Synthesis and characterization of chiral benzylic ether-bridged periodic mesoporous organosilicas

Article abstract of DOI:10.1002/chem.200800239

The first synthesis of a chiral periodic mesoporous organosilica (PMO) carrying benzylic ether bridging groups is reported. By hydrolysis and condensation of the new designed chiral organosilica precursor 1,4-bis(triethoxysilyl)-2-(1 -methoxyethyl)benzene

Full text of DOI:10.1002/chem.200800239

FULL PAPER
DOI: 10.1002/chem.200800239
Synthesis and Characterization of Chiral Benzylic Ether-Bridged Periodic
Mesoporous Organosilicas
Jürgen Morell,[b] Sangam Chatterjee,[c] Peter J. Klar,[d] Daniel Mauder,[e]
Ilja Shenderovich,[e] Frank Hoffmann,[a] and Michael Frçba*[a]
Abstract: The first synthesis of a chiral
periodic mesoporous organosilica
(PMO) carrying benzylic ether bridging
groups is reported. By hydrolysis and
condensation of the new designed
chiral organosilica precursor 1,4-bis-
ACHTREUNG(triethoxysilyl)-2-(1-methoxyethyl)ben-
zene (BTEMEB) in the presence of
the non-ionic oligomeric surfactant Brij
76 as supramolecular structure-direct-
ing agent under acidic conditions, an
ordered mesoporous chiral benzylic
ether-bridged hybrid material with a
high specific surface area was obtained.
The chiral PMO precursor was synthe-
sized in a four-step reaction from 1,4-
dibromobenzene as the starting com-
pound. The evidence for the presence
of the chiral units in the organosilica
precursor as well as inside the PMO
material is provided by optical activity
measurements.
Keywords: chirality · mesoporous
materials · optical activity · organo-
silicas · structure-directing agents
Introduction
ities to deliberately tune the chemical and physical proper-
ties of the PMOs by varying the organic spacer groups of
the organosilica precursors, which makes these materials in-
teresting for applications such as catalysis, adsorption, chro-
matography, or host–guest chemistry.[4,5] The organic bridg-
ing groups of the organosilica precursors that have been suc-
cessfully converted into PMO materials include, for in-
stance, methylene,[6] ethylene,[1,3] ethenylene,[2,3] phenylene,[7]
biphenylene,[8] thiophene,[7,9] and divinylphenylene[10,11] units.
It was also possible to prepare PMOs possessing molecular-
scale periodicity inside the channel walls instead of the
common observed amorphous pore walls as a result of a
highly cooperative process.[12,13] To obtain larger pore sizes
in PMO materials, non-ionic triblock copolymers can be em-
ployed as structure-directing agents instead of the common-
ly used ionic alkylammonium halides.[9,14] Moreover, it was
possible to prepare PMOs with defined morphologies such
as spherical particles, which have proved to be suitable for
chromatography applications.[15] Furthermore, the synthesis
of bifunctional PMO materials has been reported.[16] How-
In 1999, a new class of organic–inorganic hybrid materials
called periodic mesoporous organosilicas (PMOs) was devel-
oped by three groups working independently.[1–3] The synthe-
sis of this kind of materials can be carried out by hydrolysis
and condensation of bridged organosilsesquioxane precur-
sors [(R’O)3Si-R-SiACHTREUNG(OR’)3] in the presence of supramolec-
ular structure-directing agents. There are enormous possibil-
[a] Dr. F. Hoffmann, Prof. Dr. M. Frçba
Institute of Inorganic and Applied Chemistry
University of Hamburg
Martin-Luther-King-Platz 6, 20146 Hamburg (Germany)
Fax : (+49)40-428-38-6348
E-mail: michael.froeba@chemie.uni-hamburg.de
[b] Dr. J. Morell
Institute of Inorganic and Analytical Chemistry
Justus Liebig University Giessen
Heinrich-Buff-Ring 58, 35392 Giessen (Germany)
[c] Dr. S. Chatterjee
AHCTREeUNG ver, so far only a few attempts have been undertaken to
Faculty of Physics and Material Sciences Center
Philipps University Marburg
Renthof 5, 35032 Marburg (Germany)
link molecular chirality and organic–inorganic hybrid mate-
rials, which would make these materials even more interest-
ing for potential applications such as enantioselective cataly-
sis or chromatography. Hitherto, either chiral mesoporous
organosilicas with a limited loading of chiral organic groups
