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

Abstract

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.