Lewis Acidity of Si6Cl12 and Its Role as Convenient SiCl2 Source

Article abstract of DOI:10.1021/acs.inorgchem.5b01703

The free cyclohexasilane Si₆Cl₁₂ (1) was obtained in 66% yield from the corresponding Cl^- diadduct [nBu₄N]₂[1·2Cl] and AlCl₃ in C₆H₆. The substituted cyclohexasilane 1,1-(Cl₃Si)₂Si₆Cl₁₀ (2), however, cannot be liberated from [nBu₄N]₂[2·2Cl] under comparable reaction conditions. Instead, a mixture of several products was obtained, from which the oligosilane Si₁₉Cl₃₆ (3) crystallized in low yields. X-ray crystallography revealed 3 to consist of two Si₅ rings, bridged by one silicon atom. Compound 1 possesses Lewis acidic sites above and below the ring centroid. Competition experiments reveal that their corresponding acid strengths are comparable to that of BCl₃. The reaction of 1 with 6 equiv of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (Idipp) leads to a complete breakdown of the cyclic scaffold and furnishes the dichlorosilylene adduct Idipp-SiCl₂.

Full text of DOI:10.1021/acs.inorgchem.5b01703

Article
pubs.acs.org/IC
Lewis Acidity of Si Cl and Its Role as Convenient SiCl Source
6
12
2
Jan Tillmann, Maximilian Moxter, Michael Bolte, Hans-Wolfram Lerner, and Matthias Wagner*
Institut fu
Germany
̈
̈
r Anorganische Chemie, Goethe-Universitat Frankfurt am Main, Max-von-Laue-Straße 7, 60438 Frankfurt am Main,
*
S Supporting Information
ABSTRACT: The free cyclohexasilane Si Cl (1) was obtained in 66% yield from the
6
12
−
corresponding Cl diadduct [nBu N] [1·2Cl] and AlCl in C H . The substituted
cyclohexasilane 1,1-(Cl Si) Si Cl (2), however, cannot be liberated from
4
2
3
6
6
3
2
6
10
[
nBu N] [2·2Cl] under comparable reaction conditions. Instead, a mixture of several
4 2
products was obtained, from which the oligosilane Si Cl (3) crystallized in low
19
36
yields. X-ray crystallography revealed 3 to consist of two Si rings, bridged by one
5
silicon atom. Compound 1 possesses Lewis acidic sites above and below the ring
centroid. Competition experiments reveal that their corresponding acid strengths are comparable to that of BCl . The reaction of
3
1
with 6 equiv of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (Idipp) leads to a complete breakdown of the cyclic scaffold
and furnishes the dichlorosilylene adduct Idipp−SiCl2.
INTRODUCTION
The use of monodisperse samples greatly facilitates
investigations of macromolecular species. We therefore focused
■
Owing to their chemical inertness, saturated hydrocarbons have
been referred to as the “noble gases of organic chemistry”. The
homologous polysilanes, in contrast, are characterized by a
pronounced dynamic covalent chemistry of their Si−Si bonds,
which results in facile skeletal rearrangements and silylene
1
our attention on homocyclic silanes Si
Cl2n. As alluded to
n
above, the classic synthesis approach relies on the reductive
15−18
dechlorination of Ph SiCl ,
followed by the (Lewis) acid-
2
2
mediated exchange of all phenyl substituents for chlorine atoms
8,19
(
Scheme 1). The crude product resulting from the first step
2
extrusion/insertion reactions. This reactivity can be triggered
contains four-, five-, and six-membered rings Si Ph (n = 4−6)
n
2n
by external stimuli such as light, heat, and the presence of Lewis
with Si Ph generally being the most abundant constituent of
3
5
10
bases or acids. With regard to photochemical reactions it is
the reaction mixture. In order to obtain cyclosilanes Si Cl
n
2n
important to note that the UV/vis absorption bands of
polysilanes are bathochromically shifted compared to those of
structurally comparable alkanes. This observation is generally
attributed to a certain degree of σ-bond delocalization present
in Si chains, rings, and cages.
So far, the organyl derivatives SinR2n+2 and SinR with R =
with uniform ring sizes, it is best to separate already the primary
mixture of Si Ph rings by extraction and fractional
n
2n
crystallization. Ring-size-selective synthesis strategies, specifi-
cally for perchlorinated cyclohexasilanes, have recently been
4
developed by Boudjouk and our group through (i) HSiCl
3
9
2n
oligomerization with PEDETA or (ii) the reaction of Si Cl
2
6
11
2
Me and Ph constitute the most thoroughly investigated class of
with [nBu N]Cl in CH Cl (Scheme 1; PEDETA = 1,1,4,7,7-
4
2
3
,5−7
polysilanes.
Such compounds are available from R SiCl /
2 2
pentaethyldiethylenetriamine). In both cases, the six-membered
−
R3SiCl and alkali metals through Wurtz-type coupling
protocols. For obvious reasons, this convenient approach falls
short on the synthesis of perchlorinated polysilanes Si Cl
rings are obtained as Cl diadducts. While Boudjouk’s protocol
2−
furnishes exclusively the unsubstituted species [1·2Cl] , our
n
2n+2
approach also provides access to trichlorosilylated derivatives
2
−
2−
and Si Cl . The preparation of these species therefore requires
such as [1,1-(Cl Si) Si Cl ·2Cl] ([2·2Cl] ; Figure 1,
3 2 6 10
10,11
n
2n
a detour via the corresponding perphenylated organosilanes,
which readily undergo a Ph/Cl exchange reaction upon
treatment with HCl/AlCl (for an example, see Scheme 1).
