1,2-Dialkynyldisilenes: Silicon analogues of (E)-enediyne

Article abstract of DOI:10.1039/c0cc00406e

Reductions of bulky alkynyl-substituted dihalosilanes, BbtSiX ₂(CCR) (Bbt = 2,6-bis[bis(trimethylsilyl)methyl]-4- [tris(trimethylsilyl)methyl]phenyl, R = SiMe₃ or Ph groups, X = halogen), with 2.0 equiv. of lithium naphthalenide gave

Full text of DOI:10.1039/c0cc00406e

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1,2-Dialkynyldisilenes: silicon analogues of (E)-enediynew
Takahiro Sato, Yoshiyuki Mizuhata and Norihiro Tokitoh*
Received 16th March 2010, Accepted 22nd April 2010
First published as an Advance Article on the web 13th May 2010
DOI: 10.1039/c0cc00406e
Reductions of bulky alkynyl-substituted dihalosilanes,
BbtSiX2(CRCR) (Bbt = 2,6-bis[bis(trimethylsilyl)methyl]-4-
[tris(trimethylsilyl)methyl]phenyl, R = SiMe3 or Ph groups,
X = halogen), with 2.0 equiv. of lithium naphthalenide gave the
first stable 1,2-dialkynyl-substituted disilenes, which constituted
extended p-conjugated systems between SiQSi bond and
ethynylene units.
substituent(s) has been reported to date.5 Most likely, the
difficulty of the synthesis and isolation of such metallenes is
due to the inherent high reactivity of the dimetallene moiety
toward an alkynyl substituent6 and the sterically small size of
an alkynyl group as a steric protection group. We have
synthesized and isolated the stable 1,2-dibromodimetallenes
[Bbt(Br)MQM(Br)Bbt, M = Si7 or Ge8] and 1,2-diaryl-
substituted dimetallynes (BbtMRMBbt, M = Si7 or Ge9)
by taking advantage of the effective steric protection group,
2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]-
phenyl (Bbt). All of them are highly reactive low-coordinated
heavier group 14 element compounds. These results strongly
suggest that the Bbt group can stabilize disilenes bearing a
rather small substituent such as an alkynyl group. Based on
this observation, we have initiated the syntheses of stable
1,2-dialkynyldisilenes, 1a and 1b, bearing Bbt groups.
Since the isolation of the first molecular compound with a
SiQSi bond (disilene) by West et al. in 1981,1 numerous
disilenes that are kinetically stabilized by bulky substituents
have been reported.2 Recently, several new disilene systems
have been designed and investigated to evaluate the conjuga-
tion between the SiQSi unit and the connected aryl sp2-carbon
conjugated systems.3 Although those disilenes exhibited
optically and electrochemically unique properties, such
p-conjugated systems are structurally limited; on the other
hand, a diverse array of all-the-carbon conjugated systems has
been reported: for example, conjugated 3-ene-1,5-diyne units
are important building blocks in the area of synthetic polymers
and nanostructures using the electronic structures of alkynyl
groups. Therefore, the synthesis and isolation of the alkynyl-
substituted disilenes––the silicon analogues of enediynes––are
attractive from the viewpoint of elucidation of the conjugation
between the SiQSi and sp-carbon units. We present herein the
syntheses, structures, and properties of novel p-conjugated
1,2-dialkynyldisilenes.
To investigate the influence of the terminal groups,
trimethylsilyl- and phenylethynylacetylene were used to
synthesize the suitable precursors, dihalosilanes 7 and 8, the
synthetic procedures of which are summarized in Scheme 1. In
the reaction of 6a with NBS in CCl4 [conditions (f)], we
expected the generation of dibromosilane 8a, but obtained
only dichlorosilane 7a. In addition, the reaction of 6a with
NCS in CCl4 did not afford 7a.10 On the other hand, the
reactions of 6a and 6b with NBS in benzene afforded the
corresponding dibromosilanes 8a and 8b. As the final step,
according to the well-established procedure for the syntheses
of disilenes, dihalosilanes 7a, 8a, and 8b were reduced with
2.0 mol equiv. of lithium naphthalenide in THF at ꢀ78 1C to
afford the title compounds, 1,2-diethynyldisilenes 1, as stable
crystals (1a, yellow, 23% from 7a, 53% from 8a; 1b, orange,
41% from 8b) (Scheme 2).z In their 13C NMR spectra, the
signals assignable to the CRC units of 1a (112.2, 116.7 ppm)
and 1b (95.0, 119.6 ppm) appeared in the lower field
compared to the those of related enediyne frameworks
[e.g., (E)-R–CRC–(H)CQC(H)–CRC–R: 102.9, 103.9 ppm
(R = SiMe3), 88.1, 94.9 ppm (R = Ph)].11 In their 29Si NMR
spectra, the signals attributable to the sp2 Si atoms of 1a and
1b appeared at 44.6 and 42.6 ppm, respectively. To the best of
our knowledge, these values are the most high-field shifted
among those of the carbon-substituted and acyclic disilenes.
