A remarkable base-stabilized bis(silylene) with a silicon(I)-silicon(I) bond

Article abstract of DOI:10.1002/anie.200902995

Singles party: A compound with a Si¹-Si¹ bond was prepared by the reduction of amidinatotrichlorosilane with potassium graphite. There is no multiple-bond character in the Si - Si bond, and the X-ray structural analysis (see picture)

Full text of DOI:10.1002/anie.200902995

Communications
DOI: 10.1002/anie.200902995
Silicon Chemistry
A Remarkable Base-Stabilized Bis(silylene) with a Silicon(I)–Silicon(I)
Bond**
Sakya S. Sen, Anukul Jana, Herbert W. Roesky,* and Carola Schulzke
Dedicated to Professor Dietmar Seyferth on the occasion of his 80th birthday
Double and triple covalent bonds are ubiquitous in carbon
chemistry and have been studied for more than two centuries
but were unusual with its congener in the periodic table,
silicon. Initial attempts to synthesize such compounds were
unsuccessful, resulting in the formation of polymeric sub-
stances. This situation changed when West and co-workers in
In addition to disilenes and disilynes, a number of other
unusual stable compounds with low-coordinate Si atoms have
been described.[9] To our knowledge, only one compound
À
consisting of a Si Si single bond and a lone pair of electrons
on each Si atom was reported to date.[8] The compound was
stabilized by an N-heterocyclic carbene, and one chlorine
atom was attached to each silicon center making the formal
oxidation state of the silicon atoms + I. This situation is
unique, because each Si center which features a lone pair of
electrons is simultaneously involved in bonding. These two
attributes are usually associated with extreme instability. In
view of this chemistry, we became interested in synthesizing a
À
1981 synthesized a compound containing a Si Si double bond
=
(R2Si SiR2, R = Me3C6H2), in which each Si atom has a
formal oxidation state of + II.[1] Key to the discovery of stable
À
compounds containing Si Si double bonds was the protection
of the double bonds by bulky substituents, which provide
kinetic stability. Apeloig et al. showed that silylene has a
singlet ground state, and the 3B1 triplet state lies significantly
higher in energy.[2] Moreover, recent calculations indicate that
the energy difference between the singlet and triplet states of
silylene is around 18–21 kcalmolÀ1.[3] This singlet–triplet
energy difference of the silylene fragments is the main
À
compound with a Si Si single bond stabilized by a mono-
anionic ligand, thus avoiding the lone pair of electrons taking
part in any bonding. We were recently successful in using an
amidinate ligand with tBu substituents on the nitrogen atoms
to stabilize heteroleptic silylenes and a GeI dimer.[10,11] It
seems that such a ligand might also stabilize a SiI compound
=
reason for the weakness of the Si Si double bond. This now
I
I
À
generally accepted model originated from Carter, Goddard,
Malrieu, and Trinquier (CGMT), who described the double-
bond topology as being a function of the energy difference
between the singlet and the triplet state of the carbene-like
fragments formally constituting the double bond.[4,5] Com-
pounds with double-bonded silicon are established and have
been studied in detail during the last decade.[6] In 2004,
Sekiguchi and co-workers as well as Wiberg et al. were
with a Si Si single bond. Our preliminary results in this
direction are reported herein.
The reaction of tert-butylcarbodiimide with one equiv-
alent of PhLi in diethyl ether and subsequent treatment with
SiCl4 afforded [PhC(NtBu)2]SiCl3 (1, Scheme 1). Compound 1
À
successful in isolating compounds containing Si Si triple
ꢀ
bonds (RSi SiR; R = Si(iPr){CH(SiMe3)2}2 and R = SiMe-
(SitBu3)2) in which the formal oxidation state of Si is + I.[7]
Subsequently, Robinson and co-workers synthesized two
À
compounds, one with a Si Si single bond having formal
À
oxidation state + I and another with a Si Si double bond in
À
which the formal oxidation state of Si is 0 (RClSi SiClR and
RSi SiR, R = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-yli-
=
dene).[8]
[*] S. S. Sen, A. Jana, Prof. Dr. H. W. Roesky
Institut fꢀr Anorganische Chemie, Universitꢁt Gꢂttingen
Tammannstrasse 4, 37077 Gꢂttingen (Germany)
Fax: (+49)551-39-3373
Scheme 1. Preparation of compound 2.
was obtained as a colorless crystalline solid in 47% yield, and
its structure was confirmed by NMR spectroscopy, EI mass
spectrometry, and elemental analysis.[10]
E-mail: hroesky@gwdg.de
Prof. Dr. C. Schulzke
School of Chemistry, Trinity College, Dublin
Dublin 2 (Ireland)
Treatment of 1 with two equivalents of potassium in THF
afforded monomeric chlorosilylene.[10] We mentioned earlier
[**] Support of the Deutsche Forschungsgemeinschaft is highly
acknowledged. We also thank the referees for their valuable
suggestions.
