Dibenzosilanorbornadienyl cations and their fragmentation into silyliumylidenes

Article abstract of DOI:10.1021/ja400306h

The terphenyl-substituted dibenzosilanorbornadienyl cation 11 was synthesized and isolated in the form of its [B(C₆F₅) ₄]^- salt. The salt was characterized by NMR spectroscopy supported by quantum mechanical computations and by an XRD analysis of a corresponding acetonitrilium salt. The thermal fragmentation of 11[B(C ₆F₅)₄] in benzene results in the high-yield formation of diphenylterphenylsilylium borate 17[B(C₆F ₅)₄]. High-lying intermediates in this process are solvent-complexed terphenylsilyliumylidene 8 and the hydrogen- and phenyl-substituted silylium ion 20. The formation of silylium ion 20 by reaction of silyliumylidene 8 with the solvent benzene demonstrates the high potential of this four valence electron species in C-H bond activation reactions. In addition, the instability of the hydrogen-substituted silylium ion 20 in benzene opens new mechanistic perspectives particular for dihydrogen activation by silyl cationic frustrated Lewis pairs and in general for the dihydrogen activation by strong Lewis acids.

Full text of DOI:10.1021/ja400306h

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Article
Dibenzosilanorbornadienyl cations and
their Fragmentation into Silyliumylidenes
Claudia Gerdes, Wolfgang Saak, Detlev Haase, and Thomas Mueller
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 20 Jun 2013
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Dibenzosilanorbornadienyl cations and their Fragmentation
into Silyliumylidenes
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Claudia Gerdes, Wolfgang Saak, Detlev Haase and Thomas Müller*
Institut für Chemie, Carl von Ossietzky Universität Oldenburg, Carl von Ossietzky-Strasse 9-11, D-26129 Oldenburg,
Federal Republic of Germany
Silicon, Silyl Cations, DFT Calculations, Bond Activation, NMR Spectroscopy
Supporting Information Placeholder
ABSTRACT: The terphenyl substituted dibenzosilanorbornadienyl cation 11 was synthesized and isolated in the form of
its [B(C6F5)4]- salt. The salt was characterized by NMR spectroscopy supported by quantum mechanical computations and
by a XRD analysis of a corresponding acetonitrilium salt. The thermal fragmentation of 11[B(C6F5)4] in benzene results in
the high yield formation of diphenyl-terphenyl silylium borate 17[B(C6F5)4]. High-lying intermediates in this process are
solvent-complexed terphenylsilyliumylidene 8 and the hydrogen-phenyl substituted silyliumion 20. The formation of
silylium ion 20 by reaction of silyliumylidene 8 with the solvent benzene demonstrates the high potential of this four
valence electron species in C-H-bond activation reactions. In addition, the instability of the hydrogen substituted silylium
ion 20 in benzene opens new mechanistic perspectives particular for dihydrogen activation by silylcationic frustrated
Lewis Pairs and in general for the dihydrogen activation by strong Lewis acids.
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Introduction.
The extreme Lewis acidity of silylium ions, R3Si+, silicon
cations with silicon in its oxidation state IV,1 recently found
beneficial applications in catalysis,2 in bond activation reac-
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tions,3 and in activation processes of small molecules.4,
Already in 2004, Gaspar pointed out that the combination of
the high Lewis acidity of silylium ions with the amphiphilic
character of non-stabilized silylenes as accomplished in the
shape of silyliumylidenes RSi:+, 1, creates silicon cations with
an extraordinary high reactivity and synthetic potential.6
Since then several groups reported the successful preparation
and isolation of stabilized silyliumylidenes, such as cations 2-
4.7-9 For the synthesis of each of these highly electron-
deficient species bulky and strongly electron donating sub-
stituents were applied, which also extent the coordination
number of the silicon cation. Some of these compounds
show already a distinct and highly interesting reactivity.7, 8
The strongly stabilizing substituents which are needed to
pacify the electron demand of the silicon atom however
significantly influence the chemical behaviour of the sili-
con(II) cation and mask their reactivity to a certain extent.
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Scheme 1. Synthesis of silyliumylidenes, 1, from silylium
ions 9, 10 by α-elimination.
Results and Discussion.
