One-dimensional chain copper(II) complex: Synthesis, X-ray crystal structure and catalytic activity in the epoxidation of styrene

Article abstract of DOI:10.1016/j.poly.2011.12.039

Two new copper(II) complexes, [Cu(HL¹)(NO₃)] (1) (H₂L¹ = 1-(N-ortho-hydroxyacetophenimine)-ethane-2-ol) and {[Mg(H₂O)₆][Cu(pydca)₂]·2H ₂O}_n (2) (H₂pydca = 2,5-pyridine dicarboxylic acid) have been synthesized and characterized. X-ray crystal structure analysis reveals that the geometry of complex 1 is square planar, where copper(II) centers are linked through a weak coordination by the nitrate O atoms occupying two axial positions, resulting a helical chain structure. Complex 1 crystallizes in the chiral space group P2₁. There is no chiral center in the Schiff-base ligand of 1, its helical structure develops due to the formation of a NO₃^- bridged chain through an asymmetric arrangement. Complex 2 is a 1D chain in which the copper(II) ions are connected by bridging carboxylato ligands. The [Mg(H₂O)₆]²⁺ moiety occupies the inter-chain space. Epoxidation reactions of styrene and substituted styrenes are homogeneously catalyzed by complexes 1 and 2 with H ₂O₂ as the oxidant.

Full text of DOI:10.1016/j.poly.2011.12.039

Polyhedron 35 (2012) 55–61
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
One-dimensional chain copper(II) complex: Synthesis, X-ray crystal structure
and catalytic activity in the epoxidation of styrene
⇑
Debraj Saha, Tanmoy Maity, Tirthankar Dey, Subratanath Koner
Department of Chemistry, Jadavpur University, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
1
1
Article history:
Two new copper(II) complexes, [Cu(HL )(NO )] (1) (H L = 1-(N-ortho-hydroxyacetophenimine)-ethane-
3
2
Received 1 November 2011
Accepted 27 December 2011
Available online 17 January 2012
2
-ol) and {[Mg(H
2
O)
6
][Cu(pydca)
2
]ꢀ2H
2
O}
n
(2) (H
2
pydca = 2,5-pyridine dicarboxylic acid) have been syn-
thesized and characterized. X-ray crystal structure analysis reveals that the geometry of complex 1 is
square planar, where copper(II) centers are linked through a weak coordination by the nitrate O atoms
occupying two axial positions, resulting a helical chain structure. Complex 1 crystallizes in the chiral
Keywords:
Copper(II)
Helical chain compound
Schiff-base complex
Hydrogen peroxide
Epoxidation
space group P2
due to the formation of a NO
1
. There is no chiral center in the Schiff-base ligand of 1, its helical structure develops
ꢁ
3
bridged chain through an asymmetric arrangement. Complex 2 is a 1D
2+
chain in which the copper(II) ions are connected by bridging carboxylato ligands. The [Mg(H
ety occupies the inter-chain space. Epoxidation reactions of styrene and substituted styrenes are homo-
geneously catalyzed by complexes 1 and 2 with H as the oxidant.
Ó 2012 Elsevier Ltd. All rights reserved.
2
O)
6
]
moi-
2 2
O
Styrene
1
. Introduction
it the most interesting oxidant after molecular oxygen and stimu-
lating its use in liquid-phase oxidations, especially for fine-chemi-
cals production [3].
Epoxidation is one of the most important C@C bond functional-
ization methods, and the epoxide group is an active intermediate
for laboratory syntheses as well as in chemical industries [1–4].
