Peralkylated four- and five-membered cyclosilanes containing a heteroatom: Synthesis, structure, and properties

Article abstract of DOI:10.1002/1099-0682(200207)2002:7<1772::AID-EJIC1772>3.0.CO;2-F

Seventeen peralkylated four- and five-membered heteroatom-containing silacycles [R¹R²Si]_nX [R¹ = iPr and/or R² = tBuCH₂; n = 3; X = CH₂ (1a, 1b), NR³ (R³ = C₆H₁₁) (3a-d), O (5a-c), and n = 4; X = CH₂ (2a, 2b), NR³ (R³ = C₆H₁₁ or Pr) (4a-d), and O (6a, 6b)] have been synthesized and characterized. The structural features of twelve compounds - 1b, 3a-d, and 5b (Si₃X) and 2a, 4a-c, 6a, and 6b (Si₄X) - determined by X-ray analysis have been investigated and geometrical differences between the silacycles bearing neopentyl groups and those with isopropyl groups as bulky substituents on silicon atoms have been identified. The ring strain energies for the compounds [R₂Si]_nX [n = 3, 4; R = Me, iPr; X = CH₂, N(iPr), NMe, O, SiH₂], estimated by theoretical calculations at the PM3 level, are discussed in relation to experimentally ascertained structural features of the silacycles [R₂Si]_nX (n = 3, 4; R = iPr; X = CH₂, NC₆H₁₁, O, SiR₂). ²⁹Si NMR spectra for 1-6 showed that the resonances for the α-silicon atoms attached to the heteroatoms X in the two ring-size series were shifted downfield by a deshielding effect due to an electronegative atom X; larger electronegativity (En) of X caused this effect at lower field. The chemical shifts of the Si^α atoms showed a good linear correlation with the electronegativities of the heteroatoms X in the [R₂Si]_nX cycles. In the UV spectra for 1-6, little difference in the longest-wavelength absorption (LWA) maxima was observed for [(iPr)₂Si]_nX and [(tBuCH₂)₂Si]_nX of the same ring size. The LWA bands for 1-6 and related compounds could be roughly classified into four groups: (a) [R₂Si]₃X (X = SiR₂, GeR′₂) λ = 286-300 nm; (b) [R₂Si]₃X (X = CH₂, NR³, O) (1, 3, 5), λ = 230-250 nm; (c) [R₂Si]₄X (X = SiR₂, GeR′₂) λ = 274-280 nm; (d) [R₂Si]₄X (X = CH₂, NR³, O) (2, 4, 6), λ = 250-260 nm. With the aid of the ionization potentials and the transition energies (E_T), the HOMO and LUMO levels for the series of compounds and the related ones were experimentally evaluated. The HOMO and LUMO levels thus obtained for [R₂Si]₃X and [R₂Si]₄X (R = iPr and tBuCH₂; X = SiR₂, GeR′₂, CH₂, NC₆H₁₁, O) are discussed in terms of the calculated values and frontier orbitals on the basis of PM3 levels. The theoretical prediction for bond scission in the silacycles [(iPr)₂Si]_nX (n = 3, 4; X = CH₂, NMe, O) in which the HOMOs and LUMOs have Si-Si bonding and antibonding character, respectively, was corroborated by the good agreement with experimental results from photolysis of the silacycles [R₂Si]_nX (n = 3, 4; R = iPr and tBuCH₂; X = CH₂, NC₆H₁₁, O), with bond scission occurring only at the Si-Si bonds, and not at the Si-X bonds. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.

Full text of DOI:10.1002/1099-0682(200207)2002:7<1772::AID-EJIC1772>3.0.CO;2-F

FULL PAPER
Peralkylated Four- and Five-Membered Cyclosilanes Containing a Heteroatom:
Synthesis, Structure, and Properties
Hamao Watanabe,*[a] Hideo Suzuki,[a] Syuji Takahashi,[a] Kiichiro Ohyama,[a]
Yuji Sekiguchi,[a] Hideki Ohmori,[a] Michio Nishiyama,[a] Michihiro Sugo,[a]
Minoru Yoshikawa,[a] Nobuo Hirai,[a] Yoichi Kanuma,[a] Takahiro Adachi,[a]
Masami Makino,[a] Katsura Sakata,[b] Kentaro Kobayashi,[b] Takako Kudo,*[b]
Haruo Matsuyama,[c] Nobumasa Kamigata,[c] Michio Kobayashi,[c] Masashi Kijima,*[d]
Hideki Shirakawa,[d] Kazumasa Honda,[e] and Midori Goto*[e]
Keywords: HOMOϪLUMO interactions / Photolysis / Small ring systems / Silicon / Strained molecules
Seventeen peralkylated four- and five-membered het-
the longest-wavelength absorption (LWA) maxima was ob-
served for [(iPr)2Si]nX and [(tBuCH2)2Si]nX of the same ring
size. The LWA bands for 1−6 and related compounds could
be roughly classified into four groups: (a) [R2Si]3X (X = SiR2,
GeRЈ2) λ = 286−300 nm; (b) [R2Si]3X (X = CH2, NR3, O) (1, 3,
eroatom-containing silacycles [R1R2Si]nX [R1 = iPr and/or
R2 = tBuCH2; n = 3; X = CH2 (1a, 1b), NR3 (R3 = C6H11
)
(3a−d), O (5a−c), and n = 4; X = CH2 (2a, 2b), NR3 (R3
C6H11 or Pr) (4a−d), and O (6a, 6b)] have been synthesized
=
and characterized. The structural features of twelve com- 5), λ = 230−250 nm; (c) [R2Si]4X (X = SiR2, GeRЈ2) λ =
pounds − 1b, 3a−d, and 5b (Si3X) and 2a, 4a−c, 6a, and 6b
(Si4X) − determined by X-ray analysis have been investig-
274−280 nm; (d) [R2Si]4X (X = CH2, NR3, O) (2, 4, 6), λ =
250−260 nm. With the aid of the ionization potentials and the
ated and geometrical differences between the silacycles transition energies (ET), the HOMO and LUMO levels for the
bearing neopentyl groups and those with isopropyl groups
as bulky substituents on silicon atoms have been identified.
The ring strain energies for the compounds [R2Si]nX [n = 3,
4; R = Me, iPr; X = CH2, N(iPr), NMe, O, SiH2], estimated
by theoretical calculations at the PM3 level, are discussed in
relation to experimentally ascertained structural features of
series of compounds and the related ones were experiment-
ally evaluated. The HOMO and LUMO levels thus obtained
for [R2Si]3X and [R2Si]4X (R = iPr and tBuCH2; X = SiR2,
GeRЈ2, CH2, NC6H11, O) are discussed in terms of the calcu-
lated values and frontier orbitals on the basis of PM3 levels.
The theoretical prediction for bond scission in the silacycles
[(iPr)2Si]nX (n = 3, 4; X = CH2, NMe, O) in which the HOMOs
and LUMOs have Si−Si bonding and antibonding character,
respectively, was corroborated by the good agreement with
experimental results from photolysis of the silacycles
the silacycles [R2Si]nX (n = 3, 4; R = iPr; X = CH2, NC6H11
O, SiR2). 29Si NMR spectra for 1−6 showed that the reson-
,
ances for the α-silicon atoms attached to the heteroatoms X
in the two ring-size series were shifted downfield by a deshi-
elding effect due to an electronegative atom X; larger elec- [R2Si]nX (n = 3, 4; R = iPr and tBuCH2; X = CH2, NC6H11, O),
tronegativity (En) of X caused this effect at lower field. The with bond scission occurring only at the Si−Si bonds, and not
chemical shifts of the Siα atoms showed a good linear correla- at the Si−X bonds.
tion with the electronegativities of the heteroatoms X in the
( Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany,
[R2Si]nX cycles. In the UV spectra for 1−6, little difference in
2002)
Introduction
[a]
Department of Materials Science, Faculty of Engineering,
Gunma University,
Kiryu, Gunma 376-8515, Japan
Fax: (internat.) ϩ 81-277/47-5453
E-mail: hwatanab@chem.gunma-u.ac.jp
The chemistry of silacycles containing a heteroatom such
as oxygen, nitrogen, etc. comprises an intriguing subject
Department of Fundamental Studies, Faculty of Engineering, worthy of study regarding their properties. Although a large
[b]
[1]
Gunma University,
Kiryu, Gunma 376-8515, Japan
number of studies of homosilacycles Sin and of het-
eroatom-containing silacycles such as SinGe,[2] SinC,[3]
Department of Chemistry, Faculty of Science, Tokyo
[c]
SinN,[4] and SinO[5] have appeared, there are only limited
Metropolitan University,
Hachioji, Tokyo, 192-0397, Japan
systematic studies focusing on the series of heteroatom-con-
[d]
[e]
Institute of Materials Science, University of Tsukuba,
taining silacycles SinX from the perspective of properties
arising from the presence of X as a heteroatom.[6] We have
previously studied some properties of various peralkylated
cyclosilanes [R1R2Si]n (n ϭ 3Ϫ7)[7] and have also reported
Tsukuba, Ibaraki, 305-8571, Japan
National Institute of Materials and Chemical Research,
Tsukuba, Ibaraki, 305-8565, Japan
Supporting information for this article is available on the
WWW under http://www.eurjic.com or from the author.
1772
 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 1434Ϫ1948/02/0707Ϫ1772 $ 20.00ϩ.50/0 Eur. J. Inorg. Chem. 2002, 1772Ϫ1793

Peralkylated Four- and Five-Membered Cyclosilanes Containing a Heteroatom
FULL PAPER
preliminary results for [R2Si]3O.[8] Recently, we described compounds 1؊6 obtained were found to be air- and mois-
the synthesis, molecular structure, and photolysis of the ture-stable liquids (1a, 5c) or crystals (the others) and were
[R2Si]nGeRЈ2 series [n
ϭ
2:
R
ϭ
tBuCH2, RЈ
ϭ
identified in the usual manner (NMR, IR, and MS data, as
[3a]
Me3SiCH2;[9] n ϭ 3: R ϭ iPr, tBuCH2, RЈ ϭ Me3SiCH2 well as elemental analysis), while [Me2Si]3CH2
and
(C, D);[10] and n ϭ 4: R ϭ iPr, tBuCH2, RЈ ϭ Me3SiCH2, [Ph2Si]4X (X ϭ NMe, NEt, O)[6e] have been reported to be
Ph (GϪI)[11]].
air- and moisture-sensitive compounds.
This paper deals with a full account of the series of het-
eroatom-containing four- and five-membered cyclosilanes
[R1R2Si]nX [n ϭ 3, 4; R1, R2 ϭ iPr, tBuCH2; X ϭ CH2 (1,
2), NR3 (R3 ϭ C6H11 or Pr) (3, 4), O (5, 6)], as shown in
Scheme 1, including molecular structures determined by X-
ray crystal analysis, some properties such as 29Si NMR and
UV spectra, and oxidation and ionization potentials. We
have performed semiempirical PM3[12] molecular orbital
calculations for a series of heterosubstituted silacycles
[R2Si]nX [n ϭ 3, 4; R ϭ Me, iPr; X ϭ SiH2, CH2, NMe,
N(iPr), O] in order to estimate the ring strain energies. The
energy levels and character of their frontier orbitals are also
discussed in relation to their molecular structures, oxidation
and ionization potentials, and photochemical reactions of
the series of heterosilacycles obtained experimentally in the
current study.
Results and Discussion
Synthesis
Compounds 1 and 2 were synthesized in good yields
(72Ϫ92%) by reductive coupling of the corresponding α,ω-
dichlorocarbosilanes with lithium in THF at room temper-
ature [Scheme 2, (a)]. Compounds 3 and 4 were synthesized
in moderate yields (35Ϫ69%) by treatment of the corres-
ponding α,ω-dichlorosilanes with lithium alkylamides
[Scheme 2, (b) and (c)]. Compounds 5 and 6 were synthe-
sized by hydrolytic cyclization with the corresponding α,ω-
dichlorosilanes, in good yields (88Ϫ96%) except in the case
of 5a (28%) [Scheme 2, (d)]. Compound E was also pre-
Scheme 2. (aϪe) Synthetic methods for heteroatom-containing sila-
cycles SinX [n ϭ 3, 4; X ϭ CH2, NR3 and O; n ϭ 4, X ϭ Si(iPr)2]
Molecular Structures
Except for Si3C[3a] and Si4O,[5g] there are no precedents
pared from the corresponding 1,4-dilithiotetrasilane and for structural studies on the ring systems Si4C, SinN (n ϭ
dichlorosilane in a reasonable yield [Scheme 2, (e)]. All the 3, 4), and SinO (n ϭ 3, 4). The molecular structures of 1b,
Scheme 1. Molecular formula of heteroatom-containing silacycles and related compounds
Eur. J. Inorg. Chem. 2002, 1772Ϫ1793
1773

H. Watanabe, T. Kudo, M. Kijima, M. Goto et al.
FULL PAPER
˚
2a, 3a؊d, 4a؊c, 5b, 6a, and 6b were determined by X-ray
crystal analysis; structural data for 1a and 5a could not be
obtained by X-ray analysis. Selected bond lengths and bond
and torsion angles for these compounds are listed in
Tables 1Ϫ12, and the pertinent geometrical data needed for
structural discussion of [R2Si]3X and [R2Si]4X (X ϭ C, N
O) and the related compounds AϪE, H, and I are summar-
ized in Tables 13 and 14.
Table 4. Selected bond lengths [A], bond angles [°], and torsion
angles [°] in 3b
(1) Bond Lengths and Angles
The Si3C ring of 1b was found to be moderately
puckered, with a dihedral angle of 25.5°, and the Si4C ring
of 2a to be a slightly deformed envelope conformation. The
structural data for four known Si3C ring compounds[3bϪ3d]
˚
Table 5. Selected bond lengths [A], bond angles [°], and torsion
angles [°] in 3c
˚
Table 1. Selected bond lengths [A], bond angles [°], and torsion
angles [°] in 1b
˚
Table 6. Selected bond lengths [A], bond angles [°], and torsion
angles [°] in 3d
˚
Table 2. Selected bond lengths [A], bond angles [°], and torsion
angles [°] in 2a
˚
Table 7. Selected bond lengths [A], bond angles [°], and torsion
angles [°] in 4a
˚
Table 3. Selected bond lengths [A], bond angles [°], and torsion
angles [°] in 3a
1774
Eur. J. Inorg. Chem. 2002, 1772Ϫ1793

