Emergence of Symmetry and Chirality in Crown Ether Complexes with Alkali Metal Cations

Crown ethers provide a valuable benchmark for the comprehension of molecular recognition mediated by inclusion complexes. One of the most relevant crown ethers, 18-crown-6 (18c6), features a flexible six-oxygen cyclic backbone that is well-known for its selective cation binding. This study employs infrared spectroscopy and quantum mechanical calculations to elucidate the structure of the gas-phase complexes formed by the 18c6 ether with the alkali metal cations. It is shown that symmetric and chiral arrangements play a dominant role in the conformational landscape of the 18c6 alkali system. Most stable 18c6-M+ conformers are found to have symmetries C-3v and C-2 for Cs+, D-3d for K+, C-1 and D-3d for Na+, and D-2 for Li+. Remarkably, whereas the bare 18c6 ether is achiral, chirality emerges in the C-2 and D-2 18c6-M+ conformations, both of which involve pairs of stable atropoisomers capable of acting as enantiomeric selective substrates.