Imidazolium-functionalized SBA-15 type silica: Efficient organocatalysts for Henry and cycloaddition reactions

Article abstract of DOI:10.1002/aoc.2974

We report the synthesis of mesoporous SBA-15 type silica bearing ionic imidazolium substructures. Surface functionalization was achieved via post-synthesis grafting reactions using bis-silylated imidazolium precursors onto a mesoporous SBA-15 type silica

Full text of DOI:10.1002/aoc.2974

Full Paper
Received: 7 June 2012
Revised: 12 December 2012
Accepted: 10 January 2013
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.2974
Imidazolium-functionalized SBA-15 type silica:
efficient organocatalysts for Henry and
cycloaddition reactions
a
b
b
Blanca Motos-Pérez , Jérôme Roeser , Arne Thomas * and
Peter Hesemann *
a
We report the synthesis of mesoporous SBA-15 type silica bearing ionic imidazolium substructures. Surface functionalization was
achieved via post-synthesis grafting reactions using bis-silylated imidazolium precursors onto a mesoporous SBA-15 type silica
support. The grafting reactions were monitored via solid-state NMR spectroscopy, nitrogen sorption, transmission electron mi-
croscopy and thermogravimetry. Post-synthesis grafting is the most convenient way to achieve highly stable functionalized
solids displaying excellent accessibility of the immobilized functional groups combined with high chemical stability. The solids
obtained via post-synthesis grafting reactions appeared as highly efficient and reusable heterogeneous organocatalysts for
Henry reactions and the cycloadditions of CO to epichlorohydrin. Copyright © 2013 John Wiley & Sons, Ltd.
2
Supporting information may be found in the online version of this article.
2
Keywords: mesoporous functional organosilica; heterogeneous organocatalysis; Henry reaction; CO -cycloaddition to epoxides
[
18,19]
constitution of the solid.
For this reason, the organic entities
Introduction
have a large influence on the physical and chemical properties of
Mesoporous and nanostructured silica-based materials, obtained
by template-directed synthesis, were introduced in the early
PMO type materials. However, the accessibility of the organic
groups within the walls of the materials may be limited.
[
20,21]
[1,2]
1
990s.
Since that time, this type of material has gained consid-
In the field of functional organosilicas, silica-based materials
bearing ionic groups have recently been particularly considered.
These materials show unique properties due to their mixed ionic
and mineral composition. Besides their interesting properties
erable interest owing to its high thermal stability, high porosity
[
3]
and narrow pore size distribution. These features, combined
with the possibility to introduce a large variety of functional or-
ganic groups, make mesoporous silica-based materials candi-
[
22]
allowing various applications in the fields of catalysis,
op-
[
4]
[23,24]
[25]
dates of choice for numerous applications ; for example, in the
tics,
separation
and controlled synthesis of nanoparti-
the use of ionic precursors allows researchers to gain a
[5–8]
[9]
[10]
[26]
fields of catalysis,
and detection.
separation, molecular recognition
cles,
[11]
detailed insight into the structuring process in template-directed
Organosilicas can be divided into two classes of materials:
surface-functionalized silicates and periodic mesoporous organo-
silicas (PMOs).
hydrolysis–polycondensation procedures due to specific tem-
[
27–29]
plate–precursor interactions.
Following our recent work on silica hybrid materials containing
[
28,30,31]
Surface-functionalized silicates can be obtained either by ’one-pot’
hydrolysis polycondensation reactions of monosilylated precursors
with silica precursors such as tetraethyl-ortho-silicate (TEOS) or via
post-synthesis grafting reactions of silylated precursors on the silica
ionic groups,
we studied here the use of mesoporous silica
bearing surface-tethered imidazolium groups as heterogeneous
organocatalysts. Functional silica materials have already been used
as heterogeneous organocatalysts, for example for Knoevenagel
[12,13]
[32]
surface.
The synthetic pathway, ’one-pot’ condensation or
reactions. In this study, we focused in particular on the rational
post grafting, has significant influence on the properties of the
formed surface-functionalized silica-based materials. Whereas
one-pot hydrolysis–polycondensation reactions yield materials
with a homogeneous distribution of the organic groups but often
low hydrothermal stability, post-synthesis grafting reactions afford
materials with higher hydrothermal stability and better accessibility
of the functional groups, which are nevertheless less homoge-
design of functional silica-based materials displaying both high
organocatalytic activity and high chemical and morphological
*
Correspondence to: P. Hesemann, Institut Charles Gerhardt de Montpellier, UMR
CNRS 5253 CNRS-UM2-ENSCM-UM1, Université de Montpellier 2, Place Eugène
Bataillon, 34095 Montpellier Cedex 05, France. Email: peter.hesemann@univ-montp2.fr
[14]
neously distributed.
Contrary to these strategies, the use of di- or oligosilylated pre-
a Institut Charles Gerhardt de Montpellier, Université de Montpellier 2, 340951
Montpellier, France
[
15–17]
cursors opens the door to PMOs.
PMOs are characterized by
the fact that the organic substructures are incorporated into the
walls of the materials and therefore are an inherent part of the
b Technische Universität Berlin, Institut für Chemie, Funktionsmaterialen, 106231
Berlin, Germany
Appl. Organometal. Chem. 2013, 27, 290–299
Copyright © 2013 John Wiley & Sons, Ltd.

