Synthesis, characterization and catalytic properties of multinuclear copper(II) complexes

Article abstract of DOI:10.1016/j.poly.2014.04.028

Four tetranuclear [Cu₄(O)(Lⁿ)₂(CH ₃COO)₄] (1, 2, 4 and 5) and one pentanuclear [Cu ₅(OH)₂(L³)₂(CH₃COO) ₆] (3) with N₂O-dono

Full text of DOI:10.1016/j.poly.2014.04.028

Polyhedron 78 (2014) 85–93
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterization and catalytic properties of multinuclear
copper(II) complexes
Shibashis Halder a,1, Sudipto Dey a,1, Corrado Rizzoli b, Partha Roy a,
⇑
a Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700 032, India
b Universita’ degli Studi di Parma, Dipartimento di Chimica, Parco Area delle Scienze 17/A, I-43124 Parma, Italy
a r t i c l e i n f o
a b s t r a c t
Four tetranuclear [Cu4(O)(Ln)2(CH3COO)4] (1, 2, 4 and 5) and one pentanuclear [Cu5(OH)2(L3)2(CH3COO)6]
(3) with N2O-donor Schiff-base ligands have been synthesized, where HL1 = 4-methyl-2,6-bis(2-hydrox-
yethyliminomethyl)phenol for complex 1, HL2 = 4-methyl-2,6-bis(3-hydroxypropyliminomethyl)phenol
Article history:
Received 17 December 2013
Accepted 11 April 2014
Available online 20 April 2014
for complex 2, HL3 = 4-methyl-2,6-bis(4-hydroxybutyliminomethyl)phenol for complex 3, HL4
=
4-methyl-2,6-bis(5-hydroxypentyliminomethyl)phenol for complex
4
and HL5 = 4-methyl-2,6-bis
Keywords:
Copper
Multinuclear
Crystal structure
Homogeneous catalysis
Epoxidation
(6-hydroxyhexyliminomethyl)phenol for complex 5. These complexes have been characterized by
elemental analysis, FT-IR, UV–Vis spectroscopy. The structures of 1, 3, 4 and 5 have been determined
by single crystal X-ray diffraction studies. X-ray analysis reveals that complexes 1, 4 and 5 are
l
4-oxido-bridged tetrameric copper(II) complexes, where four copper atoms arrange themselves around
an oxidooxygen atom at the vertices of a distorted tetrahedron. The pentanuclear complex, 3, has been
found to have two 3-hydroxido bridging ligands each connecting three copper atoms. These complexes
l
have been employed as catalyst for the epoxidation of olefins in the presence of tert-butyl hydroperoxide
(TBHP) as the oxidant under mild conditions.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
connect two metal ions. These metal ions extend their coordination
with linkage to other ancillary ligands such as halides, carboxyl-
Multinuclear transition metal complexes of Schiff-base ligands
have been drawing attention of researchers over few decades
because of their diverse structures and their applications in various
fields e.g. magnetism, biology, and catalysis [1–7]. Schiff-base
ligands can induce substrate chirality, alter the metal center elec-
tronic factor or increase the solubility and stability of metal com-
plexes. In 1970, Robson reported for the first a dinucleating N,O
donor Schiff-base ligand involving 4-methyl-2,6-diformylphenol
as the aldehyde and 2-aminophenol as the amine [8,9]. After that,
a huge number of Schiff-base ligands, cyclic or acyclic, involving
this aldehyde have been reported [9–13]. Acyclic N,O donor ligands
of this aldehyde have been used to prepare oligo or polynuclear
metal complexes [12]. Tetranuclear complexes of such donor
systems can adopt different coordination geometries [14,15]. In
ates, or both to get extra stability. The magnetic properties of these
ligands with copper(II) metal ion have been studied extensively. In
general, it has been found that there are antiferromagnetic interac-
tions among copper atoms in their
[1,2,15].
l4-oxido bridge complexes
Multinuclear copper compounds have been used as catalysts in
different types of organic transformations, e.g. epoxidation, sulfox-
idation, olefin aziridination, atom transfer radical addition (ATRA)
[4,5,16–20] etc. in homogeneous as well as heterogeneous media.
Many mono- and multinuclear copper(II) complexes have been
employed as active catalysts for peroxidative oxidation of alkanes.
Recent reviews nicely describe the employment of transition metal
complexes as the catalyst for alkane oxidation [21,22]. Different
transition metal compounds have been used as catalysts for the
epoxidation of olefins in the past few decades [23]. Schiff-base
complexes of manganese(III), iron(III), cobalt(II), nickel(II), cop-
per(II) and zinc(II) have been described in the literature where they
were used as active catalysts for the oxidation of alkanes and
alkenes. A Ga(III) complex has been used recently as homogeneous
catalyst for the epoxidation of alkenes at mild temperatures and
under optimum conditions [24]. Apart from metal compounds,
gold nanoparticles supported on gold doped titania have been used
l4-oxido bridge complexes, it has been seen that four metal atoms
arrange themselves in the vertices of a tetrahedron around the
bridging oxygen atom at the center. Deprotonation of phenolic
oxygen atom of such Schiff-base ligand makes it suitable to
⇑
Corresponding author. Tel.: +91 3324572267; fax: +91 3324146414.
E-mail address: proy@chemistry.jdvu.ac.in (P. Roy).
Contributed equally.
1
http://dx.doi.org/10.1016/j.poly.2014.04.028
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

86
S. Halder et al. / Polyhedron 78 (2014) 85–93
as the active catalysts for epoxidation of stilbene [25]. Even though
copper(II) Schiff-base complexes have been well known for a long
time, they have barely been used as catalysts in olefin epoxidation
reactions in homogeneous medium. Recently many groups are
using copper(II) complexes for the epoxidation of various alkenes.
