Two copper (II) complexes derived from anthranilic acid and 4-iodo-anthranilic acid Schiff bases: Structural elucidation, halogen bonding interactions and catalytic study using 3,5-DTBC

Article abstract of DOI:10.1016/j.molstruc.2020.128398

Two monomeric copper complexes [Cu(Pymab)(CCl₃COO)(H₂O)].H₂O (1) and [Cu(Pymaib)(H₂O)Cl]₂ (2) were synthesized by reacting copper (II) trichloroacetate and copper(II) chloride with the monobasic tridentate Schiff-Base ligands (HPymab = (E)-2-((pyridine-2-yl)methyleneamino)benzoic acid and HPymaib = 4-iodo-2-{(E)-[(pyridin-2-yl) methyleneamino}benzoic acid) and characterized by elemental analysis, IR, UV–Vis, mass spectrometry, CV and single crystal XRD analysis. Both complexes are monomeric in the solid state and in the solution as well. For both complexes, single crystal XRD indicates a distorted square pyramidal coordination geometry of the metal atoms. Both 1 and 2 behave as effective catalysts towards the oxidation of 3,5-di-tert-butyl catechol to the corresponding quinone derivative in aerial oxygen. The reactions follow the Michaelis-Menten enzymatic reaction kinetics with (k_cat) values of 1452 h^?1 and 1458 h^?1 respectively in DMSO and 1590 h^?1 and 1636 h^?1 in THF respectively. In the solid state both compounds participate in halogen bonding interactions that have been studied using DFT calculations and the molecular electrostatic potential (MEP) and the noncovalent interaction plot index (NCI plot) computational tools.

Full text of DOI:10.1016/j.molstruc.2020.128398

Journal of Molecular Structure 1217 (2020) 128398
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Two copper (II) complexes derived from anthranilic acid and 4-iodo-
anthranilic acid Schiff bases: Structural elucidation, halogen bonding
interactions and catalytic study using 3,5-DTBC
,
Barun Kumar Biswas a, Sandeepta Saha a b, Niladri Biswas a, Manas Chowdhury a,
Antonio Frontera c, Corrado Rizzoli d, Ruma Roy Choudhury e,
,
*
Chirantan Roy Choudhury a
a Department of Chemistry, West Bengal State University, Barasat, Kolkata 700126, India
b Sripur High School, Madhyamgram Bazar, Madhyamgram, Kolkata, 700130, India
c Departament de Química, Universitat de les Illes Balears, Crta. De Valldemossa km 7.5, 07122, Palma de Mallorca, Baleares, Spain
d Dipartimento S.C.V.S.A., University of Parma, Parco Area delle Scienze 17/A, 43124 Parma, Italy
e Department of Chemistry, Heritage Institute of Technology, Chowbaga Road, Anandapur, Kolkata, 700107, West Bengal, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Two monomeric copper complexes [Cu(Pymab)(CCl3COO)(H2O)].H2O (1) and [Cu(Pymaib)(H2O)Cl]2 (2)
were synthesized by reacting copper (II) trichloroacetate and copper(II) chloride with the monobasic
Received 16 February 2020
Received in revised form
3 May 2020
tridentate Schiff-Base ligands (HPymab
¼
(E)-2-((pyridine-2-yl)methyleneamino)benzoic acid and
HPymaib ¼ 4-iodo-2-{(E)-[(pyridin-2-yl) methyleneamino}benzoic acid) and characterized by elemental
analysis, IR, UVeVis, mass spectrometry, CV and single crystal XRD analysis. Both complexes are
monomeric in the solid state and in the solution as well. For both complexes, single crystal XRD indicates
a distorted square pyramidal coordination geometry of the metal atoms. Both 1 and 2 behave as effective
catalysts towards the oxidation of 3,5-di-tert-butyl catechol to the corresponding quinone derivative in
aerial oxygen. The reactions follow the Michaelis-Menten enzymatic reaction kinetics with (kcat) values
of 1452 hꢀ1 and 1458 hꢀ1 respectively in DMSO and 1590 hꢀ1 and 1636 hꢀ1 in THF respectively. In the
solid state both compounds participate in halogen bonding interactions that have been studied using DFT
calculations and the molecular electrostatic potential (MEP) and the noncovalent interaction plot index
(NCI plot) computational tools.
Accepted 5 May 2020
Available online 10 May 2020
Keywords:
Copper
XRD
Catalyst
DFT
NCI plot
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
having NeO donor ligands are the main focus of a number of
studies [6e24]. Copper dimers easily oxidizes phenols and cate-
For mimicking the structural property, or for reactivity study of
biological metalloenzymes, biomimetic models are extensively
designed [1]. For example, the copper-containing type-III metal-
loenzyme, catechol oxidase is responsible for the catalytic oxida-
tion of catechols to quinones, which is called catecholase activity
[2]. Being one of the essential micronutrients, copper is used by
nature to play a crucial role in many biological processes [3], like
oxygenation reaction, oxygen transport, electron transfer etc [4,5].
Because of the dinuclear copper(II) moiety present in the active site
structure of catechol oxidase, dinuclear Copper(II) complexes
chols and it is supported by various methods [25]. Besides dinuclear
complexes, mononuclear copper (II) complexes are also able to
show catecholase activity and follow different mechanisms [26,27].
From these reports, it is clear that for mononuclear complexes,
when bulkiness around the central metal are less then Kcat values
are high enough but the value becomes very low, when central
metal ions are sterically crowded [28e31]. Mainly, two types of
mechanisms are mentioned for the catalytic oxidation of substrates
by these compounds [28e30]. So, study of newly synthesized
model complexes having prominent catecholase activity and
finding the correlation between structural and catalytic properties
are considered as promising area of current day research [32]. Schiff
bases are still one of the important types of coordinating ligands.
Metal complexes with tridentate Schiff base ligands having N, N, O
* Corresponding author.
E-mail address: crchoudhury2000@yahoo.com (C. Roy Choudhury).
https://doi.org/10.1016/j.molstruc.2020.128398
0022-2860/© 2020 Elsevier B.V. All rights reserved.

