Synthesis, characterization, theoretical calculations and enzymatic activities of novel diimine-dioxime ligand and its homodinuclear Cu(II) complex

Article abstract of DOI:10.1016/j.molstruc.2018.03.015

A novel ligand [2-(biphenyl-4-yl)-2-(2-(1-(biphenyl-4-yl)-2-(hydroxyimino) ethylideneamino)ethylimino)acetaldehyde oxime] (BPHEO) and its homodinuclear Cu(II) complex [Cu₂(L) (H₂O) (phen)](ClO₄)₂ were prepared starting from biphenyl and characterized by different physical techniques. Elemental analysis, ICP-OES, FT-IR, UV–vis molar conductivity, magnetic moment measurements and thermal analyses studies were used for the characterization of the complexes. The free ligand was also characterized by ¹H and ¹³C NMR spectra. Elemental analyses, stoichiometric and spectroscopic data of the metal complex indicated that the metal:ligand ratio of homodinuclear Cu(II) complex was found to be 2:1. The structural and vibrational spectroscopic data of the BPHEO and its homodinuclear Cu(II) complex were calculated and confirmed by DFT method. Chemical shifts (¹H and ¹³C NMR) of the ligand were also calculated using the gauge-independent atomic orbital (GIAO) method. Furthermore the synthesized complex was tested as catalyst for the catalytic oxidation of 3,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone at aerobic medium (catecholase-like activity) and disproportionation of hydrogen peroxide to the water and molecular oxygen in the presence of 1-methylimidazole (catalase-like activity). It was found that the complex showed moderate catalytic activity and suitable as a catalyst for the selected enzymatic reactions.

Full text of DOI:10.1016/j.molstruc.2018.03.015

Accepted Manuscript
Synthesis, characterization, theoretical calculations and enzymatic activities of novel
diimine-dioxime ligand and its homodinuclear Cu(II) complex
Bülent Dede, Nurten Özen, Güvenç Görgülü
PII:
S0022-2860(18)30298-9
10.1016/j.molstruc.2018.03.015
MOLSTR 24954
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 27 October 2017
Revised Date: 28 February 2018
Accepted Date: 6 March 2018
Please cite this article as: Bü. Dede, N. Özen, Güç. Görgülü, Synthesis, characterization, theoretical
calculations and enzymatic activities of novel diimine-dioxime ligand and its homodinuclear Cu(II)
complex, Journal of Molecular Structure (2018), doi: 10.1016/j.molstruc.2018.03.015.
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ACCEPTED MANUSCRIPT

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SYNTHESIS, CHARACTERIZATION, THEORETICAL
CALCULATIONS AND ENZYMATIC ACTIVITIES OF NOVEL
DIIMINE-DIOXIME LIGAND AND ITS HOMODINUCLEAR Cu(II)
COMPLEX
Bülent DEDE1*, Nurten ÖZEN1, Güvenç GÖRGÜLÜ2
1Süleyman Demirel University, Faculty of Sciences and Arts, Department of Chemistry,
Isparta, Turkey
2Mehmet Akif Ersoy University, Faculty of Education, Department of Science Education,
Burdur, Turkey
*e-mail: bulentdede@sdu.edu.tr
Tel: +90 246 2114153
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ABSTRACT
A
novel
ligand
[2-(biphenyl-4-yl)-2-(2-(1-(biphenyl-4-yl)-2-(hydroxyimino)
ethylideneamino)ethylimino)acetaldehyde oxime] (BPHEO) and its homodinuclear Cu(II)
complex [Cu2(L)(H2O)(phen)](ClO4)2 were prepared starting from biphenyl and characterized
by different physical techniques. Elemental analysis, ICP-OES, FT-IR, UV-vis molar
conductivity, magnetic moment measurements and thermal analyses studies were used for the
1
characterization of the complexes. The free ligand was also characterized by H- and 13C-
NMR spectra. Elemental analyses, stoichiometric and spectroscopic data of the metal complex
indicated that the metal:ligand ratio of homodinuclear Cu(II) complex was found to be 2:1.
The structural and vibrational spectroscopic data of the BPHEO and its homodinuclear Cu(II)
complex were calculated and confirmed by DFT method. Chemical shifts (1H- and 13C-NMR)
of the ligand were also calculated using the gauge-independent atomic orbital (GIAO)
method. Furthermore the synthesized complex was tested as catalyst for the catalytic
oxidation of 3,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone at aerobic medium
(catecholase-like activity) and disproportionation of hydrogen peroxide to the water and
molecular oxygen in the presence of 1-methylimidazole (catalase-like activity). It was found
that the complex showed moderate catalytic activity and suitable as a catalyst for the selected
enzymatic reactions.
Keywords: Imino-oxime molecule, dinuclear complex, DFT calculations, catecholase-like
activity, catalase-like activity
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1. Introduction
Protein and non-protein metallobiomolecules play major structural and functional roles in
every organism [1]. Understanding the roles that metallic and nonmetallic elements play in
biological systems is the goal of biological inorganic (bioinorganic) chemistry. Understanding
the geometry and other details of metal binding sites of metalloproteins enabled to synthesize
molecules mimicking the natural compounds [2]. Today, synthesizing ligands and its metal
containing complexes, changing metal ions in the center, increase the number of metal ions
(homo/heteromultinuclear) in a complex is a great era of bioinorganic chemistry. Metal
complexes, with their comparable advantages to natural enzymes in delivery and stability, are
the candidates to be the solution of many problems in medicinal pharmacology and other
disciplines. Besides, Schiff base and oxime group containing Cu(II) complexes are excellent
precedents for biological systems due to their physical and chemical behaviours [3,4].
Various techniques have been used to illuminate the structures of these synthesized
compounds. Combination of experimental and theoretical study gives better results for
understanding of molecular and bonding properties of related compounds [5]. Technological
developments in computing science enabled to study more complex structures. Density
functional theory (DFT) has been a major quantum chemical method for the prediction of
basic, thermodynamic and electronic properties [6].
Oxidation–reduction processes continue to be a central theme of biological inorganic
chemistry [7]. Most of the metal complexes are potential agents with a high capacity for
oxidation-reduction reactions. They mimic a number of enzymes especially, oxido-reductases
which are a broad group of enzymes. Catalase (EC 1.11.1.6) and catecholase (catechol
oxidase: EC 1.10.3.1) are the important members of this group. Catalase contains Fe(II) and
catecholase contains Cu(II) in their active sites [8].
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In this study, a novel diimine-dioxime ligand and its homodinuclear Cu(II) complex were
synthesized and characterized using various spectroscopic and analytical methods. Also, DFT
studies were performed to illuminate electronic and molecular structures of the ligand and
complex. Theoretical studies were used to compare and correlate with experimental results. In
addition, catecholase and catalase-like activities of homodinuclear metal complex were tested.
2. Experimental
2.1. Materials and Methods
Chemicals and solvents were purchased from the commercial sources and used without
further purification. 1H- and 13C-NMR spectra were recorded in CDCl3 on a Bruker 400 MHz
spectrometer using TMS as an internal standard. Elemental analyses (C, H, N) were
performed using a Leco 932 CHNS analyser and ICP-OES analysis were done on a Perkin
Elmer Optima 5300 DV ICP-OES spectrometer. The UV–Vis measurements were carried out
on a PG T80+ UV–Vis spectrometer. A Shimadzu IRPrestige-21 FT-IR spectrometer was
used to record infrared spectra of the compounds using KBr disks (4000-400 cm−1).
Conductivity measurement of 10-3 M N,N-dimethylformamide solution of prepared complex
was determined at room temperature using a Optic Ivymen System conductivity meter.
Melting points of the compounds were measured using an electrothermal model IA 9100. The
magnetic moment values were taken from a Sherwood Scientific Magnetic Susceptibility
Balance (Model MX1) at room temperature.
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2.2. Synthesis of Ligand, [2-(biphenyl-4-yl)-2-(2-(1-(biphenyl-4-yl)-2-(hydroxyimino)
ethylideneamino)ethylimino)acetaldehyde oxime] (BPHEO)
1-(biphenyl-4-yl)ethanone and 2-(biphenyl-4-yl)-2-oxoacetaldehyde oxime was synthesized
according to the literature [9-11]. BPHEO was synthesized originally by solving 2-(biphenyl-
4-yl)-2-oxoacetaldehyde oxime (30 mmol) in absolute ethanol (10mL). Then, this solution
was added to ethylenediamine (15 mmol) which was also dissolved in absolute ethanol. The
mixture was stirred for 2 hrs at room temperature. Final solution was filtered and the
precipitation rinsed several times with Et2O and dried over P2O5.
2.3. Synthesis of Complex [Cu2(L)(H2O)(phen)](ClO4)2
Caution: Perchlorate salt of metal complex is potentially explosive and should be handled in
small quantities.
A solution of Cu(ClO4)2.6H2O (1 mmol) in acetone (25 mL) was added to the ligand solution
(1 mmol) in 30 mL of EtOH, and this mixture was refluxed with stirring for 1h. After
stripping off the excess solvent under reduced pressure, a crude oily product was obtained.
The mononuclear Cu(II) complex was used without further purification.
The mononuclear Cu(II) complex (1 mmol) was added to Et3N (101 mg, 1 mmol) in MeOH
(25 mL) and the mixture was stirred for 0.5 h. The solution of Cu(ClO4)2.6H2O (370 mg, 1
mmol) in MeOH (10 mL) and 1,10-phenanthroline monohydrate (397 mg, 2 mmol) in MeOH
(10 mL) was successively added to the resulting mixture, which was refluxed for 3 h. The
product was filtered off, washed with H2O, MeOH and Et2O and dried over P4O10.
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2.4. Computational Methods
Gaussian 09 software package was used for theoretical calculations [12] and GaussView 5.0.9
software package was used for molecular visualizations [13]. Geometry optimizations and
frequency calculations were performed in gas phase at the level of density functional theory
(DFT) using Becke’s three parameter exchange-functional (B3) [14] combined with gradient-
corrected correlation functional of Lee, Yang and Parr (LYP) (B3LYP) [15,16] using basis set
6-311++G(d,p) and 6-31G(d,p) for BPHEO and its homodinuclear Cu(II) complex,
respectively.
UV-Vis spectra and electronic properties like HOMO and LUMO energies were determined
1
by TD-DFT method at the B3LYP level with same basis set. H- and 13C-NMR shielding
constants were calculated by applying GIAO method.
2.5. Catecholase-like activity
Homodinuclear Cu(II) complex’s catecholase-like activity was tested on the reaction in
which, the substrate 3,5-DTBC (3,5-di-tert-butylcatechol) converted to the quinone product
3,5-DTBQ (3,5-di-tert-butyl-o-benzoquinone). The catalytic oxidation of the model substrate
was evaluated spectrophotometrically in dioxygen-saturated methanol by monitoring the
increase in absorbance at 400 nm, corresponding to the formation of the product [17]. The
observed rate constant (kobs) value of the metal complex for the 3,5-DTBQ formation was
obtained from eq (1).
ln(A∞/A∞-At)=kt
eq (1)
A∞ and At are the absorbance of the formed 3,5-DTBQ at time = ∞ and time = t, respectively.
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2.6. Catalase-like activity
Evolved dioxygen measured volumetrically from the disproportionation reaction of H2O2 with
homodinuclear Cu(II) complex by a method applied and modified from Kaizer et al.[18].
0.005 mmol of solid complex was dissolved in a 50 mL three-necked round-bottom flask with
o
10 mL N,N-dimethylformamide and placed in a water bath at 25 C. One of the necks was
connected to a burette and the others were stoppered by a rubber septum. 1-methylimidazole
(50 mg) was introduced into the reaction vessel as accelerator just before the addition of
H2O2. 1.33 mmol, 0.150 mL of H2O2 was injected into the reaction medium through the
rubber septum using a micro syringe, while the solution was being stirred. Amount of evolved
dioxygen were measured at 1 min intervals by volumometry.
3. Results and Discussion
Novel diimine-dioxime ligand was performed in three steps from biphenyl as shown in
Scheme 1. 1-(Biphenyl-4-yl)ethanone was prepared from acetyl chloride and biphenyl in the
presence of aluminum chloride according to Friedel-Crafts acylation. 2-(Biphenyl-4-yl)-2-
oxoacetaldehyde oxime was obtained by reacting 1-(biphenyl-4-yl)ethanone with alkyl nitride
in the presence of metallic sodium. Condensation of 2-(biphenyl-4-yl)-2-oxoacetaldehyde
oxime with ethylenediamine gave the novel ligand (BPHEO) 2-(biphenyl-4-yl)-2-(2-(1-
(biphenyl-4-yl)-2-(hydroxyimino)ethylideneamino)ethylimino)acetaldehyde
oxime.
The
homodinuclear Cu(II) complex was prepared by reaction of the ligand with Cu(ClO4)2.6H2O
and 1,10-phenanthroline monohydrate.
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Scheme 1. Synthetic plan for the synthesis of the BPHEO
Newly synthesized diimine-dioxime ligand and its homodinuclear Cu(II) complex were
characterized by several analytic and spectroscopic method.
3.1. 1H- and 13C-NMR Spectroscopic Analysis
1
In order to characterize the structure of the BPHEO, H- and 13C-NMR were recorded in
CDCl3. Furthermore 1H- and 13C-NMR chemical shift calculations of the ligand were made by
using gauge-including atomic orbital (GIAO) method. Optimized geometry was used for the
NMR chemical shift calculations. The experimental and calculated spectra of the BPHEO for
1H-NMR are given in Fig. S1 (Suppl. Mat.) and values are in Table 1. According to
experimental 1H-NMR spectrum, a singlet peak is observed at 8.60 ppm which corresponds to
the proton of oxime groups since the compounds are in symmetrical conformation. Calculated
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value for the oxime protons (H47 and H58) was observed at 7.98 ppm. Multiplet peaks
corresponding to the C-H protons in aromatic groups of ligand were observed between 7.25-
8.05 ppm interval. The chemical shifts for all aromatic protons in the ligand molecule were
calculated theoretically as signals in the range of 7.12-7.78 ppm. The proton bonded to the
imine carbon observed at 6.80 ppm as a singlet peak corresponds to two protons while
theoretically this signal was observed at 6.99 ppm. Aliphatic protons bonded to
ethylenediamine carbons exhibited a chemical shift as a singlet peak at 4.15 ppm. The
corresponding signal was calculated at 3.83 ppm for B3LYP level.
Table 1. The experimental and calculated chemical shift values in 1H-NMR spectrum of the
ligand at the B3LYP/6-311++G(d,p) level δ(ppm)
Experimental
(in CDCl3)
4.15
Theoretical
(in CDCl3)
Atom
H59-H62
H46, H57
3.83
6.99
(t , 4H)
6.80
(s, 2H)
H37-H45
H48-H56
7.25-8.05
(m, 18H)
8.60
7.12-7.78
7.98
H47, H58
(s, 2H)
13
1H-NMR results were confirmed by C-NMR spectral data for the BPHEO. The spectrum
13
was obtained from the ligand dissolved in CDCl3 solution. Experimental and calculated C-
NMR spectra of the BPHEO are shown in Fig. S2 (Suppl. Mat.) and chemical shift values are
listed in Table 2. Peak observed lower region at 163.75 ppm depicts the imine carbon while
the peak corresponding oxime carbon was observed at 155.59 ppm. The corresponding signals
were calculated at 154.06 and 142.46 ppm, respectively. The chemical shift observed at
163.75 ppm corresponding to the imine carbon reveals that the condensation reaction between
ketooxime and ethylenediamine occurred during the synthesis. The experimental chemical
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shift values for aromatic carbons were between 125.12-143.37 ppm range. Chemical shifts for
the aromatic carbons were theoretically calculated in the range of 112.38-130.98 ppm for the
BPHEO. The aliphatic ones in ethylenediamine were observed at 39.04 ppm. Calculated value
1
of methylene carbons by DFT observed at 47.96 ppm. H- and 13C-NMR spectra of
synthesized ligand were in accordance with the previous data recorded for the similar
molecules. Spectroscopic and stoichiometric analysis results of this ligand also supported the
data obtained [19-22].
Table 2. The experimental and calculated chemical shift values in 13C-NMR spectrum of the
ligand at the B3LYP/6-311++G(d,p) level δ(ppm)
Experimental
(in CDCl3)
Theoretical
(in CDCl3)
Atom
C35, C36
39.04
47.96
C1-C12
C18-C29
125.12-143.37
112.38-130.98
C14, C31
C13, C30
155.59
163.75
142.46
154.06
3.2. FT-IR Spectroscopic Analysis
The FT-IR spectra of the BPHEO and its homodionuclear Cu(II) complex were performed in
the range 4000-400 cm−1. Furthermore, IR spectra of the free ligand and its homodinuclear
Cu(II) complex were calculated at the B3LYP/6-311++G(d,p) and B3LYP/6-31G(d,p) level,
respectively. The spectra belong to the ligand and complex both experimental and calculated
are given in Fig. S3. and S4., respectively (Suppl. Mat.). Significant experimental and
theoretical vibrational wavenumbers were selected and these values are given in Table 3.
Theoretically obtained vibrational frequencies are generally higher than the experimental
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ones, if they are unscaled. The reason for the higher frequencies may be the systematic errors,
exclusion of anharmonicity, partially involved correlation energy and trimmed basis sets [23].
Therefore, values of 0.983 and 0.958 were used for frequencies lower and higher than 1700
cm-1, respectively for scaling the calculated values with B3LYP/6-311++G(d,p) basis set
[24,25].
FT-IR spectrum of ligand has a broad band appeared in 3205 cm-1, attributable to ν(O-H), was
lost by the complex formation. The DFT computation predicts this vibrational mode at 3127
cm-1. However, in complex spectrum at 3439 cm-1, new broad band observed indicating
coordinated H2O molecule. ν(O-H) due to the coordinated water was calculated as 3407 cm-1
at the mentioned level.
Stretching vibrations belong to the carbonyl group of 4-acetyl biphenyl seen around 1675 cm-1
were disappeared by the joining of diamine to the structure and C=N stretching vibrations
belong to Schiff base were appeared at 1649 cm-1. The corresponding value was found as
1647 cm-1 in our calculation. C=N stretching vibrations belong to the imine group of ligand
shifted to higher frequencies about 9 cm-1 as a result of complex formation (1658 cm-1 found
and 1637 cm-1 calculated for the complex). However, C=N stretching vibrations belongs to
oxime group appeared at 1602 cm-1 (calc. 1614 cm-1) was shifted to 1598 cm-1 (calc. 1605 cm-
1) by the complex formation. These shifts occurred as a result of complexion reveals that the
ligand coordinated to metal ion through imine and oxime nitrogen [26,27].
