The family of ferrocene-stabilized silylium ions: Synthesis, 29Si NMR characterization, lewis acidity, substituent scrambling, and quantum-chemical analyses

Article abstract of DOI:10.1002/chem.201302885

The purpose of this systematic experimental and theoretical study is to deeply understand the unique bonding situation in ferrocene-stabilized silylium ions as a function of the substituents at the silicon atom and to learn about the structure parameters

Full text of DOI:10.1002/chem.201302885

FULL PAPER
DOI: 10.1002/chem.201302885
29
The Family of Ferrocene-Stabilized Silylium Ions: Synthesis, Si NMR
Characterization, Lewis Acidity, Substituent Scrambling, and Quantum-
Chemical Analyses
Kristine Mꢀther,[a, b] Peter Hrobꢁrik,*[a, c] Veronika Hrobꢁrikovꢁ,[a, d]
Martin Kaupp,*[a] and Martin Oestreich*[a, b]
Abstract: The purpose of this systemat-
ic experimental and theoretical study is
to deeply understand the unique bond-
ing situation in ferrocene-stabilized si-
lylium ions as a function of the sub-
stituents at the silicon atom and to
learn about the structure parameters
that determine the 29Si NMR chemical
shift and electrophilicity of these
strong Lewis acids. For this, ten new
members of the family of ferrocene-
stabilized silicon cations were prepared
by a hydride abstraction reaction from
silanes with the trityl cation and char-
acterized by multinuclear 1H and 29Si
NMR spectroscopy. A closer look at
the NMR spectra revealed that addi-
tional minor sets of signals were not
impurities but silylium ions with substi-
tution patterns different from that of
the initially formed cation. Careful as-
signment of these signals furnished ex-
perimental proof that sterically less
hindered silylium ions are capable of
exchanging substituents with unreacted
silane precursors. Density functional
theory calculations provided mechanis-
tic insight into that substituent transfer
in which the migrating group is ex-
changed between two silicon fragments
in a concerted process involving a ferro-
cene-bridged intermediate. Moreover,
the quantum-chemical analysis of the
29Si NMR chemical shifts revealed
a linear relationship between dACHTUNGTRENNUNG
(29Si)
values and the Fe···Si distance for sub-
sets of silicon cations. An electron lo-
calization function and electron localiz-
ability indicator analysis shows a three-
center two-electron bonding attractor
between the iron, silicon, and C’ipso
atoms, clearly distinguishing the silicon
cations from the corresponding carbe-
nium ions and boranes. Correlations
between 29Si NMR chemical shifts and
Lewis acidity, evaluated in terms of
fluoride ion affinities, are seen only for
subsets of silylium ions, sometimes with
non-intuitive trends, indicating a com-
plicated interplay of steric and elec-
tronic effects on the degree of the
Fe···Si interaction.
Keywords: density functional theo-
ry · donor–acceptor interactions ·
Lewis acids · silylium ions · sub-
stituent scrambling
Introduction
The pronounced electron deficiency of tricoordinate silicon
cations (silylium, silylenium, or silicenium ions) coerces
these strong Lewis acids to seek stabilization by electron
density in their proximity.[1] Even p-basic arene solvents[2]
and weakly s-coordinating counteranions, such as Reedꢀs
[HCB11H5Br6]ꢀ anion,[3] are not innocent and coordinate to
[a] K. Mꢁther, Dr. P. Hrobꢂrik, Dr. V. Hrobꢂrikovꢂ, Prof. Dr. M. Kaupp,
Prof. Dr. M. Oestreich
Institut fꢁr Chemie, Technische Universitꢃt Berlin
Strasse des 17. Juni 115, 10623 Berlin (Germany)
E-mail: martin.kaupp@tu-berlin.de
ꢀ
the silicon atom. The Si H bond of a remaining precursor
martin.oestreich@tu-berlin.de
will also interact with the empty orbital at the silicon atom,
a
three-center two-electron (3c2e) [Si···H···Si]+
[b] K. Mꢁther, Prof. Dr. M. Oestreich
Organisch-Chemisches Institut
forming
bridge.[4] These intermolecular Lewis acid–base interactions
had thwarted the isolation of the free cation,[2–4] and it was
steric bulk around the trigonal planar silicon atom that
Westfꢃlische Wilhelms-Universitꢃt Mꢁnster
Corrensstrasse 40, 48149 Mꢁnster (Germany)
[c] Dr. P. Hrobꢂrik
ACTHNUTRGNEUNG
eventually resulted in success (Mes3Si+[B(C6F5)3]ꢀ with
Institute of Inorganic Chemistry, Slovak Academy of Sciences
Dfflbravskꢂ cesta 9, 84536 Bratislava (Slovakia)
E-mail: peter.hrobarik@savba.sk
29Si NMR (C6D6): d=225.5 ppm).[5]
Conversely, the same effects were deliberately utilized to
intramolecularly stabilize silylium ions, and several beautiful
molecules were designed in which the Lewis basic group is
cleverly positioned to allow for controlled taming of the
Lewis acidity (Figure 1). An elegant implementation using
an alkene p donor is seen in the preparation of a silicon var-
[d] Dr. V. Hrobꢂrikovꢂ
Department of Organic Chemistry
Faculty of Natural Sciences, Comenius University
Mlynskꢂ dolina, 84215 Bratislava (Slovakia)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302885.
Chem. Eur. J. 2013, 19, 16579 – 16594
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
16579

erating ferrocene (Fc) stabilized silylium ions had not been
new at that time as there were reports by Corey[14] and Man-
ners[15] of anion- and solvent-coordinated systems, respec-
tively. A purely intramolecularly stabilized, that is, free, a-
ferrocenylsilylium ion had, however, been elusive until we
accomplished its preparation and spectroscopic characteriza-
tion ([FcSiACTHNUTRGNEUNG
(tBu)Me]+; Figure 1, upper left).[16]
We were recently able to determine the molecular struc-
ture and the bonding situation of our ferrocene-stabilized
silicon cation [FcSiACTHNUTRGNEUNG
(tBu)Me]+ by crystal structure analysis
and quantum-chemical calculations (Figure 2, upper).[17] The
Figure 1. Selected intramolecularly stabilized silylium ions with 29Si NMR
chemical shifts.
Figure 2. Molecular structure of our ferrocene-stabilized silylium ion
[FcSiACHTNUGRTNEUNG
(tBu)Me]+ with relevant data (boranate counter anion [B12Cl12]2ꢀ is
omitted for clarity).
iant of the norbornyl cation (1; Figure 1, upper right).[6] The
Baldridge/Siegel team recently introduced a new class of sil-
icon cations with various terphenyl substituents. The roof-
like structure not only provides kinetic stabilization through
steric shielding of the silicon cation but also thermodynamic
stabilization through fast oscillation of the cationic silicon
atom between the p systems of the flanking arenes (2;
Figure 1, middle left).[7] A similar effect was achieved by in-
strong interaction between the cationic silicon atom and the
ferrocene backbone is reflected in short distances between
the silicon atom and the iron atom (d=2.492 Š) as well as
the ipso carbon atom of the lower Cp ring (d’=2.784 Š). To
accommodate these bonds, a large dip angle (a*=44.88) of
the silicon atom out of the plane of the upper Cp ligand re-
sults (Figure 2, lower). For comparison, that dip angle is sub-
stantially smaller (a*=20.78) in the phenyl-substituted a-
ferrocenylcarbenium ion, in which the positive charge is par-
tially delocalized into the upper Cp ligand, now showing ful-
vene character.[12,18]
teraction with neutral halogen atoms (3, middle right).[8]
A
conceptually novel approach to intramolecular silicon cation
stabilization was presented by Mꢁller (4 and 5, lower left
and middle).[9] Intramolecular 3c2e [Si···H···Si]+ bridges de-
localize the positive charge in such a way that steric shield-
ing is not required anymore. Several congeners are known
today (not shown),[1] and the structure of 6 (Figure 1, lower
right) is particularly noteworthy.[10] In all these cases, coordi-
nation of neither solvent nor counteranion is an issue. Incor-
poration of the silicon atom into cyclic p-conjugated systems
is another general strategy to stabilize silylium ions in their
free form (not shown).[11]
We follow a different approach to intramolecular stabili-
zation of the tricoordinate silicon cation. The well-under-
stood stabilization of the isoelectronic carbenium ions[12] and
related boranes[13] by an electron-rich metallocenyl group at-
tached to it stimulated us a few years ago to consider the re-
lated metallocene-substituted silylium ions. The idea of gen-
Based on our previous work on a single member, we in-
troduce here a large family of ferrocene-stabilized silylium
ions bearing both aryl (including additional ferrocenyl) and
alkyl substituents at the cationic silicon atom. Our account
comprises full spectroscopic characterization with experi-
mental 29Si NMR chemical shifts confirmed by quantum-
chemical calculations. A detailed analysis of chemical shifts,
stability and Lewis acidity, and their correlation with struc-
tural and electronic factors is provided. Moreover, careful
1
analysis of H and 29Si NMR spectra reveals scrambling of
substituents at the silicon atom, and it is possible to assign
the additional sets of signals to silicon cations with redistrib-
uted substitution patterns. Supported by quantum-chemical
16580
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ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 16579 – 16594