Dibenzo-7-silanorbornadienes 12 were prepared according
a procedure published by Tokitoh and coworkers starting
from Dichlorosilanes 13.13 The central step in this reaction
sequence involves regioselective deprotonation of 9-
silylanthracene 14 at carbon atom C10, followed by an intra-
molecular substitution reaction (Scheme 2). The regio-
selectivity of the C10-deprotonation increases with the steric
bulk of the applied base and of the substituent R at the sili-
con atom.13 Therefore, this route is particularly well suited
for the synthesis of dibenzo-7-silanorbornadienes with bulky
terphenyl substituents.14 Dibenzosilanorbornadienes 12 were
obtained in satisfying isolated yields (Scheme 2) and were
fully characterized by NMR spectroscopy and X-ray diffrac-
tion analysis of suitable crystals (see Figure 1, Table 1 and
Supplementary Material). The relatively low field shifted
resonances in the 29Si NMR spectra (δ29Si=31.6 (12a); 31.5
(12b)) are characteristic for dibenzo-7-silanorbornadienes. 14-
This prompted us to search for substituents R in silylium-
ylidenes, 1, which extend their lifetime in condensed phase
without spoiling their original reactivity. In this respect, the
1,3-diarylphenyl (terphenyl) substituents seemed to be a
good compromise between the requirements for thermody-
namic stabilization and steric protection on one hand and,
on the other hand, for preserved reactivity of the silicon(II)
cation.10 Isolobal neutral compounds of group 13 metals in
their oxidation state +I, 5, stabilized by terphenyl substitu-
ents exist10b-d and Power and coworkers reported on the
synthesis of the toluene complex of terphenyl substituted
mono-coordinated lead cation, 6.11 In line with these experi-
mental achievements, the results of quantum mechanical
computations suggested, that in the case of silicon the ther-
modynamic stability of terphenyl-substituted silyliumyli-
denes such as 7 and 8, approaches those of cations 2 and 3.12
The same theoretical study suggested that in general α-
elimination from silylium ions, 9, (Scheme 1) is a suitable
synthetic approach to silyliumylidenes. In particular, the
elimination of benzene from dibenzo-7-silanorbornadienyl
cations 10 is particularly well suited for the generation of ter-
phenyl substituted silyliumylidenes 7 or 8 (Scheme 1).12 Fol-
lowing these theoretical guidelines, we report here on the
From a structural point of view the long endocyclic Si-C1/4
16
bonds (d(SiC1/4) = 192.7 pm (12a); 192.4 pm (12b)) are notice-
able. Both features, the deshielded 29Si nuclei and the long
endocyclic SiC bonds are typical for the dibenzo-7-
silanorbornadiene subunit and they are indications for the
occurrence of σ-π*-conjugation in these compounds.14,15
Scheme 2. Synthesis of dibenzo-7-silanorbornadienes 12
from 9-dihydroanthracenylsilanes 14. (a: R= 2,6-bis-(2,4,6-tri-
methylphenyl)-phenyl (Ter); b: R=2,6-bis-(2,4,6-tri-iso-
propylphenyl)-phenyl
propylamide).
(Ter*),
LDA
Lithium-di-iso-
synthesis of
a
7-terphenylsubstituted-dibenzo-7-silanor-
bornadienyl cation 11 by hydride transfer from the corre-
sponding silane 12a and on the investigations of its thermal
decomposition, which provides evidence for the intermediate
formation of the terphenyl substituted silyliumylidene 8.
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Figure 2. a) 500 MHz H NMR spectrum of 11[B(C6F5)4] in
benzene-d6 (# : residual proton signal of the solvent). b) 100
MHz 29Si{1H} NMR spectra of 11[B(C6F5)4] in benzene-d6 at
room temperature (upper trace) and after heating for 2h to
70°C (lower trace).
Figure 1. Molecular structure of dibenzo-7-silanorborna-
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diene 12b in the crystal (thermal ellipsoid presentation
drawn at the 50% probability level. Only one of two inde-
pendent molecules in the crystal is shown). All hydrogen
atoms but the SiH hydrogen atom (red) are omitted (bond
lengths in pm, angles in °). Si-C(1) 192.04(14), Si-C(4)
192.74(16), Si-C(ter) 189.09(16), C(1)-C(2) 151.73(23), C(2)-C(3)
140.09(22), C(1)-Si-C(4) 81.44(7).
Addition of benzene to an equimolar mixture of trityl
tetrakispentafluorophenyl borate ([Ph3C][B(C6F5)4]) and
dibenzo-7-silanorbornadiene 12a results in a biphasic reac-
tion mixture, characteristic for benzene solutions of the
1
B(C6F5)4 salts. Monitoring of the reaction progress by H
NMR spectroscopy of the lighter non-polar phase indicates a
slow hydride transfer and selective formation of the ter-
phenylsubstituted 7-silanorbornadienyl borate 11[B(C6F5)4],
which was obtained as the sole product after 14 h at r.t.
