Zeolite-Y immobilized Metallo-ligand complexes: A novel heterogenous catalysts for selective oxidation

Article abstract of DOI:10.1016/j.inoche.2016.08.017

Transition metal [M?=?Co(II), Cu(II)] complexes of H₂L¹ and H₂L² ligands have been prepared as neat and nanohybrid zeolite-Y immobilized complexes. The various analytical tools such as FTIR, ICP-AES, elemental analysis, UV–vis, Brunauer, Emmett and Teller (BET) surface area analysis, Thermal analysis, scanning electron micrographs, Powder XRD, conductivity, magnetic moment, and AAS were employed for the characterization of the prepared catalysts. Among all catalysts, the [Cu(L¹)]-Y (heterogeneous) and [Cu(L¹)] (homogeneous) have offered high activity and selectivity over oxidation of cyclohexene. Moreover, the [Cu(L¹)] and [Cu(L¹)]-Y were employed as catalyst over various organic substrates at identical reaction condition. The immobilized catalyst [Cu(L¹)]-Y is found to be moderate active over oxidation of cyclohexane (75.2%,), benzene (8.21%), phenol (14.5%), styrene (87.5), benzyl alcohol (21.5%), limonene (11.2%), α-pinene (9.15%), and cyclooctane (76.8%) with high TON values (21942-2054). The mechanistic study using UV–vis and FTIR suggests the participation of active metalperoxo species, which is reinforced by its high catalytic activity over limonene (16.3%) in the absence oxidant.

Full text of DOI:10.1016/j.inoche.2016.08.017

Inorganic Chemistry Communications 72 (2016) 105–116
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Feature article
Zeolite-Y immobilized Metallo-ligand complexes: A novel heterogenous
catalysts for selective oxidation
⁎
Dinesh R. Godhani , Haresh D. Nakum, Digvijaysinh K. Parmar, Jignasu P. Mehta, Nisheeth C. Desai
Department of Chemistry, (UGC NON-SAP & DST-FIST sponsored) Mahatma Gandhi Campus, Maharaja Krishnakumarsinhji Bhavnagar University, Bhavnagar 364 002, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Transition metal [M = Co(II), Cu(II)] complexes of H2L1 and H2L2 ligands have been prepared as neat and
nanohybrid zeolite-Y immobilized complexes. The various analytical tools such as FTIR, ICP-AES, elemental anal-
ysis, UV–vis, Brunauer, Emmett and Teller (BET) surface area analysis, Thermal analysis, scanning electron micro-
graphs, Powder XRD, conductivity, magnetic moment, and AAS were employed for the characterization of the
prepared catalysts. Among all catalysts, the [Cu(L1)]-Y (heterogeneous) and [Cu(L1)] (homogeneous) have of-
fered high activity and selectivity over oxidation of cyclohexene. Moreover, the [Cu(L1)] and [Cu(L1)]-Y were
employed as catalyst over various organic substrates at identical reaction condition. The immobilized catalyst
[Cu(L1)]-Y is found to be moderate active over oxidation of cyclohexane (75.2%,), benzene (8.21%), phenol
(14.5%), styrene (87.5), benzyl alcohol (21.5%), limonene (11.2%), α-pinene (9.15%), and cyclooctane (76.8%)
with high TON values (21942-2054). The mechanistic study using UV–vis and FTIR suggests the participation
of active metalperoxo species, which is reinforced by its high catalytic activity over limonene (16.3%) in the ab-
sence oxidant.
Article history:
Received 29 June 2016
Received in revised form 20 August 2016
Accepted 27 August 2016
Available online 29 August 2016
Keywords:
Cu(II) complexe
Co(II) complexe
Immobilization
Selective oxidation
TON
© 2016 Elsevier B.V. All rights reserved.
Contents
1.
2.
Introduction .
Experimental.
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106
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112
112
2.1.
2.2.
2.3.
Materials
Physical methods and analysis.
Syntheisis section
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. . . . .
2.3.1.
2.3.2.
2.3.3.
2.3.4.
2.3.5.
Preparation of ligands
Preparation of Co (II) and Cu(II) based neat complexes
Preparation of metal exchanged M (II)–Y (M = Cu, Co) .
Preparation of zeolite-Y immobilized metal complexes
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Catalytic activity .
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3.
Results and discussion
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3.1.
3.2.
3.3.
3.4.
3.5.
3.6.
3.7.
Elemental analysis .
Brunauer, Emmett, and teller (BET) surface area analysis.
X-ray diffraction (XRD) study .
Scanning electron microscopy (SEM) analysis.
Fourier transform infrared (FTIR) spectroscopy .
Electronic spectra and magnetic moments
Thermal analysis .
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4.
5.
Catalytic activity . . . .
4.1.1.
4.1.2.
Oxidation of cyclohexene .
Catalyst stability and reusability .
Oxidation of the other organic substrates .
⁎
Corresponding author.
E-mail address: drgodhani@yahoo.com (D.R. Godhani).
http://dx.doi.org/10.1016/j.inoche.2016.08.017
1387-7003/© 2016 Elsevier B.V. All rights reserved.

