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 (TS1-1) 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 (TS1-2) 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 (TS2-1) 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 (TS2-2) 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 (TS3-1) 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 (kH/kD) for the electronic processes of TS1-1, TS2-1, and TS3-1 are computed and analyzed

Femtosecond Dynamics of the Methane−Methanol and Benzene−Phenol Conversions by an Iron−Oxo Species

Femtosecond dynamic behavior of the methane−methanol conversion by the bare iron−oxo complex (FeO+) is presented using the B3LYP density-functional-theory (DFT) method. We propose that the reaction pathway for the direct methane−methanol conversion is partitioned into the H atom abstraction via a four-centered transition state and the methyl migration via a three-centered transition state. It is demonstrated that both the H atom abstraction and the methyl migration occur in a concerted manner in a time scale of 100 fs. The concerted H atom abstraction and the direct H atom abstraction via a transition state with a linear C−H−O(Fe) array are compared. The direct H atom abstraction of methane is predicted to occur in a time scale of 50 fs. Isotope effects on the concerted and the direct H(D) atom abstractions are also computed and analyzed in the FeO+/methane system. Predicted values of the kinetic isotope effect (kH/kD) for the H(D) atom abstraction of methane are 9 in the concerted mechanism and 16 in the direct abstraction mechanism at 300 K. Dynamics calculations are also carried out on the benzene−phenol conversion by the FeO+ complex. The general profile of the electronic process of the benzene−phenol conversion is identical to that of the methane−methanol conversion with respect to essential bonding characters. It is demonstrated that the concerted H atom abstraction and the phenyl migration require 200 and 100 fs to be completed, respectively, in the FeO+/benzene system