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An Unsaturated -Dianionic Oligosilane.

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Angewandte
Chemie
Low-Valent Silicon Compounds
DOI: 10.1002/anie.200503975
An Unsaturated a,w-Dianionic Oligosilane**
Kai Abersfelder, Deniz Gcl, and
David Scheschkewitz*
Silyl anions are widely applied as reagents in organic and
inorganic chemistry.[1] In this context, interest in a,w-dianionic oligosilanes as precursors for cyclic and acyclic homoand heterosilanes has been considerable.[2] Curiously, whereas
the chemistry of low-valent silicon compounds is otherwise
highly developed,[3] only two anionic compounds with a lowvalent silicon backbone had been reported until recently: the
cyclobutenide analogue 1[4] and the cyclobutadiene dianion
analogue 2 (Scheme 1).[5]
Scheme 1. Anionic low-valent silicon compounds; 1, 2: R = SiMetBu2 ;
3 a: R = R’ = Tip = 2,4,6-iPr3C6H2 ; 3 b: R = SiMetBu2, R’ = 2,4,6-Me3C6H2.
In 2004, we and Sekiguchi et al. independently succeeded
in the isolation of the monoanionic disilenides 3 a and 3 b,
which are disila analogues of vinyl anions.[6] These disilenides
were expected to combine the reactivity of the Si=Si double
bond with the nucleophilicity of silyl anions. Derivative 3 a
had been previously proposed by Weidenbruch et al. as an
intermediate in the synthesis of their tetrasilabutadiene.[7] In
the meantime, further applications of 3 a as a ligand in a
zirconium complex[8] and as a building block for a silicon
cluster with a “naked” vertex have emerged.[9]
Herein, we report the synthesis and isolation of the first
a,w-dianionic unsaturated oligosilane. Reaction of the lithium salt of disilenide 3 a with an equimolar amount of
TipSiCl3 (Tip = 2,4,6-triisopropylphenyl) in tetrahydrofuran
(THF) yielded the moderately air- and moisture-sensitive
(dichlorosilyl)disilene 4, quantitatively (Scheme 2). Although
single crystals could not be obtained, comparison of the
characteristic 29Si NMR spectrum of 4 (d = 99.1, 54.7, and
[*] K. Abersfelder, D. G3cl3, Dr. D. Scheschkewitz
Institut f3r Anorganische Chemie
Bayerische Julius-Maximilians-Universit<t W3rzburg
Am Hubland, 97074 W3rzburg (Germany)
Fax: (+ 49) 931/888-4623
E-mail: scheschkewitz@mail.uni-wuerzburg.de
[**] This work was funded by the DFG (Sche 906/3-1) and the University
of W3rzburg. The authors thank Dr. R. Bertermann and M. Sch<fer
for NMR measurements, Dr. C. Burschka for the X-ray data
collection, and Prof. H. Braunschweig for generous support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 1643 –1645
11.7 ppm) with that of the related compound Tip2Si=Si(Tip)SiMe3[6a] allowed unambiguous identification of 4.
Reduction of 4 with an excess of active magnesium (Mg*)[10]
in THF then afforded the magnesium salt 5 of the trisilene1,3-diide in 42 % yield after repeated crystallizations, as very
air-sensitive, but thermally highly stable (m.p. 209 8C) orange
blocks (Scheme 2).
Scheme 2. Preparation of 5.
The dianion of 5 was characterized by means of multinuclear NMR and UV/Vis spectroscopy. The 29Si NMR
spectrum shows three signals of equal intensity at d = 143.9,
134.5, and 44.4 ppm. The 2D Si–H correlation gave the first
indication that the double bond in 5 is shifted with respect to
its position in 4. Whereas the two lowfield resonances
(assignable to the Si=Si double bond) couple to only one
Tip group each, the signal at d = 44.4 ppm shows cross peaks
to two such groups. Another prominent feature of the
otherwise complex 1H and 13C NMR spectra is the appearance
of two distinct, albeit broad, signals (1H NMR: d = 3.67,
3.19 ppm; 13C NMR: d = 70.16, 69.36 ppm) for the methylene
groups bonded to the oxygen atoms of the two coordinating
THF molecules, indicating the presence of a contact ion pair
in [D6]benzene solution.
