Copper(II) complexes with a benzimidazole functionalized Schiff base: Synthesis, crystal structures, and role of ancillary ions in phenoxazinone synthase activity

Article abstract of DOI:10.1002/aoc.6211

This research study reports the synthesis, structural characterization and phenoxazinone synthase-like activity of two structurally similar copper(II) complexes developed with a benzimidazole functionalized Schiff base (L). The ligand, L, was designed and

Full text of DOI:10.1002/aoc.6211

Received: 24 December 2020
DOI: 10.1002/aoc.6211
Revised: 20 January 2021
Accepted: 6 February 2021
F U L L P A P E R
Copper(II) complexes with a benzimidazole functionalized
Schiff base: Synthesis, crystal structures, and role of
ancillary ions in phenoxazinone synthase activity
Prafullya Kumar Mudi1
Madhusudan Shit3
Bhaskar Biswas1
| Rajani Kanta Mahato1
| Angshuman Roy Choudhury2
| Mayank Joshi2
| Hari Sankar Das1
|
|
1Department of Chemistry, University of
North Bengal, Darjeeling, India
2Department of Chemical Sciences, Indian
Institute of Science Education and
Research, Mohali, Punjab, India
This research study reports the synthesis, structural characterization and
phenoxazinone synthase-like activity of two structurally similar copper(II)
complexes developed with a benzimidazole functionalized Schiff base (L). The
ligand, L, was designed and synthesized in high yield by the reaction of
p-methoxy benzaldehyde with o-phenylenediamine. The reaction of L with
CuCl2 and Cu(NO3)2 leads to the formation of two isostructural complexes,
[Cu(L)2Cl2]2 (1) and [Cu(L)2(NO3)2]2 (2). Single crystal X-ray structural study
reveals that both the Cu(II) centre in 1 and 2 adopts a square planar geometry.
An attempt has also been made to understand the role of coordinated co-
ligands on the catalytic oxidation of 2-aminophenol (2-AP) to 2-amino-3H-
phenoxazine-3-one (2-APX) in methanol. The presence of coordinated nitrate
to Cu(II) ions imparts a more labile character to complex 2, and the catalytic
efficiency (kcat/KM) for complex 2 (1.50 × 107) was determined almost double
compared with that of complex 1 (8.78 × 106). Electro-chemical and
electrospray ionization mass spectrometry studies of 1 and 2 with 2-AP sug-
gests that the square planar geometries of the Cu(II) centres remain the driving
force to develop enzyme-substrate adducts and excellent catalytic performance
of the complexes. Electrochemical and EPR spectral analysis of the reaction
mixture confirm the presence of active 2-AP−/2-AP•− redox species in the
course of catalytic oxidation and suggest the radical driven oxidative coupling
of 2-AP in an aerobic environment. Temperature-dependent kinetic measure-
ments were carried out to evaluate the activation parameters (Ea, ΔH‡, ΔS‡),
which favours the higher rate of catalytic oxidation of 2-AP for complex 2 than
complex 1.
3Department of Chemistry, Dinobandhu
Andrews College, Kolkata, India
Correspondence
Bhaskar Biswas, Department of
Chemistry, University of North Bengal,
Darjeeling 734013, India.
Email: bhaskarbiswas@nbu.ac.in;
icbbiswas@gmail.com
Funding information
Science and Engineering Research Board,
Grant/Award Number: TAR/2018/000473
K E Y W O R D S
copper(II), crystal structure, electrochemical analysis, phenoxazinone synthase activity,
Schiff base
Prafullya Kumar Mudi and Rajani Kanta Mahato have equal contributions in this work.
Appl Organomet Chem. 2021;35:e6211.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6211

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MUDI ET AL.
1 | INTRODUCTION
(L) ligand under ambient conditions. To elucidate the
use of such type of ligand in coordination and applied
chemistry research field, we have prepared two
structurally similar mononuclear copper(II) complexes
with L, namely, [Cu(L)2Cl2]2 (1) and [Cu(L)2(NO3)2]2
(2) and structurally characterized by X-ray diffraction
technique with other spectroscopic methods. Attempts
have also been made to illustrate the presence of
Nowadays, copper(II)-based coordination compounds
represent a promising class of smart molecules with
significant contributions in designing functional
materials like catalytic, magnetic, optic and conducting
materials.[1–8] In the biological world, copper(II) ion
remains one of the most prevalent metal ions, and
its compounds play significant roles in different
metal-dependent proteins such as the active site in
haemocyanin[9] as an oxygen carrier and perform
aromatic ring oxidations in tyrosinase,[10] catechol
ancillary
ligands
in
the
primary
zone
of
coordination of Cu(II) centre for the oxidative catalysis
of 2-aminophenol in methanol medium. Both the com-
plexes were found to show impressive phenoxazinone
synthase mimicking activity towards oxidative coupling
of 2-aminophenol.
