Silyl Chalconium Ions: Synthesis, Structure and Application in Hydrodefluorination Reactions

Article abstract of DOI:10.1002/chem.201700995

The synthesis of two series of silylated chalconium borates, 9 and 10, which are based on the peri-naphthyl and peri-acenaphthyl framework, is reported (chalcogen (Ch): O, S, Se, Te). NMR investigations of the selenium- and tellurium-containing precursor silanes 3 d–f and 8 d, f revealed a significant through-space J-coupling between the chalcogen nuclei and the Me₂SiH group. Experimental and computational results typify the synthesized cations 9 and 10 as chalconium ions. The imposed ring strain weakens the Si?Ch linkage compared to acyclic chalconium ions. This attenuation of the Si?Ch bond strength is more pronounced in the acenaphthene series. Surprisingly, the Si?O bonds in oxonium ions 9 a and 10 a are the weakest Si?Ch linkage in both series. The synthesized silyl chalconium borates are active in hydrodefluorination reactions of alkyl fluorides with silanes. A cooperative activation of the silane by the Lewis acidic (silicon) and by the Lewis basic side (chalcogen) is suggested.

Full text of DOI:10.1002/chem.201700995

A Journal of
Accepted Article
Title: Silyl Chalconium Ions - Their Synthesis, Structure and Application
in Hydrodefluorination Reactions
Authors: Thomas Müller, Kordts Natalie, Künzler Sandra, Saskia
Rathjen, Thorben Sieling, Henning Großekappenberg, and
Marc Schmidtmann
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Silyl Chalconium Ions – Their Synthesis, Structure and
Application in Hydrodefluorination Reactions
Natalie Kordts,[a,b] Sandra Künzler,[a,b] Saskia Rathjen,[a] Thorben Sieling,[a] Henning
Großekappenberg,[a] Marc Schmidtmann[a] and Thomas Müller*[a]
Abstract: The synthesis of two series of silylated chalconium borates,
9 and 10, that are based on the peri-naphthyl and peri-acenaphthyl
framework is reported (chalcogen (Ch): O, S, Se, Te). NMR
investigations of the selenium and tellurium containing precursor
silanes 3d – f and 8d, f revealed a significant through – space J –
coupling between the chalcogen nuclei and the Me2SiH group.
Experimental and computational results typifies the synthesized
cations 9 and 10 as chalconium ions. The imposed ring strain
weakens the Si – Ch linkage compared to acyclic chalconium ions.
This attenuation of the Si – Ch bond strength is more pronounced in
the acenaphthene series. Surprisingly, the Si – O bonds in oxonium
ions 9a and 10a are the weakest Si – Ch linkage in both series. The
synthesized silyl chalconium borates are active in hydrodefluorination
Figure 1. Silyl cationic compounds (LB Lewis basic group, Ch = O, S, Se, Te).
reactions of alkyl fluorides with silanes. A cooperative activation of the
silane by the Lewis acidic (silicon) and by the Lewis basic side
(chalcogen) is suggested.
Introduction
Silyl cations and in particular tricoordinated silylium ions I are
strong Lewis acids (LA) and as such they are relevant species for
organic synthesis and in catalytic reactions.[1-3] The prerequisite
for the beneficial use of silylium ions in these fields is however a
clear control over this Lewis acidity. Previous studies by the
Siegel group have shown that in cases in which an intramolecular
Lewis basic group (LB), as in tetra-coordinated cation II, pacifies
the Lewis acidity of the cationic silicon atom their structures,
spectroscopic properties, and reactivities are greatly determined
by the electron donating ability of the LB group.[4] As the control
over the Lewis acidity is accomplished by LA/LB interaction in a
spatially confined reaction compartment, the analogy to
Figure 2. Selected LA/LB pairs based on the naphthene and acenaphthene
framework.
Based on our previous experience with stabilized silyl cations,[7]
we utilized the 1,8-naphthalenediyl-, IV, and the 5,6-acenaph-
thenediyl, V, scaffold to study the intramolecular interaction
between the dimethylsilyl cation unit and a series of chalcogena
ether groups (Chalcogen (Ch) = O, S, Se, Te) (Figure 1).
Beginning with the landmark investigations by Alder and by Katz,
intramolecular frustrated Lewis Pairs (FLP) is obvious.[5]
A
possible amphiphilic reactivity of the stabilized silyl cation as
suggested by the equilibrium II / III shown in Figure 1 is a matter
of the strength of silyl cation LB interaction.
[8, 9]
many groups have shown that these naphthalene-derived
frameworks are ideal for the study of cooperative phenomena
between spatially confined groups. Recent systematic studies by
the groups of Gabbai, Woollins and Beckmann are here
particularly noteworthy.[10, 11] Several intramolecular naphthalene-
linked LA/LB systems including B/P, B/N, Si/N and P/P systems
have been studied, and some were tested in bond activation
chemistry (Figure 2).[12–14] Tamao and coworkers have used peri-
Ch / Si (Ch = O, S, Se) interactions to form silaylides.[15] The use
of chalcogena ether units as electron donating LB substituents in
cations IV and V seems to us very attractive, as we expected to
[a]
Dr. N. Kordts, M.Sc. S. Künzler, M.Sc. S. Rathjen, M.Sc. T. Sieling,
Dr. H. Großekappenberg, Ph.D. M. Schmidtmann, Prof. Dr. T. Müller
Institute of Chemistry
Carl von Ossietzky University Oldenburg
Carl von Ossietzky Str. 9-11, 26211 Oldenburg, Federal Republic of
Germany, European Union
E-mail: thomas.mueller@uni-oldenburg.de
Both authors contributed equally.
[b]
Supporting information for this article is given via a link at the end of
the document
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vary its electron donor capability with the chalcogen atom over a
wide range. The Oestreich and Landais groups have recently
disclosed that silyl cations, which are stabilized by intramolecular
interactions with remote ether or thioether substituents are stable
and are in some cases active Lewis acidic catalyst.[16, 17] In
addition, Jutzi and coworkers achieved the synthesis of penta-
coordinated siliconium ions using pincer type substituents with
flanking ether and thioether groups.[18] Based on these previous
results, we report here on the synthesis and the NMR
spectroscopic and structural characterization of a series of
silylated chalconium ions (chalcogen = O, S, Se, Te) based on the
1,8-naphthalene and 5,6-acenaphthene scaffolds. The influence
of the diverse donor atoms and the different carbon-backbones
on the bonding situation in these cations is discussed. First
experimental studies disclose their catalytic activity in HDF
reactions and indicate the amphiphilic reactivity of these silylated
chalconium ions.
