Mn(III)–pentadentate Schiff base complex supported on multi-walled carbon nanotubes as a green, mild and heterogeneous catalyst for the synthesis of tetrahydrobenzo[b]pyrans via tandem Knoevenagel–Michael cyclocondensation reaction

Article abstract of DOI:10.1002/aoc.3690

Mn(III)–pentadentate Schiff base complex supported on multi-walled carbon nanotubes as a recyclable and reusable, green and nano-heterogeneous catalyst was designed and fully characterized using infrared spectroscopy, X-ray diffraction, scanning electron

Full text of DOI:10.1002/aoc.3690

Received: 7 September 2016
DOI 10.1002/aoc.3690
Revised: 17 October 2016
Accepted: 25 October 2016
F U L L PA P E R
Mn(III)–pentadentate Schiff base complex supported on multi‐
walled carbon nanotubes as a green, mild and heterogeneous
catalyst for the synthesis of tetrahydrobenzo[b]pyrans via
tandem Knoevenagel–Michael cyclocondensation reaction
Jamshid Rakhtshah | Sadegh Salehzadeh | Mohammad Ali Zolfigol | Saeed Baghery
Faculty of Chemistry, Bu‐Ali Sina University,
Hamedan 6517838683, Iran
Mn(III)–pentadentate Schiff base complex supported on multi‐walled carbon
nanotubes as a recyclable and reusable, green and nano‐heterogeneous catalyst
was designed and fully characterized using infrared spectroscopy, X‐ray diffraction,
scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, inductively
coupled plasma mass spectrometry, elemental analysis and thermogravimetric
Correspondence
Sadegh Salehzadeh, Faculty of Chemistry, Bu‐Ali
Sina University, Hamedan 6517838683, Iran.
Email: saleh@basu.ac.ir; ssalehzadeh@gmail.com
analysis. A facile, eco‐friendly, mild and green procedure was developed for the
one‐pot three‐component synthesis of tetrahydrobenzo[b]pyrans via tandem
Knoevenagel–Michael cyclocondensation reactions between aromatic aldehydes,
1,3‐diones and malononitrile using a catalytic amount of Mn(III)–pentadentate
Schiff base complex supported on MWCNTs as an efficient recyclable heteroge-
neous catalyst under solvent‐free conditions at room temperature. This process has
the advantages of easy availability, stability, recyclability and eco‐friendliness of
the catalyst, short reaction times, high to excellent yields and simple work‐up
procedure.
KEYWORDS
azo–azomethine, manganese(III) complex, solvent‐free, supported complex,
tetrahydrobenzo[b]pyrans
1
|
INTRODUCTION
in MWCNTs is chemical functionalization. The
functionalization of CNTs may expand the range of their
potential applications.[22] Metal complexation has been a
recent choice for functionalization of CNTs and fabrication
of CNTs/inorganic hybrid materials. Although a few
examples have been demonstrated for complexation,[23] this
field has not been fully investigated. A general procedure
for surface functionalization is to affect the reaction of
surface hydroxyl groups with organosilane coupling reagents,
such as tri(alkoxy)organosilanes or trichloroorganosilanes.
The nature of the linker can also influence the
enantioselectivity, stability and performance of a supported
catalyst.[24]
Multi‐component reactions (MCRs) are those in which three
or more substrates react together to provide products in one
pot under appropriate reaction conditions. MCRs suggest
the benefit of ease and synthetic efficacy over conventional
chemical reactions. MCRs have the further benefits of
synthetic convergence, selectivity and atom‐economy.[1–3]
As a novel type of nanomaterial, carbon nanotubes
(CNTs) have attracted much attention during the past two
decades.[4,5] Multi‐walled CNTs (MWCNTs) are composed
of a series of concentric single‐walled CNTs with an inter‐
tube distance of 0.34 nm.[6,7] Since CNTs are insoluble in
most solvents, these materials can be used as catalyst
supports.[8–21] A good approach that has opened up a large
number of research opportunities and potential applications
To the best of our knowledge, homogeneous catalysts of
transition metals are often more difficult to prepare and
expensive to purchase. Also, some industrial problems such
Appl Organometal Chem 2017; e3690;
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
1 of 10
DOI 10.1002/aoc.3690

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RAKHTSHAH ET AL.
