Intramolecular O-arylation using nano-magnetite supported N-heterocyclic carbene-copper complex with wingtip ferrocene

Article abstract of DOI:10.1002/aoc.5066

Nano-magnetite supported N-heterocyclic carbene-copper complex with wingtip ferrocene has been prepared via multi-step procedure. The complex has been characterized by various analytical techniques such as fourier transform infrared (FT-IR), fourier transform Raman (FT-Raman), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) analysis. The catalytic activity of the complex has been exploited in intramolecular O-arylation of o-iodoanilides under heterogeneous conditions. The complex could be successfully recycled up to twelve consecutive cycles.

Full text of DOI:10.1002/aoc.5066

Received: 26 December 2018
DOI: 10.1002/aoc.5066
Revised: 5 May 2019
Accepted: 8 May 2019
F U L L P A P E R
Intramolecular O‐arylation using nano‐magnetite supported
N‐heterocyclic carbene‐copper complex with wingtip
ferrocene
Altafhusen Naikwade | Megha Jagadale | Dolly Kale | Shivanand Gajare | Prakash Bansode |
Gajanan Rashinkar
Department of Chemistry, Shivaji
Nano‐magnetite supported N‐heterocyclic carbene‐copper complex with
University, Kolhapur 416004, M.S., India
wingtip ferrocene has been prepared via multi‐step procedure. The complex
Correspondence
Gajanan Rashinkar, Department of
has been characterized by various analytical techniques such as fourier trans-
form infrared (FT‐IR), fourier transform Raman (FT‐Raman), X‐ray photoelec-
Chemistry, Shivaji University, Kolhapur,
416004, M.S., India.
tron spectroscopy (XPS), X‐ray diffraction (XRD), transmission electron
Email: gsr_chem@unishivaji.ac.in
microscopy (TEM) and vibrating sample magnetometer (VSM) analysis. The
Funding information
catalytic activity of the complex has been exploited in intramolecular O‐
arylation of o‐iodoanilides under heterogeneous conditions. The complex could
be successfully recycled up to twelve consecutive cycles.
Science and Engineering Research Board,
Grant/Award Number: SB/FT/CS‐060/
2014
KEYWORDS
ferrocene, intramolecular O‐arylation, nano‐magnetite, N‐heterocyclic carbenes, reusability
1 | INTRODUCTION
accessible and can be installed by simple synthetic
manipulation in backbone of NHCs.[6] These unique
N‐Heterocyclic carbenes (NHCs) have emerged as a ver-
satile class of ligands with enormous applications.[1] The
selection of NHCs as best ligands can be rationalized by
their coordinative unsaturation, incomplete electron
octet, which promise comparatively weak π‐acceptor
property, strong σ‐donor capability, strong binding with
transition metals and sterically demanding structure.[2]
In addition, owing to flexible design of NHCs and ease
of convenience to their azolium salt precursors, tailor
made NHCs are readily offered.[3] Moreover, the steric
and electronic properties of NHCs can be systematically
fine‐tuned with the assistance of wingtip substituents on
the ligand backbone.[4] It is well recognized that tether-
ing organometallic fragment like ferrocene at wingtip
site in ligand backbone can have notable influence on
steric and spatial properties of NHCs.[5] Ferrocene, by
virtue of its unique cylindrical shape and powerful
electron donor capacity causes expedient stabilization
of NHCs. Additionally, ferrocene is non‐toxic, easily
properties of ferrocene have focused a quest towards
progress of new methods for synthesis of ferrocenyl
NHC‐transition metal complexes in the field of
catalysis.[7a–f]
In recent years, magnetic nanoparticles (MNPs) are
intensely researched in the catalysis arena, as
magnetic based nanocatalysts have been contributing
significantly to meet requirements of green chemistry
standpoints.[7g–o] The bridge between heterogeneous and
homogeneous catalysts can be accomplished through
the use of MNPs.[7p] Out of various MNPs, nano‐
magnetite finds a myriad of applications in the diverse
areas including catalysis,[8] bioengineering,[9a–d] drug
delivery,[9e] sensing,[9f] environmental treatment,[10]
bioseparation,[11] biomaterials[12] and food analysis.[13]
The ease of surface functionalization of nano‐magnetite
with desired precursors have provided significant versatil-
ity to design task specific catalytic systems.[14a] Amongst
various precursors, organosiloxane precursors have been
Appl Organometal Chem. 2019;e5066.
