Robust alkyl-bridged bis(N-heterocyclic carbene)palladium(II) complexes anchored on Merrifield's resin as active catalysts for the selective synthesis of flavones and alkynones

Article abstract of DOI:10.1002/aoc.6195

Highly active and efficient propylene-bridged bis(N-heterocyclic carbene)palladium(II) complexes covalently anchored on Merrifield's resin were synthesized and characterized using various physical and spectroscopic techniques. The two anchored Pd(II) complexes consist of the system: Merrifield's resin-linker-bis(NHC)Pd(II), the linkers being benzyl and benzyl-O-(CH₂)₃ for (Pd-NHC1@M) and (Pd-NHC2@M), respectively. The short linker anchored bis-benzimidazolium ligand precursor (PBBI-1@M) was synthesized via direct carbon–nitrogen alkylation of a propylene-bridged bis(benzimidazole) (PBBI-1) by Merrifield's resin chlorobenzyl group. The longer linker anchored bis-benzimidazolium ligand precursor (PBBI-2@M) was obtained in a two-step reaction involving first alkylation of (PBBI-1) with 3-chloro-1-propanol followed by a nucleophilic substitution at Merrifield's resin chlorobenzyl group. Both supported ligand precursors (PBBI-1@M and PBBI-2@M) reacted with palladium acetate to produce the two heterogeneous catalysts (Pd-NHC1@M) and (Pd-NHC2@M). ¹³C NMR palladation shift of the benzimidazole N–C–N (C2) carbon was found very similar in both the liquid NMR spectra of the homogeneous complexes and the CP/MASS spectra of the corresponding covalently anchored complexes. The catalytic activity, stability, and the recycling ability of the supported catalysts have been investigated in the carbonylative Sonogashira coupling reactions of aryl iodides with aryl alkynes and alkyl alkynes and also in the cyclocarbonylative Sonogashira coupling reactions of aryl iodides with aryl alkynes via one pot reactions. The longer linker catalyst Pd-NHC2@M demonstrated excellent catalytic activity, stability, and very high recycling ability in the two carbonylative coupling reactions. These systems exhibit the hypothesized thermodynamic stability offered by the chelate effect in addition to the strong sigma donor ability of a bis(NHC) ligand system generating electron-rich palladium centers that favor the oxidative addition step of the aryl halide.

Full text of DOI:10.1002/aoc.6195

Received: 16 October 2020
DOI: 10.1002/aoc.6195
Revised: 20 January 2021
Accepted: 22 January 2021
F U L L P A P E R
Robust alkyl-bridged bis(N-heterocyclic carbene)palladium
(II) complexes anchored on Merrifield's resin as active
catalysts for the selective synthesis of flavones and
alkynones
Waseem Mansour1
Bassam El Ali1
| Mohammed Fettouhi1
| Qasim Saleem2
|
1Chemistry Department, King Fahd
University of Petroleum & Minerals,
Dhahran, Saudi Arabia
2Research & Development Center, Saudi
Aramco, Dhahran, Saudi Arabia
Highly active and efficient propylene-bridged bis(N-heterocyclic carbene)palla-
dium(II) complexes covalently anchored on Merrifield's resin were synthesized
and characterized using various physical and spectroscopic techniques. The
two anchored Pd(II) complexes consist of the system: Merrifield's resin-
linker-bis(NHC)Pd(II), the linkers being benzyl and benzyl-O-(CH2)3 for
(Pd-NHC1@M) and (Pd-NHC2@M), respectively. The short linker anchored
bis-benzimidazolium ligand precursor (PBBI-1@M) was synthesized via
direct carbon–nitrogen alkylation of a propylene-bridged bis(benzimidazole)
(PBBI-1) by Merrifield's resin chlorobenzyl group. The longer linker anchored
bis-benzimidazolium ligand precursor (PBBI-2@M) was obtained in a two-
step reaction involving first alkylation of (PBBI-1) with 3-chloro-1-propanol
followed by a nucleophilic substitution at Merrifield's resin chlorobenzyl
group. Both supported ligand precursors (PBBI-1@M and PBBI-2@M)
reacted with palladium acetate to produce the two heterogeneous catalysts
(Pd-NHC1@M) and (Pd-NHC2@M). 13C NMR palladation shift of the benz-
imidazole N–C–N (C2) carbon was found very similar in both the liquid NMR
spectra of the homogeneous complexes and the CP/MASS spectra of the
corresponding covalently anchored complexes. The catalytic activity, stability,
and the recycling ability of the supported catalysts have been investigated in
the carbonylative Sonogashira coupling reactions of aryl iodides with aryl
alkynes and alkyl alkynes and also in the cyclocarbonylative Sonogashira
coupling reactions of aryl iodides with aryl alkynes via one pot reactions. The
longer linker catalyst Pd-NHC2@M demonstrated excellent catalytic activity,
stability, and very high recycling ability in the two carbonylative coupling
reactions. These systems exhibit the hypothesized thermodynamic stability
offered by the chelate effect in addition to the strong sigma donor ability of a
bis(NHC) ligand system generating electron-rich palladium centers that favor
the oxidative addition step of the aryl halide.
Correspondence
Bassam El Ali, Chemistry Department,
King Fahd University of Petroleum &
Minerals, Dhahran, Saudi Arabia.
Email: belali@kfupm.edu.sa
Funding information
Saudi Aramco, Grant/Award Number:
CHEM2432
Appl Organomet Chem. 2021;35:e6195.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6195

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MANSOUR ET AL.
K E Y W O R D S
alkynones, carbonylative Sonogashira coupling reaction, flavones, Merrifield's resin, Supported
Pd-NHC catalysts
1 | INTRODUCTION
contributing to the stability of the different intermediates
of the catalytic cycle and preventing deactivation pathways
Palladium catalyzes the carbonylative Sonogashira cou-
pling reactions of aryl iodides or 2-iodophenols with ter-
minal alkynes, in the presence of carbon monoxide, to
produce alkynones or flavones via one-step reactions.[1–8]
These represent essential industrial processes for the pro-
duction of monomers, pharmaceuticals, agrochemicals,
and natural products.
such as the formation of inactive palladium black.[41,42]
The stability of these complexes can be further enhanced
by the thermodynamic chelate effect such as the case of
the chelating bis(NHCs).[43–47]
More attention has been paid to the immobilization of
(NHC)palladium(II) complexes on solid supports, and
several supported catalytic systems have been reported
in literature.[48] They were used to catalyze various
organic reactions such as Heck–Mizoroki coupling
reactions,[49,50] Suzuki–Miyaura coupling reactions,[28,51,52]
and Sonogashira–Hagihara reactions.[53]
However, the reported systems suffer from some
weaknesses in their practical applications due to the
high loading of the catalyst, the leaching out of the
metal, the low recyclability, the harsh experimental
conditions, low efficiency, and low stability. Hence, it
is necessary to design and develop more efficient and
stable supported catalysts that can be recycled several
times and can be considered for potential industrial
applications.
We have assumed that the chelating bis(NHC)
ligand system would offer a thermodynamic stability to
the palladium complexes by chelate effect. On the
other hand, the strong sigma donor ability of the
bis(NHC) ligand system would generate electron-rich
palladium centers that favor the oxidative addition step
of the aryl halide. To the best of our knowledge, the
applications of supported palladium(II)-bridged bis
(N-heterocyclic carbene) catalysts in carbonylative
coupling reactions, especially in the carbonylative
Sonogashira coupling reactions to produce “alkynones”
or “flavones” have not been explored.
A plethora of literature publications describe active
and selective homogeneous palladium catalysts for the
carbonylative Sonogashira coupling reactions.[9–12] How-
ever, the recovery of the catalysts after the coupling reac-
tions is a tedious and costly process due to the difficulty
to separate by simple separation methods like filtration or
decantation. Subsequently, most homogeneous palladium
catalysts contaminate the final products, which leads to
complication during the industrial implementation for
these homogeneous catalysts.[13–15] On the other hand,
heterogeneous catalysts are recoverable easily and ther-
mally stable but have lower activity and require sever
conditions than their homogeneous counterparts.[16,17]
Therefore, in order to combine advantages of both hetero-
geneous and homogeneous catalysis, the catalyst species
can be immobilized onto insoluble solids, leading to
supported catalysts.[13,18–20] Apart from the easy separa-
tion of the catalyst from the coupling products, the
supported catalyst is effectively recyclable and can be
reused,[20,21] which makes it an economically viable alter-
native in the industry.[22–24] Taking into consideration
the intrinsic advantages of the improved supported cata-
lysts, there are increasing interests in immobilizing the
palladium catalysts onto different solid supports such
as functionalized silica,[25] functionalized organic
polymers,[26,27] clay,[13,20] magnetic nanoparticles,[28]
ZIF-,[29] MOF-,[30] and Pd/C.[31–35]
In continuation of our interest in exploring the chem-
istry and the catalytic activities of (N-heterocyclic
carbene)palladium(II) complexes,[37,45,54–56] this work
reports on strong covalent bond and chemical
anchoring of chelate bis(NHC)palladium(II) complexes
on Merrifield's resin. The stability of newly synthesized
supported catalysts (Pd-NHC@M) is enhanced by the
thermodynamic chelate effect of the bridged bis(NHC)
ligands.
Recently, N-heterocyclic carbene (NHCs) metal com-
plexes have emerged as versatile species in organometallic
chemistry with a rich potential in catalytic applications in
organic transformations. (NHC)palladium(II) complexes
were used efficiently in different applications including
carbonylative coupling and alkoxycarbonylation and
aminocarbonylation reactions.[36–40] The activity of (NHC)
palladium(II) complexes is connected to the bonding
nature of the NHC ligands including a strong σ-electron
donation, as well as a significant π acidity and π basicity,
The new anchored catalytic systems (Pd-NHC@M)
were evaluated successfully for the first time in
carbonylative Sonogashira coupling reactions of aryl

MANSOUR ET AL.
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iodides with terminal alkynes to produce alkynones and
in cyclocarbonylative Sonogashira coupling reactions of
ortho-functionalized aryl iodides with aryl alkynes to pro-
duce flavones. These reactions were conducted under CO
pressure and in the absence of phosphines ligands. The
catalytic efficiency, stability, and recovering ability of the
newly synthesized supported catalysts (Pd-NHC@M)
have been studied. In addition, the role of the chain
length that connects the NHC ligand and Merrifield's
resin on the catalytic activities of these catalysts was also
studied.
complexes were studied using scanning electron micro-
scope, JEOL JSM6610LV SEM. Thermogravimetric analysis
(TGA) using Perkin-Elmer TGA 7, USA, at a heating rate
of 10ꢀC min−1 up to 700ꢀC under nitrogen atmosphere was
used to establish the thermal stability of the supported
ligands and complexes. Elemental analyses were performed
on Perkin Elmer Series 11 (CHNS/O) Analyzer 2400.
Inductively coupled plasma-mass spectrometer, X-series
2 ICP-MS, Thermo Scientific, was used to estimate the
palladium loading on Merrifield's resin.
X-ray photoelectron spectroscopy (XPS) (Thermo
Scientific, ESCALAB-250Xi) was employed to find the
chemical composition of the supported bridged bis
(N-heterocyclic carbene)palladium(II) catalysts. The XPS
spectra were recorded using Al Kα radiation (1486.6 eV)
as excitation source. The take-off angle θ of the emitted
photoelectrons was adjusted to 45ꢀ with respect to the
surface normal.
2 | EXPERIMENTAL
2.1 | Materials
Merrifield's peptide resin (50–100 mesh, extent of label-
ling: 2.5–4.0 mmol/g Cl loading, 1% crosslinked with
divinylbenzene) was purchased from Sigma-Aldrich.
Materials for the synthesis of propylene-bridged
N-heterocyclic carbene ligand precursors and their
supported palladium(II) complexes were also purchased
from Sigma-Aldrich company and were used directly as
received. All solvents (reagent grade) used in the prepara-
tion of catalysts and their evaluation were distilled and
dried before use. Purification of the products was con-
ducted using flash column chromatography packed with
silica gel 170–400 mesh, Fisher chemical (Fisher scien-
tific, USA). Thin-layer chromatography (TLC) analyses
were conducted using Merck 60 F254 silica gel plates
(250-mm layer thickness).
2.3 | Synthesis of the propylene-bridged
bis(1H-benzo[d]imidazol-1-yl) (PBBI-1)
A dry and clean round bottom flask was charged with
benzimidazole (10.0 mmol), excess amount of
1,3-dibromopropane (5.50 mmol), and potassium hydrox-
ide (20.0 mmol) as a base. Then, 100 ml of distilled ace-
tonitrile was added, and the mixture was stirred at 80ꢀC
for 24 h. The reaction was monitored by TLC (50% hex-
ane: 50% ethyl acetate) until all benzimidazole was con-
sumed. After the reaction is complete, the solvent was
evaporated under vacuum using a rotary evaporator.
Oily products were obtained as residues, which were
purified by extracting three times with 30 ml of ethyl
acetate and 20 ml of distilled water. The separated
organic layers were washed several times by n-hexane,
and the aqueous layer was separated and then washed
with ethyl acetate.
2.2 | Instrumentation
1H and 13C NMR spectral data were collected using
500-MHz NMR spectrometer (Joel 1500 model) and
400-MHz NMR spectrometer (Bruker Avans). Chemical
shifts were reported in ppm downfield of tetramethylsilane,
as an internal standard, using different deuterated solvents
like CDCl3 and DMSO-d6. Solid-state NMR spectral
data were recorded using CP/MAS on a Bruker Avans
400-MHz machine and CP/MAS on a Varian NMRS
500-MHz spectrometer. IR spectra were recorded in wave
numbers (cm−1) using FT-IR spectrometer (PerkinElmer
16F model). The products were analyzed by gas
chromatography–mass spectrometry (Agilent GC-MS; GC
6890N and MS 5975B; 30-m HP-5 capillary column).
Agilent 6890 gas chromatography (GC) was used to moni-
tor the reactions and analyze the products. The morphol-
ogy of the supports, supported ligands, and supported
2.3.1 | 1,3-Bis(1H-benzo[d]imidazol-1-yl)
propane (PBBI-1)
1
Isolated yield = 73%; sticky brown oil product; H NMR
(500 MHz, DMSO-d6) δ (ppm): 8.14 (s, 2H, NCHN), 7.65
(2H, m, Ar-H), 7.63 (2H, m, Ar-H), 7.53–7.35 (4H, m,
3
Ar-H), 4.62 (4H, t, J = 7.32 Hz, NCH), 2.02 (2H, m,
NCH2(CH2)2); 13C NMR (125 MHz, DMSO-d6) δ (ppm):
143.1 (NCN), 133.8, 122.7, 121.7, 119.7, 110.5, (Ar-H),
41.9 (NCH2), 29.8 [NC (CH2)2]; Anal. Calcd for C17H16N4
(276.34), C, 73.89; H, 5.84, N; 20.27; Found: C, 74.83; H,
5.24; N, 19.93.

