Two mononuclear cobalt(III) complexes exhibiting phenoxazinone synthase activity

Article abstract of DOI:10.1002/aoc.4336

Two mononuclear cobalt(III) complexes, namely [LCo(tmtp)(H₂O)]ClO₄?MeOH (1) (tmtp = tri(m-tolyl)phosphine) and [LCo(PPh₃)(H₂O)]PF₆ (2), have been prepared from a polydentate ligand, N,N′-bis(3-methoxy

Full text of DOI:10.1002/aoc.4336

Received: 1 November 2017
DOI: 10.1002/aoc.4336
Revised: 29 January 2018
Accepted: 3 February 2018
F U L L P A P E R
Two mononuclear cobalt(III) complexes exhibiting
phenoxazinone synthase activity
Mamata Mahato1 | Kristof Van Hecke2 | Hari Pada Nayek1
1 Department of Applied Chemistry,
Two mononuclear cobalt(III) complexes, namely [LCo(tmtp)(H2O)]
Indian Institute of Technology (Indian
School of Mines), Dhanbad 826004,
Jharkhand, India
2 XStruct, Department of Chemistry,
Ghent University, Krijgslaan 281‐S3, B‐
9000 Ghent, Belgium
ClO4⋅MeOH (1) (tmtp = tri(m‐tolyl)phosphine) and [LCo(PPh3)(H2O)]PF6 (2),
have
been
prepared
from
a
polydentate
ligand,
N,N′‐bis(3‐
methoxysalicylidehydene)cyclohexane‐1,2‐diamine (H2L). Standard analytical
techniques such as elemental analysis and UV–visible and Fourier transform
infrared spectroscopies were used to characterize both complexes. The solid‐
state molecular structures of both complexes were confirmed from single‐crys-
tal X‐ray diffraction analysis. Structural analyses show that the Co(III) ion
occupies the centre of a distorted octahedron in a complex cation: [LCo(tmtp)
(H2O)]+ and [LCo(PPh3)(H2O)]+ for 1 and 2, respectively. Phenoxazinone syn-
thase activities of both complexes were screened. Kinetic studies and other
experimental observations reveal that the reaction follows rate saturation kinet-
ics and proceeds through the formation of a catalyst (complex)–substrate
adduct. The turnover number (Kcat) of complex 2 is 54.07 h−1, exhibiting better
catalytic activity compared to 1 (Kcat = 45.11 h−1).
Correspondence
Hari Pada Nayek, Department of Applied
Chemistry, Indian Institute of Technology
(Indian School of Mines), Dhanbad –
826004, Jharkhand, India.
Email: hpnayek@iitism.ac.in
Funding information
Science and Engineering Research Board
(SERB), DST, India, Grant/Award Num-
ber: SB/EMEQ 013/2014
KEYWORDS
cobalt(III) complex, phenoxazinone synthase activity, Schiff base, single‐crystal X‐ray diffraction,
turnover number
1 | INTRODUCTION
most widely explored among all Schiff bases in coordina-
tion chemistry.[8] Modification of salicylaldimine ligands,
Schiff bases cover an enormous area in coordination
chemistry predominantly due to their smooth synthesis,
controllability of steric and electronic environment and
diversity in applications.[1] Schiff bases can easily accom-
modate single or multiple metal ions of various oxidation
states.[2] Judicial selection of Schiff bases and metal ions
often results in homo‐ or heteronuclear metal complexes
including those of transition metals and main group ele-
ments.[3] Depending on the nature of the ligands and
metal ions, these complexes exhibit structural diversities
and open up applications in several areas such as biol-
ogy,[4] catalysis,[5] materials science[6] and magnetism.[7]
Interestingly, salen‐type ligands, which are prepared from
various diamines and salicylaldehyde derivatives, are
for example by anchoring a methoxy group (―OMe) on
the salicylaldimine moiety, increases their denticity mak-
ing the ligands more prone to form polynuclear com-
plexes.[9] Numerous examples of metal complexes of
Schiff base ligands have been reported in the literature.
