Ion and Solvent Modulation of Ferrocene and Decamethylferrocene Oxidation Potentials in Organic Electrolytes as Predicted by Molecular Dynamics Simulations

Ferrocene is commonly used as an internal redox couple in electrochemical measurements. Therefore, understanding how the absolute oxidation potential of ferrocene is modulated by different solvents and ion concentrations is important for the comparison of experimental measurements between different electrochemical systems. While standard implicit solvation models may provide relatively good predictions in bulk solvents, they lack the ability to describe ion coordination effects that can substantially alter redox potentials in practical electrolyte systems. In this work, we utilize molecular dynamics simulations to compute absolute oxidation potentials for the ferrocene and decamethylferrocene redox couple in bulk solvents of water, acetonitrile, 1,2-dichloroethane, and trichloromethane, as well as organic electrolytes consisting of mixtures of [BMIM⁺][BF₄^–] ionic liquid and acetonitrile and 1,2-dichloroethane solvents, for a wide range of ion concentrations. The goals are twofold: first, for the bulk solvents, we compare and evaluate the consistency of redox potential predictions for polarizable and nonpolarizable force fields from explicit solvent, free energy simulations, with predictions from an implicit solvent model. Second, we evaluate how ion coordination within the organic electrolytes modulates the redox potential of ferrocene and decamethylferrocene as a function of the ionic concentration and solvent dielectric constant. Utilizing linear response theory, we analyze the solvation contribution to the redox potential in terms of distributions of anion coordination number and how the anion coordination modulates the vertical ionization energy. We show that inclusion of liquid-vacuum interfacial potentials is essential for consistent prediction/interpretation of redox potentials across different solvents and force fields in order to compensate for the artificial quadrupole trace contribution to the solute cavity interfacial potential; this important consideration was previously proposed by Harder and Roux [J. Chem. Phys. 2008, 129, 234706].