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MANSOUR ET AL.
2.4 | Synthesis of hydroxyl propyl
functionalized propylene-bridged bis(N-
heterocyclic carbene) ligand salt (PBBI-2)
2.5.1 | 1,3-Bis(1H-benzo[d]imidazol-1-yl)
ium chloride propane supported on
Merrifield's resin (PBBI-1@M)
1,3-Bis(1H-benzo[d]imidazol-1-yl)propane
(PBBI-1)
Isolated yield = 96%; Golden-brown solid; CP-MAS 13C
NMR: δ 28.2, 41.3, 46.8, 58.7, 64.6, 128.3, 130.6, 145.8; IR:
vmax (cm−1) 3441, 2923, 1634, 1400, 1118, 453.
(2.5 mmol) and 3-chloropropanol (5.0 mmol) were added
into a dried 100-ml round bottom flask. The mixture was
refluxed in 25 ml of 1,4-dioxane solvent with stirring at
103ꢀC for 24 h. The product appeared as a pink-white salt
precipitate. After decanting the solvent, the product was
washed three times using 10 ml of 1,4-dioxane followed
by 10 ml of toluene to remove any traces of the starting
materials. The product was dried under vacuum and col-
lected as a white precipitate. The characterization of the
1,10-(propan-1,3-diyl) bis(3-(2-hydroxyethyl)-1H-benzo[d]
imidazol-3-ium) chloride salt (PBBI-2) was conducted
using various spectroscopic techniques including 1H
NMR, 13C NMR, and elemental analysis.
2.6 | Synthesis of supported 1,10-(propan-
1,3-diyl)-bis(3-(3-(benzyloxy)propyl)-1H-
benzo[d]imidazol-3-ium) chloride ligand
precursor salt on Merrifield's resin
(PBBI-2@M)
In a dry flask, NaH (0.50 mmol) was added in one portion
to
a
stirred solution of 1,10-(propan-1,3-diyl) bis
(3-(2-hydroxypropyl)-1H-benzo[d]imidazol-3-ium) chlo-
ride (1.5 mmol) in dry DMF. The mixture was stirred for
2 h at room temperature under nitrogen atmosphere.
Merrifield's resin (1.2 g) was added, and the mixture was
stirred at 90ꢀC for 12 h. The solid product was filtered
and washed successively with DMSO, DMF, methanol,
water, acetone, and dichloromethane. The product was
dried at room temperature under vacuum and the
supported propylene-bridged bis(NHC) on Merrifield's
resin (PBBI-2@M) was obtained as a preligand. Many
physical and spectroscopic techniques were used to char-
acterize the (PBBI-2@M) supported preligand salts such
as NMR, FT-IR, SEM, TGA, and ICP-MS.
2.4.1 | 1,10-(Propan-1,3-diyl)bis
(3-(2-hydroxypropyl)-1H-benzo[d]imidazol-
3-ium) chloride salt (PBBI-2)
Isolated yield = 87%; pink-white product; 1H NMR
(500 MHz, DMSO-d6) δ (ppm): 9.92 (s, 2H, NCHN),
8.13–8.06 (4H, m, Ar-H), 7.79–7.67 (4H, m, Ar-H),
4.71–4.68 (4H, m, NCH2), 4.63–4.53 (4H, m, NCH2), 3.4
(4H, m 2CH2OH overlapping with H2O signal), 2.76 (2H,
broad OH), 2.6 (2H, m, CH2), 2.06–2.04 (4H, m, 2× CH2);
13C NMR (125 MHz, DMSO-d6) δ (ppm): 142.51 (NCN),
131.3, 131.2, 131.1, 126.8, 126.6, 113.8, 113.7, (Ar-H), 66.9
(NCH2), 57.6 (NCH2), 44.5, 44.1 (NCH2)2, 31.3, 30.8, 28.7,
28.3 CCH2; Anal. Calcd for C23H30N4O2Cl2 (465.41): C,
59.36; H, 6.50; N, 12.04; Found: C, 58.63; H, 5.54; N, 11.73.
2.6.1 | Supported 1,10-(propan-1,3-diyl)bis
(3-(3-(benzyloxy)propyl)-1H-benzo[d]
imidazol-3-ium) chloride on Merrifield's
resin (PBBI-2@M)
2.5 | Synthesis of supported propylene-
bridged bis(N-heterocyclic carbene) ligand
precursor on Merrifield's resin (PBBI-1@M)
Isolated yield = 93%; Golden-brown solid; CP-MAS 13C
NMR: δ 28.2, 41.1, 44.8, 114.1, 128.1, 130.6, 145.3; IR:
vmax (cm−1) 3431 (broad), 3024, 2922, 1639, 1565, 1491,
1448, 1206, 1022, 952, 756, 699.
Merrifield's resin (0.12 g) and 1.5 mmol (0.414 g) of 1,3-bis
(1H-benzo[d]imidazol-1-yl)propane (NHC1) were intro-
duced into a cleaned and dried 10-ml round bottom flask.
The mixture was refluxed in 25 ml of dimethyl sulfoxide
(DMSO) under stirring at 120ꢀC for 48 h. After completion
of the reaction, the dark brown precipitate was collected
by filtration and washed three times successively with
10 ml of DMSO, distilled water, and dichloromethane
(DCM) to remove any traces of the starting materials. The
golden-brown solid (S-NHC-1-@M) was dried under vac-
uum and characterized using different spectroscopic tech-
niques including CP-MAS NMR, FT-IR, TGA, and SEM.
2.7 | General procedure for the synthesis
of propylene-bridged bis(N-heterocyclic
Carbene) (NHC)palladium(II) supported
catalysts
The Merrifield resin-supported propylene-bridged bis
(N-heterocyclic carbene) ligand precursor (PBBI-1@M)
or (PBBI-2@M) (3.0 mmol, 1.0 g) were stirred in DMSO
for 30 min. Palladium(II) acetate (3.0 mmol, 0.67 g) was
then added, and the resulting mixture was stirred at 70ꢀC

MANSOUR ET AL.
5 of 19
for 24 h. The dark solid product was collected by filtra-
tion and washed thoroughly with THF, toluene,
1,4-dioxane, DCM, ethyl acetate, diethyl ether, and H2O
followed by a drying step under vacuum.
removed and dialyzed in acetonitrile to extract the prod-
ucts. The solid catalyst in the dialysis bag was dried in
the oven at 100ꢀC to be reused in the next experiments.
The organic mixture was extracted three times with 5 ml
of distilled water and 10 ml of ethyl acetate. The ethyl
acetate extracts were combined and concentrated in a
rotary evaporator under reduced pressure. Several tech-
niques were used to fully characterize the final carbonyl
2.7.1 | Merrifield's resin-supported
dichlorido-bis(1,3-bis(1H-benzo[d]imidazol-
1-yl)propane palladium(II) catalyst
(Pd-NHC1@M)
1
products including H, 13C NMR (Figures S1–S18), GC,
and GC-MS. The spectral data of the alkynones prepared
in this study were in full agreement with those reported
in the literature.[4,11,45,54,57,58]
Isolated yield = 83%; Dark brown solid; 13C NMR: δ
(ppm): 40.5, 127.9, 134.8, 145.9, 180.7 (Pd-C) carbene sig-
nal; IR: vmax (cm−1) 3430 (broad), 3024, 2921, 1960, 1938,
1602, 1565, 1490, 1448, 1022, 952, 700, 548; Metal loading
from ICP-MS: 1.2% corresponding to 0.12 mmol/g.
2.9 | General procedure for the
evaluation of the supported catalysts
Pd-NHC1@M and Pd-NHC2@M in
cyclocarbonylative Sonogashira coupling
reactions of 2-iodophenols and aryl
terminal alkynes
2.7.2 | Merrifield's resin-supported
dichlorido-bis(1,10-(propan-1,3-diyl) bis
(3-(3-(benzyloxy)propyl)-1H-benzo[d]
imidazol-1-yl)palladium(II) catalyst
(Pd-NHC2@M)
Chromones were synthesized via cyclocarbonylative
Sonogashira coupling reactions that were accomplished
in a 45-ml stainless steel autoclave equipped with a glass
liner, gas inlet valve, and pressure gauge. The
immobilized bridged bis(N-heterocyclic carbene)palla-
Isolated yield = 96%; Dark brown solid; CP-MAS 13C
NMR δ (ppm): 24.5, 40.8, 46.1, 112.3, 127.6, 134.0, 146.5,
178.4 (Pd-C) carbene signal; IR: vmax (cm−1) 3438, 3022,
2918, 2850, 1696, 1601, 1491, 1449, 1307, 1327, 1211,
1176, 1022, 829, 754, 699, 544; Metal loading from
ICP-MS: 8.7% corresponding to 0.83 mmol/g.
dium(II) catalyst contained in
a
dialysis bag
(0.015 mmol), substituted 2-iodophenol (0.50 mmol),
alkyne (0.55 mmol), base (1.0 mmol), and an anhydrous
solvent (2 ml) were charged into the glass liner of the
autoclave. Then the autoclave was vented carefully three
times with CO and then pressurized to 100 psi of
CO. The mixture was stirred and heated to the required
temperature for a specific period. At the end of the reac-
tion period, the autoclave was cooled down to room tem-
perature and the excess of CO was discharged carefully
under fume hood. The dialysis bag with catalyst was
removed and dialyzed in acetonitrile to extract all the
products. The solid catalyst in the dialysis bag was dried
in the oven at 100ꢀC to be used in the next experiment.
The organic mixture was extracted three times with 5 ml
of distilled water and 10 ml of ethyl acetate. The ethyl
acetate extracts were combined and concentrated in a
rotary evaporator under reduced pressure. Flash chroma-
tography was used to purify the reaction mixture using
silica gel and an eluent (pentane/ethyl acetate = 7/1) to
afford the corresponding chromones or flavones. Various
2.8 | General procedure for the
evaluation of the supported catalysts
Pd-NHC1@M and Pd-NHC2@M in
carbonylative Sonogashira coupling
reactions of aryl iodides and alkynes
The carbonylative Sonogashira coupling reactions were
conducted in a 45-ml stainless steel autoclave equipped
with a glass liner, a gas inlet valve, and a pressure gauge.
The immobilized bridged bis(N-heterocyclic carbene)pal-
ladium(II) catalyst (0.0050 mmol) was placed in a dialysis
bag, and aryl iodide (1.0 mmol), alkyne (1.2 mmol), Et3N
as a base (2.0 mmol) and a solvent (3 ml) were introduced
into the glass liner. The later was then carefully placed in
the autoclave. The autoclave was vented three times with
carbon monoxide and then pressurized to 200 psi of
CO. The mixture was heated to the required temperature
and stirred for the required time. After completion of the
reaction, the mixture was cooled down to room tempera-
ture and the CO excess was vented slowly and carefully
under fume hood. The dialysis bag with catalyst was
1
physical and spectroscopic techniques such as H and
13C NMR (Figures S19–S30), GC, and GC-MS were used
to fully characterize the products. The spectral data of
the chromones and flavones prepared in this study
were in full agreement with those reported in the
literature.[8,10,45,59–62]

