A molybdenum(VI) Schiff base complex immobilized on functionalized Fe3O4 nanoparticles as a recoverable nanocatalyst for synthesis of 2-amino-4H-benzo[h]chromenes

Article abstract of DOI:10.1007/s11243-018-0204-x

A dioxomolybdenum(VI) complex of a Schiff base, immobilized on the surface of modified Fe₃O₄ with a silica coating, has been synthesized and characterized by spectroscopic and microscopic techniques including FTIR, TGA, ICP, SEM, EDX, VSM, and XRD analyses. The catalytic performance of this material has been evaluated for the preparation of 2-amino-4H-benzo[h]chromenes via the one-pot, three-component reaction of aldehydes, malononitrile, and 1-naphthol under solvent-free conditions. The benefits of this protocol are short reaction time, simple workup procedure, and high yields of products. Also, the synthesized nanocatalyst could be separated easily from the reaction mixture using an external magnet and reused for four consecutive times with only minor degradation of its catalytic performance.

Full text of DOI:10.1007/s11243-018-0204-x

Transition Metal Chemistry
https://doi.org/10.1007/s11243-018-0204-x
A molybdenum(VI) Schif base complex immobilized on functionalized
Fe3O4 nanoparticles as a recoverable nanocatalyst for synthesis
of 2‑amino‑4H‑benzo[h]chromenes
Niaz Monadi1 · Elham Moradi1
Received: 2 October 2017 / Accepted: 28 December 2017
© Springer International Publishing AG, part of Springer Nature 2018
Abstract
A dioxomolybdenum(VI) complex of a Schif base, immobilized on the surface of modifed Fe3O4 with a silica coating, has
been synthesized and characterized by spectroscopic and microscopic techniques including FTIR, TGA, ICP, SEM, EDX,
VSM, and XRD analyses. The catalytic performance of this material has been evaluated for the preparation of 2-amino-4H-
benzo[h]chromenes via the one-pot, three-component reaction of aldehydes, malononitrile, and 1-naphthol under solvent-
free conditions. The benefts of this protocol are short reaction time, simple workup procedure, and high yields of products.
Also, the synthesized nanocatalyst could be separated easily from the reaction mixture using an external magnet and reused
for four consecutive times with only minor degradation of its catalytic performance.
Introduction
eco-friendly, and efcient procedures is an important chal-
lenge. An efective strategy to overcome these problems is
the use of supported nanocatalysts. Nanoscale materials can
provide excellent catalytic activities because of their large
surface area. Nanocatalysis can be made of various materi-
als such as metallic nanoparticles, metal oxides, polymers,
and carbon-based materials [14–17]. These materials can
have special properties including high surface area, excel-
lent catalytic activity, and high absorption capacity, which
make them suitable as nanocatalysis [18, 19]. A convenient
method for the preparation of nanocatalysis is the hetero-
genization of homogeneous catalysts by immobilization onto
an insoluble nanosupport such as iron oxide nanoparticles
(Fe3O4). Fe3O4 nanoparticles have attracted much attention
due to their useful properties including large surface area,
superparamagnetism, low toxicity, biocompatibility, and
their potential applications in various felds [20–24]. Moreo-
ver, this nanosupport can be easily separated from reaction
media by the use of an external magnet, giving more efec-
tive separation than conventional methods such as fltration
or centrifugation.
Chromenes (benzopyrones) are an important class of com-
pounds which serve as intermediates in the biological and
pharmaceutical chemistry. They are typically prepared by
multicomponent reactions (MCRs) [1–4]. MCRs are one
of the most important processes for the synthesis of bio-
logically active compounds in modern synthetic chemistry,
in which three or more reactants are combined together to
generate a desired product. Such compounds are gener-
ally prepared by reacting aldehydes, activated methylene
compounds, and activated phenols (three-component reac-
tion) in organic solvents [5–8]. Several methods have been
reported for the synthesis of these compounds by homoge-
neous catalysts [9–13], but most of them sufer from draw-
backs such as the use of toxic catalysts and bases, difcult
recovery and recycling of the catalyst, and lack of general
applicability. Therefore, for the synthesis of biological com-
pounds such as chromenes, the development of economical,
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-018-0204-x) contains
supplementary material, which is available to authorized users.
Schif base ligands and their complexes are versatile com-
pounds synthesized from the condensation of amines with
carbonyl compounds, and widely used for industrial pur-
poses, biological activities, and catalysis of various reactions
[25–28]. Despite their high catalytic activity and selectivity,
these homogeneous complexes have some problems such as
difcult separation of the catalyst, deactivation during the
* Niaz Monadi
Nimonadi@umz.ac.ir
1
Department of Inorganic Chemistry, Faculty of Chemistry,
University of Mazandaran, Babolsar 47416-95447, Iran
Vol.:(0123456789)
1 3

