Silicon clusters with six and seven unsubstituted vertices via a two-step reaction from elemental silicon

Unsaturated silicon clusters with only partial substitution, and thus, "naked" Si atoms are well studied species as they are proposed intermediates in gas-phase deposition processes. Although a remarkable number of stable molecular clusters has been reported, they are typically still obtained by multi-step syntheses. Herein we introduce a newly developed synthetic approach which led to the formation of the anionic species {Si(TMS)₃}₃Si₉ ^- (1a) and {Si(TMS)₃}₂Si₉ ^2- (1b), and an extension of this synthetic protocol resulted in the first covalent attachment of ligands through metal atoms to these clusters, (SnCy₃)₃Si₉ ^- (2a) and (SnCy₃)₂Si₉ ^2- (2b). The influence of the substituents on the electron localization in the central Si₉ unit is analyzed by means of intrinsic bond orbital (IBO) analysis and partial atomic charge distribution. The IBO analyses reveal a new type of delocalization including 5-center-6-electron besides 3-center-2-electron bonds. The Raman spectra of 1b and 2b allow an assignment of the Si-Si intra-cluster vibrations by comparison to calculated (DFT-PBE0) spectra. The anions are formed in a one-step synthesis from binary K₁₂Si₁₇ which can easily be obtained by fusing the elements K and Si. The anions are characterized by ESI mass spectrometry and comprehensive NMR studies (¹H, ¹³C, ²⁹Si, ¹¹⁹Sn). Attempts to crystallize 1a and 2a as their (K-222crypt)⁺ salts yielded after the loss of one of the substituents single crystals containing 1b and 2b. The single crystal X-ray structure analyses reveal the presence of anionic siliconoids with surfaces of seven unsubstituted silicon atoms.