Influence of anions and solvents on distinct coordination chemistry of cobalt and effect of coordination spheres on the biomimetic oxidation of o-aminophenols

Article abstract of DOI:10.1016/j.mcat.2018.02.013

The present work reports the synthesis and structural characterizations of five new cobalt complexes (1–5) resulting from a N₃O donor ligand, a Schiff base condensation product of N,N-dimethyldipropylenetriamine and 3-ethoxysalicylaldehyde, and their catalytic activity for the aerobic oxidation of various substrates, namely o-aminophenols and catechol. X-ray structural studies reveal that the Schiff base ligand can bind the metal centre either in the tetradentate fashion using all the available donor sites of the monoanionic deprotonated form (in 2 and 3) or in the tridentate fashion using the zwitterionic form of the Schiff base ligand with pendent quaternary amine nitrogen (in 1 and 4). Additionally, the monoanionic deprotonated ligand can bind the metal centre in the tridentate fashion (in 4 and 5) where the pendent tertiary amine nitrogen is involved in the intramolecular hydrogen bonding. All these adaptabilities associated with this triamine make it appealing candidate for exploration of the coordination chemistry with diverse structures. All complexes except compound 1 are efficient functional models for phenoxazinone synthase, and as expected the availability of labile sites at the first coordination sphere for the substrate, o-aminophenol, binding is the governing factor for higher catalytic activity in 2 and 3. Requisite of the hydrogen bond acceptor centre as well as proton abstraction centre at the second coordination sphere behind the facile oxidation of the substrate were also explored (reactivity of 1 versus 4 and 5). Moreover, the broader catalytic ability of these complexes was examined with substituted aminophenol and catechol as substrates, and the results were assembled and analysed. Furthermore, emphasis was given to get insight into the mechanistic pathway of functioning phenoxazinone synthase activity, well supported by the mass spectral study.

Full text of DOI:10.1016/j.mcat.2018.02.013

MolecularCatalysis449(2018)49–61
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Influence of anions and solvents on distinct coordination chemistry of cobalt
and effect of coordination spheres on the biomimetic oxidation of o-
aminophenols
Anangamohan Panjaa,⁎, Narayan Ch. Janaa, Paula Brandãob
a
Postgraduate Department of Chemistry, Panskura Banamali College, Panskura RS, WB 721152, India
Department of Chemistry, CICECO-Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Cobalt complexes
Structural diversity
Biomimetic catalytic study
Influence of coordination sphere on catalytic
activity
The present work reports the synthesis and structural characterizations of five new cobalt complexes (1–5)
resulting from a N3O donor ligand, a Schiff base condensation product of N,N-dimethyldipropylenetriamine and
3-ethoxysalicylaldehyde, and their catalytic activity for the aerobic oxidation of various substrates, namely o-
aminophenols and catechol. X-ray structural studies reveal that the Schiff base ligand can bind the metal centre
either in the tetradentate fashion using all the available donor sites of the monoanionic deprotonated form (in 2
and 3) or in the tridentate fashion using the zwitterionic form of the Schiff base ligand with pendent quaternary
amine nitrogen (in 1 and 4). Additionally, the monoanionic deprotonated ligand can bind the metal centre in the
tridentate fashion (in 4 and 5) where the pendent tertiary amine nitrogen is involved in the intramolecular
hydrogen bonding. All these adaptabilities associated with this triamine make it appealing candidate for ex-
ploration of the coordination chemistry with diverse structures. All complexes except compound 1 are efficient
functional models for phenoxazinone synthase, and as expected the availability of labile sites at the first co-
ordination sphere for the substrate, o-aminophenol, binding is the governing factor for higher catalytic activity
in 2 and 3. Requisite of the hydrogen bond acceptor centre as well as proton abstraction centre at the second
coordination sphere behind the facile oxidation of the substrate were also explored (reactivity of 1 versus 4 and
5). Moreover, the broader catalytic ability of these complexes was examined with substituted aminophenol and
catechol as substrates, and the results were assembled and analysed. Furthermore, emphasis was given to get
insight into the mechanistic pathway of functioning phenoxazinone synthase activity, well supported by the
mass spectral study.
Mass spectrometry and plausible mechanism
1. Introduction
oxygen is a challenging tusk because of its kinetic restriction that is
associated with conversion of triplet ground state to singlet state of
Oxidation reaction plays a vital role in organic chemistry for the
synthesis of fine organic chemicals as well as in manufacture of large-
scale petrochemical compounds [1,2]. The majority of these industrial
processes involve the stoichiometric oxidation with traditional oxi-
dants, such as permanganate, dichromate and heavy metal oxides,
thereby producing vast wastes which are causing environmental pol-
lution. Pressure from society has placed restrictions on such industrial
oxidation methods, with emphasis on the need for sustainable and en-
vironmentally friendly processes. As a result, a greater attention has
been given on the development of novel and efficient catalytic pro-
cesses with use of molecular oxygen as a sole oxidant, which is en-
vironmentally benign and show a high efficiency per weight of oxidant.
However, the direct oxidation of organic substrates by molecular
oxygen, and most of the time selectivity does not reach to the desired
level [3–6]. Nature has developed several metallo-oxidases to make
controlled aerobic oxidations under highly mild conditions in which
metalloenzymes activate molecular oxygen for the synthesis of nu-
merous biochemically important compounds with great regio- and
stereo-selectivity [7–9]. Bioinorganic chemists have paid a lot attention
in unravelling the catalytic mechanism of these enzymes by means of a
synthetic analogue approach [10–12]. This approach helps to get in-
sight into the structures of the active sites and reactive intermediates
and the mechanistic details of dioxygen activation and oxidation re-
actions occurring at the active sites [13–19]. Mechanistic studies are
clearly a means of developing better synthetic methods, which in turn
help in searching efficient catalysts as alternatives of traditional toxic
⁎
Corresponding author.
E-mail address: ampanja@yahoo.co.in (A. Panja).
https://doi.org/10.1016/j.mcat.2018.02.013
Received 2 August 2017; Received in revised form 12 February 2018; Accepted 14 February 2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

A. Panja et al.
MolecularCatalysis449(2018)49–61
industrial catalysts. Therefore, the application of various oxidative
transformations involving metal catalysis to specific transformations of
different organic substrates, such as aromatic hydrocarbons, alkanes,
and oxygen-containing compounds, by molecular dioxygen and their
mechanistic exploration is the subject of growing interest [20].