106
D.R. Godhani et al. / Inorganic Chemistry Communications 72 (2016) 105–116
6.
7.
References.
Plausible reaction pathway of catalysts
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113
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116
Conclusion .
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. .
1. Introduction
property of the neat complexes was measured by the magnetic suscep-
tibility balance (Johnson Metthey and Sherwood model). BET surface
area analysis was measured by a multipoint BET method using
Micromeritics, ASAP 2010 surface area analyzer. The powder XRD was
executed by Bruker AXS D8 Advance X-ray powder diffractometer
with a CuKα (λ = 1. 54,058) target and movable detector. Scanning
electron micrographs (SEMs) of [Cu(HL1)]-Y were carried out using
SEM instrument (model-JSM-5610LV), JEOL. FTIR (4000–400 cm−1) of
were recorded with KBr on a FTIR-8400S Shimadzu. UV–Vis spectra
was recorded on “SHIMADZU” UV-2450 spectrophotometer using a
quartz cell of 1 cm3 optical path in 10−3 M methanol, and/or aqueous
HF. TG analysis was carried out in air atmosphere in the temperature
range 30–700 °C using Shimadzu (TGA-500) Instrument. Atomic ab-
sorption spectra was recorded on a PerkinElmer 4100-1319. The prod-
ucts during catalytic oxidation were identified by GC–MS having BP-5
capillary column (30 m × 0.25 mm × 0.25 m) 95% silicoxane surface
and FID detector.
The development of the environmentally benign catalyst and the
new catalytic pathway has grown much interest of researcher in the
current research scenario. Especially, the use of H2O2 as an oxidant for
oxidation reactions has found many advantages such as it strongly oxi-
dize the substrate, it is quite cheap, and it affords water as a byproduct
[1]. The catalytic oxidation of cyclohexene, cyclohexane, benzene, phe-
nol, styrene, benzyl alcohol, limonene, α-pinene, and cyclooctane usual-
ly affords oxyfunctionalized derivatives, which are very essential
intermediates in the preparation of fragrances, drugs, food additives, ag-
rochemicals, and in industrial reactions [2–4].
It is most desirable that oxyfunctionalized derivatives could be pro-
duced by environmentally benign oxidants like hydrogen peroxide
using the ecofriendly and recyclable heterogeneous catalyst. Usually,
the mineral acids, transition metals, and neat metal complexes are
being used as a traditional catalyst over the oxidation reactions due to
their high selectivity and activity towards the anticipated product. But
the practical utilization of these catalysts is rather provoking due to its
difficulty in the separation and retrieval of catalyst from the reaction
medium. On the other hand, the uses of heterogeneous catalyst which
are fairly easy to operate under severe conditions, highly stable, usually
prepared at comparatively low budgets, and can be simply isolated from
the products without tedious experimental work has gained consider-
able interest over the last few decades.
Nowadays, the researchers are trying to merge the edge of both the
homo and heterogeneous systems by immobilization of homogeneous
catalyst on or into the polymers, MOFs, hydrotalcites, microporous
and the mesoporous materials [5–18]. Out of these dynamic methods,
immobilization of metallo-ligand complexes inside the zeolite-Y
nanovoids has been found to be proper and pleasant since the metal
complex once formed inside the nanovoids of the zeolite-Y, is too
bulky to spread out and cannot leach into the liquid-phase during the
catalytic study [19–27]. Moreover, these heterogeneous catalysts can
be easily salvaged due to their high stability as compared to the corre-
sponding homogeneous counterparts. In connection with our previous
work [28], we have prepared the metal (Cu2+, Co2+) complexes of
H2L1 and H2L2 ligands as neat and zeolite-Y immobilized complexes. Pri-
marily, these prepared catalysts were scrutinized over oxidation of cy-
clohexene to catch the most active homo and heterogeneous catalysts
and then these most active catalysts were further tested over various or-
ganic substrates.
2.3. Syntheisis section
The graphical representation of the ligands and neat complexes is
manifested in scheme 1.
2.3.1. Preparation of ligands
Ligands viz. H2L1 and H2L2 were prepared according to reported
method and are well characterized in our previous article [28].
2.3.2. Preparation of Co (II) and Cu(II) based neat complexes
A dropwise addition of the aqueous metal salt (2.6 mmoL
CoSO4·7H2O and/or CuSO4·5H2O) solution to a 25 mL of an ethanolic
Schiff bases (2.6 mmoL, H2L1, and/or H2L2) solution leads to the forma-
tion of neat metal complexes. The subsequent mixture was heated at
80 °C in a water bath for 4–5 h. After cooling, the solid product was iso-
lated by vacuum filtration and dried for 3 h at 60 °C.
2.3.3. Preparation of metal exchanged M (II)–Y (M = Cu, Co)
A much lower concentrate metal salt solution (0.003 M of
CoSO4·7H2O and/or CuSO4· 5H2O in 250 mL deionized water) having
a pH range between 4 and 4.5 were stirred at 90 °C for 24 h after the ad-
dition of 3.0 g pure zeolite-Y into it. As a result, the slurry was isolated,
washed with deionized water to confiscate the excess of metal ions
present on the surface of zeolite-Y (confirmed by AAS), and then it
was dried for 12 h at 120 °C.
2. Experimental
2.3.4. Preparation of zeolite-Y immobilized metal complexes
2.1. Materials
A 0.5 g of activated Co(II)-Y and/or Cu(II)-Y was refluxed for 16 h
with stochometrically excess amount of ligand (H2L1 or H2L2). The slur-
ry was isolated and treated for Soxhlet extraction with acetonitrile,
methanol, chloroform, acetone and DMF to confiscate vexatious excess
ligands and the complexes formed on the external surface of M(II)-Y.
Subsequently, the material was moderately stirred with 0.01 M NaCl
for 6 h to consent the back-exchange of excess uncordinated Co(II)
and/or Cu(II) ions with Na+ ions. Then it was washed with deionized
water to wipe out the chloride ions (confirmed by AgNO3) from it and
dried for 12 h at 140 °C.
All chemicals were commercially obtainable (Sigma-Aldrich,
Rankem, Hi-media) and were used as received. The zeolite-Y (Si/Al =
5.62) was acquired from Hi-media, India. The compound 5-chloro-2-
hydroxyacetophenone, 2-hydroxyacetophenone, ethelenediamine,
phenol, R-(+)-limonene, benzene, styrene, benzyl alcohol, α-Pinene,
cyclohexane, cyclohexene, and cyclooctane were purchased from
sigma Aldrich Ltd. (India). 30% H2O2, CuSO4·5H2O and CoSO4·7H2O in
extra pure form were acquired from Rankem (India).
2.2. Physical methods and analysis
2.3.5. Catalytic activity
Liquid phase oxidation reactions were performed in a 25 mL RBF
fitted with water condenser. The movement of the reaction was super-
vised as a function of time by withdrawing a small aliquot after certain
The quantitative analysis of Si, Al, Na, Co, and Cu was executed by
ICP-AES using a model Perkin Elmer optima 2000 DV. The magnetic

