Reaction pathways for the oxidation of methanol to formaldehyde by an iron-oxo species

The reaction mechanism and energetics for the conversion of methanol to formaldehyde by an iron-oxo species, FeO⁺, is investigated. Three competitive reaction pathways for the catalytic reaction are analyzed from DFT computations at the B3LYP level of theory. In Path 1, the H atom of the OH group of methanol is first abstracted by the oxo group of FeO⁺ via a four-centered transition state (TS_1-1) leading to the intermediate complex HO-Fe⁺-OCH₃, and after that one of the H atoms of the OCH₃ group is shifted to the OH ligand via a five-centered transition state (TS_1-2) to form the final product complex H₂O-Fe⁺-OCH₂. In Path 2, one of the H atoms of the CH₃ group of methanol is abstracted by the oxo group via a five-centered transition state (TS_2-1) leading to the intermediate complex HO-Fe⁺-OHCH₂, and then the H atom of the OHCH₂ group is shifted to the OH ligand via a four-centered transition state (TS_2-2) to give the product complex. Unlike Paths 1 and 2, which involve a hydrogen shift, the first step in Path 3 involves a methyl migration that takes place via a four-centered transition state (TS_3-1) resulting in the formation of the intermediate complex HO-Fe⁺-OCH₃ and the second half of Path 3 is identical to that of Path 1. From B3LYP computations, Path 1 and Path 2 are competitive in energy and Path 3 is unlikely from the energetic viewpoint. Kinetic isotope effects (k_H/k_D) for the electronic processes of TS_1-1, TS_2-1, and TS_3-1 are computed and analyzed.