Hypercrosslinked Polymer Platform-Anchored Single-Site Heterogeneous Pd-NHC Catalysts for Diverse C-H Functionalization

Article abstract of DOI:10.1021/acs.organomet.1c00182

We demonstrate a new class of hypercrosslinked polymer (HCP) platform-Anchored single-site heterogenized Pd-NHC catalysts for multipurpose C-H functionalization reactions. This new class is represented by a set of three catalysts, viz., HCP-B-MeNHC-Pd, HCP-B-BnNHC-Pd, and HCP-TPM-MeNHC-Pd, having a variation on the structural feature of the anchoring polymeric platform. All three catalysts were fully characterized via diverse solid-state characterization and analytical techniques such as X-ray photoelectron spectroscopy, 13C cross-polarization magic-Angle-spinning nuclear magnetic resonance, field-emission scanning electron microscopy, energy-dispersive X-ray analysis, thermogravimetric analysis, and inductively coupled plasma-optical emission spectrometry. Three types of regularly practiced and very useful C-H functionalization reactions, viz., C-H halogenation, acetoxylation, and arylation, are tested with the new catalysts and found to be highly compatible and successful. With the HCP-TPM-MeNHC-Pd catalyst, up to 85, 75, and 70% yields of the functionalized products were achieved for the halogenation, acetoxylation, and arylation reactions, respectively. Demanding attributes such as enhanced activity, heterogeneity, and recyclability are offered by this new system, making it a promising candidate in the field of heterogeneous C-H functionalization, where only a few efficient catalysts are available.

Full text of DOI:10.1021/acs.organomet.1c00182

pubs.acs.org/Organometallics
Article
Hypercrosslinked Polymer Platform-Anchored Single-Site
Heterogeneous Pd−NHC Catalysts for Diverse C−H
Functionalization
Tanmoy Mandal, Moumita Mondal, and Joyanta Choudhury*
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ABSTRACT: We demonstrate a new class of hypercrosslinked
polymer (HCP) platform-anchored single-site heterogenized Pd−
NHC catalysts for multipurpose C−H functionalization reactions.
This new class is represented by a set of three catalysts, viz., HCP-
B-MeNHC-Pd, HCP-B-BnNHC-Pd, and HCP-TPM-MeNHC-Pd,
having a variation on the structural feature of the anchoring
polymeric platform. All three catalysts were fully characterized via
diverse solid-state characterization and analytical techniques such
as X-ray photoelectron spectroscopy, 13C cross-polarization magic-
angle-spinning nuclear magnetic resonance, field-emission scanning
electron microscopy, energy-dispersive X-ray analysis, thermogravi-
metric analysis, and inductively coupled plasma−optical emission
spectrometry. Three types of regularly practiced and very useful C−H functionalization reactions, viz., C−H halogenation,
acetoxylation, and arylation, are tested with the new catalysts and found to be highly compatible and successful. With the HCP-
TPM-MeNHC-Pd catalyst, up to 85, 75, and 70% yields of the functionalized products were achieved for the halogenation,
acetoxylation, and arylation reactions, respectively. Demanding attributes such as enhanced activity, heterogeneity, and recyclability
are offered by this new system, making it a promising candidate in the field of heterogeneous C−H functionalization, where only a
few efficient catalysts are available.
heterogenized catalytic systems for oxidative C−H trans-
formation reactions needs not only new self-supporting
platforms with enhanced stability but also robust Pd-platform
bonding (Figure 1B). Scaffolds such as metal−organic frame-
works,4 multiwalled carbon nanotubes,5 and graphene oxide6
have been used hosting Pd complexes and Pd nanoparticles to
carry out oxidative C−H functionalization catalysis, albeit with
still-existing issues on complicated catalyst preparation, limited
substrate applicability, and/or compromised catalyst activity
and product selectivity. More recently, metal-coordination
polymers7 and organic polymers (OPs)8 have been utilized to
chemically graft well-defined Pd complexes for C−H halogen-
ation and acetoxylation catalysis, with certain improvements of
the above-mentioned limitations.