as deposition on reactor walls and difficulty in recovery and
separation of catalysts from reaction products are associated
with homogeneous catalysts. One way to overcome these
disadvantages is the immobilization of homogeneous
catalysts on solid supports.[20] Tetrahydrobenzo[b]pyran
derivatives have lately been the subject of much consider-
ation as a significant class of heterocycles having useful
biological and pharmacological activities, such as anticancer,
anticoagulant, diuretic and anti‐anaphylactic activities.[25–30]
Correspondingly, they have a broad range of application as
cognitive enhancers, for the therapy of neurodegenerative
illnesses, including Alzheimer's disease, Down's syndrome,
Parkinson's disease and AIDS‐associated dementia, as well
as for the therapy of schizophrenia.[31] Recently, many proce-
dures have been described for the synthesis of 4H‐benzo[b]
pyran derivatives via one‐pot three‐component tandem
Knoevenagel–Michael cyclocondensation involving the use
of several media or catalysts such as ionic liquids,[32–35]
Fe3O4@SiO2@TiO2,[36] SO42−@MCM‐41,[37] KF‐alu-
SCHEME
1
Synthesis of tetrahydrobenzo[b]pyrans using Mn(III)–
pentadentate Schiff base complex supported on MWCNTs
mina,[38]
Fe3O4@MWCNTs,[39]
tetrabutylammonium
bromide[40] and Zn(Phen)2Cl2.[41]
2.1 | Characterization of Mn(III)–pentadentate schiff
base complex supported on MWCNTs as heterogeneous
catalyst
Different approaches have been used for immobilization
of complexes on CNTs to prepare heterogeneous
catalysts.[15–22] Among the synthesized catalysts are a few
examples of Schiff base complexes. Baleizao and co‐workers
have reported efficient cyanosilylation of aldehydes catalysed
by vanadyl salen complexes covalently anchored to single‐
walled CNTs.[16] Also, Salavati‐Niasari et al. have synthe-
sized a novel hybrid material composed of MWCNTs and
cobalt(II) salen complex and investigated its catalytic activity
in liquid‐phase epoxidation of cyclohexene by air.[42]
Increasing demand for eco‐friendly, green and mild
methods using heterogeneous recyclable and reusable cata-
lysts encouraged us to improve a safe alternative process for
the synthesis of tetrahydrobenzo[b]pyrans via tandem
Knoevenagel–Michael cyclocondensation reactions in the
presence of a recyclable and reusable heterogeneous catalyst
under solvent‐free conditions at room temperature. With this
aim and following our studies on design and synthesis of
Schiff base complexes,[43,44] complex supported on
MWCNTs[45] and pyran derivatives,[46,47] in the work
reported here we decided to immobilize Mn(III)–pentadentate
Schiff base complex on the surface of MWCNTs using
(3‐chloropropyl)trimethoxysilane as a Si linker and spacer.
The designed MWCNT@Si–H2azosaldien system could
be a novel and simple approach for loading metals in a
variety of coordination modes (Scheme 1).
The surface structure of the materials was confirmed using
Fourier transform infrared (FT‐IR) spectroscopy. Figure 1
shows the FT‐IR spectra obtained for HOOC@MWCNTs,
Cpm@MWCNTs,
MWCNTs@Si–H2azosaldien
and
Mn(III)–pentadentate Schiff base complex@MWCNTs. The
FT‐IR spectrum of HOOC@MWCNTs (Figure 1a) shows a
strong band at 1709.14 cm−1 and a weaker band at
1228.58 cm−1 attributed to C═O and C─OH groups in
oxidized MWCNTs, respectively, as well as a broad band at
about 3440–3480 cm−1 assigned to O─H stretching vibra-
tions which are not observed in the spectrum of pristine
MWCNTs. As can be seen in Figure 1(b), in the FT‐IR
spectrum of Cpm@MWCNTs there is a broad band at about
2
| RESULTS AND DISCUSSION
The reactions leading to the nanomaterial of Mn(III)–
pentadentate Schiff base complex supported on MWCNTs
are summarized in Scheme 2. This synthesis was carried
out as described in Section 2.
