Synthesis of nitroaldols through the Henry reaction using a copper(II)–Schiff base complex anchored on magnetite nanoparticles as a heterogeneous nanocatalyst

Article abstract of DOI:10.1080/00958972.2021.1921748

A Cu(II)-Schiff base complex supported on functionalized Fe₃O₄ magnetic nanoparticles (MNPs@Salen-Cu(II)) was obtained as a new heterogeneous nanocatalyst. The nanocomposite materials were characterized by vibrating sample magnetometer (VSM), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA). The catalyst was used in the Henry reaction in order to accomplish one-pot synthesis of nitroaldol derivatives in green conditions. This nanocatalyst can be easily separated from media of reaction using an external magnet and reused several times without loss of its catalytic activity. Furthermore, the non-toxicity of the catalysts and the high yield of the products are other advantages of this method.

Full text of DOI:10.1080/00958972.2021.1921748

Journal of Coordination Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Synthesis of nitroaldols through the Henry
reaction using a copper(II)–Schiff base complex
anchored on magnetite nanoparticles as a
heterogeneous nanocatalyst
Niloufar Parandeh-Khoozani & Mohsen Moradian
To cite this article: Niloufar Parandeh-Khoozani & Mohsen Moradian (2021) Synthesis of
nitroaldols through the Henry reaction using a copper(II)–Schiff base complex anchored on
magnetite nanoparticles as a heterogeneous nanocatalyst, Journal of Coordination Chemistry,
74:12, 2035-2054, DOI: 10.1080/00958972.2021.1921748
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JOURNAL OF COORDINATION CHEMISTRY
2021, VOL. 74, NO. 12, 2035–2054
https://doi.org/10.1080/00958972.2021.1921748
Synthesis of nitroaldols through the Henry reaction using
a copper(II)–Schiff base complex anchored on magnetite
nanoparticles as a heterogeneous nanocatalyst
Niloufar Parandeh-Khoozani and Mohsen Moradian
Department of Organic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, Iran
ABSTRACT
ARTICLE HISTORY
Received 30 September 2020
Accepted 8 April 2021
A Cu(II)-Schiff base complex supported on functionalized Fe3O4
magnetic nanoparticles (MNPs@Salen-Cu(II)) was obtained as a
new heterogeneous nanocatalyst. The nanocomposite materials
were characterized by vibrating sample magnetometer (VSM), X-
ray diffraction (XRD), scanning electron microscopy (SEM), energy
dispersive X-ray (EDX), Fourier transform infrared spectroscopy
(FT-IR), and thermogravimetric analysis (TGA). The catalyst was
used in the Henry reaction in order to accomplish one-pot syn-
thesis of nitroaldol derivatives in green conditions. This nanocata-
lyst can be easily separated from media of reaction using an
external magnet and reused several times without loss of its cata-
lytic activity. Furthermore, the non-toxicity of the catalysts and
the high yield of the products are other advantages of this
method.
KEYWORDS
Heterogeneous nanocata-
lyst; copper(II)-complex;
Henry reaction; green
conditions; magnetite
1. Introduction
The Henry reaction is a significant reaction in organic chemistry that was discovered
in 1895 by Henry [1]. The coupling between nitroalkanes and carbonyl compounds in
the presence of base gives C-C bond formation to produce nitroaldols [2–5].
Nitroaldols are useful primary substrates to synthesize other compounds [6–11]. Also,
these materials are used as an efficient intermediate in preparation of drugs such as
CONTACT Mohsen Moradian
m.moradian@kashanu.ac.ir
Department of Organic Chemistry, Faculty of
Chemistry, University of Kashan, Kashan, Iran
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2021.1921748.
ß 2021 Informa UK Limited, trading as Taylor & Francis Group

