Palladium-Schiff Base Complexes Encapsulated in Zeolite-Y Host: Functionality Controlled by the Structure of a Guest Complex

Article abstract of DOI:10.1021/acs.inorgchem.8b03031

A series of palladium complexes of tetradendate Schiff base ligands L1 (N,N′-bis(salicylidene)phenylene-1,3-diamine) and its derivatives L2 and L3 have been synthesized by using the "flexible ligand method" within the supercage of zeolite-Y. These complex

Full text of DOI:10.1021/acs.inorgchem.8b03031

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Palladium−Schiff Base Complexes Encapsulated in Zeolite‑Y Host:
Functionality Controlled by the Structure of a Guest Complex
†
†
‡
,†
Susheela Kumari, Karthik Maddipoti, Bidisa Das, and Saumi Ray*
†
Birla Institute of Technology and Science, Pilani, Rajasthan 333031, India
‡
Technical Research Center, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
S Supporting Information
*
ABSTRACT: A series of palladium complexes of tetradendate Schiff base ligands L1
N,N′-bis(salicylidene)phenylene-1,3-diamine) and its derivatives L2 and L3 have been
(
synthesized by using the “flexible ligand method” within the supercage of zeolite-Y. These
complexes in both their free and encapsulated states have been thoroughly characterized
with the help of different characterization tools such as XRD, SEM-EDS, BET, thermal
analysis, XPS, IR, and UV−vis spectroscopic studies. All these encapsulated complexes are
identified with a dramatic red shift of the d−d transition in their electronic spectra when
compared with their free states. Theoretical as well as experimental studies together suggest
a substantial modification of the structural parameters of square planar Pd(II)−Schiff base
complexes upon encapsulation within the supercage of zeolite-Y. Encapsulated complexes
are also subject to show modified catalytic activities toward the Heck reaction. These
heterogeneous catalysts can easily be separated from the reaction mixture and reused.
INTRODUCTION
environment provided by host materials and ease of separation.
It is a unique route for site isolation of the desired catalyst.
■
Cross-coupling reactions are of great importance and
significance, as these coupling reactions are widely used in
organic synthesis for the preparation of natural products,
biologically active compounds, conducting polymers, pesti-
16,17
Starting from the revolutionary work of Herron,
many
reports exist in literature encompassing encapsulation of
transition metal complexes inside the supercage of zeolite-Y.
Vasudevan and co-workers have emphasized the geometries
adopted by guest metal complexes upon encapsulation within
the cavities of the host framework. From our group we have
reported the design of different metal complexes with salen
1
−5
cides, pharmaceutical intermediates, and liquid crystals. Very
commonly, these reactions are catalyzed by palladium based
compounds. A number of homogeneous palladium complexes
have been reported as efficient catalysts for Heck coupling
reactions; however, they certainly have some limitations and
disadvantages specifically when separation, purification of the
products, thermal stability, and recyclability of catalyst are
concerned, and in many cases these catalyst suffer from loss of
18−22
Schiff base ligands inside the zeolite-Y and MCM-41.
Although several reports explore the encapsulation of metal
complexes inside zeolite-Y, very few actually focus on
investigating the modified catalytic functionality of the
encapsulated metal complex, originated from its structural
modifications. Recently, we have reported the synthesis of a
series of palladium salen complexes inside zeolite-Y. These
encapsulated palladium complexes have been perceived to be
efficient catalysts for sulfoxidation of methyl phenyl sulfide
compared to their neat analogues, and toward the betterment
of catalysis, the role of the distorted structure of the
6
,7
catalytic activity. In recent years, many researchers are
8
pursuing several techniques such as support of ionic liquids,
9
10
immobilized on silica, grafted on polymers, anchored on
1
1
12
activated carbon or carbon nanotubes and supported on
1
3
14,15
zeolites and MCM-41,
catalyst successfully.
etc., especially to recover the
22
Among various host materials, zeolite-Y is a well-known
microporous crystalline aluminosilicate material with a pore
diameter of 7.4 Å and supercage dimension of 12.47 Å. It
provides high surface area and excellent chemical stability with
quite low toxicity. The specific architecture of the framework
makes the zeolite a very attractive host for transition metal
complexes, organometallics, organic dyes, and polymer within
their voids, which in turn, makes these hybrid systems effective
and competent catalysts with size and shape selectivity.
Encapsulation of a metal complex inside the porous zeolite
definitely enables the coupling between the reactivity of the
host complex and the stability, with a specific electronic
encapsulated complexes has been established.
The current work deals with the synthesis of encapsulated
palladium−Schiff base complexes inside the supercage of
zeolite-Y via the “flexible ligand method”. This method is
found to be appropriate for the encapsulation of palladium
salophen complexes, as the salophen ligand having desired
flexibility can be introduced into the supercage of zeolite
through the channel. Once, the complex formed, it loses the
flexibility and thereby becomes immobilized inside the
Received: October 26, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b03031
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Inorganic Chemistry
Article
Figure 1. Molecular dimensions of palladium complexes.
supercage unless the zeolite framework is destroyed.
Encapsulation of these complexes inside the host makes it
altogether a heterogeneous catalyst. This is a convenient and
efficient route to combine the reactivity of palladium complex
with the robustness and stereochemistry of the zeolite
framework.
The palladium complexes of interest with the L1, L2, and L3
ligands follow the order of molecular dimension as PdL1 <
PdL2 < PdL3, where L1 is N,N′-bis(salicylidene)phenylene-
EXPERIMENTAL SECTION
■
Materials. The Na-zeolite-Y, salicylaldehyde, 2-hydroxy-5-me-
thoxy benzaldehyde, 2-hydroxy-5-nitro benzaldehyde, 1,3- Phenyl-
enediamine, styrene, and bromobenzene are purchased from Sigma-
Aldrich, India. Palladium acetate is purchased from TCI chemicals,
India. All the solvents such as methanol, ethanol, acetone, and ether
are purchased from SD fine, India.
