From an N-Heterocyclic silacyclopropene to Donor-Supported Silacyclopropenylium Cations

Article abstract of DOI:10.1021/om801178g

Addition of equimolar amounts of B(C₆F₅)₃ to the N-heterocyclic silacyclopropenes [LSi( C₂H₃ )](2;L= CH{(C=CH₂)-(CMe)(2,6-ⁱPr₂C₆H ₃N)₂

Full text of DOI:10.1021/om801178g

1610
Organometallics 2009, 28, 1610–1612
From an N-Heterocyclic Silacyclopropene to Donor-Supported
Silacyclopropenylium Cations
Shenglai Yao,† Yun Xiong,† Christoph van Wu¨llen,‡ and Matthias Driess*,†
Institute of Chemistry: Metalorganic and Inorganic Materials, Technische UniVersita¨t Berlin, Sekr. C2,
Strasse des 17. Juni 135, 10623 Berlin, Germany, and Fachbereich Chemie, Technische UniVersita¨t
Kaiserslautern, Erwin-Schro¨dinger-Strasse, D-67663 Kaiserslautern, Germany
ReceiVed December 12, 2008
Scheme 1. Resonance Structures of Silylene 1 and of the
Summary: Addition of equimolar amounts of B(C6F5)3 to the
N-heterocyclicsilacyclopropenes[LSi(C2H2)](2;L)CH{(CdCH2)-
(CMe)(2,6-iPr2C6H3N)2}) affords the first isolable, zwitterionic
silacyclopropenylium-boranide compounds 3. Protonation of
2 with the conVenient Brønsted acid H(OEt2)2+B(C6F5)4- leads
to quantitatiVe formation of the corresponding silacycloprope-
nylium salt [4-B(C6F5)4].
N-Heterocyclic Silacyclopropenes 2a and 2b (R
)
2,6-iPr2C6H3)
Silicon-containing small-ring compounds are of interest
because of their versatile role as building blocks in organosilicon
chemistry.1 Among those, silacyclopropenes and silacyclopro-
penylium cations, in which the skeletal carbon atoms are fully
or partially replaced by silicon, represent the most strained
cycloorganosilanes. While silacyclopropenes have been inves-
tigated extensively,2 little is known about the corresponding
silacyclopropenylium cations, owing to synthetic difficulties. In
contrast, cyclopropenylium cations, the smallest members of
Hu¨ckel aromatic systems, have been considerably studied.3
Recently, even heavier congeners of the cyclopropenylium
cations have been synthesized by Sekiguchi and co-workers.
Striking examples comprise the cyclotrigermenylium cation
4b-d
(tBu3Si)3Ge3
,
the
,
cyclotrisilenylium
and the disilacyclopropenylium
cation
+
4e
{(tBu2MeSi)(SitBu3)2}Si3
+
cation {(tBu3Si)2}Si2CR+ (R ) adamantyl).4f To our knowledge,
neither an isolable monosilacyclopropenium cation nor related
donor-supported silacyclopropenylium cations have been re-
ported to date.
* To whom correspondence should be addressed. Tel: +49(0)30-314-
22265. Fax: +49(0)30-314-29732. E-mail: matthias.driess@tu-berlin.de.
Web: http://www.driess.tu-berlin.de.
Very recently, we reported on the N-heterocyclic silacyclo-
propenes 2a and 2b5 (Scheme 1) as new types of SiC2 cycles
which are formed by the [2 + 1] cycloaddition of the
zwitterionic silylene 16a,b with alkynes. Remarkably, the elec-
tronic nature of 2a and 2b implies σ* aromaticity7 and a
significant contribution of the ylide-like resonance structures
2a′ and 2b′, respectively, in the electronic ground state (Scheme
1). In the preliminary communication,5 we proposed that the
resonance ylide structures 2a′ and 2b′ might play a crucial
(autocatalytic) role in their formation from 1 and the respective
acetylene. According to the calculation (see the Supporting
Information), the proton affinity of the terminal H2C group in
the model compound of 2a, in which the 2,6-iPr2C6H3 groups
on the nitrogen atoms were replaced by phenyl groups, amounts
to 1111.6 kJ mol-1. This implies that the basicity of 2a is even
† Technische Universita¨t Berlin.
‡ Technische Universita¨t Kaiserslautern.