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Table 3. Significant experimental and theoretical IR values of the BPHEO and complex (cm-
1
)
Compounds
Exp
O-H C=N(im) C=N(ox) N-O
ClO4
M-O M-N
3205b 1649m 1602m 1382s
-
-
-
-
-
-
BPHEO
Theo
Exp
3127 1647
3439b 1658w 1598w 1390m
3407 1637 1605 1430
1614
1417
1091m-1147m,
623w
1095-1134
621
487w 435w
495 443
COMPLEX
Theo
COMPLEX: [Cu2(L)(H2O)(phen)](ClO4)2
b:broad, s:sharp, m:medium, w:weak
Characteristic band for the ligand appeared at 1382 cm-1 can be attributed to N-O stretching
vibrations of oxime group [28]. The same vibration appeared in the theoretical calculations at
1417 cm-1. By the complexion of the ligand to metal salts, the band belongs to N-O stretching
vibrations shifted higher frequencies (1390 cm-1 found and 1430 cm-1 calculated).
FT-IR spectrum of synthesized complex has antisymmetrical stretching band with medium
intensity observed at 1091 and 1147 cm-1 and antisymmetrical bending band with weak
intensity at 623 cm-1 and calculated as 1095, 1134, 621 cm-1 which are attributed non-
coordinated perchlorate anions [29].
By the complexion of the ligand, weak stretching bands appeared at 487 and 435 cm-1 which
were calculated as 495 and 443 cm-1 at the B3LYP level with 6-31G(d,p) basis set. These
bands correspond to M-O and M-N bonds, respectively [30,31]. Data obtained from the
spectra of the ligand and complex conclusively indicate that Cu(II) ions have been
coordinated with two imine nitrogen and two oxime nitrogen and oxygen atom.
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3.3. UV-vis Spectroscopic Analysis
The electronic absorption spectra data of the BPHEO and its homodinuclear Cu(II) complex
were recorded and compared to each other. The Uv-vis spectra of the compounds are also
given in Fig. S5. and S6. (Suppl. Mat.). BPHEO displayed 3 main peaks: at 281, 295 and 314
nm. The first and second peaks can be attributed to π ꢀπ* transitions of the benzene and
imine group, respectively. The first peak of the ligand was not significantly affected by
complexation. But the π ꢀπ* transition for the imine group was shifted to a shorter
wavelength (288 nm). This shift may be attributed to the chelation of the ligand to the Cu(II)
ion. Chelation is also supported by the shifting of the peak at 314 nm to a longer wavelength
(332 nm) which was attributed to the n ꢀπ* transitions. The strong and weak bands appeared
in the electronic spectra of the metal complex at 455 and 572 nm were assigned to charge
transfer and d ꢀd transition bands, respectively.
The electronic absorption spectrum of the BPHEO was calculated using the TD-DFT/B3LYP
method in ethanol. The calculated and the experimental absorption data, oscillator strength (f)
and the character of electronic transitions are listed in Table 4. The calculation for the BPHEO
predicts three sharp peaks at 309, 323 and 337 nm, which was compatible with the measured
experimental data. According to the calculated absorption spectra, the maximum absorption
wavelength corresponds to the electronic transition from the HOMO-2 to LUMO with 82%.
The transition at 323 nm occurred between HOMO and LUMO with 93% contribution. The
major contributions for the transition at 309 which was the lowest calculated wavelength, are
also given in Table 4.
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Table 4. The experimental, theoretical electronic transitions, excitation energies (eV),
oscillator strengths (f) and major contributions of the BPHEO in ethanol
Theoretical
Transitions
Experimental
B3LYP/6-311++G(d,p)
Major
λ (nm)
E (eV) λ (nm) E (eV)
f
contributions
H-3→L+1 (35%)
π ꢀπ* (benzene)
281
4.41
309
4.01 0.0218 H-5→L+1 (17%)
H-1→L (10%)
π ꢀπ* (imine)
n ꢀπ*
295
314
4.20
3.95
323
337
3.84 0.2971
3.68 0.0090
H→L (93%)
H-2→L (82%)
The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) are called as the frontier molecule orbitals (FMOs). Furthermore, the properties of
the molecules such as the chemical reactivity, kinetic stability, polarizability, chemical
hardness and softness, aromaticity and electronegativity can be determined by using this
energy gap [36,37] between the HOMO and LUMO orbitals.
