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

Abstract

AbstractWater 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−FeIII(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 FeV‐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.