Fe3O4-supported N-pyridin-4-amine-grafted graphene oxide as efficient and magnetically separable novel nanocatalyst for green synthesis of 4H-chromenes and dihydropyrano[2,3-c]pyrazole derivatives in water

Article abstract of DOI:10.1007/s11164-018-3597-4

Abstract: Fe₃O₄-magnetized N-pyridin-4-amine-functionalized graphene oxide [Fe₃O₄@GO-N-(pyridin-4-amine)] was readily prepared via a three-step procedure. The synthesized nanofilms were characterized by scanning

Full text of DOI:10.1007/s11164-018-3597-4

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-018-3597-4
Fe3O4‑supported N‑pyridin‑4‑amine‑grafted graphene
oxide as efcient and magnetically separable novel
nanocatalyst for green synthesis of 4H‑chromenes
and dihydropyrano[2,3‑c]pyrazole derivatives in water
Davood Azarifar1 · Masoud Khaleghi‑Abbasabadi1
Received: 9 July 2018 / Accepted: 10 September 2018
© Springer Nature B.V. 2018
Abstract
Fe3O4-magnetized N-pyridin-4-amine-functionalized graphene oxide [Fe3O4@GO-
N-(pyridin-4-amine)] was readily prepared via a three-step procedure. The synthe-
sized nanoflms were characterized by scanning electron microscopy, powder X-ray
difraction analysis, Fourier-transform infrared spectroscopy, energy-dispersive
X-ray spectroscopy, vibrating-sample magnetometry, and elemental analysis. The
prepared magnetic amine-functionalized graphene oxide exhibited high catalytic
activity for one-pot three-component synthesis of dihydropyrano[2,3-c]pyrazole
and tetrahydrobenzo[b]pyran (4H-chromene) derivatives. Excellent product yield,
short reaction time, use of water as green solvent, and easy workup procedure are
the main advantages of the present protocol. In addition, the catalyst could be easily
separated by magnetic decantation and reused for six cycles without signifcant loss
of catalytic activity.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-018-3597-4) contains supplementary material, which is available to authorized users.
*
Davood Azarifar
azarifar@basu.ac.ir
1
Department of Chemistry, Bu-Ali Sina University, Hamedan 65178, Iran
Vol.:(0123456789)
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D. Azarifar, M. Khaleghi-Abbasabadi
Graphical abstract
Keywords Magnetite-supported · N-pyridin-4-amine-grafted · Graphene oxide ·
Magnetic nanocatalyst · Dihydropyrano[2,3-c]pyrazoles · Tetrahydrobenzo[b]pyrans
Introduction
Due to increasing concerns regarding the environment, green chemistry has emerged
as an important issue, including development of green approaches and efcient
recoverable heterogeneous catalysts for use in industrial processes and organic syn-
theses [1–3]. In recent years, heterogeneous catalysts have found wide applications
as environmentally compatible materials, resulting in great interest for use in vari-
ous industrial and organic transformations and syntheses [4]. Among the heteroge-
neous catalysts, diferent nanoparticles (NPs) have attracted enormous interest due
to their high catalytic activity and interesting physical and chemical properties, such
as high selectivity, nontoxicity, reusability, long-term stability, and high surface area
to volume ratio that enables high catalyst loading capacity [5–12]. In addition, such
nanosized particles are easily dispersible in solution, forming stable suspensions
[13, 14]. In spite of these advantages, such small particles require tedious processes
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Fe3O4‑supported N‑pyridin‑4‑amine‑grafted graphene oxide…
for recycling and separation from the reaction mixture by fltration or expensive
ultracentrifugation, limiting their application [15]. This problem has been tackled
by using magnetic nanoparticles (MNPs) as excellent supports amenable to simple
magnetic separation [16–21]. Such MNP-supported heterogeneous catalysts have
found wide applications in organocatalysis and industrial catalytic processes [22,
23]. In this context, Fe2O3 and Fe3O4 nanoparticles are the most extensively stud-
ied core magnetic supports, owing to their high surface area, high catalyst loading
ability, conductivity, magnetic susceptibility, catalytic activity, and striking stability
[24–26].
Among the well-documented nanomaterials, nanostructured carbon materials
have attracted enormous interest as environmentally compatible and efcient cata-
lysts for use in diferent chemical and industrial processes, due to their interesting
properties such as high corrosion resistance, air stability, and low cost [27–32]. Car-
bon-based nanomaterials exist in diferent allotropic forms such as fullerene, carbon
nanotubes (CNTs), graphene, diamond, and porous carbon on zero-, one-, two-, and
three-dimensional nanoscales. Graphene is a well-represented allotrope of carbon,
characterized structurally by one-atom-thick planar sheets of sp2-bonded carbon
atoms densely packed in a two-dimensional hexagonal honeycomb crystal lattice.
Graphitic materials have emerged as the most widely used carbon materials in vari-
ous felds due to their outstanding properties such as appropriate electrical conduc-
tivity, high mechanical strength, suitable transparency, high surface area, and proper
biocompatibility [33–38]. Graphene oxide (GO) has been reported to possess oxi-
dative catalytic activity and has found versatile applications as an environmentally
friendly and air-stable heterogeneous metal-free catalyst for use in various chemical
reactions [39–41]. The structural model suggested for graphene oxide (GO) on the
basis of its solid-state 13C nuclear magnetic resonance (NMR) spectrum is shown
in Fig. 1 [42]. Accordingly, GO is built of layers of six-membered aromatic rings
of variable sizes, carrying diferent functional groups such as carboxyl, hydroxyl,
and epoxy groups. Such functional groups enable the carbon layers of GO to medi-
ate ionic and nonionic interactions with a wide range of molecules and also exhibit
oxidizing and acidic efects in diferent reactions [40].
Fig. 1 Structural model of
H
O
O
H
O
H
O
O
graphene oxide (GO)
H
O
O
H
O
H
O O
O
O
H
O
H
O
H
O
H
O
H
O
O
O
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D. Azarifar, M. Khaleghi-Abbasabadi
Polyfunctionalized 4H-chromenes have been found to exhibit intriguing bio-
logical and pharmaceutical properties such as antimicrobial [43], antileishma-
nial [44], antitumor [45], hypotensive [46], anti-human immunodefciency virus
(HIV) [47, 48], and local anesthetic [49] activities. 4H-Chromenes carrying
amino and nitrile functionalities are also favored medicinal compounds show-
ing potential anticancer activity [50, 51]. Moreover, a variety of functionalized
4H-chromene moieties appear as the key building block of numerous oxygen-
containing natural products that exhibit diverse biological and pharmacological
activities such as antitumor [52], antiallergenic [53], antiinfammatory [54], and
antineurodegenerative efects [55]. In addition, chromenes have found applica-
tions in other felds such as cosmetics, pigments, and potentially biodegradable
agrochemicals [56].
Conventional methods reported for synthesis of chromenes generally employ
three-component condensation reaction between aldehydes, malononitrile,
and 1,3-diketones. These reactions utilize diferent catalytic systems including
hexadecyltrimethyl ammonium bromide [57], ionic liquids [58], Mg/La mixed
metal oxides [59], nanosilica [60], and silica-bonded propylpiperazine-N-sul-
famic acid [61], including reactions performed under various conditions such
as electrochemical condition [60] and microwave radiation [62]. In addition, a
novel magnetically immobilized organocatalyst fabricated by covalently anchor-
ing 2-aminomethylphenol moiety on the surface of hydroxyapatite-encapsulated
maghemite nanoparticles has been explored for preparation of benzo[b]pyrans
and dihydropyrano[c]chromenes [63].
4H-Pyran-containing molecules form another important class of heterocy-
clic compounds that show diverse biological activities. Pyrano[2,3-c]pyrazoles
have attracted considerable interest owing to their signifcant pharmacological
and therapeutic values as antibacterial, anticoagulant, anticancer, diuretic, and
insecticidal agents [64, 65]. These derivatives also constitute a structural unit