Peralkylated Four- and Five-Membered Cyclosilanes Containing a Heteroatom
FULL PAPER
˚
˚
Table 8. Selected bond lengths [A], bond angles [°], and torsion
angles [°] in 4b
Table 12. Selected bond lengths [A], bond angles [°], and torsion
angles [°] in 6b (molecule A and molecule B)
˚
Table 9. Selected bond lengths [A], bond angles [°], and torsion
angles [°] in 4c
have been reported previously (dihedral angles: 0Ϫ14°,
SiϪSiϪSi: 77.2Ϫ82.9°, SiϪCϪSi: 100.8Ϫ107.8°, SiϪSi dis-
˚
tances: 2.389Ϫ2.500 A, SiϪC(ring) distances: 1.936Ϫ1.983
˚
A). If the data listed in Tables 1 and 13 for 1b are compared
with those for the known compounds,[3bϪ3d][6a] the dihedral
angle in 1b is seen to be the largest, and both the SiϪSiϪSi
angle and SiϪC(ring) distance in 1b are the smallest. The
˚
Table 10. Selected bond lengths [A], bond angles [°], and torsion
angles [°] in 5b
˚
SiϪSi bond length (2.394 A) in 1b is in the range between
the known Si3C cycles.[3bϪ3d] Among the four inner angles
for the Si3C ring of 1b, SiϪSiϪSi (75.9°) is the smallest
and SiϪCϪSi (102.3°) the largest. A similar trend was also
observed for the five inner angles of the Si4C ring of 2a.
˚
The bond lengths of SiϪSi (2.393 A) and SiϪC(ring)
(1.888 A)[13] in the Si4C of 2a are within the normal ranges.
˚
The four-membered Si3N cycles of 3a؊d are planar or
puckered structures (dihedral angles 21Ϫ23°). The five-
membered Si4N cycles of 4a؊c are in distorted half-chair
or envelope conformations. The SiϪSi bond lengths in
˚
3a؊d are 2.370Ϫ2.384 A. Compound 3a has nearly the
˚
˚
Table 11. Selected bond lengths [A], bond angles [°], and torsion
same distance as in cyclotetrasilane A (2.373 A), while those
in compounds 3b؊d (average 2.381 A) are shorter than
those in cyclotetrasilane B (2.409) by 0.028 A. On the other
angles [°] in 6a
˚
˚
˚
hand, the SiϪSi bond lengths (2.397Ϫ2.402 A) in the five-
membered compounds 4a؊c are, except for the Si2ϪSi3
˚
bond (2.372 A) in 4c, very close to each other. The three
distances are longer than those in the corresponding four-
membered compounds 3a؊d, but shorter than that
[14]
˚
(2.422 A) in the related homosilacycle E.
The SiϪN
˚
bond lengths in the two sets Si3N (3a؊d, 1.757Ϫ1.768 A)
and Si4N (4a؊c, 1.760Ϫ1.775 A) are comparable with the
˚
[13]
˚
typical value of 1.70Ϫ1.76 A,
those involving neopentyl
groups on the silicons being longer than those with isopro-
Eur. J. Inorg. Chem. 2002, 1772Ϫ1793
1775

H. Watanabe, T. Kudo, M. Kijima, M. Goto et al.
Table 13. Geometrical data of four-membered heteroatom-containing silacycles [R2Si]3X
FULL PAPER
[a]
[b]
[c]
[d]
Non-bonding distances. Ref.[7f]
.
Ref.[16]
.
Ref.[10]
Table 14. Geometrical data of five-membered heteroatom-containing silacycles [R2Si]4X
[a]
[b]
[c]
[d]
Average value of the distances of Si(1)ϪSi(2) and Si(3)ϪSi(4).
Ref.[14] [e] Ref.[11]
Distance Si(2)ϪSi(3). Average value from molecules A and B.
pyl groups in each set. Comparison of the four angles be- Table 14. It is of interest to note that the longer SiϪSi bond
tween the two ring sets identifies some interesting features. length in 6b, relative to those in 6a and in the other heteros-
The sizes of the corresponding angles fall in the order: ilacyles Si3X, is similar to the situation in the five-mem-
SiϪNϪSi (109.5°, 3a; 108.7Ϫ109.3°, 3b؊d) Ͼ SiϪSiϪN bered rings E, H, and I. The SiϪO bond lengths in 5b, 6a,
˚
(88.0°, 3a; 85.9Ϫ86.5°, 3b) Ͼ SiϪSiϪSi (74.5°, 3a; and 6b are in the 1.631Ϫ1.656 A range, around the typical
[13]
˚
˚
74.0Ϫ74.3°, 3b؊d). This trend is also observed in the five- values of 1.64Ϯ0.03 A.
The distance (1.656 A) in 5b is
˚
membered Si4N rings of 4a؊c. Interestingly, it could be slightly short relative to those (1.663Ϫ1.721 A) in the sim-
shown that all the nitrogen atoms in 3 and 4 were of almost ilar four-membered compounds cyclo-1,3-disiloxanes
sp2 geometry, from the sums (359.8Ϫ360°) of the three [R2SiO]2.[15] The SiϪSiϪSi bond angles in 5b, 6a, and 6b
angles around the nitrogen atoms, as has also been shown are 68.2, 95.3, and 92.9°, respectively, and are the smallest
in various silylamines[13] and
[H2Si]3NH.[6a]
a
calculated cycle in each set of the two types of rings (68.2Ϫ90.4°, Si3X;
92.9Ϫ106.0°, Si4X), as seen in Tables 13 and 14. Surpris-
The Si3O cycle in 5b and the Si4O cycles in 6a and 6b are ingly, the angle (68.2°) in 5b, with a planar structure, is un-
almost planar (dihedral angle 3°) and a slightly distorted usually small and much closer to the inner angle (60°) of a
envelope (dihedral angle 23°Ϫ25°), respectively, but 6b has three-membered ring rather than that (90°) of a planar four-
a distorted half-chair form. The SiϪSi bond length in 5b membered one. This compound may thus have quite large
˚
is 2.433 A, which is the longest in all the Si3X rings and angular and Pitzer strains, for which supporting results
˚
considerably longer than that (2.409 A) in the related cyclo- were obtained by theoretical calculations of the ring strain
tetrasilane B. The long SiϪSi distance in the small-mem- energies by the PM3 method (vide infra). The SiϪOϪSi
bered rings is reminiscent of the fact that the presence of bond angle in 5b (111.1°) is comparable with those (106.3
the bulky substituents on the highly strained planar Si3O and 112.1°) of the calculated [H2Si]3O,[6a] respectively.
ring should result in remarkably stretched SiϪSi distances, However, the SiϪOϪSi bond angles (133.6 and 132.3°) for
as has been discussed previously for cyclosilanes [R2Si]n 6a and 6b are significantly large compared to those
(n ϭ 3, 4) bearing bulky substituents such as Me3Si and (119.9Ϫ124.6°) in calculated [R2Si]4O (R ϭ H, Me).
tBu groups.[7f] The SiϪSi bond lengths in 6a and 6b are
Finally, the SiϪSi bond lengths in AϪI (except for F) and
˚
˚
2.392Ϫ2.414 A, which is comparable with those 1Ϫ6 in Tables 13 and 14 fall in the 2.370Ϫ2.433 A range,
˚
(2.393Ϫ2.402 A) in the other Si4X rings of 2 and 4 in and are considerably longer than the sums of the covalent
1776
Eur. J. Inorg. Chem. 2002, 1772Ϫ1793

