Ligand directed synthesis of a unprecedented tetragonalbipyramidal copper (II) complex and its antibacterial activity and catalytic role in oxidative dimerisation of 2-aminophenol

Article abstract of DOI:10.1002/aoc.5935

In pursuit of the significant contribution of copper ion in different biological processes, this research work describes the synthesis, X-ray structure, Hirshfeld surface analysis, oxidative dimerization of 2-aminophenol and antibacterial activity of a ne

Full text of DOI:10.1002/aoc.5935

Received: 24 March 2020
DOI: 10.1002/aoc.5935
Revised: 17 June 2020
Accepted: 26 June 2020
F U L L P A P E R
Ligand directed synthesis of a unprecedented
tetragonalbipyramidal copper (II) complex and its
antibacterial activity and catalytic role in oxidative
dimerisation of 2-aminophenol
1
1
2
Shreya Mahato | Nishith Meheta | Muddukrishnaiah Kotakonda
|
3
4
5
Mayank Joshi | Prasanta Ghosh | Madhusudan Shit
|
3
1
Angshuman Roy Choudhury | Bhaskar Biswas
1
Department of Chemistry, University of
In pursuit of the significant contribution of copper ion in different biological
processes, this research work describes the synthesis, X-ray structure,
Hirshfeld surface analysis, oxidative dimerization of 2-aminophenol
and antibacterial activity of a newly designed copper (II)-Schiff base complex,
North Bengal, Darjeeling-734013, India
2
Department of Pharmaceutical
Technology, Anna University, BIT
Campus Thiruchirappalli, India
3
Department of Chemical Sciences, Indian
[
Cu(L) ] (1), [Schiff base (HL) = 2-(2-methoxybenzylideneamino)phenol].
2
Institute of Science Education and
Research, Mohali, Sector 81, Knowledge
City, S. A. S. Nagar, Manauli PO, Mohali,
Punjab 140306, India
X-ray structural analysis of 1 reveals that the Cu (II) complex crystallizes in a
cubic crystal system with Ia-3d space group. The Cu (II) centre adopts an
unprecedented tetragonal bipyramidal geometry in its crystalline phase. The
Schiff base behaves as a tridentate chelator and forms an innermetallic chelate
of first order with Cu (II) ion. The copper (II) complex has been tested in the
bio-mimics of phenaxozinone synthase activity in acetonitrile and exhibits
4
Department of Chemistry, Narendrapur
Ramakrishna Mission Residential College,
Kolkata 700103, India
5
Department of Chemistry, Dinabandhu
Andrews College, Kolkata 700084, India
−1
good catalytic activity as evident from high turnover number, 536.4 h .
Electrochemical analysis exhibits the appearance of two additional peaks at
Correspondence
Bhaskar Biswas, Department of
Chemistry, University of North Bengal,
Darjeeling-734013, India.
−
0.15 and 0.46 V for Cu (II) complex in presence of 2-AP and suggests the
−
•−
•−
development of AP /AP and AP /IQ redox couples in solution, respectively.
The presence of iminobenzosemiquinone radical at g = 2.057 in the reaction
mixture was confirmed by electron paramagnetic resonance and may be
considered the driving force for the oxidative dimerisation of 2-AP. The
existence of a peak at m/z 624.81 for Cu (II) complex in presence of 2-AP in
electrospray ionization mass spectrum ensures that the catalytic oxidation
proceeds through enzyme-substrate adduct formation. The copper (II) complex
exhibits potential antibacterial properties against few pathogenic bacterial
species like Staphylococcus aureus, Enterococcus and Klebsiella pneumonia and
scanning electron microscope studies consolidates that destruction of bacterial
cell membrane accounts on the development of antibacterial activity.
Email: icbbiswas@gmail.com
Funding information
Science and Engineering Research Board,
Grant/Award Number: TAR/2018/000473
Shreya Mahato and Nishith Meheta have equal contribution.
Appl Organomet Chem. 2020;e5935.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 12
https://doi.org/10.1002/aoc.5935

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MAHATO ET AL.
K E Y W O R D S
aminophenol oxidation activity, antibacterial property, copper (II), electrochemical analysis,
X-ray structure
1
| INTRODUCTION
complex. The antibacterial property of this copper
(
II) complex towards different bacterial species has also
In this modern age of science, Schiff base as polydentate
chelators have been widely used in the development
of coordination compounds of varied functionality.
been delineated.
[
1–3]
Among the transition metal ions, copper ion has been
considered as an essential metal ion in living system and
it also plays significant role in designing advance func-
2 | EXPERIMENTAL
2.1 | Preparation of the Schiff base and
dinuclear copper (II) complex
[4–7]
tional materials.
In nature, copper ions are integrated
with different bio-ligands in the functional sites of
various metallo-enzymes such as catechol and galactose
oxidase, phenoxazinone synthase, superoxide dismutase,
2.1.1 | Chemicals, solvents and starting
materials
[8–11]
lysine oxidase, N O reductase etc.