(obtained via the co-condensation approach),[17–19] or chiral
organosilicas lacking true periodicity were reported.[20]
Polarz and Kuschel[21] reported on a PMO composed of a
[d] Prof. Dr. P. J. Klar
Institute of Experimental Physics I
Justus Liebig University Giessen
Heinrich-Buff-Ring 16, 35392 Giessen (Germany)
[e] D. Mauder, Dr. I. Shenderovich
Institute of Chemistry, Free University of Berlin
Takustrasse 3, 14195 Berlin (Germany)
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chiral silsesquioxane precursor; however, neither informa-
tion on the enantiomeric excess of the precursor was given
nor a proof of the chirality of the PMO. Very recently, Ina-
gaki et al. reported the synthesis of a new PMO material
carrying chiral phenylethylene bridging groups, for which
the enantiomeric purity was determined by eluting the or-
ganic groups from the solids.[22] Here we present the synthe-
sis of the completely novel chiral organosilica precursor 1,4-
bis(triethoxysilyl)-2-(1-methoxyethyl)benzene (BTEMEB)
carrying benzylic ether bridging groups, which was used as a
single-source precursor to prepare a homochiral genuine
PMO (see Scheme 1). Furthermore, we demonstrate that
the chirality of the PMO can be directly and in a non-de-
structive way proven by measurements of the optical activi-
ty, which is a novelty for the solid-state.
to the novel chiral organosilica precursor BTEMEB (4) was
achieved by carrying out a Grignard reaction. The new
chiral benzylic ether-bridged PMO material 5 was prepared
by hydrolysis and condensation of BTEMEB in the presence
of the non-ionic oligomeric surfactant Brij 76 as structure-
directing agent under acidic conditions (see the details of
the synthesis procedures in the Experimental Section).
The powder X-ray diffraction pattern of the solvent-ex-
tracted benzylic ether-bridged PMO material (Figure 1) ex-
hibits one intense Bragg reflection at 2q=1.528, which con-
firms the existence of an ordered mesostructure with an in-
terplanar d spacing of 5.81 nm of the pore arrangement with
approximately uniform pore sizes. The absence of any other
reflections in the low-angle region indicates limited perio-
dicity in the long-range order of the mesopore system. Thus,
Results and Discussion
The synthesis of the chiral PMO precursor 1,4-bis(triethoxy-
silyl)-2-(1-methoxyethyl)benzene (BTEMEB) (4) with
a
high enantiomeric excess of 88% ee (determined by enan-
tioselective chromatography) was achieved in a four-step re-
action from 1,4-dibromobenzene by using synthesis proce-
dures established in the literature (see Scheme 1). 2,5-Di-
bromoacetophenone (1) was synthesized from 1,4-dibromo-
benzene by Friedel–Crafts acylation[23] followed by an asym-
metric hydrogenation of 1 in the presence of the chiral
ruthenium catalyst 6 to obtain chiral 1-(2,5-dibromophenyl)-
ACHTREUNG
ethanol (2).[24] The OH group of 2 was methylated by treat-
ment with NaH and CH3I, leading to 1,4-dibromo-2-(1-
Figure 1. Powder X-ray diffraction pattern of the chiral benzylic ether-
bridged periodic mesoporous organosilica.
methoxyethyl)benzene (3).[25] The last synthesis step leading
it can be assumed that the mes-
ostructure of the material re-
sembles that of the HMS silica
phases.[26] The occurrence of
only one intense reflection indi-
cates the existence of smaller
scattering domain sizes inside
the product compared to those
of typically highly ordered
PMO materials. Furthermore,
the pore wall structure is not
crystal-like as no reflections in
the higher angle region are ob-
served.
The transmission electron mi-
croscope (TEM) image of the
PMO material (Figure 2) also
shows the presence of a worm-
hole-like structure rather than a
2D hexagonal mesostructure as
expected from the XRD pat-
tern. Thus, the mesostructural
Scheme 1. Synthesis pathway of the PMO precursor BTEMEB as well as the chiral PMO product.
order of the product occurs in
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Periodic Mesoporous Organosilicas
FULL PAPER
Figure 4. Pore size distribution of the chiral benzylic ether-bridged peri-
odic mesoporous organosilica.
