Encapsulated Schiff base nickel complex in zeolite Y: Correlation between catalytic activities and extent of distortion supported by experimental and DFT studies

Article abstract of DOI:10.1016/j.ica.2017.04.003

It is observed that planar Ni(II)–Schiff base complexes of general formulae NiL with L as N, ?-bis(4-hydroxy salicylidene)ethylenediamine (4-OH-salen) when encapsulated in the supercage of zeolite Y, the complex adopts non-planar geometry.The detailed characterization studies of the 4-OH-Ni-salen complex when compared with Ni-salen complex encapsulated in zeolite Y (Ni-salen-Y) Choudhary et al. (2016) have suggested that the complex undergoes more distortion in order to accommodate itself inside the supercage of zeolite-Y. Optical spectroscopic studies show relatively more blue shifted and intensified d-d transition for 4-OH-Ni-salen complex after encapsulation. We also report the catalytic activities and enhanced magnetization of encapsulated complex though it is diamagnetic in the ‘free’ state. DFT studies of these complexes have been carried out in different spin states and electronic spectra are simulated to have better insight about the geometries adopted by the guest complexes inside the host framework.

Full text of DOI:10.1016/j.ica.2017.04.003

Accepted Manuscript
Research paper
Encapsulated Schiff Base Nickel Complex in Zeolite Y: Correlation Between
Catalytic Activities and Extent of Distortion Supported by Experimental and
DFT Studies
Archana Choudhary, Bidisa Das, Saumi Ray
PII:
S0020-1693(17)30186-X
http://dx.doi.org/10.1016/j.ica.2017.04.003
ICA 17507
DOI:
Reference:
Inorganica Chimica Acta
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31 March 2017
3 April 2017
Please cite this article as: A. Choudhary, B. Das, S. Ray, Encapsulated Schiff Base Nickel Complex in Zeolite Y:
Correlation Between Catalytic Activities and Extent of Distortion Supported by Experimental and DFT Studies,
Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.04.003
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Encapsulated Schiff Base Nickel Complex
in Zeolite Y: Correlation Between Catalytic Activities and Extent of Distortion Supported by
Experimental and DFT Studies
Archana Choudharya, Bidisa Dasb and Saumi Ray*a
a Birla Institute of Technology and Science, Pilani, Rajasthan 333031
b Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032.
ABSTRACT: It is observed that planar Ni (II)–Schiff base complexes of general formulae NiL with L as
N, Ń-bis(4-hydroxy salicylidene)ethylenediamine (4-OH-salen) when encapsulated in the supercage of
zeolite Y, the complex adopts non-planar geometry. The detailed characterization studies of the 4-OH-Ni-
salen complex when compared with Ni-salen complex encapsulated in zeolite Y (Ni-salen-Y)[1] have
suggested that the complex undergoes more distortion in order to accommodate itself inside the supercage
of zeolite-Y. Optical spectroscopic studies show relatively more blue shifted and intensified d-d transition
for 4-OH-Ni-salen complex after encapsulation. We also report the catalytic activities and enhanced
magnetization of encapsulated complex though it is diamagnetic in the 'free' state. DFT studies of these
complexes have been carried out in different spin states and electronic spectra are simulated to have better
insight about the geometries adopted by the guest complexes inside the host framework.
1

INTRODUCTION
Zeolites are microporous materials having three dimensional long ranges ordered aluminosilicate
framework with the channels and cavities of precise and regular size [2-6]. These materials have
numerous applications in petrochemical industries [7-9] and is utilized as potential shape and size
selective heterogeneous catalysts [10, 11] for wide range of the chemical reactions. The fascinating
architecture of zeolite helps to design coupled systems by encapsulating organic, inorganic molecules,
clusters, dyes and nanoparticals inside the cavities of the host lattice so that these coupled materials can
have diverse applications [12-18]. Additionally, they are thermally stable with varied active sites and act
as eco-friendly medium for different processes. They are also known as 'solid acid' due to the presence of
Brønsted and Lewis acid sites and even can be used like sulphuric acid in the chemical reactions [19-21].
The encapsulated transition metal complexes like phthalocyanine, bipyridyl and Schiff base ligands in
zeolites are well recognized as bio mimetic systems and are known as 'zeozymes' as they have shown the
structural and functional analogy with the metallo-enzymes such as cytochrome 450 [22, 23]. These
heterogeneous systems offer a novel route to couple the reactivity of metal complex with special
structural robustness of zeolites. The systems, where the complex is synthesized inside the cavity of
zeolite, cannot escape out of the supercage due to size restriction of the window, are especially known as
'ship-in-a-bottle' complexes [24]. The modified catalytic activity and selectivity of the encapsulated
complexes originates from host–guest interaction in the hybrid system. Rudy F. Patron et al reported that
iron phthalocyanine complex in zeolites Y is a mimic of cytochrome 450 and 10 times more efficient for
oxidation of C-H bond in mono branched hydrocarbons. The authors have reported that zeolite framework
exercises the steric constraint on active site of FePc complex as does the protein mantle in cytochrome
450. In another report P.A. Jacob et.al have reported that the manganese complexes in zeolites X and Y
are selective towards alkenes oxidation reactions [25]. The zeolite encapsulated metal Schiff-base
complexes are well explored as catalysts for the various reactions like oxidation of toluene, benzyl
alcohol, cyclohexane, ethyl benzene methyl phenyl sulphide, and styrene [26-33], with mild oxidants
like H2O2, and TBHP. A recent report has also explored the other dimensions of these systems and found
2

