Strong organic bases as building blocks of mesoporous hybrid catalysts for C-C forming bond reactions

Article abstract of DOI:10.1002/ejic.201200716

1,8-Bis(tetramethylguanidino)naphthalene (TMGN), a neutral organic base that combines the properties of guanidine and the properties of proton sponges, was used as a building block to produce organic-inorganic silica-based mesoporous hybrids with strong basic properties. The TMGN-based mesoporous hybrids (TMGN/SiO₂) were prepared by a sol-gel route working at a neutral pH and low temperatures, which avoided the use of SDAs. TMGN has been modified in order to have two terminal reactive silyl groups able to perform co-condensation with a conventional organosilane (TMOS) used as a silicon source. This synthesis has allowed us to directly introduce the unmodified, functionalized TMGN as part of the walls of the mesoporous silica by a one-pot synthesis. TMGN/SiO₂ hybrid materials present excellent catalytic properties for C-C bond forming reactions: Knoevenagel, Henry (nitroaldol), and Claisen-Schmidt condensations. The activity of the hybrid materials is higher than that of the counterpart homogeneous catalyst. The TMGN/SiO₂ hybrid catalyst was synthesized using 1,8-bis(tetramethylguanidino)naphthalene (TMGN). It combines the properties of guanidine and of proton sponges. A functionalized TMGN builder was introduced directly into a nonordered mesoporous silica by the NH₄F-catalyzed sol-gel route. This catalyst shows good catalytic performances in C-C bond forming reactions. Copyright

Full text of DOI:10.1002/ejic.201200716

FULL PAPER
DOI: 10.1002/ejic.201200716
Strong Organic Bases as Building Blocks of Mesoporous Hybrid Catalysts for
C–C Forming Bond Reactions
Enrica Gianotti,[a,b] Urbano Diaz, Alexandra Velty, and Avelino Corma*
[a]
[a]
[a]
Keywords: Organic–inorganic hybrid composites / Proton sponges / Sol–gel processes / Basic catalysts / C–C coupling
1
,8-Bis(tetramethylguanidino)naphthalene (TMGN), a neu-
perform co-condensation with a conventional organosilane
(TMOS) used as a silicon source. This synthesis has allowed
us to directly introduce the unmodified, functionalized
TMGN as part of the walls of the mesoporous silica by a one-
tral organic base that combines the properties of guanidine
and the properties of proton sponges, was used as a building
block to produce organic–inorganic silica-based mesoporous
hybrids with strong basic properties. The TMGN-based
2
pot synthesis. TMGN/SiO hybrid materials present excel-
mesoporous hybrids (TMGN/SiO
2
) were prepared by a sol–
lent catalytic properties for C–C bond forming reactions:
Knoevenagel, Henry (nitroaldol), and Claisen–Schmidt con-
densations. The activity of the hybrid materials is higher than
that of the counterpart homogeneous catalyst.
gel route working at a neutral pH and low temperatures,
which avoided the use of SDAs. TMGN has been modified
in order to have two terminal reactive silyl groups able to
Introduction
with a pK of 12.1 in MeCN and PA(MP2)gas of
a
–1
245.5 kcalmol . The proton sponges are a matter of inter-
The design of organic–inorganic hybrid mesoporous ma-
terials with strong basic functionalities has attracted great
attention in the last decade because of their potential appli-
cation in heterogeneous catalysis for the synthesis of fine
est in current research investigations and the design of more
basic proton sponges has received great attention.[
14–17]
By
combining the proton sponge skeleton with two peralkyl
guanidine functions, known for their strong proton affinity
and high basicity, a neutral organic base, named 1,8-bis-
[
1]
chemicals. The basic properties can be introduced into or-
dered or nonordered mesoporous supports by incorporat-
(tetramethylguanidino)naphthalene (TMGN, Scheme 1)
ing primary, secondary, or tertiary amines.[
2,3]
In this case,
was recently obtained with an experimental pK of 25 in
a
the resultant catalyst will have Lewis basic sites, while there
are other cases in which catalytic Brønsted basic sites are
preferred. Several examples of strong basic catalysts are re-
ported in the literature by anchoring an organic ammonium
–1 [18]
MeCN and PA(MP2)gas of 257.5 kcalmol .
[
4,5]
[6]
quaternary salt
or guanidine on different mesoporous
silica supports. Recently, a proton sponge, 1,8-bis(dimethyl-
amino)naphthalene, with strong basicity, was used as an or-
ganic building block to produce organic–inorganic hybrid
[
7]
mesoporous materials with good catalytic properties. The
proton sponges are diamines with neighboring atoms at
short distances and aromatic frames, such as naphthal-
Scheme 1. Scheme of 1,8-bis(dimethylamino)naphthalene (DMAN)
and 1,8-bis(tetramethylguanidino)naphthalene (TMGN).
