Two pentasilahousanes fused together

Article abstract of DOI:10.1002/chem.201403375

The organosilicon cluster 1 with two pentasilahousanes fused together was synthesized by the reduction of 1,1,3-trichlorocyclotetrasilane. This reaction involves dimerization and rearrangement of the silicon skeleton. The structure and properties of 1 wer

Full text of DOI:10.1002/chem.201403375

DOI: 10.1002/chem.201403375
Communication
&
Organosilicon Chemistry
Two Pentasilahousanes Fused Together
Akihiro Tsurusaki,[a] Makoto Koganezono,[a] Kyohei Otsuka,[a] Shintaro Ishida,[b] and
Soichiro Kyushin*[a]
Abstract: The organosilicon cluster 1 with two pentasila-
housanes fused together was synthesized by the reduc-
tion of 1,1,3-trichlorocyclotetrasilane. This reaction in-
volves dimerization and rearrangement of the silicon skel-
eton. The structure and properties of 1 were studied by X-
ray crystallographic and spectroscopic analyses.
Organosilicon clusters with unique structures have been stud-
ied intensively for the last few decades. Their studies are im-
portant because 1) organosilicon clusters can be regarded as
Figure 1. Organosilicon clusters synthesized by the reductive oligomeriza-
tion of polyhalosilanes.
partial structures of bulk crystalline or amorphous silicon and
2) organosilicon clusters containing a large number of silicon
atoms may open new organosilicon chemistry.[1] Various types
of silicon analogues of polyhedranes have been reported.[2–4]
Nowadays, organosilicon clusters expand into more complicat-
ed and sophisticated ones such as decasilaadamantane,[5]
pentasila[1.1.1]propellane,[6] persila[n]staffanes,[7] organosilicon
clusters with bicyclo[1.1.0]tetrasilane substructures,[8] and
others.[9] The construction of organosilicon clusters is still chal-
lenging, and one of the most fundamental approaches is re-
ductive oligomerization of polyhalosilanes. For example,
(tBu3Si)6Si8 (I),[8a] an isomer of hexasilabenzene II,[10] and charge-
separated cyclotetrasiladiene III,[11] were synthesized by the re-
duction of 1,1,2,3,4-pentaiodocyclotetrasilane, 1,1,2-trichlorocy-
clotrisilane, and tribromosilane, respectively (Figure 1). Com-
pounds I–III are intriguing from the viewpoints of the unique
skeletons, the characteristic configurations around the silicon
atoms, and/or peculiar SiÀSi bonds. In addition, some organo-
silicon clusters such as I and II have unsubstituted silicon ver-
texes, for which Scheschkewitz and co-workers have recently
proposed the term “siliconoids”.[3b]
labenzvalene V[13] by the reduction of 1,1,2,2-tetrachlorocyclo-
tetrasilane and 1,1,2,2-tetrachlorocyclopentasilane, respectively.
Compounds IV and V are the first examples of octasilacuneane
and hexasilabenzvalene derivatives, respectively. They have un-
substituted silicon atoms with highly deformed trigonal mo-
nopyramidal structures. These results prompted us to examine
the reduction of different types of polyhalocyclooligosilanes to
form unknown organosilicon clusters. We report herein the
synthesis, structure, and properties of tBu8Ph2Si8 (1), an orga-
nosilicon cluster with two pentasilahousanes fused together by
the reduction of 1,1,3-trichlorocyclotetrasilane 4.
The synthetic route to 1 is outlined in Scheme 1. The Wurtz-
type coupling of dichlorodisilane 2[12] with sodium in THF gave
a mixture of cyclotetrasilanes 3a and 3b.[14] The head-to-tail
adduct 3a was isolated in 13% yield after washing with etha-
nol and purification by column chromatography on silica gel.
