Mononuclear and dinuclear salen type copper(II) Schiff base complexes: Synthesis, characterization, crystal structures and catalytic epoxidation of cyclooctene

Article abstract of DOI:10.1016/j.ica.2014.01.047

A series of salen type Schiff base ligands and their mononuclear (CuL ⁿ, n = 1-4) or dinuclear Cu(II) complexes (Cu₂(L ⁵)₂) were synthesized and characterized by ¹H NMR, IR and UV-Vis spectroscopy as well as elemental analyses. The ligands were synthesized from the condensation of meso-1,2-diphenyl-1,2-ethylenediamine with various salicylaldehyde derivatives (x-salicylaldehyde for H₂L ⁿ, x = H (n = 1), 5-Br (n = 2), 5-Br-3-NO₂ (n = 3), 3-OMe, (n = 5)) and 2′-hydroxyacetophenone (n = 4). Crystal structures of two complexes (CuL⁴ and Cu₂(L⁵)₂) were determined. Catalytic performance of the complexes was studied in the epoxidation of cyclooctene using tert-butylhydroperoxide (TBHP) as oxidant. Various factors including reaction temperature, solvent type, time, catalyst amount and substrate to oxidant ratio were optimized. High catalytic activity and epoxides selectivity was found. Solvent free epoxidation of cyclooctene was also studied and higher catalytic activity, epoxide selectivity and lower reaction times were observed.

Full text of DOI:10.1016/j.ica.2014.01.047

Accepted Manuscript
Mononuclear and dinuclear salen type Copper(II) Schiff base complexes: syn-
thesis, characterization, crystal structures and catalytic epoxidation of cyclooc-
tene
Zeinab Abbasi, Mahdi Behzad, Abolfazl Ghaffari, Hadi Amiri Rudbari,
Giuseppe Bruno
PII:
S0020-1693(14)00077-2
http://dx.doi.org/10.1016/j.ica.2014.01.047
ICA 15852
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
2 November 2013
18 January 2014
22 January 2014
Please cite this article as: Z. Abbasi, M. Behzad, A. Ghaffari, H.A. Rudbari, G. Bruno, Mononuclear and dinuclear
salen type Copper(II) Schiff base complexes: synthesis, characterization, crystal structures and catalytic epoxidation
of cyclooctene, Inorganica Chimica Acta (2014), doi: http://dx.doi.org/10.1016/j.ica.2014.01.047
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Mononuclear and dinuclear salen type Copper(II) Schiff base complexes: synthesis,
characterization, crystal structures and catalytic epoxidation of cyclooctene
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Zeinab Abbasia, Mahdi Behzada1, Abolfazl Ghaffaria, Hadi Amiri Rudbarib, Giuseppe Brunoc
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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
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Abstract
A series of salen type Schiff base ligands and their mononuclear (CuLn, n = 1-4) or dinuclear Cu(II)
complexes (Cu2(L5)2) were synthesized and characterized by HNMR, IR and UV-Vis spectroscopy as
1
well as elemental analyses. The ligands were synthesized from the condensation of meso-1,2-diphenyl-
1,2-ethylenediamine with various salicylaldehyde derivatives (x-salicylaldehyde for H2Ln, x = H (n = 1),
5-Br (n = 2), 5-Br-3-NO2 (n = 3), 3-OMe, (n = 5)) and 2'-hydroxyacetophenone (n = 4). Crystal
structures of two complexes (CuL4 and Cu2(L5)2) were determined. Catalytic performance of the
complexes was studied in the epoxidation of cyclooctene using tert-butylhydroperoxide (TBHP) as
oxidant. Various factors including reaction temperature, solvent type, time, catalyst amount and substrate
to oxidant ratio were optimized. High catalytic activity and epoxides selectivity was found. Solvent free
epoxidation of cyclooctene was also studied and higher catalytic activity, epoxide selectivity and lower
reaction times were observed.
