Contraction and expansion of the silicon scaffold of stable Si 6R6 isomers

Article abstract of DOI:10.1021/ja307344f

The reactivity of two stable Si₆R₆ clusters (4 and 5, R = 2,4,6-ⁱPr₃C₆H₂) with unsymmetrical substitution patterns (including Si, SiR, and SiR₂ vertices) is reported. In order to account for the importance of such clusters as model systems for transient intermediates in the deposition of elemental silicon, we here propose the term siliconoids for silicon clusters with unsaturated valencies. With the hexasilaprismane 8a, a saturated-i.e., non-siliconoid-Si₆R₆ isomer is accessible from a suitable Si₃ precursor. Thermal redistribution of the substituents converts 1,1,2-trichlorocyclotrisilane 6 into the corresponding 1,2,3-derivative 7 prior to the requisite reductive coupling step leading to 8a. On the other hand, a stable expanded Si₁₁-siliconoid 9 was isolated as a minor side product of the thermal isomerization of 4 to 5, thus providing a first example of siliconoid cluster expansion in the condensed phase. In the solid-state structure, the two unsubstituted vertices of 9 strongly interact in a staggered propellane-like fashion. Oxidative cluster contraction of a siliconoid scaffold is observed upon treatment of siliconoid 5 with a large excess of iodine in refluxing toluene, thus providing access to a highly functionalized hexaiodocyclopentasilane 11 in high yield. Conversely, chlorination of the isomeric 4 with BiCl₃ as a mild source of Cl₂ results in a complex mixture of products from chlorination of the unsubstituted vertices as well as σ-bonds of the cluster framework of 4. The main product, 1,2-dichlorotricyclo[2.2.0.0^2,5]hexasilane 12, undergoes thermal cluster contraction to give tricyclo[2.1.0.0^2,5]pentasilane 14 with an exohedral chlorosilyl group.

Full text of DOI:10.1021/ja307344f

Article
pubs.acs.org/JACS
Contraction and Expansion of the Silicon Scaffold of Stable Si6R6
Isomers
Kai Abersfelder,#,§ Adam Russell,§ Henry S. Rzepa,§ Andrew J. P. White,§ Peter R. Haycock,§
and David Scheschkewitz*,#,§
#Krupp-Chair of General and Inorganic Chemistry, Saarland University, D-66125 Saarbrucken, Germany
̈
§Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom
S
* Supporting Information
ABSTRACT: The reactivity of two stable Si6R6 clusters (4 and
5, R = 2,4,6-iPr3C6H2) with unsymmetrical substitution
patterns (including Si, SiR, and SiR2 vertices) is reported. In
order to account for the importance of such clusters as model
systems for transient intermediates in the deposition of
elemental silicon, we here propose the term “siliconoids” for
silicon clusters with unsaturated valencies. With the hexasilap-
rismane 8a, a saturatedi.e., non-siliconoidSi6R6 isomer is
accessible from a suitable Si3 precursor. Thermal redistribution
of the substituents converts 1,1,2-trichlorocyclotrisilane 6 into
the corresponding 1,2,3-derivative 7 prior to the requisite
reductive coupling step leading to 8a. On the other hand, a stable expanded Si11-siliconoid 9 was isolated as a minor side product
of the thermal isomerization of 4 to 5, thus providing a first example of siliconoid cluster expansion in the condensed phase. In
the solid-state structure, the two unsubstituted vertices of 9 strongly interact in a staggered propellane-like fashion. Oxidative
cluster contraction of a siliconoid scaffold is observed upon treatment of siliconoid 5 with a large excess of iodine in refluxing
toluene, thus providing access to a highly functionalized hexaiodocyclopentasilane 11 in high yield. Conversely, chlorination of
the isomeric 4 with BiCl3 as a mild source of Cl2 results in a complex mixture of products from chlorination of the unsubstituted
vertices as well as σ-bonds of the cluster framework of 4. The main product, 1,2-dichlorotricyclo[2.2.0.02,5]hexasilane 12,
undergoes thermal cluster contraction to give tricyclo[2.1.0.02,5]pentasilane 14 with an exohedral chlorosilyl group.
siliconoids. The presence of at least one unsubstituted silicon
vertex in a neutral molecule with a three-dimensional silicon
backbone remains as the sole criterion.
Stable siliconoidswhich would allow for the application of
the powerful analytical tools of synthetic chemistry to the
investigation of structure and reactivitywere unknown until
recently. In 2005, the serendipitous synthesis of Si5R6 siliconoid
1 was reported by one of us (Chart 1).10 Just a few months later,
1. INTRODUCTION
Transient silicon clusters with unsaturated vertices are potential
intermediates of the gas-phase deposition of elemental silicon.1
The free valences, also referred to as “dangling bonds” in the
context of surface chemistry,2 are preferred reaction sites for
cluster expansion during deposition processes.3 Furthermore,
some of the band-gap-determining defects in hydrogenated
amorphous silicon (a-Si:H) closely resemble such unsaturated,
low-coordinate sites.4 The relevance of the defect structure for
the undesirable decline in the performance of thin-film solar cells
based on a-Si:H, the so-called Staebler−Wronski effect, has been
pointed out repeatedly.4,5
a
Chart 1. Schematic Representation of Stable Siliconoids
We now propose to refer to unsaturated molecular silicon
clusters as “siliconoids” 6,7 for simplicity and in order to account
for their intermediacy in the deposition of elemental silicon from
molecular precursors. The term is inspired by the definition of
metalloid clusters as introduced by Schnockel in the context of
̈
Group 13 clusters.8 Accordingly, such metalloid clusters require
an average oxidation state between 0 and 1 for the cluster atoms,
a notion that has been upheld by Schnepf et al. for the
corresponding germanium and tin derivatives.9 Although the
unsubstituted vertex in Group 14 clusters itself certainly requires
a formal oxidation state of zero, the average value for the whole
cluster is not considered in our proposed definition of
a
1, 4, 5: R = Tip = 2,4,6-iPr3C6H2. 2: R = SitBu3. 3: R = Mes = 2,4,6-
Me3C6H2.
Received: July 26, 2012
Published: August 25, 2012
© 2012 American Chemical Society
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Wiberg and Veith et al. disclosed the synthesis of an Si8R6
derivative, 2, with two unsaturated vertices.11 Recently, a
persilapropellane, 3, was isolated by Breher et al.,12 completing
the series of group 14 propellanes.13 The present article describes
the reactivity (including cluster expansion) of the Si6R6
siliconoids 4 and 5, which are readily accessible via reduction
of 1,1,2-trichlorocyclotrisilane 6.14,15 The reaction chemistry of 4
and 5 seemed particularly worth exploring in view of the very
unusual electronic situation of both compounds.15,16 In addition,
we now report the thermal isomerization of 6 to the
symmetrically substituted 1,2,3-trichloro derivative 7. Reduction
of 7 affords only the second stable hexasilaprismane 8 and thus a
non-siliconoid stable isomer within the Si6Tip6 manifold.17
2. RESULTS AND DISCUSSION
2.1. Non-siliconoid Si6Tip6 Isomer. The reduction of 1,1,2-
trichlorocyclotrisilane 6 afforded the dismutational isomer of
hexasilabenzene 4.14 An alternative, shorter method for the
synthesis of 4 employing the reaction of Tip2SiSi(Tip)Li with
Jutzi’s Cp*Si+ cation was recently reported.18 Both of these
approaches preserve the unsymmetrical substitution pattern of
the cyclotrisilane precursor 6 inasmuch as silicon atoms with
zero, one, and two substituents are present in 4, thus constituting
the formal dismutation from a hypothetical hexasilabenzene.