Transition Metal Chemistry
reaction, and difcult separation of the product. Immobi-
lization of the homogeneous catalyst onto a solid support
such as Fe3O4 nanoparticles can overcome these drawbacks
[29–32]. With these considerations in mind, we here report
the synthesis of 2-amino-4H-benzo[h]chromenes catalyzed
by a dioxomolybdenum(VI) complex supported on Fe3O4
nanoparticles modifed with a silica coating, as a magneti-
cally recoverable nanocatalyst.
deionized water (20 mL) and ethanol (30 mL) under soni-
cation (1 h). Next, tetraethyl orthosilicate (TEOS, 2 mL)
and NH4OH (2.4 mL, 25% w/w) were added dropwise and
the mixture was stirred for 3 h at room temperature. The
black suspension was decanted using an external magnet
and washed several times with deionized water/ethanol. The
product SiO2@Fe3O4 was dried at 60 °C for 4 h.
Functionalization of Fe3O4@SiO2 with amine
Experimental section
In a typical procedure, Fe3O4@SiO2 (0.5 g) was dispersed
in ethanol (20 mL) by ultrasonic irradiation and then 1.5 mL
3-aminopropyltriethoxysilane (APTES) was added drop-
wise. The reaction mixture was refuxed for 12 h under
N2 atmosphere. The fnal product designated as Fe3O4@
SiO2–NH2 was decanted using an external magnet and dried
at 80 °C for 4 h [36].
Materials and physical measurements
All reagents and solvents were purchased from Aldrich,
Merck, or Fluka and used without further purification.
MoO2(acac)2 was synthesized according to a literature
procedure [33]. FTIR spectra were recorded using KBr
pellets on a Bruker Vector 22 spectrometer in the range
of 400–4000 cm−1. Thermogravimetric (TGA) analysis
was performed on a Perkin Elmer analyzer under nitrogen
atmosphere at a heating rate of 10 °C/min over a temperature
range of 25–800 °C. Ultrasonic irradiation was carried out
with a SONICA-2200 Ep instrument, input 50–60 Hz/305w.
The nanoparticles were analyzed using a Holland Philips X
Pert X-ray difraction difractometer with Cu Kα radiation
(λ = 1.5404 Å) in the 2θ range of 10°–70° at room tem-
perature. Scanning electron microscopy (SEM) images were
recorded with a VEGA/TESCAN KYKY-EM3200 micro-
scope. The metal content of the catalysts was measured by
inductively coupled plasma atomic emission analysis (ICP-
AES, Varian company VISTA-PRO model). Magnetic prop-
erties of samples were measured using a vibrating sample
magnetometer (VSM, Model MDKB, Meghnatis Danesh
Pajoh Kashan Company, Iran).
Synthesis of the nanocatalyst
For preparation of the supported dioxomolybdenum(VI)
complex, a mixture of Fe3O4@SiO2–NH2 (0.3 g),
MoO2(acac)2 (1 mmol, 0.33 g), and ethanol (30 mL) was
refuxed under N2 for 12 h. After completion of the reaction,
the green solid was separated with an external magnet and
washed three times with deionized water/ethanol. Subse-
quently, Soxhlet extraction was carried out with ethanol to
remove excess/nonreacted metal salt and the fnal product
was dried at 80 °C for 10 h. The dried pale green solid was
denoted as Fe3O4@SiO2@MoO2(VI) nanocatalyst.
General procedure for the synthesis
of 2‑amino‑4H‑benzo[h]chromenes
The catalytic activity of Fe3O4@SiO2@MoO2(VI) for the
synthesis of various chromene derivatives was investigated
using the following procedure. A mixture of the required
aldehyde (1 mmol), 1-naphthol (1 mmol), malononitrile
(1 mmol), and nanocatalyst (0.1 g) was heated to 90 °C
under solvent-free conditions. The progress of the reaction
was monitored using TLC until the aldehyde spot had dis-
appeared. The mixture was then cooled to room tempera-
ture. 5 mL of ethanol was added to the reaction mixture,
and it was stirred for 5 min. The nanocatalyst was decanted
with the help of an external magnet, washed with ethanol
(3 × 15 mL), dried, and then reused in the next reaction. The
obtained products were fltered of and washed with ethanol
three times. The synthesized 2-amino-4H-benzo[h]chromene
derivatives were recrystallized from hot ethanol.
Synthesis of Fe3O4 nanoparticles
Fe3O4 nanoparticles were prepared according to the reported
method [20, 34]. In a typical procedure, FeCl3·6H2O (5.83 g)
and FeCl2·4H2O (2.147 g) were dissolved in deionized
water (150 mL) under an N2 atmosphere. Aqueous ammo-
nia (15 mL, 25% w/w) was added dropwise under N2, and
the mixture was refuxed for 1 h. After completion of the
reaction, the black precipitate was gathered with an external
magnet, washed with deionized water (3 × 10 mL), and dried
in vacuum.
Synthesis of silica‑coated Fe3O4
All products were characterized by FTIR and melt-
ing point, which were in agreement with the litera-
Fe3O4@SiO2 nanoparticles were prepared according to the
reported method, using the Stöber process [35]. Typically,
Fe3O4 nanoparticles (1 g) were dispersed in a mixture of
1
ture [37, 38]. Also, H and 13C NMR spectra of some
1 3

