Sustainable Synthesis of Biaryls Using Silica Supported Ferrocene Appended N-Heterocyclic Carbene-Palladium Complex

Article abstract of DOI:10.1007/s10562-020-03480-w

Abstract: A novel silica supported ferrocene appended N-heterocyclic carbene-palladium complex (SilFemBenzNHC@Pd) has been prepared and characterized by using fourier transform infrared (FT-IR), fourier transform Raman (FT-Raman), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and energy dispersive X-ray analysis (EDX). This novel complex served as a robust heterogeneous catalyst for the synthesis of biaryls via homocoupling of aryl boronic acids under base-free conditions in water. Recyclability experiments were executed successfully for six successive runs. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-020-03480-w

Catalysis Letters
https://doi.org/10.1007/s10562-020-03480-w
Sustainable Synthesis of Biaryls Using Silica Supported Ferrocene
Appended N‑Heterocyclic Carbene‑Palladium Complex
Sharanabasappa Khanapure1 · Dattaprasad Pore1 · Megha Jagadale1 · Vaishali Patil2 · Gajanan Rashinkar1
Received: 28 April 2020 / Accepted: 27 November 2020
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature 2021
Abstract
A novel silica supported ferrocene appended N-heterocyclic carbene-palladium complex (SilFemBenzNHC@Pd) has been
prepared and characterized by using fourier transform infrared (FT-IR), fourier transform Raman (FT-Raman), X-ray photo-
electron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and energy dispersive
X-ray analysis (EDX). This novel complex served as a robust heterogeneous catalyst for the synthesis of biaryls via homo-
coupling of aryl boronic acids under base-free conditions in water. Recyclability experiments were executed successfully
for six successive runs.
Graphic Abstract
Keywords N-Heterocyclic carbenes · Homocoupling · Aryl boronic acids · Biaryls · Ferrocene · Reusability
1 Introduction
N-Heterocyclic carbenes (NHCs) have emerged as one of
the most extraordinary class of ancillary ligands that form
stable complexes with a large number of metals including
lanthanides, actinides as well as main group elements [1–5].
The selection of NHCs as leading ligands can be rational-
ized by their coordinative unsaturation, incomplete electron
octet, strong σ-donor capacity and weak π-acceptor property.
In addition, NHCs are remarkably stable toward air, mois-
ture and heat and the structural diversity in these privileged
Supplementary Information The online version contains
supplementary material available at https://doi.org/10.1007/s1056
2-020-03480-w.
* Gajanan Rashinkar
gsr_chem@unishivaji.ac.in
1
Department of Chemistry, Shivaji University,
Kolhapur 416004, M.S, India
2
Vishwakarma Institute of Information Technology,
Pune 411046, M.S, India
Vol.:(0123456789)
1 3

S. Khanapure et al.
motifs can be easily achieved by employing simple synthetic
routes [6]. Moreover, the electronic and steric properties of
NHCs can be modulated with the help of suitable substit-
uents on the nitrogen atoms rendering tailor made NHC-
metal complexes for task specifc applications [7–9]. Owing
to these fascinating properties, NHC-metal complexes fnd
myriad of applications in the wide areas including organo-
metallic catalysis [10, 11]. The scrutiny of literature reveals
that most of the NHC-metal complexes used in catalysis
are homogeneous in nature [12, 13]. An insight into homo-
geneous NHC-metal complexes have revealed basic issues
in terms of the separation and recycling. This factor cou-
pled with their ability to induce contamination of the ligand
residue in the products has sparked increased interest in
heterogenization of homogeneous NHC-metal complexes
[14–16]. The heterogeneous-NHC metal complexes allow
robust recycling thus minimizing environmental pollution
caused by residual metals in the waste. This factor is cru-
cial for the overall economic performance when precious
metals are employed in NHC-metal complexes. In addition,
heterogeneous-NHC metal complexes ofer advantages such
as increased reaction rates with signifcant enhancement in
selectivity [17]. In view of this, design and synthesis of new
heterogeneous NHC-metal complexes for catalytic applica-
tions has been a major driver of current ground-breaking
research especially in the domain of sustainable synthesis
[18–22]. Amongst these, heterogeneous NHC-Pd complexes
have garnered special attention as a robust catalytic systems
for various organic transformations [23–37].
represents a robust strategy for the synthesis of biaryls [62].
