Facile Synthesis of Cooperative Acid-Base Catalysts by Clicking Cysteine and Cysteamine on an Ethylene-Bridged Periodic Mesoporous Organosilica

Article abstract of DOI:10.1002/ejic.201501179

A periodic mesoporous organosilica (PMO) that contains ethylene bridges was functionalized to obtain a series of cooperative acid-base catalysts. A straightforward, single-step procedure was devised to immobilize cysteine and cysteamine on the support material by means of a photoinitiated thiol-ene click reaction. Likewise, PMO materials capped with hexamethyldisilazane (HMDS) were used to support both compounds. This resulted in different materials in which the amine site was promoted by carboxylic acid groups, surface silanol groups, or both. The catalysts were tested in the aldol reaction of 4-nitrobenzaldehyde and acetone. It was found that silanol groups have a stronger promoting effect on the amine than the carboxylic acid group. The highest turnover frequency (TOF) was obtained for an amine-functionalized material that contained only silanol promoting sites. The loading of the active sites also had a significant effect on the activity of the catalysts, which was rationalized on the basis of a computational study.

Full text of DOI:10.1002/ejic.201501179

DOI: 10.1002/ejic.201501179
Full Paper
CLUSTER
ISSUE
Immobilized Catalysts
Facile Synthesis of Cooperative Acid–Base Catalysts by Clicking
Cysteine and Cysteamine on an Ethylene-Bridged Periodic
Mesoporous Organosilica
[
a]
[b]
[a]
[a,c]
Judith Ouwehand, Jeroen Lauwaert, Dolores Esquivel, Kevin Hendrickx,
[c]
[b]
[a]
Veronique Van Speybroeck, Joris W. Thybaut, and Pascal Van Der Voort*
Abstract: A periodic mesoporous organosilica (PMO) that con- both. The catalysts were tested in the aldol reaction of 4-nitro-
tains ethylene bridges was functionalized to obtain a series of benzaldehyde and acetone. It was found that silanol groups
cooperative acid–base catalysts. A straightforward, single-step have a stronger promoting effect on the amine than the carb-
procedure was devised to immobilize cysteine and cysteamine oxylic acid group. The highest turnover frequency (TOF) was
on the support material by means of a photoinitiated thiol-ene obtained for an amine-functionalized material that contained
click reaction. Likewise, PMO materials capped with hexameth- only silanol promoting sites. The loading of the active sites also
yldisilazane (HMDS) were used to support both compounds. had a significant effect on the activity of the catalysts, which
This resulted in different materials in which the amine site was was rationalized on the basis of a computational study.
promoted by carboxylic acid groups, surface silanol groups, or
Introduction
groups contained in PMO materials have previously been ex-
ploited to incorporate spatially separated amino and sulfonic
acid groups. However, these approaches make use of several
synthetic steps, including protection and deprotection steps or
The incorporation of antagonistic acid and base groups on mes-
oporous materials for cooperative catalysis has been an impor-
tant research topic in recent years. This type of material is espe-
cially useful for carbon–carbon coupling reactions such as
[
2,5]
epoxidation and amination steps.
In this work, we took ad-
[
1]
[1c,2]
[3]
vantage of ethylene functionalities embedded in a PMO mate-
rial to incorporate carboxylic acid and amine groups by means
of a single thiol-ene click reaction using the amino acid
cysteine. A similar approach was employed for the incorpora-
tion of the sole amine functionality through the grafting of
cysteamine. In this type of click reaction, a thiol and an ethylene
aldol, nitroaldol,
and Knoevenagel reactions. These cou-
pling reactions are catalyzed by an amine, but the presence of
an acid function can promote the activity. The literature holds
examples of how several different types of acid and amine
groups were incorporated onto silica gel or ordered mesopo-
rous silicas with the formation of cooperative acid–base cata-
lysts. The incorporation of the acid and base groups usually
occurs through grafting or co-condensation, but these synthe-
sis methods often require several reaction steps including the
protection and deprotection of the amine or an oxidation reac-
[
6]
moiety are linked to form a thioether bond. Thus, in this case,
the catalytically active sites are not linked to the carrier material
through siloxane bonds, as is the case when classical grafting
[
1f]
or co-condensation procedures are used, but through hydro-
thermally more stable thioether bonds. In this way, the thiol-
ene click reaction provides a reliable procedure for a straightfor-
ward, single-step grafting method.
