Cu(II)–metformin immobilized on graphene oxide: an efficient and recyclable catalyst for the Beckmann rearrangement

Article abstract of DOI:10.1007/s11164-019-03985-z

Abstract: In this study, for the first time, the copper(II) nanoparticles (NPs) have been immobilized on metformin-functionalized graphene oxide and then its catalytic applications have been investigated in synthesis of amides from aldoximes (Beckmann rearrangement). The chemical structure of prepared catalyst has been characterized by various analyses like FT-IR, TGA, TEM, SEM, EDX, and ICP. All analyses confirm the successful and stable immobilization of copper NPs on functionalized graphene oxide. This synthesized heterogeneous nanocatalyst showed excellent catalytic activity with high product yields and short reaction times. Also, the suggested catalyst could be recycled ten times without a drastic decrease in its catalytic activity. Graphic abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s11164-019-03985-z

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03985-z
Cu(II)–metformin immobilized on graphene oxide:
an efcient and recyclable catalyst for the Beckmann
rearrangement
Ahmed Solaiman Hamed1 · Ehab Mohammad Ali1
Received: 13 July 2019 / Accepted: 13 September 2019
© Springer Nature B.V. 2019
Abstract
In this study, for the frst time, the copper(II) nanoparticles (NPs) have been immo-
bilized on metformin-functionalized graphene oxide and then its catalytic applica-
tions have been investigated in synthesis of amides from aldoximes (Beckmann rear-
rangement). The chemical structure of prepared catalyst has been characterized by
various analyses like FT-IR, TGA, TEM, SEM, EDX, and ICP. All analyses confrm
the successful and stable immobilization of copper NPs on functionalized graphene
oxide. This synthesized heterogeneous nanocatalyst showed excellent catalytic activ-
ity with high product yields and short reaction times. Also, the suggested catalyst
could be recycled ten times without a drastic decrease in its catalytic activity.
Graphic abstract
Keywords Copper · Graphene oxide · Amides · Oximes · Beckmann rearrangement
Extended author information available on the last page of the article
Vol.:(0123456789)
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A. Solaiman Hamed, E. Mohammad Ali
Introduction
Graphene is an important allotrope of carbon including a single layer of carbon atoms
which is interconnected in a hexagonal structure [1–4]. Graphene is one of the strong-
est materials in conducting electricity and heat and has a transparent structure with one
atom thick layer [5–8]. The oxide form of graphene is named graphene oxide (GO),
which is prepared by the oxidation of graphite and has many applications [9–11].
Hydroxyl, epoxide, and carboxyl groups are three special functional groups on basal
planes and edges of GO. Due to the existence of these functional groups, the immobili-
zation of organic ligands, and consequently, metalation is available [12–15].
The conversion of oximes to corresponding amides has been carried out by vari-
ous methods, but the most important one is Beckmann rearrangement [16]. This reac-
tion provides an efcient method for the preparation of ε-caprolactam, which is a key
intermediate in the manufacture of fbers and resins [17]. The reaction was performed
by diferent methods and conditions [18–23]. Recently, the use of metals is a favorable
strategy to achieve primary amide transformations [24]. Asencio et al. [25] studied the
application of copper acetate in the synthesis of amides from aldoximes. Their pro-
tocol fulflled the points of green chemistry, including did not apply organic solvents
and poison reagents. In 2011, Saidulu et al. [26] supported copper on silica (SBA-15)
and investigated its catalytic activity for the rearrangement of benzaldoxime into ben-
zamide. This method was carried out in solvent- and acid-free conditions. Mahajan
et al. [27] directly converted ketones to amides via Beckmann rearrangement cata-
lyzed by FeCl3·6H2O in the presence of NH2OH·HCl. Their catalyst was inexpensive,
eco-friendly, easy to handle, and stable. Kumari et al. [28] used the complex of chi-
tosan–zink for the synthesis of primary amides. They believed that the catalyst can be
separated from the reaction mixture easily and reused several times without consider-
able loss of its catalytic activity. Despite the applying of these reported methods for the
conversion of oximes to amides, the mentioned studies have some disadvantages such
as long reaction time, use of microwave power and high temperature, lack of general-
ity, and use non-environmentally friendly conditions. Thus, to succeed in dealing with
these disadvantages, the introduction of new methods and metal complex is essential.
