Tetradentate N 2O 2 type Nickel(II) Schiff base complexes derived from meso -1,2-diphenyle-1,2-ethylenediamine: Synthesis, characterization, crystal structures, electrochemistry, and catalytic studies

Article abstract of DOI:10.1080/00958972.2013.867031

A series of Ni(II) complexes of salen type Schiff base ligands were synthesized and characterized. The ligands were synthesized from the condensation of meso-1,2-diphenyle-1,2-ethylenediamine with salicylaldehyde (H2L¹), 5-bromosalicylaldehyde (H2L²), 5-bromo-3-nitrosalicylaldehyde (H2L³), and 2′- hydroxyacetophenone (H2L⁴). The complexes were characterized by means of ¹HNMR, IR, and UV-Vis spectroscopy and elemental analysis. Crystal structures of NiL³ and NiL⁴ were also determined by x-ray crystallography. Electrochemistry of the complexes was studied by means of cyclic voltammetry. Catalytic performance of the complexes was studied in the oxidation of cyclooctene using tert-butylhydroperoxide (TBHP) as oxidant. Various factors including solvent type, reaction temperature, time, catalyst amount, and substrate to oxidant ratio were optimized. Solvent free oxidation of cyclooctene with these catalysts was also studied. Increased catalytic activity and higher epoxide selectivity was achieved in solvent-free conditions. NiL⁴ with more electron donating substituents on the ligand was the most efficient oxidation catalyst. 2013

Full text of DOI:10.1080/00958972.2013.867031

This article was downloaded by: [Moskow State Univ Bibliote]
On: 14 December 2013, At: 23:20
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Journal of Coordination Chemistry
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gcoo20
Tetradentate N2O2 type Nickel(II)
Schiff base complexes derived
from meso-1,2-diphenyle-1,2-
ethylenediamine: synthesis,
characterization, crystal structures,
electrochemistry, and catalytic studies
Mahsa Pooyana, Abolfazl Ghaffaria, Mahdi Behzada, Hadi Amiri
Rudbarib & Giuseppe Brunoc
a Department of Chemistry, Semnan University, Semnan, Iran
b Faculty of Chemistry, University of Isfahan, Isfahan, Iran
c Università degli Studi di Messina, dip. Scienze Chimiche. Viale
Ferdinando S. d’Alcontres, Messina, Italy
Published online: 10 Dec 2013.
To cite this article: Mahsa Pooyan, Abolfazl Ghaffari, Mahdi Behzad, Hadi Amiri Rudbari &
Giuseppe Bruno (2013) Tetradentate N2O2 type Nickel(II) Schiff base complexes derived from
meso-1,2-diphenyle-1,2-ethylenediamine: synthesis, characterization, crystal structures,
electrochemistry, and catalytic studies, Journal of Coordination Chemistry, 66:24, 4255-4267, DOI:
10.1080/00958972.2013.867031
To link to this article: http://dx.doi.org/10.1080/00958972.2013.867031
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &
Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Journal of Coordination Chemistry, 2013
Vol. 66, No. 24, 4255–4267, http://dx.doi.org/10.1080/00958972.2013.867031
Tetradentate N2O2 type Nickel(II) Schiff base complexes
derived from meso-1,2-diphenyle-1,2-ethylenediamine:
synthesis, characterization, crystal structures,
electrochemistry, and catalytic studies
MAHSA POOYAN†, ABOLFAZL GHAFFARI†, MAHDI BEHZAD*†,
HADI AMIRI RUDBARI‡ and GIUSEPPE BRUNO§
†Department of Chemistry, Semnan University, Semnan, Iran
‡Faculty of Chemistry, University of Isfahan, Isfahan, Iran
§Università degli Studi di Messina, dip. Scienze Chimiche. Viale Ferdinando S. d’Alcontres,
Messina, Italy
(Received 7 September 2013; accepted 14 November 2013)
A series of Ni(II) complexes of salen type Schiff base ligands were synthesized and characterized.
The ligands were synthesized from the condensation of meso-1,2-diphenyle-1,2-ethylenediamine
with salicylaldehyde (H2L1), 5-bromosalicylaldehyde (H2L2), 5-bromo-3-nitrosalicylaldehyde (H2L3),
and 2′-hydroxyacetophenone (H2L4). The complexes were characterized by means of 1HNMR, IR,
and UV-Vis spectroscopy and elemental analysis. Crystal structures of NiL3 and NiL4 were also
determined by x-ray crystallography. Electrochemistry of the complexes was studied by means of
cyclic voltammetry. Catalytic performance of the complexes was studied in the oxidation of
cyclooctene using tert-butylhydroperoxide (TBHP) as oxidant. Various factors including solvent
type, reaction temperature, time, catalyst amount, and substrate to oxidant ratio were optimized.
Solvent free oxidation of cyclooctene with these catalysts was also studied. Increased catalytic
activity and higher epoxide selectivity was achieved in solvent-free conditions. NiL4 with more
electron donating substituents on the ligand was the most efficient oxidation catalyst.
Keywords: Schiff base; Nickel(II); Crystal structure; Epoxidation; Cyclooctene
1. Introduction
Fine tuning the electronic and steric properties of transition metal complexes to control the
redox properties and catalytic activity of these compounds has been the subject of several
studies [1–5]. In this regard, Schiff base ligands have obtained considerable attention, since
the steric and electronic effects on these ligands could be easily achieved by the possibility
of the introduction of various substituents on both the amine and the carbonyl moieties
[6–8, 10]. Transition metal complexes of salen type tetradentate Schiff base ligands have
been shown to catalyze various industrially and laboratory important reactions such as poly-
merization, ring opening polymerization, oxidation, reduction, alkylation reactions, etc.
[11–15]. Epoxidation of olefins is a pivotal reaction in organic chemistry [16–30]. Perhaps,
*Corresponding author. Emails: mbehzad@semnan.ac.ir; mahdibehzad@gmail.com
© 2013 Taylor & Francis

