Synthesis of a Counteranion-Stabilized Bis(silylium) Ion

Article abstract of DOI:10.1002/anie.202003799

The preparation of a molecule with two alkyl-tethered silylium-ion sites from the corresponding bis(hydrosilanes) by two-fold hydride abstraction is reported. The length of the conformationally flexible alkyl bridge is crucial as otherwise the hydride abstraction stops at the stage of a cyclic bissilylated hydronium ion. With an ethylene tether, the open form of the hydronium-ion intermediate is energetically accessible and engages in another hydride abstraction. The resulting bis(silylium) ion has been NMR spectroscopically and structurally characterized. Related systems based on rigid naphthalen-n,m-diyl platforms can only be converted into the dications when the positively charged silylium-ion units are remote from each other (1,8 versus 1,5 and 2,6).

Full text of DOI:10.1002/anie.202003799

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Accepted Article
Title: Synthesis of a Counteranion-Stabilized Bis(silylium) Ion
Authors: Qian Wu, Avijit Roy, Guoqiang Wang, Elisabeth Irran, Hendrik
F.T. Klare, and Martin Oestreich
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Angewandte Chemie International Edition
10.1002/anie.202003799
COMMUNICATION
Synthesis of a Counteranion-Stabilized Bis(silylium) Ion
+
+
Qian Wu , Avijit Roy , Guoqiang Wang, Elisabeth Irran, Hendrik F. T. Klare, and Martin Oestreich*
Dedicated to Professor Paul Knochel on the occasion of his 65th birthday
Abstract: The preparation of a molecule with two alkyl-tethered
silylium-ion sites from the corresponding bis(hydrosilanes) by two-
fold hydride abstraction is reported. The length of the conformatio-
nally flexible alkyl bridge is crucial as otherwise the hydride abstrac-
tion stops at the stage of a cyclic bissilylated hydronium ion. With an
ethylene tether, the open form of the hydronium-ion intermediate is
energetically accessible and engages in another hydride abstraction.
The resulting bis(silylium) ion has been NMR spectroscopically and
structurally characterized. Related systems based on rigid naphtha-
len-n,m-diyl platforms can only be converted into the dications when
the positively charged silylium-ion units are remote from each other
before (Scheme 2, bottom).[3a] We anticipated that smaller or lar-
ger ring sizes would make the formation of the Si–H–Si unit less
favorable (gray box), thereby allowing for donor-mediated ring
opening and eventually facile hydride abstraction (Scheme 1,
bottom). The aliphatic linker would further equip the dication with
the necessary conformational flexibility to accommodate the
positive charges and Lewis pair formation with bulky donor mo-
lecules. We disclose here the synthesis of a bis(silylium) ion, a
new class of bidentate silicon superelectrophiles[6] for potential
[
5]
application in catalysis or as receptors.[8]
[7]
(
1,8 versus 1,5 and 2,6).
A common way of stabilizing silylium ions is by Lewis pair forma-
tion with σ-basic molecules to yield onium ions.[1] Under certain
circumstances, that is with no such Lewis base present in the re-
action medium, even an Si–H bond can lend stabilization to a
silylium ion in the form of a three-center, two-electron (3c2e) Si–
[
2–4]
H–Si bond (Scheme 1, top).
If intermolecular, this is a weak
interaction, and the hydrosilane in these hydronium ions is usu-
[2]
ally supplanted by the arene used as the solvent. Conversely,
the situation changes when a tethered Si–H bond is geometri-
cally accessible (Scheme 1, bottom). Several cyclic hydronium
ions have already been described, and their spectroscopic and
crystallographic characterization established that these are free
in the sense that neither the solvent nor the counteranion coor-
dinates to either of the silicon atoms.[3]
These systems are actually fairly stable, and the abstraction
of another hydride with the trityl salt Ph C[B(C F ) ] to access a
3 6 5 4
bis(silylium) ion is, if at all, slow. A likely reason could be the
stereoelectronic inaccessibility of the Si–H bond engaged in the
aforementioned 3c2e bond. An attempt by Müller and co-wor-
Scheme 1. Inter- and intramolecular hydronium ions, and the generation of
bis(silylium) ions from those cyclic systems. WCA = weakly coordinating anion.
