TMEDA in Iron-Catalyzed Kumada Coupling: Amine Adduct versus Homoleptic 'ate' Complex Formation

The reactions of iron chlorides with mesityl Grignard reagents and tetramethylethylenediamine (TMEDA) under catalytically relevant conditions tend to yield the homoleptic "ate" complex [Fe(mes)3 ](-) (mes=mesityl) rather than adducts of the diamine, and it is this ate complex that accounts for the catalytic activity. Both [Fe(mes)3 ](-) and the related complex [Fe(Bn)3 ](-) (Bn=benzyl) react faster with representative electrophiles than the equivalent neutral [FeR2 (TMEDA)] complexes. Fe(I) species are observed under catalytically relevant conditions with both benzyl and smaller aryl Grignard reagents. The X-ray structures of [Fe(Bn)3 ](-) and [Fe(Bn)4 ](-) were determined; [Fe(Bn)4 ](-) is the first homoleptic σ-hydrocarbyl Fe(III) complex that has been structurally characterized.