[Diaquo{bis(p-hydroxybenzoato-κ1O1)}(1-methylimidazole- κ1N1)}copper(II)]: Synthesis, crystal structure, catalytic activity and DFT study

Article abstract of DOI:10.1016/j.molstruc.2021.131323

Metal-organic hybrid complexes often exhibit large surface area, pore volume, fascinating structures and potential applications including catalytic applications. Hence a new metal-organic hybrid complex [Diaquo{bis(p-hydroxybenzoato-κ¹O¹)}(1-methylimidazole- κ¹N¹)}copper(II)] was synthesized using conventional method. Physico-chemical characterization of the complex was performed with FTIR spectroscopy, single crystal X-ray diffraction, TGA, EPR and FESEM. Single crystal X-ray diffraction study suggests it to be three dimensional with space group P2₁2₁2₁ (orthorhombic). The crystal achieves its three-dimensional structure and stability through extensive intermolecular hydrogen bonding. Hirshfeld surface analysis, catalytic activity and DFT study of the complex was also performed. The synthesized complex acts as good catalyst in benzimidazole synthesis with good recyclability as catalyst up to 5th run.

Full text of DOI:10.1016/j.molstruc.2021.131323

Journal of Molecular Structure 1247 (2022) 131323
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
1
1
[
Diaquo{bis(p-hydroxybenzoato-κ O )}(1-methylimidazole-
1
1
κ N )}copper(II)]: Synthesis, crystal structure, catalytic activity and
DFT study
a
b
b
c
a,∗
Amarjit Kamath , Dhiraj Brahman , Sailesh Chhetri , Patrick McArdle , Biswajit Sinha
a
Department of Chemistry, University of North Bengal, Darjeeling 734013, India
b
Department of Chemistry, St. Joseph’s College, Darjeeling 734101, India
c
Crystallography Centre, National University of Ireland, Galway, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history:
Metal-organic hybrid complexes often exhibit large surface area, pore volume, fascinating structures
and potential applications including catalytic applications. Hence a new metal-organic hybrid complex
Received 27 June 2021
Revised 11 August 2021
Accepted 15 August 2021
Available online 18 August 2021
1
1
1
1
[
Diaquo{bis(p-hydroxybenzoato-κ O )}(1-methylimidazole- κ N )}copper(II)] was synthesized using con-
ventional method. Physico-chemical characterization of the complex was performed with FTIR spec-
troscopy, single crystal X-ray diffraction, TGA, EPR and FESEM. Single crystal X-ray diffraction study sug-
gests it to be three dimensional with space group P212121 (orthorhombic). The crystal achieves its three-
dimensional structure and stability through extensive intermolecular hydrogen bonding. Hirshfeld surface
analysis, catalytic activity and DFT study of the complex was also performed. The synthesized complex
acts as good catalyst in benzimidazole synthesis with good recyclability as catalyst up to 5th run.
Keywords:
Single crystal
DFT
Hirshfeld
1
-methylimidazole
©
2021 Elsevier B.V. All rights reserved.
Benzimidazole
Catalysis
1
. Introduction
Amongst the transition metals, copper is probably the most fa-
vorite element for designing and synthesis of metal-organic hybrid
materials; since its ions [Cu(I) and Cu(II)] in combination with var-
ious organic linkers can form fascinating network structures and
packing motifs [18–21]. Again the architecture of low dimensional
metal organic framework can easily be formed by various benzoic
acid derivatives, since these derivatives can serve both as hydro-
gen bond donor and hydrogen bond acceptor for the construction
of such frameworks [22,23]. One of the key features of the organic
carboxylate ligand is that they can bind a metal ion through vari-
ous coordination modes and this unique feature of the carboxylate
ligands have been employed to prepare a large number of coor-
dination polymers of different dimensionality and different prop-
erties [24,25]. Moreover, many metal organic hybrid complexes of
Cu(II) ion containing derivatives of benzoic acid have been re-
ported in the literature, where the substituent were halogen, ni-
tro, hydroxy and cyano groups [26,27]. Therefore, herein this pa-
per the synthesis, crystal structure, Hirshfeld surface analysis, cat-
alytic activity and DFT study of a new Cu(II) complex derived from
4-hydroxybenzoic acid and 1-methylimidazole using conventional
method has been reported.
