Synthesis, crystal structure and catecholase activity of a vanadium(V) Schiff base complex

Article abstract of DOI:10.1016/j.poly.2016.03.033

An X-ray structurally characterized vanadium(V) complex, [VO₂(L-H)] [L-H = H₂L derived ligand (H₂L = N,N′-(salicyaldimine)-1,3-diaminopropan-2-ol], was found to behave as an effective catalyst towards the oxidation of 3,5-

Full text of DOI:10.1016/j.poly.2016.03.033

Accepted Manuscript
Synthesis, crystal structure and catecholase activity of a vanadium(V) Schiff
base complex
Sunit Kumar Mal, Merry Mitra, Hareram Yadav, Chandra Shekhar Purohit,
Angshuman Roy Choudhury, Rajarshi Ghosh
PII:
S0277-5387(16)30023-7
DOI:
http://dx.doi.org/10.1016/j.poly.2016.03.033
POLY 11898
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
24 November 2015
12 March 2016
Please cite this article as: S.K. Mal, M. Mitra, H. Yadav, C.S. Purohit, A.R. Choudhury, R. Ghosh, Synthesis, crystal
structure and catecholase activity of a vanadium(V) Schiff base complex, Polyhedron (2016), doi: http://dx.doi.org/
1
0.1016/j.poly.2016.03.033
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1
Synthesis, crystal structure and catecholase activity of a vanadium(V)
Schiff base complex
a
a
b
c
Sunit Kumar Mal, Merry Mitra, Hareram Yadav, Chandra Shekhar Purohit, Angshuman
b
a,*
Roy Choudhury and Rajarshi Ghosh
a
Department of Chemistry, The University of Burdwan, Burdwan 713 104, India.
b
Department of Chemical Sciences, Indian Institute of Science Education & Research,
Mohali, Sector 81, S.A.S. Nagar, Knowledge City, Manauli PO, Mohali 140 306, India.
c
Department of Chemical Sciences, National Institute of Science Education and Research,
Bhubaneswar 751 005, India.
Abstract
An X-ray structurally characterized vanadium(V) complex, [VO
2
(L-H)] [L-H = H
2
L derived
ligand (H L = N-(salicyaldimine)-1,3-diaminopropan-2-ol)], was found to behave as an
2
effective catalyst towards the oxidation of 3,5-di-tert-butylcatechol in methanol to its
corresponding quinone derivative in aerial oxygen. The reaction follows Michaelis–Menten
3
-1
enzymatic reaction kinetics with a turnover number (Kcat) of 2.063 × 10 h .
Keyword: Vanadium, Schiff base, catecholase activity
_
__________________________________________________________________________
Corresponding author, E-mail: rghosh@chem.buruniv.ac.in
*

2
1
. Introduction
The ubiquitous plant enzyme catechol oxidase catalyzes exclusively the oxidation
of catechols to the corresponding
The resulting highly reactive
o
-quinones in the presence of molecular oxygen.
-quinones auto-polymerize to form brown
o
polyphenolic catechol melanins, a process thought to protect the damaged tissues of
plants from pathogens or insects. The active site of catechol oxidase contains an
antiferromagnetically coupled (EPR silent) dicopper(II) centre [1]. It is well
documented that the active site of catechol oxidase consists of a hydroxo-bridged
dicopper(II) centre in which each copper(II) centre is coordinated to three histidine
nitrogen atoms and adopts a trigonal pyramidal environment with one nitrogen atom
in the apical site [1]. Since the publication of the crystal structure of the active site
of catechol oxidase [2] several model complexes have been reported [3-6] to
elucidate the structure-function relationship of this biocatalytic catechol-quinone
conversion. Moreover, some monometallic complexes of copper(II) [7] and some
other metal ions, such as Mn(II/III/IV) [7e,8], Fe(III) [9], Co(III) [10], Ni(II) [11]
and Zn(II) [12], are also found to show catecholase activity. The catecholase
activity of vanadium(V) was first reported by Chakravarty et al [13] about a decade
ago. Some other groups also reported vanadium(V) complexes showing catecholase
activity [14]. Here in this present work, we report the synthesis and X-ray structural
characterization of a mononuclear vanadium(V) complex with an (N,O) donor
Schiff base ligand, [VO
2
(L-H)] (
2 2
1) [L-H = H L derived ligand (H L = N-
(
salicyaldimine)-1,3-diaminopropan-2-ol); alcoholic and phenolic protons are
+
2 3
removed from H L, and the primary amine is protonated as -NH to derive L-H)]
[15] which show catecholase activity in methanol. The behaviour of the complex as
a catecholase mimic is described in this present report.

