Role of Water in Modulating the Fe³⁺/Fe²⁺ Redox Couple in Iron-Based Complexes and Single-Atom Catalysts

A key challenge in modeling electrocatalysis with single-atom catalysts (SACs) is accurately capturing the redox behavior of transition metals across oxidation states. This is particularly true for iron, a widely used element in such systems. Iron phthalocyanine (FePc) serves as a model compound for graphene-based Fe SACs and is commonly used in reactions like the oxygen evolution reaction (OER). While FePc initially contains Fe(II), the active species under oxidative conditions is Fe(III), with an Fe(II)/Fe(III) transition occurring at intermediate potentials. Density functional theory (DFT) simulations must reflect this redox change. However, standard DFT predicts that oxidation removes an electron from the ligand, leaving the iron in the II state. This limitation arises not from DFT itself, but from an incomplete model. We show that adding at least one (preferably two) water molecules to the axial coordination sites of iron corrects this issue. The water ligands raise the energy of iron orbitals, making electron removal from the metal more favorable. This finding has two key implications: (1) the redox properties of transition metal complexes and graphene-based SACs are strongly influenced by the coordination environment, including solvent molecules; and (2) accurate description of the atomistic structure of the catalyst requires the explicit inclusion of axial water ligands, not just the in-plane ligands, to capture the true redox behavior.