Supramolecular capsules, i.e., non-covalently associated molecules forming a capsule-like structure, could be useful in functional materials. A controlled assembly is important for the creation of useful capsules. Thus, understanding interactions stabilizing the capsules and their reversible formation/breaking induced by simple stimuli is useful. Calixarenes, for example, have a bowl-lie shape, are easy to functionalize, and show chemical stability. This makes them valuable building blocks for supramolecular assemblies.

Nikola Bregović, University of Zagreb, Croatia, and colleagues have used urea-carboxylate interactions in order to accomplish a “handshake” between two complementary p–tert-butylcalix[4]arenes in acetonitrile. The high affinity of the prepared compounds towards acetate encouraged them to prepare diacetate-calix[4]arenes and use them as the complementary part of the capsule.

Stable heterodimeric capsules containing ureido- and acetate-calixarenes were formed. The team observed that the “firmest handshake” was achieved between tetraureido-calixarene and simple diacetate-calixarene, leading to practically quantitative association upon mixing the two calixarene counterparts. The researchers also found that control over the (dis)assembly of the calixarene-heterodimer can be achieved simply by adding acid or base.

The concept demonstrated in this work could be applied to the design and fabrication of larger supramolecular polymers featuring specific properties for various tasks, while maintaining strict control over their formation.

• Supramolecular handshakes: characterization of urea‐carboxylate interactions within calixarene frameworks,
  Marija Cvetnić, Nikola Cindro, Edi Topić, Nikola Bregovic, Vladislav Tomišić,
  ChemPlusChem 2024.
  https://doi.org/10.1002/cplu.202400130