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Near-UV excitation of non-heme Fe IV O complexes results in light intensity dependent increase in reaction rates for the oxidation of C–H bonds even at low temperature (−30 °C). The enhancement of activity is ascribed to the ligand-to-[Fe IV O] charge transfer character of the near-UV bands to generate a highly reactive [(L + ) Fe III –O*] species. The enhancement is not observed with visible/NIR excitation of the d–d absorption bands. 

Abstract Thesynandantiisomers of [Fe^IV(O)(TMC)]²⁺(TMC=tetramethylcyclam) represent the first isolated pair of synthetic non‐heme oxoiron(IV) complexes with identical ligand topology, differing only in the position of the oxo unit bound to the iron center. Both isomers have previously been characterized. Reported here is that thesynisomer [Fe^IV(O_syn)(TMC)(NCMe)]²⁺(2) converts into itsantiform [Fe^IV(O_anti)(TMC)(NCMe)]²⁺(1) in MeCN, an isomerization facilitated by water and monitored most readily by¹H NMR and Raman spectroscopy. Indeed, when H₂¹⁸O is introduced to2, the nascent1becomes¹⁸O‐labeled. These results provide compelling evidence for a mechanism involving direct binding of a water moleculetransto the oxo atom in2with subsequent oxo–hydroxo tautomerism for its incorporation as the oxo atom of1. The nonplanar nature of the TMC supporting ligand makes this isomerization an irreversible transformation, unlike for their planar heme counterparts.