New Copper Complex on Fe3O4 Nanoparticles as a Highly Efficient Reusable Nanocatalyst for Synthesis of Polyhydroquinolines in Water

Article abstract of DOI:10.1007/s10562-019-02986-2

Abstract: In this work, we present a simple, environmentally friendly and economical route for the preparation of a novel copper-Schiff-base organometallic complex on Fe₃O₄ nanoparticles (Fe₃O₄@Schiff-base-Cu) u

Full text of DOI:10.1007/s10562-019-02986-2

Catalysis Letters
https://doi.org/10.1007/s10562-019-02986-2
New Copper Complex on Fe3O4 Nanoparticles as a Highly Efcient
Reusable Nanocatalyst for Synthesis of Polyhydroquinolines in Water
Muhammad Aqeel Ashraf1,2 · Zhenling Liu3 · Wan‑Xi Peng1 · Caixia Gao4
Received: 21 August 2019 / Accepted: 27 September 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
In this work, we present a simple, environmentally friendly and economical route for the preparation of a novel copper-Schif-
base organometallic complex on Fe3O4 nanoparticles (Fe3O4@Schif-base-Cu) using an inexpensive and simple method
and available materials. This magnetic nanocatalyst was comprehensively characterized using Fourier transform infrared
spectroscopy (FT-IR), X-Ray Difractometer (XRD), inductively coupled plasma atomic emission spectroscopy (ICP), energy-
dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), X-ray mapping, thermogravimetric analysis
(TGA) and vibrating sample magnetometer (VSM) analysis. In the second stage, the catalytic activity of this catalyst was
studied in the synthesis of polyhydroquinoline derivatives via Hantzsch reaction in water as a green solvent. In this sense,
simple preparation of the catalyst from the commercially available materials, high catalytic activity, simple operation, short
reaction times, high yields and use of green solvent can be regarded as some advantages of this protocol. In addition, it is
worth mentioning that this nanocatalyst was easily recovered using external magnet and reused for several times without
signifcant loss of its catalytic efciency. Finally, the leaching, heterogeneity and stability of Fe3O4@Schif-base-Cu were
studied by hot fltration test and ICP technique.
Graphic Abstract
A green and novel Fe3O4@Schif-base-Cu catalyst sucssesfully was prepared and characterized. This catalyst can be used
for the Synthesis of polyhydroquinolines in water as the green solvent. This catalyst could be recovered easily and reused
many times without important decrease in efciency.
Keywords Fe3O4@Schif-base-Cu · Polyhydroquinolines · Hantzsch reaction · Reusable catalyst
* Wan-Xi Peng
pengwanxi163@gmail.com; pengwanxi@163.com
Extended author information available on the last page of the article
Vol.:(0123456789)
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M. A. Ashraf et al.
chemistry, due to their unique properties of forming stable
complexes [57, 58]. During the last two decades, schif
base ligands have been leading the world of catalysis in
an unprecedented manner [59–62]. Schif base-transition
metal complexes have been extensively used as catalysts
for various organic reactions such as cross- coupling reac-
tions, multicomponent reactions, degradations, substitu-
tion reaction, elimination reaction, addition reaction, radi-
cal reactions and oxidation–reduction reactions [50, 55,
63–70]. Thus, for the researchers, schif base ligands have
become of vital interest to understand the fundamental
reasons behind their catalytic behavior [71–78]. Rational
catalysts design, by proper functionalization of schif base
ligands to explore their utility in a host of catalytically
relevant syntheses of interest to contemporary organic
transformations, is thus central to industrial and academic
research [77, 79]. Various synthetic methodologies have
been developed using heterogenized organometallic schif
base catalysis [80–91]. However, in this report, copper
complex supported on surface-modifed Fe3O4 nanoparti-
cles (Fe3O4@Schif-base-Cu) is introduced as a new and
efcient reusable catalyst for the synthesis of polyhydro-
quinoline derivatives.
1 Introduction
polyhydroquinoline derivatives have a wide range of bio-
logical activities such as bronchodilator, antiatheroscle-
rotic, antitumor and antidiabetic and vasodilator proper-
ties [1–6] i.e. polyhydroquinoline derivatives which can be
considered as a signifcant class of the well-known Ca2+
channel blockers establish the skeletons of drug molecules
utilized in the treatment of hypertension and cardiovascu-
lar diseases [7, 8]. Therefore, the synthesis of polyhydro-
quinoline derivatives can be viewed as an area of remark-
able attention in organic chemistry. The use of copper
catalysts for this transformation is still interesting from an
industrial perspective because of the fact that copper is less
toxic and inexpensive than other transition metals [9–16].
Homogenous Cu complexes which are among the most
representative Cu catalysts are coordinated with difer-
ent organic ligands and provide excellent yields [17–21].
However, the homogenous Cu complexes sufer from sig-
nifcant drawbacks; including, the difculty of separation
and recovery of Cu complexes from the reaction system,
high costs and the potential environmental pollution [20,
22–24]. In order to circumvent these issues and regarding
the rapid advancement of nanotechnology, heterogeneous
copper catalysts have been proposed by supporting Cu
nanoparticles on the surface of the insoluble hosts [25–29].
Recently, the nanomaterials have widely attracted much
attention due to the fact that they are applied as a pre-
cursor of inorganic–organic hybrid materials [24, 30–35].