[
8,9]
[10]
[11]
[12]
ene,
their analogues.
basicity, where
fluorine, heterofluorene, phenanthrene, and
[
13]
These molecules exhibit high unusual
1,8-bis(dimethylamino)naphthalene
From theoretical studies, the relatively high proton affin-
ity of TMGN is a consequence of the inherent basicity of
(DMAN, Scheme 1) is the archetype of a proton sponge
the guanidine fragment and a relatively strong intramo-
+
[
a] Instituto de Tecnología Química (UPV-CSIC) Universidad lecular hydrogen bond (IMHB) in [TMGN]H
Politécnica de Valencia, Consejo Superior de Investigaciones
Scheme 2).[19–22]
(
Científicas,
Because of this strong basicity, the proton sponges have
found applications in the field of synthetic organic chemis-
try as homogeneous catalysts for C–C forming bond reac-
Avenida de los Naranjos s/n., 46022 Valencia, Spain
E-mail: acorma@itq.upv.es
Homepage: http://itq.upv-csic.es/
[
b] Dipartimento di Scienze e Innovazione Tecnologica, Centro
Interdisciplinare Nano-SiSTeMI, Università del Piemonte
Orientale,
tions such as Knoevenagel or aldol condensations and
[23]
Michael additions.
Moreover, different transition-metal
V. T. Michel 11, 15100 Alessandria, Italy
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200716.
complexes of proton sponges, such as Pd, Pt, Re, and Mn
[
24]
4,9-dichloroquino[7,8-h]quinoline
and more recently,
5175
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

E. Gianotti, U. Diaz, A. Velty, A. Corma
FULL PAPER
co-condensation with
a
conventional organosilane
(TMOS), through a sol–gel route. The level of functionali-
zation of the organic bases was followed using liquid NMR
spectroscopy (see Supporting Information, Figures S1–S4).
1
More specifically, the H NMR spectrum of the silylated
TMGN and DMAN shows a signal at δ = 6.11 ppm as-
[
7]
signed to NH groups attached to aromatic rings. The in-
corporation of disilylated TMGN and disilylated DMAN
in the silica mesoporous framework was carried out by a
Scheme 2. Intramolecular hydrogen bond (IMHB) formation in
TMGN.
one-pot synthesis following a NH F-catalyzed sol–gel route
at neutral pH and room temperature, without the use of
structure-directing agents.
4
Pd–TMGN complexes[25] have been synthesized. The latter
was tested in Heck reactions between phenyliodide and
styrene to obtain trans-styrene.
These soft synthetic conditions allow covalent binding of
the functionalized organic fragments within the walls of the
nonordered inorganic silica, preserving the basic properties
of the organic function in the resultant hybrid and avoiding
the use of post-synthetic treatments to remove the SDAs.
The functionalized TMGN-Sil and DMAN-Sil have two
terminal reactive silyl groups that are able to perform hy-
drolysis and co-condensation with a conventional organo-
silane (TMOS), used as a silicon source, as is represented in
Scheme 3. Hybrids with low organic (TMGN or DMAN)
loading have been synthesised to obtain isolated and well-
dispersed basic sites.
1,8-Bis(tetramethylguanidino)naphthalene
(TMGN),
which combines the properties of guanidine and the proper-
ties of proton sponges, is an interesting organic molecule
that could be used as building blocks to prepare organic–
inorganic silica-based hybrid materials with strong basic
features. Organic–inorganic hybrid materials can be synthe-
sised following several routes.[ However, for introducing
a base as a building block to form mesoporous organic–
inorganic hybrid materials, it is necessary to select and opti-
mize the synthesis methodology in order to avoid degrada-
tion of the base either during the synthesis or during the
removal of the template. In our case, we have used a pro-
cedure that allows us to carry out the synthesis at neutral
pH, room temperature, and in the absence of structural di-
recting agents. Using this method, it should be possible to
achieve mesoporous hybrid organic–inorganic materials
that, although will not have a long-range order, will have
mesopores in a narrow pore-size distribution and be ther-
1]
[
26–29]
mally stable.
In our case, TMGN is the organic builder
of the hybrid and the inorganic part corresponds to silica
tetrahedra. The organic moiety may be a part of the solid
network if the organosilane contains more than one silicon
center. Therefore, TMGN has been modified in order to
have two terminal reactive silyl groups able to perform co-
condensation with a conventional organosilane (TMOS),
through sol–gel routes. This method has allowed the direct
introduction of the functionalized TMGN builders into
nonordered mesoporous silica by a one-pot synthesis. The
synthesized TMGN hybrid materials present interesting
properties as solid base catalysts and they have been used
for catalyzing Knoevenagel and nitroaldol condensations
and their activity has been compared with the DMAN-
based hybrids and the corresponding homogeneous cata-
lysts. Moreover, these organic–inorganic hybrids were also
used as catalysts in the Claisen–Schmidt condensation reac-
tions that produce valuable chemicals of biological and
pharmaceutical importance, such as chalcones.[30–33]
Scheme 3. Schematic representation of the synthesis of the TMGN/
SiO hybrids.
2
The presence of the organic moieties in the hybrids was
evidenced by means of elemental analysis. Then, to confirm
that TMGN and DMAN are not just adsorbed but chemi-
cally stabilized and covalently bound into the silica frame-
work, leaching tests were performed by washing the as-syn-
thesized hybrids with ethanol at 298 K for 3 days and then
performing a series of spectroscopic analyses, as will be de-
scribed later.