1. INTRODUCTION
Catalytic functionalization of C−H bonds present on arene and
heteroarene backbones of commonly used organic building
blocks is a powerful transformation in the modern era of step-
economic chemical synthesis.1 Extensive research efforts,
dedicated over several decades, blossomed a myriad of highly
efficient homogeneous transition-metal catalysts for such an
important transformation. Within the impressive library of metal
complexes, the PdII−N-heterocyclic carbene (NHC) systems
recently gained extraordinary attention for oxidative C−H
transformation catalysis involving PdII/PdIV cycles, probably
due to the strong σ-donation effect of NHC ligands in favoring
efficient turn-over of high-valent Pd intermediates.2 Impor-
tantly, to exploit the diverse practical advantages primarily
associated with the catalyst’s reusability, separation, and
compatibility to flow technology, a current focus intends to
mould many Pd complexes including Pd−NHCs into single-site
heterogeneous versions.3 However, it is clearly evident from
these developments that the progress is not so impressive in the
area of oxidative C−H functionalization (Figure 1A), in
comparison to the Suzuki−Miyaura and Sonogashira cross-
coupling reactions. Issues related to the oxidative stability of the
heterogeneous scaffold might be an impeding factor toward
achieving a parallel success. Therefore, developing site-isolated
Recently, a new class of microporous OPsnamely, hyper-
crosslinked polymers (HCPs), have been successfully utilized to
anchor Pd complexes for various cross-coupling reactions owing
Received: March 25, 2021
Published: July 16, 2021
https://doi.org/10.1021/acs.organomet.1c00182
© 2021 American Chemical Society
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halogenation, acetoxylation, and arylation (Figure 1C).
Interestingly, triphenylmethane (TPM)-derived HCP delivered
benefits over benzene-derived HCP systems not only in terms of
Pd-metallation footprint but also in catalytic reactivity.
2. RESULTS AND DISCUSSION
Following the pioneering method developed by Li and Tan for
the preparation of NHC-precursor-knitted HCPs through the
FeCl3-catalyzed Friedel−Crafts reaction,9b three such HCPs
HCP-B-MeNHC, HCP-B-BnNHC, and HCP-TPM-MeNHC
were synthesized, followed by their palladation to access three
target catalystsHCP-B-MeNHC-Pd, HCP-B-BnNHC-Pd, and
HCP-TPM-MeNHC-Pd (Figure 1C and the Supporting
Information). The three systems vary in the self-linking and
co-linking patterns originated from the variation of the NHC
precursor (methyl-substituted, MeNHC- or benzyl-substituted,
and BnNHC benzimidazolium salts) and arene monomer
(benzene, B, or TPM). Interestingly, as per inductively coupled
plasma−optical emission spectroscopy (ICP−OES) analyses,
the Pd-metallation footprint in the case of the TPM-derived
branched HCP system (HCP-TPM-MeNHC-Pd, Pd weight % =
11.43%) was found to be much superior in comparison to that of
the benzene-derived HCP systems (HCP-B-MeNHC-Pd, Pd
weight % = 0.37% and HCP-B-BnNHC-Pd, Pd weight % =
0.61%). The branching via co-linking of the TPM (HCPh3) core
probably provided less congested and more accessible pockets
for metallation in the case of HCP-TPM-MeNHC-Pd, over the
other two systems, in line with similar advantages obtained via
the usage of the PhPh3-based hypercrosslinked framework.11
Thermogravimetric analysis (TGA) showed that these materials
were stable on heating up to a minimum of 300 °C. The X-ray
photoelectron spectroscopic (XPS) investigation of all the three
metallated HCPs revealed the Pd 3d core-level binding energy
values in the range of 337.2−337.5 eV for 3d5/2 and 342.3−
342.6 eV for 3d3/2, in agreement with the energy values expected
for a PdII species,9b,12−15 confirming the presence of PdII in the
material (Figure 2A,C,E). Moreover, these values are also
indicative of the formation of PdII−NHC bonds from the
reaction of the imidazolium moieties present in the HCPs and
Pd(OAc)2, in line with previous reports on similar hetero-
genized PdII−NHC systems.7,9a,16−18 In comparison with the Pd
3d core-level binding energies in Pd(OAc)2 (338.4−338.7 eV
for 3d5/2 and 343.8 eV for 3d3/2),12,16,19 the respective binding
energy values in HCP-B-MeNHC-Pd, HCP-B-BnNHC-Pd, and
HCP-TPM-MeNHC-Pd systems shifted to lower energies by
0.9−1.5 eV for the 3d5/2 level and by 1.2−1.5 eV for the 3d3/2
level. It signified the enhancement of electron density at the PdII
center caused due to the strong electron donation from the
NHC ligands in the HCP-Pd systems. While the presence of free
Pd(OAc)2 in HCP-B-MeNHC-Pd and HCP-B-BnNHC-Pd was
ruled out by XPS analyses (Figure 2A,C and the Supporting
Information), a negligible amount of encapsulated free Pd-
(OAc)2 might be present in the HCP-TPM-MeNHC-Pd system,
as indicated from a deconvolution analysis of the corresponding
XPS spectrum (Figure 2E). The absence of peaks corresponding
to the binding energies of Pd in the metallic state or Pd0 complex
suggested the absence of Pd0 species in the catalysts, which may
form during synthesis.20 Field-emission scanning electron
microscopy (FESEM) images displayed different morphologies
of the three Pd systems (Figure 2B,D,F). Energy-dispersive X-
ray spectroscopic (EDX) analysis of the polymeric catalysts
confirmed the presence of Pd in addition to the other elements
Figure 1. (A) General scheme for single-site heterogeneous catalysis in
C−H functionalization reactions. (B) Examples of previously explored
heterogenized PdII-complex-based catalysts on different platforms. (C)
Key aspects of the present work. Only idealized structures of the
polymeric backbone and catalysts are shown.
to the much-needed advantages such as good stability, low cost,
and the easy preparation method of the HCPs.9 These
advantages of the HCPs could be well exploited in developing
Pd-based C−H functionalization catalysts as well. Indeed,
during our ongoing work, Li, Zhu, and coworkers demonstrated
the same by supporting a Pd-salen-based catalyst onto a
benzene-derived HCP system.10 As expected, clear benefits were
evident in terms of improved reactivity, recyclability, and
substrate scope, even though only catalytic C−H halogenation
reactions were explored. As part of our ongoing program of
heterogenizing the homogeneous Pd−NHC-based C−H
activation-functionalization catalysts for widened applicabili-
ty,2h,i,7 herein, we report the successful demonstration of the first
Pd−NHC catalyst anchored on the HCP platform used for
multiple oxidative C−H transformation reactions, namely, C−H
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Figure 2. (A) XPS and (B) SEM image of HCP-B-MeNHC-Pd. (C) XPS and (D) SEM image of HCP-B-BnNHC-Pd. (E) XPS and (F) SEM image of
HCP-TPM-MeNHC-Pd.
which were common in all three cases (Supporting Informa-
tion).
