An inorganic–organic hybrid material based on a Keggin-type polyoxometalate@Dysprosium as an effective and green catalyst in the synthesis of 2-amino-4H-chromenes via multicomponent reactions

Article abstract of DOI:10.1002/aoc.5793

A novel inorganic–organic hybrid, [Dy₄(PDA)₄(H₂O)₁₁(SiMo₁₂O₄₀)]·7H₂O denoted as (POM@Dy-PDA), based on a lanthanide cluster, a Keggin-type polyoxomolybdate, and PDA (1,10-phenant

Full text of DOI:10.1002/aoc.5793

Received: 19 February 2020
DOI: 10.1002/aoc.5793
Revised: 18 April 2020
Accepted: 19 April 2020
F U L L P A P E R
An inorganic–organic hybrid material based on a Keggin-
type polyoxometalate@Dysprosium as an effective and
green catalyst in the synthesis of 2-amino-4H-chromenes via
multicomponent reactions
Sara Hosseinzadeh-Baghan1 | Masoud Mirzaei1
| Hossein Eshtiagh-Hosseini1
|
Vahideh Zadsirjan2 | Majid M. Heravi2
| Joel T. Mague3
1Department of Chemistry, Faculty of
A novel inorganic–organic hybrid, [Dy4(PDA)4(H2O)11(SiMo12O40)]·7H2O den-
oted as (POM@Dy-PDA), based on a lanthanide cluster, a Keggin-type poly-
oxomolybdate, and PDA (1,10-phenanthroline-2,9-dicarboxylic acid) was
prepared and fully characterized by elemental analysis, Fourier-transform
infrared and UV–Vis spectroscopies, thermogravimetric analysis, powder X-ray
diffraction (PXRD), and single-crystal X-ray diffraction. The structural analysis
study showed that the [SiMo12O40]4− ions reside in the interspace between two
cationic layers as discrete counterions and are not coordinated to the rare-
earth ions. Significantly, this hybrid catalyst is a rare case of an inorganic–
organic hybrid polyoxometalate (POM) with a PDA ligand based on CSD sea-
rch (CSD version 5.40 m2 g−1 and 51.3 m2 g−1, respectively. The catalytic activ-
ity of the hybrid catalyst was successfully examined in the synthesis of
2-amino-4H-chromene derivatives through a multicomponent reaction. A
three-component, one-pot reaction involving differently substituted benzalde-
Science, Ferdowsi University of Mashhad,
Mashhad, 917751436, Iran
2Department of Chemistry, School of
Science, Alzahra University, PO Box
1993891176, Tehran, Vanak, Iran
3Department of Chemistry, Tulane
University, New Orleans, LA, 70118, USA
Correspondence
Masoud Mirzaei, Department of
Chemistry, Faculty of Science, Ferdowsi
University of Mashhad, 917751436
Mashhad, Iran.
Email: mirzaeesh@um.ac.ir
Majid M. Heravi, Department of
Chemistry, School of Science, Alzahra
University, PO Box 1993891176, Vanak,
Tehran, Iran.
hydes,
resorcinol/α-naphthol/β-naphthol/4-hydroxycoumarin/3-methyl-4H-
Email: mmh1331@yahoo.com;
mmheravi@alzahra.ac.ir
pyrazole-5(4H)-one, and malononitrile or ethyl cyanoacetate in the presence of
a catalytic quantity of the aforementioned hybrid catalyst in EtOH/H2O under
reflux condition gave the corresponding highly functionalized 2-amino-4H-
chromenes in satisfactory yields. The catalyst can be reused several times with-
out appreciable loss in its catalytic activity.
Funding information
Ferdowsi University of Mashhad, Grant/
Award Number: 3/42203
Highlights
• A mild, simple, convenient, and efficient synthetic strategy was developed
for the synthesis of biologically and pharmacologically active products.
• A novel inorganic–organic hybrid, 2[Dy4(PDA)4(H2O)11(SiMo12O40)]·7H2O
denoted as POM@Dy-PDA), based on a lanthanide cluster, a Keggin-type
polyoxomolybdate, and PDA (1,10-phenanthroline-2,9-dicarboxylic acid)
was synthesized.
• POM@Dy-PDA catalyzed synthesis of 2-amino-4H-chromenes via one-pot
multicomponent reactions.
Appl Organomet Chem. 2020;e5793.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5793

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HOSSEINZADEH-BAGHAN ET AL.
• This protocol is being performed under green conditions, isolating the
desired products through facile work-up procedure, in excellent yields.
K E Y W O R D S
2-amino-4H-chromenes, Heteropoly acids, inorganic–organic hybrid, Keggin, polyoxometalates
1 | INTRODUCTION
anti-inflammatory,[5]
and antiallergenic[9,10] activities, together with
antineurodegenerative activity toward disorders
antitumor,[6]
anti-HIV,[7,8]
The concept of green chemistry plays a significant role in
adapting major scientific encounters to protect our envi-
ronment. Commonly, the reaction conditions are totally
changed, modified, or improved in a way to minimize
pollution and side products. Thus, one avoids the use of
toxic and volatile organic solvents, performs the reaction
at or close to ambient conditions, uses a green heteroge-
neous catalyst, and if heating is required, one uses
unconventional sources of energy such as Microwave
irradiation (MWI) or ultrasonic irradiation. If possible,
solvent-free conditions should be used to perform useful
chemical conversions while minimizing side products
and waste material. Thus, the design and preparation of
novel green and reusable heterogeneous catalysts to ful-
fill the principles of green chemistry are still in much
demand.[1,2] Multicomponent reactions (MCRs) play a
significant role in modern synthetic organic chemistry.
Their well-established advantages such as atom economy,
convergent character, simplicity, and usefulness of a one-
pot method, performing the reaction without isolation
and purification of intermediates, use of commercially
available inexpensive or easily accessible starting mate-
rials are quite in agreement with the principles of green
chemistry.[3] Different heterocyclic systems, for example,
benzopyrans, benzoxanthene, and benzochromene have
been constructed via one-pot MCRs.[3] 4H-Pyran deriva-
tives and 4H-pyran-annulated heterocyclic moieties,
namely, 4H-chromene, which are the common frame-
work of various oxygen-containing heterocycles present
in natural products have attracted much attention. This
structural scaffold is generally present in various kinds of
alkaloids exhibiting different pharmacological and bio-
logical properties such as antimicrobial and antifungal,[4]
including Huntington’s diseases, Parkinson’s disease, and
Alzheimer’s disease.[11,12] These aspects have attracted
the attention of synthetic organic chemists to design and
develop analogs of several alkaloids, for example,
(+)-calanolide A,[13] arisugacin,[14] veprisine, and
huajiaosimuline,[15,16] that all include the main
pyran-annulated pharmacophoric scaffold (Figure 1).
2-Amino-4H-chromene derivatives are known to
be scaffolds
of
biologically
active,
naturally
occurring compounds,[17] thus various approaches for
the construction of dihydropyrano[2,3-c]chromene
derivatives have been developed.[18–22] 2-Amino-3-cyano-
4H-chromene derivatives are commonly synthesized
through MCRs involving an appropriate aldehyde,
malononitrile (ethyl cyanoacetate),[23] and enolizable
C–H acids such as barbituric acid, dimedone, resorcinol,
naphthol (α and β), Kojic acid, and 2-hydroxy-1,-
4-naphthoquinone-4-hydroxycoumarin in a one-pot fash-
ion. The reaction is assumed to proceed via
a
Knoevenagel-carba-Michael–Thorpe–Ziegler-type
cas-
cade method.[24] These MCRs are performed in the
presence of diverse heterogeneous or homogeneous
catalysts including NaOH,[25] TiCl4,[26] piperidine,[27]
K2CO3,[28]
Et3N,[31]
InCl3,
tetramethylguanidine,[30]
[29]
triethylbenzylammonium
chloride,[23]
chloride,[32]
Preyssler
cetyltrimethylammonium
heteropoly acid,[33] nano-sized magnesium oxide,[34] N,
N-dimethylaminoethylbenzyl
dimethylammonium
chloride,[35] or γ-alumina.[35] These approaches have
their own merits and drawbacks, and thus the introduc-
tion of novel effective heterogeneous and green catalysts
is still in much demand. Nowadays, the mainstream of
FIGURE 1 Selected biologically active
compounds containing the pyran-annulated
motif

