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 [Fe IV (O syn )(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 Fe III −OAr product 3 . At the same time, the Fe II (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 [Fe IV (O)(L Ph )] 2+ ( 2 ‐syn)→2 [Fe III (L OAr )] 2+ ( 3 )+[Fe II (L Ph )] 2+ ( 1 )+H 2 O. This system thus serves as a paradigm for aryl hydroxylation by Fe IV =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.