(1) For reviews, see: (a) Barton, T. J. In ComprehensiVe Organometallic
Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon
Press: Oxford, U.K., 1982; Vol. 2, p 205. (b) Aylett, B. J.; Sullivan, A. C.
In ComprehensiVe Organometallic Chemistry II; Wilkinson, G., Stone,
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p 45.
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(e) Ishikawa, M.; Horio, T.; Yuzuriha, Y.; Kunai, A.; Tsukihara, Y.; Naitou,
H. Organometallics 1992, 11, 597. (f) Kunai, A.; Matsuo, Y.; Ohshita, J.;
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Chem. Soc. 1997, 119, 3629. (h) Kunai, A.; Mihara, T.; Matuo, Y.; Ohshita,
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Tsutsui, S.; Sakamoto, K.; Kira, M. J. Am. Chem. Soc. 1998, 120, 9955. (j)
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Sekiguchi, A.; Ichinohe, M.; Yamaguchi, S. J. Am. Chem. Soc. 1999, 121,
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T.; Apeloig, Y. J. Am. Chem. Soc. 2001, 123, 347. (n) Ichinohe, M.; Tanaka,
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10.1021/om801178g CCC: $40.75
2009 American Chemical Society
Publication on Web 02/23/2009

Communications
Organometallics, Vol. 28, No. 6, 2009 1611
Figure 2. Molecular structure of compound 3b. Thermal ellipsoids
are drawn at the 30% probability level. Hydrogen atoms are omitted
for clarity. Selected interatomic distances (Å) and angles (deg):
Si1-N2 ) 1.748(3), Si1-N1 ) 1.755(3), Si1-C30 ) 1.765(4),
Si1-C31 ) 1.776(4), N1-C2 ) 1.358(4), C1-C2 ) 1.495(4),
C1-B1 ) 1.659(6), N2-C4 ) 1.371(4), C2-C3 ) 1.407(5),
C3-C4 ) 1.366(5), C4-C5 ) 1.504(5), C30-C31 ) 1.367(5);
N2-Si1-N1 ) 102.6(1), N2-Si1-C30 ) 129.3(2), N1-Si1-C30
) 120.6(2), N2-Si1-C31 ) 124.9(2), N1-Si1-C31 ) 126.3(2),
C30-Si1-C31 ) 45.4(2).
Figure 1. Molecular structure of compound 3a. Thermal ellipsoids
are drawn at the 30% probability level. Hydrogen atoms (except
for those at C30 and C31) are omitted for clarity. Selected
interatomic distances (Å) and angles (deg): Si1-N2 ) 1.736(2),
Si1-N1 ) 1.745(2), Si1-C30 ) 1.746(3), Si1-C31 ) 1.756(3),
N1-C2 ) 1.368(2), N2-C4 ) 1.366(2), C1-C2 ) 1.487(3),
C1-B1 ) 1.656(3), C2-C3 ) 1.388(3), C3-C4 ) 1.378(3),
C4-C5 ) 1.491(3), C30-C31 ) 1.330(3); N2-Si1-N1 )
102.17(8), N2-Si1-C30 ) 126.4 (1), N1-Si1-C30 ) 125.6(1),
N2-Si1-C31)123.8(1),N1-Si1-C31)126.3(1),C30-Si1-C31
) 44.7(1).
system of 3 ((NICS(0) ) 1.6 ppm; NICS(1) ) 0.4 ppm; see
the Supporting Information). In contrast, related silyliumylidene
cations with two-coordinate silicon featuring a six-π-electron
heteroaromatic C3N2Si system are aromatic (calculated NICS(0)
) -1.4 ppm; NICS(1) ) -3.9 ppm).6c The structure of 3a has
been confirmed by a single-crystal X-ray diffraction analysis
(Figure 1).
