Four μ4-oxo-bridged copper(ii) complexes: Magnetic properties and catalytic applications in liquid phase partial oxidation reactions

Article abstract of DOI:10.1039/b913556a

Four copper(ii) complexes, [Cu₄(O)(Lⁿ) ₂(CH₃COO)₄] with N₂O-donor Schiff-base ligands, where HL¹ = 4-methyl-2,6- bis(cyclohexylmethyliminomethyl)phenol for complex 1, HL²

Full text of DOI:10.1039/b913556a

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PAPER
www.rsc.org/dalton | Dalton Transactions
Four l4-oxo-bridged copper(II) complexes: magnetic properties and catalytic
applications in liquid phase partial oxidation reactions†
Partha Roy,a,b Mahasweta Nandi,c,d Mario Manassero,e Mauro Ricco´, f Marcello Mazzani, f Asim Bhaumikc
and Pradyot Banerjee*b
Received 8th July 2009, Accepted 28th August 2009
First published as an Advance Article on the web 19th September 2009
DOI: 10.1039/b913556a
Four copper(II) complexes, [Cu4(O)(Ln)2(CH3COO)4] with N2O-donor Schiff-base ligands, where
HL1 = 4-methyl-2,6-bis(cyclohexylmethyliminomethyl)phenol for complex 1, HL2 = 4-methyl-2,6-
bis(phenylmethyliminomethyl)phenol for complex 2·CH3CN, HL3 = 4-methyl-2,6-bis(((3-tri-fluoro-
methyl)phenyl)methyliminomethyl)phenol for complex 3, HL4 = 4-methyl-2,6-bis(((4-tri-fluoro-
methyl)phenyl)methyliminomethyl)phenol for complex 4, were synthesized and characterized by
elemental analysis, FT-IR, UV-vis spectroscopy and finally by single crystal X-ray diffraction study.
X-Ray analysis reveals that all of these are m4-oxo-bridged tetrameric copper(II) complexes. Four copper
atoms arrange themselves around an oxygen atom tetrahedrally. Magnetic susceptibility measurements
show the existence of very strong antiferromagnetic coupling among these ions (J = -210.1 to
-271.3 cm-1), mediated by the oxygen atoms. Catalysis of the epoxidation of cyclohexene, styrene,
a-methylstyrene and trans-stilbene by these complexes has been carried out homogeneously as well as
heterogeneously by immobilizing the metal complexes over 2D-hexagonal mesoporous silica. The
results obtained in both the catalytic conditions show that the olefins are converted to the respective
epoxides in good yield together with high selectivity.
amply reported in the literature.17–19 Schiff-base complexes with
tetranuclear {Cu4(m4-O)} cores are described in research articles
Introduction
and studied extensively.14,20 The four copper atoms are arranged
in a distorted tetrahedron keeping the m4-O at the center. The
metals are also coordinated to phenoxide, halides or carboxylate
groups to get extra stability. A considerable amount of research
has been carried out to elucidate the magnetostructural relation-
ship. Magnetic properties of Cu(II) ions in multinuclear metal–
organic complexes are widely explored because copper(II) ions
produce variable and distorted coordination geometries and have
a simple electronic configuration.21,22 There are many reports on
the magnetic properties of multinuclear complexes with Cu(II).
Ray and co-workers have reported m4-O-bridged Cu(II) complexes
and studied their magnetic properties extensively. Temperature
dependent magnetic studies reveal that there are antiferromagnetic
interactions between the metal atoms.23,24
Multinuclear copper(II) complexes of Schiff-base ligands have
been drawing special attention from researchers for a few decades
because of their interesting properties as well as their applications
in the fields of magnetism, catalysis, biology etc.1–9 A dinucleating
Schiff-base ligand, a condensate of 4-methyl-2,6-diformylphenol
with 2-aminophenol, was reported by Robson for the first time
in 1970.10,11 Many examples of similar ligands of 4-methyl-2,6-
diformylphenol have been reported.12–16 Deprotonation of the
phenolic –OH group and then bridging of two metal ions
make them a potential dinucleating ligands. Hydroxo-, alkoxo-,
and phenoxo-bridged dicopper(II) complexes involving a Cu2O2
bridging moiety with their relevance to copper enzymes are
aDepartment of Chemistry, Jadavpur University, Jadavpur, Kolkata 700 032,
India
The transition metal complexes have been used as catalysts for
the epoxidation of alkenes in the past few decades.25,26 Epoxides are
versatile synthetic compounds, constituting convenient building
bDepartment of Inorganic Chemistry, Indian Association for the Cultivation
of Science, Jadavpur, Kolkata 700 032, India. E-mail: icpb@iacs.res.in;
Fax: +91 33 2473-2805
blocks for the synthesis of many products and fine chemicals.27,28
A
cDepartment of Materials Science, Indian Association for the Cultivation of
Science, Jadavpur, Kolkata 700 032, India
large number of publications concerns about the use of complexes
of the transition metals Mn, Fe and Ni, primarily as highly
enantioselective catalysts in the epoxidation of alkenes.29–40 Several
studies have focused on the preparation of the epoxides employing
transition metal complexes as catalysts in the presence of several
terminal oxidants such as NaOCl, peracid, tert-butyl hydroper-
oxide, hydrogen peroxide, molecular oxygen, etc. The reactions
with hydrogen peroxide are attractive from the environmental
view point41 since water is produced as the only waste product
when hydrogen peroxide is used as the oxidant. Catalytic reactions
may be carried out either homogeneously or heterogeneously. The
dDepartment of Integrated Science, Visva-Bharati University, Santiniketan,
731 235, India
eDipartimento di Chimica Strutturale
e Stereochimica Inorganica
dell’Universita` di Milano, Via G. Venezian 21, 20133, Milano, Italy
f
Dipartimento di Fisica, Universita` di Parma, Via G. Usberti, 7/a, 43100,
Parma, Italy
† Electronic supplementary information (ESI) available: A view of struc-
tures of complexes 2 and 3 (Fig. S1 and S2), temperature dependence
of magnetization for complex 2, 3 and 4 (Fig. S3, S4 and S5) and CIFs
for four complexes. CCDC reference numbers 724021–724024. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b913556a
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main problem in homogeneous catalysis is the separation of the
catalyst, substrate, oxidant and product(s) because these are in a
single phase with the solvent. Poor recycling efficiencies of the
catalysts are also noted in the case of homogeneous catalysis.
A catalyst with high recycling capacity would require minimum
effort for its separation and seems to be the most desirable one
in an industrial process. For this reason, attention has been
prompted on the use of heterogeneous catalysts for over the last
few decades.42–44 The heterogenization of transition metal-based
catalysts on different solid supports, e.g. silica,45,46 polymers,47
mesoporous materials,48–50 activated carbons,51,52 pillared clays,53
and zeolites,54 has been made effectively. The immobilization and
application of copper(II) complexes in heterogeneous catalysis
have not been extensively explored.55 Recently our group has
reported the synthesis and characterization of some Schiff-base
complexes of Ni(II)56 and Cu(II),57 and explored their catalytic
activity towards the oxidation of alkenes. The complexes immo-
bilized on mesoporous silica acted as efficient catalysts for the
catalytic reaction to yield epoxides as the major products.
spectrometer with samples prepared as KBr pellets. The ESI-MS
was recorded on Qtof Micro YA263 mass spectrometer. Magnetic
measurements were performed using a Quantum Design MPMS
XL SQUID magnetometer, capable of a field as high as 5 T,
working in the temperature range 5–300 K. Characterization
of the samples by powder X-Ray diffraction was performed
by using a Bruker AXS D8 Advanced SWAX diffractometer,
where the small and wide-angle goniometers are mounted. The
X-ray source was Cu-Ka radiation (a = 0.15406 nm) with an
applied voltage and current of 40 kV and 20 mA, respectively.