4256
M. Pooyan et al.
the most celebrated complex in such studies is Jacobsen’s catalyst, which has been shown
to achieve high enantioselectivity in epoxidation of alkenes [14]. Nickel(II) complexes of
tetradentate salen type Schiff base ligands have also been investigated in both homogenous
and heterogeneous epoxidation of alkenes [17–20]. Besides, almost all of the usual
epoxidation of alkenes are performed in organic solvents and solvent free conditions have
not received necessary attention. Herein, and in continuation of our previous studies on the
factors affecting the transition metal Schiff base-catalyzed epoxidation of alkenes [21–24],
we report the synthesis, characterization, crystal structures, and catalytic performances of a
series of nickel(II) complexes of salen type Schiff base ligands in the epoxidation of cyclo-
octene. The Schiff bases were synthesized from the condensation of meso-1,2-diphenyl-1,2-
ethylenediamine with salicylaldehyde (H2L1), 5-Bromosalicylaldehyde (H2L2),
5-bromo-3-nitrosalicylaldehyde (H2L3), and 2′-hydroxyacetophenone (H2L4). The nickel(II)
central ion was deliberately chosen since we are studying the epoxidation of cyclooctene
with similar ligands but different metal ions to find the factors governing the reactivity of
such complexes due to the presence of different electronic and steric factors of the
central metal ion. The complexes were tested as catalysts in the oxidation of
cyclooctene with tert-butylhydroperoxide (TBHP) under various conditions to find the
optimum operating conditions. Moderate catalytic activity and epoxide selectivity was
achieved. Solvent free epoxidation of cyclooctene with these catalysts was also studied.
The catalysts showed increased activity and higher epoxide selectivity in the solvent-free
conditions. NiL4 with more electron donating substituents on the ligand was the most effi-
cient oxidation catalyst.
2. Experimental
2.1. Materials and methods
All chemicals were purchased from commercial sources and were used as received.
Meso-1,2-diphenyl-1,2-ethylenediamine [31] and the H2Lx (x = 1–3) ligands were synthe-
sized as described elsewhere [21, 22]. H2L4 was also synthesized following a similar proce-
dure. Melting points were obtained on a thermoscientific 9100 apparatus. Elemental
analyses were performed using a Perkin–Elmer 2400II CHNS-O elemental analyzer.
1HNMR spectra were recorded on a 500MHz Bruker FT-NMR spectrometer using CDCl3
or DMSO-d6 as solvent; chemical shifts (δ) 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 spectrophotometer 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 CCD area-detector diffractometer using Mo Kα
radiation (λ = 0.71073Å). Gas chromatography (GC) analyses were carried out on a
GC-17A Shimadzu instrument.
2.2. Synthesis of the ligands
2.2.1. Synthesis of H2L4. The ligands were readily synthesized by the reaction between
methanolic solutions of 1 equivalent of meso-1,2-diphenyl-1,2-ethyenediamine and 2 equiv-
alent of the appropriate carbonyl compound (scheme 1). In a typical experiment and for the