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MANSOUR ET AL.
3 | RESULTS AND DISCUSSION
NHC2@M). It is worth mentioning that the ether bond
is considered suitable for linking the NHC ligand to the
support as the ether link is particularly stable under
the reaction conditions, which is expected to prevent the
leaching of the ligand during the catalytic application.
On the other hand, most of other functional groups such
as esters and amides can be easily hydrolyzed under the
standard conditions of the coupling reactions that
encounter the use of relatively strong bases.
Various physical, thermal, morphological, and spectro-
scopic characterizations have been conducted on the
supported ligand salts (PBBI-1@M) and (PBBI-2@M) and
their corresponding supported catalyst (Pd-NHC1@M)
and (Pd-NHC2@M) such as ICP-MS, 13C NMR, FT-IR, SEM,
XPS, and TGA (Figures S31–S44).
3.1 | Synthesis and characterization of
the propylene-bridged bis(N-heterocyclic
carbene) (NHC)palladium(II) supported on
Merrifield's resin (Pd-NHC-@M)
The 1,3-bis(1H-benzo[d]imidazol-1-yl)propane (PBBI-1) was
prepared from the N-alkylation of 2 mol equivalent of
benzimidazole with 1 mol equivalent of 1,3-dibromopropane
in the presence of potassium hydroxide (Scheme 1).
The
1,10-(propan-1,3-diyl)-bis(3-(2-hydroxyethyl)-
1H-benzo[d]imidazol-3-ium) chloride salt ligand precursor
PBBI-2 was synthesized by the direct alkylation of
1,3-bis(1H-benzo[d]imidazol-1-yl)propane PBBI-1 with
3-chloropropanol (Scheme 2). The formation of the hydroxyl
propyl functionalized bridged bis(benzimidazolium chloride)
salt was confirmed by the presence of a downfield
singlet peak at δ 9.92 ppm PBBI-1, in its proton NMR
spectrum, which is assigned to C-2 protons of the
benzimidazolium rings.
3.1.1 | Infrared spectroscopy
The characteristic methylene (CH2) wagging mode of the
benzyl chloride group in the FT-IR spectrum of the Mer-
The propylene-bridged bis(NHC) ligand precursor
rifield resin is observed at 1264 cm .
−1 [26,34,50] This band is
1,3-bis(1H-benzo[d]imidazol-1-yl)propane
(PBBI-1)
absent in the spectra of the anchored ligand precursor
salts PBBI-1@M and PBBI-2@M.
reacted with benzyl chloride functionalized Merrifield's
resin support (PS) to obtain the immobilized NHC
ligand via N-C alkylation. The immobilized propylene-
bridged bis(NHC) preligand on Merrifield's resin
(PBBI-1@M) was reacted further with palladium(II)
acetate to afford the supported palladium(II)-NHC
catalyst (Pd-NHC1@M) (Scheme 3, I).
The benzimidazolium vibrational analysis has been
studied both experimentally and computationally.[63]
The out-of-phase N–C–N stretching was theoretically
well predictable and assigned to bands observed at 1561
and 1547 cm−1 for the dimethyl and diadamantyl
N-substituted benzimidazoliums, respectively. In the
present work, both PBBI-1@M and PBBI-2@M
show a medium intensity band at 1565 cm−1. This band
is assigned to the out of phase N–C–N stretching of
the anchored benzimidazolium. Additionally, a new
band at 1206 cm−1, observed for PBBI-2@M, is
consistent with the asymmetrical C–O–C stretching
mode of the new ether functional group.[64] All these
data are consistent with the successful alkylation and
The
hydroxyl-functionalized
propylene-bridged
bis(NHC) ligand PBBI-2 reacted with chloro-
functionalized Merrifield's resin support to afford the
immobilized NHC ligand via an ether linkage between
the ligand and the support (Scheme 3, II). This
immobilized ligand on Merrifield's resin (PBBI-2@M)
was further reacted with palladium(II) acetate to yield
the supported palladium(II)-NHC catalyst (Pd-
SCHEME 1 Synthesis of 1,3-bis(1H-benzo
[d]imidazol-1-yl)propane (PBBI-1)
SCHEME 2 Synthesis of hydroxyl propyl
functionalized propylene-bridged bis(N-
heterocyclic carbene) ligand precursor salt
((PBBI-2)

MANSOUR ET AL.
7 of 19
SCHEME 3 Synthesis of
anchored ligands (PBBI-1@M
and PBBI-2@M) and their
dichloridopalladium(II) catalysts
(Pd-NHC1@M and Pd-
NHC2@M) (PS = Merrifield's
resin)
substitution reactions leading to the anchored ligand
precursors in PBBI-1@M and PBBI-2@M, respectively
(Figures S39–S42).
A significant palladation downfield shift relative to the
benzimidazolium ligand precursor salts is observed, in
consistence with the strong sigma donation of the NHC
ligands. Pd-NHC1@M and Pd-NHC2@M spectra show
the Pd-C2 carbon chemical shift at 180.7 and 178.4 ppm,
respectively, in a similar range as the literature reported
palladium bridged bis(NHC) complexes and supported
bridged bis(NHC) palladium complexes.[43,45,55,67,68] Inter-
estingly, a very similar palladation shift of 35 ppm was
recorded in liquid NMR, for benzyl substituted ligand salt
precursor and its palladium(II) complex, and solid state
NMR for Merrifield's resin-anchored ligand precursor and
its palladium(II) complex (Figures 1 and S35–S38).
3.1.2 | 13C CP/MAS NMR spectroscopy
The Merrifield resin-anchored ligand precursors and
their palladium complexes were further characterized
using solid state 13C CP/MAS NMR spectroscopy. The
spectra of the anchored ligand precursors and their palla-
dium complexes were consistent with those reported for
similar free bridged bis(NHC) ligands and their
corresponding complexes.[55] The 13C NMR chemical
shift of the benzimidazole N–C–N (C2) carbon is a well-
established probe of palladation and correlates with the
Lewis acidity of the metal.[65,66]
The palladium loading on the Merrifield resin-
supported dichlorido-bridged bis(NHC) palladium(II) cat-
alysts were estimated using ICP-MS technique and was
found to be 1.24% and 8.70% for Pd-NHC1@M and