We are now involved in developing biomimetic catalysts for the
oxidation of 3,5-di-tert-butylcatechol and o-aminophenol by molecular
dioxygen, modelling the structures and functions of catechol oxidase
[21,22] and phenoxazinone synthase [23,24]. The latter is a multi-
copper metalloenzyme that catalyses the oxidative coupling of a wide
variety of substituted o-aminophenols to phenoxazinone chromophore
in the final step for the biosynthesis of actinomycin D in aerobic con-
dition [25]. This potent antineoplastic agent actinomycin D is well
known for its wide clinical application for the treatment of many tu-
mours, including Wilm’s tumour, where the phenoxazinone chromo-
phore intercalates with DNA base-pairs, thereby abnormal functioning
can be stopped by preventing the DNA dependent RNA synthesis
[26,27]. The active site structures of several oxidase enzymes and the
structures of their efficient synthetic analogues with transition metals
[21–25,28–36] clearly suggest that the functional models must have
labile or vacant coordination sites available for substrate binding as a
primary requirement for the efficient in vitro catalysis. Therefore, the
ligands with fewer donor centres could be the buffeting choice for the
development of synthetic analogues. Transition metal complexes with
Schiff base ligands have been provided a large number of enzymatic
models mainly because of their synthetic simplicity, and thereby fine
tuning of the catalytic activity may be achieved by modification of the
coordination environment at the metal centre by judicious choice of
amine and carbonyl parts of the Schiff bases. It is also well known that
the extraordinary catalytic activity of metalloenzymes is mainly regu-
lated by the participation of protein chain through the substrate re-
cognition and stabilisation of the intermediates exploiting various
noncovalent forces [37–39]. Therefore, the design and synthesis of
coordination compounds beyond the first coordination sphere could be
the right way of developing efficient biomimetic catalysts.
We have now started developing coordination chemistry with the
Schiff base ligands derived from the triamine N,N-dimethyldipropyle-
netriamine as the coordination chemistry with this amine was almost
unexplored [40]. Moreover, this amine may readily produce N3O donor
Schiff base ligands when reacts with salicylaldehyde or its derivative.
Such ligands by the reaction with transition metal ions could produce
synthetic analogues having either vacant or labile position(s) available
for substrate binding. Furthermore, the presence of different kind of
functionality in the Schiff base ligands derived from this amine, namely
imine, amines (both secondary and tertiary), and phenol (from salicy-
laldehyde part), may allow fine tuning of the ligand field around the
metal centre, leading to significant modification of the catalytic effi-
ciency. In continuation to our long standing interest in searching better
biomimetic catalysts [21–24], in this present work, Schiff base HL
(Scheme 1) derived from N,N-dimethyldipropylenetriamine and 3-
ethoxysalicylaldehyde was allowed to react with cobalt(II) salts in the
presence of different counter ions, and finally we ended up with the
isolation of five new cobalt complexes, [Co(HL)2](ClO4)3·2H2O (1), [Co
(L)(N3)2] (2), [Co(L)(NCS)2] (3), [Co(HL)(L)][Co(NCS)4]·0·5CH3OH (4)
and [Co(L)2]2[Co(NCO)4] (5). Their structural diversities and biomi-
metic catalytic activities with various substrates such as o-amino-
phenol, 2-amino-5-methylphenol and 3,5-di-tert-butylcatechol in
aerobic condition have been critically analysed. Remarkably, the in-
fluence of solvents and counter ions on the diverse coordination
chemistry of cobalt was observed. Moreover, the flexible donor prop-
erty associated with N,N-dimethyldipropylenetriamine part of the
Schiff base was also explored. Further emphasis was also given to get
insight into the structure-property correlation to justify the reactivity
trend of the complexes.
2. Experimental section
2.1. Materials and physical measurements
Reagent or analytical grade chemicals such as cobalt(II) nitrate
hexahydrate, cobalt(II) perchlorate hexahydrate, o-aminophenol
(OAPH), 2-amino-5-methylphenol, 3-ethoxysalicylaldehyde, and N,N-
dimethyldipropylenetriamine were purchased from commercial sources
and used as received. Solvents were also reagent grade and used
without further purification.
Caution! Azide and perchlorate salts of metal complexes especially
with organic ligands are potentially explosive. Only a small quantity of
material should be prepared at a time and it should be handled with
great care.
Elemental analyses for carbon, hydrogen and nitrogen were per-
formed using a Perkin-Elmer 240C elemental analyser. IR spectra were
recorded on a PerkinElmer Spectrum Two FTIR spectrophotometer in
the range 400–4000 cm−1 with the samples prepared as KBr pellets.
UV–vis spectrophotometric studies were carried out in an Agilent Carry-
60 diode array UV–vis spectrophotometer at room temperature. Cyclic
voltammetric studies were performed at room temperature in methanol
with tetrabutylammonium perchlorate as a supporting electrolyte on a
CH Instrument electrochemical workstation model CHI630E with three-
electrodes assembly comprising of a platinum working electrode, a
platinum wire auxiliary electrode and a Ag/AgCl reference electrode.
Electrospray ionization mass spectra (ESI–MS positive) were recorded
in a Micromass Q-tof-Micro Quadruple mass spectrophotometer.
2.2. Synthesis of the Schiff-base ligand (HL)
Tetradentate N3O donor Schiff base ligand (HL) was synthesized by
the
condensation
reaction
of
1.0 mmol
of
N,N-
Dimethyldipropylenetriamine (159 mg) and 1.0 mmol of 3-ethox-
ysalicylaldehyde (166 mg) in 20 ml of methanol (or acetonitrile).
Mixture of these reactants in methanol was allowed to reflux for ca. 1 h,
and then cooled. The in situ prepared Schiff base ligand was used di-
rectly for the synthesis of the subsequent metal complexes as described
below.
2.3. Synthesis of [Co(HL)2](ClO4)3·2H2O (1)
Addition of Co(ClO4)2·6H2O (0.730 g, 2.0 mmol) dissolved in 20 ml
of methanol to the Schiff base ligand HL (2.0 mmol) solution at room
temperature instantly produced a dark-brown solution. The mixture
was stirred in air for 30 min during which time light brown powders
separated out from the solution. It was collected by filtration and wa-
shed with mother liquor followed by methanol/ether and finally air
dried. Yield: 0.867 g (85%). Dark-brown crystals suitable for X-ray
analysis were obtained from slow evaporation of acetonitrile-methanol
mixture of the complex at ambient temperature within few days. Anal.
Calcd. for C70H128Co2N12O35Cl6: C 41.45%, H 6.36%, N 8.29%. Found:
C 41.28%, H 6.42%, N 8.35%. FTIR (KBr, cm−1): ν(N3) 2050 vs; ν(C]
N) 1632 s; ν(ClO4) 624 s, 1104 vs.
2.4. Synthesis of [Co(L)(N3)2] (2)
Co(NO3)2·6H2O (291 mg, 1.0 mmol) and Schiff base ligand HL
(1.0 mmol) were mixed together in 40 ml acetonitrile, and to the mix-
ture 2 ml aqueous solution of sodium azide (130 mg, 2.0 mmol) was
added with stirring. The resulting solution was then heated to reflux for
about 30 min during which time colour of the solution changed to dark
brown. The reaction mixture was then filtered and kept at ambient
temperature for slow evaporation. Analytically pure dark-brown crys-
tals suitable for X-ray diffraction study were separated out from the
solution after few days, which was collected by filtration and washed
with methanol/ether and air dried. Yield: 392 mg (86%). Anal. Calcd.
