Immobilization of a Metal Complex in Y-Zeolite Matrix: Synthesis, X-ray Single-Crystal, and Catalytic Activities of a Copper (Schiff-Base)-Y Zeolite Based Hybrid Catalyst

Article abstract of DOI:10.1246/bcsj.77.709

A new zeolite-immobilized copper(II) complex catalyst has been prepared by entrapping [CuL] [LH₂ = N,N′ -(1,1-dimethylethylene)bis(salicylaldiimine)] on NaY zeolite matrix. The reaction of Cu-NaY and molten LH₂ affords a green mass of crude catalyst that upon a treatment with CH₃CN gives a gray-colored hybrid catalyst (CuL-NaY). The prepared catalyst has been characterized by IR and UV-vis spectroscopic and EPR spectrometric measurements, TG-DTA analysis, powder X-ray diffraction, and surface-area measurements. To ascertain the structure of the immobilized complex, a single-crystal X-ray diffraction analysis of [CuL] was performed. Spectroscopic measurements showed that the green crude of CuL-NaY contains a penta- or hexa-coordinated copper(II) moiety, while the CuL-NaY catalyst contains a distorted square-planar [CuL] complex moiety. A remarkable catalytic activity of the prepared hybrid catalyst has been observed in oxidation reactions of 1-naphthol and norbornene.

Full text of DOI:10.1246/bcsj.77.709

Ó 2004 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 77, 709–714 (2004)
709
Immobilization of a Metal Complex in Y-Zeolite Matrix:
Synthesis, X-ray Single-Crystal, and Catalytic Activities
of a Copper (Schiff-Base)–Y Zeolite Based Hybrid Catalyst
Pratap Kumar Saha, Surajit Banerjee,1 Sandip Saha, Alok Kumar Mukherjee,1
ꢀ
Subramanian Sivasanker,2 and Subratanath Koner
Department of Chemistry, Jadavpur University, Jadavpur, Calcutta 700 032, India
1Department of Physics, Jadavpur University, Jadavpur, Calcutta 700 032, India
2Catalysis Division, National Chemical Laboratory, Pune 411 008, India
Received July 22, 2003; E-mail: skoner55@hotmail.com
A new zeolite-immobilized copper(II) complex catalyst has been prepared by entrapping [CuL] [LH2 = N,N0-(1,1-
dimethylethylene)bis(salicylaldiimine)] on NaY zeolite matrix. The reaction of Cu–NaY and molten LH2 affords a green
mass of crude catalyst that upon a treatment with CH3CN gives a gray-colored hybrid catalyst (CuL–NaY). The prepared
catalyst has been characterized by IR and UV–vis spectroscopic and EPR spectrometric measurements, TG–DTA analy-
sis, powder X-ray diffraction, and surface-area measurements. To ascertain the structure of the immobilized complex, a
single-crystal X-ray diffraction analysis of [CuL] was performed. Spectroscopic measurements showed that the green
crude of CuL–NaY contains a penta- or hexa-coordinated copper(II) moiety, while the CuL–NaY catalyst contains a dis-
torted square-planar [CuL] complex moiety. A remarkable catalytic activity of the prepared hybrid catalyst has been ob-
served in oxidation reactions of 1-naphthol and norbornene.