D.R. Godhani et al. / Inorganic Chemistry Communications 72 (2016) 105–116
107
Scheme 1. Graphical presentation of neat complexes and ligands.
time intervals and analyzing them quantitatively by GC–MS. Primarily,
an oxidation of cyclohexene was carried out to catch the highly active
catalysts (homo and heterogeneous) among all the prepared catalyst
at identical reaction condition and then most active catalysts were fur-
ther tested over oxidation of various organic substrates such as cyclo-
hexane, benzene, phenol, styrene, benzyl alcohol, limonene, α-pinene,
and cyclooctane at identical reaction conditions.
based materials are different due to the different water content in
each catalyst (See TGA), however, the similar Si/Al ratio in zeolite-Y
based materials and similar C/N ratio in immobilized complexes com-
pared to their respective neat complexes clearly indicates the absence
of dealumination during the modification and the existence of metal
complexes inside the zeolite-Y nanovoids, respectively. Moreover, the
high M/C ratio in immobilized complexes compared to the respective
neat complexes clearly specify the presence of uncoordinated extra
Cu(II) and/or Co(II) trapped in zeolite-Y; however, it was treated with
0.01 M NaCl during the preparation of immobilized complexes in
order to back-exchange the uncoordinated Cu(II) and/or Co(II) ions by
Na+. In the immobilized complexes, the lesser amount of metal cations
compared to the Cu(II)-Y, Co(II)-Y is may be due to the participation of
the metal cations in the formation of transition metal complexes within
zeolite-Y, or it may be due to trivial leaching of metal ions during the im-
mobilization procedure.
3. Results and discussion
3.1. Elemental analysis
The elemental analysis data of neat complexes (Table 1) reveal that
the formation of chelate complexes took place in 1:1 (M:L) metal (Co,
Cu) to the ligand mole ratio. Furthermore, the similarities of finding re-
sults with the theoretical values clearly support the formation of non-
ionic metal complexes in 1:1 (M:L) fashioned through deprotonation
of both the phenolic –OH group of ligands, making the coordination en-
tities neutral. The non-ionic nature of neat complexes was further rein-
forced by its lower molar conductivity values (Table 1) [29]. As
presented in Table 2, the metal content (Si, Al%) in each zeolite-Y
3.2. Brunauer, Emmett, and teller (BET) surface area analysis
The surface textural properties of the pure Na-Y, Co(II)-Y, Cu(II)-Y,
and zeolite-Y immobilized complexes are presented in Table 3. The
Table 1
Elemental analysis, molar conductivity, and magnetic moment of neat metal complexes.
Compound
Elemental analysis
% found (calculated)
Ratio
(%)
Ratio (%)
M/C
Yield (%)
M.P (°C)
Molar conductivityb
μeff
(B.M)
(S cm2 mol−1
)
C/N
C
H
N
Ma
O
Cl
[Co(L1)]
[Cu(L1)]
[Co(L2)]
[Cu(L2)]
61.19
(61.20)
61.48
(60.41)
50.91
(51.21)
50.41
5.19
(5.14)
4.96
(5.07)
3.92
(3.82)
3.64
7.92
(7.93)
7.94
(7.83)
6.57
(6.64)
6.52
16.64
(16.68)
17.76
(17.76)
14.11
(13.96)
14.96
9.06
(9.06)
7.86
(8.93)
7.64
(7.58)
7.87
–
7.72
(7.71)
7.74
(7.71)
7.74
(7.71)
7.73
0.27
0.27
0.28
0.28
0.27
0.27
0.29
0.29
56.19
95.11
67.51
85.39
N300
N300
N300
N300
2.19
1.30
2.30
1.11
4.30
1.78
4.40
1.74
–
16.85
(16.79)
16.60
(50.66)
(3.78)
(6.56)
(14.89)
(7.50)
(16.61)
(7.72)
a
Respective transition metal Co(II) or Cu(II).
Conductivity of neat metal complex was measured using DMSO as solvent at 30 °C.
b
Table 2
ICP-AES analysis data of zeolite-Y based materials with their probable unit cell formulae.
Compound
ICP-AES elemental analysis (%)
Percentage ratio
Unit cell formulae
C
N
Na
Ma
Si
Al
Si/Al
M/C
C/N
Na-Y
Co(II)-Y
Cu(II)-Y
–
–
–
2.07
1.63
1.92
1.94
1.75
–
–
–
5.07
4.46
4.25
4.61
4.64
4.60
4.71
4.98
–
33.46
33.70
32.91
33.59
33.03
33.29
33.91
35.45
5.95
5.99
5.85
5.97
5.87
5.92
6.03
6.30
5.62
5.62
5.62
5.62
5.62
5.62
5.62
5.62
–
–
–
0.29
0.31
0.29
0.30
0.31
–
–
–
7.72
7.71
7.71
7.72
7.70
Na30[(AlO2)30(SiO2)162
Na26.2Co1.90[(AlO2)30(SiO2)162
Na25.6Cu2.20[(AlO2)30(SiO2)162
Na27.2[Co1.40(L1)1.30(AlO2)30(SiO2)162
Na27.8[Cu1.1(L1)1.04(AlO2)30(SiO2)162
Na27.4[Co1.31(L2)1.22(AlO2)30(SiO2)162
Na27.5[Cu1.25(L2)1.21(AlO2)30(SiO2)162
Na27.8[Cu1.1(L1)1.04(AlO2)30(SiO2)162
]
0.829
1.011
0.609
0.507
0.564
0.592
0.544
]
]
[Co(L1)]-Y
[Cu(L1)]-Y
[Co(L2)]-Y
[Cu(L2)]-Y
[Cu(L1)]-Y b
0.268
0.211
0.249
0.251
0.227
]
]
]
]
]
a
Respective transition metal Co(II) or Cu(II).
b
Before ICP-OES analysis, recycled catalyst [Cu(L1)]-Y was dehydrated at 300 °C for 2 h.

