Silicon Tetrakis(trifluoromethanesulfonate): A Simple Neutral Silane Acting as a Soft and Hard Lewis Superacid

Article abstract of DOI:10.1002/anie.202103414

A facile synthesis and isolation of pristine silicon tetrakis(trifluoromethanesulfonate), Si(OTf)₄, is reported, acting as the first neutral silicon-based Lewis superacid suitable towards soft and hard Lewis bases. Its OTf groups have a dual function: they are excellent leaving groups and modulate the degree of reactivity towards soft and hard Lewis bases. Exposed to soft Lewis donors, Si(OTf)₄ leads to [L₂Si(OTf)₄] complexes (L=isocyanide, thioether and carbonyl compounds) with retention of all Si?OTf bonds. In contrast, it can cleave C?X bonds (X=F, Cl) of hard organic Lewis bases with a high tendency to form SiX₄ (X=F, Cl) after halide/triflate exchange. Most notable, Si(OTf)₄ allows a gentle oxydefluorination of mono- and bis(trifluoromethyl)benzenes, resulting in the formation of the corresponding benzoylium species, which are stabilized by the weakly coordinating [Si(OTf)₆] dianion.

Full text of DOI:10.1002/anie.202103414

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Main Group Complexes
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Silicon Tetrakis(trifluoromethanesulfonate): A Simple Neutral Silane
Acting as a Soft and Hard Lewis Superacid
Andrꢀ Hermannsdorfer and Matthias Driess*
Dedicated to Professor Gottfried Huttner
Abstract: A facile synthesis and isolation of pristine silicon
tetrakis(trifluoromethanesulfonate), Si(OTf)4, is reported,
acting as the first neutral silicon-based Lewis superacid
suitable towards soft and hard Lewis bases. Its OTf groups
have a dual function: they are excellent leaving groups and
modulate the degree of reactivity towards soft and hard Lewis
bases. Exposed to soft Lewis donors, Si(OTf)4 leads to
[L2Si(OTf)4] complexes (L = isocyanide, thioether and car-
achieved during the last years with reports on the first neutral
silicon LSAs in form of the perhalogenated bis(catecholato)-
silanes Si(catX)2 (X = Cl, Br, Figure 1).[5,6] Other silicon(IV)-
based LAs are routinely used in organic synthesis, with the
commercially available TMSOTf (TMS = Me3Si) arguably
being prevalent,[7] but far from being a LSA.[3] Bis- and
tris(triflate) silanes potentially represent stronger LAs, but
have mainly served as building blocks for polysilane synthe-
ses[8,9] or as precursors for silicon-based polycations[10,11] with
ꢀ
bonyl compounds) with retention of all Si OTf bonds. In
ꢀ
ꢀ
contrast, it can cleave C X bonds (X = F, Cl) of hard organic
cleavage of the Si OTf bonds. Donor-free Si(OTf)4 has not
Lewis bases with a high tendency to form SiX4 (X = F, Cl) after
halide/triflate exchange. Most notable, Si(OTf)4 allows a gentle
oxydefluorination of mono- and bis(trifluoromethyl)benzenes,
resulting in the formation of the corresponding benzoylium
species, which are stabilized by the weakly coordinating
[Si(OTf)6] dianion.
been isolated to date,[9] but recently, a high FIAwas calculated
for Si(OTf)4 suggesting its Lewis superacidity.[12]
M
ain group Lewis acids (LA) are valuable functional
species and mediators in many branches of chemistry,
including homogeneous catalysis, synthesis of frustrated
Lewis pairs (FLP) and weakly coordinating anions
(WCA).[1] LAs that are stronger than monomeric SbF5 in
the gas phase are classified as Lewis superacids (LSA),
a feature that is generally assessed by calculation of the
fluoride ion affinities (FIA).[2] While the FIA scales the Lewis
acidity towards the hard Lewis base Fꢀ, the related hydride
ion affinity (HIA) defines the scale of soft Lewis acidity. In
this regard, it has been proposed that a LA that exceeds the
HIA of B(C6F5)3 as a reference compound is considered as
a soft LSA.[3] Silicon-based LAs are particularly attractive
because of the high abundance and low toxicity of this
element. Accordingly, Lewis superacidic silylium ions, R3Si+,
have become valuable catalysts in organic synthesis but
require the use of potent WCAs.[4] Large progress has been
Figure 1. Previously studied silane Lewis superacids (LSA) and the
hard and soft LSA Si(OTf)4 presented herein (OTf=CF3SO3, WCA=
weakly coordinating anion).
