The axial ligand effect on aliphatic and aromatic hydroxylation by non-heme iron(IV)-oxo biomimetic complexes

Iron(IV)-oxo heme cation radicals are active species in enzymes and biomimetic model complexes. They are potent oxidants in oxygen atom transfer reactions, but the reactivity is strongly dependent on the ligand system of the iron(IV)-oxo group and in particular the nature of the ligand trans to the oxo group (the axial ligand). To find out what effect the axial ligand has on the reactivity of non-heme iron(IV)-oxo species, we have performed a series of density functional theory (DFT) calculations on aliphatic and aromatic hydroxylation reactions by using [Fe^IV=O(TMC)(L)]ⁿ⁺ (TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, and L=acetonitrile or chloride). The studies show that the regioselectivity of aliphatic over aromatic hydroxylation is preferred. The studies are in good agreement with experimental product distributions. Moreover, the system with the acetonitrile axial ligand is orders of magnitude more reactive than that with a chloride axial ligand. We have analyzed our results and we have shown that the metal-ligand interactions influence the orbital energies and as a consequence also the electron affinities and hydrogen atom abstraction abilities. Thermodynamic cycles explain the regioselectivity preferences. Out of orbit: A series of density functional theory (DFT) calculations on aliphatic and aromatic hydroxylation reactions have been performed by using [Fe^IV=O(TMC)(L) ]ⁿ⁺. These studies predict regioselective aliphatic hydroxylation over aromatic hydroxylation (see scheme). The observed trends and product distributions have been rationalized by using thermodynamic cycles and orbital assignments, and explain the reasons for the regioselectivity preference. (TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, and L=CNCH ₃ or Cl^-).