Novel nickel(II) and manganese(III) complexes with bidentate Schiff-base ligand: synthesis, spectral, thermogravimetry, electrochemical and electrocatalytical properties

Article abstract of DOI:10.1007/s11164-015-2325-6

An unsymmetrical bidentate Schiff base ligand, ethane 2-(4-methoxyphenyl)-1-iminosalicylidene, and its novel two mononuclear complexes, Nickel(II) [Ni(II)-2L] and Manganese(III) [Mn(III)Cl-2L] where L represents the ligand, have been synthesized and characterized by various physicochemical methods. The Ni(II) ion is coordinated by two nitrogen and two oxygen atoms with both the bidentate Schiff base ligands in an approximately square planar coordination geometry, while the manganese complex, the Mn(III) ion, is involved in an additional contact with one chloride anion for which the coordination sphere appears as a square pyramidal arrangement. The thermogravimetric analyses of the synthesized compounds revealed three different stages of decomposition for NONO bis-bidentate manganese and nickel complexes. The cyclic voltammetry studies of these complexes in N,N-dimethylformamide showed a redox couple for each one of them, such as Ni(II)/Ni(I) and Mn(III)/Mn(II), which are quasi-reversible. Their catalytic behaviors were tested showing that the nickel complex is an effective electrocatalyst in the reduction of bromocyclopentane. Regarding the manganese complex, it was revealed that it is an efficient catalyst in the activation of molecular dioxygen, currently applied in oxidation reactions of hydrocarbons according to the monooxygenase enzymes as those of cytochrome P450 model.

Full text of DOI:10.1007/s11164-015-2325-6

Res Chem Intermed
DOI 10.1007/s11164-015-2325-6
Novel nickel(II) and manganese(III) complexes
with bidentate Schiff-base ligand: synthesis, spectral,
thermogravimetry, electrochemical
and electrocatalytical properties
Brahim Bouzerafa2 Ali Ourari1 Djouhra Aggoun1
•
•
•
Ramiro Ruiz-Rosas3 Yasmina Ouennoughi1
•
•
Emilia Morallon4
Received: 23 July 2015 / Accepted: 17 October 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract An unsymmetrical bidentate Schiff base ligand, ethane 2-(4-methox-
yphenyl)-1-iminosalicylidene, and its novel two mononuclear complexes, Nickel(II)
[Ni(II)-2L] and Manganese(III) [Mn(III)Cl-2L] where L represents the ligand,
have been synthesized and characterized by various physicochemical methods. The
Ni(II) ion is coordinated by two nitrogen and two oxygen atoms with both the
bidentate Schiff base ligands in an approximately square planar coordination
geometry, while the manganese complex, the Mn(III) ion, is involved in an addi-
tional contact with one chloride anion for which the coordination sphere appears as
a square pyramidal arrangement. The thermogravimetric analyses of the synthesized
compounds revealed three different stages of decomposition for NONO bis-biden-
tate manganese and nickel complexes. The cyclic voltammetry studies of these
complexes in N,N-dimethylformamide showed a redox couple for each one of them,
such as Ni(II)/Ni(I) and Mn(III)/Mn(II), which are quasi-reversible. Their catalytic
behaviors were tested showing that the nickel complex is an effective electrocatalyst
in the reduction of bromocyclopentane. Regarding the manganese complex, it was
revealed that it is an efficient catalyst in the activation of molecular dioxygen,
&
&
1
Brahim Bouzerafa
bouzer.brah@gmail.com
Djouhra Aggoun
aggoun81@yahoo.fr
´
Laboratoire d’Electrochimie, d’Ingenierie Moleculaire et de Catalyse Redox (LEIMCR),
Faculte de Technologie, Universite Ferhat ABBAS Setif-1, 19000 Setif, Algeria
´
´
´
´
´
2
´
´
Laboratoire de Preparation, Modification des Materiaux Polymeriques Multiphasiques
(LMPMP), Faculte de Technologie, Universite Ferhat ABBAS Setif-1, 19000 Setif, Algeria
´
´
´
´
´
3
´
Departamento de Quımica Inorganica, Instituto Universitario de Materiales, Universidad de
Alicante, Ap. 99, 03080 Alicante, Spain
´
4
´
Departamento de Quımica Fısica, Instituto Universitario de Materiales, Universidad de
Alicante, Ap. 99, 03080 Alicante, Spain
´
123

B. Bouzerafa et al.
currently applied in oxidation reactions of hydrocarbons according to the
monooxygenase enzymes as those of cytochrome P450 model.
Keywords Bidentate Schiff base ligand Á Nickel and manganese complexes Á
Cyclic voltammetry Á Thermogravimetric analysis Á Catalytic behavior
Introduction
Schiff base ligands have been connected with the progress of coordination
chemistry since the late nineteenth century. They have played a major role in the
development of the coordination chemistry, providing various effects and steric
interactions including diverse geometries [1–4]. These complexes may be consid-
ered as new class of coordination chemistry compounds when compared to
porphyrinic complexes [5].
The methoxy substituent is interesting in the field of Schiff base complexes, [6,
7] since it offers the opportunity to study it as functional group inducing an electron
donor effect. Consequently, its position isomers in the monosubstituted aromatic
compounds either as primary amines [8] or in the carbonyls compounds [9] have
previously been well discussed. The bidentate Schiff base ligands containing the
methoxy group have been found to be useful for versatile applications covering a
wide range of areas such as catalysis, electrocatalysis, biochemistry, electrochem-
istry, and spectroscopy [10–13].
In the case of electrocatalysis knowledge, the role played by such structural and
electronic effects to control the electrochemical properties of these complexes has
been proved to be critical [14]. However, Schiff base complexes are efficient as
electrode modifiers for anodic oxidation or cathodic reduction. The anodic oxidation
reactions are suitable for electrosynthesis of polymers owing to their high ability to
efficiently stabilize the radical-cation, currently involved in most electropolymer-
ization processes useful for the fabrication of modified electrodes [15]. On the other
hand, the methoxylated complexes may be considered as catalysts of choice for the
electroreduction reactions, seeing that, hypothetically, their ligands are not easily
reduced. This is due to an excess of negative charge induced by the methoxy group
on the metallic center [16].
Coordination chemistry offers a wide range of 3d and 4f metals with several
oxidation degrees and different geometries associated to the complexes dia- or
paramagnetic [17]. Thus, manganese and nickel have received greatest interest for
their redox active role in different chemical, electrochemical, biochemical and
electrocatalytical processes.
So, manganese has played an important role for its redox active character in
catalytic processes particularly with olefins epoxidation [18] or hydrocarbons
oxidation [19]. Taking into account their low cost, easy preparation and large
availability, the epoxidation of olefins is among the most important reactions in
organic chemistry, since it provides an effective way to produce several compounds,
among them epoxy resins [20] and chiral compounds, obtained with the use of
Jacobsen [21] and catsuki catalysts [22]. Additionally, the complexes containing
123