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https://doi.org/10.1002/aoc.5066

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NAIKWADE ET AL.
extensively employed for coating of silica on nano‐
magnetite which serves as a shell and hydroxyl‐rich
surface for high degree of organofunctionalization.[8p]
Owing to unique physicochemical properties such as
good dispersivity, high surface area, low toxicity,
superparamagnetic behavior and biocompatibility, nano‐
magnetite has emerged as promising support
material.[14b, c] Consequently, nano‐magnetite supported
catalytic systems have received significant attention in
synthetic chemistry.[15] In addition, facile retrievability
by external magnet have sparked the interest of scientific
community in the recognizing the goal of magnetically
separable catalysis and sought to become a promising
option for application in fixed‐bed technologies.[16]
Despite remarkable progress, research toward applica-
tions of nano‐magnetite in the synthesis of magnetically
retrievable NHCs is still in its infancy and therefore
warrants immediate attention.
2 | RESULTS AND DISCUSSION
The synthesis of nano‐magnetite supported NHC‐Cu
complex is outlined in Scheme 1. Initially, bare nano‐
magnetite (1) was prepared by conventional co‐
precipitation followed by coating with a silica layer using
sol–gel method to afford silica coated nano‐magnetite (2).
The tendency of surface Si‐OH groups of 2 to form stable
Si‐O‐Si bonds with alkoxy groups of (3‐chloropropyl)
triethoxy silane (3) allowed a facile synthesis of 3‐
chloropropyl modified nano‐magnetite (4) with high
degree
of
organofunctionalization.
The
1‐N‐
ferrocenylmethyl benzimidazole (5) was covalently teth-
ered on 4 via quaternization to afford heterogeneous
azolium salt abbreviated as [NMagFemBenz]Cl (6). The
complexation of 6 with copper iodide in the presence of
Benzoxazoles have acquired more importance in recent
years due to their many applications in medicinal
chemistry. They possess a wide range of pharmaceutical
properties
including
anticancer,[17]
antibiotic,[18]
antimycobacterial,[19] antiparasitic[20] and inhibitory activ-
ities.[21] In addition to these, they are used as key synthons
for the synthesis of bioactive molecules[22] and natural
products.[23] Moreover, they are renowned as a central
scaffold to build materials,[24] novel ligands[25] as well as
exploited in agrochemicals, metal sensors, engineering
plastics and as optical brightener for textiles.[24a, 26] Based
on their significance, the development of new methods
for the ecofriendly and facile synthesis of benzoxazoles is
highly anticipated. A plethora of strategies have been
reported for the synthesis of benzoxazoles, which
involve condensation of 2‐aminophenol with alde-
hydes,[27] transition metal‐mediated direct arylation of
bezoxazoles with aryl halides,[3b, 28] intramolecular cycli-
zation of o‐halobenzanilides[29] and ring‐opening‐cou-
pling‐recyclization of benzoxazoles with benzoyl chloride
or aromatic aldehydes.[30] Amongst these, transition metal
catalyzed intramolecular cyclization of o‐haloanilides rep-
resents most elegant protocol due to potential synthetic
utility, wide substrate scope and mild reaction conditions.
A large number of protocols for the synthesis of 2‐aryl
benzoxazoles via intramolecular cyclization have been
developed.[31] However, there is a still scope to develop
an efficient protocol especially using robust heterogeneous
catalyst.
Considering aforementioned discussion and in contin-
uation of our studies related to green chemistry,[32] we
report herein the preparation of nano‐magnetite
supported NHC‐copper complex with wingtip ferrocene
and its application as a heterogeneous catalyst in intra-
molecular O‐arylation of o‐iodoanilides.
SCHEME 1 Preparation of NMagFemBenzNHC@Cu complex
(7)

NAIKWADE ET AL.
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K2CO3 afforded the desired nano‐magnetite supported
NHC‐Cu complex with wingtip ferrocene acronymed as
NMagFemBenzNHC@Cu complex (7) (Scheme 2).
