Crystal-like periodic mesoporous organosilica bearing pyridine units within the framework

Article abstract of DOI:10.1039/c0cc01944e

Periodic mesoporous organosilica with densely packed pyridine units within the framework and crystal-like molecular-scale periodicity was synthesized. The framework pyridines were chemically active and fully accessible for protonation and Cu²⁺

Full text of DOI:10.1039/c0cc01944e

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Crystal-like periodic mesoporous organosilica bearing pyridine units
within the frameworkw
Minoru Waki,ab Norihiro Mizoshita,ab Tetsu Ohsuna,ab Takao Taniab and Shinji Inagaki*ab
Received 18th June 2010, Accepted 24th September 2010
DOI: 10.1039/c0cc01944e
Periodic mesoporous organosilica with densely packed pyridine
units within the framework and crystal-like molecular-scale
periodicity was synthesized. The framework pyridines were
chemically active and fully accessible for protonation and
mesoporous framework, due to the difficulty in synthesizing
1–14
the precursors.
1
Here we report the synthesis of the first crystal-like PMO
bearing heterocyclic organic groups from a newly designed
divinylpyridine (v-Py)-bridged alkoxysilane precursor in the
presence of a cationic template surfactant. The v-Py groups in
the walls are packed regularly with an interval of 1.16 nm
along the channel direction. Most of the pyridine units in the
walls were accessible for protonation and deprotonation
with acid and base treatment. v-Py–PMO exhibited higher
2
+
Cu adsorption.
Periodic mesoporous organosilicas (PMOs), synthesized from
up to 100% organic-bridged alkoxysilane precursors
0
0
(
R–[Si(OR )
3 n 3
] , n Z 2, R = organic group, R = CH ,
C H , etc.) by surfactant-templated supramolecular assembly,
2
5
2
+
are a new class of functional hybrid materials in which organic
groups are densely and covalently embedded within the silica
1
framework. A broad spectrum of functionalities can be
adsorption capacity for copper ions (Cu ) than that for
conventional divinylbenzene–PMO with similar meso-scale
and molecular-scale periodicities.
incorporated into the framework by the appropriate design
of organosilane precursors with specific functional groups
In our preliminary experiments, direct silylation at the 2,5-
1
positions of pyridine by both Rh-catalyzed cross-coupling
3
2
14
such as hydrocarbons, heteroaromatics and metal complexes.
and Heck reaction of 2,5-diiodopyridine (Scheme S1, ESIw)
was unsuccessful. However, we succeeded in the Rh-catalyzed
hydrosilylation of 2,5-diethynylpyridine, which was derived
from 2,5-diiodopyridine, to form 2,5-bis[(E)-2-(triethoxysilyl)-
vinyl]pyridine (1) (Scheme 1). The isomer with trans-b addition
In addition, some PMOs bearing interactive organic groups
3
4
5
6
such as benzene, biphenyl, ethene, divinylbenzene and
7
naphthalene have been found to show a crystal-like ordering
of the bridges within the pore walls. The formation of
molecular-scale periodicity in the PMO provides the potential
to exhibit unique chemical and physical properties due to
specific interaction between the organic groups. However,
there have been no reports on the synthesis of crystal-like
PMOs bearing heterocyclic organic groups within the pore
2
was mainly obtained in this study using [Rh(cod)Cl] as a
1
5
catalyst in acetonitrile, although the hydrosilylation of
terminal alkynes usually afforded three types of isomers with
different additions (a-, cis-b and trans-b). Selective formation
1
of the trans-b product was confirmed by H nuclear magnetic
8
walls, despite their high functionalities.
resonance (NMR) spectroscopy and nuclear Overhauser
effect (NOE) measurements (Fig. S1 and S2, ESIw). The
v-Py-bridged precursor was polycondensed in an aqueous
solution containing octadecyltrimethylammonium chloride
(template surfactant) and sodium hydroxide (basic catalyst)
at 96 1C for 24 h to yield a white precipitate (Scheme 1).
The dried white powder was treated with a HCl/ethanol
a
Pyridine is a weak organic base (pK = 5.22) that is both a
proton acceptor and an electron-pair donor, and acts as a
nucleophilic reagent for various reactions and a typical ligand
9
for binding metals. Immobilization of pyridyl groups on the
surface of mesoporous silica and organosilica has been carried
out by post-synthesis grafting or co-condensation using
0
various monosilylated precursors [R–Si(OR ) ] with a pyridine
3
solution for extraction of the surfactant to obtain a
unit as R for applications such as heterogeneous catalysts
1
0
and adsorbents. However, these approaches have been
unsuccessful in immobilizing a large amount of pyridine units
and organizing a crystal-like ordered structure within the pore
walls. On the other hand, there have been no reports on
the synthesis of PMOs from 100% bridged alkoxysilane
precursors with a pyridine unit, although it is an appropriate
approach for the dense accumulation of pyridine units in the
a
Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan.
E-mail: inagaki@mosk.tytlabs.co.jp; Fax: +81 561-63-6507;
Tel: +81 561-71-7393
Core Research for Evolutional Science and Technology (CREST),
b
Japan Science and Technology Agency (JST), Kawaguchi, Saitama
332-0012, Japan
Scheme 1 Synthesis of precursor 1 and v-Py–PMO. TMSacetylene =
trimethylsilyl)acetylene, C18TMACl = octadecyltrimethylammonium
chloride.
w Electronic supplementary information (ESI) available: Experimental
details, characterization for 1, additional data for v-Py–PMO and
Cu adsorption of v-Py–PMO. See DOI: 10.1039/c0cc01944e
(
2+
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Chem. Commun., 2010, 46, 8163–8165 8163