[
1f,1h,3,4]
tion.
corporated by means of successive addition of the organo-
Alternatively, the antagonistic groups have been in-
[
1a–1c]
silanes during a co-condensation synthesis
of their successive grafting.
Periodic mesoporous organosilicas (PMOs) are materials de-
rived from a bridged silane of the form (R′O) Si–R–Si(OR′) in
or by means
[
1e]
In this work, the catalytic activity of the functionalized PMO
materials in the aldol reaction of 4-nitrobenzaldehyde and acet-
one was investigated. The acid promoting properties of the
carboxylic acid groups were compared to that of the silanol
groups, which are intrinsically present on the PMO surface. Sub-
sequently, the influence of the loading of the catalytic center
was investigated, as well as the potential of inducing chirality
in the reaction products.
3
3
which R represents a functional organic bridge. The functional
[
a] Department of Inorganic and Physical Chemistry, Ghent University
Krijgslaan 281-S3, 9000 Ghent, Belgium
E-mail: pascal.vandervoort@ugent.be
www.comoc.ugent.be
[
[
b] Department of Chemical Technology, Ghent University
Technologiepark 914, 9052 Ghent, Belgium
c] Center for Molecular Modeling, Ghent University
Technologiepark 903, 9052 Zwijnaarde, Belgium
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201501179.
Results and Discussion
Synthesis of the Materials
Ethylene-bridged PMO materials were prepared by means of
the condensation of (E)-1,2-bis(triethoxysilyl)ethylene (BTEE) un-
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Scheme 1. Overview of the catalyst functionalization (TMS = trimethylsilyl).
der acidic conditions by using P123 as template and butanol as
additive. The double bonds of the obtained ethylene PMO
(
ePMO) were used as an anchoring point for the grafting of
cysteine and cysteamine by means of a photoinitiated thiol-ene
click reaction to result in Cys-ePMO and Amine-ePMO samples,
respectively (Scheme 1). The reaction time of the thiol-ene click
reaction was varied to obtain a series of these materials with
different loadings of the active sites. A second type of catalyst
was synthesized by treating the ePMO with hexamethyldisil-
azane (HMDS) to cap the surface silanol groups. Subsequently,
the capped samples were also grafted with cysteine and cys-
teamine. These samples are indicated with the suffix -HMDS.
Through this approach, catalysts that contained a primary
amine combined with carboxylic acid groups, surface silanol
groups, or a combination of both were obtained.
Figure 1. DRIFTS spectra of the materials: (a) ePMO, (b) ePMO-HMDS, (c) Cys-
ePMO, and (d) Amine-ePMO.
Characterization of the Materials
The structure and functionalities of the PMO materials were an-
alyzed by means of diffuse reflectance infrared Fourier trans- different loadings were obtained by varying the reaction time
form spectroscopy (DRIFTS), nitrogen sorption, X-ray diffraction of the thiol-ene click reaction. Thus, the loadings of the cata-
lysts range from 0.08 to 0.84 mmol g– of nitrogen.
1
(
XRD), and elemental analysis.