Ferrocene-Stabilized Silylium Ions
FULL PAPER
Table 1. NMR spectroscopic data of all ferrocenylsilanes 7a–7k and silylium ions 9a–9k as well as an over-
view of observed substituent scrambling products (Fc=ferrocenyl).
calculations, we present a con-
certed reaction mechanism for
the substituent exchange be-
tween the initially formed silyli-
um ion and remaining silane
precursor.
Silane precursor[a]
Silylium ion[b]
Substituent
Entry
R
R’
Cmpd
1H NMR
29Si NMR
[ppm]
Cmpd
29Si NMR
ACHTUNGTNER[NUNG ppm]
scrambling
products
Computational Methods
ꢀ
Si H [ppm]
G
The ground-state structures of all sys-
tems under investigation were fully
optimized (without counterion) at the
PBE0 level of theory,[19] including an
atom-pairwise correction for disper-
sion forces (Grimmeꢀs D3 model)[20]
and employing def2-TZVPP basis
sets[21] for all atoms. For comparative
purposes, the selected structures were
also optimized at the PBE0/def2-
TZVPP level without dispersion cor-
rections and at the TPSS-D3/def2-
1
2
3
4
5
6
7
8
tBu
iPr
tBu
iPr
Me
Ph
Ph
Fc
tBu
iPr
Me
Me
Me
tBu
Me
Fc
7a
7b
7c
7d
7e
7f
7g
7h
7i
4.16
4.29
4.36
4.46
4.69
4.83
5.18
5.73
5.21
5.72
5.78
7.6[c]
1.4[c]
9a
9b
9c
9d
9e
9 f
9g
9h
9i
120.3[d]/120.9[e]
114.1[d]/114.7[e]
113.9[d]/114.4[e]
113.5[d]/113.4[e]
111.0[d]/110.7[e]
98.8[d]/98.6[e]
93.8[e]
–
–
ꢀ2.7[c]
9l[f]
ꢀ7.6[c]
9m[g]
ꢀ18.8[c]
ꢀ0.4[c]
9i
9l[f]
ꢀ18.5[c]
ꢀ21.4[c]
ꢀ20.5[c]
ꢀ17.7[c]
ꢀ18.3[c]
9e, 9i, 9j, 9k
–
9h
9k
9h
91.3[d]/91.3[e]
88.5[d]/88.3[e]
81.4[d]/81.0[e]
77.5[d]/77.4[e]
9
10
11
Fc
Ph
Fc
Me
Ph
Ph
7j
7k
9j
9k
[a] NMR data for all silanes were measured in C6D6. [b] Silylium ions were generated according to general
procedure 2 (GP 2, see the Experimental Section) as a solution (175–236 mm) in 1,2-Cl2C6D4 and directly sub-
jected to NMR spectroscopic analysis. [c] 29Si DEPT NMR spectroscopy. [d] 1H,29Si HMQC NMR spectrosco-
py. [e] 29Si NMR spectroscopy. [f] Silylium ion 9l (R=Fc, R’=tBu) was only obtained from substituent scram-
bling; 29Si NMR: d=104.5 ppm. [g] Silylium ion 9m (R=Fc, R’=iPr) was only obtained from substituent
scrambling; 29Si NMR: d=100.4 ppm.
TZVPP[22] level used in
a previous
study.[17] The optimized structures
were characterized as true minima on
the potential energy hypersurface by
harmonic vibrational frequency analy-
ses. Transition-state structures were lo-
cated as follows: initially detailed ge-
ometry scans between the two local
synthesized in good yields (49–91%) by monolithiation of
ferrocene using tBuLi and a catalytic amount of KOtBu[37,38]
and subsequent treatment with an appropriate monochloro-
silane. Ferrocenylmethylphenylsilane (7g) was obtained in
a slightly lower yield of 48% from monolithiated ferrocene
and dichloromethylphenylsilane followed by reduction with
LiAlH4. Using an excess of monolithiated ferrocene and the
corresponding dichlorosilane, diferrocenylsilanes 7i and 7k
were accessible in the same way in 66 and 76% yield, re-
spectively. For bulkier substituents (iPr and tBu), the synthe-
sis of diferrocenylsilanes was not successful, probably due to
the steric bulk around the silicon atom. Nevertheless, we
succeeded in the preparation of the sterically very hindered
triferrocenylsilane (7h) in a moderate yield of 31%.[39]
minima were performed. The maxima obtained were consequently used
as starting points of full transition-state optimizations. The transition
states were verified by subsequent harmonic vibrational frequency calcu-
lations. All these calculations were carried out with Turbomole.[23] Calcu-
lations of NMR nuclear shieldings were performed in the Gaussian09
program package[24] using gauge-including atomic orbitals (GIAO)[25] at
the PBE0 level, employing modified TZVP all-electron basis set of
(15s11p6d)/ACHTUNGTRENNUNG[9s7p4d] quality for iron and Huzinaga–Kutzelnigg-type
IGLO-III basis set for all other atoms.[26,27] The calculated 1H and
29Si NMR shieldings were converted to chemical shifts (d in ppm) relative
to the shieldings of tetramethylsilane (TMS). In these calculations, bulk
solvent effects were simulated by means of the integral equation formal-
ism of the polarizable continuum model (IEF-PCM)[28] whereas a conduc-
tor-like screening solvation model (COSMO)[29] was employed in struc-
ture optimizations and energy evaluations. In both cases, 1,2-Cl2C6H4
with er =9.8 was considered as the solvent. The structures of the selected
silylium ions remained essentially unaffected when a continuum solvation
model was applied in the optimization procedure (this does not hold for
the energies though) and, therefore, only the gas-phase structures were
used throughout this work. The wave functions were analyzed by means
of the electron localization function (ELF)[30] and the electron localizabil-
ity indicator (ELI-D),[31] respectively, using the DGrid program[32] with
grids using ten points per Bohr. Mayer bond orders[33] were calculated
using the program BORDER.[34] The results of ELF and ELI-D analyses
were visualized by using the ParaView program.[35]
The NMR spectroscopic characterization of ferrocenylsi-
lanes 7a–7k was carried out in C6D6. Of particular interest
1
ꢀ
are the chemical shifts of the Si H functionality in the H
and 29Si NMR spectra (Table 1, columns 5 and 6). The hy-
dride shifts are observed in the range of d=4.16–5.78 ppm
depending on the substitution pattern at the silicon atom.
For dialkyl-substituted ferrocenylsilanes 7a–7e a shift be-
tween d=4.16–4.69 ppm is obtained, whereas mono- and
diaryl-substituted silanes 7 f–7k lead to a more deshielded
signal between d=4.83–5.78 ppm. 29Si NMR spectroscopy
shows an opposite trend: more aryl substituents at the sili-
con atom result in a low-frequency shifted signal.
Results and Discussion
Silylium ion precursors; silane preparation: Silylium ions
are commonly prepared by hydride abstraction from the cor-
responding silane precursor using [Ph3C]+[B
For the generation of ferrocene-stabilized silicon cations, we
prepared a series of differently substituted ferrocenylsilanes
7a–7k (Table 1). Monoferrocenylsilanes 7a–7 f and 7j were
(C6F5)4]ꢀ (8).[36]
Silylium ions: With these ferrocenylsilanes (7a–7k) in hand,
we were able to synthesize a new family of ferrocene-stabi-
lized silicon cations.[40] All silylium ions 9a–9k were rapidly
generated from the corresponding silanes at room tempera-
Chem. Eur. J. 2013, 19, 16579 – 16594
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16581

