Crystal structure and bio-catalytic potential of oxovanadium(IV) Schiff base complexes derived from 2-hydroxy-4-(prop-2-yn-1-yloxy)benzaldehyde and alicyclic/aromatic diamines

Article abstract of DOI:10.1016/j.ica.2017.09.060

Syntheses, structures, electronic properties and bio-catalytic activities of oxovanadium(IV) complexes [VOL¹,VOL²] containing N,O-donor Schiff base ligands (L¹ = 2,2′-{cyclohexane-1,2-diylbis[nitrilo(E)methylylidene]}bis[5-(prop-2-yn-1-yloxy)phenol] and L² = 2,2′-{1,2-phenylenebis[nitrilo(E)methylylidene]}bis[5-(prop-2-yn-1-yloxy)phenol]) are presented. The single crystal XRD analyses show distorted square pyramidal based geometry for VOL¹ and VOL². The structure and electronic properties of the complexes were analyzed using DFT and TD-DFT calculations. The topological analysis of the electron density was carried out to analyze the bonding nature in the complexes. The bio-relevant catalytic activities (catecholase, phenoxazinone synthase and haloperoxidases) of the complexes have been investigated. The X-band EPR studies show that the oxidation of 3,5-DTBC and 2-aminophenol catalyzed by VOL¹ and VOL² are due to the formation of ligand-centered radical pathway. DFT studies support the ligand-centered radical formation in the reduced metal complexes with significant elongation of the imine bond length.

Full text of DOI:10.1016/j.ica.2017.09.060

Accepted Manuscript
Crystal Structure and bio-catalytic potential of oxovanadium(IV) Schiff base
complexes derived from 2-hydroxy-4-(prop-2-yn-1-yloxy)benzaldehyde and
alicyclic/aromatic diamines
Chithiraivel Balakrishnan, M.A. Neelakantan
PII:
S0020-1693(17)31316-6
https://doi.org/10.1016/j.ica.2017.09.060
ICA 17919
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
19 August 2017
25 September 2017
26 September 2017
Please cite this article as: C. Balakrishnan, M.A. Neelakantan, Crystal Structure and bio-catalytic potential of
oxovanadium(IV) Schiff base complexes derived from 2-hydroxy-4-(prop-2-yn-1-yloxy)benzaldehyde and
alicyclic/aromatic diamines, Inorganica Chimica Acta (2017), doi: https://doi.org/10.1016/j.ica.2017.09.060
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Crystal Structure and bio-catalytic potential of oxovanadium(IV) Schiff base
complexes derived from 2-hydroxy-4-(prop-2-yn-1-yloxy)benzaldehyde and
alicyclic/aromatic diamines
*
Chithiraivel Balakrishnan and M.A. Neelakantan
Chemistry Research Centre, National Engineering College, K.R. Nagar, Kovilpatti 628 503, Thoothukudi District,
Tamil Nadu, India
*Corresponding Author
E-mail Address :
drmaneelakantan@gmail.com; maneels@rediffmail.com
1

Abstract
Syntheses, structures, electronic properties and bio-catalytic activities of
1
2
1
oxovanadium(IV) complexes [VOL ,VOL ] containing N,O-donor Schiff base ligands (L =
2
2
,2'-{cyclohexane-1,2-diylbis[nitrilo(E)methylylidene]}bis[5-(prop-2-yn-1-yloxy)phenol] and L
2,2'-{1,2-phenylenebis[nitrilo(E)methylylidene]}bis[5-(prop-2-yn-1-yloxy)phenol])
presented. The single crystal XRD analyses show distorted square pyramidal based geometry for
=
are
1
2
VOL and VOL . The structure and electronic properties of the complexes were analyzed using
DFT and TD-DFT calculations. The topological analysis of the electron density was carried out
to analyze the bonding nature in the complexes. The bio-relevant catalytic activities (catecholase,
phenoxazinone synthase and haloperoxidases) of the complexes have been investigated. The X-
1
band EPR studies show that the oxidation of 3,5-DTBC and 2-aminophenol catalyzed by VOL
2
and VOL are due to the formation of ligand-centered radical pathway. DFT studies support the
ligand-centered radical formation in the reduced metal complexes with significant elongation of
the imine bond length.
Keywords: Oxovanadium(IV) Schiff base complexes; Crystal structure; Topology analysis of
electron density; Bio-catalytic potential
2

1
.
Introduction
The extensive variety of chemical, biological and catalytic properties of vanadium has
created constant interest towards its coordination chemistry [1]. Among the transition metals,
vanadium is incredibly precise due to its broad range of coordination numbers and geometries,
available redox states, low toxicity, and involved in many enzymatic and metabolic processes
[
2]. Schiff bases and their metal complexes have useful properties in different fields, ranging
from biochemistry to industrial processes [3]. Vanadium complexes with ligands bearing oxygen
and nitrogen donors have been studied as structural and functional mimics of biological systems
[
4]. Also, vanadium complexes have attracted much attention because of their useful applications
in both chemical and biological processes [5]. Vanadium is capable to oxidize catechol to its
corresponding quinones, which defend the damage of tissues against pathogens or insects [6].
Vanadium-dependent haloperoxidase enzymes (VHPO) catalyse the oxidative halogenation of
organic compounds in the presence of halide ions and hydrogen peroxide under physiological
conditions [7,8]. The catalytically promiscuous metal complexes show low activities, and others
require precise synthetic conditions or have high toxicity. Hence, the development of novel,
efficient, eco-friendly bioinspired metallo-catalysts is still a tough challenge in the field of bio-
catalysis.
In the present work we studied two vanadium complexes containing N,O-donor Schiff
base ligands, derived from 2-hydroxy-4-(prop-2-yn-1-yloxy)benzaldehyde and alicyclic/aromatic
diamines, from the following aspects: (i) synthesis and characterization by using spectral and
single crystal XRD techniques; (ii) molecular geometry and electronic transition calculations by
DFT and TD-DFT; (iii) topological analysis of the electron density and nature of bonding by
Bader’s AIM method and (iv) catecholase, phenoxazinone synthase and vanadium
haloperoxidase like activity.
2
.
Experimental Section
2
.1. Physical Methods and Materials
Elemental analyses were performed on a Thermo Finnigan Flash EA 1112 series carbon,
hydrogen and nitrogen analyzer. IR spectra were recorded with KBr pellets on a Shimadzu
3

