Synthesis and catalytic activity of a chiral periodic mesoporous organosilica (ChiMO)

Article abstract of DOI:10.1039/b304814d

A chiral vanadyl salen complex having two peripheral trimethoxysilyl groups has been used to obtain a chiral periodic mesoporous organosilica having MCM-41 periodicity and the two Si-CH₂ groups anchored on the framework; this solid induces 30% enantioselectivity in the cyanosilylation of benzaldehyde.

Full text of DOI:10.1039/b304814d

Synthesis and catalytic activity of a chiral periodic mesoporous
organosilica (ChiMO)
ab
a
b
b
b
Carlos Baleizão, Bárbara Gigante,* Debasish Das, Mercedes Alvaro, Hermenegildo Garcia* and
b
Avelino Corma*
a
INETI- Departamento de Tecnologia das Industrias Químicas, Estrada do Paço do Lumiar, 22, 1649-038
Lisboa, Portugal
b
Instituto de Tecnología Química/CSIC-Av. de los Naranjos, s/n, 46022 Valencia, Spain.
E-mail: acorma@itq.upv.es; Fax: +343877809; Tel: +34963877800
Received (in Cambridge, UK) 30th April 2003, Accepted 27th May 2003
First published as an Advance Article on the web 18th June 2003
A chiral vanadyl salen complex having two peripheral
trimethoxysilyl groups has been used to obtain a chiral
periodic mesoporous organosilica having MCM-41 periodic-
ity and the two Si–CH groups anchored on the framework;
2
this solid induces 30 % enantioselectivity in the cyanosilyla-
tion of benzaldehyde.
1
The synthesis of ordered mesoporous silicas has opened a new
2
field for the preparation of materials. This has been recently
expanded by the introduction of hybrid organic–inorganic
materials comprising two siloxane units bridged by an organic
component and containing ordered mesopores.3 These solids
are coined as periodic mesoporous organosilicas (PMOs).
Polysilsesquioxanes with ordered pore structure and con-
trolled porosity can be obtained by using surfactants as structure
directing agents. This procedure allows to introduce a large
variety of functional groups within the walls generating
functional frameworks. PMOs could complement and add extra
functionality to the one achieved previously by grafting organic
functions on the walls of the structured inorganic mesoporous
materials by a post-synthesis treatment.
–6
However it is known that it is not possible to achieve PMO
materials with any desired functionality if they are synthesized
only from bridged organosilica precursors. This is the case
when very large organic groups are involved in the organosilica
precursor since then it becomes difficult to achieve the proper
assemblage with the surfactant. On the other hand the
possibilities for preparing hybrid organic inorganic materials
will increase if a surfactant assembled condensation of silica
and the organic silane precursor is carried out. We will illustrate
this here by carrying out the synthesis of, as far as we know, the
first chiral mesostructured organosilica (ChiMO) in where the
chiral organic component is a vanadyl salen complex.
Scheme 1 a) Br , CH Cl , 0 °C, 1 h; b) 4-vinylphenylboronic acid,
2
2
2
3 4 2 3
[Pd(PPh ) ], 2 M Na CO , THF, 70 °C, 3 h; c) (1R,2R)-diaminocyclohex-
ane, EtOH, reflux, 1 h; d) VOacac, MeOH, r.t., overnight; e) 3-mercaptopro-
pyltrimethoxysilane, AIBN, CHCl (degassed), 70 °C, 20 h;
3
complexes. In the precedents reported on the literature so far
much simpler organic components have been used.
The resulting PMO solid was characterized by powder X-ray
diffraction where the typical pattern of hexagonal MCM-41-like
ordering was observed. Figure 1 shows the powder X-ray
diffraction corresponding to as-synthesized VOsalen@ChiMO.
The synthesized ChiMO contains the channels completely filled
with cetyltrimethylammonium bromide (CTABr) used as
structure directing agent for the synthesis of the solids. This was
removed by exhaustive solid liquid extraction at 40 °C of the as-
The preparation of ChiMO required the synthesis of a chiral
vanadyl salen complex having two terminal trimethoxysilyl
groups attached to the ligand. This disilylated precursor 3 was
obtained from a symmetric distyryl derivative of a chiral
vanadyl salen complex 2 that was submitted to a CNC double
bond addition reaction of 3-mercaptopropyltrimethoxysilane
initiated by azoisobutyronitrile (AIBN) (Scheme 1). The
distyryl vanadyl salen complex 2 that is the key intermediate in
our synthetic scheme is analogous to the manganese complex
7
previously reported in the literature. This intermediate 2 and
the disilylated precursor 3 were fully characterized by analytical
and spectroscopic means with all the data being compatible with
the proposed structure.
The precursor 3 was soluble in organic solvents such as
halogenated hydrocarbons, acetone, acetonitrile and ethanol.
The synthesis of the corresponding VOsalen@ChiMO was
accomplished following the general procedure previously
reported8 introducing ethanol as co-solvent to increase the
solubility of the precursor in aqueous solution. † The presence
of O, N and V atoms achieves a suitable balance between
hydrophilicity and hydrophobicity to permit the synthesis of a
PMO material in spite of the large molecular size of the
–9
Fig. 1 X-ray diffraction of VOsalen@ChiMO before (a) and after extracting
the CTAB (b).
1
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CHEM. COMMUN., 2003, 1860–1861
This journal is © The Royal Society of Chemistry 2003