FT‐IR and FT‐Raman spectroscopy were used to mon-
itor the progress of the reactions involved in the synthesis
of NMagFemBenzNHC@Cu complex (7). The FT‐IR
spectrum of pristine nano‐magnetite (1) demonstrated
Fe‐O stretching vibration band at 592 cm−1. The FT‐IR
spectrum of nano‐magnetite (2) showed peaks at 792
(Si‐O‐Si symmetric stretching modes), 941 (Si‐O symmet-
ric stretching modes) and 1060 cm−1 (Si‐O‐Si asymmetric
stretching modes) suggesting the formation of silica
coated nano‐magnetite (2) (Figure 1a).[33] The FT‐IR
spectrum of 3‐chloropropyl modified nano‐magnetite (4)
(Figure 1b) displayed characteristic peak at 2948 cm−1
attributed to the C‐H stretching vibration of propyl
group.[34] The covalent tethering of 1‐N‐ferrocenylmethyl
benzimidazole (5) on 4 was evident from strong bands at
1637 (C=C stretching) and 477 (Fe‐Cp stretching)
observed in the FT‐IR spectrum of [NMagFemBenz]Cl
FIGURE 1 FT‐IR Spectra of (a) silica coated nano‐magnetite (2);
(b) 3‐chloropropyl modified nano‐magnetite (4); (c)
[NMagFemBenz]Cl (6); (d) NMagFemBenzNHC@Cu complex (7);
(e) reused NMagFemBenzNHC@Cu complex (7) after twelfth
catalytic run
(6)
(Figure
1c).
The
formation
of
NMagFemBenzNHC@Cu complex (7) was confirmed by
FT‐IR spectroscopy on the basis of strengthening of
C═N ring stretching bands in the range of 1350–
1500 cm−1 suggesting binding of carbenic carbon with
Cu metal (Figure 1d), which is in accordance to the liter-
ature.[35] In addition, FT‐Raman spectrum of 7 exhibited
signals at 472 cm−1 (Fe‐Cp stretching band), 1238, 1341,
1412, 1466 cm−1 (ring stretching modes of
benzimidazolium ring), 3141 and 3105 cm−1 (C‐H
stretching of Cp rings) confirming the proposed structure.
The energy dispersive X‐ray (EDX) analysis was
employed to investigate elemental loading. The analysis
revealed 0.28 mmol of Cu per gram of 7.
The
thermogravimetric
analysis
(TGA)
of
NMagFemBenzNHC@Cu complex (7) was carried out
over the temperature range of 25–1000 °C at a heating
SCHEME 2 Structure of
NMagFemBenzNHC@Cu complex (7)

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NAIKWADE ET AL.
NMagFemBenzNHC@Cu complex (7) and is shown in
Figure 3a‐c. The TEM micrographs appeared to have
spheres with non‐smooth surface. Moreover, encapsu-
lated dark magnetite nanocores surrounded by grey shell
were seen in TEM micrographs (Figure 3a‐b).[36] A bright
dotted pattern assured the single crystalline nature of
magnetite nanocores in the selected area electron diffrac-
tion (SAED) pattern (Figure 3(c)).
The powder X‐ray diffraction (XRD) pattern was
employed to ascertain retention of crystal structures of
nano‐magnetite in NMagFemBenzNHC@Cu complex
(7). The well indexing of diffractogram with JCPDS card
no. 19–629 affirmed the single‐phase inverse cubic spinel
structure of standard magnetite nanocore with high
phase purity and crystallinity (Figure 4a). The character-
istic peaks at 2θ values of 18.38o, 30.28o, 35.62o, 57.15o,
62.77o, 66.21o, 73.83o and 86.70o assigned to (1 1 1), (2 2
0), (3 1 1), (5 1 1), (4 4 0), (5 3 1), (5 3 3), (6 4 2) crystal-
lographic planes of magnetite nanocore respectively were
observed. The most intense peak at 2θ value 35.62o is
ascribed for (3 1 1) plane. The crystallite size calculated
with respect to most intense peak is found to be 21 nm
by using Debye–Scherrer equation. The XRD analysis
revealed preservation of crystallographic structure of
magnetite in 7 even after multi‐step functionalization.
