Synthesis and characterization of dimeric μ-oxidovanadium complexes as the functional model of vanadium bromoperoxidase

Article abstract of DOI:10.1002/aoc.5508

Two vanadium (IV) complexes [V^IVO(Haeae-sal)(MeOH)]⁺ (1) and [V^IVO(Haeae-hyap)(MeOH)]⁺ (2) were prepared by reacting [VO(acac)₂] with ligands [H₂aeae-sal] (I) and [H₂aeae-hyap] (II) respectively. Condensation of 2-(2-aminoethylamino)ethanol with salicylaldehyde and 2-hydroxyacetophenone produces the ligands (I) and (II) respectively. Both vanadium complexes 1 and 2 are sensitive towards aerial oxygen in solution and rapidly convert into vanadium(V) dioxido species. Vanadium(V) dioxido species crystalizes as the dimeric form in the solid-state. Single-crystal XRD analysis suggests octahedral geometry around each vanadium center in the solid-state. To access the benefits of heterogeneous catalysis, vanadium(V) dioxido complexes were anchored into the polymeric chain of chloromethylated polystyrene. All the synthesized neat and supported vanadium complexes have been studied by a number of techniques to confirm their structural and functional properties. Bromoperoxidase activity of the synthesized vanadium(V) dioxido complexes 3 and 4 was examined by carrying out oxidative bromination of salicylaldehyde and oxidation of thioanisole. In the presence of hydrogen peroxide, 3 shows 94.4% conversion (TOF value of 2.739 × 10² h^?1) and 4 exhibits 79.0% conversion (TOF value of 2.403 × 10² h^?1) for the oxidative bromination of salicylaldehyde where 5-bromosalicylaldehyde appears as the major product. Catalysts 3 and 4 also efficiently catalyze the oxidation of thioanisole in the presence of hydrogen peroxide where sulfoxide is observed as the major product. Covalent attachment of neat catalysts 3 and 4 into the polymer chain enhances substrate conversion (%) and their catalytic efficiency increases many folds, both in the oxidative bromination and oxidation of thioether. Polymer supported catalysts 5 displayed 98.8% conversion with a TOF value of 1.127 × 10⁴ h^?1 whereas catalyst 6 showed 95.7% conversion with a TOF value of 4.675 × 10³ h^?1 for the oxidative bromination of salicylaldehyde. These TOF values are the highest among the supported vanadium catalysts available in the literature for the oxidative bromination of salicylaldehyde.

Full text of DOI:10.1002/aoc.5508

Received: 3 October 2019
DOI: 10.1002/aoc.5508
Revised: 2 January 2020
Accepted: 2 January 2020
F U L L P A P E R
Synthesis and characterization of dimeric μ-oxidovanadium
complexes as the functional model of vanadium
bromoperoxidase
Abhishek Maurya1
| Arun Kumar Mahato1 | Nikita Chaudhary2
|
Neha Kesharwani1 | Payal Kachhap1 | Vivek Kumar Mishra1
Chanchal Haldar1
|
1Department of Chemistry, Indian
Two vanadium (IV) complexes [VIVO(Haeae-sal)(MeOH)]+ (1) and
[VIVO(Haeae-hyap)(MeOH)]+ (2) were prepared by reacting [VO(acac)2] with
ligands [H2aeae-sal] (I) and [H2aeae-hyap] (II) respectively. Condensation of
Institute of Technology (Indian School of
Mines), Dhanbad, 826004, Jharkhand,
India
2Department of Chemistry and Polymer
Science, Stellenbosch University,
Matieland, 7602, Stellenbosch,
South Africa
2-(2-aminoethylamino)ethanol
with
salicylaldehyde
and
2-hydrox-
yacetophenone produces the ligands (I) and (II) respectively. Both vanadium
complexes 1 and 2 are sensitive towards aerial oxygen in solution and rapidly
convert into vanadium(V) dioxido species. Vanadium(V) dioxido species crys-
talizes as the dimeric form in the solid-state. Single-crystal XRD analysis sug-
gests octahedral geometry around each vanadium center in the solid-state. To
access the benefits of heterogeneous catalysis, vanadium(V) dioxido complexes
were anchored into the polymeric chain of chloromethylated polystyrene. All
the synthesized neat and supported vanadium complexes have been studied by
a number of techniques to confirm their structural and functional properties.
Bromoperoxidase activity of the synthesized vanadium(V) dioxido complexes
3 and 4 was examined by carrying out oxidative bromination of salicylaldehyde
and oxidation of thioanisole. In the presence of hydrogen peroxide, 3 shows
94.4% conversion (TOF value of 2.739 × 102 h−1) and 4 exhibits 79.0% conver-
sion (TOF value of 2.403 × 102 h−1) for the oxidative bromination of
salicylaldehyde where 5-bromosalicylaldehyde appears as the major product.
Catalysts 3 and 4 also efficiently catalyze the oxidation of thioanisole in the
presence of hydrogen peroxide where sulfoxide is observed as the major prod-
uct. Covalent attachment of neat catalysts 3 and 4 into the polymer chain
enhances substrate conversion (%) and their catalytic efficiency increases many
folds, both in the oxidative bromination and oxidation of thioether. Polymer
supported catalysts 5 displayed 98.8% conversion with a TOF value of
1.127 × 104 h−1 whereas catalyst 6 showed 95.7% conversion with a TOF value
of 4.675 × 103 h−1 for the oxidative bromination of salicylaldehyde. These TOF
values are the highest among the supported vanadium catalysts available in
the literature for the oxidative bromination of salicylaldehyde.
Correspondence
Chanchal Haldar, Department of
Chemistry, Indian Institute of Technology
(Indian School of Mines), Dhanbad
826004, Jharkhand, India.
Email: chanchal@iitism.ac.in
Funding information
Science and Engineering Research Board,
Grant/Award Number: SB/EMEQ-055/
2014
Appl Organometal Chem. 2020;e5508.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 21
https://doi.org/10.1002/aoc.5508

2 of 21
MAURYA ET AL.
K E Y W O R D S
elemental mapping, heterogeneous catalysis, single crystal XRD, vanadium bromoperoxidase
model, μ-oxidovanadium complexes
1 | INTRODUCTION
a large fraction of industrial and chemical organo-
bromine compounds.[17] Molecular bromine is a toxic,
The field of the coordination chemistry of vanadium is
becoming a popular choice among the researchers gradu-
ally. Vanadium and its complexes are found to be an
active component in various biological processes like hal-
operoxidation,[1] phosphorylation,[2] nitrogen fixation,[3]
glycogen metabolism,[4] and insulin mimicking.[5] Espe-
cially, the appearance of vanadium in the active site of
vanadium haloperoxidases[1] and its active involvement
in the various important catalytic reactions[6] has encour-
aged participation in the research of coordination chem-
istry of vanadium. Vanadium bromoperoxidase, isolated
from marine algae and fungus, is the most discussed
member in the vanadium dependent enzymes,[6b] which
is actively involved in the biosynthesis of brominated nat-
ural products. Vanadium bromoperoxidase promotes the
oxidation and oxidative bromination of organic com-
pounds in the presence of hydrogen peroxide or molecu-
lar oxygen.[7] During the catalytic cycle, in situ generated
peroxido vanadium(V) is believed to be the active species
responsible for the catalytic oxidation of bromide where a
change in geometry between trigonal bipyramidal and
square pyramidal is suggested by X-ray crystallography.[8]
Hence, to understand the structural and functional
behavior of vanadium in biological systems, fundamental
research on vanadium oxo complexes with various oxida-
tion states such as +3, +4 and + 5 has become essential.
Because of higher stability and tunable electronic envi-
ronment, vanadium(V) complexes are frequently devel-
oped as the structural and functional models of
vanadium haloperoxidase.[9] In association with catalytic
oxidation of halides, the mononuclear vanadium cofac-
tor[10] is capable of accelerating the oxidation of organic
sulfides to sulfoxides and sulfones in the presence of
hydrogen peroxide.[11] Besides these, vanadium com-
plexes are found to catalyze a number of oxidative
organic transformations like alcohol oxidation, oxidation
of hydrocarbons, halide oxidation, etc.[12]
volatile and corrosive chemical that restricts its use as a
brominating agent. Whereas, the formation of in situ bro-
mine from the oxidation of bromide by oxygen or hydro-
gen peroxide is much more economical and green. But
surprisingly at normal pH, the oxidation of bromide by
peroxides is very slow. Nature uses vanadium
bromoperoxidase to catalyze the oxidation of bromide in
the presence of hydrogen peroxide.
Herein, we report two new vanadium (IV) oxido com-
plexes [VIVO(Haeae-sal)(MeOH)]+ (1) and [VIVO(Haeae-
hyap)(MeOH)]+ (2) of Schiff-base [H2aeae-sal] (I) and
[H2aeae-hyap] (II). Upon dissolution, in the presence of
air, these complexes spontaneously convert into the
corresponding vanadium(V) dioxido complexes. In 1988
Li et al. first reported the structure and electrochemical
properties of the complex [{VVO(Haeae-sal)}μ-O]2 (3),
and later in 2010 Xie et al. reported its antidiabetic prop-
erties, whereas In 2013 Zhao et al. reported the structure
and antimicrobial properties of the complex
[{VVO(Haeae-hyap)}μ-O]2 (4).[18] But none of the
research papers focused on the structural uniqueness of
the complexes [{VVO(Haeae-sal)}μ-O]2 (3) and
[{VVO(Haeae-hyap)}μ-O]2 (4) and examined their cata-
lytic potential. Single-crystal XRD analysis suggests that
both the complexes 3 and 4 exist in the dimeric form in
solid-state where each vanadium center adopts an octa-
hedral geometry. We have observed that in solution both
vanadium(V) complexes exist in the monomeric form
where the vanadium center acquires a geometry similar
to the active form of vanadium haloperoxidase, which
convinced us to use them as functional models of vana-
dium bromopeoxidase. Thus complexes [{VVO(Haeae-
sal)}μ-O]2 (3) and [{VVO(Haeae-hyap)}μ-O]2 (4) have
been applied towards the catalytic oxidation of bromide
and sulfide. Oxidative bromination of salicylaldehyde in
the presence of reported catalysts proceeds with high sub-
strate conversion (%) and TOF values. To make the cata-
lytic process more green, economic and for easy catalyst
separation, heterogeneous catalysts were developed by
covalent anchoring of the neat catalysts into the poly-
There is a steady demand for organobromines in the
area of organic synthesis and the chemical industry.[13]
Organobromine compounds are applied in various fields
such as materials,[14] flame retardants, emulsifiers,[15]
agrochemicals, pharmaceuticals,[16] and intermediates
which makes them one of the important classes of com-
pounds in our society. Despite the hazardous nature of
liquid molecular bromine, it is used for the production of
meric
chain
of
chloromethylated
polystyrene.
Heterogenization enhances the substrate conversion (%)
as well as the TOF values greatly. Hence we present the
synthesis, detailed characterization, and catalytic activity
of the neat and polymer anchored vanadium(V)