SCHEME 2 Sequence of stages in preparation of Mn(III)–pentadentate
Schiff base complex supported on MWCNTs

RAKHTSHAH ET AL.
3 of 10
FIGURE 1 FT‐IR spectra of (a) HOOC@MWCNTs, (b) Cpm@MWCNTs, (c) MWCNTs@Si‐H2azosaldien and (d) Mn(III)–pentadentate Schiff base
complex@MWCNTs
1000–1100 cm−1 assigned to Si─O stretching vibrations
which is not present in the spectra of pristine MWCNTs
and/or HOOC@MWCNTs. The presence of anchored propyl
chain is confirmed from C─H stretching vibrations appearing
at about 2927 cm−1. The above bands suggest that during the
anchoring of CPTMS, a condensation reaction occurs
between the OH surface groups of the oxidized MWCNTs
and methoxy groups of CPTMS to form the stable covalent
Si─O─C linkage, leading to the attachment of chloropropyl
groups on the surface of MWCNTs. The most informative
spectroscopic data to support the covalent anchoring of the
[Mn(H2azosaliden)] complex on the modified MWCNTs
were obtained from a comparison of the set of FT‐IR data cor-
responding to Cpm@MWCNTs, MWCNTs–Si–H2azosaldien
and Mn(III)–pentadentate Schiff base complex@MWCNTs.
The appearance of azomethine peak at 1642 cm−1 in
Figure 1(c) suggests binding of the Schiff base to the
MWCNTs through a covalent bond. This sharp band in the
of MWCNTs is similar to that of highly oriented pyrolytic
graphite.[42] The MWCNT pattern shows typical peaks of
(002), (110) and (400) at 2θ = 26°, 43° and 53°, respectively.
Indeed, the crystalline structure of the matrix is only slightly
affected due to the reaction of the MWCNTs with acid.
The structural and morphology study of the
nanomaterial was conducted using field emission scanning
electron microscopy (SEM). A representative image of
immobilized complex supported on MWCNTs is shown in
Figure 3. From the SEM micrograph it can be concluded
that there is a difference between the nanotube structure
for pristine MWCNTs[48] and immobilized complex
supported on MWCNTs. The smooth surface of pristine
MWCNTs has been altered to a rough surface after the
immobilization process, which can be associated with the
complex moieties attached to the nanotube surface. From
the SEM micrograph of immobilized complex supported
on MWCNTs, it can be seen that there is no structural
damage to the MWCNTs. This is probably due to the mild
conditions of aqueous acid solution used for MWCNT
functionalization.[49]
spectrum
of
Mn(III)–pentadentate
Schiff
base
complex@MWCNTs (Figure 1d) shifts to a lower wavenum-
ber, and appears at 1622 cm−1, which indicates the involve-
ment of the azomethine nitrogen in coordination. It should
be noted that in the FT‐IR spectra of neat Schiff base
H2azosaliden and its neat metal complex the vibrational
stretching frequency of C═N bond appears at 1647 and
1622 cm−1, respectively (Figs 110S and 111S). Thus the
attachment of H2azosaliden and/or its Mn(III) complex to
the MWCNTs has a small effect on stretching frequency of
imine bond.
The functionalization process of the MWCNTs with the
Schiff base and the formation of its complex were also
confirmed by the presence of C, O, N, Cl, Si and the
Figure 2 shows the X‐ray diffraction (XRD) patterns of
MWCNTs and Mn(III)–pentadentate Schiff base complex
supported on MWCNTs. The XRD pattern of MWCNTs bear-
ing the Mn(III) complex is similar to that of MWCNTs. This
indicates that the crystallinity and morphology of MWCNTs
are preserved during the grafting process. The XRD pattern
FIGURE 2 XRD patterns of (a) MWCNTs and (b) Mn(III)–pentadentate
Schiff base complex@MWCNTs

4 of 10
RAKHTSHAH ET AL.
weight loss within the temperature range 240–275 °C.[49]
However, for the corresponding supported complex, weight
losses of 0.47, 0.63, 3.18, 8.42, 18.76 and 43.93% appear at
147, 243, 340, 438, 532 and 629 °C, respectively, and of
68.79% at 725 °C.