Novel nickel(II) and manganese(III) complexes with…
Mn(III) centers have excellent magnetic properties because of their high spin of the
metallic ion, Mn(III).
As for the nickel redox centers, their complexes showed high catalytic activity
towards the oxidation of small organic compounds such as methanol (fuel cells)
[23]. For this reason, particular attention has been focused for a long time on nickel
complexes used as modifying agents, such as porphyrins and Schiff base.
Furthermore, electrogenerated nickel(I) complexes have also been used extensively
as catalysts for the reductive cleavage of carbon–halogen bonds in a wide variety of
organic compounds [24].
This work is a continuation of our previous studies on the preparation of Schiff
base ligands with different electronic and/or steric properties using transition metal
complexes Ni(II)-2L and Mn(III)Cl-2L. The reaction ways leading to the synthesis
of the ligand (HL) and its corresponding complexes are shown in Scheme 1.
OCH3
O
CH2CH2NH2
C
H
Methanol
+
HC N
OH
OH
H3CO
OCH3
Nickel: Ni(II)-2L
Manganese: Mn(III)Cl-2L
O
HC N
O
M =
M
N CH
Methanol
MCl2.nH2O
H3CO
OCH3
OCH3
Cl
Mn
O
HC N
O
O
HC N
Ni
N CH
N CH
O
H3CO
H3CO
Scheme 1 Proposed synthesis and structures of the Schiff base ligand (HL) and the corresponding nickel
and manganese complexes
123

B. Bouzerafa et al.
Experimental
Physical measurements
All the chemicals were purchased from commercial sources and used without any
further purification. The purity of the synthesized compounds was checked by TLC
1
using glass plates, precoated with silica gel (60F; Merck). H NMR and 13C NMR
spectra were recorded on a Bruker 400 MHz spectrometer with CDCl3 as solvent and
tetramethylsilane (TMS) as the internal standard. Chemical shifts are expressed in d
(ppm) units and coupling constants are given in Hertz (Hz). High-resolution mass
spectra (HRMS) were acquired by the electrospray ionization (ESI) technique with the
aid of a Bruker APEX-2 instrument. FT-IR spectra were recorded as KBr disks on a
Perkin-Elmer 1000 FTIR Spectrophotometer while the UV–visible spectra were
obtained on a UNICAM UV-300 spectrophotometer with DMF solutions (1 cm, cell).
Thermal analysis (TG and DTG) were obtained in a nitrogen atmosphere using a
TGA Perkin Elmer thermal analyzer. The heating rate was set at 10 °C min-1 and
the weight loss was measured from ambient temperature up to 950 °C.
Cyclic voltammetry experiments were carried out in an undivided Metrohm cell
of 5 cm3 using a Voltalab 40 (potentiostat/galvanostat) PGZ 301. A planar, circular
glassy carbon (GC) electrode (Ø, 3 mm) was employed as the working electrode
and a platinum wire as the auxiliary (counter) electrode. All potentials are quoted
with respect to the saturated calomel electrode (SCE). An amount of 0.1 M Tetra-n-
butylammonium tetrafluoroborate (TBABF4) was used as the supporting electrolyte
and DMF as the solvent in all the electrochemical experiments.
Preparation of 2-(40-methoxyphenyl)-1-iminosalicylideneethane (HL)
An amount of 122 mg (1 mmol) of salicylaldehyde was dissolved in 5 mL of
methanol. This solution was added drop-wise to 5 mL of methanol containing 151 mg
(1 mmol) 4-methoxyphenylethyl amine. The mixture obtained turned rapidly yellow
and was then refluxed for 1 h under continuous stirring. The solvent was removed
under reduced pressure yielding the expected ligand (HL) as suitable crystals. These
crystals were recovered by filtration, washed with cold methanol and dried under
vacuum (yield 86 %). Selected FT-IR: (KBr, cm-1): m (3650–3290) (OH); 1630
(C=N); 1240 (C–O); UV–Vis [kmax (nm)/-e (M cm-1)]: 314 (14,000).
Preparation of complexes
The metallic chloride salts react with the compound in 2:1 (L:M) molar ratio using a
methanolic solution. This synthesis procedure can be employed for the production
of both metal chelates with appreciable yields.
Ni(II)-2L complex
An amount of 125.5 mg (1 mmol) of Ni(Cl)2.6H2O was dissolved in 10 mL of
methanol. This solution was added drop-wise to a hot solution of HL ligand
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Novel nickel(II) and manganese(III) complexes with…
containing 512 mg (2 mmol). The mixture rapidly turned green and the reaction was
continued with stirring and refluxed for 6 h. The resulting mixture was kept at room
temperature for 24 h and the expected compound was recovered by filtration as a
green precipitate (yield: 75 %). Microanalysis of C32H32N2O4Ni found (calc.) was:
C 67.75 % (67.61); H 5.69 % (5.66); N 4.94 % (4.91). Selected FT-IR: (KBr, cm-1
)
m: 1610 (C=N); 1246 (C–O); 518 (M–O); 453 (M–N); UV–Vis [kmax(nm)/-e
(M cm-1)]: 314 (18,900); 390 (13,700).
Mn(III)Cl-2L complex
An amount of 198 mg (1 mmol) of Mn(Cl)2.4H2O was dissolved in 10 mL of
methanol. This solution was added drop-wise to a hot solution of HL ligand
containing 512 mg (2 mmol). The synthesis procedure was kept identical to that
described in the previous section (yield: 40 %). Microanalysis of C32H32N2O4-
MnCl found (calc.) was: C 64.16 % (63.78); H 5.38 % (5.29); N 4.68 % (4.65).
Selected FT-IR: (KBr, cm-1) m: 1622 (C=N); 1237(C–O); 568 (M–O); 455 (M–N);
UV–Vis [kmax(nm)/-e (M cm-1)]: 321 (13,700), 375 (64,700).
Results and discussion
We report here on two new nickel(II) and manganese(III) complexes using bidentate
Schiff base ligand (HL), derived from salicylaldehyde and methoxyphenylethy-
lamine. The electrochemical properties of these two complexes were studied by
cyclic voltammetry. In addition, the catalytic performances of these complexes were
examined in the electroreduction of dioxygen and bromocyclopentane under diverse
experimental conditions. The thermal decomposition and relationship between
structure of ligand and their metal complexes were also discussed.
The stoichiometric mixture obtained from salicylaldehyde and 4-methoxyphenyl
ethylamine in methanol solution gave the desired bidentate ligand (HL) [25].This
ligand is soluble in all common organic solvents like alcohols, dimethylformamide
(DMF), dimethylsulfoxide (DMSO) and acetonitrile. The elemental analysis along
with the FT-IR and UV–Vis measurements demonstrate that all the obtained
complexes have (1:2) metal–ligand stoichiometry and show a composition in good
agreement with the proposed formulae. The chelates were proven to be stable under
the atmospheric conditions either when stored in solution or as pure solids.
Spectroscopic properties
UV–Visible spectroscopy
Figure 1 shows the UV–Vis spectra of the ligand and their metal complexes. The
electronic spectra of the ligand (HL) and the two complexes were recorded in DMF
solutions (10-5 M) at room temperature. In the UV–Vis spectra of the parent ligand,
an intense absorption band at around 314 nm was assigned to the n ? p* of
azomethine groups. This band is present in the electronic absorption spectra of the
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B. Bouzerafa et al.
2,5
2,0
1,5
1,0
0,5
0,0
-0,5
(a) HL
(b) Ni(II)-2L
(c) Mn(III)Cl-2L
(c)
(b)
(a)
300
400
500
600
700
(nm)
Fig. 1 UV–Vis spectra of ligand (HL) and its metal complexes Ni(II)-2L and Mn(III)Cl-2L
complexes showing no significant shift for the nickel complex and only a reduced
shift (7 nm) in the case of the manganese one. The electronic spectra of the two
complexes also give information about metal–ligand binding in the complexes.
Their electronic absorption spectra showed a peak between 350 and 400 nm which
could be assigned to the charge transfer band, while the weak band observed at
about 420 nm for the manganese complex is assignable to the MLCT, demonstrat-
ing the coordination of the metallic center to the ligand with coordinating bonds
between the (Ph-O) phenoxy and the (C=N) azomethine functions [26].
FT-IR spectroscopy
Figure 2 presents the FT-IR spectra of the ligand and the metal complexes. The
infrared spectra are consistent with the formation of the Schiff base ligand. The IR
spectrum of the free ligand exhibits a broad absorption band in the range of
3650–3290 cm-1 centered at 3442 cm-1, characteristic of the stretching vibration
of the OH phenolic groups fixed on the phenyl ring. A second set of characteristic
bands observed around 3010 cm-1 can be related to the presence of aromatic m (C–
H). The fundamental stretching mode of azomethine m (C=N) can be readily
assigned by comparison between the infrared spectrum of the reagents, salicylalde-
hyde and 2-(4-methoxyphenyl)ethanamine, and the resulting product [27]. The
absorption bands of the –NH2 function in 2-(4-methoxyphenyl)ethanamine and the m
(C=O) carbonyl function of the salicylaldehyde disappear to the benefit of the m
(C=N) azomethine group, confirming that the condensation reaction has occurred.
The most intense absorption band, observed at 1630 cm-1, is assigned to the –C=N–
stretching frequency of the azomethine moiety of the bidentate ligand. Finally, the
band located at 1240 cm-1 is attributed to the m (=C–O) phenolic groups [28].
In the case of the complexes, the strong absorption band corresponding to the m
(C=N) group is shifted to the lower frequencies by 20–30 cm-1, indicating the
involvement of the nitrogen atom of the m (C=N) imine function in the coordination
123