2036
N. PARANDEH-KHOOZANI AND M. MORADIAN
antibiotics, anthracycline, norephedrine and ephedrine [12, 13]. Furthermore, nitroole-
fins are obtained [14, 15] via dehydration of nitroaldol intermediates [16]. In these
processes, different catalysts were used such as BINOL [17], alkaloid [18], metal com-
plexes [19–21], alkali metal hydroxide [22], alkoxides [23], ammonium acetate [24], car-
bonates [25] and amines [26, 27]. The copper and their ions in different oxidation
states play a crucial role as a catalyst in the synthesis of nitroaldols through the Henry
reaction. Various reports of copper ions as a core of a catalyst for both homogeneous
or heterogeneous systems work very well in this reaction [28–53].
Heterogeneous catalysts have attracted interest because of their advantages in easy
separation from products and reusability which is vital for effective and eco-friendly
organic transformations [54–57].
One of the most important magnetite materials is iron oxides (Fe3O4) due to the
2þ
superparamagnetic properties [58]. These particles have iron in two valences, Fe
and Fe3þ, in the crystal structure that causes unparalleled magnetic structure. Fe3O4
magnetite nanoparticles (MNPs) are the ideal support for a new kind of efficient cata-
lyst owing to low toxicity, easy preparation, high surface area, and unique magnetic
properties. The superiorities of these nanocatalysts are a very active surface for adsorp-
tion, suitable immobilization of metals and ligands, and also easy separation of these
nanocomposite materials by an external magnet [59, 60]. Significantly, a silica shell
both prevents the aggregation of MNPs and can protect the core in order to prevent
direct contact [61, 62]. Moreover, silica as a shell causes active hydroxyl groups on the
surface for reaction with complexes and metals [63]. In this work, we present a new
catalytic system based on magnetite material supporting a copper(II) salen complex.
This nanocomposite material is used as an efficient catalyst in the Henry reaction
toward nitroaldols. Some advantage of this catalyst is the activity, selectivity, hetero-
geneity, high yield of the desired products, magnetic separation, reusability, and easy
work-up.
2. Materials and methods
2.1. Materials
The materials were used containing benzaldehyde derivatives, nitromethane, ethanol,
toluene, iron(III) chloride hexahydrate, iron(II) chloride tetrahydrate, polyethylene gly-
col, ammonia, tetraethylorthosilicate, and trimethoxysilyl propylamine prepared from
Merck Company.
2.2. Apparatus
Melting points were measured by a Yanagimoto Micro Melting Point apparatus in
open capillary tubes. Fourier transform infrared (FT-IR) spectra were obtained (in KBr)
by a Nicolet FT-IR spectrophotometer. The UV-vis absorption spectra were recorded in
a solution using a Perkin-Elmer double beam model UV-550S spectrophotometer using
1
10 mm quartz cells. The H and 13C NMR spectra were recorded on a Bruker DRX-400
spectrometer with CDCl3 as solvent at 25 ꢀC and chemical shifts are given in ppm rela-
tive to Me4Si. The XRD patterns were obtained by an X’PertPro (Philips) instrument

JOURNAL OF COORDINATION CHEMISTRY
2037
with 1.54 Å wavelengths of the X-ray beam and Cu anode material. The microscopic
morphology of the nanoparticles was visualized by SEM (MIRA 3 TESCAN). Energy-dis-
persive X-ray spectroscopy (EDX) of the nanoparticles was imaged by a Sigma ZEISS,
Oxford Instruments Field Emission. The amount of Cu in the solid nanocomposite
material was determined using inductively coupled plasma atomic emission spectros-
copy (ICP-AES, Optima 8000 Perkin Elmer). The purity of the substrates and reaction
monitoring was accomplished by TLC on silica gel polygram SILG/UV 254 plates (from
Merck Company). The conductivity measurement was performed using a PC 7000 con-
ductivity cell (EUTECH, Tuas) in a 10ꢁ3 M complex solution of DMSO at room tempera-
ture. Magnetic susceptibility measurement on a powder sample was performed at
room temperature with
tetrathiocyanatocobaltate(II).
a
Gouy balance calibrated against mercury
2.3. General procedure for preparation of magnetite nanoparticles
Magnetite nanoparticles were synthesized via the reported method. FeCl3ꢂ6H2O (1.8 g)
and FeCl2ꢂ4H2O (0.6 g) were dissolved in 50 ml deionized water at 70 ꢀC. Then 1.0 g of
the polyethylene glycol was added, the solution was stirred and the reaction mixture
was poured slowly into an ammonium hydroxide aqueous solution under vigorous
stirring for 45 minutes, before cooling to room temperature. The black Fe3O4 product
was magnetically separated and washed several times with deionized water and etha-
nol. The precipitates were dried at 70 ꢀC and Fe3O4 nanoparticles were obtained.
2.4. General procedure for preparation of magnetite nanoparticles@SiO2
core-shell
Magnetite nanoparticles (0.5 g) were dispersed in a mixture of ethanol (60 ml) and
water (20 ml). Next, nitrogen gas was introduced slowly, and then 2 ml NH3 and 1 ml
TEOS were added and stirred for one day at room temperature. The resulting solution
was washed several times with deionized water and ethanol and dried at 70 ꢀC.
Finally, the product, Fe3O4@SiO2, was obtained.
2.5. Synthesis of schiff base ligand
The 1:1 molar ratio of salicylaldehyde (10 mmol, 0.9 mL) and trimethoxysilyl propyl-
amine (10 mmol, 3.5 mL) was refluxed in 200 ml ethanol at 70 ꢀC. The obtained solution
was yellow due to imine bond formation. The progress of the reaction was followed
by thin-layer chromatography (TLC). Finally, the synthesized Schiff base was purified
1
and characterized by FT-IR and H NMR.
2.6. Synthesis of Cu(II)-schiff base complex
Cu(OAc)2ꢂH2O (0.50 mmol, 0.10 g) was added to a solution of 0.50 mmol (0.142 g) syn-
thesized Schiff base in ethanol (5 mL) and the mixture was refluxed for 2 days at 70 ꢀC.
After completion of the reaction (checked by TLC), the mixture was cooled to 0 ꢀC and

