The Reactivity of Bispidine Ligand Based Iron(IV) and Iron(V) Oxido Species for the Demethylation of Acetic Acid

The high valent bispidine iron-oxido complexes LFe^IV═O and LFe^V═O show exceptionally high reactivity for C-H abstraction reactions. While the reactivity, electronics of the reactive species and general mechanisms have been studied in detail before, differences in reactivity of [(L)Fe^IV═O]²⁺ and [(L)Fe^V═O]³⁺, specifically for the demethylation of acetic acid, remained unexplored. Following experimental work of the C-H abstraction reaction by ferryl complexes of tetradentate bispidine ligands with acetic acid as substrate, this reaction was investigated with computational methods. A density functional theory (DFT) study of the iron bispidine catalyzed mechanism with hydrogen peroxide as oxidant and acetic acid as substrate and precursor of various small organic products, specifically methane, formaldehyde and formic acid, is used to analyze details of the reaction mechanism and specifically also to investigate the influence of different oxidation states of iron on this reaction. Therefore, two pathways, catalyzed by bispidine based [(L)Fe^IV═O]²⁺ and [(L)Fe^V═O]³⁺ species with acetic acid were considered. The computational analysis confirms that the rate-determining step of the demethylation is C-H abstraction at C2 of acetic acid, leading to a radical intermediate that leads to the various products. The [(L)Fe^V═O]³⁺ catalyzed reaction is found to be more efficient than that based on [(L)Fe^IV═O]²⁺. Reasons for the different reactivities have been explored by the WB index, EDA and an NBO analysis.