This protocol provides easy and efcient access to biaryls
under environmentally benign conditions. In addition, high
reactivity, low toxicity, and extensive commercial availabil-
ity of aryl boronic acids have substantially expanded the
generality of this methodology. Several elegant protocols
using Pd, Cu and other metal-based catalysts have been
reported for homocoupling of aryl boronic acids [63–70].
However, despite impressive progress, an efcient protocol
using highly robust heterogeneous catalyst operating under
environmentally benign conditions can be noteworthy con-
tribution to an umbrella of sustainable synthesis.
In continuation of our work related to green chemistry
[71, 72], we report herein preparation of silica supported fer-
rocene appended NHC-palladium complex and its catalytic
activity in the sustainable synthesis of biaryls from homo-
coupling of aryl boronic acids in water.
2 Experimental Section
All reagents and solvents were purchased from
Sigma-Aldrich/Fischer and were used as received.
1-N-(Ferrocenylmethyl)benzimidazole was prepared follow-
ing the literature procedure [73]. Column chromatography
was performed with silica gel (120 mesh) and thin layer
chromatography (TLC) was carried out by using Merck sil-
ica gel GF254 plates. FT-IR spectra were recorded on a Per-
kin-Elmer FT-IR spectrometer. The samples were examined
as KBr discs (ca 5% w/w). Raman spectra were recorded
by using Bruker FT-Raman (Multi-RAM) spectrometer. 1H
NMR and 13C NMR spectra were recorded with a Bruker
Biaryls are core structural motifs found in commer-
cially important natural products, agrochemicals, molecu-
lar switches, motors, polymers and ligands for catalysis
[38–44]. Biaryls have received increased attention as
privileged structures in the medicinal chemistry as they are
key components of the top-selling drugs such as Losartan
(angiotensin II receptor blocker used to treat hypertension),
Boscalid (succinate dehydrogenase inhibitor used as broad-
spectrum fungicide), Biphenomicyn B (natural cyclic pep-
tide antibiotic), Etalocib (leukotriene B4 receptor antagonist
used in cancer therapy), LY503430 (alpha-amino-3-hy-
droxy-5-methylisoxazole-4-propionic acid (AMPA) receptor
potentiator), OSU-6162 (dopaminergic stabilizer), Mibam-
pator (positive allosteric modulator (PAM) of the AMPA
receptor used in the treatment of Parkinson’s diseases) and
Felbinac (non-steroidal anti-infammatory drugs) [45, 46].
Owing to the wide applications in various felds, a myriad
of synthetic strategies have been developed for the efcient
synthesis of biaryls. Some of the prominent methods for
the synthesis of biaryls include Stille coupling [47, 48],
Ullmann reaction [49–51], Suzuki–Miyaura cross-coupling
[52–58] and C-H activation [59, 60]. In addition, biaryls
have been conveniently obtained using aryne intermedi-
ates [61]. The aerobic homocoupling of aryl boronic acids
1
AC (300 MHz for H NMR and 75 MHz for 13C NMR)
spectrometer using CDCl3 as solvent and tetramethylsilane
(TMS) as an internal standard. Mass spectra were recorded
with a Shimadzu QP2010 GCMS. The elemental compo-
sitions of material were analyzed by an energy-dispersive
X-ray spectra (EDX) attached to the feld emission scan-
ning electron microscope (FE-SEM, Model Hitachi S 4800,
Japan). The morphology of materials was analyzed by scan-
ning electron microscope (SEM) with a JEOL model JSM
with 5 kV and 20 kV accelerating voltage. Melting points
were determined using MEL-TEMP capillary melting point
apparatus.
