Facile Chan-Lam coupling using ferrocene tethered N-heterocyclic carbene-copper complex anchored on graphene

Article abstract of DOI:10.1002/aoc.4915

Ferrocene tethered N-heterocyclic carbene-copper complex anchored on graphene ([GrFemImi]NHC@Cu complex) has been synthesized by covalent grafting of ferrocenyl ionic liquid in the matrix of graphene followed by metallation with copper (I) iodide. The [GrFemImi]NHC@Cu complex has been characterized by fourier transform infrared (FT-IR), fourier transform Raman (FT-Raman), CP-MAS ¹³C NMR spectroscopy, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), energy dispersive X-ray (EDX) analysis, X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) surface area analysis and X-ray diffractometer (XRD) analysis. This novel complex served as a robust heterogeneous catalyst for the synthesis of bioactive N-aryl sulfonamides from variety of aryl boronic acids and sulfonyl azides in ethanol by Chan-Lam coupling. Recyclability experiments were executed successfully for six consecutive runs.

Full text of DOI:10.1002/aoc.4915

Received: 3 January 2019
DOI: 10.1002/aoc.4915
Revised: 24 February 2019
Accepted: 27 February 2019
F U L L P A P E R
Facile Chan‐Lam coupling using ferrocene tethered
N‐heterocyclic carbene‐copper complex anchored on
graphene
Shivanand Gajare | Megha Jagadale | Altafhusen Naikwade | Prakash Bansode |
Gajanan Rashinkar
Department of Chemistry, Shivaji
Ferrocene tethered N‐heterocyclic carbene‐copper complex anchored on
University, Kolhapur 416004, MS, India
graphene ([GrFemImi]NHC@Cu complex) has been synthesized by covalent
Correspondence
Gajanan Rashinkar, Department of
grafting of ferrocenyl ionic liquid in the matrix of graphene followed by
metallation with copper (I) iodide. The [GrFemImi]NHC@Cu complex has
been characterized by fourier transform infrared (FT‐IR), fourier transform
Raman (FT‐Raman), CP‐MAS 13C NMR spectroscopy, transmission electron
microscopy (TEM), thermogravimetric analysis (TGA), energy dispersive X‐
ray (EDX) analysis, X‐ray photoelectron spectroscopy (XPS), Brunauer–
Emmett–Teller (BET) surface area analysis and X‐ray diffractometer (XRD)
analysis. This novel complex served as a robust heterogeneous catalyst for
the synthesis of bioactive N‐aryl sulfonamides from variety of aryl boronic
acids and sulfonyl azides in ethanol by Chan‐Lam coupling. Recyclability
experiments were executed successfully for six consecutive runs.
Chemistry, Shivaji University, Kolhapur,
416004, MS, India.
Email: gsrchem@unishivaji.ac.in
KEYWORDS
ferrocene, graphene, N‐aryl sulfonamides, N‐heterocyclic carbene, reusability
1 | INTRODUCTION
that majority of the NHC‐TM complexes are homogenous
in nature. The insight into these complexes has revealed
N‐Heterocyclic carbenes (NHCs) have emerged as the
powerful spectator ligands in transition metal (TM) catal-
ysis.[1] NHCs are electron–rich, excellent σ–donors and
moderate π–acceptors, which make them ideal ligands
for complexation with transition metals. An important
attribute of NHC‐TM complexes is their unexceptional
stability that is often cited as one of the key advances of
these ligands over their phosphine counterparts.[2] In
addition, they are remarkably stable towards air, heat
and moisture. Moreover, their steric and electronic prop-
erties can be fine‐tuned with the help of wingtip
substituents in the ligand backbone thereby making them
process compatible.[3] The scrutiny of literature reveals
some basic problems in terms of separation and
recycling.[4] In addition, the contamination of ligand res-
idue in the product causes inevitable pollution and unfa-
vorable economics when the precious metals are
employed during complexation. These inherent limita-
tions associated with homogenous NHC‐TM complexes
have sparked major research activity in the field of heter-
ogenous analogs of these complexes.[5] The heteroge-
neous NHC‐TM complexes are usually prepared by
immobilizing NHCs on high area support materials. A
plethora of distinct protocols for synthesis of heteroge-
neous NHC‐TM complexes by employing structurally
diverse supports have been reported.[6] However, despite
Appl Organometal Chem. 2019;e4915.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4915

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GAJARE ET AL.
tremendous advances, there is considerable scope for the
further development especially using nanomaterials as a
support in order to accomplish the aim of sustainable
and environmentally benign processes.
most elegant protocol due to wide substrate scope, mild
reaction conditions and potential synthetic utility. A vari-
ety of catalytic systems have been reported to improve effi-
ciency of this protocol. However, there is a still scope to
explore new methodology especially using a robust hetero-
geneous catalyst.
