Robust 1D open rack-like architecture in coordination polymers of Anderson POMs [{Na4(H2O)14}{Cu(gly)}2] [TeMo6O24] and [{Cu(en)2}3{TeW 6O24}]: S

Article abstract of DOI:10.1039/c0dt00426j

Two novel organic-inorganic hybrid tungsto- and molybdo- telurates having formula [{Na₄(H₂O)₁₄}{Cu(gly)} ₂][TeMo₆O₂₄] (1){gly = glycine} and [{Cu(en)₂}₃{TeW₆

Full text of DOI:10.1039/c0dt00426j

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PAPER
www.rsc.org/dalton | Dalton Transactions
Robust 1D open rack-like architecture in coordination polymers of Anderson
POMs [{Na (H O) }{Cu(gly)} ][TeMo O ] and [{Cu(en) } {TeW O }]:
4
2
14
2
6
24
2 3
6
24
synthesis, characterization and heterogeneous catalytic epoxidation of
olefines†
a
b
a
c
d
a
Daipayan Dutta, Atish D. Jana, Mainak Debnath, Asim Bhaumik, Jaromir Marek and Mahammad Ali*
Received 5th May 2010, Accepted 8th September 2010
DOI: 10.1039/c0dt00426j
Two novel organic-inorganic hybrid tungsto- and molybdo- telurates having formula
[
{Na
4
(H
2
O)14}{Cu(gly)}
2
][TeMo
6
O
24] (1){gly = glycine} and [{Cu(en)
2
}
3
{TeW
6
O
24}]·6H O {en =
2
ethyline-diamine} (2) based on Anderson type heteropolyoxometalates (POMs) have been synthesized
and characterized by X-ray crystallography. Common structural feature of both 1 and 2 is the presence
of a unique 1D open rack-like architecture, where the disc shaped Anderson POMs act as steps and
cationic Cu-organic complexes act as handles of the rack. In 1 the independent structural unit is a 1D
coordination polymer with the above mentioned rack type architecture, while in 2, these independent
rack-like architectures are further extended to a 2D coordination polymer. Heterogeneous catalysis for
the epoxidation of cyclohexene and styrene by complexes 1 and 2 showed very good catalytic efficiency
resulting epoxides of ~60% yield, with dialcohol formed by the hydrolysis of epoxides, as the other
major product (~28%). Cyclic voltammetric studies of [{Na
4
(H
2
O)14}{Cu(gly)}
2
][TeMo O24] (1) in
6
aqueous KCl solution indicates that the redox changes occur only on the copper centers and supported
2
+
6-
by carrying out parallel experiments on the precursors like ([Cu(gly)
2
]
and [TeMo
6
O
24] , under the
identical experimental conditions. The E1/2 = 0.662, -0.142 and -0.332 V(vs. SCE) correspond to Cu(III)
→
Cu(II), Cu(II) → Cu(I) and Cu(I) → Cu(0) reductions, respectively. Thermal analyses reveal identical
◦
phase transition reactions with an exothermic peak in the DTA curve at 380 C for 1 and an
◦
endothermic peak appears at comparatively higher temperature (408 C) for 2 manifesting the higher
stability of tungstane based POM over the molybdenum ones. EPR as well as magnetic moment results
indicate that both the complexes 1 and 2 are paramagnetic with one unpaired electron per copper(II) ion.
Introduction
type polyoxoanions as inorganic building block is only in its
9
infancy. Anderson type POMs being inherently planar, act as
Framing and generation of metal-oxide based new materials with
value added properties is a challenging task in modern inorganic
chemistry. Polyoxometalates (POMs), metal-oxide based materi-
als, are gaining more and more interest due to their unlimited
structural varieties and widespread applications in numerous fields
like catalysis, biochemical separations and medicinal chemistry.
They play an important role for the design of new materials
with novel electronic, magnetic and topological properties. Up
until now, various organic/metallo-organic/inorganic linkers have
been utilized to construct extended metal-oxide based materials
10
promising templates for the organization of layered structures.
Only very recently, some Anderson-type heteropolyanions based
on molybdenum oxide cluster linked to metal complexes including
9
transition metals and lanthanides have been reported but in
contrast the use of corresponding tungsten oxide based Anderson
1
11
POM is very rare.