8
400S spectrometer. Electronic spectra were recorded using
a
Shimadzu-2600
Spectrophotometer. Emission spectra were measured on a Jasco FP-8300 Spectrofluorometer.
Electrospray ionization mass spectrometry (ESI-MS) spectra of the samples were recorded on a
BrukermicroTOF-Q II mass spectrometer in CH CN solvent medium. The X-band EPR (9.78
3
GHz) spectra were measured with a Bruker EMX Plus spectrometer, using DPPH radical as
reference. Cyclic voltammetry studies for the complexes (0.001 M) were performed on a
CHI620E spectroelectrochemical workstation in acetonitrile solutions containing 0.1M
tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. The experiments were done
in a conventional three-electrode cell composed of a reference electrode Ag/AgCl, a platinum
wire as auxiliary electrode and a glassy carbon as working electrode. All the chemicals were
purchased from Aldrich.
2
.2. Synthetic Procedures
1
The ligands, L (2,2'-{cyclohexane-1,2-diylbis[nitrilo(E)methylylidene]}bis[5-(prop-2-
2
yn-1-yloxy)phenol]) and L (2,2'-{benzene-1,2-diylbis[nitrilo(E)methylylidene]}bis[5-(prop-2-
yn-1-yloxy)phenol]) were synthesized as reported in our earlier work [9a].
1
2
.2.1. Synthesis of VOL
An aqueous solution of VOSO (0.082 g, 0.5 mmol) (10 mL) was added to a solution of
4
1
L (0.43g, 1.0 mmol) in hot methanol (10 mL). The resulting solution was continuously stirred
and refluxed for 3 h, affording a dark green solution. After reducing the solvent volume to 5 mL
and cooling to room temperature, a green crystalline product was slowly separated (Scheme S1).
It was filtered, washed with cold methanol and dried in a vacuum desiccator over silica gel.
1
Crystals of VOL suitable for X-ray diffraction analysis were grown in its DMSO solution at
room temperature. Yield: 85%. Dark green color solid. M.p.: 284°C. Anal. Calcd. for
C
26
H
24
N
2
5
O V.Found (Calculated) (%) C: 65.12 (65.14), H: 5.04 (5.05), N: 5.82 (5.84). UV-Vis
-1
(
CH CN): λmax = 296, 375, 595 nm. FT-IR (KBr): ν, cm 3283 (–C≡C–H), 2915 (asymmetric –
3
C–H), 2845 (symmetric –C–H), 2166 (–C≡C–), 1630 (–C=N).
2
2
.2.2. Synthesis of VOL
4

2
A hot methanolic solution of L (0.42g, 1.0 mmol) (10 ml) was added to an aqueous
solution of VOSO (0.082 g, 0.5 mmol) (5.0 mL) followed by the addition of 1 ml of Et N. The
4
3
solution mixture was refluxed with stirring for 5 h, affording a dark brown solution. The
resulting solution was cooled and kept in air for slow precipitation of brown crystalline
compound (Scheme S1). It was filtered, washed with methanol and dried in a vacuum desiccator
2
over silica gel. Crystals of VOL suitable for X-ray diffraction (XRD) analysis were grown in
DMSO solution at room temperature. Yield: 87%. Brown color solid. M.p.: 292°C. Anal. Calcd.
for C26
H
18
N
2
5
O V. Found (Calculated) (%) C: 65.94 (65.97), H: 3.81 (3.83), N: 5.91 (5.92). UV-
-1
Vis (CH
3
CN): λmax = 317, 369, 568 nm. FT-IR (KBr): ν, cm 3276 (–C≡C–H), 2910
(
asymmetric –C–H), 2844 (symmetric –C–H), 2163 (–C≡C–), 1631 (–C=N).
2
.3. X-ray Crystallography
Single crystal X-ray data were collected for VOL and VOL on a Bruker Kappa Apex II
1
2
X-ray diffractometer at 296(2) K, using MoKα radiation (0.71073 Å). Data were corrected for
absorption effects using the numerical method SADABS solved by direct methods and refined
2
by full-matrix least squares on F using SHELX-97 [10].The relevant cell was determined and
refined by nonlinear least squares and Bravais lattice methods. The unit cell was confirmed by
checking the h k l overlays on various frames of data by comparing with both the orientation
matrices. The integrated intensity data for each reflection was collected by reduction of the of the
data frames with the program Apex II. The non-hydrogen atoms were refined with anisotropic
displacement parameters. All hydrogen atoms were located at calculated positions and refined as
riding atoms using isotropic displacement parameters. Final difference Fourier maps showed no
–
3
1
–3
2
residual density outside: 6.647 and –1.562 e.Å for VOL , 0.304 and –0.309 e.Å for VOL . A
1
disordered DMSO molecule is present in the crystal packing of VOL . This disorder was refined
using 79 restraints (SADI, SIMU and DELU restraints were used). Crystallographic data is
presented in Table 1. Selected bond distances and bond angles are listed in Table S1 and S2.
2
.4. DFT Calculations
Density functional theory (DFT) study is a significant tool for understanding the
geometry, electronic structure, optical properties and spin density of the system. DFT
5