Based on aforementioned discussion and in continua-
tion of our studies related to heterogeneous catalysis,[20]
we report herein preparation of ferrocene tethered
N‐heterocyclic carbene‐copper complex anchored on
graphene and its application in the synthesis of N‐aryl
sulfonamides.
Nanomaterials play a pivotal role in green chemistry
as they enhance the environmental sustainability of
processes producing negative externalities. They exhibit
exciting features such as small size, large surface to
volume ratio and ease of surface functionalization. Recog-
nition of these inherent attributes has sparked flourishing
research activity especially in the area of nanomaterial
supported catalysis. Recently, graphene oxide (GO) has
emerged as one of the most promising nanomaterial that
has been widely applied in diverse areas including cancer
therapy,[7] catalysis,[8] organic synthesis, electrochemis-
try,[9] tissue engineering,[10] imaging, diagnostics,[11]
sensors,[12] drug delivery[13] etc. Significant increase in
research in the GO based nanomaterials is predominantly
due to its extraordinary chemical, physical and electronic
properties such as surface functionalizability, electrocon-
ductivity, surface enhanced Raman scattering (SERS)
property, amphiphilicity, fluorescence quenching ability
and excellent aqueous processability. The large surface
area and chemical stability[14a–c] of GO facilitate high
loading of active sites and contribute to resistance to deg-
radation in both acidic and basic media. These attributes
have made GO as unparalleled 2D support in the synthe-
sis of heterogeneous catalysts. The recent applications of
GO supported catalysts in the realm of catalysis have
demonstrated tremendous potential in the development
of economical, efficient and environmentally benign
organic synthesis.[14d–f]
2 | EXPERIMENTAL
2.1 | Materials and methods
All reactions were carried out under air atmosphere in
dried glassware. FT‐IR 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 using a Bruker:RFS 27 spectrometer. The
thermal gravimetric analysis (TGA) curves were obtained
using instrument TA SDT Q600 V20.9 Build 20 in the
presence of static air at linear heating rate of 10 °C/min
from 25 °C to 1000 °C. Elemental analyses were per-
formed in a Perkin–Elmer 2400, Series II, CHNS/O ana-
lyzer and using an energy‐dispersive X‐ray spectroscopic
facility attached to the field emission SEM instrument
1
(Hitachi S 4800, Japan). H NMR and 13C NMR spectra
1
N‐Aryl sulfonamides are privileged motifs that find
prominent place in medicinal chemistry owing to their
intriguing chemical properties and potential biological
activities. They exhibit anticonvulsant, anticancer, anti-
bacterial and HIV protease inhibitory activities. They are
also employed in the treatment of diabetes mellitus,
oedema and hypertension.[15] Due to broad therapeutic
potential, synthesis of N‐aryl sulfonamides represents sig-
nificant area of research in organic synthesis. Some of
the most prominent methods for their synthesis include
copper catalyzed reaction of sulfonyl azides or sulfon-
amides with boronic acids,[16] Pd(OAc)2 mediated
N‐arylation of sulfonamides using aryl halides or aryl
triflates,[17] copper promoted reaction of chloramine‐T
and arylboronic acids,[18] Pd assisted cross‐coupling of
methanesulfonamide with aryl halides,[19a] transition
metal free reaction of o‐silylaryl triflates with
sulfonamides,[19b] Pd promoted aminosulfonylation of aryl
iodides[19c] and iron mediated coupling of nitroarenes and
sodium sulfonate.[19d] Amongst these, copper catalyzed
reaction of sulfonyl azides and boronic acids represents
were recorded with a Bruker AC (300 MHz for H NMR
and 75 MHz for 13C NMR) spectrometer using CDCl3 as
solvent and tetramethylsilane as an internal standard.