Peralkylated Four- and Five-Membered Cyclosilanes Containing a Heteroatom
FULL PAPER
radii (2.34 A), suggesting that the four- and five-membered the electronegativities[18] of the heteroatoms X in the rings.
rings are fairly strained because of steric repulsion between Thus, if this idea is applicable to the current systems for
the bulky substituents (iPr or tBuCH2) on the silicons and explaining the geometrical properties of such types of small
˚
the angular strains.
rings, the results shown in Tables 13 and 14 seem virtually
to substantiate this concept for the relationship between
parameters such as the electronegativity of X, the SiϪX
bond length, and the SiϪXϪSi angle.
(2) Geometries
Firstly, the geometrical features show the following be-
havior with increasing bulkiness of substituent from an iso-
propyl to a neopentyl group in each pair of heterosilacycles
[R2Si]nX (R ϭ iPr, tBuCH2; X ϭ CH2, NC6H11, O, SiR2,
GeRЈ2) (Tables 13 and 14):
i. The dihedral angles in the four-membered cycles in-
crease (3a, 3b; A, B; C, D).
ii. In the five-membered cycles, the ring torsion angles
increase (4a, 4b; 6a, 6b; H, I), whereas the sums of the in-
ternal angles decrease (4a, 4b; 6a, 6b; H, I).
iii. In all cases, the SiϪSi and SiϪX bond lengths in-
crease, except for SiϪGe in Si3Ge, SiϪSi in Si4N, and SiϪX
in Si4O cycles.
In terms of the molecular structures of heterosilacycles
[R2Si]nX (n ϭ 3, 4; R ϭ iPr, tBuCH2; X ϭ CH2, NC6H11,
O) (1؊6), it is of considerable interest to discuss the mo-
lecular shapes on the basis of the steric repulsions arising
from the bulky substituents on the ring silicon atoms and
the steric requirements around the heteroatoms X, includ-
ing their atomic sizes. Scheme 3 shows schematic diagrams
of four-membered silacycles consisting of X ϭ CH2,
NC6H11, O (see Table 13). Silacycles 3a (X ϭ NC6H11; R ϭ
iPr) and 5b (X ϭ O; R ϭ tBuCH2) are planar rings, while
1b (X ϭ CH2; R ϭ tBuCH2) and 3b؊d (X ϭ NC6H11; R ϭ
tBuCH2) are folded ones. The results show that when bulky
neopentyl groups (steric substituent constant, Es
ϭ
Ϫ1.74)[17] are attached to the two α-silicon atoms (1b,
3b؊d, 5b), heterosilacycle 5b containing (the smallest) oxy-
iv. The SiϪSiϪSi, SiϪSiϪX, and SiϪXϪSi bond angles
decrease, except for SiϪSiϪO in the Si4O cycle.
˚
Consequently, the above results probably reflect the fact
that the compounds bearing neopentyl groups release the
steric repulsions between the bulky substituents through
elongation of the bond lengths and expansion of the dihed-
ral angles or torsion angles, while the compounds carrying
isopropyl groups avoid steric repulsion (Pitzer strains)
through expansion of the internal bond angles and distor-
tion of the molecular shape. The bond elongation with in-
creasing bulkiness of the substituents on the silicon atoms
was also confirmed by the fact that the SiϪSi bond lengths
in the four- and five-membered [R2Si]3X and [R2Si]4X are
longer Ϫ except in only one case (X ϭ SiR2)[14,16] Ϫ than
those in the corresponding calculated cycles [R2Si]3X (R ϭ
H, Me, etc., X ϭ SiH2, CH2, NH, O).[6a]
gen (covalent atomic radius, 0.66 A) as X (or X group), is
planar, whereas the other silacycles, containing nitrogen
˚
˚
(0.70 A; 3b؊d) and carbon (0.77 A; 1b), are folded. How-
ever, in 3a, which contains the medium-sized nitrogen (0.70
˚
A) as X and less bulky isopropyl substituents (Es ϭ
Ϫ0.47[17]) on the α-silicon atoms, the ring shape is also
planar. In addition, the strongly electronegative X atoms,
such as X ϭ O (En ϭ 3.50[18]) and N (En ϭ 3.07[18]) in
silacycles 3aϪd and 5b strongly attract the neighboring sil-
icon atoms (En ϭ 1.74[18]) to produce an SiϪX bond length
shorter than that calculated on the basis of the normal co-
valent radii of silicon and X when there is a large electrone-
gativity difference between them.[19] Furthermore, so-called
(pϪd)π bonding produced by back-donation of the lone-
pair electrons from X to the d-orbitals of α-silicon atoms is
also possible. From these observations, it is thus reasonable
to consider that the ring shapes of the heterosilaycles may
Secondly, the types of heteroatoms X in the four- and
five-membered cycles [R2Si]nX also affect the geometrical
parameters as follows:
i. The dihedral angles vary depending on the different be attributable to the result of a compromise between para-
combinations of heteroatom X, substituent R, and ring size; meters: steric repulsions due to the congestion imposed by
that is, SinX rings with NC6H11 or O as X prefer planar the bulky substituents on the silicon atoms, especially α-
conformations (3a, 5b), while the other silacyles have silicon atoms, the steric requirement around the het-
puckered (n ϭ 3) or distorted conformations (n ϭ 4).
eroatoms X (or X groups) in SiϪXϪSi systems, the ring
ii. For heterosilacycles [R2Si]nX [n ϭ 3, 4; R ϭ iPr, strain energies, and the electronegativity differences be-
tBuCH2; X ϭ Ge(CH2SiMe3)2, CH2, NC6H11, O], as can tween heteroatom X (or heteroatom group) and silicon.
be seen in Tables 13 and 14, the SiϪSiϪSi bond angles and
For five-membered rings (R2Si)nX (n ϭ 4; R ϭ iPr;
Siα···SiαЈ nonbonding distances in Si3X decrease with de- tBuCH2) (2, 4, 6), on the other hand, detailed investigation
creasing atomic radii of the heteroatoms X and SiϪX bond of the molecular shapes is apparently necessary to explain
lengths, whereas the SiϪXϪSi bond angles increase. Altern- their structural features in Table 14 fully.
atively, with increasing electronegativity[18] of X, decreases
29Si NMR Spectra
The 29Si NMR spectroscopic data for 1؊6 are summar-
in the SiϪSiϪSi angles and the Siα···SiαЈ nonbonding dis-
tances and increases in the SiϪXϪSi angles were observed.
Interestingly, similar results from calculations for [H2Si]3X ized in Table 15, together with those for the related com-
(X ϭ SiH2, PH, S, CH2, NH, O) have previously been re- pounds A؊I (except for F). Various features relating to
ported by another group of workers,[19] who explained their variation in ring size, heteroatom X, silicon atom (Siα or
results in terms of a concept, the σ-bridged-π-bonding con- Siβ), and substituents on the silicon atoms [see Scheme 1,
cept, by which the structural properties were correlated with (a)Ϫ(e)] are observed, as follows:
Eur. J. Inorg. Chem. 2002, 1772Ϫ1793
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H. Watanabe, T. Kudo, M. Kijima, M. Goto et al.
FULL PAPER
Scheme 3. Schematic diagrams of four-membered heteroatom-containing silacycles 1b, 3aϪd, and 5b (side views)
Table 15. 29Si NMR spectroscopic data for [(R1R2Si)p(R2Si)2]X[a]
and related compounds
1) The resonances for the α-silicon atoms attached to the
heteroatoms X are shifted downfield by a deshielding effect
due to the electronegative atom X, and Ϫ as shown in Fig-
ure 1 Ϫ the larger the electronegativity (En)[18] of X, the
lower the field of the chemical shifts.
2) The resonance for the β-silicon atoms generally occurs
at higher fields than those for the α-silicon atoms, and there
is no relationship between the chemical shift of the β-silicon
atoms and the electronegativity[18] of X.
3) In the [(iPr)2Si]nX and [(R1R2Si)n{(tBuCH2)2Si}2]X
types (n ϭ 1, 2), the resonances for the α- and β-silicon
atoms appeared at higher fields with increasing ring size,
due to a shielding effect with decreasing ring strain, as has
previously been shown in various cyclosilanes.[20]
4) In each system of SinX cycles, the resonances for both
α- and β-silicon atoms of the compounds bearing neopentyl
groups are shifted to higher fields than those bearing isop-
ropyl groups, since the silicon atom at the γ-position from
the terminal methyl carbon atom in an SiCH2C(CH3)3
group is subject to a high-field shielding effect due to the
terminal carbon atom.[21]
[a]
[b]
[c]
R2 ϭ iPr or tBuCH2. In CDCl3. In C6D6.
Interestingly, it can be seen from Table 15 that the trend
towards an increasing deshielding effect on the Siα chemical
shifts with increasing electronegativity[18] of X is substan-
tially present, regardless of the presence or absence of a
substituent on the heteroatom X. Indeed, as shown in Fig-
ure 1, the chemical shifts of the Siα atoms in each series
showed a good linear correlation with the electronegativ-
ity[18] of the heteroatom X, with good correlation coeffi-
cients (γ ϭ 0.95Ϫ0.99). This is the first example in which a
good linear correlation of δSiα with the electronegativity[18]
of X in the [R2Si]nX cycles has been found.
in Table 16. In the UV spectra of 1a and 1b and of 2a and
2b, the longest-wavelength absorptions occur at λ ϭ 234,
231 and 261, 254 nm, respectively, with varying extinction
coefficients. The absorption maxima for the Si3C cycles (1)
consisting of peralkylated ring silicon atoms are substan-
tially different from those (λ ϭ 502Ϫ538 nm) for known
Si3C cycles with rather particular substituents,[3b][3c] such
as trisilacyclobutaimines [tBu2Si]3CϭNAr. The absorption
maxima of Si4C (2) occur in almost the same region as that
[3j]
(λ ϭ 258 nm) of peralkylated compound [Me2Si]4CH2
and are fairly different from that (λ ϭ 443 nm) of per-
Electronic Spectra
The UV spectra for [R2Si]nCH2 (1, 2), [R1R2Si]nNR3 phenylated tetrasilacyclopentaimine [Ph2Si]4CϭNPh[3f] and
(R3 ϭ C6H11 or Pr) (3, 4), and [R1R2Si]nO (5, 6) are listed that (λ ϭ 325 nm) of partially phenylated allenic tetrasilacy-
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Peralkylated Four- and Five-Membered Cyclosilanes Containing a Heteroatom
FULL PAPER
In the UV spectra of the O-substituted four- and five-
membered compounds 5 and 6, the longest-wavelength ab-
sorption bands occurred at λ ϭ 245Ϫ250 and 255Ϫ260 nm,
respectively. The absorption maxima for the Si4O cycles (6)
are comparable with those (λ ϭ 253Ϫ258 nm) for [Me2-
Si]4O,[6d] [Ph2Si]4O,[6e] and [Me(tBu)Si]4O.[5d] The UV data
for the four-membered compounds 3 and 5 are the first ex-
amples for the Si3N and Si3O ring systems, respectively.
The UV data for 1؊6 in Table 16, together with those
for the related compounds A؊I, showed various interesting
features, as follows:
i. Little difference was observed in the UV data for
[iPr2Si]nX and [(tBuCH2)2Si]nX of the same ring size.
ii. With respect to the longest-wavelength absorption
bands, compounds 1؊6 and A؊I could be roughly classi-
fied by a combination of ring size and heteroatom X into
four groups: (a) [R2Si]3X [X ϭ SiR2, Ge(CH2SiMe3)2]
(A؊D), λ ϭ 280Ϫ300 nm; (b) [R2Si]3X (X ϭ CH2, NC6H11,
O) (1, 3, 5), λ ϭ 230Ϫ250 nm; (c) [R2Si]4X (X ϭ SiR2,
GeRЈ2; RЈ ϭ CH2SiMe3, Ph) (E, G؊I), λ ϭ 270Ϫ300 nm;
(d) [R2Si]4X [X ϭ CH2, NR3 (R3 ϭ C6H11, Pr), O] (2, 4,
6), λ ϭ 250Ϫ260 nm. Compounds belonging to the same
group showed absorption bands in almost the same region
with nearly the same molar extinction coefficients, regard-
less of the nature of X and the substituents on the silicon
atoms.
iii. As the ring size increased, the absorption maxima for
the Ge-substituted compounds C؊D and G؊I hypsoch-
romically shifted by 8Ϫ10 nm with increased extinction co-
efficients, as expected by analogy with the peralkylcyclosil-
anes such as [R2Si]n (R ϭ Me[22a,22b] and Et[22c]), while
those for the C-, N-, and O-substituted compounds 1Ϫ6
bathochromically shifted by 10Ϫ20 nm with decreased ex-
tinction coefficients. This trend resembles that observed
with increasing chain length for linear permethylpolysilanes
Me(Me2Si)nMe,[23] rather than the cyclosilanes[22] men-
tioned above. Similarly, bathochromic shifts with increasing
ring size have previously been observed for the N-substi-
tuted compounds [Me2Si]nX (n ϭ 4Ϫ6).[6b]
Figure 1. Correlation of 29Siα chemical shifts with the electronegat-
ivities (En, see ref.[18]) of heteroatom X in [R2Si]nX (n ϭ 3, 4; R ϭ
iPr, tBuCH2)
clopentane [Me2Si]4CϭCϭCPh2.[3e] It is worth noting that
the UV absorption spectra of 1 are the first results for the
peralkylated four-membered Si3C cycle, since no such spec-
troscopic data have been described even for the only known
peralkylated compound [Me2Si]3CH2.[3a]
Table 16. UV data for [R2Si]nX and related compounds
iv. The molar extinction coefficients (ε ϭ 200Ϫ4100
dm3cmϪ1molϪ1) for compounds AϪI, 2, 4, 6, containing
one germanium or four silicon atoms in the ring systems,
were very small relative to those (ε ϭ 8800Ϫ12000
dm3·cmϪ1·molϪ1) for 1, 3, and 5, containing three silicon
atoms. The large difference in the ε values between the two
sets of ring systems may be due to differences in the forbid-
den σϪπ* or allowed σϪσ* transitions[1a][3j] (vide infra).
Ring Strain Energies by Theoretical Calculation
[a]
[b]
In c-C6H12.
Ref.[7d] [c] Ref.[16] [d] Ref.[10] [e] See also ref.[14] [f]
The ring strain energies of homosilacycles or silicon clus-
ter compounds are a very interesting subject, not only ex-
perimentally but also theoretically. We have previously ex-
Our data. Ref.[7d]
[g]
The N-substituted four- and five-membered cycles 3 and perimentally determined the ring strain energies for a series
4 showed the longest-wavelength absorption bands at λ ϭ of peralkylated homosilacycles [R1R2Si]n as 41, 23, 6, and 0
233Ϫ240 nm and 252 nm (sh), respectively. The absorption kcal/mol for n ϭ 3Ϫ6, respectively.[7e] From the theoretical
(252 nm) for 4 occurred at a longer wavelength region than perspective, on the other hand, the homodesmotic ring
that (235 nm) for [Me2Si]4NMe,[6bϪ6d] but shorter than that strain energies for (SiH2)4 and (SiH2)5 at HF
(282 nm) for [Ph2Si]4NR3 (R3 ϭ Me, Et).[6e]
(HartreeϪFock) or MP2 levels with the basis sets of double
Eur. J. Inorg. Chem. 2002, 1772Ϫ1793
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H. Watanabe, T. Kudo, M. Kijima, M. Goto et al.
FULL PAPER
˚
zeta plus polarization functions quality were calculated to SiϪO (1.65 A), and the small ring-torsion angle (23°),
be ca. 17.0 and 6.0 kcal/mol, respectively.[24] Similar results would be a fairly strained molecule. It is worthwhile to note
(18.7 and 5.7 kcal/mol) have been obtained at the higher that a similar consideration may be applicable to the large
MP4SDTQ/6-31G** level.[25] The calculated values thus strain energy (11.4 kcal/mol) for the heterosilacycles con-
seem to be slightly underestimated for the four-membered taining X ϭ N(iPr) if the X group can accommodate the
ring, although the data for the five-membered ring agree NC6H11 group of 4a (see Table 17, footnote).
well with the above experimentally determined values.
Although some theoretical studies on the types of
[24,25]
homosilacycles (SiH2)n
mentioned above and on
Table 17. Estimated strain energies [kcal/mol] of [R2Si]nX at PM3
(SiH2)nX (X ϭ PH and S)[19,26] are thus available, alkyl- level
substituted homosilacycles (SiR2)n and alkyl-substituted
heteroatom-containing silacycles have not so far been re-
ported. In a preliminary study,[27] we estimated the ring
strain energies for the alkylated four- and five-membered
heterosilacyles [R2Si]nX [R ϭ Me, iPr; n ϭ 3, 4; X ϭ CH2,
NMe, N(iPr), O, SiH2], on the basis of the homodesmotic
reaction energies.[28] The magnitudes of the calculated
strain energies for the series R ϭ iPr, listed in Table 17, were
found to be in the order: n ϭ 3: X ϭ O (5a, 12.8 kcal/mol)
[a] For convenience, the bulkiness of the two β-carbon atoms of the
isopropyl group is assumed to be close to those of the cyclohexyl
group; polar substituent constant σ*(iPr) ϭ Ϫ0.190, σ*(C6H11) ϭ
Ͼ CH2 (1a, 8.7) Ͼ SiH2 (8.1) Ͼ N(iPr) (6.4) Ͼ NMe (2.2);
n ϭ 4: X ϭ N(iPr) (11.4) Ͼ O (6a, 8.3) Ͼ SiH2 (3.9) Ͼ CH2
(2a, 3.1) Ͼ NMe (Ϫ2.1). In the four-membered cycle with
SiH2 as X, it was shown that the PM3 strain energy (8.1
kcal/mol) was much less than the above result (18.7 kcal/
mol) obtained by the ab initio method,[25] and also the ex-
perimentally determined one (23 kcal/mol). The strain ener-
gies calculated for the four-membered rings are much larger
Ϫ0.15 (see ref.[17]
)
Oxidation Potentials
The oxidation potentials (Epa vs. SCE) of the homo- and
than those for the five-membered rings for all kinds of het- heterosilacycles were determined by cyclic voltammetry in
eroatom (or heteroatom group) X except for X ϭ N(iPr), MeCN or CH2Cl2. The results are summarized in
in which the two kinds of silacycles are reversed in the mag- Table 18[7h,29] and Table 19, respectively, together with those
nitude of their strain energies. This may be explained in for the related compounds AϪI (except for E). Firstly, it
terms of the steric repulsions between the bulky groups in- should be noted that this is the first report describing the
cluding the SiϪN(R3)ϪSi system (R3 ϭ iPr; see Table 17, oxidation potentials for silacycles containing a heteroatom
footnote) in the Si4N cycle, as such an effect is apparently in the ring system [R2Si]nX (X ϭ CH2, O, NR3, GeRЈ2).
more severe in the five-membered cycle than in the four- The silacycles also gave irreversible voltammograms similar
membered one, as shown and discussed previously for to those obtained for the homosilacycles, as shown in the
homosilacycles.[7f] This order of the magnitude in the two previous work,[7h] suggesting the formation of their unstable
series of silacycles seems to be reasonable, although more cation-radicals produced from the starting silacycles. Inter-
advanced calculations are necessary.[27] In the four-mem- estingly, the oxidation potentials thus obtained from the
bered rings, it is noteworthy that the strain energy (12.8 four- and five-membered silacycles 1, 3, 5, AϪD and 2, 4,
kcal/mol) for 5a (X ϭ O) is the largest and that the results 6, FϪI roughly fall into nearly the same potential ranges as
[7h]
agree well with prediction from the structural features of found for the homosilacycles Si4 and Si5 in the two solv-
this molecule, that is, the highly strained conformation, as ents. Thus, the potentials for the Si3X rings were 0.8Ϫ1.0 V
shown in Table 13 and discussed in the previous section. in MeCN and 0.96Ϫ1.24 V in CH2Cl2, while those for the
For the silacycles with X ϭ CH2 and SiH2 as X, the strain Si4X rings were 1.0Ϫ1.24 V in MeCN and 1.16Ϫ1.53 V in
energies of Si3X (8.7, 8.1 kcal/mol) and Si4X (3.1, 3.9) are CH2Cl2. As described in the previous work,[7h] solvation of
fairly close to each other, while those for Si3X and Si4X the cation radicals produced by the oxidations may account
cycles where X ϭ NMe (2.2, Ϫ2.1, respectively) are the for the lower potentials in MeCN than in CH2Cl2 for 1Ϫ6.
smallest. However, the considerable strain energy for Si3X, Furthermore, it is also apparent from Table 19 that the po-
X ϭ N(iPr) (6.4 kcal/mol) is of interest, because the con- tentials for the Si3X system decreased relative to those for
formation of the silacycle, including the steric requirements the Si4X system in each solvent system. The results are in
around the SiϪN(R3)ϪSi system, seems to be similar to accord with the trend of the oxidation potentials in a series
that of X ϭ NC6H11 (3a, see also Table 17, footnote). The of homosilacycles [R1R2Si]n (n ϭ 3Ϫ7).[7h,29] The silacycles
large strain energy calculated for Si4X (X ϭ O, 6a) (8.3 k/ of smaller ring size have larger ring strain energies and more
cal) may also be in agreement with the experimental results strongly electron-donating natures than those with larger
shown in Table 2, in which the conformation, consisting of ring sizes.[7e] It was thus found that the oxidation potentials
the relatively small angles SiϪSiϪSi (95°) and SiϪSiϪO were primarily affected by the ring size, regardless of the
(102°), the large angle SiϪOϪSi (134°), the short distance types of heteroatom X and the substituents on the silicon
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Peralkylated Four- and Five-Membered Cyclosilanes Containing a Heteroatom
FULL PAPER
atoms, and Ϫ in addition Ϫ the presence or absence of a
substituent on the heteroatom X.
Table 19. Oxidation and ionization potentials and HOMO and
LUMO levels for four- and five-membered silacycles [R2Si]nX
Table 18. Oxidation and ionization potentials and HOMO and
LUMO levels for homosilacycles [R1R2Si]m and related silacycles
[a]
The first anodic peak potentials; scan rate, 250 mV/s; see also
[a]
The first anodic peak potentials; scan rate, 250 mV/s; see also
ref.[7h] [b] Values calculated using the regression line (Figure 2) ob-
tained from the experimental data of the oxidation (Epa, CH2Cl2)
and ionization potentials (Table 18). Values calculated by using
the HOMO levels and the transition energies obtained from the
ref.[7h] [b] Longest-wavelength absorption band. Reported values
[c]
from UPS measurements. [d] Calculated values based on the regres-
sion line (Figure 2) obtained from the relationship between the ion-
ization (UPS) and oxidation potentials (CV, Epa in CH2Cl2) deter-
mined experimentally for runs number 1, 3, 5, 7, 10, and 16, re-
[c]
[d]
longest-wavelength absorption bands (Table 16).
Transition en-
ergy. Ref.[31]
[e]
[e]
spectively. Values calculated by using the HOMO levels and the
transition energies obtained from the longest-wavelength absorp-
tion bands. [f] Transition energy. [g] Ref.[31] [h] Ref.[29]; scan rate, 500
[i]
[j]
mV/s. RЈ ϭ Me3SiCH2. Ref.[32] [k] Ref.[16] [l] Ref.[34] [m] Ref.[35]
though a better correlation (γ ϭ 0.92) can be obtained if
the former value is disregarded, the reason why the rela-
tively large discrepancy occurred could not be clarified and
remains to be investigated.
Molecular Frontier Orbitals
Generally, it is well known that oxidation potentials ob-
tained by cyclic voltammetry (CV) for various compounds
are linearly correlated to their ionization potentials,[30]
which are useful for evaluation of HOMO levels. Accord-
ingly, the oxidation potentials for this series of heteroatom-
containing silacycles are applicable to estimation of their
HOMO levels, from which their LUMO levels can also be
estimated.
(1) Observed Results
Firstly, in order to determine the HOMO levels for a
series of homosilacyles, we obtained a regression line for
the relationship, with a good correlation coefficient (γ ϭ
0.86 in Figure 2), between the reported ionization potentials
(eV) of some homosilacycles measured by photoelectron
spectroscopy (PES)[31Ϫ33,35] and their oxidation potentials
(Epa, in V vs. SCE in CH2Cl2) determined by the CV
Figure 2. Correlation of the ionization potentials (PES) with the
oxidation potentials (CV) of peralkylated homosilacycles [R2Si]n
(n ϭ 3Ϫ7)
Next, with the aid of the regression line (Figure 2), the
method, as listed in Table 18 (numbers 1, 3, 5, 7, 10, and frontier HOMO levels of all the homosilacycles (R2Si)m
16). Interestingly, it should be pointed out that, in the cur- were estimated, from the oxidation potentials listed in
rent study, the Epa value of (Me2Si)5 was shown to be 1.26 Table 18, as approximately Ϫ6.8, Ϫ7.3, Ϫ7.7, Ϫ7.8,
V (number 10), whereas the reported value is 1.54 V.[29] Al- Ϫ7.6 eV for m ϭ 3Ϫ7 (numbers 1Ϫ18), respectively. The
Eur. J. Inorg. Chem. 2002, 1772Ϫ1793
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H. Watanabe, T. Kudo, M. Kijima, M. Goto et al.
FULL PAPER
Figure 3. HOMO and LUMO levels for four- and five-membered heteroatom-containing silacycles [R2Si]nX (n ϭ 3, 4; R ϭ iPr, tBuCH2)
results clearly exhibit decreasing levels with increasing ring LUMO levels of the former group in comparison to those
size, except for m ϭ 7, which is similar to that of m ϭ 5. of the latter may be attributable to the contribution of the
However, the LUMOs of m ϭ 3Ϫ6 fall within a narrow heteroatom (or heteroatom group) X to the frontier or-
range (numbers 1Ϫ17, Ϫ2.81 to Ϫ3.29 eV), and the level bitals, especially the LUMOs, associated with the con-
for the ring size m ϭ 7 (number 18, Ϫ2.5 eV) appears to be formations due to the four-membered ring systems. In addi-
slightly higher than those for m ϭ 3Ϫ6.
tion, the transition probabilities from the HOMOs to the
In a similar manner, the two frontier levels for the series LUMOs (ε ϭ 8800Ϫ12000 dm3·cmϪ1·molϪ1for 1, 3, 5; ‘‘al-
of heteroatom-containing silacycles [R2Si]nX (R ϭ iPr, lowed’’ transition[36]) for the former definitely appear differ-
tBuCH2; n ϭ 3, 4; X ϭ CH2, O, NR3, SiR2, GeRЈ2) were ent from those for the latter (ε ϭ 200Ϫ590 dm3·cmϪ1·molϪ1
then estimated as listed in Table 19 and shown in Figure 3. for A؊D; ‘‘forbidden’’ transition[36]), as shown in the long-
Of interest is the fact that, in the four-membered cycles (n ϭ est-wavelength UV absorption bands in Table 16.
3), the HOMO levels were found to be very close to each
For the five-membered Si4X (X ϭ SiR2, GeRЈ2, CH2,
other (ca. 7.2Ϫ7.4 eV), due to the close values of their ox- NR3, O), on the other hand, the LUMOs are close to each
idation potentials (Epa ϭ 0.96Ϫ1.24 V). The results suggest other in all ring types, showing lower levels than those seen
that the HOMOs of the [R2Si]3X series lie at very close for the corresponding Si3X, except for X ϭ SiR2. Thus, the
levels, probably arising from similar bonding character in contribution of the heteroatom (or heteroatom group) X to
the four-membered heterosilacycles at the HOMO levels. the LUMOs mentioned above cannot be observed in Fig-
For the frontier orbitals of the five-membered cycles Si4X, ure 5 in the heterosilacycles containing X ϭ CH2, NR3, O
on the other hand, the HOMO levels are also very close to (2, 4, 6), which is related to their conformations. The values
each other (Figure 3), but, as would be expected from their for the transition probabilities (ε
oxidation potentials, are at slightly lower levels than those dm3cmϪ1molϪ1 for 2, 4, 6, E, F, G; ‘‘partially allowed’’
for the Si3X cycles.
transition[36]) in Table 16 fall in the mid-range between the
ϭ
1100Ϫ4100
With regard to the LUMO levels for the Si3X cycles, the two cases in the transitions, including compounds H and I
experimentally determined results can apparently be classi- (ε ϭ 5700Ϫ7600 dm3cmϪ1molϪ1) which contain phenyl
fied by the size of the energy gaps (ET) into two groups: rings that provide resonance effects on the germanium atom
X ϭ CH2, NR3, O (1, 3, 5) and X ϭ SiR2, GeRЈ2 (A, D), in the rings.
as shown in Table 19 and Figure 3. Interestingly, as can be
From the above observations it should therefore be noted
seen in Figure 4 at the LUMOs, including both the first that the concept of a contribution through the heteroatom
LUMO and the second LUMO levels, which are in a very (or heteroatom group) X in the SiϪXϪSi systems to the
narrow range (vide infra), it is likely that the enhanced LUMOs derived from theoretical calculations for the
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Peralkylated Four- and Five-Membered Cyclosilanes Containing a Heteroatom
FULL PAPER
Figure 4. Orbital pictures of the frontier orbitals of [(iPr)2Si]3X (X ϭ SiH2, CH2, NMe, and O)
HOMOs and LUMOs of the heterosilacycles Si3X and Si4X ponding (or similar) heterosilacycles [R2Si]nX [n ϭ 3, 4;
(X ϭ CH2, NR3, O) may account for the above experi- X ϭ CH2 (1a; 2a), NC6H11 (3a; 4a), O (5a; 6a), SiR2 (A;
mental results, although detailed studies are necessary.[27]
E)], respectively (see Table 19 and Figure 3).
The frontier orbitals for the four- and five-membered
rings containing X ϭ CH2, NMe, SiH2, and O are displayed
(2) Theoretical Calculations
The frontier orbital levels determined by PM3 are listed in Figures 4 and 5, respectively. The orbital levels of the
in Table 20. There seems to be no clear trend in the trans- HOMO and the second HOMO, and the LUMO and se-
ition energies or energy gaps (ET) of the molecules of inter- cond LUMO are very close to each other, so that the order
est. In the case of X ϭ SiH2, both HOMO and LUMO of levels can vary depending on the kinds of heteroatom (or
levels lie slightly lower in energy than in the other com- heteroatom group). However, the character of the orbitals is
pounds with heteroatoms (or heteroatom groups). Both en- independent of the natures of X, as the HOMOs are almost
ergy levels are almost the same in both sizes of rings, but localized on the ring frames. This is also true even if the
the energy gaps seem to be slightly smaller in the five-mem- kinds of substituents are changed. Interestingly enough, it
bered rings than in the four-membered ones. Interestingly, was shown from Figures 4 and 5 that photochemical bond
the trend of the calculated ET values in Table 20 for the scission in the heteroatom-containing silacycles [(iPr)2Si]nX
series of compounds [(iPr)2Si]nX [n ϭ 3, 4; X ϭ CH2, NMe, would occur at the SiϪSi bonds and not at the SiϪX bonds,
N(iPr), O, SiH2] is in good agreement with those for the as the HOMOs and LUMOs have SiϪSi bonding and anti-
series of experimental results obtained from the corres- bonding character, respectively.
Eur. J. Inorg. Chem. 2002, 1772Ϫ1793
1783

