Polymer-grafted and neat vanadium(V) complexes as functional mimics of haloperoxidases

Article abstract of DOI:10.1016/j.poly.2013.09.021

The monobasic tridentate ONN donor ligand 1-(2-pyridylazo)-2-naphthol [Hpan (I)] reacts with [V^IVO(acac)₂] in dry methanol to yield the oxidovanadium(IV) complex [V^IVO(acac)(pan)] (1). The dioxidovanadium(V) complex [{V^VO(pan)}₂(μ-O) ₂] (2) is obtained by aerial oxidation of 1 in methanol. Complex 2 can also be prepared directly by reacting [V^IVO(acac)₂] with I followed by aerial oxidation in methanol. Treatment of 1 or 2 in methanol with H₂O₂ yields the oxidomonoperoxidovanadium(V) complex [V^VO(O₂)(pan)(MeOH)] (3). Reaction of imidazolomethylpolystyrene cross-linked with 5% divinylbenzene (PS-im) with 2 in DMF resulted in the formation of the polymer-grafted dioxidovanadium(V) complex PS-im[V^VO₂(pan)] (4). All these complexes are characterized by various spectroscopic techniques (IR, electronic, NMR ( ¹H and ⁵¹V) and electron paramagnetic resonance (EPR)), thermal, field-emission scanning electron micrographs (FE-SEM) as well as energy dispersive X-ray (EDX) studies. The crystal and molecular structure of 3 has been determined, confirming the ONN binding mode of I. The polymer-grafted complex 4 has been used for the oxidative bromination of styrene, salicylaldehyde and trans-stilbene. Various parameters, such as amounts of catalyst, oxidant (aqueous 30% H₂O₂), KBr and aqueous 70% HClO₄ have been optimized to obtain the maximum oxidative bromination of the substrates. Under the optimized reaction conditions, styrene gave a maximum of 99% conversion after 2 h of reaction, with the main products having a selectivity order of: 1-phenylethane-1,2-diol (75%) > 2-bromo-1- phenylethane-1-ol (20%) > 1,2-dibromo-1-phenylethane (1.2%). With nearly same conversion in same time, the oxidative bromination of salicylaldehyde gave three products with the selectivity order: 5-bromosalicylaldehyde > 2,4,6-tribromophenol > 3,5-dibromosalicylaldehyde. A maximum of 91% conversion of trans-stilbene has been obtained in 2 h of reaction time, where selectivity of the obtained reaction products varied in the order: 2,3-diphenyloxirane (trans-stilbene oxide) > 1,2-dibromo-1,2-diphenylethane > 2-bromo-1,2-diphenylethanol. The catalytic activity of the non-polymer grafted complex 2 is lower than that of the polymer-grafted one. In addition, the recycling ability of the grafted complex makes it better over the neat complex.

Full text of DOI:10.1016/j.poly.2013.09.021

Polyhedron 67 (2014) 436–448
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Polymer-grafted and neat vanadium(V) complexes as functional mimics
of haloperoxidases
a
b
Mannar R. Maurya a, , Nikita Chaudhary , Fernando Avecilla
⇑
a Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India
b Departamento de Química Fundamental, Universidade da Coruña, Campus de A Zapateira, 15071 A Coruña, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The monobasic tridentate ONN donor ligand 1-(2-pyridylazo)-2-naphthol [Hpan (I)] reacts with
[VIVO(acac)2] in dry methanol to yield the oxidovanadium(IV) complex [VIVO(acac)(pan)] (1). The
Received 22 June 2013
Accepted 15 September 2013
Available online 8 October 2013
dioxidovanadium(V) complex [{VVO(pan)}2(
l-O)2] (2) is obtained by aerial oxidation of 1 in methanol.
Complex 2 can also be prepared directly by reacting [VIVO(acac)2] with I followed by aerial oxidation
in methanol. Treatment of 1 or 2 in methanol with H2O2 yields the oxidomonoperoxidovanadium(V)
complex [VVO(O2)(pan)(MeOH)] (3). Reaction of imidazolomethylpolystyrene cross-linked with 5% divi-
nylbenzene (PS-im) with 2 in DMF resulted in the formation of the polymer-grafted dioxidovanadium(V)
complex PS-im[VVO2(pan)] (4). All these complexes are characterized by various spectroscopic
techniques (IR, electronic, NMR (1H and 51V) and electron paramagnetic resonance (EPR)), thermal,
field-emission scanning electron micrographs (FE-SEM) as well as energy dispersive X-ray (EDX) studies.
The crystal and molecular structure of 3 has been determined, confirming the ONN binding mode of I. The
polymer-grafted complex 4 has been used for the oxidative bromination of styrene, salicylaldehyde and
trans-stilbene. Various parameters, such as amounts of catalyst, oxidant (aqueous 30% H2O2), KBr and
aqueous 70% HClO4 have been optimized to obtain the maximum oxidative bromination of the substrates.
Under the optimized reaction conditions, styrene gave a maximum of 99% conversion after 2 h of reac-
tion, with the main products having a selectivity order of: 1-phenylethane-1,2-diol (75%) > 2-bromo-1-
phenylethane-1-ol (20%) > 1,2-dibromo-1-phenylethane (1.2%). With nearly same conversion in same
time, the oxidative bromination of salicylaldehyde gave three products with the selectivity order:
Keywords:
Vanadium complexes
Polymer-grafted vanadium complex
Crystal structure
Oxidative bromination of styrene
Oxidative bromination of salicylaldehyde
Oxidative bromination of trans-stilbene
5-bromosalicylaldehyde > 2,4,6-tribromophenol > 3,5-dibromosalicylaldehyde.
A maximum of 91%
conversion of trans-stilbene has been obtained in 2 h of reaction time, where selectivity of the obtained
reaction products varied in the order: 2,3-diphenyloxirane (trans-stilbene oxide) > 1,2-dibromo-1,2-
diphenylethane > 2-bromo-1,2-diphenylethanol. The catalytic activity of the non-polymer grafted
complex 2 is lower than that of the polymer-grafted one. In addition, the recycling ability of the grafted
complex makes it better over the neat complex.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
However, such complexes are coordinatively unsaturated and have
a tendency to attain a six coordination environment [12]. This is
The trigonal–bipyramidal coordination environment of the
vanadium center in vanadate-dependent haloperoxidases (VHPO)
enzymes, irrespective of their origin [1–4], has stimulated the
coordination chemistry of vanadium, with particular emphasis on
the model character of vanadium(V) complexes having O and N
functionalities. These structural models have also been extended
to functional similarities including other catalytic potentials in
oxygen transformations [5–11]. VVO2 complexes with tridentate
(ONO or ONN) ligand systems provide mostly a five-coordinate
environment to the vanadium center, similar to the enzymes.
mostly achieved through the coordination of a solvent molecule,
if a suitable functional group of a neighboring molecule for the for-
mation of bridging complex is not available [13–15]. The replace-
ment of a coordinated solvent molecule in such model complexes
by an imidazole moiety, one of the binding sites in VHPO, pre-
grafted in a polymer produces immobilized structural models that
can be modified to functional models for continuous working with-
out losing their catalytic activities [8,16,17].
Herein, we have prepared the dioxidovanadium(V) complex of a
tridentate ONN donor ligand, 1-(2-pyridylazo)-2-naphthol [Hpan
(I)], and have grafted the complex through coordination of an imid-
azole functionalized chloromethylated polystyrene (cross-linked
with 5% divinylbenzene). The characterization of the immobilized
⇑
Corresponding author. Tel.: +91 1332 285327; fax: +91 1332 273560.
E-mail address: rkmanfcy@iitr.ac.in (M.R. Maurya).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.09.021

