Isolable Silicon-Based Polycations with Lewis Superacidity

Article abstract of DOI:10.1002/anie.202011696

Molecular silicon polycations of the types R₂Si²⁺ and RSi³⁺ (R=H, organic groups) are elusive Lewis superacids and currently unknown in the condensed phase. Here, we report the synthesis of a series of isolable terpyridine-stabilized R₂Si²⁺ and RSi³⁺ complexes, [R₂Si(terpy)]²⁺ (R=Ph 1²⁺; R₂=C₁₂H₈ 2²⁺, (CH₂)₃ 3²⁺) and [RSi(terpy)]³⁺ (R=Ph 4³⁺, cyclohexyl 5³⁺, m-xylyl 6³⁺), in form of their triflate salts. The stabilization of the latter is achieved through higher coordination and to the expense of reduced fluoride-ion affinities, but a significant level of Lewis superacidity is nonetheless retained as verified by theory and experiment. The complexes activate C(sp³)?F bonds, as showcased by stoichiometric fluoride abstraction from 1-fluoroadamantane (AdF) and the catalytic hydrodefluorination of AdF. The formation of the crystalline adducts [2(F)]⁺ and [5(H)]²⁺ documents in particular the high reactivity towards fluoride and hydride donors.

Full text of DOI:10.1002/anie.202011696

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Accepted Article
Title: Isolable Silicon-Based Polycations with Lewis Superacidity
Authors: Matthias Driess and André Hermannsdorfer
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10.1002/anie.202011696
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Isolable Silicon-Based Polycations with Lewis Superacidity
André Hermannsdorfer[a] and Matthias Driess*[a]
Dedicated to Professor Helmut Schwarz
[a]
M. Sc. A. Hermannsdorfer, Prof. Dr. M. Driess.
Department of Chemistry: Metalorganics and Inorganic Materials
Technische Universität Berlin
Strasse des 17. Juni 115, Sekr. C2, 10623 Berlin (Germany)
E-mail: matthias.driess@tu-berlin.de
Supporting information for this article is given via a link at the end of the document.
Abstract: Molecular silicon polycations of the types R2Si2+ and RSi3+
(R = H, organic groups) are elusive Lewis superacids and currently
unknown in the condensed phase. Here, we report the synthesis of a
series of isolable terpyridine-stabilized R2Si2+ and RSi3+ complexes,
[R2Si(terpy)]2+ (R = Ph 12+; R2 = C12H8 22+, (CH2)3 32+) and
[RSi(terpy)]3+ (R = Ph 43+, cyclohexyl 53+, m-xylyl 63+), in form of their
triflate salts. The stabilization of the latter is achieved through higher
coordination and to the expense of reduced fluoride ion affinities, but
a significant level of Lewis superacidity is nonetheless retained as
verified by theory and experiment. The complexes activate C(sp3)–F
bonds, as showcased by stoichiometric fluoride abstraction from 1-
fluoroadamantane (AdF) and the catalytic hydrodefluorination of AdF.
The formation of the crystalline adducts [2(F)]+ and [5(H)]2+ documents
in particular the high reactivity towards fluoride and hydride donors.
Figure 1. Previously studied silicon-based monocations and polycations (L =
terpyridine; this work) with their fluoride ion affinities (FIA [kJ mol-1]).
Main group Lewis acids (LA) have garnered increasing attention
over the last two decades with the emerging concept of frustrated
Lewis pairs contributing significantly to this development.[1,2,3]
With the need for potent LAs, the focus often lies on Lewis
superacids (LSA), that are defined as “molecular LAs, which are
stronger than monomeric SbF5 in the gas phase”, according to
Krossing.[4] Recent reports on Lewis superacidic perhalogenated
bis(catecholato)germanes[5] and -silanes[6] emphasize the great
potential of molecular main group LAs, with silicon-based LAs
being particularly attractive due to the high abundance and low
toxicity of this element. Furthermore, silylium ions, R3Si+, are well-
established as valuable catalysts in organic synthesis[7,8] and
exhibit high fluoride ion affinities (FIAs), which categorize them as
LSAs.[3,9] Expectedly, molecular silicon(IV) dications of the type
Their Lewis superacidity is confirmed theoretically (FIA > SbF5)
and experimentally (F- abstraction from SbF6 ). Further reactivity
-
investigations revealed that they are one of the strongest silicon-
based LAs known to date.