that the amine functionalized ionic liquid encapsulated in zeolites Y shows capability for CO2 capture
[34]. Catalysis of zeolite encapsulated complexes is well investigated research area however the impact
of encapsulation on the structural alteration of the guest complex still remains relatively less studied,
though the enhanced selectivity and modified reactivity might have strong correlation with altered
geometry adopted by the encapsulated complex. There are only few reports available in the literature [35-
38], which have explored the influence of the topology of the void on the geometry of encapsulated
complex and its consequences in terms of change in electronic, physical, catalytic and redox properties of
encapsulated system. A series of Fe2+ complexes with different ligands encapsulated in zeolite Y, have
been reported and with the help of spectroscopic techniques it is observed that the smaller complexes like
[Fe(en)3]2+ and [Fe(amp)3]2+ stay inside the supercage without any distortion, but complexes with slightly
larger size with respect to the zeolite cavity like [Fe(bpy)3]2+ and [Fe(phen)3]2+, have adopted
significantly distorted geometry on encapsulation, and further attempt to encapsulate bigger complexes
like [Fe(dmbpy)3]2+ and [Fe(dmphen)3]2+ is a failure. In other report, exciting magnetic behaviour of
encapsulated complex has been observed where authors have reported that the [Co(bpy)3]2+ complex after
encapsulation in zeolite Y shows the temperature driven spin crossover phenomena, whereas the complex
ion in its free form retains a high spin ground state in solution as well as in solid state [37]. Ray et al have
reported that the square planar cobalt phthalocyanine of D4h symmetry, under encapsulation in zeolites
Y, adopts a distorted geometry and as a result encapsulated complex shows altered magnetic behaviour
with enhanced magnetic moment [35]. R.C. Deka et al have observed that cobalt phenanthroline complex
after encapsulation in zeolites Y exhibits difference in electrochemical response and shifted positions of
frontier orbitals of the molecule, further explored by DFT studies [38]. Earlier we have reported that the
diamagnetic nickel Schiff-base complex when encapsulated in zeolite Y, adopts the non-planar geometry
due to space constraint where nickel metal is out of the plane of N2O2 moiety, exhibiting paramagnetism,
however the same complex, when encapsulated in another host with much larger pore diameter such as
MCM-41, doesn’t show this behaviour [39]. Recently it is observed that the nickel (II) Schiff-base
complexes having different molecular dimensions when encapsulated within the zeolite supercage, higher
3

the molecular dimension, greater are the degree of distortion of the structure of the encapsulated complex
and the catalytic activity [1]. In the present manuscript, we report the encapsulation of the 4-OH nickel
Schiff base complex with nearly same ‘end to end’ distance (~12.98 Ǻ) of nickel salen complex[1]
encapsulated within the supercage of zeolite-Y with a pore diameter of 12.47 Ǻ via flexible ligand
synthesis method. The difficulty therefore lies in how to fit the planar complex into the spherical cavities.
We have carried out the detailed characterization of the system with the help of comparative
spectroscopic (IR, UV-Vis), morphological (XRD, SEM), magnetic and theoretical studies to understand
the confinement of the complex in zeolite Y supercage. DFT studies have provided the detailed
information of different adopted geometries of the guest complexes under space constraint within zeolite.
Furthermore, optical spectra are simulated employing TD-DFT method to have a greater insight of the
experimentally observed shifts in UV-Vis spectra of the zeolite encapsulated complexes. Additionally, we
have employed complex as catalyst both in free and entrapped states on the styrene oxidation reaction to
comprehend the extent of distortion and modified reactivity of the complexes after encapsulation in
zeolite Y.
RESULT AND DISSCUSSIONS
Powder XRD Analysis
The powder X-ray diffraction patterns of Na-zeolite Y, Ni-zeolite Y, and 4-OH-Ni-salen complex in
zeolite Y (4-OH-Ni-salen–Y) systems are shown in Figure 1. The powder XRD pattern of 4-OH-Ni-salen-
Y is essentially similar with that of the pure and nickel exchanged zeolite Y, indicating the retention of
integrity of host zeolite lattice after the encapsulation processes. However a noticeable intensity reversal
is observed at 2θ = 10° and 12° positions in the XRD patterns for the encapsulated complex. Pure and
nickel exchanged zeolite Y hold the intensities relation as I220> I311, but after the encapsulation of complex
in the supercage, the XRD pattern of the hybrid system holds the reverse relation of intensities as I220<
I311. In the literature, empirical correlation of such observation with the presence of a large complex inside
the supercage of zeolite Y already exists.[40-43] Similar observation in XRD patterns is observed for the
nickel salen-Y complex [1]. Such type of experimental observation does not appear in case of tethering
4

