Synthesis, characterization and heterogeneous catalytic application of a nickel(II) Schiff base complex immobilized on MWCNTs for the Hantzsch four-component condensation

Article abstract of DOI:10.1080/00958972.2016.1253068

A nickel(II) Schiff base complex immobilized on multi-wall carbon nanotubes (MWCNTs) surface as a highly efficient heterogeneous catalyst was synthesized and characterized by IR, X-ray diffraction patterns, scanning electron microscopy, energy-dispersive

Full text of DOI:10.1080/00958972.2016.1253068

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis, characterization and heterogeneous
catalytic application of an immobilized nickel(II)
Schiff-base complex supported on MWCNTs for the
Hantzsch four-component condensation
Jamshid Rakhtshah, Sadegh Salehzadeh, Mohammad Ali Zolfigol & Saeed
Baghery
To cite this article: Jamshid Rakhtshah, Sadegh Salehzadeh, Mohammad Ali Zolfigol
& Saeed Baghery (2016): Synthesis, characterization and heterogeneous catalytic
application of an immobilized nickel(II) Schiff-base complex supported on MWCNTs for
the Hantzsch four-component condensation, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2016.1253068
To link to this article: http://dx.doi.org/10.1080/00958972.2016.1253068
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Date: 26 October 2016, At: 12:12

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2016.1253068
Synthesis, characterization and heterogeneous catalytic application of an
immobilized nickel(II) Schiff-base complex supported on MWCNTs for the
Hantzsch four-component condensation
JAMSHID RAKHTSHAH, SADEGH SALEHZADEH*, MOHAMMAD ALI ZOLFIGOL and SAEED BAGHERY
Faculty of Chemistry, Bu-Ali Sina University, Hamedan 6517838683, Iran
Immobilized nickel(II) Schiff-base complex supported on multi-wall carbon nanotubes
(MWCNTs) as a highly efficient heterogeneous catalyst was synthesized and characterized by IR,
X-ray diffraction patterns (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray
spectroscopy (EDX), inductively coupled plasma (ICP), elemental analysis (CHN) and thermal
gravimetric analysis (TGA). Then a facile and environmentally benign procedure was developed
for synthesis of polyhydroquinoline derivatives via Hantzsch one-pot condensation reaction of
aromatic aldehydes, 1,3-diones, ethyl acetoacetate and ammonium acetate in the presence of an
immobilized nickel(II) Schiff-base complex supported on MWCNTs as a reusable heterogeneous
catalyst under solvent-free conditions. This protocol has the advantages of stability, easy
availability, recyclability and eco-friendly catalyst, simple experimental, high to excellent yields
and work-up procedure. Considering the solvent free condition and also temperature, time and
yield of the model reaction the nano-catalyst reported here is among the best catalysts reported so
far for synthesis of polyhydroquinolines.
Keywords: Immobilized Ni(II)-Schiff base complex supported on MWCNTs;
Polyhydroquinolines; Hantzsch condensation; Solvent-free
*Corresponding author. Emails: ssalehzadeh@gmail.com; saleh@basu.ac.ir
1

1. Introduction
Schiff bases can be easily synthesized and form complexes with almost all metal ions. Many
Schiff base complexes demonstrate excellent catalytic activity in various reactions at high
temperature (up to 100 °C) and in the presence of moisture [1]. Schiff bases stabilize many
different metals in various oxidation states, controlling the performance of metals in a wide range
of practical catalytic transformations [2]. Over the past few years, there have been many reports on
their applications as catalyst in reactions such as polymerization reaction [1], reduction [3],
oxidation of organic compounds [4], hydrosilylation of ketones [5], synthesis of bis(indolyl)
methanes [6] and Diels–Alder reactions [7]. Specifically, Schiff base complexes of nickel(II) and
copper(II) have increased enantioselectivity in alkylation of enolates [8].
Due to unusual properties of multi-wall carbon nanotubes (MWCNTs) such as structural
and thermal stability and also insolubility in the most solvents, they are promising supports for
immobilizing catalytic species and have considerable benefits [9-14]. Metal complexation has
been a recent choice for functionalization of CNTs and fabrication of CNTs/inorganic hybrid
materials. Although a few examples have been established for complexation, this field has not
been fully investigated [15]. However, some efforts have been made to immobilize nickel
complexes on the surface of various supports. For example, a paper by Salavati and co-workers
introduces the immobilization of a Ni(II)-salen complex on MWCNTs and reports the resulting
heterogeneous catalysts for oxidation of phenol with hydrogen peroxide [12]. Khatri and Farahani
reported a rhenium-oxo complex such as methyltrioxorhenium (MTO) immobilized on modified
graphene oxide (GrO) and also a molybdenum–Schiff base complex supported on MWCNTs via
covalent bonding [16, 17].
We report here the synthesis and characterization of a nickel(II) Schiff base complex,
which is covalently attached to the MWCNTs, and its application on the green, sequential, one-pot
four-component protocol for synthesis of polyhydroquinoline derivatives via the Hantzsch
reaction between aromatic aldehydes with 1,3-diones, ethyl acetoacetate and ammonium acetate
under solvent-free conditions at room temperature (scheme 1).
150 years after the first multi-component reaction (MCR), there is still an ongoing and
widespread interest in this field, having been re-invigorated with the development of high-speed
synthesis and the necessity for new highly flexible chemical starting points in drug discovery.
Increase of MCRs has broadly resulted in the rapid assembly of arrays with high-molecular
2

