Elucidating the mechanism of a stable and re-usable Mn12-based electrocatalyst for homogenous water oxidation

Mn12O12(O2R)16(H2O)4 clusters are mimetic models of the oxygen-evolving center near photosystem II. A few water-soluble Mn12 clusters were recently developed as water oxidation (WO) electrocatalysts. Although some understanding regarding their composition-activity relationships was acquired, their full catalytic mechanism was never studied. In this work, we report on Mn12O12(O2CC6H2(OH)2Br)16(H2O)4 (Mn12Br), which is a stable and, for the first time, re-usable electrocatalyst for homogeneous WO in low overpotential (124 mV). Based on mechanistic investigations, we propose, also for the first time, the catalytic mechanism of Mn12Br. Thus, multiple electrochemical experiments provided evidence for a proton-coupled electron transfer event, and kinetic isotope effect studies suggested a water nucleophilic attack (WNA) pathway for O–O bond formation. We also demonstrated that although WNA is triggered by the first Mn oxidation, a second oxidation has a significant role in WO, as seen from the faster kinetics of WO after the second Mn oxidation occurs.