of complex and even if complex does not form in the cavity of supercage [38, 44]; XRD experimental
data suggest the encapsulation of guest complex is successfully carried out in the pores of zeolite.
Appearance of any extra peak is not observed as expected because the loading levels of the metal
complex maintained in zeolite are very low.
Elemental Analysis
The parent host material, Na-zeolite Y has the unit cell formula as Na58Al58Si136O388•yH2O and Si/Al ratio
is 2.34. EDS spectra and data of 4-OH-Ni-salen-Y [supporting information Figure S1] suggests that Si/Al
ratio (2.30) remains unaffected in encapsulated complex , which signifies that the leaching of aluminium
doesn’t take place during encapsulation processes. Presence of both the sodium and nickel metal ions, in
the hybrid system of zeolite with the encapsulated complex within indicates the partial exchange of
sodium metal ions during exchange reactions. The nickel ion concentration in encapsulated complex is
observed to be lesser (0.50 wt %) than that in nickel exchanged zeolite Y (0.92 wt %) as expected because
of possible leaching of some of the metal ions during the synthesis of complex which in turn, can lead in
lowering the nickel ion concentration.
Scanning Electron Microscopy Analysis
Encapsulation of metal complex within the supercage of zeolite-Y essentially targets the complex
formation only inside the cavities of supercage, however such a precise method of synthesis not feasible
practically. Inevitable adsorptions of some complexes and ligand on the surface of zeolite lattice have
been removed by extensive Soxhlet extraction. SE micrographs of pure zeolite Y, 4-OH-Ni-salen -Y after
Soxhlet extraction is shown (Figure 2 a-b). After Soxhlet extraction, surface boundaries of host lattice are
much clearly visible and differentiable, providing a clear indication of the removal of surface species
from surface, as well as preservation of crystallinity of zeolite framework [42, 45]. Eventually the SEM
analysis gives the indirect evidence about the complex formation, mostly occurring inside the host cavity
only.
5

X-Ray Photoelectron Spectroscopy
The existence of metal complex in the zeolite framework has been further confirmed by the X-ray
photoelectron spectroscopy (XPS). The survey spectra and high resolution XP spectra of C (1s), N (1s), O
(1s) and Ni (2p) of 4-OH-Ni-salen-Y are presented in Figure 3. The observed spectra and binding energy
data (Figure 3 and Table 1) clearly indicates the presence of C, N, O, Na, Si, Al, Ni(II) in their respective
chemical states, which is accordance to the literature [1, 46-50]. The loading level of metal is lowin the
complex within the host lattice, therefore the weak XPS peak intensities are observed for nickel (2p)
signals for the encapsulated complex (4-OH-Ni-salen-Y) and it is perfectly in agreement with the other
concentration dependent studies obtained from EDX, IR and UV-Visible spectroscopies. It already has
been observed that Ni-salen in both its free and zeolite encapsulated forms show the appearance of 2p3/2
and 2p1/2 peaks in the XPS spectra at the binding energies at around 855 and 872 eV suggesting nickel
metal present in +2 oxidation state in both the forms [1]. As well as absence of any shake up and satellite
peak around 878 eV confirms that the complex stays in square planar geometry not in the octaheadral
geometry [46, 49]. Intense and broad carbon (1s) XPS spectra have observed for nickel salen complex in
both states which further deconvoluted into two peaks and validated the presence of sp3 and sp2 carbon
atoms in complex. Similarly the complex have shown the nitrogen (1s) and oxygen (1s) XPS traces for
(M-N), (N=C) and (C-O), (M-O) respectively [1]. In the present study, encapsulated 4-OH-Ni-salen-Y
also has shown the essentially similar patterns of XPS signals for all constituent atoms of the complex
with the minor shifts in the signals. The encapsulated 4-OH-Ni-salen complex has demonstrated 2p3/2 and
2p1/2 peaks at 853.40 and 872.95 eV binding energies since Ni in the complex is present in +2 oxidation
state with square planar geometry. Furthermore, carbon (1s) XPS spectra of 4-OH-Ni-salen-Y have
confirmed the presense of sp3 and sp2 carbon atoms and XPS traces of nitrogen (1s) are deconvoluted into
two signals associated to (M-N) and (N=C) peaks. Oxygen (1s) XPS traces are deconvuluted into two
peaks which are attributed to (M-O) and (C-O) peaks. All observed signals along with the Si (2p) and Al
(2p) peaks in 4-OH-Ni-salen-Y hybrid system are in accordance with the literature [1]. Presence of all
6