The X‐ray photoelectron spectroscopy (XPS) was
employed for structural investigations of immobilized
species in NMagFemBenzNHC@Cu complex (7). The
FIGURE 2 TGA curve of NMagFemBenzNHC@Cu complex (7)
rate of 10 °C/min (Figure 2). The thermogram displayed
an initial weight loss of 7.83% centered at 113 °C ascribed
to evaporation of physically adsorbed water followed by a
cramped weight loss of 3.90% up to 163 °C and a steep
weight loss of 10.97% at 554 °C owing to the thermal
decomposition of covalently anchored organic moieties.
Finally, the residual weight is ascribed to the formation
of silica and metallic oxides which possess high thermal
stability.
Transmission electron microscopy (TEM) analysis
was employed to warrant the morphology of the
FIGURE 3 TEM images of (a‐c) NMagFemBenzNHC@Cu complex (7) with SAED pattern; (d‐f) reused NMagFemBenzNHC@Cu
complex (7) after twelfth catalytic run

NAIKWADE ET AL.
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at room temperature (Figure 6). The magnetic saturation
of 1 is found to be 30 emug−1, whereas that of 7 is 14
emug−1. The lower saturation magnetization of 7 is
attributed to functionalization of non‐magnetic layers
on surface of 1 during multistep‐functionalization.[42]
Despite of decline in the saturation magnetization of 7,
it can be still efficiently and easily removed with external
magnet.
Our next task was to evaluate the catalytic efficiency of
NMagFemBenzNHC@Cu complex (7) in intramolecular
O‐arylation of o‐iodoanilides. Intramolecular O‐arylation
of N‐(2‐iodophenyl)benzamide was chosen as a model
reaction for the optimization of reaction conditions. Ini-
tially, the effect of catalyst loading on model reaction
was investigated. To standardize the reaction conditions,
a series of experiments were carried out by varying
amounts of 7 in ethanol using Cs2CO3 as a base at
70 °C. The reaction was sluggish in the presence of
25 mg of 7 affording the corresponding product, 2‐
phenylbenzimidazole (9a) in 47% yield after prolonged
reaction time of 6 hr (Table 1, entry 1). Increasing the
quantity to 50 mg elevated the yield significantly
affording 9a in 94% in 4 hr (Table 1, entry 2). However,
further increase in the quantity of 7 did not found to have
profound effect on the yield of product (Table 1, entries 3,
4). In addition, control experiments demonstrate that the
model reaction does not occur in the absence of 7 signify-
ing that complex has definite role in cycle.
The base is a crucial parameter for intramolecular O‐
arylation reaction. Therefore, the effect of different bases
on model reaction was assessed (Table 2). Excellent yields
were observed with Cs2CO3 (Table 2, entry 4). All tested
inorganic bases including K3PO4, KOH, tBuOK and
K2CO3 give much better yields (Table 2, entries 1, 2, 5
and 7) in comparison with other organic bases DIPEA
and TEA (Table 2, entries 3,6). These results implied that
among all the screened bases Cs2CO3 was found to be
superior in view of reaction time and product yield.
Next, we investigated the effect of various solvents on
model reaction. An array of solvents was employed for
this purpose. Reactions in polar‐aprotic solvents such as
DMF, DMSO and acetonitrile afforded desired product
with moderate yields (Table 3, entries 1–3), whereas
non‐polar solvents like 1,4‐dioxane, toluene provided
the better yields (Table 3, entries 4–5). On the contrary,
the polar solvents such as ethanol, DMAC improved the
yield significantly (Table 3, entries 6–7). Ethanol is found
to be the best solvent (Table 3, entry 6) as the yield of
desired product was elevated among all the screened
solvents.
FIGURE
4
XRD pattern of (a) NMagFemBenzNHC@Cu
complex (7); (b) reused NMagFemBenzNHC@Cu complex (7)
after twelfth catalytic run
XPS survey spectrum of 7 displayed peaks due to Cu, C,
N, O, Fe and Si (Figure 5a). The core level XPS spectrum
of Cu 2p displayed peaks at 933.0 eV (2p3/2) and 952.8 eV
(2p1/2) respectively (Figure 5b). These peaks corroborate
existence of +1 oxidation state of Cu in 7. In the core
level XPS spectrum of C1s, the main peak is observed at
283.2 eV which is again deconvoluted into three peaks
at 283.1, 284.4, 286.6 eV (Figure 5c). The peak at
283.1 eV shows bonding interactions of carbon and sili-
con. This fact is again confirmed by a peak around
100.6 eV in the Si 2p region (Figure 5g).[37] The peaks
exhibited with binding energies at 284.4 and 286.6 eV
are assigned to ferrocenyl carbons and carbenic carbon
(C2 carbon of benzimidazolium ring) bonded with Cu
respectively.[38] In addition, the large peak area with
binding energy 284.4 eV reaffirms existence of bulky
ferrocenyl group. Moreover, the core level spectrum of
Fe 2p demonstrate peaks with binding energy 708.1 eV
and 719.6 eV, also designates existence of wingtip
ferrocenyl group (Figure 5f).[39] A set of two isolated
peaks is perceived in core level XPS spectrum of N 1 s.
