Imidazol-2-ylidene-Based NCCN Ligands for Chiral-at-Iron Catalysis

A new class of achiral tetradentate NCCN ligands containing two imidazol-2-ylidene moieties tethered by a tetraphenylene linker is reported. Upon coordination to iron-(II), these ligands enforce a C ₂-symmetric cis-α coordination geometry, where the helical twist of the tetradentate ligand induces metal-centered chirality. The racemic complexes were resolved into their individual enantiomers using a chiral auxiliary approach. Notably, the tetradentate framework provides significantly enhanced configurational stability compared to previously reported bis-bidentate analogues, showing negligible racemization in the presence of air and water. Finally, the catalytic utility of these chiral-at-iron complexes was demonstrated across three distinct asymmetric transformations, namely an intramolecular Cannizzaro reaction (up to 82% ee), an enantioselective C-H amidation (up to 81% ee), and a hetero-Diels-Alder reaction (up to 94% ee under open air conditions). Collectively, these results demonstrate the versatility of the new tetradentate NCCN architecture in stabilizing iron-based catalysts and facilitating efficient asymmetric induction through purely metal-centered chirality.