CuI incorporated cobalt ferrite nanoparticles as a magnetically separable catalyst for oxidative amidation reaction

Article abstract of DOI:10.1039/c9dt03440d

A Cu-incorporated magnetic nanocatalyst (CoFe₂O₄?SiO₂-SH-CuI) has been developed by immobilizing CuI on the modified surface of CoFe₂O₄ magnetic nanoparticles. The surface of the silica coated cobalt

Full text of DOI:10.1039/c9dt03440d

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
CuI incorporated cobalt ferrite nanoparticles as
a magnetically separable catalyst for oxidative
amidation reaction†
Cite this: Dalton Trans., 2019, 48,
16041
Mintu Maan Dutta, Hrishikesh Talukdar and Prodeep Phukan
*
A Cu-incorporated magnetic nanocatalyst (CoFe2O4@SiO2-SH-CuI) has been developed by immobilizing
CuI on the modified surface of CoFe2O4 magnetic nanoparticles. The surface of the silica coated cobalt
ferrite magnetic core was first treated with 3-mercaptopropyl triethoxysilane to produce the thiol functio-
nalized nanoparticle CoFe2O4@SiO2-SH. Further treatment of the nanoparticle with CuI produced the
desired magnetic nanocatalyst. The versatility of the catalyst has been demonstrated for the synthesis of
N-(pyridin-2-yl) benzamide via oxidative amidation of aryl aldehydes with 2-amino pyridine in the pres-
ence of an oxidant. The reaction was carried out in DMSO at 80 °C using TBHP (70% aqueous) in the
presence of 20 wt% of the catalyst. Notably, the catalyst could be separated from the reaction mixture in
the presence of an external magnetic field. During the study, a new compound N-(pyridin-2-yl)anthra-
cene-2-carboxamide has been synthesized and its turn on fluorescence sensing properties towards
various metal ions have been studied. Fluorescence experiments and theoretical studies indicated that the
newly synthesized carboxamide molecule can be used for fluorescence sensing of the Hg2+ ion in
aqueous solution.
Received 23rd August 2019,
Accepted 25th September 2019
DOI: 10.1039/c9dt03440d
rsc.li/dalton
the treatment of osteoporosis12,13 and possess antiulcer14,15
and tumor growth inhibition properties.16 Development of
1. Introduction
In the recent past, the use of magnetic nanoparticles (MNPs) new methods for obtaining this particular functionality has
in various applications has attracted extensive attention from received extensive attention recently because of its wide-scale
the scientific community. They have found applications in the application in various fields.17,18
fields of technology, catalysis, medical science, environmental
Traditional methods for the synthesis of N-heteroaryl
remediation etc.1,2 In particular, MNP supported catalysts have amides involve the reaction of activated carboxylic acid with
been widely employed as catalysts for various organic amine derivatives.19 There are several limitations associated
reactions.3–5 The ease of recovery of the catalysts by simple with these methods. To overcome those limitations alternative
magnetic separation is the major advantage of these MNPs. synthetic pathways have been developed which include classi-
This can solve the problems associated with isolation and re- cal named reactions such as Staudinger,20–22 Ritter,23 and
usability that are faced in many catalyst promoted Beckmann24,25 and hydrative coupling reactions of alkynes
transformations.1,3–5 Another important feature of the MNP- and azides.26,27 A few other synthetic pathways using alde-
supported catalysts is their high catalytic activity due to their hydes, alcohols, alkynes and amines have also been found in
high surface-to-volume ratio and high degree of stability in the literature for the synthesis of amides.28 Among them,
both aqueous and organic media.6
organic chemists have focused more on the oxidative amida-
The amide bond is the basic structural framework of many tion of aldehydes and amines in the presence of transition
natural polymers, bioactive molecules, proteins, fine chemicals metal catalysts. Cu, Rh, Ru, La, Pd, Ni, Au, Fe etc. are a few
and drugs.7,8 N-Heteroaryl amides in general and N-(pyridin-2- transition metals the complexes of which along with an extra
yl) benzamide in particular are important building blocks for oxidant have been utilized for this amidation reaction.29
many bioactive molecules.9–11 They have wide application in
Maheswari et al. reported a metal free approach for oxi-
dative amidation of aldehydes and alcohols for the synthesis
of N-(pyridin-2-yl)benzamide.30 Li and co-workers initially
reported the reaction of aldehydes and aliphatic amine-
hydrochlorides using CuI as the catalyst and tert-butyl hydro-
peroxide (70% aqueous) as the oxidant in the presence of
AgIO3 and CaCO3.31 Later, a modified version of Li’s work was
Department of Chemistry, Gauhati University, Guwahati 781014, Assam, India.
E-mail: pphukan@yahoo.com, pphukan@gauhati.ac.in
†Electronic supplementary information (ESI) available. CCDC 1941942. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
C9DT03440D
This journal is © The Royal Society of Chemistry 2019
Dalton Trans., 2019, 48, 16041–16052 | 16041

View Article Online
Paper
Dalton Transactions
reported by Chen for the synthesis of primary, secondary and this dispersion, 8 mL of aqueous ammonia solution (25 wt%)
tertiary amides from aldehydes and amine hydrochloride salts and 100 mL of ethanol were added sequentially and sonicated
using copper sulfate and copper oxide as catalysts.32 for another 1 h (during ultrasound irradiation, the tempera-
Thereafter, various other catalytic systems have been reported ture of the bath gradually increases from room temperature to
for this particular transformation, which include the use of 60 °C). Thereafter, 1.0 mL of tetraethyl orthosilicate (TEOS)
heteropolyanion-based ionic liquids under microwave-pro- was added dropwise to this mixture under continuous ultra-
moted conditions,33 CuI/TBHP,34 CuI/air,29 Rh(III)/Ag2CO3,35 sonication. Ultrasonication was continued with the reaction
and Cu(OTf)2/I2-SDS.36 Some of these protocols have some mixture for 5 h, followed by vigorous stirring (using a magnetic
limitations such as use of toxic and expensive metal reagents stirrer) at room temperature for 24 h. The resulting reaction
and oxidants and narrow substrate scope and are usually non- mixture was centrifuged and washed several times with water
recyclable. Therefore, there is a necessity for easy and efficient and ethanol. Finally, it was dried in an oven at 50 °C for 6 h to
catalytic processes for the synthesis of amides.