8 of 19
MANSOUR ET AL.
FIGURE 1 13C NMR signals for NCN (C2) carbon showing the effect of palladation. (a) Liquid 13C NMR chemical shift of the benzyl
substituted ligand precursor and its bridged bis(N-heterocyclic carbene)palladium(II) complex; (b,c) 13C CP/MAS NMR chemical shifts of
Merrifield's resin (PS)-anchored ligand precursor salts and their bridged bis(N-heterocyclic carbene)palladium(II) complexes

MANSOUR ET AL.
9 of 19
FIGURE 2 Scanning electron micrograph of
(a) Merrifield's resin, (b) Merrifield's resin-
supported (PBBI-2@M) ligand, and
(c) Merrifield's resin-supported Pd-NHC2@M
catalyst

10 of 19
MANSOUR ET AL.
Pd-NHC2@M, respectively. This corresponds to 0.12
and 0.83 mmol of palladium per gram of supported cata-
lyst, respectively.
Thermogravimetric analysis (TGA) was used to study
the thermal stability of the Merrifield resin NHC-
supported ligand precursors and their palladium com-
plexes.[26,34] The data are consistent with a high thermal
stability and decomposition temperatures in the range of
370–375ꢀC (Figure S44).
ligand and the Pd-NHC2@M complex. The morphologi-
cal changes observed were similar to those reported for
other polystyrene supported ligands and their
corresponding palladium complexes.[26,34,52,70]
3.2 | Carbonylative Sonogashira coupling
reactions of aryl iodides with aryl and alkyl
terminal alkynes by supported bridged
bis(NHC)-Pd complexes on Merrifield's
resin
The X-ray photoelectron spectroscopy (XPS) spectrum
of the palladium(II) supported catalysts showed peaks in
the range of 330.08 to 345.08 eV. For instance, for cata-
lysts (Pd-NHC1@M) two distinctive 3d peaks were iden-
tified. The first peak with binding energy of 336.08 eV
(Pd3d5/2) and the second peak with binding energy of
341.08 (Pd3d3/2), whereas for catalyst (Pd-NHC2@M),
other two distinctive 3d peaks were identified. The first
peak is with binding energy of 336.08 eV (Pd3d5/2) and
the second peak is with binding energy of 340.28 eV
(Pd3d3/2).[26,34] These data are consistent with
palladium(II) species, being the main form of palladium
in the supported catalyst.[29,50,68,69]
The morphology of the Merrifield resin, the Merrifield
resin-supported ligands PBBI, and the Merrifield's resin
supported with dichloridopalladium(II)-NHC complexes
were studied by scanning electron microscopy (SEM)
micrographs for a single bead of pure Merrifield's resin
(Figure 2a), the supported PBBI-2 ligand (Figure 2b),
and the supported dichloridopalladium(II) complex
Pd-NHC2@M (Figure 2c). As expected, the SEM micro-
graphs of the Merrifield resin showed smooth and flat
microsphere surfaces that have been broken to a rough
and irregular surface after incorporation of the PBBI
The recycling ability of the new supported propylene-
bridged bis(N-heterocyclic carbene) palladium(II) cata-
lysts was investigated in the carbonylative Sonogashira
coupling of 2-iodophenol (1a) and phenylacetylene (2a)
at 100ꢀC for 12 h in toluene as a solvent. The supported
Pd-(NHC)@Merrifield's resin catalyst was packed in a
membrane tube fabricated from a commercially available
dialysis bag. Only the reactants and products can easily
pass through the membrane allowing the migration of
low molecular weight molecules and keeping the catalyst
inside the dialysis bag. The transport of the reactants and
products is driven by the concentration gradient and tem-
perature of the reaction (100ꢀC).[71,72] The adopted model
reaction for the carbonylative Sonogashira coupling
includes 4-iodoanisole with phenylacetylene under CO
pressure in the presence of a catalytic amount of the
supported bridged bis(NHC)-Pd catalyst (Pd-NHC1@M
or Pd-NHC2@M) (Equation 1). The results of the opti-
mization reactions are summarized in Table 1. When the
reactions were conducted with 1.0 mol% of palladium
catalyst
Pd-NHC1@M
or
Pd-NHC2@M
and
TABLE 1 Optimization of the
carbonylative Sonogashira coupling
reactions of 4-iodoanisole (1a) and
phenylacetylene (2a)
Entry
NHC-Pd@M
Pd-NHC2@M
Pd-NHC1@M
Pd-NHC2@M
Pd-NHC1@M
Pd-NHC2@M
Pd-NHC1@M
Pd-NHC2@M
Pd-NHC1@M
Pd-NHC2@M
Pd-NHC1@M
Pd-NHC2@M
Pd-NHC2@M
Cat (mol%)
1.0%
T (ꢀC)
100
100
100
100
100
100
100
100
100
100
80
Time (h)
Yield %a
99
1
18
18
18
18
18
18
12
12
9
2
1.0%
99
3
0.50%
0.50%
0.25%
0.25%
0.50%
0.50%
0.50%
0.50%
0.50%
0.50%
99
4
76
5
70
6
35
7
99
8
60
9
80
10
11
12
9
43
12
12
82
50
52
Note: Reaction conditions: 4-iodoanisole (1.0 mmol), phenylacetylene (1.5 mmol), base (2.0 mmol), solvent
(5 ml), CO (200 psi).
aIsolated yield.

