Oxidovanadium (V and IV) complexes incorporating coumarin based O^N^O ligand: Synthesis, structure and catalytic activities

Article abstract of DOI:10.1016/j.poly.2019.114241

The tridentate ligand H₂L₁, [(E)-7-Hydroxy-8-[(2-hydroxy-phenylimino)-methyl]-4-methyl-chromen-2-one] has been used in the present work towards the synthesis of mononuclear oxidovanadium complexes. Three mononuclear complexes [VOL₁(OMe)(MeOH)], 1; [VO(L₁)(8-Hq)], 2 and [VO(L₁)(1,10-phen)], 3 have been successfully synthesized with high yields by reacting [VO(acac)₂] with H₂L₁ in 1:1 ratio in methanol under refluxing conditions where 8-hydroxyquinoline and 1,10-phenanthroline were used as co-ligands in the synthesis of complex 2 and 3. X-ray crystallographic studies reveal that in all the complexes synthesized in the present study, the ligand H₂L₁ binds as O^N^O coordinating ligand. The synthesized complexes were well characterized by using UV–Vis, IR, NMR and Mass spectral techniques. The physiochemical properties have been well interpreted by density functional theory (DFT) and time dependent density functional theory (TDDFT) calculations. The synthesized complexes were established to show some distinctive properties e.g. oxidative bromination of aromatic aldehyde with high conversion rate and enhanced selectivity as well as high TON and TOF. The above properties were all well matched and demonstrated by using UV–visible and fluorescence as well as quenching studies. Complex 1 reacts with 3,5-DTBC catalytically in presence of molecular oxygen to generate corresponding ortho-benzoquinone.

Full text of DOI:10.1016/j.poly.2019.114241

Journal Pre-proofs
Oxidovanadium (V and IV) complexes incorporating coumarin based O^N^O
†
ligand: synthesis, structure and catalytic activities
Mitali Majumder, Kajal Krishna Rajak
PII:
S0277-5387(19)30686-2
DOI:
https://doi.org/10.1016/j.poly.2019.114241
POLY 114241
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
22 August 2019
6 November 2019
7 November 2019
Please cite this article as: M. Majumder, K. Krishna Rajak, Oxidovanadium (V and IV) complexes incorporating
†
coumarin based O^N^O ligand: synthesis, structure and catalytic activities , Polyhedron (2019), doi: https://doi.org/
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0.1016/j.poly.2019.114241
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Polyhedron
019
2
Oxidovanadium (V and IV) complexes incorporating coumarin
based O^N^O ligand: synthesis, structure and catalytic activities†
[a]
[a]*
Mitali Majumder and Kajal Krishna Rajak
Inorganic Chemistry Section, Department of Chemistry, Jadavpur University, Kolkata- 700032,
Email id: kajalrajak@gmail.com

Abstract:
The tridentate ligand H L , [(E)-7-Hydroxy-8-[(2-hydroxy-phenylimino)-methyl]-4-methyl-
2
1
chromen-2-one] has been used in the present work towards the synthesis of mononuclear
oxidovanadium complexes. Three mononuclear complexes [VOL (OMe)(MeOH)], 1;
1
[
VO(L )(8-Hq)], 2 and [VO(L )(1,10-phen)], 3 have been successfully synthesized with high
1 1
yields by reacting [VO(acac) ] with H L in 1:1 ratio in methanol under refluxing conditions
2
2
1
where 8-hydroxyquinoline and 1,10-phenanthroline were used as co-ligands in the synthesis of
complex 2 and 3. X-ray crystallographic studies reveal that in all the complexes synthesized in
the present study, the ligand H L binds as O^N^O coordinating ligand. The synthesized
2
1
complexes were well characterized by using UV-Vis, IR, NMR and Mass spectral techniques.
The physiochemical properties have been well interpreted by density functional theory (DFT)
and time dependent density functional theory (TDDFT) calculations. The synthesized complexes
were established to show some distinctive properties e.g. oxidative bromination of aromatic
aldehyde with high conversion rate and enhanced selectivity as well as high TON and TOF. The
above properties were all well matched and demonstrated by using UV-visible and fluorescence
as well as quenching studies. Complex 1 reacts with 3,5-DTBC catalytically in presence of
molecular oxygen to generate corresponding ortho-benzoquinone.
Keywords: Multi-dentate ligand; Oxidovanadium complex; Bromoperoxidase; DFT studies;
Catechol oxidation.

1
. Introduction:
3
2
Vanadium (3d 4s ) displays all oxidation states in the interval of -3 to +5. The
states +2 to +5 can be sustained in aqueous solution and on the whole the most stable states
are +4 and +5. The coordination chemistry of vanadium (IV) as well as Vanadium (V) with
multidentate ligand has considered the great attention of many researchers for decades.
Vanadium is an essential bio-metal which shows remarkable activities such as insulin
mimetic agent, 1-3 catechol oxidize activity, anti cancer activity and catalytic activity in
haloperoxidase 4-6 and it also possess pharmacological properties . Many marine algae 8-10,
some lichens 11-12 and fungi 13-18 use vanadium-dependent haloperoxidase (VHPO) for the
catalytic synthesis of organobromo compounds. It is assumed that the marine organisms use
7
1
0
the organobromo compounds for their self defense against predators . Vanadium
haloperoxidase actually catalyse the halogenations of the organic compounds in presence of
halide ions and peroxide under feasible reaction condition. Vanadium (V) acts as a strong
Lewis acid to activate the peroxo group, which then undergoes nucleophilic attack by
+
bromide to form the reactive “Br ” intermediate. This intermediate then reacts with organic
compounds containing alcohols or sulfide to form organobromo compounds. As per the
above fact some new flexible vanadium Schiff base compounds were synthesized and that
complexes show excellent catalytic activities and a little change from reported species.
Monobromination of salicylaldehyde with relatively high yield is reported by many research
groups but dibromination of salicylaldehyde with high yield up to 72% is rare. Herein, we
report the formation of dibromo and tribromo salicylaldehyde via catalysis by complex 2 and
3
. The oxidation of catechols to quinones in presence of molecular oxygen is an important
biochemical transformation usually catalyzed by binuclear copper. In such copper complexes
the rate of formation of quinones from catechol is in the order of 10 to 10 min-1 19-20.
Various model systems of Vanadium complexes have been published to illuminate the
mechanism of the catechol-quinone conversion. In this present work, the vanadium (V)
complexes show relatively higher rate of conversion than other vanadium containing
-2
-1
2
7
complexes have been reported .
The present work is concerned with the successful synthesis of mononuclear
oxidovanadium complexes bearing tridentate O^N^O coordinating Schiff base synthesized
from 7-hydroxy-4-methyl-2-oxo-2H-chromene-8-carbaldehyde and 2-aminophenol. The
complexes were well characterized using different spectral techniques such as UV-Vis, IR,
NMR, Mass etc. Crystal structure of two complexes has been determined. The

