A One-Step Germole to Silole Transformation and a Stable Isomer of a Disilabenzene

Article abstract of DOI:10.1002/chem.201703955

An unusual germole-to-silole transformation is described. As key intermediates hetero-fulvenes are formed which rearrange to more stable bicyclic carbene analogues. The so-formed germylenes undergo a reductive elimination yielding elemental germanium and siloles. In contrast, the analogous silylenes are stable at ambient conditions and were identified by MS spectrometry and NMR spectroscopy supported by the results of quantum mechanical calculations. These bicyclic silylenes are stable derivatives of the global minimum of the C₄Si₂H₆ potential energy surface.

Full text of DOI:10.1002/chem.201703955

A Journal of
Accepted Article
Title: A One-Step Germole to Silole Transformation and a Stable
Isomer of a Disilabenzene
Authors: Thomas Müller, Crispin Reinhold, Zhaowen Dong, Jan
Winkler, Henning Steinert, and Marc Schmidtmann
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A One-Step Germole to Silole Transformation and a Stable Isomer
of a Disilabenzene
Crispin R. W. Reinhold, Zhaowen Dong, Jan M. Winkler, Henning Steinert, Marc Schmidtmann and
Thomas Müller*[a]
Abstract: An unusual germole-to-silole transformation is described.
As key intermediates hetero-fulvenes are formed which rearrange to
more stable bicyclic carbene analogues. The so-formed germylenes
undergo a reductive elimination yielding elemental germanium and
siloles. In contrast, the analogous silylenes are stable at ambient
conditions and were identified by MS spectrometry and NMR
spectroscopy supported by the results of quantum mechanical
calculations. These bicyclic silylenes are stable derivatives of the
4 2 6
global minimum of the C Si H potential energy surface.
6 6 2 6 3
Figure 1. Isolated Si R Isomers (I: R = Dipp = 2,6-iPr C H ; II, III: R= Tipp =
26 – 28]
[
3 6 2
,4,6-iPr C H ).
2
Introduction
4 2 6
In the case of the C Si H potential energy surface (PES),
computational explorations prior to 2012 concluded that planar
Benzene and the concept of aromaticity is central to Chemistry.[1]
Generations of researchers have undertaken significant efforts to
extend this concept to other elements than carbon.[2-10] Naturally,
a particular focus has been on the heavier members of group 14
in particular silicon and germanium.[2,3] Over the last decades, the
groups of Okazaki, Tokitoh, Sekiguchi and Saito developed
synthetic methods that allowed the isolation of cyclic aromatic
compounds with atoms of all four heavier group 14 elements at
ambient conditions.[2, 3, 11, 12] In the case of heavier congeners of
benzene, mono- and di-sila and germa-derivatives have been
prepared at ambient conditions that have an almost planar cyclic
[24]
disilabenzenes correspond to the most stable isomers.
In
qualitative agreement with these theoretical predictions, the
groups of Sekiguchi and Tokitoh reported the synthesis of
thermally stable substituted 1,2-disila- and also 1,2-digerma-
.[16 – 17]
benzenes by reaction of ditetrylynes with alkynes
ingly, a recent global minima search on the C
Surpris-
2 6
Si H PES by
4
Ivanov and Boldyrev revealed that the bicyclo[2.1.1]hexene
BCH) silylene 4 is the global minima, which is significantly
stabilized compared to all three disilabenzene regioisomers 5 – 7
Figure 2).[25] In this communication, we report on an unexpected
(
(
germol / silol transformation, which proceeds via BCH silylenes or
germylenes and the isolation and spectroscopic characterization
of a stable derivative of the alleged global minimum on the
[13-21]
delocalized structure.
Computational studies clearly indicate
however that with increasing number of heavy atoms the relative
stability of nonplanar structures increases.[23-25] Experimentally,
4 2 6
C Si H PES.
6 6
this was demonstrated by the landmark investigations on Si R
[
26,
isomers by the groups of Sakurai and Scheschkewitz (Figure 1).
-
28]
These studies revealed the clear thermodynamic preference
of non-planar three-dimensional arrangements such as 1 – 3 over
planar, cyclic delocalized structures.
-
1
Figure 2. Relative energies of selected
4 2 6
C Si H isomers (in kJ mol , at
CCSD(T)/CBS//B3LYP/6-311++G(d,p), from ref. 25) .
