Tris(trichlorosilyl)tetrelide Anions and a Comparative Study of Their Donor Qualities

Article abstract of DOI:10.1002/chem.201806298

Trichlorosilylated tetrelides [(Cl₃Si)₃E]^? have been prepared by adding 1 equiv of a soluble Cl^? salt to (Cl₃Si)₄Si (E=Si) or 4 Si₂Cl₆/GeCl₄ (E=Ge). To asse

Full text of DOI:10.1002/chem.201806298

A Journal of
Accepted Article
Title: Tris(trichlorosilyl)tetrelide Anions and a Comparative Study of
Their Donor Qualities
Authors: Julian Teichmann, Chantal Kunkel, Isabelle Georg, Maximilian
Moxter, Tobias Santowski, Michael Bolte, Hans-Wolfram
Lerner, Stefan Bade, and Matthias Wagner
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To be cited as: Chem. Eur. J. 10.1002/chem.201806298
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Tris(trichlorosilyl)tetrelide Anions and a Comparative Study of
Their Donor Qualities
Julian Teichmann,[a]# Chantal Kunkel,[a]# Isabelle Georg,[a] Maximilian Moxter,[a] Tobias Santowski,[a]
Michael Bolte,[a] Hans-Wolfram Lerner,[a] Stefan Bade,[b] Matthias Wagner*[a]
Abstract: Trichlorosilylated tetrelides [(Cl3Si)3E]− have been
prepared by adding 1 equiv of a soluble Cl− salt to (Cl3Si)4Si (E = Si)
or 4 Si2Cl6/GeCl4 (E = Ge). To assess their donor qualities, the
anions [(Cl3Si)3E]− (E = C, Si, Ge) have been treated with BCl3, AlCl3,
and GaCl3. BCl3 and GaCl3 give 1:1 adducts with the anionic centers.
AlCl3 leads to Cl− abstraction from [(Cl3Si)3E]− with formation of
(Cl3Si)4E (E = Si or Ge). (Cl3Si)4Ge is cleanly converted to the
perhydrogenated (H3Si)4Ge by use of Li[AlH4]. Another case of Cl−
abstraction was observed for [(Cl3Si)3Ge·GaCl3]−, which reacts with
GaCl3 to afford the neutral dimer ((Cl3Si)3Ge−GaCl2)2.
germanium compound was obtained through catenation of the
Ge(II) adducts GeCl2(IDipp) and GeCl2(dioxane), which stops at
the stage of ((IDipp)Cl2Ge)Ge(GeCl3)2 (B; IDipp = 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene).[22]
A
comparable
compound, [(Cl3Si)2P]− ([C]−), has also been reported for the
pnictogens.[23]
Oligosilanes are precursors in the chemical vapor deposition
(CVD) of amorphous Si-based materials for use in thin-film solar
cells. The spectrum of absorbed light can be widened to shorter
or longer wavelengths through the incorporation of C or Ge
atoms, respectively.[1] Yet, the simultaneous deposition of
individual silanes and germanes suffers from separation effects,
which lead to Si- and Ge-enriched areas and thereby reduce the
photovoltaic efficiency of the device. Mixed-element precursors
containing Si and Ge covalently linked to each other offer a
solution to the problem, but the number of available compounds
is very limited.[2–5] Future progress in the field will therefore
depend on the development of additional access routes to well-
defined Si-Ge oligomers.[6]
Figure 1. Cl3Si/Cl3Ge substituted main-group compounds.
Beckmann, Ketkov, and Mebs characterized the silanide [2]− as
weakly coordinating anion with an inert lone pair. Likewise,
Rivard’s GeCl2 catenation does not proceed beyond B, which
indicates
a
low nucleophilicity of this compound, too.
Oligotetrels are also interesting in their own right, because of the
Considering the fundamental importance of such low-valent Si
and Ge species, a more thorough assessment of their Lewis
basicities is nevertheless worthwhile. Herein, we first extend the
series of tetrelides [(Cl3Si)3E]− (E = C, Si) to the germanide
[(Cl3Si)3Ge]− ([3]−). We then show that all three anions [1]−−[3]−
are surprisingly reactive toward various electrophiles and
structurally characterize their adducts [(Cl3Si)3E·GaCl3]− (E = C,
Si, Ge]. Finally, we disclose the neutral species
((Cl3Si)3Ge−GaCl2)2 and (Cl3Si)4Ge, which are relevant for the
preparation of single-source, mixed-element CVD precursors
such as (H3Si)4Ge.
−* conjugation along the oligomer backbones[7] and
a
tendency for skeletal rearrangements. To manipulate the
electronic structures and reactivities of oligotetrels, one can
decorate the chains with strongly electron-withdrawing
substituents or incorporate low-valent, lone-pair bearing tetrel
centers. Still today, most oligotetrels carry hydrogen or organyl
substituents.[8] Far fewer examples are known of perhalogenated
derivatives,[9] which means lost opportunities given that already
the simplest member of the family, Si2Cl6, reacts substantially
different from its methylated relative Si2Me6: (i) Tertiary amines
suffice to catalyze the disproportionation of Si2Cl6 according to 4
Si2Cl6 → (Cl3Si)4Si + 3 SiCl4 and thereby open an access route
to the perchlorinated neo-pentasilane.[10,11] (ii) In the presence of
soluble chloride salts, Si2Cl6 undergoes heterolytic cleavage of
The methanide [1]− is accessible in >90% yield from CCl4 and in
situ-generated [SiCl3]− (Scheme 1).[18] The silanide [2]− was first
identified on the basis of 29Si NMR spectroscopy[13] and later
structurally characterized by Beckmann, Ketkov, and Mebs,[19]
who isolated [2]− as a side product of the synthesis of
SiCl2(IDipp).[24] Their subsequent targeted protocol involves two
steps: (i) (Cl3Si)4Si in SiCl4 is subjected to a protodesilylation
reaction using 1 equiv of ethereal HCl.[25] (ii) The obtained
(Cl3Si)3SiH is then deprotonated with PMP to obtain [PMP−H][2]
in yields of 96% (PMP = 1,2,2,6,6-pentamethylpiperidine; less
bulky, more nucleophilic amines were found not suitable for this
reaction).[19] To save the deprotonation step, we performed the
desilylation of (Cl3Si)4Si using Cl− ions without H+ present: The
reaction of 1 equiv of [R4N]Cl with (Cl3Si)4Si resulted in the
heterolytic cleavage of one Si−Si bond and directly furnished the
silanide salts [R4N][2] (R = Et, nBu).[9,20] The germanide salt
[nBu4N][3] was prepared by adding 4 equiv of Si2Cl6 to an
equimolar mixture of GeCl4 and [nBu4N]Cl in CH2Cl2.[26] A 1:1:1
mixture of Si2Cl6, GeCl4, and [nBu4N]Cl in CH2Cl2 cleanly gave
SiCl4 and [nBu4N][GeCl3] (Scheme 1), as proven by X-ray
crystallography.[26,27] Treatment of [nBu4N][GeCl3] in CH2Cl2 with
3 equiv of Si2Cl6 again resulted in the formation of [nBu4N][3],
which confirmed the role of [GeCl3]− as the first intermediate of
its Si−Si bond to form SiCl4 and the [SiCl3]− anion,[12,13]
a
versatile intermediate for the in situ generation of higher oligo-
and organosilanes.[14] (iii) The challenging activation of Si2Me6,
on the other hand, requires the presence of powerful transition-
metal catalysts.[15] Representatives of exhaustively Cl3Si-
substituted tetrelides are the methanide [(Cl3Si)3C]− ([1]−)[16–18]
and the silanide [(Cl3Si)3Si]− ([2]−).[19,20]
A corresponding
uncharged species A also exists (Figure 1).[21] The related
[a] [*] M. Sc. J. Teichmann, M. Sc. C. Kunkel, M. Sc. I. Georg, Dr. M.
