A dimer of silaisonitrile with two-coordinate silicon atoms

Article abstract of DOI:10.1002/anie.201101320

A specialty of silicon: A stable dimeric silaisonitrile (ArNSi:) ₂ (see picture; Ar=2,6-bis(2,4,6-triisopropylphenyl)phenyl) was prepared by the reduction of dichlorosilaimine IPrCl₂SiNAr with KC₈. The dimer is the first b

Full text of DOI:10.1002/anie.201101320

Communications
DOI: 10.1002/anie.201101320
Silicon Chemistry
A Dimer of Silaisonitrile with Two-Coordinate Silicon Atoms**
Rajendra S. Ghadwal, Herbert W. Roesky,* Kevin Prꢀpper, Birger Dittrich, Susanne Klein, and
Gernot Frenking*
Dedicated to Professor Raymundo Cea-Olivares on the occasion of his 60th birthday
Carbenes and silylenes are among the most important
reactive intermediates of carbon[1] and silicon.[2,3] These
species play key roles in numerous thermal and photochem-
ical reactions and are extremely important in synthetic
chemistry.[1–3] The first N-heterocyclic carbene (NHC) that
is stable at room temperature was isolated by Arduengo et al.
in 1991.[4] The silicon analogue of NHC was reported three
years later by West and co-workers[5] as an N-heterocyclic
silylene (NHSi) and using the same stabilization concept.
Since then, a number of stable carbene[6,7] and silylene[8]
compounds have been isolated with various electronic
structures and properties. Synthesis and stabilization of the
reactive species are not just a simple desire of academic
interest to realize highly reactive molecules at normal
laboratory conditions; they have also found many useful
applications.[7–10] The extraordinary electronic properties of
NHCs as strong s-donor and weak p-acceptor ligands have
been shown to bind more strongly with transition metals than
classical ligands.[7,9] The unusual stability of transition-metal
complexes supported by NHC as ligands has led to a major
breakthrough in the second generation of Grubbꢀs cata-
lysts.[10]
ular and electronic structures of unsaturated compounds of
silicon are very different from those of the corresponding
carbon compounds in which s/p hybridization models are
valid.[16,17] To understand the physical and chemical properties
of unsaturated silicon compounds, additional experimental
investigations are required. Further progress and understand-
ing of the field depend on the availability of appropriate
stable silicon compounds.
Organic nitriles and isonitriles are very stable, but their
silicon analogues are only detected as transient species in a
low-temperature argon matrix.[18–20] The first transient silai-
=
sonitrile, HN Si, was described in 1966 by Ogilvie and
Cradock;[19] this species was generated by photolysis of
H3SiN3 in an argon matrix at 4 K. The formation of
silaisonitrile[20] was shown by PE and IR spectroscopic studies
in an argon matrix. Quantum chemical calculations have
shown that silaisonitriles are more stable than silanitriles.[21]
This result is in contrast to the carbon analogues.[22] Despite
the remarkable theoretical contributions by Apeloig and
others[20,21] on the relative stability of silanitrile (RSiN) and
silaisonitrile (RNSi), no silaisonitrile or silanitrile has been
reported to date that is stable at room temperature.
The properties of silicon and carbon are rather different
from each other although both have the same number of
valence electrons. These differences are most pronounced for
compounds with low-coordinate silicon and carbon atoms.
Among unsaturated compounds, a wide range of carbon
compounds are known that are stable, whereas analogous
silicon compounds are very limited and quite reactive. The
syntheses of stable silicon compounds have been inspired
almost exclusively by their relationship with organic con-
geners. Silicon analogues of alkenes,[11] alkynes,[12] allenes,[13]
Very recently, we reported NHC-stabilized dichlorosilai-
mine[23] IPr·Cl2Si NAr (1; IPr=1,3-bis(2,6-diisopropylphenyl)-
=
imidazol-2-ylidene,
Ar= 2,6-bis(2,4,6-triisopropylphenyl)-
[24]
phenyl), which was prepared by the reaction of IPr·SiCl2
with ArN3. Herein, we present the reduction of dichlorosilai-
mine 1 with KC8 to afford a dimeric silaisonitrile (ArNSi:)2 (3;
Scheme 1). The mechanism for the formation of 3 is unknown.
In view of the reluctance of silicon to form compounds with
multiple bonds, the formation of 3 is assumed to proceed by
the initial formation of the unstable monomeric silaisonitrile
2. Dimerization of 2 by [2+2] cycloaddition under elimination
of two molecules of IPr affords 3 as yellow crystalline blocks
in 21% yield. To the best of our knowledge, an analogous
carbon compound is not known.
=
and other unsaturated organic compounds with a C X (X = O
or S) functional group[14,15] have been prepared. The molec-
[*] Dr. R. S. Ghadwal, Prof. Dr. H. W. Roesky, K. Prꢀpper, Dr. B. Dittrich
Institut fꢁr Anorganische Chemie
Treatment of 3 with trimethylsilyl azide (Me3SiN3) gives
the first bis(silaimine) 4 (Scheme 2) with three-coordinate
silicon atoms as a colorless crystalline solid in 62% yield.
Formation of 4 confirms the presence of a reactive lone pair of
electrons on each of the silicon atoms of 3.
Georg-August-Universitꢂt Gꢀttingen
Tammannstrasse 4, 37077 Gꢀttingen (Germany)
Fax: (+49)551-393-373
E-mail: hroesky@gwdg.de
S. Klein, Prof. Dr. G. Frenking
Compounds 3 and 4 are stable under an inert atmosphere
and are soluble in common organic solvents. The molecular
structures of 3 and 4 were established by single-crystal X-ray
diffraction studies. Refinement of 3 benefited from non-
spherical scattering factors of the invariom approach.[25] The
1H and 13C NMR spectra of 3 show resonances for terphenyl
((2,4,6-iPr3-C6H2)2C6H3) groups on amino nitrogen atoms.
Fachbereich Chemie, Philipps-Universitꢂt Marburg
Hans-Meerwein-Strasse, 35032 Marburg (Germany)
E-mail: frenking@chemie.uni-marburg.de
[**] Support of the Deutsche Forschungsgemeinschaft is highly
acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101320.
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Angew. Chem. Int. Ed. 2011, 50, 5374 –5378