electrochemical behaviour of the complexes was also examined. The bromoperoxidase
activity and the catecholase were studied.
Here, we also present a full density functional theory (DFT) and time-dependent
density functional theory (TDDFT) investigation to get better insight into the geometry,
electronic structure, and optical properties of these systems, with good accuracy. Geometry
optimizations of the singlet ground-state were carried out by means of DFT calculations.
TDDFT calculations of several singlet states have been performed for better understanding
of the electronic origin of the absorption spectra.
2
. Results and Discussion
2
.1. Syntheses and characterization.
The journey towards the synthesis of tridentate (O^N^O) ligand (H L ) was
2
1
accompanied by two steps as depicted in scheme 1. Initially 4-methyl-7-hydroxy coumarin
was formylated according to the literature reported procedure.21 In the second step, the
formylated product was allowed to react with the methanolic solution of 2-aminophenol for
8
h under refluxing condition to afford the desired tridentate Schiff base ligand as an orange
coloured solid in good yield. The synthesized ligand (Scheme 1) was well characterized by
1
using H NMR, IR, Mass spectra and the data are listed in experimental section.
CH3
CH3
Glacial CH COOH
3
(
CH ) N
2 6
4
O
O
OH
Reflux, 8h
O
O
OH
CHO
OH
CH3
CH3
O
NH2
OH
O
O
MeOH
N
O
OH
Reflux,8h
OH
CHO
Ligand, H L1
2
Scheme 1: Synthesis of the ligand, H L
2
1
The complexes are synthesized by the stoichiometric reaction of Schiff base ligand and
vanadyl acetylacetonate in methanol to give complex 1. Further addition of 8-
Hydroxyquinoline and 1, 10-phenanthroline in methanolic solution of complex 1 gives

complexes 2 and 3 under refluxing condition. Scheme 2 depicts the detailed synthetic
routes towards the syntheses of all the complexes used in the present study.
Black coloured crystals suitable for single crystal X-Ray diffraction studies were
obtained with good yields (81% and 75% respectively) from the slow evaporation of the
methanolic solutions of complex 1 and 2. Despite our several attempts we were unable to
grow good quality crystals suitable for X-ray diffraction from the solution of complex 3.
CH3
O
O
O
O
OMe
V
N
,
2
OMe
H
cac
)
O
(
a
O
V
ux
efl
CH3
O
,
R
eOH
Complex 1
M
CH3
O
O
OH
VO(acac) , 8-Hq
2
O
N
O
O
O
N
OH
MeOH, Reflux
V
N
O
V
O
(
a
c
a
c
)
M
2 ,
Complex 2
1
e
,1
0
O
CH3
H
-p
h
,
Refl
e
n
u
x
8
-Hq :
O
N
N
O
O
O
N
V
OH
N
O
N
1,10-phen:
N
Complex 3
Scheme 2: Synthetic routes towards complex 1, 2 and 3
2
.2. Spectral characteristics:
IR Spectroscopy:
The IR spectra of the ligand and its corresponding metal complexes were recorded
in a KBr disk. The IR data of the ligand and the complexes were given in the
-1
-1
experimental section. The appearance of the spectral bands near 1622 cm and 1603 cm
clearly indicates the presence of imine, C=N and lactone moiety respectively. The broad
-1
absorption band at 3043 cm attributes to the stretching frequency of “free OH” moiety.
This spectral data matches satisfactorily with the synthesized ligand.
All the vanadium complexes exhibited spectral bands in the range of 1630-1620
-1
-1
cm and 1608-1603 cm arising from the imine and lactone moiety vibrations. The
-1
-1
spectral bands in the range of 1000 cm – 850 cm indicates the presence of V=O moiety
in the Vanadium (IV/V) complexes3h and the small band near 500 cm relates the
-1
2
2
-1
-1
presence of V-O moiety. The strong vibrations at 1451 cm and 743 cm are due to
ν(C-N stretch) of the co-ordinated 1,10-phenanthroline moiety in complex 3, the non-

-1
-1
coordinated ν(C-N stretch) of 1,10-phenanthroline appears at 1421 cm and 731 cm
respectively.22
NMR Spectra of ligand, complex 1 and 2:
The synthesized ligand and all the complexes except complex 3 are diamagnetic
in nature and display well resolved NMR spectra in CDCl solution and the spectral data
3
are given in the experimental section. The assigning of NMR peaks is done on the basis
of the intensity and spin–spin splitting pattern. The appearance of singlet peak at 10.26
ppm which disappears upon the addition of D O confirms the presence of phenolic
2
hydrogen. A singlet corresponding to the azomethine hydrogen atom was observed at
9
.33 ppm in the ligand, complexes 1 and 2.
EPR spectra of complex 3:
Complex 3 exhibits one unpaired electron due to the presence of vanadium in it’s
IV oxidation state having the magnetic moment 1.75 Bohr magneton (μ ). The
+
B
paramagnetism corresponds to 3dxy1 configuration. Complexes 3 exhibits hyperfine EPR
5
1
spectra owing to the V(I = 7/2) nucleus, and the simulated g values are 1.9696, which is
consistent with the existence of oxidovanadium(IV) ions in complex 3 depicted in figure
1
. The average hyperfine splitting (A) is 95.25 G and line width of 2.1 mT.40 The
calculated hyperfine splitting value 76.67 G, from DFT study is usually lower than the
experimental value.41
Figure 1: X-band EPR spectra of complex 3 in CH Cl (dichloromethane) solution at 298
2
2
K (black, experimental; red, simulated).