H. Watanabe, T. Kudo, M. Kijima, M. Goto et al.
FULL PAPER
Figure 5. Orbital pictures of the frontier orbitals of [(iPr)2Si]4X (X ϭ SiH2, CH2, NMe, and O)
investigated photochemical decomposition of the series of
Table 20. Frontier orbital levels [eV] of [(iPr)2Si]nX at PM3 level
silacycles in a hydrocarbon solvent at room temperature or
at 77 K under various reaction conditions in the absence or
presence of reactants to trap reactive intermediates pro-
duced during the reactions.[1b] With a low-pressure mercury
lamp (λ ϭ 254 nm light)[8,37a] or a halogen lamp (λ Ͼ
390 nm light) in the presence of a photosensitizer such as
9,10-dicyanoanthracene,[37b] various photochemical decom-
positions were carried out to give the corresponding prod-
ucts, which were characterized, identified, and quantitat-
ively analyzed (1, 3, 5, 7, 8, 9, 10, and final products). The
results, summarized in Scheme 4, clearly show that the
bond scissions occurred exclusively at the SiϪSi bonds, and
not at the SiϪX bonds, which indicated good agreements
with the above predictions. Finally, it should be noted that
the strain energies in the series of silacycles SinX (n ϭ 3, 4;
X ϭ CH2, NR3, O, GeRЈ2) probably affect the differences
between their reaction rates, as has been previously shown
for thermal reactions of homosilacycles.[1b,38]
[a]
[b]
Transition energy.
For convenience, the bulkiness of the two
β-carbon atoms of the isopropyl group is assumed to be close to
that of the cyclohexyl group; polar substituent constant σ*(iPr) ϭ
Ϫ0.190, σ*(C6H11) ϭ Ϫ0.15 (see ref.[17]).
It seemed quite interesting to compare the theoretical
prediction with experimental results obtained by photo-
chemical cleavage of the heteroatom-containing silacycles
SinX (n ϭ 3, 4; X ϭ CH2, NC6H11, O).[8,37] We therefore
1784
Eur. J. Inorg. Chem. 2002, 1772Ϫ1793

