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-iPr2C6H3). 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.