Synthesis, crystal structure, catecholase and phenoxazinone synthase activities of a mononuclear cobalt(III) complex containing in situ formed tridentate N-donor Schiff base

Article abstract of DOI:10.1016/j.molstruc.2017.06.077

Synthesis and structural characterization of a mononuclear cobalt(III) Schiff base complex is reported. It crystallizes with monoclinic crystal system with P21/n space group with a = 9.9793(4) ?, b = 28.2907(12) ? and c = 13.1233(6) ?, and β = 97.532(3)°. The compound is active to catecholase and phenoxazinone synthase activities in MeOH, and MeOH and MeCN solvents, respectively at room temperature. Each of the reactions was found to be of first order with reaction rate 8.08 × 10^?3 min^?1 (MeOH) for the catecholase activity and 1.05 × 10^?3 min^?1 (MeOH) and 3.82 × 10^?3 min^?1 (MeCN) for the phenoxazinone synthase activity. The turn over numbers for the catecholase activity is 5.02 × 10³ h^?1 (MeOH) and for the phenoxazinone synthase activity is 4.59 × 10³ h^?1 (MeOH) and 5.12 × 10³ h^?1 (MeCN). Substrate-catalyst adduct was tried to be trapped in each case using mass spectrometry.

Full text of DOI:10.1016/j.molstruc.2017.06.077

Accepted Manuscript
Synthesis, crystal structure, catecholase and phenoxazinone synthase activities of a
mononuclear cobalt(III) complex containing in situ formed tridentate N-donor Schiff
base
Ashis Kumar Maji, Arnab Chatterjee, Sumitava Khan, Barindra Kumar Ghosh,
Rajarshi Ghosh
PII:
S0022-2860(17)30858-X
10.1016/j.molstruc.2017.06.077
MOLSTR 23963
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 7 May 2017
Revised Date: 14 June 2017
Accepted Date: 17 June 2017
Please cite this article as: A.K. Maji, A. Chatterjee, S. Khan, B.K. Ghosh, R. Ghosh, Synthesis, crystal
structure, catecholase and phenoxazinone synthase activities of a mononuclear cobalt(III) complex
containing in situ formed tridentate N-donor Schiff base, Journal of Molecular Structure (2017), doi:
10.1016/j.molstruc.2017.06.077.
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ACCEPTED MANUSCRIPT
Synthesis, crystal structure, catecholase and phenoxazinone synthase activities of a
mononuclear cobalt(III) complex containing in situ formed tridentate N-donor
Schiff base
Ashis Kumar Maji, Arnab Chatterjee, Sumitava Khan¶, Barindra Kumar Ghosh*, Rajarshi
Ghosh*
Department of Chemistry, The University of Burdwan, Burdwan 713104, India
Synthesis and structural characterization of a mononuclear cobalt(III) Schiff base
complex is reported. It crystallizes with monoclinic crystal system with P21/n space group. The
compound is active to catecholase and phenoxazinone synthase activities in MeOH, and MeOH
and MeCN solvents, respectively at room temperature. The turn over number for the catecholase
activity is 5.02 × 103 h-1 and for the phenoxazinone synthase activity is 4.59 × 103 h-1 (MeOH)
and 5.12 × 103 h-1 (MeCN). Substrate-catalyst adduct was tried to be trapped in each case using
mass spectrometry.

ACCEPTED MANUSCRIPT
Synthesis, crystal structure, catecholase and phenoxazinone synthase activities of a
mononuclear cobalt(III) complex containing in situ formed tridentate N-donor
Schiff base
Ashis Kumar Maji, Arnab Chatterjee, Sumitava Khan¶, Barindra Kumar Ghosh*, Rajarshi
Ghosh*
Department of Chemistry, The University of Burdwan, Burdwan 713104, India
Abstract
Synthesis and structural characterization of a mononuclear cobalt(III) Schiff base
complex is reported. It crystallizes with monoclinic crystal system with P21/n space group with a
= 9.9793(4) Å, b = 28.2907(12)
Å and c = 13.1233(6) Å, and β = 97.532(3)º. The compound is
active to catecholase and phenoxazinone synthase activities in MeOH, and MeOH and MeCN
solvents, respectively at room temperature. Each of the reactions was found to be of first order
with reaction rate 8.08 × 10-3 min-1 (MeOH) for the catecholase activity and 1.05 × 10-3 min-1
(MeOH) and 3.82 × 10-3 min-1 (MeCN) for the phenoxazinone synthase activity. The turn over
numbers for the catecholase activity is 5.02 × 103 h-1 (MeOH) and for the phenoxazinone
synthase activity is 4.59 × 103 h-1 (MeOH) and 5.12 × 103 h-1 (MeCN). Substrate-catalyst adduct
was tried to be trapped in each case using mass spectrometry.
____________________
¶Present address: Department of Chemistry, Burdwan Raj College, Burdwan 713 104
*Corresponding author
Key words: Cobalt, Schiff base, catecholase activity, phenoxazinone synthase activity