Scheme 1).
[nBu N] [1·2Cl] can easily be separated
4 2
from the substituted congeners, because its crystals float on
8
the surface of the CH Cl solution, whereas the substituted
3
2
2
Our group is particularly interested in perchloro polysilanes,
because an increase in the substituents’ electronegativity
influences the stability of the Si−Si bonds and raises the
Lewis acidity of the individual Si atoms. For example, addition
rings can be harvested from the bottom of the reaction flask.
Large quantities of [nBu N] [1·2Cl] are best prepared from the
4
2
primary mixtures of substituted Si
rings by temperature
Si
6
treatment, which results in the stripping of all Cl
3
1
1
−
of two Cl ions to the cyclohexasilane Si Cl (1) results in the
substituents.
6
12
According to in situ 29Si NMR spectroscopy and quantum-
2−
2−
inverse sandwich structure [Si Cl ·2Cl] ([1·2Cl] ; Figure
6
12
9
−11
−
chemical calculations, the addition of Cl anions to Si Cl
creates an equilibrating mixture of several species. It is the Cl
1),
whereas the permethylated Si Me does not form
2
6
6
12
−
9
adducts with halide ions. In terms of applications, perchloro
polysilanes serve as key precursors for the preparation of
analogous hydrogenated polysilanes, which are widely used for
Received: July 28, 2015
1
2−14
the deposition of silicon thin films.
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b01703
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Known Syntheses of Perchlorinated Cyclosilanes
Scheme 2. Synthetic Routes to Si Cl (1) via Boudjouk’s
6
12
2−
Protocol or the Decomplexation of [1·2Cl] Reported
a
Herein
Figure 1. Molecular structures of [1·2Cl] and [2·2Cl]2− obtained by
2
−
11
X-ray crystallography. Cations are omitted for clarity (Si: blue, Cl:
green).
complexation that finally provides the thermodynamic thrust
1
1
toward cyclohexasilane formation. Yet, for a thorough
investigation of perchlorinated cyclohexasilanes, as well as for
many applications, it is indispensable to use the uncomplexed
Si Cl ring (1; Scheme 2). Until to date, the preparation of 1
6
12
a
+
+
Cation: [pedeta·H SiCl] or [nBu N] .
2
4
via chlorodephenylation of Si Ph constitutes the only viable
6
12
option, but it is neither time- nor atom-economical (Scheme 1).
Boudjouk et al. obtained 1 from [pedeta·H SiCl] [1·2Cl]
2
2
upon hydration with LiAlH and subsequent chlorination of the
4
RESULTS AND DISCUSSION
pyrophoric liquid intermediate Si H with Cl in cold (−89
■
6
12
2
2
0
°
C) CH Cl . Herein we report on the convenient and
Preparation of Si Cl (1) through Decomplexation of
2
2
6
12
quantitative direct decomplexation of [nBu N] [1·2Cl] to 1 by
[nBu N] [1·2Cl]. We have chosen AlCl for the decomplex-
4
2
4
2
3
means of AlCl . We also provide an experimental estimate of
ation of [nBu N] [1·2Cl] because this Lewis acid is also
4 2
3
the Lewis acidity of the acceptor sites above and below the
employed in the chlorodephenylation of Si Ph with HCl/
6
12
8
centroid of the Si ring. Finally, we show that 1 reacts with the
AlCl . When the latter reaction is complete, the crude mixture
6
3
−
N-heterocyclic carbene 1,3-bis(2,6-diisopropylphenyl)imidazol-
contains free Si Cl (1) together with [AlCl ] ions. Thus, the
6
12
4
2
1,22
−
2-ylidene (Idipp) to give the literature-known
adduct
Cl ion affinity of AlCl is apparently larger than that of 1 or [1·
3
−
−
Idipp−SiCl , which proves that Si Cl is an oligomeric storage
Cl] (experimental gas-phase dissociation energy for [AlCl ] :
4
23
2
6
12
−1
form of dichlorosilylene.
ΔH° = 321 ± 12 kJ mol ).
B
DOI: 10.1021/acs.inorgchem.5b01703
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
A mixture of crystalline [nBu N] [1·2Cl] and 2 equiv of
added in several portions at room temperature (rt) to a
suspension of 2.5 equiv of AlCl in C H . After the addition was
4
2
AlCl in CD Cl gave a clear, colorless solution within minutes.
3
2
2
3
6
6
29
The Si NMR spectrum of the solution confirmed the
complete, the reaction mixture was stirred for 1 h, all volatiles
were removed under reduced pressure, and 1 was extracted into
n-hexane at elevated temperature. Crystals of 1 were grown
from the hexane extract upon slow evaporation of the solvent
quantitative consumption of the starting material and showed
exclusively one resonance at δ = −3.0, assignable to the free Si
6
2
4,25
ring 1.
