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Hydroxy-Substituted Oligosilane Dendrimers Controlling the Electronic Properties through Hydrogen Bonding.

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DOI: 10.1002/anie.200601598
Hydroxy-Substituted Oligosilane Dendrimers:
Controlling the Electronic Properties through
Hydrogen Bonding
Ulrike Jger-Fiedler, Martin Kckerling, Ralf Ludwig,
Alexander Wulf, and Clemens Krempner*
Poly and oligosilanes have received considerable attention
owing to their unique electronic properties arising from the
extensive delocalization of s-electrons along the silicon
backbone.[1] Their intense electronic absorption in the near
UV was found to be sensitive to the electronic nature of the
substituents and the conformation of the silicon backbone, as
reflected in the thermochromism,[2] solvatochromism,[3] and
ionochromism[4] of some oligosilanes. Recent studies on linear
derivatives with discrete conformations have shown that sconjugation is more effective in an anti-conformation (Si-SiSi-Si dihedral angle w = 1808) than in conformations with
small dihedral angles, such as syn, cisoid, or even gauche (w =
0?608).[5] We report herein the synthesis, structure, and
remarkable thermochromism of the first hydroxy-substituted
oligosilane dendrimers.[6] IR spectroscopic studies supported
by density functional theory (DFT) calculations clearly show
the electronic properties of these compounds to be determined by the conformation of the silicon backbone, which is
controlled by hydrogen bonding.
The hydroxy-substituted dendrimer 3, which has three
identical stereogenic silicon centers has been prepared in two
steps (Scheme 1). MeSi(SiMeCl2)3 (1) was treated with
3 equivalents of KSi(SiMe3)2Me at 78 8C. The reaction
proceeds regio- and diastereoselectively yielding the l,l-form
of 2 as the main product.[7] Hydrolysis of 2 in the presence of
NH4(NH2COO) gave a racemic mixture of the two possible
diastereomers l,u-3 and l,l-3 (see Supporting Information).
The molecular structures of l,l-3 and l,u-3 have been
determined by X-ray crystallography (Figure 1 and 2).[8]
Suitable crystals were grown from THF and n-heptane
solutions, respectively. The results clearly reveal both compounds to be dimeric in the solid state.[9] In l,l-3, two
molecules are connected to each other by six intermolecular
hydrogen bonds, forming a cage structure that consists of
eight-membered rings (Figure 1). The measured intermolecular O1иииO1? distance and O1-HиииO1? angle are found to be
279.9 pm and 135.38, respectively. In l,u-3, the steric demand
of the dendrimer wings (-Si(SiMe3)2Me) enforces an arrange-
[*] U. Jger-Fiedler, Prof. Dr. M. K$ckerling, Prof. Dr. R. Ludwig, A. Wulf,
Dr. C. Krempner
Institut f2r Chemie der Universitt Rostock
A.-Einstein-Strasse 3a, 18059 Rostock (Germany)
Fax: (+ 49) 381-498-6382
Supporting information for this article is available on the WWW
under or from the author.
Angew. Chem. Int. Ed. 2006, 45, 6755 ?6759
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Intermolecular hydrogen bonding between O3? and O2 or O3
(O2иииO3? 277.5 pm and O3иииO3? 285.2 pm), results in the
formation of eight-membered ring motifs (Figure 2). Struc-
Figure 2. Molecular structure of the hydrogen-bonded dimer l,u-3. The
thermal ellipsoids are set at 50 % probability (methyl groups omitted
for clarity). Selected bond lengths [B] and angles [8]: Si2?O1
1.6757(15), Si3?O2 1.6782(14), Si4?O3 1.6715(14); O1-Si2-Si1
105.49(6), O2-Si3-Si1 104.82(5), O3-Si4-Si1 107.40(5), Si3-Si1-Si2
101.05(2), Si3-Si1-Si4 109.45(3), Si2-Si1-Si4 113.15(3); Si6-Si5-Si2-Si1
151.1, Si5-Si2-Si1-Si4 115.8, Si2-Si1-Si4-Si11 155.0, Si1-Si4-Si11-Si13
tural parameters indicate only slight strain in both molecules;
the Si?Si distances with 233?238 pm, are in the normal range
and the Si O bond lengths (167?168 pm) are very similar to
those found in other OH containing oligosilanes.[10]
Unexpectedly, both diastereomers exhibit different UV
absorption behavior; the absorption curves of l,u-3, l,l-3 and
of the structural analogue MeSi[SiMe2Si(SiMe3)2Me]3 (4) are
shown in Figure 3. For l,u-3, the intense peak at 282 nm (e =
3.7 A 104) with a shoulder near 260 nm is significantly shifted
to longer wavelength compared to that of 4 (lmax = 269 nm,
e = 5.2 A 104).[11] This red-shift is attributable to reduction in
the optical-band-gap resulting from effective interactions
between silicon s and oxygen lone pair nonbonding orbi-
Scheme 1. Synthesis of l,l-3 and l,u-3.