However, metal complexes containing cobalt(II/III) ions
have recently started to receive attention.[10] Cobalt(II/
III) complexes of Schiff base ligands find important appli-
cations in magnetism,[11] biochemistry[12] and cataly-
sis.[13] These complexes have been used in mimicking
important biological co‐factors such as phenoxazinone
synthase,[14] photosynthesis,[15] methionine aminopepti-
dase,[16] cobalamin, etc. Among these, phenoxazinone
synthase facilitates the conversion of o‐aminophenol
Appl Organometal Chem. 2018;e4336.
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Copyright © 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4336

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MAHATO ET AL.
(OAPH) to 2‐aminophenoxazine‐3‐one (APX) during the
synthesis of actinomycin D which is renowned for the
treatment of gestational choriocarcinoma, Wilms tumour,
rhabdomyosarcoma, etc.[17] Although the active site of
phenoxazinone synthase contains five Cu(II) ions respon-
sible for conversion of OAPH to APX,[18] several catalysts
(complexes) containing Co(II/III) centres have been devel-
oped as model systems to understand the role of
in all experimental work. The ligand H2L was synthesized
following a reported process.[23] Elemental (C, H and N)
analyses were performed with a FLASH EA 1112 series
(Thermo Finnigan, Italy). FT‐IR spectra were recorded
with a Cary 600 series FT‐IR spectrometer in the range
400–4000 cm−1. A Shimadzu UV‐1800 spectrometer was
used to record the absorption spectra of all complexes in
methanol. Electrospray ionization mass analyses were
carried out with a Waters UPLC‐TQD mass spectrometer.
phenoxazinone synthase in such
a
conversion.[19]
Scheme 1 shows the conversion of OAPH to APX. Several
mechanistic pathways are available for such a conversion.
However, the most widely accepted one involves a quinone
imine intermediate (I), which combines with a second
molecule of OAPH resulting in an intermediate (II), which
is then converted to p‐quinoneimine (III) upon two‐elec-
tron oxidation processes. Finally, a phenoxazinone chro-
mophore is obtained by another conjugate addition and
subsequent two‐electron oxidation of it.[20]
While working with a Schiff base ligand, N,N′‐bis(3‐
methoxysalicylidehydene)cyclohexane‐1,2‐diamine (H2L),
we have recently reported several homo‐ or heteronuclear
Ni(II)–Na(I), Co(III)–Na(I) and Co(III) complexes. We
have also investigated the biological activity of the
Ni(II)–Na(I) complex.[21] In addition, we were able to
show phenoxazinone synthase activities of two Co(III)
complexes of H2L.[22] In the work reported in the present
contribution, we used the ligand H2L to synthesize two
new mononuclear Co(III) complexes, [LCo(tmtp)(H2O)]
ClO4⋅MeOH (1) (tmtp = tri(m‐tolyl)phosphine) and
[LCo(PPh3)(H2O)]PF6 (2) (PPh3 = triphenylphosphine).
The solid‐state molecular structures of both complexes
were investigated using single‐crystal X‐ray diffraction
analyses. The complexes were further characterized using
elemental analyses and UV–visible and Fourier transform
infrared (FT‐IR) spectroscopies. Phenoxazinone synthase
activities of complexes 1 and 2 were investigated. Various
important kinetic parameters such as Vmax, KM and Kcat
were evaluated.
2.2 | Synthesis of 1
A solution of H2L (0.1911 g, 0.5 mmol) in 10 ml of metha-
nol was added to a solution of Co(OAc)2⋅4H2O (0.1245 g,
0.5 mmol) in methanol (10 ml). The mixture immediately
turned brown and was stirred for 15 min. Tri(m‐tolyl)phos-
phine (0.1522 g, 0.5 mmol) was added to it and stirred for
1 h with bubbling air. Solid NaClO4 (0.0702 g, 0.5 mmol)
was added to the mixture and stirred for an additional
0.5 h. The resulting mixture was filtered and kept for slow
evaporation at room temperature in open atmosphere.
