Intramolecular Halo Stabilization of Silyl Cations—Silylated Halonium- and Bis-Halo-Substituted Siliconium Borates

Article abstract of DOI:10.1002/chem.202004838

The stabilizing neighboring effect of halo substituents on silyl cations was tested for a series of peri-halo substituted acenaphthyl-based silyl cations 3. The chloro- (3 b), bromo- (3 c), and iodo- (3 d) stabilized cations were synthesized by the Corey protocol. Structural and NMR spectroscopic investigations for cations 3 b–d supported by the results of density functional calculations, which indicate their halonium ion nature. According to the fluorobenzonitrile (FBN) method, the silyl Lewis acidity decreases along the series of halonium ions 3, the fluoronium ion 3 a being a very strong and the iodonium ion 3 d a moderate Lewis acid. Halonium ions 3 b and 3 c react with starting silanes in a substituent redistribution reaction and form siliconium ions 4 b and 4 c. The structure of siliconium borate 4 c₂[B₁₂Br₁₂] reveals the trigonal bipyramidal coordination environment of the silicon atom with the two bromo substituents in the apical positions.

Full text of DOI:10.1002/chem.202004838

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Accepted Article
Accepted Article
Title: Intramolecular Halo Stabilization of Silyl Cations - Silylated
Halonium- and Bis-Halo Substituted Siliconium Borates
Authors: Thomas Müller, Anastasia Merk, Lukas Bührmann, Natalie
Kordts, Katharina Görtemaker, and Marc Schmidtmann
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To be cited as: Chem. Eur. J. 10.1002/chem.202004838
Link to VoR: https://doi.org/10.1002/chem.202004838
01/2020

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RESEARCH
ARTICLE
Intramolecular Halo Stabilization of Silyl Cations - Silylated
Halonium- and Bis-Halo Substituted Siliconium Borates
Anastasia Merk, Lukas Bührmann, Natalie Kordts, Katharina Görtemaker, Marc
Schmidtmann and Thomas Müller*[a]
[a]
MSc A. Merk, MSc L. Bührmann, Dr. N. Kordts, MSc K. Görtemaker, Dr. M. Schmidtmann, Prof. Dr. T. Müller
Institute of Chemistry
Carl von Ossietzky University Oldenburg
Carl von Ossietzky-Str. 9-11, D-26129 Oldenburg, Federal Republic of Germany, European Union
E-mail: thomas.mueller@uni-oldenburg.de
Supporting information for this article is given via a link at the end of the document.
Abstract: The stabilizing neighboring effect of halo substituents on
silyl cations was tested for series of peri-halo substituted
state structures of the corresponding salts of the silyl cation
solvent complexes[4] and leads to distinctly different reactivity of
the cation, for example versus carbon dioxide.[5] The recent
synthesis of the parent silylium ion [SiH3]+ by the Oestreich group
highlights the role of halogenated counter anions and halo-
genated arenes as solvents.[6] In o-dichlorobenzene solution
[SiH3]+ exists as close ion pair with the brominated carborane
anion with the silicon cation coordinated via a bromine atom to the
anion (Figure 1). The solid-state structure of the silylium car-
borane revealed a one-dimensional polymeric structure with a
trigonal-bipyramidal-coordinated silicon atom with two bromine
substituents from two different carborane anions in the apical
positions (Figure 1).
a
acenaphthyl-based silyl cations 3. The chloro- (3b), bromo- (3c) and
iodo- (3d) stabilized cations were synthesized by the Corey protocol.
Structural and NMR spectroscopic investigations for cations 3b-d
supported by the results of density functional calculations indicate
their halonium ion nature. According to the fluorobenzonitrile (FBN)
method, the silyl-Lewis acidity decreases along the series of halonium
ions 3, the fluoronium ion 3a being a very strong and the iodoniumion
3d a moderate Lewis acid. Halonium ions 3b and 3c react with starting
silanes in a substituent redistribution reaction and form siliconium ions
4b and 4c. The structure of siliconium borate 4c2[B12Br12] reveals the
trigonal bipyramidal coordination environment of the silicon atom with
the two bromo substituents in the apical positions.
