Geometric and Symmetric Adaptations of Crown Ether upon Its Coordination to Alkali Metal Cations on the Surface

Abstract

Crown ethers (CRs), known for their selective affinity toward alkali metal cations, play a crucial role in supramolecular chemistry. In this study, scanning tunneling microscopy and qPlus-atomic force microscopy are employed to visualize 18-crown-6 (18C6) complexes with various alkali metal cations (Li⁺, K⁺, and Cs⁺) on Cu(110) at a submolecular level. These visualizations are supplemented by theoretical calculations and simulations that reveal the geometric and symmetrical adaptations occurring upon the cation accommodations. It is observed that upon its coordination to the K⁺ cation, the 18C6 molecule transforms its crown ring into a regular triangular feature in D_3d symmetry with a binding energy of 1.21 eV. In contrast, the 18C6 molecule maintains a similar triangular feature in C_3v symmetry upon its coordination to the Cs⁺ cation, but the formed complex loses its planar symmetry, and the Cs⁺ cation is about 110 pm vertically off the central oxygen plane of the 18C6 molecule, resulting in a reduced binding energy of 0.57 eV. However, the most striking change occurs upon its coordination to the Li⁺ cation; the 18C6 molecule depicts two configurations, an elongated triangle and a distorted one, in C₁ and C_s symmetry, with significantly lower binding energies of 0.08 and 0.31 eV, respectively. These symmetrical variations in the coordination complexes highlight the optimal compatibility of the 18C6 molecule with the K⁺ cation. Our experimental study provides a model case with atomic insights into the host–guest recognition mechanisms of the formed alkali cation–CR complexes.