Functional hybrid materials such as metal organic framework by
virtue of their applications as catalyst, optical material, gas storage,
separations, magnetism and chemical sensors have gained interest
worldwide [1–9]. One of the advantages of this type of functional
hybrid material is due to the presence of both the tunable inor-
ganic and organic component. Thus by varying the organic and in-
organic components in such materials, the diversity in the prop-
erties of such material can be tuned for their potential uses in
many fields [10]. In last few decades a variety of flexible organic
linkers consisting of carboxylate and heterocyclic tethers such as
pyridine, imidazole, 1,10-phenanthroline, etc, have shown the most
significant development in the field of synthesis of tunable metal
organic framework and supramolecular structure [11–14]. Among
these organic linkers, carboxylate ligands have great importance
in the construction of coordination architectures because such lig-
and possesses special metal coordinating ability, versatile coordina-
tion modes and also the diverse topologies [15–17]. On the other
hand, copper is one of the essential biological elements found in a
number of enzymes which catalyzes certain biological processes.
∗
Corresponding author.
E-mail address: biswachem@gmail.com (B. Sinha).
https://doi.org/10.1016/j.molstruc.2021.131323
0
022-2860/© 2021 Elsevier B.V. All rights reserved.

A. Kamath, D. Brahman, S. Chhetri et al.
Journal of Molecular Structure 1247 (2022) 131323
Table 1
2. Experimental section
Crystal data collection and structure refinement for copper complex 1.
2
.1. Materials and methods
Crystal data
Chemical formula
Mr
[Cu(C18 H20 N2 O8).2H2O]
491.93
Copper nitrate [Cu(NO ) ·3H O, 99%], 4-hydroxy benzoic acid
3
2
2
Crystal system, space group
Temperature (K)
Colour, habit
Orthorhombic, P212121
(
99%), 1-methylimidazole (98.5%) and triply distilled de-ionized
298.8 (7)
water (with specific conductance < 1·10− S cm
6
−1
at 25 °C)
Blue, Parallelepiped
were used in this synthesis. All these chemicals were of analyti-
cal reagent (A. R.) grade and used without further purification. All
the solvents used were of spectroscopic grade. Elemental micro-
analysis of the complex (C, H and N) was carried out using Perkin–
Elmer (Model 240C) analyzer. Cu-content in the complex was de-
termined using AAS (Varian, SpectrAA 50B), calibrated with stan-
dard Cu-solution (Sigma-Aldrich, Germany). IR spectrum was ob-
tained from a Perkin-Elmer FT-IR spectrometer (RX-1) in the re-
Size, mm
a, b, c ( A˚ )
0.50 × 0.40 × 0.20
7.0603 (3), 12.1658 (5), 24.2757 (8)
α, β, γ (°)
90
V ( A˚ 3
)
2085.14 (14)
4
Z
Radiation type
Density (calculated), Mg/m3
Mo Kα
1.567
−
1
Absorption coefficient, mm
Data collection
Diffractometer
1.106
xcalibur, sapphire3
0.608, 0.809
29.271, 3.453
-9→7
−1
gion 4000 to 400 cm
at ambient temperature. The sample was
Tmin, Tmax
diluted with IR grade KBr (Sigma-Aldrich, Germany) and pressed
into a pellet. EPR data of the single crystal of the complex in the
X-band was obtained from EPR magnetometer (JEOL, JES-X3 FA200)
for the field range of -10 to 990 mT. The mass spectra of the syn-
thesized compounds have been analyzed using WATERS Q-TOF Pre-
mier HAB-213 mass spectrophotometer in methanol solvent. Single
crystal X-ray diffraction study was performed using Xcalibur, sap-
phire3 (manufactured by Oxford diffraction, Poland) equipped with
CCD camera. X-ray powder diffraction (PXRD) study has been car-
ried out in analytical X’pert powder X-ray diffractometer using Cu-
Kα radiation of wavelength 1.54 Ǻ and the data were recorded for
the 2θ range varied from 5° to 90° at room temperature. The melt-
ing point of the synthesized compounds was determined by open
capillary method. Thermal stability of the complex was studied by
thermogravimetric (TGA) and differential thermogravimetry (DTG)
analysis. The thermal data were recorded using TGA instrument
θ
h
k
l
max,θmin
-16 →15
-25 →32
6703
Reflections collected
Unique reflections
4513
Observed reflections (>2σ(I))
Rint
4110
0.0363
Refinement
Refinement method
Data/restraints/parameters
Final R indices [I>2σ(I)]
Full-matrix least-squares on F2
451 /12/308
R1 = 0.0363, wR2 = 0.0859
[Cu(C18 H20N O ).2H O]: C 43.94, H 4.92, N 5.69, Cu 19.91; found:
2
8
2
C 43.77, H 4.69, N 5.55, Cu 12.96.
2.3. X-ray crystallographic measurements
(
Q500 V20.10 Build 36) in N2 atmosphere with an initial weight of
the complex (6.619 mg) subjected to a heating rate of 10 °C min 1.