2038
N. PARANDEH-KHOOZANI AND M. MORADIAN
the precipitate was filtered under reduced pressure. The collected solid was washed
with absolute ethanol (3 ꢃ 10 mL) and dried in vacuum for 2 days at 30 ꢀC to give a
dark green solid. The final product was further purified by recrystallization from a 1,4-
dioxane solution to achieve the respective product; yield 81% (0.17 g). The Cu(II) com-
plex was identified using spectroscopic data such as FT-IR, ultraviolet-visible (UV-Vis)
spectra, molar conductance, and magnetic susceptibility measurements.
2.7. Immobilization of Cu(II)-schiff base complex on magnetite nanoparticles
(MNPs@Salen-Cu (II))
The synthesized Fe3O4 nanoparticles were dispersed in ethanol under ultrasonic condi-
tions. After that, the dispersed solution was added to the synthesized Cu(II)-Schiff base
complex under reflux for two days in toluene as a solvent. The MNPs@Salen-Cu(II) was
separated by external magnet and washed several times with ethanol, and then dried
at 60 ꢀC in a vacuum oven.
2.8. Synthesis of nitroaldols using MNPs@Salen-Cu(II) nanocatalyst
1 mmol of benzaldehyde and 2 mmol of nitromethane (0.12 ml) in the presence of
0.05 g MNPs@Salen-Cu(II) nanocatalyst in ethanol was stirred at room temperature.
The progress of the reaction was monitored by TLC. After completion of the reaction,
the solution was filtered and the solid catalyst was separated and washed carefully
with ethanol. The filtered solution was concentrated by a rotary evaporator and the
desired nitroaldol was obtained and purified by column chromatography. All of the
1
products were identified via FT-IR and H-NMR spectroscopy and their structures were
checked by authentic samples and were confirmed [64–74].
1-(4-methoxyphenyl)-2-nitroethan-1-ol (3a): IR (KBr) V (cmꢁ1): 3580, 3122, 2850, 1563,
1
1378, 1127, 1110, 658; H NMR (CDCl3) d (ppm): 2.44 (1H; OH), 3.74 (3H; CH3), 4.37
(dd, J ¼ 3.2, 9.5 Hz, 1H; CH2NO2), 4.41 (dd, J ¼ 3.2, 9.5 Hz, 1H; CH2NO2), 5.30 (dd, J ¼ 3.2,
3.2 Hz, 1H; CHOH), 6.80-6.97 (m, 2H; ArH), 7.22 (dd, J ¼ 4.7, 4.7 Hz, 2H; ArH).
2-nitro-1-(p-tolyl)ethan-1-ol (3b): IR (KBr) V (cmꢁ1): 3552, 3028, 2923, 1554, 1378,
̅
1
1120, 818; H NMR (CDCl3) d (ppm): 2.33 (3H; CH3), 2.36 (1H, OH), 4.73 (dd, J ¼ 4.0 Hz,
1H; CH2NO2), 4.27 (dd, J ¼ 4.0 Hz, 1H; CH2NO2), 4.76 (dd, J ¼ 4 Hz, 1H; CHOH), 7.19–7.28
(m, 4H; ArH).
2-nitro-1-(4-nitrophenyl)ethan-1-ol (3c): IR (KBr) V (cmꢁ1): 3544, 3070, 2898, 1564,
1369, 1120, 1104, 974; H NMR (CDCl3) d (ppm): 3.30 (1H; OH), 4.59 (d, 2H; CH2NO2),
̅
1
5.6 (dd; J ¼ 8.0, 4.8 Hz, 1H, CHOH), 8.22 (d, J ¼ 8.4 Hz, 2H; ArH), 7.61 (d, J ¼ 8.4 Hz,
2H; ArH).
2-nitro-1-(2-nitrophenyl)ethan-1-ol (3d): IR (KBr) V (cmꢁ1): 3446, 2923, 1553, 1376,
̅
1088, 1012, 824; 1H NMR (CDCl3) (ppm): 3.23 (1H; OH), 4.76 (dd, J ¼ 2, 8.2 Hz, 1H;
CH2NO2), 4.93 (d, J ¼ 9.5, Hz, 1H; CH2NO2), 6.07 (d, J ¼ 8.4 Hz, 1H; CHOH), 7.66 (t,
J ¼ 7.2 Hz, 1H; ArH), 7.76 (t, J ¼ 7.3 Hz, 1H; ArH), 7.98 (d, J ¼ 7.2 Hz, 1H; ArH), 8.20 (t,
J ¼ 8.0 Hz, 1H; ArH).
2-nitro-1-(3-nitrophenyl)ethan-1-ol (3e): IR (KBr) V (cmꢁ1): 3064, 3114, 2854, 2853,
̅
1
2402, 1322, 1302, 1248; H NMR (CDCl3) d (ppm): 3.28 (1H; OH), 4.76 (m, 2H; CH2NO2),