for synthesis of some pharmaceutical agents, promising drugs, and photoactive
materials [66–69].
Generally, synthesis of dihydropyrano[2,3-c]pyrazole derivatives involves
three-component condensation reaction between 3-methyl-1-phenylpyrazolin-
5-one, malononitrile, and aldehydes under catalysis by various catalytic systems
such as ionic liquids [70], piperidine [71], cupreine [72], MgO [73], and hexade-
cyldimethylbenzyl ammonium bromide (HDMBAB) [74]. We recently described
a facile, high-yielding, one-pot synthesis of pyrano[2,3-c]pyrazole derivatives
using γ-Fe2O3@Cu3Al-LDH as efcient and reusable magnetic nanocatalyst
[75].
As part of our ongoing eforts towards development of newer and benign het-
erogeneous nanocatalysts, and their applications in catalytic organic reactions
and synthesis of heterocyclic compounds [75–80], we describe herein the syn-
thesis and characterization of Fe3O4-supported N-pyridin-4-amine-functional-
ized graphene oxide and its application as efcient and recyclable heterogeneous
magnetic nanocatalyst for one-pot three-component synthesis of 4H-chromenes
(4a–h) and dihydropyrano[2,3-c]pyrazoles (5a–h) (Scheme 1).
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Fe3O4‑supported N‑pyridin‑4‑amine‑grafted graphene oxide…
Scheme 1 Synthesis of 4H-chromenes 4a–h and pyrano[2,3-c]pyrazoles 5a–h under catalysis by
Fe3O4@GO-N-(pyridin-4-amine) nanoflms
Experimental
General
Chemicals including natural fake graphite (−325mesh, 99.95 %) were purchased
from Merck chemical company and used as received. Reaction progress was moni-
tored by thin-layer chromatography (TLC) on Silica Gel PolyGram SIL G/UV 254
plates. Melting points were measured in open capillary tubes using an Electrother-
mal 9100 apparatus. Fourier-transform infrared (FT-IR) spectra were recorded from
KBr pellets using a PerkinElmer Spectrum 65 FT-IR spectrophotometer. NMR
spectra were recorded for samples in CDCl3 or dimethyl sulfoxide (DMSO)-d6 on
90, 300, and 400 MHz Bruker spectrometers using Me4Si as internal standard. The
magnetic properties of the catalyst sample were measured with a vibrating-sample
magnetometer (VSM, model LBKFB, Meghnatis Daghigh Kavir Co., Iran) at room
temperature. Energy-dispersive X-ray (EDX) analysis of the prepared catalyst was
performed on a VG ESCALAB-200R spectrometer equipped with a hemispheri-
cal electron analyzer. Powder X-ray difraction (XRD) analysis was conducted on
a PANalytical X’PertPRO X-ray difractometer. Ultrasonication was performed in a
2200 ETH-SONICA ultrasound cleaner at frequency of 45 kHz. Scanning electron
microscopy (SEM) images were obtained using a Tescan model MIRA 3 feld emis-
sion gun-scanning electron microscope (FEG-SEM).
Preparation of graphene oxide (GO)
Graphene oxide was prepared following the modifed Hummers method [81, 82].
Initially, 250 mL H2SO4 was added to 5 g graphite powder, and the mixture was
stirred for 24 h. Next, to the resulting mixture was slowly added 30 g KMnO4, and
the reaction mixture was continuously stirred at 50 °C for 72 h. The resulting mix-
ture was then poured into a beaker containing 500 mL ice, followed by addition of
50 mL H2O2 (30 %) in 500 mL deionized (DI) water. As a result, the mixture turned
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D. Azarifar, M. Khaleghi-Abbasabadi
from brown color to bright yellow. The reaction product was separated by centrifu-
gation, washed with DI water and 10 % HCl solution repeatedly, and dried at 60 °C.
Preparation of Fe3O4‑supported N‑pyridin‑4‑amine‑grafted graphene oxide
magnetic nanoflms
Functionalization of graphene oxide with N‑pyridin‑4‑amine
Functionalization of graphene oxide (GO) with 4-aminopyridine takes place via
nucleophilic ring-opening of epoxide groups [83]. To a dispersion of 1 g GO in
50 mL ethanol in a three-necked round-bottomed fask was added 3 g 4-aminopyri-
dine, and the mixture was stirred for 24 h under refux condition. Then, the reac-
tion mixture was allowed to reach room temperature, followed by fltration to sepa-
rate the product GO-N-(pyridin-4-amine). Finally, the solid product was repeatedly
washed with deionized (DI) water and ethanol three times, and dried in an oven at
70 °C.
Nanomagnetization of N‑pyridin‑4‑amine‑functionalized graphene oxide
(NMAP‑GO)
Fe3O4-supported N-pyridin-4-amine-functionalized graphene oxide [Fe3O4@GO-N-
(pyridin-4-amine)] was prepared by coprecipitation of FeCl2·4H2O and FeCl3·6H2O
in presence of 4-aminopyridine-grafted graphene oxide. Aqueous solution of
FeCl2·4H2O and FeCl3·6H2O was prepared with molar ratio of 1:2. Nanomagnetiza-
tion of N-pyridin-4-amine-functionalized graphene oxide was performed following a
modifed procedure reported by Kassaee et al. [84]. First, 20 mg of previously pre-
pared N-pyridin-4-amine-functionalized graphene oxide [GO-N-(pyridin-4-amine)]
in 20 mL DI water was ultrasonicated for 30 min. Then, to the resulting mixture was
added 25 mL solution of FeCl3 (400 mg) and FeCl2 (150 mg) in DI water (20 mL)
at room temperature. The feeding weight ratio of FeCl3 to GO (mFeCl3:mGO) in the
prepared mixture was 20: 1. Afterwards, the pH value of the mixture was increased
to 11 by adding 30 % aqueous solution of ammonia (15 mL), and the mixture was
stirred at 75 °C for 30 min. After the resulting mixture was allowed to reach room
temperature, the precipitated black nanocomposite was centrifuged and washed
six times with DI water, then fnally dried at 70 °C to obtain pure Fe3O4@GO-N-
(pyridin-4-amine) nanoflms.
General procedure for synthesis of 4H‑chromene derivatives
To a mixture of malononitrile (66 mg, 1 mmol), aromatic aldehyde 1 (1 mmol), and
dimedone (14 mg, 1 mmol) in DI water (5 mL) was added Fe3O4@GO-N-(pyridin-
4-amine) (10 mg), and the resulting mixture was stirred at refux condition for an
appropriate time (Table 3). After reaction completion as monitored by TLC, 15 mL
hot EtOH was added to the resulting reaction mixture, and the catalyst was mag-
netically separated by using an external magnet. The solvent was removed under
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Fe3O4‑supported N‑pyridin‑4‑amine‑grafted graphene oxide…
reduced pressure to leave crude residue, which was recrystallized from absolute
EtOH to aford pure product. The synthesized products 4a–h are all known com-
pounds and were characterized based on their melting point and spectral (FT-IR, 1H
NMR, 13C NMR) data, in comparison with corresponding reported data (Table 3).
General procedure for synthesis of 1,4‑dihydropyrano[2,3‑c]pyrazole derivatives
To a mixture of aldehyde 1 (1 mmol), malononitrile (66 mg, 1 mmol), and 3-methyl-
1-phenyl-1H-pyrazol-5(4H)-one (174 mg, 1 mmol) in DI water (5 mL) was added
the catalyst Fe3O4@GO-N-(pyridin-4-amine) (10 mg), and the mixture was stirred
under refux for an appropriate time (Table 5). Reaction progress was monitored by
TLC. After reaction completion, the reaction mixture was diluted with hot ethanol
(15 mL), and stirred for 5 min. Then, the catalyst was isolated by using an external
magnet. The remaining supernatant liquid was evaporated to leave crude product,
which was purifed by recrystallization from absolute EtOH. All the synthesized
products 5a–h are known compounds and were characterized by their physical prop-
erties and spectral (FT-IR, 1H NMR, 13C NMR) analysis, in comparison with cor-
responding reported data (Table 5).
Selected data
2‑Amino‑7,7‑dimethyl‑5‑oxo‑4‑phenyl‑5,6,7,8‑tetrahydro‑4H‑chromene‑3‑carbonitrile
(4a) White solid; m.p. 235–237 °C; IR (KBr, ν, cm−1): 3394, 3324, 3251, 3212,
2960, 2889, 1680, 1604, 1413, 1371, 1249, 1214, 1159, 1138, 1071, 1036, 838, 695,
1
578. H NMR (90 MHz, CDCl3) δ: 1.04 (s, 3H, CH3), 1.11 (s, 3H, CH3), 2.23 (s,
2H, CH2), 2.45 (s, 2H, CH2), 4.41 (s, 1H, CH), 4.45 (s, 2H, NH2), 7.24 (s, 5H, H–
Ar) ppm; 13C NMR (75.5 MHz, DMSO-d6) δ: 26.8, 28.4, 31.8, 35.5, 49.9, 58.3,
112.7, 119.7, 127.1, 128.3, 144.7, 158.4, 162.5, 195.6 ppm.
2‑Amino‑7,7‑dimethyl‑4‑(p‑fluorophenyl)‑5‑oxo‑5,6,7,8‑tetrahydro‑4H‑chromene‑3‑car‑
bonitrile (4b) White solid; m.p. 188–190 °C; IR (KBr, ν, cm−1): 3354, 3178, 3100,
2960, 2190, 1674, 1636, 1603, 1504, 1410, 1366, 1261, 1215, 1160, 1139, 1033,
1
1015, 859, 561, 530, 487. H NMR (300 MHz, DMSO-d6) δ: 0.93 (s, 3H, CH3),