yield the desired CoFe2O4@SiO2 nanocomposite (yield:
Although several homogeneous Cu-based catalysts have 1.035 g).
been reported, the use of recyclable catalysts for oxidative ami-
In the next step, we modified the surface of the
dation has received scant attention, with a few methods out- CoFe2O4@SiO2 nanocomposite by incorporating a linker with
lined.37 There are only a few reports available in the literature a thiol end functionality. Thus, the thiol-functionalized cobalt
on the use of heterogeneous catalyst for oxidative amidation of ferrite silica composite was prepared by refluxing 250 mg of
aldehydes using 2-aminopyridine. For example, the use of the CoFe2O4@SiO2 nanocomposite with 2 mmol 3-mercapto-
Cu2O/MWCNTs and CuI/MWCNTs as heterogeneous catalysts propyl triethoxysilane (MPTES) (M. Wt: 238.42 g mol−1
,
has been reported by Kathyayini.37a Rostamnia and co-workers density: 0.987 g mL−1) in the ethanol−water mixture in a ratio
reported the use of graphene oxide supported Pd-nano- of 1 : 1 for 5 h. The products were separated by centrifugation,
particles for oxidative amidation of aldehydes in the presence washed 3 times with 20 mL of water, and dried overnight in
of H2O2 as the oxidant.37b Heteropolyanion-based ionic liquids the oven at 60 °C to obtain 410 mg of the thiol functionalized
have been found to be active catalysts under microwave cobalt ferrite silica nanocomposite (CoFe2O4@SiO2-SH).47
irradiation for oxidative amidation reaction in the presence of
The last step of catalyst synthesis involves the binding of
TBHP as the oxidant.33 Literature search revealed that there is the active metal species with the thiol end-group. To accom-
no report on the use of magnetic nanoparticle derived catalysts plish this, a solution of CuI (95 mg, 0.5 mmol) in acetone
for such transformations. Hence, magnetic nanoparticles func- (20 mL) was prepared by ultrasonic irradiation (30 min),
tionalized with different spacer groups and loaded with metals 200 mg of the CoFe2O4@SiO2-SH nanocomposite was added
may be an important substitute.1,38 In continuation of our under continuous ultrasonication and finally the mixture was
work on functionalized magnetic nanoparticles,39–46 we wish kept under ultrasonic irradiation for 30 min. The mixture was
to report a new magnetically recoverable copper iodide catalyst stirred at room temperature for 6 h. The final product (the
supported over thiol functionalized cobalt ferrite nanoparticles CoFe2O4@SiO2-SH-CuI nanocomposite) was separated by cen-
for oxidative amidation of aldehydes and 2-amino pyridines. trifugation, washed with water followed by acetone and dried
During the course of our investigation, we synthesized a new overnight in the oven at 60 °C (yield: 235 mg).
compound N-(pyridin-2-yl) anthracene-9-carboxamide which is
2.2. Characterization
useful for turn on selective fluorescence sensing of the Hg2+
ion in solution.
The synthesized CoFe2O4@SiO2-SH-CuI nanocomposite was
characterized using various analytical techniques. The FT-IR
spectra (using Affinity-1 SHIMADZU) of CoFe2O4@SiO2-SH-CuI
samples were recorded in the form of KBr pellets in the mid
2. Materials and methods
2.1. Preparation of the CoFe2O4@SiO2-3-mercaptopropyl-
IR-region of 4000–500 cm−1 with a resolution of 4 cm−1
.
Powder X-ray diffraction (XRD) patterns were obtained using a
Rigaku X-ray diffractometer with Cu-Kα radiation at a wave-
siloxane CuI composite
The CoFe2O4@SiO2-3-mercaptopropylsiloxane CuI composite length of 1.5418 Å operated at a voltage of 40 kV and a current
catalyst was synthesized following a four-step sequential pro- of 30 mA. FESEM and EDX analyses were performed using a
cedure. Initially the co-precipitation method was followed for ZEISS Sigma 300 scanning electron microscope equipped with
the synthesis of cobalt ferrite under constant ultrasonic an energy dispersive X-ray spectrometer (EDX). For FESEM ana-
irradiation conditions.39,40
lysis, the nanoparticles were affixed uniformly over the alu-
Bare cobalt ferrite MNPs are very unstable and often get minium stub and then mounted in the instrument. TEM ana-
agglomerated. To prevent cobalt ferrite MNPs from agglomera- lyses were performed using a JEOL JEM-2100 electron micro-
tion, a coating of silica using tetraethyl orthosilicate (TEOS) scope. For TEM analysis, the sample was first dispersed in
was applied over the nanoparticles to form
a stable ethanol and a drop of dilute ethanolic dispersion of the par-
CoFe2O4@SiO2 nanocomposite.41 For silica coating, 200 mg of ticles was added on the copper grid and then dried in an oven.
cobalt ferrite MNPs was dispersed in water under ultrasound The thermogravimetric analysis (TGA) of copper incorporated
irradiation (53 kHz) for 1 h using an ultrasonicator bath. To magnetic samples was performed on a Mettler Toledo TGA/
16042 | Dalton Trans., 2019, 48, 16041–16052
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
DSC 1, STAR system analyzer in the temperature range of 2.4. General procedure for UV and fluorescence
300 K–1023 K at a heating rate of 10 K min−1 in a nitrogen gas measurements
atmosphere. The magnetic properties of the functionalized
Various metal salts used in fluorescence titrations were in the
form of sulphates (Cu, Cd, and Mg), chlorides (Fe, Ni, Hg, Al,
and Ca) and nitrates (Ag and Pb). All the metal solutions (Ag+,
Al3+, Ca2+, Cu2+, Fe3+, Hg2+, Mg2+, Cd2+, Ni2+ and Pb2+) and
compound 3l were prepared in acetonitrile and water (1 : 1 v/v).
The stock solutions were prepared with 1 × 10−3 M concen-
tration. All the experiments were performed at room tempera-
ture. UV-vis spectra were recorded in the wavelength range of
200–500 nm and fluorescence was recorded at a fluorescence
excitation wavelength of 320 nm. For fluorescence titration a
solution of the compound in CH3CN–H2O (v/v, 1 : 1) (200 μL)
was taken in a quartz cell and 2 μL of metal solution was added
for each measurement into the quartz cell (0–9.9 × 10−3 M) by
using a micropipette.