JOURNAL OF COORDINATION CHEMISTRY
2039
Scheme 1. Synthesis of Fe3O4 magnetic nanoparticles.
5.77 (d, J ¼ 6.8 Hz, 1H; CHOH), 7.65 (t, J ¼ 8.0 Hz, 1H; ArH), 7.88 (d, J ¼ 7.6 Hz, 1H; ArH),
8.34 (d, J ¼ 8.0 Hz, 1H; ArH), 8.45 (s, 1H; ArH).
1-(2-chlorophenyl)-2-nitroethan-1-ol (3f): IR (KBr) V (cmꢁ1): 3520, 3068, 2924,1696,
̅
1554, 1378, 1130, 1036, 760; 1H NMR (CDCl3) d (ppm): 3.00 (1H; OH), 4.45 (dd,
J ¼ 8.0 Hz, 1H; CH2NO2), 4.65 (dd, J ¼ 12.0 Hz, 1H; CH2NO2), 5.83 (d, J ¼ 9.6 Hz, 1H;
CHOH), 7.60 (d, 1H, ArH), 7.3–7.40 (m, 3H, ArH).
1-(2,4-dichlorophenyl)-2-nitroethan-1-ol (3g): IR (KBr) V (cmꢁ1): 3515, 3093, 2926,
̅
1557, 1380, 1087, 1048, 963, 784; 1H NMR (CDCl3) d (ppm): 3.47 (1H, OH), 7.62 (d,
J ¼ 8.4 Hz, 1H, ArH), 7.20–7.40 (m, 2H, ArH), 5.75 (dd, J ¼ 7.6, 9.2 Hz, 1H, CHOH), 4.62
(dd, 1H, CH2NO2), 4.12 (dd, J ¼ 8.0, 12.0 Hz, 1H, CH2NO2).
2-nitro-1-(thiophen-2-yl)ethan-1-ol (3h): IR (KBr) V (cmꢁ1): 3446, 2923, 1556, 1378, 1138,
̅
1
1078, 709; H NMR (CDCl3) d (ppm): 7.30–7.32 (m, 1H, ArH), 6.99 (m, 2H, ArH), 4.81 (dd,
J ¼ 8.0, 16.0 Hz, 2H, CH2NO2), 4.65 (dd, J ¼ 8.0, 12.0 Hz, 1H, CHOH), 6.98 (1H, OH).
3-(1-hydroxy-2-nitroethyl)benzene-1,2-diol (3i): IR (KBr) V (cmꢁ1): 3443, 2924, 1550,
̅
1383, 1460, 1647; 1H NMR (CDCl3) d (ppm): 2.67 (2H, OH), 8.04 (1H, OH), 4.42 (1H;
CHOH), 4.86 (2H; CH2NO2), 6.52 (1H; ArH), 6.81 (1H; ArH), 6.69 (d, J ¼ 16 Hz, 1H, ArH).
1-(2,3-dimethoxyphenyl)-2-nitroethan-1-ol (3j): IR (KBr) V (cmꢁ1): 3520, 2938, 2840,
̅
1
1554, 1377, 1170, 748; H NMR (CDCl3) d (ppm): 3.85 (3H; OCH3), 3.94 (3H; OCH3), 3.91
(1H; OH), 4.51–4.57 (m, 1H; CH2NO2), 4.80 (d, J ¼ 8.0 Hz, 1H; CH2NO2), 6.7 (d, J ¼ 8.0 Hz,
1H; CHOH), 6.90–7.06 (m, 3H; ArH).
1-(4-bromophenyl)-2-nitroethan-1-ol (3k): IR (KBr) V (cmꢁ1): 3441, 2924, 1553, 1378,
1
1283, 1073, 822; H NMR (CDCl3) d (ppm): 4.55 (1H; OH), 4.48 (d, 1H; CH2NO2), 4.51 (d,
1H; CH2NO2), 5.45 (dd, J ¼ 8.0 Hz, 1H; CHOH), 7.30 (d, J ¼ 8.0 Hz, 2.0 H; ArH), 7.55 (d,
J ¼ 8.0 Hz, 2H; ArH).
1-(4-fluorophenyl)-2-nitroethan-1-ol (3l): IR (KBr) V (cmꢁ1): 3543, 3078, 2922, 1555,
̅
1378, 1229, 1105, 837; 1H NMR (CDCl3) d (ppm): 4.5 (1H, OH), 4.75–4.81 (m, 2H;
CH2NO2), 5.4 (d, J ¼ 8.8 Hz, 1H; CHOH), 7.09–7.40 (m, 4H; ArH).
3. Results
3.1. Synthesis of Fe3O4 magnetite nanoparticles
As discussed in the Experimental section, magnetite nanoparticles were synthesized
(Scheme 1) and their magnetic property was investigated by vibrating sample magnet-
ometer analysis (VSM). The amount of saturation magnetization for Fe3O4 is 53.50 emu
gꢁ1 (Figure 1).
The infrared spectrum of the magnetite nanoparticles is shown in Figure 2. The Fe-
O stretch appears at 570 cmꢁ1 that indicates magnetite nanoparticles are formed.