Synthesis of Ligands and Palladium Complexes. The Schiff
bases L1, L2, L3, and corresponding palladium complexes are
23
prepared by following the literature report. In a round-bottom flask,
10 mmol of 1,3-phenylenediamine and 20 mmol of salicylaldehyde or
its derivatives in ethanol are mixed under constant stirring at 70 °C
for 4 h. After the end of the reaction, the yellow solid product is
vacuum filtered, washed with ethanol, and finally dried at room
temperature (Scheme S1 is given in the Supporting Information (SI)).
To synthesize the palladium−Schiff base complex, the ethanolic
solution of the ligand is heated at 80 °C for 30 min and palladium
acetate is added in a 1:1 ligand-to-metal molar ratio. The reaction
mixture is refluxed for 3 h. The solid palladium−Schiff base complex
is washed with ethanol and finally dried. All three palladium
complexes are synthesized using the same procedure (Scheme S2 in
the Supporting Information).
1,3-diamine, L2 is N,N′-bis(5-methoxysalicylidene)phenylene-
1,3-diamine, and L3 is N,N′-bis(5-nitrosalicylidene)phenylene-
1,3-diamine (given in Figure 1). The metal in three different
complexes with different substituents essentially experience the
electronic effect differently, along with being subjected to
different steric constraints imposed by the topology of the
zeolite cavity. The host framework induces the structural
modification to the guest complex and alters the electronic,
magnetic, and redox properties of the guest metal complex.
Therefore, the topology of the host framework has substantial
control over the functionality and selectivity of the guest
molecule. These hybrid systems are employed as a catalyst for
the cross-coupling Heck reaction of bromobenzene with
styrene in the presence of sodium carbonate as a base to
investigate the steric interaction and electronic effect of the
substituent’s group attached to the complex.
Preparation of Pd(II) Exchanged Zeolite-Y and Encapsu-
1
9
lated Pd(II)−Schiff Base Complexes in Zeolite-Y. Pure zeolite-
NaY (10 g) is allowed to react with 0.01 M palladium salt
[
Pd(CH COO) = 0.224 g] in 100 mL of distilled water to acquire
3 2
the required loading level of palladium ions and stirred at room
temperature for 24 h. The slurry is filtered, washed repeatedly with
water, and then desiccated for 12 h at 150 °C.
B
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The Pd(II) exchanged zeolite-Y is treated with a stoichiometric
excess of respective Schiff base ligand. Being a flexible ligand, it
diffuses through the channels of the host when the reaction mixture is
refluxed for 24 h at 150−200 °C under constant stirring. The color of
the solid reaction mass changes from light brown to dark brown. The
reaction mass is then recovered and dried at room temperature. The
resultant product is further purified by Soxhlet extraction maintaining
the sequence of solvents as acetone, methanol, and finally diethyl
ether to remove any unreacted ligand and palladium complex
adsorbed on the surface of the host. The obtained product is further
dried in a muffle furnace at 150 °C for 12 h. The product is allowed to
react with 0.01 M NaCl solution to remove the unreacted palladium
ions, followed by filtration and washing with distilled water
thoroughly until the filtrate is negative for the chloride ion test.
Finally, zeolite encapsulated palladium−Schiff base complexes
X-ray Diffraction and Scanning Electron Microscopy
Analysis. The powder X-ray diffraction patterns have been
recorded for pure zeolite-Y, palladium(II)-exchanged zeolite-Y,
and zeolite encapsulated complexes (presented in Figure 2)
(
represented as EPdL1-Y, EPdL2-Y, and EPdL3-Y) are obtained as
dark brown powders (Scheme S2 in the Supporting Information).
Catalytic Reaction. A mixture of 0.70 mmol % of catalyst, 10 mL
of DMF solvent, 5 mmol of bromobenzene, 10 mmol of styrene, and
1
.06 g of base is then refluxed for 20 h at 140 °C under constant
stirring. After the completion of the reaction, the catalyst is recovered
by filtration and washed with solvents for reuse. The filtrate is
collected, and then after centrifugation, the supernatant solution of
the reaction mixture is analyzed by FID-GC, using n-heptane as the
internal standard, to determine the % conversion (Scheme 1). The
reaction is monitored by collecting a small amount of the reaction
mixture after 1, 4, 8, 12, and 20 h of reaction time.
Figure 2. XRD pattern of (a) pure zeolite-Y, (b) Pd-exchanged
zeolite-Y, (c) EPdL1-Y, (d) EPdL2-Y, and (e) EPdL3-Y.
with 2θ varying from 8 to 50 to study the crystallinity and the
integrity of the host framework and to ensure encapsulation
inside the supercage of zeolite-Y. The appearance of very
similar XRD patterns for pure zeolite-Y, Pd(II)-Y, and zeolite
encapsulated palladium−Schiff base complexes positively
indicates that the framework of the host has not undergone
any structural modification after encapsulation of the metal
complexes inside the cavity and the crystalline nature of host is
Scheme 1. Schematic Representation of the Heck Coupling
Reaction
24
also not affected. However, there is an interesting observation
with the relative intensities of peaks appearing at 2θ values 10°
and 12° corresponding to the planes 220 and 311 respectively.
For pure and Pd(II)-Y zeolite samples, the XRD peak at 10° is
more intense than the peak at 12°, whereas, for encapsulated
metal Schiff base complexes, the relation between intensities is
just the reverse, i.e. I220 < I311. A quite well-established
semiempirical fact is that when a large molecule is entrapped
within the supercage of zeolite-Y, an intensity reversal of the
XRD peaks of pure zeolite at 2θ = 10° and 12° takes
RESULTS AND DISCUSSION
■
Elemental Analysis. The chemical composition of
palladium(II)-exchanged zeolite-Y and encapsulated palladium
complexes (EPdL1-Y, EPdL2-Y, and EPdL3-Y) are analyzed by
the EDX technique. The Si/Al ratio for unit cell
Na (AlO ) (SiO ) ·yH O of pure zeolite-Y is 2.7. The Si/
5
2
2 52
2 140
2
Al ratio has not been significantly affected during the
palladium(II)-exchanged reaction as well as during the process
of encapsulation, as the dealumination during encapsulation is
minimal. Results obtained from EDX analysis are given in the
form of weight % in Table 1 and Figure S1 in the Supporting
25
place. Our observation is in accordance with this fact. This
change in the relative intensities may be related with the
rearrangement of randomly coordinated free cations in zeolite-
2
6,27
Y.