The Lewis acidity of dications R2Si2+ and their terpy complexes
[R2Si(terpy)]2+ was assessed by determining isodesmic FIAs
using the TMS-reference system[14] (Table 1, for computational
details see Supporting Information). Expectedly, dicationic R2Si2+
(FIA > 1450 kJ mol-1) are stronger LAs than R3Si+ (700 – 950 kJ
mol-1)[3,15]. Notably, through terpy-coordination, the FIAs of
[R2Si(terpy)]2+ are about 35 – 40 % lower than those of “free”
R2Si2+, but still well above the SbF5 threshold of 500 kJ mol-1. An
estimation of solvation correction in CH2Cl2 was included (FIAsolv
)
using the C-PCM solvation model,[16] as gas phase FIAs of cations
often contain strong contributions of charge neutralization and
electrostatic attraction.[3,17] This resulted in a pronounced FIA-
damping for R2Si2+ and [R2Si(terpy)]2+. The reduced positive
charge around the Si centers in the terpy-ligated R2Si2+ moieties
can be visualized by mapping of the electrostatic potential on a
0.025 e– Bohr–3 isodensity surface (Figure 2). Areas with blue
color-coding represent sections which are more positively
charged, whereas the delocalization of the positive charges in
[R2Si(terpy)]2+ results in areas with lower potential (red color-
coding = less positively charged), notably also around the Si
centers.
R2Si2+ represent stronger LAs (e.g., FIA(SiF22+) = 2167 kJ mol-
1
)
[10], but such species are unknown in the condensed phase;
merely SiH22+ and SiF22+ were observed in mass spectrometry.[11]
The only experimental evidence for a RSi3+ species reported to
date is about SiH3+ generated from Si3+ and H in ion beams.[12]
Further stabilization is required in order to make such gas-phase
molecules accessible in the condensed phase. In this regard, the
formation of complexes with neutral Lewis bases represents a
viable approach towards isolable silicon-based polycations (cf.
Figure 1). However, increased stability comes at the cost of lower
electrophilicity and Lewis acidity. Accordingly, numerous di- and
tetracationic silicon complexes with hexacoordinated SiIV atoms
are even water stable.[13]
In contrast to the reported synthesis of the [PhP(terpy)]2+
phosphorus dication from PhPCl2,[18] attempts to obtain
analogous cations directly from dichlorosilanes failed. However,
treatment of the corresponding R2Si(OTf)2 ditriflates (R2 = Ph2,
(CH2)3, C12H8) with terpy afforded the stable complex salts 1[OTf]2,
2[OTf]2 and 3[OTf]2 in 73 – 93 % yields (Scheme 1), which were
Here, we demonstrate that silicon-based polycations retain Lewis
superacidic features upon stabilization with the terpyridine ligand
(terpy) and in the presence of the OTf (CF3SO3; triflate) counter
anion.
1
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Table 1. DFT-derived fluoride ion affinities (FIA) and solvent corrected FIAs
In order to avoid ligand scrambling and other side-reactions, the
compounds were exposed to 1-fluoroadamantane (AdF) as a F-
source. Furthermore, C(sp3)–F bond activation is a crucial step
for catalytic hydrodefluorination reactions (HDF) and silylium ions
readily abstract F- from fluorocarbons.[2,7,20] 19F NMR spectra of
the reactions of 12+, 22+ and 32+ with AdF showed complete
consumption of AdF in less than 1 h. With 12+, the generation of
(FIAsolv) of the cations R2Si2+ and their terpyridine complexes in kJ mol-1.