2040
N. PARANDEH-KHOOZANI AND M. MORADIAN
Figure 1. Magnetization curve (VSM) of Fe3O4 nanoparticles.
Figure 2. FT-IR spectra of Fe3O4 magnetite nanoparticles.
The XRD pattern of Fe3O4 is shown in Figure 3. The data confirm the formation of
the magnetite nanoparticles and match them with the magnetite phase.
3.2. Preparation of Fe3O4@SiO2
The SiO2 shell was coated on the surface of the Fe3O4 core (Scheme 2), which pre-
vents oxidation of Fe2þ to Fe3þ and losing the superparamagnetism of nanoparticles.
Magnetic properties of Fe3O4@SiO2 nanoparticles were determined by VSM (Figure 4)
as 41.55 emu/g. The reduction of the saturation magnetization is due to the silica-
coated on the magnetite nanoparticles.
3.3. Synthesis of the Schiff base ligand
As shown in Scheme 3, the Schiff base was prepared via reaction of salicylaldehyde
and trimethoxysilyl propylamine in ethanol at 70 ꢀC under reflux. In this reaction, the

JOURNAL OF COORDINATION CHEMISTRY
2041
Figure 3. The XRD of magnetite nanoparticles.
Scheme 2. Synthesis of magnetite nanoparticles Fe3O4@SiO2 core-shell.
imine bond was formed by elimination of water that caused the color of the mixture
to become yellow. The progress of the reaction was followed by TLC. In order to iden-
1
tify the Schiff base ligand, FT-IR and H NMR spectroscopies were studied.
As can be seen in Figure 5, FT-IR analysis of the Schiff base indicates absorptions at
1632 cmꢁ1 and 3435 cmꢁ1 that referred to an imine band formation and O-H stretch.
1
Furthermore, H NMR spectra show the following signals: a singlet at 8 ppm refers to
the imine group (CH ¼ N), the signal for O-H is at 13 ppm, signals at 7.24 and 6.87 are
Ar-H, the OCH3 appears at 3.70 ppm and resonances at 1.79 ppm, 1.22 ppm, and
0.69 ppm are related to the aliphatic hydrogens (Figure 6).
3.4. Synthesis of the Schiff base copper(II) complex
By adding Cu(OAc)2ꢂH2O to the Schiff base, the related complex was formed (Scheme
4). In this complex, Cu was coordinated to N and a hydroxyl functional group. For

2042
N. PARANDEH-KHOOZANI AND M. MORADIAN
Figure 4. Comparison magnetism curve of Fe3O4 and Fe3O4@SiO2 core-shell.
Scheme 3. Synthesis of the Schiff-base ligands.
Figure 5. FT-IR spectrum of the Schiff base ligand.
confirmation of the complex structure, the FT-IR of the Schiff base copper(II) complex
is presented in Figure 7. In the complex, the azomethine band shifts to lower fre-
quency (1611 cmꢁ1) due to coordination of nitrogen with copper. The appearance of
two additional bands (not present in the ligand) at 637 V(Cu-O) and 509 cmꢁ1 V(Cu-N)
confirms coordination of phenolate oxygen to the metal ion. In the complex, a broad
band at 3274–3610 cmꢁ1 shows the presence of a coordinated water.

JOURNAL OF COORDINATION CHEMISTRY
2043
Figure 6. 1H NMR (CDCl3, 400 MHz) of the Schiff base ligand.
Scheme 4. Synthesis of the Cu(II)-Schiff base complex.
Figure 7. FT-IR spectrum of Cu(II)-Schiff base complex.
The ultraviolet-visible (UV-Vis) spectra of the ligand and complex were recorded in
dry DMSO, in which they are readily soluble (Figure 8). The UV-vis spectrum of the lig-
ꢄ
and exhibits peaks at 287 nm assigned to p-p transition localized on the aromatic
ꢄ
rings and a low-intensity band at 331 nm can be attributed to the n-p excitation of
ꢄ
the lone pair on the imine N to the p orbital on the C ¼ N bond. Further, the UV-Vis

