Unusually selective synthesis of chlorohydrooligosilanes

Article abstract of DOI:10.1039/d0cc06506d

New pathways towards molecular chlorohydrooligosilanes enable their one-pot synthesis in preparative amounts either by the selective chlorination of the corresponding perhydrosilanes with HCl/AlCl3 or by the partial hydrogenation of perchlorooligosilanes

Full text of DOI:10.1039/d0cc06506d

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Unusually Selective Synthesis of Chlorohydrooligosilanes
Thomas Lainer,a Roland Fischer,a Mario Leypold,a Michael Holthausen,b Odo Wunnicke,b Michael
Haasa* and Harald Stuegera*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
New pathways towards molecular chlorohydrooligosilanes enables will outperform the state-of-the-art precursors concerning their
their one-pot synthesis in preparative amounts either by the film deposition properties. A recently published study actually
selective chlorination of the corresponding perhydrosilanes with showed the formation of higher quality films at higher
HCl/AlCl3 or by the partial hydrogenation of perchlorooligosilanes deposition rates in atomic layer deposition (ALD) processes for
with substoichiometric amounts of iBu2AlH. The unexpected the precursor molecule HSi2Cl5 as compared to Si2Cl6.6
selective formation of Cl3Si-substituted species in the partial However, the state-of-the-art synthetic protocols of
hydrogenation reactions could be related to mechanistic aspects.
chlorohydrooligosilanes are not straightforward. Partial
halogenation reactions of di- tri- and tetrasilane with BX3,
HX/AlX3, AgX, SnX4, HgX2 or X2 (X = Cl, Br, I) afforded product
mixtures which only could be analyzed after further
derivatization of the initial products.7 The partial hydrogenation
of perchlorooligosilanes by treatment with substoichiometric
amounts of LiAlH4 or Bu3SnH also was found to be rather
unselective.8,9 Additionally, selected halohydrooligosilanes
were prepared by the halodephenylation of appropriately
substituted phenylated precursors with anhydrous HX.10 The
challenge of this method is the preparation of the starting
materials which frequently involves tedious multi-step
procedures. In a more recent study we observed the clean
Chlorosilanes, especially chlorohydrosilanes, are ideal
precursors for the generation of hyperpure polycrystalline
silicon. Because of the dramatic rise of photovoltaic
applications, the production capacity of polysilicon reached 600
kton/year at the end of 2018.1 The vast majority of
polycrystalline Si produced worldwide are made by chemical
vapor deposition of HSiCl3.2 Less than 10% of all solar grade
silicon is currently manufactured from silicon hydrides such as
SiH4 in spite of their lower decomposition temperatures and the
resulting lower energy cost incurring during the deposition
process. Major drawbacks of silicon hydrides primarily are their
high production costs and their difficult handling because of
their pyrophoric nature. Therefore, the search for alternative
precursor materials for silicon deposition is still an issue of
primary importance.3
Based on calculations two mechanisms for the deposition of
silicon from HSiCl3 are possible. The disilane mechanism
proposed by Swihart and Carr et al.4 and the radical mechanism
by Cavallotti et al.5 In both mechanisms, the formation of
disilanes as reaction intermediates plays a crucial role by
enhancing the gas phase reactivity. This is due to their increased
reactivity, determined by the relatively low Si−Si bond energy
and small barriers for Cl and H intramolecular transfers.
Therefore, it can be anticipated that chlorohydrooligosilanes
formation of 1,2,3,4-tetrachloroneopentasilane
2
after
treatment of neopentasilane 1 with 3.5 equivalents of SnCl4.11
To the best of our knowledge this was the first example for a
direct and selective functionalization of a higher silicon hydride
on a preparative scale.
