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

Abstract

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.

Crown ether–electrolyte interactions permit nanopore detection of individual DNA abasic sites in single molecules

“…In the case of K þ , the 18c6-K þ complex exists as a single quasiplanar symmetric D 3d conformer, resulting from the ideal size matching between the cation and the polyether cavity. Additionally, this conformer provides ideal solvation, and consequently, stabilizes the system and maximizes its size (∼5.3 Å in radius) (41). Given the high K þ concentration and the high-energy cost for 18c6-K þ to change conformation (>13 kJ mol −1 ), we anticipated that the passage of the 18c6-K þ through the constriction of the α-HL would be difficult.…”

Vibrational Analysis of Azacrown Ether Complex with Li Metal Cation

“…Therefore it is not likely to be seen clearly as reported in recent study of Li + ion complex with 18-Crown-6. 3 The benzo ring C~C~C deformation modes are supposed to appear at around 850 cm . The torsion modes of pyridyl and benzo ring system are anticipated to appear with broadening, but the ether and amide groups would not.…”

“…3 This proposition would be feasible in the case of plenty of flexibility in the cyclic system. Then, the infrared absorption is likely to show less broadening or coalescence in the spectral features.…”

“…Crown ether species have drawn considerable attention because of their binding selectivity with alkali metal cations, [1][2][3][4] Li + , Na , with alkaline-earth metal cations, 4,5 with heavy metal ions, 4,6 or organic functional groups. 7,8 To understand the nature of crown ether-metal cation complexes, the spectroscopic or theoretical studies of complexes in the gas phase, solid phase and solution have been performed in the scope of the coordination structures and conformations.…”

“…The 18-crown-6 backbone has been most studied partly because it is quite flexible and prone to various conformations of symmetry upon complex formation. 3 It is known to form open structures to host the heavier alkali cations, such as Rb + or Cs + , and to adopt folded cage-like conformations for the lighter alkali cations, Li + or Na + . The infrared spectra of these alkali complexes of 18-crown-6 backbone show some peculiarities, for example, continuum features in the mid range of 1500 to 3500 cm -1 or in the far-infrared range, and line broadening or coalescing features of specific infrared absorption bands.…”