Chemical shifts are expressed in parts per million (ppm)
and coupling constants are expressed in hertz (Hz). The
CP‐MAS 13C NMR spectrum was recorded with a JEOL‐
ECX400 type FT‐NMR spectrometer under prescribed
operating conditions. Mass spectra were recorded with a
Shimadzu QP2010 GC–MS. The materials were analyzed
by TEM using a PHILIPS CM 200 model with 20–
200 kV accelerating voltages. Melting points were deter-
mined using MEL‐TEMP capillary melting point appara-
tus and are uncorrected. BET (Brunauer‐Emmet‐Teller)
Quantachrome Nova Win surface area analyzer was used
for surface area and pore size measurements by N2
adsorption–desorption analysis. 1‐N‐Ferrocenylmethyl
imidazole was synthesized following the literature proce-
dure.[21] Graphite (1) and all other chemicals were
obtained from local suppliers and used without further
purification. XPS was conducted with X‐ray Photo Spec-
trometer and UPS equipped with Al Kα radiation.

GAJARE ET AL.
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(3 × 50 ml) and CH2Cl2 (3 × 50 mL) and dried under
vacuum at 50 °C for 24 hr to afford [GrFemImi]Cl (6).
IR (KBr, thin film): υ = 3800, 3410, 2921, 2852, 1642,
2.2 | Preparation of graphite oxide (2)
The graphite oxide was synthesized by using modified
Hummers and Offeman's method.[22,23] A mixture of 1
(5.0 g), NaNO3 (2.5 g) and conc. H2SO4 (115 ml) was
stirred in an ice bath. Afterwards, KMnO4 (15 g) was
added gradually with constant stirring maintaining the
temperature below 20 °C. The reaction mixture was then
shifted to water bath at 35 °C and stirred for 30 min. to
produce thick paste. Afterwards, distilled water
(230 mL) was added slowly, which enhances the temper-
ature to 98 °C and kept for 15 min. Further, reaction
mixture was treated with distilled water (700 ml) and
30% H2O2 (50 mL). The residue of graphite oxide (2)
was washed with water until the pH 7, separated by
centrifugation and dried under vacuum at 50 °C.
1575, 1463, 1405, 1385, 1097, 1023, 657, 467 cm−1
.
Raman: υ = 3418, 2869, 2127, 1599, 1293 cm−1. Elemental
analysis observed: % C 59.34, % H 2.04, % N 4.65. Loading:
0.68 mmol functional group per gram of 6.
2.6 | Preparation of [GrFemImi]NHC@Cu
complex (7)
A mixture of 6 (9.0 g), CuI (1.9 g, 10 mmol) and NaOtBu
(0.096 g, 10 mmol) in THF (50 ml) was stirred at room
temperature under nitrogen for 6 hr. Afterwards, the
mixture was separated by centrifugation, washed with
THF (3 × 20 ml) and dried under vacuum for 24 hr to
afford [GrFemImi]NHC@Cu complex (7).
IR (KBr, thin film): υ = 3400, 1711, 1611, 1215, after
1037 cm−1
.
FT‐IR (KBr, thin film, υ): 3410, 2921, 2852, 1642, 1575,
1463, 1385, 1022, 1023, 727, 470 cm−1. Raman: υ = 3418,
2869, 2127, 1602, 1293 cm−1
. Elemental analysis
2.3 | Preparation of graphene oxide GO (3)
observed: % C 67.27, % O 27.91, % Si 1.56, % Fe 1.43, %
Cu 1.69, % I 0.15. 13C CP‐MAS NMR (500 MHz): δ 193
(C=O groups of graphene), 169 (O=C‐O), 101 (O‐C‐O),
130 (graphitic sp2‐C), 70 (C‐OH), 62 (epoxide), 177
(s, imidazolium C2), 135 (s, imidazolium C4), 120
(s, imidazolium C5), 72 (s, non‐substituted Cp ring carbon
of ferrocene), 79 (s, substituted Cp ring carbon of
ferrocene), 49 (s, 1C, =N‐CH2CH2‐CH2), 28 (s, 1C, =N‐
CH2CH2‐CH2), 8 (s, 1C,‐CH2Si). Loading of copper:
0.26 mmol functional group per gram of 7 of graphene.
A mixture of (2) (1.0 g) and distilled water (250 ml) was
sonicated for 30 min. Subsequently, 50% hydrazine
hydrate (5 ml) was added slowly. The resulting mixture
was refluxed in an oil bath. After 4 hr, insoluble product
was separated by centrifugation, washed with water
(6 × 20 ml) and dried under vacuum at 50 °C to afford
graphene oxide GO (3).
IR (KBr, thin film): υ = 3437, 1710, 1060; Raman:
υ = 3036, 2129, 1974, 1596, 1293 cm−1
.