elements in the expected chemical states and nickel metal ion in +2 oxidations state undeniably signifies
the successful formation of 4-OH-Ni-salen complex within the supercage of zeolite-Y.
Infrared Spectroscopy
The IR spectra of pure zeolite Y, 4-OH-Ni-salen, and 4-OH-Ni-salen-Y are shown in Figure 4, and
corresponding data are provided in the Table 2. Pure-zeolite-Y mainly shows bands in the region of 450-
1200 cm-1 and also at around 1643 cm-1 and 3500 cm-1 region. The major characteristics bands at 560,
717, 786, and 1018 cm-1 are identified as (Si/Al-O)4 bending mode, double ring, symmetric stretching and
asymmetric stretching vibration respectively [51]. Another two broad bands at 3500 cm-1 and 1643 cm-1
are originated from the surface hydroxylic group and lattice water molecules respectively [45]. The
appearance of these bands even for the integrated host-guest systems (Figure 4) indicates the preservation
of host lattice integrity, during the process of encapsulation. However, the characteristic IR bands of the
encapsulated complexes are majorly masked by strong zeolitic bands over most of the range. The signal
intensity is very low, because of very low concentration of metal complex inside the framework, the only
region of 1600-1200 cm-1 is found to be suitable for the study because in that region bands appear only
from the guest complex.; Within 1600-1200 cm-1, encapsulated complex shows main characteristics C=C,
C=N and C-O stretching [52] and C-H deformation bands with small shifts. The shift in these bands can
be due to the different environment imposed by the cavity on the of guest complex. The C-H deformation
band of the complex has shifted from 1375 cm-1 to 1381 cm-1 under encapsulation [1, 53]. These
observed IR data point towards the successful synthesis of the nickel Schiff-base complex inside the pores
of zeolite-Y. The overall experimental FTIR spectral data (Table 2) suggests the successful synthesis of
the complex in the ‘free’ as well as in encapsulated state inside the zeolite Y cavities.
UV-Visible Spectroscopy
Electronic spectra of the 4-OH-salen ligand and 'free state' 4-OH-Ni-salen complex have been recorded in
7

chloroform (supporting information, Figure S2). Chloroform is preferred over the other solvents because
it is already an established fact that in chloroform, such complexes experience minimum structural
deviation from planarity when dissolved in chloroform [39, 54]. The comparative solid state UV-Vis
studies of the complex in 'free state' complex and zeolite entrapped state have been carried out (shown in
Figure 5 and Table 3) to explore the change of electronic environment around the metal centre after
encapsulation. Not only that, comparative studies of this system with those of Ni-salen become relevant in
this point to investigate the effect of the substituent groups. The free state complexes Ni-salen (Figure 5a),
4-OH-Ni-salen (Figure 5b) show the high energy strong bands at 251 nm and 241 nm respectively, which
are attributed to π—π* transitions. Further two bands at 323 and 413 nm in the spectrum of Ni-salen, and
at 325 nm and 397 nm in the spectrum of 4-OH-Ni-salen are the n—π* transitions [1, 45]. For both Ni-
salen and 4-OH-Ni-salen, relatively weak yet broad absorption bands adjacent to n—π* bands, at 453 nm
and 439 nm respectively must have originated from charge transfer transitions. The broad d-d band
centered at ~(555-570) nm for Ni-salen [1] and ~(560-580) nm for 4-OH-Ni-salen can be attributed
majorly to the transition from dz2 to dxy orbital [55]. The appearance of charge transfer bands and d-d
bands directly proves the formation of the complex. The UV-Vis spectral bands of encapsulated
complexes are not so distinctly visible, due to very low concentration of complex inside zeolite, but it is
quite apparent that, the high energy π—π* and n—π* transitions for both the encapsulated complexes Ni-
salen-Y (Figure 5d), 4-OH-Ni-salen-Y (Figure 5c) secure nearly same positions as their corresponding
'free state' complexes. However on encapsulation, lower energy d-d bands, are considerably intensified
[53] and blue shifted [1, 39] with respect to the corresponding 'free state' d-d bands. Such type of
electronic behaviour reveals that the complex is not able to sustain its planar geometry especially around
the metal ion due to space restrictions imposed by the host cavity. Though both Ni-salen and 4-OH-Ni-
salen complexes have nearly same end-to-end distances as 12.94 Ǻ and 12.99 Ǻ respectively, two
hydroxyl substituent’s on both the benzene rings of 4-OH-Ni-salen complex lead in increase of the width
of the complex essentially requiring more space to be accommodated inside the nearly spherical
supercage (cavity diameter is 12.47Ǻ). The extent of observed blue shift in d-d bands is roughly double
8

