Phosphomolybdic acid supported on Schiff base functionalized graphene oxide nanosheets: Preparation, characterization, and first catalytic application in the multi-component synthesis of tetrahydrobenzo[a]xanthene-11-ones

Article abstract of DOI:10.1002/aoc.4881

New Schiff base (SB) functionalized graphene oxide (GO) nanosheets containing phosphomolybdic counter-anion H₂PMo₁₂O₄₀^ˉ (H₂PMo) were successfully prepared by grafting of 3-aminopropyltriethoxysilane (

Full text of DOI:10.1002/aoc.4881

Received: 12 November 2018
DOI: 10.1002/aoc.4881
Revised: 31 January 2019
Accepted: 1 February 2019
F U L L P A P E R
Phosphomolybdic acid supported on Schiff base
functionalized graphene oxide nanosheets: Preparation,
characterization, and first catalytic application in the multi‐
component synthesis of tetrahydrobenzo[a]xanthene‐
11‐ones
Marziyeh Rohaniyan1 | Abolghasem Davoodnia1
Amir Khojastehnezhad2
| S. Ali Beyramabadi1 |
1 Department of Chemistry, Mashhad
New Schiff base (SB) functionalized graphene oxide (GO) nanosheets contain-
Branch, Islamic Azad University,
Mashhad, Iran
2 Young Researchers and Elite Club,
Mashhad Branch, Islamic Azad
University, Mashhad, Iran
¯
ing phosphomolybdic counter‐anion H2PMo12O40 (H2PMo) were successfully
prepared by grafting of 3‐aminopropyltriethoxysilane (APTS) on GO nano-
sheets followed by condensation with benzil and finally reaction with
phosphomolybdic acid (H3PMo12O40, denoted as H3PMo) and characterized
using Fourier transform infrared (FT‐IR) spectroscopy, field emission scanning
electron microscopy (FESEM), transmission electron microscopy (TEM),
atomic force microscopy (AFM), particle size distribution, energy‐dispersive
X‐ray (EDX) analysis, EDX elemental mapping, and inductively coupled
plasma optical emission spectrometry (ICP‐OES). The prepared new
nanomaterial, denoted as GO‐SB‐H2PMo, was shown to be an efficient
heterogeneous catalyst in one‐pot, three‐component reaction of β‐naphthol,
aldehydes, and dimedone, giving high yields of tetrahydrobenzo[a]xanthene‐
11‐ones within short reaction times. The catalyst is readily recovered by simple
filtration and can be recycled and reused several times with no significant loss
of catalytic activity.
Correspondence
Abolghasem Davoodnia, Department of
Chemistry, Mashhad Branch, Islamic
Azad University, Mashhad, Iran.
Email: adavoodnia@mshdiau.ac.ir;
adavoodnia@yahoo.com
KEYWORDS
functionalized graphene oxide, phosphomolybdic acid, Schiff base, tetrahydrobenzo[a]xanthene‐
11‐ones
1 | INTRODUCTION
conductivity, high thermal stability, and high structural
strength,[2–4] and therefore has diverse applications in
Graphene, one of the allotropes of carbon, is two‐
dimensional monolayers of carbon atoms arranged in a
honeycomb lattice that can be functionalized easily
with acidic moieties to give new materials that can be
used in acid‐catalyzed processes.[1] It possesses unique
physical properties, such as high thermal and electronic
various fields such as electrochemical sensors,[5] compos-
ite materials,[6] and energy technology.[7] Graphene is
easily produced from graphene oxide (GO) by reduction
using reducing agents such as NaBH4 or hydrazine.[9]
[8]
In recent years, GO which can be prepared from graphite
powder using Hummers method,[10] has attracted great
Appl Organometal Chem. 2019;e4881.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4881

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ROHANIYAN ET AL.
interest as a promising ideal support for a number of
metals, nanoparticles and organic compounds due to its
extremely high specific surface area, high hydrophilic
nature, high adsorption capacity, high chemical stability
and good accessibility.[11–13] The presence of hydrophilic
oxygen containing groups on the surface of GO nano-
sheets such as hydroxyl, epoxy, and carboxyl groups
makes surface modification of GO further efficiently
and allow construction of various novel functional groups
attached at the GO surface.[13–15] Functionalized GO‐
based materials are attractive for various application
areas such as solar cells,[16] adsorption of organic
dye,[17] and also serve as efficient heterogeneous catalysts
in various chemical transformations such as oxidation of
olefins,[18] synthesis of N‐aryl‐2‐amino‐1,6‐naphthyri-
dines,[19] and synthesis of indazolophthalazinetriones.[20]
The synthesis of xanthenes has received significant
attention because of their wide range of biological and
pharmaceutical properties.[35–37] They are also known
as laser dyes.[38] Among xanthene based compounds,
tetrahydrobenzo[a]xanthene‐11‐ones are generally syn-
thesized by one‐pot, three‐component reaction of β‐
naphthol, aldehydes, and dimedone in the presence of a
variety of catalysts.[39–49] Nevertheless, development of
new efficient recyclable catalysts for the synthesis of
these compounds was of certain demand.