Keywords: Copper(II); Schiff base; Crystal structure; dinuclear; epoxidation
1 Corresponding author email: mbehzad@semnan.ac.ir; mahdibehzad@gmail.com, tel: +98-231-3366195
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1. Introduction
Schiff base complexes of transition metal ions have received great attention for their biological,
medicinal, industrial and catalytic activities [1-10]. These complexes show great catalytic activity in
various fundamentally and industrially important reactions [11-15]. Among the most extensively studied
such reactions are the oxidation of hydrocarbons to a variety of value-added oxygen containing
compounds such as alcohols, aldehydes, ketones and epoxides [16-18]. Catalytic epoxidation of alkenes
is an important industrial reaction since the epoxides are used as suitable precursors for the production of
a wide variety of fine chemicals. Various copper salts have been used as catalysts for the epoxidation of
olefinic compounds [19]. Among them, copper (II) complexes of salen type Schiff base ligands have
received considerable attention [19-23]. Hence, and in continuation of our previous studies, we herein
report the synthesis, characterization, crystal structures, and catalytic performance of a series of copper(II)
complexes of Schiff base ligands derived from the condensation meso-1,2-diphenyl-1,2-ethylenediamine
with salicylaldehyde derivatives as well as 2'-hydroxyacetophene. The new complexes were tested as
catalysts in the epoxidation of cyclooctene with TBHP under various conditions to find the optimum
operating conditions. The complexes showed high catalytic activity and epoxide selectivity. On the other
hand, Schiff base catalyzed epoxidation reactions are usually performed in organic solvents and solvent
free conditions are rarely studied. Therefore, we herein report the results of our solvent free epoxidation
studies of cyclooctene to cycloocteneoxide using Cu(II) Schiff base complexes. Higher catalytic
activities, epoxide selectivity and lower reaction times were achieved.
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 [24], H2Ln (n = 1-5) ligands [25-27] and CuL1 complex [28] were synthesized as
described elsewhere. Melting points were obtained on a thermoscientific 9100 apparatus. Elemental
analyses were performed using a Perkin-Elmer 2400II CHNS-O elemental analyzer. IR spectra were
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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).
2.2. Synthesis of the complexes
The complexes were synthesized in good yields by equimolar reaction between the ligands (H2L1-5) and
Cu(OAc)2.H2O in ethanol following similar procedure as described elsewhere for the parent complex,
CuL1 [26]. In a typical experiment, a solution of 0.46 g (1 mmol) of H2L4 in 15 mL of ethanol was placed
in a round bottom two-necked flask equipped with a magnetic stirrer, a dropping funnel and a reflux
condenser. This solution was heated to about 60 °C while being vigorously stirred and then a solution of
0.20 g Cu(OAc)2.4H2O (1mmol) in 15 mL of ethanol was added drop-wise from the dropping funnel. The
color of the solution gradually turned green. The reaction mixture was heated for 3 h and then was left
undisturbed overnight. The resulting precipitate was filtered off, washed with 10 mL of ethanol and air
dried. Recrystallization from acetonitrile yielded single crystals of CuL4 suitable for x-ray
crystallography. Scheme 1 shows the schematic representation of the synthetic procedure for the
preparation of the ligands and complexes and spectroscopic and analytical data for the complexes are
collected in table 1.
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Ph
N
N
Ph
OMe
O
Cu
MeO
O
O
OMe
N
Cu
N
O
MeO
x
y
Ph
Ph
HO
N
z
Cu2(L5)2
R
R
N
+
Cu(OAc)2
OH
z
y
x
x
y
y
x
O
O
z
z
Cu
N
N
R
R
Ph
Ph
CuLn
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Ligands and Complexes
H2L1, CuL1
R
x
H
y
z
H
H
H
H
H
H
H
H
H
Br
Br
H
H2L2, CuL2
H
H2L3, CuL3
NO2
H
H2L4, CuL4
CH3
H
H2L5, Cu2(L5)2
OCH3
H
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Scheme 1. Synthetic procedure for the preparation of ligands and complexes. Ethanol was used as
solvent.
