The first silastannene 〉Si=Sn〈: A new doubly-bonded system of heavier group 14 elements

Article abstract of DOI:10.1021/ja021077l

The reaction of bis[di-tert-butyl(methyl)silyl]dilithiosilane 1 with dichlorobis(2,4,6-triisopropylphenyl)stannane in THF at room temperature yielded highly air- and moisture-sensitive deep violet crystals of 1,1-bis[di-tert-butyl(methyl)silyl]-2,2-bis(2,

Full text of DOI:10.1021/ja021077l

Published on Web 11/20/2002
The First Silastannene >SidSn<: A New Doubly-Bonded System of Heavier
Group 14 Elements
Akira Sekiguchi,* Rika Izumi, Vladimir Ya. Lee, and Masaaki Ichinohe
Department of Chemistry, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
Received August 12, 2002
After the breaking of the so-called “double-bond rule”1 by
the preparation of the first heavy alkene analogues stable both in
the solid state and solution, silene >SidC<2 and disilene >Sid
Si<3 in 1981, many stable metallenes >EdC< and dimetallenes
>EdE< of group 14 elements (E ) heavier group 14 element)
have been synthesized to date.4 However, the chemistry of the stable
heteronuclear dimetallenes >EdE′< composed of different group
14 elements heavier than carbon is still not very common,5,6
although the homonuclear dimetallenes (i.e., disilenes, digermenes,
distannenes, diplumbenes) are represented by a number of examples
reported by several research groups.4 Recently, we reported the
synthesis of a 1,2-disila-3-germacyclopenta-2,4-diene derivative,
which has SidGe and CdC double bonds with a silole structure,
and determined the SidGe double bond length for the first time.7
Very recently, we have also demonstrated the utility of 1,1-
dilithiosilanes in the preparation of a variety of doubly bonded
derivatives of heavier group 14 elements.8 We now report on a
new application of the dilithiosilane derivative, resulting in the
successful isolation of the first silastannene with a >SidSn<
double bond. Compounds of this type, both stable and transient,
were completely unknown until our study.9
The reaction of bis[di-tert-butyl(methyl)silyl]dilithiosilane 18 with
dichlorobis(2,4,6-triisopropylphenyl)stannane10 in dry THF pro-
ceeded cleanly to form the corresponding coupling product, 1,1-
bis[di-tert-butyl(methyl)silyl]-2,2-bis(2,4,6-triisopropylphenyl)-1-
sila-2-stannaethene 2, as deep violet crystals in 50% isolated yield
(Scheme 1).11
Figure 1. ORTEP drawing of 2. Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å): Sn(1)-Si(1) ) 2.4188(14), Sn(1)-C(19) )
2.190(4), Sn(1)-C(34) ) 2.181(4), Si(1)-Si(2) ) 2.3647(19), Si(1)-Si-
(3) ) 2.3692(19); Selected bond angles (deg): C(34)-Sn(1)-C(19) )
102.64(17), C(34)-Sn(1)-Si(1) ) 126.54(13), C(19)-Sn(1)-Si(1) )
129.55(13), Si(2)-Si(1)-Si(3) ) 138.71(8), Si(2)-Si(1)-Sn(1) ) 110.61-
(6), Si(3)-Si(1)-Sn(1) ) 106.27(6).
Scheme 1
Figure 2. The trans-bent angles (δ) around Si and Sn atoms in silastannene
2 (R ) SiMetBu2, Tip ) 2,4,6-triisopropylphenyl).
sp2 Si atoms connected with Si-substituents and downfield-shifted
signals for sp2 Si atoms connected with aryl substituents.8b
The crystal structure of 2 was determined by X-ray crystal-
lography (Figure 1).14 The most important result is the SidSn
double bond length (2.4188(14) Å), which was determined for the
first time. This value is intermediate between the typical SidSi
(2.138-2.289 Å)15 and SndSn (2.59-3.087 Å)4e double bond
lengths, being ca. 7% shorter than the usual Si-Sn single bond
length of 2.60 Å.16 The twisting angle is 34.6°, as determined by
the angles between the mean plane of Si1-Si2-Si3 and Sn1-
C19-C34. As expected, the SidSn double bond is trans-bent, but
the bending angles around the Si and Sn atoms are quite unusual:
26.2° for the Si atom and 9.6° for the Sn atom (Figure 2).