for 4-OH-Ni-salenas compared to that of Ni-salen under entrapment. The observed blue shift in the d-d
band of Ni-salen-Y is around 30 nm, whereas that in 4-OH-Ni-salen-Y is approximately 60 nm. The
2
2
observed blue shift definitely in turn points towards the reduction of the gap between dxy and dx -y
orbitals. This observation is in line with the argument that extent of distortion in the case of entrapped 4-
OH-Ni-salen complex is more than that for Ni-salen complex under confinement. It is explicable, as the
complex with comparatively bulkier ligand experiences more space constraint, imposed by the host
framework, which makes the complex more distorted and is also well supported by the electronic
spectroscopic studies.
Theoretical Studies & Results
Structure & Geometry
The ground states of Ni-salen and 4-OH-Ni-salen complexes are planar, diamagnetic and exist as singlet
states without any unpaired electrons (The details of theoretical studies on Ni-salen and 4-OH-Ni-salen
complexes in isolated, singlet and encapsulated triplet forms are discussed in SI section).The structure of
the Ni-salen and 4-OH-Ni-salen complexes in the free state and then in zeolite encapsulated triplet states
are relaxed and the optimized structures are presented in Figure 6. The structure of the encapsulated
complexes shows deviations from those of the neat complexes and few important results have been
tabulated in Table 4. The variations in the bond distances and bond angles upon encapsulation are small
and are due to the influence of the zeolite framework, however, the Ni-salen/4-OH-Ni-salen complexes as
a whole are deformed and puckered when packed inside the pore. The encapsulation process changes the
ordering of the molecular orbitals in such a way that the magnetic triplet state becomes stable. The
switching of a diamagnetic planer molecule to a magnetic puckered molecule when encapsulated is the
focus of the present work.
The neat Ni-salen and 4-OH-Ni-salen complexes though are planar, diamagnetic and are more stable in
singlet states, they may also exist in high energy triplet states with two unpaired electrons. The energy
differences calculated between the singlet state and the triplet state for free neat Ni-salen is 12.8 kcal and
9

11.8 kcal for 4-OH-Ni-salen.The non planer triplet state predominates when encapsulated as the
experimental magnetic studies indicate. The theoretical spin densities on Ni atom is 1.56 (total moment:
2.35 BM, [n(n+2)]1/2, where n is number of unpaired electrons) for Ni-salen and 1.68 (total moment: 2.49
BM) for 4-OH-Ni-salen. From the results tabulated in Table 3, we find that Ni-N distances change from
1.87Å for the neat complexes to 1.96 Å for the encapsulated complexes and Ni-O distances change from
~1.86 Å in neat states to ~1.89Å when encapsulated both for Ni-salen and 4-OH-Ni-salen. The O-C and
N-C distances are 1.29Å for both free and encapsulated cases of Ni-salen and 4-OH-Ni-salen. The O-Ni-
N bond angle decreases from 93.9° to 92.3° and the angle C-Ni-C (atoms shown in rectangular boxes as
marked in Figure 6, give an idea about the non-planarity of the molecule), , changes from 179.99° to
169.7° in case of the Ni-salen after encapsulation. The HOMO for neat Ni-salen has a dyz character
hybridized with the p bonding character of the salen ligand, and the LUMO has Ni dxz character
hybridised with p antibonding type orbitals from C and N. The HOMO and LUMO energies are -5.28 and
-1.83 eV respectively with a HOMO-LUMO energy gap of 3.45 eV. The overall distortion of the complex
4-OH-Ni-salen after encapsulation is indicated from the same bond-angles O-Ni-N; which changes from
93.9° to 92.2° and C-Ni-C which changes from 179.99° to 169.3°. The HOMO for the isolated 4-OH-Ni-
salen complex has a dyz character and the LUMO has Ni dxz character. The HOMO and LUMO energies
are -5.26 and -1.60 eV respectively with a HOMO-LUMO energy gap of 3.66 eV. The change in the bond
distances and bond angles on encapsulation may be attributed to the influence of the zeolite framework,
where the zeolite valence electrons are distributed all over as a delocalized electron cloud and interacts
strongly with the encapsulated Ni complex.
Optical transitions from TD-DFT studies
TD-DFT methods are employed to calculate the UV-Vis spectra for the singlet states of the neat
complexes and triplet spin states for the encapsulated Ni-salen and 4-OH-Ni-salen complexes, as given in
Figure 7. In case of isolated Ni-salen, the d-d transition is calculated at 633 nm, a transition from HOMO
orbital (Ni dyz) to LUMO+2 (Ni dxy hybridized with O and N p orbitals). The transition around 555 nm
occurring from HOMO-2(Ni dz2) to LUMO+2(Ni dxy) is also a d-d transition. Another d-d transition is
10

