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

Article abstract of DOI:10.1016/j.molstruc.2014.01.052

Three new nickel(II) complexes of a series of methoxy substituted salen type Schiff base ligands were synthesized and characterized by IR, UV-Vis and ¹H NMR spectroscopy and elemental analysis. The ligands were synthesized from the condensation of meso-1,2-diphenyl-1,2-ethylenediamine with n-methoxysalicylaldehyde (n = 3, 4 and 5). Crystal structures of these complexes were determined. Electrochemical behavior of the complexes was studied by means of cyclic voltammetry in DMSO solutions. Catalytic performance of the complexes was studied in the epoxidation of cyclooctene using tert-butylhydroperoxide (TBHP) as oxidant under various conditions to find the optimum operating parameters. Low catalytic activity with moderate epoxide selectivity was observed in in-solvent conditions but in the solvent-free conditions, enhanced catalytic activity with high epoxide selectivity was achieved.

Full text of DOI:10.1016/j.molstruc.2014.01.052

Journal of Molecular Structure 1063 (2014) 1–7
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
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
a
b
c
Abolfazl Ghaffari a, Mahdi Behzad a, , Mahsa Pooyan , Hadi Amiri Rudbari , Giuseppe Bruno
⇑
a Department of Chemistry, Semnan University, Semnan 35351-19111, Iran
b Faculty of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
c Università degli Studi di Messina, dip. Scienze Chimiche, Viale Ferdinando S. d’Alcontres, 98166 Messina, Italy
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
Catalytic epoxidation of cyclooctene was studied with three new structurally characterized salen type
Ni(II) Schiff base complexes and enhanced catalytic activity and epoxide selectivity was achieved in sol-
vent free conditions.
ꢀ
ꢀ
ꢀ
Three new Ni(II) Schiff base
complexes were synthesized.
Crystal structures of these complexes
were determined.
Physico-chemical properties were
correlated with position of
substituents.
ꢀ
ꢀ
Epoxidation of cyclooctene with these
catalysts was studied.
Solvent free conditions gave
enhanced catalytic activity and
epoxide selectivity.
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new nickel(II) complexes of a series of methoxy substituted salen type Schiff base ligands were
synthesized and characterized by IR, UV–Vis and 1H NMR spectroscopy and elemental analysis. The
ligands were synthesized from the condensation of meso-1,2-diphenyl-1,2-ethylenediamine with
n-methoxysalicylaldehyde (n = 3, 4 and 5). Crystal structures of these complexes were determined. Elec-
trochemical behavior of the complexes was studied by means of cyclic voltammetry in DMSO solutions.
Catalytic performance of the complexes was studied in the epoxidation of cyclooctene using tert-butyl-
hydroperoxide (TBHP) as oxidant under various conditions to find the optimum operating parameters.
Low catalytic activity with moderate epoxide selectivity was observed in in-solvent conditions but in
the solvent-free conditions, enhanced catalytic activity with high epoxide selectivity was achieved.
Ó 2014 Elsevier B.V. All rights reserved.
Received 17 December 2013
Received in revised form 21 January 2014
Accepted 21 January 2014
Available online 28 January 2014
Keywords:
Schiff base
Nickel(II)
Crystal structure
Catalysis
1. Introduction
electrochemistry, and spectroscopy [11–19]. In the case of cataly-
sis, knowledge of the role played by such structural and electronic
The steric and electronic properties of salen type Schiff base
ligands could be fine tuned by the introduction of appropriate
substituents on the phenyl rings of their salicylaldehyde moieties
[1–10] and many metal complexes of such ligands have been syn-
thesized. These metal complexes have found versatile applications
effects to control the electrochemistry of such complexes has been
proven to be critical [20]. Schiff base complexes have been used as
catalysts in different industrially and laboratory important reac-
tions such as oxidation, reduction and polymerization [21–27].