Even though this chemical shift value did not change
29
upon cooling to −30 °C, storage of the sealed NMR tube at
this temperature led to the formation of colorless crystals. An
X-ray analysis revealed that the crystals consisted of the
(yield: 66%). The compound gave rise to a Si NMR signal at
δ = −3.0; X-ray crystallography also confirmed the identity of 1
as Si Cl (the molecular structure of Si Cl is already literature
6
12
6
12
2
5
known ).
The clean transformation of [nBu N] [1·2Cl] to 1 by AlCl
4
2
3
could not be taken for granted in the first place, because
treatment of permethylated cyclohexasilane, Si Me , with a
6
12
catalytic amount of AlCl3 (C H , rt) leads to skeletal
6
6
rearrangement furnishing (trimethylsilyl)nonamethyl-
2
8
cyclopentasilane, (Me Si)Si Me , in 90% yield. A reaction
3
5
9
mechanism has been proposed in which the chloropolysilane
Si Me Cl, formed by initial chlorodemethylation of the starting
6
11
2
8
material, acts as a key intermediate. This scenario would
suggest that 1 might be even more amenable to corresponding
ring-contraction reactions than Si Me . We therefore moni-
6
12
2
9
tored an equimolar mixture of 1 and AlCl in C D by Si
3
6
12
NMR spectroscopy, but found the cyclohexasilane inert toward
the Lewis acid in the temperature regime between rt and 80 °C
(
1 h).
When they revisited the AlCl -mediated rearrangement of
3
Si Me , West et al. did not observe chlorosilanes at any stage
6
12
29
of the reaction sequence. Instead, they showed that the AlCl3
catalyst needed to contain some Fe in order to be efficient. We
therefore investigated next an equimolar mixture of 1 and FeCl3
in C D after it had been heated to reflux temperature for 1 h.
6
12
Apart from 1, we detected only trace amounts of decom-
position products in the NMR spectrum. On the other hand, all
our attempts to generate 1 from [nBu N] [1·2Cl] using FeCl
3
Figure 2. Molecular structure of [nBu N] [1·2AlCl ] in the solid state.
4
2
4
Cations are omitted for clarity; displacement ellipsoids are shown at
the 30% probability level. Selected bond lengths [Å], atom···atom
distance [Å], bond angles [deg], and torsion angles [deg]: Si(1)−Si(2)
4
2
−
as the Cl -abstracting agent failed and rather resulted in
2
9
complex, inseparable product mixtures (CD Cl ; Si NMR
=
2.369(3), Si(1)−Si(3A) = 2.364(3), Si(2)−Si(3) = 2.367(3),
2
2
Al(1)−Cl(1) = 2.192(3), Al(1)−Cl(2) = 2.114(3), Al(1)−Cl(3) =
spectroscopic control).
2
.116(3), Al(1)−Cl(4) = 2.117(3); Cl(1)···Cl(1A) = 4.467(3);
We found it impossible to achieve a similarly selective
2−
Si(1)−Si(2)−Si(3) = 120.0(1), Si(2)−Si(1)−Si(3A) = 120.4(1),
decomplexation reaction as in the case of [1·2Cl] with its 1,1-
2 29
−
Si(2)−Si(3)−Si(1A) = 119.6(1), Si(1)−Si(2)−Si(3)−Si(1A) =
disilylated congener [2·2Cl] . According to Si NMR
spectroscopy, a variety of products are generated when AlCl3
is added at rt to a solution of [nBu N] [2·2Cl] in CD Cl . After
−
0.5(2), Si(2)−Si(3)−Si(1A)−Si(2A) = 0.6(2), Si(3)−Si(2)−
Si(1)−Si(3A) = 0.6(2). Symmetry transformation used to generate
equivalent atoms: A: 0.5−x, 0.5−y, −z.
4
2
2
2
the NMR sample had been stored for 30 d, crystals of the
highly branched oligosilane Si Cl (3) could be harvested. 3 is
19
36
diadduct [nBu N] [1·2AlCl ], in which the cyclohexasilane
only very sparingly soluble in inert solvents, and it decomposes
in THF solution. We therefore had to restrict its character-
ization to X-ray crystallography (Figure 3). The C -symmetric
2
molecule 3 consists of two five-membered rings, connected by
4
2
4
−
moiety binds two [AlCl ] anions (Figure 2). Similar to
4
11,26
[
nBu N] [1·2Cl],
the Si ring in [nBu N] [1·2AlCl ] is
4
2
6
4
2
4
25
planar and does not adopt the chair conformation of 1 in the
solid state. The shortest contact between the [AlCl ] anion
−
one silicon atom (Si(10)). The Si(10) bridge carries two
4
and the centroid (COG) of the six-membered ring in the
additional Cl
Si substituents; each Si ring is equipped with
3 5
2
−
centrosymmetric aggregate [1·2AlCl4] amounts to Cl(1)···
three Cl Si substituents.
3
COG = 2.233(2) Å and is thus larger by 0.315 Å than the
The perchlorinated oligosilane 3 shares common structural
motifs with the permethylated species cis-1,1,2-tris-
(trimethylsilyl)-4-tris(trimethylsilyl)silylhexamethylcyclopenta-
silane (4; Figure 4), which was isolated as one of four products
−
2−
corresponding distance Cl ···COG = 1.918(1) Å in [1·2Cl] .