[
a]
Dr. C. R. W. Reinhold, MSc Z. Dong, BSc J. M. Winkler, BSc H.
Steinert, PhD M. Schmidtmann, Prof. Dr. T. Müller
Institut für Chemie
Carl von Ossietzky Universität Oldenburg
D-26129 Oldenburg, Carl von Ossietzky-Str. 9 – 11, Federal
Republic of Germany, European Union
E-mail: thomas.mueller@uni-oldenburg.de
Supporting information for this article is given via a link at the end of
the document.
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Results and Discussion
driving force for the formation of germylene 14. Although, its
decomposition into silole 9 and atomic triplet germanium is
-1
calculated to be endothermic by 96 kJ mol , the condensation of
germanium atoms to form bulk germanium provides the required
thermodynamic driving force.[34] The recent report on the isolation
of the stable isolobal hafnocene substituted BCH-germylene and
silylene 18, 19 provides further support for germylene 14 as an
intermediate in the observed germole / silole transformation
(
Figure 4).[32, 35]
Scheme 1. Synthesis of silole 9.
The reaction of germole 8 with tris-(trimethylsilyl)silylpotassium[29]
in THF gave silole 9 in isolated yields of 60% (Scheme 1, path
Scheme 2. Suggested reaction path for the formation of silole 9 from germole
(
a)). The expected product of a salt metathesis reaction was not
8.
detected. During the reaction, a black precipitate formed and
tetrakis-(trimethylsilyl)silane was identified as the only THF
soluble by-product. The identity of silole 9 was confirmed by its
independent synthesis from dimethoxysilole 10 (Scheme 1, path
(
(
b)). Silole 9 was characterized by an X-ray diffraction analysis
XRD) from suitable single crystals (see Supporting Information)
and by NMR spectroscopy. 29Si NMR spectroscopy proved to be
2
9
most diagnostic as it shows for silole 9 three silicon signals ( Si
-11.3 (C-SiMe ), -15.2 (Si(SiMe ), -17.1 (Si(SiMe )), the high
field resonance being typical for silyl-substituted siloles.
=
3
3
)
2
3 2
)
[
30, 31]
The
amorphous black residue was insoluble in all organic solvents. X-
ray fluorescence revealed its relative composition as Ge : K : Cl
as 1.1 : 1.1 : 1.0 (calculated for an equimolar mixture of elemental
germanium and potassium chloride: Ge / KCl : 2.0 : 1.1 : 1.0).
The by-product tetrakis-(trimethylsilyl)silane suggests that the
initiating step for the reaction sequence is the cleavage of a Si –
Si bond with formation of silylanion 11 (Scheme 2). This is
supported by the observation that addition of t-BuOK to germole
Scheme 3. Formation of silole 17 from dipotassium germoldiide K [15].
2
8
yields silole 9 along with elemental Germanium and the
silylether t-BuOSiMe , although in a less clean reaction (Scheme
, path (c)). The polarized germasilafulvene 12 is then formed
3
1
from silylanion 11 by loss of chloride and it is transformed by two
subsequent rearrangement steps via the zwitterion 13 into the
bicyclic germylene 14 (Scheme 2). The idea that germasila-
fulvenes such as 12 are intermediates in this rearrangement
reaction finds support by the formation of silole 17 by reaction of
Figure 3. Calculated relative energies of fulvenes 12, 21 and their isomers 13,
25 and 14, 22 (at M062X/6-311+G(d,p)).
2
dipotassium germolediide K [15] with diphenyldichlorosilane via
fulvene 16 (Scheme 3). [31,32] The mechanistic scenario outlined
in Scheme 2 is further supported by the results of density
functional (DFT) calculations at the M06-2X/6-311+G(d,p)
level.[33] These computations revealed that the bicyclic zwitterion
13 and in particular the bicyclohexene-type (BCH) germylene 14
are more stable than the isomeric germasilafulvene 12 (see
Figure 3). Consequently, there is a significant thermodynamic
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inverse gated 29Si{ H} NMR spectra recorded with relaxation
delays of 60 s showed five 29Si NMR signals with an approximate
intensity ratio of 1 : 2 : 1 : 1 : 1 (high to low frequency). The field
desorption mass spectrum for 22b with the peak corresponding to
the molecular ion at m/z = 634 is in agreement with the expected
1
6 34
C H
58. The most characteristic features in the 29Si
formula Si
NMR spectra of compounds 22 are one relatively deshielded
Figure 4. Isolobal BCH tetrylenes 14, 18 and 19.
2
9
resonance ( Si = 32.2 (22a), 33.3 (22b)) and a second signal at
2
9
remarkably high field (( Si = -210.6 (22a), -211.8 (22b)). The low
field resonance was assigned to the central silicon atom of the
1
29
Si(SiMe
3
)
2
group based on H Si HMBC NMR spectra.
Tentatively, we assigned the high-field signals to the
1
dicoordinated silicon atoms Si by comparison with the isolobal
BCH-silylene 19 for which a 29Si NMR chemical shift of  Si(Si )
=
29
1
-155 was reported.[35] GIAO Si NMR chemical shift calculations
for DFT-optimized structures of BCH silylenes 22 further support
this assignment (see Table 1). The 29Si NMR chemical shifts of
the silylene silicon atoms are very unusual when compared for
example to that of the cyclic dialkylsilylene from the Kira group
2
9
[36]
(
 Si = 567) and indicate already a higher coordination number
for this silicon atom. A 29Si NMR resonance at even lower
frequency is reported for Jutzi’s decamethyl silicocene with a
2
9
[37]
deca-coordinated Si(II) atom ( Si = -398).
Table 1. Comparison between experimental (underlined) and calculated 29Si
NMR chemical shifts of BCH silylenes 22.
2
2a[a]
22a[b]
-231.7
31.2
22b[a]
-211.8
33.3
22b[b]
-219.9
30.4
Si1
Si2
Si3
Si4
Si5
-210.6
32.2
Scheme 4. Formation of BCH silylenes 22. (a: R = Me; b: R = Et).
-16.7
-25.5
-6.4
-17.1
-23.8
-5.8
-16.3
-25.8
0.2
-15.0
-25.5
-4.1
3 3
In contrast to the reaction of germole 8 with KSi(SiMe ) , the
reaction of the chlorosiloles 20 did not result in the formation of
the corresponding degraded siloles 23 and no precipitation of
elemental silicon was observed (Scheme 4). Instead, red
solutions were formed from which 29Si{ H} NMR spectra were
1
[
a] 99 MHz in benzene-d
6
at r.t.. [b] Calculated at GIAO/M06L/6-311G(2d,p)//
recorded that indicated the formation of (Me
new product with five magnetically non-equivalent silicon atoms
see Table 1 and Figure 5). The synthesized compounds showed
3 4
Si) Si along with a
M06-2X/6-311+G(d,p).
(
high thermal stability as heating of the NMR samples to 70 °C for
several hours did not lead to significant changes of the 29Si NMR
spectrum or to precipitation of elemental silicon. The reaction is
however sensitive to the steric bulk of the substituents as the
reaction is much cleaner for the triethylsilyl-substituted compound
2
9
2
0b. The same Si NMR spectrum consisting of five resonances
for the new compound was obtained when 20b was treated with
potassium tert.-butylate, or when dipotassium silolediide K [24]
was reacted with (Me Si) SiCl
(Scheme 4).[31, 35] For all three
2
3
2
2
preparation methods, disilafulvenes 21 are the most reasonable
common intermediates (Scheme 4). The number of observed 29Si
NMR signals are however not compatible with the structure of
disilafulvenes 21. In addition, the fulvenes 21 and bicyclic zwitter
ions 25 are calculated to be significantly less stable than the
isomeric BCH silylenes 22 (Figure 3), which points to BCH
silylenes 22 as possible candidates for the observed species. An
Figure 5. 29Si{ H} NMR spectrum (99.3 MHz, 305 K, benzene-d
mixture of silole 20b with KSi(SiMe
. Signals at 29Si = -10.2; -135.7 are due
to (Si(SiMe . (* signals from unknown side products).
1
6
) of the reaction
3 3
)
3 4
)
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2
2b by homoconjugation is substantial (E = -137 kJ mol-1 at
M06-2X/6-311+G(d,p)).
