Catalytic activity tuning of a biomimetic HO-FeV=O oxidant for methane hydroxylation by substituents on aromatic rings: theoretical study

HO-(TPA)FeV=O (TPA = tris(2-pyridylmethyl)amine) has been proposed in the literature as the key high-valent iron-oxo intermediate involved in alkane hydroxylation. Here the structure of this species is investigated theoretically in the framework of density functional theory (DFT). A detailed electronic structure analysis leads to the presumption that the properties of the FeV=O bond can be modified by introducing substituents to the aromatic rings of TPA and thus the reactivity of HO-(TPA)FeV=O for the hydrogen atom abstraction of methane hydroxylation can be tuned on the quartet potential energy surface. The validity of our presumption is verified by DFT calculations. According to the rebound mechanism, the H-abstraction step is examined by using five complexes with TPA and TPA-derivative ligands and the corresponding reaction energies and energy barriers are obtained and compared with each other. The results are fully in agreement with our qualitative model, showing that electron-withdrawing groups are able to lower the barrier and facilitate the reaction, whereas the electron-donating groups increase the barrier and reduce the reactivity.