Transition Metal Chemistry
2-amino-4H-benzo[h]chromene derivatives are reported (see
In the FTIR spectrum of Fe3O4 (Fig. 1a), an intense broad
peak at 578 cm−1 is attributed to the Fe–O stretching mode,
while another broad band around 3390 cm−1 is assigned to
O–H stretching of the hydroxyl group attached to Fe. The
spectrum of Fe3O4@SiO2 (Fig. 1b) shows the Fe–O stretch-
ing vibrations at 577 cm−1 and also two bands at 799 and
1089 cm−1 corresponding to the symmetric and asymmet-
ric stretching vibrations of Si–O–Si groups, respectively. In
the spectrum of Fe3O4@SiO2–NH2, a peak at 2924 cm−1 is
ascribed to the –CH2 group of APTES; this peak is absent
from the spectrum of Fe3O4@SiO2 (Fig. 1c). The FTIR spec-
trum of the Fe3O4@SiO2@MoO2(VI) nanocatalyst exhibits
absorption peaks at 584 and 1091 cm−1, corresponding to
Fe–O and Si–O–Si vibrations, respectively. Furthermore,
a new peak at 1635 cm−1 is assigned to the C=N group,
While another new peak at 944 cm−1 (stretching vibration
of O=Mo=O) is evidence of successful grafting of the metal
complex onto the surface of the FeO4@SiO2–NH2 nanopar-
ticles (Fig. 1d) [35].
supporting information).
Selected spectroscopic data
2‑Amino‑4‑(4‑chlorophenyl)‑4H‑benzo[h]chromene‑3‑car‑
bonitrile (Table 2, entry 3) νmax (KBr) 3468, 3327,
2192,1669, 1599, 1407,1374, 1101 cm−1, δH (400 MHz,
CDCl3): 4.82 (br s, 2H, NH2), 4.88 (s,1H, CH), 7.01–8.20
(10H, Ar).
2‑Amino‑4‑(3‑nitrophenyl)‑4H‑benzo[h]chromene‑3‑carbon‑
itrile (Table 2, entry 5) νmax (KBr) 3470, 3328, 2192,1666,
1601,1525,1406,1375, 1102 cm−1, δH (400 MHz, CD3CN):
5.73 (br s, 2H, NH2), 5.09 (s,1H, CH), 7.04–8.30 (10H, Ar).
Results and discussion
Preparation and characterization
of the nanocatalyst
In order to confrm immobilization of the molybdenum
complex onto the surface of the magnetic support, TGA
analysis was also carried out. The TGA curves of Fe3O4
and Fe3O4@SiO2@MoO2(VI) are depicted in Fig. 2. The
Fe3O4 nanoparticles show a weight loss near 100 °C, of
about 3%. This might be due to evaporation of physically
adsorbed water. However, there is no further signifcant
weight loss in the range of 100–800 °C (Fig. 2a). On the
other hand, in the TGA curve of Fe3O4@SiO2@MoO2(VI)
(Fig. 2b), signifcant weight loss was observed in the range
of 250–500 °C (10.28%), due to the thermal decomposi-
tion of the organic groups and metal complex, consistent
The route for the preparation of the nanocatalyst is shown in
Scheme 1. Initially, functionalized magnetic nanoparticles
were prepared from Fe(III) and Fe(II) salts via a co-precipi-
tation method, subsequently encapsulated with a silica shell,
and functionalized with APTES. Finally, the Fe3O4@SiO2@
MoO2(VI) nanocatalyst was formed by reaction of the pre-
cursors with dioxomolybdenum(VI) acetylacetonate. Both
the precursors and fnal nanocatalyst were characterized by
FTIR, TGA, SEM, XRD, EDX, ICP, and VSM.
O
Fe3O4
Fe3O4
TEOS
O
O
Si
Fe3O4
APTES
NH2
Fe3O4
Fe3O4@SiO2
Fe3O4@SiO2-NH2
MoO2(acac)2
O
N
O
O
O
O
O
O
Mo
Fe3O4
Si
O
Fe3O4@SiO2@MoO2(VI)
Scheme 1 General route for the preparation of magnetically recoverable nanocatalyst
1 3

