Polymer supported vanadium and molybdenum complexes as potential catalysts for the oxidation and oxidative bromination of organic substrates

Article abstract of DOI:10.1039/b600822d

The Schiff base (H₂fsal-ohyba) derived from 3-formylsalicylic acid and o-hydroxybenzylamine has been covalently bonded to chloromethylated polystyrene cross-linked with 5% divinylbenzene (abbreviated as PS-H ₂fsal-ohyba, I). Treatmen

Full text of DOI:10.1039/b600822d

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PAPER
www.rsc.org/dalton | Dalton Transactions
Polymer supported vanadium and molybdenum complexes as potential
catalysts for the oxidation and oxidative bromination of organic substrates
a
a
b
Mannar R. Maurya,* Umesh Kumar and P. Manikandan
Received 19th January 2006, Accepted 21st April 2006
First published as an Advance Article on the web 28th April 2006
DOI: 10.1039/b600822d
The Schiff base (H
covalently bonded to chloromethylated polystyrene cross-linked with 5% divinylbenzene (abbreviated as
PS-H fsal-ohyba, I). Treatment of [VO(acac) ] with PS-H fsal-ohyba in dimethylformamide (DMF)
gave the oxovanadium(IV) complex PS-[VO(fsal-ohyba)·DMF] (1). Complex 1 can be oxidized into the
dioxovanadium(V) species, PS-K[VO (fsal-ohyba)] (2) on aerial oxidation in the presence of KOH or
into the oxoperoxo species, PS-K[VO(O )(fsal-ohyba)] (3) in the presence of H and KOH in DMF
suspension. Similarly, PS-[MoO
(fsal-ohyba)·DMF] (4) has been isolated by the reaction of
MoO (acac) ] with PS-H fsal-ohyba. All these complexes have been characterised by various
2
fsal-ohyba) derived from 3-formylsalicylic acid and o-hydroxybenzylamine has been
2
2
2
2
2
2
O
2
2
[
2
2
2
techniques. These complexes catalyse the oxidation of styrene, ethylbenzene and phenol efficiently.
Styrene gives five reaction products namely styrene oxide, benzaldehyde, 1-phenylethane-1,2-diol,
benzoic acid and phenylacetaldehyde, while ethylbenzene gives benzaldehyde, phenyl acetic acid,
styrene and 1-phenylethane-1,2-diol. The oxidation products of phenol are catechol and
p-hydroquinone. These catalysts are also able to catalyse the oxidative bromination of salicylaldehyde to
5
-bromosalicylaldehyde with ca. 80% selectively in the presence of aqueous 30% H
2
2
O /KBr, a reaction
similar to that exhibited by vanadate-dependent haloperoxidases. Their corresponding neat complexes
have also been prepared and their catalytic activities have been compared.
Introduction
Discovery of vanadium(V) in the active site of vanadate-
dependent haloperoxidases (V-HalPO) and its importance in
1–8
catalytic reactions has stimulated research on the coordination
9
chemistry of vanadium. These enzymes catalyse the oxidation of
halides, by peroxide, to hypohalous acid which further halogenates
4,5
hydrocarbons non-enzymatically. Many vanadium complexes
show catalytic activity towards oxidative halogenations along
10–13
with other oxidation reactions.
However, these complexes
Scheme 1
generally decompose during the catalytic reaction and thus
make their recovery from the catalytic reaction mixture difficult.
Immobilization of these materials onto polymer supports through
covalent attachment is one of the important means to solve
this problem. Chloromethylated polystyrene cross-linked with
divinylbenzene is one of the most widely employed macromolecu-
evidence has been presented for peroxide binding, in the presence
of H
2
2
O , to the vanadium centre. The catalytic potential of these
complexes has been demonstrated by studying the oxidation of
styrene, ethylbenzene and phenol. The ability of these complexes
to mimic oxidative bromination in the presence of H
2
2
O and
14–17
lar supports.
These supported materials offer advantageous
bromide has been demonstrated by considering salicylaldehyde
as an example. The corresponding polymer-anchored as well as
neat dioxomolybdenum(VI) complexes have also been prepared
for comparison of their catalytic properties.
features of heterogeneous catalysis such as thermal stability,
selectivity and recyclability to homogeneous systems and ease their
separation from the reaction products also leading to operational
18,19
flexibility.
Herein we report the preparation and characterisation of
polymer-anchored oxovanadium(IV) and dioxovanadium(V) com-
Experimental
plexes of polymer-anchored ligand PS-H
The corresponding neat complexes i.e. complexes of ligand H
2
fsal-ohyba (I), Scheme 1.
fsal-
sal-ohyba (III) have also been prepared. Spectral
Materials and methods
2
Chloromethylated polystyrene [18.9% Cl (5.3 mmol Cl per gram
of resin)] cross-linked with 5% divinylbenzene was obtained as
a gift from Thermax Limited, Pune, India. Analytical reagent
ohyba (II) and H
2
a
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee,
grade V
2
O
5
, ethylbenzene (Loba Chemie, India), ammonium
2
47667, India. E-mail: rkmanfcy@iitr.ernet.in
b
heptamolybdate, acetylacetone (E. Merck, India), aqueous 30%
Catalysis and Inorganic Chemistry Division, National Chemical Laboratory,
Pune, 411008, India
H
2
O
2
, salicylaldehyde, phenol (Ranbaxy, India), styrene (Acros
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organics, USA), and other chemicals were used as obtained.
PS-K[VO
2
(fsal-ohyba)] 2. Complex 1 (1.5 g) was suspended in
20
21,22
23
24
H
2
fsal-ohyba,
H
2
sal-ohyba,
[VO(acac)
2
], [MoO
2
(acac)
2
],
aqueous KOH (1.12 g, 20 mmol in 15 ml) and air was bubbled
through the suspension for ca. 48 h. During this period the color
of the beads slowly changed to dark red. They were filteredoff ,
25
26
3
-formylsalicylic acid, and o-hydroxybenzylamine were pre-
pared according to the methods reported in the literature.
◦
Elemental analyses of the ligands and complexes were ob-
tained by an Elementar model Vario-EL-III. IR spectra were
recorded as KBr pellets on a Nicolet NEXUS Aligent 1100
FT-IR spectrometer. Electronic spectra of the polymer-anchored
compounds were recorded in Nujol on a Shimadzu1601 UV-Vis
spectrophotometer by a layering mull of the sample on the inside
of one of the cuvettes while keeping the other one layered with
Nujol as reference. Other ligands and complexes were recorded in
washed with water and dried at 120 C in an air oven.
PS-K[VO(O
pended in 15 ml water and to this KOH (0.56 g, 10 mmol) and 30%
aqueous H (1.5 ml) ware added and the suspended reaction
mixture was stirred at room temperature. After ca. 10 h a further
2
)(fsal-ohyba)] 3. Complex 1 (1.5 g) was sus-
2
O
2
1
1
ml of H
2
O was added and stirring continued for an additional
2
0 h. During this period the colour of the beads slowly changed
to orange. They were filtered off, washed with H
room temperature over silica gel.
1
2
O and dried at
methanol. H NMR spectra were obtained on a Bruker 200, and
5
1
V NMR spectra on a Bruker Avance 400 MHz spectrometer at
4.73 MHz with common parameter settings. NMR spectra were
9
PS-[MoO
the method reported by Syamal and Singh. Polymer-anchored
ligand PS-H fsal-ohyba (2.0 g) was allowed to swell in DMF
20 ml) for 2 h. [MoO
above suspension and the reaction mixture was heated at 90 C
in an oil bath for 10 h with stirring. After cooling to ambient
temperature, the orange-red polymer-anchored complex 4 was
filtered off, washed with hot DMF followed by hot methanol and
2
(fsal-ohyba)·DMF] 4. This was prepared following
5
1
recorded in DMSO-d , and d( V) values are referenced relative to
6
20
VOCl
temperature on a Bruker EMX X-band spectrometer operating at
00 kHz field modulation at room temperature. The microwave
3
as external standard. EPR spectra were recorded at room
2
(
2
(acac)
2
] (7.28 g, 22 mmol) was added to the
1
◦
frequency was calibrated using a frequency counter of the
Microwave Bridge ER 041 XG-D. The Bruker Simfonia software
package was used in the spectral simulations and to calculate
hyperfine coupling constants. The magnetic susceptibilities of
the oxovanadium(IV) complexes were measured at 298 K with
a Vibrating Sample Magnetometer model 155, using nickel as
standard. Diamagnetic corrections were carried out using Pascal’s
◦
dried at 120 C in an air oven.
[
VO(fsal-ohyba)] 5.
1.355 g, 5 mmol) in dry methanol (50 ml) was treated with
VO(acac) ] (1.33 g, 5 mmol) dissolved in dry methanol (25 ml)
A
stirred solution of
2
H fsal-ohyba
(
[
27
increments. Thermogravimetric analyses of the complexes were
carried out using a TG Stanton Redcroft STA 780 under oxygen
atmosphere. Scanning electron micrographs (SEM) of catalysts
were recorded on a Leo instrument model 435 VP. The samples
were coated with a thin film of gold to prevent surface changing
and to protect the surface material from thermal damage by the
electron beam.
