Supramolecular Recognition. Terpyridyl Palladium and Platinum Molecular Clefts and Their Association with Planar Platinum Complexes

Molecular receptors, consisting of either two parallel cofacially disposed terpyridyl−Pd−Cl+ or terpyridyl−Pt−Cl+ units, are described. Concerted rotation of these units about the molecular spacer can alter their separation between 6.4 and 7.2 Å to accommodate the dimensions of molecular guests. Neutral and anionic planar complexes of platinum(II) were investigated as guests to determine if metal−metal interaction between the host and guest metals could stabilize host−guest association. With a neutral guest, it was found that host−guest formation is signaled by a color change from light yellow to deep red. For one of the anionic guests, a visible absorption band appears upon host−guest formation with the platinum receptor that is ascribed to transitions associated with a Pt−Pt interaction. The association constants found for the neutral guest with the palladium and platinum receptors are large, suggesting that metal−metal interaction contributes to the molecular recognition. The structures of the host−(neutral)guest complexes in solution have been determined by 1H NOESY spectra. A crystal structure of the platinum host−(neutral)guest complex is the same as that found in solution and confirms the presence of a Pt−Pt interaction. Temperature-dependent 195Pt NMR spectra in solution provide a quantitative estimate of the conformational interconversions of the free platinum receptor.