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1. Introduction
Synthetic models of different enzymes are important as they can be used as
catalysts for the respective organic conversions which are not possible in a straight
forward way [1-3]. Catecholase and phenoxazinone synthase, both the copper proteins,
are available in plant tissues and the bacterium Streptomyces antibioticus, respectively
[1(a), 1(b)]. These respectively catalyses the oxidation of catechol to quinone and
coupling of two molecules of 2-aminophenol to give phenoxazinone chromophore
(Schemes 1 and 2). Oxidation reactions of alcohols to carbonyls are important both in
laboratory as well as industry due to its wide range of utility of the products as important
precursors and intermediates in drugs, vitamins and fragrances [4]. Synthesis and
structural characterization of a number of catecholase [5-7] and phenoxazinone synthase
active [8] copper(II) complexes are reported in literature. In addition to that a number of
non-copper complexes are also found to show these activities [9-12]. In each of the
reported ones [5-12] Michaelis-Menten enzyme kinetic measurements were done to prove
the biological mimic. Among the non-copper complexes cobalt(II/III) is a good choice as
is found in literature [3c, 12]. We are also active in this field during last few years in
designing different non-copper complexes which are showing catecholase and/or
phenoxazinone synthase activities. In this effort, we have synthesized and
crystallographically characterized one mononuclear cobalt(III) [Co(L)2](ClO4)3 [L = N-
(pyridine-2-yl)-benzylidine-1,3-propanediamine (Scheme 3)] (1) complex which is
showing catecholase activity in MeOH and phenoxazinone synthase activity in both
MeOH and MeCN at room temperature.

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Scheme 1. Catecholase activity
Scheme 2. Phenoxazinone synthase activity
Scheme 3. Na, Ni, Np donor sites of the Schiff base (L)
2. Experimental
2.1. Materials and method:
High purity 2-benzoylpyridine and propane-1,3-diamine (Lancaster) were used as
received. Cobalt(II) perchlorate hexahydrate was prepared [13] by treatment of the
cobalt(II) carbonate (Merck) with perchloric acid (Merck) followed by slow evaporation

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on a steam-bath, filtration through a fine glass-frit and preserved in a desiccator
containing concentrated sulfuric acid (Merck) for subsequent uses. All other chemicals
and solvents used were AR grade. The synthetic reactions and work-up were done in
open air
Caution! Perchlorate salts of metal ions are potentially explosive [14] especially in the
presence of organic ligands. Only a small amount of these materials should be prepared
and handled with care.
Elemental analyses (carbon, hydrogen and nitrogen) were done using a Perkin-Elmer
2400 CHNS/O elemental analyzer. IR spectra (KBr discs, 4000-400 cm-1) were recorded
using a Perkin-Elmer FTIR model RX1 spectrometer. Molar conductances were
measured using a Systronics conductivity meter where the cell constant was calibrated
with 0.01M KCl solution and dry MeCN was used as solvent.
2.2. Preparation of [Co(L)2](ClO4)3 (1):
Methanolic solution (20 ml) of 1,3-propanediamine (296 mg, 4 mmol) and 2-
benzoylpyridine (732 mg, 4 mmol) was refluxed for 3 h, followed by the addition of a
methanolic solution (30 ml) of Co(ClO4)2.6H2O (474 mg, 2 mmol). The resulting solution
was refluxed again for an additional 2 h. The final orange-red solution was filtered and left
for slow evaporation in open air at room temperature. After 3 days fine green
microcrystalline product of 1 that separated, was washed with toluene and dried in vacuo
over silica gel indicator. Yield 0.65 g (75%). Found: Calcd. C30H34N6O12Cl3Co (1): C,
43.06%; H,4.06%; N, 10.04%. Found, C, 42.1; H, 4.1; N, 10.06%. IR (KBr, cm-1): ν(NH2)
3245; ν(C=N) 1584; ν(ClO4) 1143, 1121, 1079, 624. ΛM (MeCN, Ω-1 cm2 mol-1): 330. UV-
Vis in DMF (λ, nm): 602, 449, 386, 269.

