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

Polyhedron 67 (2014) 436–448

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Polyhedron

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

of haloperoxidases

a

b

Mannar R. Maurya ^a,, Nikita Chaudhary , Fernando Avecilla

⇑

^aDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India

^bDepartamento 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

[V^IVO(acac)₂] in dry methanol to yield the oxidovanadium(IV) complex [V^IVO(acac)(pan)] (1). The

Received 22 June 2013

Accepted 15 September 2013

Available online 8 October 2013

dioxidovanadium(V) complex [{V^VO(pan)}₂(

l-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 H₂O₂yields the oxidomonoperoxidovanadium(V)

complex [V^VO(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[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% HClO₄have 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.

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]. V^VO₂complexes 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).

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. [{V^VO(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. [V^IVO(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-d₄or DMSO-

d₆, and d(⁵¹V) values are referenced relative to neat VOCl₃as 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, cm^ꢁ1): 1359 (N@N_azo), 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[V^VO₂(pan)] (4)

Imidazolomethylpolystyrene (PS-im) [16] (1.00 g) was allowed

to swell in DMF (20 mL) for 2 h. A solution of [{V^VO(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 [V^IVO(acac)(pan)] (1)

A stirred solution of [V^IVO(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 Å^ꢁ3

. A weighting scheme, w = 1/[r

²(F_o²) + (0.057800-

P)²+ 0.602000P], where P = (|F_o|²+ 2|F_c|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% CH₂Cl₂in 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)

D_calc(g cm^–3

)

l

(mm^ꢁ1

)

h (°)

R_int

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 Å^ꢁ3

)

a

R₁

wR₂= {

=

R

j|F_o| ꢁ |F_c|j/ jF_oj.

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|F_o|²ꢁ |F_c|²j)²]j/

R

[w(F_o⁴)]}^1/2

.

2.5. Catalytic activity studies

Warning: HClO₄is potential oxidant, hence must be handled

3. Results and discussion

carefully.

The polymer-grafted complex PS-im[V^VO₂(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 [V^IVO(acac)₂] and

Hpan (I) in dry, reﬂuxing methanol yielded the oxidovanadium(IV)

complex [V^IVO(acac)(pan)] (1) (Eq. (1)). On aerial oxidation of 1 in

methanol, the dioxidovanadium(V) complex [{V^VO(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 [V^IVO(acac)₂] with I followed by aerial oxidation. Addi-

tion of H₂O₂to a methanolic solution of 1 yielded the oxidomonop-

eroxidovanadium(V) complex [V^VO(O₂)(pan)(MeOH)] (3) (Eq. (3)).

Same complex could also be isolated by the reaction of 2 with

H₂O₂in 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% CH₂Cl₂in n-hexane as an eluent. The products were identiﬁed

by GC–MS and ¹H NMR spectroscopy.

½V^IVOðacacÞ₂ꢂþHpan ! ½VO^IVðacacÞðpanÞꢂþHacac

ð1Þ

ð2Þ

1

2½VO^IVðacacÞðpanÞꢂþ O₂þH₂O ! ½V^VOðpanÞ ð

l

ꢁOÞ₂ꢂþ2Hacac

2

2½VO^IVðacacÞðpanÞꢂþ3H₂O₂þMeOH ! ½V^VOðO₂ÞðpanÞðMeOHÞꢂþ2Hacacþ2H₂O

ð3Þ

1=2½V^VOðpanÞ₂ð

l

ꢁOÞ ꢂþH₂O₂þMeOH ! ½V^VOðO₂ÞðpanÞðMeOHÞꢂþH₂O

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[V^VO₂(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 [V^VO(O₂₎(pan)(MeOH] (3)

The complex [V^VO(O₂₎(pan)(MeOH)] (3) crystallizes from meth-

anol as dark red prisms (crystal dimensions 0.22 ꢀ 0.21 ꢀ 0.20).

Fig. 1 shows an ^ORTEPrepresentation 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 O_naphthol, one N_azo

and one N_pyridineatom, and with one O_oxide(terminal), one O_methanol

and two O_peroxide, 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 [V^VO(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 [V^VO(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

_methanolbond, 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 [VO^IV(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 [V^VO(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 [V^VO(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

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[V^VO₂(pan)] (4). This observation suggests the

immobilization of the metal complex onto the polystyrene beads.

Fig. 4. IR spectra of (a) [V^IVO(acac)(pan)] (1), (b) [{V^VO(pan)}₂(

[V^VO(O₂)(pan)(MeOH)] (3) and (d) PS-im[V^VO₂(pan)] (4).

l-O)₂] (2), (c)

Table 4

IR spectral data (cm^ꢁ1) of the ligand and the complexes.

Compounds

m

(N@N_azo

)

m(C@N_pyridyl/C@C)

m(V@O)

Hpan (I)

1436

1360

1359

1364

1566, 1620

1549, 1586

1550, 1587

1550, 1585

1560, 1600

–

955

936, 848

960

940, 917

[V^IVO(acac)(pan)] (1)

[{V^VO(pan)}₂(

l-O)₂] (2)

[V^VO(O₂)(pan)(MeOH)] (3)^a

PS-im[V^VO₂(pan)] (4)

3.5. IR spectral study

Bands due to the peroxido group: 923, 721 and 565 cm^ꢁ1

.