To confirm the presence of the organic moieties inside the
PMO material, 29Si and 13C MAS NMR spectroscopic meas-
urements were carried out. The 13C CPMAS NMR spectrum
(Figure 5) shows the characteristic signals of the benzylic
Figure 2. TEM image of the chiral benzylic ether-bridged periodic meso-
porous organosilica.
the range between unordered compounds and highly or-
dered periodic mesoporous organosilicas.
The N2 physisorption measurements on the PMO material
reveal an isotherm, which can best be assigned to type IV,
but which also exhibits features of type I, indicating the
presence of both meso-and micropores inside the material
(Figure 3). The isotherm shows a not very distinctive capilla-
ry condensation step at a p/p0 of about 0.22, and further-
more no hysteresis loop is observed that is typical of meso-
porous materials with small pore sizes.[27] The pore size dis-
tribution actually shows the existence of both mesopores
with an average pore diameter of 2.3 nm and also micro-
pores inside the product (Figure 4). The specific surface
area of the PMO is 820 m2 gꢀ1 (BET) with a total mesopore
volume of 0.46 cm3 gꢀ1.
Figure 5. 13C CPMAS spectrum of the chiral benzylic ether-bridged peri-
odic mesoporous organosilica.
ether bridging group, which can be attributed to the C spe-
cies as follows: d=23.8 (Caryl-CH-CH3); 55.9 (O-CH3); 78.5
(Caryl-CH-CH3); 126.6, 130.0, 130.1, 137.1, 145.6, 154.0 ppm
(Caryl). The 29Si CPMAS NMR spectrum (Figure 6) exhibits
three Tn signals, which can be assigned to the following Si
species covalently bonded to the carbon atoms: T1 [C-Si-
A
ACHTRE(UGN OSi)2(OH), d=
ꢀ72.0 (38.8%)], and T3 [C-Si
(OSi)3, d=ꢀ79.3 (43.9%)]. As
A
Qn signals (n=2–4, d=ꢀ92 to ꢀ113) only appear with a
proportion of less than 15%, the MAS NMR spectroscopic
measurements clearly indicate that almost all benzylic ether
units are still intact inside the pore walls of the PMO mate-
rial after the synthesis and extraction procedure.
Figure 3. Nitrogen adsorption/desorption isotherm of the chiral benzylic
ether-bridged periodic mesoporous organosilica.
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5937

M. Frçba et al.
Figure 8. Results of the optical activity measurements of the chiral PMO
material. Anticlockwise rotation of the polarization plotted against the
PMO concentration as well as the resulting linear fit.
Figure 6. 29Si CPMAS spectrum of the chiral benzylic ether-bridged peri-
odic mesoporous organosilica.
that is, 1=1.135 gmLꢀ1. Therefore, the concentration of
8 mgmLꢀ1 corresponds to a volume ratio VPMO:Vtot of about
1:110. Furthermore, assuming that the suspension is isotrop-
ic, the effective length of the light path through the PMO,
LPMO, is related to the length of the light path through the
Figure 7 and Figure 8 depict the results of the optical ac-
tivity measurements of solutions of the precursor BTEMEB
in chloroform and suspensions of the PMO product in etha-
cuvette,
L
tot =0.1 dm,
by
L
PMO =(VPMO/Vtot)1/3·Ltot
ꢁ0.021 dm. Finally, we obtain the value of ꢀ2758dmꢀ1 for
the specific optical activity at 633 nm for the synthesized
solid PMO product. Our results provide unambiguous proof
that both the precursor BTEMEB and the corresponding
PMO material carrying benzylic ether bridging groups, are
chiral compounds.
Conclusion
In summary, we have synthesized the first chiral PMO mate-
rial bearing benzylic ether bridging groups. The PMO mate-
rial was prepared from the novel chiral organosilica precur-
sor
1,4-bis(triethoxysilyl)-2-(1-methoxyethyl)benzene
(BTEMEB) in the presence of the surfactant Brij 76 as
structure-directing agent. The PMO precursor can be syn-
thesized with an enantiomeric excess of 88% ee. We have
clearly proved the chirality of the organic moieties inside
the organosilica precursor as well as in the PMO material
by measuring the optical activity of the samples. With
regard to potential applications such as enantioselective
chromatography, the chiral PMO product represents a very
promising new kind of periodic mesoporous hybrid material
with interesting adsorption properties, which are still under
further investigation.