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2.3. X-ray crystallographic analysis
Suitable single crystals of 1 was mounted on a thin glass fiber and X-ray single crystal
structural data was collected on a Bruker Smart-CCD diffractometer using graphite
monochromated Mo Kα radiation (0.71073 Å) at 293 K. The program SAINT [15] was used
for integration of diffraction profiles and absorption correction was made with SADABS
[16] program. The structure was solved by SIR 92 [17] and refined by full matrix least
squares method using SHELXL 97 [18]. The non-hydrogen atoms were refined
anisotropically. All hydrogen atoms were located by Fourier analysis. Crystal data and
structure refinement parameters for 1 are summarized in Table 1.
3. Results and discussion
3.1. Synthesis and formulation
The hexacoordinated mononuclear compound 1 was obtained as a green crystalline
product by refluxing 2-benzoylpyridine and 1,3-propanediamine with Co(ClO4)2.6H2O in
methanol. The ligand hydrolysed at one side was produced in the reaction mixture in situ.
The complex was characterized using microanalytical, spectroscopic and other
physicochemical results. The microanalytical data is in good conformity with the
formulation of 1. The air-stable moisture-insensitive compound is crystalline and soluble
in a range of common organic solvents such as methanol, ethanol, acetonitrile,
dimethylformamide and dimethylsulphoxide but is insoluble in water. In MeCN solution
the compound behaves as 3:1 electrolyte [19] as reflected in their conductivity value (ΛM
: 330 Ω-1 cm2 mol-1). In IR, the ν(C=N) stretching vibrations of the metal bound Schiff
base are seen at 1584 cm-1. The bands corresponding to the ν(ClO4) stretches [20] at
1143, 1121, 1079 and 624 cm-1 are also seen as expected. Several bands in the range

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2800-3000 cm-1 are assigned to the aliphatic ν(C-H) stretching vibration of the ligand. All
other characteristic ligand vibrations are seen in the 1600-600 cm-1 range. Green DMF
solution of 1 shows a d-d transition band at 602 nm and several charge transfer bands are
1
1
observed in the region 449-386 nm. The d-d band at 602 nm is due to the A1g → T1g
transition for the distorted octahedral cobalt(III) ion in the complex [21]. The higher
1
1
energy band, A1g → T2g, is obscured by ligand to metal charge transfer. The band
around 269 nm may be assigned to the intraligand n-π*/π-π* charge transfer transition
[22].
3.2. X-ray structure
An ORTEP view with atom numbering scheme of the mononuclear compound 1,
is displayed in Fig. 1. Selected bond lengths and angles pertaining to the coordination
sphere are set in Table. 2. The coordination polyhedron around cobalt(III) is best
described as a distorted octahedron with a CoN6 chromophore ligated by six nitrogen
atoms of two tridentate Schiff base . Distortion from the ideal octahedral geometry is
due to the asymmetric nature of the bound tridentate Schiff base blocker (L) and the
deviations of the refine angles (90º/180º) formed at the metal centre. The
hexacoordinated cobalt(III) center is bonded to two tridentate Schiff base ligands (L; Na,
Ni, Np) and each ligand molecule offers one pyridine nitrogen, one imine nitrogen and
one amine nitrogen atom as the coordination sites. The equatorial plane is constructed
by two imine nitrogens [N(2), N(5)], one pyridine nitrogen N(1) and one amine nitrogen
N(3) while the axial position are occupied by the remaining one pyridine nitrogen N(4)
and one amine nitrogen N(6). The degrees of distortion of the coordination sphere are
reflected in the cisoid [82.8(4)-96.0(4)º] and the transoid angles [175.4(4)-178.9(4)º].

