A one-step co-condensation method for the synthesis of well-defined functionalized mesoporous SBA-15 using trimethallylsilanes as organosilane sources

Article abstract of DOI:10.1039/c5cc07286g

A new method for the preparation of well-defined functionalized mesoporous SBA-15 has been developed by a one-step co-condensation method using trimethallylsilanes as organosilane sources. This new method enables the incorporation of various bulky organic functional groups with long alkyl chain tethers into the mesoporous silica network.

Full text of DOI:10.1039/c5cc07286g

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A one-step co-condensation method for the
synthesis of well-defined functionalized mesoporous
SBA-15 using trimethallylsilanes as organosilane
sources†
Cite this:DOI: 10.1039/c5cc07286g
Received 31st August 2015,
Accepted 29th September 2015
Ye Ri Han, Jung-Woo Park, Hanil Kim, Hyejeong Ji, Soo Hyun Lim and Chul-Ho Jun*
DOI: 10.1039/c5cc07286g
www.rsc.org/chemcomm
A new method for the preparation of well-defined functionalized processes, and their intrinsic lability under acidic conditions,
mesoporous SBA-15 has been developed by a one-step co-condensation trimethallylsilanes (1) serve as ideal precursors of functionalized
method using trimethallylsilanes as organosilane sources. This new mesoporous silicas (SBA-15s).9,10 In the studies described below,
method enables the incorporation of various bulky organic functional we developed a new co-condensation method using trimethallyl-
groups with long alkyl chain tethers into the mesoporous silica network. silanes containing bulky organic functional groups linked
through long tethers (Fig. 1). A notable feature of this approach
Fabrication of high performance, organo-functionalized meso- is its ability to produce high performance functionalized SBA-15s
porous silicas has received considerable attention owing to the through a direct, one-pot co-condensation protocol. Specifically, we
high recyclabilities and enhanced stabilities of these materials.1–4 observed that the reaction of properly structured trimethallylsilanes
Among many approaches devised for this purpose, co-condensation with TEOS under acidic conditions generates long alkyl chain-
methods employing alkoxysilanes have been most often used tethered functionalized mesoporous silicas that are otherwise
to introduce organic functional groups into silica networks. difficult to prepare. Finally, in this effort we employed the
Advantageous features of the functionalized silicas generated functionalized, organic and organometallic group-impregnated
in this manner include high loading efficiencies and homo- SBA-15s as recyclable catalysts to promote highly efficient reactions.
geneous surface coverage.5 However, alkoxysilanes, commonly
The first phase of this investigation was designed to demonstrate
used in this technique, are difficult to functionalize and purify.1–6 the practical utility of a new, one step method for synthesizing
As a consequence, applicable substrates employed in this approach functionalized SBA-15s. As a model trimethallylsilane substrate
are limited to distillable short chain alkoxysilanes containing a
small range of functionalities.2–4 And often inefficient, multi-
step functional group transformations must be performed on
the mesoporous materials to generate desired functional meso-
porous silica materials. Another formidable challenge when
alkoxysilane-based protocols are utilized is the difficulty in
preparing materials that contain functional groups attached to
the silica surface through long tethers. This is an important
feature because long tethers enable ideal interactions to take
place between the immobilized catalytic moieties with substrates,
a requirement for high catalytic performance.7
In earlier studies, we developed an efficient grafting method,
which utilizes methallylsilanes as alkoxysilane surrogates, for
installation of organic functional groups on the silica surface.7b,8
Considering their unique properties, including high stability
in functional group transformation reactions and purification
Department of Chemistry, Yonsei University, 50 Yonsei-ro, Seodaemun-gu,
Seoul 120-749, Republic of Korea. E-mail: junch@yonsei.ac.kr
† Electronic supplementary information (ESI) available: Experimental details and
Fig. 1 Advantage of one-step synthesis of functionalized SBA-15 using
characterization of new materials. See DOI: 10.1039/c5cc07286g
functional organotrimethallylsilanes with an undecyl tether.
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for this process, we selected 4-(pyren-1-yl)-1-(11-(tris(2-methylallyl)-
Thus, the amount of pyrenyl groups on the functionalized
silyl)undecyl)-1H-1,2,3-triazole (1a), which contains pyrenyl mesoporous SBA-15 can be controlled by simply regulating the
groups tethered through a long alkyl chain to the methallylsilane ratio of TEOS and 1a. TEM (Fig. 2c) and SAXS (Fig. 2d) of
moiety. We reasoned that successful incorporation of pyrenyl the Pyr@Si(15%) gave data that correspond directly to those of the
groups on the silica surface would be easily demonstrated original SBA-15.5b The 13C CP-MAS solid-state NMR spectrum of
by using fluorescence techniques. The desired silane 1a was Pyr@Si(15%) clearly displays peaks that are representative of the
prepared by using the Cu(I)-promoted [3+2]-cycloaddition reaction11 pyrene-triazolylundecyl group (see ESI,† Fig. S2).
