Experimentally formulated and theoretically rationalized alumina immobilized copper catalyst for alcohol oxidation

Article abstract of DOI:10.1080/00958972.2020.1749273

Our present study deals with the synthesis of a heterogeneous catalyst (CuL@Al₂O₃). Initially [CuL], having the formula [Cu(L)(H₂O)](NO₃) (HL = 2-[(3,5-dichloro-2-hydroxy-benzylidene)-amino]-2-hydroxymethyl-propane-1,3-diol, obtained from condensation between 3,5-dichloro-2-hydroxybenzaldehyde and 2-amino-2-(hydroxymethyl)propane-1,3-diol), has been synthesized, followed by immobilization of [CuL] on alumina to generate the catalyst. The catalyst has been characterized by SXRD, UV-vis, FT-IR and SEM. Ground-state structure of the homogeneous catalyst [CuL] was optimized by Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) calculations. This catalyst exhibits appreciable catalytic activity towards oxidation of alcohols by using hydrogen peroxide as oxidant. Through ESI-MS study, it becomes possible to isolate intermediate species of the catalytic oxidation. Finally, a plausible mechanistic pathway has been proposed to elucidate the role of each substituent of the catalytic system. Prepared (CuL@Al₂O₃) offers a number of advantages such as easy work-up and separation of the catalyst from the reaction mixture by centrifugation, recyclability for six times with minimal loss of activity along with the high yield of product.

Full text of DOI:10.1080/00958972.2020.1749273

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Experimentally formulated and theoretically
rationalized alumina immobilized copper catalyst
for alcohol oxidation
Tania Chowdhury, Sourav Chatterjee, Priyabrata Banerjee, Dipankar Sukul,
Madhulata Shukla & Tanmay Chattopadhyay
To cite this article: Tania Chowdhury, Sourav Chatterjee, Priyabrata Banerjee, Dipankar Sukul,
Madhulata Shukla & Tanmay Chattopadhyay (2020): Experimentally formulated and theoretically
rationalized alumina immobilized copper catalyst for alcohol oxidation, Journal of Coordination
Chemistry, DOI: 10.1080/00958972.2020.1749273
To link to this article: https://doi.org/10.1080/00958972.2020.1749273
View supplementary material
Published online: 23 Apr 2020.
Submit your article to this journal
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
https://www.tandfonline.com/action/journalInformation?journalCode=gcoo20

JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2020.1749273
Experimentally formulated and theoretically rationalized
alumina immobilized copper catalyst for
alcohol oxidation
a
b
c
d
Tania Chowdhury , Sourav Chatterjee , Priyabrata Banerjee , Dipankar Sukul ,
e
f
Madhulata Shukla and Tanmay Chattopadhyay
a
b
Department of Chemistry, University of Calcutta, Kolkata, India; Department of Chemistry,
c
Panchakot Mahavidyalaya, Sarbari, Purulia, India; Department of Surface Engineering and Tribology,
d
CSIR-CMERI, Durgapur, India; Department of Chemistry, NIT, Durgapur, West Bengal, India;
e
Department of Chemistry, G.B. College, Veer Kunwar Singh University, Arrah, Bihar, India;
f
Department of Chemistry, Diamond Harbour Women’s University, Sarisha, West Bengal, India
ABSTRACT
ARTICLE HISTORY
Our present study deals with the synthesis of a heterogeneous cata-
lyst (CuL@Al O ). Initially [CuL], having the formula [Cu(L)(H
2 3 2
Received 26 June 2019
Accepted 7 March 2020
O)](NO ) (HL ¼ 2-[(3,5-dichloro-2-hydroxy-benzylidene)-amino]-2-
3
KEYWORDS
hydroxymethyl-propane-1,3-diol, obtained from condensation
between 3,5-dichloro-2-hydroxybenzaldehyde and 2-amino-2-
Schiff base; copper;
alumina; heterogeniza-
tion; catalyst
(hydroxymethyl)propane-1,3-diol), has been synthesized, followed
by immobilization of [CuL] on alumina to generate the catalyst. The
catalyst has been characterized by SXRD, UV-vis, FT-IR and SEM.
Ground-state structure of the homogeneous catalyst [CuL] was
optimized by Density Functional Theory (DFT) and Time-Dependent
DFT (TD-DFT) calculations. This catalyst exhibits appreciable cata-
lytic activity towards oxidation of alcohols by using hydrogen per-
oxide as oxidant. Through ESI-MS study, it becomes possible to
isolate intermediate species of the catalytic oxidation. Finally, a
plausible mechanistic pathway has been proposed to elucidate the
role of each substituent of the catalytic system. Prepared
(
2 3
CuL@Al O ) offers a number of advantages such as easy work-up
and separation of the catalyst from the reaction mixture by centrifu-
gation, recyclability for six times with minimal loss of activity along
with the high yield of product.
CONTACT M. Shukla
Veer Kunwar Singh University, India; T. Chattopadhyay
Diamond Harbour Women’s University, Diamond Harbour Road, India
madhu1.shukla@gmail.com
Department of Chemistry, G.B. College, Ramgarh, Kaimur,
tanmayc2003@gmail.com Department of Chemistry,
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2020.1749273.
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
T. CHOWDHURY ET AL.
1. Introduction
The conversion of alcohols into aldehydes and ketones is of paramount significance
and widely used in organic chemistry both for laboratory-scale experiments and in the
manufacturing processes [1], because aldehydes are potential intermediates for several
organic syntheses and are employed in the manufacture of odorants, flavors, vitamins,
and drugs [2–6]. Many attempts have been executed to design and develop reagents
for the selective oxidation of primary alcohols to aldehydes. This is synthetically a very
difficult transformation because of the marked propensity towards over-oxidation to
the respective carboxylic acid even under mild conditions [7, 8]. Use of oxides and
complexes of chromium and lead has become obsolete nowadays due to their poten-
tial hazards, toxicity, scarcity of selectivity in oxidation, difficulties in product isolation
and separability. Instead, hydrogen peroxide is considered as an ecologically ‘clean’,
flexible, non-toxic, “green” oxidant. H O is much easier to handle after oxidation [9]
2
2
as it creates only water as a byproduct. Thus it is more preferred oxidant to be used
in the chemical industry from the environmental point of view [10]. Efficiency of
expensive transition metal-based catalysts like ruthenium [11], palladium [12], platinum
[13], etc. for catalyzing the oxidation of alcohols has already been reported. Thus quest
for relatively low cost 3d-transition metal-based catalysts has received enormous inter-
est. Copper complexes are found to be much promising in catalyzing several organic
transformations [14] and more cost effective and more sustainable than precious tran-
sition metal catalysts. The use of copper complexes as catalysts for the alcohol oxida-
tion is scant [15–20]. In spite of having chemo- and regioselectivity of such
homogeneous catalyst, aldehyde purification, catalyst recovery becomes the main obs-
tacle in the path of commercialization. Immobilization of the homogeneous catalyst
on a solid support [21, 22] is a smart technique to get rid of product isolation and
separation problem. Moreover this will affect catalyst recyclability and eventually
increases catalyst turnover number.
Presently, we are deeply involved in the field of catalysis to find out new cost
effective, efficient, selective, transition metal complex mediated catalytic system.
Herein, we have explored catalytic activity for selective oxidation with copper(II) com-
plex [CuL] comprising of Schiff base ligand and hydrogen peroxide as oxidant. This
Cu-complex has been characterized by SCXRD, FT-IR, UV-vis and ESI-MS studies.
Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) calculations are
performed to optimize the ground-state structure of the homogeneous catalyst [CuL].
FT-IR and UV-vis experimental data are well rationalized and correlated with the TD-
DFT calculations. We have planned to use alumina as solid support and prepared a
novel heterogeneous catalyst (CuL@Al2O3) (Schemes 1 and 2). Chemical inertness,
strength, hardness, large surface area and narrow pore size distribution of alumina
make it a judicious choice for the heterogenization of our homogenous copper-Schiff
base complex. The synthesized catalyst (CuL@Al O ) has been characterized by FT-IR
2
3
and SEM. The catalyst shows very high catalytic efficiency for the selective oxidation
of alcohols to carbonyl compounds with 90-98% product yield even after reusing it for
6
times. Furthermore this solid supported copper complex provides exquisite selectiv-
ꢀ
ꢀ
ity of oxidation of 1 over 2 alcohols that can definitely substitute the conventional
precarious techniques in an environmentally benign way. Plausible mechanistic