2.4 | Preparation of chloropropyl
graphene (4)
2.7 | General method for synthesis of
N‐aryl sulfonamides
A mixture of 3 (8.0 g) and (3‐chloropropyl)triethoxysilane
(10.8 ml, 45 mmol) in xylene (50 mL) was refluxed in an
oil bath. After 24 hr, the mixture was cooled and the
residue was separated by centrifugation washed with
THF (3 × 5 ml) and dried under vacuum at room temper-
ature to afford chloropropyl graphene (4).
IR (KBr, thin film, υ): 2945, 2885, 2375, 1698, 1426,
995, 792, 682 cm−1; Raman: υ = 3036, 2129, 1974, 1597,
1293 cm−1; Loading: 0.26 mmol of functional group per
gram of 4.
A mixture of phenyl boronic acid (1.2 mmol), sulfonyl
azide (1 mmol) and [GrFemImi]NHC@Cu complex (7)
(70 mg) in ethanol (5 ml) was stirred at 70 °C. After com-
pletion of the reaction as monitored by the TLC, the reac-
tion mixture was centrifuged to remove complex 7.
Evaporation of solvent in vacuuo followed by column
chromatography over silica gel using petroleum ether‐
ethyl acetate afforded pure products.
3 | RESULTS AND DISCUSSION
The preparation of the ferrocene tethered N‐heterocyclic
carbene‐copper complex anchored on graphene is out-
lined in Scheme 1. Initially, purified graphite (1) was
oxidized to graphite oxide (2) following modified
Hummers and Offeman method.[22,23] The ultrasound
assisted exfoliation of 2 using hydrazine hydrate in
2.5 | Preparation of [GrFemImi]Cl (6)
A mixture of 4 (8.0 g) and 1‐N‐ferrocenylmethyl imidaz-
ole (5) (3 g, 11 mmol) in DMF (25 ml) was heated at
80 °C in an oil bath. After 72 hr, the solid was separated
by centrifugation, washed with DMF (3 × 50 ml), MeOH

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GAJARE ET AL.
SCHEME 1 Preparation of [GrFemImi]NHC@Cu complex (7)
aqueous medium afforded GO (3) which was subse-
quently treated with (3‐chloropropyl)triethoxysilane to
form chloropropyl graphene (4). The synthetically fertile
chloro group in 4 allowed installation of azolium group
in the graphene matrix through quaternization with
1‐N‐ferrocenylmethyl imidazole (5) to yield heteroge-
neous salt acronymed as [GrFemImi]Cl (6) that served
as precursor for preparation of NHC‐Cu complex. Finally,
the complexation of 6 with CuI yielded the desired
ferrocene tethered N‐heterocyclic carbene‐copper com-
plex anchored on graphene acronymed as [GrFemImi]
NHC@Cu complex (7).
Fourier transform infrared (FT‐IR), FT‐Raman and
cross polarization magic angle spinning (CP‐MAS) 13C
NMR spectroscopy were used to monitor reactions
involved in the preparation of [GrFemImi]NHC@Cu com-
plex (7). The FT‐IR spectrum (Figure 1c) of 4 displayed
characteristic peaks at 3437 cm−1 (O‐H stretching),
2885 cm−1 (C‐H stretching), 995 cm−1 (Si‐O stretching),
682 cm−1 (C‐Cl stretching).[24] The quarternization of 4
with 5 was monitored by FT‐IR spectroscopy. The
vanishing of peak at 682 cm−1 (C‐Cl stretching) and
appearance of peaks at 467 cm−1 (Fe‐Cp stretching band),
1385, 1463 and 1575 cm−1 (ring stretching modes of

GAJARE ET AL.
5 of 13
imidazolium ring), 2921 and 2852 cm−1 (C‐H stretching of
Cp rings), 1642 cm−1 (C=C stretching), 1575 cm−1 (N‐C‐N
stretching), 3410 cm−1 (O‐H stretching) demonstrated for-
mation of 6 (Figure 1d). The shifting of characteristic IR
absorption peaks to lower frequency values affirmed the
formation of complex with copper (Figure 1e).[25]
The structural modifications in the preparation of
[GrFemImi]NHC@Cu complex (7) were also reflected
in the FT‐ Raman spectroscopy. Raman spectra of GO
and 7 (Figure 2a) displayed characteristic D and G band.