This substitution pattern persists even during the thermal
rearrangement of 4 to the global minimum isomer 5, which can
be distilled at 340 °C and reduced pressure with only minor signs
of decomposition.15
We anticipated that symmetrization of the substitution pattern
might be possible at the stage of the halogenated precursor 6
(solid-state structure in Figure 1a).14 A strong Lewis acid could
facilitate this process through (intermediate) chloride abstrac-
tion in view of the ready occurrence of aryl shifts in cationic
silicon compounds.19 Incidently, aluminum(III) chloride had
proven suitable to initiate the rearrangement of cyclic silanes.20
Indeed, the addition of an equimolar amount of AlCl3 to 6 in
C6D6 led to the appearance of 29Si NMR signals at δ = −13.9 and
−18.7 ppm in a ratio of 1:2 after a few hours at ambient
temperature, which indicated the formation of a new species with
higher symmetry. Conversion to a clean product 7 was
completed by heating the mixture to 60 °C for a few hours.
The separation of AlCl3 from the product, however, proved
tedious. These difficulties prompted us to reinvestigate the
decomposition product mixture obtained upon melting of 6.14
Indeed, the major product (approximately 70%) present in the
melt turned out to be 7.
Figure 1. (a) Structure of 6 in the solid state (thermal ellipsoids at
30%).14 Hydrogen atoms omitted. Selected bond lengths [Å] and angles
[°]: Si1−Si2 = 2.2970(6), Si2−Si3 = 2.3908(7), Si3−Si1 = 2.3661(7),
Si1−Cl1 = 2.0594(7), Si1−Cl2 = 2.0534(7), Si2−Cl3 = 2.0866(7);
Si1−Si2−Si3 = 60.59(2), Si3−Si1−Si2 = 61.67(2), Cl1−Si1−Cl2 =
105.89(3), Si2−Si3−Si1 = 57.744(19). (b) Structure of one of four
crystallographically independent molecules of 7 (thermal ellipsoids at
30%). Hydrogen atoms omitted. Selected bond lengths [Å] and angles
[°]: Si1−Si2 = 2.3507(15), Si2−Si3 = 2.3490(15), Si3−Si1 =
2.3821(15), Si1−Cl1 = 2.0885(15), Si2−Cl2 = 2.0864(14), Si3−Cl3
= 2.0788(15).
Scheme 1. Thermal Rearrangement of 6 to 7 and Reductive
a
Dimerization to Hexasilaprismane 8
By heating a toluene solution of 6 to reflux for 4 days, a more
selective conversion of 6 to 7 was achieved (Scheme 1). The
resulting crude product contained approximately 6% impurities
by NMR. Pale yellow single crystals of the product 7 were
obtained at room temperature from an oversaturated hexane
solution and analyzed by X-ray diffraction.
a
R = Tip = 2,4,6-iPr3C6H2.
The structure in the solid state confirmed the anticipated
constitution of a 1,2,3-trichlorocyclotrisilane 7 in the sterically
most favorable cis,trans-conformation (Figure 1b). Due to the
almost identical structural parameters, only one of the four
crystallographically independent molecules will be discussed. All
Si−Cl bonds are close to the average value of Si−Cl bond of 2.08
Å. The Si−Si bonds are also in the expected region for Si−Si
single bonds and vary just by 0.033 Å. The negligible variation of
the bond lengths is a result of the equal substitution pattern on
each silicon atom in 7. In unsymmetrically substituted cyclo-
trisilanes, the substituents exert a considerable electronic
influence on the endocyclic Si−Si bonds.21
Reduction of 7 with magnesium metal in THF at room
temperature resulted in a color change from pale yellow to dark
red after stirring overnight. Hexasilaprismane 8 can be isolated as
orange crystals in 31% yield from hexane (Scheme 1).
With 8b a very similar hexasilaprismane, Si6R6 (R = Dip =
2,6-iPr2C6H3), had been reported by Sekiguchi and Sakurai et al.
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in 1993.17 In that case, the reported yields were just 1% and 7%,
depending on the starting material, DipSiCl3 and (DipSiCl2)2,
respectively. We conclude that the use of the larger Si3 precursor
improves the selectivity of reductive coupling of fragments.
Hexasilaprismane 8a is thermally stable up to temperature of
approximately 350 °C. Much like 8b, 8a is stable to extended
exposure to air and water. The 29Si NMR spectrum of 8a shows
just one chemical shift at δ = −23.1 ppm, as expected from the
Scheme 2. Isolation of 9 as a Side Product in the Synthesis of
the Global Minimum Isomer 5
a
D3h symmetry of the molecule, in the region for tetracoordinate
silicon atoms. The electron impact mass spectrum confirmed the
expected formula C90H138Si6 for 8a by showing the molecular ion
peak at 1388 m/z.
The structure of 8a was confirmed by X-ray analysis on suitable
single crystals obtained from a concentrated hexane solution
(Figure 2). All Si−Si bond lengths are in the typical region for
a
Indicated yields are isolated. R = Tip = 2,4,6-iPr3C6H2.
The constitution of 9 was determined by X-ray diffraction of a
single crystal (Figure 3). The expanded siliconoid 9 contains a
Figure 2. Structure of 8a in the solid state (thermal ellipsoids at 15%).
Hydrogen atoms omitted. Selected bond lengths [Å]: Si1−Si2 =
2.3906(14), Si1−Si3 = 2.3656(14), Si1−Si4 = 2.3789(13), Si2−Si3 =
2.3833(15), Si2−Si5 = 2.3667(14), Si3−Si6 = 2.3718(14), Si4−Si6 =
2.3799(14), Si4−Si5 = 2.3777(14), Si5−Si6 = 2.3868(15).
Si−Si single bonds. There are no differences in the Si−Si bond
length of the bonds of the three- and four-membered faces of the
polyhedron, which is in line with the experimental structure of
8b17 as well as with theoretical predictions.22
So far the non-siliconoid isomer 8a has resisted attempts to
rearrange it to other valence isomers of hexasilabenzene or
hexasilabenzene itself. Sakurai et al. had suggested the formation
of a Dewar benzene isomer from hexasilaprismane 8b on the
basis of a strongly red-shifted longest wavelength absorption after
UV irradiation.17
Figure 3. Structure of 9 in the solid state (thermal ellipsoids at 30%).
Hydrogen atoms except H3, co-crystallized hexane, and minor position
of disorder of the entire molecule omitted. Selected bond lengths [Å]:
Si1−Si2 = 2.4976(9), Si1−Si5 = 2.4267(9), Si1−Si9 = 2.4175(9), Si2−
Si3 = 2.3196(9), Si2−Si6 = 2.3201(10), Si2−Si8 = 2.3219(9), Si3−Si4 =
2.3580(9), Si3−Si10 = 2.3747(9), Si4−Si5 = 2.3923(9), Si4−Si11 =
2.3888(9), Si5−Si6 = 2.3993(9), Si6−Si7 = 2.3818(9), Si7−Si8 =
2.3816(9), Si8−Si9 = 2.4006(9), Si9−Si10 = 2.3746(9), Si10−Si11 =
2.3926(8).