Salen type Nickle complexes as oxidation catalysts
4257
synthesis of the new derivative, H2L4, 20 mL of a methanolic solution of meso-1,2-diphe-
nyl-1,2-ethyenediamine (1 mM, 0.21 g) was placed in a round-bottom two-necked flask
equipped with a reflux condenser, a dropping funnel, and a magnetic stirrer. This solution
was stirred vigorously and heated to reflux and then from the dropping funnel was added a
solution of 2 mM (0.27 g) 2′-hydroxyacetophenone in 10 mL of methanol and the reaction
mixture was heated at reflux for a further 3 h during which the color of the solution gradu-
ally turned yellow. The progress of the reaction was checked by TLC. The reaction mixture
was then left undisturbed overnight to yield a yellow powder, which was recrystallized from
THF to give 0.33 g of yellow crystals of the target compound (73.2%). M.P. = 243 °C.
1
Selected IR (KBr, cm−1): 3460 (νO–H), 1608 (νC=N). HNMR (δ, ppm): 15.73 (2H, s, OH),
7.51–6.69 (18H, m, HAr), 5.52 (2H, s, CHPh), 2.13 (6H, s, CH3). UV–Vis in DMSO: λ,
x
y
HO
N
z
y
x
H2N
Ph
NH2
Ph
Methanol
Reflux
2 z
OH
O
R
R
N
H
H
z
OH
R
y
x
y
x
x
y
Ni(OAc)2.H2O, Methanol
Reflux
z
O
N
O
z
Ni
N
R
R
Ph
x
Ph
H
H
Ligands and Complexes
H2L1, NiL1
R
H
H
H
y
z
H
H
H
H
H2L2, NiL2
H Br
H2L3, NiL3
NO2 H Br
H2L4, NiL4
CH3
H
H
H
Scheme 1. Synthetic procedure for the ligands and complexes.

4258
M. Pooyan et al.
nm; (ε, M−1 cm−1): 261 (~85000), 323 (~15800). Anal. Calcd for C30H28N2O2: C, 80.36;
H, 6.25; N, 6.25. Found: C, 80.23; H, 6.31; N, 6.20.
2.3. Synthesis of the complexes
The complexes were synthesized following a similar procedure as described elsewhere for
NiL1 (scheme 1) [32].
2.3.1. Synthesis of NiL2.DMF. In a typical experiment, a solution of 0.58 g (1 mM) of
H2L2 in 30 mL of methanol was placed in a round bottom two-necked flask equipped
with a magnetic stirrer, a dropping funnel and a condenser. This solution was heated to
about 60 °C while being vigorously stirred and then a solution of 0.19 g Ni(OAc)2.H2O
(1mM) in 30 mL of methanol was added drop-wise from the dropping funnel. The color of
the solution gradually turned red. The reaction mixture was heated for 3 h and then it was
left undisturbed overnight. The resulting precipitate was filtered, washed with 10 mL of
methanol and air dried. Recrystallization from DMF yielded red crystals of the target com-
pound. Attempts to prepare single crystals of this compound failed. Yield: 0.56 g (88%).
1HNMR (δ, ppm): 7.63 (2H, s, HC=N); 7.426.70 (m, 16H, HAr); 5.08 (s, 2H, CHPh).
Selected IR (KBr, cm−1); 2920–3070 (νC–H), 1612 (νC=N), 1519 (νC=C), 1184 (νC–O).
UV-Vis in DMSO: λ, nm; (ε, M−1 cm−1): 260 (~60600), 339 (~7700), 366 (~4400), 432
(~5400), 470 (~2000), 550 (~220). Anal. Calcd for C31H27Br2N3NiO3: C, 52.58; H, 3.84;
N, 5.93. Found: C, 52.62; H, 3.90; N, 5.89.
2.3.2. Synthesis of NiL3.DMF. This complex was synthesized following a similar
procedure as described for NiL2 except H2L3 was used instead of H2L2. Recrystallization
from DMF gave red single crystals of the target complex suitable for x-ray crystallography.
1HNMR (δ, ppm): 7.37 (2H, s, HC=N); 7.30–6.91 (m, 14H, HAr); 4.88 (s, 2H, CHPh).
Selected IR (KBr, cm−1); 2900–3070 (νC–H), 1616 (νC=N), 1527 (νC=C), 1195 (νC–O).
UV-Vis in DMSO: λ, nm; (ε, M−1 cm−1): 258 (~55300), 342 (~7100), 430 (~8300), 452
(~7200), 544 (~230). Anal. Calcd for C31H25Br2N5NiO7: C, 46.61; H, 3.13; N, 8.77.
Found: C, 46.70; H, 3.18; N, 8.68.
2.3.3. Synthesis of NiL4.2CHCl3. This complex was also synthesized following a similar
procedure as described for NiL2 except H2L4 was used instead of H2L2. Recrystallization
from chloroform gave single crystals of the target complex suitable for x-ray crystallogra-
1
phy. HNMR (δ, ppm): 7.78–6.45 (m, 18H, HAr); 5.00 (s, 2H, CHPh), 2.10 (s, 6H, CH3).
Selected IR (KBr, cm−1); 2900–3070 (νC–H), 1598 (νC=N), 1560-1527 (νC=C), 1164 (νC–O).
UV-Vis in DMSO: λ, nm; (ε, M−1 cm−1): 262 (~61700), 333 (~10700), 414 (~9900), 450
(~5000), 547 (~400). Anal. Calcd for C32H28Cl6N2NiO2: C, 51.61; H, 3.76; N, 3.76. Found:
C, 51.68; H, 3.80; N, 3.70.
2.4. X-ray crystallography
Diffraction data were collected at room temperature with a Bruker APEX II CCD
area-detector diffractometer using Mo Kα radiation (λ = 0.71073 Å). Data collections, cell
refinements, data reductions, and absorption corrections were performed using multi-scan

