Tris(imidazolin-2-ylidenamino)phosphine: A Crystalline Phosphorus(III) Superbase That Splits Carbon Dioxide.

We report the synthesis and remarkable properties of the phosphine P(NIiPr) (NIiPr=1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidenamino), a crystalline solid accessible through a short and scalable route. Evaluation of the electron-donor properties reveals a Tolman electronic parameter (TEP) value of 2029.7 cm for the new phosphine that is significantly lower than those of all known phosphines or even N-heterocyclic carbenes. Moreover, P(NIiPr) is more basic [pK (MeCN)=38.8] than Verkade's proazaphosphatranes, thus being the strongest reported nonionic phosphorus(III) superbase. The coordination chemistry of the new phosphine towards different metal centers has been explored, and due to its unique electron-releasing character, the phosphine is capable of capturing and cleaving the CO molecule.