In Table 1, the C, H, N content and the calculated per-
centage in weight of the organic species present in the as-
synthesized samples and in the hybrids after the leaching
tests is reported. The efficiency of the covalent TMGN/
DMAN incorporation in the silica matrix, evaluated from
the organic content of the as-synthesized and washed hy-
brids, is also reported in Table 1.
Results and Discussion
Synthesis and Characterization of the Hybrid Materials
The C/N/H ratio, obtained by elemental analysis, con-
To become part of the silica network of a mesoporous firms that the organic part corresponds to the TMGN or
material, the organic bases have to be functionalized with DMAN builders and that no decomposition of the organic
two terminal reactive silyl groups that are able to perform content has occurred during the synthesis. However, it is
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Building Blocks of Hybrid Catalysts
Table 1. Elemental analysis of the as-synthesized hybrids before and after the leaching tests and efficiency of TMGN/DMAN incorpora-
tion into the SiO
2
.
Hybrids
As-synthesized hybrids
N, C, H Organic content
%] (EA) [%]
Washed hybrids
N, C, H Organic content Organic loss Efficiency of TMGN/DMAN incorporation
[
[%]
(EA) [%]
[%]
[%]
TMGN/SiO
TMGN/SiO
DMAN/SiO
DMAN/SiO
2
-0.5 0.44, 2.6, 1.2
-0.2 0.20, 1.7, 1.1
4.24
3.00
2.90
2.30
0.2, 2.5, 1.1
0.12, 1.6, 0.9
0.19, 1.6, 0.7
0.17, 1.2, 0.6
3.80
2.62
2.57
1.97
0.44
0.38
0.33
0.31
89
87
88
86
2
2
-0.5 0.30, 1.8, 0.8
-0.2 0.20, 1.4, 0.7
2
observed that the experimental C/N ratio is somewhat
Thermogravimetric analysis was performed in order to
higher than the theoretical value, this is probably because gain insight, not only into the organic content of the solids,
of the entrapment of some methanol molecules that occurs but also into the thermal stability of the inserted TMGN
during the formation of the hybrids.[
7]
or DMAN units with respect to the pure TMGN and
In the TMGN/SiO hybrids, the organic content is larger DMAN. The weight loss (TGA) and their respective deriva-
2
than the organic content in DMAN/SiO because of the tives (DTA) for the TMGN/SiO and DMAN/SiO hybrids
2
2
2
higher molecular weight of the former organic superbase. and pure TMGN and DMAN molecules are reported in
In the washed hybrids, the loss of organic content goes from Figure 1. The first weight loss for the hybrids is observed
0
.4% for the TMGN/SiO hybrids and from 0.3% for the at around 80–150 °C and can be associated with the re-
2
DMAN/SiO hybrids, confirming that in the hybrid materi- moval of physisorbed water and methanol. At higher tem-
2
als most of the functionalized TMGN or DMAN is at- peratures, the main weight loss is associated with the or-
tached to silica, as the incorporation of the organic moiety ganic moieties (TMGN or DMAN). The DTA curves re-
is very efficient under our experimental conditions.
veal that pure TMGN (Section A, curve e) decomposes at
a higher temperature (350 °C) than DMAN (250 °C, Sec-
tion B, curve f). The thermal stability of the organic bases
incorporated into the silica network is very similar to the
isolated bases. Indeed, the organic moieties in the hybrids
start to decompose at around 280 °C. From the DTA curves
it is possible to observe that DMAN/SiO loses a higher
2
quantity of water with respect to TMGN/SiO , suggesting
2
that DMAN hybrids are more hydrophilic than the TMGN
ones, because of the lower organic content present in the
silica (Table 1).
Spectroscopic Characterization
To confirm that TMGN and DMAN are covalently in-
serted into the network of the mesoporous silicas, solid-
state NMR spectroscopy was used. More specifically, the
13
C CP/MAS NMR spectra of the hybrids (Figure 2, curves
a and b) present typical signals of methyl groups belonging
to –N(CH ) at 40–45 ppm, whilst in the range between
3
2
13
1
50–110 ppm, where the
C NMR signals from the
[
14,18,22]
naphthalene groups should appear,
very weak sig-
nals are detected because of the low loading of the organic
bases inside the silica network. Nevertheless, the signals of
the naphthalene groups are visible in the FTIR spectra of
the hybrids (see Figure S5). In fact weak bands at 3045 and
–1
at 1530 cm , from the stretching mode of C–H and C=C of
the aromatic rings, respectively, are present. These features
indicate that the basic organic molecules are preserved in
the mesoporous hybrids.
In addition to the 13C CP/MAS NMR signals of the
methyl groups, other signals at 158, 61, 22, and 10 ppm are
observed that do not correspond to TMGN or DMAN mo-
lecules. In particular, the peak at δ = 158 ppm is assigned
to C=O groups and the other signals are from C1* (δ =
Figure 1. Section A: TGA and DTA curves of TMGN/SiO
curves a and c) TMGN/SiO -0.5 (curves b and d). Section B: TGA
and DTA curves of DMAN/SiO -0.2 (curves a and c) and DMAN/
SiO -0.5 (curves b and d). DTA of pure TMGN (curve e) and
2
-0.2
(
2
2
2
DMAN (curve f) are also reported.