2
.3. Crystal structure
The molecular structures of the complexes [V O(OMe)(MeOH)(L )], 1 and
V
1
V
[
V O(L )(8-HQ)], 2, have been determined by using single crystal X-ray diffractometer.
1
The molecular structures of 1 and 2 are shown in Fig.2. The selected bond lengths and
bond angles are depicted in Table 1.
Complex 1 crystallizes in the triclinic crystal system with P-1 space group. Here
^
^
the ligand binds with the metal centre as an O N O coordinating dianionic ligand in
tridentate mode. The ligand possesses two phenoxide groups and imine nitrogen, which
are directly bound to the metal centre in a meridional fashion. The fourth coordination
site of vanadium is occupied by oxo oxygen atom. The remaining two vacant
coordination sites of vanadium are occupied by methoxy group and methanol
respectively owing to the formation of distorted octahedral geometry.3f,3h The V1-O1
bond length (1.58 Å) characterizes the presence of V=O moiety. The elongated V-O bond
(2.27 Å) indicates the presence of the attached solvent moiety with the metal centre.
Figure 2: ORTEP plot of complex 1 & 2, showing essential numbering; H atoms and solvent
moiety are omitted for clarity.

Figure 3: Optimized structure of complex 3.
Complex 2 also crystallizes in the orthorhombic crystal system with space group
Pca2 . Here also the four coordination sites of the metal were occupied in the similar
1
fashion, whereas the remaining two vacant sites were occupied by the nitrogen and
oxygen atom of the 8-Hq moiety. The nitrogen atom of 8-hyroxyquinoline lies trans to
the oxo oxygen atom. The metal complex thus attains distorted octahedral geometry as in
the previous case. For complex 3, we have tried several methods but suitable crystal for
X-Ray diffraction study was not obtained. Although we have performed DFT studies to
get an optimized stable geometry. However, the proposed structure of complex 3 is
optimized with the help of DFT and the structural parameters are given in Table 1 and the
depicted structure in [fig.3].
2
.4. Geometrical studies:
Geometrical optimizations for complexes 1 and 2 are accomplished in
presence of solvent. The main geometrical optimized parameters (bond length and
bond angle) of complexes 1, 2 and 3 are given in the ESI (Table S1†).
The geometries retain a distorted octahedral arrangement around the vanadium
(V) metal centre. The optimized structures of these complexes are well matched to the

experimentally observed structures, which were determined using single crystal XRD
studies. A slight variation in structural parameters may occur due to crystal lattice
2
3
distortion in real molecules. The V–N and V–O bond lengths are in the range of 1.58–
.27 Å in both the theoretical calculation and experimental observation. The isodensity
2
plot of some selected frontier molecular orbitals in their singlet ground state (S ) is listed
0
†
in Table S2 . A partial molecular orbital diagram with the HOMO and LUMO for all the
three complexes is shown in Fig. 4. In the ground state (S ), the HOMO of complexes 1
0
and 2 are similar in energy whereas that of complex 3 is slightly higher. But we can see
the variation in the energies of LUMO for the complexes in their ground state. The
LUMO of complex 3 is energetically much higher than complex 1 and 2. The HOMO -
LUMO energy gaps in all the mononuclear complexes are in the close resemblance to
each other and the values are 2.89 eV, 2.51 eV and 3.09 eV for complexes 1, 2 and 3
respectively. The corresponding orbital contribution for the three complexes is givenin
†
Table S3 . The electron density in the HOMO of all the complexes mainly resides on the
coumarin moiety (in the rangeof 20–21%) and phenol moiety (in the range of 68-69%).
On the other hand, for the LUMO, the electron densitymainly resides on the vanadium
centre (in the range 65-68%)and the π* orbitals of the attached methanol (complex 1) and
8
-hydroxyquinoline (complex 2) (in the range of 14-19%). For complex 3 the LUMO is
distributed over the metal centre (9%), imine bond (18%), phenanthroline (48%) and
coumarin moiety (17%). This data depicts a very low density around the metal centre.

