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)+H₂ 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.