Koner et al. have synthesized copper(II) complexes with Schiff-
base ligands and employed them as catalyst in the epoxidation of
styrene [18], cyclooctene [19], etc. in homogeneous medium. Cop-
per(II) complexes with N,O donor ligands have been used for the
epoxidation of different alkenes in homogeneous as well as heter-
ogeneous media in the presence of different oxidant [17,26,27].
In this context, we report here the synthesis, characterization
and catalytic properties of multinuclear copper(II) complexes with
amine (0.6 mmol) (0.045 g of 3-amino-1-propanol for complex 2;
0.053 g of 4-amino1-butanol for complex 3; 0.062 g of 5-amino-
1-pentanol for 4; 0.070 g of 6-amino-1-hexanol for complex 5).
The mixture was stirred for 1 h and after that it was refluxed for
4 h. It was then cooled to room temperature. This ligand was used
to synthesize copper(II) complexes without any purification or
identification. Copper(II) acetate monohydrate (0.6 mmol,
0.120 g) was added and the solution stirred again for 45 min. The
mixture was then refluxed for about 1 h, after which it was cooled
to room temperature and filtered. The filtrate was kept at ambient
temperature. Single crystals of complexes 3, 4 and 5 suitable for
X-ray diffraction were grown from the filtrate upon slow evapora-
tion within few days.
N2O donor dinucleating Schiff-base ligands. Four
l
4-oxido bridged
Data for 2. (Yield = 0.45 g, 70%). Anal. Calc. for C38H54Cu4N4O15
C, 43.02; H, 5.13; N, 5.28. Found: C, 42.98; H, 5.10; N, 5.23%.
Data for 3. (Yield = 0.48 g, 60%). Anal. Calc. for C46H74Cu5N4O22
C, 40.84; H, 5.51; N, 4.14. Found: C, 40.78; H, 5.55; N, 4.17%.
Data for 4. (Yield = 0.47 g, 65%). Anal. Calc. for C48H73Cu4N5O15
C, 47.48; H, 6.06; N, 5.77. Found: C, 47.44; H, 5.99; N, 5.72%.
Data for 5. (Yield = 0.55 g, 75%). Anal. Calc. for C50H78Cu4N4O15
C, 48.85; H, 6.40; N, 4.56. Found: C, 48.81; H, 6.44; N, 4.57%.
:
:
:
:
tetranuclear and one 3-hydroxido bridged pentanuclear copper(II)
l
complexes have been synthesized and characterized by elemental
analysis, FTIR, UV–Vis spectroscopy and X-ray diffraction analysis.
These complexes have been used as the catalysts for epoxidation of
cyclohexene, styrene, a-methyl styrene and trans-stilbene in aceto-
nitrile in the presence of tert-butyl hydroperoxide as the oxidant.
2. Experimental
2.4. X-ray data collections and structure determinations
2.1. Materials and physical methods
Details of the data collection and refinement parameters for
complexes 1, 3, 4 and 5 are summarized in Table 1. The diffraction
experiments were carried out on a Bruker APEX-2 CCD diffractom-
Ethanolamine, 3-amino-1-propanol, 4-amino1-butanol, 5-
amino-1-pentanol, 6-amino-1-hexanol, copper(II) acetate mono-
hydrate, styrene,
a-methyl styrene, trans-stilbene and tert-butyl
eter using graphite monochromated Mo Ka radiation at 296 K for 1
hydroperoxide were purchased from Sigma Aldrich and used
without purification. Other reagents were purchased from
commercial source and used without further purification.
4-methyl-2,6-diformylphenol was synthesized following a pub-
lished procedure [28]. Solvents used for spectroscopic studies were
purified and dried by standard procedures before use [29]. Elemen-
tal analysis was carried out in a 2400 Series-II CHN analyzer, Perkin
Elmer, USA. FT-IR spectra were obtained on a Nicolet MAGNA-IR
750 spectrometer with samples prepared as KBr pellets. Absorp-
tion spectra were studied on a Shimadzu UV 2100 spectrophotom-
eter. Gas chromatography analysis was performed with an Agilent
Technologies 7890A network GC system equipped with a fused sil-
ica capillary column (30 m  0.32 mm) and a FID detector. All
experiments were carried out at room temperature in air unless
reported otherwise.
and 3 and 150 K for 4 and 5. Data were processed using the Bruker
SAINT package [31]. Absorption corrections based on multi scans
using the SADABS software [31] were applied to all intensity data.
The structures were solved by direct methods using SHELXS-97
[32] and refined with full-matrix least-squares on F2 on all unique
reflections using SHELXL-97 [32]. All the non-hydrogen atoms of the
complexes were refined anisotropically. In 1 the acetonitrile
solvent molecule was found to be disordered over two sets of ori-
entations with refined site occupancy ratio of 0.760(8):0.240(8). In
complex 4 an acetonitrile solvent molecule is disordered over two
orientations sharing the nitrogen atom close to an inversion center
with refined site occupancy ratio of 0.310(2):0.190(2). The disorder
of the solvent molecule compels the methyl group of an acetato
anion and the pentyl group of a pentanol side chain to be also
disordered over two sets of orientations with refined site
occupancy ratio of 0.619(5):0.381(5). During the refinement of
the disordered molecules and groups soft restraints on bond
lengths and angles to regularize their geometry were applied and
the anisotropic displacement parameters for paired components
of disorder were constrained to be equivalent. The water H atoms
in 3 were located in a difference Fourier map and refined as riding
on the oxygen atoms, with the OÀH bond lengths and HÁ Á ÁH sepa-
rations restrained to be 0.86(1) and 1.36(1) Å, respectively. All
other H atoms were placed geometrically and refined using a riding
model approximation, with Uiso(H) = 1.2 Ueq(C) or 1.5 Ueq(C, O) for
methyl and hydroxyl H atoms.
Ligand, HL1, was synthesized following a literature procedure
[30].
2.2. Synthesis of [Cu4(L1)2(O)(CH3COO)4]Á3H2OÁCH3CN (1)
To an acetonitrile solution (10 mL) of ligand, HL1, (0.3 mmol,
0.075 g) was added copper(II) acetate monohydrate (0.6 mmol,
0.120 g). The mixture was stirred for 45 min and refluxed for 1 h
on appearance of a green color. The mixture was finally cooled
and filtered to remove any undissolved or suspended materials.
The filtrate was kept at ambient temperature. Green single crystals
suitable for X-ray diffraction study were produced within few days.
Data for 1. (Yield = 0.43 g, 72%). Anal. Calc. for C34H46Cu4N4O15
C, 40.64; H, 4.61; N, 5.58. Found: C, 40.58; H, 6.57; N, 5.62%.
:
2.5. Epoxidation of olefins
Typically, 10.0 mmol of the substrate was taken in a magneti-
cally stirred two necked round-bottomed flask fitted with a
condenser in 5 ml acetonitrile, followed by the addition of
0.05 mmol of the complex. The mixture was heated to 50 °C. The
reaction was started with the addition of tert-butyl hydroperoxide
(20 mmol). Aliquots from reaction mixtures were collected at reg-
ular intervals. 10 ml of diethylether were added for extracting the
reactants and products. The substrate and product(s) from the
2.3. Syntheses of [Cu4(L2)2(O)(CH3COO)4] (2),
[Cu5(L3)2(OH)2(CH3COO)6]ÁH2O (3), [Cu4(L4)2(O)(CH3COO)4]ÁCH3CN
(4) and [Cu4(L5)2(O)(CH3COO)4] (5)
Complexes 2, 3, 4 and 5 were synthesized following a similar
procedure. Typically, to an acetonitrile solution (10 mL) of
4-methyl-2,6-diformyl (0.3 mmol, 0.049 g) was added respective

S. Halder et al. / Polyhedron 78 (2014) 85–93
87
Table 1
Crystal data for complexes 1, 3, 4 and 5.
Complex
1
3
4
5
Formula
Formula weight
T (K)
C
34H46Cu4N4O15Á3H2OÁC2H6N
C46H70Cu5N4O20Á2H2O
1352.79
296(2)
green
monoclinic
P21/c
11.4395(13)
14.4005(16)
18.053(2)
90.00
98.513(3)
90.00
C46H70Cu4N4O15Á1.5C2H3N
C50H78Cu4N4O15
1229.32
150
green
monoclinic
P21/c
14.8592(8)
24.1904(13)
16.7730(10)
90.00
107.252(2)
90.00
1100.01
296(2)
green
triclinic
P1
9.596(4)
12.147(5)
20.975(9)
103.830(12)
94.967(13)
103.193(13)
2285.3(16)
2
1234.80
150
Colour
green
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
ꢀ
ꢀ
P1
12.5810(9)
14.8446(11)
15.1852(11)
87.720(2)
85.554(2)
82.089(2)
2799.3(4)
2
a
(°)
b (°)
c
(°)
V (Å3)
2941.2(6)
2
5757.8(6)
4
Z
Crystal dimensions (mm)
Minimum and maximum transmission factors
F(000)
0.45 Â 0.25 Â 0.15
0.311–0.885
1132
1.598
0.71073
0.40 Â 0.25 Â 0.15
0.302–0.935
1398
1.528
0.71073
0.40 Â 0.23 Â 0.12
0.649–0.829
1286
1.465
0.71073
0.45 Â 0.25 Â 0.15
0.211–0.901
2568
1.522
0.71073
Dc (g cmÀ3
k (Mo K ) (Å)
)
a
h Range (°)
Reflection collected/unique/observed
Absorption correction
Rint
1.8–23.8
19255, 6961, 5067
multi-scan
0.0493
1.8–24.1
31177, 4609, 3628
multi-scan
0.0622
1.4–23.1
22633, 7818, 5896
multi-scan
0.0552
1.5–23.3
45827, 8303, 5340
multi-scan
0.0830
Final R1 index [I > 2
r(I)]
0.0455
0.0374
0.0413
0.0432
Final wR2 index (all reflections)
0.1309
0.0983
0.0998
0.1125
Goodness-of-fit
1.025
1.048
1.040
1.012
reaction mixture were analyzed by gas chromatography and iden-
tified by the comparison with known standards.
Magnetic properties of the complexes have been analyzed with
powdered samples at 298 K. Complexes 1, 2, 4 and 5 have been
expected to show a behavior similar to that of complexes with
comparable metal-donor connectivities. The reported complexes
with similar N,O-donor ligands exhibited strong antiferromagnetic
coupling among the four copper atoms. The calculated magnetic
moment of four isolated copper(II) ions is 4.9 BM, but the
measured magnetic moment has been found to be smaller than
the calculated value indicating antiferromagnetic coupling among
four copper atoms at 298 K. The pentanuclear complex 3 should
differ from the other complexes. The observed magnetic moment
of 3 at 298 K has been found to be 1.79 BM. This value is smaller
than that of 5.9 BM expected for five isolated Cu(II) ions (d9,
S = ½), indicating strong antiferromagnetic interactions amongst
copper atoms at 298 K, and is very close to that of 1.73 BM
expected for one paramagnetic copper(II) ion. The observed mag-
netic moments of complexes 1, 2 and 5 are found to be 2.16,
1.77 and 1.75 BM respectively at 298 K. These values are remark-
ably small as compared to the value of 4.9 BM expected for four
isolated Cu(II) ions indicating strong antiferromagnetic interac-
tions amongst copper atoms at 298 K.
A blank experiment for the epoxidation of cyclohexene, as the
representative case, was performed without addition of any cata-
lyst under the same experimental conditions. Another blank reac-
tion for the epoxidation of cyclohexene was carried out in the
presence of copper(II) chloride keeping all other parameters
unaltered.
3. Results and discussion
3.1. Synthesis and characterization
The general reaction route adopted for the synthesis of all com-
plexes is schematically shown in Scheme 1. Complex 1 was synthe-
sized by the reaction between the appropriate ligand and
copper(II) acetate in acetonitrile. For all the complexes other than
1, the ligand was prepared by simple Schiff-base condensation
between 4-methyl-2,6-diformyl phenol and respective amine in
1:2 M ratio in acetonitrile. These in situ Schiff-base ligands in ace-
tonitrile reacted with copper(II) acetate to produce the complexes.
The acetate ion from copper(II) acetate probably deprotonated the
phenolic proton of the ligand and attached to copper to give extra
stability to the complex.
3.2. Description of crystal structures of complexes
FTIR spectra of the complexes were recorded with samples pre-
pared as KBr pellets. All the complexes show strong mC–H bands at
2800–3000 cmÀ1 [11,33]. Complexes 1, 2, 3, 4 and 5 show IR bands
at 1627, 1625, 1633, 1638 and 1632 cmÀ1 respectively confirming
the presence of C@N bonds. All complexes except 3 show a sharp
band of medium intensity at 560–570 cmÀ1 indicating that the
ligand is coordinated to the metal atoms [34,35].
Selected bond lengths and bond angles are listed in Tables 2–4 for
1, 4 and 5, respectively, and in Table 6 for 3. Perspective views of the
complexes of with partial atom labeling are shown in Figs. 1–4,
respectively. Due to the similarity of their crystal structures, com-
plexes 1, 4 and 5 are described hereafterjointly. The asymmetric unit
of complexes 1, 4 and 5 consists of four crystallographically inde-
pendent Cu2+cations, two
l
2-bridging tridentate ligands LnÀ, one
UV–Vis spectra of all the complexes were recorded in acetoni-
trile at room temperature. All of them behave similarly in solution.
They exhibited peaks in the range 252–257 nm which can be
l
4-oxido anion and four 2-bridging acetato anions. In 1 and 4 water
l
and/or acetonitrile solvent molecules are also present in a complex/
water/acetonitrile molar ratio of 1:3:1 for 1 and in a complex/aceto-
nitrile molar ratio of 1:1.5 for 4. The main feature of these structures
is the presence of a Cu4O11N4 inner core, where each copper metal
displays a distorted square pyramidal coordination geometry, with
the basal planes provided by one nitrogen and one oxygen atom of
attributed to
p ?
p⁄ transitions within the ligands part. Their
peaks in the range of 382–386 may be assigned to charge transfer
transitions (LMCT). All of them show typical d-d transitions in
range of 570–650 nm.

88
S. Halder et al. / Polyhedron 78 (2014) 85–93
Scheme 1. Schematic presentation of synthesis of the complexes.
a ligand, the
l
4-oxido oxygen atom and one oxygen atom of an ace-
complexes 1, 4 and 5. The CuÀO and CuÀN bond distances are in
good agreement with those reported for related ( 4-oxo)-( 2-2,
6-bis(alkyl)-4-methylphenolato)-tetrakis( 2-acetato)-tetracopper
complexes [16,17,30,35,47,48]. The CuÁ Á ÁCu separations in complex
1 are remarkablydifferent, ranging from2.9732(15)to 3.3631(13) Å,
whereas in 4 and 5 these distances fall in a narrow range of values
(3.0107(7)–3.2145(7) Å and 3.0168(9)–3.2046(10) Å, respectively).
It may be noted that the shortest metalÁ Á Ámetal separation increases
with the increase in the number of carbon atom of the alkyl side
chain.
tato anion (r.m.s. deviations in the range 0.100–0.195, 0.137–0.377
and 0.057–0.112 for 1, 4 and 5, respectively; metal displacement
in the range 0.0453(8)–0.1378(8), 0.0282(5)–0.0915(5) and
0.1311(6)–0.2286(6) Å for 1, 4 and 5, respectively), and the apical
positions occupied by the oxygen atom of a different acetato anion.
The magnitude of the distortion from the ideal geometry may be
l
l
l
esteemed from the value of the trigonal index, s. This may be defined
as the difference between the two largest donor-metal-donor angles
divided by 60, which results in a value of 1 for the ideal trigonal
bipyramid coordination and 0 for the square pyramid coordination
[36]. Table 5 shows the trigonal index value of the copper atoms in
A perspective view with partial atom labeling of complex 3 is
shown in Fig. 4. The pentanuclear discrete complex molecule has

S. Halder et al. / Polyhedron 78 (2014) 85–93
89
Table 2
Table 4
Selected bond lengths (Å) and angles (°) of complex 1.
Selected bond lengths (Å) and angles (°) of complex 5.
Cu1ÀO1
1.929(4)
1.961(3)
1.945(4)
1.975(4)
2.283(4)
1.908(3)
1.975(4)
1.939(4)
Cu3ÀO1
1.933(3)
1.963(4)
1.944(4)
1.962(5)
2.464(4)
1.929(3)
1.992(4)
1.948(5)
Cu1ÀO1
1.919(3)
1.978(3)
1.947(3)
1.975(3)
2.269(4)
1.925(3)
1.967(3)
1.961(3)
Cu3ÀO1
1.901(3)
1.994(3)
1.949(3)
1.976(3)
2.282(4)
1.903(3)
1.988(3)
1.955(4)
Cu1ÀO2
Cu3ÀO3
Cu1ÀO2
Cu3ÀO2
Cu1ÀO8
Cu3ÀO14
Cu1ÀO8
Cu3ÀO13
Cu1ÀN1
Cu3ÀN3
Cu1ÀN1
Cu3ÀN2
Cu1ÀO10
Cu3ÀO9
Cu1ÀO10
Cu3ÀO15
Cu2ÀO1
Cu4ÀO1
Cu2ÀO1
Cu4ÀO1
Cu2ÀO2
Cu4ÀO3
Cu2ÀO3
Cu4ÀO3
Cu2ÀO12
Cu4ÀO11
Cu2ÀO14
Cu4ÀO11
Cu2ÀN2
1.984(4)
2.508(5)
Cu4ÀN4
2.003(4)
2.295(4)
Cu2ÀN3
1.976(4)
2.264(3)
Cu4ÀN4
1.961(4)
2.294(3)
Cu2ÀO15
Cu4ÀO13
Cu2ÀO12
Cu4ÀO9
O1ÀCu1ÀO2
O1ÀCu1ÀO8
O2ÀCu1ÀO8
O1ÀCu1ÀN1
O2ÀCu1ÀN1
O8ÀCu1ÀN1
O1ÀCu1ÀO10
O2ÀCu1ÀO10
O8ÀCu1ÀO10
N1ÀCu1ÀO10
O1ÀCu2ÀO2
O1ÀCu2ÀO12
O2ÀCu2ÀO12
O1ÀCu2ÀN2
O2ÀCu2ÀN2
O12ÀCu2ÀN2
O1ÀCu2ÀO15
O2ÀCu2ÀO15
O12ÀCu2ÀO15
N2ÀCu2ÀO15
O1ÀCu3ÀO3
O1ÀCu3ÀO14
O3ÀCu3ÀO14
79.31(15)
92.32(15)
170.84(16)
163.30(17)
91.80(17)
97.22(17)
95.54(15)
87.88(16)
89.23(16)
98.25(16)
79.47(15)
99.64(16)
165.59(16)
169.05(16)
90.69(18)
91.09(19)
90.05(14)
91.01(16)
103.48(16)
85.31(17)
79.73(14)
95.15(15)
169.46(17)
O1ÀCu3ÀN3
O3ÀCu3ÀN3
O14ÀCu3ÀN3
O1ÀCu3ÀO9
O3ÀCu3ÀO9
O14ÀCu3ÀO9
N3ÀCu3ÀO9
O1ÀCu4ÀO3
O1ÀCu4ÀO11
O3ÀCu4ÀO11
O1ÀCu4ÀN4
O3ÀCu4ÀN4
O1ÀCu4ÀN4
O1ÀCu4ÀO13
O3ÀCu4ÀO13
O11ÀCu4ÀO13
N4ÀCu4ÀO13
Cu3ÀO1ÀCu4
Cu3ÀO1ÀCu1
Cu4ÀO1ÀCu1
Cu3ÀO1ÀCu2
Cu4ÀO1ÀCu2
Cu1ÀO1ÀCu2
162.82(16)
91.13(16)
96.05(17)
96.32(14)
83.46(15)
88.00(17)
97.05(17)
79.11(15)
97.55(16)
176.20(17)
162.99(16)
90.48(16)
92.32(17)
94.56(15)
88.55(15)
93.56(18)
98.63(17)
102.01(15)
105.56(16)
108.55(16)
115.52(16)
122.50(17)
101.60(15)
O1ÀCu1ÀO2
O1ÀCu1ÀO8
O2ÀCu1ÀO8
O1ÀCu1ÀN1
O2ÀCu1ÀN1
O8ÀCu1ÀN1
O1ÀCu1ÀO10
O2ÀCu1ÀO10
O8ÀCu1ÀO10
N1ÀCu1ÀO10
O1ÀCu2ÀO3
O1ÀCu2ÀO14
O3ÀCu2ÀO14
O1ÀCu2ÀN3
O1ÀCu2ÀN3
O14ÀCu2ÀN3
O1ÀCu2ÀO12
O3ÀCu2ÀO12
O14ÀCu2ÀO12
N3ÀCu2ÀO12
O1ÀCu3ÀO2
O1ÀCu3ÀO13
O2ÀCu3ÀO13
78.39(13)
94.75(13)
170.57(13)
160.40(14)
91.17(15)
93.45(16)
95.65(13)
87.24(12)
99.98(13)
100.41(14)
78.52(13)
94.42(13)
170.77(13)
157.66(14)
91.13(15)
93.41(16)
96.31(13)
86.02(13)
100.79(13)
102.72(14)
78.43(13)
95.95(13)
173.12(13)
O1ÀCu3ÀN2
O2ÀCu3ÀN2
O13ÀCu3ÀN2
O1ÀCu3ÀO15
O2ÀCu3ÀO15
O13ÀCu3ÀO15
N2ÀCu3ÀO15
O1ÀCu4ÀO3
O1ÀCu4ÀO11
O3ÀCu4ÀO11
O1ÀCu4ÀN4
O3ÀCu4ÀN4
O11ÀCu4ÀN4
O1ÀCu4ÀO9
O3ÀCu4ÀO9
O11ÀCu4ÀO9
N4ÀCu4ÀO9
Cu1ÀO1ÀCu2
Cu1ÀO1ÀCu3
Cu1ÀO1ÀCu4
Cu2ÀO1ÀCu3
Cu2ÀO1ÀCu4
Cu3ÀO1ÀCu4
163.93(16)
89.78(14)
94.93(14)
99.39(13)
88.06(13)
96.84(13)
91.00(13)
78.53(13)
95.54(13)
172.97(13)
163.48(16)
90.44(14)
94.52(14)
98.69(13)
87.71(13)
96.98(13)
93.05(14)
110.94(16)
104.33(15)
111.81(15)
111.43(15)
103.74(15)
114.79(16)
Table 3
Selected bond lengths (Å) and angles (°) of complex 4.
Table 5
values for the coordination polyhedra about the copper atoms in complexes 1, 4 and
5.
s
Cu1ÀO1
1.921(3)
1.971(3)
1.958(3)
1.963(3)
2.383(3)
1.917(3)
1.975(3)
1.934(3)
Cu3ÀO1
1.914(3)
1.977(3)
1.933(3)
1.968(3)
2.332(3)
1.913(3)
1.981(3)
1.955(3)
Cu1ÀO2
Cu3ÀO3
Complex 1
Cu1
0.13
Cu2
0.06
Cu1ÀO10
Cu3ÀO14
Cu3
Cu1
Cu3
Cu1
Cu3
0.11
0.13
0.21
0.17
0.15
Cu4
Cu2
Cu4
Cu2
Cu4
0.22
0.16
0.18
0.22
0.16
Cu1ÀN1
Cu3ÀN4
Complex 4
Complex 5
Cu1ÀO8
Cu3ÀO11
Cu2ÀO1
Cu4ÀO1
Cu2ÀO3
Cu4ÀO2
Cu2ÀO9
Cu4ÀO13
Cu2ÀN3
1.955(3)
Cu4ÀN2
1.961(3)
2.293(3)
Cu2ÀO12
2.308(3)
Cu4ÀO15
O1ÀCu1ÀO2
O1ÀCu1ÀO10
O2ÀCu1ÀO10
O1ÀCu1ÀN1
O2ÀCu1ÀN1
O10ÀCu1ÀN1
O1ÀCu1ÀO8
O2ÀCu1ÀO8
O10ÀCu1ÀO8
N1ÀCu1ÀO8
O1ÀCu2ÀO3
O1ÀCu2ÀO9
O3ÀCu2ÀO9
O3ÀCu2ÀN3
O9ÀCu2ÀN3
O1ÀCu2ÀN3
O1ÀCu2ÀO12
O3ÀCu2ÀO12
O9ÀCu2ÀO12
N3ÀCu2ÀO12
O1ÀCu3ÀO3
O1ÀCu3ÀO14
O3ÀCu3ÀO14
78.27(11)
93.67(11)
169.68(11)
162.13(13)
91.45(13)
97.95(13)
94.58(10)
82.29(10)
92.05(11)
98.50(11)
78.51(11)
96.59(12)
170.74(11)
160.97(13)
90.67(12)
96.10(12)
94.90(11)
85.77(11)
86.84(11)
99.91(13)
78.52(11)
97.54(12)
171.25(12)
O1ÀCu3ÀO3
O3ÀCu3ÀO3
O14ÀCu3ÀO3
O1ÀCu3ÀO11
O3ÀCu3ÀO11
O14ÀCu3ÀO11
N4ÀCu3ÀO11
O1ÀCu4ÀO2
O1ÀCu4ÀO13
O2ÀCu4ÀO13
O1ÀCu4ÀN2
O2ÀCu4ÀN2
O13ÀCu4ÀN2
O1ÀCu4ÀO15
O2ÀCu4ÀO15
O13ÀCu4ÀO15
N2ÀCu4ÀO15
Cu1ÀO1ÀCu2
Cu1ÀO1ÀCu3
Cu1ÀO1ÀCu4
Cu2ÀO1ÀCu3
Cu2ÀO1ÀCu4
Cu3ÀO1ÀCu4
158.85(12)
89.86(15)
96.19(15)
94.01(10)
85.71(10)
86.79(11)
102.76(13)
78.21(11)
95.69(11)
173.16(12)
162.09(13)
90.70(13)
95–96(13)
96.42(11)
86.28(11)
91.39(11)
96.85(12)
109.28(14)
113.47(13)
103.87(12)
103.59(13)
114.11(14)
112.77(13)
of crystallization are also present in a complex/solvent molar ratio
of 1:2. Contrarily to what observed in complexes 1, 4 and 5, where
all copper atoms display a similar coordination geometry, the three
independent metals of the Cu5O12N4 inner core of 3 assume differ-
ent coordination modes. In fact, the central Cu2 atom, which lies on
a centre of symmetry, exhibits a distorted octahedral coordination
provided by the two l3-hydroxido anions and by the oxygen atoms
of four different acetato anions, whereas atom Cu3 displays a dis-
torted square pyramidal geometry, with the basal plane defined by
one nitrogen and one oxygen atom of the ligand, the l3-hydroxido-
oxygen atom and one oxygen atom of an acetato anion (r.m.s. devi-
ation 0.046; Cu3 is displaced by 0.0922(5) Å from the least-square
basal plane), and the apical position occupied by the oxygen atom
of a different acetato anion. The trigonal index
s is only 0.02, sug-
gesting an almost perfect square pyramidal geometry about Cu3,
but the coordination polyhedron is in fact rather distorted as indi-
cated by the dihedral angle of 13.71(8)° between the Cu3–O9i line
(i = Àx, 1 À y, 1 À z) and the normal to the mean basal plane. Coor-
dination about atom Cu1 should be best described as distorted
square planar (r.m.s. deviation 0.176, with the metal atom dis-
placed by 0.0505(5) Å from the least-square mean plane through
the donor atoms) provided by one nitrogen and one oxygen atom
crystallographically imposed inversion symmetry and consists of
five Cu2+ cations, two binucleating ligands 2,6-bis(4-hydroxybu-
tyl)iminomethylphenolate, L3À, two
l3-hydroxido and six acetato
of the ligand, the
l3-hydroxidooxygen atom and one oxygen atom
anions, two of which adopt a
l2-bridging mode. Water molecules
of an acetato anion. As observed for complexes 1, 4 and 5, the

90
S. Halder et al. / Polyhedron 78 (2014) 85–93
Table 6
Selected bond lengths (Å) and angles (°) of complex 3.
Cu1ÀN1
1.932(3)
1.934(3
Cu2ÀO9
Cu2ÀO9i
Cu3ÀO1
Cu3ÀO2
Cu3ÀN2
Cu3ÀO7
Cu3ÀO9
2.231(3)
2.231(3)
1.936(2)
1.975(2)
1.948(2)
1.926(3)
2.364(3)
Cu1ÀO1
Cu1ÀO2
1.975(2)
1.934(3)
1.976(2)
1.976(2)
2.227(3)
2.227(3)
77.16(10)
95.78(11)
166.47(12)
165.12(12)
92.24(12)
96.40(12)
180(-)
Cu1ÀO5
Cu2ÀO1
Cu2ÀO1i
Cu2ÀO8
Cu2ÀO8i
O1ÀCu1ÀO2
O1ÀCu1ÀO5
O1ÀCu1ÀN1
O2ÀCu1ÀO5
O2ÀCu1ÀN1
O5ÀCu1ÀN1
O1ÀCu2ÀO1i
O1ÀCu2ÀO9i
O1ÀCu2ÀO9
O1iÀCu2ÀO9i
O1ÀCu2ÀO9i
O9ÀCu2ÀO9
O1ÀCu2ÀO8i
O1ÀCu2ÀO8
O8ÀCu2ÀO9
O8iÀCu2ÀO9
O1iÀCu2ÀO8i
O1ÀCu2ÀO8i
O9ÀCu2ÀO8i
O9iÀCu2ÀO8i
O8ÀCu2ÀO8i
O1ÀCu3ÀO2
O1ÀCu3ÀO7
O2ÀCu3ÀO7
O1ÀCu3ÀN2
O2ÀCu3ÀN2
O7ÀCu3ÀN2
O1ÀCu3ÀO9
O2ÀCu3ÀO9
O7ÀCu3ÀO9
N2ÀCu3ÀO9
Cu1ÀO1ÀCu2
Cu1ÀO1ÀCu3
Cu2ÀO1ÀCu3
88.47(10)
91.48(10)
88.52(10)
180(-)
77.13(10)
94.29(10)
168.50(11)
169.54(12)
92.68(11)
95.55(12)
78.88(9)
86.73(10)
93.27(10)
93.27(10)
86.73(10)
180(À)
86.11(9)
88.47(10)
91.53(10)
88.52(10)
91.48(10)
91.53(10)
99.88(10)
102.57(11)
109.62(11)
104.32(11)
99.31(11)
Fig. 2. A perspective view of complex 4. Hydrogen atoms and solvent molecule
were omitted for clarity.
3.3. Catalysis
Complexes 1, 2, 3, 4 and 5 have been used as catalyst for the
epoxidation of cyclohexene, styrene,
a-methylstyrene and trans-
stilbene using tert-butyl hydroperoxide as the oxidant at 50 °C.
The results of the oxidation reactions are shown in Table 7. It can
be evidently seen from the table that all substrates are converted
in good yield with the formation of the corresponding epoxides
as the major product for the each reaction. Among all the sub-
strates, the conversion of cyclohexene has been found to be the
highest with 84% yield in the presence of complex 4 as the catalyst.
Selectivity of the corresponding epoxide in this case is 86%. How-
ever, conversions of cyclohexene by catalysts 1, 2, 3 and 5 are
80%, 83%, 80% and 82% respectively with formation of the corre-
sponding epoxide with high selectivity. 2-Cyclohexen-1-ol and
2-cyclohexen-1-one are detected as the minor products. Epoxida-
tion of styrene draws a special attention of researchers from the
view point of academics and industry. Complexes 1, 2, 3, 4 and 5
can act as catalyst to convert styrene with 76%, 74%, 72%, 73%
and 75% yields respectively into its products. Apart from the for-
mation of epoxide as the major product, a small amount of benzal-
dehyde has been detected as the by-product. a-Methyl styrene has
been transformed with 76%, 74%, 72%, 73% and 75% yields into its
products by 1, 2, 3, 4 and 5 respectively. It can be clearly noticed
from Table 7 that these complexes are better catalyst for the epox-
idation of cyclohexene and trans-stilbene in comparison to styrene
and its derivative. Trans-stilbene is converted in 82%, 78%, 75%, 82%
and 77% yields in the presence of 1, 2, 3, 4 and 5 respectively. It is
worth noting that for each reaction the corresponding epoxide has
been obtained as the major product.
Fig. 1. A perspective view of complex 1. Hydrogen atoms and solvent molecules
were omitted for clarity.
It has been observed that during the catalytic reaction, the yield
of the reaction increases with time and after a certain time the con-
version seems to reach its saturation. Samples were collected from
the reaction mixtures at regular time interval and analyzed by GC
to calculate the yield of reaction at different time intervals. A plot
CuÀO and CuÀN bond lengths are in good agreement with the lit-
erature values. The shortest and longest CuÁ Á ÁCu separations
within the Cu5O12N4 inner core are 2.982(À) and 6.3919(8 Å,
respectively.

S. Halder et al. / Polyhedron 78 (2014) 85–93
91
Fig. 3. A perspective view of complex 5. Hydrogen atoms were omitted for clarity.
Table 7
Epoxidationa of olefins by different catalysts.
Catalysts
1
Olefins
Conversionb
Selectivity
TOFc
cyclohexene
Styrene
80
76
71
82
83
74
70
78
80
72
70
75
84
73
76
82
82
75
69
77
86
82
78
80
88
80
79
78
84
75
76
83
85
82
78
88
85
78
78
84
66.7
63.3
59.2
68.3
69.2
61.7
58.3
65.0
66.7
60.0
58.3
62.5
70.0
60.8
63.3
68.3
68.3
62.5
57.5
64.2
a
-Methyl styrene
trans-stilbene
Cyclohexene
Styrene
2
3
4
5
a
-Methyl styrene
trans-stilbene
cyclohexene
Styrene
a
-Methyl styrene
trans-stilbene
cyclohexene
styrene
a
-methyl styrene
trans-stilbene
cyclohexene
styrene
a
-methyl styrene
trans-stilbene
a
Solvent: Acetonitrile; temperature: 50 °C; oxidant: tert-butyl hydroperoxide;
conversions were measured after 24 h of the reaction, average of two mea-
b
surements done under identical conditions);
c
TOF: turnover number = moles of substrate converted per mole of metal com-
plex per hour.
Fig. 4. A perspective view of complex 3. All hydrogen atoms except hydrogen atoms
of hydroxide ions and solvent molecule were omitted for clarity.
has been chosen since maximum conversion has been achieved
with it. The result for that reaction shows a significant decrease
in the conversion (6%) of the substrate as well as the epoxide selec-
tivity (25%) with no metal complex. Another blank reaction was
carried out in the presence of copper(II) chloride under the same
experimental conditions. The results show a slight improvement
of the conversion (12%) and epoxide selectivity (20%) in compari-
son to the conversion of cyclohexene with no metal salt, but a
much lower conversion yield in comparison to all the complexes
of conversion vs time for complex 2 is shown in Fig. 5. Similar plots
(Figs. s1–s4) for other copper(II) complexes are given in the Sup-
plementary information. It can be seen from the figure that the
conversion yield followed a sigmoid curve. A blank reaction was
carried out without any catalyst using cyclohexene as the model
substrate under the same experimental conditions. Cyclohexene

92
S. Halder et al. / Polyhedron 78 (2014) 85–93
olefins at different temperature, showing the best activity in the
temperature region near 50 °C.
One of the main drawbacks of homogeneous catalysis is the
recovery of the catalyst. In this regard, we have tried to recover cat-
alysts after completion of the catalytic reactions. After recovery we
have tried to confirm whether the recovered materials were the
same as they were before catalytic reaction by elemental analysis,
FT-IR and mass spectroscopy. The data of analysis of the recovered
materials were not similar to that of materials before catalysis. In
conclusion, the complexes acted as catalyst for epoxidation but
they were destroyed after the reaction.
3.4. Mechanism
Copper(II) complexes have been found to be active catalysts for
epoxidation of olefins while copper-hydroperoxo species has been
detected in the reaction mixture [26]. Metal complexes formed
metal-hydroperoxo species in the presence of TBHP as evidenced
from the UV–Vis spectra. The UV–Vis spectrum of complex 2 in
the presence of TBHP was recorded as a representative case. It
showed an intense peak at around 400 nm, with a shoulder in
the range of 415–440 nm. This fact could be attributed to the for-
mation of Cu-peroxo or Cu-hydroperoxo species [26,46]. This metal
peroxo intermediate may be the active species in the conversion of
alkenes. It is difficult to identify the exact active species which is
responsible for the catalytic transformations. In order to detect
the active species or number of copper atom(s) of the complexes
in solution we have recorded ESI mass spectra of the complexes
in acetonitrile. The spectrum of complex 4 showed peaks at m/z
395.15 and 457.08 along with a peak at 363.27 corresponding to
the existence of one and two copper atoms coordinated to the
ligand along with the presence of only ligand respectively. Because
of fragmentation of the complex, different species were detected
during recording of mass spectrum of 4. Fragmentation of other
complexes during mass spectra analysis has also been noticed.
Thus, it became difficult to detect the actual active center by mass
spectroscopy. As copper-peroxo or copper-hydroperoxo has been
detected in this study like previously reported research, so a simi-
lar pathway of epoxide formation is expected here and accordingly
a probable mechanism has been proposed (Scheme 2). This reac-
tion may proceed with the formation of copper-peroxo species
and tert-butyl alcohol since we can detect tert-butyl alcohol during
the course of reaction for all the catalysts.
Fig. 5. A plot of conversion vs time for complex 2.
leading to the relevance of the presence of N,O donor ligands with
the metal centers.
The catalytic activities of the complexes in homogeneous med-
ium are lower or comparable in conversion of the substrate and
epoxide selectivity in comparison with W(VI) complexes or Mo
catalysts [37,38], Mn catalysts [39], and Fe–porphyrin complexes
[40], but these are more active catalysts in comparison to previ-
ously reported Cu complexes [17–20,26,27,41].
Epoxidation reactions are carried out using different oxidants
like hydrogen peroxide [38], tert-butyl hydroperoxide [42], molec-
ular oxygen [43], sodium hypochlorite [44], iodosylbenzene [45]
etc. The ability to oxidize cyclohexene in the presence of different
oxidants such as hydrogen peroxide, TBHP and sodium hypochlo-
rite has been examined in the presence of 1 as the catalyst under
identical reaction conditions. The results show 63%, 80% and 48%
conversion yields of cyclohexene using hydrogen peroxide, TBHP
and sodium hypochlorite as the oxidants respectively. The yield
of the reactions has been analyzed after 24 h. It is clear from the
results that TBHP serves as the best oxidant among all the oxidants
whereas other oxidants show distinctly low activity. Although
hydrogen peroxide would have been the most desired oxidant
from an environmental view point, TBHP has been selected as
the oxidant due to its better activity.
Acetonitrile has been used as the solvent since all the com-
plexes are soluble in acetonitrile. Apart from this, it is difficult to
oxidize acetonitrile in the present reaction conditions. In all the
reactions, all substrates were converted into their products in good
yield.
The catalytic reaction was also performed at different tempera-
tures to study the effect on the catalytic efficacy of the complexes.
All the complexes behave similarly for their ability to oxidize
4. Conclusions
Five multinuclear copper(II) complexes with N,O-donor
Schiff-base ligands have been synthesized and characterized using
various techniques. These complexes have been used as catalysts
for the epoxidation of olefins using tert-butyl hydroperoxide as
Scheme 2. Probable mechanism for the epoxidation of cyclohexene (representative case).

S. Halder et al. / Polyhedron 78 (2014) 85–93
93
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