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E. Gianotti, U. Diaz, A. Velty, A. Corma
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Figure 3. 29Si BD/MAS NMR of TMGN/SiO
DMAN/SiO -0.5 (curve b), and DMAN/SiO -10 (curve c). In the
inset, the Si CP/MAS NMR of TMGN/SiO -0.5 (curve aЈ),
DMAN/SiO -0.5 (curve bЈ), and TMGN/SiO -0.5 after washing
curve cЈ) with pure TMGN-Sil.
-0.5 (curve a),
2
2
2
2
9
2
2
2
(
Figure 2. 13C CP/MAS NMR of TMGN/SiO
-0.5 (curve a),
2
2
DMAN/SiO -0.5 (curve b).
–40 ppm, which could be assigned to the pure TMGN-Sil
or DMAN-Sil unreacted bis-silylated species, since this
band disappears after washing the sample (curve cЈ). The
incorporation of the disilylated organic bases into the
framework of nonordered mesoporous silica is also con-
firmed by comparing the 29Si NMR spectra of the hybrids
and the pure disilylated TMGN (see inset of Figure 3). The
TMGN-Sil exhibits one peak centered at –45 ppm typical
of Si–C bonds. When the TMGN-Sil or DMAN-Sil build-
ers are finally inserted into the silica framework, the signal
from the silicon atoms bound to the carbon units shifts to
the range –60 to –80 ppm, supporting the covalent incorpo-
ration of the disilylated organic species into the silica frame-
work.
1
0 ppm), C2* (δ = 22.7 ppm), and C3* (δ = 61.3 ppm) of
the –CH2 groups belonging to the terminal propyl-silyl
fragments of the functionalized TMGN and DMAN (see
the inset in Figure 2 for the labels of the C atoms).
the case of the TMGN/SiO -0.5 hybrid, a signal at δ =
[
34]
In
2
159 ppm is also visible that can be assigned to the carbon
[18]
belonging to the guanidine functions. The presence of all
these signals also confirms that the TMGN and DMAN
molecules were successfully functionalized.
To further confirm that the TMGN and DMAN frag-
ments are not only intact after the synthesis of the hybrids
but are also incorporated covalently into the nonordered
porous network bound to the inorganic silica units, the hy-
2
9
brids were characterized by Si BD/MAS NMR spec-
troscopy. The spectra of the hybrids (Figure 3) show three
Textural Properties of the Hybrid Materials
2
peaks at –92, –100, and –110 ppm, from the Q [Si(OH) -
2
3
4
The textural properties of the hybrids, measured by N2
(
OSi) ], Q [Si(OH)(OSi) ], and Q [Si(OSi) ] silicon units,
2 3 4
[
35]
adsorption, are reported in Table 2.
respectively.
The typical signals from –50 to –80 ppm, assigned to T-
type silicon species having a Si–C bond, are not visible in
the TMGN/SiO -0.5 and DMAN/SiO -0.5 spectra, because
of the low organic loading present in these hybrids, whilst
they are well visible when the organic loading is higher
Table 2. Textural properties of the TMGN/SiO
hybrids.
2 2
and DMAN/SiO
2
2
Samples
SSABET
SSA [m g ]
External SA Pore size Pore volume
2
–1
2
–1
3 –1
[m g ]
[Å]
[cm g ]
Pure SiO
TMGN/SiO
TMGN/SiO
2
797
795
792
752
725
791
790
787
750
718
27
41
39
28
27
0.3
0.8
0.7
0.28
0.28
(
DMAN/SiO -10, curve c). To identify better the T-type
2
2
-0.2
-0.5
2
9
silicon species present in the hybrid materials, Si CP/MAS
2
NMR spectra have also been recorded (inset of Figure 3). DMAN/SiO -0.2
There we can see two bands at –40 and –66 ppm, assigned DMAN/SiO
2
2
-0.5
to T-type silicon species having a Si–C bond for TMGN/
SiO -0.5 (curve aЈ) and DMAN-SiO -0.5 (curve bЈ). The
signal at –66 ppm confirms that the hydrolysis and polycon- is around 800 m g , whilst in the case of DMAN/SiO it
densation of silylated TMGN and DMAN has occurred is ca 750 m g . However, in all cases, the SSAs and pore
through alkoxy terminal groups of the silylated moieties. volumes obtained are in the mesoporous range; the micro-
Moreover, the as-synthesized hybrids also show a peak at porous contribution is practically negligible (Table 2). This
The BET specific surface area (SSA) of the TMGN/SiO
2
2
2
2
–1
2
2
–1
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Building Blocks of Hybrid Catalysts
can also be deduced from the shape of the adsorption iso- bution is around 30–40 Å, estimated from textural property
therms (Figure 4, A) that are typical of nonordered meso- measurements, indicates that intracrystalline mesoporous
porous systems. The pore distribution of the hybrid samples cavities are present in the individual organosilica nanocrys-
(Figure 4, B) are narrow and the mean pore diameters are tals, although interparticle porosity is also observable in
centered at 40 Å for TMGN/SiO hybrids, while DMAN/ this range.
