Solid poly-N-heterocyclic carbene catalyzed CO2 reduction with hydrosilanes

Article abstract of DOI:10.1016/j.jcat.2015.09.009

The utility of solid poly-N-heterocyclic carbene (poly-NHC) materials as organocatalysts for carbon dioxide reduction was studied. These poly-NHC particles were demonstrated to be useful as heterogeneous organocatalysts for the reduction of carbon dioxide to methanol and for the formylation of N–H bonds with hydrosilanes as a hydride donor. These solid catalysts could potentially be useful in large-scale syntheses due to the ease of catalyst recycle and reuse, providing cost savings and environmental sustainability in the long run.

Full text of DOI:10.1016/j.jcat.2015.09.009

Journal of Catalysis xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Solid poly-N-heterocyclic carbene catalyzed CO2 reduction
with hydrosilanes
⇑
⇑
Siti Nurhanna Riduan, Jackie Y. Ying , Yugen Zhang
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
The utility of solid poly-N-heterocyclic carbene (poly-NHC) materials as organocatalysts for carbon diox-
ide reduction was studied. These poly-NHC particles were demonstrated to be useful as heterogeneous
organocatalysts for the reduction of carbon dioxide to methanol and for the formylation of N–H bonds
with hydrosilanes as a hydride donor. These solid catalysts could potentially be useful in large-scale syn-
theses due to the ease of catalyst recycle and reuse, providing cost savings and environmental sustain-
ability in the long run.
Received 6 July 2015
Revised 14 September 2015
Accepted 16 September 2015
Available online xxxx
Keywords:
Ó 2015 Elsevier Inc. All rights reserved.
Carbon dioxide transformation
Heterogeneous organocatalysis
N-heterocyclic carbene
Hydrosilylation
1. Introduction
solid organic materials have been reported for use in carbon diox-
ide adsorption and sequestration [21], they have not been exam-
It is of great interest to develop methods to utilize carbon
dioxide as a C1 source [1] for organic synthesis [2–4] and material
applications [5]. The conversion of carbon dioxide into energy
storage materials via traditional chemical [6] and electrochemical
[7] methods, as well as frustrated Lewis pairs [8], has also attracted
substantial interest in recent years. Most of these reports involve
homogeneous and heterogeneous transition metal complexes [9],
metal oxides [10] or nanostructured materials [11] as catalysts.
In comparison, relatively inexpensive organocatalysts have been
employed in organic synthesis [12] and carbon dioxide utilization
[13], but little efforts have been devoted toward heterogeneous
organocatalysis [14,15]. We have recently developed a main-
chain poly-N-heterocyclic carbene (poly-NHC) system as an excel-
ined as catalysts for reactions involving carbon dioxide. Herein,
we examined the use of main-chain poly-NHC system for the con-
version of carbon dioxide to methanol under ambient conditions.
The solid poly-NHC microparticles were demonstrated as novel,
recyclable heterogeneous catalysts. Their use as organocatalysts
was also extended to the synthesis of formamides from amines,
hydrosilanes and carbon dioxide.
2. Experimental
2.1. Preparation of heterogeneous catalysts
1 mmol equivalent of poly-imidazolium A [16], an equimolar
amount of sodium hydride, and 10 mL of anhydrous dimethylfor-
mamide (DMF) were placed in a crimp-top vial (20 mL). The vial
was sealed, and the suspension was stirred for 1 h before CO2
was introduced via a balloon. The reaction mixture was allowed
to stir overnight before the suspension was centrifuged and the
supernatant was removed. The remaining solid was then washed
with three portions of dichloromethane (10 mL each), and left
under the Schlenk line to dry overnight.
lent heterogeneous catalyst, both as
a support material for
organometallic catalysis and as a solid organocatalyst [16]. Since
then, several studies on the synthesis and applications of main-
chain NHC polymeric materials as self-supporting organocatalysts
have been reported [17].
As part of our efforts to develop new CO2 chemistries, we have
examined the overall reduction of carbon dioxide to methanol with
NHC catalysts and hydrosilanes as a hydride donor [18,19].
Hydrosilanes have been widely used as reduced agents. Cantat
and co-workers have also reported the synthesis of formamides
from amines, CO2 and silanes under ambient conditions [20]. While
2.2. Hydrosilylation of CO2 to methanol with solid poly-NHC and
catalyst recycle
⇑
Corresponding authors.
E-mail addresses: jyying@ibn.a-star.edu.sg (J.Y. Ying), ygzhang@ibn.a-star.edu.sg
0.1 mmol equivalent of poly-imidazolium carboxylate, 2 mL of
DMF, and 1 mmol of silane were added in a crimp-top vial
(Y. Zhang).
http://dx.doi.org/10.1016/j.jcat.2015.09.009
0021-9517/Ó 2015 Elsevier Inc. All rights reserved.
