Metal Oxidation States for the O-O Bond Formation in the Water Oxidation Catalyzed by a Pentanuclear Iron Complex

Understanding the water oxidation mechanism, especially how the O-O bond formation takes place, provides crucial implication for the design of more efficient molecular catalysts for water oxidation in artificial photosynthesis. Density functional calculations have here been used to revisit the mechanism of O-O bond formation catalyzed by a pentanuclear iron complex. By comparing energetics for O-O bond formation at different oxidation states, it is suggested that the formally Fe-5(III,III,III,IV,IV) state is the best candidate for the coupling of two oxo groups, with a barrier of 17.3 kcal/mol, rather than the previously suggested lower oxidation state of Fe-5(II,II,III,IV,IV). Importantly, the first water insertion into the Fe-5(III,III,III,III,III) complex is associated with a barrier of 18.8 kcal/mol. The calculated barrier is somewhat overestimated as discussed in the text. Other possible reaction pathways, including water attack at the Fe-5(III,III,III,IV,IV) state, coupling of oxo and hydroxide at the Fe-5(III,III,III,III,IV) state, and coupling of two oxo groups at the Fe-5(III,III,IV,IV,IV) state, were found to have much higher barriers.