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.