Since homogenous systems cannot be recycled and cause contamination of the prod-
uct [29, 30], this problem can be overcome by the design of heterogeneous systems
[31–35]. Thus, in the present study, for the frst time, metformin has been covalently
immobilized on the surface of GO, and then, copper acetate was coordinated into the
amine groups of metformin and the Cu(II) coordinated on metformin linked to gra-
phene oxide (GO–Met–Cu) was prepared (Scheme 1). After full characterization by
diferent analyses, the synthesized nanocatalyst was applied for the conversion of dif-
ferent aldoximes to primary amides in water at 100 °C (Scheme 2).
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Cu(II)–metformin immobilized on graphene oxide: an efcient…
Scheme 1 Preparation of catalyst (GO–Met–Cu)
Scheme 2 Synthesis of primary amides from aromatic aldehydes in presence of GO–Met–Cu
Results and discussion
Synthesis and characterization of the GO–Met–Cu
The nano Cu complex catalyst was prepared according to Scheme 1. In the frst step,
the metformin ligand was anchored covalently on the surface of synthesized acylated
GO by the nucleophilic reaction of amino groups of metformin and carboxyl groups
of GO. Then, copper was complexed into the amino ligands of metformin and the
catalyst (GO–Met–Cu) was totally prepared. The chemical structure of catalyst was
analyzed using diferent techniques such as FT-IR, SEM, EDX, TEM, ICP, and
TGA.
TEM and SEM analyses
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM)
analyses were applied for deep understanding of the morphology, size, and structure
of prepared nanocatalyst (Fig. 1). SEM photograph of GO–Met–Cu shows that the
GO nanosheets have a uniform surface, including sheet-like structures from one µm
to few microns in the lateral dimension (Fig. 1a). TEM image also demonstrates the
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A. Solaiman Hamed, E. Mohammad Ali
Fig. 1 a SEM and b TEM images of GO–Met–Cu
same results and shows the sheet-like monolayer structures of GO with relatively
transparent view (Fig. 2a). These photographs also confrm the successful linkage
of organic and inorganic ligands on GO [36, 37]. Moreover, analysis of energy-dis-
persive X-ray analysis (EDX) of GO–Met–Cu (Fig. 2) approved the successful car-
bon nanotubes functionalization. The EDX spectrum clearly shows the presence of
carbon, nitrogen, oxygen, chlorine, and copper elements, approving the successful
CuCl2 anchoring on the surface of GO–Met [38].
FT‑IR analysis
The FT-IR spectrum of all steps of the catalyst preparation including GO, GO
with organic ligand (GO–Met), and GO with organic complex and inorganic com-
plex (GO–Met–Cu) are shown in Fig. 3. In the FT-IR spectrum of GO (Fig. 3a),
the peaks appeared at 1715 cm−1, 1590 cm−1, 1222 cm−1, and 1040 cm−1 are
Fig. 2 EDX pattern of GO–Met–Cu
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Cu(II)–metformin immobilized on graphene oxide: an efcient…
Fig. 3 FT-IR spectra of (a) GO, (b) GO–Met, and (c) GO–Met–Cu
correspond to C=O, C=C, C–O–C, and –OH groups, respectively, and as well as,
the broad peak at 3400 cm−1 attributed to the O–H stretching bands of hydroxy and
carboxylic acid sections [39]. The second spectrum (Fig. 3b) is related to immo-
bilization of the organic ligand (metformin) on the surface of GO. The sharp band
appeared at around 3390 cm−1 ascribed to the stretching vibrations of amino groups
(N–H) of the metformin molecule. Plus, the peak positioned at around 2940 cm−1 is
assigned to C–H starching of methyl groups on the metformin structure. Moreover,
the peaks located at 1635 and 1528 cm−1 can be attributed to the stretching vibra-
tion of carbonyl (C=O) and amine (N–H) groups of amide structures. Thus, these
bands approve that the metformin ligand is successfully immobilized to the surface
of the GO sheets through the amidation reaction [36, 37]. In the third spectrum of
fnal catalyst (Fig. 3c), the peaks correspond to C=O and N–H groups shifted to
1628–1522 cm−1 because of the coordination of copper to the amine groups of met-
formin molecule. This evidence confrms the proper coordination of Cu ion to the
metformin ligands.
TGA analysis
Thermogravimetric analysis (TGA) is a way of thermal analysis in that the weight
of a sample is analyzed over time during the temperature changes. This technique
is applied to estimate the functionalization percentage of GO surfaces. As shown
in Fig. 4, two main weight losses are observed. The frst one about 20% begins
at 44 °C and continues to about 530 °C is assigned to the decomposition of oxy-
genic functional groups (epoxy, hydroxy, and carboxyl) on GO and organic ligands
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A. Solaiman Hamed, E. Mohammad Ali
Fig. 4 TGA diagram of GO–Met–Cu
(metformin). Another weight loss (35%) between 530 and 600 °C is attributed to the
decomposition of GO nanosheets. According to the obtained results of TGA analy-
sis, ligand grafting has been successfully accomplished.