In general, most of the POMs form 1D chain like architecture
with transition metal ions and especially with lanthanides as
2
12
linkers. On the other hand, Keggin type POMs and only in few
cases Anderson POMs were found to form 2D and 3D frameworks
3
4
5
of the type Keggin, Wells-Dawson, Lindquist polyoxoanions,
and isopoly anions;
+
+
in presence of Na or Na and transition metal or lanthanide
6–8
but in contrast, the use of Anderson-
13–15
ions as linkers.
Recently, Das et al. reported an extended
Al(OH) Mo O with rare earth cation
polymer [La(H
2
O)
7
6
6
O
18
]
n
·4H
2
a
16
Department of Chemistry, Jadavpur University, Kolkata, 700 032, India.
E-mail: mali@chemistry.jdvu.ac.in
as linkers. They also reported a spiral-shaped inorganic–organic
n- 17
hybrid chain [Cu(2,2¢-bipy)(H
2
O)
2
–Al(OH)
6
Mo
6
O
18
]
n
.
On the
)–
Mo12O35(OH)] complex from spontaneous conversion of an
b
Department of Physics, Sripat Singh College, Jiaganj, Murshidabad, W.B.,
other hand, Eda et al. prepared an a-Keggin type [(GaO
4
7
42123, India
4
-
c
Department of Materials Science, Indian Association for the Cultivation of
Science, Jadavpur, Kolkata, 700 032, India
3
- 18
Anderson type anion [Ga(OH)
6
Mo O18] .
6
d
Laboratory of Functional Genomics and Proteomics, Institute of Experi-
Although POMs are regarded as promising candidates as
mental Biology, Faculty of Science, Masaryk University, Kamenice 5/A2,
CZ-625 00, Brno, Czech Republic
19–21
green catalysts,
their application as heterogeneous catalysts
is very limited due to their inherent tendency to conglomerate
†
CCDC reference numbers 775686 (1) and 775687 (2). For crys-
tallographic data in CIF or other electronic format see DOI:
0.1039/c0dt00426j
22
resulting deactivation of the POMs. Efforts have been taken
to overcome such limitations through the incorporation of the
1
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2
3–28
POMs into the metal–organic frameworks (MOFs).
Though,
Synthesis of the complexes: [{Na (H O) }{Cu(gly)} ][TeMo O ]
4
2
14
2
6
24
a great variety of POM-based coordination polymers in the MOFs
(1)
2
3–28
have successfully been assembled,
remains practically unexplored;
their application as catalysts
particularly the Anderson-
POM based materials are totally absent from this scenario.
Very recently, we have reported a crystal structure where due to
the templating action of the Anderson POMs, the tris-chelated
29–30
A mixture of H
6
TeO
6
(0.08 g, 0.35 mmol), Na
·2H O (0.060 g, 0.35 mmol), glycine
0.053 g, 0.7 mmol), 4(M) HCl (8–10 drops) and H O (25 mL)
2
MoO
4
·2H
2
O
(0.508 g, 2.1 mmol), CuCl
2
2
(
2
were taken together in a 100 mL beaker. The pH of the resultant
2
+
solution was maintained at 5.50 by the addition of 4.0 (M) HCl
[Cu(phen)
3
]
units are organized in a layer with honeycomb
network topology instead of their preferred tendency to or-
◦
10
solution. Then, the mixture was stirred with heating at ~ 80 C
31
for 1 h to get a blue solution. It was cooled to room temperature
and filtered. The resultant sea-blue filtrate was kept in open air for
slow evaporation, which resulted to blue, rhombic single crystals in
ganize through multifold embrace motifs In our continuing
effort to synthesize Anderson POM based novel materials we
further envisaged that a suitable design strategy can lead to
rational construction of Anderson based 1D and 2D polymeric
frameworks. In contrast to the spherical or oval shape of other
type of POMs, Anderson POMs have disk-shaped structure and
has the potentiality to give rise to ladder topology, if these discs
could be connected by linear spacers (Scheme 1). An obvious
design strategy in this regard is to use transition metal complexes
with octahedral coordination where the terminal oxygen atoms of
Anderson POM units can satisfy the axial sites and the equatorial
sites have to be blocked by suitable chelating ligands. Here, we
report the successful employment of this strategy to generate rack
type architecture of Anderson based POMs, for the first time,
1
0 days. The crystals were filtered off, washed with distilled water,
and dried in a desiccator at room temperature to give an yield of
0% based on Mo. Anal. Calcd for 1, C Mo TeN
M.W. 1706.2): C, 2.81; H, 2.11; N, 1.64%. Found: C, 2.78; H, 2.07;
5
(
4
H
36Na
4
O42Cu
2
6
2
N, 1.62%.
