Synthesis of silylium and germylium ions by a substituent exchange reaction

Article abstract of DOI:10.1021/om400366z

The reaction of diarylalkylsilanes and -germanes with trityl cation in the presence of a weakly coordinating anion to give the corresponding triarylsilylium or -germylium ions was investigated. This reaction provides a facile access to a larger range of these sterically highly hindered Lewis acids. The factors that promote the substituent exchange were studied, and significant effects of the substituent, of the solvent, and of the group 14 element were revealed. A combined solid-state XRD and NMR investigation of the tris(pentamethylphenyl) silylium borate [(Me₅C₆) ₃Si]₂[B₁₂Cl₁₂] disclosed the trigonal planar coordination environment of the silicon atom in this silylium ion. NMR investigations indicate for 2,4,6-triisopropylphenyl-substituted silylium and germylium ions the onset of C-H···E ⁺ three-center interactions (E = Si, Ge) between the distant CH bond of the isopropyl group and the positively charged group 14 element atom.

Full text of DOI:10.1021/om400366z

Article
pubs.acs.org/Organometallics
Synthesis of Silylium and Germylium Ions by a Substituent Exchange
Reaction
Andre Schafer,† Matti Reißmann,† Sebastian Jung,† Annemarie Schafer,† Wolfgang Saak,† Erica Brendler,‡
́
̈
̈
and Thomas Muller*,†
̈
†Institut fur Chemie, Carl von Ossietzky Universitat Oldenburg, Carl von Ossietzky-Straße 9-11, D-26129 Oldenburg, Federal
̈
̈
Republic of Germany
‡Institut fur Analytische Chemie, Technische Universitat Freiberg, Leipziger Straße 29, D-09599 Freiberg, Federal Republic of
̈
̈
Germany
S
* Supporting Information
ABSTRACT: The reaction of diarylalkylsilanes and -germanes
with trityl cation in the presence of a weakly coordinating
anion to give the corresponding triarylsilylium or -germylium
ions was investigated. This reaction provides a facile access to a
larger range of these sterically highly hindered Lewis acids. The
factors that promote the substituent exchange were studied,
and significant effects of the substituent, of the solvent, and of the group 14 element were revealed. A combined solid-state XRD
and NMR investigation of the tris(pentamethylphenyl) silylium borate [(Me5C6)3Si]2[B12Cl12] disclosed the trigonal planar
coordination environment of the silicon atom in this silylium ion. NMR investigations indicate for 2,4,6-triisopropylphenyl-
substituted silylium and germylium ions the onset of C−H···E+ three-center interactions (E = Si, Ge) between the distant CH
bond of the isopropyl group and the positively charged group 14 element atom.
Scheme 1. Synthesis of Triarylsilylium Ions 1 from
Diaryl(methyl)silanes 2
INTRODUCTION
■
a
The extreme Lewis acidity of silyl cationic species1 recently
found beneficial applications in catalysis,2 in bond activation
reactions,3 and in activation processes of small molecules.4,5
The relevant silyl cations are stabilized by either the solvent,
anion, or an intramolecular electron-donating substituent.
Consequently their Lewis acidity is clearly controlled by the
electron-donating group or partner.6 Applications of the
extreme Lewis acidity of tricoordinated silylium ions, R3Si+,
however, remained mostly unexplored. The only exception so
far is the use of silylium ions in frustrated Lewis pairs for
dihydrogen activation.4 One reason for this obvious absence of
application is certainly their high reactivity by implication of
their extreme electron deficiency. Another aspect is the lack of a
straightforward synthesis. Previous to our investigations only a
few examples of silylium ion salts that feature a strictly
tricoordinated silicon atom and that could be isolated in
substance were reported, and in these cases quite elaborate
synthetic procedures are required.7−9 In this respect our recent
report4 on the surprising reaction course of the Bartlett−
Condon−Schneider (BCS) hydride transfer10 reaction applying
diarylmethylsilanes 2 as starting materials is of interest. We
found that the reaction of three equivalents of diarylsilanes 2
with two equivalents of trityl tetrakispentafluorophenyl borate,
[Ph3C][B(C6F5)4], in arenes or haloarenes results in the
formation of two equivalents of the triarylsubstituted silylium
borates 1[B(C6F5)4] and one equivalent of trimethylsilane
(Scheme 1).4 Our preliminary results suggested that the
reaction proceeds via an initiating slow hydride transfer to give
diarylmethylsilylium ions 3 and a faster subsequent substituent
a
a: Ar = 2,6-dimethylphenyl (Xylyl); b: Ar = 2,4,6-trimethylphenyl
(Mes); c: Ar = 2,3,5,6-tetramethylphenyl (Duryl); d: Ar = 2,3,4,5,6-
pentamethylphenyl (Pemp); e: Ar = 2,4,6-triisopropylphenyl (Tipp).
The weakly coordinating borate ion, [B(C6F5)4]−, is not shown.4
exchange between the cation 3 and starting silane 2 (Scheme
2). As the thus produced silane 4 is sterically less congested
than the starting silane 2, the following, third reaction step to
give trimethylsilane and a second equivalent of the
triarylsilylium ion 1 is faster than both preceding steps. It was
also demonstrated that the intermolecular substituent exchange
is not restricted to methyl groups as ethyldimesitylsilane,
Mes2EtSiH, 5a, cleanly yields trimesitylsilylium, Mes3Si+, 1b.
On the other hand, the formation of bis(triisopropylphenyl)-
ethyl silylium, Tipp2EtSi+, 6, from the corresponding precursor
silane Tipp2EtSiH, 7, instead of tris(triisopropylphenyl)-
silylium, Tipp3Si+, 1e, indicated that the progress of the
substituent exchange depends on a delicate balance of steric
effects (Scheme 3).4
In an effort to examine the scope and limitations of this
reaction, we probed the influence of the nature of the alkyl
Received: May 2, 2013
© XXXX American Chemical Society
A
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In contrast to the clean reactions observed in the cases of the
alkylsilanes 2b, 5a,b, the reaction of disilane 5d with trityl
cation led only to a complicated mixture of products, both at
Scheme 2. Suggested Reaction Course for the Formation of
Triarylsilylium Ions 1 from Diarylmethylsilanes 24
1
room temperature and at −10 °C. Although H and 13C NMR
spectroscopy of the product mixture indicated the formation of
triphenylmethane, Ph3CH, 29Si NMR spectroscopy provided no
indication for the formation of silylium ion 1b in this case.
Steric protection of the incipient diaryl(methyl)silylium ion
3, formed in the first reaction step (Scheme 2) by the aryl
substituent, is mandatory for the reaction to occur. Therefore, a
series of diarylmethyl silanes 2 substituted with bulky aryl
groups yields the corresponding triarylsilylium ions 1 upon
treatment with trityl cation (Scheme 1). This is demonstrated
in each case by the low-field 29Si NMR resonance detected for
silylium borates 1[B(C6F5)4] in the region of δ29Si = 216.2−
229.9 (see Table 1). The low-field position of the 29Si
resonance is for most of the silylium ions independent of the
solvent as long as arenes are used (see Table 1). The largest
solvent effect was detected for Tipp2EtSi+, 6, for which a
slightly more shielded 29Si NMR signal was found in
chlorobenzene[D5] than in benzene[D6] (Δδ29Si = −4.1, see
Table 1).
Scheme 3. BCS Hydride Transfer Reaction with
Diaryl(ethyl)silanes 5a and 7
a
Interestingly, dixylyl-substituted silane 2a cleanly underwent
the substituent exchange to give silylium ion Xylyl3Si+, 1a, while
di-ortho-tolylmethyl silane, o-Tol2MeSiH, 2f (and likewise
diphenylmethylsilane, Ph2MeSiH, 2g, or mesityldimethylsilane,
MesMe2SiH, 9), failed to give the corresponding silylium ion.