Herein, we describe a facile access to pristine Si(OTf)4
and demonstrate that the OTf substituents are not only
excellent leaving groups but also enable an unprecedented
Lewis superacidic reactivity of a silane towards both hard and
soft donors.
The easy access to Si(OTf)4 in multi-gram scale succeeds
in 81% yields through the reaction of SiI4 with four molar
equivalents of AgOTf in CH2Cl2. The colorless liquid
solidifies below 08C and can be stored indefinitely at
ꢀ308C under an inert atmosphere. The observed singlet
29Si NMR resonance at d = ꢀ121.7 ppm (CD2Cl2) is in good
agreement with the reported value of ꢀ118.2 ppm for in situ
generated Si(OTf)4 in CH2Cl2.[9]
In order to assess the Lewis acidity also in the “soft
dimension”, the isodesmic HIA of Si(OTf)4 in addition to the
FIA was calculated using the TMS-reference system[12,13]
(Table 1, for computational details see Supporting Informa-
tion). The obtained FIA is in good agreement with the value
that was determined by Greb (519 kJmolꢀ1).[12] Both the FIA
and HIA are well above the respective thresholds of SbF5 and
B(C6F5)3, qualifying Si(OTf)4 as a hard and soft LSA. Notably,
also the second ion affinities are particularly high (approx.
[*] M. Sc. A. Hermannsdorfer, Prof. Dr. M. Driess
Department of Chemistry: Metalorganics and Inorganic Materials
Technische Universitꢀt Berlin
Strasse des 17. Juni 115, Sekr. C2, 10623 Berlin (Germany)
E-mail: matthias.driess@tu-berlin.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202103414.
ꢁ 2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution Non-Commercial
License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited and is not used
for commercial purposes.
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Dd1H of the adduct [(CA)B(C6F5)3] (1.05 ppm).[18]
In contrast, it was shown by Ghosez that addition of
TMSOTf to CA does not lead to a measurable Dd1H
and a 1:1 mixture of HOTf and CA gave a Dd1H of
only 1.28 ppm.[19]
Table 1: DFT-derived first and second ion affinities (fluoride: FIA, hydride: HIA) for
selected Lewis acids (LA) in kJmolꢀ1
.
Lewis acid (LA)
FIA[a]
HIA[a]
ꢀ
ꢀ
2ꢀ
ꢀ
ꢀ
2ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
LA/LA F LA F /LA F2
LA/LA H LA H /LA H2
SiF4
324
495
497
521
500
443
ꢀ180
315
491
492
547
–
ꢀ258
Si(catCl)2
Si(catBr)2
Si(OTf)4
SbF5
69
21
Furthermore, we applied a scaling method
developed recently by Mꢀller, based on the weak
p-fluorobenzonitrile donor (FBN).[20] From an equi-
molar mixture of Si(OTf)4 and FBN only the bis-
adducts cis- and trans-[(FBN)2Si(OTf)4] are formed
(molecular structure of the latter in Figure S124).
79
216
–
32
212
–
B(C6F5)3
–
514
–
[a] PW6B95-D3BJ/def2-QZVPP//B3LYP-D3BJ/def2-SVP.
The induced shifts (d19F = ꢀ89.7, ꢀ90.1 ppm; 1JCF
=
267, 267 Hz) are similar to those for the FBN adduct
400 kJmolꢀ1 above [XSiF4]ꢀ/[X2SiF4]2ꢀ) and indicate a high
tendency for the formation of hexacoordinate species.
of a weakly iodo-stabilized silylium ion.[20] This emphasizes
that Si(OTf)4 acts as a strong LA with retention of the Si OTf
ꢀ
The Lewis superacidity was confirmed experimentally by
reaction with [PPh4][SbF6], which led to fluoride abstraction
with subsequent formation of (TfO)SiF3 and SiF4 (19F and
29Si NMR) among other, unidentified products. In agreement
with the high HIA, addition of Si(OTf)4 to the hydridoborate
salt [Mes3PH][HB(C6F5)3] in CD2Cl2 led to hydride abstrac-
tion. Consequently, the SiH species [HSi(OTf)4]ꢀ and [H2Si-
(OTf)3]ꢀ were detected by 1H and HSi HMQC NMR
spectroscopy (see Supporting Information for details). To
the best of our knowledge, Si(OTf)4 represents the first
isolable neutral silicon compound that is a soft LSA.
