Heteroligand bivalent transition metal complexes with an azo-oxime ligand and 1,10-phenanthroline: Synthesis, spectroscopy, thermal analysis, DFT calculations and SOD-mimetic activities

Article abstract of DOI:10.1016/j.molstruc.2020.129579

Novel mononuclear heteroligand transition metal complexes: [M(HL)(phen)₂]ClO₄, (M: Mn(II) for 1, Ni(II) for 2), [M(HL)(phen)(ClO₄)], (Ni(II) for 3, Cu(II) for 4, Zn(II) for 5) with 2-[(E)-(hydroxyimino)methyl]-4-[(E)-pheny

Full text of DOI:10.1016/j.molstruc.2020.129579

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Heteroligand bivalent transition metal complexes with an azo-oxime
ligand and 1,10-phenanthroline: synthesis, spectroscopy, thermal
analysis, DFT calculations and SOD-mimetic activities
Kerim Serbest , Turan Dural , Mustafa Emirik , Ali Zengin ,
¨
Ozlem Faiz
PII:
S0022-2860(20)31893-7
DOI:
Reference:
https://doi.org/10.1016/j.molstruc.2020.129579
MOLSTR 129579
To appear in:
Journal of Molecular Structure
Received date:
Revised date:
Accepted date:
21 July 2020
22 October 2020
2 November 2020
¨
Please cite this article as: Kerim Serbest , Turan Dural , Mustafa Emirik , Ali Zengin , Ozlem Faiz ,
Heteroligand bivalent transition metal complexes with an azo-oxime ligand and 1,10-phenanthroline:
synthesis, spectroscopy, thermal analysis, DFT calculations and SOD-mimetic activities, Journal of
Molecular Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2020.129579
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Highlights
 Mixed ligand complexes were prepared.
 DFT calculations were performed to assign the experimental spectral data.
 Cu(II) complex shows the most SOD-mimicking activity.
1

Heteroligand bivalent transition metal complexes with an azo-oxime
ligand and 1,10-phenanthroline: synthesis, spectroscopy, thermal analysis,
DFT calculations and SOD-mimetic activities
Kerim Serbesta, *, Turan Durala, Mustafa Emirika, Ali Zenginb and Özlem Faiza,
a Department of Chemistry, Recep Tayyip Erdogan University, 53100 Rize, Turkey
b Pazar Vocational School, Recep Tayyip Erdogan University, 53300 Pazar/Rize, Turkey
Abstract
Novel mononuclear heteroligand transition metal complexes: [M(HL)(phen)2]ClO4, (M:
Mn(II) for 1, Ni(II) for 2), [M(HL)(phen)(ClO4)], (Ni(II) for 3, Cu(II) for 4, Zn(II) for 5) with
2-[(E)-(hydroxyimino)methyl]-4-[(E)-phenyldiazenyl]phenol, H2L as primary ligand and
1,10- phenanthroline as bidentate co-ligand(s) in different mole ratios have been synthesized
and characterized by using elemental analysis, FTIR, UV-Vis, NMR, MALDI-TOF mass
spectrometry and thermal analysis. The complexes (1, 2) have distorted octahedral geometry
while the complexes (3-5) have distorted square-pyramidal coordination geometry. In the
complexes, the metal ion is coordinated to the deprotonated azo-oxime ligand through the
phenolic oxygen atom and nitrogen of the imine. 1,10-Phenanthroline is coordinated to the
metal ion through its two N-donors. The thermograms of all the complexes were confirmed
the proposed structures. Time-dependent (TD) DFT-based calculations have been also
performed for geometric optimization and to assign the experimental vibrational and
electronic transition of the complexes. The superoxide-scavenging activities of the complexes
were also investigated and IC50 values were evaluated. Among the complexes studied, the
Cu(II) complex (4) exhibits the most activity with the lowest IC50 value (2.02 ± 0.15).
2

Keywords: Vibrational analyses; Electronic transition, TDDFT; Azo dye; SOD.
______________________
*Corresponding author. Department of Chemistry, Recep Tayyip Erdogan University,
53100, Rize, Turkey Tel.: +90 464 2236126.
E-mail address: kerimserbest@yahoo.com (K.Serbest).
1. Introduction
The azo-imine ligands contain both azo and imine group and their complexes have been
studied for their variety of applications in laser, liquid crystalline displays, textile, leather and
plastic industries, corrosion prevention, catalysis, analytical fields, optical storage
technologies as well as to treat nuclear wastes [1–3]. The azo ligands have —N=N— linkage
between two phenyl rings. Two isomeric forms, energetically stable trans-form and less stable
cis-form, are presented in these ligands due to the double bond between the nitrogen atoms
[4]. A great number of azo compounds are used in pharmaceuticals and cosmetics although
some azo compounds were reported as toxic [5]. The azo-imine complexes also find
important applications in medicine due to their useful biological activities including anti-
microbial, antitumor, anticancer, anti-fungicidal [1,6].
The transition metal complexes based on 1,10-phenanthroline (phen) and its derivatives
are widely studied due to antitumor, catalytic, redox, photochemical, and photophysical
properties [7–10]. Superoxide radical anions (O2˙−) cause oxidative damage of the cells
resulting DNA damage, aging, and many other disease formations as cancer [11].
Phenanthroline containing complexes has been reported to be effective for the scavenging of
the superoxide radicals [12,13]. Superoxide dismutases (SODs) are metalloenzymes
3