108
D.R. Godhani et al. / Inorganic Chemistry Communications 72 (2016) 105–116
Table 3
BETa surface analysis of zeolite-Y based materials.
Surface
area
Pore
volume
Langmuir Surface area
Loss in pore
volume (%)
Compound (m2/g)
(cm2/g)
(m2/g)
Na-Y
Co(II)-Y
Cu(II)-Y
539.0
495.6
482.9
0.327
0.301
0.293
0.228
0.207
0.217
0.249
0.218
861.2
794.0
771.6
581.1
620.3
572.1
657.1
651.7
–
08.0
10.4
30.5
36.5
33.5
23.7
33.4
[Co(L1)]-Y 386.2
[Cu(L1)]-Y 407.9
[Co(L2)]-Y 358.0
[Cu(L2)]-Y 411.3
[Cu(L1)]-Yb 435.3
a
Before analysis, all samples (except last entry) were de-gasified at 110 °C for 2 h to
remove any adsorbed gases.
b
Before BET analysis, recycled catalyst [Cu(L1)]-Y was dehydrated at 300 °C for 2 h.
formation of Co(II) and Cu(II) complex inside the nanovoids of zeolite-Y
significantly reduced the pore volume, surface area and Langmuir Sur-
face area of the zeolite-Y immobilized complexes. This favors the exis-
tence of Cu(II) and Co(II) complexes inside the nanovoids zeolite-Y
and not on the peripheral surface of zeolite-Y framework [31,32]. This
type of lowering in surface area and pore volume upon immobilization
of metal complexes has also been described previously by some re-
searcher [30–32].
3.3. X-ray diffraction (XRD) study
The X-ray diffraction (XRD) patterns of pure Na-Y, Co(II)-Y, Cu(II)-Y,
and zeolite-Y immobilized complexes are displayed in Fig. 1. The
immobilized complexes demonstrate analogous patterns that can be
indexed to zeolite-Y; apart from a slight change in the intensity of the
peaks, no new crystalline patterns have appeared. This directs that the
crystallinity and morphology of the zeolite-Y are conserved upon im-
mobilization and the complexes are well distributed in the nanovoids
of zeolite-Y [33]. The relative peak intensities of the 220, 311 and 331 re-
flections have been supposed to be allied with the locations of cations.
In pure zeolite-Y, Co(II)-Y and Cu(II)-Y, the order of peak intensity is:
I331 ≫ I220 N I311, whereas in the immobilized complexes, the order of
peak intensity becomes I331 ≫ I311 N I220. This difference in the relative
intensities of the peaks is may be due to the reallocation of arbitrarily
coordinated free cations (Cu+2, Co+2) in the zeolite-Y. This observation
supports the successful immobilization of metal complexes within the
zeolite-Y nanovoids without any kind of demolition in the framework
of zeolite-Y. Besides, the indistinguishable XRD pattern of recycled cat-
alyst [Cu(L1)]-Y (Fig. 1h) to that of fresh catalyst evidently support the
fact that crystallinity and morphology of zeolite-Y framework is well-
maintained even after the catalytic study.
Fig. 1. XRD pattern of the (a) Na-Y, (b) Co(II)-Y, (c) Cu(II)-Y, (d) [Co(L1)]-Y, (e) [Cu(L1)]-Y,
(f) [Co(L2)]-Y, (g) [Cu(L2)]-Y and (h) Recycled and dehydrated [Cu(L1)]-Y.
3.5. Fourier transform infrared (FTIR) spectroscopy
The ligands H2L1 and H2L2 mainly show bands nearly at 3400–3300
and 1616 cm−1 due to ʋ(O\\H) (weak band due to hydrogen bonding)
and ʋ(C_N) present in both ligands, respectively [28]. As displayed in
Fig. 3, a shift of ʋ(C_N) band towards lower wavenumbers (1600-
1587 cm−1) and the disappearance of the weak ʋ(O\\H) broad band in
all the neat complexes (Fig. 3f–l) proposes a coordination of
azomethyne nitrogen and phenolic oxygen of ligands to the metal ion,
respectively. The coordination of metal ions to the ligand through the
“N′ and “O″ atoms is further reinforced by the appearance of new
bands at 570–510 and 460–420 cm−1 region due to the ʋ(M-O) and
ʋ(M\\N) modes, respectively [34]. The FTIR spectra of Na-Y, Co(II)-Y,
Cu(II)-Y, and zeolite-Y immobilized complexes (Fig. 3a–e and g–i)
mainly shows the zeolitic bands at 3700–3300, 1635, 445, 580, 800
and 1050 cm−1 due to surface hydroxyl groups, lattice water molecules,
TO4 (T = Si, Al) bending mode, symmetric stretching, ʋsymT–O (inter-
nal), and antisymmetric vibrations, respectively [35]. No broadenings
or shift of these vibrations is spotted upon the immobilization of the
complexes inside the zeolite-Y which further provision the fact that
the zeolite-Y framework conserves upon immobilization of metal com-
plexes [36,37]. The presence of weaker bands in immobilized complexes
within the range of 1620–1210 cm−1 (the range where zeolite-Y mate-
rials does not absorb) not only confirm the formation of metal
3.4. Scanning electron microscopy (SEM) analysis
The SE Micrograph of immobilized complexes before and after
soxhlet extraction is displayed in Fig. 2. As shown in SEM of [Cu(L1)]-Y
taken prior to the soxhlet extraction, the excess redundant particles
such as uncoordinated ligands and metal complexes formed on the pe-
ripheral surface of zeolite-Y are observable, whereas no redundant par-
ticles can be seen in the SEM of [Cu(L1)]-Y taken after the soxhlet
extraction, demonstrating the successful removal of the surface redun-
dant particles during soxhlet extraction. The particle boundaries on
the peripheral surface of zeolite-Y are distinctive and no new crystalline
patterns are appearing in the SEM of [Cu(L1)]-Y taken after the soxhlet
extraction due to the impeccable distribution of complexes inside the
nanovoids of zeolite-Y [30]. Moreover, the absence of metal ion
leaching, no color change in reaction medium and catalyst during the
catalytic study approves a successful removal of extra redundant parti-
cles from the external boundaries of zeolite-Y by soxhlet extraction.

D.R. Godhani et al. / Inorganic Chemistry Communications 72 (2016) 105–116
109
Fig. 2. SE micrograph of (a) [Cu(L1)]-Y taken before Soxhlet extraction (b) [Cu(L1)]-Y
taken after Soxhlet extraction.
complexes inside the zeolite-Y but also suggest its lower concentration
within zeolite-Y.
3.6. Electronic spectra and magnetic moments
The electronic spectrum of H2L1 and H2L2 displayed five (390, 320,
284, 272, and 256 nm) and six bands (401, 332, 287, 277, 263, and
258 nm) respectively. The former two bands in both the ligands are
due to n → π* transition occurring in azomethyne group, whereas the re-
maining bands are due to π → π* transition occurring in the phenyl rings
of ligands [28]. As displayed in Fig. 4a and c, the neat Co(II) complexes of
both ligands exhibited bands at 282–233, 306–307, 384–373 and 869–
867 nm due to π → π*, n → π*, charge transfer (MLCT), and 4A2
(F) → 4 T1(P) transition (d-d), respectively. The presence of d-d bands
nearly at 870 nm and magnetic moment value (Table 1) nearly at 4.3–
4.4 B.M of both the Co(II) complexes is a characteristic value of cobalt
complex with tetrahedral geometry [38]. The Cu(II) complexes of both
ligands (Fig. 4b and d) exhibit bands at 287–223, 306–304, 363–352,
and 551–550 nm due to π → π*, n → π*, charge transfer (MLCT), and
Fig. 3. FTIR spectra of (a) Na-Y, (b) Co(II)-Y, (c) Cu(II)-Y, (d) [Co(L1)]-Y, (e) [Cu(L1)]-Y, (f)
neat [Cu(L1)], (g) Recycled and dehydrated [Cu(L1)]-Y, (h) [Co(L2)]-Y, (i) [Cu(L2)]-Y, (j)
[Co(L1)], (k) [Co(L2)], and (l) [Cu(L2)].

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D.R. Godhani et al. / Inorganic Chemistry Communications 72 (2016) 105–116
2A1g → 2B1g transition (d-d), respectively. The lower magnetic moment
value (Table 1) at about 1.7 B.M and appearance of d-d bands at 551–
550 nm is a characteristic value of Cu(II) complexes with square-planar
geometry [38,39]. As displayed in the Fig. 4e–k, pure Na-Y, and each
modified zeolite-Y have exhibits one intense band at 306 nm and the
other very weak band nearly at 345 nm due to the charge transfer tran-
sition for oxygen to aluminium atoms of two different Al\\O units
existing in the host zeolite-Y [40,41]. The electronic spectrum of
Cu(II)-Y (Fig. 4g) exhibits extra two bands nearly at 740 and 899 nm,
which could be attributed to the octahedral geometry of Cu(II) ion
surrounded by oxygen atoms of zeolite-Y, whereas no extra bands are
observed (Fig. 4f) in Co(II)–Y. The UV–vis spectrum of both Co(II)
immobilized complexes (Fig. 4h and j) displayed about five or six
bands at 277–228, 291–288 and 365–346 nm, attributed to π → π*,
n → π*, charge transfer transition (MLCT), respectively. The electronic
spectrum of both the Cu(II) immobilized (Fig. 4i and k) complex also ex-
hibited about five or six bands 277–250, 293–284 and 330–324 nm due
to π → π*, n → π* and charge transfer transition (MLCT), respectively.
Furthermore, the presence of the very less intense d-d band in both
the immobilized Co(II) complex at 856–851 and at 542–521 nm in
both the immobilized Cu(II) complexes, hypsochromic shifted to that
of their corresponding neat complexes, is characteristic values of the
distorted tetrahedral and square planar geometry around the center
metal ion, respectively. This hypsochromic shift in immobilized com-
plexes clearly suggests an extra splitting of the energy levels from the
barycenter under the impact of zeolite-Y framework. The details of the
peak assignment are given in Table 4.
3.7. Thermal analysis
The ligand H2L1 and H2L2 ligands decompose in two stages. In the
first stage, nearly 80.11 and 76.56% of weight loss is detected at 30–
300 °C due to a major breakdown in the carbon skeleton of ligands
H2L1 and H2L2, respectively, In the second stage, both the ligands
completely decomposes and form CO, CO2, NO, NO2 gases with weight
loss of about 19.89 and 23.44%, respectively [28]. The TG of neat com-
plexes (Fig. 5a–d) reveals no considerable weight loss (only 1–2%)
below 160 °C owing to the absence of outer-sphere and/or inner sphere
water molecule in neat complexes. Both the neat Cu(II) complex shows
only one decomposition stage within the temperature range 161–450 °
C with assessed weight loss of 77.20 (calc. 77.89%) and 79.71% (calc.
81.43%) in [Cu(L1)] and [Cu(L2)], respectively, corresponding to the
elimination of ligands from complexes and leaving behind a cupric
oxide as a residue (21.3–19.1%). Similarly, both the neat Co(II) com-
plexes have shown only one stage weight loss of 52.30 (Calc.
52.03%) and 59.29% (calc. 60.61%) in [Co(L1)] and [Co(L2)], respec-
tively, within the range of 161–500 °C, which could be ascribed to
the breakdown of the complexes and leaving behind cobalt oxide
as a residue (44.9–39.1%). As displayed in Fig. 5f–l, the thermal de-
composition in Na\\Y, Co(II)-Y, Cu(II)-Y and zeolite-Y immobilized
complexes mainly occurs only in one stage in the temperature
range of 50–400 °C owed to the elimination of the physically and
chemisorbed water molecules existing in it. Besides, one more de-
composition stage of 2–3% of weight loss is detected in each
immobilized complexes (Fig. 5h-l) beyond 400 °C which indicates
the existence of the metal complexes in lower concentration within
the zeolite-Y nanovoids. The weight loss in immobilized complexes
is extended up to 400 °C compared to the respective neat complexes,
which indicates that the thermal stability of complexes is greatly en-
hanced upon immobilization. The TG data of neat complexes and ze-
olite-Y based material are presented in Table 5.
Table 4
electronic spectroscopy of neat complexes and zeolite-Y based materials.
Compound
Electronic transition (nm)
π → π*
n → π*
MLCT
d-d transition
[Co(L1)]
[Cu(L1)]
[Co(L2)]
[Cu(L2)]
Na-Y
Co(II)-Y
Cu(II)-Y
[Co(L1)]-Y
[Cu(L1)]-Y
[Co(L2)]-Y
[Cu(L2)]-Y
282, 272, 266, 233
285, 249, 223,
276, 270, 258, 247
287, 258, 241
–
306
304
307
306
–
–
–
288
284
291
293
373
352
384
363
306, 345
306, 343
307, 343
365, 306, 343,
324, 306, 343
346, 306, 337
330, 306, 343
869
550
867
551
–
–
–
–
740, 899
856
521
851
542
274, 256, 228
277, 245, 222
281, 254, 228
261, 248, 226
Fig. 4. Electronic spectrum of the (a) [Co(L1)], (b) [Cu(L1)], (c) [Co(L2)], (d) [Cu(L2)], (e)
Na-Y, (f) Co(II)-Y, (g) Cu(II)-Y, (h) [Co(L1)]-Y, (i) [Cu(L1)]-Y, (j) [Co(L2)]-Y, and (k)
[Cu(L2)]-Y.

D.R. Godhani et al. / Inorganic Chemistry Communications 72 (2016) 105–116
111
Fig. 5. TGA of the (a) [Co(L1)], (b) [Cu(L1)], (c) [Co(L2)], (d) [Cu(L2)], (e) Na-Y, (f) Co(II)-Y, (g) Cu(II)-Y, (h) [Co(L1)]-Y, (i) [Cu(L1)]-Y, (j) [Co(L2)]-Y, (k) [Cu(L2)]-Y and (l) dehydrated
recycled catalyst [Cu(L1)]-Y.
4. Catalytic activity
inside the zeolite-Y nanovoids is found most active compared to the
one having higher (%) metal content. This might be due to an easy effec-
tive collision of cyclohexene and H2O2 with intensely suppressed metal
complexes present in the nanovoids of catalyst having lower metal con-
tent compared to the one having higher metal content (%). The above
activity trends of the catalysts is also reinforced by the data of H2O2 con-
version and H2O2 efficiency carried out separately at identical reaction
condition. As presented in the Table 6, the H2O2 conversion is 14.1% at
reaction temperature (80 °C), even in the absence of the catalyst due
to self-decomposition, whereas the presence of catalysts such as H2L1,
H2L2, and Na-Y has no considerable effect on H2O2 conversion, signify-
ing no role of ligands and zeolite-Y framework in the catalytic activity
of immobilized and neat complexes. The use of a catalyst that contains
transition metal ion (metal exchanged, immobilized, and neat com-
plexes) drastically increases the conversion of H2O2, demonstrating
the imperative role of transition metals present in the catalyst. The
lower H2O2 efficiency and cyclohexene conversion in the presence of
Co(II)-Y and/or Cu(II)-Y catalysts could be ascribed to the lower activity
of H2O2 to react with cyclohexene or quick decomposition of H2O2 or
the inability of the metal ion to form an active intermediate, which
4.1.1. Oxidation of cyclohexene
The catalytic oxidation of cyclohexene was performed at optimized
reaction condition (25 mmol cyclohexene, 25 mmol 30% H2O2,
12.5 mg Catalyst, 6 mL of acetonitrile, 353 K, 18 h) using each prepared
catalysts. As presented in Table 6, the oxidation of cyclohexene is found
trivial in the presence of only ligands [28], Na-Y [28], Co(II)-Y and
Cu(II)-Y as catalyst, whereas immobilized metal complexes and the
neat complexes extensively catalyzed the cyclohexene in the presence
of H2O2. This observation signifies that the metal complexes present in
of zeolite-Y nanovoids are playing a vigorous role in catalyzing the sub-
strate. Nevertheless, the immobilized complexes have lesser metal con-
tent (%) compared to the respective neat complexes, it efficiently
catalyzes the cyclohexene with higher conversion (higher TON) with
higher selectivity of allylic products. The conversion (%) of cyclohexene
catalyzed by zeolite-Y based catalysts upsurges in the order: Na-
Y b Co(II)-Y b Cu(II)-Y b [Co(L2)]-Y b [Cu(L2)]-Y b [Co(L1)]-Y [Cu(L1)]-
Y. According to the trends, the catalyst having lower metal content
Table 5
Thermogravimetric results of neat complexes and zeolite-Y based materials.
Compound
[Co(L1)]
TG range (°C)
Mass loss %
obs. (calc.)
Assignment
30–160
161–500
501–700
30–160
161–450
451–700
30–160
161–500
501–700
30–160
161–450
451–700
30–400
30–400
30–400
30–400
401–700
30–400
401–700
30–400
401–700
30–400
401–700
30–400
401–700
01.80
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
No considerable mass loss
Loss of ligands due to complex breakdown
Cobalt oxide as residue
No considerable mass loss
Loss of ligands due to complex breakdown
CuO as residue
No considerable mass loss
Loss of ligands due to complex breakdown
Cobalt oxide as residue
No considerable mass loss
Loss of ligands due to complex breakdown
CuO as residue
Loss of physically and chemisorbed water
Loss of physically and chemisorbed water
Loss of physically and chemisorbed water
Loss of physically and chemisorbed water
Loss of ligands due to complex breakdown
Loss of physically and chemisorbed water
Loss of ligands due to complex breakdown
Loss of physically and chemisorbed water
Loss of ligands due to complex breakdown
Loss of physically and chemisorbed water
Loss of ligands due to complex breakdown
Loss of physically and chemisorbed water
Loss of ligands due to complex breakdown
52.30 (53.03)
45.90 (46.97)
1.50
77.20 (77.89)
21.30 (22.10)
1.55
59.29 (60.61)
39.16 (39.39)
1.12
79.71 (81.43)
19.17(18.56)
10.32
09. 82
12.12
06.63
02.10
08.22
02.31
06.10
02.76
[Cu(L1)]
[Co(L2)]
[Cu(L2)]
Na-Y
Co(II)-Y
Cu(II)-Y
[Co(L1)]-Y
[Cu(L1)]-Y
[Co(L2)]-Y
[Cu(L2)]-Y
[Cu(L1)]-Ya
04.24
02.45
00.56
02.26
a
Before TG analysis, recycled catalyst [Cu(L1)]-Y was dehydrated at 300 °C for 2 h.

