A debut for base stabilized monoalkylsilylenes

Article abstract of DOI:10.1039/c2cc31041d

The first base stabilized monoalkylsilylenes LSitBu (2) and LSi[C(SiMe ₃)₃] (3) (L = PhC(NtBu)₂) were synthesized by the facile metathesis reactions of LitBu and KC(SiMe₃)₃ with LSiCl (1). The reactio

Full text of DOI:10.1039/c2cc31041d

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A debut for base stabilized monoalkylsilyleneswz
Ramachandran Azhakar, Rajendra S. Ghadwal,* Herbert W. Roesky,* Hilke Wolf and
Dietmar Stalke*
Received 13th February 2012, Accepted 14th March 2012
DOI: 10.1039/c2cc31041d
The first base stabilized monoalkylsilylenes LSitBu (2) and
LSi[C(SiMe3)3] (3) (L = PhC(NtBu)2) were synthesized by
the facile metathesis reactions of LitBu and KC(SiMe3)3 with
LSiCl (1). The reaction of LSitBu (2) with N2O afforded the
dimer [LSitBu(l-O)]2 (4) which contains a four-membered
Si2O2 ring.
addition reactions with organic substrates14 and Lewis bases.15
Recently, we were able to stabilize the fluorosilylenes as LSiFꢀ
M(CO)5 (M = Cr, Mo, W) in the coordination sphere of
transition metals.13e A literature survey revealed that so far
two stable dialkylsilylenes A (by Kira et al. in 1999)16 and B
(Driess et al. in 2011)17 are known (Chart 1), which were
synthesized by KC8 reduction of the corresponding
dibromosilanes. A single-crystal X-ray analysis of B and its
further derivatives have not been reported. In both A and B,
sterically demanding groups such as SiMe3 and PR3 (R = Ph,
m-tol) are present which provide protection at the low
coordinate silicon atoms. Furthermore, B benefitted from
neighbouring phosphorus ylide functionalities, which
contribute electron density towards the silicon atom.
Stabilization of highly reactive species under normal laboratory
conditions has been an ever growing field of research in main
group chemistry.1 Among group 14 intermediates, carbenes and
silylenes R2E: (where R = alkyl, aryl, H, or halogen, E = C or Si)
are highly reactive and attracted attention of theoreticians as well
as experimentalists.2–5 Their first stable analogues were isolated by
Arduengo et al. (1991)6 and West et al. (1994)7 as N-heterocycles
with a singlet ground state. Singlet carbenes have been used as
ligands for main group and transition metals to stabilize unusual
oxidation states and also find applications in activation of small
molecules.8,9 Gaseous dichlorosilylene Cl2Si: has been known for
many years.10 However, it is unstable and readily condenses to
polymeric (Cl2Si)x or disproportionates to Si and SiCl4. The
properties of gaseous Cl2Si: and polymeric (Cl2Si)x have been
studied early in the 1970s by Timms, Margrave and others.10,11 In
2006, the first stable chlorosilylene was isolated as LSiCl (L =
PhC(NtBu)2) (1) supported by an amidinato ligand in less than
10% yield using potassium as a reducing agent.5d After that it was
possible to stabilize dichlorosilylene as NHCꢀCl2Si: (NHC =
1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene or 1,3-bis-
(2,4,6-trimethylphenyl)imidazol-2-ylidene) in high yield by
utilizing a new synthetic strategy comprising the reductive
elimination of HCl from trichlorosilane in the presence of NHC
under mild reaction conditions.12 Subsequently a modified high
yield method for 1 was also reported which further facilitates to
explore its chemistry.5e We utilized silylenes as s-donor ligands
for transition metal complexes13a–f as well as for its oxidative
In continuation of our interest in developing safer and more
convenient methods for the synthesis of silylenes, we report
here a facile route to monoalkylsilylenes LSitBu (2) and
LSi[C(SiMe3)3] (3) by metathesis reaction of LitBu and
KC(SiMe3)3 with LSiCl (1).
Compounds 2 and 3 were obtained as neat products by facile
reactions of LitBu18a and KC(SiMe3)3 with 1 in a 1 : 1 ratio
(Scheme 1). Compounds 2 and 3 are soluble in common organic
solvents. They were fully characterized by spectroscopic techniques
and by elemental analysis. Furthermore, the molecular structure of
3 was established unequivocally by single-crystal X-ray structural
analysis. However, the reaction of 1 with MeLi or PhLi resulted in
a mixture of uncharacterizable products.
The 29Si NMR spectrum of 2 shows a single resonance at
d 61.5, which is downfield shifted compared to LSiCl (d 14.6).5e
The tBu protons attached to the nitrogen atom of compound 2 in
Institut fur Anorganische Chemie, Universitat Gottingen,
¨
¨
¨
¨
Tammannstrasse 4, 37077 Gottingen, Germany.
Chart 1 Stable dialkylsilylenes A and B.
E-mail: hroesky@gwdg.de, rghadwal@uni-goettingen.de,
dstalke@chemie.uni-goettingen.de
w This article is part of the ChemComm ‘Frontiers in Molecular Main
Group Chemistry’ web themed issue. (Dedicated to Professor Gunter
¨
Schmid on the occasion of his 75th birthday.)
z Electronic supplementary information (ESI) available: Experimental
Section. CCDC 866116 (3) and 866117 (4). For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/
c2cc31041d.
Scheme 1 Synthesis of monoalkylsilylenes 2 and 3.
Chem. Commun., 2012, 48, 4561–4563 4561
c
This journal is The Royal Society of Chemistry 2012