2.2. Expanded-Core Siliconoid. The core expansion of
silicon clusters is a pivotal process of the chemical vapor
deposition of elemental silicon. Despite a large number of
experimental gas-phase studies1 and theoretical investigations,23
the absence of stable siliconoids prevented the investigation of
such processes in the condensed phase until recently.10−12,14,15
During the thermal rearrangement of the dismutational isomer
4 to the presumed global minimum 5, a few side products are
formed in very small quantities. The core-expanded Si11-
siliconoid 9 was thus isolated as orange crystals by crystallization
from the hexane mother liquor of 5 in 2.6% yield (Scheme 2) and
characterized by multinuclear NMR and UV/vis.
severely disordered Si11-cluster core (see Supporting Informa-
tion). Eight carbon-substituted silicon atoms formally constitute
a strongly puckered eight-membered ring about the three central
silicon atoms without Si−C connectivity.
Two presumably unsubstituted vertices, Si1 and Si2, are
arranged in a propellane-like unit, which in contrast to the global
minimum isomer 515 and Breher’s propellane 312 is arranged in a
staggered rather than in an eclipsed manner. Possibly as a
consequence of this staggering the distance between the two
bridgehead atoms is considerably shorter (7, 2.4976(9); 5,
270.76(8); and 3, 2.636(1) Å), indicating a much stronger
bonding interaction. In order to accommodate this shorter bond,
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the bridgehead atoms in 9 show a distinctly planarized
coordination (sum of angles Si1 343.5°; Si2 354.7°) Conversely,
due to the pronounced pyramidalization at Si3 (sum of angles Si3
294.8°) we tentatively assigned H3 as bonded to this atom,
although it was not located on the difference electron density
map. The position of the Si−H functionality was confirmed by a
series of 2D NMR spectra as well as DFT calculations (vide
infra).
the para-isopropyl groups of the Tip substituents were replaced
by hydrogen atoms, and optimization was halted after
convergence of the total energies to the third decimal place.
Only the siliconoid 9Dip bearing the hydrogen atom at Si3
(Figure 4b) matches the structure found experimentally in the
solid state (Figure 4a). In contrast to this, the silicon scaffold of
9Dip* and 9Dip** visibly distorts as the comparison of the
crucial Si−Si distances clearly illustrates (Figure 4c,d).
The UV/vis spectrum of 9 shows two absorption maxima at λ
= 399 nm (ε = 1650 M−1 cm−1) and λmax = 451 nm (ε = 2455 M−1
cm−1). The longest wavelength absorption is blue-shifted by
about 20 nm in comparison with those of 3 and 5, which is in line
with a larger HOMO−LUMO gap and the shorter and therefore
presumably stronger bond between the bridgehead atoms Si1
and Si2 as observed in the solid state. The large size of the core-
expanded siliconoid 9, however, precluded the use of TD-DFT to
support this assumption.
Up to now the mechanism of the formation of 9 has remained
obscure. Formally, an SiTip4 unit is expelled from an entirely
hypothetical dimerization product, Si12Tip12. This would be
followed by insertion of one of the resulting three unsubstituted
vertices into a CH bond of the substituent. The extrusion of
silicon from the Si6 backbone of 4 or 5, however, is in principle
feasible, as the following results suggest.
2.3. Cluster Contraction of 5 by Iodination. We
previously reported that bromination and iodination of the
global minimum isomer 5 affords the 1,5-dihalogenated
persilatricyclo[2.2.0.02,5]hexane derivatives 10a,b by addition
of halogen to the bridgehead silicon atoms (Scheme 3).15
The 29Si NMR spectrum of 9 exhibits the expected 11
resonances of equal intensity at δ = +26.2, +0.1, −0.6, −4.0,
−41.2, −43.0, −43.5, −56.0, −62.7, −64.2, and −78.7 ppm. In
1
the complicated H NMR spectrum the isolated singlet at δ =
5.68 ppm can be assigned to the Si-bonded hydrogen H3 on the
basis of isotopic satellite peaks with a 1J(Si,H) coupling constant
of 194 Hz. Indeed the connectivity of 9 in the solid state proves
an insertion reaction of a silicon atom into a methyl C−H bond
of an ortho-isopropyl group under formation of a spiro-cyclic Si−
C bond at Si11. In order to determine the location of H3, full
assignment of the 29Si signals by means of 2D NMR
spectroscopic analysis was done (see Supporting Information).
For further corroboration of the position of H3, the geometry
of the silicon scaffold was partially optimized at the ωB97XD/6-
31G(d,p) level of theory with the hydrogen atom H3
alternatively being located on Si3 (9Dip), Si2 (9Dip*), or Si3
(9Dip**, Figure 4).24 To economize computational resources
Scheme 3. Halogenation of Siliconoid 5 to 1,5-Dihalo
Derivatives 10a,b15 and Contraction to
Hexaiodocyclopentasilane 11
a
a
R = Tip = 2,4,6-iPr3C6H2
Surprisingly, 2 equiv of halogen are required to complete
conversion in these reactions. In order to shed light on the nature
of the side products during the synthesis of 10b, the reaction was
carried out with a 10-fold excess of iodine in refluxing toluene
(instead of benzene at ambient temperature). Notably, a new
species, 11, formed almost quantitatively with three 29Si NMR
signals at δ = −35.9, −45.3, and −65.4 ppm in a 1:2:2 ratio, which
served as a first indication for the expulsion of one silicon vertex
(Scheme 3). Three consecutive crystallizations from toluene
were necessary to completely remove excess iodine. Con-
sequently, only 28% of 11 was isolated as colorless crystals.
The constitution of 11 as hexaiodinated cyclopentasilane was
determined by an X-ray diffraction study on single crystals
obtained from hexane at room temperature (Figure 5). Four Tip
substituents are arranged in an all-trans fashion on four of the five
ring atoms of the silicon backbone. The five-membered ring
exhibits a typical envelope confirmation. Si3 lies 0.8281(14) Å
out of the plane through Si1, Si2, Si4, and Si5 (max. deviation
0.0848(6) Å for Si1). In comparison with the solid-state
structures of cyclopentasilanes Si5I10,25 all Si−Si bonds of 11
are slightly elongated, reflecting the larger steric bulk of the Tip
substituents.
Figure 4. (a) Experimentally determined structure of 9 in the solid state.
(b−d) Optimized structures of 9Dip, 9Dip*, and 9Dip** at the
ωB97XD/6-31G(d,p) level of theory. Distances [Å]: Si1−Si2 = 2.498
(9), 2.509 (9Dip), 3.302 (9Dip*), 3.455 (9Dip**); Si1−Si3 = 3.601
(9), 3.643 (9Dip), 2.625 (9Dip*), 3.835 (9Dip**).
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a
Scheme 4. Products Obtained from Reaction of 3 with BiCl3
a
Estimated percentage in crude product; isolated yield in brackets. R =
Tip = 2,4,6-iPr3C6H2.
Si6Tip6Cl2. According to H NMR and 2D H/29Si correlation
spectra, the silicon scaffold of 12 is formed of six vertices
substituted with two, one, and no Tip substituents, respectively.
In agreement with the yellow color, the longest wavelength
absorption in the UV/vis spectrum of 12 is observed as a
shoulder at 396 nm (ε = 850 M−1 cm−1) and thus in a similar
region as those of 10a,b.15 The spectroscopic data therefore
initially suggested the formation of the 1,5-substituted derivative
10c as the main product.
Single crystals suitable for X-ray analysis were grown from a
concentrated solution in benzene. The unexpected 1,2-dichloro
isomer was identified as the main product 12 by an X-ray
diffraction study (Figure 6).
The tricyclic silicon scaffold of 12 resembles that of the global
minimum isomer 5 rather than the dismutational isomer 4. The
chloro-substituted silicon atoms are adjacent to one another, i.e.,
in the 1,2-position rather than in the 1,5-position as in 10a,b. The
Si−Cl distances of Si1−Cl1 = 2.0665(6) Å and Si2−Cl2 =
2.0683(6) Å are typical for tetrahedral coordination environ-
1
1
Figure 5. Structure of 11 in the solid state (thermal ellipsoids at 30%).
Hydrogen atoms and hexane omitted. Selected bond lengths [Å]: Si1−
I1 = 2.4736(9), Si1−I2 = 2.4678(9), Si1−Si5 = 2.4339(13), Si1−Si2 =
2.4369(12), Si2−Si3 = 2.4277(13), Si3−Si4 = 2.3935(13), Si4−Si5 =
2.4145(12).
In order to check whether the 1,5-diiodo derivative 10b is an
intermediate during the formation of 11, a sample of 10b was
exposed to identical reaction conditions as during the synthesis
of 11, but no reaction occurred even after heating to 110 °C for
15 h. Obviously, the formal expulsion of a “SiTip2” fragment
takes place prior to iodination of the unsubstituted vertices.
Analysis of the frontier orbitals of 5 reveals that both HOMO and
HOMO−1 belong to the cluster’s σ-framework.15 Accordingly,
an initial oxidative expulsion of one of the SiTip2 bridges
followed by hexaiodination and rearrangement of the cluster
scaffold to the five-membered ring of 11 is plausible.
Compound 11 is the first 1,2,3,4-tetraaryl-substituted cyclo-
pentasilane and therefore inherently suitable for subsequent
studies regarding the synthesis of heavier persila analogues of
cyclopentadiene and related compounds, which are currently
being investigated in our laboratory. It should be noted that the
parent cyclopentasilane has been used as a convenient and safe
starting material for an inkjet-printable formulation of
amorphous silicon precursors.26
2.4. Reaction of 4 with BiCl3. Due to its peculiar electronic
structure (dismutational aromaticity), we anticipated that the
halogenation of 4 might lead to entirely different products.
Indeed, the halogenation of 4 with elemental bromine or iodine,
unlike that of 5, resulted in complex reaction mixtures. A milder
source of halogen was thus required. Bismuth trichloride (BiCl3)
is commonly used as oxidizing agent in synthetic organic
synthesis27 and should act as a mild Cl2 source.
Similar to the use of elemental halogens, the reaction of 4 with
1 equiv of BiCl3 in benzene at ambient temperature resulted in
multiple products. Extraction of the mixture with hexane
followed by fractional crystallization, however, allowed for the
isolation of most components in this case (Scheme 4).
The main product of the reaction, 12, which could be isolated
in pure form in 30% yield, shows three signals in the 29Si NMR
spectrum in the ratio 1:1:1 at δ = 19.1, −25.8, and −69.9 ppm.
The electron impact mass spectrum of this compound exhibits
the highest mass at 1458.7 m/z, in agreement with the formula
Figure 6. Structure of 12 in the solid state (thermal ellipsoids at 30%).
Hydrogen atoms and benzene molecules omitted. Selected bond lengths
[Å]: Si1−Si4 = 2.3906(6), Si1−Si2 = 2.3298(7), Si1−Si6 = 2.3428(6),
Si2−Si3 = 2.3294(6), Si2−Si5 = 2.3938(6), Si3−Si4 = 2.4157(6), Si4−
Si5 = 2.4730(7), Si5−Si6 = 2.4634(7), Si1−Si5 = 2.8229(6), Si2−Si4 =
2.8145(6).
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ments of silicon. Most Si−Si bonds are in the range of Si−Si
single bonds in saturated small rings.17,20,21 In marked contrast,
the bond between Si4 and Si5 is substantially longer (2.4730(7)
Å), presumably due to steric congestion. The two through-space
distances of the chlorinated silicon atoms to the opposing silicon
atoms (Si1−Si5 = 2.8229(6) Å and Si2−Si4 = 2.8145(6) Å) are
approximately 11 pm longer than the distance between the
bridgehead atoms in 5.
Fractional crystallization of the product mixture of the reaction
of 3 with BiCl3 also allowed for the isolation of 13 and 10c as side
products in 10% and 2% yield, respectively (Scheme 4). In the
electron impact mass spectrum of 13, the highest mass is
observed at 1528.8 m/z, which is in agreement with the formula
Si6Tip6Cl4. The additional incorporation of two chlorine atoms
in 13 could proceed by oxidative cleavage and chlorination of one
Si−Si σ-bond of 12. However, this assumption was disproven
since a solution of 12 and BiCl3 in benzene shows no reaction
even after heating to 70 °C for 48 h. Similarly to the reaction of 5
with excess iodine (cf. section 2.3), the chlorination of a Si−Si σ-
bond apparently takes place prior to the formation of 12 and
therefore possibly in a more reactive transient intermediate.
Colorless crystals of 13, suitable for X-ray analysis, were grown
from a concentrated benzene solution. The solid-state structure
contains one molecule benzene in the unit cell (Figure 7). It
The observed Si−Si bonds of 13 are all on the upper end of the
typical range observed for Si−Si single bonds, reflecting the high
steric strain in the molecule. The two four-membered rings are
distorted from planarity, as indicated by the sum of inner angles
(Si1−Si2−Si3−Si4 = 350.96°; Si1−Si2−Si5−Si6 = 358.31°).
In the 29Si NMR spectrum six signals at δ = 27.7, 19.0, 18,5,
15.8, 13.2, and −7.8 ppm are observed, reflecting the C1
symmetry of the molecule. With the help of 2D 1H/29Si
correlation, the signals at δ = 27.7 and −7.8 ppm are
unequivocally assigned to the SiTip2 moieties (Si3 and Si6).
The additional product 10c (Figure 8) could only be isolated
in a mixture with siliconoid 5. After several crystallizations, the
Figure 8. Structure of 10c in the solid state (thermal ellipsoids at 30%)
determined on a crystal with co-crystallized 5 in approximate 1:1 ratio.
Hydrogen atoms and hexane molecules omitted.
ratio of the mixture was still approximately 1:1, and no further
purification could be achieved in this way. Given that the NMR
data of the global minimum siliconoid 3 are known,15 the
assignment of the 29Si NMR signals was straightforward. The
chemical shifts belonging to 10c were identified at δ = 21.1, 17.5,
9.8, and −80.3 ppm. They are, with the exception of the signal at
17.5 ppm, very similar to those of 10a,b.15 As expected, the latter
resonance can be assigned to the halogenated silicon atoms Si1
and Si1′ by 2D 1H/29Si correlation. In the electron impact mass
spectrum of the mixture of compounds 5 and 10c, the highest
peak, at 1459 m/z, is the molecular ion peak of 10c (Si6Tip6Cl2).
