Novel color isomerism and catalytic activities of Cu(salen) complex encapsulated in a zeolitic matrix

Article abstract of DOI:10.1039/a707681i

The green zeolite encapsulated Cu(salen) complex on treatment with MeCN turns red and is found to be catalytically active towards oxidation reactions whereas the green species is inactive.

Full text of DOI:10.1039/a707681i

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Novel color isomerism and catalytic activities of Cu(salen) complex
encapsulated in a zeolitic matrix
Subratanath Koner*
Catalysis Division, National Chemical Laboratory, Pune 411 008, India
The green zeolite encapsulated Cu(salen) complex on
treatment with MeCN turns red and is found to be
catalytically active towards oxidation reactions whereas the
green species is inactive.
[Cu(salen)] are 2.189, 2.178, 2.176 and 2.041, 2.039, 2.044,
respectively.
All the above results convincingly demonstrate the presence
of the [Cu(salen)] chromophore in both varieties of Cu(salen)-
NaY catalysts. The shifting of the d–d transition band to shorter
wavelength in electronic spectra of the red species in compar-
ison to the green species clearly indicates that the axial ligand
The development of efficient biomimetic oxidation catalysts
containing metal complexes of porphyrins, phthalocyanines,
Schiff-bases, etc. which mimic the active sites of metalloenzy-
II
6b,8b
field around the Cu ion is weaker in the former.
spectra are in accord with this as g values of the red species are
higher than that of the green species whereas the g values are
EPR
1
mes has received a lot of attention. The oxidation reactions
∑
catalyzed by metal complexes are often impeded due to
oxidative degradation of the complex and/or the formation of
4
almost the same. It is well documented that a decrease of axial
1c
II
m-oxo dimers. Several strategies, viz. encapsulation of those
ligand interaction with Cu in these types of complexes leads to
2
,3
4
6b,8
complexes within zeolitic or polymeric matrices or interca-
a red-shift of d–d transition bands and an increase in g
∑
.
It is
2
lation in clays, have been adopted to enhance the stability and
also noteworthy that the resolution of IR spectral bands for
ligand vibrations of the red species is higher than the green
species suggesting that the Cu(salen) moiety in the red species
is more comfortably enclosed inside the zeolitic supercage than
in the green species; which also indirectly indicates that the
interaction between the Cu(salen) moiety and the zeolitic matrix
in the red species is much weaker than in the green species.
reactivity of such catalysts. It is now well understood that
encapsulation of these complexes in zeolitic hosts can enhance
the catalytic performance of the complexes in comparison to
their homogeneous counterparts used in solution.1c
Here, we report the preparation, novel physicochemical
properties and unique catalytic behavior of a zeolite (NaY)
encapsulated Cu(salen) [salen = N,NA-bis(salicylaldehyde)-
ethylenediimine] complex [Cu(salen)-NaY].
II
Therefore, it can be proposed that in the green species, Cu
To prepare Cu(salen)-NaY, 2 g of calcined Cu-NaY† zeolite
was mixed with 2 g of salen and placed in a sealed glass tube
containing dry N
for 5–6 h. The green mass thus formed was then pulverized and
subjected to Soxhlet extraction with MeOH and Me CO. The
2
and heated at 150 °C with occasional stirring
2
extracted green solid was then stirred with the portions of fresh
MeCN several times. To our surprise during the stirring with
MeCN the green mass transformed in to a red solid within a few
min whereas treatment of Cu(salen) with MeCN or re-
crystallization of the complex from the same solvent gave no
such color change. Although there is ample evidence of metal
complexes recrystallized from different solvents giving differ-
ent colors owing to the formation of different isomers or
260
380
500
l / nm
620
740
conformers5,6 the present type of color isomerism among zeolite
encapsulated metal complexes is rarely observed. No color
Fig. 1 UV–VIS diffuse reflectance spectra of Cu(salen)-NaY(red) (———)
Cu(salen)-NaY(green) (-·-·-·-) and Cu(salen) (- - - -)
change is observed when the green species is treated with other
i
sovlents such as EtOH, Pr OH, CH
2
Cl
2
, CHCl
3
, etc. Elemental
analysis showed the Cu content of this solid was ca. 0.06%,
which corresponds to encapsulation of ca. 26 molecules of
g = 2.189
[
Cu(salen)] per 100 supercages in this solid. The Cu content of
(
c)
b)
the green solid is virtually the same.
A relatively weak band in the visible region in the electronic
spectra of the prepared catalysts and pure complex {590 nm for
g = 2.178
g = 2.176
(
[
Cu(salen)] and Cu(salen)-NaY(green); 558 nm for Cu(salen)-
NaY(red)} is assigned to d–d transitions whereas two fairly
strong bands appearing in the range 410–280 nm are assigned to
ligand charge transfer bands {370 and 286 nm for the red and
(a)
green species and 406 and 355 for the [Cu(salen)] complex}
5
(
Fig. 1). All the prominent IR bands for ligand vibrations
2
1
appearing in the region 1700–1250 cm of the [Cu(salen)]
species are also present in Cu(salen)-NaY. The ligand vibration
bands in the other regions are obscured by the presence of
zeolitic vibration bands. The principal g values calculated by
usual methods from EPR spectra (Fig. 2) are in agreement with
0.0138 T
II
7
those reported for Cu Schiff base complexes. The g
∑
and g
4
Fig. 2 EPR spectra of Cu(salen) (a), Cu(salen)-NaY(green) (b) and
Cu(salen)-NaY(red) (c)
values of the red and green species of Cu(salen)-NaY and for
Chem. Commun., 1998
593