Transition Metal Chemistry
(a)
(b)
O-H
Fe-O
Si-O-Si
(sym)
Fe-O
(c)
Si-O-Si
(asym)
Fe-O
Fe-O
(d)
Mo-O
Si-O-Si
C=N
Si-O-Si
Wavenumber (cm-1)
Fig. 1 FT-IR spectra for Fe3O4 (a), Fe3O4@SiO32 (b), Fe3O4@SiO2–NH2 (c), and Fe3O4@SiO2@MoO2(VI) (d)
measurements, which indicated a molybdenum loading on
the magnetic nanocatalyst of 0.34 mmol g−1.
100
95
90
85
80
(a)
(b)
In order to obtain information about the magnetic prop-
erties of the prepared materials, their magnetization curves
were determined by VSM under N2 atmosphere at room
temperature (Fig. 3). The magnetization curves exhibit no
remanence efects, which clearly indicates their superpara-
magnetic behavior. The magnetization saturation values for
Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2@MoO2(VI) are 55,
35, and 25 emu g−1, respectively. The observed decrease in
the magnetization values is consistent with immobilization
of the metal complex on the surface of Fe3O4@SiO2 [35].
Also, by using an external magnetic feld, the nanocatalyst
can be separated from the reaction mixture; when the mag-
netic feld is removed, it is able to redisperse rapidly, indi-
cating that the nanocatalyst possesses good redispersibility
and responsivity.
0
200
400
600
800
Temperature, ºC
Fig. 2 TGA diagram for Fe3O4 (a) and Fe3O4@SiO2@MoO2(VI) (b)
The XRD patterns of Fe3O4 and Fe3O4@SiO2@
MoO2(VI) are depicted in Fig. 4. The XRD pattern of
Fe3O4 (Fig. 4a) shows six characteristic peaks at 2θ = 30.2°
(220), 35.5° (311), 43.2° (400), 53.6° (422), 57.1° (511),
and 62.7°(440), in agreement with magnetite standard data
(reference JCPDS card number 19-0629), with an inverse
with successful grafting of the metal complex onto the
support. Furthermore, based on the weight loss in the
TGA curve of Fe3O4@SiO2@MoO2(VI), the loading of
MoO2(VI) complex on the surface was estimated to be
0.29 mmol g−1. This is in good agreement with inductively
coupled plasma atomic emission spectroscopy (ICP-AES)
1 3

