Bioinspired catalysis and bromoperoxidase like activity of a multistimuli-responsive supramolecular metallogel: Supramolecular assembly triggered by pi-pi stacking and hydrogen bonding interactions

Article abstract of DOI:10.1039/c9nj05732c

We report the synthesis and characterization of a new self-assembled VO₂-L metallogel. Gel formation was investigated by dissolving VO₂-L in various solvents and it was found that water/methanol (1?:?9 (v/v) ratio) induces gel formation. The single crystal X-ray structure of the VO₂-L metallogel exhibits C-H?O and N-H?O hydrogen bonding interactions and pi-pi stacking. The VO₂-L xerogel obtained after removing the solvents was found to exhibit outstanding performance in catalysis. Bromoperoxidase-like activity of the VO₂-L metallogel was also reported. The present catalytic studies are simple and proceed under mild conditions.

Full text of DOI:10.1039/c9nj05732c

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Bioinspired catalysis and Bromoperoxidase like activity of Mu
D
lt
O
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I:
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9NJ05732C
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Responsive Supramolecular Metallogel: Supramolecular assembly
triggered by pi-pi stacking and hydrogen bonding interactions
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Sunshine Dominic Kurbah and Ram A. Lal
Centre for Advanced Studies, Department of Chemistry, North-Eastern Hill University,
Shillong-793022, India.
ABSTRACT
We report the synthesis and characterization of new self-assemble VO -L metallogel.
2
Gel formation was investigated by dissolving VO -L in various solvents and it was found that
2
water/methanol (1:9 (v/v) ratios) induces gel formation. Single X-ray crystal structure of
VO -L metallogel exhibits C-H---O, N-H---O hydrogen bonding interaction and pi-pi
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stacking. The VO -L Xerogel obtained after removing the solvents were found to exhibit
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outstanding performance in catalysis. Bromoperoxidase-like activity of VO -L metallogel
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was also reported. The present catalytic studies are simple and proceed under mild condition.
Keywords: Pi-pi stacking, metallogel, catalysis and bromoperoxidase
INTRODUCTION
Oxidative bromination of organic molecules constitutes one of the most important
and fundamental research in organic chemistry, because of its precursors for various
transformations employed in organic and pharmaceutical synthesis [1-3]. Conventional
method for bromination of organic substrates utilized toxic and hazardous elemental
bromine, which is both health hazard and harmful from environmental point of view, in
which half of the bromine ends up as hydrogen bromide wastes [4-6]. Alternative safer
method for oxidative bromination which are less harmful and toxic can be developed by
using suitable brominating agents such as, N-bromoacetamide (NBA), N-bromosuccinimide
(
NBS), or bromodimethylsulfonium bromide [7, 8]. However, these protocols find many
drawbacks due to high cost and generation of organic waste associated with these reagents.
However, re-oxidation of HBr using hydrogen peroxide can solve this problem.
Hence, for developing new and efficient method that are environmentally and
economically favorable, efforts have been made using bromide ion as a bromide source
instead of bromine. Using various oxidants such as hydrogen peroxide [9, 10], cerium
ammonium nitrate [11, 12], oxone [13], lead tetra-acetate [14], and sodium periodate [15, 16]
in presence of tetrabutylammonium bromide or alkali metal bromide has been shown to

