On the Edge of the Known: Extremely Electron‐Rich (Di)Carboranyl Phosphines.

The syntheses of the first B9‐connected carboranylphosphines (B9‐Phos) featuring two carboranyl moieties as well as access to B9‐Phos ligands with bulky electron‐donating substituents, previously deemed unattainable, is reported. The electrochemical properties of the B9‐Phos ligands were investigated, revealing the ability of the mesityl derivatives to form stabilized phosphoniumyl radical cations. The B9‐Phos ligands display an extremely electron‐releasing character surpassing that of alkyl phosphines and commonly used N‐heterocyclic carbenes. This is demonstrated by their very small Tolman electronic parameters (TEPs) as well as extremely low P−Se coupling constants. Cone angles and buried volumes attest to the high steric demand exerted by the (di)carboranyl phosphines. The dicarboranyl phosphine AuI complexes show superior catalytic performance in the hydroamination of alkynes compared to the monocarboranyl phosphine analogs. [ABSTRACT FROM AUTHOR]