The creation of synthetic receptors with high affinity and selectivity is required for the sustainable management of anions. With an increasing global demand for clean water, the development of high-affinity receptors can facilitate new directions in the effective control of waste ions in water. High-affinity anion receptors have traditionally been associated with NH hydrogen bonds, but the recent discovery of attomolar affinity for chloride, using a CH hydrogen bonding triazolo cage has helped to overturn this previous consensus. The elevated affinity of the cage, while being its most impressive asset, inhibits its synthesis in the chloride-free form hindering further study. Moreover, the one-pot synthesis limits the scope of the modularity in the design that can be investigated. Motivated by the need to address these challenges we conducted a retrosynthesis. This analysis led us to an interesting intermediate that served as the starting point for a new and unexpected class of CH hydrogen bond receptors called cyclopods. This receptor is designed by disconnecting one of the cage’s three linkages leaving behind a flexible macrocycle (aka cyclo) with two customizable arms (aka pods). Unlike the parent cage, the cyclopod can be prepared in a modular manner. Solution-phase anion binding studies for the cyclopods showed a binding affinity that was less than the parent cage but greater than a flexible 3D tripod receptor with three arms. The cyclopods affinity was found to be comparable to a rigidly preorganized triazolophane, which does not have any arms. Cyclopods, therefore, lay the groundwork for the development of receptors for ion binding that incorporate rigidity, which is essential for affinity, and flexibility, which affords a high degree of modular control. The modularity in the receptor design will enable further customization and incorporation of functional arms to expand our understanding of properties of this new series of receptors. Specifically we aim to extend the design enabling attachment of a variety of foldable arms to test the role of controllable organization on affinity, selectivity and switching.

Cyclopods: Understanding the structural and recognition properties of a new family of receptors

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ACS Fall 2022

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