Absolute configurational assignment of serf-organizing asymmetric tripodal ligand-metal complexes

The solution configuration of labile coordination complexes may be difficult to determine, even in cases in which the solid state structure is known. We have previously synthesized a series of chiral ligands which form pseudo-C₃-symmetric complexes with Zn(II) and Cu(II) salts that possess an available electrophilic coordination site. Molecular modeling of Zn(II) complexes of the chiral ligand N,N-bis[(2-quinolyl)methyl]1-(2- pyridyl)ethanamine (α-MeBQPA) showed that the spatial arrangement of the heterocyclic arms is controlled by a substituent on one methylene arm, resulting in the adoption of an enantiomeric conformation displaying a propeller-like asymmetry. In this paper we report the application of the exciton chirality method to the determination of the conformation of asymmetric metal-ligand complexes in solution. There is a good correlation between the predicted and the observed Cotton effects, demonstrating that the geometry in solution closely resembles that predicted by computational simulations and those obtained by X-ray crystallographic studies of metal complexes with racemic and enantiomerically pure ligands. The X-ray crystallographic structure of the first optically pure complex in this series is reported.