Synthesis, crystal structure and catecholase activity of [Co(SCN)2(L)] [L?=?N,N′-(bis(pyridine-2-yl)benzilidene)-1,2-ethanediamine]

Article abstract of DOI:10.1016/j.molstruc.2017.04.105

Synthesis and structural characterization of a mononuclear cobalt(II) complex [Co(SCN)₂(L)] [L?=?N,N′-(bis(pyridine-2-yl)benzilidene)-1,2-ethanediamine] (1) is reported. Molecular geometry of the complex is found to be distorted octahedron with

Full text of DOI:10.1016/j.molstruc.2017.04.105

Accepted Manuscript
Synthesis, crystal structure and catecholase activity of [Co(SCN) (L)] [L = N,N′-
2
(bis(pyridine-2-yl)benzilidene)-1,2-ethanediamine]
Ashis Kumar Maji, Sumitava Khan, Ayon Kanti Ghosh, Chia -Her Lin, Barindra Kumar
Ghosh, Rajarshi Ghosh
PII:
S0022-2860(17)30557-4
10.1016/j.molstruc.2017.04.105
MOLSTR 23728
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 3 April 2017
Revised Date: 26 April 2017
Accepted Date: 26 April 2017
Please cite this article as: A.K. Maji, S. Khan, A.K. Ghosh, C.-H. Lin, B.K. Ghosh, R. Ghosh, Synthesis,
crystal structure and catecholase activity of [Co(SCN) (L)] [L = N,N′-(bis(pyridine-2-yl)benzilidene)-1,2-
2
ethanediamine], Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.04.105.
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ACCEPTED MANUSCRIPT
Synthesis, crystal structure and catecholase activity of [Co(SCN) (L)] [L = N,N'-
2
(bis(pyridine-2-yl)benzilidene)-1,2-ethanediamine]
a
a,ξ
a
b
Ashis Kumar Maji , Sumitava Khan , Ayon Kanti Ghosh , Chia -Her Lin , Barindra Kumar
a,*
a,*
Ghosh , Rajarshi Ghosh
Department of Chemistry, The University of Burdwan, Burdwan 713 104
Department of Chemistry, Chung Yuan Christian University, Chung Li 32023, Taiwan, ROC
a
b
Crystallographically characterized [Co(SCN) (L)] [L
=
N,N'-(bis(pyridine-2-
2
yl)benzilidene)-1,2-ethanediamine] (1) as the functional model of catecholase activity was
synthesized and characterized. The catechol-quinone conversion in presence f aerial oxygen and
1
was found in methanol and acetonitrile. In each case Michaelis-Menten kinetics were followed
to prove the biomimicking nature of 1.

ACCEPTED MANUSCRIPT
1
Synthesis, crystal structure and catecholase activity of [Co(SCN) (L)] [L = N,N'-
2
(bis(pyridine-2-yl)benzilidene)-1,2-ethanediamine]
a
a,ξ
a
b
Ashis Kumar Maji , Sumitava Khan , Ayon Kanti Ghosh , Chia -Her Lin , Barindra Kumar
a,*
a,*
Ghosh , Rajarshi Ghosh
Department of Chemistry, The University of Burdwan, Burdwan 713 104
Department of Chemistry, Chung Yuan Christian University, Chung Li 32023, Taiwan, ROC
a
b
Abstract
Synthesis and structural characterization of a mononuclear cobalt(II) complex
Co(SCN) (L)] [L = N,N'-(bis(pyridine-2-yl)benzilidene)-1,2-ethanediamine] (1) is reported.
[
2
Molecular geometry of the complex is found to be distorted octahedron with CoN4S2
chromophore. 1 is shown to respond in catecholase activity in methanol and acetonitrile. In each
solvent Michaelis-Menten reaction kinetics were followed. The reaction mechanism of the
catecholase activity is investigated using mass spectrometry.
Key words: Cobalt, Schiff base, X-ray structure, catecholase activity
Corresponding author: barin_1@yahoo.co.uk (BKG), rghosh@chem.buruniv.ac.in (RG)
ξ
Present address: Department of Chemistry, Burdwan Raj College, Burdwan 713 104
1
. Introduction
The active site structure of the plant enzyme catecholase which catalyzes oxidation of
catechol to quinone in presence of molecular oxygen appeared in 1998 [1].
Antiferromagnetically coupled dinuclear Cu(II) in the active site of the enzyme encouraged the
researchers round the globe since then to model small dinuclear Cu(II) complexes to mimic the