Epoxidation of olefins is among the most important reactions in or-
ganic chemistry since it provides an expedient and effective way to
produce several invaluable new compounds. Various metal com-
plexes of salen type Schiff base ligands have been studied in such
reactions [28]. Nickel(II) complexes have also been used as catalyst
for epoxidation of olefins [23]. In continuation of our previous
in
a wide range of areas such as catalysis, biochemistry,
⇑
Corresponding author. Tel.: +98 2313366195; fax: +98 2313354110.
E-mail addresses: mbehzad@semnan.ac.ir, mahdibehzad@gmail.com
(M. Behzad).
0022-2860/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2014.01.052

2
A. Ghaffari et al. / Journal of Molecular Structure 1063 (2014) 1–7
studies on the preparation of Schiff base ligands with different
electronic and/or steric properties and metal complexes, and the
study of their potential application in epoxidation of alkenes, here-
in we report the synthesis and characterization of three new salen
type Schiff base complexes of Ni(II). The ligands were synthesized
from the condensation of meso-1,2-diphenyl-1,2-ethylenediamine
with n-methoxysalicylaldehyde (n = 3, 4 and 5). The nickel central
atom was chosen to compare the effect of different central metal
atoms in epoxidation studies since we have previously reported
the results of the epoxidation of cyclooctene with similar ligands
but different metal ions, namely VO(IV) and Cu(II) [2,13,29] . The
methoxy substituent is interesting in the field of Schiff base com-
plexes since different isomers of methoxysalicylaldehyde are com-
mercially available and hence, it readily provides the opportunity
of studying the positional effects of a single substituent. The new
complexes were characterized by different spectroscopic and ana-
lytical methods. The positional effects of the methoxy substituents
were studied and discussed. Crystal structures of these three com-
plexes have also been determined. Electrochemical properties of
these complexes were studied by cyclic voltammetry. Catalytic
performance of these complexes was examined in the epoxidation
of cyclooctene under various conditions. Several factors such as
solvent type and amount, reaction temperature, time, catalyst
amount and oxidant to substrate ratio were optimized. Solvent-
free epoxidation of cyclooctene with these complexes has also
been studied. It was found that under optimized in-solvent condi-
tions, these complexes showed low catalytic performance with
moderate epoxide selectivity which was similar to previously
reported analogous [23,25] but under solvent-free conditions,
enhanced catalytic activity and high epoxide selectivity was
achieved.
The reaction mixture was heated for 3 h and then it was left undis-
turbed overnight. The resulting precipitate was filtered off, washed
with 10 mL of ethanol and air dried. Recrystallization from aceto-
nitrile yielded single crystals of the target compound suitable for
X-ray crystallography. Yield: 0.46 g, 80%. 1H NMR (d, ppm): 7.26
(2H, s, HC@N); 7.41–6.23 (m, 16H, HAr); 4.98 (s, 2H, CHPh); 3.77
(s, 6H, OCH3). Selected IR (KBr, cmꢂ1); 1604 (
mC@N); 432 (mNiAN);
543 ( NiAO). UV–Vis in DMSO, k nm; (
m
e
Mꢂ1 cmꢂ1): 261 (53,100),
345 (7500), 414 (4800), 562 (140). Anal. Calcd. for C32H29N3NiO4;
C: 66.40, H: 5.01, N: 7.26, found; C: 66.32, H: 5.05, N: 7.33.
2.2.2. Synthesis of NiL2
This complex was prepared following a similar procedure as de-
scribed for NiL1 except H2L2 was used instead of H2L1. Recrystalli-
zation from acetonitrile yielded single crystals of the target
compound suitable for X-ray crystallography. Yield: 0.48 g, 85%.
1H NMR (d, ppm): 7.29 (2H, s, HC@N); 7.38–6.08 (m, 16H, HAr);
4.83 (s, 2H, CHPh); 3.73 (s, 6H, OCH3). Selected IR (KBr, cmꢂ1);
1605 (
mC@N); 455 (mNiAN); 532 (mNiAO). UV–Vis in DMSO, k nm;
(e
Mꢂ1 cmꢂ1): 268 (52,200), 313 (13,300), 400 (7500), 543 (440).