Epoxides can be converted into alcohols, polyethers and aldehydes,
which show widespread applications in chemical and pharmaceu-
tical industries [5]. Olefin epoxidation catalyzed by transition
metal complexes has been a subject of great interest in the past
few decades [6]. Immobilization of transition metal Schiff-base
complexes onto zeolitic, polymeric or clay matrices has been
adopted to prepare heterogeneous catalysts. Copper(II) Schiff-base
complexes, upon immobilization into microporous or mesoporous
aluminosilicates, have been found to be capable of catalyzing olefin
epoxidations [7–9]. However, attempts of using Schiff-base cop-
per(II) complexes in homogeneous catalytic oxidation are rare in
the literature [10–12]. Several oxidants, 30% hydrogen peroxide,
tert-butyl-hydroperoxide (tert-BuOOH), sodium hypochlorite and
even molecular oxygen are used for the production of epoxides
from alkenes. Amongst them, 30% hydrogen peroxide attracts
increased attention in industrial use because it is environmental
Styrene oxide is an important organic intermediate for the
production of a number of fine/specialty chemicals and pharma-
ceuticals. The epoxidation of styrene, being a terminal olefin, to
styrene oxide is quite difficult and in many cases benzaldehyde
or phenylacetaldehyde are produced to a large excess. In the
course of our continuing investigation on epoxidation of olefins
using heterogeneous and homogeneous catalytic processes, we
found transition metal based catalysts demonstrate desirable cat-
alytic efficacy [8,9,11,12]. We report here the synthesis and crys-
tal structure of a 1D helical copper(II) schiff-base complex,
1
1
namely [Cu(HL )(NO
mine)-ethane-2-ol) and a 1D copper(II) carboxylato complex,
namely {[Mg(H O) ][Cu(pydca) O} (2) (H pydca = 2,5-pyri-
]ꢀ2H
dine dicarboxylic acid), along with their catalytic activity towards
3 2
)] (1) (H L = 1-(N-ortho-hydroxyacetopheni-
2
6
2
2
n
2
the epoxidation of styrene using H
homogeneous conditions.
2 2
O as the oxidant under
friendly. In fact, as recently outlined by Beller [13], H
2
O
2
is charac-
2. Experimental
terized by unique features and advantages [14], such as high
atom-efficiency [15], moderate cost, safe handling and storage,
and production of water as the only by-product [16,17], making
2.1. Materials
Ethanolamine, o-hydroxyacetophenone and 2,5-pyridine dicar-
boxylic acid were purchased from Sigma–Aldrich and were used
as received. Copper nitrate, magnesium nitrate and solvents used
were purchased from Merck (India).
⇑
Corresponding author. Tel.: +91 33 2457 2778; fax: +91 33 2414 6414.
E-mail address: snkoner@chemistry.jdvu.ac.in (S. Koner).
0
277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.12.039

5
6
D. Saha et al. / Polyhedron 35 (2012) 55–61
1
2
.2. Synthesis of the Schiff base (H
2
L )
Table 1
Crystal data and structure refinement parameters for complexes 1 and 2.
This Schiff-base was prepared as reported previously, with some
modifications [18]. In this process a 10 mL methanolic solution of
-ethanolamine (0.061 g, 1 mmol) and o-hydroxyacetophenone
0.136 g, 1 mmol) were mixed together in a flat-bottomed flask.
The resulting solution was then refluxed for 30 min, and then cooled
to room temperature. Methanol was then separated almost com-
pletely from the mixture by using a rotary evaporator. On storing
in a refrigerator overnight, a crystalline yellow solid product was
Complex
1
2
Formula
Formula weight
Crystal system
C
10
H
N
12 2
O
5
Cu
C
14 2 16
H22CuMgN O
2
(
303.77
monoclinic
P2
562.20
triclinic
P1ꢀ
7.2742(3)
7.5120(4)
10.7141(5)
Space group
1
0
8
.3180(10)
.0526(10)
a ( ÅA )
0
8
b ( AÅ )
0
9.6984(2)
90.00
114.0590(10)
90.00
c ( AÅ )
obtained and characterized. Anal. Calc. for C10
.31; N, 7.83. Found: C, 67.1; H, 7.1, N, 7.7%. IR (KBr disk, cm ):
(azomethine) 1610.
H13NO
2
: C, 66.98; H,
a
(°)
b (°)
(°0)
80.059(2)
ꢁ1
7
m
74.5000(10)
69.4630(10)
526.29(4)
c
3
593.179(16)
V ( AÅ )
1
Z
D
2
1
2
3 n
.3. Synthesis of the complex [Cu(HL )(NO )] (1)
calc (g/cm3)
1.701
1.856
1.774
1.155
Absorption coefficient
ꢁ
1
A
10 mL methanolic solution of Cu(NO
mmol) was added dropwise to a 10 mL methanolic solution of
L (0.179 g, 1 mmol). The solution was then stirred vigorously
for only 5 min at room temperature. The resulting deep green mix-
ture was then filtered. The filtrate thus obtained was kept undis-
turbed at room temperature. On slow evaporation of the filtrate
deep bluish green block crystals appeared after 2 days. The crystals
were collected by filtration. The compound is hygroscopic so it was
3
)
2
ꢀ3H
2
O
(0.241 g,
(mm )
1
H
F(000)
Intervals of reflection
indices
310
289
1
ꢁ10 ꢂ h ꢃ 10,
ꢁ9 ꢂ h ꢃ 9,
ꢁ9 ꢂ k ꢃ 9,
ꢁ13 ꢂ l ꢃ 13
6979
2124
2330
2
ꢁ10 ꢂ k ꢃ 9,
ꢁ
12 ꢂ l ꢃ 12
Measured reflections
Reflections with [I > 2
Independent reflections
Final R indices [I > 2
9601
2485
2591
r
(I)]
r
(I)]
R
1
= 0.0395,
wR = 0.1146
= 0.0408,
R
1
= 0.0287,
wR = 0.0786
1
R = 0.0319,
2
2
stored in CaCl
2 12 2 5
desiccator. Anal. Calc. for C10H N O Cu: C, 39.50;
R indices (all data)
R
1
H, 3.98; N, 9.21. Found: C, 39.7; H, 3.6; N, 9.3%.
