Self-Recognition Based on Atropoisomerism with New Chiral Bidentate Ligands and Copper( i ) Jean-Marc Vincent,* [a] Christian Philouze, [b] Isabelle Pianet, [c] and Jean-Baptiste Verlhac* [a] Abstract: A new family of atropoiso- meric bidentate ligands that have a dissymmetric benzimidazole-pyridine binding site has been synthesized. Aro- matic rings, that is, naphthyl, tolyl, and cumyl, were introduced in order to fine tune the complexation properties of the ligands. The tetrahedral copper( i ) com- plexes L 2 Cu were prepared and the structure of the complex with the naph- thyl-substituted ligand was established by X-ray diffraction. The behavior of the L 2 Cu complexes in solution was studied by 1 H NMR spectroscopy. With the most crowded cumyl-derived ligand, ligand self-recognition based on chirality oc- cured: 95% of the complex was present in solution as a racemate RRD/SSL, the heterochiral RSD/SRL isomers repre- sented only 5% of the mixture, and the RRL/SSD isomers were not detected. Owing to lower steric repulsions within the other L 2 Cu 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. Keywords: atropoisomerism ´ chir- ality ´ copper ´ N ligands ´ self- assembly Introduction In 1993, Lehn and co-workers [1] introduced the ligand self- recognition process as a new paradigm for the selective synthesis of supramolecular architectures. [2] The synthesis of a well-defined superstructure can be programmed by using a mixture of ªinstructedº components that contain steric and electronic information. By treating copper( i ) ions with a mixture of oligo-2,2'-bipyridine strands, which differ in the number of binding sites, self-assembly of the double helicates with two di-, tri-, tetra-, or penta-topic [oligo(2,2')-bipyridine] occured spontaneously. Other examples of ligand homorec- ognition in helicates have been reported based on 1) the preferred coordination geometry of the metal ions in the presence of an appropriate structure of the ligand, [1] 2) the differences in distances between the metal-chelating groups of ligands, [3] or 3) the nature of the countercation that binds to the tetra-anionic complexes formed. [4] Ligand recognition based on chirality was shown to play an important role in the non-linear effects in asymmetric reac- tions. [5] However, in such cases the heterorecognition is favored; this leads to the formation of heterochiral, oligomeric metal species. Recently, Stack and co-workers reported the first example of ligand homorecognition based solely on chirali- ty. [6] Starting from a racemic mixture of a tetradentate amino ligand that contained a trans-1,2-diaminocyclohexane back- bone, a racemic mixture of stereospecific, homochiral, binu- clear complexes L,L-[{Cu( RR L)} 2 ] 2 and D,D-[{Cu( SS L)} 2 ] 2 was formed selectively. The homochiral complexes produced a more compact and stable cubic structure than the heterochiral ones. A more intricate example of self-recognition based on chi- rality was described in the self-assembly of [2  2] grids from ditopic ligands that possess terpyridine-like binding sites. [7] We were interested in studying the self-recognition process for a structure encountered widely in coordination chemistry: two bidentate ligands arranged in a tetrahedral complex. Here, we report that a racemate of an atropoisomeric bidentate ligand can self-assemble stereoselectively in the presence of copper( i ) ions in such a way as to favor the formation of homochiral complexes. Results and Discussion Preparation of the ligands : The ligands 4, 5, and 6, which contain a dissymmetric binding site with a pyridine and a benzimidazole ring, were prepared as racemic mixtures (Scheme 1). The pyridine-benzimidazole moiety has proved [a] Dr. J.-M. Vincent, Prof. J.-B. Verlhac Laboratoire de Chimie Organique et Organome Âtallique CNRS, UMR 5802 351, cours de la Libe Âration, 33405 Talence Cedex (France) Fax: ( 33)556-84-69-94 E-mail: j-b.verlhac@lcoo.u-bordeaux.fr [b] Dr. C. Philouze Institut de Chimie des Substances Naturelles CNRS Ba Ãt. 27, Avenue de la Terasse 91198 Gif sur Yvette (France) [c] Dr. I. Pianet Cesamo, UMR 5802, Universite  Bordeaux I 351 Cours de la Libe Âration, 33405 Talence Cedex (France) FULL PAPER Chem. Eur. J. 2000, 6, No. 19  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 0947-6539/00/0619-3595 $ 17.50+.50/0 3595