Salen type Nickle complexes as oxidation catalysts
4259
Table 1. Crystal data, data collection and structure refinement parameters for NiL3 and NiL4.
Compound
NiL3
NiL4
Empirical formula
Formula weight
C31H25Br2N5NiO7
798.09
C32H28Cl6N2NiO2
743.97
Temperature (K)
298(2)
298(2)
λ (Å)
Crystal system
space group
0.71073
Triclinic
P–1
0.71073
Monoclinic
P21/c
Unit cell dimensions (Å, °)
a (Å)
b (Å)
c (Å)
α, °
10.4565(7)
11.4382(7)
14.5234(9)
72.446(3)
14.3642(3)
9.4268(2)
23.9211(3)
90.00
β, °
γ, °
69.185(3)
83.042(3)
1547.88(17), 2
1.712
3.265
96.906(5)
90.00
3215.62(10), 4
1.537
1.135
1520
Volume (Å3), Z
Calculated density (Mg m−3
)
Absorption coefficient (mm−1
F (000)
)
800
θ Range for data collection (°)
Limiting indices
2.07 to 26.00
–12≤h≤12,
–14≤k≤14,
–17≤l≤17
6070/0/415
52518
2.32 to 26.00
–17≤h≤17,
–11≤k≤11,
–29≤l≤29
6312/0/391
99703
Data/restraints/parameters
Total reflections
Unique reflections (Rint
)
6070 (0.1127)
99.9%
6312 (0.1185)
99.9%
Completeness
Refinement method
Goodness-of-fit on F2
Full-matrix least-squares on F2
0.969
Full-matrix least-squares on F2
1.057
Final R index [I > 2σ(I )]
R index [all data]
R1 = 0.0396, wR2 = 0.1143
R1 = 0.0472, wR2 = 0.1203
0.711 and –0.580
R1 = 0.0826, wR2 = 0.2198
R1 = 0.1018, wR2 = 0.2434
1.581 and –0.773
Largest difference peak and hole (e Å−3
)
methods with Bruker software [33]. The structures were solved by direct methods using
SIR2004 [34]. The non-hydrogen atoms were refined anisotropically by the full matrix least
squares method on F2 using SHELXL [35]. 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.
2.5. Cyclic voltammetry
A Metrohm 757 VA computerace instrument was employed to obtain cyclic voltammograms
in DMSO solutions at room temperature (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.
2.6. General oxidation reactions
2.6.1. 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 times for the starting materials and the products were

4260
M. Pooyan et al.
determined by comparison with authentic samples. In the absence of the complexes, no or
very few oxidation products were observed. Oxidation of cyclooctene gave
cycloocteneoxide as the major product of the reaction. The conversion percentages (%) and
the TONs were calculated by the following equations, in which Ci and Cf are initial and
final concentration of the substrate, respectively, and [Q] is the concentration of the catalyst.
In a typical experiment, 10 μM of NiL4 catalyst was dissolved in 10 mL of freshly distilled
acetonitrile and then 15 mM of cyclooctene and 30 mM of TBHP were added. The reaction
mixture was refluxed while being stirred and the reaction progress was monitored at 30 min
intervals.
Conversion ð%Þ ¼ ½ðCi ꢀ Cf Þ ꢁ 100ꢂ=Ci
(1)
(2)
Turn over number ðTONÞ ¼ ½ð%ConversionÞ ꢁ Ciꢂ=½Qꢂ
2.6.2. Solvent free oxidations. Typically, a mixture of 15 μM of the catalysts, 15 mM of
cyclooctene, and 45 mM of TBHP were refluxed and the progress of the reaction was
monitored every 30 min by GC.
3. Results and discussions
3.1. Spectroscopic characterizations
In the IR spectra of the previously reported ligands (H2L1−3) [23] and the new H2L4 ligand,
the presence of an intense band at around 1610 10 cm−1 was assigned to the stretching
vibration of the C=N group. This band was present in the IR spectra of the complexes but
had shifted to lower wave numbers upon coordination, indicating that the nitrogen atoms of
the imine groups had participated in coordination. The electronic absorption spectra of
H2L4 showed two intense bands at 261 and 323 nm corresponding to the π→π* transitions
of the phenyl rings and azomethine groups, respectively [32]. These peaks were present in
the electronic absorption spectra of the complexes. The red shift of the latter band (π→π*
transitions of the C=N groups) further confirmed the coordination of the nitrogen atoms of
the azomethine groups. The electronic absorption spectra of the complexes showed a band
at around 420 10 nm with a shoulder at higher wavelengths which could be assigned to
the LMCT comprised of a transition between the metal centered HOMO to the ligand cen-
tered LUMO (C=N(π*)) [23]. The weak bands at about 545 nm are also assignable to the
1
d-d transitions. In the HNMR spectra of the ligands, the presence of broad (br) bands at
above 13 ppm was assigned to the presence of phenolic protons. These signals were absent
1
in the HNMR spectra of NiLx complexes, which meant that the quadridentate H2Lx ligands
were doubly deprotonated at the phenolate oxygen atoms [21–23]. The azomethine protons
of the free Schiff base ligands (H2L1−3) gave sharp singlet signals (s) at around 8 ppm,
which were shifted up-field to about 7.50 0.15 ppm in the diamagnetic nickel(II) com-
plexes. This observation was indicative of the incorporation of the nitrogen atoms of the
azomethine groups in coordination to the Ni(II) center. Other signals, which appeared as
multiplets (m) in the aromatic region at around 6.50 to 7.50, could be assigned to the aro-
matic protons of the Schiff base ligands. Aliphatic protons of the diamine part of the Schiff
base ligands appeared at around 5 ppm with appropriate signal intensity. A sharp singlet