ACCEPTED MANUSCRIPT
2
enzyme and its activity [2-5]. In addition to this some mononuclear Cu(II) complexes [6] and
complexes of some transition metals other than Cu(II) like manganese [6e, 7], nickel [8], iron
[9], cobalt [10], zinc [11], vanadium [12], etc also appeared in literature. Schiff bases are
important and popular class of ligands because of their preparational accessibilities, varied
structural motifs, etc. Chemistry of Cobalt-Schiff base complexes have attracted much attention
in the field of coordination chemistry as they exhibit different structural features and varied
applications covering catalysis, photochemistry, electrochemistry and biological inorganic
chemistry [13-15]. In continuation to our interest in synthesizing a number of non-copper
transition metal complexes particularly the mononuclear cobalt complex here we present
synthesis and X-ray structural characterization of a mononuclear cobalt(II) complex with neutral
(
N,N) donor ligand [Co(SCN) (L)] [L = N,N'-(bis(pyridine-2-yl)benzilidene)-1,2-ethanediamine,
2
Scheme 1] (1) which did show catalyzing the aerial oxidation of 3,5-ditertiary butyl catechol to
corresponding quinone. The catalytic results are found in methanol and acetonitrile solvents and
in each case reaction rates and turn over numbers were measured. The measurement Michelis-
Menten kinetics in each solvent supported the biomimicking nature of the synthesized
compound.
Ni
Ni
Np
Np
p
i
i
p
Scheme 1: (N ,N ,N , N ) donor set in L

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3
2
. Experimental
2
.1.Materials
High purity 2-benzoylpyridine (Sigma Aldrich, India), ethylenediamine (Sigma Aldrich,
India), ammonium thiocyanate (E. Merck India), cobalt(II) perchlorate hexahydrate (Sigma
Aldrich, India), 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
procedure and distilled prior to use.
Caution! Perchlorate compounds are potentially explosive [16] especially in the presence
of organic ligands. Only a small amount of these materials should be prepared and handled with
care.
2
.2. X-ray diffraction
Single crystals of 1 for X-ray crystallographic analysis were selected following examination
under a microscope. Diffraction data at 296 K 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 as C 2/c space group. The structure was solved by
2
direct methods, and the structure solution and refinement were based on |F| . The final
differences Fourier map showed the maximum and minimum peak heights at 0.318 and -0.254
-3
eÅ with no chemical significance. All calculations were carried out using SHELXL-97 [17] and
were refined using SHELSL-97 [17] available within Olex-2 [18] suite. All the figures have been
generated using ORTEP-32 [19].
2
.3. Physical measurements

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4
Elemental analyses (carbon, hydrogen and nitrogen) were performed on a Perkin-Elmer 2400
-1
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 RX1
1
spectrometer, respectively. The HNMR spectral data of the quinine were collected in CDCl on
3
a Bruker 400 MHz spectrometer.
2
1
.4. Synthesis of L
To a solution of 2-benzoylpyridine (0.499 g, 2.73 mmol) in 100 mL methanol, ethane-
,2-diamine (0.816 g, 1.36 mmol) was added. The mixture was refluxed for a period of 7-8 hours
until a light yellow color developed. The resultant solution was partially dried in a water bath
until a sticky yellow mass was obtained. Yield: 0.273 g (70%). Anal. calc. for C H N (L): C,
2
6
22
4
-
7
9.97; H, 5.68; N, 14.35; Found: C, 79.70; H, 5.60; N, 14.28. Selected IR bands (KBr pellet, cm
1
): ν(C=N) 1616. UV-Vis (λ, nm, MeOH): 254.
2
.5. Synthesis of 1
The solution mixture resulted from the gradual addition of Co(ClO ) .6H O (0.0370 g;
4
2
2
0
.1 mmol) to a methanolic solution of L (0.039g, 0.1 mmol) followed by addition of NH NCS in
4
methanol (0.0152 g, 0.2 mmol) was stirred for 5 hours. The final solution mixture was filtered
and kept undisturbed at room temperature. After 7 days, dark blue X-ray quality single crystals
were obtained which were filtered off through a fine glass frit, washed with toluene and dried in
a vacuum dessicator. Yield (on the basis of metal salt): 0.0267g (73%). Found C, 59.60; H, 4.00;
-1
N,15.00. Anal. Calc. for C H N S Co: C, 59.46; H, 3.92; N, 14.86. IR (KBr, Cm ): ν(C=N)
28
22
6 2
1
596; ν(NCS) 2110. UV-Vis (CH CN, λ, nm): 496, 264.
3
3
. Results and discussion