Results and Discussion
The naphthyl phenyl ether 3a was prepared in 66% isolated yield
from dibromide 1 via naphthyl bromide 2 by an Ullmann-type
reaction and a subsequent metalation / salt metathesis sequence
using dimethylsilyl chloride as electrophile (Scheme 1).[19] Direct
peri-lithiation of 1-methoxynaphthalene 4 and reaction with di-
methylsilyl chloride gave 8-methoxysilylnaphthalene 3b in
moderate yields (57%, Scheme 1).[15a] The synthesis of 1,8-disub-
stituted naphthyl compounds 3c-f was accomplished by two sub-
sequent lithiation / salt metathesis reaction sequences starting
from dibromide 1. After installation of the dimethylsilyl group to
give 8-dimethylsilylnaphthylbromide 5, the aryl chalcogenyl group
was introduced by halogen metal exchange and reaction of the
lithiated intermediate with the dichalcogenyl compound in
moderate to good yields (66 – 99%) (Scheme 1).[20] In close
analogy, the acenaphthene derivatives 8 were prepared from
dibromide 6 either via the 6-phenoxy-acenaphtene-5-bromide or
via the 6-silyl-acenapthene-5-bromide 7 (Scheme 1). The com-
position and identity of silanes 3 and 8 was established by NMR
spectroscopy. In addition, X-ray diffraction analysis were
performed for selenylethers 3e and 8d. The most important NMR
parameter for these compounds are summarized in Table 1. The
investigated silanes 3, 8, are characterized by 1H NMR chemical
shifts for the silane hydrogen atom between 1H = 4.62 – 5.50 and
corresponding 29Si NMR chemical shifts between 29Si = -12.4
- -22.0. The collected 29Si NMR data show a noticeable trend for
the silicon resonance towards higher field with increasing atomic
weight of the remote chalcogen atom (see Table 1). The 77Se
NMR chemical shifts of compounds 3d,e and 8d (77Se = 304 –
397) are in the expected range for diarylselenides and also the
position of the 125Te NMR resonance of compounds 3f, 8f are
unremarkable. [21, 22]
.
Scheme 1. Synthesis of precursor silanes. a) 1 equiv. PhONa, Cu2O, diglyme,
160° C; b) 1 equiv. n-BuLi / THF / hexane, -78° C; c) 1 equiv. Me2SiHCl /
THF, -78° C; d) 1.1 equiv. t-BuLi, hexane, r.t.; e)
1 equiv. Me2SiHCl /
Et2O, -20° C; f) 1 equiv. n-BuLi / Et2O / hexane, -70° C; g) 1 equiv. Ar2Ch2,
Et2O, -70° C. h) 1 equiv. n-BuLi / Et2O / hexane, -70° C; i) 1 equiv. Ar2Ch2,
Et2O, -30° C.
Figure 3. Left: Part of the 499.9 MHz 1H NMR spectra of silylnaphthyltellane 3f
(Si - H region, 1H = 5.70 – 5.15 ppm, # unidentified by-product). Right: Part of
the 157.8 MHz 125Te NMR spectra of silylnaphthyltellane 3f (125Te = 693 – 690
ppm).
A quite unexpected feature in the NMR spectra of the selenium
and tellurium containing compounds 3d-f, 8d,f is the detection of
J couplings between the chalcogenyl atom and the hydrogen
atom of the silyl group. As an example, relevant parts of the H
NMR and of the 125Te NMR spectrum of compound 3f are shown
1
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in Figure 3. The J(TeH) coupling constant of 74 Hz is clearly
visible in both spectra. The detected chalcogen/hydrogen
coupling is largest for the tellurium compounds 3f and 8f (Table
1). In the series of selenium compounds it decreases from the
mesityl-substituted compound 3e (J(SeH) = 35 Hz) to the phenyl
selane 3d (J(SeH) = 18 Hz). The change from the naphthyl
skeleton (3d) to the acenaphthene backbone (8d) is associated
with a further reduction of the J(SeH) coupling constant (J(SeH)
(8d) = 10 Hz). The relative large size of the coupling constants
Figure 4. Left: Molecular structure of silylnaphthylselane 3e in the crystal
(thermal ellipsoids at 50% probability; only the Si-H hydrogen atom is shown)
pertinent bond lengths [pm], bond angles and dihedral angles [°]: Se – Si
322.86(4), Se – C1 193.22(9), Si – C8 189.71(9), Si – H 140(1), Se / H 276(1),
excludes
a through-bond coupling mechanism. Instead, it
suggests that the spin-spin interaction is transferred through
24]
space rather than along the carbon skeleton.[23,
For the
Se - C1 - C10 122.45(6), Si – C8 - C10 131.06(6), C1 - C10 – C8 126.05(6), Cipso
-
Se – C1 99.51(3). Right: Molecular structure of silylacenaphthylselane 8d in the
crystal. (thermal ellipsoids at 50% probability; only the Si-H hydrogen atom is
shown) pertinent bond lengths [pm], bond angles and dihedral angles [°]: Se –
Si 331.61(5), Se – C5 192.87(12), Si – C6 189.30(12), Si – H 137(2), Se / H
289(2), Se – C5 - C10 122.44(2), Si – C6 - C10 128.55(9), C6 - C10 – C5 129.56(1),
Cipso - Se – C1 99.80(6).
selenium compound 3e and for tellurium compounds 3f, 8f also
through space spin-spin couplings between the 77Se / 125Te and
the 29Si nuclei are detectable (J(SeSi) = 31 Hz (3e), J(TeSi) =
93 Hz (3f), 103 Hz (8f)). These data suggests through space
interaction between the Si-H group and the chalcogen atom.
Further investigations concerning its nature are currently under
investigation in our laboratories. Previously, the group of Woollins
has reported similar through space J-coupling between heavy
group 15 and 16 elements in peri-substituted naphthalene and
acenaphthene derivatives and quite recently Beckmann and
coworkers disclosed
a
related Si-HP interaction in
acenaphthene based compounds.[25, 26]
Single crystals suitable for X-ray diffraction (XRD) analysis have
been obtained for selanes 3e and 8d. Their overall molecular
structures are unremarkable (Figure 4). Not unexpected, the peri-
substitution in both compounds is accompanied with steric strain
which is manifested in their molecular structures by relative large
bond angles in the bay region and displacements of the
heteroatoms from the plane of the naphthalene/acenaphthene
rings.[27] In particular, for compound 3e the sum of the bay angles
(Se - C1 - C10; C1 - C10 - C8, C10 - C8 - Si)  is 379.6°, significant
more than in unstrained naphthalenes ( ~ 360°). The silicon
atom is placed 13 pm above the plane spanned by the ten carbon
atoms of the naphthalene ring, while the selenium atom is pushed
by 7 pm below this plane. These distortions lead to a distance
between the peri-atoms, selenium and silicon, of d(Se - Si) =
322.9 pm which is markedly larger than in unstrained
naphthalenes (244 pm). Similar distortions have been noticed for
compound 8d: ( ~ 380.6°; displacement d(Si) = 20.4 pm, d(Se)
= 25.6 pm, d (Se - Si) = 331.6 pm, compared to 270 pm for
unstrained acenaphthenes). The increased Se / Si separation
going from the naphthalene compound to the acenaphtene
system parallels the decreased through-space coupling between
the Si-H hydrogen atom and the selenium atom.
.
Scheme 2. Synthesis of silyl borates 9 and 10 (Ch = O, S, Se, Te; Ar = Ph, Mes).
a) 1 equiv. [Ph3C][B(C6F5)4], benzene, r.t..
Reaction of silanes 3a,c-f and 8 with trityl tetrakispentafluoro-
phenyl borate yielded the corresponding silyl borates 9a,c-f
[B(C6F5)4] and 10a,c,d,f [B(C6F5)4] in nearly quantitative yields
(Scheme 2).[28] At ambient conditions, solutions of the silyl borates
in aromatic hydrocarbons or halogenated aromatic hydrocarbons
are stable and once formed most of the borates are stable also in
methylene chloride solution. One exception is oxonium borate
10a[B(C6F5)4] which decomposes in methylene chloride at r.t.
within one day. All prepared silyl borates were fully characterized
by multinuclear NMR spectroscopy and pertinent NMR data are
summarized in Table 1. In all cases, the 29Si NMR chemical shift
was found to be invariable with the tested solvent, with nearly
identical 29Si NMR chemical shifts in benzene and in toluene. The
effected ionization is apparent by the significant low field chemical
shift of the 29Si NMR resonance of the product. The low field shift
is largest for the oxygen substituted compounds 9a[B(C6F5)4]
( 29Si = 71.4) and 10a[B(C6F5)4] ( 29Si = 77.5). For the
naphthalene and for the acenaphthene systems, the  29Si NMR
chemical shift decreases along the series of sulfur (9c, 10c),
selenium (9d,e, 10d) and tellurium (9f, 10f) substituted silyl
cations (Table 1). The smallest 29Si NMR chemical shift was
detected for the tellurium containing compound 9f[B(C6F5)4] ( 29Si
= 45.4). The change from the naphthalene to the acenaphthene
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system is associated with a low field shift of the 29Si NMR
resonance of  29Si = 6.1 – 8.4. The analysis of the 29Si NMR
chemical shift data reveals the expected trend that the 29Si NMR
chemical shift decreases with the size and polarizability of the
chalcogen atom. It is largest for the oxygen donor and smallest
for the tellurium substituent. The observed deshielding for the
acenaphthene based cations 10 indicates a slightly smaller
interaction between the chalcogen and the silicon atom.
the tellurium-substituted cations, 9f, two-dimensional exchange
spectroscopy (EXSY) at T = 90°C indicated magnetization
transfer between the two diastereomeric methyl groups due to a
slow exchange process. Similar results were obtained for borates
10d,f[B(C6F5)4] (see Figures S68, S73). As underlying process for
the magnetization transfer between the methyl groups, we
suggest the inversion of the configuration at the chalcogen atom,
which is slow on the NMR time scale at room temperature.
(Scheme 3).
The significant high field shift of the 77Se NMR resonance in silyl
cations 9d,e and 10d compared to the respective precursor
silanes ( 77Se (9d/3d) = -149;  77Se (9e/3e) = -86;  77Se
(10d/8d) = -123) gives a clear indication for the increase of the
coordination number at the selenium atom due to interaction with
the positively charged silicon atom. As expected this effect is even
more pronounced for the tellurium substituted silyl cations 9f and
10f ( 125Te = -411 (9f/3f), -210 (10f/8f)). The detection of J
coupling constants of considerable size between the silicon atom
and the chalcogen atoms selenium and tellurium in cations 9d,e,f
and 10d, f provides additional strong evidence for a direct bonding
1
interaction. The observed J(SiSe) coupling constants in cations
9d,e and 10d fall in a close range of 1J(SiSe) = 58 – 66 Hz, with
the smallest coupling constant detected for the acenaphthene
cation 10d. These values are however significantly smaller than
reported for silylselenoethers with dicoordinated selenium atoms
.
such as trimethylsilylphenylselane (1J(SeSi)
=
107 Hz).[29]
Scheme 3. Planar coordination of the oxygen atom in cations 9a, 10a and
inversion of the configuration of the chalcogen atom in chalcogenyl substituted
silyl cations 9 and 10 (Ch = S, Se, Te; Ar = Ph, Mes).
1
Similarly, the detected J(SiTe) constants in cations 9f and 10f
are substantial (1J(SiTe) = 161 Hz (9f), 169 Hz (10f)) but are small
compared to that of model compounds with dicoordinated
tellurium atoms such as bis-trimethylsilyltelluride (1J(SiTe) = 278
1
Hz).[30] Finally, also the non-equivalence of the H NMR and 13C
At this point, we note that the aryloxy-substituted cations 9a and
10a showed only one signal for both methyl groups in the
temperature range from T = -90°C – 30°C (see Figure S74). This
suggests that either the above described inversion process is fast
on the NMR time scale even at T = -90°C or that cations 9a, 10a
adopt a ground state conformation of higher symmetry than its
heavier congeners 9c-f, 10c,d,f.
NMR signals for the methyl groups at the silicon atom of the sulfur,
selenium and tellurium containing cations 9c-f, 10c,d,f indicated
the formation of a direct linkage between the chalcogen and the
silicon atom. In these cases, the pyramidalization of the
chalcogen atom implicates a syn/anti relation of the aryl-
substituent at the chalcogen atom relative to the methyl groups at
the silicon atom (Scheme 3). NOE spectroscopy indicated that the
1H and 13C NMR signals for the methyl group syn to the aryl
substituent appear in cations 9c-f, 10c,d,f at higher field.
Interestingly, we noticed only for the 13C NMR signal of the anti-
methyl group in the selenium, 9d,e, 10d, and tellurium substituted
Single crystals suitable for X-ray diffraction analysis were
obtained from a dichloromethane solution of 9a[HCB11H5Br6] and
from a benzene solution of 9d[B(C6F5)4]. The selenium substituted
silyl borate 9d[B(C6F5)4] crystallizes in the triclinic space group P-
1 as benzene solvate. Cation and anion are well separated with
the shortest silicon fluorine distance well above the sum of the
van der Waals radii, vdW (Si / F = 377 pm vs. vdW(Si / F) =
357 pm).[31] The closest contact between selenium and fluorine is
333 pm, close to the vdW(Se/F) = 337 pm.[31] The molecular
structure of cation 9d (Figure 5) reveals the expected short Si Se
separation (Se – Si = 240.6 pm), which is only slightly longer than
a regular Si – Se single bond (232 pm).[32] Consequently, the
silicon atom is tetra-coordinated. The sum of the bond angles
between the silicon atom and its carbon substituents, (Si), is
345.6° and indicates a significant trigonal flattening of the
tetrahedral coordination of the silicon atom. The coordination
environment of the selenium atom is trigonal pyramidal with a
mean bond angle around the selenium atom of 96.5° ((Se) =
289.5) and mean Se – C bond lengths of 192.8 pm. The attractive
Lewis acid / base interaction between the two atoms in peri-
position of the naphthalene moiety does not lead to significant
2
compounds 9f satellite signals corresponding to a J(CCh) spin
spin coupling (Ch = Se, Te, see Table 1)). While the presence of
these satellite signals provide further support for the direct
bonding relation between the silicon and the chalcogen atom, its
exclusive detection for the anti-methyl group suggests that it is a
valuable tool for conformational analysis of selenium and tellurium
ring compounds. In the case of the mesityl substituted cation 9e
broad signals for the ortho-methyl groups and meta-hydrogen
atoms are detected even at room temperature indicating hindered
rotation around the C(Mesityl) – Se bond. The 1H NMR signals of
the diastereotopic methyl groups at the silicon atom in cations 9c-
f, 10d,f show no significant kinetic line broadening ( < 3Hz) up
to temperatures of T = 70°C. In contrast, for the sulfur containing
cation 10c coalescence between both signals is detected at T =
105°C in toluene-d8, which corresponds to an approximate free
energy barrier of Gǂ(378K) = 74 kJ mol-1 (see Figure S74). For
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strain in cation 9d.[27] The sum of the bay angles  is 357.3°,
close to the ideal value of 360°. The silicon atom is placed 15 pm
above the plane of the naphthalene ring, while the selenium atom
is pushed by merely 2 pm below this plane. In summary, the
molecular structure of cation 9d shows all features of a
selenonium ion, similar to the classical Ph3Se+.[33] There are no
structural indications for increased strain imposed by the
formation of the new Si – Se bond. The crystal structure of the
carborate 9a[HCB11H5Br6] (monoclinic space group P1 21/c)
reveals separated cations and anions as well. The closest
contacts between the bromine substituents of the carborane cage
and silicon or oxygen atoms of the cation are larger than van der
Waals contacts (Si / Br = 404 pm; O / Br = 395 pm, vdW(Si/Br)
= 393 pm, vdW(O/Br) = 335 pm).[31] Despite the close chemical
relation between the cations 9a and 9d, the molecular structure of
the aryloxy substituted cation 9a is in several aspects distinctively
different from that of the selenium cation 9d (Figure 5). The Si / O
distance in cation 9a (183.8 pm) is by 20 pm longer than the mean
Si – O bond length for tetra-coordinated silicon atoms and di-
coordinated oxygen atoms (163 pm). It is also longer than the Si-
O bonds in silylated oxonium ions (Si – O 177.7 – 178.5 pm).[34 –
Figure 5. Left: Molecular structure of cation 9a in the crystal of 9a[HCB11H5Br6]
(thermal ellipsoids at 50% probability, hydrogen atoms are not shown) pertinent
bond lengths [pm], bond angles and dihedral angles [°]: O – Si 183.79(17), O –
C8 143.99(29), Si – C1 185.05(25), O – C8 - C10 109.920(191), Si – C1 - C10
109.294(168), C1 - C10 – C8 118.709(206), Cipso - O – C8 121.322(176), C1 - C8
– O - Si 1.660(122), Cipso - O – C8 – C10 -177.911(191). Right: Molecular
structure of cation 9d in the crystal of 9d[B(C6F5)4]. (thermal ellipsoids at 50%
probability; hydrogen atoms are not shown) pertinent bond lengths [pm], bond
angles and dihedral angles [°]: Se – Si 240.56(5), Se – C1 192.02(15), Si – C8
186.02(14), Se – C1 - C10 117.64(11), Si – C8 - C10 117.52(11), C8 - C10 – C1
122.13(13), Cipso - Se – C1 101.72(7), C8 – C1 – Se - Si 2.490(53), Cipso - Se –
C1 – C10 -96.00(13).
36]
Remarkable is the pronounced trigonal flattening of the tetra-
coordinated silicon atom. The measured (Si) is 351.3°, which
is well removed from tetrahedrality (ideal value (Si)= 328.2°).