The electronic spectra were recorded with a JascoV-530 double
beam Spectrophotometer in methanol. 1H-NMR spectra of the syn-
thesized benzimidazole derivatives (1–18) were recorded at room
temperature on a FT-NMR (Bruker Avance-II 400 MHz) spectrome-
ter by using DMSO-d6 as solvents and chemical shifts are quoted in
ppm downfield of internal standard tetramethylsilane (TMS). The
morphology of the crystal was studied using Field Emission Scan-
ning Electron Microscopy (FESEM, INSPECT F50, FEI, The Nether-
land). The magnetic moment measurements of the copper com-
plex were carried out on Sherwood Scientific magnetic suscepti-
−
Crystal data collection and structure refinement are summa-
rized in Table 1. Data collection of the synthesized copper(II) com-
plex were carried out on an Oxford Diffraction Xcalibur 3/Sap-
phire3 CCD diffractometer (manufactured by Oxford diffraction,
Poland) equipped with CCD (with Mo Kα, λ = 0.71073 Ǻ) at
299 K using ω scan technique. Data refinement and reduction
were carried out using CrysAlisPRO(Agilent Technologies, version
1,171.37.35, 2014). The crystal system was determined by Laue sym-
metry and the space groups was assigned on the basis of sys-
tematic absences using XPREP. The structure was solved by direct
method using the SHELXS program of the SHELXTL package and
refined on F2 by full-matrix least-squares methods with SHELXL-
bility balance (UK) using Hg[Co(NCS) ] as the calibrant.
4
2
014 crystallographic software package [28]. All the non-hydrogen
1
1
2
.2. Synthesis of [Diaquo{bis(p-hydroxybenzoato-κ O )}(1-
atoms were anisotropically refined and all hydrogen atoms were
placed in the calculated positions and refined isotropically with
a rigid model. The hydrogen atoms bound to water molecules
were found via Fourier difference map and these hydrogen atoms
were then restrained at a fixed positions and refined isotropically.
The crystal of copper (II) complex was twinned (inversion); its
twin law was found via TWIN/BASF instructions included in the
SHELXL2014 instruction file [28]. All calculations and molecular
graphics were carried out with the SHELXL-2014/7 PC programme
package. The data collection, structure refinement parameters and
crystallographic data for the complex is given in Table 1. Some se-
lected bond lengths, bond angles and non-covalent interaction pa-
rameters (H-bond) are presented in Tables 2 and 3, respectively.
1
1
methylimidazole- κ N )]copper(II)] dry CH OH
3
To a 50 mL round bottom flask containing a solution of 4-
hydroxy benzoic acid (0.277 g, 2 mmol) in dry methanol (20 mL),
copper nitrate (0.2417 g, 1 mmol) was added. The light blue so-
lution was then stirred at room temperature for 30 min. To this
solution 1-methyl imidazole (0.082 g, 1 mmol) was added drop-
wise. The color of the solution changed from light blue to dark
blue. To this dark blue solution, few drops of triethyl amine were
added and then refluxed for 4 h at 160 °C. The mother liquor
was then transferred in 50 mL beaker and allowed to stand at
room temperature. After few days the compound was obtained as
a blue crystal. The compound was then collected by filtration and
washed repeatedly with triply distilled de-ionized water followed
by diethyl ether. Single crystals suitable for X-ray diffraction were
collected by hand picking under a microscope (40x). The synthe-
sized copper complex was soluble in most of the polar solvent ex-
2.4. Catalytic activity for benzimidazole synthesis
In
a typical procedure a mixture of 1,2-phenylenediamine
(1.0 mmol), substituted benzaldehyde (1.0 mmol) and TLC grade
silica gel (silica gel G) were thoroughly grinded and mixed with
the help of motor and pastle to make a homogenous mixture. The
cept water and it did not melt up to 280 °C. Yield: 0.383 g, 78%
based on Cu), ESI-MS 491.95 (M ). Elemental analysis calcd (%) for
+
(
2

A. Kamath, D. Brahman, S. Chhetri et al.
Journal of Molecular Structure 1247 (2022) 131323
Table 2
Selected bond lengths ( A˚ )andbond angles (°) for (1).
Cu1–O7
1.956 (3)
1.961 (2)
1.961 (2)
1.966 (3)
2.351 (4)
1.328 (6)
1.385 (7)
1.355 (4)
1.494 (5)
88.96 (12)
88.89 (12)
174.44 (11)
167.72 (14)
91.06 (13)
92.13 (13)
O1–C7
O2–C7
O4–C14
O5–C14
C15–N1
C17–N1
C4–O3
C1–C7
1.279 (5)
1.243 (5)
1.274 (5)
1.245 (5)
1.317 (6)
1.361 (6)
1.362 (4)
1.496 (5)
Cu1–O1
Cu1–O4
Cu1–N1
Cu1–O8
C15–N2
C16–N2
C11–O6
C8–C14
O7–Cu1–O1
O7–Cu1–O4
O1–Cu1–O4
O7–Cu1–N1
O1–Cu1–N1
O4–Cu1–N1
O4–Cu1–O8
N1–Cu1–O8
Cu1–O7H1O7
Cu1–O7H2O7
O7–Cu1–O8
O1–Cu1–O8
87.67 (13)
95.78 (15)
128 (3)
Fig 1. Coordination geometry of Cu(II) ion in the complex. Color scheme: green, Cu;
red, O; yellow, S; blue, N; grey, C and sky blue, H.