M.R. Maurya et al. / Polyhedron 67 (2014) 436–448
437
as well as neat complexes, their reactivity and catalytic potential as
2.3.2. [{VVO(pan)}2(
l-O)2] (2)
functional mimics for the oxidative bromination of organic sub-
strates have been reported.
Complex 1 (0.414 g, 1 mmol) was dissolved in hot methanol
(100 mL) and air was passed through the solution with stirring
and occasional heating. After ca. 3 days the solution slowly chan-
ged to a wine red colour. After reducing the volume and keeping
the solution at room temperature, complex 2 slowly precipitated,
which was filtered and dried at ca. 100 °C. Yield 85%. Anal. Calc.
for C30H20N6O6V2 (662.41): C, 54.40; H, 3.04; N, 12.69. Found:
C, 53.8; H, 2.9; N, 12.5%. 1H NMR (DMSO-d6/ d in ppm): 7.15–
7.17 (d, 1H), 7.49–7.52 (t, 1H), 7.72–7.75 (q, br, 2H), 7.90–7.92(d,
1H), 8.12–8.14 (d, 1H), 8.26–8.28 (d, 1H), 8.38–8.41 (t, 1H),
9.33–9.34 (d, 1H), 9.36–9.37 (d, 1H). 51V NMR (MeOD-d4/d in
ppm): ꢁ542 (major and sharp), ꢁ549 (minor).
2. Experimental
2.1. Materials
V2O5 (Loba Chemie, Mumbai, India), acetylacetone (Hacac),
1-(2-pyridylazo)-2-naphthol (Aldrich Chemicals Co., USA), salicyl-
aldehyde (Sisco research, India), styrene (Acros Organics, USA),
trans-stilbene (Lancaster, England) and aqueous 30% H2O2 (Quali-
gens, India) were used as obtained. Chloromethylated polystyrene
[18.9% Cl (5.35 mmol Cl per gram of resin)] cross-linked with 5%
divinylbenzene was obtained as a gift from Thermax Limited, Pune,
India. [VIVO(acac)2] was prepared according to the methods re-
ported [18].
2.3.3. [V VO(O2)(pan)(MeOH)] (3)
Aqueous 30% H2O2 (ca. 2 mL) was added dropwise to complex 1
(0.414 g, 1 mmol) suspended in methanol (30 mL) with constant
stirring at ambient temperature. After ca. 8 h, the insoluble com-
plex slowly dissolved and gave a pinkish red solution. The solution
was kept for ca. 24 h after reducing the solution volume to ca.
10 mL. The precipitated solid was filtered, washed with cold meth-
anol (2 ꢀ 4 mL) and dried in air. Yield 50%. Anal. Calc. for C16H14
N3O5V (379.25): C, 50.67; H, 3.72; N, 11.07. Found: C, 51.1; H,
2.2. Physical methods and analysis
Elemental analyses of the ligand and complexes were obtained
with an Elementar model Vario-EL-III. The vanadium content in the
polymer-grafted complex was checked by Inductively Coupled
Plasma spectrometry (ICP; Labtam 8440 plasma lab). Thermogravi-
metric analyses of the complexes were carried out using a Perkin-
Elmer Pyris Diamond under an oxygen atmosphere. The energy
dispersive X-ray analyses (EDX) of the anchored ligand and the
complex were recorded on an FEI Quanta 200 FEG. The samples
were coated with a thin film of gold dust to protect the surface
material from thermal damage by the electron beam and to make
the sample conductive. IR spectra were recorded as KBr pellets on a
Nicolet NEXUS Aligent 1100 FT-IR spectrometer after grinding the
sample with KBr. The electronic spectrum of a solid sample was re-
corded in Nujol using a Shimadzu 1601 UV–Vis spectrophotometer
by layering the mull of the sample to the inside of one of the cuv-
ettes while keeping the other one layered with Nujol as a reference.
Spectra of neat complexes were recorded in methanol or DMSO. 1H
and 51V NMR spectra were obtained on Bruker Avance III 500 and
400 MHz spectrometers, respectively, with the common parameter
settings. NMR spectra were usually recorded in MeOD-d4 or DMSO-
d6, and d(51V) values are referenced relative to neat VOCl3 as an
external standard. The EPR spectrum was recorded with a Bruker
EMX EPR X-band spectrometer. The spin Hamiltonian parameters
were obtained by simulation of the spectrum with the computer
program of Rockenbauer and Korecz [19]. A Shimadzu 2010 plus
3.8; N, 11.2%. IR (KBr,
m
max, cmꢁ1): 1359 (N@Nazo), 935 (V@O),
848 (O–O), 771 [V(O)2 asym], 562 [V(O)2 sym]. 1H NMR (DMSO-d6/d
in ppm): 3.16–3.17 (d, 3H), 4.07–4.10 (q, 1H), 6.65–6.67 (d, 1H),
7.24–7.31 (t, br, 1H), 7.56–7.59 (t, 1H), 7.67–7.69 (d, 1H), 7.90–
7.92 (d, 1H), 7.99 (s, 2H), 8.38–8.92 (t, 2H). 51V NMR (MeOD-d4/d
in ppm): ꢁ597.
2.3.4. Preparation of PS-im[VVO2(pan)] (4)
Imidazolomethylpolystyrene (PS-im) [16] (1.00 g) was allowed
to swell in DMF (20 mL) for 2 h. A solution of [{VVO(pan)}2(
l-
O)2] (2) (1 g, 3.02 mmol) in DMF (20 mL) was added to the above
suspension and the reaction mixture was heated at 90 °C for 14 h
with slow mechanical stirring. After cooling to room temperature,
the dark black polymer-grafted complex 4 was separated by filtra-
tion, washed with hot DMF, followed by hot methanol and dried at
120 °C in an air oven. Found: V, 1.12%.
2.4. X-ray crystal structure determination
Three-dimensional room temperature X-ray data for 3 were col-
lected on a Bruker Kappa Apex CCD diffractometer at low temper-
gas-chromatograph fitted with
a
Rtx-1 capillary column
(30 m ꢀ 0.25 mm ꢀ 0.25 m) and an FID detector was used to ana-
l
ature by the /–
hemisphere of data collected from frames, each of them covering
0.3° in . Of the 15840 reflections measured for 3, all were cor-
x scan method. Reflections were measured from a
lyze the reaction products. The percent conversion of substrate and
the selectivity of the products were made on the basis of the rela-
tive peak area of the substrate/respective product in the GC using a
formulae presented elsewhere [20]. The identity of the products
was confirmed using a GC–MS model Perkin-Elmer, Clarus 500
by comparing the fragments of each product with the library
available.
x
rected for Lorentz and polarization effects and for absorption by
multi-scan methods based on symmetry-equivalent and repeated
reflections, 1575 independent reflections exceeded the significance
level (jFj/ jFj) > 4.0. Complex scattering factors were taken from
r
the program package SHELXTL [21]. The structure was solved by
the direct method and refined by full matrix least-squares on F2.
Hydrogen atoms were located in the difference Fourier map and
freely refined, except for the hydrogen atoms of C(7) and O(1M),
which were included in calculation positions and refined in the rid-
ing mode. Refinement was done with allowance for thermal anisot-
ropy of all non-hydrogen atoms. Further details of the crystal
structure determination are given in Table 1. A final difference Fou-
rier map showed no residual density outside: 0.628 and
2.3. Preparations
2.3.1. Preparation of [VIVO(acac)(pan)] (1)
A stirred solution of [VIVO(acac)2] (0.53 g, 2 mmol) dissolved in
methanol (20 mL) was added to a hot solution of Hpan (0.498 g,
2 mmol) in methanol (25 mL) and the resulting reaction mixture
was refluxed for ca. 8 h in an oil bath. The obtained dark brown
precipitate was filtered off, washed with methanol and dried in a
vacuum. Yield 72%. Anal. Calc. for C20H17N3O4V (414.31): C,
57.98; H, 4.14; N, 10.14. Found: C, 57.5; H, 4.0; N, 10.2%.
ꢁ0.852 e Åꢁ3
. A weighting scheme, w = 1/[r
2(Fo2) + (0.057800-
P)2 + 0.602000P], where P = (|Fo|2 + 2|Fc|2)/3, was used in the later
stages of the refinement.