The oxygen atom in cation 9a adopts a nearly perfect trigonal
planar coordination environment ((O)= 360°). This is in
agreement with the previously reported structural data for
silylated oxonium ions 11 – 13 (Figure 6)..[34 – 36] Furthermore, it
suggests that the appearance of only one signal for the methyl
1
groups in the H and 13C NMR spectra of 9a[B(C6F5)4] is not a
result of a fast equilibrium between syn/anti isomers, but is due to
the symmetric ground state structure with a trigonal planar
coordinated oxygen atom. Our finding contrasts the results of
Ducos et al. who proposed for the silylated oxonium ion 14 a
pyramidal structure based on NMR measurements.[17] In contrast
to the relaxed structure of the selenonium ion 9d, the molecular
structure of oxonium ion 9a indicates the consequences of the
attractive Lewis acid base interaction. To accomplish the short Si
– O peri-distance of 184 pm both atoms are forced nearly into one
plane with the naphthalene system, the silicon atom is placed 3.7
pm above and the oxygen atom 9.8 pm below the plane of the
naphthalene ring. More noticeable are however the acute bond
angles in the bay region of cation 9a ( = 337.9°, see Figure 5).
.
Figure 6. Selected structural parameter of silylated oxonium ions 11 - 13.
.
The reaction of methoxy-substituted naphthylsilane 3b with trityl
cation took a quite unexpected course. Reaction of trityl borate
with silane 3b at r.t. resulted in noticeable heat evolvement and in
gas evolution. NMR analysis revealed that only small amounts of
triphenylmethane were formed and most of the trityl borate was
still present in the reaction mixture. Although naphthylsilane 3b
was completely consumed, there was no indication for the
formation of the expected cyclic methoxonium ion 9b. Instead, the
cyclic siloxane 15 was formed in nearly quantitative yield as
indicated by NMR spectroscopy ( 29Si = 27.9) and GC/MS
analysis (m/z = 200) (see Supporting Information for further
analytic details). Siloxane 15 is formed in an autocatalytic reaction
with methoxonium ion 9b as the key intermediate (Scheme 4), a
process which can be regarded as an intramolecular variant of the
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Piers-Rubinsztajn reaction.[37, 38] Cation 9b is formed as expected
by the reaction of silane 3b with trityl cation. It reacts however with
the starting silane 3b with loss of methane and recovery of cation
9b. Trityl cation serves in this reaction sequence only as initiator.
Additional runs with only 0.01 and 0.03 equivalents of trityl borate
confirmed this. In both cases, siloxane 15 was isolated in
quantitative yields. Methane formation was confirmed by its 1H
NMR resonance at  1H = 0.20 in benzene-d6. Attempts to
synthesize methoxonium ion 9b at low temperatures under
careful controlled conditions avoiding excess of silane 3b gave
strong indications for the formation of cation 9b (in CD2Cl2
at -70°C:  29Si = 70, 1H(OMe) = 4.64). The isolation and definite
characterization of the borate 9b[B(C6F5)4] was not possible due
to its beginning decomposition.
.
Scheme 4. Reaction of silane 3b with catalytic amounts of trityl cation. a) 0.01
- 0.03 equiv. [Ph3C][B(C6F5)4], benzene, r.t..
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Table 1. Selected NMR parameter of naphthylsilanes 3, 8 and silyl borates 9, 10
(C6D6, r.t. if not noted otherwise)
compd[a]

29Si
 1H(SiH) [b]
nJ(HX) [Hz]

77Se

125Te
 1H(Si(CH3)2)
0.55

13C(Si(CH3)2)
nJ(CX) [Hz]
nJ(SiX) [Hz]
nJ(SeX) [Hz]
nJ(TeX) [Hz]
3a
-12.4
1J(SiH) = 192
5.11
1J(HSi) = 192
-1.0
-0.9
0.8
1.0
0.8
3b
-12.7
1J(SiH) = 188
5.09
1J(HSi) = 188
0.52
3c
-17.1
1J(SiH) = 199
5.36
1J(HSi) = 199
0.60
3d
Ar = Ph
-18.6
1J(SiH) = 199
5.50
1J(HSi) = 199
397.1
J(SeH) = 18
0.60
3e
Ar = Mes
-17.9
1J(SiH) = 196
J(SiSe) = 31
5.75
1J(HSi) = 196
J(HSe) = 35
304.0
J(SeH) = 35
J(SeSi) = 31
0.68
3f
Ar = Ph
-22.0
1J(SiH) = 191
J(SiTe) = 93
5.43
1J(HSi) = 191
J(HTe) = 74
690.5
J(TeH) = 74
0.57
0.7
8a[b]
8c
-14.0
-17.7
4.62
0.40
0.63
0.64
0.69
-1.5
0.2
0.6
0.8
1J(HSi) = 191
5.39
1J(HSi) = 198
8d
Ar = Ph
-18.7
1J(SiH) = 198
5.53
1J(HSi) = 198
376.0
J(SeH) = 10
8f
-21.6
1J(SiH) = 188
J(SiTe) = 103
5.68
1J(HSi) = 188
J(HTe) = 82
425.9
J(TeH) = 82
J(TeSi) = 103
Ar = Mes
9a
9c
71.4
57.4
0.32
-1.3
-0.02 (syn);
0.41 (anti)
-3.7 (syn);
-1.0 (anti)
9d
Ar = Ph
55.7
1J(SiSe) = 60
55.7[a]
248.0
1J(SiSe) = 60
247.9[a]
0.03 (syn);
0.45 (anti)
-2.9 (syn)
-0.6 (anti)
2J(CSe) = 18
9e
Ar = Mes
48.7
1J(SiSe) = 66
218.1
1J(SiSe) = 66
0.26 (syn);
3J(HSe) = 7
0.49 (anti)
3J(HSe) = 8
-2.3 (syn)
-0.3 (anti)
2J(CSe) = 22
9f
45.4
1J(SiTe) = 161
280.0
1J(SeTe) = 161
0.11 (syn);
0.50 (anti)
3J(HTe) = 17
-1.9 (syn)
-0.2 (anti)
2J(CTe) = 38
10a[a]
10c
77.4
0.41
-0.6
77.5[a]
65.8
-0.01 (syn);
0.47 (anti)
-3.0 (syn);
-0.3 (anti)
10d
Ar = Ph
64.0
1J(SiSe) = 58
64.2[a]
253.4
1J(SeSi) = 58
253.4[a]
0.05 (syn);
0.51 (anti)]
-2.3 (syn)
-0.1 (anti)
2J(CSe) = 19
64.6[b]
259.2[b]
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10f
Ar = Mes
49.1
1J(SiTe) = 169
215.9
1J(SeTe) = 169
0.28 (syn);
0.57 (anti)
-0.1 (syn);
0.9 (anti)
[a] In C7D8 at r.t.. [b] In CD2Cl2 at r.t..
