Halide Coordination of Perhalocyclohexasilane Si6X12 (X = Cl or Br)

The addition of halide anions (X′ = Cl−, Br−, or I−) to perhalocyclohexasilane Si6X12 (X = Cl or Br) led to the formation of complexes comprising [Si6X12X′2]2− dianions. An upfield shift in the 29Si NMR spectra was noted upon coordination, and structural determination by X-ray crystallography showed that the dianions adopt an “inverse sandwich” structure where the six cyclic silicon atoms form a planar hexagon with the two halide anions X′ located on the 6-fold axis equally disposed above and below the plane of the Si6 ring. Additionally, these apical X′ atoms are within the van der Waals bonding distance to the silicon ring atoms, indicating a strong interaction between X′ and silicon atoms. These results detail crystallographic variations within the halogen series providing further insight into the nature of the Lewis acid sites above and below the Si6X12 ring, where interactions with hard Lewis bases such as halide anions are observed. Interestingly, the stereochemistry of the silicon atoms in [Si6X12X′2]2− is not affected much by the size or electronegativity of the halogen atoms.