oxidase,[11]
oxygenase
enzyme
in
quercetin
2,3-dioxygenase,[12–14] hydrogen peroxide producer in
galactose and glyoxal oxidases[15–18] and methane
oxidation as methane monooxygenase, pMMO.[19–21]
Among the different copper-based metalloproteins and
metalloenzymes in the living system, phenoxazinone
synthase acts as an important metalloenzyme.[22]
Phenoxazinone synthase catalyses the oxidation of
2-aminophenols to 2-aminophenoxazinone species
actinomycin D, a naturally occurring antineoplastic
agent, which inhibits DNA-directed RNA synthesis and is
also used clinically for the treatment of certain types of
cancer.[23–25]
2 | EXPERIMENTAL
2.1 | Preparation of the Schiff base and
dinuclear copper(II) complex
2.1.1 | Chemicals, solvents and starting
materials
Highly pure o-phenylenediamine (Sigma Aldrich, USA),
p-anisaldehyde (Sigma Aldrich, USA), copper(II)
chloride dihydrate (Merck, India), copper(II) nitrate
trihydrate (Merck, India) and o-aminophenol (Sigma
Aldrich, USA) are purchased from the respective out-
lets. All the materials used in this investigation were of
high purity.
On the other hand, benzimidazole and its
derivatives are considered as a significant class of
N-containing heterocyclic compounds because of their
broad range of biological activities such as antimicro-
bial,
antivirus,
antifungals,
anti-inflammatory,
anticancer, antiulcer, proton pump inhibitors and
anticoagulants activities.[26–37] They are also the key
intermediate in many organic reactions and are used
for the industrial synthesis of numerous products.[38]
Therefore, various scientific groups have been
engrossed in the synthesis and metallation of benz-
imidazoles with novel functionality to develop smart
molecules with improved biomimetic and pharmacolog-
ical activities that remain an urgent priority. Until to
date, several methods are employed for the synthesis
of different substituted benzimidazoles using either a
catalyst or high temperature, which makes the process
economically and environmentally less attractive.[39–42]
Because copper(II) and benzimidazole play a signifi-
cant role in many disciplines, it is important to study
the copper–benzimidazole interaction, and chemists
have made use of this interaction to build up some
fascinating classes of molecules that show many
interesting properties.[43–45] As a part of our interest
in developing copper-based coordination compounds
for mimicking the bio-inspired oxidation reactions,
2.1.2 | Synthesis of the Schiff base, L
The Schiff base ligand, L, was prepared following the
reported procedure with slight modification.[46–48]
The Schiff base was prepared by refluxing
o-phenylenediamine (0.108 g,
1
mmol) with
p-anisaldehyde (0.272 g, 2 mmol) in 25-ml ethanol for
6 h. The brown coloured compound was extracted from
the solution and stored in vaccuo over CaCl2. Yield:
0.219 g (ꢀ85.2%). Anal. Calc. for C22H20N2O2 (L): C,
76.72; H, 5.85; N, 8.13; O, 9.29; Found: C, 76.70; H,
5.80; N, 8.11; O, 9.28. IR (KBr, cm−1; Figure S1): 3433
(νOH), 3058 (νCH2), 1612 (νC N); UV-vis (λmax, nm;
1
Figure S2): 258, 289, 335; H NMR (δ ppm, 400 MHz,
CDCl3; Figure S3) δ = 6.844–7.847 (Ar-H, 12H), 5.386
(s, 2H), 3.786–3.852 (s, 6H), ppm.13C NMR (400 MHz,
CDCl3; Figure S4): 160.887 (HC N); 159.113, 154.129,
148.89 (Ar-OCH3); 136.108 (Ar-N C); 130.706, 128.512,
127.212, 122.708, 122.497, 119.735, 114.420, 114.176,
110.394 (Ar-C).
we
report
a
straightforward
synthesis
of
1-(4-methoxybenzyl)-2-(4-methoxyphenyl)benzimidazole

MUDI ET AL.
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2.1.3 | Synthesis of copper(II) complexes
copper(II) complexes with graphite monochromated Mo-
Kα radiation (λ = 0.71073 Å) at 100 K for 1 and at 150 K
for 2 using ω scans. The data were reduced and the space
group was determined using CrysAlisPro 1.171.39.35c
and 1.171.39.7f.[49] The crystal structures were solved
by dual space method, and the space group was
redetermined using SHELXT-2015.[50] The crystallo-
graphic data were refined by full-matrix least-squares
procedures using the SHELXL-2015[51] software package
through OLEX2 suite.[52]
A methanolic solution of CuCl2ꢁ2H2O (0.170 g, 1 mmol)/
Cu(NO3)2ꢁ3H2O (0.241 g, 1 mmol) was mixed dropwise to
dichloromethane solution of L (0.688 g, 2 mmol). The
yellow coloured solution of the Schiff base immediately
turned green for both the reactions, and the resulting
solutions were further stirred for 45 min followed by
filtering. The filtrate was kept for slow evaporation in an
open atmosphere. After 5–7 days, microcrystalline green
coloured crystals for 1 and 2 were separated, which were
dried over silica gel.