diversity [18, 19]. In 1882, Arthur Hantzsch described first synthesis of symmetrically substituted
1,4-dihydropyridine derivatives via the one-pot, four component condensation of two molecules of
ethylacetoacetate, aromatic aldehyde and ammonia [20]. Polyhydroquinoline derivatives are an
important group of nitrogen heterocycles of importance because of their different therapeutic and
pharmacological properties, antitumor, vasodilator, calcium channel blockers, bronchodilator,
anti-atherosclerotic, geroprotective, hepatoprotective and antidiabetic activity [21-25]. Various
methods have been described for the synthesis of polyhydroquinoline derivatives, due to the
biological significance of these compounds. The classical method includes three-component
coupling of an aldehyde with ethyl acetoacetate and ammonia in acetic acid or in refluxing alcohol
[26-30]. These methods suffer from disadvantages such as lower yields, an excess of organic
solvent, a long reaction time and harsh refluxing conditions. Therefore, chemists have developed
more efficient methods for synthesis of polyhydroquinoline derivatives, which involve the use of
microwave irradiation [31], solar thermal energy [32], solvent-free conditions on grinding [33],
refluxing at high temperature [34], and using various catalysts such as bismuth(III) bromide [35],
L-proline [36], {[HMIM]C(NO2)3} [37], K7[PW11CoO40] [38], HY-zeolite [39], [pyridine-
SO3H]Cl [40], nanoparticles [41], ceric ammonium nitrate [42], [2-MPyH]OTf [43], boronic acids
[44], Cu(II) and Mn(III) Schiff base complex [45], HClO4-SiO2 [46], Baker's yeast [47], polymers
[48], PTSA-SDS [49], and iodine [50].
Nanoscale heterogeneous catalysts propose higher surface area and low-coordinate metal
sites, which are responsible for higher catalytic activity. Furthermore, heterogeneous catalysis has
the advantage of easy product purification, high atom-efficiency and reusability of the catalyst
[51, 52].
3

Scheme 1. Hantzsch four-component condensation using immobilized nickel(II) Schiff-base
complex supported on MWCNTs.
2. Experimental
2.1. Materials and instrumentation
All chemicals were attained from Merck, Fluka and Sigma Aldrich chemical companies. Multi-
walled carbon nanotubes of >85% purity and length 1-10 mm, outer diameter,
5–20 nm were purchased from Kermanshah petrochemical industries company. The identified
compounds were analyzed by comparison of their physical properties and spectral data with
reliable samples in the literature. Reaction progress and purity determination of the compounds
were tested via TLC using silica gel SIL G/UV 254 plates. The 1H NMR (250) and 13C NMR
(62 MHz) spectra were recorded on a Bruker spectrometer (δ in ppm). Melting points are
uncorrected and were recorded on a Buchi B-545 apparatus in open capillary tubes.
2.2. Immobilized nickel(II) Schiff-base complex supported on MWCNTs, preparation and
purification
The raw MWCNTs were first immersed in an aqueous solution of HF to remove SiO2, then
filtered and washed with deionized water and THF and dried in an oven. Following careful
4

