Photolytic Ring Expansion of Phenylphosphinidene and Its Hydrogen‐Bonded Complexes

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–Cl2) 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.