This observation can be rationalized given the usual
classification of [AlCl4] as a weakly coordinating anion
and the reversibility of [AlCl ] complexation in solution
according to Si NMR spectroscopy. Nevertheless, the
−
27
−
in the Al(Fe)Cl -catalyzed rearrangement of octakis-
4
3
2
9
30
(trimethylsilyl)cyclotetrasilane. Even corresponding Si−Si
−
interaction between 1 and [AlCl ] must still be sufficiently
bond lengths and Si−Si−Si bond angles of 3 and 4 are
4
2
8
29
strong to compensate for the energy penalty for the
planarization of the Si Cl ring (calculated value: 50 kJ
strikingly similar. Ishikawa et al., West et al., and more
30
recently Marschner et al. reported that the cyclohexasilane
Si Me can selectively be converted into the monosilylated
6
12
−
1
26
mol ).
6
12
For the liberation of the free cyclohexasilane 1 on a
preparative scale, finely divided solid [nBu N] [1·2Cl] was
cyclopentasilane (Me Si)Si Me by treatment with Al(Fe)Cl .
3 5 9 3
The silylated cyclohexasilane (Me Si)Si Me gave the
4
2
3
6
11
C
DOI: 10.1021/acs.inorgchem.5b01703
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 3. Molecular structure of Si Cl in the solid state;
1
9
36
displacement ellipsoids are shown at the 30% probability level.
Selected bond lengths [Å], bond angles [deg], and torsion angles
[
deg]: Si(1)−Si(2) = 2.345(1), Si(1)−Si(5) = 2.360(1), Si(1)−Si(10)
2.365(1), Si(2)−Si(3) = 2.339(1), Si(3)−Si(4) = 2.352(1), Si(4)−
Si(5) = 2.359(1); Si(1)−Si(2)−Si(3) = 103.8(1), Si(1)−Si(5)−Si(4)
=
=
107.5(1), Si(1)−Si(10)−Si(1A) = 105.7(1), Si(2)−Si(1)−Si(5) =
9
8.1(1), Si(2)−Si(1)−Si(10) = 120.9(1), Si(2)−Si(3)−Si(4) =
103.8(1), Si(3)−Si(4)−Si(5) = 104.5(1); Si(1)−Si(2)−Si(3)−Si(4)
=
40.7(1), Si(1)−Si(5)−Si(4)−Si(3) = −12.9(1), Si(2)−Si(3)−
Si(4)−Si(5) = −16.6(1), Si(2)−Si(1)−Si(5)−Si(4) = 36.5(1),
Si(3)−Si(2)−Si(1)−Si(5) = −46.9(1). Symmetry transformation
used to generate equivalent atoms: A: 1−x, y, 0.5−z.
disubstituted cyclopentasilane 1,1-(Me Si) Si Me . As a general
3
2
5
8
feature, rearrangement reactions of permethylated cyclosilanes
favored products with a geminal bis(trimethylsilyl) substitution
pattern. Obviously, the formation of 3 proceeded along
analogous guiding lines: The disubstituted cyclohexasilane
starting material 2 furnished trisubstituted cyclopentasilane
fragments. We also note that 3 contains a number of geminally
0
substituted Si centers, i.e., Si(4), Si(4A), and Si(10).
Recently, our group discovered a one-step synthesis of the
siladodecahedrane [R N][5] (Figure 4; R = Et, nBu), which
4
3
1
starts from Si Cl and nBu N/[nBu N]Cl. Now, the
2
6
3
4
generation of compound 3 provides further indication that
reaction cascades beginning with the Lewis base-induced
disproportionation reaction of Si Cl can ultimately be taken
2
6
far beyond the preparation of perchlorinated neo-pentasilane
3
2
Figure 4. Molecular structures of Si19Cl36 (3), Si12Me24 (4), and the
(
trigger: R N) or (substituted) cyclohexasilanes (trigger:
3
10,11
−
−
silafullerane [Si Cl ] ([5] ) in the solid state. Hydrogen atoms of 4
−
32 45
Cl ).
With regard to the reaction mechanism leading to the
+
−
and the [Et N] cation of [5] are omitted for clarity.
4
formation of [R N][5], it is revealing that the Si ring of 2 can
4
6
readily rearrange to furnish Cl Si-substituted Si rings and that
there appears to be a tendency of the resulting cyclo-
pentasilanes to cross-link via Si bridges.
3
5
of the respective Lewis acid. Addition of Et PO to free 1 in
3
C D led to the formation of various silicon-containing species
6
6
Assessment of the Lewis Acidity of Si Cl (1). In the
together with Et P (NMR spectroscopic control). This result
6
12
3
planarized Si Cl moiety, 12 electron-withdrawing chloro
was to be expected, because it is well known that phosphine
oxides are readily reduced by Si Cl to their corresponding
6
12
substituents lead to strong Lewis acid sites above and below
2
6
36,37
the positively polarized Si core. Coordination of the two extra
phosphines.
6
−
2−
Cl ions in [1·2Cl] is therefore partly due to Coulomb
We next performed competition experiments to narrow
2
6,33
−
attraction.