Table 2. Selected calculated structural parameter and 29Si NMR chemical shifts of silyl borates 9, 10
(at M06-2X/def2tzvp, NMR chemical shifts at GIAO/M06L/def2tzvp). Experimental data are given in parenthesis for comparison.
compd.
9a
Ch
O
Si - Ch
[pm]
Si)
[°]
Ch)
[°]

[°]

29Si

183.2a
351.6
359.5
337.7
65.0
(183.8)
(351.3)
(360.0)
(337.9)
(71.4)
9c
S
229.6 a
343.4
299.3
353.6
63
(57.4)
9d
Se
Te
O
242.3 a
(240.6)
347.1
(345.6)
289.2
(289.5)
357.3
(357.3)
58
(55.7)
9f
260.0 a
185.4
232.1
244.7 a
261.4
345.3
352.1
349.3
347.9
345.4
276.9
360.0
299.5
289.0
293.0
360.9
337.3
353.4
359.1
361.9
51
(45.4)
10a
10c
10d
10f
70
(77.5)
S
71
(65.8)
Se
Te
67
(64)
48
(49.1)
[a] Single bond lengths [pm] calculated from self-consistent covalent radii: Si – O 179; Si – S 219, Si – Se 232; Si – Te 252.[32]
[b] vdW [pm]: Si / O 362; Si / S 390; Si / Se 400; Si / Te 416.[31]
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Table 3. Calculated characteristic parameter for the Si – Ch linkage in cations 9, 10 and related compounds (at M06-2X/def2tzvp, BDE: bond dissociation energy;
WBI: Wiberg bond index, Charge: Charge according to the NBO analysis,  electron density at the bond critical point (bcp), 2: Laplacian of the electron density
at the bcp; H: total energy at the bcp).
compd.
Ch
O
Si - Ch
[pm]
BDE (Si - Ch)a
[kJ mol-1]
WBI
Charge(Si)
Charge(Ch)

[a.u.]
2
[a.u.]
H
[a.u.]
9a
183.2
185.4
166.9
182.4
136
104
0.36
0.36
0.58
0.36
1.93
1.92
1.94
1.95
-0.56
-0.56
-0.83
-0.59
0.085
0.081
0.128
0.086
+0.405
+0.369
+0.759
+0.409
-0.025
-0.024
-0.044
-0.026
10a
18a
19a
214
9c
S
229.6
232.1
216.0
229.7
188
144
0.64
0.63
0.87
0.63
1.64
1.64
1.56
1.64
0.47
0.47
-0.18
0.46
0.074
0.071
0.093
0.072
+0.057
+0.047
+0.102
+0.061
-0.039
-0.037
-0.054
-0.037
10c
18c
19c
215
9d
Se
Te
242.3
244.7
230.7
244.3
190
151
0.69
0.68
0.90
0.67
1.60
1.60
1.51
1.60
0.61
0.61
-0.09
0.60
0.072
0.068
0.085
0.067
-0.003
-0.006
+0.005
-0.002
-0.041
-0.037
-0.054
-0.037
10d
18d
19d
211
9f
260.0
261.4
251.8
263.3
191
149
0.74
0.75
0.93
0.73
1.52
1.52
1.44
1.52
0.87
0.86
0.07
0.88
0.069
0.068
0.080
0.065
-0.063
-0.069
-0.108
-0.064
-0.040
-0.040
-0.051
-0.037
10f
18f
19f
212
[a] Approximate bond energy of the Si – Ch according to isodesmic equations (Scheme 5).
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The interaction between the chalcogen and the silicon atom in
cations 9a,c-f and 10a,c,d,f was studied in detail using quantum
mechanical calculations at the DFT M06-2X/def-2tzvp level of
theory.[39, 40] This model chemistry provided calculated molecular
structures for cations 9a and 9d that are close to those deter-
mined in the solid state, i.e. the deviation of the computed Si – Ch
bond lengths from the experimentally determined is less than 1%
in both cases (see Table 2 for a comparison of pertinent structural
parameter). The Si – Ch distances in the naphthalene-based
cations 9 are all significantly smaller than the corresponding sum
of the van der Waals radii but they are larger by 11% (Ch = O) –
4% (Ch = Te) than calculated for the corresponding silylaryl
chalcogenides 18. They are almost identical to the predicted Si –
Ch bond lengths in acyclic diarylsilylchalconium ions 19 (see
Tables 2 and 3). The change from the naphthalene based cations
9 to the acenaphthene systems 10 is connected with a slight
increase of the Si – Ch bond lengths by approximatively 2 pm.
The molecular structures of sulfur, selenium and tellurium
stabilized cations 9c, d, f and 10c, d, f show only minor indi-
cations of steric stress.[27] For example, the sum of the bay angles,
, are close to 360° (Table 2). In contrast, the computed
molecular structures of the oxygen stabilized cations 9a and 10a
show very short Si – O distances and consequently small bond
angles in the bay region ( = 337.7° (9a), 337.3° (10a), Table 2).
Cations 9a and 10a differ also from their heavier homologues by
the planar coordination environment of the tricoordinated oxygen
atom ((O) = 360°) in agreement with the XRD results for
9a[HCB11H5Br6]. All heavier homologues adopt the trigonal
pyramidal coordination expected for a chalconium ion ((Ch) =
277 - 300°).
are shown in Scheme 5 are a good first approximation for the
strength of the Si-Ch linkage in cations 9 and 10.
.
Scheme 5. Isodesmic reactions for the assessment of the strength of the Si –
Ch bond in naphthalene and acenaphthene based silyl cations 9 and 10.
The calculated bond dissociation energies of the Ch – Si bond for
the acyclic chalconium ions 19 are nearly independent of the
chalcogen atom and fall in a narrow range between 211 (Ch = Se)
and 215 kJ mol-1 (Ch = S, Table 3). The BDEs predicted for the
naphthalene-based cations 9 are significantly smaller. The Si –
Ch bonds in cations 9c, d, f are weaker by 21 – 27 kJ mol-1. This
weakening of the Si – Ch is due to the formation of the five-
membered rings. Remarkably, the Si – O bond in 9a is weaker
than the Si – O linkage in oxoniumion 19a by even 78 kJ mol-1.
This is in agreement with the computed and observed structural
indications of strain in cation 9a. For the acenaphthene series 10,
an additional decrease of the BDE by 32 – 44 kJ mol-1 relative to
the corresponding naphthalene compound is computed, which
agrees with the longer calculated Si – Ch bonds for cations 10. At
this point, we notice that in the series of cations 9 and 10 the
oxonium ions 9a and 10a show the weakest Si – Ch interactions
(136 kJ mol-1 (9a), 104 kJ mol-1 (10a)) and that the Si – Te bond
in cation 9f is the strongest Si – Ch linkage in this series (191 kJ
mol-1 (9f)). In general, the differences between the calculated
bond energies for the three series of cations 9, 10 and 19 confirm
the trend inferred from the structural parameter.
The bond dissociation energy for the newly formed Si – Ch bond
is estimated using the isodesmic equations shown in Scheme 5
and the results are given in Table 3.[41] These isodesmic
equations are not ideal for two reasons: silanes 3, 8 are
destabilized compared to their isomers 16, 20 by the peri-
disubstitution. This leads to the prediction of too strong Si – Ch
bonds in cations 9 and 10 by the isodesmic equations. In addition,
the conformation of the SiMe2 units in cations 17, 21 relative to
the naphthalene / acenaphthene backbone allow significant
conjugation between the 3p(Si) orbital and the -system of the
arene unit, which is not possible in the chalcogenyl stabilized
cations 9, 10. This latter imbalance of the isodesmic equations
leads to the prediction of too weak Ch – Si bonds. These opposing
and in consequence cancelling effects in mind, we suggest that
the calculated reaction enthalpies of the isodesmic equations that
The comparison of the NBO results for the cations 9, 10 and 19
with that of the corresponding aryl-silyl chalcogenides 18 indicate
that for each chalcogen atom the Wiberg Bond Index (WBI) of the
Si – Ch bond decreases significantly upon formation of an onium
ion (by app 0.19 – 0.24, see Table 3).[42, 43] Notably, for a given
chalcogen atom the WBI of the Si – Ch bond is constant for all
three types of onium ions. A particularly low WBI is computed for
the oxonium ions 9a, 10a and 19a. In these three cations, the
charge at silicon and the charge difference to the oxygen atom is
the largest calculated in the whole series of chalcogenyl-stabilized
cations. Remarkably, the charge at the silicon atom in cations 9a,
10a and 19a is the same as calculated for the silyl ether 18a, only
the negative charge at the oxygen atom decreases upon
formation of the oxonium ions (Table 3). As expected the polarity
of the Si – Ch bond decreases markedly from the oxygen-
containing cations to the sulfur-substituted species and
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.
decreases further along the series of selenium and tellurium
containing compounds (Table 3). For each series of S-, Se- and
Te-substituted compounds the calculated charge at the silicon
atoms remains nearly constant upon onium ion formation and only
the positive charge at the chalcogen atom increases, as shown
by comparison between disubstituted chalcogenides 18 and
cations 9, 10, 19 (Table 3). This comparison of calculated charge
distributions in silylchalcogenides 18 and cations 9, 10, 19
suggest that the cations are best termed onium ions. The analysis
of the computed electron density of the cations 9, 10 and 19 in the
framework of Bader’s quantum theory of atoms in molecules
(QTAIM) reveals for all cations topological molecular graphs that
enclose bond paths between the chalcogen and the silicon
atoms.[44] 2D-Laplacian contour plots of the S-, Se- and Te-
substituted cations are very similar and Figure 6 provides the
relevant part of the Laplacian contour plot of selenonium cation
9d as a representative example. The direct comparison with that
of oxonium cation 9a reveals obvious differences in the Laplace
distribution. The bonding interaction between the oxygen and the
silicon atom in cation 9a is typified as being strongly polar
covalent by the properties at the bond critical point (bcp).[46] The
bcp is shifted to the electropositive silicon atom and it shows a
relative large electron density (bcp), a positive Laplacian
2(bcp) and a negative total energy density H(bcp) (see Table
3). The 1D-Laplacian profile along the Si – O bond path reveals
the expected pronounced valence shell charge concentration
(VSCC) close to the oxygen atom and a second weak VSCC in
the direction of the silicon atom (Figure 7). Both VSCCs belong to
the atomic basin of the oxygen atom. Similarly, the bcp of the Si
– Se linkage is shifted to the more electropositive silicon atom as
expected from the different electronegativities of silicon and
selenium (Figure 7). The bcp is located close to a nodal surface
of the Laplace distribution and both weak VSCCs along the bond
path have slightly negative Laplacian values. These properties
characterize the Si – Se bond as a covalent bond with only small
polarity.[46]
Figure 7. 1D profiles of the calculated Laplacian of the electron density, 2 (r),
along Si – Ch bond path of cations 9a (top) and 9d (bottom). Bond critical points
(bcp’s) are shown as circles.
The synthesized stabilized silyl cations are typical silyl Lewis
acids and can be used as catalyst in organic synthesis. To provide
an example, we concentrate here on one particular and prominent
feature of strong silyl Lewis acids, on the catalytic
hydrodefluorination (HDF) reaction.[47-49] In
a standardized
reaction, we used 5 mol% of the tetraaryl borates of cations 9a, c
- f as catalyst for the HDF reaction of decyl fluoride as shown in
Scheme 6. Moderate turn over numbers (TONs) between 13 and
124 indicate for all investigated cations a catalytic reaction (Table
4). In our hand, the sulfonium borate 9c[B(C6F5)4] is the most
efficient catalyst for our test reaction while the selonium (9d) and
oxonium (9a) borates show comparable TONs. A significant
smaller TON was measured for the tellonium borate 9a[B(C6F5)4]
(Table 4). Increase of the steric congestion at the chalcogen atom
decreases the TON, as shown by comparison of the catalytic
activity of the borates of selonium ions 9d and 9e. Exchange of
the anion to the more robust brominated closo-carboranate
[CB11H6Br6]- leads for the sulfonium ion 9c to a significant lower
activity. This is an unexpected result, in particular in view of
literature reports on the exquisite performance of silyl-closo-
borates in HDF reactions.[47b] We note however that the closo
.