Moreover, they have excellent applications in catalytic
reactions, adsorption materials, optical and environmen-
tal problems, food processing, medical industry, energy
production and they have also been used in various felds
of chemistry, physics, engineering and science [36–41].
Besides, magnetic Fe3O4 nanoparticle has attracted great
attention due to its intrinsic magnetic property, facile syn-
thesis and being environment-friendly with high surface
area, low cost and good chemical stability [42–45]. Among
the various methods for the synthesis of Fe3O4 MNPs,
the coprecipitation method has several advantages such as
production in a one-pot process, green conditions, and low
time of synthesis [46–49]. The Schif bases are essential
class of organic ligands, and their metal complexes have
been extensively exploited in a wide variety of organic
reactions especially in the multicomponent reactions
[50, 51]. In this sense, they can become possible alterna-
tives to other ligands [41] owing to the advantages such
as their cost-efectiveness, availability, ease of synthesis,
and chemical and thermal stability [50–56]. Researchers
have developed these ligands and their metal complexes,
which aforded more efective and easier oxidative addi-
tion. Schif bases are common ligands in coordination
The results of this study differ from the previously
reported literatures due to the less number of catalyst synthe-
sis steps. Also, the fnal complex can be synthesized using
a stable interaction between the amine and ether groups of
the prepared Schif base ligand and the Cu atom. In addi-
tion, the presence of both of these electron-donor functional
groups in the prepared Schif base structure, which can form
a stable complex with Cu, can also reduce the leaching of
Cu into the reaction media. Moreover, water was used as a
green solvent under aerobic reaction conditions. Besides,
the catalyst could be easily separated by applying a simple
magnet and could also be reused in several consecutive runs
without appreciable change in its catalytic activity. Notewor-
thy, features of this catalyst are high conversion yields (less
reactive substrates) and good selectivity.
2 Experimental
2.1 Preparation of Fe3O4@Schif‑Base‑Cu Catalyst
The Fe3O4 magnetic nanoparticles were prepared by the
coprecipitation technique as it was previously reported
[92]. For the synthesis of the supported Fe3O4@Schif-Base-
Cu complex, 1 g of the prepared Fe3O4 nanoparticles was
dispersed in 50 mL ethanol/water (1:1) by sonication for
15 min. and, then, 1.5 mL 3-aminopropyltrimethoxysilane
(APTMS) was added to the reaction mixture. The reaction
mixture was stirred under the N2 atmosphere at 40 °C for
24 h. Then, the obtained Fe3O4@APTMS MNPs product
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New Copper Complex on Fe3O4 Nanoparticles as a Highly Efcient Reusable Nanocatalyst…
was separated by magnetic decantation, washed with eth-
anol and, then, dried at 90 °C in an oven for 4 h. After-
wards, the Schif-Base ligand supported on heterogeneous
Fe3O4 was obtained by the reaction of O-bisMe-furaldehyde
(5,5′-Oxybis(5-methylene-2-furaldehyde)) (2.5 mmol) with
the amine groups on the surface of Fe3O4@APTMS (1 g) in
ethanol (20 mL) overnight and under nitrogen atmosphere
and refux conditions. The target Fe3O4@Schif-base was
obtained by magnetic separation, washed with ethanol, and
dried at 80 °C in an oven for 4 h. Finally, In order to pre-
pare Fe3O4@Schif-Base-Cu organometallic complex, the
obtained Fe3O4@Schif-base (0.1 g) was dispersed in 30 mL
ethanol by sonication for 30 min and, then, Cu(NO3)2·3H2O
(0.5 g) was added to the reaction mixture. The reaction mix-
ture was stirred at 80 °C for 24 h. Then, the fnal product
(Fe3O4@Schif-Base-Cu) was separated using an external
magnet, washed with water and ethanol and dried at 80 °C
in an oven for 4 h.
J=7.2 Hz, 2H), 3.67 (s, 3H, OCH3), 2.43 (d, J=16.8 Hz, 1H),
2.29 (m, 4H), 2.17 (d, J=16.0 Hz, 1H), 1.98 (d, J=16.0 Hz,
1H), 1.15 (t, J=7.2 Hz, 3H), 1.00 (s, 3H), 0.85 (s, 3H). 13
C
NMR (100 MHz, DMSO-d6) δ=194.2, 166.9, 157.3, 149.2,
144.6, 140.0, 128.4, 113.1, 110.1, 103.9, 59.0, 54.8, 50.3, 34.9,
32.1, 29.1, 26.5, 18.2, 14.2.
2.3.3 Ethyl 4‑(3,4‑dimethoxyphenyl)‑2,7,7‑trimethyl‑5‑
oxo‑1,4,5,6,7,8‑hexahydroquinoline‑3‑carboxylate
(Table 2 Entry 4)
1H NMR (400 MHz, DMSO-d6) δ = 9.02 (s, 1H, NH),
6.77–6.74 (m, 2H), 6.63 (dd, J=2.0 & 8.4 Hz 1H), 4.79 (s,
1H), 4.02 (q, J=7.2 Hz, 2H), 3.66 (s, 3H, OCH3), 3.65 (s,
3H, OCH3), 2.44 (d, J=17.2 Hz, 1H), 2.30–2.26 (m, 4H),
2.19 (d, J = 16.4 Hz, 1H), 2.00 (d, J = 16.4 Hz, 1H), 1.17
(t, J = 7.2 Hz, 3H), 1.01 (s, 3H), 0.88 (s, 3H). 13C NMR
(50 MHz, DMSO-d6) δ=194.2, 166.8, 149.3, 147.9, 146.9,
144.5, 140.4, 119.2, 111.7, 111.4, 110.0, 103.8, 59.0, 55.4,
55.3, 50.3, 35.1, 32.1, 29.2, 26.4, 18.2, 14.2.
2.2 General Procedure for the Synthesis of Polyhyd‑
roquinolines
2.3.4 Ethyl 2,7,7‑trimethyl‑5‑oxo‑4‑(4‑(trifuoromethyl)
phenyl)‑1,4,5,6,7,8‑hexahydroquinoline‑3‑carboxy‑
late (Table 2 Entry 5)
A mixture of aldehyde (1 mmol), dimedon (1 mmol), ethy-
lacetoacetate (1 mmol) and ammonium acetate (1.3 mmol)
was dissolved in 3 mL water in the presence of Fe3O4@
Schif-Base-Cu catalyst (0.25 mol% of Cu) and stirred at
refux conditions. The progress of the reaction was mon-
itored by TLC. After the completion of the reaction, the
catalyst was separated by a magnet and washed with etha-
nol. Then, the solvent was evaporated and, fnally, all the
products were recrystallized in ethanol.
1H NMR (400 MHz, DMSO-d6) δ=9.17 (s, 1H, NH), 7.57
(d, J = 7.6 Hz, 2H), 7.37 (d, J= 8.0 Hz, 2H), 4.93 (s, 1H),
3.99 (q, J=7.2 Hz, 2H), 2.45 (d, J=16.4 Hz, 1H), 2.32 (m,
4H), 2.19 (d, J = 16.4 Hz, 1H), 2.00 (d, J= 16.0 Hz, 1H),
1.13 (t, J=7.2 Hz, 3H), 1.00 (s, 3H), 0.83 (s, 3H). 13C NMR
(50 MHz, DMSO-d6) δ=194.1, 166.6, 151.8, 149.8, 145.7,
128.2, 124.7, 124.6, 109.3, 102.8, 59.1, 50.1, 36.3, 32.1,
29.0, 26.5, 18.3, 14.1.
2.3 Selected Spectral Data
2.3.1 Ethyl 4‑(4‑chlorophenyl)‑2,7,7‑trimethyl‑5‑oxo‑1,4,5
,6,7,8‑hexahydroquinoline‑3‑carboxylate (Table 2
Entry 1)
2.3.5 Ethyl 4‑(4‑cyanophenyl)‑2,7,7‑trimethyl‑5‑oxo‑1,4,
5,6,7,8‑hexahydroquinoline‑3‑carboxylate (Table 2
Entry 6)
1H NMR (400 MHz, DMSO-d6) δ=9.12 (s, 1H, NH) 7.25
(d, J = 8.0 Hz, 2H), 7.16 (d, J= 8.4 Hz, 2H), 4.83 (s, 1H),
3.99 (q, J=7.2 Hz, 2H), 2.43 (d, J=17.2 Hz, 1H), 2.25–2.28
(m, 4H), 2.19 (d, J=16.0 Hz, 1H), 1.99 (d, J=16.0 Hz, 1H),
1.13 (t, J=7.2 Hz, 3H), 1.00 (s, 3H), 0.83 (s, 3H). 13C NMR
(50 MHz, DMSO-d6) δ=194.1, 166.5, 149.5, 146.5, 145.3,
130.2, 129.2, 127.6, 109.6, 103.0, 59.0, 50.1, 35.6, 32.1,
29.0, 26.4, 18.3, 4.1.
1H NMR (400 MHz, DMSO-d6) δ=9.19 (s, 1H, NH), 7.68
(d, J = 8.0 Hz, 2H), 7.34 (d, J= 8.0 Hz, 2H), 4.90 (s, 1H),
3.98 (q, J=7.2 Hz, 2H), 2.44 (d, J=17.2 Hz, 1H), 2.30–2.26
(m, 4H), 2.19 (d, J=16.4 Hz, 1H), 1.99 (d, J=16.4 Hz, 1H),
1.12 (t, J=7.2 Hz, 3H), 1.00 (s, 3H), 0.81 (s, 3H).13C NMR
(50 MHz, DMSO-d6) δ=194.1, 166.3, 152.8, 149.9, 146.0,
131.8, 128.5, 119.0, 109.1, 108.5, 102.4, 59.1, 50.1, 36.7,
32.1, 29.0, 26.4, 18.3, 14.1.
2.3.2 Ethyl 4‑(4‑methoxyphenyl)‑2,7,7‑trimethyl‑5‑oxo‑1,4,
5,6,7,8‑hexahydroquinoline‑3‑carboxylate (Table 2
Entry 3)
2.3.6 Ethyl 4‑(3,4‑dihydroxyphenyl)‑2,7,7‑trimethyl‑5‑oxo‑
1,4,5,6,7,8‑hexahydroquinoline‑3‑carboxylate
(Table 2 Entry 10)
1H NMR (400 MHz, DMSO-d6) δ=9.00 (s, 1H, NH), 7.05 (d,
J=8.4 Hz, 2H), 6.74 (d, J=8.4 Hz, 2H), 4.78 (s, 1H), 3.99 (q,
1H NMR (400 MHz, DMSO-d6) δ=8.94 (s, 1H, NH), 8.56
(s, 1H, OH), 8.45 (s, 1H, OH), 6.57 (d, J=2.0 Hz, 1H), 6.51
1 3