Yield of 1: 0.1310 g (ꢀ77.0% metal salt based) Anal.
calc. for C44H40N4O4CuCl2: C, 64.47; H, 5.17; Cl, 8.46;
Cu, 7.58; N, 6.68; O, 7.63; Found: C, 64.45; H, 5.16; Cl,
2.4 | Hirshfeld surface analysis of the
copper(II) complexes
8.42; Cu, 7.54; N, 6.63; O, 7.60. IR (KBr pellet, cm−1
;
Hirshfeld surfaces[53] and 2D fingerprint plots[54] for the
copper(II) complexes have been generated by Crystal
Explorer 17.5[55] program package employing their X-ray
diffraction data. The location of intermolecular interac-
tions within crystal packing was examined through
Hirshfeld Surface analysis. The details of Hirshfield
surface analysis were described elsewhere.[53–55]
Figure S5): 3021, 2939(νCH2), 1620 (νC N); UV-vis
(1 × 10−4 M, λmax (abs), nm, MeOH; Figure S6): 286, 605.
Yield of 2: 0.1850 g (ꢀ76.80% metal salt based) Anal.
calc. for C44H40N6O10Cu: C, 60.30; H, 4.60; Cu, 7.25; N,
9.59; O, 18.26; Found: C, 60.28; H, 4.59; Cu, 7.21; N,
9.55; O, 18.24. IR (KBr pellet, cm−1; Figure S5): 3019,
2938(νCH2), 1617(νC N); UV-vis (1 × 10−4 M, λmax (abs),
nm, MeOH; Figure S6): 282, 611.
2.5 | Catalytic oxidation studies of
2-aminophenol
2.2 | Physical measurements
The catalytic oxidation of 2-aminophenol was performed
by treatment of 1 × 10−4 M solution of copper(II) com-
plexes with 1 × 10−3 M of 2-aminophenol (2-AP) solution
in methanol (MeOH). The wavelength scans for the
course of catalysis were monitored with a spectrophotom-
eter for 2 h from the 300- to 700-nm wavelength
range.[24]
FT-IR spectra of the Schiff base and copper compounds
were recorded using FTIR-8400S SHIMADZU spectrome-
ter in the range of 400–3600 cm−1. 1H and 13C NMR spec-
trum of Schiff base were obtained on a Bruker Advance
400-MHz spectrometer in CDCl3 at 298 K. Steady-state
absorption and other spectral data were recorded with a
JASCO V-730 spectrophotometer. ESI-MS spectral
measurements were performed with a Q-TOF-micro
quadruple mass spectrometer. A Perkin Elmer 2400 CHN
microanalyser was employed to carry out the elemental
analyses of the compounds. The pH values of different
solutions were measured by Labman pH meter at room
temperature. X-band EPR spectral measurements for the
copper complexes were done with a Magnettech GmbH
MiniScope MS400 spectrometer where the microwave
frequency was measured with an FC400 frequency
counter.
Kinetic experiments were also carried out spectropho-
tometrically to comprehend the catalytic efficacy and
nature of oxidation of aminophenol by the copper(II)
complexes in MeOH at 298 K[56]; 0.04 mL of 1 × 10−4
M
constant concentration of copper(II) complexes the solu-
tions were mixed with 2 ml of 10-fold concentrated solu-
tion of 2-AP to attain the final concentration as
1 × 10−4 M. The catalytic conversion of 2-aminophenol to
its oxidation product was examined with the variation of
time at 433 nm for 1 and 430 nm for 2 (time scan) in
MeOH.[57,58] The rate of catalytic oxidation based on sub-
strate concentration was determined by following the ini-
tial rate law in triplicate. The kinetics of the catalytic
oxidation reactions were carried out at different tempera-
ture to determine the activation parameters.
2.3 | X-ray structural studies and
refinement
The product, 2-amino-3H-phenoxazine-3-one (APX)
developed by catalytic oxidation of 2-AP has been
extracted with column chromatographic method for both
the copper(II) complexes. Neutral alumina as column
A Rigaku XtaLABmini diffractometer equipped with
Mercury 375R (2 × 2 bin mode) CCD detector was
employed to collect X-ray diffraction data for the

4 of 12
MUDI ET AL.
support and benzene-ethyl acetate an eluant mixture was
employed in the chromatographic separation. Proton
NMR spectroscopy was evidenced to check the purity of
aminophenoxazinone compound (APX). 1H NMR data
for APX, (CDCl3, 400 MHz,) δH: 7.61 (m, 1H), 7.45
(m, 3H), 6.46 (s, 1H), 6.37 (s, 1H), 6.25 (s, 1H).
3 | RESULTS AND DISCUSSION
3.1 | Synthesis and structural
formulation of the Schiff base (L) and
copper(II) complexes
The benzimidazole functionalized ligand, L, was
synthesized by refluxing o-phenylenediamine with
p-anisaldehyde in ethanol. The copper(II) complexes
were obtained by adding hydrated copper(II) chloride
and nitrate salts to the ligand, L, in 1:2 mole ratio in
MeOH-DCM under slow stirring conditions (Scheme 1).