DIMERIC μ-OXIDOVANADIUM COMPLEXES AS EFFICIENT MODELS OF VBPO'S
3 of 21
complexes towards the oxidative bromination of
salicylaldehyde and oxidation of thioanisole in the pres-
ence of hydrogen peroxide.
damage by the electron beam. Vanadium content was
estimated by Lab India, AA 8000 atomic absorption spec-
trometer, after digesting the polymer-bound metal com-
plexes by conc. HNO3 and subsequent diluting the
filtrate. For AFM imaging of polymers, a scanning probe
microscope from DIMENSION iCON with ScanAsyst was
used. Thermogravimetric analysis (TGA) was performed
by using Perkin Elmer, Diamond TG/DTA instrument.
An Agilent 7890 B gas-chromatograph fitted with an HP-
5 capillary column (30 m × 0.25 mm × 0.25 μm) and an
FID detector was used to analyze the reaction products.
Substrate conversion (%) was calculated using the calibra-
tion plot. The identity of the oxidation products was con-
firmed by Thermo ISQ QD single quadrupole mass
analyzer. Substrate conversion (%) and product selectivity
(%) were calculated by using the following formulae:
2 | EXPERIMENTAL
2.1 | Materials
Analytical grade reagent V2O5 (Loba Chemie, Mum-
bai, India), acetylacetone (Merck, India), 2-(2-amino-
ethylamino) ethanol (Alfa-Aesar, Mumbai, India),
salicylaldehyde (SRL, India), 2-hydroxyacetophenone
(Loba Chemie, Mumbai, India), chloromethylated poly-
styrene (1% cross-linked with divinylbenzene 50–100
mesh, extent of labeling: 2.5–4.0 mmole/g Cl− loading)
(Sigma-Aldrich, New Mumbai, India), Nujol (paraffin
wax heavy oil, SRL, India), KI (Rankem, Thane, India),
30% aqueous H2O2 (Merck, Mumbai, India), 70% HClO4
(Rankem, Thane, India) and thioanisole (Alfa-Aesar,
Mumbai, India) were used as obtained. Other chemicals
and solvents were of analytical reagent grade. HPLC
grade solvents (Spectrochem, Mumbai, India) were used
for GC and GC–MS analysis. [VIVO(acac)2][19] was pre-
pared according to the reported method.
Area of substrate
Substrate Conversion ð%Þ = 100−
× 100, ð1Þ
Total area of substrate + Area of products
Area of product
Total area of products
Similarly,product selectivity ð%Þ = 100−
× 100,
2.3 | Computational method
Gaussian 09[20] program pack was used to perform all
the theoretical calculations with the help of an HP Z440
work station. For visualization, GaussView 5.0[21] and
Chemcraft[22] molecular graphical programs were used.
X-ray refinement data were taken as an initial gauss for
the optimization of the molecular structures. The solu-
tion phase and gas phase molecular structure of the
ligands and metal complexes in the ground state was
optimized by Density Functional Theory (DFT) in its
restricted and unrestricted forms, without any symmetry
constraint. All the calculations were carried out with 6-
31G (d,p)[23,24] basis set for the C, H, N, O elements and
DFT/UB3LYP employing LANL2DZ basis set with an
effective core potential for the V atom.[25] In the DFT
calculation, hybrid functional was also used. The Becke's
three-parameter functional (B3)[26] was used in combina-
tion with the correlation functional of Lee et al.[27] Den-
sity functional theory was used to evaluate the chemical
reactivity and some selective global parameters such as
electronegativity (χ) hardness (η) and softness (σ) Thus
for an electron system with total electronic energy
(E) and an external potential V(r), chemical potential (ρ)
is known as the first derivative of the E with respect to
N at V(r).[28]
2.2 | Physical methods and analysis
Elemental analysis was carried out by Electron Probe
Microanalyser SX, CAMECA (France) instrument. FT-IR
spectra were recorded in ATR mode by using Agilent
600 series FT-IR spectrophotometer. Electronic spectra
were measured in a Shimadzu UV-1800 spectrophotome-
ter using methanol as a solvent. Nujol was used for
recording the electronic spectra of polymer-bound metal
1
complexes. H NMR spectra of the ligands were recorded
in CDCl3 and 13C NMR spectra of the ligands were
recorded in DMSO-d6 solvent on a Bruker Avance II
400 MHz NMR spectrometer with the common parame-
ter settings. 51V NMR spectra of vanadium(V) oxido com-
plexes were recorded in
a JEOL-ECZ 500 MHz
spectrometer at 131.55 MHz frequency in DMSO-d6 by
using VOCl3 as an external standard. The X-band EPR of
vanadium (IV) complexes were recorded in methanol
with a JES-FA200 spectrometer operating at 100 kHz at
liquid nitrogen temperature.
Elemental mapping and surface morphology of poly-
mer anchored vanadium complexes were analyzed by
Energy Dispersive X-ray Analysis (EDX) and scanning
electron microscope (SEM) on a Hitachi S-3400 N instru-
ment after coating the polymer bead surface with a thin
film of gold to block the surface charging and thermal
χ = ρ
ð2Þ
= −ð∂Ε=∂ΝÞÞζðρÞ

4 of 21
MAURYA ET AL.
Hardness (η) has been defined as the second derivative of
the E with respect to N at V(r) property, which measures
both the stability and reactivity of a molecule.[29]
such as time, amounts of catalyst, amounts of oxidant,
amounts of solvent (acetonitrile), and nature of sol-
vents were monitored and optimized to get the efficient
oxidative desulfurization of thioanisole. Identities of the
reaction products were confirmed from GC–MS
analysis.
À
Á
= − ∂2Ε=∂Ν2 ςðρÞ
= −ð∂ρ=∂ΝÞςðρÞ
ð3Þ
Where E is the electric energy, N is the number of elec-
trons and V(r) is the external potential due to the nuclei
and ρ is chemical potential. For the compounds 3, 3(a),
4, and 4(a) vibrational frequencies were calculated by
using DFT/UB3LYP/6-31G (d) level of theory. Lack of
Imaginary frequency confirms the optimized structure as
local minima in the potential energy surface.
3.3 | X-ray crystallography
The data of complexes [{VOV(Haeae-sal)}μ-O]2 (3) and
[{VOV (Haeae-hyap)}μ-O]2 (4) were collected by Rigaku
Oxford Diffraction system equipped with state of the art
CCD Eos S2 detector using Mo K\α radiation (wavelength
0.71073 Å) at room temperature. The data collection and
reduction were executed with the Oxford diffraction
Crysalis system. The structures of complexes 3 and 4 were
solved by direct methods (SIR92)[30] and refined on F2 by
full-matrix least-squares methods employing SHELXS–
2013 and SHELXL–2013 programs.[30,31] The molecular
graphics were haggard using the PLATON program.
Non-hydrogen atoms(C, O, N, and V) were anisotropi-
cally refined. All H atoms were put in calculated location
riding on their carrier atoms. The function minimized
3 | CATALYTIC ACTIVITY
3.1 | Oxidative bromination of
salicylaldehyde
Oxidative bromination of salicylaldehyde was carried
out
at
room
temperature.
Conventionally,
was [Σw (Fo - Fc ) ] (w = 1/[σ2(Fo ) + (aP)2 + bP]),
2
2 2
2
salicylaldehyde (10 mmol, 1.22 g) was reacted with 30%
aqueous dilute H2O2 (15 mmol, 1.70 g), 70% aqueous
HClO4 (20 mmol, 2.86 g) and KBr (20 mmol, 2.38 g) in
the presence of a polymer-supported catalyst in 20 ml
of water at room temperature. The reaction continued
for two hours while precipitate separates from the solu-
tion. By using DCM as a solvent, unreacted substrate
and brominated products were isolated from the reac-
tion mixture by solvent extraction. Substrate conversion
(%) and product selectivity (%) were estimated by ana-
lyzing the DCM extract on a Gas Chromatography. To
improve the catalytic efficiency addition of perchloric
acid in three equal portions with a fixed time interval
(15 min) is necessary. The identities of the brominated
products were confirmed by the available external com-
mercial standards as well as GC–MS.
where P = (Max (Fo ,0) + 2Fc )/3 with σ2(Fo ) from cou-
nting statistics. The function R1 and wR2 were (σ||Fo|-|
2
2
2
Fc||)/σ|Fo|and [σw (Fo -Fc ) /σ (wFo )]1/2, respectively.
CCDC No. 1957124 for complex 4 contains the supple-
mentary crystallographic data for this paper. The crystal-
lographic details are summarized in Table 1 and
Table S1.
2
2 2
4
4 | SYNTHETIC PROCEDURES
4.1 | Preparation of ligands [H2aeae-sal]
(I) and [H2aeae-hyap] (II)
Both ligands [H2aeae-sal] (I) and [H2aeae-hyap] (II)
were prepared according to the method shown in
Scheme 1. In short, 20 ml methanolic solution of 2-(2-
aminoethylamino)ethanol (10 mmol, 1.04 g) was added
dropwise into 20 ml methanolic solution of
salicylaldehyde (10 mmol, 1.22 g) or 2-hydrox-
yacetophenone (10 mmol, 1.36 g) with constant stirring.
The resulting reaction mixture was refluxed for 2 hr. The
progress of the reaction was checked by TLC. After 2 hr,
the volume of the resulting yellow reaction mixture was
reduced to ~15 ml and kept in a refrigerator overnight.
The yellow precipitate thus obtained was filtered, washed
with cold methanol (2 × 5 ml) followed by petroleum
ether and dried over silica gel under vacuum.
3.2 | Oxidation of Thioanisole
Oxidation of thioanisole was catalyzed by 5 and 6 at
room temperature. In a typical reaction, thioanisole
(5 mmol, 0.627 g) and 30% H2O2 (12.5 mmol, 1.417 g)
were reacted in the presence of 0.020 g pre-swelled
(in 4 ml acetonitrile for 6 hr) catalyst with constant stir-
ring for 4 hr at room temperature. Periodically a small
aliquot was withdrawn from the reaction mixture and
analyzed quantitatively through a Gas Chromatograph.
The impact of different parameters on this reaction

