Electrochemical characterization and electrocatalysis of high valent manganese meso-tetrakis(N-methyl-2-pyridyl)porphyrin

Electrochemical oxidation of water-soluble manganese(III) meso-tetrakis(N-methyl-2-pyridyl)porphyrin (Mn^III(2-TMPyP)) generates stable Mn^IV and Mn^V porphyrins. Speciation of various oxidation states of the porphyrin are characterized by spectroelectrochemical methods. The acid dissociation constants (pK_as) for Mn^III(2-TMPyP)(H₂O)₂ are 9.6 and 10.7, respectively. Spectroelectrochemical results of the one-electron oxidation of Mn^III(2-TMPyP) exhibit different forms of oxomanganese(IV) porphyrin, depending on the pH of the solution and the applied potential. The pK_a for OMn^IV(2-TMPyP)(H₂O) is 10.5. The axial oxygen atom ligated to the Mn(IV) center is protonated in acidic solution (pK_a 3.4). Further one-electron oxidation generates dioxomanganese(V) porphyrin, (O)₂Mn^V(2-TMPyP), which is stable in alkaline solution at room temperature. No oxidation wave is observed in the cyclic voltammograms, indicating the slow heterogeneous electron transfer rate of these oxidation reactions. The electrogenerated dioxomanganese(V) porphyrin exhibits higher reactivity toward olefin oxidation than oxomanganese(IV) porphyrin in basic solutions.