Speciation, Catalysis and Transmetallation of Manganese Tetrakls(<i>N</i>‐Methyl‐4‐Pyridyl)Porphlne in Aqueous Media

Abstract

Abstract The electrochemistry and spectroelectrochemistry of manganese tetrakis( N ‐methyl‐4‐pyridyl)porphine (Mn‐TMPyP) in aqueous media have been studied. For Mn III TMPyP two water molecules ligate to the metal center and the formation constant(β 2 ) is 0.11 in acetonitrile solution. The pKa 1 and pKa 2 for Mn III IMPyP(H20)2 are 10.9 and 12.3, respectively in aqueous media. There is only one pKa at 11.7 for Mn II IMPyP(H 2 O) 2 . The pKa of the oxo‐manganese(IV) porphyrin, O = Mn IV TMPyP(H 2 O), is 11.3. Mn II TMPyP demetallates rapidly to free base in acidic aqueous solution. Mn II TMPyP also undergoes electrocatalysis for oxygen reduction. In acidic conditions, demetallation and catalysis rates are competitive. The transmetallation of MnTMPyP by Zn 2+ 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.