containing transition metals such as Fe, Mn, Cu/Zn, and scavenge catalytically the superoxide
radicals generated by so many spontaneous and enzymatic oxidations in aerobic organisms
[14,15]. SODs are inefficient in case of oxygen burst in vivo while they are very efficient in
the dismutation of superoxide radicals into hydrogen peroxide and oxygen, under normal
conditions [16]. SOD as a metalloprotein has too high molecular weight to pass the cell
membranes [17]. Therefore, we are interested in developing synthetic SOD mimics. The
critical factors for the effectiveness of mimics are stability, membrane permeability, and low
molecular weight. Phenanthroline has two nitrogen donors to coordinate and form stable
structures. The azo-imine ligands have a great ability to coordinate with the transition metals
and form high stable structures [1]. So, the mixed ligand complexes with their low molecular
weight and stabilities of azo-imine and phenanthroline ligands have been seen as good
candidates for SOD mimics.
In the present work, we report the synthesis, characterization, quantum chemical
assignment of experimental IR and UV–Vis spectra and SOD mimetic activities of a series of
Mn(II), Ni(II), Cu(II), and Zn(II) complexes (1-5) derived from azo-imine ligand 2-[(E)-
(hydroxyimino)methyl]-4-[(E)-phenyldiazenyl]phenol, H2L and 1,10-phenanthroline (phen)
as co-ligand (Figure 1).
O
(ClO4)
O
O
O
Cl
O
N
N
N
N
O
N
M
N
M
N
N
N
N
N
N
M
OH
OH
M
Ni(II), 3
Cu(II), 4
Zn(II), 5
Mn(II), 1
Ni(II), 2
4

Figure 1. The proposed structures of the complexes (1-5).
2. Experimental
2.1. Materials and Methods
1,10-Phenanthroline monohydrate (Merck), metal perchlorates (Merck), and all the
solvents were of reagent grade and used without any further purification. 2-[(E)-
(hydroxyimino)methyl]-4-[(E)-phenyldiazenyl]phenol, H2L was synthesized in two steps by
previously reported procedure [18].
2.2. Measurements
Fourier transform infrared (FT-IR) spectra were recorded on a Perkin Elmer Spectrum
100 spectrometer equipped with an ATR apparatus. Elemental analyses were performed using
a LECO truspect analyzer at the Central Research Laboratory of Recep Tayyip Erdogan
(RTE) University. MALDI-TOF mass spectroscopy in a DHB matrix was investigated on a
1
Bruker Microflex LT at the Gebze Institute of Technology for the complex. The H NMR
spectra were recorded on an Agilent Technologies 400/54 spectrometer at the Central
Research Laboratory of RTE University. UV–Vis spectra were recorded on a SpectroScan
60DV UV-vis. spectrophotometer. Magnetic susceptibility and thermogravimetric data were
collected by using Sherwood MK-1 and SII 6300 TG/DTA, respectively.
2.3. Determination of SOD-like activity
The SOD-like activities of the heteroligand transition metal complexes at different
concentrations were evaluated by the indirect NBT-DMSO method in which alkaline DMSO
was used as a source of superoxide radical (O2˙−) and nitroblue tetrazolium chloride (NBT)
was used as O2˙− scavenger [19–21]. 0.2 mL sample was mixed with a solution containing 1.0
5

mL of 0.2 M potassium phosphate buffer (pH 8.6) and 0.5 mL of 5 mM of alkaline DMSO
solution (prepared just before use) to give a final volume of 1.7 mL. The absorbance was
measured at 540 nm against a sample containing all the reagents above except DMSO was
used instead of alkaline DMSO.
2.4. Computational Details
All calculations were performed using the Gaussian 09 [22] program. Density functional
theory (DFT) were used for all the theoretical calculations. Geometry optimizations were
carried out at the B3LYP functional combined with basis sets 6–311ꢀ++ꢀG(d,p) for nonmetal
atoms and LANL2DZ for metal atoms and an effective core potential (ECP) was used for
metal atoms [23]. The gas-phase vibrational frequencies were calculated at the same level of
theory. Frequency analysis calculations have characterized the structures to be the minima (no
imaginary frequency). The fundamental vibrational modes were analyzed employing the
Potential Energy Distribution (PED) using the VEDA4X program [24]. The Time Dependent-
Density Functional Theory (TD-DFT) calculations of both ligands and metal complexes were
performed using Gaussian 09. The electronic excitations were calculated at the same level of
theory by using TD-DFT combined with a conductor-like polarizable continuum model
(CPCM) in the implicit solvent of DMSO (εꢀ=ꢀ46.826) [25]. GAUSSSUM 3.0 [26] was used
to attribute the excitation energies of experimental UV-Vis bands and analyze the fractional
contributions to each molecular orbital. The optimized geometries have been considered for
the TD-DFT calculation. All generated molecular orbitals and optimized geometry were
visualized by Gauss view 5.0.
6