Moxter, M. Sc.T Santowski, Dr. M. Bolte, Dr. H.-W. Lerner, Prof. Dr.
M. Wagner
Institut für Anorganische Chemie
Goethe-Universität Frankfurt
Max-von-Laue-Strasse 7, 60438 Frankfurt (Main) (Germany)
E-mail: Matthias.Wagner@chemie.uni-frankfurt.de
[b]
#
Dr. S. Bade
Evonik Resource Efficiency GmbH, Untere Kanalstraße 3, 79618
Rheinfelden, Germany
These authors contributed equally
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the reaction cascade leading to [3]−: analogous to the case of
CCl4,[18] chlorophilic attack of [SiCl3]− on GeCl4 removes a Cl+
cation. However, while [CCl3]− is nucleophilic enough to bite
back into SiCl4 and form Cl3C−SiCl3,[18] [GeCl3]− behaves inert
toward SiCl4.[28] Toward the more reactive Si2Cl6, [GeCl3]− likely
acts as a Cl− donor to afford GeCl2 and [SiCl3]−/SiCl4, which
subsequently combine to establish a first Ge−Si bond in a
putative [Cl2GeSiCl3]− anion. A repetition of analogous steps
ultimately furnishes the anion [(Cl3Si)3Ge]− ([3]−). [R4N][1] and
[R4N][3] are stable over the long term under inert conditions,
whereas [R4N][2] decomposes in CH2Cl2 solution at room
temperature over
cyclohexasilane-chloride
(Cl3Si)2Si6Cl10⋅2Cl][12] as the main product.
a
period of several days to give the
diadduct [R4N]2[1,1-
Scheme 1 compiles the solid-state structures of the anions
[1]−−[3]−. For a discussion of the trigonal-planar [1]−, see
references[17,18] and the Supporting Information of this paper.
The Si and Ge centers in [2]− and [3]− are strongly pyramidalized
((SiSiSi) = 290.0°, (SiGeSi) = 285.5°). The average Si−Si
(2.321 Å) and Ge−Si (2.373 Å) bond lengths are alike those in
the permethylated analogs [(Me3Si)3Si]− (2.331 Å) and
[(Me3Si)3Ge]− (2.384 Å).[29,30] Contrary to the sp2-hybridized C
atom of [1]− with its π-donation into the σ*(Si–Cl) bonds,[31] the
anionic centers of [2]−/[3]− avoid hybridization and keep their
lone pairs essentially localized in 3s/4s orbitals.[32]
Scheme 2. Adduct formation of [1]−−[3]− with GaCl3 (rt, CH2Cl2). Solid-state
structures of [Ph4P][1·GaCl3], [nBu4N][2·GaCl3]·0.5 SiCl4, and [Ph4P][3·GaCl3]
(shown as ball-and-stick models; for ORTEP plots, see the ESI;
[Ph4P]+/[nBu4N]+ cations and co-crystallized SiCl4 molecules are omitted for
clarity).
It is known that [1]− and [2]− accept protons to afford H[1] and
H[2].[18,19] We now disclose that they also react with the group III
Lewis acids E’Cl3 (E’ = B, Al, Ga). Of those, only GaCl3 (1 equiv)
gave adducts with all three anions that were sufficiently stable
and well-ordered in the solid state to allow X-ray crystallography
([Ph4P][1·GaCl3], [nBu4N][2·GaCl3], [Ph4P][3·GaCl3]; Scheme
2).[33] We observe fully pyramidalized GaCl3 moieties, indicating
strong donor-acceptor interactions (Cl–Ga–Cl
= 107.5(1)°
−110.8(3)°). Also the [1]− moiety is no longer planar (av. Si–C–Si
= 110.9°), because the lone pair is now localized between the C
and Ga atoms. In contrast, the Si–Si–Si and Si–Ge–Si angles in
[2·GaCl3]− (av. 106.0°) and [3·GaCl3]− (av. 106.1°) are wider than
those of the free Lewis bases [2]− (av. 96.7°) and [3]− (av. 95.2°)
and approach the value of the ideal tetrahedral angle (109.5°; cf.