Table 1: Selected bond lengths and bond angles of complexes 1, 2 and 3.
Complex 1
Complex 2
Complex 3
optimized)
(
Bond Length (Å)
V1-O1
V1-O2
V1-O3
V1-O4
V1-O5
V1-N1
C19-N1
1.5843 (19)
V1-O1
V1-O2
V1-O3
V1-O6
V1-N1
V1-N2
C7-N1
1.5920 (11) V1-O1
1.6000
1.8964 (18)
1.9106 (19)
2.272 (2)
1.8860 (12) V1-O2
1.8890 (11) V1-O3
1.8500 (11) V1-N1
2.1110 (13) V1-N2
2.3500 (12) V1-N3
1.2980 (18) C7-N1
1.9823
1.9540
2.0841
2.4152
2.1815
1.3054
1.7651 (19)
2.157 (2)
1.279 (3)
Bond angles (°)
O1-V1-O2
O1-V1-O3
O1-V1-O4
O1-V1-O5
O1-V1-N1
O2-V1-O3
O2-V1-O4
O2-V1-O5
O2-V1-N1
O3-V1-O4
O3-V1-O5
O3-V1-N1
O5-V1-O4
O4-V1-N1
O5-V1-N1
97.95 (10)
100.85 (10)
172.25 (9)
102.19 (10)
93.31 (9)
152.92 (9)
77.03 (8)
101.74 (9)
82.05 (8)
82.04 (8)
93.16 (9)
77.60 (8)
84.73 (8)
80.23 (7)
163.27 (9)
O1-V1-O2
O1-V1-O3
O1-V1-O6
O1-V1-N1
O1-V1-N2
O2-V1-O3
O2-V1-O6
O2-V1-N1
O2-V1-N2
O3-V1-O6
O3-V1-N1
O3-V1-N2
O6-V1-N1
O6-V1-N2
N1-V1-N2
100.9 (6)
96.8 (6)
101.2 (6)
98.7 (6)
177.6 (6)
156.3 (5)
95.9 (5)
78.9 (5)
80.6 (5)
95.9 (5)
83.0 (5)
82.2 (5)
160.1 (5)
76.7 (5)
83.5 (4)
O1-V1-O2
100.0
103.3
103.6
165.1
93.5
O1-V1-O3
O1-V1-N1
O1-V1-N2
O1-V1-N3
O2-V1-O3
O2-V1-N1
O2-V1-N2
O2-V1-N3
O3-V1-N1
O3-V1-N2
O3-V1-N3
N1-V1-N2
N1-V1-N3
N2-V1-N3
150.5
86.9
78.9
93.9
80.6
80.2
91.6
91.0
162.3
72.0

Figure 4: Partial orbital molecular diagram of complex 1, 2 and 3
.5. UV-Vis spectra and DFT studies:
2
The UV- Visible spectral behaviour of the ligand and its corresponding
complexes 1, 2 and 3 are recorded in dichloromethane solution at room temperature. The
spectral parameters with the experimental molar extinction coefficient (ε) value of H L
2
1
and complexes 1–3 are listed in Table 2. The relevant electronic spectra are depicted in
SI (Fig. S1 - S4). Ligand shows only one characteristic peak near 350 nm which may
arise due to intramolecular n–π* and π–π* charge transfer transitions. The complexes 1, 2
and 3 display a strong peak varying between 300 and 525 nm having molar extinction
−
1
−1
coefficient in the range of 6400 – 42810 M cm . The bands in the longer wavelength
region of these complexes can be attributed to the ligand to metal charge transfer
[
H L →V(dπ)] transition. Only complex 3 shows low intense intra-ligand charge transfer
2 1
transition.

Compound
Electronic
transition
Composition
Excitation
Oscillator
Strength
Configuration
Interaction
(CI)
Assign
λexp in
Energy(eV)
nm ( in
-
1
-1
(f)
M cm )
S
0
S
7
HL-3
H-2L+2
H-1L+5
H-1L
3.1578
392 nm)
0.2672
0.3001
0.0977
0.2574
0.1893
0.1150
0.0905
0.0751
0.2672
0.5942
1LMCT
1LMCT
1LMCT
1LMCT
1LMCT
1LMCT
1LLCT
1LMCT
1LMCT
400
(22502)
(
(
(
(
(
(
(
(
(
1
S
0
0
S13
S31
3.6936
335 nm)
0.6102
0.4261
0.4971
0.6341
0.6622
0.8131
0.4781
0.5942
327
(42810)
S
4.6699
265 nm)
261
(19575)
S
0
S
3
2.3615
525 nm)
505
(7845)
2
3
S
S
0
0
S10
S20
HL+3
H-3L+2
HL+1
HL+5
HL-3
3.0645
404 nm)
401
(10955)
3.6924
335 nm)
330
(29225)
S
0
S
6
2.2661
472 nm)
472
(6410)
S
0
S32
3.6534
339 nm)
336
(12485)
S
0
S
7
3.1578
392 nm)
400
(22502)
Table 2: Main calculated optical transitions for complexes in DCM:
2
.6.
Electrochemical studies:
All the complexes are electroactive in acetonitrile solution. The cyclic
voltammogram were recorded with scan rate of 50 mV/sec. Vanadium (V) exhibits one quasi-
reversible vanadium (V)/(IV) couple. Complex 1 displays 0.674V (ΔE = 90 mV) where as
p
that of 2 displays at 0.0V (ΔE = 200 mV). The more negative value for 2 is associated with
p
higher stabilization of (+V) oxidation state than the (+IV) state. In case of complex 3 the
quasi-reversible vanadium (IV)/(V) couple displays at 0.375V (ΔE = 75 mV). The respective
p
cyclic voltammogram are shown in Figure 5.

Figure 5: Cyclic voltammogram plots of complex 1, 2 and 3. (Scan rate: 50 mV/s) in CH
Conditions: 0.20M [N(n-Bu) ]PF used as supporting electrolyte and Pt working electrodes.
3
CN solvent at 298K.
4
6
2
.7.
Oxidative bromination of Salicylaldehyde; Bromoperoxidase activities:
In the present study oxidative bromination of salicylaldehyde was carried out in
presence of synthesized vanadium complexes as catalysts in presence of H O (120 mmol),
2
2
Water (40 mL) in each case was used as solvent.Other conditions were taken as standard:
Salicylaldehyde (20 mmol), KBr (50 mmol), HClO (80 mmol), Vanadium complexes 1-3 as
4
catalyst (15 mg). The catalytic oxidative bromination reaction afforded three compounds viz.
5
-Bromosalicylaldehyde, 3,5-dibromo salicylaldehyde and 2,4,6-tribromophenol. The
addition of HClO was done in four equal portions during the progress of the reaction, which
4
was found to be necessary to prevent the decomposition of the complex and maximize the
conversion of the substrate. Using similar reaction conditions, maximum 99 percent
conversion was accomplished with complex 1 giving mono-bromo derivative as the main
product. Selectivity order is as follows; 5-Bromosalicylaldehyde (75%)> 3,5-
dibromosalicylaldehyde (24%)> 2,4,6- tribromophenol (1%). Complex 2 and 3 also possesses
very high conversion with dibromo derivative (72% and 65% respectively) as the main
product. It was also depicted that, if complex 2 was used in the bromination reaction; 2,4,6-
tribromophenol was formed as bromination product with low yield (24%). Table 3 provides
the percentage conversion of salicylaldehyde and selectivity of the products formed using
equivalent reaction conditions for complexes 1, 2 and 3.Interestingly, when the reaction was
performed in the absence of catalyst, the reaction mixture gave very low conversion of