MANSOUR ET AL.
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triethylamine as a base at 100ꢀC for 18 h, high isolated
yield (99%) of 1-(4-methoxyphenyl)-3-phenylprop-2-yn-
1-one (3aa) (Table 1, entries 1 and 2) was obtained as a
product. The isolated yield of the product 3aa is
maintained very high (99%) when the catalyst loading of
Pd-NHC2@M decreased to 0.50 mol%; however, the
yield decreased to 76% with 0.50 mol% of Pd-NHC1@M
(Table 1, entries 3 and 4). These results confirmed the
crucial role of the propylene linker between the support
and N-heterocyclic carbene in Pd-NHC2@M catalyst.
This linker enhanced the free motion of the
palladium active sites inside the solution, whereas the
Pd-NHC1@M was more rigid and the steric effect
reduced the catalytic efficiency as compared with
Pd-NHC2@M. Similarly, the isolated yields of 3aa
dropped to 70% and 35%, respectively, when the loading
of the catalysts was decreased to 0.25 mol% for
Pd-NHC2@M and Pd-NHC1@M (Table 1, entries 5 and 6).
It is important to note that when the reaction time
was decreased from 18 to 12 h with 0.5 mol% of
Pd-NHC2@M, the isolated yield of the product 3aa
remained excellent (99%). However, a decrease from 99%
to 80% was observed after 9 h of reaction under the same
optimized conditions (Table 1, entries 7 and 9). On the
other hand, 0.5 mol% of Pd-NHC1@M produced lower
isolated yields of 3aa after 12 and 9 h (60% and 43%,
respectively) (Table 1, entries 8 and 10).
turnover frequencies were estimated as 192 cycles/h. In
order to confirm the activity of the new supported cata-
lysts, we have conducted an experiment with a ratio of
4-iodoanisole (12.0 mmol) to supported palladium-bis
(NHC) catalyst (0.005 mmol) for 12 h. The turnover of
Pd-NHC2@M was estimated to 2364. A complete con-
version of 4-iodophenol and excellent isolated yield (99%)
was obtained for the Pd-NHC2@M.
3.4 | Carbonylative Sonogashira coupling
of various aryl iodides with different aryl
alkynes and by Pd-NHC2@M, scope of
substrates
The supported catalyst Pd-NHC2@M was evaluated in
the carbonylative Sonogashira coupling reactions of var-
ious aryl iodides with different alkynes. We have con-
ducted the carbonylative coupling reactions under the
optimized experimental conditions (0.50 mol% of
Pd-NHC2@M, 2.0 equivalent of Et3N, 3 ml of toluene,
200 psi CO, 100ꢀC, 12 h). Various aryl iodides reacted
with different functionalized aryl and alkyl alkynes
(Equation 2, Table 2). Important alkynones were pro-
duced in excellent isolated yields via carbonylative
Sonogashira coupling reactions of various aryl alkynes
with different electron-withdrawing and electron-
donating substituted aryl iodides. The carbonylative
Sonogashira coupling reaction of iodobenzene with
phenylacetylene in the presence 0.50 mol% of
Pd-NHC2@M produced the corresponding alkynone in
excellent isolated yield (95%) (Table 2, entry 1). The
carbonylative Sonogashira coupling reaction of
As expected, the reduction of temperature from 100ꢀC
to 80ꢀC and 50ꢀC under the optimized conditions
(toluene/Et3N/Pd-NHC1@M, 0.50 mol%/12 h) decreased
the isolated yields of the alkynone 3aa to 82% and 52%,
respectively (Table 1, entries 11 and 12). Based on the
results of the optimization experiments, the subsequent
reactions were carried out using Et3N as a base, toluene
as a solvent, and at 100ꢀC for 12 h.
4-iodoanisole
“activated
aryl
iodide”
with
phenylacetylene (Table 2, entry 2) and other activated
3.3 | Carbonylative Sonogashira coupling
reaction of 4-iodoanisole with
phenylacetylene by Pd-NHC2@M, recycling
ability of the catalyst
substituted aryl acetylenes (Table 2, entries 3 and 4)
afforded the corresponding alkynones in excellent yields
(93%–98%). The carbonylative Sonogashira coupling
reactions of activated aryl acetylenes with various
deactivated aryl iodides were also very successful lead-
ing to excellent yields (90%–92%) (Table 2, entries
5 and 6).
The investigation of the carbonylative Sonogashira
coupling reactions of aryl iodides with alkyl alkynes have
been conducted in the absence of any phosphine ligands.
Interestingly, Pd-NHC2@M was an active catalyst in the
The recycling ability of the supported catalyst
Pd-NHC2@M was carefully studied (Figure 3, Equa-
tion 1). The supported catalyst Pd-NHC2@M could be
recycled up to 12 times without bearing a significant loss
in its catalytic activity. The turnover of Pd-NHC2@M
was estimated for the 12 cycles as 2297, whereas the

12 of 19
MANSOUR ET AL.
FIGURE 3 Merrifield's resin-
supported palladium-bis(NHC) as
catalyst for carbonylative Sonogashira
coupling reaction of 4-iodoanisole with
phenylacetylene, recycling ability of the
catalyst system
carbonylative coupling reactions of different aryl iodides
with various alkyl alkynes with a catalyst loading of
0.50 mol% under the optimized conditions (Et3N/
Toluene/100ꢀC/200 psi of CO/24 h) (Equation 2). The
results of these reactions are summarized in Table 2. For
instance, the carbonylative coupling reactions of
iodobenzene with 1-heptyne (Table 2, entry 7) and
1-pentyne (Table 2, entry 8) produced the corresponding
alkynones in very good yield (79%–83%). As expected, the
results showed that the linear alkynes such as 1-heptyne
(79%) and 1-pentyne (83%) (Table 2, entries 7 and 8) are
relatively more reactive than the branched aliphatic
alkynes such as prop-2-yn-1-ylcyclohexane (58%)
(Table 2, entry 9) under the optimized conditions likely
due to the steric effect.
publications[10,50] with palladium(II) catalysts as homog-
enous catalysts were adopted in this study. The new
supported bridged bis(NHC)-palladium(II) catalyst
Pd-NHC2@M
was
used
to
catalyze
the
cyclocarbonylative Sonogashira coupling reaction of
2-iodophenol with phenylacetylene at the same opti-
mized conditions but with a catalyst loading of 1.5 mol
% (DMF/Et2NH/100 psi/100ꢀC/24 h) (Equation 3). The
recycling ability of Pd-NHC2@M was also examined
(Figure 4). The supported palladium catalyst Pd-
NHC2@M could be recycled up to 10 times with only
a small loss in the catalytic activity. The turnover of
Pd-NHC2@M was estimated for 10 cycles as
627, whereas the turnover frequency was estimated as
26/h. In order to confirm the efficiency of Pd-
3.5 | Cyclocarbonylative Sonogashira
coupling reactions of 2-iodophenols with
aryl terminal alkynes by Pd-NHC2@M,
recycling ability of the catalyst
NHC2@M, we have conducted an experiment with
2-iodophenol (10.0 mmol) using 0.015 mmol of Pd-
NHC2@M for 24 h. The turnover of Pd-NHC2@M
was calculated as 647, whereas the turnover frequency
was found to be 30/h. A complete conversion of
2-iodophenol and excellent isolated yield of product
(97%) using Pd-NHC2@M was observed.
The optimized conditions for the cyclocarbonylative
coupling reactions that were used in previous

MANSOUR ET AL.
13 of 19

14 of 19
MANSOUR ET AL.

MANSOUR ET AL.
15 of 19
3.7 | Characterization of the recovered
supported bridged bis(NHC)palladium
(II) catalyst (Pd-NHC2@M)
The ability to recover and reuse the supported bridged
bis(NHC)palladium(II) catalyst, Pd-NHC2@M, for sev-
eral times and in different carbonylative coupling reac-
tions without significant loss in the catalytic activity
confirms its high stability. The interesting results
obtained with the supported catalysts require further
investigations to asses any possible change in the struc-
ture of the used catalysts as compared with the fresh cata-
lysts. The catalysts recovered after the 12th cycle from
the carbonylative Sonogashira coupling and from the
10th cycle from the cyclocarbonylative Sonogashira cou-
pling reactions were carefully washed with distilled
water, acetone, DMF, and toluene. After drying in the
oven at 100ꢀC, the recovered catalyst was analyzed by
FT-IR and XPS, and the quantitative analysis of palla-
dium was established using ICP-MS.
3.6 | Cyclocarbonylative Sonogashira
coupling of various 2-iodophenols with
different aryl alkynes by Pd-NHC2@M,
scope of substrates
The active supported catalyst Pd-NHC2@M was used
efficiently in the cyclocarbonylative Sonogashira coupling
reactions of various 2-iodophenols with different alkynes
under optimized conditions (Equation 4, Table 3). The
reactions were conducted using Pd-NHC2@M under the
optimized experimental conditions (1.5 mol%, 2.0 equiv.
Et2NH, 3 ml of DMF, 100 psi, 100ꢀC, 24 h). Various chro-
mones and flavones were produced in excellent to very
good yields via cyclocarbonylative Sonogashira coupling
reactions of aryl alkynes having different electron with-
drawing substituted 2-iodophenols. For instance, the
cyclocarbonylation reaction of 2-iodophenol with alkynes
that have activating electron releasing groups afforded
the corresponding flavones in very good to excellent iso-
lated yields (84%–90%) (Table 3, entries 2 and 3). The
cyclocarbonylative Sonogashira coupling reactions of
phenylacetylene and aryl alkynes having electron donat-
The FT-IR spectrum of the recovered supported cata-
lyst Pd-NHC2@M was found to be similar to the spec-
trum of the freshly prepared sample (Figures S45a
and S45b).
The percentage of palladium in the recovered
Pd-NHC2@M was estimated using ICP-MS and was
found to be 8.2% and 7.7%, respectively, after the 12th
cycle of the carbonylative and the 10th cycle of the
cyclocarbonylative coupling reactions. The fresh catalyst
has a palladium loading of 8.7%. This corresponds to a
loss of 5.7% in the carbonylative reaction against 11.5% in
the cyclocarbonylative counterpart. These data are con-
sistent with the observed higher recycling ability of
Pd-NHC2@M in the carbonylative coupling reaction
compared with the cyclocarbonylative analog.
ing substituents with activated and deactivated
2-iodophenols were also very successful with excellent
isolated yields (88%–96%) (Table 3, entries 4–6).
The XPS of the freshly prepared supported
palladium(II) catalyst Pd-NHC2@M and the 12th cycle
recovered material were analyzed (Figure 5). The 3d
FIGURE 4 Merrifield's resin-
supported palladium-bis(NHC) as
catalyst for cyclocarbonylative
Sonogashira coupling reaction of
2-iodophenol with phenyl acetylene,
recycling ability of the catalyst

