Experimental and quantum computational study of two new bridged copper(II) coordination complexes as possible models for antioxidant superoxide dismutase: Molecular structures, X-band electron paramagnetic spectra and cryogenic magnetic properties

Article abstract of DOI:10.1016/j.poly.2019.07.015

In the present study, we use a dual approach comprising experimental and quantum computational studies of two new bridged copper(II) coordination complexes with NNO donor ligands, viz., [Cu₂(μ-sulfato)(L)₂(2H₂O)]·1.5H

Full text of DOI:10.1016/j.poly.2019.07.015

Accepted Manuscript
Experimental and quantum computational study of two new bridged copper(II)
coordination complexes as possible models for antioxidant superoxide dismu-
tase: Molecular structures, X-band electron paramagnetic spectra and cryogenic
magnetic properties
Yogendra Singh, Ram N. Patel, Satish Kumar Patel, Abhay Kumar Patel, Neetu
Patel, Rita Singh, R.J. Butcher, Jerry P. Jasinski, A. Gutierrez
PII:
S0277-5387(19)30485-1
https://doi.org/10.1016/j.poly.2019.07.015
POLY 14065
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
1 June 2019
12 July 2019
13 July 2019
Please cite this article as: Y. Singh, R.N. Patel, S. Kumar Patel, A. Kumar Patel, N. Patel, R. Singh, R.J. Butcher,
J.P. Jasinski, A. Gutierrez, Experimental and quantum computational study of two new bridged copper(II)
coordination complexes as possible models for antioxidant superoxide dismutase: Molecular structures, X-band
electron paramagnetic spectra and cryogenic magnetic properties, Polyhedron (2019), doi: https://doi.org/10.1016/
j.poly.2019.07.015
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Experimental and quantum computational study of two new bridged copper(II)
coordination complexes as possible models for antioxidant superoxide dismutase:
Molecular structures, X-band electron paramagnetic spectra and cryogenic
magnetic properties
a
a
a
b
b
c
d
e
Yogendra Singh , Ram N. Patel , Satish Kumar Patel , Abhay Kumar Patel , Neetu Patel , Rita Singh , R.J. Butcher , Jerry P. Jasinski , A.
Gutierrezf
aDepartment of Chemistry, A.P.S. University, Rewa (M.P.) 486003 India
b
Department of Chemistry, Faculty of Science, The MS University of Baroda, Vadodara 390002, India
cDepartment of Physics, Govt. Model Science College, Rewa (M.P.) 486001 India
dDepartment of Inorganic & Structural Chemistry, Howard University, Washington DC, 22031 USA
eDepartment of Chemistry, Keene State College, 229 Main Street, Keene, NH 03435-2001
fDepartamento de Quimica Inorganica, Universidad Complutense, 28040 Madrid, Spain
Graphical Abstract
Two new bridged copper(II) coordination complexes with NNO donor ligands, viz.,[Cu (μ-sulfato)
2
(
HL) (H O)].1.5 H O (1) and [Cu (μ-succinato)(L)(HL)(H O)]ClO (2), where HL/L =N'-[(E)-pyridin-
2 2 2 2 2 4
2
-ylmethylidene]benzohydrazide, have been synthesized and characterized using various physico-
chemical techniques. Both complexes are structurally characterized using single crystal X-ray diffraction
studies and belong to the triclinic crystal system having space group P 1 .
Abstract
In the present study, we use a dual approach comprising experimental and quantum computational studies of two new bridged
copper(II) coordination complexes with NNO donor ligands, viz.,[Cu (μ-sulfato)(L) (2H O)].1.5H O (1) and [Cu (μ-
succinato)(L)(HL)(H O)]ClO (2), where HL/L =N'-[(E)-pyridin-2-ylmethylidene]benzohydrazide, have been synthesized and
2
2
2
2
2
2
4
characterized using various physico-chemical techniques. Both complexes are structurally characterized using single crystal X-ray
diffraction studies. The distances between two copper centers are 3.270(2) Å and 3.178(1) Å, for 1 and 2, respectively. On the basis of
quantum computational DFT study, electronic excitations involve transitions mainly from metal-ligand bonding MO’s to the β-LUMO
within the dominant Cu atom exhibiting dxy character and to the β-LUMO + 1. EPR spectra for these polycrystalline samples were
determined for the copper(II) hyperfine structures as well their zero-field splitting which are appropriate for the triplet state of such
dimers. Cryomagnetic behavior is consistent with weak antiferromagnetic interactions between the Cu(1)

Cu(2) centers in both
complexes. The magnetic exchange coupling constant (J) between the Cu(1) Cu(2) centers for 1 and 2 were determined to be J = –

-1
1
.50(1) and J = –7.7(1)cm , respectively. CH, , and lone pair  interactions which have gained attention and their role in
bimolecular structure analysis has been recognized. In addition, antioxidant superoxide dismutase activity measurements have showed that

homodinuclear complexes give significant scavenging effects against superoxide free radicals. Complex 2 is more antioxidant superoxide
dismutase active than 1.
Keywords: Copper(II)-hydrazone complexes; Molecular structures; Antioxidant superoxide; Cryogenic
magnetic properties ; Quantum computational study.
1
. Introduction
The tridentate coordination mode of hydrazone ligands are very efficient as binucleating ligands
for metal ions. Binuclear complexes using tridentate Schiff bases and bridging ligands have been
receiving considerable interest due to their biological and industrial relevance[1]. They have received
attention as model compounds for the active site of biological copper enzymes[2]. The enzymatic
activity of these enzymes is associated with a coordination sphere of the metal active site[3]. Superoxide
dismutase (SOD) enzymes are ubiquitous in living systems and serve a vital role in defending oxygen
utilizing life forms from oxidative damage[4]. SOD enzymes catalyze the dismutation of superoxide
free radical anions, giving rise to a one electron reduction product of molecular oxygen to non-radical
products (Scheme 1). The above mentioned features have attracted attention to the design new
copper(II) antioxidant biomimetic compounds[5]. The reaction mechanism of a Cu-Zn SOD or a
copper(II), biomimetic system consists of a disproportionation of O2∙ ─ as in the following steps of
Scheme 1.
Schiff bases are well known chelating ligands and are used in the synthesis of various mono- bi-
and polynuclear metal(II) complexes[6]. The present Schiff base is an NNO donor ligand and acts as a
tridentate neutral and mono negative donor for binuclear copper(II) complexes. We have been
interested in synthesizing two homodinuclear copper(II) complexes with a specific anion to see the
effect of the amide protonation state on the coordination sphere, redox properties and magnetic
interactions between the two paramagnetic metal ions[7]. The binding group anions of a
sulfate/succinate can act as a 1, 2 bridge and leads to formation of a binuclear complex. It is well known
the sulfate/carboxalate groups are able to generate H-bonds leading to formation of a supramolecular
architecture which can play an important role in transmission of a magnetic interaction. Both bridging
ligands can bridge two metal ions to give rise to a variety of polynuclear carboxalate bridged[7c, 8] and
sulfate bridged[9] complexes. The inspiration of this kind of study is not only to know the role of
fundamental interactions in both chemical and biological molecular recognition, but also to find out their

potential for technological applications in a number of areas of material science[10]. The Schiff-base
used in the present work, is so designed that it can participate in hydrogen bonding interactions. In order
to gain insights into the coordination chemistry of such binuclear complexes and an extension of our
recent studies in understanding their magnetic behavior, electrochemical and quantum computational
density functional studies[7c], we report the synthesis, structural characterization and quantum
computational calculations of two binuclear copper(II) complexes [Cu (μ- sulfato)(L) (2H O)].1.5H O
2
2
2
2
(1) and [Cu2(

-succinato)(L)(HL)(H O)]ClO (2) obtained by a biomimetic synthesis strategy,
2 4
catalyzing the Cu(II) N O /N O structural motifs as their inner sphere structure(Scheme 2).
2
2
2
3
Computational density functional theory has been used to predict the most stable geometrical form of
the ligand and its complexes. Quantum computational density functional theory also yields significant
electronic structural data. In addition, antioxidant assay superoxide dismutase activity data for these
complexes was also measured. The coordination site available for the binding of O2∙ ─ is clearly shown
by the X-ray structures, whereby both have a similar topology.
2
. Experimental
2
.1. Materials and methods
Copper(II) sulfate pentahydrate, copper perchlorate hexahydrate, benzyolhydrazide and 2-
pyridinecarboxaldehyde were purchased from Aldrich, Qualigens and Across Organics, respectively. All
other chemicals were of synthetic grade and used without further purification. Solvents were purified by
standard methods before use[11].
Caution! Perchlorate salts of metal complexes with organic ligands are potentially explosive.
Experiments were carried out at the mmol scale and prepared in small quantities.
2
.2. Physical measurements
Microanalysis was performed on an Elementar Vario EL III Carlo Erba 1108 analyser. The H
1
NMR spectrum of the ligand was obtained in CH OD with a Bruker Advance 400 (FT-NMR)
3
Instrument. Chemical shifts were reported in parts per million (ppm) using tetramethylsilane (TMS) as
internal standard. UV-vis absorption spectra were recorded at room temperature on a Shimadzu UV-vis

recording Spectrophotometer UV-1601 in 1cm quartz cuvettes. The Fourier Transform Infrared (IR)
-1
spectra were recorded in KBr on a Perkin-Elmer spectrophotometer in the rage 4000-400 cm region. X-
band electron paramagnetic resonance (EPR) spectra were measured with a Varian E-line Century Series
(
X-band 9.1 GH ) Spectrometer equipped with a dual cavity and operating at X-band with 100 kHz
Z
modulation frequency. Varian quartz tubes were used for measuring EPR spectra of polycrystalline
samples and frozen solutions. Tetracyanoethylene (TCNE) was used as a marker (g = 2.00277). Cyclic
e
-1
voltammetry measurements were performed on solutions containing 0.1 mol L tetrabutyl ammonium
perchlorate (TBAP) with a BAS-100 Epsilon Electrochemical Analyzer having an electrochemical cell
with a three-electrode configuration. Ag/AgCl was used as a reference electrode, glassy carbon as the
working electrode and platinum wire as an auxiliary electrode. The concentrations of the complexes
-3
used for redox potential measurements were 310 M DMSO solutions. All measurements were carried
out at room temperature under nitrogen atmosphere. Ferrocene (Fe) was added to the solution as an
-3
internal standard. Molar conductivities of freshly prepared complexes 310 M DMSO solutions were
measured on a Systronics conductivity TDS meter.
2
.3. Cryogenic magnetic susceptibility studies
The variable temperature magnetic susceptibility of complexes 1 and 2 were measured by using a
Quantum Design SQUID magnetometer MPMS-XL5 with liquid helium cryostat in the temperature
range of 2-300 K at an applied field of 0.1 T. All data were corrected for the diamagnetic correction
done by using Pascals constants. The Magnetic susceptibility data were fitted with least-square
techniques. The experimental data were also corrected for temperature independent paramagnetism
(TIP) of the metal center.
2
.4. X-ray crystallography
X-ray diffraction data were performed on Bruker APEX-II diffractometer using graphite
monochromated Mo-Kα radiation (λ= 0.71073 Å). Crystals suitable for single crystal X-ray analysis of
all complexes were grown by slow evaporation of the reaction mixtures at room temperature. Single-
crystals suitable for single-crystal X-ray analysis were mounted on glass fibers and used for data
collection. The crystal orientation, cell refinement, and intensity measurements were made using a
CAD-4PC performing
-scan measurements. The structures were solved by direct methods by using
SHELXT[12] and refined with SHELXL-2014[13]. All non-hydrogen atoms were refined

anisotropically by full-matrix least squares. All hydrogen atoms were located in difference Fourier maps
and allowed to refine using a riding model.
2
.5. Quantum computational density functional theory
All quantum computational calculations were performed using the GUASSIAN09 package
program[14] by the DFT/B3LYP method. The input files of the copper(II) complexes were prepared
with Gauss View 5.0.9[15]. In the computational model, the anion was ignored and the cationic
complexes were taken into account using crystallographic data as starting input. Time dependent
quantum computational DFT calculations were performed and compared to the electronic spectra of
complexes with the LANL2DZ basis set in the gas phase[16]. Vertical electronic excitations based on
B3LYP optimized geometries were computed using the time dependent density functional theory (TD-
DFT) formalism[17] in DMSO using a conductor-like polarizable continuum model[18].
2
.6. Antioxidant superoxide dismutase (SOD) assay
The in-vitro antioxidant superoxide dismutase (SOD) activity was measured using alkaline
DMSO as a source of the superoxide radical (O2∙ ─) and the SOD activity was evaluated using the nitro
blue tetrazolium (NBT) method[19]. A unit of antioxidant SOD activity is the concentration of the
complex, which causes 50% (IC ) inhibition of alkaline DMSO and mediated reduction of NBT. The
5
0
catalytic rate constants were calculated as kMcCF = kNBT [NBT] /IC , where k
(pH = 7.8) = 5.94  104
NBT
50
-1
-1
M S [20].
2
.7. Synthesis of the ligand
The Schiff base was prepared by a standard literature procedure[21] and synthesized by reacting
benzoylhydrazide (136.1 mg, 10.0 mmol) and 2-pyridenecarboxaldehyde (107.1 mg, 10.0 m mol) in
ethanol. The resulting solution was refluxed for 4 hours during which a brown precipitate was formed
(Scheme 3). The reaction mixture was then cooled to room temperature and the solid formed was
filtered and washed with ethanol. The product was recrystallized from hot ethanol. Yield: (83%).
Anal.Calcd for C H N O (%): C, 69.32; H, 4.92; N, 18.66. Found (%): C, 69.31; H, 4.92; N, 18.65. IR
1
3
11
3
-1
data (KBr disc, cm ): 3438, 2920, 2852,1970,1595,1456, 1379, 1324, 1253, 1154, 1102, 1030, 763, 703
1
and 651(Fig. S1). H NMR (CH OD); 3.31(s, 1H, OH); 7.39–8.56 (m, 9H, Ar–H) [Fig. S2].
3
2
.8. Synthesis of complexes

2
.8.1. Synthesis of [Cu (μ-sulfato)(L) (2H O)].1.5H O 1
2 2 2 2
To a MeOH solution (20 mL) of CuSO .5H O (0.250 g, 1.0 mmol) was added a MeOH solution
4
2
(20 mL) of the ligand (0.500 g, 2.0 mmol) with stirring for 30 min at 25˚C. The resulting green solution
was allowed to slowly concentrate by evaporation at room temperature for a period of one week to yield
a green solid. A green microcrystalline solid deposited was collected by filtration, washed with methanol
and stored in CaCl desiccator at RT. Yield: 70%; Anal. Found (%): C, 43.51; H, 3.62; N, 11.57; Calcd
2
-1
(
%) for: C H Cu N O S : C, 43.53; H, 3.52; N, 11.73; IR data (KBr disc, cm ):3440, 2920, 2853,
52 50 4 12 17 2
1
628, 1464, 1437, 1381, 1316, 1285, 1105, 1022, 748, 697, 660, 572,511 and 443(Fig. S3). Electronic
-1
-1
absorption spectra in DMSO [λmax.,nm( , M cm )]: 704(30),390 (7100). Molar conductivity (ΛM ) 17
-2
-1
Ω cm mol in DMSO solution.
.8.2. Synthesis of [Cu2 ( -succinato)(L)(HL)(H O)] ClO 2
To a MeOH solution (20 mL) of Cu(ClO ) .6H O (0.642 g, 1.0 mmol) was added a MeOH
2
μ
2
4
4
2
2
solution (20 mL) of ligand (HL) (0.500 g, 2.0 mmol). The resulting green solution was stirred with
succinic acid (0.118g, 1.0 mmol) and a few drops of triethyl amine were added to give a dark green
solution. The dark solution was allowed to slowly concentrate by evaporation at room temperature for 3
days. Dark green microcrystalline crystals were deposited and collected by filtration, washed with
methanol and stored in CaCl desiccator at RT. Yield: 72%; Anal. Found (%): C, 44.41; H, 3.49; N,
2
-
1
1
3
7
7
0.34; Calcd (%) for: C H Cl Cu N O :C, 44.42; H, 3.47; N, 10.36. IR data (KBr disc, cm ):3856,
30 28 2 6 11
750, 3447, 3067, 2927, 2860, 2715, 2613, 2427, 2364, 2075, 1593, 1489, 1323, 1271, 1153, 1101, 996,
-1
-1

63,703, 660 and 655(Fig. S4). Electronic absorption spectrum in DMSO [λmax. nm( , M cm )]:
-2
-1
12(38), 390 (6300). Molar conductivity (ΛM ) 125 Ω cm mol in DMSO solution.
3
. Results and discussion
3
.1 Synthesis of HL, 1 and 2
Both copper(II) complexes were synthesized from the reaction of a copper(II) salt with a Schiff
base ligand(HL). The target complexes were first characterized by elemental analysis followed by a
conventional solution method for the synthesis and formation of crystalline products suitable for single-
crystal X-ray diffraction analysis. The synthetic routes of the ligand and its corresponding binuclear
copper(II) complexes are described in Scheme 4. After 3-6 days, crystals of 1 and 2 suitable for single-
crystal X-ray analysis were isolated. The IR spectra of complexes 1 and 2 display IR absorption bands at
-1
1
595 and 1590 cm respectively, (Fig. S3, S4) which can be assigned to the v(>C=N) of the coordinated
-1
ligand(HL/L), whereas for the free ligand (HL) the same band is observed at 1628 cm .The shift of this

band on complexation towards lower wave numbers indicates the coordination of the azomethine
nitrogen (>C=N) to the copper centre[22]. In both complexes intense absorptions of sulfato 1 and
-1
succinato bridges stretching vibrations at 1379 and 1378 cm respectively[23]. Complex 2 shows an
-1
-
absorption band at 1488 cm which could be assigned to HC=N-C-O moiety of a deprotonated ligand
-3
-1
-2
[
24]. The molar conductivity value for complex 1 in DMSO solution (3×10 mol L ) was 17Ω cm
-1
mol indicating non-electrolytic behavior[25]. However, complex 2 shows a molar conductivity ( M
)
-2
-1
of 125Ω cm mol in DMSO solution corresponding to a 1: 1 electrolyte[25, 26].
3
.2. X-ray crystallography structural characterization of complexes 1 and 2
The molecular structures of both complexes were crystallographically characterized and the
structures are shown in Fig. 1a. Selected structural parameters are presented in Table 1 and important
bond distances and angles are shown in Table 2. Both complexes are binuclear in nature.
In complex 1 two oxygen atoms form sulfate anion and two oxygen atoms form carbonyl moiety of the
Ligand(HL), act as bridging units. In this complex Cu1 is square pyramidal and Cu2 is hexacoordinated.
The basal plane of the Cu1 centre is occupied by a Schiff base ligand and one oxygen (O1) atom of a
sulfate anion, thus completing a square pyramidal structure. The distortion index (
pentacoordinated Cu1 centre can be estimated by Addison distortion index ( ) as
60°)[27], in which and are two largest coordinating angles , and = 0 for ideal square
pyramidal and = 1 for ideal trigonal pyramidal geometry. The estimated  5 value for this centre is
.16, indicating distortion in five coordinate geometry. For the Cu2 centre the degree of distortion from

5
) for a

5
5
 
= (  -
/




0
octahedral coordination can be defined through a tetragonality parameter (T) as: T = R /R , in which
int out
R /R , are the average in plane and out plane bond distances respectively and a T value < 0.9, indicates
int out
static and T =1 dynamic distortion in octahedral geometry[28]. The degree of distortion the geometry
around Cu2 was estimated to be 0.78. Therefore, the geometry around Cu2 is ascribed as a static
distortion. The Cu

Cu distance (3.270(1) Å) is comparable to that of reported complexes[29]. The

copper(II) coordination sphere of 1 is shown in Fig. 1b. Models of this complex constitute infinite 1-D
chains along the b-axis through hydrogen bonds involving the C-H groups of HL and free coordinated
water molecules (Fig. 1c). Two adjacent units are connected by such hydrogen bondings. Distances of
various hydrogen bonds and weak intermolecular interactions are summarized in given in Table 3. These
hydrogen bonds and weak intermolecular interactions (C6-H6BA

O2W, O2W-H2W2

O1W and
3
O1W–H1W1

N3B) for one
R
(8), graph-set ring motif Similarly (C6B)-H6BA...O2W, O2W-H2W2
3
3

O1W and O1W-H1W1

N3B for other R3 (8)) results in a dimer like association between pairs of
3
complexes in an extended two heterosynthon (motif) R3 (8) graph-set ring motif in Fig.1c. These types
of distances and angles are generally employed to describe the unusual molecular shape a complex has
that is determined from a complex system of inter- and intramolecular non-covalent interactions
comprising face-to-face and edge-to-face aromatic interactions as well as lone pair interactions[30].
Additional stabilization of these motifs is provided by the above non-covalent interactions. Crystal
packing is further supported by additional weak π

π stacking interactions and CH

π inter- and
intramolecular contacts. Two pairs of weak intermolecular interactions are present between a CH group
from the benzoyl rings (C13A--H13A and C13B--H13B) with π electrons of the pyridyl rings (Fig.1c),
with distances 2.941 (15) Å and 3.334 (16) Å, respectively, while a CH

π intermolecular contact
remains between the C4B--H4B group of a pyridyl ring and the π electrons of nearby benzoyl rings with
a distance of 3.501(14) Å. Chelate rings of metal complexes with delocalized π electrons can be
involved in non-covalent interactions in similar ways to aromatic molecules. In the presence of a chelate
ring with delocalized π electrons there is a close contact between a copper atom and a carbon atom from
nearby benzoyl rings. This contact survives between a C6B--H6BA group from a benzoyl ring with a
chelate ring of the complex with distance of 3.535(13) Å. Inter- and intramolecular H
from H1W1/H1W2 and π electrons from nearby chelate rings, with distances displayed, are shown in
Fig. 1d. Crystal packing is also affected by π π stacking interactions between planar molecular units
and are formed due to a stacking of the molecular chains. These interesting observations involving
molecules from complex 1, therefore, exhibit effective π π (aryl-aryl, aryl-metal chelates and metal

π interactions


chelate-metal chelates) stacking interactions. Regarding intermolecular stacking, a pair of pyridyl-benzyl
stacking interactions are observed with a centeroid-centeroid distance, dC-C = 3.989 Å. Along with these
intermolecular interactions, an additional metal-aryl ring pi...pi stacking interaction is observed with a
distance, dC-C = 4.515 Å. So, the metallo aromaticity in these types of aryl chelate rings has been
structurally characterized[31]. In this complex intermolecular π

π (aryl-aryl, aryl-metal chelates and

metal chelate-metal chelates) are also observed with distances, dC-C = 4.331 Å and dC-C = 4.298 Å. Many
of these types of contacts can be seen in Fig. 1(e). Uncoordinated solvent molecules and long pairs from
the oxygen atom of coordinated water molecules with metal chelates along with lone pair interactions is
also shown in Fig. 1(f). Recently lone pair (lp) interactions have gained renewed attention and their
role in bimolecular structure is being recognized[32].
The molecular structure of complex 2 is shown in Fig. 2a .The structure consist of a succinate group,
two HL and copper(II) binuclear cations and one perchlorate anion. The carbonyl oxygen atoms of HL
link the binuclear units, in a bis(monodentate) way, building one-dimensional chains of Cu(II) linked by
succinate and carbonyl groups from HL as spacers. They are assembled in a three-dimensional network.
The selected bond lengths and angles are given in Table 3. In this complex Cu1 is in a distorted
octahedral geometry and coordinated to four oxygen atoms and two nitrogen atoms. The degree of
distortion (T) is 0.79, which indicates static distortion in the geometry [28]. The Cu2 centre has distorted
square pyramidal geometry with a distortion index  5 = 0.23, showing distorted square pyramidal
geometry around the Cu2 centre. The distances of four equatorial bonds Cu2-N2B, Cu2-O1B, Cu2-O2
and Cu2-N1B are 1.922(3), 1.969(3), 1.922(3), and 2.008(3) Å, respectively. However, the axial bond
distance Cu2-O1A was found to be somewhat longer. The copper(II) coordination sphere of 2 is
presented in Fig. 2b. The Cu
reported complexes[29]. In the asymmetric unit, the catainic and perchlorate anion are linked by
internuclear hydrogen bonds, where the O1W-H1W1 Cl1 and O1W-H1W1 O12 interactions

Cu distance (3.270(1) Å) is comparable to that of 1 and other similar


involving the H atoms of bonded water molecules of one binuclear complex from a pair of bifurcated
2
donor bonds yielding a heterosynthon (motif) with a R1 (3) graph-set ring motif. The O1W- H1W1

O12 and O1W-H1W1

Cl1 interactions (involving an H atom of a coordinated water molecule of an
2
adjacent binuclear unit) together form a pair of donor bonds defining the heterosynthon (motif) R1 (3)
2
2
graph-set ring motif. Similarly, two more heterosynthon (motif)
R
(8) graph-set ring motifs are
constituted by two binuclear units into a three dimensional network by a collection of O1W-H1W2
N3A, C6A-H6AA O12, O1W-H1W...O12 and O1W-H1W...Cl1 hydrogen bonds and weak
intermolecular interactions. These interactions are displayed in Fig. 1c. A careful analysis of the crystal



packing also revealed that CHπ
this complex. In complex 2, in addition to hydrogen bonding interactions, further several non-covalent
interactions viz., weak CH π. H π, π π and lone pair π interactions similar to complex 1 have
been observed. These interactions are shown in Fig. 2 (d-f) and distances of different non-covalent
interactions as well as an extended network of weak π π stacking interactions between nearby benzyl
and pyridine rings including the centeroid-centeroid distances of the π π stacking interactions are
presented in Table S1. Both complexes exhibit similar intramolecular stacking parameters and they are
appropriate enough to consider that effective "metal chelate-aryl rings" exhibiting weak π π stacking
interactions are operative. Such intramolecular "metal chelate-aryl rings" π π stacking represent new
,
ππ and lone pair π interactions are an additional feature of








structural evidence of the metalloaromaticity. In addition, these complexes show different molecular
structural patterns to recognize themselves and build upon the crystal packing. A further interesting
observation is that in both complexes inter- and intramolecular π

π and CH

π stacking interactions
coexist with extensive weak 3-D hydrogen bonding intermolecular interaction network. Uncoordinated
perchlorate ions are shown in Fig. 2(f) which is responsible for additional weak interactions to form
these supramolecular structures. Oxygen atoms of coordinated water molecules showed lone pair of
interactions with metal chelates (Fig. 2(f). Studies on lone pair (lp)π interactions and their importance
in biomolecular structures have already been reported [33]. Although, the nature of (lp)π interactions
in general are weak and attractive.
3
.3. Cryogenic magnetic properties
The susceptibility of both complexes has been measured in the range 2-300 K. The temperature
dependence of  and T for complexes 1 and 2 are shown in Fig. 3 and Fig. 4, respectively. The T
3
–1
room temperature value for 1 per dimeric unit is 0.83 cm Kmol and corresponds to a total magnetic
moment of 2.57, typical for two non interacting S = ½ ions. This value is kept approximately constant
on lowering the temperature to 30 K and then abruptly decreases to a minimum value of 0.556
3
–1
cm Kmol at 2 K. This behaviour is consistent with an antiferromagnetic interaction between the copper
ions in the dinuclear compound and can be interpreted by using the spin Hamiltonian H = –J·S ·S ,
1
2
whose analytical expression is the well-known Bleaney-Bowers equation[34].

2
2
2

 Ng  / kT·
(1)
J /kT

3
 e
where N, g,  and k have their usual meaning and J is the coupling constant between the two interacting
–
1
2
copper spins. The best fit has been obtained for g = 2.102(3) and J = –1.50(1) cm , with R = 0.9974
Fig.3).
(
These data are indicative of a small coupling between the two copper ions. The crystal structure supports
this weak interaction since the coupling path through one brigding oxygen involves a long Cu–O
distance product of the Jahn-Teller distortion in the Cu(II) coordinative environment. The theoretical
magnetic orbitals for both complexes 1 and 2 are shown in Scheme 5.
The magnetic behaviour of 2 is similar to that of complex 1. It shows a room temperature T value of
3
–1
0
.587 cm Kmol per dimeric unit, somewhat lower from the expected value for two copper ions. This
3
–1
value slightly increases to 0.596 cm Kmol at 250 K, a fact probably due to some error in the
3
–1
experimental values. On lowering the temperature, the T value steadily decreases to 0.478 cm Kmol
3
–1
at 25 K followed by a drop to 0.0375 cm Kmol at 2 K. The behaviour between 25 and 250 K can be
interpreted as a contribution of a temperature independent paramagnetism [35], while the drop at lower
temperatures is indicative of the antiferromagnetic coupling expected for the two copper ions present in
the molecule.
The room temperature T value found for this compound corresponds to a magnetic moment of
2
.17 , lower than the predicted value of 2.44  for two uncoupled S = ½ spins. The interacting
binuclear magnetic cluster does not give any substantial fit of data (with some errors), so it may be
assumed that intercluster interaction, if any, must be very small. Based on this assumption, the
paramagnetic susceptibility has been considered as the sum of the copper dimer contribution
(represented by equation 1) and a temperature independent paramagnetism. The best fit of the data
–
1
–6
3
–1
2
below 250 K gave g = 2.070(2), J = –7.7(1) cm , TIP = 290(6)×10 cm mol , with R = 0.9969
(Fig.4). No significant changes were observed on fitting the T values between 2 and 300 K.
The fitting of the susceptibility variation using the same parameters (Fig.4 inset)
reproduces well the maximum observed at 8 K, a consequence of the antiferromagnetic coupling
between the copper atoms. This maximum is not observed for 1 since the smaller coupling constant

would shift it to ~2 K, the lower limit of the measurements. These fittings indicate that both dimers have
a similar magnetic behavior, differing only in a somewhat more intense coupling for 2. There are two
short Cu–O distances (1.97 and 1.99 Å in both derivatives) and two long ones (2.47 and 2.74 Å for 1 and
2
.38 and 2.70 Å for 2), leading to a Cu–Cu distance of 3.27 Å and 3.18 Å, respectively. The shorter
distances in 2 could account for the observed higher J value. Alternatively, a three atom bridge path
Scheme4) is possible, since both copper atoms are bridged by one sulfate anion in 1 and one
(
carboxylate in 2. Never the less, this path is less prone to be responsible for the magnetic coupling, but it
could have and indirect influence, since the smaller size of the O–C–O fragment relative to the O–S–O
fragment should be responsible for the shorter distances found in 2.
3
.4. Electronic spectral studies
The electronic spectra of complexes 1 and 2 have been recorded in DMSO solutions. The UV-
3
visible spectrum of complexes 1 and 2 (3 10 M) in DMSO shown in Fig. S3 indicates a broad d–d
band at 704 nm for 1 and at 712 nm for 2. In the visible region, each spectrum involves ligand field
bands, appearing in the ~ 700 nm regions, corresponding to transitions d  d
and dxz
,
2
2
2
z
x y
d  d 2
respectively, as expected for square-planar Cu(II) complexes and confirming the
d
x y
2
2
2
yz
x y ,
based ground state[36]. Square-planar Cu(II) complexes may formed by the decomposition or
dissociation of complexes to mononuclear species in DMSO, as expected for Square-planar
environment around Cu(II) centre. Such types of absorption bands between the Cu(II)–Cu(II)
homobinuclear copper(II) complexes were observed in homobinuclear complexes reported in the
literature[37]. In addition, the UV-visible spectra of both complexes at 390 nm are attributed to a L → M
charge transfer associated with the nitrogen and oxygen donors [38] as shown in Fig. S5.
3
.5. EPR Experiments
The EPR spectra of complexes 1 and 2 in polycrystalline material at RT and in DMSO solutions
-3
(
310 M) at LNT are recorded and shown in Fig. 5. The estimated EPR parameters are presented in
Table 4. The EPR spectra of complexes 1 and 2 in polycrystalline are typical of S = 1 systems. The
polycrystalline EPR spectra of both complexes were recorded at room temperature (RT) and are
displayed in Fig. 5. The room temperature polycrystalline spectra exhibit the expected spectral feathers

of a triplet state with a low field (∆M = ±2) signal [39]. The spin Hamiltonian parameter (

) for an S
s
=
1 state interacting with a magnetic field (H) is defined as:
2
Z
2
x
2
y

 gHs  DS  E S  S  2 / 3 D




where D and E are zero field splitting parameters. The polycrystalline spectra of binuclear complexes 1
and 2 are quite similar in appearance (Fig. 5), with parallel and perpendicular signals unresolved at X-
band frequencies. The EPR spectra are in agreement with the geometry confirmed from molecular
structures (Fig. 1(a) and 2(a). The EPR spectrum of complex 1 consists of a g∥ absorption at g = 2.237
and a signal corresponding to g⊥ signal at g = 2.060. Similar spectral feathers were also shown by
complex 2. The half-field ∆M = ±2 transitions near 1560 G for both complexes are present in EPR
s
spectra. This signal is comparatively strong in complex 2 [Fig. 5(b)]. These kinds of observations at RT
suggest that the two copper(II) ions are antiferromagnetically spin-spin coupled, as demonstrated
through variable temperature magnetic measurements[40]. The exchange interaction parameter G, of
complexes (3.410 for 1 and 2.924 for 2) suggest that there is an exchange interaction between two
copper centers which may be propagated through bridging contacts as shown in molecular structures
[41]. Zero- field splitting parameter (D) of present complexes shown in Table 4 and are consistent with
other reported binuclear copper(II) complexes [39]. Unfortunately, the forbidden transition (∆M = ±2)
s
is not defined in 1 and poorly resolved in 2 DMSO solution at LNT. It may be caused by the
decomposition or dissociation of complexes to mononuclear species in DMSO, as a result EPR spectra
in DMSO at LNT are typical for usual mononuclear copper(II) complexes with square planar
geometry[42]. EPR spectra of both complexes exhibit three
g lines well-defined and one overlapped

with g component at high field part of the spectra at LNT [Fig. 5]. EPR spectra of both complexes 1
and 2 were also recorded in DMSO solution at RT in both cases poorly resolved forbidden transition
(
∆M = ±2) was observed (Fig. S6 and S7). The symmetric shape of spectra with a narrow high field
s
peak (g ) and low field components (
g ) is a characteristic for a g-factor anisotropy with g > ,
g
 
┴
where indicates
║
and
┴
denote the direction of the magnetic field parallel and perpendicular to the
and g of complex 1 and 2 are consistent with a d 2 based ground
anisotropy axis. The values of
g

┴
2
x y
state[43]. EPR parameters and d-d transition energies were used to estimate in-plane,

-bonding
parameters ( 2), in-plane-  2) and out-of plane-

(

(
 2) parameters, which may be known as

measures of co-valency in metal complexes. The values of these parameters are consistent with strong
in-plane - and in-plane -bonding. The orbital reduction factors ( and K ) were also evaluated in


K


the present complexes
bonding.
K value is less than K in both complexes employs considerable in-plane


3
.6. Electrochemical studies
The redox behavior of these complexes was studied using cyclic voltammetry (CV) and
3
differential pulse voltammetry (DPV). Cyclic voltammograms of 1 and 2 in DMSO(310 M) were
recorded at ambient temperature with TBAP as a supporting electrolyte (0.1M) in a nitrogen
atmosphere. Cyclic voltammograms (scan rate 300 mv/s) are shown in Fig. 6(a) and redox properties are
summarized in Table 5. In both complexes two reduction processes (
II
II

II
I
II
I

I
I
Cu Cu  e  Cu Cu andCu Cu  e  Cu Cu ) vs. Ag/AgCl references, both involving an
identical number of electrons, are revealed from DPV experiments (Fig. 6b). It has been observed that
DPV is a very good technique for resolving redox responses having small differences in peak potentials,
provided the two peaks differ in their formal potential by more than 180 mV. The Cu(II)/Cu(I) couple
appears at ~0.018 V with respect to the Ag/AgCl electrode and in most cases in the CV experiments the
two Cu(II)/Cu(I) couples are not resolved, though in DPV these two couples are sometimes resolved.
The cyclic voltammograms for both 1 and 2 were also recorded in acetonitrile (Fig. S8). In acetonitrile
some differences appear in redox behaviour which may associated with changes in the coordination
spheres of complexes 1 and 2. The difference in electrochemical behaviour in both complexes could be
due to the distribution of the electron density of coordination sphere.
A comparison of the electrochemical data (Table 5) of these complexes indicates that process I in
II
II
II
I
1
and 2 is due to a one-electron Cu Cu

I
Cu Cu reduction the involving the square planar copper
II
I
I
site, while the reduction Cu Cu

Cu Cu at more negative potential (Process II) also involves a
square planar copper center [44]. The oxidation potentials of complex 1 are more negative than 2 and

could be due to some structural changes involved [Fig. 1(a) and 2(a)]. Reduction potentials for
complexes 1 and 2 are in agreement with reported binuclear complexes in the literature[45]. The
reduction process of complexes 1 and 2 were irreversible in these reduction processes where two
electrons are involved. The redox process of two copper centers seems to occur over a broad range with
two reduction and oxidation peaks (Fig. 6a). These reduction waves are confirmed by DPV experiments
(Fig. 6b). The stability of the mixed valent complexes is expressed by the conproportionation constant
(
K ) for the following equilibrium:
con
K
II
II
I
I
com
II
I