The careful chlorodephenylation of 3a with hydrogen chloride
and aluminum chloride gave a mixture of 1,1,3-trichlorocyclo-
We have recently reported the synthesis of cyclotetrasilane-
fused octasilacuneane IV[12] and cyclopentasilane-fused hexasi-
[a] Dr. A. Tsurusaki, M. Koganezono, Dr. K. Otsuka, Prof. Dr. S. Kyushin
Division of Molecular Science
Graduate School of Science and Technology
Gunma University
Kiryu, Gunma 376-8515 (Japan)
E-mail: kyushin@gunma-u.ac.jp
[b] Dr. S. Ishida
Department of Chemistry
Graduate School of Science
Tohoku University
Aoba-ku, Sendai 980-8578 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403375.
Scheme 1. Synthesis of 1.
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tetrasilane 4 and 1,1,3,3-tetrachlorocyclotetrasilane 5.[15,16] Pu-
rification by column chromatography on silica gel gave 4 in
41% yield. The reduction of 4 with potassium graphite afford-
ed 1 in 61% yield as pale yellow crystals. The formation of 1 in-
volves dimerization and rearrangement of the silicon skeleton
(vide infra).
such as I,[8] IV,[12] and V.[13] This deformation implies that the Si2
orbital of the Si2ÀSi3 bond has increased s character, whereas
the Si2 orbital of the Si2ÀSi4 bond has increased p character.
In addition, the Si4 orbital of the Si2ÀSi4 bond has increased p
character. As a result, the Si2ÀSi3 bond becomes short, where-
as the Si2ÀSi4 bond becomes long.
The X-ray crystallographic analysis of 1 showed the unique
tetracyclic silicon skeleton with two cyclotrisilane rings and
two cyclotetrasilane rings with all-anti configuration
(Figure 2).[16] Compound 1 has a twofold axis through a mid-
The dihedral angles between the cyclotrisilane and cyclote-
trasilane rings and between the two cyclotetrasilane rings are
129.4 and 145.28, respectively. These values are much larger
than those of tricyclo[3.1.0.02,4]hexasilane 6 (113.548)[17] and
bicyclo[2.2.0]hexasilane 7 (119.38) (Figure 4).[18] To obtain in-
Figure 4. Compound 6 and ladder oligosilanes 7–9.
sight into the large dihedral angles, theoretical calculations of
1 and the derivative of 1, in which all tert-butyl groups are re-
placed by methyl groups (represented as 1calc and 1Me, here-
after) were carried out at the B3LYP/6-311G(3d) level for silicon
atoms and at the B3LYP/6-31G(d) level for carbon and hydro-
gen atoms. The optimized structure of 1calc well reproduces
the X-ray structure except that all SiÀSi bond lengths are
slightly longer (see Figure S22 and Table S4 in the Supporting
Information). The dihedral angles between the cyclotrisilane
and cyclotetrasilane rings of 1Me (116.68) is smaller than that of
1calc (128.38) and similar to those of 6 and 7, indicating that
the large dihedral angle of 1 is due to the steric hindrance be-
tween tert-butyl groups. The dihedral angle between the two
cyclotetrasilane rings of 1Me (135.38) is still larger. This large di-
hedral angle may be related with the Si1-Si2-Si3 bond angles.
If the dihedral angle became smaller, the large Si1-Si2-Si3 bond
angle (142.20(4)8) would become larger. To prevent an unusu-
ally large Si(1)-Si(2)-Si(3) bond angle, the dihedral angle should
be large.
In the 1H and 13C NMR spectra of 1, four sets of tert-butyl sig-
nals and one set of phenyl signals are observed, supporting
the C2 symmetry of 1. The 29Si NMR spectrum exhibits four sig-
nals at d=À84.5 (Si2), À48.0 (Si4), À20.1 (Si3), and 26.1 (Si1)
ppm.[19] The three signals observed in the upfield region are as-
cribed to the 29Si nuclei of the cyclotrisilane ring. The 29Si
chemical shift of the Si2 nucleus (d=À84.5 ppm) is similar to
those of IV (d=À71.8, À62.4, and À56.6 ppm).[12] The others
are normal compared with those of cyclotrisilanes and cyclote-
trasilanes with tert-butyl and phenyl groups.[20]
Figure 2. Molecular structure of 1. Thermal ellipsoids are drawn at the 50%
probability level. Hydrogen atoms and included hexane are omitted for clari-
ty. Selected bond lengths [ꢁ] and angles [8]: Si1ÀSi2 2.3642(9), Si1ÀSi4*
2.3812(9), Si2ÀSi3 2.3179(9), Si2ÀSi4 2.4005(9), Si2ÀSi2* 2.369(1), Si3ÀSi4
2.3703(9); Si2-Si1-Si4* 84.40(3), Si1-Si2-Si3 142.20(4), Si1-Si2-Si4 145.09(4),
Si1-Si2-Si2* 96.31(4), Si3-Si2-Si4 60.28(3), Si3-Si2-Si2* 118.00(3), Si4-Si2-Si2*
83.87(3), Si2-Si3-Si4 61.59(3), Si2-Si4-Si3 58.13(3), Si2-Si4-Si1* 95.03(3), Si2-Si4-
C17 134.58(8), Si3-Si4-Si1* 127.73(4), Si3-Si4-C17 114.84(8), Si1*-Si4-C17
115.18(8).
point of the Si2ÀSi2* bond. The cyclotetrasilane rings have
a slightly folded structure with the fold angle of 6.4 and 7.18.
They also have a slightly rhombic structure: the Si1-Si2-Si2*
(96.31(4)8) and Si2-Si4-Si1* (95.03(3)8) bond angles are large,
and the Si2-Si1-Si4* (84.40(3)8) and Si4-Si2-Si2* (83.87(3)8)
bond angles are small. The SiÀSi bond lengths range from
2.3642(9) to 2.3812(9) ꢁ except for the short Si2ÀSi3 bond
(2.3179(9) ꢁ) and the long Si2ÀSi4 bond (2.4005(9) ꢁ). The
bridgehead Si2 and Si4 atoms exhibit deformed trigonal mo-
nopyramidal geometries with the sum of three Si-Si-Si bond
angles of 356.5 and 357.88, respectively (Figure 3). Similar
structures have previously been reported in silicon clusters
In the UV/Vis spectrum of 1, the broad absorption tailing to
about 450 nm was observed with a few shoulders at 247, 267,
303, and 376 nm (Figure 5). The absorption without clear
bands is similar to that of IV[12] and is in contrast with those of
ladder oligosilanes 7–9, which show the lowest energy absorp-
tion as a distinctive band.[21] Thus, this result suggests that
1 shows the electronic properties as an organosilicon cluster.
The TD-DFT calculation of 1calc shows that the lowest energy
Figure 3. Geometries around the deformed silicon atoms of 1.
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duction of 10, followed by the ring expansion by double 1,2-
silyl migration to give 12. Further reduction of 12 leads to the
formation of two cyclotrisilane rings by the cleavage of the Si=
Si double bond to give 1. On the other hand, path B includes
the formation of 13 by the reduction of 10, followed by the
double 1,2-silyl migration to give 14. Further reduction of 14
gives 1. Although it is difficult to determine whether the reac-
tion proceeds by path A or path B at this moment, we believe
path B may be favorable because of the steric congestion
around the internal SiÀCl bonds. In addition, the coefficients of
the s* orbitals of the terminal SiÀCl bonds in the LUMO of 10
are somewhat larger than those of the internal ones (Fig-
ure S25), implying high reactivity of the terminal SiÀCl bonds
compared with the internal ones.[24]
Figure 5. UV/Vis spectra of 1 in hexane at room temperature with transitions
calculated at the B3LYP/6-31+G(2d,p) level (gray bars).
