structure–activity stud-
ies of vanadium-hydrazone chelate complexes. There are few re-
ports on the structures and solution state properties of vanadium
complexes of hydrazone Schiff bases derived by the condensation
of an aroyl hydrazide with an aromatic carbonyl compound [33].
A very few have been investigated for their catalytic potential
[34]. In this paper we are concerned about the structural investiga-
tion and catalytic activity of a vanadium complex chelated by a
hydrazone ligand, which is the condensation product of a less
investigated alkyl hydrazide with an aromatic ketone.
In search of catalysts for organic transformations we have direc-
ted our research on immobilization of complexes on various solid
supports. As benefits of this process, in many cases these catalysts
(or catalyst precursors) are easily recyclable and maintain their
activity after several catalytic cycles. Although the solid support
can constitute a protective environment around the metal com-
plex, the physical restrictions imposed by the matrix can make
the access of reactants to the metal centre more difficult compared
to homogeneous system affecting the conversion and sometimes
selectivity [35].
⇑
Corresponding authors. Tel.: +91 9836883638; fax: +91 033 2414 6414.
E-mail addresses: smitra_2002@yahoo.com (S. Mitra), aupatel_chem@yahoo.
com (A. Patel).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.11.007

2
D. Sadhukhan et al. / Polyhedron 69 (2014) 1–9
Catalytic oxidation of styrene using H2O2 may give rise to sev-
purchased from Aldrich Chemical Company and used as received.
VO(acac)2 was prepared by the procedure described below. In a
solution containing sodium carbonate (5.0 g) and acetylacetone
(5 mL) in distilled water was added a second solution of VO(SO4) -
ꢀ H2O (5 g) in 15 mL distilled water. The mixture was warmed in a
steam bath for precipitation of VO(acac)2 and then allowed to cool
in ice. The solid was then filtered, washed with ice cold water,
dried on a steam bath and kept in CaCl2 desiccator for further
use. Neutral active Al2O3 (Activity I-II, according to Brockmann,
Loba chemie, Mumbai) was used as received. Dichloromethane,
30% aqueous H2O2 and styrene were obtained from Merck and
used as received.
eral products. Formation of at least five products such as styrene
oxide, benzaldehyde, benzoic acid, phenylacetaldehyde and 1-
phenylethane-1,2-diol has been reported [36]. Benzaldehyde is ob-
tained with the highest yield due to fast conversion via nucleo-
philic attack of H2O2 on styrene oxide followed by the cleavage
of hydroperoxystyrene intermediate.
A recent review by Chaudhury et al. [37] admits the existence of
at least thirty structurally characterized l-oxido divanadium com-
pounds containing a [V2O3]n+ (n = 4, 3 and 2) core where both me-
tal centers are chelated by identical ligand molecules [33a,38–42].
The V–O–V bridging angle and the orientation of the terminal V@O
groups with respect to each other as well as to the bridging oxo li-
gand are characteristic features of these species. With octahedral
vanadium centers most of these complexes have a linear V–O–V
bridge with the terminal V@O groups mutually trans located
(anti-linear structure) [38]. On the other hand, when the vanadium
centers have square pyramidal geometry [33a,39–42], the terminal
and bridging oxo atoms have diverse range of arrangements
(Scheme 1), hovering between anti-linear (i) [41] and syn-angular
(iv) [39] through anti-angular (ii) [33a,40] and twist angular (iii)
[42] conformations. In the present case the [V2O3]4+ core structure
is quite unconventional. Both the vanadium centers are square-
pyramidal with the terminal V@O groups cis to the bridging oxo
ligand but acquire anti-angular orientation with respect to each-
other.