These results were corroborated by Lewis acidity scaling
using different Lewis base NMR probes. Employing one
molar equivalent of the hard phosphine oxide OPEt3 (Gut-
mann–Beckett method),[14] a strong low-field shift was
observed for d31P (Dd31P = 51.2 ppm), which is well above
the Dd31P reported for the OPEt3 adducts of Si(catBr)2
(33.2 ppm)[5] and silylium ions (43.9 ꢁ 2.7 ppm).[15,16] Higher
Dd31P values were only obtained with cationic LSAs such as
[(m-(CH3)2C6H4)Si(terpy)]3+ and [catB]+ (61.1 and
60.7 ppm).[11,16] Upon cooling, crystals of the ion pair
[(OPEt3)2Si(OTf)2][Si(OTf)6] are formed illustrating the
bonds.
In this context, the reaction of Si(OTf)4 with soft neutral
donors afforded unprecedented [L2Si(OTf)4] complexes (L =
isocyanide, thioether or carbonyl compounds, Figure 2) in 56–
91% yield, which were fully characterized. The only crystallo-
graphic evidence for a SiIV complex with an aldehyde
compound exists for the silylium adduct [(PhCHO)SiEt3]+,[21]
while, to the best of our knowledge, SiIV complexes with
external thioether and isocyanide ligands were unknown. In
accordance with the downfield-shifted d13CCO = 208.4 ppm for
the benzophenone complex [(Ph2CO)2Si(OTf)4] in CD2Cl2
(cf. d13CCO = 196.8 ppm for Ph2CO),[22] the C O distance of
=
1.275(4) ꢁ is strongly elongated in the solid-state structure
compared to Ph2CO (1.2229(17) ꢁ).[23] A shortening of the
ꢂ
N C distance is observed for [(XylNC)2Si(OTf)4]
(1.145(3) ꢁ) in comparison to the “free” isocyanide
(1.160(3) ꢁ),[24] whereas, in SiII isocyanide complexes, back-
[25]
ꢀ
bonding causes an elongation of the N C bond.
Investigation of the thermochemistry of [L2SiX4] complex
formation (X = OTf, F) by DFT calculations elucidates that
the extraordinary Lewis acidity of Si(OTf)4 in comparison to
SiF4 is not due to inherently different reorganization energies
ꢀ
expected lability of the Si
OTf bonds. With two molar
equivalents of OPEt3, the
bis-adduct
[(OPEt3)2Si-
(OTf)4] (Dd31P = 33.5 ppm)
was isolated in 48% yield
(VT-NMR and molecular
structures in Supporting
Information).
After addition of the soft
Lewis base crotonaldehyde
(CA, Childs method)[17] to
Si(OTf)4, the formation of
cis- and trans-[(CA)2Si-
(OTf)4] could be clearly
detected by multinuclear
NMR spectroscopy. The
high induced shift for the
Figure 2. Top: Synthesis of the [L2Si(OTf)4] complexes; bottom: their molecular structures (ellipsoids at 50%
probability; H atoms are omitted and OTf reduced to wireframe).[34] Selected distances (ꢂ) for L=Ph2CO: C–
O 1.275(4), Si–O1 1.796(2), PhCHO: C–O 1.263(2), Si–O1 1.7816(12), XylNC (two independent molecules):
N–C 1.145(2)/1.146(2), Si–C 1.9631(17)/1.9602(18), THT: S–Si 2.3593(4), PQ: Si–O1 1.828(2), Si–O2
1.845(2).
H3
protons
(Dd1H =
1.52 ppm) complies with
the soft Lewis superacidity
of Si(OTf)4 and exceeds the
2
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Ereorg (for details see SI, Table S5). In the gas phase, the
association of [L2Si(OTf)4] (DEass) is exothermic and exer-
gonic, which is in stark contrast to the corresponding [L2SiF4]
complexes where DGass > 0. This is in keeping with our results
and experimental reports for SiF4.[26] The Ereorg necessary to
deform tetrahedral SiX4 to the square-planar geometry
adapted in trans-[L2SiX4] decrease in the order SiF4 >
SiCl4 > SiBr4 and are considered crucial for complex forma-
tion [Eq. (1)].[27]
ꢀDEass ¼ Eint ꢀ ðErSeiXor4g þ 2ErLeorg
Þ
ð1Þ
However, the here calculated Ereorg[Si(OTf)4] (288–
329 kJmolꢀ1) are in a similar range as Ereorg[SiF4] (298–
304 kJmolꢀ1). Consequently, the high affinity of Si(OTf)4
towards the L donors is due to much higher interaction
energies Eint between the prepared monomers for
[L2Si(OTf)4] (466–569 kJmolꢀ1) against Eint[L2SiF4] (128–
329 kJmolꢀ1).