The different stoichiometric ratios between hydrated
copper(II) salts and L were applied to divulge the mode
of coordination of the benzimidazole functionalized
ligand with Cu(II) ion.
However, we were unsuccessful to prepare the copper
complexes with different structural compositions. Other
copper(II) salts like perchlorate, bromide, acetate and sul-
phate were also reacted with L to develop the copper(II)
complexes of varied dimensions and nuclearities but did
not able to do so. Complexes 1 and 2 displayed good solu-
bility in polar solvents like methanol, acetonitrile and
dichloromethane. The compounds may be successfully
synthesized in methanol–dichloromethane or methanol–
acetonitrile medium.
2.6 | Detection of hydrogen peroxide in
the course of catalysis
The participation of atmospheric oxygen in the oxidative
coupling of 2-AP was verified from the detection of
hydrogen peroxide with reported literature.[59,60] In the
course of oxidation of 2-AP in MeOH, H2SO4 was added
to attain pH 2. After a certain time, an equal volume of
water was mixed to stop further oxidation. The oxidation
product, phenoxazinone compound was extracted three
times with DCM; 10% solution of KI (1 ml) and three
drops of a 3% solution of ammonium molybdate were
−
mixed with the aqueous layer. The development of I3
species at λmax = 353 nm was monitored through a
spectrophotometer, which may be assignable to the
production of hydrogen peroxide.
2.7 | Electrochemical analysis
The BASi Epsilon-EC instrument containing 0.2-M
tetrabutylammonium hexafluorophosphate as supporting
electrolyte was employed for carrying out electrochemical
experiments in the DCM medium. The BASi platinum
working electrode, platinum auxiliary electrode and
Ag/AgCl reference electrode were used for all the
measurements.
3.2 | Description of crystal structures
and Hirshfeld surface analysis
The X-ray structural analysis indicates that both com-
plexes 1 and 2 crystallize in the triclinic space group P-1
and are isomorphous and isostructural, differing for the
coordinating chlorine and nitrate anions in 1 and 2,
SCHEME 1 Synthesis of 1,2-disubstituted
benzimidazole (L) and complexes 1 and 2

MUDI ET AL.
5 of 12
respectively. Crystallographic refinement details are
shown in Table 1. Selected bond lengths and bond angles
for complexes 1 and 2 are tabulated in Tables S1 and S2,
respectively. The ORTEP representations of complexes
1 and 2 are shown in Figures 1 and 2, respectively. The
asymmetric unit of both compounds 1 and 2 comprises
two half complexes located on a centre of symmetry hav-
ing a slight different conformation. The metals in all
cases exhibit an almost regular square planar geometry,
as evidenced by the coordination bond angles close to 90ꢂ
(Tables S1 and S2). The copper(II) ion in 1 is coordinated
by two ligand units through the benzimidazole N atom
and by two chloride ions (Figure 1), whereas in 2, the
metal is similarly coordinated by two benzimidazole N
atoms from organic ligands and by two monodentate
nitrate anions (Figure 2).
The Cu–N bond lengths in all the copper(II)
complexes are close comparable falling in the range 1.956
(2)–1.963(2) Å, whereas the Cu–Cl bond lengths in
1 average to 2.2837(8) Å, and Cu–O in 2 are similar, of
−
2.003(2) Å. The NO3 ligands in the two complexes
of
2 have approximate coplanar atoms with the
uncoordinated O5/O5’ and O7/O7’ atoms located above
and below the coordination plane. This is also reflected
by the Cu-O3-N-O5 and Cu-O7-N-O9 torsion angles close
to 20ꢂ (Table S2). The monodentate coordination mode of
−
the NO3 ligands in 2 is confirmed by the criteria listed
in Table S3.[61] All the values, Δd, Δd0, Δθ and θ3 of
0.609 Å, 0.037 Å, 28.19ꢂ and 119.30ꢂ, respectively, are in
the range of monodentate coordination mode (Table 2).
TABLE 1 Crystallographic data and structure refinement
parameters for the copper complexes
−
Bond length analysis within the NO3 ligands show a
Parameters
Crystal data
Empirical formula
Formula weight
T (K)
1
2
relatively longer N–O bond length for the coordinated
oxygen, of 1.265(3) and 1.273(3) Å, to be compared with
the other N–O bond lengths in the range 1.237(3)–1.249
(3) Å, due to the coordination. The benzimidazole and
the para-methoxyphenyl mean planes are not coplanar
as indicated by the torsion angles N1–C8–C5–C4/
N4-C29-C30 in 1 and N1–C7–C8–C9/N3-C30-C27-C28 in
2 (φ) that increases from 36.7ꢂ in free L[66] to 46.30 and
51.34ꢂ in 1, 48.57 and 51.34ꢂ in 2, likely due to packing
requirements. The structures of 1 and 2 are very close as
evident from the identical space group and crystal
system, and the unit cell dimensions exhibit a very small
difference in cell axes (Table 1).
