Elucidating the Role of Aqueous Solvent in an Iron-Based Water Oxidation System by DFT-based Molecular Simulation

Water solvent plays an important role in catalytic water oxidation to dioxygen, in particular in the O-O bond formation process. In this work, we revisit the mechanism of O-O bond formation catalyzed by a mononuclear iron catalyst [Cl-Fe^III(dpa)-Cl]⁺, in a DFT-based molecular dynamics (DFT-MD) study that incorporates explicit solvent and thermal fluctuations. Two possible mechanisms for the crucial O-O bond formation, namely water nucleophilic attack (WNA) and nitrate nucleophilic attack (NNA) on the high-valent Fe-V-oxo moiety, were considered and found to have similar barriers (15 kcal/mol vs 16 kcal/mol). Comparison with static DFT calculations demonstrated the important role of water solvent molecules, especially for the NNA pathway. For this mechanism, the interaction of the negatively-charged nitrate with solvent molecules is substantial, giving rise to a free energy barrier increase of 7.7 kcal/mol compared with static DFT calculations. The study suggests that for molecular water-oxidation catalysts, the local aqueous solvation structure and its thermal fluctuations plays a significant role in the crucial O-O bond formation step. The study also elucidates the role of the nitrate ion as a co-catalyst, a notion that may serve as a potential design rule for developing improved water oxidation catalysts.