Figure 7. Results of the optical activity measurements on the chiral PMO
precursor. Anticlockwise rotation of the polarization plotted against the
PMO precursor concentration as well as the resulting linear fit.
nol/chloroform, respectively. In both cases the rotation
angle Dq depends linearly on the concentration of the chiral
compound, as expected by theory. The specific optical activi-
ty of BTEMEB determined at 633 nm is about
ꢀ2008(dmmLgꢀ1)ꢀ1. A value for the specific optical activity
in 8 per dm (8dmꢀ1) of the solid PMO powder can be rough-
ly estimated from the measurements of the corresponding
suspension as follows. We obtain a rotation of the polariza-
tion by 68 for a PMO concentration of 8 mgmLꢀ1 of 1:1 eth-
anol/chloroform from the linear fit to the experimental data
in Figure 8. As the PMO material is floating in the ethanol–
chloroform solution, we assume that its density is the same
as the average density of the ethanol–chloroform mixture,
Experimental Section
Powder X-ray diffaction measurements: The powder X-ray diffraction
(P-XRD) pattern was recorded at room temperature with a PANalytical
X’Pert PRO diffractometer using filtered CuKa radiation.
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Periodic Mesoporous Organosilicas
FULL PAPER
TEM analysis: The transmission electron micrograph was obtained with
a Philips C 30 microscope operating at 300 kV.
hexane (150 mL) was added. The precipitated magnesium salts were re-
moved by filtration under an argon atmosphere, and then hexane and re-
sidual TEOS were evaporated. The remaining brown crude product was
distilled in vacuo to obtain BTEMEB (6.7 g, 0.0146 mol, yield: 34%) as a
colorless oil. The ee value of BTEMEB was determined to 88% by ena-
N2-physisorption measurements: N2-physisorption data were recorded
with a Quantachrome Autosorb 6 at 77 K. The BET surface area was cal-
culated from p/p0 =0.03–0.3 in the adsorption branch, and the BJH pore
size distribution was calculated from the desorption branch.
1
tioselective gas chromatography. H NMR (400 MHz, CDCl3): d=1.26 (t,
MAS NMR measurements: The 13C and 29Si NMR measurements were
performed on a Bruker MSL-300 instrument, operating at 7 T, equipped
with a Chemagnetics-Varian 6 mm pencil CPMAS probe. The samples
were spun at 6.0–8.0 kHz under magic angle spinning (MAS) conditions.
The {1H}–29Si CPMAS spectrum was recorded by using a relatively long
cross polarization (CP) contact time of 8 ms, which ensures sufficient po-
larization transfer to all different species of silica; further parameters
were a recycle delay of 5 s and 2000 scans. The 13C NMR measurement
was performed by employing the {1H}–13C CPMAS technique with a CP
contact time of 2 ms, a recycle delay of 15 s and 3000 scans. The typical
p/2-pulse width in the CP experiments was 3.5 ms for 1H. 29Si and 13C
chemical shift values are referenced to solid TSP.
J=7.11 Hz, 18H, O-CH2-CH3), 1.41 (d, J=6.23 Hz, 3H, Caryl-CH-CH3),
3.22 (s, 3H, O-CH3), 3.89 (q, J=7.11 Hz, 12H, O-CH2-CH3), 4.79 (q, J=
6,23 Hz, 1H, Ar-CH-CH3), 7.56 (d, J=7.68 Hz, 1H, Caryl-H), 7.77 (d, 1H,
J=7.68 Hz,
C
aryl-H) 8.04 ppm (s, 1H, Caryl-H); 13C NMR (100 MHz;
CDCl3): d=16.9 (O-CH2-CH3), 22.9 (Caryl-CH-CH3), 54,9 (O-CH2-CH3),
57.4 (O-CH3), 77.2 (Cayl-CH-CH3), 123.2, 127.0, 127.2, 136.2, 141.2,
151.9 ppm (Caryl).