The development of new functional materials by immobiliz-
ing selective chemical compounds in zeolitic or molecular-
sieve matrices, which are expected to have exotic properties,
like chromotropism, photoredox, photocatalysis or catalysis,
has received much attention.1–5 The immobilization of certain
types of organic compounds in zeolite matrices has demonstrat-
ed a brilliant color change.6 Recently, organic dye molecules
have been immobilized in zeolite or a molecular sieve to pre-
pare sensors.7 Studies on the photochemical and photophysical
properties of ruthenium complexes encapsulated in zeolite ma-
trices have been undertaken by several groups.8 It has been
found that the said properties of neat metal complexes changed
such catalysts. It is now well understood that the encapsulation
of these complexes in zeolitic hosts can enhance the catalytic
performance of the complexes compared to their homogeneous
counterparts used in solution.9,14 Heterogenization of homoge-
neous catalysts in a porous solid support, like zeolite, makes the
hybrid materials industrially important as new catalysts: a) can
be reused in a catalytic process with its minimum loss; b) can be
easily separated from the reaction mixture; c) they will be more
robust than their homogeneous counterparts.2,16 In an earlier
communication we showed that the complex Cu(salen) [salen
= N,N0-(ethylene)bis(salicylaldiimine)] encapsulated in NaY
zeolite exhibits a novel color isomerism from green to red upon
a treatment of the prepared hybrid material, Cu(salen)–NaY,
with a specific solvent.14 Subsequently, more reports concern-
ing this type of color change of zeolite-immobilized metal-
complex hybrid materials have been appeared in the litera-
ture.19 The color change of the chromophoric groups originates
from the steric obligations of the moiety inside the zeolite
pores, most often due to the interaction of an axial ligand with
the central metal ion. Interestingly a marked difference in the
catalytic properties of the red and green species of Cu(salen)–
NaY was observed in the oxidation reactions.14 It has been
postulated that the vacant trans axial positions facilitate a nu-
cleophilic attack of the oxidant during a catalytic reaction.19
However, such reports published so far have rarely stressed on
understanding the stereo-electronic properties of the complex
present in the zeolite supercage after immobilization. Knowl-
edge regarding the structure of the immobilized complex is im-
portant to delineate the mechanism of the catalytic reactions
and to design new catalysts of this kind. Such studies are scanty
dramatically upon immobilization in zeolite matrices. Sykora et
2þ
al. showed that the zeolite immobilized [Ru(bpy)n(bpz)3ꢁn
]
complex (where bpz is 2,20-bipyrazine and bpy is 2,20-bipyri-
dine) type hybrid materials can be used to store solar energy
by a photochemical reaction.8b Studies on a zeolite-immobi-
lized metal-complex based heterogeneous catalysts has re-
ceived immense attention by a number of researchers in last
two decades. Several interesting systems of this kind have been
discovered.1,2,9–11 To this end, metal complexes of porphyrins,
phthalocyanines, Schiff-bases, etc. have been encapsulated into
a zeolitic matrix for the development of efficient biomimetic
oxidation catalysts, which have acted as a functional mimic
of metalloenzymes.12–15 The oxidation reactions catalyzed by
metal complexes are often impeded due to oxidative degrada-
tion of the complex and/or the formation of ꢀ-oxo dimers.9
Several strategies, viz. encapsulation of those complexes within
zeolitic1,16 or polymeric matrices17 or intercalation in clays,1,18
have been adopted to enhance the stability and reactivity of
Published on the web April 2, 2004; DOI 10.1246/bcsj.77.709

710 Bull. Chem. Soc. Jpn., 77, No. 4 (2004)
Immobilization of Copper (Schiff-Base) in NaY
tometer ! scan mode with graphite-monochromated Mo Kꢄ radi-
ation. Data were corrected for Lorentz polarization and empirical
absorption based on the scan (Tmin=Tmax: 0:447=0:628) applied.
Out of 2720 unique reflections (Rint ¼ 0:0636) 2161 reflections
with I > 2ꢅðIÞ were considered. The structure was solved by the
heavy atom method, and refined by full-matrix least-squares using
SHELX-97.21 The final refinement with all non-hydrogen atoms
anisotropic and hydrogen atoms (placed at geometrical positions)
held fixed with isotropic temperature factors converged to
R1 ¼ 0:0591, !R2 ¼ 0:1512 for 2161 observed reflections
(R1 ¼ 0:0768, !R2 ¼ 0:1647 for all reflections). The weighing
in the literature. Here we report the synthesis of [CuL] [LH2 =
N,N0-(1,1-dimethylethylene)bis(salicylaldiimine)], its X-ray
crystal structure, immobilization of the [CuL] complex in the
NaY zeolite matrix, and characterization of the prepared hybrid
catalyst and its catalytic efficacy in oxidation reactions.
