Novel polysilanols by selective functionalizations of oligosilanes

Article abstract of DOI:10.1002/anie.200460874

As part of a succinct synthesis, a new functionalization method leads to hitherto unknown polysilanols with an oligosilane backbone, a class of compounds with remarkable electronic properties. The trifluoroacetolysis of phenyl-substituted hydroxyoligosila

Full text of DOI:10.1002/anie.200460874

Communications
Synthetic Methods
Novel Polysilanols by Selective Functionalizations
of Oligosilanes
Clemens Krempner,* Jürgen Kopf, Constantin Mamat,
Helmut Reinke, and Anke Spannenberg
Dedicated to Professor H.Oehme
on the occasion of his 65th birthday
Stable polysilanols constructed of oligosilane frameworks,
which can be conceived as silicon analogues of polyalcohols,
are hitherto unknown. Due to the comparatively high acid-
ities and basicities of the SiOH functionalities[1] in these
derivatives, unique supramolecular arrangements stabilized
by hydrogen-bonding interactions can be expected.[2] In
addition, such polysilanols might be promising precursors
for the synthesis of M-O-Si-Si-containing materials[3] with
exceptional optoelectronic properties arising from the
extended delocalization of s electrons along the oligosilane
backbone (s-conjugation).[4] Based on a novel synthetic
concept for the selective functionalization of oligosilanes,
we report herein the first synthesis of a stable 2,3,4,5-
tetrahydroxyhexasilane and its supramolecular organization
through hydrogen bonding into a 21-membered macrocycle.
The starting point of our investigations was the oligosi-
lanes 2a,b, which could be produced in high yields by
treatment of Ph(Me3Si)2Si-K·3THF (1) with MeSiCl3 and
MeCl2Si-SiMeCl2, respectively (Scheme 1).[5] Compounds
2a,b possess nucleophilic, easily exchangeable chloro func-
tionalities and two phenyl groups attached to silicon, which
can normally be modified by protodesilylation reactions with
strong acids. However, all attempts to cleave the Si–phenyl
bond selectively by treatment of 2a,b with trifluoromethane-
sulfonic acid failed. Instead skeletal rearrangements typical
for oligosilanes in the presence of strong acids were
observed.[6]
Scheme 1. Synthesis of 2a,b–5a,b (a: n=1; b: n=2; R=SiMe3).
However, in the presence of the markedly less acidic
trifluoroacetic acid (TFA), which served as both the proto-
desilylation reagent and the solvent, 2a,b proved to be nearly
insoluble, and thus no reaction occurred. In contrast, silanols
3a,b, obtained quantitatively by hydrolysis of 2a,b, are more
soluble and can be transformed by cleavage of the phenyl
groups into the moisture-sensitive trifluoroacetoxyosilanes
4a,b in excellent yields (Scheme 1). This reaction sequence is
unique insofar as apart from smooth protiodesilylation of
3a,b a complete nucleophilic exchange of the OH function-
alities by trifluoroacetate groups also takes place. Moreover,
the trifluoroacetolysis of 3b proceeds diastereoselectively,
giving meso-4b exclusively, regardless whether meso- or rac-
3b was used. The molecular structure of meso-4b was
confirmed unequivocally by X-ray crystallography (Fig-
ure 1).[7a]
Finally, by hydrolysis of 4a,b in the presence of equimolar
amounts of ammonium carbaminate the tri- and tetrasilanoles
5a,b could be isolated in good yields. The tetrasilanol 5b was
[*] Dr. C. Krempner
Department of Chemistry
The Ohio State University
100 West 18th Avenue, Columbus, OH 43210 (USA)
Fax : (+1)614-292-0368
E-mail: ckrempne@chemistry.ohio-state.edu
Dipl.-Chem. C. Mamat, Prof. Dr. H. Reinke
Fachbereich Chemie
Universität Rostock
Einsteinstrasse 3a, 18051 Rostock (Germany)
Prof. Dr. J. Kopf
Institut für Anorganische und Angewandte Chemie
Universität Hamburg
Martin-Luther-King-Platz 6, 20146 Hamburg (Germany)
Dr. A. Spannenberg
Leibniz-Institut für Organische Katalyse
Universität Rostock e.V.
Figure 1. Molecular structure of 4b in the crystal (30% probability
level, H atoms are omitted for clarity and the disordered CF3 groups
are depicted only in one orientation). Selected bond lengths [Š] and
angles [8]:O1-Si2 1.736(4), O2-Si1 1.764(4), Si1-Si2 2.354(2), Si1-Si3
2.354(2), Si1-Si4 2.363(2), Si2-Si2 2.372(3), Si2-Si1-Si3 121.78(8), Si1-
Si2-Si2 117.25(10), C2-O2-Si1 122.7(3), C11-O1-Si2 121.7(4).
Buchbinderstrasse 1–3, 18055, Rostock (Germany)
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200460874
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Angewandte
Chemie
obtained as a diastereomeric mixture (rac/meso = 2:1) as
shown by 1H NMR spectroscopy. Both compounds 5a,b
exhibit broad IR absorptions for the OH groups typical for
silanols. In the solid state they prove to be relatively stable;
only in solution upon gentle heating do they tend to undergo
condensation reactions to form polymeric siloxanes. The
results of an X-ray structure analysis of 5b are in agreement
with the structure proposed and the fact that the racemic form
crystallized from the mixture of diastereomers (Figure 2).[7b]
Figure 3. Ring structure of rac-5b in the crystal (30% probability level,
all methyl groups are omitted for clarity).
In conclusion, trifluoroacetoxyoligosilanes, which are
easily accessible in high yields, have considerable synthetic
potential,[10] as we have demonstrated for the synthesis and
structure of novel tri- and tetrasilanols. Moreover, the
Figure 2. Structure of 5b in the crystal (30% probability level, H atoms
bonded to C atoms are omitted for clarity). Selected bond lengths [Š]
and angles [8]:Si1-O1 1.6837(14), Si2-O2 1.6683(17), Si1-Si2 2.3744(7),
Si2-Si3 2.3727(6), Si3-Si4 2.3785(7), Si4-O4 1.6826(15), Si3-O3
1.6646(16), Si12-Si1-Si2 114.31(3), Si2-Si3-Si4 117.43(3), Si11-Si1-Si2-
Si3 À172.76(3), Si1-Si2-Si3-Si4 161.00(3), Si2-Si3-Si4-Si41 À170.40(3).
trifluoroacetolysis of oligosilanes might be
a valuable
method for the incorporation of functional groups into
structurally more complex oligo- and polysilanes having
different substitution patterns and potentially promising
electronic properties.[4]
Received: June 4, 2004
À
À
The Si Si and Si O bond lengths and the Si-Si-Si angles in
rac-5b show the typical values for oligosilanes. The hexasilane
chain Si11-Si1-Si2-Si3-Si4-Si41 adopts an all-anti conforma-
tion (AAA), which is believed to be optimal for the
delocalization of s electrons.[4] As confirmed by 1H NMR
investigations, differently bonded water molecules are incor-
porated into the crystal; two of them interact with the two
internal OH groups of the tetrasilanol moiety through
hydrogen bonds.
Keywords: hydrogen bonds · polysilanols · silanes · silicon ·
.
structure elucidation
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Remarkably, additional intermolecular hydrogen-bonding
interactions between the terminal OH groups of three
molecules of rac-5b result in the formation of a novel 21-
membered macrocycle (Figure 3). Two molecules of water,
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[2]For structures of tetra- and hexasilanols based on carbosilane
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Angew. Chem. Int. Ed. 2004, 43, 5406 –5408
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5407