Anal. Calcd. for C30H26N2NiO4; C: 67.01, H: 4.84, N: 5.21, found;
C: 67.12, H: 4.75, N: 5.20.
2.2.3. Synthesis of NiL3ꢁCH3CN
This complex was prepared following a similar procedure as de-
scribed for NiL1 except H2L3 was used instead of H2L1. Recrystalli-
zation from acetonitrile yielded single crystals of the target
compound suitable for X-ray crystallography. Yield: 0.49 g, 87%.
1H NMR (d, ppm): 7.27 (2H, s, HC@N); 7.43–6.04 (m, 16H, HAr);
5.06 (s, 2H, CHPh); 3.58 (s, 6H, OCH3). Selected IR (KBr, cmꢂ1);
1605 (
mC@N); 462 (mNiAN); 547 (mNiAO). UV–Vis in DMSO, k nm;
(e
Mꢂ1 cmꢂ1): 261 (51,800), 336 (9300), 440 (8700), 563 (280).
Anal. Calcd. for C32H29N3NiO4; C: 66.40, H: 5.01, N: 7.26, found;
C: 66.44, H: 5.07, N: 7.21.
2. Experimental
2.1. Materials and methods
2.3. X-ray crystallography
All chemicals were purchased from commercial sources and
were used as received. Meso-1,2-diphenyl-1,2-ethylenediamine
[30] and the H2Lx (x = 1–3) ligands were synthesized as described
elsewhere [2]. Melting points were obtained on a thermoscientific
9100 apparatus. Elemental analyses were performed using a Per-
kin–Elmer 2400II CHNS-O elemental analyzer. 1H NMR spectra
were recorded on a 500 MHz Bruker FT-NMR spectrometer using
CDCl3 as solvent; chemical shifts (d) are given in ppm. IR spectra
were obtained as KBr plates using a Bruker FT-IR instrument.
UV–Vis spectra were obtained on a Shimadzu UV-1650PC spectro-
photometer in DMSO solutions. A Metrohm 757 VA computerace
instrument was employed to obtain cyclic voltammograms. X-ray
data were collected at room temperature with a Bruker APEX II
Diffraction data were collected at room temperature with a
Bruker APEX II CCD area-detector diffractometer using Mo/K
a
radiation (k = 0.71073 Å). Data collections, cell refinements, data
reductions and absorption corrections were performed using mul-
ti-scan methods with Bruker software [32]. The structures were
solved by direct methods using SIR2004 [33]. The non-hydrogen
atoms were refined anisotropically by the full matrix least squares
method on F2 using SHELXL [34]. All the hydrogen (H) atoms were
placed at the calculated positions and constrained to ride on their
parent atoms. Details concerning collections and analyses are
reported in Table 1.
2.4. Cyclic voltammetry
CCD area-detector diffractometer using Mo/K
a
radiation
(k = 0.71073 Å). Gas chromatography (GC) analyses were carried
out on a GC-17A Shimadzu instrument.
A Metrohm 757 VA computerace instrument was employed to
obtain cyclic voltammograms in DMSO solutions at room temper-
ature (25 °C) under nitrogen atmosphere using 0.1 M tetra-n-
octylammonium bromide (TOAB) as supporting electrolyte. A
platinum working electrode, a platinum auxiliary electrode and
an Ag/AgCl reference electrode was used to obtain cyclic
voltammograms.
