Theoretical Studies on Stability of Oxygen-Evolving Center Model Complex Mn₄CaO₄ in Oxidative Condition

The photosynthetic oxygen-evolving center (OEC) is a unique Mn₄CaO₅ cluster catalyzing the water oxidation into electrons, protons and O₂ through a five-intermediate state cycle (Sn, n=0, 1, 2, 3, 4). The modeling of OEC is essential for understanding the water oxidation mechanism and developing high efficient water oxidation catalysts. Recently, a series of Mn₄CaO₄ model complexes have been synthesized, which have very similar structures to OEC and also show reactivity of water oxidation. In this work, we employed DFT method to investigate the stability of Mn₄CaO₄ model complex in oxidative conditions, aiming to figure out whether it decomposes itself to release O₂ during the catalytic water oxidation process. We discovered that the barrier for the O−O bond formation is quite high in the S1 and S2 states, while decreases to 40.2 kcal/mol in the S3 state, indicating the good stability in all oxidation states under normal conditions. Acetonitrile and pyridine can effectively reduce the barrier to 36.5 kcal/mol and 29.9 kcal/mol, respectively. Therefore, strong Lewis base, such as pyridine, could be harmful to the stability of Mn₄CaO₄ and shall be avoided when designing such a catalytic system. Once the O−O bond is formed, Mn₄CaO₄ in the S3 state can readily decompose to O₂, solvated Ca²⁺ and Mn₄O₂ complex, with the assistance of acetonitrile or pyridine. As a comparison, the O₂ decomposition in the S2 state is kinetically hindered and thermodynamically disfavored. The S4 state Mn₄CaO₄ has a much lower barrier for O–O bond formation, and is unstable. However, the S4 state Mn₄CaO₄ is difficult to achieve under water oxidation condition, as evidenced by the calculated redox potential of 2.3 V for S3→S4 transition.