Halide-Ion Diadducts of Perhalogenated Cyclopenta- and Cyclohexasilanes

Article abstract of DOI:10.1002/zaac.201800145

The halosilicates(II) [nBu₄N]₂[Si₆Br₁₂·2Br] and [nBu₄P]₂[Si₆I₁₂·2I] were prepared by mere addition of the appropriate halide salt to the corresponding disilane Si₂X₆ (X = Br, I). In the first case, the Br₃Si-substituted derivative [nBu₄N]₂[Si₇Br₁₄·2Br] formed as a second product. We have been able to obtain single crystals of [Ph₄P]₂[Si₇Br₁₄·2Br] by switching the Br^– salt from [nBu₄N]Br to [Ph₄P]Br. All three compounds, [nBu₄N]₂[Si₆Br₁₂·2Br] (monoclinic, P2₁/c), [nBu₄P]₂[Si₆I₁₂·2I] (triclinic, P1), and [Ph₄P]₂[Si₇Br₁₄·2Br] (triclinic, P1) were structurally characterized by X-ray crystallography and found to form “inverse sandwich complexes”, in which two X^– ions are located above and below a planarized Si₆ ring. The free periodated cyclohexasilane Si₆I₁₂ is accessible from [nBu₄N]₂[Si₆Cl₁₂·2Cl] and BI₃ (1:5 molar ratio; CH₂Cl₂) via a decomplexation/halide-exchange cascade. Si₆I₁₂ (monoclinic, C2/c) adopts a puckered chair conformation in the solid state. The cyclopentasilane diadduct [nBu₄P]₂[Si₅Cl₁₀·2Cl] forms at –78 °C from Si₅Cl₁₀ and 2 equiv. of [nBu₄P]Cl in CH₂Cl₂. An X-ray analysis of [nBu₄P]₂[Si₅Cl₁₀·2Cl]·2CH₂Cl₂ (monoclinic, C2/c) again revealed the structure of an “inverse sandwich complex”. Crystalline [nBu₄P]₂[Si₅Cl₁₀·2Cl]·2CH₂Cl₂ is stable at room temperature. However, in CH₂Cl₂ solution the compound quantitatively undergoes ring-expansion reactions to furnish cyclohexasilane derivatives. This behavior is in striking contrast to that of [nBu₄N]₂[Si₆Cl₁₂·2Cl], which persists at room temperature both in solution and the solid state. Competition experiments revealed free Si₅Cl₁₀ and Si₆Cl₁₂ to have comparable Cl^– affinities, even though the distance between the two apical Cl^– ions in [Si₅Cl₁₀·2Cl]^2– (4.292 ?) is larger by 0.456 ? than that in [Si₆Cl₁₂·2Cl]^2– (3.836 ?).

Full text of DOI:10.1002/zaac.201800145

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.201800145
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Halide-Ion Diadducts of Perhalogenated Cyclopenta- and
Cyclohexasilanes
Julian Teichmann,[a] Benedikt Köstler,[a] Jan Tillmann,[a] Maximilian Moxter,[a] Robin Kupec,[a] Michael Bolte,[a]
Hans-Wolfram Lerner,*[a] and Matthias Wagner*[a]
Dedicated to Prof. Alexander C. Filippou on the Occasion of his 60th Birthday
Abstract.
The
halosilicates(II)
[nBu4N]2[Si6Br12·2Br]
and puckered chair conformation in the solid state. The cyclopentasilane
[nBu4P]2[Si6I12·2I] were prepared by mere addition of the appropriate
halide salt to the corresponding disilane Si2X6 (X = Br, I). In the first
diadduct [nBu4P]2[Si5Cl10·2Cl] forms at –78 °C from Si5Cl10 and
2 equiv. of [nBu4P]Cl in CH2Cl2. An X-ray analysis of
case, the Br3Si-substituted derivative [nBu4N]2[Si7Br14·2Br] formed as [nBu4P]2[Si5Cl10·2Cl]·2CH2Cl2 (monoclinic, C2/c) again revealed the
a second product. We have been able to obtain single crystals of
[Ph4P]2[Si7Br14·2Br] by switching the Br– salt from [nBu4N]Br to
structure of an “inverse sandwich complex”. Crystalline
[nBu4P]2[Si5Cl10·2Cl]·2CH2Cl2 is stable at room temperature. How-
[Ph4P]Br. All three compounds, [nBu4N]2[Si6Br12·2Br] (monoclinic, ever, in CH2Cl2 solution the compound quantitatively undergoes ring-
¯
P21/c), [nBu4P]2[Si6I12·2I] (triclinic, P1), and [Ph4P]2[Si7Br14·2Br]
expansion reactions to furnish cyclohexasilane derivatives. This behav-
¯
(triclinic, P1) were structurally characterized by X-ray crystallography ior is in striking contrast to that of [nBu4N]2[Si6Cl12·2Cl], which per-
and found to form “inverse sandwich complexes”, in which two
sists at room temperature both in solution and the solid state. Competi-
X– ions are located above and below a planarized Si6 ring. The tion experiments revealed free Si5Cl10 and Si6Cl12 to have comparable
free periodated cyclohexasilane Si6I12 is accessible from
Cl– affinities, even though the distance between the two apical Cl– ions
in [Si5Cl10·2Cl]2– (4.292 Å) is larger by 0.456 Å than that in
[nBu4N]2[Si6Cl12·2Cl] and BI3 (1:5 molar ratio; CH2Cl2) via a decom-
plexation/halide-exchange cascade. Si6I12 (monoclinic, C2/c) adopts a [Si6Cl12·2Cl]2– (3.836 Å).