Figure 1. Molecular structure of the hydrogen-bonded dimer l,l-3. The
thermal ellipsoids are set at 50 % probability (methyl groups are
omitted for clarity). Selected bond lengths [B] and angles [8]: Si2?O1
1.6750(12); O1-Si2-Si1 109.68(5), Si2-Si1-Si2 108.94(2); Si2-Si1-Si2-Si3
83.11(3), Si2-Si1-Si2-Si3 157.43(3), Si1-Si2-Si3-Si4 92.36(3), Si1-Si2-Si3Si5 147.51(3).
ment in which the non-equivalent hydroxy groups are linked
differently to each other by hydrogen bonds. As a result, O1
and O2 are linked by intramolecular hydrogen bonds
(O1иииO2 280.0 pm), forming a six-membered ring structure.
Figure 3. Room temperature UV spectra of l,l-3, l,u-3, and 4 in
n-heptane (c = 10 5 m).
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 6755 ?6759
tals.[12] In striking contrast, l,l-3 exhibits two distinct peaks at
260 nm and 290 nm with almost the same intensity (e = 2.5 A
104). The absorption maximum at 290 nm is not only
significantly shifted to longer wavelengths relative to that of
the stereoisomer l,u-3, it is also the longest observed to date
for heptasilanes. To our knowledge, 3 is the first stereochemical active oligo or polysilane for which the UV spectroscopic
behavior differs depending on the configuration.
It is well known that upon increasing the Si-Si-Si-Si
dihedral angles, s-conjugation becomes more effective, and
hence the wavelength and intensity of the absorption
maximum increase significantly. Consequently, the energy of
the s?s* transition depends on both electronic and conformational factors. Therefore, the conformation of the heptasilane
chains of l,u-3 and l,l-3, which are defined by four dihedral
angles, has been determined on the basis of the X-ray data
(Supporting Information). Owing to the trigonal space group
of l,l-3, there are only four different combinations of conformers; D-D-O-D, D-D-O-O, D-O-D-O, and O-D-O-O
(D = deviant, w 1508; O = ortho, w 908).[13] Note that the
dihedral angles found in the heptasilane chains of l,l-3 are
similar to those of the nonfunctionalized dendrimer 4[11]
However, in l,u-3, the conformational arrangement is slightly
different because a D-D-E-D conformer (E = eclipsed, w
1208) was found which has the largest dihedral angles in
the molecule. Clearly, this conformer arises from intramolecular hydrogen bonding between O1 and O2 that enforces a
twisting of the Si5-Si2-Si1-Si4 dihedral angle to a value of
115.88. However, none of the conformers found in l,u-3 and
l,l-3 are close to an all-anti conformer (A, anti, w 1808) that
is believed to be optimal for s-conjugation.[1]
The UV spectra of l,l-3 in n-heptane, as a function of
temperature, are shown in Figure 4. Remarkably, upon
increasing the temperature, the intensity of the twin peaks
slowly decreases. The peaks completely disappear at 343 K,
and above 333 K, a new relatively broad signal appears at
280 nm. In contrast, the UV absorption curves of l,u-3 and 4
remain unchanged at 343 K. Upon cooling l,l-3 to room
temperature, the twin peaks appear again at the original
wavelengths, clearly indicating the process to be reversible;
structural irreversible changes such as epimerization of the
stereogenic silicon centers as well as skeletal rearrangements
Figure 4. UV spectra of l,l-3 in n-heptane as a function of temperature.
Angew. Chem. Int. Ed. 2006, 45, 6755 ?6759
or condensation reactions to siloxanes did not occur according to NMR spectroscopic measurements. However, temperature dependent 1H NMR spectroscopic investigations of l,l-3
in [D8]toluene (c = 10 2 m) suggest the occurrence of intermolecular hydrogen exchange processes. The broadened OH
signal (d = 5.2 ppm at 303 K) sharpens with increasing
temperature and shifts upfield (d = 1.6 ppm at 353 K),
whereas for l,u-3 the chemical shifts of the three nonequivalent and sharp OH signals remain unchanged.
