Synthesis and characterization of highly pure azido-functionalized polyhedral oligomeric silsesquioxanes (POSS)

Article abstract of DOI:10.1039/b909802j

Novel azido-functionalized POSS were synthesized, which could serve as nanoprecursors for higher-order molecular construction of organic-inorganic hybrids via"click" chemistry. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b909802j

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Synthesis and characterization of highly pure azido-functionalized
polyhedral oligomeric silsesquioxanes (POSS)w
Vuthichai Ervithayasuporn, Xin Wang and Yusuke Kawakami*
Received (in Cambridge, UK) 18th May 2009, Accepted 29th June 2009
First published as an Advance Article on the web 20th July 2009
DOI: 10.1039/b909802j
Novel azido-functionalized POSS were synthesized, which could
serve as nanoprecursors for higher-order molecular construction
of organic–inorganic hybrids via ‘‘click’’ chemistry.
methyldichlorosilane (Scheme 1). The azidation at room
temperature to 45 1C proceeded very slowly and it was difficult
to obtain a complete conversion. Moderately high temperature
(about 70 1C) and less than three times excess of NaN3 in
DMF–THF mixed solvent (7 : 2) were key to achieve a high
yield of 3 by clean reaction. Conversion of 2 was accelerated in
the presence of THF probably because of improved solubility
of the starting material. When the temperature was raised
above 80 1C, materials was observed to decompose, and to
change into higher molecular weight compounds.
Since Huisgen reported that the 1,3-dipolar cycloaddition
reaction between azide and terminal or internal alkynes can
smoothly form a stable 1,2,3-triazole linkage in high yield
and purity, many reports have widely used this reaction for
constructing diverse molecular structures.1
Meanwhile, much attention has been given to polyhedral
oligomeric silsesquioxanes (POSS), especially to octahedral
octasilsesquioxanes (T8), having an octacyclic inorganic silicon
and oxygen framework covered externally by various organic
substituents.2 The octasilsesquioxanes have shown a great
potential for many applications such as electronics,3 optics,4
surface-modified supports,5 catalyst,6 and OLEDs.7
Almost quantitative azidation of 2 to 3 (93% yield) could be
1
achieved within 24 h, as confirmed by H NMR by the shift
from 3.53, 1.82 and 0.75 (3-chloropropyl) to 3.27, 1.74 and
0.69 (3-azidopropyl) ppm. Conversion of 5 to 6 was similarly
carried out to give almost quantitative yield. These reactions
smoothly proceeded without any cage rearrangement.
Novel precursor 5 exists as a mixture of cis and trans
isomers, as indicated by 29Si NMR at ꢀ18.35, ꢀ78.51,
ꢀ79.46 ppm (trans), and ꢀ18.35, ꢀ78.51, ꢀ79.40, ꢀ79.49 ppm
(cis) isomers. So far this mixture was unable to be seperated
by column chromatography. Accordingly, azidation of 5
gave an isomeric mixture of 6. Careful recrystallization
of 6 using THF–methanol (1 : 1) gave pure crystalline trans-
isomer, the structure of which was confirmed by single-crystal
X-ray structural analysis (Fig. 1).z
To further widen aspects of the application of POSS
chemistry it is very important to utilize new types of reactions.
Complete functionalization with novel reactive groups, thus
providing the opportunity to design and build materials with
well-defined dimensions possessing nanophase behaviour is a
case in point. Introduction of substituents to T8 has been
mostly achieved by hydrosilation,8 Heck,9 and cross-metathesis10
reactions. We have been working on the azide function to
combine organic substituents and the POSS core, but only a
few reports have attempted to introduce the azide function to
POSS derivatives.11 Further, some important aspects of the
reaction were not well-established.
Meanwhile, we noticed extensive rearrangement of the T8
cage under azidation condition of octakis(3-chloropropyl)-
silsesquioxane (7)12 to produce
a
thermodynamically
stable mixture of octakis(3-azidopropyl)silsesquioxane (8),
deca(3-azidopropyl)silsesquioxane (9), and dodeca(3-azido-
propyl)silsesquioxane (10) (Scheme 2), probably because of
the necessary use of pure DMF of higher polarity as the
solvent under severer reaction condition and longer reaction
time to achieve a complete conversion of eight functional
groups.
We herein report on the clean and quantitative introduction
of azide groups from a newly synthesized hepta(isobutyl)-
substituted-T8 (2) and double-decker silsesquioxane (5) to
