Enantioselective henry reaction catalyzed by "ship in a bottle" complexes

Article abstract of DOI:10.1021/ic400599c

Two chiral Schiff-base complexes of copper(II) have been successfully encapsulated inside the cavity of zeolite-NaY via a "ship in a bottle" synthesis method. The presence of the two complexes inside the cages of zeolite-Y has been confirmed based on vari

Full text of DOI:10.1021/ic400599c

Article
pubs.acs.org/IC
Enantioselective Henry Reaction Catalyzed by “Ship in a Bottle”
Complexes
Kusum K. Bania,† Galla V. Karunakar,‡ Kommuru Goutham,‡ and Ramesh C. Deka*,†
†Department of Chemical Sciences, Tezpur University, Napaam, Assam, India 784028
‡Division of Crop Protection Chemicals, Indian Institute of Chemical Technology, Uppal Road, Hyderabad, Andhra Pradesh, India
500007
S
* Supporting Information
ABSTRACT: Two chiral Schiff-base complexes of copper(II) have been
successfully encapsulated inside the cavity of zeolite-NaY via a “ship in a bottle”
synthesis method. The presence of the two complexes inside the cages of zeolite-Y
has been confirmed based on various spectrochemical and physicochemical
techniques, viz. FTIR, UV−vis/DRS, ESR, XPS, CV, EDX, SEM, and TGA.
Zeolite-encapsulated chiral copper(II) Schiff-base complexes are found to give a
high-enantioselective (84% ee, R conformation) nitro-aldol product at −20 °C.
The encapsulated copper complexes are found to show higher catalytic efficiency
than their homogeneous counterparts under identical conditions. Density
functional theory (DFT) calculation has been implemented to understand the
effect of the zeolite matrix on structural, electronic, and reactivity properties of the
synthesized complexes. Theoretical calculation predicts that upon encapsulation
into the zeolite matrix the Cu center becomes more susceptible to nucleophilic
attack, favoring a nitro-aldol reaction. A plausible mechanism is suggested based on the experimental and theoretical results. The
structures of reaction intermediates and transition state(s) involved in the catalytic cycle are derived using DFT.
important C−C bond-formation reactions whose ultimate
product has been used for various drug design.19 Starting
from Shibasaki et al.20 and Evans et al.,21 various chiral catalytic
systems have been designed to bring out the enantioselective
Henry reaction. Still, there remains a lot of scope to improvise
this reaction.
1. INTRODUCTION
Zeolite-encapsulated transition-metal and organometallic com-
plexes have now emerged as some of the potent competitors for
their homogeneous counterpart, in terms of catalytic behavior
and stability.1−6 The walls of the zeolite framework impart a
constrained environment to the encapsulated, so-called “ship in
a bottle”, complexes.7,8 The boundary or space constraint
imposed by the zeolite walls changes the structural and
electronic behavior of the encapsulated complexes in
comparison to their homogeneous analogue.9,10 These changes
further influence their catalytic activities and, consequently,
have fueled researchers to design more and more heteroge-
neous catalysts. In turn, this has resulted in a wide area of
catalysis called “heterogenization of homogeneous catalyst”.11,12
There are several reports in which zeolite-encapsulated
complexes have been found to exhibit better catalytic activity,
high turnover number, and high thermal stability in comparison
to their homogeneous counterpart.13−16 Besides having the
advantage of these hybrid materials in catalysis,17 zeolite-
encapsulated complexes have been currently studied to mimic
the biosystem and hence are also named zeozymes.18
Chiral Schiff bases and their complexes with transition metals
are one of the most studied chiral catalysts22 in asymmetric
synthesis.23,24 because of their ability to act as chiral catalysts or
as cocatalysts, Schiff-base ligands as well as their complexes
have been extensively applied either in the homogeneous phase
or by anchoring them in various inorganic supports, viz., zeolite,
MCM-41, clay, LDH, etc.15,25−30 However, to best of our
knowledge, the enantioselective Henry reaction performed by
incorporating such types of chiral catalysts in zeolite-Y is sparse.
So, considering the potentiality of chiral Schiff-base complexes,
the advantages of the heterogeneous catalytic system, and the
importance of the enantioselective Henry reactions, we have
synthesized two Schiff-base complexes of copper (Scheme 1)
inside zeolite-Y and tested these complexes for the
enantioselective Henry reaction. Excellent yield and high
enantioieselectivity have been obtained. Density functional
theory (DFT) calculation has been applied to understand the
nature of the structural and electronic changes that occur in the
complexes under the influence of the zeolite framework. On the
Finding a commercially viable pathway for the stereoselective
organic synthesis of biologically important chemicals may be
considered as one of the major thrust areas of chemistry today.
Among the many routes attempted, those involving the use of
chiral reagents and chiral catalysts are very important in the
synthesis of effective drugs. The Henry reaction, commonly
known as the nitroaldol condensation reaction, is one of the
Received: March 13, 2013
Published: July 2, 2013
© 2013 American Chemical Society
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Scheme 1. Molecular Structures of (a) Ligand L1, (b) Ligand
L2, (c) Cu(L1), and (d) Cu(L2) and Schematic
Representation of Zeolite-Y-Incorporated (e) Cu(L1)-Y and
(f) Cu(L2)-Y Complexes
Figure 1. (a) XRD patterns of (a) pure zeolite-NaY, (b) Cu2+-Y, (c)
Cu(L1)-Y, and (d) Cu(L2)-Y.
basis of theoretical and experimental evidence, a plausible
mechanism has been suggested for the reaction catalyzed by
such complexes.
Figure 2. FTIR spectra of (a) NaY, (b) Cu2+-exchanged zeolite, (c)
Cu(L1)-Y, (d) Cu(L2)-Y, (e) Cu(L1), and (f) Cu(L2). The inset
shows the peak values for the neat complexes in the region of 1650−
500 cm−1.
