Persilastaffanes: Design, synthesis, structure, and conjugation between silicon cages

Article abstract of DOI:10.1002/anie.201106422

By stepwise catenation of bicyclo[1.1.1]pentasilane units persila[n]staffanes (n=1, 2, and 3) were synthesized as air-stable colorless crystals. A remarkable red-shift of the UV/Vis absorption bands with increasing number of bicyclo[1.1.1]pentasilane unit

Full text of DOI:10.1002/anie.201106422

.
Angewandte
Communications
DOI: 10.1002/anie.201106422
Polysilanes
Persilastaffanes: Design, Synthesis, Structure, and
Conjugation between Silicon Cages**
Takeaki Iwamoto,* Daisuke Tsushima, Eunsang Kwon, Shintaro Ishida, and
Hiroyuki Isobe
Angewandte
Chemie
2
340
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 2340 –2344

Angewandte
Chemie
[
1–3]
Polycyclic oligosilanes
are interesting silicon compounds
because they have s electrons of SiꢀSi bonds delocalized over
a three-dimensional silicon framework which are often found
in crystalline or amorphous silicon and related important
[4–6]
silicon clusters.
Although various types of polycyclic
oligosilanes have been synthesized and investigated so far,
there have been very few studies on catenated polycyclic
oligosilanes (oligomers of polycyclic oligosilanes), which
should show significant interactions between polycyclic
[
7–9]
oligosilane cages.
Persila[n]staffanes A are one of the fascinating unknown
catenated polycyclic oligosilanes because they have highly
symmetric rodlike structures with bicyclo[1.1.1]pentasilane
units catenated at the bridgehead positions and they are
predicted by Yamaguchi to have small band gaps because of
the intrinsic electronic structure of bicyclo[1.1.1]pentasilane
frameworks and extension of bicyclopentasilane units at the
bridgehead positions. Compound 3 was synthesized according
to the reaction sequence shown in Scheme 1. Reaction of
iBu SiCl2 with 1 equiv of (Me Si) SiK in toluene gave
[
7]
delocalization of s electrons along the silicon cages. The
corresponding all-carbon [n]staffanes B have been studied
[
10,11]
extensively by Michl and co-workers.
Although stable
2
3
3
[
12]
bicyclo[1.1.1]pentasilanes bearing aryl substituents 1
and
[
13]
[14]
2
have been synthesized by the Masamune and Breher,
no catenated bicyclo[1.1.1]pentasilanes (persila[n]staffanes,
n ꢁ 2) have been reported. Herein, we would like to report
the synthesis and structure of a series of persila[n]staffanes 3
(n = 1), 4 (n = 2), and 5 (n = 3) and their remarkable
conjugation between bicyclo[1.1.1]pentasilane units.
Scheme 1. Synthesis of persila[1]staffane 3.
(
Me Si ) SiSi(iBu) Cl (6) in 87% yield, which was treated
3 3 3 2
with 1 equiv of tBuOK to afford 1,1,3,3-tetrasilylcyclotetra-
[
16]
To synthesize persila[n]staffanes, we designed novel
bicyclo[1.1.1]pentasilane 3 (persila[1]staffane) as a unit for
persila[n]staffanes. Compound 3 has 1) alkyl substituents
silane 7 in 57% yield. Then, treatment of 7 with 2 equiv of
tBuOK giving bicyclotetrasilane-1,3-diide 8 followed by
iBu SiCl afforded 3 as air-stable colorless crystals in 70%
2
2
[
17,18]
(iBu) on the bridge silicon atoms that cause least electronic
yield.
The structure of 3 was determined by NMR
perturbation to silicon frameworks and 2) silyl substituents on
spectroscopy, MS spectrometry, and X-ray analysis (Figure 1).
Synthesis of persila[2]staffane 4 and persila[3]staffane 5
are accomplished by stepwise catenation of the bicyclo-
the bridgehead silicons (Me Si) that can be easily function-
3
[
15]
alized, and hence, 3 should be suitable for investigation of
[
1.1.1]pentasilane units through functionalization of their
bridgehead silicons of 3 as shown in Schemes 2 and 3.