P. Hrobꢂrik, M. Kaupp, M. Oestreich et al.
ture using Coreyꢀs hydride abstraction reaction (Table 1, 7!
9).[36] The samples were prepared in 1,2-Cl2C6D4 and directly
subjected to multinuclear NMR spectroscopy.
bonds through participation of both the upper and the lower
Cp rings of the ferrocene sandwich structure. The unusual
bonding situation is apparent also in the Mayer bond orders
(MBO) between the silicon atom and its neighbors (Table 2
and Table 3). Furthermore, the Fe···Si distance in [FcSi-
Structure, bonding, and stabilization energies: An unprece-
dented bonding motif in ferrocene-stabilized silylium ions
had already been indicated in the previous structural charac-
AHCTUNGTRENNUNG
(tBu)Me]+ determined experimentally and computationally,
d
(Fe···Si)ꢁ2.49 Š, is close to the sum of iron and silicon
terization of the first member of this series, [FcSi
(tBu)Me]+
single-bond covalent radii (1.16+1.16=2.32 Š).[41] We note
in passing that the Fe···Si distances and dip angles are close-
ly correlated in the studied series (Figure S1 in the Support-
ing Information).
[BACHTUNGTRENNUNG
(C6F5)4]ꢀ, on the basis of localized molecular orbitals
(LMOs).[17] Specifically, the extreme dip angle of the silicon
atom toward the iron center was rationalized by two 3c2e
In an attempt to analyze the nature of the inter-
actions between the silicon and iron atoms and the
Table 2. Computed interatomic distances, dip angles, stabilization energies, and Mayer
bond orders (MBO) in ferrocene-stabilized electrophiles (Scheme 1).[a]
Cp ligands further, we examined here the real-
space functions ELF (electron localization function)
and ELI-D (electron localizability indicator). The
results of the ELI-D analysis on silicon cation
[FcSiMe2]+ and carbocation [FcCMe2]+ are shown
in Figure 3 (the very similar ELF picture is given in
Figure S2 in the Supporting Information; see also
Table S1 for the values of ELF and ELI-D attrac-
tors). A three-center-bonding attractor between
iron, silicon and the C’ipso atom of the lower Cp ring
can be clearly identified. The presence of this at-
tractor clearly distinguishes the ferrocene-stabilized
silylium ions from the corresponding carbenium
ions, consistent with the large dip angle between
Distances
d(Fe···E) d(E···C’ipso
MBO
Fe···E E···C’ipso
System[b]
)
a*
[8]
DEr
[kJmolꢀ1
DEstab
[kJmolꢀ1
]
[c]
[d]
[Š]
[Š]
G
]
N
FcBMe2
2.921
2.989
2.513
2.888
3.294
3.061
2.596
2.718
2.823
2.987
13.4 ꢀ26.1
32.5 ꢀ32.1
29.1 ꢀ85.8
45.0 ꢀ106.8
43.0 ꢀ89.6
41.6 ꢀ79.1
37.3 ꢀ58.7
ꢀ3.5
0.09
0.12
0.27
0.44
0.34
0.31
0.26
0.01
0.07
0.03
0.26
0.20
0.17
0.15
FcAlMe2
ꢀ12.5
ꢀ16.9
ꢀ58.9
ꢀ45.6
ꢀ40.9
ꢀ24.8
ACHTUNGTRENNUNG
[FcCMe2]+
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
[a] PBE0-D3/def2-TZVPP results (see the Computational Methods for more detail).
[b] E atoms are given in bold. [c] See Scheme 1 for the reaction; R=R’=Me. [d] Sta-
bilization energy calculated as a difference between energies of the fully optimized
structure and corresponding structure optimized with a fixed dip angle a*=0.08.
Table 3. Optimized structural parameters, relative stabilities (DEr and DEstab), fluoride ion affinities (FIA), Mayer bond orders (MBO), and computed
29Si NMR chemical shifts in a series of ferrocene-stabilized silylium ions [FcSiR(R’)]+.[a,b]
R
R’
d
[Š]
(Fe···Si)
d(Si···C’ipso
[Š]
)
a*
[8]
DEr
[kJmolꢀ1
DEstab
[kJmolꢀ1
FIA
[kJmolꢀ1
MBO
Si···C’ipso
dACHTUNGTRENNUNG
G
]
G
]
G
]
Fe···Si
ACHTUNGTRENNUNG
Me
iPr
tBu
iPr
tBu
Ph
Ph
Mes
Me
Me
Me
iPr
tBu
Me
tBu
Me
2.462
2.477
2.487
2.491
2.539
2.488
2.493
2.593
2.688
2.515
2.535
2.534
2.528
2.599
2.730
2.656
2.569
2.472
2.463
2.585
2.545
2.559
2.627
2.589
2.615
2.596
2.604
2.616
2.603
2.706
2.640
2.606
2.865
3.018
2.688
2.706
2.702
2.692
2.866
3.076
2.821
2.734
2.645
2.637
2.732
2.682
2.706
2.777
2.759
2.762
45.0
44.6
44.2
44.1
42.3
44.1
44.1
40.3
36.7
43.2
42.4
42.5
42.8
40.1
35.6
37.9
41.2
44.8
45.1
40.4
42.0
41.5
38.8
40.4
39.3
ꢀ106.8
ꢀ105.0
ꢀ104.9
ꢀ106.0
ꢀ96.6
ꢀ90.5
ꢀ95.8
ꢀ59.6
ꢀ62.2
ꢀ77.1
ꢀ69.7
ꢀ71.5
ꢀ67.4
ꢀ58.2
ꢀ38.2
ꢀ43.4
ꢀ58.2
ꢀ89.4
ꢀ93.7
ꢀ38.5
ꢀ37.3
ꢀ27.9
ꢀ37.0
ꢀ27.9
ꢀ27.0
ꢀ58.9
ꢀ56.1
ꢀ54.3
ꢀ52.0
ꢀ41.7
ꢀ45.6
ꢀ50.4
ꢀ18.7
ꢀ19.2
ꢀ36.6
ꢀ31.3
ꢀ33.0
ꢀ30.9
ꢀ21.5
ꢀ6.7
816.6
814.8
816.0
809.2
818.3
806.7
810.0
815.1
802.0
798.8
786.7
783.0
792.1
810.2
786.3
744.3
775.1
852.4
856.3
789.9
785.4
792.2
784.0
779.4
780.3
0.436
0.427
0.420
0.425
0.423
0.449
0.440
0.416
0.390
0.457
0.444
0.449
0.450
0.456
0.429
0.360
0.418
0.488
0.496
0.384
0.383
0.400
0.444
0.403
0.409
0.255
0.279
0.293
0.299
0.285
0.264
0.273
0.220
0.200
0.281
0.270
0.277
0.272
0.212
0.162
0.210
0.255
0.305
0.308
0.239
0.270
0.297
0.276
0.241
0.245
114.7
116.3
115.9
118.3
125.3
95.2
98.7
114.1
128.7
85.8
86.6
86.8
87.8
88.7
108.1
92.9
86.8
81.4
81.0
89.1
102.3
107.7
73.6
99.2
87.7
Mes
Ph
tBu
Ph
4-Tolyl
3,5-Xylyl
2-Naphthyl
1-Naphthyl
Mes
p-NMe2-Ph
p-OMe-Ph
p-CN-Ph
p-NO2-Ph
Fc
4-Tolyl
3,5-Xylyl
2-Naphthyl
1-Naphthyl
Mes
p-NMe2-Ph
p-OMe-Ph
p-CN-Ph
p-NO2-Ph
Me
ꢀ11.6
ꢀ22.7
ꢀ44.2
ꢀ47.2
ꢀ39.8
ꢀ38.9
ꢀ35.0
ꢀ31.4
ꢀ24.3
ꢀ21.1
Fc
Fc
iPr
tBu
Fc
Ph
Fc[d]
Fc[d]
Fc[d]
Fc[d]
[a] PBE0-D3/def2-TZVPP results in the gas-phase if not stated otherwise (see the Computational Methods for more detail). [b] Only structure parame-
ters corresponding to the primary Lewis acid–base Fc···Si interactions are given here (see Table 4 for secondary through-space interactions in compounds
containing more than one Fc unit). [c] PBE0/IGLO-III//PBE0-D3/def2-TZVPP results using an IEF-PCM solvation model. [d] Data reported for the two
minima found for [FcSiFc2]+.
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Ferrocene-Stabilized Silylium Ions
FULL PAPER
the fully optimized silylium ions and the corresponding par-
tially optimized structures with a fixed dip angle a*=0.08.
In case of the multiply ferrocene-stabilized systems, DEstab in
Table 3 refers to only the primary interaction obtained indi-
rectly by fixing a* to 0.08 for all Fc moieties and subtracting
the energy of the secondary interaction(s) given in Table 4.
Among the group 14 elements, the largest stabilization is
indeed obtained for the silylium system. A decrease in the
stabilization energy with increasing atomic number from sili-
con to lead is accompanied by decreased dip angles a*
(from 45.08 to 37.38). The most pronounced stabilization of
silylium ion can be explained by: 1) easier bending of the
Figure 3. ELI-D analysis of bonding in the ferrocene-stabilized a) silicon
cation [FcSiMe2]+, and b) in the corresponding carbocation [FcCMe2]+.
The selected planes include the iron (in red), C’ipso and positively charged
silicon (in yellow) or carbon (in dark grey) atoms.
ꢀ
Cipso Si bond out of the Cp ring plane compared to the car-
ꢀ
the Si Cipso bond and the upper
Table 4. Computed interatomic distances, dip angles, and stabilization energies of the secondary through-
space interactions in silylium ions [FcSiR(R’)]+ with more than one Fc unit.[a]
Cp plane. Going down in the
periodic table, a corresponding
bonding attractor is also found
in the analogous ferrocene-sta-
bilized germanium(IV) cation
but not in the tin(IV) and
lead(IV) congeners (Figure S3
in the Supporting Information).
A second 3c2e bond between
iron, silicon, and Cipso of the
upper ring is not evident from
Unit B
)
Unit C
(Si···Fe) d(Si···C’ipso) dAHCTUNRGTENUNN(G Si-Cipso) a* DEstab
R
R’
d
[Š]
G
d
[Š]
E
)
a*
[8]
d
G
[b]
[Š]
[Š]
[Š]
[8]
[kJmolꢀ1
ACHTUNGTERNNUNG ]
Fc
Fc
Fc
Fc
Me 3.093
iPr 3.295
tBu 3.340
3.324
3.605
3.656
3.285
3.661
3.393
1.800
1.813
1.825
1.801
1.809
1.804
18.4
8.6
6.4
19.8
6.2
16.6
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
4.5
1.5
1.0
4.4
Ph
3.065
Fc[c] Fc[c] 3.332
Fc[c] Fc[c] 3.135
3.458
3.521
3.907
4.056
1.819
1.822
1.7 1.4
4.5 4.6
[a] PBE0-D3/def2-TZVPP results. [b] Secondary “through-space” stabilization energy DEstab evaluated as a dif-
ference between energies of the fully optimized silylium ions and the corresponding partially optimized struc-
the ELF and ELI-D analysis. tures with a fixed dip angle a* in Fc unit B (and unit C where applicable) to 0.08. [c] Data reported for the
two minima found for [FcSiFc2]+.
However, the presence of such
an interaction cannot be ex-
cluded as the corresponding at-
tractor could overlap with that
ꢀ
of the Si Cipso bond. The moderate Mayer bond orders for
a Fe···E+ pair in [FcEMe2]+ ions (Table 2) as well as the
non-negligible dip angle and stabilization energies DEstab in
all these systems (see below) hint at the attractive intramo-
lecular donor–acceptor interaction between the electrophilic
center and the metallocene backbone.
benium system, and 2) a smaller atomic radius of silicon
compared to the heavier congeners leading to somewhat
more favorable interactions (see also Mayer bond orders in
Table 2).
Isoelectronic neutral species containing boron or alumi-
num instead of the positively charged silicon atom display
much smaller dip angles (a*=13.48 and 32.58, respectively)
and lower stabilization energies. No ELF or ELI-D bonding
attractors between iron and the electrophilic center are
found in these species.[42]
To quantify the strength of the ferrocene–electrophile in-
teraction, we calculated the energy of the isodesmic reac-
tion, DEr, where a Ph moiety in the [PhER(R’)]q series (E=
B, Al/C, Si, Ge, Sn, Pb; q=0/+1; R and R’ stand for alkyl
or aryl) is replaced by an Fc unit (Scheme 1). Note that DEr
contains also contributions from stabilization due to a some-
what stronger electron-donating effect of a metallocene
backbone compared to the Ph substituent, in addition to the
possible presence of a direct Fe···E donor–acceptor interac-
tion. For this reason, we calculated additionally a stabiliza-
tion energy DEstab as a difference between the energies of
Among the silylium ions (Table 3), the largest ferrocenyl
stabilization energies (DEr and DEstab) are generally found
for the dialkyl-substituted species, the weakest for the diaryl
analogues, particularly for those containing more than one
Fc unit. The latter derivatives possess only one short Fe···Si
distance ranging between about 2.54 and 2.63 Š, and one or
two weaker Fc···Si+ interactions with d
ACTHNUTRGNEUNG(Fe···Si) varying from
approximately 3.06 to 3.52 Š (Table 4).
For the monoferrocenyl-substituted systems, DEr is about
30–60 kJmolꢀ1 more negative than DEstab. This provides
a rough estimate of the stabilization due to the stronger in-
ductive and mesomeric (+I/+M) effects of ferrocene rela-
tive to phenyl. In case of more than one Fc substituent (R
and/or R’=Fc), the single primary donor–acceptor interac-
ꢀ
tion with the smallest Fe Si distance is present on both
sides of the isodesmic reaction (Scheme 1), and DEr thus in-
Scheme 1. Isodesmic reaction used to determine the stabilization energy
(DEr) due to the metallocene–electrophile interaction.
Chem. Eur. J. 2013, 19, 16579 – 16594
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16583