Synthesis of the complex [Cu(en)
2
]
3
[TeW
6
O
24]·6H
2
O (2)
The synthetic procedure is similar to that of 1, except
Na WO O (0.693 g, 2.1 mmol), Cu(NO ·4H O (0.242 g,
·2H
.0 mmol) and ethylenediamine (0.18 g, 3.0 mmol) were added
instead of Na MoO O, CuCl O and glycine, respectively.
2
4
2
3
)
2
2
1
2
4
·2H
2
2
·2H
2
in two complexes [{Na
4
(H
2
O)14}{Cu(gly)}
2
][TeMo
6
O24] (1) and
The violet rectangular crystals, formed within few days, were
filtered off, washed with distilled water, and dried in a desiccator
at room temperature to give an yield of 40% based on W. Anal.
[
Cu(en) [TeW (2). This also comprises the first report where
2
]
3
6
O
24 2
]
Anderson POMs based MOFs have been found to be efficient
heterogeneous catalyst for the liquid phase partial oxidation of
olefins.
Calcd for 2, C12
H
48Cu
3
N
12
O
24TeW (M. W. 2261.92) : C, 6.37; H,
6
2.12; N, 7.43%. Found: C, 6.08; H, 2.28; N, 7.34%
Physical measurements
Elemental analyses were carried out using a Perkin-Elmer 240
elemental analyzer. Room temperature magnetic susceptibility
was recorded on a PAR vibrating sample magnetometer using
[
Hg{Co(NCS) }] as the calibrant. EPR was recorded from Bruker
4
EMX-220 digital X-Band (n = 9.43 GHz) spectrophotometer.
TGA and DTA study were carried out using a Perkin-Elmer
Pyris Diamond TGA/DTA Analyser. Catalysis were carried out
using an Agilent 6890A, FID GC. Electrochemical measure-
ments were carried out using a computer controlled AUTOLAB
(model: 263A VERSASTAT) electrochemical system with glassy
carbon as working electrode, Pt-wire as counter electrode and
saturated calomel as reference electrode. Cyclic voltammograms
◦
were recorded at 25 C in deionized water from Millipore - Milli-Q
Water Purification Systems under pure N
KCl as supporting electrolyte.
2
atmosphere with 0.10 M
Scheme 1 Framing of Racks from Anderson POM and transition metal
complexes
Crystal data collection and refinement
Intensity data for complexes 1 and 2 were collected on a four-
circle diffractometer Kuma KM-4 equipped with a CCD detector
and an Oxford Cryostream Cooler. The w-scan technique with
different k and j offsets for covering the whole independent
Experimental
Materials and methods
◦
Sodium molybdate (Merck, Germany), Sodium tungstate (Merck,
Germany), copper chloride (A. R. Loba, India), copper nitrate
part of reflections in the 2 < 2q < 25 for monochromated
(monochromator Enhance, Oxford Diffraction) Mo-Ka radiation
˚
(
(
Merck, Germany), glycine (Merck, Germany), ethylenediamine
Merck, Germany), telluric acid (Alfa Aesar, Germany) and
(l = 0.71073 A) was performed. Data collection, cell param-
eter refinement and data reduction were carried out using a
32
hydrochloric acid were of reagent grade and used as received.
De-ionised water was used as the solvent.