29Si NMR signals in the range typical for silylated arenium ions
(δ29Si = 80−100) indicated in these cases the formation of
benzenium ions 8 (Scheme 5).9b,11 These results suggest that
the formation of relatively stable arenium ions in the case of the
reaction of the sterically less hindered silanes 2f,g with trityl
cation inhibits the intermolecular substituent exchange. This is
further supported by the results of quantum mechanical
computations,12,13 which indicate that for silylium ion
Xylyl2MeSi+, 3a, produced in the first step of the reaction
sequence leading to cation 1a (Scheme 2) complexation with
benzene is an endergonic process at room temperature (ΔG298
= +8 kJ mol−1), while for the related silylium ion o-Tol2MeSi+,
3f, benzenium ion formation is thermodynamically favored
(ΔG298 = −30 kJ mol−1, all values computed at M05-2X/6-
311+G(d,p)). The significantly different Si−C separations in
the computed molecular gas phase structures of benzenium
ions 8f and in the solvent complex 1a/benzene reflect their
different thermodynamic stability at room temperature (see
Figure 1).
a
(a) Ph3C+, rt, benzene, Ar = Tipp; (b) Ph3C+, rt, benzene, Ar = Mes.4
group, the size of the aryl substituent, and the nucleophilicity of
the solvent on the reaction course. In addition we investigated
the possible extension of this chemistry for the synthesis of
triarylgermylium ions.
RESULTS AND DISCUSSION
■
Reaction of dimesitylalkylsilanes 2b and 5a−c with [Ph3C][B-
(C6F5)4] in benzene results in the formation of a biphasic
mixture, which is typical for solutions of the [B(C6F5)4]− anion
in aromatic hydrocarbons (Scheme 4). The more dense ionic
Scheme 4. BCS Hydride Transfer Reaction with
Dimesitylsilanes 2b and 5a−d
a
a
2b: R = Me, 5a: R = Et; 5b: R = i-Pr; 5c: R = t-Bu; 5d: R = SiMe3.
As a consequence, the formation of stable solvent complexes
is expected to hamper the intermolecular substituent exchange
reaction also for more bulky substituted diarylsilylium ions 3.
To verify this conclusion, the reaction of dimesitylmethylsilane
2b with trityl cation was conducted in the significantly stronger
donor solvent acetonitrile. In this case exclusively the
dimesitylmethylsilyl nitrilium ion 10 was formed, establishing
that the substituent exchange is inhibited by the formation of
strong silylium ion/solvent complexes. The silylnitrilium ion 10
was identified by its high-field 29Si NMR resonance (δ29Si =
10.6), which appears at somewhat higher field than those
reported for related trialkylsilylnitrilium ions.1c,14 The structural
integrity of the dimesitylmethylsilyl unit is demonstrated by the
1H/29Si-HMBC NMR spectrum, which shows a cross signal for
the resonances of the methyl protons and the 29Si nuclei
(Figure 2).
phase contains the trimesitylsilylium borate 1b[B(C6F5)4], and
the corresponding neutral trialkylsilane is detected in the lighter
phase. In each case the silylium ion 1b was identified by
multinuclear NMR spectroscopy. Particularly useful for this
purpose was its characteristic low-field 29Si NMR chemical shift
at δ29Si = 225.3 (see Table 1 and Figure S10, Supporting
Information (SI)). In case of dimesityl-tert-butylsilane, 5c, the
hydride abstraction appears to be very slow, probably due to
the high steric demand. In this case even after 24 h of stirring at
room temperature with excess silane (5c: [Ph3C][B(C6F5)4] =
2:1), major amounts of trityl cation were detected in the
mixture by NMR spectroscopy. A signal at δ 29Si = 225.3 was
however observed, indicating the formation of Mes3Si+, 1b
(Figure S10d). The reaction time was not extended to
completeness of the reaction, since prolonged stirring at
room temperature resulted in slow decomposition of cation 1b.
B
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Table 1. Products from the BCS Hydride Transfer Reaction between Trityl Cation and Different Silanes and Germanes
According to Eqs 4 and 5 along with Characteristic 29Si NMR Chemical Shifts for Formed Silyl Cations
starting silane
Ar
R
E
reaction according to equation
cation
δ
29Si (cation)
solvent
C6D6
ref
2a
2b
2b
2c
2d
Xylyl
Mes
Me
Si
Si
Si
Si
Si
4
4
5
4
4
1a
1b
229.9
225.3
10.6
4
Me
Me
Me
Me
C6D6
4, 7
a
Mes
10
C6D6/CH3CN
C6D6
this work
e
Duryl
Pemp
1c
226.5
216.2
216.4
216.8
229.8
this work, 7
4
1d
C6D6
C7D8
this work
this work
4
C6D5Cl
C6D6
2e
2f
Tipp
o-Tol
Ph
Me
Me
Me
Et
Si
Si
Si
Si
Si
Si
Si
Si
4
5
5
4
4
4
1e
b
8f
C6D6
this work
this work
b
2g
5a
5b
5c
5d
7
8g
C6D6
e
Mes
Mes
Mes
Mes
Tipp
1b
1b
225.3
225.3
225.3
C6D6
4
e
i-Pr
t-Bu
SiMe3
Et
C6D6
this work
c
e
1b
d
C6D6
this work
C6D6
this work
4
5
6
244.7
240.6
C6D6
C6D5Cl
C6D6
this work
e
11a
11b
Mes
Me
Me
Ge
Ge
4
5
12a
13
this work
e
Tipp
C6D6
this work
a
b
c
No substituent exchange occurred. More than one signal was observed in the 29Si NMR spectra, but no triarylsilyl cation could be detected. Only
d
e
minor conversion of silane after 24 h, due to the high steric demand. Product mixture obtained. See also the SI.
Scheme 5. BCS Hydride Transfer Reaction of
a
Diaryl(methyl)silanes 2f,g with trityl cation
a
f: Ar = 2-methylphenyl (o-Tol); g: Ar = phenyl (Ph).
Figure 2. H, 29Si-HMBC NMR spectrum of 10[B(C6F5)4] (499.87
MHz/99.31 MHz, 305 K, C6D6).
1
Figure 1. Calculated molecular structures for complexes (a) [o-
Tol2MeSi+/C6H6] (8f) and (b) [Xylyl2MeSi+/C6H6] at the M05-2X/
6-311+G(d,p) level of theory. Atomic distances are given in pm.
Scheme 6. Formation of Dimesitylmethylsilyl Nitrilium Ion
10 from Dimesityl(methyl)silane 2b
The possible extension of the investigated intermolecular
substituent exchange reaction on diarylmethylgermanes was
tested for the dimesitylmethyl-, Mes2MeGeH, 11a, and
bis(triisopropylphenyl)methylgermane, Tipp2MeGeH, 11b.
