Fe3O4-Methylene diphenyl diisocyanate-guanidine (Fe3O4–4,4′-MDI-Gn): A novel superparamagnetic powerful basic and recyclable nanocatalyst as an efficient heterogeneous catalyst for the Knoevenagel condensation a

Article abstract of DOI:10.1002/aoc.3905

In this paper, guanidine groups (Gn) supported on modified magnetic nanoparticles (Fe₃O₄–4,4′-MDI) were synthesized for the first time. The catalyst synthesized was characterized by various techniques such as SEM (Scanning Electron M

Full text of DOI:10.1002/aoc.3905

Received: 13 March 2017
DOI: 10.1002/aoc.3905
Revised: 24 April 2017
Accepted: 6 May 2017
F U L L PA P E R
Fe3O4‐Methylene diphenyl diisocyanate‐guanidine (Fe3O4–4,
4′‐MDI‐Gn): A novel superparamagnetic powerful basic and
recyclable nanocatalyst as an efficient heterogeneous catalyst
for the Knoevenagel condensation and tandem Knoevenagel‐
Michael‐cyclocondensation reactions
Razieh Maleki | Nadiya Koukabi | Eskandar Kolvari
Department of Chemistry, Semnan
University, P.O. Box 35195‐363, Semnan,
Iran
In this paper, guanidine groups (Gn) supported on modified magnetic nanoparticles
(Fe3O4–4,4′‐MDI) were synthesized for the first time. The catalyst synthesized was
characterized by various techniques such as SEM (Scanning Electron Microscopy),
TEM (Transmission electron microscopy), XRD (X‐ray Diffraction), TGA (Ther-
mogravimetric ananlysis), EDS (Energy‐dispersive X‐ray spectroscopy) and VSM
(vibrating sample magnetometer). The catalyst activity of modified MNPs–MDI‐
Gn, as powerful basic nanocatalyst, was probed through the Knoevenagel and Tan-
dem Knoevenagel–Michael‐cyclocondensation reactions. Conversion was high
under optimal conditions, and reaction time was remarkably shortened. This
nanocatalyst could simply be separated and recovered from the reaction mixture
by simple magnetic decantation and reused many times without significant loss of
its catalytic activity. Also, the nanocatalyst could be recycled for at least seven
(Knoevenagel condensation) and six (Knoevenagel and Tandem Knoevenagel–
Michael‐cyclocondensation) additional cycles after they were separated by magnetic
decantation and, washed with ethanol, air‐dried, and immediately reused.
Correspondence
Eskandar Kolvari, Department of Chemistry,
Semnan University, P.O. Box 35195‐363,
Semnan, Iran.
Email: kolvari@semnan.ac.ir
KEYWORDS
basic nanocatalyst, Fe3O4‐Methylene diphenyl diisocyanate‐guanidine, Knoevenagel condensation,
nanocatalyst, tandem Knoevenagel–Michael‐cyclocondensation reaction
1 | INTRODUCTION
applications.[4] Efficient catalysts that can be easily and sim-
ply separated from the reaction media were produced. It is
Heterogeneous catalysts have been widely involved in various
organic reactions, because the reuse of catalysts is highly
favourable for economy.[1] Solid‐supported catalysts are a
significant and growing arena in heterogeneous catalysis.[2]
The nano‐magnetic catalyst may be a better choice of hetero-
geneous catalyst because the magnetic separation generally
avoids loss of catalyst and increases its reusability in compar-
ison to filtration or centrifugation.[3] Magnetic nanoparticles
have been studied widely for disparate biological and medical
known that iron oxides, magnetite (Fe3O4) and maghemite
(γ‐Fe2O3), are intrinsically biocompatible and are amenable
to post‐synthesis surface modification, which makes them
great candidates for many important applications.[5] The
insoluble and paramagnetic nature of the nano magnetic
enables trouble‐free separation of this catalyst from the reac-
tion mixture by using an external magnet, which eliminates
the necessity of catalyst filtration. Although iron oxide mag-
netic nanoparticles are non‐toxic and could be easily
Appl Organometal Chem. 2017;e3905.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.3905

