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S.N. Riduan et al. / Journal of Catalysis xxx (2015) xxx–xxx
(8 mL). The vial was then evacuated, and CO2 was introduced via a
balloon. After the reaction was completed, the reaction mixture
was centrifuged, and the solution was decanted. This procedure
was repeated at least three times using DMF as the washing sol-
vent. The combined solution was collected for further analysis.
The recovered catalyst was used directly for the next run.
tylene was added. An aliquot was then taken for NMR spec-
troscopy. For catalyst recycling, the remaining solution was
decanted, and 2 mL of fresh DMF was added. The reaction was
allowed to stir for 30 min before centrifuging and removal of
DMF. This procedure was repeated at least three times using
DMF (1ꢀ) and dichloromethane (3ꢀ) as the washing solvent. The
recovered catalyst was then dried under the Schlenk line, and used
directly for the next run.
2.3. Hydrolysis reaction mixture to release methanol
To produce methanol via hydrolysis of the reaction mixture, the
reaction was quenched after a certain time by adding 2 equivalents
of NaOH/H2O solution. It was stirred for another 24 h before an ali-
quot of isopropanol was added as an internal standard. An aliquot
of 1 mL was removed from the sample and diluted with dichloro-
methane before the resulting mixture was subjected to GC analysis
with an Agilent HP-5 column ((5%-phenyl)-methylpolysiloxane
bonded phase).
3. Results and discussions
Poly-imidazolium salt A was synthesized by condensation of
bisimidazole and 2,4,6-tris (bromomethyl) mesitylene in N,N0-
dimethylformamide (DMF) at 110 °C for 24–72 h (see Fig. 1), as
previously reported [16]. Spherical particles of poly-imidazolium
A were obtained in quantitative yield (see ESI). The white powder
A was treated with a strong base, such as sodium hydride, to gen-
erate dark red poly-NHC particles B. After the introduction of CO2
into the reaction system, poly-imidazolium carboxylate C was
obtained as white precipitates (see Fig. 1). The generation of B
and C was clearly indicated by 13C solid-state nuclear magnetic res-
onance (NMR) spectra (see Fig. 2). The weak peak observed at
218 ppm in Fig. 2(b) corresponded to the carbene C2 carbon, which
was not observed in the spectrum of Fig. 2(c). Instead, a new peak
was found at 165 ppm, which was attributed to the carboxylate
group of C. The resulting material was also confirmed in composi-
tion with photoacoustic Fourier-transform infrared (PA-FTIR) spec-
troscopy (see ESI).
We have earlier reported the CO2 hydrosilylation reduction
reaction to yield silyl methoxide end products under ambient con-
ditions with NHC catalysts, and that the reaction proceeded via a
three-step cascade reaction (Scheme 1) [18b]. The reduction of
CO2 with hydrosilanes, catalyzed by solid poly-NHC particles was
conducted with reaction conditions similar to that of our
previously reported paper. An aliquot of 1 mmol of silane was
stirred with 0.1 mmol equivalent of poly-imidazolium
carboxylates in 2 ml of DMF in a crimp-top vial (8 mL). The vial
was then evacuated, and CO2 atmosphere was introduced via a
balloon. It was found that the poly-NHC particles were effective
catalysts for the reaction, achieving complete consumption of
Ph2SiH2 in 12 h, as monitored with gas chromatography–mass
spectrometry (GC–MS) (see Supplementary Information).
2.4. Catalyst regeneration
An equimolar amount of sodium hydride and 10 mL of anhy-
drous DMF were added to the recovered polymer catalyst. The mix-
ture was stirred overnight before it was used for the next reaction
run.
2.5. Reactions with in situ generated catalyst for hydrosilylation of CO2
to methanol
0.1 mmol equivalent of poly-imidazolium, an equimolar
amount of sodium hydride, and 2 mL of anhydrous DMF were
placed in a crimp-top vial (8 mL). The vial was sealed and the sus-
pension was stirred for 1 h before CO2 was introduced via a bal-
loon. The reaction mixture was then allowed to stir for 1 h before
1 mmol of silane was added. Aliquots were withdrawn from the
sample at 2-h intervals, and subjected to GC–MS analyses with
mesitylene as an external standard.
2.6. Formylation of N–H bonds with CO2 and hydrosilanes with solid
poly-NHC and catalyst recycle
0.05 mmol equivalent of poly-imidazolium, an equimolar
amount of sodium hydride, and 2 mL of anhydrous DMF were
placed in a crimp-top vial (8 mL). The vial was sealed, and the sus-
pension was stirred for 2 h before CO2 was introduced via a bal-
loon. Amines (1 mmol) were subsequently added, and the
reaction mixture was then allowed to stir for 5 min before 2.5
equivalents of silane were added. The reaction was allowed to stir
for 18 h. After the reaction was completed, the reaction mixture
GC–MS and NMR studies showed that similar Si-OMe products
were formed with the poly-NHC catalyst as with the homogeneous
1,3-bis(2,4,6-trimethylphenyl) imidazolium (IMes) catalyst (see
Fig. 3 and ESI). However, a lower methanol yield (40% based on
silane, after hydrolysis of supernatant) was achieved over the
poly-NHC catalyst as compared to the homogeneous IMes catalysts
was centrifuged, a 400-lL aliquot was sampled, and 20 lL of mesi-
Fig. 1. Preparation of polymer catalysts for CO2 hydrosilylation. A: poly-imidazolium bromide; B: poly-NHC; C: poly-imidazolium carboxylate; D: after centrifuging C.
Reaction conditions: (i) DMF, 110 °C, 2 days; (ii) NaH, DMF, room temperature, 24 h; (iii) CO2, 1 atm, room temperature, 24 h; and (iv) centrifugation at 2000 rpm, 5 min.
Please cite this article in press as: S.N. Riduan et al., Solid poly-N-heterocyclic carbene catalyzed CO2 reduction with hydrosilanes, J. Catal. (2015), http://dx.