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.
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2
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.
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S.N. Riduan et al. / Journal of Catalysis xxx (2015) xxx–xxx
3
A
B
Fig. 3. 13C NMR spectra of NMR tube reaction of 13CO2, diphenylsilane, poly-NHC
catalyst (5 mol%) in DMF-d7. Spectra (A) and (C) show the conversion of 13CO2 (#) to
C
13CH2(OSiR3)2 (.) to 13CH3O-SiR3
( ). Spectrum (B) was obtained in the absence of
proton decoupling. Reaction in the NMR tube was observed to be slower due to the
solid catalyst employed and the static nature of the experiment.
260 240 220 200 180 160 140 120 1000 80 60 40 20
0
ppm
Fig. 2. 13C solid-state NMR spectra of (A) poly-imidazolium bromide, (B) poly-NHC,
and (C) poly-imidazolium carboxylate. Arrows indicate the signals from free
carbene carbon (218 ppm) in B and carboxylate carbon (165 ppm) in C.
increased activity of the catalyst after regeneration were made
by treating the catalyst after the fourth run with the catalyst regen-
eration conditions (Fig. 4B). We observed that the poly-NHCs
would have to be reactivated every 4 runs to remain highly active.
This regeneration process suggested that the catalyst deactivation
could be due to the minor impurities from CO2 gas or silane, such
as water. The impurities could protonate carbene to form imida-
zolium and cause the color change in the catalyst. Like the homo-
geneous IMes carbene reaction system, the reaction with in situ
generated poly-NHC B catalyst gave the same results as the iso-
lated poly-imidazolium carboxylate catalyst C.
(over 95% of methanol yield) [18]. Similar experiments were also
performed with a controlled amount of CO2 as previously reported
[18] to drive the reaction forward toward the formation of silyl
methoxide end-products. With the addition of 1 mmol (instead
of an excess amount) of CO2 into the reaction mixture via a gas-
tight syringe, the maximum methanol yield achieved was only
47%. In a similar setup, 2 equivalents of excess silanes were added
to the reaction after several days, and the methanol yield was
improved to ꢁ60%. The lower yield of methanol from these reac-
tions could be due to the incomplete reaction of the intermediates
over the heterogeneous poly-NHC catalyst to form the silane oligo-
mers bearing the methoxide moiety, or the inaccessibility of the
oligomers that were trapped in the pores of the poly-NHC catalyst.
This solid catalyst was easily recycled, and remarkably, the sub-
sequent runs were much faster than the first run (Fig. 4A). This
might indicate that catalyst particles were swollen during the first
run, and that mass transfer became more effective in the subse-
quent runs. The solid catalyst was reused for 3 runs, and the
methanol yield of each recycled run was consistent with the fresh
catalyst. Catalyst deactivation was observed after 4 runs; incom-
plete consumption of Ph2SiH2 was obtained in subsequent runs,
even after 12 h of reaction. In the first 4 runs, the color of the cat-
alyst gradually changed from reddish brown to pale yellow. We
attempted to regenerate the catalyst via reaction with a base
(NaH) after the 6th run. The original reddish brown color of cata-
lyst was recovered, and the activity of poly-NHC catalyst was fully
regained, such that the hydrosilane was completely consumed in
4 h in the 4 subsequent runs. The process could be repeated to keep
the solid poly-NHC catalyst highly efficient (Fig. 4A). Attempts to
preempt catalyst deactivation after 4 runs and to study the
Buoyed by the modest success of poly-NHC particles as cata-
lysts for the overall reduction of CO2 to methanol, we expanded
the application of these catalyst particles to the formylation of
Fig. 4. Recyclability of poly-NHC catalyst in CO2 hydrosilylation to methanol. (A)
The first set of recycling experiments showed the deactivation of poly-NHC catalyst
at runs #5 and #6. Upon regeneration by NaH after run #6, the activity of poly-NHC
was recovered in runs #7 to #10. (B) The second set of recycling experiments
involved the catalyst regeneration after run #4. The catalyst then remained active
to run #8. The catalyst was found to be deactivated at run #9, and was subsequently
regenerated. The activity of poly-NHC was recovered in runs #10 to #12.
Scheme 1. NHC-catalyzed CO2 reduction with hydrosilane.