Electrostatic interactions likely contribute also to
down the Cl affinity of 1 (Table 1), because in assessing a pair
2
−
the bonding in [1·2AlCl ] , and we note in this context that
of Lewis acids for comparative Lewis acid strength toward a
given Lewis base, a competition experiment usually provides
4
comparable aggregates do not form when 1 cocrystallizes with
−
38
CCl , a charge-neutral analogue of [AlCl ] (see the SI for
the most accurate information. First, we determined the
4
4
details of the crystal structure of [1/CCl ]). The apical anion
relative affinities of 1 and 2. The 29Si NMR spectrum of a
solution of 1 and [nBu N] [2·2Cl] in CD Cl showed an
4
2
−
coordination in [1·2Cl] is further stabilized through nCl
→
4
2
2
2
29
2−
σ*
interactions, by which charge density is donated from
intense signal at δ( Si) = −21.7, assignable to the [1·2Cl]
ion. Correspondingly, [nBu N] [1·2Cl] was later isolated from
Si−Cl
−
11,33
Cl lone pairs into Si−Cl antibonding orbitals.
4
2
The Gutmann−Beckett acceptor number is a commonly
the sample in almost quantitative yield. According to NMR
spectroscopy, the reaction mixture did not contain notable
amounts of free 2, but a number of other, unidentified follow-
up products. These results lead to the following conclusions: (i)
used measure for the classification of relative Lewis acid-
3
4,35
ities.
The calculation of acceptor numbers requires the
determination of the P NMR shift of Et PO in the presence
3
1
3
D
DOI: 10.1021/acs.inorgchem.5b01703
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Competition Experiments Performed to Gauge the Lewis Acidity and Stability of 1
entry no.
reactant
reagent
reaction conditions
CD Cl , rt
result
free 1 obtained
2
2
2
2
2
2
2
2
2
2
2
2
2
2
−
1
2
3
4
5
6
7
8
9
[1·2Cl]
[1·2Cl]
[1·2Cl]
[1·2Cl]
[1·2Cl]
[1·2Cl]
[2·2Cl]
[2·2Cl]
[1·2Cl]
[1·2Cl]
[1·2Cl]
[1·2Cl]
[1·2Cl]
[2·2Cl]
1
AlCl (2 equiv)
3
2
2
−
−
−
−
−
−
−
−
−
−
−
−
−
AlCl (2 equiv)
C D , 80 °C
free 1 obtained
free 1 obtained
66% 1 isolated
3
6
12
AlCl (2 equiv)
C D , rt
6 6
3
AlCl (2 equiv)
C H , rt
6 6
3
AlCl (1 equiv)
CD Cl , rt
rearrangement/fragmentation
rearrangement/fragmentation
rearrangement/fragmentation
rearrangement/fragmentation
Cl/Br scrambling
3
2
2
AlCl (1 equiv)
C D , rt
6 6
3
AlCl (2 equiv)
CD Cl , rt
2 2
3
AlCl (2 equiv)
C D , rt
6 6
3
AlBr (2 equiv)
C D , rt
6 6
3
10
11
12
13
14
15
16
17
18
19
20
21
FeCl (2 equiv)
CD Cl , rt
rearrangement/fragmentation
rearrangement/fragmentation
no reaction
3
2
2
BCl (exc.)
CD Cl , rt
2 2
3
BPh (2 equiv)
CD Cl , rt
2 2
3
2−
1 (1 equiv)
1 (1 equiv)
CD Cl , rt
91% [1·2Cl] isolated
2
2
2−
CD Cl , rt
92% [1·2Cl] isolated
2
2
AlCl (1 equiv)
CD Cl , 50 °C, 2 h
no reaction
3
2
2
1
AlCl (1 equiv)
C D , 80 °C, 1 h
no reaction
3
6
12
1
FeCl (1 equiv)
CD Cl , 50 °C, 2 h
no reaction
3
2
2
1
FeCl (1 equiv)
C D , 80 °C, 1 h
no reaction
3
6
12
1
BCl (exc.)
CD Cl , rt
no reaction
3
2
2
1
[nBu N][BCl ] (exc.)
CD Cl , rt
rearrangement/fragmentation
4
4
2
2
1
Et PO
C D , rt
Et P observed
3
3
6
6
−
The unsubstituted cyclohexasilane 1 is a stronger Lewis acid
than 2, which can be understood on steric as well as electronic
grounds, because 2 offers fewer Si−Cl antibonding orbitals for
stable than [1·Cl] , which is why 2 is not accessible from [2·
2Cl]2 and AlCl3.
−
−
So far, it is obvious that the Lewis acidities of [1·Cl] and 1
n → σ*
donation. (ii) The degradation of free 2 already
are lower than that of AlCl
2Cl] was also treated with BPh
first case, the only resonances detectable appeared at chemical
. To narrow the gauge, [nBu
3
N] [1·
4 2
Cl
Si−Cl
observed earlier is not solely attributable to the action of AlCl3.