2. EXPERIMENTAL AND THEORETICAL METHODS
The details of the materials used in this study, methods of synthesis of
ligands L1 and L2 (Scheme S1), preparation of neat Cu(L1) and
Cu(L2) and encapsulated copper chiral Schiff-base complexes Cu(L1)-
Y and Cu(L2)2-Y (Scheme S2), method of characterization, and
theoretical calculations are provided in the Supporting Information.
due to lattice water molecules and surface hydroxylic groups,
respectively. The parent NaY zeolite shows characteristic bands
at 552, 690, and 1009 cm−1 (Figure 2a) that are attributed to
T−O bending mode, symmetric stretching, and antisymmetric
vibrations, respectively.32 No shift is observed upon introduc-
tion of the metal ions and upon encapsulation of the metal
complexes (Figure 2b−d), which further implies that the zeolite
framework has remained unchanged upon encapsulation of the
complexes. The IR bands of all encapsulated complexes are
weak because of their low concentration in the zeolite cage and
thus can only absorb in the region of 1200−1600 cm−1, where
the zeolite matrix does not show any absorption band. The
peak intensity of the zeolite-encapsulated complex may also
occur because of strong interference of the bands associated
with water. In other words, the bands of water might mask
some of the weak bands in the encapsulated complex. In order
to confirm this, we have taken the FTIR spectrum of the solid
samples under vacuum after dehydration. The FTIR spectra of
the dehydrated samples are shown in Figure 3. The FTIR
spectra of the dehydrated samples are found to be much more
intense, and no additional peak is observed in the region of
1200−1600 cm−1 (Figure 3, inset). IR spectra of the neat
Schiff-base copper(II) complexes (Figure 2e,f) show major
bands at 1625 (CC), 1530 (CN), 1450, 1325 (C−O), 753
3. RESULTS AND DISCUSSION
3.1. X-ray Diffraction (XRD) Study. The powder XRD
pattern of the neat NaY, Cu2+-exchanged zeolites and that of
encapsulated complexes are shown in Figure 1. It can be
observed from Figure 1 that, for pure zeolite-Y and for M2+-
exchanged zeolite-Y, the intensity of the I220 plane is greater
than that of I311, but for the encapsulated complex, the reverse
is obtained, i.e., I311 > I220. This reversal in the intensities has
been empirically correlated with the presence of a large
complex within the zeolite-Y supercage.31 The above
observation may therefore be construed as evidence for the
successful encapsulation of copper Schiff-base complexes within
the supercage of zeolite-Y.
3.2. Fourier Transform Infrared (FTIR) Spectroscopy.
The FTIR spectra of NaY, metal-exchanged zeolite-Y, and neat
and encapsulated Schiff-base complexes are shown in Figure 2.
FTIR spectra of NaY and metal-exchanged zeolites show strong
zeolite lattice bands in the range 500−1200 cm−1. The strong
and broad bands in the region 1010−1045 cm−1 could be
attributed to the asymmetric stretching vibrations of (Si/Al)O4
units. The broad bands in the regions 1650 and 3500 cm−1 are
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Figure 3. FTIR spectra of dehydrated samples taken under vacuum: (a) NaY; (b) Cu2+-Y; (c) Cu(L1)-Y; (d) Cu(L2)-Y. The inset shows the
expanded IR spectra of the solid samples in the region of 2000−500 cm−1.
(νC−H aromatic ring) cm−1. Similar frequencies are also
observed in the case of zeolite-Y-encapsulated complexes with
a little shift in the CO, CN, and C−O bands to
wavenumbers 1634, 1532, 1454, and 1335 cm−1, respectively,
indicating nitrogen and oxygen coordination inside the cavity of
the zeolite framework (Figures 2c,d and 3c,d). In addition to
these bands, the encapsulated complexes also show a band at
1395 cm−1 corresponding to C−H deformation. These results
give information for the formation of copper Schiff-base
complexes in zeolite-Y.
3.3. UV−Vis/DRS Analysis. The UV−vis spectra of the
Schiff-base ligands and their corresponding neat and zeolite-
encapsulated copper complexes are depicted in Figure 4. The
peaks are assigned in Table S1 in the Supporting Information.
It is observed that the Schiff-base ligands give almost similar
patterns of UV−vis peaks in water. Both ligand systems show
five peaks nearly at 232, 262, 274, 340, 366, and 389 nm
(Figure 4a,b). The first two higher-energy peaks are due to
ligand-based π → π*, and the four lower-energy peaks are due
to n → π* transitions. The lower-energy bands at 262 and 274
nm in the case of ligands L1 and L2, respectively, correspond to
the azomethyne group. The neat copper(II) complexes with
ligands L1 and L2 show peaks above and below 250 nm due to
π → π* and n → π* originating from the ligand system. Besides
these, it also shows weak bands at 323, 377, and 384 nm, which
can be attributed to metal-to-ligand charge transfer (MLCT).
The comparison of the UV−vis spectra of the complexes with
those of the free ligands indicates a blue shift in the π → π* and
n → π* transitions. The shifting of the peaks toward higher-
energy values and the appearance of MLCT transitions suggest
formation of the copper complex.
The dehydrated Cu2+-Y sample exhibits absorption bands at
636 and 710 nm, characteristic of the e → t2 and e → a1
transitions of the CuII (3d9) ion in the trigonal site, respectively
(see Figure S1 in the Supporting Information). The intense UV
absorption component is attributed to charge-transfer ex-
1
citation. The band at 253 nm is assigned to the (3d10) S0 →
2
(3d94sl) D5/2 configurational transition or may be due to
interaction of the Cu2+ ion with the O atoms of zeolite. The
second derivative of the peak gives absorption at 253 nm, which
occurs as a shoulder in the DRS spectrum. This spectral
behavior of the Cu2+-exchanged zeolites indicates that the Cu2+
ion maintains a pseudotetrahedral environment of the type
(O1)3-Cu2+-L (where O = oxygen of the supercage of zeolite-
Y).8
The diffuse-reflectance spectrometry (DRS) spectra of the
encapsulated complexes are shown in Figure 4. It is observed
from Figure 4d,e that the peaks are less intense than those of
the corresponding neat complex, which may be due to the
presence of a low concentration of metal complex inside the
cavities. However, both complexes retain the same spectral
regions as those in the neat complex. The only difference that
can be observed from the peak values is that upon
encapsulation the MLCT transitions significantly shifted to
higher wavelengths (Figure 4g). The shifting of the spectral
region toward lower-energy values may be due to the influence
of the zeolite matrix on the electronic and structural properties
of the encapsulated complex.
3.4. Electron Spin Resonance (ESR) Spectra. ESR
spectra of the Cu2+-exchanged zeolite, neat Schiff-base
complexes, and encapsulated copper Schiff-base complexes
are shown in Figure 5. ESR spectra for the neat complex as
polycrystals are characterized by an axial g tensor. Hyperfine
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Figure 4. UV−vis/DRS spectra of (a) L1, (b) L2, (c) Cu(L1), (d) Cu(L2), (e) Cu(L1)-Y, and (f) Cu(L2)-Y and (g) a comparison of the absorption
spectra of the neat and encapsulated complexes.