calculated at 493 nm which is HOMO-3 (Ni dyz) to LUMO+2 (Ni dxy), but it is hybridized with ligand
orbitals. Strong bands around 420 and 350 nm are due to transitions; HOMO to LUMO+1, HOMO-2 to
LUMO and HOMO-1 to LUMO respectively which could correspond to the n-π* bands as observed
experimentally. Several intra ligand charge transfer bands (π-π*) are calculated around 280 nm. For
encapsulated Ni-salen-Y complex, the d-d transition appears at 589 nm which is a transition from
HOMO-6 orbital (Ni dz2) to LUMO+3 orbital (Ni dxy). Next moderately intense transition is observed at
499 nm which is HOMO-1 to LUMO transition and is a ligand to metal charge transfer transition. The
intense transitions occurring at 450-390 nm (HOMO to LUMO, HOMO-1 to LUMO, HOMO to
LUMO+1 orbitals) are primarily intra ligand transitions with minimal involvement of metal d orbitals and
correspond to n-π* bands experimentally.
In case of isolated 4-OH-Ni-salen complex, the d-d transitions are found at 627 nm, a transition from
HOMO (Ni dyz) to LUMO+2 (Ni dxy hybridized with O and N p orbitals) and at 556 nm from HOMO-
4(Ni dz2) to LUMO+2(Ni dxy). Ligand to metal charge transfer transition is calculated at 451 nm (HOMO-
5 to LUMO+2) and at 396 nm the transition from HOMO to LUMO+1 is an intra ligand transition. Strong
transitions around 340-310 nm are n-π* transitions. Several π-π* bands occur around 270 nm. For
encapsulated 4-OH-Ni-salen complex, the d-d transition appears at 596 nm which is a transition from
HOMO-6 orbital (Ni dz2) to LUMO+3 orbital (Ni dxy hybridized with O and N p orbitals). Moderately
intense transitions are observed at 475, 462 nm which are HOMO-1 to LUMO and HOMO-4 to LUMO+3
for beta orbitals, which are mainly ligand to metal charge transfer transition. Theoretically there are
intense transitions occurring at 350-320 nm are primarily intra ligand transitions with very small
involvement of metal d orbitals. Theoretically intense π-π* intra ligand transitions occur around 270-290
nm for both complexes.
From the theoretical study of optical transitions, it is clear that after encapsulation, the d-d transition is
shifted to lower wavelengths for both complexes by 30-40 nm, but the n-π* and the π-π* transitions
remain largely unaltered, similar to experimental observations. It is important to note that the observed d-
11

d shifts in the spectra of encapsulated complexes have major contributions from dz2 and dxy atomic
orbitals of Ni as expected for square planer complexes. DFT study also reveals that every transition
associated with the dxy orbital is essentially blue shifted, which points towards the reducing energy gap
between the dxy and dz2 orbitals and slightly higher lying triplet state becomes comparatively more
accessible which also leads to a significant increase in the magnetic moment after encapsulation.
Magnetic Studies
Optical spectroscopic studies as well as the theoretical studies clearly indicate the distortion in geometry
of 4-OH-Ni-salen complex upon encapsulation, therefore to comprehend the geometry adopted by the
encapsulated complex especially around the metal centre, magnetic studies have been carried out and then
compared with that of Ni-Salen (without any substitution) entrapped in zeolite-Y [1]. As mentioned in
elemental analysis section, the loading level of nickel in 4-OH-Ni-salen-Y is 0.50 wt % whereas in Ni-
salen-Y is 0.59 wt %. The planar d8 nickel complexes are diamagnetic. Several literature reports are
available, which clearly suggest that these metal complexes are diamagnetic in nature, without any effect
of the substituent on the benzene rings [56-58]. From the observed experimental magnetic data in the
temperature range of 4K-300K, the Ni-salen complex, is found to be almost diamagnetic with magnetic
moment near to zero (0.45 BM), which essentially indicate planarity of N2O2 moiety around the Ni(II)
centre [1, 56]. However, in the same temperature range, both the encapsulated 4-OH-Ni-salen and Ni-
salen complexes display markedly different behaviour. The plots of molar susceptibility vs., temperature
(5-300) K for both the encapsulated complexes along with that of the neat Ni-salen are shown in Figure 8.
The origin of such radically different behavior of the hybrid system is apparently from the guest nickel
complexes, as the host is diamagnetic [40, 59]. On the other hand, the paramagnetic behaviors of the
encapsulated Ni complexes are quite evident. This enhanced paramagnetism must be instigated from the
structural deformation of the encapsulated complex where Ni ion experiences an electronic environment
different from that of the neat complex, and the non- planar NiN2O2 moiety results in an altered ordering
of molecular energy levels. As a result, the slightly higher lying excited triplet state becomes
12