The HOMO and LUMO orbitals of free ligand were calculated by using B3LYP/6-
31++G(d,p) level. Frontier molecular orbitals of the BPHEO are given in Fig. 1. The HOMO-
LUMO energy gap for free BPHEO molecule was found as 5.439 eV. HOMO orbitals of
BPHEO were located on two benzene rings and the imine group, while the LUMO orbitals
were generally on a benzene, imine and oxime groups. In addition, calculated HOMO and
LUMO energies of the homodinuclear Cu(II) complex were found to be -3.701 and -1.932 eV
in gas phase, respectively forming an energy gap of 1.769 eV (Fig. 2). The energy gap
between HOMO and LUMO orbitals is an important criterion for deciding the stability of the
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molecules. Therefore, the calculated energy gap between BPHEO and its homodinuclear
Cu(II) complex at the DFT level reveals that the ligand is more stable.
Fig. 1. Molecular orbital surfaces and energy levels for the main orbitals which contribute to
the electron transitions of the BPHEO computed at B3LYP/6-311++G(d,p) level.
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Fig. 2. Molecular orbital surfaces and energy levels for the HOMO and LUMO of the
[Cu2(L)(H2O)(phen)](ClO4)2 computed at B3LYP/6-31G(d,p) level.
3.4. Thermal Analysis
Thermogravimetric (TG) and differential thermogravimetric (DTG) analyses for the BPHEO
and its homodinuclear Cu(II) complexes were performed within the temperature range from
room temperature up to 1000oC. The relationship between the decomposition steps of the
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compounds with the corresponding weight losses were investigated in terms of the proposed
formula of the compounds.
The BPHEO with the molecular formula [C30H26N4O2] was thermally decomposed in two
steps. The first step occurs within the temperature range 105-380oC with an estimated mass
loss 18.57% (calculated mass loss = 18.12%), which was reasonably accounted for the loss of
two OH and CN group. The second estimated mass loss of 64.21% (calculated mass loss =
64.48%) within the temperature range 380-675oC could be attributed to the liberation of two
biphenyl groups. Total estimated mass loss was 82.78% (calculated mass loss = 82.60%).
The thermal decomposition of the homodinuclear complex with the molecular formula
[C42H34N6O11Cu2Cl2] proceeded with three main degradation steps. The first estimated mass
loss of 1.91% (calculated mass loss = 1.80%) within the temperature range 170-205oC could
be attributed to the liberation of one H2O group. The second step occurred within the
temperature range 205-615oC with an estimated mass loss 38.45% (calculated mass loss =
38.03%), which was reasonably accounted for loss of two perchlorate and one 1,10-
phenanthroline groups. The last step probably accounted for the decomposition of remaining
part of the molecule. TG analysis of the complex showed that the residual was two mol of Cu
which corresponded to a theoretical mass of 12.75% of the homodinuclear Cu(II) complex of
BPHEO.
Physical properties namely; magnetic susceptibility, molar conductivity, color, yield, melting
point and elemental analyses data were given in Table 5.
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Table 5. Selected physical properties of the BPHEO and complex with elemental analysis data
Melting
Point
(oC)
Calculated(Found) %
Conductivity
ꢀeff
Color
Compounds
(BM) Ω-1cm2mol-1 (Yield %)
C
H
N
Cu
Orange
75.93
5.52
11.81
BPHEO
-
-
130
-
(49.34)
Green
(75.62) (5.74) (11.58)
50.61 3.44 8.43
(50.46) (3.53)
12.75
COMPLEX
2.53
157
>300
(54.46)
(8.74) (12.81)
COMPLEX: [Cu2(L)(H2O)(phen)](ClO4)2
3.5. Magnetic Susceptibility
Synthesized Schiff based complex tested for magnetic moment at room temperature and
showed paramagnetic property. Magnetic susceptibility value for homodinuclear Cu(II)
complex was 2.53 BM. This value was higher than the Cu(II) ion having d9 electronic
configuration and yet, lower than the value expected for dinuclear Cu(II) complexes [32].
Lower value for magnetic susceptibility for homodinuclear Cu(II) complex can be explained
by antiferromagnetic effect. Di- and trinuclear complexes of ligands having oximato and
oximidato bridges show relatively higher intermolecular spin change effect at room
temperature. This is explained briefly as the weak antiferromagnetic effect between two
copper atoms [33]. Such type of magnetic interaction was proved using e.p.r. observations of
the same type complex by Patel et.al. [34].
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3.6. Conductivity Measurement
Conductivity measurements performed in N,N-dimethylformamide (1x10-3 M) solution at
room temperature resulted that the synthesized complex was an electrolyte since it contained
perchlorate salt in its structure (Fig. 3). Molar conductivity value of the complex was 157
[ΛMS(ꢀ-1cm2mol-1)]. This result showed that the complex and electrolyte ratio was 1:2
indicating that there were three kinds of electrolytes in the solution [35].
Fig. 3. Structure of the synthesized homodinuclear Cu(II) complex
3.7. Computational Studies
3.7.1. Molecular structure
The molecular geometry of the dimine-dioxime ligand (BPHEO) was completely optimized in
the gas phase using DFT based B3LYP method with the 6-311++G(d,p) basis set and the
optimized structure of the BPHEO was shown in Fig. 4. The calculated bond distances, bond
angles and dihedral angles are given in Table 6.
