A unique transition metal-stabilized silicon cation

Article abstract of DOI:10.1021/ja205538t

Trivalent silicon cations are exceptionally strong electron pair acceptors that react, either desired or undesired, with almost any σ and π basic molecule. One way of intramolecular attenuation of the Lewis acidity of these superelectrophiles is by installation of a ferrocene unit at the electron-deficient silicon atom. While well-understood for isoelectronic α-ferrocenyl-substituted carbenium ions and also boranes, the stabilizing interactions between the ferrocene backbone and a positively charged silicon atom are not clear due to the challenge of crystallizing such cations. The structural characterization of our ferrocene-stabilized silicon cation now reveals an unprecedented bonding motif different from its analogues. An extreme dip angle of the silicon atom toward the iron atom is explained by two three-center-two-electron (3c2e) bonds through participation of both the upper and the lower aromatic rings of the ferrocene sandwich structure. The positive charge is still localized at the silicon atom that also retains a quasi-planar configuration.

Full text of DOI:10.1021/ja205538t

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A Unique Transition Metal-Stabilized Silicon Cation
Kristine M€uther,† Roland Fr€ohlich,†,‡ Christian M€uck-Lichtenfeld,†,§ Stefan Grimme,†,§ and
Martin Oestreich*,†
†Organisch-Chemisches Institut, ‡R€ontgenstrukturanalyse, and §Theoretische Organische Chemie, Westf€alische Wilhelms-Universit€at
M€unster, Corrensstrasse 40, 48149 M€unster, Germany
S Supporting Information
b
electron-poor R-substituent lends these electrophilic com-
pounds increased stability. It was a seminal paper by Behrens21
ABSTRACT: Trivalent silicon cations are exceptionally
strong electron pair acceptors that react, either desired or
that established direct ironꢀR-carbon bonding in the molecular
undesired, with almost any σ and π basic molecule. One way
of intramolecular attenuation of the Lewis acidity of these
superelectrophiles is by installation of a ferrocene unit at the
electron-deficient silicon atom. While well-understood for
isoelectronic R-ferrocenyl-substituted carbenium ions and
also boranes, the stabilizing interactions between the ferro-
cene backbone and a positively charged silicon atom are not
clear due to the challenge of crystallizing such cations. The
structural characterization of our ferrocene-stabilized silicon
cation now reveals an unprecedented bonding motif differ-
ent from its analogues. An extreme dip angle of the silicon
atom toward the iron atom is explained by two three-center-
two-electron (3c2e) bonds through participation of both the
upper and the lower aromatic rings of the ferrocene
sandwich structure. The positive charge is still localized at
the silicon atom that also retains a quasi-planar configura-
tion.
structure of a ferrocene-stabilized methylium ion. The ferrocene
backbone is bent by 9.3°, and the former substituted Cp ligand
shows fulvene character, indicating delocalization of the positive
charge. There was an early (unsuccessful) attempt to access a
silicon analogue (with R = Ph),23 that is an R-ferrocenylsilylium
ion (III, Figure 1), but that structural motif24 had been un-
precendented until our laboratory recently reported its synthesis
and spectroscopic characterization [1 f 3+ [B(C6F5)4]ꢀ, left,
3
Scheme 1].25
For the present investigation, we replaced the [B(C6F5)4]ꢀ by
the easy-to-access [B12Cl12]2ꢀ counteranion26,27 [1 f (3+)2
3
[B12Cl12]2ꢀ, right, Scheme 1] and we now accomplished the
crystallization of that ferrocene-stabilized silylium ion. We wish
to report its unique molecular structure with both the bonding
situation and the charge distribution being distinctly different
from the related carbenium ion.21 Quantum-chemical calcula-
tions assist the interpretation of the experimental data and allow
for comparison of the silylium ion and the carbenium ion.
