Ate Complexes in Iron-Catalyzed Cross-Coupling Reactions.

Iron-catalyzed cross-coupling reactions have an outstanding potential for sustainable organic synthesis, but remain poorly understood mechanistically. Here, we use electrospray-ionization (ESI) mass spectrometry to identify the ionic species formed in these reactions and characterize their reactivity. Transmetalation of Fe(acac)3 (acac=acetylacetonato) with PhMgCl in THF (tetrahydrofuran) produces anionic iron ate complexes, whose nuclearity (1 to 4 Fe centers) and oxidation states (ranging from -I to +III) crucially depend on the presence of additives or ligands. Upon addition of iPrCl, formation of the heteroleptic FeIII complex [Ph3 Fe(iPr)]- is observed. Gas-phase fragmentation of this complex results in reductive elimination and release of the cross-coupling product with high selectivity.