An EPR study on the enantioselective aziridination properties of a CuNaY zeolite

Article abstract of DOI:10.1039/b010083h

Zeolites have been used as solid catalysts for large scale industrial processes such as the production of gasoline and related petrochemicals. The enantioselective aziridination of styrene was studied using CuNaY catalyst, with PhI=NT as the nitrogen sour

Full text of DOI:10.1039/b010083h

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An EPR study on the enantioselective aziridination properties of a
CuNaY zeolite
Yvonne Traa,¤ Damien M. Murphy,* Robert D. Farley and Graham J. Hutchings
Department of Chemistry, Cardi† University, PO Box 912, Cardi†, UK CF10 3T B.
E-mail: MurphyDM=cardi†.ac.uk
Received 18th December 2000, Accepted 19th January 2001
First published as an Advance Article on the web 20th February 2001
A CuNaY catalyst was prepared and used to study the enantioselective aziridination of styrene, with PhI2NTs
as the nitrogen source, in the presence of a bis(oxazoline) chiral modiÐer. The chiral modiÐer used was a
diimine ligand, (S)-([)-2,2@-isopropylidenebis(4-phenyl-2-oxazoline). EPR spectroscopy provides the Ðrst direct
experimental evidence for the formation of a copper(II)-bis(oxazoline) complex inside the Y zeolite pores after
stirring the calcined catalysts with the chiral ligand using acetonitrile as solvent. The copper complexes possess
square pyramidal and square planar symmetries, with spin Hamiltonian parameters analogous to those of the
equivalent homogeneous complex dissolved in solution. These copper(II) complexes accounted for at least 40%
of all available copper within the ion exchanged CuNaY catalyst and represent one Cu(II)-bis(oxazoline)
complex per supercage. The remaining uncomplexed Cu(II) ions remain solvated to the acetonitrile molecules.
After the aziridination reaction was carried out in the presence of styrene and PhI2NTs, EPR evidenced the
selective loss of the signal due to the copper(II)-bis(oxazoline) complex with square pyramidal and square
planar symmetries but practically no loss in overall Cu(II) content. This was explained on the grounds of a
changing co-ordination environment of the encapsulated complex. However when PhI2NTs was added
separately to the catalyst a dramatic loss in Cu(II) signal intensity was observed. These results are discussed in
terms of the reaction mechanism in operation.
ane (hereafter abbreviated to PhI2NTs) as nitrogen source.9,10
In the heterogeneous system Langham et al.,6h8 evidenced the
successful use of a copper exchanged zeolite Y (CuHY) cata-
lyst for aziridination again using PhI2NTs as the nitrogen
source, and found high yields for alkene aziridination without
the need for an excess of the alkene. Nevertheless, despite the
considerable potential of these catalysts to provide a synthetic
method for the asymmetric aziridination of alkenes, the struc-
ture of the complex and the mechanistic steps involved in the
transformation in the CuY zeolite remain unclear and require
further investigation.
Understanding the fundamental mechanistic features of
these reactions, and ultimately designing new and better
asymmetric solid catalysts, requires detailed knowledge on
how the chiral information is transferred to the reactant(s) and
whether there is any interaction between the chiral modiÐer
and the catalytically active species. In the current work, we
have used EPR spectroscopy to study this problem and to
explore at the molecular level the interactions of a CuNaY
catalyst with the solvent, the chiral modiÐer bis(oxazoline) and
the reactants themselves. EPR has been widely used for many
years to study the bonding, co-ordination and location of
copper ions11h18 and encapsulated copper complexes19h21
inside the pores of zeolites. However, while EPR is sensitive to
changes in the local co-ordination environment of the Cu(II)
ion, multiple resonance techniques such as electron nuclear
double resonance (ENDOR) or pulsed techniques such as
electron spin echo envelope modulation (ESEEM) are
required to detect the long range interactions with the sur-
rounding or distant ligand nuclei which are too weak to detect
in the normal EPR spectrum. While Kevan et al.,22h24 have
successfully applied ESEEM spectroscopy to understand the
interactions and spatial co-ordination of transition metal ions
1
. Introduction
For many years zeolites have been successfully used as solid
catalysts for large scale industrial processes such as the pro-
duction of gasoline and related petrochemicals. However in
recent years zeolites have taken on a new role as novel enan-
tioselective catalysts for Ðne chemical synthesis.1h3 Indeed
with increasing demand for optically pure compounds for
pharmaceutical and agrochemical applications, enantio-
selective catalysis is now becoming more and more important.
Many experimental approaches have subsequently been devel-
oped for the design of new enantioselective systems for heter-
ogeneous catalysis, such as the use of a chiral support for an
achiral metal catalyst, immobilisation of an asymmetric cata-
lytically active complex onto an achiral support and modiÐ-
cation of an achiral catalyst using a chiral cofactor. The latter
approach has been successfully demonstrated by one of us in
recent years, for example, in the enantioselective dehydration
of butan-2-ol using a chiral dithiane 1-oxide modiÐed HY
zeolite.4,5 Another demonstration of the induced enantio-
selective properties of modiÐed zeolites, and the purpose of
the present investigation, is the enantioselective aziridination
of alkenes based on copper exchanged zeolite Y.6h8
The importance of developing an e†ective enantioselective
aziridination catalyst is underpinned by the extensive applica-
tions of chiral aziridines in Ðne chemical synthesis since they
can be easily converted into chiral diamines and amino-
alcohols. Copper catalysts modiÐed with chiral bis(oxazolines)
have recently proven to be efficient for the homogeneously
catalysed reaction using N-(p-tolylsulfonyl)iminophenyliodin-
¤
Permanent address: Institute of Chemical Technology, University
of Stuttgart, 70550 Stuttgart, Germany.
DOI: 10.1039/b010083h
Phys. Chem. Chem. Phys., 2001, 3, 1073È1080
This journal is ( The Owner Societies 2001
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in microporous materials with various inorganic and organic
probe molecules, ENDOR has been less widely applied to
these systems.25 Nevertheless the additional information
obtained from these multiple resonance techniques is vital for
studies in heterogeneous catalysis since analysis of the
ENDOR and ESEEM spectra amounts to identifying the
magnetic components of the ligand superhyperÐne tensor and
thereby provides information on the electronic and geometri-
cal arrangement of the ligands and reactants around the Cu(II)
active site.
gaussmeter calibrated using the perylene radical cation in con-
centrated H SO with g \ 2.002 56. The spin Hamiltonian
2
4
parameters were determined by simulation of the spectra
using the SIM14S software by G. P. Lozos et al. (modiÐed by
J. M. Lagan, DOS version of 06/09/1991).
Absolute spin concentrations of Cu(II) ions were determined
by double integration of the Ðrst derivative EPR spectrum,
with reference to two di†erent sets of standards
(CuSO É 5H O and TiCl É 3THF) for accurate comparisons.
4
2
3
In the present paper we have used EPR to probe the co-
ordination of a CuNaY zeolite in the presence of a chiral
bis(oxazoline) modiÐer and to examine the e†ects of the reac-
tion solvent (acetonitrile) and di†erent substrates such as
PhI2NTs (the nitrogen source) and styrene (the reactant) on
the EPR spectra.
Results
Catalytic tests
Before the spectroscopic measurements were performed, an
experiment using 1.4CuNaY (the numerical preÐx indicates
the metal content in wt.% of the dry catalyst) as a catalyst in
the aziridination of styrene (Scheme 1) was carried out in
order to conÐrm the catalytic activity of the zeolite sample. An
isolated yield of aziridine of 75% (based on styrene) and an ee
of 52% were observed, which compares well to the extensive
results obtained previously by our group in this system6h8
and demonstrates the appropriate catalytic nature of the
samples used in this study.
Methods
CuNaY sample preparation
CuNaY was prepared using a conventional ion exchange
method in which zeolite NaY (ex Zeolyst International,
CBV100, n /n \ 2.5) was treated with an aqueous copper(II)
Si Al
acetate (ex Aldrich) solution. The solid was then washed with
distilled water and calcined for several hours at 823 K in air
prior to use. Elemental analysis of the sample was performed
by ICP-AES on a Perkin Elmer Plasma 400 spectrometer.
Catalytic activity
The aziridination reaction was carried out in a batch reactor
at 298 K. The chiral modiÐer or diimine complex used in the
current work was (S)-([)-2,2@-isopropylidenebis(4-phenyl-2-
oxazoline and will hereafter be referred to as bis(oxazoline).
Scheme 1
The catalyst (m
\ 0.329 g) and the chiral modiÐer (with
cat., dry
EPR spectra of calcined CuNaY
a n /n
ratio of B2) were stirred together in 2.5 cm3
Cu chiral modifier
anhydrous acetonitrile (ex Aldrich) for 20 min. Then 0.101 g
styrene (ex Aldrich) and 0.563 g PhI2NTs (prepared according
to Yamada et al.),26 were added. After stirring for 24 h, the
products were isolated by column chromatography (Matrex
Silica 60, Fisher) and analysed using chiral HPLC (column:
The CuNaY catalyst was prepared as described in the Experi-
mental section and above all in a manner strictly similar to
that reported by Langham et al.,6h8 in their aziridination
studies on a similar catalyst. The catalyst was calcined in air
at 823 K for several hours, evacuated on a conventional
vacuum manifold (10~3 Torr) at 298 K for ca. 1 h, and Ðnally
the EPR spectrum was recorded at 100 K (Fig. 1). An identical
spectrum was also obtained at 10 K. This spectrum is typical
of a d9 Cu(II) ion in an axial ligand Ðeld. The high resolution
of both the g and A components is due to the well-deÐned
co-ordination states of the fully dehydrated ion. Because Cu(II)
has a nuclear spin of 3/2, four well-resolved hyperÐne lines are
25 cm Chiralcel OJ; eluent system: 82 vol.% hexane, 18 vol.%
isopropanol; Ñow rate: 3 cm3 min~1; wavelength: 235 nm), as
described in detail elsewhere.6h8
EPR spectroscopy
The zeolite samples for the EPR experiments were prepared
according to the procedure used in the catalytic experiments
above, i.e., the zeolite was calcined, stirred in anhydrous ace-
tonitrile with or without the chiral modiÐer, and in some
experiments the substrates were added. Afterwards, the sus-
pension was Ðltered, and the solid was gently dried at 298 K
for ca. 15 min, placed into an EPR cell and then gently evac-
uated on a conventional vacuum manifold (10~3 Torr).
expected in the g region. The additional lines observed in the
A
low Ðeld region of Fig. 1, indicate the presence of two di†erent
The homogeneous copper complexes were prepared by
adding the appropriate amounts of copper(II) acetate (ex
Aldrich) or copper(II) perchlorate (ex Aldrich) and two di†er-
ent bis(oxazoline) ligands, (S)-([)-2,2@-isopropylidenebis(4-
phenyl-2-oxazoline)
butyl-2-oxazoline] (ex Aldrich) (n /n
ethanol (ex Hayman Limited) containing a few drops of water.
To obtain a clear solution, the mixture was gently heated (ca.
or
2,2@-isopropylidenebis[(4S)-4-tert-
B 1), to
Cu chiral modifier
313 K) and/or Ðltered through a plug of cotton wool. The
frozen solution EPR spectra were recorded at 10 K (obtained
after adding a drop of dimethyl sulfoxide (ex Aldrich) to
obtain a good glass).
The EPR spectra were recorded on a Varian E109 and a
Bruker ESP 300-E Series X-band spectrometer at ca. 100 or
Fig. 1 EPR spectrum of 1.4 CuNaY: calcined at 823 K and degassed
at 298 K.
10 K. The g values were determined using a Bruker NMR
1074 Phys. Chem. Chem. Phys., 2001, 3, 1073È1080

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Cu(II) sites in the calcined/dehydrated CuNaY sample. This
assignment will be discussed in detail later.
EPR spectra of calcined CuNaY with acetonitrile and chiral
bis(oxazoline)
After calcination, the CuNaY sample was stirred in acetonitri-
le at 298 K for 30 min. The suspension was then Ðltered, the
solid was gently dried at 298 K for ca. 15 min and a small
amount of the sample (ca. 20 mg) placed into an EPR cell. The
sample was subsequently evacuated (10~3 Torr) at progres-
sively higher temperatures, while the low temperature EPR
spectra (100 K) were recorded at each successive step. The
resulting spectra are shown in Fig. 2, while the corresponding
g and hyperÐne coupling values (A) are listed in Table 1
(
2
along with selected literature values11h13,18,27). Evacuation at
98 K creates a single Cu(II) species which is clearly evident in
the low temperature spectrum (100 K) and characterised by
the g value of 2.37 and A value of 12.7 mT. The A hyper-
A
A
A
Ðne splitting is illustrated with the “stick diagramÏ in Fig. 2.
