Reversible base coordination to a disilene

Article abstract of DOI:10.1002/anie.201202277

Si=Si activation: Reversible formation of a donor-acceptor complex between an N-heterocyclic carbene and a cyclotrisilene with carbon-based substituents shifts the electron density of the double bond and thus induces strong polarization, as shown by the significantly pyramidal tricoordinate silicon atom. Copyright

Full text of DOI:10.1002/anie.201202277

Angewandte
Chemie
DOI: 10.1002/anie.201202277
À
Si Si Double Bonds
Reversible Base Coordination to a Disilene**
´
Kinga Leszczynska, Kai Abersfelder, Andreas Mix, Beate Neumann, Hans-Georg Stammler,
Michael J. Cowley, Peter Jutzi,* and David Scheschkewitz*
Dedicated to Prof. Theophil Eicher on occasion of his 80th birthday
During the last few decades, alkene and alkyne analogues of
the heavier Group 14 elements have attracted considerable
interest. Their isolation as stable derivatives has become
possible by the use of carefully designed bulky substituents
that provide kinetic (and to some extent thermodynamic)
stabilization. The considerable differences in structure, bond-
ing, and reactivity of such compounds in comparison to the
carbon-based species have prompted various experimental
and theoretical studies.[1–4] By and large, the lower electro-
negativity of heavier elements and the increasing spatial
extension of their valence electron shells are responsible for
many of these differences.[5] One of several rationalizations
for structure and reactivity of such doubly and triply bonded
species is based on zwitterionic (Ib and IIb in Scheme 1) and
germanium, tin, and lead, and less pronounced in compounds
of silicon.[1–4]
Conversely, heteronuclear silicon multiple bonds, in
particular those involving chalcogen atoms, are inherently
polar owing to the differences in electronegativity. The strong
ylidic contribution to the ground-state structure becomes
apparent by facile base coordination to the positively
polarized silicon atom.[6] Similarly, silylenes as the formal
=
constituents of the Si Si bond are readily coordinated by
various bases owing to their vacant p orbital.[7]
In marked contrast, the first experimentally observed
example showing the importance of polar contributions (Ib in
À
Scheme 1) in species with a homonuclear Si Si triple bond
was reported only very recently.[8] Reaction of the disilyne
1 with the N-heterocyclic carbene (NHC) 1,3,4,5-tetramethyl-
imidazol-2-ylidene irreversibly afforded the corresponding
polar disilyne–NHC complex 2 with trans geometry of the
=
remaining Si Si moiety and a lone pair of electrons residing
on the dicoordinate silicon atom (Scheme 2).
ꢀ
Scheme 1. Zwitterionic and diradical contributions to RE ER and
=
R2E ER2 (E=Si, Ge, Sn, Pb; R=sterically demanding substituent).
Scheme 2. R=SiiPr[CH(SiMe3)2]2.
diradical contributions (Ic and IIc in Scheme 1) to the ground
state structures, which illustrate both the weakness and the
trans-bent structure of heavier multiple bonds. Such contri-
butions are, however, more prominent in compounds of
Despite a plethora of fully characterized disilenes,[9] no
disilene–NHC adduct analogous to 2 has been reported to
date. Herein, we describe the reversible formation and
isolation of a Lewis acid–base adduct between a disilene
and an NHC and thus demonstrate experimentally the
importance of polar contributions to the electronic structure
´
[*] Dr. K. Leszczynska, Dr. A. Mix, B. Neumann, Dr. H.-G. Stammler,
Prof. Dr. P. Jutzi
Faculty of Chemistry, University of Bielefeld
33613 Bielefeld (Germany)
À
of Si Si double bonds (IIb in Scheme 1). Reaction of
cyclotrisilene 3 with 1,3-di(isopropyl)-4,5-dimethylimidazol-
2-ylidene leads to the formation of the cyclotrisilene–NHC
complex 4 by nucleophilic attack of the carbene carbon atom
E-mail: peter.jutzi@uni-bielefeld.de
Dipl.-Chem. K. Abersfelder, Dr. M. J. Cowley,
Prof. Dr. D. Scheschkewitz
Krupp-Chair of General and Inorganic Chemistry
Saarland University, 66125 Saarbrꢀcken (Germany)
E-mail: scheschkewitz@mx.uni-saarland.de
=
at the Cp*-substituted silicon atom of the Si Si bond.
(Scheme 3).