Figure 1. Molecular structure of 3. Ellipsoids set at 50% probability;
hydrogen atoms, isopropyl groups, and benzene molecules are omitted
for clarity. Selected bond lengths [ꢃ] and angles [8]: Si1–N1 1.756(1),
Si1–N2 1.754(1), N1–C1 1.408(3), N2–C20 1.408(3); N1-Si1-N2
86.02(6), Si1-N1-Si2 94.02(9), C20-N2-Si1 132.99(5), C1-N1-Si1
133.03(5).
Scheme 1. Synthesis of dimeric silaisonitrile 3. IPr=1,3-bis(2,6-diiso-
propylphenyl)imidazol-2-ylidene; Ar’=2,4,6-triisopropylphenyl.
The molecular structure of 3 is shown in Figure 1. Single
crystals of 3 were grown from a benzene solution at room
temperature. 3 crystallizes in the monoclinic space group C2/
c. The asymmetric unit of 3 contains half a molecule with one
and a half molecules of benzene. Each of the silicon atoms in 3
is two-coordinate and it is the first example of a base-free
disilylene.[26] The four-membered Si2N2 ring in 3 is almost
planar. The endocyclic N-Si-N (86.02(6)8) and Si-N-Si
(94.02(9)8) angles of the Si2N2 ring are nearly orthogonal to
each other. Bonds to divalent silicon are expected to be longer
by (0.1 ꢁ 0.02) ꢁ in comparison to those of tetravalent
silicon.[27] The Si N bond lengths (av. 1.755(1) ꢁ) in 3 are in
ꢀ
agreement with those observed for heterocycles[26] with low-
valent silicon. The phenyl rings on the amino nitrogen are
almost perpendicular to one another, with a dihedral angle of
83.078.[28a]
Scheme 2. Synthesis of bis(silaimine) 4. Ar’=2,4,6-triisopropylphenyl.
=
A few silicon compounds with Si N bonds have been
1
Compound 4 exhibits H and 13C NMR resonances for the
reported; however, most of them are stabilized with a Lewis
base.[23,29] Very recently, a base-free monosilaimine was
prepared[29] with a terphenyl group on the imine nitrogen
atom. Compound 4 is the first example of a base-free
bis(silaimine) with two three-coordinate silicon atoms. The
molecular structure of 4 is shown in Figure 2. Bis(silaimine) 4
Me3Si groups along with the resonances for terphenyl groups.
The 29Si NMR chemical shift at d = + 183.29 ppm for 3
indicates strong deshielding, as expected for compounds
with low-valent silicon atoms. The value is quite far downfield
when compared with those of reported NHSi compounds.[5,8]
The relatively high downfield shift in the 29Si NMR spectrum
of 3 in comparison to those observed for two-coordinate
silicon[5,8] in NHSi compounds may be due to the presence of
two silylene moieties in the ring. However, the reason for this
strong deshielding is presently unknown. The molecular ion
[M+] observed at m/z 1046 in the EI mass spectrum of 3 with
the base ion at 1003 [(MꢀC3H7)+] further supports the
dimeric nature of 3. The 29Si NMR spectrum of 4 exhibits two
resonances at d = 2.76 and ꢀ56.82 ppm for trimethylsilyl
¯
crystallizes in the triclinic space group P1 with half a molecule
of 4 and a molecule of toluene in the asymmetric unit. Each of
the silicon atoms in the four-membered Si2N2 ring of 4 is
three-coordinate. As expected for compounds with tetrava-
lent silicon, the endocyclic Si N bond lengths in 4 (av.
1.724(2) ꢁ) are slightly shorter in comparison to those
ꢀ
ꢀ
observed for 3 (av. 1.755(1) ꢁ). The exocyclic Si N bond
lengths (1.564(2) ꢁ) are in agreement with those reported for
compounds with silicon–nitrogen double bonds.[23,29] The
internal Si2N2 bond angles are almost the same as those
found in 4. Interestingly, the phenyl rings on the amino
nitrogen of the Si2N2 ring are arranged in the same plane, but
perpendicular to the plane of the Si2N2 ring.[28b]
=
(SiMe3) and silaimine (ArNSi N) moieties, respectively. The
slightly downfield shift in the 29Si NMR of 4 compared to
those of reported compounds[23] may be due to the lower
