Self-recognition based on atropoisomerism with new chiral bidentate ligands and copper(I)

A new family of atropoisomeric bidentate ligands that have a dissymmetric benzimidazole-pyridine binding site has been synthesized. Aromatic rings, that is, naphthyl, tolyl and cumyl, were introduced in order to fine tune the complexation properties of the ligands. The tetrahedral copper(I) complexes L2Cu were prepared and the structure of the complex with the naphthyl-substituted ligand was established by X-ray diffraction. The behavior of the L2Cu complexes in solution was studied by 1H NMR spectroscopy. With the most crowded cumyl-derived ligand, ligand self-recognition based on chirality occured: 95% of the complex was present in solution as a racemate RRdelta/SSlambda, the heterochiral RSdelta/SRlambda isomers represented only 5 % of the mixture, and the RRlambda/SSdelta isomers were not detected. Owing to lower steric repulsions within the other L2Cu complexes (i.e., with the naphthyl- and tolyl-based ligands) the homorecognition is less pronounced, as diastereomeric excesses of 6 and 26% were measured, respectively.