In 2001, Boudjouk et al. reported an alternative synthesis of
Introduction
Si6Cl12, starting from HSiCl3 and pedeta in CH2Cl2 (40 °C,
48 h; pedeta = 1,1,4,7,7-pentaethyldiethylenetriamine).[9] The
compound was isolated in the form of the chlorosilicate(II)
“inverse sandwich complex” [(pedeta)H2SiCl]2[Si6Cl12·2Cl],
in which two additional Cl– ions are coordinated in μ6 modes
above and below the planarized Si6 ring. The chloride diadduct
can be converted to parent cyclohexasilane, Si6H12, by em-
ploying LiAlH4 as the hydride source.[9,10] Subsequent treat-
ment of Si6H12 with elemental Cl2 or Br2 at low temperatures
results in exhaustive H/X exchange to afford Si6Cl12 or
Si6Br12, respectively.[2,5,10,11]
Recently, our group has established convenient access to
[Si6Cl12·2Cl]2– and to its Cl3Si-substituted congeners
[Si7Cl14·2Cl]2– and [Si8Cl16·2Cl]2– via the chloride-induced
disproportionation of Si2Cl6 (Scheme 1).[12–14] We also found
ways to liberate the free cyclohexasilane Si6Cl12 from
[Si6Cl12·2Cl]2– by means of AlCl3.[15] It turned out that the
Lewis acidity of Si6Cl12 is comparable to that of BCl3,[15] and
a number of stable diadducts with Lewis bases (L) other than
the Cl– ion have been reported.[10,11]
Perhalogenated oligosilanes are of great technological im-
portance, because they find use in the synthesis of silicon-con-
taining polymers and also as precursor molecules for the depo-
sition of amorphous or crystalline silicon.[1] A deeper under-
standing of the intrinsic properties of perhalogenated oligosil-
anes can best be gained by investigating samples of monodis-
perse, cyclic species to avoid spurious chain-length and end-
group effects. The classical route to cyclic oligosilanes SinX2n
(n = 4, 5, 6; X = Cl, Br, I) proceeds via phenyl/halogen ex-
change on SinPh2n using HX in the presence of AlX3.[2–5] The
required perphenylated cyclosilanes are typically prepared
through Wurtz-type coupling of Ph2SiCl2 with alkali met-
als.[6,7] However, the coupling reaction is not size-selective and
mixtures of Si4Ph8, Si5Ph10 (main product), and Si6Ph12 are
usually obtained, which poses problems with regard to product
separation and reduces the product yields.[6–8]
* Dr. H.-W. Lerner
Fax: +49 69 79829260
E-Mail: lerner@chemie.uni-frankfurt.de
* Prof. Dr. M. Wagner
E-Mail: matthias.wagner@chemie.uni-frankfurt.de
[a] Institut für Anorganische Chemie
Goethe-Universität Frankfurt
The information gathered so far on SinX2n immediately
raises the following questions, which are answered in this pa-
per:
(1) Is it possible to prepare also the bromo- and iodosili-
cates(II) [Si6X12·2X]2– (X = Br, I) in ways similar to the
synthesis of [Si6Cl12·2Cl]2– from Si2Cl6/Cl–?
Max-von-Laue-Str. 7
60438 Frankfurt, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201800145 or from the au-
thor.
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[Si7Br14·2Br]2– (Scheme 1). [nBu4N]2[Si6Br12·2Br] could be
separated from [nBu4N]2[Si7Br14·2Br] and SiBr4 by crystalli-
zation. [nBu4N]2[Si6Br12·2Br] crystallizes in the monoclinic
space group P21/c and represents a polymorph of a published
structure (the latter sample had been prepared through the ad-
dition of [nBu4N]Br to free Si6Br12).[11] In the solid state,
[nBu4N]2[Si6Br12·2Br] reveals an essentially planar Si6 ring
(rms deviation: 0.026 Å; average Si–Si bond length =
2.342 Å). Two Br– anions are located above and below the
ring centroid (COG) with distances of 2.061 Å and 2.059 Å
for COG···Br(1) and COG···Br(2), respectively (Scheme 1; se-
lected bond lengths and angles are listed in the caption of Fig-
ure 1S, Supporting Information). The distances of the apical
Br(1) and Br(2) ions to the Si atoms within the six-membered
ring (average value: 3.12 Å) are shorter than the sum of the
van der Waals radii [rvdw(SiBr): 3.95 Å],[20] but longer than
the sum of the covalent radii [rcov(SiBr): 2.31 Å].[21]
Overall, the structural parameters of our [Si6Br12·2Br]2– di-
anion are close to those found in the polymorphic structure
[11]
¯
(triclinic, P1).
The main difference between the two poly-
morphs is that the triclinic structure has just half a Si6Br12
molecule, one Br– anion and one [nBu4N]+ cation in the asym-
metric unit, whereas the asymmetric unit of the monoclinic
polymorph contains one complete Si6Br12 molecule, two Br–
anions and two [nBu4N]+ cations. In the triclinic polymorph,
Scheme 1. (a) Solid-state structures of [nBu4N]2[Si6Cl12·2Cl] and the planes of all Si6 rings are exactly coplanar; in the mono-
its
Cl3Si-substituted
congeners
[Ph4P]2[Si7Cl14·2Cl]
and
clinic structure, half of the ring planes are tilted by 43.9° with
respect to the other half. The distance between the Br– anion
and the N atom of the [nBu4N]+ cation is shorter in the triclinic
(4.71 Å) than in the monoclinic form (average value: 5.05 Å).
In the triclinic form, all four N–C–C–C torsion angles adopt a
trans conformation, whereas in the monoclinic form, one of
the eight N–C–C–C torsion angles adopts a gauche con-
formation.
[Ph4P]2[Si8Cl16·2Cl] (cations omitted for clarity).[13] (b) Synthesis of
[Si6X12·2X]2– through disproportionation of Si2X6 (X = Cl, Br, I) with
the corresponding X– ions; in the cases of X = Cl, Br, several X3Si-
substituted derivatives are formed in addition to the perhalogenated
cyclohexasilanes (i: CH2Cl2, room temperature or 80 °C). (c) Solid-
state structures of [nBu4N]2[Si6Br12·2Br], [Ph4P]2[Si7Br14·2Br], and
[nBu4P]2[Si6I12·2I] (cations omitted for clarity).
(2) Is the stability of chlorosilicates(II) ring-size dependend
and can, apart from [Si6Cl12·2Cl]2–, also the cyclopentasilane
diadduct [Si5Cl10·2Cl]2– be isolated and fully characterized?
The reaction of Si2Br6 with a solution of [Ph4P]Br in
CD2Cl2 again led to the bromosilicate(II) anions
[Si6Br12·2Br]2– and [Si7Br14·2Br]2– and to SiBr4 as necessary
byproduct (Scheme 1). The disilane was quantitatively con-
sumed after 1 h at room temperature (29Si-NMR spectroscopic
Results and Discussion
¯
control). Single crystals of [Ph4P]2[Si7Br14·2Br] (triclinic, P1,
Z = 2), grew at room temperature in the NMR tube directly
from the reaction solution. The structure of the dianionic moi-
ety [Si7Br14·2Br]2– is shown in Scheme 1 (selected bond
lengths and angles are listed in the caption of Figure 2S,
Supporting Information). As was the case for
Synthesis and Characterization of [Si6X12·2X]2–, Starting
from Si2X6/X– (X = Br, I)
[16]
The precursor compounds Si2Br6
and Si2I6,[17] were pre-
pared according to literature procedures. From a broad variety
of available soluble halide salts, [nBu4N]Br, [Ph4P]Br, or
[nBu4P]I were chosen such as to generate the reaction product
with the highest degree of crystallinity.
First, we investigated the reactions of Si2Br6 with
[nBu4N]Br and [Ph4P]Br, respectively. Treatment of Si2Br6
with [nBu4N]Br (8:3 molar ratio; CD2Cl2) resulted in an im-
mediate reaction, whereupon the mixture quickly became
heterogeneous. A 29Si NMR spectrum showed that the reso-
nance of Si2Br6 [δ(29Si) = –35.9 ppm][16] had completely van-
ished. Newly developed signals were assignable to SiBr4
[nBu4N]2[Si6Br12·2Br],
the
six-membered
ring
of
[Ph4P]2[Si7Br14·2Br] is by and large planar (rms deviation
0.054 Å). The distances between the two coordinated bromide
anions and the ring centroid amount to COG···Br(1) = 2.071 Å
and COG···Br(2) = 2.078 Å, which is slightly longer than in
the case of [nBu4N]2[Si6Br12·2Br] and likely attributable to the
steric demand of the Br3Si substituent.
Next, we prepared the iodosilicate(II) [nBu4P]2[Si6I12·2I] by
heating an equimolar mixture of Si2I6 and [nBu4P]I in CD2Cl2
for 20 h to 80 °C (flame-sealed NMR tube; Scheme 1). We
[δ(29Si) = –90.2; lit: –92.7 ppm[18]] and the bromosilicates(II) applied the elevated temperatures and used the significantly
[Si6Br12·2Br]2– [δ(29Si) = –38.1; lit: –37.4 ppm[11,19]] and overstoichiometric amount of [nBu4P]I (cf. Scheme 1) with the
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aim to promote a reaction of the poorly soluble Si2I6. After at ambient temperature and the colorless periodated cyclohexa-
cooling to room temperature, colorless single crystals of silane (Si6I12) precipitated from the mixture. The precipitate
[nBu4P]2[Si6I12·2I] formed (90% yield). In the crystal lattice was allowed to age for 6 d to grow single crystals suitable for
2–
¯
(triclinic, P1), [Si6I12·2I] shows a planar six-membered ring X-ray crystallography.
(rms deviation 0.021 Å), located on a crystallographic center
of inversion, with an average Si–Si bond length of 2.382 Å.
Two I– anions are coordinated to establish an “inverse sand-
wich complex” with a COG···I distance of 2.285 Å (Scheme 1;
selected bond lengths and angles are listed in the caption of
Figure 3S, Supporting Information). Similar to the case of
[nBu4N]2[Si6Br12·2Br], the average distance between the Si
Scheme 2. Synthesis and solid-state structure of the periodated cyclo-
hexasilane Si6I12.
atoms and the apical halide ions of [nBu4P]2[Si6I12·2I]
(3.30 Å) is smaller than the sum of the van der Waals radii
[rvdw(SiI): 4.08 Å],[20] but larger than the sum of the covalent
radii [rcov(SiI): 2.50 Å] of Si and I.[21]
Not only the I– diadduct [nBu4P]2[Si6I12·2I], but also the
free cyclohexasilane Si6I12 is not sufficiently soluble to record
29Si NMR spectra. An IR spectrum of Si6I12 revealed two char-
acteristic bands at 405 cm–1 (s) and 468 cm–1 (w).[5] Neverthe-
less, the above-mentioned caveat regarding the possibility of
minor Cl contamination of the iodosilicate(II) also applies to
Si6I12.
A
29Si NMR spectroscopic investigation of the mother
liquor revealed the presence of chlorinated side products, such
as SiCl4, which likely originate from I/Cl-exchange reactions
between the iodosilanes and the solvent CD2Cl2[22] (see Figure
7S, Supporting Information, for a 13C{1H} NMR spectrum of
the reaction solution showing the resonance of CD2ICl). This
observation is a caveat concerning a possible chloride contami-
nation of [nBu4P]2[Si6I12·2I]. Unfortunately, the poor solubil-
ity of [nBu4P]2[Si6I12·2I] in all common inert solvents pre-
cluded its characterization by solution-state NMR spec-
troscopy, which would otherwise have been a powerful diag-
nostic tool to distinguish between SiI2 and SiICl/SiCl2 frag-
ments within the sample. The key quality criteria of the X-ray
crystal-structure analysis of [nBu4P]2[Si6I12·2I] (i.e., Si–I bond
lengths, shapes and sizes of the thermal ellipsoids, residual
electron density) gave no indication for significant chloride
contamination. To finally resolve the issue, we repeated the
synthesis of [nBu4P]2[Si6I12·2I] in 1,2-dichlorobenzene. This
time, an NMR spectroscopic investigation of the reaction mix-
ture did not reveal any halide-exchange reaction with the sol-
vent. Again, we obtained single crystals of [nBu4P]2[Si6I12·2I].
X-ray diffraction gave precisely the same results as before.
We emphasize at this point that the addition of [nBu4N]I to
Si6Cl12 in CH2Cl2 cleanly affords the I– diadduct
[nBu4N]2[Si6Cl12·2I] with no indication of Cl/I scrambling at
the silicon atoms.[11] It is thus barely surprising that also in
the case of the synthesis of [nBu4P]2[Si6I12·2I] in CD2Cl2 the
chlorosilane side products are not converted back to corre-
sponding iodosilanes by the excess amount of [nBu4P]I. On
the other hand, when we generated [SiCl3]– from the
Si2Cl6/[R4N]Cl system[12,13,15,25] and attempted to trap
the anion in situ with BI3, we did not detect any
[Et4N][I3B–SiCl3], but rather isolated the [SiI3]– adduct
[Et4N][I3B–SiI3].[22] According to quantum-chemical cal-
culations, the quantitative Cl/I exchange between [SiCl3]– and
BI3 is overall strongly exergonic and therefore provides a
thermodynamic thrust toward [I3B–SiI3]– formation.[22]
The cyclohexasilane Si6I12 crystallizes in the monoclinic
space group C2/c (Scheme 2; selected bond lengths and angles
are listed in the caption of Figure 4S, Supporting Information).
The molecule is located on a crystallographic center of inver-
sion. The Si6 ring adopts a regular chair conformation with
approximate D3d symmetry. The average dihedral angle be-
tween the best planes through four central Si6-ring atoms and
the corresponding flap planes can be taken as a measure of the
ring puckering. In the solid-state structure of centrosymmetric
Si6I12, there are three such dihedral angles: Si(2)Si(3)Si(2A)
Si(3A)//Si(2)Si(1)Si(3A) = 42.9°, Si(1)Si(2)Si(1A)Si(2A)//
Si(2)Si(3)Si(1A) = 43.2°, and Si(1)Si(3A)Si(1A)Si(3)//Si(1)
Si(2)Si(3) = 44.3°. Their average value amounts to 43.5° (cf.
Scheme 2 and Figure 4S, Supporting Information). A chair
conformation is the intuitive minimum for cyclohexane ana-
logues such as Si6X12, because it helps to avoid angular strain.
In a series of thorough theoretical studies, a pseudo-Jahn-
Teller (PJT) effect has been identified as an important factor
inducing ring puckering.[11,23,24] An interaction of the unoccu-
pied MOs of Si6X12, which are basically composed of
σ*(Si–X) orbitals, with electron lone pairs of apically coordi-
nated X– ions suppresses the PJT distortion, induces a planari-
zation of the Si6 ring and leads to a stabilizing effect in the
“inverse sandwich complexes” [Si6X12·2X]2–.[11,23,24] The
bonding of X– ions to the six-membered ring in [Si6X12·2X]2– is
not entirely covalent in nature, but also benefits from Coulomb
attraction between the positively polarized Si ring atoms and
the anionic guests.[23] It has been computed that, irrespective
of the halogen pattern, the degree of covalency in complexes
[Si6X12·2X]2– is approximately 45%.[23]
With the molecular structures of [nBu4P]2[Si6I12·2I] and
Si6I12 at hand, we are now in a position to compare the key
structural parameters of the entire series of compounds
[Si6X12·2X]2– and Si6X12 (X = Cl, Br, I; Table 1 and Table 2).