I
I
À
that we are interested in preparing a compound with a Si Si
single bond. Therefore, we reacted 1 in a ratio of 2:6
8536
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 8536 –8538

Angewandte
Chemie
(Scheme 1) with potassium graphite (KC8) in THF, which
afforded air-sensitive, orange-red crystals of 2 (5.21% yield).
Compound 2 is highly soluble in solvents such as diethyl ether,
toluene, and THF. It has been characterized by elemental
analysis, spectroscopic methods, and X-ray structural analysis.
equivalents hydrazine and diphosphine.[16] It is interesting to
I
I
À
mention that a bis(carbene) with a C C bond has not been
reported to date.
In conclusion, we have prepared and fully characterized a
I
I
À
compound with a Si Si bond, which is stabilized by bulky
1
The H and 13C NMR spectra of 2 (in [D8]THF) display one
amidinate ligands. Compound 2 exhibits a gauche-bent
À
set of resonances that result from the amidinate ligand. The
resonances show a downfield shift relative to that of 1 and also
from chlorosilylene.[10] The shift is probably due to the lower
oxidation state of the silicon center (+ I in 2) compared to
silylene, where normally a formal oxidation state at the Si
geometry, and the Si Si bond shows no multiple-bond
character. Currently we are exploring the reactivity of
compound 2.
center of + II is observed. The resonance of 2 in the 29Si NMR Experimental Section
All manipulations were carried out in an inert atmosphere of
spectrum in [D8]THF (75.71 ppm) is further downfield than
dinitrogen using standard Schlenk techniques or in a dinitrogen-
filled glove box. Solvents were dried over molecular sieves and
distilled prior to use. The NMR spectra were recorded in C6D6 and
[D8]THF. The chemical shifts d are given relative to SiMe4. The EI
mass spectrum was obtained using a Finnigan MAT 8230 instrument.
Elemental analyses were performed by the Institut fꢁr Anorganische
Chemie, Universitꢂt Gꢃttingen. The melting point was measured in a
sealed glass tube on a Bꢁchi B-540 melting point apparatus and was
uncorrected.
À
that of RClSi SiClR, (d = 38.4 ppm; R = 1,3-bis-(2,6-diiso-
propylphenyl)imidazol-2-ylidene).[8] We were also curious to
measure the NMR spectrum in nonpolar solvent (C6D6), and
in the 29Si NMR spectrum, 2 resonates at d = 76.29 ppm. So
there is very little downfield shift in the 29Si NMR spectrum.
The molecular ion of 2 appeared as the most abundant peak in
the EI mass spectrum at m/z 518.3.
The molecular structure of 2 is shown in Figure 1.[12]
Compound 2 crystallizes in the monoclinic space group
C2/c. The coordination environment of each of the Si atoms
exhibits a distorted tetrahedral geometry. The coordination
2: THF (50 mL) was added to a mixture of 1 (1.49 g, 4.08 mmol)
and potassium graphite (1.65 g, 12.24 mmol) at À788C. The resulting
red mixture was stirred overnight. The solvent was then removed in
vacuo, and the residue was extracted with toluene (50 mL). The
insoluble precipitate was filtered off and the red filtrate was
concentrated to yield orange-red crystals of 2 (0.11 g, 5.21%).