Do = donor, typically solvent molecule.
kers did not enable the isolation of such a species (Scheme 2,
top).[3c] The targeted dication was too reactive and immediately
decomposed the borate counteranion. The close proximity of the
cationic centers attached to the rigid naphthalen-1,8-diyl plat-
form is presumably part of the problem. Müller had reported
another six-membered ring system with an aliphatic backbone
Müller and co-workers (2006): attempted synthesis of bis(silylium) ion
H
Ph C+[B(C F ) ]–
Me2Si
SiMe2
Me Si
2
SiMe2
3
6 5 4
(
1.0 equiv)
C6H6
7
5 °C
2 [B(C6F5)4]–
[B(C F ) ]–
6
5 4
slow reaction
(29
not stable
Si) 54.4 ppm
in C6D6 (Ref. [3c])
[
*]
Dr. Q. Wu,[+] A. Roy,[+] Dr. G. Wang, Dr. E. Irran,[++] Dr. H. F. T.
Klare, Prof. Dr. M. Oestreich
Müller (2001)
this work
Institut für Chemie, Technische Universität Berlin
Strasse des 17. Juni 115, 10623 Berlin (Germany)
E-mail: martin.oestreich@tu-berlin.de
+
Ph3C [WCA]–
H
WCA WCA
iPr2Si SiiPr2
H
iPr2Si
SiiPr2
Me2Si
SiMe2
(1.0 equiv)
(
n = 0–2)
n
Homepage: http://www.organometallics.tu-berlin.de
B(C6F5)4]–
(29Si) 76.7 ppm
[
WCA]–
n
[
+
[
[
]
These authors contributed equally to this work.
X-ray crystal-structure analysis.
++
]
in C6D6 (Ref. [3a])
Supporting information for this article is given via a link at the end of
the document.
Scheme 2. Attempted and planned synthesis of counteranion-stabilized
bis(silylium) ions.
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Angewandte Chemie International Edition
10.1002/anie.202003799
COMMUNICATION
+
The choice of substituents at the silicon atom is crucial for
the successful generation of heteroleptic silylium ions as substi-
tuent redistribution reactions often produce complex mixtures.[9]
The alkyl-substituted precursors required for the present study
are inherently afflicted with this problem but isopropyl and tert-
sed-5 , respectively. In turn, the silicon atom in ethylene-tethe-
+
red silylium ion 6 (n = 0) is significantly more deshielded with δ
( Si) 107.3 ppm. This value lies between those of related hydro-
nium ions and δ ( Si) 130.4 ppm of iPr
29
29
+
3
Si[CHB11Cl11] (7 ) in 1,2-
6
C D
4
Cl and could therefore be an average resonance signal. To
For this reason, learn whether this is due to an equilibrium between the closed
2
[9c,9d]
butyl group usually hamper such processes.
we opted for the bis(hydrosilanes) 1–3 with iPr
Scheme 3, top); their straightforward preparation is outlined in
the Supporting Information. The chemical stability of the counter-
+
2
Si moieties
and counteranion- or solvent-stabilized open forms of 6 , we per-
formed theoretical calculations at the B3LYP-D3(BJ)/cc-PVTZ+
(PCM, benzene)//B3LYP-D3(BJ)/6-31G(d,p)+(PCM, benzene)
level of theory.
out of those three, the closed forms represent the energetic
minimum while the solvent adducts of the open forms are higher
in energy than those of the counteranion (C
versus [CHB11Cl11] ). Importantly, a low free energy difference
was only found between closed-6 and open-6+ (ΔG = 1.3 kcal
mol ). This means that the counteranion-stabilized open form of
6 is energetically accessible in C
and open-5 do not exist in solution state (ΔG = 3.9 and 5.5
kcal mol , respectively). This trend was the same but more pro-
(
[
10]
[14–16]
anion is of utmost importance in silylium-ion chemistry,
and
Generally, these computations revealed that,
‒
[B(C
6
F
5
)
4
] previously used by Müller is not suitable for the given
[3c,11]
– [12]
challenge.
least coordinating and most robust carborate counteranions.