For example, tetraaryl- and tetraalkyl-substituted, acyclic
disilenes exhibited 29Si NMR signals of their sp2 Si atoms at
57–73 and 90–103 ppm,2 respectively. Probably, the noticeable
high-field shift of the central Si atoms in 1 results from the
magnetic anisotropy effect of their CRC moieties. Actually,
the high-field shift caused by the effect of an alkynyl group
was observed for 29Si NMR signals of sp3-silicon compounds
[e.g., trimethylsilylacetylene (ꢀ17.3 ppm in C6D6) compared
with tetramethylsilane (0.0 ppm in C6D6)].
Only one example of the heavier group 14 element
(Si, Ge, Sn, Pb) analogues of eneyne derivatives has been
reported in the literature. Weidenbruch et al. reported the
synthesis and isolation of ethynylene-bridged bis(germaethene)s
[Ar2GeQC(R)CRCC(R)QGeAr2; Ar = 4,5,6-trimethyl-2-
tert-butylphenyl, R = n-Bu or Ph] and experimentally
revealed the p-conjugation between the GeQC and CRC
units.4 However, no dimetallene systems bearing (an) alkynyl
Institute for Chemical Research, Kyoto University, Gokasho, Uji,
Kyoto, 611-0011, Japan. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp;
Fax: +81-774-38-3209; Tel: +81-774-38-3200
w Electronic supplementary information (ESI) available: Experimental
procedures and physical properties for new compounds and the results
of the theoretical calculations. CCDC 770194 and 770195. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0cc00406e
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Fig. 1 Thermal ellipsoid plots of 1a (left) and 1b (right) are shown at
the 50% probability level. The hydrogen atoms and one part of
disorders on the phenylethynyl groups in 1b were omitted for clarity.
Selected bond lengths (A) and bond angles (1) for 1a: Si1–Si1*
2.202(2), Si1–C1 1.811(4), Si1–C(Ar) 1.881(4), C1–C2 1.213(6),
C2–Si2 1.842(5), Si1*–Si1–C1 116.04(15), Si1*–Si1–C(Ar) 131.80(14),
C(Ar)–Si1–C1 111.18(17), Si1–C1–C2 176.4(4), C1–C2–Si2 176.2(4).
For 1b: Si1–Si1* 2.1871(10), Si1–C1 1.8029(19), Si1–C(Ar) 1.8899(17),
C1–C2 1.214(3), Si1*–Si1–C1 115.98(7), Si1*–Si1–C(Ar) 131.53(6),
C(Ar)–Si1–C1 110.94(8), Si1–C1–C2 175.65(17).
Scheme 1 Preparation of alkynyl-substituted dihalosilanes 7 and 8.
Reagents and conditions: (a) tert-BuLi (2.1 eq), THF, ꢀ78 1C.
(b) HSi(OMe)3, THF, ꢀ78 1C to r.t. (c) BBr3 (1.0 eq), hexane, r.t.
(d) RCRCLi, THF, ꢀ50 1C. (e) LiAlH4, Et2O, 0 1C. (f) NBS, CCl4,
r.t. (g) NBS, benzene, 6 1C.