Dark green single crystals suitable for single‐crystal X‐ray
diffraction analysis were achieved after 13 days. Yield
0.3532 g (79%). Anal. Calcd for C44H51ClCoN2O10P (molec-
ular weight: 893.21) (%): C, 59.16; H, 5.75; N, 3.14. Found
(%): C, 58.21; H, 3.96; N, 3.05. FT‐IR (ATR, cm−1): 3354
(m), 2932 (m), 2855 (w), 1623 (s), 1550 (w), 1445 (s), 1312
(s), 1221 (s), 1078 (s), 976 (m), 860 (w), 777 (w), 737 (m),
690 (m), 618 (w), 553 (m), 452 (m).
2.3 | Synthesis of 2
A solution of H2L (0.1911 g, 0.5 mmol) in 10 ml of
methanol was added to a solution of Co(OAc)2⋅4H2O
(0.1245 g, 0.5 mmol) in methanol (10 ml). The mixture
immediately turned brown and was stirred for 15 min.
Triphenylphosphine (0.1311 g, 0.5 mmol) was added to
it and stirred for 1 h with bubbling air. Solid KPF6
(0.092 g, 0.5 mmol) was added to the mixture and stirred
for an additional 0.5 h. The resulting mixture was fil-
tered and kept for slow evaporation at room temperature
in open atmosphere. Dark green single crystals suitable
for single‐crystal X‐ray diffraction analysis were obtained
after 18 days. Yield 0.3312 g (74 %). Anal. Calcd for
C40H41CoF6N2O5P2 (molecular weight: 864.62) (%): C,
55.56; H, 4.78; N, 3.24. Found (%): C, 54.50; H, 3.41; N,
3.53. FT‐IR (ATR, cm−1): 3370 (m), 2934 (w), 2867 (w),
2 | EXPERIMENTAL
2.1 | General and Analytical Techniques
All chemical reactions were executed under aerobic con-
ditions. All reagents employed in the present work were
of analytical grade and obtained from Sigma Aldrich,
India, or Alfa Aesar, India. Distilled solvents were used
SCHEME 1 Conversion of o‐aminophenol (OAPH) to 2‐aminophenoxazine‐3‐one (APX)

MAHATO ET AL.
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1624 (s), 1604 (s), 1551 (m), 1470 (m), 1438 (s), 1317 (s),
1248 (s), 1220 (s), 1090 (s), 1030 (m), 977 (m), 836 (s),
734 (m), 693 (m), 556 (m), 518 (m).
10−5 M for catalysts in a solution of total volume of
2.5 ml. Initial rate was calculated from the absorbance
value recorded at 150 s after addition of the solutions.
2.5 | X‐ray Crystallography
2.4 | Phenoxazinone Synthase Activity
Study
Single crystals of 1 and 2 suitable for X‐ray diffraction were
mounted on an Oxford Diffraction XCalibur Eos instru-
ment equipped with Cu Kα radiation (λ = 1.54184 Å)
and Mo Kα radiation (λ = 0.71073 Å) at T = 100(2) K,
respectively. CrysAlisPro software was used for data col-
lection and data reduction. The structure was solved by
direct methods (SIR92)[24] followed by refinement on F2
by full‐matrix least‐squares methods using SHELXL‐
2013.[25] Non‐hydrogen atoms such as C, P, N, O, Cl, F
and Co were refined anisotropically. Hydrogen atoms
were included using appropriate AFIX command. The
The phenoxazinone synthase activities of complexes 1
and
2 were investigated by reacting them (1 ×
10−5 M) with OAPH (1 × 10−3 M) in methanol
(25 °C). The progress of the reaction was monitored by
measuring the increase in absorbance at 433 nm as a
function of time. To determine important kinetic
parameters and to evaluate dependence of the reaction
rate on substrate concentration, a solution of catalyst
(1 × 10−5 M) in methanol was treated with a set of solu-
tions of the substrate in methanol (1 × 10−3–10 ×
10−3 M). Initially, stock solutions of OAPH as well as
complexes of desired concentrations were prepared by
dissolving calculated amounts of them in 10 ml of
methanol. Then, 2 ml of each OAPH solution was
added to 0.5 ml of catalyst solution to achieve concen-
trations of 1 × 10−3–10 × 10−3 M for OAPH and 1 ×
h
i
ꢀ
ꢁ
ꢂ
ꢀ
ꢁ
2
function minimized was Σw F2o−Fc2
(w ¼ 1= σ2 F2o
ꢀ
ꢀ
ꢁ
ꢁ
2
þðaPÞ þ bPꢀ), where P ¼ Max F2o; 0 þ 2Fc2 =3 with
ꢀ
ꢁ
σ2 F2o from counting statistics. The functions R1 and
wR2 were (σ||Fo|
−
|Fc||)/σ|Fo| and
F2o−F2c =σðwFo4ꢀ1=2, respectively. Crystallographic data
½σw
ꢀ
ꢁ
2
for the structures reported in this paper have been
deposited at the Cambridge Crystallographic Data Cen-
tre as supplementary publication no. CCDC 1579029‐
1579030. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cam-
bridge CB2 1EZ, UK (fax: +(44)1223‐336‐033; email:
deposit@ccdc.cam.ac.uk).
