Immobilized copper-layered nickel ferrite on acid-activated montmorillonite, [(NiFe2O4?Cu)(H+-Mont)], as a superior magnetic nanocatalyst for the green synthesis of xanthene derivatives

Article abstract of DOI:10.1039/c9ra04320a

In this study, the immobilization of copper-layered nickel ferrite on the surface and in the cavities of acid-activated montmorillonite (H⁺-Mont) was investigated. In this context, magnetic nanoparticles (MNPs) of NiFe₂O₄

Full text of DOI:10.1039/c9ra04320a

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Immobilized copper-layered nickel ferrite on acid-
activated montmorillonite, [(NiFe2O4@Cu)(H+-
Mont)], as a superior magnetic nanocatalyst for the
green synthesis of xanthene derivatives
Cite this: RSC Adv., 2019, 9, 28038
*
Behzad Zeynizadeh
and Soleiman Rahmani
In this study, the immobilization of copper-layered nickel ferrite on the surface and in the cavities of acid-
activated montmorillonite (H+-Mont) was investigated. In this context, magnetic nanoparticles (MNPs) of
NiFe2O4 as the prime magnetic cores were prepared. Next, through the reduction of Cu2+ ions with
sodium borohydride, the nanoparticles of Cu0 were immobilized on the nanocore-surface of NiFe2O4,
and the constituent NiFe2O4@Cu MNPs were obtained. Moreover, through the activation of
montmorillonite K10 (Mont K10) with HCl (4 M) under controlled conditions, the H+-Mont constituent
was prepared. The nanostructured NiFe2O4@Cu was then intercalated within the interlayers and on the
external surface of the H+-Mont constituent to afford the novel magnetic nanocomposite
(NiFe2O4@Cu)(H+-Mont). The prepared clay nanocomposite was characterized using FTIR spectroscopy,
SEM, EDX, XRD, VSM and BET analyses. The obtained results showed that through acid-activation, the
stacked-sheet structure of Mont K10 was exfoliated to tiny segments, leading to a significant increase in
the surface area and total pore volume of the H+-Mont constituent as compared to those of
montmorillonite alone. SEM analysis also exhibited that the dispersion of NiFe2O4@Cu MNPs in the
interlayers and on the external surface of acid-activated montmorillonite was carried out successfully,
and the nanoparticle sizes were distributed in the range of 15–25 nm. The BET surface analysis also
indicated that through the immobilization of NiFe2O4@Cu MNPs, the surface area and total pore volume
of the H+-Mont system were decreased. The catalytic activity of (NiFe2O4@Cu)(H+-Mont) was further
studied towards the synthesis of substituted 13-aryl-5H-dibenzo[b,i]xanthene-5,7,12,14(13H) tetraones
3(a–k) and 3,3,6,6-tetramethyl-9-aryl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H) diones 5(a–k) via the
pseudo-one-pot three-component cyclocondensation of 2-hydroxy-1,4-naphthoquinone (Lawsone)/
dimedone and aromatic aldehydes in a mixture of H2O–EtOH (1 : 1 mL) as a green solvent at 80–90 ꢀC.
The (NiFe2O4@Cu)(H+-Mont) MNPs can be easily separated from the reaction mixture by an external
magnetic field and reused for seven consecutive cycles without significant loss of catalytic activity.
Received 9th June 2019
Accepted 22nd August 2019
DOI: 10.1039/c9ra04320a
rsc.li/rsc-advances
capabilities indicate the signicant impact of xanthenes as the
source of valuable drugs and rationalize the immense interest of
1. Introduction
numerous researchers towards their synthesis; a literature review
has shown that using Fe3O4,15 ZrO2,16 silica sulfuric acid,17 (HPO3)n,18
HOAc,19 HClO4/SiO2,20 PPA/SiO2,20 SbCl3/SiO2,21 silica chloride,22
NaHSO4$SiO2,22 FeCl3/SiO2,23 InCl3,18 InCl3$4H2O/ionic liquid,24
Among O-containing heterocyclic compounds, xanthene derivatives
are very important. These materials have attracted signicant
attention of many pharmacologists and synthetic chemists due to
their broad spectrum of biological properties such as insecticidal,
antifungal and urease activity,1 analgesic,2 antiinammatory,2 anti-
bacterial,3 antioxidant,4 and antiproliferative activity,5 trypanothione
reductase (TryR) inhibition,6 chloroquine (CQ) potentiation,6 and
antimalarial6 and anticancer activity.7 Moreover, some xanthene
materials have been used as antagonists for paralyzing the action of
zoxalamine8 and dyes,9–11 in laser technology,12 as pH-sensitive
uorescent agents13 and as bactericides in agriculture.14 These
2ꢁ
In(OTf)3,25 SO4 /TiO2,26 Fe3O4@SiO2@SO3H,27 Fe3O4@TiO2@SO3-
H,28
ZrO2@SO3H,29
n-Bu4HSO4,30
[Hmim]TFA,31
p-
dodecylbenzenesulfonic acid,32 p-dodecylbenzenesulfonic acid/
ultrasound,33 Dowex-50W,34 amberlyst-15,35 montmorillonite K10,36
Fe3+-montmorillonite,37 1,1,3,3-N,N,N0,N0-tetramethylguanidinium
triuoroacetate/TFA,38 Et3(PhCH2)NBr,39 TMSCl,40 DABCO,41 CuI/
poly(4-vinylpyridine),42 graphene oxide-incorporated strontium
NPs,43 choline chloride/itaconic acid,44 (NH4)2HPO4,45 and polyani-
line-p-toluenesulfonate46 1,8-dioxo-octahydroxanthenes have been
prepared. In this context, some procedures involving Dowex-50W,34
Faculty of Chemistry, Urmia University, Urmia 5756151818, Iran. E-mail:
bzeynizadeh@gmail.com
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1,1,3,3-N,N,N0,N0-tetramethylguanidinium
triuoroacetate/TFA,38 and in continuation of our research program towards the
graphene oxide-incorporated strontium NPs,43 choline chloride/ synthesis and application of magnetic nanocatalysts,65,72–76
itaconic acid,44 [Msim]Cl,47 [Hmim][HSO4],47 [Et3N–SO3H]Cl,47 herein, we wish to introduce the immobilization of copper-
[Et3NH][HSO4],47 H2SO4,48 [Bmim][HSO4],48 [Bmim][BF4],49 N- layered nickel ferrite on acid-activated montmorillonite,
sulfonic acid poly(4-vinylpyridinium) hydrogen sulfate,50 p-toluene (NiFe2O4@Cu)(H+-Mont) (Fig. 1), with the aims of (i)
sulfonic acid,51 amberlyst-15,52 and FeCl3$6H2O53 have also been combining the catalytic activity of the copper-layered nickel
reported for the preparation of 5H-dibenzo[b,i]xanthene tetraone ferrite constituent with the Brønsted and Lewis acidity of
materials. Aligned with the manifold capabilities of xanthene the H+-Mont clay mineral, affording prominent catalytic
derivatives, therefore, the development of simple, efficient and activity of the nal catalyst system and (ii) avoiding aggre-
clean synthetic protocols using green and highly reactive gation and improving the stability of the copper nano-
environmental benign catalysts is still needed.
particles as well as the shelf life capability of the
Previously reported studies indicate the usefulness of Cu NiFe2O4@Cu MNPs via immobilization in the interlamellar
nanoparticles because of their low cost, high thermal and spaces of the clay mineral. Further examination indicated
electrical conductivity, antifungal and antibacterial properties, that the prepared nanostructured (NiFe2O4@Cu)(H+-Mont)
and catalytic activity in organic synthesis; Cu is the most exhibited excellent catalytic activity towards the one-pot
commonly investigated transition metal elements.54–59 In spite Knoevenagel reaction of 2-hydroxy-1,4-naphthoquinone
of the great convenience of Cu NPs, however, they have a strong (Lawsone)/dimedone with aromatic aldehydes in a mixture
tendency to agglomerate as well as a high affinity to react with of H2O–EtOH (1 : 1 mL) as a green solvent to afford 5H-
air oxygen. These characteristics extensively diminish the dibenzo[b,i]xanthene tetraones and hexahydroxanthene
surface areas and catalytic activities of copper nanoparticles. In diones (Fig. 2).
order to overcome the mentioned drawbacks, the immobiliza-
tion of Cu NPs on the surface of solid supports, especially
magnetically nanoparticles (MNPs) of metal oxides, may be
2. Experimental
2.1. Instruments and reagents
a prime choice. Among the various magnetic metal oxides,
NiFe2O4 MNPs have attracted the attention of scientists due to
their excellent magnetic, physical and electrical properties and
high thermal stability (up to 900 ꢀC) as well as their usefulness
in drug delivery, ferrouids, pigments, microwave devices, gas
sensors and catalysts.60–64 In this area, the immobilization of
copper nanoparticles on nickel ferrite (NiFe2O4@Cu) was
successfully reported for the efficient reduction of nitroarenes
to arylamines with NaBH4.65 Observation of the results reveals
that because of the synergic inuences of copper element and
the core magnetic material, this type of immobilization
dramatically accelerates the rates of reduction reactions.
Moreover, feasible and efficient separation of the applied cata-
lyst system from the reaction mixture can be carried out using
an external magnetic eld.
Clay minerals of the smectite group, such as montmorillonite
K10 (Mont K10), are considered to be ecofriendly and suitable solid
supports on which metal nanoparticles can be stabilized in the
interlamellar spaces.66–68 Mont K10 is a stacked-sheet clay mineral
that belongs to the family of 2 : 1 phyllosilicates. It is composed of
one sandwiched-Al3+ octahedral sheet between two Si4+ tetrahedral
sheets69 and has several advantages in terms of high surface area,
swelling, cation exchange ability, low cost, ease of handling and
non-corrosiveness.70 This micro/mesoporous material, in both its
natural and modied forms, possesses both Brønsted and Lewis
acid characteristics. Although montmorillonite K10 possesses
inherent acidity, further activation of montmorillonite with strong
acids such as HCl can exfoliate the adjacent sheets to tiny segments,
leading to dramatic increases in surface area and Brønsted acidity
as well as the generation of numerous pores with micro/meso
dimensions.71
All chemicals were purchased from chemical companies, were
of the best quality and were used without further purication.
1H, 13C NMR and FT-IR spectra were recorded on 300 MHz
Bruker Avance and Thermo Nicolet Nexus 670 spectrometers.
Melting points were recorded on an Electrothermal 9100
melting point apparatus and are uncorrected. TLC was applied
to monitor the reactions over silica gel-coated 60 F254
aluminum sheets. The magnetic properties of the nanocatalysts
were measured using vibration sample magnetometer (VSM,
Meghnatis Daghigh Kavir Co., Iran) analysis under an external
magnetic eld of up to 20 kOe. The morphologies and size
distributions of the nanoparticles were examined by scanning
electron microscopy (SEM) using an FESEM-TESCAN MIRA3
instrument. X-ray diffraction (XRD) measurements were carried
out on X'PertPro Panalytical diffractometer (Holland) at 40 kV
˚
and 30 mA with Cu Ka radiation (l ¼ 1.5418 A). Signal data were
recorded from 2q ¼ 10ꢀ to 80ꢀ with a step interval of 0.05ꢀ.
2.2. Synthesis of NiFe2O4 MNPs
A mixture of Ni(OAc)2$4H2O, Fe(NO3)3$9H2O, NaOH and NaCl in
a molar ratio of 1 : 2 : 8 : 2, respectively, was ground for 60 min in
A literature review shows that the synthesis of xanthene
materials catalyzed by copper nanoparticles has not yet been
investigated. Therefore, in line with the outlined strategies Fig. 1 Magnetic nanoparticles of (NiFe2O4@Cu)(H+-Mont).
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Fig. 2 Synthesis of xanthene materials catalyzed by (NiFe2O4@Cu)(H+-Mont) MNPs.
the absence of solvent. The reaction was exothermic and was water to remove NaCl and residual contaminants, followed by
ꢀ
accompanied by the release of heat. When the reaction was drying at 80 C for 10 h. The resulting powder was calcinated at
complete, the mixture was washed several times with deionized 900 ꢀC for 2 h to obtain magnetic nanoparticles of NiFe2O4.65
Fig. 3 Preparation of the (NiFe2O4@Cu)(H+-Mont) MNPs.
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acid-activated clay was collected and dried in an oven (50 ꢀC) to
afford acid-activated montmorillonite (H+-Mont).76
2.6. Synthesis of (NiFe2O4@Cu)(H+-Mont) MNPs
Individual suspensions of H+-Mont (4.2 g) in deionized water (200
mL; the pH value of the resulting suspension was adjusted to ꢂ5 by
3 wt% HCl) and NiFe2O4@Cu (2.1 g) in deionized water (200 mL; the
pH value of the resulting suspension was adjusted to ꢂ9 by 3 wt%
aqueous ammonia) were irradiated by ultrasound for 30 min. The
two suspensions were combined, followed by stirring for 2 h. The
clay nanocomposite, (NiFe2O4@Cu)(H+-Mont), was separated by
magnetic decantation and then washed with EtOH. Drying under
air atmosphere afforded the nal product in a weight ratio of 2 : 1
for NiFe2O4@Cu : H+-Mont.
2.7. Determining the cation-exchange capacity (CEC) of the
clay minerals
Fig. 4 XRD patterns of (a) NiFe2O4 and (b) the NiFe2O4@Cu MNPs.