2.5. Synthesis of the complexes (1-5)
A general method for the synthesis of complexes has been followed. An ethanolic
solution of metal(II) perchlorates (Mn(ClO4)2·6H2O, Ni(ClO4)2·6H2O, Cu(ClO4)2·6H2O and
Zn(ClO4)2·6H2O) (1 mmol) to a solution of 2-[(E)-(hydroxyimino)methyl]-4-[(E)-
phenyldiazenyl]phenol, H2L (1 mmol) in 15 mL anhydrous ethanol, neutralized with an
ethanolic solution of NaOH and the mixture was stirred for an hour at room temperature.
Then, the solution of 1,10-phenanthroline monohydrate (2.0 mmol for 1 and 2; 1.0 mmol for
3-5) in 10 mL anhydrous ethanol was dropped to the reaction mixture, the color change was
observed immediately. The mixture was stirred for two days at room temperature and the
resulting precipitate was collected by filtration. It was recrystallized from hot DMF-EtOH (ca.
1:5) mixture, washed with water and ethanol, respectively, and dried in an oven 50 °C and
then over CaCl2 in vacuo. It wasn’t obtained any suitable crystals for X-ray diffraction studies
though our great efforts.
2.5.1. Synthesis of [Mn(HL)(phen)2](ClO4), (1)
Analytical and physical data; Yield 0.465 g (62%). mp 299-301 °C (dec.). Color:
Greenish brown. Anal. Calc. for C37H26ClN7O6Mn: C, 58.86; H, 3.47; N, 12.99. Found C,
59.04; H, 3.55; N, 13.16. UV-Vis. λmax, nm (ε, M-1 cm-1) in DMF: 337 (21960); 397 (25800).
FT-IR (cm-1): 1626, 1592 ν(C=N); 1519, 1496 ν(-C═C-); 1423 ν(N═N); 1304 ν(C-O); 1073
ν(ClO4)-; 724, 842 (phen). Molar conductivity (Ω−1cm2mol−1) 63. μeff B.M. (298 K): 5.88 (for
per metal ion). MALDI-TOF MS (m/z): Calcd for C37H26ClMnN7O6: 754.1; Found: 754.3
[M]+, Calcd for C25H18MnN5O: 459.4; Found: 459.7 [M-(phen+ClO4+OH)]+.
7

2.5.2. Synthesis of [Ni(HL)(phen)2](ClO4), (2)
Analytical and physical data; Yield 0.485 g (64%). mp 269-273 °C (dec.). Color: Green.
Anal. Calc. for C37H26ClN7O6Ni: C, 58.57; H, 3.45; N, 12.92. Found C, 58.28; H, 3.49; N,
13.09. UV-Vis. λmax, nm (ε, M-1 cm-1) in DMF: 268 (23050); 351 (24490); 435 (4810). FT-IR
(cm-1): 3600-3200 ν(OH); 1626, 1604 ν(C=N); 1518, 1475,1422 ν(-C═C-); 1410 ν(N═N);
1301 ν(C-O); 1076 ν(ClO4)-; 721, 842 (phen). Molar conductivity (Ω−1cm2mol−1) 76. μeff B.M.
(298 K): 3.19 (for per metal ion). MALDI-TOF MS (m/z): Calcd for C37H26N7NiO2: 658.1;
Found: 657.7 [M-ClO4]+, Calcd for C31H24N6NiO2: 570.2; Found: 570.7 [M-(C6H5N+ClO4)]+.
2.5.3. Synthesis of [Ni(HL)(phen)(ClO4)], (3)
Analytical and physical data; Yield 0.274 g (48%). mp 267-274 °C (dec.). Color:
Goldenroad. Anal. Calc. for C25H18ClN5O6Ni: C, 51.90; H, 3.14; N, 12.10. Found C, 51.68;
H, 3.32; N, 11.96. UV-Vis. λmax, nm (ε, M-1 cm-1) in DMF: 295 (31790); 365 (20460); 448
(1360). FT-IR (cm-1): 3640-3200 ν(OH); 1644, 1604 ν(C=N); 1548, 1475 ν(-C═C-); 1405
ν(N═N); 1301 ν(C-O); 1145, 1110 ν(ClO4)-; 763, 825 (phen). Molar conductivity
(Ω−1cm2mol−1) 25. μeff B.M. (298 K): 2.81 (for per metal ion). MALDI-TOF MS (m/z): Calcd
for C25H19ClN5NiO6: 578.0; Found: 577.8 [M+H]+.
2.5.4. Synthesis of [Cu(HL)(phen)(ClO4)], (4)
Analytical and physical data; Yield 0.563 g (96%). mp 253-259 °C (dec.). Color: Khaki.
Anal. Calc. for C25H18ClN5O6Cu: C, 51.47; H, 3.11; N, 12.00. Found C, 51.74; H, 3.32 N,
11.81. UV-Vis. λmax, nm (ε, M-1 cm-1) in DMF: 331 (29220); 366 (32290); 437 (26700). FT-IR
(cm-1): 3650-3180 ν(OH); 1648, 1613 ν(C=N); 1522, 1483, 1432 ν(-C═C-); 1405 ν(N═N);
1315 ν(C-O); 1150, 1111, 1067 ν(ClO4)-; 765, 827 (phen). Molar conductivity (Ω−1cm2mol−1)
8

