ZnCl2 supported on Fe3O4@SiO2 core–shell nanocatalyst for the synthesis of quinolines via Friedl?nder synthesis under solvent-free condition

Article abstract of DOI:10.1002/aoc.3566

A magnetic nanocatalyst of Fe₃O₄@SiO₂/ZnCl₂ was prepared by supporting ZnCl₂ on silica-coated magnetic nanoparticles of Fe₃O₄. This recoverable catalyst was used for the synthesi

Full text of DOI:10.1002/aoc.3566

Received: 25 May 2016
DOI 10.1002/aoc.3566
Revised: 26 June 2016
Accepted: 1 July 2016
F U L L PA P E R
ZnCl2 supported on Fe3O4@SiO2 core–shell nanocatalyst for the
synthesis of quinolines via Friedländer synthesis under solvent‐free
condition
*
Ebrahim Soleimani | Mahbobeh Naderi Namivandi | Heshmatollah Sepahvand
Department of Chemistry, Razi University,
A magnetic nanocatalyst of Fe3O4@SiO2/ZnCl2 was prepared by supporting ZnCl2
on silica‐coated magnetic nanoparticles of Fe3O4. This recoverable catalyst was
used for the synthesis of quinolines via Friedländer synthesis from 2‐aminoaryl
ketones and α‐methylene ketones under solvent‐free condition. The prepared cata-
lyst was characterized by FT‐IR, TEM, SEM, XRD, EDX, ICP‐OES, VSM and
BET. It was found that Fe3O4@SiO2/ZnCl2 showed higher catalytic activity than
homogenous ZnCl2, and could be reused several times without significant loss of
activity.
Kermanshah 67149‐67346, Iran
Correspondence
Ebrahim Soleimani, Department of Chemistry,
Razi University, Kermanshah 67149‐67346, Iran.
E‐mail: e_soleimanirazi@yahoo.com
KEYWORDS
magnetite nanocatalyst, ZnCl2, quinolines, Friedländer
1
|
INTRODUCTION
syntheses.[5] It has been reported that the heterogeneous cat-
alysts supported on MNPs reveal excellent performance in
In recent years, much attention has been focused on heteroge-
neous catalysts due to their potential applications for replac-
ing homogenous catalysts in organic chemistry and
industry; they exhibit advantages of easy separation of the
catalyst from the reaction medium, minimal corrosion,
simplify recovery, reusability, green chemical processes,
and enhanced product selectivity.[1] Among heterogeneous
catalysts, supported catalysts on a solid support has attracted
significant attention due to a number of advantages as their
available active sites, stability, and product separation and
their recovery, which are all important factors in organic
chemistry and industry.[2] Although supported catalysts are
available on different supports such as charcoal, alumina, sil-
ica and polymers, since silica displays many advantages
properties such as excellent stability (chemical as well as
thermal), no swelling, high surface area, good accessibility
and in addition organic groups can be robustly anchored to
the surface to provide catalytic center.[3]
many reactions e.g., hydrolysis, hydrogenation, oxidation,
carbon–carbon coupling and reduction.[6]
The use of ZnCl2 has received considerable attention as
an inexpensive, nontoxic, commercially available catalyst
for various organic transformations. Due to the numerous
advantages associated with this eco‐friendly compound,
ZnCl2 has been explored as a powerful Lewis acid catalyst
for different reactions.[7] However, in spite of their potential
utility, this homogeneous catalyst present limitations due to
the use of toxic and corrosive reagents and the tedious
work‐up procedure that thus making the process economi-
cally and environmentally undesirable. Therefore, in this
research, ZnCl2 was immobilized on the surface of
Fe3O4@SiO2 core–shell nanoparticles, and then used a mag-
netic heterogeneous nanocatalyst for the synthesis of quino-
lines via Friedlander annulation reaction under solvent‐free
conditions. Advantages and novelty of this new catalyst is
easy separation of from reaction medium by applying exter-
nal magnetic field and reused several times.