Treatment of bicyclopentasilane 3 with tBuOK in the
presence of [18]crown-6 (18-c-6) in toluene resulted in the
cleavage of bridgehead SiꢀSi bond to form potassium
[
*] Prof. Dr. T. Iwamoto, D. Tsushima, Dr. S. Ishida, Prof. Dr. H. Isobe
Department of Chemistry, Graduate School of Science
Tohoku University, Aoba-ku, Sendai 980-8578 (Japan)
E-mail: iwamoto@m.tohoku.ac.jp
bicyclo[1.1.1]pentasilanide 9(18-c-6) in 55% yield. Although
Dr. E. Kwon
[19]
a few polycyclic silyl anions have been synthesized,
Research and Analytical Center for Giant Molecules
Graduate School of Science, Tohoku University
Aoba-ku, Sendai 980-8578 (Japan)
compound 9 is the first silyl anion with bicyclo-
[1.1.1]pentasilane skeleton. When the solution of 9 was
treated with an excess amount of 1,2-dibromoethane, 1-
bromobicyclo[1.1.1]pentasilane 10 was obtained in 76% yield
[
**] This work was supported by the Ministry of Education, Culture,
Sports, Science, and Technology of Japan (Grant-in-Aid for Scientific
Research, grant number 20685004, T.I.) and the JST (Research
Seeds Quest Program, T.I.). We thank Prof. Dr. Masahiro Hirama
and Prof. Dr. Masahiro Terada (Tohoku University) for measure-
ment time of solid-state NMR spectra and Dr. Toshihito Nakai
[
20]
(
from 3). Then, coupling reaction of 9 and 10 provided
persila[2]staffane 4 as air-stable colorless crystals in 53%
yield. In a similar manner, persila[3]staffane 5 was obtained
[21]
2
9
from 4 in 2% yield in two steps. Structures of persilastaf-
fanes 4 and 5 were determined by NMR spectroscopy, MS
(
JEOL RESONANCE) for recording the solid-state Si NMR spectra.
D.T. thanks the Global COE program (Molecular Complex Chemis-
try) for a predoctoral fellowship.
[
22]
spectrometry, and X-ray analysis.
Molecular structures of persilastaffanes 3–5 determined
by X-ray single-crystal analysis are shown in Figure 1.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106422.
Angew. Chem. Int. Ed. 2012, 51, 2340 –2344
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2341

.
Angewandte
Communications
Persila[2] and [3]staffanes 4 and 5 adopt almost linear
structures. The silicon atoms of the terminal SiMe group and
3
the bridgehead silicon atoms of the bicyclopentasilane cages
in 4 lie on the crystallographic three-fold axis, whereas those
of 5 deviate slightly from perfect linearity probably because of
[23]
the crystal packing.
The neighboring bicyclo-
[
1.1.1]pentasilane cages are almost perfectly staggered with
the averaged dihedral angle Si(bridge)ꢀSi(bridgehead)ꢀ
Si(bridgehead)ꢀSi(bridge) of 1808 for 4 and 178.88 for 5.
All the tetrasilane units adopt the all-transoid and all-anti
conformation suitable for conjugation between SiꢀSi bonds in
the silicon cages. The endocyclic SiꢀSi distances of
2
.3602(13)–2.3788(13) ꢀ for 4 and 2.355(3)–2.394(3) for 5
are slightly longer than those of 3. Similarly to 3, the intercage
bridgehead SiꢀSi distances of 4 (2.360(3) ꢀ for Si4ꢀSi4’) and
5
(2.348(3) and 2.363(3) for Si6ꢀSi7 and Si11ꢀSi12, respec-
tively) as well as the Si(bridgehead)ꢀSiMe distances
3
(
2.341(2) ꢀ) are shorter than the endocyclic SiꢀSi bond.
The shorter exocyclic SiꢀSi bonds observed in 3–5 are
explained in terms of Bent’s rule: an increased s character
of the bridgehead silicon orbital in the exocyclic SiꢀSi bond
resulting from the acute intracage SiꢀSiꢀSi angles would be
responsible for the shorter exocyclic SiꢀSi bond distance.
Figure 1. ORTEP drawings of a) persila[1]staffane 3, b) persila[2]staf-
fane 4, and c) persila[3]staffane 5. All hydrogen atoms are omitted for
clarity. Thermal ellipsoids are shown at the 30% probability level.