2
B.K. Biswas et al. / Journal of Molecular Structure 1217 (2020) 128398
donor sites are employed for decoding the structural features of
non-heme protenes [33,34]. In recent times, Schiff bases synthe-
sized from pyridine 2-carboxaldehydes are widely studied because
of its various possible applications in the field of chemotherapeu-
tics and bio-mimetic catalysis on super oxide dismutase activities
[35e46].
method [47].
Anthranilic acid (0.137 g, 1 mmol) was dissolved in 20 ml of
methanol and a solution of pyridine-2-carboxaldehyde (1 mmol) in
20 ml methanol were added dropwise to it with continuous stir-
ring. The Schiff base ligand was prepared according to published
procedure [47]. (Scheme 1A).
All these facts prompted us to synthesize two novel mono-
nuclear copper (II) Schiff base complexes using pyridine-2-
carboxaldehyde and anthranilic acid or 4-iodo-anthranilic acid
respectively. The resulting complexes [Cu(Pymab)(CCl3COO)(-
H2O)].H2O (1) and [Cu(Pymaib)(H2O)Cl]2 (2) were evaluated for
conversion of 3,5- DTBC to corresponding quinone. From the results
it can be concluded that complexes 1 and 2 are very effective cat-
alysts with kcat values 1452 hꢀ1 and 1458 hꢀ1 respectively in DMSO
and 1590 hꢀ1 and 1636 hꢀ1 in THF respectively.
2.3.2. Synthesis of (HPmyaib ¼ 4-iodo-2-{(E)-[(pyridin-2-yl)
methyleneamino}benzoic acid)
Schiff base ligand (HPymaib) was prepared using the same
method as HPymab using 4-iodo anthranilic acid (1 mmol) instead
of anthranilic acid (Scheme-1B).
2.3.3. Preparation of copper trichloroacetate
Copper trichloro acetate was prepared using the standard
literature procedure [48].
2. Experimental section
2.3.4. Synthesis of [Cu(Pymab)(CCl3COO)(H2O)].H2O (1)
2.1. Chemicals and methods
A methanolic solution (20 ml) of Copper (II) trichloroacetate
(0.390 g, 1 mmol) was taken in a 100 ml beaker, then 20 ml of
HPymab (1 mmol) was added dropwise to it. After addition of
HPymab the solution turned blue to light green. Then, the solution
was stirred for 5 min over a magnetic stirrer. Then the solution was
filtered and left for crystallization. After a weak, green coloured
Anthranilic
acid,
4-iodo-anthranilic
acid,
pyridine-2-
carboxldehyde, cupric carbonate, copper(II) chloride were ob-
tained from Sigma-Aldrich (USA). 3,5-di tert-butyl catechol, tri-
chloroacetic acid were purchased from Spectrochem (India). All
chemicals were used without further purification. All solvents were
dried according to standard procedures and distilled before to use.
crystals (yields
characterization.
~ 65%) separated out and it was used for
Yield: 65% (0.360 g), Anal. Calc. For [C15H11Cl3CuN2O5$H2O] (1)
C, 36.94; H, 2.67; N, 5.74%. Found: C, 36.91; H, 3.59; N, 5.67%. IR
(KBr, cmꢀ1): 1691(s), 1568-1586(s), 1324-1363(s) and 743-778(b).
2.2. Physical characterization
Elemental analyses were performed on a PerkinElmer 2400
CHNS O element analyzer. Infrared spectra of complexes were
recorded in the solid state as a KBr pellet on a PerkinElmer SPEC-
TRUM Two FT-IR spectrophotometer within the range of
400e4000 cmꢀ1. Electronic spectra of 1 and 2 were recorded at
300 K on a PerkinElmer Lambda-35 UVeVis spectrometer using
DMSO as solvent. Cyclic voltammograms of 1 and 2 were recorded
using CH-electrochemical analyzer with DMSO as solvent. We have
done catecholase activity study by UV spectrometer (PerkinElmer
Lamda-35) using 1 cm path length using the same solvent.
2.3.5. Synthesis of [Cu(Pymaib)(H2O)Cl]2 (2)
Complex 2 was synthesized using Copper (II) chloride instead of
Copper (II) trichloroacetate and HPymaib instead of HPymab
(1 mmol) by the same procedure as in complex 1.
Yield: 67% (0.315 g), Anal. Calc. For [C13H10ClCuIN2O3] (2) C,
33.32; H, 2.14; N, 5.98%. Found: C, 33.22; H, 2.18; N, 5.89%. IR (KBr,
cm-1): 1685(s), 1582-1592(s), 1444-11490(s) and 749-741(b).
2.4. Single-crystal X-ray diffraction study
2.3. Synthesis
Good quality single crystals of 1 and 2 were mounted on a
Bruker APEX2 CCD diffractometer respectively, equipped with
2.3.1. Synthesis of (Hpymab ¼ (E)-2-((pyridine-2-yl)
methyleneamino)benzoic acid)
graphite monochromatized Mo K
focus sealed tubes. Intensity data were collected at 294 K using
scans. Crystal data of both complexes were collected using Apex2
a
radiation (
l
¼ 0.71073 Å) fine-
u
Schiff base ligand HPymab was prepared according to literature
A
B
Scheme 1.