Peralkylated Four- and Five-Membered Cyclosilanes Containing a Heteroatom
FULL PAPER
freshly distilled before use. Butyllithium solution in pentane (ca.
1.78 ) was commercially available. Spectrochemical grade cyclo-
hexane was used for the determination of UV spectra. For cyclic
voltammetry, spectrochemical grade acetonitrile and dichlorome-
thane were dried by heating under reflux with P2O5 and freshly
distilled under nitrogen before use. Other solvents and materials
were commercially available.
Synthesis of C-Substituted Compounds 1 and 2: Compounds 1 and
2 were synthesized by reductive coupling of the corresponding α,ω-
dichlorocarbosilanes with lithium as shown in Scheme 2, (a).
Synthesis of 2,2,3,3,4,4-Hexaisopropyl-2,3,4-trisilacyclobutane (1a):
Typically,
a solution of 1,4-dichloro-1,1,2,2,4,4-hexaisopropyl-
1,2,4-trisilabutane Cl(R2Si)2CH2SiR2Cl (R ϭ iPr) (1a-1, 0.161 g,
0.38 mmol) in a solvent mixture of THF (1.9 mL) and hexane
(10.1 mL) was slowly added over 8 min at room temperature with
stirring to a green suspension of lithium (69 mg, 2.98 mmol) and
biphenyl (23 mg, 0.15 mmol) in THF (3.8 mL). After 2 h of stirring
and addition of hexane, the unchanged lithium was filtered off.
Purification by short column chromatography (silica gel, hexane)
gave 1a (0.134 g, 93% purity by GLC) as a colorless liquid, which
1
was isolated by preparative GLC. H NMR (CDCl3): δ ϭ 0.03 (s,
2 H, C(ring)H2), 1.09Ϫ1.12 [m, 24 H, SiαCH(CH3)2], 1.24 [d, 12 H,
SiβCH(CH3)2], 1.13Ϫ1.20 (m, 4 H, SiαCHMe2), 1.41Ϫ1.50 (m, 2 H,
SiβCHMe2). 13C NMR (CDCl3) Ϫ4.0 (C(ring)H2), 13.6 (SiβCHMe2),
14.9 (SiαCHMe2), 19.7 and 20.0 [SiαCH(CH3)2], 22.5
[SiβCH(CH3)2]. MS: m/z (%) ϭ 356 (100) [Mϩ], 313 (60) [M Ϫ
(iPr)]ϩ, 271 (80) [M Ϫ (iPr) Ϫ C3H6]ϩ. C19H44Si3: calcd. C 63.96,
H 12.43; found C 64.18, H 12.63. The above 1,4-dichlorotrisilabut-
ane (1a-1) was prepared by a method similar to that used for the
preparation of the methyl analogue Br(Me2Si)2CH2SiMe2Br,[3a]
starting from R2SiHCl (R ϭ iPr) via HR2SiCH2Cl (GLC yield,
36%), HR2SiCH2SiR2Ph (74%), ClR2SiCH2SiR2Ph (100%),
PhR2SiSiR2CH2SiR2Ph (100%), and then ClR2SiSiR2CH2SiR2Cl
(1a-1) (70%) (overall yield based on R2SiHCl used, 19%); Colorless
Scheme 4. Photochemical decompositions of heteroatom-con-
taining silacycles SinX (n ϭ 3, 4; X ϭ CH2, NR3, and O) to give
products; the paths shown with a dotted arrow do not seem to exist
Experimental Section
General Procedure: All reactions were carried out in dry flasks un-
der an inert gas (N2 or Ar). A commercially available 30% lithium
dispersion in mineral oil was employed and was usually washed
with the same solvent as used in subsequent reactions unless other-
wise noted. In all preparations, the progress of the reaction in each
step was monitored by GLC analysis. All melting points (uncorrec-
ted) were determined in sealed tubes, and for high melting point
measurements a block heating apparatus equipped with a micro-
1
liquid; b.p. 103Ϫ105 °C/0.2 Torr. H NMR (CDCl3): δ ϭ 0.28 (s,
2 H, SiCH2Si), 1.11Ϫ1.50 [m, 42 H, SiCH(CH3)2]. 13C NMR
(CDCl3): δ ϭ 7.6 (SiCH2Si), 13.5 (Si2CHMe2), 16.5 (Si1CHMe2),
17.0 (Si4CHMe2), 17.7 [Si2CH(CH3)2], 18.1 and 18.6
[Si1CH(CH3)2], 19.6 [Si4CH(CH3)2]. MS: m/z (%) ϭ 383 (5) [M Ϫ
(iPr)]ϩ, 341 (3) [M Ϫ (iPr) Ϫ C3H6]ϩ, 274 (100) [M Ϫ (iPr)2SiCl]ϩ.
C19H44Cl2Si3: calcd. C 53.35, H 10.37; found C 53.27, H 10.36.
1
scope were used. H NMR spectra were recorded with Varian EM
360 A (60 MHz) (for 3b and 5), Hitachi 90H (90 MHz) and Varian
Gemini 200M (200 MHz) spectrometers (for 1, 2, 3a, 3c, 3d, 4, and
6) in CDCl3 or C6D6 with Me4Si as an internal standard. 13C NMR
spectra were recorded with Hitachi 90H (for 3a, 3b, 5, and 6) and
Varian Gemini 200M (for 1, 2, 3c, 3d, and 4) spectrometers in
CDCl3 or C6D6 with Me4Si as an internal standard. For conveni-
ence, some spectroscopic data signals include the multiplicities in
parentheses determined by the off-resonance technique. 29Si NMR
spectra were recorded in CDCl3 or C6D6 with Hitachi 90H (for
3Ϫ6) and JEOL ALPHA 500 (for 1 and 2) spectrometers. Mass
spectra were recorded with a JEOL DX 302 spectrometer (Ip ϭ 30
or 70 eV). UV spectra were obtained with a Hitachi 200-10 spectro-
meter. GLC analysis was performed with an Ohkura GC-103 gas
chromatograph equipped with a glass column (1 m) packed with
SE-30 (10%) on Celite 545-AW (60Ϫ80 mesh). Cyclic voltammetry
was performed with a Hokuto Denko Model HB-107A function
generator and a Hokuto Denko Model HA-101 potentiostat. A
cyclic voltammogram obtained at a scan rate of 250 mV/s was re-
corded with a Rikadenki X-Y recorder Model BW 133. For high-
speed recording, a two-channel wave memory (NF Model MW-
812A) was employed.
Synthesis of 2,2,3,3,4,4-Hexaneopentyl-2,3,4-trisilacyclobutane (1b):
This compound was synthesized by a method similar to that used
for 1a, by treatment of 1,4-dichloro-1,1,2,2,4,4-hexaneopentyl-
1,2,4-trisilabutane Cl(R2Si)2CH2SiR2Cl (1b-1:
R ϭ tBuCH2;
0.599 g, 1.00 mmol, THF 5 mL/hexane 15 mL) with lithium
(0.220 g, 9.5 mmol) and biphenyl (33 mg, 0.21 mmol, THF 10 mL)
for 2.5 h. Compound 1b (recrystallized from ethanol/pentane):
1
0.379 g, 72% yield; m.p. 138Ϫ140 °C. H NMR (CDCl3): δ ϭ 0.44
(s, 2 H, C(ring)H2), 1.05 [s, 18 H, SiβCH2C(CH3)3], 1.06 [s, 36 H,
SiαCH2C(CH3)3], 1.15Ϫ1.19 [m, 12 H, SiCH2(tBu)]. 13C NMR
(CDCl3)
δ
ϭ
Ϫ6.7 (C(ring)H2), 29.5 [SiβCH2(tBu)], 31.4
(SiβCH2CMe3), 32.0 (SiαCH2CMe3), 33.6 [SiαCH2(tBu)], 33.8
[SiαCH2C(CH3)3], 33.9 [SiβCH2C(CH3)3]. MS: m/z (%) ϭ 524 (28)
[Mϩ], 453 (45) [M Ϫ (tBuCH2)]ϩ, 397 (13) [M Ϫ (tBuCH2) Ϫ
C4H8]ϩ, 341 (10) [M Ϫ (tBuCH2) Ϫ (C4H8)2]ϩ. C31H68Si3: calcd.
C 70.90, H 13.10; found C 70.52, H 13.05. The above 1,4-dichloro-
trisilabutane (1b-1) was prepared by a method similar to that used
for 1a-1, starting from R2SiPhCl (R ϭ tBuCH2) via PhR2SiCH2Cl
Materials: Tetrahydrofuran and benzene used in synthesis were
dried with sodium wire and then freshly distilled out from a flask (GLC yield, 86%), PhR2SiCH2SiR2Ph (61%), ClR2SiCH2SiR2Cl
containing benzophenone/ketyl radical before use. Hexane, cyclo- (78%), PhR2SiSiR2CH2SiR2Cl (100%), and then ClR2SiSiR2CH2-
hexane, and pentane were dried with lithium aluminum hydride and SiR2Cl (1bϪ1) (84%) (overall yield based on R2SiPhCl used, 34%);
Eur. J. Inorg. Chem. 2002, 1772Ϫ1793
1785