50

A. Panja et al.
MolecularCatalysis449(2018)49–61
Scheme 1. Schiff base HL.
for C17H28CoN9O2: C 45.43%, H 6.28%, N 28.05%. Found: C 45.28%, H
attached (1.5 times for methyl hydrogens and 1.2 times for others).
While hydrogen atoms connected to nitrogen atoms were located on the
difference Fourier map and isotropically treated with given thermal
parameters equivalent to 1.2 times of the parent atoms. Molecular
graphics of 1–5 were generated with the programs like MERCURY-3.9.
Detailed information related to crystallographic data collection and
structure refinements are available in Table 1.
6.25%, N 27.90%. FTIR (KBr, cm−1): ν(N3) 2050 vs; ν(C]N) 1632 s.
2.5. Synthesis of complex [Co(L)(NCS)2] (3)
Complex 3 was synthesized from an acetonitrile/water (20:1; v/v)
mixture following a very similar procedure as described for 2, except
that NaSCN was used instead of NaN3. Colour: Dark brown, Yield:
395 mg (83%). Anal. Calcd. for C19H28CoN5O2S2: C 47.39%, H 5.86%,
N 14.54%. Found: C 47.48%, H 5.80%, N 14.43%. FTIR (cm−1, KBr):
ν(NCS) 2107 s, ν(C]N) 1625 s.
2.9. Catalytic oxidation of o-aminophenols/catechol
The catalytic oxidation of various substrates was carried out by the
reaction of 1 × 10−4 M or 5 × 10−5 M of the complexes with
1.0 × 10−2 M substrates in air saturated methanol at room tempera-
ture. Kinetics of the aerobic oxidation of o-aminophenols catalysed by
1–5 were investigated monitoring increase of the absorbance as a
function of time at their respective absorption maxima [43]. In order to
determine dependence of the rate on the substrate concentration and to
evaluate various kinetic parameters, 1 × 10−5 M solution of the com-
plexes were subjected to react with at least 10 equivalents of the sub-
strate maintaining the pseudo-first-order condition. All the kinetic
measurements were carried out for the period of 10 min and the initial
rate of the reaction was determined by the linear regression from slope
of the absorbance verses time plots.
2.6. Synthesis of complex [Co(HL)(L)][Co(NCS)4]·0·5CH3OH (4)
Complex 4 was synthesized from a methanol/water (20:1; v/v)
mixture following the same methodology that applied for the synthesis
of 2, but NaSCN was used in place of NaN3. Colour: Dark brown, Yield:
358 mg (72%). Anal. Calcd. for C77H118Co4N20O9S4: C 50.49%, H
6.49%, N 15.29%. Found: C 50.34%, H 6.50%, N 15.20%. FTIR (cm−1
,
KBr): ν(NCS) 2064 s, ν(C]N) 1618 s.
2.7. Synthesis of complex [Co(L)2]2[Co(NCO)4] (5)
Complex 5 was synthesized from an acetonitrile/water (20:1; v/v)
mixture adopting the identical procedure as described for 2, except that
NaNCO was used instead of NaN3. Colour: Dark brown, Yield: 295 mg
(78%). Anal. Calcd. for C72H112Co3N16O12: C 55.06%, H 7.19%, N
14.27%. Found: C 55.19%, H 6.98%, N 14.28%. FTIR (cm−1, KBr):
ν(NCO) 2201 vs, ν(C]N) 1629 s.
3. Results and discussion
3.1. Syntheses and general characterizations
Schiff base ligand HL was synthesized by the reaction of 3-ethox-
ysalicylaldehyde and N,N-dimethyldipropylenetriamine in a 1: 1 molar
ratio in methanol (Scheme 1). This Schiff base ligand by the reaction
with cobalt(II) perchlorate hexahydrate in a 2:1 molar ratio produced
[Co(HL)2](ClO4)3·2H2O (1) in high yield. Again treatment of this in situ
generated Schiff base with cobalt(II) nitrate hexahydrate in the pre-
sence of pseudohalides (N3− for 2, NCS− for 3 and 4, and NCO− for 5)
in a 1:1:2 molar ratio produced [Co(L)(N3)2] (2), [Co(L)(NCS)2]· (3),
[Co(HL)(L)][Co(NCS)4·0·5CH3OH (4) and [Co(L)2][Co(NCO)4] (5), in
high yield. Although the formation of these complexes was not de-
pendent on the stoichiometric ratio of the reactants, the influence of
solvents for the synthesis of distinct thiocyanate complexes (3 versus 4)
was observed. Whereas the reaction of cobalt nitrate hexahydrate,
Schiff base ligand, and sodium thiocyanate in acetonitrile/water mix-
ture resulted only isolable product 3; the same reaction from methanol/
water mixture afforded only compound 4 in high yield. Starting with
same reactants isolation of both 3 and 4 suggests that formation of the
complexes is highly dependent on the solvents used for the synthesis,
2.8. X-ray crystallography
X-ray diffraction data for 1–5 were collected using monochromated
Mo-Kα radiation (λ = 0.71073 Å) on a Bruker Smart Apex-II dif-
fractometer, equipped with a CCD area detector. Multiple scans in φ
and ω directions were made to increase the number of redundant re-
flections and were averaged during the refinement cycles. The images
obtained during the data collection for all complexes were processed
using the software SAINT-plus, and the multi-scan absorption correc-
tion method was then applied to minimize the absorption effects using
SADABS program [41]. The structures were solved by the direct method
and refined by the successive full-matrix least-squares cycles on F2
using SHELXL-v.2013 or SHELXL-v.2014 [42]. All the non-hydrogen
atoms were refined with anisotropic thermal parameters. Hydrogen
atoms connected to carbon atoms were placed in geometrically idea-
lized positions with fixed thermal parameters related to those they are
51

A. Panja et al.
MolecularCatalysis449(2018)49–61
Table 1
Crystal data and structure refinement parameters of complexes 1 to 5.
1
2
3
4
5
Empirical formula
C35.5H64CoN6O17.5Cl3
1014.21
302(2)
Monoclinic
P21/n
14.624(6)
13.434(5)
24.005(7)
90
C17H28CoN9O2
449.41
150(2)
Monoclinic
P21/n
14.919(2)
7.8906(11)
17.811(3)
90
96.642(5)
90
2082.6(5)
4
1.430
0.857
C19H28CoN5O2S2
481.51
150(2)
Orthorhombic
Pca21
25.8819(5)
8.7010(4)
9.7736(11)
90
90
90
2201.0(3)
4
1.453
C38.5H59Co2N10O4.5S4
980.05
150(2)
Monoclinic
P21/n
11.944(5)
16.115(6)
24.762(10)
90
C72H112Co3N16O12
1570.56
298(2)
Tetragonal
P42/n
17.2441(11)
17.2441(11)
12.8049(8)
90
90
90
3807.7(5)
2
1.370
Formula weight (g mol−1
)
Temperature (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
b (°)
100.29(3)
90
4640(3)
98.66(2)
90
4712(3)
γ (°)
volume (Å3)
Z
4
4
Dcalc (mg m−3
)
1.437
0.618
2104
1.382
0.931
2056
μ (mm−1
F(000)
)
0.995
1008
0.716
1666
940
θ Range (°)
Reflections collected
Independent reflections (Rint
2.147–28.346
109337
11555(0.0346)
7208
0/591
1.052
1.687–25.851
15172
3946(0.0456)
2978
0/269
1.061
2.341–27.162
14693
4784(0.0559)
3721
1/268
1.010
1.513–25.543
42476
8687(0.0750)
5370
1/564
1.066
2.848–26.394
51321
3893(0.0710)
3241
0/271
1.183
)
Observed reflections [I > 2σ(I)]
Restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R1 = 0.0809
wR2 = 0.2109
R1 = 0.1274
wR2 = 0.2502
0.599/–0.964
R1 = 0.0483
wR2 = 0.1094
R1 = 0.0708
wR2 = 0.1179
0.532/–0.416
R1 = 0.0410
wR2 = 0.0762
R1 = 0.0645
wR2 = 0.0823
0.370/–0.438
R1 = 0.0923
wR2 = 0.2004
R1 = 0.1496
wR2 = 0.2273
1.294/–0.866
R1 = 0.0850
wR2 = 0.1725
R1 = 0.1029
wR2 = 0.1828
0.693/–1.298
R indices (all data)
Largest diff. peak/hole (e Å−3
)
where the relative stabilities in a particular solvent system governs the
identity of the species that crystallizes from that solvent. These ob-
servations establish that both of them are thermodynamic products and
formation of them is controlled by the solvents used for their synthesis.
As usual the metal centre connected to the Schiff base ligand was air-
oxidized to cobalt(III) as found in the related complexes with similar
ligand systems. All these complexes are sufficiently soluble in solvents
like methanol, acetonitrile and DMF. The elemental analyses ensure the
purity of the bulk samples, which are well matched with the X-ray
crystallographic results.
In the IR spectra of all complexes, a sharp band is observed in the
range 1615–1626 cm−1, which are characteristic stretching vibrations
of azomethine bond of the Schiff base. IR spectrum of 1 additionally
shows board absorption bands at around 1080 and 624 cm−1, which
are assigned to the stretching and bending vibrations of perchlorate
counter anion, respectively [44]. The broadening of ν(CleO) band is
presumably due to the involvement of ClO4− ions in hydrogen bonding
in the solid-state. IR spectrum of complex 2 displays a strong absorption
band at 2033 cm−1, which can be assigned to the stretching band of
azide ion [45]. Complexes 3 and 4 show two stretching bands for
thiocyanate ions at 2108 and 2067 cm−1, respectively [46,47]. Lower
stretching frequency of the thiocyanate ion in 4 is consistent with the
greater π acceptor ability of thiocyanate from Co(II) centre compared to
its π acceptor ability from Co(III) centre in 3. Presence of cyanate ion in
5 was also confirmed by the observed stretching band at 2217 cm−1
[45].
Fig. 1. Crystal structure of one of the crystallographically independent complex cations of
1 with selected atom numbering scheme. Thermal ellipsoids are drawn at 30% prob-
ability, and solvents are omitted for clarity.
crystallographically independent complex cations of formula [Co
(HL)2]3+ on inversion centres, and two lattice water molecules and
three perchlorate counter anions on general positions. Protonation of
the tertiary amine nitrogen atom of the Schiff base is required to bal-
ance the charge of the system, and thus the zwitterionic form of the
ligand is connected to the metal centre in 1. The structure of both the
crystallographically independent complex cations are eventually iden-
tical in that both the metal centres are hexacoordinated with distorted
octahedral geometry. The coordination environments of the metal
centres are fulfilled by one phenolate-O atom and both imine and sec-
ondary amine nitrogen atoms from each zwitterionic Schiff base ligand.
The average Co−O and Co−N bond lengths are 1.895(3) and 1.982(4)
Å which are typical for the low spin octahedral Co(III) complexes. The
solid state structure of the complex is stabilised by strong hydrogen
3.2. Structural descriptions
Crystal structures of all complexes were determined by the single-
crystal X-ray diffraction studies. Crystal structure of 1 along with se-
lected atom numbering scheme is depicted in Fig. 1, the structures of 2
and 3 are shown in Fig. 2, while Figs. 3 and 4 represent the molecular
structures of 4 and 5, respectively. Structural parameters related to
important bond distances around the metal coordination sphere for all
structures are given in Table 2. Complex 1 crystallizes in the monoclinic
P21/c space group where the asymmetric unit consists of two
52

A. Panja et al.
MolecularCatalysis449(2018)49–61
Fig. 2. Crystal structures of 2 and 3 showing selected atom numbering schemes. Thermal ellipsoids are drawn at 30% probability.
bonds involving the secondary and quaternary amines and lattice water
molecules and perchlorate counter anions (Fig. S1).
Both 2 and 3 are neutral complexes and sit on general positions, but
crystallize in the monoclinic P21/n and orthorhombic Pca21 space
groups, respectively. Both structures are eventually similar in which the
central metal ions are hexacoordinated with slightly distorted octahe-
dral geometry as suggested by the deviation of cisoid
(84.07(11)–94.35(13)°) and transoid (173.66(11)–175.37(13)°) angles
from the ideal values. The monoanionic Schiff base ligand (L) binds the
metal centre through phenolate-O and three nitrogen atoms (one each
of imine, secondary amine and tertiary amine nitrogen atoms) and two
remaining coordination positions of the metal centre are connected
with two nitrogen atoms of the terminal azides (in 2) and thiocyanates
(in 3). In both cases, one meridional position is occupied by three ni-
trogen atoms from the triamine part of the Schiff base, while other one
is occupied by two nitrogen atoms of pseudohalides and the phenolate-
O atom of the Schiff base. The Co–N distances of pseudohalides and the
Co–O distances of phenolate group of the tetradentate L ligand in 2 and
3 are found in the range 1.844(3)–1.895(2) and 1.912(5)–1.964(3) Å,
Fig. 4. Crystal structure of the complex cation of 5 with selected atom numbering
scheme. Ellipsoids are drawn in 30% probability.
Fig. 3. Crystal structure of complex 4 displaying both crystallographically independent complex cations with selected atom labelling scheme. Ellipsoids are drawn in 30% probability.