Cu Cu  Cu Cu  2Cu Cu
which was estimated using the relationship log Kcon = 16.9(∆E )[46]. The observed value of K for
1/2
con
both complexes is given in Table 5. From Table 5, it is evident that the large Kcon values indicate that the
II
I
addition of a second electron is most difficult than the first electron and the Cu Cu mixed valence
species is stable with respect to conproportionation.
Similar observations were reported earlier in many binuclear systems [46,5c]. The value of Kcon for
5
6
complex 1 was found to be 3.6×10 compared to the value of K
( 4.9  10 ) for complex 2 which
con
confirms that complex 1 is more stable.
3
.7. Quantum computational density functional theory based geometry optimization and
molecular orbital analysis
The B3LYP /LAN2DZ level of theory was used to calculate bond angles and bond lengths. The
selected bond lengths and angles are given along with the crystal data (Table 2).The quantum
computational DFT calculated bond lengths and angles are similar to that of experimentally observed
data. The calculated bond lengths and angles are slightly larger than the experimental (XRD) values
since the quantum computational DFT calculation were performed on isolated molecules in the gas
phase, whereas the XRD results were obtained in solid state[47].The six important

and  molecular
orbitals (MO's) where analyzed for complexes 1 and 2. These MO's with energy level diagrams are
shown in Fig. 7 and 8. MO's play important roles in the electrical, optical and chemical reactions [48].
As these MO's can decide the way in which a molecule interacts with other chemical species, therefore,

these are known as the frontier molecular orbitals(MO's). These MO's may be highest occupied
molecular orbitals (HO MO's) and lowest unoccupied orbitals (LUMO) and are important parameters for
.
illustrating the chemical behavior (Fig. 7) of these molecular species[49] The derived energy of six

spin states (HOMO-2 to LUMO+2) for complex 1 are -6.538,-6.461,-5.461,-2.262,-0.950 and -0.818 eV
respectively and energy gaps ( E) between HOMO-LUMO, HOMO-1-LUMO+1 and HOMO-2-

LUMO+2 are 3.199,5.51 and 5.721eV, respectively. Similarly, six MO energies of the β spin state have
been calculated. HOMO-2 to LUMO+2) are also calculated along with energy gaps as shown in Fig. 7.
MO energies and the energy gaps were calculated for complex 2 as well and are depicted in Fig. 7. The
energy gap in complex 1 is found to be larger than for complex 2. The energy gap between the HOMO
and LUMO orbitals indicate molecular chemical stability [18a]. A large HOMO-LUMO gap suggests a
stable molecule with low chemical stability [18b]. Also, the energy gaps between the HOMO-LUMO
orbitals are important parameters to decide the electrical transport properties of molecules [50].
Therefore complex 2 is more active than 1.
The unpaired electrons remain in the HOMO and hence the HOMO are also known as singly
occupied orbitals (SO MO's). In HOMO-LOMO structures of 1, HOMO-1 and LUMO of

spins states
are mainly constructed on the d 2 magnetic orbital of the copper(II) atoms and sp hybrid orbitals of
2
2
x -y
the oxygen atoms bearing the lone pairs in the bridging region. The exchange pathway is of the
Moreover, SOMO's show that the atomic orbitals with a higher contribution to these HOMO's and
LUMO's are those of the non-bridging atoms (oxygen and nitrogen atoms) of the Schiff base and the
bridging moieties of the oxygen atoms in HOMO-2 and the spins states. In 2, the electron spin

type.

densities on non- bridging and bridging atoms are have a positive electron spin density (Fig. 8). In 2,
HOMO and LUMO+1 also showed electron spin density in the C = C moiety of the Schiff base (Fig.
8
).This type of electron delocalization from copper centers to donor atoms is in agreement with the
molecular structures obtained from single crystal X-ray analysis.

The molecular orbital analysis is used to predict the delocalization of electron density between occupied
Lewis type natural molecular orbitals (NBO's), which correlate to stability donor acceptor (D-A)
interactions[55]. As per molecular orbital analysis, all kind of interactions between the copper(II) atoms
and coordinating atoms are referred to as coordinate bonds of the type N
Cu or O Cu. Such
 
types of D-A mechanisms are shown in Fig. 9. In such interactions electron density is transferred from
the lone pair orbitals on the nitrogen or oxygen atoms, LP(N) or LP(O) to the antibonding LP* (Cu).
SOMO selected orbitals showing LP (N) → LP* (Cu) and LP (O) → LP* (Cu) interactions are shown in
Fig. 9.
The spin density distribution was observed using the quantum computational DFT using the
B3LYP/LANL2DZ basis set for the present complexes. The spin density is mainly delocalized into the
copper(II) atom and those atoms which are bonded to copper(II) atom[5c]. The spin density of
complexes is given as an example in Fig. 10. The positive sign of the densities are spread over the
copper(II) atom and the negative sign of the densities were distributed over the donor atoms. These
distributions were also supported with the HOMO-LUMO shapes observed in these copper(II)
complexes.
3
.8. Antioxidant superoxide dismutase (SOD) assay
The antioxidant SOD mimetic activities of these complexes has been evaluated and discussed.
The antioxidant SOD activities of the complexes were evaluated by an NBT assay[52] following the
+
reduction of NBT to MF kinetically at 560 nm. These complexes exhibit significant catalytic activity
toward the dismutation of superoxide anions. The concentration to attain 50% inhibition of the NBT
reduction defined as IC were estimated for complexes 1 and 2(Table 6). The observed IC values of
5
0
50

the present complexes were compared with earlier SOD mimics that have been reported[1f, 5c, 53, 54]
(Table 6) and the value fo the native SOD is also given for the comparison. The IC value for complex 2
50
(24 μmol) is less than 1 (35 μmol), thus, complex 2 is more antioxidant SOD active than 1. The
difference in IC value between 1 and 2 may be ascribed to structural variation, thus favouring ligand
5
0
exchange and interaction with the substrate. The frontier molecular orbitals suffer a sustained change in
coordination atmosphere around the metal centres in complex 2. In complex 2, Cu1 is six coordinate and
Cu2 is five coordinate. Water molecules also play an important role for enhancing antioxidant SOD
activity by guiding the O2∙ ─ ion to enter and allow H O to leave rapidly from the copper centre. The
2
2
same trend of biological activity is also verified from the quantum computational DFT calculations. The
catalytic rate constant (kMcCF) was also evaluated for these complexes. The kinetic catalytic constant
−
4
–1 -1 –1
kMcCF was obtained by the equation kMcCF = kNBT  [NBT] / IC , where k
= 5.94 × 10 (mol L ) s
50
NBT
is the second order rate constant for NBT[55]. The kinetic catalytic constant (kMcCF) of 1 and 2 are 9.50
−
4
–1 -1 –1
−4
–1 -1 –1
×
10 (mol L ) s and 13.84 × 10 (mol L ) s , respectively. The values of catalytic rate constants
for the superoxide disproportiation constant (kMcCF) clearly indicate that the present complexes can be
used as an antioxidant superoxide scavenger. Comparisons of kMcCF for complexes 1 and 2 reveal that
the geometry around copper contributes to the modulation of antioxidant SOD activity, with 2 being the
best suited to react with superoxide free radicals. On the basis of antioxidant results, it is clear that
complexes 1 and 2 are more efficient antioxidant than vitamin c, which is the standard for antioxidant
superoxide dismutase[56].
3
.9. Structure activity relationship
The catalytic role of the stable sulfate/succinato bridge can be well understood according to a
mechanism suggested in the literature[57] (Scheme 6). The dismutation of O2∙ ─ may take place in the
following steps: First, O2∙ ─ displaces an H O molecule and binds directly to a copper(II) ion and gives
2
up its electron. Second, the O2∙ ─ binding directly to the copper(II) ion can be exchanged rapidly between
the axial and the planar position of the distorted square pyramid, leading it to give up its electron and
form an O molecule. Third, the sulfate/succinato bridge breaks and the electrically neutral oxygen
2
molecule leaves. The O atom of sulfate/succinato bridge gets its proton from the bulk solvent and
copper(II) reduces to copper(I) [Scheme 6 (iv)]. Fourth, a second O2∙ ─ binds again to the copper(I) ion
and accepts an electron. Since the proton exchange between the substrates and buffer is a rapid process,