For the diadducts [Si6X12·2X]2–, computed bond lengths
Given this background, we decided to examine the reaction
between [nBu4N]2[Si6Cl12·2Cl] and 5 equiv. of BI3 with the
aim to obtain free Si6I12 in a combined decomplexation/halide-
exchange process (Scheme 2). In CH2Cl2 solution, an immedi- [B3LYP/6-311++g(d,p)][23] are available in addition to the X-
ate reaction occurred between both starting materials already ray crystallographically determined ones. As a general trend,
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Table 1. Comparison of computed and experimentally determined structural parameters of [Si6X12·2X]2– (X = Cl, Br, I). To ensure maximum
comparability we only included examples with [nBu4N]+ or [nBu4P]+ counterions. Note: In this table, XЈ denotes an apical X– ion.
Si–Si (avg) /Å
Si–X (avg) /Å
Si···XЈ (avg) /Å
X···XЈ (avg) /Å
XЈ···XЈ /Å
a)
a)
[Si6Cl12·2Cl]2– calcd.
2.372
2.322
2.368
2.342
2.343
2.123
2.078
2.290
2.251
2.247
3.085
3.010
3.090
3.118
3.125
3.729
3.635
3.805
3.786
3.789
b)
[Si6Cl12·2Cl]2– exp.
[Si6Br12·2Br]2– calcd.
[Si6Br12·2Br]2– exp.
3.836
b)
b)
b)
b)
b)
4.119
4.136
c)
c)
c)
c)
c)
[Si6I12·2I]2– calcd.
2.447
2.382
2.571
2.496
3.429
3.300
4.147
4.018
a)
b)
[Si6I12·2I]2– exp.
4.570
a) Vedha et al.[23] b) Prepared through the halogen-induced disproportionation of Si2X6 (this work and reference[13]). c) Prepared by Boudjouk
[11,24]
et al. through the addition of 2 equiv. of [nBu4N]Br to Si6Br12
.
the calculated Si–Si and Si–X bond lengths are slightly too
A solution of the perhalogenated cyclopentasilane in CH2Cl2
long ( Յ 0.065 Å for Si–Si; 0.045 Å for Si–Cl, Յ 0.043 Å for was added at –78 °C to a solution of [nBu4P]Cl in the same
Si–Br, 0.075 Å for Si–I), but still agree well with the averaged solvent. The mixture was stored at –72 °C for 7 d to grow
experimentally determined parameters. Along both series colorless single crystals of [nBu4P]2[Si5Cl10·2Cl]·2CH2Cl2 in
[Si6X12·2X]2– and Si6X12, the nature of the halogen atom has a a yield of 76% (monoclinic, C2/c). Similar to [Si6Cl12·2Cl]2–,
surprisingly small influence on the Si–Si bond lengths. The the dianion [Si5Cl10·2Cl]2– forms an “inverse sandwich com-
coordination of X– ions to Si6X12 leads to a slight contraction plex” with a planar Si5 ring (rms deviation: 0.011 Å) located
of the Si–Si bonds in the case of X– = Cl– and to an elongation on a twofold rotation axis (Scheme 3; selected bond lengths
in the case of X– = I–; Si6Br12 and [Si6Br12·2Br]2– possess the and angles are listed in the caption of Figure 5S, Supporting
same average Si–Si bond length. We hypothesize that these Information). All Si–Si bond lengths fall in the narrow range
structural changes are necessary to maintain an optimal inter- between 2.340(1) Å and 2.346(1) Å. The average value of
action of the respective Si6 ring with the small Cl– the me- 2.344 Å is close to the average Si–Si bond length of the cyclo-
dium-sized Br–, and the large I– ion. An expansion is evident hexasilane diadduct [nBu4N]2[Si6Cl12·2Cl] (2.322 Å). The
for the Si–X bonds of the diadducts, likely due to electron same is true for the average Si–Cl bond lengths in the Si5
donation from the apical X– ions into antibonding σ*(Si–X) diadduct (2.094 Å) compared to the Si6 diadduct (2.078 Å).
orbitals.
The distance between the two apical Cl– ions, however, is
larger by as much as 0.456 Å in [Si5Cl10·2Cl]2– (4.292 Å) than
in the Si6 analog (3.836 Å). These decisive structural differ-
ences likely result from the fact that the void spaces above and
below the ring planes are more limited in the case of the five-
membered ring species.
Table 2. Comparison of structural parameters of Si6X12 (X = Cl, Br,
I).
Si–Si
(avg) /Å
Si–X
(avg) /Å
Si–Si–Si
(avg) /°
Flap angle
(avg) /°
a)
a)
Si6Cl12
Si6Br12
2.342
2.341
2.355
2.029
2.196
2.435
112.9
113.2
113.5
45.4
44.2
43.5
b)
Si6I12
[10]
a) Prepared through exhaustive H/X exchange on Si6H12 using X2
.
b) Prepared through decomplexation and exhaustive Cl/I exchange on
[Si6Cl12·2Cl]2– using BI3.