CoFe2O4 i.e. CoFe2O4@SiO2-SH-CuI nanoparticles were deter-
mined using a vibrating sample magnetometer (Lakeshore
7410). The samples were analysed at room temperature
by fixing 35 mg of the solid sample to the tip of the vibrating
rod.
The PXRD pattern confirmed the coating of silica over
cobalt ferrite. The surface functionalization over cobalt ferrite
with propyl and thiol moieties was confirmed using IR spec-
troscopy. FESEM and energy dispersive X-ray (EDX) analyses
were performed to determine the morphology and the compo-
sition of various elements present in the nanocomposite. TEM
analysis was carried out to determine the particle size of the
composite. In addition to the use of these characterization
techniques, the thermal stability and magnetic properties of
the nanocomposite were determined using thermogravimetric
analysis (TGA) and vibrating sample magnetometer (VSM)
analysis.
The products obtained from amidation reaction were
characterized using 1H and 13C-NMR spectroscopy (using a
Bruker 300 MHz instrument). For fluorescence sensing experi-
ments, UV-Vis spectroscopy and fluorescence spectroscopy
were carried out on a Shimadzu UV 1800 spectrophotometer
and a Hitachi F-7000 fluorescence spectrophotometer.
Single crystal X-ray diffraction data were collected on a
Bruker SMART APEX II CCD diffractometer equipped with a
graphite monochromator and a Mo Kα fine-focus sealed tube
(λ = 0.71073 Å). The crystallographic cif file of the new com-
pound was deposited with the CCDC 1941942.†
2.5. DFT calculations
The structure of the compound N-(pyridin-2-yl) anthracene-9-
carboxamide was optimized using density functional calcu-
lations with the Gaussian 09 series of programs.48 The B3LYP
formulation49,50 of density functional theory (DFT) was
applied using the LANL2DZ basis set.
3. Results and discussion
3.1. Synthesis of the catalyst and characterization
The synthetic steps for the preparation of the CoFe2O4@SiO2-
SH-CuI magnetic nanocatalyst are shown in Scheme 1. Cobalt
ferrite NPs used as the magnetic core were synthesized by fol-
lowing
a
reported procedure, via the co-precipitation
method.39,40 To impart stability and prevent agglomeration,
the cobalt ferrite magnetic core was coated with TEOS and sub-
sequently surface functionalized with MPTES. Finally, copper
iodide was immobilized onto the thiol functionalized cobalt
ferrite to yield the desired CoFe2O4@SiO2-SH-CuI nano-
composite catalyst. The synthesized catalyst was characterized
using various characterization techniques such as FT-IR, XRD,
FE-SEM, EDX, TEM, TGA and VSM analysis.
2.3. General procedure for the amidation reaction
In a typical reaction, aldehyde (1.2 mmol), 2-amino pyridine
(1 mmol), catalyst (20 wt%), and TBHP (70% aqueous,
2 mmol) were mixed in DMSO (2 mL) in a 10 mL Schlenk tube.
The reaction mixture was tightly capped and subjected to con-
tinuous stirring at 80 °C for 12 h. After completion of the reac-
tion (monitored by TLC), the catalyst was separated using an
external magnet and the reaction mixture was taken in ethyl
acetate. The organic layer was washed using brine solution
and dried over sodium sulfate. The solvent was evaporated
under reduced pressure and the crude product was purified by
column chromatography using 230–400 silica mesh with 15%
ethyl acetate and petroleum ether as the eluent to obtain the
desired product.
Experimental data for the new compound. N-(Pyridin-2-yl)
anthracene-9-carboxamide (3l): Light yellow crystals,
m.p. 117–119 °C; 1H NMR (300 MHz, CDCl3) δ: 9.88 (s, 1H),
8.54 (d, 2H, 6 Hz), 8.13 (d, 2H, 8.1 Hz), 8.03 (d, 8.1 Hz, 2H),
7.71–7.65 (m, 1H), 7.57–7.47 (m, 4H), 7.21 (d, 4.2 Hz, 1H),
6.78–6.74 (m, 1H); 13C NMR (75 MHz, CDCl3) δ: 168.4, 151.4,
147.2, 138.4, 130.9, 128.7, 128.5, 127.9, 127.2, 126.9, 125.6,
124.7, 119.8, 114.3; HRMS (ES) m/z calculated for C20H15N2O
[M + H] 299.1184; found: 299.1188.
Scheme 1 Synthesis of the CoFe2O4@SiO2-SH-CuI nanocomposite.
Dalton Trans., 2019, 48, 16041–16052 | 16043
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
Dalton Transactions
Fig. 3 The SEM image of CoFe2O4@SiO2-SH-CuI MNPs.
Fig. 1 The IR spectra of the CoFe2O4@SiO2-SH-CuI nanocatalyst.
Fig. 1 shows the FT-IR spectrum of the CoFe2O4@SiO2-
SH-CuI nanocomposite. The IR spectrum exhibits a strong
peak at 594 cm−1 and a broad band at around 3300–3500 cm−1
which are due to the stretching vibration of the Fe–O bond
and the O–H bond, respectively. Other peaks at about
1095 cm−1, 1550 cm−1, 2426 cm−1 and 2980 cm−1 correspond
to Si–O–Si stretching, O–H stretching of adsorbed water, and
the S–H and C–H bond stretching of the mercapto and propyl
groups of MPTES respectively.51–53 These data indicate the
presence of the mercapto propyl group on the surface of cobalt
ferrite nanoparticles.
Fig. 4 The EDX image of CoFe2O4@SiO2-SH-CuI MNPs.