CrysAlis program package. The cell parameters were refined
from all strong reflections. During data reduction of less absorbing
1
1552 | Dalton Trans., 2010, 39, 11551–11559
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Table 1 Crystal data and refinement parameters
1
2
Formula
Formula weight
Wavelength ( A˚ )
Crystal system
Space group
a, b, c ( A˚ )
C
4
H
36Na
4
O
42Cu
2
Mo
6
TeN
2
C
12
H
48Cu
3
N
12
O
24TeW
6
·6(H
2
O)
1706.6
0.71073
Triclinic
P1(No.2)
7.1526 (5)
2274.04
0.71073
Triclinic
P1(No.2)
8.5534(3)
¯
¯
1
1
0.6481(6)
4.2288 (9)
11.5294(4)
13.1841(5)
67.382(4)
85.597(3)
72.689(3)
1144.75(8)
1
3.299
16.932
1039
0.30 ¥ 0.34 ¥ 0.44
100(2)
0.71073
3.0, 25.0
-10: 9; -10: 13; -12: 15
5959, 4013, 0.019
3947
◦
a, b, g ( )
73.573(5),
7
7
1006.74(11)
1
2.808
3.720
812
0.10 ¥ 0.30 ¥ 0.30
120(2)
0.71073
3.8, 25.0
8.822(5)
8.197(5)
V ( A˚ 3)
Z
-3
D(calc) [g cm ]
m(Mo-Ka) [1/mm]
F(000)
Crystal Size/mm
T/K
Mo-Ka[ A˚ ]
Min, qMax [ ]
◦
q
Dataset
-8: 8; -12: 12; -16: 15
6905, 3549, 0.016
3348
Tot., Uniq. Data, Rint
Observed data [I > 2.0 s(I)]
Nref, Npar
3549, 323
4013, 310
R, wR
2
, S
0.0246, 0.0635, 0.90
0.00, 0.00
0.0289, 0.0786, 1.26
0.00, 0.00
Max. and Av. Shift/Error
complex 1 an empirical absorption correction using spherical
Results and discussion
32
harmonics was applied, while an analytical numeric absorption
33
correction using a multifaceted crystal model has been applied on
Solid state structure
heavily absorbing complex 2. The both structures were determined
34
Complexes 1 and 2 were prepared by simple one-pot reaction
by direct methods (SHELXS-97) and refined anisotropically on
6
-
2
34
between [TeM
6
O
24
]
(H
6
TeO
6
+ Na
2
MO ) (M = Mo or W)
4
F using full matrix least-squares procedure by SHELXL-97 All
C and N bonded hydrogen atoms in 1 and 2 were placed in their
geometrically calculated positions, while positions for O-bonded
H-atoms were localized in difference electron density maps and
refined with restrained the O–H distances. An only exception is
the terminal oxygen atom attached to Na2 in complex 1: this atom
is disordered over two positions (O24a and O24b) and hydrogen
atoms attached to these oxygen positions could not be located.
Table 1 Comprises the detail of crystallographic data collection
and refinement parameters for complexes 1 and 2.
2
+
Cu and appropriate ligand in aqueous media at pH 5.50.
The main intention was to generate open shelf-like architec-
ture with Anderson POM as base and copper(II) complexes as
linker as shown in Scheme 1. Single crystal X-ray structure
of compound 1 reveals that it consists of one dimensional
polymeric chains having open rack-like architecture (Fig. 1a).
6
-
10
It is formed by Anderson type [TeMo
6
O
24
]
polyoxoanion
cationic complexes (Fig. 1c). The
in 1 is similar to the structures reported
for other Anderson type anions. This consists of six coplanar
MoO octahedra which are arranged in a ring sharing edges
leaving an octahedral pocket at the center of the ring for the
6
+
and [{Na
4
(H
2
O)14}{Cu(gly)}
2
]
6
-
structure of [TeMo
6
O ]
24
6
Catalysis
6
+
The catalytic epoxidation reactions of cyclohexene and styrene
by Tert-butylhydroperoxide (TBHP) were carried out heteroge-
neously in MeCN using 0.02 g of complex 1 or complex 2, 0.5 g
substrate, 5 ml acetonitrile under stirring in a two-neck round-
bottom flask fitted with a water condenser and placed in an oil bath
at 333 K, 70 wt% aqueous TBHP, equimolar with respect to the
substrate, was added immediately before the start of the reaction.
Aliquots of the reaction mixture were withdrawn at various
time intervals to study the progress of the reaction. In a typical
operation, 0.5 mL solution was taken out with the help of a micro-
pipette and the solution was subjected to multiple ether extraction
and the extract was concentrated. From this, 1 mL solution was
withdrawn with the help of a gas tight syringe and injected to
the GC port (Agilent 6890A, FID). For the identification of
products the GC retention times of different reaction products
were compared with those of commercial standards.
heteroatom, Te , in the present case, which occupies a center
of inversion (Fig. 1b). The Mo–O bond lengths are as follows:
˚
Mo–O
t
(terminal oxygens), 1.703(3)–1.955(3) A; Mo–O (bridging
b
˚
oxygens), 2.257(3)–2.300(3) A and that for Te–O ranges between
˚
1.922(3) and 1.934(3) A.
