Zinc complexes of bipyrrolidine-based diamine-diphenolato and diamine-diolato ligands: Predetermination of helical chirality around tetrahedral centres

The coordination chemistry of chiral tetradentate dianionic ligands of the diamine-diphenolato and diamine-diolato families having the (R,R)-2,2′- bipyrrolidine core around Zn^II was investigated. Reactions with diethylzinc led to complexes of the type [{ONNO}Zn] for most ligands and to bridging dinuclear complexes of the type [μ-Lig⁴(ZnEt) ₂] for one of the diamine-diolato ligands. Reactions with bis(hexamethyldisilazide)zinc led to complete conversions. Most complexes were obtained as mononuclear complexes. The least bulky Salan ligand, Lig ³, led to an equilibrium between mononuclear and dinuclear complexes in noncoordinating solvents. All ligands were found to wrap around the tetrahedral zinc with very high diastereoselectivities supporting predetermined chiral induction from the bipyrrolidine core to the helical ligand wrapping. Molecular structures determined by single-crystal X-ray diffraction for two complexes of the type [{ONNO}Zn] substantiated the predicted Δ wrapping of the (R,R)-based ligands. In contrast, representative Salan and diamine-diolato ligands assembled around the trans-1,2-diaminocyclohexane core led to diastereomer mixtures of the corresponding complexes. Chiral tetradentate dianionic ligands of the diamine-diphenolato and diamine-diolato families based on the (R,R)-2,2′-bipyrrolidine core led to chiral-at-metal complexes of the type [{ONNO}Zn] as single diastereomers supporting a predetermined chiral induction from the bipyrrolidine core to the helical ligand wrapping.