DIMERIC μ-OXIDOVANADIUM COMPLEXES AS EFFICIENT MODELS OF VBPO'S
5 of 21
TABLE 1 Crystal and refinement parameters for complexes 4
4.1.1 | Data for [H2aeae-sal] (I)
Identification code
(4)
Compound isolated 1.98 g (yield 95.5%). Anal. calcd. For
C11H16N2O2 (M.W. 208.26): C, 63.44%; H, 7.74%; N,
13.45%. Found: C, 63.55%; H, 7.80%; N, 13.51%. FT-IR
(ATR method, cm−1): 3444(νOH), 3300(νN-H), 1626(νC=N),
1295(νC-O). UV–Vis [λmax (nm), ε (Lmol−1 cm−1)]:
400 (8.63 × 102), 315 (3.38 × 103), 280 (2.60 × 103),
Empirical formula
C24H34N4O8V2
608.44
Formula weight
Temperature/K
293(2)
Crystal system
Monoclinic
P 21/c
Space group
1
255 (1.03 × 104), 215 (2.09 × 104). H NMR (CDCl3, δ in
a(Å)
b(Å)
c(Å)
11.2062(9)
11.0681(10)
11.0883(8)
90.000
ppm): 8.33(s, 1H), 7.26–7.30(dt, 1H), 7.21–7.23(dd, 1H),
6.91–6.93(d, 1H), 6.83–6.87(dt, 1H), 3.68(t, 2H), 3.60–
3.63(t, 2H), 2.91–2.94(t, 2H), 2.74–2.76(t, 2H). 13C-NMR
(DMSO-d6, δ in ppm): 166.52, 161.09, 132.27, 131.66,
118.87, 118.45, 116.76, 60.42, 58.57, 51.78, 49.84.
α(ꢀ)
β(ꢀ)
γ(ꢀ)
109.950(9)
90.000
Volume (Å3)
1292.8(2)
2
4.1.2 | Data for [H2aeae-hyap] (II)
Z
ρcalc (g cm−3
)
1.5629
Compound isolated 2.13 g (yield 95.94%). Anal. calcd. For
C12H18N2O2 (M.W. 222.28): C, 64.84%; H, 8.16%; N,
12.60%. Found: C, 64.78%; H, 8.18; N,12.69%. FT-IR (ATR
method, cm−1): 3419(νOH), 3293(νN-H), 1609(νC=N),
1299(νC-O). UV–Vis [λmax (nm), ε (Lmol−1 cm−1)]:
390 (3.28 × 103), 324 (2.43 × 103), 272 (6.22 × 103),
μ (mm−1
)
0.779
F(000)
Crystal size/mm3
633.6
0.15 × 0.12 × 0.1
0.71073
Radiation Mo Kα (λ)
2θ range for data
3.86 to 54.00
1
255 (7.46 × 103), 216 (2.04 × 104). H NMR (CDCl3, δ in
collection (ꢀ)
ppm): 16.44(s, b), 7.52–7.54(d, 1H), 7.22–7.26(t, 1H),
6.79–6.81(d, 1H), 6.71–6.74(t, 1H), 3.67–3.70(t, 2H), 3.59–
3.61(t, 2H), 2.98–3.01(t, 2H), 2.75–2.78(t, 2H), 2.38(s, 3H).
13C-NMR (DMSO-d6, δ in ppm): 173.22, 165.35, 132.86,
128.98, 118.95, 118.65, 116.33, 60.57, 51.71, 49.46, 48.68,
14.52.
Index ranges
−15 ≤ h ≤ 12, −14 ≤ k ≤ 15,
−9 ≤ l ≤ 14
Reflections collected
6207
Independent reflections
2768 [Rint = 0.0254,
Rsigma = 0.0428]
Data/restraints/parameters
Goodness-of-fit on F2
2768/0/173
1.0553
Final R indexes
R1(I > 2σ(I)), wR2
0.0376, 0.0781
5 | SYNTHESIS OF VANADIUM
COMPLEXES
Final R indexes [all data]
R1, wR2
0.0584, 0.0866
5.1 | Preparation of [VIVO(Haeae-sal)
(MeOH)]+ (1)
Largest diff. Peak/hole/e
Å−3
0.42/−0.43
Deaerated 20 ml methanolic solution of [H2aeae-sal]
(I) (10 mmol, 2.08 g) was mixed with deaerated 30 ml
SCHEME 1 Synthetic scheme for the
preparation of ligands and metal complexes
along with the heterogenization of vanadium
complexes

6 of 21
MAURYA ET AL.
methanolic solution of [VO (acac)2] (2.65 g) under a
nitrogen atmosphere with constant stirring. The
reaction mixture was refluxed for one hour. Afterward,
the brown-colored solution was reduced to dryness
under vacuum, during which a light brown colored
compound 1 was separated which was filtered off,
washed with methanol and finally dried under
vacuum.
Method B: 5.0 mmol (1.53 g) [VIVO(Haeae-sal)(MeOH)]+
(1) or 5.0 mmol (1.60 g) [VIVO(Haeae-hyap)(MeOH)]+
(2) was dissolved in the minimum amount of DMSO
(10 ml) and kept for slow evaporation in the open air.
Within 6–8 days, crystals suitable for X-ray analysis
appeared in the solution.
5.1.4 | Data for [{VVO(Haeae-sal)}μ-O]2
(3)
5.1.1 | Data for [VIVO(Haeae-Sal)
(MeOH)]+ (1)
Yield 0.819 g (30%). Anal. calcd. For C22H30N4O8V2 (M.
W. 580.39): C, 45.53%; H, 5.21%; N, 9.65%. Found: C,
45.48%; H, 5.42%; N, 9.55%. FT-IR (ATR method, cm−1):
3397(νOH), 3211(νN-H), 1597(νC=N), 1276(νC-O), 937(νV=O),
Yield 2.45 g (80.01%). Anal. calcd. For C12H19N2O4V (M.
W. 306.23): C, 47.07%; H, 6.25%; N, 9.15% Found: C,
47.38%; H, 6.50; N, 9.05%. FT-IR (ATR method, cm−1):
770(νV-(μ-O)-V asymmetric
)
,
840(νV-(μ-O)-V symmetric
)
.
UV–Vis
3397(νOH),
3210(νN-H),
1591(νC=N),
1278(νC-O),
[λmax (nm), ε (Lmol−1 cm−1)]: 385 (1.84 × 103),
316 (5.97 × 103), 277 (1.21 × 104), 254 (2.15 × 104),
215 (4.36 × 104).
936(νV=O). UV–Vis [λmax (nm), ε (Lmol−1 cm−1)]:
802 (20), 372 (3.39
×
103), 272 (1.92
×
104),
232 (2.47 × 104).
Using a similar method, complex [VIVO(Haeae-hyap)
(MeOH)]+ (2) was isolated by reacting [VO (acac)2]
(10 mmol, 2.65 g) with [H2aeae-hyap] (II) (10 mmol,
2.22 g) in deaerated methanol under nitrogen
atmosphere.
5.1.5 | Data for [{V(V)O(Haeae-hyap)}μ-
O]2 (4)
Yield 1.022 g (35.6%). Anal. calcd. For C24H34N4O8V2 (M.
W. 608.44): C, 47.48%; H, 5.63%; N, 9.21%. Found: C,
47.43%; H, 5.79%; N, 9.15%. FT-IR (ATR method, cm−1):
3374(νOH), 3271(νN-H), 1581(νC=N), 1233(νC-O), 932(νV=O),
5.1.2 | Data for [VIVO(Haeae-hyap)
(MeOH)]+ (2)
743(νV-(μ-O)-V asymmetric
)
,
822(νV-(μ-O)-V symmetric
)
.
UV–Vis
[λmax (nm), ε (Lmol−1 cm−1)]: 387 (1.70 × 103),
321 (1.10 × 104), 276 (1.22 × 104), 250 (3.83 × 104),
212 (7.95 × 104).
Yield 2.674 g (83.49%). Anal. calcd. For C13H21N2O4V (M.
W. 320.26): C, 48.75%; H, 6.61%; N, 8.75%. Found: C,
48.88%; H, 6.85; N, 8.65%. FT-IR (ATR method, cm−1):
3368(νOH), 3289(νN-H), 1576(νC=N), 1234(νC-O), 936(νV=O).
UV–Vis [λmax (nm),
ε
(Lmol−1 cm−1)]: 786 (28),
5.2 | Synthesis of polymer-supported
vanadium complexes
372 (8.10 × 102), 305 (3.97 × 104), 272 (5.38 × 104),
231 (5.61 × 104).
Preparation of PS-[VVO2(Haeae-sal)] (5) and PS-
[VVO2(Haeae-hyap)] (6): Covalent attachment of
[{VVO(Haeae-sal)}μ-O]2 (3) and [{VVO(Haeae-hyap)}μ-
O]2 (4) into the chloromethylated polystyrene bead was
5.1.3 | Preparation of [{VVO(Haeae-
sal)}μ-O]2 (3) and [{V(V)O(Haeae-hyap)}μ-
O]2 (4)
done by
a
conventional procedure described in
Scheme 1. In brief, chloromethylated polystyrene
(1.0 g) was allowed to swell in DMF (20 ml) for
12 hours. DMF solution (40 ml) of 3 (6.89 mmol, 4.0 g)
or 4 (6.57 mmol, 4.0 g) was added to the above suspen-
sion in the presence of KI (6.69 mmol, 1.11 g) and
triethylamine (2.19 gm). The reaction mixture was
heated at 90 ꢀC for 48 hr with gentle stirring
(150 RPM) in an oil bath fitted with a water-cooled
condenser. The cooled reaction mixture was filtered
and washed with hot DMF (4 × 20 ml) followed by hot
methanol (4 × 20 ml). The brown-colored polymer-
Method A: 30 ml methanolic solution of [VO (acac)2]
(10 mmol, 2.65 g) was reacted with 15 ml methanolic
solution of [H2aeae-sal] (I) (10 mmol, 2.08 g) or [H2aeae-
hyap] (II) (10 mmol, 2.22 g) with constant stirring at
room temperature and pressure. The solution was
refluxed for 3 hr in the presence of air. After 3 hr, the red-
dish-brown colored solution was reduced to ~15 ml and
kept in a refrigerator overnight. Within 4–5 days, light
orange colored crystals suitable for X-ray analysis were
isolated from the solution.