The D band is attributed to structural disorder at defect
sites resulting from lattice vibrations and breathing mode
of sp2 carbon related to graphene edge or defect, whereas
the G band arises from the vibration of the sp2 bonded
carbon atoms in planar configuration that constitutes
graphene. It is observed that the G band shifts from
1596.51 in GO to 1599.14 cm−1 in [GrFemImi]Cl (6) with
a further blue shift to 1602.85 in [GrFemImi]NHC@Cu
complex (7). This upshift is probably due to gradually
increased compressive local stress caused by covalent
binding chains.[26] This phenomenon also suggest an
effective charge transfer from GO to 6 and 7.
The thermal stability profile of [GrFemImi]NHC@Cu
complex (7) was assessed by TGA analysis over the
temperature range of 25–1000 °C at heating rate of
10 °C/min (Figure 3a). The initial weight loss of 5.3%
up to 185 °C is due to desorption of physically adsorbed
water. The second key weight loss of 10.5% up to 308 °C
is attributed to the weight loss of pendant ferrocenyl
group and surface bound organic scaffolds.[27] The third
weight loss of 13.8% corresponds to the decomposition
FIGURE 1 FT‐IR spectra of (a) Graphite oxide; (b) Graphene
oxide; (c) Chloropropyl graphene; (d) [GrFemImi]Cl; (e)
[GrFemImi]NHC@Cu complex (7); (f) reused [GrFemImi]
NHC@Cu complex (7)
FIGURE 2 (a) Raman spectra of GO (3) (black), [GrFemImi]Cl (6) (red), [GrFemImi]NHC@Cu complex (7) (green); (b) Raman spectrum
of reused [GrFemImi]NHC@Cu complex (7) (blue)
FIGURE 3 (a) TGA curve of [GrFemImi]NHC@Cu complex (7) (b) TGA curve of reused [GrFemImi]NHC@Cu complex (7)

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GAJARE ET AL.
of the graphene by oxidative mode up to the 380.20 °C.
The two combined weight losses of 30.6% and 18.1% are
assigned to complete combustion of GO and carbon
skeleton. The observations are in good agreement with
TGA profile of GO reported in the literature.[28a]
The X‐ray diffraction (XRD) pattern of [GrFemImi]
NHC@Cu complex (7) (Figure 4), displays two noticeable
diffraction peaks at 26.0o and 43.3o corresponding to the
(002) and (101) reflections of graphitized carbon, respec-
tively, suggesting structural integrity of support.[28b]
The amount of Cu in [GrFemImi]NHC@Cu complex
(7) was quantified by using energy dispersive X‐ray
(EDX) analysis. The analysis revealed presence of
0.26 mmol of Cu per gm of 7.
Brunauer–Emmett–Teller (BET) surface area analysis
and was performed to evaluate the textural properties of
[GrFemImi]NHC@Cu complex (7). The 7 showed BET
surface area of 20.23 m2/g with the average pore diameter
of 43 A°. The surface area, pore volume and pore size of 7
was significantly smaller as compared to pristine
graphene oxide (77 m2/g, 23A°) indicating loss of micro-
pores due to substantial grafting of functional groups on
the surface.[29]
Transmission electron microscopy (TEM) was
employed to warrant morphological aspects of
[GrFemImi]NHC@Cu complex (7) and GO (Figure 5a‐c,
5f). The TEM micrographs displayed crumpling features
with twisted nanosheets in disordered phase. The fold
edges by virtue of sp3 carbon in nanosheets of GO carry
immobilized NHC‐Cu complex on both sides facilitating
easy access of catalytic sites to the reactants. The selected
area electron diffraction (SAED) pattern that exhibit
bright dotted pattern persuades crystalline nature of GO
nanosheets.[30]
The X‐ray photoelectron spectroscopy (XPS) was
employed for structural investigations of immobilized
species on GO. The XPS survey spectrum of
[GrFemImi]NHC@Cu complex (7) displayed peaks of
Fe, O, Si, C, N and Cu (Figure 6a). The core level XPS
FIGURE 4 XRD of [GrFemImi]NHC@Cu complex (7)
FIGURE 5 TEM images of (a‐c) [GrFemImi]NHC@Cu complex (7) with SAED pattern; (d‐e) reused [GrFemImi]NHC@Cu complex (7);
(f) TEM image of graphene oxide (GO)

GAJARE ET AL.