The orange crystals of the mixture of 5 and 10c which were
obtained from a concentrated hexane solution were analyzed by
X-ray diffraction to confirm the presence of 10c and to get an
impression of the structural features. The obtained bond length
and angles should just serve as approximate values due to the co-
crystallization of two components at the same crystallographic
positions.
Although the mechanism for this chlorination reaction
remains obscure, it is assumed that the presence of subvalent
bismuth compounds in the reaction mixture might play an
important role in the formation of the reaction products. The
assumed “BiCl” intermediate could aggregate, as has been
described for the reductive dehalogenation of BiCl3,28 which
would explain the precipitation of an unidentified black material
from the reaction mixture.
Figure 7. Structure of 13 in the solid state (thermal ellipsoids at 30%).
Hydrogen atoms and benzene molecules as well as minor occupation
site omitted. Selected bond lengths [Å]: Si1−Cl1 = 2.0831(6), Si1−Si4
= 2.3305(7), Si1−Si2 = 2.4313(6), Si1−Si6 = 2.3749(6), Si2−Cl2 =
2.0797(6), Si2−Si3 = 2.4559(7), Si2−Si5 = 2.4405(6), Si4−Cl3 =
2.1692(8), Si3−Si4 = 2.4102(6), Si5−Cl4 = 2.1015(8), Si5−Si6 =
2.4951(6).
displays significant disorder of the whole molecule, which was
accounted for by refinement on split positions (Supporting
Information), preventing a detailed discussion of bonding
parameters.
The constitution of 13 as hexasilabicyclo[2.2.0]hexane is
formally derived from the tricyclic scaffold of 12 by cleavage of
Si4−Si5, i.e., the most congested and thus longest Si−Si σ-bond.
2.5. Thermal Rearrangement of 12. The clean thermal
isomerization observed upon melting of the 1,2-chlorinated
tricyclo[2.2.0.02,5]hexasilane 12 is suitable to shed some further
light on the complex rearrangements of the silicon scaffold that
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take place during the reactions of siliconoids 4 and 5 described
above.
Heating of neat 12 to 230 °C until completely melted results in
its clean isomerization to a new compound, 14 (Scheme 5). The
14 constitutes a derivative with one SiR2 moiety, four SiR units,
and one silicon atom with no directly attached substituent. In
other words, 14 features a substitution pattern halfway between
hypothetical hexasilabenzene and its dismutational isomer 4. In
addition, the chlorine functionalities on the cluster core as well as
in the side chain provide ample opportunity for further
functionalization and modification of the cluster size.
a
Scheme 5. Thermal Rearrangement of 12 to 14
3. CONCLUSION
The contraction and expansion of unsaturated silicon clusters is
pivotal to nucleation processes in the gas-phase deposition of
elemental silicon materials. We therefore propose the term
“siliconoids” for such clusters. The reactivity of stable siliconoids
4 and 5 in solution phase has been investigated in this regard
taking advantage of the powerful preparative and analytical tools
of synthetic chemistry. We have thus provided preparative
examples of a cluster expansion process of a stable siliconoid in
the condensed phase, as well as the thermal or oxidative
contraction.
The structure elucidation of transient siliconoidsevident
intermediates in important industrial processes (Siemens
process, Direct process, CVD of amorphous silicon)until
very recently relied on computational and gas-phase spectro-
scopic studies alone. Structure−reactivity relationships begin to
emerge from the present study. The unusual electronic situation
encountered in siliconoids gives rise to a surprisingly rich
chemistry that we will continue to study in our laboratories.
a
R = Tip = 2,4,6-iPr3C6H2.
highest mass in the electron impact mass spectrum of 14 at
1458.7 m/z is identical to that of 12, confirming that no
fragmentation occurred during the melting process. The product
of thermal rearrangement shows a new set of 29Si NMR signals at
δ = 59.4, 5.8, −96.4, −113.2, −151.2, and −151.9 ppm. On the
basis of a 2D H/29Si correlation NMR experiment, the 29Si
1
signal at δ = 5.8 ppm is assigned to the exocyclic silicon atom. All
other 29Si signals are observed at unusual chemical shifts. Four
resonances are located at relatively high field, whereas one is
significantly deshielded. In the Si6 siliconoid 5, the strong
deshielding of a tetracoordinate silicon atom had been correlated
to the presence of magnetically induced cluster currents.15
An X-ray diffraction study on single crystals of 14 allowed for
the determination of the molecular structure in the solid state,
confirming the contraction of the Si6-scaffold to the tricyclo-
[2.1.0.02,5]pentasilane 14 (Figure 9). The remaining silicon atom
is located in an exocyclic chlorosilyl group. It is noteworthy that
4. EXPERIMENTAL SECTION
General Remarks. All manipulations were carried out under a
protective atmosphere of argon applying standard Schlenk techniques or
in a drybox. Ethereal solvents were refluxed over sodium/benzophe-
none; toluene over sodium; pentane, hexane, and (deuterated) benzene
over sodium/potassium alloy. All solvents were distilled and stored
under argon and degassed prior to use. 1,1,2-Trichlorocyclotrisilane (6)
was prepared following our published procedure.14 Silicon tetrachloride
was purchased from Aldrich, stored over Mg turnings, and distilled prior
to use. 1H and 13C{1H} NMR spectra were referenced to external SiMe4
using the residual signals of the deuterated solvent (1H) or the solvent
itself (13C). 29Si{1H} NMR spectra were referenced to external SiMe4.
UV/vis spectra were recorded on a Perkin-Elmer Lambda 20 UV/vis
spectrometer. Melting points were determined under argon in closed
NMR tubes and are uncorrected.
1,2,3-Trichlorocyclotrisilane (7). Method 1: In an oil bath, 1.50 g
(1.87 mmol) of neat 1,1,2-trichlorocyclotrisilane (6) was heated to 185
°C for 5 min. The red melt was cooled to room temperature and
dissolved in 5 mL of hexane. The filtrate was reduced to 3 mL, and at
room temperature 0.84 g (56%) of 1,2,3-trichlorocyclotrisilane (7)
crystallized as pale yellow crystals. Method 2: A solution of 7.94 g (9.92
mmol) of 6 in 60 mL of toluene was refluxed for 16 h under a slight
overpressure of about 0.2 bar. 1H NMR shows full conversion to 7 along
with 6% unidentified side products. 1H NMR (500.13 MHz, C6D6, 25
°C): δ = 7.22, 7.14 (each s, altogether 6H, Tip-H), 4.11, 3.39, 2.77, 2.74
(each hept, altogether 9H, iPr-CH), 1.50, 1.34, 1.26, 1.19, 1.16, 1.15
(each d, altogether 54H, iPr-CH3). 13C NMR (125.76 MHz, C6D6, 25
°C): δ = 157.38, 156.79, 153.37, 152.92 (Tip-Co/p), 128.62, 127.01
(Tip-Ci), 122.36, 122.16 (Tip-CH), 37.76, 37.09, 34.83 (iPr-CH),
26.44, 25.80, 25.02, 23.98, 23.95, 23.89 (iPr-CH3). 29Si NMR (99.36
MHz, C6D6, 25 °C): δ = −13.9 (1Si), −18.7 (2Si). Exact mass (EI):
calcd m/z for C45H69Cl3Si3+ (M+), 798.3773; found, 798.3787. MS (EI):
m/z (%) = 800.4 (14.2, M+), 757.3 (5.2, M+ − iPr), 533.2 (16.6, M+ −
iPr), 497.3 (100.0, M+ − SiCl2 − Tip), 469.3 (43.5), 455.2 (19.5), 433.3
(38.8), 391.3 (10.5), 265.1 (42.9), 251.1 (27.5), 231.1 (88.7), 215.1
(37.0), 203.2 (35.8), 187.1 (42.5), 145.0 (21.4), 43.0 (64.9).