HOSSEINZADEH-BAGHAN ET AL.
3 of 23
the research has been focused on the preparation of new
active catalysts with economic feasibility for being
conducted at an industrial level as well as complying
with green chemistry principles.[36,37] Heteropolyacids
(HPAs) and polyoxometalates (POMs) are well-known
anionic metal–oxygen clusters with a wide range of
sizes, nuclearities, shapes, and varied redox properties.
HPAs and POMs have been employed in photochro-
mism, separation, electrochemistry, magnetism, and
especially catalysis.[38–42] Among the POMs, Keggin-
type polyoxomolybdates have recently found many
applications.[43–46] The application of POMs as catalysts
originated from their inherent resistance to oxidative
decomposition, high thermal stability, impressive
sensitivity to light and electricity,47 and having differ-
ent active sites such as protons, oxygen atoms, and
metals. The presence of protons and metal ions with
unoccupied orbitals give higher acidic character to
POMs, which guarantee their catalytic activity espe-
cially in acid-catalyzed reactions. In addition, the basic
properties of some surface oxygen atoms of POM
anions, in particular those on the lacunary sites of
lacunary POM anions with a high negative charge, are
suitable to be used in base-catalyzed reactions.[48–52]
POMs have been extensively used as homogeneous cat-
alysts, owing to their good solubility in water and sev-
eral organic solvents. Although homogenous catalysis
is often preferred over heterogeneous catalysis in many
organic transformations, they suffer from some
drawbacks such as difficulty in separation of the
catalyst from the reaction mixture, which hampers
their reusability as well as purification of products.
Heterogenization of homogeneous catalysts can circum-
vent the aforementioned problems which are of great
importance from economic and environmental points
of view in academia and particularly in chemical
industries.[53–55] By contrast, the specific surface areas
of pure bulk POMs are relatively small which prevent
accessibility to active sites, thus affecting their catalytic
activity. For these reasons, the catalyst engineering of
POMs is in much demand. Traditional methods for the
catalyst engineering of POMs include increasing their
specific surface areas and immobilization of POMs on
various porous silica and zeolite supports.[56,57]
However, the hydrophilic frameworks of the support
largely restrict their catalytic activities because of the
low accessibility of the active sites for hydrophobic
reactants. A promising approach to overcome such
problems is the heterogenization of POMs, that is,
combination with metal–organic coordination com-
plexes to construct crystalline inorganic–organic
hybrids with high-dimensional supramolecular net-
works. In this way, the high catalytic performance is
achieved by increasing the specific surface area and
number of active sites. In addition, synergistic
interaction of two moieties in inorganic–organic hybrid
compounds results in effective recyclability of
heterogeneous catalyst that agrees with the principles
of green chemistry.[58–62] Hybrid POMs have been
studied as catalysts in different reactions such as
oxidation of alcohols and carbonyl compounds, alkene
polymerization, and the hydrolysis of esters.[63–68] For
example, Fe(salen)–POM and Co(salen)–POM were
successfully used in oxidation of alkanes and primary
and secondary benzyl halides. In these transformations,
M(salen)–POM gave better yields in shorter reaction
times compared with those of its parent, M(salen)
complex.[69,70]
In the last two decades, our research group
has employed HPAs,[71] POMs[,72] and inorganic–organic
hybrids based on PMOs[73–79] as highly effective and
green homogeneous and heterogeneous catalysts.[65,80–82]
Among POMs, Keggin-type [XM12O40]n- ions have been
mostly employed for constructing inorganic–organic
hybrids materials because of their suitable size and struc-
tural stability. They can interact with metal–organic com-
plexes through covalent and noncovalent bonds which
play an inorganic ligand and a charge-balancing anion
role, respectively.[83,84] Lanthanoid coordination com-
plexes are particularly valuable in this context because of
their Lewis acid properties.[85–88] They are hard Lewis
acids in which hybridization with POMs increase cata-
lytic activity. They also have a tendency to form bonds
with ligands containing hard donor atoms such as oxy-
gen, nitrogen, or fluorine. Expectedly, the suitable selec-
tion of metal–organic compounds for combination with
POMs gives rise to efficient catalytic properties. There-
fore, the logical choice of organic ligand is one of the
important points in designing a polymer that can eventu-
ally determine the dimensionality of the final solid state
structure. One of the best kinds of ligands that require
the lowest energy upon complexation to metal ion and
that give more thermodynamic stability and greater
selectivity without any change in conformation are
preorganized ligands.[89–91] Phenanthroline ligands
with donor groups at the 2- and 9-positions, that is,
alcohols, amides, and carboxylates, especially PDA
(1,10-phenanthroline-2,9-dicarboxylic acid), are well-
known examples of preorganized ligands. Some
metal-ion complexing properties of PDA have been
reported by Hancock et al.[92–94] A CSD search on
1,10-phenanthroline (phen) showed that much research
has been done with this ligand (more than 9000 com-
pounds so far). However, according to the CSD, PDA, as
an important phen derivative, has just[64] compounds, in
which there are metal–organic complexes and only four

