Diversity in synthetic perchlorate anion receptors: Challenges and opportunities

The present review highlights the synthetic development of structurally diverse receptors for perchlorate oxyanion. The recognition of tetrahedral perchlorate is important as this anion creates fundamental health, environmental, or energy related problems. Owing to its lower charge density and directional oxygen-atom-arrangements, a high degree of design is required to develop a suitable receptor. The neutral receptors (cavitands) which feature deep cavities are proficient in perchlorate binding. The charged receptors which can be obtained by the protonation at specific sites or by forming cationic metal-organic frameworks, equipped with geometrical complementarities are also capable of binding perchlorate and these properties are described throughout the manuscript. Various dipodal, tripodal, macromonocyclic, macrobicyclic, covalent organic cage molecules, and metal-organic cages are presented as perchlorate receptors. The binding properties of the receptor molecules towards perchlorate anion are delineated in both the solid and solution states. In order to understand the binding of perchlorate with the receptor molecules, several non-covalent interactions such as electrostatic attraction, hydrogen bonding, anion-π interaction, and hydrophobic interaction are described along with the solvent effect which is one of the major determining factors in solution state binding. Emphasis is also given to the hard-acid/hard-base pairing while discussing hydrogen bonding. To sum up the findings, a statistical approach has been taken and the data are provided based on the structural frameworks, types, hydrogen-bonding-sites of the various reported receptors and their perchlorate anion binding abilities. Finally, the current trend in the construction of perchlorate receptors and the possibilities of their usefulness in remediating environmental and toxicological problems are narrated.