Synthesis, characterization, and electrochemical study of a new tetradentate nickel(II)-Schiff base complex derived from ethylenediamine and 5′-(N-methyl-N-phenylaminomethyl)-2′-hydroxyacetophenone

Article abstract of DOI:10.1016/j.poly.2013.08.056

A tetradentate Schiff base ligand (3) has been synthesized via the reaction of 5′-(N-methyl-N-phenylaminomethyl)-2′-hydroxyacetophenone (2) with a stoichiometric amount of ethylenediamine in absolute ethanol. Compound 2 was prepared by reaction of 5′-chlo

Full text of DOI:10.1016/j.poly.2013.08.056

Polyhedron 67 (2014) 59–64
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Polyhedron
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Synthesis, characterization, and electrochemical study of a new
tetradentate nickel(II)-Schiff base complex derived from ethylenediamine
and 50-(N-methyl-N-phenylaminomethyl)-20-hydroxyacetophenone
a
a
b
c
Ali Ourari a, , Yasmina Ouennoughi , Djouhra Aggoun , Mohammad S. Mubarak , Erick M. Pasciak ,
⇑
Dennis G. Peters c,
⇑
a Laboratoire d’électrochimie, d’Ingénierie Moléculaire et de Catalyse Rédox (LEIMCR), Faculté de Technologie, Université Sétif-1, Route de Béjaia, 19000, Algeria
b Department of Chemistry, The University of Jordan, Amman 11942, Jordan
c Department of Chemistry, Indiana University, Bloomington, IN 47405, USA
a r t i c l e i n f o
a b s t r a c t
A tetradentate Schiff base ligand (3) has been synthesized via the reaction of 50-(N-methyl-N-phenylami-
nomethyl)-20-hydroxyacetophenone (2) with a stoichiometric amount of ethylenediamine in absolute
ethanol. Compound 2 was prepared by reaction of 50-chloromethyl-20-hydroxyacetophenone (1) with
N-methylaniline, in the presence of sodium bicarbonate in tetrahydrofuran. Compound 1, on the other
hand, was obtained through a reaction between a hydrochloric acid–formaldehyde mixture and 20-
hydroxyacetophenone. Refluxing a mixture of the Schiff base (3) and a stoichiometric amount of nickel(II)
acetate tetrahydrate in absolute ethanol at 50 °C under a nitrogen atmosphere afforded the expected tet-
radentate nickel(II)-Schiff base coordination compound (4). Compounds 1–4 have been characterized
with the aid of a number of techniques: UV–Vis spectrophotometry; FT-IR, 1H NMR, 13C NMR, and mass
spectrometry; and elemental analysis. Cyclic voltammetry has been employed to investigate the redox
behavior of compound 4 as well as the ability of the electrogenerated nickel(I) form of 4 to catalyze
the reduction of 1-iodooctane.
Article history:
Received 30 July 2013
Accepted 23 August 2013
Available online 3 September 2013
Keywords:
Chloromethylation reaction
Tetradentate Schiff base
Nickel(II)-Schiff base complex
Cyclic voltammetry
Electrocatalysis
Reduction of 1-iodooctane
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
electrode. Accordingly, metal-Schiff base complexes could be
used, together with monomers such as aniline, pyrrole, and thio-
There has been considerable effort in recent years to prepare
new Schiff bases (imines) similar in their structures to porphyrins;
these compounds are well known as good ligands to coordinate
metal ions [1]. Their transition-metal complexes have been found
to be useful as antibacterial [2] and anticancer [3,4] agents, as effi-
cient drugs in the arena of pharmacology [5], as excellent homoge-
neous- and heterogeneous-phase catalysts [6], as precursors for
electrocatalytic processes [7–10], and as chemical sensors [11].
