N-Heterocyclic carbene containing element organic frameworks as heterogeneous organocatalysts

Article abstract of DOI:10.1039/c1cc10268k

A bifunctional imidazolium linker was used for the production of highly crosslinked element organic frameworks by Suzuki-coupling with tetrafunctional boronic acids. The resulting porous materials are good heterogeneous organocatalysts in the N-heterocyclic carbene-catalyzed conjugated umpolung of α,β-unsaturated cinnamaldehyde. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc10268k

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COMMUNICATION
N-Heterocyclic carbene containing element organic frameworks as
heterogeneous organocatalystsw
Marcus Rose,a Andreas Notzon,b Maja Heitbaum,b Georg Nickerl,a Silvia Paasch,c
Eike Brunner,c Frank Glorius*b and Stefan Kaskel*a
Received 14th January 2011, Accepted 24th February 2011
DOI: 10.1039/c1cc10268k
A bifunctional imidazolium linker was used for the production of
highly crosslinked element organic frameworks by Suzuki-
coupling with tetrafunctional boronic acids. The resulting porous
materials are good heterogeneous organocatalysts in the
N-heterocyclic carbene-catalyzed conjugated umpolung of
a,b-unsaturated cinnamaldehyde.
species on different substrates another method is the cross-
linking of multifunctional imidazolium-based linkers by
convenient synthetic routes to achieve highly crosslinked,
porous framework materials. As a side effect they should show
a large number of accessible carbene functionalities due to an
open framework structure. So far, only the approach for the
production of periodic mesoporous organosilicas (PMOs)11
was used for the immobilization of bifunctional imidazolium
linkers. Nguyen et al. have synthesized a bifunctional bis-
aryl-imidazolium linker with triethoxysilyl groups for cross-
linking.12 Polycondensation of this linker in the presence of
different surfactants as structure directing agents resulted in
porous materials with low specific surface areas of maximal
104 m2 gÀ1. The co-condensation with tetraethoxysilane led to
materials with a specific surface area of up to 1170 m2 gÀ1. So
far, no results from catalytic tests of these materials have been
published.
N-Heterocyclic carbenes (NHCs) are known for their attractive
properties, being electron-rich and sterically demanding,
however, also tunable over a wide range.1 Consequently, they
have found numerous applications as ligands in transition-
metal catalysis1 and as organocatalysts in their own right.2
A preferred method for their preparation is the deprotonation
of the corresponding azolium salts.
Most of this research was focussing on homogeneous
processes. However, handling and separation of homogeneous
catalysts exhibits significant economical and ecological
disadvantages and efficient and selective heterogeneous catalyst
systems would be highly desirable, especially for large scale
applications. The immobilization of imidazolium-based NHCs
has been investigated, like the immobilization on the surface of
nanoparticles,3 polymers4–6 or silica-based, porous materials
like SBA-15.7 All of those publications show catalytic activity
of metal species coordinated to the NHC-functionality. To our
knowledge, no results have been published so far, dealing
with heterogeneous NHC-compounds and their application in
metal-free organocatalysis.2
Recently, we have shown the production of highly porous
element organic framework materials by the Pd-catalyzed
Suzuki coupling reaction of multifunctional aryl bromides
and aryl boronic acids.13 In this work, our interest was the
incorporation of imidazolium linkers based on bifunctional
aryl bromides by crosslinking with tetrafunctional boronic
acids (Scheme 1). In the following, the resulting three-
dimensional, porous element organic framework materials
are used as heterogeneous organocatalysts in the NHC
catalyzed conjugated umpolung of a,b-unsaturated cinnam-
aldehyde for the diastereoselective synthesis of g-butyrolactones
as model reaction.
In the last few years a lot of effort has been made in the
development of metal-free organic framework materials.8–10
The huge variety of possible reactions to crosslink multi-
functional linkers for the production of porous polymers
has opened new routes for the immobilization of functional
organic molecules. Besides the immobilization of imidazolium
The imidazolium-based linker is synthesized in two steps
from 2,6-dimethylaniline similar to the previously described
procedure14 as well as the corresponding tetrafunctional
linkers (silane-) and (methane-tetrayltetra-4,1-phenylene)
tetrakisboronic acid.15 The Suzuki coupling reaction of the
a Department of Inorganic Chemistry, Dresden University of
Technology, Bergstraße 66, D-01069 Dresden, Germany.
E-mail: stefan.kaskel@chemie.tu-dresden.de;
Fax: +49 351 46337287; Tel: +49 351 46334885
b Organic Chemistry Institute, WestfalischeWilhelms-University of
¨
Munster, Corrensstraße 40, D-48149 Munster, Germany
¨
¨
c Department of Bioanalytic Chemistry, Dresden University of
Technology, Bergstraße 66, D-01069 Dresden, Germany
Scheme 1 Syntheses of EOF-15 (E = Si) and -16 (E = C) from
tetrahedral boronic acids and the bifunctional imidazolium linker by
Suzuki coupling.
w Electronic supplementary information (ESI) available: Experimental
procedures, DRIFT spectra, 1H and 13C solid state NMR spectra and
DTA/TG analysis. See DOI: 10.1039/c1cc10268k
c
4814 Chem. Commun., 2011, 47, 4814–4816
This journal is The Royal Society of Chemistry 2011