H. Watanabe, T. Kudo, M. Kijima, M. Goto et al.
m.p. 55Ϫ57 °C. 1H NMR (CDCl3): δ ϭ 0.50 (s, 2 H, SiCH2Si), Synthesis of N-Substituted Compounds 3 and 4: Compounds 3 and
FULL PAPER
1.06, 1.08 and 1.11 [54 H, SiCH2C(CH3)3], 1.13Ϫ1.32 [m, 12 H,
4 were synthesized by treatment of the corresponding α,ω-dichloro-
SiCH2(tBu)]. 13C NMR (CDCl3): δ ϭ 4.8 (SiCH2Si), 32.0, 34.4, silanes with lithium alkylamide as shown in Scheme 2, (b) or (c).
and 37.3 [CH2(tBu)], 31.6, 31.7, and 32.0 (CH2CMe3), 33.2, 33.3,
Synthesis of 1-Cyclohexyl-2,2,3,3,4,4-hexaisopropyl-1-aza-2,3,4-tris-
ilacyclobutane (3a) [Scheme 2, (b)]: Typically, a pentane solution of
butyllithium (1.59 mL, 2.83 mmol) was added dropwise to a solu-
and 34.0 [CH2C(CH3)3]. MS: m/z (%) ϭ 523 (9) [M Ϫ (tBuCH2)]ϩ,
389 (100) [M Ϫ (tBuCH2)2SiCl]ϩ, 333 (10) [M Ϫ (tBuCH2)2SiCl
Ϫ C4H8]ϩ. C31H68Cl2Si3: calcd. C 62.47, H 11.50; found C 62.20,
tion of cyclohexylamine (0.25 mL, 2.18 mmol) in THF (11 mL) at
H 11.42.
Ϫ75 to Ϫ60 °C. With stirring at the same temperature, the reaction
Synthesis of 2,2,3,3,4,4,5,5-Octaisopropyl-2,3,4,5-tetrasilacyclopen-
tane (2a): This compound was synthesized by a similar method
to that used for 1a, by treatment of 1,5-dichloro-1,1,2,2,4,4,5,5-
progress was monitored by GLC for the peak intensity of the
amine. When the intensity had decreased to a minimum, a solution
of 1,3-dichloro-hexaisopropyltrisilane[5a] (0.50 g, 1.21 mmol) in
THF (1 mL) was added to the reaction mixture at the same temper-
ature. After additional stirring for 1 h at that temperature, 1 h at
room temperature, and then heating under reflux for 10 h, the solv-
ents were evaporated to give a solid, to which cyclohexane was
added. After filtration, the resulting solution was concentrated,
treated with water, and extracted with cyclohexane. The extracts
were dried with calcium chloride, filtered, and concentrated to give
a solid product, which was recrystallized from ethanol to afford
colorless crystals of 3a (0.36 g, 57% based on the dichlorotrisilane
used); m.p. 208Ϫ213 °C. 1H NMR (CDCl3): δ ϭ 1.08Ϫ1.15 [m, 24
H, SiαCH(CH3)2], 1.29Ϫ1.32 [m, 12 H, SiβCH(CH3)2], 1.50Ϫ1.77
(m, 6 H, SiCHMe2), 0.7Ϫ2.0 (m, 10 H, C2Ϫ5H2 in C6H11), 2.7Ϫ2.9
(m, 1 H, C1H in C6H11). 13C NMR (CDCl3): δ ϭ 14.8 (d,
SiβCHMe2), 17.0 (d, SiαCHMe2), 19.2 [q, SiαCH(CH3)2], 19.8 [q,
SiαCH(CH3)2], 23.3 [q, SiβCH(CH3)2]; 25.9 (t, C4 in C6H11), 26.7
(t, C3,5 in C6H11), 37.8 (t, C2,6 in C6H11), 57.6 (d, C1 in C6H11).
MS: m/z (%) ϭ 439 (8) [Mϩ], 396 (100) [M Ϫ (iPr)]ϩ, 354 (7) [M
Ϫ (iPr) Ϫ C3H6]ϩ. C24H53NSi3: calcd. C 65.52, H 12.14; found C
64.95, H 11.97.
octaisopropyl-1,2,4,5-tetrasilapentane
Cl(R2Si)2CH2(SiR2)2Cl
(2aϪ1: R ϭ iPr; 0.175 g, 0.32 mmol, THF 6 mL/hexane 5 mL) with
lithium (0.127 g, 5.5 mmol) and biphenyl (42 mg, 0.28 mmol) in
THF (3.5 mL) for 30 min. Compound 2a (recrystallized from eth-
anol): 0.130 g, 87% yield; m.p. 249 Ϫ263 °C. 1H NMR (CDCl3):
δ ϭ Ϫ0.09 (s, 2 H, C(ring)H2), 1.14 [d, 24 H, SiαCH(CH3)2],
1.21Ϫ1.26 [m, 28 H, SiαCHMe2 and SiβCH(CH3)2], 1.41Ϫ1.56 (m,
4 H, SiβCHMe2). 13C NMR (CDCl3) δ ϭ Ϫ5.9 (C(ring)H2), 13.6
(SiβCHMe2), 15.3 (SiαCHMe2), 20.3 and 20.4 [SiαCH(CH3)2], 22.5
and 22.8 [SiβCH(CH3)2]. MS: m/z (%) ϭ 470 (20) [Mϩ], 427 (100)
[M Ϫ (iPr)]ϩ, 385 (35) [M Ϫ (iPr) Ϫ C3H6]ϩ. C25H58Si4: calcd. C
63.74, H 12.41; found C 63.79, H 12.25. The 1,5-dichlorotetrasilap-
entane (2a-1) mentioned above was prepared by HCl bubbling in
the presence of AlCl3 catalyst (2.33 mmol) in benzene (17 mL), by
chlorination of Ph(R2Si)2CH2(SiR2)Cl (R ϭ iPr; 2.27 mmol), which
was obtained in quantitative yield by treatment of 1a-1 (2.27 mmol
in 7 mL of THF) with PhR2SiLi (R ϭ iPr; 3.76 mmol in 4 mL of
THF). Compound 2a-1: 1.27 g, purity 86%; yield 89%; Preparative
1
GLC separation gave analytical samples; viscous liquid. H NMR
(CDCl3): δ ϭ 0.25 (s, 2 H, SiCH2Si), 1.1Ϫ1.4 [m, 56 H,
SiCH(CH3)2]. 13C NMR (CDCl3) δ ϭ Ϫ7.9 (SiCH2Si), 14.5
Synthesis of 1-Cyclohexyl-2,2,3,3,4,4-hexaneopentyl-1-aza-2,3,4-tri-
silacyclobutane (3b): This compound was synthesized by a method
similar to that used for 3a, by treatment of 1,3-dichloro-1,1,2,2,3,3-
(CH2SiCHMe2),
17.2
(ClSiCHMe2),
18.3
and
18.8
[CH2SiCH(CH3)2], 19.8 and 20.2 [ClSiCH(CH3)2]. MS: m/z (%) ϭ
391 (100) [M Ϫ (iPr2SiCl)]ϩ, 349 (40) [M Ϫ (iPr2SiCl)Ϫ C3H6]ϩ.
C25H58Cl2Si4: calcd. C 55.40, H 10.79; found C 54.97, H 10.80.
hexaneopentyltrisilane Cl(R2Si)3Cl (R
ϭ
tBuCH2; 2.00 g,
3.44 mmol)[10] with lithium cyclohexylamide (twice), prepared from
cyclohexylamine (0.79 mL, 6.88 mmol in 20 mL of THF) and bu-
tyllithium (6.2 mL, 11.25 mmol in pentane). Compound 3b (recrys-
tallized from ethanol): 1.43 g, 69% yield; m.p. 228Ϫ238 °C. 1H
NMR (CDCl3): δ ϭ 1.08 [s, 36 H, SiαCH2C(CH3)3], 1.11 [s, 18 H,
SiβCH2C(CH3)3], 1.22 [broad s, 8 H, SiαCH2(tBu)], 1.32 [broad s,
4 H, SiβCH2(tBu)], 1.0Ϫ1.9 (m, 10 H, C2Ϫ5H2 in C6H11), 2.5Ϫ2.9
(m, 1 H, C1H in C6H11). 13C NMR (CDCl3): δ ϭ 31.2 [t,
SiβCH2(tBu)], 31.4 (s, SiαCH2CMe3), 33.4 (s, SiβCH2CMe3), 33.9
[q, SiαCH2C(CH3)3], 34.2 [q, SiβCH2C(CH3)3], 37.5 [t,
SiαCH2(tBu)], 26.8 (t, C4 in C6H11), 28.7 (t, C3,5 in C6H11), 37.8 (t,
C2,6 in C6H11), 59.2 (d, C1 in C6H11). MS: m/z (%) ϭ 607 (60) [Mϩ],
536 (100) [M Ϫ (tBuCH2)]ϩ, 436 (45) [M Ϫ (tBuCH2)2SiH]ϩ, 365
(45) [M Ϫ (tBuCH2)3SiH]ϩ. C36H77NSi3: calcd. C 71.09, H 12.76;
found C 71.13, H 12.62.
Synthesis of 2,2,3,3,4,4,5,5-Octaneopentyl-2,3,4,5-tetrasilacyclopen-
tane (2b): This compound was synthesized by a method similar
to that used for 1a, by treatment of 1,5-dichloro-1,1,2,2,4,4,5,5-
octaneopentyl-1,2,4,5-tetrasilapentane
Cl(R2Si)2CH2(SiR2)2Cl
(2bϪ1: R ϭ tBuCH2; 0.153 g, 0.20 mmol, THF 1 mL/hexane 3 mL)
with lithium (44 mg, 1.9 mmol) and biphenyl (7 mg, 0.047 mmol) in
THF (2 mL) for 1.5 h. Compound 2b (recrystallized from ethanol):
1
0.107 g, 77% yield; m.p. 354Ϫ364 °C. H NMR (CDCl3): δ ϭ 0.47
(s, 2 H, C(ring)H2), 1.07 [s, 36 H, SiβCH2C(CH3)3], 1.08 [s, 36 H,
SiαCH2C(CH3)3], 1.14Ϫ1.36 [m, 16 H, SiCH2(tBu)]. 13C NMR
(CDCl3):
δ
ϭ
1.5 (C(ring)H2), 30.1 [SiβCH2(tBu)], 32.3
(SiβCH2CMe3), 32.6 (SiαCH2CMe3), 33.1 [SiαCH2(tBu)], 34.1
[SiαCH2C(CH3)3], 34.5 [SiβCH2C(CH3)3]. MS: m/z (%) ϭ 694 (31)
[Mϩ], 623 (21) [M Ϫ (tBuCH2)]ϩ, 453 (13) [M Ϫ (tBuCH2)3Si]ϩ.
C41H90Si4: calcd. C 70.81, H 13.04; found C 70.40, H 12.94. The
Synthesis of 1-Cyclohexyl-3-isopropyl-2,2,3,4,4-pentaneopentyl-1-
1,5-dichlorotetrasilapentane (2b-1) mentioned above was prepared aza-2,3,4-trisilacyclobutane (3c): This compound was synthesized
by method similar to that used for 2a-1, from by a method similar to that used for 3a, by treatment of 1,3-
Ph(R2Si)2CH2(SiR2)2Cl (R ϭ tBuCH2; 0.74 mmol), AlCl3 catalyst dichloro-2-isopropyl-1,1,2,3,3-pentaneopentyltrisilane ClSiR2-
SiRRЈSiR2Cl (3c-1: R ϭ tBuCH2, RЈ ϭ iPr; 0.679 g, 1.23 mmol)
a
(1.23 mmol in 6 mL of benzene), and HCl gas, as a crude product,
0.820 g (purity 63% by GLC; yield 91%). Recrystallization from
hexane afforded analytical samples; m.p. 128Ϫ129 °C. 1H NMR
with lithium cyclohexylamide (twice), prepared from cyclohexylam-
ine (0.278 mL, 2.46 mmol in 15 mL of THF) and butyllithium
(CDCl3): δ ϭ 0.31 (s, 2 H, SiCH2Si), 1.07 and 1.12 [72 H, (2.2 mL, 3.20 mmol in pentane). Compound 3c (recrystallized from
SiCH2C(CH3)3], 1.14Ϫ1.49 [m, 16 H, SiCH2(tBu)]. 13C NMR
ethanol): 0.386 g, 54% yield; m.p. 219Ϫ225 °C. 1H NMR (CDCl3):
(CDCl3): δ ϭ 2.9 (SiCH2Si), 33.0 and 34.2 [CH2(tBu)], 31.7 and δ ϭ 1.03 [s, 2 H, SiβCH2(tBu)], 1.09 [s, 36 H, SiαCH2C(CH3)3],
32.8 (CH2CMe3), 33.4 and 34.1 [CH2C(CH3)3]. MS: m/z (%) ϭ 693 1.20Ϫ1.35 [m, 34 H, CH(CH3)2, SiαCH2(tBu), SiβCH2C(CH3)3,
(6) [M
Ϫ
(tBuCH2)]ϩ, 559 (100) [M
Ϫ
(tBuCH2)2SiCl]ϩ.
C
2Ϫ6H2 in C6H11], 2.7Ϫ2.9 (m, 1 H, C1H in C6H11). 13C NMR
C41H90Cl2Si4: calcd. C 64.25, H 11.84; found C 64.16, H 11.79.
(CDCl3):
δ
ϭ
13.6 (SiCHMe2), 21.2 [SiCH(CH3)2], 28.6
Eur. J. Inorg. Chem. 2002, 1772Ϫ1793
1786