3
2
. Experimental
2
.1.Materials
High purity salicyaldehyde (Sisco Research Laboratories Pvt Ltd, India), 1,3-diamino-
2
-propanol (Aldrich, UK), ammonium vanadate (Aldrich, UK), 3,5-di-tert-butylcatechol
(
Aldrich, UK) and all other solvents were purchased from the respective concerns and used as
received. Solvents were dried according to standard procedures and distilled prior to use.
The ligand H L was prepared using a reported method [16]. For the catecholase
2
-
4
-3
-2
-3
activity study, 1 × 10 mol dm solutions of 1 (0.0003 g) were treated with 1 × 10 mol dm
(
100 equivalents) of 3,5-DTBC (0.0222 g) under aerobic conditions.
2
.2. X-ray diffraction
Single crystals of 1 for X-ray crystallographic analysis were selected following
examination under a microscope. Diffraction data for 1 at 293 K for 1 were collected on a
Bruker SMART APEX ΙΙ CCD diffractometer using Mo-Kα radiation (λ = 0.71073 Å). The
crystal data and refinement details are listed in Table 1. 1 was identified having the Pna2
1
space group. The structure was solved by direct methods, and the structure solution and
2
refinement were based on |F| . The final differences Fourier map showed the maximum and
-3
minimum peak heights at 0.267 and -0.289 e Å with no chemical significance. All
calculations were carried out using SHELXL-97 [17] and were refined using SHELSL-97
[
17], which are available within the Olex-2 [18] suite. All the figures have been generated
using ORTEP-32 [19].
2
.3. Physical measurements
Elemental analyses (carbon, hydrogen and nitrogen) were performed on a Perkin-Elmer
-1
2
400 CHNS/O elemental analyzer. UV-Vis and IR spectra (KBr discs, 4000-300 cm ) were
recorded using a Shimadzu UV-Vis 2450 spectrophotometer and Perkin-Elmer FT-IR model

4
1
RX1 spectrometer, respectively. The HNMR spectral data of the quinine were collected in
1
13
CDCl
recorded in DMSO-d
recorded in DMSO-d
3
on a Bruker 400 MHz spectrometer. H and C NMR spectra of compound 1 were
5
1
6
using a Bruker 300 MHz spectrometer. V NMR data of 1 were also
6
with a Bruker 131 MHz spectrometer. The mass spectrometry was
recorded using a wQtof Micro instrument.
2
.4. Preparation of 1
A warm methanolic solution of NH
4
VO (0.0116 g, 0.1 mmol) was added over half an
3
hour to a stirring solution of the ligand (0.0298 g, 0.1 mmol) in methanol. The yellow
solution was filtered and the supernatant liquid was kept in air for slow evaporation. The
product was obtained as yellow crystals.
Yield: (based on metal salt) 0.0073 g (23.62 %). Anal. calc. for C10
16 2 6
H N O V (1): C,
38.59; H, 5.18; N, 8.99; Found: C, 39.69; H, 5.43; N, 8.94% (Table S1, Supplementary
-
1
file). Selected IR bands (KBr pellet, cm ): 3329 (s), 1633 (s), 898 (s). UV-Vis (λ, nm,
1
MeOH): 318, 250, 217. H NMR (300 MHz, DMSO-
d
6
)
δ
, ppm: 2.94 (t, J = 9.6 Hz, 1H),
3
.07 (d, J = 9.6 Hz, 1H), 3.22-3.61 (m, 2H), 3.68-3.75 (m, 1H), 4.09 (dd, J = 13.8, 4.5
Hz, 1H), 4.50 (brs, 1H), 6.77 (t, J = 8.1 Hz, 2H), 7.40 (t, J = 8.1 Hz, 1H), 4.41 (d, J = 7.8
1
3
Hz, 1H), 8.80 (s, 3H). C NMR (75 MHz, DMSO-
d
6
)
δ
, ppm: 42.72, 63.83, 71.31,
5
1
1
15.55, 120.08, 120.65, 132.98, 133.86, 165.58, 166.30. V NMR (131 MHz, DMSO-
d
6
) δ, ppm: -531.14
3
. Results and discussion
3
.1. Synthesis and formulation
The reaction between NH
4
VO
3
and the ligand H L in methanol afforded the
3
pentacoordinated mononuclear complex 1. The complex was characterized by
microanalytical (C, H and N), spectroscopic and other physicochemical results. The