Ph2Si2+
12+
(C12H8)Si2+
22+
(CH2)3Si2+
32+
FIA[a]
1459
673
934
263
1541
767
982
306
1622
691
1000
296
[b]
FIAsolv
-
Ph2SiF2 was observed similarly to the results obtained with SbF6 .
[a] B3LYP-D3BJ/def2svp//PW6B95-D3BJ/def2qzvpp [b] B3LYP-D3BJ/def2svp,
gas phase geometries with C-PCM solvation model in CH2Cl2.
For 22+, SiF HMQC measurements showed the clean formation of
1
[2(F)]+ (δ29Si –135.9 ppm and δ19F –112.5 ppm, JSiF = 224 Hz),
which was isolated in 42 % yield. SC-XRD analysis revealed a
hexacoordinate Si center with a Si–F bond length of 1.6693(14) Å
(Figure 4). The same species was detected in the reaction mixture
of 2[OTf]2 and [PPh4][SbF6]. Exposure of 3[OTf]2 to AdF gave a
complex mixture of products. Most likely, the strained
silacyclobutane fragment is not stable under the reaction
conditions. In fact, a ring-cleavage reaction was observed for the
reaction of 1,1-difluorosilacyclobutane with KF.[21]
Figure 2. Projection of the calculated electrostatic potential (B3LYP-
D3BJ/def2svp) onto the isodensity surface (0.025 e– Bohr–3) of the dications
R2Si2+ (top) and their terpyridine complexes [R2Si(terpy)]2+ (bottom).
fully characterized by NMR, IR, elemental analysis and SC-XRD.
The Si centers are penta- (32+) or hexacoordinated (12+ and 22+)
in the solid state (Figure 3), but 1H, 13C and 19F NMR spectra imply
that the Si–OTf interactions are labile in solutions.
Figure 4. Reaction of [R2Si(terpy)][OTf]2 with [PPh4][SbF6] or 1-
fluoroadamantane (AdF) and the molecular structure of [2(F)]+ (thermal
ellipsoids at 50% probability; Si-F 1.6693(14) Å).
The Lewis superacidity of 1[OTf]2, 2[OTf]2, and 3[OTf]2 was
confirmed by reactions with [PPh4][SbF6] in CD3CN. In all cases,
-
Compared to R2Si2+, the determined FIAs for RSi3+ suggest a
tremendous increase in Lewis acidity and reach almost 2000 kJ
mol-1 (cf. Table 2). The transition to the four-coordinate
[RSi(terpy)]3+ results in FIAs decreased by only ca. 20 % (> 1500
kJ mol-1), which lie above the FIA of the two-coordinate borenium
cation [(catecholato)B]+ (1210 kJ mol-1).[3,22] Solvation damping is
pronounced for the triply charged species but the obtained FIAsolv
for 43+, 53+ and 63+ surpass the FIAsolv of SbF5 by more than 200
kJ mol-1. Electrostatic potential maps illustrate the “smearing” of
the positive charge over the larger complex cations when
comparing RSi3+ and [RSi(terpy)]3+ (Figure 5). However, regions
with high potential remain around the silicon centers also for
[RSi(terpy)]3+ (blue color-coding), which is in agreement with the
high FIAs despite the additional coordination.
fluoride abstraction from SbF6 leads to the formation of ‘SiF’
species as observed by 19F NMR (Figure 4). For 12+, the formation
1
of Ph2SiF2 (δ19F –142.9 ppm, JSiF = 291 Hz)[19] was observed
after 20 h, while the product mixtures resulting from 22+ and 32+
could not be identified unambiguously.
R = Ph
RR = C12H8
RR = (CH2 3
1
2
3
R
[OTf]2
[OTf]2
[OTf]2
N
RRSi
(OTf)2
N
N
R
N
Si
N
)
N
[OTf]
OTf
Scheme 1. Synthesis of the triflate salts of the dications 12+, 22+ and 32+
.