of 1-ethynylpyrene with 11-azidoundecyltrimethallylsilane, and
We also determined the fluorescence properties of the Pyr@Sis
purified by using silica-gel chromatography. The modified containing different pyrene loading levels (Fig. 2e). The emission
procedure described by Gupta and coworkers was applied in spectrum of the low pyrene group-embedded Pyr@Si(1%) contains
this co-condensation method.5c The reaction of TEOS (tetraethyl maxima at 398 nm with a weak peak at 485 nm. In contrast,
orthosilicate) with 1a (TEOS/1a) in the presence of aqueous HCl Pyr@Sis containing greater than 5% loading have fluorescence
and P123 followed by solvent extraction gave the pyrene func- spectra in which the band at 485 nm becomes more intense
tionalized SBA-15, Pyr@Sis. Under the reaction conditions, the relative to the 398 nm band in a manner that is dependent on
trimethallylsilane moiety is completely consumed, as determined the %-loading. These observations are consistent with the
1
by using H NMR spectroscopy.12 The one-pot co-condensation assignment of the maximum at 485 nm to emission from
process was carried out using different ratios (99/1, 95/5, 90/10 excimers which become more dominant as the surface is more
and 85/15) of TEOS and 1a (Fig. 2a). The loading of pyrene populated by pyrene fluorophores (Fig. 2f). This phenomenon
triazolyl groups on the surfaces of the resulting materials, was also observed earlier by Stack et al.5a,b Therefore, the results
determined by using elemental analysis, was found to increase of this analysis show clearly that the extent of pyrene group
linearly as the amount of 1a used in the reaction increases up loading on the surface of mesoporous silica can be readily
to 15% (0.12 mmol gꢀ1 for 1% 1a; 0.40 mmol gꢀ1 for 5% 1a; determined by using fluorescence spectroscopy.
0.77 mmol gꢀ1 for 10% 1a; 1.0 mmol gꢀ1 for 15% 1a, Fig. 2b).
The one-step co-condensation reaction was performed using
the 4-(dimethylamino)azobenzene-40-sulfonyl (dabsyl) group
containing trimethallylsilane 1b (Fig. 3a). Reactions utilizing
various ratios of TEOS and 1b lead to the formation of the
dabsyl group linked SBA-15s with loading levels in the range of
0.11–0.78 mmol gꢀ1. The loading amount on SBA-15 was readily
determined by using elemental analysis and measuring the
intensity of the red color associated with the dabsyl group
(Fig. 3b and c). Noteworthily, the analysis of TEM images shows
that the mesoporous structures of Dabsyl@Si are retained up to
15% loading of dabsyl group (see ESI†).
Fig. 2 (a) Pyrenetriazolyl functionalized SBA-15s (Pyr@Sis) prepared using
different ratios of TEOS and 1a (99/1, 95/5, 90/10, and 85/15) using the one-
step co-condensation method. (b) Loading of pyrene triazolyl groups in each
Pyr@Si. (c) TEM image of Pyr@Si(15%), (d) SAXS of Pyr@Si(15%), (e) fluorescence
spectra of Pyr@Si(1%), Pyr@Si(5%), Pyr@Si(10%) and Pyr@Si(15%) (normalized
at 414 nm). (f) Illustrational rationale for the excimer peak of 485 nm.
Fig. 3 (a) Preparation of Dabsyl@Sis by using different ratios of TEOS
and 1b (99/1, 95/5, 90/10, 85/15) utilizing the co-condensation method.
(b) Intensity change of the red color and (c) loading rate of the dabsyl group in
Dabsyl@Si(1%), Dabsyl@Si(5%), Dabsyl@Si(10%) and Dabsyl@Si(15%).
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The SBA-15s, prepared by using the approach described
Communication
above, contain functional groups that are tethered to the silica
surface through long alkyl chains. Consequently, it is anti-
cipated that these functionalized mesoporous silicas will display
much higher catalytic activities than conventional functionalized
SBA-15 with short tethers owing to the fact that the pendant
functional groups can better interact with substrates.7 To evaluate
this hypothesis, a study was conducted to probe the use of an amine
containing catalyst of this type to promote a Knoevenagel reaction.
For this purpose, we prepared primary amine-functionalized
SBA-15s from the corresponding azide-functionalized silicas,
Pr-N3@Si and Und-N3@Si, which were synthesized by using
one-pot co-condensation reactions of the respective 3-azido-
propyltrimethallylsilane (1c) and 11-azidoundecyltrimethallylsilane
(1d) with TEOS (TEOS/1c, TEOS/1d = 90/10) (Fig. 4a). The amine
functionalized SBA-15s, Pr-NH2@Si and Und-NH2@Si were then
generated from the corresponding azides through Staudinger
reactions,13 involving treatment with a mixture of PPh3 and
water. The success of these processes was demonstrated by
analysis of the IR spectra of the products, which showed that
peaks at 2110 cmꢀ1 corresponding to the azide moiety were
absent (see ESI,† Fig. S3).