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The sum (359.9º) of the equatorial angles [N(1)-Co(1)-N(5) 93.9(4)º, N(1)-Co(1)-N(2)
82.8(4)º, N(2)-Co(1)-N(3) 95.7(4)º, N(3)-Co(1)-N(5) 87.5(3)º] are very close to 360.00º,
this reflects that the equatorial nitrogens and metal centers are almost in a same plane.
The fact that Co-N(NH2) bonds are longer then the Co-N(imine) and Co-N(pyridine)
bonds is quite evident from the different in hybridization states (sp2 for imine as well as
pyridine nitrogens and sp3 for free NH2).
3.3. Catecholase activity
3.3.1. Spectrophotometric study
The catechol-quinone oxidation reaction was carried out using 3,5-di-tert
-
butyl catechol (3,5-DTBC) as substrate. The oxidized product 3,5-di-tert-butyl
quinone (3,5-DTBQ) is highly stable and shows a maximum absorption at 401 nm
in methanol [23]. Spectral bands at 602, 449, 386 and 269 nm appear in the
electronic spectrum of
Methanolic solution of
1
, whereas 3,5-DTBC shows a single band at 282 nm.
1
was added in 100 equivalents of 3,5-DTBC under aerobic
conditions and the repetitive UV-Vis spectral scan was recorded (Fig. 2).
Conversion of 3,5-DTBC to 3,5-DTBQ was indicated by slow colour change from
light yellow to deep brown. In UV-Vis spectrophotometry, after this addition, the
spectral scan shows very smooth increase of a quinone band at around ~400 nm as
reported by Krebs et al [24]. 3,5-DTBQ produced in the reaction was purified by
column chromatography. Chromatographically pure 3,5-DTBQ was characterized
by determining its melting point (~110°C) which agreed well with that reported in
literature [25].

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The time dependent change in absorbance at a wavelength of 401 nm,
characteristic of 3,5-DTBQ in methanol, was observed for 25 min to comprehend the
reaction kinetics and find out the reaction rate between 3,5-DTBC and 1. The difference
in absorbance ꢀA at 401 nm, was plotted against time to obtain the rate/velocity for that
particular catalyst to substrate concentration ratio (Fig. 3). A first-order catalytic reaction
is observed, with rate 8.08 × 10-3 min-1.
3.3.2.Enzyme kinetics study
Kinetic experiments were performed spectrophotometrically with
1 and the
substrate 3,5-DTBC in methanol, thermostated at 20°C. 0.04 ml of the complex
solution, with a constant concentration of 1 × 10-4 M, was added to 2 ml of 3,5-
DTBC of a particular concentration (varying its concentration from 1 × 10-3 M to 1
× 10-2 M) to achieve the final concentration of the complex as 1 × 10-4 M. The
conversion of 3,-5-DTBC to 3,5-DTBQ was monitored with time at a wavelength
of 401 nm for solutions 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, KM and Kcat. The observed rate vs. [substrate] plot as well as Lineweaver-
Burk plot in methanol is given in Fig. 4 and inset to Fig. 4, respectively. The
kinetic parameters are listed in Table 3. The turnover numbers (Kcat) are 5.02 × 103
h-1.

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3.3.3. Reaction mechanism
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 (Scheme S1; Supporting information). 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 1 in methanol (Figs. S1(a) and S1(b);
Supporting information) and of a 1:100 mixture of 1 and 3,5-DTBC of mixing them
together (Figs. S1(c)-S1(f); Supporting information). The signal at m/z = 835.92 and
858.94 (Figs. S1(a) and S1(b); Supporting information) represents the metal complex
and its sodium adduct, respectively. Signal at 239.25 (Fig. S1(c); Supporting
information) is due to the ligand N-(pyridine-2-yl)-benzylidine-1,3-propanediamine in
the mixture. The peak at m/z = 243.32 can be assigned to sodium aggregate of quinone
[3,5-DTBQ]-Na+ (Fig. S1(d); Supporting information). The 1 exhibits a peak at m/z =
834.66 in the mixture (Fig. S1(e); Supporting information). The formation of a
complex-substrate adduct intermediate species is identified by the peak at m/z =
1096.87 (Fig. S1(f); 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 H2O2. H2O2 thus liberated was identified
and characterized spectrophotometrically (S2; Supporting information) [26]. Using the
experimental data a probable catalytic cycle is proposed (Scheme 4).