The peak at 398.2 eV corresponds to N atom bonded with
sp3 hybridized C atoms (N‐ sp3C), whereas the peak at
400.9 eV is attributed to N atom bonded with sp2 hybrid-
ized C atoms (N‐ sp2C) (Figure 5d).[40] These observations
strongly support the coordination of NHC with Cu. The
core level spectrum of oxygen displays peaks with binding
energies 528.9 and 530.7 eV which are indicative for oxy-
gen in magnetite and oxygen bonded with Si (Figure 5
e).[41] Consequently, these structural investigations affirm
the successful formation of 7.
The magnetization measurements of pristine nano‐
magnetite (1) and NMagFemBenzNHC@Cu complex (7)
were assessed by vibrating sample magnetometer (VSM)
The active site in NMagFemBenzNHC@Cu complex
(7) responsible for catalytic cycle was explored by
performing several catalytic runs with varying amounts

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NAIKWADE ET AL.
FIGURE 5 (a) XPS survey spectrum of NMagFemBenzNHC@Cu complex (7); (b) Core level XPS spectrum of Cu in (7); (c) Core level XPS
spectrum of C in (7); (d) Core level XPS spectrum of N in (7); (e) Core level XPS spectrum of O in (7); (f) Core level XPS spectrum of Fe in (7);
(g) Core level XPS spectrum of Si in (7)
of ferrocene, nano‐magnetite (1), silica coated nano‐
magnetite (2) and [NMagFemBenz]Cl (6). Excitingly,
the reaction failed to proceed in all the catalytic runs.
Under the scenario of these observations, we believe that
Cu is the active site in 7 for catalytic run.
After the optimization of reaction conditions, the
generality of protocol was explored by reacting series
of o‐iodoanilides under the optimized reaction condi-
tions. As shown in Table 4, the protocol was proved to
be effective for both o‐iodoanilides bearing electron‐
donating groups (Table 4, entries 8b, 8e, 8f, 8h, 8i and
8l) as well as electron‐withdrawing groups (Table 4,
entry 8c) affording anticipated products in good to
excellent yields. The electronic effect on the o‐
iodoanilides has no influence on the reaction rate as
well as yield. Sterically hindered o‐iodoanilides also
FIGURE 6 Magnetic curves of a) bare nano‐magnetite (1); b)
NMagFemBenzNHC@Cu complex (7)

NAIKWADE ET AL.
7 of 13
TABLE 1 Catalyst optimization in NMagFemBenzNHC@Cu complex (7) promoted intramolecular O‐arylation of o‐iodoanilidea
Entry
Catalyst mg (mol%)
Time (h)
Yieldb (%)
1
2
3
4
25 (0.70 mol%)
50 (1.40 mol%)
75 (2.11 mol%)
100 (2.81 mol%)
6
4
4
4
47
94
95
95
aReaction conditions: 8a (1 mmol), Cs2CO3 (2 mmol), NMagFemBenzNHC@Cu complex (7) and ethanol (5 ml).
bIsolated yield after column chromatography.
TABLE 2 Screening of bases in NMagFemBenzNHC@Cu complex (7) catalyzed intramolecular O‐arylation of o‐iodoanilidea
Entry
Base
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
K3PO4
KOH
15
14
24
4
47
72
44
94
61
39
79
DIPEA
Cs2CO3
tBuOK
TEA
19
21
7
K2CO3
aReaction conditions: 8a (1 mmol), base (2 mmol), NMagFemBenzNHC@Cu complex (7) (50 mg; 1.40 mol%) and ethanol (5 ml).
bIsolated yield after column chromatography.
