A Masked Phosphinidene Trapped in a Fluxional NCN Pincer

The trapping of a phosphinidene (R-P) in an NCN pincer is presented. Stabilized phosphinidene 1 was characterized by ³¹ P{¹ H}, ¹ H, and ¹³ C{¹ H} NMR spectroscopy, exhibiting an averaged C_2v symmetry in solution between -60 and 60 °C. In the solid state, the phosphinidene is coordinated by one adjacent N atom featuring a formal P-N bond (1.757(2) Å) to give a five-membered ring with some aromatic character, confirmed by DFT calculations (B3LYP-D3/6-311G**++) to be the ground-state structure. Equilibration of the two N ligands occurs rapidly in solution via a "bell-clapper"-type process through an associative symmetric transition state calculated to lie 4.0 kcal mol^-1 above the ground state.