Heterogeneous organocatalysis
stability in order to achieve reusable heterogeneous organocata-
lysts. In the area of organometallic catalysis, we recently observed
that materials obtained via post-synthesis grafting reactions show
better long-lasting performance and recyclability compared to their
p-Nitrobenzaldehyde and p-anisaldehyde were purchased
from Aldrich. p-Hydroxybenzaldehyde was purchased from Acros
Organics. The imidazolium precursors 1 and 2 and the SBA-15 type
support were synthesized following previously published proce-
[
33]
[21] 1
13
counterparts obtained via co-condensation reactions.
These
dures.
H and C spectra in solution were recorded on Bruker
differences were attributed to (i) enhanced stability of post-grafted
materials and (ii) better accessibility of the tethered organic entities,
as the grafting reaction only occurs on accessible sites on the
surface of the materials. For this reason, post-grafting reactions
appear more atom-economic compared to one-pot hydrolysis–
polycondensation processes as nearly 100% of the surface-grafted
organic substructures are accessible. On the other side, the nature
of the silica scaffold also plays an important role in the stability of
mesoporous silica supports.
AC 250 or Bruker Avance 400 spectrometers at room temperature.
Deuterated chloroform was used as solvent for liquid NMR experi-
ments and chemical shifts are reported as d values in parts per mil-
13
29
lion relative to tetramethylsilane. Solid-state C and Si magic an-
gle spinning (MAS) NMR experiments were performed on a Varian
VNMRS 400 MHz solid spectrometer using a two-channel probe
29
2
with 3.2 or 7.5mm ZrO rotors. The Si Solid NMR spectra were
recorded using both cross-polarization (CP) MAS and single-pulse
sequences with samples spinning at 6 kHz. CP MAS was used to ob-
tain high signal-to-noise ratio with 5 ms contact time and 5 s recy-
cling delay. The number of scans was in the range 1000–3000.
For single-pulse experiments, p/6 pulse and 60 s recycling delay
were used to obtain quantitative information on the silane–silanol
It has been shown that SBA-15 type materials show considerably
higher hydrothermal stability compared to the related MCM-41
[34]
type material. These differences are due, in particular, to greater
wall thickness in these materials: whereas MCM-41 type materials
often show wall thicknesses of 2–3 nm, corresponding to only
several atomic layers, the walls of SBA-15 type materials are consid-
13
condensation degree. The C CP MAS spectra were obtained using
3 ms contact time, 5 s recycling delay and 6 kHz spinning rate.
Nitrogen sorption isotherms were obtained at 77 K with a Micro-
meritics ASAP 2020 apparatus. Prior to measurement, the samples
[
35]
erably thicker, typically between 4 and 6 nm, and consequently
more resistant. Furthermore, the larger pore diameter of SBA-15
type materials facilitates diffusion of reactants and reaction
ꢀ
were degassed for 18 h at 60 C. The surface areas (SBET) were
[
36]
products within the pores.
obtained according to the equation introduced by Brunauer,
Emmett and Teller (BET), in appropriate p/p0 ranges and
assuming a surface coverage of nitrogen molecule estimated to
Silica-derived materials bearing imidazolium groups have
already been used for the formation of silica supported palladium
[
37–39]
2
N-heterocyclic carbene complexes.
still remains a challenge in the area of silica-supported metallic
NHC complexes.
However, metal leaching
be 13.5 Å . Pore size distributions were calculated from the adsorp-
tion branch of the isotherms using the Barrett–Joyner–Halenda
(BJH) method. The pore width was estimated at the maximum of
the pore size distribution. X-ray diffraction (XRD) experiments
were carried out with an Xpert-Pro (PanAnalytical) diffractom-
eter equipped with a fast X’celerator detector using Cu-Ka
radiation. TGA was performed using a TA Instruments Q50 V6.7
[37]
For these reasons, we focused here on the
elaboration of heterogeneous organocatalysts via post-synthesis
grafting reactions using the bis-silylated diaryl imidazolium precursor
1
and the bis-alkyl imidazolium precursor 2 (Scheme 1) using SBA-15
type silica as support. The use of disilylated precursors for post-
synthesis grafting reactions may also have a favorable effect towards
the formation of highly stable heterogeneous organocatalysts.
Thus, the aim of this work was to synthesize surface-modified
SBA-15 materials via post-synthesis grafting using bis-silylated
imidazolium precursors and to study the organocatalytic properties
of these materials in two different test reactions: Henry reactions
and cycloadditions of carbon dioxide to epichlorohydrin.
ꢀ
Build 203 instrument over a temperature range of 25–800 C
using air as carrier gas and a temperature ramping rate of
ꢀ
ꢁ1
10 C min
.
Epichlorohydrin was used as purchased from Sigma-Aldrich.
Caution: epichlorohydrin is a carcinogen and mutagen. Handle with
extreme caution! CO (99.995%) was supplied by Air Liquide. The
2
reactor set-up consisted of a 100 ml stainless steel Berghof BR-100
high-pressure reactor with simple heating and stirring using a lab-
oratory heating plate with heater block. The reactor was fitted with
a Teflon insert, a pressure indicator, a temperature probe
submersion tube to measure the internal reactor temperature, a
pressure relief valve and a metal rupture disc to safely limit
the maximum pressure. Quantitative analysis was conducted by
GC-MS, for which an Agilent Technologies 7890A GC system
running with an HP-5MS capillary column and an Agilent Technol-
ogies 5975C mass selective detector were employed with a known
amount of toluene as an external standard.