Figure 5. Powder EPR spectra of (a) Cu2+Y taken at different temperatures, (b) neat Cu(L1), (c) neat Cu(L2), (d) encapsulated Cu(L1)-Y, and (e)
Cu(L2)-Y complexes.
1
3
features for copper (S = /2 and I = /2) are observed at 77 K
due to intermolecular spin−spin coupling for both the neat and
encapsulated complexes. The ESR spectra of the neat copper
Schiff-base complexes are shown in Figure 5b,c with two g
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values [gII = 2.450 and g⊥ = 2.223 for Cu(L1)2 and gII = 2.420
and g⊥ = 2.218 for Cu(L2)] that are characteristics of a normal
tetragonal environment. The gII > g⊥ > 2.0023 indicates that the
Supporting Information. The presence of Cu2+ is confirmed
by the Cu 2p3/2 and Cu 2p1/2 peaks at 932.4 and 955.2 eV,
respectively, accompanied by a relatively low intense satellite
peak at 942.3 eV. These values are in accordance with the
previously reported binding energy value for copper.34 It can be
observed from Table 1 that in all of the samples the binding
energies of Si 2s, Al 2p, and Na 1s remain unchanged. However,
small shifts toward higher energies for M 2p and toward lower
energy for O 1s and N 1s are observed in all of the encapsulated
complexes. The high M 2p3/2 binding energy found in [M(L)]Y
indicates the presence of Schiff-base complexes inside zeolite-Y.
This is attributed to the fact that upon encapsulation the charge
density on the metal centers decreases, which could be due to
impairment of delocalization of the π electrons of the ring when
the complexes are confined inside the zeolite cavity.35
3.6. Cyclic Voltammetry (CV) Study. The electron-
transfer processes associated with the electroactive species
located in the interior part of the microporous material have
gained substantial interest in recent years.36 However, the exact
electron-transfer mechanism is still a matter of debate in some
cases.37 Shaw et al.38 first proposed two possible mechanisms,
intrazeolite and extrazeolite (eqs 1 and 2, respectively), for
electron transfer associated with encapsulated transition-metal
complexes within the supercage of zeolite and surface-bound
metal complexes, respectively, based on zeolite-modified
electrodes (ZMEs).
2
2
unpaired electron is present in the dx −y orbital, which is
characteristic of Cu ions that undergo tetragonal elongation.
The four-line hyperfine signals in Cu2+-Y at 250−400 mT are
3
characteristic of the Cu nucleus with I = /2. The encapsulated
complexes show more than four lines of hyperfine signals due
to interaction between the Cu nuclei and two equivalent 14N
atoms, indicating the presence of a copper(II) mononuclear
complex inside zeolite (Figure 5d,e). The gII values are found to
be 2.227 for Cu(L1)-Y and 2.229 for Cu(L2)-Y and are less
compared to the copper-exchanged NaY (gII= 2.367). It is
known that when the symmetry of the ligand field of the copper
complex changes from octahedral via square-pyramidal
coordination to a planar environment, the gII value decreases.
The decrease in the g value suggests the formation of a square-
planar copper complex within the framework of zeolite-Y.
3.5. X-ray Photoelectron Spectroscopy (XPS). The
location of the complexes in the zeolite cages can also be
confirmed by XPS because it provides information about the
relative concentrations of elements on the surface of ca. 40−50-
Å-thick layers of the sample (ca. 1% of the crystal).33 The XPS
measurements are carried out for various copper Schiff-base
complexes. It is found from a comparison of the signal
intensities of the M2p level (M = Cu) for the encapsulated
samples and the metal-exchanged samples that the encapsulated
complexes contain a lower concentration of the metal ions than
NaY-metal-exchanged samples. The results obtained are in
accordance with our energy-dispersive X-ray (EDX) and UV−
vis studies. The decrease in the metal content in the
encapsulated metal complexes can be attributed to the
migration of noncomplexed metal ions during complex
formation in the ship-in-a-bottle synthesis process. In addition
to the information about the location of the complexes, some
preliminary information about the oxidation states of the metal
ion in both the neat and zeolite complexes can be obtained
from the XPS data. Table 1 lists the binding energies for M 2p,
Intrazeolite mechanism
m
Ez + ne− + nC(s)+ ↔ Ez(m−n)+ + nC(z)+
(1)
Extrazeolite mechanism
m+
Ez + mC(s)+ ↔ E(s)m+ + mC(z)+
E(s)m+ + ne− ↔ E(s)(m−n)+
(2)
where Em+ = an electroactive probe, z = zeolite, s = solution
phase, and C+ = an electrolyte cation.
Besides the two possible electron-transfer mechanisms, Dutta
and Ledney39 have proposed three distinct pathways of charge
transfer in ZMEs, which is an extension of the original models
Table 1. XPS Analysis of the Metal-Exchanged Zeolite, Neat,
and Encapsulated Complexes (M = Cu)
proposed by Shaw et al. Domen
́
ech et al.40 have also proposed
state
Cu(L1) Cu(L2) Cu(L1)-Y Cu(L2)-Y
Cu2+-Y
the following mechanism for a zeolite-Y-associated Mn(salen)-
N3 complex.