wR2 = 0.1157
0.0210
0.273
wR2 = 0.0805
0.0218
0.367
R
D
D
int
ꢁ
3
q
q
max (e Å
)
2.4. Synthesis of the complex {[Mg(H
2 6
O) ][Cu(pydca)
2
2 n
]ꢀ2H O} (2)
min (e Åꢁ3)
ꢁ0.256
ꢁ0.294
Goodness-of-fit (GOF) on
0.988
1.082
Complex 2 was prepared though a hydrothermal route. It was
2
F
grown as a green block crystal (yield 55%) in a 20 mL Teflon-lined
Parr acid digestion bomb at 170 °C for 5 days, followed by slow
cooling at the rate of 10 °C/h to room temperature. For digestion,
final difference Fourier maps there were no remarkable peaks,
except for the ghost peaks surrounding the metal centers. A
summary of crystal data and relevant refinement parameters for
compound 1 and 2 are given in Table 1.
Cu(NO
pydca), LiOH and Milli-Q water were added in the molar ratio
of 2:1:2:1:500. The crystals thus formed were collected and dried
in air. Anal. Calc. for C14 22CuMgN 16: C, 29.88; H, 3.94; N, 4.98.
Found: C, 30.1; H, 3.7; N, 4.8%.
3
)
2
ꢀ6H
2
O, Mg(NO
3
)
2 2
ꢀ6H O, 2,5-pyridine dicarboxylic acid
(H
2
H
2
O
2.7. Catalytic reactions
2.5. Physical measurements
The catalytic reactions were carried out in a glass batch reactor
according to the following procedure. Substrate (10 mmol), solvent
5 mL) and catalysts (2 mg) were first mixed. The mixture was then
Elemental analysis was performed using
a Perkin-Elmer
(
2
4
40C elemental analyzer. Fourier transformed IR spectra (4000–
ꢁ1
equilibrated to the desired temperature in an oil bath. After addi-
tion of hydrogen peroxide (14.4 mmol, 1.5 equiv.), the reaction
mixture was stirred continuously. The reactions were performed
in the open air. The products of the epoxidation reactions were col-
lected at different time intervals and were identified and quanti-
fied by gas chromatography.
00 cm ) of KBr pellets were recorded at 300 K on a Perkin-Elmer
RX1 FT-IR spectrometer. TG-DT (Thermo-Gravimetric and Differen-
tial Thermal) analysis was made using a Perkin-Elmer (SINGAPORE)
Pyris Diamond TG/DTA unit. The heating rate was programmed at
ꢁ
1
5
1
°C min with a protecting stream of N
2
flowing at a rate of
ꢁ
1
50 mL min . The products of the catalytic reactions were identi-
fied and quantified by a Varian CP-3800 Gas Chromatograph using
a CP-Sil 8 CB capillary column.
3. Result and discussion
2.6. X-ray crystallography
3.1. IR spectroscopic study
ꢁ
X-ray diffraction data for 1 and 2 were collected on a Bruker
For compound 1 a strong peak appears at 1605 cm 1 (see Sup-
plementary material, Fig. S1) which is attributed to the vibrational
band for the azomethine (>C@CN–) group. Strong peaks appear in
SMART APEX CCD X-ray diffractometer using graphite-monochro-
mated Mo K radiation (k = 0.71073 Å). Determination of inte-
a
ꢁ1
grated intensities and cell refinement were performed with the
SAINT [19] software package using a narrow-frame integration algo-
rithm. An empirical absorption correction (SADABS) [20] was applied.
