Evidence for an alternative to the oxygen rebound mechanism in C-H bond activation by non-heme Fe(IV)O complexes.

The hydroxylation of alkanes by heme Fe(IV)O species occurs via the hydrogen abstraction/oxygen rebound mechanism. It has been assumed that non-heme Fe(IV)O species follow the heme Fe(IV)O paradigm in C-H bond activation reactions. Herein we report theoretical and experimental evidence that C-H bond activation of alkanes by synthetic non-heme Fe(IV)O complexes follows an alternative mechanism. Theoretical calculations predicted that dissociation of the substrate radical formed via hydrogen abstraction from the alkane is more favorable than the oxygen rebound and desaturation processes. This theoretical prediction was verified by experimental results obtained by analyzing iron and organic products formed in the C-H bond activation of substrates by non-heme Fe(IV)O complexes. The difference in the behaviors of heme and non-heme Fe(IV)O species is ascribed to differences in structural preference and exchange-enhanced reactivity. Thus, the general consensus that C-H bond activation by high-valent metal-oxo species, including non-heme Fe(IV)O, occurs via the conventional hydrogen abstraction/oxygen rebound mechanism should be viewed with caution.