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spectrum of Cu 2p displayed peaks at 933.52 eV and
953.44 eV respectively (Figure 6b). The energy difference
of 19.92 eV between these peaks confirms existence of
+1 oxidation state of Cu in 7.[31] In the core level XPS
spectrum of C1s, the main peak is observed at
283.5 eV which is again deconvoluted into four peaks
at 284.4, 285.3, 286.7, 287.6 eV (Figure 6c). The large
peak area with binding energy 284.4 eV is ascribed to
presence of bulky ferrocenyl group.[32] A pronounced
peak displayed with binding energy 286.7 eV confirms
carbenic carbon bonded with copper metal (C2 carbon
of imidazolium ring).[33] The peak at 285.3 eV desig-
nates C‐C bonds (carbon sp3). Moreover, the peak at
287.6 represents C=O (carbonyl) and C‐O‐C (epoxy)
interactions.[34] A set of two isolated peaks is observed
in core level XPS spectrum of N 1 s. The peak at
399.3 eV correlates with N atom bonded with sp3
hybridized C atoms (N‐ sp3C) whereas the peak at
400.6 eV is ascribed to N atom bonded with sp2 hybrid-
ized C atoms (N‐sp2C) (Figure 6d). These observations
strongly suggest the coordination of NHC with Cu.
The core level spectrum of oxygen displays peak with
binding energy 531.5 eV which is indicative for oxygen
bonded with Si (Figure 6e).[35] Consequently, these
structural investigations confirm the successful forma-
tion of 7.
FIGURE 6 (a) XPS survey spectrum of [GrFemImi]NHC@Cu complex (7); (b) Core level XPS spectrum of Cu in [GrFemImi]NHC@Cu
complex (7); (c) Core level XPS spectrum of C in [GrFemImi]NHC@Cu complex (7); (d) Core level XPS spectrum of N in [GrFemImi]
NHC@Cu complex (7); (e) Core level XPS spectrum of O in [GrFemImi]NHC@Cu complex (7)

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GAJARE ET AL.
Our next task was to access the catalytic activity of
believe that GO support in combination with ferrocene
tethered N‐heterocyclic carbene‐copper complex exhibit
synergistic effect in catalysis.
[GrFemImi]NHC@Cu complex (7) in the synthesis of
N‐aryl sulfonamides (Scheme 2). In order to optimize
reaction conditions, we executed an extensive screening
of various parameters using phenyl boronic acid (8a;
1.2 mmol) and 4‐toluenesulfonyl azide (9a; 1.0 mmol) as
model substrates. In order to examine active site in
[GrFemImi]NHC@Cu complex (7) responsible for cata-
lytic cycle, the model reaction was performed by using
varying amounts of CuI, ferrocene, 1‐N ferrocenylmethyl
imidazole (5) and [GrFemImi]Cl (6) (Table 1, entry 1–4).
Interestingly, the reaction failed to proceed in all the
catalytic runs. In the light of these observations, we
Initially, the influence of catalyst loading on N‐aryl
sulfonamides synthesis was investigated by employing
various amounts of [GrFemImi]NHC@Cu complex (7).
Use of 50 mg (0.013 mmol) and 60 mg (0.015 mmol) of
7 provided low yield of corresponding product viz
4‐methyl‐N‐phenylbenzenesulfonamide (10a) even after
prolonged reaction time (Table 1, entry 5–6). More
efficient results were observed by increasing quantity to
70 mg (0.018 mmol), 80 mg (0.020 mmol), 90 mg
(0.023 mmol) and 100 mg (0.026 mmol) as the yield of
SCHEME 2 [GrFemImi]NHC@Cu
complex (7) catalyzed synthesis of N‐aryl
sulfonamides
TABLE 1 Optimization of catalyst loading in synthesis of N‐aryl sulfonamidesa
Entry
Catalyst
Solvent
Amount of catalyst
Time (Min.)