2.1 Preparation of 3‑Chloropropyl silica (2)
A mixture of activated silica (3.0 g) and 3-chloropropyltri-
ethoxy silane (1) (3.0 mL, 25.5 mmoL) in toluene (25 mL)
was refuxed at 100 °C in an oil bath. After 24 h, reaction
mixture was cooled and the insoluble solid was fltered and
washed with toluene (3 × 5 mL) and dried under reduced
pressure at 100 °C for 8 h to yield 3-chloropropyl silica
1 3

Sustainable Synthesis of Biaryls Using Silica Supported Ferrocene Appended N-Heterocyclic…
(2) (2.8 g). FT-IR (KBr, thin flm): υ = 3345, 2986, 1635,
1426, 1311, 1171, 967, 879, 712, 575 cm−1; FT-Raman:
υ = 3404, 2891, 1471, 1365, 1297, 1117, 1089, 898, 619,
519, 377 cm−1.
2.4 General Procedure for Synthesis of Biaryls
A suspension of aryl boronic acid (5) (0.5 mmol, 0.0610 g)
and SilFemBenzNHC@Pd complex (4) (50 mg) in water
(5 mL) was stirred at room temperature. After completion
of the reaction as monitored by thin layer chromatography
(TLC), the insoluble complex was fltered and washed with
copious amount of water. The fltrate was extracted with
ethyl acetate (3×5 mL). The combined extracts were dried
over Na2SO4 and evaporated under reduced pressure. The
residue was purifed by column chromatography over silica
gel to give pure biaryls which were characterized by FT-IR,
1H-NMR, 13C-NMR and mass spectroscopy.
2.2 Preparation of [SilFemBenz]Cl (3)
A mixture of chloropropyl silica (2) (4 g) and 1-N-ferroce-
nylmethyl benzimidazole (2.5 g, 8 mmol) in DMF (25 mL)
was heated at 80 °C in an oil bath for 72 h. The insoluble
solid was fltered, washed with DMF (3× 50 mL), MeOH
(3×50 mL), CH2Cl2 (3×50 mL) and dried under vacuum
at 50 °C for 24 h to afford [SilFemBenz]Cl (3). FT-IR
(KBr, thin flm): υ=3383, 3275, 2939, 1660, 1508, 1453,
1396, 1204, 1059, 799, 616 cm−1. FT-Raman: υ = 3140,
3100, 2122, 1457, 1410, 1338, 1280, 776, 624, 580, 482,
450 cm−1. Elemental analysis observed: C 9.01%, H 1.31%,
N 0.51%. Loading: 0.18 mmol of benzimidazolium units
g−1 of 3.
3 Result and Discussion
A multi-step route for the preparation of silica supported
ferrocene appended NHC-palladium complex using com-
mercially available silica is shown in Scheme 1. Initially,
activated silica was functionalized with 3-chloropropyltri-
ethoxy silane (1) to give 3-chloropropyl silica (2). The het-
erocyclic unit was covalently grafted on 2 by quaternization
with 1-N-ferrocenylmethyl benzimidazole to form heteroge-
neous azolium salt acronymed as [SilFemBenz]Cl (3). The
in situ deprotonation followed by complexation of 3 with
palladium acetate in THF aforded the desired silica sup-
ported ferrocene appended NHC-palladium complex acro-
nymed as SilFemBenzNHC@Pd complex (4).
2.3 Preparation of SilFemBenzNHC@Pd Complex (4)
[SilFemBenz]Cl (3) (3.5 g) was added in a solution of
Pd(OAc)2 (0.500 g, 2.21 mmol) in THF (15 mL). The
resulting mixture was heated at refux for 48 h and cooled to
room temperature. Afterwards, insoluble solid was fltered,
washed with THF (3 ×20 mL) and dried under vacuum at
50 °C for 48 h to aford SilFemBenzNHC@Pd complex
(4). FT-IR (KBr, thin flm): υ = 3366, 2913, 1646, 1506,
1450, 1394, 1200, 1151, 1050, 800, 661 cm−1; FT-Raman:
υ=3143, 3114, 2124, 1470, 1418, 1340, 1263, 1040, 774,
560, 484, 450 cm−1. Elemental analysis observed: C 9.01%,
H 1.31%, N 0.51%. Loading: 0.063 mmol Pd g−1 of 4.