It pursuit of deep
2
understanding of different biological oxidation processes
in nature, synthetic coordination chemists have been
stepped forward to tune electronic and geometric factors
of the ligands in engineering bio-inspired coordination
Highly pure o-anisidine (Sigma Aldrich, Missouri,
Texas, USA), salicylaldehyde (Sigma Aldrich, Missouri,
Texas,USA) and cupric acetate monohydrate (SRL,
Gurugram, Haryana, India) were purchased from the
respective concerns and used as received. All other
chemicals and solvents were of analytical grade and used
as received without further purification.
[12–15]
driven compounds.
It is well documented that
the bio-inspired synthetic analogues also serve as magic
catalysts in several catalytic oxidation reactions of
[14–16]
laboratory and industrial significance.
Among the
different oxidase enzymes, phenoxazinone synthase has
[14,15]
drawn special attention
as the oxidized product,
2.1.2 | Synthesis of the Schiff base and
aminophenoxazinone acts as an antineoplastic agent
named actinomycin D (questiomycin A) which is usually
recommended in the treatment of certain types of
copper (II) complex
The Schiff base, HL was synthesized following a reported
[17–20]
[32]
cancer.
Furthermore, 2-aminophenoxazin-3-one
method.
The Schiff base was synthesized through con-
and 3-aminophenoxazin-2-one compounds show potential
antimicrobial, antiviral, and antitumor activities against
densation reaction between o-anisidine (0.123 g, 1 mmol)
and salicylaldehyde (0.122 g, 1 mmol) in ethanol under
reflux for 8 hr. Then, the yellowish brown coloured
gummy product was extracted and stored in vaccuo over
[21–24]
different pathogens.
Aminophenoxazinone com-
pounds have also been reported in different forms like
Streptomyces parvulus and wood rotting fungi metabolites,
oxidative coupling products of o-aminophenols with
bovine erythrocyte hemolyzate, and bio-conversion prod-
CaCl for use. Yield: 0.201 g (ꢀ88.5%). Anal. Calc. for
2
C H NO (HL): C, 73.99; H, 5.77; N, 6.16; Found:
1
4
13
2
−
1
C, 73.93; H, 5.72; N, 6.19. IR (KBr, cm ; Figure S1): 3372
[24–28]
ucts of Pseudomonas putida grown on nitroarenes.
(νOH), 1615, 1590 (νC=N); UV–Vis (λmax, nm; Figure S2):
1
In light of incessant emergence for new antibiotics
with potential resistance against microorganisms, it is of
great importance to design novel antibiotics, which
would destroy the lipoid layer as well as the cell mem-
230, 270, 346; H NMR (δ ppm, 400 Mz, CDCl3;
Figure S3) δ = 13.88 (s, 1H), 8.63 (s, 1H), 7.26–6.82
13
(Ar-H, 7H), 3.79–3.83 (t, 3H) ppm.
C
NMR
(400 MHz,CDCl ; Figure S4): 162.06 (HC=N); 153.03,
3
[29,30]
brane of the pathogen with high selectivity.
In this
(Ar-OH); 137.05 (Ar-N=C); 132.94, 132.05, 127.95,
127.13, 119.64, 118.46, 117.40, 115.05, (Ar-C); 77.49,
77.17, 76.85 (-OCH3).
The copper (II)-Schiff base complex was prepared
by drop wise addition of acetonitrile solution of Cu (OAc)2
(0.199 g, 1 mmol) to the methanolic solution of HL
(0.454 g, 2 mmol). The yellow coloured Schiff base solution
was instantly turned to green coloured solution.
perspective, copper based coordination compounds hold
a great promise to provide future alternatives to the
[30,31]
existed antibiotics.
In the context of newly designed
copper (II) complexes with high catalytic activities and
potential therapeutic values, this research study deals
with the synthesis, structural characterization and cata-
lytic oxidase activity of a new copper (II)-Schiff base

MAHATO ET AL.
3 of 12
[
33c]
Thereafter, the reaction mixture was kept on a magnetic
stirrer for 20 mins and kept in open atmosphere for slow
evaporation. After 7–10 days, green coloured single
crystals of the compound were separated out from the
solution. The crystalline compound was washed with
toluene and dried over silica gel. Finally, different
spectroscopic analysis was carried out to determine the
structural formulation of the Cu (II)-Schiff base complex.
The results are summarized as follows.
least-squares procedures using the SHELXL-2015
software package through OLEX2 suite.
[
33d]
2.4 | Hirshfeld surface calculations of
copper (II) compound
[
34a]
Crystal Explorer 17.5
progam package was employed
and 2D fingerprint
of 1 using its single crystal X-ray diffraction
[
34b]
to generate Hirshfeld surfaces
plots
[
34c]
Yield of 1: 0.415 g (ꢀ63.5% based on metal salt) Anal.
calc. For C H N O Cu (1): C, 65.17; H, 4.69; N, 5.43;
data. Hirshfeld Surface analysis is an important tool to
study and locate intermolecular interactions within
The function dnorm is a ratio of the
distances of any surface point to the nearest interior (di)
2
8
24 2 4
−1
Found: C, 65.13; H, 4.61; N, 5.38. IR (KBr pellet, cm ;
Figure S1): 1605,1585 (νC=N); UV–Vis (1 × 10 M, λmax
abs), nm, MeCN; Figure S2): 238, 283, 400.