438
M.R. Maurya et al. / Polyhedron 67 (2014) 436–448
Table 1
CH2Cl2. The organic layer was analyzed by gas chromatography
and the reaction products were identified by GC–MS analysis.
Crystal data and structure refinement for 3.
Compound
3
Formula
Formula weight
T (K)
C
16H14N3O5V
2.5.3. Oxidative bromination of trans-stilbene
379.24
100(2)
The catalyst (0.005 g), trans-stilbene (0.90 g, 5 mmol), KBr
(3.56 g, 30 mmol), 30% H2O2 (3.39 g, 30 mmol) and 70% HClO4
(5.72 g, 40 mmol) (added in four equal portions as mentioned be-
fore) were taken in a two phase CHCl3–H2O (40 mL, 50%, v/v) mix-
ture) system and stirred at 40 °C. After 2 h the reaction mixture
was washed with water and the organic layer was separated. The
obtained reaction products were analyzed as mentioned above
using this organic layer. For the identification of the reaction prod-
ucts the crude mass was suspended in CH2Cl2; the insoluble trans-
stilbene oxide was separated by filtration, and then the solvent was
evaporated. Other reaction products were separated using a silica
gel column. Elution of the column with 1% CH2Cl2 in n-hexane first
separated the mono bromo derivative, followed by the dibromo
derivative. The products were identified by GC–MS and 1H NMR
spectroscopy.
k (Å)
0.71073
monoclinic
P21/c
8.827(7)
9.970(7)
17.633(14)
91.39(2)
1551(2)
4
776
1.624
0.674
2.31–27.69
0.2179
Crystal system
Space group
a (Å)
b(Å)
c(Å)
b (°)
V (Å3)
Z
F (000)
Dcalc (g cm–3
)
l
(mmꢁ1
)
h (°)
Rint
Crystal size (mm3)
0.20 ꢀ 0.14 ꢀ 0.05
Goodness-of-fit (GOF) on F2
1.007
a
R1
0.0819
0.1874
0.647 and ꢁ0.876
1H NMR spectral data of reaction products are as follows:
2,3-Diphenyloxirane: 1H NMR (CDCl3/d in ppm): 7.35–7.55 (m,
10H, aromatic); 5.5 (d, 2H, CH).
wR2 (all data)b
Largest differences peak and hole (e Åꢁ3
)
a
R1
wR2 = {
=
R
j|Fo| ꢁ |Fc|j/ jFoj.
R
2-Bromo-1,2-diphenylethanol: 1H NMR (CDCl3/d in ppm): 8.0 (s,
1H, OH); 7.37–7.52 (m, 10H, aromatic); 5.5 (d, 2H, CH).
1,2-Dibromo-1,2-diphenylethane: 1H NMR (CDCl3/d in ppm): 7.1–
7.4 (m, 10H, aromatic); 6.15 (d, 2H, CH).
b
R
[w(j|Fo|2 ꢁ |Fc|2j)2]j/
R
[w(Fo4)]}1/2
.
2.5. Catalytic activity studies
Warning: HClO4 is potential oxidant, hence must be handled
3. Results and discussion
carefully.
The polymer-grafted complex PS-im[VVO2(pan)] (4) was used
for the oxidative bromination of styrene, salicylaldehyde and
trans-stilbene. All reactions were carried out in a 50 mL round bot-
tom flask at 40 °C. The polymer beads were allowed to swell in
methanol for 2 h prior to the start of the catalytic reaction.
3.1. Synthesis, characterization and solid state characteristics
The reaction between equimolar amounts of [VIVO(acac)2] and
Hpan (I) in dry, refluxing methanol yielded the oxidovanadium(IV)
complex [VIVO(acac)(pan)] (1) (Eq. (1)). On aerial oxidation of 1 in
methanol, the dioxidovanadium(V) complex [{VVO(pan)}2(
l-O)2]
2.5.1. Oxidative bromination of styrene
(2) was obtained (Eq. (2)). Complex 2 can also be prepared directly
by reacting [VIVO(acac)2] with I followed by aerial oxidation. Addi-
tion of H2O2 to a methanolic solution of 1 yielded the oxidomonop-
eroxidovanadium(V) complex [VVO(O2)(pan)(MeOH)] (3) (Eq. (3)).
Same complex could also be isolated by the reaction of 2 with
H2O2 in methanol (Eq. (4)).
The catalyst (0.010 g), styrene (1.04 g, 10 mmol), KBr (2.38 g,
20 mmol), aqueous 30% H2O2 (2.27 g, 20 mmol) and 70% aqueous
HClO4 (5.72 g, 40 mmol, added in four equal portions at t = 0, 15,
30 and 45 min) were stirred at 40 °C in a two phase CH2Cl2–H2O
(40 mL 50%, v/v mixture) system for 1 h. Every 15 min a small ali-
quot was withdrawn and after addition of 5 mL of CH2Cl2, it was
washed with distilled water in a separating funnel and the organic
layer was analyzed by gas chromatography. At the end of the reac-
tion, the organic layer after processing as above, was evaporated
and the residue was purified by column chromatography using
1% CH2Cl2 in n-hexane as an eluent. The products were identified
by GC–MS and 1H NMR spectroscopy.
½VIVOðacacÞ2ꢂþHpan ! ½VOIVðacacÞðpanÞꢂþHacac
ð1Þ
ð2Þ
1
2½VOIVðacacÞðpanÞꢂþ O2 þH2O ! ½VVOðpanÞ ð
l
ꢁOÞ2ꢂþ2Hacac
2
2
2½VOIVðacacÞðpanÞꢂþ3H2O2 þMeOH ! ½VVOðO2ÞðpanÞðMeOHÞꢂþ2Hacacþ2H2O
ð3Þ
1=2½VVOðpanÞ2ð
l
ꢁOÞ ꢂþH2O2 þMeOH ! ½VVOðO2ÞðpanÞðMeOHÞꢂþH2O
ð4Þ
2
1,2-Dibromo-1-phenylethane: 1H NMR (CDCl3/d in ppm): 7.29–
7.39 (m, 5H, aromatic), 5.11–5.13 (q, 1 H, CH), 3.97–4.06 (septet,
2H, CH2).
All the complexes are fairly soluble in methanol, CHCl3, DMF
and DMSO; complex 1 is additionally soluble in CH2Cl2. Scheme 1
presents the structures proposed for these complexes, which are
based on the spectroscopic characterization (IR, electronic, EPR,
1H and 51V NMR), elemental analyses, thermogravimetric patterns
and single crystal X-ray analysis of 3. The ligand coordinates
through its monoanionic (ONN) functionalities.
1-Phenylethane-1,2-diol: 1H NMR (CDCl3/d in ppm): 7.29–7.39
(m, 5H, aromatic), 4.9 (q, 1H, CH), 3.5 (q, 1H of CH2), 3.6 (q, 1H
of CH2), 2.7 (br, 1H, OH).
2-Bromo-1-phenylethane-1-ol: 1H NMR (CDCl3/d in ppm): 7.29–
7.39 (m, 5H, aromatic), 5.1 (q, 1H, CH), 3.9 (septet, 2H, CH2).
These data matched well with those reported earlier [22].
2.5.2. Oxidative bromination of salicylaldehyde
The catalyst (0.005 g), salicylaldehyde (0.61 g, 5 mmol), KBr
(1.18 g, 10 mmol), 30% H2O2 (1.13 g, 10 mmol) and 70% HClO4
(1.42 g, 10 mmol, added in four equal portions at t = 0, 30, 60 and
90 min) were stirred at 40 °C in distilled water (20 mL) for 2 h. After
completion of the reaction, it was transferred to a separating funnel
and washed several times with water after addition of 10 mL of
Scheme 1. Proposed structures of the complexes prepared. Only the idealized
structure of 2 is shown.

M.R. Maurya et al. / Polyhedron 67 (2014) 436–448
439
Immobilization of imidazole through covalent attachment onto
chloromethylated polystyrene, cross-linked with 5% divinylben-
zene (imidazolomethylpolystyrene, PS-im) has been achieved in
acetonitrile in the presence of KI and triethylamine, following the
literature procedure (Scheme 2) [16,17]. The remaining chlorine
content of 0.166 mmol/g in imidazolomethylpolystyrene (PS-im)
suggests ca. 97% substitution of imidazole. Reaction of PS-im with
complex 2 in DMF resulted in the formation of the polymer-grafted
dioxidovanadium(V) complex PS-im[VVO2(pan)] (4). The free chlo-
romethyl groups of PS do not coordinate with the vanadium pre-
cursor. Scheme 2 presents the whole synthetic procedure. The
metal ion loading calculated from the obtained vanadium content
(0.21 mmol/g of resin) for PS-im[VO2(pan)] is also close to the va-
lue determined by thermogravimetric analysis (0.18 mmol/g of
resin).
3.2. Structure description of [VVO(O2)(pan)(MeOH] (3)
The complex [VVO(O2)(pan)(MeOH)] (3) crystallizes from meth-
anol as dark red prisms (crystal dimensions 0.22 ꢀ 0.21 ꢀ 0.20).
Fig. 1 shows an ORTEP representation of 3 and Table 2 contains se-
lected bond lengths and angles. The vanadium atom is in the oxida-
tion state V. In the molecular structure, the vanadium center adopts
a distorted seven-coordinated pentagonal bipyramidal geometry
with the (pan) ligand coordinated through one Onaphthol, one Nazo
and one Npyridine atom, and with one Ooxide (terminal), one Omethanol
and two Operoxide, O(2) and O(3), atoms also coordinated to the
vanadium centre. The peroxide O–O distance of 1.443(5) Å lies
within the range (1.38–1.45 Å) of the majority of peroxide com-
pounds [23–25]; the distances V(1)–O(2) of 1.889(4) Å and V(1)–
O(3) of 1.856(4) Å also indicate the presence of a peroxide group.
The V@O bond [V(1)–O(4): 1.600(4) Å] is characteristic of an
Fig. 1. ORTEP plot of the complex [VVO(O2)(pan)(MeOH)] (3). All non-hydrogen atoms
are presented by their 30% probability ellipsoids. Hydrogen atoms are omitted for
clarity.
Table 2
Bond lengths (Å) and angles (°) for [VVO(O2)(pan)(MeOH)] (3).
3
Bond lengths (Å)
V(1)–O(1)
V(1)–O(2)
V(1)–O(3)
V(1)–O(4)
V(1)–O(1M)
2.000(4)
1.889(4)
1.856(4)
1.600(4)
2.241(4)
O(2)–O(3)
V(1)–N(1)
V(1)–N(3)
N(1)–N(2)
1.443(5)
2.135(5)
2.115(5)
1.275(6)
oxido-type O atom with strong
p bonding (see Table 2). The V–
Bond Angles (°)
O(4)–V(1)–O(3)
O(4)–V(1)–O(2)
O(3)–V(1)–O(2)
O(4)–V(1)–O(1)
O(3)–V(1)–O(1)
O(2)–V(1)–O(1)
O(4)–V(1)–N(3)
O(3)–V(1)–N(3)
O(2)–V(1)–N(3)
O(1)–V(1)–N(3)
O(4)–V(1)–N(1)
O
methanol bond, which is trans to the oxo atom, is significantly long-
105.5(2)
104.1(2)
45.30(17)
97.7(2)
80.05(17)
124.62(18)
91.8(2)
127.6(2)
82.79(19)
146.95(18)
91.3(2)
O(3)–V(1)–N(1)
O(2)–V(1)–N(1)
O(1)–V(1)–N(1)
N(3)–V(1)–N(1)
O(4)–V(1)–O(1M)
O(3)–V(1)–O(1M)
O(2)–V(1)–O(1M)
O(1)–V(1)–O(1M)
N(3)–V(1)–O(1M)
N(1)–V(1)–O(1M)
152.30(19)
151.2(2)
75.91(19)
72.3(2)
166.82(19)
87.52(17)
86.32(18)
82.47(17)
81.34(18)
75.95(18)
er [V(1)–O(1M): 2.241(4) Å] [11]. Intermolecular hydrogen bonds
occur between the bonded methanol group and the two oxygen
atoms of the peroxide group of another molecule (see Fig. 2 and Ta-
ble 3). The pan ligand has a planar structure in the complex. The
vanadium atom is displaced by about 0.2743 Å from the plane con-
stituted for all the atoms of the ligand [C(1), C(2), C(3), C(4), C(5),
C(6), C(7), C(8), C(9), C(10), C(11), C(12), C(13), C(14), C(15), N(1),
N(2) and N(3)], which presents a deviation from planarity of
0.0498(56) Å.
3.3. Thermogravimetric analysis (TGA) studies
The complex [VOIV(acac)(pan)] (1) decomposes exothermically
in two major steps. Between 200 and 480 °C, the weight loss corre-
sponds to removal of the acac ligand (Found: 23.6, Calc.: 23.9%). On
Scheme 2. Synthetic procedure for the isolation of the polymer-grafted dioxido-
vanadium(V) complex, PS-im[VO2(pan)] (4). The ball represents the polystyrene
back-bone.
Fig. 2. Hydrogen bonding scheme of the compound [VVO(O2)(pan)(MeOH)] (3). All
the non-hydrogen atoms are presented by their 30% probability ellipsoids.

440
M.R. Maurya et al. / Polyhedron 67 (2014) 436–448
Table 3
Hydrogen bonds for [VVO(O2)(pan)(MeOH)] (3) (Å and °).
D–Hꢃ ꢃ ꢃA
d(D–H)
d(Hꢃ ꢃ ꢃA)
d(Dꢃ ꢃ ꢃA)
<(DHA)
O(1M)–H(1M)ꢃ ꢃ ꢃO(2)#1
O(1M)–H(1M)ꢃ ꢃ ꢃO(3)#1
0.93
0.93
2.05
2.36
2.694(6)
3.157(6)
125.1
143.4
Symmetry transformations used to generate equivalent atoms:
#1 ꢁx + 2,y + 1/2,ꢁz + 1/2.
further heating the remaining residue decomposes sharply in the
range 480–550 °C to give V2O5 (Found: 29.0, Calc.: 30.0%) as the
end product. The polymer-grafted complex PS-im[VO2(pan)] (4)
starts losing weight at ca. 200 °C, but major exothermic decompo-
sition occurs in two overlapping steps in the temperature range
400–500 °C. It was difficult to distinguish the decomposition of
the polymer and organic ligand fragments. However, the remaining
vanadium content of 0.18 mmol/g of resin suggests the formation
of V2O5.
3.4. Field emission-scanning electron microscopic (FE-SEM) and
energy dispersive X-ray analysis (EDX) studies
Field emission-scanning electron micrographs (Fe-SEM) for sin-
gle beads of pure chloromethylated polystyrene and the polymer-
grafted complex were recorded to see the morphological changes.
Some of these images are reproduced in Fig. 3. As expected, graft-
ing of the vanadium complex resulted in the roughening of the top
layer of the polymer-supported beads. Energy dispersive X-ray
analyses (EDX) estimate a vanadium content of ca. 0.18 mmol/g
of resin for PS-im[VVO2(pan)] (4). This observation suggests the
immobilization of the metal complex onto the polystyrene beads.
Fig. 4. IR spectra of (a) [VIVO(acac)(pan)] (1), (b) [{VVO(pan)}2(
[VVO(O2)(pan)(MeOH)] (3) and (d) PS-im[VVO2(pan)] (4).
l-O)2] (2), (c)
Table 4
IR spectral data (cmꢁ1) of the ligand and the complexes.
Compounds
m
(N@Nazo
)
m(C@Npyridyl/C@C)
m(V@O)
Hpan (I)
1436
1360
1359
1359
1364
1566, 1620
1549, 1586
1550, 1587
1550, 1585
1560, 1600
–
955
936, 848
960
940, 917
[VIVO(acac)(pan)] (1)
[{VVO(pan)}2(
l-O)2] (2)
[VVO(O2)(pan)(MeOH)] (3)a
PS-im[VVO2(pan)] (4)
3.5. IR spectral study
Bands due to the peroxido group: 923, 721 and 565 cmꢁ1
.
Fig. 4 presents the IR spectra of the complexes. The complex
a
[VIVO(acac)(pan)] (1) shows one sharp band at 955 cmꢁ1 in the IR
spectrum due to the
[{VVO(pan)}2( -O)2] (2) shows one sharp band at 936 cm–1 and a
broad band at 848 cm–1 corresponding to
(V@O) and
modes, respectively. This observation hints towards dimerization
of complex 2 in the solid state. This dimer breaks into a monomer
m(V@O) stretch, while the dioxido complex
The IR spectrum of Hpan (I) displays a sharp band at 1436 cm–1
l
m
m
(V@Oꢃ ꢃ ꢃV)
due to the m(–N@N–) group. In the complexes, this band shifts to a
lower wave number and appears at 1359–1364 cmꢁ1, confirming
the coordination of the azo nitrogen to the metal centre [26]. The
ligand also exhibits two sharp bands at 1566 and 1620 cm–1 due
upon grafting onto the polymer and exhibits masym(V@O) and msym
(O@V@O) modes at 940 and 917 cm–1, respectively (Table 4). The
peroxo complex [VVO(O2)(pan)(MeOH)] (3) exhibits three IR active
vibrational modes associated with the peroxido moiety [V(O2)]2+ at
923, 721 and 565 cm–1, which are assigned to the O–O intra
stretching, asymmetric V(O2) stretching and symmetric V(O2)
stretching, respectively. In addition, a band at 960 cmꢁ1 is assigned
to
m(C@C/C@N) and both bands appear at lower wavenumbers
(1549–1560/1585–1600 cm–1) due to the coordinated ring nitro-
gen to the metal. The absence of a band in the 3400 cm–1 region
(in 1, 2 and 4) indicates the coordination of the phenolic oxygen
after proton replacement. Thus, the IR data confirm the monobasic
tridentate ONN coordination behavior of the ligand in these
complexes.
due to the m(V@O) stretch.
3.6. Electronic spectral studies
As the ligand I is an azo dye with a phenolic oxygen, complexes
of this ligand are highly coloured and show several bands in the
visible region. We have recorded the electronic spectra of the
complexes in MeOH and DMSO. In general, the complexes
[{VVO(pan)}2( -O)2] (2) and [VVO(O2)(pan)(MeOH)] (3) show a
l
good resolution of bands in MeOH, while the complex
[VIVO(acac)(pan)] (1) shows better resolved bands in DMSO. Table 5
provides details of the spectral data in both solvents, but the dis-
cussion written below is based on the best spectra obtained for
these complexes. The absorption bands appearing at 502, 540
and 567 nm in 1 and at 486, 551 and 580 nm in 2 are due to
ligand-to-metal charge transfer type transitions (LMCT). Such
bands in 3 appear at 552 and 580 nm. The band appearing at
Fig. 3. Field emission-scanning electron micrographs of (a) chloromethylatedpoly-
styrene (PS) and (b) PS-im[VO2(pan)] (4).

M.R. Maurya et al. / Polyhedron 67 (2014) 436–448
441
Table 5
Electronic spectral data of the ligand and the complexes.
Compounds
Solvent
kmax/nm (e/Mꢁ1 cmꢁ1
)
Hpan (I)
MeOH
DMSO
MeOH
DMSO
MeOH
DMSO
MeOH
Nujol
224, 304, 463
[VIVO(acac)(pan)] (1)
257 (19327), 294 (10113), 340 (5097),415 (6478), 502 (7425)(sh), 540 (11209), 567 (12414), 745 (85)
225 (20491), 292 (7845), 415 (7790)(sh), 462 (10291), 574 (604)
[{VVO(pan)}2(
l
-O)2] (2)
274 (13500)(sh), 309 (8107)(sh), 331 (6290)(sh), 410 (6242), 552 (13107), 573 (13708)
228 (19225), 304 (4886)(sh), 435 (4842)(sh), 486 (4047), 551 (5970), 580 (6259)
271 (14491)(sh), 307 (7959)(sh), 326 (7167)(sh), 414 (6946), 550 (169450), 581 (18482)
229 (16286), 269 (7368)(sh), 307 (3498)(sh), 326 (3213)(sh), 421 (3719), 552 (8322), 580 (8973)
252, 333, 435, 584, 627
[VVO(O2)(pan)(MeOH)] (3)
PS-im[VVO2(pan)] (4)
415, 435 and 421 nm in 1, 2 and 3, respectively is due to the ligand-
to-metal charge-transfer from the phenolate O atom to the d orbi-
tals of vanadium for monomeric dioxidovanadium(V) complexes
[27]. Complex 1 also displays a weak shoulder band at 745 nm
due to a d–d transition. The higher energy UV region bands,
occurring in the range 200–330 nm, are assigned to intra-ligand
transitions. The electronic spectrum of polymer-grafted PS-im
[VVO2(pan)] (4) recorded in Nujol (Fig. 5) exhibits a spectral pat-
tern exactly the same as that of 2, but with weaker intensities,
and thus confirms that the supported complex maintains the same
coordination atmosphere as found for the non-grafted one.
2.5
2.3
2.1
1.9
1.7
1.5
g
Fig. 6. First derivative EPR spectrum of a frozen solution of [VIVO(acac)(pan)] (1) in
MeOH at 77 K.
3.8. 1H and 51V NMR studies
3.7. EPR spectroscopy study
The 1H and 51V NMR chemical shift values for the complexes are
included in the experimental section. The 1H NMR spectra of com-
plexes 2 and 3 show all the expected signals. The shielded doublets
at 9.33–9.34 (d, 1H) and 9.36–9.37 (d, 1H) ppm in 2 are assignable
to the protons nearest to the pyridine-nitrogen and are normally
considered as characteristic of a coordinated pyridinic nitrogen
[26]. This signal is not very clear in complex 3 as the signals of
two protons overlap in this region. Assignments of other aromatic
protons are difficult. The ligand Hpan (I) displays a signal at
d = 15.8 ppm due the naphtholic proton and the absence of this sig-
nal in the complexes confirms the coordination of the naphtholato-
O atom.
Fig. 6 presents the EPR spectrum of a ‘‘frozen’’ (77 K) solution
(in MeOH) of [VIVO(acac)(pan)] (1). The hyperfine features and
spectrum itself are consistent with binding modes involving (Oacac
,
O
phenolate, Nimine, Npy equatorial and (Oacac axial. Once a particular
)
)
binding mode is assumed, the value of Ak can be estimated using
the additivity relationship proposed by Wüthrich [28] and Chas-
teen [29], with an estimated accuracy of 3 ꢀ 10–4 cm–1. However,
for the potential donor groups under consideration, their predicted
contributions to the parallel hyperfine coupling constant are rather
similar (Oacac ꢄ41.7, Ophenolate ꢄ38.9, Nimine ꢄ41.6, Npy ꢄ40.7,
OMeOH ꢄ45.5, all A|| contributions in cm–1 ꢀ 10–4) [29,30], hence
it is not possible to distinguish between the several plausible bind-
ing modes. The spectrum of [VIVO(acac)(pan)] was simulated [16]
In the 51V NMR spectra, the line widths at half height are typi-
cally about 200 Hz. The complex [{VVO(pan)}2(
l-O)2] (2) in MeOD-
and the spin Hamiltonian parameters obtained are g 1.952, A
k
k
163.5 ꢀ 10–4 cmꢁ1, A\ 63.0 ꢀ 10–4 cmꢁ1 and g\ 1.981. Thus, the
EPR spectrum of [VIVO(acac)(pan)] in MeOH is in good agreement
with an O2N2 binding mode. However, the coordination of solvent
cannot be ruled out, though this implies the substitution of one of
the equatorial donor atoms by an O-atom of the solvent.
d4 displays a sharp resonance at d = ꢁ542 and a minor one at
d = ꢁ549 ppm. These chemical shifts are within the values ex-
pected for dioxidovanadium(V) complexes containing an O/N do-
nor set [27,31]. The minor signal at d = ꢁ549 ppm gains intensity
with time in MeOD-d4 (24 h), and therefore we assigned this reso-
nance to [VVO2(pan)(MeOD)].
0.14
0.12
0.10
0.08
0.06
3.9. Catalytic activity studies
Vanadium haloperoxidases catalyze the oxidative bromination
of organic substrates in the presence of H2O2 and bromide ions;
vanadium(V) complexes have been reported to show functional
similarities to these enzymes. During the catalytic action vana-
dium(V) coordinates with 1 or 2 equivalents of H2O2 to give
oxido-monoperoxido, [VVO(O2)]+, or oxido-diperoxido, [VO(O2)2]–,
species that oxidize bromide ions in the presence of acid, most
–
likely to Br2, Br3 and/or HOBr, which ultimately brominate the
organic substrates [4,32].
0.04
200 300 400 500 600 700 800 900
3.9.1. Oxidative bromination of styrene
Wavelength/ nm
The dioxidovanadium(V) complexes (polymer grafted as well as
non-polymer grafted) reported here satisfactorily catalyze the oxi-
dative bromination of styrene, salicylaldehyde and trans-stilbene.
Fig. 5. Electronic spectrum of PS-im[VVO2(pan)] (4) recorded after dispersing in
Nujol.

442
M.R. Maurya et al. / Polyhedron 67 (2014) 436–448
HO
Similarly, three different amounts of KBr, i.e. 10, 20 and
Br
HO
Br
Br
OH
30 mmol, were taken for styrene (1.04 g, 10 mmol), catalyst
(0.010 g), CH2Cl2–H2O (40 mL, v/v), 30% aqueous H2O2 (2.27 g,
20 mmol) and 70% HClO4 (2.86 g, 20 mmol, added as mentioned
above) and the reaction was carried out at 40 °C. The obtained con-
version was comparatively low (41%) with 10 mmol KBr, but
increasing the KBr amount from 10 to 20 mmol improved this con-
version to 65%, which further improved to 81% at 30 mmol KBr, as
shown in Fig. 7(b). Therefore, 30 mmol KBr was used to optimize
the remaining reaction conditions.
Catalyst
H2O2, KBr, HClO4
+
+
1,2-dibromo-1-
phenylethane
Styrene
2-bromo-1-
phenylethane-1-ol
1-phenylethane-
1,2-diol
Scheme 3. Oxidative brominated products of styrene.
As also observed earlier [6,22,33], oxidative bromination of sty-
rene in the presence of KBr, HClO4 and 30% H2O2 under a bi-phasic
system (CH2Cl2–H2O) gave mainly three major products, 1,2-dibromo-
1-phenylethane, 2-bromo-1-phenylethane-1-ol and 1-phenylethane-
1,2-diol; Scheme 3. Other minor products, like benzaldehyde, styrene
epoxide, benzoic acid and 4-bromostyrene, have also been identi-
fied, but their overall percentage is extremely low compared to the
total of the main products. All major products were separated, iden-
tified and confirmed by 1H NMR spectroscopy as well as GC–MS.
Various reaction parameters, viz. amounts of catalyst, oxidant
(30% aqueous H2O2), KBr and 70% aqueous HClO4, were optimized
to obtain the maximum conversion of styrene. For a fixed amount
of styrene (1.04 g, 0.010 mol), H2O2 (2.27 g, 0.020 mol), KBr (2.38 g,
0.020 mol) and HClO4 (2.86 g, 0.020 mol, added in four equal por-
tions at t = 0, 15, 30 and 45 min of the reaction time) in a CH2Cl2–
H2O (40 mL, v/v) solvent system, three different amounts of cata-
lyst, i.e. 0.010, 0.015 and 0.020 g, were taken and the reaction
was carried out at 40 °C for 1 h. As shown in Fig. 7 (a), a maximum
of 65% conversion was achieved with 0.010 g of catalyst. The con-
version of styrene improved only marginally to 68% and 70% on
increasing the catalyst amount to 0.015 and 0.020 g, respectively.
Therefore, 0.010 g amount of catalyst was considered optimum
for the maximum conversion of styrene.
The effect of the amount of oxidant was studied by considering
substrate to oxidant ratios of 1:1, 1:2 and 1:3 for a fixed amount of
styrene (1.04 g, 10 mmol) under the above reaction conditions. A
maximum 64% conversion was achieved with a substrate to oxi-
dant ratio of 1:1. Increasing the substrate to oxidant ratio from
1:1 to 1:2 increased the conversion from 64% to 81%. This conver-
sion further improved considerably (91%) at a substrate to oxidant
ratio of 1:3. Only a slight improvement in conversion was obtained
upon further increasing this amount. Therefore, an oxidant amount
of 30 mmol was considered to be an adequate one [Fig. 7(c)] for the
maximum conversion of styrene in 1 h of reaction time.
Acid (here HClO4) was found to be essential to carry out the cat-
alytic bromination. The amount of HClO4 was optimized by taking
substrate to HClO4 ratios of 1:1, 1:2 and 1:3 under the above reac-
tion conditions; Fig. 7(d). For each condition, HClO4 was added in
four equal portion at t = 0, 15, 30 and 45 min reaction time. Only
47% conversion was obtained with 10 mmol of HClO4 (i.e. a 1:1
substrate to HClO4 ratio), but the conversion reached 91% with
20 mmol of HClO4. On increasing this ratio to 1:3, the conversion
improved to 98% in 1 h of reaction time. At this ratio, the formation
of 1-phenylethane-1,2-diol is relatively more at the expense of 2-
bromo-1-phenylethane-1-ol. However, a substrate to HClO4 ratio
80
(a)
(b)
0.010 g
0.01 mol
0.02 mol
0.03 mol
70
60
50
40
30
20
10
0
0.015 g
0.020 g
80
60
40
20
0
0
15
30
45
60
0
15
30
45
60
Time/ min
Time/ min
100
80
60
40
20
0
100
(c)
(d)
0.01 mol
0.02 mol
0.03 mol
0.01 mol
0.02 mol
0.03 mol
80
60
40
20
0
0
15
30
45
60
0
15
30
45
60
Time/ min
Time/ min
Fig. 7. Effect of (a) amount of catalyst PS-im[VO2(pan)] (4), (b) amount of KBr, (c) amount of oxidant (i.e. 30% aqueous H2O2) and (d) amount of 70% HClO4 on the oxidative
bromination of styrene in 1 h of reaction time. For reaction conditions see text and Table 6.

