Characterization and Reactivity of an Incredibly Reactive Intermediate in the Protonation Reaction of Dioxo-Manganese(V) Porphyrin with Acid

A monooxo-manganese(V) porphyrin complex, Mn(V)O(P), has been proposed as a reactive intermediate in the catalytic reactions, but Mn(V)O(P) is still a long-sought intermediate probably because of its high reactivity. To characterize Mn(V)O(P), we examined a new strategy, where Mn(V)O(P) is generated by the protonation of the oxo ligand of dioxo-manganese(V) tetramesitylporphyrin complex (1) with various acids by using a rapid-mixing stopped-flow technique at low temperatures. Rapid mixing of the solution of 1 containing 100 equiv of tetra-n-butylammonium hydroxide (TBAOH) with 25–50 equiv of trifluoroacetic acid (TFA-H) at −40 °C generates a new intermediate (2), having the absorption peaks at 423, 516, and 643 nm. When 100 equiv of TFA-H is mixed with the solution of 1 containing 100 equiv of TBAOH, another new intermediate (3), having absorption peaks at 404, 418, and 660 nm, is observed immediately after mixing. The Mn(V) oxidation states of 2 and 3 are confirmed by the reaction with excess TBAOH and tris(p-bromophenyl)amine, respectively. 2 and 3 can be generated by other acids. 3 can be produced from the reaction of 2 with TFA-H. The kinetic analysis of the reaction of 3 with various substrates shows that 3 can catalyze epoxidation, hydrogen abstraction, and hydroxylation reactions with incomparably faster reaction rates than the corresponding corrole and corrolazine complexes. The reaction rates of 3 are also 103–105-fold higher than those of oxoiron(IV) porphyrin π-cation radical complexes, known as the most powerful metal-oxo porphyrin complexes. 2 is less reactive than 3. These results indicate that 2 and 3 are the most reliable candidate for the long-sought Mn(V)O(P) having hydroxide and aqua axial ligands, respectively.