SPECIATION, CATALYSIS AND TRANSMETALLATION OF MANGANESE TETRAKIS(N-METHYL-4-PYRIDYL)PORPHINE IN AQUEOUS-MEDIA

The electrochemistry and spectroelectrochemistry of manganese tetrakis(N‐methyl‐4‐pyridyl)porphine (Mn‐TMPyP) in aqueous media have been studied. For MnIIITMPyP two water molecules ligate to the metal center and the formation constant(β2) is 0.11 in acetonitrile solution. The pKa1 and pKa2 for MnIIIIMPyP(H20)2 are 10.9 and 12.3, respectively in aqueous media. There is only one pKa at 11.7 for MnIIIMPyP(H2O)2. The pKa of the oxo‐manganese(IV) porphyrin, O = MnIVTMPyP(H2O), is 11.3. MnIITMPyP demetallates rapidly to free base in acidic aqueous solution. MnIITMPyP also undergoes electrocatalysis for oxygen reduction. In acidic conditions, demetallation and catalysis rates are competitive. The transmetallation of MnTMPyP by Zn2+ can be achieved in the presence of thiols. The UV‐Visible spectra in the reaction process suggest that the formation of some reactive intermediate is essential for the transmetallation. Copyright © 1990 The Chemical Society Located in Taipei & Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim, Germany