3.5 SnCl4
pentane
SiH3
H3Si Si SiH3
SiH3
SiH2Cl
ClH2Si Si SiH2Cl
- 3.5 SnCl2
- 3 HCl
SiH2Cl
- 0.5 H2
1
2
SiH2Cl
SiH2Cl
HClg
+
ClH2Si Si SiH3
ClH2Si Si SiH2Cl
benzene
cat. AlCl3
SiH2Cl
SiH2Cl
2
3
15 %
a. Institute of Inorganic Chemistry, Technische Universität Graz
Stremayrgasse 9/IV, 8010 Graz (Austria). E-mail: michael.haas@tugraz.at,
harald.stueger@tugraz.at
85 %
Scheme 1. Partial chlorination of neopentasilane with SnCl4
b. Evonik Creavis GmbH
and HCl/AlCl3.
Paul-Baumann-Strasse 1, 45772 Marl (Germany).
†Electronic Supplementary Information (ESI) available: NMR spectra, experimental
procedures. See DOI: 10.1039/x0xx00000x
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A major disadvantage of SnCl4 as a halogenating reagent is the This new synthetic method can be performed in the absence of
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DOI: 10.1039/D0CC06506D
formation of huge amounts of SnCl2, which are difficult to any solvent at room temperature, which enabled the isolation
remove completely in particular on a larger scale. Thus, we of 4 - 7 along with the corresponding perhydrogenated species
investigated the chlorination of neopentasilane with gaseous and small amounts of other Cl/H silanes by trap-to-trap
HCl in the presence of AlCl3 as a catalyst. When dry HCl gas was distillation in vacuum. The structures and relative quantities of
bubbled through
a benzene solution of neopentasilane the individual species present in the volatile fraction were
containing catalytic amounts of AlCl3 at 70 °C for 15 minutes, assigned by NMR spectroscopy. (The corresponding
compound 2 was obtained with remarkable selectivity along experimental data and procedures are included in the Electronic
with 1,2,3-trichloroneopentasilane 3 as a by-product (Scheme Supplementary Information). If Si2Cl6 was used as a precursor
1). Removal of the solvent and the catalyst and recondensation the volatile fraction additionally contained considerable
of the crude product afforded a mixture of 2 (85 %) and 3 (15 %) amounts of 1,1,1,2-tetrachlorodisilane. In all other cases higher
in > 60 % yield. Attempts to separate both components by chlorinated species could not be separated from iBu2AlCl and
distillation or crystallization were unsuccessful.
ended up in the high boiling residue of the vacuum
An even more appealing approach to chlorohydrooligosilanes is condensation along with some unreacted starting material. For
the partial hydrogenation of perchlorooligosilanes with Si3Cl8 the addition of excess iBu2AlH to the higher boiling
substoichiometric amounts of iBu2AlH. This pathway enables product fraction afforded further 0.6 g of Si3H8 which means a
the synthesis of chlorohydrooligosilanes on a broader scope. In more or less quantitative conversion of the starting material to
recent studies it has been found that iBu2AlH is suitable for the chlorohydrotrisilanes. Extensive side reactions such as Si-Si
hydrogenation of linear and branched chlorooligosilanes under bond scission or oligomerization, thus, can be ruled out. Yields
mild reaction conditions without the formation of any Si-Si bond calculated for 4 - 7 on the basis of the relative intensities of 1H-
scission products.12 When we reacted hexachlorodisilane NMR signals range between 28 and 39 %.