JOURNAL OF COORDINATION CHEMISTRY
3
Scheme 1. Schematic representation of preparation of homogeneous catalyst [CuL].
Scheme 2. Schematic representation of preparation of heterogeneous catalyst CuL@Al O .
2
3
pathway has also been proposed by obtaining evidence for the formation of inter-
mediate species through ESI-MS, UV-vis and EPR studies.
2
. Experimental
.1. Materials and physical methods
All reagents and chemicals were of reagent grade and copper(II) nitrate, 3,5-dichloro-
-hydroxybenaldehyde and 2-amino-2-hydroxymethyl-propane-1,3-diol were purchased
2
2
from commercial sources and used as received. Solvents used for spectroscopic studies
were redistilled and dried before use. UV-vis spectra were recorded on a SHIMADZU
UV-2450 spectrophotometer. FT-IR spectra of the samples were recorded in the 4000-
ꢁ
1
4
00 cm range using a Perkin-Elmer 1600 FTIR spectrometer. Elemental analyses (car-
1
bon, hydrogen, and nitrogen) were performed using a Perkin-Elmer 240 C analyzer. H
and C NMR spectra (400 MHz and 500 MHz) were recorded in CDCl at 25 C on a
1
3
ꢀ
3
Tm
Tm
Bruker Ascend 400 and Bruker UltrashieldPlus 500 NMR spectrometer. Electrospray
mass spectra were recorded on a XEVO G2 SQ-TOF mass spectrometer. Scanning elec-
tron microscope (SEM) measurement was carried out with JEOL JSM-6700 F. X-Ray
powder diffraction (XRPD) was carried out on a XPERT-PRO Diffractometer monochro-
mated Cu-Ka radiation (40.0 kV, 30.0 mA) at room temperature. Thermo Scientific iCAP
7400 ICP-OES was used to detect leaching of copper ions from the heteroge-
neous catalyst.