Transition Metal Chemistry
to Fe3O4, which is probably due to the shielding efects of
silica and the metal complex on the surface [36].
The morphology and size of the prepared materials
were evaluated by SEM (Fig. 5). As shown in Fig. 5a, the
Fe3O4 nanoparticles have approximately spherical morphol-
ogy with some aggregation and vary from 30 to 40 nm in
size distribution. The morphology of the nanocatalyst was
evaluated by FE-SEM (Fig. 5b), revealing the formation of
uniform and mono-dispersed nanoparticles in the range of
90–100 nm. It is obvious that the complexation process has
not signifcantly changed the structure or morphology of the
primary support.
The presence of C, N, O, Si, Fe, and Mo in the structure
of the Fe3O4@SiO2@MoO2(VI) nanocatalyst was confrmed
by EDX analysis (Fig. 6). The element atom ratios of C, N,
O, Si, Fe, and Mo are 8.55, 2.84, 62.75, 12.03, 9.58, and
4.27 wt%, respectively. This observation provides further
evidence for successful attachment of the dioxomolybdenum
complex to the magnetic support.
Fig. 3 Magnetization curves for Fe3O4 (a), Fe3O4@SiO2 (b), and the
Fe3O4@SiO2@MoO2(VI) nanocatalyst (c)
Catalytic performance
After synthesis and characterization of the Fe3O4@SiO2@
MoO2(VI) nanocatalyst, we have investigated its catalytic
activity for the synthesis of 2-amino-4H-benzo[h]chromenes
via the one-pot, three-component reaction of diferent alde-
hydes, malononitrile, and 1-naphthol. Initially, we selected
the reaction of 2-chlorobenzaldehyde with malononitrile and
1-naphthol as a model for optimization of the conditions
(Scheme 2).
The efects of various parameters on catalytic activity
such as solvents, amount of the catalyst, and temperature
were examined, and the results are collected in Table 1. A
control experiment without any catalyst showed negligi-
ble conversion even after 24 h (Table 1, entry 1). Hence,
the nanocatalyst is essential for this synthesis. Similarly,
the use of Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2–NH2, and
MoO2(acac)2 in place of the Fe3O4@SiO2@Mo(VI) nano-
catalyst gave no reaction even after 24 h (Table 1, entries
2–5).
The efects of various solvents such as H2O, ethanol,
and methanol were then investigated (Table 1, entries 6–8).
Moreover, solvent-free condition was also tested; signifcant
yield and shorter reaction time were observed when this con-
dition was employed (Table 1, entry 10).
Fig. 4 XRD pattern for Fe3O4 (a) and Fe3O4@SiO2@MoO2(VI) (b)
To examine the efect of the quantity of nanocatalyst on
the catalytic activity, diferent quantities of Fe3O4@SiO2@
Mo(VI) were employed (Table 1, entries 9–12). It was found
that 0.1 g of nanocatalyst gave maximum yield (96%) under
solvent-free conditions at 90 °C (Table 1, entry 10). Lower
amounts gave poorer yields, but higher amounts of the nano-
catalyst gave no further improvement.
cubic spinel structured without impurity phases. The XRD
pattern of the Fe3O4@SiO2@MoO2(VI) nanocatalyst also
shows six peaks related to Fe3O4 (Fig. 4b). Therefore, the
magnetic support does not change phase during immobiliza-
tion of the metal complex. The peak intensities in the XRD
pattern of the nanocatalyst are slightly decreased compared
1 3

Transition Metal Chemistry
Fig. 5 SEM image of Fe3O4 (a), Fe3O4@SiO2@MoO2(VI) (b), and reused nanocatalyst after 4th run (c)
We next investigated the efect of temperature on the
activity of the nanocatalyst; these experiments suggested
90 °C as the optimum temperature. Upon raising the temper-
ature to 125 °C, the yield was decreased (Table 1, Entry 15).
Conversely, on running the reaction at room temperature,
the yield was decreased to trace (Table 1, entries 13–14).
The optimum conditions were thus 0.1 g of the nanocatalyst,
under solvent-free conditions at 90 °C.
The results are summarized in Table 2. Both electron-rich
and electron-poor aldehydes reacted well in this method
to obtain the corresponding products in good to excellent
yields, although the best results were obtained with the
electron-withdrawing substituents.
In order to evaluate the present system in more depth, a
comparison was made between the current procedure and
several published methods for the reaction of 2-chlorobenza-
ldehyde, malononitrile, and 1-naphthol, as shown in Table 3
[39–44]. The results show that the present method has some
merits compared to the previous literature reports in terms
of yield, reaction time and temperature. Furthermore, the
In order to investigate the versatility of this method, dif-
ferent aromatic aldehydes were treated with malononitrile
and 1-naphthol for the synthesis of corresponding 2-amino-
4H-benzo[h]chromenes under the optimized conditions.
1 3