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DOI: 10.1039/C9NJ05732C
enhance the oxidative bromination. Additionally, mimicking bromination reaction of a
naturally occurring enzyme vanadium bromoperoxidase found in marine algae has attracted
much attention in the past years [17-19]. In the presence of H O , bromoperoxidase catalyzes
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two-electron oxidation of bromide ion, forming a bromonium-like species, which induces the
bromination of organic compounds [20, 21]. Metals like vanadium [22-24], tungsten [25],
molybdenum [26] and rhenium [27] have been used as a bromination catalyst in the presence
of stoichiometric amount of hydrogen peroxide. On the other hand, using molecular oxygen
as a terminal oxidant in place of a strong oxidant have been reported by few groups by
developing the more advanced catalytic systems rather than the enzyme for bromination
reactions [28, 29].
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Furthermore, supramolecular gels have potential applications in catalysis [30].
Moreover, when the catalyst is immobilised within the network, the reagents can diffuse into
gels, whereas the products diffuse out. Supramolecular gels can also provide catalyst
longevity, protection from product contamination and easy separation [30]. Hence, it has
been challenges to design and develop a simple catalytic system that can both remediate
waste, and subsequently act as active catalysts. In this paper, we report the oxidative
bromination of some organic substrate using newly synthesized VO -L metallogel derived
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from hydrazone ligand in presence of hydrogen peroxide.
EXPERIMENTAL METHODS
Materials
Solvents were reagents grade purchased from Himedia. Other chemicals such as aniline,
acetophenone, 2-nitrophenol, 2-hydroxyacetophenone, 2-hydroxybenzaldehyde, 4-
methoxyacetophenone,
2-hydroxyethylbenzoate,
3-methylacetophenone,
4-
methylacetophenone and 4-chlorophenol were purchased from Sigma Aldrich and were used
as received without any further purification. Vanadium pentoxide and other metal salts were
purchased from Himedia and were used as received.
Single X-ray crystallography
Single crystal X-ray data were collected at 291.8 K, using Xcalibur, Eos, Gemini
diffractometer equipped with a monochromated Mo K radiation ((λ = 0.71073 Å). The
CrysAlis PRO; Agilent, 2013 software packages were used for data collection and reduction.
The intensity data were corrected using semi empirical absorption method. In all cases
absorption corrections based on multiscan using SADABS software were applied [31]. The
crystal structure was solved and refined using SHELXL-2014 [32]. All non-hydrogen atoms

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DOI: 10.1039/C9NJ05732C
were refined anisotropically, whereas the hydrogen atoms were place at a calculated position
and refined in the final refinement.
Scanning Electron Microscope (SEM)
The morphologies of the synthesized VO -L metallogel were characterized using a
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scanning electron microscope (JSM-6360, Joel, with oxford EDS detector) operating at 1-30
kV. For sample preparation, diluted sample of VO -L metallogel was put into the thin
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aluminum sheet by using capillary tube and then allowing it to dry in air. The sample was
also coated with a thin layer of Au before the experiment to minimize sample charging.
Procedure for synthesis of complex (VO -L)
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A suspension of vanadium pentoxide (0.2 g, 1 mmol) in methanol solution was added
dropwise to a methanol solution 20 ml containing the ligand (1 mmol). The reaction mixture
was refluxed for 30 min at 70 °C. The reaction mixture was allowed to stand at room
temperature, filtered, washed with warm methanol and dried to obtain the desired complex.
Single crystal of complex was successfully obtained by slow evaporation of Water/DMF
1
solution at ambient temperature. Colour: yellow; M.p: >300 °C; H NMR (400 MHz,
DMSO-d , Me Si): δ (ppm): 8.22-8.32 (d, 2H, H-C=N), 6.85-7.78 (m, 9H, Ar-H).
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Typical procedure for the synthesis of VO -L Metallogel
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In a typical gel formation reaction, a required amount of VO -L was dissolved in 2
2
mL of water/methanol (1:9; v/v) in a 5 mL vial and sonicated for 5 min to get a clear
solution. The initial brown solution turned into either yellow or green. After that, the solution
was kept undisturbed until the gel was formed. Then, the formation of gel was confirmed by
the glass vial inversion method at room temperature (Scheme 2).
General experiment for oxidative bromination
In a 50 mL round bottom flask equipped with a magnetic stirring bar, substrates (10
mmol) were added. To this solution, KBr (40 mmol) dissolved in 20 mL water was added
and then dissolved by stirring at ambient temperature followed by addition of a 15% H O (4
2
2
mmol). The reaction mixture was stirred and VO -L metallogel (1 mmol) and 50% HClO (4
2
4
o
o
mmol) were added at 0 C for 10 minutes and then at 24 C. An additional 4 mmol of 50 %
HClO was further added in three equal portions after every 10 minutes with continuous
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stirring. All the reactions were monitored using thin layer chromatography.
Typical procedure for catalyst separation
At the end of every reaction, the aqueous solution was removed on a rotary
evaporator. Then solid were washed with water to remove the VO -L metallogel, extracted
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with CH Cl , dried over anhydrous MgSO and then evaporated to afford the desired
DOI: 10.1039/C9NJ05732C
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products. The NMR spectrum was recorded using the isolated product directly. The catalyst
can be used for the next cycle of reactions without any decrease in products formation.
RESULTS AND DISCUSSION
Synthesis and characterization
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The VO -L was synthesized by reaction of the ligand with vanadium pentoxide in 2:1
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molar ratios in methanol (Scheme 1). VO -L is soluble in methanol, ethanol, acetonitrile,
2
DMF and DMSO. Alternatively, VO -L was also synthesized using methanol/water as a
2
1
solvent. The H NMR spectral data of the ligand and VO -L were recorded using DMSO-d
2
6
1
and the data are given in the experimental section (Fig S7, S8). The H NMR spectra of the
ligand exhibits resonances at δ = 12.15 and 12.17 ppm which are assigned to -OH proton
attached to phenyl ring, the signal at δ = 11.45 is assigned to -NH proton, both these peaks
disappeared upon complexation, confirming the ligand coordinate to metal ions in
deprotonated form. The aromatic proton appeared at the expected position in the ranges δ =
6.85-8.32 ppm.
O
O
OH
O
V
O
O
V O
2
5
N
N
N
Br
N
Br
H
Methanol, reflux
Scheme 1. Schematic diagram showing the preparation of VO2-
Molecular structure
The molecular structure of VO -L was determined by Single Crystal X-ray
2
crystallography (Fig 1). The crystal data and structure refinement parameters for VO -L are
2
given in Table 1. Selected bond lengths and bond angles are presented in Table 2. VO -L
2
crystallized in monoclinic P2 /c space group. The ligand coordinates to metal ion in
1
tridentate fashion through -ONO- donor atom at the basal position, together with two oxido
ligand at the terminal position forming penta-coordinate geometry around the metal center.
The metal bond distances in VO -L are 1.889(9) Å (V1-O1), 1.957(8) Å (V1-O2), 1.648(7) Å
2
(
V1=O3), 1.571(8) Å (V1=O4), 1.884(9) Å (V2-O5), 1.961(8) Å (V2-O6), 1.661(8) Å
(V2=O7), 1.562(8) Å (V2=O8), 2.128(10) Å (V1-N1) and 2.117(9) Å (V2-N3). The
o
o
geometrical bond angles around the vanadium ions are 73.9(3) (O2-V1-N1), 91.8(4) (O3-
o
o
o
o
V1-O2), 96.0(4) (O3-V1-O1), 144.2(4) (O3-V1-N1), 149.0(4) (O1-V1-O1), 82.5(4) (O1-
o
o
o
o
V1-N1), 105.7(4) (O4-V1-O2), 107.6(5) (O4-V1-O3), 100.4(5) (O4-V1-O1), 107.7(4)