ICP‑OES analysis
One of the best and excellent techniques for estimating metal amount in catalyst
structure is inductively coupled plasma (ICP) analysis. Therefore, in order to fnd
the exact amount of the copper content in the prepared catalyst, the ICP technique
has been applied. Based on ICP result, the amount of copper ion complexed into the
metformin ligand is 4.05%, which is another evidence for the successful grafting of
Met–Cu over the surface and the edge of GO nanosheets.
Catalytic application of GO–Met–Cu
The application of the prepared catalyst has been investigated in the synthesis of
primary amides via Beckmann rearrangement. In order to fnd the optimum reaction
conditions, the conversion of benzaldoxime to benzamide was selected as a model
reaction. The reaction did not proceed in the absence of catalyst in solvent-free con-
ditions even after 5 h (entry 1). Then, diferent polar and nonpolar solvents in pres-
ence of 25 mg of catalyst were examined (entries 2–9). The yield of the reaction
was better in polar solvents like methanol, ethanol, and water (entries 7–9), and the
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Cu(II)–metformin immobilized on graphene oxide: an efcient…
best one was water as a green solvent (entry 9). With increasing the temperature to
120 °C, the yield of the reaction was not changed (entry 12), while decreasing the
temperature from refux to room temperature, decreased the yield of the reaction
to 32% (entries 13, 14). Moreover, the amount of catalyst has been optimized. The
yield of the reaction was decreased to 83% with lower amount of catalyst (15 mg,
0.96 mol%) (entry 15) and using of higher amount of catalyst (30 mg, 1.92 mol%)
did not infuence on the yield (entry 16). Thus, the optimized reaction conditions
were using 25 mg (1.6 mol%) of catalyst at 100 °C in water as a green solvent (entry
11). Moreover, to fnd the efect of copper, other parts of catalyst (GO, GO–Met,
Met), as well as copper acetate on the reaction, the role of them was also investi-
gated in the model reaction under the optimized reaction conditions (entries 17–20).
The yield of the reaction in the presence of bare GO and GO linked to metformin
(GO–Met) was very low (entries 17, 19), and the reaction was failed in the presence
of only metformin as an organic ligand (Met) (entry 18), while the copper acetate
can catalyze the reaction with good yield (entry 20) (Table 1).
After fnding the optimum reaction conditions (amount of catalyst, temperature,
and solvent), the scope of the reaction was examined in a vast range of diferently
substituted aromatic aldoximes in presence of GO–Met–Cu catalyst (Table 2).
Table 1 Optimization of solvent, temperature, and amount of catalyst for the synthesis of 3a
Entry
Conditions
Catalyst (mg, mol%)
Temperature Time (min)
(°C)
Yield (%)a
1
2
3
4
5
6
7
8
Solvent free
Solvent free
n-Hexane
CHCl3
CH2Cl2
MeCN
MeOH
EtOH
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
–
100
100
68
61
40
82
65
78
100
100
100
120
50
300
180
180
180
180
180
120
120
120
60
30
30
30
30
60
60
60
60
–
GO–Met–Cu (25, 1.6)
GO–Met–Cu (25, 1.6)
GO–Met–Cu (25, 1.6)
GO–Met–Cu (25, 1.6)
GO–Met–Cu (25, 1.6)
GO–Met–Cu (25, 1.6)
GO–Met–Cu (25, 1.6)
GO–Met–Cu (25, 1.6)
GO–Met–Cu (25, 1.6)
GO–Met–Cu (25, 1.6)
GO–Met–Cu (25, 1.6)
GO–Met–Cu (25, 1.6)
GO–Met–Cu (25, 1.6)
GO–Met–Cu (15, 0.96)
GO–Met–Cu (30, 1.92)
GO (25)
Trace
–
12
15
50
63
79
95
95
95
95
60
32
83
96
Trace
–
Trace
9
10
11
12
13
14
15
16
17
18
19
20
rt
100
100
100
100
100
100
Met (25)
GO–Met (25)
Cu(OAC)2 (25)
60
60
86
aBased on isolated yield
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A. Solaiman Hamed, E. Mohammad Ali