2044
N. PARANDEH-KHOOZANI AND M. MORADIAN
Figure 8. UV-Vis spectra of the ligand (a) and copper(II) complex (b) in DMSO. [Inset: visible spec-
trum of copper(II) complex in DMSO at higher concentration].
spectrum of the complex indicated a slight shift in position and intensity of bands at
314 and 367 nm. The visible region spectrum of the copper(II) complex displays an
2
2
additional broad band at 572 nm due to d-d transitions assignable to B1g! A1g,
suggesting distorted square planar geometry around copper(II).
The molar conductance of the metal complex was determined from the KM ¼ j/C
equation, where j is the measured conductivity and C is the concentration of the
solution. The observed molar conductance KM ¼ 10.92 Xꢁ1cm2molꢁ1 of the Cu(II)
complex solution indicates the non-electrolytic nature of the material.
The magnetic behavior of the copper(II) complex was measured at room tempera-
ture and the value obtained for the gram susceptibility (vg) was then converted to
molar susceptibility (vM). The effective magnetic moment value, leff, was calculated
from the expression leff ¼ 2.83 (vMT)1/2 where T is the absolute temperature. The
number of unpaired electrons, n, in the copper atom was then calculated by compar-
ing the theoretical spin-only value for leff given by the leff ¼ [n (n þ 2)]1/2 equation.
The copper(II) ion has an unpaired electron in the 3d shell and magnetic moment
must be close to the spin-only value, 1.73 B.M., but because of spin-orbit coupling,
higher values are often detected. The magnetic moment value for the copper complex
was 1.87 B.M., compatible with the square-planar copper(II) system. The leff value
shows that complex is paramagnetic. This value is indicative of the 3d9 electronic con-
figuration of the Cu(II) ion and characteristic of monomeric species.
3.5. Synthesis of the MNPs@Salen-Cu(II) nanocomposite
As shown in Scheme 5, nanocomposite MNPs@Salen-Cu(II) was synthesized by immo-
bilizing Cu(II)-Schiff base complex on magnetite nanoparticles. Characterization of syn-
thesized nanocatalyst MNPs@Salen-Cu(II) was performed with FT-IR, VSM, XRD, TGA,
SEM and EDX techniques.
The FT-IR spectrum of the MNPs@Salen-Cu(II) nanocomposite is shown in Figure 9.
The peak at 1630 cmꢁ1 is from the imine group (CH ¼ N), the clear peak at 585 cmꢁ1

JOURNAL OF COORDINATION CHEMISTRY
2045
Scheme 5. Immobilization of Cu(II)-Schiff base complex on magnetite nanoparticles
(MNPs@Salen-Cu(II)).
Figure 9. FT-IR spectrum of MNPs@Salen-Cu(II) nanocomposite material.
is related to Fe-O bond, and the peak at 3431 cmꢁ1 corresponds to the hydroxy group.
Overall, the FT-IR spectrum suggests MNPs@Salen-Cu(II) nanocatalyst formation.
Additionally, the magnetic property of the MNPs@Salen-Cu(II) nanocomposite was
detected by using VSM analysis (Figure 10). The saturation magnetization of the nano-
composite material was 27 emu/g, indicating the magnetism of the MNPs@Salen-
Cu(II) particles.
Figure 11 shows the XRD pattern of MNPs@Salen-Cu(II) to find out the type of crys-
tal structure and size of the nanocomposite. Sequence, apparent peaks at 18.22, 30.32,

2046
N. PARANDEH-KHOOZANI AND M. MORADIAN
Figure 10. VSM analysis of MNPs@Salen-Cu(II) nanocomposite material.
Figure 11. XRD pattern of synthesized MNPs@Salen-Cu(II) nanocomposite.
35.67, 43.34, 53.74, 57.26 and 62.95 angles are matched to the standard card (card 70-
003) and the results confirm the production of magnetite nanocomposite.
The TGA graph of the MNPs@Salen-Cu(II) nanocomposite is shown in Figure 12. The
weight reduction from 250 to 500 ꢀC is due to the presence of organic compounds.
Also in the range of 550–600 ꢀC weight loss occurs.

JOURNAL OF COORDINATION CHEMISTRY
2047
Figure 12. TGA graph of the MNPs@Salen-Cu(II) nanocomposite.
Figure 13. SEM pattern of MNPs@Salen-Cu(II) nanocomposite material.
The size and morphology of the MNPs@Salen-Cu(II) nanocomposite were deter-
mined by scanning electron microscopy (Figure 13). The particle distribution is equal
and the size of the nanoparticle is 500 nm, confirmed with the Scherer theory.
Moreover, to support the above observation, MNPs@Salen-Cu(II) was investigated
by energy-dispersive X-rays (Figure 14). The existence of the C, N, Cu, O, Fe, and Si ele-
ments in the EDX analysis of nanocomposite proves the structure of the copper(II)
salen complex anchored on the Fe3O4@SiO2 magnetic nanoparticles.