DIMERIC μ-OXIDOVANADIUM COMPLEXES AS EFFICIENT MODELS OF VBPO'S
7 of 21
supported complex 5 orꢀ 6 was recovered and dried in a
hot air oven at ca. 110 C for 24 hr.
in the corresponding vanadium complexes with slight
shifting from their original position. Additionally,
[VIVO(aeae-sal)(MeOH)]+ (1) and [VIVO(aeae-hyap)
(MeOH)]+ (2) exhibits a broad band, of low intensity and
low energy, at 802 nm (ε = 20 Lmol−1 cm−1) and 786 nm
(ε = 28 Lmol−1 cm−1), respectively, which can be
assigned to d-d transition. The polymer anchored vana-
dium complexes 5 and 6 displayed low-intensity absor-
bance bands because of the low concentration of 3 and
4 in the polymer matrix. However, both 5 and 6 showed
absorbance bands characteristic of their respective
ligands (Figure S2), which confirms successful attach-
ment of the vanadium complexes in the chlor-
omethylated polystyrene chain.
5.2.1 | Data for PS-[VVO2(Haeae-sal)] (5)
Recovery yield 96%. FT-IR (ATR method, cm−1):
3390(νOH), 1587(νC=N), 1261(νC-O), 953(νV=O), 907(νV=O);
UV–Vis [λmax (nm)]: 230, 264, 304, 340.
5.2.2 | Data for PS-[V(V)O2(Haeae-hyap)]
(6)
Recovery yield 94.8%. FT-IR (ATR method, cm−1):
3393(νOH), 1587(νC=N), 1212(νC-O), 975(νV=O), 904(νV=O);
UV–Vis [λmax (nm)]: 220, 263, 312, 341 nm.
6.2 | Solution behaviour of [VIVO(aeae-
Sal)(MeOH)]+ (1) and [VIVO(aeae-hyap)
(MeOH)]+ (2)
6 | RESULT AND DISCUSSION
6.1 | Electronic spectral studies
Both vanadium (IV) complexes 1 and 2 are stable in the
solid-state but in solution, they spontaneously oxidize in
the presence of aerial oxygen and quickly convert into
vanadium(V) dioxido species. Absorbance spectra of the
methanolic solution of 1 (1.165 × 10−2 M) and
2 (2.132 × 10−2 M) were recorded periodically (intervals
of 2 minutes) in the presence of air at room temperature
and pressure while stirring the solution continuously. A
representative plot of change in absorbance spectra of
2 due to the aerial oxidation is shown in Figure 1. It is
observed that the broad bands at 802 nm in 1 and 786 nm
in 2 gradually disappear with time (Figure 1 and
Electronic spectral data of all the ligands and metal com-
plexes are enlisted in Table 2 and spectra are shown in
Figure S1. Absorbance bands appearing in the range of
215–216 nm for both ligands I and II are due to σ-σ*
transition. Both ligands show absorbance bands in the
range of 255–272 nm because of π-π1* and π-π2* transi-
tion whereas two low-intensity absorbance bands in the
range of 315–400 nm can be assigned to n1-π* and n2-π*
transition. All these electronic transitions also appeared
TABLE 2 Electronic spectral data of the ligands and their vanadium complexes
SN
Compound
Solvent
λ )
max (nm) /(ε)(Lmol−1 cm−1
1
[H2aeae-sal] (I)
MeOH
400(8.63 × 102), 315(3.38 × 103), 280(sh),
255(1.03 × 104), 215(2.09 × 104)
2
3
4
5
6
[H2aeae-hyap] (II)
MeOH
MeOH
MeOH
MeOH
MeOH
390(3.28 × 103), 324(2.43 × 103), 272(6.22 × 103),
255(7.46 × 103),216(2.04 × 104)
802(20)$, 372(3.39 × 103), 272(1.92 × 104),
232(2.47 × 104)
786(28)$, 372(8.10 × 102), 305(3.97 × 104),
272(5.38 × 104), 231(5.61 × 104)
385(1.84 × 103), 316(5.97 × 103), 277(1.21 × 104),
254(2.15 × 104), 215(4.36 × 104)
[VOIV(Haeae-sal)(MeOH)]+ (1)
[VOIV(Haeae-Hyap)(MeOH)]+ (2)
[{VOV(Haeae-sal)}μ-O]2 (3)
[{VOV(Haeae-hyap)}μ-O]2 (4)
387(1.70 × 103), 321(1.10 × 104), 276(1.22 × 104),
250(3.83 × 104), 212(7.95 × 104)
7
8
PS-[VVO2(Haeae-sal)] (5)
PS-[VVO2(Haeae-hyap)] (6)
Nujol
Nujol
340, 304, 264, 230
341, 312, 263, 220
$= d-d transition.
sh = shoulder band.

8 of 21
MAURYA ET AL.
Figure S3) which suggests the formation of corresponding
vanadium(V) dioxido species in solution as proposed in
the scheme 1.
The methanolic solution of 3.210 × 10−5 M of
[{VOV(Haeae-sal)}μ-O]2 (3) and 1.550 × 10−4 M of
[{VOV(Haeae-hyap)}μ-O]2 (4) was reacted with dilute
aqueous hydrogen peroxide and the corresponding spec-
tral changes are plotted in Figure 2 and Figure S4. The
gradual addition of one drop portion of 8.82 × 10−3
M
(diluted in methanol) H2O2 into the methanolic solution
of 3 decreases the intensities of absorbance bands at
254 nm and 316 nm, and a new band appears at 364 nm
whereas the 277 nm band intensifies. Similar changes
can also be seen while reacting 4 with 8.82 × 10−3 M of
hydrogen peroxide. All these changes support the genera-
tion of vanadium peroxido species in solution.
FIGURE 2 Observed spectral changes of methanolic solution
of 3.210 × 10−5 M [{VOV(Haeae-sal)}μ-O]2 (3) by the addition of
methanolic solution of 1 drop portion of 8.82 × 10−3 M dilute
aqueous H2O2
6.3 | IR spectral studies
Changes in the functional groups, during the
heterogenization of metal complexes 3 and 4 into the
polymeric chain, were examined by FT-IR spectral analy-
sis in the range of 400–4000 cm−1. Both ligands [H2aeae-
sal] (I) and [H2aeae-hyap] (II) display one broad ν(O-H)
band in the range of 3419–3444 cm−1, the N-H stretching
frequency in the range of 3293–3300 cm−1 and one sharp
band characteristic of ν(C=N) in the range of 1609–
1626 cm−1. A partial list of stretching frequencies of
ligands and the metal complexes of primary interest is
given in Table S2. Also, selected FT-IR spectra are shown
in Figure 3 and Figure S5.
and ν(C=N) stretching frequencies in their expected regions.
A sharp and strong band, in the range of 932–937 cm−1,
characteristic of V=O bond is present in the FT-IR spectra
of all vanadium complexes 1–4. Moreover, all the metal
complexes show slight shifting in the ν(N-H) and ν(C=N)
stretching frequencies towards the lower wavenumber
with respect to their corresponding ligands due to the
coordination of amine N and azomethine N to the vana-
dium center.[32] Stretching frequencies in the range of
743–770 cm−1 (in complex 3) and 822–840 cm−1
(in complex 4) appear due to the symmetric and asymmet-
ric stretching of V-O-V bond vibrations. Polymer anchored
metal complexes PS-[VVO2(Haeae-sal)] (5) and PS-
[VVO2(Haeae-hyap)] (6) revealed a similar spectral pattern
to that of complexes 1 and 2. However, both complexes
exhibit two sharp stretching bands, one in the range of
953–975 cm−1 and another in the range of 904–907 cm−1,
Metal complexes [VIVO(aeae-sal)(MeOH)]+ (1),
[VIVO(aeae-hyap)(MeOH)]+ (2), [{VOV(Haeae-sal)}μ-O]2
(3) and [{VOV(Haeae-hyap)}μ-O]2 (4) produce ν(O-H), ν(N-H)
due
to
the
unique
(O=V=O)asymmetric
and
(O=V=O)symmetric bond vibrations. These bands indicate
the presence of cis-VVO2 group in the polymer-bound
vanadium complexes.[33] Absence of ν(N-H) stretching fre-
quency, in the polymer-bound metal complexes 5 and 6,
supports the covalent attachment of neat complexes
(3 and 4) with the polymer matrix.
6.4 | EPR spectral studies
The true oxidation state of vanadium (IV) oxido com-
plexes 1 and 2 was established through EPR spectral
analysis. X-band EPR spectra of complexes 1 and 2 were
FIGURE 1 Change in UV–Vis Spectra observed during the
oxidation of 15 ml methanolic solution of 2.132 × 10−2 M complex
2 in the presence of aerial oxygen at room temperature

