Synthesis and characterization of a novel non-symmetrical bidentate Schiff base ligand and its Ni(II) complex: electrochemical and antioxidant studies

Article abstract of DOI:10.1007/s11696-020-01200-7

Abstract: This paper describes the synthesis and properties of a new nitro-substituted bidentate Schiff base ligand (HL) and its nickel complex (Ni(II)-2L). Its ligand was derived from the condensation of 2′-methoxyphenyl-2-ethylamine with 5-nitro-2-hydroxybenzaldehyde. This ligand and its nickel complex were characterized through elemental analysis, UV–Vis, IR, ¹H NMR and ¹³C NMR. The molecular structure of the nickel complex was confirmed using single-crystal X-ray diffraction. This complex crystallizes in dichloromethane in triclinic system, space group of P-1 with a = 6.6803 (2), b = 9.6702 (2), c = 12.1836 (3) and 1 formula unit in the cell. The obtained data revealed that the Ni(II) center is tetra-coordinated by two oxygen and two nitrogen atoms involving two ligands. Cyclic voltammograms of the nickel complex, recorded in DMF solutions, showed one quasi-reversible redox couple related to the Ni(II)/Ni(I) couple, demonstrating its electro-activity. This indicates a slow redox transition and/or electron exchange process, which implies that this electrochemical process is mainly diffusion-controlled. The antioxidant activity of the synthesized compounds was evaluated from the reactivity with the free radical DPPH, showing IC₅₀ values between 0.4 and 4.5?mg/mL. Graphic abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s11696-020-01200-7

Chemical Papers
https://doi.org/10.1007/s11696-020-01200-7
ORIGINAL PAPER
Synthesis and characterization of a novel non‑symmetrical
bidentate Schiꢀ base ligand and its Ni(II) complex: electrochemical
and antioxidant studies
Imène Bougossa · Djouhra Aggoun1,2 · Ali Ourari · Raúl Berenguer · Sofane Bouacida · Emilia Morallon
1
1
3
4
3
Received: 5 November 2019 / Accepted: 15 May 2020
©
Institute of Chemistry, Slovak Academy of Sciences 2020
Abstract
This paper describes the synthesis and properties of a new nitro-substituted bidentate Schiꢀ base ligand (HL) and its
nickel complex (Ni(II)-2L). Its ligand was derived from the condensation of 2′-methoxyphenyl-2-ethylamine with 5-nitro-
2
-hydroxybenzaldehyde. This ligand and its nickel complex were characterized through elemental analysis, UV–Vis, IR,
1
13
H NMR and C NMR. The molecular structure of the nickel complex was conꢁrmed using single-crystal X-ray diꢀrac-
tion. This complex crystallizes in dichloromethane in triclinic system, space group of P-1 with a=6.6803 (2), b=9.6702
(
2), c=12.1836 (3) and 1 formula unit in the cell. The obtained data revealed that the Ni(II) center is tetra-coordinated by
two oxygen and two nitrogen atoms involving two ligands. Cyclic voltammograms of the nickel complex, recorded in DMF
solutions, showed one quasi-reversible redox couple related to the Ni(II)/Ni(I) couple, demonstrating its electro-activity.
This indicates a slow redox transition and/or electron exchange process, which implies that this electrochemical process is
mainly diꢀusion-controlled. The antioxidant activity of the synthesized compounds was evaluated from the reactivity with
the free radical DPPH, showing IC values between 0.4 and 4.5 mg/mL.