Fig. 5 (a) One-step synthesis of imidazolium salt functionalized SBA-15
(Imid@Si) and preparation of Pd-NHC@Si. (b) TEM image of Imid@Si.
(c) Suzuki reaction using Pd-NHC@Si as a catalyst and isolated yields of
recycled catalytic reactions.
The amine-functionalized silicas, Pr-NH2@Si and Und-NH2@Si,
were tested as catalysts for the Knoevenagel reaction of malono-
nitrile (2) with benzaldehyde (3) (Fig. 4b).14 Reaction between 2
This finding suggests that interactions between the catalytic
and 3 carried out using Und-NH2@Si as a catalyst at ambient groups in Und-NH2@Si and substrates are superior to those
temperature for 2 h resulted in the formation of 2-benzylidene- occurring with Pr-NH2@Si.
malononitrile (4) in a quantitative yield. Under identical conditions,
A new approach was also employed to prepare a silica based
the condensation reaction promoted by Pr-NH2@Si required 8 h catalytic system containing a supported transition metal.15
to complete (Fig. 4c). The results show that Und-NH2@Si, in Because imidazolium groups are readily transformed into
which the amine group is tethered to the silica surface by a 4-imidazoline (NHC) ligands for Pd when treated with Pd(OAc)2,16
longer alkyl chain, is a superior catalyst for the Knoevenagel an imidazolium group containing trimethallylsilane 1e was selected
reaction.
as the reagent for the preparation of the modified mesoporous
silica. Trimethallylsilane derivative 1e was prepared by the reaction
of 11-chloroundecyltrimethallylsilane with 1-mesityl-1H-imidazole
at 120 1C for 24 h.17 Co-condensation of TEOS with 1e (TEOS/1e =
90/10) then generated the imidazolium-bound SBA-15, Imid@Si
(1.13 mmol gꢀ1 loading, Fig. 5a). A TEM image of the modified
silica shows that it contains a uniform distribution of mesopores
(Fig. 5b, pore size: 6.7 nm), and inspection of its 13C CP-MAS
NMR spectrum shows that it contains surface imidazolium
groups (see ESI,† Fig. S4). In addition, the solid state 29Si NMR
spectra of Imid@Si shows Q3 (single silanol), Q4 (silicate) and T3
[R(SiO)3Si] signals, which are consistent with previously reported
data of functionalized SBA-15 prepared by the classical alkoxy-
silane protocol (see Fig. S5 in ESI†).5c
The Pd–NHC complex-tethered SBA-15, Pd-NHC@Si, was
prepared by first blocking residual surface silanol groups in
Imid@Si by treatment with trimethylchlorosilane (TMCS) and
then carrying out the reaction with Pd(OAc)2 in toluene at
reflux.18 The loading of Pd in the resulting Pd-NHC@Si was
determined to be 2.9% by using ICP-MS. In addition, the
presence of the transition metal in its Pd2+ state was demon-
strated by analysis of the XPS spectrum (see ESI†), which
contains a Pd(3d) peak at 337 eV that is identical to the
previously reported value for the Pd2+ state.18
Fig. 4 (a) Synthesis of amine functionalized SBA-15s (Pr-NH2@Si
and Und-NH2@Si) through the Staudinger reaction of Pr-N3@Si and
Und-N3@Si with PPh3 and H2O. (b) Knoevenagel reactions of malononitrile
and benzaldehyde using Pr-NH2@Si and Und-NH2@Si as catalysts. (c) GC
yield (%) of products 4 versus reaction time.
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metal remains in the Pd2+ state.
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In the study described above, we have devised a new method
for the preparation of functionalized mesoporous silicas that
take advantage of the characteristic properties of methallyl-
silanes, notably their high stabilities during functional group
transformations and purification, and their labilities under
acidic conditions. These unique properties have been used to
develop a method for the one-step synthesis of high-performance,
functionalized mesoporous silicas from appropriately modified
organo-trimethallylsilanes that contain functional groups tethered
through long alkyl chains. Employing this method, a variety of
bulky functional groups such as a fluorophore, chromophore,
and even an ionic imidazolium moiety for recycling transition
metal catalysts can be incorporated into mesoporous silica
networks with loading extents of up to 15% of the organosilane
(ca. 1.0 mmol gꢀ1). The new method developed in this effort
overcomes the limitations of the conventional alkoxysilane
approach and the accompanying requirement for inefficient
multistep, solid-phase transformations.
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This work was supported by a grant from the National
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silica surface. For comparison, when the Pd-adsorbed silica with no
NHC ligand, prepared by treatment of SBA-15 with TMCS and Pd(OAc)2,
was applied for the recycling experiment, no reaction occurred after the
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Suzuki coupling product was obtained in the first run).
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