The structures were solved by direct methods and refined using the
the range 1300–1200 cm
(–NO ) group. A broad medium intensity band appears in the range
3500–3100 cm , ascribed to the
methyl group. The peaks appearing around 2920 cm
to the vibration of methylene (–CH –) groups. The band at
528 cm is ascribed to the Cu–O vibration of compound 1.
due to the presence of the nitrate
3
ꢁ1
m
(OH) vibration of the hydroxy-
ꢁ
1
are due
2
full-matrix least-squares technique against F with anisotropic dis-
2
ꢁ
1
placement parameters for non-hydrogen atoms with the programs
SHELXS97 and SHELXL97 [21]. Hydrogen atoms were placed at calcu-
lated positions using suitable riding models with isotropic
displacement parameters derived from their carrier atoms. In the
ꢁ
1
Compound 2 shows a broad band at 3472 cm (see Supple-
mentary material, Fig. S1), which is attributed to (OH) of water
(CH) vibrations
m
m
ꢁ1
molecules. The peaks near 3068 cm are due to

D. Saha et al. / Polyhedron 35 (2012) 55–61
57
ꢁ
1
of the pyridine ring. The sharp absorption band at 1626 cm is due
square plane (CuONOO) of the complex (Fig. 1). The four atoms
of the square-plane are essentially coplanar, with a slight tetrahe-
dral distortion. The two weakly coordinating oxygen atoms, O3 and
O3a (a = ꢁ1 + x, y, z) from nitrate groups occupy the two axial posi-
tions, above and below the CuONOO plane with longer bond dis-
tances of 2.587(4) and 2.577(4) Å, respectively (Table 2, and Figs.
1 and 2). Such longer bond distances are consistent with weak
coordination of the nitrate groups, reported previously [24]. Two
weakly coordinated axial oxygen atoms, O3 and O3a (a = ꢁ1 + x,
y, z) from nitrate groups form helical chains parallel to the crystal-
lographic b-axis (Fig. 2b). The bond distances and bond angles are
comparable to other similar types of copper(II) complexes [25,26].
The copper(II) centers of complex 1 are linked through weak coor-
dination by the nitrate O atoms resulting in a helical chain struc-
ture, Fig. 2. The crystal packing of complex 1 is further stabilized
by intramolecular O2–H6ꢀ ꢀ ꢀO1 hydrogen bonding, with an Oꢀ ꢀ ꢀO
distance of 2.585(5) Å, between the alcoholic oxygen atoms (O2)
and hydroxy oxygen atoms (O1) (Fig. 2).
to a combination of
ligand. Several strong
the region 1607–1556 cm
indicating the participation of the carboxylic group of H
m
(C@C) and
m
(C@CN) vibrational bands of the
ꢁ
ꢁ
m
as(COO ) and
m
s
(COO ) bands appear in
ꢁ1
ꢁ1
and at 1392 and 1358 cm
for 2
2
pydc in
coordination with the Cu(II) ion through deprotonation. The band
ꢁ
1
ꢁ
at 1607 cm is assigned to the
m
as(COO ) vibration and the sharp
ꢁ1
ꢁ
bands at 1392 and 1358 cm are ascribed to the
m
s
(COO ) vibra-
tions. The difference between the asymmetric and symmetric car-
m (COO )] is often used for
s
the correlation of the infrared spectra with the structures of metal
ꢁ
ꢁ
boxylate stretching [
D
=
m
as(COO ) ꢁ
ꢁ1
carboxylates [22]. These values of
D
are approximately 228 cm
ꢁ
1
ꢁ1
for monodentate, 164 cm for ionic and 42 cm for bidentate
ꢁ
1
carboxylate groups [23]. The value of
D
for 2 is 249 cm (1607–
ꢁ1
1
1
358 cm
287, 1268, 1043 and 768 cm
) showing monodentate coordination. IR bands at
(C–O) and d(O–C–O)
vibrations of the pydca ligand. The band at 513 cm may be
ꢁ
1
are due to
m
2ꢁ
ꢁ1
ꢁ1
ascribed to Cu–O vibrations and the band at 419 cm is attributed
to Cu–N vibrations of compound 2. The coordination mode of the
1ꢁ
2ꢁ
ligands HL and pydca in complexes 1 and 2, respectively, are
3
.3. Crystal structure of {[Mg(H
2
O)
6
][Cu(pydca)
2 2 n
]ꢀ2H O} (2)
shown in Scheme 1.