2.2. Physical measurements
C, H and N microanalyses were carried out with a Perkin-Elmer
2400 II elemental analyzer. The Fourier Transform Infrared spectra
were recorded in the range 4000–400 cmꢁ1 on a Perkin-Elmer RX I
FT-IR spectrophotometer with solid KBr pellets. The electronic
spectra in HPLC grade acetonitrile were recorded at 300 K on a Per-
kin–Elmer Lambda 40 (UV–Vis) spectrometer in a 1 cm quartz cuv-
ette in the range 800–200 nm. Thermogravimetric analyses of the
complex and the catalyst was carried out on a SII EXSTAR6000
TG/DTA 6300. The experiments were performed in N2 at a heating
rate of 10 °C minꢁ1 in the temperature range 25–500 °C using an
alumina pan. The powder X-ray diffraction (XRD) patterns of the
samples were recorded with a Scintag XDS-2000 diffractometer
Here we report the synthesis and characterization of a
l-oxido
divanadium compound [(VOL)2( -O)] with the aliphatic hydrazone
l
using Cu Ka radiation. Adsorption–desorption isotherms of sam-
ligand LH2 = (E)-N0-(1-(2-hydroxyphenyl)ethylidene)acetohydraz-
ide, along with its catalytic activity for oxidation of styrene using
H2O2 as oxidant under heterogeneous condition. The synthesized
complex was heterogenized by supporting onto anhydrous alu-
ples were recorded on a micromeritics ASAP 2010 surface area ana-
lyzer at ꢁ196 °C.
2.3. Syntheses
mina (Al2O3). The catalyst, [(VOL)2(l-O)]/Al2O3 was characterized
by thermogravimetric analysis, IR spectroscopy, powder X-ray dif-
fraction and BET surface area analysis. Catalytic activity for the oxi-
dation of styrene was evaluated by varying different parameters
such as reaction temperature, time, molar ratio of styrene to
H2O2 and amount of the catalyst. The recycled catalyst has also
been thoroughly characterized and its activity was evaluated under
optimized conditions.
2.3.1. Synthesis of the hydrazone ligand [LH2]
LH2 [(E)-N0-(1-(2-hydroxyphenyl)ethylidene)acetohydrazide]
was prepared by the condensation of acetic hydrazide (0.74 g,
10 mmol) in 200 mL of methanol with 2-hydroxyacetophenone
(1.362 g, 10 mmol) in presence of a single drop of glacial acetic acid
(catalyst). On refluxing the methanolic solution for 5 h a colorless
solution was obtained. The solvent was removed under reduced
pressure and the white residue was purified by crystallization from
MeOH. Colorless shiny crystals were obtained. Yield 0.177 g (92%).
Anal. Calc. for C10H12N2O2 (M = 192.1 g/mol): C, 62.49; H, 6.29; N,
14.57. Found: C, 62.58; H, 6.23; N, 14.49%. FT-IR bands (KBr,
2. Experimental
2.1. Materials
cmꢁ1):
m(C@N) 1606, m(C@O) 1667.
All solvents were of reagent grade and used without further
purification. Acetic hydrazide and 2-hydroxyacetophenone were
2.3.2. Synthesis of the complex [(VOL)2(
l
-O)]
The complex was synthesized by adding solid VO(acac)2
(0.270 g, 1 mmol) to 30 mL acetonitrile solution of LH2 (0.192 g,
1 mmol). The mixture was stirred for half an hour at 60 °C. The
dark brown solution was then cooled to room temperature, filtered
and kept for slow evaporation of solvent. Dark brown square crys-
tals appeared on standing overnight. Yield 0.296 g (56%). Anal. Calc.
for C20H20N4O7V2 (M = 530.28 g/mol): C, 45.30; H, 3.80; N, 10.56.
O
O
V
V
O
O
V
O
V
O
(i)
V
V
(ii)
O
O
O
O
Found: C, 45.38; H, 3.75; N, 10.60%. FT-IR bands (KBr, cmꢁ1): mC
@
@
?
O
N
O
V
(v)
1542,
mC–O(phenolate) 1383,
mC–O(enolate) 1301, mN–N 1082, mV
O
p⁄
V
V
O
V
988 and mV–(
875. UV–Vis bands (acetonitrile, nm):
p
l
–O)–V
O
O
272, LMCT 370.