PMO synthesis: The chiral PMO material was synthesized as follows: In
a typical synthesis, Brij 76 (0.23 g, 0.32 mmol) was dissolved in a mixture
of 2m HCl (8.3 g, 0.014 mol) and distilled water (1.7 g, 0.094 mol). After
addition of BTEMEB (0.77 g, 1.7 mmol), the reaction mixture was kept
for 20 h at 508C under vigorous stirring. After additional hydrothermal
treatment for 24 h at 908C in a teflon-lined stainless steel autoclave, the
obtained white precipitate was filtered, washed with distilled water (3”
50 mL), and dried in air overnight. Removal of the surfactant was accom-
plished by extraction with a mixture of ethanol/HCl (conc.) (100:3 v/v)
using a Soxhlet apparatus for 48 h with subsequent drying of the product
in air. The molar ratios of the components in the reaction mixture were:
BTEMEB/Brij 76/HCl/H2O=1:0.19:8.24:55.
PMO precursor synthesis: 1,4-Bis(triethoxysilyl)-2-(1-methoxyethyl)ben-
zene (BTEMEB) was synthesized in a four-step reaction starting from
1,4-dibromobenzene as follows:
Step 1: Synthesis of 2,5-dibromoacetophenone: The synthesis was carried
out from 1,4-dibromobenzene in accordance with the synthesis protocol
given in reference [23].
Step 2: Synthesis of chiral 1-(2,5-dibromophenyl)ethanol: The synthesis
was carried out following a slightly modified version of the synthesis pro-
tocol given in reference [24]: In a three-necked flask with a condenser,
[RuIICl2-(p-cymene)] (0.37 g, 0.624 mmol), 1S,2S-N-p-tosyl-1,2-diphenyle-
thylenediamine (0.2 g, 1.24 mmol), and triethylamine (2.3 mL) were dis-
solved in 2-propanol (57.5 mL) under an argon atmosphere. After the so-
lution had been heated for 1 h at 858C, 2-propanol and triethylamine
were evaporated, whereon the preformed chiral catalyst remained. Then
2,5-dibromoacetophenone (34.6 g, 0.125 mol) and HCOOH/NEt3
(62.1 mL) were added and the solution was stirred for 48 h under ambi-
ent conditions. After addition of saturated NaHCO3 (100 mL), the crude
product was extracted with ethyl acetate, and the organic layer was dried
over MgSO4. The ethyl acetate was evaporated, and the crude product
was distilled in vacuo to obtain chiral 1-(2,5-dibromophenyl)ethanol
(28.4 g, 0.102 mol, yield: 81%). 1H NMR (400 MHz, CDCl3): d=1.43 (d,
J=6.4 Hz, 3H, Caryl-CH-CH3), 2.47 (s, 1H, -OH), 5.13 (q, J=6.4 Hz, 1H,
CAryl-CH-CH3), 7.44–7.64 ppm (m, 3H, Caryl-H); 13C NMR (100 MHz,
CDCl3): d=23.7 (Caryl-CH-CH3), 68.7 (Cayl-CH-CH3), 121.4, 122.0, 128.0,
130.9, 134.9, 143.8 (Caryl).
Optical activity measurements: The optical activity, that is, the rotation
of the polarization of linearly polarized light passing through the sample,
was measured by a difference method as depicted schematically in
Figure 9. The light of a HeNe laser (633 nm) was linearly polarized to an
Step 3: Synthesis of chiral 1,4-dibromo-2-(1-methoxyethyl)benzene: The
synthesis was carried out following a slightly modified version of the syn-
thesis protocol given in reference [25]: In a three-necked flask with a
condenser, chiral 1-(2,5-dibromophenyl)ethanol (28.4 g, 0.102 mol) was
dissolved in THF (305 mL) under an argon atmosphere, and then NaH
(6.1 g, 0.15 mol, 55–65%) was added. After addition of methyl iodide
(25.4 mL, 0.406 mol), the resulting suspension was stirred for 20 h under
ambient conditions. Then the THF was evaporated and the crude product
was extracted with dichloromethane. After the organic layer had been
dried over MgSO4, the dichloromethane was evaporated, and the remain-
ing crude product was distilled in vacuo to obtain chiral 1,4-dibromo-2-
(1-methoxyethyl)benzene (24.5 g, 0.083 mol, yield: 82%). 1H NMR
(400 MHz, CDCl3): d=1.37 (d, J=6.4 Hz, 3H, Caryl-CH-CH3), 3.25 (s,
3H, -O-CH3), 4.65 (q, J=6.4 Hz, 1H, CAryl-CH-CH3), 7.34 (d, J=8.3 Hz,
1H, Caryl-H), 7.47 (d, J=8.3 Hz, 1H, Caryl-H), 7.68 ppm (s, 1H, Caryl-H);
13C NMR (100 MHz; CDCl3): d=23.7 (Caryl-CH-CH3), 56.8 (-O-CH3),
77.9 (Cayl-CH-CH3), 121.1, 122.9, 128.2, 131.2, 134.7, 142.1 ppm (Caryl).