2
SiO shows a distribution centered around 30 Å. Hence,
2
TMGN/SiO also has a larger total pore volume. These fea-
2
tures could be because of the larger molecular dimensions
of TMGN in comparison with the DMAN proton sponge.
The above conclusion is also supported by the presence of
inflexion points in the adsorption isotherms. In the hybrid
materials containing TMGN fragments, the inflexion point
is close to p/p≈0.5, whilst in the case of DMAN builders
inserted in the network, the inflexion point appears at lower
relative pressures (p/p≈0.3).
Catalytic Activity
In order to investigate the accessibility to the reactants
and the activity of the organic strong base that has been
inserted into the wall of the mesoporous silica, the catalytic
performance of the hybrid materials was tested for base cat-
alyzed C–C bond formation reactions such as Knoevenagel,
Henry (nitroaldol), and Claisen–Schmidt condensations.
The activity was then compared with that of DMAN/SiO2
and with the activity of the molecular proton sponges
TMGN and DMAN in the homogeneous phase.
Knoevenagel Condensation
The Knoevenagel condensation of carbonyl compounds
is widely used in organic synthesis to produce important
intermediates and end products for perfumes, pharmaceuti-
[
5,37]
cals, and polymers.
The reaction can be catalyzed by
strong and weak bases depending on the level of activation
of the reactant containing methylenic activated groups. The
kinetics of the Knoevenagel reaction is generally considered
to be first order with respect to each reactant and the cata-
[
36–40]
lysts.
Here, the Knoevenagel reaction between benzal-
dehyde (1) and different methylene compounds (Scheme 4),
with increasing pK values, was performed on TMGN/SiO2
a
and DMAN/SiO hybrids.
2
Figure 4. Section A: N
size distribution, calculated by the BJH method, of TMGN/SiO
DMAN/SiO hybrids, and pure SiO
2
adsorption isotherms and Section B: Pore
2
,
2
2
.
Scheme 4. Scheme of the Knoevenagel reaction of benzaldehyde
In accordance with the nitrogen adsorption isotherms and methylene compounds.
that are characteristic of nonordered conventional mesopo-
rous materials synthesized through sol–gel routes catalyzed
Ethyl cyanoacetate (2a ECA, pK ≈9), ethyl acetoacetate
a
by fluoride ions, TEM micrographs show the formation of (2b EAA, pK ≈10), and diethyl malonate (2c DEM,
a
organosilica particles with inhomogeneous sizes (20– pK ≈13) were used. In the case of ECA, one polar protic
a
60 nm). This makes it impossible to identify long-range or- solvent, such as ethanol, was used as this is the most effec-
der in the hybrid materials (Figure S6). The pore size distri- tive solvent for the formation of the trans-α-ethyl cyano-
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E. Gianotti, U. Diaz, A. Velty, A. Corma
FULL PAPER
cinnamate (product 3a),[7,42] whilst toluene was used as a Table 3. Yields, turnover frequencies (TOF) calculated at 30 min,
and initial rate.
solvent when Knoevenagel condensation was performed
using EAA and DEM.
Catalysts
Yield [%]
(after 30 min)
TOF
[min ]
r
0
[min ]
–
1
–1
In Figure 5, the yields for product 3a, obtained with the
TMGN/SiO hybrids (Section A) and DMAN/SiO hybrids
2
2
Pure TMGN
77
69
63
75
60
56
256
230
210
250
200
187
1.56
1.17
0.95
1.15
0.82
0.48
(Section B), are reported, and compared to the yields of TMGN/SiO
2
-0.5
-0.2
Pure DMAN
TMGN/SiO
2
pure TMGN and DMAN. TMGN/SiO -0.5 is the most
2
active catalyst and reaches the yield of homogeneous
TMGN in only 1 h with 100% selectivity. In the case of
DMAN-containing hybrids, the yield observed is lower, and
both the catalysts reach the yield of homogeneous DMAN
DMAN/SiO
DMAN/SiO
2
-0.2
-0.5
2
after a 4 h reaction time. The turnover frequencies calcu- SiO hybrid material (Figure 6 A and B). In both hybrids,
2
lated after 30 min and the initial rate are reported in Table 3 the selectivity towards the 3b and 3c products is 100%. The
(
Figure S7). The TMGN/SiO -0.5 hybrid shows the highest TOF and the initial rate, reported in Table 4, evidence the
2
TOF, close to the one of the homogeneous base.
superior activity of the TMGN/SiO -0.5 hybrid catalyst.
2
Figure 5. Knoevenagel condensation of benzaldeyde with ECA
using TMGN/SiO (Section A) and DMAN/SiO (Section B) hy-
2 2
brids at 333 K with ethanol and using 1 mmol-% of proton
sponges, in the silica, with respect to ECA. The yields of homogen-
eous TMGN and DMAN are also reported for comparison.
Figure 6. Knoevenagel condensation of benzaldehyde with EAA
(
2 2
A) and DEM (B) using TMGN/SiO -0.5 and DMAN/SiO -0.5 hy-
When more demanding reactants (EAA and DEM) were
used and the Knoevenagel reactions were carried out at
brids and homogeneous TMGN and DMAN at 353 K and 383 K,
respectively.
