Spherosilicates with peripheral malonic acid and vinyl end groups

Article abstract of DOI:10.1039/c3cc40533h

Two novel spherosilicates comprised of an octahedral Si₈O ₁₂ core, [Si₈O₁₂]-(OSiMe₂CH ₂CH₂CH₂CH(COOH)₂)₈ and [Si₈O₁₂]-(OSiMe₂CH₂CH ₂CH₂CH(COOSi(CHCH₂)₃) ₂)₈, were synthesized from [Si₈O ₁₂]-(OSiMe₂H)₈. These new structures have high densities of peripheral functional groups, and the second structure also possesses silyl ester bonds that are easily cleavable under mild conditions. These functionalities enable these structures to be modified further and to have many potential applications. We demonstrated one by cross-linking the vinyl-terminated spherosilicate to form nanospheres with a narrow size distribution and utilizing the hydrophilic interior to accommodate a Pd salt in a toluene solution. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc40533h

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Spherosilicates with peripheral malonic acid and vinyl
end groups†
Cite this: Chem. Commun., 2013,
49, 3357
Zhongliang Shen,a Jongsik Kim,z Jingmei Shen,a Christopher M. Downing,b
a
Received 21st January 2013,
Accepted 2nd March 2013
Sungsik Lee,c Harold H. Kunga and Mayfair C. Kung*a
DOI: 10.1039/c3cc40533h
www.rsc.org/chemcomm
Two novel spherosilicates comprised of an octahedral Si8O12 core, low yield processes,15a–c and there is no commercially available
[Si8O12]-(OSiMe2CH2CH2CH2CH(COOH)2)8 and [Si8O12]-(OSiMe2CH2- [SiO1.5R]8 where R contains a malonic acid. Here we report a simple,
Q
CH2CH2CH(COOSi(CH CH2)3)2)8, were synthesized from [Si8O12]- high-yield procedure to synthesize spherosilicate with 8 malonic acid
(OSiMe2H)8. These new structures have high densities of peripheral groups [Si8O12]-(OSiMe2CH2CH2CH2CH(COOH)2)8 IV. In addition,
functional groups, and the second structure also possesses silyl we describe a one-step conversion of this compound into a dendri-
ester bonds that are easily cleavable under mild conditions. These mer [Si8O12]-(OSiMe2CH2CH2CH2CH(COOSi(CHQCH2)3)2)8 V with
functionalities enable these structures to be modified further and to 48 peripheral vinyl groups and labile silyl ester bonds. The structure
have many potential applications. We demonstrated one by cross- of V permits intramolecular cross-linking of the vinyl groups to form
linking the vinyl-terminated spherosilicate to form nanospheres with a core–shell nanosphere, or intermolecular linking to form organic–
a narrow size distribution and utilizing the hydrophilic interior to inorganic hybrid polymers. The silyl ester bonds can be cleaved by
accommodate a Pd salt in a toluene solution.
hydrolysis to detach the [Si8O12] core from the rest of the structure.
Other templates with cleavable bonds have been used for molecular
Functionalized polyhedral oligosilsesquioxanes (POSS) form a imprinting18,19 and drug delivery.20
versatile platform to build macromolecules for various potential
The syntheses of IV and V are shown in Scheme 1. One of the
applications.1 For example, they have been designed to anchor starting compounds was dibenzyl allylmalonate I,21 which was
fluorescent emitter molecules,2 for capping noble metal nano- synthesized by condensation of allylmalonic acid and benzyl alcohol
particles,3 as building blocks for amphiphilic,4 porous,5 or thermoset (BzOH) with a catalytic amount of H2SO4 and with constant removal
polymers,6 and for potential biocidal activities.7 To date, various of H2O (eqn (1), its structure was confirmed by NMR and
[SiO1.5R]8 molecules have been reported with chemically stable, elemental analysis, see ESI†). The other starting compound
functionalized R groups at the Si corners, where R is alkyl for POSS
and alkoxy or siloxy for spherosilicates. Examples of functionality at
the R groups include methacrylates,8 epoxides,9 alcohols,10 amino-
phenyl,11 isocyanate,12 azide,13 ferrocenyl,14 and carboxylic acid/
carboxylate.15 The polyacids are interesting due to their dual ability
to donate and accept hydrogen in hydrogen bonding interactions,
and are particularly useful in crystal engineering16 and as building
blocks in supramolecular structures.17 Yet, the few reports on the
synthesis of octacarboxy POSS or spherosilicate involve multi-step,
a Department of Chemical & Biological Engineering, Northwestern University, 2145
Sheridan Road, Evanston, Illinois 60208, USA. E-mail: m-kung@northwestern.edu;
Fax: +1-8474671018; Tel: +1-8474917620
b Department of Chemistry, Northwestern University, 2145 Sheridan Road,
Evanston, Illinois 60208, USA
c X-ray Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
† Electronic supplementary information (ESI) available: Synthesis, NMR, MS, IR,
XRD of the compounds; DLS, SAXS, GPC of the core–shell nanosphere. See DOI:
10.1039/c3cc40533h
‡ Current address: Department of Chemistry, Dong-A University, Busan, 604-714,
Scheme 1 Synthesis of carboxyl and vinyl functionalized spherosilicates.
