Energy Landscape for the Electrocatalytic Oxidation of Water by a Single-Site Oxomanganese(V) Porphyrin

A cationic manganese porphyrin, Mn-III-TDMImP, is an efficient, homogeneous, single-site water oxidation electrocatalyst at neutral pH. The measured turnover frequency for oxygen production is 32 s(-1). Mechanistic analyses indicate that Mn-V(O)(OH2), the protonated form of the corresponding trans-Mn-V(O)(2) species, is generated from the Mn-III(OH2)(2) precursor in a 2-e(-) two-proton process and is responsible for O-O bond formation with a H2O molecule. Chloride ion is a competitive substrate with H2O for the Mn-V(O)(OH2) oxidant, forming hypochlorous acid with a rate constant that is 3 orders of magnitude larger than that of water oxidation. The data allow the construction of an experimental energy landscape for this water oxidation catalysis process.