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.

Experimental and computational studies support an inner-sphere radical pathway for iron-catalysed C–H activation/functionalisation with allyl electrophiles.