−
4
[34c,d]
crystal packing.
(
and exterior (d ) atom, and the van der Waals radii of the
e
[
35,36]
atoms.
The normalized contact distance (dnorm
)
2.2 | Physical measurements
could be expressed following the equation 1.
vdW
vdW
FTIR-8400S SHIMADZU spectrometer (Shimadzu,
Nakagyo-ku, Kyoto, Japan) was employed to record IR
spectrum (KBr) of Schiff base and 1 in the range of
di −ri
r
de −re
dnorm
=
+
ð1Þ
vdW
i
vdW
r
e
−1
1
13
vdW
vdW
4
00–3,600 cm . H and C NMR spectra of the ligand
Where, re
and ri
denote the corresponding van der
(
HL) were obtained on a Bruker Advance 400 MHz
Waals radii of atoms. The negative value of dnorm indi-
cates that the sum of d and d is shorter than the sum of
spectrometer (Bruker, MA, USA) in CDCl at 298 K.
3
i
e
Steady-state absorption and other spectral data
were recorded with a JASCO V-730 UV–Vis spectro-
photometer (Jasco, Hachioji,Tokyo, Japan). Electrospray
ionization (ESI) mass spectral measurements were
performed with a Q-TOF-micro quadruple mass spec-
trometer (Waters, Milford,USA). Elemental analyses
the relevant van der Waals radii, which is considered to
be a closest contact and is visualized in red colour. The
white colour denotes intermolecular distances close to
van der Waals contacts with dnorm equal to zero whereas
contacts longer than the sum of van der Waals radii with
positive dnorm values are coloured with blue. A plot of di
were performed on
a
Perkin Elmer 2400 CHN
versus d is a fingerprint plot that identifies the presence
e
microanalyser (Perkin Elmer, Waltham, Massachusetts,
USA). X-band EPR spectra were recorded on a Magne-
ttech GmbH MiniScope MS400 spectrometer (equipped
with temperature controller TC H03, Magnettech,
Berlin, Germany), where the microwave frequency was
measured with an FC400 frequency counter. The EPR
spectrum of Cu (II) complex was simulated using Easy
Spin software.
of different types of intermolecular interactions.
2.5 | Aminophenol oxidation activity of
copper (II) complex (1)
Aminophenol oxidation activity was studied by treating
1 × 10− M solution of Cu (II) complex with 1 × 10
4
−3
M
of 2-aminophenol (2-AP) solution in acetonitrile under
aerobic conditions at room temperature. Absorbance
vs. wavelength (wavelength scans) of the solution was
monitored through spectrophometer at a regular time
interval of 6 min for 1 hr in the wavelength range from
2
.3 | Crystal structure determination and
refinement
[
13,15]
X-ray diffraction data of 1 were collected using a
Rigaku XtaLABmini diffractometer equipped with
Mercury 375R (2 × 2 bin mode) CCD detector. The data
were collected with graphite monochromated Mo-Kα
radiation (λ = 0.71073 Å) at 296 (2) K using ω scans. The
300–700 nm.
Kinetic experiments were also carried out spectropho-
tometrically to specify the efficacy of catalytic oxidation
of aminophenol by the Cu (II) complex in MeCN at
[
13,15]
298 K.
0.04 ml of the complex solution with a
[
33a]
−4
data were reduced using CrysAlisPro 1.171.39.35c
constant concentration of 1 × 10 M was added to 2 ml
and the space group determination was done using
Olex2. The structure was resolved by dual space method
solution of 2-AP of a particular concentration (varying its
concentration from 1 × 10− M to 1 × 10 M) to achieve
3
−2
[33b]
−4
using SHELXT-2015
and refined by full-matrix
the ultimate concentration as 1 × 10 M. The conversion

4
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MAHATO ET AL.
of 2-aminophenol to 2-aminophenoxazine-3-one was
monitored with time at a wavelength 407 nm (time scan)
2.8.1 | Bacterial strains (clinical bacterial
cultures), culture media
[13,15,37,38]
in MeCN.
To determine the dependence of
rate on substrate concentration, kinetic analyses were
performed in triplicate.
The oxidized product was extracted by column chro-
matography. Neutral alumina was employed as column
support and benzene-ethyl acetate solvent mixture was
treated as an eluant mixture in this chromatographic
The antimicrobial property of the Cu (II) complex was
examined against clinical Staphylococcus aureus, Entero-
coccus and Klebsiella. Microbial cultures were procured
from government medical college from Tiruchirappalli,
Tamil Nadu. Muller-Hinton agar media of Himedia Pvt.