16 of 19
MANSOUR ET AL.
TABLE 3 Cyclocarbonylative Sonogashira coupling reactions of 40-hydroxy-30-iodoaryls (6a–6c) with aryl alkynes (2a–2d) catalyzed by
the Pd-NHC2@M
Entry
40-Hydroxy-30-iodoaryl 6a–6c
Aryl alkyne 2a–2d
Product 7aa–7bd
Yield (%)a
1
2a
96
6a
7aa
2
90
6a
6a
2b
7ab
3
84
2d
2a
7ad
4
96
6b
7ba
5
2a
88
6c
6b
7dc
6
2d
92
7bd
Note: Reaction conditions: Pd-NHC2@M (1.50 mol%), 40-hydroxy-30-iodoaryl (0.50 mmol), aryl alkyne (0.60 mmol), Et2NH (1.0 mmol), DMF (2.5 ml), CO
(100 psi), 100ꢀC, 24 h.
aIsolated yield.

MANSOUR ET AL.
17 of 19
FIGURE 5 XPS spectrum of Pd-
NHC2@M recovered from carbonylative
Sonogashira coupling reaction showing
the Pd3d (left). Spectrum of fresh
catalyst is displayed on the right
spectra resolved into 3d5/2 and 3d3/2 spin orbit pairs with
binding energies of 336.08 and 341.08 eV. The spectral
data are consistent with robust palladium(II) species after
12 cycles of the carbonylative Sonogashira coupling
reaction.[29,68,69]
chelate effect of a chelating ligand. This chelate effect is
added to the strong sigma donor ability of a bis(NHC)
ligand system to generate electron-rich palladium centers
that favor the oxidative addition step of the aryl halide.
These systems are highly active catalysts in the formation
of various alkynones and flavones via carbonylative and
cyclocarbonylative Sonogashira coupling reactions of var-
ious aryl iodides with a wide range of aryl alkynes and
alkyl alkynes. They also show an excellent stability and
recycling ability.
3.8 | Palladium leaching test
The main purpose for supporting a homogeneous catalyst
is to enable its easy separation from the product, to
recover the catalyst, and to reduce the metal contamina-
tion levels. ICP-MS was used to analyze the possible pal-
ladium leaching into the solution. After the 12th cycle of
the carbonylative Sonogashira coupling reactions and
10th cycle of the cyclocarbonylative Sonogashira coupling
reactions, the filtrates of the reaction mixtures were col-
lected and the solvent was evaporated. The residues were
digested using concentrated nitric acid, and the solutions
were then analyzed using ICP-MS. The results showed
that the average concentration of palladium is less than
10 ppb for the carbonylative reaction against 127 ppb for
the cyclocarbonylative reaction. This corresponds to a
leaching of 0.475% and 1.34% of the total palladium in
the supported catalyst for the two reactions. These data
are consistent with the lower recycling ability encoun-
tered during the cyclocarbonylative Sonogashira coupling
reaction and the higher leaching, in this case, is likely
due to the reaction conditions, such as a solvent (DMF)
coordination effect and the longer reaction time (24 h).
ACKNOWLEDGMENTS
Saudi Aramco is acknowledged for funding this project
under the award number CHEM2432. We also acknowl-
edge the chemistry department at King Fahd University
of Petroleum & Minerals (KFUPM) for all support.
AUTHOR CONTRIBUTIONS
Waseem Mansour: Conceptualization; data curation; for-
mal analysis; investigation; methodology. Mohammed
Fettouhi: Conceptualization; data curation; formal
analysis; funding acquisition; investigation; methodology.
Qasim Saleem: Data curation; investigation; methodology.
Bassam El Ali: Conceptualization; data curation; formal
analysis; funding acquisition; investigation; methodology;
project administration; resources; software; supervision;
validation; visualization.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are avail-
able in the supporting information of this article.
4 | CONCLUSION
ORCID
Waseem Mansour https://orcid.org/0000-0002-9023-
0971
Mohammed Fettouhi https://orcid.org/0000-0003-2969-
7924
Qasim Saleem https://orcid.org/0000-0002-4260-6639
Bassam El Ali https://orcid.org/0000-0003-4005-6386
In this work, we report the successful covalent anchoring
of chelating bis(N-heterocylic carbene) ligand precursors
and their corresponding palladium(II) complexes on
Merrifield's resin. The anchored complexes exhibit the
hypothesized thermodynamic stability offered by the