5
0
*
Djouhra Aggoun
aggoun81@yahoo.fr
1
Laboratoire d’Electrochimie, d’Ingénierie Moléculaire et
de Catalyse Rédox (LEIMCR), Département Génie des
Procédés, Faculté de Technologie, Université Ferhat ABBAS
Sétif-1, 19000 Sétif, Algeria
2
3
Département de Chimie, Faculté des Sciences, Université
Ferhat ABBAS Sétif-1, 19000 Sétif, Algeria
Departamento de Química Física, Instituto Universitario
de Materiales, Universidad de Alicante (UA), Ap. 99,
0
3080 Alicante, Spain
4
Département Sciences de la Matière, Faculté des Sciences
Exactes et Sciences de la Nature et de la Vie, Université Oum
El Bouaghi, Oum El Bouaghi, Algeria
Vol.:(0
1
123456789)
3

Chemical Papers
Graphic abstract
Keywords Bidentate Schiꢀ base · Nickel(II) complex · X-ray diꢀraction · Electrochemical reduction · Antioxidant activity
Introduction
Schiꢀ base ligands containing N and O atoms or functional
groups with electrodonor capability is then very interesting
for metal complexation (Alturk et al. 2015). Particularly,
the chemistry of nickel Schiꢀ base complexes has been
the subject of extensive investigation in the past decades
(Kuchtanin et al. 2016). Many of these studies were essen-
tially focused on the electrocatalytic reduction activity of
the Schiꢀ base complexes in a large number of homogene-
ous (Luca et al. 2012) and heterogeneous reactions (Bedi-
oui et al. 1991). It was generally found that the nature of
the ligand determines the properties of the complex, so the
design and synthesis of novel ligands become a hot topic
in this ꢁeld. Among diꢀerent types, the ortho-hydroxylated
Schiꢀ bases, incorporating another e-donating functionality,
have received a special attention to build these new bidentate
structures. Taking into account of these facts, a novel Schiꢀ
base ligand and its nickel(II) complex were synthesized and
characterized using diꢀerent spectroscopies and analytical
Obtained by condensation of aldehydes with primary
amines, Schiꢀ bases are versatile ligands in the ꢁeld of
coordination chemistry currently containing nitrogen, oxy-
gen and other donor atoms (Djebbar et al. 1997; Tai et al.
2
003; Wu et al. 2001; Bhattacharya et al. 1998), able to be
applied in a wide range of applications (Gungor and Kara
2
011; Yahsi et al. 2011; Hopa et al. 2010; Odabasioglu et al.
009; Kara and Naturforsch 2008; Dey et al. 2003; Amirnasr
2
et al. 2001; Chang et al. 2016; Dey et al. 2002; Nassimbeni
et al. 1976).
In parallel, considerable interest has aroused in the com-
parative donor properties of oxygen and nitrogen when
both are present in the same molecule (Al-allaf and Sheet
1
995). In fact, many organic ligands containing these donor
atoms have been systematically studied in recent years with
very successful results in the synthesis of high nuclear-
ity of complexes. This may be attributed to their relative
high stability, biological activity (Highꢁeld et al. 2000; Das
et al. 1999) and potential applications in many ꢁelds such
as oxidation catalysis (Djebbar et al. 1998), electrochemis-
try (Gorczyński et al. 2016) and analytical chemistry (Dalia
et al. 2018; Altun et al. 2009). Hence, the design of new
1
13
methods, such as IR, H NMR, C NMR, UV–Vis. The
crystal structure of the nickel(II) complex has also been
determined using crystallographic studies. Electrochemical
behavior of the nickel complex was investigated by cyclic
voltammetry. Moreover, these compounds were found be
endowed with antioxidant activity after testing them by the
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3

Chemical Papers
DPPH (1,1-diphenyl-2-picrylhydrazyl) showing that IC50
values were also found to be between 0.5 and 4.0 mg/mL.
was immediately observed and, then, the mixture was again
reꢂuxed for 4 h for completion of the reaction. The formed
solid was again recovered by ꢁltration and ꢁnally washed
with small portions of cooled ethanol.
Experimental
Physicochemical and electrochemical
characterization
Materials
All chemicals and solvents used in this work were obtained
from commercial sources and were used as received without
any further puriꢁcation. The purity of the synthesized com-
pounds was checked by means of thin-layer chromatography
The uncorrected melting points of the ligand and its complex
were measured with a Koꢂer Bench (model WME) appara-
tus. Elemental analysis (C, H and N) of the new compounds
was carried out with the aid of a vario EL III elemental
1
13
(
TLC). In this analysis, glass plates precoated with silica gel
60F, Merck) were used.