No new peak is detected for Pd(II)−Y as well as for
zeolite with encapsulated palladium complexes confirming a
low loading level of metal in the host framework.
Table 1. Concentration of Palladium (wt %) Content in the
Different Samples
From scanning electron microscopy analysis, it has been
observed that some impurities are present at the surface of the
host in the form of complex and unreacted ligand but these
impurities have been completely washed from the surface of
zeolite-Y by extensive Soxhlet extraction. SEM images are
given in Figure 3. After Soxhlet extraction, clear boundaries of
the host framework are manifest in the SEM micrograph. This
observation further suggests that the crystalline nature of the
host is conserved during the encapsulation process.
BET Surface Area Analysis. The BET surface area analysis
has been carried out to determine the surface area and pore
volume for the pure zeolite-Y and zeolite with encapsulated
complexes. The comparative adsorption−desorption isotherms
for all encapsulated complexes and zeolite-Y are shown in
Figure 4 and the surface area and pore volume data are given in
s. no.
samples
palladium (wt %)
Si/Al ratio
1
2
3
4
5
Zeolite-Y
Pd-Y
EPdL1-Y
EPdL2-Y
EPdL3-Y
−
2.90
2.79
2.75
2.84
2.80
0.65
0.38
0.41
0.40
Information. The concentration of the palladium metal in the
encapsulated complexes as compared to palladium(II)-
exchanged zeolite-Y is found to be lower. The decrease in
palladium content can probably be attributed to the formation
of the palladium complex inside the cavity of the host, as
leaching of some of the palladium ion during the encapsulation
process is commonly observed phenomenon.
C
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Figure 3. SEM images (A) EPdL2-Y (before Soxhlet extraction) and (B) EPdL2-Y (after Soxhlet extraction).
Figure 4. BET isotherms for pure zeolite-Y and zeolite encapsulated complexes: (a) pure zeolite-Y and (b) EPdL1-Y, EPdL2-Y, and EPdL3-Y.
Table 2. The pattern of adsorption−desorption isotherms for
Thermal Analysis. Differential thermal analysis and
thermogravimetric analysis have been used in characterization
for all neat and encapsulated palladium complexes. The TG/
DTA curves for pure zeolite-Y, free state, and encapsulated
PdL3 complexes are presented in Figure 5,and thermal analysis
spectra for neat complexes PdL1, PdL2 and corresponding
encapsulated complexes are represented in Figure S2 in the
Supporting Information. Pure zeolite shows only weight loss
due to loss of adsorbed water in the temperature range 70−250
all the samples are found to be nearly identical (Figure 4),
Table 2. BET Surface Area and Pore Volume of Pure Zeolite
Y and Encapsulated Complexes EPdL1-Y, EPdL2-Y, and
EPdL3-Y
2
3
s. no.
sample
BET surface area (m /g) pore volume (cm /g)
1
2
3
4
Pure zeolite-Y
EPdL1-Y
EPdL2-Y
535
355
342
340
0.3456
0.2025
0.1961
0.1920
°
C. All three neat complexes exhibit sharp weight loss in a
single step. There is a sharp decomposition with the highest
weight % change at 272 °C for the PdL3 neat complex, and
this decomposition corresponds to the loss of the organic
moiety of the complex. However, for the corresponding zeolite
encapsulated palladium complexes weight loss occurs in two
steps. The first step in the temperature range 30−150 °C is
certainly due to the desorption of physically adsorbed water
EPdL3-Y
suggesting that the host framework remains unaffected during
the whole process of encapsulation. However, there exists a
considerable difference in the surface area and pore volume of
zeolite-Y and all hybrid systems. In all the encapsulated
systems, the BET surface area and pore volume are found to be
significantly lower in comparison to those for pure zeolite-Y,
which directly indicates the presence of the metal complex
inside the supercage of zeolite-Y rather than adsorbed on the
31
molecules from the host framework, which appears as an
32
endothermic peak in the DTA curve. The second step in the
wide temperature range 300−800 °C is associated with the
decomposition of the organic part of the catalyst as an
31
2
8,29
exothermic phenomenon. The TG/DTA curves show a sharp
weight loss occurring for all neat complexes in the exothermic
external surface.
All the catalysts have shown type I
adsorption−desorption isotherms, which is a characteristic of
32
30
mode, but all encapsulated complexes are characterized by a
plateau-type plot, indicating slow and continuous weight loss
and that the process of decomposition extends toward higher
temperature. It is quite apparent that, upon encapsulation, the
the microporous material. The lowering of BET surface areas
and pore volumes for the hybrid systems are largely dependent
upon the loading level of metal in zeolites along with the
molecular dimension and geometry of the complex encapsu-
lated inside the zeolite supercage.
33
thermal stability of the complexes is enhanced significantly.
D
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Figure 5. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) results for (A) pure zeolite-Y, (B) PdL3, and (C) EPdL3-Y.
Figure 6. FTIR spectra of (A) pure zeolite-Y, PdL1, and EPdL1Y; (B) zeolite-Y, EPdL1-Y, EPdL2-Y, and EPdL3-Y; and (C) enlarged view in the
−
1
−1
range of 500 cm to 2000 cm for pure zeolite-Y, EPdL1-Y, EPdL2-Y, and EPdL3-Y.