16. μeff B.M. (298 K): 2.04 (for per metal ion). MALDI-TOF MS (m/z): Calcd for
C25H18CuN5O2: 483.0; Found: 482.7 [M-ClO4]+, Calcd for C32H24CuN5O6: 637.1; Found:
637.8 [M+DHB-ClO4]+.
2.5.5. Synthesis of [Zn(HL)(phen)(ClO4)], (5)
Analytical and physical data; Yield 0.285 g (49%). mp 295 °C (dec.). Color: Brownish
yellow. Anal. Calc. for C25H18ClN5O6Zn: C(51.30%) H(3.10%) Cl(6.06%) N(11.97%) C,
51.30; H, 3.10; N, 11.97. Found C, 51.27; H, 3.16; N, 12.13. UV-Vis. λmax, nm (ε, M-1 cm-1)
in DMF: 350 (12310); 435 (13530). FT-IR (cm-1): 3650-3180 ν(OH); 1624 1587 ν(C=N);
1518, 1475, 1432 ν(-C═C-); 1396 ν(N═N); 1293 ν(C-O); 1143, 1102, 1070 ν(ClO4)-; 721,
1
817 (phen). H NMR (CDCl3, δ ppm): 10.98 s. (1H, OH), 9.02 s. (1H, HC=N), 8.49 s. (2H,
Ar), 7.96-7.79 m. (5H, Ar), 7.46-7.21 m. (8H, Ar), 6.62 d. (1H, Ar, J=8,0 Hz), 1.58 s. (H2O in
solvent); Molar conductivity (Ω−1cm2mol−1) 44. μeff B.M. (298 K): dia. MALDI-TOF MS
(m/z): Calcd for C25H18ClN5O6Zn: 583.0; Found: 583.3 [M]+.
MALDI-TOF MS, m/z (%): 583.3 (100 a.u.) [M-2H]+. C25H18ClN5O6Zn
3. Results and discussion
3.1. NMR spectra
The proton NMR spectra of Zn(II) complex, 5 has been recorded in CDCl3 solution using
tetramethylsilane (TMS) as an internal standard (Figure 2). The chemical shifts of the
different types of protons of Zn(II) complex is given in the experimental section and the
9

spectrum was compared with that of primary 2-[(E)-(hydroxyimino)methyl]-4-[(E)-
phenyldiazenyl]phenol, H2L. While the signal of C=N-OH at 11.60 ppm shifted to 10.98 ppm,
the phenolic OH signal at 10.96 ppm is completely disappeared. The singlet of oxime proton
at 10.98 ppm was disappeared after D2O was added. The singlet of HC=N at 8.39 ppm shifted
to 9.02 ppm. Lowfield shift of the imine proton signal, a decrease in the acidity of oxime
proton, and the disappearance of the phenolic proton signal indicate the coordination of H2L
to Zn(II) ion through the nitrogen of imine and the phenolic oxygen atoms, respectively.
Comparing with the proton NMR spectra of the primary ligand, an increase in the integrated
intensities at the region of 6.5-8.5 ppm was observed. The integrated intensities of the
aromatic protons show the presence of a phenanthroline co-ligand.
Serbest-Zn-fen1-1_PROTON_01
b)
0.20
0.15
0.10
0.05
a)
0
1.05
1.00 2.00 5.017.65 0.98
16
14
12
10
8
6
4
2
0
Chemical Shift (ppm)
Figure 2. 1H NMR spectra of Zn(II) complex, 5 in CDCl3 (a) before the addition of D2O, and
(b) after the addition of D2O.
10