The presence of the functional groups was confirmed by
The nitrogen-sorption isotherms are shown in Figure 2. All
DRIFTS measurements. As shown in Figure 1, all non-capped materials exhibit type IV isotherms and H1 type hysteresis loops
–1
samples show characteristic silanol peaks around 3720 cm . at relative pressures in the range of 0.4–0.8, indicative of cylin-
The peaks at 2950 cm–1 represent the C–H stretching from the drical mesopores. The specific BET surface area (SBET), average
ethylene bridge and the carbon atoms in the cysteine or pore size (d ), pore volume (V ), and wall thickness are summa-
p
p
cysteamine groups. For ePMO-HMDS, the silanol peak is rized in Table 1. The starting ePMO and ePMO-HMDS materials
2
–1
strongly reduced, thus indicating that the majority of free silan- have large surface areas of 890 and 751 m g , respectively.
ols were capped with HMDS. The increased C–H signal at After functionalization with cysteine and cysteamine, the spe-
–
1
2
950 cm for the same sample is due to the presence of the cific surface area of the samples decreases, which is due to an
trimethylsilyl groups that originate from HMDS.
effective decrease in surface area as well as to an increase in
In the Cys-ePMO spectrum, a carbonyl signal can be ob- weight after the grafting procedure. All samples have pore
–
1
served at 1730 cm . In the Amine-ePMO spectrum, two peaks diameters around 5 nm and a wall thickness between 4 and
at 3315 and 3365 cm–1 can be assigned to N–H stretching. In 6 nm. This large wall thickness provides stability to the PMO
the Cys-ePMO and Amine-ePMO spectra, the signal at materials.
1
615 cm–1 is due to N–H bending.
The XRD patterns of the materials before and after grafting
After grafting cysteine and cysteamine, the loading of the of cysteine and cysteamine are shown in Figure 3. All the mate-
functional groups was determined by elemental analysis of rials show the characteristic (100), (110), and (200) reflections
nitrogen (Table 1). For Cys-ePMO and Amine-ePMO, catalysts of of a hexagonal pore ordering, which confirms that after func-
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Table 1. Structural properties and functional-group loadings of the PMO materials.
BET [(m g–1]
[
a]
2
d
p
[b]
[nm]
V
p
[c]
[mL g–1
]
a
0
[d]
[nm]
Wall thickness[e] [nm]
Loading[f] [mmol g–1
]
Sample
S
ePMO
ePMO-HMDS
Cys-ePMO-1
Cys-ePMO-2
890
751
610
576
496
604
518
373
633
5.2
0.98
0.85
0.71
0.80
0.39
0.83
0.62
0.63
9.4
4.2
5.2
4.8
4.7
5.6
4.8
4.3
4.6
5.5
–
–
5.4
5.4
5.5
5.0
5.0
4.8
5.5
5.3
10.6
10.2
10.2
10.6
9.8
9.1
10.1
10.8
0.08
0.41
0.84
0.15
0.49
0.65
0.46
Cys-ePMO-3
Cys-ePMO-HMDS
Amine-ePMO-1
Amine-ePMO-2
Amine-ePMO-HMDS
0.861
[
a] Specific Brunauer–Emmett–Teller surface area. [b] Barrett–Joyner–Halenda pore diameter determined from desorption isotherm. [c] Pore volume determined
0 0 p
from desorption isotherm. [d] Lattice parameter calculated by a = 2d100/√3. [e] Calculated as the difference between a and d . [f] Loading of nitrogen from
elemental analysis.
Figure 2. Nitrogen-sorption isotherms of the non-capped (left) and capped (right) samples.
Figure 3. XRD patterns of the non-capped (left) and capped (right) samples: (a) ePMO, (b) Cys-ePMO, (c) Amine-ePMO, (d) ePMO-HMDS, (e) Cys-ePMO-HMDS,
and (f) Amine-ePMO-HMDS.
tionalization, all samples retain their ordered pore structure of (Figure 4). In each experiment, the amount of catalyst added to
the P6mm point group.
the reaction mixture was adjusted such that the desired
amount of amines was present (i.e., 4 mol-% with respect to
the 4-nitrobenzaldehyde concentration) to be able to directly
compare the activity of the different materials to each other.