(iii)
(iv)
2.3.3. Synthesis of the catalyst
Scheme 1. Possible conformations of the [V2O3]n+ (n = 4, 3, and 2) cores with
respect to bridging and terminal oxygen atoms in -oxido divanadium complexes,
(i) anti-linear, (ii) anti-angular, (iii) twist-angular, (iv) syn-angular and (v) an
The catalyst containing 15% of the complex [(VOL)2(l-O)] was
l
synthesized by impregnating 1 g of Al2O3 with a solution of
[(VOL)2( -O)] (0.15 g in 15 ml of acetonitrile) with stirring for
35 h and dried at 100 °C for 10 h. The obtained material was desig-
nated as [(VOL)2( -O)]/Al2O3.
l
unconventional anti-angular conformation with syn orientation of the bridging
oxygen with respect to both terminal oxygens in the present [(VOL)2(
complex.
l-O)]
l

D. Sadhukhan et al. / Polyhedron 69 (2014) 1–9
3
2.4. Crystallographic data collection and structure refinements
square pyramidal geometry at the vanadium centers (
s
= 0.095
for V1 and 0.093 for V2) [47]. The ONO donor ligand LH2 undergoes
enolization at the amido (HNAC@O) fragment followed by double
deprotonation to chelate three of the equatorial positions via the
amido oxygen, imino nitrogen and the phenoxo oxygen atoms
(O3, N1, O4 to V1 and O6, N3, O7 to V2). The fourth equatorial po-
sition of both metal centers is occupied by the bridging oxygen
atom O2. A terminal oxygen takes up the apical position (O1 to
V1 and O5 to V2). V1 and V2 atoms are displaced towards the ter-
minal oxygen atoms by 0.464 and 0.449 Å, respectively, from their
mean equatorial planes (O3/N1/O4/O2 plane for V1 and O6/N3/O7/
O2 for V2). The trichelating ligand mean planes show large distor-
tions from planarity (max displacements for C4 +0.224(8), O3
ꢁ0.304(6) Å in the first plane, C14 ꢁ0.409(7), O6 0.374(6) Å for
the second) and they form a dihedral angle of 56.2°.
Intensity data were collected on a Cu rotating anode equipped
with a Brucker CCD2000 detector (k = 1.54178 Å) 293 K. Data col-
lection was performed with the ‘Nonius, Supergui’ program [43].
Programs Denzo and Scalepack [44] were used for data reduction.
The structure was solved by direct methods [45] and refined by
full-matrix least-squares method with program SHELXL-97 [46]. All
non hydrogen atoms were refined anisotropically by full-matrix
least-squares based on F2. The H atoms were generated geometri-
cally and included in the final cycles of refinement in the riding
model approximation. Selected crystallographic data, experimental
conditions and relevant features of the structural refinement for
the complex are summarized in Table 1.
Of particular interest in the structure is the conformation of the
[V2O3]4+ core, which is a function of the steric requirement of the
ligand and the coordination geometry of the metal center [37].
The V1–O2–V2 bridge angle is 110.7(3)°, second to the lowest re-
ported value (109.3(6)°) of that angle in a single oxido bridged
divanadium compound [48]. The terminal V@O groups have a rare
anti-angular conformation with respect to each-other (Scheme 1)
although both are syn to the bridging oxygen atom. The torsion an-
gle between the two V@Ot bonds is 99.61° which indicates that the
2.5. Catalytic reaction procedure
The oxidation reaction was carried out in a borosilicate glass
reactor provided with a double walled condenser. The desired cat-
alyst, styrene and H2O2 mixtures were intensively stirred in the
reactor at the set constant temperature for the whole duration of
the reaction. The same reaction was carried out by varying the
parameters such as the molar ratio of substrate to H2O2, amount
of the catalyst, reaction time and reaction temperature. After com-
pletion of the reaction, catalyst was removed and the product was
extracted with dichloromethane. The product was dried with mag-
nesium sulfate and analyzed on Gas Chromatograph using BP-1
capillary column. Product was identified by comparison with the
authentic samples and finally by Gas Chromatography–Mass Spec-
troscopy (GC–MS).
terminal V@O groups are almost orthogonally twisted. The V1ꢂ ꢂ ꢂV2
0
distance is 2.957 ÅA, slightly shorter than beforehand reported
l-
oxido [V2O3]4+ complexes synthesized with hydrazone ligands
0
[33a,45]. The V@O bond distances [1.571(6) and 1.567(6) ÅA], the
vanadium to bridging oxygen bond distances [1.789(5) and
0
0
1.806(6) ÅA] and V–N ones [2.117(7) and 2.082(8) ÅA] are within
the range of analogous oxovanadium-hydrazone complexes [33a].