2.2. Synthesis of the complexes
2.2.1. Synthesis of NiL1ꢁCH3CN
The complexes were synthesized following a similar procedure
as described elsewhere for the un-substituted parent complex
(Scheme 1) [31]. In a typical experiment, a solution of 0.48 g
(1 mmol) of H2L in 30 mL of ethanol was placed in a round bottom
two-necked flask equipped with a magnetic stirrer, a dropping fun-
nel and a condenser. This solution was heated to about 60 °C while
being vigorously stirred and then a solution of 0.25 g Ni(OAc)2ꢁ4H2-
O (1 mmol) in 30 mL of ethanol was added drop-wise from the
dropping funnel. The color of the solution gradually turned red.
2.5. General oxidation reactions
2.5.1. In-solvent oxidation of cyclooctene
These nickel(II) Schiff-base complexes were used as catalysts
for oxidation of cyclooctene using TBHP as oxidant in different
solvents and various reaction conditions. The progress of the
reaction was monitored by GC in 30 min intervals. The retention

A. Ghaffari et al. / Journal of Molecular Structure 1063 (2014) 1–7
3
Scheme 1. Synthetic procedure for the ligands and complexes.
times for the starting materials and the products were deter-
mined by comparison with authentic samples. In the absence of
the complexes, no or very little oxidation products were ob-
served. Oxidation of cyclooctene gave cycloocteneoxide as the
major product of the reaction. The conversion percentages (%)
and the TONs were calculated by Eqs. (1) and (2), in which Ci
and Cf are initial and final concentration of the substrate, respec-
tively, and [Q] is the concentration of the catalyst. In a typical
experiment, 10
freshly distilled acetonitrile and then 15 mmol of cyclooctene
l
mol of NiL1 catalyst was dissolved in 10 mL of
Table 1
Crystal data, data collection and structure refinement parameters for NiL1–3
.
Compound
NiL1
NiL2
NiL3
Empirical formula
Formula weight
C32H29N3NiO4
578.29
C30H26N2NiO4
537.24
C32H29N3NiO4
578.29
T (K)
298(2)
298(2)
298(2)
k (Å)
Crystal system
Space group
Unit cell dimensions (Å, °)
0.71073
Monoclinic
P21/n
0.71073
Triclinic
Pꢂ1
0.71073
Monoclinic
P21/n
a (Å)
b (Å)
c (Å)
aꢁꢃ
15.5622(5)
10.7402(4)
17.3074(6)
90.00
10.8302(2)
10.8986(2)
12.5082(3)
109.6430(10)
100.1540(10)
11.8708(2)
18.7418(4)
12.6293(3)
90.00
bꢁꢃ
105.009(2)
98.9600(10)
cꢁꢃ
90.00
2794.09(17), 4
1.375
110.2130(10)
1231.34(4)
1.449
90.00
2775.48(10)
1.384
0.741
Volume (Å3), Z
Calculated density (g cmꢂ3
)
Absorption coefficient (mmꢂ1
)
0.730
0.828
F(000)
1208
560
1208
h Range for data collection (°)
Limiting indices
2.25–26.00
ꢂ19 6 h 6 19
ꢂ13 6 k 6 13
ꢂ21 6 l 6 21
5480/0/362
89,468
2.30–28.00
2.19–28.00
ꢂ14 6 h 6 14
ꢂ14 6 k 6 14
ꢂ16 6 l 6 16
5945/0/336
ꢂ15 6 h 6 15
ꢂ24 6 k 6 24
ꢂ16 6 l 6 16
6686/0/364
Data/restraints /parameters
Total reflections
140,952
50,821
Unique reflections (Rint
Completeness
)
5480 [R(int) = 0.0548]
99.9% (h = 26.00)
5945 [R(int) = 0.0260]
99.8% (h = 28.00)
Full-matrix least squares on F2
1.089
6686 [R(int) = 0.0806]
99.7% (h = 28.00)
Full-matrix least squares on F2
1.064
Refinement method
Full-matrix least squares on F2
1.040
Goodness-of-fit on F2
Final R index [I > 2
R index [all data]
r
(I)]
R1 = 0.0457, wR2 = 0.1434
R1 = 0.0622, wR2 = 0.1663
0.549 and ꢂ0.349
R1 = 0.0251, wR2 = 0.0768
R1 = 0.0277, wR2 = 0.0811
0.281 and ꢂ0.348
R1 = 0.0350, wR2 = 0.0962
R1 = 0.0479, wR2 = 0.1048
0.536 and ꢂ0.336
Largest difference peak and hole (e Åꢂ3
)

4
A. Ghaffari et al. / Journal of Molecular Structure 1063 (2014) 1–7
and 30 mmol of TBHP were added. The reaction mixture was
refluxed while being stirred and the reaction progress was mon-
itored at 30 min intervals.