Magnetic silica nanoparticles have recently received sig-
nificant attention due to their chemical inertness, magnetic
properties, no toxicity, and excellent thermal stability.[4]
These favorable properties allow nanoparticles (NPs) to be
widely used as a catalyst or supported catalyst for organic
It is well known that quinolines exhibit a wide range of
biological activities,[8–10] and are valuable reagents for the
synthesis of nano‐ and mesostructures with enhanced elec-
tronic and photonic properties.[11] The classic version of the
Appl. Organometal. Chem. 2016; 1–8
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
E. SOLEIMANI ET AL.
Friedländer quinoline synthesis, which combines a 2‐
aminoaryl ketone and a carbonyl compound containing an
activated α‐CH acid under acidic or basic conditions by
refluxing in an aqueous or alcoholic solution to give quino-
line, has been extensively explored.[12] In a typical procedure,
the catalyst is varied from strong protic inorganic liquid
acids,[13] such as HCl, H2SO4, and polyphosphoric acid, to
Lewis acids,[14] such as NaAuCl6, and AuCl3.3H2O, and
transition‐metals,[15] such as ruthenium and palladium. More-
over, the synthesis of these heterocycles has been usually car-
ried out in a polar solvent such as THF, DMF, or DMSO
leading to complex isolation and recovery procedures. There-
fore, the discovery of a novel and inexpensive catalyst, which
can be easily separated and reused, without undesirable con-
tamination, is of prime importance. Therefore, we have pre-
pared a magnetic heterogeneous catalyst of Fe3O4@SiO2/
ZnCl2by supporting ZnCl2 on silica‐coated magnetic
nanoparticles of Fe3O4.
functional groups. Eventually, the nanocatalysts were charac-
terized by different techniques such as XRD, TEM, SEM,
EDX, VSM, BET and ICP‐OES.
The crystalline structure of magnetite nanoparticles was
identified with XRD technique. Figure 1 shows the XRD pat-
terns for Fe3O4 MNPs, Fe3O4@SiO2 MNPs and
Fe3O4@SiO2/ZnCl2 MNPs. The XRD pattern of Fe3O4
(Figure 1a) clearly showed six reflection peaks (2θ) at 30°,
35.5°, 43°, 53.8°, 56.9° and 62.5° refer to (220), (311),
(400), (422), (511) and (440), confirmed the formation of a
cubic spinel ferrite structure, and the crystallite size was cal-
culated from the Scherrer's formula, found to be an average
diameter of about 20 nm (Figure 1a). Furthermore, no change
observed in the crystalline structure of the Fe3O4 core upon
coating and immobilization of the magnetite surface by
TEOS (Figure 1b) and ZnCl2 (Figure 1c). After immobiliza-
tion of ZnCl2 onto the surface of magnetic nanoparticles, new
diffraction peaks were appeared at 21.1 °, 27.4 °, 46.0 ° and
53.5 ° attributed to the ZnCl2.[19] Furthermore, no change
observed in the crystalline structure of the ZnCl2 after immo-
bilization on surface of Silica. Therefore, there is a possible
electronic interaction between Zn2+ and hydroxyl or surface
oxide species on surface in which O atom donates electron
In continuation of our efforts to develop new methods in
the
synthesis
of
quinolines,[16]
and
magnetic
nanocatalysts,[17] herein, we wish to report a mild and effi-
cient approach for the synthesis of polysubstituted quinolines
via Friedländer annulation using a catalytic amount of
Fe3O4@SiO2/ZnCl2 under solvent‐free conditions at 60°C.
Accordingly, treatment of 5‐chloro‐2‐aminobenzophenone 1
with α‐methylene ketones 2 in the presence of 20 mol% of
Fe3O4@SiO2/ZnCl2 resulted in the formation of quinolines
3 in high yields (Scheme 1).
to Zn2+ [18,19]
.
Figure 2 shows the FT‐IR spectra of Fe3O4 MNPs,
Fe3O4@SiO2 core‐shell MNPs and Fe3O4@SiO2/ZnCl2
MNPs.