Taking all these facts into consideration, and in line
with our interest in catalysis,[50–57] in this paper, for
the first time, a new SB functionalized GO containing a
¯
phosphomolybdic counter‐anion H2PMo12O40 (H2PMo)
was prepared by grafting of APTS on GO nanosheets
followed by condensation with benzil and finally reac-
tion with phosphomolybdic acid (H3PMo12O40, denoted
as H3PMo) and fully characterized (Scheme 1). The cata-
lytic activity of this new material which was denoted as
GO‐SB‐H2PMo was also investigated in one‐pot synthesis
of tetrahydrobenzo[a]xanthene‐11‐ones by reaction of β‐
naphthol, aldehydes, and dimedone (Scheme 2).
There are
a number of methods developed for
functionalization of GO including amidation reaction
between the carboxylic acid group of GO and amine,[21]
esterification reaction between the carboxylic acid group
of GO and alcohols,[22] reaction between the hydroxyl
and carboxyl groups of GO and isocyanates,[23] and
many others. Silylation of the hydroxyl groups of GO
using 3‐aminopropyltriethoxysilane (APTS), as a silane
coupling agent, is another method to covalently modify
the surface of GO.[14,15]
2 | RESULTS AND DISCUSSION
Condensation of an aldehyde or ketone with a primary
amine that was introduced for the first time by Hugo
Schiff[24] give a compound with an imine group
(–C=N–) that is known as a Schiff base (SB). These com-
pounds, either alone or in a metal chelated form, are
widely applied as antioxidant,[25] anticonvulsant,[26] anti-
hypertensive α‐blocking agent,[27] and in cancer diagno-
sis.[28] In recent years, SBs derived from aromatic
(benzene) rings containing carbonyl groups such as
benzil or benzil like compounds, salicyaldehyde, and
benzoin are widely used in functionalization of nano-
materials.[18,29–33] These SB functionalized nanomaterials
are used for different kinds of applications in cataly-
sis,[18,29,30] CO2 capture for environmental clean‐up,[31]
antibacterial agents,[32] fabrication of polymer nanocom-
posites,[33] and electromagnetic studies.[34]
2.1 | Preparation and characterization of
the catalyst GO‐SB‐H2PMo
At first, GO nanosheets were prepared from natural
graphite powder by modified Hummers method.[10]
Grafting of APTS on GO nanosheets gave GO‐NH2 to pro-
vide amino moieties for further functionalization. These
amino moieties undergo condensation reaction with
carbonyl groups in benzil to form GO‐SB. Finally, interac-
tion of obtained GO‐SB with H3PMo gave GO‐SB‐H2PMo
as
a
new SB functionalized GO containing
a
phosphomolybdic counter‐anion. The latter was charac-
terized using Fourier transform infrared (FT‐IR) spectros-
copy, field emission scanning electron microscopy
(FESEM), transmission electron microscopy (TEM),
atomic force microscopy (AFM), particle size distribution,
SCHEME 1 Preparation of GO‐
SB‐H2PMo nanosheets

ROHANIYAN ET AL.
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SCHEME 2 GO‐SB‐H2PMo nanosheets
catalyzed synthesis of tetrahydrobenzo[a]
xanthene‐11‐ones
energy‐dispersive X‐ray (EDX) analysis, EDX elemental
mapping, and inductively coupled plasma optical emis-
sion spectroscopy (ICP‐OES).
the FT‐IR spectrum of GO‐NH2 (Figure 1(b)). The vibra-
tional bands at 2927 and 2854 cm−1 are attributed to the
symmetric and asymmetric CH2 of the alkyl chains and
the bands located at around 3428 and 1572 cm−1 are
assigned to the vibrations of the amino groups in
aminosilane moieties. Moreover, the appearance of bands
at 1120 and 1040 cm−1 which are attributed to character-
istic absorption of Si‐O bonds acquired during the
silylation process, provided more evidence for the suc-
cessful grafting of APTS onto GO through chemical bond-
ing. The loading of benzil on GO‐NH2 leads to GO‐SB
formation, which is corroborated by a sharp band at
1658 cm−1 attributed to the C=N (imine) stretching mode
(Figure 1(c)). Finally, the successful preparation of
GO‐SB‐H2PMo is confirmed by the appearance of the
new bands at 797, 914, and 1056–1104 cm−1 (overlapped
with Si‐O stretching vibration bands) (Figure 1(e)) which
originates from the H2PMo stretching vibration bands
similar to H3PMo (Figure 1(d)). Other bands in the
FT‐IR spectrum of GO‐SB‐H2PMo are similar to those of
GO‐SB with a slight shift for some of them.
