Supramolecular recognition: association of palladium molecular clefts with planar platinum complexes

The crystal structures of two new molecular recognition adducts formed between a dicationic, di-terpyridyl-Pd–Cl molecular cleft and two square planar platinum complexes are reported. In both structures, the planar platinum-containing guests are located within the molecular cleft formed by the two parallel disposed terpyridyl-Pd–Cl+ units of the receptor. The crystal structure of the adduct formed between the molecular cleft and a neutral platinum complex has interplanar distances between the host and guest of 3.24 Å, a distance shorter than that usually ascribed to π-stacking interaction (∼3.45 Å). The short distance is likely the result of metal–metal interaction between the host and guest. The second adduct, that between the dicationic molecular receptor and an anionic platinum complex, also bears the guest within the molecular cleft. The interplanar distances between the cationic terpyridyl-Pd–Cl units of the host and the anionic guest (3.21 and 3.29 Å) are also shorter than typical π-stacking distances but no metal–metal interaction is present. Coulombic attraction between the host and guest is believed to be responsible for the short interplanar separation. These data are discussed in relation to analogous systems that associate through π–π and metal–metal interaction.