A cyclic silylone ("siladicarbene") with an electron-rich silicon(0) atom

Article abstract of DOI:10.1002/anie.201302537

King of donor: The chelating bis(carbene) 1 enabled the synthesis of the new silyliumylidene salt 2. This salt can be dechlorinated with sodium naphthalenide to give the first cyclic siladicarbene 3, which bears a single Si⁰ atom with strikingl

Full text of DOI:10.1002/anie.201302537

Angewandte
Chemie
DOI: 10.1002/anie.201302537
Silicon Chemistry
A Cyclic Silylone (“Siladicarbene”) with an Electron-Rich Silicon(0)
Atom**
Yun Xiong, Shenglai Yao, Shigeyoshi Inoue, Jan Dirk Epping, and Matthias Driess*
Dedicated to Professor Werner Uhl on the occasion of his 60th birthday
There is a continuing increase in interest regarding the
synthesis and reactivity of low-valent silicon species. Besides
the successful access to two-coordinate cyclic[1] and acyclic
silylenes,[2] even several donor-supported three-coordinate
silicon(II) compounds could be isolated and structurally
characterized. Striking examples are the chlorosilylene
PhC[N(tBu)]2SiCl[3] and the dichlorosilylene complex NHC-
SiCl2 A (Scheme 1; NHC = N-heterocyclic carbene, in A: 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene).[4] The latter
donor-stabilized chlorosilylenes are long-sought-after con-
venient building blocks for the syntheses of new silicon-
containing functional groups. For instance, the NHC ligand in
A can be replaced by a chelate ligand 1,8-bis(tributylphos-
phazenyl)naphthalene to yield the first chlorosilyliumylidene
complex B (Scheme 1).[5] Remarkably, the SiII center in B
remains three-coordinate and prefers ion-pair formation.
Along with those divalent silicon species, two-coordinate
!
Si0 species L:!Si :L, termed as silylones, have also attracted
significant attention (L = N-heterocyclic carbenes, phos-
phanes, etc.). This new class of silicon compounds and their
heavier homologues have been theoretically investigated by
the group of Frenking.[6] Results from calculations predict
that, in contrast to linear allenes, silylones adopt a bent
molecular geometry, which is in accordance with the VSEPR
theory because the central Si atom has two lone pairs of
electrons and two dative electron pairs from the donor ligands
L:. Accordingly, the previously reported trisilaallene C and
digermasilaallene D (Scheme 1)[7,8] with E-Si-E angles of
136.58 (E = Si) and 125.78 (E = Ge) should rather be silylones
with a central Si atom in the formal oxidation state zero that is
stabilized by two silylene or germylene moieties, respectively.
The results from calculations have inspired experimentalists
to synthesize other types of silylones and examine their
fascinating chemical behavior. Very recently, Roesky and co-
workers prepared and isolated the siladicarbene (silylone) E
(Scheme 1),[9a] which represents a Si0 species stabilized by two
cyclic alkyl amino carbene (cAAC) molecules.
The seminal work by Robinson and co-workers has
demonstrated that even NHCs can serve as efficient donor
ligands for the stabilization of Si0 species, as shown by the
successful dechlorination of NHC!SiCl4 (in this case NHC is
1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene), affording
Scheme 1. Selected low-valent silicon and germanium complexes stabi-
lized through donor-acceptor coordination: the dichlorosilylene A,
chlorosilyliumylidene B, silylones C–E, and the germylone F. Dip=2,6-
diisopropylphenyl.
=
the fascinating isolable disilicon(0) species NHC!Si
!
Si NHC.[10] Inspired by the latter results and Roeskyꢀs
siladicarbene E, we employed the chelate N-heterocyclic
carbene 1 (Scheme 2) for the synthesis of the first cyclic bis-
NHC-stabilized silylone. Recently we have shown that the
bis(carbene) 1 is suitable for the synthesis of an isolable cyclic
germadicarbene F (Scheme 1).[11] We wish to report herein
straightforward access to the new chlorosilyliumylidene
precursor 2, which can be reduced to the desired cyclic
siladicarbene (silylone) 3 (Scheme 2).