which was in line with a similar observation for some previously
reported PdII−NHC-based polymeric materials.8a,17
The structural backbone of the HCP-supported PdII−NHC
catalysts was also verified by 13C cross-polarization magic-angle-
spinning (13C CP-MAS) solid-state nuclear magnetic resonance
(NMR) spectroscopy (Figure 3). All the HCP-B-MeNHC-Pd,
The performance of the HCP-supported Pd−NHC catalysts
HCP-B-MeNHC-Pd, HCP-B-BnNHC-Pd, and HCP-
TPM-MeNHC-Pd was tested in the catalytic oxidative C−H
halogenation reaction of a set of different directing group-
appended arene compounds using N-halosuccinimides (NXS, X
= Cl and Br) as the oxidizing-halogenating agents in CH3CN at
95 °C (Figure 4A). All the catalysts showed monohalogenation
products as the major one with traces of bishalogenation
products. Interestingly, the benzene-derived HCP-B-MeNHC-
Pd and HCP-B-BnNHC-Pd provided comparatively less yields
than the TPM-based HCP-TPM-MeNHC-Pd catalyst. This was
true for both chlorination and bromination reactions with all
three substrates having different directing groups (pyridine,
pyrazole, and thiophene). The reason may be ascribed to the
three-dimensional structure of TPM in HCP-TPM-MeNHC-Pd,
resulting in enhanced porosity and thereby allowing facile
exposure of the catalytically active Pd centers for the arene
substrates in comparison to the benzene-based systems. The
superior catalyst, HCP-TPM-MeNHC-Pd, was further used for
the C−H halogenation of some other arenes with a variety of
directing groups (pyrimidine, imine, and azo) as well as
substituents (OMe, CF3, and F) (Figure 4B). The catalyst was
found to be equally efficient in terms of overall yield as well as
selectivity toward monohalogenation. It is important to mention
here that for the rigid directing group-containing substrate such
as benzo[h]quinoline, the present HCP-TPM-MeNHC-Pd
system furnished 85% yield of the monochlorinated product.
Figure 3. Solid-state 13C CP-MAS NMR spectra of (A) HCP-
TPM-MeNHC and (B) HCP-TPM-MeNHC-Pd.
HCP-B-BnNHC-Pd, and HCP-TPM-MeNHC-Pd systems dis-
played broad resonance signals at around 30−40 ppm, assigned
to the −CH3 (of N−CH3 in the case of HCP-B-MeNHC-Pd and
HCP-TPM-MeNHC-Pd) and −CH2− (of the methylene linkers
for all systems and of the N−CH2 group for the HCP-B-BnNHC-
Pd system) carbons in the organic backbone.8a,9b The aromatic
carbons in all the materials appeared at around 128−141 ppm as
broad signals.8a,9b In addition, the HCP-TPM-MeNHC-Pd
system showed a characteristic peak at around 56 ppm due to
the tertiary carbon atom (HCAr3), which was also present in the
spectrum of the precursor HCP HCP-TPM-MeNHC.8a,9b The
resonance signal due to the PdII−NHC-based carbene carbon
either overlapped within the aromatic region or did not appear,
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Figure 5. Examples of HCP-TPM-MeNHC-Pd-catalyzed C−H
acetoxylation and arylation reactions. The numbers in the parenthesis
indicate the ratio of the monofunctionalized (as shown) and
difunctionalized (not shown) products.
Figure 4. (A) Comparison of the activity of the three catalysts in C−H
halogenation reactions. The corresponding product yields are ex-
pressed in percentage based on the substrate. (B) Examples of HCP-
TPM-MeNHC-Pd-catalyzed C−H halogenation reactions. Reaction
conditions were the same as shown in (A) above. Yields (%) are shown
in parantheses.
In comparison, Wendt’s OP-based Pd catalyst provided 73%
overall yield with a regioselectivity of 73:27 for mono- and
dichlorinated products under similar reaction conditions,8b
while this substrate was not explored by Zhu and coworkers with
their HCP-Salen-Pd catalyst.10 The catalyst HCP-
TPM-MeNHC-Pd was also explored for directed oxidative
acetoxylation and arylation reactions with some representative
arene substrates (Figure 5). In the case of acetoxylation, the
overall yields were found to be in the range of 50−75% with
regioselectivity varying from 1.8:1 to 5:1 for mono- and
bisacetoxylated products. Similarly, the catalyst was effective in
arylation as well, providing 30−70% yields with a regioselectivity
of 3:1−5.5:1 toward mono- and bisarylated products. Some
substrates such as phenylthiophene and azobenzene were
ineffective (only 5−15% for acetoxylation and nil for arylation)
even after prolonged reactions with excess acetoxylating or
arylating sources.