purification, MWCNTs were oxidized in a mixture of concentrated sulfuric and nitric acids (3:1,
98 and 70%, respectively) by ultrasonication for 24 h to obtain MWCNT-COOH [53]. In order to
increase the reactivity and/or population of OH surface groups, the above material was further
treated with sodium borohydride in methanol and carboxyl groups were reduced to CH2OH
groups. The obtained MWCNT-OH (2 g) was suspended in xylene (100 ml) and then
3-aminoropropyltrimethoxysilane (APTMS) (2 ml) was added under dry nitrogen. The mixture
was refluxed for 24 h and the resultant solid (ApmMWCNTs) was separated and washed with
methanol to remove the unreacted residue of silylating reagent and then dried at 80 °C. An
ethanolic solution of excess terephthalaldehyde was added to the resulting nanomaterial and was
refluxed for 24 h at 60 °C. The product was filtered and washed with ethanol to remove unreacted
terephthalaldehyde. Afterwards, the obtained tereph-MWCNTs were suspended in ethanol
(100 ml) and 2-aminothiophenol (4 mmol) was added under dry nitrogen. The mixture was
refluxed for 24 h and the resulting solid (thiosal-MWCNTs) separated and was washed with
methanol to remove the unreacted reagents and then vacuum dried at 80 °C. Finally, a methanolic
solution of Ni(OAc)2 and thiosal-MWCNTs was refluxed for 24 h to give Ni(OAc)2thiosal-
MWCNTs nanomaterial. The solution was filtered and the precipitate was washed several times
with dichloromethane to remove any residues, and then dried in an oven at 100 °C overnight
(scheme 2).
2.3. Synthesis of neat thiosal Schiff base and its complex for catalytic application
A methanolic solution of terephthalaldehyde was added to a methanolic solution of
2-aminothiophenol and stirred for ~9 h in a closed flask maintaining the temperature at ~60 °C.
The yellow precipitate formed was filtered and recrystallized from hot benzene. A concentrated
methanolic solution of nickel(II) acetate was added to the ligand in benzene and the mixture was
refluxed for ~2 h. The precipitated complex (green solid) on cooling to room temperature was
filtered, washed with hot benzene and dried in vacuum.
5

Scheme 2. The sequence of events in the preparation of Ni(OAc)2thiosal-MWCNTs nanomaterial.
2.4. General procedure for the synthesis of polyhydroquinoline derivatives
The immobilized nickel(II) Schiff-base complex supported on MWCNTs as a heterogeneous
catalyst (0.005 g) was added to a mixture of aromatic aldehyde (1 mmol), 1,3-dione (1 mmol),
ethyl acetoacetate (1 mmol) and ammonium acetate (1.5 mmol) in a round bottom flask and the
resulting mixture was stirred magnetically under solvent-free conditions at room temperature.
After reaction, as observed by TLC (n-hexane/ethyl acetate: 5/2), ethyl acetate (5 mL) was added
to the reaction mixture, stirred and refluxed for 10 min, washed with ethanol (5 mL) and decanted
to separate catalyst from other materials (the reaction mixture was soluble in hot ethyl acetate and
nano-catalyst was insoluble). The solvent of organic layer was evaporated and the crude product
was purified by recrystallization from ethanol. In this study, nano heterogeneous catalyst was
recycled and reused for seven times without significant loss of its catalytic activity.
6