P. Hrobꢂrik, M. Kaupp, M. Oestreich et al.
cludes only the “pure” metallocene-backbone electronic ef-
fects and one or two weak (ca. 1.0–5.0 kJmolꢀ1, Table 4) sec-
ondary “through-space” Fc···Si interactions. Incidentally,
DEr and DEstab are very similar in these cases (Table 3).
To be able to better distinguish electronic from steric sub-
stituent effects, the computational study was further extend-
ed to systems combining diverse aryl and alkyl groups, and
by introducing additional electron-donating/-withdrawing
substituents into the para position of the silicon-bonded Ph
substituent (this latter modification is not easily studied ex-
perimentally due to the likely occurrence of intermolecular
donor–acceptor interactions). The data in Table 3 show that
sterically less demanding substituents (Me in the alkyl
series, Ph in the aryl series) lead to shorter Si···Fe and
Si···C’ipso distances and to larger dip angles, and thus to more
effective interactions between the Fc moiety and the silyli-
um center. On the other hand, sterically more demanding
groups, such as tBu or mesityl (Mes), diminish the interac-
tion. Note that DEstab for [FcSiMes2]+ is about one order of
magnitude lower than that for the dimethyl analogue.
mers (Table 3; the Boltzmann-averaged value is used in this
case for comparison with the experiment).
From Table 3 it is also evident that the stabilization
energy is affected by both electronic and steric effects, and
in most cases both act in the same direction (DEstab decreas-
es with increasing +I/+M effect and with increasing steric
bulk of the alkyl/aryl substituents). This is demonstrated by
plotting the stabilization energy against both electronic and
steric Hammett–Taft parameters[43] for the simplest case of
two alkyl substituents (Figure 5).
In derivatives bearing two Fc substituents, secondary
through-space interactions to the second Fc unit affect the
resulting structure and NMR properties (see below). In gen-
eral, differences between primary and secondary Fe···Si dis-
tances are much smaller in compounds with sterically less
demanding groups, such as Me or Ph (Table 4). In the case
of the silylium ion [FcSiFc2]+ with three Fc units, two virtu-
ally isoenergetic minima were found. They differ in the ori-
entation of the third Fc moiety (unit C) with respect to the
FeCp2 unit featuring the primary Fe···Si contact (Figure 4,
the structure on the right is energetically favored by
0.9 kJmolꢀ1) as well as in the distances between the silicon
atom and the other atoms in question (Table 4). These struc-
tural differences result also in remarkably different
29Si NMR chemical shifts calculated for the two stereoiso-
Figure 5. Dependence of DEstab in a dialkyl series of ferrocene-stabilized
silylium ions [FcSiR(R’)]+ (R and R’=Me, iPr, and tBu) on electronic
and steric parameters of R and R’ substituents (expressed as a sum of
their Hammett–Taft s* and Es constants, respectively).
The situation becomes, however, more complicated when
dealing with mixed alkyl/aryl systems. Here, silylium ions
containing a bulkier tBu group are somewhat more stabi-
lized by a donor–acceptor interaction with the Fc group
than those bearing a Me group (this is in contrast to the dia-
Figure 4. Schematic structures of the two virtually isoenergetic minima found for the silylium ion with three ferrocenyl units, [FcSiFc2]+.
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Ferrocene-Stabilized Silylium Ions
FULL PAPER
lkyl-substituted series but differences in DEstab are less pro-
nounced here). This behavior can be attributed to a much
more pronounced twisting of the Ph ring from the SiC3
plane in the tBu-substituted derivatives due to steric conges-
tion (t=8.58 in [FcSiPhMe]+ whereas t=67.78 in [FcSiPh-
ACHTUNGTRENNUNG
the somewhat shorter Si···C’ipso distance in [FcSiPhACTHNUTRGNEUNG
(tBu)]+
when compared to [FcSiPhMe]+.
Analysis of 29Si NMR chemical shifts: For the determination
of the 29Si NMR chemical shifts, a direct measurement was
in most cases supported by a H,29Si HMQC measurement
1
Figure 6. Correlation between experimental and calculated 29Si NMR
chemical shifts in a series of ferrocene-stabilized silylium ions (PBE0/
IGLO-III//PBE0-D3/def2-TZVPP results with an IEF-PCM solvation
model).
due to a higher sensitivity for the latter technique. The
29Si NMR chemical shifts of our new directly accessed silyli-
um ions 9a–9k (Table 1, column 8) as well as 9l and 9m ob-
tained from substituent scrambling cover a broad range be-
tween d=77.4 and 120.9 ppm. Note that the 29Si NMR
chemical shifts in our ferrocenyl-substituted silylium ions do
not alter upon changing the counterion.[17] Interestingly, we
see a strong dependence of the 29Si NMR chemical shift on
the choice of substituents at the silicon atom. In general, di-
alkyl substituted silylium ions 9a–9e tend to show more
high-frequency shifted signals (d=110.7–120.9 ppm) than
aryl-substituted cations 9 f–9m (d=77.4–104.5 ppm), as sup-
ported by our DFT calculations (see below). This behavior
is opposite to that observed for the hydride shifts of the si-
lanes (Table 1, column 5), but overall the 29Si NMR chemical
by several other factors. Therefore, such correlations with
electron density (the electron-withdrawing/-donating charac-
ter of the substituents) tend to be very limited.[44] This is
borne out also by the present results:[45] although the di-
AHCTUNGTREGaNNUN lkyl-substituted species exhibit generally more high-fre-
quency shifted 29Si NMR signals than those complexes with
at least one aryl or a second Fc substituent (with the notable
exception of R=Mes and R’=tBu; see below), the overall
trends tend to be influenced in a complicated way by vari-
ous factors (see also Figure S7 in the Supporting Informa-
tion, showing a rather poor correlation, if any, between
29Si NMR chemical shifts and Mulliken charges on the silyli-
um center). This holds true despite the fact that, apparently,
both steric and electronic substituent effects tend to work,
at least to a certain extent, in the same direction (Figure 7).
For example, the most downfield 29Si NMR chemical shift
within the studied ferrocene-stabilized silylium ions is found
1
shifts of the cations and the H NMR chemical shifts of the
corresponding silanes correlate only poorly (Figure S4 in the
Supporting Information). This may be understood easily
from the fact that the direct Fe···Si donor–acceptor interac-
tions affect the former but not the latter. Moreover, ring
current effects due to aromatic substituents can affect both
shifts by up to about d=1–2 ppm (see Figure S5 in the Sup-
porting Information for a NICS analysis). Although this is
an essentially negligible contribution to the 29Si NMR chem-
for [FcSi
ACHTUNGTRENNUNG
predicted for [FcSiAHCTUNGTRENNUNG
substituted analogue (Table 3). This points already toward
1
ical shifts, it is important for the H NMR chemical shifts.
The agreement between measured and computed
29Si NMR chemical shifts is excellent and consistent with
a good quality of the PBE0-D3/def2-TZVPP optimized
structures (Tables 1 and 3 as well as Figure 6). We note in
passing that omission of the dispersion corrections from the
optimizations results in structures with slightly longer Fe···Si
and Si···C’ipso distances (up to 0.04 Š), and consequently in
inferior agreement with experimental NMR chemical shifts
of the silylium ions (Table S2 and Figure S6 in the Support-
ing Information provide comparisons of different computa-
tional methods). The good quality of the computed chemical
shifts allows us to further analyze the relations between
29Si NMR chemical shifts and molecular and electronic
structure, including also systems for which no experimental
data are available.
the importance of steric effects of R and R’ substituents for
NMR chemical shifts are still often interpreted as indica-
tive of the atomic charge at the nucleus of interest, even
though it is well known that the paramagnetic part of the
Ramsey expression for the chemical shift may be influenced
Figure 7. Dependence of calculated 29Si NMR chemical shifts in a dialkyl
series of ferrocene-stabilized silylium ions [FcSiR(R’)]+ (R and R’=Me,
iPr, and tBu) on steric and electronic (Hammett–Taft) parameters.
Chem. Eur. J. 2013, 19, 16579 – 16594
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16585