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Fig. 4. Optimized structure of the BPHEO at the B3LYP/6-311++G(d,p) level
According to our calculations, we found that the longest bond in the molecule is among the
C35-C36 atoms in the ethylenediamine moiety. There are two different localized double
bonds in the molecule and these bonds are in the oxime and imine groups. The length of the
bond between the C14-N16 and C31-N33 atoms in the oxime structures is found to be 1.256
and 1.257 Å, respectively and the length of this bond is slightly shorter than the C13-N15 and
C30-N32 double bonds in the imine groups.
From Table 6 C26-C27-C30-N32, C13-C14-N16-O17, C30-C31-N33-O34, N15-C35-C36-
N32 dihedral angle values were calculated as 15.1o, 179.7o, 178.8o and -176.9o, respectively.
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Therefore, it can be said that these atoms almost lie in same plane since the angle between
these four adjacent atoms is equal to 0o or 180o.
Table 6. Some selected calculated bond lengths (Å), bond and dihedral angles (o) for the
BPHEO and [Cu2(L)(H2O)(phen)](ClO4)2 at the B3LYP/6-311++G(d,p) and B3LYP/6-
31G(d,p) level, respectively.
BPHEO
Parameters
COMPLEX
B3LYP/
6-311++G(d,p)
B3LYP/
6-31G(d,p)
Parameters
Bond lengths (Å)
C4-C7
1.489
1.488
1.492
1.495
1.259
1.260
1.483
1.490
1.256
1.257
1.443
1.441
1.467
1.471
1.529
C32-C38
C16-C21
C43-C44
C26-C27
C44-N46
C27-N29
C44-C45
C27-C28
C45-N48
C28-N47
N48-O51
N47-O50
C31-N46
C30-N29
C30-C31
N29-Cu49
N48-Cu49
O50-Cu52
N11-Cu52
1.487
1.486
1.539
1.522
1.332
1.325
1.502
1.507
1.289
1.296
1.413
1.402
1.491
1.504
1.563
1.903
1.919
1.872
1.886
C21-C24
C10-C13
C27-C30
C13-N15
C30-N32
C13-C14
C30-C31
C14-N16
C31-N33
N16-O17
N33-O34
C35-N15
C36-N32
C35-C36
Bond angles (o)
C11-C10-C13
C26-C27-C30
120.5
117.7
C41-C43-C44
C25-C26-C27
119.7
119.1
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C14-C13-N15
C31-C30-N32
O17-N16-C14
O34-N33-C31
N15-C35-C36
N32-C36-C35
115.2
120.8
110.1
109.8
107.5
107.2
C45-C44-N46
C28-C27-N29
O51-N48-C45
O50-N47-C28
N46-C31-C30
N29-C30-C31
N29-Cu49-N46
N46-Cu49-N48
O50-Cu52-O51
O51-Cu52-N11
106.5
113.2
118.9
119.8
108.1
106.7
85.5
84.4
106.1
109.4
Dihedral angles (o)
C5-C4-C7-C12
C20-C21-C24-C25
C11-C10-C13-N15
C26-C27-C30-N32
C13-C14-N16-O17
C30-C31-N33-O34
N15-C35-C36-N32
50.8
49.6
C31-C32-C38-C39
C15-C16-C21-C22
C41-C43-C44-N46
C25-C26-C27-N29
C44-C45-N48-O51
C27-C28-N47-O50
N46-C31-C30-N29
N29-Cu49-N47-O50
N48-O51-Cu52-N6
41.2
41.7
-56.7
15.1
-23.2
-43.1
178.6
179.1
-32.8
178.1
163.6
179.7
178.8
-176.9
Mulliken atomic charges of the ligand were calculated at the B3LYP/6-311++G(d,p) level. C7
atom in the benzene ring has the maximum positively charge (0.696992). The very less
negative charge was found on the N16 nitrogen atom (-0.645688). It can be said that this low
negatively charge was caused by the high electronegative oxygen atom adjacent to the
nitrogen atom
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The optimized and numbered molecular structure at B3LYP/6-31G(d,p) basis set of
[Cu2(L)(H2O)(phen)](ClO4)2 is shown in Fig. 5. Geometrical parameters such as bond length,
bond angle and dihedral angle of the homodinuclear Cu(II) complex are given in Table 6.
Fig. 5. Optimized structure of the [Cu2(L)(H2O)(phen)](ClO4)2 at the B3LYP/6-31G(d,p)
level
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A slight elongation in bond lengths C13-N15 and C30-N32 was noted after complexation.