2.2 | Application of Mn(III)–pentadentate schiff base
complex supported on MWCNTs as heterogeneous
catalyst
After the synthesis of Mn(III)–pentadentate Schiff base
complex supported on MWCNTs as a green heterogeneous
catalyst, in order to optimize the reaction conditions we
considered the efficacy of the catalyst in the synthesis of
tetrahydrobenzo[b]pyrans via tandem Knoevenagel–Michael
cyclocondensationreactions(Scheme1).Forthisaim,asamodel
reaction, the condensation reaction of 1‐naphthaldehyde,
dimedone and malononitrile was studied using various
amounts of the catalyst at room temperature to 100 °C under
solvent‐free conditions (Table 1). As evident from Table 1,
the best results are achieved when the reaction is conducted
using 0.002 g of nanocatalyst at room temperature (Table 1,
entry 5). In the absence of catalyst a low yield of the product
is obtained after 30 min (Table 1, entries 1 and 2). Increasing
the reaction temperature and catalyst loading does not increase
the rate of the reaction (Table 1, entries 6–12).
We also investigated the effects of solvent. As a model
reaction, the reaction of 1‐naphthaldehyde, dimedone and
malononitrile catalysed using 0.002 g of Mn(III)–pentadentate
Schiff base complex supported on MWCNTs as a green
heterogeneous catalyst in various solvents was conducted.
The results are summarized in Table 2. It is evident that
solvent‐free reaction conditions are obviously the best choice
for this reaction.
Encouraged by the important results and with the aim of
investigating the scope of this original procedure, numerous
tetrahydrobenzo[b]pyrans were synthesized from tandem
Knoevenagel–Michael cyclocondensation reactions of
aromatic aldehydes, 1,3‐diones and malononitrile using a
catalytic amount of Mn(III)–pentadentate Schiff base
complex supported on MWCNTs as a green recyclable
heterogeneous catalyst under solvent‐free conditions at room
temperature. The results are summarized in Table 3. The
effect of substituents on the aromatic ring shows estimated
strong effects in terms of yields under these reaction condi-
tions. All aromatic aldehydes including those with electron‐
releasing substituents and electron‐withdrawing substituents
on their aromatic ring give the linked products in high to
excellent yields in short reaction times. The reaction times
of aromatic aldehydes having electron‐withdrawing groups
are rather faster than those having electron‐donating groups.
Additionally, the reaction under the same conditions in the
presence of dimedone goes faster than in the presence of
1,3‐cyclohexanedione.
FIGURE
3
SEM image of Mn(III)–pentadentate Schiff base
complex@MWCNTs
corresponding Mn signals in the energy‐dispersive X‐ray
spectroscopy (EDX) pattern of the nanocatalyst. The
presence of the manganese is confirmed by inductively
coupled plasma (ICP) analysis as well as the EDX results as
can be clearly seen in Figure 4.
The nitrogen and hydrogen contents of Mn(III)–
pentadentate Schiff base complex@MWCNTs were
determined using CHN elemental analysis, which shows
values of 2.28 and 4.16%, respectively. The Mn content of
the immobilized complex supported on MWCNTs was also
measured using ICP analysis, the value for which is about
0.82 mmol g−1
.
Thermogravimetric analysis (TGA) was used to charac-
terize the Mn(III)–pentadentate Schiff base complex
supported on MWCNTs. These studies were made in the
temperature range 50–950 °C in air. The decomposition
behaviour of the raw MWCNTS (Figure 5a) and grafted
complex (Figure 5b) has been compared in order to under-
stand the effect of the support. The free complex shows a
FIGURE
4
EDX diagram of Mn(III)–pentadentate Schiff base
complex@MWCNTs

RAKHTSHAH ET AL.