Mesophases of different samples were analyzed using a JEOL,
JEM 2010 transmission electron microscope at an accelerating
voltage of 200 kV. N2 adsorption measurements were carried
out using a Bel Japan Inc. Belsorp-HP surface area analyzer at
77 K. Pre-treatment of the samples was done at 473 K for 3 h
under high vacuum. A Shimadzu AA-6300 double beam atomic
absorption spectrophotometer (AAS) was used for determining
the percentage loading of Cu through wet chemical analysis.
Absorption spectra were studied on a Shimadzu UV 2100
spectrophotometer. UV-visible diffuse reflectance spectra (DRS)
for the immobilized catalysts were recorded on a Shimadzu
2401PC UV-visible spectrophotometer with an integrating
sphere attachment using BaSO4 as the background standard.
Gas chromatography analysis was performed with an Agilent
Technologies 6890 N network GC system equipped with a fused
silica capillary column (30 m ¥ 0.32 mm) and a FID detector. All
experiments were carried out in air and room temperature unless
reported otherwise.
We report here the synthesis, characterization, magnetic and
catalytic properties of four copper(II) complexes [Cu4(O)(Ln)2-
(CH3COO)4] with N2O-donor Schiff-base ligands, where
HL1
=
4-methyl-2,6-bis(cyclohexylmethyliminomethyl)phenol
for complex 1, HL2 = 4-methyl-2,6-bis(phenylmethylimino-
methyl)phenol for complex 2·CH3CN, HL3 = 4-methyl-2,6-
bis(((3-tri-fluoromethyl)phenyl)methyliminomethyl)phenol
for
complex 3, HL4 = 4-methyl-2,6-bis(((4-tri-fluoromethyl)phenyl)-
methyliminomethyl)phenol for complex 4. Single crystal X-
ray analysis has revealed that all complexes have similar
structures. Homogeneous catalytic reactions for the epoxidation
of cyclohexene, styrene, a-methylstyrene and trans-stilbene are
carried out using hydrogen peroxide as the oxidant in presence
of the complexes as catalysts. The complexes 1–4 have been
found to be highly active catalysts. The catalytic conversion could
be achieved heterogeneously. The heterogeneous catalysts have
been prepared by immobilizing the complexes on 2D-hexagonal
mesoporous silica. The immobilized catalyst is employed for
the epoxidation of cyclohexene, styrene, a-methylstyrene and
trans-stilbene using hydrogen peroxide as the oxidant. The results
show that the conversion of the alkene occurs with high yield and
high selectivity towards the corresponding epoxide.
The Schiff-base ligands, 4-methyl-2,6-bis(cyclohexylmethyl-
iminomethyl)phenol (HL1)16 and 4-methyl-2,6-bis(phenylmethyl-
iminomethyl)phenol (HL2),60,61 were synthesized following the
published procedures.
Synthesis of 4-methyl-2,6-bis(((3-tri-fluoromethyl)phenyl)-
methyliminomethyl)phenol (HL3) and 4-methyl-2,6-bis(((4-tri-
fluoromethyl)phenyl)methyliminomethyl)phenol (HL4)
These two ligands were prepared following the same pro-
cedure with a slight modification.60 To a solution of 4-
methyl-2,6-diformylphenol (0.656 g,
4 mmol) in 15 mL
of acetonitrile was added the respective amine (0.857 g,
8 mmol) (3-trifluoromethyl-1-phenylmethanamine for HL3 and
4-trifluoromethyl-1-phenylmethanamine for HL4) in 10 mL ace-
tonitrile. The reaction mixture was refluxed for 3 h. The solution
was filtered, concentrated on a rotary evaporator to dryness and
kept overnight at 4 ◦C. The resulting Schiff base is solid and it has
been recrystallized from acetonitrile.
Data for HL3. Yield = 1.68 g, 88%. Found: C, 62.70; H, 4.16;
N, 5.90%; C25H20N2OF6 requires C, 62.76; H, 4.21; N, 5.86%; FT-
IR (KBr phase) nmax/cm-1: 2921 (CH), 1639 (CN); dH (300 MHz,
CDCl3, Me4Si) 2.32 (3H, s, Ar-CH3), 4.86 (4H, s, CH), 7.32–
Experimental
Materials and physical methods
Cyclohexylmethylamine, 1-phenylmethanamine, 3-trifluorome-
thyl-1-phenylmethanamine, 4-trifluoromethyl-1-phenylmethan-
amine, cyclohexene, styrene, a-methylstyrene, trans-stilbene,
cycloheptanone, copper(II) acetate monohydrate, tetramethyl-
ammonium hydroxide (TMAOH) and tetraethyl orthosilicate
(TEOS) were purchased from Aldrich and used without
purification. Other reagents were purchased from commercial
sources and used without further purification. 4-Methyl-
=
7.52 (10H, m, Ar-CH), 8.70 (2H, s, HC N). m/z (ESI) 479.4370
(C25H21N2OF6 requires 479.4375).
+
Data for HL4. Yield = 1.72 g, 90%. Found: C, 62.71; H, 4.15;
N, 5.82%; C25H20N2OF6 requires C, 62.76; H, 4.21; N, 5.86.%; FT-
IR (KBr phase) nmax/cm-1: 2896 (CH), 1643 (CN); dH (300 MHz,
CDCl3, Me4Si) 2.31 (3H, s, Ar-CH3), 4.87 (4H, s, CH), 7.36–
2,6-diformylphenol was synthesized following
a published
procedure.58 Solvents used for spectroscopic studies were purified
and dried by standard procedures before use.59 Elemental 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
=
7.62 (10H, m, Ar-CH), 8.69 (2H, s, HC N). m/z (ESI) 479.4371
(C25H21N2OF6 requires 479.4375).