In summary, we have synthesized the organosilicon cluster
1 with two pentasilahousanes fused together by the reduction
of 1,1,3-trichlorocyclotetrasilane 4 via dimerization and rear-
rangement of the silicon skeleton. The X-ray crystallography,
absorption band at 376 nm is due to the HOMO–LUMO transi-
tion (lcalcd =371 nm, f=0.0130). The major contribution to the
HOMO is s conjugation of SiÀSi bonds, that is, an out-of-phase
interaction among s orbitals along the Si2*ÀSi1*ÀSi4ÀSi3ÀSi2À spectroscopy, and theoretical calculations revealed the unique
Si2*ÀSi3*ÀSi4*ÀSi1ÀSi2 bonds (Figures S23 and S24). The
major contribution to the LUMO is pseudo p* orbitals of the
cyclotrisilane rings, consisting of the s* orbitals of six SiÀSi and
SiÀC bonds attached to the cyclotrisilane rings. It is interesting
to compare the lowest energy absorption maximum of 1 with
those of related compounds. The lowest energy absorption
maximum of 1 (lmax 376 nm) is larger than those of 6 (lmax
320 nm),[17] 7 (lmax 310 nm),[21b] and 8 (lmax 345 nm).[21] It is
close to that of 9 (lmax 380 nm),[21b] in which two terminal cy-
clotetrasilane rings are fused into bicyclo[2.2.0]hexasilane.
Possible reaction mechanisms of the formation of 1 from 4
are shown in Scheme 2. On the basis of the previous reports
on the double 1,2-silyl migration during the formation of
bicyclo[3.3.0]octasil-1(5)-ene[22,23] and (tBu3Si)6Si9Cl2,[8b] the fol-
lowing reaction pathways are conceivable. Initially, bis(cyclote-
trasilanyl) 10 is formed by the reductive coupling of 4 at the
less crowded and more reactive silicon atom with two chlorine
atoms. Path A includes the formation of disilene 11 by the re-
structure and properties of 1, such as the bridgehead silicon
atoms with the deformed trigonal monopyramidal geometries
and the s conjugation of the three-dimensional organosilicon
cluster. Further studies on the synthesis of unique organosili-
con clusters are currently under way.
Acknowledgements
This work was supported in part by the Advanced Catalytic
Transformation Program for Carbon Utilization of the Japan
Science and Technology Agency, Japan and the Element Inno-
vation Project of Gunma University, Japan. We thank Dr.
Eunsang Kwon, Satomi Ahiko, and Hiroyuki Momma, Tohoku
University, Japan for the measurement of the high-resolution
mass spectrum and elemental analysis. Theoretical calculations
were performed in the Research Center for Computational Sci-
ence, Japan.
Keywords: organosilicon clusters
conjugation theoretical calculations
monopyramidal geometry
·
pentasilahousane
·
s
·
·
trigonal
[1] For reviews on cyclic oligosilanes and silicon cage compounds, see:
a) E. Hengge, R. Janoschek, Chem. Rev. 1995, 95, 1495–1526; b) A. Seki-
guchi, H. Sakurai, Adv. Organomet. Chem. 1995, 37, 1–38; c) A. Sekigu-
chi, S. Nagase in The Chemistry of Organic Silicon Compounds, Vol. 2
(Eds.: Z. Rappoport, Y. Apeloig), Wiley, Chichester, 1998, pp. 119–152;
d) E. Hengge, H. Stꢂger in The Chemistry of Organic Silicon Compounds,
Vol. 2 (Eds.: Z. Rappoport, Y. Apeloig), Wiley, Chichester, 1998,
pp. 2177–2216.
[2] For tetrasilatetrahedranes, see: a) N. Wiberg, C. M. M. Finger, K. Polborn,
Angew. Chem. 1993, 105, 1140–1142; Angew. Chem. Int. Ed. Engl. 1993,
32, 1054–1056; b) M. Ichinohe, M. Toyoshima, R. Kinjo, A. Sekiguchi, J.
