# The Role of Ion Size and π‐Interaction in Stabilizing Calix[4]arene Crown Ether Metal Complexes

## TL;DR

This study shows how ion size and π-interactions affect the stability of MAXCalix complexes, with a preference for larger ions like cesium.

## Contribution

The study reveals the structural and stabilizing role of π-interactions in MAXCalix metal complexes, particularly for large cations.

## Key findings

- MAXCalix forms stable complexes with both large and small ions, with distinct structural subtypes identified.
- π-Interactions between cations and benzene rings significantly enhance complex stability, especially for larger ions like Cs+.
- Complex stability increases with ionic radius, as confirmed by competitive NMR studies.

## Abstract

This study systematically investigates the influence of ion size on the structure and stability of complexes formed between the calix[4]arene crown ether 1,3‐alt‐25,27‐bis(3,7‐ dimethyloctyl‐1‐oxy)calix[4]arenebenzocrown‐6 (MAXCalix) and mono‐ and divalent ions from the alkali and alkaline earth metal series. NMR spectroscopy studies revealed that while MAXCalix efficiently coordinates large ions such as Cs+, it also forms complexes with smaller ions like Na+, highlighting the ligand's versatility. The size of the ion directly influences the complex structure, with two distinct structural subtypes identified via NMR and DFT calculations. In addition, π‐interactions between the cation and the cation‐facing benzene rings of the calix[4]arene backbone play an important role in stabilizing the complex. Larger ions like Cs+ benefit from π‐interactions with both cation‐facing rings, whereas smaller ions like K+ interact with only one ring, if any. These π‐interactions are primarily drivers of the enhanced affinity for Cs+ and the resulting higher complex stability. Competitive NMR studies further confirmed that the complex stability increases with increasing ionic radius, and ions of similar size show comparable stability of their complexes.

This work investigates MAXCalix's ability to coordinate alkali and alkaline earth metal ions, showing a preference for larger ions like Cs⁺. Structural analysis confirms that π‐interactions with benzene rings enhance stability, particularly for large cations that engage both rings. Competitive NMR experiments validate that stability increases with ionic radius, demonstrating MAXCalix's strong affinity for cesium.