2
and the resulting reaction mixture was refluxed in an oil bath for
h. After cooling to ca. 10 C overnight, a green precipitate of 5
was filtered off, washed with methanol and dried in vacuo. Yield
0%.
◦
2
8
K[VO(O)
suspended in methanol (50 ml) and after addition of KOH (0.112 g,
mmol) and 30% H (2 ml) air was bubbled through for about
36 h with stirring. During this period, 0.5 ml portions of H were
2
(fsal-ohyba)] 6. Complex 5 (0.672 g, 2 mmol) was
2
2
O
2
Preparation of ligand
2
O
2
added ca. every 9 h and the green suspension slowly disappeared.
After keeping the solution at 10 C overnight the yellow solid that
separated was filtered off, washed with cold methanol and dried
in vacuo. Yield 63%.
Polymer anchored ligand, PS-H
2
fsal-ohyba I. Polymer-
◦
anchored ligand, PS-H fsal-ohyba was prepared following the
2
20
procedure developed by Syamal and Singh. Chloromethylated
polystyrene (3.0 g) was allowed to swell in DMF (30 ml) for 2 h. A
solution of H
2
fsal-ohyba (12.2 g, 45 mmol) in DMF (40 ml) was
[K(H O)][VO (fsal-ohyba)] 7. A solution of PPh (0.393 g,
2
2
3
added to the above suspension followed by triethylamine (4.50 g)
in ethylacetate (100 ml). The reaction mixture was heated at 90 C
1.5 mmol) in 10 ml of acetonitirile was added to 6 (0.407 g,
1 mmol) dissolved in 15 ml acetonitrile and the reaction mixture
was refluxed on a water bath for 6 h. After cooling and reducing
the volume of the solvent to ca. 5 ml it was kept at 10 C overnight
and a greenish solid separated out. This was filtered off, washed
◦
for 15 h with mechanical stirring in an oil bath. After cooling
to room temperature, the yellow resins were filtered off, washed
thoroughly with hot DMF followed by methanol and dried in an
◦
◦
air oven at 120 C.
with cold methanol and dried in vacuo. Yield 87%.
[
MoO
ohyba (0.271 g, 1 mmol) in 30 ml of methanol was added
MoO (acac) ] (0.33 g, 1 mmol) with vigorous shaking and a clear
2
(fsal-ohyba)·MeOH] 8. To a stirred solution of H
2
fsal-
Preparation of complexes
[
2
2
PS-[VO(fsal-ohyba)·DMF] 1. Polymer-anchored ligand PS-
solution was obtained. After filtering, the solution was refluxed
on a water bath for 6 h and thereafter the volume was reduced to
ca. 10 ml and kept in a refrigerator overnight. The yellow solid
which separated was filtered off, washed with methanol and dried
in vacuo. Yield 75%.
H fsal-ohyba (1.50 g) was allowed to swell in DMF (25 ml) for 2 h.
2
A solution of [VO(acac) ] (5.30 g, 20 mmol) in 20 ml DMF was
2
added to the above suspension and the reaction mixture was heated
◦
at 90 C in an oil bath for 15 h with slow mechanical stirring. After
cooling to ambient temperature, the dark green polymer-anchored
complex was filtered off, washed with hot DMF followed by hot
[VO(sal-ohyba)] 9. This complex was prepared analogously
◦
methanol and dried at 120 C in an air oven.
to 5, replacing H
2
fsal-ohyba with H sal-ohyba. After 5 h of
2
3
562 | Dalton Trans., 2006, 3561–3575
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refluxing, the precipitated blackish solid was filtered off, washed
with methanol and dried in vacuo over silica gel. Yield 71%.
analysed using gas chromatography after specific time intervals by
withdrawing a small aliquot.
[
K(H
was suspended in methanol (40 ml) and after addition of KOH
0.561 g, 1 mmol) air was bubbled through the suspension
2
O)][VO
2
(sal-ohyba)] 10. Complex 9 (0.292 g, 1 mmol)
Oxidative bromination of salicylaldehyde. In a typical reaction,
aqueous 30% H O (2.27 g, 20 mmol) was added to the mixture
2
2
(
of salicylaldehyde (1.22 g, 10 mmol) and KBr (2.38 g, 20 mmol)
taken in 10 ml of water. An appropriate catalyst (30 mg) and
H SO (0.490 g, 5 mmol) were added to it and the reaction mixture
for 2 days with occasional shaking. During this period, the
blackish suspension slowly dissolved. After filtering and reducing
the volume of the solvent to ca. 5 ml, it was kept at room
temperature where a brownish-yellow solid slowly precipitated.
This was filtered off, washed with methanol and dried in vacuo.
Yield: 69%.
2
4
was stirred at room temperature. An additional 15 mmol H SO
2
4
was added to the reaction mixture in three equal portions at
20 min intervals under continuous stirring. After 2 h, the separated
white product was filtered off, washed with water followed by
diethyl ether and dried in air. The crude mass was dissolved in
CH Cl ; the insoluble solid, if any, was filtered off and the solvent
[MoO
2
(sal-ohyba)·MeOH] 11. Complex 11 was prepared as
21
2
2
reported previously. Yield 60%.
evaporated. A CH Cl solution of this material was subjected
2
2
to gas chromatography and the identity of the products was
confirmed by GC-MS.
Reactivity of PS-K[VO(O
2
)(fsal-ohyba)] with PPh
3
Peroxo complex 3 (1.5 g) was suspended in 20 ml of acetonitrile
Results and discussion
and after adding PPh
was heated at 80 C for 12 h with slow stirring. During this period
the colour of the beads slowly changed to dark red. They were
3
(5.25 g, 20 mmol), the suspended mixture
◦
Synthesis, reactivity and solid state characteristics
◦
The chloromethylated polystyrene, cross-linked with 5% divinyl-
filtered off, washed with acetonitrile and dried at 120 C in an
benzene, reacts with ligand II in DMF in presence of triethylamine
air oven. Spectral data of the obtained complex matched well
to give polymer-anchored ligand PS-H
2
fsal-ohyba (I). Carring out
with 2.
◦
the reaction at 90 C was required for effective anchoring. At
this temperature the ligand did not decompose due to its high
melting point. Miller and Sherrington have used refluxing toluene
to carry out the anchoring of organic ligands to chloromethylated
Catalytic reactions
Oxidation of styrene. Oxidation of styrene was carried out
in a 50 ml reaction flask fitted with a water condenser. The
catalysts were swelled in acetonitrile for 2 h prior to use. In a
typical oxidation reaction, styrene (1.04 g, 10 mmol) and aqueous
28
polystyrene. During this process the –COOH group of 3-
formylsalicylic acid reacts with the –CH Cl group as shown in
2
eqn (1) (for I and II, see Scheme 1). The remaining chlorine content
of 1.5% (0.42 mmol Cl per gram of resin) in the anchored ligand
suggests roughly 92% loading of the ligand.
30% H
2
O
2
(2.27 g, 20 mmol) were mixed in 20 ml acetonitrile
◦
and the reaction mixture was heated to 80 C. An appropriate
catalyst (0.040 g) was added to the reaction mixture and stirred for
(C2H5)3N
PSꢀCl + II −− −− −→ I + HCl
(1)
6
h. The progress of the reaction was monitored by withdrawing
[
DMF]
reaction samples at different time intervals and analysing them
quantitatively by gas chromatography. The identities of the
products were confirmed by GC-MS (Perkin-Elmer, Clarus 500).
The effect of various parameters such as temperature, amount of
oxidant and catalyst were checked to optimise the conditions for
the best catalyst performance.
The anchored ligand on reaction with [VO(acac)
2
] in DMF
gave the oxovanadium(IV) complex PS-[VO(fsal-ohyba)·DMF] (1).
On aerial oxidation of 1 in the presence of KOH in DMF, the
dioxovanadium(V) complex PS-K[VO
tained. Similarly, addition of H
to PS-[VO(fsal-ohyba)·DMF]
suspended in acetonitrile in the presence of KOH gave the oxo-
monoperoxo complex PS-K[VO(O )(fsal-ohyba)] (3). A change in
2
(fsal-ohyba)] (2) was ob-
2
O
2
Oxidation of ethylbenzene. Polymer anchored catalysts after
swelling in acetonitrile as stated above, were used for the oxidation
of ethylbenzene. Ethylbenzene (0.42 g, 4 mmol), 30% aqueous
2
the colour of the beads can also be visualized during the reactions.
Eqn (2), (3) and (4) represent the synthetic procedures.
H
2
O
2
(0.906 g, 8 mmol) and catalyst (0.050 g) in 10 ml acetonitrile
◦
PS-H
2
fsal-ohyba + [VO(acac)
]
2
were heated at 80 C and the reaction was monitored as mentioned
above. Various parameters such as amount of oxidant and catalyst,
and temperature were considered in order to study their effect on
the reaction products. However, the basic procedure was the same
as outlined above.
(
2)
IV
→
PS-[V O(fsal-ohyba) · DMF] 1 + 2Hacac
IV
1
2
PS-[V O(fsal-ohyba) · DMF] + /
2
O
2
+ 2KOH
(3)
V
→
2PS-K[V O
2
(fsal-ohyba)] 2 + DMF + H
2
O
Oxidation of phenol. In a typical experiment, phenol (4.7 g,
IV
0
.05 mol) and 30% aqueous H
2
O
2
(5.67 g, 0.05 mol) were
PS-[V O(fsal-ohyba) · DMF] + KOH + H O
2
2
(
4)
suspended in 2 ml of acetonitrile and the reaction mixture was
V
O + H+
→
PS-K[V O(O
2
)(fsal-ohyba)] 3 + DMF + H
2
◦
heated in an oil bath with stirring at ca. 80 C. When an appropriate
catalyst (0.025 g), after swelling in acetonitrile, was charged into it,
the reaction was considered to begin. The reaction products were
The dioxo complex 2 can also be obtained by the reaction of
peroxo complex 3 with PPh ; eqn (5). During this process PPh
3
3
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Dalton Trans., 2006, 3561–3575 | 3563

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abstracts one of the oxygen atoms from the peroxo group of 3.
V
PS-K[V O(O
2
)(fsal-ohyba) + PPh
3
(
5)
V
→
PS − K[V O
2
(fsal-ohyba)] 2 + OPPh
3
The reaction between [MoO
straight forward, where acetylacetonate groups are replaced by
a strong ONO donor ligand to give PS-[MoO
(fsal-ohyba)·DMF]
4). Similarly, reaction of [VO(acac) ] with an equimolar amount of
2
(acac)
2
] and PS-H
2
fsal-ohyba is
2
(
2
II and III in refluxing methanol yielded the oxovanadium(IV) com-
plexes [VO(fsal-ohyba)] (5) and [VO(sal-ohyba)] (9), respectively.
These complexes exhibit effective magnetic moments of 1.41 (for
1
5
1
) and 1.12 l
B
(for 9) at 298 K, which is subnormal for d (S =
29
/2) systems. Syamal and Kale have suggested antiferromagnetic
exchange in 9 on the basis of low temperature magnetic suscep-
tibility studies. We studied the magnetic susceptibility of 5 as a
2 2
Fig. 1 Titration of [VO(fsal-ohyba)] (5) with 30% H O ; the spectra were
function of temperature in the 298–90 K range. The magnetic
recorded after the successive addition of 1-drop portions of H
2
O
2
dissolved
corr
moment shows the dependence of v
M
on temperature and the
−4
in methanol to 10 ml of ca. 10 M solution of 5 in methanol.
observed magnetic moment values vary in the range 1.41–0.84 lB.
corr
The Curie–Weiss plot [1/v
M
vs. T] is a straight line with a Weiss
constant of ca. −20 indicating an antiferromagnetic interaction
possibly through dimerisation.
Treatment of a methanolic solution of 5 with H
7 could not be obtained under the above conditions. However, this
could be isolated through an oxygen transfer reaction between
29,30
2
O
2
in the
6 and PPh (eqn (5)). Physico-chemical data of the isolated
3
presence of an equimolar amount of KOH gave oxo-monoperoxo
complex, 6. This is slightly stable and loses oxygen slowly at room
temperature in a week period to give the respective dioxo complex.
complexes are presented in Table 1.
Scheme 2 presents the proposed structures of the polymer-
anchored complexes prepared in this contribution. These are
based on various physico-chemical studies (vide infra) of neat as
The formation of the oxo-monoperoxo vanadium(V) complex,
−
[
VO(O
2
)(fsal-ohyba)] has also been established by electronic
well as polymer-anchored complexes. A similar dioxovanadium(V)
−
absorption spectroscopy in methanol. In a typical reaction 10 ml
complex, e.g. [VO
2
(sal-ea)] (H
2
sal-ea = Schiff base derived
−
4
of ca. 1 × 10 M solution of 5 was treated with one drop portions
from salicylaldehyde and 2-amino-2-methylpropane-1-ol) having a
of 30% H dissolved in methanol and the resulting spectral
2
O
2
changes are presented in Fig. 1. Thus, the intensity of the 397 nm
band increases quickly with the addition of the first two drops of
H
2
O
2
solution. Further addition of H
2
2
O causes a shift of this band
to 392 nm along with a partial decrease in intensity. The 336 nm
band does not change its position but undergoes an increase in
intensity, while the band at 253 nm broadens initially and later
disappears with an increase in intensity. The final spectral pattern
is very similar to that obtained for the isolated peroxo complex 6
in methanol.
Aerial oxidation of a methanolic solution of 9 in the presence of
one equivalent of KOH, gave dioxovanadium(V) complex 10 while
Scheme 2
Table 1 Physical and analytical data
Found (Calc.) (%)
a
Compound
Colour
C
H
N
V/Mo
PS-H
PS-[VO(fsal-ohyba)·DMF], 1
PS-K[VO (fsal-ohyba)], 2
PS-K[VO(O )(fsal-ohyba)], 3
PS-[MoO
(fsal-ohyba)·DMF], 4
2
fsal-ohyba, I
Yellow
75.44
68.03
67.34
64.57
7.42
6.91
6.82
6.71
2.50
2.17
2.07
2.05
—
6.2
5.7
5.7
Greenish-black
Dark red
Orange
Orange-red-red
Green
Yellow
Greenish-yellow
Yellow
2
2
2
64.74
6.59
2.11
6.9
[
VO(fsal-ohyba)], 5
53.38 (53.59)
44.16 (44.23)
44.13 (44.01)
44.43 (44.55)
57.48 (57.53)
46.59 (46.03)
46.32 (46.51)
3.31 (3.30)
2.81 (2.72)
3.18 (3.20)
3.64 (3.51)
3.82 (3.80)
3.11 (3.59)
4.03 (3.91)
4.12 (4.17)
3.29 (3.44)
3.31 (3.42)
3.17 (3.25)
4.65 (4.80)
3.96 (3.84)
3.51 (3.62)
14.9 (15.15)
12.3 (12.52)
12.1 (12.46)
22.1 (22.72)
16.8 (17.45)
13.5 (13.96)
24.80 (25.30)
K[VO(O
2
)(fsal-ohyba)], 6
(fsal-ohyba)], 7
(fsal-ohyba)·MeOH], 8
VO(sal-ohyba)], 9
K(H O)][VO (sal-ohyba)], 10
MoO
(sal-ohyba)·MeOH], 11
[
[
[
[
[
K(H
MoO
2
O)][VO
2
2
Blackish
Brown-yellow
Yellow
2
2
2
a
Calculated from TGA data.
3
564 | Dalton Trans., 2006, 3561–3575
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9
,31
Table 2 IR spectral data (cm−1
)
cis-VO
ture of [MoO
a single crystal X-ray study.
2
structure has been structurally characterised. The struc-
2
(sal-ohyba)·MeOH] has already been confirmed by
a
22,32
Compound
m(C=O)
m(C=N)
m(M=O) (M = V, Mo)
I
1
2
3
4
1685
1671
1660
1668
1673
1668
1661
1661
1661
1672
—
1611
1600
1605
1606
1604
1612
1601
1601
1600
1603
1656
1630
1638
1632
—
970
927, 888
932
953, 906
—
988
946
969, 918
942, 907
—
Scanning electron micrograph
b
Scanning electron micrographs (SEM) for a single bead of pure
chloromethylated polystyrene, polymer-anchored ligand and the
vanadium complexes were recorded to observe the morphological
changes. Some of these images are reproduced in Fig. 2. As
expected the pure polystyrene bead has a smooth and flat surface
while the anchored ligand and complexes show a very slight
roughening of the top layer. This roughening is relatively larger
in the complex possibly due to the interaction of vanadium with
the anchored ligand that resulted in the formation of a complex
with a fixed geometry. Accurate information on the morphological
changes in terms of exact orientation of ligands coordinated to the
metal ion has not been possible due to poor loading of the metal
complex.
II
5
c
6
7
8
III
9
—
—
—
913
896, 860
933, 911
10
1
1
a
c
b
−1
For details see Table 1. Bands due to peroxo group: 860, 773, 591 cm
Bands due to peroxo group: 885, 761, 621 cm
.
−1
.
2
+
ciated with the {V(O
2
)} moiety, namely the symmetric V(O
) (m
) stretch at 860 cm , characteristic of g -
2
−1
)
−
1
stretch (m
2
) at 591 cm , the antisymmetric V(O
2
3
) at 773 cm ,
−
1
2
and the O–O (m
coordination of the peroxo group. In addition, this displays the
1
3
3
−
1
m(V=O) mode at 932 cm . The dioxomolybdenum(VI) complex 4
−
1
2+
exhibits two bands at 953 and 906 cm due to its cis-[MoO ]
2
3
4
structure. As shown in Table 2, the corresponding neat (oxo,
dioxo and peroxo) complexes also exhibit characteristic bands
well within the expected region.
The polymer-anchored ligand exhibits a sharp band at
−
1
1
685 cm due to the m(C=O) of the carboxylic group. Existence of
this band suggests covalent bond formation between the ligand
Fig. 2 Scanning electron micrographs (SEM) of (a) PS-H fsal-ohyba (I)
2
and the polymer through a carboxylic acid group. A strong
and (b) PS-K[VO
2
(fsal-ohyba)] (2); magnification ∼200×.
−
1
band appearing at 1611 cm due to the m(C=N) of the ligand
−
1
undergoes a negative shift by 5–11 cm in the complexes. This
observation suggests the coordination of the azomethine nitrogen
atom to the metal ion. The coordination of phenolic oxygen
could not be assigned unequivocally in most complexes because
TGA study
Thermogravimetric analysis under an oxygen atmosphere shows
−
1
the good stability of polymer-anchored complex 2 up to ca.
of the appearance of a weak broad band in the ca. 3400 cm
◦
2
1
50 C and thereafter decomposes in several steps. Complexes
region. However, the absence of such band in 9 suggests the
coordination of phenolic oxygen to the vanadium. Neat vanadium
and molybdenum complexes display spectral patterns very close
◦
and 4 start losing weight at ca. 175 C, possibly due to the
loss of coordinated DMF followed by weight loss in multiple
steps as noted above. Quantitative measurement of weight loss
at various stages was not possible due to their overlapping nature.
20,22,29
to those above and are similar to those reported in literature.
◦
However, the stability of the final residues at ca. 650 C suggests
Electronic spectral study
the formation of V
2
O
5
(10.1% for 1), KVO (15.4% for 2) or
3
The electronic spectral studies of ligands H
ohyba and PS-H
described earlier in detail.
exhibited by the ligands and complexes in the UV region are similar
2
sal-ohyba, H
fsal-ohyba, and their metal complexes have been
2
fsal-
MoO
3
(10.35% for 4). The peroxo complex 3, as expected, starts
◦
2
decomposing at ca. 100 C due to rupture of the peroxo group
followed by decomposition of the organic moiety to give KVO
20–22
The electronic spectral patterns
3
◦
(
15.4%) as the final product at ca. 650 C. Estimation of final
20–22
to those observed earlier (Table 3).
dium(IV) complexes, [VO(sal-ohyba)], [VO(fsal-ohyba)] and PS-
VO(fsal-ohyba)·DMF] display one weak shoulder between 517
In addition, oxovana-
residues was helpful in approximating the percentage of metal
ions in the polymer-anchored complexes.
[
and 630 nm due to a d-d transition. A band appearing at ca. 400 nm
in neat dioxovanadium(V) and peroxo-oxovanadium(V) complexes
is assigned to a ligand-to-metal charge transfer band (LMCT).
In some dioxovanadium(V) and both dioxomolybdenum(VI) com-
plexes, the n → p* and LMCT bands merge together and appear
as a broad band at ca. 400 nm. As a LMCT band generally
requires a high concentration of metal complexes, such a band
could not be located in the polymer-anchored dioxovandium(V)
IR spectral study
A partial list of IR spectral data of the polymer-anchored ligand
and complexes are listed in Table 2. Complex 1 exhibits a
−
1
sharp band at 970 cm due to m(V=O) while dioxo complex
−
1
2
exhibits two such bands at 927 and 888 cm corresponding
9
to mantisym(O=V=O) and msym(O=V=O) modes, respectively. The
peroxo complex shows three IR active vibrational modes asso-
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Table 3 Electronic spectral data
a
Compound (solvent)
kmax/nm
I (Nujol)
296, 216
624, 321, 281, 224
292.5, 275, 229
1
2
3
4
(Nujol)
(Nujol)
(Nujol)
(Nujol)
295, 280, 225
304, 271, 219, 207
401, 260, 279(sh), 221, 208
630, 397, 336, 253, 214
398, 341, 258, 206.5
401, 342, 255, 208
347(br), 281, 209
314, 277, 255, 212
517, 496.5, 351.5, 206
406.5, 325, 281, 254, 213.5
360, 305, 272,
II (methanol)
5
6
7
8
(methanol)
(methanol)
(methanol)
(methanol)
III (methanol)
9
1
1
(methanol)
0 (methanol)
1 (methanol)
Fig. 4 Electronic spectra of free complexes.
nitrogen atom. The methylene and aromatic protons appear at
almost the same position in the ligands as in their complexes.
All these data are consistent with the conclusion drawn from
the IR spectral studies and support the dibasic tridentate ONO
coordination mode of the ligands.
a
For details see Table 1.
and dioxomolybdenum(VI) complexes. The electronic spectra of
some anchored and neat complexes are reproduced in Fig. 3 and
4
, respectively.
51
The d( V) values of the dioxovanadium(V) complexes (also
presented in Table 4) display one strong resonance at −515.9 (in
7
) and at −521.0 ppm (in 10) in DMSO-d
6
; an expected value
for dioxovanadium(V) complexes having mixed O/N coordinating
atoms.
35–37
The resonances are somewhat broadened due to
5
1
quadrupolar interactions ( V: nuclear spin = 7/2, quadrupole
−
28
2
moment = −0.05 × 10 m ); the line width at half height is
approximately 200 Hz, which is still considered comparatively
5
1
36,37
narrow in V NMR spectroscopy.
The peroxovanadium(V)
complex 6 displays a signal at −528.5 ppm. This upfield shift with
respect to dioxovanadium(V) complexes is commonly observed
as an oxo group is replaced by the side-on coordinated peroxo
38
group.
Fig. 3 Electronic spectra of polymer-anchored complexes.
EPR studies
Room temperature X-band EPR spectra of the polymer an-
1
51
chored complex, PS-[VO(fsal-ohyba)·DMF] and the neat complex
H and V NMR studies
[
VO(fsal-ohyba)] are reproduced in Fig. 5. The EPR spectrum
The coordinating modes of the ligands H
2
fsal-ohyba and H
2
sal-
of the polymer anchored sample (Fig. 5a) is characteristic of
monomeric V(IV) complexes with an axial pattern. The hyperfine
features are well resolved in both parallel and perpendicular
regions. The resolved EPR pattern suggests that the vanadium
centers are well separated. The simulation of the EPR spectrum
was carried out in order to extract the Hamiltonian parameters.
1
ohyba were also confirmed by comparing H NMR spectra of
the ligands with the neat complexes 6, 7, 8, 10 and 11. The
relevant spectroscopic data are collected in Table 4. A broad
signal appearing at 12.50–13.24 ppm in the ligands, due to a
phenolic proton, disappears in the spectra of the complexes and
this suggests the coordination of a phenolic oxygen atom. A sig-
nificant down field shift (Dd = 0.33–1.39 ppm) of the azomethine
Accordingly, the values are: g
1.985, A
GHz. The hyperfine lines are due to the interaction of an unpaired
ꢀ
= 1.946, A
ꢀ
= 184 (± 2) G, g =
⊥
⊥
= 65 (± 2) G where the microwave frequency was 9.4458
(
–CH=N–) signal in the complexes with respect to the corres-
ponding ligand demonstrates the coordination of the azomethine
electron of a V(IV) center with its own nucleus (V, I = 7/2). The
1
51
Table 4 H NMR and V NMR spectral data (d in ppm)
51
Compound –OH
–CH=N–
–CH
2
Aromatic H
V NMR
H
2
fsal-ohyba
13.24(br)
—
—
—
12.50(br)
—
—
10.17(s)
10.73(s)
10.60(s)
10.50(s)
9.01(s)
10.40(s)
10.30(s)
4.87(s)
4.86(s)
4.86(s)
4.86(s)
4.70(s)
4.77(s)
4.77(s)
8.80–6.60(m)
7.90–6.67(m)
7.92–6.67(m)
7.94–6.65(m)
8.60–6.65(m)
8.88–6.61(m)
8.85–6.60(m)
K[VO(O )(fsal-ohyba)], 6
2
−528.5
−515.9
[
K(H
MoO
sal-ohyba
K(H O)][VO
MoO
2
O)][VO
2
fsal-ohyba)], 7
sal-ohyba)], 10
[
2
(fsal-ohyba)·MeOH], 8
H
2
[
2
2
−521.0
2
(sal-ohyba)·MeOH], 11
3
566 | Dalton Trans., 2006, 3561–3575
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products are common and have also been identified previously by
40
others.
Amongst these catalysts, PS-K[VO
a representative and three different parameters viz. amount of
(moles of H per mole of styrene), and catalyst, and
2
(fsal-ohyba)] was taken as
H
2
O
2
2
O
2
temperature of the reaction mixture were varied to obtain suitable
reaction conditions for the maximum oxidation of styrene. For
three different molar ratios (1 : 1.5, 1 : 2 and 1 : 3 of styrene to
aqueous 30% H
catalyst (0.040 g) in 10 ml of CH
2
O
2
), the amount of styrene (1.04 g, 10 mmol) and
◦
3
CN at 80 C were fixed and the
results after 6 h of contact time were analysed. As illustrated in
Fig. 6, the oxidation of styrene improved from 40.2% to 79.7%
on increasing from 1 : 1.5 to 1 : 2 (styrene to H
2
2
O ) molar ratios.
This oxidation remained nearly constant on further increasing
the molar ratio to 1 : 3, which suggests that a large amount of
oxidant is not an essential condition to improve the oxidation
of styrene. Similarly for three different amounts (viz. 0.030 g,
Fig. 5 X-Band EPR spectra of PS-[VO(fsal-ohyba)·DMF] (a) and
VO(fsal-ohyba)] (b).
0
2
4
.040 g and 0.070 g) of catalyst and styrene to H
under the above reaction conditions, 0.030 g catalyst gave only
5.2% conversion while 0.040 g and 0.070 g catalyst have shown
2
O ratio of 1 :
2
[
above parameters possibly indicate that the geometry is distorted
square pyramidal where the V=O bond is along the z-axis.
a maximum conversion of 79.7 and 81.3%, respectively; Fig. 7.
However, at the expense of H , 0.040 g catalyst can be considered
sufficient enough to carry out the reaction. Further, the turnover
2
O
2
The EPR spectrum of the neat complex 5, unlike the polymer an-
chored complex 1, exhibits broad signals as shown in Fig. 5(b). The
spectrum could not be simulated with a monomeric V(IV) model,
in combination with magnetic susceptibility data, indicating the
presence of a dimeric species with a magnetic superexchange
interaction between the two V(IV) centers. Such a dimeric structure
has been confirmed e.g. for [VO(sal-ea)]
2
(H
2
sal-ea = Schiff base
derived from salicylaldehyde and 2-aminoethanol) by a X-ray
39
single crystal study.
Catalytic activity study
The catalytic potential of polymer-anchored as well as neat
complexes were studied for the oxidation of styrene, ethylbenzene
and phenol, and for the oxidative bromination of salicylaldehyde.
Oxidation of styrene. Oxidation of styrene catalysed
by polymer-anchored complexes, PS-[VO(fsal-ohyba)·DMF],
2 2
Fig. 6 Effect of amount of H O on the oxidation of styrene. Reaction
PS-K[VO
2
(fsal-ohyba)] and PS-[MoO
2
(fsal-ohyba)·DMF] gave
conditions: styrene (1.04 g, 10 mmol), PS-K[VO
2
(fsal-ohyba)] (0.040 g),
◦
CH
3
CN (10 ml) and temp. (80 C).
styrene oxide, benzaldehyde, benzoic acid, phenylacetic acid and
1
-phenylethane-1,2-diol as mentioned in Scheme 3. These reaction
2
Fig. 7 Effect of amount of catalyst PS-K[VO (fsal-ohyba)] on the
oxidation of styrene. Reaction conditions: styrene (1.04 g, 10 mmol), H
2
O
2
◦
Scheme 3
(2.27 g, 20 mmol), CH
3
CN (10 ml) and temp. (80 C).
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−
1
rates (TOF h = moles of substrate converted per mole of catalyst
per hour) is also higher for 0.040 g catalyst to achieve 79.7%
conversion of styrene. The temperature of the reaction mixture
has also influenced the performance of the catalyst. Amongst three
We have compared the catalytic activity of these polymer-
anchored complexes with their respective neat ones. The conver-
sion percentage for each catalyst as a function of time is presented
in Fig. 9, Table 5 presents selectivity details of various products
obtained after 6 h of reaction time. For 0.040 g each of the neat
vanadium complex and keeping the other reaction conditions fixed
as above, the obtained percentage conversion varies in the range
◦
different temperatures (60, 70 and 80 C), running the reaction at
0 C gave much better conversion; Fig. 8. Moreover, the time
required to achieve the maximum conversion was also reduced on
carrying out the reaction at 80 C.
◦
8
◦
53.3 to 63.7% with the order: [K(H
2
O)][VO
(sal-ohyba)] (60.5%) > [VO(fsal-ohyba)] (57.2%) >
VO(sal-ohyba)] (53.3%). Thus, the catalytic performances of the
2
(fsal-ohyba)] (63.7%) >
[
[
K(H
2
O)][VO
2
neat complexes are also good. However, the TOF values are much
less in comparison to the polymeric ones. Moreover, easy recovery
of the anchored catalyst, no leaching and recycle ability make
them better over the neat complexes. Similarly, the immobilized
dioxomolybdenum(VI) complex is a better catalyst over the neat
[
MoO
2
(fsal- ohyba)·MeOH]. Independent of catalyst, the product
selectivity follows the order: benzaldehyde > 1-phenylethane-1,2-
diol > benzoic acid > phenylacetaldehyde > styrene oxide. The
formation of benzaldehyde in highest yield is understandable
because the styrene oxide formed in the first step may convert
into benzaldehyde by a nucleophilic attack of H
2
2
O on styrene
oxide followed by cleavage of the intermediate hydroperoxistyrene;
Scheme 4. Benzaldehyde formation may also be facilitated by
direct oxidative cleavage of the styrene side chain double bond
via a radical mechanism. Other products e.g. benzoic acid
formation through benzaldehyde is rather slow in all reactions.
Fig. 8 Effect of temperature on oxidation of styrene. Reaction conditions:
styrene (1.04 g, 10 mmol), PS-K[VO
0 mmol) and CH CN (10 ml).
2
(fsal-ohyba)] (0.040 g), H
2
O
2
(2.27 g,
40
2
3
Other catalysts e.g. PS-[VO(fsal-ohyba)·DMF] and PS-[MoO
fsal-ohyba)·DMF] were also tested under the above optimized
reaction conditions. Thus, for 10 mmol of substrate, 20 mmol
2
-
(
aqueous 30% H
of CH
results presented in Table 5 show that PS-K[VO
2
O
2
and 0.040 g catalyst were taken in 10 ml
◦
3
CN and the reaction was carried out at 80 C. The
2
(fsal-ohyba)]
exhibits better catalytic activity (79.7% conversion of styrene)
than PS-[VO(sal-ohyba)·DMF]; here only 73.2% conversion was
achieved, though one should expect nearly the same conversion
as both would form peroxo species easily in the presence of H
vide supra). The PS-[MoO
(sal-ohyba)·DMF], on the other hand,
gave only 17.5% conversion. The difficulty in transferring oxygen
from the intermediate peroxo complex, PS-[MoO(O )(sal-ohyba)],
formed by the reaction between PS-[MoO
(sal-ohyba)·DMF] and
may be the possible reason for the poor oxidation ability of
2
O
2
(
2
2
2
H
2
O
2
Fig. 9 Catalytic comparison of catalysts for the oxidation of styrene.
the catalyst. No significant improvement in conversion was noted
beyond 6 h of reaction time in all cases.
Reaction conditions: styrene (1.04 g, 10 mmol), catalyst (0.040 g), H O
(1.27 g, 20 mmol), CH CN (10 ml), temp. (80 C).
3
2
2
◦
Table 5 Percent conversion of styrene and selectivity of various oxidation products
Selectivity (%)
a
−
1
Catalyst
Conversion (%)
TOF/h
SO
Phaa
Bza
Bzac
Phed
Others
PS-K[VO
2
(fsal-ohyba)]
79.7
73.2
17.5
57.2
63.7
53.3
60.5
10.2
9.6
29.5
25.1
10.1
8.2
10.8
6.7
10.5
1.8
1.5
3.54
3.21
4.19
4.27
3.82
3.86
3.22
3.69
2.95
2.97
4.15
4.82
5.39
4.22
3.77
3.92
4.28
4.53
60.65
60.67
64.01
53.36
57.31
58.17
57.59
60.66
61.29
9.82
8.46
8.67
11.51
9.63
9.91
10.82
9.43
10.53
20.61
18.83
16.42
21.16
20.5
19.91
20.34
18.09
17.72
2.45
4.68
1.89
4.31
4.52
4.38
4.11
3.85
2.98
PS-[VO(fsal-ohyba)·DMF]
PS-{[MoO
2
(fsal-ohyba)·DMF]
VO(fsal-ohyba)]
K(H O)][VO (fsal-ohyba)]
VO(sal-ohyba)]
K(H O)][VO (sal-ohyba)]
(fsal-ohyba)·MeOH]
(fsal-ohyba)·MeOH]
[
[
[
[
[
[
2
2
2
2
MoO
MoO
2
2
a
SO = styrene epoxide, Phaa = phenylacetaldehyde, Bza = benzaldehyde, Bzac = benzoic acid, Phed = 1-phenylethane-1,2-diol.
3
568 | Dalton Trans., 2006, 3561–3575
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of benzoic acid is highest (67.9%) followed by benzaldehyde
(29.7%), and selectivity of styrene oxide and phenylacetaldehyde
is 0.6 and 1.6%. The highest selectivity of benzoic acid is clearly
42
due to further oxidation of benzaldehyde.
Oxidation of ethylbenzene. Selective oxidation of ethylbenzene
to acetophenone has been considered an important chemical
reaction because of its use as an intermediate in pharmaceuticals,
resins, alcohols, esters, aldehydes, and tear gas. For the oxidation of
Scheme 4
59
ethylbenzene, homogeneous (e.g. metal acetylacetonates, [Co(2-
Similarly, the formation of phenylacetaldehyde, a product formed
by isomerisation of styrene oxide is less in all cases. Water present
O is probably responsible for hydrolysis of styrene oxide to
-phenylethane-1,2-diol to some extent.
The oxidation of styrene at the side chain of industrially
important fine chemicals has also been obtained previously using
different types of catalysts. Mainly homogeneous catalysts such as
6
0
61
pyridinecarboxamide)
3
],
metalloporphyrins,
macrocyclic
62
complexes etc.) as well as heretogeneous catalysts (e.g. metal
salt or complex supported on alumina/silica, heteropolyacids
in H
1
2
2
63
64
65
66
M-APO-11 (M = Co, Mn and V), Mn-MCM-41, polymer-
anchored complexes, zeolite encapsulated metal complexes
etc.) have been employed. Mostly molecular oxygen has been used
5
6,67
68
59–68
as oxidant
and there are only limited reports where tert-butyl
41
III
ꢁ
cobalt chloride, [Mn (salen)] (H
2
salen = N,N -bis(salicyledene)-
56,67,68
hydroperoxide/H
2
O
2
has been used as oxidant.
With mild
4
2
III
III
ethane-1,2-diamine),
[Cr (dpa)(dipy/ophen)Cl], [Fe (dpa)-
oxidants usually acetophenone is obtained as the major product
while strong oxidants give several oxidized products.
We have used polymer-anchored complexes to catalyse the
(
2
dipy/ophen)Cl] (H
2
dpa = pyridine-2,6-dicarboxylic, dipy =
ꢁ
43
,2 -dipyridyl, ophen = 1,10, phenanthroline), mixed-metal
44
III
45
carboxylate, Mn -complexs attached to crown ethers, vana-
oxidation of ethylbenzene, by H
2
2
O , to give benzaldehyde, phenyl-
4
6
47
dium peroxo complexes, and metalloporphyrins, and hetero-
geneous catalysts such as heteropolytungstates, nano-sized spinel
type mixed metal oxides, zeolites, metal complexes encapsu-
lated in zeolites or anchored to mesoporous materials, metal
acetic acid, styrene and 1-phenylethane-1,2-diol as shown in
Scheme 5. Reaction conditions have also been optimised for the
maximum oxidation of ethylbenzene by varying the amount of
48
4
9
50
51
oxidant (moles of H
2
2
O per mole of ethylbenzene), catalyst and
containing TM-MCM-41 and TM-MCM-48 (TM = Al, Ti, V,
temperature of the reaction considering PS-K[VO
a representative catalyst.
2
(fsal-ohyba)] as
5
2
Cr, Mn, Fe, Co and Ni), titanium containing molecular sieves
5
3
54
40
55
(
TS-1, BTS, Ti-MCM-41 ) and silica have been used for
the oxidation of styrene. The literature, however, contains very
limited citations on polymer supported catalysts for the oxidation
56,57
of styrene.
Within the polymer supported catalysts, the catalytic activity of
these anchored vanadium complexes is better than that reported
for the similar polymer-anchored PS-[VO(hmbmz) ] [where PS-
2
Scheme 5
Hhmbmz = polymer-anchored 2-(a-hydroxymethyl)benzimida-
56
zole] under similar reaction conditions. Here, the highest
selectively of benzaldehyde (78%) and lowest of styrene oxide
The effect of temperature on the oxidation of ethylbenzene was
studied at three different temperatures (viz. 50, 60 and 80 C)
while keeping ethylbenzene (0.424 g, 4 mmol), aqueous 30% H
0.906 g, 8 mmol), and catalyst (0.050 g) in 10 ml of CH CN. It is
clear from the profile presented in Fig. 10 as a function of time that
performance of the catalyst is much better at 80 C compared to
◦
(
4.4%) were maintained while conversion was limited to 48%.
For the polymer-anchored peroxovanadium(V) complex PS–
K[VO(O
(2-pybzm)] [where 2-pybzm = 2-(2-pyridyl)benzimida-
zole], the percent conversion as well as the selectivity of all five
2
O
2
(
3
)
2 2
◦
58
products (Scheme 3) were nearly same. Oxidation of styrene
catalysed by a poly(maleic acid-co-styrene)-triruthenium cluster
gave three major products: benzaldehyde (65.4%), phenylacetalde-
hyde (15.3%) and acetophenone (11.5%). As molecular oxygen
was used as oxidant, the Wacker-type oxidation of styrene was
57
suggested to follow for the formation of these products. The
ꢁ
neat vanadium complex [VO(salphen)] [where H
2
salphen = N,N -
bis(salicyledene)phenyl-1,2-diamine] exhibits only 10.7% conver-
sion using 70% tert-butylhydroperoxide and among the only two
products identified, the selectivity of benzaldehyde is 32.2 and of
styrene oxide is 9.4%. Encapsulation in zeolite-Y reverses this
selectivity along with 34.8% conversion. This conversion was
further improved to 40.6% on anchoring [VO(salphen)] to Al-
MCM-41 with an improvement in selectivity to 45.0% towards the
51a
+
formation of styrene oxide. Catalytic activity of [Mn(salen)]
encapsulated in zeolite-Y has been tested using molecular oxygen
as oxidant in excess tert-butylhydroperoxide as initiator. While
activity of the catalyst is about 30%, the selectivity of the formation
Fig. 10 Effect of temperature on the oxidation of ethylbenzene. Reaction
conditions: ethylbenzene (0.424 g, 4 mmol), H O (0.906 g, 8 mmol),
2
2
PS-K[VO
2
(fsal-ohyba)] (0.050 g) and CH
3
CN (10 ml).
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◦
5
0 and 60 C along with the maximum conversion in considerably
◦
less time. Thus, a minimum of 80 C is the required temperature to
cross the energy barrier to get maximum conversion of substrate.
The influence of H
ethylbenzene as a function of time has been studied considering
ethylbenzene : H molar ratios of 1 : 1, 1 : 2 and 1 : 3, and the
2
2
O concentration on the oxidation of
2
O
2
results are illustrated in Fig. 11 as a function of time. For a fixed
amount of ethylbenzene (0.424 g, 4 mmol), catalyst (0.050 g) and
CH
3
CN (10 ml), the conversion of ethylbenzene was 22.6% in 6 h
◦
of reaction time at 80 C for the substrate to H
2
O molar ratio of 1 :
2
1
. Increasing this ratio to 1 : 2 increases the conversion to 40% and
reaches completion within 6 h while further enhancement of H
2
O
2
hardly affects the conversion. The effect of amount of catalyst
on the oxidation of ethylbenzene is illustrated in Fig. 12. It is
clear from the plot that reaction proceeds slowly and gives only
Fig. 12 Effect of amount of catalyst on the oxidation of ethylbenzene.
Reaction conditions: ethylbenzene (0.424 g, 4 mmol), H
2
O
2
(0.906 g,
1
4
6
9.2% conversion with 0.030 g of catalyst, ethylbenzene (0.424 g,
mmol) and H (0.906 g, 8 mmol) in 10 ml of CH CN after
h of reaction time. Increasing the catalyst amount to 0.050 g,
◦
8
mmol), CH
3
CN (10 ml) and temperature (80 C).
2
O
2
3
improves this conversion to 40% within 6 h of reaction time.
Further increment of catalyst amount has no further effect on
the oxidation of ethylbenzene.
Fig. 13 Comparison of catalytic potential of catalysts on the oxidation of
ethylbenzene under optimised conditions. Reaction conditions: ethylben-
zene (0.424 g, 4 mmol), H
2 2 3
O (0.906 g, 8 mmol), catalyst (0.050 g), CH CN
◦
(
10 ml) and temperature (80 C).
2 2
Fig. 11 Effect of amount of H O on the oxidation of ethylbenzene. Re-
action conditions: ethylbenzene (0.424 g, 4 mmol), PS-K[VO
2
(fsal-ohyba)]
very comparative results and give 40 and 36.9% conversion,
◦
(
0.050 g), CH
3
CN (10 ml) and temperature (80 C).
respectively, while PS-[MoO (fsal-ohyba)·DMF] shows very poor
2
performance, here only 12.8% conversion was achieved. The
Table 6 presents the results of ethylbenzene oxidation under
optimised reaction conditions after 6 h of reaction time while
Fig. 13 presents the conversion as a function of time. Cata-
products formed are independent of catalyst and follow the order:
benzaldehyde > phenylacetic acid > styrene > 1-phenylethane-
1,2-diol. A very small amount of unidentified product was
also noticed (see Table 6). No significant improvement in the
lysts PS-K[VO
2
(fsal-ohyba)] and PS-[VO(fsal-ohyba)·DMF] show
Table 6 Effect of oxidant on percentage conversion of ethylbenzene and selectivity of various oxidation products
a
Selectivity (%)
−
1
Catalyst
Conversion (%)
TOF/h
Phac
Bza
Styrene
Phed
Others
PS-K[VO
2
(fsal-ohyba)]
40.0
36.9
12.83
32.5
34.8
30.4
33.6
8.2
4.7
4.1
2.4
1.5
1.9
1.2
1.9
0.5
0.4
7.61
7.05
8.64
9.81
8.28
8.59
8.75
8.26
8.61
87.51
85.42
83.84
82.18
85.77
84.62
83.49
82.52
81.49
2.13
4.37
3.52
4.36
2.86
2.91
3.61
3.94
4.27
1.21
1.13
1.19
1.08
1.29
1.36
1.32
1.21
1.35
1.54
2.23
2.93
2.47
1.8
2.52
2.83
4.07
4.28
PS-[VO(fsal-ohyba)·DMF]
PS-{[MoO
2
(fsal-ohyba)·DMF]
VO(fsal-ohyba)]
K(H O)][VO (fsal-ohyba)]
VO(sal-ohyba)]
K(H O)][VO (sal-ohyba)]
(fsal-ohyba)·MeOH]
(sal-ohyba)·MeOH]
[
[
[
[
[
[
2
2
2
2
MoO
MoO
2
2
7.5
a
Phac = phenylacetic acid, Bza = benzoic acid, Phed = 1-phenylethane-1,2-diol.