Salen type Nickle complexes as oxidation catalysts
4261
Figure 1. ORTEP representation of NiL3.DMF. Thermal ellipsoids are drawn at the 50% probability level, while
the hydrogen size is arbitrary. Solvent (DMF) has been omitted for clarity.
Table 2. Selected bond lengths and angels for NiL3 and NiL4.
Complex
NiL3
Selected bond lengths (Å)
Selected bond angels (°)
Ni(1)–O(1)
Ni(1)–O(2)
Ni(1)–N(1)
Ni(1)–N(2)
1.8467(17)
O(1)–Ni(1)–O(2)
O(1)–Ni(1)–N(1)
O(2)–Ni(1)–N(1)
O(1)–Ni(1)–N(2)
O(2)–Ni(1)–N(2)
N(1)–Ni(1)–N(2)
O(1)–Ni(1)–O(2)
O(1)–Ni(1)–N(1)
O(2)–Ni(1)–N(1)
O(1)–Ni(1)–N(2)
O(2)–Ni(1)–N(2)
N(1)–Ni(1)–N(2)
84.54(7)
94.56(8)
178.90(8)
177.59(9)
94.74(8)
86.13(9)
82.35(19)
95.17(19)
172.7(2)
174.3(2)
93.95(19)
89.0(2)
1.8496(16)
1.833(2)
1.842(2)
NiL4
Ni(1)–O(1)
Ni(1)–O(2)
Ni(1)–N(1)
Ni(1)–N(2)
1.823(4)
1.818(4)
1.872(5)
1.859(4)
signal was observed at 2.10 ppm for NiL4 due to CH3 protons, which had also been seen in
the same region for H2L4. Elemental analysis also confirmed the synthesis of the H2L4
ligand and the complexes.
3.2. Description of the crystal structures
3.2.1. Description of the crystal structure of NiL3. Red crystals of NiL3 suitable for
X-ray crystallography were obtained by recrystallization from a DMF solution after one
week. An ORTEP drawing of this compound is shown in figure 1 along with a common

4262
M. Pooyan et al.
Figure 2. ORTEP representation of NiL4.2CHCl3. Thermal ellipsoids are drawn at the 50% probability level,
while the hydrogen size is arbitrary.
atom numbering scheme. All interatomic distances can be considered as normal and a sum-
mary of the crystallographic data, selected bond lengths, and bond angles are listed in tables
1 and 2. The neutral species contains one Ni(II) ion connected to the doubly deprotonated
ligand (L3)2–. As can be seen from table 2, both of the N(1)–Ni–N(2) and O(1)–Ni–O(2)
bond angels deviate from and are smaller than the ideal 90° whereas the cisoid N–Ni–O
angels are both larger than 90°. The transoid N–Ni–O angels are also both smaller than
180° causing a slightly distorted square planar geometry around the metal center, which is
similar to previously reported analogue [17, 36, 37]. The angle between the two NCCCO
chelating ring systems is only 2.59(3)°, which indicates a flat arrangement around the metal
center. The metal atom is well located at the center of the N2O2 coordination sphere with
the average Ni–N bond distance of 1.838 Å compared to the average Ni–O bond distance
of 1.848 Å. No important hydrogen bond or π–π stacking was observed for this complex.
3.2.1. Description of the crystal structure of NiL4. The geometry around the metal center
in the crystal structure of NiL4 is similar to that of NiL3 in which the geometry is distorted
square planar where the N2O2 donating atom of the tetradentate Schiff base ligand forms the
coordination sphere. The angle between the two NCCCO chelating ring systems is 15.28(2)°,
which indicates more distortion towards the tetrahedral geometry. The average Ni–O bond
length (1.820 Å) is shorter than the average Ni–N bond distance (1.866 Å), which confirms
more distortion. Figure 2 shows the ORTEP representation of NiL4. As can be seen in
figure 2, two CHCl3 molecules have hydrogen bonds with both oxygen atoms of the ligand.
Table 3. Redox potential data for 10−3 M L−1 solutions of NiLx (x = 1–4)
complexes in DMSO solutions containing 0.1 M L−1 TOAB and scan rate
100 mV s−1. Data are in volts.
Complex
Epa
Epc
E0
ΔE
NiL1
NiL2
NiL3
NiL4
–1.40
–1.36
–0.98
–1.49
–1.50
–1.41
–1.07
–1.56
–1.45
–1.38
–1.02
–1.52
0.10
0.05
0.09
0.07