Figure 1 shows the FT‐IR spectra of GO, GO‐NH2,
GO‐SB and GO‐SB‐H2PMo. For GO (Figure 1(a)), the
intense and broad peak at 3437 cm−1 is attributed to the
stretching vibration of OH groups at the surface of GO,
which is major binding site for trialkoxysilane materials.
The adsorption band at 1720 cm−1 is assigned to the car-
bonyl group (C=O) of carboxylic acid. Other characteris-
tic bands at 1639 and 1061 cm−1 are related to C=C and
C‐O stretching vibrations, respectively. Subsequent,
grafting of APTS with hydroxyl groups of GO was con-
firmed by the appearance of many vibrational peaks in
The surface morphology of the GO‐SB‐H2PMo was
characterized using FESEM image. As seen in Figure 2,
the FESEM image exhibits a nanosheet‐like structure in
disordered phase with crumpled and wrinkled edges.
These folded edges owing to sp3‐carbon in GO‐SB‐H2PMo,
carry SB‐H2PMo on both sides of the GO nanosheets.
These sites may serve as reactive catalytic sites to provide
the desired chemical transformation. On the other hand,
the granular‐like particles on the surfaces of GO nano-
sheets, confirm efficient grafting of new SB functionalized
GO containing H2PMo anion on the GO nanosheets.
In the TEM image of the GO‐SB‐H2PMo shown in
Figure 3, some recognizable nanoparticles supported on
the surface of the GO nanosheets can be seen.
Furthermore, the AFM topography images (two‐
dimentional (2D), Figure 4a, and three‐dimentional (3D),
Figure 4b) were employed to observe the morphology of
the GO‐SB‐H2PMo catalyst and measure its thickness.
In these images, there is some prominence which confirm
successful immobilization of organic and inorganic
ligands onto the surface of GO. Moreover, according to
3D image, the thickness of the functionalized GO layer
is about 4.5 nanometers which approve the nanosheet‐
like structure of the catalyst.
FIGURE 1 FT‐IR spectra of (a) GO, (b) GO‐NH2, (c) GO‐SB, (d)
H3PMo, and (e) GO‐SB‐H2PMo

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ROHANIYAN ET AL.
FIGURE 5 Particle size distribution of GO‐SB‐H2PMo
the catalyst has a size between 40 to 600 nm which prob-
ably the small parts are related to the organic and inor-
ganic ligands attached to the GO nanosheets and the big
ones are correspond to the GO nanosheets support.
In order to get more evidences about the chemical struc-
ture of GO‐SB‐H2PMo catalyst, EDX analysis was pro-
vided. As depicted in Figure 6, the EDX spectrum clearly
shows the presence of oxygen, carbon, silicon, nitrogen,
phosphorus and molybdenum elements in the catalyst,
confirming the successful functionalization of GO.
FIGURE 2 FESEM image of GO‐SB‐H2PMo
In addition, EDX elemental mapping was used to
investigate the elements distribution in GO‐SB‐H2PMo.
As can be seen in Figure 7, the uniform distribution of
Si, P, and Mo, indicates that the SB‐H2PMo is uniformly
immobilized on GO, which is very important to perform
efficient catalytic organic transformations.
Based on ICP‐OES, the amounts of P and Mo incorpo-
rated into the GO‐SB‐H2PMo are 2.076% and 7.131%,
respectively, which is another conformation for immobili-
zation of SB‐H2PMo over the surfaces of GO nanosheets.
2.2 | Evaluation of catalytic activity of
GO‐SB‐H2PMo in the synthesis of
tetrahydrobenzo[a]xanthene‐11‐ones
FIGURE 3 TEM image of GO‐SB‐H2PMo
To determine the size of the nanoparticles, the particle
size distribution of the GO‐SB‐H2PMo catalyst was mea-
sured and the results are shown in Figure 5. As depicted,
The catalytic efficiency of GO‐SB‐H2PMo was tested in the
three component coupling of β‐naphthol, aldehydes, and
FIGURE 4 2D (a) and 3D (b) AFM topography images of GO‐SB‐H2PMo

ROHANIYAN ET AL.
5 of 11
solvents used such as H2O, EtOH, EtOH/H2O, MeOH,
CH3CN, CH2Cl2, CHCl3, THF, and neat, the reaction
proceeded most readily to give the highest yield of the
product 4b in shorter reaction times under solvent‐free
conditions. Increases in the amount of the catalyst and
reaction temperature up to 5.1 mol% and 120 °C, respec-
tively, led to an increase in the yield of the product 4b.