Reacting precursors 1–3 with the corresponding trityl salt
We decided to go with [CHB11Cl11] , one of the
[
10,13]
6
H
6
or 1,2-C
6
H
Cl
4 2
–
+
Ph
3
C[CHB11Cl11] in C
6
D
6
+
resulted a biphasic suspension that
+
–1
contained silylium ions 4 –6 (Scheme 3, top). For characteri-
+
zation, the phases were separated and the remaining slurry was
6
H
6
solution. Conversely, open-
+
+
washed with a few drops of C
was fully dissolved in 1,2-C
6
D
6
and n-pentane. The remainder
, and the homogeneous solu-
4
–1
6
D
4
Cl
2
+
tion was subjected to NMR analysis (Scheme 3). Running those
nounced for the NMR solvent 1,2-C
6
D
4
Cl
2
(ΔG = 8.1 for 4 , 9.4
+
–1
+
reactions directly in 1,2-C
6
D
4
Cl
2
was far less clean.
for 5 , and 4.7 kcal mol for 6 ). This is in line with VT NMR
measurements which did not indicate any change in the stabili-
zation mode (closed forms only). The computed NMR chemical
shifts agree with those obtained experimentally (see the Suppor-
ting Information).
+
–
Cl10B11CH
Cl
SiiPr2
H
H
Ph3C [CHB11Cl11]
H
iPr2Si
SiiPr2
H
iPr2Si
SiiPr2
(1.0 equiv)
iPr2Si
C6D6
n
n
RT for 24 h
–
+
[
CHB11Cl11]
n
+
open-4 –6+
+
1
–3 (n = 0–2)
closed-4 –6
Based on the above reasoning, we predicted that ethylene-
+
tethered 6 would be more likely to participate in another hydride
H
H
iPr2Si
SiiPr2
abstraction than 4 and 5 with longer alkyl bridges. Indeed, 4+
+
+
iPr2Si
SiiPr2
H
iPr2Si
SiiPr2
+
and 5 did not react with Ph
3
C[CHB11Cl11] any further. However,
[
CHB11Cl11]–
4+
(n = 2): 82%
(1
H) 1.76 ppm
[CHB11Cl11]–
5+ (n = 1): 62%
(1H) 1.29 ppm[a]
JSi–H–Si = 45.0 Hz[a]
(29Si) 85.2 ppm
[CHB11Cl11]–
6+ (n = 0): 63%[b]
(1H) 2.16 ppm
+
the reaction of 6 with the trityl salt was extremely slow, fur-
nishing the desired bis(silylium) ion 9 in small quantities along
with byproducts (Scheme 4). In turn, direct treatment of bis-
3 11 11
(hydrosilane) 3 with 2.0 equiv of Ph C[CHB Cl ] afforded the
dication 9 in 78% yield (gray box). We explain this dramatic
difference by the poor solubility of the reactants in C ; pre-
formed, solid 6 is less prone to undergo the hydride abstraction
than in-situ-formed, dissolved 6 .
2+
JSi–H–Si = 45.1 Hz
(29
JSi–H–Si = 45.0 Hz
(29Si) 107.3 ppm
Si) 82.6 ppm
2+
reference compounds
iPr iPr
6 6
D
iPr
iPr
Si
iPr
CHB11Cl11]–
7+
(this work)
Si
iPr
+
[Me3Si
H
SiMe3]+
+ [17]
The new bis(silylium) ion
–
[CHB11Cl11]–
[
[CHB11Cl11]
was fully characterized by NMR spectroscopy, and its molecular
structure was determined by X-ray single-crystal analysis (Figu-
7+
8+
(29
(29Si) 114.4 ppm
(29Si) 85.4, 82.2 ppm
Si) 130.4 ppm
[
18]
in 1,2-C6D4Cl2
in toluene-d8 (Ref [12b])
CPMAS (Ref. [2a])
re 1). Different from the previously reported solvent-stabilized
iPr
Si+ ion 7+ (Scheme 3, bottom), the cationic silicon centers
3
Scheme 3. Synthesis and NMR spectroscopic characterization of cyclic hydro-
nium ions. Unless noted otherwise, all NMR spectra were recorded in 1,2-
are each stabilized by one of the counteranion’s chlorine atoms.