its 1H NMR spectrum appear as a singlet (d 1.11), which is
downfield shifted compared to that of LSiCl (d 1.08).5e The tBu
protons on the silicon atom resonate at d 1.30. Moreover, 2
shows its molecular ion for [M+] in its mass spectrum at m/z 316.
Compound 3 exhibits two resonances at d 72.7 and d ꢁ0.4
for the silylene SiC(SiMe3)3 and silyl SiC(SiMe3)3 moieties,
respectively, in its 29Si NMR spectrum. In the 1H NMR
spectrum, the tBu protons attached to the silicon atom exhibit
a singlet at d 0.57. The tBu protons on the nitrogen atom
resonate at d 1.23. The molecular structure of 3 is shown in
Fig. 1. Compound 3 crystallizes in the monoclinic space group
P21/n. The silicon atom is three-coordinate and features a
trigonal pyramidal geometry, with the lone pair of electrons
residing on the apex. The coordination environment of silicon
is derived from two nitrogen atoms of the amidinato ligand
and one tertiary carbon atom of the alkyl group. The
Si(1)–C(16) bond length in 3 is 2.0173(11) A. This is longer
than the value of 1.9075 (2) A in compound A (Chart 1),
presumably due to the higher coordination number of silicon
in 3. The bond length between the chelating nitrogen atoms
and the silicon is nearly similar (Si1–N1, 1.9145(11) A and
Si1–N2, 1.9387(10) A). The bite angle of N1–Si1–N2 is
68.30(4)1. The Si atom is shifted out of the plane defined by
N1–C1–N2 by 0.3329(0.0003) A.
trimer [LSi(m-O)Cl]3, which contains a Si3O3 six-membered
ring.15b Driess et al. prepared the NHC-supported silanone
upon reaction of N2O with the adduct of NHC-silylene.18b In
contrast, Severin et al. isolated a covalent bonded nitrous
oxide of NHC.19 Obviously the affinity of silicon to oxygen is
much higher than that of carbon. From the above reactivity
studies with N2O, we presume that the formation of a dimer
might be due to the presence of a bulkier tBu group on the
silicon atom.
Compound 4 is soluble in common organic solvents and it is
stable both in the solid state and in solution for a long time
without any decomposition under an inert gas atmosphere.
The 29Si NMR spectrum of 4 exhibits a single resonance at
d ꢁ74.3 consistent with the compounds containing five coordinate
silicon. In the 1H NMR spectrum of 4, the tBu protons attached to
the nitrogen atoms show a single resonance at d 1.35 and the tBu
protons on the silicon atom resonate at d 1.68. In its mass
spectrum, compound 4 exhibits its molecular ion at m/z 665 [M+].
%
Compound 4 crystallizes in the triclinic space group P1. The
molecular structure of 4 is shown in Fig. 2. In 4 the silicon
atom is five-coordinate and arranged in a distorted trigonal
bipyramidal geometry with a t value20 (t = 1 for perfect
trigonal bipyramidal; t = 0 for perfect square based pyramid) of
0.64 (Si1), comprising two nitrogen atoms from the supporting
amidinato ligand, one carbon and two oxygen atoms. The four-
membered Si2O2 ring in 4 is slightly distorted from planarity.21