analyzer, whereas H NMR and C NMR spectra were
recorded by using a 400 MHz Bruker NMR spectrometer
(
and CDCl as solvent. In this characterization, the chemi-
3
Synthesis of the ligand and its Ni(II) complex
cal shifts (δ) were referenced to tetramethylsilane (TMS),
used as internal reference and expressed in ppm. Infrared
transmission spectra were obtained, after preparing KBr
pellets using a Bruker IR instrument. UV–Vis spectra of
the compounds were obtained in CH Cl with the aid of an
The synthesis of the bidentate Schiꢀ base ligand and its
Ni(II) complex is summarized in Scheme 1. Basically,
2
2
-methoxyphenylethylamine (1 mmol, 151 mg) and 5-nitro-
-hydroxybenzaldehyde (1 mmol, 167 mg) were dissolved
2
2
UNICAM UV-300 instrument.
in absolute ethanol (16 mL). The mixture was reꢂuxed for
The cyclic voltammograms were recorded a 10 ml cell
equipped with three conventional electrodes containing
DMF solutions and 0.1 M tetra-n-butylammonium perchlo-
rate (TBAP) as supporting electrolyte. For the preparation
of working electrodes, the compounds were supported on a
rod of glassy carbon (3 mm diameter) previously polished
by diamond paste and rinsed with acetone and then DMF.
The counter electrode was a platinum wire, whereas a satu-
rated calomel electrode (SCE) was used as the reference.
3
h. A yellow precipitate was observed as soon as the ꢁrst
contacts have been took place, and the expected product was
then recovered by ꢁltration after its washing several times
small portions of cold ethanol. To synthesize this complex,
a solution of Ni(OCOCH ) ·4H O (0.5 mmol, 124 mg) in
3
2
2
ethanol (8 mL) was added and mixed with the previous
ethanolic solution of ligand. After contact of these two
solutions, a green precipitate of the complex (Ni(II)-2L)
Scheme 1 Experimental proce-
H
dure showing the synthesis of
H CO
C
O
3
the bidentate Schiꢀ base ligand
H CO
3
Ethanol
Reflux
(
(
HL) and its nickel complex
+
Ni(II)-2L)
O N
OH
2
HC N
H N
2
O N
OH
2
H CO
3
NO Ni(OAc)
.4H
O
2
2
HC N
O
O
2
Ni
Ethanol, Reflux
O N
N CH
2
OCH3
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Chemical Papers
ꢀAbs
− Abstestꢁ
Zero-grade Ar (Air Products) was used to deaerate all elec-
trolytic solutions.
control
%inhibition =
Abscontrol
where Abs
and Abs are the absorbance of the stand-
test
control
X‑ray crystallography
ard (ascorbic acid) and the sample, respectively. The pro-
vided activity was given as the IC value (50% inhibitory
5
0
Green prismatic single crystals of the complex (Ni(II)-2L)
were obtained after their redissolution followed by slow
evaporation in/of dichloromethane. The crystal was coated
concentration) in mg/mL.
®
with paratone oil and mounted on loops for data collection.
Results and discussion
X-ray data were collected with a Bruker Apex II CCD area
detector diꢀractometer by using a graphite monochromated
Mo-Kα radiation source (λ = 0.71073 Å) at 150 K. The
crystallographic data and experimental details for structural
analysis are summarized in Table 3. The reported structure
was solved by direct methods with SIR2004 (Burla et al.
Physicochemical properties and molecular formulae
Some of the chemical and physical properties of the synthe-
sized Schiꢀ base ligand (HL) and Ni complex (Ni(II)-2L)
are presented in Table 1. Both the ligand and its complex
of nickel were stable at room temperature and were soluble
in DMF, DMSO, ACN and dichloromethane. The retard-
2
005) to locate all the non-H atoms, which were reꢁned
anisotropically with SHELXL97 (Sheldrick 2008) and using
full-matrix least squares on F2 procedure from within the
WINGX suite of software (Farrugia 2012) used to prepare
material for publication. All the H atoms were placed in the
calculated positions and constrained to ride on their parent
atoms. Drawings of molecules were produced with the pro-
gram ORTEP-3 and Diamond (Farrugia 1997; Brandenburg
and Berndt 2001).
ing factors (R ) were found to be 45 and 56, respectively,
f
whereas their melting points are ranged between 100 and
2
00 °C. Elemental analysis of the ligand showed that it con-
tains ca. 64% of carbon and almost 5 and 9% of hydrogen
and nitrogen, respectively. This composition agrees perfectly
with molecular formulae (C H O N ) of the ligand. In the
1
6
16
4
2
case of the complex, the C% was found to be practically
the expected value whereas those of hydrogen and nitrogen
showed very small diꢀerences that are practically accept-
able. Therefore, its molecular formula is consistent with
C H O N Ni.