Figure 9. Structure of one of the two independent molecules of 14 in the
solid state (thermal ellipsoids at 15%). Second molecule, hydrogen
atoms, and hexane molecules omitted. Selected bond lengths [Å]: Si1−
Si6 = 2.347(4), Si1−Si2 = 2.355(4), Si1−Si4 = 2.356(4), Si1−Si5 =
2.312(4), Si2−Si5 = 2.334(4), Si2−Si3 = 2.366(4), Si3−Si4 = 2.366(4),
Si4−Si5 = 2.334(4).
Hexasilaprismane (8a). To a mixture of 7.94 g (9.92 mmol) of 7
and 0.36 g (14.81 mmol) of magnesium powder was added 90 mL of
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THF, and the reaction mixture was stirred for 16 h at room temperature.
All volatiles were removed under reduced pressure, and the residue was
digested in 65 mL of hexane. Insoluble parts were filtered off hot, and the
filtrate was reduced until a solid precipitated, which was redissolved by
gentle heating. Standing overnight at room temperature gave 2.14 g
(31%) of 8a as small orange crystals (mp >250 °C). 1H NMR (500.13
MHz, C6D6, 25 °C): δ = 7.15, 7.03 (each br, altogether 12H, Tip-H),
4.91, 3.54, 2.75 (each hept, altogether 18H, iPr-CH), 1.75, 1.22, 1.18,
1.12, 0.73 (each d, altogether 108H, iPr-CH3). 13C NMR (100.61 MHz,
C6D6, 25 °C): δ = 156.95, 153.87, 150.37 (Tip-Co/p), 133.26 (Tip-Ci),
121.65, 121.33 (Tip-CH), 37.18, 36.73, 34.73 (iPr-CH), 27.00, 26.87,
25.60, 24.14 (iPr-CH3). 29Si NMR (99.36 MHz, C6D6, 25 °C): δ =
−23.1. UV/vis (hexane): λmax (ε) = 264 nm (5550 L mol−1 cm−1), 242
nm (7623 L mol−1 cm−1), 205 nm (18325 L mol−1 cm−1). MS (EI): m/z
(%) = 1388 (100.0, M+), 587 (12.0), 433 (17.8), 231 (46.4), 175 (44.5),
111.0 (63.8), 91 (45.4), 44.0 (38.7).
(iPr-CH3). 29Si NMR (79.49 MHz, C6D6, 70 °C): δ = −35.9 (1Si, SiI2),
−45.4 (2Si, SiTipI), −65.4 (2Si, SiTipI). MS (EI): m/z (%) = 1716 (4,
M+), 1588 (27, M+ − I), 1489 (49), 1461 (19, M+ − 2I).
1,2-Dichlorotricyclo[2.2.0.02,5]hexasilane (12), 1,2,3,5-
Tetrachlorobicyclo[2.2.0]hexasilane (13), and 1,5-
Dichlorotricyclo[2.2.0.02,5]hexasilane (10c). A solution of 25.00 g
(17.04 mmol) of 3 in 600 mL of benzene was warmed to 50 °C, and 5.39
g (17.09 mmol) of BiCl3 was added in small portions as a solid. Stirring
was continued overnight before all solvent was distilled off under
reduced pressure. The residue was digested with 750 mL of hexane, and
the black precipitate (“BiCl”) was filtered off carefully. The filtrate was
reduced to 30 mL until 13 started to precipitate as a white solid, which
was collected by filtration and recrystallized from benzene, resulting in
2.66 g (10%) of pure 13 as colorless crystals (mp 244 °C). The filtrate
was reduced to dryness to yield pure 12 as a pale yellow solid.
Crystallization from a concentrated hexane solution afforded 7.54 g
(30%) of 12 as pale yellow crystals (mp 219−222 °C, dec.). The initial
mother liquor was reduced to 20 mL and stored at room temperature,
resulting in the formation of 0.99 g (4%) of orange crystals, which
consisted of 10c and 5 in a 1:1 ratio. A second crop of 0.41 g (2%) of
Si11-Siliconoid (9). Siliconoid 9 was formed as a side product during
the synthesis of propellane 5 that was described previously.15 The
mother liquor (40 mL) separated from the second batch in the synthesis
of 5 was reduced in volume to 25 mL. After 48 h at room temperature,
1
1
pure 5 was collected from the mother liquor. 12: H NMR (500.13
0.51 g (2.6%) of pure 9 was obtained as orange crystals. H NMR
MHz, C6D6, 25 °C): δ = 7.15, 7.05, 6.93 (each br, altogether 12H, Tip-
H), 4.25, 4.17, 3.60, 3.27, 3.01, 2.76, 2.70 (each br, altogether 18H, iPr-
CH), 1.62, 1.40, 1.33, 1.25, 1.20, 1.15, 1.10, 0.74, 0.38 (each br,
altogether 108H, iPr-CH3). 13C NMR (125.76 MHz, C6D6, 25 °C): δ =
158.19, 156.90 (br), 155.45, 155.18, 154.92, 154.57 (br), 150.25, 150.15,
150.04, 136.74, 136.40 (Tip-C), 124.04, 123.82, 123.69, 123.06, 122.31
(Tip-CH), 39.05, 37.46, 36.70 (br), 36.53, 36.35, 35.97, 34.50, 34.48,
34.46 (iPr-CH), 30.15, 29.08 (br), 28.59 (br), 26.26 (br), 25.80, 25.56,
25.38, 24.88, 24.75, 24.52 (br), 24.10, 24.06, 24.00, 23.99, 23.90, 23.88
(iPr-CH3). 29Si NMR (99.36 MHz, C6D6, 25 °C): δ = 19.1 (2Si, SiTip),
−25.8 (br, 2Si, SiCl), −69.9 (2Si, SiTip2). UV/vis (hexane): λmax (ε) =
396 nm (850 L mol−1 cm−1). MS (EI) m/z (%) = 1458.7 (24.0, M+),
(500.13 MHz, C6D6, 25 °C): δ = 7.122, 7.119, 7.10, 7.08, 7.05, 7.03,
6.80, 6.75, 6.73, 6.72, 6.68, 6.66, 6.65, 6.64 (each s, altogether 16H, Tip-
H), 5.68 (s, 1H, 1J(Si,H) = 194 Hz, Si−H), 4.88, 4.80, 4.66, 4.51, 4.37,