3 | RESULTS AND DISCUSSION
3.1 | Synthesis
The procedure for the synthesis of the Schiff base ligand
SCHEME 2 Synthesis of complexes 1‐2
H2L was previously established.[23] H2L was prepared
FIGURE 1 Molecular structures of complexes 1 and 2. Hydrogen atoms of the ligands, as well as a methanol solvent molecule are omitted
for clarity

4 of 8
MAHATO ET AL.
by refluxing o‐vanillin with 1,2‐diaminocyclohexane in
ethanol. Two mononuclear Co(III) complexes 1 and 2
were prepared by reacting H2L with cobalt(II) acetate
tetrahydrate in the presence of tmtp and PPh3, respec-
tively (Scheme 2). Additionally, anions perchlorate
(ClO4 ) or hexafluorophosphate (PF6 ) were employed
for neutralizing the charge of cationic complexes in these
compounds. Detailed synthetic processes for complexes 1
and 2 are described in Section 1.
Both complexes were dark green in colour, stable in
air and soluble in standard organic solvents. Successful
syntheses of 1 and 2 were established using various
analytical techniques, for instance, FT‐IR spectroscopy,
elemental analysis and UV–visible spectroscopy. In
addition, solid‐state molecular structures of 1 and 2
were confirmed using single‐crystal X‐ray diffraction
analyses.
3.2 | FT‐IR Spectroscopy
The FT‐IR spectra of complexes 1 and 2 show a character-
istic band of azomethine (―C¼N) stretching at 1623 and
1624 cm−1, respectively (Figures S1 and S2).[26] These
bands are shifted to lower energy compared to the free
ligand (1631 cm−1; Figure S3) confirming the coordina-
tion of the azomethine nitrogen to the cobalt(III) ion in
these complexes. A broad band due to ―OH (of a coordi-
nated water molecule) stretching appears at approxi-
mately 3350 cm−1 for both complexes. An intense band
observed at around 1078 cm−1 is assigned to the
–
–
−
stretching vibration of a non‐coordinated ClO4 anion in
1.[19c, 26c] The spectrum of complex 2 exhibits a moder-
ate band at 836 cm−1 caused by stretching vibration of
the PF6 anion.[27] In addition, aliphatic C―H stretching
−
resonances are observed in the range 2933–2937 cm−1 for
both complexes.