[*] Dr. Y. Xiong, Dr. S. Yao, Prof. Dr. S. Inoue, Dr. J. D. Epping,
Prof. Dr. M. Driess
Technische Universitꢀt Berlin, Institut fꢁr Chemie
Metallorganische Chemie und Anorganische Materialien
Sekr. C2, Strasse des 17. Juni 135, 10623 Berlin (Germany)
E-mail: matthias.driess@tu-berlin.de
Homepage: http://www.metallorganik.tu-berlin.de/menue/home/
Analogous to the synthesis of B and (cAAC)2SiCl2[9b] (the
precursor of E), we examined the reactivity of the ligand
1 toward NHC-SiCl2 A. The experiment revealed that the
reaction of equimolar amounts of 1 with NHC-SiCl2 in THFat
room temperature leads to the desired new chlorosilyliumy-
[**] Financial support from the Deutsche Forschungsgemeinschaft (DR-
17/2) and the Alexander von Humboldt foundation (Sofja Kova-
levskaja Program for S.I.) are gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302537.
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Figure 1. Molecular structure of the chlorosilyliumylidene cation of 2.
Ellipsoids are set at 50% probability. H atoms and one acetonitrile
lattice solvent molecule are omitted for clarity. Selected interatomic
distances [ꢂ] and angles [8]: Si1–Cl1 2.139(2), Si1–C1 1.960(4), Si1–C2
1.963(4), C1–N1 1.348(5), C1–N2 1.355(5), C2–N3 1.356(5), C2–N4
1.342(5); C1-Si1-C2 88.7(2), C1-Si1-Cl1 95.8(1), C2-Si1-Cl1 97.6(1), Si1-
C1-N1 126.4(3), Si1-C1-N2 127.9(3), N1-C1-N2 105.2(3), Si1-C2-N3
127.3(3), Si1-C2-N4 127.1(3), N3-C2-N4 104.9(3).
Scheme 2. Synthesis of the cyclic bis-NHC stabilized silylone 3,
starting from the bis-NHC ligand 1 and the chlorosilyliumylidene
precursor 2.
lidene precursor 2 (Scheme 2). Compound 2 is insoluble in
THF, but soluble in acetonitrile, and could be isolated in 57%
yield. It has a similar electronic nature as B, but is remarkably
different from that of the neutral biradical tetravalent
(cAAC)2SiCl2,[9b] which results from the reaction of two
molar equivalents of cAAC with NHC-SiCl2 A. Unexpect-
edly, the three-coordinate SiII atom in 2 exhibits a drastic
upfield shift in the 29Si NMR spectrum at d = ꢀ58.4 ppm in
CD3CN, and similarly in the solid-state CP/MAS 29Si NMR
spectrum at d = ꢀ57.8 ppm, in comparison to those of B (d =
ꢀ3.30 ppm in CD2Cl2)[5] and other related three-coordinate
silicon(II) compounds LSi(Ar)Cl (L = 1,3,4,5-tetramethyl-
imidazol-2-ylidene, Ar= 2,6-Mes2C6H3, d = 1.34 ppm in
C6D6; Ar= 2,6-Trip2C6H3 (Trip = 2,4,6-iPr3C6H2), d =
0.77 ppm in C6D6,[12] d = 19.1 ppm in C6D6 for A,[4] and d =
14.6 ppm for PhC[N(tBu)]2SiCl in C6D6),[3] respectively. The
calculated 29Si chemical shift (GIAO/B3LYP/6-311(d) for (H,
C, N), and 6-311G(3d) for Si) of the bare silyliumylidene
cation of 2 confirms the relatively large shielding of the 29Si
nucleus (d = ꢀ66.3 ppm), which is in reasonable agreement
with the experimental value. Remarkably, the 29Si NMR
chemical shift of 2 falls even in the range of four-coordinate Si
species (d = ꢀ20 to ꢀ80 ppm).[13] The strong upfield shift of
the 29Si NMR signal of 2 indicates a much stronger electron-
donation effect of the bNHC chelate ligand 1 to the SiII atom
than those of 1,8-bis(tributylphosphazenyl)naphthalene in B
and by other ligands mentioned above.
plane defined by N2, C3, N3 and the plane defined by C1, N2,
N3, C2 is 35.88. The sum of angles around each carbene
carbon atom in 2 is 359.538 and 359.318, respectively,
indicating nearly ideal trigonal-planar coordination geometry.
ꢀ
The Si1 Cl1 distance of 2.139(2) ꢁ in 2 is slightly shorter than
those in A (2.158(2), 2.174(2) ꢁ),[4] in B (2.172(2) ꢁ),[5] and in
PhC[N(tBu)]2SiCl (2.156(1) ꢁ).[3] The two long Si C distan-
ꢀ
ces in 2 (average 1.961(4) ꢁ vs. 1.985(4) ꢁ in A)[4] fall in the
normal range for dative C!Si donor–acceptor bonds.