provide 3 and 6 as mono- and diazido-functionalized POSS,
respectively. Moreover, the independent report of cage
rearrangement of the T8 structure (7) under azidation reaction
toward the multi azido-functionalized T8 (8), T10 (9) and T12
(10) POSS is presented for the first time.
This phenomenon strongly suggested that the Si–O–Si
bonds in the T8 silsesquioxane cores were cleaved by strongly
nucleophilic N3ꢀ, resulting in cage opening and silsesquioxane
fragmentation. Reassembly of the T8 cage and other size-cage
silsesquioxanes has been found to occur under certain reaction
conditions.2n Herein, we first report that sodium azide may
play a dual role as a supplier of azide groups and as a
nucleophile to induce cage rearrangement. Less steric hindrance,
compared with other substituent groups (isobutyl or phenyl),
on the silsesquioxane core of highly reactive 3-chloropropyl
groups led to cage cleavage and rearrangement in the presence
of sodium azide.
The novel precursors 2 and 5 were prepared following the
reaction of known hepta(isobutyl)-T7-trisilanol (1)2f with
3-chloropropyltrichlorosilane and double-decker octasilses-
quioxane-tetraol tetrasodium salt (4)2m with (3-chloropropyl)-
School of Materials Science, Japan Advanced Institute of Science and
Technology (JAIST), 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan.
E-mail: kawakami@jaist.ac.jp; Fax: +81-761-51-1635;
Tel: +81-761-51-1630
w Electronic supplementary information (ESI) available: Detailed
experimental procedures, 1H, 13C, 29Si NMR, Maldi-TOF MS,
elemental analysis, and analytical data for all new compounds.
CCDC 731874. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b909802j
Careful separation using 1 g of sample by use of column
chromatography (run through twice), led not only to 8 but
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Scheme 1 Introduction of mono- and diazide functions to T8 and double-decker silsesquioxane cages. (a) Conversion of 2 to 3 in DMF–THF
(7 : 2), 0.025 mol Lꢀ1 for functional group with 3 eq. NaN3 at 70 1C for 24 h; (b) conversion of 5 to 6 in DMF–THF (5 : 2), 0.020 mol Lꢀ1 with 3 eq.
NaN3 at 60 1C for 24 h.
and could not be separated. A second separation using ethyl
acetate–hexane (2 : 9) gave 9 (Rf = 0.35, 0.45 g), and 10
(Rf = 0.30, 0.18 g) as pure compounds. 29Si NMR of the cage
8 (T8) and 9 (T10) showed singlets at ꢀ67.04 and ꢀ68.94 ppm,
respectively. The signal of 9 (T10) consisting of eight- and ten-
membered rings resulted in an upfield shift about ꢀ1.90 ppm
from that of 8. The 29Si chemical shifts of 10 (T12) appeared
at ꢀ68.69 and ꢀ71.36 ppm with an integral ratio of 1 : 2
corresponding the presence of two different types of silicon
atom in the cage of D2d symmetry. Four 10-membered rings
were obviously connected and rolled up to each other by
sharing two disiloxane linkages (in total eight silicon atoms),
leaving one silicon atom in each ring unconnected (four Si
atoms). Upon connection of a pair of silicon atoms via a
siloxane linkage, two eight-membered rings would be
formed (so in total four eight-membered rings).13 Such cage
rearrangement was reported by Rikowski and Marsmann.14
Ge et al. apparently reported the azidation reaction of 7
under more rigorous conditions, which might lead to various
products.11c
Fig. 1 X-Ray crystallographic structure of trans-6.
In conclusion, novel mono- and diazido-functionalized POSS
also 9 and 10 being isolated in pure form. The first separation
using ethyl acetate–hexane (2 : 5) as eluent could isolate only 8
(Rf = 0.60, 0.25 g, the actual yield of 8 is probably a little
higher) in pure form; 9 and 10 gave very close Rf values (0.55),
precursors (3 and 6) were successfully synthesized, as well as a
cage rearrangement for the less sterically hindered T8 cage (7) to
T8 (8), T10 (9), and T12 (10) cages. Functionalization of POSS in
the presence of sodium azide is thus demonstrated.
Scheme 2 Cage rearrangement of T8 cage 7 upon full introduction of azide functions leading to 8, 9 and 10; DMF, 0.077 mol Lꢀ1, 1.5 eq. NaN3,
at 60 1C for 16 h.
ꢁc
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Chem. Commun., 2009, 5130–5132 | 5131

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We are very grateful to Dr Yuriko Kakihana for discussions,
Dr Motoo Shiro for single-crystal X-ray analysis, and the
Ministry of Education, Culture, Sports, Science, and
Technology (MEXT), Japanese Government for scholarships
to V. Ervithayasuporn and X. Wang.
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