Bombay, India was used for the media for the microbial
test. The antibacterial activity was evaluated by using the
Himedia zone reader.
separation. The purity of catalytic oxidation product of
1
2
-AP was examined by proton NMR spectroscopy. H
NMR spectral analysis reveals to identify the final
1
product. H NMR data for 2-amino-3H-phenoxazine-
3
-one (APX), (CDCl , 400 MHz,) δ : 7.61 (m, 1H), 7.45
2.8.2 | Agar well diffusion method
3
H
(m, 3H), 6.46 (s, 1H), 6.37 (s, 1H), 6.25 (s, 1H).
The antibacterial activity of Cu (II)-Schiff base complex
and a standard drug (Amikacin-100 mg/2 ml) was
studied initially by using a well plate method. Staphylo-
coccus aureus, Enterococcus and Klebsiella pneumoniae
inoculums were prepared by using nutrient broth media.
Double strength sterile Mueller Hinton agar media were
prepared by autoclaving 7.6 gm in 100 ml. Inoculate
the test microorganisms on the Mueller Hinton agar
plates by using sterile cotton swabs. Formulations of
Cu (II) complex and Amikacin were placed on agar well.
Plates were incubated for 30 min at the refrigerator to
diffuse the formulation into the agar plate, and finally,
2
.6 | Detection of presence of hydrogen
peroxide in the catalytic oxidation of
-aminophenol
2
The involvement of aerobic oxygen in the course of oxida-
tive dimerization of 2-AP was examined by testing the
presence of hydrogen peroxide following a reported proce-
[37,38]
dure.
In the oxidation of aminophenol in MeCN, the
solution was acidified with H SO till the pH of the solution
2
4
became 2. After a certain time, an equal volume of water
was added to stop further oxidation. The phenoxazinone
species were extracted three times with dichloromethane.
ꢁ
plates were again incubated at 37 C for 24 hr.
Antibacterial activity was evaluated by using the Himedia
zone reader.
1
ml of 10% solution of KI and three drops of a 3% solution
of ammonium molybdate were added to the aqueous layer.
−
The formation of I3 was monitored through spectropho-
tometer to examine the development of the characteristic
I3 band (λmax = 353 nm) which may be assignable to the
production of hydrogen peroxide.
2.8.3 | Determination of MIC and MBC for
Cu (II) complex against clinical Klebsiella
pneumoniae
−
The minimum inhibitory concentration (MIC) and mini-
mum bactericidal concentration (MBC) was determined
by selecting Klebsiella pneumonia. Among the three
pathogenic bacteria, Klebsiella pneumoniae is a very
well-known opportunistic pathogen that accounts for
ꢀ10% of nosocomial bacterial infections, including sepsis,
pneumonia, urinary tract infections, and hepatic abscess.
The organism can invade almost all part of the human
body, although the most frequently affected urinary and
respiratory tracts.
2
.7 | Electro-chemical analysis
The electroanalytical instrument, BASi Epsilon-EC
was employed for electrochemical experiments in
CH Cl solutions containing 0.2 M tetrabutylammonium
hexafluorophosphate as supporting electrolyte. The
BASi platinum working electrode, platinum auxiliary
electrode, Ag/AgCl reference electrode were used for
the measurements.
2
2
The method of micro-dilution was used to establish
the antibacterial potential of the copper (II)-Schiff base
2.8 | Antimicrobial susceptibility studies
complex and respective controls. A spectrophotometer
8
(
OD595 = 0.22) equivalent to 10 CFU/mL used to fix the
Antibacterial activity of the Cu (II) complex was tested
against few clinical pathogenic bacteria by well plate and
serial dilution method.
bacterial cultures to 0.22 optical density at 595 nm.
Different concentrations of Cu (II) complex and the
respective controls in 2.0 ml centrifuge tubes at 37 C for
ꢁ

MAHATO ET AL.
5 of 12
2
hr, were incubated with an inoculum of 10 μL of the
hydrated copper (II) chloride also produced same Cu (II)
complex. Single crystal form of the Cu (II) complex
was obtained by slow evaporation of reaction mixture at
room temperature. This Cu (II) complex is highly
soluble in polar solvents like methanol, acetonitrile,
chloroform etc.
above bacteria culture. Next, bacteria were diluted
serially and placed 10 μL of each dilution on nutrient
agar plates. Such plates were incubated overnight at
ꢁ
[39]
3
7 C, followed by a viable count of CFU bacteria.
Preparation of stock solutions for MIC was done
according to following equation 2.
Weight of the powder ðmgÞ
3.2 | Description of crystal structure
Volume of solution ðmLÞ × Concentration ðmg=LÞ
=
X-ray structural analysis indicates that the Cu (II)
complex crystallizes in a cubic crystal system with Ia-3d
space group. The thermal ellipsoidal plot of Cu (II)
complex is shown in Figure 1. The structural refinement
parameter for this Cu (II) complex is presented in
Table 1. The bond angles and bond distances are given in
Table 2. Two units of HL coupled with one Cu (II) ion
and lead to mononuclear Cu (II) complex of inner-
metallic chelate of first order. In accordance with the dis-
position of coordination centres around metal centre as
well as M-L bond angle values, it is confirmed that
the Cu (II) centre adopts an irregular six coordinate
geometry. The coordination geometry of copper (II)
ion is described as tetragonal bipyramidal geometry.