Peralkylated Four- and Five-Membered Cyclosilanes Containing a Heteroatom
[SiβCH2(tBu)], 31.1 and 31.6 (SiαCH2CMe3), 31.9 (SiβCH2CMe3), (tBuCH2)]ϩ, 285 (100) [M Ϫ (tBuCH2)2SiH]ϩ. C26H60Si3: calcd. C
FULL PAPER
33.7 [SiβCH2C(CH3)3], 34.2 [SiαCH2C(CH3)3], 37.1 and 38.1
[SiαCH2(tBu)], 26.1 (C4 in C6H11), 26.9 (C3,5 in C6H11), 39.7 (C2,6
in C6H11), 59.5 (C1 in C6H11). MS: m/z (%) ϭ 579 (5) [Mϩ], 536
(3) [M Ϫ (iPr)]ϩ, 508 (100) [M Ϫ (tBuCH2)]ϩ. C34H73NSi3: calcd.
C 70.38, H 12.68; found C 69.90, H 12.69. The 1,3-dichlorotrisilane
(3c-1) mentioned above was prepared by chlorination of the corres-
ponding trisilane HSiR2SiRRЈSiR2H (3c-2; R ϭ tBuCH2, RЈ ϭ
iPr). Phosphorus pentachloride (0.37 g, 1.74 mmol) was added por-
tionwise and slowly to a solution of 3c-2 (0.27 g, 0.56 mmol in
10 mL of benzene), and the mixture was stirred for ca. 18 h at room
temperature. After evaporation of the solvent, hexane was added
to the resulting mixture, which was then filtered. The filtrate, on
evaporation of the solvents, gave 3c-1, viscous liquid: 0.26 g, 0.83%
yield. MS: m/z (%) ϭ 552 (1) [Mϩ], 509 (1) [M Ϫ (iPr)]ϩ, 481(100)
[M Ϫ (tBuCH2)]ϩ, 347 (100) [M Ϫ (tBuCH2)2SiCl]ϩ. The trisilane
3c-2 was prepared by lithium-mediated (0.46 g, 66 mmol in 50 mL
of THF) cross-coupling with chlorodineopentylsilane (5.16 g,
25 mmol) and dichloro(isopropyl)(neopentyl)silane (2.37 g,
11 mmol) in THF (80 mL) (with magnetic stirring and ultrasound
irradiation in a hot water bath). Compound 3c-2 (recrystallized
68.33, H 13.23; found C 68.39, H 13.39.
Synthesis of 1-Cyclohexyl-2,2,3,3,4,4,5,5-octaisopropyl-1-aza-
2,3,4,5-tetrasilacyclopentane (4a): This compound was synthesized
by a method similar to that used for 3a [Scheme 2, (c)], by treat-
ment of 1,4-dichloro-1,1,2,2,3,3,4,4-octaisopropylcyclotetrasil-
ane[11] Cl(R2Si)4Cl (R ϭ iPr; 1.4 g, 2.6 mmol) with lithium cyclo-
hexylamide, prepared from cyclohexylamine (0.9 mL, 7.9 mmol in
5 mL of THF) and butyllithium (6.5 mL, 10.2 mmol in hexane).
Compound 4a (recrystallized from 2-propanol): 0.51 g, 35% yield;
m.p. 346Ϫ350 °C. 1H NMR (δ, C6D6) 1.22Ϫ1.34 [m, 24 H,
SiαCH(CH3)2], 1.36Ϫ1.48 [m, 24 H, SiβCH(CH3)2], 1.60Ϫ1.76 (m,
8 H, SiCHMe2), 1.2Ϫ2.0 (m, 10 H, C2Ϫ6H2 in C6H11), 3.0Ϫ3.2 (m,
1 H, C1H in C6H11). 13C NMR (C6D6) δ ϭ 15.2 (SiβCHMe2), 17.8
(SiαCHMe2), 20.4 and 20.6 [SiαCH(CH3)2], 24.1 and 24.4
[SiβCH(CH3)2]; 26.3 (C4 in C6H11), 27.9 (C3,5 in C6H11), 38.9 (C2,6
in C6H11), 59.0 (C1 in C6H11). MS: m/z (%) ϭ 559 (7) [Mϩ], 510
(100) [M Ϫ (iPr)]ϩ, 468 (20) [M Ϫ (iPr) Ϫ C3H6]ϩ, 446 (20) [M Ϫ
(iPr) Ϫ (C3H6)2]ϩ. C30H67NSi4: calcd. C 65.01, H 12.19; found C
64.50, H 11.98.
1
from ethanol): 1.1 g, 20% yield; m.p. 57Ϫ58 °C. H NMR (C6D6)
Synthesis
of
1-Cyclohexyl-2,2,3,3,4,4,5,5-octaneopentyl-1-aza-
δ
ϭ
1.13(s), 1.15(s), and 0.85Ϫ2.1(m) [62 H, SiCH(CH3)2,
2,3,4,5-tetrasilacyclopentane (4b): This compound was synthesized
by a method similar to that used for 3a, by treatment of 1,4-
SiCH(CH3)2, SiCH2C(CH3)3, SiCH2(tBu)], 4.38 (m, 2 H, SiH). 13
C
dichloro-1,1,2,2,3,3,4,4-octaneopentyltetrasilane[11]
Cl(R2Si)4Cl
NMR (C6D6) δ ϭ 13.6 (SiCHMe2), 21.3 [SiCH(CH3)2], 29.0
[Si2CH2(tBu)], 30.1 (Si2CH2CMe3), 30.7 [Si1,3CH2(tBu)], 31.6
(Si1,3CH2CMe3), 32.9 [Si1,3CH2C(CH3)3], 33.8 [Si2CH2C(CH3)3].
MS: m/z (%) ϭ 484 (7) [Mϩ], 441 (4) [M Ϫ (iPr)]ϩ, 413 (10) [M Ϫ
(tBuCH2)]ϩ, 313 (100) [M Ϫ (tBuCH2)2SiH]ϩ. C28H64Si3: calcd. C
69.33, H 13.30; found C 69.01, H 13.32.
(R ϭ tBuCH2; 0.58 g, 0.77 mmol) with lithium cyclohexylamide,
prepared from cyclohexylamine (1.4 mL, 12.3 mmol in 3 mL of
THF) and butyllithium (10.1 mL, 16.0 mmol in pentane). Com-
pound 4b (recrystallized from 2-propanol): 0.23 g, 38% yield; m.p.
1
379Ϫ383 °C. H NMR (CDCl3): δ ϭ 1.12 (s), and 1.13 (s) [72 H,
SiCH2C(CH3)3], 0.7Ϫ1.0 (m), 1.15Ϫ1.30 (m), and 1.30Ϫ2.2 (m) [26
H, C2Ϫ6H2 in C6H11, SiCH2(tBu)3], 3.0Ϫ3.2 (m, 1 H, C1H in
C6H11). 13C NMR (CDCl3) (tentative) δ ϭ 30.3 [t, SiβCH2(tBu)],
31.7 (s, SiβCH2CMe3), 32.1 (s, SiαCH2CMe3), 34.4 [q,
SiβCH2C(CH3)3], 34.5 [q, SiαCH2C(CH3)3], 36.9 [t, SiαCH2(tBu)],
26.0 (t, C4 in C6H11), 27.1 (t, C3,5 in C6H11), 37.8 (t, C2,6 in C6H11),
59.5 (d, C1 in C6H11). MS: m/z (%) ϭ 777 (3) [Mϩ], 706 (100) [M
Ϫ (tBuCH2)]ϩ, 536 (30) [M Ϫ (tBuCH2)3Si]ϩ. C46H99NSi4: calcd.
C 70.78, H 12.82; found C 70.96, H 12.82.
Synthesis of 1-Cyclohexyl-3,3-diisopropyl-2,2,4,4-tetraneopentyl-1-
aza-2,3,4-trisilacyclobutane (3d): This compound was synthesized
by a method similar to that used for 3a, by treatment of 1,3-
dichloro-2,2-diisopropyl-1,1,3,3-tetraneopentyltrisilane
ClSiR2-
SiRЈ2SiR2Cl (3d-1: R ϭ tBuCH2, RЈ ϭ iPr; 0.55 g, 1.0 mmol) with
lithium cyclohexylamide, prepared from cyclohexylamine (1.2 mL,
10.4 mmol in 7 mL of THF) and butyllithium (8.2 mL, 13.5 mmol
in pentane). Compound 3d (recrystallized from 2-propanol): 0.23 g,
42% yield; m.p. 291Ϫ297 °C. 1H NMR (CDCl3): δ ϭ 1.09 [s, 36
Synthesis of 1-Cyclohexyl-3,3,4,4-tetraisopropyl-2,2,5,5-tetraneo-
H, CH2C(CH3)3], 0.76Ϫ1.95 [m, 32 H, CH(CH3)2, CH2(tBu), pentyl-1-aza-2,3,4,5-tetrasilacyclopentane (4c): This compound was
2Ϫ6H2 in C6H11], 2.70Ϫ2.85 (m, 1 H, C1H in C6H11). 13C NMR
synthesized by a method similar to that used for 3a, by treatment
(CDCl3): 12.9 (SiCHMe2), 21.6 [SiCH(CH3)2], 31.1 of 1,4-dichloro-2,2,3,3-tetraisopropyl-1,1,4,4-tetraneopentyltetrasi-
(CH2CMe3), 34.0 [CH2C(CH3)3], 37.6 [SiCH2(tBu)], 25.9 (C4 in lane ClSiR2(SiRЈ2)2SiR2Cl (4c-1: R ϭ tBuCH2, RЈ ϭ iPr; 0.59 g,
C
δ
ϭ
C6H11), 26.8 (C3,5 in C6H11), 38.2 (C2,6 in C6H11), 59.0 (C1 in
0.90 mmol) with lithium cyclohexylamide, prepared from cyclohex-
C6H11). MS: m/z (%) ϭ 651 (10) [Mϩ], 508 (25) [M Ϫ (iPr)]ϩ, 482 ylamine (3.46 mL, 28.7 mmol in 8 mL of THF) and butyllithium
(100) [M Ϫ (tBuCH2)]ϩ. C32H69NSi3: calcd. C 69.61, H 12.60; (17.1 mL, 27.9 mmol in pentane). Compound 4c (recrystallized
found C 69.42, H 12.46. The 1,3-dichlorotrisilane (3d-1) mentioned
1
from 2-propanol): 0.28 g, 47% yield; m.p. 364Ϫ366 °C. H NMR
(C6D6) δ ϭ 1.09 [s, 36 H, CH2C(CH3)3], 0.78Ϫ2.10 [m, 51 H,
above was prepared by chlorination of the corresponding trisilane
HSiR2SiRЈ2SiR2H (3d-2:
R ϭ tBuCH2, RЈ ϭ
iPr; 0.52 g, CH(CH3)2, CH2(tBu), C2Ϫ6H2 in C6H11], 3.10Ϫ3.25 (m, 1 H, C1H
1.13 mmol) with phosphorus pentachloride (1.67 g, 7.97 mmol) in
benzene (7 mL). Compound 3d-1: viscous liquid; 0.55 g, 92% yield.
MS: m/z (%) ϭ 481 (4) [M Ϫ (iPr)]ϩ, 453 (20) [M Ϫ (tBuCH2)]ϩ,
319 (100) [M Ϫ (tBuCH2)2SiCl]ϩ. The trisilane 3d-2 was prepared
by lithium-mediated (0.62 g, 89 mmol in 13 mL of THF) cross-
coupling with chlorodineopentylsilane (5.71 g, 28 mmol) and
dichlorodiisopropylsilane (2.38 g, 13 mmol) in THF (15 mL). Com-
pound 3d-2 (recrystallized from ethanol): 2.37 g, 40% yield; m.p.
in C6H11). 13C NMR (CDCl3): δ ϭ 14.5 (SiCHMe2), 23.3 and 23.8
[SiCH(CH3)2], 31.7 (CH2CMe3), 34.1 [CH2C(CH3)3], 36.7
[CH2(tBu)], 26.0 (C4 in C6H11), 27.0 (C3,5 in C6H11), 37.6 (C2,6 in
C6H11), 60.0 (C1 in C6H11). MS: m/z (%) ϭ 665 (3) [Mϩ], 622 (10)
[M Ϫ (iPr)]ϩ, 594 (100) [M Ϫ (tBuCH2)]ϩ. C38H83NSi4: calcd. C
68.49, H 12.55; found C 68.49, H 12.46. The 1,4-dichlorotrisilane
(4c-1) mentioned above was prepared by chlorination of the corres-
ponding tetrasilane HSiR2(SiRЈ2)2SiR2H (4c-2: R ϭ tBuCH2, RЈ ϭ
67Ϫ68 °C. 1H NMR (CDCl3): δ ϭ 1.02 [s, 36 H, CH2C(CH3)3], iPr; 1.00 g, 1.75 mmol) with phosphorus pentachloride (1.30,
0.78Ϫ1.50 [m, 22 H, CH(CH3)2, CH2(tBu)], 4.10Ϫ4.17 (m, 2 H, 6.19 mmol) in benzene (15 mL). Compound 4c-1 (recrystallized
SiH). 13C NMR (CDCl3): δ ϭ 14.7 (CHMe2), 23.0 [CH(CH3)2], from hexane): 0.53 g, 47% yield; m.p. 65Ϫ66 °C. MS: m/z (%) 569
31.8 [CH2(tBu)], 33.2 (CH2CMe3), 34.5 [CH2C(CH3)3]. MS: m/z
(10) [M Ϫ (tBuCH2)]ϩ, 433 (100) [M Ϫ (tBuCH2)2SiCl]ϩ. The
(%) ϭ 456 (3) [Mϩ], 413 (6) [M Ϫ (iPr)]ϩ, 385 (8) [M Ϫ tetrasilane 4c-2 was prepared by lithium-mediated (1.51 g,
Eur. J. Inorg. Chem. 2002, 1772Ϫ1793
1787

H. Watanabe, T. Kudo, M. Kijima, M. Goto et al.
FULL PAPER
270 mmol in 80 mL of THF) cross-coupling with 1,2-dichlorote-
traisopropyldisilane (7.36 g, 24.6 mmol) and chlorodineopentylsil-
ane (14.24 g, 68.3 mmol) in THF (20 mL). Compound 4c-2 (recrys-
34.0 [q, SiβCH2C(CH3)3], 36.5 [t, SiαCH2(tBu)]. MS: m/z (%) ϭ 526
(30) [Mϩ], 511 (25) [M Ϫ Me]ϩ, 455 (90) [M Ϫ (tBuCH2)]ϩ, 399
(80) [M Ϫ (tBuCH2) Ϫ C4H8]ϩ. C30H66OSi3: calcd. C 68.36, H
tallized from pentane/ethanol): 10.6 g, 69% yield; m.p. 94Ϫ95 °C. 12.62; found C 68.24, H 12.63.
1H NMR (CDCl3): δ ϭ 1.04 [s, 36 H, CH2C(CH3)3], 0.82Ϫ1.50 [m,
Synthesis of 2,4-Diisopropyl-2,3,3,4-tetraneopentyl-1-oxa-2,3,4-trisi-
lacyclobutane (5c): This compound was synthesized by a method
similar to that used for 5a, by hydrolysis of 1,3-dichloro-1,3-diiso-
36 H, CH(CH3)2, CH2(tBu)], 4.20Ϫ4.28 (m, 2 H, SiH). 13C NMR
(CDCl3): δ ϭ 13.9 (CHMe2), 22.2 [CH(CH3)2], 29.8 (CH2CMe3),
31.5 [CH2(tBu)], 32.7 [CH2C(CH3)3]. MS: m/z (%) ϭ 570 (1) [Mϩ],
527 (10) [M Ϫ (iPr)]ϩ, 499 (20) [M Ϫ (tBuCH2)]ϩ, 399 (100) [M Ϫ
(tBuCH2)2SiH]ϩ. C32H74Si4: calcd. C 67.28, H 13.06; found C
67.06, H 13.07.
propyltetraneopentyltrisilane ClSiRRЈSiR2SiRRЈCl (5c-1:
R ϭ
tBuCH2, RЈ ϭ iPr; 0.50 g, 0.95 mmol) with H2O (1 mL) in triethyl-
amine (15 mL), followed by workup of the resulting mixture. Com-
pound 5c: 0.39 g (purity: 92%) 87% yield, liquid; analytical samples
were isolated by GLC purification. 1H NMR (C6D6) δ ϭ 1.05Ϫ1.30
(m) (composed of four peaks at 1.16, 1.19, 1.22, 1.25) [48 H,
SiCH(CH3)2, SiCH2C(CH3)3], 1.48 (broad s, 2 H, SiCHMe2), 1.60
[broad s, 8 H, SiCH2(tBu)]. C26H58OSi3: calcd. C 66.30, H 12.41;
found C 66.10, H 12.42. The 1,3-dichlorotrisilane (5c-1) mentioned
above was prepared by chlorination of the corresponding trisilane
Synthesis of 1-Propyl-2,2,3,3,4,4,5,5-octaneopentyl-1-aza-2,3,4,5-
tetrasilacyclopentane (4d): This compound was synthesized by a
method similar to that used for 3a, by treatment of 1,4-dichlorooc-
taneopentyltetrasilane[11] Cl(R2Si)4Cl (R
ϭ tBuCH2; 0.52 g,
0.69 mmol) with lithium propylamide, prepared from propylamine
(0.44 g, 5.34 mmol in 3 mL of THF) and butyllithium (4.0 mL,
7.01 mmol in pentane). Compound 4d (recrystallized from 2-pro-
panol): 0.18 g, 35% yield; m.p. 396Ϫ399 °C. 1H NMR (CDCl3):
δ ϭ 0.83 (t, 3 H, NCH2CH2CH3), 1.07 [s, 36 H, SiαCH2C(CH3)3],
1.12 [s, 36 H, SiβCH2C(CH3)3], 1.17 (s), 1.24 (s), 1.31 (s), 1.38 (s),
1.43 (s), and 1.46Ϫ1.75 (m) [18 H, NCH2CH2CH3, SiαCH2(tBu),
SiβCH2(tBu)], 2.90Ϫ3.10 (m, 2 H, NCH2CH2CH3). 13C NMR
HSiRRЈSiR2SiRRЈH (5c-2:
R ϭ tBuCH2, RЈ ϭ iPr; 6.5 g,
14 mmol/CCl4 30 mL) by chlorine gas bubbling. Compound 5c-1:
b.p. 185Ϫ195 °C/3 Torr; 3.8 g, 52% yield. 1H NMR (C6D6) δ ϭ
1.16 (s), and 1.18 (s) [48 H, CH(CH3)2, CH2C(CH3)3], 1.35 [m, 4
H, Si1,3CH2(tBu)], 1.50 [broad s, 4 H, Si2CH2(tBu)], 1.73 (m, 2 H,
CHMe2). C26H58Cl2Si3: calcd. C 59.38, H 11.12; found C 59.41, H
11.15. The trisilane 5c-2 was prepared by lithium-mediated (fine
cut, 0.4 g, 58 mmol in 40 mL of THF) cross-coupling with di-
chlorodineopentylsilane (2.4 g, 10.0 mmol) and chloro(isopropyl)-
(neopentyl)silane (4.2 g, 23.6 mmol) in THF (15 mL). Compound
(CDCl3):
δ ϭ
30.0 [SiβCH2(tBu)], 31.6 (SiαCH2CMe3), 32.0
(SiβCH2CMe3), 34.2 [SiβCH2C(CH3)3], 34.4 [SiαCH2C(CH3)3], 36.5
[SiαCH2(tBu)], 11.7 (C3 in C3H7), 27.8 (C2 in C3H7), 51.6 (C1 in
C3H7). MS: m/z (%) ϭ 737 (3) [Mϩ], 666 (100) [M Ϫ (tBuCH2)]ϩ,
497 (15) [M Ϫ (tBuCH2)4Si2]ϩ. C43H95NSi4: calcd. C 69.93, H
12.96; found C 69.57, H 12.80.
1
5c-2: b.p. 124Ϫ127 °C/5 Torr; 1.8 g, 39% yield. H NMR (CDCl3):
Synthesis of O-Substituted Compounds 5 and 6: Compounds 5 and
6 were synthesized by treatment of the corresponding α-ω-dichloro-
silanes with H2O or NaOH as shown in Scheme 2, (d).
Table 21. Crystal data, data collection, and refinement for four-
and five-membered silacycles [R2Si]nX (X ϭ CH2)
Synthesis of 2,2,3,3,4,4-Hexaisopropyl-1-oxa-2,3,4-trisilacyclobut-
ane (5a): Typically, H2O (1 mL, 56 mmol) was added at 53 °C with
stirring to
a
solution of 1,3-dichlorohexaisopropyltrisilane[5a]
(3.50 g, 8.5 mmol) in degassed triethylamine (20 mL). After the
mixture had been stirred for 1 h at this temperature, degassed cyclo-
hexane was added, and the mixture was extracted and dried with
calcium chloride. After filtration, the filtrate was concentrated to
give a solid product, which was recrystallized from methanol to
afford colorless crystals of 5a (see also ref.[8]) (0.85 g, 28%); m.p.
100Ϫ103 °C. 1H NMR (C6D6) δ ϭ 1.0Ϫ1.6 [m, 42 H, SiCH(CH3)2,
SiCH(CH3)2]. 13C NMR (C6D6) δ ϭ 13.7 (d, SiβCHMe2), 16.6 (d,
SiαCHMe2), 17.9 [q, SiαCH(CH3)2], 22.7 [q, SiβCH(CH3)2]. MS: m/
z (%) ϭ 358 (40) [Mϩ], 315 (100) [M Ϫ (iPr)]ϩ, 273 (35) [M Ϫ (iPr)
Ϫ C3H6]ϩ. C18H42OSi3: calcd. C 60.26, H 11.80; found C 59.58,
H 11.72.
Synthesis of 2,2,3,3,4,4-Hexaneopentyl-1-oxa-2,3,4-trisilacyclobut-
ane (5b): 1,3-Dichlorohexaneopentyltrisilane[10] Cl(R2Si)3Cl (R ϭ
tBuCH2; 0.59 g, 1.0 mmol in 10 mL of THF) was added to a mix-
ture of NaOH (0.16 g, 4.0 mmol), THF (5 mL), and EtOH (5 mL).
The mixture was stirred for 20 min at room temperature. After ad-
dition of hexane and filtration, the filtrate was concentrated and
cyclohexane was then added. The solution was washed with dilute
hydrochloric acid solution and water, dried, and then concentrated
to give a crude product. Compound 5b (recrystallized from ethanol/
1
pentane) (see also ref.[8]): 0.41 g, 77% yield; m.p. 151Ϫ153 °C. H
NMR (C6D6) δ ϭ 0.97 [s, 18 H, SiβCH2C(CH3)3], 1.03 [s, 36 H,
SiαCH2C(CH3)3], 1.22 [s,
4 8 H,
H, SiβCH2(tBu)], 1.28 [s,
SiαCH2(tBu)]. 13C NMR (CDCl3): δ ϭ 29.0 [t, SiβCH2(tBu)], 31.5
(s, SiαCH2CMe3), 31.8 (s, SiβCH2CMe3), 33.7 [q, SiαCH2C(CH3)3],
[a]
[b]
w ϭ 1/σ2 |Fo|. w ϭ 1/σ2 |Fo|.
1788
Eur. J. Inorg. Chem. 2002, 1772Ϫ1793