comparatively more accessible. This argument is even strongly supported by the theoretical studies and
experimentally obtained optical data indicating the fact that encapsulation causes significant reduction of
2
2
the energy gap between the highest lying metal dx -y and next highest dxy orbitals. Hence encapsulation
enables a considerable fraction of the complex molecules to exist in high energy triplet states and is
consequently responsible for the substantial increase in the magnetization of the encapsulated complex.
Comparing the magnetic behaviour of both the encapsulated complexes, we have found that encapsulated
4-OH-Ni-salen-Y complex has much higher magnetization than the encapsulated Ni-salen-Y complex
establishing the fact that 4-OH-Ni-salen-Y complex has larger fraction of molecules in higher lying
excited triplet states upon encapsulation. It is reasonable, because the bulkier 4-OH-Ni-salen-Y complex
does not fit into the spherical cavity of host and hence suffers from more steric strain to accommodate
itself into the cavity thus adopts more distorted geometry than encapsulated Ni-salen-Y complex.
Catalytic activity and structural correlation
Catalytic activities of the complex both in free and encapsulated states have been investigated for styrene
oxidation reaction using H2O2 as oxidant (Table 5) and Ni-salen system is employed as catalyst [1]. In
this reaction, benzaldehyde is identified as major product in each of the cases whereas styrene oxide and
phenyl acetaldehyde are the minor products. Nickel Schiff-base complexes are known to be less reactive
than other transition metals for the oxidation reactions [33, 60]. On comparison the percentage conversion
data, of both state complexes, neat complexes seems to have high conversion but there is a difference in
concentration of active centers (nickel metal)as encapsulated complexes have poor loading level (
concentrations of nickel metal in the encapsulated complex is 0.50 wt % and 59 wt % for the 4-OH-Ni-
salen-Y and Ni-salen-Y respectively) to get maximum free zeolite surface for the reaction so relative
reactivity of catalysts has been calculated in term of Turnover number, TON. It has been observed that the
encapsulated complexes are more selective for benzaldehyde than their corresponding neat state [60]. Ni-
salen-Y and 4-OH-Ni-salen-Y complexes in their neat form show comparatively similar reactivity for the
oxidation reaction in terms of Turn Over Number (TON). Interestingly, the 4-OH-Ni-salen-Y complex
13

shows greater reactivity than Ni-salen-Y and UV-Vis, magnetic and TD-DFT studies evidently support
the greater extent of distortion of planar geometry in 4-OH-Ni-salen-Y complex. In literature, it has been
reported [61, 62] that distortion in planar complexes induces the better reactivity for the oxidation
processes because, distorted planar structure induces more electropositive character on the metal, which
act as more sensitive site for nucleophilic attack of the reagent. Study of relation in modified functionality
and structural behaviour surely provide a new gateway to design the more effective heterogeneous
catalyst which will be useful in various industrial processes.
Conclusion
This manuscript presents detailed characterizations of complex 4-OH-Ni-salen encapsulated in zeolite -Y.
The comparative morphological, spectroscopic, magnetic, catalytic and theoretical studies of both
complexes have been carried out to explore the effect of the host framework on the guest complexes.
XRD pattern, SEM and IR studies have indicated successful formation of the complex inside the zeolite Y
cavities. Solid state UV-Visible studies have supported the complex formation in the neat and also in the
encapsulated states in zeolite Y, however the d-d transitions of the encapsulated complexes are observed
to be blue shifted. Interestingly, observed blue shift in d-d bands is more when compared with Ni-salen-Y
TD-DFT simulated electronic spectra have supported the experimental observations and indicated that
encapsulated complexes are distorted and exist in triplet states. Further the magnetic studies have
presented an evident exposure about the distortion in geometry around the Ni center of encapsulated
complexes, because both diamagnetic complexes show paramagnetism whereas 4-OH-Ni-salen- complex
shows relatively enhanced magnetism in comparison to Ni-salen-Y complex, which again proves the
effect of substituent groups in the encapsulated complex. Theoretical studies also have indicated a higher
magnetic moment for 4-OH-Ni-salen-Y complex, with respect to Ni-salen-Y complex, showing good
agreement with experimental magnetic behavior. The electronic, magnetic and DFT studies altogether
indicate that encapsulation forces the complex to adopt a distorted geometry which drives the Ni atom out
of the plane of NiN2O2 moiety. This alteration in geometry lowers the energy gap between the triplet and
14