Experimental
Materials. NaY zeolite was procured from Tosoh Corporation,
Japan. The ligand, N,N0-(1,1-dimethylethylene)bis(salicylaldi-
imine) (LH2), was prepared from 1,1-dimethylethylenediamine
and salicylaldehyde, following the standard condensation reaction
procedure.20 All other chemicals were of AR quality.
2
scheme, ! ¼ 1=½ꢅ2ðF0 Þ þ 0:1088P2 þ 0:9744Pꢅ, where P ¼
2
2
ðF0 þ 2Fc Þ=3, gave satisfactory agreement.
Synthesis of Complex [CuL]. A solution of (0.503 g, 2 mmol)
Cu(NO3)2 3H2O in methanol (20 mL) was slowly added to a
Physical Measurements. Infrared and electronic spectra were
measured on a Perkin Elmer 783 and Hitachi U 3400 spectropho-
tometer, respectively. EPR spectra were measured on a Varian E-
112 EPR spectrometer at room temperature. Carbon, hydrogen,
and nitrogen analyses were carried out with a Perkin Elmer
240C elemental analyzer. A TG–DTA study was performed on
Shimadzu TG-50 thermogravimetric analyzer, a DTA-50 differen-
tial thermal analyzer and a Mettler Toledo Thermal Analyzer sys-
tem. The X-ray powder-diffraction measurements were made on a
PW 1730/1710 Philips XRD diffractometer. The copper content of
the sample was estimated on a Varian Techtron AA-ABQ atomic
absorption spectrometer. The catalytic reaction products were an-
alyzed by an AIMIL 5700 NUCON and a Varian 3400 Gas Chro-
matograph equipped with an FID detector, and verified by a
Shimadzu QP 5000 GCMS. Nitrogen adsorption measurements
were made at 77 K using a Coulter Omnisorb 100 CX instrument.
Catalytic Reactions. The catalytic reactions were undertaken
in a glass batch reactor. In a typical reaction, 1 g of the substrate
was taken in 10 mL of CH3CN, to which the catalyst (50 mg)
was added. One mL of a 30% H2O2 solution was then added with
continuous stirring. The products were collected at different time
intervals, identified and quantified by GC, and verified by GC–MS.
ꢂ
methanolic (20 mL) solution of the Schiff base, LH2 (0.613 g, 2
mmol), with constant stirring. The stirring was continued for an-
other half an hour while violet-brown crystals were precipitated
out. The crystals were collected by filtration, and were recrystal-
lized from dichloromethane. Single crystals for X-ray analysis
were grown from dichloromethane by slow evaporation technique.
(Found: C, 60.82; H, 4.78; N, 7.80%. C18H18N2O2Cu requires C,
60.41; H, 5.07; N, 7.83%); ꢁmax/cmꢁ1 (KBr) 1630, 1605, 1535,
1460, 1449, 1380, and 1312. ꢂmax/nm (Nujol) 535 and 370.
Incorporation of Copper(II) in NaY. An aqueous solution of
Cu(NO3)2 3H2O (4.002 g) was stirred continuously for 5 h with
ꢂ
NaY (4.107 g) in suspension. The white NaY zeolite turned to a
bluish mass. The solid mass was then filtered and washed thor-
oughly with a large amount of deionized water using Soxhlet,
and dried under vacuum to give a light-blue powder of Cu–NaY.
The zeolite, thus produced, was then used for immobilization
work.
Immobilization of [CuL] in NaY. The immobilization was
done by the flexible ligand method (Scheme 1).1 Cu–NaY (1.031
g) and 3.991 g of the required Schiff base (LH2) were taken into
a mortar and repeatedly ground and mixed intimately with each
other. The whole mass then turned to a green powder. This was tak-
en into a petridish and kept at 85 ꢃC for half an hour. During heat-
ing the ligand, a Schiff base, was melted. The mixture was then
cooled down when it was solidified. The melting process was re-
peated for 3–4 times mixing with some more ligand (LH2) (ca.
0.5 g) each time. Finally, a green solid was obtained. This was thor-
oughly washed with acetonitrile using a Soxhlet. The solid residue
was then dried under vacuum to give a light-gray powder hybrid
catalyst (CuL–NaY).