4
T. CHOWDHURY ET AL.
2.2. Synthesis of metal complex [CuL]
A copper complex was prepared by adding copper(II) nitrate to the Schiff-base 2-[(3,5-
dichloro-2-hydroxy-benzylidene)-amino]-2-hydroxymethyl-propane-1,3-diol, formed in
situ in ethanolic medium via condensation between 3,5-dichloro-2-hydroxybenalde-
hyde and 2-amino-2-hydroxymethyl-propane-1,3-diol. Red crystals suitable for X-ray
diffraction were obtained. Anal. Calcd., C H C CuN O : C, 30.25; H, 3.23; N, 6.41%.
1
1
14 l2
2 8
ꢁ
1
Found: C, 30.19; H, 3.16; N, 6.37%. FT-IR data (KBr pellet): m(C ¼ N) 1626 cm ; m(skeletal
ꢁ
1
-
ꢁ1
ꢁ1
vibration) 1515 cm
(
;
m(NO ) 1380 cm
;
m(O-H) 3400 cm
;
UV-vis kmax(nm)
3
ꢁ
1
ꢁ1
e, M cm ): 380(1866); 664 (50); 723 (41).
2.3. X-ray data collection and structure determination
Diffraction data for 1 was collected at room temperature (293 K) on a Bruker Smart
CCD diffractometer equipped with graphite-monochromated MoKa radiation
(
k ¼ 0.71073 Å). Cell refinement, indexing and scaling of the data set were carried out
using Bruker SMART APEX and Bruker SAINT package [23]. Experiment of compound 1
was performed at 100 K at the Elettra Synchrotron (Trieste, Italy) with a monochro-
matic wavelength of 0.700 Å. The diffraction data were indexed, integrated and scaled
using XDS [24]. The structure was solved by direct methods and subsequent Fourier
2
analyses [25] and refined by the full-matrix least-squares method based on F with all
observed reflections using SIR-92 and SHELX-97 [26] software. For the complex, all
non-hydrogen atoms were refined with anisotropic thermal parameters and the hydro-
gen atoms were fixed at their respective positions riding on their carrier atoms and
refined anisotropically. All calculations were performed using the WinGX System,
Ver1.80.05 [27], PLATON99 [28], ORTEP3 [29] programs. Selected crystallographic data
and details of refinements are displayed in Table 1.
2.4. Computational details
The Gaussian 16 program [30] package employed for the DFT calculations at the
Becke’s three parameter functional and Lee-Yang-Parr hybrid functional (B3LYP) level
[
31] of calculation and using LANL2DZ basis set for Cu atoms and 6-31þþG(d,p) basis
set for C, H, N, O and Cl atoms. Initially, DFT calculations were carried out on single-
crystal X-ray diffraction structures (SCXRD) of copper complex to find the single-point
energy and after that geometry optimization was carried out in gas phase to fully
optimize the ground-state structure. The IR frequency calculation done using DFT and
UV-vis spectroscopic and electronic properties of these optimized complexes have also
been computed by TD-DFT calculations at the same B3LYP level of calculation.
2
.5. Synthesis of heterogeneous catalysts [CuL@Al O ]
2 3
1
g of Al O was dispersed in 10 mL acetonitrile. Then 0.25 g acetonitrilic solution of
2 3
[CuL] was added dropwise with continuous stirring for 24 h. The greenish white solid
mass obtained was collected by centrifugation and the residue was washed several
times with acetonitrile and dried in air.

JOURNAL OF COORDINATION CHEMISTRY
5
Table 1. Crystallographic data and details of refinements
for [CuL].
CCDC No.
Empirical formula
Fw
System
Space group
a (Å)
1922313
C H14Cl CuN O
11 2 2 8
432.67
Monoclinic
P2 /c
1
8.030(3)
31.614(11)
6.607(2)
90
113.375(12)
90
b (Å)
c (Å)
ꢀ
a ( )
ꢀ
b ( )
ꢀ
c ( )
3
V (Å )
1539.6(9)
Z
D
l (M
4
ꢁ
3
calcd (g.cm
)
1.884
1.809
ꢁ1
o
-K
a
) (mm
)
F(000)
h range ( )
Collected reflections
Indep. reflections
884
ꢀ
2.58-25.62
19100
2684
R
int
0.062
1871
Obs. reflections [I > 2r(I)]
Parameters
228
a
R
I
[I > 2r(I)]
0.0777
0.1042
1.237
ꢁ0.81, 0.74
a
wR
GOF on F
2
[I > 2r(I)]
2
ꢁ3
Residuals (e Å
)
3. Results and discussion
3.1. Characterization of homogeneous catalysts
The synthesis of the Schiff base ligand was carried out by refluxing the ethanolic solu-
tion of 3,5-dichloro-2-hydroxybenaldehyde and 2-amino-2-hydroxymethyl-propane-1,3-
diol. In situ addition of copper(II) nitrate to the reaction mixture produced complex
[CuL]. After a few days of keeping undisturbed, red crystals of the complex were
obtained. The complex was characterized by several physiochemical methods. The
complex was initially characterized by the FT-IR spectroscopy (Figure S1, supplemen-
ꢁ
1
tary material). A sharp band at 1626 cm
was observed as a signature of C ¼ N
ꢁ
1
stretching, which confirms formation of the Schiff base. The band at 1515 cm corre-
sponds to the benzene skeletal vibration. A broad band in the region 3568-3400 cm
was due to O-H stretching of H O moiety and O-H group. The band at 1380 cm cor-
responds to the NO3 antisymmetric stretching mode. The electronic absorption spec-
ꢁ
1
ꢁ
1
2
-
tra of the complex in the UV-visible range were carried out in acetonitrile (Figure S2,
supplementary material). The complex showed a broad absorption band at 380 nm
ꢁ
1
ꢁ1
(
e ¼ 1866 M cm ) which is due to charge-transfer from pp orbital of the phenolate
ꢂ
to the dk orbital of Cu(II), a band at 391 nm is for the transition of benzene (k) to
-
ꢂ
ꢁ1
ꢁ1
ꢁ1
ꢁ1
NO3 (k ) and d-d bands at 664 nm (e ¼ 50 M cm ) and 723 nm (e ¼ 41 M cm ). In
order to have an idea about the solution phase structure of the complex, the electro-
spray ionization mass (ESI-MS positive) spectra study was performed in acetonitrile.
ESI-MS spectrum (Figure S3, supplementary material) consisted of a base peak at
m/z ¼ 356.9445 amu which is consistent with the mononuclear species
þ
C H Cl CuNO
(calc. m/z ¼ 356.9510 amu). This gave evidence that H O moiety
1
1
12
2
4
2