−
1
IR Spectroscopic Study. The FTIR spectral data of
ligands, palladium−Schiff base complexes, pure zeolite-Y and
zeolite encapsulated palladium complexes are shown in Figure
Information). The strong IR peak at 1018 cm of pure zeolite-
Y corresponds to the asymmetric stretching vibrations of (Si/
Al)O units of host framework. Few important peaks observed
at 560, 717, and 786 cm , are the signature of T-O bending
4
−
1
6
and Table 3 (Figures S3−S4 are given in Supporting
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Table 3. FTIR Data of Neat and Encapsulated Complexes
(2p), and Na (1s) and XPS spectra of PdL1, EPdL1-Y, and
EPdL2-Y are presented in Table 4 and Figure 7 respectively
CN
CC
νC−H
C−O
(
Figures S5−S6 in the Supporting Information). The binding
s. no.
samples
stretching
stretching
deformation stretching
energies of Pd 3d and Pd 3d confirm the palladium metal
3
/2
5/2
1
2
3
4
5
6
PdL1
EPdL1-Y
PdL2
EPdL2-Y
PdL3
EPdL3-Y
1605
1636
1589
1643
1605
1628
1528,1439
1535,1462
1528,1466
1497,1443
1551,1481
1520,1481
1358
1369
1366
1378
1380
1383
1278
1257
1257
1265
1256
1234
present in the +2 oxidation state in all complexes. For the
PdL1 complex, these signals appear at 343.6 (Pd 3d3/2) and
3
6
3
38.2 eV (Pd 3d5/2), whereas, for the encapsulated
complexes, these signals are at approximately the same binding
energies; for the EPdL1-Y, signals have appeared at 342.4,
337.2 eV, and for EPdL2-Y complex, the signals are observed
at 342.5, 336.3 eV with one new XPS signal at a higher binding
energy of 347.9 eV. The appearance of the new peak after
encapsulation is a mere indication of removal of electron
density around the palladium center because of obliteration of
the delocalization environment driven by the adaptation of
mode, double ring and symmetric stretching vibrations
3
4
respectively. In addition, the prominent IR bands appeared
−
1
at 1643 and 3500 cm are assigned to lattice water molecules
35
and surface hydroxylic group, respectively. All these charac-
teristics IR bands of zeolite-Y remain unaltered after palladium
exchange and even after encapsulation reaction, concluding
that the host framework maintains its integrity during the
whole process of encapsulation of palladium−Schiff base
37−39
square planar geometry of the encapsulated complexes.
For the PdL1 complex, C (1s) XPS signals appear at 286.4 and
3
2
2
84.5 eV corresponding to sp and sp carbon atoms
respectively whereas two different type of N (1s) signals
appeared, confirming the presence of M−N and CN species.
−
1
complexes. The IR spectral range of 1200 to1600 cm appears
to be a suitable range for the study of zeolite encapsulated
metal complex as in this range, prominent IR bands of the
zeolite are absent. Therefore, the IR peaks observed are only
because of the complexes though the intensity of these IR
peaks is very weak definitely indicating a low loading level of
palladium inside the zeolite cage.
The O (1s) XPS signal is observed at a binding energy value of
20
5
34.0 eV. Encapsulated complexes (EPdL1-Y and EPdL2-Y)
secure these XPS peaks almost at the identical positions. In
both encapsulated complexes, the binding energy signals for Pd
(
3d), C (1s), N (1s), O (1s), Si (2p), Al (2p) and Na (1s) are
observed. The EPdL1-Y system has shown zeolitic Si 2p, Al 2p,
and Na 1s signals at 103.2, 75.5, and 1073.3 eV respectively,
whereas, in EPdL2-Y, zeolitic signals appear at binding energies
at 103.5 eV (Si 2p), 74.8 eV (Al 2p), and 1072.6 eV (Na 1s).
The binding energy data of the Pd 3d peak for all encapsulated
complexes ensure the successful encapsulation of the
palladium−Schiff base complex inside the zeolite-Y.
UV−visible Spectroscopy. The UV−vis spectra of the
ligands and free state palladium−Schiff base complexes
recorded in CHCl have shown a few interesting features
The FT-IR peaks of the ligand, observed as two strong bands
−
1
−1
at 1612−1620 cm and 1273−1296 cm , are attributed to
the CN and C−O stretching vibrations, and these bands are
slightly shifted toward lower frequency upon complexation,
indicating nitrogen and oxygen coordination inside the cavity
of zeolite-Y. The IR spectra of free state palladium complexes
−
1
show important peaks at 1528, 1439 cm (CC stretch.),
−
1
−1
1
1
605 cm (CN stretch.), 1278 cm (C−O stretch.), and
deformation). Similar IR bands with little
C−H
−
1
23
358 cm (ν
shift are also observed in all encapsulated complexes providing
indirect evidence for the presence of palladium−Schiff base
complexes inside the supercage of the host. Shifts in some
characteristics peaks upon encapsulation are attributed to the
effect of the host matrix on the metal complex. The shift in
νC−H deformation frequencies obtained in all encapsulated
complexes also provides clear evidence for the encapsulation of
the metal complex inside the cavity of the host.
X-ray Photoelectron Spectroscopy (XPS). XPS studies
also provided explanation regarding the existence of the
palladium complex inside the supercage of zeolite-Y. XPS data
confirm the relative concentration of elements and their
oxidation states in the neat and encapsulated complexes. It is
noticed that the XPS signal for palladium metal in the complex
is very weak when it is encapsulated, definitely indicating a very
low concentration of palladium in the host framework.
Observations are in accordance with those obtained from
EDX, IR, and UV−vis spectroscopic studies. The XPS
measurements are carried out for PdL1 (neat state) as well
as EPdL1-Y and EPdL2-Y (encapsulated states). The binding
energies (eV) for Pd (3d), C (1s), N (1s), O (1s), Si (2p), Al
3
(
presented in Figure 8; spectral data given in Table 5).
An intense band observed in the UV−vis spectra of the
Schiff base ligand in the range 232−278 nm is assigned as the
π−π* transition, and the n−π* transition is observed in the
range 302−375 nm. The lowest energy band appearing at
4
01−468 nm is identified as a transition involving a metal, i.e. a
charge transfer transition or d−d transition. The appearance of
lowest energy bands, therefore, strongly supports the formation
of the complex.
The solid-state UV−vis spectrum of PdL1 exhibits bands at
200, 246, 308, and 360 nm. The first two bands are assigned as
the π−π* transition, and the next two bands are originated
from the n−π* transition. The peak at 439 nm is assigned as
the d−d transition and is blue-shifted to a large extent so that
eventually it is merged with charge transfer. This observation is
in line with fact that being a 4d transition metal Pd always
40
undergoes large d orbital splitting.