(Si2Cl6), octachlorotrisilane (Si3Cl8), nonachloroisotetrasilane Neopentasilane and its derivatives just recently attracted
ClSi(SiCl3) or dodecachloroneopentasilane Si(SiCl3)4 with sub- considerable attention as precursors for the deposition of
stoichiometric quantities of iBu2AlH we observed the silicon and silicon heterostructures.13 Thus, we decided to study
predominant formation of the corresponding 1,1,1- compound 7 in more detail. 7 could be isolated from the crude
trichlorooligosilanes 4 - 7 with unexpected selectivity (Table 1). product mixture obtained according to Scheme 2 from the
SiHCl2- and SiH2Cl- moieties were only detected in minor side reaction of Si(SiCl3)4 with 9 equiv. of iBu2AlH by fractional
products. Based on screening experiments we determined that distillation as a colorless oil in about 20 % yield and
the ideal amount of iBu2AlH equals the number of chlorine characterized by comparison of its NMR data with literature
atoms minus three. If more equivalents of iBu2AlH were used values.14 The resulting samples were reasonably pure but still
the completely hydrogenated species were formed as the major contained small amounts of neopentasilane 1 and iBu2AlCl. For
products. Application of less equivalents of iBu2AlH afforded the isolation of pure 7 the application of more sophisticated
increasing portions of higher chlorinated polysilanes.
distillation techniques would be necessary which was beyond
the scope of this study.
+ (m - 3) iBu2AlH
For nucleophilic substitution reactions of chlorosilanes a
reactivity order of R3SiCl < R2SiCl2 < RSiCl3 < SiCl4 is usually found
due to the impact of the electronegative Cl substituents at the
silicon center.15 Attempts to partially hydrogenate
polychlorosilanes with substoichiometric amounts of LiAlH4,
thus, only afforded more or less statistical mixtures of various
chlorohydrosilanes.8
neat, 25 °C
other Cl/H
oligosilanes
SinClm
Cl3SiSi(n-1)H(m-3)
SinHm
+
+
- (m - 3) iBu2AlCl
4
7
-
n = 2 - 5
major products
m = 2n + 2
Scheme 2. General scheme for the partial hydrogenation of
perchlorooligosilanes with iBu2AlH.
Table 1: Summary of experimental results of the partial hydrogenation of perchlorooligosilanes with iBu2AlH
equiv.
iBu2AlH
yield of
Cl3SiSinHm
Precursor
molar ratio of main productsa
a
Si2Cl6
Si3Cl8
3
Cl3SiSiH3 (4) (54 %); ClH2SiSiCl3 (40 %); Si2H6 (6 %)b
Cl3SiSiH2SiH3'(5) (67 %); Si3H8 (33 %)b
39 %
36 %
28 %
2 %
5
6
9
ClSi(SiCl3)3
Si(SiCl3)4
Cl3SiSiH(SiH3)2 (6) (57 %); HSi(SiH3)3 (43 %)c
Cl3SiSi(SiH3)3 (7) (11 %); Si(SiH3)4 (89 %)d
Cl3SiSi(SiH3)3 (7) (67 %); Si(SiH3)4 (33 %)d
28 %
a calculated by integration of 1H-NMR signals of the volatile fraction after trap-to-trap distillation. b trap-to-trap distillation at room temperature and 0.01 mbar.
c fraction 1: trap-to-trap distillation at 50°C and 0.01 mbar. d fraction 1: trap-to-trap distillation at 50°C and 0.01 mbar. e fraction 2: subsequent trap-to-trap distillation of high boiling
fraction at 80°C and 0.01 mbar.
2 | J. Name., 2012, 00, 1-3
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The unexpected selectivity of the hydrogenation reactions with ether does not show any selectivity. When 7 was reacted with
iBu2AlH towards the formation of the 1,1,1-trichlorosilanes 4 - sub-stoichiometric amounts of iBu2AlH in diethyl ether solution
7 as described above can be easily rationalized if mechanistic only small amounts of neopentasilane 1 were formed along
aspects are taken into account. Based on stereochemical with insoluble polymeric material and SiH4. In this case the
arguments it has been proposed that the hydrogenation of hydrogen atom in iBu2AlH-Et2O containing formally tetravalent
chlorosilanes with iBu2AlH in non-coordinating solvents like aluminum is far more hydridic and nucleophilic and the reaction
hexane follows a SNi-Si-mechanism involving a four-center proceeds by a SN2 mechanism.16b As a consequence the
transition state A. In diethyl ether, on the contrary, the same reduction is accompanied by Si-Si bond scission and
hydrogenation reaction proceeds via
a pentacoordinate redistribution reactions and occurs rather unselective just as
transition state B typical for SN2 reactions (Scheme 3).16
found for the hydrogenation of chlorosilanes with LiAlH4 in
diethyl ether.