Experimental
General Remarks
All reagents purchased from commercial sources were used
without purification. Nitroethane, p-bromobenzaldehyde
and p-cyanobenzaldehyde were purchased from Alfa-Aesar.
(EtO)3Si
Si(OEt)3
N
N+
Synthesis of the SBA-15 Materials Bearing Surface-Tethered
Imidazolium Groups
OTf-
1
Synthesis of organosilica materials A and C by grafting reactions on
mesoporous SBA-15
N
N+
(EtO)3Si
Si(OEt)3
Into a two-neck, round-bottomed flask attached to a condenser
Cl-
with a nitrogen bubbler, 0.180 g SBA-15 (previously degassed
2
ꢀ
under reduced pressure at 80 C for 24 h) was suspended in
dry toluene (25 ml) under nitrogen atmosphere. To this suspen-
Scheme 1. Imidazolium precursors 1 and 2 used for post-grafting
reactions with SBA-15 type nanostructured silica.
sion, 0.200 mmol of the organo-ionic precursor 1 or 2 were
Appl. Organometal. Chem. 2013, 27, 290–299
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

B. Motos-Pérez et al.
added. The resulting suspension was stirred for 24 h under re-
flux. After cooling to room temperature, the solvent was filtered
off and the solids were washed with toluene (2 ꢂ 10 ml), ethanol
Cycloaddition reactions
In a typical reaction, 100 mg catalyst and 18 mmol epichlorohy-
drin were charged in a 100 ml Teflon insert. The reactor was
sealed and flushed five times at room temperature with CO to
2
remove the air from the vessel. The pressure was adjusted to
(
2 ꢂ 10 ml) and acetone (2 ꢂ 10 ml) for removing the unreacted
precursor. Finally, the slightly brownish solids A (0.215 g) and
C (0.222 g) were dried under reduced pressure for 18 h.
ꢀ
6.9 bars and the reactor heated to 130 C. After 4 h reaction time,
the reactor was cooled to room temperature and the pressure
was released. The collected sample was centrifuged, filtered
and analyzed by GC-MS and NMR spectroscopy.
Synthesis of organosilica materials B and D by passivation reactions of
mesoporous materials A and C
Into a two-neck, round-bottomed flask attached to a condenser
with a nitrogen bubbler, 0.150 g of material A or C (previously
ꢀ
degassed under reduced pressure at 60 C for 20–24h) was sus-
Results and Discussion
pended in dry toluene (7ml) under nitrogen atmosphere. To the
stirred suspension, an excess of 1,1,1,3,3,3-hexamethyldisilazane
Synthesis of SBA-15 Type Support and the Silylated Imidazolium
Precursors
(
HDMS) was added (0.20 ml). The resulting suspension was
stirred for 24 h under reflux. After cooling to room temperature,
the solvent was filtered off and the solids were washed with tol-
uene (2 ꢂ 10 ml), methanol (2 ꢂ 10 ml), dichloromethane
The two bis-silylated imidazolium precursors 1 and 2 were synthe-
[
31,40,41]
sized following previously published procedures.
The bis-
0
silylated N,N -diaryl imidazolium precursor 1 was obtained via a
four-step sequence starting from 4-iodo-2,6-dimethylaniline. The
ring-closing reaction was realized with chloromethylpivalate in
the presence of silver triflate. The silylation was finally achieved
via an Rh(I)-catalyzed coupling of the diiodo intermediate with
(
(
2 ꢂ 10 ml) and diethyl ether (2 ꢂ 10 ml). Finally, the solids B
0.180 g) and D (0.160 g) were dried under reduced pressure
for 18 h.
Preparation of organosilica materials E and F by anion exchange reactions
from mesoporous material B
[42]
0
triethoxysilane. The bis-silylated N,N -dialkyl imidazolium precur-
sor 2 was prepared by coupling N-(3-triethoxysilylpropyl)imidazole
and (3-chloropropyl)triethoxysilane.
Into a one-neck, round-bottomed flask, material B (0.100 g) was
suspended in an ethanolic solution of sodium acetate (0.3 M) or
in an ethanolic solution of sodium chloride 0.2 M, stirred at room
temperature for 1 h and filtered. This procedure was repeated
four times. After filtration, material E (0.095 g) or F (0.090 g) was
washed with ethanol and dried under vacuum at room tempera-
ture for 18 h.
The mesoporous nanostructured SBA-15 was prepared by tem-
plate-directed hydrolysis–polycondensation reaction of tetraethyl
orthosilicate (TEOS) using a P123 type (EO) (PO) (EO) triblock
20
70
20
copolymer as structure-directing agent according to a method in-
[35]
[34]
troduced by Zhao et al.
and modified by Galarneau et al.
This material was used for the preparation of functional silica
materials via post-grafting reactions using the bis-silylated imida-
zolium precursors 1 and 2.