353 K and 383 K, respectively, using toluene as the solvent,
the TMGN/SiO -0.5 hybrid displayed a higher yield
The catalytic activity of the heterogeneous hybrids is
2
towards the desired products (3b and 3c) than the DMAN/ higher than that observed using homogeneous TMGN and
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Building Blocks of Hybrid Catalysts
Table 4. Yields, turnover frequencies (TOF), and initial rate for the Knoevenagel condensation of benzaldehyde with EAA and DEM.
Catalysts
EAA[a]
Yield [%] of 3b
TOF
[min ]
r
0
DEM[b]
Yield [%] of 3c
TOF
[min ]
r
0
[min ]
–
1
–1
–1
–1
[min ]
TMGN/SiO
DMAN/SiO
2
-0.5
-0.5
81
51
13.5
8.5
0.68
0.35
28
18
1.6
1
0.16
0.1
2
[a] 1 h reaction time. [b] 3 h reaction time.
DMAN as organocatalysts that show very low yields. This higher organic loadings (TMGN/SiO -0.5 and DMAN-
2
behavior is due to the polarity of the solvent used that has SiO -0.5) as they showed the highest activity for the Knoev-
2
a strong effect on the reaction rate. In fact, in the case of enagel reaction. From the yields of nitrostyrene (product 2)
homogeneous proton sponges, the solvent influences the re- reported in Figure 7, one can see that the TMGN/SiO -0.5
2
action mechanism and not the capacity of the organocata- material shows the best catalytic performance and reaches
lysts for proton transfer. Thus, when polar solvents are 100% conversion and 100% selectivity towards product 2
used, the activation energy of the Knoevenagel reaction in 6 h, whilst DMAN/SiO -0.5 reaches 80% conversion and
2
should be lower than when the reaction occurs in an apolar 100% selectivity for the same reaction time. In the case of
[37]
solvent (chlorobenzene, toluene).
In fact, the polar sol- homogeneous TMGN and DMAN molecules, the conver-
+
vent polarizes the N–H –N bond and the intramolecular sions in the Henry reaction are lower than with the hetero-
hydrogen bond of the protonated proton sponge loses geneous hybrid catalysts and, in addition, the formation of
strength, facilitating the release of the proton. When proton byproducts (1 and 3 in Scheme 5) are detected when work-
sponges are introduced into the silica network to form or- ing in the homogeneous phase (see Table 5, Figure S8 for
ganic–inorganic hybrid catalysts, the reaction rate is less af- the yields of byproducts and benzaldehyde conversion of
fected by the solvent polarity and a role of the surface po- the homogeneous bases). In particular, it is possible to ob-
larity of the silica support on the reaction rate has to be serve that at the beginning of the reaction (up to 1 h), the
considered. Indeed, in heterogeneous catalysis, the reaction homogeneous catalysts show conversions higher than
occurs on the surface and when polar reactants are involved heterogeneous hybrids and that after 1 h the conversion
(as in Knoevenagel condensation), the transition-state com- stops and remains almost the same until the end of the reac-
plex should be stabilized by polar surfaces, such as silica tion. In the Henry reaction, the heterogeneous hybrid cata-
surfaces, which leads to an increase in the reaction rate. In
this case, only a weak solvent effect is observed[ and the
41]
reaction occurs both in polar and in apolar solvents.
Henry Reaction
The TMGN- and DMAN-based hybrids were tested in
the Henry reaction, i.e, a condensation reaction of nitroal-
kanes with carbonyl compounds to generate nitroalkenes
(
Scheme 5), which are of importance for the synthesis of
[42,43]
pharmaceutical products.
However, the selective for-
mation of a nitroalkene (product 2 in Scheme 5) using con-
ventional strong bases is difficult to achieve since the conju-
gate addition of the nitroalkane to the C–C double bond
of the nitrolkene gives bis-nitro compounds that results in
poor yields because of their dimerization or polymerization
product 3, Scheme 5).[
44–47]
(
Figure 7. Henry condensation of benzaldeyde with nitromethane
using TMGN/SiO-0.5 and DMAN/SiO -0.5 hybrids at 363 K. The
2
conversions of homogeneous TMGN and DMAN are also re-
ported for comparison.
Table 5. Conversion and yields (%) of products 1, 2, and 3 of the
heterogeneous hybrids and homogeneous bases calculated after 6 h.
Catalysts
% conv. Yield 1 Yield 2 Yield 3 TOF[a] [min–1]
Pure DMAN
Pure TMGN
28
36
80
11
13
0
4
7
80
100
13
16
0
2
Scheme 5. Scheme of the Henry reaction of benzaldehyde and ni-
tromethane.
2.6
2.3
4.4
DMAN/SiO
TMGN/SiO
2
-0.5
-0.5
100
0
0
The Henry reaction between benzaldehyde and nitro-
2
methane (pK = 10.2) was performed with hybrids with [a] TOF calculated after 2 h of reaction time.
a
Eur. J. Inorg. Chem. 2012, 5175–5185
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5181

E. Gianotti, U. Diaz, A. Velty, A. Corma
FULL PAPER
lysts, and in particular TMGN/SiO , have evidenced a supe-
2
rior activity both in conversion and selectivity towards the
desired product 2 compared with the homogeneous bases.