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Table 1 Catalytic performance of zeolite encapsulated Cu(salen)
Conversion
(mass%)
Catalyst
Substrate
t/h
Product
Cu(salen)-NaY(red)
Norbornene
3
2
12
12
12
23.1
66.7
16.8
0.1
exo-Epoxynorbornanea
a
1
exo-Epoxynorbornane
a
1
-Naphthol
Norbornene
-Naphthol
1,4-Naphthoquinone
b
Cu(salen)-NaY(green)
—
b
1
0.2
—
a
Selectivity = 100 (mass%). b No expected oxidized products are detected.
forms weak Cu–O bonds with zeolitic oxygen to give either
square-pyramidal or octahedral geometry around the Cu ion,
thanks Dr S. Sivasanker for his encouragement and interest in
this work.
II
which upon treatment with MeCN becomes square planar. The
II
formation of weak Cu–O bonds between the Cu ion and
zeolitic oxygen can not be ruled out since recently it has been
shown by single crystal X-ray analysis that Cu(salen) forms
dimers through weak Cu–O (O-salen) bonds in the solid
state.5
The epoxidation of norbornene and hydroxylation of 1-naph-
thol were carried out in a glass batch reactor using TBHP (tert-
butyl hydroperoxide) as oxidant. MeOH was used as solvent for
Cu(salen)-NaY(green) while MeCN was used for Cu(salen)-
NaY(red). In a typical reaction, 0.1 g of catalyst was slurried in
a batch reactor with 10 g of MeCN or MeOH. To this, 0.5 g of
oxidant was added and the mixture allowed to equilibrate at
Notes and References
*
Present Address and Address for Communication: Biomolecular Engi-
neering Department, National Institute of Bioscience and Human Technol-
ogy, 1-1 Higashi, Tsukuba, Ibaraki 305, Japan. Fax: +81-298-54-6161;
E-mail: skoner@ccmail.nibh.go.jp
† Cu-NaY was prepared by stirring a slurry of NaY in Cu(NO
g NaY with 0.25 g Cu(NO dissolved in 200 ml of water] at room temp.
In order to control the quantity of Cu exchange in NaY-zeolite a relatively
dilute Cu(NO solution was used and the stirring time was fixed at 30 min.
3 2
) solution [1
3 2
)
II
3 2
)
The Cu content in the solid is found to be ca. 0.075%. Cu(salen)-
NaY(green) was prepared following the same procedure as above (main
text) avoiding treatment with MeCN. The catalysts were calcined at 120 °C
5
0 °C in oil bath. After ca. 10 min the substrate was added and
2
for 8–10 h in dry N before use in reactions. To re-exchange the unreacted
products collected at different time intervals and identified and
quantified by GC and verified by GC–MS.
II
+
Cu present in both the green and red solids by Na ions they were
repeatedly stirred in NaNO solution for several hours.
3
Results for both reactions (Table 1) established that the
Cu(salen)-NaY(red) catalyst showed excellent product selec-
tivity as well as activity towards oxidation reactions with
exclusively one oxidized product being obtained in each case.
Selectivity of products to this extent is rare among zeolite
encapsulated metal complex catalysts. For the 1-naphthol
hydroxylation reaction 1,4-naphthoquinone is selectively ob-
tained among the three possible products (1,4-naphthoquinone,
