Iron Catalyzed Aryl-Aryl Kumada Cross-Coupling: A Mechanistic and Computational Investigation

The widespread use of precious metal catalysts in C-C bond-forming reactions is increasingly challenged by concerns over toxicity, cost, and limited availability. As a sustainable alternative, iron offers distinct advantages in cross-coupling chemistry, but its broader application has been hindered by limited mechanistic understanding. Here, we report a mechanistically driven investigation of aryl-aryl Kumada cross-coupling catalyzed by our previously reported iron complex [(PC_NHCP)FeCl₂] (2). Through a combination of multinuclear NMR, ⁵⁷Fe Mössbauer spectroscopy, single-crystal X-ray diffraction, and reactivity studies, we identify and characterize key in situ formed intermediates, including mono- and bis-arylated iron species, along the catalytic pathway. While PCP-ligated Fe(II) complexes support two-electron chemistry, our findings uncover a distinct radical mechanism responsible for the efficient formation of the biaryl products. Furthermore, we demonstrate that small coordinating molecules, such as N₂, significantly influence the speciation and reactivity of the iron catalyst. These insights advance fundamental understanding of iron-mediated cross-coupling and provide new design principles for sustainable C(sp²)-C(sp²) bond construction.