The cation-exchange capacities (CEC) of the clay minerals were
measured through a reported procedure.77 In this context, a clay
mineral (such as Mont K10) (0.5 g) was dispersed in a standard
alcoholic solution of CaCl2 (10 mL, 0.05 M) and the prepared
mixture was stirred at room temperature for 24 h. The
suspension was ltered, washed with EtOH (20 mL) and dried at
room temperature for 12 h. The Ca2+-exchanged clay mineral
2.3. Synthesis of NiFe2O4@Cu MNPs
A mixture of NiFe2O4 (1 g) containing an aqueous solution of
CuCl2$2H2O (0.25 g in 30 mL H2O) was prepared. The resulting
mixture was stirred for 30 min at room temperature; NaBH4
(0.11 g in 30 mL H2O) was then added dropwise over 20 min (N2
atmosphere). The mixture was then stirred for 2 h. Aer that,
the magnetic nanoparticles of NiFe2O4@Cu were separated by
magnetic decantation and then washed with ethanol and
deionized water. Drying under air atmosphere afforded NiFe2-
O4@Cu MNPs.65
2.4. Preparation of homoionic Na+-exchanged
montmorillonite (Na+-Mont)
To a beaker containing an aqueous solution of NaCl (2 M, 200
mL), montmorillonite K10 (10 g) was added, and the mixture
was stirred vigorously for 2 h at room temperature. The mixture
was then allowed to settle, and the aqueous phase was dec-
anted. Then, the solid residue was again charged with an
aqueous solution of NaCl (2 M, 200 mL). The procedure was
repeated 3 more times. Finally, the aqueous phase was decanted
and the solid residue was washed frequently with distilled water
until the conductivity of the liquid ltrate reached the
conductivity of distilled water. The resulting product was then
+
ꢀ
dried at 50 C in an oven to afford homoionic Na -exchanged
montmorillonite (Na+-Mont).74
2.5. Preparation of acid-activated montmorillonite (H+-
Mont)
To a round-bottom ask containing an aqueous solution of HCl
(4 M, 100 mL), Na+-Mont (5 g) was added. The resulting
dispersion was stirred under reux conditions for 2 h. Aer
cooling, the mixture was ltered, and the resulting montmo-
rillonite was washed frequently with distilled water to remove
Fig. 5 XRD patterns of (a) Mont K10, (b) Na+-Mont, (c) H+-Mont, (d)
any Clꢁ ions from the clay mineral (tested with AgNO3). The NiFe2O4@Cu and (e) (NiFe2O4@Cu)(H+-Mont).
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Fig. 6 SEM images of NiFe2O4 (a and b), NiFe2O4@Cu (c and d), Mont K10 (e and f), H+-Mont (g and h) and (NiFe2O4@Cu)(H+-Mont) (i and j).
was then transferred to a volumetric ask (250 mL). By adding titration with a standard solution of EDTA. The difference in the
distilled water, the volume of the ask was increased to the concentrations of Ca2+ before and aer cation-exchange pro-
standard limit (250 mL). The amount of Ca2+ was determined by cessing afforded the CEC (meq. gꢁ1) of the clay mineral.
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Fig. 7 N2 adsorption–desorption isotherms of the (a) Mont K10, (b) H+-Mont and (c) (NiFe2O4@Cu)(H+-Mont) systems.
CDCl3): d (ppm) 6.09 (s, 1H, CH), 7.12–7.25 (m, 2H, Ar), 7.28–
7.38 (m, 2H, Ar), 7.74–7.86 (m, 4H, Ar), 7.86–8.06 (m, 4H, Ar).
2.8.3. 5H-Dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraone
(3d). FT-IR (KBr, n cmꢁ1): 3071, 1676, 1602, 1351, 1263, 1209,
968, 942, 770, 735; 1H NMR (300 MHz, CDCl3): d (ppm) 2.49 (s,
2H, CH2), 7.71–7.85 (m, 4H, Ar), 7.95–7.97 (m, 4H, Ar).
2.8.4. 13-(4-Methoxyphenyl)-5H-dibenzo[b,i]xanthene-
5,7,12,14(13H)-tetraone (3e). FT-IR (KBr, n cmꢁ1): 3345, 3071,
2932, 1654, 1594, 1468, 1361, 1275, 1041, 730, 464; 1H NMR (300
MHz, CDCl3): d (ppm) 3.77 (s, 3H, OCH3), 6.35 (s, 1H, CH), 6.84–
6.94 (m, 2H, Ar), 7.11–7.18 (m, 1H, Ar), 7.21–7.29 (m, 1H, Ar),
7.64–7.80 (m, 4H, Ar), 8.04–8.15 (m, 4H, Ar); 13C NMR (75 MHz,
CDCl3): d (ppm) 33.12, 54.1, 122.8, 125.5, 126.2, 126.6, 128.1,
128.6, 129.5, 132.9, 135.1, 154.1, 157.4, 181.5, 184.1.
2.8.5. 13-(p-Tolyl)-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-
tetraone (3g). FT-IR (KBr, n cmꢁ1): 3174, 3013, 1638, 1585, 1382,
1347, 1275, 1218, 973, 880, 736, 666; 1H NMR (300 MHz, CDCl3):
d (ppm) 3.79 (s, 3H, CH3), 6.18 (s, 1H, CH), 6.83 (d, 2H, Ar), 7.21
(d, 2H, Ar), 7.65–7.79 (m, 4H, Ar), 8.07–8.15 (m, 4H, Ar); 13C
NMR (75 MHz, CDCl3): d (ppm) 36.9, 113.8, 122.9, 126.3, 127.2,
128.1, 129.6, 129.8, 132.7, 132.2, 135.0, 154.7, 158.4, 181.3,
184.7.
2.8. General procedure for the synthesis of 13-aryl-5H-
dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraones
In a round-bottom ask (10 mL) equipped with a magnetic
stirrer, a mixture of 2-hydroxy-1,4-naphthoquinone (2, 2 mmol),
aromatic aldehyde (1, 1 mmol) and (NiFe2O4@Cu)(H+-Mont) (20
mg) in H2O–EtOH (1 : 1 mL) was prepared. The resulting
mixture was stirred at 80 ꢀC for a particular time, as mentioned
in Table 4. The progress of the reaction was monitored by TLC
(eluent, n-hexane/EtOAc: 10/4). Aer completion of the reaction,
EtOAc (3 mL) was added, and stirring of the reaction mixture
was continued for 5 min. The nanocatalyst was then separated
from the reaction mixture by an external magnetic eld. Aer
that, the organic layer was separated from the aqueous solution.