Both germanes gave with [Ph3C][B(C6F5)4] biphasic reaction
mixtures in benzene. In the case of mesitylgermane 11a 1H and
13C NMR spectroscopy of the lower ionic phase showed the
clean formation of trimesitylgermylium borate, 12a[B(C6F5)4]
(see Figures S4, S5, SI).15 This borate had been synthesized
previously by the Lambert group,7b and the comparison
data. The side product in the substituent exchange reaction,
Me3GeH, was detected in the nonpolar phase by GC-MS
analysis. Further support for the identification of Mes3Ge+, 12a,
came from its derivatization reaction with tributyltin and the
subsequent detection of trimesitylgermane, Mes3GeH, by GC-
MS analysis. The attempted synthesis of Tipp3Ge+, 12b, under
the same reaction conditions however failed. Instead, reaction
1
revealed an almost perfect match of the H and 13C NMR
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of Tipp2MeGeH, 11b, with [Ph3C][B(C6F5)4] yielded the
regular hydride transfer product bis(triisopropylphenyl)methyl
germylium, 13, without rearrangement (Scheme 7). The
Scheme 9. Intermolecular Mesityl/Methyl Transfer to Give
Trimesitylsilylium 1b via the Mesitylenium Cation 14b
a
Scheme 7. BCS Hydride Transfer Reaction of Germanes 11
with Trityl Cation
Tri- and diaryl tetrylium borates [Ar3E][B(C6F5)4] and
[Ar2RE][B(C6F5)4] (E = Si, Ge) are stable in the solid state at
room temperature. In arene solution slow decomposition
occurs even at −10 °C, in particular in the case of sterically less
hindered cations such as 1a,b.4 The decomposition is
significantly slower for highly substituted cations such as
tris(pentamethylphenyl)silylium, Pemp3Si+, 1d. While for the
less bulky substituted cations all attempts to grow crystals
suitable for X-ray diffraction (XRD) analysis resulted in crystals
of decomposition products,4 crystallization of a salt of cation 1d
was achieved by replacing the tetrakis(pentafluorophenyl)-
borate anion by the dodecachloro-closo-dodecaborate dianion
[B12Cl12]2−. Recent work from the Knapp, Reed, Ozerov, and
Oestreich laboratories revealed the superior crystallization
ability of this robust, weakly coordinating anion.16 Crystals
suitable for XRD analysis were obtained from an ortho-
dichlorobenzene solution of [Pemp3Si]2[B12Cl12] at −15 °C
after four weeks.13 The asymmetric unit contains one cation,
three ortho-dichlorobenzene molecules, and one-half of the
anion. The crystal structure solution indicates some positional
disorder of one of the solvent molecules, which was resolved by
using a split model with a 5:2 occupation. The crystal structure
of the salt reveals well-separated cations, anions, and solvent
molecules (Figure 3). In particular, no chlorine atom of the
closo-borate dianion or solvent molecules approaches the silicon
atom at a distance smaller than 489 pm. Silylium ion 1d adopts
the propeller-shaped conformation that is typical for triaryl-
substituted compounds of the general AB3 composition. The
trigonal planar coordination environment for the silicon atom is
indicated by the summation of all bond angles α around the
silicon atom to 360.0° (Figure 4a). Si−Cipso bond lengths in
Pemp3Si+ (average Si−Cipso: 184.72(57) pm) are slightly longer
than found for Mes3Si+ in the crystal structure of [Mes3Si]-
[HCB11Me5Br6] (181.7 pm).7c Also the dihedral angles β
between the central Cipso3Si+ plane and the plane of the aryl
rings of the substituents (β = 51.4−65.1°, average value of β =
60.0°, see Figure 4b) are larger than reported for Mes3Si+
(average β = 49.2°).7c Both structural differences between the
closely related cations 1b and 1d can be attributed to the
somewhat larger steric effects of the Pemp substituent due to
the buttressing influence of the meta and para methyl groups.
On the other hand, the average Si−Cipso bond in cation 1d is
significantly shorter than found experimentally for the
tetracoordinated silanes tris(pentamethylphenyl)silane,
Pemp3SiH (194.96(42) pm),4 and bis(pentamethylphenyl)-
methylsilane, Pemp2(Me)SiH, 2d (190.18(32) pm).13 This
bond length shortening results from the superposition of two
effects. The reduced steric congestion in the trigonal planar
coordinated cation 1d compared to the tetrahedral silanes
allows for shorter Si−Cipso bonds. In addition, the smaller
covalent radii of tricoordinated silicon and the onset of π-
conjugation between the Pemp substituent and the central
silicon atom contribute to the shorter Si−Cipso bond in silylium
ion 1d in particular in comparison with the diaryl-substituted
a
a: Ar = 2,4,6-trimethylphenyl (Mes); b: Ar = 2,4,6-triisopropylphenyl
(Tipp).
identity of cation 13 was confirmed by H and 13C NMR
1
spectroscopy.13 Two-dimensional NMR spectroscopy was
particularly important for the structure elucidation. Decisive
for the identification of the TippGeMe linkage in cation 13 was
the detection of cross-signals in the 1H,13C-HMBC between the
methyl hydrogen atoms and the ipso-carbon atoms of the Tipp
substituent (see Figure S8a,b, SI). Additional support came
from 1H,1H-NOESY spectroscopy, which revealed a close
spatial relation between the isopropyl groups of the aryl
substituent and the methyl group at germanium (see Figure S9,
SI) This behavior of germane Tipp2(Me)GeH, 11b, in the BCS
hydride transfer reaction, which is remarkably different from
that of silane Tipp2(Me)SiH, 2e, emphasizes that the
intermolecular substituent exchange as observed in the case
of the silane crucially depends on subtle steric and electronic
effects.
The general reaction course of the substituent exchange
reaction as shown in Scheme 2 is supported by cross
experiments, by the applied stoichiometry, and by the
identification of the byproducts as outlined in a previous
communication.4 The actual mechanism of the key step, the
intermolecular aryl/methyl exchange between the diaryl-
methylsilylium ion 3 and the corresponding silane 2 is however
less clear (Scheme 8). With respect to similar intramolecular
Scheme 8. Proposed Reaction Mechanism for the
Intermolecular Alkyl−Aryl Exchange
substituent exchange reactions studied earlier in our group,3g
we suggested arenium ions 14 as possible intermediates, which
transform via a methonium-like transition state into the
products.4 The results of preliminary calculations at the M06-
2X/6-311G(d,p) level of theory support this view.12,13 The
formation of trimesitylsilylium 1b by reaction of silylium ion 3b
with silane 2b is predicted to be exergonic at ambient
temperature by ΔG298 = −40.5 kJ mol−1, and also the
formation of intermediate 14b from the reactants is slightly
exergonic (ΔG298 = −8.3 kJ mol−1, Scheme 9).
D
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Figure 3. Ellipsoid presentation of the double asymmetric unit of [Pemp3Si]2[B12Cl12]·6 o-Cl2C6H4 in the crystal (H atoms omitted for clarity;
thermal ellipsoids at 50% probability level). Color code: gray, carbon; blue, silicon; yellow, boron; green, chlorine.
Figure 4. Ellipsoid presentation of the molecular structure of Pemp3Si+, 1d, in the crystal: (a) view perpendicular to the SiCipso3 plane; (b) view along
one Si−Cipso vector (H atoms omitted for clarity; thermal ellipsoids at 50% probability level). Color code: gray, carbon; blue, silicon. Pertinent angles
[deg] and bond lengths [pm]: Si−Cipso: 184.2(6), 184.5(4), 185.5(4); Cipso−Cortho (av): 140.47(72); Cortho−Cmeta (av): 140.18(80); Cmeta−Cpara (av):
139.81(79); α: 117.9(2), 119.7(2), 122.4(2); β: 51.4(5), 63.5(2), 65.1(2).
Figure 5. Calculated molecular structure of silylium ions at B3LYP/6-311G(d,p) (pertinent bond lengths and interatomic distances are given [pm];
all H atoms except methine H atoms are omitted for clarity. Color code: gray, carbon; blue, silicon, red, hydrogen. (a) Pemp3Si+, 1d (C3 molecular
symmetry); (b) Tipp2EtSi+, 6. (Si−H1: 238.2, Si−H2: 256.5, Si−H3: 252.0, Si−H4: 255.8).
silane 2d. DFT computations at the B3LYP/6-311G(d,p) level
of theory predict a structure of molecular C3 symmetry for
Pemp3Si+, 1d, which is very close to the experimentally
determined structure in all relevant parameters (see Figure 5a).