Results and Discussion
Immobilization of the complex in a zeolite matrix was done
by the ‘‘ship-in-the-bottle’’ technique.1 A flexible Schiff base
ligand (in the molten state) is allowed to react with copper(II)
incorporated NaY zeolite (Scheme 1). It is expected that the
[CuL] complex is formed in the supercage as well as on the sur-
face of the Cu–NaY zeolite. Because the complex [CuL] is a
neutral moiety, only a weak force, like electrostatic attraction,
van der Waals interaction, etc. is operative here to catch hold of
the complex on the surface of the zeolite. It seems that the
Soxhlet extraction of the crude hybrid material with CH3CN ef-
fectively washes away the complex molecule attached to the
surface of the zeolite. However, the possibility of the presence
of a small amount of copper(II) complex on the outer surface of
the catalyst can not be ruled out, even after prolong washings.
Nevertheless, complex molecules formed inside the zeolite su-
X-ray Crystallography. Crystal data: C18H18N2O2Cu, M ¼
357:88, orthorhombic, space group Pbca, Z ¼ 8, a ¼ 15:991ð4Þ,
ꢀ
ꢀ 3
b ¼ 11:497ð3Þ, c ¼ 17:512ð4Þ A, V ¼ 3219:4ð14Þ A (by least-
square refinement of diffractometer setting angles for 57 automati-
cally centered reflections ꢂ ¼ 0:71073), Dcalcd ¼ 1:477 g cmꢁ3
,
Fð000Þ ¼ 1480, ꢀ ¼ 1:367 mmꢁ1, dark brown crystals, dimen-
sions = 0:72 ꢄ 0:64 ꢄ 0:34 mm, T ¼ 293 K.
Structure Determination: Intensities for 3424 reflections
(2.55ꢃ < ꢃ < 24.98ꢃ) were collected using a Siemens P4 diffrac-
Scheme 1.

P. K. Saha et al.
Bull. Chem. Soc. Jpn., 77, No. 4 (2004)
711
percage appear to remain in the matrix after prolong washings.
A piece of evidence in support of this was obtained in surface-
area measurements of the hybrid material. A significant de-
crease in the total surface area of the zeolite was observed upon
immobilization of the complex (vide infra). Therefore, it can be
postulated that most of the complex molecules in CuL–NaY are
physically trapped inside the supercage of the zeolite. It is note-
worthy that a sharp color change (light blue to green) of the Cu–
NaY was observed during mixing with the melted ligand (vide
supra). Upon washing (Soxhlet) with CH3CN, the green-col-
ored crude CuL–NaY catalyst turned to a gray powder. This
type of color change of the hybrid materials upon a treatment
with a certain solvent was also observed earlier.14,19
The elemental analysis of CuL–NaY showed the copper con-
tent of the catalyst to be 0.211% (by weight), whereas result of
estimating the carbon and nitrogen in the prepared material
supports the formation [CuL] in the zeolite matrix. This shows
that about 50% of the supercages of NaY are occupied by the
complex [CuL]. The copper complex present in the hybrid ma-
terial acts as active centers of the catalyst.
X-ray powder diffraction measurements were performed for
the NaY and CuL–NaY catalysts. A comparison of the XRD
pattern of NaY and CuL–NaY shows that the crystallinity of
the zeolite matrix almost remained intact after the formation
of the complex in it. This indicates that the basic structure of
the mother zeolite remain unaffected after immobilization; only
the catalytically active copper complex moieties are sterically
assembled in the zeolite matrix.
Fig. 1. UV–vis spectra of the pristine complex [CuL] (a);
CuL–NaY (b); green crude product of CuL–NaY (c).
IR spectral measurements of [CuL] and CuL–NaY showed
that all of the prominent bands for the azomethine group of
the ligand appearing in the region 1600–1300 cmꢁ1 of the
[CuL] species are also present in CuL–NaY. These bands can
be regarded as being a signature of the complex, because no
zeolitic IR band appears in this region of the spectra, except
for the band for HOH bending of H2O at 1650 cmꢁ1. Upon
careful drying of the prepared catalyst, this band (at ca. 1650
cmꢁ1) disappears. The vibration bands of the host zeolite ob-
scured the IR bands of the complex in other regions.