2.5. Spectral data for compounds
Ethyl-2,7,7-trimethyl-5-oxo-4-(pyridin-4-yl)-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate
(table 3, entry 8):Yellow solid; M.p: 218-220 °C; Yield: 95%; IR (KBr): υ 3285, 3203, 3073,
2941, 1689, 1622, 1480, 1223 cm-1; 1H NMR (250 MHz, DMSO-d6): δ 1.44 (t, 3H, J = 7.2 Hz, —
CH3), 1.65 (s, 3H, —CH3), 1.87 (s, 3H, —CH3), 2.10 (s, 2H, —CH2), 2.34 (s, 2H, —CH2), 2.83
(q, 2H, J = 6.8 Hz, —CH2), 3.65 (s, 3H, —CH3), 4.82 (s, 1H, —CH), 7.30 (d, 2H, J = 7.4 Hz,
ArH), 8.62 (d, 2H, J = 7.4 Hz, ArH), 9.20 (s, 1H, —NH); 13C NMR (62 MHz, DMSO-d6): δ 26.7,
28.3, 28.7, 29.4, 31.5, 32.5, 36.2, 38.8, 39.1, 39.5, 39.8, 40.1, 40.5, 40.8, 50.4, 59.6, 102.3, 109.1,
123.2, 146.5, 146.7, 149.6, 150.5, 150.6, 155.8, 166.8, 194.7; MS: m/z = 340 [M]+.
Ethyl-2-methyl-5-oxo-4-(1-phenylprop-1-en-2-yl)-1,4,5,6,7,8-hexahydroquinoline-3-
carboxylate (table 3, entry 17):Yellow solid; M.p: 199-201 °C; Yield: 93%; IR (KBr): υ 3289,
3219, 2958, 1703, 1608, 1485, 1217 cm-1; 1H NMR (250 MHz, DMSO-d6): δ 0.79 (t, 3H, J =
5.2 Hz, —CH3), 1.14 (s, 3H, —CH3), 1.51 (s, 3H, —CH3), 2.45 (m, 6H, —CH2), 4.32 (q, 2H, J =
7.0 Hz, —CH2), 4.48 (s, 1H, —CH), 6.27 (s, 1H, —CH), 7.22 (m, 5H, ArH), 9.02 (s, 1H, —NH);
13C NMR (62 MHz, DMSO-d6): δ 16.3, 18.4, 18.5, 21.2, 26.7, 36.4, 37.3, 38.8, 39.2, 39.5, 39.8,
40.2, 40.5, 40.8, 59.4, 111.4, 124.8, 124.9, 126.2, 128.4, 129.0, 138.4, 142.1, 145.5, 152.1, 152.5,
152.6, 167.6, 195.4; MS: m/z = 351 [M]+.
3. Results and discussion
3.1. Characterization of immobilized nickel(II) Schiff-base complex supported on MWCNTs as
a heterogeneous catalyst
The structure of immobilized nickel(II) Schiff-base complex supported on MWCNTs as a
heterogeneous catalyst was studied and identified by FT-IR, XRD, SEM, EDX, ICP, CHN and
TGA analysis. In order to confirm the surface modification of MWCNTs, the FTIR spectra of the
prepared materials were obtained and are shown in figure 1. The FT-IR spectrum of HOOC-
MWCNTs (figure 1b) shows a strong band at 1708 cm-1 and a weaker band at 1229 cm-1 attributed
to asymmetric stretching of COO of carboxylic acids and C-OH groups in oxidized MWCNTs,
respectively, as well as a broad band about 3440–3480 cm−1 assigned to O-H stretches which were
not observed in the spectrum of pristine MWCNTs. As can be seen in figure 1b, in the IR
spectrum of ApmMWCNTs there is a broad band at 1000–1100 cm−1 assigned to Si-O stretching
vibrations which were not present in pristine MWCNTs and/or HOOC-MWCNTs. The presence
7

of anchored propyl chain was confirmed by CH2 stretching vibrations appearing at 2922 cm-1
(asymmetric CH2 stretch) and 2855 cm-1 (symmetric stretch). The presence of these bands
suggested that during the anchoring of APTMS, a condensation occurred between the OH surface
groups of the oxidized MWCNTs and methoxy groups of APTMS to form the stable covalent Si-
O-C linkage, leading to attachment of aminopropyl groups on the surface of MWCNTs. Thus, this
treatment leads to disappearance of the bands at 1708 and 1229 cm-1 and provides further evidence
for the desired reaction. In the FT-IR spectrum of TerphApmMWCNTs (figure 1d), the bands
observed at 1630 and 1705 cm-1 were assigned to C=N and C=O vibrations due to the anchoring of
the terephthaldehyde on the modified MWNTs with APTMS. In the IR spectrum of the
Ni(OAc)2thiosal-MWCNTs a sharp band at 1618 cm-1 due to υC=N (azomethine) was shifted to
lower frequencies with respect to corresponding ligand. The shift indicates involvement of
azomethine nitrogen in coordination (figure 1e). In IR spectra of neat thiosal Schiff base and its
neat metal complex the vibrational stretching frequency of C=N bond appears at 1632 and
1614 cm-1, respectively (figures S9 and S10). Thus the attachment of thiosal Schiff base and/or its
Ni(II) complex to the MWCNTs have had a small effect on stretching frequency of the imine
bond.
The thermal behavior of the Ni(OAc)2thiosal-MWCNTs was investigated by thermal
gravimetric analysis (TGA). Figure 2 shows the TGA curve for the as-prepared Ni(OAc)2thiosal-
MWCNTs, presenting several weight losses from 150-630 °C. The small mass loss below 300 °C
probably reflects loss of humidity. The main weight loss at 300-400 °C was assigned to the
combustion of organic compounds and removing the Schiff-base from MWCNTs system which is
in agreement with endothermic DTA peak in this region [12]. DTA curve demonstrates the
stepwise loss in weight, where one distinct weight loss has been observed at 355 °C.
Field emission scanning electron microscopy (FE-SEM) imaging was employed to
investigate the morphology of the unfunctionalized MWCNTs and products to determine the
degree of nanostructured properties of the materials (figure 3). Also, figure 3b shows a SEM
image of Ni(OAc)2thiosal-MWCNTs in which nanotubes are aggregated and have retained their
nanotube nature. These data also confirmed that the morphology change has not occurred during
the reaction and the average diameters of these nano compounds are 150-250 nm.
The energy-dispersive X-ray (EDX) of the raw MWNTS (figure 4a) and grafted complex
(figure 4b) have been compared to understand the effect of the support. The functionalization of
8

