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.