An acyclic imino-substituted silylene: Synthesis, isolation, and its facile conversion into a zwitterionic silaimine

Article abstract of DOI:10.1002/anie.201203257

A new type of Si(II): A novel silylene stabilized by a Cp* and an imidazolin-2-iminato ligand has been prepared using two different methods (see scheme). The X-ray crystallographic structure shows that the silicon(II) center is coordinated to an ?2-Cp* ligand and the nitrogen atom of an imidazolin-2-iminato ligand. This silylene easily reacts with B(C6F5)3 to give a stable borane adduct having a zwitterionic resonance structure.

Full text of DOI:10.1002/anie.201203257

Angewandte
Chemie
DOI: 10.1002/anie.201203257
Silylene Compounds
An Acyclic Imino-Substituted Silylene: Synthesis, Isolation, and its
Facile Conversion into a Zwitterionic Silaimine**
´
Shigeyoshi Inoue* and Kinga Leszczynska
Dedicated to Professor Peter Jutzi
Silylenes, heavier analogues of carbenes, have attracted
enormous research interest because of their interesting
properties and reactivity.[1] Since the synthesis of the first
N-heterocyclic silylene (NHSi) in 1994,[2] a number of isolable
silylenes have been synthesized by taking advantage of
thermodynamic stabilization of coordinating ligands and by
the introduction of kinetic stabilization.[1,3] Despite significant
advances made in the area of stable cyclic silylenes, which
include donor-free silylenes,[4,5] acyclic silylenes are notori-
ously difficult and challenging synthetic targets. The acyclic
diaminosilylene [(Me3Si)2N]2Si: was reported by West and co-
workers and is stable in solution at low temperatures,
although it decomposes at 08C.[6] In fact, only a few examples
of stable acyclic silylenes are known. For example, Jutzi and
co-workers reported that the two acyclic silicon(II)
compounds (I and II, Scheme 1) could be synthesized by
tivity was investigated.[9,10] However, the chemistry of acyclic
silylenes is largely unexplored.
Recently, the isolation of small reactive molecules, such as
B2H2,[11] Si2,[12] Ge2,[13] P2,[14] As2,[15] and PN,[16] was accom-
plished by employing elegant procedures using N-heterocy-
clic carbenes (NHCs).[17] One such reactive species, which has
yet to be isolated, is the elusive silylene–nitrene species,
[18]
ꢀ ꢀ
ꢀ ꢁ
R Si N, which is isoelectronic with silanitrile R Si N.
This silylene–nitrene species has only been studied by
theoretical calculations but a stable derivative may be
accessible using carbenes, which have stabilizing properties.
Generally, the free carbene is used when generating these
types of species; however, in this case, an alternative
approach using the imidazolin-2-iminato ligand as a precursor
to the N-carbene moiety may be more convenient. This is
because there are two coordination sites for a free carbene,
one at the silicon atom (R(NHC)SiN) and the other at the
!
nitrogen atom (R SiN NHC).[19] Preliminary computa-
tional studies showed that coordination to the nitrogen
atom yields a product that is more stable than that derived
from coordination to the silicon atom (for details see the
Supporting Information). It is certain that this order of
ꢀ
=
stability exists because of the strong C N bond that is formed
upon coordination as well as the steric congestion at the
silicon center. Regardless, by using the imidazolin-2-iminato
ligand, the preexisting bond assures that the carbene will be
coordinated to the nitrogen atom in the product.
Scheme 1. Stable acyclic, monomeric silicon(II) compounds I–IV.
Cp*=Me5C5, Trip=2,4,6-iPr3C6H2, Dipp=2,6-iPr2C6H3, Mes=2,4,6-
Me3C6H2.
Tamm and co-workers have isolated and characterized
numerous imidazolin-2-iminato transition-metal com-
plexes.[20] In these cases the imidazolin-2-iminato ligands act
as a 2s- and either a 2p- or a 4p-electron donor, which results
in some multiple-bond character in the interaction between
the nitrogen atom and the metal center.[20,21] Therefore, we
reasoned that the bis(2,6-diisopropylphenyl)imidazolin-2-imi-
nato ligand would be an ideal bulky electron-donating group
for the synthesis of an an NHC-stabilized silylene–nitrene
compound. Notably, the resultant compound A can be
described by several mesomeric structures, which include
silylene–nitrene AI, the acyclic silylene AII, the zwitterionic
silylene amide AIII, and the 1-sila-2-azaallene AIV (Scheme 2).