Figure 6. 2D contour plots of the calculated Laplacian of the electron density,
2 (r), in the Si - C10 – Ch plane of cations 9a (left) and 9d (right). Relevant
parts of the molecular graphs of the cations are projected onto the respective
contour plot. Solid black lines show the bond paths, which follow the line of
maximum electron density between bonded atoms. The corresponding bond
critical points (bcp’s) are shown as green circles. Ring critical points are shown
as red circles. Red contours indicate regions of local charge accumulation
(2 (r) < 0); blue contours indicate regions of local charge depletion (2 (r) >
0).
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carborate 9c[CB11H6Br6] is hardly soluble in the solvent mixture o-
dichlorobenzene / Et3SiH.
Scheme 6. Hydrodefluorination reaction of decyl fluoride using borates
9[B(C6F5)4] as catalyst.
Table 4. TON of the HDF reaction of decyl fluoride using with different
tetraaryl borates as catalysts.
Anion[c]
TON
Scheme 7. Suggested cycle for the HDF reaction of primary alkyl fluorides
catalyzed by silylchalconium ions 9.
cation
Chalcogen
Ar
9a
9c
9c
9d
9e
9f
O
S
Ph
Ph
[B(C6F5)4]
[B(C6F5)4]
[CB11H6Br6]
[B(C6F5)4]
[B(C6F5)4]
[B(C6F5)4]
82
124
28
Conclusions
S
Ph
We report the synthesis and characterization of two series of
silylated chalconium borates, 9 and 10, that are based on the peri-
naphthyl and peri-acenaphthyl framework. Their synthesis was
accomplished using the standard Corey - hydride transfer
reaction with the corresponding silanes, 3 and 8 as starting
materials. NMR investigations of the selenium and tellurium
containing silanes 3d – f and 8d, f revealed a significant through–
space J – coupling between the chalcogen nuclei and the Me2SiH
group. Experimental (NMR spectroscopic and structural) and
computational (DFT-based bonding analysis) results typifies the
Se
Se
Te
Ph
109
52
Mes
Ph
13
Based on mechanistic investigations by the Oestreich group on
related HDF reactions using the cationic ruthenium complex 22
as catalyst,[50] we suggest the catalytic cycle shown in Scheme 7
for the HDF reaction of decyl fluoride. This mechanistic proposal
is in agreement with the observed different reactivity of the
chalconium ions 9a < 9c > 9d > 9f. It reflects the expected Lewis
basicity of the different chalcogen centers (O < S > Se > Te),
which is a prerequisite for the activation of the silane (step A,
Scheme 7). In addition, the absence of rearranged hydrocarbons
in the product mixture, suggests a concerted -bond metathesis
for the product formation (step B, Scheme 7). This mechanistic
proposal and other reactions that are mediated by cations 9 and
10 are currently subject of intensive experimental and
computational investigations in our laboratories.
synthesized cations
9 and 10 as chalconium ions, best
represented by the canonical structure B (Scheme 8). The
imposed ring strain weakens the Si – Ch linkage compared to
acyclic chalconium ions. This attenuation of the Si – Ch bond
strength is more pronounced in the acenaphthene series.
.
Scheme 8. Resonance structures of silylated chalconium ions 9 and 10.
The oxonium ions 9a and 10a differ from their heavier
homologues in having a trigonal planar coordination for the
oxygen atom instead the expected trigonal pyramidal
configuration (Scheme 3, Figure 5). In addition, the small size of
the oxygen atoms results in significant distorted molecular
structures of the silylated oxonium ions 9a and 10a. This imposed
strain weakens the Si – O bond and as a surprising result the Si
– O bonds in oxonium ions 9a and 10a are the weakest Si – Ch
linkage in both series. The heavier chalconium ions 9c - f and 10c,
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[6]
[7]
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Organometallics 2013, 32, 6736–6744; b) A. Schäfer, M. Reißmann, A.
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d, f (Ch = S, Se, Te) show the expected trigonal pyramidal
coordination environment for the chalcogen atom (Scheme 3). In
these cases, variable temperature NMR investigations and two
dimensional exchange spectroscopy (EXSY) indicate an
inversion process of the trigonal pyramidal coordinated chalcogen
atom, similar to related processes in isoelectronic phosphanes
and arsanes.
Although borates of the synthesized cations are stable
compounds in the absence of moisture and air, they are highly
reactive. They catalyze HDF reactions of alkyl fluorides with
silanes with TON between 13 and 124. For the activation process,
we suggest a cooperative activation of the silane by the Lewis-
basic chalcogen center and the Lewis acidic silyl group (Scheme
7).
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Acknowledgements
This
work
was
supported
by
the
Deutsche
Forschungsgemeinschaft (DFG-Mu1440/12-1) and by the Carl
von Ossietzky University Oldenburg. The simulations were
performed at the HPC Cluster HERO (High End Computing
Resource Oldenburg), located at the University of Oldenburg
(Germany) and funded by the DFG through its Major Research
Instrumentation Program (INST 184/108-1 FUGG) and the
Ministry of Science and Culture (MWK) of the Lower Saxony State.
Keywords: silicon • chalcogens • cations • NMR • X-ray
diffraction • bond activation
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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Oxygen
structural,
computerchemical characterization of
two series of silylated chalconium ions
Breaks
Ranks.
The
and
Natalie Kordts,[a,b] Sandra Künzler,[a,b]
Saskia Rathjen,[a] Thorben Sieling,[a]
Henning Großekappenberg,[a] Marc
Schmidtmann[a] and Thomas Müller*[a
spectroscopic
(Ch
=
O, S, Se, Te) revealed
Page No. – Page No.
significant differences between the
second row element oxygen and its
higher congeners.
Silyl Chalconium Ions – Their
Synthesis, Structure and Application
in Hydrodefluorination Reactions
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