Simple Acyclic Molecules Containing a Single Charge-Assisted O–H Group Can Recognize Anions in Acetonitrile:water Mixtures

Hydroxypyridinium and hydroxyquinolinium compounds containing acidic O–H groups attached to a cationic aromatic scaffold were synthesized, i.e. N-methyl-3-hydroxypyridinium (1+) and N-methyl-8-hydroxyquinolinium (2+). These very simple compounds are capable of binding to chloride very strongly in CD3CN and with moderate strength in 9:1 CD3CN:D2O. Comparison with known association constants reveal that 1+ and 2+ bind chloride in CD3CN or CD3CN:D2O with comparable affinities to receptors containing significantly more hydrogen bond donors and/or higher positive charges. Crystal structures of both compounds with coordinating anions were obtained, and feature short O–H∙∙∙anion hydrogen bonds. A receptor containing two hydroxyquinolinium groups was also prepared. While the low solubility of this compound caused difficulties, we were able demonstrate chloride binding in a competitive 1:1 CD3CN:CD3OD solvent mixture. Addition of sulfate to this compound results in the formation of a crystallographically-characterised solid state anion coordination polymer.