The FT-IR spectrum of SilFemBenzNHC@Pd com-
plex (4) displayed characteristic peaks at 3366 cm−1 (C-H
stretching of alkyl group), 1614 cm−1 (NCN-Pd stretch-
ing), 1506 cm−1 (C=C stretching vibrations of Cp ring of
Scheme 1 Preparation of silica supported ferrocene appended heterogeneous NHC-Pd complex (4)
1 3

S. Khanapure et al.
ferrocene), 1394 cm−1 (C-N stretching of benzimidazolium
ring), 1050 cm−1 (Si–O stretching vibrations) and 800 cm−1
(Si–C stretching vibrations) confrming the successful for-
mation of the 4.
organic functional groups immobilized on silica matrix. The
large residual weight of 69.64% is due to formation of silica
and metallic oxides. The analysis reveals signifcant thermal
stability of the 4.
The amount of Pd in SilFemBenzNHC@Pd complex (4)
was quantifed by using energy dispersive X-ray (EDX) anal-
ysis (Fig. 1). The analysis revealed presence of 0.063 mmol
of Pd per gm of 4.
To examine morphological changes occurred on the sur-
face of silica at various stages of preparation of SilFem-
BenzNHC@Pd complex (4), scanning electron microscopy
(SEM) was employed. The micrographs of silica, 3 and 4
clearly displayed particles of micron size with irregular mor-
phology (Fig. 3a–c). The morphology of 4 resemble with
pristine silica indicating no alteration in the morphology
during multi-step synthesis, signifying the efectiveness of
silica as a support.
The thermogravimetric analysis (TGA) of SilFemBen-
zNHC@Pd complex (4) was carried out to investigate its
thermal stability (Fig. 2). The thermogram displayed three
characteristic decomposition stages. The initial weight
loss of 5.74% up to 213 °C is due to desorption of physi-
cally adsorbed water. The second and third weight loss of
22.57% in the temperature range 213–891 °C is ascribed
to the decomposition tethered ferrocenyl moiety and other
The X-ray photoelectron spectroscopy (XPS) was used to
investigate the elemental coordination state of Pd species in
SilFemBenzNHC@Pd complex (4) (Fig. 4). The core level
Fig. 1 EDX spectra of a [SilFemBenz]Cl (3); b SilFemBenzNHC@Pd complex (4)
Fig. 2 TGA spectrum of Sil-
FemBenzNHC@Pd complex (4)
1 3

Sustainable Synthesis of Biaryls Using Silica Supported Ferrocene Appended N-Heterocyclic…
Fig. 3 SEM images of a silica; b [SilFemBenz]Cl (3); c SilFemBenzNHC@Pd complex (4)
Fig. 4 XPS core level spectrum of Pd in SilFemBenzNHC@Pd com-
plex (4)
XPS spectrum of Pd 3d displayed peaks of binding energies
Fig. 5 N2 adsorption isotherm of SilFemBenzNHC@Pd complex (4)
at 341.2 eV (for Pd 3d3/2) and 336.1 eV (for Pd 3d5/2). These
values corroborate existence of palladium(II) in 4 reafrm-
ing the proposed structure.
ranging from 16° to 40° indicating that amorphous nature of
silica is retained even after multi-step functionalization. In
addition, the difractogram displayed two shoulders around
the either sides of characteristic hump of silica and cannot be
indexed unambiguously with this data quality. In our view,
these additional refections are due to successful grafting
of NHC-Pd complex on the silica thereby modifying the
structural properties of support.