6
-
The [TeMo
6
O ] anions act as steps of the rack-like assembly
24
which are joined on both sides by a complex of two sodium atoms
and a Cu atom. The Cu atom (Cu1) and the sodium atoms (Na1
and Na2) each possesses octahedral coordination environment.
Among four equatorial sites of the Cu(II) octahedron, two are
occupied by water oxygen atoms (O4 and O5) and the rest two
are contributed by a glycine molecule which acts as a chelating
agent for the Cu(II) center by binding it through the carboxylate
O atom (O1) and amine N atom (N3). The trans axial sites of the
two Cu(II) octahedra are occupied by O6 oxygen atoms that are
6
-
contributed by two [TeMo
6
O
24
] units from opposite sides. In the
This journal is © The Royal Society of Chemistry 2010
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Fig. 1 (a) The rack-like architecture of compound 1. (b) Structure of X-Te metal cluster (X = Mo for compound 1 and X = W for compound 2. (c) The
ORTEP diagram of 1 (30% ellipsoidal probability) with atom numbering scheme. (i) = -1+x,y,z; (ii) = 1-x, 1-y, 1-z.
equatorial plane the Cu(II) octahedron shares an edge with the
adjacent Na1 octahedron; as oxygen atoms O5 of water and O1
of glycine occupy two of the equatorial cis-positions of the Na1
octahedron. Two other equatorial sites of the Na1 octahedron are
occupied by water oxygen atoms (O21 and O22). The trans axial
sites of the Na1 octahedron are constituted by oxygen atoms (O11)
6
-
of two [TeMo
6
O
24
]
units from opposite directions. Thus both
6
-
Cu1 and Na1 take part in joining successive steps [TeMo
6
O ]
24
of the rack on both sides due to centrosymmetric arrangement
of the system to give rise to an open rack-like architecture of the
complex. Na1 atom is further connected to Na2 atom. They share
O21 and O22 water oxygen atoms in the equatorial plane of their
respective octahedral coordination environments. Two terminal
water oxygen atoms, (O23) and a disorder water oxygen atom
over two sites (O24a and O24b) satisfy the other two equatorial
coordination sites of the Na2 octahedron. The trans axial sites
of the Na2 octahedron are occupied by a water molecule (O20)
and the glycine O2 atoms from an adjacent chain. The Na2–O2
Fig. 2 The packing diagram of individual ladders of 1 viewed down the
a-axis.
˚
˚
distance is rather larger 2.886(4) A as compared to 2.290(3) A for
Na2–O20. There is no unusual bonding geometry for the Cu1
and Na1 coordination and the bond distances and angles are
described in Table 2. The inter ladder Cu ◊ ◊ ◊ Cu separation is 7.153
Table 3 displays selective bond distances and bond angles of
6
-
˚
A. These anion chains are arranged to form layers along the ab
complex 2. Crystal structure of 2 contains [TeW
6
O
24
]
heteropoly
2
+
˚
plane, with the inter-chain distance of about 10.171 A (in the same
anions and [Cu(en) ] cations. Te atom and one of the Cu (Cu2)
2
layer). Individual polymeric chains pack (Fig. 2) in a compact
fashion due to hydrogen bonding interaction between adjacent
polymers.
atoms occupy the center of inversion.
The architecture of the complex is a 2D polymeric sheet
6
-
(Fig. 3) in which [TeW
6
O
24
]
ions are bridged doubly (along
1
1554 | Dalton Trans., 2010, 39, 11551–11559
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◦
Table 2 Selected bond distances ( A˚ ) and bond angles ( ) of compound 1
Bond distances ( A˚ )
Na1–O21
Na1–O22
Na1–O12_a
Na1–O1
Na2–O20
Na2–O21
Na2–O22
Na2–O23
Na2–O24A
Na2–O24B
Cu–O1
2.372(4)
2.318(3)
2.346(3)
2.334(4)
2.290(3)
2.382(3)
2.458(5)
2.369(5)
2.355(8)
2.31(2)
1.967(3)
1.993(3)
2.006(3)
2.346(3)
1.969(3)
2.405(3)
Te–Mo1
Te–Mo2
Te–Mo3
Te–O10
Te–O14
Te–O15
Mo1–O6
Mo1–O7
Mo1–O8
Mo1–O9
Mo1–O10
Mo1–O15
Mo2–O8
Na1–O5
Na1–O11
3.2703(4)
3.2674(4)
3.3051(4)
1.930(2)
1.934(3)
1.922(3)
1.718(3)
1.703(3)
1.955(3)
1.940(3)
2.257(3)
2.307(3)
1.931(2)
2.476(3)
2.347(3)
Cu–O4
Cu–O5
Cu–O6
Cu–N3
Cu–O8_a
Fig. 3 The 2D polymeric sheet (ii) of the complex 2 which is the
◦
Bond angles ( )
result of joining of successive ladders (shown in [i]) that running along
O6–Mo1–O7
O6–Mo1–O8
O6–Mo1–O9
O6–Mo1O10
O6–Mo1O15
O1–Cu–O4
107.87(15)
99.42(12)
96.77(13)
157.69(13)
88.45(13)
178.21(13)
90.87(12)
92.42(11)
O1–Cu–N3
83.45(12)
89.58(11)
172.69(16)
99.60(13)
87.25(15)
86.0(3)
2+
2
crystallographic a-axis by [Cu(en) ] units.