Catalytic Tests: General Procedures
Henry reactions
Grafting Reactions
To a solution of p-nitrobenzaldehyde (99.0 mg, 0.33 mmol,
1
equiv.) in nitroethane (0.94 ml, 20 equiv.) was added material
The post-grafting reactions were carried out as previously de-
ꢁ
1
[21]
B (98.3 mg, 1.0 mmol imidazolium derivate g material accord-
ing to TGA, 0.15 equiv.) under nitrogen atmosphere. The hetero-
geneous mixture was stirred for 4 h at room temperature. After
that time, the reaction mixture was filtered and the excess of
nitroethane was eliminated by evaporation. The product was re-
covered as a slightly yellow solid. The conversion (99%) was cal-
scribed for related oligosilylated ion pairs
Si and C solid-state NMR spectroscopy and thermogravimetry
and monitored via
29
13
(see supporting information). In this way, we obtained materials
A and C with covalently attached imidazolium substructures
(Scheme 2, step a). The obtained grafted materials A and C were
subsequently passivated via end-capping reactions with
1,1,1,3,3,3-hexamethyldisilazane (HDMS), giving rise to the
corresponding trimethylsilylated materials B and D (Scheme 2,
step b). The grafting was monitored via thermogravimetry and
solid-state NMR spectroscopy. The results of these analyses are
given in Table 1.
1
culated from the H NMR spectrum on the basis of the remain-
ing nitrobenzaldehyde. On the other hand, the recovered
catalyst was washed with ethanol (3 ꢂ 10 ml) and dried at room
temperature overnight (18 h). In the following runs, the same
molar ratio of reagents and catalysts were used.
Scheme 2. Post-grafting reaction for the synthesis of SBA-15 material with the imidazolium precursors 1 or 2 (step a) and subsequent end-capping of
residual silanol groups (step b).
wileyonlinelibrary.com/journal/aoc
Copyright © 2013 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2013, 27, 290–299

Heterogeneous organocatalysis
Table 1. Surface properties and loadings of the SBA-15-derived materials A–D bearing doubly anchored imidazolium groups
ꢁ
1
2
ꢁ1
3
ꢁ1
Entry
Material
Grafted precursor
Loading (mmol g SiO
2
)
Surface area SBET (m g
)
Pore diameter (Å)
Pore volume (cm g
)
1
2
3
4
5
SBA-15
–
1
1
2
2
–
875
515
315
388
327
82
65
60
67
65
1.00
0.47
0.33
0.45
0.39
a
A
B
1.0
b
0.92
a
b
C
D
1.4 /1.02
a
(1.5 )
a
Values obtained by TGA measurements.
b
29
Values obtained from the integral of the signals of the T-series obtained from the Si SP-MAS-NMR spectrum.
2
9
Si solid-state NMR spectroscopy allows monitoring the post-
thermogravimetry gave additional quantitative information re-
garding the functionalization degree of the materials. When
synthesis grafting reaction of the imidazolium precursors onto
the silica scaffold and the end-capping reaction of the silanol
groups. The Si CP-MAS technique allows monitoring in a quali-
tative manner the incorporation of the ionic groups within the
the organic grafting was estimated from TGA measurements,
2
9
ꢀ
the decomposed weight/residual (SiO ) weight at 700 C stems
2
from organic grafted species on silica based on 1 g of silica. We
used the following equation for determination of the amount
of immobilized organic entities:
2
9
materials. The Si CP-MAS spectra of materials A and B are given
as examples in Fig. 1. The spectrum obtained with material A
shows signals at ꢁ71.9 ppm, ꢁ101.1 ppm and ꢁ109.6 ppm.
2
3
4
nimidaz ¼ org:wt %=½100 ꢁ ðorg:wt % ꢁ H O wt %Þꢃ
(1)
2
These signals are characteristic of T , Q and Q resonances and
reflect the presence of silicon species in R-Si(OSi) OH, Si(OH)
OSi) and Si(OSi) environments, respectively. The presence of
T [R-Si(OSi)(OH)
2
ðMimidaz ꢁ 2H2OÞ
(
3
4
1
2
] silicon species in material A is also suspected
The obtained results are given in Table 1. The results of the TGA
experiments are in good agreement with the results obtained from
from the shoulder at ꢁ63 ppm. The spectrum of material B shows
an additional signal at 12.9 ppm indicating the presence of
trimethylsilyl groups. This M-resonance was introduced via end-
capping reaction of the residual silanol groups with hexamethyl
disilazane. The higher intensity of highly condensed silicon
centers in material B (T and Q centers) compared to material
A suggests that the passivation reaction increases the condensa-
tion degree of the materials. Further information about the exact
loading of the materials was obtained from Si single pulse (SP-)
MAS spectra of the materials. In contrast to the cross-polarization
CP) technique, the Si SP-MAS technique allows one to quantify
the different silicon species in the materials. From the Si SP-MAS
spectra of materials (see supporting information), we could deter-
mine a T/Q-ratio of 9.9/90.1 for material B and 10.9/89.1 for material
C. Supposing that the passivation reactions do not affect the
functionalization of the materials with imidazolium groups, we
2
9
Si SP-MAS NMR spectroscopy. The observed discrepancies may
be due to the utilization of different characterization techniques.
29
Si SP-MAS solid-state NMR spectroscopy gives more precise
3
4
information and allows distinguishing M, D, T, and Q silicon centers
within the materials. TGA analyses do not allow this distinction. The
29
loading of materials B and C, obtained by Si SP-MAS NMR spec-
troscopy (Table 1, entries 3 and 4), reflect the amount of immobi-
lized ionic substructures, without taking into account residual
triethoxysilyl groups or trimethylsilyl groups (M-sites), introduced
through the passivation reaction. The higher loadings determined
from TGA measurements (Table 1, entries 2 and 4) can be explained
by the presence of residual ethoxy groups in the materials. This
trend has already been observed in other functional silica-based
29
2
9
(
29
[
33]
materials obtained via post-grafting methods. The increase in or-
ganic loading in material D compared to the parent material C (Ta-
ble 1, entries 4/5) can by explained by the trimethylsilylation of re-
sidual silanol groups in the passivation reaction.
