Unmasking the Iron-Oxo Bond of the [(Ligand)Fe-OIAr] Complexes.

ArIO (ArI = 2-(BuSO)CHI) is an oxidant used to oxidize Fe species to their Fe-oxo state, enabling hydrogen-atom transfer (HAT) and oxygen-atom transfer (OAT) reactions at low energy barriers. ArIO, as a ligand, generates masked Fe═O species of the type Fe-OIAr. Herein, we used gas-phase ion-molecule reactions and DFT calculations to explore the properties of masked iron-oxo species and to understand their unmasking mechanisms. The theory shows that the I-O bond cleavage in [(TPA)FeO(ArIO)] (, TPA = tris(2-pyridylmethyl)amine)) is highly endothermic; therefore, it can be achieved only in collision-induced dissociation of leading to the unmasked iron(VI) dioxo complex. The reduction of by HAT leads to [(TPA)FeOH(ArIO)] with a reduced energy demand for the I-O bond cleavage but is, however, still endothermic. The exothermic unmasking of the Fe═O bond is predicted after one-electron reduction of or after OAT reactivity. The latter leads to the 4e oxidation of unsaturated hydrocarbons: The initial OAT from [(TPA)FeO(ArIO)] leads to the epoxidation of an alkene and triggers the unmasking of the second Fe═O bond still within one collisional complex. The second oxidation step starts with HAT from a C-H bond and follows with the rebound of the C-radical and the OH group. The process starting with the one-electron reduction could be studied with [(TQA)FeO(ArIO)] (, TQA = tris(2-quinolylmethyl)amine)) because it has a sufficient electron affinity for electron transfer with alkenes. Accordingly, the reaction of with 2-carene leads to [(TQA)FeO(ArIO)] that exothermically eliminates ArI and unmasks the reactive Fe-dioxo species.