A major difference between the molecular structures derived
from XRD and from DFT calculations12,13 can be found for the
E
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dihedral angle β, which is computed to be 51.8°, 8.2° smaller
than the average value in the experimental structure.
Accordingly, 29Si NMR chemical shift calculations based on
this molecular structure resulted in a predicted isotropic 29Si
NMR chemical shift for silylium ion 1d of δ29Sicalc = 221.6
(Table 2), in good agreement with the NMR chemical shift
found in solution.
located in the SiCipso3 plane of 1d, and the shielded component
δ33 is oriented perpendicular to that plane and along the local
C3 axis (see Figure 7). This orientation of the 29Si NMR
Table 2. 29Si NMR Chemical Shift Data of Silylium Ion 1d in
Solution and in the Solid State and Its Experimental and
Figure 7. Orientation of the principal axes of the 29Si NMR chemical
shift tensor in silylium ion 1d.
a
Calculated (italic) 29Si NMR Chemical Shift Tensor
chemical shift tensor relative to the molecular structure and also
the size of the principal components are confirmed by the
results of NMR chemical shift computations (see Table 2). A
more detailed analysis22,23 of the calculations suggests that the
strong deshielding of the in-plane components δ11 and δ22 is
mainly the result of paramagnetic currents induced by the
magnetic field that correlates the degenerate Si−C σ-bonding
orbitals with the LUMO, which is dominated by contributions
from the 3p(Si). These deshielding currents are very efficient
due to the low energy of the LUMO and the consequentially
small energy difference between Si−C σ-bonds and the LUMO.
1H and 13C NMR data obtained for cations 1 and 12a are in
general unremarkable. The 1H and 13C NMR data obtained for
the Tipp-substituted cations 1e, 6, and 13 reveal, however, clear
indications for multicenter bonding between the positively
charged central atom and the ortho-CH groups of the Tipp
b
c
d
Pemp3Si+, 1d
δiso
δ11
δ22
δ33
Ω
κ
C6D6
216.2
223.5
221.6
solid state
329
312
30
299
0.89
a
calculated
320
318
27
293
0.99
a
b
Calculated at GIAO/B3LYP/IGLOIII//B3LYP/6-311G(d,p). δiso
=
c
d
1/3(δ11 + δ22 + δ33). Span Ω = δ11−δ33. Skew κ = 3(δ22−δiso)/Ω.
The measured 29Si NMR chemical shift of 1d[B(C6F5)4] is
insensitive to changes of the arene solvent (see Table 1).
CPMAS solid-state NMR spectroscopy (see Figure 6) showed a
1
substituent (Table 3 and Figure S11, SI). In particular, the H
NMR resonance of the ortho methine protons are high field
shifted by Δδ1H = −1.11 to −1.37, and the ortho methine
carbon atoms are deshielded by Δδ13C = 8.4 to 9.6 compared
to the corresponding reference silanes (see Table 3). Similar
1
significant changes in the H and 13C NMR chemical shifts
were not detected for the para isopropyl groups. In addition,
1
the J(ortho-CH) coupling constants in cations 1e, 6, and 13
Figure 6. 29Si CPMAS NMR spectra of [Pemp3Si]2[B12Cl12]·6 o-
Cl2C6H4 at 4 kHz rotation frequency (lower trace) and at 2.5 kHz
(upper trace).
are markedly smaller than those detected for the para methine
groups (Δ1JCH = 14 Hz). Although these effects appear to be
relatively small compared to prototypical compounds that
exhibit three-center M−H−C bonding,24−26 the NMR
spectroscopic data for 1e, 6, and 13 indicate the onset of a
multicenter E+···H−C interaction (E = Si, Ge). While the
consequences of these E+···H−C interactions are clearly
detectable by NMR spectroscopy, IR spectroscopic inves-
tigation of the borate 6[B(C6F5)4] reveals no indications for
E+···H−C bonding; in particular no significant bathochromic
shift of the C−H stretch vibrations was detected. In the absence
of experimental structural data we tested molecular structures
of these cations optimized with DFT methods for possible
indications of Si+···H−C interactions (see Figure 5b and SI).
The results of the computations predict for all three cations
Tipp3Si+, 1e, Tipp2EtSi+, 6, and Tipp2MeGe+, 13, separations,
d(E+HC), between the positively charged central atom and the
ortho methine hydrogen atoms (d(E+HC): 253.5−255.3 pm
(1e); 238.2, 252.0−256.5 pm (6); 245.0, 252.8−253.4 pm
(13)) that are far longer than the sum of the covalent radii of
the tetrel atom and the hydrogen atom (SiH, 148 pm; GeH,
153 pm).27 They are, however, significantly smaller than the
sum of the van der Waals radii (SiH, 330 pm; GeH, 331 pm).28
Interestingly, an analysis of the B3LYP/6-311++G(3df,3pd)
wave function based on the quantum theory of atoms in
molecules (QTAIM)29 for silylium ion 1e predicts no bond
paths between the silicon atom and the methine hydrogen
atoms of the ortho-isopropyl groups despite their relatively
slightly more deshielded 29Si resonance for the salt
1d2[B12Cl12] (δiso29Si = 223.5). Analysis of the slow spinning
29Si CPMAS NMR spectra revealed within the limits of
accuracy (standard deviation for the principal components: 3
ppm) an almost oblate 29Si NMR chemical shift tensor (δ11
=
δ22 > δ33 and skew κ close to +1) in agreement with the axial
molecular symmetry of silylium ion 1d (see Table 2).13,17,18
The high anisotropy of the 29Si NMR chemical shift tensor is
indicated by its large span Ω = δ11−δ33 = 299. A similar highly
anisotropic chemical shift tensor of axial symmetry was
determined for [Mes3Si][HCB11Me5Br6].7c Therefore, the
anisotropy of the 29Si chemical shift tensor is significantly
larger for triarylsilylium ions than for tetrahedral-coordinated
silicon compounds (i.e., for triphenylsilane, Ph3SiH, Ω = 52).19
In fact, the spans Ω for triaryl silylium ions approach those
determined for disilenes (Ω = 161−514)20 but are still
somewhat smaller than measured for N-heterocyclic silylenes
(Ω = 328−376).21
The origin of this high anisotropy of the 29Si NMR chemical
shift tensor of silylium ion 1d is the strongly deshielded δ11 and
δ22 components (δ11 = 329; δ22 = 312), while the high-field
component δ33 is in the chemical shift range for tetracoordi-
nated silicon atoms (δ33 = +30 to −60; see also Table 2).19 For
symmetry reasons both deshielded principal components are
F
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Article
Table 3. Selected 1H and 13C NMR Parameters for Tipp-Substituted Silylium ions 1e and 6, Related Silanes 2e and 7, and Tipp-
Substituted Germylium Ion 13 and Related Germane 11b
a
NMR parameter
2e
1e
7
6
11b
13
δ
29Si
−36.7
3.62
229.8
2.31
−28.5
3.62
244.7
2.25
1
δ H(CHortho
)
3.25
2.14
δ
13C(CHortho
)
33.6
43.2
33.6
44.1
33.7
42.1
1J(CHortho
)
128 Hz
2.82
116 Hz
2.72
125 Hz
2.82
115 Hz
2.73
126 Hz
2.81
116 Hz
2.73
δ H(CHpara
)
1
δ
13C(CHpara
)
34.7
35.0
34.7
35.0
34.1
34.9
1J(CHpara
)
128 Hz
130 Hz
125 Hz
129 Hz
127 Hz
129 Hz
a
C6D6, 305 K.
close spatial proximity. Only for the Si/H1C1 pair in 6 with the
smallest separation (238.2 pm) was an weakly pronounced
bond path with a bond critical point (bcp) of low electron
density located (see Figure 8). Comparison of the computed
distant methyl hydrogen atom. In addition the close proximity
of the bcp of the SiH1 bond and the ring critical point (rcp) of
the SiCipsoCorthoC1H1 cycle with nearly identical electron density
at rcp and bcp describes a situation near to fissure of that bond
(bcp: ρ(r) = 0.01815 e bohr−3, rcp: ρ(r) = 0.01813 e bohr−3;
see Figure 8). Complementary information comes from a
comparative analysis of the charge density properties of the
C1−H1 bond in cation 6 with those of CH bonds of the
standard compounds Me3CH and cation 15 (see Table 4 and
Figure 8). This comparison classifies the C1−H1 bond in cation
6 as a rather regular C−H bond with a calculated electron
density at the bcp that is not markedly smaller than that
predicted for the C−H bond in Me3CH (see Table 4).