1
2
calculations for VOL and VOL were carried out using Gaussian 09 [11] at the B3LYP/6-
11G(d,p) level, while for vanadium the LANL2DZ basis set was used. Vibrational frequency
3
calculations were done to ensure that the optimized geometries represent the local minima. The
topological analysis of the electron density was carried out using the Atoms in Molecules (AIM)
method [12], as employed in the Multiwfn program [13] to analyze the bonding nature of the
compounds.
3
.
Result and Discussion
3
.1. Synthesis and Stability
1
2
Schiff bases (L and L ) were prepared by condensing 2-hydroxy-4-(prop-2-yn-1-
yloxy)benzaldehyde with trans-diaminocyclohexane and o-phenylenediamine (2:1) in dehydrated
1
2
alcohol as reported in our earlier work [9a]. The vanadium complexes (VOL and VOL ) were
1
2
obtained as dark green colored solid by the reaction of methanolic solution of L and L with
aqueous VOSO . All the complexes were characterized by various spectral techniques and
4
single-crystal X-ray diffraction analysis. Stability of the complexes in solvent or buffer medium
was studied as reported in our earlier work [9a] (Fig. S1).
3
.2. Spectroscopic Characterization
Synthesized vanadium complexes were characterized using elemental analyses and UV-Visible,
FT-IR, EPR and ESI-MS spectral techniques. In the FT-IR spectra of ligands a broad band
–
1
–1
attributed to O–H···N appears around 3425-3427 cm is shifted to 3430-3433 cm in vanadium
complexes explains the breaking of intramolecular hydrogen bonding and the coordination of
vanadium ion with phenolic oxygen and azomethine nitrogen atoms. The alkyne stretching
–
1
1
–1
frequency of metal complexes exhibits at 3283 & 2166 cm (VOL ) and at 3276 & 2163 cm
2
–1
(
VOL ), respectively. The azomethine (−C=N−) stretching vibrations appeared around 1635cm
–
1
1
–1
2
is shifted towards lower values (1630 cm for VOL and 1631 cm for VOL ) upon
complexation with vanadium ions (Fig. S2) show the coordination of azomethine nitrogen to the
–
1
vanadium ion [9]. The vanadium complexes exhibit bands in the range of 645-570 cm and 554-
–
1
5
65 cm have been assigned as (V–O) and (V–N), indicate that the deprotonated phenolic
oxygen and azomethine nitrogen atoms are coordinated to the metal ion. The V=O stretching
6

−
1
frequencies of the complexes observed in the range of 950-980 cm is reliable with the
IV
previously reported V complexes having square pyramidal structure [14].
1
2
Electronic spectra of VOL and VOL exhibit three bands. The two bands appear at 296
1
2
and 375 nm (VOL ) and 317 and 369 nm (VOL ) correspond to intraligand transitions. The
2
2
2
2
broad band around 595 nm and 568 nm [dxy → dx −y ( B
2
→ B
1
)] corresponds to the d–d
transition of the vanadium complexes (Fig. S3). DFT study was carried out to understand the
1
2
1
2
better insight of the electronic properties of VOL and VOL . The VOL and VOL exhibit high
energy absorption bands in the range of 300-400 nm can be attributed to the ILCT or LLCT
charge transfer transitions. The lowest energy absorption band at 597-606 nm is due to the d-d
transitions (Fig. S4 & Table 2). The calculated values are in reasonable agreement with the
experimental values.
1
2
The X-band EPR spectra of VOL and VOL in acetonitrile were recorded at frozen
•
temperature using DPPH as internal standard. The spectra of the complexes show a hyperfine
1
splitting consistent with axial type spectra of monomeric metal-bound species with d xy ground
5
1
state configuration. The spectra display well resolved V (I = 7/2) hyperfine lines. The spin
Hamiltonian parameters determined from the spectra sensitively reflect changes in the local
2
+
environment of the respective single VO site and the values are, g
x
, g
y
= 1.976 (±0.005), g =
z
−
4
−1
−4
−1
1
.939 (±0.003), A , A
x y
= 57.2 (±0.6) × 10 cm and A
z
= 161.2 (±0.5) × 10 cm . In addition,
1
the bond valence sum (BVS) calculated (3.89 and 3.97 ) for the vanadium center in VOL and
2
VOL clearly suggest an oxidation state of +4 for vanadium within the fixed error limit [17,18].
1
2
The redox property of VOL and VOL has been studied by cyclic voltammetry. The
vanadium complexes display reversible waves (+0.53 to +0.47 V) and quasireversible peaks (–
1
0
0
.29 to –0.50 V) for VOL and reversible waves (+0.60 to +0.40 V) and quasireversible waves (–
2
.30 to –0.52 V) for VOL vs. SCE in acetonitrile solution (Fig. S5). The oxidative peak at 0.53
V
1
+
IV
1
V
2
+
IV
2
V is due to the [V O(L )] /[V O(L )] / [V O(L )] /[V O(L )] process, while the reductive
IV
III
–
reaction may be assigned to [V O(salen)]/[V O(salen)] process [15]. However the reductive
IV
III
–
2+
process, [V O(salen)]/[V O(salen)] has not been reported for related (VO) (salen)
complexes, but these types of redox couples were reported in various type of vanadium
complexes [16].
7

