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 (1)H-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(3)O 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.