Hot filtration experiments with all the three catalysts, HCP-
B-MeNHC-Pd, HCP-B-BnNHC-Pd, and HCP-TPM-MeNHC-
Pd, were performed for the C−H chlorination of 2-phenyl-
pyridine under standard reaction conditions to probe the nature
of the catalysis whether it was truly heterogeneous or
homogeneous triggered by PdII leaching from the polymer
support. As evident from the time versus yield profile shown in
Figure 6A, no further catalysis was observed over time after
filtering the solid catalysts off the ongoing reactions after 6 h in
each case, suggesting the heterogeneous nature of all the present
catalysts. It was noteworthy to mention that even though a
Figure 6. (A) Hot filtration experiments with all three catalysts. The
traces labeled with A−II, B−II, and C−II represent the undisturbed
catalytic reactions performed with HCP-TPM-MeNHC-Pd, HCP-
B-BnNHC-Pd, and HCP-B-MeNHC-Pd, respectively, while those
labeled with A−I, B−I, and C−I represent the equivalent reactions
with the catalysts filtered off after 6 h of the reaction. (B) Reusability
experiments with the three catalysts. The blocks labeled with A, B, and
C represent the reactions performed with HCP-TPM-MeNHC-Pd,
HCP-B-BnNHC-Pd, and HCP-B-MeNHC-Pd, respectively. (C) SEM
image and (D) XPS of the reused catalyst HCP-TPM-MeNHC-Pd.
negligible amount of free Pd(OAc)2 was suspected to be present
in the HCP-TPM-MeNHC-Pd catalyst as per the XPS analysis,
the corresponding hot-filtered homogeneous solution, likely
containing the leached-out Pd(OAc)2, could not catalyze the
chlorination reaction. It suggested that indeed the amount of
encapsulated but leachable Pd(OAc)2 in this catalyst was
negligible, which was insufficient to be effective in catalysis. After
confirming the heterogeneous nature of the catalysts, we tested
their potential in terms of recyclability considering the practical
utility. Taking the chlorination of 2-phenylpyridine as the model
reaction, it was found that the catalysts could be successfully
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an Oxford Instrument with an X-MaxN detector. TGA was carried out
using a PerkinElmer TGA-6000 instrument. XPS analysis was
conducted in a PHI 5000 Versa Probe II instrument from FEI Inc. at
IIT Kanpur, India. ICP−OES was performed using a PerkinElmer
Optima 5300 DV instrument at IIT Madras, India. Gas chromatog-
raphy−mass spectrometry (GC−MS) was performed using an Agilent
7890A GC coupled with an Agilent 5975C MS system. Solid-state
NMR spectroscopy was performed on JEOL; Model: ECX400 Proton
freq: 400 MHz instrument at IISc Bangalore, India.
reused in subsequent fresh reactions for at least 10 cycles
without any significant loss in the catalytic activity (Figure 6B).
This remarkable reusability profile might be attributed to the
impact of the framework stability in terms of both crosslinking
and Pd− polymer bonding. Furthermore, the nature of the
recovered catalyst HCP-TPM-MeNHC-Pd was analyzed by
FESEM, EDX, and XPS techniques. Interestingly, the FESEM
image displayed a similar morphology to the original material
(Figure 6C), EDX showed the existence of Pd and other
expected elements (Supporting Information), and XPS
confirmed the presence of PdII−NHC after catalysis, showing
the PdII binding energy values at 337.2 for 3d5/2 and 342.3 eV for
3d3/2 levels (Figure 6D). An ICP-OES analysis of the recovered
HCP-TPM-MeNHC-Pd catalyst as well as the separated
homogeneous postreaction supernatant solution showed only
a negligible (<2%) loss of Pd from the fresh unused catalyst. A
comparison of the line-widths of the Pd XPS peaks pertinent to
the fresh catalyst (Figure 2E) and the recovered catalyst (Figure
6D) also suggested only 1.8% loss of the relative peak intensity at
343.7 eV in the recovered catalyst, which is a typical region of
Pd(OAc)2 (Supporting Information). Such a very low level of
leaching was also reported by Wendt in oxidative C−H
halogenation catalysis with OP-supported Pd(II)−NHC
catalysts.8b
4.2. Synthesis of Catalysts. 4.2.1. Step-I: NHC Precursors
(Benzimidazolium Salts). N,N′-Dimethyl-benzimidazolium iodide
and N,N′-dibenzyl-benzimidazolium bromide were prepared according
to the literature procedures.21
N,N′-Dimethyl-benzimidazolium iodide: 1H NMR (400 MHz,
DMSO-d6) 9.66 (1H, s), 8.02 (2H, dd, J = 6.2, 3.1), 7.71 (2H, dd, J
= 6.3, 3.1), and 4.08 (6H, s).
N,N′-Dibenzyl-benzimidazolium bromide: 1H NMR (400 MHz,
DMSO-d6) 10.08 (1H, s), 7.97 (2H, dd, J = 6.2, 3.1), 7.64 (2H, dd, J =
6.2, 3.1), 7.53 (4H, d, J = 6.8), 7.47−7.37 (6H, m), and 5.80 (4H, s).
4.2.2. Step-II: HCPs. HCP-B-MeNHC and HCP-B-BnNHC were
prepared according to the literature procedure reported by Li and
Tan.9b HCP-TPM-MeNHC was prepared using the similar procedure.
4.2.2.1. HCP-B-MeNHC. N,N′-Dimethyl-benzimidazolium iodide (1
mmol, 274.1 mg) and benzene (3 mmol, 234 mg) were mixed with
formaldehyde dimethyl acetal (684 mg, 9 mmol) in 4 mL of 1,2-
dichloroethane (DCE). After mixing properly, FeCl3 (9 mmol, 1460
mg) was added to it. The whole mixture was next stirred at room
temperature (25 °C) for 2 h, followed by heating at 45 °C for 5 h and
later at 85 °C for the next 72 h. The resulting precipitate was filtered and
washed with methanol until the yellow-colored filtrate turned colorless.
After that, the solid was collected and washed again with methanol in a
Soxhlet for 24 h. At last, the product was dried properly and used for the
next step.
3. CONCLUSIONS
In summary, we exploited the power of properly designed HCP-
anchored single-site PdII−NHC-based C−H activation-func-
tionalization catalytic centers in developing highly effective and
reusable practical heterogeneous catalysts. Solid-state character-
ization and analytical techniques such as XPS, 13C CP-MAS
NMR, FESEM, EDX, TGA, ICP−OES, and so forth
underscored the features of these catalysts. Out of three designs
that we investigated, the TPM-based catalyst HCP-
TPM-MeNHC-Pd proved to be the most beneficial in terms of
both Pd-metallation footprint and catalytic performance.
Multiple oxidative arene C−H functionalization reactions such
as chlorination, bromination, acetoxylation, and arylation were
found to be catalyzed effectively by HCP-TPM-MeNHC-Pd,
thus demonstrating the wider scope of applicability than the
existing similar catalysts. Two important aspects such as the
reusability of the catalyst for a minimum of 10 times without any
significant loss of activity and highly heterogeneous nature of the
catalyst as supported by the hot filtration test are truly
encouraging and could open up the possible applications in
continuous flow catalysis. In this regard, a recent study was
noteworthy wherein Wendt demonstrated the application of an
OP-supported Pd(II)−NHC catalyst in the continuous flow
oxidative C−H chlorination of benzo[h]quinoline.8b Work is
underway in our laboratory toward this direction.
4.2.2.2. HCP-B-BnNHC. It was prepared using the similar procedure as
described above for HCP-B-MeNHC by using N,N′-dibenzyl-
benzimidazolium bromide (1 mmol, 379.3 mg), benzene (3 mmol,
234 mg), formaldehyde dimethyl acetal (9 mmol, 684 mg), and FeCl3
(9 mmol, 1460 mg) in 4 mL of DCE.