Figure 2 shows the shapes of the frontier orbitals of the
chlorosilyliumylidene cation of 2. The HOMO has mainly
s lone-pair orbital character at the SiII center. It becomes
obvious that compound 2 has at the same time silylium-like
and silylene character. On the other hand, the LUMO shows
Figure 2. HOMO (left) and LUMO (right) of 2.
The yellow compound 2 crystallizes in the orthorhombic
space group Pbca. Akin to B, the SiII atom of the silyliumy-
lidene cation of 2 is three-coordinate, namely by the two
carbene carbon atoms of the chelate ligand 1 and one chlorine
atom (Figure 1), featuring a trigonal-pyramidal coordination
geometry with the sum of bond angles around Si atom of
282.138. The counteranion (Clꢀ) is located far away from the
silicon atom, with the smallest distance of 6.424(2) ꢁ. Owing
to the chelate coordination, the Si atom resides in a puckered
six-membered C3N2Si ring with boat conformation, and the
C1-Si1-C2 bond angle is 88.7(2)8. The dihedral angle between
the plane defined by C1, Si1, C2 and the plane defined by C1,
N2, N3, C2 is 8.18, whereas the dihedral angle between the
significant p-type contribution at the Si atom. A similar p-
type LUMO was also determined by DFT calculation of the
cyclic chlorogermyliumylidene analogue,[11] indicating that
compound 2 could be an excellent precursor for the synthesis
of the target silylone 3 (Scheme 2) through dechlorination.
When a freshly prepared THF solution of sodium
naphthalenide was cooled down to ꢀ608C and added into
a suspension of 2 in THF (ꢀ608C) in a 2:1 mol ratio, the
reaction mixture immediately turned dark red, indicating that
a reduction process took place. In fact, the desired silylone 3
could be isolated from the reaction mixture in the form of
a dark red powder in 68% yield. The 1H and 29Si NMR spectra
and also the IR spectrum clearly rule out the existence of
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ꢀ
a Si H bond in 3 (see the Supporting Information). Strikingly,
the chemical shift of the two-coordinate silicon atom in 3 in
the 29Si NMR spectrum appears at much higher field (d =
ꢀ83.8 ppm in [D8]THF and d = ꢀ80.1 ppm in C6D6) than that
of the acyclic siladicarbene Si(cAAC)2 E (d = 66.7 ppm in
C6D6). The enormous upfield shift observed for 3 is in
accordance with GIAO calculations (d = ꢀ78.7 ppm). We
reasoned that the remarkable shielding of the 29Si nucleus in 3
results from a stronger s-donor but weaker p-acceptor ability
of the two NHC moieties towards silicon and the acute C-Si-C
angle caused by the cyclic structure. To gain insights into the
electronic differences, we carried out additional computa-
tional investigations of the related model compounds 3’ (with
a chelate bis-NHC), 3’’ (with two NHCs), and E’ (bearing two
cAAC ligands). Selected results of the calculations are
Figure 3. Molecular structure of 3. Ellipsoids are set at 50% proba-
bility. H atoms are omitted for clarity. Selected interatomic distances
[ꢂ] and angles [8]: Si1–C1 1.864(1), Si1–C2 1.874(1), C1–N1 1.383(2),
C1–N2 1.382(2), C2–N3 1.374(2), C2–N4 1.377(2); C1-Si1-C2 89.1(1),
Si1-C1-N2 128.9(1), Si1-C1-N1 127.9(1), N1-C1-N2 103.1(1), Si1-C2-N3
128.4(1), Si1-C2-N4 127.8(1), N3-C2-N4 103.7(1).
ꢀ
summarized in Table 1. Whereas the C-Si C angle of the
cyclic silylone model 3’ is smaller than that of acyclic silylone
ꢀ
3’’ and E’, the Si C bond length of 3’ is slightly longer than
Table 1: Calculated distances and angles, 29Si chemical shifts, NBO
charges at Si, and the first and second proton affinities (PAs) of model
compounds 3’, 3’’, and E’.[a]
Crystals of 3 in the monoclinic space group C2/c could be
obtained in toluene solutions at ꢀ208C. The X-ray diffraction
analysis (Figure 3) reveals that the Si atom in 3 is two-
coordinate with a C-Si-C angle of 89.1(1)8, which is much
smaller than those in C (136.58),[7] D (125.78),[8] and E
(117.98),[9] but is comparable to those in the germylone
analogue F (86.68)[11] and in its precursor 2 (88.7(2)8).
