A theoretical study of alcohol oxidation by ferrate

The conversion of methanol to formaldehyde mediated by ferrate (FeO42-), mo noprotonated ferrate (HFeO4-), and diprotonated ferrate (H2FeO4) is discuss ed with the hybrid B3LYP density functional theory (DFT) method. Diprotonat ed 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-eliminatio n mechanism that involves coordination of methanol to diprotonated ferrate; (2) a direct abstraction mechanism that involves H atom abstraction from t he O-H or C-H bond of methanol. Within the framework of the polarizable con tinuum model (PCM), the energetic profiles of these reaction mechanisms in aqueous solution are calculated and investigated. In the addition-eliminati on 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 formaldehy de is branched into four reaction pathways. The most favorable reaction pat hway 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 sta te to lead to formaldehyde complex. In the direct abstraction mechanism, th e oxidation reaction can be initiated by a direct H atom abstraction from e ither the O-H or C-H bond, and it is branched into three pathways for the f ormation of formaldehyde. The most favorable reaction pathway in the direct abstraction mechanism is initiated by C-H activation that leads to organom etallic intermediate containing an Fe-C bond, followed by a concerted H ato m transfer from the OH group of methanol to an oxo ligand of ferrate. The f irst 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.