Trans-dioxo Manganese(V) Porphyrins

The relatively stable oxoMn(V) porphyrins generated by oxidation of Mn(III) porphyrins in alkaline solutions using peroxides are unambiguously identified as trans -dioxo species. Raman spectra revealed symmetric O Mn V O stretching frequencies between 741 and 744 cm -1 for five porphyrin complexes while the IR of dioxo-Mn V -tetrapentafluoroporphyrin [Mn V (O 2 )(TPFPP)] - displayed an antisymmetric stretch at 805 cm -1 . Both half-labeled and fully labeled 18 O−Mn V porphyrin complexes were prepared, and the Raman and IR shifts correspond well with a linear, three-body O Mn O oscillator model. Terminal, monooxo formulations such as HO−Mn V O or five-coordinate Mn V O are excluded by the data. The 1 H NMR spectrum of [Mn V (O 2 )(TMP)] -, which shows a single, sharp resonance for the two ortho methyl groups, is also consistent with octahedral coordination of D 4 h symmetry. The force constant F for the Mn O double bond was calculated to be 454 N/m with a stretch−stretch force constant k = 67.2 N/m. Comparison with other Mn−O force constants revealed a very good agreement with Badger's rule spanning five Mn oxidation states. These dioxo-Mn V porphyrins were shown to not exchange the oxo oxygens with bulk water solvent. Dioxo Mn V porphyrins, which are anionic species [Por-Mn V (O) 2 ] -, are unreactive toward olefins in the presence of excess base but efficiently epoxidize cyclooctene upon protonation at −70 °C. These compounds are the only trans-dioxomanganese complexes of any type to be characterized as such, thus extending the known π-bonding arrangements for first-row transition metals.