Formation and characterization of the photochemically interconvertible side-on and end-on bonded dioxygen-iron dioxide complexes in solid argon

Two interconvertible iron dioxide-dioxygen complexes were prepared and characterized by matrix isolation infrared absorption spectroscopy as well as theoretical calculations. Iron atoms react with O2 to form the inserted FeO2 molecule in solid argon only upon UV-visible light irradiation. Annealing allows the dioxygen molecules to diffuse and to react with FeO2 and form the side-on and end-on bonded dioxygen-iron dioxide complexes, (eta2-O2)FeO2 and (eta1-O2)FeO2. The side-on bonded structure is a peroxide complex having a singlet ground state with a nonplanar C2v symmetry. The end-on bonded isomer is characterized to be a superoxide complex with a planar 3A' ' ground state. These two isomers are photoreversible, that is, near-infrared light (lambda > 850 nm) induces the conversion of the side-on bonded (eta2-O2)FeO2 complex to the end-on bonded (eta1-O2)FeO2 isomer and vice versa with red light irradiation (lambda > 600 nm).