Antioxidant activity
The antioxidant activity, given as the ability of the synthe-
sized compounds to scavenge hydrogen peroxide, was deter-
mined according the method described by the literature (Al
Zoubi et al. 2016). For this analysis, an equal volume of
each fraction of the both synthesized compounds was added
to a methanolic solution of 2,2-diphenyl-1-picrylhydrazyl
3
2
30
8
4
IR spectra
The IR spectrum of the bidentate Schiꢀ base ligand (HL)
(Fig. 1) exhibits a broad absorption band, centered at
−
1
(
DPPH) using diꢀerent concentrations of this free radical.
around 3445 cm that can be assigned to the O–H inter-
actions like inter- and intramolecular hydrogen bonds. The
free hydroxyl group is characterized by its own stretching
This mixture is vigorously shaken at room temperature dur-
ing 30 min. The absorbance of the solution is then measured
at 517 nm. The radical scavenging activity, expressed as the
inhibition percentage, was calculated according the follow-
ing relation:
−
1
vibration appearing at higher energy around 3700 cm
(Ebrahimi et al. 2014). The presence of an intense band
−
1
at 1657 cm was attributed to the stretching vibration of
Table 1 Physicochemical properties and proposed molecular formulae of the synthesized compounds
a
Yield (%)b,c
Elemental analysisd
Compound
Color
R
M.p. (C°)
Molecular formula
C H O N
f
C (%)
H (%)
N (%)
HL
Ni(II)-2L
Yellow
Green
45
56
70
25
106
194
64.18 (64.00)
58.46 (58.47)
5.41 (5.38)
4.69 (4.56)
8.81 (9.33)
8.02 (8.53)
1
6
16
4
2
C H O N Ni
32 30 8 4
a
Retarding factor values obtained from mixture of dichloromethane/ethanol (9/0.5, v/v) as eluent
Mole number of ligand/mole number of aldehyde
Mole number of complex/mole number of ligand, M.p.: melting point
Microanalysis C, H, N, found and (calculated) values
b
c
d
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Chemical Papers
HL
Ni(II)-2L
4
000
3000
2000
1000
0
Fig. 2 UV-Vis spectra of 10− M CH2Cl2 solutions of HL (a) and
5
Wavenumber / cm-1
Ni(II)-2L (b).The inset shows the MLCT and d-d transitions,
−3
recorded in the 450–700 nm region using 10 M solutions of the
complex
Fig. 1 IR spectra of the ligand (HL) and its nickel complex (Ni(II)-
L)
2
of the (C=N) azomethine group (Keypour et al. 2013; Ourari
et al. 2014). This band is also present in the electronic spec-
trum of the complex Ni(II)-2L displaying small shifting to
higher energy at 388 nm. This is due of the coordination
of the nitrogen atom of the azomethine groups to the metal
center. The electronic absorption spectrum of the complex
shows a shoulder at 498 nm which could be assigned to the
charge transfer between the metallic center and the coordi-
nating atoms of the ligand (i.e. the so-called metal ligand
charge transfer, MLCT) (Khalaji et al. 2011). The absorp-
tion bands of Ni(II)-2L complex observed at around 600 nm
(see inset of Fig. 2), with its characteristic weaker intensities
the azomethine group (HC=N). This band is also present
in the IR spectrum of the Ni(II)-2L complex (Fig. 1) but,
in this case, it has been shifted to lower energy appearing
−
1
at 1609 cm . This shifting is explained by an extension
of the electronic delocalization through the nickel center
and the both azomethine groups (Ilhan et al. 2007). Further-
more, this Schiꢀ base ligand displays also two other bands
−
1
−1
at around 1540 cm and 1431 cm , respectively. These
absorption bands have been assigned to the ν and ν
asym
sym
(
NO ) modes, respectively (Cisterna et al. 2015). The band
2
−
1
located at 1305 cm for the ligand HL is attributed to the
phenolic ν (C–O) stretching vibration. This band is shifted
3
−1
−1
(ε = 0.288.10 mol L cm ), are attributable to the d–d
−
1
to the higher frequencies (1317 cm ) in the spectrum of
the nickel complex. This shifting suggests that the hydroxyl
groups of the both Schiꢀ base ligands have taken part in the
formation of nickel complex. These observations suggest
that the metallic ion was eꢀectively coordinated to the four
coordinating heteroatoms (NONO) (VimalKumar and Rad-
hakrishnan 2011). Other conclusive absorption bands of this
complex (Ni(II)-2L) were also observed between 420–480
transitions (Pooyan et al. 2013).