Table 1 Elemental composition
Fig. 2 displays the XRD pattern of the CoFe2O4@SiO2-
SH-CuI nanocomposite, which indicates the crystalline nature
of the synthesized composite. Peaks at 2-theta 25.6°, 50.1°,
52.5°,67.5°, 69.5° and 77.3° represent the characteristic peaks Element
Weight%
Atomic%
of cubic CuI and other peaks at 2 theta 29.6°, 35.75°, 42.3°
C K
25.13
28.31
15.47
1.65
12.75
7.13
43.17
36.50
11.36
1.06
2.07
2.63
and 61.3° resemble the characteristics peaks of cobalt ferrite
nanoparticles. This indicates the successful incorporation of
CuI over cobalt ferrite.54 The average particle size of
CoFe2O4@SiO2-SH-CuI was found to be 49.10 nm, based on
the Debye−Scherrer formula.
The structure and morphology of the synthesized nano-
composite were studied by SEM analysis. The SEM image of
the sample is presented in Fig. 3. The SEM image reveals that
the particles are of uniform size with spherical morphology.
Fig. 4 shows the EDX (Energy Dispersive X-ray) spectrum of the
O K
Si K
S K
I L
Fe K
Co K
Cu K
3.64
5.93
1.28
1.92
CoFe2O4@SiO2-SH-CuI nanocomposite. Table 1 shows the dis-
tribution of various elements obtained from EDX analysis of
the synthesized product. From the table, the iron/cobalt ratio
in the nanoparticles determined by EDX was found to be
2.05% which is very close to the atomic ratio in the formula
CoFe2O4 with 1.92 atomic% of copper incorporation.
The presence of various elements in the nanocomposite
determined by FESEM-EDX was further confirmed using EDX
elemental mapping. Fig. 5 shows the elemental mapping and
the results confirm the presence of C, O, Si, S, Fe, Co, Cu and I
in the CoFe2O4@SiO2-SH-CuI nanocomposite.
Fig. 6 displays the TEM images of the synthesized CoFe2O4
and CoFe2O4@SiO2-SH-CuI nanocomposite. TEM images
reveal the uniform coating of silica and the spacer group
(thiol) over the cobalt ferrite MNPs. Thus, uniform coating of
the thiol group is accountable for the effective binding of
copper iodide nanoparticles. The images indicate that the par-
Fig. 2 The XRD pattern of CoFe2O4@SiO2-SH-CuI MNPs.
16044 | Dalton Trans., 2019, 48, 16041–16052
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
Fig. 6 TEM images of (a) cobalt ferrite and (b) CoFe2O4@SiO2-SH-CuI
MNPs. (c) The HRTEM image of CoFe2O4@SiO2-SH-CuI MNPs. (d) The
SAED pattern of CoFe2O4@SiO2-SH-CuI MNPs.
Fig. 5 Energy dispersive X-Ray (EDX) mapping analysis of
CoFe2O4@SiO2-SH-CuI MNPs.
ticles were nearly spherical in shape and most of the
particles are in the range of 15–20 nm. Fig. 6(c) shows the
HRTEM image of CoFe2O4@SiO2-SH-CuI MNPs. From the
HRTEM image, the interplanar distance was found to be
0.31 nm which corresponds to the spinel phase of the polycrys-
talline nanoparticles. Various concentric rings in the SAED
pattern indicate the crystalline nature of the synthesized
composite.
Fig. 7 TGA plots of (a) CoFe2O4; (b) CoFe2O4@SiO2; (c) CoFe2O4@SiO2-
SH; and (d) CoFe2O4@SiO2-SH-CuI MNPs.
The target CoFe2O4@SiO2-SH-CuI nanocomposite was syn-
thesized in four consecutive steps and in each step, the nano- cobalt ferrite. A further weight loss of 25.13% was observed in
composite was isolated before proceeding to the next step. the last step above 450 °C for the CoFe2O4@SiO2-SH-CuI
Patterns resulting from thermogravimetric analysis (TGA) of nanocomposite which is attributed to the crystallization
each intermediate nanocomposite i.e.,
CoFe2O4, process.51,52
CoFe2O4@SiO2 and CoFe2O4@SiO2-SH along with the CuI Fig.
8
shows the DTA curve of the synthesized
incorporated composite catalyst are presented in Fig. 7. The CoFe2O4@SiO2-SH-CuI nanocomposite. From the DTA curve
TGA pattern shows that nanoparticles undergo an initial weight three distinct exothermic and one endothermic peaks are
loss within 100 °C in all the four steps, which corresponds to observed. The endothermic peak at around 250 °C can be
the loss of moisture and volatile components in the sample. ascribed to the loss of water. The exothermic peaks at 340 °C,
With the subsequent modification of the surface of cobalt 400 °C and 450 °C may be due to the rearrangement of the
ferrite, the weight loss percentage increases due to the crystal structure or the degradation of CoFe2O4@SiO2-SH-CuI
decomposition of the various organic functional groups of MNPs. Thus, the nanocomposite is thermally stable below
TEOS and MPTES that were functionalized over the surface of 340 °C.55
This journal is © The Royal Society of Chemistry 2019
Dalton Trans., 2019, 48, 16041–16052 | 16045

View Article Online
Paper
Dalton Transactions
Magnetic property measurements of the synthesized
CoFe2O4@SiO2-SH-CuI nanocomposite were performed using
the VSM measurement at room temperature in the range of
1.5 T. Fig. 9 shows the M–H loop obtained at the maximum
applied field. The magnetization curve of the CoFe2O4@SiO2-
SH-CuI nanocomposite ascertains the ferromagnetic behaviour
with coercivity (HC) and saturation magnetization (MS) values
of 222 Oe and 1.9324 emu g−1 respectively. Magnetization
retention (Mr) of the nanomaterial was found to be 0.24875
emu g−1
.
Thus, the above analysis of the data clearly demonstrates
that CuI species has been successfully loaded over the surface
of thiol functionalized cobalt ferrite. These results explain that
the synthesized material is magnetic and crystalline in nature
with the size in the range 15–20 nm and the particles are ther-
mally stable up to 300 °C.
Fig. 8 The DTA curve of CoFe2O4@SiO2-SH-CuI MNPs.
