Copper(II) hydrazone complexes with different nuclearities and geometries: Synthesis, structural characterization, antioxidant SOD activity and antiproliferative properties

Article abstract of DOI:10.1016/j.poly.2020.114624

New copper(II) hydrazone complexes with (Z)-2-(phenyl(2-(pyridin-2-yl)hydrazono)methyl)pyridine (L) were synthesized and characterized using various physicochemical methods. The geometries of the complexes can be classified as mononuclear and binuclear. T

Full text of DOI:10.1016/j.poly.2020.114624

Journal Pre-proofs
Copper(II) hydrazone complexes with different nuclearities and geometries:
Synthesis, structural characterization, antioxidant SOD activity and antiproli‐
ferative properties
A.K. Patel, R.N. Jadeja, H. Roy, R.N. Patel, S.K. Patel, R.J. Butcher, M.
Cortijo, S. Herrero
PII:
S0277-5387(20)30281-3
DOI:
Reference:
https://doi.org/10.1016/j.poly.2020.114624
POLY 114624
To appear in:
Polyhedron
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Revised Date:
Accepted Date:
20 April 2020
17 May 2020
19 May 2020
Please cite this article as: A.K. Patel, R.N. Jadeja, H. Roy, R.N. Patel, S.K. Patel, R.J. Butcher, M. Cortijo, S.
Herrero, Copper(II) hydrazone complexes with different nuclearities and geometries: Synthesis, structural
characterization, antioxidant SOD activity and antiproliferative properties, Polyhedron (2020), doi: https://
doi.org/10.1016/j.poly.2020.114624
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Copper(II) hydrazone complexes with different nuclearities and geometries:
Synthesis, structural characterization, antioxidant SOD activity and
antiproliferative properties
A.K. Patela, R.N. Jadejaa, H. Royb, R.N. Patelc, S.K. Patelc, R.J. Butcherd, M. Cortijoe, S. Herreroe.
aDepartment of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390002, India.
bDepartment of Zoology,Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390002, India.
cDepartment of Chemistry, A.P.S. University, Rewa, M.P 486003, India.
dDepartment of Inorganic & Structural Chemistry, Howard University, Washington, DC 22031, USA.
eDepartamento de Química Inorgánica, Facultad de Ciencias Químicas, Universidad Complutense, 28040, Madrid, Spain.
Abstract
New
copper(II)
hydrazone
complexes
with
(Z)-2-(phenyl(2-(pyridin-2-
yl)hydrazono)methyl)pyridine (L) were synthesized and characterized using various physicochemical
methods. The geometries of the complexes can be classified as mononuclear and binuclear. The
complex 1, [Cu(L)Cl2], is mononuclear whereas the solid-state structure of complex 2 contain a
mixture of co-crystals of the mono- and binuclear complexes 2a, [Cu(L)(H2O)(SO4)], and 2b,
[(L)Cu-(SO4)2-Cu(L)]. The molecular structure of 2 contains two units of the mononuclear complex
2a and two units of the binuclear complex 2b. The copper atoms contained in all the mono- and
binuclear complexes are in a distorted square pyramidal geometry. The present study indicates that
complexes having different nuclearities and geometries can be achieved by changing the synthetic
conditions and methods. Variable temperature magnetic susceptibility measurements of the
complexes have shown the presence of weak anti-ferromagnetic interactions. These interactions are
mediated by intermolecular hydrogen bonding in 1 and through a symmetric sulfate bridge in 2. The
EPR spectra in the polycrystalline state for 1 and 2 exhibited a broad signal at ~2.149 due to spin-
spin interactions between two copper(II) ions. The cyclic voltammograms of complexes 1 and 2 in
DMSO gave two irreversible redox waves. Density functional theory (DFT) calculations were
evaluated in the study, involving the molecular specification with the use of B3LYP/LANL2DZ
formalism for the copper atoms and B3LYP/6-31G for the remaining atoms. Both complexes
catalyzed the dismutation of superoxide ( ∙ - ). Furthermore, the copper complexes and the ligand
O2
were tested to explore their anticancer properties. Promising cytotoxicity of the synthesized
compounds was observed against the selected cancerous cell lines of neuroblastoma, lung carcinoma,
hepatocellular carcinoma and breast cancer.
1

Keywords: Copper(II) complexes, electron paramagnetic resonance (Epr), magnetic measurements,
cyclic voltammograms, biological studies.
1. Introduction
The chemistry of hydrazones and their transition metal complexes is an interesting area of
research due to their wide variety of chemical structures with different physicochemical properties1-4.
They have potential applications in molecular recognition, catalysis and nanomaterials due to their
mild reaction conditions and high synthetic rates for Schiff base complexes5-8. Additionally, copper
and Schiff base complexes have potential biological applications owing to their stability and
biocompatibility9. Copper(II) Schiff base complexes have been receiving considerable attention. The
flexibility of the coordination sphere around the copper(II) ion along with other atoms leads to
structural diversity10-14. Copper(II) complexes provide models for metalloprotein activities and
insight towards the design of new catalysts. Hydrazone Schiff bases are organic compounds with
suitable structural features and can form metal chelates either as a neutral ligand (HL) or in the
mono-anionic form (L-). The coordination possibilities of hydrazones are introduced by using
different substituted aldehydes or ketones, which includes additional donor atoms15-20. Coordination
complexes containing such ligands with transition metals yield monometallic, bis-chelated, bi- and
multi-metallic complexes21. Hydrazones also have continued attention due to their structural diversity
and coordination properties. Recently, the design and application of low molecular weight copper
complexes as superoxide dismutase mimics have received great attention. Several copper(II)
complexes have been designed and characterized as potent models of enzymes, including Schiff base
complexes22.
The present paper is devoted to structural, magneto-chemical and quantum chemical (DFT)
calculations using the (Z)-2-(phenyl(2-(pyridin-2-yl)hydrazone)methyl)pyridine ligand. The extent of
the deprotonation depends on the reaction conditions and the metal23,24. The hydrazone Schiff base
inspired us to investigate the nature of the coordination as well as the structural properties of
copper(II) complexes with (Z)-2-(phenyl(2-(pyridin-2-yl)hydrazono)methyl)pyridine (HL), viz. [Cu
(Cl)2(L)] (1) and [Cu2(μ- SO4)2(L)2] (2). The molecular structures of these complexes were
determined using single-crystal X-ray analysis. Both complexes were further character using other
physicochemical techniques (UV-vis, IR, CV and DPV). These two complexes were also studied
using electron paramagnetic resonance (EPR) spectroscopy. This technique is widely used for the
2

characterization of paramagnetic species25-27. The technique enables important insight into the
description of the chemical environment and binding pattern around the metal center. These
complexes have the catalyzing copper(II) N3Cl2/N3O2 structural motifs as their inner sphere structure
O2∙ -
(Scheme 1). The coordination sites available for the binding of
are shown by single-crystal X-
ray structures. Additionally, the antioxidant superoxide dismutase activity data for both complexes
were collected and compared with reported SOD models. The anticancer activities of the compounds
toward human IMR 32 (neuroblastoma), MCF 7 (breast cancer), HepG2 (hepatocellular carcinoma)
and A549 (lung cells) cancer cell lines have been examined and compared against cisplatin under
identical conditions using an MTT assay.
Scheme 1. Synthesis of complexes 1 and 2 based on the active site structure of Cu-Zn SOD. [A] The
structure of Cu-Zn SOD. [B] Structures of complexes 1 and 2.
2. Experimental
2.1. Materials and instrumentation
3

All solvents were dried and distilled before use following standard procedures28. Reagent
grade chemicals were used throughout and HPLC grade solvents were employed for spectroscopic
studies. The metal salts were purchased from Sigma-Aldrich Chemical Co. Pvt. Ltd. and used as
received.
2.2. Synthesis of the ligand (HL)
The Schiff base ligand HL = (Z)-2-(phenyl(2-(pyridin-2-yl)hydrazono)methyl)pyridine was
synthesized by taking 2-hydrazinopyridine (1.091 g, 10 mmol) in absolute ethanol (50 ml) and then
2-benzoylpyridine (0.78 g, 10 mmol) was added with a few drops of glacial acetic acid as a catalyst.
The synthesis of the ligand is shown in Scheme 2. The resultant mixture was stirred at room
temperature for 30 min and then refluxed at 75 °C for 3 h. The yellowish solution was filtered and the
filtrate was kept for slow evaporation at room temperature to yield a light yellow polycrystalline
sample. The Schiff base was washed with ethanol and dried over fused CaCl2. Yield 1.10g (75%).
Anal. Calc. for C17H14N4 (274.33): C, 74.52; H, 5.54; N, 20.04 %. Found: C, 74.43; H, 5.14; N, 20.42
1
%. FT-IR (KBr, cm−1): 1592 υ(C=N). H NMR (CDCl3) , ppm: 8.00-7.29 (s, 3H, CH
benzylidenimine), 8.861–7.89 (s, 8H, CH 2-pyridine), and 13.0 (s, 1H, NH hydrazide). The 1H-NMR
spectrum is presented in (Fig. S1).
Scheme 2. Synthetic route to the ligand HL.
4

2.3. Synthesis of the complex [Cu(Cl)2(L)] (1)
The Schiff base ligand (0.274 g, 1.0 mmol) was dissolved in methanol (20 mL). A solution of
CuCl2.2H2O (0.134 g, 1.0 mmol) in methanol (20 mL) was added dropwise to the above solution,
followed by stirring for 5 h to give a green clear solution. The resulting solution was filtered. The
filtrate was left for slow evaporation at room temperature. Plate-like crystals were formed from the
solution two weeks later. These crystals were washed with hot distilled water and then ethanol to
remove impurities. The crystals were dried under vacuum. Yield 0.380 g (80%). Anal. Calc. for
C17H15Cl2CuN4 (408.77): C, 49.80; H, 3.65; N, 13.61 %. Found: C, 49.83; H, 3.69; N, 13.67 %. FT-
IR (KBr, cm−1): 1564 υ(C=N), 417 (s) υ(Cu–N).
2.4. Synthesis of the complex [Cu2(μ-SO4)2(L)2] (2)
The Schiff base ligand (0.274 g, 1.0 mmol) was dissolved in methanol (20 mL). A solution of
CuSO4.5H2O (0.159 g, 1.0 mmol) in methanol (20 mL) was added dropwise to the above solution,
followed by stirring for 5 h to give a green clear solution. The resulting solution was filtered. The
filtrate was left for slow evaporation at room temperature. Plate-like crystals were formed from the
solution two weeks later. These crystals were washed with hot distilled water and then ethanol to
remove impurities. The crystals were dried under vacuum. Yield 0.380 g (80%). Anal. Calc. for
C68H80Cu4N16O28S4 (1951.88): C, 46.82; H, 3.75; N, 12.35 %. Found: C, 46.95; H, 3.48; N, 12.88 %.
FT-IR (KBr, cm−1): 1568 υ(C=N), 462 (s) υ(Cu-O), 417 (s) υ(Cu–N).
2.5. Methods and physical measurements
Elemental analyses were carried out using an Elementar Vario EL III Carlo Erba 1108
1
Analyzer. The H NMR spectrum of the ligand was recorded in CDCl3 on a Bruker Advance 400
(FT-NMR) multinuclear spectrometer. Chemical shifts are reported in parts per million (ppm) using
tetramethylsilane (TMS) as an internal standard. The accelerating voltage was 10 kV and the spectra
were recorded at room temperature. UV-vis spectra were recorded at room temperature using a
Shimadzu UV-vis spectrophotometer UV-1601 in quartz cells. Infrared (IR) spectra (4000-400 cm-1)
were collected using the KBr pellet technique on a Perkin-Elmer spectrophotometer. The low and
room temperature electron paramagnetic resonance (EPR) spectra were recorded using a Varian E-
line Century Series spectrometer equipped with a dual cavity and operating at the X-band with 100
kHz modulation frequency. Varian quartz tubes were used for measuring EPR spectra of
5

polycrystalline samples and frozen solutions. The EPR spectra were calibrated with
tetracyanoethylene (TCNE) as a marker (g = 2.00277). The EPR parameters for the copper(II)
complexes were determined accurately from the computer simulation program19. The mass spectrum
of the ligand was recorded on a Trace GC ultra DSQ II. Cyclic voltammetry was performed using a
BAS-100 Epsilon Electrochemical Analyzer on the complexes in DMSO solutions using Ag/AgCl
and glassy carbon as reference electrodes. All measurements were carried out at room temperature
under a nitrogen atmosphere. The concentrations of the solutions were 10-3 mol dm-3 with respect to
the complexes and 0.1 mol dm-3 with respect to tetrabutylammonium perchlorate (TBAP) as a
supporting electrolyte. Ferrocene (Fe) was added to the solution as an internal standard. Magnetic
measurements at variable temperatures were performed on crushed single crystals of the complexes
using a Quantum Design MPMSXL SQUID (Superconducting Quantum Interference Device)
magnetometer. The measurements were performed over the temperature range 2-300 K under a
magnetic field of 0.5 T. The data were corrected for the intrinsic diamagnetic contributions based on
Pascal’s constants, the Temperature-Independent Paramagnetism (TIP) and the sample holder
contribution. X-ray crystallographic data of the complexes were collected on a Bruker APEX-II
diffractometer using graphite monochromated MoKα radiation (λ = 0.71073 Å). The crystal
orientation, cell refinement and intensity measurements were made using CAD-4PC performing C-
scan measurements. The structures were solved by direct methods using SHELXS-97 and refined by
full-matrix least-squares using SHELXL-9729-32. Crystallographic data of the complexes were
collected on a Rigaku-Oxford Diffraction Gemini Eos diffractometer using graphite monochromated
CuKα radiation (λ = 1.54184 Å) for L and graphite monochromated MoKα radiation for 1 and 2,
respectively. For both systems, all non-hydrogen atoms were refined anisotropically and all hydrogen
atoms were geometrically fixed in calculated positions. Crystals suitable for single-crystal X-ray
analysis of both complexes were grown by slow evaporation of the reaction mixtures at room
temperature. Single crystals of suitable dimensions for single-crystal X-ray analysis were mounted on
glass fibers for the Bruker instrument and polymer loops for the Gemini instrument and used for data
collection. A simultaneous TGA was performed using a TG-DTA 6300 INCARP EXSTAR 6000 at a
heating rate of 10 °C/min in the temperature range 25-600 °C with a nitrogen atmosphere maintained
throughout the measurement.
2.6. Computational study
6