Bent’s rule[34]).[27] The C–Ga, Si–Ga, and Ge–Ga bond lengths
amount to 2.094(9) Å,[35] av. 2.422 Å,[36] and 2.460(4) Å,
respectively. The first value is larger by 0.09 Å than that of the
corresponding bond in [Li(thf)2][(PhMe2Si)3C·GaCl3] (2.00(1)
Å),[37] which contains a more Lewis basic methanide donor. The
length of the dative Si–Ga bond in [2·GaCl3]− compares well to
that of the covalent Si–Ga bond in the sterically crowded dimer
(tBu3Si–GaCl2)2 (av. 2.413 Å).[36,38] Suitable systems for
comparison with [3·GaCl3]− are lacking.[27]
Scheme 1. Synthesis of [Ph4P][1], [Et4N][2], and [Et4N][3] in CH2Cl2 solutions.
Solid-state structures of [Ph4P][1], [Et4N][2], and [Et4N][3] (shown as ball-and-
stick models; for ORTEP plots, see the ESI).
Differences in the bond orders between the C–Si bond and the
(Si/Ge)–Si bonds of [1]−−[3]− also become evident from the SiCl3
NMR resonances: [1]− resonates at δ(29Si) = –10.9 ppm,
whereas the signals of [2]− and [3]− are detected at lower field
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strengths with almost identical chemical shift values of δ(29Si) =
29.5 and 29.7 ppm, respectively. In the case of [1·GaCl3]− vs.
[1]−, the SiCl3 resonance undergoes a downfield shift to δ(29Si) =
–3.4 ppm (cf. H[1]: –2.6 ppm).[18] The resonances of the other
two adducts experience almost identical upfield shifts to δ(29Si) =
11.9 ppm ([2·GaCl3]−; cf. H[2]: 5.5 ppm)[19] and 11.3 ppm
([3·GaCl3]−). The smaller spread in the chemical shift values of
the three adducts compared to the free anions reflects a smaller
difference in the bond orders between the respective central
atom and its SiCl3 substituents.
yield. The analogous experiment repeated with [Ph4P][3] led to
29Si NMR signals at 14.2 and 3.8 ppm. The first signal
possesses a similar chemical shift value as the corresponding
resonances of [3·BCl3]− and [3·GaCl3]− and is thus tentatively
assigned to a Ge→Al adduct [3·AlCl3]−. The signal at 3.8 ppm
arises from the previously unknown compound (Cl3Si)4Ge (5).
Removal of all volatiles under reduced pressure and extraction
of the solid residue with n-hexane ultimately gave single crystals
of (Cl3Si)4Ge in 24% yield (unoptimized; Scheme 3). Compound
5 possesses a tetrahedral structure with an average Ge−Si bond
Compounds of the type [2·GaCl3]− and [3·GaCl3]− are promising
precursors for the fabrication of Ga-doped Si and Si/Ge layers.
However, the gas-phase deposition of such materials requires
length of 2.366 Å[36] (cf. (Me3Si)4Ge: av. Ge−Si = 2.371 Å[39]).