Novel nickel(II) and manganese(III) complexes with…
160
(c)
(a) (HL)
(b) Ni(II)-2L
120
(c) Mn(III)Cl-2L
(b)
80
(a)
40
4000 3500 3000 2500 2000 1500 1000
500
Wavenumber (cm-1)
Fig. 2 FT-IR spectra of ligand (HL) and its metal complexes Ni(II)-2L and Mn(III)Cl-2L
of the metal ion (Fig. 2). In the reverse case, the m (=C–O) absorption bands are
instead shifted to the higher frequencies around 1310–1340 cm-1 [29] by a
strengthening of electronic density provided from the metal ion. Furthermore, in
coordination chemistry, the complexes exhibit the formation of metal–nitrogen
bonds. This is further supported by the appearance of an absorption band
corresponding to the metal nitrogen m (M–N) stretching vibration at around
440–470 cm-1 [30]. Further conclusive evidence for the formation of metal–oxygen
bonds is the appearance of new bands around 540–570 cm-1, which are
attributable to the metal–oxygen bonds stretching vibrations [31].
1H and 13C NMR spectroscopy
The 1H NMR spectrum of the ligand (HL) in CDCl3 is shown in Fig. 3 in which the
phenolic proton of the salicylidene moieties may be observed at chemical shifts
more than 10 ppm. The 1H NMR spectra of the ligand have shown a sharp singlet
signal at 7.240 ppm due to the azomethine protons. Other signals which have
appeared as multiplets (m) in the aromatic region at around 5.861–6.348 ppm are
assignable to the aromatic protons. The (OCH3) methoxy protons absorb at
2.813 ppm with the expected signal integration. The CH2–NH2 protons of the
ethylene group are also observed as a triplet at 1.983 ppm. Regarding the CH2-
phenyl, it resonates at weak fields due to the presence of an adjacent aromatic ring
inducing a significant deshielding effect on the chemical shift value obtained at
2.840 ppm.
The 13C NMR spectrum of the ligand (HL) was also recorded in CDCl3 and is
presented in Fig. 4. The results obtained for this compound are consistent with those
reported in the literature [32] describing similar structures. The carbon of the
methoxy group is responsible of the peak observed at 55.43 ppm while the
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B. Bouzerafa et al.
e
H
a
CH3O
CH2
CH2
N
C
a
c
d
g
b
HO
f
b and g
e
c
d
Fig. 3 1H NMR spectrum of the ligand (HL)
Fig. 4 13C NMR spectrum of the ligand (HL)
azomethinic carbon is located at 165.339 ppm [33]. The signals observed at 161.40
and 158.33 ppm are assigned, respectively, to the phenolic (C–OH) and phenoxy
(C–O–C) groups [34]. Also, the spectrum showed peaks at 132–114 ppm
corresponding to carbons of the phenyl ring. As for the CH2–NH2, it resonates at
36.71 ppm whereas the CH2-phenyl is shifted to the weaker fields and observed at
61.47 ppm. These signals are fully compatible with the number of their carbon
atoms.
123