(Scheme 3, Figure 2a). In contrast, the bulkier Ter* substitu-
ent of dibenzosilanorbornadiene 12b efficiently hampers the
hydride transfer reaction and its reaction with trityl cation
yields after 24h an intractable mixture of different com-
pounds as judged from 29Si NMR spectroscopy.
Figure 3. Molecular structure of nitrilium ion 16 in the
crystal of 16[B(C6F5)4]•2•C6F6. (thermal ellipsoid presentation
drawn at the 50% probability level). All hydrogen atoms are
omitted (bond lengths in pm, angles in °). Si-C(1) 190.28(44),
Si-C(4) 189.18(39), Si-C(ter) 185.89(40), C(1)-C(2) 152.49(75),
C(2)-C(3) 139.23(66), C(1)-Si-C(4) 82.840(176), Si-C(2)
254.52(50), Si-C(3) 253.85(57), Si-N 184.80(35).
The 7-silanorbornadienyl borate 11[B(C6F5)4] was character-
ized by multinuclear NMR spectroscopy (Table 1). The 29Si
NMR resonance of cation 11 is practically independent from
the solvent used (δ29Si= 1.3 (benzene-d6) (Figure 2b), 1.2 (tol-
uene-d8), 2.2 (chlorobenzene-d5)). This suggests only insig-
nificant solvent cation interactions and it discards the possi-
bility of the formation of an arene complex. The 29Si NMR
signal for cation 11 is, however, detected at an unusual low
frequency compared to triarylsilylium ions (δ29Si = 215-230)4
or even to the terphenyl substituted dimethylsilylium ion 15
and closely related cations (δ29Si = 59-80).17 The silyl cation 15
greatly benefits from intramolecular π-donation from the
flanking aryl groups, which is accompanied by a significant
shielding of the silicon atom. Similar intramolecular interac-
tions between the positively charged silicon atom and the
flanking aryl groups, and consequently comparable shielding
effects on the silicon nuclei, are expected to be operative in
cation 11 (Scheme 4). In addition homoconjugative effects
between the π-system of the benzonorbornadiene part of
cation 11 and the positively charged silicon atom add a se-
cond shielding influence on the 29Si NMR chemical shift.
This shielding effect, which accompanies homoconjugation is
well documented for 7-norbornadienyl cations18 and is pre-
dicted also for their silicon analogues.16 In summary, both
shielding effects account for the unusual high field shifted
29Si NMR resonance of the 7-silanorbornadienyl cation 11.
Scheme 3. Synthesis of 7-silanorbornadienyl cation 11 and
nitriliumion 16 .
a)
b)
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cant parameter very close to the experimental solid state
structure.19 Therefore, the isolation of nitrilium borate,
16[B(C6F5)4], provides further clear evidence for the molecu-
lar structure and the identity of cation 11.
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Upon heating of the biphasic benzene solution of borate
11[B(C6F5)4] to 70°C for 2h the complete degradation of cati-
on 11 was observed. Anthracene was detected in the non-
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polar phase by H NMR spectroscopy and GC/MS, while in
the ionic phase a new silicon containing compound was
formed in 70-80% yield (see Figure 2b). This new compound
was characterized by one single 29Si resonance at δ29Si = 52.7.
The presence of the intact [B(C6F4)]- anion was confirmed by
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19F NMR and 13C NMR spectroscopy. H and C NMR spectra
obtained from the reaction product at room temperature
indicated a symmetric terphenyl substituted species. In addi-
Scheme 4. Intramolecular interaction in terphenylsub-
stituted 7-silanorbornadienyl cation 11 and the degenerate
equilibrium between different equivalent conformers of
cation 11.
1
tion, a set of three signals in the H NMR spectra was detect-
ed (δ1H = 6.66, 6.91 and 7.14), which are typical for phenyl
protons.21, 22 These signals were not found when the thermal
fragmentation of cation 11 was performed in deuterated ben-
zene. In this case, the 29Si NMR chemical shift of the frag-
mentation product was δ29Si = 53.0. In addition, degradation
of cation 11 in toluene-d8 gave rise to a mixture of three iso-
meric silicon compounds as indicated by their very similar
29Si NMR chemical shifts of δ29Si = 56.8, 57.8 and 58.7. The
identity of the cationic species 17 formed by the thermal
degradation of cation 11 in benzene was finally established by
its conversion into a neutral compound by reaction with n-
Bu3SnH (Scheme 5). Addition of the tin hydride to the polar
phase again gave a biphasic reaction mixture. The ionic
product was identified as [n-Bu3Sn(C6D6)][B(C6F5)4] by the
119Sn NMR chemical shift of δ119Sn = 262 and its comparison
with literature data.23 The newly formed neutral product was
unequivocally characterized as silane 18 by NMR spectrosco-
1
13
The H and C NMR spectra obtained from 11[B(C6F5)4] are
notable in that respect that only one set of signals (δ1H = 1.72,
1.96, 6.57, Figure 2a) for the flanking mesityl groups of the
1
terphenyl substituent in the H NMR spectrum and only five
resonances of methin carbon atoms in the aromatic region of
the 13C NMR spectra indicate a highly symmetric structure
for cation 11 on the NMR time scale at room temperature.
This time-averaged symmetry of cation 11 is rationalized by a
degenerate equilibrium between equivalent conformers of
cation 11 (Scheme 4). A similar dynamic behaviour was estab-
lished for the strongly related silyl cation 15 and was put
forward as reason for the isochronicity of the mesityl substit-
uents in the NMR experiment.17 The computed molecular
structure of cation 11 (at M06-2X/6-311+G(d,p), see Figure
4a)19 suggests intramolecular π-donation from the flanking
mesityl group to the electron deficient silicon atom. Most
significant features are (i) the short distance between the
ortho carbon atom C(o) and the silicon atom d(SiC(o))
(d(SiC(o)) = 238.0 pm) which is markedly smaller than the
sum of the van der Waals radii (Σ(vdW) = 380 pm)20; and (ii)
the notable pyramidalization of the silicon atom as judged
from the sum of the bond angles around Si, Σα(Si), which
deviates significantly from 360° (Σα(Si) = 348°). GIAO 29Si
NMR chemical shift calculations predict for cation 11 in ben-
zene solution a 29Si NMR resonance at somewhat higher field
than observed experimentally (δ29Si(calc) = -14).19
In solution the salts 11[B(C6F5)4] and 11[HCB11H5Br6] are on-
ly of limited stability, which severely hampered all attempts
to grow crystals suitable for X-ray diffraction analysis. Previ-
ously, it was shown that the formation of complexes with
solvents of higher donicity increases the thermal stability of
7-silanorbornadienyl cations.16 Addition of acetonitrile to a
benzene solution of the borate 11[B(C6F5)4] results in the
formation of the nitrilium salt 16[B(C6F5)4] as indicated by
NMR spectroscopy (Scheme 3, Table 1). In particular, the 29Si
NMR chemical shift of δ29Si = 6.4 is very close to that of a
strongly related silanorbornadienylnitrilium borate reported
previously (δ29Si = 8.9).16 Moreover, the increased thermal
stability of the nitrilium borate allowed its crystallization
from hexafluorobenzene/pentane as its hexafluorobenzene
solvate. The small size of the needle-shaped crystals prevents
a structure determination to high accuracy, the overall mo-
lecular structure and the topology of cation 16 is however
secured (see Figure 3). In addition, DFT calculations at the
M06-2X/6-311+G(d,p) level of theory predict for nitrilium ion
16 a molecular structure (Figure 4b), which is in all signifi-
1
py (δ29Si = -22.4, δ1H(SiH) = 5.14, J(SiH) = 203 Hz) and mass
spectrometry (m/z = 496.3) and finally by its independent
synthesis (see Scheme 5 and Supporting Information). This
result clearly indicates that the cationic species formed by
thermal degradation of the 7-silanorbornadienyl cation 11 in
benzene is terphenyldiphenyl silylium ion 17. Similarly, in
toluene as solvent, a mixture of at least three tolyl-
substituted silyl cations 19 is formed (see Scheme 6). This
conclusion is suggested from 29Si NMR investigations of the
reaction mixture and from GC/MS analysis of the product
mixture after derivatization with n-Bu3SnH (see Supporting