6
T. CHOWDHURY ET AL.
ꢀ
Table 2. Coordination bond lengths (Å) and angles ( ) for [CuL] obtained from XRD and com-
pared with DFT-calculated data.
Parameter
Bond length/angle (XRD data)
DFT calculated values
Cu1-O3
Cu1-O5
Cu1-O6
Cu1-N1
1.927(4)
1.876(3)
1.969(3)
1.911(3)
90.22(15)
92.36(16)
166.28(16)
167.28(15)
96.66(14)
83.52(14)
125.1(3)
111.1(3)
113.00
1.925
1.891
1.972
1.915
90.62
O3-Cu1-O5
O3-Cu1-O6
O3-Cu1-N1
O5-Cu1-O6
O5-Cu1-N1
O6-Cu1-N1
Cu1-O5-C5
Cu1-O6-C8
Cu1-O3-H8
Cu1-O3-H9
Cu1-O6-H7
Cu1-N1-C9
Cu1-N1-C7
92.61
165.70
167.92
96.64
83.93
126.01
110.96
113.99
121.65
109.69
112.15
124.66
120(5)
108(4)
113.4(2)
124.2(3)
departs from the metal center in solution phase. Another base peak at m/
þ
z ¼ 434.9538 amu is due to the species C H Cl CuN O (calc. m/z ¼ 434.9523 amu),
11
14
2
2 8
attributed to the H O containing Cu complex with its counter anion nitrate.
2
The asymmetric unit of [CuL] contains monomeric copper complex and one nitrate
as a counter anion where the copper center is coordinated with one nitrogen atom
from imine and three oxygen atoms, two from phenoxo group and Schiff base ligand
and another from aqua ligand. The central metal center is tetra-coordinate with dis-
torted square planar geometry and the associated tetrahedrality index (s ’) [32] is 0.18,
4
ꢀ
verified from the trans angle of N(1)-Cu(1)-O(3) and O(5)-Cu(1)-O(6) are 166.28 and
ꢀ
1
67.28 , respectively. The bond lengths of Cu(1)-N(1), Cu(1)-O(5), Cu(1)-O(3) and Cu(1)-
O(6) are 1.911 Å, 1.876 Å, 1.927 Å and 1.968 Å, respectively. This mononuclear complex
crystallizes in the space group P2 /c. It is observed that O atom of the counter anion
1
nitrate part is involved in intermolecular H bonding with the H atom of the H O moi-
2
ety that is ligated to Cu center with the bond distance of H(9)ꢃꢃꢃꢃO(2) is 1.991 Å. A
selection of coordination bond lengths and angles is reported in table 2 and the
ORTEP is given in Figure 1.
3.2. Geometry optimization
DFT calculations were carried out on single-crystal X-ray diffraction structures (SCXRD)
of copper complex in gas phase to fully optimize the ground-state structure.
Optimized structures of copper complex are shown in Figure 2. DFT-calculated main
geometrical parameters for copper complex have been compared with the XRD data
and have been tabulated in table 2. Data presented in table 2 reveal that DFT-calcu-
lated data for Cu-complex report the experimental data very closely. Calculated bond
lengths and angles are in reasonably good agreement with the experimental values.
Copper atom is associated with three oxygen atoms and one nitrogen atom in a dis-
torted square planar manner. Bond lengths and angles reported in Table 2 justified
the square planar structure of the complex. Slight differences observed between XRD

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 1. ORTEP drawing (55% probability ellipsoid) of [CuL].
Figure 2. Optimized structure of [CuL].
and DFT calculated parameters is practical and is also acceptable because experimen-
tal geometry is for the atoms packed in a lattice in which intermolecular interactions
play a significant role, whereas in the case of DFT calculation geometry is in isolated
state. Density of state plot for [CuL] is given in Figure 3.
3.3. Vibrational frequency calculation and assignment of bands
Figure 4 shows the experimental IR spectrum of complex [CuL] recorded in the range
ꢁ
1
4000-400 cm
and has been correlated with the DFT-calculated IR frequency

8
T. CHOWDHURY ET AL.
Figure 3. Density of state plot for [CuL].
Figure 4. Comparison between TD-DFT calculated and experimental data of IR frequencies
of [CuL].
calculation at B3LYP level of calculation. The experimental results are interpreted and
rationalized in light of the theoretical investigation based on the DFT calculation.
Calculated IR bands with the corresponding assignments are presented in table 3.
Vibrational frequencies corresponding to different vibrations such as stretching, scis-
soring, twisting, in-plane bending, rocking, ring breathing, etc. were assigned based
on the predominant motion of the atoms and magnitude of the displacement vector
associated with the atoms involved in concerned vibration and are also included in
the tables. Different O-H stretching frequencies were observed to be analyzed above
ꢁ
1
ꢁ1
3
3
400 cm . C-H stretching was observed between 2900-3200 cm as shown in table
ꢁ
1
. Scaling factor of 0.982 applied in higher wavenumber region (above 2900 cm ) to
compensate the anharmonicity presents in higher wavenumber region for experimen-
tal frequencies. Scissoring in H-O-H and C ¼ N stretching was observed at 1682 and
ꢁ
ꢁ
1
1
1
656 cm , respectively. N-O asymmetric stretching in NO3 was observed at
1
ꢁ1
190 cm . Different C-O stretching was observed at 1041 and 1029 cm . Other