Close inspection on coordination geometry suggests that
Cu (II) centre forms a tetragonal plane based on O1,O1,
N1,O2 and bipyramidal geometry appears from apical
existence of two donor sites, N1 and O2.
The potency of powder ðmg=gÞ
ð2Þ
2
(
.8.4 | Antimicrobial activity of Cu
II) complex using scanning electronic
microscope
To examine the mode of action of Cu (II) complex on
bacterial species, bacterial cultures were obtained from
MIC and MBC samples and centrifuged. Thereafter, the
bacterial cells were collected and sputter-coated with a
thin layer of gold–palladium. The coated bacterial cells
were fixed on a glass coverslip and observed under a
[40]
scanning electron microscope.
3
3
| RESULTS AND DISCUSSION
.1 | Synthesis and formulation of the
Schiff base (HL) and copper (II) complex(1)
The tridentate Schiff base was prepared by refluxing
o-aminophenol with o-anisaldehyde in 1:1 mole ratio
in ethanol. The synthetic route is given in Scheme 1.
The Cu (II) complex was synthesized by adding
copper (II) acetate to HL in 2:3 mole ratio in methanol-
acetonitrile under slow stirring on magnetic stirrer.
Different ratio of Cu (II) acetate and HL was also used to
produce copper (II) complex of varied nuclearity, how-
ever same molecular composition,[Cu(L) ] was observed
FIGURE 1 X-ray structure of the copper (II)-Schiff base
2
in each cases. Replacement of copper (II) acetate by
complex with 30% ellipsoid probability
SCHEME 1 Synthetic route
for the formation of 1

6
of 12
MAHATO ET AL.
2
3
TABLE 1 Crystallographic data and structure refinement
parameters for 1
628.31 Å and surface area is determined as 491.0 Å . The
red highlighted area shows the dnorm area and close
non-covalent interactions of 1 with its surrounding
within the 3D crystal. Percentage share of each element
in close interaction with others is given in Table S1. In
Parameters
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
1
C
28 24 2 4
H N O Cu
516.03
296
…
this dnorm, the blue area is showing the weak C-H π
interactions between aromatic H attached to aromatic
C and aromatic rings of the ligands. Red area indicates
very weak intermolecular C-H O H-bonding between
Cubic
Ia-3d
…
C-H of benzene ring and O of OPh/OCH . This molecule
3
31.2952(17)
31.2952(17)
31.2952(17)
30,650(3)
48
…
is interacted by surrounding molecules through C-H O
b (Å)
…
hydrogen bond, and C-H π interactions as display in
c (Å)
Fingerprint plots (Figure S6). Quantitative information of
different intermolecular interactions by each pair of
elements is given in Table S1.
3
Volume (Å )
Z
ρ (gcm−
μ (mm−
3
)
)
1.571
1
0.890
3
.4 | Solution property of the Schiff base
F (000)
12,816
and copper (II) complex
R
int
0.179
ꢁ
θ ranges ( )
2.4–32.9
4,617
The electronic transitions for the Schiff base and its
Cu (II) complex were recorded in acetonitrile medium
(MeCN) from 200 to 900 nm at room temperature. The
Schiff base, HL displayed electronic transitions at
230, 270 nm and 346 nm and the Cu (II) complex
exhibited electronic bands at 238, 283 and 400 nm. The
electronic spectra are displayed in Figure S2. Electronic
bands at 230, 270 and 346 nm in the UV region for the
Schiff base may be attributed to π ! π* and n ! π*
Number of unique reflections
Total number of reflections
69,708
Final R indices
Largest peak and hole (eAꢁ−3)
0.0984, 0.3585
1.38, −0.34
TABLE 2 Bond angles and bond distances value of Cu (II)
complex
[
41]
electronic transitions.
The appearance of electronic
Bond distances
bands at 238 and 283 nm may be corresponded to π ! π*
[41]
and n ! π* electronic transitions of ligand origin
while the optical band at 400 nm may be assigned as
Cu1-O1
1.903(4)
1.981(4)
2.727(4)
Cu1-O2
2.727(4)
1.903(4)
Cu1-N1
Cu1-O1a
[
42b,c]
phenoxo to Cu (II) ion electronic transition.
Cu1-O2a
Cu1-N1a
1.981(4)
The electrolytic behaviour of the Cu (II) complex has
been checked through measurement of molar conductiv-
ity in MeCN medium at room temperature. The molar
Bond angles
O1-Cu1-O2
N1-Cu1-O1a
O1-Cu1-N1a
O1a-Cu1-O2
O2-Cu1-N1a
O2a-Cu1-N1
O1a-Cu1-O2a
O2a-Cu1-N1a
O11-Fe2-O3
O1-Cu1-N1
O1-Cu1-O2a
O2-Cu1-N1
O2-Cu1-O2a
O1a-Cu1-N1
N1-Cu1-N1a
O1a-Cu1-N1a
94.9(3)
128.81(15)
88.84(18)
149.01(17)
88.74(15)
82.15(14)
82.15(14)
128.81(15)
65.86(14)
93.62(17)
88.74(15)
65.86(14)
129.91(11)
149.01(17)
99.77(16)
93.62(17)
−
3
conductance value was recorded for 1.15 × 10
M
2
−1
solution of 1 as 05 Sm mol . The molar conductance
value suggests about the non-electrolytic nature of the
[
43]
Cu (II) complex in MeCN medium.
