Ortho-Substituted Catechol Derivatives:  The Effect of Intramolecular Hydrogen-Bonding Pathways on Chloride Anion Recognition

This paper reports a series of chloride anion receptors containing two catechol head groups connected through their ortho-positions via a spacer chain. The linking group chosen to attach the spacer chain to the catechol units has a major impact on the anion-binding potential of the receptor. Linking groups that are capable of forming stable six-membered intramolecular hydrogen-bonded rings with the catechol O−H groups significantly inhibit the ability of the catechol units to hydrogen bond to chloride anions. However, where the linking groups are only capable of forming five- or seven-membered intramolecular hydrogen-bonded rings, then anion binding via hydrogen bonding through the catechol O−H groups becomes a possibility. This process is solvent dependent; the presence of competitive solvent (e.g., DMSO-d₆) disrupts the intramolecular hydrogen-bonding pattern and enhances anion binding relative to simple unfunctionalized catechol. The most effective receptor is that in which the hydrogen-bonding linker (−CH₂CONH−) is most distant from the catechol units and can only form a seven-membered intramolecular hydrogen-bonded ring. In this case, the receptor, which contains two catechol units, is a more effective chloride anion binder than simple unfunctionalized catechol, demonstrating that the two head groups, in combination with the N−H groups in the linker, act cooperatively and enhance the degree of anion binding. In summary, this paper provides insight into the hydrogen-bonding patterns in ortho-functionalized catechols and the impact these have on the potential of the catechol O−H groups to hydrogen bond to a chloride anion.