After the azomethine nitrogen is coordinated to the metal, the charge density decreases and
the double bond between the carbon atom and the nitrogen atom in the azomethine group
elongates. Nitrogen and oxygen single bond lengths in the oxime groups of the BPHEO are
1.443 and 1.441 Å, while there is a slight shortening in the complexes for these bonds and
calculated as 1.413 and 1.402 Å, respectively. Some selected bond angles including the Cu(II)
ions namely; N29-Cu49-N46, N46-Cu49-N48, O50-Cu52-O51 and O51-Cu52-N11 was found
to be 85.5, 84.4, 106.1 and 109.4°, respectively. The four equatorial positions of Cu49 were
occupied by azomethine and oxime nitrogen atoms, while axial position was occupied by
water molecule. On the other hand two oxime oxygen atoms and two 1,10-phenanthroline
nitrogen atoms were coordinated with Cu52 to form tetrahedral geometry. The values of the
bond length and bond angles calculated for the metal complex showed a distorted square
pyramidal geometry around the Cu49 ion while a distorted tetrahedral geometry around the
Cu52 ion. Dihedral angle for N29-Cu49-N47-O50 was calculated as 178.1°, so in the
homodinuclear Cu(II) complex, azomethine nitrogen, Cu49 ion, oxime nitrogen and oxime
oxygen atoms were almost on the same plane.
3.7.2. Molecular electrostatic potential
Molecular electrostatic potential (MEP) is used for identifying the chemical reactivity as well
as the presence of intra- or intermolecular interactions of the molecule [38]. MEP surface of
the BPHEO is given in Fig. 6 in which, different colors are indicative of different values of
electronic potential. Red colour is correlated with electron-rich area while blue one represents
electropositive sites.
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Fig. 6. Molecular electrostatic potential of the BPHEO at the B3LYP/6-311++G(d,p) level.
According to the MEP surface of the ligand molecule, electron-rich area was located on the
oxygen atom of oxime group and partially nitrogen atom of oxime and imine groups. The
electropositive regions were prominently cantered on the proton of the oxime groups. This
map of the BPHEO also explains the easy deprotonation of the molecule and its coordination
to the metal via the oxygen and nitrogen atoms present in the oxime and imine groups
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3.8. Enzyme-like Activity Studies
Synthesized homodinuclear Cu(II) complex tested for both catecholase and catalase-like
activities. Catecholase-like activity was measured spectrophotometrically by the oxidation of
3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylquinone (3,5-DTBQ) (Fig. 7) while
catalase activity was measured volumetrically by the determination of the volume of evolved
oxygen gas from the disproportion reaction of hydrogen peroxide.
3.8.1. Catecholase-like activity
Fig. 7. Oxidation reaction of the 3,5-di-tert-butylcatechol by catecholase enzyme to the 3,5-
di-tert-butylquinone in the presence of O2
Catecholase-like activity studies of the [Cu2(L)(H2O)(phen)](ClO4)2 was carried out using N,N-
dimethylformamide solvent solution owing to the good solubility of the complex as well as
the substrate and product. Formation of 3,5-DTBQ was monitored by the increase in
absorbance at 400 nm as a function of time (Fig. 8).
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Fig. 8. 3,5-DTBQ increase, recorded 30 sec. intervals at 400 nm. in the presence of
[Cu2(L)(H2O)(phen)](ClO4)2
[3,5-DTBC]=5x10-3 M, [[Cu2(L)(H2O)(phen)](ClO4)2]=1x10-4 M
Blank experiments indicated that the oxidation reaction of 3,5-DTBC to 3,5-DTBQ did not
occur in the absence of synthesized complex. In order to determine the catalytic activity of the
complex, the observed rate constant, kobs value for the 3,5-DTBQ formation was obtained.
The experiments were repeated several times to ensure consistency of the results. The slope of
the line showed the rate of the complex. Obtained rate constant value of the oxidation reaction
was found to be as 0.0422 min-1 (Fig. 9).
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1.4
1.2
1
y = 0.0422x - 0.0023
0.8
0.6
0.4
0.2
0
0
5
10
15
20
25
30
35
-0.2
Time (min)
Fig. 9. Catecholase-like activity of the [Cu2(L)(H2O)(phen)](ClO4)2 complex
In the literature, generally dinuclear complex systems were used for their higher activities
compared to mononuclear ones probably due to the good proximity and orientation of
bimetallic Cu(II) structure with the dioxygen system of the o-catechol and compatible redox
potential of the complex.
3.8.2. Catalase-like activity
The catalase-like activity of the [Cu2(L)(H2O)(phen)](ClO4)2 towards hydrogen peroxide was
investigated by volumetric determination of evolved dioxygen.
The catalytic activity of complex in disproportion reaction of hydrogen peroxide was
investigated in N,N-dimethylformamide solutions in the presence and absence of 1-
28