Previously, only cyclotrisilenes with silyl groups, cyclo-
Si3(SiR3)4, have been reported, by the groups of Kira and
Sekiguchi.[10–12] For the synthesis of 3,[13] the first cyclotrisilene
with carbon-based substituents (Cp* = C5Me5; Tip = 2,4,6-
iPr3C6H2), we developed a novel synthetic strategy. Reaction
of the silicon(II) tetrakis(pentafluorophenyl)borate 5[14] with
the lithium disilenide 6[15] in DME at À658C visibly resulted in
[**] This work was funded by the EPSRC (EP/H048804/1), the Alfried
Krupp Foundation, the DFG (Deutsche Forschungsgemeinschaft)
and the European Commission (Marie Curie Postdoctoral Fellow-
ship for M.J.C.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202277.
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1518C) and characterized by multinuclear NMR spectrosco-
py.
The molecular structure of 8 based on X-ray crystal
structure analysis confirmed the constitution of a cyclotrisi-
lene (Figure 1, Table 1).[13] An apparent disorder of the
Scheme 3. Tip=2,4,6-iPr3C6H2.
Scheme 4. Tip=2,4,6-iPr3C6H2; DME=1,2-dimethoxyethane.
salt elimination to produce a mixture containing 85% of 3
(Scheme 4). Formation of 3 is readily explained by intra-
molecular attack of the SiII center of a transient silylene
species 7 at the b-silicon atom of the double bond. This step is
followed by the cyclic rearrangement of the electrons in the
Si3 fragment and the change of the coordination mode of the
Cp* system. To a small extent, the tetrasilabutadiene
Si4Tip6,[16] decamethyl silicocene,[17] and elemental silicon
were detected as byproducts. Apparently, competing electron
transfer processes generate radical fragments that subse-
quently homocouple. The latter two products would thus be
produced by disproportionation of transient Cp*2Si2.[18]
Si4Tip6 and Cp*2Si were separated for the most part by
crystallization from hexane solution. The cyclotrisilene was
finally obtained as an orange-brown oil, still containing some
residual contamination (see the Supporting Information).
Compound 3 was characterized by 29Si, 1H, and 13C NMR
spectroscopy and derivatization to NHC complex 4.
To validate the general applicability of the new synthetic
procedure and for corroboration of the structural assignment
of cyclotrisilene 3 by means of comparison, we have also
prepared the first fully aryl-substituted cyclotrisilene 8. On
the basis of the successful synthesis of 3, we concluded that 8
might occur as an intermediate of the previously reported
synthesis of the magnesium salt of a trisilendiide 9.[19] Indeed,
the use of Et2O as a solvent instead of THF during the
reduction of the dichlorosilyl disilene precursor with magne-
sium powder halts the reaction at the stage of neutral species
8 (Scheme 5). Cyclotrisilene 8 was isolated as orange crystals
from a concentrated pentane solution in 51% yield (mp. 150–
Figure 1. Molecular structure of 8 in the solid state. Ellipsoids are set
at 30% probability; hydrogen atoms are omitted for clarity.[13]
Table 1: Comparison of structural parameters for 4 and 8 (X-ray data)
and 3Dip and 4Dip (B3LYP/6-31G(d)).[20]
3Dip
8
4
4Dip
À
Si1 Si2 [ꢁ]
2.1336
2.3370
2.3539
1.9619
2.7264/
2.7792
À12.37
358.91
360.00
2.118(1)
2.289(1)
2.289(1)
1.880(3)
n/a
2.2700(5)
2.4332(5)
2.3389(5)
1.992(1)
2.857(1)/
2.906(1)
À86.54(7)
317.0
2.2839
2.4882
2.3504
2.0425
2.9224/
2.9804
À83.97
314.5
À
Si1 Si3 [ꢁ]
À
Si2 Si3 [ꢁ]
À
Si1 Ca/ipso [ꢁ]
À
Si1 Cb
C-Si1-Si2-C [8]
S angles Si1 [8]
S angles Si2 [8]
9.07
359.46
359.98
340.2
345.4
central Si3 ring was ignored owing to the relatively low
intensity of residual electron density peaks. Despite the
resulting less-than-ideal refinement, an acceptable bond
precision allows a cautious discussion of bond parameters.
=
À
The Si1 Si2 bond (2.1182(12) ꢀ) is at the short end of Si Si
double bonds. The deviation of the Tip substituents from the
Si3 plane is small, as illustrated by the sums of angles about Si1
(359.458) and Si2 (359.988).
Compared to the silyl-substituted cyclotrisilenes,[10–12] the
29Si NMR signals of the unsaturated silicon atoms of 3 and 8
are observed significantly upfield (Table 2), which is in line
with the general observations for open-chained disilenes.[9]
À
Unlike most unsymmetrically substituted Si Si double
bonds,[21,22] however, the downfield resonance of 3 is observed
for the silicon atom with the less electronegative substituent
(Cp*). From the NMR spectra of 3, it is concluded that the
Scheme 5. Tip=2,4,6-iPr3C6H2; THF=tetrahydrofuran.
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Table 2: Comparison of the NMR data of 3, 8, and 4 with the data
calculated for 3Dip.[a]
d29Si/13C
3
3Dip
4
8
SiAr2
SiAr
SiCp* or SiAr
NCN
À15.3
37.1
57.7
–
3.3
51.8
92.6
–
À85.9
À61.5
À85.6
161.4
À23.2
42.5
42.5
–
Scheme 6. Tip=2,4,6-iPr3C6H2.
[a] NMR spectroscopy for 3 ([D10]-1,2-dimethoxyethane, 298 K, Ar=Tip),
8 ([D6]benzene, 293 K, Ar=Tip), and 4 ([D8]toluene, 298 K, Ar=Tip);
calculations for 3Dip with Ar=Dip=2,6-iPr2C6H3 (B3LYP/6-311+G-
(2df) on Si, 6-31G(d) on C,H).[20]
HOMO (Figure 2a) and HOMOÀ1 (Figure 2b) formally
result from the antibonding and bonding linear combination
=
of the Si Si p orbital with a p orbital of the Cp* ligand. The
resulting weak p donation approximates a h3 coordination
Cp* substituent shows fast exchange processes at ambient
temperature. Such sigmatropic rearrangements of cyclopen-
tadienyl silanes have been investigated in detail previously.[23]
With the purpose of clarifying the influence of the Cp*
mode and apparently stabilizes a partial positive charge at Si1.
=
To further probe the polarization of the Si Si bond
experimentally, we treated 3 with an N-heterocyclic carbene
as a strong s donor. Addition of a hexane solution of the NHC
1,3-di(isopropyl)-4,5-dimethylimidazol-2-ylidene to a hexane
solution of 3 at À208C resulted in an immediate color change
to deep red caused by the formation of the 1:1 adduct 4
(Scheme 3), which was isolated as red crystals in 48% yield
and characterized by elemental analysis, X-ray crystallogra-
phy, and multinuclear NMR spectroscopy (see the Supporting
Information).
À
ligand on the Si Si double bond in the absence of an
experimental solid-state structure, calculations of the model
system 3Dip (Dip = 2,6-iPr2C6H3 instead of Tip) were per-
formed at the B3LYP/6-31G(d) level of theory (Figure 2,
Table 2).[20] The trends in the 29Si NMR shifts of 3 are well-
The molecular structure of 4 in the solid state as
determined by X-ray diffraction on single crystals (Figure 3,
Figure 2. Contour plots of a) HOMO and b) HOMOÀ1 of 3Dip at an
isovalue of 0.03.[20]
reproduced in the calculated data for 3Dip (B3LYP/6-311 +
G(2df) on Si; 6-31G(d) on C,H), lending support to the
validity of the model. Similarly to 8 and the persilyl-
substituted cyclotrisilenes,[10–12] the Si1 Si2 bond (2.1336 ꢀ)
=
in the fully optimized structure of 3Dip is relatively short. The
sums of the bond angles at the respective silicon atoms
indicate slight pyramidalization of the Cp*-substituted Si1
atom (358.91), while the coordination environment of Si2
with the Dip group is perfectly planar (360.008). In general,
unsymmetrical substitution patterns in open-chained disilenes
lead to a more pronounced pyramidalization of the silicon
atom with the more electropositive substituents and thus
a partial negative charge at this atom (resonance structure 3’
in Scheme 6).[22] In the case of 3 and 3Dip, however, despite
the slightly pyramidal Si1 atom, the Mullikan atomic charges
(Si1 + 0.227, Si2 À0.012) in concert with the 29Si NMR data
Figure 3. Molecular structure of 4 in the solid state. Ellipsoids are set
at 30% probability; hydrogen atoms and methyl groups of the Tip
substituents are omitted for clarity.[13]
Table 1)[13] reveals NHC coordination by the carbenic carbon
=
atom to the Si Si bond of 3. The coordination site is the Cp*-
substituted silicon atom, which is in line with reversed polarity
discussed above. The NHC ligand and the Tip substituent are
positioned at the same side of the Si3 ring plane. The newly
formed donor–acceptor bond, however, appears to be rela-
À
À
suggest the reverse polarization of the Si Si double bond
tively weak: the Si C(NHC) bond distance (1.9843(14) ꢀ) is
according to resonance structure 3’’.