Table 1. Spectroscopic and analytical data for the new copper complexes
Anal. Calcd. (found)
Complex
Yield%
90.6
Selected
IR, cm-1
UV-Vis, nm (ε,
[for]
H
M-1 cm-1)
C
N
CuL2.DMF
1620
259 (52600),
379 (15900),
52.18(52.25)
3.79(3.84)
5.89(5.82)
(νC=N
)
[C31H27Br2N3O3Cu]
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587 (290)
259 (35300),
403 (13700),
610 (245)
CuL3.CH3CN
CuL4.0.5CH3CN
Cu2(L5)2.2CH2Cl2
92.4
71.4
68.5
1627
46.70(46.78)
70.11(70.01)
59.33(59.28)
2.72(2.81)
9.08(8.98)
6.60(6.73)
4.66(4.52)
(νC=N
)
[C30H21Br2N5O6Cu]
1573
259 (30600),
367 (12100),
600 (280)
5.18(5.20)
(νC=N
)
[C31H27.5N2.5O2Cu]
1616
285 (2150),
4.66(4.60)
(νC=N
)
381 (910),
587(220)
C62H56Cl4Cu2N4O8
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2.3. X-ray crystallography
Diffraction data were collected at room temperature with a Bruker APEX II CCD area-detector
diffractometer using Mo/Kα radiation (λ = 0.71073Å). Data collections, cell refinements, data reductions
and absorption corrections were performed using multi-scan methods with Bruker software [29]. The
structures were solved by direct methods using SIR2004 [30]. The non-hydrogen atoms were refined
anisotropically by the full matrix least squares method on F2 using SHELXL [31]. 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 2.
2.4. Cyclic voltammetry
A Metrohm 757 VA computerace instrument was employed to obtain cyclic voltammograms in DMSO
solutions at room temperature (25ºC) 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.
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2.5. General oxidation reactions
2.5.1. Oxidation of cyclooctene in different solvents
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These Copper(II) Schiff-base complexes were used as catalysts for the study of the oxidation of
cyclooctene using TBHP as oxidant in different solvents and various reaction conditions. The progress of
the reaction was monitored by GC at 60 min intervals. The retention times for the starting materials and
the products were determined by comparison with authentic samples. In the absence of the complexes,
very little 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.
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Eq. 1. Conversion % = [(Ci – Cf)/Ci] × 100
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Eq. 2. Turnover = (Conversion % × Ci)/[Q]
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In a typical experiment, 10 µmol of CuL2 catalyst was dissolved in 10 mL of solvent 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 60 min intervals by GC.
2.5.2. Solvent free oxidations
Typically, a mixture of 10 µmol of the catalysts, 15 mmol of cyclooctene, and 30 mmol of TBHP were
refluxed and the progress of the reaction was monitored at 30 min intervals by GC.
3. Results and discussions
3.1. Spectroscopic characterizations
In the IR spectra of the complexes (table 1), intense bands at around 1600 cm-1 were due to the stretching
vibrations of the C=N groups. Compared to the free ligands, these bands were shifted to lower wave
numbers upon coordination which was indicative of the participation of these groups in coordination [25].
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Furthermore, the IR spectra of the ligands showed broad bands above 3300 cm-1 due to the stretching
vibrations of the hydroxyl groups which were not observed in the IR spectra of the complexes. This meant
that the ligands have acted as dianionic ligands with the deprotonation of the phenolic OHs. The UV-Vis
spectra of the complexes showed three bands. The two lower wavelength bands, which were present in
the UV-Vis spectra of the ligands, were assigned to the π→π* transitions. The first bands at around 270
nm which appeared without considerable shifting compared to their parent ligands were assigned to the
π→π* transitions in the phenyl rings. The second lower wavelength bands which were considerably red
shifted compared to the free ligands were assigned to the π→π* transitions of the C=N groups. These red
shifts further confirmed the coordination of the azomethine groups to the Cu(II) ions. The third weak
bands at above 550 nm were attributed to the d-d transitions. Analytical data were also in good agreement
with the proposed structures.