117 (4)
96.48 (14)
87.47 (13)
Table 3
Hydrogen bonded geometries in (1).
D - H···A
D - H
H···A
D···A
D - H···A
154.4
i
O3–H1–O3···O10
0.82
1.99
2.751 (5)
2.685 (6)
2.643 (5)
2.671 (5)
i
O6–H1–O6···O9
0.82
1.92
155.4
ii
O7–H1–O7···O5
0.885 (13)
0.886 (13)
1.764 (16)
1.790 (17)
172 (6)
172 (2)
ii
O7–H2–O7···O2
Symmetry codes: (i) -x+1/2, -y+1, z-1/2, (ii)-x+3/2, -y+1, z-1/2.
mixture was transferred to a 50 mL round bottomed flask. To this
mixture 5 mol% of the title complex was added. The reaction mix-
ture was stirred for 20 min at room temperature. TLC was used to
monitor the progress of the reaction. After completion of the reac-
tion, the reaction mixture was extracted with dry ethylacetate and
filtered. The filtrate was evaporated in vacuum and subsequently
dried to afford crude product which was purified by column chro-
matography using n-hexane/ethylacetate (v/v, 80:20) as eluent to
afford pure benzimidazole derivatives (2a-2s) as shown in Scheme
Fig 2. Crystal packing and hydrogen bonding (green dotted lines) within the frame-
work.
two coordinated water molecule. The asymmetric unit also con-
tains two guest water molecules (Fig. 1). The penta-coordinated
Cu(II) adopts a square pyramidal geometry comprising of two oxy-
gen atoms from the two carboxylate ligand (O1, O4), one nitro-
gen atom from the 1-methyl imidazole (N1) and two oxygen atom
from the two coordinated water molecule (O7, O8). Furthermore,
the bond lengths of the metal atom Cu and the ligands are Cu1-
O1 1.961(2), Cu1-O4 1.961 (3), Cu1-N1 1.966 (3), Cu1-O7 1.956 (3)
and Cu1-O8 2.351 (4), respectively. The bond length of coordinated
water molecule Cu1-O8 at the apical position is 2.351 (4) and it
is larger than the other water molecule Cu1-O7, 1.956 (3) coordi-
nated to the square plane. Thus the water molecule at the api-
cal position could be described as a weakly coordinated as the
other water molecule of the square plane. The three dimensional
packing arrangement of the title compound has been shown in
Fig. 1. The complex forms the supramolecular framework through
intramolecular and intermolecular hydrogen bonding between co-
ordinated water molecules and the guest water molecules as evi-
dent by Fig. 2.
1.
2
.5. Theoretical study
All DFT calculations were carried out using Gaussian 16, Revi-
sion A.03 programme package [29]. The molecular structure of the
representative complexes in the ground state were optimized by
the DFT method using B3LYP hybrid functional [30,31] combined
with LANL2DZ basis sets for ligand and Cu(II) ion, respectively. One
of the prominent features of the LANL2DZ basis set is its inclusion
of relativistic effect that is prerequisite for heavy elements. The to-
tal energy calculations within the DFT framework were carried out
to ascertain the interaction of copper ion with the O and N co-
ordination environment. The optimized structures, vibrational fre-
quencies, electronic spectra, HOMO-LUMO energies, chemical po-
tential (μ), global hardness (η) and global electrophilicity power
3
.2. FTIR spectroscopic study
(
ω), Mulliken charge and MESP [32] properties are the main theo-
The Infra–red spectra of the synthesized compound give valu-
retical assessments that have been emphasized herein this work.