Evaporation of the solvent under reduced pressure afforded the
crude 5H-dibenzo[b,i]xanthene tetraone. Further purication
could be carried out by recrystallization from hot EtOH,
affording the products 3(a–k) in 70% to 98% yields. Selected
spectral data for the products are given as follows:
2.8.1. 13-Phenyl-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-
tetraone (3a). FT-IR (KBr, n cmꢁ1): 3346, 1651, 1594, 1360, 1267,
1043, 725; 1H NMR (300 MHz, CDCl3): d (ppm) 6.26 (s, 1H, CH),
7.21–7.34 (s, 4H, Ar), 7.65–7.79 (m, 4H, Ar), 8.08–8.19 (m, 5H,
Ar); 13C NMR (75 MHz, CDCl3): d (ppm) 32.2, 122.6, 129.6, 132.7,
137.9, 154.8, 181.3, 184.7.
2.8.6. 13-(2-Hydroxyphenyl)-5H-dibenzo[b,i]xanthene-
5,7,12,14(13H)-tetraone (3h). FT-IR (KBr, n cmꢁ1): 3222, 1641,
1589, 1480, 1354, 1237, 1041, 975, 728; 1H NMR (300 MHz,
DMSO-d6): d (ppm) 5.74 (s, 1H, CH), 7.08–7.29 (m, 4H, Ar), 7.70–
7.98 (m, 8H, Ar), 11.48 (bs, 1H, OH).
2.8.2. 13-(2-Chlorophenyl)-5H-dibenzo[b,i]xanthene-
5,7,12,14(13H)-tetraone (3b). FT-IR (KBr, n cmꢁ1): 3267, 1638,
1591, 1464, 1354, 1267, 1035, 971, 725, 643; 1H NMR (300 MHz,
Table 1 Surface properties of the examined clay composite systems
Average pore
diameter (nm)
Total pore volume
Sample
SBET (m2 gꢁ1
)
Vm (cm3 gꢁ1
)
Vp (cm3 gꢁ1
)
(cm3 gꢁ1
)
Mont K10
250.11
280.52
119.52
14.84
51.835
50.313
27.461
3.41
6.427
0.4016
0.419
0.2326
0.128
0.4298
0.5081
0.2368
0.129
H+-Mont
6.7483
7.9242
34.74
(NiFe2O4@Cu)(H+-Mont)
NiFe2O4@Cu
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Fig. 8 FT-IR spectra of the Mont K10, H+-Mont and (NiFe2O4@Cu)(H+-Mont) systems.
2852, 1671, 1597, 1499, 1351, 1268, 1159, 944, 803, 724, 574; 1H
NMR (300 MHz, CDCl3): d (ppm) 2.94 (s, 1H, CH), 6.78 (bs, 2H,
Pyridyl), 7.64–8.01 (m, 8H, Ar), 8.62 (d, 2H, Pyridyl).
2.8.7. 13-(4-Nitrophenyl)-5H-dibenzo[b,i]xanthene-
5,7,12,14(13H)-tetraone (3i). FT-IR (KBr, n cmꢁ1): 3330, 2923,
2859, 1654, 1592, 1519, 1346, 1265, 1038, 724; 1H NMR (300
MHz, CDCl3): d (ppm) 6.38 (s, 1H, CH), 7.31–7.36 (m, 1H, Ar),
7.44–7.51 (m, 1H, Ar), 7.71–7.84 (m, 5H, Ar), 8.08–8.18 (m, 5H, 2.9. General procedure for the synthesis of 3,3,6,6-
Ar).
2.8.8. 13-(3-Nitrophenyl)-5H-dibenzo[b,i]xanthene-
tetramethyl-9-aryl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-
diones
5,7,12,14(13H)-tetraone (3j). FT-IR (KBr, n cmꢁ1): 3337, 2923,
1654, 1594, 1526, 1349, 1267, 1035, 726; 1H NMR (300 MHz,
CDCl3): d (ppm) 6.25 (s, 1H, CH), 7.42–7.48 (m, 1H, Ar), 7.63–
7.84 (m, 6H, Ar), 8.10–8.19 (m, 5H, Ar).
Synthesis of hexahydroxanthene diones was carried out using
the procedure described in Section 2.8 with dimedone (4, 2
mmol) at 90 ꢀC. Selected spectral data for the products are given
as follows:
2.9.1. 3,3,6,6-Tetramethyl-9-phenyl-3,4,5,6,7,9-hexahydro-
1H-xanthene-1,8(2H)-dione (5a). FT-IR (KBr, n cmꢁ1): 2949,
2.8.9. 13-(4-Pyridyl)-5H-dibenzo[b,i]xanthene-
5,7,12,14(13H)-tetraone (3k). FT-IR (KBr, n cmꢁ1): 3463, 2918,
Fig. 9 Magnetization curves of (a) NiFe2O4, (b) NiFe2O4@Cu and (c) the (NiFe2O4@Cu)(H+-Mont) MNPs.
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Table 2 Optimization experiments for the synthesis of 13-(4-chlor-
ophenyl)-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraone (3c) cata-
lyzed by (NiFe2O4@Cu)(H+-Mont) MNPsa
2.9.6. 3,3,6,6-Tetramethyl-9-(4-nitrophenyl)-3,4,5,6,7,9-hex-
ahydro-1H-xanthene-1,8(2H)-dione (5j). FT-IR (KBr, n cmꢁ1):
1
2950, 2875, 2631, 1599, 1514, 1363, 1259, 1155, 851; H NMR
(300 MHz, CDCl3): d (ppm) 0.81 (s, 6H, 2Me), 0.95 (s, 6H, 2Me),
2.01–2.16 (s, 4H, 2CH2), 2.47 (s, 4H, 2CH2), 4.54 (s, 1H, CH), 7.73
(s, 2H, Ar), 8.01 (s, 2H, Ar).
Solvent
Entry Nanocatalyst (mg) (2 mL)
Temp. (ꢀC) Time (min) Yieldb (%)
1
2
3
4
5
—
—
20
20
20
EtOH
H2O
EtOH
H2O
EtOH–H2O 60
(1 : 1)
60
60
60
60
240
240
45
30
30
5
5
60
70
85
2.9.7. 3,3,6,6-Tetramethyl-9-(3-nitrophenyl)-3,4,5,6,7,9-hex-
ahydro-1H-xanthene-1,8(2H)-dione (5k). FT-IR (KBr, n cmꢁ1):
1
2954, 2882, 1668, 1600, 1527, 1456, 1359, 1192, 1145, 752; H
NMR (300 MHz, CDCl3): d (ppm) 0.99 (s, 6H, 2Me), 1.11 (s, 6H,
2Me), 2.13–2.28 (s, 4H, 2CH2), 2.51 (s, 4H, 2CH2), 4.83 (s, 1H,
CH), 7.37–8.03 (m, 4H, Ar).
6
7
8
9
20
20
20
20
DMF
60
60
60
60
120
120
120
120
40
50
55
60
EtOAc
CH3CN
Solvent-
free
EtOH–H2O r.t.