Figure 9. Experimental setup for the measurements of the optical activity.
angle q of 458 with respect to the horizontal x-direction using the linear
polarizer. The laser was modulated at 8 kHz using an optical chopper to
enable the lock-in detection technique. The light then passed through the
sample in a cuvette at normal incidence, and then through an iris dia-
phragm, which was used to suppress scattered light. Next, a Wollaston
prism spatially separated the polarized light into a horizontally polarized
component (x-component of the electric field vector Ex) and a vertically
polarized component (y-component of the electrical field Ey). The differ-
2
ence in intensity of the two corresponding beams, Ix =jEx j and
2
Iy =jEy j , was measured by a pair of identical, balanced photodiodes by
using the lock-in technique. A sample which is optically active changes
the polarization angle by Dq. Thus Equation (1) holds:
Step 4: Synthesis of the PMO precursor 1,4-bis(triethoxysilyl)-2-(1-me-
thoxyethyl)benzene (BTEMEB): In a three-necked flask with a condens-
er and dropping funnel under an argon atmosphere, a mixture of TEOS
(88.6 g, 0.426 mol), THF (65 mL), magnesium turnings (3.1 g 0.129 mol),
and a small crystal of iodine was heated to 958C, and then a solution of
chiral 1,4-dibromo-2-(1-methoxyethyl)benzene (12.5 g, 0.0425 mol) in
THF (22 mL) was added dropwise over 2 h. After the mixture had been
refluxed for an additional 3.5 h, THF was evaporated in vacuo, and
Iy
Ix
tan2q ¼
ð1Þ
where q is the angle between the x-direction and the polarization direc-
tion of the light after passing the sample. Expanding Equation (1) into a
Taylor series about q=458 (=p/4) yields Equation (2):
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5939

M. Frçba et al.
ꢀ
ꢁ
Iy
Ix
p
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¼ tan2
þ Dq ꢁ 1 þ 4 ꢂ Dq
ð2Þ
ð3Þ
4
and thus for Dq, in degrees, Equation (3) holds:
180 ꢃ
IxꢀIy
Dq ¼
ꢂ
p
4 Ix
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The chiral benzylic ether-bridged PMO product is of powder form. Thus
its optical activity is difficult to measure because multiple scattering
within the powder leads to a loss of polarization. This effect can be mini-
mized by making a suspension of the powder with a non-chiral solvent of
preferably the same refractive index. In this experiment a 1:1 mixture of
ethanol/chloroform was used. The optical activity of the suspension was
then measured with the latter inside a cuvette, where interfaces between
air–cuvette–suspension are well defined, and, therefore, normal incidence
of the light on the sample can be realized. Usually the refractive index
matching between the constituents of a suspension is not perfect, that is,
multiple scattering by the solid particles in the suspension still takes
place. One preferentially detects photons which are still on axis, that is,
mostly have gone through scattering events at normal incidence only by
using a laser as the light source and the iris diaphragm after the sample
as mentioned above. The high sensitivity of the differential measurement
in combination with lock-in technique allows a very short optical path
through the sample. Here, a 1 cm optical path is used to detect the opti-
cal activity, compared to the 10 cm used in standard set-ups. We mea-
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chloroform and suspensions of the PMO product in ethanol/chloroform,
respectively, to verify the linear dependence of the rotation of the polari-
zation on the concentration of the chiral compounds. Prior, to each mea-
surement the two photodiodes were balanced, that is, the difference
signal was set to zero with an empty cuvette in the place of the sample.
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Received: February 7, 2008
Published online: May 19, 2008
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