However, after evacuation at higher temperatures (323 and
3
73 K) a second Cu(II) species (hereafter labelled species B@)
becomes visible in the low temperature spectrum, indicating
the presence of a second co-ordination environment for the
exchanged copper ions, with a gA value of 2.32 and an A
A
value of 16.5 mT (Table 1). The intensity of species B@ grows as
the evacuation temperature increases.
The spin concentration of Cu(II) ions in the CuNaY sample
after stirring in acetonitrile was determined by double integra-
tion of the Ðrst derivative EPR signal (Fig. 2(a)). This value
was compared to the integrated intensities from two di†erent
sets of standards. Reproducible and similar results were
obtained using both standards. The analysis indicated that
Fig. 2 EPR spectra of 1.4 CuNaY: calcined at 823 K, stirred in ace-
tonitrile and degassed at di†erent temperatures (a)È(c).
2.18 ] 10~4 moles of EPR visible Cu(II) ions are present per
gram of catalyst. This corresponds to 1.39 wt.% copper in the
catalyst and demonstrates that practically all of the available
copper ions exchanged into the NaY zeolite exist as Cu(II)
ions. The spectra were also recorded at 10 K and a similar
value of the integrated Cu(II) intensity was observed.
two new sets of peaks can now be identiÐed. The Ðrst peak
(labelled BA in Table 1) produces g and A values which are
very similar to those of species B@, while the second peak
(labelled C and identiÐed with the asterisk in Fig. 3) has
values which are quite di†erent from the A and B type species.
The new peaks were only observable in the presence of the
bis(oxazoline) ligand, as no analogous peaks were observed in
either the calcined or acetonitrile treated CuNaY sample.
Nevertheless, despite the marked changes to the proÐle of the
EPR spectrum, the overall integrated signal intensity
remained constant at 100 or 10 K (i.e., no loss in Cu(II) con-
centration upon addition of the chiral ligand).
The CuNaY sample was then stirred in the acetonitrile
solvent containing the chiral bis(oxazoline) modiÐer,
(
n /n
B 2). Prior to recording the spectrum, the
Cu chiral modifier
suspension was Ðltered and the solid was allowed to dry in air
at 298 K. The resulting EPR spectra of the sample evacuated
at di†erent temperatures are shown in Fig. 3. Each spectrum
was recorded at 100 K. The addition of the chiral modiÐer
clearly produces a more complex EPR proÐle (compared to
Fig. 2 for CuNaY and acetonitrile only) because, in addition
to the peaks ascribed above (arising primarily from species A),
Table 1 Measured g and A values for Cu(II) species in ZSM-5 and Y zeolites
Species
gA
AA/mT
gM
AM/mT
Ref.
Pseudo-octahedrally co-ordinated copper in ZSM-5
Pseudo-square-pyramidally co-ordinated copper in
ZSM-5
Pseudo-square planar co-ordinated copper in ZSM-5
Pseudo-octahedrally co-ordinated copper in Y
Pseudo-square-pyramidally co-ordinated copper in Y
Pseudo-square planar co-ordinated copper in Y
2.37È2.40
2.31È2.36
12.0È13.6
14.1È15.6
2.08È2.09
2.06È2.07
0.9
1.4È1.9
13, 18
13, 18
2.27È2.29
2.38È2.39
2.33
2.24
2.37
2.32
2.37
2.37
2.31
15.6È17.4
12.4È14.5
15.7È18.2
16.8È17.5
12.6È14.3
16.5
12.7
12.6È13.9
16.5
2.05
2.07
2.07
2.2È2.9
1.3
1.9È2.0
13, 18
12, 27
12, 27
11
a
a
a
a
a
1
.4 CuNaY (calcined and degassed at 298 K)
A
2.04
2.05
2.07
2.06È2.08
2.08
2.05
2.06
1.3
2.0
1.1
1.3
2.0
1.7
0.5
B@
1
1
.4 CuNaY (stirred in acetonitrile, degassed at 298 K)
.4 CuNaY (stirred in acetonitrile with the chiral
ligand, (S)-([)-2,2@-isopropylidenebis-
A
A
BA
(
4-phenyl-2-oxazoline), and degassed at 298 K)
C
2.26
2.29
17.3
16.0
a
a
Cu[(S)-([)-2,2@-isopropylidenebis-
D
(
4-phenyl-2-oxazoline)] (OR)
2
Cu[2,2@-isopropylidenebis-
D
2.29
14.9
2.06
0.5
a
(
(4S)-4-tert-butyl-2-oxazoline)] (OR)
2
a This work.
Phys. Chem. Chem. Phys., 2001, 3, 1073È1080
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Fig. 4 (a) EPR spectrum of 1.4 CuNaY: calcined at 823 K, stirred in
acetonitrile with bis(oxazoline). The spectrum was then recorded after
addition of PhI2NTs (b) or water (c) to the catalyst. Each sample was
degassed at 298 K.
Fig. 3 EPR spectra of 1.4 CuNaY: calcined at 823 K, stirred in ace-
tonitrile with the bis(oxazoline) and degassed at di†erent temperatures
(
a)È(c).
In order to obtain accurate g and A values of the copper
species, computer simulations of the EPR spectra were carried
out. The experimental and resulting simulated spectra of
In order to explore the changes to the co-ordination and
oxidation state of the Cu(II) ions, the EPR spectra were mea-
sured after various substrates, including PhI2NTs and styrene,
had been added to the suspension of the CuNaY catalyst in
acetonitrile containing the bis(oxazoline) ligand. The quan-
tities of catalysts and substrates used in these tests were identi-
cal to those employed in the catalytic experiments. After 1 h of
reaction, the catalyst was Ðltered and gently dried before evac-
uation at 298 K in the EPR cell. The resulting EPR spectra
are shown in Fig. 4 for the case in which PhI2NTs and styrene
only were added. The most pronounced change in the spectra
was the complete disappearance of the peaks assigned to
species BA and C and the partial loss of the Cu(II) signal inten-
sity. However, the Ðnal Cu(II) concentrations were di†erent
depending on the adsorbate used. Based on the initial Cu(II)
1.4CuNaY sample after stirring in acetonitrile and after stir-
ring in acetonitrile with the bis(oxazoline) ligand, are ahown
in Fig. 5(a) and (b) respectively.