Figure 4. Oxydefluorination of trifluoromethylbenzenes (A/B),
together with X-ray structures of [PhCO]2[Si(OTf)6] (C) and
[m-(CO)(COOTf)C6H4]2[Si(OTf)6] (D) (ellipsoids at 50% probability;
H atoms are omitted and Si-bound OTf reduced to wireframe; only
one of the respective cations is depicted).[34]
We further investigated the applicability of Si(OTf)4 as
ꢀ
a halide and hydride acceptor. Due to the lability of the Si
OTf bond, hydride/triflate exchange was observed when
exposing Si(OTf)4 to Et3SiH with formation of HSi(OTf)3 and
Et3SiOTf (1H and 29Si NMR). Remarkably, Hꢀ/OTfꢀ
exchange even occurs with the weaker hydride donor
PhSiH3,[28] affording a mixture of HSi(OTf)3 and PhSiH2-
(OTf) (d29Si = ꢀ8.5 ppm).[9] The formation of tritylium with
could be isolated in 30% yield from the reaction mixture and
ꢀ1
ꢂ
fully characterized (n˜CO = 2224 cm , d(C O) = 1.124(5) ꢁ,
consistent with literature values,[29] Figure 4a,c). [PhCO]2[Si-
(OTf)6] was independently synthesized from Si(OTf)4 and
benzoyl triflate, PhCOOTf, in 92% yield. The latter is a high
boiling liquid and not dissociated in contrast to [PhCO]2-
[Si(OTf)6], which illustrates the weakly coordinating nature
of the [Si(OTf)6]2ꢀ anion.[30]
1
two equivalents of trityl chloride was monitored by H and
13C NMR spectroscopy (d13C = 211.1 ppm) after mixing in
CD2Cl2 (Figure 3). The 29Si NMR spectra show the formation
of SiCl4 and [Si(OTf)6]2ꢀ (d29Si = ꢀ18.8 and ꢀ215.6 ppm).
Layering of the solution with n-hexane afforded yellow
crystals suitable for a single-crystal X-ray diffraction (SC-
XRD) analysis; the latter confirmed the formation of [Ph3C]2-
[Si(OTf)6] (Figure 3). Likewise, [Si(OTf)6]2ꢀ and SiX4 are
formed from the reaction of Si(OTf)4 with two molar
equivalents of [NBu4]X (X = Cl, Br, I) as evidenced by
29Si NMR. Accordingly, Si(OTf)4 can act both as a strong
halide and triflate acceptor.
This feature allows for the oxydefluorination (ODF) of
trifluoromethyl benzenes with concurrent stabilization of
resulting benzoylium species through the [Si(OTf)6]2ꢀ WCA
(Figure 4). When exposing PhCF3 to Si(OTf)4, Fꢀ/OTfꢀ
exchange to PhCF2(OTf) and FSi(OTf)3 took place within
1 h at 208C. However, 19F NMR spectra indicated the
formation of increasing amounts of Tf2O and FnSi(OTf)4ꢀn
(n = 2–4). Accordingly, the [PhCO]2[Si(OTf)6] ODF product
In the literature, the ODF of ArylCF3 species with the
Brønsted superacid HOTf and subsequent Friedel–Crafts-
type arylations afforded aromatic ketones, but the electro-
philic intermediates could not be detected.[31] Perfluoroto-
luene is at the threshold of deactivation for the reaction with
Si(OTf)4 as there was no evidence for ODF and only partial
conversion to C6F5CF2(OTf) and FSi(OTf)3 took place after
14 days (66% by 19F NMR, Figure 4a). A related Fꢀ/OTfꢀ
exchange reaction between C6F5CF3 and the LSA B(OTf)3
was reported by Petrov.[32]
In
contrast,
m-bis(trifluoromethyl)benzene,
m-
(CF3)2C6H4, was gradually oxydefluorinated with Si(OTf)4
in CD2Cl2 as monitored by NMR (Figure 4b, for more details
about observed intermediates see Supporting Information).