On complexation, metal complexes show these transitions
slightly shifted; the π−π* transition was observed at 240−245
nm, the n−π* transition was observed at 294−370 nm. The
Table 4. Binding Energy (eV) of Neat and Encapsulated Complexes
s. no.
samples
Si (2p)
Al (2p)
Na (1s)
C (1s)
N (1s)
O (1s)
Pd (3d5/2)
Pd (3d3/2)
1
2
3
PdL1
EPdL1-Y
EPdL2-Y
−
103.27
103.53
−
−
286.41, 284.57
287.70, 285.08
286.37, 285.01
400.79, 398.59
399.63, 398.49
400.84, 399.28
534.08
533.10
532.92
338.24
337.22
336.30
343.65
342.40
342.59, 347.96
70.40, 75.50
74.87, 76.30
1073.31
1072.68
F
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Figure 7. (A) XPS survey spectra of PdL1 and encapsulated complexes EPdL1-Y and EPdL2-Y. High resolution XPS signals of Pd (3d) for (B)
PdL1, (C) EPdL1-Y, and (D) EPdL2-Y complex.
Figure 8. Solution UV−vis spectra of (A) L1 and PdL1, (B) L2 and PdL2, and (C) L3 and PdL3.
G
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Table 5. Solution UV-visible Data of Ligand and Neat
Complexes
Table 6. Solid-State UV−visible Data of Neat and
Encapsulated Complexes
π−π*
n−π*
CT transitions/d−d
π−π*
n−π*
CT transitions/d−d
s. no. samples
transitions
transitions
transitions
s. no.
samples
transitions
transitions
transitions
1
2
3
4
5
6
L1
PdL1
L2
PdL2
L3
PdL3
237, 273
245
245, 278
240
232, 268
241
302, 347
294, 340
306, 375
302, 370
305, 340
300, 350
−
429
−
468
−
1
2
3
4
5
6
PdL1
EPdL1-Y
PdL2
EPdL2-Y
PdL3
EPdL3-Y
246
259
250
252
246
256
308, 360
328, 362
301, 386
292, 387
309, 375
318, 371
439
468, 598
471
480, 605
402
476, 618
401
4
1
presence of similar electronic transitions in the encapsulated
states as its neat analogues confirms the complex formation
inside the supercage of zeolite-Y quite evidently. However,
upon encapsulation inside the zeolite matrix, the d−d
transitions are found to be red-shifted clearly indicating a
different electronic environment experienced by the square
planar complex especially around the metal. The PdL1
complex, simply upon encapsulation, shows the shift of the
d−d transition from 439 to 468 nm. Similar observations are
found for all three encapsulated palladium complexes (shown
in Figure 9 and Table 6), so the structural modification of all
the encapsulated complexes must be very alike. The red shift in
d−d transition is practically unexpected and hence noteworthy,
as it demands an enhanced π-delocalization around the metal
center after encapsulation. Encapsulation by some means
induces planarity around the central metal atom, thereby,
supplementing π-delocalization.
suite of ab initio quantum chemistry programs. The details of
the theoretical methods are included in the Supporting
Information (SI).
Structure of Pd-Complexes in Neat and Encapsulated
States. We theoretically first studied the free ligands, L1, L2,
and L3 (details are presented in SI), and observed that the
three aromatic rings in the molecule are not coplanar, but the
middle ring of m-phenylenediamine is placed at an angle of
40°−50°, from the two side rings, depending on the
substituents (shown in Figure 10a) and the molecule is
flexible and stretched. In the presence of the Pd ion, the
ligands readily form metal complexes and encompass the metal
atom in a half-circular fashion, where the two side rings of the
ligand and the metal atom are coplanar, but the middle
aromatic ring of the ligand is now bent almost perpendicular to
the two terminal aromatic rings (see Figure 10b). The neat
PdL1, PdL2, and PdL3 exist as singlet states, without any
unpaired electrons; however, there exists higher energy triplet
states with two unpaired electrons for all complexes studied.
Structural optimizations are carried out for PdL1, PdL2, and
PdL3 complexes in the neat singlet states and then in zeolite
encapsulated triplet states, and the optimized structures are
2
+
Theoretical Methods. The electronic structure calcula-
tions based on Density Functional Theory (DFT) were used to
study and analyze the structural and optical properties of the
Pd complexes in neat and zeolite encapsulated states. All
results presented here are obtained using the GAUSSIAN 09
Figure 9. Solid state UV−vis spectra of (A) PdL1 and EPdL1-Y, (B) PdL2 and EPdL2-Y, and (C) PdL3 and EPdL3-Y.
H
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unlike the neat molecules, which indeed were largely planar.
Moreover, the central phenylene-1,3-diamine ring is no longer
symmetrically placed behind the metal center, and thus Pd−N
and NC distances are different now. The Pd−N distances
are 2.07 and 2.13 Å, and the NC distances are 1.29 and 1.33
Å respectively (Table 7). In contrast, the Pd−O and O−C
distances for the singlet and the triplet states are very similar.
Thus, the structural studies show, in the free ground state,
PdL1, PdL2, and PdL3 complexes are partially planar with the
central aromatic ring aligned almost perpendicular and are
diamagnetic and singlet; however, the complexes are deformed
and bowl-shaped when encapsulated inside the zeolite pore
and the distortion helps to stabilize the triplet state. This, in
turn, alters the relative ordering of the molecular orbitals, and
consequently, the optical and catalytic properties of the
complexes are modified.
Frontier Molecular Orbtials of the Pd-Complexes. To
understand the optical properties and related transitions of the
Pd-complexes it is imperative to look into the nature of their
frontier molecular orbitals. The present study observes that the
HOMO for neat PdL1 has a modest Pd dxz character
hybridized with the π-bonding character of the ligand, and
Figure 10. (a) Ligand N,N′-bis(5-methoxysalicylidene)phenylene-
1
,3-diamine: frontal and side views are shown. The −OMe group is
the LUMO has Pd d character hybridized with π-antibonding
xy
marked by a red circle for L2, which is replaced by −NO for L3 and
2
type orbitals from the central “out-of-plane” aromatic ring of
the ligand (Figure S12 in Supporting Information). The
HOMO and LUMO energies for PdL1 are −5.55 and −2.67
eV respectively with a HOMO−LUMO energy gap of 2.89 eV.
