No. 5] Proc. Jpn. Acad., Ser. B 88 (2012) 167 Review Bonding and structure of disilenes and related unsaturated group-14 element compounds † By Mitsuo KIRA*1, (Communicated by Hitosi NOZAKI, M.J.A.) Abstract: Structure and properties of silicon-silicon doubly bonded compounds (disilenes) are shown to be remarkably different from those of alkenes. X-Ray structural analysis of a series of acyclic tetrakis(trialkylsilyl)disilenes has shown that the geometry of these disilenes is quite flexible, and planar, twist or trans-bent depending on the bulkiness and shape of the trialkylsilyl substituents. Thermal and photochemical interconversion between a cyclotetrasilene and the corresponding bicyclo[1.1.0]tetrasilane occurs via either 1,2-silyl migration or a concerted electrocyclic reaction depending on the ring substituents without intermediacy of the corresponding tetrasila-1,3-diene. Theoretical and spectroscopic studies of a stable spiropentasiladiene have revealed a unique feature of the spiroconjugation in this system. Starting with a stable dialkylsilylene, a number of elaborated disilenes including trisilaallene and its germanium congeners are synthesized. Unlike carbon allenes, the trisilaallene has remarkably bent and fluxional geometry, suggesting the importance of the :-<* orbital mixing. 14-Electron three-coordinate disilene- palladium complexes are found to have much stronger :-complex character than related 16-electron tetracoordinate complexes. Keywords: silicon, germanium, double bond, synthesis, structure, theoretical calculations types of organosilicon compounds; a number of Introduction reviews have been published for their experimental4) So-called “double-bond rule” states that unlike and theoretical aspects.5) Now studies in this research carbon, the elements with a valence principal field look heading for two directions, in addition quantum number of three or greater do not to further synthetic development of new types of effectively participate in : bonding.1) In accord with unsaturated silicon compounds; (1) application of this rule, multiply bonded compounds of silicon had their unique electronic properties towards the mate- been believed to be non-existent or highly unstable rial science and (2) restructuring of a general theory until the first synthesis of stable disilene (Mes2SiF of bonding and structure of heavy main group 2) SiMes2; Mes F 2,4,5-trimethylphenyl) and silaeth- elements including silicon. The former is just its 3) ene [Ad(Me3SiO)CFSi(SiMe3)2,AdF 1-adamantyl] beginning but the latter looks biding its time. were achieved by West et al. and Brook et al., Actually, thanks to interplay between theory and respectively, in 1981. Since then, much attention experiment, remarkable distinctions of bonding has been focused on various aspects of the chemistry and structure between silicon unsaturated com- of silicon unsaturated compounds including their pounds and their carbon congeners have been bonding and structure, spectroscopic properties, accumulated. reactivity, and application to the synthesis of novel We have created a number of thermally stable silicon unsaturated compounds with SiFSi, SiFC, *1 Department of Chemistry, Graduate School of Science, and SiFX(XF S, Se, Te, etc.) bonds and elucidated Tohoku University, Sendai, Japan. 6) † their unique properties since 1994. In this account, Correspondence should be addressed: M. Kira, Depart- the results are discussed with a focus on the ment of Chemistry, Graduate School of Science, Tohoku Univer- ff sity, 6-3 Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan (e-mail: di erences in bonding and structure between carbon [email protected]). and silicon unsaturated compounds and their origin. doi: 10.2183/pjab.88.167 ©2012 The Japan Academy 168 M. KIRA [Vol. 88, A A A si Br Na in toluene si si Si Si Si si A Br Reflux siA si A ½1 A t 2a, si = BuMe2Si A t 1a, si = BuMe Si A i 2 2b, si = Pr2MeSi A i 1b, si = Pr2MeSi A si A si A si Br LiNaph in THF Si − ° Si Si si A Br 78 C ½2 si A si A A i A i 2c, si = Pr3Si 1c, si = Pr3Si siA siA Si h v (λ> 300 nm) LiNaph in THF 1a, 1b 2a, 2b A siA ½3 −78 °C si Si Si rt siA siA 3a, 3b A siA si B si Br Na in toluene Si Si Si B Reflux si Br si B si A ½4 A t 2d, si = BuMe Si, A t 2 (E)- and (Z)-1d, si = BuMe Si, B i 2 si = Pr MeSi B i 2 si = Pr2MeSi A B si si si A si B KC8 in THF A Si Si B si si −78 ∼ −10°C Si Si ½5 Br Br si A si B A t B i A t B i 1e, si = BuMe Si, si = Pr MeSi 4e, si = BuMe2Si, si = Pr2MeSi 2 2 A t B i A t B i 4f, si = BuMe2Si, si = Pr3Si 1f, si = BuMe2Si, si = Pr3Si 1. Stable acyclic tetrakis(trialkylsilyl)disilenes si A si A si A si A 1.1. Synthesis. A series of acyclic tetra- Ge Ge