Crystals of 1a and 1b suitable for X-ray diffraction analysis
were obtained by slow evaporation from benzene/THF and
hexane solution, respectively. The molecular structures of 1a
and 1b are presented in Fig. 1.y Their structures have a centre
and an axis of symmetry at the middle point of the Si1–Si1*
bond. For 1b, the phenylethynyl groups were disordered
with a 1 : 1 ratio (See Fig. S4, ESIw). The Si1–Si1* bonds
[1a: 2.202(2) A, 1b: 2.1871(10) A] are shorter than those
reported for typical Si–Si single-bond compounds (2.33–2.37 A)12
and within the range reported for carbon-substituted disilenes.2
The trans-bent angles, which are defined as angles between
the respective axes of SiQSi bonds and the plane of
C(Aripso)–Si(1)–C(1), were 8.91 (1a) and 11.31 (1b). The bond
lengths of C(1)–C(2) [1a, 1.213(6) A; 1b, 1.214(3) A] are almost
equal to that of the C(sp)–C(sp) bond in bis(trimethylsilyl)-
acetylene [1.208(3) A].13 The structural parameters for the
CRC–SiQSi–CRC skeletons of 1a and 1b are almost
identical and neither of them shows apparent elongation of
their SiQSi and CRC bonds. It is noteworthy that 1b has
sufficient planarity, giving rise to conjugation between the
SiQSi bond and phenyl moieties.
In the UV-vis spectra of 1a and 1b in hexane, the longest
absorption maxima (lmax) of 1a and 1b appeared at 437 nm
(e = 2.4 ꢂ 104) and 469 nm (e = 3.1 ꢂ 104), respectively
(Fig. 2). Compared with lmax of trimethylsilylethynyl-
substituted disilene 1a, that of the phenylethynyl-substituted
disilene 1b showed 32 nm of red-shift together with increased
molar absorptivity. In addition, the lmax of 1b is longer
than those of tetraaryl-substituted disilenes. For example,
Mes2SiQSiMes2 and Tip2SiQSiTip2 exhibited their lmax
values at 420 and 432 nm, respectively.2 These results strongly
demonstrate the effective p-conjugation between the SiQSi unit
and two phenyl groups of 1b through the ethynylene moieties.
To evaluate the conjugation between the SiQSi and CRC
(and Ph in 1b) moieties, DFT calculations were conducted
Fig. 2 UV-vis spectra of 1a (grey) and 1b (black) in hexane at room
temperature.
using simplified models 10 bearing 2,6-dimethylphenyl
groups instead of Bbt groups. The HOMO and LUMO of 10
are shown in Fig. 3. The HOMOs of 1a0 and 1b0 were found
to localize not only on their p orbitals of SiQSi bonds,
but also on the two ethynylene units. The HOMO and
LUMO of 1b0 spread to the phenyl rings through the
ethynylene units. The TD-DFT calculations14 for 1a0
and 1b0 [TD-B3LYP/6-311+G(2df,2p)//B3LYP/6-31G(d)]
indicated values of 449 nm (f = 0.4042; 1a0) and 511 nm
(f = 0.7021; 1b0) for their HOMO–LUMO transitions,
respectively, supporting the experimental results of UV/vis
spectroscopy for 1a and 1b.15
In conclusion, we synthesized and isolated the first
stable 1,2-dialkynyldisilenes 1 and elucidated their structures
using X-ray crystallography. In the UV/vis spectrum of 1, the
absorption maximum of 1b was remarkably more red-shifted
than that of 1a, indicating the p-conjugation between the
SiQSi bond and phenyl groups through the CRC bonds.
This result was supported by the results of MO calculations
of 1b. Notwithstanding the different energy levels of p-orbitals
between SiQSi and CRC bonds, these showed that conjuga-
tion was possible between SiQSi and CRC units. With a view
to developing the chemistry of novel p-conjugated systems
containing (a) disilene unit(s), substitution reactions of the
Scheme 2 Syntheses of 1,2-diethynyldisilenes 1.
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y Crystal data for 1a: C70H152Si18, M = 1499.54; triclinic; space group
Pꢀ1 (no. 2); a = 13.4469(16), b = 13.4846(13), c = 15.106(2) A; a =
69.666(5), b = 68.255(8), g = 72.742(8)1; V = 2340.9(5) A3; Z = 1;
Dcalcd. = 1.064 g cmꢀ3; m = 0.277 mmꢀ1; 2ymax = 501; T = 103 K;
R1 [I > 2s(I)] = 0.0706; wR2 (all data) = 0.2022; GOF = 1.128 for
17115 reflections and 421 parameters.
Crystal data for 1b: C76H144Si16, M = 1507.35; monoclinic;
space group C2/c (no. 15); a = 32.0708(5), b = 13.4092(2), c =
23.4405(9) A; b = 114.0243(16)1; V = 9207.2(3) A3; Z = 4; Dcalcd.