3. Results and discussion
3.1. Crystal structure description
3.2. Characterization of the complex [(VOL)2(l-O)] and the catalyst
[(VOL)2( -O)]/Al2O3
l
The ORTEP diagram of the complex is depicted in Fig. 1 and the
bond lengths and angles are listed in Table 2. The complex is a l-
The synthesized complex and the catalyst were characterized
by FT-IR, TGA and BET surface area. The FT-IR spectra of the com-
plex and of the catalyst are presented in Fig. 2. The spectrum of the
oxido divanadium compound containing a [V2O3]4+ core. Two non
equivalent [LV = O]+ moieties centered at V1 and V2 are connected
by an oxo bridge group (O2). Both halves of the molecule have
complex (Fig. 2a) exhibits imine stretching
(mC@N) band at
1542 cmꢁ1 indicating the coordination of the imine nitrogen atom
to the metal center [32]. The absorption at 1383 and 1301 cmꢁ1 are
Table 1
due to the stretching of mC–O (phenolate) and mC–O (enolate) groups,
Crystal data and details of crystal structure refinement.
respectively. N–N stretching band of the azo-fragment emerged at
1082 cmꢁ1. A strong band appearing at 988 cmꢁ1 is the signature of
Empirical formula
Formula weight
T (K)
C20H20N4O7V2
530.28
293
terminal V@O stretching, whereas a moderately strong V–(l-O)–V
antisymmetric stretching was observed at 875 cmꢁ1 [37]. FT-IR
spectrum of the catalyst (Fig. 2b) shows bands at 1600, 1395 and
k (Å)
1.54180
triclinic
P1 (No. 2)
8.2470(12)
8.4830(11)
17.7830(17)
94.35(2)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
1304 cmꢁ1 corresponding to mC
@
N, mC–O (phenolate) and
late) respectively, indicating the presence of the complex over the
surface of catalyst. Terminal V@O stretching and V–( -O)–V anti-
mC–O (eno-
ꢀ
l
symmetric stretching bands are averaged in the catalyst, may be
due to overlapping of bands of the complex with those of Al2O3.
The shifting in the band values may be due to interaction of the
complex with oxygen atoms of the Al2O3 surface.
TGA of the complex (Fig. 3a) shows mass loss of 3.36% within
the range of 150–290 °C, corresponding to the loss of one oxygen
atom (calculated value 3.02%), which implies cleavage of the V–
a
(°)
b (°)
97.49(2)
c
(°)
116.70(2)
1089.2(3)
2
1.617
7.644
V (Å3)
Z
Dcalc (Mg/m3)
l
(mmꢁ1
)
F(000)
540
Crystal size (mm3)
h range (°)
0.21 ꢀ 0.15 ꢀ 0.11
(l-O)–V bond and dissociation of the molecule in two monomeric
2.5–54.1
units. The decomposition of the complex was observed in several
steps from 250 °C. The data indicate that the complex is stable
up to 250 °C. The TGA of the catalyst (Fig. 3b) shows initial mass
loss 2.1% up to 150 °C likely due to the removal of adsorbed water
molecules. No significant loss occurs up to 450 °C, indicating that
the stability of the complex increases after supporting on Al2O3.
Reflections collected
Independent reflections (Rint
Data/parameters
S
19414
)
2592 (0.085)
2592/302
0.99
Final R indices [I > 2
r(I)]
R1 = 0.0562, wR2 = 0.1535
0.43, ꢁ0.44
D
q
(e Åꢁ3
)

4
D. Sadhukhan et al. / Polyhedron 69 (2014) 1–9
Fig. 1. ORTEP drawing of the complex (ellipsoids at 40% probability level).