Introduction
Recent years have witnessed the transformation of silyl
cations from laboratory curiosities to valuable reagents and
catalysts in synthetic organic chemistry.[1] These positively
charged organosilicon compounds are of interest due to their high
electrophilicity and Lewis acidity, properties that severely
hampered their isolation and application.[2] Significant progress in
the preparation methods for silyl cations was needed before their
synthetic potential could be exploited. In particular, the quest for
non-coordinating reaction conditions has been omnipresent in
silyl cation chemistry. This was guaranteed by the use of weakly
Figure 1. 29Si NMR chemical shifts of triethylsilyl cation in different solvents and
of silyl cation in solution and the solid state.[2b, 6]
coordinating
anions
(nearly
exclusively
halogenated
tetraarylborates, aluminates, boranes and carboranes) and
solvents such as arenes and halogenated arenes.[3] The halo
substitution is of significant importance for the stability of the
anions as it increases their oxidation potential and lowers their
nucleophilicity. Halogenated arenes as solvents differ in their
reactivity versus silyl cations from aromatic hydrocarbons in so far
as the most nucleophilic side is the halo substituent and not the
aromatic -system. This difference is obvious from the recorded
29Si NMR chemical shifts[2b] (Figure 1), it is manifested in solid
In addition, the interaction between halo substituents and silyl
cations is the first step of each C-F or C-Cl activation by silyl
cations or of the decomposition of silyl cations in halogenated
solvents.[2b, 7] Bissilylated halonium ions have been studied by our
group[8] and, more extensively, by the groups of Schulz and
Wagner.[9] The interaction between aryl halides and silyl cations
has not been studied systematically. In view of the importance of
1
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halo stabilization in silyl cation chemistry, we included these sub-
stituents in our general study on neighboring effects on the
properties of silylium ions.[10] We synthesized the acenaphthyl
silanes 1 and naphthyl silane 2 and studied their reactivity versus
trityl cation (Figure 2).
Figure 2. Acenaphthyl silanes 1 and naphthyl silane 2.
Results and Discussion
Scheme 1. Synthesis of bromonium borate 3c[B(C6F5)4] and of siliconium
borate 4c[B(C6F5)4] from acenaphthyl silane 1c applying the Corey protocol.
peri-Halo-silyl-substituted acenaphthenes 1 and the 8-silyl-
substituted bromonaphthalene 2 were synthesized according to
established protocols and were fully characterized by standard
analytical technics especially multinuclear NMR spectroscopy
(see Supporting Information). The molecular structures of silanes
1a and 1c were obtained from X-ray diffraction analysis of suitable
single crystals. Their molecular structures in the crystal are
inconspicuous (Figure 3). Remarkable is that the fluoro
substituted acenaphthyl silane 1a shows the on-set of an
intramolecular Si/F interaction.[11] This is indicated by an almost
linear alignment of the F/Si-H atoms (177.9°) and a Si/F
separation of 300.9 pm that is significant shorter than the sum of
the van der Waals radii (357 pm).[12] In addition, a through space
coupling of TSJ = 5.7 Hz was detected in both 29Si NMR (29Si =
12.4) and 19F NMR spectra (19F = -119.4).[13] The bromo
substituted acenaphthyl silane 1c adopts a conformation that
brings the Si-H in a syn-arrangement to the bromine atom. The
H/Br separation is 292 pm, almost exactly the expectation value
from the sum of the van der Waals radii (293 pm).[12] The sum of
the bond angles  involving the atoms defining the bay region of
silane 1c (Si, C6, C12, C5, Br) is 381.7°, which is larger than the
value reported for acenaphthene ( = 368°).[14] This indicates
repulsion between the silyl and the bromo substituent in silane
1c.[15]
Next, we attempted the synthesis of cyclic halonium borates
3[B(C6F5)4] using the standard Corey protocol (Scheme 1).[16]
Surprisingly, the reaction of silane 1c with [Ph3C][B(C6F5)4] at
room temperature gave two silicon containing products that are
identified by 29Si NMR chemical shifts of 29Si = 107.9 and 29Si =
82.6 in a ratio 65 : 35 (Scheme 1, path a), Figure 4a). The trityl