Interestingly, the long axis of persilastaffanes 3–5 are
arranged almost parallel in the single crystals (see Figure S15
[18]
in the Supporting Information). Distances between SiMe3
silicon atoms at the bridgehead position are 7.6626(7),
1
2.979(3), and 18.311(6) ꢀ for 3, 4, and 5, respectively. The
molecular structures of 3–5 show that each addition of
bicyclopentasilane units increases the length of the persilas-
taffane rod by about 5.3 ꢀ, but catenation of the bicyclopen-
tasilane units causes no remarkable change in the structural
characteristics of each bicyclopentasilane unit.
Scheme 2. Functionalization of 3 (R=iBu).
The NMR spectra show that compounds 3–5 have highly
1
13
symmetric structures in solution. The H and C NMR
spectra of 3–5 show a singlet-signal assigned to Me Si
3
groups on the bridgehead silicon atoms and one (for 3 and
4) or two (for 5) set(s) of signals due to iBu groups on the
bridge silicon atoms suggesting facile rotation around the
[
22]
interbridgehead SiꢀSi bonds in solution.
The most striking spectral feature was found in UV/Vis
absorption spectra. As shown in Figure 2, compounds 3–5 in
hexane show two absorption bands I and II. The intense
band I and weak band II of 3 appear at around 220 nm (e =
Scheme 3. Synthesis of persila[2]staffane 4 and persila[3]staffane 5
(
R=iBu).
Bicyclo[1.1.1]pentasilane 3 (Figure 1a) has a three-fold axis
through the bridgehead silicon atoms in the solid state.
Similarly to Masamune’s bicyclopentasilanes 1 and all-carbon
[
10]
bicyclo[1.1.1]pentanes, 3 has a short nonbonded distance
between bridgehead silicon atoms (Si2···Si2*) of 2.9768(5) ꢀ,
which is much shorter than the sum of van der Waals radii of
[12]
two silicon atoms (4.20 ꢀ) and close to that of 1a (2.98 ꢀ),
and considerably acute intracage SiꢀSiꢀSi angles (Si3ꢀSi2ꢀ
Si4(*) and Si4ꢀSi2ꢀSi4* angles) at the bridgehead silicon
atom Si2 (84.68 on average). The exocyclic Si1ꢀSi2 distance of
3
(2.3429(6) ꢀ) is slightly shorter than the endocyclic SiꢀSi
distances of 3 (2.3622(6)–2.3626(6) ꢀ for Si2ꢀSi3 and Si2ꢀ
Si4), which are lying in the range of those of 1a (2.32(2)–
Figure 2. UV/Vis absorption spectra of persila[n]staffanes 3 (n=1,
dotted line), 4 (n=2, broken line), and 5 (n=3, solid line) in hexane
at room temperature.
2
.38(1) ꢀ).
2
342
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 2340 –2344

Angewandte
Chemie
4
ꢀ1
ꢀ1
3
6
1
.8 ꢁ 10 cm mol dm ) and 255 nm (shoulder, e = 5.4 ꢁ
and saxis orbitals become higher in energy, while the scage* and
saxis* orbitals become lower in energy. These orbital features
would be ascribed to the conjugation between s orbitals
3
ꢀ1
ꢀ1
3
[24]
0 cm mol dm ), respectively. Both bands I and II are
remarkably red-shifted and increased with increasing the
number of bicyclopentasilane cages (lmax/nm of bands I and
II: 263 and 300 for 4, 285 and 320 for 5). Although the red-
cage
9,28]
[
and between the linearly arranged saxis orbitals.
The experimental absorption spectra of 3–5 are qualita-
[25]
shift is similar to those observed for s!s* transitions of 12
and 13 having all-anti and all-transoid pentasilane and
octasilane moieties similar to 3 and 4, the spectral features of
tively reproduced by the calculated absorption band positions
[
26]
[27]
and oscillator strengths of 3’–5’. According to the compar-
3
and 4 are different from those of 12 and 13.
ison of the experimental and calculated absorption spectra,
bands I and II are assignable to the saxis!saxis* transition and
the scage!s * transition with a significant contribution of
cage
[29]
To elucidate the absorption bands of 3–5, we performed
scage!s * transition, respectively. The red-shifts of both
axis
DFT calculations for model compounds 3’–5’. The molecular
structures of 3–5 determined by X-ray analysis were well-
reproduced by the structures of 3’–5’ optimized at the B3LYP/
bands I and II on going from 3 to 5 indicate the remarkable
delocalization of s electrons between the catenated bicyclo-
[
7]
[1.1.1]pentasilane cages.