3.2. Catalytic activity studies
The catalytic activity of the CoFe2O4@SiO2-SH-CuI magnetic
nanoparticles was tested for the oxidative amidation reaction
of benzaldehyde and 2-amino pyridine in the presence of an
oxidant. In order to find the optimum conditions, the initial
study was carried out by taking benzaldehyde and 2-amino pyr-
idine as model substrates. The results are summarized in
Table 2. The initial experiment using an equimolar amount of
substrates (1 mmol each) in DMSO (2 mL) in the presence of
20 wt% of the catalyst did not yield any positive result after car-
rying out the reaction for 12 h at 100 °C (entry 1). However, the
addition of 1 mmol tert-butyl hydroperoxide (TBHP, 70%
aqueous) to the previous reaction mixture resulted in the for-
mation of the desired N-(pyridin-2-yl)benzamide in 47% yield.
The column chromatographic technique was used for the
isolation of the desired product. In order to improve the yield,
Fig. 9 The M–H loop in the VSM measurement of CoFe2O4@SiO2-
SH-CuI MNPs.
Table 2 Optimization of reaction conditions
Entry
Solvent
Catalyst (wt%)
Aldehyde (mmol)
Oxidant (mmol)
Time (h)
Temp. (°C)
Yield (%)
1
2
3
4
5
7
9
10
11
12
13
14
15
16
17
18
DMSO
DMSO
DMSO
DMSO
DMF
Dioxane
MeCN
EtOAc
Toluene
Hexane
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
20
20
20
20
20
20
20
20
20
20
20
20
20
10
1
1
1
No oxidant
TBHP (1)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
H2O2 (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
12
12
12
12
12
12
12
12
12
12
12
6
100
100
100
100
100
100
85
80
80
70
100
100
80
80
100
100
NR
47
69
89
63
54
68
17
28
Trace
72
63
87
58
NR
NR
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
12
12
24
24
No catalyst
CoFe2O4 (20)
Reaction conditions: 2-Amino pyridine (1 mmol), solvent (2 mL), oxidant, isolated yield.
16046 | Dalton Trans., 2019, 48, 16041–16052
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
Table 3 Substrate scope for oxidative amidation reaction
a series of experiments were performed by changing the reac-
tion conditions. Further reaction with the increase in the con-
centration of TBHP to 2 mmol could improve the reaction
yield to 69%. The reaction performed with an equimolar
amount of reactants yielded the desired product in a compara-
tively lower amount as the aldehyde source was totally con-
sumed and the unreacted amine was left behind in the reac-
tion. So the amount of aldehyde was increased to 1.2 mmol,
which could improve the yield of the reaction to 89%.
Thereafter, the reaction was examined using different solvents
such as DMF, dioxane, acetonitrile, ethyl acetate, toluene and
n-hexane. However, results were not found to be encouraging.
Hydrogen peroxide was found to be inferior to TBHP for
this particular reaction (Table 2, entry 11). Lowering the temp-
erature or the catalyst amount produced diminishing results.
Moreover, a blank reaction under the optimized conditions
without the catalyst was found to be ineffective. In order to
examine the effect of CuI on the catalytic activity of the
CoFe2O4@SiO2-SH-CuI MNPs, the reaction was performed
using cobalt ferrite CoFe2O4 as the catalyst (Table 2, entry 15).
The reaction did not proceed at all even after continuing the
reaction for 24 h.
With the optimized reaction conditions in hand, the scope
and generality of this oxidative amidation were explored using
various aryl aldehydes. The results are described in Table 3.
From the table it can be seen that aryl aldehydes bearing func-
tional groups such as –Me, –Br, –Cl, –NO2 and –OMe on the
aromatic ring could be transformed into the corresponding N-
(pyridin-2-yl) benzamide in moderate to high yield. Substrates
with electron donating groups show higher reactivity compared
to those with electron withdrawing groups. Moreover, the pres-
ence of a substituent at the para-position produces higher yields.
For example, the reaction produces higher yield in the case of
4-bromo benzaldehyde (Table 3, entry 3f) than that resulting
from 2-bromo benzaldehyde (Table 3, entry 3g). This may be due
to steric hindrance caused by the bromo-substituent at the ortho-
position. Other aldehydes such as anthracene carboxaldehyde
and furfural produced the product in moderate yield.
Reaction conditions: 2-Amino pyridine (1 mmol), aldehyde
(1.2 mmol), catalyst 20 wt%, DMSO (2 mL), 70% aq. TBHP (2 mmol).
These experiments revealed that the catalytic activity of the
nanocomposite significantly decreases after the first cycle of
reuse (Fig. 11), which is due to decrease of Cu-content in the
reused catalyst. Hence the catalyst is not suitable for repetitive
reuse as the third run provides significantly lower yields.
3.3. Catalyst recycling test
The recyclability of the CoFe2O4@SiO2-SH-CuI magnetic nano-
catalyst was also investigated by carrying out repeated runs of
the used material. The reusability test was performed for the
reaction of benzaldehyde with 2-aminopyridine under the opti-
mized conditions. Upon completion of the reaction, the cata-
4. Fluorescence sensing properties
lyst was separated from the reaction mixture via magnetic After having success in synthesizing a variety of N-(pyridin-2-
decantation and washed with excess of ethyl acetate to remove yl) aryl-amides, we also examined the optical properties of a
the traces of the previous reaction mixture. It was then dried in newly synthesized compound (Table 3, entry 3l), namely N-
an oven at 60 °C for 6 h and kept in a vacuum desiccator for (pyridin-2-yl) anthracene-9-carboxamide (L). The crystal struc-
further use. A comparative analysis of SEM images and EDX ture of compound L (CCDC 1941942†) is shown in Fig. 12. The
spectra of both fresh and reused catalysts was performed. single crystals were grown using the pure product from
Fig. 10 shows the SEM image and EDX analysis of the reused aqueous acetonitrile. The collected X-ray data were refined in
catalyst. The SEM image of the reused catalyst indicates no sig- the P21/c monoclinic space group and the ORTEP diagram
nificant deformation of the catalyst after reuse. However, a indicated that the asymmetric unit contains four molecules.
slight decrease in Cu concentration was observed in the case Compound L shows strong absorption in the UV region. Upon
of the reused catalyst (Table 4).
excitation at 320 nm, the compounds showed fluorescence in
This journal is © The Royal Society of Chemistry 2019
Dalton Trans., 2019, 48, 16041–16052 | 16047

View Article Online
Paper
Dalton Transactions
Fig. 11 Reusability of the catalyst.
Fig. 10 The EDX spectra and the SEM image of the reused
CoFe2O4@SiO2-SH-CuI catalyst.