112
D.R. Godhani et al. / Inorganic Chemistry Communications 72 (2016) 105–116
Table 6
Oxidation of cyclohexene over various neat complexes and zeolite-Y based catalysts at optimized conditions.
Catalyst
H2O2
Cyclohexene
Selectivity (%)c
Metal (μmol) in catalysth
TONi
Conversionb (%)
Conv.
(%)
effi.a
(%)
CyOld
CyOnee
CyOxf
Cydiolg
Nil
Co(II)-Y
14.1
45.0
100
58.3
100
45.1
99.3
100
100
78.1
94.8
100
100
100
25.4
25.2
13.2
86.7
76.7
89.8
62.5
75.5
86.6
72.4
66.9
83.5
81.1
80.2
03.6
11.3
13.2
50.6
76.1
40.5
62.1
75.5
86.6
56.5
63.4
85.5
84.1
83.2
28.90
39.77
32.41
35.33
48.29
36.14
41.20
36.73
39.02
29.07
35.49
38.78
39.12
38.90
09.18
41.40
42.18
46.82
48.50
58.52
56.48
59.95
59.40
66.49
58.62
60.46
59.12
60.34
45.66
13.55
14.44
–
–
–
–
–
16.26
05.28
10.97
17.85
03.21
05.34
02.32
03.32
01.58
00.61
04.03
00.76
01.76
00.76
–
–
1.75
1.98
38.0
34.9
29.9
29.4
1.29
0.99
1.19
1.25
1.07
1.07
1.07
1606
1641
Cu(II)-Y
[Co(L1)]
332.3
545.3
338.1
527.6
14,612
21,708
11,808
12,623
19,566
18,990
18,792
[Cu(L1)]
[Co(L2)]
[Cu(L2)]
[Co(L1)]-Y
[Cu(L1)]-Y
[Co(L2)]-Y
[Cu(L2)]-Y
[Cu(L1)]-Yj
[Cu(L1)]-Yk
[Cu(L1)]Yl
–
03.83
01.86
–
–
–
a
%H2O2 efficiency = (moles of product formed/mol of H2O2 reacted × 100.
Conversion means fraction of starting material expended in the reaction.
Selectivity is the production rate of one component per production rate of another component.
2-Cyclohexen-1-ol.
2-Cyclohexen-1-one.
Cyclohexene oxide (1,2-epoxycyclohexane).
Cyclohexene-1,2-diol
Amount of metal atom in μmol present per 12.5 mg of catalyst.
TON (turnover number): Moles of cyclohexene transformed per mole of active metal ion
First reuse of catalyst after dehydrated at 300 °C for 2 h.
Second reuse of catalyst after dehydrated at 300 °C for 2 h.
b
c
d
e
f
g
h
i
j
k
l
Third reuse of catalyst after dehydrated at 300 °C for 2 h. Reaction condition: 25 mmol of substrate (cyclohexene), 25 mmol of 30% H2O2, 12.5 mg of Catalyst, 6 mL of CH3CN, 353 K, 18 h.
can easily attack the substrates and provides its oxygen to substrates
immediately. In the case of neat and immobilized chelate complexes,
higher H2O2 efficiency, higher cyclohexene and H2O2 conversion were
observed, which might be facilitated via formation of an active interme-
diate that can easily attack to the cyclohexene and leads to the higher
activity. Moreover, the presence of electron withdrawing groups (chlo-
rine) at the para position to the hydroxyl group in complexes of H2L2 li-
gand decreases an electron density at ligating “O” atom in complexes,
which leads to the electron deficient metal center compared to the com-
plex of H2L1 ligand having no electron withdrawing group (R = H) [42–
44]. Consequently, the complex of H2L1 can be simply oxidized to M(III)
due to low redox potentials of M(n + 1)/M(n). Therefore, the complexes
bearing H2L1 ligand have been found highly active (higher TON) com-
pared to the complexes of H2L2.
5. Oxidation of the other organic substrates
Additionally, an afford have been made to sightsee the activity of
[Cu(L1)] and [Cu(L1)]-Y catalysts over benzene, phenol, styrene, benzyl
alcohol, limonene, α-Pinene, cyclohexane and cyclooctane at identical
reaction condition. As presented in Table 7, heterogeneous [Cu(L1)]-Y
catalyst is more active and selective (Table 7, results out of the bracket)
as compared to its homogeneous (Table 7, results in the bracket) coun-
terpart [Cu(L1)].
The [Cu(L1)]-Y is highly selective (allylic product: 98.4%,) and active
(86.6% of conversion with TON; 21,708) for oxidation of cyclohexene
and therefore, the higher activity and selectivity was anticipated for
similar kind of organic substrate having C_C and weak allylic C\\H
bonds such as limonene and α-Pinene. However the catalyst can attacks
on C_C and weak allylic C\\H of limonene or α-pinene and can pro-
duce allylic and epoxide products simultaneously, the presence of the
bulky methyl group at C_C may hinder the attack of bulky catalyst at
C_C as well as fairly at allylic C\\H bond, which reduces the activity
of the catalyst for the limonene (11.9%) and α-pinene (9.2%). Moreover,
the high selectivity of allylic products (100%) upon oxidation of limo-
nene and α-pinene could be accredited to the preference of catalyst to
attack at a less hinder site of the substrates (allylic C\\H). Because of
the absence of bulky group and activated allylic C\\H bond in the sty-
rene, mainly epoxide (87.5%) product is obtained due to epoxidation,
which immediately hydrolyzed to styrene glycol (66.7%) owed to acidic
nature of the host Material [45,46]. The production of benzaldehyde
(33.33%) is owing to the later nucleophilic attack of H2O2 on the pre-
formed styrene oxide [47,48].
4.1.2. Catalyst stability and reusability
The reusability of a heterogeneous catalyst has great importance in
designing the appropriate catalyst. The homogeneous [Cu(L1)] was
readily degraded and could not be recovered even once, whereas zeo-
lite-Y immobilized [Cu(L1)]-Y catalyst was easily isolated and reused
trice without substantial loss of activity and the selectivity of allylic
products (Table 6 last three entries). The catalyst [Cu(L1)]-Y was easily
isolated from the reaction mixture after first catalytic cycle (oxidation
of cyclohexene) by simple filtration, washed with acetonitrile and
dehydrated at 300 °C for 2 h before being analyzed by various physico-
chemical techniques and/or used for successive run. The analogous pat-
tern of XRD (Fig. 1h), FTIR (Fig. 3g) and TGA (Fig. 5l) of recycled
[Cu(L1)]-Y to the fresh catalyst confirms the conservation of zeolite-Y
framework without any kind of destruction during the catalytic oxida-
tion. Moreover, the similar M/C ratio (Table 2) of recycled [Cu(L1)]-Y
catalyst (0.31) and fresh catalyst (0.31) suggest the absence of metal
ion leaching during the catalytic activity and it is further buttressed by
the absence (tested by AAS) of metal ion in oxidation products.
As presented in Table 7, the catalyst [Cu(L1)]-Y is found extremely
active for the oxidation of cycloalkanes with the higher selectivity of
corresponding cycloalkanone (80–75%) and it is also highly selective to-
wards the formation of hydroquinone (100–87%) from benzene and
phenol. Moreover, the catalyst is abstemiously active (21.51%) for oxi-
dation of benzyl alcohol with 55.45% of the benzaldehyde. However,
this catalyst is lower active (conversion), it offers higher selectivity