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MALEKI ET AL.
synthesized by co‐precipitation methods,[6] their applications
have been limited due to low chemical and thermal stability
in environmental conditions. Thus, the coating of these nano-
particles with an oxygen‐impermeable scabbard is a neces-
sary prerequisite for their potential use in biomedical and
catalyst support applications.[7] Magnetite‐supported cata-
lysts have emerged as the viable alternatives to existing
solid‐supported heterogeneous catalysts, as they are inert,
inexpensive, easy to prepare, and most importantly could be
separated by an external magnet and reused multiple times
for the several reaction cycles.[8] In addition, because of their
large ratio of surface area to volume, superparamagnetic
behaviour, and low toxicity, magnetic nanoparticles have
attracted much attention in manifold technological fields.
The magnetically separable nanocatalysts are valuable addi-
tion to sustainable methodologies as the demand for benign
nanocatalyst and their applications in synthesis is on the
rise. For example MNPs supported catalysts are used in
asymmetric synthesis of organic compounds.[9]
inorganic chemistry commonly as organic bases.[10] During
the recent increasing interest in the field of organocatalysis,
guanidines have also been shown to act as organic bases and
nucleophilic catalysts.[11] However, the major disadvantage
of catalysts based on guanidines is their separation from
the product, which needs solid–liquid or liquid–liquid tech-
niques in many reactions. This problem can be overcome
by immobilizing these catalysts on MNPs, which can be eas-
ily removed from the reaction mixture by magnetic
separation.
On the other hand, organic reactions should be fast and
facile, and the target products should be easily separated
and purified in high yields.[12] One‐pot multicomponent
reaction strategies propose significant advantages over con-
ventional linear‐type syntheses by virtue of their conver-
gence, productivity, facile execution, and high yield.[13] In
the other hand, condensation reaction is a chemical reaction
in which two, three or four molecules or moieties, often
functional groups, combine to form a larger molecule,
together with the loss of a small molecule.[14] Condensa-
tion reactions, known to be catalysed by base, are of great
Guanidines are important classes of compound that are
found throughout nature that also have many uses with
SCHEME 1 Preparation of MNPs–MDI‐Gn nanocatalyst
FIGURE 1 XRD spectrum of MNPs–MDI‐Gn

MALEKI ET AL.
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importance for the synthesis of pharmaceutical and fine
chemicals.
Catalyst can be easily separated with an external magnet,
without using extra organic solvents.
Herein, we have applied 4,4′‐Methylene diphenyl
diisocyanate (4,4′‐MDI) as a coupling agent which is a type
of bifunctional‐group organic chemicals and guanidine
hydrochloride which is a feasible amination reagent and
report a catalytic system based on magnetic nanocatalyst with
4,4′‐MDI and guanidine, which proved to be highly efficient
for low temperature Knoevenagel and MCRs reactions.
2 | EXPERIMENTAL
2.1 | Materials and instruments
All starting materials were purchased from Merck and
Sigma‐Aldrich and used as received without further purifica-
tion. Thermogravimetric analyses (TGA) was done using a
DUPONT 951 thermal analysis Instruments heated from
25 °C to 1000 °C at ramp 5 °C/min under N2 atmosphere.
The morphology of the catalyst were studied using scanning
electron microscopy (SEM) with a Philips XL30 field
TABLE 1 Scherrer data information for MNPs‐MDI‐Gn nanocatalyst
Crystal
size (nm)
B1/2
(rad)
B1/2
(°)
cos B
2
Sample
10
0.951872 0.01396 0.80 17.8481 35.6962
FIGURE 2 SEM spectrum of MNPs–
MDI‐Gn
FIGURE 3 TEM spectrum of MNPs–
MDI‐Gn

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MALEKI ET AL.
emission scanning electron microscope (Royal Philips Elec-
tronics, Amsterdam, the Netherlands) instrument operating
at 25 kV and transmission electron microscopy (TEM,
CM30, Philips, the Netherland). The magnetic measurements
were carried out in vibrating sample magnetometer (VSM,
Lakeshore 7407) at room temperature. The energy dispersive
X‐Ray spectroscopy (EDS or EDX) (Philips XL‐30) was
used for determination of elemental composition of catalyst.
Wide‐angle X‐ray diffraction (XRD) measurements were per-
formed at room temperature on a Siemens D5000 (Siemens
AG, Munich, Germany) using Cu‐Ka radiation of wavelength
1.54 °A. The purity of products was checked by thin layer
chromatography (TLC) on commercial plates coated with sil-
ica gel 60 F254 using n‐hexane/ethyl acetate mixture as
mobile phase. Melting points were recorded on THERMO
SCIENTIFIC 9100 Instrument.
reported procedure.[15] 3 ml FeCl3(2 M dissolved in 2 M
HCl) was added to 10.33 ml double distilled water, and 2 ml
Na2SO3 (1 M) was added to the former solution dropwise in
1 min under magnetic stirring. Just after mixing of Fe3+ and
SO3−2, the colour of the solution in the smaller beaker could
be seen to alter from light yellow to red, indicating formation
of complex ions. This solution was added to 80 ml NH3 solu-
tion (0.85 M) under vigorous stirring when the colour
changed from red to yellow again. A black precipitate quickly
formed, which was allowed to crystallize completely for
another 30 min under magnetic stirring. The resulting black
MNPs were isolated by applying an external magnet, washed
several times with deionized water until the pH was less than 7
and then dried under vacuum at 60 °C for 12 h.
2.3 | Functionalization of magnetic
nanoparticles with MDI
2.2 | Preparation of the magnetic Fe3O4
nanoparticles
A mixture of 1.5 g of synthesized Fe3O4 nanoparticles was
dispersed in 15 ml of dried toluene. Then 2.25 g of MDI
was added and placed in an ultrasonic bath for 15 min. Then,
the reaction mixture was maintained at the temperature of
The Fe3O4 magnetic nanoparticles were synthesized by
reduction–precipitation method according to the previously
FIGURE 4 EDS spectrum of MNPs–MDI‐Gn
FIGURE 5 TGA spectrum of MNPs–MDI‐Gn

MALEKI ET AL.
5 of 10
100 °C and refluxed for 22 h under nitrogen atmosphere. The
modified nanoparticles with MDI were magnetically sepa-
rated and washed with dried toluene to remove the unreacted
MDI. The product was dried in a vacuum at 100 °C for 8 h.
2.5 | General procedure for the Knoevenagel
condensation by using of (n‐Fe3O4‐MDI‐Gn)
A mixture of aldehyde (0.5 mmol), malononitrile (0.5 mmol)
or dimedone (0.5 mmol) and (n‐Fe3O4‐MDI‐Gn (0.05 g) in
round bottom flask containing 1 ml ethanol:water was stirred
at RT using a magnetic stirrer for the time specified. After ter-
minus of the reaction monitored by TLC, the catalyst was sep-
arated magnetically. The reaction mixture was placed at room
temperature until solidification occurred. In order to further
purification, the solid was recrystallized from 96% ethanol.
2.4 | Preparation of magnetic nanocatalyst (n‐
Fe3O4‐MDI‐Gn)
The modified nanoparticles with MDI (0.75 g) were dis-
persed in dry toluene (10 ml) by ultrasonication for 15 min.
Subsequently, guanidine hydrochloride (0.3 g,) and sodium
bicarbonate (0.5 g) were added and the mixture was refluxed
for 24 h. Then, the final product, which named as Fe3O4‐
MDI‐Gn, was separated by magnetic decantation and washed
twice by dry CH2Cl2, MeOH and CH2Cl2 respectively. The
synthesized nanocatalyst was dried in a vacuum at 70 °C
for 6 h.
2.6 | General procedure for the synthesis of
tetrahydrobenzo[b]pyrans using of (n‐Fe3O4‐
MDI‐Gn)
A mixture of aldehyde (0.5 mmol), malononitrile (0.5 mmol),
dimedone (0.5 mmol) and (n‐Fe3O4‐MDI‐Gn (0.05 g) in
FIGURE 6 (a) Room‐temperature
magnetization curve of magnetic Fe3O4. (b)
Room‐temperature magnetization curve of
MNPs–MDI‐Gn