A further increase in temperature and catalyst amount
did not improve the product yield. To show the merit of
the GO‐SB‐H2PMo catalyst, the effect of Graphite, GO,
and GO‐SB was also investigated in the above model reac-
tion in optimized conditions. The results are shown in
Table 1 (entries 24–26). As depicted, GO‐SB‐H2PMo
proved to be the better catalyst than others in terms of
reaction time and yield.
Thereafter, to evaluate the applicability of the method,
a variety of other aromatic aldehydes were used for
the synthesis of a range of tetrahydrobenzo[a]xanthene‐
11‐ones under optimized conditions. As shown in
Table 2, all electron‐rich as well as electron‐poor aro-
matic aldehydes reacted successfully and gave the prod-
ucts in high yields within short reaction time. However,
using aliphatic aldehydes, good yields of the products
were obtained in longer reaction times.
The obtained results using GO‐SB‐H2PMo as a
heterogeneous catalyst were compared with those of the
other methods reported for the synthesis of tetrahy-
drobenzo[a]xanthene‐11‐ones in the presence of various
homogeneous, heterogeneous, and supported catalysts.
This comparison is shown in Table 3. As can be seen,
our reaction conditions showed a shorter reaction time
with high yields of the desired products.
FIGURE 6 EDX analysis of GO‐SB‐H2PMo
dimedone, giving tetrahydrobenzo[a]xanthene‐11‐ones.
For optimization of the reaction conditions, the reaction
of β‐naphthol 1, 4‐chlorobenzaldehyde 2b, and dimedone
3 was investigated as a model. Different reaction parame-
ters like catalyst amount, effect of media and influence of
temperature were studied for the formation correspond-
ing product 4b. These results are summarized in Table 1.
When the reaction was carried out without catalyst
(Table 1, entry 1) under solvent‐free conditions at high
temperature, a low yield of the product was formed even
after 120 min. However, when the reaction was performed
in the presence of GO‐SB‐H2PMo, the desired product was
obtained in moderate to high yields. Among the various
FIGURE 7 EDX elemental mapping of GO‐SB‐H2PMo

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ROHANIYAN ET AL.
TABLE 1 Synthesis of 4b under different reaction conditionsa
Entry
Catalyst (mol%)
Solvent
Temp. (°C)
Time (min)
Yield (%)
TONb
TOF (min−1 c
‐‐‐‐‐
2.78
)
1
‐‐‐‐‐
‐‐‐‐‐
120
120
9
15
55
60
68
60
69
78
75
79
85
85
91
95
94
95
25
80
61
48
77
79
75
30
22
81
84
‐‐‐‐‐
2
GO‐SB‐H2PMo (2.2)
GO‐SB‐H2PMo (2.2)
GO‐SB‐H2PMo (2.2)
GO‐SB‐H2PMo (3.7)
GO‐SB‐H2PMo (3.7)
GO‐SB‐H2PMo (3.7)
GO‐SB‐H2PMo (4.4)
GO‐SB‐H2PMo (4.4)
GO‐SB‐H2PMo (4.4)
GO‐SB‐H2PMo (5.1)
GO‐SB‐H2PMo (5.1)
GO‐SB‐H2PMo (5.1)
GO‐SB‐H2PMo (5.1)
GO‐SB‐H2PMo (5.5)
GO‐SB‐H2PMo (5.1)
GO‐SB‐H2PMo (5.1)
GO‐SB‐H2PMo (5.1)
GO‐SB‐H2PMo (5.1)
GO‐SB‐H2PMo (5.1)
GO‐SB‐H2PMo (5.1)
GO‐SB‐H2PMo (5.1)
GO‐SB‐H2PMo (5.1)
Graphite (5.1)
‐‐‐‐‐
80
25.00
27.27
30.91
16.22
18.65
21.08
17.05
17.95
19.32
16.67
17.84
18.63
18.43
17.27
4.90
3
‐‐‐‐‐
100
9
3.03
3.86
2.03
2.66
3.51
2.44
2.99
3.86
4.17
5.95
18.63
18.43
17.27
0.02
0.17
0.10
0.09
0.14
0.14
0.13
0.02
0.04
0.64
0.82
4
‐‐‐‐‐
120
8
5
‐‐‐‐‐
80
8
6
‐‐‐‐‐
100
7
7
‐‐‐‐‐
120
6
8
‐‐‐‐‐
80
7
9
‐‐‐‐‐
100
6
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
‐‐‐‐‐
120
5
‐‐‐‐‐
80
4
‐‐‐‐‐
100
3
‐‐‐‐‐
120
1
‐‐‐‐‐
140
1
‐‐‐‐‐
120
1
H2O
EtOH
EtOH/H2O
MeOH
CH3CN
CH2Cl2
CHCl3
THF
‐‐‐‐‐
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
120
240
90
120
100
110
110
110
240
120
25
20
15.69
11.96
9.41
15.10
15.49
14.71
5.88
4.31
GO (5.1)
‐‐‐‐‐
120
15.88
16.47
GO‐SB (5.1)
‐‐‐‐‐
120
aReaction conditions: β‐naphthol (1; 1 mmol), 4‐chlorobenzaldehyde (2b; 1 mmol), and dimedone (3; 1 mmol).