Table 2
benzaldehyde possibly because of the fast conversion of styrene
oxide to benzaldehyde.
Bond lengths (Å) and angles (°) of the complex.
Bond lengths
V1–O1
V1–O2
V1–O3
V1–O4
V1–N1
1.571(6)
1.789(5)
1.938(7)
1.804(6)
2.117(7)
V2–O2
V2–O5
V2–O6
V2–O7
V2–N3
1.806(6)
1.567(6)
1.946(6)
1.810(6)
2.082(8)
3.4. Effect of temperature
The effect of temperature on the oxidation of styrene was inves-
tigated at three different temperatures viz. 60, 80 and 100 °C, keep-
ing the other parameters fixed: namely styrene amount (10 mmol),
30% H2O2 (30 mmol), catalyst (30 mg) and reaction time (24 h). The
results shown in Fig. 4 reveal that 16.9%, 99.7% and 99.6% of con-
version was found corresponding to the temperature of 60, 80
and 100 °C, respectively. At 80 °C, the conversion of styrene was
99.7% with 88% selectivity of benzaldehyde and 12% selectivity of
benzoic acid. On the other hand at elevated temperature (100 °C),
99.6% styrene conversion was found with 79% selectivity of benz-
aldehyde and 21% selectivity of benzoic acid, i.e. a decrease in
selectivity for benzaldehyde was observed. This is due to over oxi-
dation of benzaldehyde to benzoic acid at elevated temperature.
Hence, further optimization of the conditions was carried out at
80 °C temperature.
Bond angles
O1–V1–O2
O1–V1–O3
O1–V1–O4
O1–V1–N1
O2–V1–O3
O2–V1–O4
O2–V1–N1
O3–V1–O4
O3–V1–N1
O4–V1–N1
108.0(3)
O2–V2–O5
O2–V2–O6
O2–V2–O7
O2–V2–N3
O5–V2–O6
O5–V2–O7
O5–V2–N3
O6–V2–O7
O6–V2–N3
O7–V2–N3
V1–O2–V2
106.7(3)
105.0(3)
104.8(3)
97.7(3)
85.1(3)
102.8(3)
150.6(3)
144.9(3)
74.3(3)
83.8(3)
85.6(3)
103.9(3)
151.1(3)
104.6(3)
104.2(3)
98.3(3)
145.5(3)
74.1(3)
83.3(3)
110.7(3)
3.5. Effect of the amount of H2O2
In order to determine the effect of H2O2 on the oxidation of sty-
rene to benzaldehyde, we studied four different styrene to H2O2
molar ratios (1:1, 1:2, 1:3, and 2:1), keeping other parameters
fixed: namely catalyst (30 mg), temperature (80 °C) and reaction
time (24 h). The results are shown in Fig. 5. Styrene to H2O2 molar
ratio of 1:1 and 1:2 resulted in 27.3 and 68.8% conversion, respec-
tively, and with styrene to H2O2 molar ratio of 1:3, conversion in-
creased to be nearly 99.7%, keeping fixed all other conditions.
However, conversion was found to remain almost the same (99%)
when the styrene to H2O2 molar ratio was further changed to
1:4. Therefore, 1:3 M ratio of styrene to H2O2 was found to be
the most favorable in terms of conversion of styrene.
3.6. Effect of the amount of catalyst
Fig. 2. FT-IR spectra of (a) [(VOL)2(
l-O)] and (b) [(VOL)2(l-O)]/Al2O3.
The amount of catalyst has a significant effect on the oxidation
of styrene. Six different amounts of catalyst viz., 10, 15, 20, 25, 30
The surface area of the catalyst (93.19 m2/g) expands signifi-
cantly when the complex (3.23 m2/g) is supported over Al2O3
(81 m2/g).
and 35 mg which contain a vanadium concentration (mole ꢀ 10ꢁ5
)
of 0.50, 0.75, 1.01, 1.26, 1.51, 1.76 were used, keeping fixed all
other reaction parameters, namely temperature (80 °C), styrene
amount (10 mmol), 30% H2O2 (30 mmol) and reaction time
(24 h). The results are shown in Fig. 6, indicating 76.1%, 80.0%,
89.1%, 93.4%, 99.7% and 99.8% conversion corresponding to an
amount of 10, 15, 20, 25, 30 and 35 mg of catalyst, respectively.