53

A. Panja et al.
MolecularCatalysis449(2018)49–61
Table 2
Selected bond distances (Å) for complexes 1–5.a
Bond
1
2
3
4
5
A
B
A
B
Co(III)–N(imine)
1.934(4)
2.045(4)
–
1.942(3)
2.005(4)
–
1.924(3)
2.012(3)
2.104(3)
1.895(2)
1.964(3)
1.948(3)
–
1.932(3)
1.996(4)
2.085(4)
1.844(3)
1.916(4)
1.912(5)
–
1.938(5)
2.004(6)
–
1.886(5)
–
1.927(6)
2.047(6)
1.940(4)
2.012(5)
–
1.888 (4)
–
Co(III)–N(secondary amine)
Co(III)–N(tertiary amine)
Co(III)–O(phenolate)
1.886(3)
1.904(3)
1.863(4)
Co(III)–N(pseudohalide)
Co(II)–;1;;1;N(pseudohalide)
1.941(10)
1.955(10)
1.956(7)
1.960(8)
1.956(5)
a
A and B denote two crystallographically independent components.
Fig. 5. A part of packing showing hydrogen bonding and C–H⋯π interactions in 2.
respectively, which are usual values for low-spin octahedral Co(III)
complexes [44,48,49]. In both complexes, the Co–N distances of imine,
secondary and tertiary amines vary in the range 1.924(3)–1.932(3),
1.996(4)–2.012(3) and 2.085(4)–2.104(3) Å, respectively. Notably, the
secondary amine bonds are somewhat longer than the imine moieties in
both complexes (see Table 2), which is in accord with their respective
states of hybridization. But significantly longer bond distance of tertiary
amine over secondary amine group observed in these structures prob-
ably arises from the increased steric crowding at the metal centre due to
the presence of two methyl substitutions at tertiary nitrogen, thereby
leading to the weaker binding ability of the tertiary amine in these
complexes [46]. Furthermore, the N–N distances of azide ions
[1.199(4)–1.202(5) Å] that between the central nitrogen and the ni-
trogen connected to the metal centre are slightly longer than the
terminal N–N bonds [1.157(4)–1.163(5) Å], which is the signature of
the terminal azide ion [48,49]. Again, as found in similar complexes,
the thiocyanate is coordinated through the nitrogen end in 3 [46,47].
The solid state structure of 2 is stabilised by hydrogen bonding inter-
action involving secondary amine and one of the terminal nitrogen
atom of coordinated azide ion from a neighbouring molecule (see
Fig. 5). The C–H⋯π interactions between the adjacent molecules pro-
vide further stability in the solid state in 2. Similar type of hydrogen
bonding interaction is also present in the crystal packing of complex 3
where the secondary amine hydrogen is bonded to the sulphur atom of
coordinated thiocyanate ion from a neighbouring molecule. The solid
state stability of the complex is further reinforced by the multiple
C–H⋯π interaction involving two adjacent molecules (see Fig. 6).
Both 4 and 5 are ionic compounds. Complex 4 crystalizes in the
monoclinic P21/c space group in which the asymmetric unit includes
two crystallographically independent complex cations of formula [Co
(HL)(L)]2+ on inversion centres, a Co(NCS)42−ion on a general position
and a lattice water molecule with half occupancy on a general position.
On the other hand, complex 5 crystalizes in the tetragonal space group
P42/n where the complex cation, [Co(L)2]+, resides on an inversion
centre, while the complex anion, Co(NCO)42−, sits on a 4-fold axis. The
geometry of both Co(NCS)42− and Co(NCO)42− is tetrahedral where all
the pseudohalide ions are coordinated to cobalt(II) ions through the
nitrogen atoms [50–52] with the N–Co–N bond angles ranging from
105.9(3)–114.6(4) and 107.47(15)–113.6(3)° for 4 and 5, respectively.
The structures of 4 and 5 are although apparently similar, the distinc-
tion of these systems originates from the charge neutralization point of
2−
view. Crystallization of the Co(NCS)4
anion on a general position
requires the protonation of tertiary amine-N of one of the Schiff base
ligands for balancing the charge of the system in 4, i.e., the metal centre
is coordinated to one zwitterionic and one monoanionic Schiff base li-
gands in the complex cation of 4. But in complex 5, the tertiary amine-N
atoms of both Schiff base ligands are in the monoanionic form. Inter-
estingly, in both cases, whether the Schiff base ligands are in the
zwitterionic form or in the monoanionic state, coordinated to the metal
centres in the tridentate fashion. In these complex cations, the geometry
of cobalt(III) centre is best described as a pseudo-octahedral and it is
coordinated by two Schiff base ligands, each of them occupies a facial
position and binds the metal centre through phenolate-O, imine-N and
secondary amine-N. The Co–N distances of imine and secondary amine
and the Co–O distances of phenolate group of the Schiff base ligand in 4
and 5 span in the range 1.926(6)–1.940(4), 2.005(6)–2.050(7) and
1.862(4)–1.888(4) Å, respectively, which are comparable to bond dis-
tances observed in 1−3 and also to the reported low-spin octahedral Co
(III) complexes with similar coordination environments [44,48,49].
Like 1−3, the imine bonds are found somewhat shorter than the sec-
ondary amine nitrogen in 4 and 5 as expected. Various types of non-
covalent interactions are present in the solid state in both compounds.
54

A. Panja et al.
MolecularCatalysis449(2018)49–61
Fig. 6. A part of crystal packing of 3 showing hydrogen bonding and C–H⋯π interactions.
The most striking difference in the noncovalent interactions in these
compounds mainly arises from the hydrogen bonding interactions. As
the tertiary amine nitrogen in the Schiff base ligand is not protonated in
5, thereby it involves in the intramolecular hydrogen bonding with
secondary amine group from the same ligand (Fig. S2). On the other
hand, both protonated and free tertiary amine nitrogen of the Schiff
base ligands together with lattice methanol molecules engage in hy-
drogen bonding in 4 (Fig. S3).
organic compounds in the living systems. Therefore, the electro-
chemical study of the synthetic analogues especially under the identical
condition of the catalytic study is important as that could provide sig-
nificant information from which one can assess the potential of the
compounds as catalysts. In this regard, the cyclic voltammetric data of
all complexes were recorded in methanol with 0.1 M tetra-
butylammonium perchlorate as a supporting electrolyte at room tem-
perature. The representative cyclic voltammograms of 2 and 3 are
displayed in Fig. 7, while Fig. 8 represents the electrochemical beha-
viour of 4 and 5. Cyclic voltammograms of 1 − 3 consist of an irre-
versible reduction response in the range −0.81 to −1.10 V, which is
associated with the reduction of Co(III) to Co(II) at the electrode sur-
face. Unlike 1 − 3, the cyclic voltammograms of 4 and 5 consist of both
an irreversible oxidative response (0.96 V for 4 and 0.93 V for 5) and a
reductive response (−0.97 V for 4 and −1.09 V for 5). The reduction
process is responsible for the reduction of Co(III) to Co(II) of the
complex cations at the electrode surface, whereas the oxidative re-
sponse is due to the oxidation of Co(II) to Co(III) of the complex anions
of general formula [CoX4]2− (X = SCN for 4 and NCO for 5). Electro-
chemical irreversible responses for both oxidation and reduction
Overall, several important information regarding the versatile co-
ordination ability of the Schiff base ligand can be achieved from the X-
ray structural studies in these systems. It is observed that the Schiff base
ligand can either bind the metal centre in the tetradentate fashion using
all the available donor sites of the monoanionic deprotonated form (in 2
and 3) or in the tridentate fashion using the zwitterionic form of the
Schiff base ligand with pendent quaternary amine nitrogen (in 1 and 4).