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(
O4-V1-N1), 91.5(4) (O7-V2-O6), 96.3(4) (O7-V2-O5), 142.0(4) (O7-V2-N3), 74.1(4)
DOI: 10.1039/C9NJ05732C
(O6-V2-N3), 108.4(5)o (O8-V2-O7), 103.9(5)o (O8-V2-O6), 100.1(5)o (O8-V2-O5),
o
o
o
1
09.2(4) (O8-V2-N3), 150.9(4) (O5-V2-O6) and 82.8(4) (O5-V2-N3). Their geometrical
o
o
trans angle in VO -L are 144.2(4) (O3-V1-N1) and 149.0(4) (O1-V1-O2) around V1, and
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42.0(4) (O7-V2-N3) and 150.9(4) (O5-V2-O6) around the vanadium ion (V2). Their
corresponding geometrical index are τ = 0.08 and τ = 0.15. Hence, vanadium atom adopted
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a distorted square pyramidal geometry with slight distortion.
Fig 1. ORTEP diagram of VO -L with 50% probability
2
Table 1. Crystal data and structure refinement
Empirical formula
Formula weight
Temperature/K
Crystal system
C H BrN O V
18 21 3 4
474.23
297.3(5)
monoclinic
Space group
P2 /c
1
a/Å
b/Å
c/Å
α/°
β/°
γ/°
Volume/Å
Z
8.371(3)
22.101(10)
22.370(14)
90
90
90
4138(4)
8
3
3
ρ g/cm
1.522
calc