The superior catalytic activity shown by the hetero-
geneous hybrids with respect to the homogeneous ones can
be inferred to the active role of surface silanol groups,
which are weakly acidic, in performing a cooperative acti-
vation of the reactants producing a higher conversion
towards the desired product.[
48–51]
Claisen–Schmidt Condensation
Claisen–Schmidt condensation between benzaldehyde (1)
and
acetophenone
(2),
produces
trans-chalcones
(
Scheme 6), which are important compounds in many phar-
Figure 8. Claisen–Schmidt condensation between benzaldehyde
maceutical applications. They are the main precursors for
the biosynthesis of flavonoids and exhibit various biological
activities, such as anticancer, anti-inflammatory, and anti-
hyperglycemic agents.[
2 2
and acetophenone using the TMGN/SiO -0.5 and DMAN/SiO -
0.5 hybrid materials at 403 K.
30–33,52–55]
Table 6. Yields and turnover frequencies (TOF) calculated after 3 h.
–
1
Catalysts
Yield [%]
TOF [min ]
TMGN/SiO
DMAN/SiO
2
-0.5
-0.5
56
19
31
11
2
Reusability
Catalyst deactivation and reusability was studied with
TMGN/SiO -0.5 and DMAN/SiO -0.5 hybrids by recycling
2
2
Scheme 6. Scheme of the Claisen–Schmidt reaction of benzalde-
hyde and acetophenone.
the used catalysts after each run, after being washed with
CH Cl . The yields of the recycled catalysts in the Knoev-
2
2
enagel condensation and Henry reaction are reported in
Figure 9 after 6 h of reaction. The yields are almost similar
during three catalytic runs. After the third run, the yield
slightly decreases for both types of hybrids, probably be-
cause of the deactivation of the basic moieties due to pro-
tonation that occurs during the reactions. No leaching of
the organic bases was observed by performing elemental
analysis on the used hybrids.
In Figure 8, the yields towards trans-chalcone of TMGN/
SiO and DMAN/SiO hybrids with reaction time are pre-
2
2
sented. TMGN/SiO -0.5 reaches almost 100% conversion
2
and selectivity in 10 h, while a 68% yield with 100% selec-
tivity was observed using DMAN/SiO -0.5. The TOF val-
2
ues are reported in Table 6. This result confirms that the
high catalytic activity of TMGN/SiO is due to the particu-
2
larly high basicity of this organic base (pK = 25) that has
a
been inserted into the network of mesoporous silica. In the
case of the DMAN containing hybrid, the fact that the pK
a
of acetophenone (pKa = 20)[
55]
is higher than that of
DMAN (pKa = 12.1) suggests that not only the basic
strength of the organic molecule is responsible for the cata-
[41]
lytic activity. As has been reported, the silanol groups of
the inorganic support are able to interact with the carbonyl
group of the acetophenone, polarizing the carbon–oxygen
bond and increasing the density of the positive charge of
the carbon. Thus, the acidity of the hydrogens in the α-
position of the carbonyl group is enhanced and the abstrac-
tion by the catalyst becomes easier. The role of the silanols
could then explain the catalytic activity observed when sub-
strates with higher pK values than that of DMAN are
used. In the case of the TMGN hybrid, the role of the silan-
a
Figure 9. Knoevenagel condensation and Henry reaction recycling
tests using TMGN/SiO
2 2
-0.5 and DMAN/SiO -0.5 as catalysts. The
ols is less required since the pK of TMGN is very high.
yield (%) is reported after 6 h of reaction.
a
5182
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 5175–5185

Building Blocks of Hybrid Catalysts
Conclusions
Table 7. Acronyms of the hybrid materials and TMGN/DMAN
loadings.
TMGN/DMAN loading x[a]
The organic base 1,8-bis(tetramethylguanidino)naphth- Hybrid acronyms
alene (TMGN) has been successfully inserted into the wall
TMGN/SiO
2
-0.2
0.002
0.005
0.002
0.005
of nonordered mesoporous silica for the first time, to the TMGN/SiO -0.5
2
best of our knowledge, to produce organic–inorganic hybrid DMAN/SiO
materials. By using a fluoride-catalysed sol–gel process, at
2
2
-0.2
-0.5
DMAN/SiO
neutral pH and low temperatures, the use of SDAs was avo- [a] x = mol of the TMGN-Sil or DMAN-Sil with respect to the
2
total mol of SiO .
ided. The characterization of the hybrids has revealed that
this organic base is preserved inside the mesoporous silica
network providing strong basic properties to the hybrid ma- To remove unreacted TMGN-Sil or DMAN-Sil molecules, the hy-
terials. These functional hybrid materials, which contain brids were washed for 3 d in ethanol at 298 K.
stable, isolated, and active basic sites, are able to carry out
condensation processes to form carbon–carbon bonds. In ing of TMGN/DMAN are reported.
