Diiron(IV)-Oxo Species and Water Oxidation: How Crucial is Electronic Cooperativity?

Water splitting, crucial for generating oxygen and hydrogen, remains a central challenge in chemistry due to its importance in developing sustainable energy sources and addressing environmental concerns. Consequently, numerous complexes have been developed to split water and release oxygen and hydrogen, albeit typically requiring external sources such as thermal, photo, or electrochemical methods. In this context, the discovery of a (μ-oxo)bis(μ-carboxamido) diiron(IV) complex, [Fe^IV₂O(L)₂]²⁺ (L=N,N-bis-(3',5'-dimethyl-4'-methoxypyridyl-2'-methyl)-N'-acetyl-1,2-diaminoethane), which activates both C-H and O-H bonds without external stimuli, has attracted significant attention. Notably, this complex generates hydroxyl radicals (⋅OH) without O₂ evolution and displays termolecular kinetics, presenting a rare and intriguing mechanistic puzzle. In this work, we explore the catalytic mechanism of water oxidation by this diiron(IV) complex using DFT methods. Our computational findings validate experimental observations regarding the necessity of a second water molecule in the reaction, revealing a bifurcated electron-proton transfer (BEPT) pathway driven by termolecular reactivity. Moreover, we highlight the crucial role of excess water molecules in stabilising the reaction intermediates, particularly via interaction with the -OMe groups to form a water cluster model. The inclusion of explicit water molecules was found to reduce the activation barrier to 23.5 kJ/mol from the reactant and 62.7 kJ/mol from the reactant complex, whereas, with only one water molecule present, the barrier was 344.3 kJ/mol, highlighting the critical role of the adventitious water molecule at the active site. Our study underscores the importance of metal-metal cooperativity, ligand design, spin-state modulation, and second-sphere effects in shaping the catalytic behaviour. These insights provide a detailed understanding of the electronic structure and reactivity, offering valuable guidelines for future catalyst design in water oxidation and beyond.