Accessing Cationic α-Silylated and α-Germylated Phosphorus Ylides

The synthesis and full characterization of α-silylated (α-SiCPs; 1-7) and α-germylated (α-GeCPs; 11-13) phosphorus ylides bearing one chloride substituent R₃ PC(R¹ )E(Cl)R² ₂ (R=Ph; R¹ =Me, Et, Ph; R² =Me, Et, iPr, Mes; E=Si, Ge) is presented. The molecular structures were determined by X-ray diffraction studies. The title compounds were applied in halide abstraction studies in order to access cationic species. The reaction of Ph₃ PC(Me)Si(Cl)Me₂ (1) with Na[B(C₆ F₅ )₄ ] furnished the dimeric phosphonium-like dication [Ph₃ PC(Me)SiMe₂ ]₂ [B(C₆ F₅ )₄ ]₂ (8). The highly reactive, mesityl- or iPr-substituted cationic species [Ph₃ PC(Me)SiMes₂ ][B(C₆ F₅ )₄ ] (9) and [Ph₃ PC(Et)SiiPr₂ ][B(C₆ F₅ )₄ ] (10) could be characterized by NMR spectroscopy. Carrying out the halide abstraction reaction in the sterically demanding ether iPr₂ O afforded the protonated α-SiCP [Ph₃ PCH(Et)Si(Cl)iPr₂ ][B(C₆ F₅ )₄ ] (6 dec) by sodium-mediated basic ether decomposition, whereas successfully synthesized [Ph₃ PC(Et)SiiPr₂ ][B(C₆ F₅ )₄ ] (10) readily cleaves the F-C bond in fluorobenzene. Thus, the ambiphilic character of α-SiCPs is clearly demonstrated. The less reactive germanium analogue [Ph₃ PC(Me)GeMes₂ ][B{3,5-(CF₃ )₂ C₆ H₃ }₄ ] (14) was obtained by treating 11 with Na[B{3,5-(CF₃ )₂ C₆ H₃ }₄ ] and fully characterized including by X-ray diffraction analysis. Structural parameters indicate a strong C_Ylide -Ge interaction with high double bond character, and consequently the C-E (E=Si, Ge) bonds in 9, 10 and 14 were analyzed with NBO and AIM methods.