3
.5 | Phenoxazinone synthase mimicking
activity of the copper (II) complex (1)
The oxidation of 2-aminophenol (2-AP) was studied by
addition of catalytic amount of copper (II) complex
3
.3 | Hirshfeld analysis of copper
(1 × 10− M) to 2-AP (1 × 10 M) under aerobic atmo-
4
−3
(II)-Schiff base complex
ꢁ
sphere at 25 C (Scheme 2).
Crystal Explorer software was employed to demonstrate
the Hirshfeld surface of Cu (II) complex (Figure S5) over a
definite dnorm. The surface volume is calculated as
The changes of absorbance in the course of catalysis
were monitored through a UV–Vis spectrophotometer.
The wavelength scan was recorded for 1 hr with a time

MAHATO ET AL.
7 of 12
increasing absorbance was developed (Figure 2).
Switching of the electronic band from 400 nm to 407 nm
is a definite signature for the oxidation of 2-AP in
[
13,15,44]
MeCN.
This spectrophotometric evidence may
correspond to the development of aminophenoxazinone
species in solution.
The kinetics for the oxidative coupling of 2-AP
was also carried out to understand the catalytic efficacy
of the copper (II) catalyst. The method of initial rates
was followed to unveil the kinetic parameter of
this oxidative dimerization of 2-AP. The growth of
oxidation product was monitored at 407 nm as a function
SCHEME 2 Catalytic oxidation of substituted aminophenol
by phenoxazinone synthase
interval of 5 min (Figure 2). The substrate, 2-AP exhibits
a characteristic single electronic transition at 267 nm
in MeCN and indicates its high purity in solution.
Addition of the solution of Cu (II) complex to the
solution of 2-AP in MeCN, the optical band
corresponding to phenoxo to Cu (II) electronic transition
at 400 nm was initiated to exhibit a bathochromic shift.
As a result, a new electronic band at 407 nm with
[
13,15,37,38,44]
of time (Figure S7).
The rate constants
vs. substrate concentration displayed the nature of
kinetics (Figure S7). The first order saturation kinetics
reveals that Michaelis–Menten model seems to be
applicable in this case and may be presented according to
the following equation 3:
Vmax½Sꢂ
V =
ð3Þ
KM + ½Sꢂ
Where, V is rate of the reaction, Km is denoted as
Michaelis–Menten constant, Vmax is the maximum
velocity of the reaction, and [S] is concentration of
the substrate.
The
values
of
kinetics
parameters
were
determined from Michaelis–Menten equation as
−
1
−5
−4
Vmax (MS ) = 1.49 × 10 ; K = 9.46 × 10 [Std. Error
M
−1
−6
for Vmax (MS ) = 2.55 × 10 ; Std. Error for
K (M) = 1.72 × 10 ].
−
5
m
A comparison of catalytic oxidation of 2-AP by this
Cu (II) complex with some other reported Cu
[
26c,45–47]
(
II) complexes is tabulated in Table 3.
The cata-
lytic efficiency for the oxidative dimerization of 2-AP by
FIGURE 2 Development of new electronic band at 407 nm
upon addition of Cu (II) complex to of 2-AP in MeCN. (The spectra
are recorded after every 6 min). Inset: Time vs Absorbance plot at
defined wavelength
this Cu (II) complex was found high as k /
cat
5
K = 5.67 × 10 .
M
TABLE 3 Comparison of kcat (h− ) values for catalytic oxidation of 2-AP by reported copper (II) compounds and 1
1
cat (h− )(Solvent)
1
CCDC No
Ref
Complex
k
1
1
[45]
[
L Cu(μ-Cl)
2
CuL ]
1,065 (CH
13 (CH CN)
86.3 (CH OH)
340.26 (CH OH)
OH)
3
OH)
1,572,023
2
3
2
[46]
[
[
[
[
[
[
Cu
Cu
Cu
4
4
2
(L )
4
4
3
]
]
]
3
1,507,035
1,507,036
1,957,033
1,524,681
1,513,638
1,957,033
3
[46]
(L )
3
4
[26c]
[15c]
[47]
(L )
78.14 (CH
3
0
3
Cu(2,2 -bpy)Cl
2
]
2.08 × 10 (CH
3
OH)
OH)
CN)
0
+
3
Cu(2,2 -bpy)
2
(OAc)]
1.83 × 10 (CH
3
2
Cu(L)
2
]
5.364 × 10 (CH
3
This work
1
2
3
L
2
= 2-(a-Hydroxyethyl)benzimidazole (Hhebmz), L = (E)-4-Chloro-2-((thiazol-2-ylimino)methyl)phenol, L = (E)-4-Bromo-2-((thiazol-
4
-ylimino)methyl)phenol, L = (Z)-2-methoxy-6-(((2-methoxyphenyl)imino) methyl)phenol].

