Photolytic Ring Expansion of Phenylphosphinidene and Its Hydrogen‐Bonded Complexes

ABSTRACT The ring‐expansion of the parent phenylphosphinidene ( 2 ) to 1‐phospha‐1,2,4,6‐cycloheptatetraene ( 3 ) has been theoretically predicted; however, this isomer remains yet unobserved in previous experimental studies. Herein, we report the observation of 2 and 3 during photolytic (193 nm) dehydrogenation of phenylphosphine ( 1 ) in an Ar‐matrix at 10 K. Upon subsequent photoexcitation at 365 nm, 3 prefers association with molecular hydrogen in the cryogenic matrix by reformation of 1; whereas, no interconversion between 3 and 2 could be observed under the irradiation conditions. When using phenylphosphine chloride ( 4 ) as the precursor, the 1:1 complex of 2 with HCl ( 2 –HCl) stabilized by intramolecular ClH•••π hydrogen bond is generated in the matrix, and it undergoes photo‐induced phosphinidene insertion into HCl by reformation of 4 upon photoexcitation at 365 nm. By photolyzing the matrix‐isolated complex of 1 with molecular chlorine ( 1 –Cl 2 ) at 310 nm, the 1:2 hydrogen‐bonded complexes of 2 with HCl are efficiently generated. Further photoexcitation of 2 –2HCl at 193 nm causes ring expansion in yielding of 3 –2HCl, and the reverse ring contraction from 3 –2HCl to 2 –2HCl occurs upon subsequent photoexcitation at 365 nm. The identification of 3 and the hydrogen‐bonded complexes 2 –HCl, 2 –2HCl, and 3 –2HCl with matrix‐isolation IR and UV–vis spectroscopy is supported by quantum chemical calculations at the B3LYP‐D3/6‐311++G(3df,3pd) level of theory.