Brunauer–Emmett–Teller (BET) specifc surface area
of SilFemBenzNHC@Pd complex (4) was determined by
N2 adsorption–desorption isotherms in the relative pressure
range P/P0 0.05 to 1.0. A typical irreversible-type VI adsorp-
tion isotherm with H1 hysteresis loop was observed due to
layered structure and presence of inter-connected as well as
structurally open pores of mesoporous nature of the silica
support (Fig. 5). The analysis revealed BET specifc surface
area of 107 m2/g of 4. The high surface area of 4 is advanta-
geous for the catalytic applications as it assures enhanced
adsorption sites required for the reactants.
Our next task was to investigate the efciency of SilFem-
BenzNHC@Pd complex (4) as a heterogeneous catalyst in
the synthesis of biaryls via homocoupling of aryl boronic
acids (Scheme 2). Phenylboronic acid (5a) was selected as a
model substrate for the optimization of reaction conditions.
Initial studies were focused on the optimization of the suit-
able solvent for the homocoupling (Table 1). Accordingly,
5a (1 mmoL) was stirred in the presence of catalytic quantity
of 4 (50 mg) in diferent solvents (5 mL). It was observed
The pristine silica and SilFemBenzNHC@Pd complex
(4) were characterized by X-ray difraction profle (Fig. 6).
The difractogram of silica shows a hump peak at 2θ ranging
from 16° to 40° which is a characteristic of amorphous mate-
rial. The difractogram from 4 displayed hump peak at 2θ
1 3

S. Khanapure et al.
the model reaction aforded the corresponding homocou-
pling product viz biphenyl (6a) in moderate yields (Table 1,
entries 2–4). Non polar solvent such as toluene aforded
slightly better yield (Table 1, entry 5), whereas CH3CN
and DMSO resulted in the moderate yields of 6a (Table 1,
entries 6,7). The use of polar protic solvent such as MeOH,
EtOH and water resulted in the good yields of 6a (Table 1,
entries 8–10). Out of all screened solvents, water ofered
the highest yield of the 6a and hence it was a chosen for
further studies.
Our subsequent task was to investigate the infuence of
catalytic loading of SilFemBenzNHC@Pd complex (4) on
the homocoupling of 5a (Table 2). The homocoupling reac-
tion did not occur in the absence of 4 (Table 2, entry 1)
whereas the loading of diferent quantities of 4 exhibited sig-
nifcant infuence on the product yield. The loading of 50 mg
of 4 was proved to be suitable quantity that ofered highest
yield (92%) of 6a within shortest reaction time (Table 2,
entry 5). Surprisingly, there was no signifcant increase in
the product yield by increasing or decreasing the amount of
4 beyond 50 mg (Table 2, entries 2–4, 6–8). Hence 50 mg
of 4 was chosen as optimum quantity of catalyst loading for
further studies.
