Calculations of One-Electron Redox Potentials of Oxoiron(IV) Porphyrin Complexes

Density functional theory calculations have been performed to calculate the one-electron redox potential for a series of oxoiron(IV) porphyrin complexes of the form [(TMP)Fe-IV(O)(L)] (TMP = 5,10,15,20-tetramesitylporphyrinate). Different axial ligands were chosen (L = none, Im, ClO4-, CH3CO2-, Cl-, F-, SCH3-) in order to compare the results with recent electrochemical experiments. The redox potentials were calculated with a Born-Haber cycle and the use of an internal reference, i.e. the absolute redox potential of ferrocene. Diverse methodologies were tested and show that the computed redox potentials depend strongly on the functional, the basis set, and the continuum models used to compute the solvation energies. Globally, BP86 gives better results for the geometries of the complexes than B3LYP and M06-L as well as more consistent values for the redox potentials. Although the results fit the experimental data for L = Im and L = ClO4-, the addition of the other anionic axial ligands to the oxoiron(IV) porphyrin complex strongly lowers the redox potential, which is in disagreement with experimental observations. This important discrepancy is discussed.