B.K. Biswas et al. / Journal of Molecular Structure 1217 (2020) 128398
3
[41]. Data refinement and reduction were performed using SAINT
(Bruker, 2008) [41]. Multiscan absorption corrections were applied
empirically to the intensity values (Tmin ¼ 0.334 and Tmax ¼ 0.745
for 1 and Tmin ¼ 0.581 and Tmax ¼ 0.746 for 2) using SADABS [49].
The structures were solved by direct methods using the program
SHELXT [42a,50a] for 1 and 2, and refined with full-matrix least-
squares based on F2 using program SHELXL-2014/7 [50b] for the
complexes. All non-hydrogen atoms were refined anisotropically. In
both complexes the water H-atoms were placed in chemically
sensible positions on the basis of hydrogen bonding interactions
and refined with Uiso(H) ¼ 1.5Ueq(O). The OeH bond lengths and
H/H separations were constrained to 1.30(1) and 0.82(1) Å,
respectively. The minimum H/H separation between the inde-
pendent water molecules was also constrained to be 2.20(1) Å. All
other H atoms were placed geometrically and refined using a riding
yellow-green-blue scale with red for rþ (repulsive) and blue for
cut
rꢀ
(attractive). Yellow and green isosurfaces correspond to
cut
weakly repulsive and attractive interactions, respectively.
3. Results and discussion
3.1. Synthetic pathway of complex formation
Copper(II) complexes were prepared by the reaction of copper
trichloroacetate and copper chloride with methanolic solution of
NNO donor Schiff base ligands HPymab and HPymaib respectively in
1:1 M proportions. The details pathway is given in Scheme 2A
(Complex 1) and Scheme 2B (Complex 2). Both the complexes 1 and
2 possess distorted square pyramidal geometry and are stable in air
at room temperature. Complexes 1 and 2 are characterised by IR,
UVeVis spectroscopy, cyclic voltammetry and single crystal X-ray
crystallography. All the spectroscopic data are consistent with the
crystal structures of the complexes 1 and 2 discussed in this paper.
atom approximation, with CeH
¼
0.93 Å, and with
Uiso(H) ¼ 1.2Ueq(C). The molecular graphics and crystallographic
illustrations for 1 and 2 were prepared using Olex2 [51a] POV ray
[51b] and ORTEP 3 [52] programs. Crystallographic data and
structure refinement parameters for 1 and 2 are summarized in
Table 1.
3.2. IR spectral study
IR Spectra of 1 and 2 are shown in Figs. S1A and S1B respectively.
Both complexes exhibit strong bands in the region of 1691 cmꢀ1
2.5. Theoretical methods
and 1685 cmꢀ1 respectively, due to presence of n(C
]
[56]. Broad
N)
peaks due to
g
(NeH) wagging are observed at around 743-778 cmꢀ1
The energetic and geometric features of the complexes included
in this study were calculated at the B3LYP-D3/def2-TZVP level of
theory using the crystallographic coordinates (only the positions of
the H-atoms were optimized). For the calculations, the GAUSSIAN-
16 program has been used [53]. The basis set superposition error for
the calculation of interaction energies has been corrected using the
counterpoise method [54]. Molecular electrostatic potential (MEP)
surfaces have been computed at the same level of theory and
represented using the 0.001 a.u. isosurface. The NCI plot index [55]
computational tool has been used to characterize non-covalent
interactions using the B3LYP-D3/def2-TZVP level wave function.
They correspond to both favorable and unfavorable interactions, as
differentiated by the sign of the second density Hessian eigen value
and defined by the isosurface color. The color scheme is a red-
and 749-741 cmꢀ1 respectively [17]. Both complexes showed nassym
(COOꢀ) and nsym (COOꢀ) peaks due to the presence of carboxylate
ions. Due to the presence of carboxylate groups, nassym (COOꢀ)
[1568-1586 and 1582-1592 cmꢀ1] and nsym (COOꢀ) [1324-1363 and
1444-1490 cmꢀ1] peaks are observed [57e59]. The large difference
between the two bands indicates the monodented nature of the
active group [57e59].
3.3. Electronic spectral study
Electronic spectra of the complexes 1 and 2 were recorded in
DMSO and display (Figs. S2A and S2B) board absorption bands at
740 and 709 nm respectively, which are in agreement with the d-
Table 1
Crystallographic data and structural refinement of complexes 1 and 2.
1
2
empirical formula
formula weight (g molꢀ1
temperature
crystal system
space group
a (Å)
C15H11Cl3CuN2O5$H2O
487.16
294
Monoclinic
P21/c
14.173(8)
9.830(6)
13.642(8)
90
106.298(5)
90
1824.2(19)
4
1.774
1.673
C13H10ClCuIN2O3
468.12
294
Triclinic
P-1
9.2267(16)
9.3259(16)
18.008(3)
87.333(2)
89.982(3)
77.508(3)
1511.1(4)
4
)
b (Å)
c (Å)
a
b
g
(deg)
(deg)
(deg)
V(Å3)
Z
dcalc (g cmꢀ3
)
2.058
3.674
m
(mmꢀ1
)
F(000)
crystal size (mm3)
range (deg)
980
900
0.07 ꢁ 0.17 ꢁ 0.22
1.5e25.2
3270
2196
0.087
0.03 ꢁ 0.06 ꢁ 0.15
1.1e25.2
5430
4682
0.031
q
measured reflections
independent reflections
R(int)
goodness-of-fit on F2
1.050
1.039
final R indices[I > 2
s
(I)]
R1 ¼ 0.0786, wR2 ¼ 0.1979
R1 ¼ 0.0527, wR2 ¼ 0.1678
wR2 ¼ 0.1063
R indices (all data)
R1 ¼ 0.1122, wR2 ¼ 0.2196
ꢀ0.75 and 0.98
R1 ¼ 0.0590, wR2 ¼ 0.1736 wR2 ¼ 0.0984
ꢀ1.45 and 2.53
Drmin and Drmax (e Åꢀ3
)