Scheme 5. Isodesmic hydrogen transfer reaction used to evaluate the
2
3
stabilization of BCH silylene 22b by interaction of the remote C =C double bond
1
and the Si atom.
As expected, there are close structural similarities between
silylenes 22 and the hafnocene-substituted compound 19 and the
relation to the isolobal germylene 14 is apparent. Stabilization of
electron deficient compounds by homoconjugation is a well-
Figure 6. Calculated molecular structure of BCH-silylene 22b (M06-2X/6-
established phenomenon for neutral boron compounds and
3
11+G(d,p). Hydrogen atoms are omitted for clarity. Selected atom distances
[40]
carbocations.
Selected examples were reported also for
1
1
1
2
1
3
1
4
[
pm] and angles: Si – C 205.0, Si – C 212.7, Si – C 210.1, Si – C 204.4,
1
– C 147.7, C – C3 141.1, C – C4 148.3, C – Si 190.1, C – Si2 190.2,
2
2
3
1
2
4
anionic silicon / boron compounds and silicon and germanium
cations.[41] Of special interest is a report by Maier and Reisenauer
on the observation of silylene 26 in low temperature matrices,
which is stabilized by homoconjugation (Figure 7).[
C
1
2
(Si ) 84.3, (Si ) 139.9.
42]
All our attempts to grow single crystals that are suitable for XRD
analysis resulted in the isolation of samples that showed no
refraction. Therefore, our structural information is based on DFT
calculations at the M06-2X/6-311+G(d,p) level of theory.[33] The
quality of the computed structural data can be measured by the
good agreement between the experimental 29Si NMR chemical
shifts and the computed values (Table 1). The main structural
Figure 7. Maiers BCH-silylene, an isomer of silabenzene.[42]
features are discussed for silylene 22b that adopts
bicyclo[2.1.1]hexene-like molecular structure (Figure 6). The
Me Si) Si fragment and the four carbon atoms form a five-
a
(
3
2
membered ring with a regular envelope conformation (flap angle
2

(Si ) = 139.4°, Figure 6). The bonds between the silicon atom
2
1
4
Si and the carbon atoms C and C are slightly longer than typical
Si - C single bonds (191 pm (22b) vs. regular Si - C = 186 - 188
[
38]
pm).
The former silole ring adopts an extreme envelope
conformation, with the dicoordinated silicon atom Si1 located
above the plane spanned by the four carbon atoms (flap angle
1
1
a)
b)

(Si ) = 84.3°, Figure 6). The Si - C distances in compound 22b
1
1/4
1
2/3 =
(
Si - C = 204, 205 pm; Si - C
210, 212 pm) are all signifi-
Figure 8. a) Molecular graph of BCH silylene 4 based on QTAIM analysis (M06-
X/def2-qzvppd//M06-2X/6-311+G(d,p)). Black lines indicate bond paths, green
cantly longer than regular Si - C single bonds involving dico-
ordinated silicon atoms and tetracoordinated carbon atoms
2
circles represent bond critical points, and small red circles are ring critical points.
b) Part of the molecular graph of silylene 4 projected on a contour plot of the
Laplacian of the electron density in the plane spanned by the midpoint of the
(
Si - C = 190 - 191 pm).[36, 39] The C - C distances in the central C
4
1
2
3
4
unit are alternating with relative short C - C /C - C single bonds
(
2
3
C - C bond, and both silicon atoms. Regions of local charge accumulation are
marked in red solid contours; regions of local charge depletion are marked in
blue dashed contours (VSCC: valence shell charge concentration).
2
3
148 pm) and a long C =C double bond (141 pm). The close
1
2
3
approach of the silicon atom Si to the carbon atoms C and C
and the long formal C=C double bond suggest electron donation
from the -orbital of the C=C double bond into the empty 3p orbital
of the silicon atom. Overall, based on the calculated molecular
structure, compound 22b can be described as a dicoordinated
silylene with significant homoconjugative interaction between the
silicon atom and the remote C =C double bond that increases its
coordination number. According to the isodesmic hydrogen
transfer reaction shown in Scheme 5, the stabilization of silylene
For further evaluation of the bonding situation in BCH silylenes 22
and for a detailed computational investigation of their formation
from disilafulvenes 21, we refer to the model compound 4. This
approach is justified since the computed structural differences
between compounds 22 and 4 are only marginal (differences in
atomic distances less than 2%) (see Table S2, SI material). An
2
3
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1
analysis of the computed charge density based on the quantum
1.67 e, occ(Si -C1/4) = 1.75 e), while the formally empty -orbital
2
3
theory of atom in molecules (QTAIM) revealed for compound 4
of the C =C double bond and the 3p(Si) orbital are significantly
occupied (occ(-C =C ) = 0.39 e, occ(3p(Si )) = 0.31 e).
