A Fragile Zwitterionic Phosphasilene as a Transfer Agent of the Elusive Parent Phosphinidene (:PH)

The simplest parent phosphinidene, :PH (1), has been observed only in the gas phase or low temperature matrices and has escaped rigorous characterization because of its high reactivity. Its liberation and transfer to an unsaturated organic molecule in solution has now been accomplished by taking advantage of the facile homolytic bond cleavage of the fragile Si=P bond of the first zwitterionic phosphasilene LSi=PH (8) (L = CH[(C=CH2)CMe(NAr)(2)]; Ar = 2,6-(Pr2C6H3)-Pr-i). The latter bears two highly localized lone pairs on the phosphorus atom due to the LSi=PH <-> LSi+-PH- resonance structures. Strikingly, the dissociation of 8 in hydrocarbon solutions occurs even at room temperature, affording the N-heterocyclic silylene LSi: (9) and 1, which leads to oligomeric [PH](n) clusters in the absence of a trapping agent. However, in the presence of an N-heterocyclic carbene as an unsaturated organic substrate, the fragile phosphasilene 8 acts as a :PH transfer reagent, resulting in the formation of silylene 9 and phosphaalkene 11 bearing a terminal PH moiety.