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3
.1. Synthesis and formulation
The Schiff base, N,N′-(bis(pyridine-2-yl)benzylidine)-1,2-ethanediamine (L) belonging
p
i
i
p
p =
i
to symmetrical N ,N ,N ,N type backbone [N N(pyridine) and N = N(imine)], was prepared by
condensation of 1:2 molar ratio of ethane-1,2-diamine and 2-benzoylpyridine in dehydrated
ethanol. The hexacoordinated mononuclear complex 1 was isolated as dark blue crystalline
product through one-pot synthesis of 1:1:2 molar ratio of Co(ClO ) .6H O, L and NH NCS in
4
2
2
4
methanol solution at room temperature. The compound is stable over long periods of time in
powder or crystalline form and are soluble in common organic solvents such as methanol and
-1
acetonitrile but are insoluble in water. In IR, ν(C=N) in the ligand appears at 1616 cm (Fig.
S1(a); Supporting information) whereas the stretching frequency of the imine in the compound
-1
-1
appears at 1596 cm (Fig. S1(b); Supporting information) [20]. Peak at 2110 cm represents the
thiocyanato group in the compound (Fig. S1(b); Supporting information) [20]. The electronic
spectrum of the complex in methanol solution exhibits a distinct strong and broad absorption
4
4
bands at ~500 nm due to the T (F) → T (P) transition, which is characteristic of distorted
1
g
1g
octahedral cobalt(II) center [21]. The bands at ~264 nm may be assigned to a ligand based
transition [22].
3
.2. X-ray structure
In order to define the coordination sphere conclusively, single-crystal X-ray diffraction
study in 1 was made. The molecular view of the complex is shown in Fig. 1. Selected bond
lengths and angles pertaining to the coordination sphere are set out in Table 2. The coordination
polyhedron around cobalt(II) is best described as distorted octahedron with CoN
4 2
S
chromophore. Two imine N atoms (N1 and N1*) and two pyridine N atoms (N2 and N2*) of L

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6
occupy the equatorial sites while two terminal S atoms (S1 and S1*) of two thiocyanates in
mutual trans orientation [S1-Co- S1*: 175.13(4)°] are housed at the axial positions. The Co-N
bond distances in the basal plane span the range 1.962(3)-1.866(2) Å and the difference ∆
between the longest and shortest bond amounts 0.096 Å. Each of the axial bond is 2.3009(7) Å
long.
In solid-state, the molecular units are packed through weak C-H…N hydrogen bond
i
(
Table 3) involving C8-H8A of the aromatic ring and N3 of thiocyanate (Fig. 2).
3
.3. Catecholase activity of 1: spectrophotometric study
In order to study the catecholase activity of the complex 1, 3,5-DTBC with two bulky t-butyl
substituents on the ring and low quinone-catechol reduction potential has been chosen as
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. 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 and MeCN (Figs. 3 and 4). Spectral bands at 264 and 496 nm
appear in the electronic spectrum of complex 1, whereas 3,5-DTBC shows a single band at 282
nm.
After addition of 3,5-DTBC, the time dependent spectral scan shows very smooth growing of
quinone band at 400 nm, as reported by Krebs et al [23], indicates the formation of the respective
quinone derivative, 3,5-DTBQ which was purified by column chromatography. The product
obtained from each solvent was isolated in high yield (65% and 70%) by slow evaporation of the
eluant and was identified by measuring melting point (~110°C) [24].