4
B.K. Biswas et al. / Journal of Molecular Structure 1217 (2020) 128398
potential range from þ2.0 to ꢀ2.0 V using tetrabutylammonium
perchlorate as supporting electrolyte. The voltammograms of
complexes 1 and 2 are shown in Fig. 1. In both complexes one
prominent oxidative peak is observed at 0.85 and 0.77 V for 1 and 2,
respectively which corresponds to Cu(II) to Cu(III) oxidation. Fig. 1
shows reduction peaks for
1 and 2 with cathodic wave
potential ꢀ0.44 and ꢀ0.56 V, respectively corresponding to the
Cu(II) to Cu(I) redox couple.
3.5. X-ray crystal structure description
3.5.1. Crystal structure of 1
A thermal ellipsoid drawing of the structure of complex 1
indicating atom numbering scheme is shown in Fig. 2. Selected
bond lengths and bond angles are given in Tables 2a and 2b. The
complex crystallizes as a five-coordinate monomeric species. The
copper(II) ion adopts a distorted square pyramidal geometry
(
t
¼ 0.23) [61] with the donor atoms forming the basal plane being
the pyridine nitrogen atom (N1), the imine nitrogen atom (N2) and
the carboxylate oxygen atom from anthranilic acid part (O1) of the
Schiff base HPymab plus the oxygen (O3) from the trichloroacetate
anion. One water molecule coordinates weakly in the apical site
through (O5).
The Schiff base is coordinated to the copper(II) ion in its imi-
nocarboxylate form. While coordinating in this form, the negative
charge generated upon deprotonation of the COOH proton is
delocalized across the six membered ring Cu1/O1/C13/C12/C7/N2
as indicated by the intermediate value of C7eN2 {1.431(4) Å} and
C12eC13 bond distances {1.520 Å}. These bonds may be compared
with the well with those of the analogous five-coordinate square-
pyramidal copper(II) complex, {[Cu(Pymab)(H2O)](NO3)}4 [47,62],
whose structure has been elucidated by X-ray diffraction. The ge-
ometry of the complex is distorted square-pyramidal conceivably
as a consequence of the restricted bite of the tridentate Schiff base
ligand. The five- and six-membered chelation rings are approxi-
mately planar (r.m.s. deviation is 0.0115 and 0.0914 for rings Cu1/
N1/C5/C6/N2 and Cu1/N2/C7/C12/C13/O1), the mean planes
through them forming a dihedral angle of 13.40(15)ꢂ.In the crystal
the complex molecules and the water molecules of crystallization
are linked by classical OeH/O, nonclassical CeH/O and CeH/Cl
A
hydrogen bonds into
Table S3A). Cohesion of the crystal structure is further stabilized by
stacking interactions (centroid-to-centroid distance 3.646(5)
a three-dimensional network (Fig. 3,
p
… p
Å) occurring between the benzene and pyridine rings of adjacent
molecules.
3.5.2. Crystal structure of 2
The asymmetric unit of 2 (Fig. 4) contains two independent
mononuclear complexes having similar conformation. An overlay
of structures of the independent complexes is shown in Fig. 5. For
sake of simplicity the molecule containing atom Cu1 and Cu2 will
be hereafter referred as molecule A and B respectively. The cop-
per(II) metals are five-coordinated by the N, O and Cl donor atoms
of a tridentate HPymaib ligand, a water molecule and a chloride
B
Scheme 2. A (Synthesis of Complex 1). B (Synthesis of Complex 2).
anion. Based on the trigonality index
t [61], which assumes a value
of 0.0 and 1.0 for ideal square pyramidal and trigonal bipyramidal
geometries, the coordination geometry about the copper(II) metal
d transition bands of copper (II) [60]. Two higher energy bands
appeared in the spectra at 341, 260 nm and 333, 252 nm for com-
in A can be described as distorted square pyramid (
whereas a somehow more regular square pyramidal coordination
geometry is observed in B (
t
¼ 0.205),
plexes 1 and 2 respectively, which are assigned for n to
* transitions respectively. .
p* and p to
p
t
¼ 0.16). In both coordination geome-
tries the basal plane is provided by the N and O atoms and the
apical position is occupied by the chloride anion. The 5-membered
chelate ring is approximately planar in both molecules (r.m.s. de-
viation is 0.274 and 0.0144 Å for rings Cu1/N1/C5/C6/N2 and Cu2/
N3/C18/C19/N4, respectively). The six-membered chelate rings
3.4. Cyclic voltammetry
Electrochemical study was carried out for complexes 1 and 2 in
Tris-HCl buffer medium under nitrogen atmosphere in the

B.K. Biswas et al. / Journal of Molecular Structure 1217 (2020) 128398
5
Fig. 1. (a): Cyclic voltammogram of complex 1, (b) Cyclic voltammogram of Complex 2.
group of molecule A and the water molecule and the chloride anion
of molecule B (Table S3B). The Cu/Cu separation within the
dimeric unit is 5.7069(13) Å. In the crystals, the dimers are further
linked into layers parallel to the ab plane (Fig. 6) through OeH/O,
OeH/Cl and CeH/Cl hydrogen interactions.
3.6. Catecholase activity of complexes 1 and 2
For the study of the catecholase activity of complexes 1 and 2,
3,5-DTBC, with two bulky tert-butyl substituents on the ring and
low quinine-catechol reduction potential, has been chosen as the
substrate [65]. In the presence of bulky substituent, further
oxidation and polymerization are stopped. Both 1 and 2 exhibit
extensive catalytic competence toward the oxidation of 3,5 eDTBC
to 3,5- DTBQ in the solvent DMSO and THF [66]. This catecholase
reaction was investigated using UVeVis spectroscopy studies in
DMSO and THF solutions (as illustrated in Scheme 3) [67]. To
perform the kinetic studies, 1 ꢁ 10ꢀ4 mol dmꢀ3 solutions of 1 and 2
were added to 100 equivalent of the substrate in DMSO or in THF
under aerobic condition at 25 ꢂC [68]. The time dependent UVeVis
spectral scan was executed after addition of catalyst 1 at 5 min time
intervals up to 90 min for complex 1 and 70 min for complex 2. As
the reaction progressed, new bands at ~392 nm for complex 1 and
390 nm for complex 2 were generated which clearly indicate the
formation of corresponding quinone derivative 3,5-DTBQ [Figs. 7
and 8]. When THF were used as solvent, new bands at ~394 nm
for complex 1 and 396 nm for complex 2 were generated which
clearly indicate the formation of corresponding quinone derivative
3,5-DTBQ [Figs. 9 and 10].
Fig. 2. Crystal Structure of Complex 1 with displacement ellipsoids drawn at the 50%
probability level.
assume an envelope conformation, with atoms Cu1 and Cu2 dis-
placed by 0.8363(8) and 0.7290(8) Å respectively from the mean
plane through the rest of the rings. In A and B the dihedral angle
between the mean planes through the five- and six-membered
rings is 34.75(13) and 38.88(13)ꢂ, respectively, and the benzene
ring is oriented with respect to the pyridine ring to form a dihedral
angle of 37.9(2) and 24.8(2)ꢂ respectively. Bond distances (Table 2a)
and angles (Table 2b) are unexceptional and in agreement with
those observed in related compounds [63].
It was observed that often molecules related to 2 form centro-
symmetric dimers via Cu‒X bridges (X ¼ halogens or pseudo hal-
ogens) [64]. In 2, dimeric units are generated through classical
hydrogen bonds involving the water molecule and the carboxyl
The kinetics of the oxidation of 3,5-DTBC was determined by the
method of initial rates by monitoring the growth of the product 3,5-
DTBQ, the experimental conditions were the same as reported
Table 2a
Selected bond lengths (Å) for 1 and 2.
Complex 1
Complex 2
Cu1eO1
Cu1eO3
Cu1eO5
Cu1eN1
Cu1eN2
1.916(5)
1.955(6)
2.265(6)
2.005(5)
2.009(6)
Cu1eCl1
Cu1eO1
Cu1eN1
Cu1eN2
Cu1eO3
Cu2eN3
Cu2eN4
Cu2eO6
Cu2eCl2
Cu2eO4
2.252(2)
2.007(6)
2.162(6)
2.132(5)
2.002(6)
2.132(6)
2.165(5)
2.016(6)
2.256(2)
1.986(5)

