Hydrogen-bonding effects in calix[4]arene capsules

The synthesis and spectroscopic characterization of self-assembling calix[4 ]arene based capsules 1a(.)1a and 1b(.)1b are described. These compounds fe ature four urea substituents at the upper rims and four secondary amide fra gments at the lower rims that can participate in inter- and intramolecular hydrogen bonding in apolar solution. Communication between the calixarene r ims in 1a, b influences the self-assembled cavity's size and shape. Specifi cally, dimerization results in a perfect cone conformation of the calixaren e skeleton in 1a, b and stabilizes a seam of intramolecular amide C=O . . . H-N hydrogen bonds at the lower rim. This seam is cycloenantiomeric, with e ither clockwise or counterclockwise arrangements of the head-to-tail amides . Complexation of Na+-cation breaks hydrogen bonds at the lower rim but mai ntains the capsular assembly. Encapsulation properties of 1a(.)1a and 1b(.) 1b were studied in nonpolar solvents and their binary mixtures as well as t hrough heterodimerization experiments. The presence of amide groups at the lower rim causes notable differences in the capsule's binding affinities wh en compared to the corresponding tetraester capsules 1c(.)1e and 1d(.)1d. I n the monomeric state calixarenes 1a, b are in a pinched cone conformation. The solid state X-ray crystallographic studies with monomeric la reveal on ly two intramolecular C=O . . .H-N hydrogen bonds between the adjacent amid es at the lower rim, and an extensive network of intermolecular hydrogen bo nds between urea groups at the upper rim.