5 of 10
FIGURE 5 TGA comparative analysis curves of (a) MWCNTs and (b) Mn(III)–Schiff base complex@MWCNTs
A suggested mechanism is shown in Scheme 3. Initially,
malononitrile (3) reacts with carbonyl group of aldehyde 1
which is activated via the Mn(III)–pentadentate Schiff base
complex supported on MWCNTs as a heterogeneous catalyst
to give arylidene malononitrile (5) after eliminating one
molecule of water. 1,3‐Dione (2) converts to enol form (6)
after tautomerization and attacks arylidene malononitrile 5
as Michael acceptor to provide intermediate 7. Finally,
cyclocondensation of intermediate 7 produces intermediate
8 which is converted to preferred product 4.[36]
Moreover, the recyclability of the Mn(III)–pentadentate
Schiff base complex supported on MWCNTs catalyst was
established using the condensation of 1‐naphthaldehyde,
dimedone and malononitrile. At the end of the reaction, ethyl
acetate was added to the reaction mixture and heated to
extract product and remaining starting materials. This
solution was washed with water to separate the catalyst from
other materials (the product is soluble in hot ethyl acetate and
nanocatalyst is insoluble). The aqueous layer was decanted,
separated and used for alternative reaction after removal of
water. It is found that the catalytic activity of the catalyst
remains within the limits of experimental error for six contin-
uous runs (Figure 6). The general methods to check the pres-
ence or absence of leached soluble Mn in the reaction
solution are a hot filtration test or elemental analysis of the
recovered catalyst. The heterogeneity of the nanocatalyst in
solvent‐free condition and at room temperature is obvious
and in such condition the hot filtration is useful. Analysis
using ICP‐AES of manganese in the product after isolation
of catalyst shows no loss of metal during the catalytic reac-
tion, indicating that no metal leaching occurs. Also, the
XRD pattern of the catalyst after six cycles shows that the
structure of the catalyst is maintained during the reaction.
2.3 | Comparison of catalytic activity of Mn(III)–
pentadentate schiff base complex@MWCNTs with that of
known catalysts
To compare the applicability and efficiency of Mn(III)–
pentadentate Schiff base complex@MWCNTs with that of
other reported catalysts in the synthesis of tetrahydrobenzo[b]
pyrans, we evaluated the catalytic activity of this catalysts in

6 of 10
RAKHTSHAH ET AL.
TABLE 1 Optimization of reaction conditions for condensation between 1‐naphthaldehyde, dimedone and malononitrile catalysed by Mn(III)–pentadentate
Schiff base complex@MWCNTs under solvent‐free conditionsa
Entry
Catalyst loading (g)
Reaction temperature (°)c
Reaction time (min)
Yield (%)b
1
Catalyst‐free
Catalyst‐free
0.001
r.t.
100
r.t.
30
30
30
30
4
31
35
75
75
97
97
97
97
97
97
97
97
2
3
4
0.001
100
r.t.
5
0.002
6
0.002
50
4
7
0.002
75
4
8
0.002
100
r.t.
4
9
0.005
4
10
11
12
0.005
100
r.t.
4
0.01
4
0.01
100
4
aReaction conditions: 1‐naphthaldehyde (1 mmol), dimedone (1 mmol), malononitrile (1 mmol).
bIsolated yield.
cThis is the temperature of oil bath and not inside the reaction medium.
the condensation reaction of dimedone and malononitrile with
4‐chlorobenzaldehyde (Table 4). As is evident, the catalytic
activity of both discrete manganese(III) Schiff base complex
and MWCNTs@COOH is very good. Therefore the excellent
catalytic activity and high efficiency of Mn(III)–pentadentate
Schiff base complex @MWCNTs is quite expected.
chlorophenylazosalicylaldehyde)) was prepared as described
previously.[69] MWCNTs of purity >85%, length of 1–10 mm
and outer diameter of 5–20 nm were purchased from
Kermanshah Petrochemical Industries Company.
