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.
Bibliographical noteFunding Information:
We thank the National Science Foundation for support of this work (CHE1665391 to L.Q.) and Dr. Rahul Banerjee for his insightful comments on the kinetic analyses. XAS data were collected at the Stanford Synchrotron Radiation Lightsource, which is supported by the U.S. DOE under Contract No. DEAC02‐76SF00515. Use of Beamline 9‐3 is supported by the DOE Office of Biological and Environmental Research and the National Institutes of Health, National Institute of General Medical Sciences (including P41GM103393).
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- aromatic hydroxylation
- intramolecular vs. intermolecular oxidation
- nonheme iron
- reaction mechanism