EPR of copper bis(oxazoline) complex
A series of experiments was also performed on a freshly pre-
pared copper-bis(oxazoline) complex dissolved in acetonitrile,
in order to compare the EPR proÐle (including g and A
values) of the homogeneous copper complex in frozen solution
with the spectra obtained from the heterogeneous CuNaY
system. The dissolved samples were prepared according to the
method described in the experimental section. The EPR
spectra were recorded at 10 K for two slightly di†erent com-
plexes,28 namely Cu[(S)-([)-2,2@-isopropylidenebis(4-phenyl-
signal intensity for the CuNaY sample stirred in CH CN plus
3
bis(oxazoline) (Fig. 4(a)), the signal intensity decreased after
the addition of the various substrates. The extent of reduction
in signal intensity is given in Table 2. These measured inten-
sities varied slightly from experiment to experiment and have
a ^10% degree of accuracy.
2-oxazoline)](OR) and Cu[2,2@-isopropylidenebis((4S)-4-tert-
2
butyl-2-oxazoline)](OR) (Fig. 6(a) and (b) respectively). The
2
two bis(oxazoline) ligands di†er in the nature of the substit-
uents, which are either the phenyl groups (see structure in
Table 2 Changes to the Cu(II) EPR signal intensity, and absolute Cu(II) concentrations, after addition of various substrates to the CuNaY
catalyst stirring in solvent (CH CN) and chiral bis(oxazoline) ligand
3
EPR signal intensity
(%)
Cu(II) concentration
moles (g CuNaY)~1
Sample
1) CuNaY stirred in
(
100
2.2 ] 10~4
CH CN with bis(oxazoline)
3
(
(
(
1) ] PhI2NTs
55
75
90
1.2 ] 10~4
1.6 ] 10~4
2.0 ] 10~4
1) ] styrene
1) ] PhI2NTs/styrene
1076
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Fig. 5 Experimental and simulated EPR spectra of 1.4 CuNaY: cal-
cined at 823 K: (a) stirred in acetonitrile and degassed at 298 K; (b)
stirred in acetonitrile with the bis(oxazoline) ligand and degassed at
Fig. 6 Experimental and simulated EPR spectra of (a) Cu[(S)-([)2,
2@-isopropylidene-bis(4-phenyl-2-oxazoline)](OR)
and (b) Cu[2,
2
2@isopropylidenebis((4S)-4-tert-butyl-2-oxazoline)](OR) . Note; the
2
2
98 K.
simulated linewidths were kept to a minimum to highlight the super-
imposed superhyperÐne structure from the 14N nuclei.
Scheme 1) or the tert-butyl groups. The corresponding com-
puter simulations are also shown in Fig. 6. The g and A values
obtained from the simulations are slightly di†erent compared
to those observed in the case of CuNaY catalyst containing
the chiral diimine modiÐer. These di†erences likely arise from
the two di†erent environments of the copper ions (i.e., inside
the zeolite supercage and in frozen homogeneous solution),
indicating that the ion experiences local perturbations which
are dissimilar in the two cases. A more accurate analysis of
this e†ect will be studied using 2D-ESEEM experiments to
compare the di†erence in structure of the Cu-complex from
the encapsulated NaY zeolite complex to the frozen solution
complex.
Only one set of hyperÐne components was resolved for the
dissolved complexes in frozen solution, indicating the presence
of a single Cu(II) co-ordination state and this species will here-
after be labelled D. Owing to the high resolution and anisot-
ropy in the spectra, additional superhyperÐne structure
associated with the interacting 14N nuclei can be resolved in
the parallel and perpendicular features.
In addition, well-resolved c.w. ENDOR spectra were
recorded for the homogeneous complex (spectra not shown
for brevity) conÐrming the coordination of the bis(oxazoline)
ligand to the Cu(II) ion. In particular the di†erent resonances
of the phenyl and tert-butyl groups on the ligand (which play
an important role in directing the enantioselectivity) were
easily identiÐed. The c.w. ENDOR spectra were also recorded
on the CuNaY sample containing the bis(oxazoline) ligand for
comparison, in an attempt to conÐrm the formation of a Cu-
bis(oxazoline) complex in the Y zeolite supercages. Although
of CuNaY were insufficiently resolved to allow an unam-
biguous assignment. Other EMR methods such as ESEEM or
pulsed ENDOR will be required to improve the quality of the
spectra.
Discussion
Co-ordination mode of CuNaY: evidence for encapsulated
Cu-bis(oxazoline) complexes
The EPR proÐles of Cu(II) exchanged zeolites have been thor-
oughly investigated for many years.11h18,22h24 Generally,
broad structureless spectra are reported at 298 K for the fresh
Cu(II) exchanged samples, due to the presence of fully
hydrated six-fold co-ordinated Cu(II) ions. Although such a
hydrated complex experiences a tetragonally elongated co-
ordination environment (due to a JahnÈTeller e†ect), which
produces an asymmetric proÐle, rapid tumbling of the
complex at 298 K results in an averaged and unresolved spec-
trum. At low temperatures, this rapid tumbling is quenched,
so that an anisotropic (axial) EPR proÐle is instead obtained,
with well-resolved g and A values. As water is progressively
removed from the sample, the tumbling complex becomes
immobilized on the zeolite pore walls, and eventually after
extensive dehydration by thermovacuum treatments the
copper ions directly bond to the framework oxygens. This
produces low co-ordinated copper species, possessing sym-
metries such as square pyramidal or in the case of ZSM-5
square planar, and these symmetries are again manifested in
the EPR spectra with axial (tetragonal) symmetry correspond-
ing to the unpaired electron located mostly in a d(x2Èy2)
orbital of the copper ion. This situation is clearly exempliÐed
1
H resonances were detected which were similar to those
observed for the homogeneous complex, the ENDOR spectra
Phys. Chem. Chem. Phys., 2001, 3, 1073È1080
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in the spectrum of the calcined (823 K) and evacuated CuNaY
sample (Fig. 1). The well-resolved and highly anisotropic spec-
trum is due to the presence of low co-ordinated and dehy-
drated Cu(II) ions anchored onto the walls of the zeolite host,
since most of the water is removed in the calcination and sub-
sequent evacuation steps.