salicylaldehyde with the selectivity order of the products as: 5-Bromosalicylaldehyde (85%)>
,5-dibromosalicylaldehyde (28%)> 2,4,6- tribromophenol (13%).
3
OH
Br
OH
Br
OH
Br
OH
CHO Br
CHO
Br
Br
catalyst
CHO
KBr/H O /HClO
4
2
2
Scheme 3: Oxidative bromination of salicylaldehyde using model vanadium complexes as
catalyst.
Table 1: Different experimental findings of bromination reaction by vanadium complexes:
Complex
KBr
g,mmol)
H
2
O
2
HClO
(g,mmol)
4
Complex
as catalyst (mL) Conv.
g)
H
2
O
%
TON
TOF
(h )
% Selectivity
-
1
(
(g,mmol)
(
Mono
Di-
Tri-
bromo bromo bromo
1
2
3
5.95, 50
5.95, 50
5.95, 50
15, 120 4.02, 80
15, 120 4.02, 80
15, 120 4.02, 80
0.015
0.015
0.015
0.000
40
40
40
99
99
99
35
2716
3136
3140
679
784
785
75
4
24
72
67
2
1
24
8
25
85
Blank Reaction
13
2
.8. Catechol Oxidation:
All the vanadium complexes are stable in their solid state under ambient conditions.
But for complex 1, [VO(L )(OMe)(MeOH)], due to presence of two weak V-O bonds originating
1
from the attached methoxy group and the solvent moiety, the complex is slightly less stable
compared to complexes 2 and 3 and is labile to be attacked by cis-diol type of compounds. To
examine the assumption, we have treated the complex 1 with 3, 5-ditert-butyl catechol (3, 5-
DTBC), an aromatic cis-diol and accordingly a new deep blue coloured complex 1a were formed
in O saturated acetone. The deep blue colour of the complex got progressively bleached in
2
presence of molecular oxygen proves the oxidation of catechol in presence of complex 1. The
mononuclear vanadium complex is converted to the oxygen bridged dinuclear complex with
subsequent release of the bound catechol moiety as ortho- benzoquinone. To proof the reversibility
of the reaction, when the dinuclear complex is treated with 3, 5-DTBC again, the mononuclear

vanadium complex incorporating the ligand and the cis-diol moiety was formed. The detailed
reactions occurring are depicted in scheme 4.
The reaction has been quantitated for complex 1 in acetone solution. The progress
1
27
of the reaction is observable in both UV-Vis spectra and H-NMR spectra. Time dependent UV-
spectral studies were performed in dichloromethane to gain better insight into the rate of the
reaction in the presence of oxygen. The absorption band at 676 nm of 1a diminishes gradually and
the band at 395 nm increases progressively with advancement of reaction time with a presence of
an isobestic point at 469 nm. This spectral change can be attributed to the generation of o-
benzoquinone and gradual transformation of the mononuclear complex towards dinuclear
vanadium compound. The time dependent spectral plot of decolourization of the deep blue
solution to brown is given in Fig. 8. The plot of A /A vs. time (min) gives a straight line with
6
76 395
2
R value 0.99 which corroborates with the gradual decay of the mononuclear form with respect to
time. In oxygen saturated acetone solution, the reaction is found to be pseudo-first order and the
-2
-1
calculated rate constant is found to be 2.9 x 10 min . The rate is comparable with other reported
complexes.27
Consequently, proton NMR studies were also performed in O saturated acetone-d
2
6
2
7
solution to observe the progress of reaction with time. The tert- butyl signals at δ 1.22, 1.53 ppm
and δ 1.37 ppm are due to the co-ordinated ligand moiety in complex 1a and free catechol moiety
(
remains in slight excess initially) respectively. At the time of completion (after 2h) the peaks at δ
.22, 1.53 and 1.37 decays whereas the peak at δ 1.25 ppm increased significantly which clearly
states the complete conversion of 3,5-DTBC to corresponding benzoquinone.
1

Scheme 4: Probable reaction pathway of complex 1a with oxygen.
Figure 8: (Top Left) Time evolution spectra (UV-Vis) of complex 1a, [VO(L
dichloromethane solution at 298K. (Top right) Ratiometric plot of A395 / A676 vs. time (min.) and (Bottom
Left) Plot of ln [(A − A )/(A - A )] vs. time (monitored at 395 nm).
Figure 9 (Bottom Right): Time evolution of tert-butyl signals in Complex 1a in O
1 2
)(3,5-DTBC)] in O saturated
∞
0
∞
2
6
saturated acetone-d .
3
. Conclusion:

