Crystal structures and solution conformations of the meso-forms of 2,3,11,12-Tetraphenyl-[18]crown-6

The X-ray crystal stuctures of the achiral cis-anti-cis and trans-syn-trans diastereoisomers of 2,3,11,12-tetraphenyl-[18]-crown-6 and their sodium iodide 1:1 complexes are presented. In the free ligands, the vicinal phenyl groups assume a diaxial- antiperiplanar orientation in the cis-anti-cis isomer and a diequatorial-synclinal one in the trans-syn-trans isomer. The vicinal coupling constants of the benzylic protons in the diphenylethanediyl groups in the three meso-forms, derived from 13C satellites, suggest similar conformations in the solid state and in deuteriotrichloromethane solution. Conformational changes of the crown ethers from the free ligand to their complexes are discussed in the context of previously determined complex association constants.