1 (a) S. B. Ogunwumi and T. Bein, Chem. Commun., 1997, 901; (b) R.
Robert and P. Ratnasamy, J. Mol. Catal. A, 1996, 100, 93; (c) K. Balkus,
Jr., M. Eissa and R. Levedo, J. Am. Chem. Soc., 1995, 117, 10753; (d)
J. T. Groves and T. E. Nemo, J. Am. Chem. Soc., 1983, 105, 5786; (e)
B. M. Weckhuysen, A. A. Verberckmoes, I. P. Vannijvel, J. A.
Pelgrims, P. L. Buskens, P. A. Jacobs and R. A. Schoonheydt, Angew.
Chem., Int. Ed. Engl., 1995, 34, 2652.
1
,4-dihydroxynaphthalene,
1,2-dihydroxynaphthalene)9
2
3
F. Bedioui, Coord. Chem. Rev., 1995, 144, 39 and references therein.
K. Balkus, Jr., A. G. Gabrilov, S. L. Bell, F. Bedioui, L. Rou e´ and J.
Devynck, Inorg. Chem., 1994, 33, 67.
whereas for norbornene only exo-epoxynorbornane is obtained,
where exo- and endo-epoxynorbornane, cyclohexene-4-carbal-
dehyde and norcamphor are all possible products.10 On the
other hand Cu(salen)-NaY(green) does not show any activity
towards these reactions. It should be noted that Cu is not
detected in the liquid phase of the reaction mixtures (the solid
catalyst is separated from the mixture by filtration at ca. 50 °C)
after completion of experiment. Therefore, the Cu complex is
not leached from the catalysts during reaction.
4 R. F. Parton, I. F. J. Venkelecom, M. J. A. Casselman, C. P.
Bezouhanova, J. B. Uytterhoeven and P. A. Jacobs, Nature, 1994, 370,
5
41.
5
6
M. M. Bhadbhade and D. Srinivas, Inorg. Chem., 1993, 32, 6122.
(a) S. Koner, A. Ghosh and N. Ray Chaudhuri, Transition Met. Chem.,
1
988, 13, 291; S. Koner, A. Ghosh and N. Ray Chaudhuri, J. Chem.
Soc., Dalton Trans., 1990, 1563; (b) D. R. Bloomquist and R. D. Willett,
Coord. Chem. Rev., 1982, 47, 125.
In conclusion, it can be stated that the catalytic activity of
Cu(salen)-NaY changes dramatically with change in the
7
A. H. Maki and B. R. McGarvey, J. Chem. Phys., 1958, 29, 35; E. Hasty,
T. J. Colburn and D. N. Hendrickson, Inorg. Chem., 1973, 12, 2414.
II
coordination geometry around the Cu ion in the zeolite. The
8 (a) H. Yokoi, M. Sai and T. Isobe, Bull. Chem. Soc. Jpn., 1969, 42,
2232; (b) R. L. Belford, M. Calvin and G. Belford, J. Chem. Phys., 1957,
26, 1165.
9 T. K. Das, K. Chaudhari, A. J. Chandwadkar and S. Sivasanker,
J. Chem. Soc., Chem. Commun., 1995, 2495.
II
Cu(salen)-NaY(red) catalyst where Cu ion appears to possess
e,5
vacant axial positions shows activity1 towards oxidation
II
reactions whereas the green variety, where Cu ion is either
five- or six-coordinate, is inactive. Further this work is a novel
example where a metal complex encapsulated in a zeolitic
matrix shows color isomerism upon treatment with a specific
solvent.
1
0 T. L. Siddall, N. Miyaura, J. C. Huffman and J. K. Kochi, J. Chem. Soc.,
Chem. Commun., 1983, 1185; T. G. Traylor and A. R. Miksztal, J. Am.
Chem. Soc., 1989, 111, 7443.
The author acknowledges CSIR, India for an award of a
Senior Research Associateship (Pool Officer). The author also
Received in Cambridge, UK, 24th October 1997; 7/07681I
594
Chem. Commun., 1998