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.
Departamento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, 41013 Seville, Spain. bmarhay@upo.es
Bruno
B
Martínez-Haya
Paola
P
Hurtado
Ana R
AR
Hortal
Said
S
Hamad
Jeffrey D
JD
Steill
Jos
J
Oomens