We finally note that also the Si–Si–Si bond angles and the
flap angles of Si6X12 are independent from X, thereby pointing
toward a similar degree of PJT distortion in all three cyclo-
hexasilanes.
Scheme 3. Synthesis of [nBu4P]2[Si5Cl10·2Cl] and solid-state structure
of the dianion [Si5Cl10·2Cl]2–. [nBu4P]Cl was used as Cl– source.
Most key structural parameters of [Si5Cl10·2Cl]2– have been
correctly predicted by a recent DFT study.[26] The most signifi-
cant deviation between the computed and experimentally de-
Synthesis and Structural Characterization of
[nBu4P]2[Si5Cl10·2Cl]
The Cl–-induced disproportionation of Si2Cl6, followed by termined values is evident for the Cl–···Cl– distance (calcd:
decomplexation of the primary product [Si6Cl12·2Cl]2– with 4.215 Å vs. exp: 4.292 Å). Nevertheless, also the DFT calcula-
AlCl3, arguably provides the most time- and cost-efficient tions unequivocally confirm much farther separated Cl– ions in
route for the synthesis of Si6Cl12.[15] Unfortunately, until to- the Si5 compared to the Si6 diadduct. To rationalize the planar
date this protocol is restricted to the preparation of cyclohexa- conformation of the Si5 ring in [Si5Cl10·2Cl]2–, the authors of-
silanes; compounds of other ring sizes have never been iso- fer a similar explanation as previously discussed for the
lated from the product mixtures. For our investigations of the [Si6X12·2X]2– diadducts.[26]
Cl– diadduct [Si5Cl10·2Cl]2–, we thus had to rely on Hengge’s
Yet, contrary to the theoretical prediction of “substantial
Si5Cl10 ring stabilization due to the exocyclic chlorine”, our
classical synthesis of Si5Cl10.[3]
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experiments suggest a destabilizing effect of Cl– coordination of the nature of X, we found no indication for a destabilizing
on the Si5Cl10 scaffold:
Even though crystalline [nBu4P]2[Si5Cl10·2Cl]·2CH2Cl2 re-
mains unchanged at room temperature, its CH2Cl2 solutions Si5Cl10: Even though
effect of coordinated X– ions on the Si6X12 ring.
The opposite is true for the perchlorinated cyclopentasilane
corresponding Cl– diadduct
a
persist only at low temperatures. Upon warming to ambient [nBu4P]2[Si5Cl10·2Cl]·2CH2Cl2 can be prepared and isolated
temperature, the complex undergoes rearrangement reactions in excellent yields at low temperatures of –78 °C, the com-
to afford cyclohexasilane diadducts such as [Si6Cl12·2Cl]2– and pound readily undergoes a ring-expansion reaction to form
[Si7Cl14·2Cl]2–. In stark contrast, free Si5Cl10/Si6Cl12 as well [nBu4P]2[Si6Cl12·2Cl] and Cl3Si-substituted derivatives
as the complexed cyclohexasilane [nBu4N]2[Si6Cl12·2Cl] are thereof in CH2Cl2 solutions at ambient temperature.
stable at room temperature under inert conditions. It is also
This observation is of immediate relevance for an under-
worth mentioning that room temperature-stable diadducts standing of the reaction mechanism underlying the synthesis
[Si5Cl10·2NCR] of Si5Cl10 with nitrile ligands have been re- of perchlorinated cyclohexasilanes via the Cl–-induced
ported (R = p-tolyl, 1-adamantyl).[27] To assess the affinity of heterolysis of Si2Cl6: The exclusive formation of Si6 rings does
Si5Cl10 toward Cl– ions, we performed a number of competi- not necessarily imply a size selectivity of the actual ring-clo-
tion experiments at room temperature on an NMR scale sure process. It is well conceivable that, besides Si6 rings, also
(CD2Cl2). While the cyclopentasilane behaves inert to cyclic Si5 intermediates are formed, which subsequently un-
[nBu4N][AlCl4], it quantitatively decomposes to numerous dergo a Cl–-mediated rearrangement to furnish cyclohexasilane
unidentified products upon addition of 2 equiv. of products.
[nBu4N][BCl4].
Moreover, we found similar Cl– affinities for Si5Cl10 and
In agreement with this observation, we succeeded in the Si6Cl12. The significantly longer distances between the apical
clean liberation of Si5Cl10 by adding crystals of Cl– ions and the ring planes in [Si5Cl10·2Cl]2– compared to
[nBu4N]2[Si5Cl10·2Cl]·2CH2Cl2 to a suspension of 2.5 equiv. [Si6Cl12·2Cl]2– are therefore not indicative of pronounced dif-
of AlCl3 in CH2Cl2 (see also reference[15] in this context). ferences in the interaction between the individual components
Attempts to reproduce this reaction using BCl3 as the Lewis of the two “inverse sandwich complexes”.
acid failed and furnished complex product mixtures. We there-
Moreover, we developed an efficient decomplexation/halogen-
fore conclude that the Cl– affinity of Si5Cl10 roughly matches exchange protocol, which starts from [nBu4N]2[Si6Cl12·2Cl]
that of BCl3 but is too low to outcompete AlCl3. It should be and BI3 (5 equiv.) and provides free Si6I12 in excellent yields.