2048
N. PARANDEH-KHOOZANI AND M. MORADIAN
Figure 14. EDX analysis of the MNPs@Salen-Cu(II) nanocomposite material.
Scheme 6. Synthesis of nitroaldols by the MNPs@Salen-Cu(II) nanocatalyst.
In order to determine the loading of copper on the MNPs@Salen-Cu(II) nanocomposite
material, inductively coupled plasma atomic emission spectrometry (ICP-AES) was used.
Based on this analysis, the amount of Cu in the solid catalyst is 78.02 ꢃ 10ꢁ5 mol gꢁ1
.
3.6. Synthesis of nitroaldols from aldehydes and nitromethane in the presence
of MNPs@Salen-Cu(II) nanocomposite material
The catalytic activity of MNPs@Salen-Cu(II) was investigated in the synthesis of nitroal-
dol derivatives through the Henry reaction (Scheme 6). For this purpose, a solution of
1 mmol aldehyde and 8 mmol nitromethane in ethanol was refluxed. After 2 h, the
reaction was monitored by TLC and indicated the reaction did not occur without the
catalyst. So catalytic amounts of the MNPs@Salen-Cu(II) as a heterogeneous nanocata-
lyst were added to the resulting mixture. The desired nitroaldol was produced in
excellent yields. The Henry reaction in this protocol was performed in the presence of
the heterogeneous catalyst MNPs@Salen-Cu(II) unlike typical procedures in basic
media. Due to the superparamagnetic property of MNPs@Salen-Cu(II) composite, the
nanocatalyst can be easily separated from the reaction mixture by an external magnet
and it is reusable.
The reaction was performed in the presence of different amounts of MNPs@Salen-
Cu(II) in ethanol at room temperature. The results showed 50 mg (4.0 mol % of active

JOURNAL OF COORDINATION CHEMISTRY
2049
Table 1. Optimization of the amount of MNPs@Salen-Cu(II) catalysta.
Entry
Catalyst (mg); [mol %]b
Time (h)
Yield (%)
1
2
3
4
10; [0.78]
30; [2.35]
50; [4.0]
5
5
2.5
2.5
32
61
96
96
100; [8.0]
a4-nitrobenzaldeyde (1 mmol), nitromethane (2 mmol) in ethanol was stirred at room temperature;.
bStoichiometry of the catalyst:substrate in mol % of active copper.
Table 2. Optimization of solvent to synthesis of nitroaldol compoundsa.
Entry
Solvent (ml)
Time (h)
Yield (%)
1
2
3
4
5
THF
3.5
4
4
4.5
2.5
70
45
53
66
96
CH2Cl2
H3CCN
H3COH
C2H5OH
aThe 4-nitrobenzaldehyde(1 mmol), nitromethane (8 mmol), and 50 mg (4.0 mol% active copper) nano-
catalyst in different solvents was stirred at room temperature.
copper:substrate) of the nanocatalyst is an optimized amount for synthesis of nitroaldol
(Table 1, entry 3). After finding an optimized amount of MNPs@Salen-Cu(II) nanocatalyst,
the effect of different solvents on the reaction efficiency was investigated. According to
the results in Table 2, ethanol is an appropriate solvent in the reaction (Table 2, entry 5).
The results indicated defining the scope through the synthesis of other 2-nitro-1-
phenylethan-1-ol derivatives in the presence of MNPs@Salen-Cu(II) as the catalyst
under reflux was needed. In order to generalize optimum reaction conditions, different
derivatives of 2-nitro-1-phenylethan-1-ol (3a–3l) were synthesized from the reaction of
aldehydes (1a–1l) and nitromethane in the presence of a catalytic amount of copper(II)-
salen complex anchored on the magnetic nanoparticles. As shown in Table 3, the general-
ity of this protocol was examined with a range of aromatic aldehydes with electron-
donating or withdrawing groups under the optimized reaction conditions. All aldehyde
derivatives gave the desired products in high to excellent yields and short reaction times.
According to Table 3, aromatic aldehydes with electron-withdrawing groups on the
ortho- or para- positions gave the related products in shorter reaction time and higher
yield than electron-donating groups on ortho- or para- positions.
In accord with recently reported work on the mechanism of Henry’s reaction over
(L)2Cu(OAc) catalyst [75, 76], the proposed reaction mechanism is shown in Scheme 7.
The reaction begins with a weak interaction between nitromethane and Cu(II) from
the solid heterogeneous catalyst. In the next step, a proton from nitromethane trans-
fers to acetate, and acetic acid is released. Finally, the nucleophilic attack of nitro-
methane anion upon aldehyde produced the desired product and releases
nanocomposite MNPs@Salen-Cu(II) catalyst for the next catalytic cycle.
For consideration of the performance of the catalyst, catalyst leaching was studied.
After half completion of the reaction, the MNPs@Salen-Cu(II) nanocatalyst was separated
by centrifuge from the media, and the reaction was followed in the solution phase with-
out any fresh catalyst under the same conditions. The reaction was investigated by TLC
after 3 h and the results showed the reaction did not progress and no further conversion