cation was completely consumed during the reaction. These low
field shifted 29Si NMR signals indicated the formation of two silicon
cationic species, which suggested that one product is formed in a
consecutive reaction. To test this hypothesis, we run the reaction
at T = -10°C in toluene and observed indeed the selective
formation of the compound with 29Si = 107.5 (Scheme 1, path b),
Figure 4b). This 29Si NMR chemical shift is indicative of an
intramolecular bromo stabilized silyl cation. The recorded 29Si
NMR chemical shift is close to that reported for the silylated
chloronium ion [Et3Si(Cl-C6H5)]+ (29Si = 99.9)[5] and in the range
of related tetra-coordinated halo stabilized silyl cations 5 – 7
(Figure 5).[17] The detailed analysis of the spectroscopic NMR
data confirmed the expected formation of the silylated bromonium
ion 3c (see Supporting Information). This finding was supported
by the results of NMR chemical shift calculations at the
GIAO/M06L/def2tzvp//M06-2X/def2tzvp level of theory, which
predict for an optimized molecular structure of cation 3c a 29Si
NMR chemical shift of 29Si(calc) = 109.[18] The second species
with a 29Si NMR chemical shift of 29Si = 82.6 could be selectively
synthesized by adding additional silane 1c at room temperature
to a toluene solution of 3c[B(C6F5)4]. Although for a stoichiometric
conversion two equivalents of 1c are needed in theory, the
reaction was more selective when a threefold excess of the
starting silane 1c at room temperature was reacted with the trityl
borate (Scheme 1, path c), Figure 4c). The NMR spectroscopic
data indicate the substitution of the cationic silicon atom with only
one methyl group but with two acenaphthyl residues. Supportive
for the structure elucidation of this cation was the detection of
relative large 1J(SiC) couplings of 1J(SiC(aryl)) = 82 Hz and
1J(SiC(alkyl)) = 62 Hz compared to those found for cation 3c
Figure 3. Molecular structure of silanes 1a,c in the crystal (Thermal ellipsoids
are shown at the 50 % probability level. Hydrogen atoms are omitted for clarity;
only the hydrogen atom attached to silicon is shown). Pertinent bond lengths
[pm] and bond angles [°]: 1a: F-C6 136.22(16), Si-C5 186.83(16), F/Si
300.87(10), Si-H 138.(3), F-C6-C12 117.89(12), Si-C5-C12 126.81(11), C5-C12-C6
128.40(13), ),  373.1, (SiC3) 336.7, F-Si-H 177.9. 1c: Br-C5 190.28(6), Si-
C6 188.79(6), Si-H 140.0(12), Br-C5-C12 122.89(4), Si-C6-C12 128.69(4),
C5-C12-C6 130.14(5),  381.7, (SiC3) 327.3 H/Br 292.1.
1
(1J(SiC(aryl)) = 76 Hz and J(SiC(alkyl)) = 54 Hz). These large
1J(SiC) coupling constants indicate an higher contribution of
atomic s-orbitals to the Si-C bonds and suggest penta-
coordination for the cationic silicon atom with the carbon
substituents at the trigonal basis of the bipyramid. On the basis of
the NMR data, we assign siliconium ion structure 4c to the
2
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observed species. No significant change was observed by 1H
NMR spectroscopy at T = -40°C, which makes a dynamic
equilibrium between two tetra-coordinated species less likely. In
favor of this claim are 29Si NMR chemical shift calculations for an
optimized molecular structure of siliconium ion 4c, which predict
a 29Si NMR chemical shift of 29Si(calc) = 85. In addition, the 29Si
NMR data is in agreement with that reported for related terphenyl-
based siliconium ions 8 and 9 (Figure 5).[19] Finally, the structure
of siliconium ion 4c was determined by X-ray diffraction (XRD)
analysis from suitable single crystals of [4c]2[B12Br12]. The
electrophilicity of halonium ion 3c is high and it reacts with excess
of the starting silane 1c to undergo a substituent redistribution
reaction. These reactions between silyl cations and silanes are
well documented for aryl and alkyl substituted silylium ions.[8, 20]
We were not able to detect the volatile trimethylsilane but
tetramethylsilane, a common follow-up product of further ligand
exchange, was detected by NMR spectroscopy.[21]
chloronium borate 3b was however not possible. Both silyl cations
were identified in the mixture by their characteristic NMR
parameter (see Supporting Information). In contrast, iodonium
borate 3d[B(C6F5)4] is formed selectively and was characterized
by NMR spectroscopy (Scheme 2 and Supporting Information).
