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

Abstract

ArIO (ArI = 2-(^tBuSO₂)C₆H₄I) 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 Feⁿ═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^IVO(ArIO)]²⁺ (1²⁺, TPA = tris(2-pyridylmethyl)amine)) is highly endothermic; therefore, it can be achieved only in collision-induced dissociation of 1²⁺ leading to the unmasked iron(VI) dioxo complex. The reduction of 1²⁺ by HAT leads to [(TPA)Fe^IIIOH(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 1²⁺ or after OAT reactivity. The latter leads to the 4e^- oxidation of unsaturated hydrocarbons: The initial OAT from [(TPA)Fe^IVO(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^IVO(ArIO)]²⁺ (2²⁺, TQA = tris(2-quinolylmethyl)amine)) because it has a sufficient electron affinity for electron transfer with alkenes. Accordingly, the reaction of 2²⁺ with 2-carene leads to [(TQA)Fe^IIIO(ArIO)]²⁺ that exothermically eliminates ArI and unmasks the reactive Fe^V-dioxo species.