LUMO (C@N(p
*)) [36]. The weak bands at about 545 nm are also
assignable to the d–d transitions. Fig. 1 shows the UV–Vis spectra
of the three complexes. An interesting feature which is evident
from this figure and the comparison of the UV–Vis data is the blue
shift of the data for NiL2 compared to both NiL1 and NiL3. In the
NiL2, the methoxy substituents are in the para positions of the imi-
nic groups and hence, are able to denote the electron density to the
p* of the C@N. This increased electron density has caused the en-
ergy of the MLCT transition shift to higher energies (i.e. lower wave
Conversion ð%Þ ¼ ½ðCi ꢂ Cf Þ ꢄ 100ꢅ=Ci
ð1Þ
ð2Þ
Turn over number ðTONÞ ¼ ½ð% ConversionÞ ꢄ Ciꢅ=½Qꢅ
2.5.2. Solvent free oxidations
Typically, a mixture of 15 lmol of the catalysts, 15 mmol of
cyclooctene, and 45 mmol of TBHP were refluxed and the progress
lengths). The
p
to p* transition of the azomethine groups and the
d–d transition are also affected by this electron donation and show
similar features.
of the reaction was monitored every 30 min by GC.
3.2. Description of the crystal structures
3. Results and discussions
ORTEP drawings of NiL1, NiL2 and NiL3 are shown in Figs. 2–4,
respectively with common atom numbering schemes. A summary
of the crystallographic data for the three complexes is collected in
Table 1 and the selected bond lengths and bond angles are listed in
Table 2. The neutral species of NiL1–3 contain one Ni(II) ion ligated
to one L2ꢂ ligand and have similar structures. As it could be seen
from Table 2, in the crystal structures of NiL complexes, both of
the N(1)ANiAN(2) and O(1)ANiAO(2) bond angels deviate from
and are smaller than the ideal 90° whereas the cisoid NANiAO an-
gels are both larger than 90°. The transoid NANiAO angels are also
both smaller than 180° causing a slightly distorted square planar
geometry around the metal centers which are similar to previously
reported analogous [25,37,38]. The angle between the two NCCCO
chelating ring system for NiL1, NiL2 and NiL3 complexes are
18.39(3)°, 14.18(4)° and 6.45(3)°, respectively which indicates a
tetrahedral distortion from square planar geometry. The metal
atoms are well located at the centers of the N2O2 coordination
spheres with the average NiAN bond distance of 1.839 Å compared
to the average NiAO bond distance of 1.834 Å for NiL1, 1.849 Å
compared to 1.841 Å for NiL2 and 1.851 Å compared to 1.841 Å
for NiL3. All the bond distances and bond angels are similar to pre-
viously reported analogous [37,38].
3.1. Spectroscopic characterization of the complexes
In the IR spectra of the Schiff base ligands [2] the presence of an
intense band at around 1620 cmꢂ1 has been assigned to the
stretching vibrations of the azomethine groups. This band is pres-
ent in the IR spectra of the complexes and has shifted to lower
wave numbers upon coordination. This shift is indicative of the
coordination of the ligands through their iminic nitrogen atoms.