The nature of Hirshfeld surfaces of complexes 1 and
2 was analysed using dnorm calculation through Crystal
Explorer software (Figures S7 and S8). The dnorm area
and the area consisting of supramolecular interactions of
the complexes 1 and 2 with their neighbouring units are
displayed in red colour. The 2D fingerprint plots and ele-
mental participation in %share of close interaction with
others are displayed in Figures S9 and S10, respectively.
In the dnorm, blue areas show the engagement of π…π
interactions between phenyl centroid of the ligand,
whereas the red area highlights intermolecular
C–H…O/Cl/N interactions and displays in fingerprint
plots (Figures S9 and S10). It is also observed that the
self-assembled supramolecular architectures for complex
1 were principally dominated by C–H…O/Cl interactions
(Figure S11), whereas the self-assembled 3D network for
complex 2 was constructed by C–H…O interactions
(Figure S12). The H…O/Cl interactions are found of mod-
erate strength (2.42–2.56 Å; Table S4) in 1; however,
extensive but moderate to weak ranged H…O interactions
are noted in the construction of the supramolecular
architecture of complex 2 (2.42–2.56 Å; Table S5).
2046275
2046276
C44H40N4O4Cl2Cu C44H40N6O10Cu
823.24
100
876.36
150
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
0.71073
Triclinic
P-1
0.71073
Triclinic
P-1
9.1963(3)
10.0533(6)
21.9806(7)
100.666(4)
90.164(3)
102.356(4)
1948.90(15)
2
9.0794(4)
10.2760(5)
22.4587(11)
79.878(4)
89.780(4)
77.426(4)
2012.02(17)
2
b (Å)
c (Å)
α (ꢂ)
β (ꢂ)
γ (ꢂ)
V (Å3)
Z
ρ (gm cm−3
)
1.403
1.447
Absorption
coefficient (mm−1
0.747
0.612
)
F(000)
Crystal size (mm3)
854
910
0.1 × 0.2 × 0.21
2.5 to 32.8ꢂ.
0.18 × 0.27 × 0.32
3.1 to 27.6ꢂ.
Theta range for data
collection
Goodness-of-fit on F2 1.057
1.074
25,004
9226
Reflections collected
34,507
12,780
Independent
reflections
Final R indexes
[I ≥ 2σ (I)]
R1 = 0.0650,
wR2 = 0.1491
R1 = 0.0592,
wR2 = 0.1767
Largest diff. peak/
hole/e Å−3
0.90/−0.70
2.53/−0.50

6 of 12
MUDI ET AL.
FIGURE 1 ORTEP view (thermal ellipsoid
probability at 30%) of the two independent
complexes of 1 located on a center of symmetry
FIGURE 2 ORTEP view (thermal ellipsoid
probability at 30%) of the two independent
complexes of 2 located on a center of symmetry
TABLE 2 Comparison of kcat (h−1) values for catalytic oxidation of 2-AP by reported copper(II) compounds and copper complexes
1 and 2
Complex
kcat (h−1) (solvent)
1.06 × 104 (CH3OH)
86.3 (CH3OH)
CCDC no.
1572023
1507035
1507036
1524680
1940162
1957033
2024056
1981345
2046275
2046276
Ref.
[L1Cu(μ-Cl)2CuL1]
[Cu4(L2)4]
[Cu4(L3)4]
Ghosh et al.[62]
Sagar et al.[63]
Sagar et al.[63]
Garai et al.[56]
Dutta et al.[64]
Mudi et al.[47]
Mahato et al.[5]
Mahato and Biswas[65]
This work
340.26 (CH3OH)
1.69 × 104(CH3OH)
11.1 (CH3OH)
[Cu(μ-Cl)(Phen)Cl]
[(CH3CN)Cu(Ls)2Cu]2+
[Cu2(Lb1)3]ClO4
[Cu(Lb2)2]
[Cu(Lb3)](H2O)
[Cu(L)2Cl2]2 (1)
[Cu(L)2(NO3)2]2 (2)
78.14 (CH3CN)
536.4 (CH3CN)
5.89 × 102
(CH3OH) 2.31 × 104
(CH3OH) 6.3 × 104 (CH3OH)
This work
Note: L1 = 2-(α-hydroxyethyl)benzimidazole (Hhebmz), L2 = (E)-4-chloro-2-((thiazol-2-ylimino)methyl)phenol, L3 = (E)-4-bromo-2-((thiazol-2-ylimino)methyl)
phenol, Lb1 = (Z)-2-methoxy-6-(((2-methoxyphenyl)imino)methyl)phenol, Lb2 = 2-(2-methoxybenzylideneamino) phenol.

MUDI ET AL.
7 of 12
3.3 | Catalytic oxidation studies of
2-aminophenol and mechanistic inferences
The catalytic oxidation of 2-aminophenol (2-AP) was
studied with the copper(II) complexes. In the course of
oxidation reaction, 2-aminophenol (2-AP) was considered
as a standard substrate in methanol (Scheme 2). The
bio-mimics of catalytic oxidation of 2-AP were monitored
with a spectrophotometer at an interval of 12 min for 2 h.