Am. Chem. Soc. 2003, 125, 13328–13329.
[3] For hexasilaprismanes, see: a) A. Sekiguchi, T. Yatabe, C. Kabuto, H. Sa-
kurai, J. Am. Chem. Soc. 1993, 115, 5853–5854; b) K. Abersfelder, A. Rus-
sell, H. S. Rzepa, A. J. P. White, P. R. Haycock, D. Scheschkewitz, J. Am.
Chem. Soc. 2012, 134, 16008–16016.
[4] For octasilacubanes, see: a) H. Matsumoto, K. Higuchi, Y. Hoshino, H.
Koike, Y. Naoi, Y. Nagai, J. Chem. Soc. Chem. Commun. 1988, 1083–
1084; b) K. Furukawa, M. Fujino, N. Matsumoto, Appl. Phys. Lett. 1992,
Scheme 2. Possible reaction mechanisms of the formation of 1.
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60, 2744–2745; c) A. Sekiguchi, T. Yatabe, H. Kamatani, C. Kabuto, H. Sa-
kurai, J. Am. Chem. Soc. 1992, 114, 6260–6262; d) H. Matsumoto, K. Hi-
guchi, S. Kyushin, M. Goto, Angew. Chem. 1992, 104, 1410–1412;
Angew. Chem. Int. Ed. Engl. 1992, 31, 1354–1356; e) K. Furukawa, M.
Fujino, N. Matsumoto, J. Organomet. Chem. 1996, 515, 37–41; f) M.
Unno, T. Matsumoto, K. Mochizuki, K. Higuchi, M. Goto, H. Matsumoto,
J. Organomet. Chem. 2003, 685, 156–161.
[19] The GIAO calculation of 1calc at the B3LYP/6–311+G(2d,p) level predicts
the 29Si chemical shifts at d=À62.4 (Si2), À32.2 (Si4), À19.5 (Si3), and
39.8 (Si1) ppm.
[20] For example, 29Si signals of cyclotrisilanes have been reported to be ob-
served at d=À4.0 ppm for (tBu2Si)3,[25a] d=À34.81 ppm for cis,cis-[(iPr-
Me2C)PhSi]3,[25b] and d=À41.25 and À35.98 ppm for cis,trans-[(iPr-
Me2C)PhSi]3.[25b] 29Si signals of di-tert-butyl-substituted silicon nuclei of
cyclotetrasilanes have been reported to be observed at d=31.7 and
41.8 ppm for tBu7Si4H,[26a] d=32.0 ppm for trans-tBu6Si4H2,[26a] d=
37.3 ppm for tBu2(tBu2MeSi)3Si4Br3,[26b] d=24.7 ppm for trans-
tBu6Si4Br2,[26c] d=16.0 ppm for tBu4Si4Cl4,[12] d=20.4 ppm for
tBu4iPr2Ph2Si,[26d] d=23.5 ppm for 3a, d=29.9 ppm for 3b,[12] d=
18.0 ppm for 4, and d=16.3 ppm for 5.
[5] J. Fischer, J. Baumgartner, C. Marschner, Science 2005, 310, 825.
[6] D. Nied, R. Kçppe, W. Klopper, H. Schnçckel, F. Breher, J. Am. Chem. Soc.
2010, 132, 10264–10265.
[7] T. Iwamoto, D. Tsushima, E. Kwon, S. Ishida, H. Isobe, Angew. Chem.
2012, 124, 2390–2394; Angew. Chem. Int. Ed. 2012, 51, 2340–2344.
[8] a) G. Fischer, V. Huch, P. Mayer, S. K. Vasisht, M. Veith, N. Wiberg, Angew.
Chem. 2005, 117, 8096–8099; Angew. Chem. Int. Ed. 2005, 44, 7884–
7887; b) T. M. Klapçtke, S. K. Vasisht, P. Mayer, Eur. J. Inorg. Chem. 2010,
3256–3260.