Therefore, the QTAIM analysis provides no resilient indication
for significant multiple-center bonding between the ortho-CH
groups of the Tipp substituent and the central atoms in cations
1e, 6, and 13.30 This is remarkable in view of the clear NMR
spectroscopic evidence for such an interaction. A somewhat
related situation was reported recently for dmpeTiEtCl3 (dmpe:
bis(dimethylphosphino)ethane).24
Figure 8. Contour plot of the calculated Laplacian of the electron
density ∇2ρ(r) of silylium ion 6 in the Si−bcp(SiH1)−H1 plane.
Pertinent parts of the molecular graph of cation 6 are projected onto
the contour plot. The bond paths that follow the line of maximum
electron density between bonded atoms are shown by solid (ρ(bcp) >
0.025 e bohr−3) or dashed (ρ(bcp) < 0.025 e bohr−3) black lines, and
the corresponding bcp’s are shown as green, and rcp’s as red circles.
Red contours indicate regions of local charge accumulation (∇2ρ(r) <
0); blue contours indicate regions of local charge depletion ((∇2ρ(r) >
0).
CONCLUSION
■
The scope and limitations of the BCS hydride transfer reaction
with subsequent substituent exchange of diarylalkyl silanes to
give triarylsilylium ions are reported. The major advantage of
this procedure is the straightforward and easy synthesis of the
starting silanes. In addition, this methodology can also be
applied for the synthesis of triarylgermylium ions starting from
the appropriate germanes. The here presented results indicate a
delicate balance between steric and electronic effects of the
starting diarylmethylsilanes and −germanes. In addition
essentially non-nucleophilic reaction conditions are necessary
for the substituent exchange to take place. In detail, the use of
arenes and haloarenes as solvents is mandatory, and weakly
coordinating anions such as the borates [B(C6F5)4]− and
[B12Cl12]2− or carborane monoanions1e,f must be applied.
Accepting these boundary conditions, the here described
variant of the BCS hydride transfer reaction10 of diarylalkyl-
silanes and -germanes provides an undemanding synthetic
route to triarylsilylium and triarylgermylium ions. This opens
the field for application of these highly Lewis acidic species in
bond activation chemistry and catalysis.2−5 In addition, the
simple generation of these carbocation analogues under
essentially stable ion conditions calls for a broader structural
investigation of this so far neglected class of compounds. To
this end, the solid-state structure of the silylium borate
[Pemp3Si]2[B12Cl12], [1d]2[B12Cl12], is presented. The XRD
analysis revealed a propeller-shaped triarylsilylium ion with a
central tricoordinated silicon atom. In agreement with the
obtained solid-state structure, a CP/MAS NMR investigation of
properties of the charge density at this bcp in 6 with those of
SiH bonds of standard compounds such as Me3SiH (two-
electron, two-center (2e-2c) bond) and the [Me3Si−H−
CMe3]+ cation, 15 (2e-3c bond, see Table 4), suggests only
small interactions between the central silicon atom and the
Table 4. Properties of the Computed Electron Density at the
Bond Critical Points of E−H Bonds in Cation 6 and Model
Compounds According to QTAIM Analysis (see also Figures
5b and 8)
atomic
distance
[pm]
bond
type
ρ(r) [e
bohr−3
∇2ρ [e
compd
Me3Si−H
]
bohr−5
]
Si−H
+0.1206
+0.0536
+0.0181
+0.2831
+0.2696
+0.2737
+0.2165
+0.1496
+0.0322
+0.0407
−0.9944
−0.9006
−0.9272
−0.5820
149.2
175.9
238.2
109.4
110.8
110.3
115.9
[Me3Si−H−CMe3]+, 15
Tipp2(Et)Si+, 6 (Si+···H1)
Me3C−H
C−H
Tipp2(Et)Si+, 6 (C1−H1)
Tipp2(Et)Si+, 6 (C2−H2)
[Me3Si−H−CMe3]+, 15
G
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−15 °C. 1H NMR (500.13 MHz, 300 K, CD3CN): δ = 7.72 (d, 3JHH
=
this compound disclosed a highly anisotropic, nearly oblate 29Si
NMR chemical shift tensor, which is characteristic for the local
C3 symmetry of the silylium ion. The close accordance between
experimental solid-state 29Si NMR chemical shift of compound
[1d]2[B12Cl12] and solution-state 29Si NMR chemical shift of its
[B(C6F5)4]− salt indicates that also in arene or haloarene
solution the tricoordinated silylium ion structure is preserved.
Furthermore, the good agreement between experimental data
and the computed 29Si NMR chemical shift obtained for
silylium ion 1d allows verifying molecular structures of silylium
ions obtained by theoretical methods. In this respect it is of
interest that NMR investigations of the Tipp-substituted
cations 1e, 6, and 12 indicate the onset of a multiple-center
bonding between the positively charged group 14 element
cation and the ortho-methine CH groups of the Tipp
substituents. This observation underscores the high electron
deficiency of tricoordinated group 14 element cations, and it
indicates their high potential in bond activation chemistry.
8 Hz, 6H, o-H), 7.88 (t, 3JHH = 8 Hz, 6H, m-H), 8.29 (t, 3JHH = 8 Hz,
3H, p-H). 13C{1H} NMR (125.70 MHz, 302.8 K, CD3CN): δ = 131.3,
141.4, 144.2, 213.3 (C+). 11B{1H} NMR (160.46 MHz, 296.9 K,
CD3CN): δ = −13.3.
2c, Duryl2(Me)SiH. A 5.3 g (46.07 mmol) portion of dichlor-
omethylsilane was added at 0 °C to a Grignard reagent freshly
prepared from 19.66 g (92.25 mmol) of bromodurene and 2.25 g
(92.55 mmol) of magnesium in 60 mL of THF. The resulting white
suspension was allowed to warm to room temperature and stirred
overnight. After refluxing for an additional hour, the mixture was
poured into an aqueous solution of ammonium chloride. The organic
phase was separated, and the aqueous phase extracted three times with
pentane. The organic fractions were combined and concentrated, and
the resulting colorless residue was recrystallized from ethanol to yield a
colorless solid (43%, 6.2 g). 1H NMR (499.87 MHz, 305 K, CDCl3): δ
3
= 0.71 (d, 3H, JHH = 5 Hz, Si-CH3), 2.17 (s, 12H, m-CH3), 2.26 (s,
12H, o-CH3), 5.35 (q, 3JHH = 5 Hz, 1JSiH = 193 Hz, 1H, Si-H), 6.95 (s,
2H, p-H). 13C{1H} NMR (125.71 MHz, 305 K, CDCl3): δ = 1.6 (Si-
CH3), 19.5 (o-CH3), 20.5 (m-CH3), 131.0, 133.6, 136.5, 139.8.