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HOSSEINZADEH-BAGHAN ET AL.
inorganic–organic hybrid architecture based on Keggin-
type POMs. Based on the points mentioned earlier, in this
article we wish to reveal the preparation of a novel
inorganic–organic hybrid catalytic system using PDA as
the organic ligand, [SiMo12O40]4− Keggin-type POM as
the inorganic building unit, and dysprosium(III) as the
lanthanide ion by hydrothermal methods and our suc-
cessful attempts to examine it as an effective and green
catalyst in the synthesis of 2-amino-4H-chromenes via
MCR. We are interested in heterocyclic chemistry,[95–98]
particularly in the synthesis of heterocyclic derivatives
through MCR[99–101] in the presence of heterogeneous
catalysts,[102,103] especially in the presence of POMs as
heteropoly acids (HPAs) and salts of POM. Herein, we
describe the synthesis of a novel POM@Dy-PDA as a
unique organic–inorganic hybrid, which is fully charac-
terized using Fourier-transform infrared (FT-IR) spec-
troscopy, elemental analysis, UV–Vis, thermogravimetric
analysis (TGA), powder X-ray diffraction (PXRD), and
single-crystal X-ray diffraction. Its catalytic activity was
successfully examined in the synthesis of 2-amino-4H-
chromene derivatives via one-pot, three-component reac-
tions of aromatic aldehydes with resorcinol/α-naphthol/
β-naphthol/4-hydroxycoumarin/3-methyl-4H-pyrazole-5
(4H)-one, and malononitrile or ethyl cyanoacetate
(Scheme 1).
2 | EXPERIMENTAL
2.1 | Materials
All chemicals were purchased from Merck Company and
used as received, except for PDA, which was synthesized
according to a reported procedure.[104] IR spectra were
recorded in KBr pellets in the 4000–400 cm−1 region
using a Buck 500 IR and Tensor 27 spectrophotometer.
Elemental analysis (CHN) was performed using a
SCHEME 1 One-pot and three-component
synthesis of 2-amino-4H-chromenes 5–7
catalyzed by POM@Dy-PDA under reflux
condition in EtOH/H2O. Dy, dysprosium; PDA,
1,10-phenanthroline-2,9-dicarboxylic acid;
POM, polyoxometalate

HOSSEINZADEH-BAGHAN ET AL.
5 of 23
FIGURE 4 The UV–Vis absorption spectra of the POM@Dy-
PDA in dimethyl sulfoxide. Dy, dysprosium; PDA,
FIGURE 2 Fourier transform infrared spectrum of POM@Dy-
PDA (green) and pure [SiMo12O40]4− (red). Dy, dysprosium; PDA,
1,10-phenanthroline-2,9-dicarboxylic acid; POM, polyoxometalate
1,10-phenanthroline-2,9-dicarboxylic acid; POM, polyoxometalate
Thermo Finnigan Flash-1112EA instrument (German
Merck Company). Thermal gravimetric analysis (TGA)
was carried out under an air atmosphere from ambient
temperature up to 950 ꢀC with a heating rate of
10 ꢀC min−1 on a Shimadzu TGA-50 instrument. Melting
points were measured by an electro thermal 9200 appara-
tus (German Merck Company). UV–Vis spectroscopy
(German Merck Company) was conducted in the wave-
length range 200–850 nm using a Perkin Elmer (Lambda
35) spectrophotometer. All products were identified by
comparison of their physical (melting points) and spec-
tral (FT-IR spectra) properties with those of authentic
samples and found to be identical. The PXRD pattern
was recorded with an ASENWAREV (AW-XDM300)
(German Merck Company) diffractometer at voltage
45 kv and current 40 mA with Cu- radiation ( = 1.54184
). Brunauer–Emmett–Teller (BET) (German Merck Com-
pany) surface area analysis was performed using
BELSORP-mini II instrument to obtain the nitrogen
adsorption–desorption isotherms at 77 K. Samples were
degassed under vacuum at 313 K for 24 h. The average
pore diameter was calculated using the Barrett–Joyner–
Halenda method.
2.2 | Synthesis and characterization of
[Dy4(PDA)4(H2O)11(SiMo12O40)]·7H2O hybrid
catalyst)POM@Dy-PDA(
A mixture of H4[SiMo12O40]·xH2O (182 mg, 0.1 mmol),
Dy(NO3)3·6H2O (109 mg, 0.25 mmol), PDA (67 mg,
0. 25 mmol), and deionized water (15 mL) was stirred in
air for about 30 min and the pH value was adjusted to
about 1.8 with aqueous NaOH (1 M). The mixture was
transferred to a Teflon-lined autoclave (30 mL) and kept
ꢀ
at 130 C for 3 days. After slow cooling for 2 days, the
solution was filtered off and light yellow crystals were
obtained in 51.44% yield (based on Mo). Anal. Calcd. For
C112H106Dy8Mo24N16O147Si2: C 17.50, H 1.39, N 2.92%;
found: C 17.16, H 1.62, N 2.94%. IR (KBr pellet, cm−1):
3381, 1600, 1568, 1464, 1389, 1309, 952, 904, 799, 716.
FIGURE 3 Thermogravimetric analysis (TGA) plots of the
POM@Dy-PDA. Dy, dysprosium; PDA, 1,10-phenanthroline-
2,9-dicarboxylic acid; POM, polyoxometalate
FIGURE 5 The powder X-ray diffraction pattern of catalyst:
green, calculated from single-crystal X-ray data; red,
experimental data