Information). Final evidence for the formation of silylium
borate 17[B(C6F5)4] came from its independent synthesis
along the standard synthetic route by reaction of silane 18
with [Ph3C][B(C6F5)4] (see Scheme 5).24 The detected 29Si
NMR chemical shift for silyl cation 17 (δ29Si = 52.7) can be
related to that reported for terphenyl substituted silyl cation
15 (δ29Si = 79.1)17 in particular when the shielding influence
on the 29Si NMR chemical shift of the phenyl substituents in
17 compared to methyl groups in cation 15 is taken into ac-
count.25 As indicated by the high field 29Si NMR resonance
and similar to the situation found for terphenyl substituted
silyl cation 15,17 cation 17 is stabilized by intramolecular in-
teraction between the positively charged silicon atom and
the flanking aryl groups of the terphenyl substituent. This is
also shown by the results of computations at the M06-2X/6-
311+G(d,p) level.19 The predicted molecular structure for silyl
cation 17 (Figure 4c) is dominated by intramolecular electron
donation from the flanking aryl group to the positively
charged silicon atom, as revealed by the relatively small Si-
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C(o) distance (d(SiC(o)) = 230.4 pm, see Figure 4c) and the
found to be by only 21 kJ mol-1 higher in energy. This compu-
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trigonal pyramidal coordination environment for the silicon
atom as indicated by the sum of the bond angles around
silicon: Σα(Si) = 352°. These structural features characterize
cation 17 as a silylarenium ion. The computationally predict-
ed NMR chemical shift for cation 17 in benzene solution is
δ29Si = 63.2 (GIAO/B3LYP/IGLOIII//PCM/M06-2X/6-311+G-
(d,p)),19 in reasonable agreement with the experimental
value. At ambient conditions cation 17 is a highly dynamic
molecule. The C2 symmetric transition state for the intercon-
version of two equivalent structures of cation 17, 17(C2), is
tational result suggests that the high symmetry of cation 17
1
in solution as indicated by H and 13C NMR spectroscopy is
only time averaged and can be explained by a degenerated
equilibrium between equivalent silylarenium structures of
cation 17 which is fast on the NMR time scale (Scheme 7).
Similar equilibria are operative in cation 15 and related silyl
cations.17
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Table 1. Selected experimental, theoretical (italic) NMR and structural parameter for dibenzo-7-silanorbornadienes 12,
dibenzo-7-silanorbornadienyl cations 11, silyliumylidenes 8 and related compounds.
1
2
3
4
5
6
7
8
d(SiC1),
[pm]
d(SiC4)
ref.
14
δ 29Si (1J(SiH) [Hz] )
31.6 (209.4)
31.5 (207.0)
1.3
δ
13C1/4
δ H1/4
α(C1SiC4)
[°]
1
Cp
R
[a]
[a]
[a]
d
12a
Ter[c
Ter*
44.3
44.4
44.2
2.82
192.75, 192.60
81.36
[b]
]
12
2.70
192.74(16),
192.04(16)
81.44(7)
this
work
this
[d]
b
11
Ter
3.68
9
[c]
work
10
11
12
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1.2[e]
2.2[f]
-14.3[g]
6.4
190.5,189.5[h]
190.28(44),
189.18(38)
84.5[h]
82.84(18)
16
17
Ter[c
Ter[c
43.8
2.66
this
work
]
]
-6.2[g]
52.7
191.0, 191.0[h]
83.2[h]
this
work
63.2[g]
381[g]
8
Ter[c
Ter[c
this
work
this
]
]
24
285[g]
work
[a] In benzene-d6 at room temperature. [b] Data from ref [14]. [c] Ter: 2,6-bis-(2,4,6-trimethylphenyl)-phenyl. [d] Ter*: 2,6-bis-
(2,4,6-tri-iso-propylphenyl)-phenyl. [e] In toluene-d8 at room temperature. [f] In chlorobenzene-d5 at room temperature. [g] 29Si
NMR chemical shifts calculated at GIAO/B3LYP/IGLOIII//PCM/M06-2X/6-311+G(d,p). [h] Structural parameter obtained at
M06-2X/6-311+G(d,p).19
(a)
(b)
(c)
Figure 4. Calculated molecular structures of terphenylsubstituted dibenzo-7-silanorbornadienyl cation 11 (a), nitrilium ion 16
(b) and triarylsilylium ion 17 (c). (M06-2X/6-311+G(d,p)) Structural parameter, pertinent for the discussion (bond lengths in pm,
bond angles in [°]): Cation 11: Si-C(1) 190.5, Si-C(4) 189.5, Si-C(ter) 185.0, C(1)-C(2) 151.9, C(2)-C(3) 141.0, C(1)-Si-C(4) , 84.5, Si-C(2)
235.4, Si-C(3) 236.2, Si-C(o) 238.0, C(o)-C(m) 141.2, C(m)-C(p) 138.3, C(p)-C(m’) 140.4, C(m’)-C(o’) 138.7, C(o’)-C(i) 141.1, C(i)-C(o)
142.6. Cation 16: Si-C(1) 191.0, Si-C(4) 191.0, Si-C(ter) 185.7, C(1)-C(2) 152.1, C(2)-C(3) 140.7, C(1)-Si-C(4) 83.2, Si-C(2) 248.2, Si-C(3)
248.2, Si-N 189.4. Cation 17: Si-C(o) 230.4, Si-C(ter) 185.8, C(o)-C(m) 141.6, C(m)-C(p) 138.2, C(p)-C(m’) 140.3, C(m’)-C(o’) 139.1,
C(o’)-C(i) 140.5, C(i)-C(o) 143.7.