synthesized PMO using acidified ethanol solution as solvent.
After extraction, powder X-ray diffraction showed that the
periodic structure of the ChiMO was preserved (see Figure 1).
The presence of the salen complex in the ChiMO has been
proved by diffuse-reflectance UV–Visible spectra.
assuming that the signal at ~ 290 ppm corresponds to the
silane groups.
If we accept at this point that a ChiMO has been obtained that
includes the salen complex in its structure, the material should
be chiral. To check this, we have determined directly the optical
activity of the solid by measuring the deviation angle of plane
polarised light using a conventional polarimeter of a suspension
of as-synthesized and extracted VOsalen@ChiMO. Direct
measurement of optical activity is possible since the suspension
of the VOsalen@ChiMO in 1,2-dichloroethane is sufficiently
transparent and lasts sufficiently long before settling down to
allow this measurement. The value of specific optical activity
From the Ar adsorption isotherm and following the Hovarth–
Kavazoe equation it is found that the pore diameter of the
mesoporous ChiMO after template removal is 4.5 nm with a
2
21
BET surface area of 900 m g (Figure 2).
2
1
measured was 22.54 ° g for the extracted VOsalen@ChiMO.
Control experiments with achiral MCM-41 do not give
measurable optical activity. The above observation constitutes a
direct evidence of the chirality of VOsalen@ChiMO.
The resultant material (0.25 mmol% (0.25 mmol% with
respect to benzaldehyde)) shows a high TON (320) for the room
temperature cyanosilylation with TMSCN (3 eq.) of benzalde-
hyde with an enantiomeric excess (ee) of 30%. The material as
catalyst is extremely stable and no leaching was observed. In
contrast, a MCM-41 in which tert-butyl VOsalen was adsorbed
leached a significant amount of the complex to the solution. On
the other hand, the activity of VOsalen@ChiMO compares well
to a MCM-41 in which a VOsalen complex having a
Fig. 2 Isothermal gas adsorption of VOsalen@ChiMO. The insert shows the
Hovarth–Kavazoe plot of the data.
1
0-undecenyloxy substituent was covalently grafted to the walls
2
1
(V content 0.04 mmol 3 g ) through a 3-mercaptopropyl
The covalent linkage of the salen complex to the silicate
group giving a TON after 72 h reaction time of 287 and an ee
value of 63%. We are now studying what causes the lower than
expected ee observed for VOsalen@ChiMO.
29
framework is demonstrated from the Si MAS NMR spectra
presented in Figure 3. As can be seen there, the spectrum
contains three bands appearing at ~ 2110, ~ 2100 and ~ 290
ppm that can be associated to Si(4Si), Si(3Si1OH), and
Si(2Si2OH) or Si(3Si1C). Cross polarization of 1H to Si
strongly enhances the signal at ~ 2100ppm corresponding to
Si(3Si1OH), but affects proportionally much less the 29Si MAS
NMR signal appearing at ~ 290 ppm, indicating that this peak
also corresponds to groups Si(3Si1C), that will result from the
inclusion of the salen in the silica wall of the mesoporous
material.
Financial support to C. Baleizão from Fundação para a
Ciência e Tecnologia, Portugal (PRAXIS XXI/BD/21375/99) is
gratefully acknowledged. The Spanish DGES (MAT2000-
29
1
768-CO2-01) has financed part of this work. We want to thank
to Dr Teresa Blasco for useful discussions.
Notes and references
†
Compound 3 was used in combination with TEOS in the synthesis of
VOsalen@ChiMO and cetyltrimethylammonium bromide (CTABr) as the
structure-directing agent. The molar proportions of the components in the
precursor gel were: 1.0 Si :0.12 CTABr :8.0 NH (20 %) :114 H O :10
3 2
The intensity of the NMR signal was strongly expanded in
order to see if partially condensed species of the type
Si(2Si,1OH,1C) and Si(1Si,2OH,1C) appearing at ~ 240 and
EtOH. TEOS and compound 3 were used as the source of Si in various
proportions from 85:15 and 95:5. After mixing the reactants, the resulting
clear gel was transferred to a polyethylene container and heated at 90 °C for
4 days. The solid obtained was washed with water and dried in air at 60 °C.
The structure-directing agent was removed by extracting the solid with
dilute ethanolic HCl acid solution at 40 °C for 2 h (20 ml of 0.5 M ethanolic
HCl for 0.5 g of solid). The vanadium content after extraction was 0.014
~
260 ppm respectively were also formed. From this study it
can be concluded that a very small number of Si(2Si,1OH,1C)
may exist. Therefore it can be concluded that the majority of the
precursor is covalently grafted through both Si atoms to the
inorganic framework.
From the chemical vanadium analysis of the final material, it
is estimated that 2.5% of the Si present should correspond to the
silane groups of the salen complex. This value corresponds
quite well with the 2.4% calculated from 29Si NMR by
21
mmol g
and the specific optical rotation was measured using a
conventional JASCO polarimeter and the yellow Na line in a 1 dm cell.
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Fig. 3 MAS 29Si-NMR spectra of VOsalen@ChiMO after CTABr removal.
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