+
9544 | Dalton Trans., 2009, 9543–9554
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Table 1 Crystal data for complex 1, 2·CH3CN, 3 and 4
Complex
1
2·CH3CN
3
4
Empirical formula
Formula weight
Crystal system
Space group
Color
C54H78Cu4N4O11
1213.40
Tetragonal
I41/a
C56H57Cu4N5O11
1230.27
C58H50Cu4F12N4O11
1461.21
C58H50Cu4F12N4O11
1461.21
Triclinic
Triclinic
Triclinic
¯
¯
¯
P1
P1
P1
Green
Green
Green
Green
˚
a/A
13.2184(7)
13.2184
34.169(2)
90
90
90
5970.2(6)
4
1.350
0.15 ¥ 0.15 ¥ 0.30
1.465
0.857, 1.000
2536
296
0.71073
30.00, 3.00
47 777, 4866
0.0410
12.6557(7)
14.5524(8)
15.7123(9)
109.029(1)
91.840(1)
92.010(1)
2731.0(16)
2
1.496
0.10 ¥ 0.50 ¥ 0.50
1.603
0.748, 1.000
1264
150
0.71073
28.00, 3.00
50 607, 13 996
0.0226
12.5687(7)
15.4115(9)
17.0077(10)
111.009(1)
96.640(1)
95.800(1)
3018.5(3)
2
1.608
0.15 ¥ 0.25 ¥ 0.45
1.490
0.833, 1.000
1476
296
0.71073
28.00, 3.00
36 534, 14 676
0.0314
12.4065(7)
15.4152(8)
16.9396(9)
99.450(2)
91.150(2)
93.010(2)
3190.0(3)
2
1.521
0.24 ¥ 0.30 ¥ 0.45
1.410
0.876, 1.000
1476
296
0.71073
29.00, 3.00
61 577, 22 297
0.0402
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
Dcalc (g cm-3)
Crystal dimensions/mm
m(Mo-Ka)/mm-1
Min. and max. transmission factors
F(000)
T/K
l(Mo-Ka)
Max. and min. q/◦
Total data, unique data
Rint
Reciprocal space explored
Final R2 and R2w indicesa (F2, all reflections)
Conventional R1 index (I > 2s(I))
Reflections with (I > 2s(I))
No. of variables
Full sphere
0.064, 0.093
0.035
2575
190
Full sphere
0.047, 0.080
0.028
11 002
685
Full sphere
0.078, 0.117
0.048
7998
802
Full sphere
0.106, 0.145
0.051
8481
802
Goodness-of-fitb
0.979
1.069
1.070
0.961
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2
2
b
2
2
a R2 = [ (|Fo - kFc2|/ Fo2], R2w = [ w(Fo - kFc2)2/ w(Fo2)2]1/2
.
[
w(Fo - kFc2)2/(No - Nv)]1/2, where w = 4Fo2/s(Fo2)2, s(Fo2) = [s (Fo2) +
(0.04Fo2)2]1/2, No is the number of observations, and Nv the number of variables.
Syntheses of complexes 1, 2·CH3CN, 3 and 4
collected frames were processed with the software SAINT,62 and
an empirical absorption correction was applied (SADABS63) to
the collected reflections. The calculations were performed using
the Personal Structure Determination Package64 and the physical
constants tabulated therein.65 The structures were solved by direct
Complexes 1, 2·CH3CN, 3 and 4 were synthesized using a common
procedure. Typically, to an acetonitrile solution (10 mL) of ligand
(0.3 mmol), 0.106 g for HL1, 0.102 g for HL2, 0.144 g for HL3
and 0.144 g for HL4) was added copper(II) acetate monohydrate
(0.6 mmol, 0.120 g). The mixture was stirred for 45 min. The color
of the mixture was green which was refluxed for 1 h. It was cooled
and filtered. The filtrate was kept at ambient temperature. Green
single crystals suitable for X-ray diffraction study were produced
within a few days.
Data for 1. Yield = 0.36 g, 70%. Found: C, 53.38; H, 6.44; N,
4.57%; C54H78Cu4N4O11 requires C, 53.45; H, 6.48; N, 4.62%.
Data for 2·CH3CN. Yield = 0.24 g, 65%. Found: C, 54.58; H,
6.64; N, 4.57%; C56H57Cu4N5O11 requires C, 54.68; H, 4.67; N,
4.55%.
methods (SHELXS66) and refined by full-matrix least-squares
ꢀ
using all reflections and minimising the function w(Fo - kFc )2
(refinement on F2). In compound 1 the three toluylic hydro-
gen atoms (which are disordered by crystallographic symmetry)
bonded to atom C5 were not detected in the final Fourier maps
and were ignored. Again in compound 1, the hydrogen atom
H15, bonded to C6, and the three methylic hydrogen atoms of
the acetato ligand, were detected in the final Fourier maps and
included in the structure factor calculations, but not refined. In
compounds, 2·CH3CN, 3, and 4, all the hydrogen atoms of the
CH3 groups either toluylic or belonging to an acetato ligand
or to CH3CN, were detected in the final Fourier maps, and
included in the structure factor calculations, but not refined. All
the other hydrogen atoms of four complexes were placed in their
2
2
Data for 3. Yield = 0.32 g, 72%. Found: C, 47.61; H, 3.43; N,
3.88%; C58H50Cu4F12N4O11 requires C, 47.68; H, 3.45; N, 3.83%.
Data for 4. Yield = 0.33 g, 75%. Found: C, 47.63; H, 3.40; N,
3.88%; C58H50Cu4F12N4O11 requires C, 47.68; H, 3.45; N, 3.83%.
˚
ideal positions (C–H = 0.97 A), with the thermal parameter U
being 1.10 times that of the atom to which they are attached,
and included in the structure factor calculations, but not refined.
All the non-hydrogen atoms of the four complexes were refined
X-Ray data collections and structure determinations
with anisotropic thermal parameters. In the final Fourier maps
Crystal data of complexes 1, 2·CH3CN, 3 and 4 are summarized
in Table 1. The diffraction experiments were carried out on a
Bruker SMART CCD area-detector diffractometer at 296 K for
complexes 1, 3 and 4, 150 K for 2·CH3CN. No crystal decay
was observed, so that no time-decay correction was needed. The
-3
˚
˚
the maximum residuals were 0.98(23) e A at 0.08 A from Cu1,
-3
-3
˚
˚
-3
˚
˚
0.49(12) e A at 0.85 A from Cu1, 1.33(27) e A at 1.13 A from
˚
˚
O10, and 1.44(54) e A at 0.53 A from Cu4, in compounds 1,
2·CH3CN, 3, and 4, respectively. Minimum peaks (holes) in the
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final Fourier maps of the four compounds, in the same order, were
of hydrogen peroxide (equimolar with respect to the substrate).
Aliquots from reaction mixtures were collected at regular intervals.
After cooling, cycloheptanone as the internal standard and 10 ml
of diethyl ether for extracting the reactants and products were
added. The substrate and product(s) from the reaction mixture
were analyzed by gas chromatography. They were identified by the
comparison with known standards.
-3
-3
-3
˚
˚
˚
-0.42(23) e A , -0.45(12) e A , -1.04(27) e A , and -0.83(54)
-3
˚
e A , respectively.
CCDC 724021, 724022, 724023 and 724024 contain the supple-
mentary crystallographic data of complexes 1, 2·CH3CN, 3 and 4,
respectively. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b913556a.
A blank experiment for the epoxidation of cyclohexene, as
the representative case, was carried out homogeneously and
heterogeneously without addition of any catalyst under the same
experimental conditions.