3
.3. Description of Crystal Structure
1
2
IV
The complexes (VOL and VOL ) consist of mononuclear species containing one V
1
2
IV
center with the ligand L /L (Fig. 1a,b). The V atom is five coordinate in a distorted square
pyramidal geometry. The basal square plane is formed by the N,N'-bis(salicylidene)-cyclohexane
diamine/N,N'-bis(salicylidene)-o-phenylenediamine molecule, which acts as a tetradentate ligand
through its trans-cyclohexanediamine/o-phenylenediamine -N atoms and deprotonated phenolic -
O atoms. The apical position is occupied by the oxo ligand; the V–Ooxo bond distance of 1.609
1
2
(
5) Å for VOL and 1.5920 (14) Å for VOL is characteristic for five coordinated vanadyl
species. The distances of the V1 position from ONNO mean planes of the square pyramid in
1
2
2+
VOL and VOL are close to 0.6 Å and these values are characteristic for VO monomeric salen
systems [19]. The geometric parameter τ = (β−α)/60, where β and α are the two largest L–M–L
angles, describes a coordination environment of five coordinated atoms; it has a value of 1 if the
1
2
geometry is trigonal bipyramidal and 0 if it is square pyramidal. For VOL and VOL (αO2–V1–N2
=
138.0º, βO1–V1–N1 = 150.9º and αO1–V1–N2 = 144.85º, βO2–V1–N1 = 145.75º), the value of τ is 0.215
1
and 0.015, respectively indicating a distorted square pyramidal geometry. The L binds with
vanadium through the amine-N atoms [V1–N1, 2.061(5) Å and V1–N2, 2.058(6) Å], two
phenolic-O atoms [V1–O1, 1.931(5) Å and V1–O2, 1.933(4) Å] and equatorial O–atom [V1–O3,
2
1
2
.609(5) Å]. Similarly L binds through the, amine-N atoms [V1–N1, 2.0495(5) Å and V1–N2,
.0645(6) Å], two phenolic–O atoms [V1–O1, 1.9380(12) Å and V1–O2, 1.9267(12) Å] and
equatorial O-atom [V1–O5, 1.5920(14) Å]. The dihedral angles between the planes containing,
O3–V1–N2, O3–V1–N1, O3–V1–O1 and O3–V1–O2 are 87.5º, 107.7º, 113.7º and 86.0º
2
respectively. For VOL , the torsion angles between the planes containing O3–V1–N2, O3–V1–
N2, O3–V1–N2 and O3–V1–N2 are 109.45º, 108.97º, 104.18º and 105.62º respectively.
1
Crystal packing diagrams and intermolecular hydrogen bonding interactions of VOL and
2
VOL are given in Fig. S6 and S7 and the hydrogen bonding parameters are listed in Table S3.
1
The supramolecular dimmers of VOL are stacked by the hydrogen bonding of vanadylic oxygen
atom O3 with H27B and H28C atoms (–CH
3
group) of dimethyl sulphoxide solvent molecule.
Moreover, aromatic oxygen atoms O1 and O5 take part in hydrogen bond with H28C and H28B
atoms (–CH
3
group) of DMSO solvent and the same continues in the 1D-polymeric unit (Fig.
8

S6a). The intermolecular hydrogen bond reveals that, the entire layer is linked via various types
of hydrogen bonding, (i) uncoordinated oxygen atom O6 of solvent molecule and the hydrogen
atom (H3A) of chair form cyclohexane moiety; (ii) uncoordinated hydrogen atom H27B and
H28C of DMSO and vanadylic oxygen atom O3 and phenolic oxygen atom O1; (iii) coordinated
aromatic oxygen atom O5 and uncoordinated methyl carbon atom C28 and hydrogen atom
(
H28B) of solvent; (iv) alkyne arm carbon atom C16 and alkyne arm hydrogen atom H24A; (v)
propargyl arm hydrogen atom H14A and phenolic oxygen atom O2 and are responsible for the
2
arrangement of 2D layer formation (Fig. S6b). In VOL , the intermolecular hydrogen bonding
formation to the adjacent layers is through vanadyl oxygen atom O5 with alkyne arm carbon
atom C14, hydrogen atom H14A and phenolic hydrogen atom H5. Also, aromatic oxygen atoms
O1 and O2 take part in the hydrogen bond with alkyne arm carbon atom C16 and hydrogen atom
H16 are responsible for the 2D layer formation (Fig. S7b).
1
.4. Nature of Bonding in VOL and VOL
2
3
1
Bader’s AIM method is used to explain the nature of bonding in the symmetrical VOL
2
and VOL structures. The bond critical point (BCP) corresponds to the point where the electron
density becomes a minimum along the bond path between the interacting atoms. Molecular
1
2
graphs and contour maps for VOL and VOL show the presence of BCPs (blue dots) and ring
critical points (orange dots) (Fig. S8). The magnitude of electron density ρ(r) at the BCP
2
indicates the strength of the interaction. Laplacian electron density ∇ ρ(r) indicates the nature of
2
the interaction; ∇ ρ(r) < 0 indicates the regions of local electronic charge concentration
2
corresponding to a covalent type interaction, whereas ∇ ρ(r) > 0 indicates the density depletion
corresponding to an ionic or closed-shell type interaction [20]. Moreover, the sum of the gradient
kinetic electron density G(r) and the potential energy density V(r) gives the total electron energy
density H(r), which further explains the nature of the bond. The bond ellipticity gives a
quantitative measure of the π-bond character and predicts the bond stability; high ellipticity
values indicate instability of the bond [21]. The largest ρ(r) values show that the V=O bonds are
1
2
stronger (Table 3). In addition, the V–N bond strength of VOL is weaker than the VOL is due
2
+
2
2+
to the VO binding with N atoms of chair form of cyclohexane moiety but in VOL , the VO
binds with N atoms of phenyl ring. The bonding nature between vanadium and donor atoms can
9