P. Hrobꢂrik, M. Kaupp, M. Oestreich et al.
the stabilizing Fc···Si interaction (the large d
(Fe···Si) for
to significantly more deshielded d
largest for systems with shorter secondary Fe···Si contacts
(Dd
(29Si)=26.4 ppm for [FcSiFcPh]+ but only Dd(29Si)=
2.0 ppm for [FcSiFc
(tBu)]+). An even larger deshielding
(29Si) values. This effect is
ACHTUNGTRENNUNG
[FcSi
ACHTUNGTRENNUNG
Because of the diversity of the substituents attached to
the silylium center and the various steric and electronic in-
fluences, we found it necessary to subdivide the series of cat-
ions into five subsets: 1) compounds with two alkyl substitu-
ents [FcSiAlk2]+, 2) compounds with one aryl and one alkyl
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
effect (more than 120 ppm) is predicted upon complete can-
cellation of all “through-space” metallocene–silylium inter-
actions (setting all dip angles within a given system to zero).
Then we obtain 29Si NMR chemical shifts comparable to or
even more deshielded than that of the trimesityl silylium
substituent [FcSiAr
ACHTUNGTRENNUNG
(Alk)]+, 3) compounds with two aryl
substituents [FcSiAr2]+, 4) compounds with two Ph substitu-
ents with varying para-substituent [FcSi
(p-X-Ph)2]+, and
ACTHNGUTERNNUG
ion[5] (d(29Si)>200 ppm; see Table S3 in the Supporting In-
formation).
5) compounds with more than one Fc unit [FcSiFcR]+.
Using this division, a correlation of 29Si NMR chemical
shift and d
(Fe···Si) (or dip angle) becomes meaningful, and
Lewis acidity: The strength of the Lewis acidity (electrophi-
licity) of silylium ions has been established on the basis of
their fluoride ion affinities (FIA;[46] see Scheme 2 for the as-
sociated isodesmic reaction). The FIA of a Lewis acid (A+)
the established relationships may, in principle, be used for
an indirect determination/estimation of the Fe···Si distances
within the ferrocene-stabilized silylium ions using 29Si NMR
spectroscopy (Figure 8 and Figure S8 in the Supporting In-
formation). Within a given subset, a shorter Fe···Si distance
(larger dip angle) signals a more upfield 29Si NMR chemical
shift, consistent with a closer Fc···Si interaction shielding the
silylium center. Systems with more than one Fc unit (blue
diamonds) are a notable exception to this rule: here the sec-
ondary interactions with the second Fc unit reverse the
trend! The longer the primary contact between the iron and
silylium center becomes, the shorter the secondary interac-
tion with the Fc unit B (Tables 3 and 4). In contrast to DEstab
Scheme 2. Reactions used to evaluate the Lewis acidity.
is defined as the enthalpy
change associated with the het-
ꢀ
erolytic cleavage of an A F
bond to give A+ and Fꢀ: the
more positive the FIA, the
stronger the Lewis acid.
In this study, FIA calcula-
tions were performed along the
lines of Christeꢀs method using
F2CO as
a standard, which
avoids the need to calculate
free Fꢀ in the gas phase.[46,47]
The FIA of each silicon cation
was then calculated by ad-
ding the FIA of F2CO,
which is known experimentally
(209 kJmolꢀ1).[46] Alternatively,
we could have used the hydride
Figure 8. Correlation of computed 29Si NMR chemical shifts with Fe···Si distance in the ferrocenyl-substituted
silylium ions [FcSiR(R’)]+.
(see above), the secondary through-space interaction plays
an important role for the 29Si NMR chemical shifts and
causes an additional shielding of the silylium center. This re-
sults, for example, in the most low-frequency 29Si NMR
chemical shift being observed for the silylium ion
ion affinity (Scheme 2) with trityl cation as competing elec-
trophile, closely related to the actual experimental genera-
tion of our silylium ions (see above). As we found an almost
perfect correlation between these two quantities (Figure S9
in the Supporting Information), we will concentrate only on
the FIAs in the following.
ACHTUNGTRENNUNG
[FcSiFcPh]+ (d(29Si)=77.5 ppm) with the shortest secondary
Fe···Si contact. The effect of Lewis acid–base interactions on
29Si NMR chemical shifts is also demonstrated in Table S3 in
the Supporting Information. Switching off the secondary in-
teraction by fixing the dip angle within unit B to 0.08 leads
In general, all silylium ions studied here display enormous
electrophilicity, larger than their carbenium congeners (for
instance, the computed FIAs of [PhSiMe2]+ and [FcSiMe2]+
are 905 and 817 kJmolꢀ1, respectively, whereas the FIAs in
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Ferrocene-Stabilized Silylium Ions
FULL PAPER
the carbon cations [PhCMe2]+ and [FcCMe2]+ are 750 and
672 kJmolꢀ1, respectively). The range of FIAs in the ferro-
cene-stabilized silylium ions (ca. 740–850 kJmolꢀ1; Table 3)
is smaller than that in the corresponding phenyl series
[PhSiR(R’)]+ (ca. 770–920 kJmolꢀ1; Table S4 in the Support-
ing Information), providing an indication that the Fc···Si in-
teractions reduce the electrophilicity of the “free” silylium
ion. However, we note also that this “Fc shielding effect” on
the Lewis-acidity is much less dominant (e.g., compared to
other substituent effects) than for the 29Si NMR chemical
shifts in which the ranges of the Fc- and Ph-substituted
series are completely disjoint (Table 3 and Table S4 in the
Supporting Information).
In both series, the highest Lewis acidity is found for deriv-
atives bearing two alkyl substituents, and it decreases upon
replacing alkyl by aryl substituents. The lowest electrophilic-
ity is found for silylium ions with three aryl groups, especial-
ly when some of them include a ferrocene backbone (the
more ferrocene units, the lower the Lewis acidity).
At first glance, one would expect that electronic +I ef-
fects of alkyl substituents lead to an increase of Lewis acidi-
ty in the order tBu<iPr<Me, as demonstrated, for exam-
ple, for the all-alkyl series [MeSiR2]+ (Table S4 in Support-
ing Information). Surprisingly, however, the highest FIA in
each of the ferrocenyl and phenyl series is found for deriva-
tives with two tBu groups. In case of [FcSi
be attributed to a weakening of the Fc···Si interaction due to
the steric bulk of the tBu groups (d(Fe···Si) values in
Table 3). On the other hand, in [PhSi
(tBu)2]+ the steric re-
(tBu)2]+, this can
ACHTUNGTRENNUNG
Figure 9. Correlation of computed fluoride ion affinities (FIAs) and
29Si NMR chemical shifts in the: a) phenyl-silylium series [PhSi
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Ph)2]+, and b) ferrocenyl-silylium series [FcSi
H, OMe, NMe2).
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
pulsion between the tBu groups and ortho phenyl hydrogen
atoms causes a pronounced twisting of the Ph ring out of
the SiC3 plane. The corresponding dihedral angle, t, increas-
es along the series [PhSiR2]+ (R=Me<iPr<tBu as 0.0,
10.3, 31.08), thereby also destabilizing the silicon cation and
increasing its electrophilic character (tBu derivatives also
(note the overall much lower-frequency silicon shifts within
a narrow range for the ferrocenyl systems compared to the
phenyl analogues; Figure 9) but not in case of the FIAs.
These findings demonstrate already how different factors
influence the ferrocenyl stabilization energies, the Lewis
acidities, and the 29Si NMR chemical shifts in a complicated
fashion. Although steric and electronic effects go in the
same direction for the ferrocenyl stabilization energies
(Figure 5) and the 29Si NMR chemical shifts of the ferrocen-
yl systems (Figure 7), these may work in opposite directions
in other cases, for example, for the FIAs and chemical shifts
of the “ferrocenyl-free” silylium ions (Figures S10 and S11
in the Supporting Information). For example, the Lewis
acidity of the [PhSiR(R’)]+ series with R, R’=Me, iPr is pri-
marily driven by electronic effects, whereas derivatives with
tBu groups deviate due to dominant steric effects.
In the ferrocenyl-substituted systems, the steric and elec-
tronic effects on the FIAs also appear to work in opposite
direction (Figure 10). This may explain why the various sub-
stituents can have such different effects on 29Si NMR chemi-
cal shifts and Lewis acidity.
The extreme influence of Fc···Si interactions and steric ef-
fects on Lewis acidity may be seen best when comparing de-
rivatives with Ph and Mes groups: the difference in FIA of
[LSiPh2]+ and [LSiMes2]+ is about 12 kJmolꢀ1 in the ferro-
cenyl series (L=Fc; see Table 3) but approximately
ꢀ
exhibit somewhat longer Si C bonds, that reduce the induc-
tive +I effects to some extent). Note that closer inspection
of the structures of our ferrocene-stabilized silylium ions did
ꢀ
ꢀ
not reveal any C H and/or C C hyperconjugation effects,
which play a role, for example, in the structure and NMR
properties of more electron-deficient all-alkyl silicon cati-
ons.[1e] The partly counterintuitive influence of the Fc···Si in-
teractions in the ferrocenyl-substituted cations is best seen
from plots of FIAs against 29Si NMR chemical shifts while
varying the para-substituent X on two Ph rings (Figure 9
and Table 3). In case of the [PhSiACHTUNGTRNEUNG
(p-X-Ph)2]+ series, we find
the expected correlation of larger FIA with more downfield
29Si NMR chemical shifts upon going to more electron-with-
drawing substituents, X (Figure 9a). Conversely, in the
[FcSiACHTUNGTRENNUNG
(p-X-Ph)2]+ series, the FIAs also increase but the
29Si NMR chemical shifts actually decrease from X=NMe2
to X=NO2 (Figure 9b)! The latter trend can be understood
from increased Fc···Si interactions for the more electron-
withdrawing aryl groups (Fe···Si distances in Table 3). For
this trend, the intramolecular Lewis acid–base interactions
overcompensate the direct p donor (+M) effect of the aryl
groups on the silylium center in case of the 29Si NMR shifts
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16587

P. Hrobꢂrik, M. Kaupp, M. Oestreich et al.
Figure 11. Correlation of calculated 29Si NMR chemical shifts with FIAs
in the series of dimethyl-substituted silylium ions (see Figure 1 for the
structures).
Figure 10. Dependence of fluoride ion affinities (FIAs) on steric and
electronic (Hammett–Taft) parameters in a ferrocene-stabilized series
[FcSiR(R’)]+ (R and R’=Me, iPr, and tBu).
33 kJmolꢀ1 in the phenyl series (L=Ph; see Table S4 in the
Supporting Information). In the ferrocenyl series, the steric
bulk of the Mes substituent diminishes the Fc···Si interaction
and thus enhances the FIA, which approaches the value of
[FcSiPh2]+. In contrast, in the ferrocenyl-free series, the
stronger electron-donating effect of Mes compared to Ph
substituents dominates, leading to the much lower electro-
philicity of [PhSiMes2]+ as compared to the “free” triphenyl-
silylium ion.
series, due to the various steric and electronic factors work-
ing partly in opposite direction (Figures 9 and 10). Fig-
ure S12 in Supporting Information shows the overall very
poor correlation between FIAs and 29Si NMR chemical
shifts, even after subdividing the ferrocenyl-substituted sily-
lium ion series.
Furthermore, we have to note that both [FcSiMe2]+ and 4
are stronger Lewis acids than the trimesityl silylium ion
[Mes3Si]+[5], which on the other hand, has a much more de-
shielded silicon nucleus (Table 5).
We may extend our comparisons also to other experimen-
tally known dialkyl-substituted silylium ions with rather dif-
ferent stabilization mechanisms (Figure 1). Table 5 shows
Substituent scrambling: During our NMR spectroscopy
studies of new ferrocene-stabilized silylium ions 9a–9k we
1
observed a second set of signals in the H NMR spectra in
Table 5. Calculated fluoride ion affinities (FIAs) and 29Si NMR chemical
shifts in a series of dimethyl-substituted silylium ions and [Mes3Si]+.[a,b]
some cases (note that NMR spectra were recorded 30 min
and 6 h after mixing the reagents and the ratio between the
expected and substituent-scrambled products did not change
significantly during this period). This observation was ac-
companied by a second high-frequency shifted signal in the
29Si NMR spectrum. When silane 7g (R=Ph, R’=Me,
Table 1, entry 7) was used as the precursor, even more cat-
ionic silicon species were generated. A careful analysis of
these spectra revealed that more than one cationic silicon
species produced by a scrambling of substituents was pres-
ent in these cases (Table 1, column 9). Similar processes are
not unprecedented in the literature.[48,49] The group of
Mꢁller reported the exchange of substituents of silylium
ions with silanes both in an intra-[50] and intermolecular[51]
manner. Related substituent exchange reactions of silanes[52]
and skeletal rearrangements in polysilanes[53] initiated by
aluminum Lewis acids are also known.
For our family of silylium ions, we noticed a substituent
scrambling mainly when small substituents were attached to
the silicon atom (Table 1, column 9). This is rationalized by
the better accessibility of the silylium ion. We identified Me,
Ph, and Fc groups as the only scrambling substituents,
whereas a migration of iPr and tBu groups was not detected.
This observation is in accord with the ability of the former
groups to act as bridging groups between two silicon atoms
Cmpd
FIA [kJmolꢀ1
]
dACTHNGUTERNNUG
(29Si) [ppm][c]
A
816.6
807.8
796.6
811.3
815.3
756.8
680.7
800.0
113.7
84.7
77.4
81.5
82.4
61.0
17.3
229.5
1[6]
2[7]
3[8]
4[9a]
5[9b]
6[10]
Mes3Si+ [5]
[a] PBE0-D3/def2-TZVPP results in the gas-phase (see the Computation-
al Methods for more detail). [b] See Figure 1 for the structures. [c] PBE0/
IGLO-III//PBE0-D3/def2-TZVPP results in the gas-phase; see Figure 1
for the experimental values.
that the by far lowest Lewis acidity is found for compound
6,[10] which possesses a rigid structure and a cationic silicon
center stabilized by two additional interactions with hydro-
gen atoms from the neighboring silyl groups. In contrast, the
highest FIAs in this series are predicted for the hydrogen-
bridged disilyl cation, 4,[9a] and [FcSiMe2]+.
For this series the FIAs correlate quite well with the
29Si NMR chemical shifts (Figure 11). As demonstrated
above, this is not the case for the ferrocenyl-substituted
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Chem. Eur. J. 2013, 19, 16579 – 16594

Ferrocene-Stabilized Silylium Ions
FULL PAPER
in a cationic species.[54] Similar to Mꢁllerꢀs report,[51] sterical-
ly crowded silicon cations 9l (R=Fc, R’=tBu) and 9m (R=
Fc, R’=iPr) were only accessible through substitutent
scrambling from cations 9c, 9d and 9 f (Table 1, entries 3, 4
and 6, respectively).
silane 7j (Table 1, entry 10) as an example of a monoferroce-
nylsilane (Scheme 3, case A) and silane 7k (Table 1,
entry 11) as an example of a diferrocenylsilane (Scheme 4,
case B). The treatment of monoferrocenylsilane 7j with
ACTHNUTRGNEUNG
[Ph3C]+[B(C6F5)4]ꢀ (8) affords the primary hydride abstrac-
To further illustrate the observed product mixtures as
a basis for a subsequent quantum-chemical analysis of the
mechanism, we will focus on the discussion of two cases:
tion product 9j and silylium ion 9k by the exchange of a Ph
and a Fc substituent (Scheme 3, upper). Both products were
1
1
clearly identified in the H and H,29Si HMQC NMR spectra
(Scheme 3,
lower).
The
1H NMR spectrum shows two
sets of signals consisting of
three signals each for the pro-
tons of the upper and the lower
Cp rings. In addition to the two
silicon cations 9j (d=81.4 ppm)
and 9k (d=77.3 ppm) a third
silicon species (d=ꢀ14.7 ppm)
was
detected
in
the
1H,29Si HMQC NMR, assigned
as Ph3SiH. A further hydride
abstraction
reaction
from
Ph3SiH with still unreacted
trityl cation is not observed at
short reaction times and room
temperature.
In a second example, diferro-
cenylsilane 7k is treated with
ACTHNUTRGNEUNG
[Ph3C]+[B(C6F5)4]ꢀ (8) yielding
the direct hydride abstraction
product 9k and, by exchange of
a Ph and a Fc group, silylium
ion 9h (Scheme 4, upper). Both
silylium ions 9k and 9h were
identified by their two sets of
signals in the 1H NMR spec-
trum and their two signals in
Scheme 3. Silylium ions 9j and 9k generated from silane 7j. The relevant parts of the 1H NMR (left) and
1 H,29Si HMQC NMR (right) spectra are shown as proof for substituent scrambling.
the
29Si NMR
spectrum
(Scheme 4, lower). Another ex-
pected ferrocene-stabilized sili-
con cation 9j derived from sub-
stituent scrambling was not de-
tected although treatment with
unreacted trityl salt 8 ought to
be possible as it would also be
a ferrocene-stabilized silylium
ion. Presumably, silylium ion
9h is easier to detect in the
1H NMR spectrum (Scheme 4,
lower left) due to three identi-
cal Fc substituents. Signals of
traces of 9j in the 1H NMR
spectrum might either be
hidden under signals of the
other silicon cations 9k and 9h
or were not detected.
Scheme 4. Silylium ions 9k and 9h generated from silane 7k. The relevant parts of the 1H NMR (left) and
To elucidate the mechanism
and energetic feasibility of the
29Si NMR (right) spectra are shown as proof for substituent scrambling.
Chem. Eur. J. 2013, 19, 16579 – 16594
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16589

P. Hrobꢂrik, M. Kaupp, M. Oestreich et al.
dimers HB-Me or HB-Ph as
the lowest-energy minimum
(however, the free energies of
these species are slightly above
those of the reactants due to
entropic reasons).[4] Yet, ferro-
cenyl-bridged species FcB-Me
or FcB-Ph are energetically ac-
cessible and present the oppor-
tunity for the methyl shift (Fig-
ure 12a) or phenyl shift (Fig-
ure 12b), respectively. The tran-
sition states TS-Me and TS-Ph
may be considered asymmetri-
cal and “early” regarding the
ꢀ
formation of the new Si C
bond. Nevertheless, we observe
a concerted substituent-transfer
process starting from the ferro-
cenyl-bridged
intermediates
FcB-Me or FcB-Ph. The ferro-
cenyl bridge opens simultane-
ously with the partial formation
ꢀ
of the new Si C bond, leading
to the methyl- or phenyl-
bridged intermediates CB-Me
and CB-Ph, respectively. These
open finally to the respective
substituent scrambling products.
The transition states are com-
puted to lie at 145 kJmolꢀ1, rel-
ative to the reactants, in the
methyl transfer case, and at
about 110 kJmolꢀ1 in the
phenyl transfer case. The over-
all reaction energies are slightly
exothermic
by
about
10 kJmolꢀ1. The computed bar-
riers are somewhat higher than
those reported by Mꢁller et al.
for an intramolecular exchange
of substituents in disilyl cations
Figure 12. Computed (PBE0-D3/def2-TZVPP/COSMO) reaction mechanisms for substituent scrambling in
two selected ferrocenyl-substituted silylium ions in reactions with the silicon hydride precursors. a) Methyl-
group transfer in [FcSiMe2]+ (9e); b) phenyl-group transfer in [FcSiPh2]+ (9j). Relative energies [kJmolꢀ1
]
with
diyl
a rigid naphthalene-1,8-
with respect to the energy of reactants are given in parentheses.
backbone
(80–
90 kJmolꢀ1),[50] but they are still
in a range indicating feasible
substituent-scrambling process, DFT calculations were per-
formed (see Figure 12 for the PBE0-D3/def2-TZVPP/
COSMO results and Figure S13 in the Supporting Informa-
tion for corresponding gas-phase results). Note that counter-
ion effects were neglected and the computed energies thus
probably provide only a rough estimate of the barriers. We
chose [FcSiMe2]+ (9e, Table 1, entry 5) as an example for
a methyl-transfer and [FcSiPh2]+ (9j, Table 1, entry 10) for
a phenyl-transfer process. Treatment of these silylium ions
with the corresponding silicon hydride precursor 7e and 7j,
respectively, provides in both instances the hydride-bridged
substituent scrambling processes in both cases, with a some-
what faster reaction for the phenyl transfer.
Conclusion
We have recently reported [FcSiACHTUNGTRNEUNG
(tBu)Me]+ as an example of
a ferrocene-stabilized silylium ion in which donor–acceptor
ꢀ
Fe Si interactions contribute to a “taming” of its Lewis
acidity and reactivity, providing an interesting starting point
for the design of tailor-made Lewis-acid catalysts. Here we
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Chem. Eur. J. 2013, 19, 16579 – 16594

Ferrocene-Stabilized Silylium Ions
FULL PAPER
extend this new class of compounds to a series of more than
ten members with various substitution patterns, all charac-
terized by detailed NMR spectroscopy studies in solution,
covering a large range of 29Si NMR chemical shifts (d=
77.4–120.9 ppm). In parallel, an even larger set of such fer-
rocenyl-substituted silylium ions has been examined by de-
tailed quantum-chemical analyses of molecular and electron-
ic structure, intramolecular donor–acceptor stabilization en-
ergies, Lewis-acidity measures, as well as 29Si NMR chemical
shifts.
quantum-chemical calculations of thermochemistry and bar-
riers for two representative cases.
Experimental Section
All reactions were performed in flame-dried glassware using an MBraun
glove box (O2 <0.5 ppm, H2O<0.5 ppm) or conventional Schlenk tech-
niques under a static pressure of argon or nitrogen. Liquids and solutions
were transferred with syringes. THF was dried over potassium. Technical
grade solvents for extraction or chromatography (cyclohexane and tert-
butyl methyl ether) were distilled prior to use. 1,2-Cl2C6D4 (Amar Chem-
icals or Eurisotop) was dried over CaH2 prior to use and stored over mo-
lecular sieves in a glove box. C6D6 (Eurisotop) was used without further
purification and was stored over molecular sieves. Ferrocene (Alfa
Aesar) was used without further purification. Chlorosilanes were either
purchased from the indicated commercial suppliers and used without fur-
ther purification [tBu2HSiCl (Fluka), iPr2HSiCl (ABCR), Me2HSiCl
(Sigma–Aldrich), MePhSiCl2 (Merck), HSiCl3 (Sigma–Aldrich), MeH-
SiCl2 (Sigma–Aldrich), Ph2HSiCl (ABCR), PhHSiCl2 (Alfa Aesar)] or
prepared in analogy to reported procedures [tBuMeHSiCl from MeH-
SiCl2 and tBuLi,[16a] MeiPrHSiCl from MeHSiCl2 and iPrMgCl,[55]
The direct stabilizing Fe···Si interactions in the ferrocenyl-
substituted silylium ions are apparent from various electron-
ic-structure analyses, for example, bond orders, ELF, or
ELI-D plots, and are manifested also in the magnitude of
ꢀ
the dip angle and Fe Si distances. Notably, computations
show that this interaction is more pronounced for silylium
ions than for lighter or heavier group 14 homologues, and
much stronger than for neutral group 13 species. However,
steric and electronic properties of the substituents at the si-
lylium center combine in a subtle way to influence the
Lewis acidity (e.g., fluoride ion affinities) and the spectro-
scopic parameters (e.g., 29Si NMR chemical shifts) of the fer-
rocenyl-substituted silylium ions, leading to partly non-intui-
tive trends. For example, although in simpler model silylium
ions more electron-withdrawing substituents (e.g., in the
para position of a phenyl group) increase both the Lewis
acidity and the 29Si NMR chemical shifts, enhanced Fe···Si
interactions in ferrocenyl-substituted cations reverse the
trend for the silicon shifts (but not for fluoride ion affini-
ties). Matters become even more involved when more than
one ferrocenyl substituent is present, as both a strong pri-
mary Fe···Si interaction and weaker through-space secon-
dary Fc···Si interaction(s) have to be considered. Correla-
tions between 29Si NMR chemical shifts and Lewis-acidity
measures are thus restricted to subsets of complexes and
29Si NMR chemical shifts should be generally taken with
caution when evaluating the electrophilic character of the si-
lylium ions. Explicit quantum-chemical computations of
these properties, nevertheless, provide substantial predictive
power and appreciable insight and can help to design ferro-
cene-substituted silylium ions with specific catalytic activity.
Furthermore, a reasonable correlation of 29Si NMR chemical
shift and Fe···Si distances found for a subset of complexes
may be used for an indirect determination of Fe···Si “bond”
lengths within the ferrocene-stabilized silylium ions using
29Si NMR spectroscopy.
ACTHNUTRGNEUNG
tBuPhHSiCl from PhHSiCl2 and tBuLi[56]]. [Ph3C]+[B(C6F5)4]ꢀ (8)[57] was
prepared according to a reported procedure. Analytical thin layer chro-
matography (TLC) was performed on silica gel 60 F254 glass plates by
Merck. Flash column chromatography was performed on silica gel 60
(40–63 mm, 230–400 mesh, ASTM) by Merck using the indicated solvents.
1H, 11B, 13C, 19F and 29Si NMR spectra were recorded in C6D6 or 1,2-
Cl2C6D4 on Bruker AV300, Bruker AV400, and Varian INOVA 500 in-
struments (Westfꢃlische Wilhelms-Universitꢃt Mꢁnster) and Bruker
AV400 and AV500 instruments (Technische Universitꢃt Berlin). Chemi-
cal shifts are reported in parts per million (ppm) and are referenced to
the residual solvent resonance as the internal standard (C6D5H: d=
7.16 ppm for 1H NMR and C6D6: d=128.1 ppm for 13C NMR; 1,2-
1
Cl2C6D3H: d=6.94 and 7.20 ppm for H NMR and 1,2-Cl2C6D4: d=127.1,
130.1 and 132.5 ppm for 13C NMR). Data are reported as follows: chemi-
cal shift, multiplicity (brs=broad singlet, s=singlet, d=doublet, t=trip-
let, q=quartet, sept=septet, brm=broad multiplet, m=multiplet, mc =
centrosymmetric multiplet), coupling constants (Hz) and integration.
1H,29Si HMQC NMR spectra were measured with a coupling constant of
3
7.0 Hz for the JH,Si coupling. The peak intensities in the 1H,29Si HMQC
NMR spectra cannot be correlated to the amount of compound. Gas
liquid chromatography (GLC) was performed on a Shimadzu GC-17A
gas chromatograph equipped with a SE-54 capillary column (30 m”
0.32 mm, 0.25 mm film thickness) by CS-Chromatographie Service using
the following program: N2 carrier gas, injection temperature 2408C, de-
tector temperature 3008C, flow rate: 1.74 mLminꢀ1; temperature pro-
gram: start temperature 408C, heating rate 108Cminꢀ1, end temperature
2808C for 10 min. Infrared (IR) spectra were recorded on a Varian 3100
FT-IR spectrometer (Westfꢃlische Wilhelms-Universitꢃt Mꢁnster) and an
Agilent Technologies Cary 630 FT-IR spectrometer (Technische Universi-
tꢃt Berlin), both equipped with an ATR unit and are reported in wave-
numbers [cmꢀ1]. Melting points (m.p.) were determined with a Stuart Sci-
entific SMP20 melting point apparatus and are not corrected. Mass spec-
trometry (MS) was obtained from the Analytical Facilities at the Organ-
isch-Chemisches Institut, Westfꢃlische Wilhelms-Universitꢃt Mꢁnster and
at the Institut fꢁr Chemie, Technische Universitꢃt Berlin.
Systematic NMR spectroscopy studies have revealed sub-
stituent exchange processes in several of the less sterically
hindered ferrocenyl-substituted silylium ions, due to reaction
of the initially formed cation with unreacted silane starting
material. This “substituent scrambling” involves mostly the
transfer of smaller Me and Ph substituents and leads to the
formation of new silylium ions with different substitution
patterns, albeit not quantitatively. The feasibility of a con-
certed bimolecular mechanism for these processes, based on
ferrocenyl-bridged intermediates, is supported by explicit
General procedure for the preparation of ferrocene-stabilized silicon cat-
ions
(1.00 equiv) in 1,2-Cl2C6D4 (0.3 mL) was added to
ACTHNUGTRNENUG
[Ph3C]+[B(C6F5)4]ꢀ (8, 1.00 equiv) in 1,2-Cl2C6D4 (0.2 mL) in an 8 mL
9
(GP 2): In
a
glove box,
a
solution of
a
ferrocenylsilane
a
suspension of
vial equipped with a magnetic stir bar. The resulting red-brown solution
was stirred for 1 min, transferred to an NMR tube, and directly subjected
to NMR spectroscopic analysis.
Di-tert-butylferrocenylsilylium tetrakis(pentafluorophenyl)borate (9a):
This was prepared from di-tert-butylferrocenylsilane (7a, 32.5 mg,
ACTHNUTRGNEUNG
99.1 mmol, 1.00 equiv) and [Ph3C]+[B(C6F5)4]ꢀ (8, 91.4 mg, 99.1 mmol,
Chem. Eur. J. 2013, 19, 16579 – 16594
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www.chemeurj.org
16591