With regard to the mechanism underlying the assembly of 5, the
following facts are important: (i) A conversion of the putative
[3·AlCl3]− adduct to 5 was never observed, even at 50 °C (sealed
NMR tube). It thus seems unlikely that [3·AlCl3]− plays a
significant role for product formation. (ii) [Ph4P][AlCl4] was
repeatedly isolated as byproduct of 5, thereby indicating that
AlCl3 abstracts one [Cl]− ion directly from [3]− to generate
(Cl3Si)2Ge=SiCl2. (iii) The yield of 5 is not increased by added
SiCl4, which rules out a mere “chlorosilylation” of the Ge=Si
double bond. We rather envisage a more complex scenario
involving oligomers of (Cl3Si)2Ge=SiCl2 and their subsequent
rearrangement to afford 5. A propensity of the Ge atom to
occupy the central position of Si/Ge neo-structures is obvious
from the AlCl3-catalyzed rearrangement of (Me3Si)3Si(GeMe3) to
give (Me3Si)4Ge.[40]
uncharged, volatile species. Taking [3·GaCl3]− as
a
representative example, we tested whether one Cl− ion is
abstracted by excess GaCl3. Indeed, the neutral dimer
((Cl3Si)3Ge−GaCl2)2, (4)2, forms in a one-pot reaction from a
mixture of [Et4N][3] and 2 equiv of GaCl3 in CH2Cl2. After
extraction with n-hexane, (4)2 crystallized from the extract in 40%
yield. The centrosymmetric solid-state structure features a
Ga2Cl2 ring with tetracoordinate Ga atoms (Scheme 3).
Quantitative Cl/H exchange to furnish (H3Si)4Ge (5H; Scheme 3)
occurred upon treatment of 5 with Li[AlH4] (1H and 29Si NMR
spectroscopy, GC/MS). The only alternative route to 5H available
to-date uses finely dispersed Na metal together with pyrophoric
and
toxic
SiH4/GeH4
to
produce
silylgermanides
NaGeHn(SiH3)3− (n = 0−2; 4h, 100 °C). After silylation with
n
C4F9SO3SiH3, 5H was isolated from the resulting mixture
GeHn(SiH3)4− (n = 0−2) by fractionating condensation and
n
subsequent preparative gas chromatography.[2] Our new three-
step process GeCl4 → [3]− → 5 → 5H is more practical than the
previous protocol and thus represents a major advance toward
the development of mixed oligotetrels.
In summary, we have disclosed a facile protocol for the
synthesis of the germanide [(Cl3Si)3Ge]− ([3]−) and thereby
extended the homologous series of trichlorosilylated tetrelides
[(Cl3Si)3E]− (E = C, Si, Ge; [1]−−[3]−). Contrary to a priori
expectations, the anions readily form 1:1 adducts with BCl3 and
GaCl3. The addition of 2 equiv of GaCl3 to [3]− results in Cl−-ion
abstraction from the primary product [3·GaCl3]− and generates
the neutral dimer ((Cl3Si)3Ge−GaCl2)2, (4)2. Cl− abstraction (this
time from an SiCl3 substituent) is also key to the reactivity of
AlCl3 toward [2]− and [3]−. Here, skeletal rearrangements afford
the neo-pentatetrels (Cl3Si)4Si and (Cl3Si)4Ge (5), respectively.
Scheme 3. Cl− abstraction reactions of [3]− with GaCl3 and AlCl3 (rt, CH2Cl2)
and hydrogenation reaction of 5 to give 5H. Solid-state structures of the neutral
products (4)2 and 5 (shown as ball-and-stick models; for ORTEP plots, see the
ESI).
The covalent Ge−Ga bond (2.4071(6) Å) is shorter by 0.053 Å
than the related dative bond in [3·GaCl3]− and the Si–Ge–Si
bond angle (av. 109.9°) is expanded a little further compared to
the situation in the adduct. The 29Si NMR signal of the dimer
appears at 7.8 ppm, and is even more upfield-shifted than the
Compound
5 can quantitatively be transformed into its
perhydrogenated congener 5H, a promising precursor for the
deposition of Si/Ge-based thin films. As an outlook, we
emphasize that [nBu4N][2] reacts with C2Cl6 to give the ethylene-
dianion salt [nBu4N]2[(Cl3Si)2C−C(SiCl3)2].[18] The scope of this
Si−C bond-forming reaction, which is conceptually similar to the
use of [(Cl3Si)2P]− and chloroalkanes for the formation of P−C
bonds,[23] will be explored in the future.
signal of [3·GaCl3]−.
