A Free Phosphaborene Stable at Room Temperature

Free phosphaborenes (R–P≡B–R) are PB analogs of alkynes and their isolation is a long-sought-after goal. Herein, we demonstrate that the combination of a π-donating and a π-accepting substituent with bulky flanking arene rings enables the isolation of a crystalline free phosphaborene 5 at room temperature. This electron push-pull cooperation, combined with the kinetic protection, hinders its inherent tendency to oligomerize. This species features a PB triple bond consisting of a conventional σ bond, a delocalized π bond and a weakly overlapped “slipped” π bond. Preliminary reactivity studies show that 5 undergoes facile (cyclo)addition reactions with p-methyl benzaldehyde, p-fluoroacetophenone and carbon disulfide, the last of which results in facile PB triple bond cleavage. Our strategy has a significant impact on the future synthesis of ambiphilic heterodiatomic multiply bonded main group species.