M.R. Maurya et al. / Polyhedron 67 (2014) 436–448
443
Table 6
Results of oxidative bromination of styrene using the catalyst Ps-im[VO2(pan)] after 1 h of reaction time.
Entry No. KBr (g, mmol) H2O2 (g, mmol) HClO4 (g, mmol) Catalyst (g) CH2Cl2/H2O (v/v, mL) % Conv. % products formation
% Mono-bromo % Di-bromo % Di-ol % Others
1
2
3
4
5
6
7
8
9
2.38, 20
2.38, 20
2.38, 20
1.18, 10
3.56, 30
3.56, 30
3.56, 30
3.56, 30
3.56, 30
2.27, 20
2.27, 20
2.27, 20
2.27, 20
2.27, 20
1.13, 10
3.39, 30
3.39, 30
3.39, 30
2.86, 20
2.86, 20
2.86, 20
2.86, 20
2.86, 20
2.86, 20
2.86, 20
1.43, 10
2.86, 30
0.010
0.015
0.020
0.010
0.010
0.010
0.010
0.010
0.010
20/20
20/20
20/20
20/20
20/20
20/20
20/20
20/20
20/20
65
68
70
41
81
64
91
47
99
28
27
28
22
27
23
22
16
16
0
0
0
0
0
0
0
0
2
33
39
40
17
51
40
66
29
77
4
2
2
2
3
1
3
2
4
100
styrene
stirring the reaction mixture further affected the reaction products
(see Fig. 8) and at the end of 2 h, the selectivity of the reaction
products followed the order: 1-phenylethane-1,2-diol (85%) > 1,2-
dibromo-1-phenylethane (8.5%) > 2-bromo-1-phenylethane-1-ol
(0.5%). It seems that the monobromo derivative converts into 1-
phenylethane-1,2-diol slowly.
0
mono-bromo
di-bromo
di-ol
80
60
40
20
0
20
40
60
80
The recycling ability of 4 was also tested for the oxidative bro-
mination of styrene. The reaction mixture, after a contact time of
1 h, was filtered and the separated catalyst was washed with ace-
tonitrile, dried and subjected to further catalytic reactions under
similar conditions. No appreciable loss in the activity (see Table 7)
indicated that the catalyst was active even after the first cycle.
100
The neat complex [{VVO(pan)}2(
l-O)2] (2) showed 90% conver-
0
20
40
60
Time/ min
80
100
120
sion of styrene using 0.0007 g under the above optimized condi-
tions. However, the turnover frequency was low compared to
that of the polymer-anchored complex. The formation of only 2-
bromo-1-phenylethane-1-ol and 1-phenylethane-1,2-diol was ob-
served at the end 1 h of reaction. The 1,2-dibromo-1-phenylethane
product starts forming only after 1 h, along with a small amount of
an unidentified product, but the overall formation of both products
are very low even after 2 h. The selectivity of the reaction products
has the following order: 1-phenylethane-1,2-diol (75%) > 2-bromo-
1-phenylethane-1-ol (20%) > 1,2-dibromo-1-phenylethane (1.2%).
In the absence of catalyst, the reaction mixture gave about 55%
conversion after 1 h of reaction time under the above optimized
reaction conditions, with the selectivity order of products: 1-phen-
ylethane-1,2-diol (58%) > 2-bromo-1-phenylethane-1-ol (35%) > others
(7%) > 1,2-dibromo-1-phenylethane (0%).
Fig. 8. Percentage consumption of styrene and selectivity of the formation of
products with time using PS-im[VVO2(pan)] (4) as a catalyst precursor for 2 h of
reaction time under the optimized conditions specified in the text.
of 1:3 was considered as the best one for the maximum oxidative
bromination of styrene. A similar effect along with an equally good
conversion of styrene has also been observed using H2SO4 under
similar conditions, while the use of acetic acid was not successful.
Details of all these conditions and the corresponding conversion
of styrene along with the percent formation of the different prod-
ucts are summarized in Table 6. Thus, the optimized reaction con-
ditions (entry No. 9) as concluded for the oxidative bromination of
10 mmol (1.04 g) of styrene under a bi-phasic system are: catalyst
(0.010 g), KBr (3.57 g, 30 mmol), aqueous 30% H2O2 (3.39 g,
30 mmol), 70% HClO4 (4.28 g, 30 mmol) and CH2Cl2–H2O (40 mL,
50% v/v). The conversion of styrene and the formation of different
reaction products under the optimized reaction conditions have
been analyzed as a function of time and are presented in Fig. 8. It
is clear from the plot that the formation of all three major products
starts with the consumption of styrene. The selectivity of the for-
mation of the diol improves continuously and reaches 78% at the
end of 1 h. The selectivity of the mono bromo derivative improves
to 20% slowly in the first 45 min and then decreases. The formation
of the dibromo derivative starts only after 30 min and ends up at
2% in 1 h. Although no further increment was obtained after 1 h,
The catalytic efficiency of PS-im[VVO2(pan)] cannot be com-
pared with similar catalytic systems as no catalytic activity for
the oxidative bromination of styrene using other polymer-grafted
complexes has been reported in the literature. However, the cata-
lytic potential of PS-im[VVO2(pan)] compares well with several
other mononuclear and binuclear VV complexes [22] and even with
a manganese complex [34]. The geometry and ligand environments
(e.g. ONO or ONS donor) do not have much effect on the catalytic
potential of the complexes and exhibit almost similar conversion
along with the formation of all three products. The dioxidovanadi-
um(V) complexes with sterically hindered ONO donor ligands gave
relatively more amounts of 2-bromo-1-phenylethane-1-ol
Table 7
Conversion of styrene and the selectivity of products under the optimized reaction condition after 1 h of reaction time.
SL.No.
Catalyst
% Conv.
TOF/hꢁ1
Selectivity (%)
Mono-bromo
Di-bromo
Di-ol
Others
1.
2.
3.
PS-im[VO2(pan)]
PS-im[VO2(pan)]a
99
95
90
4648
4460
4226
16
15
24
2
2
0
78
79
73
4
4
3
[{VVO(pan)}2(
l-O)2]
a
First cycle of used catalyst.