Single crystals suitable for X-ray diffraction were obtained
from a solution of our silicon cation 3+ and the [B12Cl12]2ꢀ
counteranion in 1,2-Cl2C6D4 at room temperature (cf. Figure S1
in the Supporting Information). There are two independent
molecules of 3+ in the solid state, and one of these silicon cations
is depicted in Figure 2 (see Table S1 in the Supporting Informa-
tion for a comparison with the other one). All data obtained were
exactly reproduced in an independent experiment. The tilt angle
of the Cp rings of 11.6° is in the same order of magnitude as for
the tilted sandwich structure of the above-mentioned carbenium
ion.21 However, the dip angle of R* = 44.8° is dramatically enlarged
compared to R* = 20.7° of the carbenium ion21 (cf. Figure S2 in the
Supporting Information for calculated dip angles). The geometry at
the silicon atom also deserves attention. The average CꢀSiꢀC
angle is 117.8° (116.3° for the other molecule), and that is closer to
trigonal planarity (120.0°) than to a tetrahedron (109.5°). Despite
that quasi-planarity, there is a bonding interaction between Si and Fe
with a bond length of 2.492 Å. These angles compare well with those
of all representative classes of intermolecularly stabilized silylium
ions: Et3Si(toluene)+ [B(C6F5)4]ꢀ (114.0°),5 i-Pr3Si+ [HCB11-
nderstanding the structure and the stabilization of reactive
U
intermediates is a fundamental task for synthetic chemistry.
As such, the isolation and the characterization of trivalent silicon
cations in the condensed phase pose a perpetual challenge,
largely due to its pronounced electrophilicity.1ꢀ3 Unlike the
electron-deficient carbon atom in isoelectronic carbenium ions,
the positive charge is essentially localized on the silicon atom, and
that makes silylium ions exceptionally potent electron pair
acceptors. That voracious appetite for nucleophiles4 had fueled a
controversy over the existence of truly three-coordinate silicon
cations for years, and crystallographic as well as spectroscopic
evidence exposed π basic solvents (intermolecular)5 and groups
(intramolecular)6,7 or even weakly σ coordinating counteranions
(intermolecular)8 and halogen atoms (intramolecular)9 as donors to
the empty orbital at the silicon atom. Intra-10ꢀ12 and intermolecular13
three-center-two-electron (3c2e) [Si
H
Si]+ bridges are also
3 3 3 3 3 3
known. Aside from these beautiful examples of inter- and intramo-
lecularly stabilized silylium ions,14 Lambert’s Mes3Si+ cation15 crystal-
lized with Reed’s [HCB11Me5Br6]ꢀ anion16 remains the sole
example of a, in a strict sense of the word, free silylium ion.17
An approach to intramolecular stabilization of isoelectronic
charged and, likewise, neutral atoms with an electron sextet is
realized in R-ferrocenylcarbenium ions18 and related boranes19
(I and II, Figure 1). The concept itself dates back several
decades,20 and it had long been assumed that a bonding inter-
action between the electron-rich transition metal atom and the
3
3
3
H5Cl6]ꢀ (117.3°),28 and [Me3Si
H
SiMe3]+ [HCB11-
3 3 3 3 3 3
Cl11]ꢀ (116.7).13
Our density functional theory (DFT) calculations (TPSS-D3/
def2-TZVPP, see the Supporting Information for details)29ꢀ31 of
Received: June 15, 2011
Published: July 19, 2011
r
2011 American Chemical Society
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Figure 1. Isoelectronic charged and neutral ferrocene-based systems
with an electron sextet at the R atom.