significantly longer than the corresponding one in the adduct
2 (1.9221(16) ꢀ)[8] and in the formal Si(0) compound NHC
À
Closer analysis of the distances between the Cp* carbon
atoms and Si1 in 3Dip reveals weak contacts to the b-carbon
atoms (2.7264 and 2.7792 ꢀ). Indeed, the topologies of the
(1.9271(15) ꢀ).[24]
The
=
Si1 Si2
bond
= À
À
Si Si NHC
(2.2700(5) ꢀ) is longer than the Si Si double bond in 3
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À
Dittrich, S. Klein, G. Frenking, J. Am. Chem. Soc. 2011, 133,
17552 – 17555.
(calc. 2.13362 ꢀ), as expected, but remains shorter than Si Si
bonds in Tip substituted cyclotrisilanes.[25] Coordination of the
carbene results in strong pyramidalization of both silicon
atoms, Si1 and Si2, as seen by the sum of angles (ignoring the
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À
Si C bond between cyclotrisilene and NHC): 3178 at Si1 and
3408 at Si2. As a consequence, the electron pair at Si2 acquires
a strongly non-bonding character. Furthermore, the weak h3-
like interaction of the Cp* ligand with the adjacent silicon
atom is cancelled upon NHC-coordination, as shown by the
increased distances between silicon and the Cp* b-carbon
atoms. For adequate comparison with 3Dip, the structure of
4Dip was fully optimized, and it shows elongation of the
distances in question by approximately 0.2 ꢀ (Table 1).[20]
The coordination of the NHC to the cyclotrisilene 3 causes
an upfield shift for all of the 29Si signals of 4, which were
unambiguously assigned by 1H/29Si 2D NMR spectroscopy
(Table 2). The 13C chemical shift of the carbene center of 4 at
d = 161.4 ppm is shifted by Dd = 44.5 ppm with respect to the
free NHC (d13C = 205.7 ppm[26]). A similar upfield 13C shift
had been observed by Sekiguchi et al. for the coordinated
NHC in 2.[8] Unlike in the case of 2, however, complex
formation is reversible, as VT-NMR studies show that 4
dissociates in solution (Scheme 3). Although 3 and 4 thus
coexist at room temperature in comparable concentrations,
only signals for the adduct 4 are observed at 223 K. The
enthalpy of the dissociation process at 298 K is estimated at
DG ꢁ 9 kJmolÀ1 (see the Supporting Information).
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[13] An Experimental Section, including crystallographic details, is
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Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
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Reversibility of individual reaction steps is a prerequisite
for the experimental realization of catalytic cycles, for
example in small-molecule activation. Heavier main-group
systems are attracting increasing interest as potential replace-
ment for transition-metal systems.[3] Herein, we have shown
that a strong base, namely an N-heterocyclic carbene,
[18] P. Jutzi, A. Klipp, A. Mix, B. Neumann, H.-G. Stammler, Silicon
Chem. 2007, 3, 151 – 156.
=
reversibly coordinates the Si Si double bond of a cyclotrisi-
[19] K. Abersfelder, D. Gꢁclꢁ, D. Scheschkewitz, Angew. Chem.
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lene. We are currently exploring how the formation of such
donor–acceptor complexes influences the reactivity of cyclo-
trisilenes, and disilenes in general, in this regard.
[20] Theoretical calculations were carried out with the Gaussian 09,
Revision A.02. Geometry optimization and IR frequencies were
at the B3LYP/6-31G(d) level of theory. NMR spectroscopy shifts
were obtained using a polarizable continuum model for benzene
and with an expanded basis set (6-311 + G(2df) for Si, 6-31G(d)
for C and H). Details have been deposited and can be retrieved
free of charge at http://dx.doi.org/ under the following identi-
fiers: 10042/to-8287 (frequencies, 3Dip); 10042/to-8286 (NMR,
3Dip); 10042/to-8290 (frequencies, 4Dip).
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Received: March 22, 2012
Published online: May 25, 2012
Keywords: donor–acceptor interactions · multiple bonds ·
.
reversibility · silicon · small rings
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