TABLE 3 Screening of solvents in NMagFemBenzNHC@Cu complex (7) catalyzed intramolecular O‐arylation of o‐iodoanilidea
Entry
Solvent
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
DMF
12
13
18
16
19
4
69
71
62
50
48
94
77
DMSO
Acetonitrile
1,4‐Dioxane
Toluene
Ethanol
DMAC
7
aReaction conditions: 8a (1 mmol), Cs2CO3 (2 mmol), NMagFemBenzNHC@Cu complex (7) (50 mg; 1.40 mol%) and solvent (5 ml).
bIsolated yield after column chromatography.

8 of 13
NAIKWADE ET AL.
TABLE 4 NMagFemBenzNHC@Cu complex (7) catalyzed intra-
molecular O‐arylation of o‐iodoanilidesa
performed well and produced corresponding products in
high yields (Table 4, entries 8g, 8j). Moreover, o‐
iodoanilides bearing heteroatom such as N‐(2‐
iodophenyl)isonicotinamide,
N‐(2‐iodophenyl)nicotin-
amide, N‐(2‐iodophenyl)furan‐2‐carboxamide (Table 4,
entries 8d, 8k, 8m) underwent smooth reactions
furnishing the corresponding products in good yields.
The structures of all the products were confirmed on
the basis of FT‐IR, 1H and 13C NMR spectroscopy as well
as by mass spectrometry. The spectroscopic data is consis-
tent with the proposed structures.
Entry ‐R
Products (9) Time (h) Yieldb (%)
a
b
c
d
e
f
C6H5
9a
9b
9c
9d
9e
9f
4
94
85
82
85
87
89
82
87
79
77
75
80
78
C (CH3)3
4.5
5
4‐O2NC6H4
Pyridin‐4‐yl
4‐MeOC6H4
4‐C2H5C6H4
5
A
plausible
mechanistic
rational
for
5
NMagFemBenzNHC@Cu complex (7) promoted intra-
molecular O‐arylation of o‐iodoanilides is depicted in
scheme 3 and is based on report of Batey et al.[43] Ini-
tially, 7 coordinates with o‐iodoanilide to form intermedi-
ate (I) which is followed by a oxidative addition step
providing the oxo‐copper complex (II). Finally, reductive
elimination of (II) results into formation of desired prod-
uct. It is noteworthy to mention that the bulky ferrocenyl
group at wingtip position of NHC backbone plays a cru-
cial role in catalysis. It is anticipated that powerful donor
capacity of ferrocene generates electron rich NHC‐Cu
complex that facilitates oxidative addition which is a
key step in the mechanism. Moreover, the bulk of the
ferrocenyl group shows substantial improvements in the
permeability of the substrate into the catalyst matrix,
4.5
6.0
4.5
4.0
5.5
5.0
5.0
5.0
g
h
i
2,6‐(MeO)2C6H3 9g
4‐MeC6H4
9h
9i
3,4‐(Me)2C6H3
2‐MeC6H4
j
9j
k
l
Pyridin‐3‐yl
4‐OHC6H4
Furan‐2‐yl
9k
9l
m
9m
aReaction
conditions:
8
(1
mmol),
Cs2CO3
(2
mmol),
NMagFemBenzNHC@Cu complex (7) (50 mg; 1.40 mol%) and ethanol
(5 ml).
bIsolated yield after column chromatography.
SCHEME 3 Plausible mechanism for
NMagFemBenzNHC@Cu complex (7)
catalyzed intramolecular O‐arylation of o‐
iodoanilide

NAIKWADE ET AL.
9 of 13
thereby improving the accessibility of reactants to the
active sites which ultimately facilitates the catalyst
performance.[44]
spectroscopy of both fresh as well as recycled complex.
The TEM images of 7 after twelfth catalytic run did not
show
a
substantial alteration in its morphology
To
check
the
heterogeneity
of
(Figure 3 d‐f), which confirms the retention of the cata-
lytic activity after recycling. The matching of peaks in
the X‐ray diffraction pattern of the fresh and reused 7
with JCPDS card no. 19–629 also revealed retention of
single‐phase inverse cubic spinel structure of Fe3O4
nanocore (Figure 4a, 4b). It is noteworthy to mention
that, the peak pattern of FT‐IR spectra (Figure 1d, 1e) of
both fresh and reused 7 revealed conservation of peaks
even after twelve successive cycles. The results of FT‐IR,
XRD and TEM analysis of fresh and reused complex
revealed that 7 is stable and do not undergo any physical
or chemical alteration during recycling even after reuse
for twelve times.
