Diradicaloid or Zwitterionic Character: The Non-Tetrahedral Unsaturated Compound [Si₄ {N(SiMe₃ )Dipp}₄ ] with a Butterfly-type Si₄ Substructure

The reduction of the tribromoamidosilane {N(SiMe₃ )Dipp}SiBr₃ (Dipp=2,6-iPr₂ C₆ H₃ ) with potassium graphite or magnesium resulted in the formation of [Si₄ {N(SiMe₃ )Dipp}₄ ] (1), a bicyclo[1.1.0]tetrasilatetraamide. The Si₄ motif in 1 does not adopt a tetrahedral substructure and exhibits two three-coordinate and two four-coordinate silicon atoms. The electronic situation on the three-coordinate silicon atoms is rationalized with positive and negative polarization based on EPR analysis, magnetization measurements, and DFT calculations as well as ²⁹ Si CP MAS NMR and multinuclear NMR spectroscopy in solution. Reactivity studies with 1 and radical scavengers confirmed the partial charge separation. Compound 1 reacts with sulfur to give a novel type of silicon sulfur cage compound substituted with an amido ligand, [Si₄ S₃ {N(SiMe₃ )Dipp}₄ ] (2).