The stable 4[OTf]3, 5[OTf]3 and 6[OTf]3 triflate salts were obtained
in 83 – 91 % yields from RSi(OTf)3 precursors (Scheme 2) with no
sign of decomposition upon storage over >6 months under inert
atmosphere. X-Ray structure analyses of 4[OTf]3 and 6[OTf]3
indicate that the anions provide further stabilization through
interaction with the silicon centers (Figure 6A). NMR data show
the dynamic exchange of coordinating and “free” triflate anions for
5[OTf]3 and 6[OTf]3 in solutions, whereas two singlets are
observable for Si-OTf and OTf- in the 19F NMR spectrum of 4[OTf]3
in CD2Cl2.
Figure 3. Molecular structures of 12+, [2(OTf)]+ and [3(OTf)]+ (from left to right;
thermal ellipsoids at 50% probability; H atoms omitted for clarity and terpy ligand
reduced to wireframe).
2
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Table 2. DFT-derived fluoride ion affinities (FIA) and solvent corrected FIAs
Further Lewis acidity scaling with the Gutmann-Beckett (GB)
method, that relies on the induced 31P NMR shift Δ31P of OPEt3
upon coordination to a LA,[23] gave the lowest Δ31P for 12+∙OPEt3
(23.5 ppm, cf. Figure S26). This is in agreement with the lower
FIA compared to the strained silacyclobutane and silafluorene
derivatives 22+ and 32+ (Δ31P = 28.6, 27.7 ppm), which exceed the
reported Δ31P for B(C6F5)3∙OPEt3 in CH2Cl2 (26 ppm).[24] X-Ray
structure analyses confirmed the formation of octahedral
[2(OPEt3)]2+ and [3(OPEt3]2+ (Figures S101 and S102). With
4[OTf]3 and 5[OTf]3, mixtures of bis-adducts and other
unidentified species formed (Δ31P ranging from 35.5 to 48.9 ppm),
which hampered a meaningful scaling with the GB method (see
Supporting Information). Remarkably, the well-defined mono-
adduct [6(OPEt3)]3+ formed from an equimolar mixture of 6[OTf]3
and OPEt3. NMR spectroscopy (δ29Si –92.0 ppm) and SC-XRD
(Figure 6B) confirmed the presence of a pentacoordinate Si
center. Strikingly, the resulting Δ31P (61.1 ppm) for [6(OPEt3)]3+
not only exceeds the Δ31P for OPEt3 adducts of silylium ions
(43.9±2.7 ppm for [R3Si(OPEt3)]+ (R= alkyl or aryl)[9,22,25] but also
for the borenium adduct [(catecholato)B(OPEt3)]+ (60.7 ppm).[9,22]
While comparisons between different classes of compounds have
to be considered with caution due to the large impact of steric and
HSAB effects,[26] the GB scaling nevertheless substantiates the
ranking of 63+ among the strongest silicon LAs known to date.
As a matter of fact, 5[OTf]3 and 6[OTf]3 react with triethylsilane
(Et3SiH) under hydride abstraction (Figure 7). Conclusively, HSi-
HMQC NMR measurements showed the formation of Et3SiOTf
(δ29Si 46 – 47 ppm)[27] in both cases. The conversion to
[5(H)][OTf]2 proceeded slowly at room temperature and was
confirmed by independent synthesis from CySi(H)(OTf)2 and terpy.
SC-XRD analysis confirmed the connectivity of the
pentacoordinate dication. Contrastingly, compound 6[OTf]3 was
consumed within hours in presence of Et3SiH with clear NMR
spectroscopic evidence for the activation of the para-carbon of the
terpy ligand (Fig. 7, δ29Si –94.8 ppm, see Supporting Information).
Recently, a similar carbon-centered Lewis acidity was reported for
a series of N-heterocycle-ligated borocations[28] and for terpy-
stabilized P(V) dications.[29]
(FIAsolv) of cations RSi3+ and their terpyridine complexes in kJ mol-1.