Table 4 Elemental composition of the reused catalyst
Element
Weight%
Atomic%
Fig. 12 The ORTEP diagram of N-(pyridin-2-yl) anthracene-9-carboxa-
C K
O K
Si K
S K
18.37
34.70
22.85
4.66
30.73
43.58
16.35
2.92
mide (L) with 50% probability ellipsoids.
I L
1.36
0.21
Fe K
Co K
Cu K
9.09
4.17
4.80
3.27
1.42
1.52
the range of 350–450 nm. Compound L (Table 3, entry 3l)
exhibited selective fluorescence turn on response to a wide
variety of competing metal ions.
Fig. 13 shows the UV-vis spectra of compound L and HgCl2
solution and the effect of the ligand on different Hg2+ metal
ion concentrations. The spectra were recorded in the
CH3CN : H2O mixture (1 : 1). Ligand L shows absorption at
380 nm, 361 nm, 345 nm, 275 nm and 244 nm respectively.
The Hg salt shows absorption maxima at 230 nm. Upon
gradual addition of Hg2+ to L, the intense peak at 244 nm
shifted to 238 nm and the shoulder peak at 275 nm dis-
appeared. In addition to this, the intensity of peaks at 380 nm,
361 nm and 345 nm diminished. Thus, the interaction of L
and Hg2+ was confirmed by UV-vis spectroscopy (Fig. 13). The
spectra show an isosbestic point at 310 nm and upon exci-
tation at 320 nm in a fluorescence spectrophotometer the
ligand shows weak fluorescence emission in the range of
345 nm to 485 nm with maximum emission at 375 nm.56
Literature search reveals various turn on fluorescent probes
developed for sensing and detection of the Hg2+ ion.57–59 It
Fig. 13 UV spectra of the ligand at different amounts of Hg2+ ion
(0–9.9 × 10–3 M) in the CH3CN–H2O mixture (1 : 1).
has been observed that upon addition of the Hg2+ ion, the
fluorescence intensity of the solution increases significantly.
The fluorescence sensing properties of compound
L
towards different metal ions were investigated in a mixture of
MeCN and H2O in a 1 : 1 (v/v) ratio (200 μL). The study was
carried out by the addition of different metal ions such as Ag+,
16048 | Dalton Trans., 2019, 48, 16041–16052
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
Al3+, Hg2+, Mg2+, Fe2+, Ca2+, Cu2+, Ni2+, Cd2+ and Pb2+ to the
solution of L. The initial study was performed by adding
AgNO3 solution to the solution of L, which induced an enhance-
ment of the fluorescence intensity. The experiment was carried
out by adding 2 μL of metal solution for each measurement into
the quartz cell (0–9.9 × 10−3 M) by using a micropipette.
Fluorescence emission showed an increasing trend with addition
of metal salt solution and finally reached a saturation value.
Thereafter, the spectra exhibit a decreasing trend till it reaches a
steady value. This can be observed by plotting F/Fo versus various
metal ions (where Fo is the fluorescence of L in the absence of
the metal ion and F = Fm − Fo (Fm = fluorescence of L in the pres-
ence of the metal ion). The results are depicted in Fig. 14. It has
been observed that the F/Fo value of L in the presence of Hg2+ is
4.24, however other metal ions show a comparatively lower value
under the present experimental conditions.
Fig. 16 A plot of complexation of compound L with Hg2+
.
concentration of Hg2+ ions, respectively. The plot gives a
straight line with a slope of 1.51 and an intercept of 2.77
which indicates the binding of one Hg2+ ion per two ligands
with log β = 2.77.
Thereafter, fluorescence emission of L was tested with
different concentrations of the Hg2+ metal ion (Fig. 15). The
fluorescence emission of the compound showed an increasing
trend with stepwise addition of metal ions to L. The fluo-
rescence intensity of L increased linearly up to 2 : 1 mole ratio
and then the intensity remained constant. This indicated that
the ligand and the metal ion form a 2 : 1 complex. The for-
mation of a 2:1 complex between the ligand and Hg2+ was
further established by using the Hill Plot (Fig. 16).
4.1. DFT calculations
The optimized structure of compound L (ligand) generated
using the DFT method and the Gaussian 09 series of programs
is presented in Fig. 17. Similarly, Fig. 18 presents the opti-
mized structure of the complex with Hg2+. For all cases, the
B3LYP formulation49,50 of density functional theory was used
employing the LANL2DZ basis set. The possibility of complex
formation due to interaction of the ligand and Hg2+ was ascer-
tained by calculating the vibrational frequency of the complex.
Theoretical calculations reveal that two molecules of L take
part in complex formation with one Hg2+ ion, through their
amide nitrogen atoms (Fig. 18).
The plot of log{(Fmax − F)/(F − Fo)} vs. log[Hg2+] is shown in
Fig. 16, where Fo, F and Fmax are fluorescence intensities of the
compound at zero, at an intermediate stage and at the infinite
Fig. 19 shows the shapes of the HOMO and LUMO of L and
the Hg2+L2 complex. From the DFT optimized HOMO–LUMO
Fig. 14 F/F0 response of L towards different metal ions in 1 : 1 (v/v)
CH3CN–H2O.
Fig. 15 The fluorescence spectra of compound L at different amounts
of Hg2+ ion (0–9.9 × 10−3 M) in a mixed solution of CH3CN–H2O in a
1 : 1 ratio.
Fig. 17 (a) The chemical structure of L and (b) the DFT optimized struc-
ture of N-(pyridin-2-yl)anthracene-9-carboxamide (right).
This journal is © The Royal Society of Chemistry 2019
Dalton Trans., 2019, 48, 16041–16052 | 16049

View Article Online
Paper
Dalton Transactions
surface of a thiol functionalized magnetic nanoparticle. The
newly developed catalyst has been utilized for the synthesis of
N-(pyridin-2-yl) benzamide from an aromatic aldehyde and
2-aminopyridine in the presence of 70% aqueous TBHP as an
oxidant. The coupling reaction could yield the desired product
in moderate to excellent yields. Due to the magnetic properties
of the nanocatalyst, it could be recovered with the aid of an
external magnet. Furthermore, we have synthesized a new
derivative viz. N-(Pyridin-2-yl)anthracene-9-carboxamide, which
behaves as a fluorescent probe for the detection of the Hg2+
ion in aqueous solution.