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HOSSEINZADEH-BAGHAN ET AL.
2.3 | Synthesis of 2-amino-4H-chromenes:
General procedure
H-pyrazole-5(4H)-one (1 mmol) was refluxed in
EtOH/H2O (5 mL) in the presence of a catalytic quantity
of POM@Dy-PDA (10 mol%) for the indicated reaction
time. The progress of the reaction was monitored by thin-
layer chromatography (TLC; 7:3 n-hexane/ethyl acetate).
Upon completion of the reaction (indicated by TLC), the
A mixture of an appropriate benzaldehyde (1 mmol),
malononitrile/ethyl cyanoacetate (1 mmol), and
4-hydroxycoumarin/α/β-naphthol/resorcinol/3-methyl-4
FIGURE 6 Nitrogen adsorption
(ADS)–desorption (DES) isotherms of
POM@Dy-PDA. (b) Pore size
distribution curve of POM@Dy-PDA.
Dy, dysprosium; PDA,
1,10-phenanthroline-2,9-dicarboxylic
acid; POM, polyoxometalate
TABLE 1
Crystallographic data and refinement details for POM@Dy-PDA
Crystal data
Chemical formula
2(C56H46Dy4N8O27)·2(Mo12O40Si)·2.5(H2O)·4O
Mr
7716.85
Crystal system, space group
Monoclinic, P21/n
Temperature (K)
a, b, c (Å)
β (ꢀ)
150
13.258 (3), 23.375 (5), 33.805 (7)
99.717 (3)
V (Å3)
10326(4)
Z
2
Radiation type
Mo-Kα
μ (mm−1
)
4.38
Crystal size (mm)
0.28 × 0.19 × 0.09
Data collection
Diffractometer
Bruker Smart APEX CCD
Multi-scan SADABS
0.37, 0.69
Absorption correction
Tmin, Tmax
Number of measured, independent, and observed [I > 2σ(I)] reflections
198,935; 27,813; 21,088
0.070
Rint
(sin θ/λ)max (Å−1
)
0.687
Refinement
R[F2 > 2σ(F2)], wR(F2), S
Number of reflections
Number of parameters
Number of restraints
H-atom treatment
0.070, 0.189, 1.09
27,813
1480
882
H-atom parameters constrained
w = 1/[σ2(Fo2) + (0.0807P)2 + 169.5744P], where P = (Fo2 + 2Fc2)/3
Δρmax, Δρmin (e Å−3
CCDC no.
)
8.85, −6.80
1946762
Computer programs: APEX3 (Bruker, 2016), SAINT (Bruker, 2016), SHELXT (Sheldrick, 2015a), SHELXL-2018/1 (Sheldrick, 2015b), DIA-
MOND (Brandenburg & Putz, 2012), SHELXTL (Bruker, 2016).
Dy, dysprosium; PDA, 1,10-phenanthroline-2,9-dicarboxylic acid; POM, polyoxometalate.

HOSSEINZADEH-BAGHAN ET AL.
7 of 23
mixture was filtered under reduced pressure. The filtrate
was cooled to room temperature and the precipitated
solid was separated by filtration under reduced pressure.
The anticipated products were purified by crystallization
from a mixture of EtOH/H2O to give the respective
desired products. The products were identified by com-
parison of their melting points as well as their FT-IR
spectra with those of authentic samples (Tables 4–7)
(Supporting Information).
3 | RESULTS AND DISCUSSION
3.1 | Synthesis and characterization of
POM@Dy-PDA
The successful synthesis of POM@Dy-PDA was achieved
by the hydrothermal reaction of lanthanide nitrate,
organic ligand, and H4SiMo12O40·xH2O as the inorganic
building unit. The applied reaction conditions such as
FIGURE 7 A view of the
molecular structure of POM@Dy-PDA.
Interstitial water molecules and H
atoms have been omitted for clarity.
Color code: Dy, yellow; O, red; Mo,
purple; N, blue; Si, green; and C, gray.
Dy, dysprosium; PDA,
1,10-phenanthroline-2,9-dicarboxylic
acid; POM, polyoxometalate
FIGURE 8 Coordination modes found for
the PDA ligand in the POM@Dy-PDA. The
ligands chelating Dy1, Dy3 are found in mode
I and for ligands chelating Dy2, Dy4 mode II is
found
FIGURE 9 Partial view of the
crystal packing of POM@Dy-PDA
showing the intermolecular anion–
interactions (in Å) established between
the organic ligand and atoms O51, O64,
and O31 of the Keggin countepart.
Color code: Dy, yellow; O, red; Mo,
purple; N, blue; Si, green; and C, gray.
Dy, dysprosium; PDA,
1,10-phenanthroline-2,9-dicarboxylic
acid; POM, polyoxometalate

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HOSSEINZADEH-BAGHAN ET AL.
temperature, pH value, and molar ratio of reactants have
been optimized. The mixture was stirred in air for
30 min. Finally, the pH was adjusted to approximately
1.8 with aqueous NaOH solution. The mixture wasꢀtrans-
ferred to a Teflon-lined autoclave and kept at 130 C for
3 days. After slow cooling for 2 days, the solution was fil-
tered off and light yellow crystals were obtained. Anal.
Calcd. For POM@Dy-PDA: C 17.50, H 1.39, N 2.92%;
found: C 17.16, H 1.62, N 2.94%, which show that the
results of the analysis are in good agreement with calcu-
lated values.
The FT-IR spectra of POM@Dy-PDA and
[SiMo12O40]4− Keggin are shown in Figure 2. The pres-
ence of the Keggin-type [SiMo12O40]4− anions is
confirmed by the characteristic pattern in the region
below 1000 cm−1. Four bands are attributed to ν(Si–Oa),
ν(Mo=Ot), ν(Mo–Ob), and ν(Mo–Oc) stretching vibra-
tions, which appear at an average of 952, 904, 860, and
798 cm−1, respectively (where Ot is a terminal oxygen, Ob
is a bridging oxygen, Oa is an internal oxygen, and Oc is a
bridging oxygen within the edge-sharing octahedra).[105]
Compared with the typical parent anion in
[SiMo12O40]4−, the vibrational frequencies related to the
terminal oxide groups (Mo=Ot) and (Mo–Ob) are red
shifted. This shows that the terminal (Ot) and bridge (Ob)
oxygen atoms are most affected by hydrogen bonding
with water molecules.[106] The characteristic pattern in
the region 1000–1621 cm−1 can be attributed to the coor-
dinated PDA ligands. The absence of ν(C=O) stretching
vibrations at 1700–1720 cm−1 demonstrates the complete
deprotonation of the carboxylic groups in the PDA
ligands. Strong asymmetric and symmetric stretching
frequencies for the carboxylate groups appear at 1600 and
1389 cm−1, respectively. The bands resulting from the
vibrations of the aromatic skeleton also appear at
1464 cm−1. The band centered at 3381 cm−1 is attributed
to the ν(OH) vibrations of the lattice and coordinated
water molecules.
FIGURE 10 (a) A
polyhedral view of the 2D
supramolecular sheet in
POM@Dy-PDA. (b) A
polyhedral view of the 3D
supramolecular framework of
hybrid catalyst. (Note: Only for
more clarity, in b, Mo atoms are
represented in two different
colored polyhedra). Dy,
dysprosium; POM,
polyoxometalate
TABLE 2
Selected bond lengths (Å) of POM@Dy-PDA
Dy1–O6
Dy1–O3
Dy1–O77[i]
Dy1–O1
Dy1–O9[ii]
Dy1–O5
Dy1–N2
Dy1–N1
Dy2–O10
Dy2–O12
Dy2–O11
2.276 (7)
2.325 (8)
2.341 (7)
2.356 (8)
2.382 (7)
2.400 (8)
2.468 (8)
2.485 (8)
2.308 (8)
2.346 (7)
2.354 (9)
Dy2–O8
Dy2–O7
Dy2–O13
Dy2–N3
Dy2–N4
Dy3–O19
Dy3–O15
Dy3–O17
Dy3–O18
Dy3–O14
Dy3–N5
2.354 (7)
2.378 (8)
2.384 (6)
2.495 (7)
2.519 (7)
2.343 (7)
2.346 (7)
2.350 (7)
2.371 (7)
2.376 (6)
2.479 (7)
Dy4–O2[iii]
Dy4–O25
Dy4–O23
Dy4–O21
Dy4–O26
Dy4–O20
Dy4–O24
Dy4–N7
2.356 (9)
2.377 (9)
2.384 (9)
2.388 (8)
2.402 (9)
2.466 (8)
2.494 (12)
2.512 (7)
2.540 (7)
2.356 (9)
2.377 (9)
Dy4–N8
Dy4–O2[iii]
Dy4–O25
Symmetry codes:
i− x + 3/2, y + 1/2, −z + 1/2;
ii− x + 1/2, y + 1/2, −z + 1/2;
iii− x + 3/2, y − 1/2, −z + 1/2.
Dy, dysprosium; PDA, 1,10-phenanthroline-2,9-dicarboxylic acid; POM, polyoxometalate.