Consequently, natural resonance theory (NRT) suggests that
canonical representations
[
43]
2
3
1
the molecular graph of a dicoordinated silylene (Figure 8a). The
2
3
interaction between the C =C double bond and the silylene
silicon atom is indicated by the location of third bond path between
A
and
B
(Figure 9) are major
2
3
1
[47]
the midpoint of the C =C bond and the Si atom (Figure 8a,b).
Such T-shaped electron distributions are typical for -complexes
between Lewis acids and CC multiple bonds.[42a] Examples are
silver ethyne complexes[44] and 2-norbornyl cations.[45,46] In
addition, the calculated Laplacian of the electron density showed
a distinct valence shell charge concentration (VSCC) at the Si1
atom which indicates the silylene lone pair. The elliptical
distribution of electron density around the bond critical point of the
contributors to the electronic structure of silylene 4.
2
3
C -C bond displays its -bond nature and suggests
a
silacyclopentene structure for compound 4. In addition, the results
of the NBO analysis for silylene 4 reflect its unique bonding
Figure 9. Leading resonance structures for silylene 4 according to natural
resonance theory (at M06-2X/6-311+G(d,p), relative contributions are given
in %).
situation.[33] In particular, the -orbital of the C =C double bond
2
3
1
1/4
2
3
and the - Si -C bonds are markedly depleted (occ(-C =C ) =
4 2 6
Figure 10. Part of the C Si H potential energy surface (relative energies at CCSD(T)/cc-pvqz//M062X/6-311+G(d,p),(italic underlined). For comparison with the
data in Figure 3, relative energies at M06-2X/6-311+G(d,p) are given in italic).
The experimental investigations suggested silafulvenes such as
a strongly exothermic three-step rearrangement from silafulvene
27 to BCH silylene 4. The thermodynamic data provided for this
transformation in Figure 10 are very close to those parameter
21 as reasonable intermediates in the formation of BCH silylenes
2. The calculations for the C Si isomers (Figure 10) indicate
2
4
2 6
H
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predicted for the experimentally investigated compounds (Figure
Z.D. thanks the state of Lower Saxony for a Georg Lichtenberg
Scholarship. The computations were performed at the HPC
Cluster HERO (High End Computing Resource Oldenburg),
located at the University of Oldenburg (Germany) and funded by
the DFG through its Major Research Instrumentation Program
(INST 184/108-1 FUGG/ INST 184/157-1 FUGG) and the Ministry
of Science and Culture (MWK) of the Lower Saxony State.
3
and SI Material). The involved barriers are relatively small and
-1
the first step is rate determining with a barrier of 54 kJ mol . This
low barrier is in agreement with the completed formation of the
BCH silylenes 22 at r.t.. Both intermediates, 28 and 29, are
located in shallow minima of the PES and in particular, compound
29 has no chemical significance. In agreement with Boldyrev’s
study (Figure 2), we found that BCH silylene 4 is significant more
stable than the three isomers of disilabenzene 5 - 7 (Figure 10).[25]
In view of the recent synthesis of thermally stable 1,2-disilaben-
zenes,[16,17] the existence of a possible reaction channel on the
Keywords: carbene analogues • silicon • germanium • benzene
isomers • NMR spectroscopy • quantum mechanical calculations
4 2 6
C Si H PES that connects 1,2-disilabenzene 5 with the global
[
1]
V. I. Minkin, M. N. Glukhovtsev, B. A. Simkin in Aromaticity and
Antiaromaticity: Electronic and Structural Aspects, John Wiley & Sons,
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minimum, BCH silylene 4, is of principal interest. In particular as
the experimentally required bulky substituents at the silicon atoms
destabilizes the 1,2-disilabenzene derivative. For example, the
energy difference between the isomeric BCH-type silylene and
[2]
[3]
V. Y. Lee, A. Sekiguchi in Organometallic Compounds of Low-Coordinate
Si, Ge, Sn, and Pb: From Phantom Species to Stable Compounds; John
Wiley & Sons, Ltd.: Chichester, 2010.
