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 FeMes2(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.