H for L1. (b) The neat, singlet state of the PdL2 complex: frontal and
side views are shown. (c) The encapsulated and extracted, triplet state
of PdL2 complex: frontal and side views are shown. On the right-hand
side, two side views are shown to understand the position of the
central ring with respect to the plane of the molecule. The distances
d , d , and d are marked on the Pd-complex and are explained in the
Though HOMO−1 also has some contribution from the Pd
2
d
orbital, HOMO−2 is comprised primarily of the Pd d
1
2
3
yz
z
text. (d) PdL2 complex encapsulated within zeolite pore: two
different views are shown.
orbital. On the other hand, the LUMO+1 orbital has a minimal
contribution from Pd dyz while LUMO+2 has a little
contribution from the Pd dxy orbital. The nature of the
molecular orbitals of similar salen complex has been discussed
presented in Figure 10c with the important structural
parameters tabulated in Table 7. In the Pd-complex, the
central ring itself is no longer completely planar, unlike
aromatic rings, but it appears to be somewhat boat-shaped, and
a unique structure−property relationship is observed for this
class of complexes. To understand the positioning of this ring
with respect to the Pd atom we have defined three distance
parameters d , d , and d marked in the Figure 10b,c. The
19−21
before in literature.
A look into the molecular orbitals of
PdL2 (Figure S13 in Supporting Information) reveals that the
HOMO has a strong contribution from the Pd dxz orbital
hybridized with ligand p orbitals, and the LUMO is mainly the
Pd dxy orbital similar to the case of PdL1. The HOMO−
LUMO gap for PdL2 is 2.59 eV. The HOMO−1 orbital is
mainly ligand p with the Pd dyz orbital, and HOMO−2 is the
1
2
3
2
encapsulated Pd complexes studied are all triplet, and they are
bent longitudinally; thus, the end-to-end distances decrease by
metal d
z
orbital, with contributions from the central ring of the
ligand. It is also observed that while LUMO+1 has
∼
0.4 Å (see Table 7). The distortion of the encapsulated
contributions from the Pd d orbital, LUMO+2 and LUMO
yz
complexes as a whole, however, affects the positioning of the
central out-of-plane ring behind the Pd atom, and the d , d
+4 both have some contributions from the Pd d orbital. The
xy
molecular orbitals of PdL3 are similar to those of PdL1 and
1
2
distances now differ and are ∼2.80 and ∼3.02 Å respectively
PdL2, and the HOMO is mostly the Pd d orbital hybridized
with ligand p orbitals and the LUMO is mainly the Pd dxy
orbital (Figure S14 in Supporting Information) with the
xz
(
Table 7). The shorter distance d is now found to be longer
3
and 2.93 Å. Thus, the whole molecule now resembles a “bowl”
Table 7. Important Structural Parameters from DFT for Neat and Encapsulated Pd-Complexes
bond distances/
angles
PdL1, neat
singlet
EPdL1, encapsulated,
extracted triplet
PdL2, neat
singlet
EPdL2, encapsulated,
extracted triplet
PdL3, neat
singlet
EPdL3, encapsulated,
extracted triplet
s. no.
1
2
3
4
5
6
Pd−O (Å)
Pd−N (Å)
O−C (Å)
N−C(conj) (Å)
<O−Pd−N
2.01
2.19
1.29
1.30
85.9
13.18
2.01
2.06, 2.13
1.29
1.29, 1.33
88.9
12.72
2.00
2.19
1.30
1.30
86.1
13.74
2.00
2.07, 2.13
1.30
1.30, 1.33
88.0
13.33
2.01
2.19
1.28
1.29
85.9
13.83
2.00
2.07, 2.14
1.28
1.28, 1.32
88.4
13.53
end to end distance
(
Å)
7
8
9
d1 (Å)
d2 (Å)
d3 (Å)
2.91
2.77
2.91
2.80
2.93
3.02
2.91
2.78
2.91
2.80
2.92
3.01
2.91
2.79
2.91
2.80
2.95
3.02
I
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Figure 11. Experimental and DFT simulated UV−vis spectra of (A) PdL1, (B) EPdL1-Y, (C) PdL2, and (D) EPdL2-Y complex.
HOMO−LUMO gap being 3.0 eV. The HOMO−1 orbital is
Figure S8), which shifts to 594 in the encapsulated triplet case.
mainly ligand p with the Pd d orbital, whereas HOMO−2 is
In the case of EPdL3, important transitions are in the range
550−470 nm. One may note that in the case of EPdL3 the
prominent peaks are around 550 nm which were around 495
and 489 nm in the case of PdL3 (SI, Figure S9), indicating a
shift toward longer wavelengths upon encapsulation Exper-
imental and DFT simulated UV−vis spectra of PdL1, EPdL1-
Y, PdL2, and EPdL2-Y complex are given in Figure 11 for
better comparison.
yz
2
the metal d orbital.
z
The calculated optical spectra for the neat PdL1, PdL2, and
PdL3 complexes are given in the Supporting Information, and
here we consider only the encapsulated and extracted Pd
complexes in triplet states. The molecular orbitals in the triplet
states are shown in Figures S15−S17 and comprised of α and β
sets, due to the presence of unpaired electrons. In the triplet
state, the structural distortion of the complexes means more
hybridization with the ligand π orbitals, which makes
identification of metal d orbitals difficult. In the triplet state,
the HOMO and HOMO−1 are closely placed regarding
energies and located in two different halves of the molecule
with each exhibiting strong contributions from the Pd dxz
Catalytic Study. All neat and encapsulated palladium−
Schiff base complexes have been explored as catalysts for the
Heck coupling of bromobenzene with styrene. The results of
the Heck reaction are given in Table 8. A calibration curve of
bromobenzene is given in the SI (Figure S18). Reaction
conditions are optimized using EPdL1-Y as a catalyst by
varying the amount of catalyst and temperature. The effect of
temperature on the catalytic activity of EPdL1-Y has been
studied up to 140 °C. The % conversion of the coupling
reaction increased with temperature, but there is no conversion
obtained below 70 °C. As the temperature increased from 100
to 140 °C, the conversion increased from 21.3% to 90.8%
(Figure S19 in Supporting Information). The effect of the
amount of catalyst on the Heck reaction is studied, and
excellent conversion occurred with 0.70 mmol % of catalyst.