Different from the cases of BCl3
[33]
and GaCl3, no adducts of
[1]−−[3]− with the stronger Lewis acid AlCl3 could be isolated so
far. The 29Si NMR spectrum of equimolar mixtures of AlCl3 and
[nBu4N][2] in CH2Cl2 rather showed two resonances assignable
to (Cl3Si)4Si (3.5, −80.9 ppm),[11] which was obtained in 36%
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10.1002/chem.201806298
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COMMUNICATION
Angew. Chem. Int. Ed. 2015, 54, 5429–5433; Angew. Chem. 2015, 127,
5519-5523.
Competing interests: J.T., H.-W.-L. and M.W. are inventors on
patent applications EP 17173940.2, EP 17173959.2, EP
18206148.1, and EP 18206150.7 submitted by Evonik Industries
AG, which cover the synthesis and use of [3]−, [3·GaCl3]−, (4)2 ,
and 5.
[15] M. B. Ansell, D. E. Roberts, F. G. N. Cloke, O. Navarro, J. Spencer,
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[16] J. Tillmann, PhD thesis, Universität Frankfurt, Germany, 2015.
[17] U. Böhme, M. Gerwig, F. Gründler, E. Brendler, E. Kroke, Eur. J. Inorg.
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[18] I. Georg, J. Teichmann, M. Bursch, J. Tillmann, B. Endeward, M. Bolte,
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Acknowledgements
[19] M. Olaru, M. F. Hesse, E. Rychagova, S. Ketkov, S. Mebs, J.
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The authors are grateful to the Evonik Resource Efficiency
GmbH, Rheinfelden (Germany), for financial funding and the
generous donation of GeCl4 and Si2Cl6. I.G. wishes to thank the
Evonik Foundation for a PhD grant.
[20] M. Moxter, PhD Thesis, Universität Frankfurt, Germany, 2017.
[21] J. I. Schweizer, M. G. Scheibel, M. Diefenbach, F. Neumeyer, C.
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Keywords: basicity • germanium • hydrogenation • main group
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only be taken as
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10.1002/chem.201806298
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Julian Teichmann,[a]# Chantal Kunkel,[a]# Isabelle Georg,[a]
Maximilian Moxter,[a] Tobias Santowski,[a] Michael Bolte,[a] Hans-
Wolfram Lerner,[a] Stefan Bade,[b] Matthias Wagner*[a]
Page No. – Page No.
Tris(trichlorosilyl)tetrelide Anions and a Comparative Study of
Their Donor Qualities
Cornerstone added: The trichlorosilylated germanide [(Cl3Si)3Ge]‒ is
readily accessible from GeCl4 and Si2Cl6/Cl‒. All tetrelides [(Cl3Si)3E]‒ (E
= C, Si, Ge) were found surprisingly nucleophilic and form adducts with
BCl3 and GaCl3. The unique (Cl3Si)4Ge was obtained through an AlCl3-
induced rearrangement of [(Cl3Si)3Ge]‒.
.
[a] [*] M. Sc. J. Teichmann, M. Sc. C. Kunkel, M. Sc. I. Georg, Dr. M.
Moxter, M. Sc.T Santowski, Dr. M. Bolte, Dr. H.-W. Lerner, Prof. Dr.
M. Wagner
Institut für Anorganische Chemie
Goethe-Universität Frankfurt
Max-von-Laue-Strasse 7, 60438 Frankfurt (Main) (Germany)
E-mail: Matthias.Wagner@chemie.uni-frankfurt.de
[b]
#
Dr. S. Bade
Evonik Resource Efficiency GmbH, Untere Kanalstraße 3, 79618
Rheinfelden, Germany
These authors contributed equally
This article is protected by copyright. All rights reserved.

10.1002/chem.201806298
Chemistry - A European Journal
COMMUNICATION
This article is protected by copyright. All rights reserved.