Table 5 shows the merits of NMagFemBenzNHC@Cu
complex (7) in comparison with those of previously
reported methods for synthesis of 2‐phenylbenzoxazole
(9a). The comparison of results clearly proves that 7 is a
superior catalyst in terms of catalyst loading, short
reaction time and product yields as compared to many
of the reported catalysts.
NMagFemBenzNHC@Cu complex (7), split test was per-
formed on model reaction. The 7 was separated from reac-
tion with the aid of external magnet after 50% conversion
(GC). The resultant filtrate was subsequently allowed to
stir under similar reaction conditions for 5 hr and the
GC–MS analysis revealed no further increment in product
formation. Additionally, the ICP‐OES analysis of reaction
mixture revealed no copper leaching suggesting that reac-
tions continue under heterogeneous conditions.
Heterogeneous catalysis has become a crucial point of
synthetic interests because of the profound merits of
straightforward recovery and reuse of the catalyst. In
order to perform reusability studies, the model reaction
was carried out under optimized reaction conditions.
After each cycle, NMagFemBenzNHC@Cu complex (7)
was recovered simply with the aid of external magnet.
The recovered complex was washed with ethanol and
dried under vacuum at 50 °C and reused for next cycle.
The complex displayed appreciable reusability as the cor-
responding yields initiated at 94% and reached at 69% at
the twelfth run (Figure 7). The structural and morpholog-
ical stability of 7 was further confirmed by various analyt-
ical techniques such as XRD, TEM analysis and FT‐IR
3 | CONCLUSION
In conclusion, we have developed simple, highly efficient
and eco‐friendly strategy of nano‐magnetite supported N‐
heterocyclic carbene‐copper complex promoted intramo-
lecular O‐arylation of o‐iodoanilides. The complex could
be magnetically separated and successfully recycled for
twelve times. The fresh and recycled complex after
twelfth cycles was characterized by variety of analytical
techniques. The morphology and structure of reused
complex were conserved after twelve consecutive recycles
without leaching of copper confirming the heterogeneity
and stability of the complex. The operational simplicity,
low catalyst loading, high yields, clean reaction profile,
ecofriendly reaction conditions, facile magnetic separa-
tion and recyclability are noteworthy merits of this
strategy.
FIGURE 7 Reusability of NMagFemBenzNHC@Cu complex (7)
in intramolecular O‐arylation of o‐
iodoanilide
TABLE 5 Comparison of different catalysts for synthesis of 9a
Sr. No.
Catalyst
Quantity
Temp. (°C)
Time (h)
Yield (%)
Ref.
1
2
3
4
5
CuI
10 mol%
10 mol%
10 mol%
5 mol%
120 °C
90 °C
8
12
20
7
88
87
[31a]
CuI
[31f]
FeCl3
120 °C
110 °C
70 °C
92
[31g]
CuO nanoparticles
>99
94
[31h]
NMagFemBenzNHC@Cu complex (7)
1.40 mol%
4
This work

10 of 13
NAIKWADE ET AL.
separated by external magnet, washed with DMF (3 x
50 ml), methanol (3 x 50 ml), CH2Cl2 (3 x 50 ml) and
dried under vacuum at 50 °C for 24 hr to
yield [NMagFemBenz]Cl (6). FT‐IR (KBr, Thin Film, υ,
cm−1): 3444, 2975, 2447, 1885, 1637, 1389, 1102, 794,
583, 477. Elemental analysis observed (%): C, 10.76; H,
1.96; N, 1.11; Loading: 0.39 mmol of benzimidazolium
units g−1 of 6.