Salen type Nickle complexes as oxidation catalysts
4263
-2
-1.8
-1.6
-1.4
-1.2
-1.8
-1.6
-1.4
-1.2
-1
E(V vs.Ag/AgCl)
E(V vs.Ag/AgCl)
NiL1
NiL2
-2
-1.8
-1.6
-1.4
-1.2
-1
-1.4
-1.2
-1
-0.8
-0.6
E(V vs.Ag/AgCl)
E(V vs.Ag/AgCl)
NiL3
NiL4
Figure 3. Cyclic voltammogram of NiLx complexes in DMSO at 298 K and 100 mV s−1 scan rate.
3.3. Electrochemistry
Electrochemical studies of the complexes were performed by means of cyclic voltammetry
in the potential range of 0 to –2 volts. The CVs were obtained in DMSO solutions at room
temperature (298 K) under nitrogen atmosphere using 0.1 M tetra-n-octylammonium bro-
mide (TOAB) as supporting electrolyte. A platinum working electrode, a platinum auxiliary
electrode, and an Ag/AgCl reference electrode was used to obtain cyclic voltammograms.
The data for the complexes are collected in table 3 and the cyclic voltammogram of the
NiLx complex is shown in figure 3. The ligands were electro-inactive in the studied poten-
tial range. In the cyclic voltammogram of the NiL complexes, a reversible (for NiL2) or a
quasi-reversible (for NiL1,3,4) reduction wave due to NiII/NiI reduction process was
observed, which are in good agreement with previously reported analogues [39, 40]. As can
be seen from table 3, the presence of electronegative substituents on the ancillary ligands
has shifted the NiII/NiI redox potential to less negative values and NiL3 with two electron
withdrawing substituents on the salicylaldehyde moiety has the least negative value while
NiL4 without electron withdrawing substituents and with electron donating CH3 substituents
on the ancillary ligand has the most negative reduction potential.

4264
M. Pooyan et al.
Scheme 2. Products of the catalytic oxidation of cyclooctene with the NiLx catalysts and TBHP oxidant.
35
32.22
30
25
20.62
20
15
9.69
8.44
8.1
10
5
0
Solvent
Figure 4. The results for the optimization of solvent type for NiL4. Reaction conditions: 15 μM of NiL4, 10 mL
of solvent, 15 mM of cyclooctene, 45 mM of TBHP, reflux for 24 h.
3.4. Catalytic studies
3.4.1. In-solvent catalytic oxidation of cyclooctene. The catalytic oxidation of
cyclooctene with these new catalysts gave three products (scheme 2). Various reaction con-
ditions including reaction time, temperature, solvent type, catalyst type, solvent amount,
amount of catalyst, and oxidant to substrate ratio were optimized. In a typical experiment,
10 μM of NiL4 catalyst was dissolved in 10 mL of freshly distilled acetonitrile and then 15
mM of cyclooctene and 30 mM 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
and 24 h was found to be the optimized reaction time. To optimize the solvent type, similar
reactions were performed in five different solvents including dichloromethane, chloroform,
methanol, ethanol, and acetonitrile. The results are shown in figure 4 and as can be seen,
acetonitrile was chosen as the best solvent. The substrate to oxidant ratio (1 : 3), catalyst
amounts (15 μM), and the reaction temperature (80 °C) were also optimized. Then the effect
of the catalyst type was studied using the above-mentioned reaction conditions. Figure 5
shows the results of the optimization of the catalyst type and as can be seen, NiL4 was the
most efficient catalyst in this process. Table 4 shows the results of the catalytic data at opti-
mized conditions for different catalysts. It can be concluded from the comparison of tables
3 and 4 that there is a good correlation between the catalytic performances of the complexes
with their redox potential. NiL4 with more electron donating substituents on the ligand has
the most negative E0 and was the most efficient oxidation catalyst while NiL3 with more

