Cavitand−Porphyrins

Abstract

The synthesis and characterization of new nanoscale container molecules 7 and 8 are described. They are covalent hybrids of deepened, self-folding cavitands and metalloporphyrins. In receptor 7, the Zn-porphyrin wall is directly built onto the cavitand skeleton. Host 8 features a large unimolecular cavity containing two cavitands attached with the Zn-porphyrin wall. Its dimensions, ∼10 × 25 Å, place it among the largest synthetic hosts prepared to date. A series of adamantyl- and pyridyl-containing guests 14 − 20 of various lengths were prepared and used to determine the hosts' binding abilities in solution using UV/vis and ¹H NMR spectroscopy. Intramolecular hydrogen bonds at the upper rims of the cavitands resist the unfolding of the inner cavities and thereby increase the energetic barrier to guest exchange. The exchange is slow on the NMR time scale (at ≤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 (−ΔG²⁹⁵ up to 10 kcal mol^-1 in toluene), to which the porphyrin fragments contribute significantly (−ΔG²⁹⁵ up to 6 kcal mol^-1). The pairwise selection of two different guests by molecular container 8 is reported, and the termolecular complex formed raises the possibility of metal-catalyzed bimolecular reactions in these containers.