HOSSEINZADEH-BAGHAN ET AL.
9 of 23
To investigate the thermal stabilities of the
POM@Dy-PDA, TGA experiments wereꢀ carried out
under an air atmosphere from 20 to 950 C. The TGA
curve exhibits three main stages. The first weight loss of
8.93% (cꢀalcd. 8.50%) occurred in the temperature range of
25–140 C and corresponds to the loss of all lattice and
coordinated water molecules. The second weight loss of
28.01% between 140 and 600 ꢀC (calcd. 27.93%) is
assigned to the loss of the four PDA ligands. The third
step between 600 and 900 ꢀC with a loss of 16.20% (calcd.
16.26%) can be attributed to the loss of
Mo2O5 + 2MoO2 + SiO4. After the thermal decomposi-
tion of POM@Dy-PDA, 53.151% of its initial weight
remained (Figure 3).
with a clear H3-type hysteresis loop in the relative
pressure P/P0 between 0.4 and 0.94, indicating the forma-
tion of mesoporous materials. It is worth mentioning that
the BET surface area measured for this catalyst shows a
60% improvement from 4 m2 g–1 for commercial
H4SiMo12O40
to 6.6 m2 g−1 for POM@Dy-PDA
[108]
(Langmuir surface area of 51.3 m2 g−1
hybridization. However, pore-size analysis with the
)
after
Barrett–Joyner–Halenda (BJH) method revealed
a
heterogeneous structure.
3.2 | Crystal structures of the hybrid
catalyst (POM@Dy-PDA)
The UV–Vis absorption spectrum of the POM@Dy-
PDA is shown in Figure 4. In the ultraviolet region
(200–400 nm), there are three absorption bands at
318, 328, and 347 nm, which are assigned to ligand-to-
metal charge transfer for the POM. This represents the
transfer of an electron from the highest occupied molecu-
lar orbital to the lowest unoccupied molecular
orbital.[107]
As shown in Figure 5, the hybrid catalyst was
characterized via PXRD pattern at room temperature.
The PXRD pattern measured for the synthesized sample
was in good agreement with the PXRD pattern simulated
from the respective single-crystal X-ray data, which
indicates the good phase purity of the catalyst.
The X-ray structure determination established that
the POM@Dy-PDA crystallizes in the monoclinic space
group P21/n. Crystallographic and refinement data
for the hybrid catalyst are listed in Table 1. The
crystal structure of the catalyst contains a polynuclear
cation unit [Dy4(PDA)4(H2O)11]4+, two [SiMo12O40]4−
polyoxoanions, and seven lattice water molecules
(Figure 7). The polyoxoanion [SiMo12O40]4− ion exhibits
the well-known Keggin-type structure and is constructed
from one SiO4 unit and four trimetallic (MoO6)3 groups.
The central Si atom sits on a crystallographic center and
is surrounded by a distorted cube of eight half-occupied
(disordered over the center) oxygen atoms. This type of
disorder often appears in the Keggin structure.[109] In the
cationic component, all Dy(III) ions are crystallographi-
cally independent and the Dy3+ ions are bridged by
carboxylate oxygen atoms belonging to four carboxylic
acids. The dysprosium(III) ions are eight and nine
The BET specific surface area and porous structure of
the POM@Dy-PDA catalyst was characterized by nitro-
gen adsorption experiments. As shown in Figure 6, the
adsorption–desorption behavior of POM@Dy-PDA shows
a type IV isotherm (according to IUPAC classification)
TABLE 3
Optimization of reaction conditions for the synthesis of 2-amino-4H-chromene 4a
Entry
Solvent
Temperature
Room temperature
Reflux
Time (min)
Catalyst amount (mol%)
Yield (%)
1
EtOH/H2O
EtOH/H2O
MeOH
80
80
60
70
60
60
60
60
15
80
80
15
60
–
0
2
No catalyst
Trace
70
52
45
40
83
73
95
35
35
95
95
3
Reflux
10
10
10
10
10
10
10
10
5
4
CH2Cl2
Reflux
5
Dimethylformamide
CH3CN
Reflux
6
Reflux
7
EtOH
Reflux
8
H2O
Reflux
9
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
Reflux
10
11
12
13
Room temperature
Reflux
Reflux
10
15
Reflux

10 of 23
HOSSEINZADEH-BAGHAN ET AL.
TABLE 4
Synthesis of 2-amino-3-cyano-7-hydroxy-4-(aryl)-4H-chromene derivatives 4a–g catalyzed by POM@Dy-PDA
Entry
Aldehyde
Product
Time (min)
Yield (%)
Observed MP (ꢀC)
Literature MP (ꢀC)
1
15
95
229–232
231–233[110]
4a
4b
4c
4d
2
3
4
18
18
20
91
90
92
186–188
190–193
232–234
185–187[110]
192–194[111]
233–235[110]

HOSSEINZADEH-BAGHAN ET AL.
11 of 23
TABLE 4 (Continued)
Entry
Aldehyde
Product
Time (min)
Yield (%)
Observed MP (ꢀC)
Literature MP (ꢀC)
170–172[110]
5
25
85
171–173
4e
6
20
88
211–213
212–214[111]
4f
7
20
88
247–251
248–250[112]
4g
Dy, dysprosium; MP, melting point; PDA, 1,10-phenanthroline-2,9-dicarboxylic acid; POM, polyoxometalate.
coordinate with DyN2O6 (Dy1, Dy2, and Dy3) and
As shown in Figure 9, the terminal O51 and
bridging O64 atoms of the Keggin moieties and also the
bridging O31 atoms of another Keggin are oriented
toward the π face of one of the phen rings of a
neighboring unit. The distances between the O51, O64,
and O31 atoms and the centroid of the phen rings are
3.258, 3.291, and 3.189 Aꢀ, respectively, which are in
the range of typical anion–π contacts, and thus a clear
indication of binding. This interaction can be also
viewed as a lone pair–π interaction in which one free
lone pair of O51, O64, and O31 atoms points toward
DyN2O7 (Dy4) coordination environments, Dy-NPDA
:
2.468(8)–2.540(7), Dy-OPDA: 2.325(8)–2.466(8), Dy-Owater
:
2.308(8)–2.494(12) Å). It should be noted that all Dy(III)
ions are coordinated by PDA ligands and each PDA acts
as a chelating and a bridging ligand between two
neighboring metal centers. Two different coordination
modes of the PDA ligand in POM@Dy-PDA are shown in
Figure 8. The ligands chelating Dy1 and Dy3 are found in
mode I and for ligands chelating Dy2 and Dy4 mode II is
found.