Salen type Nickle complexes as oxidation catalysts
4265
35
30
25
20
15
10
5
32.22
14.69
12.19
8.41
0
NiL1
NiL4
NiL3
NiL2
Catalyst
Figure 5. The results of the catalytic oxidation of cyclooctene with NiL1−4. Reaction conditions: 15 μM of NiLx
(x = 1−4), 10 mL of freshly distilled acetonitrile, 15 mM of cyclooctene, 45 mM of TBHP, reflux for 24 h.
Table 4. Oxidation of cyclooctene with TBHP in the presence of NiLx (x = 1−4) in optimized reaction conditions.
Selectivity
Complex
Conversion %
Turn over number (TON)
NiL1
NiL2
NiL3
NiL4
14.69
12.19
8.41
147
122
84
44.56
50.08
51.24
42.93
15.99
16.28
19.37
25.22
39.45
33.64
29.39
31.85
32.22
322
Table 5. Oxidation of cyclooctene with TBHP in the presence of NiLx (x = 1−4) in solvent-free conditions.
Selectivity
Complex
Conversion %
Turn over number (TON)
NiL1
NiL2
NiL3
NiL4
36.88
36.51
30.16
42.47
369
365
302
425
63.35
64.10
74.88
69.26
18.64
13.09
11.09
18.56
18.00
22.81
14.03
11.88
electron withdrawing substituents on the ligand and with the most positive E0 value was
the least active catalyst. Considering the epoxide selectivity on the other hand, NiL3 with
more electron withdrawing substituents showed higher selectivity. Although the electro-
chemistry and electronic effects of the substituents are important factors, more work is nec-
essary to be done to find all the effects governing such reactivity. Our data are comparable
with the results of the oxidation of cyclooctene using nickel(II) Schiff base complexes
found in the literature [23–25].

4266
M. Pooyan et al.
3.4.2. Solvent-free catalytic oxidation of cyclooctene. Similar catalytic experiments were
performed with NiLx catalysts in the solvent-free conditions. To compare the results, we
used the optimized catalytic conditions mentioned for in-solvent oxidation reactions. A sim-
ilar trend was observed and NiL4 was found again to be the most efficient oxidation
catalyst with highest conversion percentage and NiL3 was the least active one. Considering
the epoxide selectivity, NiL3 was again the most selective catalyst. Besides, increased
catalytic activity and higher epoxide selectivity was observed for all of the complexes in
our solvent-free studies compared to in-solvent conditions (table 5).
4. Conclusion
Three new and one previously reported Ni(II) complexes of salen type Schiff base ligands
were synthesized and characterized by spectroscopic methods as well as elemental analysis
and X-ray crystallography. Catalytic performances of these complexes were studied both in
solvent and solvent-free conditions and several factors affecting the catalytic activity of the
complexes were optimized. Although high-valent complexes such as MoVI [16, 28], WVI
[29], or VIV [24] Schiff bases are reported to be highly active epoxidation catalysts, a
literature survey shows that some lower valent complexes are also active in this reaction
[26, 38]. Indeed, the oxidation number is not the only factor affecting the epoxidation effi-
ciency of the catalyst and other factors need to be studied and still more work is necessary
to find all factors governing such reactivity. In our research group, we are studying the
effect of different metal ions with similar Schiff base ligands on epoxidation performance
and selectivity, as well as the study of other important variables. In this work, we report the
results of our studies using nickel(II) Schiff base complexes as catalysts. Our complexes
showed almost similar activity (conversion percent) and epoxide selectivity compared to
many other Ni(II) Schiff bases. But we were able to achieve higher epoxide selectivity
compared to several previously reported Ni(II) Schiff base complexes in our solvent-free
conditions [17, 20].
Supplementary material
CCDC 941409 and 941410 contains the supplementary crystallographic data for NiL3 and
NiL4, respectively. These data can be obtained free of charge via http://www.ccdc.cam.
ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.
ac.uk.
Acknowledgement
We are acknowledged to the office of gifted students at Semnan University for financial sup-
ports.
References
[1] A.H. Kianfar, L. Keramat, M. Dostani, M. Shamsipur, M. Roushani, F. Nikpour. Spectrochim. Acta Part A,
77, 424 (2010).
[2] S. Zolezzi, E. Spodine, A. Decinti. Polyhedron, 21, 55 (2002).
[3] F. Zaera. J. Mol. Catal. A Chem., 228, 21 (2005).

