A Reversible Metalloporphyrin Oxygen Carrier Both in the Solid State and in Solution: Preparation, Characterization, and Kinetics of Formation of a Molybdenum(VI) 5,10,15,20-Tetramesitylporphyrin Dioxygen Complex

The molybdenum(VI) porphyrin dioxygen complex, [MoVIO(O2)(tmp)] (tmp = 5,10,15,20-tetramesitylporphyrinato), prepared from the reaction of [MoIVO(tmp)] with dioxygen at room temperature is characterized in detail. Bulky mesityl substituents on the porphyrin rings enable the isolation of the dioxygen complex. Photoirradiation of the toluene solution of the dioxygen complex with visible light regenerated [MoIVO(tmp)] immediately. Upon heating the solid sample of the dioxygen complex, dioxygen gas was liberated around 40—90 °C, leaving [MoIVO(tmp)]. The solid and a toluene solution of [MoIVO(tmp)] thus formed, again absorbed O2 from air to make [MoVIO(O2)(tmp)]. The IR and 1H NMR data confirmed that O2 was coordinated as a peroxide mode to the central molybdenum(VI) ion from the same side of the porphyrin plane with the oxo ligand and that both the axial ligands (peroxo and oxo) eclipsed the trans-N atoms of the porphyrin rings. The second-order rate constant for the formation of the dioxygen complex in toluene was evaluated to be (3.9 ± 0.1) × 10−1 dm3 mol−1 s−1 at 20 °C. The rate constant is extremely small as compared with those of the oxygenation of iron(II) porphyrins, by at least a factor of 10−5.