In Figure 2a (FT‐IR of Fe3O4), the band at 565 cm‐1 is
attributed to the vibration of Fe–O bond, while band at
3420 cm‐1 is assigned to the symmetrical stretching vibration
of hydroxyl groups (–OHs), indicating the presence of some
amount of ferric hydroxide in Fe3O4. The band at 1620 cm‐
2
| RESULTS AND DISCUSSION
1
is attributed to the bending vibration of adsorbed water.
2.1 | Preparation and characterization of the catalyst
Figure 2b shows the IR spectrum of Fe3O4@SiO2 core‐shell
MNPs. In comparing Figure 2b with Figure 2a, we have
found some new absorption peaks in Figure 2b. The bands
at 1088 cm‐1 are related to the asymmetric stretching vibra-
tion and the symmetric stretching vibration of Si–O–Si and
the band at 960 cm‐1 is assigned to the symmetric stretching
vibration of Si–OH. Figure 2c shows the IR spectrum of
Fe3O4@SiO2/ZnCl2 MNPs. In Figure 2c, in addition to bands
around 565 cm‐1 (for the stretching vibration of Fe–O bond),
around 960 cm‐1 (for the stretching vibration of Si–OH),
around 1088 cm‐1 (for the stretching vibration of Si–O–Si),
The Fe3O4@SiO2/ZnCl2 nanocatalyst was prepared in three
steps; preparation of colloidal iron oxide magnetic
nanoparticles (Fe3O4 MNPs) as a magnetic core, coating of
silica on Fe3O4 MNPs (Fe3O4@SiO2) as a shell through
sol–gel process and incorporation of ZnCl2 on the surface
by an electron interaction between Zn2+ and surface oxide
species[18] (for details see Experimental section). The catalyst
preparation steps is illustrated in Scheme 2. To prove this
claim, each step of the catalyst preparation procedure was
investigated by FT‐IR to ensure the presence of new
SCHEME 1 Synthesis of quinolines

E. SOLEIMANI ET AL.
3
SCHEME 2 The schematic pathway for the syn-
thesis of Fe3O4@SiO2/ZnCl2 core‐shell
nanoparticles
to be around ~100 nm. The SEM images (Figure 3b–d) shows
that, the as‐prepared Fe3O4@SiO2 MNPs have a spheroidal
morphology and the Fe3O4@SiO2/ZnCl2 MNPs appear more
aggregated and less defined, but they still have a spheroidal
shape, with an average size of 90 15 nm.
Furthermore, the Energy dispersive X‐ray spectroscopy
(EDX) in Figure 4 and Table 1 confirmed the presence of
ZnCl2 on the surface of Fe3O4@SiO2.
The magnetization hysteresis for Fe3O4@SiO2/ZnCl2
nanoparticle at room temperature is shown in Figure 5.
The low Ms value of Fe3O4@SiO2/ZnCl2 nanoparticles
(38 emu/g) compared to that of magnetite nanoparticles
(79.26 emu/g) can be explained by the presence of a thick
diamagnetic silica layer surrounding the magnetic core. It
is worth mentioning that the Ms value of the present pre-
pared nanoparticles is much larger than previously published
reports.[20]
FIGURE 1 XRD patterns of (a) Fe3O4 MNPs; (b) Fe3O4@SiO2 MNPs; (c)
Fe3O4@SiO2/ZnCl2 MNPs. The arrow mark indicates the (110) diffraction
peak of ZnCl2
The specific surface area of the samples was measured
using a BET method with N2 adsorption–desorption at
−196°C, and the typical results were shown in Figure 6.
The BET surface area of Fe3O4 was 38 m2 g−1. After SiO2
coating, the BET surface area of Fe3O4@SiO2 was dramati-
cally increased to 272m2 g−1. The BET surface area of
Fe3O4@SiO2/ZnCl2 was decreased to 224m2 g−1 due to the
presence of ZnCl2. Although there is still much debating on
whether the activity of a catalyst can be directly related to
the catalyst surface area, it was generally believed that the
adsorption on the catalyst surface would at least help to
concentrate the reactant molecules for the reactions.