Theoretical calculations by density functional theory (DFT) were performed with regard to
molecular structure optimization and HOMO-LUMO energies etc. of complexes 1 and 2. Full
geometry optimizations were carried out using the density functional theory (DFT) method at the
B3LYP level for the complexes33. All DFT calculations were carried out starting from the
experimental single-crystal X-ray data for the input geometries. All elements except Cu were
assigned the LANL2DZ basis set34. LANL2DZ with an effective core potential was used for the Cu
atom35. In the computational model, the cationic complex was taken into account. All calculations
were carried out with the GAUSSIAN09 program,36 with the aid of the Gauss View visualization
program. Vertical electronic excitations based on B3LYP optimized geometries were evaluated using
the time-dependent density functional theory (TDDFT) formalism37 in DMSO, using a conductor-like
polarizable continuum model (CPCM)38.
2.7. Antioxidant SOD activity
The antioxidant SOD activities of complexes 1 and 2 were assessed using alkaline DMSO as
a source of superoxide radical ( ∙ ― ) and nitro blue tetrazolium chloride (NBT) as a scavenger39-41
.
O2
∙ ―
The quantitative reduction of NBT to formazan by
was followed spectrophotometrically at 450
O2
nm. The SOD activity was obtained by plotting the percentage inhibition of NBT reduction (%) vs
the concentrations of the complexes. The unit of SOD activity is the concentration of the enzyme or
complex that induces a 50% inhibition (IC50) in the reduction of NBT. Two assays were carried out
for each concentration of the metal complex.
2.8. Anti-cancer activity
The antiproliferative properties of the present complexes were studied in vitro on various
carcinoma cell lines.
2.8.1. Cell culture conditions
For the cytotoxicity determination, we used four different human cancer cell lines, viz. IMR
32 (neuroblastoma), MCF 7 (breast cancer), HepG2 (hepatocellular carcinoma) and A549 (lung
cells), which were procured from the National Center for Cell Science (NCCS), Pune. Cells were
cultured in Dulbecco’s Modified Eagle Medium (DMEM) supplemented with 10% FBS and 1%
antibiotics. All the cell lines were maintained at a temperature of 37 °C with 100% relative humidity
and 5% CO2.
7

2.8.2. Cytotoxicity assay
Cell viability was measured using an MTT assay (MTT = 3-(4,5-dimethyl-2-thiazolyl)-2,5-
diphenyl-2H-tetrazolium bromide). Cells were seeded in 96 well plates at a density of 8000 cells/well
and incubated overnight for cell adhesion. Cells were treated with the synthesized compound for 24 h
and incubated at 37 °C temperature in a CO2 incubator. Cisplatin was used as a reference drug. After
the incubation time, the cells were washed with PBS and exposed to MTT (5 mg/ml) for 4 h in the
dark at 37 °C. The medium with MTT was removed and formed formazan crystal was dissolved in
200 ml of DMSO. At the end of the reaction, the optical density was measured at 540 nm using an
ELISA plate reader (ELX800 Universal Microplate Reader, USA). The % inhibition concentration
was calculated using the formula: % inhibition concentration = 100 _ (OD of treated/OD of Control)×
100.
3. Results and discussion
The copper(II) complexes, having the general composition [Cu(Cl)2(L)] 1 and [Cu2(μ-
SO4)2(L)2] 2, were synthesized by a general procedure. These complexes have been characterized by
FT-IR, UV–Vis, CV and X-ray analysis. The SOD and anti-cancer activities of the complexes have
also been evaluated. The complexes are insoluble in water and non-polar solvents, but are soluble in
MeOH, EtOH, DMF, DMSO, acetonitrile and chlorinated solvents.
3.1. Synthesis and structural characterization
The reaction of the Schiff base (HL) from the condensation of 2-hydroxy-1-naphthaldehyde
and acethydrazide with either CuCl2.2H2O and Cu(SO4).5H2O in a 1:1 molar ratio yields
[Cu(Cl)2(L)] 1 and [Cu2(μ-SO4)2(L)2] 2, in which the NH group of HL remains deprotonated. The
synthetic routes used for the synthesis of 1 and 2 are depicted in Scheme 3. These complexes were
synthesized in good yield. Both complexes 1 and 2 are air-stable. All general characterizations were
carried out with crystalline samples. Microanalyses showed that the components of both complexes
are consistent with the results of the molecular structural analysis. Single crystal X-ray analysis
showed that complex 1 is mononuclear, whereas 2 is a binuclear complex. In the IR spectrum of
complex 1, the peaks at 3433 and 1459 cm-1 are characteristic for N-H stretching and bending modes
respectively. Similarly for complex 2, peaks at 3406 and 1464 cm-1 are observed. The sharp >C=N
stretching vibration bands, corresponding to the imine group of Schiff base framework, appear at
1568 cm-1 and the skelton vibrations of the phenyl groups are at 1480 and 1542 cm-1. The redshifts in
8

the vibrational absorption bands of the >C=N group, compared to 1592 cm-1 for the free Schiff base,
are in agreement with the coordination of the Schiff base to the copper(II) center42. The observed
vibrational bands at 427 and 462 cm-1 are due to υ(M-N) and υ(M-O) stretching frequencies in
complex 2. Other bands associated with the Schiff base showed minor shifts, suggesting that the
electron density of the bonds have been altered on coordination43. The FT-IR spectra are given in
Figs. S2-S4.
Scheme 3. Synthetic routes to the metal complexes 1 and 2.
3.2. Molecular structure characterization
The molecular structures of both complexes are shown in Figs. 1 and 2 respectively. Complex
1 is mononuclear, whereas 2 is a binuclear complex. The crystal data and structure refinement details
are shown in Table 1. Selected bond distances and angles are presented in Table 2. In both
complexes, the Schiff base ligand acts tridentate via the NNN donor atom set and it is mono
diprotonated ( - ).
L
9

(a)
(b)
10

(c)
(d)
Fig. 1 (a) Molecular structure of complex 1, (b) the co-former Him interaction with the hydrogen
bonded motif in 1, (c)
(metal chelate) interactions, (d) stereoscopic view of the cell of
C ― H⋯π
complex 1 down the b-axis (the a-axis is vertical).
11

The molecular structure of complex 1 consists of a discrete mononuclear [CuL(Cl2)] unit. A
perspective view of the structure, together with the atom labelling scheme of complex 1, is shown in
Fig. 1a. The copper(II) ion is five coordinated with a geometry that is intermediate between a trigonal
1
bipyramid (c v) and a square-based pyramid ( ). The distortion in the geometry is described by
c4v
3
Addison and his coworker.44 According to the procedure for the Addison parameter (τ5), the
copper(II) atom has a τ value of 0.088 (
τ5 =
(β ― α), where β and α are the largest angles in the
60
coordination sphere), thus the coordination environment can be better described as a distorted square
pyramidal structure in which the four-fold polyhedron is comprised of the N1, N2, N3 and Cl1 atoms,
with the axial atom Cl2. The Cu-Cl2 bond length is longer than equatorial bonds, which results from
the Jahn-Teller effect. The slight distortion in the basal plane of the coordination polyhedron may be
due to the strain imposed by the Schiff base to the copper(II) center during coordination. The axially
coordinated Cl2 atom has longer bond distance [2.5116(6) Å] than equatorially coordinated Cl1
[2.2429(5) Å] atom. The copper(II) ion is shifted by 0.249 Å towards the axially bound Cl2 atom
from the basal plane. In addition to H-bonding, various C-H···π (aryl or metal chelates) interactions
are detected, having ring centroid (H···Cg) distances in the range 2.508-3.491 Å (Fig. 1c). The C-
H···π interactions are responsible for extra stabilization in the solid-state. The axially coordinated Cl2
atom is involved in a hydrogen bond interaction with the H atoms of an adjacent molecule. The
R12(6)
involved Cl2 atom showed bifurcated interactions, thus forming two
membered
1
heterosynthons (Fig. 1b). The hydrogen bond parameters are presented in Table 3. The two
R2(6)
motifs are responsible for the formations of a homodimer. The intermolecular C-H···π (aryl and
metal chelate) interactions are also responsible for the formation of the homodimer (two co-crystals).
Table 1. Crystal data and structure refinement for complexes 1 and 2.
1
2
Empirical formula
Formula weight
Temperature (K)
C17H14Cl2CuN4
408.76
100(2)
C68H80Cu4N16O28S4
1951.88
100(2)
Wavelength (Å)
Crystal system
Space group
0.71073
Triclinic
P -1
0.71073
Triclinic
P -1
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
Volume (Å3)
8.4475(3)
9.6940(3) Å
11.4276(3) Å
93.352(2)
110.877(2)
105.187(2)
831.78(5)
2
8.2118(2)
14.8543(3)
17.2565(4)
74.1250(10)
80.5990(10)
74.2080(10)
1939.25(8)
1
Z
Density (calculated) (Mg/m3)
1.632
1.671
12

Absorption coefficient (mm-1)
F(000)
1.640
414
1.284
1004
Crystal size (mm3)
0.23 x 0.16 x 0.12
2.652 to 33.256
-13<=h<=13, -14<=k<=14, -17<=l<=17
23733
Theta range for data collection (°)
Index ranges
Reflections collected
2.547 to 28.328
-10<=h<=10, -19<=k<=19, -23<=l<=23
28965
Independent reflections
Completeness to theta = 25.242°
Refinement method
6350 [R(int) = 0.0594]
99.9 %
9599 [R(int) = 0.0212]
99.8 %
Full-matrix least-squares on F2
6350 / 0 / 217
Full-matrix least-squares on F2
9599 / 0 / 597
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I>2sigma(I)]
R indices (all data)
1.058
1.062
R1 = 0.0426, wR2 = 0.0918
R1 = 0.0665, wR2 = 0.1008
1.124 and -1.049
R1 = 0.0252, wR2 = 0.0614
R1 = 0.0307, wR2 = 0.0649
0.428 and -0.490
Largest diff. peak and hole (e Å-3)
Table 2. Bond lengths [Å] and angles [°] for complexes 1 and 2.
1
Bond lengths
XRD data
DFT data
XRD data
DFT data
Cu-N(3)
Cu-N(1)
Cu-N(4)
1.9856(16)
2.0065(17)
2.0177(18)
1.9856
2.0065
2.0177
Cu-Cl(1)
Cu-Cl(2)
2.2429(5)
2.5116(6)
2.2429
2.5116
Bond angles
N(3)-Cu-N(1)
N(3)-Cu-N(4)
N(1)-Cu-N(4)
N(3)-Cu-Cl(1)
N(1)-Cu-Cl(1)
2
79.14(7)
79.33(7)
155.75(7)
161.04(5)
98.39(5)
79.14
79.36
156.13
161.38
98.39
N(4)-Cu-Cl(1)
N(3)-Cu-Cl(2)
N(1)-Cu-Cl(2)
N(4)-Cu-Cl(2)
Cl(1)-Cu-Cl(2)
98.35(5)
96.27(5)
95.15(5)
98.15(5)
102.68(19)
98.35
96.27
95.18
98.23
103.69
Bond lengths
Cu(1)-O(1A)
Cu(1)-N(3A)
Cu(1)-N(1A)
Cu(1)-N(4A)
Cu(1)-O(1A)#1
Sulfur ion
1.9303(11)
1.9557(13)
2.0087(13)
2.0136(13)
2.2806(11)
1.9303
1.9557
2.0086
2.0136
2.2806
Cu(2)-O(1B)
Cu(2)-N(3B)
Cu(2)-N(1B)
Cu(2)-N(4B)
Cu(2)-O(1W)
1.9214(11)
1.9593(13)
2.0032(13)
2.0163(13)
2.1841(12)
1.9214
1.9601
2.0032
2.0163
2.1841
S(1)-O(2A)
S(1)-O(4A)
S(1)-O(3A)
S(1)-O(1A)
1.4538(11)
1.4653(11)
1.4684(11)
1.5400(11)
1.4538
1.4652
1.4684
1.5399
O(1B)-S(2)
O(2B)-S(2)
O(3B)-S(2)
O(4B)-S(2)
1.5029(11)
1.4685(12)
1.4714(12)
1.4697(12)
1.5031
1.4685
1.4714
1.4697
Bond angles
O(1A)-Cu(1)-N(3A)
O(1A)-Cu(1)-N(1A)
N(3A)-Cu(1)-N(1A)
O(1A)-Cu(1)-N(4A)
N(3A)-Cu(1)-N(4A)
N(1A)-Cu(1)-N(4A)
O(1A)-Cu(1)-O(1A)#1
N(3A)-Cu(1)-O(1A)#1
N(1A)-Cu(1)-O(1A)#1
N(4A)-Cu(1)-O(1A)#1
Sulfur ion
176.68(5)
103.74(5)
79.57(5)
97.11(5)
79.58(5)
157.67(5)
78.59(5)
101.25(5)
99.58(5)
92.24(5)
176.68
103.89
79.57
97.11
79.58
158.37
78.59
101.25
99.58
92.24
O(1B)-Cu(2)-N(3B)
O(1B)-Cu(2)-N(1B)
N(3B)-Cu(2)-N(1B)
O(1B)-Cu(2)-N(4B)
N(3B)-Cu(2)-N(4B)
N(1B)-Cu(2)-N(4B)
O(1B)-Cu(2)-O(1W)
N(3B)-Cu(2)-O(1W)
N(1B)-Cu(2)-O(1W)
N(4B)-Cu(2)-O(1W)
173.08(5)
97.29(5)
79.53(5)
101.95(5)
80.02(5)
157.36(5)
89.11(5)
97.50(5)
100.44(5)
91.74(5)
173.08
97.29
79.53
102.31
80.02
158.11
89.24
97.50
100.44
91.74
O(2A)-S(1)-O(4A)
O(2A)-S(1)-O(3A)
O(4A)-S(1)-O(3A)
O(2A)-S(1)-O(1A)
O(4A)-S(1)-O(1A)
O(3A)-S(1)-O(1A)
111.95(7)
112.77(7)
111.41(7)
107.66(7)
106.30(7)
106.30(6)
111.95
112.77
111.41
106.84
106.30
106.30
O(2B)-S(2)-O(4B)
O(2B)-S(2)-O(3B)
O(4B)-S(2)-O(3B)
O(2B)-S(2)-O(1B)
O(4B)-S(2)-O(1B)
O(3B)-S(2)-O(1B)
111.63(7)
110.98(7)
109.98(7)
109.38(7)
107.22(7)
107.48(7)
111.63
110.98
109.98
109.59
107.22
107.48
Symmetry transformations used to generate equivalent atoms: #1 -x+1,-y+1,-z+1
Table 3. Hydrogen bonds [Å and °] for complexes 1 and 2.
13

