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

Abstract

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 R and R (R =Ar**NSi(OtBu) , where Ar**=2,6-bis[bis(4-tert-butylphenyl)methyl]-4-methylphenyl; R =Ar***NSi(iPr) , where Ar***=2,6-bis[bis(3,5-di-tert-butylphenyl)methyl]-4-methylphenyl) facilitates the formation of diphosphene-type dimers, [(RGe)P] and [(RSn)P] . In an effort to circumvent dimerisation, the bulkier R substituent (R =Ar***NSi(4-tert-butylphenyl) ) 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 R substituent, which in case of the stannylene could act as a trap for another R -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.

N-Heterocyclic Carbene-Stabilized Germanium and Tin Analogues of Heavier Nitriles: Synthesis, Reactivity, and Catalytic Application

“…This can be viewed as increased contribution of the valence isomer tetrylene–phosphinidene resonance structure IB into the ground state of the molecule descending the group 14. Although sufficiently bulky aryl substituents were predicted to stabilize the heavier nitrile form IA (E = Si, Sn), , reported experimental attempts to access these compounds using this strategy alone were not successful so far . Most recently, an approach using very sterically demanding amido ligands resulted in tetrylene–phosphinidenes dimers, diphosphenes III (Chart B)…”

A Bismuth–Arene σ‐Complex – On the Edge of Menshutkin‐Type Complexes

“…Thus, strong Lewis acids can easily attack the Mes* moiety and it is therefore not robust enough to stabilize the desired bismadiazonium or bismuthenium cations. The limitations of protecting highly labile functionalities by steric bulk alone was recently discussed by Hinz and Giocoechea for sterically demanding R Bhp derivatives ( R Bhp = 4‐R‐2,6‐bis(benzhydryl)phenyl, R = Me, i Pr, t Bu) …”

Pseudo‐One‐Coordinate Tetrylenium Salts Bearing a Bulky Carbazolyl Substituent

“…The buried volume (for a sphere with r =3.5 Å) for RH ( d =1.937 Å) amounts to 74.8 %, but drops to only 61.6 % for RGeCl. With these values, the carbazolyl substituent provides comparable bulk to Jones’ amides (62.4 % Ar*(Me 3 Si)N) and modified variations thereof (up to 77.7 %), while the bulky Tip Ter only gives 48.9 %. Even for the recently reported Mes* TerSnCl, only a buried volume of 51.4 % was found .…”