M 2p3/2
M 2p1/2
satellite
O 1s
931.5
955.1
942.3
531.4
400.2
284.6
286.5
931.6
955.2
942.3
531.4
400.2
284.6
286.5
932.4
955.3
943.1
529.9
399.4
284.6
286.5
154.0
102.9
74.4
932.4
955.3
943.1
529.9
399.4
284.6
286.5
154.0
102.9
74.4
931.3
955.1
942.1
531.4
MnIII(salen)N3(sol) + e− = MnII(salen)N3(sol)
(3)
The reduction of boundary-associated complexes can yield
species in solution
N 1s
C 1s (C−C)
(O−CO)
Si 2s
MnIII(salen)N3(b) + M+(sol) + e−
154.0
103.0
74.5
= MnII(salen)N3−(sol) + M+(b)
Si 2p
(4)
Al 2p
or involve the electron-transfer process to a zeolite-associated
species:
Na 1s
1073.1
1073.1
1073.2
MnIII(salen)N3(b) + M+(sol) + e−
O 1s, N 1s, C 1s, Si 2s, Si 2p, Al 2p, and Na 1s in various
copper Schiff-base complexes. In all of the Schiff-base
complexes (both neat and encapsulated), two different kinds
of C atoms (C−C, 284.6 eV; C−O, 286.5 eV) and only one
kind of N atom (399.2 eV) are observed (Figure S2(ii) in the
Supporting Information). The core-level photoelectron peaks
of the neat and encapsulated complexes as well of the metal-
exchanged zeolites are assigned in Figure S2(i) in the
= MnII(salen)N3−(b) + M+(b)
(5)
where (sol) represents species in the solution phase and (b) is
for boundary-associated species. M+ represents the charge-
balancing cation of the supporting electrolyte. On the basis of
the above mechanism and electrochemical behavior, they
propose an extrazeolite electron-transport process and the
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Table 2. Elemental Analysis for NaY, Cu2+-Y, Cu(L1)-Y, and Cu(L2)-Y
EDX analysis (wt %)
AAS (wt %)
Na
CHN analysis (wt %)
sample
Si
Al
Na
M
C
N
Al
M
C
N
H
NaY
21.42
21.46
21.80
21.40
8.57
8.44
8.20
8.80
7.55
6.74
5.65
5.35
8.59
8.48
8.23
8.76
7.58
6.56
5.45
5.38
Cu2+-Y
0.94
0.72
0.76
0.93
0.73
0.77
Cu(L1)-Y
Cu(L2)-Y
20.80
19.75
1.44
1.49
21.80
20.15
1.46
1.52
18.70
21.85
Table 3. Calculated Energies of the HOMO and LUMO Levels (in eV), Chemical Potential (μ, in eV), Global Hardness (η, in
a
eV), Electrophilicity Index (ω, in eV), and Softness (S, in eV)
complex
IP
EA
HOMO
LUMO
μ
η
ω
S
Cu(L1)(↑)
Cu(L1)(↓)
Cu(L2)(↑)
Cu(L2)(↓)
Cu(L1)-Y(↑)
Cu(L1)-Y(↓)
Cu(L2)-Y(↑)
Cu(L2)-Y(↓)
2.419
1.221
−4.438
−4.478
−4.171
−4.664
−3.917
−3.322
−3.936
−3.268
−2.341
−2.633
−2.440
−3.052
−2.440
−2.118
−2.416
−1.963
−3.389 (−1.820)
−3.555
−3.305 (−2.339)
−3.858
−3.178 (−4.802)
−2.72
−3.17 (−4.537)
−2.615
1.048 (0.599)
0.9225
5.478 (2.764)
6.851
0.476 (0.834)
0.542
2.843
5.176
4.922
1.834
4.428
4.151
0.865 (0.504)
0.806
6.312 (5.423)
9.233
0.577 (0.991)
0.620
0.738 (0.374)
0.602
6.840 (30.838)
6.144
0.677 (1.336)
0.830
0.76 (0.385)
0.652
6.636 (26.690)
5.242
0.657 (1.296)
0.766
a
The values given in parentheses are those obtained by using the IP and EA values.
existence of topological redox isomers of the Mn(salen)N3
complex.
cages.45 Electrochemical analysis of zeolite-encapsulated
complexes based on ZME depends on the preparation of the
modified electrode. It may so happen that during modification
or pressing of the sample the crystallinity of the zeolite may get
ruptured and the voltammogram so obtained may not be
apparent because of encapsulated complexes. This may be due
to disintegration of the complexes over the zeolite surface. In
order to confirm this, we again perform XRD and DRS analysis
of the samples after the CV study. We obtain the same pattern
of XRD and DRS spectra as we obtained before it was subjected
to ZME. So, it can be concluded that during the preparation of
ZME the zeolite structures are not ruptured and the redox
behavior is due to the presence of a redox-active copper Schiff-
base complex present in the cavities of zeolite-Y.
3.7. Elemental Analysis. Elemental (Cu, C, N, O, Al, Na,
and Si) detection has been performed by EDX (Figure S4 in
the Supporting Information), atomic absorption spectroscopy
(AAS), and CHN analyses. The amount of metal content in the
synthesized complexes has also been determined by Vogel’s
method46 for comparison with the above elemental analyses.
Complete elemental analysis data are given in Table 2. It is
found from elemental analyses that the Si/Al ratio in the virgin
NaY is found to be ∼2.5. This ratio remains almost constant in
all of the prepared samples, indicating the absence of
dealumination either upon metal exchange or upon encapsu-
lation of the copper Schiff-base complexes. Chemical analyses
of the encapsulated samples reveal the presence of a metal
complex, Cu(L), with Cu/C and Cu/N ratios roughly similar to
the theoretical values of 0.035 and 0.5, respectively, for pure
complexes. It can be observed from Table 3 that the loadings of
the metal into the ion-exchanged zeolite and zeolite-
encapsulated complexes are quite close in value to each
other, which suggests that almost all of the metal ions present
in the zeolite lattice get complexed and the uncomplexed metal
ions are removed during back-exchange by refluxing with 0.01
M NaCl. The small percentage of metal that remains
uncomplexed may be metal ions trapped in the cage by the
complex formed around it or may be leached out during
encapsulation of the complex. This is quite similar to that of
manganese leaching after the introduction of salen ligands into
Although these mechanistic assignments are controversial,
there are various reports demonstrating the redox behavior of
transition-metal complexes encapsulated in zeolite cavities.41−43
Cyclic voltammograms of zeolite-encapsulated complexes
taken as a part of ZME in the presence of 0.1 M TBAP as the
supporting electrolyte are shown in Figure S3 in the Supporting
Information. It has been reported that electrochemical analysis
of chemically modified zeolite is much more reliable unless the
crystallinity of the zeolites get ruptured during the preparation
of ZME.44 The cyclic voltammograms of the neat complexes
Cu(L1) and Cu(L2) taken in solution mode are shown in parts
a and b of Figure S3 in the Supporting Information,
respectively. It is observed that Cu(L1) shows two reduction
potentials (Table S2 in the Supporting Information). The first
wave corresponds to the electrochemically quasi-reversible
CuII/CuI transition, whereas the second one features a
reduction of CuI to the corresponding Cu0, followed by
removal of the metal center from the Schiff-base ligand. It also
shows two oxidation peaks corresponding to Cu0/CuI and CuI/
CuII couples. Similar to Cu(L1), the Cu(L2) complex also
shows two reduction and two oxidation peaks (Figure S3b in
the Supporting Information). The difference in redox potential
values in the two complexes indicates that substitution at the
phenyl ring greatly influences the electrochemical behavior. The
cyclic voltammograms of encapsulated complexes as a part of
ZME are shown in Figure S3c in the Supporting Information.