D-H...A
d(D-H)
d(H...A)
d(D...A)
<(DHA)
Symmetry
transformations
1
N(2)-H(2B)...Cl(2)#1
C(1)-H(1A)...Cl(1)
C(4)-H(4A)...Cl(2)#1
C(16)-H(16A)...Cl(1)#2
C(17)-H(17A)...Cl(1)
0.88
2.37
2.84
2.78
2.73
2.90
3.1579(16)
3.392(2)
3.531(2)
3.669(2)
3.428(2)
149.6
117.8
136.8
168.6
116.4
#1 -x+1,-y+1,-z+1
#2 -x,-y+2,-z+1
0.95
0.95
0.95
0.95
2
N(2A)-H(2A)...O(3A)#1
C(1A)-H(1AA)...O(2A)
C(4A)-H(4AA)...O(3A)#2
O(1W)-H(1W1)...O(4W)
O(1W)-H(1W2)...O(4A)
N(2B)-H(2B)...O(5W)
C(2B)-H(2BA)...O(3B)#3
C(10B)-H(10B)...O(4W)#4
C(11B)-H(11B)...O(3B)
O(2W)-H(2W1)...O(3B)
O(2W)-H(2W2)...O(6W)
O(3W)-H(3W1)...O(4B)#5
O(3W)-H(3W2)...O(2W)#6
O(4W)-H(4W1)...O(6W)#3
O(4W)-H(4W2)...O(2W)#7
O(5W)-H(5W1)...O(2B)#5
O(5W)-H(5W2)...O(3W)
O(6W)-H(6W1)...O(4B)#4
O(6W)-H(6W2)...O(3B)#8
0.83(2)
0.95
0.95
0.82(2)
0.81(3)
0.82(2)
0.95
1.97(2)
2.48
2.57
1.96(2)
1.93(3)
1.94(2)
2.59
2.7291(17)
3.360(2)
153(2)
153.6
123.7
167(2)
177(3)
168(2)
127.0
#1 -x+1,-y+1,-z+1
#2 -x,-y+1,-z+1
3.1928(19)
2.7573(19)
2.7382(17)
2.7460(19)
3.253(2)
#3 x+1,y,z
#4 x-1,y,z
0.95
0.95
2.56
2.56
3.406(2)
3.403(2)
148.9
147.8
0.78(3)
0.81(3)
0.80(3)
0.80(3)
0.84(3)
0.82(3)
0.77(3)
0.77(3)
0.79(3)
0.84(3)
1.98(3)
1.97(3)
1.99(3)
2.03(3)
2.05(3)
2.09(3)
2.05(3)
2.03(3)
2.01(3)
1.98(3)
2.7596(18)
2.767(2)
2.7832(18)
2.8173(19)
2.874(2)
2.8720(19)
2.8268(18)
2.774(2)
2.7824(19)
2.8051(18)
175(2)
171(3)
172(3)
168(3)
166(3)
159(3)
177(3)
161(3)
164(3)
169(3)
#5 -x+1,-y+2,-z
#6 -x,-y+2,-z
#7 -x+1,-y+1,-z
#8 -x,-y+1,-z
The single crystal X-ray structures were determined for all co-crystals for a better
understanding of the molecular packing and the non-covalent interactions amongst the molecules, in
particular the supramolecular architectures. The molecular structure of 2 reveals co-crystals of
monomer and binuclear copper(II) complexes in a 1:1 ratio. The binuclear co-crystal of complex 2
has a noncentrosymmetric structure with a binuclear Cu2(Cu-O-R)2 rectangular core. One oxygen
atom of the sulfate anion acts as a bridge between two copper(II) metal centers. The coordination
geometry around each copper(II) center is distorted pyramidal. The relative amounts of the distortion
of the square pyramids are given by the Addision factor (τ5).45 The τ5 values for two copper(II)
centers are estimated as (τ5)1 = 0.3 and (τ5)2 = 0.2. Thus, the coordination environment of each
copper(II) center is a slightly distorted square pyramidal structure in which the four equatorial sites
are comprised of O(1A), N(3A), N(1A) and N(A) atoms, with the axial position being occupied by
one of the oxygen atoms of the sulfate anion which also coordinates to the second copper(II) center,
⋯
acting as bridging ligand. In the binuclear motif, the Cu Cu distance is 3.266 Å. The two copper
atoms are separated by 1.930 Å through the bridging oxygen atoms of the sulfate anions, with Cu-
(M-O)-Cu angles of 101.41 and 89.10 °, respectively. The values of these angles exhibit anti-
ferromagnetic coupling. The copper(II)-copper(II) distance is within the usual range for this type of
complex46,47. The monomer unit of this binuclear complex is shown in Fig. 2. The coordination
environment of the copper(II) center is distorted square pyramidal as suggested by the τ5 factor ( β ― α
60°
14

