Halide-Ion Diadducts of Perhalogenated Cyclopenta- and Cyclohexasilanes: Halide-Ion Diadducts of Perhalogenated Cyclopenta- and Cyclohexasilanes

The halosilicates(II) [nBu(4)N](2)[Si(6)Br(12)2Br] and [nBu(4)P](2)[Si(6)I(12)2I] were prepared by mere addition of the appropriate halide salt to the corresponding disilane Si2X6 (X = Br, I). In the first case, the Br3Si-substituted derivative [nBu(4)N](2)[Si(7)Br(14)2Br] formed as a second product. We have been able to obtain single crystals of [Ph4P](2)[Si(7)Br(14)2Br] by switching the Br- salt from [nBu(4)N]Br to [Ph4P]Br. All three compounds, [nBu(4)N](2)[Si(6)Br(12)2Br] (monoclinic, P2(1)/c), [nBu(4)P](2)[Si(6)I(12)2I] (triclinic, P), and [Ph4P](2)[Si(7)Br(14)2Br] (triclinic, P) were structurally characterized by X-ray crystallography and found to form inverse sandwich complexes, in which two X- ions are located above and below a planarized Si-6 ring. The free periodated cyclohexasilane Si6I12 is accessible from [nBu(4)N](2)[Si(6)Cl(12)2Cl] and BI3 (1:5 molar ratio; CH2Cl2) via a decomplexation/halide-exchange cascade. Si6I12 (monoclinic, C2/c) adopts a puckered chair conformation in the solid state. The cyclopentasilane diadduct [nBu(4)P](2)[Si(5)Cl(10)2Cl] forms at -78 degrees C from Si5Cl10 and 2 equiv. of [nBu(4)P]Cl in CH2Cl2. An X-ray analysis of [nBu(4)P](2)[Si(5)Cl(10)2Cl]2CH(2)Cl(2) (monoclinic, C2/c) again revealed the structure of an inverse sandwich complex. Crystalline [nBu(4)P](2)[Si(5)Cl(10)2Cl]2CH(2)Cl(2) is stable at room temperature. However, in CH2Cl2 solution the compound quantitatively undergoes ring-expansion reactions to furnish cyclohexasilane derivatives. This behavior is in striking contrast to that of [nBu(4)N](2)[Si(6)Cl(12)2Cl], which persists at room temperature both in solution and the solid state. Competition experiments revealed free Si5Cl10 and Si6Cl12 to have comparable Cl- affinities, even though the distance between the two apical Cl- ions in [Si(5)Cl(10)2Cl](2-) (4.292 angstrom) is larger by 0.456 angstrom than that in [Si(6)Cl(12)2Cl](2-) (3.836 angstrom).