O1–Cu–O8_a
O20–Na2–O21
O20–Na2–O22
O20–Na2–O23
O20–Na2–O24A
O20–Na2–O24B
O1–Cu–O5
O1–Cu–O6
87.2(10)
Symmetry a = -1+x,y,z; c = 1-x,1-y,1-z.
◦
Table 3 Selected bond distances ( A˚ ) and bond angles ( ) of 2
Bond distances ( A˚ )
W1–O1
W1–O2
W1–O3
W1–O4
W1–O5
W1–O6
Te1–O4
Te1–O5
Te1–O10
Te1–O4_b
Te1–O5_b
1.744(6)
1.749(6)
1.933(6)
2.268(6)
2.271(6)
1.936(5)
1.920(5)
1.929(6)
1.932(6)
1.920(5)
1.929(6)
Te1–O10_b
Cu1–O12
Cu1–N1
Cu1–N2
Cu1–N3
Cu1–N4
Cu1–O1_a
Cu2–O7
Cu2–N5
Cu2–N6
1.932(6)
2.409(6)
2.031(8)
2.009(8)
2.009(7)
2.015(8)
2.848(6)
2.521(6)
2.008(8)
2.031(7)
◦
Bond angles ( )
O1–W1–O2
O1–W1–O3
O1–W1–O4
O1–W1–O5
104.8(3)
97.2(3)
157.7(2)
89.4(3)
99.9(3)
88.5(3)
90.3(3)
89.6(2)
97.9(3)
161.59(19)
83.8(3)
175.9(3)
92.4(3)
N2–Cu1–N4
O1_a–Cu1–N2
N3–Cu1–N4
O7–Cu2–N5
O7–Cu2–N6
O7–Cu2–O7_c
O7–Cu2–N5_c
O7–Cu2–N6_c
N5–Cu2–N6
O7_c–Cu2–N5
N5–Cu2–N5_c
O5–Te1–O5_b
O5–Te1–O10_b
170.9(3)
80.7(2)
84.2(3)
85.1(3)
93.8(3)
180
94.9(3)
86.2(3)
85.3(3)
94.9(3)
180.00
180
Fig. 4 Packing of successive polymeric sheets of 2 that give rise to intra
layer channels that accommodate solvent water molecules.
O1–W1–O6
O12–Cu1–N1
O12–Cu1–N2
O12–Cu1–N3
O12–Cu1–N4
O1_a–Cu1–O12
N1–Cu1–N2
N1–Cu1–N3
N1–Cu1–N4
Catalysis
Typically, the oxidation reactions were carried out using 0.02 g
of complex 1 or complex 2, 0.5 g substrate, 5 mL acetonitrile at
3
33 K with TBHP, 70 wt% aqueous, equimolar with respect to the
85.5(3)
substrate as oxidant. Identification of products were carried out by
comparing the GC retention times of different reaction products
with those of commercial standards.
a = -1-x,-y,1-z; b = -x,-y,1-z; c = -x,-y,2-z.
In Table 4 results of the catalytic activity as well as TON
(Turn Over Number) for selected catalytic runs are given for
complexes 1 and 2. Acetonitrile has been used as the solvent in all
these liquid phase reactions. The major products for the partial
oxidation of both styrene and cyclohexene were their respective
epoxides. It is clear from the Fig. 5 that the selectivity of the
epoxide for both the substrates was high at the initial stage of
the reaction. However, with time as the conversion reaches the
respective maxima, epoxide selectivities go down to some extent.