the O2∙ ─ further combines with another proton from the solution to form an H O molecule. Finally, the
2
2
sulfate/succinato bridge re-forms and electrically neutral H O leaves from the system, completing a
2
2
catalytic cycle.
The reduction potentials of the present complexes are compared with the bovine and human SOD
metalloenzyme[55c]. Fortunately, reduction potentials of complexes 1 and 2 are lower than the reduction
potential values of 0.115 and 0.075V/SHE for the bovine and human SOD respectively, but in a very
good range for promoting dismutation of the O2∙ ─ anion as shown by the high catalytic activity. High
SOD activity observed for complexes 1 and 2 could be explained in terms of fast exchange of axial
water molecules and less steric hindrance to the O2∙ ─ moiety in these complexes. These complexes have
potent SOD activity and may be considered as therapeutic agents with a small molecular weight
compared to native SOD. Furthermore, the moderately high SOD activities of complexes 1 and 2 may
also be related to possible cooperation of two copper(II) centres, in close proximity, in free radical
binding and electron transfer through the sulfate/succinato bridge. In native SOD, one Cu(II) may
resemble the role of a mimic through the imidazolato bridge, in controlling the electron density of the
redox action copper center[55d]. The relatively high SOD activity of complex 2 could be due to some
structural variation.
4
. Conclusions
A dual approach comprising the experimental and quantum computational study of two new
bridged binuclear copper(II) coordination complexes 1 and 2 that have been synthesized by metal N'-
(E)-pyridin-2-ylmethylidene] benzohydrazide appear to be good models for superoxide dismutase. For
[
complexes 1 and 2, we were able to appropriately fit magnetic susceptibility data. Complex 1 has
bridging sulfate and two oxygen atoms of two O, N, N’ ligands. Complex 2 has bridging succinate and
also two oxygen atoms of two same ligands. The extensive ligand based hydrogen bonding, CH

π
,
ππ and lpπ (metal chelate) interactions in both complexes have resulted into new intriguing 3-D
supramolecular structures. The cryomagnetic susceptibility data revealed that the copper(II) centers in 1
-1
-1
and 2 are antiferromagnetically coupled (J =
 
1.50) cm for 1 and (J = 7.7) cm for 2).
Electrochemical studies of binuclear complexes 1 and 2 showed two redox waves. Coordinated water
molecules in both complexes create well directed H-bonding activity. Bond lengths and angles from
computational DFT calculations have been compared with those obtained from single X-ray techniques

and agreements between them have been observed. In addition, antioxidant (SOD) measurements
indicate that both complexes behave as superoxide dismutase bio mimetic systems. In both complexes
coordination sites available for the biding of O2∙ ─ as shown by the X-ray structures show antioxidant
SOD activity.
Acknowledgements
Authors thanks RSIC (SAIF) IIT, Bombay for EPR measurements SAIF, Central Drug Research
Institute, Lucknow, are thankfully acknowledged for providing analytical, spectral facilities. Authors
acknowledge Dr. Alok Banerjee, UGC-DAE CSR, Indore, for providing the low temperature magnetic
measurement facility. The authors wish to acknowledge the assistance of Dr. Matthias Zeller in the
collection of diffraction data and NSF Grant CHE 0087210, Ohio Board of Regents Grant CAP-491, and
by Youngstown State University for funds to purchase the X-ray diffractometer.

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Fig. 1. (a) Molecular structure of 1 showing the atom labelling scheme of the asymmetric unit, (b)
Coordination sphere of 1, (c) Hydrogen bonding, weak intermolecular interactions and heterosynthon
graph-set ring motifs for 1 (blue dashed lines), (d) Weak CH

π and H
π stacking inter and intramolecular interactions for 1
dashed lines), (f) Uncoordinated solvent molecules along with lone pair interactions for 1 (dashed lines)

π intermolecular interactions
(dashed lines) for 1, (dashed lines), (e) Weak π

(
Fig. 2. (a) Molecular structure of 2 showing the atom labelling scheme of the asymmetric unit, (b)
Coordination sphere of 2, (c) Hydrogen bonding, weak intermolecular interactions and heterosynthon
graph-set ring motifs for 2 (blue dashed lines), (d) Weak CH

π and H
π stacking inter- and intramolecular interactions for 2,
dashed lines), (f) Uncoordinated perchlorate ion along with lone pair interactions for 2, (dashed lines)

π intermolecular interactions
(dashed lines) for 2, (dashed lines),(e) Weak π

(
Fig. 3. Temperature dependence of T and  (inset) for complex 1. The solid line represents the fit using
the equation and parameters described in the text
Fig. 4. Temperature dependence of T and  (inset) for complex 2. The solid line represents the fit using
the equation and parameters described in the text
Fig. 5. (a) EPR spectra of complex 1 in polycrystalline state (RT) and DMSO solution at liquid nitrogen
temperature (LNT), inset half-field at RT and LNT. (b) EPR spectra of complex 2 in the polycrystalline
state (RT) and in DMSO solution at liquid nitrogen temperature(LNT), inset half-field at RT and LNT
Fig. 6. (a) Cyclic voltammograms for 1 and 2 in DMSO at an Ag/AgCl electrode with scan rate 100 mV
-
1
s and temperature 25
℃. (b) Differential pulse voltammogram (DPV) for 1 and 2 at room temperature
-1
using a scan rate 20 mV s in DMSO. The pulse amplitude is 50 mV
Fig. 7. HOMO-LUMO structure with energy level diagram for complex 1
Fig. 8. HOMO-LUMO structure with energy level diagrams for complex 2
Fig. 9. The donor and acceptor orbitals involved in the donor-acceptor (D-A) mechanism interactions in
complexes 1 and 2
Fig. 10. (a) Spin density of complex 1; (b) Spin density of complex 2

Scheme 1. Half reactions for the dismutation of thesuperoxide radical mediated by the Cu-SOD enzyme.
Scheme 2. Synthesis of complexes 1and 2 is based on the active site structure of Cu-Zn SOD. (A) The
structure of Cu-Zn SOD. (B) Structures of complexes 1 and 2.
Scheme 3. Condensation of salicylhydrazide with 2-pyridine caboxyldehyde which subsequently leads
to the formation of N'-[(E)-pyridin-2-ylmethylidene]benzohydrazide
Scheme 4. Synthetic routes of complexes 1and 2
Scheme 5. Schematic representations of the orientations of d and p atomic orbital’s for the metal and
bridged moieties in complexes 1 and 2
Scheme 6. Schematic diagram of the proposed O dismutation reaction catalyzed by 1 steering the O2∙
∙
─
2
─
2+.
to Cu
Table 1. Crystal data and structure refinement parameters for the structures 1 and 2
Complex
1
2
Empirical formula
Formula weight
Temperature
Wavelength
C
26
H
25 Cu
2
N
6
O
8.50
S
C
30 2 6 11
H28 Cl Cu N O
716.66
811.11
150(2) K
0.71073 Å
Triclinic
100(2) K
0.71073 Å
Triclinic
Crystal system

Space group
P
1
P
1
Unit cell dimensions Å,°
a
b
c
9.6229(19)
12.665(3)
14.924(3)
9.882(4)
12.373(5)
15.488(6)
97.829(6)
6
9.20(3)