ꢁ
1
can assume imidazolium loadings of 0.92 meq g for the materials
ꢁ
1
A and B and of 1.02 meq g for materials C and D. The presence of
the organo-ionic groups in materials A and C was also monitored
1
3
After having ascertained the degree of incorporation of the
ionic groups within the materials, we focused on the determina-
tion of the morphological and textural properties of the materials.
The X-ray diffractograms of the parent SBA-15 silica together
with the grafted and grafted/passivated materials A and B are
shown as examples in Fig. 2. XRD experiments with the materials
clearly showed that the regular hexagonal architecture of the
parent SBA-15 type material is not strongly affected by the graft-
ing reactions. The diffractograms of the SBA-15 silica nicely show
the (1 0 0), (1 1 0) and (2 0 0) reflections, which are characteristic
for structured materials displaying hexagonal architectures on a
mesoscopic length scale. The (1 1 0) and (2 0 0) reflections still ap-
pear in the diffractograms of materials A and B, indicating that
the structural ordering is maintained after functionalization.
Characterization of the materials via nitrogen sorption experi-
ments showed the influence of the grafting reactions on the sur-
face properties of the materials. The nitrogen sorption isotherms
of SBA-15 and materials A and B are given as examples in Fig. 3.
The surface properties of materials A–D are summarized in
via C CP-MAS solid-state NMR spectroscopy. The obtained spectra
see supporting information) are in nice agreement with previously
(
[28,31]
obtained results.
The characterization of the materials by
2
9
Figure 1.
Si CP-MAS spectra of materials A (top) and B (bottom).
Appl. Organometal. Chem. 2013, 27, 290–299
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

B. Motos-Pérez et al.
Figure 2. X-ray diffractograms of the parent SBA-15 material together
with the material A and the passivated material B.
Figure 3. Nitrogen sorption isotherms of the parent SBA-15 materials to-
gether with the material A obtained by grafting of precursor 1 and the
passivated material B.
Table 1. It is clearly shown that the grafted materials A and C
display reduced specific surface area, pore diameter and pore
volume compared to the parent SBA-15 type silica which was
used as scaffold in the post-grafting reactions. The specific
evaluate to what extent pore blocking occurs. The grafting and
passivation reactions strongly affect the adsorption coefficient C
of the materials. For the parent SBA-15 material we found C-values
in the range of 200–300. The grafting and passivation reactions led
to a strong decrease of the C-values to 90 of the grafted materials
A and C and 50–60 in the case of the grafted and passivated
materials B and D.
Finally, we performed TEM experiments with our materials. The
TEM images of the parent SBA-15 silica and the materials A and B
containing bis-aryl imidazolium substructures are shown in Fig. 4.
The TEM images of materials C and D are given in the supporting
information. TEM analyses show that the whole series of materi-
als displays highly regular architectures on a mesoscopic length
scale. However, on the surface of the particles in materials
A and B, some dark spots indicate the formation of deposits on
the surface of the silica support. These deposits are probably
2
ꢁ1
surface areas SBET of the materials decreased from 875 m g for
2
ꢁ1
2
ꢁ1
the used SBA-15 to 515 m g for material A and to 388m g
for material C. The passivation reaction of surface located Si—OH
groups with hexamethyl disilazane led to a further reduction of
2
ꢁ1
the specific surface areas to 315 and 327 m g in the case of
materials B and D. Similar trends were found for the pore diameters
and pore volumes of the materials (Table 1). Similar results have
already been obtained with related materials obtained via post-
synthesis grafting reactions of guanidiniums sulfonamide ion
[
21]
pairs on mesoporous MCM-41 and SBA-15 type silicas.
The
decrease of both pore diameter and pore volume indicates that
both surface tethering of the organo-ionic precursor and pore
blocking are responsible for the modifications of the nitrogen
sorption properties of the materials. However, it is difficult to
Figure 4. TEM images of the parent SBA-15 (top left) and materials A (top right) and B (bottom).
wileyonlinelibrary.com/journal/aoc
Copyright © 2013 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2013, 27, 290–299

Heterogeneous organocatalysis
formed by hydrolysis–polycondensation reactions of the silylated
precursors. These deposits are responsible for partial pore block-
ing and lead to the decrease of the mesopore volume observed
by nitrogen sorption experiments. Similar results were obtained
with materials C and D, obtained via post-synthesis grafting
reactions with the dialkyl imidazolium precursor 2.
from material B containing triflate anions via anion exchange
reactions by repeated washing with 0.2–0.3 M ethanolic solutions
of sodium acetate and sodium chloride, respectively. Whereas the
material E containing acetate anions shows similar catalytic activity
and selectivity compared to material B (Table 2, entry 19), the anion
exchange with sodium chloride gave rise to the formation of
material F which showed lower catalytic activity (Table 2, entry 20).
[47]
These results are in agreement with the works of Burguete et al.,
Catalytic Tests
who reported similar results with imidazolium derivatives supported
on Merrifield resins containing chloride or acetate as counteranion.
A similar trend was observed for the Henry reaction between p-
nitrobenzaldehyde and nitromethane with materials containing
acetate or chloride counterions.
The catalytic properties of the materials A–D were evaluated in
two different organocatalytic reactions: (i) the Henry reaction of
nitroalkanes with aromatic aldehydes giving b-nitroalcohols;
and (ii) the cycloaddition of carbon dioxide to epichlorohydrin
yielding 4-chloromethyl-1,3-dioxolan-2-one.