In Table 7, the acronyms for the synthesized hybrids and the load-
particular, TMGN-based hybrids have shown higher con-
versions in the Knoevenagel, Henry (nitroaldol), and
Claisen–Schmidt condensations with respect to DMAN-
based hybrids, because of the higher basicity of TMGN. In
addition, the organic–inorganic hybrids have shown higher
catalytic performances compared with the homogeneous or-
ganocatalysts because of the presence of silanol groups be-
longing to the silica support that are able to activate the
reactants by cooperative effects. The role of silanols is par-
ticularly effective in the case of the DMAN hybrid in which,
Characterization of Hybrid Materials
2
All the hybrids were characterized by N adsorption, thermogravi-
1
3
metric and elemental analyses, and solid-state MAS NMR ( C,
2
9
Si).
C, N, and H contents were determined with a Carlo–Erba 1106
elemental analyzer. Thermogravimetric and differential thermal
analyses (TGA-DTA) were recorded in a nitrogen stream with a
Metler–Toledo TGA/SDTA 851E instrument. Volumetric analyses
were performed by nitrogen adsorption isotherms at 77 K with a
Micromeritics ASAP2010. Before measurements were performed,
the samples were outgassed for 12 h at 100 °C. The BET specific
in some cases, the pK of the substrates is higher than that
a
of DMAN. In this view, TMGN/SiO2 hybrids represent surface area[56] was calculated from the nitrogen adsorption data
promising organic–inorganic base catalysts and open up a in a relative pressure range from 0.04 to 0.2. The total pore vol-
route for the design and synthesis of multifunctional hy- ume[ was obtained from the amount of N
57]
adsorbed at a relative
2
pressure of about 0.99. The external surface area and micropore
volume were estimated with the t-plot method in the t range 3.5 to
. The pore diameter and the pore size distribution were obtained
following the Barret–Joyner–Halenda (BJH) method on the ad-
sorption branch of the isotherms.
brids by combining acidic and basic sites or basic and redox
functionalities within the mesoporous silica network.
5
[58]
Solid-state MAS NMR spectra were recorded at room temperature
under magic angle spinning (MAS) with a Bruker AV-400 spec-
Experimental Section
2
9
trometer. The single pulse Si spectra were acquired at 79.5 MHz
with a 7-mm Bruker BL-7 probe using pulses of 3.5 μs correspond-
ing to a flip angle of 3/4 p radians, and a recycle delay of 240 s.
Synthesis of the Hybrid Materials
TMGN and DMAN were purchased by Sigma–Aldrich.
1
3
For the C NMR cross-polarization (CP) spectra, a 7-mm Bruker
The 1,8-bis(tetramethylguanidino)naphthalene (TMGN) was func-
tionalized according to the procedure described in ref.[ All the
steps required for TMGN and DMAN functionalization (see Sup-
13
BL-7 probe was used at a sample spinning rate of 5 kHz. C NMR
7]
29
and Si NMR were referenced to adamantane and tetramethylsil-
ane, respectively.
porting Information, Scheme S1) were followed by 1H, C, Si
13
29
FTIR spectra were obtained with a Nicolet 710 spectrometer
liquid NMR (Figures S1–S4).
–
1
(
4 cm resolution) using a conventional greaseless cell. Wafers of
–
2
TMGN/SiO
2
and DMAN/SiO
2
hybrids were synthesized using a
ca. 10 mgcm were outgassed at room temperature, 100, and
200 °C overnight.
NH F co-condensation route. Tetramethyl orthosilicate (TMOS)
4
and disilylated TMGN or DMAN (TMGN-Sil or DMAN-Sil)
were mixed in methanol at 298 K. After dissolution of precursors,
a water solution of NH F was added under vigorous stirring.
4
The morphology of the hybrid materials was studied by trans-
mission electron microscopy (TEM). Samples were ultrasonically
dispersed in a CH
copper grids. TEM micrographs were collected with a Philips CM-
0 microscope operating at 100 kV.
2 2
Cl solvent and transferred to carbon coated
The final reaction mixture has the following molar composition:
x)SiO /xDMAN-Sil/TMGN-Sil/4MeOH/4H
O/3.13ϫ10–4
1
(1
–
2
2
Catalytic Tests
4
NH F
The hybrid materials were studied as catalysts for the Knoevenagel,
Henry, and Claisen–Schmidt condensations. The catalysts used
were washed for 3 d in ethanol to remove the unreacted TMGN-
Sil or DMAN-Sil molecules. Before catalytic tests, all the catalysts
Hydrolysis and condensation of the silicon precursor was carried
out under vigorous stirring at 298 K until gelation occurred. Then,
the gel was aged at 36 °C for 24 h and finally dried at 60 °C for
24 h. Several hybrids with different TMGN/DMAN loadings, from were outgassed at 373 K for 12 h to remove physisorbed water. For
x = 0.002 to x = 0.005, were synthesized (Table 7). Additionally,
pure SiO was also produced following the NH F sol–gel route.
Knoevenagel reactions, a mixture of benzaldehyde (8 mmol) and
methylene compounds (7 mmol) was stirred at the desired tempera-
2
4
Eur. J. Inorg. Chem. 2012, 5175–5185
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5183

E. Gianotti, U. Diaz, A. Velty, A. Corma
FULL PAPER
ture under a N
2
atmosphere. Then, 1 to 10 mmol-% of TMGN or
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1
13
29
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Received: June 29, 2012
Published Online: September 28, 2012
2797.
Eur. J. Inorg. Chem. 2012, 5175–5185
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5185