the MWCNTs with the Schiff base and the formation of its complex were also confirmed by the
presence of C, O, N, S, Si, Ni and the corresponding metal signals in the EDX patterns of the
products. The existence of nickel is confirmed by atomic absorption spectrometry as well as EDX,
as can be clearly seen in figure 4.
The nitrogen content of the support was determined by CHN analysis which showed values
of 2.24% for Apm-MWCNTs. Nickel content of the prepared nanomaterial was determined by
inductively coupled plasma (ICP) chemical analysis which revealed the existence of 0.54
mmol.g−1 nickel in the Ni(OAc)2thiosal-MWCNTs.
Figure 5 shows the X-ray diffraction (XRD) patterns of parent MWCNTs (a) and
Ni(OAc)2thiosal-MWCNTs (b). The X-ray diffraction pattern of MWNTs bearing nickel(II)
complex is similar to that for MWCNTs, indicating that the crystallinity and morphology of
MWNTs were preserved during grafting method [13, 14]. Figure 5 shows that the XRD pattern of
MWNTs is similar with that of highly oriented pyrolytic graphite (HOPG). The MWCNTs showed
typical peak of (002), (110) and (400) at 2θ = 26, 43 and 53°, respectively. The (002) peak at 26°,
corresponding to the inter-planar spacing of 0.340 nm, is a little greater than that of HOPG of
about 0.336 nm [12].
3.2. Application of immobilized nickel(II) Schiff-base complex supported on MWCNTs as a
heterogeneous catalyst
In order to find the best efficiency of the nano-sized Schiff base catalyst in the synthesis of
polyhydroquinoline derivatives we tried to optimize the reaction conditions (scheme 1). As a
model, the condensation reaction of 2-thiophenecarbaldehyde, dimedone, ethyl acetoacetate and
ammonium acetate was studied with different amounts of the catalyst from 25-100 °C under
solvent-free conditions (table 1). As can be seen in table 1, the best results were attained when the
reaction was achieved in the presence of 0.005 g of nano-catalyst at room temperature (table 1,
entry 7). Increasing the reaction temperature and catalyst loading did not improve the rate of the
reaction (table 1, entries 8-14). In the absence of catalyst a low yield of product was acquired after
1 h (table 1, entries 1 and 2). Then we studied the solvent effect on the reaction using 0.005 g
immobilized nickel(II) Schiff base complex supported on MWCNTs in various solvents (see
table 2). From table 2 solvent-free reaction conditions are clearly the best choice for these
reactions.
9