444
M.R. Maurya et al. / Polyhedron 67 (2014) 436–448
OH
Br
OH
OH
OH
Br
Br
Br
Catalyst
O
O
O
KBr, H2O2, HClO4
Br
Br
2,4,6-tribromophenol
3,5-dibromosalicylaldehyde
5-bromosalicylaldehyde
Scheme 4. Oxidative brominated products of salicylaldehyde.
Table 8
Results of the oxidative bromination of salicylaldehyde using the catalyst PS-im[VO2(pan)] (4) after 2 h of reaction time.
Entry No.
KBr (g, mmol)
H2O2 (g, mmol)
HClO4 (g, mmol)
Catalyst (g)
H2O (mL)
% Conv.
% Mono-bromo
% Di-bromo
% Tri-bromo
1.
2
3
4
5
6
7
8
9
1.18, 10
1.18, 10
1.18, 10
1.18, 10
1.18, 10
0.59, 5
1.77, 15
1.18, 10
1.18, 10
1.13, 10
1.13, 10
1.13, 10
0.56, 5
1.69, 15
1.13, 10
1.13, 10
1.13, 10
1.13, 10
1.42, 10
1.42, 10
1.42, 10
1.42, 10
1.42, 10
1.42, 10
1.42, 10
0.177, 5
2.13, 15
0.005
0.010
0.015
0.005
0.005
0.005
0.005
0.005
0.005
20
20
20
20
20
20
20
20
20
96
99
99
46
96
53
98
78
99
80
84
84
37
78
44
83
67
85
14
12
10
8
6
7
12
11
14
0
3
5
1
12
2
3
0
0
(bromohydrin) [15]. However, in most cases 1-phenylethane-1,2-
diol has been obtained in higher amounts.
and follows the order: 5-bromosalicylaldehyde > 2,4,6-tribrom-
ophenol > 3,5-dibromosalicylaldehyde. The non-polymer bound
catalyst exhibited 80% conversion with the formation of 5-bromo-
salicylaldehyde (86%) and 3,5-dibromosalicylaldehyde (14%), and
thus also showed a good catalytic activity. In the absence of the cat-
alyst, the reaction mixture gave about 60% conversion of salicylal-
dehyde under the above optimized reaction conditions, with 82%
selectivity of the mono-bromo and 18% of the di-bromo derivative.
The catalytic efficiency of the polymer-grafted complex 4 com-
pares well with the non-grafted vanadium complexes reported in
the literature [22], but its recyclable ability and high turnover fre-
quency makes it better over the neat complexes.
3.9.2. Oxidative bromination of salicylaldehyde
The catalyst Ps-im[VVO2(pan)] successfully performed the oxi-
dative bromination of salicylaldehyde using the above mentioned
reagents in water and gave three major products, 5-bromosalicyl-
aldehyde, 3,5-dibromosalicylaldehyde and 2,4,6-tribromophenol
(Scheme 4) at 40 °C. About 2 h was required to get maximum con-
version. The oxidative bromination of salicylaldehyde has been re-
ported earlier using vanadium complexes as catalysts [22] and the
products mentioned above are common oxidative brominated
products of salicylaldehyde. The reaction conditions were opti-
mized for the maximum oxidative bromination considering differ-
ent parameters, like amounts of catalyst, aqueous 30% H2O2, KBr
and 70% HClO4. Thus for 5 mmol (0.61 g) of salicylaldehyde, three
different amounts of catalyst (0.005, 0.010 and 0.015 g), aqueous
30% H2O2 (5, 10 and 15 mmol), KBr (5, 10 and 15 mmol) and 70%
HClO4 (5, 10 and 15 mmol, added in four equal portions to the
reaction mixture, first portion at t = 0 and the other three portions
after 30 min intervals) were taken in water and the reaction was
carried out. After several trials (see Table 8), the optimized reaction
conditions for the maximum conversion of salicylaldehyde were
obtained and these were: salicylaldehyde (0.61 g, 5 mmol), KBr
(1.18 g, 10 mmol), aqueous 30% H2O2 (1.13 g, 10 mmol), catalyst
precursor (0.005 g), aqueous 70% HClO4 (1.42 g, 10 mmol)) and
water (20 mL). The conversion of salicylaldehyde under various
reactions conditions and the formation of different products after
2 h of reaction time are presented in Table 8.
3.9.3. Oxidative bromination of trans-stilbene
NH4VO3 catalyzed oxidative bromination of trans-stilbene in
aqueous medium in the presence of H2O2, KBr and HBr has been re-
ported by Hirao et al. [35] Using dodecyltrimethylammonium bro-
mide as a surfactant facilitated the bromination in 51% yield with
two reaction products, namely 1,2-dibromo-1,2-diphenylethane
and 2-bromo-1,2-diphenylethanol. We have carried out the oxida-
tive bromination of trans-stilbene in a biphasic chloroform-water
system. Chloroform was found to be a better solvent in terms of
solubility of trans-stilbene and the reaction products. The oxidative
bromination of trans-stilbene gave mainly three products:
(i) 2,3-diphenyloxirane (trans-stilbene oxide), (ii) 2-bromo-1,2-
diphenylethanol and (iii) 1,2-dibromo-1,2-diphenylethane;
Scheme 5. About 2 h was required to get the maximum conversion.
The reaction conditions for the maximum oxidative bromination of
trans-stilbene were also optimized considering all the parameters
mentioned above, such as amounts of catalyst, aqueous 30%
H2O2, KBr and 70% HClO4. Their details, the corresponding oxida-
tive bromination of substrate under various conditions and % selec-
tivity of different products are summarized in Table 10. It is clear
Table 9 compares the selectivity of the products obtained by
grafted, grafted but recycled and neat catalysts. The recycled cata-
lyst showed slightly less conversion than that obtained by a fresh
one, but the selectivity of the different products are nearly the same
Table 9
Product selectivity and % conversion under the optimized reaction conditions for salicylaldehyde.
SL.No.
Catalyst (g, mmol)
% Conv.
TOF/hꢁ1
Product selectivity (%)
Mono-bromo
Di-bromo
Tri-bromo
1.
2.
3.
PS-im[VO2(pan)]
PS-im[VO2(pan)]a
96
92
80
2254
2160
1878
81
80
86
6
6
14
13
14
0
[{VVO(pan)}2(
l-O)2]
a
First cycle of used catalyst.

M.R. Maurya et al. / Polyhedron 67 (2014) 436–448
445
OH
Br
H2O2, KBr
HClO4
O
Br
Br
trans-stilbene
2,3-diphenyloxirane
2-bromo-1,2-diphenyl
ethanol
1,2-dibromo-1,2-diphenyl
ethane
Scheme 5. Products obtained upon oxidative bromination of trans-stilbene.
Table 10
Results of oxidative bromination of trans-stilbene using the catalyst Ps-im[VO2(pan)] after 2 h of reaction time.
Entry No.
KBr
(g, mmol)
H2O2
(g, mmol)
HClO4
(g, mmol)
Catalyst (g)
Solvent (mL)
(CHCl3/H2O)
% Conv.
% Mono-bromo
% Di-bromo
% T.S.O.a
% Others
1
2
3
4
5
6
7
8
9
3.56, 30
3.56, 30
3.56, 30
1.18, 10
2.36, 20
2.36, 20
2.36, 20
2.36, 20
2.36, 20
3.39, 30
3.39, 30
3.39, 30
3.39, 30
3.39, 30
1.13, 10
2.27, 20
2.27, 20
2.27, 20
5.72, 40
5.72, 40
5.72, 40
5.72, 40
5.72, 40
5.72, 40
5.72, 40
2.86, 20
4.29, 30
0.005
0.010
0.015
0.005
0.005
0.005
0.005
0.005
0.005
20/20
20/20
20/20
20/20
20/20
20/20
20/20
20/20
20/20
96
98
99
79
92
85
91
70
86
4
5
5
6
3
7
7
5
6
28
28
29
22
28
21
22
18
22
62
63
63
48
58
55
60
45
56
2
2
2
3
3
2
2
2
2
a
T.S.O. = trans-stlbene oxide.
Table 11
Product selectivity and % conversion under the optimized reaction conditions for trans-styrene.
SL.No.
Catalyst
% Conv.
TOF/ hꢁ1
Selectivity (%)
Mono-bromo
Di-bromo
T.S.O
Others
1.
2.
3.
PS-im[VO2(pan)]
PS-im[VO2(pan)]a
91
87
72
2136
2043
1690
8
11
16
24
21
20
66
65
62
2
3
2
[{VVO(pan)}2(
l-O)2]
a
First cycle of used catalyst.
from the table that the optimized reaction conditions for this reac-
tion are different from other two reactions mentioned above and
they are (entry No. 7): trans-stilbene (0.90 g, 5 mmol)), KBr
(2.36 g, 20 mmol) aqueous 30% H2O2 (2.27 g, 20 mmol) catalyst
precursor (0.005 g), aqueous 70% HClO4 (5.72 g, 40 mmol) and
CHCl3–H2O (40 mL, 50% v/v). Under these conditions, a maximum
of 91% conversion of trans-stilbene was obtained, where selectivity
of the reaction products follows the order: 2,3-diphenyloxirane
(trans-stilbene oxide) (66%) > 1,2-dibromo-1,2-diphenylethane
(24%) > 2-bromo-1,2-diphenylethanol (8%).
Table 11 compares the conversion of trans-stilbene, obtained by
the fresh and recycled grafted catalysts, and the non-polymer
grafted catalyst, along with the selectivity of the products. The
recycled catalyst again showed slightly less conversion (87%) than
that obtained by a fresh catalyst. The non-grafted catalyst exhibits
even lower conversion (72%). However, the selectivity of the for-
mation of all three products is nearly same for all catalysts and fol-
lows the same order as mentioned above. In the absence of the
catalyst, the reaction mixture gave about 55% conversion under
above optimized reaction conditions with 81% selectivity of
trans-stilbene oxide, 15% of 1,2-dibromo-1,2-diphenylethane and
2% of 2-bromo-1,2-diphenylethanol.
H2O2, and forms a peroxido vanadium derivative. In the presence
of KBr and HClO4, it oxidizes a bromide ion, giving a bromine
equivalent (most likely HOBr) intermediate. Such an intermediate
may then brominate an appropriate organic substrate. The possi-
bility of involving an intermediate containing vanadium bound
OBr–, in the presence of H2O2, KBr and HClO4, has recently been
exploited through DFT calculations [15]. The processes occurring
in the aqueous phase require acidic conditions, probably to pro-
mote the protonation of the peroxido moiety.
We have monitored the reactivity of [VIVO(acac)(pan)] (1) with
H2O2 in DMSO by electronic absorption spectroscopy. Thus, the
stepwise addition of one drop portions of 30% H2O2 (0.058 g,
0.513 mmol) in 10 ml of DMSO to 10 ml of ca. 10–4 M solution of
1 in DMSO results in the flattening of the shoulder band appearing
at ca. 745 nm (Fig. 9 (a)). In dilute solution (ca. 10–4 M) of complex,
the intensity of the 540 and 567 nm bands decreases with a slight
red shift towards 552 and 581 nm, while the 502 and 411 nm
bands shift to 471 and 441 nm, respectively, with a slight increase
in intensity; Fig. 9(b). In the UV region, the intensity of the weak
shoulder at ca. 298 nm decreases and a new shoulder band at
307 nm is generated (same as in the peroxo complex). The inten-
sity of the band around 257 nm slowly decreases. The shape of
the final spectrum is almost the same as the one obtained after
treating 2 with H2O2, which, in turn, is similar to the spectrum of
the peroxide complex 3, thus verifying the formation of similar
oxidoperoxido species with both complexes.
3.10. Reactivity of oxidovanadium(IV) and dioxidovanadium(V)
complexes and possible mechanism of the oxidative bromination of
substrates
We have also recorded the spectral changes during stepwise
additions of two drop portions of 30% H2O2 in 5 ml of MeOH to
25 ml of a 0.51 ꢀ 10–4 M solution of 2 in CHCl3 (see Fig. S1 of sup-
porting information). The spectral changes observed are very sim-
The generally accepted mode of action of V-BrPOs involves the
presence of vanadium in their active sites, where the V ion serves
as a strong Lewis acid in the activation of the primary oxidant,