Fig. 4 presents the IR spectra of the complexes. The complex

a

[V^IVO(acac)(pan)] (1) shows one sharp band at 955 cm^ꢁ1in the IR

spectrum due to the

[{V^VO(pan)}₂( -O)₂] (2) shows one sharp band at 936 cm^–1and a

broad band at 848 cm^–1corresponding 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

(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 cm^ꢁ1, conﬁrming

the coordination of the azo nitrogen to the metal centre [26]. The

ligand also exhibits two sharp bands at 1566 and 1620 cm^–1due

upon grafting onto the polymer and exhibits m_asym(V@O) and m_sym

(O@V@O) modes at 940 and 917 cm^–1, respectively (Table 4). The

peroxo complex [V^VO(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 cm^ꢁ1is 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^–1region

(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

[{V^VO(pan)}₂( -O)₂] (2) and [V^VO(O₂)(pan)(MeOH)] (3) show a

l

good resolution of bands in MeOH, while the complex

[V^IVO(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

k_max/nm (e/M^ꢁ1cm^ꢁ1

)

Hpan (I)

MeOH

DMSO

MeOH

DMSO

MeOH

DMSO

MeOH

Nujol

224, 304, 463

[V^IVO(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)

[{V^VO(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

[V^VO(O₂)(pan)(MeOH)] (3)

PS-im[V^VO₂(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

[V^VO₂(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 [V^IVO(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 [V^IVO(acac)(pan)] (1). The hyperﬁne features and

spectrum itself are consistent with binding modes involving (O_acac

,

O

_phenolate, N_imine, N_{py equatorial}and (O_{acac axial}. Once a particular

)

binding mode is assumed, the value of A_kcan be estimated using

the additivity relationship proposed by Wüthrich [28] and Chas-

teen [29], with an estimated accuracy of 3 ꢀ 10^–4cm^–1. However,

for the potential donor groups under consideration, their predicted

contributions to the parallel hyperﬁne coupling constant are rather

similar (O_acacꢄ41.7, O_phenolateꢄ38.9, N_imineꢄ41.6, N_pyꢄ40.7,

O_MeOHꢄ45.5, all A_||contributions in cm^–1ꢀ 10^–4) [29,30], hence

it is not possible to distinguish between the several plausible bind-

ing modes. The spectrum of [V^IVO(acac)(pan)] was simulated [16]

In the ⁵¹V NMR spectra, the line widths at half height are typi-

cally about 200 Hz. The complex [{V^VO(pan)}₂(

l-O)₂] (2) in MeOD-

and the spin Hamiltonian parameters obtained are g 1.952, A

k

163.5 ꢀ 10^–4cm^ꢁ1, A_\63.0 ꢀ 10^–4cm^ꢁ1and g_\1.981. Thus, the

EPR spectrum of [V^IVO(acac)(pan)] in MeOH is in good agreement

with an O₂N₂binding 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.

d₄displays 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 [V^VO₂(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 H₂O₂and 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 H₂O₂to give

oxido-monoperoxido, [V^VO(O₂)]⁺, or oxido-diperoxido, [VO(O₂)₂]^–,

species that oxidize bromide ions in the presence of acid, most

–

likely to Br₂, Br₃and/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[V^VO₂(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

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, HClO₄and 30% H₂O₂under 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 HClO₄was optimized by taking

substrate to HClO₄ratios of 1:1, 1:2 and 1:3 under the above reac-

tion conditions; Fig. 7(d). For each condition, HClO₄was 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 HClO₄ratio), 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 HClO₄ratio

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

15

30

45

60

0

15

30

45

60

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

15

30

45

60

0

15

30

45

60

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% HClO₄on 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

1.18, 10

3.56, 30

2.27, 20

1.13, 10

3.39, 30

2.86, 20

1.43, 10

2.86, 30

0.010

0.015

0.020

0.010

20/20

65

68

70

41

81

64

91

47

99

28

27

28

22

27

23

22

16

0

2

33

39

40

17

51

40

66

29

77

4

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 [{V^VO(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[V^VO₂(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 H₂SO₄under

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[V^VO₂(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[V^VO₂(pan)] compares well with several

other mononuclear and binuclear V^Vcomplexes [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/h^ꢁ1

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

0

78

79

73

4

3

[{V^VO(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

Br

Catalyst

O

KBr, H₂O₂, HClO₄

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

0.59, 5

1.77, 15

1.18, 10

1.13, 10

0.56, 5

1.69, 15

1.13, 10

1.42, 10

0.177, 5

2.13, 15

0.005

0.010

0.015

0.005

20

96

99

46

96

53

98

78

99

80

84

37

78

44

83

67

85

14

12

10

8

6

7

12

11

14

0

3

5

1

12

2

3

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[V^VO₂(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

NH₄VO₃catalyzed 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/h^ꢁ1

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

14

13

14

0

[{V^VO(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

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

1.18, 10

2.36, 20

3.39, 30

1.13, 10

2.27, 20

5.72, 40

2.86, 20

4.29, 30

0.005

0.010

0.015

0.005

20/20

96

98

99

79

92

85

91

70

86

4

5

6

3

7

5

6

28

29

22

28

21

22

18

22

62

63

48

58

55

60

45

56

2

3

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/ h^ꢁ1

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

[{V^VO(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 [V^IVO(acac)(pan)] (1) with

H₂O₂in 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^–4M 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^–4M) 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% H₂O₂in 5 ml of MeOH to

25 ml of a 0.51 ꢀ 10^–4M 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

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. 10^ꢁ4

solution of [V^IVO(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 ꢀ 10^ꢁ4

solution of 1 in DMSO.

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 H₂O₂to a metha-

nolic solution of 2 (ca. 4 mM), a peak at d = ꢁ597 ppm emerges,

which we assigned to [V^VO(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 [V^VO(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 [V^VO(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 [V^VO(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, H₂O₂and

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 [V^VO(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

OH₂

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 H₂O₂and 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

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Article Doi

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

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