Novel nickel(II) and manganese(III) complexes with…
Mass spectrometry
The electronic impact mass spectra of the bidentate Schiff base (Fig. 5a) show the
mainly intense molecular ion peak at m/z = 256.2, which corresponds to the
expected molecular weight of the ligand. The Ni(II)-2L complex (Fig. 5b) and the
Mn(III)Cl-2L complex (Fig. 5c) also share the same fragmentation pathway. In
addition, the high resolution mass spectra (HRMS) show multiple peaks that come
from the different fragments of the parent molecule, as well as an ion peak observed
at m/z = 135.1, which is also found in the spectrum of the ligand. As can be seen in
Scheme 2, the diverse fragments observed are in good agreement with the proposed
molecular structure of the ligand. Thus, it was also confirmed that the studied
complexes were formerly constituted by two ligand units adopting a mononuclear
behavior when recording their HRMS mass spectra. These observations were found
to be in perfect agreement with the formula suggested early from the elemental
analysis.
Thermogravimetric analyses
Thermal behavior of the ligand and its metal complexes has been performed in
temperatures ranging from 25 to 950 °C under nitrogen atmosphere at a heating rate
of 10 °C.min-1. The thermogravimetric (TG-DTG) analyses of ligand (HL) and
both its metal complexes, Ni(II)-2L and Mn(III)Cl-2L, are illustrated in Fig. 6. The
main results are summarized in Table 1.
Two well-differenced mass loss steps are clearly observed showing the pathway
decomposition. The components involved in these decomposition processes relate to
the ligand structure, generating unstable intermediates that evolve as gaseous
products. In the case of the (HL) (Fig. 6a) ligand, the first weight loss step starts
from 285 up to 457 °C corresponding to the removal of the C16H16ON molecular
residue with weight loss estimated at 92.44 % (calculated 93.33 %). In the last part
of the thermogram covering the range 457–800 °C, the observed weight loss is
evaluated to 7.56 % (calculated 6.65 %) corresponding to the loss of the hydroxyl
group (OH).
For the nickel complex, the TG analysis (Fig. 6b) revealed that its decomposition
is produced in three steps. The first is accompanied by a weight loss of 2.72 %
(calculated 2.64 %) which is ascribed to the loss of the methyl (–CH3) group within
the temperature range 171–259 °C. The second step shows a weight loss within the
temperature range of 259–334 °C. This is due to the elimination of coordinated
oxygen atoms, with a weight loss of 3.12 % (calculated 2.82 %). The last step starts
from 334 and ends at 485 °C (Tmax = 375.6 °C) and is attributed to the
decomposition of the coordinated molecular form C25H23N2O with a weight loss
of 65.30 % (calculated 64.77 %).
The Schiff base manganese complex Mn(III)Cl-2L also decomposes via three
steps. This decomposition starts from 225 and ends at 765 °C (Fig. 6c). The first
decomposition step produces an observed weight loss of 35.68 % (calculated
36.04 %) that has been assigned to the loss of two C10H11NO moieties with the HCl
molecule [35]. The second step occurs at temperatures ranging from 360 to 495 °C,
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B. Bouzerafa et al.
Fig. 5 Mass spectra of HL, Ni(II)-2L and Mn(III)Cl-2L
and has been attributed to the removal of the phenylene group (C6H4),
corresponding to a weight loss of 8.21 % (calculated 7.61 %). The last step
probably represents the loss of the last C6H4 phenylene group as mentioned above
123

Novel nickel(II) and manganese(III) complexes with…
OCH3
OCH3
OCH3
Cl
Mn
O
HC N
O
O
HC N
O
HC
N
Ni
N CH
N CH
OH
HL + H+ [256.2, 100%]
H3CO
H3CO
Mn(III)Cl-2L [ 563.1735,
Ni(II)-2L [ 567.1686, 2.2%]
OCH3
6.6%]
-C9H11ON
OCH3
- C9H11OCl
+ 2H+
HC N
O
OCH3
+ H+
+ H+
O
HC N
O
O
HC N
O
Mn
- C7H5ONi
- C7H5NOMn
Ni
N CH
HC
- C7H5NO
[ 429.1008, 12.5%]
[ 419.3155, 1.5%]
[ 256.1345, 100%] [ 256.1336, 100%]
OCH3
[ 135.0807, 6.6%]
[ 135.0807, 40.7%]
H2CH2C
Scheme 2 The fragments observed in the mass spectra of HL, Ni(II)-2L and Mn(III)Cl-2L
with a weight loss of 8.14 % (calculated 7.61 %). The results obtained from
thermogravimetric analyses are given in Table 1. Therefore, it has been found that
the decomposition pathway of the Mn(III)Cl-2L corroborates the suggested
decomposition process illustrated in Scheme 1.
Electrochemistry
Electrochemical behavior of the ligand HL
The electrochemical response of the ligand was studied by cyclic voltammetry using
a scan rate of 100 mV s-1, exploring potentials from -2.0 V to ?2.0 V (Fig. 7).
The voltammogram shows two oxidation waves, at Epa1 = 0.232 and
Epa2 = 1.374 V, versus SCE, respectively. The first oxidation wave is probably
assignable to the oxidation of methoxy substituents as reported in the literature [36].
As has been proposed, the oxidation wave corresponds to the formation of radical-
cations as the first step. This intermediate is not stable due to its high chemical
reactivity [37]. The second may be assigned to the oxidation of the phenolic
function. These results may approach those reported in the literature [38] for the
123