Two copper species (labelled A and B@) can be identiÐed in
the EPR spectrum of the calcined material (Fig. 1). Two addi-
tional species were identiÐed (labelled BA and C) in the aceton-
itrile treated sample containing the bis(oxazoline) ligand. As
seen in Table 1, the g and A values of the species A, B and C
are signiÐcantly di†erent, and this indicates the presence of
three distinct groups of copper environments. Species B@ and
BA have very similar EPR parameters, and represent two
similar but slightly di†erent square pyramidal co-ordination
environments of the copper. In order to understand the origin
and symmetry of each species, it is necessary to consider
brieÑy the relationship between the observed g and A values,
and the associated molecular symmetry.
dal site, in the former case the geometry arises due to co-
ordination with the solvent molecules, while in the latter case
it clearly arises through complexation with the diimine ligand
which have a strong tendency to complex with Cu(II) ions. It
should be recalled that species B@ was only visible in the ace-
tonitrile treated CuNaY samples after evacuation at 323 K.
However, in the presence of the chiral modiÐer, the square
pyramidal species BA was visible even in the pre-evacuated
sample, indicating that the ligand is directly responsible for
the formation of species BA and C. In other words, while dif-
ferent Cu(II) locations can exist in the Y zeolite, and may
display di†erent reactivities as a function of location and
loading, the observed trends clearly arise from the preferential
interaction of one copper site with the chiral ligand. Unfor-
tunately no superhyperÐne interaction from the 1H or 14N
nuclei of the bis(oxazoline) ligand could be identiÐed in the
EPR spectra. However, given the fact that only 40% of the
entire Cu(II) signal is composed of Cu(II)-bis(oxazoline) com-
plexes (see below), and with the added complexity of line
broadening due to the heterogeneous distribution of sites
within the CuY zeolite, the absence of resolved superhyperÐne
splitting is not surprising.
Two prevailing copper species have been reported in Cu(II)
exchanged Y zeolites, which are pseudo-octahedrally co-
ordinated copper ions which exist in the supercages (g \ 2.38
A
to 2.39, A \ 12.4 to 14.5 mT, g \ 2.07, A \ 1.3 mT) and
Based on the results of the computer simulations, ca. 40%
of all the available copper ions co-ordinate with the
bis(oxazoline) ligand producing sites with square planar sym-
metry (species C; 20%) and square pyramidal symmetry
(species BA; 20%). This percentage of complexed copper also
A
M
M
square-pyramidally co-ordinated copper ions which exist in
the sodalite cages (g \ 2.33, A \ 15.7 to 18.2 mT, g \ 2.07,
A
A
M
A \ 1.9 to 2.0 mT).12,27 Similar copper species can also be
M
identiÐed in Cu(II) exchanged ZSM-5 materials, including
octahedral (g \ 2.37 to 2.40, A \ 12.0 to 13.6 mT, g \ 2.08
makes sense considering that
a
molar ratio of
A
A
M
to 2.09, A \ 0.9 mT) and square pyramidal (g \ 2.31 to
Cu(II) : bis(oxazoline) of 2 : 1 was used in the reaction, and
conÐrms the previous results from our group8 that not all the
Cu(II) cations are modiÐed by the chiral ligand. Knowing that
the overall Cu(II) concentration is 2.18 ] 10~4 mol (g
catalyst)~1, this ca. 40% contribution of the signal intensity
from species BA and C translates into approximately one
copper-bis(oxazoline) complex per supercage. It is not clear if
any copper-bis(oxazoline) complexes with octahedral sym-
metry are formed inside the zeolite, as these sites would
produce spin Hamiltonian parameters analogous to the fully
solvated copperÈacetonitrile ions (species A) and therefore
give rise to overlapping signals. Nevertheless it is evident from
the EPR data that one copper-bis(oxazoline) complex is
indeed formed per supercage either with square planar or
square pyramidal symmetry, and considering a 2 : 1 molar
ratio of Cu(II) : bis(oxazoline) was used, it is unlikely that any
pseudo-octahedral Cu(II)-bis(oxazoline) complexes are formed.
This data provides the Ðrst direct experimental evidence that a
Cu(II)-bis(oxazoline) complex, responsible for the enantio-
selective aziridination properties, actually forms inside the
zeolite supercage.
M
A M M
A
2
.36, A \ 14.1 to 15.6 mT, g \ 2.06 to 2.07, A \ 1.4 to 1.9
mT) co-ordination.18 In addition, a square planar co-
ordination mode of copper (g \ 2.27 to 2.29, A \ 15.6 to
A
A
1
7.4 mT, g \ 2.05, A \ 2.2 to 2.9 mT) has been observed in
M M
this system,13,18 whereas in faujasites this copper environment
was only identiÐed in the presence of N-containing ligands,
e.g., NH (g \ 2.24, A \ 16.8 to 17.5 mT).11 To the best of
3
A
A
our knowledge, there is only one report in the literature on a
dehydrated CuY system with spin Hamiltonian parameters
typical of square planar symmetries which was assigned to
Cu(II) ions in site SI@ co-ordinated with three lattice oxygen
atoms and one extra-framework oxygen of a hydroxide
forming a deformed tetrahedron.22 No square planar co-
ordination modes of copper are reported for the calcined
materials themselves.
It is therefore clear from the above analysis of the literature
that species A (g \ 2.37; A \ 12.6 to 14.3 mT) and B@ (g \
A
A
A
2
.32 A \ 16.5 mT) can be assigned to a pseudo-octahedral
A
and square pyramidal copper site respectively. These are the
only two sites visible in the calcined material after evacuation
at 298 K (Fig. 1 and Table 1). Only one species A was visible
in the calcined sample after stirring in acetonitrile and evac-
uation at 298 K (Fig. 2(a)) which can be assigned to a fully
solvated copper species with a pseudo-octahedral ligand
environment. At higher evacuation temperatures (323 and 373
K; Fig. 2(b) and (c) respectively) some of the acetonitrile mol-
ecules are removed from the solvated copper complex, so that
lower copper symmetries are observed (i.e., similar to the
above mentioned dehydration process; as solvating molecules
such as water or acetonitrile are removed, the co-ordination
state of the cation is reduced).
