Ring Protonation Regulates the Homogeneous Electrocatalytic Oxygen Reduction Reaction Mediated by Manganese Phthalocyanine

The development of transition-metal based catalysts for small-molecule activation is essential to the development of viable alternative energy sources to nonrenewable fossil fuels. Herein, the synthesis and characterization of Mn­(tBuphthalocyanine)Cl (1Cl) as a homogeneous catalyst for the oxygen reduction reaction (ORR) with halogenated acetic acid derivatives is reported. The activity and selectivity of 1Cl was studied electrochemically and spectrochemically. Interestingly, the activity of 1Cl as a catalyst for the ORR displays an inverse correlation with the strength of the acid. Mechanistic studies suggest that this occurs because ring protonation leads to a deactivated form of the catalyst and ultimately degradation, limiting the strength of the proton donor which may be used. These observations contrast studies on previously reported analogous Fe-based phthalocyanines, where protonation has been suggested to improve activity. The results with the Mn complex imply that an understanding of ligand basicity and resultant control over the reaction environment is generally useful for the catalytic properties of this class of macrocyclic complexes in homogeneous configurations.