Mechanistic insight into peroxo-shunt formation of biomimetic models for compound II, their reactivity toward organic substrates and the influence of N-methylimidazole axial ligation

High-valent iron-oxo species have been invoked as reactive intermediates in catalytic cycles of heme and nonheme enzymes. The studies presented herein are devoted to the formation of compound II model complexes, with the application of a water soluble (TMPS)FeIII(OH) porphyrin ([meso-tetrakis(2,4,6-trimethyl-3-sulfonatophenyl)porphinato]iron(III) hydroxide) and hydrogen peroxide as oxidant, and their reactivity toward selected organic substrates. The kinetics of the reaction of H2O2 with (TMPS)FeIII(OH) was studied as a function of temperature and pressure. The negative values of the activation entropy and activation volume for the formation of (TMPS)FeIV[DOUBLE BOND]O(OH) point to the overall associative nature of the process. A pH-dependence study on the formation of (TMPS)FeIV[DOUBLE BOND]O(OH) revealed a very high reactivity of OOH− toward (TMPS)FeIII(OH) in comparison to H2O2. The influence of N-methylimidazole (N-MeIm) ligation on both the formation of iron(IV)-oxo species and their oxidising properties in the reactions with 4-methoxybenzyl alcohol or 4-methoxybenzaldehyde, was investigated in detail. Combined experimental and theoretical studies revealed that among the studied complexes, (TMPS)FeIII(H2O)(N-MeIm) is highly reactive toward H2O2 to form the iron(IV)-oxo species, (TMPS)FeIV[DOUBLE BOND]O(N-MeIm). The latter species can also be formed in the reaction of (TMPS)FeIII(N-MeIm)2 with H2O2 or in the direct reaction of (TMPS)FeIV[DOUBLE BOND]O(OH) with N-MeIm. Interestingly, the kinetic studies involving substrate oxidation by (TMPS)FeIV[DOUBLE BOND]O(OH) and (TMPS)FeIV[DOUBLE BOND]O(N-MeIm) do not display a pronounced effect of the N-MeIm axial ligand on the reactivity of the compound II mimic in comparison to the OH− substituted analogue. Similarly, DFT computations revealed that the presence of an axial ligand (OH− or N-MeIm) in the trans position to the oxo group in the iron(IV)-oxo species does not significantly affect the activation barriers calculated for C[BOND]H dehydrogenation of the selected organic substrates.