Catalytic Results
Activity and Selectivity
The conversion increased linearly with the reaction time up
The catalytic activities of the functionalized materials were to a conversion of approximately 95 %, thus indicating that the
tested in the aldol reaction of 4-nitrobenzaldehyde and acetone reaction rate remained constant and that the materials were
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Figure 4. Aldol condensation of 4-nitrobenzaldehyde and acetone with the formation of aldol products 1 and 2.
Table 2. Catalytic data of the functionalized PMO materials.
Sample
Loading[a] [mmol g–1
]
TOF × 10–4 [s–1
]
Selectivity[b] [%]
ee[c] [%]
Cys-ePMO-1
Cys-ePMO-2
Cys-ePMO-3
0.08
0.41
0.84
0.15
0.49
0.65
0.46
5.5
2.5
1.1
0.9
3.5
2.3
0.3
92.6
95.0
96.8
95.3
96.5
96.5
91.2
<5
<5
<5
25 (±5)
<5
<5
Cys-ePMO-HMDS
Amine-ePMO-1
Amine-ePMO-2
Amine-ePMO-HMDS
<5
[
a] Loading of nitrogen, from elemental analysis. [b] Towards aldol product 1 (Figure 4). [c] Towards the (R) isomer of 1.
stable during the catalytic tests. Figure 5 shows the data points second run amounted to 55 % of the original TOF. The linear
at the start of the reaction, which were used to determine the character of the conversion behavior with time indicates that
turnover frequency (TOF) values of the different catalysts.
stable catalytic behavior was observed in a single run. More
insight into the lower slope in the second run is currently being
gained by means of time-on-stream measurements in a contin-
uous flow setup and will be reported separately.
The chirality of the aldol product was investigated by chiral
HPLC. The only sample that showed significant enantiomeric
excess (ee) was Cys-ePMO-HMDS. These catalytic data are inter-
preted in the following sections.
Influence of the Acid Promoting Site
As shown in Table 2, four different types of catalysts were ob-
tained, which each contain a primary amine combined with
different types of acid promoting sites. Cys-ePMO contains both
carboxylic acid and surface silanol groups. Cys-ePMO-HMDS
only contains carboxylic acid groups, whereas Amine-ePMO
only contains surface silanol groups. Amine-ePMO-HMDS, how-
ever, does not contain any acid promoting sites. To compare
these four types of materials while minimizing the influence of
the loading of the active sites, four different samples with simi-
Figure 5. Data points of the conversion curves used to determine the TOF
values.
–1
The catalytic data of the catalysts are shown in Table 2. All lar amine loadings (in the range of 0.15–0.5 mmol g ) were
catalysts were able to promote the reaction with a high selectiv- selected. These samples and their TOF values are shown in Fig-
ity towards aldol product 1 (>90 %) with TOF values ranging ure 6.
–
5
–4 –1
from 3.0 × 10 to 5.5 × 10 s .
As could be expected, Amine-ePMO-HMDS, which has no
The TOF values in Table 2 can be compared to activities of acid promoting sites, has the lowest activity. Amine-ePMO,
aminated porous silica materials previously reported by Lauw- which contains only silanol promoting groups, is more active
[
1e]
aert et al.,
under identical reaction conditions. Porous silica grafted with a moting groups. This suggests that, for this reaction, the silanol
as these were tested in the same aldol reaction than Cys-ePMO-HMDS, which contains only carboxylic acid pro-
primary amine showed a TOF of 7.8 × 10– s , which is slightly groups on the PMO surface are more efficient promoters to the
higher than those of the Amine-ePMO samples. A bifunctional amine group. This is also immediately apparent from the fact
silica material grafted with both amine and carboxylic acid that both capped samples have lower activities than the non-
groups showed a TOF of 9.5 × 10–5 s , which is slightly lower capped samples.
4 –1
–1
than those of the Cys-ePMO samples. Overall, the catalytic activ-
ities of the PMO material reported here are in the same order studies on functionalized porous silica materials.