Preparation of the immobilized catalysts
Highly ordered 2D-hexagonal mesoporous silica was synthe-
sized using a cationic (cetyltrimethylammonium bromide, CTAB)
and non-ionic (Brij 35, C12H25–(OC2H4)23–OH, a polyether and
aliphatic hydrocarbon chain surfactant) mixed surfactant system
as the self-assembled structure directing agent (SDA) in the pres-
ence of tartaric acid (TA) as a mineralizer. Tetraethyl orthosilicate
(TEOS) was used as the silica source. In a typical synthesis,
4.44 g CTAB and 2.5 g Brij 35 were dissolved in an acidic
aqueous solution of TA (1.17 g in 60.0 g of H2O) under vigorous
stirring at room temperature for 1 h. This was followed by the
addition of 7.0 g TEOS under continuous stirring. After 2 h of
continuous stirring, tetramethylammonium hydroxide (TMAOH,
25% aqueous) was added dropwise and the pH was maintained at
ca. 11.2. The resulting mixture was aged overnight under stirring
at room temperature and then treated hydrothermally at 353 K for
72 h without stirring. The solid product was collected by filtration,
washed several times with water and dried under vacuum at room
temperature. The resulting powder was calcined in the flow of air
at 723 K for 8 h to remove all the organic surfactants.
Results and discussion
Synthesis and IR spectra
The ligands have been prepared by simple Schiff-base conden-
sation between one equivalent of 4-methyl-2,6-diformylphenol
and two equivalent of the respective amine in acetonitrile. The
complexes have been synthesized by the reaction between the
ligand and copper(II) acetate monohydrate in acetonitrile. Acetate
ion from copper(II) acetate may deprotonate the phenolic proton
of ligand. The acetate moieties are coordinated to the metal atom
to give added stability to the complex.
FT-IR spectra of both the ligands and complexes were obtained
with samples prepared as KBr pellets. FT-IR spectra of the ligands
show a number of strong nC–H bands at 2800–3000 cm-1. The
complexes also show strong nC–H bands in this range.67,68 HL1,
HL2, HL3 and HL4 show strong bands at 1637, 1635, 1639 and
1643 cm-1 respectively, and these bands may be assigned to the
Immobilization of the metal complexes was carried out by
dispersing 0.5 g of the mesoporous silica in a solution containing
0.1 g of the metal complex dissolved in 25 ml of dry acetonitrile,
followed by vigorous stirring at room temperature for 4 h. Then
the solid was filtered, washed with acetonitrile and dried under
vacuum.
=
C N bond. The complexes 1, 2·CH3CN, 3 and 4 show IR bands
at 1629, 1625, 1630 and 1632 cm-1 respectively confirming the
=
retention of C N bond in the complex. The complexes show a
sharp band of medium intensity at around 560 to 570 cm-1 for
the characteristic T2 mode of the Cu4O core.69 The spectra of the
ligands miss these bands as expected.
Heterogeneous catalysis
Description of crystal structures of complexes
For the heterogeneous oxidation of olefin, the reactions were
performed in a magnetically stirred two necked round-bottomed
flask fitted with a condenser and placed in a temperature-
controlled oil bath. Typically, 0.5 g of the substrate was taken
in 5 ml acetonitrile (solvent), followed by the addition of 0.1 g of
the immobilized catalyst and the mixture was then preheated to
333 K. The reaction was started with the addition of hydrogen
peroxide (equimolar with respect to the substrate). Aliquots from
reaction mixtures were collected at regular intervals. After cooling,
cycloheptanone was added as the internal standard. The substrate
and product(s) from the reaction mixture were analyzed by gas
chromatography. They were identified by the comparison with
known standards.
Complex 1 crystallizes in the tetragonal space group I41/a from
acetonitrile. A perspective view of the molecule is shown in
Fig. 1. Selected bond lengths and bond angles are listed in
Table 2. The molecule has crystallographic site symmetry 4-. It is a
discrete tetrametallic monomer, with only one copper atom in the
asymmetric unit. Atom O1 lies in a site having crystallographic
symmetry 4-, and is bonded to four copper atoms in a slightly
distorted tetrahedral arrangement. The Cu atom is coordinated to
a m4-oxido oxygen atom O1, one m2-phenoxido oxygen atom O2
and one nitrogen atom N1 of the dinucleating ligand, 4-methyl-
2,6-bis(cyclohexylmethyliminomethyl)-phenolate (L1-), and two
acetato oxygen atoms O3 and O4 from two different acetato
Homogeneous catalysis
◦
˚
Table 2 Selected bond lengths (A) and bond angles ( ) of complex 1
The homogeneous catalysis was carried out at 298 K by a process
similar to that adopted for heterogeneous catalysis. Typically,
0.5 g of the substrate was taken in a magnetically stirred two
necked round-bottomed flask fitted with a condenser in 5 ml
acetonitrile, followed by the addition of 0.01 mmol of the complex
at room temperature. The reaction was started with the addition
Cu–O1
Cu–O3
1.919(1)
1.942(1)
Cu–O2
Cu–N1
1.975(1)
1.979(2)
O1–Cu–O2
O1–Cu–N1
O2–Cu–N1
78.4(1)
164.0(1)
90.9(1)
O1–Cu–O3
O2–Cu–O3
O3–Cu–N1
94.8(1)
172.1(1)
96.6(1)
9546 | Dalton Trans., 2009, 9543–9554
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◦
˚
Table 3 Selected bond lengths (A) and bond angles ( ) of complex
2·CH3CN
Cu1–O1
Cu1–O2
Cu1–O4
Cu1–O6
Cu1–N1
Cu2–O1
Cu2–O2
Cu2–O10
Cu2–N2
1.937(1)
1.983(1)
1.917(1)
2.274(1)
2.000(2)
1.910(1)
1.974(1)
1.932(1)
1.976(1)
Cu3–O1
Cu3–O3
Cu3–O5
Cu3–O7
Cu3–N3
Cu4–O1
Cu4–O3
Cu4–O9
Cu4–N4
1.924(1)
1.987(1)
2.221(1)
1.926(1)
1.998(2)
1.920(1)
1.968(1)
1.927(1)
1.963(1)
O1–Cu1–O2
O1–Cu1–O4
O1–Cu1–O6
O1–Cu1–N1
O2–Cu1–O4
O2–Cu1–O6
O2–Cu1–N1
O4–Cu1–O6
O4–Cu1–N1
O6–Cu1–N1
O1–Cu2–O2
O1–Cu2–O10
O1–Cu2–N2
O2–Cu2–O10
O2–Cu2–N2
O10–Cu2–N2
O1–Cu3–O3
O1–Cu3–O5
O1–Cu3–O7
79.9(1)
96.5(1)
92.2(1)
163.5(1)
170.1(1)
86.3(1)
90.0(1)
103.1(1)
91.2(1)
100.3(1)
80.8(1)
94.6(1)
168.5(1)
168.4(1)
89.5(1)
93.8(1)
79.8(1)
99.7(1)
95.5(1)
O1–Cu3–N3
O3–Cu3–O5
O3–Cu3–O7
O3–Cu3–N3
O5–Cu3–O7
O5–Cu3–N3
O7–Cu3–N3
O1–Cu4–O3
O1–Cu4–O9
O1–Cu4–N4
O3–Cu4–O9
O3–Cu4–N4
O9–Cu4–N4
Cu1–O1–Cu2
Cu1–O1–Cu3
Cu1–O1–Cu4
Cu2–O1–Cu3
Cu2–O1–Cu4
Cu3–O1–Cu4
164.2(1)
90.7(1)
166.2(1)
89.6(1)
102.8(1)
92.0(1)
92.2(1)
80.3(1)
93.2(1)
170.9(1)
161.8(1)
91.4(1)
93.5(1)
101.4(1)
111.9(1)
114.3(1)
119.7(1)
108.2(1)
101.8(1)
Fig. 1 A perspective view of complex 1. Hydrogen atoms omitted for
clarity.