Another important observation for the complexes is the omission
of the bands at around 3400 cmꢂ1 which were present in the li-
gands. These bands were assigned to the stretching vibrations of
the OAH groups in the ligands which are absent in the IR spectra
of the complexes. This confirms that the ligands have acted as
dianionic L2ꢂ ligands. Indeed, the ligands have provided an N2O2
coordination sphere around the Ni(II) central atoms and the com-
plexes are therefore neutral. Another conclusive evidence for the
formation of NiAN and NiAO bonds is also shown by the appear-
ance of new bands at around 450 and 550 nm which could be as-
signed to the NiAN and NiAO stretching vibrations, respectively
[35]. In the 1H NMR spectra of the ligands, the presence of signals
at above 10 ppm has been assigned to the phenolic protons of the
salicylaldehyde moieties [2]. These signals are absent in the 1H
NMR spectra of the diamagnetic Ni(II) complexes and this observa-
tion further confirms the L2ꢂ character of the ligands in complexes.
The 1H NMR spectra of the ligands have shown sharp singlet sig-
nals (s) at around 8 ppm due to the azomethine protons. These sig-
nals are present in the 1H NMR spectra of the complexes but have
shifted up-field to about 7.25 ppm which is indicative of the incor-
poration of the azomethine nitrogen atoms in coordination. Other
signals which have appeared as multiplets (m) in the aromatic re-
gion at around 7.5–6.0 ppm are assignable to the aromatic protons.
Aliphatic protons of the diamine part of the Schiff base ligands
have also been observed at around 5 ppm. The OCH3 protons are
also observed at around 3.7 ppm with appropriate signal intensity.
The electronic absorption spectra of the complexes have shown
very similar features which are representative of the UV–Vis spec-
tra of Ni(II) complexes in a square planar pocket of a tetradentate
Schiff base ligand. In the UV–Vis spectra of their parent ligands,
two intense bands at around 260 and 300 nm had been assigned
3.3. Electrochemistry
Cyclic voltammetric electrochemical studies of the complexes
were performed in the potential range of 0 to ꢂ2 volts. The cyclic
voltammograms were obtained in DMSO solutions at room
temperature (298 K) under nitrogen atmosphere using 0.1 M tet-
ra-n-octylammonium bromide (TOAB) as supporting electrolyte.
A platinum working electrode, a platinum auxiliary electrode and
to the
p ?
pꢆ transitions in the phenyl rings and azomethine
groups, respectively [31]. These peaks are present in the electronic
absorption spectra of the complexes. The former peaks are ob-
served without any significant shift but the latter ones are consid-
erably red shifted. The red shift of the latter bands (p ?
p*
transitions of the C@N groups) is also indicative of the coordination
of the nitrogen atoms of the azomethine groups to the metal cen-
ter. The electronic absorption spectra of the complexes showed
bands at around 420 20 nm with a shoulder at higher wave-
lengths which could be assigned to the MLCT comprised of a tran-
sition between the metal centered HOMO to the ligand centered
Fig. 1. UV–Vis spectra of the 10ꢂ5 M DMSO solutions of the complexes. The inset
shows the d–d transition region with 10ꢂ3 M solutions.

A. Ghaffari et al. / Journal of Molecular Structure 1063 (2014) 1–7
5
Table 2
Selected bond lengths and angels for NiL1–3 complexes.