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molecular-scale periodicity (1.16 nm) obtained by XRD, and
corresponds to the molecular length of the bridging units
(
–Si–C H –Py–C H –SiO–). These results suggest that the
2 2 2 2
v-Py groups in the walls align their molecular axes parallel
to the channel direction to form alternating organic and
silica belt-like layers with an interval of 1.16 nm (Fig. 2d);
this structure is similar to those of previously reported crystal-
like PMOs.
Nitrogen adsorption/desorption isotherms showed the type-IV
isotherm typical for mesoporous materials (Fig. S4, ESIw). The
Brunauer–Emmett–Teller (BET) surface area, pore volume,
and density functional theory (DFT) calculated pore diameter
2
ꢀ1
3
ꢀ1
Fig. 1 XRD patterns of v-Py–PMO before and after extraction of the
were 803 m g , 0.683 cm g , and 4.1 nm, respectively. The
pore wall thickness was estimated to be 1.3 nm, which
corresponds to three v-Py layers, from the pore diameter
and the d-spacing under the assumption of hexagonal packing.
template surfactant.
v-Py–PMO powder. Complete removal of the surfactant
was confirmed by infrared (IR) absorption measurement
2
9
The solid-state Si magic angle spinning (MAS) NMR
(
Fig. S3, ESIw).
The powder X-ray diffraction (XRD) pattern of v-Py–PMO
spectrum of v-Py–PMO showed two resonances at ꢀ71.6
2
3
n
and ꢀ79.3 ppm assigned to T and T [T : SiC(OH)3ꢀn(OSi)
n
]
showed an intense peak at 2y = 1.881, which corresponds to
an ordered mesostructure with a d-spacing of 4.70 nm, and
additional four peaks with d-spacings of 1.16, 0.59, 0.40 and
species, respectively, but no signals around ꢀ100 ppm, due to
n
Q [Si(OH)4ꢀn(OSi)
n
] species, formed by cleavage of the Si–C
1
3
bonds (Fig. S5A, ESIw). The C cross-polarization MAS
NMR spectrum of v-Py–PMO displayed four resonances at
0
.30 nm at medium scattering angles (2y = 5–401) that
indicate the molecular-scale periodicity of v-Py groups with
an interval of 1.16 nm in the pore walls (Fig. 1). The as-made
v-Py–PMO also showed a similar XRD pattern before
removal of the template surfactant, due to both the meso-
and molecular-scale periodicities. Fig. 2 shows transmission
electron micrographs and a structural model of v-Py–PMO.
The low magnification image (Fig. 2a) shows that v-Py–PMO
has a fibrous particle morphology with a width of several
hundred nanometres. The medium magnification image
1
22.8, 131.9, 147.7 and 153.0 ppm, which were composed of
nine overlapping signals of aromatic and vinylic carbons
Fig. S5B, ESIw). These results suggest that the Si–C bonds
(
and organic bridges of v-Py–PMO are preserved during the
synthesis and subsequent extraction processes. The IR spectrum
of v-Py–PMO showed absorption bands assignable to
ꢀ1
n(CQC)aromatic and vinylic (1610, 1480 and 1380 cm ),
ꢀ
1
ꢀ1
)
n(Si–O) (1040 cm ), and n(C–H)trans-vinylene (987 cm
Fig. S3, ESIw), which also supports preservation of the
(
(
Fig. 2b) shows one-dimensional channels with an interval of
–5 nm running in the fibrous direction. The high magnification
organic bridges. Thermogravimetric analysis indicated that