There are two types of Si–N (amidinato ligand) bond lengths
present, one being shorter (Si(1)–N(1) 1.8342(10) A) and one
longer (Si–N(2) 2.0579(10) A). Similarly, two types of Si–O bond
lengths are present; the shorter bond length is 1.6752(9) A and the
longer one is 1.7245(9) A. These shorter and longer bond lengths
correspond to the equatorial and axial positions. The Si–O–Si and
O–Si–O bond angles are 94.79(5)1 and 85.21(5)1.
In order to compare the reactivity of monoalkylsilylene with
chlorosilylene, we carried out a reaction of 2 with N2O, which
yielded the dimer [LSitBu(m-O)]2 (4) (Scheme 2). It is interesting
to mention that N2O has been considered as an ideal source of
atomic oxygen, which proceeds under elimination of dinitrogen
as the single side product. Recently we reported on the reaction
of LSiCl (1) with N2O, which resulted in the formation of the
Herein, we report the base stabilized monoalkylsilylenes
LSitBu (2) and LSi[C(SiMe3)3] (3). The formation of 2 and 3
Fig. 1 Molecular structure of 3. Anisotropic displacement para-
meters are depicted at the 50% probability level. Selected bond lengths
[A] and angles [1]: Si1–N1 1.9145(11), Si1–N2 1.9387(10),
Si1–C16 2.0173(11), Si2–C16 1.9109(13), Si3–C16 1.9200(12),
Si4–C16 1.9095(12); N1–Si1–N2 68.30(4), N1–Si1–C16 107.77(5),
N2–Si1–C16 109.77(4).
Fig. 2 Molecular structure of 4. Hydrogen atoms are omitted for
clarity. Anisotropic displacement parameters are depicted at the 50%
probability level. Selected bond lengths [A] and angles [1]:
Si1–N2 2.0579(10), Si1–N1 1.8342(10), Si1–O1 1.6752(9), Si1–O1A
1.7245(9), Si1–C16 1.9209(13); O1–Si1–O1A 85.21(5), O1–Si1–N1
121.37(5),
O1A–Si1–N1
100.54(5),
O1–Si1–C16
121.44(5).
Scheme 2 Synthesis of 4.
O1A–Si1–C16 101.05(5), N1–Si1–N2 67.42(4), Si1–O1–Si1A 94.79(5).
c
4562 Chem. Commun., 2012, 48, 4561–4563
This journal is The Royal Society of Chemistry 2012

was accomplished by the metathesis reactions of LitBu and
KC(SiMe3)3 with LSiCl (1). This approach provided easy
access to stable monoalkylsilylenes. Due to the presence of
the bulky substituents, the reactivities of the monoalkylsilylene
differ from other silylenes including monochlorosilylene.
Interestingly, the reaction of LSitBu (2) with N2O yielded
the dimer [LSitBu](m-O)]2 (4), with a four-membered Si2O2
ring, which is in contrast to the trimer [LSi(m-O)Cl]3 obtained
from the reaction of LSiCl (1) with N2O. The latter consists of
a six-membered Si3O3 ring.
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We thank the Deutsche Forschungsgemeinschaft for
supporting this work. R.A. is thankful to the Alexander von
Humboldt Stiftung for a research fellowship. D.S. and
H.W. are grateful to the DNRF funded Center for Materials
Crystallography (CMC) for support and the Land Niedersachsen
for providing a fellowship in the Catalysis of Sustainable
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Chem. Commun., 2012, 48, 4561–4563 4563