DIMERIC μ-OXIDOVANADIUM COMPLEXES AS EFFICIENT MODELS OF VBPO'S
9 of 21
FIGURE 3 FT-IR spectra of (A) [H2aeae-
sal] (I), [VIVO(Haeae-sal)(MeOH)]+ (1),
[{VVO(Haeae-sal)}μ-O]2 (3), PS-[VVO2(Haeae-
sal)] (5) and used PS-[VVO2(Haeae-sal)] (5)
recorded in methanol at liquid nitrogen temperature and
presented in Figure 4.
All the 8 lines of perpendicular orientation are clearly vis-
ible in EPR spectra of both vanadium (IV) oxido com-
plexes but only 5 lines out of 8 in the parallel orientation
are observed. However, the intensity of the spectral lines
in the parallel orientation of 2 is quite low as compared
to 1. Well resolved hyperfine spectra indicate complexes
1 and 2 exist in monomeric form in the frozen
methanolic solution, and mutual vanadium-vanadium
interaction is absent.
Frozen methanolic solutions of 1 and 2 show axially
symmetrical EPR spectra characteristic of V(IV) = O com-
plexes.[34] A well resolved hyperfine splitting pattern, in
both parallel and perpendicular regions, is observed for
both of the vanadium (IV) oxido complexes 1 and 2. Due
to the coupling of one unpaired electron (S = 1/2) of V4+
(3d1) with its own nucleus (S = 7/2), 8 lines appear in the
hyperfine splitting of both complexes (shown in Figure 4).
6.5 | NMR spectral studies
1H-NMR spectra of ligand [H2aeae-sal] (I) and [H2aeae-
hyap] (II) are presented in figure 5 and spectral data are
summarized in Table S3. Ligand [H2aeae-sal] (I) displays
the characteristic (—H-C=N) proton singlet signal at
8.33 ppm along with the four aromatic proton signals in
the range of 6.83–7.30 ppm and eight aliphatic proton sig-
nals within their expected range (2.74–3.68 ppm). Ligand
[H2aeae-hyap] (II) shows its characteristic sharp singlet
signal at 2.38 ppm due to the –CH3 protons and signals
for four aromatic protons are present in the range of
6.71–7.54 ppm. Eight aliphatic proton signals arise within
the expected range of 2.75–3.70 ppm. Both ligands are
unable to produce any detectable –NH proton signal,
although ligand (II) displays one broad signal at
16.44 ppm due to the –OH proton.[35] Thus, 1H-NMR
spectral data strongly supports the structure of ligand (I)
and (II) as suggested in scheme 1.
FIGURE 4 First derivative plot of EPR spectrum of
(A) [VIVO(Haeae-sal)(MeOH)]+ (1) and (B) [VIVO(Haeaehyap)
(MeOH)]+ (2) recorded in methanol at 77 K. Solutions were freezed
immdiately after dissolving the vanadium (IV) complexes in
methanol
13C NMR spectroscopy was used to confirm the
molecular structure of ligand [H2aeae-sal] (I) and
[H2aeae-hyap] (II). 13C NMR spectra of (I) and (II) were
recorded in DMSO-d6 and shown in Figure 6. As

10 of 21
MAURYA ET AL.
FIGURE 5 1H NMR
spectra of [H2aeae-sal] (I) and
[H2aeae-hyap] (II) recorded in
CDCl3
expected, ligand (I) shows eleven distinctive signals in its
13C NMR spectrum. The most downfield signal at
166.52 ppm appears due to the azomethine carbon (C7).
The signal at 161.09 ppm is due to the phenolic carbon
(C1). The rest of the aromatic carbons are present in the
range of 116.76–132.27 ppm. There are four signals pre-
sent in the aliphatic region, out of which the two most
downfield signals at 58.57 and 60.42 ppm are due to C11
and C8, respectively, as they are bonded to the electro-
negative atoms O and N, whereas the other two aliphatic
signals appearing at 49.84 and 51.78 ppm are due to C9
and C10, respectively. Ligand (II) exhibits a similar type
of spectra with one additional signal at 14.52 ppm due to
the methyl carbon (C8). As mentioned for ligand (I), here
also the azomethine carbon (C7) shows the most down-
field signal at 173.22 ppm and the phenolic carbon
(C1) at 165.35 ppm. All the other aromatic protons
appear in their expected range (116.33–132.86 ppm). Sig-
nals due to the rest four aliphatic carbons appear at 48.67
(C10), 49.46 (C11), 51.71 (C9), and 60.57 (C7) ppm.
Observed signals and their assignments are tabulated in
Table S4 which are in good agreement with the molecu-
lar structure proposed by FT-IR and 1H NMR analyses.
Both vanadium(V) oxido complexes 3 and 4 were
characterized in solution through 51V NMR spectroscopy.
The 51V NMR spectra of complexes [{VVO(Haeae-sal)}μ-
FIGURE 6 13C NMR spectra of
[H2aeae-sal] (I) and [H2aeae-hyap] (II)
recorded in DMSO-d6

DIMERIC μ-OXIDOVANADIUM COMPLEXES AS EFFICIENT MODELS OF VBPO'S
11 of 21
O]2 (3) and [{VVO(Haeae-hyap)}μ-O]2 (4) were recorded
with a freshly prepared solution in DMSO-d6 while
VOCl3 was used as an external reference (shown in Fig-
ure 7). Due to the quadrupole interaction, 51V NMR lines
appear slightly broad.[36] Vanadium(V) complexes 3 and
4 exhibit one strong resonance at −528.126 ppm
and − 531.467 ppm, respectively, which are within the
range of reported VVO2 form of complexes.[37] Along with
the intense −531.467 ppm signal, complex 4 shows two
additional
weaker
bands
at
−511.812
ppm
and − 488.801 ppm, which may be due to the coordina-
tion of DMSO.[36a,38] The 51V NMR studies further con-
firm the existence of the monomeric vanadium(V)dioxide
species in solution.
6.6 | Single crystal X-ray diffraction
studies
FIGURE 8 ORTEP diagram of complex [{VVO(Haeae-1
hyap)}-O]2 (4)
The molecular structure of 3 and 4 have been confirmed
by single-crystal X-ray analysis. ORTEP diagrams of
3 and 4 are shown in Figure 8 and Figure S6, respectively.
Both complexes 3 and 4 crystallizes in the centrosymmet-
ric monoclinic crystal system with space group P21/c.
The asymmetric unit of compounds 3 and 4 carry one
half of the dinuclear complex. X-ray analysis shows that
both vanadium centers in dinuclear complexes 3 and
4 acquire distorted octahedral geometry while keeping
the V1-V10 distance 3.102 Å and 3.134 Å, respectively.
However, each vanadium center in the asymmetric unit
adopts a square pyramidal geometry. The basal plane is
occupied by one O atom from phenolate moiety, one N
atom from azomethine, one N from the amine and one
oxido O atom. Another oxido O atom occupies the apical
position. One oxido O-atom from each vanadium center
coordinates to another vanadium center in the solid-state
acting as a bridging ligand and hence produces a dimer.
The mutual distance between the two bridging O atoms
is 2.566 Å in 3 and 2.596 Å in 4. The V=O bond distance
is 1.612 Å in 3 and 1.609 Å in 4, which is a characteristic
of a strong vanadium oxido (V=O) bond. While much
longer V1-O3 bond [1.685 Å in 3 and 1.684 Å in 4] in the
equatorial plane allows O atom to act as a bridging
ligand.
A partial list of all the vital bond angles and bond
lengths of the complexes 3 and 4 are enlisted in Table S5.
Both intra-molecular and inter-molecular H-bonding
is present in complexes 3 and 4. In complex 3, a strong
intramolecular hydrogen bond (1.941 Å) exists between
OH-group of the Schiff-base ligand and one of the
FIGURE 7 51V-NMR spectra of (A)
[{VVO(Haeae-sal)}μ-O]2 (3) and
(B) [{VVO(Haeae-hyap)}μ-O]2 (4) recorded in
DMSO-d6. Chemical shifts (δ) of 51V are
referenced with respected to VOCl3 as an
external standard

12 of 21
MAURYA ET AL.
bridging O-atom (O4-H4…… O3). Whereas in 4, a rela-
tively weak H-bond (2.264 Å) is observed between pheno-
late O-atom of the Schiff-base ligand and amine H (–NH)
atom of the other half of the dimer (O1…… H20-N20).
Stronger H-bonding can be a reason for the shorter V-V0
atom distance in 3 as compared to 4. Inter-molecular H-
bonding in both complexes leads to the molecular associ-
ation to form a 3D structure in solid state. Detailed H-
bonding distances and bond angles are listed in table S6.
With the help of Koopman's theorem[39] electronega-
tivity and chemical hardness of the molecule were calcu-
lated. Electrophilicity index (ψ) and global softness were
also calculated as calculated by Parr et al.[40] All the
above-mentioned quantum chemical properties of mono-
meric and dimeric vanadium complexes calculated by
using UB3LYP/LANL2DZU6-31G (d, p) method are
shown in Table 3.
As suggested by single-crystal X-ray analysis, both
3 and 4 can dissociate into corresponding monomeric
species in solution due to their elongated V-O bridging
distance. So, the theoretical UV–Vis absorption spectra of
both the complexes 3 and 4 were determined in their
dimeric and monomeric form. TD-DFT calculation was
performed on the optimized geometry employing a mixed
basis set B3LYP using LANL2DZ with an effective core
potential for V atom and 6-31G++(d, p) for the rest of
the atoms. All the calculations were performed in metha-
nol using CPCM solvation model. The experimental UV–
Vis spectra of the complexes 3 and 4 recorded in metha-
nol were plotted against the theoretically predicted UV–
Vis spectra of 3 and 4 in its dimeric as well as monomeric
form (shown in Figure 9).
Also selected theoretical absorption spectral maxima
(nm), along with their excitation energy (eV) and oscilla-
tor strength parameter (f) are listed in Table S7. Experi-
mental UV–Vis absorption spectra of 3 and 4 recorded in
methanol bear a close resemblance with the theoretically
predicted absorption maxima of the respective mono-
meric unit (shown in Figure 9). Therefore, 3 and 4 exist
in its monomeric form in solution which can be further
confirmed by the FT-IR analysis of polymer-supported
complexes 5 and 6. Heterogenization was done by dis-
solving the complexes 3 and 4 in DMF which allows easy
dissociation of the dimer into monomer and as a result
monomeric units get attached to the polymeric chain.
7 | THEORETICAL STUDIES
7.1 | Geometry optimization
Molecular structure of the dimeric vanadium complexes
[{VVO(aeae-sal)}μ-O]2 (3), [{VVO(aeae-hyap)}μ-O]2 (4)
and corresponding monomeric complexes [VVO2(aeae-
sal)] 3(a), [VVO2(aeae-hyap)] 4(a) (shown in the Fig-
ure S7), were optimized by using a mixed basis set, DFT/
UB3LYP employing LANL2DZ with an effective core
potential for V atom and 6-31G (d,p) for the rest of the
atoms. Single-crystal XRD structures were used as an ini-
tial guess. All the optimized geometrical parameters are
listed in Table S5 and shown in Figure S8–S11. There is
an excellent agreement between the solid-state data of
the single crystals and theoretically calculated data of
gas-phase as well as solution phase of the complexes. A
slight deviation of the structural parameters can be
explained by the approximate basis set chosen for the gas
phase and solution phase DFT calculation without con-
sidering lattice interactions. The absence of imaginary
frequencies indicates the local minima on the potential
energy surface. Based on these optimized structures cal-
culations of the other parameters such as polarizability,
vibrational frequencies, Natural Bond Orbital (NBO)
analysis, etc. were carried out.
Hence both supported complexes
5 and 6 carry
TABLE 3 Quantum chemical properties calculated by using DFT/UB3LYP methods with mixed basis set LANL2DZU6-31G(d, p)
Complexes
SL.No.
Parameters
3
4
[VVO2(aeae-sal)] 3(a)
−0.1329
[VVO2(aeae-hyap)] 4(a)
1
2
3
4
5
6
7
8
9
HOMO (eV)
−0.2172
−0.2191
−0.0700
−0.0651
−0.1472
0.1436
−0.2029
−0.2042
−0.0491
−0.0459
−0.1538
0.126
−0.2123
−0.2377
−0.0671
−0.0424
−0.1452
0.1397
HOMO-1 (eV)
−0.2488
LUMO (eV)
−0.0523
LUMO+1 (eV)
−0.0434
|ΔE|(energy gap) (eV)
χ (eV) (Electronegativity)
η (eV) (Chemical hardness)
ζ (eV) (softness)
−0.0806
0.0926
−0.0736
−6.7934
−0.1400
−0.0769
−6.5019
−0.1032
−0.0403
−0.0726
−6.8870
−0.1344
−12.4069
−0.1064
ψ (eV) (electrophilicity index)

DIMERIC μ-OXIDOVANADIUM COMPLEXES AS EFFICIENT MODELS OF VBPO'S
13 of 21
FIGURE 9 Comparison between
experimental and theoretical UV–Vis
plots of (A) [{VVO(Haeae-sal)}μ-O]2 (3)
and (B) [{VVO(Haeae-hyapl)}μ-O]2 (4)
characteristic FT-IR stretching vibration of five coordi-
nated cis-VO2 species in the solid-state.
In the final step, both complexes exhibit a massive mass
loss in the temperature range of 341 ꢀC to 476 ꢀC because
of the simultaneous breakdown and decomposition of
polymer structure as well as metal complexes. Complex
5 shows a mass loss of 64.89% while 52.47% weight loss is
observed in 6. TGA analysis suggests that both supported
complexes 5 and 6 are stable up to 341 ꢀC.
7.2 | TGA and AAS analysis
Thermal stability of the supported complexes PS-
[VVO2(Haeae-sal)] (5) and PS-[VVO2(Haeae-hyap)] (6)
was tested by TGA under nitrogen atmosphere at a tem-
ꢀ
perature rate of 10 C/min over a temperature range of
7.3 | SEM–EDX and AAS analysis
ꢀ
ꢀ
30 C to 850 C (shown in Figure 10). Both complexes
5 and 6 show three-stage thermal decomposition profiles
in their TGA curve which is a typical TGA pattern for
polymer-supported metal complexes.[35,41] Initially, both
complexes 5 and 6 undergo a small mass loss of 3.12%
and 6.53%, respectively, due to the loss of physically
adsorbed gases and moisture. In the next step, 7.51%
(in 5) and 14.01% (in 6) of weightꢀ loss was ꢀobserved
within the temperature range of 191 C to 341 C due to
the loss of entrapped solvent (DMF) and water molecules.
Change
in
surface
morphology
during
the
heterogenization of [{VOV(Haeae-sal)}μ-O]2 (3) and
[{VOV(Haeae-hyap)}μ-O]2 (4) into the polymeric chain
was analyzed by SEM. The change in elemental composi-
tion was examined by the energy-dispersive X-ray (EDX)
analysis. SEM and elemental mapping of PS-
[VVO2(Haeae-sal)] (5) and PS-[VVO2(Haeae-hyap)] (6)
was done and shown in Figure 11 and S12.
As expected, pure chloromethylated polystyrene bead
shows flat and smooth surface containing only the signals
of carbon, hydrogen and chlorine atoms (Figure S13, the
SEM and EDX image of pure polymer bead is reproduced
from our earlier paper).34c Both polymer-supported com-
plexes show the elemental signal of oxygen and nitrogen
along with the signal of vanadium atoms in their elemen-
tal mapping images. A semi-quantitative measurement,
EDX, indicates 5.4% and 5.5% of vanadium content in
complex 5 and 6, respectively. All this information sug-
gests successful covalent attachment of complexes 3 and
4 into the chloromethylated polystyrene bead. Exact
vanadium loading into the polymer support was esti-
mated by atomic absorption spectroscopy, which is found
to be 0.44625% (0.0876 mmol/g) and 1.043%
(0.20474 mmol/g) in complex 5 and 6, respectively
(Table S8). Elemental maps of important atoms along
with the SEM images of the recycled catalysts 5 and
6 were also analyzed. Similar SEM images to that of fresh
catalysts indicate no change in morphology as well as the
FIGURE 10 DTA-TGA plots of polymer anchored vanadium
complexes 5 and 6