Moreover, the monoanionic deprotonated ligand can also bind the
metal centre in the tridentate fashion (in 4 and 5) in which the pendent
tertiary amine nitrogen participates in the intramolecular hydrogen
bonding. Furthermore, the facial binding of the triamine part of the
Schiff base ligand to the metal centre is observed in our recently re-
ported Ni(II) systems [46], while in 2 and 3 it is meridionally co-
ordinated to the metal centre. Therefore, all these above adaptabilities
associated with this triamine suggest that the ligand derived from this
triamine could be a promising ligand for the development of the co-
ordination chemistry with diverse structures. One more interesting as-
pect of the structural studies is that the influence of the counter ions is
noticed in the structural diversity. What we have observed that the
stronger donor ability of the counter anion (for example azide ion)
helps to stabilize the system through the coordination to the metal
centre, while the weaker coordinating counter ion like perchlorate, the
zwitterionic anion of the Schiff base coordinates the metal centre for
the stabilisation of the system. A mixed impression is observed when
thiocyanate counter anion was used, and in that case influence of the
solvent used for the synthesis is also observed for the ultimate stability
of the system. As the cyanate ion preferably coordinates the metal
centre using relatively less electronegative nitrogen donor atom that is
why it could not provide sufficient electron density to the metal centre
in the stability of the system.
3.3. Electrochemical studies
Fig. 7. Cyclic voltammograms of 2 and 3 in methanol using a platinum working electrode
in the presence of tetrabutylammonium perchlorate as a supporting electrolyte at room
temperature with scan rate 100 mV s−1.
The reduction potential of the active site of a metalloenzyme is the
crucial factor behind the catalytic oxidation of biologically important
55

A. Panja et al.
MolecularCatalysis449(2018)49–61
Fig. 10. Time dependent spectral scan showing growth of 2-aminophenoxazin-3-one at
434 nm upon addition of 1.0 × 10−2 M OAPH to a solution containing 3 (1 × 10−4 M) in
dioxygen-saturated methanol. The spectra were recorded at 5 min intervals under aerobic
conditions at room temperature.
Fig. 8. Cyclic voltammograms of 4 and 5 in methanol using a platinum working electrode
in the presence of tetrabutylammonium perchlorate as a supporting electrolyte at room
temperature with scan rate 100 mV s−1.
processes are indicative of the significant modification of the co-
ordination spheres of the resulting species at the electrode surface.
3.4. Catalytic oxidation of o-aminophenol
Oxidation of o-aminophenol (OAPH) by molecular dioxygen in
presence of catalytic amount of the complexes was studied UV–vis
spectrophotometrically in dioxygen-saturated methanol at room tem-
perature [33]. In this context, 1.0 × 10−4 M methanolic solution of the
complexes were allowed to react with a 0.01 M solution of OAPH under
aerobic condition. The advancement of the reaction was examined by
UV–vis spectrophotometrically at 5 min time interval, and the time-
dependent spectral profiles are depicted in Figs. 9–11 for 2, 3 and 5,
respectively, and in Figs. S4 and S5 for 1 and 4, respectively. These
spectral scans reveal the cumulative development of peak intensity at
ca. 434 nm, characteristic of phenoxazinone chromophore, that un-
ambiguously establish that all complexes except complex 1 are efficient
catalysts for the oxidation of OAPH by molecular dioxygen. Detailed
Fig. 11. The time resolved spectral profile showing oxidation of o-aminophenol
(1.0 × 10−2 M) catalysed by
5
(1 × 10−4 M) in dioxygen-saturated methanol. The
spectra were recorded at 5 min intervals under aerobic conditions at room temperature.
kinetic study is further required to get insight into the catalytic effi-
ciency in these complexes. Accordingly, 2.0 × 10−5 M solutions of the
complexes were reacted with excess substrate at the pseudo-first-order
condition, and the reaction kinetics at the maximum band (434 nm) of
2-aminophenoxazin-3-one was carried out for a period of 10 min, and
the initial rate was determined by linear regression from the slope of
the absorbance versus time plot [43]. As can be seen from Fig. 12, the
initial rates follow the rate saturation kinetics with increasing con-
centration of the substrate. These observations suggest that a complex-
substrate aggregate is formed in a preequilibrium stage prior to the
irreversible redox transformation of the intermediate in the rate de-
termining step of the catalytic cycle. Michaelis-Menten model is ap-
propriate to analyse such kinetic data to evaluate the parameters Vmax
,
KM, and Kcat. The observed and simulated nonlinear initial rates versus
substrate concentration plot are shown in Fig. 12 and simulated kinetic
parameters are given in Table 3. Analysis of the experimental data
yielded Michaelis binding constant (KM) value of 1.02 × 10−2
,
Fig. 9. Time dependent UV–vis spectral changes for the oxidation of o-aminophenol
(1.0 × 10−2 M) catalysed by
2
(1 × 10−4 M) in dioxygen-saturated methanol. The
1.35 × 10−2, 1.24 × 10−2, 8.87 × 10−3 and 2.74 × 10−2 M for 1–5,
respectively, while Vmax values of 4.64 × 10−8 3.11 × 10−7
spectra were recorded at 5 min intervals under aerobic conditions at room temperature.
,
,
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A. Panja et al.
MolecularCatalysis449(2018)49–61
some cases even it was wrongly assigned [53]. Therefore, the present
observation with careful assignment could provide useful information
regarding the mechanistic pathway of functioning phenoxazinone
synthase activity.
3.6. Influence of first and second coordination spheres
Although the reduction potential of the cobalt(III) centre in these
complexes are very close enough, the spectral scan as well as detailed
kinetic analysis reveal that 2 and 3 are efficient catalysts for the aerobic
oxidation of o-aminophenol, and 4 and 5 are moderately reactive, while
compound 1 is feebly reactive. Higher reactivity of 2 and 3 is mainly
because of the availability of labile sites (azide ions in 2 and thiocya-
nate ions in 3) at the first coordination sphere for the substrate binding.