(1 : 1)
EtOH–H2O 80
(1 : 1)
EtOH–H2O 80
(1 : 1)
3. Results and discussion
10
11
12
13
20
20
10
30
120
20
60
95
70
75
3.1. Synthesis and characterization of (NiFe2O4@Cu)(H+-
Mont) MNPs
Synthesis of the (NiFe2O4@Cu)(H+-Mont) MNPs was carried out
through a seven-step procedure: (i) swelling (hydration) of
montmorillonite K10 by vigorous stirring of the clay mineral in
distilled water. Due to this process, Mont K10 can absorb water
between its interlamellar spaces; hence, the stacked sheets of
the clay mineral move apart. The swelling expands the surface
area of montmorillonite and exposes the cations of the inter-
layers to contribute Brønsted and Lewis acidity character;70 (ii)
stirring of the swelled montmorillonite in an aqueous solution
of NaCl to afford homoionic Na+-exchanged montmorillonite.
Through simple ion-exchange, many cations of the interlayers
in montmorillonite K10 are exchanged with Na+ ions. The
diffusion of Na+ ions endows the clay mineral with a high
capacity for exchanging Na+ ions with various guest species
(transition metals, complexes and H+ ions). In other words, the
ions are encapsulated in the interlamellar spaces; therefore, the
acidity (Brønsted and Lewis) of the clay mineral can be changed
for different purposes; (iii) stirring of the homoionic Na+-
exchanged Mont in an aqueous solution of HCl to afford acid-
activated Mont; (iv) preparation of NiFe2O4 MNPs via solid
state grinding of Ni(OAc)2$4H2O and Fe(NO3)3$9H2O in the
presence of NaOH; (v) mixing of NiFe2O4 MNPs with an aqueous
solution of CuCl2$2H2O; (vi) reduction of Cu2+ ions to Cu0 with
NaBH4 to afford NiFe2O4@Cu MNPs; and nally, (vii)
20
EtOH–H2O 80
(1 : 1)
120
a
The condensation reactions were carried out with 2-hydroxy-1,4-
naphthoquinone (2 mmol) and 4-chlorobenzaldehyde (1 mmol).
b
Isolated yield.
1662, 1650, 1363, 1350, 1197, 1163, 1143, 997, 700; 1H NMR (300
MHz, CDCl3): d (ppm) 0.99 (s, 6H, 2Me), 1.10 (s, 6H, 2Me), 2.13–
2.23 (d, 4H, 2CH2), 2.43 (s, 4H, 2CH2), 4.69 (s, 1H, CH), 6.98–
7.31 (m, 5H, Ar).
2.9.2. 9-(2-Chlorophenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-
hexahydro-1H-xanthene-1,8(2H)-dione (5b). FT-IR (KBr,
n cmꢁ1): 2952, 2636, 1723, 1628, 1381, 1241, 838, 757; 1H NMR
(300 MHz, CDCl3): d (ppm) 1.02 (s, 6H, 2Me), 1.10 (s, 6H, 2Me),
2.18 (s, 4H, 2CH2), 2.45 (s, 4H, 2CH2), 5.01 (s, 1H, CH), 7.00–7.26
(m, 4H, Ar).
2.9.3. 9-(4-Chlorophenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-
hexahydro-1H-xanthene-1,8(2H)-dione (5c). FT-IR (KBr, n cmꢁ1):
2953, 2881, 1665, 1472, 1388, 1195, 1153, 1010, 834, 759; 1H
NMR (300 MHz, CDCl3): d (ppm) 1.00 (s, 6H, 2Me), 1.11 (s, 6H,
2Me), 2.14–2.28 (s, 4H, 2CH2), 2.47 (s, 4H, 2CH2), 4.72 (s, 1H,
CH), 7.17–7.27 (m, 4H, Ar).
2.9.4. 9-(2-Methoxyphenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-
hexahydro-1H-xanthene-1,8(2H)-dione (5h). FT-IR (KBr,
n cmꢁ1): 2951, 1724, 1661, 1487, 1368, 1242, 1195, 1143, 831,
759; 1H NMR (300 MHz, CDCl3): d (ppm) 0.91 (s, 6H, 2Me), 1.04
(s, 6H, 2Me), 2.12 (s, 4H, 2CH2), 2.37 (s, 4H, 2CH2), 3.72 (s, 3H,
OCH3), 4.82 (s, 1H, CH), 6.82–7.35 (m, 3H, Ar).
Table 3 Comparison of the catalytic activity of (NiFe2O4@Cu)(H+-
Mont) MNPs with those of other potentially active species in the
synthesis of dibenzo[b,i]xanthene tetraone 3ca
Entry
Catalysta
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
Cu0 NPs
NiFe2O4
60
60
60
60
60
60
20
70
65
70
60
80
60
95
2.9.5. 9-(2-Hydroxyphenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-
hexahydro-1H-xanthene-1,8(2H)-dione (5i). FT-IR (KBr, n cmꢁ1):
2951, 2829, 1721, 1627, 1379, 1239, 1161, 1026, 877, 754; 1H
NMR (300 MHz, CDCl3): d (ppm) 1.01 (s, 6H, 2Me), 1.04 (s, 6H,
2Me), 1.96 (s, 2H, CH2), 2.34–2.38 (s, 4H, 2CH2), 2.34–2.64 (s,
4H, 2CH2), 4.68 (s, 1H, CH), 7.01–7.27 (m, 4H, Ar), 10.47 (bs, 1H,
OH).
NiFe2O4@Cu
Mont K10
H+-Mont
Na+-Mont
(NiFe2O4@Cu)(H+-Mont)
a
All reactions were carried out in H2O–EtOH (1 : 1 mL) under oil bath
b
(80 ꢀC) conditions using 20 mg of the nanocatalyst. Isolated yield.
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preparation of (NiFe2O4@Cu)(H+-Mont) MNPs via simple mix- peaks present at 2q ¼ 27.40ꢀ and 48.02ꢀ can be attributed to the
ing of the individual sonicated suspensions of NiFe2O4@Cu and reection planes of copper species. The XRD pattern of mont-
H+-Mont constituents (Fig. 3). In this context, through the morillonite K10 (Fig. 5a) also shows that along with hkl and two-
electrostatic attraction of NiFe2O4@Cu (positive charge) to H+- dimensional hk reections, the commercial clay mineral
Mont (negative charge) compartments and the repulsion within contains impurities of quartz, cristobalite and feldspar. More-
the H+-Mont layers/segments, nanosegments of NiFe2O4@Cu over, analysis of the pattern of homoionic Na+-exchanged
are dispersed on the surface and in the interlamellar spaces of montmorillonite (Fig. 5b) reveals that the intensity of the
the H+-Mont constituent.
reection peaks notably increased. This increase is attributed to
The concentration of exchangeable cations in a clay mineral the diffusion of Na+ ions in the interlamellar spaces of the clay
is called the cation exchange capacity (CEC), and it is measured mineral via ion-exchange processing.78,79 Despite this, the XRD
in milli-equivalents per 100 g of dried clay.77 The measured CEC pattern of the H+-Mont (Fig. 5c) system shows a decrease in the
results for the Mont K10 and Na+-Mont clays were 85 and 110 intensity of the reection peaks. This can be attributed to
meq. gꢁ1, respectively, representing the successful replacement exfoliation/destruction of layers in the Na+-Mont clay during
of Na+ ions instead of other cations in the interlamellar spaces HCl-activation.80 In continuation, the XRD pattern of (NiFe2-
of the clay minerals.