6
B.K. Biswas et al. / Journal of Molecular Structure 1217 (2020) 128398
Table 2b
Selected bond angles (ꢂ) for 1 and 2.
Complex 1
Complex 2
O1eCu1eO3
O1eCu1eO5
O1 eCu1eN1
O1eCu1eN2
O3eCu1eO5
O3 eCu1 eN1
O3eCu1eN2
O5eCu1eN1
O5eCu1eN2
N1eCu1eN2
92.2(2)
95.9(2)
162.0(2)
91.7(2)
88.6(2)
93.5(2)
175.6(2)
101.3(2)
92.9(2)
82.2(2)
Cl1eCu1eO1
Cl1eCu1eO3
Cl1eCu1eN1
Cl1eCu1eN2
O1eCu1eO3
O1eCu1eN1
O1eCu1eN2
O3eCu1eN1
O3eCu1eN2
N1eCu1eN2
O4eCu2eO6
O4eCu2eN3
O4eCu2eN4
Cl2eCu2eO4
Cl2eCu2eO6
Cl2eCu2eN3
Cl2 eCu2eN4
O6eCu2eN3
O6eCu2eN4
N3eCu2eN4
109.20(19)
109.92(18)
113.62(16)
102.18(16)
85.2(2)
135.6(2)
83.7(2)
90.9(2)
147.9(2)
76.6(2)
85.1(2)
138.3(2)
84.73(18)
112.22(17)
108.00(18)
108.39(18)
104.03(15)
91.2(2)
147.9(2)
76.8(2)
Fig. 4. Crystal Structure of Complex 2 with displacement ellipsoids drawn at the 50%
probability level.
Fig. 3. Crystal packing of complex 1 approximately viewed down the b axis, showing
the three-dimensional hydrogen-bonding network (dashed lines). Hydrogen atoms not
involved in hydrogen bonds are omitted.
earlier [67,69e71].
The kinetic parameters were determined by applying the
Michaelis-Menten approach and the results were obtained from
Lineweaver-Burk plots [68]. The binding constant (km), maximum
velocity (Vmax) and rate constant for the conversion of the substrate
(turnover number, Kcat) were calculated for both complexes 1 and 2
in solvents DMSO and THF respectively, using the Lineweaver-Burk
Fig. 5. Overlay diagram of the two independent molecules in complex 2 (1) (yellow
image contains Cu1).
From the data presented in Table 4 it may be inferred that the
catalytic efficiency of 1 and 2 in both the DMSO and THF solvent are
nearly comparable with copper catalysts [72e79]. From the kinetic
data, it is observed that Kcat value of complex 2 is slightly higher
than that of complex 1. Rate constants for the oxidation of catechols
graph of 1/v vs. 1/[s], using equation 1/v ¼ { KM/VMax(1/[s]) 1/VMax
}
[36] [Figs. 11a, b and 12a, b] for DMSO solvent and [Figs. 13a, b and
14a, b] for THF solvent. The kinetic parameters Vmax and Kcat of the
complexes 1 and 2 are listed in Table 3.

B.K. Biswas et al. / Journal of Molecular Structure 1217 (2020) 128398
7
Fig. 8. Change in Spectral pattern of complex
2
after reaction with 3,5- DTBC,
Observing the reaction for 70 Min in DMSO: Inset UVeVis spectrum for complex 2 in
DMSO.
Fig. 6. Crystal packing of 2 approximately viewed along the b axis showing the
hydrogen bonding network (dashed lines).
Scheme 3. Oxidation of 3,5-DTBC to 3,5-DTBQ.
Fig. 7. Change in Spectral pattern of complex
1 after reaction with 3,5- DTBC,
Observing the reaction for 90 min in DMSO: Inset UVeVis spectrum for complex 1 in
Fig. 9. Change in Spectral pattern of complex 1 after reaction with 3,5- DTBC,
DMSO.
observing the reaction for 90 min in THF. Inset UVeVis spectrum for complex 1 in THF.
Some mononuclear and dinuclear copper (II) complexes can be
correlated [80e83]. This type of correlation of relative activity is
usually depends on the Cu/Cu distance [81a,83a], coordination
geometry around metal centre [82c,82d], nature of the substituents
by complexes 1 and 2 with similar previously reported Cu(II), Ni (II),
Mn (II/III) and Zn (II) complexes using 3,5-DTBC as the substrate are
given in Table 4.

8
B.K. Biswas et al. / Journal of Molecular Structure 1217 (2020) 128398
Fig. 10. Change in Spectral pattern of complex
1
after reaction with 3,5- DTBC,
observing the reaction for 90 min in THF. Inset UVeVis spectrum for complex 1 in THF.
of the ligands [83b] and steric strain of the compounds [83b].
Another correlation for mononuclear manganese(III) catalysts is
described where the role of the apical ligand on the activity has
been discovered [84].
3.7. Mechanism of catalytic oxidation of 3,5 DTBC with complex 1
or 2
A plausible mechanism for catalysis of 3,5- DTBC by complex 1
or 2 are provided in Scheme 4. It may be suggested that in the first
step DTBCH2 displace eH2O and eOCOCCl3/-Cl from the coordi-
nation sphere and gets attached with complex 1 or 2. So in this case
DTBC2ꢀ is coordinated as a bidented ligand. A semi-quinone is
formed when DTBC2ꢀ provide one electron and consequently
semiquinone complex possess CuI-central metal ion. CuI e complex
readily reacts with available O2 from air and form CuII-superoxo
species, involving semi quinone ligand. Next step would be
regeneration of catalyst and elimination of H2O2 (Figs. S4 and S 5
Supplementary information) [32,73,85e87]. For the detection of
H2O2 during catalytic oxidation modified iodometric method was
applied [27,88]. Formation of H2O2 was confirmed by detection of I3ꢀ
band at 353 nm by UVeVis spectroscopy [2,27].
Fig. 12. a. Plot of rate vs. [substrate] (3,5-DTBC) in presence of complex 1 in DMSO;
Inset: Lineweaver-Burk plot. 12 b. Plot of rate vs. [substrate] (3,5-DTBC) in presence of
complex 2 in DMSO; Inset: Lineweaver-Burk plot.
Fig. 11. (a)A plot of the difference in absorbance (
DA) vs. time to evaluate the rate of catalysis of 3,5-DTBC by complex 1 in DMSO; (b) A plot of the difference in absorbance (DA) vs.
time to evaluate the rate of catalysis of 3,5-DTBC by complex 2 in DMSO.