bTurn Over Number.
cTurn Over Frequency.
The principle advantage of employing heterogeneous
solid catalysts in organic transformations is their reusabil-
ity which makes the method economically valuable, indus-
trially profitable and environmentally sustainable. So, the
recycling performance of GO‐SB‐H2PMo was investigated
in the model reaction. After completion of the reaction,
the mixture was cooled to room temperature and hot etha-
nol was added. The catalyst was easily separated by a sim-
ple filtration, washed with ethanol, dried at 70 °C under
vacuum for 1 hr, and used for the next cycle. The recovered
catalyst could be used at least nine times without any
noticeable reduction in catalytic activity (Figure 8).
the ortho‐quinone methide (o‐QM) intermediate [II], pre-
pared by condensation of β‐naphthol 1 with aldehydes
2a‐n via the intermediate [I]. Subsequent Michael addition
of the o‐QM intermediate [II] with enolic form of
dimedone 3 followed by addition of the phenolic hydroxyl
moiety in intermediate [III] to the carbonyl group provides
cyclic hemiketal intermediate [IV], which on dehydration
afforded final products 4a‐n. As shown in Scheme 3, we
propose that the catalyst GO‐SB‐H2PMo ≡ Cat. with
several accessible Mo sites and P‐O‐H moieties can act as
Lewis acid and Brønsted acid centres, respectively, and
therefore activates the reactants and the intermediates in
this reaction. Under these conditions, attempts to isolate
the intermediates failed even after careful monitoring of
the reactions.
To show the catalyst's role, a plausible mechanism for
this reaction may proceed as depicted in Scheme 3. It is
proposed that the reaction occurs via initial formation of

ROHANIYAN ET AL.
7 of 11
TABLE 2 GO‐SB‐H2PMo catalyzed synthesis of tetrahydrobenzo[a]xanthene‐11‐ones 4a‐na
M.P. (°C)
Find
Time
(min)
Isolated
Yield (%)
TOF
Entry
R
Product
TONb
(min−1 c
)
Reported
1
C6H5
4a
7
90
95
89
87
91
89
91
90
88
92
86
93
75
80
17.65
18.63
17.45
17.06
17.84
17.45
17.84
17.65
17.25
18.04
16.86
18.24
14.71
15.69
2.52
18.63
2.49
2.44
5.95
2.49
3.57
3.53
2.46
2.55
1.69
1.82
0.59
0.52
151–153
190–191
175–177
174–176
170–172
221–224
171–173
175–177
200–202
174–176
203–205
219–221
113–115
170–173
150–151[39]
191–193[43]
173–175[46]
175–176[43]
170–171[43]
220–222[39]
170–172[43]
175–177[39]
196–198[43]
175–177[43]
201–203[43]
216–217[34]
110–111[41]
170–173[40]
2
4‐ClC6H4
3‐ClC6H4
2‐ClC6H4
3‐BrC6H4
2‐O2NC6H4
3‐O2NC6H4
4‐O2NC6H4
2‐Pyridyl
4‐MeC6H4
4‐MeOC6H4
4‐HOC6H4
Me
4b
4c
1
3
7
4
4d
4e
7
5
3
6
4f
7
7
4 g
4 h
4i
5
8
5
9
7
10
11
12
13
14
4j
8
4 k
4 l
4 m
4n
10
10
25
30
n‐Pr
aReaction conditions: β‐naphthol (1; 1 mmol), an aldehyde (2a‐n; 1 mmol), dimedone (3; 1 mmol), GO‐SB‐H2PMo (5.1 mol%), 120 °C, solvent‐free.
bTurn Over Number.
cTurn Over Frequency.
TABLE 3 Comparison of the efficiencies of different catalysts for the synthesis of tetrahydrobenzo[a]xanthene‐11‐ones
Catalyst Conditions
amount
(mol%)
Time
(min)
Yield
(%)
Catalyst
Solvent
T (°C) Other
Ref.