2050
N. PARANDEH-KHOOZANI AND M. MORADIAN
Table 3. Products of nitroaldols (3a-3l) between aromatic aldehydes (1a-1l) and nitromethane (2)
under optimized conditionsa.
3a-3l: Time (h), Isolated yield (%)
3b: 2.5, 90 %
TON: 33
3d: 2, 93 %
TON: 32
3a: 3, 91 %
TON: 32
3c: 2, 96 %
TON: 36
TOF (h-1): 328
TOF (h-1): 407
TOF (h-1): 364
TOF (h-1): 431
3e: 2, 92 %
TON: 34
3f: 3.5, 87 %
TON: 34
3g: 3, 84 %
TON: 35
TOF (h-1): 387
3h: 4, 83 %
TON: 31
TOF (h-1): 296
TOF (h-1): 257
TOF (h-1): 232
3j: 4, 84 %
TON:31
3i: 3.5, 82 %
TON: 31
3l: 2.5, 83 %
TON: 34
TOF (h-1): 266
3k: 2.5, 89 %
TON: 33
TOF (h-1): 227
TOF (h-1): 246
TOF (h-1): 304
aAldehyde (1 mmol), nitromethane (8 mmol), 50 mg (4.0 mol % active copper) nanocatalyst were added to ethanol at
room temperature.
of substrates to the desired nitroaldol was observed. Solid nanoparticles or active metal
leached from solid MNPs@Salen-Cu(II) nanocatalyst do not remain in the filtrate.
A fundamental subject to express environmentally friendly methodology is the level of
recovery and reusability of the catalyst. In addition, the lifetime and stability of solid

JOURNAL OF COORDINATION CHEMISTRY
2051
Scheme 7. The proposed reaction mechanism.
Figure 15. Reusability of the catalyst for synthesizing nitroaldols.
Table 4. Comparison of literature reports on the Henry reaction under various conditions.
Entry
Catalyst
Reaction conditions
Time (h), Yield (%) [Lit.]
1
2
3
4
5
6
7
8
Zirconium-MOF
[Bmim]OH@agar
KI/Fe3O4
KF/NaY
Cu(II)-MOF (NbO type)
Co(II)-MOF
DABCO, THF, r.t.,
EtOH, r.t.,
Neat, r.t.,
H3COH/H2O
Neat, 50 oC
Water, 70 oC
H3CCN, r.t.,
EtOH, r.t.,
24, 90 [77]
2.0, 76 [78]
1.5, 74 [79]
48, 63 [64]
48, 58 [80]
24, 74 [81]
4, 76 [82]
LiClO4
MNPs@Salen-Cu(II)
2.5, 90 [this work]
catalysts are important factors in industry. Therefore, the reusability of the MNPs@Salen-
Cu(II) nanocatalyst was examined. The nanocatalyst was recovered by an external magnet
and was thoroughly washed with absolute ethanol (3 ꢃ 5 ml) and CH2Cl2 (3 ꢃ 5 ml) and
dried in a vacuum oven at 60 ꢀC before using for reaction without adding any fresh

2052
N. PARANDEH-KHOOZANI AND M. MORADIAN
catalyst. As shown in Figure 15, MNPs@Salen-Cu(II) nanoparticles can be reused at least
13 sequential runs without losing a significant catalytic activity.
In order to show the merit of this protocol, we compared the obtained results with
results reported. For this purpose, p-methylbenzaldehyde was chosen as a substrate in
the model reaction and comparison was carried out on the basis of reaction condi-
tions, reaction time, and percentage yields obtained (Table 4).
4. Conclusion
Synthesis of MNPs@Salen-Cu(II) nanocomposite material as a heterogeneous catalytic
system is reported. These nanoparticles were used as efficient catalysts for synthesis of
nitroaldol derivatives from reaction of aldehydes and nitromethane. Eco-friendly, easy
separation, cleanliness, simple workup, recyclability of the catalysts, and selectivity to
products are advantages of the protocol. The prepared catalysts were confirmed using
1
VSM, SEM, FT-IR, H NMR, XRD, TGA and EDX. The nanocatalyst can be easily recovered
from the reaction mixture by use of an external magnet and reused 14 times without
significant loss of catalytic activity. The leaching of the catalyst was also studied and it
was shown that the active parts of the heterogeneous catalyst do not leach.
Acknowledgements
The authors are thankful to the University of Kashan for supporting this work.
Disclosure statement
The authors declare no conflict of interest.
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