Bio-derived nanosilica-anchored Cu(II)-organoselenium complex as an efficient retrievable catalyst for alcohol oxidation

Article abstract of DOI:10.1002/aoc.6416

A new copper(II) complex supported onto rice-husk-derived nanosilica was prepared from 2,6-bis((phenylselanyl)methyl)pyridine, salicylaldehyde and copper acetate monohydrate, Cu(OAc)₂·H₂O. The as-synthesized complex Cu(II)SeNSe@imine-nanoSiO₂ (Complex I) was extensively characterized with FT-IR, powder XRD, SEM-EDX, solid-state UV-Vis, ESR, XPS, TGA and BET surface area analysis. The catalytic activity of the complex was explored for alcohol oxidation reactions using H₂O₂ as oxidant and acetonitrile as solvent. For comparison, we have also prepared an analogous homogeneous catalyst (Complex II) and characterized it with FT-IR, UV-Vis, LC-MS and ESR analyses. Its catalytic activity was also screened to the same reaction. The immobilized catalyst showed better efficiency with 75%–95% isolated yield compared with the homogeneous one for alcohol oxidation with at least five times recyclability without profound loss in activity.

Full text of DOI:10.1002/aoc.6416

Received: 29 April 2021
DOI: 10.1002/aoc.6416
Revised: 13 July 2021
Accepted: 30 July 2021
R E S E A R C H A R T I C L E
Bio-derived nanosilica-anchored Cu(II)-organoselenium
complex as an efficient retrievable catalyst for alcohol
oxidation
Rajjyoti Gogoi
| Geetika Borah
Department of Chemistry, Dibrugarh
University, Dibrugarh, India
Abstract
A new copper(II) complex supported onto rice-husk-derived nanosilica was
prepared from 2,6-bis((phenylselanyl)methyl)pyridine, salicylaldehyde and
copper acetate monohydrate, Cu(OAc)2ꢀH2O. The as-synthesized complex
Cu(II)SeNSe@imine-nanoSiO2 (Complex I) was extensively characterized
with FT-IR, powder XRD, SEM-EDX, solid-state UV-Vis, ESR, XPS, TGA and
BET surface area analysis. The catalytic activity of the complex was explored
for alcohol oxidation reactions using H2O2 as oxidant and acetonitrile as
solvent. For comparison, we have also prepared an analogous homogeneous
catalyst (Complex II) and characterized it with FT-IR, UV-Vis, LC-MS and
ESR analyses. Its catalytic activity was also screened to the same reaction. The
immobilized catalyst showed better efficiency with 75%–95% isolated yield
compared with the homogeneous one for alcohol oxidation with at least five
times recyclability without profound loss in activity.
Correspondence
Geetika Borah, Department of Chemistry,
Dibrugarh University, Dibrugarh, India.
Email: geetikaborah@dibru.ac.in
Funding information
UGC, New Delhi, India
Highlights
1. This is the first report of immobilized copper complex with Se–N–Se pincer
ligand. Moreover, we have used bio-derived nanosilica as a solid support.
2. The alcohols are oxidized under environmentally benign mild reaction con-
ditions using hydrogen peroxide as an oxidant.
3. The catalytic performance of the supported complex was compared with an
analogous unsupported complex.
K E Y W O R D S
2,6-bis((phenylselanyl)methyl)pyridine, Cu(II) organoselenium, nanosilica, oxidation,
rice-husk
1 | INTRODUCTION
are available to affect this transformation.[3–7]
Traditionally, for the oxidation of alcohols to produce
The oxidation of alcohols to the corresponding carbonyl
compounds is a highly relevant reaction in organic
chemistry and is a fundamental and pivotal transforma-
tion in the synthesis of fine/bulk chemicals and
pharmaceuticals.[1,2] Till date, numerous methodologies
corresponding aldehydes/ketones, transition metals such
as Cr(VI) and Mn(II) were widely used, which are toxic,
corrosive and expensive.[8–11] Moreover, several reports
are available, which used Cr-, Mn-, Fe-, Cu-, Mo-, Ru-,
Pt-, Pd-, Mn- and Os-based catalyst in the presence of
Appl Organomet Chem. 2021;e6416.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6416

