Green synthesis of CuO nanoflakes from copper pincer complex for effective N-arylation of benzimidazole

Article abstract of DOI:10.1016/j.catcom.2015.11.018

Nanostructured CuO is synthesized in water using copper pincer complex as precursor without any stabilizing agent and characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-

Full text of DOI:10.1016/j.catcom.2015.11.018

Green synthesis of CuO nanoflakes from copper pincer complex for effective
N-arylation of benzimidazole
P. Jerome, G. Kausalya, T. Daniel Thangadurai, R. Karvembu
PII:
DOI:
Reference:
S1566-7367(15)30152-7
doi: 10.1016/j.catcom.2015.11.018
CATCOM 4514
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Catalysis Communications
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15 October 2015
25 November 2015
27 November 2015
Please cite this article as: P. Jerome, G. Kausalya, T. Daniel Thangadurai, R. Karvembu,
Green synthesis of CuO nanoflakes from copper pincer complex for effective N-arylation
of benzimidazole, Catalysis Communications (2015), doi: 10.1016/j.catcom.2015.11.018
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ACCEPTED MANUSCRIPT
Green synthesis of CuO nanoflakes from copper pincer complex for effective N-arylation of
benzimidazole
P. Jerome,1 G. Kausalya,2 T. Daniel Thangadurai,2,* R. Karvembu,1,*
1Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India
2Department of Nanoscience and Technology, Sri Ramakrishna Engineering College,
Coimbatore 641 022, India
ABSTRACT
Nanostructured CuO is synthesized in water using copper pincer complex as precursor
without any stabilizing agent and characterized by X-ray diffraction (XRD), field emission
scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray
photoelectron spectroscopy (XPS) and energy dispersive X-ray analysis (EDAX). The selected
area electron diffraction (SAED) and magnetization measurements indicate lattice fringes
spacing of 0.25 nm for (111) plane and ferromagnetic nature of the CuO nanoflakes,
respectively. Benzimidazole undergoes N-arylation reaction with aryl halides in the presence of
CuO nanoflakes and K2CO3 in N,N-dimethylacetamide (DMAc) at 120 C. The reusability of the
CuO nanoflakes is also tested and the results are found to be good.
Keywords: Copper pincer complex; CuO nanoflakes; Catalytic studies; N-arylation
*Corresponding author.
E-mail address: danielt@srec.ac.in (T. Daniel Thangadurai), kar@nit.edu (R. Karvembu)
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1. Introduction
Metal oxide nanomaterials have attracted great interest because of many unique properties
linked to the nanometer size of the particles [1-3]. The oxides of transition metal elements are an
imperative class of semiconductors, which have applications in solar energy transformation,
magnetic storage media, electronics, catalysis, etc [4-6]. Among the metal oxides, CuO
nanomaterials are of special interest because of their versatility in catalysis [7-9].
N-arylation of heterocycles is an important reaction in organic synthesis due to the
application of N-arylated products in various fields such as pharmaceuticals, natural products
synthesis, materials chemistry, etc [10-15]. To be specific, N-arylated benzimidazoles such as
MRL-1237 and Telmisartan have been used as drug molecules, and 2,2',2"-(1,3,5-benzinetriyl)-
tris(1-phenyl-1-H-benzimidazole) was found to be an useful OLED material [16]. Many
homogenous Cu catalysts have been reported for N-arylation of heterocycles [17]. Even though
the homogeneous systems are active, they could not be commercialized due to the difficulty in
the separation of catalyst and reusability. Hence may researchers have focused on the
development of heterogeneous catalysts for the N-arylation reaction [7]. Among the various
heterogeneous catalysts, nano CuO was found to be efficient and highly reusable [18]. CuO
nanostructured materials have been prepared from various sources like CuCl25H2O, CuI,
Cu(NO3)2, etc. [19-22]. To the best of our knowledge, no one has utilized Cu pincer complex as
precursor for producing CuO nanomaterials. Cu pincer complexes have found importance mainly
in homogeneous catalysis [23-24]. We have used Cu pincer complexes as a precursor for the
preparation of nano CuO and studied its catalytic application towards N-arylation of
benzimidazole under heterogeneous conditions. Nano CuO prepared from Cu pincer complex
was found to possess a flake structure and exhibited very good catalytic efficiency and
reusability in aforesaid reaction. CuO with different morphologies have been used as catalysts in
various organic transformations but the use of nanoflake CuO in catalysis is rare [25].
2. Experimental
2.1. Synthesis of CuO
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Cu pincer complex (50 mg) was synthesized by following a literature procedure [26-27] and
then it was dissolved in 20 mL of distilled water under reflux condition. To the above solution
K2CO3 (40 mg) was added and the resulting mixture (pH = 6-7) was refluxed for 45 min. Then,
the solution was cooled to room temperature and the solid product was separated by
centrifugation, washed with distilled water and dried in hot air oven at 110 C.
