Structural elucidation of 1,3-phenylenebis(methylene)-bridged calix[6]arenes with four uncapped hydroxy groups

1,3-Phenylenebis(methylene)-bridged calix[6]arenes with four uncapped hydroxy groups at the lower rim and various kinds of functionalities on the bridging unit were synthesized. Their structures in solution and in the crystalline state were determined by NMR spectroscopy, molecular mechanics calculations, and X-ray crystallographic analyses. They were found to adopt a pinched cone conformation, where the bridging 1,3-phenylenebis(methylene) unit lies below in such a way that it forms the bottom of the cone. There is a strong intramolecular hydrogen bonding network forming a cyclic array in these molecules; this network is considered to play an important role in the stabilization of the pinched cone conformation. The conformational mobility of the bridging unit was also investigated. It was found that the steric demand of the central functionality has a great influence on the swinging motion of the bridging unit.