Biomimetic catalytic activity and structural diversity of cobalt complexes with N3O-donor Schiff base ligand

Article abstract of DOI:10.1016/j.ica.2019.03.017

This report describes the syntheses and structural characterizations of four new cobalt(III) compounds (1–4) derived from a tetradentate Schiff base ligand and their catalytic oxidation of different o-aminophenols in aerobic condition. X-ray crystal struc

Full text of DOI:10.1016/j.ica.2019.03.017

InorganicaChimicaActa490(2019)163–172
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Biomimetic catalytic activity and structural diversity of cobalt complexes
with N3O-donor Schiff base ligand
⁎
Narayan Ch. Janaa,1, Moumita Patraa,1, Paula Brandãob, Anangamohan Panjaa,
b Department of Chemistry, CICECO-Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal
a Postgraduate Department of Chemistry, Panskura Banamali College, Panskura RS, WB 721152, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
This report describes the syntheses and structural characterizations of four new cobalt(III) compounds (1–4)
derived from a tetradentate Schiff base ligand and their catalytic oxidation of different o-aminophenols in
aerobic condition. X-ray crystal structural studies reveal that both pseudohalide ions and solvents used for the
syntheses have significant influence on the structural diversity in these systems. All these compounds show the
diverse catalytic activity towards the aerial oxidation of different o-aminophenols. As expected, the sub-
stitutionally labile coordination sites available for substrate binding is responsible for significantly higher cat-
alytic activity of 1 and 2 then others. Literature reports reveal that although significant attention has been made
to the catalytic oxidation of some substituted o-aminophenols in terms of the isolations and characterizations of
their oxidised products with the native enzyme and its models, but the detailed kinetic studies were not available
in the literature. Herein, we have examined the detailed kinetic studies of the aerobic oxidation of two sub-
stituted o-aminophenols, namely 2-amino-5-methylphenol and 2-amino-4-methylphenol, using 1 and 2 as cat-
alysts. ESI mass spectrometry provides significant information to get insight into the mechanistic details and
further disclose that the methyl substation in aromatic ring does not inhibit formation of the complex-substrate
aggregate but it resulted the phenoxazine chromophore instead of phenoxazinone chormophore.
Biomimetic catalysis
Intermediate
Cobalt complexes
Crystal structures
Mass spectrometry and mechanism
1. Introduction
synthesis by intercalating the DNA base pairs [20,21]. Model studies of
such metalloenzymes are worthy as that may help in unravelling the
Metal complexes of Schiff base ligands have gained considerable
attention because of their synthetic simplicity, ease of tunability of their
stereo-electronic structures, and notable biological activities [1–3],
catalytic activities [4–9], fluorescence properties [10–12], applications
in sensors [13], etc. Remarkably, the Schiff base ligands readily form
stable complexes with most of the transition metals [14,15], Schiff base
complexes of first row transition metals provided us numerous efficient
structural and functional models of various metalloproteins and me-
talloenzymes. We have been working for last few yours in modeling the
structures and functions of two important metallo-oxidases, namely
phenoxazinone synthase and catechol oxidase. Phenoxazinone synthase
[16–18] functions in the final step of biosynthesis of natural antibiotic
actinomycin D, where two molecules of substituted o-aminophenols are
oxidatively coupled to form phenoxazinone chromophore through
catalytic activation of dioxygen [19]. That naturally occurring anti-
biotic is used clinically for the treatment of many tumors including
Wilm’s tumor, where the phenoxazinone chromophore inhibits the RNA
catalytic mechanism of action of these enzymes by means of a synthetic
analogue approach. Moreover, the efficient functional models could be
the alternative of traditional oxidants, such as permanganates, dichro-
mate and heavy metal oxides, used industrially in the production of
various desired organic and pharmaceutical compounds, resulting in
huge wastes and causing environmental pollution. Model studies of
phenoxazinone synthase [22–24] were mainly carried out with simple
o-aminophenol [25–33], while other aminophenol derivatives have not
been examined well. Recently, Tomoda and co-workers reported the
antibacterial effects of Phx-1 (2-aminophenoxazin-3-one), Phx-2 (2-
amino-4,4a-dihydro-4a-7- dimethyl-3H-phenoxazin-3-one), and Phx-3
(3-amino-1,4a-dihydro-4a-8-dimethyl-2Hphenoxazin- 2-one) (Scheme
1), which were produced by the reaction of o-aminophenol or its de-
rivatives with bovine hemoglobin [34]. Thus, the chemistry of phe-
noxazine derivatives is worthy to be further explored.
Nature has mainly opted copper and iron for the active sites of the
metallo-oxidases for their biological functions [35,36]. Although,
⁎ Corresponding author.
E-mail address: ampanja@yahoo.co.in (A. Panja).
1 These authors contributed equally to this work.
https://doi.org/10.1016/j.ica.2019.03.017
Received 4 February 2019; Received in revised form 9 March 2019; Accepted 12 March 2019
Availableonline13March2019
0020-1693/©2019ElsevierB.V.Allrightsreserved.

N.C. Jana, et al.
InorganicaChimicaActa490(2019)163–172
presence of different pseudohalide ions. Accordingly, we report herein,
the synthesis and structures of four new cobalt complexes, [Co(L)(N3)2]
(1), [Co(L)(NCS)2] (2), [Co(L)(Sal)]2[Co(NCS)2] (3) and [Co
(L)2]2[Co0.75(NCS)3]Cl0.5 (4). Remarkable coordination diversity influ-
enced by both auxiliary anions and solvents used for the synthesis has
been explored. The biomimetic catalytic activities of the synthesized
complexes using o-aminophenol and two other substituted o-amino-
phenols as substrates have also been reported. Further emphasis was
also given to explore the structure–function relationship along with the
detailed mechanistic investigation, which is not yet explored for o-
aminophenol derivatives.
2. Experimental section
2.1. Materials and physical measurements
Hexahydrated salts of cobalt(II) nitrate, cobalt(II) chloride, o-ami-
nophenol (OAPH), salicylaldehyde, N,N-dimethyldipropylenetriamine,
2-amino-5-methylphenol, and 2-amino-4-methylphenol were purchased
from commercial sources and used as received. Other chemicals and
solvents were of reagent or analytical grade and used without further
purification.
Scheme 1. Chemical structures of Phx-1, Phx-2, and Phx-3.
Caution! Metal complexes containing perchlorates and azide salts
are potentially explosive. Although no problem was encountered in the
present study, handling small amount of materials with great care is
recommended.
cobalt has comparable redox flexibility like copper and iron, but it has
not been chosen for the active site in any metallo-oxidase. The sub-
stitutionally inert character of cobalt especially in cobalt(III) state is
main reason behind its exclusion form such metalloenzymes. However,
this inert character could be helpful in detecting the intermediates
species formed in the catalytic cycle through various instrumental
techniques, which in turn may help to better understanding the me-
chanistic pathway of a catalytic reaction [37–39]. With this aim, we
intended to develop some cobalt complexes with Schiff base ligands for
in vitro catalytic studies, modeling the functions of metallo-oxidases
that may give us significant information in unraveling the catalytic
mechanism. We have started systematic development of coordination
chemistry with Schiff bases derived from a triamine N,N-dimethyldi-
propylenetriamine, protected at one end for Schiff base condensation;
the coordination chemistry of which is almost unexplored [40,41]. In
the present endeavor, we employed Schiff base ligand HL (Scheme 2),
derive from salicylaldehyde (SalH) and N,N-dimethyldipropylene-
triamine, for exploration of coordination chemistry of cobalt in
Elemental analysis of carbon, hydrogen and nitrogen were carried
out using a Perkin-Elmer 240C elemental analyser. UV-vis absorption
spectra were measured in an Agilent Carry-60 diode array UV-vis
spectrophotometer at room temperature with a 1-cm path-length quartz
cell. IR spectroscopic data in the range 400–4000 cm−1 were collected
in a PerkinElmer Spectrum Two FTIR spectrophotometer where samples
prepared in KBr pellets. Electrochemical measurements of all complexes
were conducted in a CH Instrument electrochemical workstation model
CHI630E at a scan rate of 100–200 mVS−1. The working electrode was
platinum, polished with alumina solution, and air dried before each
electrochemical run. The reference electrode was Ag/AgCl, with pla-
tinum as counter electrode. Tetrabutylammonium perchlorate was used
as a supporting electrolyte. Electrospray ionization mass spectra (ESI-
MS positive) were recorded in a Micromass Q-tof-Micro Quadruple
mass spectrophotometer.
Scheme 2. Route to synthesis of the Schiff base and its cobalt complexes.
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InorganicaChimicaActa490(2019)163–172
Table 1
Crystal data and structure refinement parameters of complexes 1 to 4.
1
2
3
4
Empirical formula
C
15H24CoN9O
C17H24CoN5OS2
437.46
C24H29Co1.50N5O3S2
588.04
C63H96Co2.75N15O8.50Cl0.50
1379.32
Formula weight (g mol−1
)
405.36
Temperature (K)
Crystal system
Space group
a (Å)
293(2)
150(2)
293(2)
150(2)
Monoclinic
P21/n
Monoclinic
P21/c
Orthorhombic
Fdd2
Tetragonal
P4/n
9.5110(3)
19.6148(5)
9.9662(3)
90
10.301(10)
12.479(5)
15.205(13)
90
47.916(6)
16.1405(9)
13.6210(9)
90
26.8475(12)
26.8475(12)
9.7200(4)
90
b (Å)
c (Å)
α (°)
β (°)
103.627(2)
90
90.56(4)
90
90
90
γ (°)
90
90
volume (Å3)
Z
1806.92(9)
4
1954(3)
4
10534.3(17)
16
7006.1(5)
4
Dcalc (mg m−3
)
1.490
1.487
1.483
1.312
µ (mm−1
)
0.975
0.975
1.150
0.726
F(0 0 0)
848
912
4872
2935
θ Range (°)
2.08–26.22
26,732
2.679–30.706
50,726
1.70–28.19
6847
2.23–26.42
72,904
Reflections collected
Independent reflections (Rint
)
3606(0.0570)
2761
6023(0.0452)
5248
3990(0.0303)
3641
14380(0.0540)
10,596
Observed reflections [I > 2σ(I)]
Restraints/parameters
0/307
0/241
1/321
1/840
Goodness-of-fit on F2
1.037
1.031
1.040
1.046
Final R indices [I > 2σ(I)]
R1 = 0.0385
wR2 = 0.0732
R1 = 0.0584
wR2 = 0.0796
0.254/ −0.240
R1 = 0.0247
wR2 = 0.0597
R1 = 0.0326
wR2 = 0.0652
0.393/–0.559
R1 = 0.0423
wR2 = 0.0962
R1 = 0.0476
wR2 = 0.1000
0.540/–0.462
R1 = 0.0643
wR2 = 0.1786
R1 = 0.0947
wR2 = 0.2023
1.620/–1.519
R indices (all data)
Largest diff. peak/hole (e Å−3
)
2.2. Synthesis of the Schiff-base ligand (HL)
of 1, but methanolic solution of NaSCN was used in place of NaN3.
Colour: Dark brown, Yield: 343 mg (69%). Anal. Calcd. for
Schiff base ligand (HL) was prepared by the reaction of N,N-di-
methyldipropylenetriamine (1.0 mmol, 159 mg) and salicylaldehyde
(1.0 mmol, 122 mg) in 20 ml of methanol (or acetonitrile) under re-
fluxing condition for a period of 1 h. This in situ generated Schiff base
ligand HL either in methanol or acetonitrile was used directly for the
synthesis of the cobalt complexes as described below.
C48H58Co3N10O6S4: C 49.02%, H 4.97%, N 11.91%. Found: C 48.96%,
H 4.84%, N 11.87%. FTIR (cm−1, KBr): ν(NCS) 2065 s, ν(C]N) 1625 s.
2.6. Synthesis of complex [Co(L)2]2[Co0.5(NCS)2][Co0.25(NCS)]Cl0.5 (4)
Similar methodology as that applied for the synthesis of 1 was
adopted for the preparation of 4 from a methanol/water (20:1, v/v)
mixture, but using chemicals CoCl2⋅6H2O and NaNCO. Colour: Dark
2.3. Synthesis of [Co(L)(N3)2] (1)
brown,
Yield:
343 mg
(69%).
Anal.
Calcd.
for
Solid cobalt(II) nitrate hexahydrate (291 mg, 1.0 mmol) was added to a
methanol solution of HL (1.0 mmol) with stirring at room temperature,
leading to a dark-brown solution. A 5 ml 4:1 (v/v) methanol/water solu-
tion of sodium azide (130 mg, 2.0 mmol) was then added dropwise to the
reaction mixture with constant stirring. The resulting solution was heated
to reflux for about 30 min. It was then filtered and left at room tempera-
ture for slow evaporation. Block shape dark-brown crystals suitable for X-
ray diffraction study were separated out from the filtrate after 3–4 days,
which were collected by filtration and washed with methanol and were
allowed to dry in air. Yield: 320 mg (79%). Anal. Calcd. for C15H24CoN9O:
C 44.45.21%, H 5.97%, N 31.10%. Found: C 44.29%, H 5.78%, N 31.03%.
FTIR (KBr, cm−1): ν(N3) 2034 vs; ν(C]N) 1616 s.
C
63H96Co2.75N15O8.50Cl0.5: C 54.86%, H 7.02%, N 15.23%. Found: C
54.79%, H 6.99%, N 15.27%. FTIR (cm−1, KBr): ν(NCO) 2215 s, ν(C]
N) 1628 s.
2.7. X-ray diffraction study
X-ray diffraction data of 1–4 were collected on a Bruker SMART
APEX-II CCD diffractometer using graphite monochromated Mo Kα
radiation (λ = 0.71073 Å). Data processing, structure solution, and
refinement were performed using Bruker Apex-II suite program. The
highly redundant data sets were reduced using SAINT-plus and cor-
rected for Lorentz and polarization effects. Absorption corrections were
applied using SADABS [42] supplied by Bruker. The structures were
solved using the direct method and refined by the successive full-matrix
least-squares cycles on F2 using SHELXL-v.2014 [43]. All nonhydrogen
atoms were refined with anisotropic displacement parameters. Hy-
drogen atoms were placed in calculated positions and refined as riding
atoms with their fixed isotropic thermal parameters which are tied 1.2
times or 1.5 times (for methyl group) with their parent atoms. The
crystal structures of all complexes were generated using MERCURY-
3.10.1 software. Details of X-ray crystallographic data collection and
structure refinements are summarized in Table 1.
2.4. Synthesis of [Co(L)(NCS)2] (2)
Complex 2 was synthesized from an acetonitrile/water (20:1, v/v)
mixture following the same procedure as applied for the synthesis of
complex 1. The only difference is that an acetonitrile solution of NaSCN
was used in place of NaN3. Colour: Dark brown, Yield: 323 mg (74%).
Anal. Calcd. for C17H24CoN5OS2: C 46.67%, H 5.53%, N 16.01%.
Found: C 46.60%, H 5.45%, N 15.95%. FTIR (cm−1, KBr): ν(NCS)
2063 s, ν(C]N) 1624 s.
2.5. Synthesis of [Co(L)(sal)]2[Co(NCS)2] (3)
2.8. Catalytic oxidation of o-aminophenols
Complex 3 was synthesized from a methanol/water (20:1, v/v)
The catalytic oxidation of substrate o-aminophenol was studied by
mixture following same methodology as that employed for the synthesis
the reaction of the substrate (1.0 × 10−2 M) in presence of catalytic
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InorganicaChimicaActa490(2019)163–172
Fig. 1. The molecular structures of 1 and 2 with selected atom numbering schemes. Thermal ellipsoids are drawn at 30% probability.
amount of the complexes (5 × 10−5 M) in air saturated methanol at
room temperature. The aerobic oxidation of OAPH catalysed by 1–4
was investigated UV-vis spectrophotometrically by monitoring increase
of the characteristic absorbance band of 2-aminophenoxazin-3-one as a
function of time at 435 nm. To evaluate various kinetic parameters, the
kinetic measurements were conducted for all complexes where a fixed
amount of the complex were subjected to react with variable con-
centrations of the substrate maintaining the pseudo-first-order condi-
tion. 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 plot. Similar
methodology was applied to check the catalytic oxidation of the sub-
stituted o-aminophenols using 1 and 2 as catalysts
vibrations of the azomethine bond of the Schiff base ligand coordinated
to the metal centre in the range 1615–1626 cm−1. In addition, strong
bands appeared at 2032 and 2215 cm−1 in the IR spectra of 1 and 4 can
be assigned to the stretching vibration of coordinated azide and cyanate
ions, respectively [41a,44]. Similarly, the strong bands observed at
2087–2125 and 2057 cm−1 in the IR spectra of 2 and 3, respectively,
are attributed to stretching vibration of the coordinated thiocyanate
ions, implying coordination of the NCS− ion through the nitrogen end
[41a,44]. Significantly lower stretching frequency of the thiocyanate
ion in 3 is probably due to greater π acceptor ability of thiocyanate
from cobalt(II) centre compared to its π acceptor ability from cobalt(III)
centre in 2.
3.2. Structural descriptions
3. Results and discussion
Crystal structures of the reported complexes were determined by the
single crystal X-ray diffraction method. The crystal structures of 1 and 2
are presented in Fig. 1, while Fig. 2 displays the solid-state structures of
3 and 4. Selected bond distances around the metal coordination sphere
for all complexes are given in Table 1. Both 1 and 2 crystallize in the
monoclinic unit cell but with different space groups P21/n and P21/c,
respectively. Asymmetric units of both 1 and 2 consist of full set of a
neutral complex molecule. In both complexes, cobalt(III) centres reveal
a distorted octahedral N5O coordination environment in which the
metal centre is coordinated with phenolate oxygen atom, imine ni-
trogen atom, secondary amine nitrogen and tertiary amine nitrogen of
the monoanionic Schiff base ligand and two remaining coordination
sites are occupied by two nitrogen atoms from two terminal azide (in 1)
and thiocyanate (in 2) ions. X-ray crystal structure determination shows
slight deviation of cisoid (83.62(7)–93.45(8)°) and transoid
(174.72(8)–176.97(4)°) angles from the ideal values which justifies the
slightly distorted octahedral geometry around the metal centre in these
complexes. The triamine part of the Schiff base ligand constitutes one
meridional position, while other meridional position is occupied by two
nitrogen atoms of pseudohalides together with the phenolate-O atom of
the Schiff base. In both complexes, the Co–O(phenolate) distances of
the tetradentate ligand and Co–N(pseudohalide) distances are in the
range1.8654(17)–1.8770(15) and 1.9094(19)–1.953(2) Å, respectively,
and are in accordance with the bond lengths as found in reported low-
spin octahedral cobalt(III) complexes with similar coordination en-
vironment [41b,41c,45–54]. The Co–N bond lengths of imine, sec-
ondary and tertiary amines span in the range 1.922(2)–1.9223(19),
1.983(2)–2.008(2) and 2.097(2)–2.1082(19) Å, respectively in both
complexes. Here, the secondary amine bonds are somewhat longer than
the imine moieties in both complexes (see Table 1), which resemble
with their standard states of hybridization. Moreover, the bond length
of tertiary amine is significantly longer than the secondary amine group
which is again consistent with the reduced Lewis basicity of tertiary
amine, leading to the weaker binding propensity to the metal centre
3.1. Syntheses and general characterizations
The Schiff base (HL) ligand used in the present study was prepared
by the reaction of salicylaldehyde and N,N-dimethyldipropylene-
triamine in a 1:1 M ratio in methanol or acetonitrile under refluxing
condition (Scheme 2). This in situ generated ligand thereafter by the
reaction with cobalt(II) nitrate hexahydrate in the presence of pseu-
dohalides in a 1:1:2 M ratio produced different compounds of compo-
sitions [Co(L)(N3)2] (1), [Co(L)(NCS)2] (2) and [Co(L)(Sal)]2[Co
(NCS)4] (3), in reasonable yields. Furthermore, a mixture of cobalt(II)
chloride and the Schiff base in presence of NaNCO yielded [Co
(L)2]2[Co0.75(NCS)3]Cl0.5 (4) as only the isolable product. It is noted
that when thiocyanate auxiliary ion was used, the resulting products
are highly dependent on the solvents used for the syntheses, but no
effect has been observed on the stoichiometric ratios of the reactants.
When the synthesis was carried out in an acetonitrile/water solvent
mixture, it produced only isolable product 2, whereas identical reaction
in a methanol/water solvent using the same reactants resulted only
isolable product 3 in reasonable yield. These diverse results clearly
justify that the reaction condition is very much crucial for the formation
of these complexes. Further, isolation of different products from the
same reactants but from different solvents clearly indicates that both of
them are thermodynamic products, and both formation and stability of
them are dependent on the solvents used for their syntheses, that
governs the identity of the species that crystallizes from a particular
solvent. However, as found in the similar ligand systems, the starting
cobalt(II) metal centre in each case is air-oxidized to cobalt(III), espe-
cially when it is coordinated to the Schiff base ligand. The solubility of
all complexes is more or less remain same and they are soluble in the
most common organic solvents like methanol, acetonitrile and DMF.
Furthermore, the purity of bulk samples was verified by the elemental
analyses, which is matched well with the X-ray crystallographic results.
IR spectra of all complexes display the characteristic stretching
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InorganicaChimicaActa490(2019)163–172
Fig. 2. The partial crystal structures of 3 and 4 displaying both complex cation and complex anions with selected atom labelling scheme. Ellipsoids are drawn in 30%
probability.
[41]. The N–N bond distances of the azide ions ranging from
1.152(3)–1.199(3) Å indicate the coordination through the anionic
terminal of azide ion [41b,41c,52–54]. Similarly, the bond distances
associated with thiocyanate ion in complex 2 are typical for coordina-
tion through the nitrogen end [41b,41c,52–54]. The solid-state struc-
ture of 1 is stabilized by the intermolecular NeH⋯N hydrogen bonding
interaction involving secondary amine and one of the terminal nitrogen
atom of coordinated azide ion with N⋯O distance of 3.238(3) Å be-
tween the adjacent molecules (Fig. S1). Stability in the solid state is
further reinforced by the multiple CeH∙∙∙π interactions between the
neighbouring molecules in 1 as depicted in the figure. Similar pattern of
hydrogen bonding interaction is also present in the solid state structure
of 2 in which the secondary amine forms hydrogen bond with the
sulphur atom of coordinated thiocyanate ion from a neighbouring
molecule with N⋯S distance of 3.428(3) Å. Apart from that the CeH∙∙∙π
interaction involving two adjacent molecules provides additional sta-
bility of compound 2 in the solid state (Fig. S2).
monoanionic in 4. Participation of free tertiary amine group in the
intramolecular hydrogen bonding as an acceptor with the secondary
amine group of the same ligand (see Fig. 2, right) further ensures the
monoanionic state of the coordinated ligand. The Co–O(phenolate),
Co–N(imine) and Co–N(secondary amine) bond lengths are in the range
1.952(4)–1.938(5), 2.016(6)–2.040(5) and 1.893(4)–1.902(4) Å, re-
spectively for complex 4. As usual, cobalt(II) centre in the complex
anions is tetrahedrally bonded with nitrogen atoms of four cyanate ions
[55–58] with bond angles (N–Co–N) varying in the range
108.0(3)–112.0(5)°.
3.3. Electrochemical studies
The electrochemical studies of all complexes carried out in me-
thanol, the same medium used for the catalytic experiments, at a scan
rate of 100 mV/s with respect to a Ag/AgCl electrode at room tem-
perature. Representative cyclic voltammograms of 2 and 3 are depicted
in Fig. 3, while the electrochemical data of all complexes are assembled
in Table 2. In the negative potential region, the complexes exhibit an
X-ray diffraction study reveals that complex 3 crystalizes in the
orthorhombic Fdd2 space group, whereas complex 4 crystalizes in the
tetragonal P4/n space group. The asymmetric unit of 3 consists of a
complex cation of composition [Co(L)(Sal)]+ on a general position and
irreversible reduction wave varying in
a
narrow range of
2−
−0.65–−0.86 V, that is consistent with the diverse coordination en-
vironments of the metal centre in these complexes. In the positive po-
tential region, only complexes 3 and 4 exhibit an irreversible oxidative
response at 0.90 V (for 3) and 0.93 V (for 4). This oxidative response is
due to the presence of complex anion of general formula [CoX4]2−
a complex anion Co(NCS)4
on a two-fold axis of symmetry. The
geometry of the metal centre of complex cation [Co(L)(Sal)]+ is
pseudo-octahedral with N3O3 coordination environment, comprising
with N3O donor set of the deprotonated Schiff base ligand and two O-
atoms from deprotonated salicylaldehyde molecule. The Schiff base li-
gand binds the metal centre exactly in similar fashion as found in the
crystal structures of 1 and 2. Therefore, the Co–N bond distances of N-
atoms of imine, secondary and tertiary amines found to be 1.935(4),
2.017(4) and 2.105(4) Å, respectively, in 3 are similar to bond distances
observed in 1 and 2. Again, they are comparable to low-spin octahedral
cobalt(III) complexes with similar coordination environments
[41b,41c,45–54]. The Co–O (phenolate) and Co–O (salicylaldehyde)
bond distances are 1.880(3) and, 1.889(3) and 1.907(4) Å, respectively,
typical for cobalt(III) coordination chemistry. In complex anion Co
(NCS)42−, the cobalt(II) centre has distorted tetrahedral geometry with
the Co–N bond lengths of 1.934(5) and 1.937(5) Å and the N–Co–N
bond angles ranging from 105.7(2)° to 116.3(4)° [55–58].
The asymmetric unit of complex 4 includes two crystallographically
independent complex cations of composition [Co(L)2]+ on general
2−
positions, two distinct Co(NCO)4
complex anions either siting on a
two-fold axis of symmetry or on a four-fold axis of symmetry along with
a chloride ion on a four-fold axis of symmetry. The structures of both
crystallographically independent complex cations are eventually si-
milar in which the metal centre has distorted octahedral geometry with
N4O2 coordination environments. In this system, the Schiff base ligands
coordinate the metal centre facially using N2O donor sites of the ligand,
but electroneutrality of the systems demands that the ligand is
Fig. 3. Cyclic voltammograms of 1 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.
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Table 2
Selected bond distances (Å) for complexes 1–4.
Bond
1
2
3
4
Co(III)–N(imine)
1.922(2)
1.983(2)
1.9223(19) 1.935(4) 1.952(4)
1.938(5)
Co(III)–N(secondary
amine)
2.008(2)
2.017(3) 2.016(6)
2.040(5)
Co(III)–N(tertiary amine) 2.097(2)
2.1082(19) 2.105(4)
–
Co(III)–O(phenolate)
1.8654(17)
–
1.8770(15) 1.880(3) 1.893(4)
1.902(4)
Co(III)–O(free aldehyde)
Co(III)–N(pseudohalide)
Co(II)–N(pseudohalide)
–
1.889(3)
1.907(4)
–
–
–
1.9098(19)
1.9094(19)
–
1.947(2)
1.953(2)
–
1.934(5) 1.983(6)
1.937(5) 1.952(6)
1.941(6)
(X = SCN for 3, NCO for 4) where the oxidation of cobalt(II) to cobalt
Fig. 5. Initial rate versus substrate concentration plot for the oxidation of OAPH
in dioxygen-saturated methanol catalysed by 1–4 at room temperature.
Symbols and solid lines represent the experimental and simulated profiles, re-
spectively.
(III) takes place.
3.4. Oxidation studies with substituted aminophenols
Oxidation of o-aminophenol (OAPH) by molecular dioxygen in
presence of catalytic amount of the complexes in aerobic condition was
studied using UV–vis spectrophotometer at room temperature. For that,
5.0 × 10−5 M methanolic solution of the complexes were allowed to
react with a 1.0 × 10−2−M solution of OAPH. The spectral change due
to formation of 2-aminophenoxazinone (Phx-1) was recorded UV–vis
spectrophotometrically. Spectral profiles for aerobic oxidation of OAPH
catalyzed by 1 and 2 for a period of 1 h (at 5 min interval) are depicted
in Fig. 4, whereas Fig. S3 represents the oxidation of OAPH by 3 and 4
respectively. The gradual increase of peak intensity at ca. 435 nm im-
plies the formation of Phx-1 as a function of time [59]. The spectral
profile further suggests that complex 1 and 2 are efficient catalysts for
the oxidation of OAPH, while 3 and 4 are less reactive. Moreover, the
detailed kinetic study is performed to get insight into the catalytic ef-
ficiency of these complexes. For that 1.0 × 10−5 M solutions of the
complexes were reacted with excess substrate at the pseudo-first-order
condition and initial rates were evaluated. Initial rate versus con-
centration of the substrate plots for oxidation of OAPH catalyzed by 1–4
show a typical rate saturation kinetics (Fig. 5). These results suggest
that an intermediate complex-substrate aggregate is formed in a pre-
equilibrium stage followed by the irreversible redox transformation of
the intermediate in the rate determining step of the catalytic cycle.
Michaelis–Menten approach was applied for evaluation of kinetic
parameters Vmax, KM, and Kcat (Table 3) for such rate saturation ki-
netics. The turnover numbers (TON) of the catalysts were then
Table 3
Cyclic voltammetric data for 1–4 at room temperature.
Complex
Reduction (CoIII/CoII)
Oxidation (CoII/CoIII
)
1
2
3
4
–0.65 V
–0.68 V
–0.65 V
–0.86 V
–
–
0.90 V
0.92 V
calculated by dividing the value of Vmax by the concentration of catalyst
used and are reported to be 47.36, 47.60, 16.46 and 10.34 h−1 for 1–4,
respectively.
A good numbers of model catalysts have been reported mimicking
the function of phenoxazinone synthase to gain clear idea about the
possible mechanism and to check efficiency of the synthetic analogues.
Comparing the present results with literature data, it is clear that al-
though 3 and 4 are less reactive but 1 and 2 are efficient catalysts for
aerial oxidation of o-aminophenol. It is to be further noted that the
catalytic oxidation of some substituted o-aminophenols in terms of
isolations and characterizations of their oxidised products were avail-
able in the literature, but the detailed kinetic studies were not reported.
Therefore, in this present study we examined the detailed catalytic
activity using two substituted o-aminophenols, namely 2-amino-5-me-
thylphenol (5MeOAP) and 2-amino-5-methylphenol (4MeOAP) to get
Fig. 4. Increase in absorption band for the oxidation of o-aminophenol (1.0 × 10−2 M) catalysed by 1 and 2 (5.0 × 10−5 M) in dioxygen-saturated methanol.
168

N.C. Jana, et al.
InorganicaChimicaActa490(2019)163–172
Fig. 6. Time resolve spectral profile showing growth of 4a,7-dimethyldihydro-2-aminophenoxazinone at 405 nm upon addition of 5.0 × 10−3 M and 1.0 × 10−2
M
2-amino-5-methylphenol to a solution containing 1 (1 × 10−5 M)(left) and 2 (5 × 10−5 M)(right) respectively in dioxygen-saturated methanol.
Fig. 7. Spectral scan showing oxidation of 2-amino-4-methylphenol (1.0 × 10−2 M) catalysed by 1 and 2 (5 × 10−5 M) in dioxygen-saturated methanol.
further insights into the reactive intermediates and the mechanistic
details of the catalytic reactions and finally to establish the structure-
reactivity correlation of the catalysts. For that the reaction kinetics of
the formation of 2-amino-4,4a-dihydro-4a-7-dimethyl-3H-phenoxazine-
3-one (Phx-2), and 3-amino-1,4a-dihydro-4a,8-dimethyl-2H-phenox-
azin-2-one (Phx-3) were carried out for a period of 10 min using the
most reactive compounds 1 and 2 as depicted in Figs. 6 and 7, re-
spectively. Similar kinetic profiles as found for the oxidation of o-
aminophenol are also obtained for the substituted o-aminophenols
(Fig. 8), indicating that all these substrates oxidations follow similar
catalytic pathway. Detailed kinetic analysis further gives us the values
of various kinetic parameters Vmax, KM, and Kcat which along with
turnover numbers are assembled in Table 4.
aminophenol itself, which is consistent with the electronic structures of
the substrates where the methyl substitution in the benzene ring facil-
itates the oxidation of methyl-substituted o-aminophenols.
Interestingly, both the rate saturation kinetics and mass spectrometry
(vide infra) clearly suggest that the methyl substitution does not inhibit
formation of the complex-substrate intermediate, but the final two-
electrons oxidation step was blocked by methyl substitution, leading to
the formation of dihydro-aminophenoxazinone chromophore instead of
aminophenoxazinone chromophore which is the final oxidation product
of o-aminophenol itself.
The mass spectrometry is quite helpful as it proves significant in-
formation regarding the product and intermediate species from which
one can schematically represent the most probable mechanistic
pathway of the catalytic action. For that, the electrospray ionization
mass spectra of a reactive compound 2 both in absence and presence of
2-amino-5-methylphenol (20 equivalents) in methanol were recorded
and are depicted in Figs S4 and S5, respectively. The mass spectrum of
compound 2 is clean enough and it consists of only one major peak at
m/z = 321.10 which is nothing but the peak of complex cation without
thiocyanate ions of formula [Co(L)]+ (calcd. m/z = 321.13). Interest-
ingly, when mass spectrum of 2 was recorded in presence of 20
equivalents of 2-amino-5-methylphenol, the base peak is shifted at m/
z = 243.10, which is matched well with the protonated species of the
3.5. Mechanistic pathway and mass spectrometry
Catalytic efficiency of the oxidase metalloenzymes and their syn-
thetic analogues depends on several factors among which the electronic
environment around the metal centers and available coordination sites
for substrate binding are the most important factors. In present systems,
although the reduction potentials of the cobalt(III) centres are com-
parable, the facile substrate binding by the replacement of coordinated
pseudohalides (azide ions in 1 and thiocyanate ions in 2) from the metal
centres, leading to the formation of stable complex-substrate inter-
mediates, are responsible for higher catalytic activity of 1 and 2 than 3
and 4. Kinetic data further show that the catalytic oxidation of methyl
substituted o-aminophenols is rather favourable than that of o-
product
2-amino-4,4a-dihydro-4a-7-dimethyl-3H-phenoxazine-3-one
(calcd. m/z = 243.11). The original peak of the complex cation at m/
z = 321.10 converted to a minor component in presence of the sub-
strate. Moreover, a minor peak at m/z = 124.11 (calcd. m/z = 124.08)
169

N.C. Jana, et al.
InorganicaChimicaActa490(2019)163–172
Fig. 8. Initial rate versus substrate concentration plot for the oxidation of OAPH in dioxygen-saturated methanol catalysed by 2 and 3 at room temperature. Symbols
and solid lines represent the experimental and simulated profiles, respectively.
was found in the mass spectrum, related to the protonated species of the
substrate itself. The appearance of a major peak at m/z 443.15 is the
most interesting one as it matches with a complex-substrate inter-
mediate species of composition [CoIII(L)(2-amino-5-methylpheno-
late)]+ (calcd. m/z = 443.19). These observations clearly support the
ease of formation of the stable complex-substrate aggregates, exploiting
the stronger chelating ability of o-aminophenolate ions by substituting
the terminal pseudohalide ions.
2H-phenoxazin-2-one for 2-amino-4-methylphenol) instead of phenox-
azinone chromophore as found for the oxidation of o-aminophenol itself
(see Scheme 3).
4. Conclusions
Four cobalt complexes were synthesized by the reaction of a new
tetradentate Schiff base ligand, derived from N,N-dimethyldipropyle-
netriamine and salicylaldehyde, and cobalt(II) salts in presence of dif-
ferent pseudohalide ions. X-ray crystallographic studies reveal that both
counter ions and solvents have significant influence on the structural
diversities of the resulting complexes. All these complexes were found
active catalysts for the aerobic oxidation of different o-aminophenols.
Among them, 1 and 2 are highly reactive as in these systems sub-
stitutionally labile metal-bound pseudohalide ions are available for
substrate binding. The detailed kinetic studies of the aerobic oxidation
of two substituted o-aminophenols, namely 2-amino-5-methylphenol
and 2-amino-4-methylphenol, were further examined using highly re-
active catalysts 1 and 2. Remarkably, the catalytic oxidation of methyl
substituted o-aminophenols is much favourable than the parent o-ami-
nophenol which is the consequence of electron donating effect of me-
thyl substitution. Mass spectrometry reveals that the methyl substitu-
tion does not inhibit the formation of a stable complex-substrate
intermediate in the catalytic cycle, but the final oxidative dehy-
drogenation step is inhibited in the presence of methyl group, leading to
the formation of dihydro-phenoxazinone chromophore instead of phe-
noxazinone chromophore.
Now we are at right position to tempt a well acceptable plausible
mechanism compiling the preceding information. Both the mass spec-
trometry and kinetic data clearly establish that the catalytic cycle for
the aerobic oxidation of o-aminophenol catalysed by 1 and 2 starts with
the formation of a stable complex-substrate aggregate through the re-
placement of coordinated pseudohalides by o-aminophenolate ion. This
complex-substrate intermediate produces o-aminophenolate radical
through the inner sphere electron transfer from o-amionophenolate to
the cobalt(III) centre in the rate determining step. Facile oxidation upon
methyl substitution at benzene ring, leading to the higher turnover
rates, further suggests that the aminophenolate oxidation is the rate
determining step. Being unstable the OAP radical thereafter is oxidised
to o-benzoquinone monoamine (BQMI) in several ways including the
disproportionation of the OAP radical itself. The intermediate (BQMI)
ultimately produces 2-aminophenoxazin-3-one through the couples of
oxidative dehydrogenations intermediate steps as shown in Scheme 3.
Similar mechanistic pathway is also applicable for catalytic oxidation of
2-amino-5-methylphenol or 2-amino-4-methylphenol in which methyl
substitution does not inhibit the formation of a stable complex-substrate
intermediate as evidenced by the mass spectrometry. Although the
methyl substitution does not inhibit the coupling of two aminophenols,
but the final oxidative dehydrogenation step was blocked by the methyl
substitution, leading to the formation of dihydro-phenoxazinone chro-
mophore (2-amino-4,4a-dihydro-4a-7-dimethyl-3H-phenoxazine-3-one
for 2-amino-5-methylphenol and 3-amino-1,4a-dihydro-4a,8-dimethyl-
Acknowledgements
A. P. also gratefully acknowledges the financial support of this work
by the University Grant Commission, India (Sanction no. F. PSW-229/
15-16(REO)). Panskura Banamali College acknowledges the grants re-
Table 4
Kinetic parameters of oxidation of o-aminophenols catalysed by 1–4.
Substrate
Catalyst
Vmax (Ms−1
)
KM (M)
kcat (h−1
)
OAPH
1
2
3
4
1
2
1
2
(1.31
(1.32
(4.57
(2.87
(2.07
(2.16
(2.00
(2.21
0.02) × 10−7
0.03) × 10−7
0.04) × 10−8
0.04) × 10−8
0.02) × 10−7
0.03) × 10−7
0.03) × 10−7
0.06) × 10−7
(3.46
(6.36
(4.04
(4.21
(3.43
(4.06
(2.59
(3.39
0.13) × 10−3
0.35) × 10−3
0.10) × 10−3
0.15) × 10−3
0.11) × 10−3
0.13) × 10−3
0.12) × 10−3
0.22) × 10−3
47.36
47.60
16.46
10.34
74.38
77.85
71.94
79.66
2-amino-5-methylphenol
2-amino-4-methylphenol
170

N.C. Jana, et al.
InorganicaChimicaActa490(2019)163–172
Scheme 3. Plausible mechanistic pathway for the aerobic oxidation of o-aminophenols catalysed by 1 and 2.
ceived from Department of Science and Technology (DST), Govt. of
India through FIST program (SR-FIST-Col/295 dated 18/11/2015). The
authors thank Jadavpur University, Kolkata, India, for providing the
Mass spectrometry facility.
Dalton Trans. 43 (2014) 6650.
[12] A. Ganguly, B.K. Paul, S. Ghosh, S. Kar, N. Guchhait, Analyst 138 (2013) 6532.
[13] W. Al Zoubi, N. Al Mohanna, Spectrochim. Acta Part A: Mol. Biomol.Spectrosc. 132
(2014) 854.
[14] P.A. Vigato, S. Tamburini, Coord. Chem. Rev. 248 (2004) 1717.
[15] R.M. Clarke, T. Storr, Dalton Trans. 43 (2014) 9380.
[16] T. Horvath, J. Kaizer, G. Speier, J. Mol. Catal. A Chem. 215 (2004) 9.
[17] L.I. Simandi, M. Tatiana, Z. May, G. Besenyei, Coord. Chem. Rev. 245 (2003) 85.
[18] T.M. Simandi, L.I. Simandi, M. Gyor, A. Rockenbauer, A. Gomory, Dalton Trans.
(2004) 1056.
Appendix A. Supplementary data
The electronic supporting information file contain Figures. S1–S5.
CCDC 1888079–1888082 contain the supplementary crystallographic
data for 1–4, respectively. Supplementary data to this article can be
found online at https://doi.org/10.1016/j.ica.2019.03.017.
[19] C.E. Barry, P.G. Nayar, T.P. Begley, J. Am. Chem. Soc. 110 (1988) 3333.
[20] E. Frei, Cancer Chemother. Rep. 58 (1974) 49.
[21] U. Hollstein, Chem. Rev. 74 (1974) 625.
[22] Alex W. Smith, Ana Camara-Artigas, Meitian Wang, James P. Allen, Wilson
A. Francisco, Biochemistry 45 (14) (2006) 4378.
[23] M. Le Roes-Hill, C. Goodwin, S. Burton, Trend. Biotechnol. 27 (4) (2009) 248.
[24] S.K. Dey, A. Mukherjee, Coord. Chem. Rev. 310 (2016) 80.
[25] R. Muthusami, M. Moorthy, K. Irena, A. Govindaraj, C. Manickam, R. Rangappan,
New J. Chem. 42 (23) (2018) 18608.
References
[1] H. Naeimi, Z.S. Nazifi, S.M. Amininezhad, M. Amouheidari, J. Antibiot. 66 (2013)
687.
[26] B. Chowdhury, B. Bhowmik, A. Sahu, M. Joshi, S. Paul, A.R. Choudhury, B. Biswas,
J. Chem Sci. 130 (12) (2018) 161.
[2] M. Hajrezaie, M. Paydar, C.Y. Looi, S.Z. Moghadamtousi, P. Hassandarvish,
M.S. Salga, H. Karimian, K. Shams, M. Zahedifard, N.A. Majid, H.M. Ali,
M.A. Abdulla, Sci. Rep. 5 (2015) 9097.
[27] M. Mahato, K. Van Hecke, H.P. Nayek, Appl. Organomet. Chem. 32 (5) (2018)
4336.
[28] A. Das, K. Bhattacharya, L.K. Das, S. Giri, A. Ghosh, Dalton trans. 47 (28) (2018)
9385.
[3] T. Mukherjee, J.C. Pessoa, A. Kumar, A.R. Sarkar, Dalton Trans. 42 (2013) 2594.
[4] A. Lehwess-Litzmann, P. Neumann, C. Parthier, S. Lüdtke, R. Golbik, R. Ficner,
K. Tittmann, Nat. Chem. Biol. 7 (2011) 678.
[29] K. Ghosh, A. Banerjee, A. Bauzá, A. Frontera, S. Chattopadhyay, RSC Adv. 8 (49)
(2018) 28216.
[5] P. Adaeo, M.L. Kuznetsov, S. Barroso, A.M. Martins, F. Avecilla, J.C. Pessoa, Inorg.
Chem. 51 (2012) 11430.
[30] A. Panja, A. Frontera, Eur. J. Inorg. Chem. 7 (2018) 924.
[31] A. Panja, N.C. Jana, M. Patra, P. Brandao, C.E. Moore, D.M. Eichhorn, A. Frontera,
J. Mol. Catal. A Chem. 412 (2016) 56.
[6] D. Gong, B. Wang, X. Jia, X. Zhang, Dalton Trans. 43 (2014) 4169.
[7] P. Das, W. Linert, Coord. Chem. Rev. 311 (2016) 1.
[32] A. Panja, Polyhedron 79 (2014) 258.
[8] W. Al Zoubi, Y.G. Ko, J. Organomet. Chem. 822 (2016) 173.
[9] A.M. Abu-Dief, I.M.A. Mohamed, J. Basic Appl. Sci. 4 (2015) 119.
[10] S.H. Li, F.R. Chen, Y.F. Zhou, J.N. Wang, H. Zhang, J.G. Xu, Chem. Commun. (2009)
4179.
[33] A. Panja, RSC Adv. 4 (2014) 37085.
[34] S. Shimizu, M. Suzuki, A. Tomoda, S. Arai, H. Taguchi, T. Hanawa, S. Kamiya,
Tohoku J. Exp. Med. 203 (2004) 47.
[35] D.J. Kosman, Metallomics 10 (3) (2018) 370.
[11] S.A. Lee, G.R. You, Y.W. Choi, H.Y. Jo, A.R. Kim, I. Noh, S.J. Kim, Y. Kim, C. Kim,
171

N.C. Jana, et al.
InorganicaChimicaActa490(2019)163–172
[36] L. Cortes, B.R. Roberts, A.G. Wedd, Z. Xiao, Inorg. chem. 56 (9) (2017) 5275.
[37] A. Panja, Polyhedron 80 (2014) 81.
[47] W. Cao, Y. Liu, T. Zhang, J. Jia, Polyhedron. 147 (2018) 62.
[48] A.T. Fiedler, A.A. Fischer, J. Biol. Inorg. Chem. 22 (2–3) (2017) 407.
[49] F.N. Karabulut, H.L. Feltham, S. Brooker, Dalton Trans. 47 (2018) 11749.
[50] A. Panja, Dalton Trans. 43 (2014) 7760.
[38] A. Panja, M. Shyamal, A. Saha, T.K. Mandal, Dalton Trans. 43 (2014) 5443.
[39] A.K. Ghosh, C.S. Purohit, R. Ghosh, Polyhedron 155 (2018) 194.
[40] A. Company, L. Gomez, R. Mas-Balleste, I.V. Korendovych, X. Ribas, A. Poater,
T. Parella, X. Fontrodona, J. Benet- Buchholz, M. Sola, L. Que Jr, E.V. Rybak-
Akimova, M. Costas, Inorg. Chem. 46 (2007) 4997.
[51] A. Panja, P. Guionneau, Dalton Trans. 42 (14) (2013) 5068.
[52] P. Chakraborty, S. Mohanta, Inorg. Chim. Acta. 435 (2015) 38.
[53] M.A. Goher, A.A. Youssef, F.A. Mautner, Polyhedron 25 (7) (2006) 1531.
[54] K. Ghosh, K. Harms, A. Bauzá, A. Frontera, S. Chattopadhyay, Dalton Trans. 47 (2)
(2018) 331.
[41] (a) A. Panja, N.C. Jana, S. Adak, P. Brandão, L. Dlháň, J. Titiš, R. Boča, New J.
Chem. 41 (2017) 3143;
(b) A. Panja, N.C. Jana, P. Brandão, Mol. Catal. 449 (2018) 49;
(c) A. Panja, N.C. Jana, P. Brandão, New J. Chem. 41 (2017) 9784.
[42] G.M. Sheldrick, SADABS, University of Go¨ttingen, Germany, 1996.
[43] G.M. Sheldrick, Acta Crystallogr., Sect. A Found Crystallogr. 64 (2008) 112.
[44] N.C. Jana, K. Pramanik, A. Bauzá, P. Brandão, M. Patra, A. Frontera, A. Panja,
Polyhedron 124 (2017) 262.
[55] L. Mandal, S. Mohanta, Dalton Trans. 43 (2014) 15737.
[56] H. Mori, N. Sakurai, S. Tanaka, H. Moriyama, T. Mori, H. Kobayashi, A. Kobayashi,
Chem. Mater. 12 (2000) 2984.
[57] S. Sen, P. Talukder, S.K. Dey, S. Mitra, G. Rosair, D.L. Hughes, G.P.A. Yap, G. Pilet,
V. Gramlich, T. Matsushita, Dalton Trans. (2006) 1758.
[58] A. Ray, G.M. Rosair, R. Kadam, S. Mitra, Polyhedron 28 (2009) 796.
[59] (a) N. Sarkar, K. Harms, A. Bauzá, A. Frontera, S. Chattopadhyay, ChemistrySelect.
2 (2017) 2975;
[45] D. Bandyopadhyay, M. Layek, M. Fleck, R. Saha, C. Rizzoli, Inorg. Chim. Acta. 461
(2017) 174.
[46] S. Banerjee, R. Patra, P. Ghorai, P. Brandão, S.G. Chowdhury, P. Karmakar, A. Saha,
New J. Chem. 42 (20) (2018) 16571.
(b) K. Ghosh, K. Harms, S. Chattopadhyay, Polyhedron 112 (2016) 6.
172