1
13
H NMR and C NMR spectra
1
13
H NMR and C NMR spectral data of the Schiꢀ base HL,
1
and Ni(II)-2L complex are listed in Table 2. The H NMR
spectrum of the Schiꢀ base ligand HL (Fig. 3a) shows a
sharp singlet (s) at 8.12 ppm attributed to the azomethine
proton (–HC=N–). The multiplet of the aromatic protons
was observed in the range 6.80–7.30 ppm. The methyl-
−
1
and 500–650 cm . These absorption bands were ascribed
to the Ni–N and Ni–O stretching vibrations, respectively
(
El-Megharbel et al. 2016).
ene group δ Ph–CH
resonates at 3.05 ppm. As for the
(8)
2
N–CH , it absorbs at 3.87 ppm while the protons of the
2
(9)
UV–Vis spectra
methoxy group (δCH O) slightly appear at stronger ꢁelds
3
resonating at 3.81 ppm.
1
3
The UV–Vis spectrum of the ligand HL (Fig. 2) showed
two intense absorption bands at 302 and 407 nm, respec-
tively. The ꢁrst band appears at higher energy with a strong
intensity may be assigned to the π−π* electronic transition
of the azomethine chromophore and the benzene ring. The
second, being weaker band, is attributed to n−π* transition
Regarding the C NMR spectrum (Fig. 3b), the HL
Schiꢀ base ligand shows the azomethine carbon peak at
164.28 ppm. The Ph–CH , N–CH
and –OCH
are
3(1)
2
(8)
2(9)
detected at 31.76, 56.58 and 55.31 ppm, respectively, as
illustrated in Fig. 3c. The aromatics carbons are observed
in the expected region between 110 and 140 ppm. In the
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Table 2 1H NMR and 13C NMR spectral data of the Schiꢀ base (HL) and its nickel complex (Ni(II)-2L) in CDCl3
1H NMR
N=CH
N–CH2(9)
Ph–CH2(8)
–OCH3(1)
H(3-6,12-13-15)
HL
8.12 (s,1H)
8.07 (s,1H)
3.87 (t,2H)
3.80 (t,2H)
N–CH2(9)
3.05 (t,2H)
3.10(t,2H)
Ph–CH2(8)
3.81 (s,3H)
4.30 (s,3H)
–OCH3(1)
7.30–6.80(m,7H)
6.40–7.80(m, 7H)
C(2–7, 11–16)
Ni(II)-2L
13
C NMR
N=C(10)
H
HL
Ni(II)-2L
164.28
157.76
56.58
64.31
31.76
29.25
55.31
55.45
100–140
100–140
Multiplicities of signals are singlet (s), triplet (t) and multiplet (m) with their chemical shifts expressed in ppm
1
3
C NMR spectra of the complex, all the carbons peaks are
The crystal packing for Ni(II)-2L can be described as
alternating layers parallel to (101) plane (Fig. 5), and mol-
ecules are also linked via C–H–O intermolecular hydrogen
bonds forming an inꢁnite two-dimensional network in one
layer (Fig. 6, Table 4).
slightly shifted to the downꢁelds when they are compared to
those of the corresponding Schiꢀ base ligand.
Description of crystal structure
The crystal structure is also supported by intermolecular
interactions of Cg–Cg (π−π stacking) interactions between
aromatic rings. The shortest centroid–centroid distance is
estimated to 3.6266 (1) Å. All these interactions link the
molecule within the layers and also link their layers together
by reinforcing the cohesion of the structure.
Structural resolution revealed that the asymmetric unit of
the complex consists of half molecule of Ni(II)-2L and the
second half is generated by symmetry, the Ni atom is located
at an inversion center (Figure 4). The nickel complex Ni(II)-
2
L crystallizes in dichloromethane in the triclinic crystal
system with P-1 space group with 1 formula unit in the cell
(
Table 3). The ORTEP (Farrugia 2012) diagram of the titled
Cyclic voltammetry of the Ni(II)‑2L
compound is shown in Fig. 4.