1
M.p.:155–1608C. H NMR (200 MHz, [D8]THF, 258C): d = 1.23 ppm
(s, 36H, tBu), 7.34–7.38 ppm (m, 10H, Ph); 13C{1H} NMR
(75.47 MHz, [D8]THF, 258C): d = 32.19 (CMe3), 53.36 (CMe3),
128.42, 129.91, 130.95, 131.95, 134.35, 136.49, (Ph), 146.32 ppm
(NCN); 29Si{1H} NMR (59.62 MHz, [D8]THF, 258C): d = 75.71 ppm;
1H NMR (200 MHz, C6D6, 258C): d = 1.18 ppm (s, 36H, tBu), 7.22–
7.30 ppm (m, 10H, Ph); 13C{1H} NMR (125.75 MHz, C6D6, 258C): d =
32.01 (CMe3), 53.73 (CMe3), 127.71, 128.19, 128.40, 129.21, 130.32,
135.98 (Ph), 145.67 ppm (NCN); 29Si{1H} NMR (99.36 MHz, C6D6,
258C): d = 76.29 ppm; EI-MS: m/z(%): 518.3 [M+] (100). Elemental
analysis (%) calcd for C30H46N4Si2 (518.33): C 69.44, H 8.94, N 10.80;
found: C 68.26, H 8.63, N 11.06.
Figure 1. ORTEP view (50% ellipsoid probability) of compound 2.
Hydrogen atoms have been removed for clarity. Selected bond lengths
[ꢃ] and angles [8] Si1–Si1A 2.413(2), Si1–N2 1.866(4), Si1–N1
1.874(4), C1–N1 1.467(6), C9–N1 1.331(6); N2-Si1-N1 69.52(18),
N2-Si1-Si1A 102.15(14), C9-N1-C1 130.09(3).
Received: June 3, 2009
Revised: July 22, 2009
Published online: October 2, 2009
Keywords: N ligands · potassium · reduction · silicon
.
sites of the Si centers are each occupied by the N atoms of the
amidinato ligand and by the other Si atom. The lone pair of
each Si atom occupies the remaining coordination site of the
tetrahedral geometry. The X-ray structural analysis of 2
[1] R. West, M. J. Fink, J. Michl, Science 1981, 214, 1343 – 1344.
[2] Y. Apeloig, R. Pauncz, M. Karni, R. West, W. Steiner, D.
Chapman, Organometallics 2003, 22, 3250 – 3256.
À
reveals that the Si Si core is sterically well-shielded by the
[3] Experimental determinations of DES–T in SiH2: a) A. Kasdan, E.
Herbst, W. C. Lineberger, J. Chem. Phys. 1975, 62, 541 – 548;
À
two amidinate ligands. The central Si Si bond (2.413(2) ꢀ) is
only about 0.07 ꢀ longer than the sum of the Si covalent radii
ˇˇ ´
b) J. L. Berkowitz, J. P. Green, H. Cho, R. Ruscic, J. Chem. Phys.
(2.34 ꢀ)[13] and about 0.05 ꢀ longer than the Si Si single bond
À
1987, 86, 1235 – 1248.
in a-silicon (2.36 ꢀ).[14] The value is 5.17% larger than the
longest disilene bond (2.29 ꢀ)[6] and 14.6% longer than the
reported disilyne bond (2.06 ꢀ),[7] and it also compares well
with that of the SiI dimer reported by Robinson et al.
(2.393 ꢀ).[8] The sum of the bond angles of the Si atoms in 2
(282.358 average) compares very well with literature values.[15]
Compound 2 possesses a gauche-bent conformation. This
structural feature of 2 is akin to that of the GeI dimer,[11] which
also favors a gauche-bent geometry, as do the isolobal
[4] a) E. A. Carter, W. A. Goddard III, J. Phys.Chem. 1986, 90, 998 –
1001; b) G. Trinquier, J.-P. Malrieu, J. Am. Chem. Soc. 1987, 109,
5303 – 5315; c) J.-P. Malrieu, G. Trinquier, J. Am. Chem. Soc.
1989, 111, 5916 – 5921.
[5] Reviews elaborating on the CGMT model: a) M. Driess, H.
Grꢁtzmacher, Angew. Chem. 1996, 108, 900 – 929; Angew. Chem.
Int. Ed. Engl. 1996, 35, 828 – 856; b) H. Grꢁtzmacher, T. F.
Fꢂssler, Chem. Eur. J. 2000, 6, 2317 – 2325.
[6] a) N. Tokitoh, H. Suzuki, R. Okazaki, K. Ogawa, J. Am. Chem.
Soc. 1993, 115, 10428 – 10429; b) H. Suzuki, N. Tokitoh, R.
Angew. Chem. Int. Ed. 2009, 48, 8536 –8538
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8537

Communications
Okazaki, Bull. Chem. Soc. Jpn. 1996, 68, 2471 – 2478; c) M. Kira,
Angew. Chem. 2006, 118, 4347 – 4348; Angew. Chem. Int. Ed.