. The less distortion is consistent with the crystallographically imposed symmetry (the plane
)
comprised of the N1B, N3B, N3B and O1B atoms), since the equatorial plane of the complex is
formed by these donor atoms together with a long axial Cu(2)-O1W bond. The guest mononuclear
motif has pentacoordination around the copper(II) center. Selected bond lengths and bond angles are
presented in Table 2. The basal plane of the copper center comprises two pyridine nitrogen atoms and
one azo nitrogen atom of L-. One perchlorate anion takes the fourth position of the basal plane. The
axial position of the metal coordination sites is occupied by one oxygen atom of a water molecule.
Thus, the geometry of the copper ion in this motif can be described by a square pyramidal structure in
which the c4 axis is composed of the N1B, N3B, N4B and O1B atoms. The relative amount of the
48
square pyramidal component is indicated by τ5 . The value of τ5 is 0.262, which describes a distorted
square pyramidal geometry. The Cu2-O1W bond length is the longest compared to the distances from
the Cu center to the other atoms, a fact which is consistent with the O1W atom being the apex of the
pseudo square pyramid, allowing for Jahn-Teller distortion.
(a)
15

(b)
(c)
16

(d)
Fig. 2 (a) Molecular structure of 2, (b) the interaction with the hydrogen bonded motif in
stoichiometric co-crystals, (c) intermolecular C-H···π (metal chelate) interactions, (d) stereoscopic
view of the cell of complex 2 down the b-axis (the a-axis is vertical).
The arrangement of the two species present in the lattice structure allows a certain degree of
hydrogen bonding throughout the crystal (Fig. 2b). The binuclear unit and two mononuclear units
present a double interaction (intermolecular H-bonding) through the H-atom of an axially
⋯
coordinated water molecule, H1W2 O1A (dHA = 1.933 Å). Similarly, a phenyl ring of the Schiff
base forms H-bonding with the oxygen atom of coordinated water molecules of the monomer of the
type C11A-H11A···O1W (dHO = 2.662 Å). There are also several intermolecular hydrogen bonding
interactions involving O/H atoms of water molecules with O atoms of the sulfate anion or the Schiff
base atoms. All kinds of interaction parameters are presented in Table 3. In complex 2, the interesting
features of the structure are the formation of a supramolecular assembly through H-bonding
interactions of coordinated/uncoordinated sulfate anions and water molecules. The hydrogen atoms,
H and ..H attached to C, (C 1) are involved in intermolecular
(aryl) interactions and the H-
C ― H⋯π
g
atoms of H……(C 1) and H….(C 2), attached to C atoms, are involved in intermolecular
S ― H⋯π
g
g
interactions with the ring centroid (H…C ) of the binuclear complex. The
interactions, along
CH⋯π
g
with other H-bonding interactions, contribute extra stabilization to the solid-state. A stereoscopic
17

projection of the molecular arrangement within the unit cell along the b-axis is shown in Fig. 2d. The
two monomers and one dimer unit constitute a 1D pattern.
3.3. Electronic spectra
The electronic spectra were recorded using DMSO (3.0 × 10-3 M) solutions of 1 and 2. UV-
visible absorption spectroscopy is a useful technique that has been frequently explored to provide
structural information. A strong absorption band in the electronic spectra of complexes 1 and 2 at 278
nm was attributed to the π-π* transition of the aromatic rings and azomethine groups. The absorption
band at 385 nm is due to a ligand to metal charge transfer (LMCT) from the >C=N group to the
copper(II) center49. Furthermore, the complexes exhibit a copper-based d-d band at ~580 nm and the
transition occurs from 2Eg→2T2g 50. The spectral properties of both complexes are very similar (Fig. 3).
Fig. 3. UV-visible spectra of the copper(II) complexes 1 and 2 in DMSO solution, 3.0 x 10-3 M.
3.4. Electrochemical studies
The electrochemical behavior of both complexes was studied using cyclic voltammetry (CV)
and differential pulse voltammetry (DPV) techniques. The CV and DPV diagrams are shown in Fig.
4. Both complexes display two reduction peaks at +0.21 (Cu2+ → Cu+1) and -0.34 V (Cu+ → Cu0) for
1 and at -0.20 (Cu2+ → Cu+1) and -0.27 (Cu+ → Cu0) for 2, and one oxidation peak at -0.28 V (Cu0
→ Cu2+) for 1 and -0.15 V (Cu0 → Cu2+) for 2. This may be due to the chemical oxidation of copper
during the complexation processes i.e. during the complexation processes the Cu+1 species is
transformed into Cu2+ species and the ligand is reduced at a more negative potential (Fig. 4). On
increasing the scan rate, the peak currents increase linearly, supporting the diffusion effect of the
electrochemical (Ec) mechanism51. Both complexes show two reduction processes (CuIICuII + e- →
18

CuIICuI and CuIICuI + e- → CuICuI) vs AgCl references, both involving an identical number of
electrons, as revealed from the differential pulse voltammography (DPV) experiments. The value of
the second reduction potential of 2 is less negative than that of 1, but the first reduction potential
values are nearly identical. Such a less negative reduction potential may be attributed due to the high
molecular weight of 2. The DPV technique is a very good electrochemical technique for resolving
reduction peak potentials with small differences in peak potentials (180 mV)52. This technique further
supports the existence of two reduction processes (Fig. 4b) and is also in agreement with the
foregoing results53.
(a)
(b)
Fig. 4. (a) Cyclic voltammograms of complexes 1 and 2 in DMSO at an Ag/AgCl electrode with a
scan rate of 300 mV s-1 and a temperature of 20 °C. (b) Differential pulse voltammogram of
complexes 1 and 2 at room temperature using a scan rate of 20 mV s-1 in DMSO. The pulse
amplitude is 50 mV.
3.5. EPR measurements
The EPR spectra of complexes 1 and 2 were recorded with a Varian E-line spectrometer
working in the X-band using polycrystalline samples at RT and in DMSO solution (3 x 10-3 M) at
liquid nitrogen temperature (LNT) (Fig. 5). The EPR spectra of the complexes in the polycrystalline
form are typical for the triplet state (S=1). The polycrystalline room temperature spectra of both
complexes are found without any hyperfine structure. The signal (half field) for ΔMs =
is weak
± 2
for both complexes. The occurrence of such a dimeric triplet state in 1 is due to dipole-dipole
interactions. Such interactions are responsible for the formation of the dimer, which has already been
explained in the single-crystal X-ray discussion. In complex 1, the dipole-dipole interactions survive
in DMSO solution at LNT (Fig. 5), whereas complex 2 decomposed in DMSO, yield spectral features
of the mononuclear complex. In the binuclear complexes 2
signals were found,
( M = ± 2)
∆
s
although in the sulfato bridged complex they are weak. The polycrystalline spectra are similar in
19