2.2. Procedure for N-arylation of benzimidazole
CuO (15 mg, 5 mol %) was added to a mixture of aryl halide (1 mmole), benzimidazole (142
mg, 1.2 mmole) and K2CO3 (276 mg, 2 mmole) in N,N-dimethyl acetamide (DMAc) (3 mL) and
stirred at 120 C. After the required time, the reaction mixture was cooled to room temperature
and the catalyst was separated by centrifugation. The product was extracted with ethyl acetate,
dried over anhydrous sodium sulphate and analyzed by GC. The recovered catalyst was washed
thoroughly with water, ethanol and toluene, and dried under vacuum before reuse.
3. Results and discussion
3.1. Catalyst characterization
The X-ray diffraction pattern of the prepared material (Fig. 1) is in good agreement with that
of reported CuO [16], which confirms the formation of CuO. The sharp and intense peaks
indicated that the CuO is pure, crystalline and well arranged in specific orientation. The XRD
reflections for CuO are observed at 2θ value of 32.17°, 35.37°, 38.80°, 48.87°, 58.36°, 61.57°
and 66.04° representing (110), ( 11), (111), ( 02), (202), ( 13) and ( 11) planes of pure
monoclinic structure (JCPDS No. 89-5895). The ( 11) peak is more intense than (111) peak,
which confirmed that the growth is more favorable in the ( 11) direction than (111) direction.
To ensure the morphological difference of newly prepared CuO in comparison with existing
reports, the FESEM and TEM images were recorded for the CuO (Fig. 2 & 3). The FESEM
images portrayed flake morphology of the CuO (Fig. 2a & 2b). The TEM images further
confirmed the flake morphology for the CuO with average size of 300-400 nm (Fig. 3a & 3b).
The CuO flakes were analyzed by Selected Area Electron Diffraction (SAED) and HRTEM
(inset of Fig. 3a) methods. The appearance of stretched out diffraction spots in the SAED pattern
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of the CuO indicated the single crystalline nature which can be indexed to the phase pure
monoclinic structure. The lattice analyses of HRTEM image confirmed the single crystalline
nature and nano dimension of the CuO flakes with a lattice fringes spacing of 0.25 nm for ( 11)
plane (inset of Fig. 3b). The SAED pattern spots the diffraction points to obtain hkl planes. The
single crystalline nature was confirmed by ( 11), (110), ( 11) and ( 02) planes and the d
spacing. The Cu and oxygen elements were observed in the EDAX spectrum of the CuO
nanoflakes (Fig. S1). There is no evidence for the presence of impurities.
The XPS spectrum of the CuO shows peaks for Cu2p, Cu3p, C1s, O1s and the Auger lines
(LMM) (Fig. S2c). High resolution spectrum of Cu2p shows four peaks at ca. 940, 949, 960 and
968 eV. The doublets at 940 and 960 eV are assigned to binding energy of Cu2p3/2 and Cu2p1/2,
respectively, with spin-orbit splitting of 8.8 eV (Fig. S2a & S2b). The other two shoulder peaks
(satellite peaks) identified at 949 and 968 eV are possibly due to unstoichiometric cupric oxide
(Cu1−δO). The satellite peaks were ~9 eV above the main Cu2p peaks and the main Cu2p peaks
indicate the formation of CuO [28].
The magnetization property as a function of magnetic field for the CuO nanoflakes was
calculated using a VSM at 25 °C (Fig. S3). The M-H loop analysis indicated less coercivity
(16.35 Oe), magnetic saturation (3.96 emu/g) and remanence (0.426 emu/g) values which
confirmed the ferromagnetic nature of the CuO nanoflakes.
During DSC analysis, the CuO nanoflakes (7.9 mg) were heated under open atmosphere at 10
°C/min rate up to 200 °C. The result revealed the endothermic decomposition nature of the CuO
nanoflakes, which includes the thermal decomposition of water molecules present on the surface
of the material without any phase transition (Fig. S4a). In TGA analysis, the CuO nanoflakes
were first held at 100 °C for 30 minutes to remove excess moisture and then the temperature was
increased from 100 to 800 °C with a heating rate of 10 °C/min. The weight percentage versus
temperature graph is provided in Fig. S4b. There are two weight losses in the temperature ranges
175-225 and 500-600 °C. The total weight loss is only about ~2% and the final remained weight
is almost constant at even above 800 °C, which indicated the stability of the CuO nanoflakes
(Fig. S4b).