Si Ge kis(trialkylsilyl)disilenes 1a–1c are synthesized si A si A si A si A typically using reductive coupling of the correspond- A t A t 5a, si = BuMe2Si 6a, si = BuMe2Si – – A i ing bis(trialkylsilyl)dibromosilanes 2a 2c (eqs [1] 5b, si = Pr2MeSi 7),8a) A i [3]). The products of the reduction of 2a–2c 5c, si = Pr3Si are dependent on the reaction conditions and steric Chart 1. bulkiness of the substituents. For example, the reduction of 2a and 2b with sodium in toluene affords disilenes 1a and 1b but the reduction of 2a affords a ca. 2:1 mixture of (E)- and (Z)-1d in and 2b with lithium naphthalenide (LiNaph) in THF solution but pure (E)-1d is obtained as single crystals at !78 °C gives the corresponding cyclotrisilanes 3a by recrystallization of the mixture from hexane.8b) and 3b, which are further converted to 1a and 1b A2SiFSiB2 type disilenes 1e and 1f are synthesized by the photolysis using a high-pressure Hg lamp by the reduction of the corresponding tetrasilyl-1,2- (eq [3]).8a) Disilene 1c with bulkier silyl substituents dibromodisilanes 4e and 4f, respectively (eq [5]).8c) is obtained directly by the reduction of 2c using Digermenes 5a–5c8d),8e) and germasilene 6a8e) LiNaph/THF at the low temperature (eq [2]), while are synthesized using similar reduction methods 1c is not formed by reducing 2c with Na in (Chart 1). toluene.8a) (E)- and (Z)-tetrakis(triakylsilyl)disilenes, All these dimetallenes are thermally stable and (E)-1d and (Z)-1d, are synthesized using a similar can be handled in a glove box at ambient temper- reductive coupling of dibromosilane 2d with two atures but are highly air- and moisture-sensitive in different silyl substituents (eq [4]). The reduction solution and in the solid state. No. 5] Bonding and structure of disilenes 169 Table 1. UV-vis absorption maxima and absorptivity (in parentheses)a) and 29Si NMR chemical shifts for unsaturated silicon nuclei of tetrasilyldimetallenes 3 29 Compound 6max/nm (C/10 ) /( Si)/ppm Ref. 1a 290 (2.5), 360 (1.9), 420 (2.8) 142.1 8a 1b 293 (2.1), 357 (1.4), 412 (7.6) 144.5 8a 1c 296 (5.4), 370 (2.4), 425 (1.9), 480 (2.2) 154.5 8a 1d —b) 141.9 8b 1e 293 (2.6), 359 (2.0), 413 (5.0) 132.4, 156.6 8c 1f —b) 142.0, 152.7 8c 8 290 (8.2), 375 (2.0), 612 (1.3) 155.5 10 5a 266 (5.3), 295 (2.6), 362 (3.1), 421 (7.0) — 8d, 8e 5b 361 (2.9), 413 (16.1) — 8d 5c 277 (3.5), 367 (2.2), 432 (2.1), 472 (2.0) — 8d 6a 359 (2.0), 413 (5.0) 144.0 8e a) In hexane or 3-methylpentane. b) Not measured. si si si si si E E si E E si E si si si si si ½6 trans -bent form twist form i E = Si, Ge; si = Pr3Si 1.2. Spectroscopic properties. Tetrasilyldi- digermene 5c with relatively bulky silyl substituents silenes 1a–1c are yellow to light orange in the solid is rather unusual. In solution, 1c and 5c show the state, and expectedly, the UV-vis spectra of disilenes longest wavelength band at much longer wavelength 1a–1c measured in KBr pellets are similar to each of 480 and 472 nm with relatively smaller absorptiv- other with the band maxima at around 420 nm.8a) ity than those for dimetallenes categorized as (1). In Table 1 are summarized the UV-vis absorption The spectra of 1c and 5c with bulky tri(isopro- spectra of acyclic tetrasilyldimetallenes 1a–1f and pyl)silyl substituents are remarkably temperature related dimetallenes at room temperature in a dependent with increasing absorptivity of the 420 nm hydrocarbon solvent. Interesting spectral features bands at lower temperatures. The behavior may be of tetrasilyldimetallenes are found in the table: (1) explained by the temperature-dependent equilibrium Whereas these tetrasilyldisilenes have no aromatic between trans-bent and twist forms in solution as substituents, the longest absorption maxima are shown in eq [6], where the twist form is supposed found at a wavelength longer than 400 nm. Sterically to have a smaller :-:* splitting energy than that for less hindered dimetallenes 1b, 1e, 5a, 5b, and 6a the trans-bent form. The explanation is consistent show the longest wavelength band maxima at around with the UV-vis spectral characteristics of a disilene 420 nm with relatively large absorptivity (C > 5000); with bulkier di(t-butyl)methylsilyl substituents 8 t t the 6max is even similar to that for Mes2SiFSiMes2 [( Bu2MeSi)2SiFSi(SiMe Bu2)2] reported by Sekiguchi (7).9) The band assignable to the :!:* transition et al.10) that has remarkably twisted geometry of normal or less distorted dimetallenes appears at around the SiFSi bond (twist angle, 55°); the longest much longer wavelengths than those of alkenes, wavelength band of 8 shows the maximum at 620 nm which are usually transparent in the wavelength with the absorptivity of 1300.