B.K. Biswas et al. / Journal of Molecular Structure 1217 (2020) 128398
9
Fig. 13. (a)A plot of the difference in absorbance (
DA) vs. time to evaluate the rate of catalysis of 3,5-DTBC by complex 1 in THF; (b) A plot of the difference in absorbance (
DA) vs.
time to evaluate the rate of catalysis of 3,5-DTBC by complex 2 in THF.
Table 3
The kinetic parameters for the catalytic oxidation of 3,5-DTBC mediated by com-
plexes 1 and 2.
Complexes
Solvent
Vmax (MSꢀ1
)
KM (M)
Kcat (hꢀ1
)
1
2
1
2
DMSO
DMSO
THF
24.20 ꢁ 10ꢀ4
24.30 ꢁ 10ꢀ4
26.50 ꢁ 10ꢀ4
27.26 ꢁ 10ꢀ4
32.94 ꢁ 10ꢀ2
33.10 ꢁ 10ꢀ2
39.33 ꢁ 10ꢀ2
34.62 ꢁ 10ꢀ2
1452.00
1458.00
1590.00
1636.00
THF
3.8. Theoretical study
The theoretical DFT study is devoted to analyze the halogen
bonding interactions observed in the solid state of both compounds
1 and 2, as detailed below. We have first computed the molecular
electrostatic potential (MEP) of both compounds in order to
examine electron rich and electron poor regions of the molecule,
paying special attention to the existence of a s-hole at the exten-
sion of the CeX bonds (X ¼ Cl, I). The MEP surfaces are shown in
Fig. 15 for both compounds. The most positive MEP value in com-
pound 1 is located at the coordinated water molecule and the most
negative at the carboxylate groups of the ligands. At the extension
of the CeCl bond the MEP value is slightly positive, thus indicating a
very small sigma hole. This is likely due to the anionic nature of the
trichloroacetate ligand. In contrast for compound 2, the MEP value
at the extension of the CeI bond is large and positive (þ16 kcal/
mol), thus more suitable for establishing halogen bonding in-
teractions. This behavior is typical in halogen bonding that is
favored for the heavier halogen atoms. In2, the most negative po-
tential is located at the carboxylate group and chlorido ligand.
In Fig. 16a we show a dimer of compound 1 retrieved from its X-
ray packing where it can be observed the formation of a quite
directional CeCl/O halogen bond. We have evaluated the dimer-
ization energy that is moderately strong,
DE1 ¼ ꢀ9.1 kcal/mol. It is
worth mentioning that the coordinated water molecule of one
complex (highlighted in yellow in Fig. 16a) is located at 3.26 Å from
the one O-atom of the coordinated carboxylate group. Since both
groups (water and carboxylate) are, according to the MEP in
Fig. 15a, the most positive and negative regions of the molecule, the
interaction energy could be influenced by this long distance H-
bond (electrostatically enhanced). Consequently, we have
computed a theoretical model where this water molecule has been
eliminated and the interaction energy is reduced to
Fig. 14. a. Plot of rate vs. [substrate] (3,5-DTBC) in presence of complex 1 in THF; Inset:
Lineweaver-Burk plot. b. Plot of rate vs. [substrate] (3,5-DTBC) in presence of complex 2
in THF; Inset: Lineweaver-Burk plot.
DE2 ¼ ꢀ3.4 kcal/mol, that corresponds to the contribution of the