29Si{1H} NMR (99.31 MHz, 305 K, CDCl3): δ = −32.2. HR-MS (EI):
calcd 310.2117; found 310.2109. IR (ATR, neat): ν = 2151 (Si−H)
cm−1.
EXPERIMENTAL SECTION
■
General Procedures. Diethyl ether and tetrahydrofuran were
distilled from sodium/benzophenone. Benzene, toluene, [D6]benzene,
[D8]toluene, pentane, and hexane were distilled from sodium.
Dichloromethylsilane, bromodurene, bromotriisopropylbenzene, and
Cs2[B12H12] were purchased from ABCR and used as received.
Methyllithium in diethyl ether (1.6 M) and tert-butyllithium (1.6 M)
in hexane were purchased from Acros Organics and used as received.
NMR spectra were recorded on Bruker Avance 500 and Avance III
2f, Ph2(Me)SiH. Compound 2f has been previously reported in the
literature,34 but was prepared analogously to compound 2c, and
purified by distillation under reduced pressure. Bp: 65−70 °C/0.1
mbar (87% yield, 8.7 g). 1H NMR (499.87 MHz, 305 K, CDCl3): δ =
3
3
1
0.79 (d, JHH = 4 Hz, 3H, Si-CH3), 5.15 (q, JHH = 4 Hz, JSiH = 194
Hz, 1H, Si-H), 7.49−7.53 (m, 6H, Ar-H), 7.72−7.74 (m, 4H, Ar-H).
13C{1H} NMR (125.71 MHz, 305 K, CDCl3): δ = −5.0 (Si-CH3),
128.0 (m-CH), 129.5 (p-CH), 134.8 (o-CH), 135.3 (ipso-Cq). 29Si-
{1H} NMR (99.31 MHz, 305 K, CDCl3): δ = −17.5. IR (ATR, neat):
ν = 2117 (Si−H) cm−1.
1
500 spectrometers at 305 K, if not stated otherwise. H NMR spectra
were calibrated using residual protio signals of the solvent (δ1H-
(CHCl3) = 7.24, δ1H(C6D5H) = 7.20, δ1H(CHD2CN) = 1.94). 13C
NMR spectra were calibrated using the solvent signals (δ13C(CDCl3)
= 77.0, δ13C(C6D6) = 128.0, δ13C(CD3CN) = 1.32). 29Si NMR spectra
were calibrated against external Me2HSiCl (δ29Si(Me2SiHCl) = 11.1),
2g, o-Tol2(Me)SiH. Compound 2g has been previously reported in
the literature,35 but was prepared in an analogous manner to
compound 2c and purified by distillation under reduced pressure.
1
Bp: 90 °C/0.2 mbar (82% yield, 9.4 g). H NMR (499.87 MHz, 305
19F NMR spectra against external CFCl3 (δ19F(CFCl3) = 0.0), and 11
B
K, CDCl3): δ = 0.88−0.90 (m, 3H, Si-CH3), 2.58−2.60 (m, 6H, o-
NMR spectra against BF3·OEt2 (δ11B(BF3·OEt2) = 0.0). High-
resolution mass spectra were recorded on a Finnigan MAT 95. IR
spectra were recorded on a Bruker Tensor 27. X-ray diffraction
analyses were performed on a Bruker Apex II. Structure solution and
refinement was done using the SHELXS97 and SHELXL97 software.31
All reactions were carried out under inert conditions, using standard
Schlenk techniques and argon (5.0), unless stated otherwise.
[Ph3C][B(C6F5)4] was synthesized according to a modified literature
procedure.32 Synthesis and complete spectroscopic data of silanes
2a,b,d,e, 5a, and 7 and [B(C6F5)4]− salts of silylium ions 1a b,d,e and
6 have been reported previously.4 Spectroscopic parameters of the
[B(C6F5)4]− anion have also been reported earlier4 and are omitted for
better overview.
CH3), 5.38−5.42 (m, JSiH = 193 Hz, 1H, Si-H), 7.37−7.40 (m, 4H,
1
Ar-H), 7.50−7.53 (m, 2H, Ar-H), 7.69−7.71 (m, 2H, Ar-H). 13C{1H}
NMR (125.71 MHz, 305 K, CDCl3): δ = −4.9 (Si-CH3), 22.5 (o-
CH3), 125.1, 129.5, 129.8, 134.2, 135.5, 144.1. 29Si{1H} NMR (99.31
MHz, 305 K, CDCl3): δ = −23.6. IR (ATR, neat): ν = 2118 (Si−H)
cm−1.
5b, Mes2(i-Pr)SiH. An excess (2 equiv) of a freshly prepared
isopropyllithium solution in hexane was added dropwise to a solution
of 10 g (33.01 mmol) of dimesitylchlorosilane in hexane. The resulting
mixture was stirred overnight and subsequently refluxed for 2 h. Under
intense stirring, 50 mL of deionized water was carefully added and
stirring was continued for 1 h. Subsequently, the phases were
separated, and the aqueous phase was extracted with hexane twice. The
combined organic layers were concentrated and dried under vacuum
Cs2[B12Cl12]. This compound was obtained by chlorination of
Cs2[B12H12] as described by Gu et al.16d 11B{1H} NMR (160.38 MHz,
298.9 K, CD3CN): δ = −13.3.
1
to yield 5b as a colorless solid (80%, 8.2 g). H NMR (499.87 MHz,
305 K, CDCl3): δ = 1.13 (d, 3JHH = 7 Hz, 6H, CH-(CH3)2), 1.73 (sep,
3JHH = 7 Hz, 1H, CH-(CH3)2), 2.25 (s, 6H, p-CH3), 2.41 (s, 6H, o-
CH3), 4.94 (d, 3JHH = 7 Hz, 1JSiH = 189 Hz, 1H, Si-H), 6.81 (s, 4H, m-
CH). 13C{1H} NMR (125.71 MHz, 305 K, CDCl3): δ = 14.3 (CH-
(CH3)2), 19.9 (p-CH3), 21.0 (CH-(CH3)2), 23.4 (o-CH3), 128.8 (m-
C), 130.4 (Cq), 138.7 (Cq) 144.5 (Cq). 29Si{1H} NMR (99.31 MHz,
305 K, CDCl3): δ = −19.3. HR-MS (EI): calcd 310.2117; found
310.2125. IR (ATR, neat): ν = 2164 (Si−H) cm−1.
[Ph3C]2[B12Cl12] (modified literature procedure, ref 33). A 10 g
amount of Cs2[B12Cl12] (12.18 mmol) and 8.04 g of [Ph3C][BF4]
(24.35 mmol) were dissolved in 1000 mL of acetonitrile. The dark red
solution was stirred overnight, and the solvent subsequently removed
under vacuum to yield an orange, amorphous solid. The solid was
charged into a Soxhlet extractor and extracted with 1,2-dichloroethane
for several hours. When only a colorless solid (Cs[BF4]) remained in
the Soxhlet filter, the extraction was stopped and the extract stored at
−15 °C for 7 days. A large amount of yellow solid had precipitated,
from which the greenish mother liquor was decanted off. The solid was
washed once with toluene and twice with pentane. The obtained
[Ph3C]2[B12Cl12] material still contained a large amount of 1,2-
dichloroethane according to 1H and 13C NMR spectroscopy. The
solvent-free trityl salt could be obtained by drying the product under
vacuum for 3 h at 120 °C (65% yield, 16.4 g). Crystals suitable for X-
ray diffraction analysis were obtained from an acetonitrile solution at
5c, Mes2(t-Bu)SiH. This compound was prepared in an analogous
manner to 5b and obtained as a colorless solid (98% yield, 10.5 g). 1H
NMR (499.87 MHz, 305 K, CDCl3): δ = 1.21 (s, 9H, C-(CH3)3), 2.24
1
(s, 6H, p-CH3), 2.41 (s, 6H, o-CH3), 4.97 (s, JSiH = 190 Hz, 1H, Si-
H), 6.80 (s, 4H, m-CH). 13C{1H} NMR (125.71 MHz, 305 K,
CDCl3): δ = 20.9 (C-(CH3)3), 21.7 (C-(CH3)3), 24.1 (o-CH3), 29.1
(p-CH3) 128.8 (m-C), 131.4 (Cq), 138.5 (Cq) 144.4 (Cq). 29Si{1H}
NMR (99.31 MHz, 305 K, CDCl3): δ = −17.3. HR-MS (EI): calcd
324.2273; found 324.2266. IR (ATR, neat): ν = 2163 (Si−H) cm−1.
