Competition and conversion between pnicogen bonds and hydrogen bonds involving prototype organophosphorus compounds

Photochemistry of phosphenic chloride (ClPO2): isomerization with chlorine metaphosphite (ClOPO) and reduction by carbon monoxide

Phosphenic chloride (ClPO₂) is an elusive congener of nitryl chloride (ClNO₂). By high-vacuum flash pyrolysis of 2-chloro-1,3,2-dioxaphospholane in the gas phase, ClPO₂ has been efficiently generated and subsequently isolated in cryogenic N₂, Ar, and CO matrices (10 K) for a first time study on its photochemistry. Upon 193 nm laser irradiation, ClPO₂ isomerizes to the novel chlorine metaphosphite (ClOPO) by initial cleavage of the Cl-P bond (→ ˙Cl + ˙PO₂) with subsequent Cl-O bond formation inside the N₂ and Ar matrix cages. The reverse transformation becomes feasible under further irradiation at 266 nm. This photochemistry is consistent with the observed absorptions of ClPO₂ and ClOPO at 207 and 250 nm, respectively. When the photolysis was performed in solid CO ice, no isomerization occurs due to CO-trapping of the initially generated ˙Cl atoms by forming caged radical pair ClCO˙⋯˙PO₂. Concomitantly, photolytic reduction of ClPO₂ to ClPO by CO has been observed, yielding a weakly bonded molecular complex consisting of ClPO and CO₂ bonded through short intermolecular C⋯O contact (2.910 Å). The characterization of ClPO, ClPO₂, ClOPO, and the molecular complexes of ClPO₂-CO and ClPO-CO₂ using matrix-isolation IR and UV-vis spectroscopy is supported by the theoretical calculations at the B3LYP/6-311 + G(3df) level, and the photochemistry of ClPO₂ is also compared with the revisited photochemistry of ClNO₂ in the N₂-matrix.