Reversibility in the formation of oxo(peroxo)porphyrinatomolybdenums

Reversibility in the formation reaction of a series of the oxo(peroxo)porph yrinatomolybdenums, [(MoO)-O-VI(tmp)(O-2)] 1, [(MoO)-O-VI(tdcpp)(O-2)] 2, [ (MoO)-O-VI(ttp)(O-2)] 3, [(MoO)-O-VI(tdmpp)(O-2)] 4, and [(MoO)-O-VI(tpp)(O -2)] 5, was studied. In these dioxygen complexes with various porphyrin rin gs, two complexes, 2 and 4, were newly prepared by the solid-state reaction s of corresponding Mo(IV) complexes with O-2. All the complexes were charac terized by IR, H-1 NMR, and UV-vis spectroscopic measurements. In the react ion of O-2 With [(MOO)-O-IV(tdcpp)] 2r, which has bulky substituents with a n electron-withdrawal character, the association rate constant was determin ed to be 1.2x10(-2) dm(3) mol(-1) s(-1) in toluene at 20 degrees C. The val ue of the rate constant is about one thirtieth of that for [(MoO)-O-IV(tmp) ] 1r, which also has bulky substituents. The result indicates that the elec tronic effects of the porphyrin rings, which reflect on the redox potential s of the central molybdenum ion, are important in determining kinetic and t hermodynamic stability of the dioxygen complexes. All the dioxygen complexe s undergo deoxygenation upon photoirradiation to give corresponding [(MoO)- O-IV(por)]. The redioxygenation profiles of [(MoO)-O-IV(por)] in the dark w ere also significantly affected by the steric bulkiness of the porphyrin li gands. While the bulky porphyrin complexes of 1 and 2 gave full recovery of the dioxygen complexes in the dark, less bulky porphyrin complexes of 3, 4 , and 5 undergo some side reactions to Mo(V) species. The rate and extent o f the Mo(V) complex formations increase with decreasing steric bulkiness.