A combined Mössbauer, magnetic circular dichroism, and density functional theory approach for iron cross-coupling catalysis: Electronic structure, in situ formation, and reactivity of iron-mesityl-bisphosphines

While iron-bisphosphines have emerged as effective catalysts for C-C cross-coupling, the nature of the in situ formed iron species, elucidation of the active catalysts and the mechanisms of catalysis have remained elusive. A combination of ⁵⁷Fe Mössbauer and magnetic circular dichroism (MCD) spectroscopies of well-defined and in situ formed mesityl-iron(II)-SciOPP species combined with density functional theory (DFT) investigations provides the first direct insight into electronic structure, bonding and in situ speciation of mesityl-iron(II)-bisphosphines in the Kumada cross-coupling of MesMgBr and primary alkyl halides using FeCl₂(SciOPP). Combined with freeze-trapped solution Mössbauer studies of reactions with primary alkyl halides, these studies demonstrate that distorted square-planar FeMes ₂(SciOPP) is the active catalyst for cross-coupling and provide insight into the molecular-level mechanism of catalysis. These studies also define the effects of key reaction protocol details, including the role of the slow Grignard addition method and the addition of excess SciOPP ligand, in leading to high product yields and selectivities.