After three weeks at 408C, the formation of m-
(COOTf)2C6H4 was observed (with approx. 10% of residual
1
m-(COOTf)(CF2OTf)C6H4, based on H NMR integrals).[33]
Cooling to 48C led to formation of crystals of [(CO)-
(COOTf)C6H4]2[Si(OTf)6] suitable for a SC-XRD analysis,
which confirmed the complete ODF (Figure 4d). In the solid
state structure, the OTf adjacent to the carbonyl group
features a C8–O2 distance of 1.20(2) ꢁ, while the linear CCO
unit comprises a much shorter C1–O1 distance of 1.119(6) ꢁ.
Independently, we obtained the same compound in 62% yield
Figure 3. Halide abstraction from Ph3CCl and Bu4NX (X=Cl, Br, I)
together with the molecular structure of [Ph3C]2[Si(OTf)6] (ellipsoids at
50% probability; H atoms are omitted and OTf reduced to wireframe;
only one of the cations is depicted).[34]
=
with two characteristic IR vibrational modes at 1807 (C O)
and 2250 cmꢀ1 (C O) by reacting m-(COOTf)2C6H4 with
ꢂ
Si(OTf)4.[30]
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involved species in CH2Cl2 (TIAsolv, for details see Supporting
Information, Table S8), the abstraction of both triflates from
m-(COOTf)2C6H4 by Si(OTf)4 is unfavorable. In agreement
with the experimental observations, the TIAsolv of Si(OTf)4
(1st/2nd 106/92 kJmolꢀ1) are in the range of PhCO+ and [m-
(CO)(COOTf)C6H4]+ (106 and 142 kJmolꢀ1), whereas the
TIAsolv of [m-(CO)2C6H4]2+ is considerably higher
(200 kJmolꢀ1).
In summary, we have demonstrated by theoretical and
experimental investigations that Si(OTf)4 represents the first
neutral silane derivative with both hard and soft Lewis
superacidity. The OTf substituents at silicon in Si(OTf)4 are
not only excellent leaving groups but can also modulate the
degree of Lewis acidity which imparts its high reactivity
towards halide sources and soft neutral donors. This is
showcased by the generation of benzoylium cations from
mono- and bis(trifluoromethyl)benzenes and their stabiliza-
tion through the weakly coordinating [Si(OTf)6]2ꢀ counterion.
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Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (Cluster of
Excellence UniSysCat of Germanyꢂs Excellence Strategy—
EXC 2008–390540038) for financial support. A.H. thanks the
Einstein Stiftung Berlin for a PhD fellowship. Open access
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Conflict of interest
The authors declare no conflict of interest.
ꢀ
Keywords: benzoylium cations · C X bond activation ·
main group complexes · silanes · weakly coordinating anions
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4
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[34] Deposition numbers 2052913, 2052914, 2052915, 2052916,
2052917, 2052918, 2052919, 2052920, 2052921, 2052922 and
2052923 contain the supplementary crystallographic data for
this paper. These data are provided free of charge by the joint
Cambridge Crystallographic Data Centre and Fachinforma-
tionszentrum Karlsruhe Access Structures service.
Manuscript received: March 9, 2021
Revised manuscript received: April 1, 2021
Accepted manuscript online: April 7, 2021
Version of record online: && &&, &&&&
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5
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Communications
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Communications
Main Group Complexes
A. Hermannsdorfer,
M. Driess*
&&&& — &&&&
Silicon
Tetrakis(trifluoromethanesulfonate): A
Simple Neutral Silane Acting as a Soft
and Hard Lewis Superacid
The title compound is the first neutral
silane acting as a hard and soft Lewis
superacid as exemplified by extensive
scaling. Its distinct reactivity towards soft
Lewis bases L leads to unprecedented
[L2Si(OTf)4] complexes, while the reac-
tions with halocarbons afford electro-
philic cations, including a straight access
to benzoylium cations from mono- and
bis(trifluoromethyl)benzenes, stabilized
by the [Si(OTf)6]2ꢀ weakly coordinating
dianion.
6
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ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 1 – 6
These are not the final page numbers!