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DOI: 10.1039/C9NJ05732C
Radiation
Reflections collected
Data/restraints/parameters 7238/0/491
Goodness-of-fit on F2
Final R indexes [I>=2σ (I)]R = 0.1055, wR = 0.2495
Final R indexes [all data] R = 0.1943, wR = 0.2975
Mo K α (λ = 0.71073)
12650
1.057
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Table 2. Selected experimental parameters obtained by X-ray crystallography and calculated
for VO -L
2
Parameters
Experimental
Theoretical (B3LYP)
6
-31G+(d, p): 6-31G+(d, p):
6-31G+(d, p):
SDD
LANL2DZ
LANL2MB
Bond lengths
V1-O1
V1-O2
V1-O3
V1-O4
1.889(9)
1.957(8)
1.648(7)
1.571(8)
2.128(10)
1.898
2.078
1.614
1.618
2.248
1.919
2.096
1.641
1.632
2.231
1.895
2.079
1.602
1.598
2.274
V1-N1
Bond angles
O2-V1-N1
O3-V1-O2
O3-V1-O1
O3-V1-N1
O1-V1-O2
O1-V1-N1
O4-V1-O2
O4-V1-O3
O4-V1-O1
O4-V1-N1
Dihedral angle
N2-N1-C7-C1
N1-N2-C8-C9
O2-C8-C9-C10
N1-C7-C1-C6
C6-C1-C2-O1
O1-C2-C3-C4
C7-N1-N2-C8
V1-O2-C8-C9
V1-O2-C2-C3
73.9(3)
71.606
71.875
98.823
106.103
148.283
144.683
79.528
92.113
110.055
102.108
99.568
71.032
92.640
101.617
146.149
142.807
78.416
91.8(4)
92.022
96.0(4)
101.279
143.034
145.833
79.164
144.2(4)
149.0(4)
82.5(4)
105.7(4)
107.6(5)
100.4(5)
107.7(4)
98.407
99.682
110.132
106.060
105.029
110.496
106.907
101.570
177.7(10)
-178.0(10)
173.2(11)
-178.6(12)
172.5(11)
-174.4(13)
179.2(10)
171.8(8)
179.213
-179.812
167.233
-177.427
177.878
-178.642
177.915
172.841
-154.966
177.464
-179.341
169.940
-176.818
178.195
-178.642
179.925
171.327
-155.597
179.113
-179.846
165.314
-177.411
177.709
-178.197
178.530
170.754
-151.758
-155.4(10)
Molecular packing diagram
The molecular packing involves C-H---O, N-H---O hydrogen bonding interaction and
pi-pi stacking between the aromatic rings. The combined effect of which is to form a layer
structure parallel ladder like structure. The N-H---O molecular synthon are parallel to each
other on both side of pi-pi stacking forming as zig-zag 1D dimensional arrangements (Fig

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A). Moreover, the presence of weak halogen bonding (Br…Br) generates a self-assembly
DOI: 10.1039/C9NJ05732C
molecular structure in the structural unit of the VO -L (Fig 2B). The diethyl-ammonium
2
hydrogen provides a crosslink by acting as a donor toward the Oxido-ligand bonded to
vanadium ions. The bond lengths are N(5) --H(5A) ..O(7) 2.761(12) Å, N(5) --H(5B)
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.
.O(3) 2.770(12) Å, N(6) --H(6A) ..O(3) 2.733(12) Å, N(6) --H(6A) ..O(3) 2.733(12) Å
and N(6) --H(6B) ..O(7) 2.752(11) with geometrical bond angle between the donor and
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acceptor 171 , 153 , 165 and 160 . The distances between the two aromatic rings forming
pi-pi stacking interaction is 3.786 Å and Br…Br interaction is 3.578 Å.
Fig 2A. Zig-zag 1D dimensional arrangements and ladder like structure connected through
hydrogen bonding interaction and pipi-stacking.