Fig. 6 XRD pattern of a Silica (red) and b SilFemBenzNHC@Pd
complex (4) (black)
that the reaction could not be initiated in the absence of
solvent even after stirring for 24 h (Table 1, entry 1). In the
presence of aprotic solvents such as THF, DMF and CH2Cl2,
Scheme 2 SilFemBenzNHC@
Pd complex (4) catalyzed syn-
thesis of biaryls
Table 1 Solvent optimization in SilFemBenzNHC@Pd (4) catalyzed homocoupling of phenyl boronic acid (5a)a
SilFemBenzNHC@Pd complex (4)
Solvent, rt, Air
2
B(OH)2
5a
6a
Entry
Solvent
Time (h)
Yieldb (%)
TON
TOF (h−1
)
1
2
3
4
5
6
7
8
9
10
Solvent-free
THF
DMF
CH2Cl2
Toluene
CH3CN
DMSO
MeOH
EtOH
24
NR
61
56
43
67
56
45
88
89
92
-
-
5.5
5.0
6.5
6.5
7.5
7.0
4.5
4.5
4.0
19,365
17,777
13,650
21,269
17,777
14,285
27,936
28,253
29,206
3520
3555
2100
3272
2370
2040
6208
6278
7301
H2O
aOptimal reaction conditions: Phenylboronic acid (5a) (1 mmol), solvent (5 mL), SilFemBenzNHC@Pd (4) (50 mg), rt
bIsolated yields after column chromatography
1 3

Sustainable Synthesis of Biaryls Using Silica Supported Ferrocene Appended N-Heterocyclic…
Table 2 Optimization of catalyst loading in SilFemBenzNHC@Pd (4) catalyzed homocoupling of phenyl boronic acid (5a)a
SilFemBenzNHC@Pd complex (4)
2
B(OH)2
H2O, rt, Air
5a
6a
Entry
Catalyst (mg)
Reaction time (h)
Yieldb (%)
TON
TOF (h−1
)
1
2
3
4
5
6
7
8
00
05
10
30
50
70
90
100
24
12
10
2.5
4.0
3.5
3.0
3.0
00
52
55
61
92
92
93
93
00
00
162500
87301
32275
29206
20861
16402
14761
13541
8730
12910
7301
5960
5467
4920
aOptimal reaction conditions: Phenylboronic acid 5a (1 mmol), H2O (5 mL), SilFemBenzNHC@Pd (4), rt
bIsolated yields after column chromatography.
After the optimization of reaction conditions, the gener-
ality of the protocol was proved by performing homocou-
pling reactions of variety of electronically and structurally
diverse arylboronic acids under the optimized reaction
conditions and the results are summarized in Table 3. In
general, arylboronic acids containing both electron donating
as well as electron withdrawing substituents aforded the
corresponding homocoupling products in good to excellent
yields with TON in the range 20,631–29,841 and TOF in
the range 1875–8163 h−1 (Table 3, entries a-q). However,
in comparison, arylboronic acids containing electron-with-
drawing groups (Table 3, entries b-k) gave higher yields of
corresponding products than those with electron donating
substituents (Table 3, entries l-q). Further, careful inspection
of the results indicates that sterically crowded arylboronic
acids such as o-chloro and o-methoxy phenyl boronic acids
were successfully converted to the corresponding products
albeit giving lower yields due to steric hindrance (Table 3,
entry b,p). It is noteworthy to mention that, no side-products
were observed.
homocoupling of aryl boronic acids is depicted in Scheme 3.
In an initial step, 4 reacts with dioxygen to generate peroxo
compound (II). Further, II reacts with arylboronic acid to
form adduct III by the co-ordination of oxygen of peroxo
group with boron. The adduct III further reacts with second
molecule of arylboronic acid to generate ArPd(OH)NHC2
complex which on transmetallation with arylboronic acid
furnishes ArPdArNHC2. Finally, the reductive elimination
generates biaryl with regeneration of Pd(0) complex (I).
The appended ferrocenyl group plays a vital role in catalytic
cycle. It is now well established that steric bulk of ligand
plays a vital role in catalytic cycle of coupling reactions
[75]. With the bulky ligands, reductive elimination is highly
favored. In the present case, owing to its cylindrical shape,
steric bulk of ferrocenyl group accelerates reductive elimina-
tion since the extrusion of biaryl relieves steric strain in the
ArPdArNHC2. In addition, the bulky ferrocenyl group shows
substantial improvements in the permeability of the substrate
into the catalyst matrix thereby improving the accessibility
of reactants to the active sites which ultimately facilitates
the catalyst performance [76–80].