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3.2. Catalysis
In order to obtain persuasive results, the reaction variables such as solvent, base, amount
of substrates and catalyst quantity were optimized. For this purpose, 4-bromotoluene and
benzimidazole were chosen as model substrates. Among the four different solvents tested,
DMAc was found to be a better one (Table 1, entries 1-3 and 4). Similarly K2CO3 (2 mmole) was
identified as efficient base (Table 1, entries 2, 5-9 and 10). 5 mol % of CuO was sufficient for the
effective N-arylation of benzimidazole (Table 1, entries 2, 12 and 13). Less than 5 mol% of CuO
decreased the yield of the product. In the absence of the catalyst no reaction took place (Table 1,
entry 11). The optimum temperature was 120 C (Table 1, entries 2, 14-15 and 16). The
conversion of benzimidazole during its N-arylation reaction with 4-bromotoluene was 98% at
120 C in DMAc after 18 h (Table 1, entry 2). Extension of reaction time did not yield any
fruitful results (Table 1, entries 18 and 19).
The scope of the present catalytic system was extended by the reaction of benzimidazole with
various aryl halides (Table 2). The conversion is reasonably high (43 – 98 %) and comparable
with already reported catalytic systems for N-arylation of benzimidazole [16, 29]. Aryl iodides
gave good conversions compared to bromides and chlorides due to their better leaving ability
(Table 2, entries 2, 4-5, 8 and 12). As the electron withdrawing effect of the substituent at the
phenyl ring increases, the reactivity towards N-arylation also increases (Table 2, entries 7, 11 and
12). Presence of more electron releasing groups at the phenyl ring hindered the reaction rate
(Table 2, entries 1 and 3) as well as the conversion (Table 2, entries 3 and 9). Interestingly,
hetero aryl halides enhance the rate of the reaction (Table 2, entries 6 and 10). Selective coupling
of aryl halide with the secondary amine restricted the formation of self-coupled product (Table 2,
entry 9). We could reuse CuO nanoflakes for at least four times for N-arylation of benzimidazole
with 4-bromotoluene and results are shown in Fig. S5.
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Conclusions
The CuO nanoflakes was successfully synthesized using pincer copper complex in water
medium. This CuO nanoflakes was found to be a promising catalyst for the N-arylation of
benzimidazoles. This report opens a new avenue for utilizing pincer complexes as precursors for
the preparation of catalytically active nanomaterials.
Acknowledgement
P.J. and R.K. are thankful to DST-SERB for the financial support (Project No. SR/S1/IC-
72/2010). The authors acknowledge the Centre for Life Sciences, DRDO–BU, Coimbatore, for
SEM-EDAX analysis, SITRA, Coimbatore for FE-SEM analysis, PSG College of Technology,
Coimbatore, for TEM analysis, IICT-Hyderabad, for XPS and thermal analysis, DRDO-DMRL
for VSM data, and SNR Sons and Charitable Trust, Coimbatore, for XRD characterization.
Appendix A. Supplementary data
Supplementary data to this article can be found online
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Table 1
Optimization of reaction conditions for the CuO nanoparticles-catalyzed N-arylation of
benzimidazole with 4-bromotoluenea
Amount of Amount of
GC
Conversion
(%)
Temperature
Time
(h)
Entry Solventb
Base
base
catalyst
(mol %)
(C)
(mmole)
1
2
DMF
DMAc
THF
K2CO3
K2CO3
K2CO3
K2CO3
KOH
2
2
2
2
2
2
2
1
3
4
2
2
2
2
2
2
2
2
2
5
5
120
120
120
120
120
120
120
120
120
120
120
120
120
80
18
18
18
18
18
18
18
18
18
18
18
18
18
18
18
18
8
83
98
3
5
15
4
DMSO
DMAc
DMAc
5
83
5
5
31
6
K3PO4
5
6
7
DMAc (CH3)3COK
5
19
8
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
5
69
9
5
92
10
11
12
13
14
15
16
17
18
19
5
55
0
No reaction
2.5
10
5
31
97
Trace
12
5
100
140
120
120
120
5
23
5
43
5
24
48
97
5
96
a All the reactions were performed with 1.2 mmole (142 mg) of benzimidazole and 1 mmole
(171 mg) of 4-bromotoluene. b 3 mL of solvent was used in all the reactions. Bold letters indicate
optimized condition (entry 2)
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Table 2
CuO nanoflakes-catalyzed N-arylation of benzimidazole with various aryl halidesa
GC
Conversion
(%)b
Ar-X
(1b)
Product
(1c)
Time
(h)
Entry
1
18
98
2
3
4
18
24
24
94
80
55
5
24
43
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6
7
18
24
96
84
8
18
95
9
36
24
68
96
10
11
12
18
24
97
88
13
36
79
a All the reactions were performed with 1 mmole of ArX, 1.2 mmole (142 mg) of benzimidazole,
2 mmole (276 mg) of K2CO3 in 3 mL of DMAc at 120 C. b GC Conversion.