14 of 21
MAURYA ET AL.
FIGURE 11 (A) SEM
image of single polymer bead of
catalyst PS-[VVO2(Haeae-hyap)]
(6) and corresponding elemental
mapping of (B) carbon,
(C) chlorine, (D) nitrogen,
(E) oxygen, and (F) vanadium
with EDX
physical form of the catalysts even after multiple uses in
a catalytic reaction. However, vanadium mapping of the
used catalysts shows slightly less density of vanadium
atoms (Figure S14 and S15) in comparison to the neat
one. This is due to the removal of excess adsorbed metal
complexes from the upper surface of the polymer bead
along with a small amount of metal leaching from the
polymer matrix during the catalytic reaction.
during the heterogenization process. The average surface
roughness of pure polymer bead along with supported
complexes and their corresponding recycled supported
complexes was analyzed by AFM and shown in Figure 12
and Figure S16 (Figure 12(A), AMF image of pure poly-
mer bead is reproduced from our earlier paper).[42] The
surface roughness and mean height of polymer resin (PS-
Cl) are 10.1 nm and 78.0 nm, respectively. AFM image
clearly shows that after covalent attachment of 3 and
4 into the polymeric chain of chloromethylated polysty-
rene surface roughness and mean height reduces. The
average surface roughness of complex 5 and 6 is 3.21 nm
and 2.88 nm, respectively. Due to the anchoring of metal
complexes into the pores of the polymer chain during
heterogenization causes the reduction in surface
roughness.[42–46] Hence, surface roughness can be
7.4 | AFM analysis
Although the SEM images are quite informative regard-
ing the change in surface morphology during
heterogenization, atomic force microscopy provides a
quantitative idea regarding changes in surface roughness
FIGURE 12 AFM pictures
and corresponding 3D-surface
maps of (A) PS-Cl, (B) PS-
[VVO2(Haeae-sal)] (5) (C) PS-
[VVO2(Haeae-hyap)] (6)

DIMERIC μ-OXIDOVANADIUM COMPLEXES AS EFFICIENT MODELS OF VBPO'S
15 of 21
correlated with the percentage of metal loading into the
polymer. The lower average surface roughness of com-
plex 6 than 5 indicates a higher percentage of metal load-
ing in complex 6 in comparison to complex 5. AAS
analysis data of complexes 5 and 6 also supports the
above facts. Detailed surface roughness along with the
Root Mean Square (RMS) roughness (Rq) and mean
height of both complexes 5 and 6 are listed in Table S9.
As mentioned in EDX analysis, the surface roughness of
the recycled complexes increases (shown in the Fig-
ure S15) to some extent due to the removal of excess
adsorbed metal complexes from the upper surface of the
polymer bead along with a small amount of metal
leaching during the catalytic reaction. The estimated
average surface roughness of the recycled catalysts 5 and
6 are 4.27 nm and 3.11 nm, respectively.
SCHEME 2 Oxidative bromination products of
salicylaldehyde [(*) major product)]
to the reaction, polymer anchored catalysts were swelled
in water for 12 hr.
Detailed results of all the reaction conditions applied
to optimize oxidative bromination of salicylaldehyde in
the presence of catalyst 5 are summarized in Table 4.
Entry No. 17 of Table 4 represents the optimized reaction
conditions for the oxidative bromination of
salicylaldehyde which are, 10 mmol of salicylaldehyde
(1.22 g), catalyst 0.005 g, 15 mmol of H2O2 (1.7025 g),
20 mmol of KBr (2.38 g), 20 mmol of HClO4 (2.86 g),
15 ml water, 2 hr of time and room temperature.
8 | CATALYTIC ACTIVITY
STUDIES
A blank reaction under the same optimized reaction
conditions shows only 47.7% substrate conversion. Neat
complexes (3 and 4) were also employed as catalysts for
the oxidative bromination of salicylaldehyde under opti-
mized reaction conditions. Catalyst 3 and 4 show 94.4%
and 79.0% conversions, respectively. Both supported and
neat catalysts produce 5-BrS as the major product. How-
ever, the decrease in product selectivity (%), as well as
substrate conversion (%), was observed while using neat
catalysts. Surprisingly, catalysts 3–6 show exceptionally
high TOF values during the catalytic oxidative bromina-
tion of salicylaldehyde.[50,52,53,55–60] The TOF values of
the neat complexes are quite low in comparison to that of
8.1 | Oxidative bromination of
Salicylaldehyde
Vanadium haloperoxidase, a vanadium-containing meta-
lloenzyme, selectively catalyzes the oxidation of halide in
the presence of hydrogen peroxide, and, if any suitable
organic substrate is available, then it can halogenate the
organic substrate also.[47–49]
The catalytic activity of PS-[VVO2(Haeae-sal)] (5) and
PS-[VVO2(Haeae-hyap)] (6) was studied for the oxidative
bromination of salicylaldehyde. It is observed that the
vanadium complexes 5 and 6 successfully catalyze the
oxidative bromination of salicylaldehyde in the presence
of hydrogen peroxide and hence become the functional
models of vanadium bromoperoxidase. Three major oxi-
dative bromination products i.e. 5-bromo-2-hydroxyl
benzaldehyde (5-BrS), 3-bromo-2-hydroxy benzaldehyde
(3-BrS) and 3, 5-dibromo-2-hydroxybenzaldehyde (3,5-
BrS), were identified during the catalytic reaction
(Scheme 2).[35,50–56] PS-[VVO2(Haeae-sal)] (5) was used as
a representative catalyst during the optimization of differ-
ent reaction parameters namely amount of catalyst,
amount of oxidant (30% aqueous H2O2), amount of
perchloric acid, amount of potassium bromide and
amount of solvent, while keeping other reaction condi-
tions such as RPM of magnetic stirrer, size of magnetic
bead, shape and size of reaction flask, etc. as identical as
possible (Figure 13). In the oxidative bromination, half of
the perchloric acid was added at the beginning of the
reaction (t = 0 min), and the rest of the amount was
added to the reaction mixture in three equal portions at a
fixed time interval (after every 15 min). In all cases prior
grafted
catalysts
(See
Table
4).
Therefore
heterogenization of the homogeneous catalysts 3 and
4 not only preserve the catalytic efficiency but also
improves the effectiveness significantly. Considering the
TOF
values,
catalyst
5
is
more
efficient
(TOF = 1.127 × 104 h−1) in comparison to the catalyst
6 (4.675 × 103 h−1). Detailed comparison of catalytic effi-
ciency of the supported catalysts 5 and 6, along with their
corresponding neat counterparts 3 and 4, for the oxidative
bromination of salicylaldehyde is presented in Figure 14.
So far several vanadium complexes are known to catalyze
the oxidative bromination of salicylaldehyde in the pres-
ence of H2O2 but none of them does the job as efficiently
as polymer-supported complexes 5 and 6.13b, 33
8.2 | Oxidation of thioanisole
Besides catalyzing the oxidation of halides in the pres-
ence of hydrogen peroxide, vanadium haloperoxidase can

16 of 21
MAURYA ET AL.
FIGURE 13 Catalytic
oxidative bromination of
salicylaldehyde by hydrogen
peroxide in the presence of 5 at
room temperature (A) Impact of
amount of catalyst, (B) Effect of
time, (C) Effect of oxidant (H2O2)
amount, (D) Impact of KBr amount
and (E) Effect of HClO4 amount and
(F) influence of solvent amount
also accelerate the oxidation and oxo transfer reaction in
aromatic compounds, including the oxidation of
thioether into sulfone and sulfoxide. Oxidative desulfuri-
zation of organic sulfides is an important reaction in the
drug and petroleum industry.11b, 61–63 Therefore, the
catalytic efficiency of synthesized catalysts, PS-
[VVO2(Haeae-sal)] (5) and PS-[VVO2(Haeae-hyap)] (6),
was evaluated for the oxidation of thioanisole. Sulfone
and sulfoxide were identified as two major products
(shown in scheme 3) for this reaction which are well
reported in the literature.[43,61,64–68]
In order to attain the maximum substrate conver-
sion(%), various reaction conditions such as reaction
time, amount of catalyst, amount of oxidant (H2O2), the
volume of solvent (acetonitrile) and nature of solvent
were optimized, while using PS-[VVO2(Haeae-hyap)] (6)
as a representative catalyst (Shown in Figure S17).
All the results of various reaction conditions to
identify the optimized reaction parameters of oxidation
of thioanisole catalyzed by catalyst 6 are listed in
Table 5. Data in Table 5 conclude that 1.417 g
(12.5 mmol) of 30% H2O2 was required to achieve maxi-
mum conversion (%) during the oxidation of thioanisole
(5 mmol, 0.627 g) in the presence of 0.020 g catalyst
6 and 4 ml of ethanol at room temperature for 4 hr of
reaction time. Under the above-optimized reaction con-
ditions oxidation of thioanisole was performed in four
different solvents like EtOH, ACN, DMF, and hexane.
A maximum of 90.7% conversion was observed in etha-
nol whereas acetonitrile, DMF and hexane showed
85.5%, 78.6%, and 72.3% conversion, respectively (shown
in Figure S17(E)). These results indicate that substrate
conversion (%) increases with the increase of polarity of
solvents. So, EtOH was chosen as the reaction solvent.
Neat catalysts [{VVO(aeae-sal)}μ-O]2 (3), [{VVO(Haeae-
hyap)}μ-O]2 (4) and supported catalyst PS-[VVO2(Haeae-
sal)] (5) show 75.2%, 84.5%, and 80.1% conversion
respectively under the above mentioned optimized reac-
tion conditions. The controlled reaction shows only
32.5% conversion.
Detailed comparison of oxidation of thioanisole by
the grafted catalysts 5, 6 and neat complexes 3, 4 is pres-
ented in Figure 15. Data in Table 5 shows that homoge-
neous oxidative desulfurization by catalysts 3 and 4 is a
little less effective in terms of substrate conversion (%),
product selectivity (%) or TOF values when compared to
their heterogeneous counterparts 5 and 6. Both homoge-
neous and heterogeneous catalytic oxidative desulfuriza-
tion produces sulfoxide as the major product
unanimously.