Table 2 Synthesis of primary amides derivatives from aldoximes in presence of GO–Met–Cu
Entry
R
Producta
Time (min)
Yields (%)b
Mp °C
Found
Reported
1
H
30
95
125–127
128–130 [41]
3a
2
3
4-Cl
3-Cl
45
55
96
90
170–172
175–178 [42]
132–134 [42]
3b
130–131
3c
4
5
6
2-Cl
3-Br
4-Br
60
45
35
94
90
95
137–139
156–157
190–192
139–141 [42]
155–157 [43]
191–192 [43]
3d
3e
3f
7
4-CF3
50
94
182–184
182–184 [42]
3g
8
9
4-CN
3-OH
30
55
96
92
223–225
169–171
222–224 [44]
170–171 [45]
3h
3i
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Cu(II)–metformin immobilized on graphene oxide: an efcient…
Table 2 (continued)
Entry
R
Producta
Time (min)
Yields (%)b
90
Mp °C
Found
Reported
10
4-MeO
60
166–168
165–167 [42]
3j
11
12
3,4-(Cl)2
3-CH3
45
60
93
92
137–138
95–96
138–139 [40]
94–95 [43]
3k
3l
aReaction condition: aldoxime (1.0 mmol), GO–Met–Cu (25 mg, 1.6 mol%), and water at 100 °C
bIsolated yields
The experimental fndings showed that the present catalyst demonstrated high ef-
ciency for both electron-donating and electron-withdrawing substituted aldoximes,
although the substrate with electron-withdrawing groups in para position exhibited
better yields than electron-donating groups (products 3b, 3f, and 3h in comparison
with 3j). All obtained products were known compounds and were identifed by the
comparison of their melting points with those synthesized in the previous literature.
Due to the heterogeneity of the catalyst, after completion of the reaction, it can
be separated from reaction mixture and reused. Thus, the reusability of the catalyst
was analyzed by the model reaction under the optimized reaction conditions. After
the completion of the frst reaction, the nanoheterogeneous catalyst (GO–Met–Cu)
was separated from the reaction mixture by centrifuging (8000 rpm for 15 min), and
then, it was washed with two solvents (water and acetone) and dried at 80 °C over
night before reusing in the second run. The catalyst can be recycled ten times with
only a little reduction in efciency (Fig. 5).
To fnd the better understanding of catalyst behavior, the complete mechanism for
the synthesis of primary amides derivatives from aromatic aldoximes is suggested
according to the literature [40]. Based on Scheme 3, the reaction starts with coordi-
nation of copper complex catalyst to aldoxime and production of metal–nitrile inter-
mediate by dehydration of aldoxime. Next, the second aldoxime is attacked to the
metal–nitrile intermediate produced in the previous step and fnally by removing one
nitrile molecule, the fnal primary amide is formed.
The hot fltration test has also been analyzed on the model reaction under
the optimized conditions in order to test catalyst leaching. So, a mixture of
GO–Met–Cu (25 mg), benzaldoxime (1.0 mmol) and water (5.0 mL) was
1 3

A. Solaiman Hamed, E. Mohammad Ali
Fig. 5 Reusability of catalyst after ten runs
Scheme 3 A proposed mechanism for conversion of aromatic aldoximes to primary amides in presence
of catalyst (GO–Met–Cu)
refuxed for 15 min. After approximately 50% conversion, the catalyst was sepa-
rated from the reaction mixture and then the reaction was continued for further
time (15 min). No product was observed which was monitored by TLC. This
result vividly demonstrates that no leaching of copper and copper–metformin
took place during the reaction and confrm the stability of catalyst structure.
The merit practical applications of the present study were compared with
other heterogeneous systems for the synthesis of benzamide 3a from benzal-
doxime (Table 3). According to Table 3, our prepared catalyst shows excellent
catalytic activity in shorter reaction time, higher isolated yields, and milder
conditions.