It is well documented that 2-AP exhibits a single band
at 267 nm. Upon addition of copper(II) complexes
(1 × 10−4 M solution) to 2-AP (1 × 10−2 M solution), the
appearance of new electronic bands at 433 and 430 nm
with increasing intensity was observed for both 1 and 2,
respectively (Figures 3 and 4).
FIGURE 4 Development of a new electronic band at 430 nm
after treatment of complex 2 to 2-AP in MeOH with a time interval
The development of the electronic bands at 433 and
430 nm for 1 and 2 is a signature for the production of
phenoxazinone compound in solution, which is well
established in the scientific literature.[67,68] Controlled
experiments were also performed using 2-AP and 2-AP
in the presence of benzimidazole functionalized
ligand L separately under identical reaction conditions
(Figure S13) up to 2 h; however, yields of control
reactions were very little and can be ignored. The
of 8 min. Inset: time vs. absorbance plot at 430 nm
production of oxidation product in control reactions
reveal that auto-oxidation of 2-AP under ambient
reaction condition.
The phenoxazinone species was extracted by column
chromatographic method. Neutral alumina as column
support and benzene-ethyl acetate as an eluant mixture
were employed for this chromatographic separation. The
oxidation product was isolated in high yield (ꢀ83% and
ꢀ76% for 1 and 2). The product was principally identified
by 1H NMR spectroscopy.[47,56,57
]
1H NMR data for 2-amino-3H-phenoxazine-3-one
(APX), (CDCl3, 400 MHz), δH: 7.61 (m, 1H), 7.46 (m, 3H),
6.48 (s, 1H), 6.39 (s, 1H), 6.27 (s, 1H).
Kinetic studies of the catalytic oxidation of 2-AP
were carried out to understand the catalytic efficacy for
the copper(II) complexes. The method of initial rates
was followed to unveil the nature of kinetic for this
catalytic oxidation of 2-AP. The kinetics of oxidative
coupling of 2-AP was monitored for the growth of
oxidized products at 433 and 430 nm as a function of
time (Figures S14 and S15).[65,69–71] The rate constants
versus concentration of the substrate plot seem to be
saturation kinetics. The values of the kinetics
parameters for the oxidation of 2-AP by 1 and 2 were
determined considering the Michaelis–Menten approach
of enzymatic kinetics and summarized in Table 2. An
attempt was also made to examine the reactivity of the
synthesized copper complexes with a comparison of
the kinetic parameter for other reported copper
complexes.[5,47,56,59,62–64] The kinetics parameter are
furnished in Table 2.
SCHEME 2 Oxidative coupling of 2-aminophenol by
phenoxazinone synthase
Michaelis–Menten equation is presented as follows:
FIGURE 3 Generation of a new electronic band at 433 nm
after addition complex 1 to 2-AP in MeOH with a time interval of
8 min. Inset: time vs. absorbance plot at 433 nm
Vmax½Sꢃ
V =
,
KM + ½Sꢃ

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MUDI ET AL.
where V indicates the reaction rate, KM is considered as
the Michaelis–Menten constant, Vmax presents maximum
reaction velocity, and [S] is the substrate concentration.
The values of kinetics parameters were determined
from Michaelis–Menten approach of enzymatic kinetics
for 1 as Vmax (MS−1) = 6.44 × 10−4; KM = 2.63 × 10−3
[std. error for Vmax (MS−1) = 7.65 × 10−5; std. error for
measurement at different temperature, the concentration
for both the copper complexes was kept constant at
1 × 10−3 M, whereas the concentration of 2-AP was fixed
at 1 × 10−2 M. It is observed that the rate of oxidation of
2-AP increases with an increase in temperature for both
the copper(II) complex catalysed reactions. The activa-
tion energy (Ea), the heat of enthalpy (ΔH‡), and entropy
change (ΔS‡) (Table S6) were determined from the
Arrhenius (Figure S16) and Eyring (Figure S17) plots.
The activation energy for the complex 2 catalysed
oxidation of 2-AP was found 9.67 kJ mol−1, which is
lower than that of complex 1 (Ea = 25.68 kJ mol−1).
Noteworthy, both the complexes exhibit negative ΔS‡
value and recommend the formation of substrate-
complex adduct at the transition state.
The redox activities of copper(II) complexes and
mixtures of the complexes with 2-AP were studied by
cyclic voltammetry in CH2Cl2. The electrochemical
potential values corresponding to active species of the
copper complexes with reference to ferrocenium/
ferrocene (Fc+/Fc) couple are displayed in Table S7. The
cyclic voltammograms for complexes 1 and 2 are illus-
trated in Figures 5 and S18. Both the copper complexes
produce quite similar nature of cathodic and anodic
waves in the presence and absence of 2-AP. Complexes
1 and 2 show reversible cathodic waves at −0.655 and
−0.730 V, which may be assigned as imine/imine anion
radical corresponding to 2e redox waves of the two C N
bonds in two benzimidazole ligands. The other cathodic
peaks at −1.53 and 1.68 V for complexes 1 and 2 appear
for the irreversible Cu2+/Cu+ redox couples.
