Two-dimensional copper-based metal-organic framework as a robust heterogeneous catalyst for the N-arylation of imidazole with arylboronic acids

Article abstract of DOI:10.1016/j.inoche.2012.10.035

N-arylation of imidazole was accomplished with a two-dimensional (2D) [Cu(ima)₂]_n metal-organic framework in methanol at room temperature. A variety of N-arylimidazoles were isolated in good yields after a short reaction time. Moreov

Full text of DOI:10.1016/j.inoche.2012.10.035

Inorganic Chemistry Communications 27 (2013) 119–121
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Feature article
Two-dimensional copper-based metal–organic framework as a robust heterogeneous
catalyst for the N-arylation of imidazole with arylboronic acids
⁎
⁎
Zhao-Hao Li, Li-Ping Xue , Lei Wang, Shuai-Tao Zhang, Bang-Tun Zhao
College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang, Henan, 471022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
N-arylation of imidazole was accomplished with a two-dimensional (2D) [Cu(ima)2]n metal–organic frame-
work in methanol at room temperature. A variety of N-arylimidazoles were isolated in good yields after a
short reaction time. Moreover, the robust MOF may be readily recovered after the reaction and reused for
many cycles without loss of catalytic activity.
Received 7 September 2012
Accepted 21 October 2012
Available online 2 November 2012
© 2012 Elsevier B.V. All rights reserved.
Keywords:
N-arylation
Imidazole
Arylboronic acids
Metal–organic framework
Contents
Acknowledgments
References .
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N-arylimidazoles are important organic compounds due to their
significant pharmaceutical, biological, and chemical activities [1–5].
During the past few years, homogeneous copper-catalyzed cross-
coupling reactions of imidazole and commercially widely-available
arylboronic acids are facile methods available for the construction of
N-arylimidazoles [6–10]. However, homogeneous processes easily
suffer from the problems concerning separation and recovery, dispos-
al of spent catalysts, and therefore their uses are greatly limited. In
principle, the use of heterogeneous catalysts could overcome some
of these drawbacks mentioned above. Thus, the practical applications
have greatly driven the need for the development of recyclable and
efficient heterogeneous copper catalysts. In this regard, heteroge-
neous copper catalysts exhibit particularly promising performances
for the reaction [11,12].
therefore all of the coordination sites around the metal centers are
occupied by organic ligands, leaving no free coordination positions
available for substrates in organic reaction. So far, only a few exam-
ples of MOFs containing coordinatively unsaturated metal ions for
heterogeneous catalysis have been investigated in comparison with
a large numbers of being discovered MOFs. Moreover, most works
focused on organic transformations such as Knoevenagel condensation,
cyanosilylation, oxidation, aldol condensation, Michael condensation,
Friedel–Crafts alkylation, and Biginelli reaction [18–29]. Indeed, reports
on heterogeneous N-arylation of imidazole using Cu-MOF catalysts are
very limited in the literature [12].
It is well known that the main drawbacks of MOFs as heteroge-
neous catalysts compared to traditional catalysts are their chemical
stabilities and lower reaction activities. Therefore, explore stable
and efficient heterogeneous catalysts become particularly desirable
for the improvement in catalytic performance. As a part of our ongo-
ing research on heterogeneous cross-coupling reactions [30], we re-
port the facile N-arylation of imidazole catalyzed by a robust two-
dimensional (2D) Cu-MOF catalyst (Fig. 1) at room temperature.
The catalyst exhibits high catalytic activity in a short reaction time
and may be readily recovered after the reaction and reused for
many cycles without loss of catalytic activity.
In recent years, metal–organic frameworks (MOFs) have received
a great deal of attention owing to their many potential applications in
gas storage, separation, magnetism, luminescence, catalysis, sensor
technology and optoelectronics [13–17]. However, the metal ions
are integral parts of the framework in many MOF structures, and
⁎
Corresponding authors. Tel./fax: +86 379 69819039.
E-mail addresses: lpxue@lynu.edu.cn (L.-P. Xue), zbt@lynu.edu.cn (B.-T. Zhao).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2012.10.035

120
Z.-H. Li et al. / Inorganic Chemistry Communications 27 (2013) 119–121
Fig. 1. Crystal structure of the 2D Cu-MOF exhibiting four-coordinate Cu2+ centers.
2D Cu-MOF catalyst was prepared using a modified literature
electron-rich substrates demonstrating higher isolated yields (Table 2,
entries 2 and 3). Slightly lower but acceptable yields were achieved
with ortho-substituted and electron-poor arylboronic acids (Table 2,
entries 4–6).
procedure and structural description of the Cu-MOF was outlined
elsewhere [31]. In brief, the local coordination environment around
the Cu2+ ion shows that each four-coordinate Cu2+ center is in a
square-planar geometry formed by two identical monodentate car-
boxylate groups and two imidazole nitrogen atoms with axial posi-
tions left uncoordinated. Each ima ligand uses both the carboxylate
oxygen and imidazole nitrogen atoms to bridge two Cu2+ ions, fur-
nishing a (4, 4) 2D wavelike rhombic layer. Adjacent 2D layers are
packed according to the ABAB sequence along the b-axis by weak
intermolecular C\H⋯O hydrogen bonding, which caused no open
channel or cavity in the 2D Cu-MOF. It should be noted, Cu2+ center
is in a distorted square-planar geometry and the axial positions
are vacant, so the exposed Cu2+ centers are chemically accessible.
Furthermore, the robust Cu-MOF is air-stable and possesses good
thermal stability up to 285 °C [30]. These interesting structural fea-
tures promoted us to study catalytic activity of the Cu-MOF. In initial
study, imidazole and phenylboronic acid were chosen as two model
substrates using the Cu-MOF as a catalyst in methanol at room tem-
perature. As shown in Table 1 [32], the 2D Cu-MOF afforded a very
good yield (Table 1, entry 1). For comparison, we also made a survey
of the effect of different homogeneous copper salts and found CuI,
CuBr, CuSO4·5H2O and Cu(OAc)2·2H2O gave very low yield under
identical conditions (Table 1, entries 2–5). Moreover, the controlled
N-arylation reaction conducted under identical conditions devoid of
Cu-MOF gave no coupled product despite prolonged reaction time
(Table 1, entry 6). In addition, the coupling reactions display substan-
tially reduced reaction times compared with literature reports (5 h in
contrast to 12–24 h) [3,12].
For practical application in the reaction, the lifetime of the MOF-
Cu catalyst and its reusability are very important factors. This catalyst
is not soluble in the reaction mixture and the reaction system is
heterogeneous unlike other homogeneous processes. After the reac-
tion, the Cu-MOF catalyst was easily recovered quantitatively from
the catalytic solution by simple filtration and reused several times
with consistent activity even after the fourth cycle (Table 2). The liq-
uid phase of the reaction mixture was collected by hot-filtration and
analyzed by ICP-AES. The absence of copper in the filtrate was con-
firmed (0.05 ppm), indicating no leaching of copper during the reac-
tion. Moreover, a filtration experiment was also run to investigate
whether the reaction proceeded in a heterogeneous or homogeneous
fashion. After 30 min of the N-arylation between imidazole and
phenylboronic acid, the solid catalyst was separated by filtration
and the filtrate was further performed for another 5 h and no further
conversion of the reactants into products was thus observed. These
results suggested that the reaction may take place on the catalyst
surface in a heterogeneous fashion.
To better verify the stability of the Cu-MOF catalyst, the powder
X-ray diffraction (PXRD) pattern of the recovered Cu-MOF catalyst
after the fifth recycling experiment was obtained. The pattern indicat-
ed that the framework was very robust and exhibited a very similar
diffraction pattern of the fresh catalyst (Fig. 2). In addition, we
employed the as-prepared Cu-MOF as a catalyst without any prior
treatment and the catalyst is quite stable in the solid state in air for
a prolonged period of time.
With these promising results, we subjected other substrates to the
reaction system in order to study the scope of the reaction (Table 2).
Similar results were obtained when different substituted phenylboronic
acids were used as arylating agent. It should be noted that electron-poor
to electron-rich arylboronic acids were tolerated, with the more
In summary, we have developed a simple and efficient heteroge-
neous 2D Cu-MOF catalyst system for the N-arylation of imidazole
under mild conditions, in which unsaturatedly coordinated Cu ions
Table 2
Recyclable N-arylation of imidazole with arylboronic acids.a
Table 1
Different copper catalysts catalyzed N-arylation of imidazole and phenylboronic acid.a
(Cycle) Yield (%)b
Entry
Catalyst
Time (h)
Yield (%)b
Entry
R
1
2
3
4
5
6
2D Cu-MOF
CuI
CuBr
CuSO4·5H2O
Cu(OAc)2·2H2O
None
5
5
5
5
5
86
25
27
14
17
0
1
2
3
4
5
6
H
(1) 90
(1) 92
(1) 90
(1) 76
(1) 88
(1) 78
(5) 89
(2) 89
(2) 91
(2) 71
(2) 85
(2) 67
4-CH3
4-OCH3
4-F
4-CF3
2-CH3
12
a
a
Reaction conditions: phenylboronic acid (1.0 mmol), imidazole (1.2 mmol), methanol
(3 mL), and catalyst (5 mol%), under air.
Reaction conditions: arylboronic acids (1.0 mmol), imidazole (1.2 mmol), and methanol
(3 mL), 5 h, under air.
b
b
Isolated yields.
Isolated yields.

Z.-H. Li et al. / Inorganic Chemistry Communications 27 (2013) 119–121
121
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(b) Synthetic procedure and characterization data for Cu-MOF catalyst. A mix-
ture of Sm2O3 (0.088 g, 0.25 mmol), CuSO4·5H2O (0.1248 g, 0.5 mmol),
and Hima (0.1250 g, 1 mmol) were added to deionized water (8.0 mL).
The mixture was added to a 23 mL Teflon-lined autoclave and kept under
autogenous pressure at 443 K for five days and then cooling to room temper-
ature at a rate of 5 Kh−1. Purple block-like crystals could be isolated in ca.
82% yield (based on Cu). Anal. calcd for C10H10CuN4O4: C, 38.3; H, 3.2; N,
17.8. Found: C, 39.0; H, 3.4; N, 18.6. IR data (KBr, cm−1): 3159vs, 3132vs,
2985m, 2948m, 1634vs, 1532m, 1358m, 1309vs, 1239m, 1218m, 1096vs,
1046m, 989m, 962m, 932m, 868m, 793m, 749vs, 701vs, 658m, 624m,
599m, 537m.
[32] Commercially arylboronic acids were provided by Adamas. Other reagents and
solvents were purchased and used without further purification or drying. A typ-
ical procedure for the arylation is as follows: to a solution of phenylboronic acid
(1 mmol) and imidazole (1.2 mmol) in methanol (3 mL) were added Cu-MOF
catalyst (15.7 mg, 5 mol%). The reaction mixture was stirred for 5 h at room tem-
perature under air. After the reaction was completed, the solid catalyst was sep-
arated by centrifuge, washed with methanol prior to being reused and the filtrate
was concentrated under reduced pressure to give the crude product. The crude
product was purified by column chromatography on silica gel to afford the
N-phenylimidazole. 1-Phenyl-1H-imidazole: 1H NMR (CDCl3, 400 MHz, ppm):
δ=7.23 (s, 1H), 7.29 (s, 1H), 7.35–7.43 (m, 3H), 7.45 (t, J=7.6 Hz, 2H), 7.88 (s,
1H).