Encouraged by the significant results, several polyhydroquinoline derivatives were
synthesized from Hantzsch four-component condensation of aromatic aldehydes, 1,3-diones, ethyl
acetoacetate and ammonium acetate in the presence of a catalytic amount of immobilized
nickel(II) Schiff-base complex supported on MWCNTs as a reusable heterogeneous catalyst under
solvent-free conditions at room temperature. The results are presented in table 3. The substituents
on the aromatic ring show strong effects in terms of yields under these reaction conditions. All
aromatic aldehydes containing electron-releasing substituents and electron-withdrawing
substituents on their aromatic ring afforded the related products in high to excellent yields in short
reaction times. The reaction times of aromatic aldehydes having electron withdrawing groups were
shorter than those with electron-donating groups. Furthermore, in the presence of dimedone the
reaction goes faster than 1,3-cyclohexanedione.
In a proposed mechanism (scheme 3), we suggest that at first 1,3-dione is converted to its
enol form by Ni(II)-Schiff base complex supported on MWCNTs and reacts with activated
aldehyde via the nano-catalyst to provide intermediate I. The activated ethyl acetoacetate (via the
immobilized nickel(II) Schiff-base complex supported on the MWCNTs) and ammonia (resulted
from ammonium acetate) give enamine II. Then, the intermediate I and enamine II react with each
other to give intermediate III. Intermediate III is converted to IV via tautomerization, and
intermediate IV provides V through intramolecular nucleophilic attack of the NH2 group to the
activated carbonyl group. Finally, polyhydroquinoline derivatives form by removing one molecule
of H2O. Garden et al. investigated this mechanism by ESI-MS using an approach including
alternate reactions with charge-tagged reagents [37-40]. According to the reaction mechanism, the
presence of carbonyl groups cannot interfere with the catalytic performance.
Ni
O
R
R
H
O
O
Ar
H2O
H
R
Ni
H
R
O
O
I
O
R
O
R
Ar
Ni
Immobilized Ni (II)-
Schiff base complex
supported on MWCNTs
Ni
=
O
H
H2N
O
NH4OAc
H
N
OEt
OEt
H
HOAc
II
O
O
Ar
O
O
O
Ar
O
Ar
O
H
O
Ar
O
OEt
Ni
R
R
R
OEt
R
OEt
OEt
N
R
R
R
HO
R10
H
N
H2O
HO
H2N
H
Polyhydroquinoline
H2N
O
III
V
IV
Ni

Scheme 3. The proposed mechanism for the synthesis of polyhydroquinoline derivatives using
immobilized nickel(II) Schiff-base complex supported on MWCNTs as a heterogeneous catalyst.
Reusability of the nano-catalyst was proved upon condensation of 2-thiophenecarbaldehyde, ethyl
acetoacetate, dimedone and ammonium acetate. At the end of reaction, ethyl acetate was added to
the reaction mixture and heated to extract the product from the substrates. This solution was
washed with ethanol to separate catalyst from other materials (the product is soluble in hot ethyl
acetate and nano-catalyst is insoluble). The catalytic activity of the nano-catalyst was restored for
eight continuous runs (figure 6). Analysis by ICP-AES of nickel in the product after isolation of
catalyst showed no loss of metal during the catalytic reaction, indicating that no metal leaching
occurred. Furthermore, the XRD of the catalyst after eight cycles exposed that the structure of the
catalyst maintains during the reaction.
3.3. Comparison of catalytic activity of Ni(OAc)2thiosal-MWCNTs with known catalysts
In order to evaluate the catalytic activity of Ni(OAc)2thiosal-MWCNTs synthesized here we
compared it with previously known catalysts. Thus all previously reported catalysts that are tested
in synthesis of polyhydroquinolines using model reaction between 4-chloro benzaldehyde,
dimedone, ethyl acetoacetate, and ammonium acetate were compared with our catalyst,
Ni(OAc)2thiosal-MWCNTs (see table 4). In addition, the catalytic activities of Ni(OAc)2, discrete
nickel(II) Schiff base complex and also MWCNTs-COOH which are the initial reactants for
synthesis of Ni(OAc)2thiosal-MWCNTs were also compared. Table 4 shows that the yield of
model reaction in the case of all catalysts (except entries 8 and 18) is lower than Ni(OAc)2thiosal-
MWCNTs and also the required reaction time for all catalysts (except entries 8 and 9) is usually
much longer than our catalyst, indicating that nano-catalyst synthesized here is one of the best
catalysts reported. In addition, considering that our nano-catalyst is used at room temperature, and
in solvent free conditions with minimum amount of mol ratio (we remember that molecular weight
of Ni(OAc)2thiosal-MWCNTs is large and herein only 0.005 g of this catalyst has been used), we
can conclude that it is in the first rank among all catalysts which are compared here.The catalytic
11