=
1.087 g cmꢀ3; m = 0.257 mmꢀ1; 2ymax = 501; T = 103 K;
R1 [I > 2s(I)] = 0.0322; wR2 (all data) = 0.0843; GOF = 1.019
for 38386 reflections and 477 parameters.
CCDC 770194 and 770195.w
1 R. West, M. J. Fink and J. Michl, Science, 1981, 214, 1343.
2 (a) M. Kira and T. Iwamoto, Adv. Organomet. Chem., 2006, 54, 73;
(b) T. Sasamori and N. Tokitoh, in Encyclopedia of
Inorganic Chemistry, ed. R. B. King, Wiley, Chichester, UK,
2nd edn, 2005, pp. 1698; (c) M. Weidenbruch, in The Chemistry
of Organic Silicon Compounds, ed. Z. Rappoport and Y. Apeloig,
Wiley, Chichester, UK, vol. 3, 2001, p. 391.
3 (a) I. Bejan and D. Scheschkewitz, Angew. Chem., Int. Ed., 2007,
46, 5783; (b) A. Fukazawa, Y. Li, S. Yamaguchi, H. Tsuji and
K. Tamao, J. Am. Chem. Soc., 2007, 129, 14164; (c) R. Kinjo,
M. Ichinohe, A. Sekiguchi, N. Takagi, M. Sumimoto and
S. Nagase, J. Am. Chem. Soc., 2007, 129, 7766; (d) T. Sasamori,
A. Yuasa, Y. Hosoi, Y. Furukawa and N. Tokitoh, Organo-
metallics, 2008, 27, 3325; (e) T. Iwamoto, M. Kobayashi,
K. Uchiyama, S. Sasaki, S. Nagendran, H. Isobe and M. Kira,
J. Am. Chem. Soc., 2009, 131, 3156.
4 (a) F. Meiners, W. Saak and M. Weidenbruch, Organometallics,
2000, 19, 2835; (b) F. Meiners, D. Haase, R. Koch, W. Saak and
M. Weidenbruch, Organometallics, 2002, 21, 3990; (c) B. Pampuch,
W. Saak and M. Weidenbruch, J. Organomet. Chem., 2006, 691,
3540.
5 A few compounds containing Si(sp2)–C(sp) bond have been
reported. (a) G. E. Miracle, J. L. Ball, D. Powell and R. West,
J. Am. Chem. Soc., 1993, 115, 11598; (b) T. Takeuchi, M. Ichinohe
and A. Sekiguchi, J. Am. Chem. Soc., 2008, 130, 16848.
6 For example, disilenes reportedly react with some acetylenes to
give the corresponding [2+2] cycloaddition products, see:
(a) D. J. De Young, M. J. Fing, J. Michl and R. West, Main
Group Met. Chem., 1987, 1, 19; (b) N. Wiberg, W. Niedermayer
and K. Polborn, Z. Anorg. Allg. Chem., 2002, 628, 1045.
7 T. Sasamori, K. Hironaka, Y. Sugiyama, N. Takagi, Y. Hosoi,
S. Nagase, Y. Furukawa and N. Tokitoh, J. Am. Chem. Soc., 2008,
130, 13856.
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metallics, 2005, 24, 3309.
9 Y. Sugiyama, T. Sasamori, Y. Hosoi, Y. Furukawa, N. Takagi,
S. Nagase and N. Tokitoh, J. Am. Chem. Soc., 2006, 128, 1023.
10 Wiberg’s group has reported the similar chlorination of Si–H
by using NBS/CCl4 or Br2/CCl4. N. Wiberg, H. Auer,
¨
W. Niedermayer, H. Noth, H. Schwenk-Kircher and K. Polborn,
J. Organomet. Chem., 2000, 612, 141.
11 K. Peter, C. Volhardt and L. S. Winn, Tetrahedron Lett., 1985, 26,
709.
12 W. S. Sheldrick, in The Chemistry of Organic Silicon Compounds
Part 1, ed. S. Patai and Z. Rappoport, John Wiley & Sons,
Chichester, New York, 1989, p. 249.
13 J. Bruckmann and C. Kruger, Acta Crystallogr., Sect. C: Cryst.
Struct. Commun., 1997, 53, 1845.
14 (a) R. E. Stratmann, G. E. Scuseria and M. J. Frisch, J. Chem.
