Water oxidation catalysis - role of redox and structural dynamics in biological photosynthesis and inorganic manganese oxides

Water oxidation is pivotal in biological photosynthesis, where it is catalyzed by a protein-bound metal complex with a Mn 4 Ca-oxide core; related synthetic catalysts may become key components in non-fossil fuel technologies. Going beyond characterization of the catalyst resting state, we compare redox and structural dynamics of three representative birnessite-type Mn(Ca) oxides (catalytically active versus inactive; with/without calcium) and the biological catalyst. In the synthetic oxides, Mn oxidation was induced by increasingly positive electrode potentials and monitored by electrochemical freeze-quench and novel time-resolved in situ experiments involving detection of X-ray absorption and UV-vis transients, complemented by electrochemical impedance spectroscopy. A minority fraction of Mn( iii ) ions present at catalytic potentials is found to be functionally crucial; calcium ions are inessential but tune redox properties. Redox-state changes of the water-oxidizing Mn oxide are similarly fast as observed in the biological catalyst (