The crystal structure of Ni(II)-2L was formed from two
units of bidentate Schiꢀ base ligand totalizing four dentate
in its coordination sphere as it can be seen in Fig. 4. The
nickel ion was found to be placed in a square-planar environ-
ment, currently adopted by this metal. This coordination was
accomplished in the trans position by two oxygen atoms of
the ligand. This crystallographic environment appears as a
coordination sphere satisfying the more stable positioning
of the both atoms of oxygen and nitrogen in phenoxy and
azomethine groups of the Schiꢀ base ligand [Ni: O3, O3i,
N2, N2i (i:-x–y-z)]. The azomethine linkage is evident from
the N2–C7 bond length which is equal 1.289 Å very close to
the typical bond length (1.29–1.32 Å) in imines. As for the
C7–N2–C8 angle, it is equal to 114.1° (Allen et al. 1987).
The bond lengths involving the phenyl ring and phenoxy
oxygen atoms, C4–O3 and C15–O4, are 1.383 and 1.372 Å,
respectively, and these values are seen in the range of the
The electrochemical behavior of the nickel(II) complex
−
1
was studied in DMF solutions containing 10 M tetra(n-
butyl)ammonium perchlorate (TBAP). This medium is used
because the radical anions of aromatic nitro-compounds are
more stable in aprotic solvents such as DMF (Grimshaw
2000; Toro et al. 2018). The nickel(II) complex showed
voltammetric responses in the cathodic and anodic regions
where the reduction and oxidation waves were observed
at Ep = − 1.51 and Ep = − 1.39 V versus SCE, respec-
c2
a2
tively. These waves are ascribed to the Ni(II)/Ni(I) redox
system (Ourari and Aggoun 2015). The voltammetric data
of nickel(II) complex were recorded using various scan rates
in the potential values ranging of −1.0 and −1.8 V versus
SCE. These electrochemical characteristics are summarized
in Table 5 and displayed in Fig. 7.
The eꢀect of the scan rate on the voltammetric responses
of the synthesized complex was also analyzed (Fig. 7b). As
it can be seen, the peak currents increase with the increase
in the square root of scan rates and vary linearly indicating
the diꢀusion redox process (Figure 7c, d). Table 5 shows
the electrochemical parameters obtained from the voltam-
mograms, and it can be observed that the potential peak-to-
peak separation appeared around 59 and 100 mV. According
to these results, it can be deduced that the redox couple was
related to a quasi-reversible expressing a monoelectronic
transfer process, controlled by diꢀusion (Ourari et al. 2017,
2018).
2
2
usually observed standard C(sp )–OH and C(sp )–O– bond
length (Kwiatkowski et al. 1990). The Ni–O and Ni–N bond
lengths for Ni(II)-2L varying in the range 1.821–1.831 Å
and 1.918–1.919 Å, respectively, and are in good agreement
with related reported complexes (Bhowmik et al. 2011).
Cisoid and transoid angles of the nickel deviate from their
ideal values of 90° and 180° and found to be in the ranges
8
6.3°–93.1° and 178.2°–176.5°, respectively. The bond
length and angle around C16 (methoxy atom) are consistent
3
with sp hybridizations of this atom (Ourari et al. 2013).