Peralkylated Four- and Five-Membered Cyclosilanes Containing a Heteroatom
δ ϭ 0.61Ϫ1.96 (m), and 1.06 (m) [58 H, CH(CH3)2, CH2C(CH3)3],
FULL PAPER
δ
ϭ
29.6 [t, SiβCH2(tBu)], 31.8 (s, SiαCH2CMe3), 32.0 (s,
3.88 (m, 2 H, SiH). C26H60Si3: calcd. C 68.33, H 13.23; found C SiβCH2CMe3), 34.0 [q, SiαCH2C(CH3)3], 34.3 [q, SiβCH2C(CH3)3],
67.80, H 13.18.
36.0 [t, SiαCH2(tBu)]. MS: m/z (%) ϭ 696 (100) [Mϩ], 625 (65) [M
Ϫ
(tBuCH2)]ϩ, 525 (50) [M (tBuCH2)3Si]ϩ, 340 (70)
Ϫ
Synthesis of 2,2,3,3,4,4,5,5-Octaisopropyl-1-oxa-2,3,4,5-tetrasilacy-
clopentane 6a: This compound was synthesized by a method similar
to that used for 5b, by hydrolysis of 1,4-dichlorooctaisopropyltetra-
silane[11] Cl(R2Si)4Cl (R ϭ iPr; 1.02 g, 1.9 mmol) with H2O (3 mL)
in THF (7 mL) at 50Ϫ60 °C for 15 h. Compound 6a (recrystallized
from methanol/ethanol): 0.88 g, 96% yield; m.p. 308Ϫ315 °C. 1H
NMR (CDCl3): δ ϭ 1.0Ϫ1.15 [m, 24 H, SiαCH(CH3)2], 1.20Ϫ1.35
[m, 24 H, SiβCH(CH3)2], 1.35Ϫ1.56 (m, 8 H, SiCHMe2). 13C NMR
(CDCl3): δ ϭ 13.4 (d, SiβCHMe2), 17.5 (d, SiαCHMe2), 18.2 [q,
SiαCH(CH3)2] and 18.8 [q, SiαCH(CH3)2], 22.6 [q, SiβCH(CH3)2]
and 22.9 [q, SiβCH(CH3)2]. MS: m/z (%) ϭ 472 (15) [Mϩ], 429
(100) [M Ϫ (iPr)]ϩ, 387 (50) [M Ϫ (iPr) Ϫ C3H6]ϩ, 345 (35) [M Ϫ
(iPr) Ϫ (C3H6)2]ϩ. C24H56OSi4: calcd. C 60.29, H 11.90; found C
60.94, H 11.93.
[(tBuCH2)4Si2]ϩ. C40H88OSi4: calcd. C 68.88, H 12.72; found C
68.58, H 12.81.
Synthesis of Decaisopropylpentasilane E: This compound was syn-
thesized (not optimized) by treatment of 1,4-dilithiooctaisopropyl-
tetrasilane (in place of 1,4-dipotassiooctairopropyltetrasilane[14]
with dichlorodiisopropylsilane [Scheme 2, (e)]. Freshly distilled an-
hydrous HMPA (1.1 mL) was added to a mixture of octaisopropyl-
cyclotetrasilane A (168 mg, 0.368 mmol) and lithium (51 mg,
0.735 mmol). The mixture was stirred at room temperature for 2 h
to afford a dark red-brown solution containing 1,4-dilithiooctaiso-
propyltetrasilane, to which benzene (5 mL) was then added. This
solution, containing the 1,4-dilithiotetrasilane, was added dropwise
by hypodermic syringe to a separate flask containing dichlorodiiso-
propylsilane (70 µL, 0.401 mmol) in benzene (2.5 mL) in an ice/
Synthesis of 2,2,3,3,4,4,5,5-Octaneopentyl-1-oxa-2,3,4,5-tetrasilacy-
clopentane (6b): This compound was synthesized by a method sim- water bath. After stirring at 0 °C for 30 min, the reaction mixture
ilar to that used for 5a, by treatment of 1,4-dichlorooctaneopentyl- was mixed well with hexane (60 mL) and water (80 mL). After
tetrasilane[11] Cl(R2Si)4Cl (R ϭ tBuCH2; 0.58 g, 0.77 mmol) with separation of the organic layer, it was washed well with water, dried
NaOH (ca. 1 mL of 1  aq. solution) in THF (15 mL) at 50Ϫ60
°C for 20 h. Compound 6b (recrystallized from hexane/ethanol):
with calcium chloride, and then filtered. The solution obtained was
concentrated to give a solid product mixture, from which the reac-
0.5 g, 93% yield; m.p. 377Ϫ380 °C. 1H NMR (CDCl3): δ ϭ 1.08 [s, tion product, decaisopropylcyclopentasilane E, was isolated by re-
36 H, SiβCH2C(CH3)3], 1.10 [s, 36 H, SiαCH2C(CH3)3], 1.16 [s, 8 peated fractional recrystallization from a mixture of pentane/eth-
H, SiβCH2(tBu)], 1.29 [s, 8 H, SiαCH2(tBu)]. 13C NMR (CDCl3): anol to afford 40 mg, 19% yield, m.p. 357Ϫ366 °C. It was found
Table 22. Crystal data, data collection, and refinement for four-membered silacycles [R2Si]3X (X ϭ NR3)
[a]
[b]
[c]
w ϭ 1/[84.2·(sin θ/λ)2 Ϫ 51.4·(sin θ/λ) ϩ 8.9].
w ϭ 1/(0.0078·|Fo|2 Ϫ 0.432·|Fo| ϩ 7.88).
w ϭ 1/(0.0078·|Fo|2 Ϫ 0.06402·|Fo| ϩ
1.54216). w ϭ 1/(0.00788·|Fo|2 Ϫ 0.21670·|Fo| ϩ 6.00345).
[d]
Eur. J. Inorg. Chem. 2002, 1772Ϫ1793
1789

H. Watanabe, T. Kudo, M. Kijima, M. Goto et al.
FULL PAPER
that crystalline materials obtained from the filtrates contained the
product E (ca. 20 mg, calculated), the unchanged starting com-
sian matrix of energy second derivatives at the same level of theory.
The orbital pictures were drawn with the Gamess program.[43] The
optimized structures for all four-membered rings are found to have
pound (A), and a small amount of unidentified material. Com-
1
pound E (see also ref.[14]): H NMR (CDCl3): δ ϭ 1.33 [d, 60 H, Cs symmetry with a σ plane involving one silicon atom and het-
SiCH(CH3)2], 1.43Ϫ1.62 (m, 10 H, SiCHMe2). 13C NMR (CDCl3): eroatom, except for X ϭ N(iPr) which is in a C1 conformation. On
δ ϭ 15.9 (SiCHMe2), 23.8 [SiCH(CH3)2]. MS: m/z (%) ϭ 570 (5) the other hand, none of the five-membered rings have a symmetry
[Mϩ], 527 (100) [M Ϫ iPr]ϩ, 485 (15) [M Ϫ iPr Ϫ C3H6]ϩ, 413 (10)
higher than C1. The ring strain energies for the four- and five-
[M Ϫ iPr Ϫ 2(C3H6)]ϩ. C30H70Si5: calcd. C 62.25, H 12.45; found membered ring compounds (R ϭ iPr) were estimated on the basis
C 63.07, H 12.35.
of the homodesmotic reaction energies[28] [Equations (1) and (2)].
Oxidation Potentials: Measurements of the oxidation poten-
tials[7h,41] for 1Ϫ6 (10Ϫ3 ) by cyclic voltammetry were performed
under N2 (purified) in an anhydrous solvent (MeCN or CH2Cl2)
containing Bu4NClO4 (TBAP, 0.1 ) as supporting electrolyte with
a specially devised cell equipped with a working electrode (Pt inlay
type, Beckmann 32273), counterelectrode (Pt wire), and saturated
calomel electrode as a reference, connected to the cell through two
salt bridges (saturated KCl solution), and the acetonitrile or dichlo-
romethane solution of the sample. All runs were made at room
temperature after a 15 min purge with N2 and within 30 min of
preparing the solution, experimental error Ϯ0.02 V. All the results
are summarized in Tables 18 and 19, together with those for the
related compounds AϪD, FϪI.
Si1R2ϪXϪSiR2Si3R2(Si1-Si3) ϩ 2 SiHR2ϪSiHR2
ϩ 2 SiHR2ϪXH Ǟ SiHR2ϪXϪSiHR2
ϩ SiHR2ϪSiR2ϪSiHR2 ϩ 2 SiHR2ϪSiR2ϪXH
(1)
Si1R2ϪXϪSiR2SiR2ϪSi4R2(Si1-Si4) ϩ 3 SiHR2ϪSiHR2
ϩ 2 SiHR2ϪXH Ǟ SiHR2ϪXϪSiHR2
ϩ 2 SiHR2ϪSiR2ϪSiHR2 ϩ 2 SiHR2ϪSiR2ϪXH
(2)
X-ray Crystal Analysis of 1؊6: Crystals obtained from alcohols
such as methanol, ethanol, and 2-propanol were used for X-ray
analysis. Intensity data were obtained with an EnrafϪNonius
CAD-4 diffractometer equipped with graphite-monochromated
˚
˚
Methods of Theoretical Calculations for Geometries, Frontier Mo-
lecular Orbitals, and Ring Strain Energies: Geometries of all molec-
ules considered here were fully optimized at the PM3 level with the
Gaussian 98 program.[42] They were then characterized as minima
or transition states by calculation and diagonalization of the Hes-
Cu-Kα (λ ϭ 1.5418 A) or Mo-Kα (λ ϭ 0.7107 A) radiation and by
the ω-2θ scan technique. Structures of 1Ϫ6 were solved by direct
methods with the MULTAN 78 program[39] and refined by full-
matrix least squares. All the calculations were performed with the
UNICS III system.[40] The molecular structures of 1b, 2a, 3aϪd,
Table 23. Crystal data, data collection, and refinement for five-membered silacycles [R2Si]4X (X ϭ NR3)
[a]
[b]
[c]
w ϭ 1/(0.047·|Fo|2 Ϫ 0.0280·|Fo| ϩ 0.4783). w ϭ 1/(0.00295·|Fo|2 ϩ 0.0781·|Fo| ϩ 1.929). w ϭ 1/(0.0068·|Fo|2 Ϫ 0.118·|Fo| ϩ 1.113).
1790
Eur. J. Inorg. Chem. 2002, 1772Ϫ1793

Peralkylated Four- and Five-Membered Cyclosilanes Containing a Heteroatom
Table 24. Crystal data, data collection, and refinement for four- and five-membered silacycles [R2Si]nX (X ϭ O)
FULL PAPER
[a]
[b]
[c]
w ϭ 1/σ2·|Fo|. w ϭ 1/(0.00341·|Fo|2 ϩ 0.0961·|Fo|ϩ 1.427). w ϭ 1/σ2·|Fo|.
4aϪc, 5b, 6a, and 6b are given in the electronic supporting informa-
Colcoat Co., Ltd., and the Gunma University Foundation for Pro-
tion. Selected bond lengths and bond and torsion angles are motion of Science and Technology.
given in Tables 1Ϫ12, respectively. The crystallographic data for
twelve four- and five-membered compounds are summarized in
[1]
[1a]
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-169801 (4b), -169802 (4c), -169803 (5b), -169804 (6a), and
-169805 (6b) contain the supplementary crystallographic data
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[I01354]
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1793