DIMERIC μ-OXIDOVANADIUM COMPLEXES AS EFFICIENT MODELS OF VBPO'S
17 of 21
TABLE 4 Summarized results of all the reaction conditions applied to optimize the maximum oxidative bromination of salicylaldehyde
by catalyst 5 at room temperature
% Selectivity
Cat.
Sub:
Oxn
HClO4
(mmol)
Time
(min)
KBr
(mmol) (ml)
Solvent
%
(%)
(%)
(%) 3,5-
SN Cat. (mg)
Conversion TOF/h
5-BrS 3-BrS di-BrS
1
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
3
4
6
-
7.5
10
15
5
1:2
1:2
1:2
1:2
1:2
1:2
2:1
1:1
2:3
2:3
2:3
2:3
2:3
2:3
2:3
2:3
2:3
2:3
2:3
2:3
2:3
2:3
20
20
20
20
20
20
20
20
20
20
20
20
10
15
30
20
20
20
20
20
20
20
120
120
120
60
20
20
20
20
20
20
20
20
20
10
15
30
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
10
15
25
15
15
15
15
98.7
98.7
98.8
88.1
95.1
98.6
36.6
61.6
98.0
60.4
84.9
98.9
81.7
94.5
98.3
98.8
98.8
96.5
94.4
79.0
95.7
47.7
7.511 × 103 77.64
1.94
20.41
21.03
7.67
7.75
16.82
19.52
0.54
0.86
3.24
0.89
1.6
2
5.633 × 103 78.15
3.759 × 103 84.92
2.011 × 104 79.66
1.447 × 104 80.40
1.125 × 104 77.02
4.178 × 103 78.6
7.031 × 103 79.11
1.118 × 104 79.47
6.894 × 103 80.7
9.691 × 103 79.75
1.128 × 104 77.56
9.326 × 103 86.8
1.078 × 104 81.99
1.122 × 104 72.23
1.127 × 104 58.63
1.127 × 104 78.24
1.101 × 104 79.42
2.739 × 102 62.09
2.403 × 102 55.82
4.675 × 103 75.16
0.81
3
7.4
4
12.30
2.24
5
5
90
6
5
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
3.46
7
5
20.85
20.02
17.28
18.4
8
5
9
5
10
11
12
13
14
15
16
17
18
19
20
21
22
5
5
18.64
7.19
5
15.24
7.81
4.63
26.56
29.25
15.03
7.9
5
5.38
5
13.37
1.2
5
5
12.10
6.72
5
5
12.67
26.80
22.08
15.75
8.95
5
5.52
1.17
4.24
0.91
5
5
5
-
37.91
8.3 | Recyclability test
After their use in completion of one catalytic cycle, poly-
mer anchored metal complexes 5 and 6 were filtered off
and washed with DMF followed by methanol, and dried
ꢀ
in an oven under 120 C for 24 hr. Recycled catalysts
were used multiple times in the catalytic oxidative bromi-
nation of salicylaldehyde and oxidation of thioanisole
FIGURE 14 Comparative plot of % conversion of
salicylaldehyde by using catalysts 3, 4, 5 and 6 with blank reaction
SCHEME 3 Oxidation of thioanisole by the catalysts in the
presence of oxidant H2O2

18 of 21
MAURYA ET AL.
TABLE 5 Results of oxidation of Thioanisole catalyzed by catalyst 6. Conversion (%) and selectivity (%) of products obtained at different
parameters in 4 hr at room temperature
% Selectivity
S.
N.
Cat.
Cat. (mg)
Substrate:
Oxidant
Solvent
(ml)
Time
Solvent (hr.)
%
Conv.
TOF/h
Sulfoxide Sulfone
1
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
3
4
5
-
20
20
20
20
10
15
20
25
30
20
20
20
20
20
20
20
20
20
20
20
20
20
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:1
2:3
2:5
2:5
2:5
2:5
2:5
2:5
2:5
2:5
2:5
2:5
2:5
10
10
10
10
10
10
10
10
10
10
10
10
4
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
EtOH
DMF
Hexane
EtOH
EtOH
EtOH
EtOH
1
2
3
5
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
44.4
58.0
64.9
69.2
49.9
55.7
68.9
68.5
67.4
20.0
38.2
85.3
85.5
87.0
87.5
90.7
78.6
72.3
75.2
84.5
81.1
32.5
5.422 × 102 56.88
3.541 × 102 54.01
2.642 × 102 51.48
1.690 × 102 63.06
3.047 × 102 58.88
2.267 × 102 65.31
2.103 × 102 67.42
1.673 × 102 67.79
1.371 × 102 65.48
6.106 × 101 79.44
1.166 × 102 73.94
2.604 × 102 64.37
2.610 × 102 64.20
2.656 × 102 64.42
2.671 × 102 64.21
2.769 × 102 79.58
2.399 × 102 78.71
2.207 × 102 75.10
1.363 × 101 74.60
1.606 × 102 79.48
5.786 × 102 84.55
43.11
45.98
48.51
36.93
41.11
34.68
32.57
32.21
34.51
20.55
26.05
35.62
35.79
36.54
35.78
20.41
21.28
24.89
25.39
20.51
15.44
12.4
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
7
13
4
4
4
4
4
4
4
-
87.59
heterogeneous nature. Recycled catalysts are also charac-
terized extensively through various techniques like FT-
IR, UV–Vis, SEM, EDX, etc. Figure 3 and Figure S5 pre-
sent the FT-IR spectra of recycled catalysts. All the
important characteristic stretching vibrations are pre-
served in the FT-IR spectra of the recycled catalysts
suggesting the existence of catalysts inside the polymer
chain. Comparable UV–Vis spectral pattern of the
recycled catalysts to that of fresh catalysts (shown in Fig-
ure S2) supports the FT-IR analysis. Unchanged physical
form and surface morphology were seen in the SEM
images (shown in Figure S14 and S15) of the recycled cat-
alysts even after two cycles of catalysis. Vanadium map-
ping (shown in Figure S14 and S15) on the surface of
recycled catalysts confirms the presence of vanadium
complexes in the chloromethylated polystyrene beads.
Although, the observed vanadium atom density is slightly
less in recycled catalysts in comparison to the fresh cata-
lysts. This is because of the inherent inhomogeneity asso-
ciated with the process of heterogenization of vanadium
complexes with the individual polymer beads as well as
FIGURE 15 Comparative plot of oxidation of thioanisole
under optimized reaction conditions in the presence of catalysts 3,
4, 5 and 6 along with blank reaction
under optimized reaction conditions, and the related data
is presented in Figure S18. Observed substrate conversion
(%) in the presence of recycled catalysts is well within the
range of experimental errors that support their

DIMERIC μ-OXIDOVANADIUM COMPLEXES AS EFFICIENT MODELS OF VBPO'S
19 of 21
the removal of few surfaces adsorbed metal complexes
during the course of the catalytic reaction.
TGA/DTA and EPR analysis. The authors acknowledge
CRF IIT (ISM) Dhanbad for providing single-crystal
XRD and AFM analysis. Authors are thankful to NMR
Research Centre (SIF) Indian Institute of Science Ban-
galore for 51V NMR analysis. The authors would like to
thank DST-FIST (project no. SR/FST/CSI-256) for
400 MHz NMR facility in the department of chemistry
IIT (ISM) Dhanbad. GC used in this study was pro-
cured from the grant given by Science and Engineering
Research Board (SERB), Department of Science and
Technology (DST), grant no SB/FT/CS-027/2014), Gov-
ernment of India, New Delhi, India, to C. H. Special
thanks to Dr. H. P. Nayek, Department of Chemistry
for solving the single crystal XRD data.
9 | CONCLUSIONS
Monomeric vanadium (IV) complexes [VIVO(aeae-sal)
(MeOH)]+ (1), [VIVO(aeae-hyap)(MeOH)]+ (2) and
dimeric vanadium(V) dioxido complexes [{VVO(aeae-
sal)}μ-O]2 (3), [{VVO(Haeae-hyap)}μ-O]2 (4) were synthe-
sized and characterized in details by various characteriza-
tion techniques. Vanadium (IV) oxido complexes are
susceptible to molecular oxygen in solution and can
quickly be oxidized into vanadium(V) dioxido complexes.
The theoretical study, solution-phase UV–vis spectra and
FT-IR spectral study of the supported vanadium com-
plexes prove the monomeric nature of vanadium(V)
dioxido complexes in solution. Both the complexes 3 and
4 isolated as a dimer in solid-state which is supported by
Single-crystal XRD analysis. Catalysts 3 and 4 efficiently
catalyze the oxidative bromination of salicylaldehyde and
oxidation of thioanisole in presence of hydrogen perox-
ide. Anchoring the complexes 3 and 4 into the polymer
chain enhances the catalytic efficiency many folds for
both the catalytic reactions. Under optimized reaction
conditions for the oxidative bromination of
salicylaldehyde, catalyst 5 shows 98.8% substrate conver-
sion with a TOF value of 1.127 × 104 h−1 whereas 95.7%
conversion was achieved by 6 with a TOF value of
4.675 × 104 h−1, which is quite high in terms of available
supported vanadium catalyst in the literature. Whereas
for the oxidation of thioanisole, catalysts 5 and 6 exhibit
CONFLICT OF INTEREST
There are no conflicts of interest to declare.
ORCID
Abhishek Maurya https://orcid.org/0000-0001-6977-
9532
Chanchal Haldar https://orcid.org/0000-0003-4642-
7918
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× ) and 90.7% (TOF
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thioanisole.
ACKNOWLEDGEMENTS
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How to cite this article: Maurya A, Mahato AK,
Chaudhary N, et al. Synthesis and characterization
of dimeric μ-oxidovanadium complexes as the
functional model of vanadium bromoperoxidase.
Appl Organometal Chem. 2020;e5508. https://doi.
org/10.1002/aoc.5508