moieties. The copper atom is in a distorted square pyramidal
geometry as revealed by the trigonal index, t, which is found to be
0.135. The value of t is defined as the difference between the two
largest donor–metal–donor angles divided by 60, a value which is
0 for the ideal square pyramid and 1 for the trigonal bipyramid.70
The O1, O2, N1 and O3 atoms form the basal plane of the square
pyramid, whereas O4 occupies the apical position. The Cu–O4
bond length is quite long compared to the other Cu–donor bond
distances. The Cu–donor bond lengths are in good agreement with
reported values.8,9,14
◦
˚
Table 4 Selected bond lengths (A) and bond angles ( ) of complex 3
Cu1–O1
Cu1–O2
Cu1–O4
Cu1–O6
Cu1–N1
Cu2–O1
Cu2–O2
Cu2–O8
Cu2–O10
Cu2–N2
1.910(2)
1.990(2)
2.328(2)
1.943(2)
1.974(3)
1.924(2)
1.984(3)
1.924(3)
2.288(3)
1.983(2)
Cu3–O1
Cu3–O3
Cu3–O5
Cu3–O7
Cu3–N3
Cu4–O1
Cu4–O3
Cu4–O9
Cu4–O11
Cu4–N4
1.918(2)
1.969(2)
1.941(2)
2.340(3)
2.000(3)
1.919(2)
1.973(2)
2.427(3)
1.960(3)
1.971(2)
Complexes 2·CH3CN, 3 and 4 crystallize in the same space
¯
group, triclinic (P1). Perspective views of the complexes are
shown in the ESI† (Fig. S1, S2) and Fig. 2, respectively.
Selected bond lengths and bond angles of complexes 2·CH3CN,
3 and 4 are given in Table 3, 4 and 5, respectively. Copper
and donor atom connectivities are the same for all the
complexes, so descriptions of the crystal structures of the three
O1–Cu1–O2
O1–Cu1–O4
O1–Cu1–O6
O1–Cu1–N1
O2–Cu1–O4
O2–Cu1–O6
O2–Cu1–N1
O4–Cu1–O6
O4–Cu1–N1
O6–Cu1–N1
O1–Cu2–O2
O1–Cu2–O8
O1–Cu2–O10
O1–Cu2–N2
O2–Cu2–O8
O2–Cu2–O10
O2–Cu2–N2
O8–Cu2–O10
O8–Cu2–N2
O10–Cu2–N2
O1–Cu3–O3
O1–Cu3–O5
O1–Cu3–O7
79.2(1)
95.6(1)
95.0(1)
169.4(1)
87.8(1)
166.3(1)
91.0(1)
105.2(1)
87.7(1)
93.8(1)
79.0(1)
93.8(1)
96.0(1)
168.3(1)
162.3(1)
87.5(1)
90.8(1)
109.5(1)
94.4(1)
89.1(1)
79.2(1)
95.0(1)
97.7(1)
O1–Cu3–N3
O3–Cu3–O5
O3–Cu3–O7
O3–Cu3–N3
O5–Cu3–O7
O5–Cu3–N3
O7–Cu3–N3
O1–Cu4–O3
O1–Cu4–O9
O1–Cu4–O11
O1–Cu4–N4
O3–Cu4–O9
O3–Cu4–O11
O3–Cu4–N4
O9–Cu4–O11
O9–Cu4–N4
O11–Cu4–N4
Cu1–O1–Cu2
Cu1–O1–Cu3
Cu1–O1–Cu4
Cu2–O1–Cu3
Cu2–O1–Cu4
Cu3–O1–Cu4
161.3(1)
172.6(1)
84.6(1)
90.4(1)
100.8(1)
94.0(1)
96.7(1)
79.0(1)
90.0(1)
93.6(1)
165.1(1)
80.5(1)
163.8(1)
90.6(1)
114.2(1)
98.9(1)
93.6(1)
103.4(1)
111.2(1)
111.3(1)
119.6(1)
108.7(1)
102.8(1)
Fig. 2 A perspective view of complex 4. Hydrogen atoms were omitted
for clarity.
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◦
˚
Table 5 Selected bond lengths (A) and bond angles ( ) of complex 4
Cu1–O1
Cu1–O2
Cu1–O6
Cu1–N1
Cu2–O1
Cu2–O2
Cu2–O8
Cu2–O10
Cu2–N2
1.910(2)
1.980(2)
1.927(2)
1.979(3)
1.923(2)
1.983(2)
1.942(2)
2.261(3)
1.982(2)
Cu3–O1
Cu3–O3
Cu3–O5
Cu3–N3
Cu4–O1
Cu4–O3
Cu4–O9
Cu4–O11
Cu4–N4
1.915(2)
1.979(2)
1.929(2)
1.989(3)
1.925(2)
1.977(2)
2.271(2)
1.929(3)
1.995(2)
O1–Cu1–O2
O1–Cu1–O6
O1–Cu1–N1
O2–Cu1–O6
O2–Cu1–N1
O6–Cu1–N1
O1–Cu2–O2
O1–Cu2–O8
O1–Cu2–O10
O1–Cu2–N2
O2–Cu2–O8
O2–Cu2–O10
O2 Cu2 N2
O8 Cu2 O10
O8 Cu2 N2
O10 Cu2 N2
Cu1 O1 Cu2
Cu1 O1 Cu4
Cu2 O1 Cu4
80.1(1)
93.4(1)
169.0(1)
164.7(1)
90.6(1)
94.2(1)
79.7(1)
95.4(1)
97.2(1)
164.5(1)
170.0(1)
89.3(1)
89.9(1)
99.9(1)
93.1(1)
94.1(1)
102.4(1)
116.1(1)
110.8(1)
O1–Cu3–O3
O1–Cu3–O5
O1–Cu3–N3
O3–Cu3–O5
O3–Cu3–N3
O5–Cu3–N3
O1–Cu4–O3
O1–Cu4–O9
O1–Cu4–O11
O1–Cu4–N4
O3–Cu4–O9
O3–Cu4–O11
O3–Cu4–N4
O9–Cu4–O11
O9–Cu4–N4
O11–Cu4–N4
Cu1–O1–Cu3
Cu2–O1–Cu3
Cu3–O1–Cu4
79.7(1)
94.6(1)
166.9(1)
167.2(1)
90.5(1)
93.1(1)
79.5(1)
95.8(1)
94.7(1)
164.5(1)
85.3(1)
168.1(1)
90.3(1)
105.8(1)
95.0(1)
93.0(1)
109.9(1)
115.8(1)
102.4(1)
Scheme 1 R = phenyl for complex 2·CH3CN, (3-trifluoromethyl)phenyl
for complex 3 and (4-trifluoromethyl)phenyl for complex 4.