[39]
Trichloroisocyanuric acid
SO3H‐functionalized ionic liquids
p‐Dodecylbenzenesulfonic acid
Ceric ammonium nitrate
Ce (SO4)2.4H2O
5
‐‐‐
110
‐‐‐
‐‐‐
25–40
55–95
74–90
75–95
63–93
[40]
[41]
[42]
[43]
[44]
[45]
10
10
5
‐‐‐
120
H2O
40–42
ultrasound 60–240
CH2Cl2/EtOH 26
ultrasound 120–144 82–87
25
10
20
‐‐‐
‐‐‐
‐‐‐
120
‐‐‐
‐‐‐
‐‐‐
8–30
10–20
8–30
85–97
87–92
80–97
Zr‐MCM‐41
80
Metal oxide nanoparticles
(TiO2, Al2O3, or Fe3O4)
110
[46]
[47]
[48]
Poly(N,N′‐dibromo‐N‐ethylnaphtyl‐2,7‐sulfonamide)
Sulfonation of carbonized xylan‐type hemicellulose
5
‐‐‐
‐‐‐
‐‐‐
110
90
‐‐‐
‐‐‐
‐‐‐
30–120
120
70–96
80–96
87–97
4.3
10
N,N′‐dibromo‐N,N′‐1,2‐ethanediyl
bis(p‐toluenesulfonamide)
90
20–95
[49]
HBF4/SiO2
10
‐‐‐
‐‐‐
80
‐‐‐
‐‐‐
55–90
1–30
83–95
GO‐SB‐H2PMo
5.1
120
75–95 This work

8 of 11
ROHANIYAN ET AL.
3.1 | Preparation of GO nanosheets
GO nanosheets were prepared from natural graphite
using Hummers method[10] with some modification. A
mixture of graphite powder (5.0 g), sodium nitrate
(NaNO3, 2.5 g), and concentrated sulfuric acid (115 ml,
98% H2SO4) was stirred in an ice bath at 0–5 °C for
15
min
and
then
potassium
permanganate
(KMnO4,15.0 g) was slowly added. The stirring was con-
tinued for 2 hr while the temperature was kept in the
range of 0–10 °C. The mixture was then transferred to
a water bath and stirred at 35 °C for 30 min, forming
a brownish grey thick paste. Afterwards, deionized
water (230 ml) was slowly added to the paste and the
suspension, now brown in color, was stirred at
95–98 °C for 15 min. The suspension was further
diluted with warm deionized water (700 ml, 40 °C),
followed with a drop by drop addition of 30% hydrogen
peroxide (H2O2, 50 ml). The mixture was centrifuged
and the isolated yellow‐brown cake was washed with
diluted HCl (5 wt%) and deionized water several times
until the pH became neutral. The solid graphite oxide
was separated by centrifugation and dried at 60 °C
under vacuum for 12 hr. The obtained graphite oxide
(0.4 g) was dispersed in distilled water (400 ml) and
sonicated in an ultrasonic bath cleaner (100 W) for
1 hr to exfoliate graphitic oxide. The complete exfolia-
tion of graphite oxide is confirmed with the formation
of light brown colored homogeneous dispersion GO.
Afterwards, the GO solution was centrifuged for
20 min and then the supernatant was removed (to
remove any unexfoliated graphitic oxide). The GO
nanosheets were obtained after drying the sediment in
a vacuum oven at 80 °C for 24 hr.
FIGURE 8 Reusability of GO‐SB‐H2PMo for the synthesis of
compound 4b
3 | EXPERIMENTAL
All chemicals were purchased from Merck and Aldrich
and used without further purification. Melting points
were recorded with a Stuart SMP3 melting point appara-
1
tus. The H NMR (300 MHz) spectra were recorded on a
Bruker 300 FT spectrometer, in DMSO‐d6 as the solvent
using tetramethyl silane (TMS) as internal standard.
FT‐IR spectra were obtained using a Tensor 27 Bruker
spectrophotometer as KBr disks. Ultrasonication was per-
formed using a Soltec sonicator at a frequency of 40 kHz
and a nominal power of 260 W. FESEM analyses was
done using a TESCAN BRNO‐MIRA3 LMU. TEM analy-
sis was performed using a Leo 912 AB microscope with
an accelerating voltage of 120 kV. EDX analysis and
elemental mapping were performed using a SAMX model
instrument. The amount of molybdenum and phosphorus
in the catalyst was determined using ICP‐OES conducted
with a Spectro Arcos model spectrometer.
SCHEME 3 Plausible mechanism for the formation of tetrahydrobenzo[a]xanthene‐11‐ones 4a‐n in the presence of GO‐SB‐H2PMo as
catalyst

ROHANIYAN ET AL.
9 of 11
of the products are given in the supporting information
(Figures S2‐S9).