Chem. Commun., 2013, 49, 3357--3359 3357
Korea.
c
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was the commercially available 1,3,5,7,9,11,13,15-octakis- the absence of moisture, V was stable upon storage at room
(dimethylsiloxy)pentacyclo-[9.5.1.13,9.15,15.17,13]octasiloxane [Si8O12]- temperature for at least a month. Furthermore, no degradation
(OSiMe2H)8 II. Hydrosilylation of I using II generated [Si8O12]- was observed in dry toluene at 100 1C after 21 h as indicated by
(OSiMe2CH2CH2CH2CH(COOBz)2)8 (III, step 1 in Scheme 1). III 1H NMR (Fig. S6, ESI†).
1
showed a characteristic H NMR resonance at d 5.17 (multiplets)
A potential alternate route to V was the dehydrosilylation
from the methylene protons of the Bz group (Fig. S2a in ESI†). A reaction between IV and a silane (trivinylsilane), using a Pd or
peak in MALDI-TOF analysis of m/z = 3648 (100%, III + K+) was Ru catalyst (eqn (2)).25 We tested its feasibility with a model
consistent with this structure. The Bz protection group was removed reaction of allyl malonic acid with trivinylsilane. Unfortunately,
to form [Si8O12]-(OSiMe2CH2CH2CH2CH(COOH)2)8 IV by hydro- the undesirable side reaction of partial hydrogenation of vinyl
genolysis at room temperature in the presence of a solid Pd/C to ethyl groups was observed. Attempts to suppress the hydro-
catalyst (step 2 in Scheme 1).22 The deprotection reaction could genation by adding 1-pentyne into the reaction mixture to
be followed by the disappearance of the characteristic proton compete for the hydrogenation site failed, although 1-pentyne
resonance of III at d 5.17, and a quantitative yield of IV was was more prone to hydrogenation than methyltrivinylsilane
achieved. The only byproduct, toluene, was removed readily under similar conditions when H2 was used. Thus, the metal
upon evacuation. To our knowledge, IV is the first example of hydride generated from oxidative addition of silane, instead of
a malonic acid-functionalized spherosilicate. It contains 16 H2, was the hydrogenation agent in the model reaction. The
carboxyl groups on the periphery, and is a white crystalline solid following mechanism was likely operating: oxidative addition
from XRD (Fig. S3a, ESI†).15d A peak in the ESI-MS analysis of of (CH2QCH)3Si–H to Pd generated Pd(H)(Si(CH2QCH)3),
m/z = 1084 (negative ion scan, (IV ꢀ 2H+)2ꢀ) was consistent with which inserted rapidly to form a four-membered palladacycle.