Somewhat greater reactivity of azide complex (2) compared to thio-
cyanate analogue (3) is perhaps due to the better lability of azide over
thiocyanate ion. At this juncture, both the mass spectral and kinetic
studies convincingly conclude that the catalytic cycle for the aerobic
oxidation of o-aminophenol catalysed by 2 and 3 begins with the for-
mation of a stable complex-substrate aggregate by the substitution of
coordinated pseudohalides by o-aminophenol. This complex-substrate
intermediate in the subsequent rate determining step produces OAP
radical through the inner sphere electron transfer from o-amionophe-
nolate to the cobalt(III) centre with the concomitant production of
hydrogen peroxide in expense of regeneration of catalyst by molecular
oxygen. The formation of hydrogen peroxide during the catalytic re-
action was verified by the iodometric method as reported earlier [54].
The generated OAP radical thereafter produces o-benzoquinone
monoamine (BQMI) in several ways including the disproportionation of
the OAP radical itself. This BQMI is then combined with another OAPH
to produce a reactive intermedia I-B as supported by the mass spectral
study, followed by two electrons oxidation with molecular oxygen to
produce another reactive intermediate II-B, which finally produces 2-
aminophenoxazin-3-one through the oxidative dehydrogenation of in-
termediate II-B as shown in Scheme 2.
Fig. 12. Initial rate versus substrate concentration plot for the oxidation of OAPH in di-
oxygen-saturated methanol catalysed by 1–5 at room temperature. Symbols and solid
lines represent the experimental and simulated profiles, respectively.
Table 3
Kinetic parameters of phenoxazinone synthase like activity for complexes 1–5.
Catalyst
Vmax (Ms−1
)
KM (M)
kcat (h−1
)
1
2
3
4
5
4.64 × 10−8
3.11 × 10−7
2.99 × 10−7
1.15 × 10−7
1.67 × 10−7
1.02 × 10−2
1.35 × 10−2
1.24 × 10−2
8.87 × 10−3
2.74 × 10−2
4.35
56.0
53.8
20.7
30.0
2.99 × 10−7, 1.15 × 10−7 and 1.67 × 10−7 Ms−1 for 1–5, respec-
tively. The turnover number (kcat) value is obtained by dividing Vmax by
the concentration of the complex used (2.0 × 10−5 M), and is found to
be 4.35, 56.0, 53.8, 20.7 and 30.0 h−1 for 1–5, respectively.
The lower reactivity of the other compounds compared to 2 and 3 is
perhaps because of unavailability of the substrate binding sites at the
metal centre, leading to the electron transfer through the outer sphere
electron transfer mechanism. Literature reports reveal that these types
of complexes are often found inactive or feebly active as observed in
compound 1 [55], but in the present case, both 4 and 5 are significantly
catalytically active towards the oxidation of o-aminophenol. We have
recently observed the influence of the positive charge at the ligand
backbone which brings significantly enhanced catalytic activity in the
system, where the positive charge of pyridinium moiety at ligand
backbone may attract the substrate OAPH towards the formation of the
complex-substrate aggregate and also provides additional stability of
the intermediate species through noncovalent interactions, leading to
the enhanced catalytic activity [56]. In the present case, the pendent
tertiary amine group is fully protonated in 1 and partially protonated in
4, while it is not protonated in 5. If simply the positive charge at the
second coordination sphere be the crucial factor, then 1 and 4 would
exhibit the enhanced catalytic activity over complex 5. But the re-
activity order of 5 > 4 > > 1 clearly suggests that some other factor
is responsible for the relative catalytic ability of these complexes. In
order to analyse the influence of the protonation/deprotonation at the
second coordination sphere on the catalytic activity, a different set of
2 × 10−5 M solutions of 1 in methanol were prepared with varying
amount of tetrabutylammonium hydroxide base, and then these re-
sulting complex solutions were combined with 0.01 M solution of the
substrate, and the reaction kinetics was monitored UV–vis spectro-
photometrically. Remarkably, the catalytic activity of 1 increases with
increase in the added base (Fig. 14) and finally the rate of the reaction
of 1 become comparable to the rate of 5. This observation suggests that
the deprotonation of quaternary amine at the second coordination
sphere favours the catalytic activity in these systems. Therefore, the
3.5. Mass spectral study
The mass spectral study could provide useful information of the
intermediate species formed in a chemical reaction from which the
most possible mechanism of the reaction pathway can be drawn.
Therefore, the electrospray ionization mass spectral (ESI–MS positive)
studies were carried out of a representative compound 3 alone and in
presence of 50 equivalents of o-aminophenol in methanol. The ESI mass
spectrum of compound 3 is displayed in Fig. S6. The basal peak of the
complex alone is found at m/z = 364.28, which is assignable to the
[CoIII(L)eH]+, consistent with isotopic distribution patterns and also to
the peak-to-peak separation of unity. The mass spectrum of the com-
plex-substrate mixture (1:50 mol ratio) in methanol is displayed in
Fig. 13. Interestingly, the base peak of the mass spectrum of complex
alone become a minor species when it was carried out in presence of
excess substrate. Remarkably, the base peak at m/z = 473.21 is a 1:1
complex-substrate intermediate of a monocationic species of formula
[CoIII(L)(OAP)]+ (calculated m/z = 473.20), ensuring the formation of
a stable complex-substrate adduct. These observations establish the
ease of formation of the complex-substrate aggregate, exploiting the
stronger chelating ability of o-aminophenolate ion by the replacement
of terminal thiocyanate ions. The peak at m/z = 213.35 is related to a
protonated species of the product 2-aminophenoxazin-3-one (calcu-
lated m/z = 213.07). The peak at m/z = 216.39 is deserved to be spe-
cial mention as that is related to the molecular ion peak of an important
reactive intermediate I-B (calculated m/z = 216.09) produced during
the course of oxidative coupling of o-aminophenol as drown in Scheme
2. This kind of information was not readily available from the literature
reports on the synthetic models of phenoxazinone synthase, and in
57

A. Panja et al.
MolecularCatalysis449(2018)49–61
Fig. 13. Electrospray ionization mass spectrum (ESI–MS positive) of a 1:50 mixture of complex 3 and o-aminophenol in methanol recorded after 10 min of mixing.
present result discloses that not the positive charge but the hydrogen
bond acceptor ability at the second coordination sphere is the essential
criteria for the substrate recognition and stability of the intermediate
for enhanced catalytic activity (see Scheme 3). Moreover, tertiary
amine nitrogen abstracts proton from o-aminophenol and thereby fa-
cilitates the oxidation of OAPH, leading to the formation of OAP radical
at the rate determining step. When tertiary amine is protonated, such
proton abstraction from the substrate is not permitted and thus oxida-
tion of o-aminophenol is not favoured. Therefore, the present study
establishes that if the strong hydrogen bond acceptor centre is present
at secondary coordination sphere that can not only recognize the sub-
strate and stabilize the complex substrate intermediate through hy-
drogen bonding, but also abstract proton from the substrate, leading to
the facile oxidation of the substrates like o-aminophenol. This result
clearly justifies the fact that the extraordinary reactivity of me-
talloenzymes in the biological kingdom is not only associated with the
availability of the labile position(s) for the substrate binding but also
due to the involvement of protein chain for the substrate recognition
and stabilisation of the intermediate by execution of various non-
covalent interactions.
clearly suggests the binding ability of catechol towards the metal
centre. Higher reduction potential i.e., a lower electron donating
character of 3,5-di-tert-butylcatechol compared to o-aminophenol sub-
strate is the main reason behind the catecholase inactivity of compound
2. On the other hand, compound 2 is found to be an efficient catalyst for
the aerobic oxidation of 2-amino-5-methylphenol. As shown in Fig. 15,
the cumulative growth of peak intensity at ca. 402 nm, characteristic for
dihydro-2-aminophenoxazinone chromophore, was observed when
5.0 × 10−5 M methanolic solution of 2 was allowed to react with a
0.01 M solution of 2-amino-5-methylphenol in aerobic condition [43].
This result indicates the catalytic formation of 4a,7-dimethyldihydro-2-
aminophenoxazinone in aerobic condition. Interestingly, the presence
of methyl substitution does not inhibit the coupling of two aminophe-
nols, but the final 2-electrons oxidation was blocked by methyl sub-
stitution at C4a, leading to the formation of 4a,7-dimethyldihydro-2-
aminophenoxazinone instead of 2-aminophenoxazinone which is the
final oxidation product of o-aminophenol itself. From the mass spectral
study, formation of the complex-substrate adduct and the final product
were clearly identified (Fig. S7). Detailed kinetic study (Fig. S8) along
with the spectral scan as shown in Fig. 15 clearly indicate that catalytic
oxidation of 2-amino-5-methylphenol (Vmax = 3.92 × 10−7 Ms−1
;
k
cat = 70.6 h−1) is even faster than o-aminophenol, which is quite
3.7. Studies with other substrates
reasonable as the methyl substitution at aromatic ring favours the
oxidation of the substituted aminophenol.
In order to check a wider scope of substrate oxidation abilities, the
reactions of 3,5-di-tert-butylcatechol and 2-amino-5-methylphenol with
dioxygen in presence of the most reactive catalyst 2 were also in-
vestigated under the identical conditions as stated earlier (Scheme 4).
From the time dependent spectral profile (Fig. 15), it is found that
compound 2 is catalytically inactive towards the oxidation of 3,5-di-
tert-butylcatechol as a cumulative increase of spectral intensity at
400 nm, characteristic for 3,5-di-tert-butylquinone, was not observed.
However, the appearance of the isosbestic points in the spectral profile
4. Conclusions
We have successfully synthesized a new tetradentate N3O donor
Schiff base ligand from the condensation of N,N-dimethyldipropylene-
triamine and 3-ethoxysalicylaldehyde. This Schiff base ligand was
thereafter utilized for the preparation of five new cobalt complexes in
the presence of different counter ions. Although stoichiometric ratios
58

A. Panja et al.
MolecularCatalysis449(2018)49–61
Scheme. 2. Plausible mechanistic pathway for the aerobic oxidation of o-aminophenol catalysed by 2 and 3.
did not have any effect in the formation of these compounds, the role of
the solvent and counter anions was noticed for their synthesis (3 versus
4). X-ray crystal crystallography disclosed that the Schiff base can co-
ordinate the metal centre either in the tetradentate fashion through
monoanionic deported form using all the available donor sites (in 2 and
3) or in the tridentate fashion using the zwitterionic form of the Schiff
base ligand with pendent quaternary amine nitrogen (in 1 and 4).
Moreover, the tridentate binding ability of the monoanionic deproto-
nated ligand to the metal centre was also observed (in 4 and 5), in
which the pendent tertiary amine nitrogen participates in the in-
tramolecular hydrogen bonding. It is to be noted that the facial binding
mode of the triamine part of the Schiff base ligand was observed in our
recent report with Ni(II) system, while it is meridionally coordinated to
the metal centre in the present systems (2 and 3). All these adapt-
abilities of this triamine make it as a promising candidate for explora-
tion of the coordination chemistry with diverse structures. All com-
plexes except 1 exhibit strong phenoxazinone synthase mimicking
activity, where the available binding sites at the first coordination
sphere of the metal centre for substrate binding is the main reason for
significantly higher catalytic activity in 2 and 3. Although, compound 1
is found almost inactive, the presence of hydrogen bond acceptor site at
the second coordination sphere in both 4 and 5 make them effective
candidates for phenoxazinone synthase activity. Interestingly, this
Fig. 14. Plot of initial rate vs concentration of an added base for the aerobic oxidation of
o-aminophenol (0.01 M) catalysed by complex 1 (2 × 10−5 M).
59

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MolecularCatalysis449(2018)49–61
Scheme 3. Stabilization of intermediate through hydrogen bonding and facilitating oxidation of o-aminophenol by proton abstraction.
Scheme 4. Reaction of substituted catechol and substituted o-aminophenol with oxygen.
Fig. 15. Reactions of 3,5-di-tert-butylcatechol (1.0 × 10−2 M) and 2-amino-5-methylphenol (1.0 × 10−2 M) with dioxygen in presence of the most reactive catalyst 2 (5.0 × 10−5 M) in
methanol.
hydrogen bond acceptor site not only stabilizes the complex-substrate
aggregate through hydrogen bonding but also abstracts proton from o-
aminophenol, thereby facilitating oxidation of the substrate. The mass
spectral study of 3 in the presence excess substrate disclosed some
important intermediates, which is helpful to get insight into the me-
chanistic pathway of functioning phenoxazinone synthase activity.
These results overall highlight the importance of the labile or vacant
positions at the first coordination sphere available for substrate binding
and the substrate recognition cum proton abstraction centre at the
second coordination sphere for the development of the better in vitro
catalysts as that have a high degree of structural resemblance with the
metalloenzymes. Finally, in order to increase scope of substrate oxi-
dation abilities, the reactions of 3,5-di-tert-butylcatechol and 2-amino-
5-methylphenol with dioxygen were investigated in presence of 2.
Although it is not an active catalyst for oxidation of substituted catechol
but is proved to be an efficient catalyst for the oxidation of substituted
o-aminophenol in aerobic condition. Interestingly, in both cases, the
substrate binding ability to the metal centre was experimentally es-
tablished.
Acknowledgements
A.P. would like to thank the University Grant Commission, New
Delhi (Sanction no. PSW-229/15-16) for financial support. The authors
thank Indian Institute of Chemical Biology, Kolkata for providing the
Mass spectral facility.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.mcat.2018.02.013.
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