Interactions of Cu-bis(oxazoline)complex in CuNaY with
adsorbates
Having established the formation of copper-bis(oxazoline)
complexes inside the supercage of the CuNaY zeolite, we sub-
sequently studied the inÑuence of adsorbates on the co-
ordination and oxidation state of the copper ions, to examine
if EPR could provide any understanding of the reaction
mechanism involved. The CuNaY zeolite was stirred in the
usual way with acetonitrile containing the chiral modiÐer, and
afterwards PhI2NTs or styrene were added to the system, as
carried out in the catalytic tests. In all cases, two pronounced
e†ects were observed in the EPR spectra. Firstly, the low sym-
metry Cu(II) species such as the square planar or square pyr-
amidal Cu(II)-bis(oxazoline) sites disappeared (Fig. 4), and
secondly the overall Cu(II) concentration decreased to di†erent
extents for all added adsorbates with the exception of the co-
addition of PhI2NTs/styrene (Table 2). If the interaction of
the adsorbate with the copper(II) ions resulted only in a
change of co-ordination mode (for example from square
However, when the chiral bis(oxazoline) modiÐer is added
to the CuNaYÈacetonitrile suspension, the EPR spectra
change dramatically (Fig. 3) and at least two new species BA
and C become visible. The g and A values of these species are
consistent with a square pyramidal (BA) and square planar (C)
co-ordination mode of copper. This strongly indicates that
complexation of the copper(II) ions has occurred with the
chiral modiÐer forming copper-bis(oxazoline) complexes with
square pyramidal and square planar symmetries. Although
species B@ and BA have both been assigned to a square pyrami-
1078
Phys. Chem. Chem. Phys., 2001, 3, 1073È1080

View Article Online
planar to square pyramidal or octahedral) without any change
in paramagnetism of the sample, then the integrated Cu(II)
signal intensity should remain constant; only the Cu(II) signal
proÐle will change. However, if the copper ions are involved in
complex catalytic steps with the reagents involving variable
copper valence states, then the Cu(II) concentration would be
expected to change.
One must also consider the additional complexity of EPR
silent Cu(II) centres, when interpreting changes to the Cu(II)
integrated signal intensities, as Cu(II) can be rendered invisible
due to spin pairing in dimeric or higher aggregate states or
due to fast relaxation processes. For example, a decrease in
the EPR signal intensity of Cu(II) in Y and ZSM-5 zeolites was
reported following gentle evacuation of the sample at 298
K.18,19 The decrease in signal intensity for CuY was
accounted for, not by reduction of Cu(II) to Cu(I) or strong
magnetically interacting states, but due to the presence of spe-
ciÐc Cu(II) symmetries such that nearly degenerate ground
states occur producing strong spinÈlattice relaxation e†ects
which broaden the signal beyond detection.29 The unde-
tectability of Cu(II) in Cu-ZSM-5 on the other hand was
ascribed to magnetic spinÈspin relaxation e†ects via
In the current Cu(II)NaY system, when the reaction was
carried out in the presence of co-added PhI2NTs/styrene,
practically no change in Cu(II) concentration was observed
(only ca. 10% loss of intensity occurred). However, when
PhI2NTs was added separately to the catalyst, a net decrease
in the Cu(II) ion concentration (ca. 45%) occurred. This ca.
45% correlates well with the ca. 40% abundance of Cu(II)-
bis(oxazoline) complexes formed in the CuNaY system.
Regardless of any interaction between PhI2NTs and the
remaining 60% of the non-complexed Cu(II), no change
occurred in the EPR proÐle for these ions. These non-
complexed cations will be completely solvated by the acetoni-
trile molecules, and may therefore be unavailable for efficient
interaction with PhI2NTs. As Li et al.,32 demonstrated
through catalytic studies of copper with tetradentate neutral
ligands such as Cu-Salen, the existence of multiple open co-
ordination sites on the copper may be crucial to catalysis of
aziridination.
It is difficult to understand how the hypothesised Cu(III)-
nitrenoid intermediate (Cu2NTs) can form in the present
system (eqn. (1)) unless we invoke an equilibrium state involv-
ing Cu(I) and Cu(III) in solution (eqn. (3)). It should be recalled
that only 40% of the copper ions are complexed with the
bis(oxazoline) so that in the acetonitrile solvent, at least some
of the remaining copper ions (60%) will be mobile in the
zeolite supercages and may establish the proposed mechanism.
Assuming the formation of Cu(I) states via eqn. (3), the addi-
tion of PhI2NTs should lead to the formation of PhI and a
copper nitrenoid species (eqn. (4)), as discussed by Li et al.10
As a result the Cu(II) signal intensity would be expected to
decrease, as indeed observed.
CuÈ(OH )É É É(OH)ÈCu
However as the EPR spectra were recorded at both 10 and
Ñuctuating
hydrogen
bonds.18
2
100 K with no apparent di†erences in intensity, another
process must be responsible for the loss of Cu(II) intensity as
the reagents are added. Furthermore, because the integrated
EPR spectra indicates that most of the Cu(II) is present as
EPR visible Cu(II) ions, the possibility of diamagnetic Cu pairs
can be eliminated.
The diimine chiral complexes of both Cu(I) and Cu(II) are
capable of mediating alkene aziridination9,10,30 and a number
of reports have appeared on the mechanism of the reaction by
these complexes. Li et al.,10 considered two possible interme-
diates in the (diimine)copper(I) catalysed asymmetric aziridina-
tion of alkenes in solution. In this Ðrst case the role of a
discrete Cu(III)-nitrenoid intermediate (Cu2NTs) in the pres-
ence of PhI2NTs as nitrogen donor was considered (eqn. (1));
2
Cu(II) 7 Cu(I) ] Cu(III)
(3)
(4)
(5)
(6)
PhI2NTs ] Cu(I) ] PhI ] Cu2NTs`
Cu2NTs` ] styrene ] Cu(I) ] aziridine
Cu(I) ] Cu(III) 7 2Cu(II)
When styrene is present in the system, the reaction of the
copper nitrenoid intermediate may proceed to produce the
aziridine product and regenerate the Cu(I) site (eqn. (5)). Again
through a similar equilibrium process as in eqn. (3), the orig-
inal Cu(II) sites may be reformed (eqn. (6) which is likely to be
a slow rate-determining step), so that the overall Cu(II) con-
centration remains constant at the end of the reaction. In fact
the integration data reveals that addition of PhI2NTs/styrene
to the catalyst only leads to a 10% loss in Cu(II) signal inten-
sity. If we assume that an equilibrium process (eqn. (3) and (6))
can be established inside the supercages of CuNaY due to the
presence of closely interacting Cu(II) pairs or dimers, then the
current results, based on eqn. (3)È(6), would support a similar
type of redox mechanism for aziridination as suggested by Li
et al.,10 since both Cu(I) and Cu(III) intermediates are EPR
silent. However it must be clearly pointed out that although
this interpretation could account for the EPR data, it is
unlikely to occur because it depends on the rate-limiting steps
in eqn. (3) and (6). While these reactions can occur in solution,
there is no evidence to suggest that they occur within the con-
Ðned spaces of the zeolite pores, so the actual mechanism
remains uncertain.
