E. Niknam, F. Panahi and A. Khalafi-Nezhad
Journal of Organometallic Chemistry 935 (2021) 121676
Table 6
Comparison between the activity of the Pd-NHC-MIL-101(Cr) catalyst and other reported catalysts for copper-free
Sonogashiracoupling reaction.
Entry
Reaction Condition
Yield (%)a
ref
1
2
3
4
5
6
MIL-101-Pd (3.5 wt%), X= Br, DMA,130 °C, KO-tBu,6 h
FDU-NHC/Pd(II)/CuI (1.0), X=I, DMF, 100 °C, K2CO3, 3 h
Pd/PdO/Cu (0.18), X=Br, Et3N, 120 °C, 5 h
80
90
63
52
77
91
Pd- TPOP (25 mg), X=Br, DMF,100 °C, Hexamine, 12h
Si–PNHC–Pd(0.5), X=Br, DMF,120 °C, TBAB, K2CO3, 1.5 h
Pd-NHC-MIL-101(Cr)(1.0), X=Br, DMF, 110 °C, K2CO3, 12 h
This work
solvent (Table 5). As you can see in table, in case of the Pd-NHC-
MIL-101(Cr) heterogeneous catalyst, the reaction yield is good and
the reaction condition is milder and better than the other catalysts.
In this work we used Pd-NHC-MIL-101(Cr) heterogeneous as highly
efficient catalyst for synthesis of distyrylbenzenes compounds.
The catalytic activity of thePd-NHC-MIL-101(Cr) system in
copper-free Sonogashira reaction was also comprised with other
catalytic Pd-heterogeneous systems (Table 6). Noticeably, the num-
ber of reported Pd@MOF catalysts system that catalyzed the
copper-free Sonogashira reaction is limited. However, the Pd-NHC-
MIL-101(Cr) catalyst catalyzed this reaction very well and superi-
ority in yields and scope of the reactions was observed in compar-
ison with other catalyst systems.
Bruker, axs) and FT-IR spectroscopy (Shimadzu FT-IR 8300 spec-
trophotometer) were employed for characterization of the cata-
lysts and products. ICP analysis was determined, using an induc-
tively coupled plasma (ICP) analyzer (Varian, Vista-Pro). The scan-
ning electron micrograph (SEM) for the catalyst was obtained by
SEM instrumentation (SEM, XL-30 FEG SEM, Philips, at 20 kV).
TEM images were obtained on a Zeiss EM10C transmission elec-
tron microscope using Acc voltage of 100 kV. X-ray photoelec-
tron spectroscopy (XPS) measurements were recorded with a Ther-
mofisher Scientific K-Alpha instrument. Melting points were de-
termined in open capillary tubes in a Buchi melting point B-545.
The reaction monitoring was accomplished by TLC on silica gel
PolyGram SILG/UV254 plates. Column chromatography was car-
ried out on columns of silica gel 60 (70−230 mesh) mesh. All
the products were characterized with IR, 1HNMR, 13CNMR and
the known compounds were compared with those reported in
literature.
3. Conclusion
In conclusion, we developed a novel Pd heterogeneous catalyst
system for application Mizoroki-Heck and copper-free Sonogashira
coupling reactions. To synthesize this catalyst system, first MIL-
101(Cr) metal organic framework was synthesized and then using
post synthetic modification approach it was functionalized with
NHC moieties (NHC-MIL-101(Cr)). Immobilization of Pd species
on the NHC-MIL-101(Cr) substrate resulted in the preparation of
a heterogeneous Pd catalyst [Pd-NHC-MIL-101(Cr)]. The catalytic
activity of this catalyst was evaluated in the Heck and copper-
free Sonogashira coupling reaction. The Heck coupling reactions
were performed very well and good substrate scope was observed.
Also, this catalyst system showed good applicability in synthe-
sis of some pi-conjugated compounds (distyryl benzenes) under
heterogeneous conditions. The Pd-NHC-MIL-101(Cr) catalyst was
also demonstrates high activity and efficiency in copper-free Sono-
gashira coupling reaction to obtain a variety of alkyne deriva-
tives under heterogeneous conditions. The catalyst was separated
from the reaction mixture by simple filtration and high degree of
reusability was observed in both coupling reactions. Based on het-
erogeneity, hot filtration and poison tests the Pd-NHC-MIL-101(Cr)
act as a heterogeneous catalyst in these Pd-catalyzed coupling re-
actions with very low Pd leaching which is highly demanded in
synthesis of pharmaceuticals and advanced materials with low Pd
contamination.
4.1. Synthesis of MIL-101- NHC-Pd catalyst
4.1.1. A procedure for the Synthesis of MIL-101(Cr)
A mixture of Cr(NO3)3.9H2O (4.5 g, 11 mmol), terephthalic acid
(10 mmol, 1.66 g), deionized water (45 mL), and hydrofluoric acid
(0.6 mL of 5M solution, 10 mmol) was charged in to a 75 mL ca-
pacity Teflon lined stainless steel autoclave. The mixture was son-
icated for 10 min, and placed in a preheated oven at 220 ˚C for
8 h. Afterward, autoclave was allowed to cool down to room tem-
perature, filtered and washed with distilled water and then dried
in an oven at 80 °C overnight. The crude product consists of green
powder of MIL-101(Cr) along with white sharp needle-type crys-
tals of unreacted terephthalic acid. The unreacted terephthalic acid
was removed in two steps. In the first step the crude product was
added to a 100 mL double-necked round bottom. Flask attached
with a reflux condenser placed on a magnetic stirrer followed by
addition of DMF (50 mL). The mixture was refluxed at 130 °C for
12 h, filtered while hot, washed with hot DMF (2 × 25 mL) and
dried. This process was repeated two times and finally the product
was dried at 70 °C for 12 h. In the second step, the resulting green
powder was added to 50 mL hot ethanol and refluxed overnight
followed by hot filtration. Finally, the material was heated at 80°C
in an oven for 12 h to complete the activation [16,33].
4. Experimental section
4.1.2. A procedure for the Synthesis of MIL-101(Cr)-CH2Cl
Chemicals were purchased from commercial chemical compa-
nies and used without further purification. The known products
were characterized by comparison of their spectral and physical
data with those reported in the literature. 1H (250 MHz) and 13C
NMR (62.9 MHz) spectra were recorded on a Bruker Avance spec-
trometer in deutrated solvents. X-ray diffraction (XRD, D8, Avance,
A mixture of MIL-101(Cr) (1.0 g), aluminum chloride hexahy-
drate (8 mmol, 1.9 g), methoxyacetyl chloride (4 mmol, 0.4 g) and
nitromethane (80 mL) was charged in to the 100 mL three necked
flask with a condenser. The reaction is carried out at 100 °C with
continuous stirring for 5 h. Then any solid product of chloromethy-
lation was washed overnight with boiling, distilled water. Followed
9