D.R. Godhani et al. / Inorganic Chemistry Communications 72 (2016) 105–116
113
Table 7
Oxidation of some other organic substrate over [Cu(L1)] (homogeneous) and [Cu(L1)]-Y (heterogeneous) catalysts.
Sr no.
1
Substrate
Conversion (%)
86.6 (76.1)
Selectivity (%)
TON
Cyclohexene
CyOne: 59.40 (48.50) CyOl: 39.03 (48.29)
Cydiol: 01.57 (3.21)
21,708 (545.3)
2
3
4
5
6
7
8
9
Cyclohexane
Benzene
Phenol
75.2 (61.2)
08.2 (7.3)
14.5 (11.3)
87.5 (70.3)
21.5 (19.2)
11.2 (05.3)
09.1 (6.8)
43.2 (35.2)
Cyaone: 80.79 (69.32) Cyaol: 19.21 (30.68)
Phenol 12.45 (54.32) HQ: 87.55 (45.68)
Cat: 00 (33.00) HQ 100 (67.00)
Bnzald: 33.30 (55.32) Sdiol: 66.70 (44.68)
Bnzald: 55.45 (76.34) Bacid: 44.55 (23.66)
Cone: 93.20 (66.55) Col: 06.80 (33.58)
Vone: 71.63 (54.22) Vol: 28.37 (45.78)
Coone: 76.89 (67.20) Cool: 23.11 (32.80)
18,846 (438.1)
2054 (68.8)
3636 (81.0)
21,942 (503.1)
4851 (137.3)
2829.6 (38.3)
2293 (48.9)
Styrene
Benzyl alcohol
Limonene
α-Pinene
Cyclooctane
10,836 (252.0)
CyOne: 2-Cyclohexen-1-one, CyOl: 2-Cyclohexen-1-ol, Cydiol: Cyclohexene-1,2-diol, Cyaone: cyclohexanone, Cyaol: cyclohexanol, HQ: hydroquinone, Cat: Catechol, Bnzald: Benzalde-
hyde, Sdiol: styrene glycol, Bacid: benzoic acid, Cone: carvone, Col: carveol, Vone: verbenone, Vol: Verbenol, Coone: cycloocatanone, Cool: cyclooctanol. TON (turnover number):
Moles of substrate transformed per mole of active metal ion, Reaction condition: 25 mmol of substrate, 25 mmol of 30% H2O2, 12.5 mg of [Cu(L1)]-Y (results out of bracket) and/or
[Cu(L1)] (results in bracket), 6 mL of CH3CN, 353 K, 18 h.
and hence there is wide scope to increase the conversion of these organ-
ic substrates by the optimization of the various reaction parameters.
Catalytic oxidation of such substrates using various homogeneous
and/or heterogeneous catalysts has been studied recently by some
other groups (Table 8) and being compared with our catalytic system
(Table 7) on the basis of their TON values [49–69]. Except one or two
catalytic systems, our catalytic system is found to be more promising
and selective for the oxidation of such type of organic substrates.
intensity of the MLCT bands, gradual vanishing of d-d bands, and appear-
ance of new isobestic point (red arrows in Fig. 6a–d) nearly at 344–
313 nm in all neat metal complexes upon consecutive addition H2O2
clearly suggests a direct interaction of M(II) center with H2O2 and not
with limonene in the first step of catalytic pathway, as reported earlier
[70,71]. This is further reinforced by the absence of an isobestic point
in electronic spectra (Fig. 6e and f) taken during to the progressive ad-
dition of 10−4 M methanolic solution of limonene to 10−4 M methano-
lic solution of neat [Co(L1)] and/or [Cu(L1)] complexes.
6. Plausible reaction pathway of catalysts
One of the intermediate having metal-oxygen interaction such as
bis(μ-oxo-MIII) or MIII–OOH (metal–hydroperoxide) are likely to be in-
volved during the catalytic oxidation which can transfer its oxygen to
the substrate and leads oxyfunctionalized products [72–74]. Out of
these two, the involvement of metal hydroperoxide (MIII–OOH) could
be left out here due to its lower stability at the present reaction temper-
ature (usually stable at −20 to −50 °C) [75,76] and the absence of
ʋ(O\\H) bands in FTIR of solid products (light pink) that is obtained
To figure out the plausible reaction pathway and intermediate spe-
cies encompass during oxidation of the limonene, we have monitored
the headway of the reaction using UV–vis spectroscopy by dropwise ad-
dition of 10−4 M methanolic 30% H2O2 to 10−4 M methanolic solution
of neat metal complexes. As displayed in Fig. 6a–d, slight upsurges in
the intensities of bands present underneath 300 nm, subsiding in the
Table 8
Literature survey of some recently published article for oxidation of organic substrate with their TON values.
Sr. No.
Catalyst
Substrate
Oxidant
Conv.
(%)
Major product
(%)
TON
Ref.
1
2
3
4
5
6
7
8
[Cu(SFCH)·H2O]-Y
Cu(II) polymer anchored
[MoO2HL(HOCH3)])
[VO(L1)(acac)]-Y
[VO(hacen)]-Y
Cyclohexane
Cyclohexene
Cyclohexene
Cyclohexene
Cyclohexene
Cyclohexene
Cyclohexene
Cyclohexene
Benzene
Benzene
Phenol
Phenol
Phenol
Phenol
Phenol
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
H2O2
TBHP
TBHP
H2O2
H2O2
TBHP
O2
45.1
83.0
100
Cyaone (84.5)
CyOne (64.0)
CyOX (100)
CyOne (48.3)
CyOne (50.2)
CyOne (90.8)
CyOl (72.5)
CyOl (74.6)
Phenol (100)
Phenol (100)
Cat (65.2)
25.20
395.2
–
86,790
53,388
–
49
50
51
52
28
53
54
55
52
56
52
57
58
59
60
52
61
62
63
50
51
50
64
65
66
67
68
50
69
74.6
91.7
92.5
53.9
68.7
7.40
27.2
23.6
52.1
41.3
32.6
44.8
15.4
58.6
97.0
100
97.0
75.0
96.0
52.6
73.2
28.2
97.7
72.0
48.0
94.0
[Mn(sal-2,6-py)]-NaY
Catalyst 5
–
–
[Ni((Benzyl)2Bzo2[14]aneN6)]2+–NaY
[VO(L1)(acac)]-Y
[TMA]5PMoV
O2
9
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
TBHP
H2O2
TBHP
TBHP
TBHP
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
H2O2
8478
–
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
[VO(L1)(acac)]-Y
[Ru(STCH) 3H2O]+-Y
([Cu-Imace-H][BF])
[VO(salen)]-Y
[VO((NO2)2-haacac)]-NaY
[VO(L1)(acac)]-Y
[Cu(bipy)Cl2]
Cu{salnptn(3-OMe)2}
[Cu(HL1)(NO3)]
Cu(II) polymer anchored
[MoO2HL(HOCH3)]
Cu(II) catalyst
27,324
244.0
62.10
11,640
–
17,832
64.00
303.0
1518
461.6
–
274.2
–
–
5412
801.6
170.4
228.5
–
Cat (93.2)
Cat (62.9)
Cat (92.7)
Cat (85.2)
Bnzald (75.9)
HAP (37.3)
Bnzald (57)
SO (86.0)
Bnzald (90.0)
SO (65.0)
Bnzald (75.0)
Bnzald (−)
Bnzald (100)
Limgly (34.37)
Limgly (45.10)
Eepo (81.0)
VOne (55.0)
CyooH (77.0)
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Limonene
Limonene
Limonene
α-Pinene
Cyclooctane
YCu(dmgH)2
[Cu(Me4[16]aneN8)]2+)NaY
[VO(L)H2O]-Y
[VO(VTCH)2]-Y
Ti4/SiO2-D-2
Cu(II) polymer anchored
PVW catalyst
Conv.: Conversion, Cyaone: cyclohexanone, Cyone: cyclohexenone, CyOx: Cyclohexene oxide (1,2-epoxycyclohexane), Cat: Catechol, Bnzald: Benzaldehyde, HAP: 2-Hydroxy
acetophenone, SO: Styrene oxide, Limgly: Limonene glycol, Eepo: endocyclic 1,2-monoepoxide, CyooH: Cyclooctyl hydroperoxide, VOne: Verbenone.