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linker molecules is carried out in THF solution with aqueous
potassium carbonate using tetrakis(triphenyl-phosphine)-
palladium(0) as a catalyst. The precipitating product is filtered
off from the solution and washed at least three times with
THF, water and ethanol, respectively. The pale grey powder is
dried at 80 1C.
With regard to the imidazolium linker yields of about 55%
could be achieved. Both compounds are amorphous in
analogy to the porous organic frameworks EOF-6 to -9
synthesized by Suzuki coupling due to the kinetic controlled
reaction path. FTIR and solid state NMR spectroscopy have
been used for structural characterization. Both methods
confirm the successful integration of the imidazolium linker
in the framework material. Comparison of IR spectra of the
linker molecules and the resulting polymer is shown in Fig. S1,
ESI.w
Fig. 1 SEM micrographs of EOF-15 (left) and -16 (right) show highly
aggregated particles of 10–50 nm in diameter.
A strong and broad band at 3420 cmÀ1 corresponds to the
hydroxy groups of the boronic acid. This band is also present
in the spectrum of the polymer EOF-16, even though with a
much lower intensity indicating only partial conversion of the
boronic acid groups. As expected from this incomplete
crosslinking, also the bromine functional groups of the
imidazolium precursor indicated by a sharp band at 567 cmÀ1
are present in the spectrum of the polymer, even though with
a much lower intensity, too. Further proof for a successful
integration is given by the bands at around 2930 cmÀ1 in the
spectrum of the imidazolium linker and the polymer, indicating
aliphatic C–H vibrations of the methyl groups. Comparison of
1H MAS and 13C CP MAS NMR spectra of the imidazolium
linker and the polymer EOF-15 (Fig. S2, ESIw) shows broad
signals for aromatic and aliphatic carbon atoms at a chemical
shift of 125–140 ppm and 16–19 ppm and the corresponding
hydrogen atoms with maxima at 6.8 and 2.8 ppm, respectively.
In the 13C spectrum of the pure linker a much better resolution
can be observed due to an ordered structure in contrast to the
non-ordered polymer structure.
Fig. 2 Nitrogen physisorption isotherms of EOF-15 (diamonds)
and -16 (triangles) measured at À196 1C. Filled symbols denote
adsorption, empty symbols denote desorption, respectively.
Nitrogen physisorption measurements result in a combination
of type I and II isotherms that are characteristic for micro-
porous polymers with an amorphous structure (Fig. 2). They
show a high uptake at relative pressure below 0.2 and a
hysteresis between the ad- and desorption branch indicating
the flexible framework structure. The approximately linear
increase of the uptake amount at p/p0 = 0.2–0.8 is a result of a
high external surface area due to small particles as shown by
SEM. All evaluation data of the isotherms are summarized in
Table 1. In comparison to other porous polymers produced by
Suzuki coupling (SBET up to 1380 m2
g
À1),13 EOF-15 and
The composition of the polymer EOF-16 was determined by
C, H and N combustion analysis. All experimental values
(wt%, C 61.65, H 4.48, N 3.06) are significantly lower in
comparison to the values calculated for an ideal structure
assuming a 100% crosslinking degree (wt%, C 76.05, H 6.27,
N 6.45). This difference might be attributed to the residual
functional groups (–B(OH)2, –Br).
-16 show rather low specific surface areas of up to 262 m2 gÀ1
.
One reason might be the large linker between the tetrahedral
nodes consisting of four 1,4-connected benzene rings and one
imidazolium ring. As shown before,13 large linkers result in
highly flexible frameworks allowing an effective packing in
space. On the other hand such a high flexibility should result in
highly swollen frameworks in certain solvents to guarantee
good accessibility for substrates to catalytic active sites.
Thus, the catalytic activity of the compounds was tested in
the conjugated umpolung of an a,b-unsaturated cinnam-
aldehyde and coupling with trifluoroacetophenone by the
in situ generated NHC (Scheme 2).16 The reusability of
the heterogeneous catalyst was demonstrated in up to four
EOF-16 shows a high thermal stability in air up to 300 1C
(Fig. S3, ESIw). Up to that temperature a slight mass loss is
observed that is attributed to volatile compounds. Under this
assumption, a total mass loss of 75 wt% is observed up to a
temperature of 800 1C. The decomposition occurs in two steps.
We assume that at lower temperature residual functional and
organic groups are decomposed, while at a higher temperature the
organic framework itself is destroyed. The large residual amount
of 25 wt% in comparison to the theoretical expected value of
6.7 wt% by boron oxide from the BF4 counter ion might be
explained by the residual boronic acid groups in the polymer.
The morphology of EOF-15 and -16 is shown to be plate-
like in the upper nano- and lower micrometre range according
to SEM micrographs (Fig. 1). But with higher magnification
one can see highly aggregated nanoparticles with a size of
10–50 nm especially for EOF-16.
Table 1 Determination of total specific surface area, external surface
area, micro- and total pore volume
EOF
15
16
Stotal(p/p0 = 0.3)a
Sextern(p/p0 = 0.4–0.7)b
Vmicro(p/p0 = 0.2)c
Vtotal(p/p0 = 0.9)c
m2 gÀ1
m2 gÀ1
cm3 gÀ1
cm3 gÀ1
253
28
0.12
0.16
262
111
0.12
0.21
a
b
Single point BET. t-plot. Gurvich.
c
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4814–4816 4815

