Mechanism and Dynamics of Formation of Bisoxo Intermediates and O-O Bond in the Catalytic Water Oxidation Process.

This work elucidates the reactivity of water molecules toward the tridentate nitrogen-containing iron complex in the water oxidation process. Here, we consider the Fe-bisoxo complex {[Fe(Metacn)(OH)(═O)]} to be responsible for the oxygen-oxygen bond formation. This O-O bond formation happens through the addition of water as a nucleophile. The transition state was determined by the synchronous transit-guided quasi-Newton method using reactants and products and verified by intrinsic reaction coordinates (IRCs). From the IRC calculations, we observe that the Fe═O moiety is attacked by water and assisted by the H-bonded interaction with the oxygen atom of the bisoxo complex. The hydrogen atom is transferred to the oxygen atom of the bisoxo complex through the transition state, and subsequently, the hydroxide is transferred to another oxygen of the bisoxo complex, resulting in the formation of the oxygen-oxygen bond. This work also explains the effect of explicit water molecules on the oxygen-oxygen bond formation. Our results also show how the formation of superoxide plays an essential role in O evolution. We used the potential energy scan method to compute the transition state in the oxygen evolution step. In the present work, we study the effect of chlorine on the formation of the oxygen-oxygen bond formation. In this study, the changes in the oxidation state, spin density, and spin multiplicity of the complexes are investigated for each successive step. Apart from these static theoretical calculations, we also studied the oxygen-oxygen bond formation through first-principles molecular dynamics with the aid of the well-tempered metadynamics sampling technique. From the observation of the free energy surfaces from metadynamics simulations, it is evident that the hydroxide transfer has a lesser free energetic reaction as compared to the proton transfer. This complete mechanistic study may give an idea to design a suitable water oxidation catalyst.