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0
1
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9
0
1
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0
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0
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8
9
0
1
2
3
4
5
6
7
8
9
0
Fig 2B. Supramolecular self-assembly form through hydrogen bonding and weak halogen
bonding interaction (Br…. Br).
Computational studies
Theoretical studies were performed in gas phase using Density functional theory
(
DFT) with hybrid functional B3LYP with different basis set for metal ions (LANL2DZ,
LANL2MB and SDD) and 6-31+G(d,p) basis set for carbon, hydrogen, oxygen, nitrogen and
bromine atoms implemented in using GAUSSIAN 09 program [35]. The geometrical bond
lengths and bond angles of the optimized structures using different basis set are presented in
Table 2. The theoretical bond lengths are 1.895-1.919 Å (V1-O1), 2.078-2.096 Å (V1-O2),
1.602-1.641 Å (V1-O3), 1.598-1.632 Å (V1-O4) and 2.231-2.274 Å (V1-N) with geometrical
o
o
trans angles 143.034-148.283 (O3-V1-N1), 142.807-145.833 (O1-V1-O2). The calculated
o
o
torsion angles are 177.464 - 179.213 (N2-N1-C7-C1), 165.314 - 169.940 (O2-C8-C9-C10),
o
-176.818 - -177.427 (N1-C7-C1-C6). Hence, the experimental geometrical parameters and
theoretical results show satisfactory agreement. As can be seen from the bond lengths and
bond angles the results obtained from theoretical slightly different from the experimental
ones. Thus, most of the calculated parameters are slightly higher compared experimental
values. It can be noted that the theoretical results depend on the isolated gaseous state,
whereas in solid state the presence of inter molecular interactions connected the molecules
together, which results in the differences of geometrical bond lengths and bond angles
between the experimental and the calculated values [36]. The charge distributions of VO -L
2

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are given in Table 3, whereas the contour plot for highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) are presented in Fig 3. The
charged distribution shows that the carbon atom attached to electronegative atoms such as
nitrogen and oxygen are positive, whereas oxygen, nitrogen and other carbon atoms are
negative. The computed charged for metal ions using different basis sets are 0.709, 0.773 and
1.539, which are lower than the expected formal charged of +5. The difference arises due to
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
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3
3
3
3
3
3
4
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4
5
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5
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0
1
2
3
4
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6
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8
9
0
1
2
3
4
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9
0
1
2
3
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8
9
0
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2
3
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9
0
1
2
3
4
5
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7
8
9
0
significant electron transfer process involved from the donor atoms such as O , O
and
oxo
phenoxo
nitrogen (azomethine) towards central metal ions. Similarly, the central metal atom has d0
electronic configuration but orbital occupations shows that the population state of d , d ,
xy
xz
2
2
and dz2 are 0.581, 0.605, 0.666, 0.552 and 0.715, respectively. These results
d , d
yz
x -y
provide evidence about the electron transfer involved from donor atoms toward metal ion.
Fig 3. Contour plots for some selected molecular orbitals of ligand and VO -L along with
2
their relative energies of HOMOs (lower orbitals) and LUMOs (upper orbitals).
Table 3. Selected Mulliken atomic charges of ligand and VO -L
2
Atom
Ligand:
6-31G+(d, p):
LANL2DZ
6-31G+(d, p):
LANL2MB
-0.701
-0.637
-0.371
-0.416
0.051
1.539
6-31G+(d, p):
SDD
6
-31G+(d, p)
O1
O2
N1
N2
Br
-0.712
-0.596
-0.291
-0.461
0.037
-
-0.568
-0.537
-0.298
-0.411
0.05209
0.77283
-0.562
-0.533
-0.294
-0.408
0.052
V
0.709

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Gelation studies
From single crystal structure analysis, we noticed that the VO -L metallogel exhibits
2
pi-pi interaction and weak hydrogen bonding. Moreover, apart from these interactions, the
VO - metallogel displays weak non-covalent interaction. Encourage by these structural
2
0
1
2
3
4
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9
0
1
2
3
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9
0
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0
1
2
3
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9
0
1
2
3
4
5
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8
9
0
properties possessed by VO -L we carried out gelation experiments. The gelation
2
experiments of VO -L were carried out using “stable to inversion of a test tube” method in
2
various solvents such as water, methanol, ethanol, acetonitrile, chloroform, toluene, DMSO,
DMF, water/methanol etc. Thus, after several attempts and by changing the solvents, gelation
in water/methanol (1:9 (v/v) ratios) was observed. We also carried out, a control experiment
to test the gelation behaviour of the VO -L metallogel and found that it could also gel in
2
water/methanol (1:9 (v/v) ratios) and the gelation behaviour of the VO -L (1:9 (v/v) ratios)
2
was thermo-reversible. The VO -L metallogel exhibits excellent gelation abilities and turned
2
out to be quite efficient displaying 1.3 wt % minimum gelator concentration, with gel-sol
dissociation temperature T , of 58 °C (water/methanol) measured by using the ‘tilted tube’
gel
method (Fig 4).
Fig 4. Gelation test carried out using different solvents combination
During the metellogel formation, various interactions present in the structural units
and crystal lattice of the VO -L metallogel such as, non-covalent interaction and pi-pi
2
stacking interactions between aromatic rings might have played crucial roles and act in
tandem to stabilize the gel structure. It may also be noted that the hydrogen bonding between
solvents molecules and gelator is very importance for molecular self-assembly required for
gelation. FT-IR spectroscopy was performed to identify the nature of participation of
functional groups present in the ligand and in their corresponding metallogel. The ligand