In addition, the disappearance of an EPR signal must be
interpreted with caution, since EPR provides no information
on the nature of any resulting diamagnetic state. While the
formation of the Cu(III)-nitrene intermediate could account for
the observed trends, or possibly the formation of a dia-
magnetic state formed directly between Cu(II) and the
PhI2NTs, one cannot disregard the possibility that a direct
interaction between the Cu(II) ions (functioning as a Lewis
acid) and the PhI2NTs also proceeds, to produce an unknown
diamagnetic intermediate which is responsible for nitrogen
LCu`PF ~ ] PhI2NTs ] [LCu2NTs]`PF ~ ] PhI (1)
6
6
where L represents the di†erent chiral diimine ligands. In this
redox process, PhI is fully dissociated from the aziridinating
species. However a possible alternative to this redox mecha-
nism involves the copper complex functioning as a Lewis acid,
in which the PhI2NTs is covalently attached to the active site;
zTs
LCu`PF ~ ] PhI2NTs ] [LCuÈN
]PF ~ (2)
6
6
}I`Ph
Li et al.,10 concluded from their results on Cu(I) that although
the catalytic aziridination with PhI2NTs can occur by more
than one mechanism, the evidence obtained for the
(
diimine)copper catalysed reaction strongly implicated the
redox mechanism. Experimental data also provided evidence
for a common oxidation state of the catalyst irrespective of
the oxidation state of the starting copper complex,31 and that
these reactions are being catalysed through Cu(II) rather than
Cu(I). It was reported that treatment of an acetonitrile solu-
tion of Cu triÑate (CuOTf) in the presence of a bis(oxazoline)
ligand with PhI2NTs generated a copper species which was
indistinguishable from one generated by an identical protocol
using Cu(OTf) suggesting that a similar catalytically e†ective
2
complex is accessible from either oxidation state.9 It was also
demonstrated that PhI2NTs oxidises Cu(I)OTf/bis(oxazoline)
to a Cu(II) complex and that beginning with either Cu(I)(OTf)
or Cu(II)(OTf)2 salts as precursors, gives identical relative
reactivities towards various pairs of alkenes.31 Although Cu(II)
can also catalyse the reaction, no details were given on the
nature of the intermediate involving Cu(II) and PhI2NTs.
Phys. Chem. Chem. Phys., 2001, 3, 1073È1080
1079

View Article Online
transfer. In conclusion, we cannot discriminate with con-
Ðdence between the two proposed reaction pathways in the
literature (eqn. (1) and (2)) based on the current EPR data, as
both pathways involve a diamagnetic intermediate between
the copper complex and the PhI2NTs.
those observed in the CuNaY catalysts, providing added con-
Ðrmation for the complexation between ion exchanged Cu(II)
and the diimine ligand inside the zeolite.
Acknowledgements
EPR investigation of chiral Cu(II)-bis(oxazoline) complexes
We thank Sophia Taylor for help with the catalytic reaction
and HPLC analysis. The authors are also grateful to Siglinde
Mierke for the elemental analysis. Yvonne Traa gratefully
acknowledges Ðnancial support from the German Science
Foundation (DFG). Financial support from EPSRC for the
National ENDOR Service is gratefully acknowledged.
The frozen solution EPR spectra of the Cu[(S)-([)-2,2@-
isopropylidenebis(4-phenyl-2-oxazoline)](OR) and the Cu[2,
2
2
@-isopropylidenebis((4S)-4-tert-butyl-2-oxazoline)](OR) com-
2
plexes are shown in Fig. 6. The EPR spectra of the two com-
plexes are similar and the g and A values (Table 1; where
g \ 2.29 and A \ 14.9 to 16.0 mT), indicate the presence of
a strong square planar pattern which is consistent with either
a square planar or square pyramidal copper geometry
A
A
References
(
labelled species D). These parameters are also in agreement
1
2
W. Ho
M. G. Clerici, G. Bellussi and U. Romano, J. Catal., 1991, 129,
59.
G. J. Hutchings, P. C. B. Page and F. E. Hancock, Chem. Brit.,
977, 33, 46.
Ž
lderich, Stud. Surf. Sci. Catal., 1989, 49, 69.
with EPR results presented by Evans et al.,28 on similar
copper complexes. By comparison, the copper species BA and
C observed in the CuNaY samples and formed only in the
presence of the bis(oxazoline) ligand are also characterised by
low g values and high A values similar to species D and
1
3
4
1
S. Feast, D. Bethell, P. C. B. Page, M. R. H. Siddiqui, D. J.
Willock, F. King, C. H. Rochester and G. J. Hutchings, J. Chem.
Soc., Chem. Commun., 1995, 2409.
A
A
indicative of low symmetry copper ions. This similarity in g
and A values for species BA and C in CuNaY and species D in
frozen solution provides further evidence for the formation of
copper-bis(oxazoline) complexes inside the zeolite pores.
5
6
7
8
9
S. Feast, D. Bethell, P. C. B. Page, M. R. H. Siddiqui, D. J.
Willock, G. J. Hutchings, F. King and C. H. Rochester, Stud.
Surf. Sci. Catal., 1996, 101, 211.
C. Langham, P. Piaggio, D. Bethell, D. F. Lee, P. McMorn,
P. C. B. Page, D. J. Willock, C. Sly, F. E. Hancock, F. King and
G. J. Hutchings, Chem. Commun., 1998, 1601.
C. Langham, D. Bethell, D. F. Lee, P. McMorn, P. C. B. Page,
D. J. Willock, C. Sly, F. E. Hancock, F. King and G. J. Hut-
chings, Appl. Catal. A, 1999, 182, 85.