1
,2-disilabenzene increases in the case of 1,2-disilyldisila-
benzene 33 to 105 kJ mol-1 (see Figure 11). Our computational
N. Tokitoh, Acc. Chem. Res. 2004, 37, 86.
[
[
[
[
[
4]
5]
6]
7]
4]
A. I. Boldyrev, L.-S. Wang, Chem. Rev. 2005, 105, 3716.
Y. Wang, G. H. Robinson, Organometallics 2007, 26, 2.
J. R. Bleeke, Chem. Rev. 2001, 101, 1205.
results for the C Si PES indicate a four-step isomerization from
4
2 6
H
the disilabenzene 5 to the more stable BCH-silylene 4 with two
silylsilylene conformers 31a,b and 1,2-disilabenzvalene 30 as
possible intermediates. The hereby-involved barriers are even for
the non-substituted compounds substantial, the rate-determining
step for rearrangement 5 → 4 is the ring opening of the
L. Nyulaszi, Chem. Rev. 2001, 101, 1229. …
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[6]
[7]
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disilabenzene 5 that involves a barrier of 191 kJ mol . This
suggests that thermal transformations of 1,2-disilabenzenes to
BCH-silylenes are not feasible.
1
04, 5693.
T. Ishii, K. Suzuki, T. Nakamura, M. Yamashita, J. Am. Chem. Soc. 2016,
38, 12787.
T. Nakamura, K. Suzuki, M. Yamashita, J. Am. Chem. Soc. 2014, 136,
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[
[
8]
9]
1
9
[
[
10] T. Nakamura, K. Suzuki, M. Yamashita, Organometallics 2015, 34, 1806.
11] M. Saito, M. Sakaguchi, T. Tajima, K. Ishimura, S. Nagase, M. Hada,
Science 2010, 328, 339.
[
[
12] T. Kuwabara, J.-D. Guo, S. Nagase, M. Minoura, R. H. Herber, M. Saito,
Organometallics 2014, 33, 2910.
13] K. Wakita, N. Tokitoh, R. Okazaki, S. Nagase, Angew. Chem. Int. Ed.
2000, 39, 634.
Figure 11. Energy differences between 1,2-disilabenzenes and BCH-silylenes
[14] K. Wakita, N. Tokitoh, R. Okazaki, N. Takagi, S. Nagase, J. Am. Chem.
Soc. 2000, 122, 5648.
-
1
(in kJ mol , calculated at M062X/6-311+G(d,p)).
[
[
15] N. Nakata, N. Takeda, N. Tokitoh, J. Am. Chem. Soc. 2002, 124, 6914.
16] R. Kinjo, M. Ichinohe, A. Sekiguchi, N. Takagi, M. Suminoto, S. Nagase
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Conclusions
[
[
17] J. S. Han, T. Sasamori, Y. Mizuhata, N. Tokitoh, Dalton Trans. 2010, 39,
9238.
In summary, we have reported a new germole 8 to silole 9
transformation which proceeds via bicyclo[2.1.1]hexene-
germylene intermediate 14, a new type of carbene analogues.
While the germylene 14 is unstable versus decomposition into
elemental germanium and the silole, the corresponding silylenes
18] T. Sasamori, T. Sugahara, T. Agou, J.-D. Guo, S. Nagase, R. Streubel,
N. Tokitoh, Organometallics 2015, 34, 2106.
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Ferao, S. Nagase, N. Tokitoh, Bull.Chem. Soc. Jpn 2016, 89, 1375.
[
[
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C. R. W. Reinhold, Z. Dong, J. M. Winkler, H. Steinert,
M. Schmidtmann, T. Müller Author(s), Corresponding
Author(s)*
unexpected. An unprece-
dented germole to silole
transformation opened the
way for the synthesis of
novel bicyclic silylenes that
are stable derivatives of the
global minimum on the
Page No. – Page No.
A One-Step Germole to Silole Transformation and
a Stable Isomer of a Disilabenzene
C
4
Si
2 6
H potential energy
surface.
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