The anti conformation of the coordinated silicon centers is likely
a consequence of both steric and charge–charge repulsion. The
average C‒Si‒C angle is 116.4° and, hence, closer to a trigonal
planar (120.0°) than a tetrahedral (109.5°) coordination geo-
metry. The Si–Cl bond (2.341 Å) is longer than that of a typical
covalent Si‒Cl bond (2.072 Å) and even longer than that in 1,2-
C
6
D
4
Cl
2
. CPMAS = cross polarization and magic-angle spinning. [a]
Determined by
a
1H/29Si-1D-CLIP-HSQMBC NMR experiment. [b] The
+
molecular structure of closed-6 was determined by an X-ray diffraction
analysis but its quality prevents its publication (see the Supporting Information).
The CIF file was deposited with The Cambridge Crystallographic Data Centre
as a personal communication (CCDC 1990361).
+
+
[2a]
6 4 2 3
C D Cl -stabilized iPr Si (7 ) (2.33 Å).
The collected 29Si NMR data were correlated with indepen-
+
+
[12b]
+
+ [2a]
dently prepared iPr
3
Si (7 )
3 2
and known [(Me Si) (μ-H)] (8 )
both having [CHB11Cl11]– as counteranion (Scheme 3, bottom).
The silylium ions 4 and 5 with a butylene (n = 2) or a propylene
+
+
(
n = 1) tether show chemical shifts similar to Reed’s acyclic
+
[2a]
29
hydronium ion 8
tem (cf. Scheme 2, bottom).
these are best represented as cyclic systems closed-4 and clo-
(approx. δ ( Si) 84 ppm) and Müller’s sys-
[
3a]
We therefore concluded that
+
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Angewandte Chemie International Edition
10.1002/anie.202003799
COMMUNICATION
+
–
Ph3C [CHB11Cl11]
HCB11Cl10 Cl10B11CH
Cl Cl
H
H
(
2.0 equiv)
iPr2Si
SiiPr2
C6D6
RT for 5 days
iPr Si
SiiPr2
2
3
92+
: 78%
(29
Si) 117.7 ppm
Ph3C+
CHB11Cl11]
Ph3C+
[CHB11Cl11]
(1.0 equiv)
–
–
[
H
(
1.0 equiv)
iPr2Si
SiiPr2
C6D6
RT for 24 h
C D
RT
extremely slow
6
6
[
CHB11Cl11]–
6+
H
C
iPr2
Si
Cl
Cl
Si
iPr2
C
H
2
+
[
iPr2Si(CH2)2SiiPr2]
2
–
2+
[CHB11Cl11] (9
)
Scheme 4. One-step and two-step approaches to bis(silylium) ion.
Scheme 5. Regioisomeric naphthalene-based bis(hydrosilanes) as precursors
for bis(silylium) ions. Unless noted otherwise, all NMR spectra were recorded
in 1,2-C D Cl .
6 4 2
As the peri substitution pattern is potentially thwarting the
establishment of another positive charge, we subjected regioiso-
meric bis(hydrosilanes) 12 and 14 to the one-step procedure
(
Scheme 5, bottom). However, we found that the counteranion
had a profound effect on the dication formation of these regio-
Figure 1. Molecular structure of bis(silylium) ion 92+ (thermal ellipsoids are
2+
2+
–
isomers; 13 and 15 did not form with [CHB Cl ] as the
11
11
shown at 50% probability; hydrogen atoms are omitted for clarity). Selected
counteranion. In turn, the reaction of 12 and 14 with 2.0 equiv of
Ph C[CHB11 Br
] afforded the bis(silylium) ions 132+ and 152+,
1
6
bond lengths [Å] and angles [°]: Selected bond length (Å): Si ‒Cl 2.3411(15),
1
2
1
3
1
6
3
H
5
6
Si ‒C 1.866(4), Si ‒C 1.855(5), Si ‒C 1.858(4). Selected bond angle (°):
2
1
3
2
1
6
3
1
6
respectively in good yields. Due to their poor solubility even in
highly polar arene solvents, these bis(silylium) ions were charac-
terized by NMR spectroscopy as the acetonitrile adducts.