In the UV/Vis spectrum of 5 in pentane, the absorption
with the highest wavelength occurs at lmax = 415 nm (e =
3200 L mol 1 cm 1), whereas in 3 a the corresponding absorption occurs at lmax = 417 nm.[6a] The lack of a significant red
shift suggests that not only the vinylic, but also the allylic
charges in 5 are essentially localized on the respective silicon
atoms. The Coulomb repulsion between the two negative
charges should certainly disfavor delocalization. However,
the monoanionic cyclotetrasilenide 1 reported by Sekiguchi
et al. also has a strictly localized, albeit fluxional, electronic
structure.[4]
An X-ray structure determination for a single crystal of
5·0.5 C5H12 confirmed that the trisilene-1,3-diide forms a
contact ion pair with the Mg2+ counterion (Figure 1).[11] The
Si1, Si2, Si3, and Mg1 atoms form an almost perfectly planar
array (largest deviation from the least-squares plane: 3.7 pm
for Si2). Whereas the Mg1 Si3 distance (262.5(1) pm) is
comparable to values usually found for magnesium silanides,[12] the Mg1 Si1 distance (255.2(1) pm) is significantly
shorter, which we tentatively attribute to the formal sp2
hybridization of Si1. To our knowledge the Mg1 Si1 bond is
the shortest Mg Si contact observed to date. The coordination sphere of the Mg2+ ion is completed by the oxygen atoms
of two molecules of THF. The Si1=Si2 double-bond length of
219.8(1) pm in 5 is similar to that in 3 a (219.2(1) pm), and the
Si2 Si3 bond (236.2(1) pm) is somewhat longer than usual
Si Si single bonds (234 pm),[13] which reflects the steric
congestion in 5 and corroborates the absence of any
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1643
Communications
Experimental Section
Figure 1. Structure of 5 in the solid state. Thermal ellipsoids set at
50 % probability. Hydrogen atoms, isopropyl groups, and the minor
disorder component of one of the THF molecules (O2) are omitted for
clarity. Selected bond lengths [pm] and angles [8]: Si1 Si2 219.8(1),
Si2 Si3 236.2(1), Si1 C1 191.5(2), Si2 C16 191.7(2), Si3 C31
193.4(2), Si3 C46 194.4(2), Si1 Mg1 255.2(1), Si3 Mg1 262.5(1),
Mg1 O1 205.1(2), Mg1 O2 203.4(2); Si1-Si2-Si3 102.52(4), C1-Si1-Si2
120.98(7), Si1-Si2-C16 127.83(7), C16-Si2-Si3 129.48(7).
delocalization of the negative charges across the three silicon
atoms. The slight elongation of the bonds between Si3 and the
ipso carbon atoms (Si3 C31: 193.4(2); Si3 C46: 194.4(2) pm)
is probably also due to the bulkiness of the Tip substituents.
To test its suitability as a precursor for unsaturated
heterocyclic oligosilanes, we treated 5 with Me2SnCl2 in
[D6]benzene. The reaction proceeded smoothly to afford the
four-membered ring 6 (Scheme 3), which was unambiguously
Scheme 3. Preparation of 6.
identified by multinuclear NMR spectroscopy. As expected,
the 29Si NMR spectrum of 6 consists of three signals at d =
105.2, 92.5, and 26.2 ppm, all of which show 117/119Sn
satellites (d(119Sn) = 39.3 ppm). Although the silicon atoms
adjacent to tin formally differ in hybridization, the coupling
constants are quite similar (sp2 : 1J(29Si,119Sn) = 479 Hz, sp3 :
1 29
J( Si,119Sn) = 497 Hz). Even the coupling to the central
silicon atom is remarkably large (2J(29Si,119Sn) = 228 Hz). In
the 1H and 13C NMR spectra of 6, two different signals are
observed for the methyl groups bonded to tin, which could be
due to a lowering of the Cs symmetry through a distortion of
the four-membered ring from planarity or a less-symmetric
arrangement of the Tip substituents.
The successful synthesis of 6 leads to the expectation that
a variety of new unsaturated homo- and heterocyclic silanes
might be accessible starting from 5. The reactivity of 5
towards other main-group and transition-metal electrophiles
is currently under investigation.
1644
www.angewandte.org
All manipulations were carried out under a protective atmosphere of
argon, using Schlenk techniques or in a glove box.
4: At 196 8C, THF (20 mL) was condensed onto the lithium salt
of 3 a[6a] (5.40 g, 6.33 mmol) and TipSiCl3[14] (2.14 g, 6.33 mmol). The
mixture was slowly brought to room temperature (30 min) and then
stirred for 15 h. All volatiles were removed under high vacuum, and
the orange residue was dissolved in hexane (50 mL). Solid LiCl was
removed by filtration, and the clear orange solution was concentrated
to dryness, affording 4 as an orange solid (6.07 g, 99 %). The product
was used for the subsequent reactions without further purification.