H
dx.doi.org/10.1021/om400366z | Organometallics XXXX, XXX, XXX−XXX

Organometallics
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5d, Mes2(Me3Si)SiH. A 0.9 g (129.68 mmol) sample of finely cut
lithium was added to a solution of 6 g (19.81 mmol) of
chlorodimesitylsilane in 40 mL of THF at 0 °C. The mixture was
stirred at that temperature for 5 h. Subsequently, the dark red
suspension was separated from excess lithium and 2.15 g (19.79
mmol) of chlorotrimethylsilane was added. The mixture was allowed
to warm to room temperature overnight. All volatiles were removed
under vacuum, and hexane was added. The suspension was filtrated,
and the filtrate was concentrated under vacuum to yield 1,1-dimesityl-
1c[B(C6F5)4], Duryl3Si[B(C6F5)4]. 1H NMR (499.87 MHz, 305 K,
C6D6): δ = 1.92 (s, 12H, m-H), 2.07 (s, 12H, o-H), 7.00 (s, 3H, p-
CH). 13C{1H} NMR (125.71 MHz, 305 K, C6D6): δ = 18.6 (o-CH3),
21.9 (p-CH3) 135.8 (Cq), 137.5 (Cq), 139.4 (Cq), 140.6 (p-CH). 13C
1
NMR (125.71 MHz, 305 K, C6D6): δ = 18.6 (q, JCH = 126 Hz, o-
1
CH3), 21.9 (q, JCH = 126 Hz, p-CH3) 135.8 (Cq), 137.5 (Cq), 139.4
(Cq), 140.6 (dm, 1JCH = 157 Hz, p-CH). 29Si{1H} NMR (99.31 MHz,
305 K, C6D6): δ = 226.5.
1
12a[B(C6F5)4], Mes3Ge[B(C6F5)4]. H NMR (499.87 MHz, 305 K,
1
2,2,2-trimethyldisilane as a colorless solid (87%, 5.87 g). H NMR
C6D6): δ = 1.99 (s, 18H, o-CH3), 2.08 (s, 9H, p-CH3), 6.63 (s, 6H, m-
CH). 13C{1H} NMR (125.71 MHz, 305 K, C6D6): δ = 21.2 (p-CH3),
23.0 (o-CH3), 130.0 (CH), 139.7 (Cq), 141.9 (Cq), 148.7 (Cq).
Me3GeH. MS (EI, 70 eV) m/z (rel intensity in %): 120 (61), 105
(100), 91 (9), 77 (13).
(500.13 MHz, 298 K, CDCl3): δ = 0.22 (s, 9H, Si-(CH3)3), 2.24 (s,
6H, p-CH3), 2.35 (s, 12H, o-CH3), 5.04 (s, 1JSiH = 173 Hz, 1H, Si-H),
6.80 (s, 4H, m-CH). 13C{1H} NMR (125.76 MHz, 298 K, CDCl3): δ
= −0.2 (Si-(CH3)3), 21.0 (p-CH3), 24.2 (o-CH3), 128.4 (m-C), 130.7
(Cq), 138.3 (Cq), 144.3 (Cq). 29Si{1H} NMR (99.36 MHz, 298 K,
CDCl3): δ = −53.6 (Mes2HSiSiMe3), 13.9 (Mes2HSiSiMe3). HR-MS
(EI): calcd 340.2043; found 340.2037. IR (ATR, neat): ν = 2138 (Si−
H) cm−1.
13[B(C6F5)4], Tipp2MeGe[B(C6F5)4]. 1H NMR (499.87 MHz, 305 K,
C6D6): δ = 1.05 (d, 3JHH = 6 Hz, 24H, o-CH-(CH3)2), 1.15 (d, 3JHH
=
6 Hz, 12H, p-CH-(CH3)2), 1.51 (s, 3H, Ge-CH3), 2.14 (sep, 3JHH = 6
3
Hz, 4H, o-CH-(CH3)2), 2.73 (sep, JHH = 6 Hz, 2H, p-CH-(CH3)2),
7.05 (s, 4H, m-CH). 13C{1H} NMR (125.71 MHz, 305 K, C6D6): δ =
20.8 (Ge-CH3), 23.2 (p-CH-(CH3)2), 24.3 (o-CH-(CH3)2), 34.9 (p-
CH3), 42.1 (o-CH3), 124.0 (m-CH), 136.1 (Cq), 153.0 (Cq), 158.8
(Cq).
11a, Mes2(Me)GeH. This compound was prepared as described by
Castel et al.36 1H NMR (499.87 MHz, 305 K, C6D6): δ = 0.81 (d, 3JHH
= 4 Hz, 3H, Ge-CH3), 2.16 (s, 6H, p-CH3), 2.37 (s, 6H, o-CH3), 5.58
3
(q, JHH = 4 Hz, 1H, Ge-H), 6.77 (s, 4H, m-CH). 13C{1H} NMR
(125.71 MHz, 305 K, C6D6): δ = 1.4 (Ge-CH3), 21.0 (p-CH3), 23.7
(o-CH3), 129.1 (m-C), 134.7 (Cq), 138.2 (Cq) 143.3 (Cq). HR-MS
(EI): calcd 328.1246; found 328.1249. IR (ATR, neat): ν = 2041
(Ge−H) cm−1.
1d2[B12Cl12], [Pemp3Si]2[B12Cl12]. In a typical reaction, 82 mg (0.079
mmol, 1.0 equiv) of [Ph3C]2[B12Cl12] and 80 mg (0.167 mmol, 2.15
equiv) of silane 2d were charged into a Schlenk tube and evacuated for
2 h. A 4 mL portion of ortho-dichlorobenzene was added and gave a
yellow suspension, which darkened slowly. After approximately 1 h the
mixture had changed into a yellow solution. To remove marginally
undissolved components, the mixture was filtrated using a filter funnel
(pore size 4). The solvent can be removed under vacuum to yield pure
1d2[B12Cl12] as a yellow solid. Crystals suitable for single-crystal X-ray
diffraction analysis were obtained from the ortho-dichlorobenzene
solution after four weeks at −15 °C.
