Substituent effect on the oxidation and reduction of electronically altered iron(II)-terpyridine derivatives – DFT study

Abstract

Structure, reactivity, and redox properties play an important role in many biological processes. The structures of two types of series of related iron(II) compounds, namely iron(II)-terpyridine and iron(II)-tris-azinyl derivatives, were optimized in the gaseous as well as solvated states by means of density functional theory (DFT). From the optimized geometries of the molecules, their frontier orbital energies (ELUMO/EHOMO), electronic chemical potential (μ), electrophilicity index, (ω), Merz-Kollman electric potential, Gibbs Free energy, Mulliken and Natural Bond Orbital charges were determined. The DFT calculated reactivity parameters were related to the experimentally reported iron-based oxidation and ligand-based reduction potential of the series of iron(II)-terpyridine and tris-azinyl derivatives. Results obtained show that the relationship between the experimental and DFT calculated Fe(II/III) redox potential gave the most accurate relationship (R2 = 0.97), though a systematic error, depending on the choice of the reference system, was obtained. Relationships between the experimental iron-based oxidation (EFe(II/III)) or ligand-based reduction (ELigand) of the iron(II) complexes, and solvent phase calculated HOMO and LUMO energies respectively (R2 = 0.96 and 0.95 respectively), lead to more accurate relationships with higher R2 values than the relationships involving the gas phase calculations (R2 = 0.87 and 0.88 respectively). More relationships, that can be used to predict the oxidation and reduction of related iron(II) derivatives containing two substituted terpyridine or tris-azinyl analogues of the terpyridine ligand, were obtained to a high accuracy.