KM (M)
=
1.29
×
10−4
]
and for
2
as Vmax
(MS−1) = 1.75 × 10−3; KM = 4.18 × 10−3 [std. error for
Vmax (MS−1
(M) = 1.97 × 10−4].
The turnover number (kcat
)
=
4.16
×
10−5
;
std. error for KM
)
for the copper(II)
complexes 1 and 2 were determined as 2.31 × 104
and 6.3 × 104 h−1, respectively. The catalytic efficiency
(kcat/KM) for 1 and 2 towards phenoxazinone synthase
activity was found high and calculated as 8.78 × 106
and 1.50 × 107. It is observed that complex 2 exhibits
approximately twofold greater rate of oxidation compare
to complex 1. In the copper(II) complex 2, the presence
of coordinated nitrate with Cu(II) centre creates higher
steric hindrance and leads to more lability, and as a
consequence, loss of the coordinated nitrate probably
occurs during the incoming approach of 2-AP
towards the copper(II) centre. For complex 1, the pres-
ence of coordinated chloride to the square plane of
Cu(II) centre facilitates a more stable/inert structure
compared to complex 2. The high catalytic efficiencies
of the copper(II) complexes may be explained from the
perspective of easy incorporation of 2-AP through the
expansion of the coordination number of copper(II)
centres.
The higher rate of catalytic oxidation for complex
2 was further evaluated in terms of determination of the
activation parameters obtained from Eyring equation/
Arrhenius equation through temperature-dependent
kinetic measurements. To understand the thermal effect
on the rate of catalytic oxidative coupling of 2-AP, kinetic
experiments were carried out with both the complexes in
the temperature range (283 K–303 K). During the kinetic
The copper complexes 1 and 2 showed the presence
of anodic peaks at 0.38 and +0.52 V, respectively. In the
mixture of individual copper complexes 1 and 2 in the
presence of 2-AP, the reversible cathodic wave of imi-
ne/imineanion radical shifted to −0.68 and −0.97 V,
respectively. Noteworthy, the irreversible cathodic peak
of Cu2+/Cu+ redox couple in 1 shifted to −1.14 V. In
addition to the presence of cathodic waves, each of the
FIGURE 5 Left: Cyclic voltammogram of
the copper(II) complex 1 in anhydrous DCM
medium; right: cyclic voltammogram of
copper(II) complex 1 in the presence of 2-AP
under molecular oxygen atmosphere in
anhydrous DCM in CH2Cl2 (0.20 M
[N(n − Bu)4]PF6) at 295 K

MUDI ET AL.
9 of 12
copper complexes exhibits reversible anodic waves at
+0.25 and +0.48 V, respectively, and suggests the devel-
opment of 2-AP−/2-AP•− redox couples in the course of
catalysis. An irreversible anodic peak at +0.98 V for
complex 1 in the presence of 2-AP was also observed and
may be attributed to the presence of 2-AP•−/2-IQ redox
couple in solution. The electrode potential values
observed for the development of 2-AP•−/2-IQ species in
the course of catalytic oxidation of 2-AP were well
corroborated with the values reported by a few previously
reported copper complexes.[5,47,61]
To find further insights into the mechanistic aspects
of the course of catalysis, X-band EPR spectral analysis of
the reaction mixture of copper complex 1 with 2-AP was
carried out in DCM medium. Complex 1 produces four
line hyperfine EPR spectra (Figure S19) for the presence
of 63Cu (I = 3/2), and the g value was found 2.116, which
may be well correlated with the reported EPR spectra of
copper(II) ion.[5,47] However, when 2-AP was added
to the complex 1 solution, the hyperfine spectra was
quenched and a new single-line spectra was found at
g = 2.001, indicating the presence of iminobenzoquinone
radical (Figure 6).[5]
Furthermore, electrospray ionization (ESI) mass spec-
tra of the copper(II) complexes 1 and 2 in the presence of
2-AP were measured after mixing of 10 min to under-
stand the labile character and the binding aspects of the
copper(II) complexes with 2-AP in MeOH medium. The
spectra are shown in Figures S20 and S21 for complexes
1 and 2, respectively. It was observed that the ESI-MS of
the reaction mixture for 1 (Figure S20) showed a base
peak at m/z 213.27, which is attributed to the existence
[(2-amino-3H-phenoxazine-3-ones) + H+] species in solu-
tion. Furthermore, a characteristics peak at m/z 823.55
was also detected, which reveals the presence of molecu-
lar ion peak. The binding adduct between complex 1 and
2-AP, [[1 + (2-AP)] + H+] was further confirmed from
the appearance of another characteristic peak at
927.41 m/z. ESI-MS spectrum of complex 2 with 2-AP
(Figure S21) displayed a characteristic peak at 213.29
FIGURE 6 EPR spectrum of the copper(II) complex 1 in the
presence of 2-AP anhydrous DCM medium
SCHEME 3 Plausible mechanistic pathway for
phenoxazinone synthase activity of the copper complexes

10 of 12
MUDI ET AL.