[
27]
6
-31G(d) level. As shown in Figure 3, frontier Kohn–Sham
In conclusion, we successfully synthesized a series of
persila[n]staffanes 3–5 (n = 1–3) through a stepwise catena-
tion and disclosed the considerable delocalization of s elec-
trons along the catenated silicon cages. Persila[n]staffanes
may be fascinating rodlike silicon molecules as linear
connectors for novel silicon-based finely defined materi-
[
30,31]
als.
Received: September 10, 2011
Published online: December 7, 2011
Keywords: conjugation · oligomerization · silanes · staffanes
.
[
1] For reviews on oligosilanes and polysilanes, see: a) M. Kumada,
K. Tamao, Adv. Organomet. Chem. 1968, 6, 19 – 117; b) R. West,
J. Organomet. Chem. 1986, 300, 327 – 346; c) R. D. Miller, J.
Michl, Chem. Rev. 1989, 89, 1359 – 1410; d) H. A. Fogarty, D. L.
Casher, R. Imhof, T. Schepers, D. W. Rooklin, J. Michl, Pure
Appl. Chem. 2003, 75, 999 – 1020.
[
2] For reviews of cyclic polysilanes and silicon cage compounds,
see: a) E. Hengge, R. Janoschek, Chem. Rev. 1995, 95, 1495 –
1526; b) A. Sekiguchi, H. Sakurai, Adv. Organomet. Chem. 1995,
37, 1 – 38; c) A. Sekiguchi, S. Nagase in The Chemistry of
Organic Silicon Compounds, Vol. 2 (Eds.: Z. Rappoport, Y.
Apeloig), Wiley, New York, 1998, chap. 5, pp. 119 – 152.
3] For reviews of ladder polysilanes, see: a) S. Kyushin, H.
Matsumoto, Adv. Organomet. Chem. 2003, 49, 133 – 166; b) H.
Matsumoto, S. Kyushin, M. Unno, R. Tanaka, J. Organomet.
Chem. 2006, 691, 52 – 63.
[
[
Figure 3. Frontier Kohn–Sham orbitals of 3’ (n=1), 4’ (n=2), and 5’
n=3) calculated at the B3LYP/def2-TZVP//B3LYP/6-31G(d) level.
(
4] For a bicyclic oligosilane with a Si=Si double bond as a model for
Si(001) surface, see: a) H. Kobayashi, T. Iwamoto, M. Kira, J.
Am. Chem. Soc. 2005, 127, 15376 – 15377. For a silaadamantane
having a partial structure of single-crystalline silicon, see: b) J.
Fischer, J. Baumgartner, C. Marschner, Science 2005, 310, 825.
For tricyclic hexasilanes as isomers of hexasilabenzene, see: c) K.
Abersfelder, A. J. P. White, H. S. Rzepa, D. Scheschkewitz,
Science 2010, 327, 564 – 566; d) K. Abersfelder, A. J. P. White,
R. J. F. Berger, H. S. Rzepa, D. Scheschkewitz, Angew. Chem.
2011, 123, 8082 – 8086; Angew. Chem. Int. Ed. 2011, 50, 7936 –
(
KS) orbitals of 3’–5’ calculated at the B3LYP/def2-TZVP
level are doubly degenerated s(SiꢀSi) orbitals in a bicyclo-
1.1.1]pentasilane cage with p symmetry relative to the three-
[
fold axis (scage) and a s(SiꢀSi) orbital delocalized mainly over
the bridgehead SiꢀSi bonds with s symmetry (s ), and the
axis
corresponding s * and s * orbitals, respectively. With
cage
axis
increasing the number of bicyclopentasilane cages, the scage
Angew. Chem. Int. Ed. 2012, 51, 2340 –2344
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2343

.
Angewandte
Communications
7
939. For a tricyclohexasilane as an isomer of hexasila-1,4-diene,
Sekiguchi, Chem. Eur. J. 2009, 15, 8401 – 8404; d) T. M. Kla-
pçtke, S. K. Vasisht, G. Fischer, P. Meyer, J. Organomet. Chem.
2010, 695, 667 – 672. See also, Ref. [8a].
see: e) T. Iwamoto, K. Uchiyama, C. Kabuto, M. Kira, Chem.
Lett. 2007, 36, 368 – 369.
[
[
5] a) N. Sandstrçm, H. Ottoson, Chem. Eur. J. 2005, 11, 5067 – 5079;
b) Y. Fujimoto, T. Koretsune, S. Saito, T. Miyake, A. Oshiyama,
New J. Phys. 2008, 10, 083001.
[20] Treatment of 9 with 1 equiv of 1,2-dibromoethane gave a com-
plex mixture with a trace amount of oxidative coupling product,
persila[2]staffane 4.
6] For a review of silicon clathrate, see: S. Yamanaka, Dalton Trans.
[21] Although 5 forms in a moderate yield (about 25%), repeated
purification of 5 using preparative gel permeation chromatog-
raphy because of the poor solubility of 5 resulted in a low yield.
2010, 39, 1901 – 1915.
[
[
7] Y. Yamaguchi, Synth. Met. 1994, 62, 23 – 26.
2
9
8] For dimethylsilylene-bridged bis(bicyclo[2.2.2]octasilanes) and
bis(bicyclo[2.2.1]heptasilanes), see: a) A. Wallner, J. Hlina, T.
Konopa, H. Wagner, J. Baumgartner, C. Marschner, Organo-
metallics 2010, 29, 2660 – 2675. For cyclohexasilanyl-substituted
bicyclo[2.2.2]octasilane, see: b) R. Fischer, T. Konopa, S. Ully, J.
Baumgartner, C. Marschner, J. Organomet. Chem. 2003, 685,
[22] The Si resonance (dSi) due to bridgehead silicon nuclei of 3
appeared at ꢀ99.7 ppm in [D ]benzene, which are close to those
6
of Masamune’s bicyclo[1.1.1]pentane 1 (dSi = ꢀ96.5, ꢀ91.9, and
[
12]
ꢀ89.9 ppm for 1a, 1b, and 1c, respectively). Because mea-
surement of dSi of 4 and 5 failed due to their poor solubility,
2
9
solid-state Si NMR (CP/MAS) spectra of 3–5 were recorded.
The dSi assignable to bridgehead silicon nuclei appeared at
ꢀ101.6 ppm for 3, ꢀ107.0 and ꢀ109.5 ppm for 4, ꢀ108.3
(overlapped) and ꢀ115.1 for 5. For details, see the Supporting
Information.
79 – 92. See also Refs. [2a] and [4b].
[
9] The electronic delocalization along the one-dimensionally
arranged SiꢀSi bonds in disilatriptycene oligomers was reported
by Tamao, Tsuji, and co-workers. S. Sase, Y.-S. Cho, A. Kawachi,
A. Wakamiya, S. Yamaguchi, H. Tsuji, K. Tamao, Organo-
metallics 2008, 27, 5441 – 5445.
[23] Similar deviation from the perfect linearity have been observed
in [n]staffanes. A. C. Friedli, V. M. Lynch, P. Kaszynski, J. Michl,
Acta Crystallogr. B 1990, 46, 377 – 389.
[
[
[
10] For reviews of [n]staffanes, see: a) P. Kaszynski, J. Michl, Adv.
Strain Org. Chem. 1995, 4, 283 – 331; b) M. D. Levin, P.
Kaszynski, J. Michl, Chem. Rev. 2000, 100, 169 – 234.
[24] Marschner
and
co-workers
reported
that
1,4-
bis(trimethylsilyl)bicyclo[2.2.2]octasilane shows two absorption
4
ꢀ1
ꢀ1
3
11] a) P. Kaszynski, J. Michl, J. Am. Chem. Soc. 1988, 110, 5225 –
bands at 210 nm (e = 6.1 ꢁ 10 cm mol dm ) and around
4
ꢀ1
ꢀ1
3
5526; b) P. Kaszynski, A. C. Friedli, J. Michl, J. Am. Chem. Soc.
240 nm (e ꢂ 2 ꢁ 10 cm mol dm ). A. Wallner, M. Hçlbling, J.
Baumgartner, C. Marschner, Silicon Chem. 2007, 3, 175 – 185.
[25] a) H. Tsuji, A. Fukazawa, S. Yamaguchi, A. Toshimitsu, K.
Tamao, Organometallics 2004, 23, 3375 – 3377; b) A. Fukazawa,
H. Tsuji, K. Tamao, J. Am. Chem. Soc. 2006, 128, 6800 – 6801.
[26] C. Marschner, J. Baumgartner, A. Wallner, Dalton Trans. 2006,
5667 – 5674.