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References
[1]
[2]
G.R. Patzke, F. Krumeich, R. Nesper, Angew. Chem. Int. Ed. 41 (2002) 24462461.
Y.Y. Wu, H.Q. Yan, M. Huang, B. Messer, J.H. Song, P.D. Yang, Chem. Eur. J. 8 (2002)
12611268.
[3]
C.N.R. Rao, F.L. Deepak, G. Gundiah, A. Govindaraj, Prog. Solid State Chem. 31 (2003)
5147.
[4]
[5]
[6]
[7]
M. Frietsch, F. Zudock, J. Goschnick, M. Bruns, Sens. Actuators B 65 (2000) 379381.
P.C. Dai, H.A. Mook, G. Aeppli, S.M. Hayden, F. Dogan, Nature 406 (2000) 965.
J.F. Deng, Q. Sun, Y.L. Zhang, S.Y. Chen, D. Wu, Appl. Catal. A 139 (1996) 7585.
M. Gopiraman, S. Ganesh Babu, Z. Khatri, W. Kai, Y.A. Kim, M. Endo, R. Karvembu,
I.S. Kim, Carbon 62 (2013) 135–148.
[8]
[9]
S. Ganesh Babu, R. Karvembu, Catal. Surv. Asia 17 (2013) 156–176.
A. Brückner, Catal. Rev. 45 (2003) 97–150.
[10] T. Wiglenda, I. Ott, B. Kircher, P. Schumacher, D. Schuster, T. Langer, R. Gust, J. Med.
Chem. 48 (2005) 6516–6521.
[11] Z. Jin, Nat. Prod. Rep. 22 (2005) 196–229.
[12] L. De Luca, Curr. Med. Chem. 13 (2006) 1–23.
[13] I.V. Seregin, V. Gevorgyan, Chem. Soc. Rev. 36 (2007) 1173–1193.
[14] T. Wiglenda, R. Gust, J. Med. Chem. 50 (2007) 1475–1484.
[15] C. Kison, T. Opatz, Chem. Eur. J. 15 (2009) 843–845.
[16] S. Ganesh Babu, R. Karvembu, Ind. Eng. Chem. Res. 50 (2011) 9594–9600.
[17] C. Fischer, B. Koenig, Beilstein J. Org. Chem. 7 (2011) 59–74.
[18] S. Ganesh Babu, R. Karvembu, Tetrahedron Lett. 54 (2013) 1677–1680
10

ACCEPTED MANUSCRIPT
[19] K. Pericherla, A. Jha, B. Khungar, A. Kumar, Org. Lett. 15 (2013) 4304–4307.
[20] E.R. Strieter, B. Bhayana, S.L. Buchwald, J. Am. Chem. Soc. 131 (2009) 78–88.
[21] J.B. Lan, L. Chen, X.Q. Yu, J.S. You, R.G. Xie, Chem. Commun. (2004) 188–189.
[22] R.J. Phipps, L. McMurray, S. Ritter, H.A. Duong, M.J. Gaunt, J. Am. Chem. Soc. 134
(2012) 10773–10776.
[23] X. Liu, P. Braunstein, Inorg. Chem. 52 (2013) 7367–7379.
[24] J. Singleton, Tetrahedron 59 (2003) 1837–1857.
[25] Y. Ma, H. Li, R. Wang, H. Wang, W. Lv, S. Ji, J. Power Sources 289 (2015) 2225.
[26] S.C. Manna, E. Zangrando, N.R. Chaudhuri, J. Mol. Struct. 877 (2008) 145–151.
[27] D. Petrovic, T. Bannenberg, S. Randoll, P.G. Jones, M. Tamm, Dalton Trans. (2007)
2812–2822.
[28] J. Zhang, J. Liu, Q. Peng, X. Wang, Y. Li, Chem. Mater. 18 (2006) 867–871.
[29] R.A. Altman, E.D. Koval, S.L. Buchwald, J. Org. Chem. 72 (2007) 6190–6199.
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Scheme 1. Synthesis of CuO nanoflakes.
Fig. 1. XRD pattern of CuO at 25 °C.
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(a)
(b)
Fig. 2. FESEM images of CuO.
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(a)
(b)
Fig. 3. TEM images of CuO nanoflakes. (inset, a) SAED pattern of CuO, (inset, b) parallel
fringes with spacing.
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Graphical abstract
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Highlights
 CuO nanoflakes were prepared from Cu pincer complex in water medium
 CuO nanoflakes catalyze N-arylation of benzimidazole
 Catalyst is efficient and reusable
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