2
+
crystallographic a-axis) and singly by [Cu(en)
2
]
cations (along
crystallographic b-axis). Due to double bridging along a-axis the
units give rise to rack-like architecture (Fig. 3(i))
which are joined side by side along c-axis by [Cu(en)
giving rise to the 2D polymeric sheet. Successive polymeric
sheets are stacked along crystallographic b axis giving rise to
I’shaped supramolecular channels which accommodate solvent
6
-
[
TeW
6
O
24
]
2
+
2
]
units
‘
water molecules (Fig. 4).
This journal is © The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 11551–11559 | 11555

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Table 4 Catalytic activity of 1 and 2 in liquid phase partial oxidation of olefins in the presence of TBHP oxidant
Cat.
Olefin
Time/h
Conv. (%)
Epoxide (%)
Diol (%)
Others (%)
TON
1
2
Styrene
Cyclohexene
Styrene
12
12
12
12
78.5
82.4
89.8
80.2
59.8
60.6
59.2
62.1
27.9
27.3
29.6
28.4
12.3
12.1
11.2
9.5
15.8
19.4
20.9
24.4
Cyclohexene
under the same conditions but with no epoxidation products
indicating the fact that it is the complex 1 or 2 which are solely
responsible for such catalytic process. Besides that there is no
leaching of reactive species during the epoxidation process. The
material separated by filtration showed more than 80% catalytic
activity strongly suggesting the reusability of the catalyst.
EPR study
EPR spectra were recorded from Bruker EMX-220 digital X-
Band (n = 9.43 GHz) spectrophotometer. Room temperature
EPR spectra on the polycrystalline powdered samples of Cu(II)
complexes 1 and 2 with one unpaired electron were recorded at
-
1
the X-band frequency (n) = 9.583 cm and for both the complexes
low resolution EPR spectra were obtained having only <geff> =
2
.014 and 1.995 respectively (Fig. 6 and 7).
Fig. 5 % Conversions of styrene (red diamond) and cyclohexene (blue
circle) at different reaction hours in liquid phase partial oxidation
by 2. Corresponding selectivity of styrene oxide (cyan, diamond) and
cyclohexene oxide (green circle) are at the right hand axis.
This could be attributed to the hydrolysis of the epoxides to the
respective diols (Scheme 2), which is quite common for the liquid
35
phase partial oxidation of olefin over a heterogeneous catalyst,
as water generated in the oxidation process promote the epoxide
ring opening. From Table 2, it is clear that TON for cyclohexene
conversion was relatively more than that for styrene. Nevertheless
high TON for the oxidation of styrene and cyclohexene suggests
the high catalytic efficiency for the Cu-hydroperoxo species that
35
could form at the active sites in the presence of TBHP oxidant.
For both the substrates, corresponding diols are the other major
products. Control experiments for the epoxidation reactions were
carried out, i.e. using only TBHP without the catalysts and it was
observed that about 2.6% and 0.7% epoxides were formed in the
oxidation of styrene and cyclohexene respectively.
Fig. 6 Room temperature solid state EPR spectrum of 1.
TGA and DTA Studies
TGA and DTA studies were carried out using a Perkin-Elmer
Pyris Diamond TGA/DTA Analyzer. TGA on 1 and 2 were
◦
-1
performed at a rate of 5 C min in the range of temperature
◦
3
(
1
9
0–500 C under N
2
gas flow. For 1, successive weight losses
Fig. 8) of 4.22%, 8.44% and 2.11%, in the range of 40–70 , 80–
00 and 105–125 C correspond to endothermic peaks at 70 ,
8 and 120 C, respectively in the DTA curve. These observed
◦
◦
◦
◦
◦
◦
Scheme 2 Liquid phase partial oxidation of olefins by TBHP to epoxides
catalyzed by compounds 1 and 2.
values are in accordance with the removal of four, eight and
two water molecules respectively, (calcd. 4.16%, 8.35% and 2.07%
respectively). Another weight loss of ~9.30% takes place in the
◦
To verify whether the observed catalysis is truly heterogeneous
or not, the complex was removed from the reaction mixture by the
filtration and the reaction was again carried out with the filtrate
range of 250–300 C and corresponds to the decomposition
of two glycine molecules (calcd. 8.67%). An exothermic peak
◦
corresponding to this change appears at ~290 C in the DTA
1
1556 | Dalton Trans., 2010, 39, 11551–11559
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Fig. 9 TGA/DTA thermogram of 2.
