The variation of coordination modes of aromatic imines in iron carbonyl complexes: Is there a correlation between bond lengths in organometallic model compounds and the reactivity of the ligands in catalytic C–C bond formation reactions?

The reaction of aromatic imines with Fe2(CO)9 proceeds via a two-step reaction sequence. A C–H activation reaction in ortho-position with respect to the exocyclic imine function is followed by an intramolecular hydrogen transfer reaction towards the former imine carbon atom. The resulting dinuclear iron carbonyl complexes show an aza-ferra-cyclopentadiene ligand which is apically coordinated by the second iron tricarbonyl moiety. Comparing the bond lengths of 43 different compounds, which were synthesized and structurally characterized in our group shows that the iron iron bond length correlates with one of the iron carbon bond lengths. The longer the iron carbon bond between the apically coordinated iron atom and the carbon atom next to the former imine carbon atom is, the shorter is the iron iron bond. The same ligands may be used as the substrates in ruthenium catalyzed C–C bond formation reactions. Whereas most of the imines react via the formal insertion of CO and/or ethylene into the C–H bond in ortho-position to the imine function, the ligands that show the longest iron carbon bond lengths in the model compounds under the same reaction conditions produce different types of isoindolones.

The molecular structures of 43 different dinuclear iron carbonyl complexes derived from aromatic imine ligands are compared. The longer the iron carbon bond length Feap–Cβ gets, the shorter is the iron iron bond. If the same ligands are used as the substrates in catalytic C–C bond formation reactions with CO and/or ethylene, only those with the longest Feap–Cβ bonds produce heterocyclic compounds whereas the other imines react via the formal acylation of the aromatic system in ortho-position with respect to the exocyclic imine substituent.Figure optionsDownload as PowerPoint slide