JOURNAL OF COORDINATION CHEMISTRY
9
ꢁ
1
Table 3. Calculated IR frequency (cm ) obtained from DFT calculation. Scaling factor of 0.982
ꢁ
1
applied in higher wavenumber region (above 2900 cm ).
-
1
IR Frequency (cm )
Intensity
Assignment of peak
3
3
3
3
3
3
3
3
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
9
8
7
6
5
4
735
604
433
144
118
103
024
017
997
959
927
682
656
552
518
463
429
412
390
316
257
201
190
041
029
72
102
19
429
1
1
2
20
36
21
43
42
99
407
109
3
O-H stretching in H O molecule
2
O-H stretching in O-H group
O-H stretching in O-H group
C-H stretching in benzene ring
C-H stretching in benzene ring
Asym. C-H stretching in CH
Sym. C-H stretching in CH
Asym. C-H stretching in another CH
Asym. C-H stretching in CH group
2
group
2
group
2
group
2
Sym. C-H stretching in CH
Sym. C-H stretching in CH
Scissoring in H-O-H
2
group
group
2
C ¼ N stretching
C ¼ C stretching in Benzene ring
Scissoring in H-C-H
78
C-H inplane bending coupled with C ¼ N and C ¼ C stretching
131
205
51
69
95
113
155
117
217
139
128
136
76
Wagging of O-H and C-H in CH
Wagging of O-H and C-H in CH
2
OH group coupled with N-O stretching
OH group
2
2
Wagging of C-H in CH group
C-H in plane bending in benzene coupled with H-C-H twisting
C-H in plane bending in benzene coupled with H-C-H twisting
C-H in plane bending in benzene coupled with benzene ring breathing
N-O asym. Stretching in NO
C-O stretching
3
C-O stretching
Wagging of C-H in CH
Twisting of O-H in H O group
Benzene ring breathing
N-O bending in NO
Out of plane bending of C-H in benzene ring
Out of plane bending of O-H in H
2
group
00
45
58
92
2
3
29
151
04
2
O
important vibrational bands and their assignments are presented in table 3. Figure 4
shows a good correlation between experimental and DFT-calculated frequencies.
3.4. Uv-visible spectra and its transitions
UV-visible spectra of [CuL] were recorded in acetonitrile. A very sharp and strong peak
appears at 210 nm with shoulders at 247, 280 and 391 nm. In order to understand the
transitions occurring in the complex, TD-DFT calculations were performed on the
ground-state optimized structure. TD-DFT calculated transition (vertical lines) com-
pared with the experimental data are shown in Figure 5. The calculated absorption
bands for [CuL] in the ground-state, their oscillator strengths and the band assign-
ments are presented in Table 4. Electronic transitions have been assigned through
Frontier molecular orbital (FMO) pictures involved in transitions. Band at 395 nm corre-
sponds to (391 nm - experimental value) transition from (d )/n(N)/p(benzene) to
z2
ꢂ
(
dx2-y2)/p NO . Shoulder at 245 nm (247 nm - experimental value) corresponds to the
3
ꢂ
transition occurring from p(benzene) to p (benzene)/(dx2-y2). Assignment of calculated
transitions was based on the contour plots of different HOMOs and LUMOs involved
in electronic transitions.

1
0
T. CHOWDHURY ET AL.
Figure 5. TD-DFT calculated transitions (vertical lines) compared with the experimental data of
absorption bands.
Table 4. TD-DFT Calculated absorption bands, oscillator strengths and the transition occurring
for [CuL].
Wavelength (nm)
Oscillator strength
Transition occurring
Assignment of transitions occurring
ꢂ
395
367
337
336
327
314
307
247
245
239
224
210
0.0634
0.0021
0.0063
0.0077
0.0179
0.0069
0.02
0.0633
0.118
0.0439
0.1
95 ! 97
93 ! 97
96 ! 106
91 ! 98
96 ! 107
94 ! 99
94 ! 99
93 ! 103
90 ! 98
88 ! 98
92 ! 103
95 ! 98
(dz2)/n(N)/p(benzene) ! (dx2-y2)/p NO
3
ꢂ
(dxy)/n(O/N) ! (dx2-y2)/p NO
3
ꢂ
(dx2-y2)/n(O/N) ! r (C-Cl)
ꢂ
p(benzene) ! p (benzene)
ꢂ
ꢂ
(dx2-y2)/n(O/N) ! p (benzene)/r (N-O)
ꢂ
(dz2)/n(O) ! p (benzene)
ꢂ
(dz2)/n(O) ! p (benzene)
ꢂ
ꢂ
(dxy)/n(O/N) ! p (benzene)/(dx2-y2)/r (C-Cl)
ꢂ
p(benzene) ! p (benzene)/(dx2-y2
)
ꢂ
n(O/NO
3
) ! p (benzene)/(dx2-y2
)
ꢂ
ꢂ
(dz2)/n(O) !p (benzene)/(dx2-y2)/r (C-Cl)
ꢂ
0.08
(dz2)/n(N)/p(benzene) ! p (benzene)/(dx2-y2
)
3.5. Synthesis and characterization of heterogeneous catalysts
Heterogenization of homogeneous catalytic systems stems from the need of modifica-
tion of homogeneous catalysts to “break” the uniformity among the molecules of the
catalyst and the product by rendering them in separable phases to allow their separ-
ation. Herein, we have supplied alumina as the solid carrier of the homogenous cata-
lyst [CuL] and our catalyst can easily be recovered after completion of the reaction by
simple centrifugation method. The FT-IR spectrum of [CuL] is mentioned earlier in
Figure S1 (supplementary material). FT-IR spectrum of alumina-supported complex
[
CuL] (CuL@Al O ) shows noticeable correspondence to that of the free complex
2 3
(Figure S1, supplementary material) with the only difference in attenuated peak inten-
sity due to low concentration of metal complex (Figure 6). In the case of CuL@Al O ,
2
3
ꢁ
1
peaks at 3454 cm can be assigned to the stretching vibrations of OH groups. In
Al O -supported complex, two characteristic vibrational peaks at 758 and 588 cm
ꢁ
1
2
3
appeared, owing to the Al-O vibration modes in AlO tetrahedra and AlO octahedra.
4
6
Heterogenization of [CuL] was further confirmed by taking UV-vis spectra of the
synthesized CuL@Al O in the solid phase. Overall spectral nature remains similar with
2
3
that of the homogeneous one. The bands are slightly red-shifted (210 nm ! 224 nm,