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S.N. Riduan et al. / Journal of Catalysis xxx (2015) xxx–xxx
Table 1
ble yields (Table 1, entries
1 and 3). In contrast, 1,3-
pNHC-catalyzed CO2 reduction to formamides with amines and hydrosilanes.a
dibenzylimidazolium (DBZIM), the monomeric unit of poly-NHC
gave a relatively low yield (45%, Table 1, entry 2). This result was
consistent with the observations in homogeneous NHC-catalyzed
system, whereby the bulkier catalysts resulted in higher reactivity
[17]. On the other hand, when paired with polymethylhydrosilox-
ane (PMHS), the yield of N-benzylformamide drastically decreased
when the reaction system was switched from a homogeneous to a
heterogeneous one. This could be due to the limited access to cat-
alytic sites in poly-NHC for the larger molecules of PMHS, as com-
pared to Ph2SiH2. As expected, the reaction was sensitive toward
steric bulk around the labile Si–H bonds. When less bulky PhSiH3
was used in the reaction with poly-NHC catalyst, slightly higher
yield (66%) was achieved (Table 1, entry 8), while the yield of N-
benzylformamide decreased to 19% with the use of Et3SiH (Table 1,
entry 9). Attempts to increase the yield of the formamide product
by extending the reaction time and raising reaction temperature
did not lead to any favorable results. It was also noted that the
intermediates from the hydrosilylation reaction of CO2 to methanol
was often observed in the GC–MS studies of such reaction
mixtures.
With the optimized reaction conditions at hand, we expanded
the substrate scope to primary and secondary amines, anilines
and heterocycles (Scheme 2). Moderate yields were achieved for
less sterically demanding primary amines, as compared to cyclo-
hexylamine, whereby low conversions to formamide were
observed. Secondary amines suffered the same fate of low conver-
sions and formamide yields. Anilines exhibited better reactivities
and yields, while heterocycles such as imidazoles gave a modest
yield of 1-imidazole-carbaldehyde. All primary amines produced
monosubstituted formamide exclusively. This was different from
the IPr/PhSiH3 system, whereby a mixture of mono- and bis-
formylanilines was attained [20]. Although the use of bulkier iPr
catalysts was reported to be beneficial for the formylation reaction,
the yields of formamides from bulky amines were markedly lower
than that of less cumbersome amines. The efficiency of the cat-
alytic process stemmed from a balance of the bulkiness of the cat-
alyst, substrate and silane reductant. While the poly-NHC catalyst
possessed a repeating unit similar to that of monomeric DBZIM, the
actual bulky environment of the active catalytic site was similar to
Entry
Catalyst
Silane
Solvent
Yield (%)b
1
2
IMes
DBZIM
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
PMHS
DMF
DMF
DMF
DMF
DMF
DMF
THF
60
45
64 (60)
56
39
24
N.R.
66
3
Poly-NHC
Poly-NHC
IMes
Poly-NHC
Poly-NHC
Poly-NHC
Poly-NHC
4c
5
6
7
8
9
PMHS
Ph2SiH2
PhSiH3
Et3SiH
DMF
DMF
19
a
Reaction conditions: 1 mmol of benzylamine, 2.5 equivalents of silane, 2 mL of
solvent, room temperature, 20 h.
b
NMR yield, isolated yield in parentheses.
Reaction performed at 80 °C.
c
N–H bonds with CO2 and silanes. The extension of the reaction sys-
tem previously reported by Cantat [20] to a heterogeneous one
would be beneficial as formamides are typically synthesized via
demanding multi-step syntheses [22]. The reaction would be envi-
ronmentally friendly when conducted at ambient temperatures
and pressures, especially with the use of recyclable catalyst
particles.
Cantat and co-workers reported excellent conversions and
yields of formamide from amines in 24 h, using hydrosilanes as a
reductant in excess, 1 atm of CO2, 5 mol% 1,3-bis(2,6-diisopropyl
phenyl)imidazolium (iPr) catalysts, and tetrahydrofuran (THF) as
solvent [20]. We initiated our studies by using benzyl amine as a
model substrate, with variations in the silane reducing agent and
solvent. DMF proved to be the optimum solvent for the reaction.
The characteristic dark orange color of poly-NHC was not observed
when THF was used for the in situ generation of poly-NHC from
poly-imidazolium, and no product was obtained from the formyla-
tion reaction (Table 1, entry 7). IMes and poly-NHC catalysts have
similar reactivities and product conversions, resulting in compara-
Scheme 2. Substrate screening for the poly-NHC-catalyzed CO2 reduction to formamides with amines and hydrosilanes. Reaction conditions: 1 mmol of amine, 2.5
equivalents of silane, 2 mL of solvent, room temperature, 20 h. NMR yields (average of 2 independent runs) are presented. aControl reactions using 5 mol% of IMes as catalyst.
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.
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S.N. Riduan et al. / Journal of Catalysis xxx (2015) xxx–xxx
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These poly-NHC particles were also examined for applicability
toward the formylation of N–H bonds with CO2 and hydrosilanes
with moderate success. While the yields of products from these
heterogeneously catalyzed reactions were lower than that for the
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This work was supported by the Institute of Bioengineering and
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doi.org/10.1016/j.jcat.2015.09.009