X-ray single-crystal analysis reveals that complex 2 has an infi-
nite 1D anionic metal-dicarboxylate ribbon-like chain extending
1
3
.2. Crystal structure of [Cu(HL )(NO
3
)]
n
(1)
parallel to the crystallographic a-axis. The 1D coordination
Compound 1 crystallizes in the chiral space group P2
1
and in-
volves the four-coordinated CuNO chromophore with a square
3
Table 2
0
planar geometry. An ORTEP diagram of 1 with the atom numbering
is given in Fig. 1. One nitrogen atom and two oxygen atoms of the
Schiff-base and one oxygen atom of the nitrate group form the
Selected bond lengths ( ÅA ) and angles (°) for complexes 1 and 2.
1
2
Cu(1)–O(1)
Cu(1)–N(1)
Cu(1)–O(2)
Cu(1)–O(4)
Cu(1)–O(3)
1.890(3)
1.932(3)
1.965(4)
1.990(3)
2.587(4)
2.577(4)
Cu1–O1
Cu1–N1
1.9468(14)
1.9757(16)
2.6484(15)
1.9468(14)
1.9757(16)
2.6484(15)
2.0605(16)
2.0605(16)
2.042(2)
c
Cu1– O3
d
Cu1– O1
d
Cu1– N1
a
e
Cu(1)– O(3)
Cu1– O3
Mg1–O4
Mg1– O4
f
Mg1–O5
f
Mg1– O5
2.042(2)
Mg1–O6
Mg1– O6
2.0543(18)
2.0543(18)
f
a
d
O(1)–Cu(1)– O(3)
83.49(13)
92.05(15)
85.02(19)
95.0(2)
O1–Cu1– O1
O1–Cu1–N1
180.00
a
O(2)–Cu(1)– O(3)
83.24(6)
96.76(6)
94.54(5)
85.47(5)
96.76(6)
83.24(6)
85.47(5)
94.54(5)
180.00
90.33(5)
89.67(5)
89.67(5)
90.33(5)
180.00
a
d
O(4)–Cu(1)– O(3)
O1–Cu1– N1
a
c
N(1)–Cu(1)– O(3)
O1–Cu1– O3
Scheme 1. Binding modes of the ligands.
e
O(1)–Cu(1)–O(3)
O(2)–Cu(1)–O(3)
O(4)–Cu(1)–O(3)
N(1)–Cu(1)–O(3)
93.53(14)
91.03(14)
54.98(18)
125.04(19)
139.91(12)
93.20(16)
85.39(17)
90.52(16)
90.87(16)
175.20(15)
176.26(18)
115.78(17)
109.0(3)
106.6(3)
127.8(3)
108.9(3)
123.2(3)
111.1(3)
79.3(3)
O1–Cu1– O3
d
O1–Cu1–N1
d
d
d
c
d
O1–Cu1– N1
c
O1–Cu1– O3
a
e
O(3)–Cu(1)– O(3)
O1–Cu1– O3
e
O(1)–Cu(1)–N(1)
N(1)–Cu(1)–O(2)
O(1)–Cu(1)–O(4)
O(2)–Cu(1)–O(4)
O(1)–Cu(1)–O(2)
N(1)–Cu(1)–O(4)
O3–Cu1– O3
O3–Cu1–N1
C9
c
c
d
O3–Cu1– N1
C10
C8
e
e
O3–Cu1–N1
d
N1
O3–Cu1– N1
d
N1–Cu1– N1
b
f
Cu(1)–O(3)– Cu(1)
O4–Mg1– O4
180.00
C7
b
N(2)–O(3)– Cu(1)
O4–Mg1–O5
92.52(7)
87.48(7)
89.38(7)
90.62(7)
87.48(7)
92.52(7)
90.62(7)
89.38(7)
180.00
90.21(9)
89.79(9)
89.79(9)
90.21(9)
180.00
O2
e
N(2)–O(4)–Cu(1)
C(7)–N(1)–Cu(1)
C(8)–N(1)–Cu(1)
C(1)–O(1)–Cu(1)
C(10)–O(2)–Cu(1)
N(2)–O(3)–Cu(1)
O4–Mg1– O5
C6
O4–Mg1–O6
f
O4–Mg1– O6
Cu1
f
C5
O4–Mg1–O5
O4–Mg1– O5
O4–Mg1–O6
f
f
f
f
C1
f
O4
N2
O4–Mg1– O6
O1
f
O5–Mg1– O5
O5–Mg1–O6
O3
C4
f
O5–Mg1– O6
f
C2
O5–Mg1–O6
f
f
O5–Mg1– O6
f
C3
O5
O6–Mg1– O6
Symmetry codes: (a) 1 ꢁ x, ꢁ1/2 + y, 2 ꢁ z; (b) 1 ꢁ x, 1/2 + y, 2 ꢁ z; (c) ꢁ1 + x, y, z; (d)
ꢁx, 2 ꢁ y, 1 ꢁ z; (e) 1 ꢁ x, 2 ꢁ y, 1 ꢁ z; (f) ꢁx, 1 ꢁ y, ꢁz.