O4@Cu)(H+-Mont) (Fig. 5e) shows that upon combining the
NiFe2O4@Cu and H+-Mont constituents, no structural changes
occurred in the clay matrix, and the pattern of the nano-
composite contains the characteristic peaks of both
compartments.
3.2.2. SEM analysis. The morphologies and size distribu-
tions of the particles in the prepared NiFe2O4, NiFe2O4@Cu,
Mont K10, H+-Mont and (NiFe2O4@Cu)(H+-Mont) systems were
investigated by scanning electron microscopy (SEM). The SEM
images of NiFe2O4 (Fig. 6a and b) and NiFe2O4@Cu (Fig. 6c and
d) present that the morphologies of the two MNPs are quite
different: NiFe2O4 is composed of polygon/irregular segments,
resulting in surfaces with low porosity and a size distribution of
the particles in the range of 10 to 25 nm. The SEM images of
NiFe2O4@Cu show that through the immobilization of copper
nanoparticles on the surface of NiFe2O4, the porosity of the
3.2. Characterization of the (NiFe2O4@Cu)(H+-Mont) MNPs
3.2.1. XRD analysis. The phase purities and structural
elucidation of the NiFe2O4, NiFe2O4@Cu, Mont K10, Na+-Mont,
H+-Mont and (NiFe2O4@Cu)(H+-Mont) systems were studied
using wide-angle X-ray diffraction analysis (Fig. 4 and 5). In the
diffractogram of NiFe2O4 (Fig. 4a), the peaks at 2q ¼ 30.40ꢀ,
35.77ꢀ, 37.40ꢀ, 43.51ꢀ, 54.21ꢀ, 57.60ꢀ, 63.11ꢀ, 71.90ꢀ, 75.20ꢀ and
76.05ꢀ correspond to the (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1
1), (4 4 0), (6 2 0), (5 3 3) and (4 4 4) reection planes of standard
cubic spinel NiFe2O4 (JCPDS 54-964), respectively, showing the
high phase purity and crystallinity of the prepared sample. In
this context, the XRD pattern of NiFe2O4@Cu MNPs (Fig. 4b)
shows that in addition to the reection planes of NiFe2O4, the
Table 4 Preparation of dibenzo[b,i]xanthene tetraones catalyzed by (NiFe2O4@Cu)(H+-Mont) MNPsa
Mp (ꢀC)43,44,48,50
Entry
R-
Product
Time (min)
Yieldb (%)
Found
Reported
1
2
3
4
5
6
7
8
C6H5
3a
3b
3c
3d
3e
3f
3g
3h
3i
40
30
20
30
40
35
70
60
70
75
80
90
92
95
97
78
75
80
70
98
95
92
305–306
308–309
331–332
306–308
319–321
>330
305–307
217–219
312–313
340–341
>320
305–307 (ref. 43 and 44)
307–309 (ref. 43 and 48)
330–332 (ref. 43 and 48)
—
320–322 (ref. 43 and 48)
—
304–307 (ref. 44)
218–220 (ref. 44)
312–313 (ref. 50)
340–342 (ref. 43 and 44)
—
2-ClC6H4
4-ClC6H4
H
4-MeOC6H4
3-MeOC6H4
4-MeC6H3
2-OHC6H4
4-O2NC6H4
3-O2NC6H4
4-Pyridyl
9
10
11
3j
3k
a
All reactions were carried out with a molar ratio of aryl aldehyde : 2-hydroxy-1,4-naphthoquinone of 1 : 2 in the presence of 20 mg of the
nanocatalyst in H2O–EtOH (1 : 1 mL, 80 ꢀC). b Yield refers to isolated pure product.
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surface was extensively increased, and the nanocomposite is of isotherm IV, with H3 hysteresis loops in the desorption proles.
constituted of agglomerated tiny spherical segments. Moreover, This type of isotherm is characteristic of micro/mesoporous
the particle sizes are distributed in the range of 10 to 40 nm.
SEM images of Mont K10 and acid-activated Mont are also
materials.81
In this context, the surface properties of the Mont K10, H+-Mont,
illustrated in Fig. 6e–h. Comparison of the images shows that (NiFe2O4@Cu)(H+-Mont) and NiFe2O4@Cu systems, including SBET
through the acid-activation, the stacked sheets of Mont K10 (m2 gꢁ1), Vm (cm3 gꢁ1), average pore diameter (nm), Vp (cm3 gꢁ1
)
were exfoliated into tiny segments. This phenomenon can be and total pore volume (cm3 gꢁ1), are summarized in Table 1.
interpreted by the major elimination of Al3+ and Fe2+/3+ contents Investigation of the results indicates that the surface area and total
from the octahedral sites when the activation of montmoril- pore volume of H+-Mont notably increased versus those of Mont K10
lonite with HCl was carried out. Moreover, through the elimi- alone. This phenomenon is attributed to the exfoliation of stacked
nation of Al3+ and Fe2+/3+ ions from the interlayers, numerous sheets to tiny segments and the formation of numerous new pores,
pores on the surface and internal sheets of montmorillonite affording increases of the pore volume and specic surface area.82,83
were generated, increasing the surface area of the H+-Mont In contrast, anchoring NiFe2O4@Cu MNPs on the H+-Mont
system. In this context, the depicted SEM images of the compartments decreases the surface area and total pore volume,
(NiFe2O4@Cu)(H+-Mont) MNPs (Fig. 6i and j) present that the while the average pore radius of the particles was found to increase.
nanocatalyst is composed of stacked-sheet structures contain- This is attributed to physical occupancy of some of the pores in
ing agglomerated rough/irregular segments. The sizes of the montmorillonite K10 by NiFe2O4@Cu MNPs.84
particles in the (NiFe2O4@Cu)(H+-Mont) system are distributed
3.2.4. FT-IR analysis. FT-IR is a sensitive tool that was
in the range of 18 to 25 nm. The SEM analysis also exhibited utilized to elucidate the structures as well as the functional
that anchoring of the (NiFe2O4@Cu) constituent on the surface groups of the Mont K10, H+-Mont and (NiFe2O4@Cu)(H+-Mont)
of the clay compartment was carried out successfully, leading to materials (Fig. 8). The absorption band at 3620 cmꢁ1 for Mont
a decrease of the surface porosity in comparison to that of the K10 is attributed to the vibration of hydroxyl groups bonded to
NiFe2O4@Cu MNPs.