noted that Si5Cl10, like Si6Cl12,[15] is a donor-free storage form This convenient reaction is remarkable, because treatment
of dichlorosilylene SiCl2. Treatment of Si5Cl10 with strong li- of [nBu4N]2[Si6Cl12·2Cl] with BCl3 fails to provide free
gands, such as NMe2Et or 1,3-bis(2,6-diisopropylphenyl)imid- Si6Cl12.[15]
azol-2-ylidene, in C6H6 results in the quantitative formation of
In summary, we have expanded the scope of the X–-induced
the corresponding dichlorosilylene adducts (NMR spectro- disproportionation reaction of Si2X6 beyond the scope of chlo-
scopic control). These adducts have already been described in rosilanes and demonstrated its versatility for generating vari-
the literature.[28,29]
ous useful silicon species.
We finally prepared
a 1:1 mixture of Si5Cl10 and
[nBu4N]2[Si6Cl12·2Cl] in CD2Cl2 and recorded 29Si NMR
spectra of the sample at room temperature. The signal of
Si5Cl10 [δ(29Si) = –2.17; lit.: –1.70 ppm[27]] had completely
vanished. Instead, we observed resonances assignable to SiCl4,
[Si6Cl12·2Cl]2–, [Si7Cl14·2Cl]2–,[13] and 1,1-[Si8Cl16·2Cl]2–,[13]
together with signals of yet unidentified products. We therefore
conclude that Si5Cl10 rivals Si6Cl12 in terms of Cl– affinity,
because it is apparently capable of abstracting Cl– ions from
[Si6Cl12·2Cl]2– and subsequently undergoes the skeletal re-
arrangement mentioned above.
Experimental Section
General Considerations: All reactions and manipulations were car-
ried out in dry, oxygen-free nitrogen or argon by using standard
Schlenk ware or an argon-filled M. Braun glovebox. CH2Cl2 and
CD2Cl2 were dried with CaH2 and freshly distilled prior to use. C6H6
and C6D6 were dried with sodium/benzophenone and freshly distilled
prior to use. Si2Br6,[16] Si2I6,[17] and Si5Cl10[3] were prepared according
to published procedures. The commercially available ammonium and
phosphonium halides, [nBu4P]Cl (Sigma Aldrich), [nBu4N]Br (Sigma
Aldrich), [Ph4P]Br (Sigma Aldrich), and [nBu4P]I (Sigma Aldrich)
were used as received. AlCl3 was freshly prepared from aluminum
metal and dry HCl gas. The NMR spectra were recorded with Bruker
Avance 300 and Avance 500 spectrometers.
Summary and Conclusions
The X– induced disproportionation of Si2X6 (X = Cl, Br, I)
provides convenient access not only to the “inverse sandwich
complex” [nBu4N]2[Si6Cl12·2Cl], but also to its heavier conge-
ners [nBu4N]2[Si6Br12·2Br] and [nBu4P]2[Si6I12·2I]. In all
cases, these halosilicates(II) are colorless, crystalline solids.
[nBu4N]2[Si6Cl12·2Cl] and [nBu4N]2[Si6Br12·2Br] persist at
room temperature in CH2Cl2 solution; [nBu4P]2[Si6I12·2I] is
Synthesis of [nBu4N]2[Si6Br12·2Br] and [nBu4N]2[Si7Br14·2Br]: An
NMR tube was charged with neat solid Si2Br6 (0.43 g, 0.80 mmol). A
solution of [nBu4N]Br (0.11 g, 0.34 mmol) in CD2Cl2 (0.8 mL) was
added at room temperature. After few minutes, the reaction mixture
turned brownish and a colorless precipitate formed. The NMR tube
was flame-sealed. The 29Si NMR spectrum of the reaction solution
revealed resonances assignable to SiBr4 (–90.2 ppm),[18]
[Si6Br12·2Br]2– (see below),[11] and [Si7Br14·2Br]2– (see below). Single
virtually insoluble in all common inert solvents. Irrespective crystals of [nBu4N]2[Si6Br12·2Br] suitable for X-ray crystallography
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were harvested after storing the reaction solution for 3 d at room tem-
perature.
Crystal Structure Determinations of [nBu4N]2[Si6Br12·2Br],
[Ph4P]2[Si7Br14·2Br], [nBu4P]2[Si6I12·2I], Si6I12, and
[nBu4P]2[Si5Cl10·2Cl]·2CH2Cl2: Data collections were performed
with a STOE IPDS II two-circle diffractometer with a Genix Microfo-
cus tube with mirror optics using Mo-Kα radiation (λ = 0.71073 Å)
and data were scaled using the frame-scaling procedure in the X-
AREA program system.[33] The structures were solved by direct meth-
ods using the program SHELXS and refined against F2 with full-ma-
trix least-squares techniques using the program SHELXL.[34] All non-
hydrogen atoms were refined anisotropically, whereas the hydrogen
atoms were treated with a riding model.
[nBu4N]2[Si6Br12·2Br]: 29Si NMR (CD2Cl2, 59.6 MHz, 298 K): δ =
–38.1 ppm (cf. reference[11,19]).