To summarize, we have successfully synthesized a new tridentate O^N^O donor
ligand to generate oxidovanadium (V and IV) complexes of our choice. Using the ligand three
complexes bearing oxidovanadium moiety have been prepared and well characterized by
single crystal X-ray diffraction studies and spectral techniques like UV-Vis, IR, NMR, ESI-
Mass etc. To attain a better insights on the electronic transition properties as well as
theoretically stable structural features, detailed density functional theoretical (DFT) as well as
time dependent density functional theoretical (TDDFT) calculations were performed and the
results are in good agreement with the experimental findings.
Present work has been aimed to generate
pentavalent and tetravalent
oxidovanadium complexes. The electrochemical as well the redox properties of the
synthesized complexes were also studied extensively. Model catalytic bromination reaction of
an aromatic aldehyde, salicylaldehyde was examined using the complexes to establish the
halo-peroxidase mimicking activities of those. The experimental findings presented herein
provide valuable insights into the planning of the preparation of strategically important
complexes in the field of oxidovanadium chemistry. Further research aimed at the synthesis of
polynuclear vanadium complexes using different polydentate ligands is currently in progress.
4
. Experimental section:
.1. Materials:
Vanadyl sulphate was purchased from S D Fine-Chem Limited, 4-
4
Methylumbelliferone and 1,10-Phenanthroline were purchased from Sigma-Aldrich. 8-
Hydroxyquinoline was purchased from Alfa-Aesar. Potassium Bromide was purchased from
Fischer scientific and Hydrogen peroxide (30% V/V) was obtained from Merck. VO(acac)2
2
8
was prepared according to the literature. Analytically pure solvents and chemicals are used
throughout the study. All the reactions with metal salts are carried out under open air
atmosphere.
4
.2.
Physical Measurements:
UV–Vis spectra were recorded on
a
Perkin–Elmer Lambda 25
spectrophotometer. IR spectra were obtained with a Perkin–Elmer L-0100 spectrophotometer.
Fluorescence spectroscopic studies were performed with Horiba Fluoromax-4
1
spectrofluorometer. H NMR spectra were measured on Bruker FT 300 and 400 MHz
1
spectrometer, where tetramethyl silane (TMS) was used as an internal reference. For H NMR
of all the complexes were measured in CDCl and DMSO-d solvent. Atom-numbering
3
6
1
scheme used for H NMR was same as that used in the crystallography. Electrospray

ionization mass spectrometry (ESI-MS) spectra were obtained on a Micromass Q-Tof YA 263
mass spectrometer. Elemental analyses (C, H, and N) were performed on Perkin–Elmer 2400
series II analyzer. Cyclic Voltametry (CV) experiments were carried out using CH
Instruments electrochemical analyser. The X-band EPR spectra were recorded with a
Magnettech GmbH MiniScope MS400 spectrometer (equipped with a TC H03 temperature
controller), and the microwave frequency was measured with an FC400 frequency counter.
The EPR spectra were simulated with the EasySpin software.
4
.3. Computational details:
The geometrical structures of the ground-state of the selected complexes were
2
9
30
optimized by the DFT method with B3LYP exchange correlation functional approach.
The geometry of the complexes was fully optimized in solution phase without any symmetry
constraints. There was a good agreement between the theoretical and experimental structures.
On the basis of the optimized ground state geometry structure, the absorption property in
dichloromethane (DCM) and methanol (MeOH) media was calculated by time-dependent
3
1
density functional theory (TDDFT)
approaches associated with the conductor-like
polarizable continuum model (CPCM).32 We computed the lowest 50 singlet – singlet
transition and results of the TD calculations were qualitatively very similar. The TDDFT
approach had been demonstrated to be reliable for calculating spectra properties of many
3
3
transition metal complexes. Due to the presence of electronic correlation in the TDDFT
B3LYP) method it can yield more accurate electronic excitation energies. Hence TDDFT
(
had been shown to provide a reasonable spectral feature for our complex of investigation. In
the calculation, the quasi-relativistic pseudo potentials of V atoms were anticipated by Hay
3
4
2
0
3
and Wadt with 5 valence electrons (outer-core [(4s 4p )] electrons and the (3d ) valence
electrons were employed, and a “double-ξ” quality basis set LANL2DZ was adopted as the
basis set for V atoms. For H we used 6-31(g) basis set and the 6-31+ G (d) basis set for C, N,
O atoms for the optimization of the ground state geometries. Figures showing MOs and the
difference density plots were prepared by using the Gauss View 5.1 software. All the
3
5
calculations were performed with the Gaussian 09W software package. Gauss Sum 2.1
3
6
program was used to calculate the molecular orbital contributions from groups or atoms.
4
.4. Crystallographic studies:
The single crystal suitable for X-ray crystallographic analysis of the complex was
obtained by slow evaporation of methanol solution of the complex. The X-ray intensity data
were collected on Bruker AXS SMART APEX CCD diffractometer (Mo K , λ = 0.71073 Å)
α

at 293 K. The detector was placed at a distance 6.03 cm from the crystal. Total 606 frames
were collected with a scan width of 0.3˚ in different settings of φ. The data were reduced in
SAINTPLUS37 and empirical absorption correction was applied using the SADABS
3
7
package. Metal atom was located by Patterson Method and the rest of the non-hydrogen
atoms were emerged from successive Fourier synthesis. The structures were refined by full
2
matrix least-square procedure on F . All non-hydrogen atoms were refined anisotropically.
All calculations were performed using the SHELXTL V 6.14program package.38
Molecular structure plots were drawn using the Oak Ridge thermal ellipsoid plot (ORTEP).39
The CCDC numbers are 1914360 and 1920471 for 1 and 2 respectively (Table 6).
Table 2: Crystal data and structure refinement parameters of complexes 1 and 2:
Complex 1
Complex 2
Formula
Mr
C H NO V
C H N O V
1
9
18
7
26 17
2
6
423.29
triclinic
P-1
504.36
Crystal system
Orthorhombic
Pca21
Space group
a/ Å
b/ Å
c/ Å
α/°
8.4009 (17)
10.7220 (2)
11.7100 (2)
107.03 (3)
91.43 (3)
90.48 (3)
1008.1 (4)
2
38.2320 (4)
8.9445 (7)
12.7294 (12)
90.00
β/°
90.00
γ/°
90.00
V/ Å3
Z
4353.0 (7)
8
Dcalcd /g cm-3
μ/mm-1
θ/°
1.497
1.539
0.531
0.504
1.820 - 27.500 1.922 – 27.148
T/K
293 (2)
273 (2)
4197
Reflns collected 3494