18 of 19
MANSOUR ET AL.
[32] R. S. Mane, B. M. Bhanage, J. Org. Chem. 2016, 81, 1223.
[33] J. Liu, J. Chen, C. Xia, J. Catal. 2008, 253, 50.
[34] M. B. Ibrahim, I. M. Abdullah, R. Suleiman, M. Fettouhi, B. El
Ali, Arab. J. Sci. Eng. 2018, 43, 271.
REFERENCES
[1] L. Cassandra, T. Y. Bolshan, J. Visualized Exp. 2015, 9, 52401.
[2] T. Kobayashi, M. Tanaka, J. Chem. Soc., Chem. Commun.
1981, 333.
[35] M. Islam, P. Mondal, A. S. Roy, K. Tuhina, S. Mondal, D.
Hossain, Synth. Commun. 2011, 41(17), 2583.
[36] A. Aktas¸a, D. Barut, C. Ruya, K. Parham, T. Yetkin, G.
[3] M. S. M. Ahmed, A. Mori, Org. Lett. 2003, 5, 3057.
[4] B. Liang, M. W. Huang, Z. J. You, Z. C. Xiong, K. Lu, R. Fathi,
J. H. Chen, Z. Yang, J. Org. Chem. 2005, 70, 6097.
[5] A. S. Karpov, E. Merkul, F. Rominger, T. J. Müller, Angew.
Chem., Int. Ed. 2005, 44, 6951.
_
Muhittin, A. I. Gülçinc, Polyhedron 2019, 159, 345.
[37] V. H. Nguyen, M. B. Ibrahim, W. W. Mansour, B. M. El Ali,
H. V. Huynh, Organometallics 2017, 36, 2345.
[38] A. Kumar, D. Yuan, H. V. Huynh, Inorg. Chem. 2019, 58, 7545.
[39] W. Tao, X. Wang, S. Ito, K. Nozaki, J. Polym. Sci., Part a:
Polym. Chem. 2019, 57, 474.
[6] R. G. Harvey, J. T. Hahn, M. Bukowska, H. Jackson, J. Org.
Chem. 1990, 5525, 6161.
~
[7] F. Ferreira, A. Fortuna, A. Falcao, G. Alves, Phytochem. Rev.
2015, 14, 233.
[40] W. W. Mansour, R. Suleiman, M. Fettouhi, B. El Ali, ACS
Omega 2020, 5(37), 23687.
[41] T. Wang, H. Xie, L. Liu, W. X. Zhao, J. Organomet. Chem.
2016, 804, 73.
[42] X. Ma, H. Wang, W. Chen, J. Org. Chem. 2014, 79, 8652.
[43] H. V. Huynh, J. Ramasamy, J. Organomet. Chem. 2011, 696,
3369.
[8] Q. Yang, H. Alper, J. Org. Chem. 2010, 75, 948.
[9] J. Liu, M. Liu, Y. Yue, N. Zhang, Y. Zhang, K. Zhuo, Tetrahe-
dron Lett. 2013, 14, 1802.
[10] S. Xu, H. Sun, M. Zhuang, S. Zheng, Y. Jian, W. Zhang, Z.
Gao, Mol. Catal. 2018, 452, 264.
[11] P. Gautam, N. J. Tiwari, B. M. Bhanage, ACS Omega 2019, 4,
1560.
[44] D. Zhang, Y. Hea, J. Tanga, Dalton Trans. 2016, 45, 11699.
[45] W. W. Mansour, M. Fettouhi, B. El Ali. ACS Omega. 2020,
https://doi.org/10.1021/acsomega.0c04706, 5, 32515.
[46] H. V. Huynh, S. S. H. Chia, N. Y. S. Leong, Organometallics
2020, 39(22), 3999.
[47] D. Zhang, J. Yu, Organometallics 2020, 39(4), 605.
[48] R. Zhong, A. C. Lindhorst, F. J. Groche, F. E. Kühn, Chem.
Rev. 2017, 117, 1970.
[49] B. Karimi, D. Enders, Org. Lett. 2006, 86, 1237.
[50] E. Mohammadi, B. Movassagh, J. Mol. Catal. 2016,
418, 158.
[51] E. Mohammadi, B. Movassagh, J. Organomet. Chem. 2016,
822, 62.
[12] J. M. Liu, X. G. Peng, W. Sun, Y. W. Zhao, C. G. Xia, Org. Lett.
2008, 10, 3933.
[13] W. Jondi, A. Zyoud, W. W. Mansour, A. Q. Hussein, H. S.
Hilal, React. Chem. Eng. 2016, 1, 194.
[14] J. Hagen, Industrial catalysis: a practical approach, John
Wiley & Sons 2015.
[15] (a)V. Polshettiwar, C. Len, A. Fihri, Coord. Chem. Rev. 2009,
253, 2599. (b)Y. Wang, S. De, N. Yan, Chem. Commun. 2016, 2,
6210. (c)W. Niu, Y. Gao, W. Zhang, N. Yan, X. Lu, Angew.
Chem. Int. Ed. 2015, 54, 8271.
[16] A. Z. Fadhel, P. Pollet, C. L. Liotta, C. A. Eckert, Molecules
2010, 15, 8400.
[17] C. H. McAteer, R. Murugan, Y. V. S. Rao, Adv. Heterocycl.
Chem. 2017, 121, 173.
[18] H. Gruber-Woelfler, P. F. Radaschitz, P. W. Feenstra, W. Haas,
J. G. Khinas, J. Catal. 2012, 286, 30.
[19] F. Rao, S. Deng, C. Chen, N. Zhang, Catal. Commun. 2014,
46, 1.
[20] A. Zyoud, W. Jondi, W. Mansour, M. A. M. Khan, H. S. Hilal,
Chem. Cent. J. 2016, 10, 12.
[21] K. Hallamn, C. Moberg, Tetrahedron: Asymmetry 2001, 12,
1475.
[22] C. Torborg, M. Beller, Adv. Synth. Catal. 2009, 351, 3027.
[23] J. Heveling, Chem. Educ. 2012, 89, 1530.
[24] W. W. Ayass, J. F. Miñambres, P. Yang, T. Ma, Z. Lin, R.
Meyer, H. Jaensch, A.-J. Bons, U. Kortz, Inorg. Chem. 2019,
589, 5576.
[25] R. S. Mane, T. Sasaki, B. M. Bhanage, RSC Adv. 2015, 5, 94776.
[26] J. J. Mannion, T. S. Wang, Spectrochim. Acta 1964, 20, 45.
[27] Y. Lei, L. Wu, X. Zhang, H. Mei, Y. Y. Gu, G. Li, J. Mol. Catal.
2015, 398, 164.
[52] J.-H. Kim, J.-W. Kim, M. Shokouhimehr, Y.-S. Lee, J. Org.
Chem. 2005, 70(17), 6714.
[53] V. N. Mikhaylov, V. N. Sorokoumov, D. M. Liakhov, A. G.
Tskhovrebov, I. A. Balova, Catalysts 2018, 8(4), 141.
[54] M. Ibrahim, I. Malik, W. Mansour, M. Sharif, M. Fettouhi, B.
El Ali, Appl. Organometal. Chem. 2018, 32, e4280.
[55] W. Mansour, M. Fettouhi, B. El Ali, Appl. Organometal. Chem.
2020, 34, 5636.
[56] M. Brahim, I. Malik, W. Mansour, M. Sharif, M. Fettouhi, B.
El Ali, J. Organomet. Chem. 2018, 859, 44.
[57] M. B. Ibrahim, I. Malik, S. M. Shakil, H. Atif, M. Fettouhi, B.
El Ali, Trans. Met. Chem. 2016, 41, 739.
[58] X. F. Jiliang, S. M. Bao, J. Chin. Chem. Soc. 2018, 65, 337.
[59] L. Xue, L. Shi, Y. Han, C. Xia, H. Huynh, F. Li, Dalton Trans.
2011, 40, 7632.
[60] X. Zhao, J. Zhou, S. Lin, X. Jin, R. Liu, R. Org. Lett. 2017,
195, 976.
[61] H. Miao, Z. Yang, Org. Lett. 2000, 212, 1765.
[62] F. Zhu, Y. Li, Z. Wanga, I.-F. Wu, Catal. Sci. Technol. 2016, 6,
2905.
[28] Z. Wang, Y. Yu, Y. X. Zhang, S. Z. Li, H. Qian, Z. Y. Lin, Green
Chem. 2015, 17, 413.
[63] K. Malek, A. Puc, G. Schroeder, V. I. Rybachenko, L. M.
Proniewicz, Chem. Phys. 2006, 327, 439.
[29] T. T. Dang, Y. Zhu, J. S. Y. Ngiam, S. C. Ghosh, A. Chen,
A. M. Seayad, ACS Catal. 2013, 3, 1406.
[64] J. N. Mahindrakar, Y. S. Patil, P. H. Salunkhe, S. S.
Ankushrao, V. N. Kadam, V. Ubale, A. A. Ghanwat,
Macromol. Sci. Pure. Appl. Chem. 2018, 55, 658.
[30] T. T. Dang, Y. Zhu, S. C. Ghosh, A. Chen, C. L. L. Chai, A. M.
Seayad, Chem. Commun. 2012, 48, 1805.
[31] R. S. Mane, B. M. Bhanage, RSC Adv. 2015, 5, 76122.

MANSOUR ET AL.
19 of 19
[65] D. Tapu, D. A. Dixon, C. Roe, Chem. Rev. 2009, 109, 3385.
[66] H. V. Huynh, Y. Han, R. Jothibasu, J. A. Yang, Organometal-
lics 2009, 28, 5395.
[67] A. J. Arduengo, R. L. Harlow, M. Kline, J. Am. Chem. Soc.
1991, 113, 2801.
SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
[68] A. I. Kovalev, A. V. Naumkin, V. S. Tyurin, N. S. Kushakova,
I. A. Khotina, INEOS Open 2018, 1(2), 103.
[69] H. Alamgholiloo, S. Zhang, A. Ahadi, S. Rostamnia, R.
Bana, Z. Li, X. Liu, M. Shokouhimeh, Mol. Catal. 2019,
467, 30.
[70] M. Bakherad, A. Keivanloo, B. Bahramian, S. Jajarmi,
J. Organomet. Chem. 2013, 15, 206.
[71] M. Gaab, S. Bellemin-Laponnaz, L. H. Gade, Chem. – Eur. J.
2009, 15, 5450.
How to cite this article: Mansour W, Fettouhi M,
Saleem Q, El Ali B. Robust alkyl-bridged bis(N-
heterocyclic carbene)palladium(II) complexes
anchored on Merrifield's resin as active catalysts
for the selective synthesis of flavones and
alkynones. Appl Organomet Chem. 2021;35:e6195.
https://doi.org/10.1002/aoc.6195
[72] J. S. Willemsen, J. C. M. van Hest, F. P. J. T. Rutjes, Beilstein.
J. Org. Chem. 2013, 9, 960.