the pyridine units were retained in the walls up to 400 1C in
air (Fig. S6, ESIw). Elemental analysis of carbon and nitrogen
in v-Py–PMO gave a carbon/nitrogen atomic ratio of 8.5 : 1,
which is primarily consistent with that expected from the
chemical formula (C NH ) of the bridges. The amount of
4
image (Fig. 2c) clearly shows lattice fringes on the pore walls in
the direction perpendicular to the channels. The basal
spacing of the lattice fringes is in good agreement with the
9
7
the pyridine units in v-Py–PMO was estimated from the
ꢀ1
nitrogen content as 3.36 mmol g , which is far higher
than those reported for mesoporous materials prepared by
ꢀ1 10
post-grafting or co-condensation (e.g. 1.14 mmol g ).
The pyridine groups incorporated in the framework of
v-Py–PMO were found to be chemically active and fully
accessible for protonation. Fluorescence spectra of v-Py–PMO
showed a fluorescence band at 390 nm with a small shoulder at
4
70 nm. Exposure of v-Py–PMO to trifluoroacetic acid (TFA)
vapor resulted in a significant increase in the fluorescence band
at 470 nm and a decrease in the fluorescence band at 390 nm
(
Fig. 3a: red), which indicates that most of the pyridine groups
are protonated, because the fluorescence bands at 390 and
4
70 nm are assignable to a pristine and protonated pyridine
1
6
species.
The protonated v-Py–PMO was subsequently
) vapor, which mostly returned
reacted with ammonia (NH
3
the fluorescence spectrum to the original. This indicates that
the pyridine groups in v-Py–PMO are reversibly protonated
and deprotonated, although a small shoulder remained at
470 nm due to the protonated species. The absorption edge
of v-Py–PMO was also reversibly red-shifted for protonation.
Fig.
2 (a–c) Transmission electron micrographs with different
magnification and (d) structural model of v-Py–PMO.
8
164 Chem. Commun., 2010, 46, 8163–8165
This journal is c The Royal Society of Chemistry 2010

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In conclusion, a new PMO with densely packed pyridine
units and a crystal-like molecular-scale periodicity in the
framework was successfully synthesized. The pyridine units
in the PMO framework were revealed to be chemically active
2
+
and primarily accessible for reversible protonation and Cu
adsorption. The pyridine group-bearing PMO has potential
applications as a solid base catalyst, metal catalyst support,
and an adsorbent for metal ions and acidic organic
compounds.
We thank Ms Keiko Fukumoto and Ms Chihiro Fujisawa
for the NOE measurements.
Notes and references
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9
2
+
2+
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(
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2+
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8
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ꢀ
1
2+
2+
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.
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This journal is c The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 8163–8165 8165