3
570 | Dalton Trans., 2006, 3561–3575
This journal is © The Royal Society of Chemistry 2006

View Article Online
oxidation of ethylbenzene was observed beyond 6 h of reaction
time. Neat complex [VO(fsal-ohyba)] shows 32.5% conversion
of ethylbenzene when 0.050 g of it was used under the above
optimised conditions. However, the turnover rates per hour are
very low compared to the respective polymer-anchored catalyst.
Again, performance of [MoO
The catalytic potentials of these polymer-anchored complexes
2
(fsal-ohyba)·MeOH] is very poor.
compare well with other polymer supported catalysts, e.g. PS-
56
[
VO(hmbmz) ]. However, the latter is more selective towards
2
the formation of acetophenone under similar conditions. At the
same time, with ca. 28% conversion PS-par-Cu (par = 4-(2-
67
pyridylazo)resorcinol) is less selective towards the formation of
acetophenone (38.3%) and more towards benzylalcohol (57.2%).
The zeolite framework in zeolite encapsulated complexes has
influence on the oxidation products of ethylbenzene in that
Fig. 14 Effect of H
Reaction conditions: phenol (4.7 g, 50 mmol), PS-K[VO
2
O
2
concentration on the oxidation of phenol.
using TBHP or H
obtained as the major product along with other minor products.
2
O as oxidant, acetophenone was always
2
2
(fsal-ohyba)]
68
◦
(
0.025 g), CH
3
CN (2 ml) and temperature (80 C).
In the polymer-anchored catalysts reported here, oxidation of
ethylbenzene does not stop at acetophenone but proceeds further
to give several oxidation products, Table 6.
0.025 and 0.040 g) for a fixed amount of phenol (4.7 g, 0.05 mol)
and oxidant (7.93 g, 70 mmol) in 2 ml of CH
3
CN, only 16.5%
◦
Oxidation of phenol. Liquid phase oxidation of phenol using
oxidation of phenol with 0.015 g catalyst was achieved at 80 C in
6 h of reaction time. However, increments of catalyst amounting to
0.025 g showed significant change and conversion reached 37.3%
within 6 h of reaction time. Further increments of catalyst hardly
improve the oxidation. Fig. 15 presents this effect as a function
of time. This has been interpreted in terms of the thermodynamic
and mass transfer limitations at higher reaction rates. Running the
reaction at various temperatures while considering a fixed amount
of phenol (4.7 g, 50 mmol), H O (7.93 g, 70 mmol), catalyst
simple oxidants such as O
2
and H
2
O
2
has been reported using
different types of heterogeneous catalyst. For example, polymeric
6
9
58
coordination complexes, polymer-anchored complexes, metal
70
complexes encapsulated in the super cages of zeolites, etc. are
well documented in the literature. Molecular sieve based catalysts,
TS-1 and TS-2 have been commercialized for the oxidation of
71
phenol using H
phenol catalysed by polymer-anchored as well as neat complexes
using H as an oxidant in CH CN. Only two major products,
2
O
2
as oxidant. We have studied the oxidation of
2
2
2
O
2
3
(0.025 g) and CH CN (2 ml) has shown a considerable effect on
3
◦
viz. catechol and p-hydroquinone as shown in Scheme 6 were
identified from the reaction mixture. These are the expected
products according to the directing effect of the phenolic OH
group.
the oxidation of phenol. As shown in Fig. 16 at least 80 C is
the required temperature to supply sufficient energy to reach the
energy barrier of phenol conversion.
Scheme 6
In order to achieve suitable reaction conditions for the max-
imum oxidation of phenol as well as better selectivity for the
formation of catechol we have studied the effect of amount of
catalyst, effect of H
2
O
2
concentration (moles of H
2
O
2
per mole of
phenol) and effect of temperature using PS-K[VO
a representative catalyst.
2
(fsal-ohyba)] as
The effect of H
illustrated in Fig. 14. For three different molar ratios of H
2
O
2
concentration on the oxidation of phenol is
(viz
0, 70 and 100 mmol), the amount of phenol (4.7 g, 50 mmol) and
2
O
2
Fig. 15 Effect of amount of catalyst on the oxidation of phenol. Reaction
5
conditions: phenol (4.7 g, 50 mmol), H O (7.93 g, 70 mmol), CH CN
2
2
3
◦
catalyst (0.025 g) were fixed in 2 ml of CH
carried out at 80 C. It is clear from the plots that a H
3
CN and the reaction was
to phenol
(2 ml) and temperature (80 C).
◦
2
O
2
ratio of 1 : 1 results in about 21% conversion in 6 h of reaction
time. The other two concentrations (i.e. 1.4 : 1, and 2 : 1) performed
better but gave comparable phenol conversions of 37.3 and 39.7%.
Thus, under the optimised conditions (i.e. 50 mmol substrate,
70 mmol 30% aqueous H
2
O
2
, 0.025 g catalyst, 2 ml CH CN and
3
◦
80 C reaction temperature), the performance of all catalysts
were studied and the percentage of phenol oxidation plotted
as a function of time are presented in Fig. 17. All catalysts
acquired a steady state in ca. 6 h of contact time and only
However, in terms of the selectivity of H
2
2
O , the 1.4 : 1 ratio would
be a suitable oxidant to phenol ratio for better performance of the
catalyst. Similarly out of three different catalyst weights (0.015,
This journal is © The Royal Society of Chemistry 2006
Dalton Trans., 2006, 3561–3575 | 3571

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substrate than the oxovanadium(IV) complex. The poor stability
of the peroxovanadium(V) complex has an added advantage in
improving the phenol oxidation while the dioxomolybdenum(VI)
forms a stable peroxomolybdenum(VI) complex and hence has
difficulty in transferring the oxygen to the substrate thereby
resulting in a poor yield of the oxidised products.
Taking 0.025 g of neat complex and keeping the other
reaction conditions fixed as above, the catalytic performances
follow the order: [K(H
ohyba)] (28.5%) > [K(H
ohyba)] (25.3%) > [MoO
2
O)][VO
O)][VO
(fsal-ohyba)·MeOH] (8.5%). Again the
2
(fsal-ohyba)] (30.7%) > [VO(fsal-
2
2
(sal-ohyba)] (26.9%) > [VO(sal-
2
vanadium based complexes have a better performance than the
neat molybdenum(VI) one. But the overall performances of the
polymer-anchored complexes are better than the corresponding
neat complexes. The turnover rates of the polymer-anchored
complexes are also higher than the corresponding neat complexes.
The percentage of phenol oxidation and the selectivity of the
catechol and p-hydroquinone formation along with the turnover
frequency of different catalysts after 6 h of contact time are
presented in Table 7. It is clear from the data that the selectivity of
neat as well polymer-anchored complexes is better for catechol for-
mation where it varies between 58.8 and 69.5% while the selectivity
of p-hydroquinone formation is between 27.5 and 39.2%.
Fig. 16 Effect of temperature on the oxidation of phenol. Reaction
conditions: phenol (4.7 g, 50 mmol), PS-K[VO (fsal-ohyba)] (0.025 g),
(7.93 g, 70 mmol) and CH CN (2 ml).
2
H
2
O
2
3
The catalytic performances of the polymer-anchored com-
plexes reported here compare well with the data reported
in the literature. For example, the reported conversions for
5
8
polymer-anchored complex, PS-K[VO(O )(2-pybzm)] (33.5%),
2
69
polymeric [–CH
lated NH [VO (sal-inh)] (where H
from salicylaldehyde and isonicotinic acid hydrazide) (26.5%)
2
{VO(salen)}–]
n
(38.6%), and zeolite encapsu-
4
2
2
sal-inh = Schiff base derived
7
0h
70g
and [VO(salen)] (32.6%), are close to that reported in Table 7.
The data suggest equally good catalytic potential for these cata-
lysts. However, their selectivity towards the formation of catechol
vary in the order: [VO(salen)]–Y (92.7%) > [–CH
2
{VO(salen)}–]
n
(
92.5%) > NH [VO (sal-inh)]–Y (84.6%) > PS-K[VO(O
4
2
2
)(2-
Fig. 17 Comparison of the catalytic activity of various catalysts on
the oxidation of phenol under optimized condition. Reaction conditions:
pybzm)]. A poor performance for all types of molybdenum based
catalysts was always noted.
phenol (4.7 g, 50 mmol), catalyst (0.025 g), H
2
O
2
(7.93 g, 70 mmol) and
◦
CH
3
CN (2 ml) and temperature (80 C).
Oxidative bromination of salicylaldehyde. As mentioned in
minor changes either in conversion or selectivity of the product
formation were observed beyond 6 h of contact time. Amongst
the polymer-anchored catalysts, the performance followed the
the Introduction, vanadium(V) complexes catalyse the oxidative
bromination of organic substrates in the presence of H
2
O
2
10,12
and bromide.