M. A. Ashraf et al.
(d, J=8.0 Hz, 1H), 6.39 (dd, J=2.0 and 8.4 Hz, 1H), 4.68
(s, 1H), 4.00 (q, J=7.2 Hz, 2H), 2.40 (d, J=17.2 Hz, 1H),
2.27 (m, 4H), 2.16 (d, J=16.0 Hz, 1H), 1.99 (d, J=16.0 Hz,
(t, J = 7.2 Hz, 3H), 1.01 (s, 3H), 0.83 (s, 3H). 13C NMR
(50 MHz, DMSO-d6) δ=194.2, 166.3, 150.3, 149.6, 147.3,
146.0, 134.2, 129.3, 121.9, 120.8, 109.2, 102.6, 59.2, 50.0,
36.4, 32.2, 29.0, 26.3, 18.3, 14.0,
1H), 1.16 (t, J=7.2 Hz, 3H), 1.00 (s, 3H), 0.87 (s, 3H). 13
C
NMR (100 MHz, DMSO-d6) δ=194.3, 167.1, 149.0, 144.6,
144.1, 143.1, 138.6, 118.1, 115.2, 114.8, 110.3, 104.1, 59.0,
50.4, 34.9, 32.1, 29.2, 26.6, 18.0, 14.2.
2.3.9 Ethyl 4‑(5‑hydroxy‑2‑nitrophenyl)‑2,7,7‑trime‑
thyl‑5‑oxo‑1,4,5,6,7,8‑hexahydroquinoline ‑3‑car‑
boxylate (Table 2 Entry 16)
2.3.7 Ethyl 2,7,7‑trimethyl‑4‑(4‑nitrophenyl)‑5‑oxo‑1,4,5,6,
7,8‑hexahydroquinoline‑3‑carboxylate (Table 2 Entry
14)
1H NMR (400 MHz, DMSO-d6) δ=10.37 (s, 1H, OH), 9.06
(s, 1H, NH), 7.69 (d, J = 8.8 Hz, 1H), 6.75 (d, J = 2.4 Hz,
1H), 6.62 (dd, J = 2.4 & 8.8 Hz, 1H), 5.77 (s, 1H), 3.94
(q, J=7.2 Hz, 2H), 2.42 (d, J=16.8 Hz, 1H), 2.29 (s, 3H),
2.25 (d, J=16.8 Hz, 1H), 2.13 (d, J=15.6 Hz, 1H), 1.92 (d,
J=15.6 Hz, 1H), 1.00 (t, J=7.2 Hz, 3H), 0.98 (s, 3H), 0.79
(s, 3H). 13C NMR (100 MHz, DMSO-d6) δ=193.9, 166.7,
161.5, 149.4, 145.8, 145.4, 140.1, 126.3, 166.5, 113.3,
100.3, 103.5, 59.0, 50.1, 32.0, 31.7, 28.8, 26.3, 18.2, 13.8.
1H NMR (400 MHz, DMSO-d6) δ=9.23 (s, 1H, NH), 8.10
(d, J = 8.8 Hz, 2H), 7.42 (d, J= 8.8 Hz, 2H), 4.96 (s, 1H),
3.98 (q, J=7.2 Hz, 2H), 2.45 (d, J=16.4 Hz, 1H), 2.27–2.31
(m, 4H), 2.20 (d, J=16.0 Hz, 1H), 1.99 (d, J=16.0 Hz, 1H),
1.12 (t, J=7.2 Hz, 3H), 1.00 (s, 3H), 0.82 (s, 3H), 13C NMR
(50 MHz, DMSO-d6) δ=194.1, 166.3, 154.9, 150.0, 146.0,
145.6, 128.7, 123.0, 109.0, 102.3, 59.2, 50.0, 36.6, 32.1,
29.0, 26.4, 18.3, 14.0.
3 Results and Discussion
2.3.8 Ethyl 2,7,7‑trimethyl‑4‑(3‑nitrophenyl)‑5‑oxo‑1,4,5,6,
7,8‑hexahydroquinoline‑3‑carboxylate (Table 2 Entry
15)
3.1 Catalyst Preparation
The presented work tries to describe a novel Cu-Schif-base
immobilized on Fe3O4 as a reusable magnetic nanocatalyst.