10
B.K. Biswas et al. / Journal of Molecular Structure 1217 (2020) 128398
Table 4
First order rate constant for the oxidation of catechols by complexes 1 and 2 with similar previously reported Cu(II), Ni (II), Mn (II/III) and Zn (II) complexes using 3,5-DTBC as
the substrate.
Complexes
Solvent
Metal Present
Oxidation state
Kcat (in hꢀ1
)
Ref.
[Cu(Pymab)(CCl3COO)(H2O)].H2O (1)
DMSO
THF
DMSO
THF
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Ni
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
(IV)
(IV)
(IV)
(IV)
(IV)
(IV)
(IV)
(II)
(II)
(II)
(II)
(II)
(II)
(II)
1452.00
1590.00
1458.00
1636.00
32400
14400
10800
14400
28800
73.70
50.50
84.60
127.20
49.60
246.80
50.70
26.90
51.80
68.30
50.10
563
564
666
365
564
1287
3800
678
244
265
199
9.27
8.48
936.64
365.34
1432.74
247.00
360.00
720.00
12.60
21.30
55.20
14.80
34.14
16.32
15.60
2160.00
1440.00
720.00
1060.00
882.00
297.00
352.00
This work
[Cu(Pymaib)(H2O)Cl]2 (2)
This work
[Cu2(H2L22)(OH)(H2O)(NO3)](NO3)3$2H2Ob
[Cu(HL14)(H2O)(NO3)]2(NO3)2$2H2Ob
[Cu(L11)(H2O)(NO3)]b2
[Cu2(L23)(OH)(H2O)2](NO3)2b
[Cu2(L21)(N3)3]b
MeOH
MeOH
MeOH
MeOH
MeOH
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeCN
MeCN
MeCN
MeOH
MeOH
MeOH
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
[65]
[65]
[65]
[65]
[65]
[66]
[66]
[66]
[66]
[66]
[66]
[66]
[66]
[66]
[66]
[66]
[67]
[67]
[67]
[67]
[67]
[67]
[67]
[67]
[67]
[67]
[67]
[68]
[68]
[69a]
[69a]
[69a]
[69b]
[69b]
[69b]
[70]
[70]
[70]
[70]
[70]
[70]
[70]
[71]
[71]
[71]
[72]
[72]
[72]
[72]
[Cu2(L11)(OH)(H2O)(NO3)2]c
[Cu2(L12)(OH)(H2O)(NO3)2]c
[Cu(L21)(H2O)(NO3)]c2
[Cu2(L13)(OH)(H2O)(NO3)2]c
[Cu2(L14)(OH)(H2O)(NO3)2]c
[Cu2(L15)(OH)(H2O)(NO3)2]c
[Cu2(L16)(OH)(H2O)(NO3)2]c
[Cu2(L17)(OH)(H2O)2(NO3)2]c
[Cu2(L19)(OH)(H2O)(NO3)2]c
[Cu2(L110)(OH)(H2O)(NO3)4]c
[Cu2(L112)(OH)(H2O)(NO3)2]c
[Cu2(Sgly)2(H2O)] 2H2O
[Cu2(D -Sala)2(H2O)] H2O
[Cu2(L -Sala)2(H2O)] H2O
Cu2(D,L-Sala)2(H2O)2] 2H2O
[Cu2(Sab2)2(H2O)2]
[Cu2(Sbal)2(H2O)2]
[Cu2(Sab4)2(H2O)2] 0.5H2O
[Cu2(Sval)2(H2O)3]
[Cu2(Shis)2(H2O)] H2O
[Cu2(Styr)2(H2O)] 2H2O
[Cu2(Stryp)2(H2O)]
[NiL1(H2O)3]I2$H2Oa
[NiL1(H2O)3]Br2$H2Oa
[MnL1(OOCH)(OH2)]e
Ni
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Zn
Zn
Zn
Zn
[MnL2(OH2)2][Mn2(L22(NO2)3]e
[Mn2L2(NO2)2]e
[MnL2Cl$4H2O]f
[MnL3Cl$4H2O]f
[MnL4Cl$4H2O]f
[Mn2IV(LA1)2(L1ꢃ)]g
[MnIV (LA2)(L2ꢃ)2]g
[MnIV(LA3)(L3ꢃ)2]g
[MnIV(LA4)(L4ꢃ)2]g
[MnIV(LA5)(L5ꢃ)2]g
[MnIV(LA6)(L6ꢃ)2]g
[MnIV(LA7)(L7ꢃ)2]g
[Mn(HL)(H2O)3](NO3)2$(H2O)]h
[Mn(HL)(SCN)2(H2O)]$0.5H2O]h
[Mn(HL)(N(CN)2)(H2O)2](NO3)$H2O]h
[Zn2(H2L1)(OH)(H2O)(NO3)](NO3)3]d
[Zn2L2Cl3]d
[Zn2L3Cl3]d
[Zn2(L4)2(CH3COO)2]d
L1 ¼ 2-[(2-piperazin-1-ylethylimino)-methyl]phenol b L22 ¼ 2,6-bis(R-iminomethyl)-4-methyl-phenolato, R ¼ N-ethylpiperazine; L14 ¼ 2-formyl-4-methyl-6R-iminomethyl-
phenolato, R ¼ N-ethylmorpholine; L11 ¼ 2-formyl-4-methyl-6R-iminomethyl-phenolato. R ¼ N-propylmorpholine; L23 ¼ 2,6-bis(R-iminomethyl)-4-methyl-phenolato,
c
R ¼ N-ethylpyrrolidine; L21 ¼ 2,6-bis(R-iminomethyl)-4-methyl-phenolato; R ¼ N-propylmorpholine. L11 ¼ 2,6-bis(R-iminomethyl)-4-chloro-phenolato, R ¼ N-ethyl-
piperidine; L12 ¼ 2,6-bis(R-iminomethyl)-4-t-butyl-phenolato, R ¼ N-ethylpiperidine; L21 ¼ 2-formyl-4-methyl-6R-iminomethyl-phenolato, R ¼ N-ethylpiperidine;
L13 ¼ 2,6-bis(R-iminomethyl)-4-chloro-phenolato, R ¼ N-ethylmorpholine; L14 ¼ 2,6-bis(R-iminomethyl)-4-t-butyl-phenolato, R ¼ N-ethylmorpholine; L15 ¼ 2,6-bis(R-
iminomethyl)-4-chloro-phenolato, R ¼ N-propylmorpholine; L16 ¼ 2,6-bis(R-iminomethyl)-4-t-butyl-phenolato, R ¼ N-propylmorpholine; L17 ¼ 2,6-bis(R-iminomethyl)-4-
chloro-phenolato, R ¼ N-ethylpyrrolidine; L19 ¼ 2,6-bis(R-iminomethyl)-4-t-butyl-phenolato, R ¼ N-ethylpyrrolidine; L110 ¼ 2,6-bis(R-iminomethyl)-4-chloro-phenolato,
d
R ¼ N-ethylpiperazine; L112 ¼ 2,6-bis(R-iminomethyl)-4-t-butyl-phenolato, R ¼ N-ethylpiperazine. L1 ¼ 2,6-bis(R-iminomethyl)-4-methyl-phenolato, R ¼ N-ethyl-
piperazine; L2 ¼ 2,6-bis(R-iminomethyl)-4-methyl-phenolato, R ¼ N-ethylpyridine; L3 ¼ 2,6-bis(R-iminomethyl)-4-methyl-phenolato, R ¼ N-ethylpyrrolidine; L4 ¼ 2,6-
e
bis(R-iminomethyl)-4-methyl-phenolato, R ¼ benzylamine. L1 ¼ 2,7-bis(2-hydroxyphenyl)-2,6-diazaocta-2,6-diene; L2 ¼ 1,7-bis(2-hydroxyphenyl)-2,6-diazahepta-1,6-
f
diene).
L2
¼
N,N0-1-methylethylenebis(3-formyl-5-methylsalicylaldimine); L3
¼
N,N0-1,1-dimethylethylenebis(3-formyl-5-methylsalicylaldimine); L4
¼
N,N0-cyclo-
hexenebis(3-formyl-5-methylsalicylaldimine). L1 ¼ 1,3-bis(4,6-di-tert-butyl-2-iminophenol)benzene; L2 ¼ 2-anilino-4,6-di-tert-butylphenol; L3 ¼ 2-(3,5-di t-butyl-ani-
g
lino)-4,6-di-tert-butylphenol; L4 ¼ 2-(3,5-di-trifluromethane-anilino)-4,6-di-tert-butylphenol; L5 ¼ 2-(3,5-di methyl-anilino)-4,6-di-tert-butylphenol; L6 ¼ 2-(3,5-di chloro-
anilino)-4,6-di-tert-butylphenol; L7 ¼ 2-(3,5-di methoxy-anilino)-4,6-di-tert-butylphenol. HL ¼ 2,6-bis{2-(N-ethyl)pyridineiminomethyl}-4-methylphenolato.
h