The six-membered C3N2Si ring is puckered and adopts
a boat conformation. The dihedral angle between the plane
defined by N2, C3, N3 and the plane defined by C1, C2, N3,
N2 is 508 (versus 46.08 in F,[11] and 35.88 in 2), whereas Si1 is
only slightly deviated from the plane defined by C1, C2, N3,
N2 with corresponding dihedral angle of 10.78 (versus 138 in
F,[11] and 8.18 in 2). The sum of angles around C1 (359.968) and
C2 (359.858) indicates almost ideal trigonal-planar coordina-
3’
3’’
E’
C-Si-C [8]
88.2
1.880
94.7
1.877
100.0
1.861
+25.5
+0.315
ꢀ
Si C [ꢂ]
d(29Si) [ppm]
NBO charge at
Si
ꢀ69.2
ꢀ67.2
+0.191
+0.163
1st PA [kcal
273.8
164.0
283.4
168.3
268.8
155.3
molꢀ1
]
2nd PA [kcal
molꢀ1
]
ꢀ
tion geometry of two carbene carbon atoms. The Si C bond
distances in 3 (average 1.869 ꢁ) are significantly shorter than
[a] Calculated at B3LYP/6-31G(d) level of theory. GIAO/B3LYP/6-311(d)
[H, C, N]: 6-311G(3d) [Si].
those in 2 (average 1.962 ꢁ), and in A (1.985(4) ꢁ).[4]
ꢀ
However, they are distinctly longer than Si C double bonds
that of acyclic silylones. The calculated 29Si chemical shifts and
NBO charges at silicon clearly support that the two NHC
ligands in 3’ and 3’’ are much stronger s-donors towards
silicon than the cAAC ligands in E’. In other words, the Si0
atoms in 3’ and 3’’ are more electron-rich, which explains their
larger first and second proton affinities (PA) in comparison to
E’ (Table 1). The presence of electron-rich silicon(0) in 3
could also explain its extreme sensitivity towards air and
moisture, which is in contrast to the behavior of E.
The UV/Vis spectrum of 3 in toluene solutions exhibits
absorption maxima at l = 345 (e = 4.6 ꢂ 103), 415 (e = 3.5 ꢂ
103), and 547 nm (e = 7.5 ꢂ 103), which are comparable to
those observed for the analogous germylone F (l = 286, 420,
564 nm),[11] the bent trisilaallene C (l = 390, 584),[7] bent
digermasilaallene D (l = 272, 383, 432, 488, and 612 nm),[8]
and E (l = 270, 327, 392, 570, 611 nm).[9] TD-DFT calculations
of compound 3 revealed the longest absorption maximum at
l = 544 nm with an oscillator strength of f = 0.0814, which can
be assigned to the electronic HOMO!LUMO transition.
in silaethenes (1.702–1.775 ꢁ).[14] In fact, they are slightly
longer than that in the siladicarbene (cAAC)2Si
E
(1.841(2) ꢁ),[9a] and close to the calculated value for Si-
(NHC)2 (1.869 ꢁ).[6]
Further insights into the electronic structure of 3 stem
from DFT calculations, which have been carried out for the
silylone 3 at the B3LYP/6-31G(d) level. The optimized
geometry of 3 in the singlet ground state closely reproduced
the experimental data. The energetic gap between singlet
ground state and triplet first excited state is 33.0 kcalmolꢀ1
.
Compound 3 exhibits similar features to those of the
germylone analogue F. For example, the HOMO represents
the silicon p-orbital, whereas the HOMOꢀ1 is the silicon s-
lone-pair orbital (Figure 4). The silicon p-bonding orbital
with Si C bonds presents an evidence for the relatively short
Si C bond length in 3 in comparison to precursor 2. This is
supported by the calculated Si C bond lengths (average
1.888 ꢁ) and the relatively large WBI (Wiberg Bond Index)
values of the Si C bonds (average 1.002). The electronic
nature of silylones has been well-described by Frenking and
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In summary, by employing the neutral bidentate bis-NHC
ligand 1 the unprecedented bis(carbene) chlorosilyliumyli-
dene complex 2 could be isolated, in which both HOMO and
LUMO are mainly localized on the silicon(II) atom according
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thalenide led to the first isolable cyclic siladicarbene (sily-
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Received: March 26, 2013
Published online: May 31, 2013
Keywords: main-group chemistry · siladicarbenes · silicon ·
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