3.2. Theoretical calculations
3.2.1. Molecular Structures
The theoretical calculations have been performed for the free ligand and the complexes
(1-5) in order to obtain satisfactory information regarding the structural characteristics and the
fundamental spectroscopic properties. The optimized molecular structures with bond lengths,
overlap populations, and bond orders between metal and donor atoms were given in Figures
3-8.
In these structures, the coordination geometry around metal atoms is described as
octahedral geometry by four nitrogens of two phenanthroline ligands and one oxygen and one
nitrogen of imine of the primary oxime ligand in compound 1 and 2. The coordination
geometry around a metal atom of complex 3, 4, and 5 are described as distorted square
pyramidal by two nitrogens of phenanthroline group, one oxygen of perchlorate, one nitrogen,
and one oxygen of oxime ligand.
Figure 3. Optimized molecular structure of the free ligand.
11

Overlap
Bond
Length(A) Population
Bond order
0.35
Mn65-N2
Mn65-O4
1.99
1.94
0.11
0.23
0.08
0.09
0.08
0.08
0.60
Mn65-N25 2.06
Mn65-N20 2.04
Mn65-N32 2.05
Mn65-N37 2.05
0.30
0.32
0.31
0.32
Figure 4. Optimized molecular structure of complex 1.
Bond
Length (A) Overlap
Bond
order
Population
Ni73-N2
Ni73-O4
Ni73-N25
Ni73-N20
Ni73-N32
Ni73-N37
2.05
1.99
2.13
2.10
2.10
2.10
0.10
0.31
0.54
0.25
0.26
0.26
0.26
0.21
0.08
0.07
0.07
0.06
Figure 5. Optimized molecular structure of complex 2.
Bond
Overlap
Length(A) Population
Bond
order
Ni56-N25
Ni56-O26
Ni56-O27
Ni56-N54
Ni56-N55
2.04
1.98
1.99
2.06
2.13
0.19
0.18
0.16
0.15
0.14
0.447
0.517
0.538
0.416
0.392
Figure 6. Optimized molecular structure of complex 3.
12

Bond
Overlap
Length(A) Population
Bond
order
0.45
Cu27-N25
Cu27-O26
Cu27-O28
Cu27-N55
Cu27-N56
Cu27-O32
1.96
1.95
2.16
2.01
2.25
2.81
0.18
0.22
0.13
0.15
0.11
0.07
0.60
0.35
0.43
0.30
0.13
Figure 7. Optimized molecular structure of 4.
Overlap
Bond
Length(A)
2.13
Population Bond order
Zn56-N25
Zn56-O26
Zn56-O31
Zn56-O27
Zn56-N54
Zn56-N55
0.15
0.19
0.13
0.10
0.12
0.11
0.32
0.42
0.27
0.21
0.28
0.26
1.99
2.24
2.38
2.17
2.22
Figure 8. Optimized molecular structure of 5.
3.2.2. Vibrational assignments
IR spectra (4000-650 cm-1) of the compounds were generally obtained using the FT-ATR
technique. The obtained spectra were evaluated considering the azo derivative oxime ligand
used as the starting compound and the IR spectrum of the 1,10-phenanthroline ligand. The
theoretical IR spectra of the ligand and all complexes were calculated using the DFT / B3LYP
method to assign the experimental signals. The selected vibrations and corresponding
functional groups were given in Table 1.
13

The vibrations observed at 3403, 1393, and 1265 cm-1 in the IR spectrum of the azo-
oxime ligand were assigned to the oscillation vibrations of the oxime protons (C=N―OH),
―N=N― and C―O, respectively. Due to the possible intermolecular and intramolecular
hydrogen bonds that can be observed in the ligand structure, the tensile vibrations of the
expected phenolic OH protons at a lower frequency than the tensile vibrations of the oxime
OH protons could not be observed.
The broad bands observed at approximately 3600-3200 cm-1 in the IR spectra of all the
complexes were attributed to the oxime O―H vibrations. Besides the phenolic OH stretch
was disappeared in all complexes, in the IR spectrum of the azo-oxime ligand, the intense of
C―O vibrations observed at 1265 cm-1 was shifted to the upper wave number in the
complexes (1315-1293 cm-1) and the intensity was decreased compared to the free ligand. It
can be seen as evidence of C―O moiety of ligand binds to metal centers. The vibration band
belonging to the vibration of the imine C=N observed at a medium intensity at 1621 cm-1 in
the spectrum of the azo-oxime ligand shifted to the upper wave number (1624-1648 cm-1) and
the intensity of this band decreased in the complexes [16,27,28]. It is obvious that in the
spectrum of the phenanthroline derivatives complexes, the imine (C=N) vibrations of the
phenanthroline ligand shifted to lower frequencies (1613-1587 cm-1). Shifts in the up and
down frequencies of the hydroxyl imine (C=NOH) and the imine of phenanthroline vibrations
were attributed to the coordination of the metal ion to the azo-oxime and phenanthroline
ligands over the imine nitrogens. In addition to the C=C vibrations observed in the region of
about 1520 and 1420 cm-1 in the phenanthroline complexes, the bands belonging to the
characteristic out-of-plane C―H bending observed in 721-765 and 817-842 cm-1 were
attributed to the phenanthroline unit [29,30]. The obtained spectral data indicate that while the
azo-oxime ligand is mostly coordinated to the metal via the imine and phenolic oxygen, 1,10-
phenanthroline is coordinated through the nitrogen atoms.
14