Yield (%)b
TON
TOF (h−1
)
1
2
3
4
5
CuI
EtOH
EtOH
EtOH
EtOH
EtOH
10 mol%
10 mol%
10 mol%
10 mol%
50 mg
24 h
24 h
24 h
24 h
130
<6
‐
‐
FemImi (5)
Ferrocene
[GrFemImi]Cl (6)
Trace
Trace
Trace
72
‐
‐
‐
‐
‐
‐
[GrFemImi]NHC@Cu
5538
2552
complex (7)
6
7
7
7
7
7
7
7
7
7
7
7
7
7
7
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
DCM
60 mg
70 mg
80 mg
90 mg
100 mg
150 mg
200 mg
70 mg
70 mg
70 mg
70 mg
70 mg
70 mg
70 mg
95
87
5577
5055
4423
3931
3538
2384
1788
3956
3791
4780
3296
3296
4450
‐
3529
7659
6701
6444
6100
4768
3576
594
7
40
92
8
40
92
9
37
92
10
11
12
13
14
15
16
17
18
19
35
92
30
93
30
93
400
540
270
660
510
45
72
DMF
69
421
THF
87
1062
299
Toluene
Xylene
Methanol
Water
60
60
387
81
5933
‐
2880
No product
aReaction condition: Phenyl boronic acid (1.2 mmol), 4‐toluenesulfonyl azide (1.0 mmol), ethanol (5 ml); bIsolated yields after chromatography.

GAJARE ET AL.
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product was boosted to 92% under identical reaction
conditions (Table 1, entry 7–10). Subsequently, further
increase in the quantity of 7 did not show a substantial
alteration in the yield of product and reaction time
(Table 1, entries 11 and 12).
entries 10 a‐n) without any side products. There was no
influence of electronic effect of substituents as electron
rich as well electron poor arylboronic acids reacted with
equal efficiency furnishing the corresponding products
in excellent yield.
The next parameter in the optimization studies was
the choice of solvent. A thorough screening of diverse
range of solvents was undertaken (Table 1). The model
reaction afforded good yields in polar aprotic solvents
such as dichloromethane (DCM), dimethyl formamide
(DMF) and polar aprotic tetrahydrofuran (THF)
(Table 1, entries 13–15), whereas moderate yields were
obtained in non‐polar solvents like toluene and xylene
(Table 1, entries 16–17). Good yields were achieved in
polar protic solvents like methanol and ethanol (Table 1
, entries 18,7). To our surprise, the reaction could not be
initiated in water (Table 1, entry 19). Amongst all the
screened solvents, ethanol gave excellent yield of desired
product and was selected for further studies. (Table 1,
entry 7).
A reasonable explanation for the notable catalytic
activity of [GrFemImi]NHC@Cu complex (7) in Chan‐
Lam coupling is attributed to the tethered ferrocenyl
group. It is now well established that electron rich
NHC‐metal complexes with the steric bulk incorporated
on nitrogen aids to promote the reductive elimination
which is the key step in coupling reaction.[36,37] One of
the reliable approach to design electron rich catalytic
systems for cross coupling reactions is to tether an
organometallic moiety like ferrocene at wingtip position
in the ligand framework. In the present case, by virtue
of its relatively steric bulk owing to unique cylindrical
shape and powerful electron donor capacity accelerate
the rate of reductive elimination, thus furnishing excel-
lent results.[38]
After the optimization of reaction conditions, the
scope and generality of protocol was investigated by
reacting sulfonyl azides with structurally diverse
arylboronic acids (Table 2). Gratifyingly, the reaction
proceeded smoothly in all the cases forming desired
N‐aryl sulfonamides in good to excellent yields (Table 2,
A plausible mechanism of [GrFemImi]NHC@Cu com-
plex (7) catalyzed Chan‐Lam coupling reaction based upon
Cu(I)/Cu (II)/Cu (III) system.[39] is outlined in Scheme 3
and is based on report by Kim and coworkers. Initially, 7
is air oxidized to afford (I), which coordinates with sulfo-
nyl azide generating intermediate (II). Further, reaction
TABLE 2 [GrFemImi]NHC@Cu complex (7) catalyzed synthesis of N‐aryl sulfonamidesa
Entry
R
R'
Product
Time (Min.)
Yieldb (%)
TON
TOF (h−1
)
a
b
c
d
e
f
H
p‐MePh
p‐MePh
p‐MePh
p‐MePh
p‐MePh
p‐MePh
p‐MePh
p‐MePh
Me
10a
10b
10c
10d
10e
10f
40
45
35
40
40
45
50
40
40
35
40
40
35
45
92
92
89
76
78
88
87
90
89
92
88
78
82
88
5055
5055
4890
4175
4285
4835
4780
4945
4890
5055
4835
4285
4505
4835
7659
6740
8383
6327
6493
6446
5736
7492
7409
8666
7326
6493
7724
6446
p‐Me
p‐Cl
p‐Br
3,5‐diMe
p‐OMe
m‐Cl
m‐OMe
H
g
h
i
10g
10h
10i
j
p‐Me
p‐Cl
Me
10j
k
l
Me
10k
10l
p‐Br
Me
m
n
m‐OMe
p‐OMe
Me
10m
10n
Me
aReaction conditions: phenyl boronic acids (1.2 mmol), sulfonyl azides (1.0 mmol), ethanol (5 ml), [GrFemImi]NHC@Cu complex (70 mg), bIsolated yields after
chromatography.