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MALEKI ET AL.
TABLE 2 Optimization of Knoevenagel condensation reaction
conditions
round bottom flask containing 1 ml ethanol:water was stirred
at RT by using of a magnetic stirrer for the time specified.
Upon completion of the reaction (monitored by TLC), the
catalyst was separated by employing an external magnet.
The reaction mixture was decanted and eluted using hot eth-
anol (5 ml). The products were obtained by recrystallization
using ethanol solution.
Entry
1
Solvent
Solvent free
Catalyst (g) Temp (°C) Yield (%)
0.05
0.05
0.05
0.05
0.05
0.02
0.03
0.04
0.05
0.06
0.05
0.02
0.04
0.05
50
80
Trace
Trace
45
2
Solvent free
3
Ethanol
50
4
Water
90
30
5
Ethanol
70
30
3 | RESULTS AND DISCUSSION
6
Ethanol: Water(2:1)
Ethanol: Water(2:1)
Ethanol: Water(2:1)
Ethanol: Water(2:1)
Ethanol: Water(2:1)
Dichloromethane
Ethanol: Water
Ethanol: Water
Acetonitrile
RT
RT
RT
RT
RT
40
50
7
63
3.1 | Preparation and characterization of
MNPs–4,4′‐Methylene diphenyl diisocyanate‐
guanidine (MNPs–MDI‐Gn) nanocatalyst
8
78
9
95
10
11
12
13
14
96
‐‐‐‐‐
40
The MNPs–MDI‐Gn was synthesized according to the con-
cise route outlined in Scheme 1. Firstly, magnetic Fe3O4
nanoparticles were prepared through reduction–precipitation
method and subsequently were coated with MDI to achieve
functionalized magnetic nanoparticles. Next, MNPs–MDI‐
Gn nanoparticles were synthesized by the reaction between
free guanidine and functionalized magnetic nanoparticles
(Scheme 1). This nanocatalyst was characterized using a vari-
ety of different techniques such as SEM, TEM, XRD, TGA,
FT‐IR, EDX and VSM.
XRD was used to recognize the crystal structure of the
superparamagnetic nanocatalyst. The XRD pattern of
MNPs–MDI‐Gn is shown in Figure 1 and Table 1. The dif-
fraction signals, positions and relative intensities of all peaks
are confirmed with the standard XRD pattern of Fe3O4.
Figure 2 shows the SEM image of the synthesized guanidine‐
functionalized magnetite nanocatalyst. According to, this Figure
synthesized nanocatalyst has particle size about 10–20 nm.
TEM, as one of the most powerful techniques to investi-
gate nanoparticles size, was used to further characterize the
morphology of the synthesized MNPs–MDI‐Gn. According
to Figure 3, the size of nanoparticles in TEM image (within
10–20 nm) was in agreement with SEM image.
50
50
47
70
‐‐‐‐‐
TABLE 3 Optimization of tandem Knoevenagel‐Michael‐
cyclocondensation reaction conditions
Entry
Solvent
Solvent free
Solvent free
Ethanol
Catalyst (g) Temp. (°C) Yield (%)
1
0.05
0.05
0.05
0.05
0.05
0.02
0.03
0.04
0.05
0.06
0.05
0.02
0.04
60
90
36
72
45
30
48
44
62
80
95
96
‐‐‐‐‐
49
64
2
3
50
4
Water
90
5
Ethanol
70
6
Ethanol: Water
Ethanol: Water
Ethanol: Water
Ethanol: Water
Ethanol: Water
Dichloromethane
Ethanol: Water
Ethanol: Water
RT
RT
RT
RT
RT
45
7
8
9
10
11
12
13
50
50
To identify the chemical composition of MNPs–MDI‐Gn,
EDS analysis was employed. According to Figure 4, chemi-
cal characterization of a typical sample shows that it is com-
posed of iron, carbon and oxygen elements. The loading of
the guanidine function on the magnetic nanocatalyst was
specified by elemental analysis of nitrogen.
The TGA was used to determine the percent of organic
functional groups chemisorbed onto the surface of magnetic
nanoparticles. In the TGA curve of nanocatalyst (Figure 5).
SCHEME 2 Fe3O4–MDI‐Gn catalyzed Knoevenagel condensation
of aromatic aldehydes with malononitrile in ethanol/water
SCHEME 3 Fe3O4–MDI‐Gn catalyzed
tandem Knoevenagel‐Michael‐
cyclocondensation reaction of aromatic
aldehydes with malononitrile and dimedone
in ethanol/water