Complex
NiL1
Selected bond lengths (Å)
Selected bond angels (°)
Ni(1)AO(1)
Ni(1)AO(2)
Ni(1)AN(1)
Ni(1)AN(2)
1.835(2)
1.832(2)
1.832(2)
1.846(2)
O(1)ANi(1)AO(2)
O(1)ANi(1)AN(1)
O(2)ANi(1)AN(1)
O(1)ANi(1)AN(2)
O(2)ANi(1)AN(2)
N(1)ANi(1)AN(2)
84.23(9)
94.73(10)
175.30(10)
175.73(10)
94.76(10)
86.61(11)
NiL2
Ni(1)AO(1)
Ni(1)AO(2)
Ni(1)AN(1)
Ni(1)AN(2)
1.8373(9)
1.8460(9)
1.8549(10)
1.8443(11)
O(1)ANi(1)AO(2)
O(1)ANi(1)AAN(1)
O(2)ANi(1)AN(1)
O(1)ANi(1)AN(2)
O(2)ANi(1)AN(2)
N(1)ANi(1)AN(2)
84.36(4)
95.41(4)
172.78(4)
172.51(4)
94.89(4)
86.27(5)
NiL3
Ni(1)AO(1)
Ni(1)AO(2)
Ni(1)AN(1)
Ni(1)AN(2)
1.8393(11)
1.8430(11)
1.8574(12)
1.8468(13)
O(1)ANi(1)AO(2)
O(1)ANi(1)AN(1)
O(2)ANi(1)AN(1)
O(1)ANi(1)AN(2)
O(2)ANi(1)AN(2)
N(1)ANi(1)AN(2)
84.80(5)
95.02(5)
176.39(5)
175.98(6)
94.36(5)
86.06(6)
Fig. 2. ORTEP representation of NiL1. Displacement ellipsoids are drawn at the 50%
probability level and H atoms are shown as small spheres of arbitrary radii. Solvent
of crystallization (CH3CN) is omitted for clarity.
Table 3
Redox potential data for 10ꢂ3 mol Lꢂ1 solutions of NiLx (x = 1–3) complexes in DMSO
solutions containing 0.1 mol Lꢂ1 TOAB and scan rate 100 mV sꢂ1. Data are in Volts.
Complex
Epa
Epc
E0
DE
NiL1
NiL2
NiL3
ꢂ1.44
ꢂ1.54
ꢂ1.55
ꢂ1.52
ꢂ1.60
ꢂ1.51
ꢂ1.48
ꢂ1.57
ꢂ1.53
0.08
0.06
0.04
collected in Table 3 and the cyclic voltammograms are shown in
Fig. 5. The ligands were electro-inactive in the studied potential
range but the blank solution containing TOAB in DMSO showed a
reversible wave at ꢂ735 mV. In the cyclic voltammogram of the
NiLx complexes, a quasi-reversible (for NiL1) or a reversible (for
NiL2 and NiL3) reduction wave was observed which were easily as-
signed to the metal centered NiII/NiI reduction processes. These
observations are in good agreement with previously reported anal-
ogous [39,40]. The proposed potential of the methoxy substituents
in the para position of the C@N groups, which was developed in the
description of the UV–Vis spectra, could also help discuss the posi-
tional effects of the substituents on the redox properties of the
complexes. As it could be seen from Fig. 5 and Table 3, the E0 value
for NiL2 is the most negative one. Actually, the increased electron
density on the metal center has resulted in the more difficult NiII/
NiI reduction and more negative E0 values.
Fig. 3. ORTEP representation of NiL2. Displacement ellipsoids are drawn at the 50%
probability level and H atoms are shown as small spheres of arbitrary radii.
3.4. Catalysis
3.4.1. In-solvent catalytic oxidation of cyclooctene
The catalytic oxidation of cyclooctene with these new catalysts
gave three products which were identified by comparison of their
retention times with those of the authentic samples (Scheme 2).
Various conditions were optimized including reaction temperature
and time, solvent type and amount, catalyst type, amount of the
catalyst and oxidant to substrate ratio. All the reactions were per-
formed in triplicates to check the reproducibility of the data. Re-
ported results are given as the average. In a typical experiment,
10 l
mol of NiL2 were dissolved in 10 mL of freshly distilled aceto-
nitrile and then 15 mmol of cyclooctene and 30 mmol of TBHP
were added. The reaction mixture was refluxed while being stirred
and the reaction progress was monitored at 30 min intervals up to
30 h. After 24 h no considerable change was observed in the con-
version percentages and 24 h was chosen as the optimized reaction
time. The catalytic reaction was performed in five different sol-
vents namely dichloromethane, chloroform, methanol, ethanol
Fig. 4. ORTEP representation of NiL3. Displacement ellipsoids are drawn at the 50%
probability level and H atoms are shown as small spheres of arbitrary radii. Solvent
of crystallization (CH3CN) is omitted for clarity.