Communications
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[7]a) Single crystals were grown from pentane. Structure analysis of
4b: STOE-IPDS diffractometer, graphite-monochromated
MoKa radiation, l = 0.71069 Š, structure solved by direct meth-
ods (SHELXS-86: G. M. Sheldrick, Acta Crystallogr.Sect.A
1990, 46, 467), the refinement calculations were performed by
the full matrix least-squares method against F2 (G. M. Sheldrick,
SHELXS-93, Program for the Solution of Crystal Structures,
Universität Göttingen, Göttingen (Germany), 1993), graphic:
XP (Bruker AXS); 0.5 ” 0.4 ” 0.3 mm, colorless prism, space
group Pbca, orthorhombic, a = 13.731(3), b = 12.942(3), c =
24.045(5) Š, V= 4273.0(16) Š3, Z = 4, 1calcd = 1.379 gcmÀ3, 9878
reflections measured, 2808 reflections independent of symmetry,
1920 reflections observed (I > 2s(I)), R1 = 0.061, wR2 (all
data) = 0.164, 224 parameters; b) Single crystals were obtained
by slow evaporation of a pentane solution of 5b at ca. 58C.
Structure analysis of 5b: BRUKER-APEX-CCD diffractome-
ter, graphite-monochromated MoKa radiation, l = 0.71069 Š,
structure was solved by direct methods (G. M. Sheldrick,
SHELXS-97, Program for the Solution of Crystal Structures,
¯
space group R3, trigonal, a = 22.4017(4), c = 34.811(1) Š, V=
15129.0(6) Š3, Z = 18, 1ber. = 1.015 gcmÀ3, 127091 reflections
measured, 9822 reflections independent of symmetry, 9096
reflections observed (I > 2s(I)), R1 = 0.043, wR2 (all data) =
0.125, 260 parameters. CCDC-240540 (4b) and CCDC-240541
(5b) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
[8]The linear hexasilane with the formula Si 6Me14 exhibits an
intense absorption maximum at 260 nm; K. Obata, M. Kira,
Organometallics 1999, 18, 2216 – 2222.
[9]C. G. Pitt, J.Am.Chem.Soc. 1969, 91, 6613.
[10]Starting with PhSi(SiMe 3)3, the synthesis of 4a,b succeeds
without further purification of intermediate products with a
total yield of ca. 70–80%. The experimental details for the
preparation of the oligosilanes 2a,b–5a,b are available online
free of charge in the Supporting Information.
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www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 5406 –5408