compared to other Cu1–donor bond length values. The Cu4 atom
is coordinated to the m4-oxido oxygen atom O1, one m2-phenoxido
oxygen atom O3 and one nitrogen atom N4 of the dinucleating
ligand, and two oxygen atoms O9 and O11 from two different
acetato ions. This atom is also in a distorted square pyramidal
geometry. Cu1–donor bond lengths are in good agreement with
the reported one.8,9,14
complexes are made in a general manner. The metal and
donor connectivities are shown in Scheme 1. These are discrete
tetranuclear complexes. The asymmetric unit consists of four
copper atoms, two N2O-donor dinucleating ligands, 4-methyl-
2,6-bis(phenylmethyliminomethyl)phenolate (L2-) for complex
2·CH3CN, 4-methyl-2,6-bis(((3-tri-fluoromethyl)phenyl)methyl-
iminomethyl)phenolate (L3-) for complex
3 and 4-methyl-
Magnetic properties of the complexes
2,6-bis(((4-tri-fluoromethyl)phenyl)methyliminomethyl)pheno-
late (L4-) for complex 4, one m4-oxido and four acetato ions. Cu1
is bonded to one m4-oxido oxygen atom O1, one m2-phenoxido
oxygen atom O2 and one nitrogen atom N1 of the same
dinucleating ligand, and two oxygen atoms O4 and O6 from two
different acetato ions. The Cu1 atom is in a distorted square
pyramidal geometry as revealed by the trigonal index value listed
in Table 6. Cu1–Oapical bond lengths are long compared to other
Cu1–donor bond distances. The Cu2 atom is linked to one oxygen
atom O1, one m2-phenoxido oxygen atom O2 and one nitrogen
atom N2 of the same dinucleating ligand, and two oxygen atoms
O8 and O10 from two different acetato ions. This copper atom
is also in a distorted square pyramidal geometry (Table 6). The
Cu3 atom is attached to one oxygen atom O1, one m2-phenoxido
oxygen atom O3 and one nitrogen atom N3 of the dinucleating
ligand, two oxygen atoms O5 and O7 from two different acetato
ions. These donor atoms make a square pyramidal environment
around Cu3. Here all the Cu1–Oapical bond lengths are longer
DC SQUID magnetometry, performed on powder samples, gave
similar results in all of the complexes. A representative curve,
recorded for complex 1, displaying the temperature dependence of
magnetization is shown in Fig. 3. Similar plots for the temperature
Table 6 Trigonal index values of the complexes 2·CH3CN, 3 and 4
Complex
Cu1
Cu2
Cu3
Cu4
2·CH3CN
0.110
0.052
0.072
0.002
0.100
0.092
0.033
0.188
0.005
0.152
0.022
0.045
Fig. 3 Temperature dependence of magnetization for complex 1, mea-
sured in a magnetic field of 1 kOe. A strong antiferromagnetic coupling,
persistent up to 325 K, characterizes the (Cu4–O) cluster.
3
4
9548 | Dalton Trans., 2009, 9543–9554
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dependence of magnetization of complexes 2–4 are given in
the ESI† (Fig. S3, S4 and S5). The oxygen-mediated exchange
interaction between Cu ions is known to be relatively strong and
of antiferromagnetic type. Thus the magnetic behavior of the
(Cu4–O) cluster, taking into account its symmetry point group, can
be properly described by an Heisenberg Hamiltonian including
three different coupling constants. Unfortunately the resulting
expression for the antiferromagnetic susceptibility18,71,72 is not
suitable for least square analysis of the data, as the three coupling
constants highly correlate. An approximate isotropic model shall
then be introduced: in the regime of strong antiferromagnetic
(AFM) coupling, only the ground state S = 0 and the first excited
state S = 1 are accessible at temperatures up to 300 K. Con-
sequently the Bleaney–Bowers formula,73 valid for a two-state
magnetic system, can be adopted. The model function for least-
square analysis is then written as:
⎛
⎜
⎜
⎞
2J
⎟
⎟
⎟
⎟
⎠
Fig. 4 Low angle X-ray diffraction patterns of (a) mesoporous silica and
(b) immobilized complex 1 (IC-1) (representative case).
exp
⎜
⎜
⎝
k T
C
T
3
rC
B
c(T ) = 6
+
+ c0.
⎛
⎞
4 T −Tc
2J
⎟
⎟
⎟
⎟
⎠
⎜
⎜
1+ 3exp
⎜
⎜
⎝
k T
B
Apart from the major AFM contribution, it includes a Curie–
Weiss component, related to a small fraction r of paramagnetic
impurities (typically 1%, except for complex 3 which contains a
much higher paramagnetic impurities concentration, 27.5%), and
a temperature-independent susceptibility c0, which accounts for
ferromagnetic impurities, the quartz sample holder contribution
and possible Van Vleck paramagnetism.
The estimated values of the antiferromagnetic coupling constant
are J = -210.1 0.1 cm-1 for complex 4, J = -219.9 0.2 cm-1
for 2, J = -227.2 0.1 cm-1 for complex 3, J = -271.3 0.2 cm-1
for complex 1. These values are in good agreement with data
previously reported for similar systems18,72 and their distribution
is likely to be due to distortions induced in the cluster geometry
by the different ligands which characterize each complex.
The amplitude of the signal, which for complex 1 is C = -1.60
0.01 emu K mol-1 Oe-1, corresponds to slightly less than the
expected value of 4 mB per formula unit. Such a reduction is
likely to be due to a small magnetic anisotropy term, suggested
by the cluster structure, which was not taken into account in the
calculations as its inclusion would largely increase the complexity
of the susceptibility calculations.74
Fig. 5 N2 adsorption/desorption isotherms for (a) mesoporous silica
material and (b) complex 1 immobilized on the silica. Inset: Pore size
distributions of (a) mesoporous silica material and (b) immobilized
complex 1.
diameter for the 2D-hexagonal mesoporous host (HMS) was 1438
m2 g-1 and 2.8 nm, respectively. The BJH pore-size distribution
suggested a very narrow range, centered at 2.8 nm for this 2D-
hexagonal mesoporous material (inset of Fig. 5). Adsorption
studies were also carried out for the complex immobilized samples.
The surface area and pore diameters for immobilized samples IC-
1, immobilized complex 2·CH3CN (IC-2), immobilized complex
3 (IC-3) and immobilized complex 4 (IC-4) were 1055 m2 g-1 and
2.49 nm; 1025 m2 g-1 and 2.47 nm; 1005 m2 g-1 and 2.43 nm;
1010 m2 g-1 and 2.44 nm, respectively. The surface areas for all
the samples were found to be somewhat less than that of the
HMS sample and also a decrease in the pore diameter could be
observed. The decrease in pore diameter can be attributed to the
metal centers adhered to the inner wall of the pores, which put
forth a reducing effect on the pore size. Pore wall thickness is quite
slim, which might help the catalytic sites to be located closer at
the surface of the catalyst. HRTEM image of the as-synthesized
sample is shown in the ESI† (Fig. S6) and it reveals hexagonal
Powder X-ray diffraction and nitrogen sorption studies
Low angle X-ray diffraction patterns of the mesoporous host as
well as the one of the representative immobilized complex 1 (IC-1)
are shown in Fig. 4. All four peaks for 100, 110, 200 and 210 planes
of the 2D hexagonal mesophase75 were observed for the calcined
mesoporous silica sample (a) and the immobilized complexes 1 (b),
suggesting the retention of highly ordered structures even after the
loading of the metal complex.