The electronic spectra of the species [CuL], CuL–NaY cata-
lyst, and green crude of CuL–NaY are given in Fig. 1. The
peaks appearing at 535 nm and 370 nm in the electronic spectra
of [CuL] are interpreted as being the d–d transition and ligand
charge-transfer bands, respectively. In the case of CuL–NaY,
the d–d band appeared at ca. 537 nm, whereas upon immobili-
zation the charge-transfer band was shifted to a shorter wave-
length region and appeared at 345 nm. This type of shifting
of the charge-transfer band after immobilization of a certain
complex in the zeolite matrix was observed earlier.14 On the
other hand, the electronic spectra of the green-colored crude
CuL–NaY showed three peaks at 550, 400(sh), and 320 nm
(Fig. 1). The shift of the d–d transition towards a longer wave-
length region in the case of crude CuL–NaY indicates a lower-
ing of the in-plane ligand field around the copper(II) ions. It is
well documented that such a type of shifting usually occurs due
to an increase of the axial ligand interaction with copper(II)
ions.22 It is noteworthy that in the case of copper(II) Schiff-base
complexes it has been observed that the green species generally
possess a penta-coordinated copper(II) moiety, whereas red
species possess a tetra-coordinated one.23 Therefore, it can be
assumed that the crude product of CuL–NaY contains a six or
five coordinated copper(II) moiety in addition to the square pla-
nar [CuL].
The EPR spectra of the species [CuL], CuL–NaY, and green
crude of the catalyst are given in Fig. 2. The principal g values,
calculated by usual methods from the EPR spectra, are in good
agreement with those reported for copper(II) Schiff-base com-
plexes.24 The g and g values of the pristine complex, CuL–
k
?
NaY catalyst, and green crude of CuL–NaY are 2.252, 2.256,
2.124 and 2.197, 2.162, 2.096, respectively. The g values of
the catalyst CuL–NaY is significantly higher than that of its
green crude product. It is well understood that a decrease in
the axial ligand interaction with the copper(II) center in these
types of complexes leads to an increase in g .22a,c,24 Therefore,
k
this difference is not inconsistent with the observations in elec-
tronic spectral measurements. The difference in the line width
of the EPR spectra of [CuL] and CuL–NaY indicates that the
copper complex in the zeolite matrix suffers a slight distortion
upon immobilization. Such an increase in the line widths is usu-
ally observed as the result of a shortening of the relaxation time
(T1) of unpaired electron of copper(II) in case the complex moi-
ety undergoes a tetrahedral distortion.25
The thermogravimetric (TG) measurement and differential
thermal analysis (DTA) of the complex [CuL], CuL–NaY,
and green crude product of CuL–NaY are shown in Fig. 3.
The DTA curve of the [CuL] shows two prominent exothermic
peaks at ca. 220 and 310 ꢃC. The corresponding weight losses
are observable in the TG curve of the thermogram of [CuL].
The TG curve of CuL–NaY shows a significant weight loss
in the temperature range 30–200 ꢃC; a gradual loss is then ob-

712 Bull. Chem. Soc. Jpn., 77, No. 4 (2004)
Immobilization of Copper (Schiff-Base) in NaY
Fig. 2. EPR spectra of the complex [CuL] (a), CuL–NaY
(b), and green crude product of CuL–NaY (c). The signal
gain for CuL–NaY (b) was 40 times higher than that of
the complex [CuL] or the green crude product.
served up to 600 ꢃC. The corresponding DTA curve shows no
observable peak up to 200 ꢃC, but a prominent exothermic peak
is observed at ca. 230 ꢃC. On the other hand, the crude product
of CuL–NaY showed a weight loss of ca. 57% in the tempera-
Fig. 3. TG–DTA curves of [CuL] (a), CuL–NaY (b), and
green crude product of CuL–NaY (c). Sample taken =
10.00 mg. Rate of heating = 10 ꢃC/min. ꢄ-Al2O3 was tak-
en as the reference solid.