FIGURE 2 FT‐IR spectra of (a) Fe3O4MNPs; (b) Fe3O4@SiO2 MNPs; (c)
Fe3O4@SiO2/ZnCl2 MNPs
newer bands at 611 cm−1, 735 cm−1 and 968 cm−1 are
assigned to the stretching vibration of Zn–O band. It reveals
that zinc chloride has been successfully grafted onto the sur-
face of Fe3O4@SiO2 core‐shell MNPs.
TEM was used to observe the morphology of
Fe3O4@SiO2/ZnCl2catalyst. Figure 3a shows that the NPs
have a core–shell structure with a distinct contrast between
silica shells and Fe3O4 cores, implying that silica shells
(Fe3O4@SiO2) successfully coated the hydrophilic Fe3O4
NPs. The average size of the cores was found to be ~25–
30 nm – a result consistent with the crystallite size of
~20 nm for Fe3O4 NPs obtained from XRD. The average size
of magnetite‐silica core–shell nanoparticles was determined
2.2 | Catalytic activity of Fe3O4@SiO2/ZnCl2 in the
synthesis of quinolines
We chose the reaction of 2‐aminobenzophenone and
acetylaceton under solvent‐free condition as a model reaction
for the optimization study. First, different amounts of the
catalyst (entries 1‐4) were used for a fixed reaction time of
two hours at 60°C. The experimental results showed that
the best yield was obtained when 0.07 g of catalyst was used
(entry 3). Next, we studied the model reaction at different

4
E. SOLEIMANI ET AL.
temperatures (Table 2, entries 5–7 and 3). The reaction rate
increased as the temperature was raised. At 60°C, the good
yield of 95% was obtained in a reaction time of 2 h
(Table 2, entry 3). Further work indicated that the best results
were obtained when the reaction was carried out at 60°C,
for 2 h under solvent‐free condition using 0.07 g of
Fe3O4@SiO2/ZnCl2 catalyst (Table 2, entry 3). It was also
found that this heterogeneous catalyst, Fe3O4@SiO2/ZnCl2
showed higher catalytic activity than the corresponding
homogenous ZnCl2.
With the optimized condition established above, we
then extended our attempt using various types of acyclic
α‐methylene ketones. The results have been summarized in
Table 3. In all cases, excellent yields were obtained. This
confirms the reliability of the synthetic method.
Finally, for further exploration of the scope and limita-
tion of the method we investigated the reaction of three
different cyclicα‐methylene ketones with 5‐chloro‐2‐
aminobenzophenone. Gratifyingly the reaction led to the
formation of the corresponding quinolines 3 h–j in high
isolated yields (Table 4).
The catalyst was successfully recycled and reused 5 times
with 20% loss of its activity, as demonstrated in Figure 7.
2.3 | Proposed mechanism
Possible mechanism for the synthesis quinolines by Lewis
acid catalysts has been proposed previously.[12] Scheme 3
shows a possible mechanism for this synthesis in the presence
of Lewis acid nano paramagnetic Fe3O4@SiO2/ZnCl2.
3
| EXPERIMENTAL
3.1 | Materials and techniques
Melting points were taken on an Electrothermal 9100 appara-
tus and left uncorrected. IR spectra were obtained on a Ray
Leigh Wqf‐510 Fourier Transform Infrared (FT‐IR) spectro-
photometer. TEM was performed on a Philips CM10 oper-
ated at an 80 kV electron beam accelerating voltage. SEM
and EDX were performed on a Philips XL‐300 instrument.