Conflicts of interest
Fig. 18 The DFT optimized structure of the Hg2+L2 complex. N atoms
(blue balls) and oxygen atoms (red balls).
There are no conflicts to declare.
Acknowledgements
Financial support from DST, India (Grant No. SR/NM/NS-18/
2011(G)) is gratefully acknowledged. MD thanks UGC (Award
No: F./2016-17/NFO-2015-17-OBC-ASS-34889) for a research fel-
lowship under the NFOBC scheme. We thank Dr Ranjit
Thakuria and SAIF, GU for single crystal X-ray analysis. We
also acknowledge the support from SAIF-NEHU and CIF-IITG
for the analytical facilities during the course of investigations.
Notes and references
1 J. Govan and Y. K. Gun’ko, Nanomaterials, 2014, 4, 222.
2 M. Al-Harahsheh, M. AlJarrah, M. Mayyas and M. Alrebaki,
J. Taiwan Inst. Chem. Eng., 2018, 86, 148.
3 (a) R. K. Sharma, S. Dutta and S. Sharma, Dalton Trans.,
2015, 44, 1303; (b) F.-C. Zheng, Q.-W. Chen, L. Hu, N. Yan
and X.-K. Kong, Dalton Trans., 2014, 43, 1220; (c) Y. Li,
Z. Zhang, J. Shen and M. Ye, Dalton Trans., 2015, 44, 16592;
(d) H. Naeimi and S. Mohamadabadi, Dalton Trans., 2014,
43, 12967.
4 (a) R. Hudson, Y. Feng, R. S. Varma and A. Moores, Green
Chem., 2014, 16, 4493; (b) S. Payra, A. Saha and S. Banerjee,
J. Nanosci. Nanotechnol., 2017, 17, 4432; (c) L. L. Chng,
N. Erathodiyil and J. Y. Ying, Acc. Chem. Res., 2013, 46,
1825.
5 (a) A. R. Sardarian, M. Kazemnejadi and M. Esmaeilpour,
Dalton Trans., 2019, 48, 3132; (b) A. Kasprzak,
M. Bystrzejewski and M. Poplawska, Dalton Trans., 2018,
47, 6314; (c) A. M. Munshi, M. Shi, S. P. Thomas,
M. Saunders, M. A. Spackman, K. S. Iyer and N. M. Smith,
Dalton Trans., 2017, 46, 5133.
Fig. 19 The HOMO and LUMO of L and Hg2+L2.
orbitals it is clear that in the Hg2+L2 complex, the electron
density of the metal atom participates in the formation of the
LUMO of the complex.
From the DFT optimized HOMO–LUMO orbitals of the
Hg2+L2 complex, the energy gap between these orbitals was
found to be positive i.e. ΔE = ELUMO–EHOMO = 0.119 eV. This
value indicates the feasibility of complex formation between
the ligand and Hg2+.
6 R. B. N. Baig and R. S. Varma, Chem. Commun., 2013, 49,
752.
7 J. M. Humphrey and A. R. Chamberlin, Chem. Rev., 1997,
97, 2243.
5. Conclusion
In conclusion, a hybrid magnetic nanocatalyst CoFe2O4@SiO2-
SH-CuI has been prepared by incorporating CuI onto the
8 S. D. Roughley and A. M. Jordan, J. Med. Chem., 2011, 54,
3451.
16050 | Dalton Trans., 2019, 48, 16041–16052
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
9 L. Ferrins, M. Gazdik, R. Rahmani, S. Varghese, 35 W. Dai, Y. Liu, T. Tong, X. Li and F. Luo, Chin. J. Catal.,
M. L. Sykes, A. J. Jones, V. M. Avery, K. L. White, E. Ryan, 2014, 35, 1012.
S. A. Charman, M. Kaiser, C. A. S. Bergströmand and 36 O. P. S. Patel, D. Anand, R. K. Maurya and P. P. Yadav,
J. B. Baell, J. Med. Chem., 2014, 57, 6393. Green Chem., 2015, 17, 3728.
10 C. Gao, B. J. Margolis, J. M. Strelow, L. R. Vidlerand and 37 (a) H. N. Hareesh, K. U. Minchitha, N. Nagaraju and
M. M. Mader, ACS Med. Chem. Lett., 2016, 7, 156.
11 S. C. Zimmermann, E. F. Wolf, A. Luu, A. G. Thomas,
M. Stathis, B. Poore, C. Nguyen, A. Le, C. Rojas,
B. S. Slusher and T. Tsukamoto, ACS Med. Chem. Lett.,
2016, 7, 520.
N. Kathyayini, Chin. J. Catal., 2015, 36, 1825;
(b) S. Rostamnia, E. Doustkhah, H. Golchin-Hosseini,
B. Zeynizadeh, H. Xin and R. Luque, Catal. Sci. Technol.,
2016, 6, 4124.
38 N. A. Frey, S. Peng, K. Cheng and S. Sun, Chem. Soc. Rev.,
2009, 38, 2532.
12 C. Petrie, M. W. Orme, N. Baindur, K. G. Robbins,
S. M. Harris, M. Kontoyianni, L. H. Hurley, S. M. Kerwin 39 C. Borgohain, K. K. Senapati, D. Mishra, K. C. Sarma and
and G. R. Mundy, WO 9715308, 1997. P. Phukan, Nanoscale, 2010, 2, 2250.
13 M. W. Orme, N. Baindur and K. G. Robbins, WO 9817267, 40 K. K. Senapati, S. Roy, C. Borgohain and P. Phukan, J. Mol.
1998.
Catal. A: Chem., 2012, 352, 128.
14 A. Robert and L. L. Skaletzky, US Pat, 3418325, 1968.
15 R. B. Moffett, A. Robert and L. L. Skaletzky, J. Med. Chem.,
1971, 14, 963.
41 M. M. Dutta, K. K. Rajbongshi and P. Phukan, Synth.
Commun., 2017, 47, 2330–2341.