SiCl3
In conclusion, we have introduced two new innovative
strategies for the synthesis of chlorohydrooligosilanes in gram
quantities. In particular, it has been discovered that
substoichiometric amounts of neat iBu2AlH react with
perchlorooligosilanes under formation of products containing
exclusively Cl3Si- and H3Si- moieties. Si-Si bond scission and
oligomerization reactions do not occur at all which contrasts the
behavior of the more common hydrogenation reagent
LiAlH4/diethyl ether described in the literature. Different
reaction mechanisms (SN2-Si and SNi-Si) operative in polar and
unpolar environments could be made responsible for this
unexpected discrepancy. The target molecules of this study are
of particular importance for potential applications as
alternative precursors for the deposition of hyperpure
polycrystalline silicon. Corresponding experiments are currently
underway in our laboratories.
This work was supported in a government financed project by
the “Austrian Klima and Energiefonds” in the framework of the
Austrian energy research project 2015 (FFG Project Nr. 858491
“Liquid Silicon 2.0”) and the NRW Ministry of Economic Affairs
of the state North Rhine-Westphalia (funding code W49, “Liquid
Silicon 2.0”). We gratefully acknowledge additional financial
support from the Evonik Creavis GmbH.
Cl
other Cl/H
oligosilanes
H3Si Si SiH3
SiH3
+
Si
Al
9 iBu2AlH
neat
H
A
(SNi-Si) mechanism
SiCl3
Cl3Si Si SiCl3
SiCl3
9 iBu2AlH
ether
SiH3
Cl
insoluble
polymer
+
+ SiH4
H3Si Si SiH3
SiH3
Si
H
B
(SN2-Si) mechanism
Scheme 3. Mechanism of partial hydrogenation of 7.
DFT calculations at the SMD(pentane) M06-2X-D3/aug-cc-pVTZ
level of theory were performed on the reaction of Cl3SiSiCl3,
H3SiSiCl3, H3SiSiHCl2, H2SiSiH2Cl with HAlMe2. For Cl3SiSiCl3
which is the best suited test system we were not able to locate
all structures (ground states and transition states) on the energy
surface. For H3SiSiCl3, H3SiSiHCl2, H2SiSiH2Cl the reaction path
via intermediate/transition state A did not show any selectivity
in terms of calculated activation energies (1. reduction step:
ΔΔGAE0 = 8.6 kcal.mol-1; 2. reduction step: ΔΔGAE0 = 9.6 kcal.mol-
1; 3. reduction step: ΔΔGAE = 11.6 kcal.mol-1). In contrast, the
0
calculated activation energies would favor a slightly preferred
reaction at the higher chlorinated Si atom, contradicting our
experimental observations. Thus, we attribute the unexpected
selectivity of our hydrogenation reactions in non-coordinating
solvents more to a steric than to an electronic effect. We
reasoned that due to the presence of the sterically demanding
iBu-groups in transition state A further substitution preferably
occurs at the sterically less encumbered Si-center, which is the
Si atom bearing already one or two hydrogens leaving the
residual SiCl3 groups rather untouched. This steric effect more
or less outperforms the electronic influence of the Cl
substituents mentioned above leading to the predominant
formation of products 4 - 7 containing exclusively Cl3Si- and
H3Si- moieties. Further support for this picture was gained from
the observation that the reduction of 7 with iBu2AlH in diethyl
Conflicts of interest
There are no conflicts to declare.
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Novel methods were developed for the synthesis of partially chlorinated higher silicon hydrides with
unexpected selectivity.