On the basis of literature report [74], a plausible mecha-
nism for SilFemBenzNHC@Pd complex (4) catalyzed
1 3

S. Khanapure et al.
Table 3 SilFemBenzNHC@Pd (4) catalyzed synthesis of biarylsa
SilFemBenzNHC@Pd complex (4)
2
B(OH)2
R
H2O, rt, Air
R
R
5
6
Entry
Phenyl boronic acid
(5)
Product
Time
(h)
Yieldb (%)
TON
TOF (h−1
7301
)
(6)
a
4.0
92
67
29206
21269
B(OH)2
b
4.0
5317
Cl
Cl
B(OH)2
Cl
c
4.5
5.5
77
89
24444
28253
5432
5137
Cl
Cl
Cl
B(OH)2
Cl
Cl
d
Cl
B(OH)2
Cl
Cl
Cl
e
f
5.0
7.5
8.6
94
84
91
29841
26666
28888
5968
3555
3359
F
F
F
B(OH)2
B(OH)2
F3C
CF3
F3C
g
O2N
O2N
B(OH)2
NO2
h
i
4.0
4.0
90
87
28571
27619
7142
6904
NC
NC
CN
NC
NC
B(OH)2
B(OH)2
CN
j
4.0
3.5
89
90
88
28253
28571
27936
7063
8163
2793
EtOOC
B(OH)2
EtOOC
COOEt
COOMe
k
l
MeOOC
B(OH)2
MeOOC
10.0
Me
Me
B(OH)2
Me
1 3

Sustainable Synthesis of Biaryls Using Silica Supported Ferrocene Appended N-Heterocyclic…
Table 3 (continued)
Entry
Phenyl boronic acid
(5)
Product
(6)
Time
(h)
Yieldb (%)
TON
TOF (h−1
2984
)
m
n
10.0
10.5
10.5
94
84
80
29841
26666
25396
Me
B(OH)2
Me
Me
OMe
2539
MeO
MeO
B(OH)2
B(OH)2
MeO
MeO
o
2418
OMe
p
q
11.0
10.5
65
82
20634
26031
1875
2479
OMe
B(OH)2
OMe
MeO
MeO
MeO
OMe
OMe
MeO
B(OH)2
MeO
aOptimal reaction conditions: aryl boronic acid 5 (1 mmol), H2O (5 mL), SilFemBenzNHC@Pd complex (4) (50 mg), rt
bIsolated yields after column chromatography. All products are characterized by IR, 1H NMR, 13C NMR and mass spectroscopy
To ascertain heterogeneous nature of SilFemBenzNHC@
Pd complex (4), a hot fltration test was performed with 5a
under optimized reaction conditions. After 50% completion
of the reaction (GC), 4 was separated from reaction mix-
ture by simple fltration. The resulting fltrate when stirred
for additional 5 h, no substantial enhancement in the yield
beyond 50% was noticed (GC–MS analysis). Moreover,
leaching of catalytically active Pd species from 4 in the solu-
tion was investigated at the midpoint of reaction by ICP-AES
analysis of fltrate. No evidence of leaching was observed
during the reaction as no palladium species was detected in
the solution. These observations indicate that catalytically
active Pd species is frmly held on the support confrming
the heterogeneous nature of 4.
The reusability of SilFemBenzNHC@Pd complex (4) was
investigated on the model substrate under optimized reaction
conditions. Upon completion of reaction, the 4 was separated
from the reaction mixture, washed several times thoroughly
with copious amount of water, dried at 60 °C and reused in
the subsequent catalytic cycle. We were delighted to note that
4 could be reused for six consecutive runs with small drop
in the product yield as the corresponding yields started with
92% and reached 84% at the sixth run (Fig. 7). The decrease
in yield at each run is attributed to the loss of catalyst quantity
1 3

