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.