C ‒Si ‒C 112.68(19), C ‒Si ‒C 116.25(19), C ‒Si ‒C 120.2(2).
We then turned towards a reinvestigation of Müller’s naph-
thalen-1,8-diyl system but were also not able to generate the
corresponding bis(silylium) ion (cf. Scheme 2, top).[3c] Precursor
The present work demonstrates that the generation of long-
sought bis(silylium) ions is possible, provided that the two posi-
tive charges can dodge each other. This can either be achieved
by conformational flexibility of the tether between those sites or
by an appropriate predefined distance in rigid systems. The
length of the flexible tether is crucial as it determines the stabi-
lity of intermediate cyclic hydronium ions. With an ethylene
brigde, the open form of such a hydronium ion exists in equili-
brium to undergo another hydride abstraction. The new bis-
1
0 converted into the stable hydronium ions 11+ in high yield
with various trityl salts Ph C[WCA] (Scheme 5, top). The aryl
3
substitution typically brings about an upfield shift of about Δδ
2
9
[9b,9c]
29
+
(
(
Si) 20 ppm.
Relative to δ ( Si) 85.2 ppm for 5 , approx. δ
29
+
Si) 63.5 ppm for 11 independent of the weakly coordinating
counteranion is in excellent agreement with this.
(
silylium) ion has been characterized by NMR spectroscopy and
X-ray diffraction. Future work will be directed towards the appli-
cation of such superelectrophilic bidentate Lewis acids in cata-
lysis or as receptors.[5]
Acknowledgements
A.R. gratefully acknowledges the Berlin Graduate School of
Natural Sciences and Engineering for a predoctoral fellowship
This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.202003799
COMMUNICATION
(
2018−2021). Q.W. and G.W. thank the Alexander von Humboldt
Foundation (2017−2019) and the China Scholarship Council
2019−2020), respectively, for postdoctoral fellowships. The
work was also funded by the Deutsche Forschungsgemeinschaft
DFG, German Research Foundation) under Germany´s
[7]
[8]
For the application of silylium ions in catalysis, see: a) J. C. L. Walker,
H. F. T. Klare, M. Oestreich, Nat. Rev. Chem. 2020, 4, 54–62; b) P.
Shaykhutdinova, S. Keess, M. Oestreich in Organosilicon Chemistry :
Novel Approaches and Reactions (Eds.: T. Hiyama, M. Oestreich),
Wiley-VCH, Weinheim, 2019, pp. 131–170; c) A. Schulz, A. Villinger,
Angew. Chem. Int. Ed. 2012, 51, 4526–4528; Angew. Chem. 2012, 124,
4602–4604.
(
(
Excellence Strategy – EXC 2008 – 390540038 – UniSysCat. All
theoretical calculations were performed at the High-Performance
Computing Center (HPCC) of Nanjing University. We also thank
Dr. Sebastian Kemper (TU Berlin) for expert advice with the
NMR measurements. M.O. is indebted to the Einstein
Foundation Berlin for an endowed professorship.
For an example, see: N. Lühmann, H. Hirao, S. Shaik, T. Müller,
Organometallics 2011, 30, 4087–4096.
[
9]
For substituent redistribution of silylium ions, see: a) A. Schäfer, M.
Reissmann, A. Schäfer, W. Saak, D. Haase, T. Müller, Angew. Chem.
Int. Ed. 2011, 50, 12636–12638; Angew. Chem. 2011, 123, 12845–
12848; b) A. Schäfer, M. Reissmann, S. Jung, A. Schäfer, W. Saak, E.