The perchlorate ion has a general tetrahedron geometry and its point group is Td, having
four normal modes of vibration (ν1-ν4). The asymmetrical stretching (ν3, 1110 cm-1) and
asymmetrical bending (ν4, 626 cm-1) modes are only IR active [31,32]. In general,
asymmetrical stretching (ν3) and asymmetrical bending (ν4) are observed in the infrared
spectra of ionic perchlorates. The former appearing as a very broad strong band is very
diagnostic and confirms the uncoordination of the perchlorate ion in complex 1, and 2 (See
the supplementary file, Fig. S12 and S13) [33]. Because the perchlorate group is involved in
partial covalent bonding between one of its oxygen atoms and central metal ion, the
asymmetrical stretching band (ν3, 1110 cm-1) splits into two bands due to the fact that the
symmetry of the perchlorate ion is lowered to C3v. The bands are 1110, 1068 cm-1 for 3,
1111, 1067 cm-1 for 4, 1102, 1070 cm-1 for 5 (Fig. S14-S16). The splitting confirms the
monodentate coordination of the perchlorate ion (Table 1) [32].
15

Table 1. Selected vibrational frequencies (observed and calculated) of synthesized compounds.
Comp.
Ligand
ν(C=N-OH)
3403
ν(C-H)
ν(C=N)
Phen
ν(C=C)
ν(N=N)
1393
1514
1423
1422
1410
1436
1405
1417
1405
1421
1396
1405
ν(C─O)
1265
1289
1304
1299
1301
1245
1301
1323
1315
1302
1293
1337
ν(ClO4)
Exp.
Theo.
Exp..
Theo.
Exp.
3150-3000
3074-2982
3100-3050
3106-3023
3100-3000
3093-3005
3150-3000
3129-3038
3200-3050
3132-3043
3100-3050
3092-2998
1621
_
1569
_
3291(Ph), 3666
_
1654*
_
1621-1590
1519,1496
1512-1634
1518,1475,1422
1634-1431
1548,1475
1544-1643
1522,1483,1432
1357-1639
1518,1475,1422
1428-1629
_
1626, 1592*
1614, 1605*
1626, 1604*
1620, 1590*
1644, 1604*
1655, 1600*
1648, 1613*
1632,1646*
1624, 1587*
1623, 1634*
724,842
735, 872
721,842
741, 873
763,825
738,805
765,827
756, 803
721,817
740, 872
1073
1
2
3
4
5
3501
_
3670-3200a
2885
1076a
Theo.
Exp.
_
3640-3200a
3493
1110,1068
Theo.
Exp.
_
3650-3180a
3164
1111,1067
Theo.
Exp.
_
3650-3180a
3240
1102,1070
_
Theo.
(a: broad, Exp.: experimentally observed, Theo.: Theoretically calculated, *: Phen ν(C=N))
16

3.2.3. UV-Vis spectra
Experimental absorption bands were interpreted using theoretical data calculated by
TDDFT/CPCM method in the implicit solvent medium of DMSO. Some theoretically
obtained wavelengths and corresponding excitations and their characters were given in
Supplementary (Table S1-S5) with experimentally observed wavelengths.
In the electronic spectrum of the Mn (II) complex (1), experimentally observed bands at
337 and 397 nm were attributed to the π→π* and d →n, π* metal to ligand charge transitions
(MLCT). The band at 337 nm was generally assigned to H-8(A,B)→L(A)/ L+1(A,B),
H(A)→L(A), H(A)→L+4(A), H-1(B)→L+6(B) transition and interpreted as π→π* character
according to molecular orbital contributions. The second band at 397 nm was theoretically
assigned to H(B)→L+2(B)/L+3(B)/L+4(B)/L+5(B), H-1(B)→L+1(B)/L+2(B)/L+3(B), H-
1(A)→L+5(A), H-2(A)→L+1(A)/L+2(A) transition and characterized as MLCT, since H-
1(A), H-2(A), H-1(B), H(B) molecular orbitals are mainly located on the metal atom. The
experimental and theoretical electronic transitions with their assignments were given in Table
S2. Three separate bands were observed at 268, 351, and 435 nm in the electronic spectrum of
the complex 2. According to theoretical data, the first band is weighted to π→π* and n→π*
transitions and minor contribution of LMCT transitions. The second and third bands attributed
to π→π * and d→d transitions, respectively. In the electronic spectrum of the complex 3, the
band observed at 295 was attributed to LMCT transitions and the bands at 448 nm and 365
nm were attributed to π→π* and minor contribution of LMCT. Three bands at 331, 366, and
437 nm observed in the electronic spectrum of the Cu(II) complex (4) were dominated by
π→π*, n→π*, and LMCT transitions. Two bands at 350 and 435 nm observed in the
electronic spectrum of the Zn(II) complex (5) were dominated by π→π* and n→π*
transitions. Experimental absorption bands, theoretical electronic transitions, and their
character assignments in detail were tabulated at Table S1-S5 in Supplementary.
17

