Synthesizing Mechanism of the Mn4Ca Cluster Mimicking the Oxygen-Evolving Center in Photosynthesis

The photosynthetic oxygen-evolving center (OEC) is a unique Mn4CaO5 cluster that serves as a blueprint to develop superior water-splitting catalysts for the generation of solar fuels in artificial photosynthesis. It is a great challenge and long-standing issue to reveal the synthesizing mechanism of this Mn4CaO5 cluster in both natural and artificial photosynthesis. Herein, efforts were made to reveal the synthesizing mechanism of an artificial Mn4CaO4 cluster, a close mimic of the OEC. Four key intermediates were successfully isolated and structurally characterized for the first time. It was demonstrated that the Mn4CaO4 cluster could be formed through a reaction between a thermodynamically stable Mn3CaO4 cluster and an unusual four-coordinated Mn-III ion, followed by stabilization process through binding an organic base (e.g., pyridine) on the "dangling" Mn ion. These findings shed new light on the synthesizing mechanism of the OEC and rational design of new artificial water-splitting catalysts.