Lower conversion of styrene into benzaldehyde at low quantity
3.3. Oxidation of styrene using hydrogen peroxide and the catalyst
Generally, styrene oxidation gives styrene oxide, benzaldehyde
and benzoic acid in perceptible yield. However under the present
reaction conditions, the major oxidation product obtained was

D. Sadhukhan et al. / Polyhedron 69 (2014) 1–9
5
Fig. 3. Thermogram of (a) [(VOL)2(l-O)] and (b) [(VOL)2(l-O)]/Al2O3.
Fig. 4. Effect of temperature;% Conversion is based on styrene; time = 24 h; molar
ratio of styrene to H2O2: 1:3; amount of catalyst = 30 mg.
Fig. 5. Effect of mole ratio;% Conversion is based on styrene; time = 24 h;
temperature = 80 °C; amount of catalyst = 30 mg.
of catalyst may be due to fewer catalytic sites. The maximum per-
centage conversion was observed with 30 mg catalyst, and this
amount was taken as optimal.
oxidation of styrene indicating that the catalytic activity occurs
in presence of the complex only. The same reaction was carried
out by taking the active amount of the complex. It was found that
the active catalyst (complex) gives 97.1% conversion of styrene
3.7. Effect of time
The time dependence of catalytic solvent free oxidation of sty-
rene was studied by performing the reaction of styrene (10 mmol)
with 30% H2O2 (30 mmol) in presence of 30 mg of catalyst at 80 °C
with constant stirring. The percentage of conversion was moni-
tored at different reaction times. Fig. 7 shows that increasing the
reaction time, the % conversion also increases. Initial conversion
of styrene increased with the reaction time that more time is re-
quired for the formation of reactive intermediate (substrate + cata-
lyst), which finally converted into the products. In addition Fig. 7
shows that maximum percentage conversion was observed for
24 h of reaction time.
Thus the best conditions for maximum % conversion of styrene
to benzaldehyde are characterized by the mole ratio of styrene to
H2O2 of 1:3, 30 mg of catalyst and 24 h reaction time at 80 °C.
3.8. Control experiment
The control experiments with Al2O3 and the complex were also
carried out under optimized conditions with styrene and H2O2. It
can be seen from Table 3 that Al2O3 is inactive towards the
Fig. 6. Effect of amount of catalyst;% Conversion is based on styrene; time = 24 h;
temperature = 80 °C; molar ratio of styrene to H2O2: 1:3.

6
D. Sadhukhan et al. / Polyhedron 69 (2014) 1–9
Table 4
Heterogeneity test.
Catalyst
% Conversion
% Selectivity
Benzaldehyde
[(VOL)2(
Filtrate (8 h)
l
-O)]/Al2O3 (6 h)
19.8
19.9
93.4
93.2
% Conversion is based on styrene; amount of catalyst = 30 mg; molar ratio of sty-
rene to H2O2: 1:3, temperature 80 °C.
3.11. Characterization of the regenerated catalyst
The catalyst, [(VOL)2(l-O)]/Al2O3 was regenerated in order to
test its stability. The regenerated material (R-[(VOL)2(
l
-O)]/
Al2O3) was characterized by FT-IR, and powder X-ray diffraction
in order to confirm the retention of the catalyst structure, after
completion of the reaction.
The FT-IR data for the fresh as well as the regenerated catalysts
are represented in Fig. 8. The characteristic FT-IR bands (Fig. 8a) for
the catalyst appeared at 1600, 1395 and 1304 cmꢁ1, and similar
bands (Fig. 8b) were shown by the regenerated catalyst. No appre-
ciable shift in the FT-IR band position of the regenerated catalyst
compared to the fresh catalyst indicates the stability of supported
catalyst. The XRD pattern of the powder sample (Fig. S1) of
Fig. 7. Effect of reaction time;% Conversion is based on styrene; amount of
catalyst = 30 mg; temperature = 80 °C; molar ratio of styrene to H2O2: 1:3.
with 87.7% selectivity of benzaldehyde. Almost the obtained same
activity (99.7% conversion of styrene with 88.1% selectivity of
benzaldehyde) for supported catalyst indicates that the complex
is the real active species. Thus, we succeeded in supporting the
complex onto Al2O3 without any significant loss in activity, hence
overcoming the traditional problems of homogeneous catalysis.