In addition, because it is well known that the Cp* group is
a good electron-donating group, sterically protects silicon(II)
compounds, and shows variable bonding modes,[22] we
employed it to stabilize the new silylene A. Herein, we
report the synthesis, structure, and reactivity of a novel
species of type A.
a salt metathesis reaction of the silyliumylidene cation
[Cp*Si]+[B(C6F5)4]ꢀ[7] with the corresponding anionic nucle-
ophiles Rꢀ (R = [Cp*Fe(CO)2], 2,6-Trip2C6H3).[8] Very
recently, two stable acyclic two-coordinate silylenes (III and
IV; Scheme 1) were successfully synthesized and their reac-
´
[*] Prof. Dr. S. Inoue, Dr. K. Leszczynska
Institute of Chemistry, Technische Universitꢀt Berlin
Straße des 17. Juni 135, Sekr. C2, 10623 Berlin (Germany)
E-mail: shigeyoshi.inoue@tu-berlin.de
[**] This work was supported by the Sofja Kovalevskaja Program
(Alexander von Humboldt foundation). We thank Prof. Jutzi
(Universitꢀt Bielefeld) for a gift of Cp*2Si.
Supporting information for this article, which includes full synthetic,
spectroscopic, and crystallographic details, is available on the
WWW under http://dx.doi.org/10.1002/anie.201203257.
The reaction of Cp*SiBr3 (1)[23] with one equivalent of the
lithium reagent LLi (L = bis(2,6-diisopropylphenyl)imidazo-
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Scheme 2. Mesomeric structures of A (R1 =Cp*, R2 =Dipp).
lin-2-iminato)[24] in diethylether gives dibromosilane 2 as
colorless crystals in 71% yield (Scheme 3). Compound 2
crystallized with two independent molecules in the unit cell
but the geometries are almost identical to each other (see also
Figure 1. Molecular structure of compound 3. Thermal ellipsoids are
shown at 50% probability. Hydrogen atoms are omitted for clarity.
ꢀ
ꢀ
Selected bond lengths (ꢁ) and angles (8): Si1 N1, 1.691(5); Si1 C28,
ꢀ
ꢀ
ꢀ
2.218(6); Si1 C29, 2.124(6); C1 N1, 1.280(6); C1 N2, 1.377(6);
ꢀ
C1 N3, 1.387(6); N1-Si1-C28, 103.9(2); N1-Si1-C29, 99.5(3);
Si1-N1-C1, 136.6(4).
(1.691(5) ꢀ) is shorter than that of the acyclic silylene III
(1.731(1) ꢀ) and those of other two-coordinate N-heterocy-
clic silylenes (1.696(3)–1.774(4) ꢀ),[4a–d,f,g,9] thus indicating
some p donation from the nitrogen atom to the silicon
atom, a type of donation that was also observed for the
imidazolin-2-iminato ligand. The Si1-N1-C1 angle of 3
(136.6(4)8) is smaller than those of compound 2 (141.2(3)8
Scheme 3. Synthesis of 3.
ꢀ
and 145.1(3)8). The N1 C1 bond length of 3 (1.280(6) ꢀ) is,
the Supporting Information, Figure S1).[25] In the solid state,
the geometry of the imidazolin-2-iminato moiety of 2 is
similar to that of reported compounds LSiMe3 and LSiMe2-
(C5Me4H).[24a,29a,c] The Cp* ligand in 2 is coordinated to the
silicon atom in an h1 mode (s bonded). The methyl groups of
the Cp* moiety appear as three broad signals in the 1H NMR
spectrum in the ratio 1:2:2. The broadening of the peaks is
presumably due to the slow rotation of the Cp* ligand on the
NMR time scale in solution. In accordance with the above
results, three signals for the methyl groups of the Cp* moiety
and three signals for the quaternary carbon atoms of the Cp*
ring were observed in the 13C NMR spectrum.
Our initial attempts to prepare imino–silylene 3 by
reacting dibromosilane 2 with various reducing reagents,
such as NaC10H8 or KC8, led to the formation of the desired
product 3 in only trace amounts (less than 10%) as observed
by NMR spectroscopy (Scheme 3). Alternatively, the direct
use of a silicon(II) precursor in the one-step reaction of
[Cp*Si]+[B(C6F5)4]ꢀ (4)[7] with LLi in a mixture of Et2O/
hexane afforded 3 as colorless crystals in a much higher yield
of 57%.
within experimental accuracy, the same as those of carbene-
stabilized phosphorus mononitride PN (1.282(3) ꢀ)[16] and in
the range of those of reported for transition-metal complexes
using imidazolin-2-iminato ligands;[20,24] however, the bond
length is slightly longer than those of LSiMe3 and LSiMe2-
(C5Me4H).[20a,c,24a]
The 1H and 13C NMR spectra of 3 show averaged
resonance signals for the methyl and ring carbon atoms of
the Cp* group; this observation is consistent with the
fluxional behavior of these types of compounds in solution.[22]
The 29Si NMR chemical shift observed for 3 (d = ꢀ43.8 ppm)
is shifted upfield relative to that found for Cp*SiN(TMS)2
(TMS = trimethylsilyl; d = ꢀ10.2 ppm), which was reported
by Jutzi and co-workers,[7,26] thus implying that there is more
electron donation from the ligand to the silicon center in 3
than in Cp*SiN(TMS)2. This hypothesis is also consistent with
ꢀ
the Si N bond length of both compounds (1.691(5) ꢀ for 3;
1.7476(10) ꢀ and 1.7483(10) ꢀ for Cp*SiN(TMS)2). Interest-
ingly, the amino–silylene compound Cp*SiN(TMS)2, exhibits
a monomeric structure only in solution, but exists as a dimer
[7,26]
=
(a disilene with a Si Si bond) in the solid state.
X-ray analysis of 3 revealed a monomeric structure
(Figure 1).[25] According to the geometry, the nitrogen atom
(N1) of the imidazolin-2-iminato ligand and the Cp* ligand
are coordinated to the silicon atom. The distances between
Furthermore, this 29Si NMR resonance of 3 is significantly
upfield relative to that of other reported silylenes,[4,5] thus
indicating that the relatively high electron density on the
silicon center is a result of not only electron donation from the
imidazolin-2-iminato ligand, but also that from the Cp*
ligand.
To gain more insight into compound 3, DFT calculations
were performed at the B3LYP/6-31G(d) level (see the
Supporting Information). The optimized structure of 3 was
ꢀ
the silicon center and the Cp* moiety in 3 (Si C28 2.218(6) ꢀ
2
ꢀ
and Si1 C29 2.124(6) ꢀ) clearly slow an h coordination of
the Cp* moiety to the silicon atom. The N1-Si1-C28 and
N1-Si1-C29 bond angles of 3 are 103.9(2)8 and 99.5(3)8,
ꢀ
respectively. Interestingly, the Si1 N1 bond length of 3
2
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in good agreement with the X-ray data. The lone-pair orbital
on the silicon atom is clearly present in the HOMO (Figure 2,
left) and the vacant 3p orbital of the silicon atom is located in
the LUMO + 4 (Figure 2, right). The HOMO-1 shows the
s-bonding interaction of the p orbitals of the Cp* group with
the vacant p orbital of the silicon(II) center (see the
Supporting Information, Figure S5). On the other hand, the
HOMO-2 indicates the presence of a s-bonding orbital
Scheme 4. Synthesis of borane adduct 5.
number of the silicon atom along with a decreased electron
density at the silicon atom, upon coordination of the boron
species. The observed signal is also significantly more down-
field than that of the carbene-stabilized silylene–borane
adducts, NHC!SiH2!BH3 (d = ꢀ55.6 ppm) and NHC!
SiCl2!BR3 (R = C6F5, d = 53.2 ppm; R = H, d = 30.7 ppm).[28]
The 11B NMR spectrum of 5 shows one signal at d =
ꢀ21.5 ppm, which is more upfield than that of [CHN-
(tBu)]2Si!B(C6F5)3 (d = ꢀ14.3 ppm).[27] Compound
5 is
Figure 2. Molecular orbitals of 3, HOMO (left, ꢀ0.1637 eV) and
LUMO+4 (right, ꢀ0.001 eV).
stable in C6D6 solution for at least one month.
The molecular structure of borane adduct 5 was also
determined by single-crystal X-ray diffraction analysis
(Figure 3).[25] The silicon(II) atom in 5 is three-coordinate: it
is bound to the boron atom of the borane, the nitrogen atom
of the imidazolin-2-iminato ligand, and the C28 atom of the
Cp* ligand. It should be noted that the Cp* ligand is
coordinated in an h1 mode with only a s bond between the
C28 and Si1 atoms (1.905(4) ꢀ). The Cp* ring and one of the
C6F5 rings in 5 are almost parallel and exhibit intramolecular
p–p stacking (the distance between the rings varies from 3.325
between C28 and Si1 (Figure S5). Furthermore, an orbital
corresponding to p donation from the nitrogen atom to the
silicon atom was also observed (HOMO-7; Figure S5), how-
ever, the calculated WBI (Wiberg Bond Index) value for the
ꢀ
Si1 N1 bond (0.798) indicates single-bond character. The
ꢀ
multiple-bond character of the N1 C1 bond in 3 is also
supported by the WBI value (1.551) and by the presence of
a p-bonding orbital between the N1 and C1 atoms (this can
also be seen in the HOMO-11; Figure S5). Although some
contribution from silylene–nitrene (AI) and silylene-ylide
(AIII and AIV) resonance forms may exist, all theoretical and
experimental results indicate that the imino-substituted
silylene structure (AII) is dominant in 3 (Scheme 2). This
result is unlike the corresponding result obtained for a car-
bene-stabilized P2 and As2,[14,15] a difference that can be
explained by the fact that a nitrogen atom can make much
stronger multiple bond with a carbon atom.
