Organometallics
Article
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
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
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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Organometallics 2021, 40, 2443−2449