In the N2 sorption isotherms for mesoporous host and the
immobilized complexes, typical type IV isotherms with a steep
rise due to capillary condensations, characteristic of mesoporous
materials,75,76 were seen. N2 adsorption/desorption isotherms for
mesoporous silica material and complex 1 immobilized on the
silica are shown in Fig. 5. BET surface area and average pore
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arrangement of the pores with different contrast than that of the
pore walls. The average pore diameter for this sample agrees well
with the N2 sorption data. Electron diffraction pattern shown in
the inset of this figure further suggested hexagonal arrangement
of the pore channels.
Similarity in the UV-vis spectra suggests loading of the metal
complex on the mesoporous materials (see later). Chemical analy-
sis (atomic absorption spectroscopy, AAS) data further indicated
6.7, 7.1, 6.5 and 6.6 wt% of copper is present the mesoporous host
for complexes 1, 2·CH3CN, 3 and 4 respectively.
conversion. In the case a-methylstyrene IC-3 is the most active
catalyst with 76% conversion. As seen from the table, TOFs for
the immobilized catalysts have been enhanced drastically vis-a`-
vis the metal complexes without any loss in epoxide selectivity.
Cu–hydroperoxo species could be formed at the active sites in
the presence of hydrogen peroxide as the oxidant (see later).
Hydrophobic aromatic ligands attached to Cu centers in the
immobilized catalyst could provide the affinity towards the olefinic
substrates. This may facilitate the adsorption of the substrates near
the vicinity of the active sites and their subsequent epoxidations.
This could be responsible for high TOFs in these liquid phase
partial oxidation reactions.
Heterogeneous catalysis
We have examined whether the heterogeneous catalysts can be
used further or not. For this purpose, one control experiment
has been performed for cyclohexene (as a representative case)
over immobilized catalyst to check the subsequent efficiency
of catalytic cycles and also to check whether Cu leaches out
from the catalyst. We have selected heterogeneous epoxidation
of cyclohexene by IC-4 as the representative case because it can be
clearly seen in Table 7 that the maximum conversion of substrate
can be achieved with this combination. The catalyst has been
recovered by filtration, washed thoroughly with acetonitrile and
then treated with 0.1 M HCl solution in ethanol for 8 h at 70 ◦C
and finally dried at 100 ◦C for 2 h. The catalytic reaction has
been performed following the same experimental procedure. The
activity of the catalyst is decreased by a small amount in the
subsequent cycles. It has been seen after repeating the catalytic
cycles that the catalyst can be reused for at least five times without
significant loss of activity (Table 7). In Fig. 6 we have plotted
the catalytic activity in five consecutive cycles for the oxidation of
cyclohexene over IC-4. As seen from this bar diagram, the catalytic
activity decreases marginally in the successive cycles. However,
epoxide selectivity remains almost the same in these repetitive
reactions (Table 7). Any Cu species present in the reaction mixture
could catalyze the olefin epoxidation. Hence, atomic absorption
spectroscopy has been used to determine the amount of copper
leached out into the reaction mixture and it was found that
there is almost no Cu leaching into the liquid phase from these
immobilized catalysts. A blank reaction in the absence of any
immobilized catalyst under identical experimental conditions has
The epoxidation reactions of cyclohexene, styrene, a-methyl-
styrene and trans-stilbene were carried out heterogeneously with
complexes 1–4 as the catalysts. In Table 7, results of the cat-
alytic activities of four different Cu complexes immobilized over
2D-hexagonal mesoporous silica material are shown. Acetonitrile
has been used as the solvent in all these liquid phase oxidation
reactions. The major products for the partial oxidation of all four
olefins are their respective epoxides. It is clear from the Table 7 that
the selectivity of the epoxide for all the substrates is quite high. The
conversion of the substrate enhances with time and after 24 h
the conversion reaches its saturation. However, a few hours after
the conversions reaching the respective maxima, epoxide selec-
tivities go down considerably and their respective diol selectivity
goes up. This could be attributed to the hydrolysis of the epoxides,
which is quite common for the liquid phase partial oxidation of
olefin over a heterogeneous catalyst77,78 as water coming from the
oxidant in the reaction mixture promotes the epoxide ring opening.
In Table 7, turnover frequencies (TOFs) for different catalytic
runs are also given. TOF for cyclohexene conversion is relatively
more than that for styrene, a-methylstyrene and trans-stilbene.
Among the immobilized catalysts used in this study, IC-4 shows
maximum reactivity in the conversion of cyclohexene and trans-
stilbene with 90 and 77% yields respectively. IC-2 is found to be
the most effective catalyst for the epoxidation of styrene with 78%
Table 7 Epoxidation of olefins over the immobilized Cu(II) complexesa
Substrate
IC-1 IC-2 IC-3 IC-4 IC-4d Blanke
Cyclohexene
Conversionb 78
85
87
88
92
90
87
78
90
30
26
Selectivity
90
TOFc
12.0 13.3 17.0 12.0
Styrene
Conversionb 75
78
88
73
85
67
90
Selectivity
91
TOFc
9.1 9.6 11.1 13.7
a-Methylstyrene Conversionb 68
73
82
76
88
70
85
Selectivity
85
TOFc
7.7 8.4 10.8 9.9
trans-Stilbene
Conversionb 72
73
92
75
89
77
90
6.8
Selectivity
87
TOFc
5.0 5.2 6.6
a Solvent: CH3CN; temperature: 333 K; oxidant: hydrogen peroxide;
catalyst: complexes immobilized on mesoporous silica. b Conversions were
measured after 24 h of the reaction. c TOF: turnover frequency = moles
of substrate converted per mole of Cu center per hour. d Re-used catalyst
(IC-4) for fifth cycle under the same conditions. e Blank reaction without
any catalyst, under identical experimental conditions.
Fig. 6 Catalytic activity of complex 4 immobilized on mesoporous silica
for epoxidation of cyclohexene. The activity of the catalyst decreases with
increasing number of catalytic cycles.
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Table 8 Epoxidationa of olefins over Cu(II) complexes
Substrate
Complex 1
Complex 2
Complex 3
Complex 4
Blank 1d
Blank 2e
Cyclohexene
Conversionb
Selectivity
TOFc
88
91
5.6
90
89
5.7
92
92
5.8
95
86
6.0
30
26
42
50
Styrene
Conversionb
Selectivity
TOFc
85
90
4.2
88
89
4.4
83
88
4.1
81
92
4.0
a-methylstyrene
trans-stilbene
Conversionb
Selectivity
TOFc
78
87
3.6
83
85
3.8
84
86
3.9
83
87
3.8
Conversionb
Selectivity
TOFc
76
88
2.2
78
93
2.3
75
89
2.2
76
92
2.2
a Solvent: CH3CN; temperature: 298 K; oxidant: hydrogen peroxide; catalyst: 0.01 mmol complexes. b Conversions were measured after 24 h of the
reaction. c TOF: turnover frequency = moles of substrate converted per mole of Cu center per hour. d Blank reaction without any catalyst, under the
same experimental conditions. e Blank reaction in the presence of CuCl2, under same experimental conditions.
also been carried out (Table 7). The result shows very poor
conversion and epoxide selectivity in this case, thereby confirming
the catalytic role of the Cu complex and the immobilized catalyst
in these epoxidation reactions. The reactivity of the immobilized
catalysts can be compared with other transition metal complexes
heterogenized on silica. The present systems are less or comparable
in catalytic activities with Mn complexes,79–81 Mo complexes,82,83
Ru complexes,84 and V complexes.85 The catalytic results obtained
by our previous immobilized Ni56 or Cu57–Schiff-base complexes
or Cu(I) complexes86 are comparable with the present systems.
of the catalyst. It is evident that the presence of N,O-donor
ligands is quite relevant. Conversion and product selectivity for the
catalyzed reactions have been improved a lot for the homogeneous
and heterogeneous catalytic reactions, suggesting the catalytic role
played by the Cu(II) centers in these partial oxidation reactions.
