Systematic Improvement of Redox Potential Calculation of Fe(III)/Fe(II) Complexes Using a Three-Layer Micro-solvation Model

Electrochemical transformations of metal ions in aqueous media are challenging to model accurately due to the dynamic solvation structure surrounding ions at different charge states. Predictive modeling at the atomistic scale is essential for understanding these solvation architectures but is often computationally prohibitive. In this contribution, we present a simple, fast, and accurate three-layer micro-solvation model to evaluate the redox potential of metal ions in aqueous solutions. Our model, developed and validated for Fe³⁺/Fe²⁺ redox potentials, combines the DFT-based geometry optimizations of the octahedral Fe complex with two layers of explicit water molecules to capture solute-solvent interactions and an implicit solvation model to account for bulk solvent effects. This approach yields accurate predictions, achieving an error of 0.02 V with the B3LYP functional for Fe³⁺/Fe²⁺ redox potentials in water. We further demonstrate the generality of our model by applying it to additional metal complexes, including the challenging Fe(CN)₆³⁻/⁴⁻ system, where our model successfully achieves close agreement with experimental values, with an error of 0.07 V. In summary, the presented simple solvation model has broad applicability and potential for enhancing computational efficiency in redox potential predictions across various chemical and industrial processes of metal ions.