The peak current remains stable more than 6 h, indicating the
stability of the complexes inside the cavity of the zeolite. These
redox potential values are completely different from that of the
Cu2+-exchanged zeolite-Y,8 indicating the formation of Schiff-
base complexes inside zeolite-Y. It can be observed from Figure
S3c and Table S2 in the Supporting Information that upon
encapsulation the peaks are broadened and the average
reduction potential values of Cu(L1)-Y get shifted toward
more negative values, whereas that of Cu(L2)-Y is shifted to
more positive values. The shifting of the reduction potential
value toward a more negative value in the case of Cu(L1)-Y
indicates stabilization of the CuII oxidation state in zeolite
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Figure 6. HOMO and LUMO orbitals of the neat and encapsulated copper(II) Schiff-base complexes. The arrows in parentheses indicate the
HOMO and LUMO orbitals with spin-up (↑) and spin-down (↓) states.
zeolite-Y.47 These traces of uncomplexed metal ions are
unlikely to cause any serious interference in the behavior of
the encapsulated complexes.
Supporting Information. The surface plot for the samples
before Soxhlet extraction, however, is found to be non-
homogeneous, indicating that the surface is being occupied by
extraneous complexes or the uncomplexed ligands. The
presence of similar surface morphology before and after
encapsulation into zeolite-Y confirms that encapsulation has
not affected the surface crystallinity. Further, it also indicates
the efficiency of the purification procedure to effect complete
removal of extraneous complexes, leading to well-defined
encapsulation in the cavity.
3.9. Thermogravimetric Analysis (TGA). TGA plots of
the neat copper(II) Schiff base and the encapsulated complexes
are shown in Figure S7 in the Supporting Information. The two
neat complexes show peaks below 100 °C, which is due to
elimination of the outer-sphere water molecule (Figure S7,
3.8. Scanning Electron Microscopy (SEM). SEM micro-
graphs of complexes taken before Soxhlet extraction and those
taken after Soxhlet extraction are shown in Figure S5 in the
Supporting Information. The SEM taken before purification
shows the presence of some unreacted or extraneous particles
on the external surface. In the SEM of finished products, no
surface complexes are seen and the particle boundaries on the
external surface of zeolite are clearly distinguishable. This is
much clearer from the surface plot shown in Figure S6 in the
Supporting Information. Homogeneous surface morphology for
the neat NaY zeolite and the samples after Soxhlet extraction
respectively are observed in parts a, d, and e of Figure S6 in the
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curves a and b, in the Supporting Information). The peaks
above 300 °C are due to partial decomposition of the
complexes. The weight losses at the higher temperature range
350−550 °C are due to complete decomposition of the
sample.48 A comparison of TGA for neat complexes with the
encapsulated ones shows that these complexes become more
stable once they get embedded inside the cavity of zeolite-Y
(Figure S7, curves c and d, in the Supporting Information). In
the case of the encapsulated metal complexes, the weight losses
due to partial decomposition occur at lower temperature;
however, they do not undergo complete decomposition up to
700 °C. This indicates that the two chiral copper(II) complexes
become somewhat more stable upon encapsulation.
3.10. Theoretical Calculation. 3.10.1. Geometries. The
geometrical parameters obtained from VWN/DN level
calculations for the neat and encapsulated complexes are
provided in Table S3 in the Supporting Information. The
calculated bond lengths and bond angles for the neat complexes
are found to be in good agreement with the similarly reported
Schiff-base complex of copper. After encapsulation of the
chelated copper Schiff-base complexes, the bond lengths and
bond angles are found to differ slightly in comparison to those
of the neat complexes. This indicates that, upon encapsulation
into the zeolite framework, the complex has not undergone
much more distortion, which may change the active site in the
complexes. Furthermore, we have also calculated the volume of
the zeolite cluster model and the neat complexes using
MaterialsStudio software.49 The calculated volume of the zeolite
supercage (1150.35 Å3) is found to be sufficient enough to hold
the two Schiff-base complexes having volumes of Cu(L1) =
410.53 Å3 and Cu(L2) = 462.70 Å3.
3.10.2. Electronic Structure, Ionization Potential (IP),
Electron Affinity (EA), and Binding Energy. A schematic
representation of the frontier molecular orbital for the neat and
encapsulated complexes is shown in Figure 6. For all of the
complexes, the patterns of the occupied and unoccupied
orbitals are qualitatively similar. Copper(II) is an open-shell
system with a d9 configuration; hence, we performed spin-
unrestricted calculations on all of the systems. The energies of
the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) orbitals corresponding
to spin-up and spin-down states are found to be different in
both the neat and encapsulated systems. The energy values for
the frontier orbital of all four systems are given in Table 3. It
can be seen from Table 3 that the HOMO and LUMO orbitals
of the encapsulated complexes lie higher in energy in
comparison to those of the neat complexes. The IP and EA
values are also found to increase upon encapsulation. However,
the HOMO−LUMO gaps and the differences between IP and
EA in the encapsulated complexes are found to decrease in
comparison to those of the free complexes. Among the
encapsulated systems, the HOMO energy of Cu(L1)-Y (with
the spin-down state) is much higher-lying and the LUMO of
Cu(L1)-Y with the spin-up state is much lower-lying. In the
neat complexes, Cu(L2) with the spin-up state has the higher-
lying HOMO, whereas its spin-down state has the lower-lying
LUMO. This change in the HOMO and LUMO energies
further signifies the effect of encapsulation on the orbital
energies of the copper Schiff-base complexes. We have also
calculated the interaction energy of the encapsulated complexes
as the energy differences between the optimized encapsulated
complexes and the corresponding fragments (neat metal
complex and the zeolite framework) optimized as isolated
molecules. The calculations of the interaction energy at the
same level of theory are found to be in the order of Cu(L1)-Y
(32.02 eV) < Cu(L2)-Y (36.83 eV). This result indicates that
the Cu(L2) complex interacts more strongly and becomes
more stabilized than the other complexes. This difference in
interaction energy can be attributed to a change in the
geometrical parameters and the presence of a bulkier group in
the phenyl ring. These differences in the interaction energy
bring a considerable change in the energies of the HOMO and
the LUMO levels.
3.10.3. Global Descriptors. We have also calculated the
DFT-based global descriptors, viz., chemical hardness (η),
chemical potential (μ), electrophilicity index (ω), and softness
(S), using a DN basis set and VWN functional for the neat and
encapsulated complexes. The values for η, μ, ω, and S given in
Table 3 are calculated based on two approximations, Koop-
mans’ and finite difference. According to Koopmans’
approximation, IP is the negative of the HOMO energy and
EA is the negative of the LUMO energy. The plot of IP and EA
against the HOMO and LUMO energies of the spin-down
state, respectively, shows a linear correlation with regression r2
values of 0.988 and 0.923 (Figure S8 in the Supporting
Information). Because of several limitations associated with
Koopmans’ approximation,50−53 especially in the case of
transition-metal complexes, numerical values of η, S, and ω
obtained from both approximations are found to be different.