3.2. Description of the crystal structure of CuL4
Single crystals of CuL4 suitable for X-ray crystallography were obtained by slow evaporation of
acetonitrile solution. The complex was crystallized in the monoclinic space group P21/c with 2 formula
units in the unit cell. The ORTEP representation of the complex with the common atom numbering
scheme is given in figure 1 and relevant X-ray diffraction data are listed in Table 2. Table 3 also collects
some of the selected bond lengths and angles. The crystal structure of CuL4 confirmed a tetradentate N2O2
coordination of the Schiff base ligand. The coordination environment of Cu was satisfied by the two
phenoxo oxygen and imine nitrogen atoms of the Schiff base ligand. The complex has adopted a distorted
square planar geometry with both the O(1)–Cu–N(2) and O(2)–Cu–N(1) bond angles smaller than 180°
measuring 171.23(10) and 171.48(10) degrees, respectively. The angle defined by the two NCCCO
chelating rings is 31.58(2) degrees which confirmed the tetrahedral distortion around the metal center.
The Cu-N bond distances are nearly identical and are slightly longer than the Cu-O bond distances. All
distances are comparable with previously reported Cu-N and Cu-O bond distances [27].
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Figure 1. ORTEP representation of CuL4. 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.
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Figure 2. ORTEP representation of Cu2(L5)2. Displacement ellipsoids are drawn are the 50% probability
level and H atoms are shown as small spheres of arbitrary radii. Solvent of crystallization (2CH2Cl2) are
omitted for clarity.
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Table 2. Crystal data, data collection and structure refinement parameters for CuL4
Compound
CuL4
Cu2(L5)2
C62H56Cl4Cu2N4O8
1253.99
298(2)
Empirical formula
2(C30H26CuN2O2), C2H3N
1061.19
Formula weight
T (K)
298(2)
λ (Å)
0.71073
0.71073
Triclinic
P-1
Crystal system
Monoclinic
P21/c
space group
Unit cell dimensions (Å, º)
a (Å)
10.9358(4)
14.8839(5)
16.6289(6)
90.00
11.5933(7)
11.9384(8)
13.2492
b (Å)
c (Å)
α (°)
63.547(3)
82.777(4)
63.638(3)
1465.09(17), 1
1.421
β (°)
99.126(2)
90.00
γ (°)
Volume (Å3), Z
Calculated density (Mg/m3)
Absorption coefficient (mm-1)
F(000)
2672.38(16), 2
1.319
0.848
0.966
1104
646
θ range for data collection (°)
Limiting indices
2.33 to 25.00
-13≤h≤13
-17≤k≤17
-19≤l≤19
4687 / 0 / 332
85421
2.06 to 27.00
-14≤ h ≤14
-15≤ k≤ 15
-16≤ l≤ 16
6384 / 0 / 363
54325
Data / restraints / parameters
Total reflections
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Unique reflections (Rint)
Completeness
4687 [R(int) = 0.1136]
6384 [R(int) = 0.0261]
99.8 % (θ = 25.00)
99.9 % (θ = 27.00)
Refinement method
Goodness-of-fit on F2
Final R index [I > 2σ(I)]
R index [all data]
Full-matrix least squares on F2 Full-matrix least squares on F2
1.214
1.154
R1 = 0.0515, wR2 = 0.1522
R1 = 0.0553, wR2 = 0.1605
R1 = 0.0364, wR2 = 0.1090
R1 = 0.0417, wR2 = 0.1161
0.741 and -0.599
Largest difference peak and hole (e Å-3) 1.534 and -0.478
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Table 3. Selected bond lengths and angles for CuL4 and Cu2(L5)2
Complex
Selected Bond lengths (Å)
Selected Bond angles (°)
CuL4
Cu(1)-O(1)
Cu(1)-O(2)
Cu(1)-N(1)
Cu(1)-N(2)
1.884(2)
1.871(2)
1.956(2)
1.942(2)
O(1)-Cu(1)-O(2)
O(1)-Cu(1)-N(1)
O(2)-Cu(1)-N(1)
O(1)-Cu(1)-N(2)
O(2)-Cu(1)-N(2)
N(1)-Cu(1)-N(2)
88.35(9)
93.42(9)
171.48(10)
171.23(10)
93.11(9)
86.40(9)
Cu2(L5)2
Cu(1)-O(1)
Cu(1)-O(2)
Cu(1)-N(1)
Cu(1)-N(2)
