Synthesis, characterisation and metal ion binding properties of crown ethers incorporating 4,5-dioxyxanthones

An efficient preparation of 4,5-dihydroxyxanthone, 5, is described, and its incorporation into crown ethers with 18-, 21- and 24-membered macrocyclic arrays, 3 (n = 3, 4 and 5), and a model compound, 4,5-diethoxyxanthone, 10. Metal ion complexation of the crowns causes a strong quenching of the highly fluorescent 4,5-diethoxyxanthone chromophore. Binding constants for complex formation with sodium, potassium, and caesium ions in methanolic solution have been measured by spectrofluorimetric and conductimetric methods and range from <102 for 3 (n = 5) binding sodium ion to 4.1 × 103 for 3 (n = 4) binding potassium ion. The behaviour of the 4,5-dioxyxanthones in aqueous sulfuric acid mixtures has been examined by UV spectroscopy and treatment of the data by the excess acidity method yields –2.6 < pKa < –2.3 for the conjugate acids with 0.33 < m* < 0.40. The behaviour of the crowns containing 4,5-dioxyxanthones is compared with that of analogues containing 1,8-dioxyxanthones.