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.