B. Bouzerafa et al.
-0,003
-0,002
-0,001
0,000
0,001
(A)100
HL
10 °C.min-1
80
60
40
20
0
100 200 300 400 500 600 700 800 900
Temperature/°C
-1,5
-1,0
-0,5
0,0
100
80
60
40
20
0
(B)
Ni(II)-2L
10 °C.min-1
100 200 300 400 500 600 700 800
Temperature (°C)
-0,08
100
80
60
40
20
0
(C)
Mn(III)Cl-2L
10 °C.min-1
-0,06
-0,04
-0,02
100 200 300 400 500 600 700 800 900
Temperature (°C)
Fig. 6 a TG-DTG curves of HL, b Ni(II)-2L and c Mn(III)Cl-2L
compounds showing a similar structure. Concerning the reduction sweep, three
reduction waves are observed at Epc1 = -1.010, Epc2 = -1.450 and
Epc3 = -1.976 V/SCE, respectively. The first is attributed to the reduction of the
oxidized function indicated earlier, while the second may most probably be due to
the reduction of the –C=N– (azomethine) group of the adsorbed molecule [39]. As
for the last wave, it seems to be an irreversible system and may be ascribed to the
reduction of the imino function.
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Novel nickel(II) and manganese(III) complexes with…
Table 1 Thermogravimetric results of ligand (HL) and its metal complexes Ni(II)-2L and Mn(III)Cl-
2L
Compound
Number Temp.
DTG Mass loss (%)
found (calcd.)
Product
Residue
of steps range (°C) Tmax
/
°C
HL
Step 1
285–457
440.6 92.44 % (93.33 %) Loss of
C16H16ON
Step 2
Step 1
Step 2
Step 3
457–800
160–259
259–334
334–485
561.1 7.56 % (6.65 %)
196.7 2.72 % (2.64 %)
269.4 3.12 % (2.82 %)
Loss of OH
Loss of CH3
Loss of O
Ni(II)-2L
NiO2 ? C6H6
375.6 65.30 % (64.77 %) Loss of
C25H23N2O
486.5 34.73 % (35.76 %) Loss of
Mn(III)Cl-2L Step 1
225–360
C20H24O2N2Cl
Step 2
Step 3
360–495
495–755
645.4 8.21 % (7.61 %)
761.5 8.14 % (7.61 %)
Loss of C6H4
Loss of C6H4
MnO2
300
(HL)
200
100
0
-100
-200
-2,5 -2,0 -1,5 -1,0 -0,5 0,0 0,5 1,0 1,5 2,0
E(V/SCE)
Fig. 7 Cyclic voltammogram of 1 mM HL on GC electrode in DMF solution, 0.1 M TBAPF4 at
100 mV s-1 from -2.2 to 2.2 V versus SCE
Electrochemical behavior of the nickel complex Ni(II)-2L
Figure 8 illustrates the voltammetric behaviour of the Ni(II)-2L complex, recorded
on the GC-electrode in DMF solutions at 100 mV/s as scan rate. Its cyclic
voltammetric curves for the anodic oxidation show two redox couples at
E1/2 = ?0.745 and E1/2 = ?1.15 V/SCE. A single electron is involved in the first
oxidation wave which can be assigned to the oxidation of Ni(II) to Ni(III). The
second one is due to the oxidation of the Schiff base ligand. As for the return sweep,
only one well-defined wave was observed. This wave expresses the monoelectronic
transfer leading to the formation of the initial form of the nickel complex according
to the redox couple Ni(III)/Ni(II). Its half-wave potential value (E1/2) was calculated
and found to be equal to -1.48 V/SCE. The electrochemical reduction peak
observed at -1.980 V/SCE is due to the irreversible reduction of the ligand [40]
(Fig. 8a).
123

B. Bouzerafa et al.
80
60
40
20
0
A
5
0
Ni(II)-2L
-5
-10
-15
-20
-2,5
-2,0
-1,5
-1,0
-0,5
0,0
E(SCE)
-20
-40
-2,5 -2,0 -1,5 -1,0 -0,5 0,0 0,5 1,0 1,5 2,0
E(SCE)
B
10
Ni(II)-2L
5
0
10 mV.s-1
25 mV.s-1
50 mV.s-1
75 mV.s-1
100 mV.s-1
200 mV.s-1
300 mV.s-1
400 mV.s-1
500 mV.s-1
-5
-10
-15
-20
-25
-2,0
-1,8
-1,6
-1,4
-1,2
-1,0
E(SCE)
Fig. 8 a Cyclic voltammogram of 1 mM Ni(II)-2L on the GC electrode in DMF solution, 0.1 M
TBABF4 at 100 mV s-1. b Cyclic voltammogram of 1 mM Ni(II)-2L on the GC electrode in DMF
solution with cycling from -1.1 to -1.8 V versus SCE, 0.1 M TBABF4 at various scan rates
The cyclic voltammogram, recorded in the range between 0.0 and -2.2 V, shows
a well-defined redox peak at Epc = -1.560 V (cathodic peak) and Epa = -1.460 V
(anodic peak) which corresponds to Ni(II)/Ni(I) system. We have followed the
evolution of the potentials of the anodic and cathodic peak currents depending on
the variation of the scan rates for the system.
Ni(II)/Ni(I) between -1.0 and -2.0 V versus SCE (Fig. 8b). In this case, we
notice that, after discounting the double layer contribution, the intensity of the peak
currents ipa, ipc follows a square-root relationship with the scan rate. It is noticeable
that their ratios are close to 1, confirming the occurrence of a reversible process,
while the peak-to-peak separation (DEp) was found to be 100 mV and to increase
slightly with the scan rate. In spite of a DEp value, corresponding to a quasi-
reversible behavior, of the couple Ni(II)L ? e- = Ni(I)L which is in agreement
with a diffusion-controlled electrochemical process [41]). This DEp value is larger
than the theoretical equal to 59.2/n mV (where n is the number of transferred
123

