Laser flash photolysis formation and direct kinetic studies of manganese(V)-oxo porphyrin intermediates.
Irradiation of porphyrin-manganese(III) perchlorate complexes in acetonitrile with 355 nm laser light gave MnV-oxo intermediates that were characterized by their UV-vis spectra and reactivities. The MnV-oxo species of tetrakis(pentafluorophenyl)porphyrin (2), tetraphenylporphyrin (4), and tetra-(4-(N-methylpyridiniumyl))porphyrin (6) were generated. Second-order rate constants for reactions of 2 with substrates were as follows: 6.1 x 105 M-1 s-1 (cis-stilbene), 1.3 x 105 M-1 s-1 (diphenylmethane), 1.3 x 105 M-1 s-1 (ethylbenzene), and 0.55 x 105 M-1 s-1 (ethylbenzene-d10). In oxidations of cis-stilbene and diphenylmethane, the order of reactivity of the MnV-oxo species was 2 > 6 > 4.