Unmasking Steps in Intramolecular Aromatic Hydroxylation by a Synthetic Nonheme Oxoiron(IV) Complex

In this study, a methyl group on the classic tetramethylcyclam (TMC) ligand framework is replaced with a benzylic group to form the metastable [FeIV(Osyn)(Bn3MC)]2+ (2‐syn; Bn3MC=1‐benzyl‐4,8,11‐trimethyl‐1,4,8,11‐tetraazacyclotetradecane) species at −40 °C. The decay of 2‐syn with time at 25 °C allows the unprecedented monitoring of the steps involved in the intramolecular hydroxylation of the ligand phenyl ring to form the major FeIII−OAr product 3. At the same time, the FeII(Bn3MC)2+ (1) precursor to 2‐syn is re‐generated in a 1:2 molar ratio relative to 3, accounting for the first time for all the electrons involved and all the Fe species derived from 2‐syn as shown in the following balanced equation: 3 [FeIV(O)(LPh)]2+ (2‐syn)→2 [FeIII(LOAr)]2+ (3)+[FeII(LPh)]2+ (1)+H2O. This system thus serves as a paradigm for aryl hydroxylation by FeIV=O oxidants described thus far. It is also observed that 2‐syn can be intercepted by certain hydrocarbon substrates, thereby providing a means to assess the relative energetics of aliphatic and aromatic C−H hydroxylation in this system. The nonheme FeIV=O complex 2‐syn (in green circle) can be generated from the reaction of its precursor [FeII(Bn3MC)]2+ (1) with an oxo‐transfer agent. At 25 °C, 2‐syn decays to form the major FeIII−OAr oxidation product 3 (in blue circle) and regenerates its FeII(Bn3MC)2+ precursor (in red circle) in a 2:1 ratio. This observed stoichiometry demonstrates for the first time the electron flow required to effect this novel transformation.