A Phosphorus Lewis Super Acid: η5-Pentamethylcyclopentadienyl Phosphorus Dication

Summary Abstraction of chloride from (Cp*PCl2) with the silylium salt [Et3Si][B(C6F5)4] afforded [(η2-Cp*)PCl][B(C6F5)4] (1), whereas further chloride abstraction using another equivalent [Et3Si][B(C6F5)4] was unsuccessful. The corresponding species [(η2-Cp*)PF][B(C6F5)4] (2) was derived by fluoride abstraction from (Cp*PF2). Treatment of (Cp*PF2) with 2 equiv of [Et3Si][B(C6F5)4] gave [(η5-Cp*)P][B(C6F5)4]2 (3), representing an η5-Cp that is bound to phosphorus. 3 behaves as a phosphorus-based Lewis super acid, abstracting fluoride from [SbF6]− anion with concurrent η5-η2-Cp* rearrangement to give 2. Similarly, the extremely high Lewis acidity and chlorophilicity of 3 is indicated by the equilibration of 3 with Et3SiCl to generate 1 and [Et3Si][B(C6F5)4]. The coordination of 2,2′-bipyridine (bipy) to the phosphorus of 3 induces the η5-η1-Cp* rearrangement, leading to a “slipped sandwich” compound [(η1-Cp*)P(bipy)][B(C6F5)4]2 (4). 3 also reacts with Et3SiH via η5-σ-Cp* rearrangement, affording [(σ-Cp*)PH2SiEt3][B(C6F5)4] (5) and [Et3Si][B(C6F5)4].