[(VOL)2(l-O)] is maintained in the catalyst [(VOL)2(l-O)]/Al2O3. A
decrease in intensity occurs with broadening of peaks upon infu-
sion of the complex with Al2O3 but the crystalline nature retains.
The XRD pattern of the powdered sample of the fresh and regener-
ated catalyst (Fig. 9) replicate each other suggesting that no struc-
tural change occurs after completion of the catalytic cycle of
reaction with active participation of the catalyst.
3.9. Heterogeneity test
Heterogeneity test was carried out for the oxidation of styrene
as an example. For a rigorous proof of heterogeneity, a test was
carried out by filtering catalyst from the reaction mixture at
80 °C after 6 h and the filtrate was allowed to react up to 8 h. This
reaction mixture and the filtrate were analyzed by gas chromatog-
raphy. No change in the % conversion as well as % selectivity was
found indicating that the present catalyst falls into category C,
i.e. active species does not leach and the observed catalysis is truly
heterogeneous in nature. The results are presented in Table 4.
3.12. Discussion
Oxidation of styrene to benzaldehyde occurs via the formation
of styrene oxide intermediate [36]. H2O2 being a strong oxidant
leads to the oxidative cleavage of the epoxide ring forming benzal-
Table 5
Recycling of the catalyst.
Catalyst
Cycle
% Conversion
% Selectivity
TON
Benzaldehyde
3.10. Recycling of the catalyst
[(VOL)2(
l
-O)]/Al2O3
Fresh
1
2
99.7
99.8
99.2
88.1
88.2
88.1
1354
1345
1347
Oxidation of styrene was carried out with the recycled catalyst,
under the optimized conditions. The catalyst was removed from
the reaction mixture after completion of the reaction by simple fil-
tration, washed with dichloromethane and dried at 100 °C. The cat-
alyst was recycled in order to test its activity as well as stability.
The obtained results presented in Table 5 point out that there
was no appreciable change in selectivity. However, a little decrease
in conversion was observed showing that the catalysts are stable
and can be regenerated for repeated use.
% Conversion is based on styrene; amount of catalyst = 30 mg; molar ratio of sty-
rene to H2O2: 1:3, time 24 h; temperature 80 °C.
Table 3
Control experiments for the oxidation of styrene:
Catalyst
% Conversion
% Selectivity
Benzaldehyde
Al2O3
–
–
[(VOL)2(
[(VOL)2(
l
-O)]
97.1
99.7
87.7
88.1
l-O)]/Al2O3
% Conversion is based on styrene; amount of catalyst = 26.1 mg (Al2O3), 3.9 mg
([(VOL)2( -O)]), 30 mg ([(VOL)2( -O)]/Al2O3); molar ratio of styrene to H2O2: 1:3,
temperature 80 °C.
l
l
Fig. 8. FT-IR spectra of (a) [(VOL)2(l-O)]/ Al2O3 and (b) R-[(VOL)2(l-O)]/Al2O3.

D. Sadhukhan et al. / Polyhedron 69 (2014) 1–9
7
dehyde. d0 Metal complexes with strong Lewis acidic character
(MoVI, VV, TiIV) catalyze the epoxide formation step. The isolation
and characterization of the actual epoxidizing species have not
yet achieved. It has been generally accepted that a metal-hydro-
peroxide intermediate is formed. Mimoun and co-workers synthe-
sized vanadium(V) alkylperoxidic complexes with several
chelating ligands [49] and suggested that the metal-(alkyl/hydro)
peroxide complexes are the most probable epoxidizing species.