C. Langham, S. Taylor, D. Bethell, P. McMorn, P. C. B. Page,
D. J. Willock, C. Sly, F. E. Hancock, F. King and G. J. Hut-
chings, J. Chem. Soc., Perkin T rans. 2, 1999, 1043.
Conclusions
A CuNaY catalyst (1.4 wt.% Cu) was prepared by convention-
al ion exchange methods and successfully used for the cata-
lytic aziridination of styrene with PhI2NTs as the nitrogen
source. The co-ordination and oxidation state of the
copper(II)-containing catalyst was investigated by EPR before
and after the catalytic reaction. The EPR spectrum of the cal-
cined and evacuated samples revealed the presence of two
copper(II) sites inside the CuNaY zeolite with pseudo-
octahedral and square pyramidal geometries. After stirring the
calcined catalyst in acetonitrile solvent, a pseudo-octahedral
species was predominantly formed due to solvent co-
ordination to the copper ion. However, upon addition of the
chiral diimine ligand, two new copper(II)-bis(oxazoline) com-
plexes were identiÐed inside the zeolite with a square pyrami-
dal and square planar geometry. Approximately 40% of all
the available copper(II) ions in CuNaY were found to exist in
these complexed states after incorporation of the chiral ligand,
corresponding to one Cu(II)-bis(oxazoline) complex per super-
cage.
After one hour of reaction with styrene and PhI2NTs, the
EPR spectra revealed the preferential loss of the peaks assign-
ed to the square pyramidal and square planar Cu(II)-bis(oxa-
zoline) complexes in the CuNaY catalyst. The rearranged
spectrum indicated the presence of pseudo-octahedrally co-
ordinated copper ions but the overall Cu(II) integrated inten-
sity remained practically unchanged at the end of the catalytic
reaction. However, a selective disappearance of the Cu(II)-
bis(oxazoline) peaks was observed after the addition of
PhI2NTs only to the catalyst. It is clear from these results that
a diamagnetic intermediate is formed which involves PhI2NTs
and the Cu(II)-bis(oxazoline) complex, which reacts with
styrene to regenerate a paramagnetic Cu(II) complex. The
nature of these observations was discussed within the context
of the current proposed intermediates involved in the catalytic
reaction, but the mechanistic steps remain unclear based on
the current data.
D. A. Evans, M. M. Faul, M. T. Bilodeau, B. A. Anderson and
D. M. Barnes, J. Am. Chem. Soc., 1993, 115, 5328.
10 Z. Li, R. W. Quan and E. N. Jacobsen, J. Am. Chem. Soc., 1995,
1
17, 5889.
1
1
2
D. R. Flentge, J. H. Lunsford, P. A. Jacobs and J. B. Uytterhoe-
ven, J. Phys. Chem., 1975, 79, 354.
1
J. C. Conesa and J. Soria, J. Chem. Soc., Faraday T rans. I, 1979,
75, 406.
13 A. V. Kucherov, A. A. Slinkin, D. A. KondratÏev, T. N. Bond-
arenko, A. M. Rubinstein and K. M Minachev, Zeolites, 1985, 5,
3
20.
1
1
4
5
Y. Sendaka and Y. Ono, Zeolites, 1986, 6, 209.
S. C. Larsen, A. Aylor, A. T. Bell and J. A. Reimer, J. Phys.
Chem., 1994, 98, 11533.
16 M. W. Anderson and L. Kevan, J. Phys. Chem., 1994, 91, 4174.
17 E. Giamello, D. Murphy, G. Magnacca, C. Morterra, Y. Shioya,
T. Normura and M. Anpo, J. Catal., 1997, 136, 510.
1
8
J. Soria, A. Mart•
and Z. Schay, J. Catal., 2000, 190, 352.
S. Deshpande, D. Srinivas and P. Ratnasamy, J. Catal., 1999,
nez-Arias, A. Mart•nez-Chaparro, J. C. Conesa
 
1
9
188, 261.
20 C. R. Jacobs, S. P. Varkey and P. Ratnasamy, Microporous
Mesoporous Mater., 1998, 22, 465.
21 S. P. Varkey, C. Ratnasamy and P. Ratnasamy, J. Mol. Catal. A,
1
998, 135, 295.
2
2
2
3
J.-S. Yu and L. Kevan, J. Phys. Chem., 1990, 94, 7620.
J. Xu, J.-S. Yu, S. J. Lee, B. Y. Kim and L. Kevan, J. Phys. Chem.
B, 2000, 104, 1307.
24 J. Y. Kim, J.-S. Yu and L. Kevan, Mol. Phys., 1988, 95, 989.
25 D. Biglino, H. T. Li, R. Erickson, A. Lund, H. Yahiro and M.
Shiotani, Phys. Chem. Chem. Phys., 1999, 12, 2887.
2
6
Y. Yamada, T. Yamamoto and M. Okawara, Chem. L ett., 1975,
61.
3
2
2
7
8
R. A. Schoonheydt, Catal. Rev.ÈSci. Eng., 1993, 35, 129.
D. A. Evans, M. C. Kozlowski, C. S. Burgey and D. W. C. Mac-
Millan, J. Am. Chem. Soc., 1997, 119, 7893.
29 J. C. Conesa and J. Soria, J. Phys. Chem., 1978, 82, 1847.
3
3
3
0
1
2
D. A. Evans, M. M. Faul and M. T. Bilodeau, J. Org. Chem.,
A comparative EPR study was also performed on the dis-
solved Cu[(S)-([)-2,2@-isopropylidenebis(4-phenyl-2-oxazol-
1
991, 56, 6744.
D. A. Evans, M. M. Faul and M. T. Bilodeau, J. Am. Chem. Soc.,
ine)](OR)2
and the Cu[2,2@-isopropylidenebis ((4S)-4-tert-
1994, 116, 2742.
butyl-2-oxazoline)](OR) derivatives in frozen solution. The
Z. Li, K. R. Conser and E. N. Jacobsen, J. Am. Chem. Soc., 1993,
115, 5326.
2
EPR parameters of the Cu(II) complex were analogous to
1080
Phys. Chem. Chem. Phys., 2001, 3, 1073È1080