S. Khanapure et al.
Fc
Fc
Cl
N
N
Pd
N
N
Cl
(4)
Fc
Fc
N
N
N
Pd
(0)
N
Ar Ar
O2
Reductive elimination
(I)
Fc
Fc
Ar
Ar
N
Fc
N
N
O
Fc
N
O
Pd
N
Pd
N
N
N
(VI)
(II)
Ar B(OH)2
Transmetallation
Ar
Fc
Fc
Ar
N
Pd
N
B
OH
OH
OH
O
Fc
N
O
Fc
Pd
N
N
N
N
N
(III)
(V)
Fc
Ar
OOB(OH)2
Fc
N
Pd
N
N
N
(IV)
Scheme 3 Plausible mechanism for SilFemBenzNHC@Pd complex (4) catalyzed synthesis of biaryls
1 3

Sustainable Synthesis of Biaryls Using Silica Supported Ferrocene Appended N-Heterocyclic…
To establish the merits of present protocol, we have com-
pared the efciency of SilFemBenzNHC@Pd complex (4)
with some of the reported catalysts employed for the syn-
thesis of biphenyl (6a) via homocoupling reaction of phe-
nylboronic acid. The results are summarized in Table 4. It
is evident that 4 is superior to many of the reported catalysts
with respect to temperature (Table 4, entries 2, 3), reaction
time (Table 4, entries 4, 5, 7 and 8), yield of the product
(Table 4, entries 3, 4, 6 and 8), and recyclability of catalyst
(Table 4, entries 1–8).
4 Conclusions
Fig. 7 The recycling performance of SilFemBenzNHC@Pd complex
In conclusion, we have successfully prepared a novel silica
supported ferrocene appended NHC-palladium complex
by multi-step procedure. The complex was successfully
employed as a heterogeneous catalyst for the homocoupling
of aryl boronic acids in water. The heterogeneity of com-
plex was confrmed by hot-fltration test and leaching stud-
ies. The complex could be reused for six consecutive times
with slight decrease in catalytic activity. The key features
of this protocol are use of environmentally benign solvent,
mild reaction conditions and base as well as oxidant-free
conditions.
(4)
due to attrition. The FT-IR spectrum of reused 4 was indis-
tinguishable from that of fresh complex which revealed that
the main characteristics of the complex are preserved even
after recycling thereby confrming the retention of structural
rigidity of the organic functional group and metal species in
the 4. In addition, the EDX analysis of reused 4 displayed no
signifcant diference in its content in comparison with the
fresh complex. Moreover, the FE-SEM analysis of fresh and
reused 4 displayed no diferences indicating that morphology
of 4 is preserved even after continuous reuse.
Table 4 Comparison studies of SilFemBenzNHC@Pd complex (4) in synthesis of biaryls
Sr No. Catalyst Catalyst loading Reaction conditions
Reaction time % yield Ref
1
2
3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2:2PPh3
1 mol%
3.6 mol%
5 mol%
Isopropyl myristate, K2CO3, water, rt, air
[bmim][PF6], water, K2CO3, 60˚C
15 min
3 h
2 h
94
92
83
[87]
[81]
[82]
DMF, K2CO3,
90˚C
4
5
6
7
Pd(PPh3)2Cl2
PdCl2
Pd(OAc)2,/AgNO3 4.5 mol%
10 mol%
3 mol%
Aq. THF, Bu4NF.H2O, rt, air
Water, Na2CO3, p-Toluenesulfonyl chloride, H2O, rt 12 h
NEt3, Grinding, rt
9 h
77
95
72
94
[83]
[84]
[85]
[86]
30 min
24 h
PdCl2(rac-BINAP) 2.5 mol%
KF, dioxane/H2O
100˚C
8
SilFemBenzNHC@ 50 mg
Pd complex (4)
Water, rt
4.0 h
92
Present work
1 3

S. Khanapure et al.
36. Jiang J, Du L, Ding Y (2020) Mini-Rev Org Chem 17:26
Acknowledgement We gratefully acknowledge Indian Institute of
Technology, Madras (IITM) and Indian Institute of Sciences (IISc),
Bangalore for providing spectral facilities.
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Compliance with Ethical Standards
41. Baran T (2019) Catal Lett 149:1496
Conflict of interest The authors declare that they have no confict of
42. Liu C, Xu W, Xiang D, Luo Q, Zeng S, Zheng L, Tan Y, Ouyang
Y, Lin H (2020) Catal Lett 150:2558
interest.
43. Feizi Mohazzab B, Jaleh B, Nasrollahzadeh M, Issaabadi Z (2019)
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