Initially, the modifed Fe3O4 nanoparticles with 3-aminoro-
propyltriethoxysilane (Fe3O4@APTMS MNPs) have been
prepared according to the reported procedure [93]. Then, the
1H NMR (400 MHz, DMSO-d6) δ = 9.24 (s, 1H, NH),
7.98 (m, 2H), 7.62–7.50 (m, 2H), 4.96 (s, 1H), 3.98 (q,
J = 7.2 Hz, 2H), 2.48 (d, J = 17.2 Hz, 1H), 2.38 (m, 4H),
2.21 (d, J = 16.0 Hz, 1H), 2.00 (d, J = 16.4 Hz, 1H), 1.13
O
Scheme 1 Stepwise preparation
of Fe3O4@Schif-Base-Cu
Si
Cl
O
O
FeCl3.6H2O
NH4OH
3-aminoropropyltrimethoxysilane
H2O:EtOH (1:1), N2, 40°C, 24 h
Fe3O4 MNPs
+
Fe3O4 MNPs
Fe3O4@APTMS MNPs
Cl
N2, r.t
FeCl2.4H2O
O
Si
O
O
O
Si
O
N
O
O
Fe3O4 MNPs
O-bisMe-furaldehyde
EtOH, Reflux, 24h
Fe3O4@APTMS MNPs
O
O
Si
O
O
O
N
Fe3O4@Schiff-Base
O
N
Si
O
O
O
O
Cu
Cu
Fe3O4 MNPs
Cu(NO3)2·3H2O
EtOH, Reflux, 24 h,
O
O
O
O
Si
N
Fe3O4@ Shiff-Cu
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New Copper Complex on Fe3O4 Nanoparticles as a Highly Efcient Reusable Nanocatalyst…
Schif-base ligand was synthesized via the substitution reac-
tion of NH2 with O-bisMe-furaldehyde. Finally, the catalyst
was synthesized by the reaction of Fe3O4@Schif-Base with
Cu(NO3)2·3H2O (Scheme 1). This catalyst has been charac-
terized by FT-IR, XRD, EDS, ICP, SEM, X-ray mapping
TGA and VSM techniques.
3.2 Catalyst Characterizations
The as-prepared Fe3O4@Schif-base-Cu catalyst was also
characterized by FT-IR, XRD, EDS, VSM, ICP, TGA, SEM,
and X-ray mapping techniques.
The FT-IR spectra for the Fe3O4 MNPs (a), Fe3O4 @
AMPTMS MNPs (b), Fe3O4@Schif-Base (c) and Fe3O4@
Schif-base-Cu (d) are shown in Fig. 1. The strong absorp-
tion at 590 and 628 cm−1 in spectrums was attributed to
the presence of Fe–O stretching vibration [94]. In addi-
tion, the peak appearing at 1624 cm−1 was attributed to
the bending vibration of OH band or the stretching vibra-
tional mode of the adsorbed water layer [95]. Moreover,
the broad band at around 3392 cm−1 is attributed to the
asymmetric and symmetric stretching vibrations of –OH
band of the Fe3O4 nanoparticles surface [96]. In the curve
of Fe3O4@APTMS MNPs (b), two peaks at 2853 and
2925 cm−1 can be attributed to the C–H stretching vibra-
tions of NH2-propyl group [93, 97, 98]. Besides, the peak
at 892 cm−1 can be attributed to the Fe–O–Si stretching
vibration. Moreover, a strong IR peak which appeared at
1648 cm − 1 corresponded to the strong bending vibration
of the amide I group and showed the successful immobi-
lization of the anchored NH2-propyl group on the Fe3O4
Support [93, 97]. Absorption peaks at 1012 and 1560 cm−1
(C–N stretching vibration) in Fe3O4@Schiff-base pro-
vide evidences confrming the formation of the desired
ligand [66, 99, 100]. The shift on the absorption peak at
Fig. 2 XRD pattern of the Fe3O4@Schif-Base-Cu
approximately 1624 cm−1 (C=N stretching vibration) in the
Fe3O4@Schif-base-Cu spectra is due to the complexion of
Cu ions with Fe3O4@Schif-base [101].
The X-ray difraction analysis (XRD) pattern of Fe3O4@
Schif-base-Cu is shown in Fig. 2, in which the peak posi-
tions of 2θ at 35.27, 41.52, 50.56, 63.24, 67.49, 74.47, corre-
spond to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4
0) refections, respectively, as they are in agreement with the
standard XRD pattern of Fe3O4 nanoparticles [102]. More
importantly, the phase of Fe3O4 support is not destroyed
during the immobilization of Schif-base-Cu organometal-
lic complex on iron oxide layers. In addition, the Crystal-
line size of Fe3O4@Schif-Base-Cu MNPs was estimated,
using the Scherrer equation from XRD pattern data, to be
20.15 1 nm.
The elemental composition of the Fe3O4@Schif-base-Cu
organometallic complex was determined using EDX analysis
(Fig. 3). The presence of Fe, O, Si, C, N, and Cu in EDX
pattern provides evidences confrming the formation of the
organometallic complex. In addition, the exact amount of
Fig. 1 FT-IR spectra for the Fe3O4 MNPs (a), Fe3O4 @APTMS
MNPs (b), Fe3O4@Schif-Base (c) and Fe3O4@Schif-base-Cu (d)
Fig. 3 EDS spectrum of the Fe3O4@Schif-Base-Cu
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M. A. Ashraf et al.
Fig. 4 SEM images of the Fe3O4 MNPs (a), Fe3O4@APTMS MNPs (b), Fe3O4@Schif-Base (c) and Fe3O4@Schif-base-Cu (d) and recovers
Fe3O4@Schif-base-Cu complex (e)
Cu, which was immobilized on Fe3O4@Schif-base, was
obtained by ICP-OSE as 0.92×10−3 mol g−1.
Figure 7 illustrates the TGA diagram of Fe3O4@Schif-
base-Cu complex. The small weight loss between 5% below
200 °C was exhibited and attributed to the evaporation of the
physically adsorbed solvents and surface hydroxyl groups
[94]. Besides, the next weight loss about 15% occurred
between 200 and 600 °C which are related to the decompo-
sition of the functional groups chemisorbed onto the surface
of the catalyst support [103]. The obtained results confrmed
the formation of the desired organometallic complex on the
Fe3O4 Support.
Figure 4 illustrates the morphology of Fe3O4 MNPs (a),
Fe3O4 @APTMS MNPs (b) Fe3O4@Schif-base (c) and
Fe3O4@Schif-base-Cu complex (d), and recovers Fe3O4@
Schif-base-Cu complex (e) as probed by the scanning elec-
tron microscopy (SEM) technique. As shown in Fig. 4, the
SEM image of Fe3O4@Schiff-Base-Cu complex shows
spherical morphology for most particles.
The size and morphology of Fe3O4@Schif-base-Cu com-
plex were investigated using TEM. As shown in Fig. 5, the
magnetic cores of the nanoparticles were uniform in both
size and shape. Moreover, the particle size was measured to
be about 15–25 nm and, consequently, the spherical mor-
phology was confrmed. The results of TEM is in agreement
with SEM and XRD results.
Magnetic measurement of the catalyst was analysed by
a vibrating sample magnetometer (VSM) at room tempera-
ture. Figure 8 exhibits the magnetization curves of Fe3O4
and Fe3O4@Schif-Base-Cu. The value of the saturation
magnetic moment of the Fe3O4@Schif-base-Cu complex
(Blue diagram) is 51.3 emu/g as compared to the saturation
magnetization of Fe3O4 which was found to be 78 emu/g.
This decrease in the saturation magnetization of Fe3O4@
Schif-Base-Cu complex is due to the successful grafting of
The X-ray mapping of Fe3O4@Schif-base-Cu nanocata-
lyst is shown in Fig. 6. The good dispersion of Cu on the
surface of the catalyst was confrmed using the elemental
map images.
1 3

New Copper Complex on Fe3O4 Nanoparticles as a Highly Efcient Reusable Nanocatalyst…
Fig. 5 TEM images of Fe3O4@Schif-base-Cu nanocatalyst
Fig. 6 X-ray mapping images of the Fe3O4@Schif-Base-Cu
1 3

M. A. Ashraf et al.
Table 1 Optimization for the synthesis of polyhydroquinolines
100
95
90
85
80
Entrya Cat.
Solvent
Temp.(°C) Time.
(min)
Yieldb(%)
(mol%)
1
2
3
4
5
6
7
8
–
H2O
H2O
H2O
H2O
H2O
H2O
Ethanol
PEG-400 100
Refux
Refux
Refux
Refux
Refux
Refux
Refux
2 day
15
15
15
15
15
15
15
15
15
15
15
NR
43
69
83
91
100
65
83
52
70
81
73
0.05
0.10
0.15
0.20
0.25
0.25
0.25
0.25
0.25
0.25
0.25
9
DMSO
Dioxane
DMF
100
Refux
100
10
11
12
25
150
275
400
525
Temperature(°C)
Solvent
100
free
Fig. 7 TGA diagram of Fe3O4@Schif-base-Cu complex
13
14
15
16
17
0.25
0.25
0.25
0.25
0.25
H2O
H2O
H2O
H2O
H2O
90
80
60
40
25
15
15
15
15
15
89
83
76
64
39
aReaction conditions: 4-chlorobenzaldehyde (1.0 mmol), dime-
done (1 mmol), ethyl acetoacetate (1 mmol), and ammonium acetate
(1.2 mmol), Catalyst and solvent (2 mL)
bIsolated yield
In order to optimize the reaction conditions, the com-
bination of p-Clbenzaldehyde (1 mmol), ethyl acetoacetate
(1 mmol), dimedone (1 mmol) and ammonium acetate
(1.2 mmol) was chosen as the model reaction to evaluate
the optimized catalytic activity. Moreover, the efect of vari-
ous experimental parameters; including, the efect of dif-
frent amounts of catalyst, difrent solvents and the efect
of various temperatures on the selected model substrate
were screened to achieve high catalytic efciency and yield.
The results are summarized in Table 1. The infuence of
the catalyst loading on the reaction efciency was studied
with diferent amounts of the catalyst.The comparison of
the results related to the efect of various amounts of the
catalyst revealed that the highest activity was observed in
the presence of 0.25 mol % of the catalyst on the basis of
Cu (Table 1, Entry 6). As can be seen from Table 1, entry
1, the reaction was not completed in the absence of the
catalyst even after 2 days. Then, the efects of solvent and
reaction temperature were evaluated on the model reaction.
Fig. 8 VSM spectrum of Fe3O4(Red) and @Schif-Base-Cu (Blue)
Schif-Base-Cu complex on the surface of Fe3O4 nanopar-
ticles (Red diagram).
All together, these analyses are indicative of the suc-
cessful immobilization of Cu (0) complex onto the Fe3O4@
Schif-base.
3.3 Catalytic Studies
After the successful characterization of the prepared nano-
material, the catalytic activities in the synthesis of polyhyd-
roquinoline derivatives were evaluated.
1 3

New Copper Complex on Fe3O4 Nanoparticles as a Highly Efcient Reusable Nanocatalyst…
Table 2 Synthesis of
Entrya
1
Aldehyde
Time (min) Yield (%)b TOF (s−1
)
M.P
polyhydroquinoline derivative
Measured Reported
15
25
20
35
100
5,760,000
244–246
192–194
256–258
216–218
243–245 [104]
2
3
4
96
3,318,290
4,234,023
2,271,215
252–255 [105]
251–252 [106]
216–218 [107]
98
92
5
6
25
20
91
95
3,145,463
4,104,410
188–190
140–143
188–190 [107]
140–142 [107]
7
8
20
30
99
93
4,320,000
2,678,400
202–204
230–232
203–205 [104]
231–234 [108]
1 3

M. A. Ashraf et al.
Table 2 (continued)
Entrya
9
Aldehyde
Time (min) Yield (%)b TOF (s−1
)
M.P
Measured Reported
35
95
2,345,276
223–225
225–227 [104]
10
20
92
3,974,797
216–218
216–218 [107]
11
12
20
15
97
91
4,190,819
5,241,600
247–249
232–235
249–251 [104]
231–233 [104]
13
14
20
35
95
94
4,104,410
2,320,589
178–180
241–243
180–182 [104]
241–243 [104]
15
15
92
5,299,200
176–177
176–177 [107]
1 3

New Copper Complex on Fe3O4 Nanoparticles as a Highly Efcient Reusable Nanocatalyst…
Table 2 (continued)
Entrya
16
Aldehyde
Time (min) Yield (%)b TOF (s−1
)
M.P
Measured Reported
15
40
35
90
94
93
5,184,000
167–169
261–263
241–243
167–169 [107]
17
18
2,030,603
2,295,902
261–263 [104]
240–242 [104]
aReaction conditions: aromatic aldehyde (1.0 mmol), dimedone (1 mmol), ethyl acetoacetate (1 mmol), and
ammonium acetate (1.2 mmol), Catalyst (0.25 mol %) and H2O (3 mL)
bIsolated yield
Considering the reaction time and the product yield, H2O at
refux conditions proved to be the best choice for the solvent
and tempereature, respectively. After an extensive screening
of the reaction parameters, as shown in Table 1, the best
yield of polyhydroquinoline product was obtained specif-
cally when the reaction was performed using 0.25 mol% of
Fe3O4@Schif-Base-Cu in H2O at 100 °C (Table 1, Entry 6).
Then, the catalytic efectiveness of the prepared Fe3O4@
Schif-Base-Cu catalyst was investigated in the synthesis of
polyhydroquinoline derivatives via the reaction of diferent
aromatic aldehydes. The results are listed in (Table 2). As
shown in Table 2, various aromatic aldehydes bearing difer-
ent electron-withdrawing group or electron-donating groups,
such as OCH3, NO2, NMe2 CH3, CF3, SMe, CN, OH, F, Cl,
and Br, produced excellent reaction yields in the presence of
Fe3O4@Schif-Base-Cu catalyst (Scheme 2).
3.4 Recyclability of the Catalyst
Reproducibility of catalysts provides an important advan-
tage in industrial applications because it reduces produc-
tion costs. Therefore, the recovery and reproducibility of
Fe3O4@Schif-Base-Cu catalyst were explored in the model
reaction. After the completion of the experiment, the reac-
tion system was cooled to room temperature and the catalyst
was magnetically separated from the solution, washed with
acetone, air-dried and, fnally, reused for the next round of
the reactions. As shown in Fig. 9, the recycling process was
repeated for fve cycles with a slight decrease in the activity
of the catalyst.
Also, in order to examine stability of the catalyst after
recycling, the recycled catalyst has been characterized by
XRD, SEM, VSM and FT-IR techniques. These charac-
terizations confrmed that the recovered catalyst is in good
agreement with the fresh catalyst. These characterizations
are strong evidences for high stability of Fe3O4@Schif-
base-Cu organometallic complex after recycling.
A plausible mechanism for the multicomponent synthe-
sis of polyhydroquinolines on the basis of previous reports
[109] has been depicted in Scheme 3. Based on this mecha-
nism, the role of Fe3O4@Schif-Base-Cu MNPS as a Lewis
acid catalyst comes in the Knoevenagel condensation of
aldehydes with active methylene compounds (dimedone or
ethyl acetoacetate) to produce an α,β-unsaturated compound.
In the next step, Fe3O4@Schif-Base-Cu MNPS catalyzed
the Michael addition of intermediates to obtain the polyhy-
droquinoline (Scheme 3).
The SEM image of the recycled catalyst is shown in
Fig. 4 (e) in which the morphology of the recovered cata-
lyst is similar to the particle form of the fresh catalyst.
Moreover, Fig. 10 shows the XRD pattern of the recov-
ered catalyst and, also, the curve intensity which indicates
the structural stability of the catalyst after 11 consecutive
cycles.
1 3

M. A. Ashraf et al.
O
Ar
O
O
Fe3O4@Schiff-Base-Cu (0.25 mol %)
H2O, reflux
O
O
OEt
+
+
+
NH4OAc
Ar CHO
OEt
N
H
O
Scheme 2 Fe3O4@Schif-Base-Cu catalyzed the synthesis of polyhydroquinolines
: Fe3O4@Schiff-Base-Cu MNPs
NH4OAc
HOAc
NH2
O
H
O
+
NH3
Ar
Ar
H
EtO
O
O
Ar
O
O
O
OEt
O
N
H
EtO
O
NH3
Ar
O
Fig. 10 XRD pattern of the recycled Fe3O4@Schif-Base-Cu
HOAc
+
H
OEt
O
H
O
NH2
Ar
NH4OAc
Scheme 3 Proposed mechanism for the synthesis of polyhydroquino-
lines in the presence of Fe3O4@Schif-Base-Cu catalyst
Fig. 9 Reusability of Fe3O4@
Schif-Base-Cu
1 3

New Copper Complex on Fe3O4 Nanoparticles as a Highly Efcient Reusable Nanocatalyst…
3.5 Leaching Study
In order to consider the leaching of Cu into the reaction
media, ICP-AES analysis was performed. In this sense,
the copper content in the reaction media and in the model
reaction was found to be 0.18%. The results show that the
leaching of Cu during the reaction process is negligible
and the catalyst is heterogeneous.
3.6 Comparison
In order to investigate the efciency of this new procedure
in comparison to the reported procedures in the litera-
ture, the results for the synthesis of polyhydroquinolines
using p-Clbenzaldehyde, as the representative example,
were compared to the best of the well-known data from
the literature outlined in the Table 3. As it is evident from
Tables 3, the synthesized Fe3O4@Schif-Base-Cu nano-
catalyst showed better results than other methods.
Fig. 11 FT-IR spectra of the recycled Fe3O4@Schif-Base-Cu
4 Conclusion
In conclusion, the Fe3O4@Schif-Base-Cu catalyst was
synthesized and successfully applied for the synthesis of
polyhydroquinoline derivatives in water as a green reaction
media. The prepared magnetic nanocatalyst was character-
ized by FT-IR, XRD, EDS, ICP, SEM, X-ray mapping,
TGA, and VSM analysis techniques. In addition, all reac-
tions were carried out in green conditions. Additionally,
the Fe3O4@Schif-Base-Cu is more economic and envi-
ronmentally friendly because of its low Cu leaching. This
method ofers several advantages; including, price, higher
catalytic efciency with lower copper content, heterogene-
ous nature, wide substrate scope, green conditions, high
yield, short reaction time, simple work-up procedure, ease
of separation, and recyclability of the magnetic catalyst.
Fig. 12 VSM spectrum of recycled Fe3O4@SiO2-Glycerol-Cu(II)
The Fig. 11 shows the FT-IR spectrum of the recycled
Fe3O4@Schif-base-Cu that points out that the catalyst is
stable during the organic reactions.
Figure 12 exhibits the magnetization curve of the recy-
cled catalyst. The magnetization curves of the recycled
Fe3O4@SiO2-Glycerol-Cu(II) points out that the catalyst
is stable during the organic reactions after 11 cycles.
1 3

M. A. Ashraf et al.
Table 3 Comparison
of the Synthesis of
Entry
Catalyst
Time (min)
Yield (%)a
References
polyhydroquinolines in the
presence of Fe3O4@Schif-
Base-Cu using p-Clbenzaldehyd
with previously reported
procedure
1
2
3
4
5
6
7
8
Fe3O4
NiFe2O4
BiFeO3
FeAl2O4
Fe2O3@HAp@Melamine
Cu-SPATBA/Fe3O4
Pd‐SBTU@Fe3O4
Fe3O4‐Adenine‐Ni
γ-Fe2O3/Cu@cellulose
RuIII @CMC/Fe3O4
Fe3O4-SA-PPCA
6
3
94
90
95
90
93
98
95
96
98
95
95
91
94
93
89
94
86
86
92
89
92
93
80
89
96
98
89
96
92
94
97
92
100
[110]
[3]
15
180
15
65
140
145
15
15
120
5
200
10
50
15
25
90
120
18
20
20
15
20
20
15
45
170
110
100
35
5
[111]
[109]
[112]
[113]
[114]
[115]
[116]
[117]
[118]
[119]
[120]
[121]
[122]
[123]
[124]
[125]
[126]
[108]
[127]
[128]
[95]
[129]
[130]
[131]
[132]
[105]
[133]
[133]
[106]
[134]
This work
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
Fe3O4/TiO2–SO3H
GSA@Fe3O4
Fe3O4@TDI@TiO2-SO3H
Fe3O4/SiO2-OSO3H
Fe3O4@PEO-SO3H
Fe3O4@FSM-16-SO3H
Fe3O4@D-NH-(CH2)4-SO3H
Fe3O4-TEDETA-Br3
Fe3O4@SiO2@(CH2)3Im}C(NO2)3
Fe3O4@SiO2@(CH2)3Im}C(CN)3
NMSMSA
AIL-SCMNPs
BIL@MNP
Fe3O4@SiO2–PEG/NH2
Fe3O4@SiO2@PPh3@Cr2O72‐
Fe3O4@GA@IG
MCM41@Serine@Cu(II)
SBA-15@Glycine- Ni
SBA-15@Glycine-Cu
SBA-15@AMPD-Co
Nicotinic acid
Fe3O4@Schif-Base-Cu
15
aIsolated yield
Intermed 43:7211–7221. https://doi.org/10.1007/s1116
4-017-3069-2
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Publisher’s Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional afliations.
Afliations
Muhammad Aqeel Ashraf1,2 · Zhenling Liu3 · Wan‑Xi Peng1 · Caixia Gao4
1
2
3
4
School of Forestry, Henan Agricultural University,
Zhengzhou 450002, China
College of Geology and Surveying Engineering, Chongqing
Vocationan Institiute of Engineering, Chongqing 402260,
China
Department of Geology Faculty of Science, University
of Malaya, 50603 Kuala Lumpur, Malaysia
School of Management, Henan University of Technology,
Zhengzhou 450001, China
1 3