A theoretical study of alcohol oxidation by ferrate

The conversion of methanol to formaldehyde mediated by ferrate (FeO₄^2-), monoprotonated ferrate (HFeO₄^-), and diprotonated ferrate (H₂FeO₄) is discussed with the hybrid B3LYP density functional theory (DFT) method. Diprotonated ferrate is the best mediator for the activation of the O-H and C-H bonds of methanol via two entrance reaction channels: (1) an addition-elimination mechanism that involves coordination of methanol to diprotonated ferrate; (2) a direct abstraction mechanism that involves H atom abstraction from the O-H or C-H bond of methanol. Within the framework of the polarizable continuum model (PCM), the energetic profiles of these reaction mechanisms in aqueous solution are calculated and investigated. In the addition-elimination mechanism, the O-H and C-H bonds of ligating methanol are cleaved by an oxo or hydroxo ligand, and therefore the way to the formation of formaldehyde is branched into four reaction pathways. The most favorable reaction pathway in the addition-elimination mechanism is initiated by an O-H cleavage via a four-centered transition state that leads to intermediate containing an Fe-O bond, followed by a C-H cleavage via a five-centered transition state to lead to formaldehyde complex. In the direct abstraction mechanism, the oxidation reaction can be initiated by a direct H atom abstraction from either the O-H or C-H bond, and it is branched into three pathways for the formation of formaldehyde. The most favorable reaction pathway in the direct abstraction mechanism is initiated by C-H activation that leads to organometallic intermediate containing an Fe-C bond, followed by a concerted H atom transfer from the OH group of methanol to an oxo ligand of ferrate. The first steps in both mechanisms are all competitive in energy, but due to the significant energetical stability of the organometallic intermediate, the most likely initial reaction in methanol oxidation by ferrate is the direct C-H bond cleavage.