Unmasking the Iron–Oxo Bond of the [(Ligand)Fe-OIAr](2+/+) Complexes

[Image: see text] ArIO (ArI = 2-((t)BuSO(2))C(6)H(4)I) is an oxidant used to oxidize Fe(II) species to their Fe(IV)-oxo state, enabling hydrogen-atom transfer (HAT) and oxygen-atom transfer (OAT) reactions at low energy barriers. ArIO, as a ligand, generates masked Fe(n)=O species of the type Fe((n-2))-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)Fe(IV)O(ArIO)](2+) (1(2+), TPA = tris(2-pyridylmethyl)amine)) is highly endothermic; therefore, it can be achieved only in collision-induced dissociation of 1(2+) leading to the unmasked iron(VI) dioxo complex. The reduction of 1(2+) by HAT leads to [(TPA)Fe(III)OH(ArIO)](2+) 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 1(2+) or after OAT reactivity. The latter leads to the 4e(–) oxidation of unsaturated hydrocarbons: The initial OAT from [(TPA)Fe(IV)O(ArIO)](2+) 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)Fe(IV)O(ArIO)](2+) (2(2+), TQA = tris(2-quinolylmethyl)amine)) because it has a sufficient electron affinity for electron transfer with alkenes. Accordingly, the reaction of 2(2+) with 2-carene leads to [(TQA)Fe(III)O(ArIO)](2+) that exothermically eliminates ArI and unmasks the reactive Fe(V)–dioxo species.