In search of elusive high-valent manganese species that evaluate mechanisms of photosynthetic water oxidation.

Significant progress in the understanding of biological water oxidation has occurred during the past 25 years. Today we have a somewhat clearer description of the structure of the Mn4Ca cluster and an idea of the appropriate oxidation states for the enzyme during catalysis. At issue is the mechanism of water oxidation. Depending on one's belief of the manganese ion oxidation levels at the catalytically active S4 configuration, one can invoke a variety of different processes that could lead to water oxidation. We have suggested that the most likely process is the nucleophilic attack of a water bound to calcium (or manganese) onto a highly electrophilic Mn(V)=O center. In this Article, we explore the difficulties of preparing Mn(V) in dimeric systems and the even more arduous task of definitively assigning oxidation states to such highly reactive species.