1992, 114, 601 – 620.
12] Y. Kabe, T. Kawase, J. Okada, O. Yamashita, M. Goto, S.
Masamune, Angew. Chem. 1990, 102, 823 – 825; Angew. Chem.
Int. Ed. Engl. 1990, 29, 794 – 796.
13] D. Nied, R. Kçppe, W. Klopper, H. Schnçckel, F. Breher, J. Am.
Chem. Soc. 2010, 132, 10264 – 10265.
[
[
14] Orudzhaeva and Suleimanov have proposed the generation of
octamethylbicyclo[1.1.1]pentasilane by using mass spectrome-
try; K. N. Orudzhaeva, G. Z. Suleimanov, Azerb. Khim. Zh.
[27] For details of the theoretical study see the Supporting Informa-
tion.
[28] Remarkable interactions between linearly arranged bridgehead
CꢀC bond orbitals in [n]staffyl radicals were observed. A. J.
2004, 1, 87.
[
[
15] C. Marschner, Eur. J. Inorg. Chem. 1998, 221 – 226.
McKinley, P. B. Ibrahim, V. Balaji, J. Michl, J. Am. Chem. Soc.
1992, 114, 10631 – 10637.
16] (Me Si) SiKSi(iBu) Cl may be an intermediate for formation of
3
2
2
8
. Marschner and co-workers reported that themolysis of
[29] The small activation energy for rotation around the interbridge-
ꢀ1
(
Me Si) SiKSi(SiMe ) F gave octakis(trimethylsilyl)cyclotetrasi-
head SiꢀSi bond in persila[2]staffane 4’ of 2.6 kJmol calculated
3
2
3 2
lane. R. Fischer, J. Baumgartner, G. Kickelbick, C. Marschner, J.
Am. Chem. Soc. 2003, 125, 3414 – 3415.
at the B3LYP/6-31G(d) level supports a facile rotation around
the SiꢀSi bond(s) in 4 and 5 in solution. TD-DFT calculations
[
17] Similarly, Masamune’s bicyclo[1.1.1]pentasilanes 1 were synthe-
sized by the reaction of cyclotetrasilane-1,3-diide with dihalosi-
lanes. See Ref. [12]. For examples of preparation of cyclo-
tetrasilane-1,3-diide, see: a) S. Kyushin, H. Kawai, H. Matsu-
moto, Organometallics 2004, 23, 311 – 313; b) R. Fischer, T.
Konopa, J. Baumgartner, C. Marschner, Organometallics 2004,
suggests that the facile rotation would not affect remarkably the
spectral feature of the absorption spectra of persilastaffanes
because the saxis!saxis* and scage!scage* transitions for 4’ in the
eclipsed form calculated at the B3LYP/def2-TZVP level at 246
and 297 nm, respectively, are similar to those of 4’ in the
optimized staggered form (251 and 298 nm).
2
3, 1899 – 1907.
[30] P. F. H. Schwab, M. D. Levin, J. Michl, Chem. Rev. 1999, 99,
1863 – 1934.
[
18] CCDC 842825 (3), 842826 (4), 842827 (5), and 842828 (7)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[31] Examples of bicyclo[1.1.1]pentanes as rigid linear carbon con-
nectors, see: a) R. Gleiter, K.-H. Pfeifer, G. Szeimeis, U. Benz,
Angew. Chem. 1990, 102, 418 – 420; Angew. Chem. Int. Ed. Engl.
1990, 29, 413 – 415; b) S. Maziꢂres, M. K. Raymond, G. Raabe,
A. Prodi, J. Michl, J. Am. Chem. Soc. 1997, 119, 6682 – 6683;
c) A. de Meijere, L. Zhao, V. N. Belov, M. Bossi, M. Noltemeyer,
S. W. Hell, Chem. Eur. J. 2007, 13, 2503 – 2516.
[
19] For examples of silicon cage anions, see: a) M. Ichinohe, M.
Toyoshima, R. Kinjo, A. Sekiguchi, J. Am. Chem. Soc. 2003, 125,
1
3328 – 13329; b) W. Setaka, N. Hamada, M. Kira, Chem. Lett.
2004, 33, 626 – 627; c) V. Y. Lee, T. Yokoyama, K. Takanashi, A.
2
344
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 2340 –2344