activity of both discrete nickel(II) Schiff-base complex and MWCNTs-COOH precursors is good
(table 4). But, a comparison between the above catalysts and MWCNTs supported complexes as
catalysts proves that the latter complex gave slightly higher yield of derivatives than its
\precursors. Also, when we consider the second very important element, the time, then we can find
that the Ni(II) Schiff-base complex immobilized on MWCNTs catalyst has better efficiency than
above precursors. It should be mentioned that immobilization of transition-metal complexes on
solid supports can prepare catalysts which are easy to be separated from and reused in the reaction
system and which may possibly exhibit improved activities because of the support environment.
Thus the excellent catalytic activity and high efficiency of Ni(OAc)2thiosal-MWCNTs is expected.
4. Conclusion
A green and efficient immobilized nickel(II) Schiff-base complex supported on MWCNTs as a
reusable heterogeneous catalyst was synthesized, considered and characterized by IR, XRD, SEM,
EDX, ICP, CHN and TGA analysis. Catalytic application of 1 was studied in the synthesis of
polyhydroquinoline derivatives via Hantzsch four-component condensation reaction of aromatic
aldehydes, 1,3-diones, ethyl acetoacetate and ammonium acetate under solvent-free conditions at
room temperature. Advantages of this study are reasonably low cost, high yield, short reaction
time, cleaner reaction profile, reusability of nano heterogeneous catalyst and simplicity of product
isolation.
Acknowledgements
We are grateful to the Bu-Ali Sina University for financial support.
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Table 1. Optimization reaction conditionsin in the synthesis of polyhydroquinoline catalyzed by immobilized
nickel(II) Schiff-base complex supported on MWCNTs under solvent-free conditionsa.
Entry
1
Catalyst loading (g)
Catalyst-free
Reaction temperature (oC)
r.t.
Reaction time (min)
60
Yield (%)b
15
2
3
4
5
Catalyst-free
0.001
100
r.t.
60
30
30
30
17
75
75
81
0.001
100
r.t.
0.002
16

6
0.002
0.005
0.005
0.005
0.005
0.01
100
r.t.
30
12
12
12
12
12
12
20
20
81
95
95
95
95
95
95
94
94
7
8
50
9
75
10
11
12
13
14
100
r.t.
0.01
100
r.t.
0.02
0.02
100
a: Reaction conditions: 2-thiophenecarbaldehyde (1mmol), dimedone (1mmol), ethyl acetoacetate (1mmol), ammonium
acetate (1.5mmol) and solvent free.
b: Isolated yield.
17

Table 2. The solvent effect in the synthesis of polyhydroquinoline catalyzed by 0.005 g immobilized Nickel(II)
Schiff-base complex supported on MWCNTsa
Entry
1
Solvent
Time (min)
12
Yield (%)b
95
Solvent-free
2
3
4
5
6
7
8
H2O
60
15
15
30
60
60
60
20
92
90
75
45
30
30
C2H5OH
CH3CN
CH3CO2Et
CH2Cl2
Toluene
Benzene
a: Reaction conditions: 2-thiophenecarbaldehyde (1mmol), dimedone (1mmol), ethyl acetoacetate (1mmol), ammonium
acetate (1.5mmol), nano catalyst (0.005 g) and room temperature.
b: Isolated yield.
18

Table 3. Synthesis of polyhydroquinoline derivatives using immobilized Ni(II)-Schiff base complex supported
on MWCNTsa
Yield
(%)b
Entry
Aldehydes
1,3-Diones
Product
Time (min)
M.p (oC) [Ref.]
1
5
98
243-245 [40]
2
7
97
245-247 [40]
3
4
5
10
12
10
96
95
96
261-263 [40]
243-245 [43]
173-175 [43]
H
N
O
O
O
NO2
19

6
7
12
15
95
95
224-226 [43]
221-223 [37]
8
9
12
10
95
96
218-220 [c]
249-251 [37]
10
10
96
203-205 [37]
11
12
95
362-364 [37]
20

12
10
96
232-234 [54]
13
14
15
16
12
15
12
15
95
93
95
93
238-240 [54]
213-215 [54]
202-204 [42]
231-233 [42]
17
18
18
12
93
95
199-201 [c]
201-203 [37]
21

a: Reaction conditions: various aldehydes (RCHO) (1mmol), dimedone (1mmol), ethyl acetoacetate (1mmol), ammonium acetate
(1.5mmol), nano catalyst (0.005 g), solvent free and room temperature.
b: Isolated yield.
c: New derivatives.
22