Magnetic study
Room temperature magnetic susceptibilities were recorded on a
Fig. 7 Room temperature solid state EPR spectrum of 2.
PAR vibrating sample magnetometer using [Hg{Co(NCS) }] as
4
the calibrant. The magnetic moments for the two complexes 1
and 2 have been measured at room temperature with meff values
1
.76 BM and 1.93 BM for 1 and 2 respectively, indicating that
copper atom in both the complexes are paramagnetic with one
unpaired electron.
Electrochemical study of compound 1
It is interesting to envisage the electrochemical properties of
a unique Anderson-type species, 1 as the probable redox be-
haviour would provide information for its further application in
electrochemistry and catalytic studies. Cyclic voltammetric (CV)
studies were carried out in aqueous 0.1 M KCl solution within
the potential range from -1.0 to 1.0 V in order to avoid any
complication from the redox processes of POM moiety. Fig. 10
shows the cyclic voltammogram of 1 in aqueous medium at a
Fig. 8 TGA/DTA thermogram of 1.
curve. Another exothermic peak observed in the DTA curve at
80 C may arise due to a phase transition, which possibly leads
-
1
scan rate of 50 mV s with two reduction peaks located at
0.130 (C ¢) and -0.450 V (C ¢) and a not well resolved peak at
0.697 V (C ¢) with corresponding anodic peaks at 0.596 (A ¢),
◦
3
+
1
2
to the formation of telluric oxide (TeO
3
) along with CuMoO and
4
-
3
1
36
MoO
3
(Eqn. 1)
Cu [TeM
3
6
O
24] → 3CuMO
4
+ 3MO
3
+ TeO [M = Mo/W]. (1)
3
For the compound 2 an initial weight loss of ~5% (calculated
.77%) in the range of 40–80 C may arrise due to the loss
◦
4
of six water molecules loosely trapped in open channel of the
◦
framework. An endothermic peak at 77 C in the DTA curve
corresponds to this change. The second weight loss of ~ 16%
◦
takes place in the range 260–320 C corresponds to the loss of six
ethylenediamine molecules (calculated 15.92%) which corresponds
◦
to an endothermic peak 280 C in the DTA curve (Fig. 9). Another
◦
endothermic peak was observed in the DTA curve at ~408 C due
to the phase transition leading to the formation of TeO along
3
with CuWO
4
and WO
3
according to Eqn. 1.
◦
In 2 the third step at 408 C in the DTA curve is endothermic
in nature which is in contrary to the exothermic peak observed in
1. Not only that, this phase transition appears comparatively at
higher temperature than that in its molybdenum analogue clearly
2
Fig. 10 Cyclic voltammograms of complex 1 in H O using a platinum
electrode and KCl (0.10 M) as supporting electrolyte at 50 mV s scan
rate.
-1
demonstrating the higher stability of 2 over 1 and conforms to the
34
previous observations.
This journal is © The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 11551–11559 | 11557

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0
.118 (A
2
¢) and 0.033 V (A
3
¢) (vs. SCE) corresponding E1/2
=
Acknowledgements
0.662, -0.142 and -0.332 V, respectively and correspond to the
MA gratefully acknowledges the financial support from UGC for
grants of JU research grant under the UGC Scheme of University
with potential for excellence and UGC (Major) (Ref. No. F.32-
reduction of Cu(III) → Cu(II), Cu(II) → Cu(I) and Cu(I) → Cu(0)
respectively. The E1/2 corresponding to Cu(III) → Cu(II) reduction
37
at ~ 0.62 V is not uncommon and comparable to literature values.
247/2006(SR), New Delhi, and DST, (ref. SR/S1/IC-35/2006)
The peak at -0.697 V (C
experiment at lower scan rate in the potential range -1.0 to +
.20 V (SUP Fig. 1) giving two cathodic peaks at -0.407 and
0.710 V and corresponding anodic peaks at -0.145 and -0.308 V,
3
¢) becomes prominent when we run the
New Delhi. JM gratefully acknowledges the financial support
from the Ministry of Education of the Czech Republic (Grant
MSM0021622415 to JM).
0
-
respectively. It is to be highlighted that the anodic peaks are shifted
significantly towards more negative potentials while the cathodic
peaks remain almost invariant on changing the scan rates from
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