3.2 | Instrumentation
Reaction progress and purity determination of the com-
pounds were checked by TLC using silica gel SIL G/UV
254 plates. The 1H NMR (400.13 MHz) and 13C NMR
(100.62 MHz) spectra were recorded with a Bruker spectrom-
eter (δ in ppm). Melting points are uncorrected and were
recorded with a Buchi B‐545 apparatus in open capillary
tubes. FT‐IR spectra were recorded with a Jasco 300 FT‐IR
3
| EXPERIMENTAL
3.1 | Materials
All of the reagents and solvents involved in synthesis
were of analytical grade and used as received without
further purification. Azo‐coupled aldehyde (5‐(4‐
TABLE 2 Solvent effect in the synthesis of tetrahydrobenzo[b]pyrans catalysed by 0.002 g of Mn(III)–pentadentate Schiff base complex@MWCNTsa
Entry
Solvent
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
Solvent‐free
H2O
4
97
45
96
96
70
60
55
50
60
10
15
30
30
60
60
C2H5OH
CH3CN
CH3CO2Et
CH2Cl2
Toluene
Benzene
aReaction conditions: 1‐naphthaldehyde (1 mmol), dimedone (1 mmol), malononitrile (1 mmol).
bIsolated yield.

RAKHTSHAH ET AL.
7 of 10
TABLE 3 Synthesis of tetrahydrobenzo[b]pyrans via tandem Knoevenagel–Michael cyclocondensation reactions in presence of Mn(III)–pentadentate Schiff
base complex@MWCNTsa
Entry
Aldehyde
R′
Time (min)
Yield (%)b
M.p. (°C)
1
4‐Nitrobenzaldehyde
Me
H
3
5
98
96
96
95
97
96
94
93
96
95
95
94
96
95
96
95
95
94
95
94
164–166[37]
254–256c
231–233[37]
267–269c
230–232[50]
176–178c
207–209c
251–253c
284–286[51]
273–275c
245–247[52]
257–259c
275–277[51]
225–227c
224–226[52]
247–249c
216–218[37]
204–206c
203–205c
249–251c
2
4‐Nitrobenzaldehyde
3
4‐Chlorobenzaldehyde
Me
H
5
4
4‐Chlorobenzaldehyde
7
5
Naphthalene‐1‐carbaldehyde
Naphthalene‐1‐carbaldehyde
α‐ Methylcinnamaldehyde
α‐ Methylcinnamaldehyde
Naphthalene‐2‐carbaldehyde
Naphthalene‐2‐carbaldehyde
4‐Hydroxybenzaldehyde
4‐Hydroxybenzaldehyde
Biphenyl‐4‐carbaldehyde
Biphenyl‐4‐carbaldehyde
3‐Nitrobenzaldehyde
Me
H
4
6
6
7
Me
H
10
12
5
8
9
Me
H
10
11
12
13
14
15
16
17
18
19
20
7
Me
H
10
12
5
Me
H
7
Me
H
5
3‐Nitrobenzaldehyde
7
4‐Methoxybenzaldehyde
4‐Methoxybenzaldehyde
2,5‐Dimethoxybenzaldehyde
2,5‐Dimethoxybenzaldehyde
Me
H
8
10
10
12
Me
H
aReaction conditions: aldehyde (1 mmol), dimedone (1 mmol), malononitrile (1 mmol), solvent‐free conditions at room temperature.
bIsolated yield.
cNew derivatives.
spectrometer using compressed KBr discs. Mass spectra of
new compounds were obtained with an Agilent Technologies
5975C VL MSD spectrometer. Thermal analyses were
performed with a PerkinElmer TG/DTA 6300 instrument.
Nanostructure observations were conducted using SEM.
XRD patterns were collected using a Bruker D8 ADVANCE
diffractometer. The samples were scanned at a rate of 0.05°
min−1 from 2θ = 10° to 90°.
3.3 | Mn(III)–pentadentate schiff base complex
immobilized on MWCNTs: preparation and purification
3.3.1
| MWCNT functionalization and purification
Following careful purification, MWCNTs (2 g) were
oxidized in a mixture of concentrated sulfuric and nitric acids
(3:1, 98 and 70%, respectively) by ultrasonication for 24 h to
obtain oxidized MWCNTs (MWCNT@COOH).[42] In order
SCHEME 3 Proposed mechanism for the synthesis of tetrahydrobenzo[b]
pyrans via tandem Knoevenagel–Michael cyclocondensation reactions
catalysed by Mn(III)–pentadentate Schiff base complex@MWCNTs
FIGURE
6
Reusability of Mn(III)–pentadentate Schiff base complex
supported on MWCNTs as a heterogeneous catalyst in 4 min

8 of 10
RAKHTSHAH ET AL.