Scheme 2. Synthesis of 3a, 3b, and 4
j
Compound 3a crystallizes in the triclinic space-group P1 and
consists of a nearly planar six-membered SiC3N2 ring with an
exocyclic B(C6F5)3 group attached to the C1 atom and a three-
membered SiC2 ring. These two cycles are perpendicular to each
other and share one (spiro) silicon atom. As expected, the
C1-C2 (1.487(3) Å) and C4-C5 distances (1.491(3) Å)
represent single bonds due to the addition of the B(C6F5)3
moiety. The Si-N bond lengths of 1.736(2) and 1.745(2) Å
are ca. 3 pm longer than those observed in the precursor 2a,
while the C2-N1 (1.368(2) Å) and C4-N2 distances (1.366(2)
Å) are about 4 pm shorter than the corresponding values in 2a,
in accordance with 6-π-electron aromatic delocalization. Slight
changes of the structural parameters can also be observed for
the three-membered ring in 3a. Thus, the C30-C31 distance
(1.330(3) Å) is slightly longer than that in 2a (1.311(5) Å).
Similarly, the silacyclopropene derivative 2b reacts gently
with equimolar amounts of B(C6F5)3, affording the analogous
adduct 3b (Scheme 2). Changing the solvent from toluene to
n-hexane allows the isolation of 3b in 88% yield in the form of
colorless crystals. The molecular structure of 3b has also been
established by X-ray diffraction analysis (Figure 2). As expected,
the characteristic metric features of 3b are similar to those of
3a, despite the additional phenyl groups attached to the C ring
atoms of the SiC2 cycle. The C30-C31 distance of 1.367 (3)
Å in 3b is slightly longer than the corresponding value in 3a,
probably due to a combination of electronic and steric factors.
In line with that, the Si1-C30 (1.765(4) Å) and Si1-C31
(1.776(4) Å) distances are 2 pm longer than those observed in
3a. Similar metric differences have been observed for the
precursors 2b vs 2a.5 Because of the π electron delocalization,
greater than that of its precursor 1 and is comparable to that of
the bent allenes calculated by Frenking8a and at the same time
synthesized by Bertrand and co-workers.8b The zwitterionic
character of the N-heterocyclic silacyclopropenes 2a and 2b
prompted us to synthesize the first silacyclopropenylium sys-
tems, simply by addition of suitable Lewis acids. Herein we
report the formation of the intramolecularly donor-supported
silacyclopropenylium cations 3a, 3b, and 4 and the striking
reactivity of 4 toward diethyl ether.
In fact, reaction of the silacyclopropene 2a with 1 molar equiv
of B(C6F5)3 in toluene at ambient temperature affords the desired
zwitterionic adduct 3a (Scheme 2). It can be isolated in the form
of air- and moisture-sensitive colorless crystals in 61% yield.
The composition and constitution of compound 3a have been
1
proven by elemental analysis and H, 13C, 29Si, 11B, and 19F
NMR spectroscopy (see the Supporting Information). The NMR
data suggest that the ring π electrons of the C3N2 moiety are
more delocalized than in the starting material 2a. Accordingly,
the γ-H atom of 3a resonates at δ 6.28, about 1 ppm downfield
shifted in comparison to that in 2a. Since the Si atom is
tetracoordinated, the six π ring electrons are merely delocalized
in the C3N2 diazapentadienylium π system: that is, the ring
system is nonaromatic. This is supportedby nucleus-independent
chemical shift (NICS) values from DFT calculation of a model
(8) (a) Tonner, R.; Frenking, G. Angew. Chem., Int. Ed. 2007, 46, 8695.
(b) Dyker, C. A.; Lavallo, V.; Donnadieu, B.; Bertrand, G. Angew. Chem.,
Int. Ed. 2008, 47, 3206.

1612 Organometallics, Vol. 28, No. 6, 2009
Communications
Scheme 3. Proposed Mechanism for the Reaction of Cation
4 with Diethyl Ether To Give 5
the metric features of the C3N2 moiety in the SiC3N2 ring in 3a
and 3b, respectively, are somewhat reminiscent of those
observed in the zwitterionic silyliumylidene-boranide
analogue.6c
Figure 3. Molecular structure of [5-B(C6F5)4]. Thermal ellipsoids
are drawn at the 30% probability level. Hydrogen atoms (except
for those at C30 and C31) are omitted for clarity. Selected
interatomic distances (Å) and angles (deg): Si1-N1 ) 1.756(2),
Si1-N2 ) 1.771(2), Si1-C30 ) 1.800(3), Si1-O1 ) 1.605(2),
N1-C2 ) 1.361(3), N2-C4 ) 1.344(3), C1-C2 ) 1.505(3),
C2-C3 ) 1.382(3), C3-C4 ) 1.392(3), C4-C5 ) 1.501(3),
C30-C31 ) 1.309(3); O1-Si1-N1 ) 111.7(1), O1-Si1-N2 )
109.77(10), N1-Si1-N2 ) 100.9(1), O1-Si1-C30 ) 108.4(1),
N1-Si1-C30)113.3(1),N2-Si1-C30)112.5(1),C31-C30-Si1
) 121.6(2).