4.2.2.3. HCP-TPM-MeNHC. This was synthesized by using N,N′-
dimethyl-benzimidazolium iodide with TPM instead of benzene for
polymerization. For this, N,N′-dimethyl-benzimidazolium iodide (1
mmol, 274.1 mg) and TPM (1 mmol, 244 mg) were taken in a 25 mL
round-bottom flask. Then, formaldehyde dimethyl acetal (3 mmol, 228
mg) and FeCl3 (3 mmol, 486 mg) were added to the mixture along with
4 mL of DCE. The reaction conditions and workup procedure were
similar to the above-mentioned as described for HCP-B-MeNHC.
4.2.3. Step-III: Metallated HCPs. 4.2.3.1. HCP-B-MeNHC-Pd and
HCP-B-BnNHC-Pd. These two catalysts were prepared according to the
same procedure reported by Li and Tan.9b
4.2.3.2. HCP-TPM-MeNHC-Pd. For the synthesis of HCP-
TPM-MeNHC-Pd, at first, the HCP-TPM-MeNHC polymer (170 mg)
was taken in a 50 mL round-bottom flask. Next, Pd(OAc)2 (20 mg) was
added with 20 mL of tetrahydrofuran (THF) as the solvent. Then, the
reaction mixture was stirred for 12 h at 80 °C under a N2 atmosphere.
After this period, the reaction mixture was cooled, and the resulting
solid was filtered and washed with THF until the filtrate became
colorless. After that, the solid was further washed with acetone and
methanol, followed by Soxhlet extraction with acetone. Finally, the solid
was isolated and dried at 60 °C under vacuum to furnish 183 mg of the
as-prepared HCP-TPM-MeNHC-Pd catalyst.
4.3. General Procedure for the Directed Halogenation of
Arenes. Arene (0.10 mmol), NBS/NCS (0.125 mmol), and the
catalyst were taken in a pressure tube. Acetonitrile (1 mL) was added,
and the pressure tube was closed tightly with a Teflon screw cap. The
resulting solution was stirred at 95 °C for 24 h. After that, the reaction
mixture was cooled, and PhCl was added as an internal standard. Then,
an aliquot (250 μL) of the reaction mixture was withdrawn, diluted with
ethyl acetate, and analyzed by GC for the calculation of yield. The
products were further verified by GC−MS analysis. The products were
4. EXPERIMENTAL SECTION
4.1. Materials and Analytical Techniques. Reactions were
performed in oven-dried glassware. NMR spectra were recorded in a
Bruker AVANCE III 400 MHz NMR spectrometer. Chemical shifts (δ)
are expressed in parts per million using the residual proton resonance of
the solvent as an internal standard [dimethylsulfoxide (DMSO): δ =
1
1
2.50 ppm for H NMR spectra; CHCl3: δ = 7.26 ppm for H NMR
spectra]. The coupling constants (J) are expressed in hertz (Hz) and
only given for 1H−1H couplings. The following abbreviations were used
to indicate multiplicity: s (singlet), d (doublet), dd (doublet of
doublets), and m (multiplet). The morphology of the polymer
materials was examined using a Carl Zeiss (Ultra Plus) field-emission
scanning electron microscope. EDX spectroscopy was performed using
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isolated by performing column chromatography in silica by taking pure
hexane or the hexane ethyl acetate mixture as a solvent.
Science Education and Research Bhopal, Bhopal 462 066,
India
4.4. General Procedure for the Directed Acetoxylation of
Arenes with HCP-TPM-MeNHC-Pd as the Catalyst. Arene (0.1
mmol), PhI(OAc)2 (0.125 mmol), and HCP-TPM-MeNHC-Pd (with a
loading of 5 mol % Pd) were taken in a pressure tube. Then, DCE (1
mL) was added to the mixture, and the pressure tube was closed tightly
with a Teflon screw cap. The resulting solution was stirred at 95 °C for
24 h. After that, the reaction mixture was cooled, and PhCl was added as
an internal standard. Then, an aliquot (250 μL) of the reaction mixture
was withdrawn, diluted with ethyl acetate, and analyzed by GC for the
calculation of yield. The products were further verified by GC−MS
analysis. The products were isolated by performing column
chromatography in silica by taking pure hexane or the hexane-ethyl
acetate mixture as a solvent.