Although there are no data available concerning the catalytic
efficiency of imidazolium-functionalized siliceous materials in
Henry reactions, the comparison with functionalized silicas con-
taining amine and ammonium groups reveals that the new mate-
rials described in this study show at least similar and sometimes
Henry reactions
The Henry reaction is a base-catalyzed C—C bond-forming reaction
between nitroalkanes and aldehydes or ketones. It is also referred
[
43]
[44,45]
[45,48]
to as the nitro aldol reaction.
Amine-functionalized silicas
even higher turnover numbers and frequencies.
[46]
and amine-containing periodic mesoporous organosilicas (PMOs)
In order to study the versatility of our materials, we used various
aldehydes as reagent in Henry reactions with material B. The high-
est conversions were observed for p-nitrobenzaldehyde and re-
lated aldehydes containing electron-withdrawing groups such as
p-bromobenzaldehyde or p-cyanobenzaldehyde, whereas alde-
hydes containing electron donating groups such as p-anisaldehyde
gave a strongly reduced conversion (Table 2, entries 15–18).
In summary, the catalytic activity of silica-derived materials as
heterogeneous Henry reaction catalysts depends on a set of para-
meters such as the nature of the ionic species, the hydrophilicity
of the surface of the mesoporous silica support, the counterion of
the immobilized imidazolium species and the synthetic pathway.
Materials synthesized via post-grafting reactions show considerably
higher catalytic activity compared to materials obtained from
co-condensation reactions. The better accessibility of the ionic
substructures may be the reason for this observation. Addition-
ally, passivation of silanol groups located at the materials’
surface is also an important parameter in order to achieve highly
active organocatalysts. In our study, the highest catalytic activi-
ties have been observed for material B synthesized via post-syn-
thesis grafting reactions with the diaryl imidazolium triflate pre-
cursor 1 and subsequent surface passivation (Table 2, entry 8).
This material also gave satisfactory results in recycling experi-
ments with only slightly reduced catalytic activity after five runs
(Table 2, entries 9–12).
are heterogeneous Henry reaction catalysts and efficiently promote
the reaction of nitroalkanes with aromatic aldehydes to yield b-
nitroalcohols, which are important intermediates for the synthesis
[43]
of pharmacologically important building blocks.
We recently
observed that PMO type materials bearing ionic ammonium sub-
structures are also able to promote Henry reactions. For this reason
we focused here on the determination of the catalytic properties
of imidazolium-functionalized SBA-15 silicas. We studied in particu-
lar the Henry reaction of p-nitrobenzaldehyde with nitroethane
(Scheme 3 and Table 2, entries 1–14). Additional experiments
were performed with other substituted benzaldehydes such as
p-bromobenzaldehyde and p-anisaldehyde (Table 2, entries 15–18).
Preliminary experiments showed that neither molecular imidazo-
liums salts such as 1-butyl-3-methylimidazolium hexachloropho-
sphate (BMIM-PF ) nor nanostructured SBA-15 type silica alone are
6
efficient Henry reaction catalysts (entries 2–6). Surprisingly, the bis-
silylated dialkyl imidazolium precursor 2 showed higher catalytic
activity compared to the ionic liquid BMIM-PF and led to the
6
formation of 2-nitro-1-(4-nitrophenyl)propan-1-ol with a conversion
of 53% after a reaction time of 72 h with a syn:anti ratio of 48:52
(Table 2, entry 5). Concerning the functionalized SBA-15 materials,
the non-passivated materials A and C show very low catalytic activity
with conversions of only 21% and 13%, respectively (Table 2, entries
7
and 13). In contrast, the corresponding passivated materials B
and D are highly active heterogeneous catalysts for Henry reactions
and led to the formation of 85% (material D) and 99% (material B)
of 2-nitro-1-(4-nitrophenyl)propan-1-ol. It clearly appears that the
passivation of imidazolium-grafted silica materials is crucial in order
to achieve high catalytic activities in Henry reactions. This phenome-
non can be explained by a facilitated diffusion of the relatively
hydrophobic reactants and substrates through the hydrophobized
pores and, as a result, a better interaction with the pore surface in
the passivated materials.
Cycloaddition of carbon dioxide to epichlorohydrin
Encouraged by the efficient heterogeneous catalysis of the Henry
reactions, we further studied the activity of the materials A–D in
the base-catalyzed cycloaddition of carbon dioxide to epoxides
[49]
(Scheme 4). This reaction allows access to cyclic organic carbo-
nates, which are industrially important chemicals that can find
application as raw material for polycarbonate or polyurethane
manufacture (phosgene substitute), fuel additive and electrolyte
We also focused on the influence of the anion on the catalytic
properties of the materials. The materials E and F were obtained
[
50,51]
for lithium-ion batteries.
cycloaddition of carbon dioxide to epoxides is efficiently promoted
It has already been reported that the
[52–54]
by imidazolium-based ionic liquids.
Different heterogeneous
OH
[55]
CHO
systems such as diaryl imidazolium-based microporous polymers,
catalyst
NO2
[56–61]
+
NO2
ionic liquid-tethered silicas
liquid
and chitosan-functionalized ionic
[62]
O2N
have also been reported as efficient catalysts for this
O2N
reaction. In the case of silica-immobilized phosphonium salts, a
synergistic effect between the supported ionic species and the
Scheme 3. Henry reaction of p-nitrobenzaldehyde with nitroethane.