coordination number of silicon in 4.
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Communications
Figure 2. Molecular structure of 4. Ellipsoids set at 50% probability;
hydrogen atoms, isopropyl groups, and toluene molecules are omitted
for clarity. Selected bond lengths [ꢃ] and angles [8]: Si1–N1 1.728(2),
Si1–N2 1.720(2), Si1–N3 1.564(2), N3–Si3 1.708(2), N1–C4 1.443(2);
N1-Si1-N2 87.31(7), Si1-N1-Si2 94.69(7), C4-N1-Si1 139.99(14), Si1-
N3-Si3 153.76(13), N3-Si1-N2 135.05(9).
We carried out quantum chemical calculations using
density functional theory at the M05-2X/TZVPP level (see
the Supporting Information) for the model compounds 2M,
3M, and 4M (Figure 3), where the substituents Arꢀ are
replaced by hydrogen atoms and where the IPr ligand is
replaced by 1,3-dimethylimidazol-2-ylidene. We also calcu-
lated free phenylsilaisonitrile (C6H5NSi). Figure 3 shows the
optimized geometries and the most important bond lengths
and angles. The calculated interatomic distances and angles of
3M and 4M are in very good agreement with the exper-
imental values of 3 (Figure 1) and 4 (Figure 2), respectively.
ꢀ
Theory and experiment agree that the equidistant Si N bonds
in 3 and 3M are distorted towards two sets of slightly different
ꢀ
Si N bonds. The elusive phenylisonitrile is predicted with
ꢀ
ꢀ
rather short C N (1.371 ꢁ) and N Si (1.557 ꢁ) bonds.
The addition of NHC ligand in 2M slightly elongates the
ꢀ
ꢀ
C N bond (1.381 ꢁ), while the N Si bond (1.654 ꢁ) is
significantly longer than in free C6H5NSi. The NHC ligand is
bonded side-on to the silicon atom in 2M, which exhibits a
rather acute bonding angle of 88.48 (Figure 3). The NBO
analysis of C6H5NSi and 2M suggests that free phenylsilaiso-
Figure 3. Calculated structures (bond lengths [ꢃ], angles [8]) of a) 2M,
b) 3M, c) 4M, and d) C6H5NSi at M05-2X/TZVPP.
ꢀ
nitrile has a strongly polarized N Si triple bond where the s
which shows that the molecule is a moderately strong donor–
acceptor complex.[30] The dimerization energy for the reaction
2C6H5NSi!3M is ꢀ48.9 kcalmolꢀ1, which indicates that the
formation of 3M from 2M that mimics the second reaction
step shown in Scheme 1 is energetically favored by 9.5 kcal
molꢀ1.
and the p components are 85.4% (s bond) and 84.1% and
84.0% (p bond) at the nitrogen end. The partial charge at
nitrogen in C6H5NSi is ꢀ1.16 e, while the silicon atom has a
positive charge of + 1.05 e. The NHC complex 2M possesses a
ꢀ
N Si double bond that is also strongly polarized toward the
nitrogen atom which carries an electron lone pair. In the latter
molecule, 82.9% of the s bond and 86.1% of the p bond are
at the nitrogen end. The partial charges in 2M are ꢀ1.17 e (at
N) and + 0.86 e (at Si). The calculated charges for the dimer
3M are ꢀ1.33 e (N) and + 1.23 e (Si).
Figure 4 shows the Laplacian[31] distributions of C6H5NSi,
2M, and 3M, which nicely exhibit the charge concentrations
of the electron lone pairs at Si in the three molecules.
Figure 4b also shows the charge concentrations of the
ꢀ
electron lone pair of nitrogen. The Si N bonds are strongly
The theoretically predicted bond dissociation energy
(BDE) for the NHC ligand of 2M is Do = 19.7 kcalmolꢀ1,
polarized towards nitrogen, which becomes visible through
the areas of charge concentration at the nitrogen end (solid
5376
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Angew. Chem. Int. Ed. 2011, 50, 5374 –5378

48.9 kcalmolꢀ1. The silicon atoms in 3M, in free C6H5NSi, and
in the complex 2M carry electron lone pairs.
Received: February 22, 2011
Published online: May 5, 2011
Keywords: density functional calculations ·
.
main-group elements · multiple bonds · silylenes ·
X-ray diffraction
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