8
of 12
MAHATO ET AL.
2
+
Electrochemical analysis and EPR studies were
to −1.11 V due to co-ordination of 2- AP to the Cu centre
of the complex. Furthermore, two new peaks at −0.15 and
0.46 V appeared which were assigned to the development
vperformed to understand the catalytic behaviour of
Cu (II) complex in the oxidative dimerization of 2-AP in
acetonitrile medium. The cyclic voltammogram of the Cu
−
•−
•−
of AP /AP and AP /IQ redox couples. The redox poten-
tial data strongly suggests the course of 2-AP oxidation
undergoes through iminobenzoquinone radical formation.
To view more insights of this copper (II) complex mediated
oxidative coupling of 2-AP, EPR spectra of the Cu
(II) complex in presence and absence of 2-AP are recorded
in MeCN and presented in Figure 4.
The EPR spectrum of copper (II) complex (1) with
X-band frequency at room temperature in MeCN medium
displays four line hyperfine spectra at g = 2.12 due to
(
II) complex was recorded in dichloromethane medium
at 298 K where N-tetrabutylammonium hexafluoro-
phosphate was employed to record the electrochemical
data in aerobic environment and shown in Figure 3. The
Cu (II)-Schiff base complex displays one irreversible
cathodic wave at −0.94 V which is assignable to electron
2+
+
transfer in Cu to Cu redox couple in solution phase.
The active participation of copper (II) centre in catalytic
oxidation of 2-AP is confirmed by measurement of redox
potentials of Cu (II) complex in presence of 2-AP under
identical reaction conditions.
63
[13,15]
presence of Cu nuclei with I = 3/2.
and the calcu-
lated EPR spectrum for the Cu (II)-Schiff base resembles
very well as evident from the simulated g value, 2.1262
(Figure 4). The mixture of copper complex in presence of
2-AP is EPR silent due to antiferromagnetic coupling
The mixture of copper (II) complex in presence of 2-AP
produces irreversible cathodic peak at −1.11 V due to
2+
+
Cu to Cu redox couple i.e. the peak shifted from −0.94
FIGURE 3 Left: Cyclic voltammogram of the Cu (II)-Schiff base complex in anhydrous DCM medium; Right: Cyclic voltammogram of
Cu (II) complex in presence of 2-AP under molecular oxygen atmosphere in anhydrous DCM in CH2Cl2 (0.20 M [N(n − Bu)4]PF6) at 295 K
FIGURE 4 X- band EPR spectra of the copper (II)-Schiff base in presence of 2-AP after 20 min in CH3CN solution at 298 K

MAHATO ET AL.
9 of 12
2
+
between Cu (S = 1/2) with iminobenzosemiquinonate
anion radical. The EPR spectral analysis of the copper
complex in presence of 2-AP in acetonitrile at room tem-
perature strongly suggests the generation of radical spe-
cies for the appearance of additional signal at g ca 2.057
mechanism via radical generation in active participation
with the metal centres favouring 6e oxidative coupling
[
49]
of substrate. T.P. Begley and co-workers
suggests
the production of 2-aminophenoxazinone through a
sequence of three consecutive 2e oxidation of 2-AP. The
tautomerization reactions were the controlling unit in
regeneration of the 2-Ap during this course of catalytic
oxidation reaction. Based upon the outcomes observed by
different analytical methods like electrochemical, EPR
and ESI-MS, a plausible mechanism is proposed in
Scheme 3. So, oxidative dimerization of 2-AP undergoes
through formation of catalyst-substrate intermediate in
the primary stage. Subsequently, Cu centre activates
molecular oxygen to produce water as a byproduct
and iminobenzosemiquinonate radical in the course of
catalytic oxidation. Iminobenzosemiquinonate radical
proceeds to couple with another unit of 2-AP and develop
aminophenoxazinone species as a final product.
(
(
Figure 4). The reported g value for oxidised 2-AP radical
iminobenzoquinone) is 2.0051 in 10 M Bu NPF
−
1
[15c,26]
6.
4
Furthermore, electrospray ionization (ESI) mass
spectrum of the copper (II)-Schiff base complex in
presence of 2-AP is recorded after mixing of 10 min
to reveal the binding aspects of the copper (II)
complex and 2-AP. It was observed that the ESI-Ms
of the reaction mixture (Figure S8) exhibits the base
peak at m/z 213.12 which is assignable to the
presence of aminophenoxazinone species, [(2-amino-3H-
phenoxazine-3-ones) + H ]. Appearance of another
important peak at m/z 624.81 corresponds to the forma-
tion of adduct between copper (II)-Schiff base complex
+
+
and 2-AP, [[1 + (2-AP)] + H ]. The experimental m/z
values correlate well with the theoretical m/z values. In
this context, involvement of molecular oxygen in the
course of oxidation of 2-AP was tested through produc-
tion of hydrogen peroxide in solution using a reported
3.6 | Antibacterial activity of the cu (II)-
Schiff base complex
[37,38]
method.
