The use of N-methylpyrrolidone (NMP) as a co-solvent in ferric salt catalyzed cross-coupling reactions is crucial for achieving the highly selective, preparative scale formation of cross-coupled product in reactions utilizing alkyl Grignard reagents. Despite the critical importance of NMP, the molecular level effect of NMP on in situ formed and reactive iron species that enables effective catalysis remains undefined. Herein, we report the isolation and characterization of a novel trimethyliron(II) ferrate species, [Mg(NMP) ][FeMe ] (1), which forms as the major iron species in situ in reactions of Fe(acac) and MeMgBr under catalytically relevant conditions where NMP is employed as a co-solvent. Importantly, combined GC analysis and Fe Mössbauer spectroscopic studies identified 1 as a highly reactive iron species for the selective formation generating cross-coupled product. These studies demonstrate that NMP does not directly interact with iron as a ligand in catalysis but, alternatively, interacts with the magnesium cations to preferentially stabilize the formation of 1 over [Fe Me ] cluster generation, which occurs in the absence of NMP.

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.

Magnetic circular dichroism studies of Fe(ii) binding to human calprotectin demonstrate the nature of Fe(ii) coordination at two different Fe(ii)-binding sites and provide insight into how Ca(ii) modulates Fe(ii) coordination.

The first direct syntheses, structural characterizations, and reactivity studies of multinuclear iron–phenyl species formed upon reaction of Fe(acac)3 and PhMgBr in THF are described.