Supramolecular recognition: Use of cofacially disposed bis-terpyridyl square-planar complexes in self-assembly and molecular recognition

A molecular receptor consisting of a molecular spacer that constrains two t erpyridyl-palladium(II) complexes to be disposed in a parallel cofacial geo metry has been prepared. The separation between the two terpyridyl-palladiu m units is enforced to be ca. 7 Angstrom, a distance sufficient to incarcer ate aromatic molecules and square-planar complexes. A number of molecules a re shown to associate with this spacer-chelator complex. In particular, 9-m ethylanthracene (9-MA) is found to form a 1:2 host-guest complex. A crystal structure of this complex shows one 9-MA in the molecular cleft formed by the two terpyridyl-palladium units and the other 9-MA molecule to lie above one of the terpyridyl-palladium units. Nuclear Overhauser effects on analo gous molecules that contain two anthracene guests tethered intramolecularly indicate that the structure found in the solid is similar to that in solut ion. Low-temperature H-1-NMR studies indicate rapid exchange between the tw o binding sites. The spacer-chelator complexes, when combined with appropri ate molecular linkers, readily form molecular rectangles, trigonal prisms, and tetragonal prisms. One molecular rectangle is shown to associate with u p to five 9-MA molecules.