1
wR2 0.0491, 0.1562 0.0812, 0.2530
R ,
[I>2σ(I)]
GOF on F2
0.988
0.970
4
.5.
Synthesis of ligand:
Synthesis of 8-formyl-7-hydroxy-4-methyl coumarin:
Initially, 8-formyl-7-hydroxy-4-methyl coumarin was synthesized from 7-
2
1
hydroxy -4-methyl-coumarin according to the literature reported method. In brief, 7-
hydroxy-4-methylcoumarin (5g, 0.0284mol) was dissolved in glacial acetic acid (45 ml)
and hexamine (10g, 0.0714mol) was added to the reaction mixture. The reaction mixture
was then refluxed at 80-85°C for 6 hours. After that 20% HCl (65ml) was added to this

mixture and refluxed for another 45 minutes. It was then cooled to room temperature and
extracted with diethyl ether twice. The combined organic layer was concentrated under
reduced pressure to afford 8-formyl-7-hydroxy-4-methyl coumarin as light yellow
powder. The analytical data are in accordance with the literature value.
Synthesis of (E)-7-Hydroxy-8-[(2-hydroxyphenylimino)-methyl]-4-methyl-
chromen-2-one [H L ]:
2
1
7
-Hydroxy-4-methyl coumarin (3.08g, 15mmol) which was prepared above was
dissolved in 15mL methanol. Then 2-aminophenol (1.64g, 15.0mmol) was added to the
solution and the mixture was heated and stirred for 6 h affording a deep orange coloured
precipitate of the targeted Schiff base ligand (H L ). After that the orange solid was
2
1
1
filtered and washed with mother liquor, and kept under vacuum. Yield: 3.11g (70%), H
NMR (CDCl , 300 MHz):δ 10.26 (s, 1H); 9.33 (s, 1H); 7.73 (d, 1H, J=9.5 Hz), 7.60 (s,
3
1
–
H), 7.17 (s, 1H), 7.08-6.92 (m, 3H), 6.79 (d, 1H, J= 9.1Hz), 6.16 (s, 1H), 2.38 (s, 3H,for
CH ), Elemental anal. calcd. for C H NO : C, 69.15; H, 4.44; N, 4.74, O, 21.67.
3
17 13
4
-1
Found: C, 69.25; H, 4.55; N, 4.60, O, 21.60. IR (KBr, cm ): ν (O–H): 3043; ν (imine
+
C=N): 1622, 1603 (lactone). ESI-MS (CH Cl ): m/z 296.092 [M+H] ,Found: 296.0958.
2
2
4
.6. Synthesis of complexes:
V
[
V O(L )(OMe)(MeOH)], complex 1.
1
IV
The ligand H L (1.5g, 5 mmol) and [V O(acac) ] (1.33g, 5 mmol) both
2
1
2
were dissolved in 7.5mL methanol respectively. Then the two solutions were mixed and
heated under reflux for 8h. The resulting clear brown coloured solution was cooled and
kept in air for slow evaporation. After two days black crystals were generated suitable for
1
X-ray diffraction analysis. Yield: 1.03 g (81%). H NMR (CDCl , 400 MHz): δ 9.79 (s,
3
1
6
H); 7.89 (s, 1H); 7.75 (s, 1H), 7. 46 (s, 1H), 7.23 (s, 1H), 7.0 (m, 1H), 6.77 (m, 1H),
.40 (d, 1H, J=8Hz), 6.19 (s, 1H), 2.50 (s, 3H, for –CH3), 1.61 (s, 6H). Elemental anal
calcd for C H NO V: Theo: C, 53.91; H, 4.29; N, 3.31, O, 26.46, V, 12.03. Found: C,
1
9
18
7
-1
5
3.45; H, 4.65; N, 3.21; O, 26.23; V, 12.21. IR (KBr, cm ): ν (O–H): 3033 (from bound
MeOH moiety); ν (imine C=N): 1624, 1604 (lactone), 933 (V=O stretch). ESI-MS
+
(
CH Cl ): m/z 423.052 Found: 408.138. [M-CH ]
2 2 3

V
IV
[
V O(L )(8-Hq)]. Complex 2: Ligand H L (1.5g, 5 mmol), [V O(acac) ] (1.33g, 5
1 2 1 2
mmol)and 8-Hydroxyquinoline (0.73g, 5 mmol) were added in 15 mL methanol and were
heated under reflux for 8h with stirring. Then the deep violet solution was cooled and
kept in air 2 days for slow evaporation, a diamond shaped black coloured crystals
were formed suitable for single crystal X-ray diffraction analysis. Yield: 0.36g
(
75%) 1H NMR (CDCl , 300 MHz): δ 8.15 (s, 1H), 7.80 (s, 1H), 7.66 (t, 2H, J = 8.85
3
Hz); 7.50 (d, 1H, J = 5.94 Hz), 7.28-7.21 (m, 5H), 6.89-6.77 (m, 2H), 6.53 (d, 1H,
J = 8.1Hz); 6.19 (s, 1H), 2.41 (s, 3H, for -CH ). Elemental anal. calcd. for
3
C H N O V: C, 61.92; H, 3.40; N, 5.55, O, 19.03, V, 10.10 Found: C, 61.98; H,
26
17
2
6
3
9
.62; N, 5.35, O, 19.12, V, 10.04. IR (cm-1): ν (imine C=N): 1622, 1599 (lactone),
+
63 (V=O stretch). ESI-MS (CH Cl ): m/z 504.053 Found for [M+H] : 505.064.
2
2
IV
[
V O(L )(1,10-phenanthroline)], Complex 3:
1
IV
Ligand H L (1.5g, 5 mmol), [V O(acac) ] (1.33g, 5 mmol) and 1,10-
2
1
2
phenanthroline (0.9g, 5 mmol) were dissolved in 20 mL methanol solution and were
heated under reflux for 12h with stirring affording a red coloured precipitate. The
precipitate was filtered and washed with methanol and kept under vacuum. After that re-
crystallisation had been done and found crystalline product. Unfortunately despite several
atoms we hardly achieved good quality single crystal which can be diffracted. Yield: 0.5g
(
77%), Elemental Anal. Calcd. for C H N O V: C, 64.45; H, 3.54; N, 7.78; O, 14.80;
29 19 3 5
V, 9.43. Found: C, 64.58; H, 3.51; N, 7.48, O, 14.65 V, 9.23; IR (cm-1): ν (imine C=N):
+
1
5
626, (lactone) 1586, 948 (V=O stretch). ESI-MS (CH Cl ): m/z 540.076 [M] , Found:
2
2
40.114. The complex is paramagnetic, having magnetic moment 1.75 BM at room
temperature.
4
.7. EPR experiment:
Room temperature EPR experiment of complex 3, having only one unpaired
electron was performed at 298K in CH Cl (dichloromethane) solution and the findings
2
2
were stated in the results and discussion portion.