1.8980(14)
1.9196(14)
O(1)-Cu(1)-O(2)
O(1)-Cu(1)-N(1)
O(2)-Cu(1)-N(1)
O(1)-Cu(1)-N(2)
O(2)-Cu(1)-N(2)
N(1)-Cu(1)-N(2)
90.16(6)
92.90(7)
176.20(6)
176.41(6)
92.85(6)
84.01(7)
1.9391(16)
1.9498(16)
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Description of the crystal structure of Cu2(L5)2
An ORTEP representation of Cu2(L5)2 complex is shown in figure 2 and the crystallographic data and
selected bond lengths and angles are collected in tables 2 and 3. In the crystal structure of this complex,
two symmetry related moieties are joined together through weak interactions from the oxygen atoms of
the phenolate groups of the Schiff base ligands to the axial position of the copper atoms at a distance of
2.736(2) Å. Although the distance is very long but is less than the sum of the van der Waals radii for
copper(II) and oxygen (2.92 Å) [32-33]. Therefore, a weak bond is considered to be formed between the
two units. Comparable distances have been regarded as bond in several other reported systems [34].
Therefore, the structure of the complex could be described as square-based pyramid with the N2O2 group
forming the basal plane of the pyramid and the bridging oxygen atom of the phenolate group occupying
the axial position.
3.3. Electrochemistry
Electrochemical studies of the complexes were performed by means of cyclic voltammetry in the
potential range of +1 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 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. The ligands were electro-inactive in the studied
range. The electrochemical data of the complexes are collected in table 4. In the cyclic voltammogram of
each of the complexes, a quasi-reversible wave was observed for the CuIILx + 1e → CuILx process. Figure
3 shows the cyclic voltammogram of CuL4 as an example.
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Figure 3. cyclic voltammogram of 10-3 mol L-1 solutions of CuL4 in DMSO solutions containing 0.1 mol
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L-1 TOAB and scan rate 100 mV s-1.
Table 4. Electrochemical data for 10-3 mol L-1 solutions of the complexes in DMSO solutions containing 0.1
mol L-1 TOAB and scan rate 100 mV s-1. Data are in Volts.
CuLn
Epa (V)
Epc (V)
∆E (V)
E0 (V)
CuL1
-1.01
-0.91
-1.14
-1.03
-1.00
-1.12
-1.03
-1.22
-1.13
-1.13
0.11
0.12
0.08
0.10
0.13
-1.06
-0.97
-1.18
-1.08
-1.06
CuL2
CuL3
CuL4
Cu2(L5)2
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3.4. Catalytic activity studies
The catalytic oxidation of cyclooctene with these new catalysts gave three products (Scheme 2). Several
reaction conditions including solvent type and amount, reaction time, catalyst type, amount of the
catalyst, and oxidant to substrate ratio were optimized. In order to find the most efficient CuLn catalyst for
the cyclooctene oxidation reaction, 10 µmol of each of the complexes were dissolved in 10 mL of freshly
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distilled methanol and then 15 mmol of cyclooctene and 30 mmol of TBHP were added and the reaction
mixtures were stirred and refluxed for 48 h. The progress of the reaction was monitored at 60 min
intervals by GC. Two important results were obtained. First, it was observed that the conversion
percentages gradually increased up to 24 h and reached their plateau, and hence, 24 h was chosen as the
optimized reaction time. Second, as it could be seen from table 5 which collects the results of the
optimization of catalyst type, CuL2 was the most efficient catalyst and was chosen to study the effect of
other variables.
