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=MnV=O stretching frequencies between 741 and 744 cm-1 for five porphyrin complexes while the IR of dioxo-MnV-tetrapentafluoroporphyrin [MnV(O2)(TPFPP)]- displayed an antisymmetric stretch at 805 cm-1. Both half-labeled and fully labeled 18O-MnV 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-MnV=O or five-coordinate MnV=O are excluded by the data. The 1H NMR spectrum of [MnV(O2)(TMP)]-, which shows a single, sharp resonance for the two ortho methyl groups, is also consistent with octahedral coordination of D4h 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-MnV porphyrins were shown to not exchange the oxo oxygens with bulk water solvent. Dioxo MnV porphyrins, which are anionic species [Por-MnV(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.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry