¹H-ENDOR evidence for a hydrogen-bonding interaction that modulates the reactivity of a nonheme Fe ^IV=O unit

We report that a novel use of 35 GHz ¹H-ENDOR spectroscopy establishes the presence in 1 of an Fe ^IV-O⋯H-O-Fe ^III hydrogen bond predicted by density functional theory computations to generate a six-membered-ring core for 1. The hydrogen bond rationalizes the difference in the C-H bond cleavage reactivity between 1 and 4 (OCH ₃) (where a CH ₃O group has replaced the HO on the Fe ^III site). This result substantiates the seemingly paradoxical conclusion that the nonheme Fe ^IV-O unit of 1 not only has the electrophilic character required for H-atom abstraction but also retains sufficient nucleophilic character to accept a hydrogen bond from the Fe ^III-OH unit.