Identification and Reactivity of Cyclometalated Iron(II) Intermediates in Triazole-Directed Iron-Catalyzed C-H Activation.

While iron-catalyzed C-H activation offers an attractive reaction methodology for organic transformations, the lack of molecular-level insight into the in situ formed and reactive iron species impedes continued reaction development. Herein, freeze-trapped 57Fe Mössbauer spectroscopy and single-crystal X-ray crystallography combined with reactivity studies are employed to define the key cyclometalated iron species active in triazole-assisted iron-catalyzed C-H activation. These studies provide the first direct experimental definition of an activated intermediate, which has been identified as the low-spin iron(II) complex [(sub-A)(dppbz)(THF)Fe]2(μ-MgX2), where sub-A is a deprotonated benzamide substrate. Reaction of this activated intermediate with additional diarylzinc leads to the formation of a cyclometalated iron(II)-aryl species, which upon reaction with oxidant, generates C-H arylated product at a catalytically relevant rate. Furthermore, pseudo-single-turnover reactions between catalytically relevant iron intermediates and excess nucleophile identify transmetalation as rate-determining, whereas C-H activation is shown to be facile under the reaction conditions.