g ∥
g ⊥
signals. The values in the polycrystalline state are
appearance, with
and
g ∥ = 2.190 g ⊥ = 2.072
exchange interaction parameter G of the complexes suggests that there is an interaction (G < 4)
between the two copper centers. is strong in complex 2. These observations are in
,
g ∥ = 2.216 g ⊥
, G = 2.56 and
,
, G = 2.93 for complexes 1 and 2, respectively. The
= 2.067
∆Ms =± 2
agreement with the antiferromagnetic interaction between copper ions in both complexes54. The
frozen DMSO solution yielded the parameters and G for
,
g ∥ = 2.219 g ⊥ = 2.063
A ∥ = 167
, indicates that the
complex 1. The nearly axial character of the g-tensor, with
g ∥ > g ⊥ > 2.023
2
푦2
ground state arises from a
orbital, which is in agreement with τ5 = 0.262 for the square
푑푥
-
pyramidal coordination sphere55. Such broad band features are consistent with the magneto-chemical
properties, which indicated a spin-spin interaction56. The frozen solution EPR spectrum of complex 2
shows four lines, typical of copper(II) ions (I = 3/2), as a result of the hyperfine interaction between
the unpaired electrons and copper(II) center. The values
= 2.284,
= 2.057 and = 167 G
A ∥
g ∥
g ⊥
were obtained. As
>
, a square pyramidal geometry can be proposed for the copper(II) centers.
g ∥ g ⊥
Such observations are suggestive of dissociation of this complex into two mononuclear copper(II)
species.
1
2
Fig. 5. X-band EPR spectra of complexes 1 and 2 in the polycrystalline state (RT) and DMSO
solution at LNT. Inset: EPR spectra showing half-field signals.
3.6. Thermal gravimetric analysis
The thermal behavior of the complexes was studied by thermal gravimetric analysis. The curves of
the thermo-gravimetric analysis (TG) are displayed in Fig. 6. Thermal decomposition of the copper
complexes occurs in the temperature range 100-530 °C, showing the high thermal stability of the
complexes. The thermal decomposition process occurs in two steps and three steps for complex 1 and
2, respectively, as shown in Scheme 4. In the TG graph of complex 1, the weight loss at 250-360 °C
20

is associated with the separation of the ligand molecule, while the next step, with a weight loss at
400-500 °C, is attributed to the loss of the Cl2 moieties, leaving behind CuO as the final product. In
the TG graph of complex 2, thermal dehydration of the complex occurs in the range 100-200 °C, due
to the loss of the water molecules. In the next step, loss of the ligand takes place, at 280-370 °C,
while in the final step, a weight loss occurs in the range 400-500 °C due to the removal of the SO4
molecule. Copper oxide CuO, which is stable up to 514 °C, was obtained as the final product57-59
.
Fig. 6. TGA curves of complexes 1 and 2.
21

Scheme 4. TG decomposition pattern of the complexes.
3.7. Cryomagnetic susceptibility studies
The thermal variation of
(
= molar magnetic susceptibility) for complexes 1 and 2
휒푀푇 휒푀
over the temperature range 300-2 K is shown in Fig. 7. The value of the
product obtained at
휒푀푇
room temperature for complex 1 is 0.43 cm3 K mol-1, which is slightly larger than the value expected
for an isolated S = ½ copper(II) ion (0.375 cm3 K mol-1). On cooling, the
product remains
휒푀푇
roughly constant until approximately 45 K when a slight decrease in this value is observed. Further
cooling of the sample leads to a sharper decrease in
interactions (Fig. 7).
, which is ascribed to antiferromagnetic
휒푀푇
22

Fig. 7. Temperature dependence of the T product of complexes 1 (squares) and 2 (circles)
measured under a magnetic field of 0.5 T. The solid lines represent the fit of the data as described in
the text.
Taking into account the crystal structure of complex 1, which is formed by mononuclear
copper(II) species, it is reasonable to consider that the interactions occur through space and hydrogen
bonds between the S = ½ ions situated at the shortest intermolecular distances. It must be pointed out
that although hydrogen bonds are medium-range interactions, they can strongly influence the
magnetic properties of compounds in some cases60. A view of the packing of the structure of 1 along
the a-axis shows alternating intermolecular Cu···Cu distances of 4.031 and 6.595 Å that form an
imaginary zigzag chain with a Cu···Cu···Cu angle of 130.07 . Moreover, the structure displays
hydrogen bonding between the mononuclear species, whose copper ions are placed at 6.595 Å (Fig.
8).
23

Fig. 8. View of complex 1 along the a crystallographic axis, showing the shortest intermolecular
Cu···Cu distances (Å) and the hydrogen bonds between the mononuclear species.
In a first approach, the
data of 1 were fitted with an alternating Heisenberg linear chain model,
휒푀푇
whose spin Hamiltonian is as follows:
푛
2
∑
,
]
2푖 2푖 + 1
퐻 =
퐽
[푆 푆
+ 푎푆 푆
-
·
·
2푖 2푖 1
-
푖 = 1
where J is the exchange coupling constant and α takes into account the distortion in the chain with
values ranging from α = 0 (isolated dimers) to α = 1 (uniform chain of S = ½ spins). The analytical
expression employed in the fit is:
푁푔2훽2 퐴 + 퐵푥 + 퐶푥2
휒푀 =
1 + 퐷푥 + 퐸푥2 + 퐹푥3
푘푇
where N, g,  and k have their usual meanings, x =
/ kT and A-F are functions of α61. The best
|퐽|
parameters obtained were g = 2.15, α = 0.68, and J = –1.8 cm−1 with σ2 = 5.77×10-4 (Fig. 8).
Therefore, antiferromagnetic coupling constants of -1.8 and -1.2 cm−1 along a 1D arrangement of S =
½ spins have been deduced from this fit. Although the value of α suggests that there are no
magnetically isolated dimers in the structure, the experimental data of 1 were fitted in a second
approach with the Bleany-Bowers equation for comparison:
2푁푔2훽2
3
휒푀 =
3퐾푇
퐽
3 + exp
(
)
⁡ -
퐾푇
(derived from the H  J S1·S2 Hamiltonian)
The best fit of the data with this model led to g and J values of 2.16 and -3.2 cm−1,
respectively (Fig. S5). This model considers only one magnetic exchange pathway in the structure
and a larger value of J in absolute value was obtained. The
product of 2 at room temperature is
휒푀푇
1.66 cm3 K mol-1. This value is close to the one expected (1.50 cm3 K mol-1) for the four S = ½
copper(II) ions that comprise the structure of this compound. A Curie-like behavior is observed in the
300-15 K range and an increase of the
value on lowering the temperature, ascribed to
휒푀푇
ferromagnetic interactions, is observed below 15 K (Fig. S5). The
vs T data were fitted using a
휒푀푇
model that takes into account the magnetic contribution of a copper(II) dimer through the Bleany
Bowers equation (see above) and the contribution of two copper(II) monomers. Therefore, it has been
considered that an intradimer ferromagnetic coupling is responsible for the magnetic behavior
observed in 2. The g value of the monomeric copper(II) ions was fixed to the value of 2 to obtain
24