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To find out the comparative reaction velocity between 3,5-DTBC and
1, the reaction
kinetics between
1 and 3,5-DTBC was studied by observing the time dependent change in
absorbance at a wavelength of 400 nm, which is characteristic of 3,5-DTBQ in solution.
The colour of the solution gradually turns deep brown both in methanol and acetonitrile
indicative of 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 (Figs. 5 and 6). A first-order catalytic
-4
-1
-4
reaction is observed, with velocity 6.35 × 10 min (MeOH) and 5.09 × 10 (MeCN).
3
.4. Engyme kinetics study
Enzymatic kinetic experiments were performed UV-Vis spectrophotometrically
thermostated at 25°C with complex 1 and the substrate 3,5-DTBC in MeOH. 0.05 ml of the
-4
complex solution, with a constant concentration of 1 × 10 M, was added to 2 ml of 3,5-DTBC
-3
-2
of a particular concentration (varying its concentration from 1 × 10 M to 1 × 10 M) to achieve
-4
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 solution in MeOH. 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 Michaelis-Menten approach of
enzymatic kinetics to get the Lineweaver-Burk (double reciprocal) plot as well as the values of
the various kinetic parameters Vmax, K and K . The observed rate vs. [substrate] plot in
M
cat
methanol and acetonitrile solutions as well as Lineweaver-Burk plots are given in Fig. 7 and Fig.
, respectively. The kinetic parameters are listed in Table 4. The turnover numbers (Kcat) are
8
4
-1
5 -1
found to be 4.15 × 10 h and 3.06 × 10 h in MeOH and MeCN, repectively.
3
.5. Mechanism of catecholase activity

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8
The catalytic process follows a two-step mechanistic pathway. The first step is the rate
determining step. Probably, in this step, the 1:1 adduct of catechol and the cobalt complex
is formed. 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 of a 1:100
mixture of
m/z = 390.83 (Fig. S2(a); Supporting information) is due to the formation of the ligand
N,N'-(bis(pyridine-2-yl)benzilidene)-1,2-ethanediamine. The metal complex exhibits a
1 and 3,5-DTBC (Figs. S2(a)-S2(d); Supporting information). The signal at
1
peak at m/z = 565.49 (Fig. S2(b); Supporting information). The peak at m/z = 243.31 can
+
be assigned to sodium aggregate of quinone [3,5-DTBQ]-Na (Fig. S2(c); Supporting
information). The formation of a complex-substrate adduct intermediate species is
identified by the peak at m/z = 669.77 (Fig. S2(d), Scheme S1; Supporting information).
The catechol derivative, 3,5-DTBC gets oxidised to quinone in presence of oxygen. The
aerial oxygen that oxidises 3,5-DTBC to 3,5-DTBQ in this process is converted to H O .
2
2
H O thus liberated was identified and characterized spectrophotometrically (S1;
2
2
Supporting information) [24].
4
. Conclusion
Synthesis and X-ray structural characterization of a mononuclear cobalt-Schiff base
complex 1 is reported. The compound shows catecholase activity in methanol and acetonitrile
with high turn over numbers. Catecholase active cobalt complexes are rare [10]. Interestingly,
the turn over number of the reported complex is better than the few reported ones [10].
Mechanistic investigation revealed that the substrate-product conversion in this case is occurred
through a substrate-catalyst adduct. This is trapped my mass spectrometry.

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9
5
. Supplementary data
CCDC 1524439 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)
1
223-336-033; or e-mail: deposit@ccdc.cam.ac.uk
Acknowledgement
RG sincerely thanks Department of Science & Technology, Government of West Bengal
No. 781(Sanc.)/ST/P/S&T/4G-4/2013 dated 04-12-2014], India for financial assistance. AKG
[
thanks CSIR, New Delhi, India for his fellowship.
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20]K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds,
part B: Applications in Coordination, Organometallic, and Bioinorganic Chemistry
[
(John Wiley & Sons Inc, New York), 1997.
[21]R. Kapoor, A. Pathak, P. Kapoor, P. Venugopalan, Polyhedron 25 (2006) 31.

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[
[
[
22]A.B.P. Lever, Inorganic Electronic Spectroscopy, second ed., Elsevier, New York,
984.
23]F. Zippel, F. Ahlers, R. Werner, W. Haase, H.-F. Nolting, B. Krebs, Inorg. Chem. 35
1996) 3409.
1
(
24](a) S. Tsuruya, S.-I. Yanai and M. Masai, Inorg. Chem. 25 (1986) 141; (b) M. Mitra, T.
Kundu, G. Kaur, G. Sharma, A.R. Choudhury, Y. Singh, R. Ghosh, RSC Adv. 6 (2016)
5
8831.
rd
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25] (a) A. I. Vogel, Textbook of quantitative inorganic analysis, (Ed.: 3 ) Longmans,
Green and Co. Ltd., London, 1961, pp-366; (b) A. Neves, L. M. Rossi, A. J. Bortoluzzi,
B. Szpoganicz, C. Wiezbicki and E. Schwingel, Inorg. Chem. 2002, 41, 1788; (c) E.
Monzani, L. Quinti, A. Perotti, L. Casella, M. Gullotti, L. Randaccio, S. Geremia, G.
Nardin, P. Faleschini and G. Tabbi, Inorg.Chem. 1998, 37, 553.
Table 1. Crystal data and structure refinement parameter for 1
Empirical formula
28 22 6 2
C H N S Co
Formula weight
Temperature
565.59
2
96(2) K
Wavelength
0.71073 Å
Crystal system
Space group
Monoclinic
C 2/c
a = 12.3213(2) Å, b = 12.7748(2) Å, c = 18.6805(3) Å
Unit cell dimensions
α = 90°, β = 99.2270(10)°, γ = 90°