and confirmed the development of [(2-amino-3H-
phenoxazine-3-ones) + H+] species in MeOH. The
enzyme-substrate adduct for complex 2 was also ensured
as [[Cu(L)2 + (2-AP)] + H+], which corroborated well
with the characteristic peak at m/z 860.29 in the spec-
trum. However, the molecular ion peak for complex
2 was absent in the spectrum. Therefore, ESI-MS spectra
for both the copper complexes suggest that both the
complexes undergo the course of catalysis through the
formation of the enzyme-substrate adduct. It is also
recommended that complex 2 in addition to 2-AP
displaced the coordinated nitrates to the solution and
more lability or coordinatively unsaturation at the
copper(II) centre facilitates higher catalytic performance
than complex 1. The participation of aerobic oxygen was
also examined to understand the fate of O2 through the
detection of hydrogen peroxide in the course of catalysis.
The development of hydrogen peroxide was detected with
a UV-vis spectrophotometer, and the observed electronic
band at 353 nm corresponds to the production of I3− spe-
cies (Figure S22)[59] and thereby confirmed the existence
of hydrogen peroxide. Therefore, based on spectrophoto-
metric, kinetics, ESI-MS spectra of the reaction mixtures,
electrochemical analysis and EPR spectral measurement,
the mechanistic pathways for the course of catalytic
oxidation may be proposed according to Scheme 3.
supports the catalytic oxidation as radical driven. The
evaluated activation parameters through temperature-
dependent kinetic measurements further support the
higher rate of catalytic oxidation of 2-AP for complex 2.
Finally, this study leads to easy preparation of benzimid-
azole derivative ligand and its copper(II) complexes and
explores the effect of ancillary ligands in the bio-mimetic
oxidation of 2-AP through details spectroscopic, ESI-MS
and electrochemical methods.
SUPPLEMENTARY DATA
Supplementary crystallographic data are available free of
charge from The Director, CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK (fax: +44-1223-336033; E-mail:
deposit@ccdc.cam.ac.uk or through http://www.ccdc.
cam.ac.uk) upon request, quoting deposition number
CCDC 2046275 and 2046276 for complexes 1 and 2,
respectively. Experimental information such FT-IR,
1
UV-vis, H and 13C NMR, supramolecular architectures,
Hirshfeld surface, fingerprint plots, rate vs. [substrate]
plot, bond distance and bond angle parameters and redox
potential data are given here.
ACKNOWLEDGEMENT
BB would like to thank the Science and Engineering
Research Board (SERB), India for financial support under
the TEACHERS ASSOCIATESHIP for RESEARCH
EXCELLENCE (TAR/2018/000473).
4 | CONCLUSIONS
This research study reports the details of synthesis,
spectroscopic and structural characterization of a newly
designed substituted benzimidazole ligand and two
structurally similar copper(II) complexes containing the
ligand. The effect of ancillary ligands (chloride and
nitrate) in the bio-mimics of phenoxazinone synthase
activity has also been investigated. X-ray structural analy-
sis exhibits that both the complex adopts a perfect square
planar geometry and exists in trans configuration. It is
observed that the catalytic efficiency (kcat/KM) for both
the copper complexes was found high towards the bio-
mimicking oxidation of 2-AP. kcat/KM values were
determined as 8.78 × 106 and 1.50 × 107 for 1 and 2,
respectively, and it can be addressed that the catalytic
oxidation efficiency is almost double that of 1. The
presence of coordinated nitrate in 2 creates steric
repulsion in the square plane and makes the complex
more labile, which facilitates the entrance of 2-AP to the
Cu(II) centre. Electrochemical analysis for both the cop-
per complexes in the presence of 2-AP confirms the
development of AP−/AP•− redox couple in the course of
oxidative coupling of 2-AP. EPR spectral analysis consoli-
dates the existence of organic radical at g = 2.01 and
AUTHOR CONTRIBUTIONS
Prafullya Kumar Mudi: Conceptualization; formal
analysis; investigation; methodology. Rajani Kanta
Mahato: Conceptualization; formal analysis; investiga-
tion; methodology. Mayank Joshi: Methodology;
validation; visualization. Madhusudan Shit: Formal
analysis; investigation; methodology. Angshuman Roy
Choudhury: Data curation; formal analysis; software;
validation. Hari Sankar Das: Formal analysis; investiga-
tion; supervision. Bhaskar Biswas: Conceptualization;
supervision.
DATA AVAILABILITY STATEMENT
The data that supports the findings of this study are
available in the supporting information of this article.
ORCID
Prafullya Kumar Mudi https://orcid.org/0000-0002-
3365-0961
Rajani Kanta Mahato https://orcid.org/0000-0003-
0765-2979
Mayank Joshi https://orcid.org/0000-0003-2463-4967

MUDI ET AL.
11 of 12
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How to cite this article: Mudi PK, Mahato RK,
Joshi M, et al. Copper(II) complexes with a
benzimidazole functionalized Schiff base:
Synthesis, crystal structures, and role of ancillary
ions in phenoxazinone synthase activity. Appl
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