1.02 (s, 3H, CH3), 2.05–2.27 (m, 2H, CH2), 2.50 (s, 2H, CH2), 4.15 (s, 1H, CH),
6.98–7.27 (m, 6H, H–Ar and NH2) ppm; 13C NMR (100 MHz, DMSO-d6) δ: 27.2,
28.8, 32.2, 35.9, 38.9, 50.4, 58.8, 113.1, 120.2, 127.1, 127.6, 128.8, 145.1, 158.9,
163.1, 196.4 ppm.
2‑Amino‑4‑(p‑chlorophenyl)‑7,7‑dimethyl‑5‑oxo‑5,6,7,8‑tetrahydro‑4H‑chromene‑3‑car‑
bonitrile (4d) White solid; m.p. 211–214 °C; IR (KBr, ν, cm−1): 3379, 3323, 3180,
2958, 2942, 2887, 2188, 1675, 1634, 1390, 1286, 1215, 1093, 1014, 884, 854, 769,
1
639, 620, 576. H NMR (90 MHz, DMSO-d6) δ: 0.94 (s, 3H, CH3), 1.03 (s, 3H,
CH3), 2.15 (s, 2H, CH2), 2.50 (s, 2H, CH2), 4.195 (s, 1H, CH), 7.05–7.30 (s, 6H,
H–Ar and NH2) ppm; 13C NMR (100 MHz, DMSO-d6) δ:14.0, 20.7, 26.8, 28.3,
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D. Azarifar, M. Khaleghi-Abbasabadi
31.8, 35.1, 49.9, 57.7, 59.7, 112.3, 119.5, 128.2, 129.1, 131.1, 143.7, 158.5, 162.6,
170.3 ppm.
2‑Amino‑4‑(p‑nitrophenyl)‑7,7‑dimethyl‑5‑oxo‑5,6,7,8‑tetrahydro‑4H‑chromene‑3‑car‑
bonitrile (4g) Cream solid; m.p. 178–181 °C; IR (KBr, ν, cm−1): 3406, 3316, 3176,
2977, 2942, 2183, 1672, 1631, 1521, 1493, 1390, 1296, 1159, 1031, 855, 729,
1
629, 617, 574, 561. H NMR (90 MHz, DMSO-d6) δ: 0.96 (s, 3H, CH3), 1.04 (s,
3H, CH3), 2.17 (s, 2H, CH2), 2.53 (s, 2H, CH2), 4.37 (s, 1H, CH), 7.18–7.18 (m,
4H, H–Ar), 8.12 (s, 2H, NH2) ppm. 13C NMR (100 MHz, DMSO-d6) δ: 26.9, 28.2,
31.8, 35.6, 39.6, 49.8, 56.9, 111.7, 119.3, 123.7, 128.6, 146.2, 152.3, 158.5, 163.1,
195.7 ppm.
6‑Amino‑3‑methyl‑1,4‑diphenyl‑1,4‑dihydropyrano[2,3‑c]pyrazole‑5‑carbonitrile
(5a) White solid; m.p. 170–173 °C; IR (KBr, ν, cm−1): 3471, 3324, 3194, 3063,
1
2917, 2198, 1659, 1593, 1516, 1385, 1265, 1066, 827, 753, 686, 651. H NMR
(90 MHz, CDCl3) δ: 1.89 (s, 3H, CH3), 4.66–4.69 (d, 3H, CH, NH2), 7.28–7.61 (m,
10H, H–Ar) ppm. 13C NMR (100 MHz, DMSO-d6) δ: 13.0, 37.2, 58.7, 99.1, 116.5,
120.4, 126.6, 127.5, 128.2, 129.0, 129.8, 138.0, 144.0, 144.3, 145.7, 159.8 ppm.
6‑Amino‑3‑methyl‑4‑(p‑chlorophenyl)‑1‑phenyl‑1,4‑dihydropyrano[2,3‑c]pyra‑
zole‑5‑carbonitrile (5c) White solid; m.p. 183–186 °C; IR (KBr, ν, cm−1): 3393,
3280, 3170, 2924, 2224, 2192, 1660, 1605, 1513, 1456, 1370, 1278, 1128, 1073,
936, 834, 759, 692, 669, 572. 1H NMR (400 MHz, DMSO-d6) δ: 1.80 (s, 3H, CH3),
4.74 (s, 1H, CH), 7.28–7.81 (m, 11H, H–Ar and NH2) ppm. 13C NMR (100 MHz,
DMSO-d6) δ: 13.0, 36.5, 58.2, 98.7, 120.3, 120.4, 120.5, 126.7, 129.0, 129.6, 130.2,
132.1, 138.0, 143.1, 144.4, 145.7, 159.9 ppm.
6‑Amino‑4‑(p‑methoxyphenyl)‑3‑methyl‑1‑phenyl‑1,4‑dihydropyrano[2,3‑c]pyra‑
zole‑5‑carbonitrile (5e) White solid; m.p. 170–173 °C; IR (KBr, ν, cm−1): 3390,
3321, 3204,3022, 2191, 1660, 1583, 1513, 1391, 1259, 11,249, 1128, 1109, 1026,
839, 759, 692, 573. 1H NMR (90 MHz, DMSO-d6) δ: 1.78 (s, 3H, CH3), 3.74 (s, 3H,
CH3), 4.62 (s, 1H, CH), 6.84–7.82 (m, 11H, H–Ar, NH2) ppm. 13C NMR (100 MHz,
DMSO-d6) δ: 12.4, 36.1, 54.6, 59.1, 98.3, 113.4, 119.8, 125.5, 128.4, 128.7, 135.0,
137.4, 143.6, 145.2, 158.0, 158.9 ppm.
6‑Amino‑4‑(m‑bromophenyl)‑3‑methyl‑1‑phenyl‑1,4‑dihydropyrano[2,3‑c]pyra‑
zole‑5‑carbonitrile (5h) White solid; m.p. 161–164 °C; IR (KBr, ν, cm−1): 3450,
3324, 3195, 2199, 1660, 1593, 1518, 1487, 1456, 1391, 1126, 1065, 859, 751, 686.
1H NMR (400 MHz, DMSO-d6) δ: 1.81 (s, 3H, CH3), 4.75 (s, 1H, CH), 7.29–7.35
(m, 5H, H–Ar), 7.47–7.52 (m, 4H, H–Ar), 7.79–7.81 (m, 2H, NH2) ppm. 13C NMR
(100 MHz, DMSO-d6) δ: 13.1, 36.8, 58.0, 98.5, 120.4, 120.6, 122.3, 126.7, 127.5,
129.8, 130.5, 130.9, 131.3, 138.0, 144.4, 145.6, 146.9, 160.0 ppm.
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Fe3O4‑supported N‑pyridin‑4‑amine‑grafted graphene oxide…
Results and discussion
Characterization of the catalyst Fe3O4@GO‑N‑(pyridin‑4‑amine)
As shown in Scheme 2, Fe3O4-supported 4-aminopyridine-functionalized graphene
oxide [Fe3O4@GO-N-(pyridin-4-amine)] was prepared using the modifed method
reported by Kassaee et al. [84]. As described above, graphene oxide (GO) was ini-
tially prepared via oxidation of graphite powder with KMnO4 and H2O2 (30 %) in
acidic solution according to the Hummers method [81]. Then, the graphene oxide
(GO) was aminated through nucleophilic ring-opening of the epoxy groups in gra-
phene oxide (GO) thin flms upon treatment with 4-aminopyridine [83]. Finally,
Scheme 2 Synthetic route to nanomagnetic Fe3O4-supported 4-aminopyridine-grafted GO [Fe3O4@GO-
N-(pyridin-4-amine)]
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D. Azarifar, M. Khaleghi-Abbasabadi
novel Fe3O4 magnetic Fe3O4@GO-N-(pyridin-4-amine) nanosheets were obtained
by coprecipitation of ferrous (Fe2+) and ferric (Fe3+) ions in the presence of the
resulting aminated graphene oxide.
The structure of the synthesized nanoflms was fully characterized by diferent
analytical techniques including Fourier-transform infrared (FT-IR) spectroscopy,
energy-dispersive X-ray (EDX) spectroscopy, scanning electron microscopy (SEM),
X-ray difraction (XRD) analysis, and vibrating-sample magnetometry (VSM).
Figure 2 illustrates the Fourier-transform (FT)-IR spectra of unfunctionalized gra-
phene oxide (GO), N-(pyridine-4-amine)-functionalized GO, and Fe3O4-magnetized
GO-N-(pyridin-4-amine) nanoflms. The O–H stretching vibration assigned
to enolic, carboxylic, and hydroxy groups of GO is observed in the range of
2700–3500 cm−1 for all three compounds. The peaks at 1717, 1620, and 1056 cm−1
in the spectrum of graphene oxide are assigned to stretching vibrations of C=O,
C=C, and C–O groups, respectively, being shifted to lower frequencies in Fig. 2b,
c. In addition, the characteristic peak at 1385 cm−1 in the spectra of pure GO can be
attributed to asymmetric stretching vibration of C–O–C moiety in epoxide groups,
disappearing in Fig. 2b, c. This result clearly confrms successful functionalization
of the GO flms with aminopyridine group. Stretching vibrations of Fe–O bond were
observed at 588 and 627 cm−1 in the spectrum of Fe3O4@GO-N-(pyridin-4-amine)
nanoflms (Fig. 2c). The peak at 1221 cm−1 in Fig. 2b and the peak at 1230 cm−1 in
Fig. 2c are attributed to stretching vibration of amino C–N bond formed by amina-
tion of the GO flms with 4-aminopyridine. Moreover, the characteristic stretching
vibrations appearing at 3300 and 3359 cm−1 in the spectra of 4-aminopyridine-func-
tionalized GO (Fig. 2b) and Fe3O4-magnetized GO (Fig. 2c) correspond respectively
to N–H bond overlapped by O–H stretching vibration. Presence of pyridine ring is
also confrmed by the appearance of the C–H stretching frequency near 3158 cm−1,
as well as the C=C and C=N stretching frequencies at 1569 and 1622 cm−1 and
near 1400 cm−1 [85]. It is seen that the stretching frequency of carboxylic group
at 1717 cm−1 for graphene oxide was shifted to lower frequency of 1622 cm−1 for
Fig. 2 Comparative FT-IR
spectra for a GO, b 4-amino-
pyridine-functionalized GO,
and c Fe3O4@GO-N-(pyridin-4-
amine) nanoflms
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Fe3O4‑supported N‑pyridin‑4‑amine‑grafted graphene oxide…
Fe3O4-magnetized GO. These results demonstrate that graphene oxide was success-
fully grafted with 4-aminopyridine groups, as further supported by magnetic Fe3O4
group.
The XRD patterns of GO and the prepared Fe3O4@GO-N-(pyridin-4-amine)
magnetic nanoflms are presented in Fig. 3. The XRD pattern of GO shows two main
difraction peaks at 2θ=11.95° and 42.45°, corresponding to oxygen functional
groups which are intercalated between the graphene sheets in the course of oxidation
[87]. These peaks are related to their corresponding indices (002) and (100), respec-
tively, and are included in the XRD pattern of the Fe3O4@GO-N-(pyridin-4-amine)
nanoflms. As evident from the XRD pattern of Fe3O4@GO-N-(pyridin-4-amine)
(Fig. 3b), the intensity of the peak at 2θ=11.8° exhibited by GO was signifcantly
decreased upon amination and magnetization with Fe3O4. However, the observation
of this peak at about 2θ=10.08° agrees well with the presence of oxygen function-
alities, even after functionalization and magnetization of graphene oxide. The rela-
tive intensities and positions of all the peaks seen at 2θ=30.39°, 35.74°, 43.36°,
57.31°, and 62.93° in the XRD pattern of Fe3O4@GO-N-(pyridin-4-amine) are in
Fig. 3 XRD patterns of a GO and b Fe3O4@GO-N-(pyridin-4-amine)
1 3

D. Azarifar, M. Khaleghi-Abbasabadi
compliance with the standard XRD pattern of magnetite Fe3O4 in Joint Committee
on Powder Difraction Standards (JCPDS) card no. 79-0417. These fve character-
istic peaks, revealing a crystalline cubic structure of iron oxide phase, are related
to their corresponding faces (220), (311), (400), (511), and (440), respectively. The
average crystalline size of Fe3O4@GO-N-(pyridin-4-amine) was calculated to be
10.7 nm by Scherrer’s equation for the (311) refection with the highest intensity.
This implies that GO flms were successfully grafted with 4-aminopyridine groups
and magnetized with Fe3O4 [86, 88].
Figure 4 presents the energy-dispersive X-ray (EDX) spectra of GO, GO-N-
(pyridin-4-amine), and Fe3O4@GO-N-(pyridin-4-amine). As illustrated by these
EDX results, the expected elemental constituents were obtained for graphene oxide
(C, O), GO-N-(pyridin-4-amine) (C, O, N,) and Fe3O4@GO-N-(pyridin-4-amine)
(C, O, N, Fe). The mass percentages of the corresponding elements calculated from
the EDX analysis are presented in Table 1. These results provide further evidence of
successful grafting of GO flms with 4-aminopyridine group as well as magnetiza-
tion with Fe3O4 MNPs.
SEM analysis was performed on the prepared flms of GO, GO-N-(pyridin-4-
amine), and Fe3O4@GO-N-(pyridin-4-amine) to determine their morphology and
size distribution. The SEM images displayed in Fig. 5 indicate that both GO-N-
(pyridin-4-amine) and Fe3O4@GO-N-(pyridin-4-amine) were composed of wrinkled
thin flms, closely associated with each other to form disordered solids and randomly
accumulated. As revealed by comparison of the SEM images of bare GO and GO-
N-(pyridin-4-amine), no prominent change in the morphology of the graphene flms
occurred as a result of functionalization. Moreover, the SEM image of the Fe3O4@
GO-N-(pyridin-4-amine) catalyst (Fig. 5c) indicates that the Fe3O4 nanoparticles
Fig. 4 EDX results for a GO, b GO-N-(pyridin-4-amine), and c Fe3O4@GO-N-(pyridin-4-amine)
1 3

Fe3O4‑supported N‑pyridin‑4‑amine‑grafted graphene oxide…
Table 1 Quantitative elemental
Element GO-N-(pyridin-4- Fe3O4@GO-N-
GO
analysis of GO, GO-N-(pyridin-
4-amine), and Fe3O4@GO-N-
(pyridin-4-amine) derived from
EDX
amine)
wt%
(pyridin-4-amine)
wt% at.%
6.35 17.5
at.%
wt%
at.%
C
O
N
Fe
51.51
35.39
13.10
–
57.68
29.75
12.57
–
46.40
53.55
46.45
–
16.76
2.12
34.16 53.60
4.94
–
–
74.77
43.65
–
Total
100.00 100.00 100.00 100.00 100.00 100.00
Fig. 5 SEM images of a GO, b GO-N-(pyridin-4-amine), and c Fe3O4@GO-N-(pyridin-4-amine)
exhibited a regularly spherical morphology on Fe3O4@GO-N-(pyridin-4-amine)
formed of nanosized flms with average diameter of about 40 nm [84].
The magnetic behavior of the prepared Fe3O4@GO-N-(pyridin-4-amine) was
examined by vibrating-sample magnetometry (VSM) analysis at 300 K. According
to the typical plot of magnetization versus applied magnetic feld of Fe3O4@GO-N-
(pyridin-4-amine) shown in Fig. 6, the saturation magnetization (Ms) value of the
Fe3O4@GO-N-(pyridin-4-amine) was found to be 48 emu g−1. This value confrms
1 3

D. Azarifar, M. Khaleghi-Abbasabadi
Fig. 6 VSM results of Fe3O4@GO-N-(pyridin-4-amine)
the superparamagnetic property of the Fe3O4@GO-N-(pyridin-4-amine) nanocata-
lyst, which can be efciently separated from the reaction mixture simply by using an
external magnet.
Catalytic activity
The activity of the synthesized Fe3O4@GO-N-(pyridin-4-amine) nanoflms as heter-
ogeneous catalyst was tested in synthesis of 4H-chromenes and dihydropyrano[2,3-
c]pyrazole derivatives. The procedure involves multicomponent one-pot cyclization
reaction between aromatic aldehyde, malononitrile, and dimedone/3-methyl-1-phe-
nyl-1H-pyrazol-5(4H)-one, as shown in Scheme 1. To investigate the feasibil-
ity of formation of the expected products and the reaction conditions, we initially
employed the one-pot three-component reaction of 4-nitrobenzaldehyde, malon-
onitrile, and 5,5-dimethyl-1,3-cyclohexanedione (dimedone) as model reaction; the
results are presented in Table 2. To study the efect of solvent on this reaction, the
reaction was performed using diferent solvents such as H2O, EtOH, dimethylforma-
mide (DMF), and tetrahydrofuran (THF) in presence of the same amount (10 mg) of
catalyst at room temperature. Among the diferent solvents used, the best result was
obtained with H2O as solvent of choice (Table 2, entry 2). It is probable that the high
polarity and ability to form hydrogen bonding make water a better solvent relative to
the other solvents tested in this reaction. In the next stage, the efect of temperature
on the reaction was studied by conducting the reaction at higher temperatures (50,
80, 100 °C, and refux point) in water with 10 mg catalyst. The best result in terms
of reaction yield and time was obtained when the reaction was carried out in H2O
under refux condition (entry 9). Then, the catalyst loading efect was investigated
in this reaction procedure; the optimal amount of catalyst was found to be 10 mg
to synthesize the highest yield of product (entry 9). From the results in Table 2, it
1 3

Fe3O4‑supported N‑pyridin‑4‑amine‑grafted graphene oxide…
Table 2 Screening reaction parameters for model synthesis of 2-amino-4-(p-nitrophenyl)-7,7-dimethyl-
5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrilea
NO2
O
O
O
O
CN
O2N
+
+
NC
CN
H
O
NH2
1
3a
2
4g
Entry
Catalyst loading (mg)
Solvent
Temperature (°C)
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
10
10
10
10
10
10
10
10
10
5
20
30
No solvent
H2O
EtOH
THF
DMF
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
25
25
25
25
25
50
80
100
Refux
Refux
Refux
Refux
Refux
Refux
240
60
90
180
240
50
45
40
15
50
45
45
40
240
10
65
45
25
35
72
85
87
98
80
82
84
87
Trace
9
10
11
12
13
14
40
No catalyst
aConditions: 4-nitrobenzaldehyde (1.1 mmol), 4-hydroxyquinolin-2(1H)-one (1 mmol), malononitrile
(1.1 mmol), solvent (5 mL)
bIsolated yield
is clear that 10 mg catalyst loading provided the best conversion. No further yield
improvement was observed when increasing or reducing the amount of catalyst in
the reaction (entries 9–13). To verify the indispensability of the catalyst Fe3O4@
GO-N-(pyridin-4-amine) in the reaction, the reaction was also conducted in absence
of catalyst, with no respective product being formed in detectable amount (entry 14).
The general applicability of the reaction was investigated by carrying out the
reaction using a diverse series of aldehydes (1a–h) bearing various substituent
groups under the optimized reaction conditions (Scheme 1). According to the
experimental results summarized in Table 3, the aldehydes all reacted efciently
to aford the respective products 4a–h in excellent yield (92–98 %), irrespective
of the nature of the substituent groups.
1 3

D. Azarifar, M. Khaleghi-Abbasabadi
Table 3 Synthesis of 2-amino-4-aryl-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile
derivatives catalyzed by Fe3O4@GO-N-(pyridin-4-amine)a
O
Ar
O
O
O
CN
(Fe3O4@GO-N-(pyridin-4-amine)
(cat.)
+
+
CN
NC
Ar
H
H2O / reflux
O
NH2
1a-h
2
3a
4a-h
Entry
Ar
Product
Time (min)
Yield (%)
M.p. (°C)
Found
Reported
1
2
3
4
5
6
7
8
C6H5
4-FC6H4
4-MeC6H4
4-ClC6H4
2-ClC6H4
3-NO2C6H4
4-NO2C6H4
4-HOC6H4
4a
4b
4c
4d
4e
4f
30
20
40
30
35
35
15
25
94
96
92
92
94
95
98
95
235–237
188–190
207–210
211–214
211–214
209–212
178–181
203–206
236–238 [89]
189–191 [90]
208–210 [77]
212–214 [91]
212–214 [91]
210–211 [92]
178–180 [92]
204–205 [93]
4g
4h
aConditions: aldehyde (1.1 mmol), dimedone (1 mmol), malononitrile (1.1 mmol), H2O (5 mL), catalyst
(10 mg), refux temperature
bIsolated yield
The versatility and catalytic potential of the prepared catalyst were further stud-
ied in one-pot condensation reaction between aromatic aldehydes, malononitrile,
and 3-methyl-1-phenyl-2-pyrazolin-5-one for synthesis of 1,4-dihydropyrano[2,3-
c]pyrazoles 5a–h (Scheme 1). Similarly, 4-nitrobenzaldehyde was chosen as test
aromatic compound in this reaction to establish the reaction conditions. First, the
efect of solvent on this reaction was studied using diferent solvents (H2O, EtOH,
DMF, and THF) in presence of 10 mg catalyst at room temperature. Considering
the experimental results summarized in Table 4, the solvent of choice was found
to be H2O in terms of reaction yield and time (entry 9). Further experiments were
carried out to study the efect of higher temperatures (50, 80, 100 °C, and refux
point), revealing that the best result was achieved at refux temperature (entry 9).
Then, the efect of the catalyst loading was studied; the optimal amount of cata-
lyst was found to be 10 mg (entry 9). Finally, the catalytic role of the catalyst was
verifed by performing the reaction without catalyst, resulting in no detectable
amount of product after a long reaction time (entry 14).
After optimization of the reaction conditions, the scope and generality of
the reaction were investigated by conducting the reaction with a diverse series
of aromatic aldehydes (1a–h) holding various substituent groups under the opti-
mized conditions (Scheme 1). The experimental results summarized in Table 5
show that all the reactions proceeded smoothly to aford the products in excel-
lent yields, irrespective of the nature of the substituents. However, aromatic
1 3

Fe3O4‑supported N‑pyridin‑4‑amine‑grafted graphene oxide…
Table 4 Screening reaction parameters for the model synthesis of 6-amino-3-methyl-4-(p-nitrophenyl)-1-
phenyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrilea
Entry
Catalyst loading (mg)
Solvent
Temperature (°C)
Time (min)
Yieldb (%)
1
2
10
10
No solvent
H2O
25
25
180
60
Trace
55
3
4
5
6
10
10
10
10
EtOH
THF
DMF
H2O
25
25
25
50
90
50
30
42
75
180
180
35
7
10
H2O
80
45
85
8
10
H2O
100
35
85
9
10
5
20
30
H2O
H2O
H2O
H2O
H2O
H2O
Refux
Refux
Refux
Refux
Refux
Refux
20
45
50
75
75
180
98
82
70
77
80
Trace
10
11
12
13
14
40
No catalyst
aConditions: 4-nitrobenzaldehyde (1.1 mmol), 4-hydroxyquinolin-2(1H)-one (1 mmol), malononitrile
(1.1 mmol), solvent (5 mL)
bIsolated yield
aldehydes carrying electron-withdrawing groups appeared to act more readily in
this reaction.
Table 6 compares the experimental results (reaction time, yield, and condi-
tions) obtained from the present protocol with results reported by other research
groups for catalytic synthesis of 2-amino-4-aryl-7,7-dimethyl-5-oxo-5,6,7,8-
tetrahydro-4H-chromene-3-carbonitrile (4a–h) and 1,4-dihydropyrano[2,3-c]
pyrazole (5a–h) derivatives. Based on this comparison, our method appears
more convenient in utilizing H2O as green solvent and an ecofriendly and ef-
cient recyclable new magnetic nanocatalyst for synthesis of 2-amino-4-aryl-
7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (entry 5) and
1,4-dihydropyrano[2,3-c]pyrazole (entry 10) derivatives in short reaction time
and high yield. The high EcoScale score of the present protocol calculated from
1 3

D. Azarifar, M. Khaleghi-Abbasabadi
Table 5 Synthesis of 1,4-dihydropyrano[2,3-c]pyrazole derivatives catalyzed by Fe3O4@GO-N-(pyridin-
4-amine)a
Entry
Ar
Product
Time (min)
Yieldb (%)
M.p. (°C)
Found
Reported
1
2
3
4
5
6
7
8
C6H5
5a
5b
5c
5d
5e
5f
30
30
35
25
40
35
20
30
94
96
97
96
93
92
98
94
170–173
145–148
183–186
173–176
170–173
190–193
186–189
161–164
170–171 [94]
145–146 [95]
184–187 [96]
174–177 [96]
171–172 [96]
190–191 [95]
186–188 [97]
160–163 [98]
2-ClC6H4
4-ClC6H4
4-FC6H4
4-MeOC6H4
3-NO2C6H4
4-NO2C6H4
3- BrC6H4
5g
5h
aConditions: aldehyde (1.1 mmol), 3-methyl-1-phenyl-2-pyrazolin-5-one (1 mmol), malononitrile
(1.1 mmol), H2O (5 mL), catalyst (10 mg), refux temperature
bIsolated yield
Table 6 Comparison of catalytic efciency of present protocol with reported catalysts for syn-
thesis of 2-amino-4-aryl-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile and
1,4-dihydropyrano[2,3-c]pyrazole based on EcoScale percentages and other reaction parameters
Product Catalyst
Conditions
Time (min) Yield (%) EcoScale Ref.
(%)
4g
4g
Pectin
Fructose
H2O:EtOH; r.t.
20
10
90
89
90
87
[99]
[100]
EtOH:H2O;
40 °C
4g
4g
NH4Al(SO4)2·12H2O
EtOH; 80 °C
H2O; 60 °C
140
10
85
96
84
90
[101]
[102]
[DABCO-PDO]
[CH3COO]
4g
Fe3O4@GO-N-(pyridin-4- H2O; refux
15
98
91
Present
work
[103]
amine)
5g
5g
5g
5g
NH4H2PO4/Al2O3
BS-2G-Ti
Glycerol; r.t.
H2O; 70 °C
EtOH; refux
EtOH; refux
12
90
10
15
73
96
90
88
83
85
87
86
[104]
[105]
[106]
BF3/MNPs-450
Silica-bonded N-propyl-
piperazine
5g
Fe3O4@GO-N-(pyridin-4- H2O; refux
20
98
91
Present
work
amine)
1 3

Fe3O4‑supported N‑pyridin‑4‑amine‑grafted graphene oxide…
Scheme 3 Possible mechanism for the synthesis of 2-amino-4-aryl-7,7-dimethyl-5-oxo-5,6,7,8-tetrahy-
dro-4H-chromene-3-carbonitrile derivatives 4a–h catalyzed by Fe3O4@GO-N-(pyridin-4-amine)
the reported method [107] demonstrates that our method is greener in comparison
with other methods shown in Table 6.
Catalytic reaction mechanism
Based on the mechanisms previously described by Rodrigo et al. [83] and other
research groups [108–110], we propose a plausible mechanism to explain the
Fe3O4@GO-N-(pyridin-4-amine)-catalyzed three-component synthesis of 2-amino-
4-aryl-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile
1 3

D. Azarifar, M. Khaleghi-Abbasabadi
Scheme 4 Possible mechanism for synthesis of 1,4-dihydropyrano[2,3-c]pyrazole derivatives 5a–h cata-
lyzed by Fe3O4@GO-N-(pyridin-4-amine)
derivatives 4a–h in Scheme 3. The frst step possibly involves deprotonation of
malononitrile promoted by the graphene oxide-bonded aminopyridine moiety, which
acts as Lewis base. Then, the resulting malononitrile anion reacts with the proto-
nated aldehyde, leading to formation of α-cyanocinnamonitrile intermediate I after
dehydration. Afterwards, aminopyridine-induced enolization of dimedone occurs to
produce its enol form. This enol undergoes nucleophilic addition to intermediate I to
1 3

Fe3O4‑supported N‑pyridin‑4‑amine‑grafted graphene oxide…
97
96
96
95
94
94
93
93
92
91
91
90
89
88
87
90
1
2
3
4
5
6
7
Reaction cycle
Fig. 7 Recyclability of the catalyst Fe3O4@GO-N-(pyridin-4-amine) in model synthesis of 2-amino-4-(p-
nitrophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4g)
aford adduct II, which subsequently undergoes catalyzed intramolecular cyclization
to yield the expected 4H-chromene derivatives 4a–h.
A similar mechanism can be postulated for the Fe3O4@GO-N-(pyridin-4-amine)-
catalyzed three-component synthesis of 1,4-dihydropyrano[2,3-c]pyrazole deriva-
tives, as depicted in Scheme 4. In this mechanism, α-cyanocinnamonitrile interme-
diate I forms as described previously in Scheme 3, then reacts with the enolized
form of methyl-1-phenyl-2-pyrazolin-5-one under catalytic efect of the catalyst.
Finally, the resulting intermediate adduct II undergoes intramolecular cyclization in
presence of the protonated catalyst to aford the expected products 5a–h.
Catalyst recyclability
The recyclability of the catalyst Fe3O4@GO-N-(pyridin-4-amine) was studied
for the model reaction under the optimized conditions repeatedly. The change of
catalytic activity in terms of yield against the number of cycles of the reaction
is presented in Fig. 7. After reaction completion in each cycle, the catalyst was
separated from the reaction mixture simply by using an external magnet. Then,
the recovered catalyst was washed with hot ethanol and dried in oven at 70 °C.
As shown in Fig. 7, the catalyst could be recycled and reused for at least fve
runs without considerable loss of activity. The slight decrease of catalytic activity
could be due to normal loss of the catalyst during the workup stage.
Conclusions
We report a novel preparation of 4-aminopyridine-grafted Fe3O4-magnetized
graphene oxide [Fe3O4@GO-N-(pyridin-4-amine)]. The structure of the pre-
pared Fe3O4@GO-N-(pyridin-4-amine) magnetic nanoflms was characterized
by EDX, FT-IR, XRD, VSM, and SEM techniques. This new nanocatalyst exhib-
ited high catalytic activity in three-component synthesis of 4H-chromenes and
1 3

D. Azarifar, M. Khaleghi-Abbasabadi
dihydropyrano[2,3-c]pyrazole derivatives. The reactions proceeded smoothly in
H2O as green solvent to aford the products in excellent yield. Green reaction condi-
tions, high yield, simple magnetic separation of the catalyst from the reaction mix-
ture, and easy recycling of the catalyst are considered as the main advantages of the
present protocol.
Acknowledgements The authors wish to thank the Research Council of Bu-Ali Sina University for fnan-
cial support to carry out this research. Also, Islamic Azad University, North Tehran Branch, is sincerely
appreciated for technical support.
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