ꢃ
ꢃ
ture range 30–300 C; the weight loss continued up to 600 C
and a total loss has been recorded for this sample, ca. 78%. This
shows the green species contains a large amount of adsorbed
H2O in addition to the copper(II) Schiff-base complexes.
Two endothermic and one exothermic peaks were observed
in the DTA curve of the green species of CuL–NaY at ca. 92,
neighbor.23 It is well established that the formation of such a di-
mer often deactivates the copper complexes in catalytic oxida-
tion reactions.9 Nevertheless, no such intermolecular interac-
tion was observed between the copper(II) ion and the phenoxo
group in the present complex.
ꢃ
250, and 372 C, respectively. The DTA profiles of the crude
CuL–NaY and CuL–NaY catalyst are completely different
from the corresponding DTA profile of the pristine complex
[CuL]. Therefore, it can be concluded that the thermal behavior
of the pristine complex changes considerably upon immobiliza-
tion in zeolite matrix.
Surface-area measurements give valuable information about
the immobilization of the complex in a zeolite matrix.27 A ni-
trogen sorption experiment showed a remarkable decrease in
the surface area of the immobilized materials compared to
the original NaY zeolite in the present case. Nitrogen is gener-
ally adsorbed in the micropores as well as the outer surface of
the zeolite. Thus, if the complex [CuL] occupies the pores of
the NaY zeolite in CuL–NaY, a smaller amount of gas will
be adsorbed by the catalyst. Indeed this happens in the case
of CuL–NaY. The total surface area of the original NaY was
770 m2 gꢁ1 upon immobilization of the complex into it this val-
An ORTEP26 diagram of the complex [CuL] along with the
atom numbering scheme is given in Fig. 4. The selected bond
distances and angles of the complex are listed in Table 1.
The coordination geometry around the copper atom can be de-
scribed as a distorted square-planar geometry. Two nitrogen
atoms of the azomethine groups of the ligand and the two oxy-
gen atoms of the phenoxo groups of the salicylaldehyde sub-
structure of the ligand occupy the four coordination sites of
the central metal ion. The copper atom is slightly displaced
from the plane consisting of N1, N2, O1, and O2 atoms. The
distortion is also apparent in the inequality of the bond lengths
of two oxygen donor atoms and two nitrogen donor atoms. The
copper(II) complexes of Schiff bases often form a dimer
through a semicoordinated axial bond between the copper(II)
ion of one complex moiety and the phenoxo group of its nearest
ue decreases to ca. 451 m2 gꢁ1
.
All the above-mentioned results convincingly demonstrate
the presence of the copper(II) Schiff-base moiety in the pre-
pared catalyst, CuL–NaY. Spectroscopic analyses indicate that
the green-colored crude of CuL–NaY most probably contains
the penta- or hexa-coordinated form of the copper(II) Schiff-
base moiety. Although virtually no difference is observed in

P. K. Saha et al.
Bull. Chem. Soc. Jpn., 77, No. 4 (2004)
713
ꢃ
ꢀ
Table 1. Selected Bond Lengths (A) and Angles ( ) for
[CuL]
ꢀ
Bond lengths/A
Cu–O(2)
Cu–O(1)
1.894(3)
1.899(3)
Cu–N(1)
Cu–N(2)
1.940(3)
1.942(4)
Bond angles/ꢃ
O(2)–Cu–O(1)
O(2)–Cu–N(1)
O(1)–Cu–N(1)
88.80(13)
170.30(14)
93.7(2)
N(1)–Cu–N(2)
O(1)–Cu–N(2)
O(2)–Cu–N(2)
85.0(2)
166.06(14)
94.83(14)
Fig. 4. ORTEP diagram of [CuL] showing atom numbering
scheme.