Novel nickel(II) and manganese(III) complexes with…
30
Ni(II)-2L, 10 Cycles
20
10
0
-10
-20
-30
-2,0
-1,5
-1,0
-0,5
0,0
0,5
E(SCE)
Fig. 9 Cyclic voltammogram showing electrodeposition of poly-[Ni(II)-2L] on the glassy carbon
electrode in DMF solution, 0.1 M TBAPF4 at 100 mV s-1
electrons in the process). This value suggests that the oxidation of nickel (II) in the
considered complex should only correspond to one electron charge transfer.
Figure 9 shows the evolution of the cyclic voltammograms of the Ni(II)-2L
complex in DMF solutions. The cathodic reduction was achieved by repetitive
cycles of the potential at the surface of the glassy carbon electrode between 0.2
and -1.8 V using a scan rate of 100 mV s-1. The build-up of the electropoly-
merized poly-[Ni(II)-2L] can be followed by an increase of the peaks associated
to the redox processes previously discussed. Thus, it is clear that, upon continuous
cycles, an electroactive polymer is deposited on the electrode surface. By
adjusting the number of CV cycles, different and controllable polymer thicknesses
can be obtained [42].
Electrochemical behavior of the manganese complex Mn(III)Cl-2L
Figure 10 depicts the voltammograms (CVs) of the manganese complex which were
recorded in the same electrolyte already used for the nickel complex. The CV
revealed three new redox systems, different to those detected for the ligand, and
appearing at E1/2(I) = -0.106 V, E1/2(II) = 0.497 V and E1/2(III) = 0.897 V (vs.
SCE). The DEp = (Epa–Epc) values were estimated to be 100, 110 and 128 mV,
respectively, pointing out again the quasi-reversible nature and the participation of
only one electron in these redox processes. The first and the second systems are,
respectively, attributed to the Mn(II)/Mn(III), Mn(III)/Mn(IV) couples. These redox
systems, when analyzed separately on CVs, and recorded with various scan rates
from 10 to 500 mV s-1, are illustrated in Fig. 11.
These voltammetric curves seem to maintain the features of a quasi-reversible
behavior process, typical for the one electron transfer process commonly known to
be diffusion controlled.
123

B. Bouzerafa et al.
125
100
75
Mn(III)Cl-2L
50
25
0
-25
-50
-1,0
-0,5
0,0
0,5
1,0
1,5
2,0
E(SCE)
Fig. 10 Cyclic voltammogram of 1 mM Mn(III)Cl-2L on the GC electrode in DMF solution, 0.1 M
TBABF4 at 100 mV s-1
Electrocatalytic properties
The electrochemical reduction of bromocyclopentane (BrCP) by the Ni(II)-2L
complex was used as a catalytic test in order to check the possible electrocatalytic
activity of both the complexes synthesized in this work. Figure 12 presents the CV
curves, recorded at 100 mV s-1 on the GC-electrode in DMF solutions containing
1.0 mM of the Ni(II)-2L complex with 0.1 M of TBABF4 as supporting
electrolyte. In each one of them, different amounts of BrCP were added prior tp
starting the CV measurement. When 1 mM of BrCP was added to the solution, a
reduction current at -1.45 V versus SCE can be seen in Fig. 12. This reduction
peak current (ipc), arising from the addition of the bromocyclopentane molecules
is due to the electrocatalytic reduction by the nickel complex used as catalyst.
Moreover, the peak potential for the reduction of Ni(II) to Ni(I) is shifted
downwards by ca. 15 mV, owing to the rapid reaction between the electrogen-
erated nickel(I) and bromocyclopentane, while no shifting was observed for the
oxidation wave of nickel(I) [43]. The intensity of the reduction peak seems to be
proportional to the concentration of BrCP, since the ipc is gradually increased as
the concentration of substrate increases. All these features are characteristic of a
catalytic reaction in which the Ni complex species act as electrochemical
mediators (or catalysts).
123

Novel nickel(II) and manganese(III) complexes with…
Mn(III)/Mn(II)
10
0
10 mV.s-1
25 mV.s-1
50 mV.s-1
75 mV.s-1
100 mV.s-1
200 mV.s-1
300 mV.s-1
400 mV.s-1
500 mV.s-1
-10
-20
-30
-40
-600
-400
-200
E(SCE)
0
200
400
10 mV.s-1
25 mV.s-1
50 mV.s-1
75 mV.s-1
100 mV.s-1
200 mV.s-1
300 mV.s-1
400 mV.s-1
500 mV.s-1
Mn (III)/Mn (IV)
8
6
4
2
0
-2
-4
200
300
400
500
600
700
E(SCE)
Fig. 11 Cyclic voltammogram of 1 mM Mn(III)Cl-2L on the GC electrode in DMF solution, 0.1 M,
TBABF4 at various scan rates
5
Ni(II)-2L
0
-5
-10
Ni(II)-2L
Ni(II)-2L + 1mM BrCP)
-15
Ni(II)-2L + 2mM BrCP)
Ni(II)-2L+ 4mM BrCP)
Ni(II)-2L+ 5mM BrCP)
Ni(II)-2L + 8mM BrCP)
-20
-25
-2,0
-1,8
-1,6
-1,4
-1,2
-1,0
E(SCE)
Fig. 12 Cyclic voltammograms recorded on the glassy carbon electrode in DMF solution containing
0.10 M TBAPF4 and 1.0 mM Ni(II)-2L at 100 mV s-1 without and with different concentrations of
bromocyclopentane (BrCP)
123