[nBu4N]2[Si7Br14·2Br]: 29Si NMR (CD2Cl2, 59.6 MHz, 298 K): δ =
–57.2 (1 Si, SiBr), –41.7 (2 Si, SiBr2), –37.6 (1 Si, SiBr2), –37.4 (2
Si, SiBr2), –18.1 ppm (1Si, SiBr3).
Crystallization of [Ph4P]2[Si7Br14·2Br]: An NMR tube was charged
with neat Si2Br6 (0.51 g, 0.95 mmol). A solution of [Ph4P]Br (0.11 g,
0.26 mmol) in CD2Cl2 (0.8 mL) was added at room temperature. After
few minutes, the reaction mixture turned brownish and a colorless pre-
cipitate formed. Single crystals of [Ph4P]2[Si7Br14·2Br] suitable for
X-ray crystallography were harvested after storing the reaction solution
for 3 d at room temperature.
Crystallographic data for the structures in this paper have been de-
posited with the Cambridge Crystallographic Data Centre, CCDC, 12
Union
Road,
Cambridge
CB21EZ,
UK.
Copies of the data can be obtained free of charge on quoting
the depository numbers CCDC-1824864 ([nBu4N]2[Si6Br12·2Br]),
CCDC-1824862
([nBu4P]2[Si6I12·2I]),
CCDC-1824865
Synthesis of [nBu4P]2[Si6I12·2I]: Neat solid Si2I6 (0.100 g,
0.122 mmol) was added to a CD2Cl2 solution (0.5 mL) of [nBu4P]I
(0.047 g, 0.122 mmol) in an NMR tube at room temperature. Even
though most of the Si2I6 remained undissolved, the color of the mixture
turned yellow. The NMR tube was flame-sealed and heated in a sand
bath to 80 °C for 20 h. After cooling to room temperature, colorless
crystals of [nBu4P]2[Si6I12·2I] had formed. Yield: 0.045 g, 0.018 mmol
(90%).
([Ph4P]2[Si7Br14·2Br]), CCDC-1824863 (Si6I12), and CCDC-1835244
([nBu4P]2[Si5Cl10·2Cl]·2CH2Cl2) (Fax: +44-1223-336-033; E-Mail:
deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
Supporting Information (see footnote on the first page of this article):
ORTEP and crystallographic data of [nBu4N]2[Si6Br12·2Br],
[Ph4P]2[Si7Br14·2Br],
[nBu4P]2[Si6I12·2I],
Si6I12,
and
[nBu4P]2[Si5Cl10·2Cl]·2CH2Cl2. 29Si NMR spectrum of the reaction
solution of the mixture of Si2Br6/[nBu4N]Br in CD2Cl2. 13C{1H} and
29Si NMR spectra of the solution phase of the reaction mixture
Si2I6/[nBu4P]I in CD2Cl2.
The reaction solution was investigated by 13C{1H} and 29Si NMR
spectroscopy. The 13C{1H} NMR spectrum contained signals of
[nBu4P]+ and CD2ICl.[30,31] CD2ICl likely originates from a Cl/I ex-
change reaction between CD2Cl2 and iodosilanes (cf. reference[22]).
Correspondingly, the 29Si NMR spectrum showed a resonance assign-
able to SiCl4 (–18.9 ppm)[13] and two more signals at –20.8 ppm and
–21.9 ppm. For comparison, we note that the chemical shift values
of [Si6Cl12·2Cl]2– and [Si6Cl12·2I]2– are –21.7 ppm[13] and
–21.9 ppm,[11,32] respectively.
Acknowledgements
The authors wish to thank Prof. Dr. N. Auner and Dr. M. Berger for
helpful discussions concerning the preparation of Si2Br6 and Si2I6.
NMR spectra of [nBu4P]2[Si6I12·2I] could not be recorded due to the
poor solubility of this compound in all common inert solvents. We
therefore picked four different crystals of the product and identified
all of them as [nBu4P]2[Si6I12·2I] by X-ray diffraction.
Keywords: Bromine; Iodine; Silicon, Cyclopentasilane;
Cyclohexasilane
Synthesis of Si6I12: A brown glass flask was charged with a solution
of [nBu4N]2[Si6Cl12·2Cl] (0.100 g, 0.087 mmol) in CH2Cl2 (10 mL).
A solution of BI3 (0.167 g, 0.426 mmol) in CH2Cl2 (10 mL) was added
dropwise at room temperature. Upon mixing, the solution turned yel-
low and very small crystals precipitated. After storing the reaction
solution for 6 d at room temperature, the crystals had grown to a size
suitable for X-ray crystallography. The crystals were isolated by fil-
tration and rinsed with CH2Cl2 (4 mL). Yield: 0.107 g, 0.063 mmol
(73%). NMR spectra of Si6I12 could not be recorded due to its poor
solubility in all common inert solvents. Instead, four different crystals
were picked and each of them identified as Si6I12 by X-ray diffraction.
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Published online: ᭿
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J. Teichmann, B. Köstler, J. Tillmann, M. Moxter, R. Kupec,
M. Bolte, H.-W. Lerner,* M. Wagner* ............................. 1–8
Halide-Ion Diadducts of Perhalogenated Cyclopenta- and Cy-
clohexasilanes
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