12 of 23
HOSSEINZADEH-BAGHAN ET AL.
TABLE 5
Synthesis of 2-amino-benzochromene derivatives 5a–h catalyzed by POM@Dy-PDA
α-Naphthol or
Entry Aldehyde β-naphthol
Time
(min)
Yield
(%)
Observed MP
(ꢀC)
Literature MP
(ꢀC)
Product
1
20
90
216–218
216–217[113]
5a
5b
2
3
25
83
239–241
231–233
240–241[114]
25
20
18
86
92
90
230–232[34]
231–234[34]
279–281[115]
5c
4
5
230–233
280–283
5d
5e

HOSSEINZADEH-BAGHAN ET AL.
13 of 23
TABLE 5 (Continued)
α-Naphthol or
Entry Aldehyde β-naphthol
Time
(min)
Yield
(%)
Observed MP
(ꢀC)
Literature MP
(ꢀC)
Product
6
25
20
20
89
90
88
239–241
206–207
185–187
238–240[33]
5f
7
8
205–206[116]
5g
186–187[116]
5 h
Dy, dysprosium; MP, melting point; PDA, 1,10-phenanthroline-2,9-dicarboxylic acid; POM, polyoxometalate.
the π-face of the phen system that is coordinated to the
metal, thus enhancing its π acidity.
of abundant intermolecular hydrogen bonding supports
the 2D sheet structure. Furthermore, extensive
hydrogen bonding between surface oxygen atoms of the
Keggin anion and neighboring layers and anion–π
interactions bind the discrete molecules into a 3D
supramolecular framework (Figure 10b). The lattice
water molecules reside as guests in the interstices of
the 3D supramolecular framework through extensive
hydrogen-bonding interactions. All of these contacts are
given in Table 2.
The overall structure of POM@Dy-PDA is best
described as composed of two cationic layers of
dysprosium(III) complexes in the crystallographic bc
plane which are stacked along the a axis (Figure 10a).
In this compound the POM units reside in the spaces
between two adjacent layers as discrete counterions and
do not show any coordinating behavior toward the dys-
prosium ions. It is worth mentioning that the existence

14 of 23
HOSSEINZADEH-BAGHAN ET AL.
TABLE 6
Synthesis of 2-amino-3,4-dihydropyrano[3,2-c]chromene 6a–k catalyzed by POM@Dy-PDA
Malononitrile/ethyl
cyanoacetate
X = CN/COOEt
Time
(min)
Yield
(%)
Observed
MP (ꢀC)
Literature
MP (ꢀC)
Entry Aldehyde
Product
1
8
97
255–259
256–258[117]
6a
2
15
91
252–255
253–256[117]
6b
3
4
18
15
90
92
241–243
248–251
242–244[118]
6c
6d
248–250[117]
5
6
12
10
94
96
258–262
261–263
259–261[119]
6e
260–262[117]
6f

HOSSEINZADEH-BAGHAN ET AL.
15 of 23
TABLE 6 (Continued)
Malononitrile/ethyl
cyanoacetate
X = CN/COOEt
Time
(min)
Yield
(%)
Observed
MP (ꢀC)
Literature
MP (ꢀC)
Entry Aldehyde
Product
7
15
93
253–255
252–254[120]
6g
8
9
20
18
90
92
256–258
259–262
257–259[21]
6h
6i
260–262[121]
10
12
95
256–259
256–258[117]
6j
11
12
20
20
91
88
209–212
200–202
210–212[122]
6k
199–201[123]
6l
14
18
86
193–195
192–194[118]
(Continues)

16 of 23
HOSSEINZADEH-BAGHAN ET AL.
TABLE 6 (Continued)
Malononitrile/ethyl
cyanoacetate
X = CN/COOEt
Time
(min)
Yield
(%)
Observed
MP (ꢀC)
Literature
MP (ꢀC)
Entry Aldehyde
Product
6m
15
15
90
232–235
232–234[123]
6n
Dy, dysprosium; MP, melting point; PDA, 1,10-phenanthroline-2,9-dicarboxylic acid; POM, polyoxometalate.
3.3 | Catalytic activity
optimize the catalyst loading, we performed the same
reaction in the presence of varying quantity of catalyst in
EtOH/H2O under reflux condition. From Table 3, it can
be concluded that 10 mol% of POM@Dy-PDA is the
required quantity of catalyst which gave 95% yield of 4a
in 15 min.
Based on the optimal reaction conditions, we investi-
gated the substrate scope of our strategy by using differ-
ently substituted benzaldehydes (comprising electron-
donating and electron-withdrawing substituents) which
were reacted with malononitrile and resorcinol to give
the corresponding 2-amino-3-cyano-7-hydroxy-4-(aryl)-
4H-chromene derivatives 4a–g in high yields (Table 4).
The products were characterized by comparison of their
melting points and FT-IR spectra with those of authentic
compounds and were found being identical. As shown in
Table 4, no noticeable substituent influence was
observed. Thus, in general POM@Dy-PDA can be consid-
ered as a tolerant, very flexible catalyst for the aforemen-
tioned one-pot three-component reaction.
Considering POM@Dy-PDA, as an ecofriendly
inorganic–organic hybrid, after its full characterization,
we decided to examine its catalytic activity in the
synthesis of 2-amino-4H-chromenes through a one-pot
MCR. For this purpose, we selected a three-component
reaction, involving benzaldehyde 1a, malononitrile 2a,
and resorcinol 3a as model reaction (Scheme 1). Initially,
we examined the model reaction in the absence of any
catalyst or additive by just refluxing the starting materials
in EtOH. The reaction was monitored by TLC using n-
hexane: ethyl acetate (7:3), which showed no conversion.
Next, the reaction was conducted in the presence of
catalytic amount of freshly prepared POM@Dy-PDA in
refluxing ethanol. This reaction was monitored by TLC,
showing consumption of starting materials along with
formation of a new product which was identified as the
corresponding 2-amino-4H-chromene 4a. For the
selection of the best solvent, the reaction was conducted
in different solvents such as H2O, EtOH, MeOH, CH2Cl2,
MeCN, dimethylformamide, and EtOH/H2O. The mix-
ture EtOH/H2O was found to be the solvent of choice in
terms of yield of the product and reaction time. To
Subsequently, the strategy was extended for the syn-
thesis of a series of 2-amino-4H-benzo[h]chromene-
3-carbonitrile 5a–d. Accordingly, differently substituted
benzaldehydes (bearing electron-withdrawing groups as

