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.