Brendler, T. Müller, Organometallics 2013, 32, 4713–4722; c) K. Müther,
P. Hrobárik, V. Hrobáriková, M. Kaupp, M. Oestreich, Chem. Eur. J.
Keywords: bidentate interaction • carboranes • density
2013, 19, 16579–16594; d) L. Omann, B. Pudasaini, E. Irran, H. F. T.
functional calculations • Lewis acids • silylium ions
Klare, M.-H. Baik, M. Oestreich, Chem. Sci. 2018, 9, 5600–5607; e) K.
Bläsing, R. Labbow, D. Michalik, F. Reiß, A. Schulz, A. Villinger, S.
Walker, Chem. Eur. J. 2020, 26, 1640–1652.
[
1]
For general reviews of silylium ion chemistry, see: a) V. Y. Lee, Russ.
Chem. Rev. 2019, 88, 351–369; b) V. Ya. Lee, A. Sekiguchi in
Organosilicon Compounds, Vol. 1 (Ed.: V. Ya. Lee), Academic Press,
Oxford, 2017, pp. 197–230; c) T. Müller in Structure and Bonding, Vol.
[
10] For recent reviews of weakly coordinating anions, see: a) S. P. Fisher,
A. W. Tomich, S. O. Lovera, J. F. Kleinsasser, J. Guo, M. J. Asay, H. M.
Nelson, V. Lavallo, Chem. Rev. 2019, 119, 8262–8290; b) I. M.
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Oestreich), Thieme, Stuttgart, 2013, pp. 1–42.
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A. Engesser, M. R. Lichtenthaler, M. Schleep, I. Krossing, Chem. Soc.
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[
[
2]
3]
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Nava, C. A. Reed, Organometallics 2011, 30, 4798–4800; c) S. J.
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[
[
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T.-P. Sze, T. C. W. Mak, D. T. W. Chan, Z. Xie, Inorg.Chem. 2000, 39,
7478–7481.
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Chem. Int. Ed. 2001, 40, 3033–3036; Angew. Chem. 2001, 113, 3123–
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[
[
4]
5]
For gas-phase studies, see: a) M. Savoca, J. Langer, O. Dopfer, Angew.
Chem. Int. Ed. 2013, 52, 1568–1571; b) M. A. R. George, M. Savoca, O.
Dopfer, Chem. Eur. J. 2013, 19, 15315–15328; c) M. A. R. George, O.
Dopfer, Int. J. Mass Spectrom. 2019, 435, 51–60.
[
[
16] J. Tomasi, M. Persico, Chem. Rev. 1994, 94, 2027–2094.
17] The one-step protocol was also applied to bis(hydrosilanes) 1 and 2 but
reactions were messy. With 1, substituent redistribution occurred to
considerable extent,[9] and iPr
3
Si[CHB11Cl11] (7[CHB11Cl11]) was found
For authoritative overviews of bidentate Lewis acids, particularly iso-
electronic bisboranes, see: a) W. E. Piers, G. J. Irvine, V. C. Williams,
Eur. J. Inorg. Chem. 2000, 2131–2142; b) M. Melami, F. P. Gabbaï,
Adv. Organomet. Chem. 2015, 53, 61–99; c) L. Schweighauser, H. A.
Wegner, Chem. Eur. J. 2016, 22, 14094–14103.
as the major product.
[
18] CCDC 1989781 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre.
[
6]
For a recent review on Lewis superacids, see: L. Greb, Chem. Eur. J.
2018, 24, 17881–17896.
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10.1002/anie.202003799
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Q. Wu, A. Roy, G. Wang, E. Irran, H. F.
T. Klare, M. Oestreich*
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Synthesis of a Counteranion-
Stabilized Bis(silylium) Ion
Supercharged. A single silylium ion is hard to handle but what about two of these
superelectrophiles in one molecule in close proximity? Depending on the distance
between these Lewis acidic sites, such a previously elusive bidentate Lewis acid
can be accessible, characterized, and stored at room temperature.
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