1
this structure in solution. The H NMR spectrum of IV in dry This intermediate reacted with allyl malonic acid to form the
DMSO-d6 contained a sharp carboxylic acid proton resonance at ester with one vinyl group hydrogenated. This phenomenon
d 12.6 (Fig. S3b, ESI†); this proton underwent fast exchange on where a vinyl group is partially hydrogenated by silane was also
the NMR time scale with those from H2O if the latter was present observed by Mizuno and coworkers.25b
in the solvent (Fig. S3c, ESI†). The IR spectrum of IV dried onto
KBr showed a split carbonyl band at 1721 and 1712 cmꢀ1
(2)
(Fig. S3e, ESI†), whereas only a single carbonyl peak at 1700 cmꢀ1
was observable for malonic acid deposited on KBr. Splitting of the
The dense peripheral vinyl groups in V can be used for
carboxylate band has been observed with diacids,23 and attributed further derivatization for different applications. Here we show
to Fermi resonance from accidental degeneracy of a fundamental an example of cross-linking the vinyl groups with 1,4-bis-
and a higher ordered vibrational transition.24
dimethylsilylbenzene via a hydrosilylation reaction to form a
soluble nanosphere VI with a spherosilicate core and carbo-
silane shell (Scheme 2). The linker was chosen for its size and
rigidity so as to minimize the linkage of vinyl groups at the
same corner, therefore maximizing the degree of cross-linking
of vinyl groups at different corners to form an intact 3-dimen-
(1)
A
pure silyl ester compound [Si8O12]-(OSiMe2CH2CH2- sional shell. The cross-linking reaction was monitored using
CH2CH(COOSi(CHQCH2)3)2)8 V was prepared from IV by reaction 1H NMR spectroscopy. The disappearance and broadening of
with chlorotrivinylsilane in dry diethyl ether in the presence of multiplets at d 6.1 (vinyl region of V) was accompanied by the
diethyl amine. The latter scavenged the HCl generated from the appearance of a broad peak at d 7.6 (aromatic H from the
reaction (step 3, Scheme 1) to form the only byproduct, diethyl- reacted linker, Fig. S7a, ESI†). Data from various runs showed
ammonium chloride, which was insoluble in ether. Thus, V was that the reactions stopped at Iaromatic/Ivinyl = 4–4.5 (I = integrated
obtained by cannula filtration, followed by the removal of solvent area), indicating that B85% of the vinyl groups were con-
and excess chlorotrivinylsilane by evacuation. We also attempted sumed. Increasing reaction time or adding fresh catalysts did
using pyridine as the base and tetrahydrofuran (THF) as the not change the degree of cross-linking. Thus, the unreacted
solvent, but the small quantity of pyridinium chloride dissolved vinyl groups were likely too sterically hindered to undergo
in THF caused degradation upon storage of not only the silyl ester hydrosilylation. There were four major peaks in the 29Si NMR
bonds but also the siloxane bonds. V is a viscous oil. As expected, spectrum of VI (Fig. S7b, ESI†), which were consistent with the
the 1H NMR spectrum of V did not show any signal of the cross-linked structure in Scheme 2.
carboxylic proton, and the 29Si NMR spectrum only contained
Characterization of VI using dynamic light scattering (DLS),
three peaks for the Si in three different environments (Fig. S4a small angle X-ray scattering (SAXS), and gel permeation chroma-
and b, ESI†), indicating complete conversion of –COOH to tography (GPC) further supported a discrete core–shell structure
–COOSi(CHQCH2)3. The ESI-MS signal of the compound at in solution.26 First, the hydrodynamic size of VI as determined
+ 2+
m/z = 1969.5 was consistent with structure V (for (V + 2 NH4 ) , by DLS was between 6.4 and 7.4 nm (Fig. S8, ESI†). Second, SAXS
Fig. S5, ESI†). From MM2 calculation, it was found that this results showed that at a concentration of 1 mg mlꢀ1 the radius of
molecule was slightly ellipsoidal, with the widest span of gyration was 2.1 nm, and this number increased to 4.9 nm and
ca. 2.6 nm and the narrowest span of ca. 2.0 nm. The presence 5.0 nm at 2.5 mg mlꢀ1 and 5.0 mg mlꢀ1, respectively (Fig. S9,
of the silyl ester bond in V made it moisture sensitive. However, in ESI†), possibly due to aggregation of nanospheres at higher
c
3358 Chem. Commun., 2013, 49, 3357--3359
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Notes and references
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observed that a toluene solution containing VI and solid PdCl2
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colorless (Fig. S11, ESI†), suggesting transfer of Pd ions into VI.
Two novel spherosilicate structures were synthesized,
[Si8O12]-(OSiMe2CH2CH2CH2CH(COOH)2)8 and [Si8O12]-(OSiMe2-
CH2CH2CH2CH(COOSi(CHQCH2)3)2)8. The first structure, with
16 peripheral carboxyl functionalities, can be used for metal
binding, crystal engineering etc. In this work, it was used to
generate the second compound, which had 16 cleavable silyl ester
linkages and 48 peripheral vinyl groups. It was demonstrated that
the vinyl groups can be cross-linked by a rigid bidentate linker to
from a soluble nanosphere with a narrow size distribution. This
nanosphere possesses silyl ester bonds that are easily cleavable
under mild conditions to form carboxylic acid–silanol pairs.
Potential applications of such a structure include binding of
metals and other molecules for delivery and catalysis, applications
that are being investigated.
This report is based on work supported as part of the Institute
for Atom-efficient Chemical Transformations (IACT), an Energy
Frontier Research Center funded by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences. We
would also like to thank Andrea Luthi from Professor Chad
Mirkin’s group for assistance with DLS, and Archana Krovi from
Professor SonBinh Nguyen’s group for assistance with GPC.
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c
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Chem. Commun., 2013, 49, 3357--3359 3359