No spectral band at λmax 353 nm
The antibacterial activity of Cu (II) complex was assessed
through the method of well diffusion against the bacteria
Staphylococcus aureus, Enterococcus and Klebsiella
pneumoniae. The results of the inhibition zone diameters
shown in Table S2. The therapeutic efficiency of this
Cu (II) complex was determined by calculating MIC and
MBC on Klebsiella pneumoniae (Table S3). MIC and MBC
values were determined as 1.250 mg/ml, 0.625 mg/ml for
corresponds to generation of hydrogenperoxide was
observed throughout the course of catalysis and ensure
the presence of water as a byproduct appeared from
molecular oxygen.
[
48]
Previously study by P. Chaudhury and co-workers
presented tetracopper complex as a bio-mimetic model
towards 2-AP oxidation and recommends an “on–off”
SCHEME 3 Plausible
mechanism for the catalytic
oxidation of 2-AP

10 of 12
MAHATO ET AL.
FIGURE 5 Electron microscope scans showing morphological changes in pathogenic bacteria. Destruction of the bacterial cell
membrane (MIC) and B growth of bacteria (MBC)
this Cu (II)-Schiff base complex. Scientific literatures
suggest that the Cu (II) complex is quite competent to
inhibit the growth of pathogenic bacterial species.
Aiming to explore the origin of antibacterial property for
this copper (II)-Schiff base complex, a study involving the
exposure of Cu (II) complex on Klebsiella pneumoniae
was carried out employing scanning electron microscope
and exhibits good catalytic activity with turnover num-
−
1
ber, 536.4 h . In view of mechanistic insights, electro-
chemical analysis of the Cu (II) complex in presence
of 2-AP was carried out and indicates the generation of
−
•−
•−
AP /AP
and AP /IQ redox couple in the course
of catalysis. EPR spectral analysis of the reaction mix-
ture confirms the existence iminobenzosemiquinone
radical at g = 2.057 which suggests the radical driven
catalytic oxidation of 2-AP. Furthermore, ESI-MS analy-
sis of the Cu (II) complex in presence of 2-AP ensures
that the catalytic oxidation of 2-AP proceeds through
the formation of enzyme-substrate adduct. Antibacterial
property of the copper (II) complex has been examined
against different pathogenic bacteria. Scanning electron
microscope images reveal that destruction of bacterial
cell membrane remains the driving force for the devel-
opment of potential antibacterial properties of the cop-
per (II)-Schiff base complex. Importantly, this Schiff
base ligand can able to isolate Cu (II) ion in an interme-
diate coordination geometry (tetragonal bipyramidal
geometry) and represents a rare example of unprece-
dented coordination geometry which is controlled by ste-
ric factors of the Schiff base ligand.
(SEM) analysis. SEM image of the Klebsiella pneumoniae
in presence of Cu (II) complex accounts on the change
of morphology on the cell wall. The recorded SEM
micrographs of Klebsiella pneumonia cells are shown in
Figure 5.
Commonly, MBC of Klebsiella pneumonia was
observed as typical rod-shape with smooth and intact
cell walls. Although, the number of Klebsiella pneumonia
was remarkably decreased, and cell walls became
wrinkled and damaged after addition of Cu (II) complex
[50,51]
to Klebsiella pneumonia.
In reality, in limited
cases, such type of observations were noted and will
definitely bring some hope in developing suitable
therapeutic agents.
4
| CONCLUSIONS
ACKNOWLEDGEMENTS
This research study provides an overview of synthesis,
X-ray structural characterization, Hirshfeld surface
analysis and bio-mimetic oxidation of 2-AP as well
BB sincerely acknowledges Science and Engineering
Research Board (SERB), India for financial support
under TEACHERS' ASSOCIATESHIP for RESEARCH
EXCELLENCE (TAR/2018/000473). MJ thanks DST
INSPIRE Programme for research fellowship. ARC
thanks Department of Chemical Sciences, IISER Mohali
for X-ray facility.
as antibacterial activity of
a
newly synthesized
copper (II)-Schiff base complex, [Cu(L) ] (1). X-ray
2
structure of the Cu (II) complex shows that the Cu
(
II) centre adopts an unprecedented tetragonal bipyra-
midal geometry in its crystalline phase. The copper
II) complex has been evaluated as a bio-inspired cata-
lyst towards oxidative coupling of 2-AP in acetonitrile
(
ORCID
Bhaskar Biswas https://orcid.org/0000-0002-5447-9729

MAHATO ET AL.
11 of 12
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
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How to cite this article: Mahato S, Meheta N,
Kotakonda M, et al. Ligand directed synthesis of a
unprecedented tetragonalbipyramidal copper (II)
complex and its antibacterial activity and catalytic
role in oxidative dimerisation of 2-aminophenol.
Appl Organomet Chem. 2020;e5935. https://doi.org/
10.1002/aoc.5935
[
[
[
[
2
017, 46, 1249.