Table 4. Comparison of various catalysts in the four‐component Hantzsch condensation for synthesis of
polyhydoquinolines.a
Time
(min)
Yieldb
(%)
Entry
Amount of catalyst
Solvent
Temp (°C)
[Ref.]
L‐proline/10 mol%
C2H5OH/reflux
C2H5OH
-
360
150
150
35
11
10
9
92
93
90
80
96
95
94
97
90
92
84
94
90
90
88
90
92
97
93
95
93
83
95
92
88
90
90
[36]
[35]
[42]
[38]
[45]
[46]
[41a]
[43]
[41b]
[55]
[56]
[57]
[58]
[59]
[60]
[61]
[62]
[63]
[64]
[65]
[66]
[67]
[68]
[69]
[70]
[71]
[72]
1
Bismuth(III) bromide/2 mol%
Ceric ammonium nitrate/10 mol%
K7[PW11CoO40]/1 mol%
r.t
r.t
-
2
C2H5OH
3
CH3CN/reflux
H2O
4
Cu(II) Schiff Base Complex/1 mol%
HClO4-SiO2/50 mg
r.t
90
r.t
r.t
r.t
90
80
90
80
75
r.t
90
80
r.t
40
r.t
60
-
5
Solvent free
C2H5OH
6
SnO2 nanoparticle /1 mol%
[2-MPyH]OTf/1 mol%
7
H2O
5
8
Nickel nanoparticle/ 10 mol%
γ-Al2O3-nanoparticle/ 0.2 g
β-Cyclodextrin–polyurethane polymer/ 0.15 g
Carbon-based solid acid/ 0.02 g
Titanium Dioxide Nanoparticles/10 mol%
FeF3/ 5 mol%
Solvent free
Solvent free
Solvent free
Solvent free
C2H5OH
1
9
8
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
10
20
120
60
15
15
45
180
12
25
180
95
50
60
120
120
150
C2H5OH
clay supported Ni0 nanoparticles/25 mg
Aluminum Phosphate [AlPO4(H)]/ 10 mg
PPA-SiO2/ 0.03 g
Solvent free
Solvent free
Solvent free
guanidine hydrochloride organocatalyst/10mol% C2H5OH
Ionic liquid/ 10 mol%
C2H5OH
ZnO-nanoparticle/ 10 mol%
Hf(NPf2)4/ 1 mol%
Solvent free
perfluorodecalin
C2H5OH/reflux
C2H5OH/reflux
C2H5OH
[(CH2)4SO3HMIM][HSO4]/0.25 mmol
Scolecite/ 200 mg
-
Zinc Oxide/ 10 mol%
80
r.t
r.t
r.t
ZrCl4/5 mol%
C2H5OH
P-TSA/ 10 mol%
C2H5OH
Cerium(IV) ammonium nitrate/ 10 mol%
C2H5OH
23

28
29
30
31
32
33
K5CoW12O40.3H2O/ 0.01 mol%
L-proline/0.1 mmol
Solvent free
C2H5OH
90
r.t
r.t
r.t
r.t
r.t
15
30
35
10
10
7
93
91
86
95
94
97
[73]
[74]
Ni(OAc)2/ 0.005 g
Solvent free
Solvent free
Solvent free
Solvent free
-
-
-
-
Ni(OAc)2thiosal Schiff-base complex/ 0.005 g
MWCNTs-COOH/ 0.005 g
Ni(OAc)2thiosal-MWCNTs/ 0.005 g
a: Reaction conditions: 4-chloro benzaldehyde (1mmol), dimedone (1mmol), ethyl acetoacetate (1mmol), ammonium acetate
(1.5mmol).
b: Isolated yield.
24

Figure 1. FT-IR spectra of (a) MWCNTs, (b) HOOC-MWCNTs, (c) ApmMWCNTs, (d)
TerphApmMWCNTs and (e) Ni(OAc)2thiosal-MWCNTs.
25

Figure 2. TGA and DTA curves of Ni(OAc)2thiosal-MWCNTs.
26

Figure 3. The SEM images of (a) MWCNTs and (b) Ni(OAc)2thiosal-MWCNTs.
27

Figure 4. EDX diagram of (a) MWCNTs and (b) Ni(OAc)2thiosal-MWCNTs.
28

Figure 5. XRD pattern of the MWCNTs (a) and the Ni(OAc)2thiosal-MWCNTs (b).
29