Limitations of Steric Bulk: Towards Phospha-germynes and Phospha-stannynes

The use of bulky aryl(silyl)amides (R) as substituents for the stabilisation of phospha-germynes and phospha-stannynes (R-Ge≡P and R-Sn≡P, respectively) is described. Such species can be transiently generated by photolysis of the phosphaketene precursors (RE(PCO); E=Ge, Sn). Utilisation of bulky amides R¹ and R² (R¹ =Ar**NSi(OtBu)₃ , where Ar**=2,6-bis[bis(4-tert-butylphenyl)methyl]-4-methylphenyl; R² =Ar***NSi(iPr)₃ , where Ar***=2,6-bis[bis(3,5-di-tert-butylphenyl)methyl]-4-methylphenyl) facilitates the formation of diphosphene-type dimers, [(RGe)P]₂ and [(RSn)P]₂ . In an effort to circumvent dimerisation, the bulkier R³ substituent (R³ =Ar***NSi(4-tert-butylphenyl)₃ ) was employed in an analogous series of experiments. This affords cyclic germylenes and stannylenes due to insertion of the terminal phosphide into Si-C bonds of the R³ substituent, which in case of the stannylene could act as a trap for another R³ -Sn≡P moiety. All attempts to isolate terminal phosphide species were unsuccessful due to the reactivity of such compounds towards the organic periphery of the bulky amides, highlighting the limitations of highly sterically demanding functionalities.