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+ H2O2
Scheme 4. Probable catalytic cycle for the catecholase activity of 1
3.4. Phenoxazinone synthase activity
3.4.1. Spectrophotometric studies
The phenoxazinone synthase activity study was carried out using the substrate o-
aminophenol (OAPH). OAPH shows bands at 286 and 232 nm in pure methanol.
Upon treatment of solution of 1 in methanol and acetonitrile, respectively into 100
equivalents of OAPH under aerobic conditions, the repetitive UV-Vis spectral scan
was recorded (Figs. 5 and 6). The colourless solution gradually turned deep brown in
each case which indicates conversion of OAPH to 2-aminophenoxazine-3-one
(APX). The spectral scan shows very smooth growing of APX band at ~427 nm [12].
APX was identified by its sodium adduct in HRMS with m/z = 236.36. (Fig. S3;

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Supporting information). The time dependent change in absorbance at a wavelength
of 427 nm, characteristic of APX in methanol, was observed for 25 min to
comprehend the reaction kinetics and find out the reaction rate between OAPH and 1.
The difference in absorbance ꢀA at 427 nm, was plotted against time in each solvent
to obtain the initial rate for that particular catalyst to substrate concentration ratio
(Figs. 7 and 8). A first-order catalytic reaction is observed, with initial rate 1.05 × 10-
3 min-1 and 3.82 × 10-3 min-1 in MeOH and MeCN, respectively.
3.4.2. Enzyme kinetics study
Enzymatic kinetic experiments were performed UV-Vis spectrophotometrically
thermostated at 25°C with 1 and the substrate OAPH in MeOH and MeCN
separately. 0.04 ml of the complex solution, with a constant concentration of 1 × 10-4
M, was added to 2 ml of OAPH of a particular concentration (varying its
concentration from 1 × 10-3 M to 1 × 10-2 M) to achieve the final concentration of the
complex as 1 × 10-4 M. The conversion of OAPH to APX was monitored with time at
a wavelength of 427 nm for solutions separately in MeOH and MeCN. 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, KM and
Kcat. The observed rate vs. [substrate] plot in methanol and acetonitrile solutions as
well as Lineweaver-Burk plot is given in Figs. 9 and 10 (and to their respective
insets), respectively. The kinetic parameters are listed in Table 4. The turnover
number (Kcat) is 4.59 × 103 and 5.12 × 103 h-1 in methanol.

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3.4.3. Reaction mechanism
To elucidate mechanistic pathway of the reaction it is found that initially OAPH
forms an adduct with 1 which is trapped by mass spectrometry at m/z = 1085.99
(OAPH-1-K+) (Fig. S3(a); Supporting information). This adduct generates 2-
amoinophenoxazine-3-one through some intermediate reaction with molecular
oxygen.
4. Conclusion
We have synthesized and crystallographically characterized one cobalt(III) Schiff
base complex. The complex is found to show catecholase as well as phenoxazinone
synthase activity. Catecholase and phenoxazinone synthase active cobalt complexes are
there in literature [3c, 12]. The turn over number of catecholase activity is comparable to
our previous reported cobalt complex [3c] and better than some other reported one [12a].
In case of phenoxazinone synthase activity of cobalt complexes our compound gives
better turn over number than some other reported ones [12c, 12d].
5. Acknowledgements
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.