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Cu(II)–metformin immobilized on graphene oxide: an efcient…
Table 3 The comparison of GO–Met–Cu with other heterogeneous catalysts in synthesis of 3a
Entry
Catalyst (mol%)
Solvent
Tempera- Time (h)
ture (°C)
Yield (%)a
References
1
2
3
4
5
6
7
Au complex (5.0)
ZnCl2 or Zn(NO3)2 (10)
Pd(OAc)2 (5.0)
Ru complex (0.5)
Rh(OH)/Al2O3 (4.0)
Cu(OAc)2 (10)
Solvent free
Heptane
DMSO
H2O
H2O
Dioxane
100
100
100
110
160
110
100
20
18
15
10
7.0
3.0b
0.5
92
88
98
97
90
99
95
[46]
[47]
[48]
[49]
[50]
[25]
This work
GO–Met–Cu (1.6)
H2O
aBased on isolated yield
bDay
Experimental
Chemicals and apparatus
Melting points of products were recorded with an Electrothermal type 9100
melting point apparatus. Fourier transform infrared (FT-IR) spectra were
recorded with a Nicolet Avatar 370 FT-IR Therma spectrometer. Transmis-
sion electron microscopy (TEM) was performed with a Leo 912AB (120 kV)
microscope (Zeiss, Germany). Scanning electron microscopy (SEM) analysis
was done using a MIRA3 TESCAN scanning electron microscope operated at an
accelerating voltage of 30 kV. Energy-dispersive X-ray (EDX) analysis was per-
formed using a SAMX instrument. Inductively coupled plasma (ICP) analysis
was carried out with a Varian VISTA-PRO, CCD (Australia). Thermogravimet-
ric analysis (TGA) was performed with a Shimadzu thermogravimetric analyzer
(TG-50) under air atmosphere at a heating rate of 10 °C min−1.
Preparation of GO–Met
The GO and acylated GO (GO–Cl) have been prepared according to literature
[39]. Then, metformin was covalently immobilized on GO nanosheets via con-
densation reaction of metformin and GO nanosheets [36]. For this purpose, GO
(50 mg) was added to a solution of DMF (50 mL) and ultrasonicated for 1 h.
After that, the sonicated solution was cooled to 0 °C, and NHS (5 mmol) and
EDC (5 mmol) were added. The reaction mixture was stirred for 2 h, and then,
metformin (5 mmol) was added to the solution and the temperature was raised
to 100 °C and stirred for another 24 h. The prepared GO–Met was collected by
centrifuging at 8000 rpm for 20 min and dried in vacuum at 50 °C.
1 3

A. Solaiman Hamed, E. Mohammad Ali
Preparation of GO–Met–Cu
The complex of copper and GO–Met was prepared by adding copper acetate (1.0 g)
into the solution of GO–Met (1.0 g) and DMF (80 mL). For the complete metala-
tion, the reaction mixture was refuxed for 6 h. The obtained GO–Met–Cu was col-
lected by centrifuging at 8000 rpm for 20 min. Finally, 20 ml of water was added
and the mixture was centrifuged at 8000 rpm for another 20 min and the Cu-coordi-
nated metformin was obtained.
General procedure for the synthesis of aldoximes
Hydroxylamine hydrochloride (0.3 g) was added to a stirred mixture of ZnO (0.15 g)
and aldehydes (1.0 mmol) at 80 °C in an oil bath. The progress of the reaction was
monitored by TLC. After the complete disappearance of the reactants, the mixture
was washed with dichloromethane (3×10 mL) and water (3×30 mL). The organic
layer was dried by anhydrous sodium sulfate and evaporated to aford the corre-
sponding aldoximes [40].
General procedure for the synthesis of primary amides derivatives 3a–l
To a mixture of GO–Met–Cu (25 mg) in water (15 mL), aldoximes derivatives
(1.0 mmol) was added and stirred at 100 °C for times mentioned in Table 2. After
completion of the reaction, as indicated by TLC, the catalyst was separated by cen-
trifuging (8000 rpm for 15 min) and after evaporated the solvent, the products 3a–l
were achieved with high purifcation. All products were characterized by melting
points (Table 2).
Conclusions
In summary, for the frst time, the heterogeneous copper complex immobilized on
GO has been applied as a novel and green catalyst (GO–Met–Cu) for the conversion
of aldoximes to primary amides via Beckmann rearrangement. Diferent techniques
(FT-IR, SEM, EDX, TEM, TGA, and ICP) were used to characterize the catalyst
structure. All analyses approved that the Cu–metformin was successfully immo-
bilized on the surface of GO as well as the presence of copper in catalyst body.
Moreover, our catalyst showed excellent catalytic activity in shorter reaction time,
higher isolated yields, and milder reaction conditions in comparison with the previ-
ous studies.
Acknowledgements The authors gratefully acknowledge the partial support of this study by the Univer-
sity of Fallujah.
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Cu(II)–metformin immobilized on graphene oxide: an efcient…
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Ahmed Solaiman Hamed1 · Ehab Mohammad Ali1
*
Ahmed Solaiman Hamed
dr.ahmedalmohammedi@uofallujah.edu.iq
1
College of Applied Sciences, University of Fallujah, Fallujah, Iraq
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