3
and BCl in CD Cl . In the
3
2
2
We therefore assumed that cyclohexasilane monoadducts,
2
9
2−
11
−
shift values of δ( Si) = −21.7 (cf. [1·2Cl] : δ = −21.7) and
generated by the transfer of Cl ions, might play a decisive role
11
39
δ( B) = 67.9 (cf. BPh : δ = 67.5), which unequivocally
for the degradation/rearrangement of the Si ring. Indeed,
3
6
−
proves that no reaction had taken place.
In the second case, a significant amount of [1·2Cl] was still
present after 2 equiv of BCl had been added. Surprisingly,
quantum chemical calculations on [1·Cl] unveiled a reaction
2
−
−
path along which the extra Cl ion leaves the acidic pocket on
top of the ring centroid to attack one of the Si atoms and open
the cyclic framework. The associated energy barrier amounts to
3
however, the BCl was already fully consumed at this stage
3
−
1
11
(NMR spectroscopic control). The sample was therefore
treated with further BCl , and we found that overall 4 equiv
of the Lewis acid are required to quantitatively transform the
Cl diadduct. Notably, we never observed the Si NMR signal
of free 1 in this experiment. In addition, we also performed the
reverse reaction between 1 and [nBu
and confirmed the presence of the Cl diadduct [1·2Cl] in a
mixture of several silicon-containing species. Even though we
have not yet identified the full palette of reaction products, it
2
2.7 kcal mol . To gain experimental insight into the
−
29
3
reactivity of [1·Cl] , we recorded Si NMR spectra on an
equimolar mixture of [nBu N] [1·2Cl] and 1 in CD Cl . We
4
2
2
2
−
29
did not detect the signal of the free cyclohexasilane 1, whereas a
2
9
strong resonance at δ( Si) = −21.7 testified to the presence of
40
−
2−
N][BCl
4
]
4
in CD Cl
2 2
the Cl diadduct [1·2Cl] . A number of further NMR signals
−
2−
indicated the formation of several new species. The most likely
−
primary product of the reaction is the monoadduct [1·Cl] ,
which subsequently reacts further to furnish the (yet
unidentified) secondary products observable in the NMR
−
can nevertheless be concluded that 1 and [1·Cl] possess Lewis
acidities similar to BCl but lower than AlCl . Seemingly
inconsistent with these results, BCl is ranked higher than AlCl
3
3
spectrum. The degradation of the Si rings continues until a
6
−
3
3
sufficient amount of Cl ions has been liberated to stabilize all
on the scale of Childs and Gutmann−Beckett acceptor
remaining cyclohexasilane molecules in the form of their
41,42
numbers.
It has, however, to be noted that different
2−
diadducts [1·2Cl] . Given the stoichiometric ratios employed,
−
reference Lewis bases (Et PO vs Cl ) can lead to different
Lewis acidity estimates (cf. gas-phase dissociation energies
ECl ] → ECl + Cl : ΔH° = 321 ± 12 kJ mol (E = Al),
89 ± 8 kJ mol (E = B) ).
3
2
−
the amount of [1·2Cl] should remain the same before and
after the addition of 1, which we found to be the case after
sample workup. The second competition experiment thus
shows that the Cl ion affinities of 1 and [1·Cl]− are
comparable, because otherwise no reaction between 1 and [1·
38
−
−
−1
[
2
4
3
−1
23
−
Reaction of Si Cl
(1) with 1,3-Bis(2,6-
6
1 2
diisopropylphenyl)imidazol-2-ylidene. Michl and West
have reported on the isolation of dimethylsilylene (SiMe2)
from photolysis of Si Me in Ar (at 10 K) or hydrocarbon
matrices (at 77 K).
strated that dihalosilylenes can be isolated and crystallo-
graphically characterized as their N-heterocyclic carbene
2
Cl]2 would have taken place. Contrary to 1 and [1·2Cl] ,
−
2−
−
the monoadduct [1·Cl] is not a stable compound. However, in
light of the successful decomplexation of [1·2Cl] to 1 by
AlCl , the lifetime of intermediate monoadducts [1·Cl] must
be sufficiently long to enable the second Cl abstraction
6
12
2−
43,44
Roesky and Filippou have demon-
−
3
−
−
21
45
reaction. We further conclude that [2·Cl] should be even less
(NHC) adducts NHC−SiX (X = Cl, Br ).
2
E
DOI: 10.1021/acs.inorgchem.5b01703
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
With the aim to establish Si Cl12 (1) as a storage form of
(n-hexane and C
6
H
6
) or CaH
2
(CH
2
Cl
2
) and freshly distilled prior to
6
use. C D , C D , and CD Cl were stored over molecular sieves (4
SiCl , we allowed 1 to react with 6 equiv of 1,3-bis(2,6-
6
12
6
6
2
2
2
Å). NMR spectra were recorded on a Bruker Avance III HD 500 MHz
spectrometer equipped with a Prodigy BBO 500 S1 probe.
Scheme 3. Synthesis of Idipp−SiCl2
1
13
1
H/ C{ H} NMR spectra were referenced against (residual) solvent
signals (C D : 1.38 ppm/26.43 ppm, C D : 7.16 ppm/128.06 ppm,
6
12
6
6
29
CD Cl : 5.32 ppm/53.84 ppm). Si NMR spectra were calibrated
against external SiMe (δ( Si) = 0); whenever present, SiCl (δ( Si)
2
2
29
29
4
4
=
−18.9) was used as internal standard.