B.K. Biswas et al. / Journal of Molecular Structure 1217 (2020) 128398
11
O
II
But
O
Catechol
O
OH2
Cu
N
N
Cu
O
N
N
OCOCCl / Cl
3
H2O + OCOCCl3 / Cl-
-
B
But
A
HO
OH
But
+ H2O2
But
I
Cu
But
O
O
H2O +
CCl3COO- / Cl-
_
N
N
O
But
But
O
O
Cu
_
C
N
N
O
O
But
O
O2
D
Scheme 4. A plausible mechanistic pathway for aerobic oxidation of catechol by complex 1 or complex 2.
Fig. 15. MEP surface plots (isosurface 0.001 a.u.) of compounds 1 (a) and 2 (b). The MEP values at selected points of the surface are indicated in kcal/mol.
halogen bonding interaction.
s for a low value of density (
coloured using red-yellow-red-blue color code according to values
of , where yellow and red correspond to weak and strongly
r). In addition, the isosurfaces are
Fig. 16b and c shows two self-assembled dimers observed in the
solid state of compound 2. In the dimer involving Cu2 complex
(Fig. 16 b), the symmetrically equivalent halogen bonds implicate
the chlorido ligand (CeI/Cl) and in the dimer involving Cu1, the
carboxylate instead of Cl participates in the halogen bonds
(CeI/O). The later are less directional than the former, which is
likely related with the stronger interaction obtained for the CeI/Cl
r
repulsive interactions, respectively, and green and blue are used for
weak and strongly attractive interactions, respectively. For a more
detailed description of NCI, see theoretical methods. The NCI plot of
compound 1 is given in Fig. 16d. It can be observed that each
halogen bond is characterized by a green (attractive) isosurface
located between the non-coordinated O-atom of trichloroacetate
and the Cleatom belonging to the trichloroacetate of the neigh-
boring molecule. The NCI plot also shows a green surface between
the O-atom os the carboxylate group and the H-atoms of the co-
ordinated water molecule, thus confirming the existence of the
long H-bond. The NCI plot of the self-assembled dimers of com-
pound 2 are represented in Fig. 16e and f. In both cases the
bonds
(
D
E3
¼ ꢀ10.2 kcal/mol) compared to CeI/O bonds
(D
E3 ¼ ꢀ6.0 kcal/mol).In order to further characterize the halogen
bonding interactions, we have used the NCIplot index. This index
and its associated isosurfaces allow to identify which regions of a
supramolecular complex interact easily. The non-covalent contacts
are revealed with the regions of small reduced density gradient (s)
at low densities. These regions are plotted by using an isosurface of

12
B.K. Biswas et al. / Journal of Molecular Structure 1217 (2020) 128398
Fig. 16. (a) Dimer of compound 1 retrieved from the X-ray structure. (b) X-ray structure of the self-assembled dimer of compound 2 corresponding to the Cu2 complex. (c) X-ray
structure of the self-assembled dimer of compound 2 corresponding to the Cu1 complex. The interactions energies at the B3LYP-D3/def2-TZVP level of theory are also indicated.
Distances in Å. (d-f) NCI surfaces of the halogen bonding complexes in 1 (d) and 2 (e,f). The gradient cut-off is s ¼ 0.35 au, and the color scale is ꢀ0.04 <
r < 0.04 au.
existence of the halogen bonds is confirmed by the presence of
small and green isosurfaces located between the I atoms and either
the Cl (Fig.16e) or O (Fig.16f). Moreover, in both dimers the NC Iplot
reveals the existence of other interactions that also contribute to
the formation of the dimers. Remarkably, in the dimer of Cu2
complex (Fig. 16e), the belt of the I-atom interacts with the C-atom
CRediT authorship contribution statement
Barun Kumar Biswas: Conceptualization, Methodology. San-
deepta Saha: Writing - original draft. Niladri Biswas: Software,
Validation, Data curation. Manas Chowdhury: Data curation,
Writing - review & editing. Antonio Frontera: Formal analysis,
Software, Validation. Corrado Rizzoli: Visualization, Investigation,
Software. Ruma Roy Choudhury: Writing - review & editing.
Chirantan Roy Choudhury: Supervision, Project administration.
of the coordinated carboxylate group, thus establishing a
pehole
interaction. In the dimer of Cu1 complex (Fig. 16e, the belt of I in-
teracts with an aromatic H-atom located between the I and
carboxylate substituents of the ring.
Acknowledgements
4. Concluding remarks
C. Roy Choudhury acknowledge DST- FIST (Project No. SR/FST/
CSI-246/2012) New Delhi, Govt. of India for instrumental support
under capital heads.
We have synthesized and X-ray characterized two new Cu(II)
complexes with Schiff base ligands derived from anthranilic acid
and 4-iodo-anthranilic acid. Both compounds exhibit catecholase
activity and we have shown their effectiveness towards the aerial
oxidation of 3,5-di-tert-butyl catechol to the corresponding quinine
in both DMSO and THF medium. Interestingly, both compounds
form dimers in the solid state governed by halogen bonding in-
teractions, which have been characterized energetically (ranging
from ꢀ3 to ꢀ5 kcal/mol) by DFT calculations and also using the NCI
plot computational tool.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molstruc.2020.128398.
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