11b, Tipp2(Me)GeH. A freshly prepared Grignard reagent from 17.5
g (61.8 mmol) of bromotriisopropylbenzene and 3.0 g (123.4 mmol)
of magnesium in 80 mL of THF was cooled to −60 °C. Then 6.4 g
(64.7 mmol) of pure, colorless copper(I) chloride was slowly added
through an addition funnel for solids, and the mixture was allowed to
warm to room temperature overnight. A 4.7 g (24.2 mmol) amount of
methyltrichlorogermane was added, and the mixture was subsequently
refluxed for 8 d. After the off-white suspension was cooled to room
temperature, 100 mL of 6 M hydrochloric acid was added. After
stirring for 10 min the organic layer was separated and the aqueous
layer was extracted with diethyl ether twice. All organic fractions were
combined, washed with deionized water once, and concentrated under
vacuum to give a colorless solid. The solid was dried under vacuum for
3 h and subsequently dissolved in diethyl ether. A 3.5 g (92.2 mmol)
portion of LiAlH4 was added, and the resulting gray suspension was
refluxed for 10 h. After the mixture was cooled to room temperature,
100 mL of hydrochloric acid was carefully added dropwise. The
mixture was stirred for 1 h, before the phases were separated and the
aqueous layer was extracted with diethyl ether twice. All organic layers
were combined and concentrated under vacuum to give 11b as a
colorless solid, still containing traces of triisopropylbenzene (53%
yield, 6.4 g). 1H NMR (499.87 MHz, 305 K, CDCl3): δ = 0.89 (d, 3JHH
= 4 Hz, 3H, Ge-CH3), 1.04 (m, 24H, o-CH-(CH3)2), 1.20 (d, 3JHH = 7
10[B(C6F5)4], Mes2(Me)Si/NCCH3[B(C6F5)4]. A 500 mg (0.54 mmol)
amount of [Ph3C][B(C6F5)4] and 154 mg (0.55 mmol) of silane were
charged into a Schlenk tube and evacuated for 2 h. A 2 mL sample of
acetonitrile was added, and the resulting mixture was stirred for 1 h. All
volatiles were evaporated, and 3 mL of benzene was added, resulting in
a two-phase reaction mixture. The upper phase was removed, and the
lower phase washed twice with small portions of benzene. The product
1
was dried under vacuum and obtained as a colorless solid. H NMR
(499.87 MHz, 305 K, C6D6): δ = 0.81 (s, 3H, Si-CH3), 1.08 (s, 3H,
NC-CH3), 2.05 (s, 12H, o-CH3), 2.13 (s, 6H, p-CH3), 6.72 (s, 4H, m-
H). 13C{1H} NMR (125.71 MHz, 305 K, C6D6): δ = 0.3 (Si-CH3), 4.5
(NC-CH3), 20.7 (p-CH3), 23.3 (o-CH3), 123.1, 128.5, 130.8, 143.6,
143.7. 29Si{1H} NMR (99.31 MHz, 305 K, C6D6): δ = 10.6.
Mes3GeH by Derivation of 12a[B(C6F5)4]. A 197 mg (0.68 mmol)
amount of n-Bu3SnH was slowly added via a Hamilton syringe to a
well-stirred biphasic reaction mixture of 832 mg (0.75 mmol) of
12a[B(C6F5)4] in benzene. During the addition the dark color of the
reaction mixture lightened. After completed addition, stirring was
stopped and the phases were allowed to separate. The top layer,
containing the neutral germane, was separated via PTFE cannula, the
solvent was evaporated, and the colorless solid residue was analyzed by
GC/MS and NMR spectroscopy. 1H NMR (CDCl3, 499.87 MHz, 305
K): δ = 2.15 (s, 18H, o-CH3), 2.25 (s, 9H, p-CH3), 5.85 (s, 1H, GeH),
6.79 (s, 6H, m-CH). 13C{1H} NMR (CDCl3, 125.69 MHz, 305 K): δ
= 21.0 (s, p-CH3), 23.6 (s, o-CH3), 128.8 (s, m-CH), 134.9 (s, p-Cq),
138.2 (s, o-Cq), 143.6 (s, ipso-Cq). IR (ATR, neat): ν [cm−1] 2031 (m,
Ge−H). MS (EI, 70 eV) m/z (rel intensity in %): 431 (1), 312 (30),
192 (100), 119 (36), 105 (83), 91 (30), 77 (18).
3
Hz, 12H, p-CH-(CH3)2), 2.81 (sep, JHH = 7 Hz, 2H, p-CH), 3.25
(sep, 3JHH = 7 Hz, 4H, o-CH), 5.45 (q, 3JHH = 4 Hz, 1H, Ge-H), 6.92
(s, 4H, m-H). 13C{1H} NMR (125.71 MHz, 305 K, CDCl3): δ = 2.9
(Ge-CH3), 23.9 (CH3), 24.3 (CH3), 24.6 (CH3), 33.7 (o-CH-
(CH3)2), 34.1 (p-CH-(CH3)2), 121.2 (m-C), 134.5 (Cq), 149.1 (Cq)
153.5 (Cq). HR-MS (EI): calcd 496.3124; found 496.3117. IR (ATR,
neat): ν = 2026 (Ge−H) cm−1.
Standard Procedure for Preparation of Silylium and
Germylium Borates. In a typical reaction, 500 mg (0.54 mmol) of
[Ph3C][B(C6F5)4] and, in the case of solid starting material, the
corresponding amount of silane or germane (1.6 equiv) were charged
into a Schlenk tube and evacuated for 2 h. A 4 mL amount of benzene
was added, resulting in a biphasic reaction mixture. In the case of liquid
reactants, the silane or germane was added after dissolving the trityl
salt via syringe. The resulting mixture was stirred for 1 h, and in the
case of Tipp-substituted precursors, for 5 h. After the allotted time,
stirring was turned off and the two layers were allowed to separate.
The upper layer was removed, and the lower layer washed twice with
small portions of benzene. The product was dried under vacuum for 5
min. The triarylsilylium or triarylgermylium borates were isolated as
yellow solids.
ASSOCIATED CONTENT
■
S
* Supporting Information
Relevant NMR spectra, X-ray crystallographic information for
compounds 1d[B(C6F5)4], 2d, and [Ph3C]2[B12Cl12]; details of
the solid-state NMR measurements and all computational
details including a table of absolute energies and Cartesian
I
dx.doi.org/10.1021/om400366z | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(11) Lambert, J. B.; Zhang, S.; Ciro, S. M. Organometallics 1994, 13,
2430.
(12) The Gaussian 03, Rev D.02 version of the program was used.
coordinates of pertinent molecular structures. This material is
available free of charge via the Internet at http://pubs.acs.org.
(13) See the Supporting Information for further details.
AUTHOR INFORMATION
■
(14) (a) Muller, T.; Bauch, C.; Ostermeier, M.; Bolte, M.; Auner, N.
̈
Corresponding Author
*E-mail: thomas.mueller@uni-oldenburg.de.
Notes
J. Am. Chem. Soc. 2003, 125, 2158. (b) Ichinohe, M.; Fukui, H.;
Sekiguchi, A. Chem. Lett. 2000, 600.
(15) Germylium salts reported previously: (a) Sekiguchi, A.;
Tsukamoto, M.; Ichinohe, M. Science 1997, 275, 60. (b) Sekiguchi, A.;
Fukawa, T.; Lee, V. Ya.; Nakamoto, M.; Ichinohe, M. Angew. Chem.,
Int. Ed. 2003, 42, 1143. (c) Schenk, C.; Drost, C.; Schnepf, A. Dalton
Trans. 2009, 773.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(16) (a) Kessler, M.; Knapp, C.; Sagawe, V.; Scherer, H.; Uzun, R.
Inorg. Chem. 2010, 49, 5223. (b) Avelar, A.; Tham, F. S.; Reed, C. A.
This work is dedicated to Prof. Werner Uhl on occasion of his
60th birthday. 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.
Angew. Chem., Int. Ed. 2009, 48, 3491. (c) Muther, K.; Frohlich, R.;
̈
̈
Muck-Lichtenfeld, C.; Grimme, S.; Oestreich, M. J. Am. Chem. Soc.
̈
2011, 133, 12442. (d) Gu, W.; Ozerov, O. V. Inorg. Chem. 2011, 50,
2726.
(17) These results also imply a prolate 29Si shielding tensor (σ11 = σ22
< σ33).
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dx.doi.org/10.1021/om400366z | Organometallics XXXX, XXX, XXX−XXX