Based on their suggestion, we have proposed a plausible mecha-
nism of styrene oxidation reaction with H2O2 catalyzed by the
of the weakly coordinated hydroperoxy oxygen or from the axial
position trans to the vanadyl oxygen. This atom arrangement
around the vanadium center leads to a nucleophilic attack of the
peroxo oxygen to the coordinated electron deficient styrene fol-
lowed by a migratory insertion of styrene between the vana-
dium–oxygen bond (r–p rearrangement) forming a pseudocyclic
dioxametallocyclopentane. A rapid decomposition of the pseudo-
cyclic moiety occurs by a 1,3 dipolar cycloreversion process result-
ing in the formation of epoxide and the vanadium(V) hydroxyl
complex. The epoxide (styrene oxide) is converted to benzaldehyde
via the nucleophilic attack of H2O2 on styrene oxide followed by
the cleavage of hydroperoxystyrene intermediate (Scheme 2).
Fig. 9 suggests that the powder XRD pattern of the regenerated cat-
alyst closely matches that of the fresh catalyst justifying the recov-
ery and recyclability of the catalyst. Unfortunately, any of the
intermediate species proposed in Scheme 2 could not be isolated.
Our group has previously reported the oxidation of styrene by a
mononuclear oxovanadium(IV) complex [50] which resulted in
99.1% selectivity for benzaldehyde with TON 1151. The present
dinuclear oxovanadium(V) complex shows a small decrease in
benzaldehyde selectivity (88.1%) but a considerable increase in
TON (1354) compared to the mononuclear complex. The increase
in TON can be justified with the fact that each molecule of the pres-
ent complex has two metal centers accessible to a hydroperoxide
anion. Hence there is a greater probability of the nucleophilic at-
tack to take place in the present dinuclear complex compared to
the previously reported mononuclear complex. This hypothesis
can further be rationalized comparing the catalytic efficacy of the
present complex with respect to a number of mononuclear oxova-
nadium complexes immobilized on polymer support, encapsulated
in aluminosilicate zeolite-NaY and also immobilized on ordered
mesoporous silica (MCM-41) reviewed by Mourya et al. [36]. Max-
imum selectivity of benzaldehyde 82.4% with 52% conversion and
TON = 8 is observed with the compound [VIVO(hpbmz)2]. Highest
TON = 2753 was obtained with [VIVO(saldien)2]-MCM-41 but only
with 41% conversion and 7.7% selectivity of benzaldehyde and
present complex [(VOL)2(l-O)] (Scheme 2).
Styrene is a good Lewis base and is attracted by vanadium(V)
(strong Lewis acid) and binds to the metal center by displacement
10
20
30
40
50
2
θ
(degree)
Fig. 9. Powder XRD pattern of fresh catalyst (black) and regenerated catalyst (red).
(Color online.)
H+ + HOO-
H2O2
O
V
O
O
O
V
O
+
OH
V L
L
V L
L
OOH
I
-OOH
II
Ph
H
C
CH2
O
O
OH
-H2O
V L
H
O
O
V
L
O
O
Ph
H
Ph
H
OH
H
+
C
CH2
V
L
II
C
CH2
O
H
O
V
O
V
L
C
O
O
L
O
CH2
Ph
H
Ph
H
C
IV
CH2
III
H
O
O
O
H2O
H2C
O
+
+
OOH
H
O OH
Scheme 2. Mechanistic approach of oxidation of styrene to benzaldehyde catalyzed by [(VOL)2(l-O)].

8
D. Sadhukhan et al. / Polyhedron 69 (2014) 1–9
the best conversion (97%) was observed with [VVO2(sal-ambmz)]-Y
with TON = 151 and benzaldehyde selectivity 54.9%. Considering
all these three factors together, the present catalyst [(VOL)2
(l-O)]/Al2O3 results in 99.7% conversion with benzaldehyde selec-
tivity 88.1% and TON = 1354, the best result observed among the
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Acknowledgements
Financial assistance from CSIR [project no. 01(2491)/11/EMR-II]
to D.S. and University Grant Commission to M.M. are highly appre-
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Appendix A. Supplementary data
CCDC 915423 contains the supplementary crystallographic data
for the complex. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2013.11.007.
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