Appl. Organometal. Chem. 2013, 27, 290–299
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

B. Motos-Pérez et al.
a
Table 2. Henry reaction catalyzed by imidazolium silica materials A–I
c
d
Entry
Aldehyde
Catalyst
Reaction time Reaction temperature Conversion syn/anti TON TOF
ꢀ
b
b
ꢁ1
(h )
(
h)
( C)
(%)
ratio
1
2
3
4
5
p-Nitro-benzaldehyde
p-Nitro-benzaldehyde
p-Nitro-benzaldehyde
p-Nitro-benzaldehyde
p-Nitro-benzaldehyde
None
4
RT
RT
RT
90
RT
0
–
–
–
–
Parent SBA-15
24
24
24
72
0
12
18
53
–
–
Passivated SBA-15
BMIM-PF
46:54
46:54
48:52
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
6
6
p-Nitro-benzaldehyde
72
45
<1
–
n.d.
n.d.
7
8
9
p-Nitro-benzaldehyde
p-Nitro-benzaldehyde
p-Nitro-benzaldehyde
p-Nitro-benzaldehyde
p-Nitro-benzaldehyde
p-Nitro-benzaldehyde
p-Nitro-benzaldehyde
p-Nitro-benzaldehyde
p-Nitro-benzaldehyde
p-Anis-aldehyde
Material A,15%
4
4
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
40
RT
RT
21
99
99
98
98
96
13
85
87
5
57:43
58:42
58:42
58:42
58:42
58:42
49:51
54:46
58:42
57:43
59:41
70:30
58:42
58:42
70
361
361
358
358
350
43
18
90
90
89
89
88
11
48
4
Material B, 15% 1st cycle
Material B, 15% 2nd cycle
Material B, 15% 3rd cycle
Material B, 15% 4th cycle
Material B, 15% 5th cycle
Material C, 15%
4
1
1
1
1
1
1
1
1
1
1
2
0
1
2
3
4
5
6
7
8
9
0
4
4
4
4
Material D, 15%
4
190
317
18
Material B, 15%
72
72
24
48
4
Material B, 15%
<1
13
3
p-Cyano-benzaldehyde
Material B, 15%
87
45
96
75
317
164
n.d.
n.d.
p-Hydroxy-benzaldehyde Material B, 15%
p-Nitro-benzaldehyde
p-Nitro-benzaldehyde
Material E, 15%
Material F, 15%
n.d.
n.d.
4
a
Reaction conditions: 1 equiv. aldehyde, 20 equiv. nitroethane, 0.15 equiv. catalyst (calculated in base of the imidazolium derivated loading for
materials A–F or equivalent mass of solid for SBA-15 and passivated SBA-15).
b
Conversion and selectivity determined by NMR.
c
TON: (%conv.) ꢂ (mol(substrate)) / mol (catalyst).
d
TOF = TON / time (h).
silica support is supposed, leading to a remarkable acceleration
of the reaction compared to the reaction in homogeneous me-
that silica-containing dialkyl imidazolium entities are overall more
stable compared to the diaryl grafted materials. Thus no significant
loss of activity was observed until at least four consecutive runs for
material C (Table 3, entries 10–13), while a gradual decrease of
conversion is observed in consecutive runs with material A
(Table 3, entries 2–5).
[63]
dium.
Here, we focused in particular on the comparison of
the catalytic activity of mesoporous silica materials containing
diaryl imidazolium and dialkyl imidazolium groups (materials A
and C), and the influence of the passivation reaction on the
catalytic activities of the materials B and D. The catalytic perfor-
mance of the different materials was evaluated in the synthesis of
Passivation of the materials affects the activities of the materi-
als to a much lower extent compared to the previously studied
Henry reaction. Comparison of the recycling test carried out with
materials A and B (Table 3, entries 2–5 and 6–9) reveals a slight,
favorable effect of passivation on catalyst stability for diaryl
imidazolium silica materials. A gradual decrease in activity
is observed in the case of grafted–passivated material B,
with slightly better conversion compared to the analogous non-
passivated material A. However, the same trend is not found for
the dialkyl imidazolium silica materials, where no loss of activity
is observed for materials C and D for the first three consecutive
runs (Table 3, entries 10–12 and 14–16). That is followed, in the
case of the material D, unlike the non-passivated analogue mate-
rial C, by a significant decrease of conversion on the fourth run
(Table 3, entries 13 and 17).
2
chloropropene carbonate from CO and epichlorohydrin (Scheme 4
and Table 3, entries 2–17). Conditions were adapted from the
procedure described by Srivastava, where catalytic activities of
several supported amines were reported under relatively mild
[64]
conditions.
First of all, materials A–D gave conversions higher than 97%
and selectivities higher than 92% for the cycloaddition of CO
2
to epichlorohydrin, showing that all of them are highly active
heterogeneous catalysts for this reaction (Table 3, entries 2, 6,
1
0 and 14). The observed catalytic properties are in the range
of previously described silica materials containing covalently teth-
[60,65]
ered imidazolium groups, both regarding conversion/selectivity
and turnover number and frequencies.