2
be determined using the combination of ∇ ρ(r), H(r) [22] and the delocalization index δ(V–L),
2
which determines the degree of covalency [23]. The positive ∇ ρ(r) and negative H(r) values
indicate that the V–L bonds are partial ionic and covalent character. This type of bonding
character has been described as “transit closed shell” bonding with some covalent contribution
[
24]. These transit closed shell interactions are characteristic in many transition metal donor
atom bonds [25]. The degree of sharing of electrons between the bonds, δ(V–L) gives an
independent measure of the extent of covalency. The δ(V–L) values range from 0.2-1.2 indicates
that the V–L bonds have considerable covalent character.
2
The smallest ∇ ρ(r) and H(r) and largest δ(V–L) values suggest that the V–N bonds have
2
most covalent character in nature. The values of ∇ ρ(r) and H(r) at the BCPs indicate that the V–
O bonds have the most ionic character (Table 3). The typical bond parameter |H(r)/ρ(r)|, which
can be taken as the total pressure per electron density unit, also shows that the V–N bond is
1
2
strongest [|H(r)/ρ(r)| = 0.329 (VOL ) and 0.398 (VOL )] and the V–O bond is weakest [|H(r)/ρ(r)|
1
2
=
0.110 (VOL ) and 0.276 (VOL )]. The ellipticity values show that the metal nitrogen bonds are
2
in the range of 0.088 - 0.221. The complex VOL has the least ellipticity values indicate that
N(1)–V(1) and N(2)–V(1) bond is more stable. The low ellipticity values indicate that the
predominant σ character and the high ε values suggest that the greater π character. The results
2
reveal that all the V–L bonds are characterized by low ρ(r), positive ∇ ρ(r) and negative H(r)
indicating partial ionic and covalent character. Also the parameters indicate that the V–N bond is
most covalent in nature and the V–O bond most ionic in nature (Fig. S9 and S10).
3
.5. Catechol Oxidase Activity
1
2
The catalytic activity of VOL and VOL has been evaluated by the oxidation of 3,5-
ditert-butylcatechol (3,5-DTBC) to o-quinone (3,5-DTBQ) in acetonitrile medium at 25°C
−
4
(
Scheme 1). The acetonitrile solution of the complex (10 M) was treated with 100 equivalents
of 3,5-DTBC under aerobic conditions. After the addition of substrate, the intensity of the
absorption band increases around ~400 nm, which is characteristic of the formation of quinone
(
3,5-DTBQ) (Fig. 2a,b). The complexes show significant catalytic oxidation of the substrate as
monitored by means of absorption spectroscopy.
1
0

The kinetics for the oxidation of substrate (3,5-DTBC) has been determined by
monitoring the increased concentration of the products, 3,5-DTBQ. At low substrate
concentrations, first order kinetics is observed, whereas at higher substrate concentrations
1
2
saturation kinetics is observed with VOL and VOL . The binding constant (K
M
), maximum
1
velocity (Vmax) and rate constant for dissociation of the substrate (kcat) were calculated for VOL
2
and VOL using the Hanes-Woolf plot of [S]/V versus [S] (Fig. 2c-f) and the order of activity is
2
1
VOL > VOL (Table 4a). In the oxidation process, it is important to identify whether dioxygen is
−
−
reduced to water or H
2
O
2
. The oxidation of I to I
2
followed by the generation of I , as is
3
noticeable from the UV-Vis spectral study of the solution (Fig. S11) obtained after suitable
1
2
workup of the mixture of catechol, complex (VOL , VOL ) and KI, clearly shows that dioxygen
is reduced to H , as reported also by other investigators [26]. To detect the formation of H
the experiment was done separately in absence of oxygen as well as in absence of 3,5-DTBC. In
both cases we did not find any indication of the formation of H suggest that the substrate and
oxygen are required for the generation of H . In order to make quantitative estimation of
, the experiment was performed at different time intervals. The results show that the amount
is almost twice of the amount of 3,5-DTBQ (Fig. S12). This inspection also supports the
2
O
2
2
2
O ,
2
O
2
2
O
2
H
2
O
2
of H
2
O
2
mechanism for the catecholase like oxidation.
EPR titration of the complex as well as mixture of complex and 3,5-DTBC in acetonitrile
at room temperature at various time intervals (within 30 min) were carried out to find out the
exact pathway of 3,5-DTBC oxidation. In the experimental conditions, the free ligand and 3,5-
DTBC are EPR-silent. EPR signal, which is an indication of formation of ligand-centered radical
species, is produced only when the complex is mixed with 3,5-DTBC and radical formation is
1
2
mainly responsible for that oxidation reaction. The complex, VOL and VOL show a weak
signal at g ~ 2 (free electron, g = 2.0023) which is characteristic for the octahedral coordination
2
+
2−
sphere of [VO] [27], i.e., the diamagnetic catecholato (L.cat ) is coordinated with the
2
+
paramagnetic [VO] ion (Fig. 3a,b).
1
2
The binding of 3,5-DTBC to VOL and VOL has been investigated by mass
1
spectrometry (ESI; positive mode). ESI-MS spectra have been recorded in acetonitrile for VOL
2
and VOL and for 1:100 [complex/3,5-DTBC] mixtures. The base peak at m/z = 496.14 and
1
1

1
2
1
+
5
07.15 observed in the mass spectra of VOL and VOL can be assigned to the species [VL (H)]
2
+
and [VL (H
2
O)] (Fig. 4a,b). After the addition of 3,5-DTBC, a new peak was observed in the
1
1
+
spectra, the peak at m/z = 734.01 for VOL corresponding to the [VL (3,5-DTBC)(H
2
O)] and
2
2
+
m/z = 711.11 for VOL corresponding to the [VL (3,5-DTBC)(H)] (Fig. 4c.d). The mass
spectrum of 3,5-DTBQ is given in Fig. S13.
The results reveal that formation of catalyst-substrate as intermediate takes part in
substrate activation during the oxidation of 3,5-DTBC to 3,5-DTBQ in presence of oxygen. The
oxygen that takes part in this process is transformed to H
2
O
2
. The liberated H
2
2
O was identified
and characterized spectrophotometrically. In the catalytic cycle, the electron transfer is mainly
facilitated by metal center and then further delocalized via C=N bond of vanadium Schiff base
1
complex to the adjacent conjugate system [28]. The time dependent UV-Vis spectra of VOL and
2
VOL were recorded in the range 500-900 nm after mixing of complex with 3,5-DTBC. After the
addition of substrate, the intensity of the d–d band of complexes is reduced in 500-600 nm range
indicates the formation of complex-substrate aggregation (Fig. S14).
3
.6. Phenoxazinone Synthase Activity
Catalytic oxidation of 2-aminophenol (OAPH) has been evaluated by
spectrophotometrically in acetonitrile solution at room temperature (Scheme S2). In order to
–
4
verify, the ability of the complexes to oxidize OAPH, 1.0×10 M solution of the complex was
treated with 0.01 M solution of OAPH under aerobic conditions. The course of the reaction was
monitored by absorption spectroscopy and the spectral scan for a period of 1 h after the addition
of OAPH (Fig. 5a,b). The spectral data reveal that the development of peak intensity at 423 nm is
characteristic of phenoxazinone chromophore suggest that the oxidized product is 2-
1
2
aminophenoxazine-3-one (APX). These spectral behaviors clearly reveal that VOL and VOL
are active catalysts for the oxidation of OAPH to the phenoxazinone chromophore under aerobic
conditions. To understand the extent of the catalytic efficiency, reaction kinetics was performed
–
4
at a constant temperature of 25°C. For this reason, the complex (1.0×10 M) was treated with
ten-fold excess of substrate (OAPH). For a complex substrate mixture, time scan of 2-
aminophenoxazine-3-one was carried out for a period of 30 min at the maximum band (423 nm)
and the rate for each concentration of the substrate was calculated by the initial rate method. The
1
2

rate versus concentration of substrate data was analyzed on the basis of Michaelis-Menten
approach of enzymatic kinetics to get the Hanes-Woolf plot as well as the values of the various
kinetic parameters such as Vmax, K
show that our complexes have better catalytic activity than the previously reported complexes
Table 4b) [29].
M
and kcat (Fig. 5c-f). The kinetic parameters calculated clearly
(
1
In the catalytic reaction, free radical and/or intermediate species are produced by VOL
2
1
and VOL . In order to identify the free radical, EPR spectra of the reaction mixture (VOL and
2
VOL and OAPH) were taken at room temperature at various time intervals in acetonitrile
medium (within 30 min). Under the experimental conditions, OAPH is EPR-silent. This involves
1
2
that oxidation of OAPH is occurring via a radical pathway only when VOL and VOL are used
as catalysts. The spectra are reliable with the existence of octahedral coordination sphere of
2
+
[
[
VO] state [30], i.e., the diamagnetic o-iminobenzoquinone coordinated with paramagnetic
2
+
VO] ion (Fig. S15). The generation of hydrogen peroxide (H
2
O ) during the oxidation of
2
OAPH has been confirmed using the same procedure as mentioned previously in the
experimental section. The results show that nearly two mol of hydrogen peroxide were produced
1
2
by the oxidation of per mole of OAPH (Fig. S16). ESI-mass spectra of VOL and VOL and o-
aminophenol (1:100) in acetonitrile were recorded after 5 minutes of mixing (Fig. S17). The
1
2
+
peak at m/z = 604.19 and 620.84 (VOL and VOL ) can be assigned as [M+OAPH+H] ,
+
[
M+OAPH+Na] , which is the complex-substrate aggregation. The peaks at m/z = 232.62,
+
2
37.18, 215.41 and 237.72 are assigned as [(2-aminophenoxazine-3-one)H
2
O] , [(2-
+
+
aminophenoxazine-3-one)Na] , [(2-aminophenoxazine-3-one)H] and [(2-aminophenoxazine-3-
+
one)Na] , respectively.
3
.7. DFT Study
DFT analysis was carried out to get a better understanding of the nature of active species
1
2
formed during catalysis process. The calculations were done on VOL and VOL at the
B3LYP/6-311G(d,p) level using the Gaussian 03 software. The catecholase and phenoxazinone
synthase like activity has been initiated by radical dependent pathway [31]. The one-electron-
1
−
2−
reduced monoanionic species VOL and VOL exhibit a doublet (S = 1/2) ground state and the
changes in net spin density display a ligand centered species involved in the course of the
1
3

reaction (Fig. S18). From the results, the HOMO is mainly located in the catechol ring and the
1
2
LUMO in the C=N bond and aromatic ring of the ligand (L and L ) (Fig. S19 and S20). Hence,
the proposed electron transfer mechanism is supported by the orbital distribution in complexes
and the electron transfer is possible by the metal center. The imine bond, C=N in the
monoanionic species is considerably elongated (~ 0.025 Å) indicating the formation of imine
radicals [32].
3
.8. Bio-mimicking Catalytic Bromination Reaction
Bromoperoxidase activity of the enzymes as well as their model complexes has been
frequently measured by the kinetic study of the catalytic bromination of phenol red to
bromophenol blue [33]. Oxovanadium complexes are capable to mimic the vanadium
haloperoxidase catalyzes the bromination of organic substrates in the presence of H
2
2
O and
1
2
bromide. Herein, the catalytic bromination reaction activities of VOL and VOL were studied
using phenol red as organic substrate, which is converted to bromophenol blue during the
reaction (Scheme S4). The catalytic ability of oxovanadium complexes for the oxidative
bromination of phenol red was examined by UV-Vis spectroscopy. The gradual decrease in
1
2
intensity of absorption band at 441 nm (VOL ) and 439 nm (VOL ) and the appearance of a new
1
2
band at 591 nm (VOL ) and 589 nm (VOL ) explains the conversion of phenol red to
1
2
bromophenol by the catalytic activity of VOL and VOL . The isosbestic points at 493 nm
1
2
(
VOL ) and 492 nm (VOL ) support that conversion of phenol red to bromophenol blue (Fig. 6).
The kinetic studies reveal saturation kinetics and a treatment based on Michaelis-Menten
1
2
model originally developed for enzyme kinetics was carried out for VOL and VOL . Michaelis-
Menten binding constant (K ), maximum velocity (Vmax), and rate constant for dissociation of
substrates (kcat) were calculated from the Hanes-Woolf plot of [S]/rate versus [S] (Table 4c). A
M
1
2
control reaction carried out without VOL and VOL shows no change in the absorbance at 498
nm (Fig. S21). During the catalytic process, the catalyst (I) is converted to a bound peroxide
intermediate (III) in presence of peroxide in mild acidic condition. This intermediate form (III) in
−
+
turn oxidizes the bromide (Br ) ion in the medium to bromonium ion (Br ), which exists in
−
reaction medium as Br
3
, Br or HOBr. Attack of a bromide ion at one of the peroxo atoms (IV)
2
and accept a proton from a surrounding water molecule leads to the formation of hypobromous
1
4

acid (HOBr) (V) followed by restoration of the native state (I). The insitu bromonium ion reacts
with the phenol red to form corresponding brominated derivatives.
4
.
Conclusion
1
2
Two oxovanadium(IV) Schiff base complexes (VOL and VOL ) have been synthesized
1
2
and characterized by various spectral techniques. Complexes, VOL and VOL have structurally
been characterized by single crystal X-ray diffraction analyses revealing distorted square
1
2
pyramidal based geometry around the vanadium ion. The VOL and VOL have been employed
to catalyze the aerobic oxidation of 3,5-di-tert-butylcatechol and 2-aminophenol as the model
substrates display efficient catecholase and phenoxazinone synthase like activities. The X-band
1
EPR studies demonstrate that the oxidation of 3,5-DTBC and 2-aminophenol catalyzed by VOL
2
and VOL is due to the formation of ligand-centered radical pathway. The DFT studies supports
ligand-centered radical in the reduced metal complexes with considerable elongation of the imine
1
2
bond length. The bromo-peroxidation activity suggests that VOL and VOL effectively
mimicking the functional models of vanadium haloperoxidase enzyme. The results achieved
IV
from these compounds are of significance for the development of V based bio-catalytic
applications.
Supplementary Materials
Experimental part on bio-catalytic activities, EPR and ESI-Mass. CCDC 1432876 and 1432873
1
2
contain the supplementary crystallographic data for VOL and VOL , respectively. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1
223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgement
Dr. M.A.N. gratefully acknowledged the financial support from Council of Scientific and
Industrial Research (CSIR), New Delhi, India (01(2736)/13/EMR-II). C.B is thankful to CSIR,
India for his fellowship. STIC, Cochin is acknowledged for performing the SCXRD and NMR
analysis.
1
5

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1
9

(
a)
(
b)
Fig. 1: ORTEP drawing (50% probability ellipsoids) of
1
2
vanadium complexes, (a) VOL and (b) VOL .
20

(
a)
(b)
(
c)
(d)
(
e)
(f)
2
1

Fig. 2: (a,b) Increase in absorbance around 400 nm, after addition of 100 equivalents of 3,5-
−
4
1
2
DTBC to a 10 M acetonitrile solution of VOL and VOL ; (c,d) Plot of rate vs. substrate
1
2
1
2
concentration for VOL and VOL ; (e,f) Hanes-Woolf plot for VOL and VOL .
(
a)
(b)
1
2
Fig. 3: EPR spectra of (a) VOL and (b) VOL after the addition of 3,5-DTBC in different time
intervals
2
2

(
a)
(b)
(
c)
(d)
1
Fig. 4: ESI-mass spectra in acetonitrile (positive mode), (a) VOL and (b) 1:100 mixture of
1
2
2
VOL and 3,5-DTBC, (c) VOL , (d) 1:100 mixture of VOL and 3,5-DTBC recorded within 5
min of mixing.
2
3

(
a)
(b)
(
c)
(d)
(
e)
(f)
2
4

Fig. 5: (a,b) Absorption spectral changes for the oxidation of o-aminophenol (0.01 M) catalyzed
1
2
−4
by VOL and VOL (10 M) for 1 h at 25°C; (c,d) Plot of rate vs. substrate concentration for
1
2
1
2
VOL and VOL ; (e,f) Hanes-Woolf plot for VOL and VOL .
(
a)
(b)
(
c)
(d)
2
5

(
e)
(f)
–
3
1
2
Fig. 6: (a,b) Oxidative bromination of phenol red (1.0×10 M) catalyzed by VOL and VOL
−
4
1
2
(
10 M) in acetonitrile; (c,d) Plot of rate vs. substrate concentration for VOL and VOL ; (e,f)
1
2
Hanes-Woolf plot for VOL and VOL .
Scheme 1: Proposed mechanism during the oxidation of 3,5-DTBC to 3,5-DTBQ in CH CN
3
1
2
medium by VOL and VOL .
26

1
2
Table 1: Crystal data and structure refinement for VOL and VOL .
1
2
VOL .DMSO
VOL
Formula
C
28
H
30
N
2
6
O SV
C
26
H
18
N
2
5
O V
Formula weight
573.54
296(2)
489.36
296(2)
T, K
λ, Å
0.71073
Monoclinic
P2 /n
9.5037(9)
0.71073
Triclinic
P-1
Crystal system
Space group
1
a/Å
b/Å
c/Å
α/º
β/º
8.3290(3)
9.6851(3)
14.3229(5)
93.279(2)
100.074(2)
108.076(2)
1073.73(6)
2
11.5968(13)
24.704(3)
90
93.330(4)
90
γ/º
3
V/Å
2718.0(5)
4
Z
F
000
1196
502
–
3
Dcalc/mg cm
1.402
1.514
–
1
µ/mm
θ/(º)
0.487
0.505
1.65 to 28.38
0.1887
1.45 to 28.20
0.0439
R
int
3
Crystal size/ mm
0.35 x 0.30 x 0.30 mm
1.793
0.40 x 0.35 x 0.35
1.116
2
Goodness–of–fit on F
a
R
1
[I>2σ(I)]
0.1468
0.0345
b
wR
2
(all data)
0.4643
0.1094
Largest differences peak and 6.647 and –1.562
0.304 and –0.309
–
3
hole (eÅ )
a
b
2
2
2
2 2 1/2
o
R1 = ΣF  – F
c
/ΣF
o
. wR
2
= {Σ[w(F
o
 –F
c
 ) ]/Σ[w(F ) ]}
o
2
7

1
2
Table 2: Theoretical electronic transition for VOL and VOL in acetonitrile.
Complex Electronic
transitions
Composition
E
Oscillator
(eV) strength
f)
2.03 0.0151
λ
(nm)
theor Assign
λ
exp
(nm)
(
S
0
→ S
2
H-1→L+2 (37%)
H-1→L+3 (47%)
H-1→LUMO (17%)
H-1→L+4 (8%)
606
d-d
595
S
S
0
0
→ S
→ S
4
7
HOMO→LUMO (85%) 2.68 0.0223
HOMO→LUMO (5%)
HOMO→L+1 (9%)
392
305
ILCT/
LLCT
375
296
1
VOL
H-2→LUMO (54%)
H-3→LUMO (36%)
H-3→L+1 (8%)
2.75 0.0121
ILCT/
LLCT
H-2→L+1 (2%)
S
S
0
0
→ S
→ S
1
5
H-6→L+1 (64%)
H-6→L+13 (2%)
H-5→LUMO (4%)
H-4→L+9 (42%)
H-4→L+13 (32%)
H-4→L+34 (22%)
H-2→L+1 (4%)
1.85 0.0010
3.27 0.0126
597
379
d-d
568
ILCT/ 369
LLCT
2
VOL
S
0
→ S
6
H-5→LUMO (95%)
H-15→LUMO (4%)
3.71 0.0072
332
ILCT/ 317
LLCT
2
8

1
2
Table 3: Topological properties of the electron density for VOL and VOL (in a.u.).
2
Structure Bond type
ρ(r)
∇ ρ(r)
Ԑ
G(r)
V(r)
H(r)
δ(V–L)
N(1)-C(1)
N(2)-C(2)
0.218 -0.254 0.043 0.106
0.240 -0.336 0.044 0.121
-0.276 -0.1700 0.459
-0.326 -0.2052 0.471
-0.125 -0.0282 0.371
-0.123 -0.0281 0.374
-0.167 -0.0112 0.247
-0.166 -0.0109 0.256
-0.428 -0.0535 1.000
-0.279 -0.1755 0.462
-0.280 -0.1768 0.485
-0.617 -0.2010 0.411
-0.656 -0.2077 0.421
-0.308 -0.1989 0.270
-0.393 -0.1833 0.287
-0.616 -0.1390 1.108
N(1)-V(1)
0.086 0.352
0.085 0.347
0.100 0.497
0.099 0.498
0.235 1.284
0.221 0.106
0.179 0.105
0.157 0.145
0.208 0.146
0.008 0.375
1
VOL
N(2)-V(1)
O(1)-V(1)
O(2)-V(1)
O(3)-V(1)
N(1)-C(1)
N(2)-C(2)
N(1)-V(1)
N(2)-V(1)
O(1)-V(1)
O(2)-V(1)
O(5)-V(1)
0.224 -0.286 0.044 0.104
0.228 -0.292 0.045 0.103
0.267 0.513
0.276 0.514
0.687 0.327
0.699 0.329
0.732 0.949
0.088 0.477
0.125 0.508
0.108 0.279
0.102 0.354
0.004 0.477
2
VOL
2
9