TABLE 4 Comparison of various catalysts in the synthesis of tetrahydrobenzo[b]pyrans with our catalyst
Entry
Catalyst
Solvent
Temp. (°C)
Time (min)
Yield (%)
Ref.
[53]
[37]
[54]
[55]
[56]
[57]
[58]
[59]
[60]
[61]
[50]
[38]
[51]
[62]
[63]
[41]
[46]
[52]
[64]
[40]
[36]
[65]
[66]
[67]
[68]
[39]
1
p‐Dodecylbenzenesulfonic acid (0.4 mmol)
SO42−@MCM‐41 (25 mg)
H2O/reflux
EtOH/reflux
DMF/reflux
EtOH/H2O/reflux
EtOH/reflux
DMSO
—
—
—
—
—
120
—
80
—
70
35
—
110
40
—
r.t.
40
—
—
—
100
r.t.
r.t.
r.t.
r.t.
—
r.t.
r.t.
r.t.
r.t.
240
60
180
30
280
210
20
30
180
10
30
120
10
15
35
5
69
80
81
84
85
88
88
88
90
90
90
92
93
94
94
94
94
94
94
95
96
96
96
97
97
97
63
90
81
96
2
3
KF–Al2O3 (250 mg)
4
Phenylboronic acid (5 mol%)
Red Sea sand (0.5 g)
5
6
Molecular iodine (10 mmol%)
Ni(NO3)2⋅6H2O (10 mol%)
NH4H2PO4–Al2O3 (0.03 g)
Sodium selenite (0.1 g)
7
H2O/reflux
Solvent free
EtOH/H2O/reflux
Solvent free
H2O
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
NaBr (42 mg)
β‐Cyclodextrin–glycerin
KF–alumina (0.2 mmol)
EtOH/reflux
Solvent free
EtOH
Acetic acid@ionic liquid
Nano‐TiO2/H14[NaP5W30O110] (25 mg)
Ce1MgxZr1−xO2 (0.2 g)
EtOH/reflux
H2O
Zn(Phen)2Cl2 (2 mol%)
Nano‐titania sulfuric acid 10 mg
Tetrabutylammonium bromide (10 mol%)
NH4Al(SO4)2⋅12H2O (0.2 g)
Tetrabutylammonium bromide (10 mol%)
Nano‐Fe3O4@SiO2@TiO2 (0.01 g)
CaCl2 (20 mol%)
EtOH
15
35
120
30
10
8
H2O/reflux
EtOH/reflux
EtOH/reflux
Solvent free
EtOH
KF/Al2O3 (145 mg)
EtOH
30
5
Fe3O4@SiO2‐Imid‐PMAn (0.015 g)
H2O
La0.7Sr0.3MnO3 (5 mol%)
EtOH
9
Fe3O4 NPs/MWCNTs/ (5 mg)
MnCl2 (0.002 g)
EtOH /reflux
Solvent free
Solvent free
Solvent free
Solvent free
8
40
7
—
—
—
—
Mn(III)–pentadente Schiff base complex (0.002 g)
MWCNTs‐COOH (0.002 g)
Mn(III)–pentadentate Schiff base complex@MWCNTs (0.002 g)
20
5
to increase the reactivity and/or population of OH surface
groups, the 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‐chloropropyltrimethoxysilane (CPTMS) (2 ml)
was added under dry nitrogen atmosphere. The mixture
diethyl ether. The resulting product was dried in air (yield:
81%).[42]
3.3.3
Mn(III) complex
| Synthesis of MWCNTs@Si–H2azosaldien and its
Triethylamine was added to a dry ethanol solution (60 ml) of
H2azosaldien and then 1 g of CpmMWCNTs was added to
the solution and the reaction mixture was refluxed at 60 °C
under ultrasonic irradiation in nitrogen atmosphere for 20 h
to produce MWCNTs@Si–H2azosaldienin. Then, an excess
amount of metal salt (MnCl2) was added to a mixture of
MWCNTs@Si–H2azosaldien (1 g) in methanol (100 ml)
containing a few drops of triethylamine and the resulting
mixture was refluxed and irradiated with ultrasound for
3 h. The resulting precipitate was filtered and washed
several times with hot water and ethanol until the solution
became colourless. The obtained product, immobilized
Mn(III)–pentadentate Schiff base complex supported on
MWCNTs (ca 1 g), was dried and stored in a vacuum
desiccator.