This observation is in agreement with previously published
[
1e,1f]
In the en-
of magnitude as those of the corresponding silica materials.
amine reaction mechanism, the role of the acid promoting sites
A recycling study performed for Cys-ePMO-1 showed that in is to activate the carbonyl species. However, it was observed
the second catalytic run the conversion still increased linearly that the grafting of carboxylic acid groups along with amine
with reaction time, albeit with a lower slope. The TOF in the groups on the silica surface lowered the activity of the materials
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owing to a stronger promotion by the silanol groups. This also
results in a lack of enantioselectivity in all samples except Cys-
ePMO-HMDS.
Influence of the Loading
In a further catalytic investigation, the influence of the loading
of the active sites was studied. For this, the Cys-ePMO and
Amine-ePMO samples with different loadings were compared.
The TOF values are shown in Figure 7, whereas the loadings
are in Table 2. For both the cysteine- and amine-functionalized
samples, the TOF increased with decreasing loading. A similar
effect was reported in the literature when grafting primary
[
1i,8]
amines onto silica materials with different loadings.
The loss
of activity at higher loadings was ascribed to a decrease in sur-
rounding silanol groups. Furthermore, it was suggested that
hydrogen bonds were formed between the functional groups
involved, thereby causing deactivation of the amine active sites.
Figure 6. Catalytic activity (TOF) of the different types of catalysts.
relative to silicas functionalized with only amines. This effect
was ascribed to two factors.
Firstly, it was proposed that the grafting procedure of the
carboxylic acid group by means of silylation of the surface silan-
ols effectively deactivates a fraction of the active silanols. How-
ever, this theory is not applicable to the PMO materials de-
scribed here, since both functional groups were incorporated
through thiol-ene click chemistry as opposed to silylation.
The second explanation for the decrease in activity is related
to the relatively high acid strength of the carboxylic acid (pK
a
[
22]
[7]
of 1.8 ) relative to the silanol groups (average pK of 7.1 ).
a
This makes the carboxylic acid group a stronger hydrogen-bond
donor and thus more likely to deactivate the amine. This sec-
ond effect can also take place on the PMO material, with the
difference that only hydrogen bonds between different cysteine
moieties can occur, as the amine and carboxylic acid on a single
cysteine moiety are not in a configuration that allows hydrogen
bonding. The degree of hydrogen-bond formation and thus the
loss of catalytic activity should then be proportional to the
Figure 7. TOFs of the samples with different loadings.
To investigate the hydrogen-bond formation on a PMO mate-
loading of the active sites. This effect is investigated in the fol- rial, a computational study based on a simplified model system
lowing section.
was performed to obtain more insight into the interactions at
The activity of Cys-ePMO, which contains both carboxylic the nanoscale level. Theoretical studies of PMO materials are
acid and silanol groups, is lower than that of Amine-ePMO. This only available in very limited examples in the literature,[9]
leads to the reasoning that a third factor should be considered, whereas modeling of mesoporous silicas is already more com-
in the form of a steric hindrance effect, caused by the fact that mon.[
10]
the carboxylic acid groups are in close proximity to the amine
To investigate the possible conformations of two cysteine
active sites. This could make the amines less accessible to the moieties grafted onto neighboring ethylene bridges, a simple
aldol reagents, which would result in a decrease in activity of model was used, in which the PMO backbone was kept rigid,
Cys-ePMO relative to Amine-ePMO.
whereas the cysteine moieties were allowed to rotate and inter-
Thus, for the PMO materials, a combination of hydrogen- act. Optimizations starting from different initial geometries
bond formation and steric hindrance can explain the reactivity showed that the potential energy surface of the system has
of the different types of catalysts.
several local minima. All localized minima are shown in Fig-
In terms of chirality of the aldol product, Cys-ePMO-HMDS ure S1 of the Supporting Information with their relative stabili-
was the only sample to show a significant enantioselectivity, ties. It was observed that geometries that allow the formation
with 25 % ee. The Cys-ePMO sample does not show any signifi- of a hydrogen bond between the amine groups are about
cant ee. This indicates that for this sample the steric hindrance 25 kJ mol– more stable than the other structures.
