(Invited) Exploration of Structure-Function Relationships for Electrocatalytic Water Oxidation By Molecular [Mn12O12] Clusters

Abstract: A series of Mn12O12(OAc)16-xLx(H2O)4 molecular clusters (L = benzoate, benzenesulfonate, diphenylphosphonate, dichloroacetate) were electrocatalytically investigated as water oxidation catalysts on an FTO glass electrode. Four of the 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 (L = diphenylphosphine), more than 200 turnovers were observed after 5 min. A known water oxidation catalyst Mn₄O₄L₆ (L=diphenylphoshine) was also explored and compared under the same electrochemical profile. Two structure-function relationships for these complexes were developed. First, these complexes must undergo at least 1-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 cubane-like Mn1 and aqua-ligated 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. Finally, these findings support utilization of cluster complexes as models for oxide-based catalysts to investigate key structural features, and to extend this approach towards the elucidation of catalytic mechanisms and intermediates to aid the design of next-generation oxide catalysts.