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Scheme 5. Thermal decomposition of dibenzo-7-silanorbornadienyl cation 11 and formation of triarylsilyl cation 17.
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Scheme 6. Formation of triarylsilyl cations 19 and 23 in toluene.
Scheme 7. Equilibration of different isomers of cation 17 via transition state 17(C2).
Scheme 8. Computed reaction path for the decomposition of dibenzo-7-silanorbornadienyl cation 11 and its subsequent
reaction to give terphenylsilylium 17. (Relative free Gibbs energy, G298rel(solv) in benzene at T = 298.15 K; p = 27.93 MPa (277
atm) computed at M06-2X/6-311+G(d,p) using the PCM model for inclusion of solvent effects).
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A mechanistic scenario which accounts for the somewhat
surprising formation of silyl cation 17 in course of the ther-
mal degradation of dibenzo-7-silanorbornadienyl cation 11, is
detailed in Scheme 5. The detection of anthracene suggests
the intended formation of terphenyl substituted silylium-
ylidene 8 by thermal fragmentation of cation 11. Reaction of
silyliumylidene 8 with the solvent benzene to give the silyl
cation 20 follows. Conceding that complexation of the ex-
tremely electron deficient silyliumylidene 8 with benzene is
expected based on the results of previous quantum mechani-
cal calculations,12, 26, 27 the formal insertion of the silyliumyli-
dene 8 into the C-H bond of benzene to give silyl cation 20 is
surprising. Although the terphenyl silyl cation 20 is stabi-
lized by intramolecular π-electron donation from the flank-
ing phenyl substituent, the formation of an intermolecular
arenium ion 21 can be envisaged. The occurrence of two
hydrogen atoms of different polarity in cation 21, the hydrid-
ic Si-H and the arenium proton, suggests facile dihydrogen
elimination from cation 21 to give terphenyldiphenyl silyl
cation 17.28, 29
Support for this mechanistic proposal came from the fol-
lowing experimental observations: (i) When toluene is ap-
plied as solvent for the thermal fragmentation of dibenzosila-
norbornadienyl cation 11 tolyl groups instead of phenyl sub-
stituents are found in the obtained product mixture, 19 (see
above and Scheme 6) (ii) The ionization of dihydrido silane
22 by one equivalent of trityl cation at room temperature in
benzene results in the quantitative formation of silyl cation
17 (Scheme 5). The expected primary product, silyl cation 20
was not detected. With toluene as solvent three different
isomers of the tolyl-phenyl substituted silyl cation 23 were
detected (Scheme 6). These results suggest that hydrido
substituted silyl cations such as 20 are not stable in arene
solvents. Remarkably, under the applied reaction conditions,
the hydrido substituent in cation 20 is replaced by an aryl
group to give either cation 17 or 23 depending on the arene
solvent used.
All attempts to directly detect cation 8, or its complex with
benzene, 24 (Scheme 8), by spectroscopic methods failed.
For example, while 29Si NMR chemical shift calculations
predict a characteristic extremely low field resonance of the
silicon nuclei in intermediate 8 at δ29Si(calc) = 381 and like-
wise a 29Si NMR resonance for the benzene complex 24 at
δ29Si(calc) = 285 (see Table 1), no 29Si NMR signal in the low
field region from δ29Si = 600 – 100 was detected during the
thermal fragmentation of dibenzosilanorbornadienyl cation
11. In addition, all attempts to intercept silyliumylidene 8
directly with reagents such as hydrido silanes or alkynes used
in the past30 for quenching silicon(II) compounds were un-
successful due to the high reactivity of the starting com-
pound 11[B(C6F5)4] versus these reagents. Similarly, all at-
tempts to stabilize cation 8 either by N-heterocyclic carbenes
(NHC)31 with different steric requirements or by crown
ethers32 only lead to inconclusive results.
