Supramolecular Recognition: Use of Cofacially Disposed Bis-terpyridyl Square-Planar Complexes in Self-Assembly and Molecular Recognition

Abstract

A molecular receptor consisting of a molecular spacer that constrains two terpyridyl-palladium(II) complexes to be disposed in a parallel cofacial geometry has been prepared. The separation between the two terpyridyl-palladium units is enforced to be ca. 7 A, a distance sufficient to incarcerate aromatic molecules and square-planar complexes. A number of molecules are shown to associate with this spacer-chelator complex. In particular, 9-methylanthracene (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 analogous molecules that contain two anthracene guests tethered intramolecularly indicate that the structure found in the solid is similar to that in solution. Low-temperature 1H-NMR studies indicate rapid exchange between the two binding sites. The spacer-chelator complexes, when combined with appropriate molecular linkers, readily form molecular rectangles, trigonal prisms, and tetragonal prisms. One molecular rectangle is shown to associate with up to five 9-MA molecules.