B. Bouzerafa et al.
Conclusion
In this paper, we have presented the synthesis and characterization of a bidentate
Schiff base ligand with its two new metal complexes, Ni(II)-2L and Mn(III)Cl-2L.
This study describes the synthetic methods of a new Schiff base chelate, which
includes a methoxy functional group, and its complexes with bivalent and trivalent
transition metals such as nickel(II) and manganese(III). These materials have been
fully characterized in terms of structure, chemical composition and electrochemical
behavior. The nickel complex was easily electropolymerized to its poly-[Ni(II)-2L]
films yielding modified electrodes. This new complex was also found to be as
efficient homogeneous electrocatalyst for the reduction of bromocyclopentane. We
also believe that this work will shed light on future work on the synthesis of mono-
or multinuclear Schiff base–metal complexes and heterocyclic compounds from
Schiff base ligands with methoxy functional groups.
References
1. H. Wang, D. Zhang, Y. Chen, Z. Hai, L. Tian, J. Jiang, Synthesis, crystal structures, and magnetic
properties of new tetranuclear Cu(II) complexes of bis-bidentate Schiff-base ligands. Inorg. Chim.
Acta 362, 4972–4976 (2009)
2. M. Khorshidifard, H.A. Rudbari, Z.K. Delikani, V. Mirkhani, R. Azadbakht, Synthesis, characteri-
zation and X-ray crystal structures of Vanadium(IV), Cobalt(III), Copper(II) and Zinc(II) complexes
derived from an asymmetric bidentate Schiff-base ligand at ambient temperature. J. Mol. Struct.
1081, 494–505 (2015)
3. M.T.H. Tarafder, K.T. Jin, K.A. Crouse, A.M. Ali, B.M. Yamin, H.-K. Fun, Coordination chemistry
and bioactivity of Ni2?, Cu2?, Cd2? and Zn2? complexes containing bidentate Schiff bases
derived from Sbenzyldithiocarbazate and the X-ray crystal structure of bis[S-benzylb-N-(5-methyl-2-
furylmethylene)dithiocarbazato]cadmium(II). Polyhedron 21, 2547–2554 (2002)
4. M. Amirnasr, A.H. Mahmoudkhani, A. Gorji, S. Dehghanpour, H.R. Bijanzadeh, Cobalt(II), nick-
el(II), and zinc(II) complexes with bidentate N,N’-bis(b-phenylcinnam aldehyde)-1,2-diiminoethane
Schiff base: synthesis and structures. Polyhedron 21, 2733–2742 (2002)
5. A. Deronzier, J.-C. Moutet, Polypyrrole films containing metal complexes: syntheses and applica-
tions. Coordin. Chem. Rev. 147, 339–371 (1996)
6. S. Saha, R.K. Kottalanka, P. Bhowmik, S. Jana, K. Harms, T.K. Panda, S. Chattopadhyay, H.P.
Nayek, Synthesis and characterization of a nickel(II) complex of 9-methoxy-2,3- dihydro-1,4-ben-
zoxyzepine derived from a Schiff base ligand and its ligand substitution reaction. J. Mol. Struct.
2014, 26–31 (1061)
7. W.I. Azeez, A.I. Abdulla, Dioxouranium(V1) thiocyanate complexes of schiff bases of p-Nitro,
chloro, bromo, hydroxy and methoxy aniline and salicylaldehyde. Inorg. Chim. Acta 110, 15–18
(1985)
8. A.A. Soliman, W. Linert, Investigations on new transition metal chelates of the 3-methoxy-sali-
cylidene-2-aminothiophenol Schiff base. Thermochim. Acta 338, 67–75 (1999)
9. C. Maxim, T.D. Pasatoiu, V.Ch. Kravtsov, S. Shova, Ch.A. Muryn, R.E.P. Winpenny, F. Tuna, M.
Andruh, Copper(II) and zinc(II) complexes with Schiff-base ligands derived from salicylaldehyde
and 3-methoxysalicylaldehyde: synthesis, crystal structures, magnetic and luminescence properties.
Inorg. Chim. Acta 361, 3903–3911 (2008)
10. E. Monzani, L. Casella, M. Gullotti, N. Panigada, F. Franceschi, V. Papaefthymiou, Cytochrome
oxidase models. Dinuclear iron/copper complexes derived from covalently modified deuteropor-
phyrins. J. Mol. Cat. A Chem. 117, 199–204 (1997)
11. T. Punniyamurthy, M.M. Reddy, S.J.S. Kalra, J. Iqbal, Cobalt(II) schiff base catalyzed biomimetic
oxidation of organic substrates with dioxygen. Pure Appl. Chem. 68, 619–622 (1996)
123

