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-m ethyl-2-pyridyl)porphyrin (Mn-III(2-TMPyP)) generates stable Mn-IV and Mn-V porphyrins. Speciation of various oxidation states of the porphyrin are ch aracterized by spectroelectrochemical methods. The acid dissociation consta nts (pK(a)s) for Mn-III(2-TMPyP)(H2O)(2) are 9.6 and 10.7, respectively. Sp ectroelectrochemical results of the one-electron oxidation of Mn-III(2-TMPy P) 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 proto nated in acidic solution (pK(a) 3.4). Further one-electron oxidation genera tes dioxomanganese(V) porphyrin, (O)(2)Mn-V(2-TMPyP), which is stable in al kaline solution at room temperature. No oxidation wave is observed in the c yclic voltammograms, indicating the slow heterogeneous electron transfer ra te of these oxidation reactions. The electrogenerated dioxomanganese(V) por phyrin exhibits higher reactivity toward olefin oxidation than oxomanganese (IV) porphyrin in basic solutions. (C) 1999 Elsevier Science S.A. All right s reserved.