TABLE 1 Single‐crystal data and structure refinement parame-
ters for complexes 1 and 2
3.3 | Description of Structures
Single‐crystal X‐ray diffraction was employed to deter-
mine the solid‐state structures of complexes 1 and 2. Both
complexes crystallize in the monoclinic space group P21/c
with four formula units per unit cell. The solid‐state
1
2
Formula
Formula mass
T (K)
C44H51ClCoN2O10P C40H41CoF6N2O5P2
893.21
100(2)
864.62
100(2)
λ (Å)
1.54184
0.71073
TABLE 2 Selected bond lengths (Å) and bond angles (°) of com-
plexes 1 and 2
Crystal dimensions
(mm)
0.22 × 0.14 × 0.05
0.22 × 0.15 × 0.11
1
2
Crystal system
Space group
a (Å)
Monoclinic
P21/c
Monoclinic
P21/c
Co1–N1
1.899(3)
1.899(3)
1.900(3)
1.899(3)
2.051(2)
2.253(11)
85.86(14)
93.11(13)
94.63(13)
85.84(11)
87.41(11)
87.77(11)
86.01(12)
87.39(12)
90.68(8)
89.92(9)
96.26(10)
94.55(10)
1.898(2)
Co1–N2
1.8933(18)
1.8825(15)
1.8880(15)
2.0609(16)
2.2456(6)
85.67(8)
93.48(8)
94.55(7)
85.87(7)
88.15(7)
87.78(7)
87.63(9)
86.61(7)
90.72(5)
88.00(5)
96.59(7)
94.55(6)
175.70(5)
12.4463(3)
20.4390(5)
18.8469(5)
119.063(2)
4190.8(2)
4
13.8631(5)
17.1662(7)
16.6104
110.385(10)
3705.3(2)
4
Co1–O2
b (Å)
Co1–O3
c (Å)
Co1–O5
β (°)
V (Å3)
Co1–P1
N1–Co1–N2
N1–Co1–O2
N2–Co1–O3
O2–Co1–O3
O2–Co1–O5
O3–Co1–O5
N1–Co1–O5
N2–Co1–O5
O2–Co1–P1
O3–Co1–P1
N1–Co1–P1
N2–Co1–P1
O5–Co1–P1
Z
Dc (g cm−3
)
1.416
1.550
μ (mm−1
)
4.667
0.629
F(000)
1872.0
1784.0
2θ range (°)
6.892–150.727
40 388
4.746–55.106
36 332
Measured reflections
Independent
reflections/Rint
8486/0.0708
8531/0.0267
Parameters
547
0.0698
531
0.0464
R1 (I > 2σ(I))
wR2 (all data)
0.2054
0.1116
Goodness‐of‐fit on F2
Δρmax,min, e Å−3
1.072
1.032
1.03, −0.62
0.83, −0.84
177.10(8)

MAHATO ET AL.
5 of 8
1
1
molecular structures of 1 and 2 are shown in Figure 1.
Detailed data for the X‐ray diffraction analysis are pro-
vided in Table 1. Each complex contains a Co(III) ion
occupying the N2O2 cavity of a deprotonated ligand
(L2−). One molecule of water and tmtp (1) or PPh3 (2)
are bonded to the Co(III) centre axially, thus resulting a
distorted octahedral geometry around it. The complex cat-
transition bands are those of A1g → T1g and A1g →
1T2g transitions, respectively.[30] A band at approximately
1
760 nm (Figure S4) is assigned to low‐energy A1g
→
1T1g transition. However, high‐energy A1g → T2g tran-
sition is not observed in the UV–visible region. Possibly,
the high‐energy charge transfer transition has masked
the expected d–d transition in that region.
1
1
ions [LCo(tmtp)(H2O)]+ and [LCo(PPh3)(H2O)]+ are
−
associated
with
a
perchlorate
(ClO4
)
and
3.5 | Phenoxazinone Synthase Activity
hexafluorophosphate (PF6−) anions, for 1 and 2, respec-
tively. Moreover, a molecule of methanol can be found
in 1 as solvent of crystallization. The Co―N and Co―O
bond lengths in the N2O2 plane of the ligand are
1.899(3) and 1.889(3)–1.900(3) Å in 1 and 1.8933(18)–
1.898(2) and 1.8825(15)–1.8880(15) Å in 2, respectively.[28]
The Co―Oaxial (O5) bond lengths are 2.051(2) and
2.0609(16) Å, in complexes 1 and 2, respectively. As antic-
ipated, the Co―Oaxial bond length is longer than the
Co―Oequitorial bond length. The Co―P bond lengths of
2.2456(6) and 2.2538(11) Å in 1 and 2 are close to reported
values.[28] Selected bond lengths and angles are given in
Table 2.
and Kinetic Studies
Conversion of OAPH to APX is generally monitored by
UV–visible spectroscopy. The change in absorbance due
to the formation of APX in the reaction mixture is
recorded at 433 nm. Therefore, a gradual increase in
absorbance with time implies the formation of APX in
solution. In the present study, the growth of the
3.4 | UV–Visible Absorption Spectroscopy
UV–visible spectra of complexes 1 and 2 were recorded
in methanol in the range 200–1100 nm at ambient
temperature. The absorption spectra (200–600 nm) of
complexes 1 and 2 are shown in Figure 2. Absorption
bands because of intra‐ligand π–π* electronic transitions
are observed in the high‐energy region of 250–307 nm
for both complexes. A high‐energy band at around
410 nm is attributed to ligand‐to‐metal charge transfer
transition.[29] In addition to that, the spectra of both
complexes show anticipated d–d transition bands. For
an octahedral Co(III) complex, the two expected d–d
FIGURE 3 Increase in absorption of 2‐aminophenoxazine‐3‐one
at 433 nm after addition of 0.01 M OAPH to a solution containing
(a) complex 1 and (b) complex 2 in methanol up to 2.5 h under
aerobic condition at 25 °C
FIGURE 2 UV–visible spectra of complexes 1 and 2

6 of 8
MAHATO ET AL.
phenoxazinone chromophore of a solution of 1 × 10−3 M
OAPH and 1 × 10−5 M of respective complexes (1 and 2)
using methanol as solvent was recorded under aerobic
conditions at temperature 25 °C for 2 h with an interval
of 10 min (Figure 3). Base was not added to avoid autoxi-
dation of OAPH. A blank test was performed in the
absence of catalyst under identical experimental
conditions. No significant amount of APX is observed
(Figure S5) supporting the role of catalyst in such
reaction.
Further, kinetic studies were performed using OAPH
as substrate and complexes 1 and 2 as catalysts. Under
aerobic conditions, a solution (1 × 10−5 M) of each cata-
lyst was mixed with 1 × 10−3–10 × 10−3 M solution of
OAPH in methanol. It was observed that the reactions
follow pseudo‐first‐order kinetics. Plots of the initial rates
of these reactions against the concentration of OAPH for
both complexes are depicted in Figure 4, which shows
rate saturation kinetics for both complexes. Such a finding
is often observed in enzyme catalysis, where a moderately
stable enzyme (catalyst)–substrate adduct is formed,
which follows a Michaelis–Menten equation. The lineari-
zation of the Michaelis–Menten equation results in a
Lineweaver–Burk plot (Figure 4, insets) from which vari-
ous important parameters such as KM (Michaelis con-
stant) and Vmax (maximum reaction velocity) are
determined. These values are listed in Table 3. The turn-
over numbers of both complexes are in the range of those
of previously reported Co(III) complexes.[19c, 22, 31] Complex
2, with a Kcat value of 54.07 h−1, exhibits better catalytic
activity compared to 1 (Kcat = 45.11 h−1).
3.6 | Mass Spectrometry
The electrospray ionization (ESI) mass spectra of a mix-
ture of OAPH and respective complexes (100:1) were
recorded using methanol as solvent within 5 min of addi-
tion of methanol to the mixture. The spectra are presented
in Figures S6–S9 in the supporting information. The for-
mation of APX chromophore was ascertained by the
appearance of a peak at m/z ~ 213.57 for both complexes.
A peak at m/z = 439.54 indicates the formation of [Co(L)]
+
for both complexes. Complex cationic fragments
[LCo(tmtp)]+ and [LCo(PPh3)]+ were observed at m/z =
743.44 and 701.50 for complexes 1 and 2, respectively.
TABLE 3 Kinetic parameters for complexes 1 and 2
Complex
Vmax
KM
Kcat (h−1
)
1
2
12.53 × 10−8
15.02 × 10−8
1.719 × 103
1.624 × 103
45.11
54.07
FIGURE 4 Plot of initial rate versus substrate concentration in
the conversion of OAPH to APX in methanol for (a) complex 1
and (b) complex 2. Respective double reciprocal or Lineweaver–
Burk plots are shown in the insets
FIGURE 5 Cyclic voltammograms of 1 and 2 in methanol

MAHATO ET AL.
7 of 8
SCHEME 3 Probable mechanistic
pathway for the conversion of
2‐aminophenol to phenoxazinone
chromophore
Moreover, the formation of catalyst–substrate adducts
[LCo(tmtp)(OAP)]+ and [LCo(PPh3)(OAP)]+ are con-
firmed by the peaks at m/z = 853.40 and 808.41 for 1
and 2, respectively.
to APX chromophore catalysed by complex 2 is proposed,
as shown in Scheme 3. Initially, complex 2 generates a
[LCo(PPh3)]+ ion in solution which combines with OAPH
to form a catalyst–substrate adduct, [LCo(PPh3)(OAP)],
which results in the formation of an OAP radical by
reacting with molecular oxygen. The OAP radical con-
verts o‐benzoquinone monoamine (BQMI), which finally
reacts with OAPH and molecular oxygen to yield APX
through many oxidative dehydrogenation processes. In
order to evaluate the role of molecular oxygen in the con-
version of OAPH to APX, the growth of APX at 433 nm
was monitored in methanol in an argon atmosphere in
the same time span using both complexes. The growth
of APX is significantly reduced under such conditions.
Therefore, it clearly demonstrates that dioxygen plays an
important role in such catalytic conversion.
3.7 | Electrochemical Studies
Being susceptible to reduction and oxidation, the electro-
chemical properties of Co(III/II) systems are of prime
interests. The electrochemical data for both complexes
were collected by maintaining a concentration of 1 mmol
for the complexes in methanol along with 0.1 M solution
of tetrabutylammonium bromide as electrolyte with a
scan rate of 100 mV s−1 in aerobic environment. A plati-
num electrode functioned as the working electrode while
Ag/AgCl served as the reference electrode. Figure 5 shows
the cyclic voltammograms of complexes 1 and 2. Peaks
were obtained at −0.62 and −0.65 V which correspond
to reduction of Co(III) to Co(II) for complexes 1 and 2,
respectively. These results indicate that both complexes
may not be suitable for catalysing any oxidation reactions
under aerobic condition in methanol. However, their elec-
trochemical behaviour may be influenced by the forma-
tion of substrate–catalyst adduct, thus favouring
oxidation of OAPH in methanol. Additionally, the nega-
tive potential favours regeneration of Co(III) catalyst via
oxidation of Co(II) under the experimental conditions.
4 | CONCLUSIONS
We have successfully synthesized two mononuclear
Co(III) complexes of a polydentate Schiff base ligand
(H2L). The Co(III) ion is positioned in the N2O2 cavity of
the ligand. Both complexes ionize in a solution of metha-
nol and exhibit excellent phenoxazinone synthase activ-
ity. Kinetic studies and ESI‐MS analysis indicate that the
conversion of 2‐aminophenol to the phenoxazinone chro-
mophore passes through the formation of a catalyst–sub-
strate adduct. Complex 2 shows better (Kcat = 54.07 h−1
catalytic activity than 1 (Kcat = 45.11 h−1).
)
3.8 | Probable Mechanism
Now, it is established that complexes 1 and 2 dissociate in
methanol to generate [LCo(tmtp)]+ and [LCo(PPh3)]+,
respectively. These are the active forms of 1 and 2 respon-
sible for catalytic conversion of OAPH to APX. It is also
evident from the kinetic studies and mass spectral analy-
sis that conversion of OAPH to APX proceeds through
ACKNOWLEDGEMENTS
The present work is financially supported by the Science
and Engineering Research Board (SERB), DST, India
(order no. SB/EMEQ 013/2014, dated 17 June 2014). The
Authors are grateful to SAIF, Panjab University for pro-
viding analytical facilities. K.V.H. thanks the Special
Research Fund (BOF)–UGent for funding.
the formation of
a catalyst–substrate adduct, i.e.
[LCo(tmtp)(OAP)]+ and [LCo(PPh3)(OAP)]+ for com-
plexes 1 and 2, respectively. Based on these experimental
observations, a mechanism for the conversion of OAPH

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MAHATO ET AL.
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SUPPORTING INFORMATION
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in the supporting information tab for this article.
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How to cite this article: Mahato M, Van Hecke
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