Sterically Crowded Tris(2-(trimethylsilyl)phenyl)phosphine - Is it Still a Ligand?

Tris(2-(trimethylsilyl)phenyl)phosphine, P(o-TMSC₆ H₄ )₃ , was synthesised and characterised in solution and in the solid state. The large steric bulk prevents most reactions of the phosphorus donor and makes the compound air stable both in the solid state as well as in solution. This shielded phosphine can still undergo three reactions, namely protonation, oxidation to the phosphine oxide under harsh conditions and complexation to Au^I , thus forming a complex with linear coordination. Unexpectedly, complexation was unsuccessful with a range of other metal cations. Neither Pd^II , Pt^II , Zn^II nor Hg^II reacted and even the remaining coinage metal cations Cu^I and Ag^I could not be coordinated. Both the parent molecule as well as the reaction products were structurally characterised by single crystal X-ray diffraction, and the conformational change of geometry required to accommodate the additional atoms was analysed in detail. Apart from chemical oxidation with H₂ O₂ , P(o-TMSC₆ H₄ )₃ displays reversible electrochemical oxidation with a potential not unlike the one of sterically unencumbered phosphines for which the oxidation is usually not reversible. P(o-TMSC₆ H₄ )₃ can thus be considered a model compound for the investigation of the electronic properties of sterically unencumbered phosphines.