42 K. K. Senapati, C. Borgohain, K. C. Sarma and P. Phukan,
J. Mol. Catal. A: Chem., 2011, 346, 111.
16 H. E. Colley, M. Muthana, S. J. Danson, L. V. Jackson,
M. L. Brett, J. Harrison, S. F. Coole, D. P. Mason, 43 K. K. Senapati, C. Borgohain and P. Phukan, Catal. Sci.
L. R. Jennings, M. Wong, V. Tulasi, D. Norman, Technol., 2012, 2, 2361.
P. M. Lockey, L. Williams, A. G. Dossetter, E. J. Griffen and 44 S. Roy, K. K. Senapati and P. Phukan, Res. Chem. Intermed.,
M. J. Thompson, J. Med. Chem., 2015, 58, 9309.
17 V. R. Pattabiraman and J. W. Bode, Nature, 2011, 480, 471.
18 T. Cupido, J. Tulla-Puche, J. Spengler and F. Albericio, Curr.
Opin. Drug Discovery Dev., 2007, 10, 768.
2015, 41, 5753.
45 C. Borgohain, D. Mishra, K. C. Sarma and P. Phukan,
J. Appl. Phys., 2012, 112, 113905.
46 C. Borgohain, K. Acharyya, S. Sarma, K. K. Senapati,
K. C. Sarma and P. Phukan, J. Mater. Sci., 2013, 48, 162.
19 E. Valeur and M. Bradley, Chem. Soc. Rev., 2009, 38, 606.
20 F. Damkaci and P. DeShong, J. Am. Chem. Soc., 2003, 125, 4408. 47 C. S. Gill, B. A. Price and C. W. Jones, J. Catal., 2007, 251,
21 E. Saxon and C. R. Bertozzi, Science, 2000, 287, 2007. 145.
22 Y. G. Gololobov and L. F. Kasukhin, Tetrahedron, 1992, 48, 48 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
1353.
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers,
K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi,
J. Normand, K. Raghavachari, A. Rendell, J. C. Burant,
S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam,
M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski,
R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth,
P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels,
O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and
D. J. Fox, GAUSSIAN 09 (Revision B.01), Gaussian Inc.,
Wallingford, CT.
23 J. J. Ritter and P. P. Minieri, J. Am. Chem. Soc., 1948, 70,
4045.
24 N. A. Owston, A. J. Parker and J. M. J. Williams, Org. Lett.,
2007, 9, 73.
25 M. Hashimoto, Y. Obora, S. Sakaguchi and Y. Ishii, J. Org.
Chem., 2008, 73, 2894.
26 S. H. Cho, E. J. Yoo, I. Bae and S. Chang, J. Am. Chem. Soc.,
2005, 127, 16046.
27 M. P. Cassidy, J. Raushel and V. V. Fokin, Angew. Chem., Int.
Ed., 2006, 45, 3154.
28 D.-Q. Dong, S.-H. Hao, H. Zhang and Z.-L. Wang, Chin.
Chem. Lett., 2017, 28, 1597.
29 S. Yang, H. Yan, X. Ren, X. Shi, J. Li, Y. Wang and
G. Huang, Tetrahedron, 2013, 69, 6431.
30 E. S. Devi, A. Alanthadka, A. Tamilselvi, S. Nagarajan,
V. Sridharana and C. U. Maheswari, Org. Biomol. Chem.,
2016, 14, 8228.
31 W.-J. Yoo and C.-J. Li, J. Am. Chem. Soc., 2006, 128, 13064.
49 A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
32 S. C. Ghosh, J. S. Y. Ngiam, A. M. Seayad, D. T. Tuan, 50 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens.
C. L. L. Chai and A. Chen, J. Org. Chem., 2012, 77, 8007. Matter Mater. Phys., 1988, 37, 785.
33 R. Fu, Y. Yang, W. Jin, H. Gu, X. Zeng, W. Chai, Y. Ma, 51 (a) J. Safari and Z. Zarnegar, Monatsh. Chem., 2013, 144,
Q. Wang, J. Yi and R. Yuan, RSC Adv., 2016, 6, 107699.
34 Y. Ding, X. Zhang, D. Zhang, Y. Chen, Z. Wu, P. Wang,
W. Xue, B. Song and S. Yang, Tetrahedron Lett., 2015, 56,
831.
1389; (b) W. Gan, L. Gao, X. Zhan and J. Li, RSC Adv., 2016,
6, 37600; (c) A. Loukanov, P. Mladenova, H. Udono,
Z. Miskolczy, A. Angelov, L. Biczók and S. Nakabayashi,
J. Chem. Technol. Metall., 2018, 53, 473; (d) J. Sherwood,
This journal is © The Royal Society of Chemistry 2019
Dalton Trans., 2019, 48, 16041–16052 | 16051

View Article Online
Paper
Dalton Transactions
Y. Xu, K. Lovas, Y. Qin and Y. Bao, J. Magn. Magn. Mater., 56 (a) J. Kumar, M. J. Sarma, P. Phukan and D. K. Das,
2017, 427, 220.
J. Fluoresc., 2015, 25, 1431; (b) J. Kumar, M. J. Sarma,
P. Phukan and D. K. Das, Dalton Trans., 2015, 44, 4576.
52 M. M. Dutta and P. Phukan, Catal. Commun., 2018, 109, 38.
53 R. A. Bohara, N. D. Thorat, H. M. Yadav and S. H. Pawar, 57 K. D. Bhatt, D. J. Vyas, B. A. Makwana, S. M. Darjee,
New J. Chem., 2014, 38, 2979. V. K. Jain and H. Shah, Chin. Chem. Lett., 2016, 27, 731.
54 Z. Ghasemi, S. Shojaei and A. Shahrisa, RSC Adv., 2016, 6, 58 J. Wang, W. Li and L. Long, Sens. Actuators, B, 2017, 245,
56213. 462.
55 L. Ajroudi, N. Mliki, L. Bessais, V. Madigou, S. Villain and 59 X. Tang, J. Han, Y. Wang, X. Bao, L. Ni, L. Wang and L. Li,
C. Leroux, Mater. Res. Bull., 2014, 59, 49. Spectrochim. Acta, Part A, 2017, 184, 177.
16052 | Dalton Trans., 2019, 48, 16041–16052
This journal is © The Royal Society of Chemistry 2019