Synthesis of Si Cl (1). A Schlenk tube was charged with AlCl3
6
12
(
145 mg, 1.09 mmol) and C H6 (5 mL). Finely ground single-
6
crystalline [nBu N] [1·2Cl] (500 mg, 0.44 mmol) was added with
4
2
stirring at rt in four portions at intervals of 30 min. The reaction
mixture was stirred for another 1 h to obtain a clear, colorless solution.
After the solvent had been removed under reduced pressure, n-hexane
(5 mL) was added to the colorless solid residue. The Schlenk tube was
immersed in liquid nitrogen in a Dewar flask until the solvent
solidified. Thereafter, the vessel was evacuated, closed, slowly heated
to 85 °C (water-bath temperature), and left at this temperature for 30
min. After the mixture had cooled to rt, it was filtered through a PTFE-
syringe filter (pore size: 0.2 μm) to obtain a clear, colorless liquid. The
solvent was removed from the filtrate under reduced pressure, and the
colorless product was dried under dynamic vacuum for 2 h. Yield: 169
mg (0.29 mmol, 66%). 29Si NMR (C D ; 99.4 MHz): δ = −3.0. Single
diisopropylphenyl)imidazol-2-ylidene in C H6 (Scheme 3).
6
Indeed, the quantitative formation of Idipp−SiCl was observed
2
(
NMR spectroscopic control; 89% isolated yield). X-ray
crystallography further confirmed the identity of the reaction
6
6
product as Idipp−SiCl , a compound that has already been
2
crystals of [1/CCl ] were grown from a solution of 1 in CCl at −30
21
4
4
described previously by Roesky et al.
°
°
C. The crystals used for X-ray analysis were selected in a cold (−40
In a general context, our experiment suggests 1 as a novel
C) stream of N .
2
source of reactive SiCl . Specifically, it provides convenient
Crystallization of [nBu N] [1·2AlCl ]. A solution of [nBu N] [1·
2
4
2
4
4
2
46
access to the promising and versatile reagent Idipp−SiCl2.
2Cl] (115 mg, 0.10 mmol) in CD Cl (0.6 mL) was prepared in an
2 2
NMR tube. AlCl (27 mg, 0.20 mmol) was added, and the NMR tube
3
CONCLUSION
was vacuum-sealed. At rt, an investigation of the clear, colorless
solution by 29Si NMR spectroscopy showed exclusively the resonance
assignable to free 1. At −30 °C, however, colorless, X-ray quality
crystals of [nBu N] [1·2AlCl ] grew within 24 h.
■
Perchlorinated cyclohexasilane (1) is readily accessible from the
Cl diadduct [1·2Cl] through abstraction of the extra Cl−
−
2−
4
2
4
ions with AlCl . Given that [nBu N] [1·2Cl] forms in one step
3
4
2
Formation of Si Cl (3). In an NMR tube, a solid mixture of
19
36
when Si Cl is treated with [nBu N]Cl, our overall two-step
2
6
4
single-crystalline [nBu N] [2·2Cl] (100 mg, 0.07 mmol) and AlCl
4
2
3
protocol provides a convenient new synthesis of 1, which is an
intensely investigated precursor for semiconductor materials.
Yet, the synthesis procedure is apparently not applicable for the
preparation of substituted derivatives, such as 1,1-
(
[
20 mg, 0.15 mmol) was dissolved in CD Cl (0.5 mL; note: AlCl or
2 2 3
nBu N] [2·2Cl] alone is very poorly soluble in CD Cl ). The NMR
4
2
2
2
29
tube was vacuum-sealed. A few hours later, Si NMR spectroscopy on
the colorless solution showed resonances assignable neither to the
(
[
(
Cl Si) Si Cl (2): Instead of 2, the reaction between
starting material [nBu N] [2·2Cl] nor to the target compound 2.
4 2
3
2
6
10
Rather, the spectrum indicated the formation of a complex product
mixture. The reaction solution was transferred to a screw-capped vial
and stored for 30 d at rt, whereupon a few colorless, cuboid crystals of
nBu N] [2·2Cl] and AlCl furnishes a number of other
4 2 3
oligo)silanes, among them Si Cl (3). On the other hand,
this result also demonstrates that it is possible to go beyond the
structural motif of Si6 rings, exploiting the intrinsic
fragmentation/rearrangement reactivity of perchlorinated oli-
gosilanes. From competition experiments between 1 and
1
9
36
3
precipitated. From the same sample, we also isolated crystals of
29
[
Bu N][AlCl ]. Si NMR data of 3 could not be acquired, because the
4 4
compound is insoluble in all common inert solvents.