10 of 13
GAJARE ET AL.
SCHEME 3 Plausible mechanism for the synthesis of N‐aryl sulfonamides using [GrFemImi]NHC@Cu complex (7)
of arylboronic acid with II followed by transmetalation
results in intermediate (III). Finally, air oxidation of III
affords higher oxidation state intermediate (IV) which
undergo reductive elimination thereby resulting in the for-
mation of desired N‐aryl sulfonamides.[40]
In order to confirm the heterogeneous nature of the
[GrFemImi]NHC@Cu complex (7), hot filtration test
was implemented employing model reaction. The
complex was filtered off when 50% conversion was
accomplished (GC). The filtrate of reaction mixture was
stirred for additional 3 hr. Interestingly, there was no
enhancement in the yield of product beyond 50%. In addi-
tion, inductively coupled plasma atomic emission spec-
troscopic (ICP‐AES) analysis of filtrate shows absence of
copper confirming the heterogeneous nature of 7.
mention that the recovered complex could be reused for
six cycles without significant loss in the yield of product.
The stability of recycled complex was studied by FT‐
Raman spectroscopy, FT‐IR spectroscopy, TGA and EDX
analysis as well as by TEM of fresh and recycled
[GrFemImi]NHC@Cu complex (7). It is noteworthy to
mention that, FT‐IR (Figure 1f) and FT‐Raman (Figure 2
The reusability is one of the significant features of
heterogeneous catalyst to determine their dynamic life
span. The recyclability of [GrFemImi]NHC@Cu complex
(7) was further scrutinized by repeating the model reac-
tion (Figure 7). In brief, after completion of the reaction,
the reaction mixture was centrifuged and recovered 7 was
washed ethanol, dried in vacuuo at room temperature
and used directly for the next cycle. It is noteworthy to
FIGURE 7 Reusability of [GrFemImi]NHC@Cu complex (7) in
the synthesis N‐aryl sulfonamides

GAJARE ET AL.
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b) spectrum of reused 7 complex still retains the prominent
peak pattern of the fresh 7. The TGA of reused 7 (Figure 3
b) displayed mimicking pattern to the thermogram of fresh
7 (Figure 3a). The elemental EDX mapping of 7 after six
catalytic cycles confirmed the integrity of the recycled
complex. Moreover, TEM analysis of fresh (Figure 5 a‐c)
and reused 7 (Figure 5 d‐e) designates that morphology is
preserved even after six successive runs. The results of
FT‐Raman, FT‐IR, TGA, EDX and TEM analysis of fresh
and reused 7 confirmed that the structural rigidity and
main characteristics of complex remain conserved,
demonstrating stability of 7 after six consecutive runs.
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In conclusion, we have reported a facile synthesis of
ferrocene tethered N‐heterocyclic carbene‐copper com-
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characterized by FT‐IR, FT‐Raman, CP‐MAS 13C NMR
spectroscopy, XPS, TEM, TGA, EDX, BET and XRD
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ACKNOWLEDGEMENTS
We gratefully acknowledge Indian Institute of Sciences
(IISc), Bangalore, Sophisticated Analytical Instrumental
Facility, Indian Institute of Technology, Bombay (IITB),
Indian Institute of Technology, Madras (IITM), Dr. P. P.
Wadgaonkar, National Chemical Laboratory (NCL), Pune
for providing spectral facilities and Shivaji University
Kolhapur for providing Golden Jubilee Research Fellow-
ship (GJRF).
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CONFLICT OF INTEREST
The authors declare no conflict of interest.
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ORCID
Gajanan Rashinkar
https://orcid.org/0000-0002-7200-
5260
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How to cite this article: Gajare S, Jagadale M,
Naikwade A, Bansode P, Rashinkar G. Facile
Chan‐Lam coupling using ferrocene tethered
N‐heterocyclic carbene‐copper complex anchored
on graphene. Appl Organometal Chem. 2019;e4915.
https://doi.org/10.1002/aoc.4915
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