2006, 45, 4241 – 4242.
T. Iwamoto, Adv. Organomet. Chem. 2006, 54, 73 – 75; d) V. Y.
Lee, A. Sekiguchi, Organometallics 2004, 23, 2822 – 2834; e) M.
Weidenbruch, Angew. Chem. 2003, 115, 2322 – 2324; Angew.
Chem. Int. Ed. 2003, 42, 2222 – 2224; f) A. Sekiguchi, V. Y. Lee,
Chem. Rev. 2003, 103, 1429 – 1448; g) M. Weidenbruch, Organo-
metallics 2003, 22, 4348 – 4360; h) R. West, Polyhedron 2002, 21,
467 – 472; i) M. Weidenbruch, J. Organomet. Chem. 2002, 646,
39 – 52; j) M. Kira, Pure Appl. Chem. 2000, 72, 2333 – 2342; k) I.
Bejan, D. Scheschkewitz, Angew. Chem. 2007, 119, 5885 – 5888;
Angew. Chem. Int. Ed. 2007, 46, 5783 – 5786; l) A. Sekiguchi, S.
Inoue, M. Ichinohe, Y. Arai, J. Am. Chem. Soc. 2004, 126, 9626 –
9629; m) D. Scheschkewitz, Chem. Eur. J. 2009, 15, 2476 – 2485;
n) T. Sasamori, K. Hironaka, Y. Sugiyama, N. Takagi, S. Nagase,
Y. Furukawa, N. Tokitoh, J. Am. Chem. Soc. 2008, 130, 13856 –
13857.
[10] a) C.-W. So, H. W. Roesky, J. Magull, R. B. Oswald, Angew.
Chem. 2006, 118, 4052 – 4054; Angew. Chem. Int. Ed. 2006, 45,
3948 – 3950; b) C.-W. So, H. W. Roesky, P. M. Gurubasavaraj,
R. B. Oswald, M. T. Gamer, P. G. Jones, S. Blaurock, J. Am.
Chem. Soc. 2007, 129, 12049 – 12054; c) S. Nagendran, H. W.
Roesky, Organometallics 2008, 27, 457 – 492.
[11] S. Nagendran, S. S. Sen, H. W. Roesky, D. Koley, H. Grubmꢁller,
A. Pal, R. Herbst-Irmer, Organometallics 2008, 27, 5459 – 5463.
[12] a) G. M. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112 – 122;
b) CCDC 734289 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.
[13] J. Y. Corey in The Chemistry of Organic Silicon Compounds,
Vol. 1 (Eds.: S. Patai, Z. Rappoport), Wiley, Chichester, 1989,
chap. 1.
[14] R. Nesper, A. Currao, S. Wengert in Organosilicon Chemistry II:
From Molecules to Materials (Eds.: N. Auner, J. Weis), Wiley-
VCH, Weinheim, 1996, p. 469.
[7] a) A. Sekiguchi, R. Kinjo, M. Ichinohe, Science 2004, 305, 1755 –
1757; b) N. Wiberg, S. K. Vasisht, G. Fischer, P. Mayer, Z. Anorg.
Allg. Chem. 2004, 630, 1823 – 1828.
[8] Y. Wang, Y. Xie, P. Wei, R. B. King, H. F. Schaefer III, P. von R.
Schleyer, G. H. Robinson, Science 2008, 321, 1069 – 1071.
[9] a) P. P. Gaspar, R. West in The Chemistry of Organic Silicon
Compounds, Vol. 2 (Eds.: Z. Rappoport, Y. Apeloig), Wiley,
Chichester, 1998, chap. 43; b) M. Weidenbruch in The Chemistry
of Organic Silicon Compounds, Vol. 3 (Eds.: Z. Rappoport, Y.
Apeloig), Wiley, Chichester, 2001, chap. 5; c) H. Ottosson, P. G.
Steel, Chem. Eur. J. 2006, 12, 1576 – 1585; d) M. Weidenbruch,
[15] G. Molev, D. Bravo-Zhivotovskii, M. Karni, B. Tumanskii, M.
Botoshansky, Y. Apeloig, J. Am. Chem. Soc. 2006, 128, 2784 –
2785.
[16] A. H. Cowley, W. D. White, M. C. Damasco, J. Am. Chem. Soc.
1969, 91, 1922 – 1928.
8538
www.angewandte.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 8536 –8538