P. Hrobꢂrik, M. Kaupp, M. Oestreich et al.
1
1.00 equiv) according to GP 2. H NMR (500 MHz, 1,2-Cl2C6D4): d=1.11
Ferrocenylmethylphenylsilylium tetrakis(pentafluorophenyl)borate (9g):
This was prepared from ferrocenylmethylphenylsilane (7g, 25.3 mg,
(s, 18H), 3.89 (brs, 2H), 4.36 (brs, 5H), 5.26 ppm (brs, 2H); 13C NMR
(125 MHz, 1,2-Cl2C6D4): d=28.3, 30.7, 65.3, 73.5, 82.3, 86.5, 124.5 (brm),
ACTHNGUTERNNUG
82.6 mmol, 1.00 equiv) and [Ph3C]+[B(C6F5)4]ꢀ (8, 76.2 mg, 82.6 mmol,
136.6 (d, JC,F =245 Hz), 138.5 (d, JC,F =243 Hz), 148.6 ppm (d, JC,F
=
1.00 equiv) according to GP 2 along with silylium ions 9e, 9i, 9j and
traces of 9k. Due to the complex mixture of silylium ions only some char-
acteristic signals in the 1H NMR spectrum could be assigned. The
13C NMR spectrum was not interpretable. 1H NMR (500 MHz, 1,2-
Cl2C6D4): d=1.07 (brs), 5.29 (brs), 7.26–7.59 ppm (m); 11B NMR
(126 MHz, 1,2-Cl2C6D4): d=ꢀ16.7 ppm; 19F NMR (470 MHz, 1,2-
Cl2C6D4): d=ꢀ166.6, ꢀ162.4, ꢀ132.6 ppm; 29Si NMR (99 MHz, 1,2-
Cl2C6D4): d=93.8 ppm.
242 Hz); 11B NMR (126 MHz, 1,2-Cl2C6D4): d=ꢀ16.2 ppm; 19F NMR
(470 MHz, 1,2-Cl2C6D4): d=ꢀ166.0, ꢀ162.1, ꢀ131.8 ppm; 1H,29Si HMQC
NMR (99 MHz, 1,2-Cl2C6D4): d=120.3 ppm; 29Si NMR (99 MHz, 1,2-
Cl2C6D4): d=120.9 ppm.
Ferrocenyldi-iso-propylsilylium tetrakis(pentafluorophenyl)borate (9b):
This was prepared from ferrocenyldi-iso-propylsilane (7b, 25.2 mg,
83.9 mmol, 1.00 equiv) and [Ph3C]+[B
1.00 equiv) according to GP 2. H NMR (500 MHz, 1,2-Cl2C6D4): d=1.16
(mc, 12H), 1.38–1.47 (m, 2H), 3.76 (brs, 2H), 4.33 (brs, 5H), 5.21 ppm
(brs, 2H); 13C NMR (125 MHz, 1,2-Cl2C6D4): d=17.8, 18.4, 20.7, 65.7,
(C6F5)4]ꢀ (8, 77.4 mg, 83.9 mmol,
1
Triferrocenylsilylium tetrakis(pentafluorophenyl)borate (9h): This was
prepared from triferrocenylsilane (7h, 52.4 mg, 89.7 mmol, 1.00 equiv)
ACTHNUTRGNEUNG
and [Ph3C]+[B(C6F5)4]ꢀ (8, 82.7 mg, 89.7 mmol, 1.00 equiv) according to
GP 2. 1H NMR (500 MHz, 1,2-Cl2C6D4): d=4.25 (brs, 15H), 4.31 (brs,
6H), 4.88 ppm (brs, 6H); 13C NMR (125 MHz, 1,2-Cl2C6D4): d=52.8,
73.4, 80.9, 86.6, 124.5 (brm), 136.6 (d, JC,F =245 Hz), 138.5 (d, JC,F
=
245 Hz), 148.6 ppm (d, JC,F =238 Hz); 11B NMR (76 MHz, 1,2-Cl2C6D4):
d=ꢀ16.2 pp; 19F NMR (282 MHz, 1,2-Cl2C6D4): d=ꢀ166.1, ꢀ162.0,
ꢀ131.8 pp; 1H,29Si HMQC NMR (99 MHz, 1,2-Cl2C6D4): d=114.1 ppm;
29Si NMR (99 MHz, 1,2-Cl2C6D4): d=114.7 ppm.
72.2, 76.5, 79.1, 124.5 (brm), 136.6 (d, JC,F =247 Hz), 138.6 (d, JC,F
=
245 Hz), 148.6 ppm (d, JC,F =245 Hz); 11B NMR (126 MHz, 1,2-Cl2C6D4):
d=ꢀ16.2 ppm; 19F NMR (470 MHz, 1,2-Cl2C6D4): d=ꢀ165.9, ꢀ162.0,
ꢀ131.8 ppm; 1H,29Si HMQC NMR (99 MHz, 1,2-Cl2C6D4): d=91.3 ppm;
29Si NMR (99 MHz, 1,2-Cl2C6D4): d=91.3 ppm.
tert-Butylferrocenylmethylsilylium
(9c): This was prepared from tert-butylferrocenylmethylsilane (7c,
23.3 mg, 81.5 mmol, 1.00 equiv) and [Ph3C]+[B(C6F5)4]ꢀ (8, 75.1 mg,
tetrakis(pentafluorophenyl)borate
Diferrocenylmethylsilylium tetrakis(pentafluorophenyl)borate (9i): This
was prepared from diferrocenylmethylsilane (7i, 35.2 mg, 85.0 mmol,
AHCTUNGTRENNUNG
81.5 mmol, 1.00 equiv) according to GP 2 along with traces of silylium ion
9l. 1H NMR (300 MHz, 1,2-Cl2C6D4): d=0.71 (s, 3H), 1.07 (s, 9H), 3.58
(brs, 1H), 4.93 (brs, 1H), 4.27 (brs, 5H), 5.21 ppm (brs, 2H); 13C NMR
(125 MHz, 1,2-Cl2C6D4): d=ꢀ4.4, 24.7, 27.7, 64.7, 73.8, 81.0, 81.4, 86.4,
1.00 equiv) and [Ph3C]+[B(C6F5)4]ꢀ (8, 78.4 mg, 85.0 mmol, 1.00 equiv) ac-
ACHTUNGTERNUNNG
1
cording to GP 2 along with traces of silylium ion 9h. H NMR (500 MHz,
1,2-Cl2C6D4): d=1.17 (s, 3H), 3.98 (brs, 4H), 4.16 (brs, 10H), 4.92 ppm
(brs, 4H); 13C NMR (125 MHz, 1,2-Cl2C6D4): d=ꢀ3.6, 52.7, 72.8, 76.2,
87,1, 124.5 (brm), 136.6, (d,
JC,F =242 Hz), 138.6 (d, JC,F =242 Hz),
J
C,F =242 Hz); 11B NMR (160 MHz, 1,2-Cl2C6D4): d=
81.7, 124.3 (brm), 136.5 (d,
148.5 ppm (d,
JC,F =246 Hz), 138.4 (d, JC,F =243 Hz),
148.6 ppm (d,
J
C,F =242 Hz); 11B NMR (126 MHz, 1,2-Cl2C6D4): d=
ꢀ16.4 ppm; 19F NMR (470 MHz, 1,2-Cl2C6D4): d=ꢀ166.1, ꢀ162.2,
ꢀ16.5 ppm; 19F NMR (470 MHz, 1,2-Cl2C6D4): d=ꢀ166.2, ꢀ162.3,
ꢀ132.1 ppm; 1H,29Si HMQC NMR (99 MHz, 1,2-Cl2C6D4): d=88.5 ppm;
29Si NMR (99 MHz, 1,2-Cl2C6D4): d=88.3 ppm.
1
ꢀ132.0 ppm; H,29Si HMQC NMR (99 MHz, 1,2-Cl2C6D4): d=113.9 ppm;
29Si NMR (60 MHz, 1,2-Cl2C6D4): d=114.4 ppm.
Ferrocenylmethyl-iso-propylsilylium
(9d): This was prepared from ferrocenylmethyl-iso-propylsilane (7d,
21.4 mg, 78.6 mmol, 1.00 equiv) and [Ph3C]+[B(C6F5)4]ꢀ (8, 72.5 mg,
tetrakis(pentafluorophenyl)borate
Ferrocenyldiphenylsilylium tetrakis(pentafluorophenyl)borate (9j): This
was prepared from ferrocenyldiphenylsilane (7j, 29.4 mg, 79.9 mmol,
AHCTUNGTRENNUNG
ACTHNUTRGNEUNG
1.00 equiv) and [Ph3C]+[B(C6F5)4]ꢀ (8, 73.7 mg, 79.9 mmol, 1.00 equiv) ac-
78.6 mmol, 1.00 equiv) according to GP 2 along with silylium ion 9m.
1H NMR (500 MHz, 1,2-Cl2C6D4): d=0.74 (s, 3H), 1.13 (mc, 6H), 1.41
(sept, J=7.6 Hz, 1H), 3.67 (brs, 1H), 3.82 (brs, 1H), 4.28 (brs, 5H),
5.22 ppm (brs, 2H); 13C NMR (125 MHz, 1,2-Cl2C6D4): d=ꢀ5.4, 17.4,