4.28, 4.20, 3.55 (each hept, altogether 10H, iPr-CH), 3.43 (m, 1H, iPr-
CH), 3.27 (m, 1H, CH2), 2.82, 2.80 (each hept, altogether 3H, iPr-CH),
2.69−2.52 (m, altogether 10H, iPr-CH), 2.11, 2.06, 1.89, 1.85 (br, 1H,
CH2), 1.82, 1.81, 1.80, 1.79, 1.77, 1.75, 1.73, 1.56, 1.53, 1.52, 1.11, 1.09,
1.08, 1.07, 1.05, 1.03, 0.45, 0.42, 0.39, 0.37, 0.33, 0.31, 0.29, 0.15, 0.13,
0.12, 0.11, 0.09, 0.07, 0.04 (each d, altogether 141H, iPr-CH3). 13C
NMR (125.76 MHz, C6D6, 25 °C): δ = 158.91, 157.13, 156.95, 156.86,
156.77, 156.25, 156.08, 155.81, 155.71, 155.59, 155.28, 153.62, 153.56,
153.18, 153.02, 152.98, 151.02, 150.50, 150.15, 149.87, 149.84, 149.79
(Tip-Co/p), 136.09, 134.19, 134.00, 133.89, 133.80, 132.45, 131.98 (Tip-
Ci), 123.56, 123.21, 123.08, 122.77, 122.68, 122.56, 122.45, 122.23,
121.69, 121.15, 120.45, 120.41 (Tip-CH), 40.58, 40.55, 37.95, 37.30,
36.71, 36.64, 36.39, 36.14, 36.06, 35.95, 35.88, 35.22, 35.17, 34.99, 34.84,
34.78, 34.75, 34.66, 34.53, 34.47, 34.42 (iPr-CH), 31.46, 30.32, 30.25,
29.30, 29.13, 28.10, 27.69, 27.63, 27.33, 26.47, 26.33, 25.98, 25.71, 25.56,
25.49, 25.44, 25.39, 25.34 (iPr-CH3), 25.24 (CH2), 25.17, 25.12, 24.88,
24.82, 24.26, 24.12, 24.09, 23.99, 23.94, 23.86, 23.79, 23.71, 23.55 (iPr-
CH3). 29Si NMR (99.36 MHz, C6D6, 70 °C): δ = 26.2 (J(Si,Si) = 58 and
12 Hz, SiTip [Si7]), 0.1 (J(Si,Si) = 71, 64, 42, and 25 Hz, SiTip [Si5/
Si9]), −0.6 (J(Si,Si) = 70, 65, 43, and 25 Hz, SiTip [Si5/Si9]), −4.0
(J(Si,Si) = 48, 44, 14, and 11 Hz, SiCH2 [Si11]), −41.2 (J(Si,Si) = 48, 43,
37, 24, and 12 Hz, SiTip [Si4/Si10]), −43.0 (J(Si,Si) = 40, 25, 18, 13,
and 8 Hz, Si [Si2]), −43.5 (J(Si,Si) = 43, 37, 24, and 13 Hz, SiTip [Si4/
Si10]), −56.0 (J(Si,Si) = 37, 16, and 10 Hz, SiH [Si3]), −62.7 (J(Si,Si) =
71, 64, 58, 23, and 18 Hz, SiTip [Si6/Si8]), −64.2 (J(Si,Si) = 71, 65, 58,
22, and 18 Hz, SiTip [Si6/Si8]), −78.7 (J(Si,Si) = 39, 23, 15, and 10 Hz,
Si [Si1]). UV/vis (hexane): λmax (ε) = 399 nm (2460 L mol−1 cm−1),
451 nm (1650 L mol−1 cm−1).
Hexaiodocyclopentasilane (11). At 110 °C a solution of 18.96 g
(12.49 mmol) of 5·1.5C6H14 in 150 mL of toluene was added dropwise
to a solution of 29.22 g (115.13 mmol) iodine in 150 mL of toluene
during 30 min. Reflux was continued for 15 h before the solution was
cooled to ambient temperature. All volatiles were distilled off under
reduced pressure. NMR analysis showed a quantitative formation of 11.
Removing the excess of I2 required three consecutive crystallizations
from toluene, resulting in 6.35 g (28%) of 11·C7H8 as colorless crystals
(mp 216 °C, dec.). Single crystals suitable for X-ray analysis were
obtained from a concentrated hexane solution as 11·1.5C6H14. 1H NMR
(400.13 MHz, C6D6, 70 °C): δ = 7.26 (br), 7.22, 6.96, 6.95 (br) (each d,
altogether 8H, Tip-H), 4.62, 4.14, 3.53, 2.93, 2.77, 2.70 (each hept,
altogether 12H, iPr-CH), 1.95, 1.57, 1.49, 1.44, 1.31, 1.19, 1.13, 1.05,
0.84, 0.29 (each d, altogether 72H, iPr-CH3). 13C NMR (100.61 MHz,
C6D6, 70 °C): δ = 161.47, 159.01, 157.51, 157.39, 152.58, 152.26 (Tip-
Co/p), 126.54 (Tip-Ci), 124.98, 123.99 (Tip-CH), 123.20 (Tip-Ci),
123.02, 120.96 (Tip-CH), 38.77, 38.27, 36.56, 36.35, 34.41 (iPr-CH),
30.85, 30.46, 28.19, 27.26, 25.81, 24.39, 24.00, 23.90, 23.86, 23.80, 23.74
+
987.4 (97.0, M+ − SiTip2 − Cl), 721.4 (43.1, Si4Tip3 ), 469.3 (67.4,
1
SiTip2Cl). 13: H NMR (400.13 MHz, C6D6, 25 °C): δ = 7.32, 7.30,
7.21, 7.13, 7.09, 7.08, 7.00, 6.93, 6.77 (each br, altogether 12H, Tip-H),
4.56, 3.91, 3.81, 3.72, 3.56, 3.53, 3.49, 3.35, 3.13, 2.97, 2.80, 2.78, 2.74,
2.73, 2.71, 2.63, 2.33, 2.26 (each hept, altogether 18H, iPr-CH), 1.86,
1.80, 1.73, 1.67, 1.61, 1.59, 1.57, 1.44, 1.38, 1.32, 1.31, 1.28, 1.26, 1.23,
1.21, 1.18, 1.16, 1.15, 1.13, 0.84, 0.80, 0.68, 0.66, 0.50, 0.48, 0.32, 0.25,
0.11 (each d, altogether 108H, iPr-CH3). 13C NMR (100.61 MHz, C6D6,
25 °C): δ = 157.31, 157.13, 156.63, 156.50, 155.58, 155.41, 155.24,
154.09, 153.63, 153.51, 153.20, 152.50, 150.16, 149.97, 149.91, 149.80,
149.70 (Tip-Co/p), 139.97, 137.62, 136.13, 136.00, 135.38, 131.83 (Tip-
Ci), 125.05, 124.49, 124.01, 123.95, 123.42, 123.36, 122.99, 122.85,
122.64, 121.99 (Tip-CH), 41.66, 40.79, 39.10, 38.06, 37.97, 37.27, 36.58,
36.34, 36.15, 36.02, 35.31, 35.15, 34.64, 34.56, 34.47, 34.34, 34.23 (iPr-
CH), 31.00, 30.24, 29.79, 28.87, 28.40, 28.34, 28.13, 28.02, 27.84, 27.74,
27.64, 27.58, 27.44, 27.39, 26.97, 26.18, 26.01, 25.97, 25.86, 24.43, 24.20,
24.08, 23.99, 23.92, 23.88, 23.83, 23.80, 23.73, 23.47, 21.92 (iPr-CH3).