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exhibits a broad band at 3448, 3270 and 1663 cm which are assigned to the phenolic OH
-1
and C=O group. Whereas, VO -L metallogel the peaks at 3448, 3270 and 1663 cm were not
2
observed, indicating that the ligand coordinated to metal ions in deprotonated form. VO -L
2
-1
metallogel shows a strong band at 1525 cm and is assigned newly NCO-group. However,
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-1
-1
the IR spectrum of VO -L metallogel show new band at 587 cm and 487cm . These bands
2
can be assign to v(M-O)(phenolic) and v(M-O)(carbonyl), respectively. The VO -L
2
metallogel exhibit strong bands in the range 909-941 cm-1 and are assigned to V=O
vibrations. These observations were consistent with that obtained from the IR performed with
the crystalline form. Moreover, the morphological features of the gel were studied using
SEM which shows a porous morphology inherited void space in the interlaced gel matrix
(
Fig. S11). The smaller particles packed up together into larger particles, with pores in
between and tend to remain in the most thermodynamically favourable state. These small
particles upon aggregation form larger particles which further form typical metallogel
architecture. The presence of H-bonding and p-p stacking in gel are supported by single X-
ray diffraction. The experimental XRD patterns of the xerogel are in good agreement with
simulated XRD patterns from single crystal X-ray diffraction results as shown in Fig. S9. The
simulated pattern of the VO2-L was calculated from single crystal structure data (cif) using
CCDC Mercury software. In order to understand the stiffness of aggregation, rheological
ʹ
measurement of the gel was also studied. The plot of elastic modulus (G) and viscous
modulus (Gʺ) against sheer stress at a constant frequency of 1 Hz are shown in Fig. S10a and
ʹ
Fig. S10b. The plot shows that above a critical stress, G and Gʺ decreases. The critical stress,
i.e. yield stress of the hydrogel, was found to be of about 312 Pa indicating its high
ʹ
mechanical strength. During the measurement, it was observed that elastic modulus (G) and
viscous modulus (Gʺ) are almost parallel to the frequency imposed. This confirms that the
material switches to the true gel state and results agree well with classical gels.
Stimuli-Responsive Metallogel
Stimuli responsive behaviors of supramolecular metallogels to various physical
factors such as light, temperature, sound and force are known up to date. Similarly, the
responsive behavior of these metallogels are not only restricted to physical factors but to
chemical factors, stimuli toward ligand, pH and anions. Subsequently, the responsive
properties of VO -L metallogel toward ions and molecules were investigated. In the first case
2
the ions sensing properties was investigated by adding and permeating various anions such as
-
-
-
-
-
-
-
-
-
-
F , Br , I , CN , HSO , ACO , H PO , ClO , N and OH into VO -L metallogel. As shown
4
2
4
4
3
2

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1
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1
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2
2
2
2
2
2
2
2
3
3
3
3
3
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3
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in Fig 5B, VO -L metallogel displayed blue colour fluorescence upon addition of OH anions.
2
The fluorescence intensity enhances gradually and showed a maximum emission peak at 475
-
nm (Fig 5A). Therefore, VO -L metallogel could sense OH anions with specific selectivity.
2
From the fluorescence titration profile, the association constant was found to be 1.68 × 104
0
1
2
3
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9
0
1
2
3
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9
0
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0
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9
0
1
2
3
4
5
6
7
8
9
0
-1
M , calculated using Benesi-Hildebrand plot. Hence, the detection limit of VO -L metallogel
2
-
-8
-1
for OH was 1.78 × 10 mL which was obtained from fluorescence titration (Figure 5C) and
-
indicates high detection sensitivity for OH anions.
-
Fig 5. Fluorescence spectra of (A) VO -L metallogel with increasing concentrations of OH
2
(
B) Fluorescence responses of VO -L metallogel in the presence of various anions (C)
2
Benesi-Hildebrand plot for calculating the association constant.
We also introduced various metal ions by adding and diffusing into VO -L
2
+
+
+
2+
2+
3+
3+
2+
2+
2+
2+
+
2+
metallogel, including Na , K , Cs , Mg , Ca , Cr , Fe , Co , Ni , Cu , Zn , Ag , Cd ,
2
+
3+
Hg and Al . The addition of these metal cations does not display any fluorescence
properties. The reactivity of VO -L metallogel toward other molecules such as HCl and H O
2
2
2
were also carried out. During the addition of HCl the VO -L metallogel collapse and show
2
light yellow color solution without retaining its original shape, whereas during the addition of
hydrogen peroxide the color of the gel turn to red color solution (Fig 7). Further evidence of
this transition was obtained by measuring UV-vis spectral changes (Fig. 6A). It was observed
that the absorption band at 392 nm is the characteristic of ligand to metal charge transfer
(
MLCT). Addition of H O (1 equivalent) to the gel solution increases the band intensity at
2 2