Fig. 1. ORTEP diagram of 1 with 50% ellipsoid probability.

5
8
D. Saha et al. / Polyhedron 35 (2012) 55–61
Fig. 2. (a) 1D chain of compound 1 with intramolecular H-bonding (yellow dotted lines for secondary interaction and green dotted lines for H-bonding). (b) Helical
arrangement of the bridging oxygen atoms of the nitrate around the copper centers in compound 1 (color online).
O1s
Cu1
O1
C2
O6
N1
O5
C8
O7
Mg1
C5
C6
C1
O2
O4
C4
O3
C3
Fig. 3. ORTEP diagram of 2 with 50% ellipsoid probability.
2 6
observed in other [Mg(H O)
]2+ containing compounds [28–30].
polymer contains copper(II) ions connected by pydca dianions,
2ꢁ
2+
forming {[Cu(pydca)
2
]
}
n
chains. An ORTEP diagram of 2 with
The [Mg(H
{[Cu(pydca)
through H-bonds mediated by [Mg(H
2
O)
6
]
}
ions occupy the space between two adjacent
chains, which are connected to each other
2
ꢁ
the atom numbering is given in Fig. 3. The coordination geometry
of copper(II) in 2 is distorted octahedral, which is satisfied by four
carboxylato oxygen atoms from four pydca dianions and two
pyridine nitrogen atoms of two pydca ligands, Fig. 4. The pyridine
nitrogen atom and one of the carboxylato oxygen atoms of the
pydca ion ligand form a five-membered ring with the metal. Two
nitrogen atoms (N1 and N1d; d = ꢁx, 2 ꢁ y, 1 ꢁ z) and two oxygen
atoms (O1 and O1d) from two chelating pydca dianions form the
basal plane of the octahedron, while the axial positions are occu-
pied by two oxygen atoms (O3c and O3e; c = ꢁ1 + x, y, z;
e = 1 ꢁ x, 2 ꢁ y, 1 ꢁ z) from two different pydca dianions. The bond
distances and angles of 2 are collated in Table 2. While the equato-
rial Cu–O and Cu–N bond distances in compound 2 are comparable,
the axial Cu–O bond distances are significantly longer, which is
commensurate with the Jahn–Teller sensitive copper(II) ion [27].
The Mg(II) ions are coordinated to six water molecules to form a
regular octahedron and afford the cationic part of compound 2
2
]
n
2
+
O)
2 6
]
ions forming 2D
layers, Fig. 2 (see also Supplementary Material, Fig. S2 and Table
S1). Additional reinforcement within the sheet is achieved by other
intermolecular O–Hꢀ ꢀ ꢀO hydrogen bonds due to the presence of
crystalline water molecules in compound 2 (see Supplementary
Material, Fig. S2 and Table S1).
3.4. Thermal analysis of compounds 1 and 2
Thermogravimetric analysis of compound 1 shows a sharp mass
loss at around 215 °C (see Supplementary Material, Fig. S3) indicat-
ing the decomposition of the complex molecule. As the compound
is hygroscopic in nature, we observed weight loss from the starting
point of heating. The corresponding DTA curve of compound 1
shows an exothermic peak centered at 215 °C. Thermogravimetric
analysis of compound 2 confirms that the compound is thermally
stable up to ꢄ70 °C. The TGA curve (see Supplementary Material,
Fig. S4) indicates that the mass loss of ꢄ22.4% in the temperature
range 70–145 °C corresponds to the loss of two crystalline water
molecules and six coordinated water molecules. The corresponding
(Fig. 4). The bond distances and angles of 2 are gathered in Table
2. The Mg–O distances vary between 2.042(1) and 2.0605(2) Å,
with the O–Mg–O angles being close to 90°. The bond lengths
and angles are in agreement with the corresponding values

D. Saha et al. / Polyhedron 35 (2012) 55–61
59
Fig. 4. 1D ribbon of compound 2.
DTA curve of compound 2 shows two endothermic peaks at ꢄ115
and ꢄ140 °C. Subsequent mass loss indicates the decomposition of
the copper-carboxylate moiety.
epoxide selectivity of ꢄ24% [32]. Selectivity of the epoxide im-
proves (to ꢄ53%) when copper-perchloro-phthalocyanine has been
anchored onto MCM-41, however, the conversion remains at 47%
[
33]. In fact, in the epoxidation of styrene with tert-BuOOH over
3
.5. Catalytic epoxidation reactions
a copper/copper complex immobilized zeolite or with molecular-
sieve catalysts, the epoxide selectivity rarely goes above 40%
[34]. In our previous attempt we have succeeded in improving
the yield of styrene epoxidation up to 86% using a copper(II)
Schiff-base anchored MCM-41 catalyst [9]. However, examples of
epoxidation reactions homogeneously catalyzed by copper
complexes are limited and in many of the cases tert-BuOOH was
Olefin epoxidation reactions catalyzed by transition metal
based catalysts under homogeneous conditions are well docu-
mented [6,31]. Seelan et al. studied the epoxidation of styrene
over a copper phthalocyanine immobilized NaY catalyst under
heterogeneous conditions, which shows over 95% conversion with
Table 3
Epoxidation of olefins catalyzed by compounds 1 and 2.a
a
Substrate
Reaction time (h)
Conversion (wt%)
Epoxide yield (wt%)
TOFb (hꢁ1
)
(
(
a) 6
b) 18
(a) 100
(b) 100
(a) 86
(b) 58
(a) 252
(b) 158
(
(
a) 6
b) 18
(a) 100
(b) 100
(a) 88
(b) 62
(a) 252
(b) 158
(
(
a) 6
b) 18
(a) 100
(b) 100
(a) 92
(b) 65
(a) 252
(b) 158
a
2 2
Reaction conditions: alkenes (10 mmol); catalyst (2 mg); 1.5 mL of 30% H O (14.4 mmol, 1.5 equiv.); solvent (ethanol for compound 1 and
acetonitrile for compound 2) (5 mL). Temperature of the reaction medium was kept at 40 °C for compound 1 and 60 °C for compound 2. The
products of the epoxidation reactions were collected at different time intervals and were identified and quantified by a Varian CP-3800 gas
chromatograph equipped with an FID detector and a CP-Sil 8 CB capillary column. Entries (a) and (b) correspond to the catalytic performance of
compounds 1 and 2, respectively.
b
Turn over frequency (TOF) = moles converted per moles of active site per unit time.

6
0
D. Saha et al. / Polyhedron 35 (2012) 55–61
used as the oxidant [10–12]. Rayati et al. studied the epoxidation of
styrene over [Cu(hnaphnptn)] (H
derived from the condensation of 2,2 -dimethylpropandiamine
2
{hnaphnptn} = Schiff-base
0
and 2-hydroxy-1-naphthaldehyde) and [Cu{salnptn(3-OMe)
2
}]
(
H
2
{salnptn(3-OMe)
2
} = Schiff-base derived from the condensation
0
of 2,2 -dimethylpropandiamine and 2-hydroxy-3-methoxybenzal-
dehyde) under homogeneous conditions using tert-BuOOH, which
shows over 95% conversion with an epoxide selectivity of ꢄ25%
[
11]. We have studied this reaction over the Cu(II) Schiff-base
2
2
complexes [Cu(L )(H
tophenimine)-2-methyl-pyridine), [Cu(L )] (HL = N,N -(2-hydro-
xy-propane-1,3-diyl)-bis-salicylideneimine) and [Cu(L )] (HL =
N,N -(2,2-dimethyl-propane-1,3-diyl)-bis-salicylideneimine), which
2
O)](ClO
4
)
(HL = 1-(N-ortho-hydroxy-ace-
3
3
0
4
4
0
gives styrene epoxide in 54–39% yield (selectivity 72–39%) under
homogeneous conditions with tert-BuOOH [11]. In this study, sty-
rene and substituted styrene react with H O to produce epoxides
2 2
with remarkable selectivity in good yield using compound 1 and 2
under homogeneous conditions. The results of the catalytic epoxi-
dation of different substrates are given in Table 3. The epoxidation
2 2
of styrene with H O gives styrene oxide in 100% conversion (selec-
Fig. 6. Kinetic plots for the epoxidation of styrenes catalyzed by compound 2.
tivity ꢄ86% for 1 and ꢄ58% for 2); along with this, a small amount
of benzaldehyde is also detected. The homogeneous oxidation of
3
-Me styrene proceeded smoothly, showing 100% conversion to
Table 4
Epoxidation of styrene catalyzed by a variety of Cu and Mg-based catalysts under
homogeneous conditions using H
form 3-Me styrene oxide as the major product with ꢄ88% and
ꢄ62% selectivity for 1 and 2 respectively; along with this, 3-Me
benzaldehyde was also generated. 4-Me styrene has also been
effectively converted to 4-Me epoxystyrene (conversion 100% for
both 1 and 2, selectivity 92% for 1 and 65% for 2); along with this,
2 2
O .