29Si NMR (79.49 MHz, C6D6, 25 °C): δ = 27.7 (SiTip2), 19.0 (SiClTip),
18.5 (SiClTip), 15.8 (SiClTip), 13.2 (SiClTip), −7.8 (SiTip2). MS (EI):
m/z (%) = 1528.8 (10.0, M+), 1510.8 (4.3), 1262.6 (7.0, M+ − SiClTip),
1227.7 (52.5, M+ − SiCl2Tip), 1059.4 (57.6, M+ − SiClTip2), 959.5
(49.2, M+ − SiTip2 − 3Cl). 10c: 1H NMR (400.13 MHz, C6D6, 70 °C):
δ = 7.22, 7.02, 7.00, 6.96, 6.83, 6.77 (each br, each 2H, Tip-H), 4.25,
3.822, 3.817, 3.35, 3.34, 3.16 (each hept, each 2H, iPr-CH), 2.77−2.56
(m, altogether 6H, iPr-CH), 1.71, 1.50, 1.49, 1.47, 1.38, 1.19, 1.16, 1.14,
1.12, 1.08, 1.06, 1.05, 0.49, 0.48, 0.28, 0.27, 0.03 (each d, altogether
108H, iPr-CH3). 13C NMR (100.61 MHz, C6D6, 70 °C): δ = 157.82,
156.34, 155.21, 154.87, 154.31, 153.38, 151.49, 150.03, 149.56, 140.00,
136.01, 128.71 (Tip-C), 124.06, 123.94, 123.78, 123.15, 122.26, 121.75
(Tip-CH), 38.17, 37.67, 37.56, 37.20, 36.63, 35.90, 34.85, 34.58, 34.32
(iPr-CH), 30.62, 29.35, 28.35, 26.69, 25.94, 25.21, 24.92, 24.07, 24.00,
23.86, 23.60, 22.68 (iPr-CH3). 29Si NMR (79.49 MHz, C6D6, 70 °C): δ
= 21.1 (1Si, SiTip2), 17.5 (2Si, SiCl), 9.8 (1Si, Si Tip2), −80.3 (2Si,
SiTip). MS (EI): m/z (%) = 1459 (33.0, M+).
Chlorosilyltricyclo[2.1.0.02,5]pentasilane (14). A sample of 12
(0.78 g, 0.52 mmol) was heated to 230 °C until all solid had liquefied.
1
After cooling to room temperature, the H NMR spectrum showed a
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quantitative rearrangement to 14. Single crystals for X-ray analysis were
obtained from a concentrated Et2O solution. H NMR (500.13 MHz,
(6) The term “siliconoid” has been used in fiction to designate
extraterrestrial life forms based on silicon instead of carbon: http://
planets4.hood-net.org/wiki/index.php/Siliconoids.
1
C6D6, 25 °C): δ = 7.30, 7.27, 7.09, 7.06, 7.03, 7.02, 7.01, 6.99, 6.93 (each
br, altogether 12H, Tip-H), 4.53, 4.26, 4.17, 4.11, 3.85, 3.52, 3.49, 3.42,
3.39, 3.34 (each hept, altogether 12H, iPr-CH), 2.83−2.65 (m, 6H, iPr-
CH), 1.71, 1.69, 1.62, 1.55, 1.54, 1.46, 1.42, 1.37, 1.26, 1.23, 1.21, 1.19,
1.18, 1.165, 1.157, 1.15, 1.14, 1.05, 1.03, 0.98, 0.76, 0.73, 0.66, 0.59, 0.57,
0.42, 0.37, 0.36 (each d, altogether 108H, iPr-CH3). 13C NMR (125.76
MHz, C6D6, 25 °C): δ = 159.27, 159.17, 158.83, 156.39, 156.37, 156.29,
156.26, 156.05, 155.00, 154.47, 153.77, 151.42, 151.18, 151.10, 151.06,
150.97, 150.78, 135.85, 134.11, 133.79, 128.57, 128.06, 124.31, 122.70,
122.54, 122.29, 122.24, 122.12, 121.98, 121.90, 121.83, 121.74, 120.06
(Tip-C), 38.36, 38.08, 37.93, 37.89, 37.78, 37.69, 37.48, 36.34, 35.69,
34.81, 34.78, 34.76, 34.67, 34.59, 31.75 (iPr-CH), 31.66, 30.25, 28.39,
28.20, 28.16, 27.85, 27.71, 27.67, 27.49, 27.30, 26.74,26.60, 26.54, 25.99,
25.74, 25.30, 25.19, 24.80, 24.46, 24.37, 24.27, 24.19, 24.16, 24.13, 24.12,
24.08, 24.01, 23.95, 23.85, 23.47, 21.71, 21.63 (iPr-CH3). 29Si NMR
(99.36 MHz, C6D6, 25 °C): δ = 59.4 (1Si, J(Si,Si) = 37, 17, and 13 Hz,
SiTip), 5.8 (1Si, J(Si,Si) = 66 and 9 Hz, SiTip2Cl), −96.4 (1Si, J(Si,Si) =
51, 36, 23, and 19 Hz, SiTip), −113.2 (1Si, J(Si,Si) = 51, 39, 22, and 17
Hz, SiTip), −151.2 (1Si, J(Si,Si) = 66, 18, 13, and 9 Hz, SiTip), −151.9
(1Si, J(Si,Si) = 23, 17, 13, and 9 Hz, Si). MS (EI): m/z (%) = 1459 (11.4,
M+), 988 (42.4, M+ − SiTip2 − Cl), 721 (21.0, Si4Tip3), 469 (37.7,
SiTip2Cl).
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ASSOCIATED CONTENT
■
S
* Supporting Information
Details of X-ray structure determinations, NMR spectroscopic
determination of the position of the Si-bonded hydrogen in
siliconoid 9, NMR and UV/vis spectra of all compounds. This
material is available free of charge via the Internet at http://pubs.
acs.org. Crystallographic data for 7, 8a, 9, 10c·5, 11, 12·benzene,
12·hexane, 13, and 14 have been deposited with the Cambridge
Structural Database under deposition numbers CCDC 893657−
893665.
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Veneziani, G. L. Chem. Commun. 2005, 6023.
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AUTHOR INFORMATION
■
(22) Nagase, S.; Nakano, M.; Kudo, T. J. Chem. Soc., Chem. Commun.
1987, 60.
Corresponding Author
scheschkewitz@mx.uni-saarland.de
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have been deposited with the SPECTRa database and can be retrieved
under http://dx.doi.org/ with the following identifiers: 9Dip, 10042/
20213; 9Dip*, 10042/20214; and 9Dip**, 10042/20215. Instructions
on how to use this database: Downing, J.; Murray-Rust, P.; Tonge, A. P.;
Morgan, P.; Rzepa, H. S.; Cotterill, F.; Day, N.; Harvey, M. J. J. Chem. Inf.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support for this study was provided by the EPSRC (EP/
H048804/1), the Alfried Krupp von Bohlen und Halbach
Foundation, and Saarland University’s start-up funds.
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