[21] a) S. Kyushin, M. Kawabata, Y. Yagihashi, H. Matsumoto, M. Goto, Chem.
Lett. 1994, 997–1000; b) S. Kyushin, H. Matsumoto, Adv. Organomet.
Chem. 2003, 49, 133–166.
[9] a) D. Scheschkewitz, Angew. Chem. 2005, 117, 3014–3016; Angew.
Chem. Int. Ed. 2005, 44, 2954–2956; b) 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, 7936–7939.
[10] K. Abersfelder, A. J. P. White, H. S. Rzepa, D. Scheschkewitz, Science
2010, 327, 564–566.
[22] H. Kobayashi, T. Iwamoto, M. Kira, J. Am. Chem. Soc. 2005, 127, 15376–
15377.
[23] Iwamoto and co-workers demonstrated the direct observation of the
ring expansion from silylenecyclotetrasilane to cyclopentasilene. Theo-
retical calculations showed that the reaction proceeds via the ring ex-
pansion giving cyclic silylene followed by the migration of a silyl group.
See: T. Iwamoto, Y. Furiya, H. Kobayashi, H. Isobe, M. Kira, Organometal-
lics 2010, 29, 1869–1872.
[11] K. Suzuki, T. Matsuo, D. Hashizume, H. Fueno, K. Tanaka, K. Tamao, Sci-
ence 2011, 331, 1306–1309.
[12] S. Ishida, K. Otsuka, Y. Toma, S. Kyushin, Angew. Chem. 2013, 125, 2567–
2570; Angew. Chem. Int. Ed. 2013, 52, 2507–2510.
[13] A. Tsurusaki, C. Iizuka, K. Otsuka, S. Kyushin, J. Am. Chem. Soc. 2013,
135, 16340–16343.
[14] Wurtz-type coupling of 2 with lithium in THF resulted in the selective
formation of the head-to-head adduct 3b.[12] The reason for the differ-
ent reactivity between sodium and lithium is unclear at present.
[15] When hydrogen chloride was bubbled rapidly, 5 was obtained selec-
tively. See the Supporting Information.
[16] CCDC-999894 (1), CCDC-999895 (4), and CCDC-999896 (5) 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.
[24] We have recently succeeded in the synthesis of 1,1’-bi(2,2,4,4-tetra-tert-
butyl-3,3-dimethylcyclotetrasilanylidene) (15), which has tert-butyl
groups instead of trimethylsilyl groups of Iwamoto and Kira’s com-
pound.[22] No reaction occurred when a solution of 15 in toluene was
refluxed for 12 h. The structural similarity between 11 and 15 may ex-
clude the possibility of the isomerization of 11 to 12 (path A). The syn-
thesis and properties of 15 will be reported in future.
[25] a) M. Weidenbruch, Chem. Rev. 1995, 95, 1479–1493; b) M. Unno, H.
Masuda, H. Matsumoto, Bull. Chem. Soc. Jpn. 1998, 71, 2449–2458.
[26] a) S. Kyushin, H. Sakurai, H. Matsumoto, J. Organomet. Chem. 1995, 499,
235–240; b) A. Sekiguchi, T. Matsuno, M. Ichinohe, J. Am. Chem. Soc.
2001, 123, 12436–12437; c) S. Kyushin, H. Kawai, H. Matsumoto, Orga-
nometallics 2004, 23, 311–313; d) T. Kuribara, S. Kyushin, Acta Crystal-
logr. Sect. E 2013, 69, o149.
[17] T. Iwamoto, K. Uchiyama, C. Kabuto, M. Kira, Chem. Lett. 2007, 36, 368–
369.
[18] H. Matsumoto, H. Miyamoto, N. Kojima, Y. Nagai, M. Goto, Chem. Lett.
1988, 629–632. The dihedral angle was calculated by using the atomic
coordinates reported in this paper.
Received: May 3, 2014
Published online on July 7, 2014
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