Table 2. Catalytic Performance of CuL–NaYa)
Catalyst
Reaction Conversion (mass/%)
time/h
Product distribution (mass/%)
Norbornene epoxide 1,4-Naphtho- 1,4-Dihydroxy-
exo- endo-
quinone
naphthalene
CuL–NaY
3
12
3
12
12
21.6
52.3
21.6
52.3
—
—
10.5
19.8
10.5
19.8
—
—
½CuL]
28.0b)
13.8
12.1
12.5b)
6.9
4.8
CuL–NaY (Green) 12
Cu–NaY 12
No detectable activityc,d)
No detectable activityd)
a) No Cu-leaching during reaction. b) This includes some other minor oxidized products. c) Reactions were performed in
CH3OH medium. d) Cu–NaY and crude CuL–NaY (Green) species showed no catalytic activity towards epoxidation of
norbornene and oxidation of 1-naphthol.
the d–d transition band of the [CuL] and CuL–NaY catalyst in
their electronic spectra, upon immobilization the ligand charge-
transfer band of the copper(II) Schiff-base moiety showed a
red-shift of the band. The difference is more apparent between
these two species in their EPR spectra, which is attributable to a
tetrahedral distortion of the copper(II) Schiff-base moiety. An
X-ray single-crystal analysis showed that the dimensions of
leached out during oxidation reactions because no copper was
detected in the liquid phase of the reaction mixture after the
completion of the reaction.
It is evident from this study that the copper(II) complex im-
mobilized in the NaY matrix possesses a distorted square-pla-
nar coordination environment. The chromotropism showed by
the green-colored CuL–NaY crude species upon a treatment
with CH3CN is due to the chromophoric change of copper(II)
ions. Most probably, a transformation from penta- or hexa-co-
ordination to a tetra-coordination environment around the cop-
per(II) ions occurrs during the chromotropic change. In an ear-
lier study we observed a similar kind of chromotropism in Cu-
(salen)–NaY. Based on the spectroscopic results it was postu-
lated that the immobilized species, which is catalytically active,
might possess a square-planar environment around the copper-
(II).14 However, we could not isolate the tetracoordinated cop-
per(II) complex in its pristine form in case of the Cu(salen) to
justify, spectroscopically or structurally, whether the red spe-
cies of Cu(salen)–NaY really contains the tetracoordinated cop-
per(II) Schiff-base moiety. This study shows that the catalyti-
cally active species indeed possesses a square-planar copper(II)
center. This strengthens our postulate that a square-planar cop-
per(II) complex moiety with its two vacant axial positions facil-
itates a neucleophilic attack by the oxidant to enhance the cat-
alytic activity of the hybrid materials in the oxidation reactions.
ꢀ
the [CuL] complex are ca. 12 ꢄ 7 A. It is well-known that
ꢀ
the size of the supercage of NaY is ca. 12 A; therefore, some
amount of distortion of the immobilized complex is not unex-
pected. However, it may be emphasized that upon immobiliza-
tion the [CuL] complex retains its tetra-coordination ligand en-
vironment around the copper(II) ions.
The epoxidation of norbornene and the hydroxylation of
naphthol were carried out to test the catalytic efficiency of
the prepared catalyst. The results of the reactions are given in
Table 2. In both reactions, the hybrid catalyst showed good cat-
alytic activity. It is interesting to note that neither the green-col-
ored crude CuL–NaY (in methanol medium) nor the Cu-ex-
changed mother zeolite (Cu–NaY) exhibited any noticeable
catalytic activity. However, the pristine [CuL] showed catalytic
activity in solution in the above-mentioned oxidation reactions,
though the catalytic efficiency was much lower than its hybrid
analogue, CuL–NaY (Table 2). It should be noted that an atom-
ic-absorption spectroscopic analysis shows that copper is not

714 Bull. Chem. Soc. Jpn., 77, No. 4 (2004)
Immobilization of Copper (Schiff-Base) in NaY
Angew. Chem., Int. Ed. Engl., 34, 2652 (1995).
14 S. Koner, Chem. Commun., 1998, 593.
15 C. A. Tolman, J. D. Droliner, M. J. Nappa, and N. Herron,
‘‘Activation and Functionalisation of Alkanes,’’ ed by C. L. Hill,
Wiley, Chichester (1989), p. 303.
The work was financially supported by the CSIR, New Delhi,
by a grant (01(1674)/00/EMR-II) to SK. PKS thanks CSIR for
a research fellowship.
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