3.3. Thermal stabilities
The thermal decomposition processes for the complexes (1-5) were studied by
TG/DTG/DTA analysis to get information about their thermal stabilities and determine the
metal/ligand ratio. The TG curves of the complexes (1-5) were shown in Figure 9 (see
Supplementary, Fig. S1-S5 for the DTAmax) and obtained data were presented in Table 2. The
overall residues in thermograms of the complexes are in agreement with the proposed
structures and their stoichiometries.
Mn(II) complex, 1 has one step decomposition at 130-580 °C with the weight lost 90.2%
(calc. 90.6%). The mass lost is assigned to the removal of HL, two phenanthroline molecules,
and ClO4-O. We assumed that the final residue left at the end of the thermal decomposition
process above 578°C is mainly MnO, the amount of the residue is 10.8% (calc. 9.4%) and
suitable for the proposed structure.
The TG curve of the Ni(II)-HL-phenanthroline complex, 2 (1:1:2) showed three separated
decomposition steps within the range of 25-685°C. The endothermic first step at 25-88°C is
assigned to dehydration of the adsorbed water with a mass loss of 2.6% (calc. 2.3%). The
complex 2 decomposed rapidly in the second step at 204-394°C with a mass loss of 23.4%
(calc. 23.2%). The mass loss is assigned to the removal of a phenanthroline molecule. The
removals of the second phenanthroline, HL, and ClO4-O occurred in the third step at 394-
685°C with a mass loss of 68.7% (calc. 69.0%). The final residue left is NiO with a mass of
9.3% (calc. 9.8%).
The TG curve of the Ni(II)-HL-phenanthroline complex, 3 (1:1:1) showed three separated
decomposition steps within the range of 25-695°C. The endothermic first step at 25-75°C is
assigned to dehydration of 0.8 mol adsorbed water with a mass loss of 2.5% (calc. 2.5%). The
complex 3 decomposed rapidly in the second step at 223-451°C with a mass loss of 29.5%
18

(calc. 29.4%). The mass loss is assigned to the removal of a phenanthroline molecule. The
removals of the HL and ClO4-O occurred in the third step at 451-695°C with a mass loss of
55.4% (calc. 52.9%). The final residue left is NiO with a mass of 12.8% (calc. 12.9%).
Cu(II) complex, 4 has three endothermic decomposition steps at 25-628 °C. The first step
at 25-58°C is assigned to dehydration of 0.8 mol adsorbed water with a mass loss of 2.6%
(calc. 2.6%). The second step at 204-293°C is assigned to the removals of phenanthroline
molecule and C6H5 with a mass loss of 43.4% (calc. 43.0%). The identification of step 2 and 3
is difficult because not only phenanthroline but also HL decomposed at the same time in the
second step. The third step at 311-628°C is assigned to the removals of C7H7ClN3O5 with
mass loss of 40.2% (calc. 41.4%). The final residue left is CuO with a mass of 15.6% (calc.
13.3%).
Zn(II) complex, 5 has one step decomposition at 217-605 °C with the weight lost
85.1.2% (calc. 86.1%). The mass loss is assigned to the removal of the HL+phenanthroline
+ClO4-O molecule. We assumed that the final residue left at the end of the thermal
decomposition process above 605°C is mainly ZnO, the amount of the residue is 14.9% (calc.
13.9%).
19

‧‧‧‧‧‧ ; 1
― ‧ ― ; 2
― ‧ ‧ ― ; 3
――― ; 4
─ ─ ─ ; 5
Figure 9. TG curves of the complexes (1-5).
Table 2. Thermal analysis data of the complexes.
Decom.
Decom.
Step(s)
Group lost,
Residue formula,
Comp.
1
Temp.,
°C
DTAmax, µV
mass loss %, exp. (calc.)
Residue %,exp. (calc.)
189(+), 466(+), HL +2phen+ClO4-O
MnO,
10.8 (9.4)
1
1
130-580
25-88
482(+)
90.2 (90.6)
H2O,
[Ni(HL)(phen)2]ClO4,
97.4 (97.7)
54(-)
2.6, (2.3)
2
3
Phen,
[Ni(HL)(phen)]ClO4,
76.0(76.8)
NiO,
9.3 (9.8)
[Ni(HL)(phen)]ClO4,
97.5 (97.5)
[Ni(HL)]ClO4,
68.0(68.1)
NiO,
12.8 (12.9)
[Cu(HL)(phen)]ClO4,
97.4 (97.4)
2
3
1
2
3
1
2
3
1
204-394 256(+), 359(+)
394-685 480(+), 614(+)
21.4 (23.2)
HL +phen+ClO4-O
67.0(68.4)
0.8H2O,
2.5, (2.5)
Phen,
29.5 (29.4)
HL +ClO4-O
55.4 (52.9)
0.8 H2O,
2.6, (2.6)
Phen+C6H5
43.4(43.0)
25-75
48(-)
223-451 300(+), 436(+)
451-695 502(+), 611(+)
25-58
48(-)
C7H7ClN3O6Cu
54.0 (54.7)
4
5
204-293 250(+), 272(+)
381(+),441(+),5 C7H7ClN3O5
311-628
CuO, 15.6 (13.3)
43(+)
40.2(41.4)
HL +phen+ClO4-O
85.1(86.1)
217-605 323(+), 545(+),
ZnO, 14.9 (13.9)
20

3.4. SOD activity studies
SOD is an antioxidant enzyme that catalyzes the dismutation of the superoxide radical
anions (O2˙−) generated during the metabolic activities of cells [34]. The catalytic activity of
the metal complexes towards superoxide radical anions were measured. IC50 is defined as the
concentration of the metal complexes produces 50% inhibition of NBT reduction [16]. SOD-
like activity provided a means to reveal potential SOD mimics [35]. Many metal complexes
have the ability to dismutate the superoxide radical anions [36–38]. The complex
concentration which is required to yield 50% inhibition of the reduction of NBT of
[Mn(HL)(phen)2]ClO4, [Ni(HL)(phen)2]ClO4, [Ni(HL)(phen)(ClO4)], [Cu(HL)(phen)(ClO4)],
[Zn(HL)(phen)(ClO4)] are 8.36 ± 0.55, 6.00 ± 0.36, 2.80 ± 0.19, 2.02 ± 0. 15 and 2.90 ± 0.11,
respectively (Table 3). The copper(II) complex has the best activity with the lowest value
(2.02 µM). All the obtained IC50 values of the complexes are higher than the value reported
for the native enzyme (IC50 = 0.04 μM) [36] but lower than the values exhibited for some
reported metal complexes [12,20,38]. The difference in activity of the complexes is
determined by the geometry and redox behavior of the metal center. So, the lower activities
than the others are expected for the Mn(II) and Ni(II) complexes (1 and 2) due to their
saturated geometries. Due to the possibility of a strong axial bond between the metal center
and superoxide ion, the greater activities are observed in the complexes (3-5) having penta-
coordination. Among them, it is not surprising that the copper(II) complex is the most active
due to the redox potential. The dismutation potential of the synthesized metal complexes was
lower than the native enzyme but instability and denaturation under environmental conditions
limit the native enzyme’s commercial applications. The synthesized metal complexes with
high stability and low molecular weight make them be good candidates as SOD mimics.
21

Table 3. The SOD-like activities of the complexes (1-5).
Complex
aIC50 (µM)
8.36 ± 0.55
6.00 ± 0.36
2.80 ± 0.19
[Mn(HL)(phen)2]ClO4, 1
[Ni(HL)(phen)2]ClO4, 2
[Ni(HL)(phen)(ClO4)], 3
[Cu(HL)(phen)(ClO4)], 4
2.02 ± 0. 15
2.90 ± 0.11
[Zn(HL)(phen)(ClO4)], 5
aIC50 is defined as the concentration of the metal complexes produces 50% inhibition of NBT
reduction
22

4. Conclusion
Five novel mononuclear heteroligand transition metal complexes with 2-hydroxy-5-[(E)-(4-
phenyl) diazenyl] benzaldehyde oxime, H2L as the primary ligand, and 1,10-phenanthroline as
bidentate co-ligand(s) in different mole ratios have been reported. The information obtained
from physical, spectroscopic, and theoretical data shows that Mn(II) complex and Ni(II)
complexes (1 and 2) have distorted octahedral geometry around metal ion while other
complexes (3-5) have distorted square pyramidal geometry. On the other hand, superoxide
radical (O2˙−) scavenging activity studies revealed that all the complexes have SOD
mimicking activity with IC50 values ranged from 2.02 to 8.36, distorted square pyramidal
complexes are more active than distorted octahedral complexes and the Cu(II) complex (4)
has found the most active. The activities of the complexes are lower than the native enzyme
but higher than some reported complexes [39,40].
Acknowledgments
The numerical calculations reported in this paper were performed at TUBITAK ULAKBIM,
High Performance, and Grid Computing Center (TRUBA Resources).
CRediT author statement
Kerim Serbest: Conceptualization, Methodology, Writing, Supervision, Reviewing and
Editing Turan Dural: Investigation, Mustafa Emirik: Software, Theoretical calculations
Ali Zengin: Investigation Özlem Faiz: Investigation of biological activity, writing
23

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
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