Transition Metal Chemistry
Fig. 6 EDX analysis of the
Fe3O4@SiO2@MoO2(VI)
nanocatalyst
NH2
CN
O
Cl
CHO
OH
CN
CN
Cl
Cat.
+
+
Scheme 2 One-pot reaction of 2-chlorobenzaldehyde, 1-naphthol, and malononitrile in the presence of nanocatalyst as model reaction
Table 1 Optimization and
Entry
Catalyst (amount)
Solvent
Temperature Time
(°C)
Yielda
screening of the nanocatalyst for
synthesis of desired product
1
2
3
4
5
6
7
8
Without catalyst
Fe3O4
Fe3O4@SiO2
Fe3O4@SiO2–NH2
No
No
No
No
No
H2O
EtOH
CH3OH
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
90
90
90
90
90
90
90
90
90
90
90
90
25
50
125
24 h
24 h
24 h
24 h
24 h
1:15 h
2 h
2:30 h
1:20 h
1 h
50 min
45 min
24 h
Trace
Trace
Trace
Trace
Trace
75%
50%
40%
60%
96%
96%
97%
Trace
65%
MoO2(acac)2
Fe3O4@SiO2@MoO2(VI) (0.1 g)
Fe3O4@SiO2@MoO2(VI) (0.1 g)
Fe3O4@SiO2@MoO2(VI) (0.1 g)
Fe3O4@SiO2@MoO2(VI) (0.05 g)
Fe3O4@SiO2@MoO2(VI) (0.1 g)
Fe3O4@SiO2@MoO2(VI) (0.2 g)
Fe3O4@SiO2@MoO2(VI) (0.3 g)
Fe3O4@SiO2@MoO2(VI) (0.1 g)
Fe3O4@SiO2@MoO2(VI) (0.1 g)
Fe3O4@SiO2@MoO2(VI) (0.1 g)
9
10
11
12
13
14
15
1:30 h
2 h
50%
Bold indicates the optimized conditions
All reactions were carried out with 2-chlorobenzaldehyde (1 mmol), malononitrile (1 mmol), and 1-naph-
thol (1 mmol)
aIsolated yield
1 3

Transition Metal Chemistry
Table 2 Synthesis of 2-amino-4H-benzo[h]chromenes in the presence
Table 2 (continued)
Entry Aldehyde Time Yield (%)a Melting
of nanocatalyst
Melting point
(literature)
(°C)
Entry Aldehyde Time Yield (%)a Melting
Melting point
(literature)
(°C)
point (our
work) (°C)
point (our
work) (°C)
7.
1 h
90
247–249
246–248 [37,
38]
1.
75 min 87
217–218
217–219 [37,
38]
CHO
OH
CHO
2.
3.
1 h
1 h
96
92
162–164
232–234
162–162 [37,
38]
All reactions were carried out with aldehyde (1 mmol), malononi-
trile (1 mmol), and 1-naphthol (1 mmol) in the presence of Fe3O4@
SiO2@MoO2(VI) (0.1 g) under solvent-free condition at 90 °C
CHO
Cl
aIsolated yield
present catalytic system ofers other advantages such as mild
reaction conditions, stability, easy recovery of the catalyst,
and low toxicity compared to other systems.
232–233 [37,
38]
CHO
Proposed reaction pathway
A plausible reaction mechanism for the synthesis of
2-amino-4H-benzo[h]chromenes using the nanocayalyst is
shown in Scheme 3 [10, 45]. In this mechanism, the nanocat-
alyst reacts with the aldehyde and the cyano group and acti-
vates it for nucleophilic attack. First, malonitrile reacts with
the aldehyde to give an α, β-unsaturated nitrile (I), which
then reacts with 1-naphthol to give the dicyano compound
(II). The phenolic OH group then undergoes rapid nucleo-
philic addition to the C=N moiety, and fnally, tautomeriza-
tion of intermediate (III) gives the 2-aminochromene (IV).
Cl
4.
45 min 94
238–240
238–240 [37,
38]
CHO
NO2
Reuse of the nanocatalyst
5.
6.
40 min 92
210–212
203–205
212–213 [37,
38]
CHO
The possibility of recycling the nanocatalyst was examined
for the preparation of 2-amino-4H-benzo[h]chromenes
under the optimized conditions (Table 4). The reaction mix-
ture was recovered each time by an external magnet, then
washed with hot ethanol (3 × 10 mL), and dried. The nano-
catalyst could be reused for at least four runs with only a
slight decrease in its catalytic activity. Also, after the fourth
catalytic run, SEM imaging was carried out (Fig. 5c). The
morphology and structure of the reused nanocatalyst were
scarcely changed, consistent with its stability.
NO2
1:10 h 85
204–206 [37,
38]
CHO
Me
To further probe the stability of the nanocatalyst, we car-
ried out leaching tests. The molybdenum leaching in the frst
and fourth runs was measured by ICP, giving values of 0.001
(0.33%) and 0.002 mmol g−1 (0.61%), respectively. Hence,
the amount of leaching of the nanocatalyst is negligible, and
the complex is strongly bonded to the surface of the support.
1 3