singlet states, which in turn, makes the energetically higher lying triplet state more accessible. Bulkier 4-
OH-Ni-salen complex suffers more space constraint imposed by spherical host cavity, thus, the extent of
distortion is higher, driving majority of molecules to the higher energy triplet state, eventually making it
more paramagnetic than Ni-salen-Y complex. Enhanced catalytic activity of 4-OH-Ni-salen-Y complex
validates the extent of distortion in adopted geometry of the guest complex as well as offers the evidences
about the connection between structural and functional alterations.
Acknowledgements
The authors acknowledge the Department of Science and Technology, New Delhi, for the financial
support (DST Project No: SR/FT/CS-038/2010) and for the instrumental facility funded by DST-FIST,
Department of Chemistry, BITS, Pilani and DST-FIST, Department of Physics, BITS, Pilani and for
theoretical studies (DST Project No: SR/FT/CS-119/2010).
SUPPORTING INFORMATION
Physical measurements, Theoretical methods, Synthesis methods, EDS spectra of pure zeolite Y and
encapsulated Ni-salen-Y, 4-OH-Ni-salen-Y complexes, UV-Vis data of neat complex and ligands, The
structure of the frontier molecular orbitals in free state and triplet encapsulated state, TD-DFT spectra for
Ni-salen-Yand 4-OH-Ni-salen-Y using B3LYP and B3PW91 exchange methods.
AUTHOR INFORMATION
Corresponding Author
Email: saumi@pilani.bits-pilani.ac.in
Contact no + 91-01596-515650
Fax no
+91-(01596) 244183
Present Addresses
15

Dr Saumi Ray
Associate professor,
Department of Chemistry
Birla Institute of Technology and Science,
Pilani Campus, Pilani, 333031, Rajasthan. INDIA.
16

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23

Figure captions
Figure 1. Powder XRD patterns of (a) pure zeolite-Y (b) Ni exchanged zeolite-Y (c) 4-OH-Ni-salen-Y.
Figure 2. SEM images of (a) pure zeolite Y, (b) 4-OH-Ni-salen-Y after Soxhlet extraction.
Figure 3. XPS spectra of 4-OH-Ni-salen-Y complex (A) survey spectra, High resolution XPS specta of
(B) nickel (2p), (C) carbon (1s), (D) nitrogen (1s) and (E) oxygen (1s).
Figure 4. (A) FTIR spectra in the range of 500 cm-1 to 4000 cm-1 of (a) 4-OH- Ni-salen (b) 4-OH-Ni-
salen-Y (c) and pure zeolite Y. (B) magnified view of IR data of (a) 4-OH- Ni-salen (b) 4-OH-Ni-salen-
Y (c) and pure zeolite Y.
Figure 5. Solid state UV-Vis spectra in the range of 190-700 nm of (a) Ni-salen (b) 4-OH- Ni-salen (c)
4-OH-Ni-salen-Y (d) Ni-salen-Y and (e) Pure zeolite Y.
Figure 6. (a) Ni-salen (singlet) free state (b) 4-OH- Ni-salen free singlet state. The atoms marked by the
squares define the C-Ni-C bond angle in different molecules. (c) The zeolite framework used for
theoretical studies. (d) The deformation of encapsulated Ni-salen (bottom) shown with respect to its neat
form (top). (e) The deformation of 4-OH-Ni-salen-Y 4-OH-Ni-salen (bottom) shown with respect to its
neat form (top). (f) 4-OH-Ni-salen-Y molecule (triplet) is encapsulated within the zeolite pore.
Figure 7. Comparative UV-Visible data of (A) Ni-salen (B) Ni-salen-Y (C) 4-OH- Ni-salen and (D) 4-
OH-Ni-salen-Y experimental data are represented by red line and TD-simulated data are represented by
black vertical line.
Figure 8. Molar susceptibility vs temperature plots in the range of 4K-300K of (a) Ni-salen (b) Ni-salen-
Y and (c) 4-OH-Ni-salen-Y.
24

Figures
Figure 1
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Figure 2
26

Figure 3
27

Figure 4
28

Figure 5
29