Salen type Nickle complexes as oxidation catalysts
[4] M. Peters, R. Breinbauer. Tetrahedron Lett., 51, 6622 (2010).
4267
[5] E.G. Jäger, K. Schuhmann, H. Görls. Inorg. Chim. Acta, 255, 295 (1997).
[6] J.A.L. da Silva, J.J.R. Fraústo da Silva, A.J.L. Pombeiro. Coord. Chem. Rev., 255, 2232 (2011).
[7] J. Costamagna, J. Vargas, R. Latorre, A. Alvarado, G. Mena. Coord. Chem. Rev., 119, 67 (1992).
[8] M. Salavati-Niasari. J. Mol. Catal. A Chem., 263, 247 (2007).
[9] V. Mirkhani, S. Tangestaninejad, M. Moghadam, I. Mohammadpoor-Baltork. H. Kargar, 242, 251 (2005).
[10] A.H. Sarvestani, A. Salimi, S. Mohebbi, R. Hallaj. J. Chem. Res., 190 (2005).
[11] K.C. Gupta, A.K. Sutar. Coord. Chem. Rev., 252, 1420 (2008).
[12] M. Bagherzadeh, M. Zare. J. Coord. Chem., 66, 2885 (2013).
[13] A. Aghmiz, N. Mostfa, S. Iksi, R. Rivas, M.D. González, Y. Díaz, F. El Guemmout, A. El Laghdach,
R. Echarri, A.M. Masdeu-Bultó. J. Coord. Chem., 66, 2567 (2013).
[14] E.N. Jacobsen, W. Zhang, A.R. Muci, J.R. Ecker, L. Deng. J. Am. Chem. Soc., 113, 7063 (1991).
[15] S. Bunce, R.J. Cross, L.J. Farrugia, S. Kunchandy, L.L. Meason, K.W. Muir, M.O. Donnell, R.D. Peacock,
D. Stirling, S.J. Teat. Polyhedron, 17, 4179 (1998).
[16] Y.-M. Cui, Y. Wang, Y.-J. Cai. X.-J. long, W. Chen, J. Coord. Chem., 66, 2325 (2013).
[17] Y. Huang, T. Liu, J. Lin, J. Lü, Z. Lin, R. Cao. Inorg. Chem., 50, 2191 (2011).
[18] M. Salavati-Niasari, M. Hasani-Kabutarkhani, F. Davar. Catal. Commun., 7, 955 (2006).
[19] S. Bhunia, S. Koner. Polyhedron, 30, 1857 (2011).
[20] D. Chatterjee, S. Mukherjee, A. Mitra. J. Mol. Catal. A Chem., 154, 5 (2000).
[21] A. Ghaffari, M. Behzad, G. Dutkiewicz, M. Kubicki, M. Salehi. J. Coord. Chem., 65, 840 (2012).
[22] D.M. Boghaei, A. Bezaatpour, M. Behzad. J. Coord. Chem., 60, 973 (2007).
[23] J. Rahchamani, M. Behzad, A. Bezaatpour, V. Jahed, G. Dutkiewicz, M. Kubicki, M. Salehi. Polyhedron, 30,
2611 (2011).
[24] A. Bezaatpour, M. Behzad, D.M. Boghaei. J. Coord. Chem., 62, 1127 (2009).
[25] S.-B. Ding, W.-H. Li. J. Coord. Chem., 66, 2023 (2013).
[26] A. Banaei, B. Rezazadeh. J. Coord. Chem., 66, 2129 (2013).
[27] R. Klement, F. Stock, H. Elias, H. Paulus, P. Pelikän, M. Valko, M. Mazür. Polyhedron, 18, 3617 (1999).
[28] X.-Q. He. J. Coord. Chem., 66, 966 (2013).
[29] M. Amini, M. Bagherzadeh, B. Eftekhari-Sis, A. Ellern, L.K. Woo. J. Coord. Chem., 66, 1897 (2013).
[30] M. Bagherzadeh, M. Zare. J. Coord. Chem., 65, 4054 (2012).
[31] M.N.H. Irving, R.M. Parkins. J. Inorg. Nucl. Chem., 27, 270 (1965).
[32] L.N. Rusere, T. Shalumova, J.M. Tanski, L.A. Tyler. Polyhedron, 28, 3804 (2009).
[33] (a) COSMO, version 1.60, Bruker AXS Inc., Madison, WI, USA (2005); (b) SAINT, version 7.06A, Bruker
AXS Inc., Madison, WI, USA (2005); (c) SADABS, version 2.10, Bruker AXS Inc., Madison, WI, USA,
(2005).
[34] M.C. Burla, R. Caliandro, M. Camalli, B. Carrozzini, G.L. Cascarano, L. De Caro, C. Giacovazzo, G. Polidori,
R. Spagna. J. Appl. Crystallogr., 38, 381 (2005).
[35] G.M. Sheldrick. SHELXL-97, University of Göttingen, Göttingen, Germany (1997).
[36] G. Hoshina, M. Tsuchimoto, S. Ohba. Acta Cryst., C56, e122 (2000).
[37] M. Mariappan, M. Suenaga, A. Mukhopadhyay, B.G. Maiya. Inorg. Chim. Acta, 390, 95 (2012).
[38] M.R. Maurya, P. Saini, C. Haldar, A.K. Chandrakar, S. Chand. J. Coord. Chem., 65, 2903 (2012).
[39] M.H. Habibi, R. Mokhtari, M. Mikhak, M. Amirnasr, A. Amiri. Spectrochim. Acta Part A, 79, 1524 (2011).
[40] E. Dunach, A.P. Esteves, M.J. Medeiros, D. Pletcher, S. Olivero. J. Electroanal. Chem., 39, 566 (2004).