1
H NMR (500 MHz, [D6]benzene, 25 8C): d = 7.09, 7.07, 7.01, 6.94 (s,
8 H, Ar-H), 4.37, 4.31 (hept, 4 H, iPr-CH), 4.03 (br, 2 H, iPr-CH), 3.59
(hept, 2 H, iPr-CH), 2.72, 2.63 (hept, 4 H, iPr-CH), 1.41, 1.22, 1.14,
1.13, 1.08, 1.07, 0.89 ppm (d, 72 H, iPr-CH3); 13C NMR (125 MHz,
[D6]benzene, 25 8C): d = 156.60, 155.97, 155.23, 154.03, 152.42, 151.74,
151.23, 151.03, 135.00, 134.12, 130.50, 130.13 (Ar-C), 122.67, 122.55,
122.36 (Ar-CH), 39.15, 38.37, 37.84, 34.74, 34.65, 34.45, 34.39, 34.22
(iPr-CH), 25.49, 25.18, 24.42, 24.31, 23.99, 23.95, 23.88, 22.69 ppm (iPrCH3); 29Si NMR (99 MHz, [D6]benzene, 25 8C): d = 99.1, 54.7,
11.7 ppm.
5: At 196 8C, THF (20 mL) was added by vacuum transfer to 4
(5.90 g, 6.09 mmol) and Mg* (0.65 g, 24.3 mmol), prepared by the
thermal decomposition of magnesium anthracene.[10] While the
suspension was brought to room temperature, it gradually darkened
until it appeared almost black after 1 h. All volatiles were then
removed under high vacuum, and the solid residue was extracted with
toluene (50 mL). Following filtration, the solution was concentrated
to approximately 5 mL, and pentane (50 mL) was added. The solution
was kept at 30 8C overnight, leading to the precipitation of 5.10 g of
an orange–red solid. After three recrystallizations, 5 was isolated as
orange blocks (2.81 g, 42 %). M.p. 209 8C (minimal decomposition);
1
H NMR (300 MHz, [D6]benzene, 25 8C): d = 7.25, 7.15 (d, 2 H, ArH), 7.05 (s, 2 H, Ar-H), 7.02, 6.99, 6.94, 6.84 (d, 4 H, Ar-H), 4.88, 4.68,
4.37, 4.10, 4.09 (hept, 7 H, iPr-CH), 3.67 (br, 4 H, THF), 3.45 (hept,
1 H, iPr-CH), 3.19 (br, 4 H, THF), 2.88, 2.81, 2.76 (hept, 4 H, iPr-CH),
1.77, 1.60, 1.59, 1.53, 1.51, 1.42, 1.40, 1.32, 1.31, 1.25, 1.24, 1.23, 1.21,
1.19, 1.06, 0.89, 0.87, 0.69, 0.63, 0.58 ppm (d, 72 H, iPr-CH3); 13C NMR
(75 MHz, [D6]benzene, 25 8C): d = 156.01, 153.85, 153.83, 153.42,
152.89, 152.26, 151.78, 148.15, 147.27, 147.24, 147.11, 145.96, 144.32,
143.65, 142.52 (Ar-C), 122.47, 122.13, 121.23, 121.02, 120.55, 120.33,
120.30 (Ar-CH), 70.16 (br, THF), 69.36 (br, THF), 37.28, 36.38, 36.19,
36.01, 35.90, 35.56, 34.93, 34.82, 34.73, 34.50, 33.25 (iPr-CH), 27.30,
26.60, 26.43, 26.34, 26.06, 25.59, 25.41, 25.09, 24.78, 24.65, 24.58, 24.56,
24.49, 24.46, 24.36, 24.32, 24.24, 24.12, 23.94 ppm (iPr-CH3); 29Si NMR
(60 MHz, [D6]benzene, 25 8C): d = 143.9, 134.5, 44.4 ppm. UV/Vis
(pentane): lmax(e) 415 nm (3200 L mol 1 cm 1).