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ability for catalyst recycling. In our view, these materials could
prove useful in the upscaling of NHC catalysed reactions
due to a higher economic and ecologic efficiency of the
heterogeneous catalysed reaction.
Financial support by the Deutsche Forschungsgemeinschaft
(SPP 1362) is gratefully acknowledged. The research of
F.G. was supported by the Alfried Krupp Prize for Young
University Teachers of the Alfried Krupp von Bohlen und
Halbach Foundation.
Scheme 2 Catalytic test reaction: conjugated umpolung of an a,b-
unsaturated cinnamaldehyde with trifluoroacetophenone by the in situ
generated N-heterocyclic carbene. DBU = Diazabicycloundecene.
Table 2 Results of catalytic test reaction with the reusable hetero-
geneous catalysts EOF-15 and -16
Notes and references
Catalyst
Cycle Isolated yielda (mol%) dr (like : unlike)b
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1
2
3
4
5
1
2
3
4
65 (86; 81)c
1.38 : 1
1.39 : 1
1.43 : 1
1.37 : 1
1.37 : 1
1.33 : 1
1.50 : 1
1.33 : 1
1.27 : 1
60 (68; 67)c
EOF-16(C) (ESI1w)
56
48
46
69
78
84
72
EOF-15(Si)
1
83
1.86 : 1
1
1
83
85
1.50 : 1
1.50 : 1
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a
b
c
Based on cinnamaldehyde. Detected by 1H NMR. Experiment
repeated twice, yields given in brackets.
recycles for proof of principle, although significantly more
cycles are necessary with regard to potential applications. The
yields obtained with EOF-15(Si) vary from 69% to 84%, while
they decrease from 65% to 46% in the case of EOF-16(C)
(Table 2). The stereoselectivity (like/unlike) of the g-butyrol-
actone formation is only slightly affected. The heterogeneity of
EOF-16(C) was investigated by removing the catalyst by
centrifugation during an ongoing reaction (ESIw). Without
the catalyst, the conversion stops and no significant product
formation could be observed. The yields obtained with
EOF-15(Si) and EOF-16(C) are comparable with the ones
obtained with homogeneous imidazolium salts.
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In summary, we have presented the successful immobil-
ization of
a bifunctional imidazolium linker in highly
crosslinked element organic framework materials by Suzuki
coupling with tetrafunctional boronic acid linkers. The porous
compounds were used as heterogeneous catalysts in an
organocatalytic test reaction with similar results as compared
to the molecular species in homogeneous catalysis but with the
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c
4816 Chem. Commun., 2011, 47, 4814–4816
This journal is The Royal Society of Chemistry 2011