Scheme 9. Reaction of dibenzo-7-silanorbornadienyl cati-
on 11 with benzene to give the intermolecular arenium ion
25.
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Computations at the M06-2X/6-311+G(d,p) level of theory
were applied to give insights into the generation of si-
lyliumylidene 8 from dibenzo-7-silanorbornadienyl cation 11
and its fate in benzene solution.19 The intramolecular π-
electron donation from the flanking aryl groups to the posi-
tively charged silicon atom in cation 11 prevents the for-
mation of intermolecular complexes with arene solvents. In
particular, the reaction of dibenzo-7-silanorbornadienyl
cation 11 with benzene to give the intermolecular arenium
ion 25 is computed to be endogonic at ambient conditions
(ꢀG298 = 37 kJ mol-1, see Scheme 9). This is in agreement with
the experimentally observed absence of solvent effects on the
29Si NMR chemical shift of cation 11 in arene solvents (see
Table 1) and it excludes the formation of arene complexes
such as 25 in the experiment. This computational result also
suggests that the decomposition of cation 11 is not triggered
by benzene complexation. The thermal decomposition of
cation 11 is predicted to be connected with a free energy
barrier of 99 kJ mol-1 and it results in the formation of the
anthracene complex 26 of silyliumylidene 8 (Scheme 8). The
subsequent replacement of anthracene by the solvent ben-
zene to give the complex cation 24 is predicted to be an
essential thermo neutral process (see Scheme 8). The compu-
tations indicate in agreement with previous theoretical inves-
tigations,12, 26 that at no point of the reaction coordinate of
the thermal decomposition of cation 11 in benzene a non-
coordinated silyliumylidene 8 is formed; instead at ambient
conditions the monocoordinated cation 8 is always bonded
to solvent and/or reactant, for example to benzene with 38 kJ
mol-1 or to anthracene with 43 kJ mol-1.
The results of this computational study suggest also a pos-
sible mechanistic scenario for the formation of diphenyl
substituted silyl cation 17 from the benzene complex 24 of
silyliumylidene 8 (see Scheme 8). In general, the overall
reaction of cation 8 with 2 eq. benzene to give terphenyl
silylium ion 17 and dihydrogen is predicted by the calcula-
tions to be strongly exergonic (ꢀG298 = -121 kJ mol-1). The
direct insertion reaction of silyliumylidene 8 into the C-H
bond of benzene to give silylium ion 20 is connected with a
substantial barrier of ꢀG298 = 163 kJ mol-1 relative to si-
‡
lyliumylidene 8 + C6H6 (Scheme 8, 24 ꢀ 20). Interestingly,
an alternative multi-step process involves a smaller overall
barrier (Scheme 8; 24 ꢀ 27 ꢀ 20). Intramolecular insertion
of the silicon cation into a benzylic C-H bond in ortho-
position of one of the flanking mesityl groups leads to the
formation of the cyclic silyl cation 27. In the second step
intramolecular protonation of the new formed Si-C bond is
accompanied with ring opening and formation of the silyl
cation 20 in a slightly exergonic process (ꢀG298 = -5 kJ
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mol-1).33 The C-H activation process 24 ꢀ 27 is the rate de-
second silylium intermediate, the hydrogen substituted silyl
1
2
3
4
5
6
7
8
termining step in that alternative sequence and its free Gibbs
energy of activation is by 11 kJ mol-1 smaller than predicted
for the direct process 24 ꢀ 20 (see Scheme 8). Therefore, the
results of the computations favour the two step process for
the formal oxidative addition of benzene to silyliumylidene
8. In course of the substitution reaction of the hydrogen
atom in cation 20 by a phenyl group in arenium ion 17, a
third cation, 21, is suggested as intermediate (see Scheme 5).