However, the trend in the change of the global descriptor
values predicted by both approximations for the considered
systems follows the same order. For example, the change in the
global hardness values predicted by eqs 1 and 2 as well as by
eqs 5 and 6 (see the Supporting Information) follows the same
order, i.e., Cu(L2) < Cu(L1) for the neat complexes and the
reverse for the encapsulated complexes.
According to the maximum hardness principle (MHP),54,55
the most stable structure has the maximum hardness. Thus, the
neat complexes with maximum hardness are chemically more
stable compared to the encapsulated ones. Further, it can be
seen from Table 3 that upon encapsulation both values of the
chemical potential (μ) increase, whereas that of the electro-
philicity index (ω) is found to increase in the case of the spin-
up state but decreases in the case of the spin-down state. This
result suggests that the occupancy of the electron, i.e., whether
it involves an α orbital or a β orbital, will also bring out a
considerable change in the reactivity of the system. In this
context, it is pertinent to mention that the electrophilicity
containing information for both electron transfer (chemical
potential) and stability (hardness) can be considered to be a
better descriptor of the global chemical reactivity. Moreover, ω
includes a hardness term in the denominator, which is a
descriptor of the stability. The electrophilicity is expected to
exhibit an inverse linear relationship with hardness, or in
correlation with MHP, we can say that under the conditions for
the existence of an MHP, there will also be a minimum
electrophilicity principle.56 We have observed from our
calculation that, in the case of neat complexes, Cu(L1) with
maximum hardness has a minimum electrophilicity and the
same is true in the case of the encapsulated Cu(L2)-Y complex
(Table 3). Thus, it is found that upon encapsulation of the
complexes into the zeolite framework the chemical potential
(μ) and global softness (S) values increase, whereas that of η
decreases. This change in the values of the global descriptors
reflects the effect of the zeolite matrix on the reactivity of the
complexes, and it can be said that complexes with high μ values
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a
Table 4. Results of the Henry Reaction in the Presence of Various Catalysts
NaY
Cu2+-Y
Cu(L1)
Cu(L2)
Cu(L1)-Y
Cu(L2)-Y
reactant product yield (%) ee (%) yield (%) ee (%) yield (%) ee (%) yield (%) ee (%) yield (%) ee (%) yield (%) ee (%)
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
48 (36)
50 (32)
54 (32)
53 (34)
49 (38)
47 (38)
63 (28)
66 (22)
68 (22)
64 (26)
58 (30)
52 (29)
82 (24)
89 (22)
87 (21)
78 (25)
82 (24)
83 (26)
62
67
76
72
64
60
80 (24)
84 (22)
88 (21)
86 (25)
77 (24)
82 (26)
61
72
74
66
60
68
92 (18)
96 (16)
94 (17)
93 (19)
88 (20)
82 (22)
82
84
84
78
74
73
90 (18)
95 (16)
93 (17)
86 (19)
82 (20)
84 (22)
80
83
82
79
76
74
a
The values given in parentheses are the time (in h) required for completion of the reaction. Reactions are conducted at 1 mmol scale employing 5
equiv of nitromethane in ethanol. Reactions are carried out at −20 °C using 10 mg of the catalyst. The values (% ee, R isomer) are calculated from
HPLC analysis.
can actively participate in electron-transfer reactions. In our
case, the Cu(L1)-Y complex with minimum η and maximum S
values will be the most reactive system.
substrate and compared it with copper(II) acetate. In the case
of heterogeneous catalysis, a 0.01 M solution of acetic acid is
used to generate the nitronate ion from nitromethane. Because
acetic acid exists as CH3COOH → CH3COO− + H+, the
conjugate base, i.e., CH3COO−, will react with some amount of
sodium species of NaY to form CH3COONa.56 The resulting
CH3COONa will act as the proton-abstracting source, thereby
3.10.4. Local Descriptors. Table S4 in the Supporting
Information presents the Fukui functions (FFs; fκ+ and fκ−) for
the selected metal atoms and the coordinated N atoms
calculated using a Hirshfeld population analysis (HPA) scheme
for the neat and encapsulated metal complexes. It is seen in
Table S4 in the Supporting Information that the values of the
FFs at the metal centers increase upon encapsulation. Thus, the
zeolite framework modifies the reactivity of the metal atom
from a less electrophilic or nucleophilic site in neat complexes
to a higher site in encapsulated ones. As far as the attack of a
nucleophile or electrophile at a particular site is concerned, the
Cu atom (in both neat and encapsulated complexes) possessing
the higher value of FFs (fκ+ and fκ−) will be the most
preferential site. A comparison of the FFs (at the metal center)
between the encapsulated and neat complexes suggests that a
nucleophile will preferentially attack the Cu center in the
encapsulated complexes. This further indicates that encapsu-
lated complexes can actively participate in a base-catalyzed
reaction.
3.10.5. Spin Density Calculation. The considered copper-
(II) complexes are open-shell systems with copper at the d9
electronic configuration. The electronic configuration suggests
that the excess spin density should be localized on the Cu
center. The spin density calculation on the two neat complexes
using DMOL3 program at the VWN/DN level of theory
indicates that, in both complexes, a high percentage (more than
50% of the total Mullikan charge density) is concentrated on
the central Cu atom (Figure S9 in the Supporting Information).
A substantial minority spin density is located on coordinated N
and O atoms (10−11% of the total Mullikan charge density).
This minority spin density results from the slight spatial offset
between the α- and β-spin forms of the dπ−pπ molecular
orbitals. The presence of the high-spin density on the metallic
center implies that the unpaired d electron is mostly
concentrated on the metallic center and is not delocalized
over the aromatic rings of the coordinating ligands.
−
generating the nitronate ion (CH2NO2 ) from nitromethane
(CH3NO2).