Catalytic data are presented in Supporting Information, Figure
S20. All catalytic reactions are monitored by gas chromatog-
raphy, and % conversion was calculated by using a calibration
curve of bromobenzene and n-heptane as an internal standard.
By using optimized reaction conditions, all neat and
encapsulated complexes containing an electron-donating and
-withdrawing substituent have been employed as catalysts. The
neat palladium complexes follow the activity order as PdL2 >
orbital. The occupied orbitals are dominated by Pd d , d , and
xz yz
2
d orbitals, whereas the unoccupied states are dominated by
z
Pd d orbitals. The LUMO is still mainly localized on the Pd
xy
dxy orbital, but strong hybridizations ensure that there are more
unoccupied orbitals with Pd dxy character. There are inner
2
occupied orbitals with Pd d and Pd d character hybridized
yz
z
with ligand p character. A closer look into the TD-DFT spectra
Figure 11 and Figure S10 in Supporting Information) readily
(
reveals that there is an overall shift toward the higher
wavelengths, but identification of the individual transitions
becomes increasingly difficult because the metal d orbitals are
mixed up with the ligand orbitals in a complicated fashion. In
the case of neat PdL1, the first prominent peak is around 521
nm (SI, Figure S7), which shifts to 568 nm in the encapsulated
EPdL1 case, and this is a HOMO−1 to LUMO transition
comprised of the Pd d to Pd d orbital. In the case of singlet
xz
xy
PdL2, we see the first important peak around 545 nm (SI,
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Table 8. Coupling Reaction between Bromobenzene and
a
Styrene Catalyzed by Palladium Complexes
mmol % of reaction temp
% yield of
b
c
s. no.
catalyst
Pd
(°C)
product
TON
1
2
3
4
5
6
7
8
Zeolite Y
Pd-Y
PdL1
PdL2
PdL3
EPdL1-Y
EPdL2-Y
EPdL3-Y
−
140
140
140
140
140
140
140
140
Nil
−
0.70
0.70
0.70
0.70
0.70
0.70
0.70
15.8
56.5
83.6
38.4
90.0
39.2
65.7
122
416
595
281
649
313
474
Figure 12. Recyclability of the EPdL1-Y catalyst for Heck coupling
reaction.
a
Reaction conditions: bromobenzene (5 mmol), styrene (10 mmol),
Na CO (10 mmol), 10 mL of DMF, catalyst, reaction time = 20 h.
modify the electronic, magnetic, and redox properties, and
hence striking changes in the functionality of the enclosed
metal complex are observed. The modified functionality for
the Heck reaction is an outcome of the modified geometry of
the palladium−Schiff base complexes.
2
3
b
c
Determined by gas chromatography. Turnover number calculated at
44
the completion of reaction (mol of bromobenzene transformed/mol
of palladium metal in catalyst).
The neat palladium−Schiff base complex has a square planar
geometry around the central metal; however, the central
aromatic ring (m-phenylenediamine) exhibits nonplanar
arrangement leading to the reduction of π-delocalization.
Geometry optimization results directly evidence this. There-
fore, [Pd(sal-1,3-phen)] type systems are subject to have a
lesser extent of conjugation from the ligand moiety in
comparison to their corresponding [Pd(sal-1,2-phen)] com-
plexes.
PdL1 > PdL3 complex. The PdL2 complex with an electron-
donating substituent (−OCH ) is the most reactive for the
3
Heck coupling, whereas PdL3 with an electron-withdrawing
substituent (−NO ) is the least reactive. The activity order of
2
free state palladium complexes is solely controlled by the
electronic effects of the substituent group attached, as
4
2
expected. After the encapsulation of the complexes inside
the supercage of zeolite-Y, a striking reversal of the reactivity
order is observed. Encapsulated palladium complexes follow
the order EPdL1-Y > EPdL3-Y > EPdL2-Y; the modified steric
environment of the guest complexes imposed by the host
zeolite lattice definitely has a significant contribution toward it.
New modified functionalities toward the Heck reaction signify
the substantial structural changes around the metal center
under encapsulation inside the host supercage. The Pd metal
center in these complexes with different substituents provides a
different electronic environment for catalysis, and additionally
on encapsulation they experience differential space constraints,
which altogether leads to the modification of the reactivity
order. Undoubtedly, the final geometry adopted by the
encapsulated complex plays a crucial role. Upon encapsulation,
the metal center in the guest complex can have a substantially
different electronic charge distribution due to the new adopted
structure. The heterogeneous catalyst can easily be separated
from the reaction mixture and reused after washing without
loss in activity (recyclability of the EPdL1-Y catalyst for Heck
coupling is presented in Figure 12).
The electronic contribution of the substituent in the
palladium complexes has been analyzed by electronic
spectroscopic studies in a series of complexes (PdL1, PdL2,
and PdL3). With the increase in the electron-withdrawing
character of the substituent attached, as observed in UV−vis
spectra, the HOMO−LUMO gap of the neat palladium
complex is subject to increase, leading to a hypsochromic shift
in absorption spectrum pertaining to the metal. The bath-
ochromic shift is in line with the increase in electron density on
4
5
the ligand. An electron-donating group (−OCH ) enhances
3
π-delocalization and maintains the planarity of the complex
whereas an electron-withdrawing group (−NO ) causes
2
nonplanarity. From electronic spectroscopic studies of the
neat complexes, we have found that the d−d transition of the
PdL2 complex is red-shifted and the d−d transition of the
PdL3 complex is blue-shifted as compared to that of the PdL1
complex. In the present study, comparative catalytic activities
of palladium (sal-1,3-phen) complexes are employed as
catalysts for the Heck reaction with aryl bromide and styrene
in the presence of Na CO as the base. Observed catalytic
Correlation between Structural Modification and
Modified Functionality. Palladium complexes are usually
efficient catalysts for the coupling reactions though they have
some limitations, such as difficulties in separation and recovery
and thermal instability. However, all these drawbacks of the
homogeneous Pd catalyst could be overcome easily by
encapsulating these systems within the pore of zeolites,
which provide easy separation, high thermal stability, and
modified functionality. Hence zeolite encapsulated metal
complexes have been studied for various organic trans-
2
3
results of neat palladium complexes indicate the reactivity
order as PdL2 > PdL1 > PdL3. An electron-donating group
(−OCH ) in the PdL2 complex increases the electron density
3
around the metal center causing the red shift of the band
appearing from the metal, and subsequently, the PdL2 complex
is most active as a catalyst toward the Heck coupling reaction.