Bromination of 1,3,5-trimethoxybenzene to 2-
order: PS-K[VO
DMF] (33.8%) > PS-[MoO
that the dioxovanadium(V) complex is more able to form the
peroxo complex easily followed by transfer of oxygen to the
2
(fsal-ohyba)] (37.3%) > PS-[VO(fsal-ohyba)·
bromo-1,3,5-trimethoxybenzene catalysed by [VO(OEt)(EtOH)-
10
2
(fsal-ohyba)·DMF] (10.3%). It seems
(sal-oap)] [H
and phenol red to bromophenol red by [VO(O
2
sal-oap = N-(2-hydroxyphenyl)salicyledenamine]
+
72
2
)(H
2
O)] species
are worthy of mention here. During this process, vanadium reacts
Table 7 Percentage conversion of phenol along with TOF values and the selectivity of the products formed after 6 h of contact time
Selectivity (%)
−
1
Catalyst
Phenol conversion (%)
TOF/h
Catechol
p-Hydroquinone
PS-[VO(fsal-ohyba)·DMF]
33.8
37.3
10.3
28.5
30.7
8.5
92.7
110.5
47.6
32.6
42.8
12.3
35.8
33.8
69.5
66.7
58.8
59.4
55.2
54.7
61.9
59.8
27.5
31.2
39.2
37.2
42.5
42.1
34.2
37.2
PS-K[VO
2
(fsal-ohyba)]
PS-[MoO
2
(fsal-ohyba)·DMF]
[
[
[
[
[
VO(fsal-ohyba)]
K(H
MoO
VO(sal-ohyba)]
K(H O)][VO (sal-ohyba)]
2
2
O)][VO (fsal-ohyba)]
2
(fsal-ohyba)·MeOH]
25.3
26.9
2
2
3
572 | Dalton Trans., 2006, 3561–3575
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with 1 or 2 equivalents of H
2
O
2
, forming oxo-monoperoxo,
catalyst. This was confirmed by comparing the IR spectrum of the
recovered catalyst at each stage with that of a fresh sample.
Under optimised reaction conditions (i.e. salicylaldehyde
+
−
[
VO(O
2
) ] or oxo-diperoxo [VO(O
2
)
2
] species, which ultimately
oxidise bromide, possibly via a hydroperoxo intermediate. The
oxidised bromine species (most likely HOBr) then brominates
the substrate. We have found that polymer-anchored complexes
(1.22 g, 10 mmol), catalyst (30 mg), H
2
O (2.27 g, 20 mmol),
2
1
H
2
SO (1.96 g, 20 mmol), KBr (2.38 g, 20 mmol) in 10 ml of
4
satisfactorily catalyse the oxidative bromination of salicylaldehyde
to give 5-bromosalicylaldehyde and 3,5-dibromosalicylaldehyde
water), the performance of the other polymer-anchored catalysts
was also tested and the results along with turnover rate per hour
and selectivity of the reaction products are summarised in Table 8.
Approximately 73% conversion of salicylaldehyde with ca. 80%
selectivity towards the formation of 5-bromosalicylaldehyde (5-
Brsal) was achieved with vanadium based catalysts under op-
timised conditions. It is interesting to note that dioxomolyb-
using H
2
O
2
/KBr in the presence of H
2
SO in aqueous solution;
4
cf. Scheme 7.
denum(VI) complex, PS-[MoO
2
(fsal-ohyba)·DMF] also mimics
haloperoxidase activity in that 92% conversion of salicylalde-
hyde with a turnover rate per hour of 213.3 was achieved.
Here the selectivity of 5-bromosalicylaldehyde is 83.4%. The
Scheme 7
In order to achieve optimum conditions, PS-K[VO
was studied in detail by varying parameters such as the volume
of sulfuric acid and H at room temperature. Thus, for a fixed
amount of salicylaldehyde (1.22 g, 10 mmol), catalyst (0.030 g),
KBr (2.38 g, 20 mmol) in 10 ml of water at least 20 mmol of H SO
1.96 g) and 20 mmol of H (2.27 g) were found to be sufficient to
give maximum conversion. Using 30 mmol of acid under the above
conditions gave only a marginal improvement in conversion. As
the presence of acid may cause decomposition of the catalyst, the
effect of acid on the rate of reaction was studied in detail under the
above operating conditions. The results after 2 h of reaction time
are illustrated in Fig. 18. It is clear from the figure that conversion
of salicylaldehyde significantly improves from 40.5% to 75% with
2
(sal-ohyba)]
performance of the previously reported polymer-anchored cat-
56
alyst PS-[VO(hmbmz)
2
] is better (100% conversion) than the
2
O
2
anchored vanadium and molybdenum complexes reported here.
The reported turnover rates for zeolite-Y encapsulated dioxo-
2
4
vanadium(V) complexes NH
oap)]-Y (where H
4
[VO
2
(sal-inh)]-Y and NH
4
[VO (sal-
2
(
2
O
2
2
sal-oap = Schiff base derived from salicy-
laldehyde and o-aminophenol) are 52 and 67, respectively for
70h
similar catalytic reactions. Simple vanadium complexes e.g.
[
K(H
pyridoxal and isonicotinic acid hydrazide) and [K(H
nah)] (H
sal-nah = Schiff base derived from salicylaldehyde and
isonicotinic acid hydrazide) have turnover rates per hour of about
2
O)
2
][VO
2
(pydx-inh)] (H
2
pydx-inh = Schiff base derived from
2
O)][VO (sal-
2
2
5
. In addition, they have a tendency towards slow decomposition
the increment of H
necessary to add the acid in batches to prevent decomposition of
the catalyst. Using 40 mmol H SO (i.e. substrate to H SO
ratio =
: 4) causes lower conversion along with the decomposition of
2
SO
4
from 10 mmol to 30 mmol but it was
73
during the catalytic reaction. The performance of the polymeric
oxovanadium(V) complexes of Schiff bases derived from 5,5 -
ꢁ
2
4
2
4
methylenebis(salicylaldehyde) and diamines are relatively better
1
69
but the turnover rates per hour is about 32.
The reaction mixture did not produce any brominated product
in the absence of these catalysts. The recovered catalysts after
regeneration with acetonitrile performed well with nearly the same
percentage conversion and this suggests that these catalysts are
recyclable.
Complex 1 generates oxoperoxo species (vide supra) and diox-
ovanadium(V) complexes are well known to form oxoperoxo
73
species. In the presence of H
2
O
2
and acid, oxovanadium(IV)
and dioxovanadium(V) may generate (proposed) intermediate
73
hydroperoxo species, which allow nucleophilic attack of the
1
bromide, followed by release of hypobromous acid, which then
brominates an organic substrate. A catalytic cycle is proposed in
Scheme 8. Dioxomolybdenum(VI) complexes may follow a similar
mechanism. A hypobromite intermediate may also be involved,
as proved by electrospray ionization mass spectra on model
74
Fig. 18 Effect of H
2
SO
4
on the oxidative bromination of salicyldehyde.
systems.
Table 8 Average percent conversion of salicylaldehyde along with TOF and product selectivity
Selectivity (%)
5-Brsal
−
1
Catalyst
Conversion (%)
TOF/h
3,5-diBrsal
Unidentified
PS-[VO(fsal-ohyba)·DMF]
73
70
92
100.0
104.3
213.3
81.36
78.56
83.44
15.82
19.17
14.03
2.82
2.27
2.53
PS-K[VO
2
(fsal-ohyba)]
PS-[MoO
2
(fsal-ohyba)·DMF]
This journal is © The Royal Society of Chemistry 2006
Dalton Trans., 2006, 3561–3575 | 3573

View Article Online
reactions suggests their heterogeneous nature. They are recyclable
up to three cycles without significant loss in their catalytic activity.
Acknowledgements
This work was supported by the University Grant Commission,
New Delhi. The authors are also thankful to Prof. Dr Dieter
Rehder, Hamburg, Germany, for recording the V NMR spectra
Scheme 8
51
and Thermax Limited, Pune, India for providing chloromethylated
polystyrene as a gift.
Test for recyclability and heterogeneity of the reaction
The recycle ability of the polymer-anchored catalysts 1, 2 and 4 has
been tested. The reaction mixture after a contact time of 6 h was
filtered. The catalysts separated from the reaction mixture after the
catalytic reaction in all three cases were washed with acetonitrile,
dried and subjected to further catalytic reactions under similar
conditions. No appreciable loss in the activity in all cases suggested
that the metal complexes were interacting with the polystyrene and
that the catalysts are active even after first cycle. During catalytic
oxidation of styrene and ethylbenzene, the filtrate collected after
separating the solid catalysts was placed into the reaction flask
and the reaction was continued after adding fresh oxidant for
another 2 h. The gas chromatographic analyses showed no further
increment in the conversion. This confirms that the reaction did
not proceed upon removal of the solid catalyst and hence the
reaction was heterogeneous in nature.
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-phenylethane-1,2-diol > benzoic acid > phenylacetaldehyde >
styrene oxide. A maximum of 40% conversion of ethylbenzene
was observed with PS-K[VO (fsal-ohyba)] which is followed by
PS-[VO(fsal-ohyba)·DMF)] with 36.9% conversion. Catalyst PS-
MoO
(fsal-ohyba)·DMF] showed very poor results and only
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[
1
2
2
2
various products follows the order: benzaldehyde > phenylacetic
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2
(fsal-ohyba)]
(fsal-ohyba)·DMF]. The
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have comparable catalytic activity. However, the recycle ability
and easy separation of the catalysts from the reaction mixture
make polymer-anchored complexes better catalysts than neat ones.
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