The reactivity and enantioselectivity of the nitroaldol
reaction is strongly dependent on the nature of the solvent
used. Therefore, a catalytic enantioselective Henry reaction is
conducted in different solvents, such as toluene, 1,2-dichloro-
methane, tetrahydrofuran, CH3CN, ethanol, and methanol,
with catalyst Cu(L1)-Y under identical reaction conditions. It
can be seen in Table S5 in the Supporting Information that
polar solvents give better yield and high enantioselectivity in
comparison to the nonpolar ones. In the present case, we found
ethanol as the best solvent system; in terms of productivity and
enantioselectivity, hence we performed all of the reaction in
ethanol by varying the catalyst. The temperature and catalytic
amount also play important roles in the yield and ee of the
reaction.57,58 We performed the nitroaldol reaction of p-
nitrobenzaldehyde as the model substrate at various temper-
atures, viz., room temperature (rt) and 0, −10, −20, −30, and
−40 °C. It can be observed from Table S6 in the Supporting
Information that p-nitrobenzaldehyde gives a maximum yield
and ee at −20 °C. However, the enantioselectivity is found to
decrease at −40 °C. This may be due to the fact that, below
−20 °C, the reaction intermediate gets stabilized and thereby
affects the enantioselectivity. Because we obtained better
efficiency of the catalyst at −20 °C, we performed all of the
other reactions at this temperature using various Schiff-base
complexes (Table 4). As far as the catalytic amount is
concerned, we have varied the catalytic amount from 5 to 25
mg, keeping the amount of the substrate constant. With an
increase in the catalytic amount from 5 to 10 mg, both the yield
(above 90%) and ee (up to 84%, R isomer) are found to
enhance. However, no significant enhancement in the yield as
well as in the enantioselectivity is observed upon further
3.11. Catalytic Study. To test the catalytic efficiency of the
synthesized complex, we choose p-nitrobenzaldehyde as the test
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Table 5. Conversion and Selectivity for the Reactions catalyzed by Neat and Encapsulated Copper Schiff-Base Complexes
Cu(L1)
Cu(L2)
Cu(L1)-Y
Cu(L2)-Y
reactant
product
% conv
R:S
% conv
R:S
% conv
R:S
% conv
R:S
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
84
92
90
82
85
86
83:17
93:7
82
86
90
88
80
85
82:18
88:12
89:11
74:26
64:37
62:38
95
100
100
97
81:19
95:5
92
100
98
84:16
93:7
90:10
81:19
84:16
86:14
92:8
97:3
90:10
94:6
87
86:14
82:18
78:22
90
87
88
75:25
85
increment of the catalytic amount. So, we choose 10 mg of the
catalyst to be the optimal amount.
catalytic activity. The enhancement of the catalytic activity
upon encapsulation into the zeolite cavity can be attributed to
chiral enhancement due to the confinement effect associated
with the supercage of zeolite-Y. It is also reported that, by
anchoring or immobilizing the catalysts onto microporous and
mesoporous materials, there is a significant increase in ee
compared to that of its homogeneous analogue.27,59−61 The
increase in the catalytic ability of these catalysts can also be
correlated with our theoretical results. Because our theoretical
calculation suggests that upon encapsulation the global
hardness decreases, according to Pearson’s HSAB principle,62,63
they become more reactive in nature. Moreover, the fκ+ value,
which indicates the preferential site for a nucleophilic attack, is
mostly concentrated at the Cu center in the encapsulated
complexes (Figure S10 in the Supporting Information). Also, as
stated by Evan et al.,21 after generation of the nitronate ion, this
nucleophile binds to the Cu center through one of the O
atoms. So, values of the FF suggest that among our systems this
step becomes more feasible in the case of the Cu(L1)-Y-
encapsulated complex having a maximum fκ+ value, followed by
Cu(L2)-Y, Cu(L2), and Cu(L1), respectively. Furthermore,
from our DFT calculation, it is evident that the HOMO−
LUMO gap (ΔE = 0.493 eV) and global hardness value (η =
0.246 eV) in the intermediate complex [Cu(L1)-Y + CH2NO2]
formed inside the cavity of zeolite-Y are much less in
comparison to those of the homogeneous counterparts (ΔE
= 1.012 eV and η = 0.506 eV; Figure 8). This result indicates
that the reaction intermediates formed in the heterogeneous
catalytic system are much more reactive than those of the
homogeneous counterparts. The decrease in the HOMO−
LUMO gap and global hardness value in the Cu(L1)-Y +
CH2NO2 complex satisfactorily explains the facts for the faster
catalytic performance of the nitroaldol reaction inside the cavity
of zeolite-Y. In other words, it can be said that the zeolite
matrix highly influences the catalytic ability of the copper
Schiff-base complexes.
In order to examine the roles of the Schiff-base moiety of the
catalytic activity, we performed the nitroaldol reaction in the
presence of NaY and Schiff-base ligands. Herein again we
consider the reaction of p-nitrobenzaldehyde as the model
system. In the presence of NaY, the maximum percent yield is
found to be 54% with a racemic mixture of the product. Upon
the addition of the chiral ligand system into the reaction media,
the yield is found to increase from 50% to 60% and the ee (%)
is found to be 45%. This revealed that the presence of a chiral
Schiff-base ligand is critical for reaction efficiency and is
essential to impart enantioselectivity of the product. We have
also tested the reaction of p-nitrobenzaldehyde with the L1
ligand without a NaY or copper source. The reaction was found
to be sluggish with a low percent yield although 40%
enantioselectivity in the product is obtained. The results of
the nitroaldol reaction performed only in the presence of a
After optimization of the solvent, temperature, and amount
of catalyst, the scope of the nitroaldol reaction with supported
catalysts is extended to various other substituted benzaldehydes
under the above optimized reaction conditions (Table 4). The
percent of conversion and selectivity (R:S ratio) of the
reactions is given in Table 5. To compare the catalytic activity
of the homogeneous and heterogeneous catalysts, time versus
conversion plots for the reactions of 2- and 4-nitro-
benzaldehyde are shown as representative cases in Figure 7.
Figure 7. Plot of (%) conversion versus time for condensation of 4-
nitrobenzaldehyde with nitromethane catalyzed by (a) Cu(L1)-Y and
(b) Cu(L1) complexes. Plot of (%) conversion versus time for the
reaction of 2-nitrobenzaldehyde with nitromethane catalyzed by (c)
Cu(L1)-Y and (d) Cu(L1).