Si4+, Al3+ or Mg2+ ions. The absorption bands at 3433 and
3.2.3. N2 adsorption–desorption analysis. Nitrogen adsorp- 1635 cmꢁ1 show the H–O–H stretching and bending vibrations
tion–desorption analysis is commonly utilized to study the specic of the adsorbed water in the interlayers of montmorillonite, and
surface areas of nanocomposite systems. In addition, information the bands at 522 and 464 cmꢁ1 correspond to the bending
regarding the pore sizes and total pore volumes of nanomaterials is vibrations of Si–O–Al and Si–O–Si, respectively.85–87 In this
accessible using this analysis. In this area, Fig. 7 shows the N2 context, comparison of the FT-IR patterns of the Mont K10 and
adsorption–desorption proles for the Mont K10, H+-Mont and H+-Mont systems reveals that through the acid-activation of
(NiFe2O4@Cu)(H+-Mont) systems. Based on BDDT IUPAC classi- montmorillonite by HCl, the position of the strong band at
cation, the shapes of the isotherms for all samples are closer to that 1054 cmꢁ1 (showing the stretching vibration of Si–O in
Table 5 Preparation of tetramethylxanthene diones using (NiFe2O4@Cu)(H+-Mont) MNPsa
Mp (ꢀC)16,19,43
Entry
Ar-
Product
Time (min)
Yieldb (%)
Found
Reported
1
2
3
4
5
6
7
8
C6H5
2-ClC6H4
4-ClC6H4
2,4-Cl2C6H3
3-OH, 4-MeOC6H3
4-OH, 3-MeOC6H3
3-MeOC6H4
2-MeOC6H4
2-OHC6H4
5a
5b
5c
5d
5e
5f
5g
5h
5i
50
35
20
15
50
90
95
96
97
80
75
75
60
75
97
98
202–204
226–227
231–233
242–244
175–177
170–172
176–177
229–230
247–249
222–224
174–176
202–204 (ref. 16)
226–228 (ref. 43)
230–232 (ref. 16 and 43)
243–245 (ref. 19)
—
50
—
100
210
80
20
20
175–178 (ref. 19)
230–231 (ref. 16)
—
221–223 (ref. 43)
175–177 (ref. 19)
9
10
11
4-O2NC6H4
3-O2NC6H4
5j
5k
a
All reactions were carried out under solvent-free conditions. b Yields refer to isolated pure products.
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tetrahedral sites) shied to 1080 cmꢁ1. This fact demonstrates
the successful ion-exchange processing of H+-Mont clay. In the
case of (NiFe2O4@Cu)(H+-Mont), the stretching vibration of Si–
O shied to 1038 cmꢁ1. This movement is attributed to
dispersion of the NiFe2O4@Cu MNPs on the surface and in the
interlamellar spaces of the clay constituent. In addition, the
bands at 2926 and 2865 cmꢁ1 conrmed the immobilization of
NiFe2O4@Cu MNPs in the clay matrix through the conforma-
tional changes in the metal-linked-clay interactions.88–90
3.3. Synthesis of xanthene derivatives catalyzed by
(NiFe2O4@Cu)(H+-Mont) MNPs
Aer the successful synthesis and characterization of the
(NiFe2O4@Cu)(H+-Mont) MNPs, the catalytic activity of the
prepared nanocomposite was further investigated towards the
tandem Knoevenagel–Michael reaction of aromatic aldehydes
with 2-hydroxy-1,4-naphthoquinone (Lawsone).
The study was primarily carried out by optimizing the
condensation reaction of 2-hydroxy-1,4-naphthoquinone (2
mmol) and 4-chlorobenzaldehyde (1 mmol) in the presence and
absence of (NiFe2O4@Cu)(H+-Mont) MNPs under different
conditions, including changes in the reaction solvent, amount
of nanocatalyst and temperature. The results of this investiga-
tion are illustrated in Table 2. Observation of the results shows
that in the absence of the nanocatalyst, the reaction did not
show appropriate efficiency (entries 1 and 2). However, by
adding a small amount of (NiFe2O4@Cu)(H+-Mont) MNPs, the
rate of the condensation reaction was dramatically accelerated.
The examinations also present that among the various solvents,
a mixture of H2O–EtOH was the best choice. Moreover, utilizing
20 mg of the nanocatalyst per 1 mmol of 4-chlorobenzaldehyde
in a mixture of H2O–EtOH (1 : 1 mL) at 80 ꢀC were the
requirements to afford 13-(4-chlorophenyl)-5H-dibenzo[b,i]
xanthene-5,7,12,14(13H)-tetraone (3c) in 95% isolated yield
(Table 2, entry 11), while the reaction was carried out with 100%
3.2.5. VSM analysis. Next, the magnetic behavior of
NiFe2O4, NiFe2O4@Cu and the (NiFe2O4@Cu)(H+-Mont) MNPs
was studied using vibrating sample magnetometer (VSM)
analysis. The magnetization curves of the samples (Fig. 9)
present nonlinear reversibility with small hysteresis loops. In
addition, the s-shapes shown in the magnetization graphs are
characteristic of so ferromagnetic materials.91 The magneti-
zation (Ms) values of NiFe2O4 (curve a) and NiFe2O4@Cu (curve
b) were found to be 43.458 emu gꢁ1 and 33.135 emu gꢁ1
,
respectively. The decrease in the value of Ms for the NiFe2-
O4@Cu MNPs is attributed to the presence of diamagnetic
copper metal. In this context, due to the existence of the non-
magnetic clay mineral constituent, the saturation magnetiza-
tion value of the (NiFe2O4@Cu)(H+-Mont) MNPs (curve c)
notably decreased to 11.57 emu gꢁ1. This level of magnetization
(11.57 emu gꢁ1) is still large enough for magnetic separation.
Fig. 10 A proposed mechanism for the Knoevenagel–Michael reaction of aryl aldehydes with 1,3-dicarbonyl compounds catalyzed by
(NiFe2O4@Cu)(H+-Mont) MNPs.
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Table 6 Synthesis of dibenzo[b,i]xanthene tetraone 3c with (NiFe2O4@Cu)(H+-Mont) MNPs and other reported promoters
Entry
Catalyst
Time (min)
Yield (%)
Conditions
Temp (ꢀC)
Reusability
Ref.
a
1
2
3
4
5
6
7
8
(NiFe2O4@Cu)(H+-Mont)
Graphene oxide-Sr NPs
Choline-Cl/itaconic acid
[Msim]Cl
[Hmim][HSO4]
[Et3N–SO3H]Cl
[Et3NH][HSO4]
[Bmim][HSO4]
N-Sulfonic acid poly(4-vinylpyridinium) HSO4
p-TSA
FeCl3$6H2O
20
105
90
5
4
5
95
89
90
95
92
98
93
86
94
80
95
EtOH–H2O
Solvent-free
Ionic liquid
Solvent-free
Solvent-free
Solvent-free
Solvent-free
EtOH
80
80
90
80
80
80
80
Reux
60
7
6
5
5
5
5
5
—
43
44
47
47
47
47
48
50
51
53
4
90
15
150
90
9
10
11
Solvent-free
Solvent-free
Solvent-free
5
—
—
100
90
a
Present work.
conversion. Investigation of the results also exhibits that
The capability of (NiFe2O4@Cu)(H+-Mont) MNPs in the
although decreasing the loading of the nanocatalyst from 20 mg synthesis of dibenzo[b,i]xanthene tetraones was further studied
to 10 mg decreased the yield of condensation reaction (entry towards the one-pot Knoevenagel–Michael reaction of
12), increasing the nanocatalyst from 20 mg to 30 mg also substituted
aromatic
aldehydes
with
2-hydroxy-1,4-
diminished the yield from 95% to 75% (entry 13). A possible naphthoquinone under the optimized reaction conditions.
explanation for the decrease of the yield (utilizing a larger The results of this investigation are illustrated in Table 4. The
amount of nanocatalyst) is encapsulation/environing of the table reveals that various aryl aldehydes, including electron
starting materials by the nanocatalyst, followed by decreasing releasing or withdrawing functionalities, participated success-
the inter-collision of the reactants to afford the nal product. In fully in the condensation reaction within 20 to 80 min at 80 ꢀC
summary, the conditions mentioned in entry 11 (Table 2) were to afford the corresponding dibenzo[b,i]xanthene tetraone
selected as the optimum reaction conditions.
materials in high to excellent yields.
In order to clarify the role of nanocatalyst in the synthesis of
The generality of this synthetic method was also studied by
dibenzo[b,i]xanthene tetraone 3c, the catalytic activity of the examining the condensation reactions of aromatic aldehydes
(NiFe2O4@Cu)(H+-Mont) MNPs was compared to those of the with dimedone as another source of 1,3-dicarbonyl compounds.
potentially active species (Cu0 NPs, NiFe2O4, NiFe2O4@Cu, The obtained results present that the utilized conditions for the
Mont K10, H+-Mont and Na+-Mont) under the optimized reac- synthesis of xanthene materials with 2-hydroxy-1,4-
tion conditions. The results in Table 3 show that performing the naphthoquinone were also efficient for the case-synthesis with
title reaction in the presence of Cu0 NPs, NiFe2O4@Cu and the dimedone material. Therefore, substituted aromatic aldehydes
modied montmorillonites led to poor yields of 3c. However, (1 mmol) were reacted with dimedone (2 mmol) in the presence
the immobilized NiFe2O4@Cu MNPs on acid-activated mont- of (NiFe2O4@Cu)(H+-Mont) (20 mg) at 90 ꢀC in a one-pot
morillonite, (NiFe2O4@Cu)(H+-Mont), dramatically accelerated procedure to afford the cyclization products of 9-aryl-3,3,6,6-
the rate of the reaction (Table 3, entry 7).
tetramethyl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H) diones.
Table 7 Synthesis of xanthene dione 5c catalyzed by (NiFe2O4@Cu)(H+-Mont) MNPs and other reported promoters
Entry
Catalyst
Time (min)
Yield (%)
Conditions
Temp (ꢀC)
Reusability
Ref.
a
1
2
3
4
5
6
7
8
(NiFe2O4@Cu)(H+-Mont)
ZrO2 NPs
HOAc
20
19
10
390
40
96
150
300
80
360
8
96
87
81
94
89
93
93
94
90
88
90
92
EtOH–H2O
Solvent-free
Solvent-free
CH3CN
Solvent-free
Solvent-free
Ionic liquid
CH3CN
Solvent-free
EtOH
Solvent-free
Solvent-free
90
100
80
7
—
—
—
4
5
4
3
—
4
8
16
19
22
28
29
31
35
36
37
42
43
NaHSO4$SiO2
Reux
110
100
80
Reux
100
Reux
80
Nano Fe3O4@TiO2@SO3H
Nano ZrO2@SO3H
[Hmim]TFA
Amberlyst-15
9
Montmorillonite K10
Fe3+-montmorillonite
CuI/poly(4-vinylpyridine)
Graphene oxide-Sr NPs
10
11
12
60
80
6
a
Present work.
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The results of this investigation are illustrated in Table 5. As can dimedone with various aryl aldehydes in a mixture of H2O–
be seen, all reactions were carried out successfully within 15 to EtOH (1 : 1 mL) as a green solvent at 80 ꢀC to 90 ꢀC, which
210 min to afford tetramethyl xanthene diones 5(a–k) in high to afforded the products in high to excellent yields. Investigation
excellent yields.
of the results exhibited that the synergic inuence of the
Although the exact mechanism of this synthetic protocol is copper-layered nickel ferrite constituent with H+-Mont resulted
not clear, a mechanism depicted in Fig. 10 presents the role of in a dramatic enhancement of the rate of the reaction. The facile
(NiFe2O4@Cu)(H+-Mont) MNPs in promoting the formation of separation of the nanocatalyst by an external magnetic eld,
xanthene materials. The mechanism shows that the carbonyl generality, short reaction times, high product yields and low
moiety of the aryl aldehyde can be activated by the clay nano- loading of the nanocatalyst as well as the excellent reusability of
catalyst through the existence of numerous Lewis and Brønsted the magnetic clay are the key features which make this synthetic
acidity sites. Then, the enolic form of the 1,3-dicarbonyl method a prominent choice for the synthesis of xanthenes as
compound readily reacts with the activated aldehyde to form biologically active materials.
the Knoevenagel intermediate I. Next, through the activation of
intermediate I with the nanocatalyst followed by Michael
addition of the second enole form of 1,3-dicarbonyl material,
Conflicts of interest
the intermediate II is produced. Through ring closing of the
The authors declare that they do not have any conict of
interest.
intermediate II, nally, the xanthene material is produced.
The usefulness of (NiFe2O4@Cu)(H+-Mont) MNPs in the one-
pot syntheses of 3c and 5c was also highlighted by a comparison
of the obtained results with current and previously reported Acknowledgements
protocols (Tables 6 and 7). A case study shows that in terms of
The authors gratefully acknowledge the nancial support of this
work by the research council of Urmia University.
reaction time, conditions, reusability of the nanocatalyst and
yield of the mentioned products, the present work presents
a more efficient process than previous systems. On the basis of
product yield, solvent-free conditions and rate of the conden-
sation reaction, however, better results with previous systems
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