4
.8. Electrochemical studies:
Cyclic voltammetry experiment was performed by CH instruments
electrochemical analyser. Pt wire was used as working electrode; TBAP was used as
supporting electrolyte. The experiment was performed by setting the scan rate to 50
mV/sec and the results are discussed under results and discussion portion.
4
.9. Vanadium bromoperoxidase activity: catalytic oxidative bromination of
salicylaldehyde:
Complexes 1, 2 and 3were used to catalyse the oxidative bromination reaction of
salicylaldehyde in aqueous medium. In this distinctive bromination reaction,
salicylaldehyde (2.44g, 20.0 mmol), KBr (5.95g, 50 mmol), H O (15g, 120 mmol) and
2
2
water (40mL) were taken followed by addition of three different catalysts viz. Complex 1,
complex 2 and complex 3 keeping the amount constant (0.015 g). To initiate the reaction
HClO (4.02 g, 80 mmol) was added and the mixture was stirred at room temperature. An
4
additional 240 mmol of HClO was added in three equal portions at regular intervals of
4
fifteen minutes. After 2h the reaction mixture was extracted with dichloromethane,
washed with brine (NaCl) solution and dried over anhydrous Na SO . The products were
2
4
analysed and identified by GC-MS.
V
4
.10. Reaction of [V O(OMe)(MeOH)(L )] with 3,5-DTBC and Oxygen: Isolation
1
of Products:27
To a stirred acetone solution (10 mL) of complex 1 (0.08 g, 0.18 mmol); 3, 5-
ditertiary butyl catechol (0.04 g, 0.18 mmol) was added. The deep blue coloured solution
formed instantly was used for further reaction with molecular oxygen. Briefly, in a two-
necked round bottomed flask the deep blue coloured solution was taken and the flask was
fitted with an oxygen filled balloon. Now the solution was stirred for 10h. At the end of
the reaction (identified by the occurrence of reddish-brown colouration of the solution);
3
,5-DTBC (0.04 g, 0.18mmol) was added again for the re-establishment of the original
deep blue colour. This deep blue solution was again bleached by O following the above-
2
mentioned steps and two more instalments (each 0.04 g) of 3, 5-DTBC were added and
the events were repeated. Solvent removal afforded blackish brown solid which was dried
in vacuum over fused CaCl . The dry mass was extracted with petroleum ether (60-80
2
°
C) leaving the dimeric vanadium complex [V O (L ) ] as the residue in analytically pure
2 3 1 2
form. Removal of solvent in vacuo from the filtrate thus obtained afforded the
corresponding quinone i.e. 3,5-ditertiary butyl quinone. The observed yields of quinone

and V O (L ) are 0.086 g and 0.095 g, respectively. Theoretical yields for complete
2
3
1 2
conversion are 0.117 g and 0.132 g, respectively. The complex [V O (L ) ] has been
2
3
1 2
characterized. Elemental Anal. Calcd. for C H N O V : C, 55.45; H, 3.01; N, 3.80; O,
3
4
22
2
11
2
-
1
2
1
3.90; V, 13.83 Found: C, 55.40; H, 3.52; N, 4.10, O, 23.48; V, 13.68 IR (KBr, cm ):
1
624 (imine C=N), 1604,933 (V=O stretch),843 (V–μ-O–V); H NMR (400 MHz,
CDCl ) δ 9.91 (s, 1H), 9.80 (s, 1H), 7.89 (d, J = 9.2 Hz, 1H), 7.78 (s, 2H), 7.46 (d, J = 7.7
3
Hz, 1H), 7.03 (s, 2H), 6.96 – 6.82 (m, 2H), 6.79 (d, J = 9.2 Hz, 2H), 6.40 (d, J = 7.4 Hz,
1
H), 6.29 (s, 2H), 6.26 – 6.14 (m, 1H), 2.57 – 2.36 (s, 6H).
Analytical data of 3,5-ditertiarybutyl quinone was already reported in literature
and experimental findings are in accordance with it.
5
6
. Conflicts of interest:
There are no conflicts of interest to declare.
. Acknowledgements:
Mitali Majumder acknowledges Jadavpur University for her doctoral fellowship.
We are also thankful to Department of Science and Technology, New Delhi, India for the
X-Ray facility in Jadavpur University under DST-FIST programme. Bruker 400 MHz
NMR facility of Indian Association for the Cultivation of Science is highly
acknowledged. We also acknowledge UGC CAS II programme, Department of
Chemistry, Jadavpur University and DST-PURSE programme for other analytical and
instrumental facilities.
7
8
. Keywords: Multi-dentate ligand; Oxovanadium complex; Bromoperoxidase; DFT
studies; Catechol oxidation.
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Oxidovanadium (V and IV) complexes incorporating coumarin based O^N^O ligand: synthesis, structure
and catalytic activities†
[a]
[a]*
Mitali Majumder and Kajal Krishna Rajak
A new tridentate ligand H L [(E)-7-Hydroxy-8-[(2-hydroxy-phenylimino)-methyl]-4-methyl-chromen-2-
1
1
one] has been used to form various catalytic vanadium complexes which shows bromoperoxidase and
catecholase activities.

No declaration of interest.