Most significant is the 29Si NMR chemical shift of 29Si = 89.5,
which agrees with the theoretically predicted value of (29Si(calc)
= 101). Finally, the reaction of the fluoro substituted acenaphthyl
silane 1a with [Ph3C][B(C6F5)4] in aromatic hydrocarbons gave
according to 29Si NMR an intractable mixture of compounds, even
at T = -40°C in chlorobenzene and at T = -80°C in methylene
dichloride. This observation suggests uncontrolled C-F activation
through the incipient silyl cation (Scheme 2).
Figure 4. 29Si{1H} NMR spectra of a) the reaction of silane 1c with 0.8 equiv.
[Ph3C][B(C6F5)4] at r.t., in benzene-d6; b) 0.9 equiv. [Ph3C][B(C6F5)4] at -10°C, in
toluene-d8; c) 1/3 equiv. [Ph3C][B(C6F5)4] at r.t., in benzene-d6 (broad signal
around 29Si = -110 is due to borosilicate glass of the NMR tube and the probe
head inlet).
Scheme 2. Reaction of silane 1a,b,d with trityl borate according to the Corey
protocol (1a: X = F, 1b: X = Cl, 1d: X = I).
The influence of the carbon-skeleton on the formation of halonium
versus siliconium ion was tested with the bromonaphthyl silane 2.
In this case, we expected a more efficient stabilization of the silyl
cation by the neighboring halo substituents.[22] The standard
Corey conditions (Scheme 3, path a)) allowed the synthesis of the
bromonium borate 10[B(C6F5)4] (29Si = 94.6, 29Si(calc) = 98).
The use of three equivalents of silane 2 (Scheme 3, path b)) did
not result in the expected selective formation of siliconium borate
11[B(C6F5)4] but gave even at higher reaction temperatures a
mixture (25:75) of the cations 10 and 11. A noticeable decomposi-
tion of both cations took place at temperatures above T = 40°C
and prevented further attempts to drive the reaction to
completeness. The experimental NMR data suggest for the silicon
atom in cation 11 (29Si = 62.9) a trigonal bipyramidal coordination
environment with one methyl group and two naphthyl
substituents. NMR chemical shift computations for an optimized
molecular structure of cation 11 support this assignment
(29Si(calc) = 61). All prepared halonium ions 3 and 10 are very
reactive in solution and decompose upon standing into non-
identified products during days. The siliconium ions 4 and 11 are
more stable in solution and in the solid state but are still highly
reactive against nucleophiles.
Figure 5. Examples of intermolecular ([5(BrC6H5)]+) and intramolecular halo
stabilized silyl cations 6-9 with different coordination environment and their 29Si
NMR chemical shifts.
The results of the Corey reaction of the other halo acenaphthyl
silanes 1a, b, d depend on the halo substituent (Scheme 2). The
chloro acenaphthyl silane 1b gave at ambient conditions in
benzene a 40:60 mixture of the chloronium borate 3b[B(C6F5)4]
(29Si = 116.2) and the siliconium borate 4b[B(C6F5)4] (29Si =
79.8). The assignments are supported by the results of 29Si NMR
chemical shift calculations for 3b (29Si(calc) = 117) and 4b
(29Si(calc) = 79). The ratio 3b/4b changed to 81:19 at T = -40°C
in chlorobenzene as solvent. The selective formation of
3
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