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2
80 nm, whereas the absorption band at 392 nm decreases its intensity, respectively. Further,
DOI: 10.1039/C9NJ05732C
after addition of the H O solution, a new band at 330 nm appeared and two isosbestic points
2
2
at 347 nm and 320 nm was observed. The results obtained may be interpreted due to the
formation of peroxovanadium species [33]. During the course of the reaction the solution
1
1
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1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
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4
4
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5
5
5
5
5
5
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5
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0
1
2
3
4
5
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9
0
1
2
3
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9
0
1
2
3
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9
0
1
2
3
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9
0
1
2
3
4
5
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8
9
0
changes it colour from yellow to red. Hence, VO -L metallogel can be used for colorimetric
2
detection of hydrogen peroxide. Similarly, reaction of VO -L metallogel with HCl were also
2
studied using UV/Vis spectroscopy (Fig 6B). During the addition of an acid to the gel
solution (HCl, 1 equivalent), the band at 312 nm increases its intensity, whereas the
absorption band at 392 nm decreases its intensity with the presence of two isosbestic point at
360 nm and 425 nm. Hence, these results obtained correspond to the formation of oxo-
hydroxo species as reported in the literature [34].
Fig 6. Spectral changes observed during the reaction VO -L metallogel with (A) HCl and (B)
2
H O
2
2

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0
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9
0
1
2
3
4
5
6
7
8
9
0
Fig 7. Reaction of VO -L metallogel towards hydrochloric acid and hydrogen peroxide
2
Bromoperoxidase-like activity
In this article, we also explore the catalytic studies for oxidative bromination of some
organic substrates using solid state structure and gel form. To achieve the best optimum
condition for oxidative bromination reactions, we carried out the reaction using phenol red as
the model substrate (Table S1). Using VO -L metallogel, as the pre-catalyst, the reaction was
2
tested at different amounts of catalyst/substrate ratios and it was found that 1:10 molar ratios
of VO - metallogel /substrates gave the optimum yield. Using VO -L metallogel /substrate
2
2
1
:10 molar ratios, the effect of bromides sources was also studied and found that VO2-
metallogel /substrate/KBr in 1:10:40 molar ratios gave the best yield. The effect of hydrogen
peroxide was also evaluated and found that 1:10:40:40 molar ratios of VO -L metallogel
2
/
substrate/KBr/hydrogen peroxide gave the maximum yield. The effects of acid for the
oxidative bromination reaction were also studied and it was found that 4 molars of HClO4
gave maximum yield (Table S1, entry 11). From the above studies, we concluded that the
present of acid is essential for catalytic bromination reaction. The oxidative bromination
reaction were also carried out using different vanadium species such VOSO , V O , NaVO
4
2
5
4
and vanadyl acetylacetonate, but it was found that VO -L metallogel gave the high yield.
2
Similarly, the reactions were also carried out in wide varieties of solvents and it was found
that the reaction proceeds smoothly in aqueous solution.
Therefore, in order to test the versatility of the catalyst, we carried out the catalytic
oxidative bromination reaction using various organic substrates. Some of the selected results