ꢀ
to 3.542 ꢀ). The B1 Si1 bond length (2.080(5) ꢀ) is shorter
than that of NHC!SiCl2!B(C6F5)3 (2.1135(6) ꢀ) and longer
To date, only a few borane adducts of silylenes have been
characterized. The only example of a silylene coordinated to
a borane was reported by Metzler and Denk, who observed
reversible formation of the adduct between [CHN(tBu)]2Si
and B(C6F5)3; however, the compound was unstable and
slowly rearranged to a silylborane.[27] There are several other
examples of silylene–borane adducts but they were synthe-
sized using coordinated silylenes or by using other forms of
reactivity.[28] However, in the case of silylene 3, its reaction
with B(C6F5)3 in hexane results in the formation of the borane
adduct 5 in 89% yield (Scheme 4).
The coordination of the boron atom to the silicon atom in
compound 5 was confirmed by the 29Si NMR spectrum where
a quartet appears at d = 114.5 ppm (1JSi,B = 70 Hz). In the
29Si NMR spectra, the resonance of 5 is significantly more
downfield shifted than that of compound 3 (ca. 158 ppm),
a fact that is attributable to the change in coordination
Figure 3. Molecular structure of compound 5. Thermal ellipsoids are
shown at 50% probability. Hydrogen atoms and solvent molecules are
ꢀ
omitted for clarity. Selected bond lengths (ꢁ) and angles(8): Si1 N1,
ꢀ
ꢀ
ꢀ
1.605(3); Si1 C28, 1.905(4); Si1 B1, 2.080(5); C1 N1, 1.302(4);
ꢀ
ꢀ
C1 N2, 1.372(4); C1 N3, 1.372(5); N1-Si1-C28, 108.69(18);
N1-Si1-B1, 129.45(17); B1-Si1-C28, 121.81(17); C1-N1-Si1, 158.7(3).
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5
ꢀ
(1.605(3) ꢀ) is significantly shorter than that of silylene 3
ꢀ
(1.691(5) ꢀ) and those of typical Si N bonds (1.713–
1.739 ꢀ).[30] The Si1 N1 bond length in 5 is in the range of
ꢀ
that reported for Si N bonds.[31,32] In addition, the p-bonding
=
interaction between the silicon and nitrogen atoms can be
observed in the HOMO-13 of the optimized structure of 5
(see the Supporting Information, Figure S6). This increased
interaction is also seen in the calculated WBI value for the
ꢀ
Si1 N1 bond of 5 (0.900), which is larger than that of 3
(0.798). Moreover, the Si1-N1-C1 angle in 5 (158.7(3)8) is
significantly larger than that of imino–silylene 3 (136.6(4)8),
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ꢀ
N1 C1 bond length of 5 (1.302(4) ꢀ) is longer than those of
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B(C6F5)3. The increase in p donation from the nitrogen atom
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atom, and the planar structure of the Si1 atom, point to a
1-sila-2-azaallene resonance form 5’. Similar p donation from
a nitrogen atom to an sp2-silicon atom and from an sp2-silicon
atom to a boron atom were also observed in the amino–
disilenes and boryl–disilenes.[29,33]
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accessible in higher yield through the salt metathesis reaction
of silyliumylidene cation 4 with the lithium reagent LLi.
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stabilized silylene–nitrene derivative AI, the structural and
spectroscopic characterization as well as DFT calculation,
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=
ꢀ
with an N C bond and a Si N bond. This silylene readily
reacts with B(C6F5)3 to form the stable silylene–borane adduct
5 having significant 1-sila-2-azaallene 5’ character. Further
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will be reported in due course.
Received: April 27, 2012
Revised: June 27, 2012
Published online: && &&, &&&&
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Keywords: carbenes · density functional calculations · silicon ·
.
silylenes · X-ray structures
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Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Silylene Compounds
S. Inoue,*
´
K. Leszczynska
&&&& — &&&&
An Acyclic Imino-Substituted Silylene:
Synthesis, Isolation, and its Facile
Conversion into a Zwitterionic Silaimine
A new type of Si(II): A novel silylene
stabilized by a Cp* and an imidazolin-2-
iminato ligand has been prepared using
two different methods (see scheme). The
X-ray crystallographic structure shows
that the silicon(II) center is coordinated
to an h2-Cp* ligand and the nitrogen
atom of an imidazolin-2-iminato ligand.
This silylene easily reacts with B(C6F5)3 to
give a stable borane adduct having
a zwitterionic resonance structure.
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!