The sample was sputtered by gold to avoid undesirable elec-
tron charging. X‐ray diffraction was conducted using a
Philips X'pert Pro (PW 3040) X‐ray diffractometer with
monochromatic Cu‐Kα radiation (k = 1.54056 Å, 40 kV,
30 mA). 1H and 13C NMR spectra were recorded on a Bruker
DRX‐250 Avance spectrometer at 250, 400 and 62,
100 MHz. NMR spectra were obtained on solutions in CDCl3
using tetramethylsilane as an internal standard. The textural
structures were measured by N2adsorption at − 196°C in a
Micromeritics TriStar ASAP 3000 system, and specific sur-
face areas of the as‐prepared samples were measured using
Brunauer–Emmett–Teller (BET) method. The concentration
of copper was estimated using Shimadzu AA‐680 flame
atomic absorption spectrophotometer and inductively
FIGURE 3 (a) TEM image of Fe3O4@SiO2/ZnCl2 MNPs, SEM images of
(b) Fe3O4@SiO2 MNPs and (c, d) Fe3O4@SiO2/ZnCl2 MNPs

E. SOLEIMANI ET AL.
5
FIGURE 4 EDX spectrum of Fe3O4@SiO2/ZnCl2 MNPs
TABLE 1 EDX data of Fe3O4@SiO2/ZnCl2 MNPs (Wt %)
Sample
C (%)
O (%)
Si (%)
Cl (%)
Fe (%)
Zn (%)
Total (%)
Fe3O4@SiO2/ZnCl2
18.44
23.75
16.06
0.19
38.73
2.82
100.00
(Imax = 150 A, P ≤ 9 kW) at room temperature. All of the
chemicals were purchased from Fluka, Merck and Aldrich,
and used without further purification.
3.2 | Preparation of Fe3O4@SiO2/ZnCl2
Magnetite (Fe3O4) nanoparticles (NPs) were prepared
according to the reported method (co‐precipitation)[21] and
coated with silica in the presence of 2‐propanol (sol–gel
method).[22] In brief, 3.0 g of FeCl3.6H2O and 1.0 g of
FeCl2.4H2O were dissolved in 120 mL deionized water under
nitrogen gas with vigorous stirring. Then, NH3 (28%) was
added dropwise into the solution until the pH of the solution
reached 11. Stirring was continued for 1 h at 60°C. The color
of the bulk solution turned from orange to black immediately.
The magnetite precipitate was separated from the solution
using a magnet, washed several times with deionized water
and ethanol, and left to dry in air. Then, 1.0 g as‐synthesized
Fe3O4 NPs were mixed with 120 mL isopropyl alcohol in a
sealed three‐neck flask by ultrasonic treatment for 30 min.
30 mL deionized water, 1.0 mL TEOS (silica precursor),
and 4.80 mL ammonia (28 wt%) were added into the mixture
of Fe3O4 and isopropyl alcohol at room temperature under
mechanical agitation. After 12 h, the final product
(Fe3O4@SiO2) was collected, washed with ethanol and
deionized water and dried at 50°C. Then 1.5 g as‐synthesized
Fe3O4@SiO2 NPs was mixed with 150 mL toluene in a
sealed three‐neck flask by ultrasonic treatment for 30 min.
Then 0.1 g ZnCl2 were added and the mixed solution was
stirred at room temperature for 24 h. The resultant magnetite
nanoparticles were collected by an external magnetic field
and washed several times with ethanol and deionized water,
respectively. Then, the product was dried and activated in a
vacuum oven at 60°C for 8 h. The loading of zinc was
obtained 6.58 mmol g−1 by inductively coupled plasma‐
optical emission spectrometry (ICP‐OES).
FIGURE 5 Magnetization loop of Fe3O4@SiO2/ZnCl2 nanoparticle
FIGURE 6 Nitrogen adsorption/desorption isotherms of Fe3O4 MNPs,
Fe3O4@SiO2 MNPs and Fe3O4@SiO2/ZnCl2 MNPs
coupled plasma optical emission spectrometer (ICP‐OES)
Varian Vista PRO Radial. The magnetic properties of parti-
cles were analyzed using Vibrating Sample Magnetometer
(VSM, Meghnatis Daghigh Kavir Co., Iran) equipment
3.3 | General procedure for the synthesis of quinolines
A mixture of 1.0 mmol 2‐aminoaryl ketone, 1.2 mmol α‐CH
acid, and 0.07 g catalyst was heated under solvent‐free