HOSSEINZADEH-BAGHAN ET AL.
17 of 23
TABLE 7
Synthesis of 2-amino-4H-chromene derivatives 7a–e catalyzed by POM@Dy-PDA
Entry
Aldehyde
Product
Time (min)
Yield (%)
Observed MP (ꢀC)
Literature MP (ꢀC)
1
15
93
176–177
175–176[124]
223–224[125]
234–236[125]
178–180[125]
248–250[125]
7a
7b
7c
7d
7e
2
3
4
5
20
18
20
20
91
90
85
88
223–225
233–235
179–181
247–250
Dy, dysprosium; MP, melting point; PDA, 1,10-phenanthroline-2,9-dicarboxylic acid; POM, polyoxometalate.
TABLE 8
The catalytic activity of POM with POM@Dy-PDA
Entry
R1
H
Carbonyl compound 3
Malononitrile 2a
Product 4
Catalyst
Yield (%)
Time (min)
1
2
3a
3a
2a
2a
4a
4a
POM@Dy-PDA
POM
95
40
60
H
180
Dy, dysprosium; PDA, 1,10-phenanthroline-2,9-dicarboxylic acid; POM, polyoxometalate.

18 of 23
HOSSEINZADEH-BAGHAN ET AL.
SCHEME 2 A reasonable reaction
mechanism for the synthesis of
2-amino-4H-chromenes catalyzed by
POM@Dy-PDA. Dy, dysprosium; PDA,
1,10-phenanthroline-2,9-dicarboxylic
acid; POM, polyoxometalate
well as electron-donating groups) were reacted with
malononitrile and 2-naphthol in the presence of a cata-
lytic amount of POM@Dy-PDA in refluxing
EtOH/H2O. In all cases, the respective products 5a–d
were obtained in satisfactory yields (Table 5). Accord-
ingly, replacement of the 2-naphthol with 1-naphthol
fruitfully gave the corresponding 2-amino-4H-benzo[h]
chromenes 5a–h (Table 5).
withdrawing or electron-releasing groups in the ortho,
meta, and para positions with malononitrile and
3-methyl-4H-pyrazole-5(4H)-one. In all cases the respec-
tive 2-amino-4H-chromenes were obtained in satisfactory
yields in relatively short times without the formation of
any by-products. The results are outlined in Table 7. As
can be seen, benzaldehydes bearing electron-donating
groups as well as electron-withdrawing moieties could be
successfully employed as substrates.
Encouraged by these results, the POM@Dy-PDA-
catalyzed synthesis of another important class of
chromene-annulated heterocycles, so-called 2-amino-3,-
4-dihydropyrano[3,2-c]chromene 6a–k, was successfully
attempted. Three-component reactions of differently
substituted benzaldehydes, 4-hydroxycoumarin, and
malononitrile in the presence of 10 mol% POM@Dy-PDA
in refluxing EtOH/H2O gave the corresponding 2-amino-
3,4-dihydropyrano[3,2-c]chromene 6a–k in excellent
yields (Table 6). To study the substrate scope of the reac-
tion differently substituted benzaldehydes having
electron-releasing or electron-withdrawing substituents
such as Me, NO2, and OMe were successfully reacted
with malononitrile and 4-hydroxycoumarin under the
same reaction conditions. The reactions proceeded
smoothly to give the products in satisfactory yields. Like-
wise, the reaction of differently substituted aldehydes
with ethyl cyanoacetate and 4-hydroxycoumarin was
performed under the similar reaction conditions and the
corresponding products were also obtained in satisfactory
yields (Table 6).
To expand our investigation, we compared the cata-
lytic activity of the commercially available POM
[SiMo12O40]4− with that of our POM@Dy-PDA catalyst.
For this purpose, the model reaction was conducted in
the presence of either [SiMo12O40]4− or POM@Dy-PDA
as catalyst in EtOH/H2O under reflux condition. The
results are outlined in Table 8. The results showed that
the POM@Dy-PDA exhibited better catalytic activity
compared with [SiMo12O40]4−
.
A reasonable reaction mechanism for the catalytic
activity of POM@Dy-PDA is suggested based on the
literature and our experimental results. It is assumed that
the reaction proceeds via a Knoevenagel condensation
In further work, the substrate scope and limitations
of the preparation of other 2-amino-4H-chromene
derivatives in the presence of POM@Dy-PDA was
examined. In this study, 3-methyl-4H-pyrazole-5(4H)-one
was employed instead of 4-hydroxycoumarin. The
generality of this strategy was examined using differently
substituted benzaldehydes having either electron-
FIGURE 11 Recyclability of catalyst for the synthesis
2-amino-4H-chromene 4a

HOSSEINZADEH-BAGHAN ET AL.
19 of 23
TABLE 9
The comparison of the catalytic activity of POM@Dy-PDA with formerly reported catalysts for the synthesis of
2-amino-4H-chromenes 4a
Catalyst
Amount
Yield
(%)
Entry Catalyst
Time
Temperature
Reflux
Solvent
Reference
[138]
[126]
[112]
[127]
1
2
3
4
No catalyst
5 h
–
2,2,2-Trifluoroethanol 90
Potassium phthalimide-N-oxyl
Mg/Al hydrotalcite
20 min 1.5 mol%
Reflux
H2O
H2O
H2O
90
95
86
4 h
15 wt%
30 mg
60 ꢀC
Tungstic acid functionalized
mesoporous SBA-15
12 h
100 ꢀC
[128]
[129]
5
6
Nanozeolite clinoptilolite
Potassium phthalimide
15 min 0.01 g
Reflux
H2O
92
97
17 min 9. 2 mg
(5 mol%)
Ball milling
technique RT
–
[130]
[131]
[110]
7
8
9
Anhydrous sodium carbonate
10 min 11 mg
(10 mol%)
125 ꢀC
Solvent free
Solvent free
Solvent free
>99
87
A triazine-based porous organic
polymer (TPOP-2)
5 h
40 mg
80 ꢀC
Ionic liquid, namely, 2-ethyl
imidazolium acetate ([2-Eim]
OAc)
27 min 10 mol%
67.5 ꢀC
92
[132]
10
Fe3O4–chitosan nanoparticles
(Fe3O4–chitosan magnetic
nanoparticles)
20 min 0.15 g
(30 mol%)
Ultrasound
irradiation
50 ꢀC
H2O
99
[133]
[134]
11
12
Amino-appended β-cyclodextrins 5 h
5 mol%
(0.05 mol)
RT
H2O
H2O
95
82
Sulfonic acid-functionalized
MIL-101(Cr) [MIL-101(Cr)–
SO3H]
3 h
0.37 mol%
100 ꢀC
[135]
13
Imidazolium ionic
5 min
0.020 g
MW 70 ꢀC
H2O
97
liquid-functionalized magnetic
multiwalled carbon nanotubes
(CNTs–Fe3O4–IL)
[136]
[111]
[139]
14
15
16
Monodisperse palladium
nanoparticles supported with
graphene oxide (Pd@GO)
15 min 10 mg
12 min 8 mg
80 ꢀC
80 ꢀC
EtOH
92
95
98
Palladium–ruthenium
nanoparticles decorated on
graphene oxide (PdRu@GO)
H2O/EtOH
H2O
–
1 min
–
Ultrasonic
conditions
60 ꢀC
(Continues)