1
effect proposed above is taking place, as the activity lowers for
Cys-ePMO relative to Amine-ePMO, but no significant ee was model in which the silanol groups and ethylene bridges were
observed.
no longer static but were also allowed to rotate and interact
These stable conformations were then introduced into a
In conclusion, the aminated ePMO with only silanol promot- with the cysteine moieties (Figure S2 in the Supporting Infor-
ers has the highest activity in the aldol condensation reaction mation). This resulted in the most stable conformation (struc-
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ture F), which is shown in Figure 8. This conformation is stabi-
lized by hydrogen bonds between the amine groups, between
the carboxylic acid groups, and between the amine and the
silanol groups.
Figure 10. Geometric model of an isolated cysteine group grafted onto the
PMO backbone (H = white, C = gray, N = blue, O = red, Si = beige, S =
yellow).
Figure 8. Most stable conformation obtained for two cysteine moieties
grafted onto neighboring ethylene bridges (H = white, C = gray, N = blue,
O = red, Si = beige, S = yellow).
reaction, by the change in conformation, as well as by the
formed hydrogen bonds.
On the contrary, an isolated cysteine moiety is not hampered
by hydrogen bonding. These findings are in accordance with
the catalytic results. At high loadings, the chance of finding two
neighboring cysteine sites increases, especially because
hydrogen bonding can occur during the grafting procedure.
The samples with higher loadings show a decrease in activity,
which can be ascribed to the effects found in the computa-
tional study. The samples with the lowest loadings have the
highest activities. This is in accordance with the model of the
isolated cysteine site, which is more likely to occur at these
lower loadings.
To confirm the geometries obtained from these static calcu-
lations, first principle molecular dynamics (MD) simulations
were performed. Within the timescale of the simulation (12 ps),
the molecule remained mostly in this stable conformation, thus
showing the strong effect of the amine hydrogen bonds. Fig-
ure 9 shows the N···H distance during the simulation. It can be
seen that the bond length stays within the range of a strong
hydrogen bond, thereby confirming that this geometry is stable
when allowed to rotate freely. In conclusion, the calculations
confirm a large stabilizing effect owing to hydrogen-bond for-
mation between two neighboring amine groups.
The Amine-ePMO samples show a similar trend in activity,
which leads us to believe that a similar reasoning is valid for
these aminated samples.
Conclusion
A straightforward, single-step procedure was developed to graft
functional groups onto an ethylene PMO. Cysteine- and amine-
functionalized materials were obtained by a photoinitiated
thiol-ene click reaction. Catalytic testing in an aldol reaction
showed that the silanol groups present on the PMO surface
were more effective as a promoting site to the amine than carb-
oxylic acid groups. The catalyst without acid promoting sites
showed the lowest activity. Only Cys-ePMO-HMDS showed ee,
which is due to the fact that for the other catalysts the reaction
was mainly promoted by the silanol groups and not by the
carboxylic acid. In terms of the loading of the active sites, a
Figure 9. Evolution of the N···H hydrogen-bond length during the MD simula-
tion. The black lines represent the interval in which the interaction is labeled
[
11]
as a strong hydrogen bond.
Figure 10 shows a similar model, this time of an isolated lower loading gave a higher activity, which can be rationalized
cysteine moiety grafted on the PMO backbone, representing by the formation of hydrogen bonds between the active sites
the materials with lower loadings. In this model, the silanol at higher loadings, as was shown in the computational study.
groups were again allowed to rotate and interact with the
cysteine group. However, in this case, no hydrogen bonding
was observed.