5
microanalytical data are in good conformity with the formulation of 1. The moisture
insensitive complex is stable over long periods of time in powdery and crystalline states, and
is soluble in methanol, ethanol etc, but is insoluble in water. In the IR spectrum, relatively
-1
intense peaks around 1630 cm due to the C=N stretching frequency appear in the complex.
5
1
The V NMR signal of 1 at -531.14 ppm (Fig. S1, Supplementary file) is characteristic of a
V(V) complex with a coordination sphere that is dominated by O functionalities [20].
3
.2. X-ray structure
The molecular view of the complex is shown in Fig. 1. Single crystal X-ray diffraction
analysis (Table 1) reveals that
The coordination geometry around the vanadium(V) centre in
distorted square pyramid with a VNO chromophore (the value for the metal centre
is calculated as 0.07) [21] Among the coordinating atoms, O3 and O4 are phenoxy and
1
crystallized in the orthorhombic space group
Pna2
1
.
1
is best described as a
4
τ
.
alkoxy oxygen atoms, O1 and O2 are oxo groups, and N1 is the imine nitrogen atom.
Considering the bond angle and bond distance data (Table 2), O1, O3, O4 and N1 form the
base of the square pyramid. The V1 and O2 are positioned on the vertical axis of the
geometry. The V-O bond distances span the range 1.909(3)-1.614(4) Å and the difference ∆
between the longest and shortest bonds amounts to 0.295 Å. The shorter V1-O1 (1.649(4) Å)
and V1-O2 (1.614(4) Å) bonds than the other bonds are indicative of double bond character
[
22].
3
.3. Thermal behaviour
The thermal behavior of the vanadium complex was followed up to 800 °C in a
static nitrogen atmosphere with a heating rate of 10 °C per minute. The complex
decomposes in three steps (Fig. S2, Supplementary file). The first two steps, from

6
3
2.11 to 112.88 and 112.88 to 216.98 °C, correspond to the loss of two lattice water
molecules and an ammonia molecule, respectively with mass losses of 11.51% (calcd.
1.56%) and 6.19% (calcd. 6.18%), respectively. The release of the organic moiety
takes place in the third step (216.98-461.58 °C). The experimental mass loss of
3.21% agrees with the calculated mass loss of 56.55%, with the formation of
vanadium(IV)oxide (VO ). However, more than 3% mass loss difference between the
1
5
2
theoretical and experimental data may suggest some other species, like vanadium(V)
oxide.
3
.4. Catecholase activity of 1: Spectrophotometric study
In order to study the catecholase activity of complex 1, 3,5-DTBC, with two bulky t-butyl
substituents on the ring and low quinone-catechol reduction potential, was chosen as the
substrate. This makes it easily oxidized to the corresponding o-quinone, 3,5-DTBQ, which is
highly stable and shows a maximum absorption at 401 nm in methanol. A solution of 1 was
treated with 100 equivalents of 3,5-DTBC under aerobic conditions. The repetitive UV-Vis
spectral scan was recorded in pure MeOH (Fig. 2). Spectral bands at 318, 250 and 217 nm
appear in the electronic spectrum of complex 1, whereas 3,5-DTBC shows a single band at
2
82 nm.
After addition of 3,5-DTBC, the time dependent spectral scan shows a very smooth
growth of the quinone band at 400 nm, as reported by Krebs et al [23], concomitant with a
decrease in the characteristic 282 nm band for 3,5-DTBC, which indicates the formation of
the respective quinone derivative, 3,5-DTBQ; this latter compound was purified by column
chromatography. The product was isolated in high yield (67 %) by slow evaporation of the
1
1
eluant and was identified by H NMR spectroscopy (Fig. S3, Supplementary file). H NMR

7
(
CDCl3, 400 MHz) δ, ppm: 1.16 (s, 9H), 1.20 (s, 9H), 6.15 (d, J = 2.4 Hz, 1H), 6.86 (d, J =
2
.4 Hz, 1H).
To find out the comparative reaction velocity between 3,5-DTBC and
1, the
reaction kinetics between
dependent change in absorbance at a wavelength of 400 nm, which is characteristic of
,5-DTBQ in methanol. The colour of the solution gradually turned deep brown,
1 and 3,5-DTBC were studied by observing the time
3
indicative of a gradual conversion of 3,5-DTBC to 3,5-DTBQ. The difference in
absorbance ∆A at 400 nm was plotted against time to obtain the velocity for that
particular catalyst to substrate concentration ratio (Fig. 3). A first-order catalytic
-
3
-1
reaction was observed, with a velocity of 1.72 × 10 min .
3
.5. Enzyme kinetics study
Enzymatic kinetic experiments were performed UV-Vis spectrophotometrically,
thermostated at 25 °C for complex 1 and the substrate 3,5-DTBC in MeOH. 0.04 ml of the
-
4
complex solution, with a constant concentration of 1 × 10 M, was added to 2 ml of 3,5-
-3
-2
DTBC of a particular concentration (varying its concentration from 1 × 10 to 1 × 10 M) to
-
4
achieve the ultimate concentration of the complex as 1 × 10 M. The conversion of 3,5-
DTBC to 3,5-DTBQ was monitored with time at a wavelength of 400 nm for a MeOH
solution. The rate for each concentration of the substrate was determined by the initial rate
method. The rate versus concentration of substrate data were analyzed on the basis of the
Michaelis-Menten approach of enzymatic kinetics to get a Lineweaver-Burk (double
reciprocal) plot as well as the values of various kinetic parameters, Vmax, K
M
and Kcat. The
observed rate vs. [substrate] plot in methanol solution as well as the Lineweaver-Burk plot
are given in Fig. 4. The kinetic parameters are listed in Table S2 (Supplementary file). The
3
-1
turnover number (Kcat) was found to be 2.063 × 10 h in MeOH.

8
3
.6. Mechanism of catecholase activity
To obtain a mechanistic inference of the catecholase activity and to get an idea
about the complex substrate intermediate, we recorded an ESI-MS spectrum (Figs. S4a
and S4b, Supplementary file) of a 1:100 mixture of complex
1
and 3,5-DTBC. The
+
signal at m/z = 195 is due to the formation of the protonated ligand [(H
2
L)H] . The
+
peak at m/z = 243 can be assigned to a sodium aggregate of quinone [3,5-DTBQ-Na] .
+
+
The complexes [VO
2
(L-H)(Na)] and [VO
2
(L-H)-(H
2
O)
2
H] exhibit peaks at m/z =
2
+
2
99 and 313, respectively. The formation of a VO species, 1a (Scheme 1), is
2+
identified by the peak at m/z = 758. The V(V) complex
1 is reduced to the VO
species 1a by 3,5-DTBC, and 3,5-DTBC itself gets oxidised to quinone in the presence
of aerial oxygen. The formation of 1a can also be identified from the appearance of a
blue colour (Fig S5, Supplementary file) in the reaction mixture after mixing
,5-DTBC. In a repetitive UV-Vis scan (Fig. S6, Supplementary file) of the mixture,
this blue coloration is supported by the appearance of a peak at 613 nm. The molecular
oxygen that takes part in the oxidation of 3,5-DTBC to 3,5-DTBQ in the presence of
is converted to H . The H thus liberated was identified and characterized
1 and
3
1
2
O
2
2 2
O
spectrophotometrically (S1; Supplementary file) [6b,24].
Scheme 1. Intermediate species 1a.
4
. Conclusions

9
In conclusion, we have synthesized and characterized one mononuclear
vanadium(V) Schiff base complex
1 which mimics the enzyme catecholase. The
synthetic methodology of the complex in this present work is simpler than described
elsewhere [15]. Simple one-pot mixing of the metal salt, organic ligand at room
temperature in methanol generates the title complex, avoiding pH control, an hour-
long reflux etc, as described in the existing report [15]. In methanol the catalytic
reaction follows a first order reaction rate. The turnover number of the reaction in
3
-1
MeOH is found to be 2.063 × 10 h , which is much higher than the few recently
reported complexes [5b,5e,6b,10a,25] showing catecholase activity. To the best of our
knowledge, our reported complex is one of the few monometallic vanadium
compounds which is found to show this activity [26].
5
. Supplementary data
CCDC 1052877 contains the supplementary crystallographic data for 1. This data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(
+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
RG sincerely thanks the Department of Science & Technology, Government of
West Bengal [No. 781(Sanc.)/ST/P/S&T/4G-4/2013 dated 04-12-2014] for financial
assistance. MM thanks The University of Burdwan for her fellowship.

1
0
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Tabbi, Inorg. Chem. 37 (1998) 553.
[
25] (a) A. Biswas, L. K. Das, M. G. B. Drew, G. Aromı´, P. Gamez, A. Ghosh,
Inorg. Chem. 51 (2012) 7993; (b) P. Seth, L. K. Das, M. G. B. Drew, A. Ghosh,
Eur. J. Inorg. Chem. (2012) 2232.
[26] C. -X. Yin, R. G. Finke, J. Am. Chem. Soc. 127 (2005) 9003.

1
4
Table 1. Crystallographic data for 1.
Emperical formula
Formula weight
Temperature, K
Wavelength, Å
Crystal system
Space group
a, Å
10 13 2 4 2
C H N O V.2(H O)
312.20
293
0.71073
orthorhombic
Pna2
1
6.9427(8)
26.568(3)
7.3384(9)
1353.6(3)
2
b, Å
c, Å
3
Volume, Å
Z
–
3
ρ, Mg m
1.527
–
1
µ, mm
F (000)
Crystal size, mm
0.756
644
3
0.2 × 0.2 × 0.2
Theta range for data collection
Limiting indices
1.533 to 25.023 deg
8 ≤ h ≤ 8, -31 ≤ k ≤ 31, -8 ≤ l ≤ 8
6867 / 3197 [R(int) = 0.0375]
26.000 (100.0 %)
Reflections collected / unique
Completeness to theta
Absorption correction
Refinement method
Multi-scan
2
Full-matrix least-squares on F
Data / restraints / parameters
2347/5/195
1.071
2
Goodness-of-fit on F
Final R indices [I>2sigma(I)]
R indices (all data)
R1 = 0.0386, wR
R1 = 0.0456, wR
0.42(2)
2
= 0.0973
= 0.1044
2
Absolute structure parameter
Largest difference peak and hole, e Å
-
3
0.267 and -0.289

1
5
Table 2. Bond distances [Å] and angles [°] for 1.
Bond distances
V(1)-N(1)
V(1)-O(1)
V(1)-O(2)
Bond angles
2.149(4)
1.649(4)
1.614(4)
V(1)-O(3)
V(1)-O(4)
1.892(4)
1.909(4)
O(1)-V(1)-O(2) 109.4(2)
O(1)-V(1)-O(3) 94.9(2)
O(1)-V(1)-O(4) 92.0(2)
O(1)-V(1)-N(1) 144.8(2)
O(2)-V(1)-O(3) 103.3(2)
O(2)-V(1)-O(4)
O(2)-V(1)-N(1)
O(3)-V(1)-O(4)
O(3)-V(1)-N(1)
O(4)-V(1)-N(1)
102.3(2)
105.4(2)
149.51(19)
81.48(15)
75.89(14)

1
6
Fig. 1 Molecular view of 1 with 20% ellipsoid probability
1.5
1.0
0.5
0.0
250
300
350
400
450
500
Wavelength (nm)
Fig. 2 Change in spectral pattern for the reaction of
1 with 3,5-DTBC in MeOH after
observing the reaction for 6 h

1
7
Fig. 3 A plot of the difference in absorbance (∆A) vs time to evaluate the initial velocity of
the catalytic oxidation of 3,5-DTBC by 1 in MeOH
Fig. 4 Plot of rate vs. [substrate] in the presence of
Burk plot
1 in MeOH; inset: Lineweaver-

1
8
Synthesis, crystal structure and catecholase activity of a vanadium(V)
Schiff base complex
Sunit Kumar Mal, Merry Mitra, Hareram Yadav, Chandra Shekhar Purohit, Angshuman Roy
Choudhury, and Rajarshi Ghosh
A structurally characterized vanadium(V) complex, [VO
2 2
(L-H)] [L-H = H L derived ligand
(H
2
L = N-(salicyaldimine)-1,3-diaminopropan-2-ol], was found to behave as an effective
catalyst towards the oxidation of 3,5-di-tert-butylcatechol in methanol to its corresponding
quinone derivative in aerial oxygen. The reaction follows Michaelis–Menten enzymatic
3
-1
reaction kinetics with a turnover number (Kcat) of 2.063 × 10 h .
Synthesis, crystal structure and catecholase activity of a vanadium(V)
Schiff base complex
Sunit Kumar Mal, Merry Mitra, Hareram Yadav, Chandra Shekhar Purohit, Angshuman Roy
Choudhury, and Rajarshi Ghosh