Structure-Function Relationships for Electrocatalytic Water Oxidation by Molecular [Mn12O12] Clusters

A series of Mn₁₂O₁₂(OAc)_16-xL_x(H₂O)₄ molecular clusters (L = acetate, benzoate, benzenesulfonate, diphenylphosphonate, dichloroacetate) were electrocatalytically investigated as water oxidation electrocatalysts on a fluorine-doped tin oxide glass electrode. Four of the [Mn₁₂O₁₂] compounds demonstrated water oxidation activity at pH 7.0 at varying overpotentials (640-820 mV at 0.2 mA/cm²) and with high Faradaic efficiency (85-93%). For the most active complex, more than 200 turnovers were observed after 5 min. Two structure-function relationships for these complexes were developed. First, these complexes must undergo at least one-electron oxidation to become active catalysts, and complexes that cannot be oxidized in this potential window were inactive. Second, a greater degree of distortion at Mn1 and Mn3 centers correlated with higher catalytic activity. From this distortion analysis, either or both of these two Mn centers are proposed to be the catalytically active site.