Thecatalytic activities ofthe complexes ina homogeneous medium
are lower or comparable in conversion of the substrate and epoxide
selectivity in comparison with W(VI) complex or Mo catalyst,87–89
Mn catalyst,90–92 and Fe–porphyrin complexes,93,94 but these are
more active catalysts in comparison to previously reported Cu
complexes.95–97
Homogenous catalysis
Spectral studies
The homogeneous catalysis for the epoxidation of the olefins
was carried out using hydrogen peroxide as the oxidant at room
temperature. The results of the conversion of cyclohexene, styrene,
a-methylstyrene and trans-stilbene are shown in Table 8. It can be
clearly seen from the table that all substrates are converted in good
yield. Respective epoxides are the major products for the every
reaction. Among the substrates, the conversion of cyclohexene
is highest with 95% yield in the presence of complex 4 as the
catalyst. Selectivity of cyclohexane epoxide in the presence of
complex 4 is 86%. However, the conversion of trans-stilbene to
its epoxide is lowest for all the catalysts. The highest conversion
of styrene is 88% in presence of complex 2·CH3CN. Complex 3
is the most active catalyst for the epoxidation of a-methylstyrene
with 84% conversion and the selectivity for epoxide is 86%. It
has been observed that during the catalytic reaction, the yield
of the reaction increases with time and after a certain time it
seems that the conversion reaches its saturation. A blank reaction
is carried out without any catalyst under same experimental
conditions. The result for that reaction shows a remarkable
decrease in the conversion of the substrate as well as the epoxide
selectivity with no metal complex. Another blank reaction is
carried out in the presence of copper(II) chloride under the same
experimental conditions. The results show an improvement of the
conversion and epoxide selectivity in comparison to the conversion
of cyclohexene with no catalyst or metal salt. But the conversion is
notably lower than the conversion of cyclohexene in the presence
UV-vis spectra for all the complexes were recorded in both solution
phase (in acetonitrile) and solid phase (immobilized catalyst) at
room temperature. All the complexes behave in a similar way. They
showed peaks in the range of 253–263 nm. These peaks may be
attributed to the p→p* transition. Their absorption peaks in the
range of 381 to 388 nm may be due a charge transfer transition.
They showed a relatively broad band at around 670 nm, which
may be assigned as the d–d transition. On the other hand, the
immobilized complexes showed a broad absorption ranging from
360–430 nm, with maxima at ca. 385 nm. This similarity of the
absorbance bands in solution and solid phase confirms that the
complexes are also present in solid silica material, i.e. the loading
of the complexes into the mesoporous silica.
The electrospray ionisation mass spectra of all the complexes
were recorded in acetonitrile at room temperature. The complexes
1, 2·CH3CN, 3 and 4 show peaks at m/z 1154.36, 1130.18,
1402.17 and 1402.17 respectively. The peaks can be assigned to
[Cu4(O)(Ln)2(CH3COO)3]+ species where n = 1, 2, 3 and 4 for
complexes 1, 2·CH3CN, 3 and 4, respectively. It is clearly evident
from the above data that each complex loses one acetate ion in
solution (complex 2·CH3CN also loses acetonitrile). Thus this
monocationic species may be the catalytically active species in
solution.
To check the effect of hydrogen peroxide, we recorded the UV-vis
spectra of complexes in the presence of H2O2 in acetonitrile. The
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effect of H2O2 on the UV-vis spectra of complex 1 is shown in Fig. 7
as the representative case. It has been observed that an intense peak
at around 400 nm with a shoulder in the range of 415–440 nm
appears. This may be due to the existence of Cu–hydroperoxo or
Cu–peroxo species.18,57 UV-vis spectra of complexes immobilized
on mesoporous silica in the presence of hydrogen peroxide were
also recorded and show similar absorption bands to those of
just the complex in the presence of H2O2. That means that
Cu–hydroperoxo or Cu–peroxo species is also generated during
the heterogeneous catalysis. This may be the active species for the
conversion of the olefins.
compounds. Magnetic properties of the complexes were measured
in the temperature range 5–300 K and the study revealed
an exceptionally strong antiferromagnetic coupling among the
copper atoms in the (Cu4–O) cluster. These have been employed as
the active catalysts for the epoxidation of olefins using hydrogen
peroxide as the oxidant. The catalytic reactions were carried
out homogeneously as well as heterogeneously. Heterogeneous
catalytic reactions were carried out over the metal complexes
immobilized on 2D hexagonal mesoporous silica as the catalyst.
The results reveal that the conversion of the olefin is quite high
and epoxides were produced with high selectivity. Conversion
of the substrate in homogeneous catalysis is more than that in
heterogeneous catalysis. But in the case of heterogeneous catalysis,
it has been evident that the catalyst can be reused for at least
five times without appreciable loss in activity after recovering it
by simple filtration. By comparing the results of catalysis we can
conclude that the heterogeneous catalyst is more effective than
the homogeneous one. The application of copper(II) complexes
as catalysts for the epoxidation of alkenes in either homogeneous
medium or heterogeneous medium has rarely been reported to
date.
Acknowledgements
PR wishes to thank Jadavpur University for providing a JU
research grant. MN acknowledges the CSIR, New Delhi, for a
Junior Research Fellowship. AB wishes to thank the Department
of Science and Technology, New Delhi, for providing a Ramanna
Fellowship and research grants. This work was partly funded by
the Nanoscience and Technology Initiative of DST, Government
of India.
Fig. 7 UV-vis spectrum of complex 1 in presence of hydrogen peroxide
in acetonitrile at room temperature.
The transfer of the oxygen atom from the peroxo species to
the olefins may be stepwise or concerted. During the catalytic
conversion, the procedure involves first coordination of alkene
to the metal center forming a metalloperoxocyclic intermediate.98
For the initial coordination of alkene, the metal center needs a
vacant coordination site. The metal atoms in all of the complexes
are five coordinated having labile sites like Cu–m4-oxo or Cu–
acetate. In solution, it has been seen by ESI mass spectroscopy that
each tetranuclear Cu(II) complex loses one acetate ion. So there
is the necessary vacant site for ligation. Then epoxide along with
the catalyst is formed by the cycloreversion of that intermediate.
Khaliullin et al. have shown that in vanadium complexes with
pyrazine-2-carboxylic acid (an N,O-donor ligand) the O atom of
the ligand takes up a proton rather than a nitrogen atom.99 Hence
it may be assumed that the proton transfer from the OH moiety to
one oxygen atom of the ligand and the breakage of the M–O bond
is the step preceding the reaction with alkene and formation of
the intermediate. One cannot rule out that the epoxidation may be
considered as a concerted one step reaction where a direct attack
of the substrate on the peroxo species occurs.100
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