It is well-known that disilenes normally prefer to have a planar
or near-planar geometry around the SidSi double bond,4e whereas
distannenes tend to have a highly pronounced trans-bent geometry
for the SndSn double bond.4e In the case of 2 we have the
Silastannene 2, representing the first example of a compound
with a >SidSn< double bond, was entirely characterized by a range
of spectral data, the most informative being the 29Si and 119Sn NMR
spectra. Thus, in the 119Sn NMR spectrum one resonance signal
was observed at +516.7 ppm, typical for an sp2 hybridized Sn
atom.12 In the 29Si NMR spectrum, two resonance signals at 27.4
and 27.6 were observed with an intensity ratio of 1:2, from which
it can be deduced that the former belongs to a doubly bonded Si
atom and the latter to the two SiMetBu2 groups. The resonance of
the sp2 Si atom is greatly shifted upfield compared with the vast
majority of other compounds with a doubly bonded Si atom.13 The
same phenomenon has been shown in the case of unsymmetrically
substituted disilenes and germasilene: upfield-shifted signals for
* To whom correspondence should be addressed. E-mail: sekiguch@
staff.chem.tsukuba.ac.jp.
9
14822
J. AM. CHEM. SOC. 2002, 124, 14822-14823
10.1021/ja021077l CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
References
(1) (a) Jutzi, P. Angew. Chem., Int. Ed. Engl. 1975, 14, 232. (b) Gusel’nikov,
L. E.; Nametkin, N. S. Chem. ReV. 1979, 79, 529.
(2) Brook, A. G.; Abdesaken, F.; Gutekunst, B.; Gutekunst, G.; Kallury, R.
K. J. Chem. Soc., Chem. Commun. 1981, 191.
(3) West, R.; Fink, M. J.; Michl, J. Science 1981, 214, 1343.
(4) For the recent reviews on metallenes and dimetallenes of group 14
elements, see: (a) Baines, K. M.; Stibbs, W. G. AdV. Organomet. Chem.
1996, 39, 275. (b) Okazaki, R.; West, R. AdV. Organomet. Chem. 1996,
39, 231. (c) Kaftory, M.; Kapon, M.; Botoshansky, M. In The Chemistry
of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y. Eds.; John
Wiley & Sons Ltd.: New York, 1998; Vol. 2, Part 1, Chapter 5. (d)
Weidenbruch, M. Eur. J. Inorg. Chem. 1999, 373. (e) Power, P. P. Chem.
ReV. 1999, 99, 3463. (f) Escudie´, J.; Ranaivonjatovo, H. AdV. Organomet.
Chem. 1999, 44, 113. (g) Weidenbruch, M. In The Chemistry of Organic
Silicon Compounds; Rappoport, Z., Apeloig, Y. Eds.; John Wiley & Sons
Ltd.: New York, 2001; Vol. 3, Chapter 5.
(5) For other examples of heteronuclear dimetallenes of heavier group 14
elements, which were stable only at low temperatures and were character-
ized by low-temperature NMR spectra and trapping reactions without
isolation: For germasilene, see: (a) Baines, K. M.; Cooke, J. A.
Organometallics 1991, 10, 3419. (b) Baines, K. M.; Cooke, J. A.
Organometallics 1992, 11, 3487. For germastannene, see: (c) Chaubon,
M.-A.; Escudie´, J.; Ranaivonjatovo, H.; Satge´, J. J. Chem. Soc., Chem.
Commun. 1996, 2621.
Scheme 3
(6) For 2-disilagermirene with a SidGe double bond, see: Lee, V. Ya.;
Ichinohe, M.; Sekiguchi, A.; Takagi, N.; Nagase, S. J. Am. Chem. Soc.
2000, 122, 9034.
completely opposite tendency. Such an unusual structural feature
can be rationalized by the polar SidSn double bond produced by
the difference in the substituent electronegativity between the silyl
and aryl groups. The electronegativities of Si and Sn atoms are
almost equal,17 but the electropositive silyl groups on the sp2 Si
atom and the electronegative aryl groups on the Sn atom cause the
great polarity of the double bond, Siδ-dSnδ+. The trans-bent
structure of dimetallenes of the heavier group 14 elements is well-
explained as a dimer of the corresponding divalent species by the
donor-acceptor interaction. If an unsymmetrical donor-acceptor
interaction operates in the doubly bonded system of heavier group
14 elements, the double bond would be polarized with different
bending angles; a negative moiety is more bent than a positive one
(Scheme 2).18 Therefore, the electronegative sp2 silicon part is more
bent than the Sn part in 2. Indeed, a calculation on the model
silastannene (H3Si)2SidSnPh2 (B3LYP/DZd level) revealed that the
SidSn double bond is highly polarized (NPA analysis): -0.536
for the Si atom and +1.400 for the Sn atom.19 The geometry of
the SidSn double bond was also well reproduced by the calcula-
tions, except for the twisting angle (5°): a SidSn double bond
length of 2.445 Å with bending angles of 17.8° around the Si atom
and 7.2° around the Sn atom.