The Hirschfield surface analysis has also been carried out using
crystal explorer 17.5 programme [33] to explore the types of in-
termolecular interactions prevailing in the molecule.
able information about the metal ligand bonding. The FTIR spec-
trum of the synthesized Cu(II) complex has been shown in
Fig. 3. From the spectra, it is evident that the band appearing at
−1
−1
1595 cm
and 1389 cm
can be attributed to the asymmetric
stretching (υas) and symmetric stretching (υs) vibrations of the
(COO-) group, respectively. The large separation ࢞υ = (υas-υs) be-
3. Results and discussion
−1
tween νas(COO-) and νs(COO-) is 206 cm
and it indicates an
3
.1. Structural description of [Cu(C18 H20N O ).2H O]
un-symmetrical monodentate mode of bonding of the carboxylate
group [34,35] in the synthesized complex which in turn substanti-
ated by the single crystal structure of the complex. Again the band
2
8
2
The single crystal X-ray analysis show that the synthesized
complex has molecular formula [Cu(C18 H20N O ).2H O] and crys-
in the region 3459–3222 cm 1 for the complex is attributed to the
−
2
8
2
tallizes in an orthorhombic space group, P2 2 2 with four for-
presence of coordinated and the lattice water molecule. The ap-
1
1 1
−
1
mula unit in the unit cell (Z = 4). The asymmetric unit contains
pearance of medium intensity band at ∼958 cm in the spectrum
one Cu(II) ion, two carboxylate anion, one 1- methyl imidazole and
of the molecule is characteristic to the carboxylate bridging (M–O–
3

A. Kamath, D. Brahman, S. Chhetri et al.
Journal of Molecular Structure 1247 (2022) 131323
of coordinated 4-hydroxy benzoic acid moieties. This stage is ac-
companied by subsequent weight loss of 16% (calcd: 16.7%) 420
to 870 °C. This weight loss is probably due to the loss of coor-
dinated 1-methyl imidazole fragment and finally the sample de-
composes to corresponding metal oxide at about 890 °C. Hence
the complex has an appreciable thermal stability at ambient to
moderately high temperatures. The complex started degrading at
9
0°C and continued to decompose to 890 °C until a metal ox-
ide residue was obtained. So the complex is thermally stable
up to 100 °C, thereafter it suffers stepwise successive thermal
degradation.
3
.5. PXRD and SEM analysis
To check the purity and homogeneity of the bulk product of the
synthesized copper complex, the X-ray powder diffraction studies
were carried out at room temperature. The simulated and experi-
mental X-ray powder diffraction (XRPD) pattern of the synthesized
complex are shown in Fig. 5. From the Fig. 5 it is evident that the
major peak positions of the diffraction pattern of the bulk solid
of Cu(II) complex matched well with that of the simulated pattern
obtained from the single crystal data. This indicates the presence
of mainly one crystalline phase and homogeneity of the synthe-
sized complex. The surface morphology of the synthesized com-
plex has been evaluated using FESEM analysis (Fig. S3). The FESEM
micrograph of the catalyst before and after the reaction (up to 5th)
run is shown in Fig. 6.
Fig 3. FTIR spectrum of the complex.
−1
M) in the complex [36]. The peak appearing at 632 cm indicates
the presence of Cu-N bond [37]. The bands in the region 1282–
−
32 cm 1 are attributed to the CH in plane or out of plane bend-
6
ing, or ring deformation absorptions of the benzene ring, respec-
tively. Again the Infra-red spectra of the complex have been cal-
culated theoretically using B3LYP/LANL2DZ basis sets [30,31]. The
theoretical infra-red spectra of the complex is shown in Fig. S1.
From a perusal of the Figs. 3 and S1 it is evident that the bands
The FESEM micrograph shows that the Copper complex has a
rectangular box like morphology with the average length of the
boxes lies between 90–160 μm. Again the FESEM micrograph of
the catalyst revealed that the surface morphology of the catalyst
has changed from rectangular boxes to a more or less 2 dimen-
sional flat wafer like structures with the length ranging from 15 to
55 μm during the reaction time (up to 5th run) (Fig. 6).
appeared at around 1595 cm 11389 cm , 1282–632 cm , 3459–
−
−1
−1
3
222 cm 1 and around 632 cm
−
−1
in theoretically calculated IR
spectra match well with the experimental spectra and hence this
validates the level of theory used for theoretical calculation.
3
.3. Magnetic moment and EPR analysis
The room temperature magnetic moments of the complex (μeff
)
3.6. DFT study
measured with magnetic susceptibility balance is found to be 1.92
B.M [38,39] and this indicates the presence of a single unpaired
electron in the title Cu(II) complex. The X-band EPR spectrum
The geometrical structure of the complex was optimized at
B3LYP level of theory using LANL2DZ basis set and the opti-
mized geometry of the complex is in consistent with the struc-
ture obtained from single crystal X-ray diffraction study. The
optimized geometry of the Cu(II) complex is shown in Fig. 7.
The Cu(II) ion in the complex is situated at the center of
the square plane which is coordinated by two carboxylate ion
(mono-dentate binding mode), two water molecule and one 1-
methylimidazole molecule. The penta-coordinated Cu(II)complex
adopts a square pyramidal geometry and this is in consistent
with the structure obtained from single crystal X-ray diffraction
study.