MALEKI ET AL.
7 of 10
four stages were observed. The initial weight loss (below
200 °C) is likely because of the removal of physically
adsorbed solvent or water and surface hydroxyl groups. The
weight loss at temperatures about 200 to 600 °C (the second
and third decomposition) was attributed to the decomposition
of the 4,4′‐Methylene diphenyl diisocyanate and the guani-
dine bases grafted onto the MNPs. The fourth decomposition
step was identified at a temperature range of 710–1000 °C
can be assigned to the oxidation reaction of magnetite to
maghemite. These explanations show that the nanocatalyst
was demonstrated to be thermally stable at the temperature
used for the Knoevenagel and MCRs reactions.
superparamagnetic. As expected, the Ms Value of MNPs–
MDI‐Gn compared to the naked MNPs (70 emu.g−1) is
decreased because of the organic materials coated to
Fe3O4. The saturation magnetization value of the functional-
ized nanoparticles was 48 emu.g−1. The images of the
MNPs–MDI‐Gn demonstrate the excellent and sufficient
magnetization for its magnetic separation with a conven-
tional magnetic.
3.2 | Applications of MNPs–MDI‐Gn for the
Knoevenagel condensation and Knoevenagel‐
Michael‐cyclocondensation reaction
The magnetic property of the final product was studied
by VSM. Magnetization curve (Figure 6) measured at room
MNPs‐MDI‐Gn was investigated as basic magnetically sepa-
temperature
showed
that
MNPs–MDI‐Gn
is
rable heterogeneous nanocatalyst for the Knoevenagel
TABLE 4 Knoevenagel condensation reaction of different aromatic aldehydes with malononitrilea
Entry
Aldehyde
Yield(%)b
Time (min)
M.p)° C)
M.p. (reported) (°C)
1
95
10
83–85
82–85[16]
2
3
92
93
15
189–191
92–95
190–191[17]
94–95[18]
10
162–163[19]
113–114[6d]
102–103[20]
< 5
4
5
6
97
98
96
162–164
113–115
106–108
5
15
7
98
91
97
90
< 5
< 10
< 5
20
159–161
135–137
139–141
187–189
160[21]
8
9
134–135[22]
136–137[23]
187–188[19]
10
aReaction conditions: Fe3O4–MDI‐Gn (0.05 g), aromatic aldehydes(1 mmol), malononitrile(1 mmol), ethanol/water (1:1), RT.
bYields are given for isolated products.