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3

Chemical Papers
Fig. 3 a H NMR, b 13C NMR spectra of the Schiꢀ base HL and c molecular structure of the Schiꢀ base HL as deduced from A) and B)
1
Antioxidant activity
tested compounds. From these results, the complex Ni(II)-
2
L exhibited an eꢃcient eꢀect with an inhibition percent-
The free radical scavenging ability of the synthesized com-
pounds was determined by useful method of DPPH, in which
the reduction of this radical in presence of antioxidant agent
led to a solution color changing from the purple to yellow-
ish. In this study, our results depicted in Fig. 8a indicate
that the scavenging eꢀect is concentration dependent of the
age of 68.47% followed by the ligand HL presenting an
inhibition percentage of 59.57%. As for the potential eꢀect
determination, we have used the IC values which seem to
5
0
be more signiꢁcant. Our ꢁndings are displayed in Fig. 8b
and demonstrate that the Ni(II)-2L complex has the high-
est scavenging eꢀect with its lower IC value estimated
5
0
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3

Chemical Papers
Table 3 Crystal data, data collection and structure reꢁnement param-
eters for and Ni(II)-2L
Compound
Ni(II)-2L
Empirical formula
Formula weight
T (K)
C32 H30 N Ni O
657.31
150
0.71073
Triclinic
P-1
4
8
λ (Å)
Crystal system
Space group
a(Å), b(Å), c(Å)
6.6803 (2), 9.6702 (2), 12.1836 (3)
(
°)α, (°)β, (°)γ
109.305 (1), 93.442 (1), 102.281 (1)
718.52 (3), 1
1.519
0.737
3
V (Å ), Z
ρ (g cm 3)
−
μ (mm−1)
F (000)
342.0
Limiting indices
Data/restraints/
Parameters
h:−6/8, k:−12/12, l: −16/16
3523
206
Fig. 4 The molecular geometry of compound Ni(II)-2L. No labeled
atoms are generated by inversion center symmetry (−x, −y, −z). Dis-
placement ellipsoids are drawn at the 50% probability level. H atoms
are represented as small spheres of arbitrary radius
Total reꢂections
Unique reꢂections (Rint)
Reꢁnement method
12,833
3523 (0.0292)
Full least-squared (Shelxl-97)
1.048
2
Goodness of ꢁt on F
Final R index [I>2σ(I)]
R index [all data]
0.0322
0.0401
0.284/−0.262
to 0.42 mg/mL, whereas the IC50 value of the ligand HL
was found to be 4.51 mg/mL. These results revealed that
they are in good accordance with previous similar studies
reported in the literature (Pramanik et al. 2015; Jamil et al.
Largest diꢀerence peak and
hole (e Å−
3
)
CCDC deposition no.
CCDC1912564
2
019; Charef et al. 2015). In this case, it was concluded that
|Fc|)2]/
the metal complexes were found be more active than their
ligands themselves.
R
=
{∑[w(|Fo|−|Fc|)]/∑w(|Fo|)}, Rw= {∑w(|Fo|
−
2
1/2
∑
w(|F | )}
o
2
2 2
2
1/2
wR2 = {∑[w(F −F ) ]/∑w(F )}
w = 1/σ (F ) + (0.0794P) + 0.6853P] where P = (F +2F )/3
o
c
o
2
2
2
2
2
0
o
c
Conclusion
nickel complex has also been investigated. In this case,
we have found that the relationship between the anodic
In this paper, we have presented the synthesis of a biden-
tate Schiff base ligand (HL) and its new nickel(II)–Schiff
base complex (Ni(II)-2L). These two compounds have
been thoroughly characterized by conventional spec-
1
/2
peak current (ip ) and the square root of scan rate (v
)
a
is linear, suggesting that the electrochemical process is
mainly governed by diffusion.
1
13
troscopic techniques, such as IR, UV–Vis, H- and C-
NMR, and elemental analysis. This structural characteri-
zation was supported by XRD diffraction analysis of our
nickel complex. The results of these analyses, including
the crystallographic studies, have as main goal to elu-
cidate the molecular structures of the synthesized com-
pounds like the nickel complex and its ligand. In compari-
son with ascorbic acid as a standard, these compounds
revealed to have a respectable inhibition activity toward
a free radical scavenging capacity assayed by DPPH free
radical method. It was found that the IC value of the
Supplementary data
Crystallographic data have been deposited at the Cambridge
Crystallographic Data Centre. CCDC 1912564 contains the
supplementary crystallographic data for Ni(II)-2L. These
data can be obtained free of charge via http://www.ccdc.cam.
ac.uk/conts/retrieving. html, or from the CCDC, 12 Union
Road, Cambridge, CB2 1EZ, UK; fax: (+44) 01223-336-
033; e-mail: deposit@ccdc.cam.ac.