(
Fig. S2) of the complex appeared to be slightly broadened with
g⊥= 2.15 and g = 2.215 and this is characteristic of a square
|
|
pyramidal geometry. Also a perusal of the g-values [ge<g⊥<g
|
|
(
2.002 < 2.15 < 2.215)] suggests that d
state and the d9 configuration is (eg) (a1g) (b2g)2(b1g)1. The ‘g’ val-
x2− y2 2
orbital is in the ground
4
ues are related to the axial symmetry and the fact, g >g⊥, sup-
|
|
ports square pyramidal geometry for Cu(II) complex [38] as men-
tioned above. The magnetic moment obtained using s = 1/2 and
g = 2.215 (best fitted curve of EPR data) was also 1.92 B.M (i.e.,
μ
= μ ). All these facts support square planar geometry and
eff
cal
presence of single unpaired electron in dx
in the title complex [38,39].
2−y2
orbital for Cu(II) ion
3
.7. Frontier molecular orbitals
3
.4. Thermogravimetric analysis (TGA)
The synthesized complex is air stable. Thermal stability of the
The Frontier Molecular Orbitals (HOMO and LUMO) can qual-
itatively predict the excitation properties and electron transport
in the studied system, therefore, the parameter HOMO and LUMO
can offer a reasonable prediction of molecular reactivity. Thus the
energies of the HOMO and LUMO orbitals of the Cu(II) complex
have been calculated using density functional theory (DFT). The
energy of HOMO and LUMO orbitals has been found to be -6.31
and -1.03 eV, respectively. Since the energy of HOMO and LUMO
orbitals are negative, it indicates that the studied complex is sta-
ble [40]. Again the energy gap (࢞E) between HOMO and LUMO
of the complex can be correlate with its stability and the energy
gap (࢞E) = (ELUMO-EHOMO) for the studied system is found to be
5.28 eV and this large HOMO-LUMO energy gap reveals that the
complex was studied using thermogravimetric technique in the
temperature range 0–890 °C. The thermogram (shown in Fig. 4)
has mainly three weight loss stages. The two sharp stages followed
by a slow but gradual weight loss stage. The sample shows an
initial mass loss of 16% (calcd: 14.6%) from around 100 to180 °C
due to loss of two coordinated and two non-coordinated water
molecules. The second weight loss stage occurs from 200 to 380 °C
with a sharp weight loss of 57% (calcd: 55.7%). The DTG curve
(
in blue) also well corroborates with the TGA curve (in green)
for the complex. This sharp weight loss is probably due to loss
4

A. Kamath, D. Brahman, S. Chhetri et al.
Journal of Molecular Structure 1247 (2022) 131323
Fig 4. Thermogram of the complex in the temperature range 0–890 °C.
Fig 5. Comparison of experimental with Theoretical PXRD Spectra.
ness (η) and global electrophilicity power (ω). The HOMO-LUMO
orbital energies are directly related to ionization energy (I = -
HOMO
E
) and electron affinity (A = -E
). Similarly, the chemi-
LUMO
cal potential (μ), Global hardness (η) and global electrophilicity
power (ω) are related to HOMO-LUMO orbital energies by the
following relations: chemical potential (μ) = (E
HOMO
+ELUMO)/2,
HOMO
Global hardness (η) = (-E
+E
)/2 and global electrophilic-
LUMO
2
ity power (ω) = μ /2η. Alternatively, chemical potential (μ) and
Global hardness (η) are given by the following relations: μ = -
Fig 6. FESEM micrograph of the catalyst after 1st run, 3rd run and 5th run of the
(
I+A)/2 and η = (I-A)/2, where I and A are the first ionization
reaction.
potential and electron affinity of the chemical species [32,41]. For
the title compound, the values of first ionization potential (I), elec-
tron affinity (A), Chemical potential (μ), Global hardness (η) and
electrophilicity power (ω) are 6.31, 1.03, -3.67, 2.64 and 2.55 eV,
respectively. Since the chemical potential (μ) of the studied com-
plex is negative and it signifies that the compound is stable and
Cu(II) complex is highly stable [40]. However, the DFT based de-
scriptors (HOMO-LUMO orbital energy) could be used in order to
understand the structure and reactivity of molecule by calculating
different properties such as Chemical potential(μ), Global hard-
5

A. Kamath, D. Brahman, S. Chhetri et al.
Journal of Molecular Structure 1247 (2022) 131323
Table 5
Charge distribution calculated by the Mulliken.
