Anionic Siliconoids from Zintl Phases: R₃Si₉ ⁻ with Six and R₂Si₉ ²⁻ with Seven Unsubstituted Exposed Silicon Cluster Atoms (R=Si(tBu)₂H)

Neutral and anionic silicon clusters (siliconoids) are regarded as important model systems for bulk silicon surfaces. For 25 years their formation from binary alkali metal silicide phases has been proposed, but experimentally never realized. Herein the silylation of a silicide, leading to the anionic siliconoids (Si(tBu)₂H)₃Si₉ ⁻ (1 a) and (Si(tBu)₂H)₂Si₉ ²⁻ (2 a) with the highest known number of ligand-free silicon atoms is reported for the first time. The new anions are obtained in a one-step reaction of K₁₂Si₁₇/NH₃(liq.) and Si(tBu)₂HCl/THF. Electrospray ionization spectrometry and ¹H, ¹³C, ²⁹Si, as well as ²⁹Si-HMBC (heteronuclear multiple bond correlation) NMR spectroscopy, confirm the attachment of three silyl groups at a [Si₉]⁴⁻ cluster under formation of 1 a, in accordance with calculated NMR shifts. During crystal growth the siliconoid di-anion 2 a is formed. The single-crystal X-ray structure determination reveals that two silyl groups are connected to the deltahedral Si₉ cluster core, revealing seven unsubstituted exposed silicon cluster atoms with a hemispheroidal coordination. The negative charges −1 and −2 are delocalized over the six and seven siliconoid Si atoms in 1 a and 2 a, respectively.