The intramolecular coordination of the silyl cation by the
flanking mesityl group in cation 20 is replaced in benzenium
ion 21 by an intermolecular complexation with the solvent
benzene. In agreement with the ad hoc assumption based on
the relative electronegativities, that the vicinal hydrogen
atoms at the Si-C arenium bond in cation 21 show different
polarity, a natural bond (NBO) analysis19 predict a signifi-
cantly different charge distribution for the Si-H- and the
arenium- Cipso-H- hydrogen atoms (calculated NBO charges:
SiH: -0.17 a.u., arenium-CipsoH: +0.30 a.u.). This antagonistic
charge distribution seems to propose facile dihydrogen elim-
ination from cation 21. The calculated free Gibbs energy of
activation for synchronous elimination of dihydrogen from 21
cation 20. The surprising formation of silylium ion 17 illus-
trates the extreme reactivity of aryl substituted silyliumyli-
denes, such as 8, in C-H-activation reactions. Moreover, it
reveals a surprising instability of hydrogen substituted si-
lylium ions, such as 20, in arene solvents. The density func-
tional study disclosed a concerted elimination of dihydrogen
from cation 21 as a feasible reaction path for the formation of
the isolated silyl cation 17. In view of the predicted almost
thermoneutral course of the reaction, the reverse reaction,
dihydrogen activation by silylium ions, seems feasible under
certain conditions. Clearly, this result influences the mecha-
nistic conceptions for dihydrogen activation by silyl cationic
frustrated Lewis Pairs and it might open a new approach for
reversible dihydrogen activation.
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ASSOCIATED CONTENT
SUPPORTING INFORMATION
All experimental details, characterization data as well as
relevant NMR spectra. X-ray crystallographic information for
compounds 12b, 14b, 16[B(C6F5)4], 18 and 22. All computa-
tional details, Tables of absolute energies and Cartesian
coordinates. This material is available free of charge via the
internet at http://pubs.acs.org.
-1
is however still appreciable (ꢀG298 = 150 kJ mol ). Neverthe-
less, this is the smallest barrier along the complete reaction
sequence shown in Scheme 8. In addition, dihydrogen elimi-
nation from silylated arenium ions is not without prece-
dence. Allen and Lampe report on the formation of si-
‡
+
labenzyl cation in the ion-molecule reaction of SiH3 with
benzene by dihydrogen elimination from the intermediate
silylbenzenium ion.34-36 The overall substitution reaction 20
ꢀ 17 is predicted by the calculations to be slightly endogonic
(ꢀG298 = 7 kJ mol-1, see Scheme 8). All three cationic interme-
diates involved in this reactions sequence, 20, 21, and 17, are,
however, all very close in energy and in view of the approxi-
mations regarding the contributions of thermal and entropy
contributions (see SI Material), a clear decision about the
thermodynamic driving force is not justified. Nevertheless, in
view of the computed positive ꢀG298 value, the evolution of
gaseous dihydrogen is certainly an important factor which
drives the reaction towards the formation of silyl cation 17.
AUTHOR INFORMATION
Corresponding Author
* thomas.mueller@uni-oldenburg.de
NOTES
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This study was supported by the CvO University Oldenburg
and by the DFG (Mu-1440/7-1). The High End Computing
Resource Oldenburg (HERO) at the CvO University is
thanked for computer time.
Conclusion.
The synthesis and NMR characterization of a terphenyl
substituted dibenzosilanorbornadienyl borate 11[B(C6F5)4] is
reported. The analysis of the experimental NMR parameter
of cation 11 in particular the 29Si NMR chemical shift in com-
bination with the results of quantum mechanical calculations
reveals the intramolecular interaction with the flanking aryl
groups of the terphenyl substituent. The identity of cation 11
is finally confirmed by a single crystal X-ray diffraction anal-
ysis of the closely related dibenzosilanorbornadienyl ni-
trilium borate 16[B(C6F5)4]. Dibenzosilanorbornadienyl bo-
rate 11[B(C6F5)4] is thermally unstable and undergoes in aro-
matic solvents a fragmentation reaction upon heating. In this
reaction anthracene is formed and the terphenyl substituted
silyliumylidene 8 is obtained as a reactive intermediate in the
form of its solvent complexes. The benzene complex of si-
lyliumylidene 8 is transformed under the applied reaction
conditions into the diphenyl-terphenyl silylium ion 17. It
could be demonstrated experimentally and it is supported by
the results of computations that this reaction proceeds via a
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(33) Dihydrogen elimination from cation 27 is a possible alter-
native process. This reaction channel is however connected with
a barrier of ꢀG298‡ = 191 kJ mol-1, compared to a barrier of ꢀG298‡
=
179 kJ mol-1 for the formation of cation 20.
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1
(21) The relative intensity of these H NMR signals and of the
signal for the meta–protons of the Mes substituent were found
to be variable, due to intensive proton/deuterium exchange
which occurred upon standing in the deuterated NMR solvent.
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