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are listed in Table 4. The bromination reaction of phenol red gave 89% yield of the product
phenol blue within 45 min and salicylaldehyde gave 90% yield of 5-bromo salicylaldehyde in
0 min (Table 4, entry 1, 2). Bromination reaction of aniline was also carried out successfully
4
giving p-bromoaniline with 87% yield as the only product within 30 min (Table 4, entry 4).
The reactions were also carried out using 2-nitro phenol and ethyl salicylate as the substrates
giving the product 5-bromo-2-nitro phenol with 90% yield and 5-bromoethyl salicylate with
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
8
6% yield as the brominated product (Table 4, entry 5, 8). Inspired by these results, we
hypothesized that the α-bromoketone might be obtained using VO -L metallogel and
2
acetophenone as the substrate. In our study, we found that bromination of acetophenone gave
α-bromoacetophenone as the only product with 88% yield within the time period of 35 min
(
Table 4, entry 6). In order to widen the substrates scope bromination reaction of different
substituted α-bromoketone was also carried out, such as 2-hydroxy acetophenone gave the
product α-bromo-2-hydroxy acetophenone with 85% yield within 25 min (Table 5, entry 3)
lower than that of unsubstituted acetophenone. Bromination reaction of p-methoxy-
acetophenone gave the product α-bromo-p-methoxy-acetophenone with 90% yield in 30 min,
higher yield is due to the electron donating nature of methoxy group present in the para
position of the benzene ring (Table 4, entry 7). Other substituted ketones such as 4-methyl-2-
hydroxy- acetophenone and 5-methyl-2-hydroxy- acetophenone were also smoothly mono-
brominated to α-bromoketone in high yields (90-93%, Table 4, entry 9, 10). The present
catalytic approach provides efficient and simple access to α-bromoketones.
Table 4. Self-assembled metal-organic metallogel catalyzed oxidative bromination under
mild condition
Br
OH
OH
O
O
O S
O
O
S
O
Br
Br
OH
OH
Br
1a
2
a, 89%
Entry
Alcohol
Product
Time Yield (%) TON
min) (isolated)
TOF
(min-1)
(
Br
40
90
900
22.5
O
O
1
OH
OH
1
b

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2
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2
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2
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3
3
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3
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3
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2
b
O
O
25
85
850
34
Br
NH2
O
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
OH
NH2
OH
1
c
2c
30
40
87
90
870
900
29
3
4
Br
Br
1
d
2
d
O
22.5
N
O
N
O
OH
OH
1
e
2e
O
O
35
30
88
90
880
900
25.14
30
Br
5
6
1f
2f
O
O
Br
O
O
1
g
2g
O
O
35
86
860
24.57
Br
7
O
OH
O
OH
1
h
2h
O
O
40
40
91
93
910
930
22.75
23.25
Br
8
9
1
i
j
2i
2j
O
O
Br
1

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OH
Br
34
91
910
26.76
OH
1
0
Cl
Cl
1
k
2
k
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
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We also carried out a standard reaction or controlled experiments in which VO -L is
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used as catalyst and p-methoxy-acetophenone as a substrate. VO -L catalysed the oxidative
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bromination of p-methoxy-acetophenone and gave α-bromo-p-methoxy-acetophenone as the
only product with 78% yield in 45 min. Therefore, the results obtained shows that VO -L
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metallogel act as a better catalyst and gave the high yield compared simple vanadium
complex (VO -L). Based on the above studies, we proposed the following mechanism
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highlight in scheme 1. VO -L metallogel on dissolution with 15% H O gives rise to the
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formation of peroxo species. In sufficiently acidic condition bromides is oxidized forming
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HOBr, Br , Br or V-OBr species. Oxidation of bromide by hydrogen peroxide in aqueous
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solution is only thermodynamically feasible under acidic or neutral condition. In the final
step the oxidized bromide species is then trapped by organic substrates to give brominated
products as shown in Scheme 2.
Scheme 2. Possible mechanism for the catalytic studies

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View Article Online
DOI: 10.1039/C9NJ05732C
CONCLUSIONS
We have developed a novel and efficient protocol for oxidative bromination of some
organic substrates using simple VO -L metallogel as catalyst. The process is appreciable due
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to safety, simplicity, availability and cheapness of the VO -L metallogel. The whole
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protocols involved are very simple and operates under mild conditions, which makes the
present procedure very attractive and prospective. Hence, VO -L metallogel catalysed the
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peroxidative bromination of phenol, phenol derivatives and other organic substrates and its
act as a potential functional model of vanadium enzymes haloperoxidases.
ACKNOWLEDGEMENTS
Sunshine D. Kurbah would like to thank Head SAIF, North-Eastern Hill University,
Shillong-793022, India for providing NMR and SEM spectra, and UGC, New Delhi for
Research Fellowship and funding.
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