Catalyst
Reaction Solvent Temperature Conversion Epoxide
time (h)
(°C)
(wt%)
yield
wt%)
(
4
-Me benzaldehyde was also produced. A high turnover frequency
Cu(NO
Mg(NO
Compound 1
[Cu(OAc) O]
Compound 2
3
)
2
ꢀ3H
2
O
2
O
6
6
6
18
18
C
C
2
H
H
5
OH 40
OH 40
10
–
100
75
12
–
86
22
58
ꢁ1
ꢁ1
has been attained (ꢄ252 h for 1 and ꢄ158 h for 2) for the epox-
ide production. A graphical representation of relative efficacy of 1
and 2 for the epoxidation of styrene/substituted styrene with time
has been given in Figs. 5 and 6, respectively. The catalytic activity
and selectivity in styrene epoxidation over various catalysts con-
taining Cu(II)/Mg(II) under homogeneous conditions are presented
in Table 4. It is well known that copper(II) can bind the peroxo-
group on treatment with peroxides [35] and the pre-catalyst
species containing LxCu–OOH (where L = ligand) type moieties
seem to be capable of transferring the oxo functionality to the
organic substrates to give the corresponding oxidized products
3
)
2
ꢀ6H
2
5
C H OH 40
2
5
H
2 2
CH
CH
3
CN
60
60
3
CN
100
ion is easily accessible for an external ligand, as a result hydroper-
oxide gets enough space to bind copper in the intermediate stages
of the catalytic cycle. In complex 2 although the copper(II) ion is
hexacoordinated, the peroxo intermediate is formed, maybe due
to the replacement at any of its coordination sites. This could be
a reason for the higher catalytic activity of compound 1 compared
to compound 2.
[
36,37,10]. In our case we propose that a similar kind of mecha-
nism is operative. The X-ray crystal structure analysis of complex
1
shows that the coordination environment around the copper(II)
2 2
The efficacy of different oxidants like tert-BuOOH, H O and
NaOCl in compound 1 and compound 2 catalyzed epoxidation
reactions of styrene under homogeneous conditions have been
studied. The results of this study are given in Table 5 and H O is
2 2
found to be the most efficient oxidant. The catalytic reaction was
performed in a variety of solvents to study the catalytic efficacy
of 1 and 2 in different solvents. The best performance of the
catalyst was observed in ethanol for 1 and in acetonitrile for 2
(
see Supplementary Material, Table S2). The catalytic efficiency
for
1
followed the order: ethanol > methanol > acetonitrile >
Table 5
Epoxidation of styrene with various oxidants over compound 1 and 2.a
a
Catalyst
Reaction
time (h)
Oxidant
Conversion
(wt%)
Epoxide yield
(wt%)
1
1
1
1
2
2
2
2
6
6
6
None
Not traceable
100
62
34
Not traceable
100
52
–
H
2
O
2
86
64
58
–
58
52
48
tert-BuOOH
NaOCl
None
6
18
18
18
18
H
2
O
2
tert-BuOOH
NaOCl
28
a
Fig. 5. Kinetic plots for the epoxidation of styrenes catalyzed by compound 1.
Reaction conditions were the same as given in the footnote of Table 4.

D. Saha et al. / Polyhedron 35 (2012) 55–61
61
acetone, but for
2
the order was: acetonitrile > acetone >
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ethanol > methanol. The catalytic reaction was also performed at
different temperatures to study the effect on the catalytic efficacy
of 1 and 2. While the best performance was observed for 1 in the
temperature range 40–45 °C, for 2 it was 60–65 °C (see Supple-
mentary Material, Table S3).
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. Conclusion
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two newly synthesized and structurally characterized copper
complexes in the olefin epoxidation reaction under homogeneous
conditions. The catalysts showed good efficiency towards the
epoxidation reactions, as reflected by the high turnover frequency
of the reactions. The copper Schiff-base compound 1 demonstrates
better selectivity towards epoxides than the copper carboxylate
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Appendix A. Supplementary data
CCDC 817491 and 817490 contain the supplementary crystallo-
graphic data for complexes 1 and 2. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.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 Supplementary data associated
with this article can be found, in the online version, at doi:10.1016/
j.poly.2011.12.039.
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