Mechanism of Benzene Hydroxylation on Tri-Iron Oxo-Centered Cluster-Based Metal-Organic Frameworks

High-valent Fe(IV)-oxo species derived upon reactions of N₂O with Fe(II) centers-embedded in the framework of tri-iron oxo-centered-based metal-organic frameworks (MOFs)- selectively affect the conversion of benzene-to-phenol via electrophilic addition to arene C-H bonds akin to oxygen transfer mechanisms in the P450 enzyme. The Fe(II) species identified by Mössbauer spectroscopy can be titrated in situ by the addition of NO to completely suppress benzene oxidation, verifying the relevance of Fe(II) centers. Observed inverse kinetic isotope effects in benzene hydroxylation preclude the involvement of H atom transfer steps from benzene to the Fe(IV)-oxo species and instead suggest that the electrophilic iron-oxo group adds to an sp² carbon of benzene, resulting in a change in the hybridization from sp²-to-sp³. These mechanistic postulates are affirmed in Kohn-Sham density functional calculations, which predict lower barriers for additive mechanisms for arene oxidation than H atom abstraction steps. The calculations show that the reaction proceeds on the pentadectet spin surface and that a non-innocent ligand participates in the transfer of the H atom. Following precedent literature which demonstrates that these Fe(IV)-oxo species react with C-H bonds in alkanes via hydrogen atom abstraction to form alcohols, it appears that iron(IV)-oxo species in MOFs exhibit duality in their reactions with inert hydrocarbon substrates akin to enzymes-if the C-H bonds are in saturated aliphatic hydrocarbons, then activation occurs via hydrogen abstraction, while if the C-H bonds are aromatic, then activation occurs by addition rearrangement.