446
M.R. Maurya et al. / Polyhedron 67 (2014) 436–448
1.0
0.8
0.6
0.4
0.2
(b)
(a)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
300
400
500
600
650
700
750
800
850
Wavelength/ nm
Wavelength/ nm
Fig. 9. Spectral changes obtained during titration of (a) stepwise addition of one drop portions of 30% H2O2 (0.058 g, 0.513 mmol) in 10 ml of DMSO to 10 ml of ca. 10ꢁ4
solution of [VIVO(acac)(pan)] (1) in DMSO and (b) stepwise additions of one drop portions of aqueous 30% (0.058 g, 0.513 mmol) in 5 ml of DMSO to 20 ml of ca. 1.58 ꢀ 10ꢁ4
solution of 1 in DMSO.
M
M
ilar to the ones recorded in DMSO, showing the formation of sim-
ilar types of species in both solutions.
2.0
Upon addition of 1.0 equiv. of a 30% aqueous H2O2 to a metha-
nolic solution of 2 (ca. 4 mM), a peak at d = ꢁ597 ppm emerges,
which we assigned to [VVO(O2)ONN(S)] (S = solvent). This signal
1.5
increases in intensity at the expense of the resonances due to the
dioxido species (d = ꢁ542 ppm) upon further addition of H2O2.
The signals due to the dioxido species finally disappear and an-
other weak signal at d = ꢁ644 ppm, in addition to the signal due
to the peroxido complex, also builds up upon addition of a total
of 4 equiv. of H2O2, which is possibly due to [VVO(O2)2S] (Fig. 10)
[36]. Leaving the NMR tube open for 24 h, the only resonance at
ꢁ549 ppm due to 2ꢃMeOD is recorded, indicating the reversibility
of the process [37].
1.0
0.5
0.0
200
300
400
500
600
In order to identify the possible intermediate forms in the cat-
alytic reaction, the reactivity of the peroxide complex [VVO(O2)
(pan)(MeOH] (3) in methanol with one drop portions of saturated
HCl solution in 5 ml of MeOH has also been studied and the spec-
tral changes are presented in Fig. 11. The bands at 420 and 490 nm
in the visible region are mainly affected in the presence of HCl.
Thus the band at 420 shifts to 410 nm with an increase in intensity,
Wavelength/ nm
Fig. 11. Spectral changes obtained during the titration of stepwise additions of one
drop portions of saturated HCl in 5 ml of MeOH to 25 ml of ca. 2.03 ꢀ 10–4
M
solution of [VVO(O2)(pan)(MeOH)] (3) in MeOH.
while the 490 nm band completely disappears. Other bands at 550
and 590 nm gain only in intensity, without changing their posi-
tions. These spectral changes have been proposed to be due to
the formation of oxidohydroperoxido species [15,38–41].
Thus, the complexes reported here show similar behaviour as
shown by other model complexes. As observed in other model
vanadium complexes [11,33,42], in the presence of KBr, H2O2 and
HClO4, the generated HOBr during the catalytic reaction may react
with styrene to give the bromonium ion as an intermediate. The
nucleophile Br– as well as H2O both may attack the
a-carbon of
the intermediate to give 1,2-dibromo-1-phenylethane and 2-bromo-
1-phenylethane-1-ol, respectively (Scheme 6). The formation of the di-
bromo product in most cases at a later stage is possibly due to (i) the
presence of a lesser amount of brominating reagent generated initially
and/or (ii) the generation of a lesser amount of Br2, another intermedi-
ate, during the catalytic action. The nucleophile H2O may further attack
the
a-carbon of 2-bromo-1-phenylethane-1-ol to give 1-phenyle-
thane-1,2-diol. All these justify the formation of 1-phenylethane-1,2-
diol in higher yield.
The formation of 5-bromosalicylaldehyde and 3,5-dibromosali-
cylaldehyde upon oxidative bromination of salicylaldehyde is ex-
pected as the –OH group of salicylaldehyde is ortho and para
directing. However, the formation of 2,4,6-tribromophenol is pos-
sible only through decarbonylative bromination. Rana et al. [43]
have recently shown that a vanadium bound OCl– species interacts
with a substrate like 2-hydroxy-1-naphthanldehyde using both
Fig. 10. 51V NMR spectra of methanolic solutions of complex 3 (ca. 4 mM): (a) after
preparation of the solution, (b) solution of (a) after addition of 1 equiv. H2O2, (c)
solution of (a) after 1 h of addition of 4 equiv. H2O2.

M.R. Maurya et al. / Polyhedron 67 (2014) 436–448
447
stilbene has been successfully carried out using 4 as a heteroge-
neous catalyst, demonstrating the functional similarity to haloper-
oxidases. The recyclable ability and better catalytic ability over the
non-grafted analog make it useful functional model of haloperox-
idases. A peroxide species similar to [VVO(O2)(pan)(MeOH] (3), that
has been isolated in the solid state, has also been demonstrated to
form in solution, which on reaction with acid possibly generates a
hydroperoxide intermediate during the catalytic action. Possible
mechanisms for the formation of different reaction products upon
oxidative bromination of the above substrates have been proposed.
Br
Br
Br
OH2
Br
Br
Br
Br
Br
H
OH
OH2
δ
HO
OH
Br
HBr
Acknowledgments
OH2
δ
OH
OH2
The authors thank the Council of Scientific and Industrial Re-
search (CSIR), New Delhi (India) for research project No. 01/
(2447)/10-EMR-II and the Department of Science and Technology,
New Delhi (India) for research project No. SR/S1/IC-32/2010. The
authors are also thankful to Prof. J. Costa Pessoa for recording the
EPR and 51V NMR spectra that appear in this paper. NC thanks CSIR
for the award of a Junior Research Fellowship.
OH
Scheme 6. Mechanism of action of HOBr on styrene.
functional groups, which in the presence of H2O2 and KCl provides
the decarbonylated chlorinated product 1-chloronaphthalen-2-ol.
Such a mechanism is partly applicable here as well to give
2-bromophenol from salicylaldehyde, which may undergo further
bromination to give 2,4,6-tribromophenol.
Appendix A. Supplementary data
The formation of 1,2-dibromo-1,2-diphenylethane and 2-bro-
mo-1,2-diphenylethanol by oxidative bromination of trans-
stilbene is again possible through a bromonium ion intermediate,
as mentioned above for the oxidative bromination of styrene, while
that of 2,3-diphenyloxirane (trans-stilbene oxide) is the expected
product by regular oxidation of trans-stilbene.
CCDC 941405 contains the supplementary crystallographic data
for 3. 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, Cambridge CB2 1EZ,
UK; fax: +44 1223 336 033; or e-mail: deposit@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.2013.09.021.
3.11. Leaching study of vanadium from the polymer-anchored catalyst
References
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e.g. for oxidative bromination of trans-stilbene [i.e. catalyst precur-
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formation of possibly hydroperoxide intermediate species during
the catalytic reaction, the added perchloric acid is consumed con-
tinuously and thus avoids the decomposition of the catalysts. How-
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4. Conclusions
The monobasic tridentate ONN donor ligand 1-(2-pyridylazo)-
2-naphthol [Hpan (I)] has been used to prepare the oxidovanadi-
um(IV) complex [VIVO(acac)(pan)] (1). Areal oxidation of 1 gave
the dioxidovanadium(V) complex [{VVO(pan)}2(
l-O)2] (2), which
can be considered to be a structural model of haloperoxidase. Com-
plex 2 has been grafted successfully through covalent bonding
with imidazole appended methylpolystyrene cross-linked with
5% divinylbenzene {now abbreviated as PS-im[VVO2(pan)] (4)}.
Oxidative bromination of styrene, salicylaldehyde and trans-

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M.R. Maurya et al. / Polyhedron 67 (2014) 436–448
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