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 sigma-hydrocarbyl Fe-III complex that has been structurally characterized.