JOURNAL OF COORDINATION CHEMISTRY
11
Figure 6. FTIR spectra of homogeneous catalyst [CuL] and CuL@Al O .
2
3
Figure 7. Solid phase UV-vis spectra of [CuL] (blue) and CuL@Al O (red).
2
3
3
91 nm !) in comparison with that of [CuL]. Figure 7 indicates that [CuL] is success-
fully coated on alumina.
The morphologies of CuL@Al O were characterized by scanning electron micro-
2
3
scopic technique, which are shown in Figure 8. Introduction of [CuL] into Al O influ-
2
3
enced the morphology of the alumina material which is supposed to be due to
complexation.
ꢀ
Powder X-ray Diffraction (PXRD) was performed in the 2Theta region of 5 to 80 for
alumina, homogeneous catalyst and alumina supported heterogeneous catalysts to
identify the phases. Figure 9a shows the PXRD patterns of [CuL] and Figure 9b depicts
PXRD patterns of alumina (bottom panel) and CuL@Al O (top panel). For [CuL],
2
3
ꢀ
ꢀ
ꢀ
ꢀ
major peaks are detected at 2h ¼ 22.24 , 23.36 and 24.84 . PXRD of Al O exhibited
2
3
ꢀ
ꢀ
ꢀ
major peaks at 2h ¼ 27.81 , 37.82 , 45.43 and 66.85 that correspond to c-Al O phase
2
3
[
22]. All the major peaks of [CuL] and alumina retains in the PXRD of CuL@Al O .
2 3

1
2
T. CHOWDHURY ET AL.
Figure 8. SEM images of CuL@Al O .
2
3
Figure 9. PXRD pattern of (A) [CuL] and (B) CuL@Al O .
2
3
Only noticeable discrepancy is that, after surface modification of alumina with the
homogeneous catalyst, peak intensity of the above-mentioned major peaks of [CuL]
decreases and peaks transformed from sharpened to broader ones.
3.6. General procedure of oxidation of alcohols
3.6.1. Using homogeneous catalyst
Initially benzyl alcohol was chosen as the model substrate to optimize the reaction con-
ditions for alcohol oxidation to aldehyde. Both the catalyst [CuL] and oxidizing agent
H O concentration varied between 1-2 mmole for 1 mmole of benzyl alcohol. The reac-
2
2
tion time was extended from 2 h to 7 h. Product yield was found to be maximized with
mmole of [CuL] and 1.25 mmole of H O on performing the reaction for 3 h. No incre-
1
2
2
ment of yield was obtained on increasing catalyst concentration or on increase of reac-
tion time. The oxidation mechanism is believed to proceed via the formation of copper-
hydroperoxo species as we got evidence from ESI-MS and UV-vis spectral studies. In
order to investigate the intermediate species in the alcohol oxidation reaction by the
assistance of [CuL] we added H O to our homogeneous catalyst [CuL] and monitored
2
2

JOURNAL OF COORDINATION CHEMISTRY
13
Figure 10. ESI-MS Spectrum of Cu-OOH in positive mode during the reaction of [CuL] and H O
2
2
ꢀ
(10 equiv) at 25 C.
the
reaction
through
ESI-MS
where
we
found
a
peak
at
m/z ¼ 410.2720 amu and 388.2227 amu along with the main peak of complex at
m/z ¼ 356.9445 amu (reported earlier in Figure S3, supplementary material). These two
peaks may be attributed to the formation of copper(II)-hydroperoxo species
þ
þ
[
CuL(OOH)Na]
and [CuL(H O )]
having the exact molecular formula
2
2
þ
þ
C H Cl CuNO Na and C H Cl CuNO , respectively, demonstrated pictorially in
1
1
13
2
6
11 12
2
6
Figure 10. A change in UV-vis spectral pattern also occurred during the reaction between
CuL] and H O . The newly generated band at 375 nm may be assigned to the ligand to
[
2
2
metal charge transfer band of the Cu-OOH species. Complex [CuL] showed 4 lines in X-
band EPR spectra due to having one unpaired electron and nuclear spin of copper being
þ3/2. On adding H O , the bands became a little bit blunt and slightly shifted with
2
2
respect to complex [CuL] (Figure S5, supplementary material). This was also in accord-
ance with the formation of Cu-OOH. By this experiment it can be inferred that copper-
hydroperoxo species is the intermediate of [CuL] catalyzed alcohol oxidation reaction
(Schemes 1 and 2). On getting an evidence of formation of this intermediate species in
the ESI-MS spectra, we have proposed a plausible mechanism showing how our homo-
geneous [CuL] help catalyzing oxidation of alcohol to aldehyde (given in Scheme 3).
3.6.2. Using heterogeneous catalyst
Previous surveys regarding Al O catalysts have shown that such catalysts are capable of
2
3
displaying excellent catalytic activities [33, 34]. Heterogenized alumina-supported [CuL]

1
4
T. CHOWDHURY ET AL.
Scheme 3. Probable mechanism for oxidation of benzyl alcohol.
was used to catalyze the alcohol oxidation reaction with the optimized concentration of
1
.25 mmole H O and 1 mmole of model substrate and methanol as solvent. The solid
2 2
catalyst concentration varied between 25 mg to 60 mg and was allowed to stir for up to
6
3
h. The optimum yield of benzaldehyde was obtained on stirring 30 mg CuL@Al O for
2 3
.5 h. No further progress in transformation was found even after performing the reac-
tion beyond 3.5 h. Transformation of benzyl alcohol to benzaldeheyde was attained with
3% and 91% conversion (table 5, entries 1-9) using [CuL] and CuL@Al O as catalyst
9
2
3
without formation of benzoic acid. Although the conversion is slightly less for alumina-
supported copper catalyst with that of [CuL] on account for having slightly decreased
surface area on heterogenization, the recyclability of the catalyst CuL@Al O ultimately
2
3
provides a greater catalytic turnover number compared to the homogeneous one.
Different aliphatic and aromatic alcohols were used for the oxidation under these opti-
mized conditions to investigate the usefulness and industrial applicability of the hetero-
genized catalyst over the homogeneous one and the yield and conversion percentage
are given in table 5; it can be observed that aliphatic and aromatic alcohols were con-
verted to respective aldehyde without formation of acid as a side-product. A comparison
of our present work with previously reported catalysts are given in table 6.
3.7. Reusability of the catalyst
Our catalyst has been recycled up to 6 times with very negligible decrease (5.5%) in
the conversion yield (table 7). The catalyst was recovered from the reaction mixture