was refluxed for 24
h
and the resultant solid
(CpmMWCNTs) was separated and washed with methanol
to remove the unreacted residue of silylating reagent and
then dried at 80 °C.
3.3.2
ligand (H2L)
| General procedure for synthesis of azo–azomethine
A solution of diethylenetriamine (1 mmol) in absolute EtOH
(20 ml) was added to a stirring solution of azo‐coupled
aldehyde (5‐(4‐chlorophenylazosalicylaldehyde); 2 mmol)
in absolute EtOH (60 ml) during a period of 15 min at
50 °C. The solution was then heated for 4 h at 80 °C with
stirring. The mixture was filtered and the obtained solid
was washed with hot ethanol (three times) and then with

RAKHTSHAH ET AL.
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3.4 | General procedure for synthesis of
tetrahydrobenzo[b]pyran derivatives
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To a mixture of aromatic aldehyde (1 mmol), 1,3‐dione
(1 mmol) and malononitrile (1 mmol) in a round‐bottom
flask was added Mn(III)–pentadentate Schiff base complex
supported on MWCNTs as a heterogeneous catalyst
(0.002 g), and the resulting mixture was stirred magnetically
under solvent‐free conditions at room temperature. After com-
pletion of the reaction, as checked via TLC (n‐hexane–ethyl
acetate, 5:2), ethyl acetate (10 ml) was added to the reaction
mixture, stirred and refluxed for 3 min, and then was washed
with water (10 ml) and decanted to separate catalyst from other
materials (the reaction mixture was soluble in hot ethyl acetate
and nanocatalyst was insoluble). The solvent of organic layer
was evaporated and the crude product was purified by
recrystallization from ethanol. In this study, the nano‐hetero-
geneous catalyst was recycled and reused six times without
sifnificant loss of its catalytic activity (spectral data analysis
for compounds are in the supporting information).
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| CONCLUSIONS
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In summary, a green, mild and eco‐friendly Mn(III)–
pentadentate Schiff base complex supported on MWCNTs
as a recyclable heterogeneous catalyst was synthesized, stud-
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EDX, ICP analysis, elemental analysis (CHN) and TGA. The
synthesized Mn(III) Schiff base complex immobilized on
MWCNTs was shown to have excellent catalytic activity in
the synthesis of tetrahydrobenzo[b]pyrans via tandem
Knoevenagel–Michael cyclocondensation reactions between
aromatic aldehydes, 1,3‐diones and malononitrile under
solvent‐free conditions at room temperature. Of the 20
synthesized derivatives of tetrahydrobenzo[b]pyrans, 12 are
new compounds. The various main advantages of this
protocol are reasonably clean reaction profile, high yield,
short reaction time, low cost and recyclability of the green
nano‐heterogeneous catalyst, and simplicity of product
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ACKNOWLEDGEMENT
We are grateful to Bu‐Ali Sina University for financial
support.
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SUPPORTING INFORMATION
Additional Supporting Information may be found online in
the supporting information tab for this article.
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How to cite this article: Rakhtshah J, Salehzadeh S,
Zolfigol MA, Baghery S. Mn(III)–pentadentate Schiff
base complex supported on multi‐walled carbon nano-
tubes as a green, mild and heterogeneous catalyst for
the synthesis of tetrahydrobenzo[b]pyrans via tandem
Knoevenagel–Michael cyclocondensation reaction,
Appl Organometal Chem. 2017;e3690. doi: 10.1002/
aoc.3690
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