Cavitand-porphyrins

The synthesis and characterization of new nanoscale container molecules 7 a nd 8 are described. They are covalent hybrids of deepened, self-folding cav itands and metalloporphyrins. In receptor 7, the Zn-porphyrin wall is direc tly built onto the cavitand skeleton. Host 8 features a large unimolecular cavity containing two cavitands attached with the Zn-porphyrin wall. Its di mensions, similar to 10 x 25 Angstrom, place it among the largest synthetic hosts prepared to date. A series of adamantyl- and pyridyl-containing gues ts 14-20 of various lengths were prepared and used to determine the hosts' binding abilities in solution using UV/vis and H-1 NMR spectroscopy. Intram olecular hydrogen bonds at the upper rims of the cavitands resist the unfol ding of the inner cavities and thereby increase the energetic barrier to gu est exchange. The exchange is slow on the NMR time scale (at less than or e qual to 300 K), and kinetically stable complexes result. When the cavities and metalloporphyrins participate simultaneously in the binding event, very high affinities for guests are found (-DeltaG(295) up to 10 kcal mol(-1) i n toluene), to which the porphyrin fragments contribute significantly (-Del taG(295) up to 6 kcal mol(-1)). The pairwise selection of two different gue sts by molecular container 8 is reported, and the termolecular complex form ed raises the possibility of metal-catalyzed bimolecular reactions in these containers.