It can be seen from Tables 4 and 5 that the heterogeneous
chiral catalyst gives almost 100% conversion with high
enantioselectivity. As far as the mass balance is concerned,
there appears to be some mass imbalance in the reaction,
especially with the reaction catalyzed by the homogeneous
catalyst. This is due to the fact that about 10−15% percent of
aldehydes remains unconverted to the desired product and
about 2−5% of the product gets decomposed or oxidized
during column separation. At a glimpse, all of the chiral
complexes encapsulated into the zeolite cavities used in the
present study give good yield and ee in the nitroaldol reaction
of a wide range of substrates. The substrates with electron-
withdrawing groups, for example, 2- and 4-nitrobenzaldehyde,
gave higher enantioselectivities in comparison to other
substrates. It can be observed from Table 4 that zeolite-
encapsulated complexes show better catalytic efficiency than
their homologous analogues. Out of the two heterogeneous
catalysts, the Cu(L1)-Y catalyst preferably shows the highest
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−
Figure 8. (a) Optimized geometry of Cu(L1)-Y with the nucleophile CH2NO2 . (b) HOMO of Cu(L1)-Y + CH2NO2. (c) LUMO of Cu(L1)-Y +
CH2NO2. (d) HOMO of Cu(L1) + CH2NO2. (e) LUMO of Cu(L1)-Y + CH2NO2. (f) HOMO−LUMO gap in neat and encapsulated intermediate
complexes.
Figure 9. Optimized geometries of possible intermediate and transition states involved in the catalytic cycle.
chiral Schiff-base ligand (L1) under different solvent conditions
are shown in Table S7 in the Supporting Information.
content losses after the sixth cycle are given in Table S8 in the
Supporting Information.
3.12. Plausible Mechanism of the Henry Reaction
Catalyzed by the Cu(L1) Complex. An attempt has been
made to rationalize the plausible pathway involved in the
catalytic conversion. The catalytic cycle is depicted in Scheme
S3 in the Supporting Information. Because Cu(L1)-Y is found
to be a better catalyst, for the mechanistic study, we choose the
neat Cu(L1) complex. The catalytic cycle is supposed to
involve three steps. The first step involves abstraction of a
The synthesized heterogeneous chiral catalysts are found to
be effective up to six cycles without any leaching of the metal
complexes. The amount of the copper content in the catalyst is
determined by Vogel’s46 method after recovery of the catalyst.
The amount of the copper content in the catalysts is found to
be the same up to six cycles. However, after the sixth cycle, the
copper content is found to be less. The amounts of metal
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methyl proton from nitromethane with acetate acting as a
base,19 generating the nitronate ion. The second step involves
coordination of the nitronate ion to the Cu center via a Cu−
O−N linkage. The third step involves the simultaneous attack
of the nucleophile to the incoming aldehyde and formation of
an activated complex followed by the release of the product. To
support this mechanism, we perform DFT calculations on the
possible intermediate and transition states. The optimized
geometry of Cu(L1), possible transition states, and reaction
intermediates are shown in Figure 9. The energy profile for this
three-step reaction is shown in Figure 10. It can be seen from
to the type proposed recently by Dhahagani et al.23 It is evident
that, for those complexes that simultaneously bind both an
electrophile and a nucleophile, the most reactive transition state
is the one in which the nucleophile lies perpendicular to the
ligand plane, while the electrophile resides in the equatorial
sites of the ligand plane.21 This statement is in good agreement
to our theoretical observation. By this argument, it can be said
that TS2 will exhibit the highest reactivity. After formation of
the reaction intermediate complex, formation of the product is
greatly favored, as indicated by a high negative value of ΔG =
−105.5 kcal/mol (Figure 10).
4. CONCLUSION
The chiral Schiff-base complexes of copper encapsulated inside
zeolite-Y are found to be better catalysts in terms of the thermal
stability and catalytic efficiency. The catalytic ability of these
complexes is found to depend on the temperature, solvent
system, and catalytic amount. Low temperatures up to −20 °C,
high polarity of the solvent, and low catalytic amount gives the
highest enantioselectivity. DFT calculation predicts that
encapsulation reduces the global hardness and increases the
softness and FF values of the complexes, resulting in higher
catalytic ability of the encapsulated complexes. Further, it is
revealed from our theoretical calculation that in the catalytic
cycle (TS2) the nucleophile must reside in the axial position
and the electrophile should be positioned in the equatorial site
of the ligand plane for better catalytic performance.
Figure 10. Simple energy profile diagram for catalytic conversion.
ASSOCIATED CONTENT
* Supporting Information
■
Figure 9 that as the nitronate ion coordinates to the Cu center
the square-planar complex undergoes a Jahn−Teller distortion
with compression of the Cu−O and Cu−N bonds. This step is
endergonic in nature, has to cross an energy barrier of 15 kcal/
mol to form the intermediate state 1 (Int-1), and is the rate-
determining step. In both the transition and intermediate states,
the incoming nucleophile is found to lie in the axial position
with Cu−O bond distances of 2.44 and 2.19 Å, respectively.
During this process, the unpaired electron on the Cu center
gets coupled with the unpaired electron of the O atom of
nitronate. This is in accordance with our ESR analysis and spin
density calculation. Once the metal complex undergoes radical
coupling, it becomes ESR-inactive. Because spin density
calculations predict the unpaired electron to be concentrated
mostly on the Cu center, the incoming nucleophile will have a
higher tendency to get attached to the Cu center. Further, from
our DFT calculation, it is predicted that if the electron spin in
the dxy orbital is in the downward direction, the complex
becomes more reactive. Therefore, the most favored situation
for d−p mixing between the Cu center and nitronate in the
TS1 or Int-1 will be the one in which the unpaired electron in
dxy aligns in the downward direction [Scheme S3(ii) in the
Supporting Information]. Upon the addition of the aldehydes,
the nucleophile bonded to the Cu center attacks the C atom of
the aldehydic group and finally leads to the formation of the
product. Upon going from Int-1 to the nitroaldol product, Int-1
passes through a transition state (TS2) and has to cross an
energy barrier of 5.3 kcal/mol. In TS2, the square-planar
copper complex undergoes a Berry pseudorotation, allowing
the O atom of the incoming aldehyde to bind from the
equatorial position. At this stage, there occurs a large elongation
of the Cu−O1 bond (due to Jahn−Teller distortion) and the
nitronate moves away from the ligand plane to lie perpendicular
to the plane. The calculated transition state geometry is similar
S
Details of the experimental and theoretical methods, character-
ization techniques, EDX spectra of zeolite-Y-encapsulated
complexes, UV−vis/DRS data, UV/DRS of Cu2+-exchanged
zeolite, redox potential values, XPS spectra, SEM, and TGA, FF
analysis, correlation between IP versus HOMO energy and EA
versus LUMO, results of the Henry reaction at different
temperatures, in different solvents, and in the presence of
1
ligand L1 alone, leaching study, mechanism of the reaction, H
and 13C NMR, and HPLC data of nitroaldol products and
synthetic ligand. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ramesh@tezu.ernet.in.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank the Department of Science and Technology,
New Delhi, India, for financial support.
■
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