JOURNAL OF COORDINATION CHEMISTRY
15
Table 5. Summarized results of various substrate oxidation with H O , catalyzed by homogeneous
2
2
catalysts [CuL] and heterogeneous catalyst CuL@Al O .
2
3
a
b
Coversion (%)
Yield (%)
Entry
1
Substrate
Product
CuL
93
Cul@Al
91
2
O
3
CuL
81
Cul@Al
79
2 3
O
2
3
4
90
88
95
89
85
92
78
72
84
75
70
80
5
6
98
88
96
85
86
76
85
73
7
8
95
95
93
93
82
83
81
80
[
a]
1
Determined by H NMR analysis of the crude reaction mixture using tetramethylsilane as internal standard.
b]Isolated yield.
[
Table 6. Comparison of our present work with other reported methods in the various oxida-
tion reactions.
Entry
Reactant
Product
Catalyst
Pt/Al
Bi-Pt/Al
Au@20Pd/SiO2
AgNPs/GOSH
Conversion (%)
Ref.
1.
2.
3.
4.
5.
6.
Cinnamyl alcohol
Cinnamyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Cinnamaldehyde
Cinnamaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
2
O
3
10
84.5
94
61
74
[35]
[35]
[36]
[37]
[38]
[38]
2 3
O
Pd/c-Al
Pd/c-Al
2
O
O
3
-fibr
-fibr
(4-(tert-butyl)phenyl) methanol
4-tert-butyl benzaldehyde
2
3
60
by centrifugation and the catalyst observed to be settled down at the bottom of the
centrifuge tube liquid product was slowly decanted. The heterogeneous catalyst was
washed with water-acetonitrile (1:1) solution three times. ICP-OES analysis was carried
out with the decanted part. But no such evidence for leaching of copper was found.

1
6
T. CHOWDHURY ET AL.
Table 7. Conversion yield (%) of optimized oxidation reaction with CuL@Al O catalyst in six cata-
2
3
lytic cycles.
Cycles
Fresh 1
2
3
4
5
6
Conversion (%)
Yield (%)
91
79
91
79
90
78.5
88
77
87
75.5
86
75
Figure 11. FTIR spectra of (I) CuL@Al O and (II) CuL@Al O after sixth catalytic cycle.
2
3
2 3
Figure 12. Reusability of CuL@Al O : catalytic conversion in 6 consecutive cycles.
2
3
After sixth time use, the intactness of chemical composition of the catalyst was
rechecked by elemental analysis, FT-IR (Figure 11), SEM images (Figure S6, supplemen-
tary material) and PXRD pattern (Figure S7, supplementary material). The results
resemble the previous characterization (Figure 12).
4. Conclusion
We have prepared a novel heterogeneous catalyst CuL@Al O for the controlled pro-
2
3
duction of aldehyde through the oxidation of alcohol. Structure of the synthesized

JOURNAL OF COORDINATION CHEMISTRY
17
homogeneous copper catalyst has been optimized by DFT calculations. Theoretically
calculated data corroborate well with the experimentally found SXRD data in support-
ing the distorted square planar geometrical orientation of the copper complex (CuL).
B3LYP level of calculation helps to get deep insight into the observed IR peaks and
appropriate bond vibrations have been assigned successfully. Here in this work, the
exact electronic transition responsible for the observed UV-Vis bands of [CuL] are
attributed in terms TD-DFT calculation. PXRD, SEM, UV-vis and FT-IR data provide
strong evidence for surface coating of alumina with [CuL]. Utility of CuL@Al O in cat-
2
3
alyzing the oxidation of alcohols is verified by corresponding optimization of trans-
formation from benzyl alcohol to benzaldehyde with the catalyst [CuL]. This results in
appreciable product yield with 89-98% catalytic conversion. UV-vis, ESR and ESI-MS
studies reveal that the catalytic reaction proceeds via the copper-hydroperoxo path-
way while the oxidation state of copper does not change during oxidation. The newly
immobilized alumina-supported heterogeneous catalyst was found to have enhanced
reusability, cost effectiveness and sustainability with desirable conversion of product.
Acknowledgements
TC is thankful to the CSIR-JRF for providing a fellowship [CSIR-sanction no: 09/028(1064)/2018-
EMR-I]. The authors wish to thank the University of Calcutta for providing the facility of the sin-
gle-crystal X-ray diffractometer by the DST FIST program. MLS thanks Computer Unit IIT-BHU for
providing computational facility.
Disclosure statement
No potential conflict of interest was reported by the author(s).
References
[1] (a) R.C. Larock, In Comprehensive Organic Transformations; VCH Publishers Inc., New York,
604 (1989); (b) G. Procter, S.V. Ley, Oxford 7, 305 (1991);(c) S.V. Ley, A. Madin, B.M. Trost,
I. Fleming, Oxford, 7, 251 (1991);(d) T.V. Lee, B.M. Trost, I. Fleming, Oxford 7, 291 (1991).
2] R. Ramesh. Inorg. Chem. Commun., 7, 274 (2004).
3] A. Bordoloi, S. Sahoo, F. Lefebvre, S. Halligudi. J. Catal., 259, 232 (2008).
4] P. Maity, C.S. Gopinath, S. Bhaduri, G.K. Lahiri. Green Chem., 11, 554 (2009).
5] V. Mahdavi, M. Mardani. J. Chem. Sci., 124, 1107 (2012).
[
[
[
[
[
[
6] C. Parmeggiani, F. Cardona. Green Chem., 14, 547 (2012).
7] I.W.C.E. Arends, R.A. Sheldon. J.E. B €a ckvall (Eds.), Modern Oxidation Methods, 2nd edn.,
Wiley-VCH: Weinheim 2011.
[
[
8] L. Boisvert, K.I. Goldberg. Acc. Chem. Res., 45, 899 (2012).
9] G. Strukul (Ed.), Catalytic Oxidations with Hydrogen Peroxide as Oxidant, Kluwer Academic
Publishers, Dordrecht, Netherlands, 1992.
[
[
10] L. Plant, M. Jeff. Chem. Eng., 16, 16 (1994).
11] C. Aliende, M. P eꢀ rez-Manrique, F.A. Jal oꢀ n, B.R. Manzano, A.M. Rodr ꢀı guez, G. Espino.
Organometallics, 31, 6106 (2012).
[12] M.M. Dell’Anna, M. Mali, P. Mastrorilli, P. Cotugno, A. Monopoli. J. Mol. Catal. A, 386, 114
(2014).
[
[
13] Y.H. Ng, S. Ikeda, T. Harada, Y. Morita, M. Matsumura. Chem. Commun., 27, 3181 (2008).
14] S.R. Chemler. Beilstein J. Org. Chem., 11, 2252 (2015).