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GOGOI AND BORAH
oxidants, such as molecular oxygen, tertiary butyl
hydroxide (TBHP), hydrogen peroxide and sodium hypo-
chlorite with toxic and costly solvents.[8,12–18] In the last
two decades, some desirable and efficient catalytic sys-
tems based on main group elements and ions, transition
metal complexes and organometallic compounds had
been developed and were performed exclusively in aque-
ous medium. However, copper had been extensively used
in homogeneous catalysis for the aerobic oxidation of
alcohols in water using Cu/TEMPO catalytic system.
Substrate scope for aliphatic alcohols using water-based
catalytic systems still poses a major challenge due to poor
water solubility of the substrates.[7] A triphasic system
(solid–liquid–liquid) without co-solvent over micropo-
rous titanium silicate showed enhanced catalytic activity
and para selectivity in the oxidation of benzyl alcohol,
cyclohexanol, etc. compared with biphasic (solid–liquid)
systems.[19] In contemporary time, heterogenization of
homogeneous system has received considerable attention
due to distinct advantages of heterogeneous systems over
homogeneous one, namely, relatively long lifetime of
catalyst, easy separability, robustness and recyclability,
and these factors form the crucial aspect of
sustainability.[2,20–26] This is why designing novel hetero-
geneous transition metal-based catalysts able to selec-
tively oxidize alcohols in eco-friendly conditions is a
continuously growing topic. In the last decade, a lot of
papers had been published reporting the use of O2 as the
oxidizing agent and water as solvent; however, a few Cu,
Au, Co and bimetallic catalysts were able to efficiently
work with these greener conditions. Some examples
where the use of ionic liquids as solvent allowed to
achieve better results than with common organic solvents
had also been reported for Cu, Pd, Fe and Co catalysts.
Looking at the substrate scope, aliphatic alcohols still
present the major challenge, except few protocols based
on Co, Au, Pd and bimetallic systems that demonstrated
high yield and good selectivity towards such substrates.
However, it is worthy to mention that the costs related to
the use of precious metals to activate oxygen remain too
high for the use of such reactions in industry. As a result,
the research community is gradually moving towards
designing of low-cost heterogeneous catalysts based on
Cu, Fe and Co, which were almost unexplored in their
heterogeneous fashion up to 2012.[6] Bhaumik and co-
workers have reported the synthesis of functionalized
mesoporous silica supported copper(II) and nickel(II) cat-
alysts as well as polymer-anchored copper(II) catalyst for
liquid-phase oxidation of olefins and aromatic alco-
hols.[27,28] Very recently, a microporous framework cop-
per silicate and Cu(II)-MOF catalyst have been reported
for mild liquid-phase and base-free aerobic oxidation of
benzylic alcohols using H2O2 as the oxidant,
respectively.[29,30] Significant advancements had been
made to synthesize immobilized copper nanoparticles
and complexes with various ligands such as Schiff bases
and N-heterocyclic carbene using silica, zeolite, clay,
charcoal, nanocellulose, etc. as solid support, and their
catalytic activity was investigated for alcohol oxidation
and other transformations as well. As per our knowledge,
there is not a single report of immobilized copper com-
plex with Se–N–Se pincer ligand and investigation of its
catalytic activity. Moreover, considering some specific
characteristics like low cost, natural abundance, high
surface area and high thermal stability, we have used
rice-husk-derived nanosilica as solid support. Rice husk
contains 90%–97% silica, the surface of silica contains
silanol (Si–OH), and siloxane (Si–O–Si) groups are
responsible for binding with other functional groups.[31]
From the view point of using low-cost metal and
agro-waste-derived nanosilica as base material and in
continuation of our preceding work as well, the authors
report the development of rice-husk-derived nanosilica-
anchored Cu(II)–Se–N–Se complex and evaluation of its
catalytic activity towards oxidation of primary and
secondary alcohols to their corresponding carbonyl
compounds.[8]
2 | EXPERIMENTAL
2.1 | Materials
All chemicals were of AnalaR grade and obtained com-
mercially. They were used as received without further
drying or purification. Solvents such as ethanol, metha-
nol, dimethyl sulfoxide, acetonitrile, ethyl acetate, tolu-
ene, chloroform and oxidant H2O2(30%) were purchased
from Merck. Copper(II) acetate monohydrate, diphenyl
diselenide and 2,6-bis(bromomethyl)pyridine were pur-
chased from Sigma-Aldrich. The alcohol substrates were
purchased from Sigma-Aldrich and Tokyo Chemical
Industry and were used as received without further
treatment.
2.2 | Physical measurements
The powder x-ray diffraction (PXRD) patterns of silica
and silica supported materials were recorded on Rigaku
Ultima IV diffractometer using Cu-Kα radiation
(λ = 1.541 Å) at 40 kV and 30 mA. The Fourier trans-
formed infrared (FT-IR) spectra were recorded as KBr
pellets on a Shimadzu IRPrestige-21 FT-IR spectropho-
tometer (200–4000 cmꢁ1). The solid-state UV–visible
(UV-Vis) spectra were obtained with a Jasco V-750
spectrometer in the wavelength range 200–800 nm. ESR

GOGOI AND BORAH
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spectra of the catalysts were recorded on JEOL
JES-FA200 ESR spectrometer. The copper content of
Complex I was determined by inductively coupled
plasma atomic emission spectrometric (ICP-AES)
analysis with an ARCOS simultaneous ICP spectrometer
at SAIF, IIT Bombay. The atomic percentage of different
elements in Complex I were evaluated from XPS survey
spectrum using casaXPS software. The Brunauer–
Emmett–Teller (BET) surface area of the materials at
liquid nitrogen temperature was recorded on
Quantachrome instrument (Boynton Beach, FL, 33426,
USA). The SEM photographs of the as-prepared materials
were performed on JEOL JSM IT-300 operating at 20 kV
accelerating voltage. The EDX spectra were also recorded
on the same instrument attached to the scanning electron
microscope. X-ray photoelectron spectra (XPS) were
recorded using XPS-AES Module, Model: PHI 5000 Versa
Prob II. The C (1s) electron binding energy corresponding
to graphitic carbon was used for calibration of the
Cu (2p) and Se (3d) core-level binding energy. GC
mass spectrograms of the products were obtained using
Agilent Technologies GC system 7820 coupled with a
mass detector 5975 and SHRXI-5MS column. ESI-MS
spectrum of the complex was recorded in a Thermo
Fisher Endura LC/MS mass spectrometer. HR-MS data
were recorded by electron spray ionization with Q-TOF
mass analyser.
the aqueous layer. The extract was washed with water
(3 ꢂ 40 ml), followed by drying over anhydrous sodium
sulfate. The solvent was evaporated under reduced
pressure on a rotary evaporator to result in a yellow
viscous oil, which upon mixing with methanol (5 ml)
and placing in a refrigerator (5ꢃC) gave crystals of
2,6-bis((phenylselanyl)methyl)pyridine ligand (L1) given
in scheme 1. The ligand L1 was characterized by FT-IR
analysis and high-resolution mass spectroscopic (HR-
MS) technique (Figures S1 and S2).[14]
2.4 | Synthesis of Cu(II)SeNSe@imine-
nanoSiO2 complex (Complex I)
The complex Cu(II)SeNSe@imine-nanoSiO2 was pre-
pared by a route given in Scheme 2. The nanosilica was
extracted from paddy waste rice husk according to the lit-
erature report without modification.[32] The procedure
was mentioned in detail in the Supporting Information.
2.4.1 | Immobilization of APTES onto
nanosilica
A 3.8 g nanosilica (dried overnight at 250ꢃC) and
aminopropyltriethoxysilane (APTES) (0.442 g, 2 mmol)
were added to a dry toluene (100 ml), followed by
refluxing for 6 h under N2 atmosphere. After cooling the
resulting mixture, a solid product formed. The solid prod-
uct was separated by filtration, washed with dry toluene
for several times through Soxhlet extraction and dried at
120ꢃC for 24 h. The as-synthesized product was desig-
nated as APTES@nanoSiO2.
2.3 | Synthesis of ligand (L1) {2,6-bis
((phenylselanyl)methyl)pyridine}
A solution of diphenyl diselenide (Ph2Se2) (0.312 g,
1 mmol) in 30 ml of ethanol was stirred under N2
atmosphere. Then, NaBH4 (0.076 g, 2 mmol) dissolved
in 5 ml of aqueous NaOH (5%) was added to it
dropwise until the solution became colourless due to
formation of PhSeNa. Thereafter, 2,6-bis(bromomethyl)
pyridine (0.264 g, 1 mmol) dissolved in 10 ml of ethanol
was added to the colourless solution with constant
stirring for 3 h. While the resulting solution was poured
into cold water (30 ml), a solid product was obtained,
which was extracted with chloroform (4 ꢂ 25 ml) from
2.4.2 | Synthesis of imine functionalized
nanosilica
A 0.77 g of APTES@nanoSiO2 was added to a 60-ml etha-
nol, followed by addition of salicylaldehyde (0.086 g,
0.707 mmol). The mixture was refluxed for 6 h with
SCHEME 1 Synthesis of 2,6-bis((phenylselanyl)methyl)pyridine (L1)

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GOGOI AND BORAH
SCHEME 2 Synthesis of Cu(II)SeNSe@imine-nanoSiO2 (Complex I)
constant stirring. A yellow solid was formed, which was
separated out and washed repeatedly through Soxhlet
extraction with ethanol and acetone. The resulting mix-
ture was dried at 120ꢃC for 24 h and designated as
imine@nanoSiO2.
followed by washing with dry toluene through Soxhlet
extraction and dried at 120ꢃC for 24 h. The solid product
was designated as Cu(II)SeNSe@imine-nanoSiO2
(Scheme 2). The Cu content of the catalyst was found to
be 0.18 mol% per10 mg from ICP-AES analysis.
2.4.3 | Immobilization of Cu(II) and 2,6-bis
((phenylselanyl)methyl)pyridine onto
imine@nanoSiO2
2.4.4 | Synthesis of Cu(II)Se-N-Se-imine
(Complex II)
A 0.127 g (0.64 mmol) of Cu(CH3COO)2ꢀH2O was dis-
solved in 50 ml of ethanol. To this solution, 0.266 g
(0.64 mmol) N-benzylideneaniline Schiff base (pre-syn-
thesized) and 0.121 g (0.64 mmol) 2,6-bis((phenylselanyl)
methyl)pyridine ligand was added and allowed to stir for
6 h at 60ꢃC (scheme 3). A reddish-brown-coloured solid
was obtained, which was filtered off and washed with
A 0.03 g (0.16 mmol) of (CH3COO)2CuꢀH2O was added to
10-ml acetone, followed by addition of 1 g imi-
ne@nanoSiO2 and 0.066 g (0.16 mmol) of 2,6-bis
((phenylselanyl)methyl)pyridine ligand. The mixture was
refluxed for 24 h under N2 atmosphere. A brown-
coloured solid was formed, which was filtered off,

GOGOI AND BORAH
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SCHEME 3 Synthesis of Cu(II)Se-N-Se-imine (Complex II)
ethanol. Molecular weight was found to be 781 g/mol
(Figure S3).
While recycling the catalyst, after each cycle, it was
isolated from the reaction mixture by simple centrifuga-
tion, washed with water and ethyl acetate for several
times and dried in an oven at 120ꢃC for overnight. The
recovered catalyst was then subjected for next run under
identical reaction conditions.
2.5 | General procedure for alcohol
oxidation
A 0.010 g (0.18 mol% of Cu per 10 mg) of the catalyst
Cu(II)SeNSe@imine-nanoSiO2 was charged to a round-
bottom flask containing a mixture of alcohol (1 mmol),
oxidant (0.3 ml) and solvent (4 ml). Reaction mixture
was stirred at 80ꢃC for the required time. The progress
of the reaction was monitored by the thin-layer chro-
matography (TLC) method. After completion of the
reaction, the mixture was cooled to room temperature
and the catalyst was separated out by centrifugal pre-
cipitation. The filtrate was extracted with water (10 ml)
and ether (3 ꢂ 10 ml). The organic layer thus collected
was washed with brine and dried over anhydrous
Na2SO4, and the solvent was evaporated under vacuum.
The residue was purified by column chromatography
on silica gel (mesh 60–120) using ethyl acetate and hex-
ane mixture (2:8) as the eluent to obtain the desired
product in pure form. The product was characterized by
GC-MS and FT-IR analysis and compared with
literature data.
3 | RESULTS AND DISCUSSION
3.1 | Characterization of the materials
3.2 | X-ray diffraction study
XRD patterns of silica, nanosilica and the catalyst are
demonstrated in Figure 1. The PXRD of silica (Figure 1a)
exhibits peaks at 2θ values of 19.05ꢃ, 20.36ꢃ, 24.75ꢃ,
28.83ꢃ, 29.25ꢃ,32.45ꢃ, 38.12ꢃ,49.33ꢃ, 55.34ꢃ and 59.60ꢃ
corresponding to (100), (101), (102), (110), (111), (202),
(202), (210), (211) and (220) planes of hexagonal unit
cell.[33] The sharp peaks of silica indicated its crystalline
nature. The XRD pattern of nanosilica (Figure 1b)
exhibits one broad peak in the range 20–23ꢃ, which can
be ascribed to amorphous silica.[34,35] The XRD pattern of

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GOGOI AND BORAH
FIGURE 1 XRD patterns of (a) silica,
(b) nanosilica and (c) Cu(II)SeNSe@imine-
nanoSiO2 (Complex I)
Cu(II)SeNSe@imine-nanoSiO2 (Figure 1c) shows 2θ
values at 15.0ꢃ, 24.7ꢃ, 25.0ꢃ, 29.02ꢃ, 30.7ꢃ, 36.4ꢃ, 47.01ꢃ,
52.7ꢃ, 59.64ꢃ and 65.17ꢃ corresponding to (100), (110),
(111), (020), (210), (211), (221), (311), (222) and (322)
planes of hexagonal Cu(II) system.[36,37]
1101 cmꢁ1, respectively, could be assigned to the asym-
metric stretching frequency of structural siloxane frame-
work (Si–O–Si), whereas the band observed at 800 and
816 cmꢁ1 corresponded to symmetric stretching of the
same group.[38,39] The bending vibration of Si–O–Si
showed peak at 476 cmꢁ1 for imine@nanoSiO2; however,
the bands are shifted slightly either higher or lower fre-
quency compared with that of Cu(II)SeNSe@imine-
nanoSiO2. The spectrum of imine@nanoSiO2 showed v
3.2.1 | FT-IR study
The FT-IR spectra of imine@nanoSiO2 and Cu(II)SeN-
Se@imine-nanoSiO2 (Complex I) provided valuable
information regarding bonding. The spectrum of imi-
ne@nanoSiO2 showed a medium-intensity peak at
1644 cmꢁ1, attributed to C=N stretching vibration of
ꢁCH=N. However, this band was shifted to 1632 cmꢁ1
in the spectrum of Cu(II)SeNSe@imine-nanoSiO2, sub-
stantiating involvement of azomethine-N to the coordina-
tion with Cu(II). The spectra of imine@nanoSiO2 and
Cu(II)SeNSe@imine-nanoSiO2 (Figure 2b,c) demon-
strated that a strong and broad peak at 1109 and
(OH) vibration of phenolic ꢁOH group at 3443 cmꢁ1
,
which was shifted to the region 3321–3645 cmꢁ1 in the
complex, confirming non-participation of phenolic oxy-
gen to the coordination with Cu(II).[5,9]
The far-infrared (IR) spectrum of Cu(II)SeN-
Se@imine-nanoSiO2 showed peaks at 522 cmꢁ1 and
511 cmꢁ1, which could be assigned to asymmetric
and symmetric stretching vibrations of C–Se bond
(Figure 3).[40] The broad and strong band at 469 cmꢁ1
might be attributed to the torsion vibration of rings,
coupled with the C–Se bending vibrations.[41] The

GOGOI AND BORAH
7 of 17
FIGURE 2 FT-IR spectra of
(a) APTES@nanoSiO2, (b) imine@nanoSiO2 and
(c) Cu(II)SeNSe@imine-nanoSiO2 (Complex I)
bending/torsional modes of Cu–N and Cu–O bonds may
submerge with this broad band. Stretching vibrational
frequencies of Cu–N and Cu–O bonds are observed at
vibrational frequencies of Cu–N and Cu–O bonds were
observed at 590, 582, 569, 569 and 545 cmꢁ1 [42,43]
The
.
bands at 343, 331 and 317 cmꢁ1 could be assigned to
565, 549 and 631 cmꢁ1 [42,43]
.
The bands at 362 cmꢁ1 and
stretching vibrations of Cu–Se bond.[44]
392 cmꢁ1 could be assigned to stretching vibrations of
Cu–Se bond.[44]
The FT-IR spectrum of Cu(II)Se-N-Se-imine
(Complex II) showed a strong intensity C=N stretching
vibration of ꢁCH=N group at 1609 cmꢁ1, whereas
v(C=O) and v(C-O) of acetate group were observed at
1751 and 1381 cmꢁ1, respectively, with other expected
peaks (Figure S4). The far-IR spectrum of Cu(II)Se-N-Se-
3.2.2 | EPR study
The ESR spectrum of Complex I clearly showed parallel
and perpendicular components of tetragonally distorted
Cu(II) complex with average g value at 2.42 and g⊥ at
k
2.02, respectively (Figure 5).[45,46] The narrow hyperfine
lines associated with perpendicular component clearly
enumerate super-hyperfine coupling of unpaired electron
spin of Cu(II) with the nuclear spin of nitrogen atoms
imine (Figure 4) showed peaks at 526, 517 and 507 cmꢁ1
,
which could be assigned to asymmetric and symmetric
stretching vibrations of C–Se bond.[40] Different stretching

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GOGOI AND BORAH
FIGURE 3 Far-IR spectra of
(a) imine@nanoSiO2 and (b) Cu(II)
SeNSe@imine-nanoSiO2
FIGURE 4 Far-FT-IR spectrum of Cu(II)Se-N-Se-imine (Complex II)

GOGOI AND BORAH
9 of 17
(IN = 1), substantiating coordination through N-atoms.
But the unsupported Complex II demonstrated only per-
pendicular component at 2.05 and could be attributed to
the formation of tetragonally distorted complex
(Figure 6).
3.2.3 | XPS study
XPS measurements have been carried out on Cu(II)SeN-
Se@imine-nanoSiO2 (Complex I) to ascertain the chemi-
cal state of Cu and Se in the catalyst (Figure 7). The Cu
2p3/2 and Cu 2p1/2 peaks at 933.1 and 952.2 eV, respec-
tively, were characteristic of Cu(II). Additionally, the
presence of Cu(II) could also be confirmed by the shake-
up peak observed at ꢄ945 eV.[47,48] The XPS spectrum of
complex showed characteristic peaks at 51.27 and
56.12 eV with a spin–orbit splitting of 4.85 eV, which
could be attributed to Se 3d5/2 and 3d3/2 of metallic sele-
nium, respectively.[49] The full XPS survey spectrum of
Cu(II)SeNSe@imine-nanoSiO2 with atomic percentage
has been incorporated in Figure S5.
FIGURE 5 ESR spectrum of Cu(II)SeNSe@imine-nanoSiO2
(Complex I)
3.2.4 | BET study
The BET surface areas of silica, nanosilica,
APTES@nanoSiO2 and Cu(II)SeNSe@imine-nanoSiO2
are shown in Table 1. It has been observed that nano-
silica has higher specific surface area than the silica as
expected. The decrease of surface area for
APTES@nanoSiO2 and Cu(II)SeNSe@imine-nanoSiO2
compared with nanosilica strongly affirmed the success-
ful grafting of APTES, imine and the metal complex on
the nanosilica.
FIGURE 6 ESR spectrum of Cu(II)Se-N-Se-imine (Complex II)
FIGURE 7 High-resolution XPS spectra of (a) Cu 2p core level and (b) Se 3d for Cu(II)Se-N-Se@imine-nanoSiO2 catalyst

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GOGOI AND BORAH
3.2.5 | SEM-EDX study
silicon in the catalyst was confirmed from SEM-EDX
analysis.
SEM micrographs (Figure 8a,b) clearly indicated that the
morphology of nanosilica was changed after successive
immobilization of APTES, imine, ligands and Cu(II) onto
the surface of nanosilica. The presence of copper, nitro-
gen and selenium along with the carbon, oxygen and
3.2.6 | TGA study
The imine ligand content was measured by TGA
analysis. The TGA analysis of APTES@nanoSiO2
(Figure S6) and imine@nanoSiO2 (Figure S7) revealed
an initial weight loss of approximately 5.80 and 7.11%,
respectively, up to temperature ꢄ250ꢃC, which could
be attributed to adsorbed water and organic
moiety. With further increase in temperature,
APTES@nanoSiO2 and imine@nanoSiO2 showed 4.75
and 4.05%wt. loss corresponding to the decomposition
of APTES and APTES-imine, respectively. Thus, the
quantity of imine ligand attached to the copper was
measured from the TGA analysis and was found to be
0.61 wt% in the catalyst (Table 2). Complex I showed
TABLE 1 BET surface area measurements of the silica- based
materials
Surface
Entry
Materials
area(m2/g)
1
2
3
4
Silica
186
280
152
123
Nanosilica
APTES@nanoSiO2
Cu(II)SeNSe@imine-nanoSiO2
FIGURE 8 (a) SEM image of nanosilica. (b) SEM image of Cu(II)SeNSe@imine-nanoSiO2. (c) SEM-EDX of nanosilica. (d) SEM–EDX of
Cu(II)SeNSe@imine-nanoSiO2

GOGOI AND BORAH
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TABLE 2 TGA-DTG analysis of the materials
Entry
Material
% weight loss (100–250ꢃC)
% weight loss (250–550ꢃC)
wt% ligand
1
2
3
APTES@nanoSiO2
Imine@nanoSiO2
Cu(II)SeNSe@imine-nanoSiO2
5.80
7.11
13.12
4.05
4.75
0.61%
TABLE 3 Optimization of reaction conditions for oxidation of 1-phenylethanola
Entry
1
Solvent
H2O
Oxidant
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
-
Catalyst (mg)
Temperature (ꢃC)
Time (min)
180
Yield (%)b
10
10
10
10
10
10
10
10
10
10
10
10
10
10
8
rt
-
2
H2O
50
rt
180
40
35
56
20
45
48
35
45
62
95
95c
Trace
Trace
92
95
95
-
3
C2H5OH
C2H5OH
MeOH
180
4
50
rt
180
5
180
6
MeOH
50
rt
180
7
MeOH (64.7 ꢃC)
CH2Cl2 (39.6)
THF (66)
Toluene
CH3CN(82)
CH3CN
-
180
8
rt
180
9
rt
180
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
80
80
80
80
80
80
80
80
80
rt
180
180
180
180
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
180
H2O2
H2O2
H2O2
H2O2
Air
180
15
20
-
180
180
180
10
10
10
10
6
24 h
24 h
120
-
TBHP
H2O2
H2O2
H2O2
H2O2
rt
45
92
90d
25e
31f
80
80
80
80
180
180
6
180
aReaction conditions: 1-phenyl ethanol (1 mmol, 0.062 ml), H2O2 (0.3 ml, 30%), TBHP (0.3 ml), solvent (4 ml), Cu(II)SeNSe@imine-nanoSiO2 (8–20 mg; 0.144–
0.36 mol% of Cu).
bIsolated yield.
cH2O2 (0.5 ml, 30%).
dH2O2 (0.2 ml, 30%).
eNanosilica as catalyst.
fAPTES@nanoSiO2 as catalyst.

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GOGOI AND BORAH
TABLE 4 Oxidation of various alcohols to corresponding carbonyl compounds
Entry
1
Reactant
Product
Yield %a
95
TON
527
2
3
85
90
472
500
4
5
6
75
80
85
416
444
472
7
85
472
416
8
75
9
82
455

GOGOI AND BORAH
13 of 17
TABLE 4 (Continued)
Entry
10
Reactant
Product
Yield %a
92
TON
511
Note: Reaction conditions: substrate (1 mmol), H2O2 (0.3 ml, 4 mmol), CH3CN (4 ml), Cu(II)SeNSe@imine-nanoSiO2 (10 mg, 0.18 mol% of Cu).
aIsolated yield.
TABLE 5 Oxidation of various alcohols to corresponding
carbonyl compounds by Cu(II)Se-N-Se-imine (Complex II)
a
13.12% weight loss in the temperature region
100–400ꢃ C (Figure S8).
3.2.7 | UV-Vis study
The solid-state UV-Vis spectra of 2,6-bis((phenylselanyl)
methyl)pyridine, imine@nanoSiO2 and Cu(II)SeN-
Se@imine-nanoSiO2 showed absorptions around 200–
400 nm, which could be attributed to intraligand π–π*
and n–π* electron transfer. The spectrum of Cu(II)SeN-
Se@imine-nanoSiO2 (Complex I) exhibited a weak
broad peak at ꢄ690 nm, which might be assigned to [2]
Entry
Substrate
Product
% yielda
1
61
2
Eg ! T2g transition of Cu2+ in a nearly octahedral envi-
2
3
52
62
48
ronment (Figure S9). Identically, the analogous
unsupported Complex II showed a characteristic band at
2
565 nm, attributed to [2]Eg ! T2g transition of Cu2+ in
a nearly octahedral environment (Figure S10).
3.3 | Catalytic activity
After characterization, catalytic efficiency of the com-
plex was explored for the oxidation of primary and sec-
ondary alcohol to their corresponding carbonyl
compounds. Initially, to optimize the reaction condi-
tions with respect to catalyst amount, solvent, oxidant,
temperature, reaction time, etc., a set of reactions were
performed taking 1-phenylethanol as a model substrate.
We have screened various solvents such as H2O,
MeOH, C2H5OH CH2Cl2, THF, toluene and CH3CN and
observed acetonitrile gave 95% product yield in 180 min
(Table 3, Entry 11). But in solvent and oxidant-free con-
ditions, the reaction produced a trace amount of prod-
uct (Table 3, Entries 13 and 14).
4
5
b
-
Note: Reaction conditions: substrate (2 mmol), H2O2 (0.3 ml, 4 mmol),
CH3CN (4 ml), Cu(II)Se-N-Se-imine (Complex II) (6 mg, 5 mol%).
aIsolated yield.
bTime 24 h.
We observed that increasing the amount of catalyst
from 10 (0.18 mol% of Cu) to 20 mg (0.36 mol% of Cu)

14 of 17
GOGOI AND BORAH
did not alter the product yield (Table 3, Entries 16 and
17), whereas decreasing the catalyst amount to 8 mg
(0.144 mol% of Cu) decreased the yield (Table 3, Entry
15). However, the reaction did not proceed in the absence
of the catalyst (Table 3, Entry 18). While applying nano-
silica and APTES@nanoSiO2 as catalyst, 25% and 31%
product yields have been isolated, respectively (Table 3,
Entries 23 and 24). The optimization of reaction tempera-
ture showed that at 80ꢃC in acetonitrile, the alcohols
were highly activated and gave excellent yield.
Using the optimized reaction conditions, a variety of
structurally different alcohols have been tested for oxida-
tion to corresponding carbonyl compounds. The results
are summarized in Table 4. It is clear that both aromatic
(primary and secondary) and aliphatic alcohols can be
activated with this protocol. However, aromatic alcohols
gave higher product yield compared with aliphatic alco-
hols in many cases.
1-phenylethanol (122 mg, 1 mmol), H2O2 (0.3 ml), H2O
(4 ml) and CH3CN (4 ml) was stirred at 80ꢃC. We have
checked percentage conversions at 20-min time interval
and found 22% and 42% conversions. After 1 h, the reac-
tion was stopped and the catalyst was separated out from
the hot reaction mixture. The product was identified by
GC-MS (50% conversion). The reaction was allowed to
continue with the filtrate without solid catalyst for
another 2 h. The percentage conversion, as examined by
GC, showed constancy at 50% even after 2 h (Figure S11).
Moreover, the ICP-AES analysis of the filtrate showed
that the content of Cu in the filtrate was less than the
detection limit. These results affirmed the non-leaching
process during the course of the catalytic reaction.
3.4.2 | Reusability
To check retrievability of the catalyst, the oxidation reac-
tion was started taking 30 mg of the catalyst (0.54 mol%
of Cu), 1-phenylethanol (1 mmol), H2O2 as oxidant
(0.3 ml) and solvent (4 ml) using optimized reaction con-
ditions. After completion of the reaction, the catalyst was
separated from the reaction mixture by centrifugal pre-
cipitation, followed by washing thoroughly with
acetonitrile–water mixture. Recovered catalyst was dried
under vacuum at 100ꢃC overnight and then subjected to
the subsequent runs. The recyclability test was carried
out under aerobic condition without any special air
3.4 | Catalyst leaching and reusability
3.4.1 | Hot filtration test
We have carried out the hot filtration test to investigate
the heterogeneous nature of the catalyst and to make
sure non-leaching of Cu during the course of catalytic
reactions. To perform the test, a mixture of Cu(II)SeN-
Se@imine-nanoSiO2 (15 mg, 0.27 mol% of Cu),
SCHEME 4 Plausible mechanism of
alcohol oxidation by Cu(II)SeNSe@imine-
nanoSiO2 (Complex I)

GOGOI AND BORAH
15 of 17
TABLE 6 Comparative study of oxidation of alcohols with supported Cu catalysts
Catalyst recycle
Yield/
Entry Catalyst
number
Reaction conditions
H2O2, CH3CN, rt
conversion (%) Reference
[43]
1
2
[CuL2-SiO2] and [CuL02-SiO2], 5 mg
5
6
30–98
[22]
Cu(II)-5CML Schiff base/
TEMPO, H2O/CH2Cl2, 45ꢃC
77–92
Fe3O4@SiO2, 17 mg, (2 mol%)
[32]
3
4
CuNPs/NC, 20 mg (0.756 mol%)
5
5
TEMPO, H2O, EG, rt
45–99
[23]
CuO-REC, 31 mg, 37.6 μmol
K2CO3, TEMPO H2O, O2 as
the oxidant at 50ꢃC
60–99
[44]
5
Cu@imine-nanoSiO2, 15 mg,
0.26 mol% Cu
5
TBHP, CH3CN, 50 ꢃC
67–98
[45]
6
7
Cu(II)/AK,5 mg
5
5
H2O2, H2O, rt
56–98
Cu(II)SeNSe@-imine-nanoSiO2,
10 mg (0.18 mol% of Cu)
H2O2, CH3CN, 80ꢃC
75–98
Our work
manipulation. A slight gradual decrease in product yield
during the course of reactions might be either physical
change or catalyst leaching due to subsequent recycling
(Figure S12). ICP-AES analysis carried out with the fil-
trate obtained after fifth cycle showed no significant Cu
leaching, substantiating that Cu was strongly embedded
on the solid support.
To check the physical change of the catalyst during
subsequent runs, we have performed SEM-EDX and
PXRD analyses of the catalyst recovered after the fifth
catalytic cycle, which revealed its initial morphology with
same chemical composition even after fifth catalytic cycle
(Figures S13–S15).
cases, hazardous reagents such as TEMPO/TBHP/CH2Cl2
were used to carry out the reactions.
4 | CONCLUSIONS
The present research work reports a novel nanosilica-
supported six-coordinated Cu(II) complex with Se–N–Se
pincer ligand. The identity of the complex has been
ascertained with different spectroscopic techniques. The
parallel and perpendicular g-components observed in
the ESR spectrum of the Complex I is indicative of tetrag-
onal distortion. In addition to this, few narrow super-
hyperfine lines have been detected with the g⊥ peak. This
significant nature of EPR spectrum clearly supports nitro-
gen coordination with the Cu(II). The catalytic activity of
as-prepared catalyst has been explored for oxidation
of primary and secondary alcohols to their corresponding
aldehydes or ketones in the presence of H2O2 as oxidant
at 80ꢃC in acetonitrile solution. The benzylic alcohols
produced excellent yields, whereas aliphatic alcohols are
less reactive and required long reaction time. This proto-
col offers several advantages such as short reaction time,
environmentally benign mild reaction conditions, good
to excellent yields and good stability of the catalyst. The
catalyst could be easily separated from the reaction prod-
ucts and reused for at least five successive runs effec-
tively. The potentiality of an analogous unsupported
Cu(II) complex as catalyst for the same reaction was
observed unsatisfactory.
3.5 | Catalytic activity of Cu(II)Se-N-Se-
imine (Complex II) for oxidation alcohols
to corresponding carbonyl compounds
A typical reaction (Table 5) was carried out with 6 mg
(5 mol % of Cu) of the catalyst, 2 mmol of substrate
(1-phenylethanol), 4 mmol oxidant and 4-ml solvent and
found the best optimized condition such as acetonitrile as
solvent and H2O2 as oxidant with 80ꢃC temperature for
3 h (Table S1, SI).
Finally, a probable mechanism of alcohol oxidation
reaction by Complex I has been proposed, which is as
given below in Scheme 4.[50–53]
The decrease of ESR signal intensity of the catalyst
recorded after completion of a catalytic cycle affirmed
conversion of Cu(II) to Cu(I) during reaction
(Figure S16).
ACKNOWLEDGMENTS
The catalytic performance of Cu(II)SeNSe@imine-
nanoSiO2 was compared with some other reported Cu-
based catalysts (Table 6), and it was observed that in most
The authors gratefully acknowledge SAIF, STIC, Kochi
University, Kochi, for ICP-AES analysis; SAIF IIT Bom-
bay for ESR analysis; IIT Kanpur for x-ray photoelectron

16 of 17
GOGOI AND BORAH
[16] T. Rahman, G. Borah, P. K. Gogoi, J. Chem. Sci. 2019, 131, 4.
[17] H. Zhao, B. Huang, Y. Wu, M. Cai, J. Organomet. Chem. 2015,
797, 21.
[18] S. Pramanik, M. R. Das, D. Das, P. Das, ChemistrySelect 2017,
2, 4577.
spectroscopic analytical facility; and DST-SAIF Cochin
for TGA analysis. The authors thank UGC, New Delhi,
for the SAP-DRS-I grant (2016–2021) awarded to the
Department of Chemistry, Dibrugarh University.
[19] A. Bhaumik, R. Kumar, J. Chem. Soc., Chem. 1995, 349.
[20] Q. Cao, L. M. Dornan, L. Rogan, N. L. Hughes, M. J. Muldoon,
Chem. Commun. 2014, 50, 4524.
[21] Z. Guo, B. Liu, Q. Zhang, W. Deng, Y. Wang, Y. Yang, Chem.
Soc. Rev. 2014, 43, 3480.
AUTHOR CONTRIBUTIONS
Rajjyoti Gogoi: Conceptualization; data curation; formal
analysis; investigation; methodology; resources; software;
visualization. Geetika Borah: Conceptualization; data
curation; formal analysis; investigation; resources;
supervision.
ꢀ
ꢀ
[22] N. Pérez-Almeida, M. Gonzalez-Davila, J. M. Santana-
ꢀ
Casiano, A. G. Gonzalez, M. S. Tangil, Environ. Sci. Technol.
2013, 47, 1239.
[23] O. Das, T. K. Paine, Dalt. Trans. 2012, 41, 11476.
[24] X. Liu, Q. Xia, Y. Zhang, C. Chen, W. Chen, J. Org. Chem.
2013, 78, 8531.
[25] M. Aghajani, N. Monadi, Appl. Organomet. Chem. 2018, 32, 1.
[26] W. Liu, J. Yang, J. Cai, Res. Chem. Intermed. 2019, 45, 549.
[27] M. Nandi, P. Roy, H. Uyamac, A. Bhaumik, Dalt. Trans. 2011,
40, 12510.
[28] S. M. Islam, P. Mondal, S. Mukherjee, A. S. Roy, A. Bhaumik,
Polym. Adv. Technol. 2011, 22, 933.
[29] P. Neves, A. A. Valente, Z. Lin, Eur. J. Inorg. Chem. 2020,
2020, 1172.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interest
regarding the publication of this article.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are avail-
able from the corresponding author upon reasonable
request.
[30] M. Cheng, M. Qin, L. Sun, L. Liu, Q. Liu, X. Tang, Dalt. Trans.
2020, 49, 7758.
[31] A. J. Lemessa, Int. J. Mater. Sci. Appl. 2017, 6, 37.
[32] E. Rafiee, S. Shahebrahimi, M. Feyzi, M. Shaterzadeh, Int.
Nano Lett. 2012, 29, 1.
ORCID
Rajjyoti Gogoi https://orcid.org/0000-0002-5555-6693
Geetika Borah https://orcid.org/0000-0002-5781-1694
REFERENCES
[33] I. M. Joni, L. Nulhakim, M. Vanitha, C. Panatarani, J. Phys.
Conf. Ser. 2018, 1080, 012006.
[34] S. Sultana, G. Borah, P. K. Gogoi, Appl. Organomet. Chem.
2019, 33, 1.
[35] S. Sultana, G. Borah, P. K. Gogoi, Catal. Lett. 2019, 149, 2142.
[36] K. Kostiantyn Kulyk, L. Liana Azizova, J. M. Cunningham, L.
Mikhalovska, M. Borysenkoa, S. Mikhalovsky, J. Mater. Chem.
B 2020, 8, 4267.
[37] P. Nyamukamba, L. Tichagwa, J. C. Ngila, L. Petrik, J. of
Photochem and Photobiol a: Chemistry 2017, 343, 85.
[38] G. Yang, J. Ma, W. Wang, J. Zhao, X. Lin, L. Zhou, X. Gao,
Catal. Lett. 2006, 112, 83.
[1] N. Gogoi, P. Bordoloi, G. Borah, P. K. Gogoi, Catal. Lett. 2017,
147, 539.
[2] P. J. Figiel, A. M. Kirillov, M. F. C. Guedes Da Silva, J. Lasria,
A. J. L. Pombeiro, Dalt. Trans. 2010, 39, 9879.
[3] B. R. Kim, J. S. Oh, J. Kim, C. Y. Lee, Catal. Lett. 2016,
146, 734.
[4] D. Datta, G. Halder, J. Polym. Environ. 2019, 27, 710.
[5] H. Unver, I. Kani, J. Chem. Sci. 2018, 130, 33.
[6] F. Cardona, C. Parmeggiani, C. Matassini, Green Chem. 2017,
19, 2030.
[7] S. Hazra, E. Malik, A. Nair, V. Tiwari, P. Dolui, A. J. Elias,
Chem. – Asian J. 2020, 15, 1916.
[39] A. Dutta, M. Chetia, A. A. Ali, A. Bordoloi, P. S. Gehlot, A.
Kumar, D. Sarma, Catal. Lett. 2019, 149, 141.
[8] R. Gogoi, R. Saikia, G. Borah, J. Organomet. Chem. 2019,
ꢀ
897, 80.
[40] F. R. Díaz, C. O. Sanchez, M. A. del Valle, L. Ugalde, L.
[9] C. Wu, B. Liu, X. Geng, Z. Zhang, S. Liu, Q. Hu, Polyhedron
2019, 158, 334.
Gargallo, Synth. Met. 1999, 105, 161.
[41] K. Helios, A. Pietraszko, W. Zierkiewicz, H. Wojtowicz, D.
ꢀ
[10] J. Kim, S. S. Stahl, ACS Catal. 2013, 3, 1652.
[11] M. B. Gawande, A. Goswami, F. X. Felpin, T. Asefa, X. Huang,
R. Silva, X. Zou, R. Zboril, R. S. Varma, Chem. Rev. 2016, 116,
3722.
Michalska, Polyhedron 2011, 30, 2466.
[42] Y. Gong, M. Zhou, Chem. Rev. 2009, 109, 6765.
[43] S. P. Sovilj, G. Vuèkovic, V. M. Leovac, D. M. Minic, Polish
J. Chem. 2000, 74, 945.
[12] Y. W. Jiang, K. Chai, Y. Q. Wang, H. D. Zhang, W. Xu, W. Li,
Y. Shi, ACS Appl. Nano. Mater 2019, 2, 4435.
[13] A. Kumar, G. Kumar Rao, A. K. Singh, RSC Adv. 2012, 2,
12552.
[14] D. Das, P. Singh, M. Singh, A. K. Singh, Dalt. Trans. 2010, 39,
10876.
[15] F. J. Morales-Leal, J. Rivera De la Rosa, C. J. Lucio-Ortiz, D. B.
Martínez, D. de Haro Del Rio, M. A. Garza-Navarro, D. X.
Martínez-Vargas, Appl. Catal. A Gen. 2018, 562, 184.
[44] K. Malek, M. Vala, J. S. Kozlowskac, L. M. Proniewiczad, New
J. Chem. 2004, 28, 477.
[45] M. Lavanya, M. Jagadeesh, J. Haribabu, R. Karvembu, H. R.
Rashmi, U. P. Maheswari Devi, A. V. Reddy, Inorganica Chim.
Acta. 2018, 469, 76.
[46] E. Garribba, G. Micera, J. Chem. Educ. 2006, 83, 1229.
[47] S. M. Lomnicki, H. Wu, S. N. Osborne, J. M. Pruett, R. L.
McCarley, E. Poliakoff, B. Dellinger, Mater. Sci. Eng., B 2010,
175, 136.

GOGOI AND BORAH
17 of 17
[48] T. Tsoncheva, G. Issa, T. Blasco, M. Dimitrov, M. Popova, S.
SUPPORTING INFORMATION
Additional supporting information may be found in the
online version of the article at the publisher's website.
ꢀ
ꢀ
Hernandez, D. Kovacheva, G. Atanasova, J. M. Lopez Nieto,
Appl. Catal. A Gen. 2013, 453, 1.
[49] J. Zhang, L. Fan, Y. Zhu, Y. Xu, J. Liang, D. Wei, Y. Qian,
Nanoscale 2014, 6, 12952.
[50] N. Gogoi, T. Begum, S. Dutta, U. Bora, P. K. Gogoi, RSC Adv.
2015, 5, 95344.
[51] T. F. S. Silva, L. M. D. R. S. Martins, Molecules 2020, 25, 748.
[52] M. Hakimi, H. Vahedi, R. Takjoo, A. Rezaeifard, Int.
J. ChemTech Res. 2012, 4, 1658.
How to cite this article: R. Gogoi, G. Borah, Appl
Organomet Chem 2021, e6416. https://doi.org/10.
1002/aoc.6416
[53] R. Mirsafaei, M. M. Heravi, T. Hosseinnejad, S. Ahmadi, Appl.
Organomet. Chem. 2016, 30, 823.