The PdL3 complex with an electron-withdrawing group
(−NO ) is the least reactive among all the neat complexes
2
as expected. Our results have been supported by literature
reports. Saleem and co-workers have already reported the
reactivity of the palladium complexes for the Heck reaction
2
2,26,43
formations.
Upon encapsulation, the steric and electro-
static constraints provided by the framework of zeolite-Y
influence the geometry of the complex and consequently
4
6
correlated with the HOMO−LUMO energy gap. The
K
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Article
palladium complex with a low HOMO−LUMO energy gap
shows higher activity toward the Heck reaction.
with their encapsulated states. These complexes are well
characterized by the help of different characterization tools
such as XRD, SEM-EDS, BET, thermal analysis, XPS, IR, and
UV−vis spectroscopic studies. These palladium complexes
when employed as catalysts for a Heck coupling reaction, all of
them differing significantly from the behavior of their
corresponding neat form. The main aim of the present study
is to explore the geometry of the palladium complex after
encapsulation as well as to rationalize the modified
functionality of the systems. The catalytic activity has been
explained in terms of the electropositivite character of the
metal center. The complex with a less electropositive palladium
center showing the lowest energy electronic band red-shifted
eventually exhibits higher activity toward the Heck coupling
reaction. Interestingly, complexes showing a blue shift in their
free state or encapsulated states are less reactive. From detailed
comparative experimental and theoretical electronic data and
catalysis studies, it can be concluded that the greater the red
shifting observed in the d−d transition, the greater the
reactivity of palladium complexes toward the Heck coupling
reaction. A red shift in the d−d transition for particularly this
series of palladium complexes can be produced in two ways,
either by addition of a strong electron-donating substituent
group on the ligand moiety or by encapsulation in zeolite-Y.
Therefore, the geometry of the catalyst complex plays a key
role in the Heck coupling reaction for the enhanced activity.
It has also been reported that the presence of an electron-
withdrawing group, the planarity of the salen ligand is
disturbed; however, an electron-donating group on the same
position maintains the planarity of the complex with
47
substantial electron density on the metal center. A higher
electron density around the metal center facilitates the
coupling reactions. Upon encapsulation, the geometry of
these palladium−Schiff base complexes is altered significantly
due to the space constraint imposed by the topology of zeolite-
Y. For all encapsulated complexes (EPdL1-Y, EPdL2-Y, and
EPdL3-Y), the charge transfer/d−d transition bands are shifted
toward higher wavelength. The optimized geometry of the
encapsulated complex obtained from theoretical studies clearly
indicates an enhanced planarity and hence enhanced π-
delocalization induced by the central aromatic ring (m-
phenylenediamine) as the topology of the zeolite-Y framework
enforces the central ring to tilt toward the molecular plane.
Consequently, a red shift in charge transfer/d−d transition is
observed. Encapsulation of the PdL2-Y complex with the
−
OCH group obstructs the electron-donating effect to some
3
extent on the other side; zeolite topology imposes planarity to
a certain extent. Hence, the effect of the substituent group and
electronic effects originating from steric hindrance due to
encapsulation oppose each other; therefore, a red shift in the
d−d transition is marginal as compared to encapsulated PdL1-
Y and PdL3-Y complexes. However, PdL1 and PdL3
complexes when encapsulated in zeolite-Y have shown
remarkable improvement in reactivity. The enhanced function-
ality of these systems are the outcome of the enhanced electron
density around the metal thereby causing a significant red shift
in the charge transfer/d−d transition. In the case of the
encapsulated EPdL3-Y complex, both the electronic effect of
the substituent group and that from the distorted geometry
function in the same direction cause a much higher red shift.
But for the EPdL1-Y complex, only steric factors contribute
and the wall of the zeolite supercage enforces the metal
complex to adopt the distorted geometry enhancing the
electron density around the metal; hence, the reactivity is also
enhanced. The functionality of the neat palladium−Schiff base
complexes is impacted by the electronic factor of the
substituent present on the phenyl ring, and the complexes
follow the activity order PdL2 > PdL1 > PdL3. However, after
the encapsulation, the activity trend is governed by the
electronic effect originated from the substituent as well as
steric hindrance. The catalytic activity of the encapsulated
complexes follow the order EPdL1-Y > EPdL3-Y > EPdL2-Y.
It appears to be an exciting approach to alteration of the
HOMO−LUMO energy gap of a guest complex by
encapsulation of the complex inside the rigid zeolite supercage,
finally leading to advancement in the field of tunable catalysis.
The electron density on palladium metal center can be
modified in two ways, either by the attachment of different
substituent groups on the ligand moiety or by encapsulating
the complexes in the cavities of a rigid host framework. Hence,
the geometry of these catalysts turns out to be a crucial factor
for the enhanced activity toward the Heck reaction.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b03031.
Physical measurements; schemes for the synthesis of
ligands and complexes in both neat and encapsulated
state; EDX spectra; TGA and DTA curves; FTIR
spectra; high-resolution XPS spectra; theoretical studies;
TD-DFT spectra; comparative optical spectra; frontier
molecular orbitals; calibration curve of bromobenzene;
%
conversion of bromobenzene for EPdL1-Y complex
with respect to the temperature of reaction and mmol of
catalyst (PDF)
AUTHOR INFORMATION
Corresponding Author
E-mail: saumi@pilani.bits-pilani.ac.in.
■
ORCID
Bidisa Das: 0000-0001-6002-6390
Saumi Ray: 0000-0002-6893-9634
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge the DST-FIST, Department of
Chemistry and Department of Physics, BITS Pilani, for the
instrumental facility. Authors also thank MNIT Jaipur, for the
XPS measurements.
CONCLUSION
■
REFERENCES
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