In conclusion, these computational studies show that the
configuration of two neighboring cysteine moieties is strongly
Experimental Section
Materials: The following chemicals were used as received:
cysteine (>99 %, Acros), cysteamine (>95 %, TCI), 2-hydroxy-4-(2-
L-
stabilized by the formation of hydrogen bonds. In this case, the hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959, 98 %, Al-
active amine site could be strongly inhibited in the catalytic drich), PEG–PPG–PEG Pluronic P-123 (Aldrich), 1-butanol (99.7 %, Al-
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drich), HCl (37 %, Roth), vinyltriethoxysilane (97 %, ABCR), Grubbs
catalyst (first generation, 97 %, Aldrich), HMDS (98.5 %, ABCR), acet-
one (99.5 %, Aldrich), dichloromethane (CH Cl , >99.5, Roth), 4-
nitrobenzaldehyde (99 %, Acros), n-hexane (extra pure, Acros), and
methyl 4-nitrobenzoate (>99 %, Aldrich).
bath at 45 °C. The reaction was monitored by taking samples (about
50 μL) of the reaction mixture. Approximately 0.5 mL of acetone
was used to wash the syringe needle and to transfer the sample to
a vial. Subsequently, the catalyst was separated from the sample by
means of centrifugation. Finally, the samples were analyzed with a
reversed-phase HPLC apparatus (Agilent 1100 series) equipped with
a UV detector with variable wavelength. The HPLC apparatus was
operated at a column temperature of 30 °C by using a gradient
method with water (0.1 % trifluoroacetic acid) and acetonitrile
2
2
Synthesis of ePMO and ePMO-HMDS: BTEE and the ePMO were
[
12]
synthesized according to previously reported procedures.
The
1
00 % (E)-ethylene precursor was synthesized from vinyltriethoxy-
silane (42.95 mL), to which Grubbs catalyst (53.5 mg) was added
under an inert gas. The mixture was stirred for 1 h and then heated
under reflux conditions at 160 °C for 3 h. Subsequent vacuum distil-
lations at 90 and 160 °C were performed to obtain pure BTEE as
the second distillate.
(HPLC grade) as solvents. In this gradient method the volumetric
percentage of acetonitrile was varied from 30 to 62 % over a period
of 7 min. The chirality of the aldol products was analyzed by chiral
HPLC (Chiralpak AS-H; hexane/ethanol, 90:10).
For the recycling experiment, the catalyst was removed by filtration
after the first run, washed in n-hexane over 3 h, filtered again, dried
at 120 °C under vacuum and reused in a second catalytic run under
identical reaction conditions.
For the synthesis of the ePMO, P123 (1 g), butanol (2.45 mL), and
HCl (3.42 mL) were dissolved in water (48.7 mL) by stirring for 3 h.
BTEE (1.89 mL) was added dropwise, and the mixture was stirred at
3
5 °C for 5 h. Butanol (18 mL) was added, and the material was
aged at 90 °C under static conditions for 16 h. The obtained white
solid was filtered, and the template was removed by Soxhlet extrac-
tion with acetone for 6 h. The PMO was dried at 120 °C under
vacuum.
Computational Methodology: Static DFT calculations were carried
[13]
[14]
out within the Gaussian 09
package using the hybrid B3LYP
triple-ꢀ Pople basis set 6-
for both optimization and frequency calculations.
functional combined with
a
[15]
311++G(d,p)
For the synthesis of ePMO-HMDS, ePMO (0.3 g) was stirred in HMDS
Empirical dispersion corrections were added by using Grimme's D3
version with Becke–Johnson damping as implemented in Gaus-
(20mL) at room temperature for 5 h. The solid was filtered and
[
16]
washed with CH Cl . It was then stirred in CH Cl overnight, filtered, sian.
A simple model was constructed of a short chain that con-
2
2
2
2
and dried under vacuum at 120 °C.
Grafting of Cysteine and Cysteamine: The solid was first dried at
20 °C under vacuum overnight. -Cysteine (0.875 g) or cysteamine
0.55 g) and Irgacure 2959 (0.425 g) were dissolved in water (50 mL),
and the solution was purged with argon. ePMO (0.425 g) or ePMO-
HMDS (0.5 g) was added, and the suspension was exposed to UV
irradiation while stirring. Different loadings of cysteine or amine
were obtained by varying the reaction times between 15 min and
sisted of two linked precursor units, representative of the PMO
backbone modified with one or two cysteine groups. The total
stoichiometry of the model is hence [Si(OH) C H (Cyst)Si(OH) ] O.