The IR spectra obtained for the O H stretching region of
l,u-3 and l,l-3 in CCl4 (c = 10 3 m), as a function of temperature, are shown in Figure 5 and Figure 6. To determine
whether the observed stretching frequencies correspond to
associated OH groups (intra- or intermolecular) or nonassociated OH groups, ab initio calculations were performed
at the density functional level (B3LYP) for the model
compound HSi(SiH2OH)3 ; the structures corresponding to
the energy minima are shown in Figure 5 and Figure 6. For l,l3, the spectra provide clear evidence for an entropically
driven dissociation process in solution. In fact, at room
temperature a very broad absorption has been observed at
3329 cm 1 which is assigned to the stretching frequency of the
intermolecularly hydrogen-bonded dimer. This is in good
agreement with the IR spectrum in the solid state (Nujol) for
which a broad signal at 3322 cm 1 has been found arising from
the dimer. However, upon increasing the temperature, a
shoulder assigned to the monomer with intramolecular
hydrogen bonds appears at 3489 cm 1. At 333 K, the spectrum
is clearly dominated by the vibrational motion of the
monomer, which results in a sharp absorption at 3664 cm 1
indicative of non-associated OH groups. In contrast, the IR
spectra of l-u-3 do not show significant temperature depend-
Figure 5. IR spectra of l,l-3 in CCl4 (c = 10 3 m) as a function of
temperature (O H stretching vibrations region). Observed stretching
frequencies for OH: 3329 (inter), 3489 (intra), 3612 (non-associated),
3664 (non-associated), and 3703 cm 1 (non-associated). At high
temperatures the spectra are dominated by the vibrational motion of
the monomer. Decreasing temperature results in the formation of
monomers with intramolecular hydrogen bonds. At room temperature
dimers with intermolecular hydrogen bonds exist as indicated by the
red shift of the bands.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
in general should have interesting and tunable electronic
properties. The delocalized s-electrons are strongly coupled
with the oxygen lone pairs, which reduces the HOMO?
LUMO band gap of these compounds significantly. We
anticipate that the absorption can be moved into the visible
region by increasing the number of oxo groups attached to
silicon backbone and by controlling the conformation in
larger dendritic oligosilanes. Further work in this area is in
Received: April 24, 2006
Revised: June 22, 2006
Published online: September 15, 2006
Figure 6. IR spectra of l,u-3 in CCl4 (c = 10 3 m) at 293 and 323 K (O
H stretching vibrations region). Independent of the temperature, the
spectra are dominated by the vibrational motion of the monomer.
Observed stretching frequencies for OH: 3498 (intra), 3615 (nonassociated), 3665 (non-associated), and 3705 cm 1 (non-associated).
ence, even at room temperature only stretching frequencies
associated with the formation of monomers are found.
From these results and DFT calculations for l,l-3 in the gas
phase that clearly show the dimeric structure to be more
stable than the monomeric one, it seems evident that l,l-3
exists as a dimer at room temperature in solution.[14]
Consequently, the remarkable red shift of the UV absorption
maximum of l,l-3 at room temperature arises from the dimer
in which the conformation of the silicon backbone is
controlled by steric repulsion of the three dendrimer wings
(-Si(SiMe3)2Me) and, more importantly, through intermolecular hydrogen bonding. Assuming the conformational
arrangement of l,l-3 in the solid-state to be similar to that in
solution, it can be concluded that the UV absorption
maximum of l,l-3 (290 nm at room temperature) most likely
corresponds to conformers with large dihedral angles, such as
D-D-O-D and D-D-O-O. The absorption at 260 nm may arise
from the O-D-O-O or from the D-O-D-O conformer, in
which both deviant segments are separated by ortho links,
resulting in an interruption of s-conjugation along the
heptasilane chain. With increasing temperatures, a dissociation process occurs that leads to the formation of the
monomer with non-associated OH groups. Clearly, this
process is accompanied by an increased conformational
flexibility of the oligosilane backbone because rotations
around Si Si bonds are not restricted through hydrogen
bonding as in the dimer. As a consequence, a variety of
different conformers exist, causing a broadened peak at
280 nm, which is blue-shifted with respect to the absorption
maximum at room temperature. Such a situation is not
evident for the stereoisomer l,u-3 at room temperature, since
the steric demand of the dendrimer wings enforces an
arrangement that weakens the overall strength of intermolecular hydrogen bonding in the molecule tremendously. This
weakening results in a rapid dissociation in solution to
monomers with mainly non-associated OH groups.
In conclusion, we believe that the preliminary findings
reported herein indicate that oxo-functionalized oligosilanes
Keywords: hydrogen bonds и IR spectroscopy и oligosilanes и
silanols и UV spectroscopy
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Engl. 1982, 21, 654.
[8] For details of X-ray structure analyses of l,l-3 and l,u-3, see
Supporting Information.
[9] For excellent reviews on solid-state structures of silanols see:
a) P. D. Lickiss, Adv. Inorg. Chem. 1995, 42, 147; b) P. D. Lickiss
in The Chemistry of Organosilicon Compounds, Vol. 3, Wiley,
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 6755 ?6759
Fuerpass, K. Renger, J. Baumgartner, Organometallics 2005, 24,
[13] The conformations are roughly classified as syn (S, w 08),
gauche (G, w 608), ortho (O, w 908), eclipsed (E, w 1208),
deviant (D, w 1508) and anti (A, w 1808); J. Michl, R. West,
Acc. Chem. Res. 2000, 33, 821.
[14] The counterpoise-corrected binding energies are 103.45 and
107.31 kJ mol 1 depending in the monomer structure (two
isomers shown in Figure 5). Gaussian 03 (Revision A.1), M. J.
Frisch et al., see Supporting Information.
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hydrogen, bonding, properties, electronica, dendrimer, oligosilane, substituted, controlling, hydroxy
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