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Volume
3
902.30(8) Å
2
8
Z
Density (calculated)
3
1
0
.294 Mg/m
Absorption coefficient
-1
.762 mm
F(000)
1164
.18 × 0.10 × 0.08 mm
2.21 to 28.31°
16<=h<=16, -17<=k<=14, -24<=l<=24
Crystal size
3
0
Theta range for data collection
Index ranges
-
Reflections collected
14405
Independent reflections
Completeness to theta = 28.31°
Absorption correction
Max. and min. transmission
Refinement method
3605 [R(int) = 0.0512]
99.9 %
Semi-empirical from equivalents
0.9416 and 0.8751
2
Full-matrix least-squares on F
Data / restraints / parameters
3605/0/168
0.933
2
Goodness-of-fit on F
Final R indices [I>2sigma(I)]
Largest diff. peak and hole
R1 = 0.0483, wR2 = 0.0970
-3
.318 and -0.254 eÅ
0
Table 2. Bond distances [Å] and angles [°] for 1.
Bond distances

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Co(1)-N(1)
Co(1)-N(1)*
Co(1)-N(2)*
Bond angles
1.866(2)
1.866(2)
1.963(2)
Co(1)-N(2)
Co(1)-S(1)*
Co(1)-S(1)
1.963(2)
2.3009(7)
2.3009(7)
N(1)-Co(1)-N(1)* 86.42(13)
N(1)-Co(1)-N(2)* 81.77(9)
N(1)*-Co(1)-N(2)* 167.14(9)
N(1)*-Co(1)-S(1)
N(2)*-Co(1)-S(1)
N(2)-Co(1)-S(1)
S(1)*-Co(1)-S(1)
92.26(7)
82.96(6)
94.25(6)
175.13(4)
N(1)-Co(1)-N(2)
N(1)*-Co(1)-N(2) 81.77(9)
N(2)*-Co(1)-N(2) 110.42(13) N(2)*-Co(1)-S(1)* 94.25(6)
167.14(9)
N(1)*-Co(1)-S(1)* 91.29(7)
N(1)-Co(1)-S(1)*
N(1)-Co(1)-S(1)
82.96(6)
91.29(7)
N(2)-Co(1)-S(1)*
82.96(6)
Table 3. Hydrogen bonding interaction parameters
Hydrogen bond parameters (Å, °)
D-H⋯A
C8-H8A⋯N3
D-H
0.9300
H⋯A
2.5100
D⋯A
3.274(5)
D-H⋯A
140.00
i
Table 4. Kinetic parameters for the oxidation of 3,5-DTBC catalyzed by 1
-1
-1
Compd Solvent Vmax (M s )
Std. error
K (M)
M
Std. error
K (h )
cat

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-
5
4
-6
-3
-3
-4
4
1
1
MeOH 1.29× 10
MeCN 4.99× 10
2.62 × 10
1.15 × 10
8.50 × 10
3.94 × 10
4.15 × 10
-
-5
-3
5
7.52 × 10
1.76 × 10
3.06 × 10
Fig. 1 Molecular view of 1 with 20% ellipsoid probability

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Fig. 2. 2D sheet structure in 1 through C-H...N hydrogen bonding

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Fig. 3. Change in spectral pattern of reaction of
1
with 3,5-DTBC in MeOH after
observing the reaction for 6 h
Fig. 4. Change in spectral pattern of reaction of
1
with 3,5-DTBC in MeCN after
observing the reaction for 6 h

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Fig. 5. 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. 6. 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 MeCN
Fig. 7. Plot of rate vs. [substrate] in the presence of 1 in MeOH; inset: Lineweaver-Burk plot

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Fig. 8. Plot of rate vs. [substrate] in the presence of 1 in MeCN; inset: Lineweaver-Burk plot

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•
•
•
•
Cobalt chemistry
Schiff base
Bioinspired catalysis
Catecholase activity