5
0
nickel complex is lower than those of its corresponding
Schiff base ligand. The electrochemical behavior of the
1
3

Chemical Papers
Fig. 5 Diagram packing of Ni(II)-2L, showing an alternating parallel layers to (101) plane
Fig. 6 Interactions of C–H–O
(
dashed line in blue) hydrogen
bonding, resulting in the forma-
tion of an inꢁnite two-dimen-
sional network in one layer
Table 4 Distances (Å) and
angles (°) of hydrogen bond for
Ni(II)-2L
D–H–A
d (D–H)
d(H–A)
d (D–A)
D–H–A
Symmetry
C8–H8a–O3
C16–H16b–O4
0.97
0. 94
2.24
2.58
2.6923 (1)
3.5055 (1)
107.0
161.0
−x, −y+1, −z
−x+1, −y+1, −z+1
1
3

Chemical Papers
Table 5 Cyclic voltammetric
parameters of the nickel
v (mV s−1
Ipa2 (μA/cm2)
Ipc2 (μA/cm2)
)
Epa2 (V)
Epc2 (V)
E1/2 (V)
ΔE (V)
ipa/ipc
complex with various scan rates
2
5
7
1
2
3
4
5
5
0
5
00
00
00
00
00
3.686
4.655
5.395
5.160
6.537
7.308
8.422
10.076
6.485
8.321
9.638
10.445
13.517
15.793
17.204
17.390
−1.352
−1.339
−1.339
−1.355
−1.341
−1.328
−1.320
−1.298
−1.472
−1.478
−1.483
−1.480
−1.486
−1.494
−1.499
−1.542
−1.412
−1.408
−1.411
−1.417
−1.413
−1.415
−1.409
−1.445
0.060
0.069
0.072
0.062
0.072
0.087
0.089
0.097
0.568
0.559
0.559
0.494
0.483
0.462
0.489
0.579
2
0
0
5
0
(A)
(B)
1
-
5
10
15
20
25
30
0
10
20
30
-
-
-
-
-
-
-
-
-40
-1.8
-1.6
-1.4
-1.2
-1.0
-1.8
-1.6
-1.4
-1.2
-1.0
-0.8
Potential (V/SCE)
Potential (V/SCE)
2
4
0
6
2
(D)
(
C)
1
0
8
6
4
2
1
1
8
4
4
8
12
16
20
24
4
8
12
16
20
24
1
/2
1/2
v1/2(mV/s)1/2
v (mV/s)
Fig. 7 a Cyclic voltammograms of the synthesized complex of
Ni(II)-2L showing the response of Ni(II)/Ni(I) redox couple,
solutions, recorded at various scan rates: from inner to outer, 25, 50,
−1
75, 100, 200, 300, 400 and 500 (mVs ), respectively. The anodic (c)
−1
−1
1/2
recorded in 0.1 mol L TBAP/DMF solution at 298 K at 100 mVs
;
and cathodic (d) currents versus square root scan rate (v
)
b cyclic voltammograms of nickel complex in 0.1 M TBAP/DMF
1
3

Chemical Papers
Fig. 8 a DPPH radical scaveng-
ing activity of the ligand (HL)
and its Ni(II) complex (Ni(II)-
100
(
A)
Ascorbic acid
HL
(B)
4
3
2
1
0
8
6
0
0
Ni(II)-2L
2
L), compared to the ascorbic
acid used as standard reference;
b IC inhibitory concentration
50
of ligand and nickel complex
for 50% of DPPH radical.
Comparison was made against
ascorbic acid
40
2
0
0
0
1
2
3
4
5
6
7
Ascorbic acid HL Ni(II)-2L
-1
Concentration (mg.mL )
Acknowledgements The authors thank the Algerian Ministry of Higher
Education and Scientiꢁc Research (MESRS) and the Directorate Gen-
eral of Scientiꢁc Research and Technological Development (DGRSDT)
for their ꢁnancial support. The authors also gratefully acknowledge the
Bhattacharya P, Parr J, Ross AT (1998) First synthesis of a unique
dilead Schiꢀ base complex. J Chem Soc Dalton 19:3149–3150.
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characterization of nickel(II) and copper(II) complexes with
tetradentate Schiꢀ base ligands. Inorg Chim Acta 366:62–67.
https://doi.org/10.1016/j.ica.2010.10.010
ꢁ
nancial help (MAT2016-76595-R) of the Ministerio de Economía y
Competitividad and FEDER. Special thanks are owed to the Spanish
Ministry of Economy and Competitiveness (MINECO) for the granting
of a “Ramón y Cajal” contract (RYC-2017-23618).
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an improved tool for crystal structure determination and reꢁne-
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