realistic values. The best data obtained from this fit were g = 2.12 and J = 1.4 cm−1, with σ2 =
1.08×10-4.
A slightly distorted square-pyramidal coordination is observed for the two copper(II) ions that
form the dimer of 2. The copper(II) ions are double bridged by one oxygen atom of each of the two
sulfate anions in the structure of the dimer. The dimer displays a Cu···Cu distance of 3.266 Å, Cu-O
distances of 2.281 and 1.930 Å and Cu-O-Cu angle of 101.41  (see the structural characterization
part for more information). These parameters are relevant because it is well known that
antiferromagnetic or ferromagnetic interactions of very different magnitude can be observed in
similar dimeric copper(II) complexes depending on the geometry, the τ parameter, the Cu-O-Cu
angle, and the Cu-O and Cu···Cu distances62. However, it is frequent to observe opposed effects for
these parameters and, indeed, it has been observed that it is particularly difficult to predict the sign
and the value of the coupling constant in similar systems in which the copper(II) ions are double
bridged, similarly to that found in 263. Nevertheless, it must be pointed out that both complexes
display antiferromagnetic interactions. To the best of our knowledge, this is the first example of a
dimeric copper(II) complex in which two oxygen atoms of two sulfate anions mediate a
ferromagnetic coupling.
3.8. DFT calculations
To help with the physicochemical data (Epr and CV), DFT calculations were executed on
complexes 1 and 2. Geometry optimization was performed using density functional theory (DFT) at
the B3LYP basic set64,65. The optimized structures of the complexes are shown in Fig. 9. Analysis of
the frontier molecular orbitals of the optimized structures shows that, in the gas phase, the highest
occupied molecular orbitals (HOMOs) and lowest occupied molecular orbitals (LUMOs) of both
complexes are similar in energy. The graphical representations (contour plots) of the frontier
molecular orbitals (FMO) are depicted in Fig. 10. It is observed that the HOMOs of both complexes
are mainly ligand centered, with the major contributions due to the p orbitals of the donor atoms.
From these observations it is easy to find out why the electrochemical potential is equal in magnitude
because the oxidation takes place on the coordinating tridentate ligand; the potentials are likely to be
slightly influenced by the nature of bridging ligand. The HOMO-LUMO energies are descriptors that
play an important role in deciding a wide range of chemical interactions66. FMOs give an insight into
the reactivity of complexes and active sites can be understood by the distribution of frontier orbitals.
The theoretical transition level energy gap (ΔE) between the HOMO and LUMO is shown in Table 4.
25

On perusal of the HOMO-LUMO energy gap, it is observed that the gap (ΔE) is high in both
complexes. This high ΔE value implies that energy is higher and a high chemical reactivity.
Complex 1
Complex 2
Fig. 9. Optimized structures of complexes 1 and 2.
Table 4. Molecular orbital energies and energy gap (ΔE).
Level
Molecular Orbital Energy (eV)
ΔE
1
2
1
2
HOMO
-3.896
-1.008
-4.519
+0.773
-5.392
-4.638
-6.335
-2.808
LUMO
2.888
3.746
0.754
3.527
HOMO-1
LUMO+1
26

LUMO
-5.025
-0.728
-6.401
-2.784
LUMO+2
4.297
3.617
Fig. 10. HOMO-LUMO structures with the energy level diagrams.
The calculated spin densities were performed using the B3LYP/LANL2DZ as basis sets for
both complexes. Fig. 10 illustrates the spin densities for the ground state. The spin density
distributions are mainly delocalized on the copper atoms and those atoms which are directly
coordinated to the copper atoms67. The spin density plots of the complexes are shown in Fig. 11. The
positive signed densities are spread over the metal center and the negative signed spin densities are
distributed over the coordinated donor atoms. Such spin density distributions are also in agreement
with the HOMO-LUMO shapes observed in both complexes. It is also shown from DFT calculations
that the DFT data agree with the EPR spectral data, from which a mainly ligand centered character of
27

2
2
the unpaired electron in the
concluded68.
orbital along with axial features of the EPR signals were
푑푥 ― 푦
Fig. 11. Spin density plots of complexes 1 and 2.
3.10. Natural bond order (NBO) analysis
NBO analysis of the synthesized complexes 1 and 2 has been carried out using the
B3LYP/LAN2DZ basis set. The computed bond lengths and bond angles are explicitly similar to the
experimentally observed values (Table 2). NBO analysis provides details about the natural charges
between occupied Lewis-type orbitals and unoccupied non-Lewis NBOs (Rydberg), which correlates
with the stabilizing donor-acceptor interactions69,70. As per the NBO analysis, all the interactions for
the Cu(II) ions and donor atoms are considered as coordination bonds (N→Cu, Cl→Cu and O→Cu ).
Such a type of interaction attributes to a donation of electron density from the lone pair orbital on the
donor atoms (N/Cl/O), LP(Cl) or LP(O) to the anti-bonding orbital on the Cu(II) LP*(Cu). The
involved selected orbitals in the interaction are shown in Fig. 12.
28

Fig. 12. The donor and acceptor orbitals involved in the LP(N)/ LP*(Cu) and LP(O)/
LP*(Cu) interactions.
The natural electronic configurations of the Cu atoms are [Ar] 3d9.36 4s0.35 4p0.64, 5p0.02 in 1
and [Ar] 3d9.46 4s0.26 4p0.37, 5p0.01 in 2. Similarly, the natural atomic charges are Cu2+ = +0.638, Cl1 = -
0.479, Cl2 = -0.622, N2 = -0.350, N3 = -0.287 and N4 = -0.507 in 1 and Cu2+ =+0.905, O1A = -1.015,
O1B = -1.011, N1 = -0.557, N2 = -0.306 and N3 = -0.312 in 2. For 1, 17.996 electrons are distributed
as core electrons, 10.341 electrons as valence on 4s, 3d and 4p orbitals and 0.025 electrons as
Rydberg electrons on the 5p orbital, giving a total electron count of 28.362. This is compatible with
the calculated natural charge of the Cu atom (+0.6382), which is correlated to the difference between
28.362 and the total number of electrons in an isolated copper atom (29 e). Likewise, the core,
valence, Rydberg electrons and the natural charge on the copper atom in 2 are 17.994, 10.089, 0.012
and +0.905 respectively. Before complexation, the charge on a copper ion is +2, despite this fact, the
charges on the copper(II) ions in complexes 1 and 2 are +0.638 and +0.905 respectively.
Second-order perturbation stabilization energies, analogous to the intermolecular charge
transfer interaction (donor-acceptor), of the complexes were obtained using the B3LYP/LAN2DZ
basis set. The computed intermolecular charge transfer energies (E) are 0.25 kcal/mol for 1 and 0.50
29