Scheme 1. Execution of the Silicon Cation Generation with
Different Counteranions
Figure 3. Localized molecular orbitals. (A) LMOA and (B) LMOB
involving silicon, iron, and ring carbon atoms (isosurfaces are displayed
at a value of 0.05 e/a03/2). The Mulliken populations of the silicon
atomic orbitals in the LMOs are 19.2% for LMOA and 12.5% for LMOB.
a 3c2e bond of Cipso of the upper Cp ring, Si and Fe (LMOA,
Figure 3A), there is another one (LMOB, Figure 3B) involving
the carbon atom C0ipso of the lower Cp ring. That involvement of
bonding interactions between the silicon atom and the lower ring
clearly distinguishes our silylium ion from the corresponding
carbenium ion.18 It also explains the extraordinarily large dip
angle between the SiꢀCipso bond and the upper Cp plane.
The intramolecular stabilization through the ferrocene donor
makes any intermolecular interaction redundant. Neither the
solvent (shortest SiꢀCl distance 5.261 Å) nor the counteranion
(shortest SiꢀCl distance 3.624 Å) coordinate to the cationic
silicon atom in the solid state (cf. sum of the van der Waals radii is
3.68 Å). These data are in accordance with 29Si NMR measure-
ments (Scheme 1) where the chemical shift is independent of
solvent (CD2Cl2 or 1,2-Cl2C6D4 with [B(C6F5)4]ꢀ as counter-
anion) and counteranion (with [B(C6F5)4]ꢀ or [B12Cl12]2ꢀ in
1,2-Cl2C6D4 as solvent). The silicon atom is significantly de-
shielded in the 29Si NMR spectrum (δ ≈ 114.5 ppm), indicating
a localized positive charge. The 29Si NMR chemical shift
calculated with GIAOꢀDFT (BP86)34ꢀ36 is δTMS = 116.4
ppm, a clear indication that the solid state structure of the silicon
cation also prevails in solution. The electrostatic potential of the
silylium ion also alludes to the presence of a positive charge at the
silicon atom (Figure 4A), especially when compared to the
related carbenium ion (Figure 4B).
Figure 2. Crystal structure analysis of 3+: Schakal plot of the molecular
structure (counteranion omitted for the sake of clarity). Selected
experimental bond lengths (Å): SiꢀCipso, 1.829(9); SiꢀFe, 2.492(2);
SiꢀC0ipso, 2.784(9). Selected calculated bond lengths (Å): SiꢀCipso
1.823; SiꢀFe, 2.461; SiꢀC0ipso, 2.647.
,
the isolated silylium ion 3+ resulted in an optimized geometry
which is in excellent agreement with the solid state structure
(Figure 2). The unusual bonding situation in the ferrocene-
32
stabilized silylium ion is apparent in the Wiberg bond orders bW
between the silicon atom and its neighbors. While a single bond is
clearly present between the silicon atom and the carbon atom
Cipso (bW = 1.03), both the SiꢀFe bond (bW = 0.51) and the
SiꢀC0ipso bond (bW = 0.26) have partial covalent bonding
character, indicative of delocalized bonding. The nature of this
bonding becomes visible when localized molecular orbitals
(LMOs)33 are calculated and drawn (Figure 3). In addition to
Quantum-chemical calculations unravelled a unique bonding
situation in our silylium ion 3+ with its ferrocene backbone.37 It is
two 3c2e bonds with participation of both the upper (expected)
and the lower (unexpected) Cp rings that account for the
unconventional molecular structure. By this mode of stabiliza-
tion, an almost planar geometry and a positive charge located at
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Journal of the American Chemical Society
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’ AUTHOR INFORMATION
Corresponding Author
martin.oestreich@uni-muenster.de
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K.M. thanks the Studienstiftung des deutschen Volkes for a
predoctoral fellowship (2009ꢀ2011). K.M. is a member of
the International Research Training Group M€unsterꢀNagoya
(GRK 1143 of the Deutsche Forschungsgemeinschaft). K.M. and
M.O. thank Jens Mohr (Universit€at M€unster) for his valuable
experimental support. This research was (in part) supported by
the Sonderforschungsbereich 858 of the Deutsche Forschungs-
gemeinschaft (Projects A3 and Z1). We acknowledge Dr. Klaus
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