20 of 23
HOSSEINZADEH-BAGHAN ET AL.
TABLE 9 (Continued)
Catalyst
Amount
Yield
(%)
Entry Catalyst
Time
Temperature
Solvent
Reference
[137]
17
Fe3O4@SiO2–Benzim–
10 min 10 mg
Ultrasonic
EtOH/H2O
95
Fc[Cl]/BiOCl nanocomposite
conditions RT
18
POM@Dy-PDA cations
15 min 10 mol%
Reflux
H2O/EtOH
95
This work
interspersed with POM anion
Dy, dysprosium; MW, Microwave irradiation; PDA, 1,10-phenanthroline-2,9-dicarboxylic acid; POM, polyoxometalate; RT, Room
Temperature.
between benzaldehyde and malononitrile with subsequent
Michael addition of resorcinol with the Knoevenagel
adduct (I). The final step involves an intramolecular cycli-
zation (II) and tautomerization to give the desired
2-amino-4H-chromenes (III) (Scheme 2). It should be men-
tioned that in the POM catalyst, metal ions on the skeleton
of POMs possess unoccupied orbital that can accept elec-
tron, thus acting as Lewis acid. Furthermore, lanthanoid
coordination complexes have Lewis acid properties. They
are hard Lewis acids in which hybridization using POMs
improves catalytic efficacy. Therefore, in the POM@Dy-
PDA catalyst, Mo atoms and Dy ions are active site in acid-
catalyzed reaction.[65] The catalyst POM@Dy-PDA con-
taining the acid sites enhances the electrophilicity of the
carbonyl group of aromatic aldehydes which are subjected
to dehydration to afford the Knoevenagel product and then
aromatization.[122]
To show the merits of our novel catalyst, its catalytic
activity was compared for the model system with that of
several other potential catalysts (Table 9), such as potas-
sium phthalimide-N-oxyl,[126] Mg/Al hydrotalcite,[112]
tungstic acid functionalized mesoporous SBA-15,[127]
nanozeolite clinoptilolite,[128] potassium phthalimide,[129]
anhydrous sodium carbonate,[130] a triazine-based porous
organic polymer (TPOP-2),[131] the ionic liquid 2-ethyl
imidazolium acetate ([2-Eim]Oac),[110] Fe3O4–chitosan
graphene oxide (PdRu@GO),[111] and Fe3O4@SiO2–
Benzim–Fc[Cl]/BiOCl nanocomposite.[137] The results
show that our catalyst is superior in that it affords the
desired products in better yield and shorter reaction
times than many of the others. From the green chemistry
point of view, the recyclability of the catalyst as well as
employing H2O/EtOH as solvent renders this catalyst
green and environmentally benign (Table 9).
3.4 | Reusability of the catalyst
A significant issue for heterogeneous catalysis is the easy
separation of the catalyst and its recyclability. To investi-
gate the catalyst reusability, the reaction of resorcinol,
benzaldehyde, and malononitrile was performed in the
presence of POM@Dy-PDA and the separated catalyst
was reused in the same reaction for at least four times
without observing any appreciable loss in its catalytic
activity (for the first run 95% and for the fourth cycle 88%
yields; Figure 11).
4 | CONCLUSIONS
We have synthesized and fully characterized a novel
inorganic–organic hybrid POM@Dy-PDA by using a
Keggin-type polyoxomolybdate (POM), the preorganized
ligand PDA, and the dysprosium(III) ion with the for-
mula 2[Dy4(PDA)4(H2O)11(SiMo12O40)]·7H2O. Its struc-
ture was unambiguously confirmed by single-crystal X-
ray diffraction. It was successfully employed as an active
heterogeneous catalyst in the efficient and green synthe-
sis of aforementioned heterocyclic systems. We have also
nanoparticles
(Fe3O4–chitosan
magnetic
nanoparticles),[132] amino-appended β-cyclodextrins,[133]
sulfonic acid-functionalized MIL-101(Cr) [MIL-101(Cr)–
SO3H],[134] imidazolium ionic liquid functionalized mag-
netic multiwalled carbon nanotubes (CNTs–Fe3O4–
IL),[135] highly monodisperse palladium nanoparticles
supported
with
graphene
oxide
(Pd@GO),[136]
palladium–ruthenium nanoparticles decorated on

HOSSEINZADEH-BAGHAN ET AL.
21 of 23
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ACKNOWLEDGEMENTS
M.M. gratefully acknowledges financial support from the
Ferdowsi University of Mashhad (Grant No. 3/42203),
the Iran Science Elites Federation (ISEF), Zeolite and
Porous Materials Committee of Iranian Chemical Society
and the Iran National Science Foundation (INSF).
M.M. also acknowledges the Cambridge Crystallographic
Data Centre (CCDC) for access to the Cambridge Struc-
tural Database. The authors also thankful Alzahra Uni-
versity and Iran Science Elites Federation for the given
grant. JTM thanks Tulane University for support of the
Tulane Crystallography Laboratory.
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ORCID
Masoud Mirzaei https://orcid.org/0000-0002-7256-4601
Majid M. Heravi https://orcid.org/0000-0003-2978-1157
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SUPPORTING INFORMATION
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article.
How to cite this article: Hosseinzadeh-Baghan S,
Mirzaei M, Eshtiagh-Hosseini H, Zadsirjan V,
Heravi MM, Mague JT. An inorganic–organic
hybrid material based on a Keggin-type
polyoxometalate@Dysprosium as an effective and
green catalyst in the synthesis of 2-amino-4H-
chromenes via multicomponent reactions. Appl
Organomet Chem. 2020;e5793. https://doi.org/10.
1002/aoc.5793
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