(7) Lee, V. Ya.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2000, 122,
12604.
(8) (a) Sekiguchi, A.; Ichinohe, M.; Yamaguchi, S. J. Am. Chem. Soc. 1999,
121, 10231. (b) Ichinohe, M.; Arai, Y.; Sekiguchi, A.; Takagi, N.; Nagase.
S. Organometallics 2001, 20, 4141.
(9) To our knowledge, there is only one paper in which the intermediate
formation of a compound with a SidSn double bond was suggested. Drost,
C.; Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F. Chem. Commun. 1997,
1845. However, neither spectroscopic nor chemical reactivity evidence
was given to prove this structure.
(10) Masamune, S.; Sita, L. R. J. Am. Chem. Soc. 1985, 107, 6390.
(11) Spectral data for 2: violet crystals; mp 78-79 °C. 1H NMR (C6D6, δ)
0.26 (s, 6 H), 1.03 (d, J ) 6.6 Hz, 12 H), 1.17 (d, J ) 6.6 Hz, 12 H), 1.27
(s, 36 H), 1.41 (d, J ) 6.6 Hz, 12 H), 2.73 (sept, J ) 6.6 Hz, 2 H), 3.74
(sept, J ) 6.6 Hz, 4 H), 7.09 (s, 4 H, ArH); 13C NMR (C6D6, δ) -3.0,
22.0, 24.1, 25.8, 30.7, 34.6, 40.4, 122.6, 150.1, 154.2, 154.6; 29Si NMR
(C6D6, δ) 27.4 (SidSn), 27.6 (SiMetBu2); 119Sn NMR (C6D6, δ) 516.7;
UV/vis (hexane) λmax/nm (ꢀ) 262 (21300), 337sh (2700), 545 (4100). Anal.
Calcd for C48H88Si3Sn: C, 66.41; H, 10.22. Found: C, 66.11; H, 10.00.
(12) The typical 119Sn NMR resonances of doubly bonded Sn atoms lie in the
region above +400 ppm, see ref 4a.
(13) The typical 29Si NMR resonances for doubly bonded Si atoms lie in the
region of +49 to +155 ppm, see ref 4b.
(14) Crystal data for 2 at 120 K: MF ) C48H88Si3Sn, MW ) 868.14,
monoclinic, P21/n, a ) 10.9180(9), b ) 18.9170(11) Å, c ) 25.0480(18)
Å, â ) 94.397(4)°, V ) 5158.1(6) Å3, Z ) 4, Dcalcd ) 1.118 g‚cm-3. The
final R factor was 0.0680 for 8168 reflections with Io > 2σ(Io) (Rw
0.1630 for all data 12279 reflections), GOF ) 1.024.
)
(15) See ref 4c and also: Schmedake, T. A.; Haaf, M.; Apeloig, Y.; Mu¨ller,
T.; Bukalov, S.; West, R. J. Am. Chem. Soc. 1999, 121, 9479.
(16) Mackay, K. M. In The Chemistry of Organic Germanium, Tin and Lead
Compounds; Patai, S., Ed.; John Wiley & Sons Ltd.: New York, 1995;
Chapter 2.
(17) The electronegativities of Si and Sn atoms according to the Pauling and
the Allred-Rochow electronegativity scales are 1.90 vs 1.96 and 1.90
vs. 1.93, respectively.
(18) The structure on the bottom in Scheme 2 represents an extreme case of
such interaction with a planar geometry around a positively polarized Sn
atom and 90° bending geometry around a negatively polarized Si atom.
The SidSn double bond in 2 is highly reactive and easily
undergoes addition reactions. Thus, 2 reacted with PhOH and PhSH
at room temperature to form the corresponding addition products
3 and 4 in 51 and 26% yields, respectively (Scheme 3).20 The
regioselectivity of PhEH (E ) O, S) addition corresponds well with
the polarity of the SidSn double bond in 2.
Supporting Information Available: Experimental procedures and
spectral data of 3 and 4, tables of crystallographic data including atomic
positional and thermal parameters for 2 (PDF/CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(19) The calculations were carried out using the Gaussian 98 program.
(20) For the experimental procedures and spectral data of 3 and 4, see the
Supporting Information.
JA021077L
9
J. AM. CHEM. SOC. VOL. 124, NO. 50, 2002 14823