Phys., 1998, 109, 8218; (b) R. Bauernschmitt and R. Ahlrichs,
Chem. Phys. Lett., 1996, 256, 454; (c) M. E. Casida, C. Jamorski,
K. C. Casida and K. D. R. Salahub, J. Chem. Phys., 1998, 108,
4439.
15 Based on results of TD-DFT calculations, the characteristic
shoulder peak around 400 nm in the UV/vis spectrum of 1b is
attributable to that of HOMO–LUMO+2 transition. Details
of results and selected MOs of 1a and 1b are summarized in the
ESI.
Fig. 3 Plots of HOMOs (a, 1a0; c, 1b0) and LUMOs (b, 1a0; d, 1b0) at
the B3LYP/6-31G(d) (isosurface at ꢃ0.02 a.u).
trimethylsilyl groups on the sp carbons of 1a are currently
being studied.
This work was supported by Grants-in-Aid for Creative
Scientific Research (No. 17GS0207), Science Research on
Priority Areas (No. 20036024, ‘‘Synergy of Elements’’), Young
Scientist (B) (No. 21750042), and the Global COE Program
(B09, ‘‘Integrated Materials Science’’) from the Ministry of
Education, Culture, Sports, Science and Technology, of
Japan. Y.M. thanks Mitsubishi Chemical Corp. and the
Society of Synthetic Organic Chemistry, Japan, for financial
support.
Notes and references
z Synthesis of diethynyldisilene 1a. To a solution of 8a (1.24 g,
1.36 mmol) in THF (15 ml) was added a THF solution of lithium
naphthalenide (0.50 M, 5.5 mL, 2.7 mmol) dropwise at ꢀ78 1C. After
stirring at ꢀ40 1C for 2 h, the reaction mixture was allowed to warm to
room temperature and concentrated in vacuo. Hexane was added to
the residue and filtered through a plug of Celites. The filtrate was
dried under reduced pressure and recrystallized from hexane several
times to afford 1a (536 mg, 0.358 mmol, 53%) as yellow crystals.
Similarly, using 7a and 8b as starting materials, 1a and 1b were
isolated, respectively, in 23% and 41% yields.
1
1a: yellow crystals; m.p. 251 1C (dec.); H NMR (300 MHz, C6D6,
298 K): d 0.14 (s, 18H), 0.40 (s, 90H), 0.48 (s, 36H), 3.28 (s, 4H), 7.04
(s, 4H); 13C NMR (75 MHz, C6D6, 298 K): d ꢀ0.15 (q), 1.59 (q), 5.70
(q) 22.56 (d), 28.95 (d), 112.16 (s), 116.73 (s), 126.25 (s), 127.68 (d),
150.31 (s), 153.16 (s); 29Si NMR (60 MHz, C6D6, 298 K): d ꢀ19.0
(CRC–SiMe3), 0.9 [C(SiMe3)3], 1.2 [CH(SiMe3)2], 2.7 [CH(SiMe3)2],
44.6 (SiQSi); HRMS (FAB) m/z calcd for C70H152Si18 1496.7741
([M+]), found: 1496.7736 ([M+]); Anal. Calcd for C70H152Si18: C,
56.07; H, 10.22. Found: C, 55.86; H, 10.05. UV/vis (hexane, r.t.):
lmax = 437 nm (e = 2.4 ꢂ 104).
1b: orange crystals; m.p. 280–282 1C; 1H NMR (300 MHz, C6D6,
323 K): d 0.40 (s, 72H), 0.43 (s, 54H), 3.39 (s, 4H), 6.86–6.96 (m, 6H),
7.08 (s, 4H), 7.41–7.47 (m, 4H); 13C NMR (100 MHz, C6D6, 323 K):
d 2.41 (q), 6.06 (q), 31.87 (d), 37.46 (d), 94.97 (s), 119.57 (s), 124.35 (s),
127.21 (d), 128.50 (d), 128.62 (d), 130.83 (s), 132.26 (d), 148.13 (s),
152.63(s); 29Si NMR (60 MHz, C6D6, 343 K): d 1.0 [C(SiMe3)3], 2.1
([CH(SiMe3)2]), 42.6 (SiQSi); HRMS m/z calcd for C76H144Si16
:
1504.7576 ([M+]), Found: 1504.7594 ([M+]). UV/vis (hexane, r.t.):
lmax = 469 nm (e = 3.1 ꢂ 104).
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This journal is The Royal Society of Chemistry 2010
4404 | Chem. Commun., 2010, 46, 4402–4404