Moumita Mondal − Organometallics & Smart Materials
Laboratory, Department of Chemistry, Indian Institute of
Science Education and Research Bhopal, Bhopal 462 066,
India; Present Address: Department of Chemistry,
National Institute of Technology (NIT) Jamshedpur,
Jamshedpur 831 014, Jharkhand, India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00182
Notes
The authors declare no competing financial interest.
4.5. General Procedure for the Directed Arylation of Arenes
with HCP-TPM-MeNHC-Pd as the Catalyst. Arene (0.05 mmol),
[Ph2I]BF4 (0.08 mmol), and HCP-TPM-MeNHC-Pd (with a loading of
5 mol % Pd) were placed in a pressure tube. A solvent mixture of
AcOH/Ac2O (1 mL, 9:1 v/v) was added to it, and the pressure tube was
closed tightly with a Teflon screw cap. The resulting solution was stirred
at 95 °C for 24 h. After that, the reaction mixture was cooled, and PhCl
was added as an internal standard. Then, an aliquot (250 μL) of the
reaction mixture was withdrawn, diluted with ethyl acetate, and
analyzed by GC for the calculation of yield. The products were further
verified by GC−MS analysis. The products were isolated by performing
column chromatography in silica by taking pure hexane or the hexane−
ethyl acetate mixture as a solvent.
4.6. Hot Filtration Test. Two parallel reactions were set containing
2-phenylpyridine (0.10 mmol), NCS (0.125 mmol), and the HCP-
based Pd catalyst (with a loading of 5 mol % Pd) in acetonitrile (1 mL).
After 6 h, from one reaction, the solid catalyst was separated quickly
under hot conditions by centrifugation and the reaction was continued,
whereas in the other case, the reaction was continued as such without
any such disturbances. The yield of the product was monitored for both
the reactions using PhCl as an internal standard.
4.7. Reusability Test. 2-Phenylpyridine (0.6 mmol), NCS (0.7
mmol), and the HCP-based Pd catalyst (with a loading of 5 mol % Pd)
were taken in acetonitrile (5 mL). The mixture was stirred at 95 °C for
24 h. After cooling to room temperature, the yield was calculated by
following the previously described procedure. The catalyst was
separated from the reaction mixture by centrifugation and washed
with acetonitrile (3 × 3 mL) by sonication. This recovered catalyst was
then reused for the next catalytic reaction. The procedure was
continued up to 10 cycles.
ACKNOWLEDGMENTS
■
This work was supported by DST-SERB (CRG/2019/006038)
and IISER Bhopal. T.M. and M.M. thank CSIR for fellowship.
NMR, Mass, SEM, and TGA facilities at CIF, IISER Bhopal, are
gratefully acknowledged. The authors thank the Sophisticated
Instruments Facility, NMR Research Centre, IISc Bangalore,
India, for recording solid-state 13C CP-MAS NMR spectra and
Surface Characterization Lab, Advanced Centre for Materials
Science, IIT Kanpur, India, for recording XPS.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00182.
■
sı
1H NMR spectra of N,N’-dimethyl-benzimidazolium
iodide and N,N’-dibenzyl-benzimidazolium bromide,
SEM images and EDX spectra, solid-state 13C CP-MAS
NMR spectra, TGA data, 1H NMR spectral data,
calibration curves, and GC−MS profiles (PDF)
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AUTHOR INFORMATION
Corresponding Author
■
Joyanta Choudhury − Organometallics & Smart Materials
Laboratory, Department of Chemistry, Indian Institute of
Science Education and Research Bhopal, Bhopal 462 066,
India; orcid.org/0000-0003-4776-8757;
Email: joyanta@iiserb.ac.in
Authors
Tanmoy Mandal − Organometallics & Smart Materials
Laboratory, Department of Chemistry, Indian Institute of
2448
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Organometallics 2021, 40, 2443−2449

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