8 of 10
MALEKI ET AL.
TABLE 5 Tandem Knoevenagel‐Michael‐cyclocondensation reaction of different aromatic aldehydes with malononitrile, and dimedonea
Entry
Aldehyde
Yield(%)b
Time
M.p)° C)
M.p. (reported) (°C)
1
93
10
228–230
228–229[6b]
2
3
96
91
< 15
< 15
172–174
211–213
171–173[3c]
212–213[3c]
4
5
6
93
95
94
10
15
10
237–239
195–197
202–204
236–238[2b]
194–196[2b]
200–201[6b]
7
8
9
98
92
97
< 5
< 20
< 10
175–177
233–234
196–197
178–179[6b]
232–234[3c]
195–197[3c]
aReaction conditions: Fe3O4–MDI‐Gn (0.05 g), aromatic aldehydes(1 mmol), malononitrile(1 mmol), dimedone(1 mmol), ethanol/water (1:1), RT.
bYields are given for isolated products.
condensation and Knoevenagel‐Michael‐cyclocondensation
reaction and compared the effect of different solvents and sol-
vent‐free conditions. Also, to investigate the optimized
amount of catalyst in this case, the reaction was accomplished
with various amounts of catalyst. Results are summarized in
Tables 2–4. On the basis of these results it can be concluded
that the best results can be obtained under the conditions
shown in Scheme 2,3. As can be seen from Tables 2–4, the
catalytic system worked exceedingly well in both
Knoevenagel condensations and Knoevenagel‐Michael‐
cyclocondensation reaction with wide range of substrates
under the optimized reaction conditions. The expected prod-
ucts were prepared in short times and in excellent to high
yields in both reactions.
conditions, were used. The expected products were obtained
in short times and in good to high yields. All results are
shown in Tables 4, 5.
After completion of the reaction, MNPs–MDI‐Gn can be
efficiently recovered easily and rapidly from the product by
TABLE 6 Recycling of Fe3O4–MDI‐Gn for Knoevenagel condensa-
tion of benzaldehyde with malononitrile in ethanol/water and RT
Cycle
1st 2nd 3rd 4th 5th 6th 7th
95 95 93 92 92 90
Converted yield (%) 95
TABLE 7 Recycling of Fe3O4–MDI‐Gn for tandem Knoevenagel‐
Michael‐cyclocondensation reaction of benzaldehyde with malononitrile
and dimedone in ethanol/water and RT
To investigate the efficiency and applicability of this cat-
alyst in the Knoevenagel condensation and Knoevenagel‐
Michael‐cyclocondensation reactions, wide range of other
substituted aldehydes under the optimized reaction
Cycle
1st
2nd
3rd
4th
5th
6th
Converted yield (%)
93
93
92
92
90
89

MALEKI ET AL.
9 of 10
TABLE 8 Comparison of n‐Fe3O4–MDI‐Gn with some of the reportedcatalysts for tandem Knoevenagel‐Michael‐cyclocondensation
Entry
Catalyst
Reaction condition
CH3CN/reflux
Solvent‐free/70 °C
Solvent‐free/100 °C
Solvent‐free/95 °C
EtOH/r.t.
Time(min)
Yield(%)
87
Ref
[24]
[25]
[26]
[26]
[27]
[28]
[29]
1
2
3
4
5
6
7
8
n‐Pd
n‐TiO2
300
35
92
n‐Fe3O4
n‐Fe3O4@SiO2@TiO2
n‐SiO2
15
81
20
93
25
94
n‐PbO
n‐ZnO
Grinding/r.t.
15
83
EtOH:H2O/r.t.
210
10
86
n‐Fe3O4–MDI‐Gn
Ethanol/water, RT
93
This work
37, 69; c) F. Osanlou, F. Nemati, S. Sabaqian, Res. Chem.
Intermed. 2016; d) F. Nemati, M. M. Heravi, R. Saeedi Rad, Chin.
J. Catal. 2012, 33; 1825.
exposure to an external magnet. The remaining nanocatalyst
was washed with EtOH, air‐dried, and used directly for the
next reaction without further purification to remove residual
products. The recycled catalyst was used for up to seven for
Knoevenagel condensation and six for Knoevenagel‐
Michael‐cyclocondensation without significant loss of cata-
lytic activity (Tables 6, 7).
In order to compare the efficiency of the prepared
nanocatalyst with other reported catalysts as well as to exhibit
the merit of the present work, our results are compared with
some other previously reported studies in Table 8.
[7] a) C. W. Lim, I. S. Lee, Nano Today 2010, 5, 412; b) Y.‐S. Li, J. S.
Church, A. L. Woodhead, J. Magn. Magn. Mater. 2012, 324, 1543;
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[11] a) Y. R. Su, S. H. Yu, A. C. Chao, J. Y. Wu, Y. F. Lin, K. Y. Lu, F.
L. Mi, Colloids Surf. Physicochem. Eng. Aspects 2016, 494, 9; b)
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We are grateful to the University of Semnan Research Coun-
cil for financial support of this work.
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