Atomic charges
Atomic charges
(Mulliken)
Atoms
(Mulliken)
Atoms
Cu1
O2
0.623
-0.503
-0.496
-0.818
-0.436
-0.440
-0.463
0.357
-0.464
0.358
-0.271
-0.701
-0.101
0.280
-0.320
0.276
-0.329
0.249
0.303
-0.373
0.209
-0.296
0.228
0.282
0.274
C26
H27
C28
H29
C30
C31
H32
C33
H34
C35
C36
H37
C38
H39
C40
H41
C42
H43
H44
H45
H46
H47
H48
H49
-0.229
0.241
-0.372
0.211
0.302
-0.329
0.248
-0.321
0.276
0.272
-0.030
0.304
-0.219
0.265
-0.130
0.280
-0.565
0.240
0.240
0.231
0.453
0.453
0.443
0.378
O3
O4
O5
O6
O7
H8
O9
H10
N11
O12
N13
C14
C15
H16
C17
H18
C19
C20
H21
C22
H23
C24
C25
Fig 7. HOMO-LUMO of the synthesized complex.
Table 4
Physical parameters obtained theoretically for
the complex.
Parameters
Energy (eV)
EHOMO
ELUMO
-6.31
-1.03
5.28
ꢀ
E
Chemical potential (μ)
Global hardness (η)
-3.67
2.64
Electrophilicity power (ω)
2.55
do not decompose spontaneously into its elemental form. The
HOMO-LUMO orbital energies, chemical potential (μ), Global hard-
ness (η) and global electrophilicity power (ω) are summarized in
Table 4.
3
.8. Mulliken charges
Fig 8. Molecular electrostatic potential surface diagram of the Cu(II) complex.
Atomic charge of a molecular system is an important parame-
ter; thus the calculation of atomic charges has an important role in
quantum mechanical calculations. The mulliken charges are a func-
tion of electron density population and thus, the mulliken charges
are usually calculated by determining the electron population of
each atom as defined in the basis function. The charge distribution
on the studied Cu(II) complex has been calculated for the equili-
birium geometry of the studied complex and the value of mulliken
charges of the complex is given in Table 5. From the table it is ev-
From the Fig. 8 it is evident that oxygen atom of the carboxylic
group is the most negative potential region (red), oxygen atom of
the water and hydroxyl group is the region of moderate negative
potential (yellow), while the hydrogen and carbon atoms of the
compound bear positive potentials (blue). The negative (red and
yellow) region of MEP [32], McKinnon et al. [41–43] is related to
the electrophilic reactivity whereas the positive (blue) region cor-
responds to nucleophilic reactivity. From the figure, it shows that
the most reactive part in the molecule is the oxygen atom of the
carboxylic group and the high reactivity of this group may be due
to the more electronegative character of oxygen atom of the car-
boxylic group.
ident that the mulliken charges in the neighborhood of C14 , C19
,
C24, C25, C30 and C35 are more positive and thus this positive value
indicates the direction of delocalization.
3
.9. Molecular electrostatic potential
The Molecular Electrostatic Potential surface diagram is used
3.10. Hirshfeld surface analysis
to understand the reactive behavior of a molecule. The nega-
tive regions of the MESP can be regarded as nucleophilic cen-
ters, whereas the positive regions are potential electrophilic sites
In order to quantify the various intermolecular interactions, Hir-
shfeld surface analysis has been carried out for the synthesized
complex and their associated two-dimensional fingerprint plots
have been used to predict the possible intermolecular interactions.
The descriptor dnorm, which incorporates two factors: (i) de, sig-
nifying the distance of any surface point nearest to the internal
[
32,42]. MESP is an Electron Density dependent property and it is
a very valuable descriptor in understanding sites for electrophilic
and nucleophilic reactions as well as hydrogen bonding interac-
tions [32,43]. Hence in order to predict the reactive sites of elec-
trophilic and nucleophilic attack for the studied Copper (II) com-
plex, MEP at the B3LYP/LANL2DZ optimized geometry was calcu-
lated. The Molecular Electrostatic Potential surface diagram of the
Cu(II) complex is shown in Fig. 8.
atoms, and (ii) d , signifying the distance of the surface point near-
i
est to the exterior atoms and also with the van der Waals radii
of the atoms has been used for mapping the Hirschfeld surface
of a molecule [32,44]. The parameter dnorm is given by a math-
6

A. Kamath, D. Brahman, S. Chhetri et al.
Journal of Molecular Structure 1247 (2022) 131323
Fig 9. Molecular Hirschfeld surface: (a) dnorm, (b) shape index, (c) curvedness and
(
d) O-H/H-O for the complex.
ematical expression as dnorm = (d − r iv dw)/r
vdw
i
+ (de − r ev dw)/r ev dw,
i
vdw
i
vdw
where r
and r
are the van der walls radii of atoms. The pa-
e
Fig 10. 2D Fingerprint plots of the complex.
