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

Abstract

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}2Si92- (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)2Si92- (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.