PhSi3+
43+
CySi3+
53+
m-XylSi3+
63+
FIA[a]
1951
764
1522
563
---[c]
---
1542
566
1890
767
1519
564
[b]
FIAsolv
[a] B3LYP-D3BJ/def2svp//PW6B95-D3BJ/def2qzvpp [b] B3LYP-D3BJ/def2svp,
gas phase geometries with C-PCM solvation model in CH2Cl2. [c] No global
minimum was found for CySi3+
.
Figure 5. Projection of the calculated electrostatic potential (B3LYP-
D3BJ/def2svp) onto the isodensity surface (0.025 e– Bohr–3) of trications RSi3+
(top) and their terpy complexes [RSi(terpy)]3+ (bottom). No minimum geometry
was found for CySi3+
.
The Lewis superacidity of 4[OTf]3, 5[OTf]3 and 6[OTf]3 was
-
1
confirmed experimentally by fluoride abstraction from SbF6 . H
NMR measurements showed the consumption of the starting
material promptly after mixing the respective complex salt with
[PPh4][SbF6] in CD3CN. Furthermore, 19F and FSi-HMQC NMR
measurements gave evidence for the formation of several species
with a SiF bond. F- abstraction from AdF gave cleaner reactions
with two unidentified major products from 4[OTf]3 and 6[OTf]3,
respectively, and
a single product from 5[OTf]3 (probably
[5(F)][OTf]2, see Supporting Information). The corresponding SiF
species were also observed in the respective reactions with
[PPh4][SbF6].
OTf
R = Ph
R = Cy
R = m-Xyl
4
5
6
[OTf]3
[OTf]3
[OTf]3
N
RSi
(OTf)3
N
N
R
Si
N
N
N
[OTf]
OTf
Scheme 2. Synthesis of the triflate salts of trications 43+, 53+ and 63+
.
Figure 7. Reactivity of 5[OTf]3 and 6[OTf]3 towards Et3SiH with depiction of the
molecular structure of [5(H)]2+
.
Furthermore, the catalytic cycle for the hydrosilylation of pyridines
to 1-silyl-4-hydropyridines is supposed to involve hydride transfer
to N-silyl-pyridinium cations.[30] We attribute the activation of the
terpy-backbone to the lower spatial accessibility of the silicon
center in 63+.
Figure 6. Molecular structures of [4(OTf)2]+, [6(OTf)]2+ (A) and [6(OPEt3)]3+ (B)
(from left to right; thermal ellipsoids at 50% probability; H atoms are omitted for
clarity and terpy ligand reduced to wireframe).
3
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COMMUNICATION
2019, 10, 7379–7388; c) R. Maskey, M. Schädler, C. Legler, L. Greb,
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Finally, the applicability of the silicon cations in the catalytic HDF
of AdF with Et3SiH was demonstrated with a catalyst loading of
10 mol% (Table 3). The low respective yields of Et3SiF may be
attributed to the formation of stable fluoride adducts and
fluorosilanes (vide supra). The catalytic efficiencies correlate well
to the scaled Lewis acidities (12+ < 22+, 32+ < 43+, 53+) with
exception of 6[OTf]3, where the concurring side-reaction with
Et3SiH probably leads to the deactivation of the catalyst.
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Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (Cluster of
Excellence UniSysCat) for financial support. A.H. thanks the
Einstein Stiftung Berlin for a PhD fellowship.
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Keywords: Coordination compounds • Hydride transfer •
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[31] CCDC 2023615 – 2023620, 2023625 and 2023626 (triflate salts of 12+
32+ 22+ [2(F)]+, 43+ 63+ [6(OPEt3)]3+ and [5(H)]2+
contain the
,
,
,
,
,
)
supplementary crystallographic data for this paper. These data are
provided free of charge by The Cambridge Crystallographic Data Centre.
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10.1002/anie.202011696
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
Down to earth with the right corset… Coordination of the terpyridine ligand to elusive gas-
phase molecules R2Si2+ and RSi3+ affords isolable polycationic complexes under retention of
Lewis superacidic features. Extensive scaling and reactivity studies reveal the highest Lewis
acidity of a silicon-based trication complex known to date for molecular Si-based cations.
5
This article is protected by copyright. All rights reserved.