rameter dnorm may have negative or positive value depending on
the types of intermolecular contacts. If the value is positive, then
the intermolecular contacts are shorter and if the value is positive
the contacts are larger than the van der walls separations [45]. The
descriptor dnorm displays a surface with bright red, white and blue
spots for shortest contact, contact around the van der walls separa-
tion and devoid of close contacts, respectively appear as a primary
interaction in the complex [Fig. 9a]. Crystals with spherical atomic
electron densities, the Hirshfeld surface is unique and it gives an
idea about the intermolecular interactions occurring in the studied
molecular crystals. In order for better visualization of the molec-
ular moiety around which they are calculated, the surfaces have
been shown as transparent. The molecular Hirschfeld surface for
Cu(II); dnorm, shape index and curvedness for the Cu(II) complex
is depicted in Fig. 9, respectively and mapped over dnorm ranges –
˚ ˚
.7184 to 1.7801 A, shape index ranges –1.0000 to 1.0000 A, and
0
Fig. 11. Recyclability of the catalyst.
˚
curvedness ranges –4.0000 to 0.4000 A, respectively. The dnorm
mapping for the studied Cu(II) complex shows a bright red area
in the Hirshfeld surfaces indicating the strong hydrogen bond in-
teractions, such as O–H···O between coordinated/lattice water and
carboxylate oxygen and these hydrogen bond interactions. The in-
termolecular interactions of the Cu(II) complex are shown in the
largest contribution to the Hirshfeld surfaces (42.3%) [32,41–46].
The O…H/H..O interactions are represented by blue spikes on top
and bottom of the pale yellow NH/HN (8.8%) interactions are near
the CH regions while the green CH/HC interactions (34.2%) are be-
tween the N—H and O—H regions.
2
D fingerprint plots shown in Fig. 10. H…H contacts make the
Table 6
Optimization of reaction condition.
Entry
Catalyst (mol%)
Time (mins)
Yield (%) of C
1
2
3
4
5
6
7
8
9
0
60
60
60
45
45
45
30
20
20
30
40
20
42
58
68
72
89
91
96
89
92
93
1
1.5
2
3
4
5
5
7
1
1
0
1
7
7
Reaction conditions: 2-itrobenzaldehyde (1 mmol), orthophenylenediamine (1 mmol),
copper complex (5 ol%) at room temperature.
7

A. Kamath, D. Brahman, S. Chhetri et al.
Journal of Molecular Structure 1247 (2022) 131323
Table 7
The yield of the catalysed products.
3
.11. Catalytic activities
Among the various nitrogen based heterocyclic compounds,
as precursors. Many of the reported methods for the synthesis of
such compound require long reaction time, expensive catalysts and
organic solvents. The recent trend towards development of green
reaction conditions encouraged us to study an efficient and eco-
friendly synthesis of 2-arylbenzimidazoles at room temperature in
solvent free condition using silica gel as a solid medium. Therefore,
in this work an attempt has been made to develop the solvent free
green reaction condition for the synthesis of 2-substituted benz-
imidazole using ortho-phenylenediamine and aromatic aldehyde as
benzimidazole scaffold because of their wide spectrum of biolog-
ical activities are ubiquitous in nature and these are often use-
ful bioactive intermediates for the preparation of pharmacological
and biological active molecules. A good number of literature meth-
ods have been reported for the synthesis of 2-substituted benz-
imidazoles using ortho-phenylenediamines and different aldehydes
8

A. Kamath, D. Brahman, S. Chhetri et al.
Journal of Molecular Structure 1247 (2022) 131323
Table 8
Sailesh Chhetri: Conceptualization, Methodology. Patrick McAr-
dle: Funding acquisition, Methodology, Software. Biswajit Sinha:
Funding acquisition, Investigation, Methodology, Project adminis-
tration, Software, Supervision, Validation, Visualization.
Recyclability of copper catalyst in model reaction.
Acknowledgments
The authors would like to acknowledge Departmental Special
Assistance Scheme under the University Grants Commission, New
Delhi (SAP-DRS-III, No. 540/12/DRS/2013) & University of North
Bengal, Department of Biotechnology, Govt. of West Bengal for
financial and instrumental support. The authors also acknowl-
edge “Centre de Diffractometrie Henri Longchambon” at Universite
Claude Bernard Lyon 1 for Single crystal X-ray diffraction studies.
Entry
No of runs
Isolated yield (%)
1
2
3
4
5
6
7
8
0
1
2
3
4
5
6
7
96
95
93
92
90
88
82
73
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2021.131323.
precursors over silica gel (TLC grade) in open air. In order to op-
timize the catalyst and reaction conditions, 1,2-phenylenediamine
(
1 mmol) and 2-nitrobenzaldehyde (1 mmol) were taken as pre-
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condition as mention above. The spectral data of the synthesized
benzimidazole derivatives are in full agreement with data reported
in the literature. The yield of the compounds (1–18) is given in
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