an Ag/AgCl reference electrode was used to obtain cyclic
voltammograms. The electrochemical data for the complexes are

6
A. Ghaffari et al. / Journal of Molecular Structure 1063 (2014) 1–7
4OMe
3OMe
5OMe
-1.8
-1.6
-1.4
-1.2
-1
-2
-1.8 -1.6 -1.4 -1.2
-1.9
-1.7
-1.5
-1.3
-1.1
E(V vs.Ag/AgCl)
E(V vs.Ag/AgCl)
E(V vs.Ag/AgCl)
Fig. 5. Cyclic voltammograms of NiLx complexes in DMSO at 298 K and 100 mV sꢂ1 scan rate.
Scheme 2. Products of the catalytic oxidation of cyclooctene with the NiLx catalysts.
Table 4
The results of the oxidation of cyclooctene with TBHP in the presence of NiLx complexes in the optimized reaction conditions.
Complex
Conversion (%)
Turn over number (TON)
Selectivity
NiL1
NiL2
NiL3
14
22
18
140
220
180
51
54
56
15
19
29
34
27
15
Table 5
The results of the solvent-free oxidation of cyclooctene with TBHP in the presence of NiLx complexes.
Complex
Conversion (%)
Turn over number (TON)
Selectivity
NiL1
NiL2
NiL3
33
52
43
330
520
430
74
73
77
14
12
10
12
15
13
and acetonitrile with NiL2 to find the optimized solvent. It was
found that acetonitrile was the optimized solvent with conversion
percent of about 20% while the other solvents gave less than 10% of
the products and then, acetonitrile was chosen as the reaction sol-
vent to study the effect of other parameters. The substrate to oxi-
Table 6
Epoxidation of cyclooctene catalyzed by a variety of nickel based catalysts.
Catalyst
Oxidant Conversion
(%)
% Yield of
epoxide
Ref.
NiL2 (in acetonitrile)
NiL2 (solvent free)
TBHP
TBHP
22
52
54
73
This
work
This
work
[25]
[25]
[41]
[40]
[40]
dant ratio (1:2), catalyst amounts (10 lmol) and the reaction
temperature (80 °C) were also optimized. Then the effect of the
catalyst type was studied using the above mentioned reaction con-
ditions. Table 4 shows the results of the optimization of the cata-
lyst type and as it could be seen, NiL2 was the most efficient
catalyst in this process with the highest conversion.
bis(Salicylaldimine)nickel(II) TBHP
33
13
25
10
8.5
33
13
36
10
8.5
Ni(NO3)2ꢁ6H2O
Nickel(III)-salen
Ni(salen)ꢁY
TBHP
H2O2
NaOCl
KHSO5
As it could be seen from Table 4, NiL2 (4-OMe derivative) was
the most effective catalyst with the highest conversion percent.
Comparison of these data with the electrochemical data in Table 3
shows another insight into the observed trend, i.e. a good correla-
tion could be found with the E0 values of the NiII/I reduction poten-
tial with the catalytic performance of the complexes. Indeed, NiL2
with the most negative E0 value was the most efficient oxidation
Ni(salen)ꢁY
catalyst while NiL1 with the least negative E0 is the least active
one. The results of cyclooctene epoxidation with these catalysts
are comparable to previously reported analogous [23,39].

A. Ghaffari et al. / Journal of Molecular Structure 1063 (2014) 1–7
7
3.4.2. Solvent-free catalytic oxidation of cyclooctene
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to previously reported complexes but we were able to achieve
higher conversion percentages and epoxide selectivity in solvent
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