7
3.93(3)
108.340(6)
74.54(3)
106.769(6)°.
Volume
3
3
3
1
2
604.9(7) Å
1666.5(11) Å
2
Z
Density (calculated)
3
1
.483 Mg/m
1.616 Mg/m
Absorption coefficient
-1
-1
1
.446 mm
1.425 mm
F(000)
730
826
Crystal size
3
3
0
.33 x 0.26 x 0.21 mm
0.250 x 0.210 x 0.090 mm
1.982 to 26.935°.
Theta range for data collection
Index ranges
2.933 to 25.252°.
-11<=h<=11, -14<=k<=15, -
7<=l<=17
-12<=h<=12, -15<=k<=15, -
19<=l<=19
1
Reflections collected
12405
21623
Independent reflections
Completeness to theta = 25.252°
Absorption correction
Max. and min. transmission
Refinement method
5812 [R(int) = 0.2687]
99.8 %
Semi-empirical from equivalents
0.7509 and 0.6466
7120 [R(int) = 0.0405]
99.3 %
Semi-empirical from equivalents
0.7455 and 0.6045
2
2
Full-matrix least-squares on F
Full-matrix least-squares on F
Data / restraints / parameters
5812 / 11 / 403
0.868
7120 / 53 / 477
1.034
2
Goodness-of-fit on F
Final R indices [I>2sigma(I)]
R indices (all data)
Extinction coefficient
R1 = 0.0950, wR2 = 0.2069
R1 = 0.2209, wR2 = 0.2885
n/a
R1 = 0.0507, wR2 = 0.1472
R1 = 0.0676, wR2 = 0.1592
n/a
Largest diff. peak and hole
-3
-3
0
.673 and -0.927 e.Å
1.880 and -0.708 e.Å
Table 2. Selected bond lengths and angles for structures 1 and 2 [Å and °].
Parameters
Single crystal
XRD data
Theoretical data*
Parameters
Single crystal XRD
data
Theoretical data*
1
Bond lengths
Cu(1)-O(1)
Cu(1)-O(1A)
Cu(2)-N(2B)
Cu(2)-N(1B)
Cu(2)-O(1W)
Bond angles
1.904(8)
1.968(7)
1.924(10)
1.978(8)
2.293(8)
2.1012
2.1832
2.1092
2.0919
2.3320
Cu(1)-N(2A)
Cu(1)-N(1A)
Cu(2)-O(2)
Cu(2)-O(1B)
Cu(2)-O(1A)
1.936(9)
2.027(9)
1.932(8)
1.993(7)
2.745(10)
2.1214
2.1912
2.1502
2.1410
2.849
O(1)-Cu(1)-N(2A)
N(2A)-Cu(1)-O(1A)
N(2A)-Cu(1)-N(1A)
N(2B)-Cu(2)-O(2)
O(2)-Cu(2)-N(1B)
O(2)-Cu(2)-O(1B)
N(2B)-Cu(2)-O(1W)
N(1B)-Cu(2)-O(1W)
2
170.2(4)
80.2(3)
80.5(4)
171.5(3)
97.2(4)
101.1(3)
95.9(3)
94.0(3)
173.4
81.8
81.9
173.2
98.7
102.7
97.0
O(1)-Cu(1)-O(1A)
O(1)-Cu(1)-(1A)
O(1A)-Cu(1)-(1A)
N(2B)-Cu(2)-(1B)
N(2B)-Cu(2)-(1B)
N(1B)-Cu(2)-(1B)
O(2)-Cu(2)-(1W)
O(1B)-Cu(2)-(1W)
99.0(3)
99.9(4)
160.6(4)
81.5(4)
78.8(4)
158.6(4)
92.6(3)
96.2(3)
100.4
101.3
161.8
82.7
79.9
159.8
93.7
95.6
97.8
Bond lengths
Cu(1)-N(2A)
Cu(1)-O(1A)
Cu(1)-O(1W)
Cu(2)-O(2)
Cu(2)-O(1B)
Cu(2)-O(1A)
Bond angles
1.936(3)
1.994(3)
2.284(3)
1.921(3)
1.969(3)
2.379(3)
2.0837
2.1385
2.3397
2.1546
2.1987
2.4927
Cu(1)-O(1)
1.943(3)
2.011(3)
2.703(3)
1.922(3)
2.008(3)
2.1084
2.1819
2.8197
2.0978
2.2280
Cu(1)-N(1A)
Cu(1)-O(1A)
Cu(2)-N(2B)
Cu(2)-N(1B)
N(2A)-Cu(1)-O(1)
O(1)-Cu(1)-O(1A)
173.00(11)
99.64(11)
174.8
100.7
N(2A)-Cu(1)-(1A)
N(2A)-Cu(1)-(1A)
78.87(12)
80.47(13)
80.7
81.5

O(1)-Cu(1)-N(1A)
100.27(12)
97.27(11)
97.13(11)
174.18(12)
79.63(12)
80.92(13)
88.10(11)
88.93(10)
101.3
98.3
99.2
175.2
80.9
81.9
90.0
90.1
O(1A)-Cu(1)-(1A)
O(1)-Cu(1)-O(1W)
N(1A)-Cu(1)-(1W)
O(2)-Cu(2)-O(1B)
O(2)-Cu(2)-N(1B)
O(1B)-Cu(2)-(1B)
N(2B)-Cu(2)-(1A)
N(1B)-Cu(2)-(1A)
158.75(12)
89.69(11)
90.35(12)
102.18(11)
96.81(12)
160.17(12)
97.48(11)
97.43(11)
160.5
91.1
91.5
103.0
98.3
161.8
98.3
N(2A)-Cu(1)-O(1W)
O(1A)-Cu(1)-O(1W)
O(2)-Cu(2)-N(2B)
N(2B)-Cu(2)-O(1B)
N(2B)-Cu(2)-N(1B)
O(2)-Cu(2)-O(1A)
O(1B)-Cu(2)-O(1A)
TD-DFT/B3LYP/LANL2DZ Calculations
98.2
*
Table 3. Hydrogen bond lengths and angles for structures 1 and 2 [Å and °].
D-H...A
d(D-H)
d(H...A)
d(D...A)
<(DHA)
Symmetry transformations used to generate
equivalent atoms:
1
O(1W)-H(1W1)...N(3B)#1
O(2W)-H(2W2)...O(1W)
O(3W)-H(3W1)...O(1)
C(6A)-H(6AA)...O(1W)#2
C(2B)-H(2BA)...O(1A)#3
C(6B)-H(6BA)...O(2W)#1
2
0.812(15)
0.82(2)
0.81(2)
0.95
0.95
0.95
2.07(2)
2.12(7)
2.45(8)
2.62
2.44
2.65
2.881(13)
2.787(12)
3.055(16)
3.454(14)
3.384(17)
3.59(2)
176(3)
139(10)
132(9)
146.0
173.1
171.6
#1 -x+1
-y+1
-z+1
#2 x+1 y,z
#3 -x+1 –y
-z+1
O(1W)-H(1W1)...Cl(1)
O(1W)-H(1W1)...O(12)
O(1W)-H(1W1)...O(14A)
O(1W)-H(1W2)...N(3A)#1
C(3)-H(3A)...O(14)#2
C(3)-H(3A)...O(11A)#2
C(2A)-H(2AA)...O(1B)#3
C(6A)-H(6AA)...O(12)#1
C(6A)-H(6AA)...O(14A)#1
C(1B)-H(1BA)...O(4)
C(2B)-H(2BA)...O(13)#4
0.835(18)
0.835(18)
0.835(18)
0.817(18)
0.99
0.99
0.95
0.95
0.95
2.914(19)
1.98(2)
2.29(4)
2.02(2)
2.40
2.29
2.57
2.45
2.26
3.741(4)
2.782(6)
3.05(2)
2.818(4)
3.276(6)
3.267(19)
3.494(5)
3.392(7)
3.18(3)
171(4)
161(3)
152(3)
164(4)
146.7
167.8
164.3
169.6
162.4
133.4
127.0
#1 -x+1,
-y+1
-z+1 #2
-x+2
-y+1
-z+2 #3
-x+1-y
-z+1#4
-x+1
-y+1
-z+2
0.95
0.95
2.62
2.62
3.345(6)
3.276(6)
Table 4. EPR parameters of copper(II) complexes 1 and 2.
EPR parameter
Polycrystalline state (298 K)
1
2
2
2
.237
.060
2.171
2.060
g

g┴
G
3.410
0.090
2.924
0.095
-1
D (cm )
Frozen solution in DMSO (77 K)
2
2
1
.200
.055
55
2.218
2.060
156
g

g┴
A
(G)

2
α
β
γ
0.659
0.220
0.227
0
0
7
0.321
0.474
0.490
0.152
2
2
.145
.149
05
K

0.157
712
K┴
max (nm)