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D.R. Godhani et al. / Inorganic Chemistry Communications 72 (2016) 105–116
Fig. 6. UV–visible spectral studies of (a) neat [Co(L1)] (b) neat [Co(L2)] (c) [Cu(L1)] (d) [Cu(L2)] taken during the sequential addition of a methanolic solution of H2O2 and (isobestic point
are observed, red arrows in Panels a–d) (e) neat [Co(L1)] (f) [Cu(L1)] taken during the consecutive addition of a methanolic solution of limonene (no isobestic point in Panels e and f). (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
from dropwise addition of H2O2 to the methanolic solution of neat
[Cu(L1)]. Moreover, the appearance of the new ʋ(Cu\\O) band at 462 nm
in the FTIR spectrum (Fig. 7b) of product clearly indicates the involve-
ment of [(L1)CuIII–O–O–CuIII(L1)] as an intermediate during the catalytic
activity.
of neat and immobilized complexes as compared to [(L1)CuIII–O–O–Cu-
III(L1)] is supposed to be due to the lower solubility of H2O2 with the
limonene.
The probable reaction mechanism for the oxidation limonene cata-
lyzed by [Cu(L1)] is demonstrated in Scheme 2. In the first step, the cat-
alyst (I) reacts with H2O2 to form [(L1)CuIII–O–O–CuIII(L1)] intermediate
(II). The [(L1)CuIII–O–O–CuIII(L1)] intermediate (II) is found to be stable
and could be isolated easily, its homolytic cleavage could only be facili-
tated when it directly interact with the less hinder site of limonene and
lead an intermediate (III). This homolytic cleavage of the peroxide bond
might have occurred due to rise in the back donation from the filled dΠ
orbitals of copper to Π* MOs of the peroxide upon the interaction of the
Furthermore, the [(L1)CuIII–O–O–CuIII(L1)] is tested as catalyst over
limonene oxidation (in the absence of H2O2) at identical reaction condi-
tion (Fig. 7a) to confirm whether the [(L1)CuIII–O–O–CuIII(L1)] is capable
to donate its oxygen to the substrate or not. Amazingly, the higher con-
version (16.3%) is achieved compared to the reaction catalyzed by neat
[Cu(L1)] (5.38%) and/or immobilized complex [Cu(L1)]-Y (11.29%) in
the presence of H2O2 at identical reaction condition. The lower activity

D.R. Godhani et al. / Inorganic Chemistry Communications 72 (2016) 105–116
115
Fig. 7. (a) Oxidation of limonene using [(L1)CuOOCu(L1)] as catalyst (b) FTIR of [(L1)CuOOCu(L1)] intermediate
Scheme 2. The merely tentative reaction mechanism for oxidation of limonene with H2O2 catalyzed by [Cu(L1)].

116
D.R. Godhani et al. / Inorganic Chemistry Communications 72 (2016) 105–116
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The immobilization Co (II) and Cu (IV) Schiff base complexes within
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