The chain was first optimized without the cysteine groups, and its
geometry was kept fixed during optimization of the complete
model system, thus accounting for the rigidity of the PMO material.
Molecular dynamics simulations were performed by using the
3
2
3
2 2
1
(
L
[
17]
3
CP2K package. A 22.0 × 22.0 × 22.0 Å box subjected to periodic
boundary conditions was used. The BLYP functional was used to-
gether with empirical dispersion corrections (DFT-D3) to describe
24 h.
[
16]
UV reactions were performed in a Metalight Classic UV reactor
equipped with 12 UV sources (360 nm) and a stirring system.
the long-distance van der Waals interactions.
plane-waves method
A Gaussian and
[
18]
was chosen for all calculations by using a
DZVP/TZVP Gaussian basis set for Si/C/S/H and O/N, respectively,
and a plane-wave basis set with a 280 Ry cutoff using Goedecker–
For Cys-ePMO and Cys-ePMO-HMDS, the obtained yellow solid was
filtered, washed with warm water and acetone, heated to reflux in
water overnight, filtered, and again washed with warm water and
acetone. Finally, the product was dried under vacuum at 120 °C.
[
19]
Teter–Hutter pseudopotentials.
After initial heating by using a
[
20]
[21]
CSVR thermostat,
a Nosé–Hoover thermostat
of five coupled
heat baths was used to keep the system at 300 K. In total, the NVT
simulations sampled 12 ps by using a 0.5 fs time step.
For Amine-ePMO and Amine-ePMO-HMDS, the obtained yellow
solid was washed by Soxhlet extraction with acetone for 6 h and
dried under vacuum at 120 °C.
Supporting Information (see footnote on the first page of this
article): Additional data concerning the computational study.
Catalyst Characterization: FT-IR measurements were performed
with a Nicolet 6700 (Thermo Scientific) with a KBr matrix, at 120 °C
under vacuum. Elemental analysis was performed with a Thermo
Flash 2000 Thermal Analyzer (Thermo Scientific). XRD was per-
Acknowledgments
formed with an ARL X'TRA (Thermo Scientific). Nitrogen-sorption This research was funded by an FWO (Fund for Scientific Re-
measurements were performed with a TriStar 3000 analyzer (Micro- search Flanders) grant with number G006813N. The work was
meritics) after drying at 120 °C under vacuum.
also supported by the Long Term Structural Methusalem Fund-
ing by the Flemish Government. K. H., V. V. S., and P. V. D. V.
Catalytic Testing Procedure: The experiments were performed in
a 25 mL two-necked round-bottomed flask equipped with a con- thank the research Board of Ghent University [BOF (Special Re-
denser and a septum. First, the supported catalyst was added to search Fund)], and UGent GOA (Concerted Research Actions)
the flask so that the desired amount of amines was present (i.e., (grant 01G00710) for financial support. The authors are grateful
4
mol-% with respect to the 4-nitrobenzaldehyde concentration).
The reaction mixture was prepared separately by mixing the desired
amounts of acetone (50 vol.-%), 4-nitrobenzaldehyde
0.03 mmol mL ), methyl 4-nitrobenzoate (internal standard,
to Tom Planckaert for the elemental analysis measurements and
Jan Goeman for the chiral HPLC analyses.
–
1
(
–1
0
5
.022 mmol mL ), and n-hexane (solvent, 50 vol.-%). Afterwards, Keywords: Heterogeneous catalysis · Click chemistry · Aldol
mL of the reaction mixture was injected into the flask that con-
reactions · Mesoporous materials · Amino acids
tained the catalyst, and the flask was immediately placed in an oil
Eur. J. Inorg. Chem. 2016, 2144–2151
www.eurjic.org
2150
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Received: October 15, 2015
Published Online: January 15, 2016
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2151
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim