A. R. Sardarian et al.
carbenes [10], a privileged class of ligands for transition-
metal catalysis [11], and for the synthesis of room tem-
perature ionic liquids, which are environmentally benign
solvents for organic synthesis [12].
functionalization, but also chemical and thermal stability to
these magnetic cores [34–39]. Therefore, among the vari-
ous magnetic core–shell structures, Fe O @SiO is a very
3
4
2
promising candidate.
Generally, N-arylimidazole compounds can be synthe-
sized via nucleophilic aromatic substitution of imidazoles
with aryl halides [13], which is limited to those aryl halides
bearing electron-withdrawing substituents or via transition-
metal catalyzed N-arylation of imidazoles [14], which has
been proven to be the most efficient and useful method for
the synthesis of N-arylimidazole derivatives. The tendency
to create inexpensive and environmentally friendly cat-
alytic systems led to develop copper-catalyzed N-arylation
of imidazoles with aryl halides, which was pioneered by
Buchwald [15]. Arylboronic acids [16], aryllead triacetates
Recently, a few examples of utilizing copper-based
magnetic nanocatalysts for imidazole N-arylation have
been reported, which, unfortunately, suffer from the need
of prolonged reaction times, excess base, high copper
loading, limited substrate scope, and/or protecting by an
inert gas [40–42].
Considering widespread applications of N-arylimidazole
derivatives and encouraged by the remarkable performance
of magnetic copper nanocatalysts [40–42], and Salen-
Cu(II) complexes (as inexpensive and air-/moisture-
stable catalysts) [43] in the N-arylation of imidazoles, we
became interested in developing a new, simple, and effi-
cient synthetic protocol for the N-arylation of imidazole
using salen complex of Cu(II) supported on superparam-
agnetic Fe O @SiO nanoparticles as illustrated in
[
[
17], hypervalent iodonium salts [18], and triarylbismuths
19] are the other types of reagents that have been
employed instead of aryl halides for Cu-mediated N-ary-
lation of imidazoles. However, the major drawbacks of
employing these kinds of aryl donors are the use of toxic,
less available, high cost, and/or unstable reagents that can
be difficult to access. For increasing the yields of the
products as well as decreasing time, temperature, toxicity,
and the cost of the imidazole N-arylation reaction, various
copper catalysts have been developed to date [20, 21].
Most of the utilized catalysts are homogeneous and, despite
their high activity, the difficulties associated with the
separation and recovery of them limit their use, especially
in pharmaceutical industry in which the final products must
be free of metal contamination [22–24]. Although hetero-
genization of the catalysts, by grafting them on a solid
support surface or trapping them inside the pores of the
support, makes them separable, reusable, and benign, it is
accepted that heterogeneous catalysts are less active than
homogeneous ones due to their less accessible active sites
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4
2
Scheme 1 to overcome above-mentioned drawbacks.
Results and discussion
The preparation steps of the Fe O @SiO /Salen-Cu(II)
3
4
2
nanocatalyst, as described in experimental section, have
been depicted in Fig. 1, briefly. Firstly, Fe O nanocore
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4
was prepared using Fe(II) and Fe(III) chloride salts, and
then coated by silica shell using TEOS as the silica source
to give Fe O @SiO core–shell structure [44].
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4
2
The Fe O , Fe O @SiO , and Fe O @SiO /Salen-
2
3
4
3
4
2
3
4
Cu(II) nanocatalyst were characterized by FT-IR, XRD,
FE-SEM, DLS, and ICP methods [44]. As can be seen in
Fig. 1, FE-SEM image of the catalyst shows the mor-
phology of the catalyst, and reveals the spherical-shaped
[
25]. With the development of the nanotechnology, creat-
ing nanosized heterogeneous catalysts, which possess the
recoverability of heterogeneous catalysts, together with the
high activity of the homogeneous ones (due to their large
surface-to-volume ratio), has become an attractive alter-
native [26–29]. However, the separation and recovery of
such catalysts by using conventional techniques (such as
centrifugation or filtration) are cumbersome because of
their nanometric size [30, 31]. To overcome this issue, the
nanocatalysts must be supported on insoluble magnetically
separable surfaces such as core–shell structures in which,
generally, iron oxides as magnetic cores have been coated
with organic or inorganic layers to prevent aggregation
phenomenon [32]. Of different iron oxides, Fe O is the
Scheme 1
3
4
most magnetic mineral in the nature and its biocompati-
bility has been proved [33, 34]. Also, of different iron
oxides coating materials, silica coating is widely reported
since the silica surfaces bestow not only facile
1
23