O
OH
Catalysts
O
+
+
TBHP, reflux
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Scheme 2. Products of the catalytic oxidation of cyclooctene with the new catalysts and TBHP oxidant
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Table 5. The results of the optimization of the catalyst type
TON
Selectivity
Other products %
Conversion %
CuLn
Epoxide %
510
1215
345
49
44
52
52
51
51
56
48
48
49
34
82
23
31
25
CuL1
CuL2
CuL3
CuL4
Cu2(L5)2
450
375
201
202
203
Figure 4 shows the results of the optimization of other parameters such as the solvent type, solvent
amount, catalyst load and the oxidant to substrate ratio.
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(a)
(b)
(d)
(c)
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207
208
209
Figure 4. The results of the optimization of solvent type solvent type (a); solvent amount (b); the oxidant
to substrate ratio (c) and catalyst load (d).
A comparison between the results of tables 4 and 5 also showed that the electrochemistry of the
complexes was an important factor affecting the catalytic performance of the Cu(II) Schiff base catalysts.
It was seen that more positive standard reduction potentials resulted in more efficiency of the catalyst.
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One of the important conclusions drawn from these studies was that as the amount of the solvent was
decreased, the conversion percentage was increased. This led us to study solvent-free oxidation of
cyclooctene using the synthesized Cu Schiff base catalysts. To the best of our knowledge, all previous
catalytic studies on the epoxidation of alkenes with tetradentate Schiff-base complexes of Copper(II) were
performed in organic solvents. Our studies showed that oxidation of cyclooctene in solvent-free
conditions with these complexes were almost complete within three hours. Table 6 shows the results of
the oxidation of cyclooctene under solvent-free conditions. Other parameters were kept similar to the
conditions obtained in the optimized solvent. As it could be seen from table 6, similar trends for the
catalytic performance of the copper complexes were observed and CuL2 was still the most efficient
catalyst with the highest conversion. Besides the reaction time was much lower than that of acetonitrile
which was the optimized solvent. Higher catalytic activity and epoxide selectivity was also achieved.
Table 6. The results of the solvent-free oxidation of cyclooctene with CuLn catalysts, using TBHP as
oxidant.
TON
Selectivity
Epoxide selectivity %
Conversion percent (%)
Catalysts
Other products %
930
1275
795
36
38
33
33
36
64
62
67
67
64
60
85
54
61
74
CuL1
CuL2
CuL3
CuL4
Cu2(L5)2
915
1110
Conditions: 10 µmmol catalyst, 15 mmol cyclooctene, 30 mmol TBHP, Reflux, 3h.
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Conclusions
Three new mononuclear, and one dinuclear salen type Schiff base complexes of copper(II), as well as a
previously reported one, were synthesized and characterized. X-ray crystal structures of two complexes, a
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mononuclear (CuL4) and a dinuclear (Cu2(L5)2) one, were also determined. These complexes were tested
as catalyst for the oxidation of cyclooctene using TBHP as oxidant both in various solvents and solvent-
free conditions. High catalytic activity and epoxide selectivity was observed. Besides, in solvent-free
conditions higher catalytic activity, epoxide selectivity and lower reaction times were achieved.
Comparison of these data with our previously published results for similar ligands but different metal ions
(VO2+ [17, 25] and Ni2+ [27]) showed that these complexes were much efficient homogeneous catalyst
than the Ni2+ analogous but slightly less efficient compared to our oxidovanadium(IV) analogous.
Besides, structural correlations with the catalytic properties of these complexes were also explained.
Appendix A. Supplementary material
CCDC 941326 and 941299 contains the supplementary crystallographic data for CuL4 and Cu2(L5)2,
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.
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A series of salen type Schiff base ligands and their mononuclear or dinuclear Cu(II) complexes were
synthesized and were tested as epoxidation catalysts. High catalytic activity and epoxide selectivity were
achieved.
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Highlights
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302
•
•
•
•
•
A dinuclear and four mononuclear salen type complexes were synthesized.
Crystal structures of two complexes were characterized.
The complexes were tested as catalyst for epoxidation of cyclooctene.
Solvent free epoxidation of cyclooctene was studied.
High catalytic activity and epoxide selectivity was achieved.
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21