6
E. SOLEIMANI ET AL.
TABLE 2 Optimization of reaction condition for synthesis of 1‐(2‐Methyl‐4‐phenyl‐quinolin‐3‐yl)‐ethanone a
b
Entry
Gram of catalyst (mmol %)
0.02 (0.13)
Temperature (°C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
60
60
2
2
46
69
95
95
60
96
96
0.05 (0.33)
0.07 (0.46)
60
2
0.10 (0.66)
60
2
0.07 (0.46)
25
12
2
0.07 (0.46)
80
0.07 (0.46)
100
2
aReaction condition: 2‐aminoaryl ketone (1 mmol), acetylaceton (1.2 mmol) and the catalyst under solvent free condition.
bIsolated yields.
SCHEME 3 Synthesis of 1‐(2‐Methyl‐4‐phenyl‐quinolin‐3‐yl)‐ethanone
TABLE 3 Synthesis of quinoline derivatives through Friedländer synthesis by catalyst Fe3O4@SiO2/ZnCl2
Entry
R1
R2
Product
3a
Yield %a (time, h)
95 (2)
Mp, oC (found)
1
2
3
4
5
6
7
Me
Me
Et
Me
148
98
OEt
3b
94 (2)
OMe
3c
92 (2)
83
Pr
OMe
3d
93 (3)
82
iPr
OMe
3e
95 (4)
98
CH2Cl
OMe
3f
92 (2)
123
124
CH2CO2Me
CH2CO2Me
3 g
90 (2)
aIsolated yields.
conditions with stirring at 60°C. After completion of the
reaction as indicated by TLC, the catalyst was separated with
a magnet from the reaction mixture and washed with hot
ethanol (2–10 mL). Then, the filtrate was concentrated, and
the solid product was recrystallized from ethanol. All
products are known compounds, which were characterized

E. SOLEIMANI ET AL.
7
TABLE 4 Reaction of different cyclic α‐methylene ketones with 5‐chloro‐2‐aminobenzophenone by catalyst Fe3O4@SiO2/ZnCl2
Entry
cyclic α‐methylene ketones
cyclohexanone
Product
Yield %a (time, h)
94 (2)
Mp, oC (found)
1
2
3
3 h
3i
157
203
130
Dimedone
97 (2)
Tetronic acid
3j
87 (3)
aIsolated yields.
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FIGURE 7 Reusability of the catalyst for the synthesis of synthesis of quin-
olines (model reaction: entry 1, Table 3)
by FT‐IR and spectral data and their mp values were com-
pared with literature reports.
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4
| CONCLUSION
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1984, Chapter 2.
This work reported on the preparation of Fe3O4@SiO2/ZnCl2
via functionalization of silica‐coated magnetic nanoparticles
with ZnCl2 by one‐pot synthesis containing continuously
three‐step reaction. The core‐shell nanoparticles were stable
and reusable, non‐toxic and inexpensive heterogeneous
nanocatalyst with great potential applications in organic syn-
theses. The Fe3O4@SiO2/ZnCl2 MNPs were used as an acid
catalyst for the synthesis of different quinolines with 2‐
aminoaryl ketone and various cyclic or acyclic β‐dicarbonyl
under solvent‐free condition with excellent yields and short
time. The major advantage of this catalyst was its ease of
the recovery, allowing it to be reused with slightly change
in its catalytic activity.
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ACKNOWLEDGMENTS
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We gratefully acknowledge financial support from the
Research Council of Razi University.
[12] C. C. Cheng, S. J. Yan, Organic Reactions, volume. 28, John Wiley, New
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How to cite this article: Soleimani, E., Naderi
Namivandi, M., and Sepahvand, H. (2016), ZnCl2
supported on Fe3O4@SiO2 core–shell nanocatalyst
for the synthesis of quinolines via Friedländer synthe-
sis under solvent‐free condition, Appl. Organometal.
Chem., doi: 10.1002/aoc.3566
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