[59,66]
The influence of the nature of immobilized imidazolium species
on catalytic activity and stability is observed during recycling tests
of the materials. Comparison between materials A and C shows
In summary, although the catalytic activity of all the imidazo-
lium silica materials is similar for the first run, material C contain-
ing dialkyl entities showed higher stability in the recycling tests.
wileyonlinelibrary.com/journal/aoc
Copyright © 2013 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2013, 27, 290–299

Heterogeneous organocatalysis
O
materials retain their morphology with a hexagonal architecture
on the mesoscopic length scale, as observed for the parent
SBA-15 material used for the grafting reactions. The grafting
reactions led to a reduction of pore volume and pore diameter.
Passivation reactions with hexamethyldisilazane led to a further
reduction in pore size and pore volume of the materials.
The main aspect of this study concerned the use of functional
silica materials as heterogeneous organocatalysts in Henry reac-
tions and cycloadditions of carbon dioxide to epichlorohydrin.
In Henry reactions, only the passivated materials showed high
catalytic activity. The materials could easily be recovered and
showed nearly unchanged catalytic activities in at least five
reaction cycles. Materials containing diaryl imidazolium substruc-
tures appear as more efficient heterogeneous catalyst systems
compared to related materials tethered with dialkyl imidazolium
species. Passivation of the materials as well as the type of coun-
terion was proven to be crucial to ensure high catalytic activities.
Indeed, the remarkable catalytic activity is suspected to be due to
a synergistic effect among the imidazolium cation, the counter-
ion present and the hydrophobic groups introduced during the
passivation of the material.
The whole series of materials show high catalytic activity in
cycloadditions of carbon dioxide to epichlorohydrin. In this specific
reaction, the passivation of the materials affects to a much lower
extent the activities of the materials compared to the previously
studied Henry reaction. However, the materials show limited recycl-
ability, probably due to more drastic reaction conditions applied in
this test reaction. Both classes of materials containing either diaryl
or dialkyl imidazolium substructures showed high catalytic activity
with a slightly higher activity for materials containing dialkyl imida-
zolium substructures. These materials also showed higher activity
in recycling tests, which may indicate that they are more stable to
drastic reaction conditions.
cat.
O
+
CO2
O
O
ClCH2
ClCH2
Scheme 4. Cycloaddition of carbon dioxide to epichlorohydrin.
In contrast, in the case of diaryl imidazolium silica materials, the
grafted–passivated material B showed better recyclability than
the homologous non-passivated A.
Finally, we investigated the origin of the decrease in catalytic ac-
tivity observed in the cycloaddition reactions. For this purpose, we
characterized the materials A and D after the fourth catalytic cycle
by means of thermogravimetry and nitrogen sorption. In both
cases, the repeated use of the materials entails a significant reduc-
tion of the specific surface area and pore volume (Fig. 5). Moreover,
the TGA of material D after the recycling tests (see supporting
information) shows modification of the weight loss curve and the
derivate loss curve associated with an increase in organic content
in the material. This result is in agreement with the observation that
the material progressively turned from an off-white powder to a
light-brown solid after four catalytic runs. These results may indi-
cate a partial loss of the porous architecture that can be attributed
[
67]
to active site pore blocking by residual carbonaceous deposits.
Conclusions
We synthesized SBA-15 type materials bearing covalently attached
ionic dialkyl and diaryl imidazolium species via post-synthesis
grafting reactions with two different bis-silylated imidazolium pre-
cursors. The loading of the materials was determined via solid-state
29
Si NMR measurements and thermogravimetry and was in the
ꢁ
1
range 0.9–1.0meq imidazolium g
2
SiO . The functionalized
Table 3. Cycloaddition reaction catalyzed by imidazolium silica materials A–D
a
c
c
c
d
ꢁ1
Entry
Catalyst
Conversion (%)
Selectivity (%)
TON
TOF (h
)
1
2
3
4
5
6
7
8
9
SBA-15, 100 mgb
13.2
97.3
81.9
76.4
65.4
100
90.5
81.6
68.1
100
100
100
98.3
100
100
100
77.8
100
–
–
Material A, 103 mg 1st cycle
Material A, 95 mg 2nd cycle
Material A, 65 mg 3rd cycle
Material A, 63 mg 4th cycle
Material B, 97 mg 1st cycle
Material B, 91 mg 2nd cycle
Material B, 64 mg 3rd cycle
Material B, 47 mg 4th cycle
Material C, 100 mg 1st cycle
Material C, 95 mg 2nd cycle
Material C, 80 mg 3rd cycle
Material C, 54 mg 4th cycle
Material D, 102 mg 1st cycle
Material D, 88 mg 2nd cycle
Material D, 75 mg 3rd cycle
Material D, 61 mg 4th cycle
92.4
94.4
94.1
95.8
93.0
94.1
94.1
94.2
94.4
94.0
95.3
95.4
94.4
94.2
95.0
77.1
170
155
212
187
202
195
249
283
176
186
221
321
118
136
160
153
43
39
53
47
50
49
62
71
44
46
55
80
29
34
40
38
10
11
12
13
14
15
16
17
a
ꢀ
ꢁ1
Conditions of the reaction: 18 mmol epichlorohydrin, 6.9 bar CO
2
, 130 C, 4 h.
b
2
ꢁ1
3
Specific surface area SBET: 856 m g ; total pore volume: 0.98 cm g , average pore diameter: 7 nm.
c
Conversion and selectivity determined by GC.
c
TON: (%conv) ꢂ (mol(substrate)) / mol (catalyst).
d
TOF = TON / time (h).
Appl. Organometal. Chem. 2013, 27, 290–299
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

B. Motos-Pérez et al.
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