Novel nickel(II) and manganese(III) complexes with…
12. K.L. Nagashree, M.F. Ahmed, Electrocatalytic oxidation of methanol on Ni modified polyaniline
electrode in alkaline medium. J. Solid State Electrochem. 14, 2307–2320 (2010)
13. R.J.P. Williams, Metal ions in biological catalysts. Pure Appl. Chem. 54, 1889–1904 (1982)
14. A. Deronzier, J.-C. Moutet, Functionalized polypyrroles. New molecular materials for electrocatal-
ysis and related applications. Acc. Chem. Res. 22, 249–255 (1989)
15. C.P. Horwitz, R.W. Murray, Oxidative electropolymerization of metal schiff base complexes. Mol.
Cryst. Liq. Cryst. 160, 389–404 (1988)
16. A. Ghaffari, M. Behzad, M. Pooyan, H.A. Rudbari, G. Bruno, Crystal structures and catalytic
performance of three new methoxy substituted salen type nickel(II) Schiff base complexes derived
from, meso-1,2-diphenyl-1,2-ethylenediamine. J. Mol. Struct. 2014, 1–7 (1063)
17. Frank. Reuter, Eva. Rentschler, Structural, magnetic and electronic characterization of an isostruc-
tural series of dinuclear complexes of 3d metal ions bridged by tpbd. Polyhedron 52, 788–796 (2013)
18. J.-C. Moutet, A. Ourari, Electrocatalytic epoxidation and oxidation with dioxygen using man-
ganese(III) Schiff-base complexes. Electrochim. Acta 42, 2525–2531 (1997)
19. J.-C. Moutet, Y. Ouennoughi, A. Ourari, S. H. -Thibault, Electrocatalytic hydrogenation on noble
metalparticules dispersed in polymer films. Enhanced catalytic activity induced by the incorporation
of bimetallic catalysts. Electrochim. Acta 40, 1827–1833 (1995)
20. B. Karlhuber, A. Fiaux, T. Schaffhauser, Ciba-Geigy Corporation, Process for producing epoxy resin
having a low-EHC-content from chlorine-substituted crude epoxy resin of high EHC-content and
apparatus for automatically controlling such process. Patent US4810776 (1989)
21. W. Zhang, J.L. Loebach, S.R. Wilson, E.N. Jacobsen, Enantioselective epoxidation of unfunction-
alized olefins catalyzed by (Sa1en)manganese complexes. J. Am. Chem. Soc. 112, 2801–2803 (1990)
22. R. Irie, K. Noda, Y. Ito, N. Matsumoto, T. Katsuki, Catalytic asymmetric epoxidation of unfunc-
tionalized olefin. Tetrahedron Lett. 31, 7345–7348 (1990)
23. A. Ourari, H. Nora, C. Noureddine, D. Aggoun, Elaboration of new electrodes with carbon paste
containing polystyrene functionalized by pentadentate nickel(II)-Schiff base complex—application to
the electrooxidation reaction of methanol and its aliphatic analogs. Electrochim. Acta 170, 311–320
(2015)
24. A. Ourari, W. Derafa, D. Aggoun, A novel copper(II) complex with an unsymmetrical tridentate-
schiff base: synthesis, crystal structure, electrochemical, morphological and electrocatalytic behav-
iors toward electroreduction of alkyl and aryl halides. RSC Adv. 5, 82894–82905 (2015)
25. A. Ourari, Y. Ouennoughi, S. Bouacida, Tris(2-{[2-(4-methoxyphenyl)ethyl]imino methyl}pheno-
lato-j2N,O1)cobalt(III), Acta Cryst. E68, Struc. Reports m803–m804 (2012)
26. Y.-G. Li, D.-H. Shi, H.-L. Zhu, H. Yan, S.W. Ng, Transition metal complexes (M = Cu, Ni and Mn)
of schiff-base ligands: syntheses, crystal structures, and inhibitory bioactivities against urease and
xanthine oxidase. Inorg. Chim. Acta 360, 2881–2889 (2007)
27. P. Singh, D.P. Singh, K. Tiwari, M. Mishra, A.K. Singh, V.P. Singh, Synthesis, structural investi-
gations and corrosion inhibition studies on Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) complexes with
2-amino-benzoic acid (phenyl-pyridin-2-yl-methylene)-hydrazide. RSC Adv. 5, 45217–45230 (2015)
28. R. Aurkie, M.G. Rosair, R. Kadam, S. Mitra, Three new mono- di-trinuclear cobalt complexes of
selectively and non-selectively condensed Schiff bases with N2O and N2O2 donor sets: syntheses,
structural variations. EPR DNA Bind Stud Polyhedron 28, 796–806 (2009)
29. A. Ourari, K. Ouari, W. Moumeni, L. Sibous, G. Bouet, M.A. Khan, Unsymmetrical tetradentate
schiff base complexes derived from 2,3-diaminophenol and salicylaldehyde or 5-bromosalicylalde-
hyde. Trans. Met. Chem. 31, 169–175 (2006)
30. A.M.O. Ali, S.M. El-Medani, D.A. Ahmed, D.A. Nassar, Synthesis, characterization, fluorescence and
catalytic activity of some new complexes of unsymmetrical Schiff base of 2-pyridinecarboxaldehyde
with 2,6-diaminopyridine, Spectrochim. Acta A Mol. Biomol. Spectrosc. 144, 99–106 (2015)
31. M. Mishra, K. Tiwari, A.K. Singh, V.P. Singh, Versatile coordination behavior of a multi-dentate
Schiff base with manganese(II), copper(II) and zinc(II) ions and their corrosion inhibition study.
Inorg. Chim. Acta 425, 36–45 (2015)
32. S. Ilhan, H. Baykara, A. Oztomsuk, V. Okumus, A. Levent, M.S. Seyitoglu, S. Ozdemir, Synthesis
and characterization of 1,2-bis(2-(5-bromo-2-hydroxybenzilidenamino)-4-chlorophenoxy)ethane and
its metal complexes: an experimental, theoretical, electrochemical, antioxidant and antibacterial
study, Spectrochim. Acta A Mol. Biomol. Spectrosc. 118, 632–642 (2014)
33. M. Shebl, S.M.E. Khalil, S.A. Ahmed, H.A.A. Medien, Synthesis, spectroscopic characterization and
antimicrobial activity of mono-, bi- and tri-nuclear metal complexes of a new Schiff base ligand.
J. Mol. Struct. 980, 39–50 (2010)
123

B. Bouzerafa et al.
34. T. Rosu, E. Pahontu, C. Maxim, R. Georgescu, N. Stanica, A. Gulea, Some new Cu(II) complexes
containing an ON donor Schiff base: synthesis, characterization and antibacterial activity. Polyhe-
dron 30, 154–162 (2011)
35. A.D. Khalaji, M. Nikookar, D. Das, Co(III), Ni(II), and Cu(II) complexes of bidentate N, O-donor
Schiff base ligand derived from 4-methoxy-2-nitroaniline and salicylaldehyde Synthesis, character-
ization, thermal studies and use as new precursors for metal oxides nanoparticles. J. Therm. Anal.
Cal. 115, 409–417 (2014)
36. V.V. Sizov, M.V. Novozhilova, E.V. Alekseeva, M.P. Karushev, A.M. Timonov, S.N. Eliseeva, A.A.
Vanin, V.V. Malev, O.V. Levin, Redox transformations in electroactive polymer films derived from
complexes of nickel with SalEn-type ligands: computational, EQCM, and spectroelectrochemical
study. J. Solid State Electrochem. 19, 453–468 (2015)
37. A.H. Said, F.M. Mhalla, C. Amatore, J.-N. Verpeaux, Mechanistic investigation of the anodic oxi-
dation of p-methoxytoluene in dry and wet acetonitrile. J. Electroanal. Chem. 464, 85–92 (1999)
38. A.A. Isse, A. Gennaro, E. Vianello, Electrochemical reduction of Schiff base ligands H2salen and
H2salophen. Electrochim. Acta 42, 2065–2071 (1997)
39. S. Yagmur, S. Yilmaz, G. Saglikoglu, M. Sadikoglu, M. Yildiz, K. Polat, Synthesis, spectroscopic
studies and electrochemical properties of Schiff bases derived from 2-hydroxy aromatic aldehydes
and phenazopyridine hydrochloride. J. Serb. Chem. Soc. 78(6), 795–804 (2013)
40. K. Aoki, K. Tokuda, H. Matsuda, Theory of linear sweep voltammetry with finite diffusion space:
part II. Totally irreversible and quasi-reversible cases. J. Electroanal. Chem. 160, 33–45 (1984)
41. K. Aoki, K. Tokuda, H. Matsuda, Theory of linear sweep voltammetry with finite diffusion space.
J. Electroanal. Chem. 146, 417–424 (1983)
42. A. Ourari, D. Aggoun, L. Ouahab, A novel copper(II)-Schiff base complex containing pyrrole ring:
synthesis, characterization and its modified electrodes applied in oxidation of aliphatic alcohols.
Inorg. Chem. Commun. 33, 118–124 (2013)
43. A. Ourari, Y. Ouennoughi, D. Aggoun, M.S. Mubarak, E.M. Pasciak, D.G. Peters, Synthesis, char-
acterization, and electrochemical study of a new tetradentate nickel(II)-Schiff base complex derived
from ethylenediamine and 50-(N-methyl-N-phenylaminomethyl)-20-hydroxyacetophenone. Polyhe-
dron 67, 59–64 (2014)
123