Comparison between the two different forms of iron(IV)-oxo porphyrin and the effects of neutral axial ligand on their catalysis: A theoretical study

Density functional theory calculations reveal that iron(IV)-oxo porphyrin cation radical (Cpd I) models are indeed much more reactive than iron(IV)-oxo porphyrin (Cpd II) models in the case of 1,3-cyclohexdiene oxidation. The geometrical parameters and electronic configuration are quite different for Cpd I and Cpd II. This is the origin of the reactivity difference. The axial ligand indeed affects the geometries of Cpd I and Cpd II and the oxidation process, even though it cannot switch the reactivity of Cpd I and Cpd II. Interestingly, the axial ligand can switch the oxidation process from dehydrogenation to epoxidation by Cpd I, however, for Cpd II, it cannot switch the reaction process, but lessen the barrier difference of dehydrogenation and epoxidation, even though the decrease of barrier is not significant. The axial ligand effect is somewhat in relation to the Hammett substituent parameter (σP) of the substituent group in the axial ligand. The more negative the σP is, the longer the FeO bond of Cpd I is, and the higher the reaction barrier is.