Symmetrical Schiff bases, particularly tetradentate species (such
as salen) with four donor heteroatoms, have been extensively stud-
ied in the literature [12], due to their ability to stabilize many dif-
ferent metals in various oxidation states. In addition, metal-Schiff
base complexes are efficient as electrode modifiers for anodic oxi-
dation or cathodic reduction [10,13–18].
phene, to form, via anodic oxidation, polymer-coated electrodes,
with imbibed electroactive metal centers for a variety of applica-
tions in the fields of electrocatalysis and electroanalysis. Alterna-
tively, it is possible to use a symmetrical metal-Schiff base
complex, structurally modified with aniline-like moieties on the
periphery of the ligand, that can be electropolymerized as a con-
ducting film onto the surface of an electrode; this strategy was
first employed by Horwitz and Murray [19] to prepare films from
aniline-modified salen complexes of nickel(II), cobalt(II) and man-
ganese(II). This latter approach, where metal complexes bearing
electropolymerizable moieties such as aniline and pyrrole are
used for the preparation of modified electrodes, is now being ac-
tively pursued in one of our laboratories. As part of this research
effort [20–22], we describe in this report the synthesis, character-
ization, and electrochemical behavior of a new tetradentate nick-
el(II)-Schiff base complex (4) bearing an aniline moiety. An
overview of the reactions involved in the synthesis of this new
complex is presented in Scheme 1. A roster of spectroscopic
techniques has been employed to characterize the nickel(II)
complex as well as the Schiff base ligand (3) and its two
building blocks—50-chloromethyl-20-hydroxyacetophenone (1)
Electropolymerization of redox-active entities, such as metal
complexes, is an effective way to modify the surface of an
⇑
Corresponding authors. Tel.: +213 776 28 15 33; fax: +213 36 61 11 54 (A.
Ourari). Tel.: +1 812 855 9671; fax: +1 812 855 8300 (D.G. Peters).
E-mail addresses: alourari@yahoo.fr (A. Ourari), peters@indiana.edu (D.G.
Peters).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.08.056

60
A. Ourari et al. / Polyhedron 67 (2014) 59–64
Scheme 1.
and
50-(N-methyl-N-phenylaminomethyl)-20-hydroxyacetophe-
High-resolution mass spectra (HRMS) were acquired with an
electrospray ion-trap (ESI) technique by collision-induced
dissociation on a Bruker APEX-4 (7 Tesla) instrument. Samples
were dissolved in acetonitrile and diluted in a spray solution
(methanol–water 1:1 v/v with 0.1% formic acid); the mobile phase
was pumped at 2 mL minꢀ1. Mass calibration was achieved with
monoprotonated arginine clusters in the mass range m/z 175–871.
none (2). Finally, we have conducted an initial exploration of
the electrochemical behavior of 4 and of the catalytic reduction
of a primary alkyl iodide (1-iodooctane) by the electrogenerated
nickel(I) analogue of 4.
2. Experimental
2.2. Synthesis of 50-chloromethyl-20-hydroxyacetophenone (1)
2.1. Reagents and instrumentation
We prepared 1 according to a procedure described by Wulff and
Akelah [23] that involved addition of 17 g (0.125 mol) of 20-
hydroxyacetophenone, in small portions with stirring, to 75 mL
of concentrated hydrochloric acid containing 6.75 g (0.225 mol)
of paraformaldehyde. This reaction mixture was maintained at
room temperature with stirring for 48 h until a precipitate formed.
Then the solid product was collected by suction filtration, washed
with an aqueous solution of sodium bicarbonate (0.5%), washed
with water, dried, and recrystallized from a toluene–petroleum
ether mixture. Yield: 18.5 g (80%) of white crystals; M.p.: 100–
102 °C. Its purity was checked by means of TLC with silica-gel
plates; the eluent was a dichloromethane–ethanol mixture (95/5,
v/v). UV–Vis (DMF) kmax(n) (nm), emax(n) [Mꢀ1 cmꢀ1]: kmax(1) (265),
emax(1) [603]; kmax(2) (327), emax(2) [1139]. IR (KBr) mx (cmꢀ1):
For the syntheses described later in this experimental section,
all chemicals were obtained from commercial sources and were
used as received without further purification. For the cyclic
voltammetry studies, dimethylformamide (DMF, EMD Chemicals,
99.9%) was employed as the solvent; tetra-n-butylammonium tet-
rafluoroborate (TBABF4, GFS Chemicals, 98%), used as the support-
ing electrolyte, was recrystallized from water–methanol and
thoroughly dried by continuous storage in a vacuum oven at
80 °C; 1-iodooctane (Aldrich, 98%) was chosen as the substrate to
test the catalytic properties of the electrogenerated nickel(I) form
of 4; and zero-grade argon (Air Products) was used to deaerate
all solutions.