Transition Metal Chemistry
Table 3 Comparison of present work with other reported methods in the reaction of aromatic aldehydes, malononitrile, and 1-naphthol
Entry
Catalyst (amount)
Solvent
Temperature
Time
Yield (%)
Ref.
1
2
3
4
5
6
7
HTMAB (10 mol%)
[bmim]OH (0.5 mmol)
H14[NaP5W30O110] (0.03 g)
Silica tungstic acid (STA) (0.1 g)
Rochelle salt (0.3 g)
Fe3O4@SiO2@Mo-Schif base (0.2 g)
(Fe3O4@SiO2@MoO2(VI) (0.1 g)
H2O
H2O
H2O
Solvent-free
Ethanol
Solvent-free
Solvent-free
100 °C
100 °C
100 °C (refux)
120 °C
78 °C (refux)
125 °C
4 h
60 min
3 h
93
91
91
90
90
95
96
[39]
[40]
[41]
[42]
[43]
[44]
This study
3 h
4-8 h
60 min
60 min
90 °C
Ar
O
O
O
O
O
Mo
CN
Fe3O4
N
O
O
Si
O
O
NH2
O
(Catalyst)
(IV)
Ar
Catalyst
O
H
Ar
H
CN
NH
O
H
Ar
CN
(III)
Catalyst
CN
Ar
H2O
CN
Catalyst
C
N
O
Ar
CN
CN
H
OH
Catalyst
Catalyst
(II)
H
(I)
Catalyst
CN
Ar
H
Catalyst
Ar
CN
CN
H
CN
O
Catalyst
Scheme 3 Plausible mechanism for the synthesis of 2-amino-4H-benzo[h]chromenes catalysed by Fe3O4@SiO2@MoO2(VI)
1 3

Transition Metal Chemistry
Chapter green nanotechnology: development of nanomaterials
for environmental and energy applications. American Chemical
Society, Washington
Table 4 Reusability test of the
Entry Cycle
Yield (%)
96
Fe
3O4@SiO2@MoO2(VI) in the
model reaction
1
2
3
4
First run
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16:307–325
Second run 92
Third run 88
Fourth run 86
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Conclusions
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loids Surf A 262:87–93
In this work, a molybdenum Schif base complex has been
anchored on the surface of Fe3O4 nanoparticles modifed
with a silica shell. This material was investigated as a
magnetically recoverable nanocatalyst for the synthesis of
2-amino-4H-benzo[h]chromenes. The supported dioxomo-
lybdenum nanocatalyst gave high yields of products under
solvent-free conditions at 90 °C. The notable advantages
of this work are the use of green, nontoxic materials, good
stability of the nanocatalyst, and facile reusability.
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Acknowledgements The authors acknowledge the University of
Mazandaran for fnancial support of this work.
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