6: In a glove box, 5 (100 mg, 0.091 mmol) and Me2SnCl2 (21 mg,
0.095 mmol) were placed in an NMR tube and dissolved in
[D6]benzene (0.5 mL). The initially orange solution changed rapidly
to yellow. After centrifugation of the NMR tube to deposit
precipitated MgCl2, NMR spectra were recorded. 1H NMR
(300 MHz, [D6]benzene, 25 8C): d = 7.22, 7.20 (d, 2 H, Ar-H), 7.03,
6.99 (s, 4 H, Ar-H), 6.96, 6.87 (d, 2 H, Ar-H), 4.69, 4.14, 4.06, 4.01, 3.64,
3.48, 2.85–2.61 (hept, 12 H, iPr-CH), 1.72, 1.63, 1.53, 1.52, 1.47, 1.42
(br), 1.36, 1.30, 1.28, 1.24–1.08, 1.01, 0.88, 0.86 (d, 60 H, iPr-CH3), 0.80
(s, 3 H, 2J(1H,117/119Sn) = 45 Hz, SnCH3), 0.73, 0.64, 0.48, 0.45 ppm (d,
12 H, iPr-CH3), 0.23 ppm (s, 3 H, 2J(1H,117/119Sn) = 43 Hz, SnCH3);
13
C NMR (75 MHz, [D6]benzene, 25 8C): d = 155.65, 154.73, 154.71,
154.59, 153.59, 152.23, 152.05, 151.14, 150.85, 149.69, 149.10, 136.92,
136.89, 136.67, 133.51 (Ar-C), 122.46, 122.36, 121.91, 121.48, 121.17,
120.77 (Ar-CH), 38.02, 37.64, 37.48, 36.90, 36.62, 35.99, 34.83, 34.79,
34.74, 34.46 (iPr-CH), 27.76, 26.98, 26.81, 25.90, 25.67 (br), 24.82,
24.75, 24.44, 24.34, 24.23, 24.16, 24.09, 23.96, 21.77 (iPr-CH3), 4.08
(SnCH3), 6.48 ppm (SnCH3); 29Si NMR (60 MHz, [D6]benzene,
25 8C): d = 105.2 (1J(29Si,117/119Sn) = 458/479 Hz, Si=SiTipSn), 92.5
(2J(29Si,117/119Sn) = 218/228 Hz, Si=SiTipSi),
26.2 ppm (1J(29Si,
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 1643 –1645
Angewandte
Chemie
117/119
Sn) = 476/497 Hz, SnSiTip2Si); 119Sn NMR (112 MHz,
[D6]benzene, 25 8C): d = 39.3 ppm (1J(119Sn,29Si) = 479, 497 Hz,
2 119
J( Sn,29Si) = 228 Hz).
Received: November 9, 2005
Published online: February 7, 2006
[12]
.
Keywords: anions · Group 14 elements · magnesium · silicon ·
stannanes
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Rev. 2005, 249, 789; b) A. Sekiguchi, V. Y. Lee, M. Nanjo, Coord.
Chem. Rev. 2000, 210, 11; c) J. Belzner, U. Dehner in The
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2002, 114, 1031; Angew. Chem. Int. Ed. 2002, 41, 989; e) R.
Fischer, D. Frank, W. Gaderbauer, C. Kayser, C. Mechtler, J.
Baumgartner, C. Marschner, Organometallics 2003, 22, 3723;
f) J. Markow, R. Fischer, H. Wagner, N. Noormofidi, J. Baumgartner, C. Marschner, Dalton Trans. 2004, 2166; g) R. Fischer, T.
Konopa, J. Baumgartner, C. Marschner, Organometallics 2004,
23, 1899.
[3] Recent reviews on low-valent silicon: a) V. Y. Lee, A. Sekiguchi,
Organometallics 2004, 23, 2822; b) M. Weidenbruch, Angew.
Chem. 2003, 115, 2322; Angew. Chem. Int. Ed. 2003, 42, 2222;
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[10] B. Bogdanovic, S. T. Liao, R. Mynott, K. Schlichte, U. Westeppe,
Chem. Ber. 1984, 117, 1378.
[11] Crystal structure determination of 5·0.5 C5H12 : orange blocks
from pentane; C70.5H114MgO2Si3, Mr = 1102.2, monoclinic, space
group P21/n; a = 1206.4(2), b = 2641.9(5), c = 2216.5(4) pm, b =
99.08(3)8, V = 6976(2) O 10 30 m3 ; Z = 4, 1calcd = 1.049 g cm 3,
crystal dimensions: 0.60 O 0.50 O 0.50 mm; diffractometer:
STOE IPDS, MoKa radiation, 173 K, 2qmax = 52.748, 71 353
reflections, 13 390 independent (Rint = 0.0520); direct methods;
semiempirical absorption correction (m = 1.17 cm 1); refinement
(against F 2o) with SHELXTL (version 5.1) and SHELXL-97,[15]
779 parameters, 0 restraints, R1 = 0.0510 (I > 2s), wR2 = 0.1362
Angew. Chem. Int. Ed. 2006, 45, 1643 –1645
[13]
[14]
[15]
(all data), GooF = 1.031, max/min residual electron density =
0.679/ 0.263 O 1030 e m 3. CCDC-288946 contains the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Tilley, J. Organomet. Chem. 2000, 603, 185; e) H.-W. Lehner, S.
Scholz, M. Bolte, N. Wiberg, H. NRth, I. Krossing, Eur. J. Inorg.
Chem. 2003, 666.
M. Weidenbruch, The Chemistry of Organic Silicon Compounds,
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C. N. Smit, F. Bickelhaupt, Organometallics 1987, 6, 1156.
G. M. Sheldrick, SHELXTL Version 5.1, Bruker AXS, Madison,
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Germany, 1997.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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