PHOTOCHEMICAL DISSOCIATION OF A GERMASILENE: PREPARATION AND PHOTOLYSIS OF Si, Si'-DI-TERT-BUTYL-TETRAMESITYLSILADIGERMIRANE

The preparation and photolysis (350 nm) of Si, Si'-di-terf-butyltetramesitylsiladigermirane, 1, has been investigated. The structure of the siladigermirane 1 has been determined by x-ray crystallography: crystals are monoclinic, space group C2/c with Ζ = 4 in a unit cell of dimensions a = 11.5830(9) A, b = 21.268(3) A, c = 17.534(2) A and β = 108.60(1)°. The structure was solved by direct methods and refined by full-matrix least squares on F on the basis of 2361 observed reflections (Ι>2σ(Ι)) to R, and wR2 values of 0.0495 and 0.0939, respectively. Photolysis of 1 in the presence of methanol gave two major products: MeSjGehKOMe) and i-Bu2Si(OMe)GeHMes2, 2 in a 1:1 ratio. Similarly, photolysis of 1 in the presence of water led to the isolation of f-Bi^SKOhOGeHMeSz, 3. The formation of compounds 2 and 3 is best explained by the regioselective photochemical cleavage of the siladigermirane to give MeSzGe: and /-Bu2Si=GeMes2, 4, followed by trapping of the germasilene, regioseledively, by water and methanol. Attempts to prepare a solution of germasilene 4 by photolysis of siladigermirane 1 in triethylsilane, a selective trapping reagent for MeSjGe:, were unsuccessful. The germasilene 4 undergoes photodissociation to yield another equivalent of Mes^e : and f-Bu2Si:, which were subsequently trapped by the silane. Introduction Although the chemistry of homonuclear, doubly-bonded compounds of the Group 14 elements beyond carbon (i.e. disilenes, digermenes and distannenes) has been extensively investigated, the chemistry of compounds containing a double bond between two different Group 14 elements beyond carbon was unknown until we reported the synthesis of tetramesitylgermasilene (Mes2Ge=SiMes2, Mes = 2,4,6-trimethylphenyl).· It has been interesting to compare the reactivity of this germasilene to that of Mes2Si=SiMes2 and Mes2Ge=GeMes2. Mes2Ge=SiMes2 adds methanol and water across the central double bond, in a manner similar to the disilene and the digermene; however, in this case the regiochemistry of the addition becomes a factor in the reaction. In the case of methanol, the addition is completely regioselective giving only the (methoxysilyl)germane. The regioselectivity of the addition of water appears to be solvent dependent: in acetone, the addition is completely regioselective giving 317 Vol. 19, No. 5, 1996 Photochemical Dissociation of a Germasilene the (hydroxysilyl)germane, whereas in toluene, the (hydroxysilyl)germane and the silyl(hydroxy)germane are formed in a 7:1 ratio. Disilenes' and digermenes add ketones and aldehydes to give 2,3-disilaor digermaoxetanes. Acetone, pivalaldehyde, benzaldehyde and benzophenone all add regioselectively across the double bond of Mes2Ge=SiMes2 to give 2-sila-3-germaoxetane derivatives. The most dramatic differences between Mes2Ge=SiMes2 and Mes2Si=SiMes2 are their relative stability and their decomposition pathways. Mes2Si=SiMes2 is stable in solution up to 180°C, at which temperature it rearranges to a benzocyclobutene derivative. Mes2Ge=SiMes2, on the other hand, is only stable in solution (toluene) at -78 °C. When a solution of the germasilene is warmed to room temperature, a facile 1,2-shift of a mesityl group from germanium to silicon occurs giving a silylgermylene. The germylene has been trapped with a variety of reagents. Mes2Ge=GeMes2 undergoes a similar rearrangement under the same conditions to give a germylgermylene. Even when heated in the presence of triethylsilane, Mes2Si=SiAr2 (Ar = 2,6-dimethylphenyl) did not give any products derived from the trapping of a silylene. The difference in the relative rate of a 1,2-aryl shift can be understood in terms of the difference in activation energies for such a shift in disilenes versus germasilenes versus digermenes. The barrier for a 1,2-hydride shift in H2Si=SiH2 is 17 kcal/mol, whereas the barriers are 13 and 12 kcal/mol for H2Ge=SiH2 and H2Ge=GeH2, respectively." Early in our investigations, we decided for two principal reasons not to attempt the crystallization and subsequent molecular structure determination of Mes2Ge=SiMes2 by x-ray crystallography. First and foremost, we believe that it is highly probable that an x-ray structure of Mes2Ge=SiMes2 will exhibit two-fold crystallographic disorder of the silicon and germanium atoms. Crystallographic disorder of the atoms is commonly observed in symmetrically substituted mixed Group 14 compounds and generally leads to unsatisfactory bond lengths. For example, the three-membered ring species SiGe2Mes„ and Si2Ge(SiMe3)e both exhibit a three-fold disorder whereas the linear compounds SiGe2Ph„ and Si2GePh8 exhibit a two-fold disorder. Secondly, on occasion, we have observed the simultaneous formation of Mes2Si=SiMes2 during the photolysis of hexamesitylsiladigermirane to generate Mes2Ge=SiMes2· We believe that if both compounds were present in solution it is highly probable that they would co-crystallize, again leading to unsatisfactory structural data. In our continuing studies on the synthesis and chemistry of germasilenes, it is desirable to prepare a germasilene that is stable in an inert solution at room temperature (i.e. that would not rearrange) to facilitate the study of the reactivity of these novel compounds. To slow the 1,2-mesityl migration in Mes2Ge=SiMes2, we decided to replace the mesityl substituents on silicon with a bulkier substituent. Our rationale was that, because of increasing steric congestion, the rate of the mesityl migration from germanium to silicon would decrease, and as a result, the germasilene may be stable at room temperature. For these reasons, we have synthesized Si.Si'-di-terf-butyldimesitylgermasilene (Mes2Ge=Sif-Bu2). Furthermore, because of the unsymmetrical substitution on the germasilene, an unambiguous structure determination should be possible.