Purity of the synthesized compounds (1–4) was checked by
means of thin-layer chromatography (TLC); glass plates, precoated
with silica gel (60F, Merck), were used. Uncorrected melting points
were measured in open-capillary tubes on a Kofler bench (model
WME) apparatus. All new compounds were analyzed for C, H,
and N with the aid of a EuroVector EA3000 elemental analyzer,
and the observed results agreed with the calculated percentages
to within 0.4%. Using CDCl3 as solvent and tetramethylsilane
(TMS) as internal standard, we obtained 1H and 13C NMR spectra
with a Bruker 400-MHz spectrometer; chemical shifts are ex-
pressed in ppm. We recorded FT-IR spectra as KBr disks on a Per-
kin-Elmer 1000 spectrophotometer, and UV–Vis spectra were
obtained with the aid of a UNICAM UV-300 instrument with
DMF solutions (1-cm cell).
mC–OH (3426), mC–H(aliphatic) (2951–3013), mC
@
(1640), mCH2–Cl
O
(1255), mC–Cl (819). 1H NMR (CDCl3) d (ppm): 12.33 (s, 1H, OH),
7.37 (m, 3H), 4.69 (s, 2H), 2.66 (s, 3H). 13C NMR (CDCl3) d (ppm):
204.3, 161.1, 137.3, 132.3, 128.9, 120.8, 118.6, 46.3, 28.4. HRMS
(ESI) m/z: calcd. for C9H935ClNaO2 [M+Na]+ 207.01888, found
207.01833; calcd. for C9H937ClNaO2 [M+Na]+ 209.01593, found
209.01538.
2.3. Synthesis of 50-(N-methyl-N-phenylaminomethyl)-20-
hydroxyacetophenone (2)
To a mixture of 13 g (0.071 mol) of 1 and 6.5 g (0.077 mol) of
NaHCO3 in 20 mL of THF, placed in a three-necked flask fitted with

A. Ourari et al. / Polyhedron 67 (2014) 59–64
61
a
condenser, was added, dropwise, 7.5 g (0.070 mol) of
2.5. Synthesis of the nickel(II) compound (4)
N-methylaniline, dissolved in 10 mL of THF. This mixture was
heated to 50 °C and maintained at this temperature with stirring
and under nitrogen for at least 2 h. Then the suspension, containing
solid NaCl, was filtered and washed with diethyl ether, and the sol-
vent was removed under reduced pressure. We recrystallized the
resulting pale yellow solid from a mixture of toluene and petro-
leum ether to obtain 2; its purity was checked by means of TLC
on silica-gel plates, with a dichloromethane–pentane mixture (7/
3, v/v) as eluent. Yield: 9.5 g (46%); M.p.: 82 °C. UV–Vis (DMF)
To prepare the desired nickel(II) salt (4), 200 mg (0.374 mmol)
of the ligand (3) was dissolved in 10 mL of absolute ethanol and
the solution was transferred to a 50-mL three-necked flask fitted
with a condenser. To the flask was added, dropwise, an ethanolic
solution (10 mL) of 93 mg (0.374 mmol) of nickel(II) acetate tetra-
hydrate. Then the stirred mixture was heated to reflux under a
nitrogen atmosphere for 20 h. After being cooled to room temper-
ature, the solution was reduced to half of its original volume and
was stored in a refrigerator overnight. Finally, the brown precipi-
tate of 4 was collected by filtration, washed with ethanol, and dried
under reduced pressure for several hours to yield 138 mg (63%) of
the desired compound. M.p.: >250 °C. UV–Vis (DMF) kmax(n) (nm),
kmax(n) (nm), e emax(1) [1500]; kmax(2)
max(n) [Mꢀ1 cmꢀ1]: kmax(1) (270),
(307), emax(2) [1231]; kmax(3) (343), emax(3) [1065]. IR (KBr) mx
(cmꢀ1): mC–OH (3425), mC–H(aliphatic) (2827–3048), mC@O (1641),
mCH2–N (1306), mN–CH3 (512). 1H NMR (CDCl3) d (ppm): 12.15 (s,
1H, OH), 7.14 (m, 8H), 4.45 (s, 2H), 2.97 (s, 3H), 2.54 (s, 3H). 13C
NMR (CDCl3) d (ppm): 204.6 (CO), 161.7 (CO), 150.0, 135.5,
129.5, 128.9, 119.8, 118.9, 117.4, 113.1, 56.4, 38.6, 28.8. HRMS
(ESI) m/z: calcd. for C16H17NO2 [M]+ 255.12593, found 255.12536.
e
max(n) [Mꢀ1 cmꢀ1]: kmax(1) (269),
emax(1) [280]; kmax(2) (309), emax(2)
[1110]; kmax(3) (413), emax(3) [443]; kmax(4) (449), emax(4) [180]. IR
(KBr) mx (cm–1): vC–H(aliphatic) 3163–3280, vC@N 1589, vCH2–N 1306,
vC–O 1328, vNi–O 521, vNi–N 498. 1H NMR (CDCl3) d (ppm): 6.68–
7.26 (m, 16H), 4.25 (s, 4H), 3.35 (s, 4H), 2.84 (s, 6H), 2.01 (s, 6H).
13C NMR (CDCl3) d (ppm): 168.2, 163.4, 150.1, 131.6, 129.1,
126.7, 124.2, 122.8, 121.1, 116.7, 112.8, 56.4, 54.6, 38.1, 18.1.
HRMS (ESI) m/z: calcd. for C34H37N4NiO2 [M+H]+ 591.22700, found
591.22568. Anal. Calc. for C34H36N4NiO2: C, 69.05; H, 6.14; N, 9.47.
Found: C, 68.87; H, 6.08; N, 9.35%. Repeated efforts to obtain a sin-
gle crystal of 4, suitable for structural analysis, were unsuccessful.
2.4. Synthesis of substituted salen ligand (3)
Schiff base 3 (a modified salen ligand) was synthesized accord-
ing to a procedure slightly modified from that described in the lit-
erature [19]. To a solution of 7.0 g (27.5 mmol) of 2 in 30 mL of
absolute ethanol in a 100-mL three-necked flask fitted with a con-
denser was slowly added a solution of 0.823 g (13.7 mmol) of eth-
ylenediamine in 20 mL of absolute ethanol. This mixture was
heated to 50 °C with stirring and under a nitrogen atmosphere
for 2 h, after which the resulting precipitate was filtered, washed
with diethyl ether, and dried under reduced pressure to afford
compound 3 as a yellow solid. Yield: 4.26 g (58%); M.p.: 156 °C.
Its purity was checked with the aid of TLC; silica-gel plates were
used and a dichloromethane–ethyl acetate mixture (8/2, v/v)
served as eluent. UV–Vis (DMF) kmax(n) (nm), emax(n) [Mꢀ1 cmꢀ1]:
kmax(1) (272), emax(1) [2950]; kmax(2) (311), emax(2) [2050]; kmax(3)
(392), emax(3) [989]. IR (KBr) mx (cmꢀ1): mC–OH (3416), mC–H(aliphatic)
(2922), mC@N (1616), mCH2–N (1295), mN–CH3 (488). 1H NMR (CDCl3)
d (ppm): 14.15 (s, 2H, OH), 6.74–7.27 (m, 16H), 4.42 (s, 4H), 3.93
(s, 4H), 2.93 (s, 6H), 2.29 (s, 6H). 13C NMR (CDCl3) d (ppm): 172.8
(CO), 162.4 (CO), 150.2, 131.6, 129.4, 127.6, 126.7, 119.5, 118.8,
117.0, 113.1, 56.4, 50.5, 38.4, 15.0. HRMS (ESI) m/z: calcd. for
2.6. Electrochemical study of the behavior of 4
Cyclic voltammograms were acquired with a Princeton Applied
Research Corporation (PARC) model 2273 multipurpose
electrochemical instrument operated by PowerSuiteÒ software
(PowerSuiteÒ-2.58, PowerSuiteÒ I/O Library-2.43.0, and PowerSui-
teCVÒ–2.46) with data processing in OriginPro 8.6. We constructed
a planar, circular working electrode with a geometric area of
0.077 cm2 by press-fitting a short length of 3-mm-diameter glassy
carbon rod (Grade GC-20, Tokai Electrode Manufacturing Co., To-
kyo, Japan) into the end of a machined Teflon tube; electrical con-
nection to the carbon was made by a 3-mm-diameter stainless-
steel rod that contacted the cathode material and extended up-
ward through the tube. Before each cyclic voltammogram was re-
corded, the working electrode was cleaned on a Master-Tex
(Buehler) polishing pad with an aqueous suspension of 0.05-lm
alumina, after which the electrode was rinsed ultrasonically in
DMF and wiped dry before being inserted into the electrochemical
cell. All potentials are reported with respect to a reference elec-
trode that consisted of a cadmium-saturated mercury amalgam
in contact with DMF saturated with both cadmium chloride and
C
34H38N4NaO2 [M+Na]+ 557.28925, found 557.28879.
A single crystal of ligand 3 was obtained by slow evaporation
from an ethanol–methylene chloride (8/2, v/v) solvent mixture.
Fig. 1 depicts the molecular geometry of 3 with displacement ellip-
soids drawn at the 50% probability level; H atoms are represented
as small spheres of arbitrary radii, and only the non-H atoms of the
asymmetric unit are labeled.
sodium chloride [24–26]; this electrode has
a potential of
ꢀ0.76 V versus an aqueous saturated calomel electrode (SCE) at
25 °C. A description of the cell used for cyclic voltammetry can
Fig. 1. ORTEP diagram for the substituted salen ligand (3).

62
A. Ourari et al. / Polyhedron 67 (2014) 59–64
be found in an earlier paper [27]; this single-compartment, three-
electrode cell incorporates a platinum wire auxiliary (counter)
electrode, along with the aforementioned working and reference
electrodes.
(1275–1295 cmꢀ1). These two observations are due to coordina-
tion of the metal cation through the oxygen atoms of ionized hy-
droxyl groups and the nitrogen atoms of azomethine groups
[35,36]. In addition, the new bands at 498 and 521 cm–1 confirm
the nature of the metal–ligand bonding; these bands are assigned
to Ni(II)–N and Ni(II)–O vibrations, respectively [37,38].
3. Results and discussion
3.3. Electronic spectra
3.1. Chemistry
Typical absorption bands of aromatic moieties were observed
for 1 and 2. Compound 1 shows two absorption bands at 265
and 327 nm, whereas three bands at 270, 307 and 337 nm (the last
being a shoulder) appear for 2. In both cases, the first two bands
correspond to phenyl-ring electronic transitions [39], whereas
the third band for compound 2 was assigned to an n ? p⁄ transi-
tion [40]. As for the other two compounds (3 and 4), the former dis-
plays three bands at 250, 309, and 414 nm, and the latter exhibits
four bands of which the first three are similar to those of the li-
gand. As for the fourth band at 449 nm for 4, it was assigned to
the Soret band [40], suggesting that nickel(II) is effectively coordi-
nated to the ligand, as previously shown by IR data [35,36].
Syntheses of compounds 1–4 were carried out via known chem-
ical reactions, as depicted in Scheme 1. Compound 1 was prepared
according to published procedures that basically involved a reaction
between
20-hydroxyacetophenone,
paraformaldehyde,
and
hydrochloric acid. Compound 2 was synthesized by alkylation of
N-methylaniline with compound 1. Reaction of compound 2 with
ethylenediamine afforded the modified salen ligand 3 which, upon
treatment with nickel(II) acetate tetrahydrate, produced 4. Purities
of compounds 1–3 were checked with the aid of TLC (with appropri-
ate mobile phases being mentioned in the experimental section) and
confirmed by determinations of their melting points. Structural fea-
tures of the compounds (including 4) were substantiated by means
of various spectroscopic techniques, including UV–Vis, IR, NMR, and
HRMS data, and an elemental analysis of 4 was carried out.
For all four compounds (1–4), the 1H NMR and 13C NMR spectral
features are in full agreement with the suggested structures; the
1H NMR spectra for compounds 1–3 showed signals corresponding
to methylene, aromatic, and OH protons, and DEPT experiments
were employed to differentiate secondary and quaternary carbons
from primary and tertiary carbons. Moreover, infrared spectra for
the prepared compounds showed absorption bands characteristic
of C@O groups, along with other absorptions correlated to the as-
signed structures. Additional support for the proposed structures
came from mass spectral data; mass spectra of the synthesized
compounds are in complete agreement with the assigned struc-
tures and showed the expected molecular ions (M+) as suggested
by their molecular formulas. Details of some spectral data are gi-
ven below.
3.4. NMR spectra
There is good agreement between the 1H NMR spectra of the
prepared compounds and their assigned structures. Spectra for 1,
2, and 3 reveal a downfield singlet signal in the range 14.15–
12.15 ppm that is due to the phenolic protons [29–31]. This
deshielding effect is ascribed to the formation of intramolecular
hydrogen bonds [31,41,42]. Aromatic protons of the four com-
pounds appear as complex multiplets between 6.86 and
7.37 ppm. Due to the difference in electronegativity between chlo-
rine and nitrogen atoms, the singlet for the methylene group shifts
from 4.69 ppm for the CH2–Cl moiety in 2 to 4.45 ppm for the
CH2–N moiety in 1. Protons of methyl groups linked to an imine
function, i.e., N@C–CH3, resonate as singlets with almost the same
chemical shifts, 2.93 ppm for 3 and 2.84 ppm for 4. Signals for phe-
nolic protons are not observed for the nickel complex (4), suggest-
ing that both phenoxy groups bind with the nickel ion as anions.
In the 13C NMR spectra for compounds 1 and 2, one sees peaks
associated with the different types of carbons. Carbon-atom reso-
nances at 204.6–150.0 ppm are associated with carbonyl, phenoxy,
and anilinic groups, whereas aromatic carbon atoms resonate in
the 137.0–113.1 ppm range. In the spectrum for 3, signals for the
aliphatic carbon atoms appear at 56.4–26.8 ppm. For 3 and 4, the
13C NMR spectra are similar to that of 2, because their molecular
structures are symmetrical and derive from compound 2 and eth-
ylenediamine. Carbon atoms associated with phenoxy groups are
more downfield and appear at 172.8 and 168.2 ppm, respectively,
and the carbon atoms of azomethine groups are observed at
162.4 ppm for 3 and at 163.4 ppm for 4. Aromatic and aliphatic
carbon atoms resonate at almost the same chemical shifts of the
same carbons in compounds 1 and 2. These results are in good
agreement with the spectra of similar compounds described in
the literature [31,37,38].
3.2. FT-IR spectra
Compounds 1–3 exhibit a broad absorption band in the region
from 3300 to 3800 cmꢀ1 that can be assigned to an intramolecular
hydrogen bond arising from the presence of hydroxyl groups in
their molecular structures. Absorption bands observed at 1640
and 1641 cm–1 are attributed to C@O stretching vibrations for
compounds 1 and 2, respectively, whereas the stretching vibration
for the CH2–Cl moiety of 1 appears at 819 cmꢀ1; this last band is
absent from the spectrum of 2, which has an absorption band at
1306 cmꢀ1 due to the newly formed CH2–N bond [28]. A significant
shift of the carbonyl group frequency to lower energy for both 1
and 2, in comparison with that of acetophenone (1720 cmꢀ1), can
be attributed to the stronger dipole–dipole interactions between
the phenolic proton and the carbonyl group. In addition, this sce-
nario could explain the significant displacement of the chemical
shift for the phenolic proton (dAr–OH) from 7.54 (phenol) to about
12–15 ppm for 20-hydroxyacetophenone and its analogues; this
behavior is well discussed for the 1H NMR spectra of similar struc-
tures [29–31].
3.5. Cyclic voltammetric behavior of the nickel(II) complex (4)
Shown in Fig. 2 is a cyclic voltammogram recorded at a scan
rate of 100 mV sꢀ1 for reduction of a 2.0 mM solution of 4 in oxy-
gen-free DMF containing 0.10 M TBABF4 at a freshly polished
glassy carbon electrode. A central feature of this cyclic voltammo-
gram is the reversible nickel(II)–nickel(I) redox couple with a
cathodic peak potential (Epc) of ꢀ1.05 V and an anodic peak poten-
In the IR spectrum for the nickel(II) complex (4), the
azomethine band is shifted to a lower frequency (1589 cmꢀ1),
which indicates that the nitrogen atom of the azomethine group
is coordinated to the metal ion. This bathochromic effect can be ex-
plained by the increase in the electronic delocalization through the
newly coordinated metal center in 4 [29,30,32–34]. Moreover, the
tial (Epa) of ꢀ0.97 V ; this 80-mV peak separation (
DEp = Epa – Epc)
(mC–O
)
absorption band shifts to higher wavenumbers
is identical to that observed for the reversible one-electron

A. Ourari et al. / Polyhedron 67 (2014) 59–64
63
Fig. 2. Cyclic voltammogram recorded at 100 mV sꢀ1 for reduction of a 2.0 mM
solution of 4 at a glassy carbon electrode (area = 0.077 cm2) in oxygen-free DMF–
0.10 M TBABF4. Scan goes from ca. 0 to ꢀ2.24 to 0 V.
Fig. 3. Cyclic voltammograms for reduction of a 2.0 mM solution of 4 at a glassy
carbon electrode (area = 0.077 cm2) in oxygen-free DMF–0.10 M TBABF4 containing
(A) 0 mM, (B) 2.0 mM, (C) 5.0 mM, and (D) 10.0 mM 1-iodooctane. Scans go from
ꢀ0.65 to ꢀ1.35 to ꢀ0.65 V.
reduction of nickel(II) salen under similar experimental conditions
[43]. At potentials more negative than ꢀ1.50 V, there are several
irreversible cathodic peaks that we believe are associated with
reduction of the ligand. In addition, on the rising portion of the
cathodic peak for reduction of 4 is a small prepeak, the origin of
which we have not yet elucidated.
We undertook a more detailed examination of the electrochem-
istry of 4 by focusing on the reversible nickel(II)–nickel(I) redox
couple in the region of potentials close to ꢀ1.00 V. Over a range
of scan rates (v) from 50 to 5000 mV sꢀ1, we found that the peak
It should be mentioned here that direct reduction of 1-iodooc-
tane (in the absence of 4) at a freshly polished glassy carbon elec-
trode in DMF-0.10 M TBABF4 and at a scan rate of 100 mV sꢀ1
shows a single irreversible cathodic peak with a peak potential of
ꢀ1.38 V [43]. Therefore, direct reduction of the alkyl iodide cannot
occur at potentials involved in the reduction of 4.
When 1-iodooctane is added to the system (curves B–D, Fig. 3),
the cyclic voltammetric behavior of the nickel(II) species is altered;
the cathodic peak current is enhanced (due to the nickel(I)-cata-
lyzed reduction of the alkyl iodide) and the anodic peak disap-
pears; however, the increase in the cathodic peak current is not
proportional to the concentration of 1-iodooctane, a phenomenon
that is also observed with nickel(II) salen.
One significant difference is observed when cyclic voltammo-
grams for the nickel(II) salen–1-iodooctane and 4–1-iodooctane
systems are compared. In previous research [43–45], it was discov-
ered that, when nickel(II) salen undergoes one-electron reduction
at a glassy carbon cathode in the presence of 1-iodooctane, a li-
gand-reduced form of nickel(II) salen is produced which reacts
with 1-iodooctane such that an imide (C@N) bond of the ligand
is octylated; the resulting alkylated nickel(II) salen shows a new
cathodic peak at a potential more negative than that for reduction
of unmodified nickel(II) salen, and the alkylated nickel(II) salen is
less effective as a precursor for catalytic reduction of the alkyl io-
dide. In contrast, we have not yet seen any voltammetric evidence
that the reduced form of 4 undergoes any octylation, so 4 may
prove to be a more robust catalyst precursor than nickel(II) salen,
because each imino (C@N) bond of the ligand of 4 is sterically pro-
tected by a methyl substituent. This phenomenon merits more
attention in a future study.
current ratio, Ipa/Ipc, for the nickel(II)–nickel(I) couple was
1.00 0.04, indicating that the nickel(I) intermediate is stable on
this cyclic voltammetric time scale. For the same range of scan
rates, the average values for the cathodic and anodic peak poten-
tials were Epc = ꢀ1.08 0.03 V and Epa = ꢀ0.96 0.01 V, respec-
tively. As the scan rate is increased, the peak separation becomes
larger, as expected, which is a reflection of intrinsic sluggishness
of the electron-transfer process. Finally, a plot of Ipc versus v1/2
was linear, revealing that reduction of 4 is a diffusion-controlled
process. As a part of future work, it would be interesting to deter-
mine the energy difference between the metal-centered and li-
gand-centered one-electron reduction of 4, as
a
further
comparison of its behavior with respect to nickel(II) salen [43].
3.6. Cyclic voltammetric behavior of the nickel(II) complex (4) in the
absence and presence of 1-iodooctane
A set of cyclic voltammograms obtained at a scan rate of
100 mV sꢀ1 with a freshly polished glassy carbon electrode for
the reduction of a 2.0 mM solution of 4 in oxygen-free DMF–
0.10 M TBABF4 in the absence and presence of 1-iodooctane is de-
picted in Fig. 3. We show just the region of potentials for which the
nickel(II) species is electroactive. In the absence of 1-iodooctane
(curve A, Fig. 3), the reversible one-electron nickel(II)–nickel(I)
process is observed; as described earlier, the cathodic peak poten-
tial (Epc) and the anodic peak potential (Epa) are ꢀ1.05 and ꢀ0.97 V,
respectively, and the ratio of anodic and cathodic peak currents
(Ipa/Ipc) is close to unity. As mentioned above, there is a small
cathodic hump that precedes the main peak for reduction of the
nickel(II) species as well as an anodic hump that follows the main
peak for oxidation of the nickel(I) species, but neither hump can be
explained at this time.
4. Conclusions
In this work, we have synthesized and characterized four com-
pounds, one among them being a Schiff-base ligand (3), possessing
in its molecular structure an aniline moiety, that might be used as
an electropolymerizable monomer to fabricate surface-modified
electrodes. In addition, the nickel(II) complex (4) of this monomer
was synthesized and characterized; its cyclic voltammetric behav-
ior has been investigated, and the corresponding electrogenerated
nickel(I) complex has been shown to catalyze the reduction of

64
A. Ourari et al. / Polyhedron 67 (2014) 59–64
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1-iodooctane. Furthermore, anodically formed films of this nickel
complex might be effective for the mediated electroreduction of al-
kyl halides. Finally, the new ligand might be useful for the prepa-
ration of other transition-metal complexes that could be
advantageously exploited as solid-phase (modified) electrodes for
heterogeneous catalysis, electrocatalysis, analysis, sensors, and
biosensors. Such work is now in progress.
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Acknowledgment
Appreciation is expressed to M. Hibert and his co-workers for
their valuable help with the spectroscopic analyses (Laboratoire
d’Innovation Thérapeutique UMR-7200, Faculté de Pharmacie,
Strasbourg, France).
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