cording to GP 2 along with silylium ion 9k. 1H NMR (500 MHz, 1,2-
Cl2C6D4): d=4.04 (brs, 2H), 4.30 (brs, 5H), 5.36 (brs, 2H), 7.39–7.42 (m,
4H), 7.51–7.60 ppm (m, 6H); 13C NMR (125 MHz, 1,2-Cl2C6D4): d=62.4,
75.5, 81.4, 87.4, 124.4 (brm), 124.6, 129.9, 135.8, 136.5 (d, JC,F =244 Hz),
136.6, 138.5 (d, JC,F =244 Hz), 148.5 ppm (d, JC,F =241 Hz); 11B NMR
(126 MHz, 1,2-Cl2C6D4): d=ꢀ16.5 ppm; 19F NMR (470 MHz, 1,2-
Cl2C6D4): d=ꢀ166.4, ꢀ162.3, ꢀ132.3 ppm; 1H,29Si HMQC NMR
(99 MHz, 1,2-Cl2C6D4): d=81.4 ppm; 29Si NMR (99 MHz, 1,2-Cl2C6D4):
d=81.0 ppm.
19.2, 19.9, 65.7, 73.9, 80.6, 80.8, 86.8, 86.9, 124.3 (brm), 136.5 (d, JC,F
=
244 Hz), 138.5 (d, JC,F =244 Hz), 148.5 ppm (d, JC,F =243 Hz); 11B NMR
(126 MHz, 1,2-Cl2C6D4): d=ꢀ16.3 ppm; 19F NMR (470 MHz, 1,2-
Cl2C6D4): d=ꢀ166.2, ꢀ162.1, ꢀ132.1 ppm; 1H,29Si HMQC NMR
(99 MHz, 1,2-Cl2C6D4): d=113.5 ppm; 29Si NMR (99 MHz, 1,2-Cl2C6D4):
d=113.4 ppm.
Diferrocenylphenylsilylium tetrakis(pentafluorophenyl)borate (9k): This
was prepared from diferrocenylphenylsilane (7k, 37.6 mg, 79.0 mmol,
Ferrocenyldimethylsilylium tetrakis(pentafluorophenyl)borate (9e): This
was prepared from ferrocenyldimethylsilane (7e, 19.7 mg, 80.7 mmol,
ACTHNUTRGNEUNG
1.00 equiv) and [Ph3C]+[B(C6F5)4]ꢀ (8, 72.8 mg, 79.0 mmol, 1.00 equiv) ac-
ACTHNUTRGNEUNG
1.00 equiv) and Ph3C+[B(C6F5)4]ꢀ (8, 74.4 mg, 80.7 mmol, 1.00 equiv) ac-
cording to GP 2 along with silylium ion 9h. 1H NMR (500 MHz, 1,2-
Cl2C6D4): d=4.05 (brs, 4H), 4.12 (brs, 10H), 4.95 (brs, 4H), 7.52 (dd,
J=7.6 Hz, J=7.6 Hz, 2H), 7.60 (t, J=7.7 Hz, 1H), 7.84 ppm (d, J=
7.4 Hz, 2H); 13C NMR (125 MHz, 1,2-Cl2C6D4): d=51.4, 73.1, 76.9, 81.8,
124.6 (brm), 125.0, 130.1, 134.4, 134.5, 136.7 (d, JC,F =244 Hz), 138.6 (d,
cording to GP 2 along with silylium ion 9i. 1H NMR (500 MHz, 1,2-
Cl2C6D4): d=0.82 (brs, 6H), 3.68 (brs, 2H), 4.21 (brs, 5H), 5.18 ppm
(brs, 2H); 13C NMR (125 MHz, 1,2-Cl2C6D4): d=ꢀ1.6, 66.4, 74.4, 80.3,
86.8, 124.3 (brm), 136.5, (d,
JC,F =246 Hz), 138.4 (d, JC,F =245 Hz),
148.4 ppm (d,
J
C,F =242 Hz); 11B NMR (126 MHz, 1,2-Cl2C6D4): d=
C,F =242 Hz), 148.7 ppm (d, J
C,F =242 Hz); 11B NMR (126 MHz, 1,2-
ꢀ16.5 ppm; 19F NMR (470 MHz, 1,2-Cl2C6D4): d=ꢀ166.3, ꢀ162.1,
J
Cl2C6D4): d=ꢀ16.1 ppm; 19F NMR (470 MHz, 1,2-Cl2C6D4): d=ꢀ165.8,
ꢀ161.9, ꢀ131.7 ppm; 1H,29Si HMQC NMR (99 MHz, 1,2-Cl2C6D4): d=
77.5 ppm; 29Si NMR (99 MHz, 1,2-Cl2C6D4): d=77.4 ppm.
1
ꢀ132.2 ppm; H,29Si HMQC NMR (99 MHz, 1,2-Cl2C6D4): d=111.0 ppm;
29Si NMR (99 MHz, 1,2-Cl2C6D4): d=110.7 ppm.
tert-Butylferrocenylphenylsilylium
(9 f): Prepared from tert-butylferrocenylphenylsilane (7 f, 41.1 mg,
118 mmol, 1.00 equiv) and [Ph3C]+[B(C6F5)4]ꢀ (8, 109 mg, 118 mmol,
tetrakis(pentafluorophenyl)borate
tert-Butyldiferrocenylsilylium tetrakis(pentafluorophenyl)borate (9l):
1H NMR (300 MHz, 1,2-Cl2C6D4): d=1.34 (s, 9H), 4.06 (brs, 4H), 4.16 (s,
10H), 4.91 ppm (brs, 4H); 13C NMR (125 MHz, 1,2-Cl2C6D4): d=25.3,
29.7, 55.0, 72.7, 78.0, 80.9, 124.5 (brm), 136.6, (d, JC,F =242 Hz), 138.6 (d,
AHCTUNGTRENNUNG
1.00 equiv) according to GP 2 along with traces of silylium ion 9l.
1H NMR (300 MHz, 1,2-Cl2C6D4): d=1.13 (s, 9H), 3.44 (brs, 1H), 4.06
(brs, 1H), 4.35 (brs, 5H), 5.18 (brs, 1H), 5.24 (brs, 1H), 7.36–7.39 (m,
2H), 7.45–7.50 ppm (m, 3H); 13C NMR (75 MHz, 1,2-Cl2C6D4): d=25.9,
28.2, 64.5, 73.8, 81.7, 83.0, 86.6, 87.4, 124.6 (brm), 126.3, 129.8, 133.7,
J
C,F =242 Hz), 148.6 ppm (d, J
C,F =242 Hz); 11B NMR (160 MHz, 1,2-
Cl2C6D4): d=ꢀ16.4 ppm; 19F NMR (470 MHz, 1,2-Cl2C6D4): d=ꢀ166.1,
ꢀ162.2, ꢀ132.0 ppm; 1H,29Si HMQC NMR (99 MHz, 1,2-Cl2C6D4): d=
104.5 ppm.
134.1, 136.7 (d, JC,F =245 Hz), 138.6 (d, JC,F =238 Hz), 148.7 ppm (d, JC,F
=
245 Hz); 11B NMR (76 MHz, 1,2-Cl2C6D4): d=ꢀ16.3 ppm; 19F NMR
(282 MHz, 1,2-Cl2C6D4): d=ꢀ166.1, ꢀ162.1, ꢀ132.0 ppm; 1H,29Si HMQC
NMR (99 MHz, 1,2-Cl2C6D4): d=98.8 ppm; 29Si NMR (59 MHz, 1,2-
Cl2C6D4): d=98.6 ppm.
Diferrocenyl-iso-propylsilylium tetrakis(pentafluorophenyl)borate (9m):
1H NMR (500 MHz, 1,2-Cl2C6D4): d=1.38–1.44 (m, 6H), 1.83 (sept, J=
7.4 Hz, 1H), 3.99 (brs, 4H), 4.17 (s, 10H), 4.92 ppm (s, 4H); 13C NMR
(125 MHz, 1,2-Cl2C6D4): d=16.8, 17.5, 53.9, 72.5, 76.7, 81.2, 124.3 (brm),
16592
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Chem. Eur. J. 2013, 19, 16579 – 16594

Ferrocene-Stabilized Silylium Ions
FULL PAPER
136.5 (d, JC,F =244 Hz), 138.5 (d, JC,F =244 Hz), 148.5 ppm (d, JC,F
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This research was (in part) supported by the Cluster of Excellence “Uni-
fying Concepts in Catalysis” (UniCat; EXC 314, 2012–2017) and the
Deutsche Forschungsgemeinschaft (Oe 249/9-1). K.M. also thanks the
Studienstiftung des Deutschen Volkes for a predoctoral fellowship (2009–
2012), and P.H. gratefully acknowledges the Alexander von Humboldt
Foundation for a research fellowship. K.M. is a member of the Interna-
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Received: July 23, 2013
Published online: October 21, 2013
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Chem. Eur. J. 2013, 19, 16579 – 16594