1
8
T. CHOWDHURY ET AL.
[
[
[
[
[
[
[
[
15] N. Jiang, A.J. Ragauskas. J. Org. Chem., 71, 7087 (2006).
16] J.M. Hoover, B.L. Ryland, S.S. Stahl. J. Am. Chem. Soc., 135, 2357 (2013).
17] H. Unver, I. Kani. J. Chem. Sci., 130, 33 (2018).
18] S. Mannam, S.K. Alamsetti, G. Sekar. Adv. Synth. Catal., 349, 2253 (2007).
19] B.L. Ryland, S.D. McCann, T.C. Brunold, S.S. Stahl. J. Am. Chem. Soc., 136, 12166(2014).
20] X. Lu, N. Wu, B. Zhang, K. Deng. J. Coord. Chem., 70, 475 (2017).
21] M. Trueba, S.P. Trasatti. Eur. J. Inorg. Chem., 2005, 3393 (2005).
22] T. Chakraborty, A. Chakraborty, S. Maity, D. Das, T. Chattopadhyay. Mol. Catal., 462, 104
(2019).
[23] Bruker, SMART, SAINT. Software Reference Manual, Bruker, AXS Inc., Madison, Wisconsin,
USA 2000
[24] W. Kabsch. Acta Crystallogr. D Biol. Crystallogr., 66, 125 (2010).
[25] G.M. Sheldrick. Acta Crystallogr. A Found. Crystallogr., 64, 112 (2008).
[26] G.M. Sheldrick. SHELXL 97, Program for the Refinement of Crystal Structures, University of
Gottingen, Germany (1997).
[27] L.J. Farrugia. J. Appl. Crystallogr., 32, 837 (1999).
[28] A.L.; Spek. PLATON, Molecular Geometry Program, University of Utrecht, The Netherlands
(1999).
[29] L.J. Farrugia. J. Appl. Crystallogr., 30, 565 (1997).
[
30] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G.
Scalmani, V. Barone, G.A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A.V. Marenich, J.
Bloino, B.G. Janesko, R. Gomperts, B. Mennucci, H.P. Hratchian, J.V. Ortiz, A.F. Izmaylov,
J.L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A.
Petrone, T. Henderson, D. Ranasinghe, V.G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W.
Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y.
Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J.A. Montgomery, Jr., J.E. Peralta, F.
Ogliaro, M.J. Bearpark, J.J. Heyd, E.N. Brothers, K.N. Kudin, V.N. Staroverov, T.A. Keith, R.
Kobayashi, J. Normand, K. Raghavachari, A.P. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi,
M. Cossi, J.M. Millam, M. Klene, C. Adamo, R. Cammi, J.W. Ochterski, R.L. Martin, K.
Morokuma, O. Farkas. J.B. Foresman, D.J. Fox, Gaussian, Inc., Wallingford, CT, (2016).
31] A.D. Becke. J. Chem. Phys., 98, 5648 (1993).
[
[
[
32] A. Okuniewski, D. Rosiak, J. Chojnacki, B. Becker. Polyhedron, 90, 47 (2015).
33] J. Adhikary, A. Datta, S. Dasgupta, A. Chakraborty, M.I. Men ꢀe ndez, T. Chattopadhyay. RSC
Adv., 5, 92634 (2015).
[
34] M.A. Uguina, J.A. Delgado, A. Rodr ꢀı guez, J. Carretero, D.G. D ꢀı az. J. Mol. Catal. A–Chem.,
2
56, 208 (2006).
[35] T. Mallat, Z. Bodnar, P. Hug, A. Baiker. J. Catal., 153, 131 (1995).
[36] H. Wang, C. Wang, H. Yan, H. Yi, J. Lu. J. Catal., 324, 59 (2015).
[37] B. Zahed, H. Hosseini-Monfared. Appl. Surf. Sci., 328, 536 (2015).
[38] L. Chen, J. Yan, Z. Tong, S. Yu, J. Tang, B. Ou, L. Yue, L. Tian. Microporous Mesoporous
Mater., 266, 126(2018).