Additionally, we examined the protonation of 2a with the
9
-
+
convenient Brønsted acid H(OEt2)2 B(C6F5)4 , in diethyl ether
at low temperature. As expected, the reaction leads to the
formation of the ion pair [4-B(C6F5)4] in quantitative yield
(Scheme 2). The constitution of [4-B(C6F5)4] has been proven
by 1H, 13C, 29Si, 11B, and 19F NMR spectroscopy (see the
1
Supporting Information). As expected, the H NMR spectrum
shows the presence of two identical terminal CH3 groups
attached to the SiC3N2 ring. The singlet resonance signal at δ
7.87 ppm can be unambiguously assigned to the two protons
attached to the olefinic carbon atoms in the SiC2 ring. Likewise,
the γ-H (δ 6.08 ppm), γ-13C (δ 102.8 ppm) and 29Si NMR
resonances (δ -84.8 ppm) are very close to the corresponding
values observed for 3a. According to DFT calculations (see the
Supporting Information), the most important geometric features
of the model cation 4′, in which the 2,6-iPr2C6H3 groups on the
nitrogen atoms were replaced by phenyl groups, are practically
identical with those of 3a. Interestingly, cation 4 reacts with
diethyl ether at ambient temperature to give [5-B(C6F5)4] and
ethene (Scheme 3). The N-heterocyclic salt [5-B(C6F5)4] forms
colorless crystals which can be isolated in 85% yield. An X-ray
diffraction analysis confirms that the latter consists of separated
ion pairs (Figure 3). The silicon atom in 5 adopts a tetrahedral
geometry and bears a vinyl and an ethoxy group. The C30-C31
distance of 1.309(3) Å lies in the normal range observed for
CdC double bonds of a vinyl group. The protons of the vinyl
group show in the 1H NMR spectrum a typical ABC spin pattern
at δ 5.55, 5.95, and 6.23 ppm with J(H,H) coupling constants
of 20.5, 15.0, and 2.6 Hz, respectively. These values are similar
to those observed for terminal vinyl groups attached to related
ꢀ-diketiminate aluminum compounds.10
Although the mechanism is still unknown, we propose that
the initial step of the ether cleavage is a silicon-(Lewis acid)
assisted ꢀ-H elimination process with release of ethene.
Subsequently, a proton-induced Si-C ring-opening reaction
occurs, presumably via silaoxonium,11 to give the final tauto-
meric cation 5 (Scheme 3). The latter reaction demonstrates that
the highly electrophilic silicon atom in the donor-supported
silacyclopropenylium cation 4 could be very useful for the
synthesis of other types of functionalized vinyl silanes, simply
by electrophilic ring-opening reactions. Respective investigations
are underway.
Acknowledgment. Financial support from the Deutsche
Forschungsgemeinschaft and the Cluster of Excellence
“Unifying Concepts in Catalysis” (EXE 314/1) are gratefully
acknowledged.
Supporting Information Available: Text, tables, and figures
giving experimental details of the synthesis and spectroscopic data
of 3a, 3b, [4-B(C6F5)4], and [5-B(C6F5)4] and CIF files giving
crystallographic data for 3a, 3b, and [5-B(C6F5)4]. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM801178G
(9) Jutzi, P.; Mu¨ller, C.; Stammler, A.; Stammler, H.-G. Organometallics
2000, 19, 1442.
(10) Zhu, H.; Chai, J.; Fan, H.; Roesky, H. W.; He, C.; Jancik, V.;
Schmidt, H.-G.; Noltemeyer, M.; Merrill, W. A.; Power, P. P. Angew. Chem.,
Int. Ed. 2005, 44, 5090.
(11) For the related reactivity of silyl oxonium ions see: Olah, G. A.;
Laali, K. K.; Wang, Q.; Surya Prakash, G. K. In Onium Ions; Wiley: New
York, 1998; Chapter 3, p 100, and references cited therein.