Synthesis of Idipp−SiCl . A solution of 1 (51 mg, 0.086 mmol) in
2
[
nBu N] [1·2Cl] or [nBu N] [2·2Cl] we further conclude (i)
4 2 4 2
C H (2 mL) was slowly added to 1,3-bis(2,6-diisopropylphenyl)-
6
6
−
−
that the monoadducts [1·Cl] and [2·Cl] are unstable and
imidazol-2-ylidene (200 mg, 0.515 mmol) in C H (6 mL). After the
6 6
subject to rearrangement and (ii) that Cl Si substitution of the
resulting yellow reaction mixture had been stirred at rt overnight, all
volatiles were removed under a dynamic vacuum. The yellow solid
residue was washed with n-hexane (1 mL), and the remaining pale
yellow crude product recrystallized from C H . Yield: 223 mg (0.457
3
−
Si ring significantly reduces its Cl affinity. According to our
current assessment, the Lewis acidity of free 1 is comparable to
6
that of BCl . While free 1 proved to be inert toward Lewis acids
6
6
3
mmol, 89%). The identity of the reaction product as Idipp−SiCl was
(
i.e., BCl , AlCl , FeCl ) even at elevated temperatures,
2
3
3
3
29
29
confirmed by Si NMR spectroscopy and X-ray crystallography. Si
NMR (C D ; 99.4 MHz): δ = 18.7.
addition of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene
leads to the instantaneous and quantitative formation of the
6
6
X-ray Crystal Structure Determination. Single crystals of
adduct Idipp−SiCl . Work is currently in progress in our
2
[
nBu N] [1·2AlCl ], Si Cl (3), and 1/CCl were measured on a
4
2
4
19 36
4
laboratories to fully exploit the potential of perchlorinated
STOE IPDS-II diffractometer with a Genix microfocus tube with
cyclic silanes as SiCl storage forms.
2
mirror optics. The data were scaled using the frame-scaling procedure
4
9
in the X-AREA program system. All structures were solved by direct
2
EXPERIMENTAL SECTION
methods and refined with full-matrix least-squares on F using the
■
5
0
General Details. All reactions were carried out under an inert-gas
atmosphere (dry argon or nitrogen) using standard Schlenk or
glovebox techniques. Commercially available substances were used as
received. [nBu N] [1·2Cl] and [nBu N] [2·2Cl] were prepared
program SHELXL97. The crystal of [nBu
nonmerohedrally twinned with a fractional contribution of 0.191(4)
for the minor domain. The chlorine atoms of CCl in 1/CCl are
4 2 4
N] [1·2AlCl ] was
4
4
disordered over two equally occupied positions. CCDC reference
4
2
4
2
1
1
according to literature procedures. AlCl3 was prepared from Al
numbers: 1414710 ([nBu N] [1·2AlCl ]), 1414711 (3), and 1414712
4
2
4
4
7,48
turnings and HCl gas.
Reaction solvents were dried over sodium
(1/CCl4).
F
DOI: 10.1021/acs.inorgchem.5b01703
Inorg. Chem. XXXX, XXX, XXX−XXX

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20.3059(13)
12.0325(5)
27.9854(17)
90
36.9602(14)
9.5908(3)
18.4902(7)
90
12.6663(11)
12.6663(11)
27.782(3)
90
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99.231(3)
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6641.5(7)
4
6469.5(4)
4
3860.1(8)
6
Dcalcd [g cm−3
]
1.417
1.858
1.930
2
680−2685.
F(000)
μ [mm−1]
2912
3512
2172
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0.984
1.873
1.977
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(
0
0.47
rflns collected
28 729
72 318
3307
(
indep rflns (Rint) 6461 (0.0988)
9065 (0.0579)
9065/0/249
977 (0.0498)
977/0/42
data/restraints/
params
GOF on F2
6461/0/281
(
1.105
1.058
1.206
(
2
(
1
R1, wR2
0.0898, 0.2577
0.0357, 0.0819
0.0375, 0.0912
[
I > 2σ(I)]
R , wR (all data) 0.1001, 0.2635
0.0406, 0.0844
0.0400, 0.0924
1
2
largest diff peak
0.846, −0.521
0.951, −1.067
0.523, −0.385
(
and hole
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3
[
e Å ]
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b01703.
■
Proepper, K.; Dittrich, B.; Jones, P. G.; Roesky, H. W. Inorg. Chem.
*
S
2
(
011, 50, 358−364.
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Details of the X-ray crystal structure analysis of 1/CCl4
and plots of the NMR spectra of [nBu N] [1·2AlCl ], 1,
4
2
4
and Idipp−SiCl (PDF)
2
Crystallographic data (CIF)
Ugrinov, A.; Schulz, D. L.; Boudjouk, P. Inorg. Chem. 2011, 50, 4047−
4
053.
AUTHOR INFORMATION
(26) Pokhodnya, K.; Olson, C.; Dai, X.; Schulz, D. L.; Boudjouk, P.;
■
Sergeeva, A. P.; Boldyrev, A. I. J. Chem. Phys. 2011, 134, 014105-1−5.
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Corresponding Author
E-mail: Matthias.Wagner@chemie.uni-frankfurt.de.
Author Contributions
J.T. and M.M. performed the synthetic work, prepared the
samples for analytics, analyzed data, and collected X-ray data;
M.B. processed and refined X-ray data; H.W.L. was involved in
study design; M.W. designed and supervised the experimental
studies; M.W., J.T., and M.M. wrote the paper; and all the
authors discussed the results and commented on the manu-
script.
(
2
(
*
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Notes
The authors declare no competing financial interest.
4
(
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DOI: 10.1021/acs.inorgchem.5b01703
Inorg. Chem. XXXX, XXX, XXX−XXX