4 | EXPERIMENTAL
4.1 | Materials and methods
All reactions were carried out under air atmosphere in
dried glassware. Infrared spectra were measured with a
Perkin‐Elmer one FT‐IR spectrophotometer. The samples
were examined as KBr discs~5% w/w. Raman spectros-
copy was done by using Bruker FT‐Raman (MultiRAM)
spectrometer. The elemental composition was analyzed
by an energy‐dispersive X‐ray spectra (EDS) attached to
the field emission scanning electron microscope (Oxford
Instruments). 1H NMR and 13C NMR spectra were
4.4 | Preparation of
NMagFemBenzNHC@Cu complex (7)
1
recorded on a Bruker AC (300 MHz for H NMR and
A mixture of 6 (2 g), copper iodide (0.176 g, 1 mmol) and
K2CO3 (0.138 g, 1 mmol) in THF (25 ml) was refluxed for
24 hr. Afterwards, the mixture was separated with exter-
nal magnet and subsequent washed with diethyl ether
to afford NMagFemBenzNHC@Cu complex (7). FT‐IR
(KBr, Thin Film, υ, cm−1): 3406, 2926, 2842, 1611, 1546,
1468, 1452, 1387, 1367, 1068, 632, 466. FT‐Raman (cm
−1): 472, 1238, 1341, 1412, 1466, 3105, 3141. Elemental
analysis observed (%): C, 9.87; O, 56.75; Si, 17.91; Fe,
9.27; Cu, 1.79; Loading: 0.28 mmol Cu g−1 of 7.
75 MHz for 13C NMR) spectrometer using CDCl3 as sol-
vent and tetramethylsilane (TMS) as an internal standard.
Chemical shifts are expressed in δ parts per million (ppm)
values and coupling constants are expressed in hertz
(Hz). Mass spectra were recorded on a Shimadzu
QP2010 GCMS. The materials were analyzed by TEM
using a JEOL JEM 2100 (200 kV). Melting points were
determined on MEL‐TEMP capillary melting point appa-
ratus and are uncorrected. X‐ray photoelectron spectrum
(XPS) was recorded on a PHI 5000 Versa Prob II, FEI
Inc. Magnetic measurements were performed on Lake-
shore magnetometer, USA, Model 7407 (Chennai,
Tamilnadu). Nano‐magnetite (1),[45] silica coated nano‐
magnetite (2)[46] and 1‐N‐ferrocenylmethyl benzimid-
azole (5)[47] were synthesized following the literature pro-
cedure. All other chemicals were obtained from local
suppliers and used without further purification.
4.5 | General method for intramolecular
O‐arylation of o‐iodoanilides
A mixture of o‐iodoanilide (1 mmol), Cs2CO3 (2 mmol)
and NMagFemBenzNHC@Cu complex (7) (50 mg;
1.40 mol%) in ethanol (5 ml) was stirred at 70 °C. The
progress of reaction was monitored by TLC. After the
completion of reaction, 7 was separated by external
magnet. Evaporation of solvent in vaccuo followed by
column chromatography over silica gel using petroleum
ether/ethyl acetate afforded pure products. The products
4.2 | Preparation of 3‐chloropropyl
modified nano‐magnetite (4)
A mixture of silica coated nano‐magnetite (2) (1 g) and
(3‐chloropropyl)triethoxy silane (3) (1 ml, 5 mmol) in
dry xylene (50 ml) was refluxed in an oil bath. After
24 hr, the reaction mixture was cooled, product was
isolated by magnetic separation and washed with xylene
(4 x 25 ml), methanol (4 x 25 ml), deionised water (4 x
25 ml) and dried under vacuum at 50 °C for 12 hr to
afford 3‐chloropropyl modified nano‐magnetite (4). FT‐
IR (KBr, Thin Film, υ, cm−1): 2948, 1665, 1488, 1443,
1333, 1064, 786, 698, 637.
1
were identified by FT‐IR, H NMR, 13C NMR and mass
spectroscopy.
ACKNOWLEDGMENTS
We gratefully acknowledge Science and Engineering
Research Board (SERB), New Delhi for providing finan-
cial support for this research work under the scheme of
Start‐Up Research Grant (Young Scientists) –Chemical
Sciences (NO. SB/FT/CS‐060/2014).
4.3 | Preparation of [NMagFemBenz]Cl
(6)
A mixture of 4 (1 g) and 1‐N‐ferrocenylmethyl benzimid-
azole (5) (0.94 g, 3 mmol) in DMF (25 ml) was heated at
80 °C in an oil bath. After 72 hr, the product was
CONFLICT OF INTEREST
The authors declare no conflicts of interest.

NAIKWADE ET AL.
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ORCID
Gajanan Rashinkar
5260
https://orcid.org/0000-0002-7200-
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