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Table 1. Crystallographic data for [Co(L)2](ClO4)3 (1)
Crystallographic parameter
1
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
D (cal) [g/cm3]
Volume (Å3)
Z
C30H34N6O12Cl3Co
835.91
293(2)
0.71073
Monoclinic
P21/n
1.570
3673.0(3)
4
Unit cell dimension
a (Å)
9.9793(4)
b (Å)
28.2907(12)
13.1233(6)
c (Å)
90, 97.532(3), 90
1784
α (° β (°), γ (°)
),
F (000)
(mm-1)
range for data collection (
0.760
µ
θ
1.44, 18.50
°
)
h/k/l
-8,8/-25,25/-11,11
18035
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
2723
2723/0/477
1.139
R = 0.0615 and wR = 0.1683
R = 0.0838 and wR = 0.2059
0.92, -0.67
Final R indices [I>2
R indices (all data)
Largest peak and hole (eÅ-3)
σ(I)]
2
2 2
2
R = |Fo| |Fc||/ |Fo, wR = [
Σ
−
Σ
Σ
w(Fo
−
Fc2)2/Σw(Fo ) ]1/2, calcd w = 1/[ 2(Fo ) +
σ
(0.1311P)2 + 9.1582P]; where P = (Fo + 2Fc2)/3.
2

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Table 2. Selected bond distances (Å) and bond angles (°) for 1
Bond distances :
Co(1)-N(1)
Co(1)-N(2)
Co(1)-N(3)
1.937(8)
1.953(7)
1.967(7)
Co(1)-N(4)
Co(1)-N(5)
Co(1)-N(6)
1.940(8)
1.930(8)
1.964(7)
Bond angles :
N(5)-Co(1)-N(1)
N(5)-Co(1)-N(4)
N(1)-Co(1)-N(4)
N(5)-Co(1)-N(2)
N(1)-Co(1)-N(2)
N(4)-Co(1)-N(2)
N(5)-Co(1)-N(6)
N(1)-Co(1)-N(6)
N(4)-Co(1)-N(6)
N(2)-Co(1)-N(6)
93.9(4)
82.9(4)
89.5(3)
175.4(4)
82.8(4)
93.8(3)
96.0(4)
90.4(3)
178.9(4)
87.3(3)
N(5)-Co(1)-N(3)
N(1)-Co(1)-N(3)
N(4)-Co(1)-N(3)
N(2)-Co(1)-N(3)
N(6)-Co(1)-N(3)
87.5(3)
178.1(4)
89.5(3)
95.7(4)
90.7(3)
Table 3. Kinetic parameters for the oxidation of 3,5-DTBC catalyzed by 1
Solvent
MeOH
V max (M s-1)
1.45 × 10-4
Std error
KM (M)
Std error
Kcat (h-1)
5.02 × 103
1.24 × 10-4
1.90 × 10-3
8.80 × 10-4
Table 4. Kinetic parameters for the oxidative coupling of OAPH catalyzed by 1
Solvent
MeOH
MeCN
V
max (M s-1)
Std error
KM (M)
Std error
Kcat (h-1)
1.27 × 10-4
1.42 × 10-4
9.15 × 10-5 3.84 × 10-3 8.47 × 10-4 4.59 × 103
9.85 × 10-5 6.04 × 10-4 1.04 × 10-4 5.12 × 103

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Fig. 1. An ORTEP diagram of [Co(L)2](ClO4)3 (1); 30% probability ellipsoids for all non-
hydrogen atoms.
Fig. 2. Change in spectral pattern of 1 after reaction with 3,5-DTBC, observing the reaction for 6
h in methanol; inset: spectrum of pure 3,5-DTBC in methanol

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

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Fig. 5. Change in spectral pattern of 1 in MeOH after reaction with OAPH, observing the
reaction for 6 h; inset: spectrum of pure OAPH in methanol
Fig. 6. Change in spectral pattern of 1 in MeCN after reaction with OAPH, observing the
reaction for 6 h; inset: spectrum of pure OAPH in methanol

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Fig. 7. A plot of the change in absorbance with time to evaluate the initial rate of the catalytic
oxidation of OAPH by 1 in MeOH
Fig. 8. A plot of the change in absorbance with time to evaluate the initial rate of the catalytic
oxidation of OAPH by 1 in MeCN

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Fig. 9. Plot of rate vs. [substrate] (OAPH) in presence of 1 in MeOH; inset: Lineweaver-Burk
plot.
Fig. 10. Plot of rate vs. [substrate] (OAPH) in presence of 1 in MeCN; inset: Lineweaver-Burk
plot.

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•
•
•
Cobalt(III) complex
Bioinspired catalysis
Catecholase and phenoxazinone synthase activities