3.2 | Preparation of GO‐NH2
The GO‐NH2 was prepared by grafting of APTS on GO
nanosheets according to methods cited in the litera-
ture.[14,15] Briefly, the synthesized GO (1.0 g) was ultra-
sonically dispersed in anhydrous toluene (30 ml) at
room temperature for 30 min and then APTS (3.0 mmol)
was added. The mixture was heated under a N2 atmo-
sphere at 110 °C for 24 hr. The resultant solid was filtered
and washed with toluene (3 × 10 ml), and dried at 50 °C
under vacuum for 24 hr to give black powder of GO‐NH2.
1
3.6 | Selected FT‐IR and H NMR data
3.6.1 | 9,9‐Dimethyl‐12‐phenyl‐8,9,10,12‐
tetrahydrobenzo[a]xanthen‐11‐one (4a)
1H NMR (300 MHz, DMSO‐d6, ppm): δ = 0.90 (s, 3H,
CH3), 1.08 (s, 3H, CH3), 2.14 (d, J = 16.1 Hz, 1H, one pro-
ton of diastereotopic protons in CH2), 2.36 (d, J = 16.1 Hz,
1H, one proton of diastereotopic protons in CH2), 2.66
(ABq, Δν = 28.5 Hz, JAB = 17.4 Hz, 2H, CH2), 5.59
(s, 1H, pyran CH), 7.07 (t, J = 7.2 Hz, 1H, arom‐H),
7.20 (t, J = 7.6 Hz, 2H, arom‐H), 7.31 (d, J = 7.4 Hz,
2H, arom‐H), 7.41–7.55 (m, 3H, arom‐H), 7.93
(d, J = 8.7 Hz, 2H, arom‐H), 8.06 (d, J = 8.3 Hz, 1H,
arom‐H); IR (KBr, cm−1) υ = 2953, 1650, 1458, 1376,
1229, 1024, 811, 698.
3.3 | Preparation of GO‐SB
The resulting GO‐NH2 (1.0 g) was dispersed in absolute
ethanol (60 ml) using an ultrasonic bath at room temper-
ature for 30 min. This is followed by addition of benzil
(3 mmol) and a few drops of glacial acetic acid. The mix-
ture was heated under reflux for 5 hr. The solid was fil-
tered and washed with absolute ethanol (3 × 10 ml) to
remove the non‐reacted benzil, and then dried at 40 °C
under vacuum for 24 hr to give GO‐SB.
3.6.2 | 12‐(4‐Chlorophenyl)‐9,9‐dimethyl‐
8,9,10,12‐tetrahydrobenzo[a]xanthen‐11‐one
(4b)
3.4 | Preparation of GO‐SB‐H2PMo
1H NMR (300 MHz, DMSO‐d6, ppm): δ = 0.90 (s, 3H,
CH3), 1.08 (s, 3H, CH3), 2.15 (d, J = 16.5 Hz, 1H, one
proton of diastereotopic protons in CH2), 2.37
(d, J = 16.5 Hz, 1H, one proton of diastereotopic protons
in CH2), 2.66 (ABq, Δν = 31.8 Hz, JAB = 17.4 Hz, 2H,
CH2), 5.61 (s, 1H, pyran CH), 7.26 (d, J = 8.6 Hz, 2H,
arom‐H), 7.33 (d, J = 8.6 Hz, 2H, arom‐H), 7.41–7.55
(m, 3H, arom‐H), 7.91–7.97 (m, 2H, arom‐H), 8.02
(d, J = 8.2 Hz, 1H, arom‐H); 13C NMR (75 MHz,
DMSO‐d6, ppm): δ = 26.72, 29.27, 32.39, 34.08, 40.70,
50.56, 113.27, 117.18, 117.65, 123.70, 125.56, 127.75,
128.61, 129.08, 129.83, 130.47, 130.99, 131.25, 131.58,
144.29, 147.63, 164.45, 196.41; IR (KBr, cm−1) υ = 3072,
2951, 2872, 1648, 1482, 1370, 1227, 1089, 1018, 817, 750.
GO‐SB (0.5 g) was ultrasonically dispersed in absolute
ethanol (20 ml) at 60 °C for 20 min. H3PMo (2 mmol)
was added to the suspension and sonication continued
for another 1 hr at same temperature. The mixture was
then refluxed for 10 hr. After cooling to room tempera-
ture, the solid was collected by filtration and repeatedly
washed with absolute ethanol and dried under vacuum
at 60 °C for 12 hr to form GO‐SB‐H2PMo.
3.5 | General procedure for synthesis of
tetrahydrobenzo[a]xanthene‐11‐ones 4a‐n
catalyzed by GO‐SB‐H2PMo
A mixture of β‐naphthol 1 (1 mmol), aldehyde (2a‐n;
1 mmol), dimedone (3; 1 mmol), and GO‐SB‐H2PMo
(70 mg) was heated in an oil bath at 120 °C. The reaction
was monitored by TLC. Upon completion of the transfor-
mation, the reaction mixture was cooled to room temper-
ature and hot ethanol was added. The catalyst was
insoluble in hot ethanol and it could therefore be recycled
by a simple filtration. The product was then collected
from the filtrate after cooling to room temperature and
recrystallized from ethanol to give compounds 4a‐n in
high yields. All the products were characterized accord-
ing to comparison of their melting points with those of
authentic samples. IR and 1H NMR spectral data for some
3.6.3 | 9,9‐Dimethyl‐12‐(3‐nitrophenyl)‐
8,9,10,12‐tetrahydrobenzo[a]xanthen‐11‐one
(4 g)
1H NMR (300 MHz, DMSO‐d6, ppm): δ = 0.89 (s, 3H,
CH3), 1.09 (s, 3H, CH3), 2.17 (d, J = 16.3 Hz, 1H, one
proton of diastereotopic protons in CH2), 2.39
(d, J = 16.3 Hz, 1H, one proton of diastereotopic protons
in CH2), 2.70 (ABq, Δν = 24.2 Hz, JAB = 17.4 Hz, 2H,
CH2), 5.81 (s, 1H, pyran CH), 7.43–7.56 (m, 4H, arom‐
H), 7.77 (dt, J = 7.9, 1.2 Hz, 1H, arom‐H), 7.93–8.02
(m, 3H, arom‐H), 8.07 (d, J = 8.2 Hz, 1H, arom‐H), 8.19

10 of 11
ROHANIYAN ET AL.
(t, J = 1.9 Hz, 1H, arom‐H); 13C NMR (75 MHz, DMSO‐
d6, ppm): δ = 26.66, 29.18, 32.44, 34.46, 40.67, 50.47,
112.82, 116.42, 117.68, 121.99, 123.03, 123.71, 125.70,
127.97, 129.13, 130.26, 130.87, 131.61, 135.32, 147.37,
147.75, 148.08, 164.92, 196.49; IR (KBr, cm−1) υ = 2955,
1649, 1529, 1467, 1353, 1224, 1086, 1026, 812, 752.
FESEM, TEM, AFM, particle size distribution, EDX anal-
ysis, EDX elemental mapping, and ICP‐OES. The new
functionalized GO performed well as a catalyst in one‐
pot synthesis of tetrahydrobenzo[a]xanthene‐11‐ones by
reaction of β‐naphthol with several aromatic or aliphatic
aldehydes and dimedone under solvent‐free conditions,
giving high yields of the products within short reaction
times. In addition, the catalyst is readily recovered by
simple filtration and can be reused for subsequent reac-
tions with no significant loss of its activity. Further appli-
cations of this new catalyst for other reaction systems are
currently under investigation.
3.6.4 | 12‐(4‐Methoxyphenyl)‐9,9‐dimethyl‐
8,9,10,12‐tetrahydrobenzo[a]xanthen‐11‐one
(4 k)
1H NMR (300 MHz, DMSO‐d6, ppm): δ = 0.91 (s, 3H,
CH3), 1.08 (s, 3H, CH3), 2.14 (d, J = 16.5 Hz, 1H, one
proton of diastereotopic protons in CH2), 2.35
(d, J = 16.5 Hz, 1H, one proton of diastereotopic protons
in CH2), 2.65 (ABq, Δν = 30.0 Hz, JAB = 17.3 Hz, 2H,
CH2), 3.64 (s, 3H, OCH3), 5.54 (s, 1H, pyran CH), 6.76
(d, J = 8.7 Hz, 2H, arom‐H), 7.20 (d, J = 8.7 Hz, 2H,
arom‐H), 7.41–7.54 (m, 3H, arom‐H), 7.90–7.95 (m, 2H,
arom‐H), 8.05 (d, J = 8.3 Hz, 1H, arom‐H); IR (KBr,
cm−1) υ = 2951, 1648, 1511, 1460, 1378, 1229, 1174,
1028, 832, 750.
ACKNOWLEDGEMENTS
This work was supported by Islamic Azad University,
Mashhad Branch (Iran) and Iran National Science
Foundation.
ORCID
Abolghasem Davoodnia https://orcid.org/0000-0002-1425-
8577
Amir Khojastehnezhad
https://orcid.org/0000-0002-9078-
7323
3.6.5 | 12‐(4‐Hydroxyphenyl)‐9,9‐dimethyl‐
8,9,10,12‐tetrahydrobenzo[a]xanthen‐11‐one
(4 l)
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